Lipid Mediators and Their Metabolism in the Brain

Lipid Mediators and Their Metabolism in the Brain

Akhlaq A. Farooqui

Lipid Mediators and Their Metabolism in the Brain

Akhlaq A. Farooqui Department of Molecular and Cellular Biochemistry The Ohio State University Columbus, OH 43210 USA [email protected]

ISBN 978-1-4419-9939-9 e-ISBN 978-1-4419-9940-5 DOI 10.1007/978-1-4419-9940-5 Springer New York Dordrecht Heidelberg London Library of Congress Control Number: 2011934260 © Springer Science+Business Media, LLC 2011 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Dedicated to my teachers for their passion to teach and stimulate the desire to learn and integrate knowledge. Akhlaq A. Farooqui

Preface

Neural membranes are highly dynamic and interactive structures composed of glycerophospholipids, sphingolipids, cholesterol, and transmembrane and peripheral proteins of various shapes, molecular masses, and functions. The binding between phospholipids and proteins is necessary for vertical positioning and tight integration of proteins into the membrane. Phospholipids and sphingolipids contribute to the lipid bilayer asymmetry, whereas cholesterol and sphingolipids form lipid rafts, which act as platforms for molecular sorting, trafficking, and signal transduction processes. Lipid mediators are chemical messengers that are released in response to cell stimulation or injury from membrane phospholipids, sphingolipid, and cholesterol. Lipid mediators play important roles in internal and external communication and modulate cellular responses such as the growth arrest, differentiation, adhesion, and migration. These processes are modulated by eicosanoids (prostaglandins, leukotrienes, thromboxanes, and lipoxins) and docosanoids (resolvins, protectins, neuroprotectins, and maresins), which are generated by the action of phospholipases A2, cyclooxygenases, and lipoxygenases on arachidonic and docosahexaenoic acids (ARA and DHA), respectively. The non-enzymic lipid mediators of ARA and DHA metabolism include isoprostanes, neuroprostanes, isoketals, neuroketals, isofurans, 4-hydroxynonenal, and 4-hydroxyhexanal. Action of sphingomyelinases on sphingomyelin generates ceramide, a metabolite closely associated with apoptotic cell death. Further degradation of ceramide generates sphingosine, which in its phosphorylated form induces cell proliferation, and thus produces the balance between cell death and cell survival. In the brain, cholesterol is hydroxylated into oxycholesterols or hydroxycholesterols (24(S)-hydroxycholesterol, 25-hydroxycholesterol, 27-hydroxycholesterol, 22-hydroxycholesterol) by cytochrome P450dependent oxygenases. Conversion of cholesterol to hydroxycholesterols is a major mechanism for the elimination of cholesterol from the brain. Collective evidence suggests that under normal conditions low levels of lipid mediators are needed for signal transduction, gene expression, and neural cell proliferation and differentiation, resulting in neural cell survival but high levels of enzymic and non-enzymic lipid mediators may cause neurodegeneration through the induction of oxidative stress, neuroinflammation, and apoptosis. Thus, neural membranes are not only vii

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simple inert barrier, but a Pandora’s box of lipid mediators; many of which have powerful neurochemical effects on cell growth, proliferation, differentiation, survival, and apoptosis. Levels of lipid mediators in neural and non-neural tissues are partly regulated by diet. The high intake of food enriched in ARA (vegetable oils) elevates levels of eicosanoids and upregulates the expression of proinflammatory cytokines. ARA and its metabolites have prothrombotic, proaggregatory, and proinflammatory properties. In contrast, diet enriched in DHA (fish and fish oil) generates docosanoids, which not only downregulate proinflammatory cytokines but also have antiinflammatory, antithrombotic, antiarrhythmic, hypolipidemic, and vasodilatory effects. At present, the threshold concentrations of lipid mediators that promote and facilitate neural cell injury and death are not known. In neurological disorders, cell death not only depends upon elevated levels of lipid mediators, but also on cross-talk (interplay) among glycerophospholipid-, glycosphingolipid-, and cholesterol-derived lipid mediators. Studies on lipid-derived mediators fall in a fastpaced research area related to neurodegeneration and provide opportunities for target-based therapeutic intervention using inhibitors of lipid mediator synthesizing enzymes. The goal of this monograph is to present readers with cutting edge and comprehensive information on lipid mediators in a manner that is useful not only to students and teachers, but also to researchers and physicians. This monograph has 11 chapters. Chapters 1 and 2 describe metabolism, roles, and involvement of eicosanoids and docosanoids in neurological disorders. Chapters 3 and 4 describe cutting edge information on the synthesis, degradation, roles, and association of lyso-glycerophospholipids and platelet activating factor in neurological disorders. Chapter 5 describes metabolism and roles of cannabinoids in brain and their relationship with neurological disorders. Chapters 6 and 7 are devoted to the metabolism of nonenzymic lipid mediators of arachidonic acid metabolism (4-hydroxynonenal and isoprostanes) and docosahexaenoic acid metabolism (4-hydoxyhexanal and neuroprostanes). Chapters 8 and 9 describe metabolism, roles, and involvement of ceramide, ceramide-1-phosphate, sphingosine, and sphingosine-1-phosphate, respectively, in neurological disorders. Chapter 10 describes metabolism, role, and association of cholesterol and hydroxycholesterols with neurological disorders. Finally, Chap. 11 provides readers and researchers with perspective that will be important for future research work on bioactive lipid mediators. My writing style and demonstrated ability to present complicated material on lipid mediators makes this monograph particularly accessible to neuroscience graduate students, teachers, and fellow researchers. It can be used as supplement text for a range of neuroscience courses. Clinicians and pharmacologists will find this book useful for understanding molecular aspects of lipid mediators in neurodegeneration in acute neural trauma (stroke, spinal cord trauma, and head injury) and neurodegenerative diseases (Alzheimer disease, Parkinson disease, and Huntington disease). To the best of my knowledge no one has written a monograph on the role of lipid mediators in the brain. This monograph will be the first to provide a comprehensive description of glycerophospholipid, sphingolipid, and cholesterol-derived mediators, their interactions with each others in normal brain and in brain tissue from neurological disorders. I also hope that this

Preface

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monograph will provide senior researchers some guidance for overcoming problems on lipid mediator research that they are encountering in their laboratories. One of the hallmark of this monograph is the presentation of a unifying concept of lipid mediator-mediated signal transduction processes associated with excitotoxicity, oxidative stress, and neuroinflammation. The presentation of this monograph is based on uniformity and logical progression of subject from one topic to another with an extensive bibliography. For the sake of explanation, simplicity, and uniformity a large number of figures and line diagrams of signal transduction pathways with chemical structures of lipid mediators are also presented. It is hoped that my attempt to integrate and consolidate the knowledge of lipid mediators and signal transduction processes in normal and diseased brain will provide the basis of more dramatic advances and developments on the determination, characterization, and roles of glycerophospholipid-, sphingolipid-, and cholesterol-derived lipid mediators in neurological disorders. Columbus, OH

Akhlaq A. Farooqui

Acknowledgments

I thank my wife, Tahira, for critical reading of the manuscript, providing colored figures, and useful discussion during the writing of this monograph. This monograph would not have been possible without her unrelenting support, constructive criticisms, and constant encouragement. I also thank Dr. Wei-Yi Ong, Department of Anatomy, National University of Singapore, Singapore, for many years of collaboration on phospholipases A2, lipid mediators, and their roles in neurodegeneration. Finally, I also express my gratitude to Ann H. Avouris and Melissa Higgs of Springer, New York, for their cooperation, rapid responses to my queries, and professional manuscript handing. It has been a pleasure working with them for many years.

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About My Teachers

Late Professor Bimal Kumar Bachhawat was my Ph.D. advisor and Chief of Neurochemistry Laboratory, Christian Medical College, Vellore, India. Later he became Director of Indian Institute of Chemical Biology, Calcutta, Professor Head, Department of Biochemistry, and Dean Faculty of Interdisciplinary and Applied Sciences, University of Delhi South Campus, New Delhi. He was elected to all of the scientific academies in India (such as Fellow of National Academy of Sciences, Indian Academy of Sciences and Indian National Science Academy). He received numerous national awards and honors, including the Shanti Swarup Bhatnagar award (1962), the Golden Jubilee Medal (1976) in India. He was elected President of Federation of Asian and Oceanian Biochemistry (1983–1985), Indian Society of Biological Chemists (1970–1972 and 1990–1994); and National Organizing Committee, International Union of Biochemistry and Molecular Biology (1994). Late Professor Bachhawat was a leader, organizer, and teacher, who nurtured a generation of biochemists not only in India, but around the world. He was a man of many admirable qualities, known for his keen interest in others with great sense of humor. Late Professor Bachhawat and his colleagues discovered that metachromatic leukodystrophy is a glycolipid storage disease that is caused by the deficiency of arylsulfatase A. This pathbreaking study set the stage for the elucidation of enzymic defects in other glycolipid storage disorders, including Gaucher’s disease and Tay–Sachs disease. He pioneered the development of carbohydrate-bearing liposomes for the site-specific delivery of drugs and enzymes to diseased organs. He was also involved in the development of liposomal formulations for treating systemic fungal infections. Professor Abdul Majid Siddiqi taught me biochemistry during my M.S. classes. He is a retired Professor and Chairman of Biochemistry Department, and first Dean of the Faculty of Life Sciences at Aligarh Muslim University (AMU), Aligarh. During a long career at AMU, Professor Siddiqi taught Biochemistry to thousands of B.Sc. and M.Sc. students and mentored 39 M.Phil. and 40 Ph.D. students, who now hold important academic positions in India and elsewhere. His research at AMU showed for the first time the utility of inhibitors of cholesterol biosynthesis, xiii

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About My Teachers

such as 3-hydroxy-3-methylglutaric acid (HMG) for treating hyperlipidemia. For this work he was awarded US Patent 36,29,44,9 titled “Process of Combating Hypercholesterolemia” in 1971, the global rights for which were taken by M/S Calbiochem-Behring Corporation, USA. Professor Siddiqi is an active member of the Society of Biological Chemists (India) and an active participant in various national and international groups on Biochemical Education. Professor V.P. Jaiswal, Retired Principal, Government Postgraduate College, Department of Higher Education, Uttar Pradesh, India, has a devoted academic, professional, and administrative career. After achieving a high merit at postgraduate level in Zoology from University of Allahabad (1956), Professor V.P. Jaiswal was appointed lecturer in Zoology at St. Andrews College Gorakhpur (Gorakhpur University, Gorakhpur), Govt. Raza Degree College Rampur (U.P.), where he taught me zoology and put me to the path of higher studies in biological sciences. He was promoted to Professor and Head of Zoology Department at Birla Government College, Srinagar, Garhwal, Government College, Pithoragarh, Gopeshwar (chamoli), Government P.G. College, Rampur, and Government P.G. College Kotdawara, Garhwal, where he was appointed as a Principal. In the year 1990 he retired as Principal, P.G. College, Hamirpur, (Bundelkhand University) where in his tenure he was a member of Executive Council Bundelkhand University, Jhansi (U.P.), India. At present he is an active member of Bombay Natural History Society, Bombay, India. Professor Mahdi Hasan is a well-known neuroscientist, neurotoxicologist and a neurogerontologist, who has inspired and mentored me in studies on neurological disorders. He has been teaching human anatomy to medical students for over 52 years and some 10,000 of his former students are globally active and productive. His publications include 135 research papers, 6 books and 14 book reviews. He is a recipient of India’s most prestigious research awards, Dr. B.C. Roy National Award for eminent teacher, Dr. Bachhawat Life Time Achievement Award of Indian Academy of Neurosciences. He is a Fellow of Alexander von Humboldt Foundation, International College of Surgeons, National Academy of Medical Sciences (India) & Indian National Science Academy (INSA). Currently, he is an Emeritus Professor and INSA Honorary Scientist at King George Medical University, Lucknow (India).

About the Author

Akhlaq A. Farooqui  is a leader in the field of signal transduction, brain phospholipases A2, bioactive ether lipid metabolism, polyunsaturated fatty acid metabolism, glycerophospholipid-, sphingolipid-, and cholesterol-derived lipid mediators, and glutamate-induced neurotoxicity. Akhlaq A. Farooqui has discovered the stimulation of plasmalogen-selective phospholipase A2 (PlsEtn-PLA2) and diacyl- and monoacylglycerol lipases in brains from patients with Alzheimer disease. Stimulation of PlsEtn-PLA2 produces plasmalogen deficiency and increases levels of eicosanoids that may be related to the loss of synapses in brains of patients with Alzheimer disease. Akhlaq A. Farooqui has published cutting edge research on the generation and identification of glycerophospholipid-, sphingolipid-, and cholesterol-derived lipid mediators in kainic acid-mediated neurotoxicity by lipidomics. Akhlaq A. Farooqui has authored six monographs: Glycerophospholipids in Brain: Phospholipase A2 in Neurological Disorders (2007); Neurochemical Aspects of Excitotoxicity (2008); Metabolism and Functions of Bioactive Ether Lipids in Brain (2008); and Hot Topics in Neural Membrane Lipidology (2009); Beneficial Effects of Fish Oil in Human Brain (2009); and Neurochemical Aspects of Neurotraumatic and Neurodegenerative Diseases (2010). All monographs are published by Springer, New York. In addition, Akhlaq A. Farooqui has edited four books (Biogenic Amines: Pharmacological, Neurochemical and Molecular Aspects in the CNS (2010) Nova Science Publisher, Hauppauge, N.Y, Molecular Aspects of Neurodegeneration and Neuroprotection, Bentham Science Publishers Ltd (2011); Phytotherapeutics and Human Health: Molecular and pharmacological Aspects, Nova Science Publisher, Hauppauge, N.Y In Press (2011) and Molecular Aspects of Oxidative Stress on Cell Signaling in Vertebrates and Invertebrates, Wiley Blackwell Publishing Company, New York in press, 2011).

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Contents

  1  Metabolism and Roles of Eicosanoids in Brain..................................... 1.1 Introduction....................................................................................... 1.2  Multiplicity of Cyclooxygenases, Lipoxygenases, and Epoxygenases in the Brain......................................................... 1.2.1 Cyclooxygenases (COXs)..................................................... 1.2.2 Lipoxygenases (LOXs)......................................................... 1.2.3 Cytochrome P450 Epoxygenases (EPOXs).......................... 1.3 Eicosanoids and Their Receptors in Brain........................................ 1.4  Interplay Among COX, LOX, and EPOX-derived Products, and Relationship to Upstream PLA2 Isoforms.................................. 1.5 Roles of Eicosanoids in the Brain..................................................... 1.5.1 Eicosanoids in Neuroinflammation....................................... 1.5.2 Eicosanoids in Neurodegeneration........................................ 1.5.3 Eicosanoids in Nociception (Pain State)............................... 1.5.4 Eicosanoids in Synaptic Plasticity........................................ 1.6 Eicosanoids in Neurotraumatic Diseases.......................................... 1.6.1 Eicosanoids in Ischemic Injury............................................. 1.6.2 Eicosanoids in Spinal Cord Trauma...................................... 1.6.3 Eicosanoids in Traumatic Brain Injury................................. 1.6.4 Eicosanoids in Epilepsy........................................................ 1.7 Eicosanoids in Neurodegenerative Diseases..................................... 1.7.1 COX, LOX, EPOX, and Eicosanoids in AD......................... 1.7.2 COX, LOX, EPOX, and Eicosanoids in PD.......................... 1.7.3  COX, LOX, EPOX, and Eicosanoids in Amyotrophic Lateral Sclerosis.................................................................... 1.7.4  COX, LOX, EPOX, and Eicosanoids in Creutzfeldt-Jakob Disease (CJD)...................................... 1.8 Conclusions....................................................................................... References..................................................................................................

1 1 4 6 11 13 15 22 25 25 27 28 28 29 30 31 31 32 32 33 35 35 36 37 37

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Contents

  2  Recent Development on the Neurochemistry of Docosanoids.............. 2.1 Introduction....................................................................................... 2.2 Importance of n-3 Polyunsaturated Fatty Acids in the Brain........... 2.3 EPA-Derived Lipid Mediators in the Brain...................................... 2.4 DHA-Derived Lipid Mediators in the Brain..................................... 2.4.1 17S D Series Resolvins......................................................... 2.4.2 Protectins and Neuroprotectin............................................... 2.5 Conclusion........................................................................................ References..................................................................................................   3  Metabolism, Roles, and Involvement of Lyso-glycerophospholipids in Neurological Disorders........................................................................ 3.1 Introduction....................................................................................... 3.2  Deacylation/Reacylation Cycle (Land’s Cycle) and Its Importance............................................................................. 3.2.1  Acyl-CoA Synthetases in the Brain...................................... 3.2.2  Acyl-CoA: Lyso-phospholipid Acyltransferase in the Brain............................................................................ 3.2.3 Phospholipases A2 (PLA2) in the Brain................................. 3.2.4  Long-Chain Acyl-CoA Hydrolase or Thioesterases in the Brain............................................................................ 3.2.5 CoA-independent Reacylation in the Brain.......................... 3.3  Effects of Lysophospholipids on Neural Membrane Metabolism........................................................................................ 3.3.1 Lysophosphatidylcholine (Lyso-PtdCho).............................. 3.3.2 Lyso-phosphatidylethanolamine (Lyso-PtdEtn).................... 3.3.3 Lyso-phosphatidylserine (Lyso-PtdSer)................................ 3.3.4 Lyso-phosphatidylinositol (Lyso-PtdIns).............................. 3.3.5 Lyso-cardiolipin (Lyso-Ptd2Gro)........................................... 3.3.6  Lyso-ethanolamine and Choline Plasmalogens (Lyso-PlsEtn and Lyso-PlsCho)............................................ 3.3.7 Lyso-phosphatidic Acid (Lyso-PtdH)................................... 3.4  Lyso-phospholipids in Neurotraumatic and Neurodegenerative Diseases............................................................................................. 3.4.1 Lyso-phospholipids in Ischemic Injury................................. 3.4.2 Lyso-phospholipids in Alzheimer Disease............................ 3.4.3 Lyso-phospholipids in MS and EAE..................................... 3.4.4 Lyso-phospholipids in NCL.................................................. 3.5 Conclusion........................................................................................ References..................................................................................................

49 49 50 52 58 60 60 68 68 73 73 74 75 77 78 80 81 82 82 85 86 87 88 89 90 93 94 95 96 97 97 97

Contents

  4  Platelet-Activating Factor in Brain: Its Metabolism, Roles, and Involvement in Neurological Disorders.......................................... 4.1 Introduction....................................................................................... 4.2 PAF Biosynthesis in the Brain.......................................................... 4.2.1 De Novo Synthesis of PAF.................................................... 4.2.2 Remodeling Pathway for PAF Synthesis.............................. 4.2.3 Oxidative Fragmentation Pathway for PAF Synthesis.......... 4.3 Catabolism of PAF in the Brain........................................................ 4.4 PAF Receptors in the Brain............................................................... 4.5 PAF in Neurological and Visceral Disorders.................................... 4.5.1 PAF in Neurological Disorders............................................. 4.5.2 PAF in Visceral Disorders..................................................... 4.5.3 PAF in Kainic Acid Neurotoxicity........................................ 4.6  Molecular Mechanism Associated with PAF-Mediated Neural Injury..................................................................................... 4.7 Conclusion........................................................................................ References..................................................................................................   5  Cannabinoids in the Brain: Their Metabolism, Roles, and Involvement in Neurological Disorders.......................................... 5.1 Introduction....................................................................................... 5.2 Cannabinoid Receptor-Mediated Signaling in the Brain.................. 5.3 Occurrence and Synthesis of Endocannabinoids in the Brain.......... 5.3.1 Metabolism of 2-Arachidonylglycerol in the Brain.............. 5.3.2 Metabolism of Anandamide in the Brain.............................. 5.4  Interplay Among Cannabinoid, Glutamate, and Dopamine Receptors in the Basal Ganglia................................. 5.5 Endocannabinoids and Neurological Disorders................................ 5.5.1 Endocannabinoids in Neurotraumatic Diseases.................... 5.5.2 Endocannabinoids in Neurodegenerative Disorders............. 5.6 Use of FAAH and MGL Inhibitors for Inflammatory Pain............... 5.7 Conclusion........................................................................................ References..................................................................................................   6  Neurochemical Aspects of 4-Hydroxynonenal...................................... 6.1 Introduction....................................................................................... 6.2 Synthesis of 4-HNE in the Brain...................................................... 6.3 Metabolism of 4-HNE in the Brain................................................... 6.4 Effect of 4-HNE on Enzyme Activities............................................ 6.4.1 Modulation of Kinases by 4-HNE........................................ 6.4.2 Modulation of Caspases by 4-HNE...................................... 6.4.3  Modulation of Glutathione S-Transferases by 4-HNE..............................................................................

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107 107 108 109 111 114 114 116 118 118 123 124 124 125 126 133 133 136 137 139 142 143 144 145 147 149 150 151 159 159 160 161 163 164 166 167

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6.4.4 Modulation of ATPases by 4-HNE.................................... 6.4.5 Modulation of Other Enzymes and Cell Cycle by 4-HNE........................................................................... 6.4.6 Modulation of Heat Shock Response by 4-HNE............... 6.5 Modulation of NMDA Receptor by 4-HNE................................... 6.6 Modulation of Gene Expression by 4-HNE.................................... 6.7 Modulation of Peroxisomes by 4-HNE........................................... 6.8 Modulation of Nucleic Acid Metabolism by 4-HNE...................... 6.9 Modulation of Phospholipid Metabolism by 4-HNE...................... 6.10 Modulation of Blood–Brain Barrier by 4-HNE.............................. 6.11 4-HNE in Neurological Disorders.................................................. 6.11.1 4-HNE in Alzheimer Diseases........................................... 6.11.2 4-HNE in Parkinson Disease............................................. 6.11.3 4-HNE in Amyotrophic Lateral Sclerosis......................... 6.11.4 4-HNE in Prion Diseases................................................... 6.11.5 4-HNE in Ischemia............................................................ 6.11.6 4-HNE in Spinal Cord Injury............................................ 6.11.7 4-HNE in Traumatic Brain Injury..................................... 6.11.8 4-HNE in Kainic Acid Neurotoxicity................................ 6.12 4-Hydroxyhexenal (4-HHE)........................................................... 6.13 Conclusion...................................................................................... References..................................................................................................   7  Isoprostanes and Other Nonenzymic Polyunsaturated Fatty Acid-Derived Lipid Mediators in the Brain................................ 7.1 Introduction..................................................................................... 7.2 Generation of IsoPs in the Brain..................................................... 7.3 Degradation of IsoPs in the Brain................................................... 7.4 IsoPs and Signal Transduction Processes....................................... 7.5 IsoPs in Neurodegenerative Diseases.............................................. 7.6 IsoPs in Neurotraumatic Diseases................................................... 7.7 NPs in Neurological Disorders....................................................... 7.8 Isoketals (Isolevuglandins) in Neurological Disorders................... 7.9 Neuroketals in Neurological Disorders........................................... 7.10 Isofurans in Neurological Disorders............................................... 7.11 Neurofurans in Neurological Disorders.......................................... 7.12 IsoPs in Kainic Acid Neurotoxicity................................................ 7.13 Conclusion...................................................................................... References..................................................................................................   8  Ceramide and Ceramide 1 Phosphate in the Brain.............................. 8.1 Introduction..................................................................................... 8.2 Synthesis of Ceramide and Ceramide 1-phosphate in the Brain...................................................................................... 8.3 Degradation of Ceramide and Ceramide 1-phosphate in the Brain......................................................................................

169 170 170 171 172 173 173 175 175 176 176 177 178 179 179 180 181 183 183 184 184 193 193 194 195 198 202 204 204 206 207 208 209 209 210 211 217 217 221 227

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8.4 Roles of Ceramide and Ceramide 1-phosphate in the Brain........... 8.5 Ceramide in Neurological Disorders............................................... 8.5.1 Ceramide in Ischemic Injury............................................. 8.5.2 Ceramide in Alzheimer Disease........................................ 8.5.3 Ceramide in Parkinson Disease......................................... 8.5.4 Ceramide in Amyotrophic Lateral Sclerosis..................... 8.5.5 Ceramide in Multiple Sclerosis......................................... 8.5.6 Ceramide in HIV-1............................................................ 8.5.7 Ceramide in Batten Disease............................................... 8.5.8 Ceramide in Major Depressive Disorders......................... 8.5.9 Ceramide in Kainic Acid Neurotoxicity............................ 8.6 Conclusion...................................................................................... References..................................................................................................

228 229 231 232 234 234 235 235 236 236 236 237 238

  9  Sphingosine and Sphingosine 1 Phosphate in the Brain...................... 9.1 Introduction..................................................................................... 9.2 Sphingosine Kinases in the Brain................................................... 9.3 Sphingosine 1 Phosphate Receptors in the Brain............................ 9.4 Sphingosylphosphorylcholine in the Brain..................................... 9.5 FTY720 and Its Neurochemical Effects.......................................... 9.6 Sphingosine 1 Phosphate in Neurological Disorders...................... 9.6.1 Sphingosine 1 Phosphate in Ischemia............................... 9.6.2 Sphingosine 1 Phosphate in Traumatic Brain and Spinal Cord Injuries.................................................... 9.6.3 Sphingosine 1 Phosphate in Alzheimer Disease............... 9.6.4 Sphingosine 1 Phosphate in Multiple Sclerosis................. 9.6.5 Sphingosine 1 Phosphate in Kainic Acid Neurotoxicity..................................................................... 9.7 Conclusion...................................................................................... References..................................................................................................

245 245 248 251 254 255 257 257

10  Cholesterol and Hydroxycholesterol in the Brain................................. 10.1 Introduction..................................................................................... 10.2  Synthesis of Cholesterol and Hydroxycholesterols in the Brain...................................................................................... 10.3 Degradation of Cholesterol in the Brain......................................... 10.4 Roles of Hydroxycholesterols in the Brain..................................... 10.4.1 Hydroxycholesterols in Neural Cell Differentiation......... 10.4.2 Hydroxycholesterols in Exocytosis................................... 10.4.3 Hydroxycholesterols in Apoptosis.................................... 10.5  Cholesterol and Hydroxycholesterols in Neurological Disorders......................................................................................... 10.5.1  Cholesterol and Hydroxycholesterol in Alzheimer Disease......................................................... 10.5.2  Cholesterol and Hydroxycholesterol in Parkinson Disease...............................................................................

267 267

259 259 260 261 261 262

269 275 277 278 279 279 281 282 284

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10.5.3  Cholesterol and Hydroxycholesterol in Huntington Disease............................................................................... 10.5.4  Cholesterol and Hydroxycholesterols in Niemann-Pick Type C................................................................................ 10.5.5  Cholesterol and Hydroxycholesterol in Multiple Sclerosis............................................................................. 10.5.6 Hydroxycholesterol in Traumatic Brain Injury................. 10.5.7  Cholesterol and Hydroxycholesterols in Cerebrotendinous Xanthomatosis.................................. 10.6  Cholesterol and Hydroxycholesterols in Kainic Acid Neurotoxicity.......................................................................... 10.7 Conclusion...................................................................................... References.................................................................................................. 11  Perspective and Direction for Future Studies on Lipid Mediators.................................................................................. 11.1 Introduction..................................................................................... 11.2 Lipid Mediators in the Brain........................................................... 11.3  Interactions Among Phospholipid-, Sphingolipid-, and Cholesterol-Derived Lipid Mediators in the Brain................... 11.4  Detection and Levels of Lipid Mediators in Neurological Disorders by Lipidomics................................................................. 11.5 Modulation of Lipid Mediators by Diet.......................................... 11.6 Conclusion...................................................................................... References..................................................................................................

285 286 287 288 288 289 290 291 299 299 300 303 306 308 309 310

Index.................................................................................................................. 315

Abbreviations

AD ALS ARA

Alzheimer disease Amyotrophic lateral sclerosis Arachidonic acid

BDNF

Brain-derived neurotrophic factor

C1P Cer COX

Ceramide 1 phosphate Ceramide Cyclooxygenase

DHA

Docosahexaenoic acid

EPOX

Epoxygenase

HD

Huntington disease

Ins-1,4,5-P3

Inositol-1,4,5-trisphosphate

LOX LTs LX

Lipoxygenase Leukotrienes Lipoxin

NPD1

Neuroprotectin D1

PD PGs PKC PLA2 PLC PLD PlsCho PlsEtn PtdCho PtdEtn PtdH

Parkinson disease Prostaglandins Protein kinase C Phospholipase A2 Phospholipase C Phospholipase D Choline plasmalogen Ethanolamine plasmalogen Phosphatidylcholine Phosphatidylethanolamine Phosphatidic acid xxiii

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Abbreviations

PtdIns PtdIns(4,5)P2 PtdIns4P PtdSer

Phosphatidylinositol Phosphatidylinositol 4,5-bisphosphate Phoshatidylinositol 4-phosphate Phosphatidylserine

ROS RvD1 RvD2 RvE1 RvE2

Reactive oxygen species Resolvin D1 Resolvin D2 Resolvin E1 Resolvin E2

S1P Sph

Sphingosine 1 phosphate Sphingosine

TXs

Thromboxanes

Chapter 1

Metabolism and Roles of Eicosanoids in Brain

1.1 Introduction Eicosanoids are signaling molecules generated through enzymic oxidation of arachidonic acid (ARA, 20:4n-6) by cyclooxygenases (COX-1 and 2) (O’Banion, 1999; Vane et  al., 1998; Phillis et  al., 2006), lipoxygenases (LOX) (Kuhn and O’Donnell, 2006; Kim et al., 2008), and epoxygenases (EPOX) (Phillis et al., 2006; Spector, 2009). Eicosanoids include prostaglandins (PGs), leukotriene (LTs), lipoxins (LXs), and thromboxanes (TXs), as well as hydroxyeicosatetraenoic acids (HETEs) and epoxyeicosatetraenoic acids (EETs), and dihydroxyeicosatrienoic acids (DHETs) (Figs. 1.1 and 1.2). Numerous eicosanoids have been detected in the nervous system in neurons, astrocytes, cerebral vascular endothelial cells, and cerebrospinal fluid (O’Banion, 1999; Schaad et al., 1991; Simmons et al., 2004; Toda and Okamura, 1993; Vila, 2004; Werz, 2002; Wolfe and Horrocks, 1994). Eicosanoids produce a wide range of biological actions including potent effects on inflammation, vasodilation, vasoconstriction, apoptosis, and immune responses. Neural membrane phospholipids are enriched in ARA and docosahexaenoic acid (DHA, 22:6n-3), which are exclusively located at the sn-2 position of glycerol moiety. ARA is released mainly by the action of cytosolic phospholipase A2 (cPLA2) (Farooqui et  al., 2000a), and DHA is hydrolyzed by plasmalogen-selective-­ phospholipase A2 (PlsEtn-PLA2) (Hirashima et al., 1992; Farooqui, 2010a) (Fig. 1.3). Free ARA is either reincorporated in neural membrane glycerophospholipids by reacylation reactions or oxidized by enzymic and nonenzymic mechanisms into various oxygenated metabolites (Farooqui et al., 2000a, b; Lee et al., 2004; Rapoport, 1999), which play important roles, not only in regulating signal transduction and gene transcription processes, but also in inducing and maintaining the acute inflammatory responses, oxidative stress, and neurodegeneration (Wolfe and Horrocks, 1994). In contrast, DHA is metabolized by 15-LOX-like enzyme into resolvins and neuroprotectins. These metabolites are collectively known as docosanoids. They not only antagonize the effects of eicosanoids (Fig. 1.3), but also modulate leukocyte

A.A. Farooqui, Lipid Mediators and Their Metabolism in the Brain, DOI 10.1007/978-1-4419-9940-5_1, © Springer Science+Business Media, LLC 2011

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OH COOH

COOH OH

Fig. 1.1  Chemical structures of prostaglandins, leukotrienes, and lipoxins. PGE2 (a), PGF2a (b), LTB4 (c), LTA5 (d), LXA4 (e) LXB4 (f), and epi-LXA4 (g)

trafficking as well as downregulating expression of cytokines (Hong et al., 2003; Marcheselli et al., 2003). ARA releasing and oxidizing enzymes (PLA2, COX, LOX, and EPOX) occur in several isoforms in brain tissue. Although little is known about the interplay among the isoforms of PLA2, COX, LOX, and EPOX enzymes, but it is becoming increasingly evident that interactions between cPLA2 and sPLA2 and among glutamate, cannabinoid, and dopamine receptors take place during prostaglandin synthesis in neural and non-neural cells (Shinohara et  al., 1999; Phillis et al., 2006). It is likely that eicosanoid synthesis through COX, LOX, and EPOX enzymes may involve different pools of ARA that may be coupled to distinct PLA2 isoforms at different cellular and subcellular levels (Farooqui et  al., 2006; Ueno et al., 2001). The action of PLA2 on phospholipid also generates 1-alkyl-2-lyso-sn-glycero-3phosphocholine. This metabolite is the immediate precursor for platelet activating factor (PAF), a potent inflammatory mediator (Fig. 1.3). PAF not only modulates transcription factors and gene expression, but is also involved in stimulation and modulation of PLA2, PLC, and PLD, and COX activities (Farooqui et al., 2008). Besides generating eicosanoids, COX, LOX, and EPOX-catalyzed reactions also produce reactive oxygen species (ROS). ROS include oxygen-free radicals

1.1  Introduction

3

O

O

O

O

OH

OH CH3

CH3

8,9-EET

5,6-EET O

OH

OH

CH3

CH3 O

O

11,12-EET OH

OH

14,15-EET O

OH

OH

O

OH

OH

CH3

5,6-DHET

CH3

8,9-DHET

Fig. 1.2  Chemical structures of epoxyeicosatetraenoic acids (EETs)

(superoxide radicals, hydroxyl, and alkoxyl radicals), and peroxides (hydrogen peroxide and lipid hydroperoxide). At low levels, ROS function as signaling intermediates in the regulation of fundamental cell activities such as growth and adaptation responses. At higher concentration, ROS contribute to neural membrane damage when the balance between reducing and oxidizing (redox) forces shifts toward oxidative stress (Fig.  1.3). The other biological targets of ROS may be membrane proteins, unsaturated lipids, and DNA (Berlett and Stadtman, 1997). The reaction between ROS and proteins or unsaturated lipids in the plasma membrane leads to a chemical cross-linking of membrane proteins and lipids and a reduction in membrane unsaturation. The depletion of unsaturation in membrane lipids is associated with decreased membrane fluidity and decreased activity of membrane-bound enzymes, ion-channels, and receptors (Farooqui et al., 2008). Nonenzymic peroxidation of ARA and DHA produces 4-hydroxynonenals (4-HNE) and 4-hydroxyhexenal (4-HHE), respectively. These reactive aldehydes are important mediators of brain damage because they form covalent bonds with lipids, proteins, and nucleic acids producing disruption of important cellular functions (Esterbauer et al., 1991; Farooqui and Horrocks, 2006). In addition, free radicalmediated nonenzymic oxidation of ARA produces isoprostanes (Roberts et al., 1998),

4

1  Metabolism and Roles of Eicosanoids in Brain Ca2+

Extracellular

A2

A1

R1

PlsEtn

PtdCho

R2

Lyso-PtdCho + Cox-1 & 2 PGE2

PAF

+

cPLA2

PM

PlsEtn-PLA 2

ARA

LXs

DHA +

15-LOX 15-LOX

n tio yla ac Re

Reacylation

Intracellular

Lyso-PlsEtn

15-LOX

5-LOX LTs

EETs

RESOLVINS

-

NEUROPROTECTINS

+

NEUROINFLAMMATION

+

RESOLUTION NEUROINFLAMMATION

-

NEURODEGENERATION

Fig. 1.3  Hypothetical diagram showing the interactions among ARA and DHA-derived enzymic and nonenzymic products. Agonist (A1 and A2), receptors (R1 and R2), plasma membrane (PM), phosphatidylcholine (PtdCho), ethanolamine plasmalogen (PlsEtn), lysophosphatidylcholine (Lyso-PtdCho), ethanolamine lysoplasmalogen (Lyso-PlsEtn), platelet activating factor (PAF), calcium (Ca2+), cytosolic phospholipase A2 (cPLA2), plasmalogen-selective phospholipase A2 (PlsEtnPLA2), cyclooxygenase-1 and 2 (COX-1 and 2), 4-hydroxynonenal (4-HNE), and reactive oxygen species (ROS)

isofurans and isoketals, whereas nonenzymic lipid mediators of DHA include neuroprostanes, neurofurans, and neuroketals (Bazan, 2009; Farooqui and Horrocks, 2007). All these mediators are reliable indices of oxidative stress in vivo.

1.2 Multiplicity of Cyclooxygenases, Lipoxygenases, and Epoxygenases in the Brain Cyclooxygenase (COX) catalyzes the conversion of ARA into prostaglandin G2 (PGG2) followed by the hydroperoxidation to prostaglandin H2 (PGH2), which is subsequently converted to PGs (PGE2, PGI2, PGF2a), and thromboxane A2 (Fig. 1.4). PGs are involved in many processes including fever, sensitivity to pain, sleep, inflammation, and oxidative stress, and are the target of aspirin-like drugs. LOXs catalyze the biosynthesis of LTs and LXs, from ARA (Fig. 1.5), and cytochrome P450 (CYP) epoxygenases convert ARA into four EET regioisomers, 5,6-, 8,9-, 11,12-, and 14,15-EET (Fig. 1.6).

Agonist PGE2

Receptor

PtdIns-4, - 5-P - 2

PLC

Ca2+

InsP3

EP

PtdCho

G

cPLA2

+

DAG lipase DAG

Lyso-PtdCho

AC cAMP

ARA COX TXA2

+

TXB2

2-AG

Intracellular signaling

PAF

PGH2

PGE2

PGD2

PGI2

PKA PGF2

Intracellular signaling

2+

Mobilization of Ca from intracellular stores

PKC

ATP

Autocrine/paracrine signaling

CELLULAR RESPONSE

Fig. 1.4  Generation of prostaglandins from ARA and their interactions with EP receptors. Phosphatidylcholine (PtdCho), lysophosphatidylcholine (Lyso-PtdCho), cytosolic phospholipase A2 (cPLA2), arachidonic acid (ARA), cyclooxygenase (COX), phospholipase C (PLC), diacylglycerol (DAG), protein kinase C (PKC), Phosphatidylinositol 4,5-bisphosphate (PtdIns 4,5-P2), inositol 1,4,5-trisphosphate (InsP3), protein kinase A (PKA), adenylate cyclase (AC), cyclic-AMP (cAMP), 2-arachidonylglycerol (2-AG)

PtdCho

cPLA2

Lyso-PtdCho

LXA4

LXB4

15-HPETE 15-

PtdIns-4,5-P - 2

2+

Ca

+

PLC

DAG-lipase

5-LOX

OX

-L

15

InsP3 DAG

ARA

2-AG

15(S)-HETE

Receptor

A

12-LOX

12-HPETE

5-HPETE H2O PKC LTA4

LTB4 Glutathione

LTC4 LTC4

+

12-HETE Glutamate LTE4

LTD4

CELLULAR RESPONSE

Fig. 1.5  Generation of leukotrienes from ARA. Phosphatidylcholine (PtdCho), lysophosphatidylcholine (Lyso-PtdCho), cytosolic phospholipase A2 (cPLA2), arachidonic acid (ARA), lipoxygenase (LOX), hydroxyeicosatetraenoic acid (HETE), phospholipase C (PLC), diacylglycerol (DAG), protein kinase C (PKC), Phosphatidylinositol 4,5-bisphosphate (PtdIns 4,5-P2), inositol 1,4,5trisphosphate (InsP3), and 2-arachidonylglycerol (2-AG)

6

1  Metabolism and Roles of Eicosanoids in Brain

Glycerophospholipid

cPLA2

Omega-terminal Omega- terminal HETES HETES

ARA

Receptor

A

Ca2+

Midchain HETES

CYP2C CYP2J BETA OXIDATION

AUTOCRINE FUNCTION Modulation of : Receptors Ion channels Signal transduction Transcription factors Regulatory proteins

EET

ELONGATION

Epoxide hydrolase

PARACRINE FUNCTION

DHET

Fig. 1.6  Generation of epoxyeicosatetraenoic acids from ARA. Cytosolic phospholipase A2 (cPLA2), arachidonic acid (ARA), epoxygenases (CYP2C and CYP2J), cytochrome P450 (CYP), hydroxyeicosatetraenoic acid (HETE), epoxyeicosatetraenoic acids (EETs), and dihydroxyeicosatrienoic acids (DHETs)

1.2.1 Cyclooxygenases (COXs) COXs are heme-containing bifunctional N-glycosylated proteins that catalyze the synthesis of eicosanoids. COXs contain two catalytic centers. COX center facilitates the bisoxygenation and cyclization of ARA to form the hydroperoxy arachidonate metabolite PGG2 and the other peroxidase center that reduces PGG2 to PGH2. Detailed mechanistic studies have indicated that a carbon-centered pentadienyl ­radical at C-11 and a carbon-centered radical at C-8 as the two radical species may lead to endoperoxide formation during the COX center-catalyzed reaction and a tyrosyl radical derived from Tyr385 during the peroxidase center-catalyzed ­reaction. Both activities are functionally coupled with each other. The overall reaction is ­initiated by the oxidation of the heme group of the peroxidase reaction by low levels of a hydroxyperoxide (PGG2) to generate a radical intermediate that start with the COX reaction (Jiang et al., 2004). PGH2 is a precursor to several prostaglandins, ­thromboxanes (TXA2), and prostacyclins (PGI2) (Fig. 1.4). Protein turnover experiments demonstrate that COX-2 is much more susceptible to degradation than COX-1 (Mbonye et  al., 2008). In murine NIH/3T3 fibroblasts, COX-2 has a half-life of approximately 2 h as a result of degradation by the cytosolic 26S proteasome under conditions in which COX-1 is very stable (t1/2 > 12  h) (Mbonye et  al., 2006). The  degradation of COX-2 protein occurs through two independent pathways.

1.2  Multiplicity of Cyclooxygenases, Lipoxygenases, and Epoxygenases in the Brain

7

One  pathway begins by post-translational N-glycosylation at Asn-594. The N-glycosyl group is then ­processed, and the protein is translocated to the cytoplasm, where it undergoes ­proteasomal degradation. Post-translational N-glycosylation of Asn-594 is modulated by a 27-amino acid instability motif (27-IM). The second pathway for COX-2 protein degradation starts with substrate-dependent suicide inactivation. Suicide-inactivated protein is then catabolized. Although the biochemical steps and mechanisms have not been determined, but substrate-dependent break down is not inhibited by proteasome inhibitors or inhibitors of lysosomal proteases (Mbonye et  al., 2008; Wada et  al., 2009). Based on detailed investigation, it is ­suggested that the 27-IM occurs at a constant rate, whereas degradation through the substrate-dependent process is coupled with the rate of substrate turnover (Mbonye et al., 2008). Three forms of COX enzymes (COX-1, COX-2, and COX-3) have been described in brain. COX-1 is constitutively expressed in brain, and is responsible for the physiological production of prostaglandins (Phillis et al., 2006). It is involved in several homeostatic processes and therefore called a “housekeeping” enzyme. Very low levels of COX-2 are detectable in brain, but this enzyme is induced by inflammatorymediators such as IL-1a/b, TNF-a, IFN-g, lipopolysaccharide (LPS), epidermal growth factor, platelet-derived growth factor, fibroblast growth factor, and oncogenes (v-src and v-ras), (Hoffmann, 2000). COX-1 and COX-2 resemble each other in structure and amino acid sequence. Both isoforms are homodimers. They transform ARA into PG and contain similar active sites. However, there are two important structural differences between COX-1 and COX-2. The active site of COX-2 is larger and more accommodating than that of COX-1 and COX-1 displays negative allosterism at low concentrations of ARA. This property may be responsible for greater eicosanoid production by COX-2 when ARA concentration is low. ARA is the preferred substrate for COX-1 and COX-2. However, these enzymes also oxidize g-linolenic acid and eicosapentaenoic acid (EPA) (Smith and Song, 2002; Serhan et  al., 2004). Thus, enzymic oxidation of EPA by COX-2 produces the 3-series of prostaglandins and thromboxanes and the 5-series of leukotrienes. These eicosanoids have different biological properties than the corresponding analogs produced by the metabolism of ARA. For example, TXA3 is less active than TXA2 in aggregating platelets and constricting blood vessels (Calder and Grimble, 2002; James et al., 2000). COX-1 and COX-2 have been cloned (DeWitt and Smith, 1988; Yokoyama and Tanabe, 1989). The COX-2 gene is 8.3 kb, whereas the COX-1 gene is much larger (22 kb) (Vane et al., 1998) (Table 1.1). The COX-2 gene contains several regulatory sites, such as the cyclic AMP response element, IL-6 response element, AP-2, nuclear factor-kB (NF-kB), Sp-1, PEA-3, GATA-1, and glucocorticoid response element (Wu, 1995). In contrast, the COX-1 gene contains Sp-1, Ap-2, NF-IL-6, GATA-1, and a shear stress response element sites. The location of these sites differs considerably in COX-1 gene than COX-2 gene. COX-1 does not respond to NF-kB as intensely as COX-2. The mRNA for COX-1 is approximately 2.8  kb, while COX-2 mRNA is approximately 4.0  kb. Homology between COX-1 and COX-2 enzymes at cDNA and amino acid levels may be responsible for similar structural and kinetic properties (Smith et al., 2000; Rouzer and Marnett, 2009). Although the

Table 1.1  Molecular properties of neural and non-neural preparations of COX-1, COX-2, and COX-3 Property COX-1 COX-2 COX-3 Expression in brain Constitutive Constitutive and inducible Constitutive Chromosomal location Chromosome 9 Chromosome 1 – Molecular mass (kDa) 68 68 60 Role Housekeeping Inflammation Fever and pain Oxidative stress Amino acid sequence Resembles COX-2 Resembles COX-1 Resembles neither COX-1 nor COX-2 Effect of vioxx No effect Inhibited – Effect of tylenol No effect No effect Inhibited//no effect FR122047 Inhibited (IC50, 28 nM) Inhibited (IC50, 65 mM) Unknown DuP-697 Inhibited (IC50, 9 mM) Inhibited (IC50, 80 nM) Unknown DuP-697 (5-Bromo-2-(4-fluorophenyl)-3-(4-(methylsulfonyl)phenyl)thiophene) Source: Modified from Phillis et al. (2006)

Carrasco et al., 2005 Chandrasekharan et al., 2002 Ochi and Goto, 2002 Peng et al., 2008

Minghetti, 2004; Snipes et al., 2005

Reference Minghetti, 2004; Snipes et al., 2005 Snipes et al., 2005 Minghetti, 2004; Snipes et al., 2005 Minghetti, 2004; Snipes et al., 2005

8 1  Metabolism and Roles of Eicosanoids in Brain

1.2  Multiplicity of Cyclooxygenases, Lipoxygenases, and Epoxygenases in the Brain

9

conformation of substrate binding sites in the catalytic regions between COX-1 and COX-2 enzymes are similar, but not identical. COX-1 contains Val at the 434 and 523 positions, whereas COX-2 has Ile at positions 434 and 523. It is proposed that these differences in amino acid sequences result in larger and more flexible substrate and inhibitor binding sites in COX-2 than in COX-1 (Kurumbail et al., 1996). Amino acid sequences of COX-1 and COX-2 also differ from each other at the N and C termini. COX-2 lacks a 17 amino acid sequence at the N-terminus but has an extra 18 amino acid sequence at the C-terminus. In spite of these differences, these proteins catalyze the same reactions. COX-1 deletion attenuates, whereas COX-2 deletion increases the neuroinflammatory response, blood–brain barrier permeability, and leukocyte recruitment during lipopolysaccharide (LPS)-mediated innate immune activation (Aid et al., 2010). Immunocytochemical studies indicate that in rat and ovine brain, COX-1 and COX-2 immunoreactivities are present in discrete neuronal populations in different areas of cerebral cortex, midbrain, and hippocampus. COX-1 immunoreactivity is enriched in midbrain, pons, and medulla (Breder et  al., 1995), whereas COX-2 immunoreactivity prevails in neurons and glial cells of hippocampus, hypothalamus, and amygdala (Yamagata et  al., 1993; O’Banion and Olschowka, 1999). In neurons, astrocytes, and microglial cells, COX-2 immunoreactivity is also localized to the perinuclear regions (Tomimoto et  al., 2000). The expression of COX-2 is markedly increased in microglial cells after intraperitoneal administration of lipopolysaccharide (LPS), whereas neuronal COX-2 remains unchanged (Elmquist et  al., 1997). In microglial cells, COX-2 expression and ability to release PGE2, TXA2 and TXB2 upon stimulation by LPS is several times higher than in astrocytes, but lower than in peripheral macrophages (Giulian et al., 1996). COX-2 induction in microglia by proinflammatory stimuli is apparently similar to peripheral macrophages and plays important roles in inflammatory and immune responses. COX-3 is a splice variant of COX-1. COX-3 is a glycoprotein product of the COX-1 gene with retention of a highly structured, G+C-rich intron 1 in its mRNA. COX-3 mRNA encodes a 127 amino acid protein with no similarity with known COX sequences (Kis et al., 2006). In canine brain, COX-1 intron 1 consists of a 93-bp sequence, and translation of this novel approximately 5.2-kb COX-3 RNA message introduces a hydrophobic 31 amino acid sequence (RECDPGARWGIF LASWWSLECQAQPLILSSA) into the signal peptide of COX-1, generating a catalytically active 65-kDa membrane-bound COX. In human brain, COX-1 intron 1 consists of a 94-bp core sequence, and because it is out of frame, alternative RNA splicing events, probably mediated by complex intron 1RNA secondary structures generate a functional COX-3 RNA and protein (Cui et al., 2004). Comparison among amino acid sequences of COX-1, COX-2, and COX-3 indicates that COX-1 (602 aa residues) and COX2 (604 aa residues) are of the same length, whereas COX-3, a putative cyclooxygenase-3 from house mouse has considerably shorter sequence (127 aa residues). COX-3 is deletion of the COX-2 but not of the COX-1 gene. So many gaps in COX-3 sequence make it difficult to have proper comparison. Thus, COX-3 has several different features in its amino acid sequence compared with COX-1 and COX-2: including (1) length, (2) too many gaps, and (3) branch distance (Fig. 1.7). To get a clear picture, it would be interesting to compare these sequences

Fig. 1.7  Alignment was produced by ClustalW (2.0.12), a general purpose multiple sequence alignment program (Thompson et al. 1994), for amino acid sequences of three cyclooxygenases. Amino acid sequences were identified by NCBI DATA bank accession number. (a) Cyclooxygenase 1 (Rattus norvegicus), COX1 (accession no. (#) AAA03465) was aligned with prostaglandin G/H

1.2  Multiplicity of Cyclooxygenases, Lipoxygenases, and Epoxygenases in the Brain

11

with cyclooxygenases from other species, including humans. COX-3 mRNA has also been cloned and sequenced from rat cerebral endothelial cells. Sequence analysis indicate that the 98-bp intron-1 of COX-1 gene remains unprocessed in COX-3, inducing a frameshift mutation and a 127-amino acid open reading frame with no sequence similarity with COX-2 (Snipes et al., 2005). This retention of intron 1 in COX-1 inserts 30–34 amino acids depending on the mammalian species. This retention may change protein folding and active site conformation. Pharmacologically, COX-3 is selectively inhibited by acetaminophen and some nonsteroidal antiinflammatory drugs (Chandrasekharan et al., 2002; Davies et al., 2004). It is likely that blockage of COX-3 may represent a primary central mechanism by which above drugs decrease pain and fever (Chandrasekharan et al., 2002; Davies et al., 2004). Highest expression of COX-3 mRNA occurs in choroid plexus and spinal cord followed by pituitary gland, hypothalamus, hippocampus, medulla, cerebellum, and cortex. In cerebrovascular system, the highest expression of COX-3 mRNA occurs in brain microvessels followed by major brain arteries. The expression pattern of COX-3 mRNA in the rat CNS primarily relates to the vascular density of a given region (Kis et al., 2003, 2004). Rat COX-3 is a cytosolic glycoprotein. Amino acid analysis also shows that COX-3 protein has a very basic character with a pI value of 12.40. In addition to the abundance of basic amino acids, the COX-3 protein is also very rich in proline (11.81%). The occurrence of high levels of proline argues against the formation of extensive a-helical domains and for the possible formation of b-turns as well as the formation of antiparallel b-sheets (Snipes et  al., 2005). Collective evidence suggests that while COX-2 is a major player in generating prostaglandins under pathological conditions, COX-3 may also play ancillary roles in membrane-based COX signaling.

1.2.2 Lipoxygenases (LOXs) LOXs convert ARA into hydroxyperoxyeicosatetraenoic acids, followed by their conversion into hydroxyeicosatetraenoic acids, leukotrienes, lipoxins, and hepoxilins (Phillis et al., 2006). LOXs are categorized with respect to their positional specificity of ARA dioxygenation into 5-LOX, 8-LOX, 12-LOX, and 15-LOX (Fig. 1.5). The substrate specificity of LOXs depends on the isoform. The human 5-LOX oxidizes free C20-fatty acids (ARA and EPA) but hardly oxygenates C18-derivatives or esterified fatty acids. In contrast, most 12/15-LOXs oxidize a variety of polyenic

Fig. 1.7 (continued)  synthase 2 (Rattus norvegicus) COX2 (accession no. (#) NP_058928); and putative cyclooxygenase 3 (Mus musculus) COX3 (accession no. (#) AAT09993). Amino acid residues in red with an asterisk (*) correspond to fully conserved region. Amino acid residues in green with a symbol (:) correspond to amino acid residues in similar groups. Amino acid residues in yellow with a symbol (∙) correspond to semiconserved substitution (similar shapes). (b) A phylogram was constructed by this program. The branch length is proportional to the number of evolutionary events that took place along this branch, representing the amount of evolutionary divergence

12

1  Metabolism and Roles of Eicosanoids in Brain

fatty acids regardless of their chain length and these enzymes also oxygenate ester lipids even if they are incorporated in biomembranes and lipoproteins (Kuhn and O’Donnell, 2006). 5-LOX and 12-LOX are most common LOXs found in the brain with their mRNA present in rat cortical neurons, astrocytes, and oligodendrocytes (Bendani et al., 1995). They predominantly generate 5- and12-hydroxyeicosatetraenoic acid (5-HETE and 12-HETE) and some 15-HETE has also been detected. Since LOX-catalyzed pathways can provide both constrictor and dilator, influencing the cerebral vasculature, they are capable of affecting the cerebral circulation in a comparable manner to COX-1 and 2 pathways (Phillis et al., 2006). Among LOXs, 5-LOX and 12-LOX are generally considered procarcinogenic, while 15-LOX-2 suppresses carcinogenesis. The 5-LOX pathway may also contribute to the mechanisms of neural plasticity ranging from neurogenesis and neural differentiation (Wada et al. 2007) to regulation of synaptic plasticity (Chabot et al., 1998; Ménard et  al., 2005). For example, metabolites derived from 5-LOX activity have been implicated in the expression of long-term depression in hippocampal slices (Chabot et  al., 1998). In addition, 5-LOX is the key enzyme in the biosynthesis of antiinflammatory lipoxins. LOX enzymes induce structural and metabolic changes in cells during a wide variety of physiological and pathological processes, such as differentiation, angiogenesis, carcinogenesis, inflammation, and atherogenesis. LOX are nonheme, iron-containing dioxygenases that insert molecular oxygen into ARA (Rådmark and Samuelsson, 2009). Their nomenclature is based on the position of oxygen insertion (carbon 5, 8, 12, or 15 of the aliphatic chain) and stereoconfiguration (R versus S) of the resulting product hydroperoxyeicosatetraenoic acid (HPETE) (Phillis et  al., 2006). LOXs have a molecular mass of 75–78 kDa. They contain a single polypeptide chain, which is folded into two domains (Brash, 2001). Like non-neural tissues, three forms of LOX are present in brain. They include 5-LOX, 12-LOX, and 15-LOX. The 5-LOX gene is highly conserved across species. It consists of 14 exons and 13 introns, and contains a promoter region that encompasses consensus regions for transcription regulators of the Egr, Sp, NF-kB, GATA, myb, and AP families. 12-LOX is the predominant LOX isoform in the CNS and its mRNA is present in rat cortical neurons, oligodendrocytes, and astrocytes (Bendani et  al., 1995). 12-LOX catalyzes the stereospecific incorporation of molecular oxygen into the C-12 position of ARA to generate 12-HPETE, which is reduced by cellular glutathione peroxidase to 12-HETE (Li et al., 1997). 12-LOX has been cloned and characterized from rat brain (Watanabe et al., 1993). Rat brain 12-LOX contains six conserved histidines and displays marked resemblance with porcine leukocyte 12-LOX (71%) and to human 15-lipoxygenase (75%), but has less identity with human platelet 12-LOX (59%) or rat leukocyte 5-LOX (41%). 5-LOX and 15-LOX have also been sequenced from several non-neural sources (Dixon et  al., 1988; Matsumoto et  al., 1988). They have a molecular mass of 78 kDa and share considerable homology with 5-LOX and 15-LOX from soybean. LOXs also catalyze a dehydration reaction, generating an unstable epoxide intermediate, leukotriene A4 (LTA4). LTA4 is metabolized to LTB4 by LTA4 hydrolase or to LTC4 by conjugation of glutathione at the sixth carbon by the action of LTC4 synthase. LTA4 is the precursor for the family of cysteinyl leukotrienes (cysLTs: LTC4, LTD4, and LTE4) (Wolfe and Horrocks, 1994) (Fig. 1.5).

1.2  Multiplicity of Cyclooxygenases, Lipoxygenases, and Epoxygenases in the Brain

13

In the brain, 5-LOX is localized in the cytoplasm. Receptor-mediated stimulation of 5-LOX results in translocation of this enzyme from cytosol to the plasma membrane and nuclear envelope, where 5-LOX transforms ARA into 5(S)-HpETE with the concerted interaction of 5-LOX- activating protein (FLAP), a 18-kDa resident integral protein that functions as a transfer protein facilitating the binding of ARA to 5-LOX. FLAP is essential for leukotriene C4 (LTC4) synthesis (Manev et  al., 2000a). Although FLAP shows homology with LTC4 synthase and other microsomal glutathione transferases, but it has no enzymic activity itself. It is suggested that FLAP acts as a scaffolding protein for the assembly of other enzymes involved in the leukotriene synthetic pathway at the nuclear envelope of leukocytes (Sampson, 2009). The regulation of LTs production occurs at various levels, including expression of 5-LOX, translocation of 5-LOX to the perinuclear region, and phosphorylation to either enhance or inhibit the activity of 5-LOX. In addition, the synthesis of LTs is also supported by the translocation of cPLA2 to the perinuclear region. LTC4 synthase is an integral membrane protein that is present at the nuclear envelope; however, LTA4 hydrolase remains cytosolic. Biologically active LTB4 is metabolized by w-oxidation carried out by specific cytochrome P450s-dependent EPOX (CYP4F) followed by b-oxidation from the w-carboxy position and after CoA ester formation. Gamma-glutamyl transpeptidase converts LTC4 into LTD4 and a membrane-bound dipeptidase that converts LTD4 into LTE4 before w-oxidation (Murphy and Gijon, 2007). These metabolic conversions of the primary leukotrienes are critical for terminating their biological activity. By blocking the formation of LTs, FLAP inhibitors may act as broad-spectrum leukotriene-modifier drugs that may have a wide range of therapeutic applications. 5-LOX also contains a Src homology 3 (SH3) binding motif, which facilitates the interaction of 5-LOX protein with growth factor receptor-bound protein 2 (Grb2) (Lepley et al., 1996).

1.2.3 Cytochrome P450 Epoxygenases (EPOXs) The cytochrome P450 (CYP) epoxygenase enzymes CYP2J and CYP2C catalyze the epoxidation of ARA to EETs, which are rapidly hydrolyzed to DHETs by soluble epoxide hydrolase (sEH) (Zeldin, 2001) (Fig. 1.6). CYP epoxygenase-derived EETs possess potent vasodilatory, anti-inflammatory, and angiogenic effects. In addition, EETs not only retard endothelial cell activation and leukocyte adhesion via attenuation of nuclear factor-kappaB activation, but inhibit hemostasis, protect against ischemia-reperfusion injury, and promote endothelial cell survival via modulation of multiple cell signaling pathways (Zeldin, 2001). In the brain, EETs are produced by astrocytes and the vascular endothelium and are involved in the control of cerebral blood flow (CBF). It is also reported that epoxygenases and sEH are present in perivascular vasodilator nerve fibers innervating the cerebral surface vasculature. Ischemic preconditioning is accompanied by the expression of cytochrome P450 2C11 epoxygenase (CYP2C11) in the brain, and that pharmacological inhibition and genetic deletion of sEH increases EETs, leading to protection against stroke-induced

14

1  Metabolism and Roles of Eicosanoids in Brain

brain damage (IIiff et al., 2007). Double-labeling experiments indicate that CYP2C11and sEH-IR are predominantly colocalized with neuronal nitric oxide synthase-IR within perivascular nerve fibres. Significant colocalization for CYP2C11 and sEH also occurs with the parasympathetic markers vasoactive intestinal peptide and choline actetyltransferase, in addition to the sensory fibre markers calcitonin generelated peptide and substance P. No colocalization is observed for either CYP2C11 or sEH with the sympathetic nerve markers dopamine b-hydroxylase or neuropeptide Y. The presence of enzymes involved in production and inactivation of EETs within extrinsic parasympathetic and sensory vasodilator fibres suggests a novel role for EETs in the neurogenic control of cerebral arteries (IIiff et al., 2007). Based on various studies, it is proposed that EETs contribute not only to the regulation of the cerebral blood flow and protection from ischemic injury, but also inhibit inflammation, release peptide hormones, and therefore are crucial for cerebrovascular homeostasis. The ability of various types of neural and non-neural cells to produce and respond to EETs-mediated effects suggests that EETs signaling may be an important integrator of cell–cell communication in the CNS, coordinating cellular responses across different cell types. Under pathophysiological conditions, such as cerebral ischemia, EETs protect neurons, astroglia, and vascular endothelium, thus preserving the integrity of cellular networks unique to and essential for proper CNS function (Iliff et al., 2010). In addition, EETs are also involved in the secretion and action of insulin and the metabolism of carbohydrates and lipids. The RhoA/Rho kinase (ROCK) signaling pathway is downstream to ARA metabolism and is reported to mediate metabolic brain dysfunctions in insulin resistance. It is proposed that pharmacological manipulation of EETs may be a useful therapeutic approach for hypertension, diabetes mellitus, and the metabolic syndrome (Mustafa et al., 2009). EETs can act like long-chain fatty acids and bind to fatty-acid-binding proteins and nuclear peroxisome-proliferator-activated receptors (PPARg and PPARa). In fact, all four EETs and their metabolite DHET can stimulate PPAR/RXR heterodimer binding to a peroxisome proliferator response element (Cowart et al., 2002; Fang et  al., 2006). Activation of peroxisome proliferator-activated receptor alpha (PPARa) by fatty acids and fibrates results in the induction of sEH, the enzyme that degrades EET into the dihydroxyeicosatrienoic acids (DHETs). In contrast, the CYP2C epoxygenases, which are responsible for EET formation, are repressed after fibrate treatment. It is proposed that P450 eicosanoids interact with PPARa and modulate PPARa-mediated gene expression (Ng et al., 2007). In endothelial cell cultures, the overexpression of EPOX not only upregulates endothelial nitric oxide synthase (eNOS), but also activates enzymes of PtdIns3-kinase/Akt pathways. Not only eNOS inhibitors, but also PtdIns3-kinase/Akt signaling pathway inhibitors, can prevent this upregulation by EPOX, suggesting that both eNOS and PtdIns3-kinase/Akt pathways may mediate the angiogenic effects of EETs. EPOX gene transfection activates the MARK pathway in endothelial cells. Collectively, these studies suggest that EPOX-derived EETs modulate angiogenesis via a nitric oxide-dependent mechanism as well as via activation of PtdIns 3 kinase and MARK pathways (Wang et al., 2005).

1.3  Eicosanoids and Their Receptors in Brain

15

1.3 Eicosanoids and Their Receptors in Brain Eicosanoids are released outside of the cells immediately after their synthesis. They act through eicosanoids receptors that are located on plasma and nuclear membranes. These receptors modulate signal transduction pathways and gene transcription. Among eicosanoids, PGs are potent autocrine and paracrine lipid mediators that contribute appreciably to physiologic and pathophysiologic responses in the CNS and PNS (Table 1.2). Among 12 PGs, the most potent are PGD2, PGE2, and PGF2. PGE2 mediate their signaling through four distinct G protein-coupled receptors, EP1, EP2, EP3, and EP4, (Sugimoto and Narumiya, 2007) which are encoded by different genes and differ in their responses to various agonists and antagonists and differentially expressed on neuronal and glial cells throughout the central nervous system. Activation of EP1 receptors disrupts Ca2+ homeostasis by impairing Na+--Ca2+ exchange, a key mechanism by which neurons cope with excess Ca2+ accumulation after an excitotoxic insult. Thus, EP1 receptors contribute to neurotoxicity by augmenting the Ca2+ dysregulation underlying excitotoxic neuronal death (Farooqui et al., 2008). EP2 activation is involved in microglial-mediated paracrine neurotoxicity as well as suppression of microglia internalization of aggregated neurotoxic peptides (Cimino et al., 2008). The activation of EP2 receptor leads to BDNF secretion through stimulation of cyclic AMP-dependent signaling involving cAMP-dependent protein kinase (PKA). The catalytic subunit of PKA stimulates gene transcription through the phosphorylation of cAMP-response element-binding (CREB) protein.This signaling may contribute to neurotoxicity or neuroprotection in microglial cells and astrocytes. EP3 receptor signaling not only involves the inhibition of adenylyl cyclase via Gi activation, but is also associated with Ca2+-mobilization through Gbg from Gi. Along with Gi activation, the EP3 receptor can stimulate cAMP formation via G(s) activation. At least three isoforms of EP3 are generated through alternative RNA splicing from a single gene and differ only in the efficiency of G protein activation and in the specificity of coupling to G proteins. Among EP receptors, EP4 utilizes phosphatidylinositol 3-kinase (PtdIns3K) as well as PKA. In addition, EP4 receptor activates the extracellular signal-regulated kinases (ERKs) 1 and 2 by way of PtdIns3K leading to the induction of early growth response factor-1 (EGR-1), a transcription factor traditionally involved in wound healing and inflammatory processes (Cimino et al., 2008). Studies on knockout mice and selective drugs indicate that PGE2 mediates fever through EP3, while both EP1 and EP3 are involved in ACTH release. Some cells and tissues express all four receptors, whereas others express some or no receptors. It is becoming increasingly evident that EPs play different roles in brain tissue, such as controlling impulsive behaviors under psychological stress, but the exact involvement and contribution of each in inflammation is not yet completely clear, and might differ between species, organs, and pathologies. Collective evidence suggests that PGs produced by various cell types in brain may integrate multiple stress stimuli through PGE receptor subtypes for adaptive responses (Furuyashiki and Narumiya, 2009). In addition, brain tissue also contains PGF receptor (FP), PGI receptor (IP), and TXA receptor (TP), which may contribute to neurotransmitter release, sleep, and vasodilation and vasoconstriction of cerebral vessels in the brain.

Table 1.2  Roles of eicosanoids in brain tissue Eicosanoids Role PGE2, PGF2a, PGI2, TxA2 Homeostasis, vasodilation, vasoconstriction, neurotransmitter release, and synaptic plasticity PGE2, PGI2 Vasodilation, vasoconstriction, inflammation, apoptosis, and gene expression Fever and pain PGD2 PGA2, PGJ2 Cell proliferation LTC4, LTD4, LTE4 Vasoconstriction, inflammation, monocyte and T-cell trafficking, apoptosis, gene expression LXA4, LXB4, Resolution, proliferation, differentiation, 15-epi-LXA4 nociception EETs Gene expression, angiogenesis, cerebral blood flow, ion transport, vasorelaxation as well as anti-inflammatory and profibrinolytic effects Hepoxilins Ca2+ release, neuro-modulation Trioxilins Vasodilation

Chandrasekharan et al., 2002 Minghetti, 2004; Smith et al., 2000 Funk, 1996; Maccarrone et al., 2001; Powell and Rokach, 2005 Chiang et al., 2005 Spector, 2009; Wang et al., 2005

Nigam et al., 2007 Pfister et al., 2003

ALX, LXA receptors PPARa receptor

Unknown Unknown

Reference Minghetti, 2004; Minghetti and Levi, 1998; Smith et al., 2000; Vane et al., 1998 Minghetti, 2004; Simmons et al., 2004

PPARg CysLT1 and CysLT2, receptors

EP1, EP2, EP3, and EP4 receptors

Receptor EP1, EP2, EP3, and EP4 receptors, TXA receptor

16 1  Metabolism and Roles of Eicosanoids in Brain

1.3  Eicosanoids and Their Receptors in Brain

17

Dehydration of the cyclopentane ring of PGE2, PGE1, and PGD2 transforms them into cyclopentenone prostaglandins PGA2, PGA1, and PGJ2. PGJ2 is further metabolized into d12-PGJ2 and 15-deoxy-d12,14-PGJ2 (15d-PGJ2). Many of these metabolites possess potent anti-inflammatory, antineoplastic, and antiviral activities (Straus and Glass, 2001). The actions of above cyclopentenone prostaglandins are not mediated through any G protein receptors, but rather caused the bioactivity of these compounds, which results from their interaction with other cellular target proteins. 15-d-PGJ2 is a reactive cyclopentenone eicosanoid formed from the dehydration of cyclooxygenase (COX)-derived PGD2. This compound possesses an a,b-unsaturated carbonyl moiety that can readily adduct thiol-containing biomolecules such as glutathione and cysteine residues of proteins via Michael addition. Due to its reactivity, 15-d-PGJ2 has been proposed to modulate inflammatory and apoptotic processes (Hardy et  al., 2010). Furthermore, 15-deoxy-d12,14-PGJ2 is a high affinity ligand for the nuclear receptor PPARg and modulates gene transcription by binding to this receptor. Unlike other PGs, which elicit their neurochemical effects through the interactions with specific G-protein-coupled receptors (GPCRs), 15-d-PGJ2 exerts its biochemical effects through interaction with key intracellular targets. As stated above, some of its activities may be mediated through the nuclear receptor PPAR-g (Forman et  al., 1995). Activation of PPAR-g by 15-d-PGJ2 is caused by covalent binding of 15-d-PGJ2 to a critical cysteine residue in the PPARg ligand binding pocket (Shiraki et al., 2005). In addition, 15-d-PGJ2 also block the inflammatory response in a number of cell types, including macrophages, by retarding NF-kB-mediated gene expression via covalent modification of a critical cysteine residue in IkB kinase and the DNA-binding domains of NF-kB subunits (Pérez-Sala et  al., 2002). 15-d-PGJ2 is also associated with resolution of inflammation by enhancing apoptosis in activated macrophages through a p38 mitogen-activated protein kinase (MAPK)-dependent increase in ROS production (Hortelano et  al., 2000). Furthermore, 15-d-PGJ2 enhances the antioxidant response pathway via covalent modification of reactive cysteine residues in Keap 1, leading to Nrf2 activation and induction of antioxidant proteins, including HO-1, peroxiredoxin 1, g-GCL, and heat shock protein 70 (Itoh et al., 2004). Finally, 15-d-PGJ2 has been reported to inhibit proliferation of vascular smooth muscle cells by inducing cell cycle arrest (Miwa et al., 2000). Other activities of the cyclopentenone prostaglandins are facilitated by the reactive a,b-unsaturated carbonyl group located in the cyclopentenone ring. NF-kB and its activating kinase (IkB kinase inhibition and blockade of NF-kB nuclear binding) are major targets for the anti-inflammatory activity of 15d-PGJ2, which blocks NF-kB-induced transcriptional activation by PPARg-dependent and independent molecular mechanisms (Fig. 1.8). Other cyclopentenone prostaglandins, such as d7-PGA1 and d12-PGJ2, show potent antitumor activity. These compounds cause cell cycle arrest or apoptosis of tumor cells depending on the cell type and treatment conditions (Straus and Glass, 2001). In addition cyclopentenone prostaglandins form covalent adducts with thiol residues within specific signaling proteins, resulting in modulation of redox-sensitive cell signaling pathways. Biotinylated 15-deoxy-Delta12,14-PGJ2 was more efficient than biotinylated PGA1 in forming adducts with components of the transcription factors

18

1  Metabolism and Roles of Eicosanoids in Brain

Growth and differentiation

Ligand for PPARy

Cytoskeletal dysfunction

Redox alterations

15d-PGJ2

Antiinflammatory

Antioxidant response

Apoptosis

Turnover of proteins

Fig. 1.8  Roles of 15d-PGJ2 in neural and non-neural tissues

NF-kB and activator protein-1 (AP-1). The presence of GSH differentially ­modulates the formation of protein-cyclopentenone prostaglandins adducts. Another signaling pathway, which is potently activated by 15d-PGJ2 is the Keap1-Nrf2-ARE pathway. As stated above, in cytoplasm, phosphorylation of Kelch-like ECH-associated protein 1 (Keap1) results in the dissociation and translocation of Nrf2 into the nucleus, where it binds to an antioxidant response element (ARE). Through transcriptional induction of ARE-bearing genes that encode antioxidant-detoxifying proteins, Nrf2 activates cellular rescue pathways against oxidative injury, inflammation/immunity, apoptosis, and carcinogenesis. ARE-driven genes include direct antioxidants (glutathione peroxidases), thiol metabolism-associated detoxifying enzymes (glutathione S-transferase), stress-response genes (hemeoxygenase), and others (proteasome subunit beta type-5). Collective evidence suggests that 15d-PGJ2 may protect cells from endogenous and exogenous stresses as well as anti-inflammatory effects (Gayarre et al., 2007; Kansanen et al., 2009). As stated above, LTs consist of dihydroxy leukotriene LTB4 and the cysteinyl leukotrienes LTC4, LTD4, and LTE4. Neurochemical effects of LTs are transduced through 7-transmembrane G-protein-coupled receptors. The two arms of the LT pathway interact with distinct receptors called as BLT receptors (activated by LTB4) and CysLT receptors (activated by the cysteinyl-LTs), respectively (Back, 2008). The BLT receptors are named as BLT1 and BLT2 for the high and low affinity receptor subtypes, respectively. These receptors activated by the cysteinyl-LTs are characterized by their sensitivity to available antagonists, and are referred as CysLT1 and CysLT2 (Back, 2008).

1.3  Eicosanoids and Their Receptors in Brain

19

Lipoxins

Inhibition of nociception

Inhibition of chemokine & cytokine gene expression

Inhibition of PMN infiltration

Reduction In edema Inhibition of inflammation

Fig. 1.9  Roles of lipoxins in neural and non-neural tissues

Lipoxins (LXs), a group of trihydroxytetraene eicosanoids, are generated by the action of lipoxygenases on hydroperoxyeicosatetraenoic acid (HPETE) and hydroxyeicosatetraenoic acid (HETE). LXs include 5S,6R,15S-trihydroxy(7E,9E,11Z,13E)-tetraenoic acid (LXA4), 5S,14R,15S-trihydroxy-6E,8Z,10E,12Eeicosatetraenoic acid (LXB4), 15 epi-LXA4, and 15 epiLXB4. They selectively regulate actions on neutrophils and monocytes. LXs also inhibit edema, pain, and inflammation (Fig.  1.9). They are generated through cell–cell interactions by the process of transcellular biosynthesis (Marcus, 1986). LXs (LXA4) also modulate the expression of many PDGF-induced genes, including transforming growth factor-b1, fibronectin, thrombospondin, matrix metalloproteinase 1, and several collagens. These genes are closely associated with matrix accumulation and profibrotic change (Rodgers et al., 2005). Mammalian non-neural cells have three major routes of transcellular biosynthesis of LXs (Romano, 2010). The first pathway of LX biosynthesis involves the interaction of platelets with PMN within the vascular lumen. In this pathway, LX biosynthesis starts with the release of the epoxide intermediate LTA4 (formed by 5-LOX in activated leukocytes), which is then converted by the platelet 12-LOX to LXA4 and LXB4 (Serhan and Sheppard, 1990; Serhan and Romano, 1995). The second pathway of transcellular LXA4 biosynthesis involves the sequential interaction of a 15-LOX with a 5-LOX. This LXA4 formation route takes place mainly in tissues in which endothelial and epithelial cells expressing 15-LOX can interact with 5-LOX-containing leukocytes (Chiang et al., 2005). The third pathway of LX biosynthesis is initiated by aspirin, which acetylates COX-2 and switches its catalytic activity from COX to 15-LOX. Following this change, PG biosynthesis is inhibited and COX-2 transforms arachidonic acid to 15(R)-HETE. 15(R)-HETE is subsequently transformed by activated leukocytes possessing 5-LOX to a new series of carbon-15 epimers of LXA4 that carry their 15 alcohol in the R configuration (15-epi-LXA4) (Clària and Serhan, 1995; Serhan, 2005; Chiang et  al., 2005).

20

1  Metabolism and Roles of Eicosanoids in Brain

Biosynthesis of LX in the tissue not only depends on cells and enzymes ­present therein, but also on other factors such as cytokines (Serhan, 2005). For example, interleukin 4 (IL-4) and IL-13, putative negative regulators of inflammatory and immune responses, promote transcellular LX generation through enhanced expression of 15-LOX in blood monocytes and epithelial cells (Munger et al., 1999). IL-3 upregulates 5-LOX transcript (Murakami et al., 1995) while IL-1b, IL-6, and TNF-a induce COX-2, thus potentially contributing to the formation of ATLs in  vivo (Parente and Perretti, 2003). The initial phase of the acute inflammatory response is accompanied by the synthesis of proinflammatory mediators followed by a second phase in which lipoxins with proresolution activities may be generated. LXs facilitate resolution not only by modulating key steps in leukocyte trafficking and preventing neutrophil-mediated acute tissue injury, but also by stimulating nonphlogistic phagocytosis of apoptotic cells by macrophages (Kantarci et al., 2003; Yacoubian and Serhan, 2007). They also reduce vascular permeability. Collective evidence suggests that a key event in resolution is the temporal “switch” in the lipid mediator class from proinflammatory (PGE2 and PGD2) to anti-inflammatory (lipoxins) eicosanoids. Drugs that disrupt this switch may delay the onset of resolution (Yacoubian and Serhan, 2007). The occurrence of aspirin-triggered LXs (ALX) has emerged as potential antifibrotic mediator that may influence profibrotic cytokines and matrix-associated gene expression in response to growth factors (Maderna and Godson, 2009). In neural and non-neural tissues, LXs and aspirin-triggered LX induce their actions at least through two major mechanisms including (a) the activation of high affinity ALXR and low affinity LXA, and (b) interaction of subclasses of cysteinyl peptide-LTs receptor. Direct activation of the lipoxin receptor mediates anti-inflammatory and proresolution activities whereas interactions with CysLT and growth-factor receptors retard angiogenesis, and mesangial cell proliferation and fibrosis. In addition, LXA4 also interact with the nuclear receptor aryl hydrocarbon receptor, which triggers expression of suppressor of cytokine signaling 2 in LX-stimulated dendritic cells (Serhan and Levy, 2003; Serhan, 2005; Chiang et al., 2005). Collective evidence suggests that LXs and aspirin-triggered LXs (ATLs) are “braking signals” molecules in inflammation, limiting the entrance of leukocytes to the site of inflammation through inhibition of neutrophil and eosinophil trafficking. In vivo studies with stable LX and ATL analogs have established that these eicosanoids also possess potent immunoregulatory properties. A LXA4 receptor has been cloned (Romano et al., 2007). It belongs to the family of chemotactic receptors and clusters with formyl peptide receptors on chromosome 19. Therefore, it is denominated formyl peptide receptor-like 1 (FPRL1). This receptor binds with high affinity and stereoselectivity LXA4 and ATL. It also recognizes a variety of peptides, synthetic, endogenously generated, or disease associated, but with lower affinity compared to LXA4. For this reason, this receptor has been renamed ALX (Romano et al., 2007). In general, TXA2 is a potent vasoconstrictor and produces vasospasm, whereas PGI2 has opposing effects (Phillis et al., 2006). PGs, LTs, and TXs are involved in many processes including fever, sensitivity to pain, sleep, inflammation, oxidative stress, and neurodegeneration (Table  1.1). TXs act through TXs receptors (TPs), which are widely distributed in the brain tissues. Like other eicosanoid receptors

1.3  Eicosanoids and Their Receptors in Brain

21

TPs are coupled with the seven-transmembrane G-protein-coupled receptor (GPCR). A single gene on chromosome 19p13.3 may be responsible for the expression of two separate TP isoforms: TPa receptor, which is broadly expressed in numerous tissues and a splice variant known as TPb, which may have a more limited tissue distribution (Huang et al., 2004). TPa and TPb differ from each other in their C-terminals. In addition, TPa and TPb are coupled with PLC and RhoGEF domain of Rho/Rac/Cdc4 like GTPase through different G proteins. They undergo hetero-dimerization, resulting in changes in intracellular traffic and receptor protein conformations (Nakahata, 2008). This signaling modulates a broad range of cellular responses including phosphoinositide metabolism, calcium redistribution, cytoskeletal arrangement, integrin activation, kinase activation, and the subsequent nuclear signaling events involved in DNA synthesis, cell proliferation, differentiation, cell survival, and cell death (Huang et al., 2004). Generation of EETs inhibits vascular smooth muscle migration, decreases inflammation, inhibits platelet aggregation, and decreases adhesion molecule expression, therefore representing an endogenous protective mechanism against atherosclerosis. The other effects of EETs include inhibition of endothelial activation and leukocyte adhesion through the attenuation of nuclear factor-kB activation, inhibition of hemostasis, protection against myocardial ischemia-reperfusion injury, facilitation of endothelial cell survival via modulation of multiple cell signaling pathways, and angiogenesis (Deng et al., 2010). Among EETs, 5,6- and 8,9-EET are powerful and selective angiogenic lipids that provide a functional link between the EET proliferative chemotactic properties and their angiogenic activity. EETs are hydrolyzed by soluble epoxide hydrolase (sEH) into less active dihydroxyeicosatrienoic acids (DHET). EETs are important for maintaining the homeostasis and in responding to stress. It is not clear whether these effects are mediated through the action of EETs on a universal receptor or through a mechanism involving second messengers (Kaspera and Totah, 2009). Recent studies indicate that EETs/DHETs represent novel autoregulatory agents that directly modulate the actions of COXderived eicosanoids following ARA mobilization (Behm et al., 2009). The vasodilatory effects of EET are largely through their ability to activate endothelial NO synthase (eNOS) and NO release (Hercule et al., 2009). The occurrence of 2-(11,12-epoxyeicosatrienoyl)glycerol (2–11,12-EG) and 2-(14,15-epoxyeicosatrienoyl)glycerol (2–14,15-EG) (Fig. 1.10) in the kidney and spleen and brain has also been reported (Chen et  al., 2008). These metabolites (2-11,12-EG and 2-14,15-EG) interact and activate cannabinoid (CB) receptor subtypes, CB1 and CB2 with high affinity and elicited biological responses in cultured cells expressing CB receptors and in intact animals (Chen et  al., 2008), suggesting that 2-11,12-EG and 2-14,15-EG may act as a novel class of endogenously produced, biologically active lipid mediators with the characteristics of endocannabinoids. This observation also indicates that ARA-derived 2-11,12-EG and 2-14,15-EG can activate G-protein-coupled CB receptors, indicating that there may be a relationship between the cytochrome P450 enzyme system and the ­endocannabinoid receptor system (Chen et  al., 2008). In addition, in brain the ­oxidation of anandamide by cytochrome P450 (P450) enzymes generates 5,6-epoxyeicosatrienoic acid ethanolamide (5,6-EET-EA), a potent, stable, and selective CB2

22

1  Metabolism and Roles of Eicosanoids in Brain O

O O

O

OH

OH

O

OH

OH O

2-14, 15-EG

2-11, 12-EG O

HO

HO OH OH

H

N

N

O

H

O

5,6-EET-EA

5,6-DHET-EA OH

OH

O

O

OH O

OH

O

2-11,12-EG Ether

2-14,15-EG Ether

Fig. 1.10  Chemical structures of EET derivatives

agonist (Fig. 1.10), which modulates signaling pathways within the endocannabinoid system (Snider et al., 2009). These results once again support the view that there may be a functional link between the cytochrome P450 enzyme system and the endocannabinoid receptor system. The synthesis of eicosanoids under physiological conditions is involved in signal transduction processes related to normal cell function. In contrast, the production of eicosanoids under pathological conditions is associated with neuroinflammation, oxidative stress, modulation of cerebrovascular blood flow, and neurodegeneration. Their active production by circulating cells such as platelets and leukocytes induces alterations in the microcirculation and ultimately to CNS dysfunction (Phillis et al., 2006).

1.4 Interplay Among COX, LOX, and EPOX-derived Products, and Relationship to Upstream PLA2 Isoforms Although mRNA, enzymic activities, and immunoreactive of COX, LOX, and EPOX are found in several regions of the brain tissue, but very little is known about their interactions with each other and linkages to upstream ARA releasing enzymes (PLA2 PLC, PLD isozymes) and downstream. PGE synthases, LT synthases, and

1.4  Interplay Among COX, LOX, and EPOX-derived Products, and Relationship…

23

TX synthases in brain tissue (Bosetti et al., 2004; Bosetti and Weerasinghe, 2003; Murakami and Kudo, 2004; Tanioka et al., 2000; Yan et al., 2005; Farooqui et al., 2007b; Farooqui, 2009). Several PGE synthases isozymes have been identified including membrane-associated PGE synthases, whose expression is induced by proinflammatory stimuli and cytosolic PGE synthases that are expressed constitutively. The conversion of PGH2 to other bioactive products, including PGD2, PGF2a, PGI2 (prostacyclin), and TxA2, via specific synthases is also biologically important (Narumiya et  al., 1999). The efficiency of coupling between isoforms of COX, LOX, EPOX, and specific PG, LT, LX, and TX synthases may be modulated by their spatial and temporal compartmentalization and by the amount of ARA released by PLA2 isoform at a moment when PG, LT, and TX synthesis is taking place (Ueno et al., 2001). These processes not only depend on the presence of regulatory cofactors and interfacial binding of PLA2, COX, LOX, and EPOX isoform to membrane glycerophospholipids, which may differ according to neural cell type, stimuli, secretory processes, and subcellular distributions, but also on levels of n-6 fatty acids present in the diet, which modulate levels of glycerophospholipid molecular species in the brain tissue. The reaction rates of COX, LOX, and EPOX with the various fatty acids and molecular species of glycerophospholipids have been reported to be quite different. For example, for LOXs, setting the rate of linoleic acid oxygenation 100%, linoleic acid-containing glycerophospholipids are oxygenated with a rate of about 20%, and biomembranes with about 5%. The oxygenation rate of low-density lipoprotein is even lower (1–2%). This suggests that oxidation rates of free fatty acids and glycerophospholipid substrate strongly depend on substrate preparation (particularly for biomembranes and lipoproteins) and on the composition of the assay system (inclusion of detergents) (Kuhn and O’Donnell, 2006; Phillis et al., 2006). In addition, substrate concentration may strongly impact the absolute catalytic activities of these enzymes. Although multiplicity of COX, LOX, and EPOX enzymes in brain tissue provides diversity in their function and the specificity of various isoforms in the regulation of enzymic activity in response to a wide range of extracellular and intracellular signals, but the interpretation of results becomes very complex when one considers the different rates of expression of isoforms of PLA2, COX, LOX, and EPOX and downstream PG, LT, and TX synthases in different stages of disease pathology (Hoozemans et al., 2008). For example in an early stage of AD, when low-fibrillar Ab deposits are present and only very few neurofibrillary tangles are seen in the cortical areas, COX-2 is elevated in neurons. The increased neuronal COX-2 expression parallels and colocalizes with the expression of cell cycle proteins. COX-1 is primarily expressed in microglia, which are associated with fibrillar Ab deposits. This suggests that in AD brain COX-1 and COX-2 are involved in inflammatory and regenerating pathways, respectively (Hoozemans et al., 2008). COX-2 and 5-LOX pathways work in parallel in the regulation of cell proliferation, differentiation, and angiogenesis. Furthermore, it is also suggested that neuronal and astrocytic 5-LOX, 12-LOX, and 15-LOX and EPOX isozymes differ considerably in responses to exogenous stimuli (Kwon et al., 2005). The complexity of cross-talk or interplay becomes obvious when one considers the coupling mechanisms of various isoforms

24

1  Metabolism and Roles of Eicosanoids in Brain

of COX, LOX, and EPOX with different receptors at cellular and subcellular levels and tries to relate them to neuronal and glial cell functions (Bosetti et al., 2004). Some isoforms of COX, LOX, and EPOX are constitutively expressed while others are inducible in response to cytokines and growth factors (Farooqui, 2009). The isoforms of COX, LOX, and EPOX may not function interchangeably but act in parallel to transducer signals. It is likely that various isoforms of COX, LOX, and EPOX are coupled with different forms of EP, LT, TX, and LX receptors through different coupling mechanisms and act on different cellular pools of ARA located in different subcellular organelles of various types of neural cells. COX, LOX, and EPOX compete for ARA. Thus, generation of PGs may reduce the synthesis of LTs, and LX biosynthesis reduces LT formation. This suggests that there may be an inverse relationship between LT and LX biosynthesis (Serhan, 2005; Chiang et al., 2005). Different coupling mechanisms may regulate these isoforms, generating common second messengers. Coordination and integration of these second messengers in various subcellular compartments is necessary for optimal functioning of signal transduction processes. In vitro studies suggest that COXs and 12/15-LOX products not only act as coactivators of peroxisomal proliferator activating-receptors (PPAR), but also regulate cytokine expression and generation, and modulate gene expression related to resolution steps of neuroinflammation. Collectively, these studies suggest that interactions among PLA2, COX, LOX, and EPOX-generated metabolites at plasma membrane, cytoplasmic and nuclear membrane levels may provide neural cells and brain tissue with great versatility in ensuring that arachidonic acid is efficiently utilized in brain tissue (Bosetti et al., 2004; Farooqui et al., 2007b; Farooqui, 2010b). It is tempting to speculate that coordinated cross talk, not only among COX, LOX, and EPOX isozymes, but also with upstream PLA2 isozymes and downstream PGE synthases, is essential for maintaining normal neuronal and glial cell growth. In the nuclear membrane and nucleus, COX, LOX, and EPOX-mediated signaling has the advantage over plasma membrane signaling in that second messengers generated by these enzymes during differentiation may directly interact with nuclear factors producing physiological responses and morphological changes. In the brain tissue, endogenous activities of COX, LOX, and EPOX isoforms may depend not only on the structural, physicochemical, and dynamic properties of neural membranes from neuronal and glial cells, but also on their metabolic state (Bosetti et  al., 2004; Farooqui, 2009). The ability of COX, LOX, and EPOX isoforms to orchestrate complex PGs, LTs, LXs, and EETs-mediated physiologic processes reflects an elaborate interplay among various receptors. Their levels not only modulate initiation, amplification, or dampening of inflammatory responses, but also influence the magnitude, duration, and nature of subsequent immune responses. The activation of COX, LOX, and EPOX isoforms at a subcellular level in neural cells is the rate-limiting step for the production of above lipid mediators. Therefore, an efficient regulation of COX, LOX, and EPOX isozymes is very important for normal brain function. Under pathological situations changes in activities of COX, LOX, and EPOX produces high levels of PGs, LTs, TXs, and LXs, which not only overstimulate their receptors, but may produce other harmful effects in neural

1.5  Roles of Eicosanoids in the Brain

25

membranes and nuclei. As stated above, the regulation of COX, LOX, and EPOX activities is complex and mediated by several factors including translocation, feedback regulation, release of cytokines and growth factors and mechanisms such as gene expression and cross talk among isoforms of these enzymes at cellular and subcellular levels. To understand the contribution of isoforms of COX, LOX, and EPOX in physiological and disease processes, a systematic approach will be necessary to identify second messengers and signaling pathways by which each isoform couples to its receptors in neuronal and glial cells at various subcellular levels (Farooqui, 2009, 2010b).

1.5 Roles of Eicosanoids in the Brain Eicosanoids are released in response to a variety of physiological and pathological stimuli and function to maintain the body homeostasis. Eicosanoids are not stored in neural cells. Because of their amphiphilic nature, they can cross cell membranes and leave the cell in which they are synthesized to act on neighboring cells. In the brain tissue, neurons and glial cells produce PGs, LTs, and LXs whereas cerebral microvessels and the choroid plexus mainly synthesize TX. Synthesis of eicosanoid is markedly increased during inflammation. LTs not only contribute to the inflammation, but also increase the microvascular permeability, and are potent chemotactic agents. LXs are associated with the resolution of inflammation, stimulation of nonphlogistic phagocytosis of apoptotic cells by macrophages, and EETs modulate ion transport and gene expression, producing vasorelaxation as well as anti-inflammatory and profibrinolytic effects (Phillis et  al., 2006). Eicosanoids act through eicosanoid receptors, which are typically G protein-coupled receptors with seven transmembrane segments that have an extracellular amino terminus and an intracellular carboxyl terminus. These receptors are involved in the generation of cyclic AMP, diacylglycerol, and phosphatidyl 1,4,5-trisphosphate and the modulation of calcium ion influx. Collective evidence suggests that eicosanoids play important roles in neural function including sleep induction (PGD2), long-term potentiation, spatial learning and synaptic plasticity (PGE2), induction of neuroinflammation (PGE2), and resolution of inflammation (lipoxins) (Farooqui, 2009). However, some PGs act neuroprotectively by elevating intracellular cAMP levels in neurons. These molecules also function as anti-inflammatory molecules by reducing the synthesis of nitric oxide and blocking the production of proinflammatory cytokines.

1.5.1 Eicosanoids in Neuroinflammation Neuroinflammation is a protective process that isolates the damaged brain tissue from uninjured area, destroys affected cells, and repairs the extracellular matrix. Inflammatory response is aimed to neutralize or eliminate the causative inflammatory

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factors and ultimately bring about the healing of the lesion by recruiting ­lymphocytes, monocytes, and macrophages of the hematopoietic system and certain glial cells of the CNS (Minghetti, 2005). Neuroinflammation is orchesterated by microglia and astrocytes. These cells are on “high-alert” for inflammatory triggers and are closely associated with the assembly and activation of the inflammasome, a cytoplasmic caspase-1 activating and self-oligomerizing signaling complex with molecular mass of greater than 700 kDa (Chakraborty et al., 2010). Inflammasome orchestrate the activation of the precursors of proinflammatory caspases, which in turn cleave the precursor forms of interleukin-1b, IL-18, and IL-33 into their active forms; the secretion of which lead to marked stimulation of PLA2, COX, LOX, and EPOX isoforms and production of ROS, proteinases, and complement proteins leading to a potent inflammatory response. Alterations in expression of inflammasome mediators may facilitate neurodegeneration in neurotraumatic and neurodegenerative diseases (Farooqui, 2010c). Based on this information, it is proposed that modulation of inflammasome machinery may be a better combating strategy than suppressing all inflammation in most neuroinflammatory situations (Farooqui et al., 2007a, b; Chakraborty et al., 2010; Farooqui, 2010c). Several anti-inflammatory drugs of various chemical ingredients have been shown not only to repress the microglial activation, but to exert neuroprotective effects in neurotraumatic and neurodegenerative situations. Neuroinflammatory response also involves the recruitment of polymorphonuclear leukocytes (PMN) from the blood stream into brain tissue. This PMN migration is a coordinated multistep process involving chemotaxis, adhesion of PMN to endothelial cells in the area of inflammation, and diapedesis (Farooqui et al., 2007a). PMN facilitate the elimination of invading antigens by phagocytosis and release free radicals and lytic enzymes into phagolysosomes. This is followed by a process called resolution which is a turning off mechanism by neural cells to limit tissue injury. Collective evidence suggests that neuroinflammation involves several converging mechanisms responsible for sensing, transducing, amplifying, and turning off mechanisms that involve the participation of eicosanoids. Some PGs and LTs produce proinflammatory effects while others induce anti-inflammatory effects (Farooqui et al., 2006; Farooqui, 2010c). During the inflammatory reaction, synthesis of proinflammatory eicosanoids (PGE2, PGF2, and LTB4) brings about the onset of neuroinflammation, secretion of proinflammatory cytokines, amplifies and maintains the inflammatory responses, and generation of LXs along with the synthesis of resolvins and protectins, breaking the intensity of neuroinflammation by reducing neutrophil entry to the neuroinflammation site, and stimulating the uptake of apoptotic polymorphonuclear leukocytes by macrophages (Yacoubian and Serhan, 2007; Lawrence and Gilroy, 2007; Serhan, 2005) (Fig.  1.3). The onset of neuroinflammation involves the expression and stimulation of iPLA2 with the generation of PGE2 and LTB4 through COX-2, and LOX reactions, respectively. The amplification and maintenence of neuroinflammation is associated with cPLA2 and sPLA2-mediated release and COX and LOX-mediated oxidation of ARA. Finally, the cPLA2, sPLA2, and PlsEtn-PLA2 coupled release of ARA and COX and LOX-mediated generation of LXs, pro­ resolving prostaglandin, PGD2 and synthesis of DHA-derived resolvins and protectins

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may be responsible for the resolution of neuroinflammation (Yacoubian and Serhan, 2007; Lawrence and Gilroy, 2007; Serhan, 2005). In addition, eicosanoids also serve as autocrine factors regulating platelet aggregation, vascular tone, and edema. PLA2, COX-2, and LOX inhibitors have been used to treat acute inflammation and pain in various animal models of inflammation-mediated pain (Farooqui, 2010c).

1.5.2 Eicosanoids in Neurodegeneration Neurodegeneration is a complex, progressive, and multifaceted process that results in neural cell dysfunction and death in the brain tissue. In brain tissue, neural cell death occurs either through apoptosis or necrosis. The necrotic cell death is characterized by the passive cell swelling, intense mitochondrial damage, rapid loss of ATP, alterations in neural membrane permeability, high calcium influx, glutathione depletion, and disruption of ion homeostasis. This type of cell death leads to membrane lysis and release of intracellular components that induce inflammatory reactions. In contrast, apoptotic cell death usually involves the participation of caspases, a group of endoproteases with specificity for aspartate residues in protein. Apoptotic cell death is accompanied by cell shrinkage, dynamic membrane blebbing, chromatin condensation, DNA laddering, loss of phospholipids asymmetry, low ATP levels, and mild calcium overload (Sastry and Subba Rao, 2000; Farooqui et al., 2004; Farooqui, 2009). Thus, apoptosis and necrosis are two extremes of a wide spectrum of cell death processes with different mechanistic and morphological features. However, there is considerable overlap in terms of some common mediators and signal transduction processes, which cannot be separated. Collective evidence suggests that PLA2, COX, LOX, and EPOX-derived eicosanoids along with caspases are involved in necrotic and apoptotic cell deaths. Apoptosis is dependent not only on an adequate ATP level but also on glutathione levels and necrotic cell death is associated with marked reduction in ATP (Farooqui, 2009). It is becoming increasingly evident that under pathological conditions, stimulation of isoforms of PLA2, COX, and LOX result in the accumulation of PGE2, a major ARA-derived PG, closely associated with apoptotic and necrotic types of cell deaths (Farooqui, 2009). Cytokines interleukin-1b (IL-1b) induces COX-2 mRNA and protein synthesis and the release of PGE2 in the human neuroblastoma cell line SK-N-SH (Farooqui and Horrocks, 2007). In addition, an uncontrolled ARA cascade sets the stage for the increased production of ROS and ROS-mediated damage to membrane proteins, ion channels, and receptors. The action of 12-LOX produces lipid hydroperoxides. These hydroperoxides are known to inhibit reacylation of phospholipids in neuronal membranes (Farooqui and Horrocks, 2007). Free radical scavengers and inhibitors of PLA2, and p38 mitogen-activated protein kinase (MAPK) reduce IL-1b-induced synthesis of COX-2, suggesting that upregulation of COX-2 may play an important role in neurodegeneration. Similarly in mutant SOD model of ALS, PGE2 signaling via the EP2 receptor functions in regulating the

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expression of a cassette of proinflammatory genes. Inhibition of EP2 signaling may represent a novel strategy to downregulate PGE2-mediated inflammatory response in the ALS (Liang et al., 2008). As stated above, dehydration of PGD2 generates the bioactive cyclopentenone prostaglandins of the J2 series (Pierre et  al., 2009). 15d-PGJ2 is a specific potent ligand for the peroxisome proliferator activator receptor-gamma (PPARg). It blocks cell growth and induces apoptosis in a number of different cancer cells (Cho et al., 2006). 15d-PGJ2-mediated ROS generation and cell death can be retarded by the antioxidant N-acetylcysteine. 15d-PGJ2 treatment also induces mitochondrial dysfunction, but the 15d-PGJ2-mediated cell death is not blocked by caspase inhibitors (Cho et al., 2006). Taken together, these observations suggest that 15d-PGJ2 triggers cell death through a caspase-independent mechanism and ROS generation and disruption of mitochondrial membrane potential play an important role in the 15d-PGJ2-mediated cell death in A172 human glioma cells (Cho et al., 2006).

1.5.3 Eicosanoids in Nociception (Pain State) Pain is a complex process involving peripheral and central mechanisms (Svensson and Yaksh, 2002). Injury to brain and spinal cord is often accompanied by induction of spontaneous pain, hyperalgesia (increased responsiveness to noxious stimuli), and allodynia (painful responses to normally innocuous stimuli). Peripheral and central mechanisms of pain are associated with inflammation and upregulation of brain and spinal cord COX-2 with a subsequent increase in central PGE2 levels associated with the development of hyperalgesia (Svensson and Yaksh, 2002). Furthermore, peripheral administration of PGI2 and PGE2 produces pain-like behavior in rats (Taiwo and Levine, 1990). COX-2 inhibitors not only prevent the increase in COX-2 activity, but also lower the intensity of allodynia. Collective evidence suggests that in inflamed brain and spinal cord, upregulation of COX-2 and increase in PGI2 and PGE2 levels contribute both to the inflammation itself and to pain hypersensitivity, acting on the peripheral terminals of nociceptors. Studies in models of “sharp” rapidly transmitted pain (hot-plate) and slowly developing, diffuse pain (writhing) with COX-isozyme-deficient mice indicate that COX-1 and COX-3 (not COX-2) play an important role in chronic pain and inflammation in dorsal root ganglia (Ballou et  al., 2000). Collective evidence suggests that all isoforms of COX enzymes are involved in pain transmission processes in brain and spinal cord.

1.5.4 Eicosanoids in Synaptic Plasticity Neuronal plasticity is a fundamental property of brain involved in adequate interactions of neuronal cells with dynamic environment. One of the most throughly investigated forms of the neuronal plasticity is a long-term potentiation (LTP), a  phenomenon associated with learning and memory processes. COX-1 and COX-2-mediated generation of PGE2, PGF2a, and PGD2 modulates the synaptic

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activity and efficacy by exerting their paracrine effect through pre-and postsynaptic glutamate receptors and their autocrine effect through intraneuronal and glial second messengers (Piomelli, 1994). Selective COX-2 inhibitors significantly decrease postsynaptic membrane excitability, back-propagating dendritic action potentialassociated Ca2+ influx, and LTP induction in hippocampal dentate granule neurons, while COX-1 inhibitors are ineffective, supporting the view that COX-2 plays a major role in the induction of synaptic plasticity and LTP (Chen et al., 2002). COX-2 inhibitors block the induction of LTP. The addition of PGE2, but not PGD2 or PGF2a, reverses COX-2-mediated suppression of LTP. These studies suggest that PGE2 is the effector of COX-2-induced synaptic plasticity. Collective evidence suggests that endogenous PGE2 dynamically regulates membrane excitability, synaptic transmission, and plasticity and that the PGE2-mediated synaptic modulation is supported through the involvement of cAMP-PKA and PKC pathways in rat hippocampal CA1 pyramidal neurons.

1.6 Eicosanoids in Neurotraumatic Diseases Neurotraumatic diseases include ischemia, spinal cord trauma, and traumatic brain injury. These injuries arise from very different kinds of initial insult to CNS and PNS. Ischemia is a metabolic insult caused by severe reduction or blockade in cerebral blood flow. This blockade decreases oxygen and glucose delivery to brain tissue and results in the build-up of potentially toxic products in brain (Farooqui et al., 2004; Farooqui, 2010b). In contrast, mechanical impact and shear forces cause spinal cord trauma and traumatic brain injury (McIntosh et al., 1998; Farooqui, 2010b). Spinal cord trauma and traumatic brain injury consists of two broadly defined components: a primary component, attributable to the mechanical insult itself, and a secondary component, attributable to the series of systemic and local neurochemical and pathophysiological changes that occur in brain and spinal cord after the initial traumatic insult (Klussmann and Martin-Villalba, 2005). The primary injury leads to a rapid deformation of brain and spinal cord tissues, resulting in the rupture of neural cell membranes, release of intracellular contents, and disruption of blood flow and breakdown of the blood–brain barrier. In contrast, secondary injury to brain and spinal cord is characterized not only by the release of glutamate from intracellular stores (Sundström and Mo, 2002), but also by excitotoxicity, inflammation, oxidative stress, and overexpression of cytokines (Hayes et  al., 2002; Ahn et  al., 2004). Collective evidence suggests that secondary injury induces edema, neutrophil infiltration, production of a range of inflammatory mediators, and apoptosis. Among neural cells, neurons and oligodendrocytes are equally susceptible to spinal cord trauma or traumatic brain injury and astrocytes and microglia become activated. Recent studies on improved understanding of the pathological changes occurring in oligodendrocytes following spinal cord trauma or traumatic brain injury indicate that the death of oligodendrocytes play a vital role in axonal demyelination of axons, which impairs the conductive capacity of surviving axons (Wu and Ren, 2009). Thus, secondary injury-mediated spinal cord or brain damage that occurs within

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Table 1.3  Activities of COX, LOX, EPOX, and levels of eicosanoids in neurotraumatic diseases COX, LOX, Eicosanoid Disease and EPOX activities levels Reference Ischemia Increased Increased Nogawa et al., 1997; Tomimoto et al., 2000; 2002; IIliff and Alkayed, 2009 Spinal cord trauma Increased Increased Resnick et al., 1998 Traumatic brain injury Increased Increased Cernak et al., 2001 Epilepsy Increased Increased Marcheselli and Bazan, 1996

hours to days after spinal cord trauma or traumatic brain injury contributes ­significantly to neurodegeneration, sensory loss, and other functional disabilities.

1.6.1 Eicosanoids in Ischemic Injury Ischemic injury is accompanied with upregulation of COX-2, and LOX immunoreactivities in neurons and endothelial cells in rodents in acute cerebral infarction models (Iadecola et al., 1999; Tomimoto et al., 2000) (Table 1.3). Intense COX-2 immunoreactivity is observed in the glial cytoplasm following ischemic injury (Tomimoto et  al., 2000). In normal human brain, COX-1 immunoreactivity is expressed in the microglia, but weakly in the neurons. COX-1-immunoreactivity in microglia is markedly increased following ischemic damage and hypoxemia. In contrast, normal human brain does not show COX-2 immunoreactivity. However, COX-2 is induced robustly in the neuronal cell bodies and dendrites during the acute stages of focal ischemic damage, and then subsided at the subacute stages (Tomimoto et  al., 2002). These COX-2-immunoreactive neurons accumulated in the peri-infarct regions, but not in the distant regions. Furthermore, in focal ischemic damage COX-2 is upregulated in the microglia. Like COX-2, neuronal immunostaining for 5-LOX is also upregulated occasionally during hypoxemia and focal ischemic damage (Tomimoto et al., 2002). Glial cells immunoreactivity for 5-LOX appears in the foci of the ischemic damage, with small blood vessels being infiltrated by 5-LOX-immunoreactive mononuclear leukocytes. Collectively, these studies suggest that COX-1 and COX-2 are differentially regulated depending on the cell type and ischemic damage, and that vascular 5-LOX may accelerate the migration of leukocytes and augment the blood–brain barrier permeability (Tomimoto et al., 2002). Transient ischemic attack also induces ischemic tolerance by a mechanism temporally linked to the upregulation of EPOX (P450 2C11) (Alkayed et al., 2002). EETs, which are generated through EPOX catalyzed reaction, exhibit a wide array of potentially beneficial actions in stroke, including vasodilation, neuroprotection, promotion of angiogenesis and suppression of platelet aggregation, oxidative stress, and postischemic inflammation (IIliff and Alkayed, 2009).The increase in COX-2 and 5-LOX, EPOX activities is coupled with the

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accumulation of PGs, LTs, and EETs in the mammalian brain during and after cerebral ischemic injury (Table 1.3) (Gaudet et al., 1980; Moskowitz et al., 1984; Nakano et al., 1990). Increase in PGs and LTs is also noticed in cerebrospinal fluid (Aktan et al., 1991).

1.6.2 Eicosanoids in Spinal Cord Trauma Spinal cord trauma is accompanied by increased expression of COX-2 and induction of 6-keto-PGF1, and TXB2 2 h following injury (Resnick et al., 1998; Harada et al., 2006) (Table 1.3). COX-2 levels peak at 48 h following spinal cord trauma. Selective inhibition of COX-2 activity with SC58125 not only results in neuroprotection from spinal cord trauma, but also improves mean BBB scores in injured animals. RT-PCR studies indicate that COX-2 transcription in the spinal cord starts to increase within 30 min, peaks at 3 h after the spinal cord trauma (Adachi et al., 2005). Western blotting analysis demonstrates that the deglycosylated COX-2 protein is significantly increased 6 h after the spinal cord trauma. Similarly, in severe clip compression model of spinal cord trauma in the rat, the expression of COX-2 and formation of 8-OHdG and protein carbonyl groups are markedly increased after spinal cord trauma while APE/Ref-1 expression is decreased (Bao et  al., 2004). Anti-CD11d mAb treatment clearly attenuates COX-2 expression and 8-OHdG and protein carbonyl formation and rescues APE/Ref-1 expression after spinal cord trauma, demonstrating that anti-CD11d mAb treatment significantly reduces intraspinal free radical formation after spinal cord trauma, thereby reducing protein and DNA oxidative damage (Bao et al., 2004). Accumulating evidence suggests that COX-2 mRNA and protein expression are induced by spinal cord trauma, and that selective inhibition of COX-2 or CD11d mAB improves functional outcome following experimental spinal cord injury. Among PGs, 15d-PGJ2, a metabolite of PGD2 has anti-inflammatory actions and its administration significantly improves the sensory and locomotor functions following spinal cord trauma. Similarly, 5-LOX, FLAP, and CysLT1 mRNAs are markedly increased in spinal cord microglia following spinal cord trauma. The increase in 5-LOX is blocked by p38 mitogen-activated protein kinase inhibitor. Continuous intrathecal infusion of the 5-LOX inhibitor or BLT1 and CysLT1 receptor antagonists retards mechanical allodynia mediated by spared nerve injury. It is proposed that the increase of LT synthesis in spinal cord microglia mediated by p38 MAPK plays a role in the generation of neuropathic pain following spinal cord trauma (Okubo et al., 2010).

1.6.3 Eicosanoids in Traumatic Brain Injury Like spinal cord trauma, traumatic brain injury (TBI) initiates a central inflammatory response, which is accompanied by stimulation in COX and LOX activities, increase in levels of PGs (PGE1, 6-keto-PGF1a, and PGF2a) not only in cerebral

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c­ ortex, but also generation of ROS in brain tissue of ROS in brain tissue, plasma, and CSF (Phillis et al., 2006) (Table 1.3). These elevations in PGs are sustained at up to 30–60 min post injury. Elevations in leukotriene levels also occur in brain after concussive brain injury (Kunz et al., 2002; Phillis et al., 2006). Detailed investigations have shown that elevation in PGs and LC4 (LTC4, LTD4, LTE4, and LTB4) following traumatic brain injury occurs in the cerebral cortex, hippocampus, and CSF in rats. It lasts for 1–2  h and is coupled with persistent accumulation of microglial cells, neutrophils, and macrophages expressing COX-1, COX-2, and LOX (Dhillon et  al., 1996; Schwab et  al., 2001, 2002; Schuhmann et al., 2003). The administration of MK-886 prevents the release of LTs and reduces brain lesion volumes (Farias et al., 2009). It is proposed that increases in leukotriene levels may be related to tissue edema, leukocyte infiltration, macrophage accumulation, microglial activation, and disruption of the blood–brain barrier.

1.6.4 Eicosanoids in Epilepsy Epileptic seizures in rat and human brain produce markedly alterations in ARA levels in focal regions compared to nonfocal regions of the cerebral cortex (Bazan et  al., 2002; Bazan, 1971) with increase in Ca2+ influx, stimulation of PLA2 and COX-2 along with the release of PGs and LTs (Bazan et al., 1986). Thus, spontaneous seizures produce elevation in PGD2 levels in the cerebral cortex, hippocampus, and striatum and of an LTC4-like substance in the cortex (Simmet et al., 1988). These areas are involved in the generation and propagation of seizures. Similarly, the formation of immunoreactive PGF2a and leukotriene-like activity is observed in the brains of spontaneously convulsing gerbils (Simmet et al., 1988). Administration of dexamethasone, a PLA2 inhibitor, attenuated bicuculline-induced free fatty acid accumulation in a dose-dependent manner (Bazan et al., 1986). In the genetically epilepsy-susceptible E1 mouse model, expression of COX-2 in the hippocampus is increased after an epileptic seizure, and indomethacin, a COX-2 inhibitor, shortens the duration from seizure onset to full recovery (Okada et  al., 2001). Collective evidence suggests that epileptic seizures induce upregulation of COX-2 and 5-LOX resulting in the increased synthesis of PGs and LTs. The increase in PGs and LTs can be blocked by selective inhibitors of COX-2 and 5-LOX.

1.7 Eicosanoids in Neurodegenerative Diseases Neurodegenerative diseases are characterized by the site-specific premature and slow death of specific neuronal populations and synapses in brain and spinal cord along with development of brain dysfunction in skilled movements, decision making, cognition, and memory (Graeber and Moran, 2002; Soto and Estrada, 2008). These diseases include Alzheimer disease (AD), Parkinson disease (PD), Huntington

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Table 1.4  Activities of COX, LOX, EPOX, and levels of eicosanoids in neurotraumatic diseases COX, LOX, Disease and EPOX activities Eicosanoid levels Reference AD Increased Increased Bazan et al., 2002; Manev et al., 2000b PD – – – PD animal/cell Increased Increased Phillis et al., 2006; culture models Bharath et al., 2002; Przybyłkowski et al., 2004 ALS Increased Increased Yasojima et al., 2001 CJD Increased Increased Minghetti et al., 2002; Minghetti and Pocchiari, 2007 HIV Increased Increase Griffin et al., 1994

disease (HD), amyotrophic lateral sclerosis (ALS), and prion diseases. It is not known when and how do neurodegenerative diseases actually start and how long does it take for neuropathological changes to appear. The most important risk factors for neurodegenerative diseases are old age, positive family history, unhealthy life style, and exposure to toxic environment (Farooqui and Farooqui, 2009; Farooqui, 2010b). In general neurodegenerative diseases are characterized by their onset in old age, extensive neurodegeneration, synaptic dysfunction, and the accumulation of intracellular or extracellular cerebral deposits of misfolded protein aggregates with a b-sheet conformation, such as b-amyloid (Ab) in AD, a-synuclein in PD, huntingtin in HD, elevation in membrane-associated oxidative stress in ALS resulting in genetic abnormalities (Cu/Zn-superoxide dismutase mutations) (Farooqui and Horrocks, 2007; Farooqui, 2010b) and accumulation of advanced glycation end-products (AGEs) (Miranda and Outerio, 2010).

1.7.1 COX, LOX, EPOX, and Eicosanoids in AD AD, the most common form of dementia, is characterized by the loss of neurons in nucleus basalis and hippocampus, accumulation of b-amyloid, formation of neurofibrillary tangles, oxidative stress, neuroinflammation, decline in cognitive responses, and consequent memory loss. Alterations in COX and LOX pathways in AD have been extensively investigated in brain tissue from AD patients and age-matched controls (Bazan et al., 2002; Manev et al., 2000b; Qin et al., 2003; Hoozemans and O’Banion, 2005). A marked increase of COX-1 and COX-2 expression and immunoreactivity in cerebral cortex and hippocampal regions of AD brains correlates with the number of senile plaques, neuronal atrophy, and increased levels of PGE2 found in AD (Table 1.4) (Pasinetti and Aisen, 1998; Qin et al., 2003). Alterations in the expression of COX-1 and COX-2 have been reported to occur in the different stages of AD pathology. In an early stage, when low-fibrillar Ab deposits are present

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and only very few neurofibrillary tangles are observed in the cortical areas, COX-2 is increased in neurons. The increased expression of neuronal COX-2 parallels and colocalizes with the expression of cell cycle proteins. COX-1 is primarily expressed in microglia, which are associated with fibrillar Ab deposits. It is suggested that in AD brain COX-1 and COX-2 are associated with inflammatory and regenerating pathways, respectively (Hoozemans et  al., 2008). Furthermore, COX-generated product, PGH2, rapidly rearranges in aqueous solution and is converted to levuglandins E2 and D2. These g-ketoaldehydes are highly reactive and rapidly adduct to accessible amine groups on macromolecules, particularly the epsilon-amine of lysine residues on proteins. The immediate LG-lysine adducts are themselves reactive, and can covalently crosslink proteins. PGH2, acting via levuglandins, accelerates the formation of the type of oligomers of Ab, which contribute to neurotoxicity and cell death (Boutand et  al., 2005). The neuroprotective effects of COX-1 and COX-2 inhibitors (NSAID) strongly support the view that upregulation of these enzymes in AD is detrimental to neuronal survival. COX-1 and COX-2 potentiate b-amyloid peptide formation through mechanisms that involve g-secretase activity (Qin et al., 2003). Interplay between increased PGE2 synthesis and g-secretase activity not only regulate b-amyloid peptide accumulation but also modulate neuroinflammation in AD brain (Gasparini et  al., 2004). Detailed investigations reveal that PGE2-mediated production of Ab involves EP4 receptor-mediated endocytosis of PS-1 followed by activation of the g-secretase, as well as EP2 receptor-dependent activation of adenylate cyclase and PKA; both of which are important in the ­inflammation-mediated progression of AD (Hoshino et al., 2009). Furthermore, COX-2 expression is also involved in the regulation of cell cycle activity, and cell cycle abnormalities are associated with the pathogenesis of AD. Re-entry into the cell cycle may underlie COX-2-mediated neuronal damage in AD (Xiang et al., 2002). Studies on 5-LOX-targeted gene disruption on the amyloid phenotype of a transgenic mouse model of AD (Tg2576) indicate that Ab deposition in the brains of Tg2576 mice lacking 5-LOX is decreased by 64–80% compared with Tg2576 controls. This decrease parallels a similar significant reduction in Ab levels. Absence of 5-LOX has no effect on amyloid-b precursor protein (APP) levels and processing, or Ab degradation. Furthermore, in vitro studies indicate that 5-LOX activation or increase in leukotrienes levels stimulates Ab deposion, whereas 5-LOX inhibition reduces Ab deposition. These observations support the view that 5-LOX activity is closely associated with the modulation of g-secretase activity and pathogenesis of AD-like amyloidosis (Firuzi et  al., 2008). Upregulation of 5-LOX expression also occurs in various regions of older rat brain compared to younger animals (Manev and Manev 2004). Collectively, these studies suggest that 5-LOX protein is upregulated in AD hippocampus, where it is primarily associated with neurofibrillary structures and Ab-containing plaques. In addition, Ab peptide fibrilization and tau overphosphorylation themselves activate LOX activity, creating a vicious cycle of pathological cascades, neuroinflammation, and oxidative stress that may perpetuate neuronal degeneration and loss of synapses in AD (Ikonomovic et al., 2008). At present, no information is available on EPOX activity and expression in AD brain. In vitro studies indicate that LOX inhibitors, nordihydroguaiaretic acid,

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AA861, and baicalein, and the EPOX inhibitors, SKF25A and metyrapone prevent cell death in neuronal cultures, suggesting that LOX and EPOX pathways may also be involved in neurodegeneration (Kwon et al., 2005).

1.7.2 COX, LOX, EPOX, and Eicosanoids in PD The gradual and selective loss of dopaminergic neurons in the substantia nigra pars compacta results in PD (Jenner and Olanow, 2006). Loss of these neurons causes pathological changes in neurotransmission in the basal ganglia motor circuit. Although the cause of dopaminergic neuronal death in PD is not fully understood, but accumulation of a-synuclein, oxidative stress, and neuroinflammation is closely associated with the pathogenesis of PD (Farooqui, 2010b). At present, nothing is known about the expression of COX-2 in patients with PD. Recent RT-PCR studies indicate that in mice COX-1 mRNA expression does not change following N-methyl,4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) administration, but COX-2 gene and protein expression in striatum increases from the 3rd to the 7th and 14th days, and diminishes on the 21st day. Production of PGs is augmented only briefly after MPTP treatment, but does not correlate with increased COX-2 mRNA and COX-2 protein production, suggesting that the increase in COX-2 expression does not follow the acute stage of cell death suggesting that COX-2 does not contribute to neurons death following MPTP administration (Przybyłkowski et  al., 2004). Nothing is known about the expression of LOX and EPOX in patients with PD or in animal models of PD.

1.7.3 COX, LOX, EPOX, and Eicosanoids in Amyotrophic Lateral Sclerosis ALS causes progressive loss of motor neurons leading to muscle loss, paralysis, and death from respiratory failure. Although the cause and molecular mechanism associated with pathogenesis of ALS remains unknown, but it is becoming increasingly evident that multiple pathophysiological mechanisms may be associated with the loss of motor neuron. These mechanisms include oxidative stress, mitochondrial impairment, protein aggregation, axonal dysfunction, and alterations in mutant superoxide dismutase expression, cytoskeletal disorganization, glutamate cytotoxicity, inflammation, and apoptotic cell death (Farooqui and Horrocks, 2007). The sporadic form of ALS is characterized by a prominent neuroinflammatory component, upregulation of COX-2 mRNA, and oxidative stress (Yasojima et al., 2001). A similar upregulation of COX-2 mRNA and increase in expression of CD40 also occurs in SOD1 transgenic mice and G93A mice, a transgenic mouse model of ALS at the onset of ALS (Almer et al., 2001). The increase in expression of CD40 and COX-2 occurs in reactive microglia and astrocytes. It is proposed that in ALS the

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upregulation of CD40 in reactive glial cells contribute to motor neuronal loss via the induction of COX-2 and treatment of ALS with COX-2 inhibitors provides neuroprotective effects (Pompl et al., 2003). Although upregulation of COX-2 and PGE2 levels may not be the root cause of ALS, alterations in these parameters may be responsible for the induction and maintenance of inflammation during the progression of ALS. At present, information on LOX and EPOX expression and their activities is not available for the ALS brain and spinal cord.

1.7.4 COX, LOX, EPOX, and Eicosanoids in Creutzfeldt-Jakob Disease (CJD) CJD is the most prevalent prion disease in humans. It is characterized by neuronal loss, astrocytosis, and spongiform degeneration, and deposition of abnormal extracellular b-helix-rich prion protein (PrPsc) (Brown, 1999; Prusiner, 2001). This abnormal protein is an isoform of a normally occurring a-helix-rich prion protein (PrPc) probably associated with synaptic function, regulation of circadian rhythms, and copper transport (Brown, 1999; Prusiner, 2001). RT-PCR and Western blotting studies indicate that both COX-1 and COX-2 are significantly increased in brains from CJD patients compared to age-matched controls (Deininger et  al., 2003). In CSF from sporadic and familial CJD, and in brain homogenates of scrapie-infected mice, the increase in COX-1 and COX-2 activities is accompanied by a several-fold increase in concentrations of PGE2 and F2-isoprostane 8-epi-prostaglandin F2a (8-epi-PGF2a) compared to age-matched controls (Minghetti et al., 2000) (Table 1.4). Similar results on PGE2 levels have been obtained in CSF of CJD variants (Minghetti et al., 2002). These findings suggest that cyclooxygenase activity may participate in the inflammatory process and have a role in CJD pathogenesis. In experimental model of prion diseases, increased COX-2 immunoreactivity is specifically localized to microglial cells and reactive astrocytes. The increase in immunoreactivity is not only related with elevation in COX-2 mRNA and protein levels, but also with the progression of disease. The induction of COX-2 parallels a substantial increase in the brain PGE2 levels. Only few scattered COX-1-positive microglia-like cells are detected in the brain suggesting that COX-2 is the major isozyme that may contribute to neuroinflammation in prion diseases (Minghetti and Pocchiari, 2007; Kim et al., 2007). Enrichment of ARA metabolism also occurs in a cell culture model of prion diseases. Thus, the treatment of primary cerebellar granule neuronal cultures with prion peptide (PrP106-126) causes an enhancement of ARA metabolism through the activation of 5-LOX pathway (Stewart et al., 2001). Nordihydroguaiaretic acid and caffeic acid, the inhibitors of 5-LOX prevent PrP106-126-mediated increase in ARA metabolism and neurodegeneration. These inhibitors also retard the PrP106126-mediated caspase-3 activation and annexin V binding associated with 5-LOXmediated apoptosis. In contrast, indomethacin, a COX-1 and COX-2 inhibitor, and baicalein, a 12-LOX inhibitor, have no affect on PrP106-126-mediated neurotoxicity in cerebellar granule neuronal cultures (Stewart et al., 2001).

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1.8 Conclusions Eicosanoids are lipid mediators that are derived from ARA, which is located at the sn-2 position of glycerol moiety of neural membrane phospholipids. In response to stimuli, ARA is mobilized from phospholipid pools and metabolized by enzymes, COX, LOX, and EPOX to form PGs, LTs, TXs, LXs, and EETs. The production of these lipid mediators is tightly regulated and coordinated in a cell-specific manner. Each mediator interacts with its receptor and performs regulatory and homeostatic functions not only in the proliferation, differentiation, and synaptogenesis, but also in the onset and resolution of inflammation, immune responses, gene expression, and tissue repair. In addition, eicosanoids act as second messengers in learning and memory and contribute to vascular function by regulating cerebral blood flow. Although it may not be the primary cause, but in neurotraumatic as well as neurodegenerative diseases the stimulation and upregulation of COX, LOX, and EPOX isozyme activities and the generation of excessive amounts of prostaglandins, leukotrienes, thromboxanes, and epoxyeicosatrienoic products may result in neuroinflammation, and oxidative stress-mediated neurodegeneration, which are closely associated with the pathogenesis of neurotraumatic and neurodegenerative diseases.

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Chapter 2

Recent Development on the Neurochemistry of Docosanoids

2.1 Introduction Neuroprotectins and resolvins are subfamilies of endogenous oxygenated metabolites derived from n-3 or w-3 fatty acids (eicosapentaenoic acid, 20:5, EPA and docosahexaenoic acid, 22:6, DHA) (Serhan et al., 2000, 2002; Hong et al., 2003; Marcheselli et al., 2003). EPA not only modulates neuronal activity, but also participates in balancing the immune function by reducing arachidonic acid (20:4, ARA) level on cell membrane and prostaglandin E2 (PGE2) synthesis (Farooqui, 2009). DHA is a major structural membrane fatty acid. At the subcellular level, highest concentration of DHA is found in synaptic membranes followed by mitochondria and microsomes (Scott and Bazan, 1989). The presence of DHA in neural membranes significantly alters many basic membrane properties, including acyl chain order and fluidity, elastic compressibility, phase behavior, permeability, fusion, flipflop, and protein activity. DHA also plays important roles in gene expression, and neurotransmission, associated with serotonin, norepinephrine and dopamine receptors. This may be related to the etiology of mood and cognitive dysfunction in neuropsychiatric and neurodegenerative diseases (Farooqui, 2009). In addition, treatment of hippocampal neuronal cultures with DHA reduces glutamate-mediated neurotoxicity (Wang et al., 2003). Changes in the polyunsaturated fatty acid (PUFA) composition of neuronal membranes may lead not only to alterations in the physical characteristics of the neural membrane, but also functional changes in the activity of receptors and other proteins embedded in the membrane glycerophospholipid bilayer. Both of these processes alter physicochemical membrane properties. Beyond their structural importance in neural and retinal membranes, DHA and EPA are involved in the synthesis and functions of neurotransmitters, and in the modulation of the immune system (Farooqui, 2009, 2010a). Neuronal membranes contain phospholipid pools that are reservoirs for the synthesis of specific lipid messengers on neuronal stimulation or injury. These messengers in turn participate in signaling cascades that can either promote neurodegeneration or neuroprotection. DHA and EPA-derived metabolites also improve

A.A. Farooqui, Lipid Mediators and Their Metabolism in the Brain, DOI 10.1007/978-1-4419-9940-5_2, © Springer Science+Business Media, LLC 2011

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2  Recent Development on the Neurochemistry of Docosanoids Modulation of growth and differentiation Modulation of learning and memory

Modulation of transcription factors and gene expression

Modulation of enzymes and ion channels

n-3 Polyunsaturated fatty acids

Generation of pro-and antiinflammatory lipid mediators

Modulation of neurotransmission

Inhibition of oxidative stress Stabilization of neural membranes

Fig. 2.1  Role of polyunsaturated fatty acids in brain

biological functions such as signal transduction, ion channeling and ligand binding to nuclear receptors (Fig. 2.1). Collective evidence suggests that dietary PUFAs regulate both eicosanoids and proinflammatory cytokine production. DHA and EPA are antiinflammatory while ARA (n-6 fatty acids) and its metabolites are proinflammatory. Inappropriate amounts of dietary n-6 and n-3 fatty acids may lead to abnormal inflammatory and immune responses because of their abundance in the brain. An adequate ratio of n-6 and n-3 fatty acids may promote a healthier balance between n-6-and n-3 PUFA-derived lipid mediators, which may maintain optimal neural membrane function (Farooqui, 2009).

2.2 Importance of n-3 Polyunsaturated Fatty Acids in the Brain n-3 polyunsaturated fatty acids are essential for brain growth and development. They play an important role throughout life, as critical modulators of neuronal function and regulation of neuroinflammatory and oxidative stress-mediated mechanisms in the normal CNS during aging and chronic neurological diseases. Inadequate levels of DHA in brain during development and old age induce cognitive deficits such as memory loss and learning disability in experimental animals. Thus, the inadequacy of DHA in neural membranes may contribute to cholinergic, dopaminergic, and glutameterigic receptor dysfunction in synapses associated with the ­hippocampal neurons, and growing evidence suggests that low levels of DHA in the brain are associated not only with neurotraumatic, neurodegenerative and neuropsychiatric diseases, but also with peroxisomal disorders (Farooqui, 2009). DHA, the

2.2  Importance of n-3 Polyunsaturated Fatty Acids in the Brain

51

major n-3 fatty acid found in neurons, has taken on a central role as a target for therapeutic intervention in neurotraumatic, neurodegenerative, and neuropsychiatric diseases (Farooqui, 2009, 2010a). DHA acts as a ligand for the PPAR-g and the RXR receptors. In conjunction with these receptors, which act as transcription factors, DHA modulates various neurochemical processes that maintain cellular homeostasis by modulating lipid metabolism, neural cell differentiation, and apoptosis (Farooqui, 2009). DHA downregulates protein kinase C, Ras, and NF-kB; activates the Jak/Stat pathway, and sustains phosphorylation of EGFR. DHA attenuates the transcription of NF-kB-dependent genes. Thereby the COX-2/PGE2-dependent generation of pro-angiogenic vascular endothelial growth factor and levels of antiapoptotic bcl-2 and bcl-X(L) are reduced. Eicosanoid-independent pro-apoptotic pathways include enhanced lipid peroxidation, modulation of mitochondrial calcium homeostasis, and enhanced production of reactive oxygen species (ROS) as well as activation of p53. In brain, DHA also restores levels of cerebellar phospho (p)-AKT, phospho-extracellular regulated kinase (p-ERK) and phospho-c-Jun N-terminal kinase (p-JNK) supporting their role in down-regulation of neuronal apoptosis (Sinha et al., 2009). n-3 PUFAs also quench gene expression of cyclooxygenase-2 and other enzymes, thereby diminishing the formation of proinflammatory eicosanoids. In addition, n-3 PUFAs scavenge of free radicals, which diminish inflammatory response and oxidation of lipoprotein particles, notably low-density lipoproteins. DHA also suppresses insulin/neurotrophic factor signaling deficits, neuroinflammation, and oxidative damage that may contribute synaptic loss and neuronal dysfunction in old age and demented subjects (Cole and Frautschy, 2010). Finally, DHA increases brain levels of neuroprotective brain-derived neurotrophic factor and reduces the (n-6) fatty acid arachidonate and its oxidative metabolites. The cross-talk among these molecular processes has distinct neuroprotective effects not only through the stabilization of neural membranes, modulation of ion channels and receptors, but also through inhibition of inflammatory processes and generation of anti-inflammatory lipid mediators (Farooqui et al., 2007; Farooqui, 2009). Dietary intake of ARA, EPA, and DHA results in incorporation of these PUFAs in neural membranes. EPA is readily incorporated into membrane phospholipids with a considerable amount present in mitochondrial cardiolipin. The incorporation of EPA in mitochondrial membranes is accompanied by a decrease in mitochondrial membrane potential, and increase in lipid peroxide and ROS generation (Colquhoun, 2009). DHA incorporates into ethanolamine and serine glycerophospholipids, which are located in the inner leaflet of lipid bilayer. The ethanolamine glycerophospholipids include phosphatidylethanolamine and ethanolamine and choline plasmalogens. These lipids are closely associated not only with the stability of synapse and functioning of various receptors but also with membrane fluidity and permeability (Farooqui et al., 2008), whereas DHA-enriched phosphatidylserine (PtdSer) is an essential cofactor for the activation of several proteins including, protein kinase C, Raf-1 kinase, Akt, which translocate from cytoplasm to the membrane for their activation, supporting the view that translocation of these kinases may target signaling events modulated by the DHA-mediated neuronal specific increase of PtdSer (Kim et al., 2010). PtdSer also modulates activities of diacylglycerol kinase, nitric

52

2  Recent Development on the Neurochemistry of Docosanoids

oxide synthase, and Na+, K+-ATPase (Ikemoto et  al., 2000; Farooqui, 2009). Ester-linked ARA, EPA, and DHA are mobilized by PLA2 to generate free ARA, EPA, and DHA (Hirashima et al., 1992; Farooqui and Horrocks, 2007; Green et al., 2008; Farooqui, 2010b), which can act as substrates for cyclooxygenase and lipoxygenase to produce lipid mediators (eicosanoids, resolvins, and protectins/neuroprotectins) (Nieves and Moreno, 2006; Cherian, 2007; Farooqui and Horrocks, 2007). Compared to DHA, levels of EPA in brain tissue are quite low. This may be due to rapid b-oxidation of EPA following its uptake by the brain tissue (Chen et  al., 2009a). In rat hepatocytes L-carnitine, a long chain fatty acid mitochondrial matrix transporter, not only increases b-oxidation of EPA, but only marginally elevates the oxidation of ARA and alleviates competitive inhibition of ARA-dependent PGE2 synthesis and COX-2 expression by EPA. It is suggested that L-carnitine modulates the competition between ARA and EPA in PG synthesis in liver cells by enhancing oxidation of EPA. This suggests that the beneficial effects of n-3 PUFA, especially EPA, are modulated by the cellular oxidation capacity (Du et al., 2010). Supplementation of diet containing EPA not only antagonizes the synthesis of PGE2 from ARA, but also reduces IL-1b-mediated increase in levels of PGE2, elevation in corticosterone, and upregulation in nerve growth factor expression in olfactory bulbectomized rat model of depression (Song et  al., 2004). Furthermore, IL-1b-mediated suppression of the antiinflammatory cytokine IL-10 is also blocked by the ethyl-EPA treatment. Collectively, these results indicate that ethyl-EPA treatment has beneficial effects on neurotraumatic, neurodegenerative, and neuropsychiatric (cognitive and affective disorders) in which inflammation and oxidative stress play a critical role (Song et al., 2004, 2009; Farooqui, 2009).

2.3 EPA-Derived Lipid Mediators in the Brain The lipid mediators derived from EPA include 3-series of prostaglandins and thromboxanes, 5-series of leukotrienes, and E series resolvins (Resolvin E1 or RvE1) (Serhan et al., 2000, 2002). The oxidized metabolites of EPA possess antiinflammatory and antiproliferative effects (Table 2.1). The oxidation of EPA by COX and LOX enzymes results in the production of 3-series of prostaglandins and thromboxanes and the 5-series of leukotrienes (Fig. 2.2). These eicosanoids have different biological properties than the corresponding analogs generated by the oxidation of ARA. For example, TXA3 is less active than TXA2 in aggregating platelets and constricting blood vessels (Calder and Grimble, 2002; Calder, 2009). In addition to generating less active lipid mediator, EPA exerts its effects on other aspects of inflammation like leukocyte chemotaxis and inflammatory cytokine production. Some of these effects are likely due to changes in nuclear factor-kB-mediated gene expression, e.g. adhesion molecule in microglia, astrocytes, and in visceral inflammatory and immune cells. In contrast, recent studies on the effects of prostaglandins (PGE2 and PGE3) and leukotrienes (LTB4 and LTB5) on endothelium permeability and mononuclear adhesion and migration across endothelial cell

2.3  EPA-Derived Lipid Mediators in the Brain

53

Table 2.1  Activities of resolvins and protectins/neuroprotections/maresin in neural and nonneural systems Parameter Resolvins and protectins Reference Aggregation Antiaggregatory Serhan et al., 2000, 2002, 2009; Serhan, 2005; Farooqui, 2009 Thrombotic activity Antithrombotic Serhan et al., 2000, 2002, 2009; Serhan, 2005; Farooqui, 2009 Inflammation Antiinflammatory Serhan et al., 2000, 2002, 2009; Serhan, 2005; Farooqui, 2009 Lipid status Hypolipidemic Serhan et al., 2000, 2002, 2009; Serhan, 2005; Farooqui, 2009 Cell death nature Antiapoptotic Serhan et al., 2000, 2002, 2009; Serhan, 2005; Farooqui, 2009 Effect on excitotoxicity Antiexcitotoxic Serhan et al., 2000, 2002, 2009; Serhan, 2005; Farooqui, 2009 Pain sensitivity Antinociceptive Serhan et al., 2000, 2002, 2009; Serhan, 2005; Farooqui, 2009

Diet

EPA PLA2?

J3-IsoP

Free EPA

(Antiinflammatory) RvE2 LTB5 ProinflamLTC5 matory LTD5

F3-IsoP

RvE1 (Antiinflammatory)

CO

X LO

X

5-

LT (5-series)

PG (3-series)

15-HPEPE

12-HPEPE

15-HEPE

12-HEPE

TXA3 PGE3

Proinflammatory

PGI3

Activation of Nrf2

Fig. 2.2  Generation of EPA-derived lipid mediators in neural and non-neural tissues. Phospholipase A2 (PLA2); 5-lipoxygenase (5-LOX); resolving E1 (RvE1); resolving E2 (RvE2); 3-series of prostaglandins and thromboxanes (PGE3 and PGI3); 5-series of leukotrienes (LTB5, LTC5, and LTD5); 15-hydroperoxy-5,8,11,13,15-eicosapentaenoic acid (15-HPEPE); 12-hydroperoxy-5,8,11,13, 12-eicosapentaenoic acid (12-HPEPE); 12-hydroxy-5,8,10,14-eicosatetraenoic acid (15-HETE), and 15-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE); cyclopentenone-IsoP moleculesderived from EPA (J3-IsoP); F-ring IsoP-like compounds derived from EPA (F3-IsoPs); and Nuclear factor (erythroid-derived 2)-like 2 (Nrf2)

54

2  Recent Development on the Neurochemistry of Docosanoids

cultures indicate that PGE3 produces more pronounced effect on trans-endothelial Evans Blue-albumin (EBA) permeability than PGE2, and these effects are antagonized by EP1 and EP2 antagonists (Moreno, 2009). LTB4 and LTB5 produce a slight effect on EBA extravasation. Collective evidence suggests that the enhancement of endothelial permeability in the presence of polymorphonuclear (PMN) cells is modulated by interplay between leukotrienes and prostaglandins (Moreno, 2009; Yin et al., 2007). n-3 PUFAs also decrease the production of the classic inflammatory cytokines tumor necrosis factor, interleukin-1, and interleukin-6 and the expression of adhesion molecules involved in inflammatory interactions between leukocytes and endothelial cells. These latter effects may occur by eicosanoid-independent mechanisms, including modulation of the activation of transcription factors involved in inflammatory processes. The anti-inflammatory actions of long chain n-3 fatty acid-induced effects may be of therapeutic use in conditions with an acute or chronic inflammatory component. EPA and ARA compete for the same COX enzymes (Zhao et al., 2004; Phillis et al., 2006), but the rate of oxidation of EPA is only 10% of the ARA. Although EPA significantly inhibits COX-1-mediated oxidation of ARA (Wada et al., 2007; Schmitz and Ecker, 2008), the oxidation of ARA by COX-2 is only modestly inhibited by EPA. EPA-derived eicosanoids antagonize the pro-inflammatory effects of ARA-derived eicosanoids. EPA and ARA are ligands/modulators for the nuclear receptors NF-kB, PPAR and SREBP-1c, which modulate various genes of inflammatory signaling and lipid metabolism. In addition, the n-3/n-6 PUFA ratio of neural membranes and microdomains strongly influences membrane function and numerous cellular processes such as cell death and survival (Wada et al., 2007; Schmitz and Ecker, 2008; Yin et al., 2007). EPA is a precursor for resolvins E series (Arita et  al., 2006, 2007), including resolvin E1 (RvE1; (5S,12R,18R)-trihydroxy-6Z,8E,10E,14Z,16E-eicosapentaenoic acid) and resolvin E2 (15S,18R-dihydroxy-EPE) (Fig.  2.3). EPA is oxidized to 18R-hydroxyeicosapentaenoic acid (18R-HEPE) by endothelial cell cyclooxygenase-2 (COX-2). Aspirin acetylates COX-2 and the acetylated enzyme no longer produces prostaglandins, but can still convert EPA to 18R-HEPE. During cell–cell interactions, 18R-HEPE is released to neighboring leukocytes for subsequent conversion by 5-LOX to RvE1 via a 5(6) epoxide-containing intermediate. RvE1 is present in human whole blood, and its levels can be increased by ingestion of aspirin (Serhan et al., 2000; Arita et al., 2006, 2007). RvE1 is transformed into several metabolic products, including 20-hydroxy-RvE1, 20-carboxy-RvE1, 19-hydroxy-RvE1, 18-oxo-RvE1 and 10,11-dihydro-RvE1 by human PMNs and whole blood as well as in murine inflammatory exudates, lungs, spleen, kidney, and liver (Seki et al., 2009, 2010). Among these products, 20-carboxy-RvE1, 18-oxo-RvE1, and 10,11-dihydroRvE1are essentially biologically inactive and may serve as inactive biomarkers of RvE1 metabolism in vivo. In contrast, 20-hydroxylated product of RvE1 has some of the activity of RvE1 suggesting that more metabolites of RvE1 are generated during inflammatory response. RvE1 and RvE2 produce potent anti-inflammation/pro-resolution effects in vivo (Arita et al., 2006) via specific G protein-coupled receptors (see below). RvE1 suppresses the activation of NF-kB by tumor necrosis factor-a (TNF-a) through

2.3  EPA-Derived Lipid Mediators in the Brain

55

OOH OH COOH

EPA

5-LO HO 5S-hydroperoxy-18-hydroxy-EPE

ida

tio

n

18-hydropxy-EPE

zy

COOH

En

5S-(6)-epoxy18-HEPE

OH

on

ma

tic

ep

COOH

cti du Re

O

ox

HOOC

HO

HO

Resolvin E2

OH Resolvin E1 OH

HO COOH

Suppression of NF-kB Prevention of pain & Inflammation, promotion of resolution

Modulation of Akt phosphorylation

Fig. 2.3  Synthesis of RvE1 and RvE2 from EPA and proposed roles of RvE1 in brain. Dietary EPA is converted to RvE1 by the action of 5-lipoxygenase (5-LOX). After aspirin treatment, RvE1 synthesis occurs even in the absence of inflammation. Aspirin inactivates COX-2 but allows the synthesis of the intermediate 18R-hydroxy-EPA which is converted to RvE1 through the action of 5-LOX. In  nonneural cells, RvE1 interacts with ChemR23 receptor (a specific seven-transmembrane G protein–coupled receptor) and inhibits inflammation by down-regulating NF-kB activation and blocking the generation of proinflammatory mediators (Adapted from Serhan et al., 2008)

binding to human polymorphonuclear leukocyte (PMN) (Arita et  al., 2007). Intrathecal RvE1 injection blocks spontaneous pain and heat and mechanical hypersensitivity evoked by intrathecal capsaicin and TNF-a. RvE1 mediates its anti-inflammatory activity not only by decreasing neutrophil infiltration, paw edema, but also by inhibiting the expression of proinflammatory cytokine (Xu et  al., 2010). In addition, RvE1 not only abolishes transient receptor potential vanilloid subtype-1 (TRPV1)- and TNF-a-mediated excitatory postsynaptic current increases, but inhibits TNF-a-mediated N-methyl-d-aspartic acid (NMDA) receptor hyperactivity in spinal dorsal horn neurons via inhibition of the extracellular signal-regulated kinase (ERK) signaling pathway (Xu et al., 2010). It is suggested that resolvins may normalize the spinal synaptic plasticity associated with pain hypersensitivity (Fig.  2.4). In addition, resolving E1 also prolongs the survival of solid organ transplants (Levy et al., 2010).

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2  Recent Development on the Neurochemistry of Docosanoids

Series D and E resolvins

Inhibition of nociception

Inhibition of hypersensitivity

Modulation of Synaptic plasticity

Inhibition of PMN infiltration Inhibition of inflammation

Fig. 2.4  Roles of resolvins in neural and non-neural tissues

RvE1 mediates its actions through specific seven-membrane spanning G proteincoupled receptors, which are expressed on dendritic cells and monocytes. These receptors are called as ChemR23 receptors (Ohira et al., 2010). RvE1 modulates the phosphorylation of Akt in a time - and dose-dependent manner in human ChemR23transfected Chinese hamster ovary cells (Ohira et al., 2010). RvE1 not only stimulates the phosphorylation of Akt, but also phosphorylates a downstream 30-kDa protein, which is identified as a ribosomal protein S6. The phosphorylation is blocked by a phosphatidylinositol 3-kinase (PtdIns 3K) inhibitor (wortmannin) and an ERK inhibitor (PD98059) but not by a p38-MAPK inhibitor (SB203580). Recent studies on RvE1 metabolome indicate that RvE1 is metabolized to several novel products by human polymorphonuclear leukocytes and whole blood as well as in murine inflammatory exudates, spleen, kidney, and liver. The new RvE1-derived products include 19-hydroxy-RvE1, 20-carboxy-RvE1, and 10,11-dihydro-RvE1. The metabolomic profiles of RvE1 were species-, tissue-, and cell type-specific (Hong et al., 2008). Direct comparisons between RvE1 and RvE1-derived products indicate that 10,11-dihydro-RvE1, 18-oxo-RvE1, and 20-carboxy-RvE1 display reduced bioactivity in vivo. At concentrations as low as 1 nM, RvE1 stimulates macrophage phagocytosis. In addition, RvE1 enhances phagocytosis of zymosan A by human macrophages, which are inhibited by PD98059 and rapamycin (mTOR inhibitor). Collectively, these studies indicate that RvE1 initiates direct activation of ChemR23 and signals receptor-dependent phosphorylation. These phosphorylationsignaling pathways identified for RvE1 receptor–ligand interactions emphasize the importance of endogenous pro-resolving agonists in resolving acute inflammation (Ohira et al., 2010). The oxidation of RvE1 by NAD+-dependent hydroxyprostaglandin dehydrogenase to the oxo product (18-oxo-RvE1) results in inactivation of RvE1 activity. Based on detailed pharmacological investigations on the effect of RvE1, it is suggested that

2.3  EPA-Derived Lipid Mediators in the Brain

57

the binding of RvE1 to BLT1 receptor mediate the resolution of inflammation (Arita et al., 2006, 2007). RvE2 is synthesized from EPA by human PMNs and produces similar actions as RvE1. When given together, the protective actions of RvE1 and RvE2 are additive at low doses, suggesting distinct receptors for RvE2 and RvE1 (Tjonahen et al., 2006). Collective evidence suggests that both RvE1 and RvE2 contribute to the beneficial actions of EPA in human diseases. Rv E1 and RvE2 dramatically reduce dermal inflammation, peritonitis, dendritic cell migration, and interleukin production. Similarly, in experimental model of pneumonia in mice, RvE1 significantly decreases lung tissue levels of several proinflammatory chemokines and cytokines, including IL-1b, IL-6, HMGB-1, MIP-1a, MIP-1b, keratinocyte-derived chemokine, and MCP-1, in a manner independent of the anti-inflammatory mediators IL-10 and lipoxin A4. In addition, animals treated with RvE1 show a marked improvement in survival suggesting that RvE1 administration may represent a novel therapeutic strategy for acute lung injury and pneumonia (Seki et al., 2010). Non-enzymic oxidation of EPA generates cyclopentenone-IsoP (A3/J3-IsoPs) (Brooks et al., 2008). Like ARA-derived IsoPs, the A3/J3-IsoPs are formed in situ esterified in phospholipids and are then released in the free form by a PAF hydrolase like PLA2. It is suggested that A3/J3-IsoPs are generated after rearrangement of their endoperoxides precursors to molecules with E- and d-prostane rings that subsequently dehydrate. The oxidation products of EPA activate the Nrf2 transcription factor (a member of the basic leucine-zipper NF-E2 family) that induces antioxidant responses in reactive electrophiles-mediated induction of gene transcription (Gao et al., 2006, 2007). These genes include glutathione S-transferase, NADPH:quinone oxidoreductase 1, UDP-glucuronosyltransferase, gamma-glutamate cysteine ligase, and hemeoxygenase-1. They mediate detoxification and/or to exert antioxidant functions, thereby, protecting cells from genotoxic damage. The non-enzymic products of EPA interact with the ubiquitin ligase adaptor protein Keap1 (a BTB-Kelch protein), the direct inhibitor of Nrf2, initiating Keap1 dissociation from Cullin3 to yield Nrf2 translocation and activation causing induction of Nrf2-directed gene expression. Binding of Keap1 to Nrf2 directs the ubiquitination and proteasomedependent degradation of Nrf2. Inhibition of Nrf2 ubiquitination results not only in Nrf2 accumulation, but also in increased ARE-directed gene expression, and an enhanced ability to respond to oxidant stress (McMahon et al., 2003). It is proposed that molecules containing an a,b-unsaturated carbonyl moiety, specifically J3-IsoPs, may be responsible for eliciting this biological activity (Gao et  al., 2007). Thus, both enzymic and non-enzymic oxidation products of EPA modulate signal transduction processes associated with anti-inflammatory responses in neural and non-neural systems. In addition, EPA is also metabolized by cytochrome P450 (CYP2C/2J)-isoforms, which convert EPA into 17,18-epoxyeicosatetraenoic (17,18-EEQ) acid. These omega-3 epoxides are highly active as antiarrhythmic agents. They suppress Ca2+induced increased rate of spontaneous beating of neonatal rat cardiomyocytes, at low nanomolar concentrations (Arnold et al., 2010). Dietary EPA/DHA supplementation of rats results in substantial replacement of ARA by EPA in membranes from

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2  Recent Development on the Neurochemistry of Docosanoids

heart, kidney, liver, lung, and pancreas, with less pronounced effects in brain. Changes in fatty acids are accompanied by concomitant changes in endogenous CYP metabolite profiles (e.g. altering the epoxyeicosatrienoic acids (EETs):17, 18-epoxyeicosatetraenoic (17,18-EEQ) and 19,20-epoxydocosapentaenoic acid (19,20-EDP) ratio from 87:0:13 to 27:18:55 in the heart). Collectively, these studies demonstrate that CYP-enzymes efficiently convert EPA to novel epoxy- and hydroxy-metabolites that may mediate some of the beneficial cardiovascular effects of dietary omega-3 fatty acids (Arnold et al., 2010).

2.4 DHA-Derived Lipid Mediators in the Brain DHA is a precursor of D-series resolvins. The action of 15-LOX-like enzyme converts endogenous DHA into 17S-hydroperoxy-DHA (17S-H(p)DHA). This biosynthetic intermediate is transformed into several bioactive compounds, including resolvin D1-D6 (RvD1, RvD2, RvD3, RvD4, RvD5, and RvD6) (Figs. 2.5 and 2.6). There are

Diet

DHA-PlsEtn Acyltransferase

( (Proinflammatory) 2

Acyl-CoA

LA

NK NK

4-HHE 4-HHE

-P

NF NF

tn

NP

Protectins Protectins (docosatriens) (docosatriens)

sE

Lyso-PlsEtn

Pl

Non-enzymic oxidation

15-LOX

15-LOX

DHA DHA

Resolvins Resolvins (RvD seies)

(Anti-inflammatory)

(Anti-inflammatory)

RvD1 RvD1

RvD2 RvD2

RvD3 RvD3

RvD4 RvD4

RvD5 RvD5

RvD6 RvD6

Fig. 2.5  Generation of enzymic and non-enzymic lipid mediators from DHA. Plasmalogen (PlsEtn); Plasmalogen-selective PLA2 (PlsEtn-PLA2); lyso-ethanolamine plasmalogen (lysoPlsEtn); neuroprostane (NP); neurofuran (NF); neuroketal (NK); 4-hydroxyhexenal (4-HHE); Resolvin D (RvD)

2.4  DHA-Derived Lipid Mediators in the Brain

59

Phospholipids containing DHA PlsEtn-PLA2 DHA Asp:COX-2 AT-protectin D1

17R-hydroperoxy-DHA

7S-hydroperoxy-17R-hydroxy-DHA

4S-hydroperoxy-17R-hydroxy-DHA

4S(5)-epoxy-17R-hydroxy-DHA

7S(8)-epoxy-17R-hydroxy-DHA

OH

HO

OH

COOH

COOH OH

HO

Aspirin triggered resolvin D1 AT-RvD1

HOOC

COOH

OH

OH OH

OH HO

OH

HO

Aspirin triggered resolvin D2 AT-RvD2

Aspirin triggered resolvin D3 AT-RvD3

Aspirin triggered resolvin D4 AT-RvD4

Suppression of PMN infiltration Downregulation of NF-KB Promotion of resolution Inhibition of apoptosis Neuroprotection following ischemia

Fig. 2.6  Generation of resolvins D series from DHA in non-neural cells. DHA is released from PtsEtn by plasmalogen-selective PLA2. Free DHA is metabolized in aspirin-COX-2 dependent manner into ATRvD1, ATRvD2, ATRvD3, and ATRvD4 through 7S(8)-epoxy-17R-hydroxy-DHA formation. Similar synthesis may occur in neural cells (Adapted from Serhan, 2005; Serhan et al., 2008)

also aspirin-triggered forms of D series resolvins (AT-Rv), which are also enzymically derived from DHA through a pathway with sequential oxygenation initiated by aspirin-acetylated COX-2 (Serhan and Chiang, 2008). For the biosynthesis of AT-RvDs, DHA is initially transformed into 17R-hydroxy-DHA. The stereochemistry at carbon 17 is maintained, so each AT-RvD carries a 17R alcohol group configuration, resulting in a distinct series of compounds. The complete stereochemistry of RvD1 (7S,8R,17S,-trihydroxy-4Z,9E,11E,13Z,15E,9Z-docosahexaenoic acid) and

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2  Recent Development on the Neurochemistry of Docosanoids

AT-RvD1 (7S,8R,17R-trihydroxy-4Z,9E,11E,13Z,15E,19Z-docosahexaenoic acid has also been studied) (Serhan, 2005; Ariel and Serhan, 2007). These lipid mediators not only antagonize the effects of eicosanoids, but also modulate leukocyte trafficking and down-regulate the expression of cytokines in glial cells. They possess potent anti-inflammatory, neuroprotective and pro-resolving properties (Hong et al., 2003; Marcheselli et al. 2003; Serhan, 2005). Biosynthesis of neuroprotectin D1 (NPD1) involves the release of DHA by PlsEtn-PLA2 or from PtdSer by Ptdser-selective PLA2 or cPLA2. Free DHA undergoes lipoxygenation and epoxidation followed by hydrolysis to form NPD1. Thus far, binding sites for NPD1 have been identified in retinal pigment epithelium cells and polymorphonuclear cells (Niemoller and Bazan, 2010) (Fig. 2.7). Like DHA, docosapentaenoic acid n-6 (DPAn-6; 22:5) also generates resolvins called 17S-HDPAn-6 (17S-hydroxydocosa-4Z,7Z,10Z,13Z,15E-pentaenoic acid) and 10S,17S-HDPAn-6 (10S,17S-dihydroxydocosa-4Z,7Z,11E,13Z,15E-pentaenoic acid). These resolvins like DHA-derived D series resolvins produce potent anti-inflammatory effects (Dangi et al., 2010). However, DPA-derived metabolites are not strong inhibitors of cytochrome-P450 enzymes. In brain tissue, initiation, intensification, and resolution of inflammation requires a distinct micro-environments composed of astrocytes, microglia endothelial cells, and highly specialized extracellular matrix (ECM) components of neural cells. At the conclusion of the inflammatory response, above neural cells also contribute to the process resolution by (a) the withdrawal of survival signals, (b) the normalization of chemokine gradients, and (c) the synthesis of lipoxins, resolvins and neuroprotectins inducing resolution programs that allow infiltrating cells to undergo apoptosis. The dysregulation of lipoxin, resolvin, and neuroprotectin may lead to persistent chronic inflammation, which can be remarkably stable.

2.4.1 17S D Series Resolvins Resolvins act through specific receptors found in neural and non-neural cells. They are called as resolvin D receptors (resoDR1) (Serhan et  al., 2006a, b; Serhan et  al. 2008a, b). ResoDR1 induce potent anti-inflammatory and immunoregulatory activities. D series resolvin not only block the production of pro-inflammatory mediators, but also regulate the trafficking of leukocytes cells at the sites of inflammation (Serhan et al., 2008; Hong et al., 2003) and inhibit the expression of cytokines and modulate inflammation. Studies on characterization of ResoDR1 are currently in progress.

2.4.2 Protectins and Neuroprotectin 15-LOX converts DHA into protectin D1 through epoxide intermediate with epoxy group at the 16(17) position. The synthesis of PD1 with complete stereochemistry

2.4  DHA-Derived Lipid Mediators in the Brain

61

Phospholipids containing containing DHA Phospholipids DHA

cPLA2

PlsEtn-PLA2

DHA 15-LOX

H(O)O

COOH

17S-hydroperoxy-DHA

O2

Reduction

Second oxygenation Reduction

HO

10S, 17S-dihydroxy-DHA COOH

O

COOH

17S-hydroxy-DHA

16(17)-epoxy-DHA Enzymic hydrolysis

OH

COOH

OH

Neuroprotectin D1

Antiapoptotic Antiapoptotic Bcl-2 proteins Bcl-2 proteins Anti-inflammatory Inhibition of NF-KB Suppression of PMN infiltration

Fig. 2.7  Generation of neuroprotection D1 from DHA in brain. DHA is hydrolyzed from PlsEtn by PlsEtn-PLA2. It undergoes lipoxygenation, epoxygenation and hydrolysis to generate NPD1 (Adapted from Serhan et al., 2008 and Marcheselli et al., 2010)

62

2  Recent Development on the Neurochemistry of Docosanoids Diet

ARA ARA

EPA EPA

Aspirin COX-2 RvE RvE 1 1 RvE RvE 2 2

COX-2 5-LOX 3-seriesPGs PGs 3-series 3-Series 3-seriesTXs TXs 5-Series LTs 5-series LTs

DHA DHA

COX-2

15-LOX

−-

−-

−-

−−-

RvD , RvD , RvD RvD11, RvD2,2 RvD3 3 RvD RvD,5,RvD RvD6 RvD 4,, RvD 4

15-LOX

Aspirin COX-2

5-LOX

5

AT-RvD1-D4 AT-RvD1-D4

NPD1 NPD1

6

Highlevels levels of 2-series High 2-series PGs,TX2,4-series 4-series LTs LTs PGs,TX2,

Oxidative stress Oxidative stress Inflammation Inflammation −-

−

Apoptosis Apoptosis

Fig. 2.8  Regulation of inflammation, oxidative stress, and apoptotic cell death by resolvins and neuroprotections

of PD1 has been established and confirmed by total organic synthesis, showing that PD1 is 10R,17S-dihydroxy-docosa-4Z,7Z,11E,13E,15Z,19Z-hexaenoic acid (Serhan et al., 2000, 2002, 2006b; Hong et al., 2003; Marcheselli et al., 2003). The reaction sequence of biosynthesis for PD1 via the epoxide intermediate distinguishes it from the formation of the double dioxygenation product 10S,17S-dihydroxyDHA. PD1 is more potent than DHA in neuroprotective action. In sharp contrast, PD1 positional isomers, including 4S,17S-diHDHA or 7S,17S-diHDHA, display less potent and non-selective actions. The occurrence of PD1 has also been reported in brain, where it is called as neuroprotectin D1 (10R, 17S-dihydroxy-docosa4Z,7Z,11E,13E,15Z,19Z-hexaenoic acid, NPD1) (Hong et  al., 2003). Tritiumlabeled NPD1 (3H]-NPD1) binds to ARPE-19 cells with high-affinity (Kd 31.3 ± 13.1  pmol/mg of cell protein). The stereospecific NPD1 interactions with these cells provide potent protection against oxidative stress-mediated apoptosis (Fig. 2.8). 3H-NPD1/PD1 also shows specific and selective high affinity binding with isolated human neutrophils (Kd approximately 25 nM). Neither resolvin E1 nor lipoxin A4 compete for 3H-NPD1/PD1 specific binding with human neutrophils. Collectively, these results indicate that stereoselective specific binding of NPD1/PD1 with retinal pigment epithelial cells as well as human neutrophils may occur through specific receptors in both the immune and visual systems (Marcheselli et al., 2010). Although the isolation and characterization of 10(S),17(S)-dihydroxy-docosahexa4Z,7Z,11E,13Z,15E,19Z-enoic acid, a main dihydroxy conjugated triene derived

2.4  DHA-Derived Lipid Mediators in the Brain

63

from the lipoxygenation of DHA has also been reported, but nothing has been reported on its interactions with neural and non-neural cells. It is an isomer of protectin/neuroprotectin D1 (PD1/NPD1) and has been named PDX (Chen et al., 2009b). This metabolite inhibits human blood platelet aggregation at submicromolar concentrations. Like resolvins, the neuroprotectins block the infiltration of PMN (Serhan et al., 2006b) (Figs. 2.4 and 2.7). They down-regulate the expression of cytokines in the glial cells (Hong et al., 2003; Serhan et al., 2006b). NPD1 reduces retinal and corneal injury (Mukherjee et al., 2004; Gronert et al., 2005) and produces neuroprotective effect in ischemic injury (Marcheselli et al., 2003). Similarly, NPD1 promotes neural cell survival via the induction of antiapoptotic and neuroprotective geneexpression programs that suppress Ab42-mediated neurotoxicity in Alzheimer disease (AD) (Lukiw et al., 2005; Bazan, 2009a, b). DHA and NPD1 protect synapses and decrease the number of activated microglia in the hippocampal system (Pomponi et al., 2008). Although the molecular mechanisms associated with the above processes are not fully understood. However, it is becoming increasingly evident that NPD1 not only inhibits IL-1b-stimulated expression of COX-2, but also regulates apoptotic signaling at the level of mitochondria, inducing the release of cytochrome c and activating effector enzyme, caspase-3 (Table 2.2). In addition in rat-infused with Ab, DHA and its oxidative metabolites attenuate elevation in levels of lipid peroxides and ROS in the cerebral cortex and the hippocampus, indicating that DHA and its metabolites facilitate neuroprotection by down-regulating g-secretase activity, an enzyme that liberates Ab from soluble amyloid precursor protein-a (Lukiw et al., 2005). Furthermore, soluble amyloid precursor protein-a stimulates the synthesis of NPD1 (Lukiw et al., 2005; Bazan, 2009a, b). Activities of key PLA2 (PlsEtn-PLA2 and cPLA2) and 15-LOX, enzymes required for NPD1 biosynthesis, are markedly increased in the AD hippocampal CA1 region (Farooqui et al., 1997, 2006; Farooqui, 2010b; Bazan, 2009a). Collective evidence suggests that DHA and its oxidative metabolites limit the generation and accumulation of the Ab peptide, which is closely associated with the pathogenesis of AD. DHA and its metabolites also suppress several signal transduction pathways induced by Ab, including two major kinases that phosphorylate the microtubule-associated protein tau and promote neurofibrillary tangle pathology (Farooqui, 2009). It is recently shown that in macrophages DHA is also metabolized through a 14-LOX pathway resulting in generation of 7,14-dihydroxydocosa-4Z,8,10,12, 16Z,19Z-hexaenoic acid. This metabolite is called as maresin (MaR1) (Fig. 2.9). This metabolite not only terminates PMN infiltration, but also stimulates macrophage phagocytosis. An isomer of MaR1, 7S,14S-diHDHA, acts less potently than MaR1. This suggests that these DHA-derived metabolites may be stereoselectively regulate catabasis and facilitate arrival of tissues to homeostasis (Serhan et al., 2009). In retina, epithelium-derived factor acts as an agonist and induces the synthesis of NPD1 and thus promoting NPD1-mediated paracrine and autocrine signal transduction processes. Also, DHA and epithelium-derived factor not only synergistically activates NPD1 generation and antiapoptotic protein expression, but also

Inflammatory bowel disease Asthma Bacterial pneumonia Acute lung injury

Inhibition of proinflammatory cytokine expression Inhibition of proinflammatory cytokine expression Inhibition of proinflammatory cytokine expression Inhibition of proinflammatory cytokine expression

Beneficial

Beneficial Beneficial Beneficial

Table 2.2  Effects of resolvins and neuroprotectins in animal models of neural and non-neural diseases Effect of resolvin/ Animal model neuroprotectin Molecular mechanism Stroke Beneficial Decrease in proapoptotic Bcl-2 expression and inhibition of caspase-3; induction of neuroprotective gene-expression Alzheimer disease Beneficial Decrease in proapoptotic Bcl-2 expression and inhibition of caspase-3; induction of neuroprotective gene-expression Inherited retinal Beneficial Decrease in proapoptotic Bcl-2 expression and inhibition of degeneration caspase-3; induction of neuroprotective gene-expression Periodontal disease Beneficial Inhibition of proinflammatory cytokine expression

Serhan et al., 2000, 2002; Serhan, 2008; Seki et al., 2009 Serhan et al., 2000, 2002; Serhan, 2008; Seki et al., 2009 Serhan et al., 2000, 2002; Seki et al., 2009 Serhan et al., 2000, 2002; Seki et al., 2010 Serhan et al., 2000, 2002; Seki et al., 2010

Reference Serhan et al., 2000, 2002; Mukherjee et al., 2007; Bazan, 2009a, b Serhan, et al., 2000, 2002; Mukherjee et al., 2007; Bazan, 2009a, b Serhan, et al. 2000, 2002; Bazan, 2009a, b

64 2  Recent Development on the Neurochemistry of Docosanoids

2.4  DHA-Derived Lipid Mediators in the Brain

65

a

COOH COOH

c OH

OH

OH HO

b HO

d COOH

COOH OH

OH

HO OH

Fig. 2.9  Chemical structures of docosahexaenoic acid and docosanoids. 16,17S-docosatriene (a); 10,17S-docosatriene (b); maresin (c); and 4S5,17S-resolvin (d). These metabolites inhibit inflammation by retarding the actions of eicosanoids

down-regulates proapoptotic Bcl-2 protein expression and activation of caspase 3 during oxidative stress (Mukherjee et al., 2007). NPD1 also promotes AKT translocation and activation and interacts with PPAR-gamma family of ligand-activated nuclear receptors, which may be involved in various aspects of neuroinflammation and neurodegeneration (Palacios-Pelaez et al., 2010; Niemoller and Bazan, 2010; Farooqui, 2010a, c). Receptors for NPD1 have not been characterized in brain tissue, but their occurrence has been suggested (Hong et al., 2003; Marcheselli et al., 2003; Mukherjee et al., 2004). Thus, NPD1-mediated regulation targets upstream events of brain cell apoptosis and modulation of neuroinflammatory signaling promote the cellular homeostasis, and restoration of brain damage through above-mentioned mechanisms. This is tempting to speculate that the generation of DHA-derived resolvins, neuroprotectins, maresin, and synthesis of ARA-derived lipoxins may be internal neuroprotective mechanisms that block neuroinflammation and apoptosismediated brain damage caused by neurotraumatic and neurodegenerative diseases (Serhan, 2005; Bazan, 2009a, b; Farooqui, 2010b). Investigators are using lipidomics, proteomics and genomics techniques to identify and determine levels of ARA, EPA and DHA-derived lipid mediators (F2-isoprostanes, eicosanoids, lipoxins, resolvins, and NPD1) (Serhan et al., 2006b,

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2008; Ariel and Serhan, 2007; Brooks et al., 2008; Bazan, 2009a, b) and developing diagnostic test in cerebrospinal fluid (CSF) from patients with acute neural trauma, neurodegenerative, and neuropsychiatric diseases. The use of proteomic strategies for characterizing resolvin and neuroprotectin metabolizing enzymes in subcellular organelles of human brain and CSF may provide new information on properties and therapeutic targets. Resolvin and neuroprotectin synthesizing and catabolizing enzymes not only control the levels of these lipid mediators in normal and diseased brain, but also modulate physiology and pathology of chronic brain diseases. Collectively, these studies suggest that combining lipidomics, proteomics, and genomics techniques may greatly enhance the existing knowledge of molecular mechanism associated with homeostasis between inflammation inducing mediators (eicosanoids and platelet activating factor) and inflammation blocking lipid mediators (lipoxins, resolvins, and neuroprotectins) in neural trauma, neurodegenerative and neuropsychiatric diseases (Serhan et  al., 2006b, 2008; Brooks et  al., 2008; Bazan, 2009a, b; Farooqui, 2010a). Information about the temporal and spatial positioning of inflammation inducing and inflammation blocking pathways in brain may not only be dictated by anti-inflammatory drugs but also by the diet. The Western diet has extremely high levels of omega-6 fatty acids with ARA to DHA of about 20:1. The Paleolithic diet on which human beings have evolved, and lived for most of their existence, had a ratio of 2–1:1, and was high in fiber, rich in fruits, vegetables, lean meat, and fish (Simopoulos, 2002, 2008; Cordain et al., 2005). The high intake of ARA enriched food in Western diet not only elevates levels of eicosanoids and platelet activating factor, but also upregulates the expression of proinflammatory genes including genes for cytokines (TNF-a and IL-1b), secretory phospholipase A2, cyclooxygenase-2, and nitric oxide synthase . In contrast, consumption of DHA-enriched diet has anti-inflammatory effects that are partly mediated by repression of genes that code for pro-inflammatory cytokines. Since Western diet is low in DHA and high in ARA, it is linked to many chronic visceral diseases as well as neurodegenerative and neuropsychiatric disorders (Simopoulos, 2002, 2008; Cordain et  al., 2005; Farooqui, 2009). Enrichment of DHA in Western diet may improve inflammation and oxidative stress in neurotraumatic and neurodegenerative diseases not only through the effects of DHA on physicochemical properties of neural cell membranes, but also through modulation of genes and generation of resolvins Ds and neuroprotectins. It is worth noting that in AD transgenic mice dietary DHA restores cerebral blood volume, reduces Ab deposition, and ameliorates Ab pathology (Green et al., 2007). Although molecular mechanisms associated with above processes are not fully understood, recent studies indicate that in 3xTg-AD mouse models and human neuronal-glial (HNG) cells in primary culture, NPD1 not only down-regulates Ab42-mediated expression of the pro-inflammatory enzyme COX-2 (Fig.  2.10), but also reduces the expression of B-94, a TNF-ainducible pro-inflammatory element resulting in inhibition of apoptosis. Moreover, NPD1 suppresses Ab42 peptide shedding by down-regulating b-secretase-1 (BACE1) while activating the a-secretase ADAM10 and up-regulating sAPPa, thus shifting the cleavage of bAPP holoenzyme from an amyloidogenic into the nonamyloidogenic pathway (Zhao et al., 2011). It is also shown that anti-amyloidogenic

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67

Excitotoxicity

Neuroprotection

PtdCho

NMDA-R

Glu

+

cPLA2

+

PlsEtn

Ca2+

PlsEtn-PLA2

ARA C OX + -2 /5Lyso-PtdCho L ROS

-

DHA

OX

sAPPα

Eicosanoids

15-LOX

PAF

ADAM10

IKB/NFKB

Inflammation

TNF-α IL-1β IL-6

+

βAPP

NPD1

IKB

COX-2 sPLA2 iNOS

-

Oxidative stress

NF-KB-RE

-

BACE1

-

-

-

Aβ42

Transcription of genes NUCLEUS

Apoptosis PPARγ

Fig. 2.10  Hypothetical diagram showing anti-inflammatory actions of neuroprotectin D1, production of Ab from holo-bAPP, and generation of ARA-derived lipid mediators in the brain. Glutamate (Glu); N-Methyl-d-aspartate receptor (NMDA-R); phosphatidylcholine (PtdCho); ethanolamine plasmalogen (PlsEtn); cytosolic phospholipase A2 (cPLA2); arachidonic acid (ARA); reactive ­oxygen species (ROS); plasmalogen-selective phospholipase A2 (PlsEtn-PLA2); 15-lipoxygenase (15-LOX); platelet activating factor (PAF); neuroprotectin D1 (NPD1); ADAM (a-secretase); ­peroxisome proliferator-activated receptor g (PPARg); amyloid precursor protein (APP); holo-bAPP (bAPP holoenzyme); nuclear factor kappa B (NF-kB); tumor necrosis factor-alpha (TNF-a); interleukin-1beta (IL-1-b); interleukin-6 (IL-6); inducible nitric oxide synthase (iNOS); secretory phospholipase A2 (sPLA2); cyclooxygenase-2 (COX-2)

effects of NPD1are mediated in part through activation of the PPARg receptor, while NPD1induced stimulation of non-amyloidogenic pathways is PPARg-independent. In summary, NPD1 potently down-regulates inflammatory signaling, amyloidogenic APP cleavage and apoptosis, supporting the potential of this lipid mediator in rescuing human brain cells in early stages of neurodegenerations (Zhao et al., 2011). In spite of above laboratory observations, to date studies on the dietary intervention of n-3 PUFA in AD have so far failed. It is proposed that long-term DHA supplementation may not only restore signal transduction processes associated with behavioral deficits and learning activity, but also ­produce several neuroendocrinological and immunological effects on brain tissue, which may be beneficial in neurotraumatic, neurodegenerative, and neuropsychiatric ­diseases (Farooqui, 2010a, c).

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2.5 Conclusion EPA and DHA-derived lipid mediator resolvins and protectins/neuroprotectins regulate immune systems by modulating signal transduction processes associated with neuroinflammation and neurodegeneration. EPA-derived E-series resolvins (i.e., RvE1 and RvE2) and DHA-derived D-series resolvins (RvD1 and RvD2) have potent anti-inflammatory and pro-resolution properties. They retard excessive inflammatory responses and promote resolution by enhancing clearance of apoptotic cells and debris from inflamed brain tissue. These actions may underlie the beneficial effects of EPA and DHA in human health and neurotraumatic and neurodegenerative diseases. Aspirin initiates resolution by triggering biosynthesis of specific epimers of resolvins and protectins. In addition to their synthesis in resolution, these lipid mediators also display potent protective roles in neural systems, liver, lung, and eye. Potent actions of resolvins and protectins in models of chronic human disease indicate that down-regulation in resolution pathways may contribute to the pathogenesis of many chronic neurodegenerative and visceral diseases.

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Scott B.L. and Bazan N.G. (1989). Membrane docosahexaenoate is supplied to the developing brain and retina by the liver. Proc Natl Acad Sci U S A. 86:2903–2907. Seki H., Tani Y., and Arita M. (2009). Omega-3 PUFA derived anti-inflammatory lipid mediator resolvin E1. Prostaglandins Other Lipid Mediat. 89:126–130. Seki H., Fukunaga K., Arita M., Arai H., Nakanishi H., Taguchi R., Miyasho T., Takamiya R., Asano K., Ishizaka A., Takeda J., and Levy B. D. (2010). The anti-inflammatory and proresolving mediator resolvin E1 protects mice from bacterial pneumonia and acute lung injury. J. Immunol. 184:836–843. Serhan C.N., Clish C.B., Brannon J., Colgan S.P., Chiang N., and Gronest K (2000). Novel functional sets of lipid-derived mediators with antiinflammatory actions generated from omega-3 fatty acids via cyclooxygenase 2-nonsteroidal antiinflammatory drugs and transcellular processing. J. Exp. Med 192:1197–1204. Serhan CN, Hong S, Gronert K, Colgan SP, Devchand PR, Mirick G, Moussignac RL. (2002). Resolvins: a family of bioactive products of omega-3 fatty acid transformation circuits initiated by aspirin treatment that counter proinflammation signals. J Exp Med. 196:1025–1037. Serhan C. N. (2005). Novel w-3-derived local mediators in anti-inflammation and resolution. Pharmacol. Ther. 105:7–21. Serhan C.N. (2008). Controlling the resolution of acute inflammation: a new genus of dual antiinflammatory and proresolving mediators. J. Periodontol. 79 (8 Suppl):1520–152. Serhan, C.N., Yacoubian, S., and Yang R. (2008a). Anti-inflammatory and proresolving lipid mediators. Annu Rev Pathol. 3:279–312. Serhan C.N., Chiang N., and Van Dyke T.E. (2008b). Resolving inflammation: dual anti-inflammatory and pro-resolution lipid mediators. Nat. Rev. Immunol. 8:349–361. Serhan C.N., Hong ., and Lu Y. (2006). Lipid mediator informatics-lipidomics: novel pathways in mapping resolution. AAPS J. 8:E284–E297. Serhan C.N., Gotlinger K., Hong S., Lu Y., Siegelman J., Baer T., Yang R., Colgan S.P., and Petasis N.A. (2006a). Anti-inflammatory actions of neuroprotectin D1/protectin D1 and its natural stereoisomers: assignments of dihydroxy-containing docosatrienes. J. Immunol. 176: 1848–1859. Serhan C.N. and Chiang N. (2008). Endogenous pro-resolving and anti-inflammatory lipid mediators: a new pharmacologic genus. Br. J. Pharmacol. 153 Suppl 1:S200–S215. Serhan C.N., Yang R., Martinod K., Kasuga K., Pillai P.S., Porter T.F., Oh S.F., and Spite M. (2009). Maresins: novel macrophage mediators with potent antiinflammatory and proresolving actions. J Exp Med. 206:15–23. Simopoulos A. P. (2002a). The importance of the ratio of omega-6/omega-3 essential fatty acids. Biomed. Pharmacother. 56:365–379. Simopoulos AP. (2008). The importance of the omega-6/omega-3 fatty acid ratio in cardiovascular disease and other chronic diseases. Exp Biol Med (Maywood). 233:674–688. Sinha R.A., Khare P., Rai A., Maurya S.K., Pathak A., Mohan V., Nagar G.K., Mudiam M.K., Godbole M.M., and Bandyopadhyay S. (2009). Anti-apoptotic role of omega-3-fatty acids in developing brain: perinatal hypothyroid rat cerebellum as apoptotic model. Int J Dev Neurosci. 27:377–383. Song C., Phillip A.G., Leonard B.E., and Horrobin D.F. (2004). Ethyl-eicosapentaenoic acid ingestion prevents corticosterone-mediated memory impairment induced by central administration of interleukin-1beta in rats. Mol. Psychiatry 9:630–638. Song C., Zhang X.Y., and Manku M. (2009). Increased phospholipase A2 activity and inflammatory response but decreased nerve growth factor expression in the olfactory bulbectomized rat model of depression: effects of chronic ethyl-eicosapentaenoate treatment. J. Neurosci. 29:14–22. Tjonahen E., Oh S.F., Siegelman J., Elangovan S., Percarpio K.B., Hong S., Arita M., and Serhan C.N. (2006). Resolvin E2: identification and anti-inflammatory actions: pivotal role of human 5-lipoxygenase in resolvin E series biosynthesis. Chem. Biol. 13:1193–1202. Wada M., DeLong C.J., Hong Y.H., Rieke C.J., Sidhu R.S., Yuan C., Warnock M., Schmaier A.H., Yokoyama C., Smyth E.M., Wilson S.J., FitzGerald G.A., Garavito R.M., Sui de X., Regan

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J.W., and Smith W.L. (2007). Enzymes and receptors of prostaglandin pathways with arachidonic acid-derived versus eicosapentaenoic acid-derived substrates and products. J. Biol. Chem. 282:22254–22266. Wang X., Zhao X., Mao Z.Y., Wang X.M., Liu Z.L. (2003). Neuroprotective effect of docosahexaenoic acid on glutamate-induced cytotoxicity in rat hippocampal cultures. Neuroreport 14:2457–2461. Xu Z.Z., Zhang L., Liu T., Park J.Y., Berta T., Yang R., Serhan C.N. and Ji R.R. (2010). Resolvins RvE1 and RvD1 attenuate inflammatory pain via central and peripheral actions. Nat. Med. 16:592–597. Yin H., Brooks J.D.., Gao L., Poeter N., and Morrow J.D. (2007). Identification of novel autoxidation products of the n-3 fatty acid eicosapentaenoic acid in Vitro and in Vivo. J. Biol. Chem. 282:29890–29901. Zhao Y., Joshi-Barve S., Barve S., Chen L.H. (2004). Eicosapentaenoic acid prevents LPS-induced TNF-alpha expression by preventing NF-kappaB activation. J. Am. Coll. Nutr. 23:71–78. Zhao Y., Calon F., Julien C., Winkler J.W., Petasis N.A., Lukiw W.J., and Bazan N.G. (2011). Docosahexaenoic acid-derived neuroprotectin D1 induces neuronal survival via secretase- and PPARg-mediated mechanisms in Alzheimer’s disease models. PLoS One. 6(1):e15816.

Chapter 3

Metabolism, Roles, and Involvement of Lyso-glycerophospholipids in Neurological Disorders

3.1 Introduction Lysophospholipids are metabolic intermediates in glycerophospholipid metabolism. In unstimulated neural cells, lysophospholipids occur at low levels (0.5–3%). They are transiently synthesized during the remodeling of glycerophospholipids (Farooqui et al., 2000a). Lysophospholipids interact with neural membrane’s lipid and enzyme components and modulate activities of membrane-associated enzymes and growth factors. The metabolism of lysophospholipid facilitates translocation of Toll-like receptor 4 to membrane and this process regulates inflammatory responses (Jacson et al., 2008a). Neural membrane lysophospholipids include lysophosphatidylcholines (lyso-PtdCho), lysophosphatidylethanolamines (lyso-PtdEtn), lysohosphatidylserines (lyso-PtdSer), lysophosphatidylinositols (lyso-PtdIns), lysocardiolipin (Lyso-Ptd2Gro), lysoplasmalogens (lyso-PlsEtn and lyso-PlsCho), and lysophosphatidic acid (lyso-PtdH) (Fig.  3.1). Many lysophospholipids are nonbilayer-­ forming lipids, which at low concentration form micelles, but at higher concentration, they tend to form cylindrical hexagonal phases (Fuller and Rand, 2001). At high concentrations, lysophospholipids also alter membrane permeability, and disturb osmotic equilibrium. In lipid bilayers, lysophospholipids modulate the gating kinetics of several membrane channels including gramicidin and TRPC5 calcium channels (Lundbaek and Andersen, 1994; Flemming et al., 2005). Lysophospholipids stabilize dimer formation between two peptide helices in apposing monolayers, forming an open channel. In neural cells, lysophospholipids are produced through the action of phospholipases A1 (PLA1), phospholipases A2 (PLA2), and plasmalogen-­selective phospholipase A2 (PlsEtn-PLA2) on PtdCho, PtdEtn, PtdSer, and plasmalogens (Farooqui and Horrocks, 2004, 2007). Lysophospholipids are quantified and separated from native phospholipids by high-performance liquid chromatography, liquid chromatography-mass spectrometry-mass spectrometry (LC-MS-MS), and electrospray ionization mass spectrometry (Lesnefsky et  al., 2000; Han et  al., 2001; Taguchi, 2009). Other lysophospholipids such as lyso-phosphatidic acid

A.A. Farooqui, Lipid Mediators and Their Metabolism in the Brain, DOI 10.1007/978-1-4419-9940-5_3, © Springer Science+Business Media, LLC 2011

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3  Metabolism, Roles, and Involvement of Lyso-glycerophospholipids…

a

b

O H2C

O

HO C

H

H2C

O

c

C R

H2C

O

HO C

H

H2C

O

H2C

O

HO C CH3 + N CH3 H2C CH3

H

O P O O

CH2

CH2

NH2

O

HO C

H

H2C

O

C R O P

O

CH2

O-

e

CH2

O

HO C

H

H2C

O

C

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O P O-

R

O P

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CH2

O-

O

f

O H2C

CH

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NH2

d

O H2C

CH

O

CH2

CH

+

NH3

COO

-

CH

CH

R

O P O O-

CH2

CH2

CH3 + N CH3 CH3

O H2C

O

HO C

H

H2C

O

C R O P

OH

O-

Fig. 3.1  Chemical structures of various lysophospholipids. Lyso-phosphatidylethanolamine (a); lysoplasmenyethanolamine (b); lyso-phosphatidylcholine (c); lyso-choline plasmalogen (d); and lyso-phosphatidylserine (e); and lyso-phosphatidic acid (f)

(lyso-PtdH) and lyso-platelet-activating factor (lyso-PAF) are also generated in ­mammalian brain. Their metabolism has been described elsewhere (Farooqui and Horrocks, 2007).

3.2 Deacylation/Reacylation Cycle (Land’s Cycle) and Its Importance Deacylation/reacylation cycle (Land’s cycle) is an important mechanism that introduces polyunsaturated fatty acids into neural membrane phospholipids by effectively bypassing the de novo synthesis of the entire phospholipid molecule. It plays a critical role in regulating availability of ARA for eicosanoid production. The deacylation arm of Land’s cycle involves phospholipase A2 (PLA2) activity, whereas reacylation arm utilizes acyl-CoA synthetase, acyl-CoA: lysophospholipid acyltransferase, and acyl-CoA hydrolase activities. These enzymes occur in multiple forms and have been purified and characterized from neural and non-neural tissues

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(Sun and MacQuarrie, 1989; Farooqui et al., 2000a). In the deacylation/reacylation cycle, multiple forms of acyltransferases and PLA2 activities control glycerophospholipid composition and the availability of free ARA and DHA (Farooqui et al., 2000a, b). Lysophosphatidylcholine acyltransferases modulate inflammatory responses to lipopolysaccharide and other microbial stimuli (Jackson et al., 2008a, b). Specific inhibition of lysophosphatidylcholine acyltransferase downregulates inflammatory cytokine production in monocytes and epithelial cells by preventing translocation of Toll-like receptor (TLR4) into membrane lipid raft domains, indicating the existence of new regulatory mechanisms that not only facilitate the innate immune responses to microbial molecular patterns, but also provide a basis for the antiinflammatory activity observed in many glycerophospholipid-derived metabolites (Jackson et  al., 2008a, b; Farooqui, 2009). Lysophospholipid acyltransferases, Phospholipipases A2, C, and D (PLA2, PLC, and PLD), cyclooxygenases, and lipoxygenases are key enzymes that regulate cellular responses to a variety of stimuli through the generation of lipid mediators that modulate oxidative stress and inflammation in brain (Farooqui et al., 2000a, b; Farooqui, 2009). Regulation or manipulation of lysophospholipid acyltransferases, PLA2, PLC, and PLD, cyclooxygenases, and lipoxygenases may thus provide important mechanisms for novel antioxidant and anti-inflammatory therapies for neurotraumatic and neurodegenerative diseases (Jackson et al., 2008b; Farooqui, 2009, 2010a). Collectively, these studies suggest that in neural membranes, the deacylation--reacylation cycle maintains a balance between free and esterified fatty acids, resulting in low levels of arachidonic acid and lysophospholipids. In addition, deacylation--reacylation cycle is necessary for not only normal membrane integrity and function, but also for the optimal activity of the membrane-bound enzymes, receptors, and ion channels involved in normal signal-transduction processes.

3.2.1 Acyl-CoA Synthetases in the Brain Enzymes catalyzing the thioesterification of fatty acids with coenzymeA and generating activated acyl-CoA derivatives are called as acyl-CoA synthetases (ACS). These enzymes not only play a fundamental role in lipid metabolism, but also contribute to lipid homeostasis (Fig. 3.2). The products of the ACS enzyme reaction (acyl-CoAs) are required for complex lipid synthesis, energy production via b-oxidation, protein acylation and fatty-acid dependent transcriptional regulation (Farooqui et  al., 2000a). Activation of fatty acids is a two-step process generating an acyl-AMP intermediate in the first step, and in the second step the enzyme facilitates the exchange of AMP with CoA to produce the activated acyl-CoA. Limited information is available on ACS in brain (Soupene and Kuypers, 2008). Some isoforms of ACS are involved in the activation of short-chain fatty acids while others are associated with the activation of long-chain fatty acids. The activation of ARA and DHA involves long-chain acyl-CoA synthetases (ACSL) in mammalian brain. The incubation of [1-14C]ARA with brain microsomes in the presence of ATP, CoA, and

76

3  Metabolism, Roles, and Involvement of Lyso-glycerophospholipids… Ca2+

Extracellular

A

Ca2+ oA

C yl-

n do hi e ac as Ar nth sy

Phospho Phospho lipids lipids

R

phos

photr

ARA-CoA

e

as

rol

yd

h oA

Lysophospholipid acyltrarnsferase

l-C

y Ac

ARA

Intracellular

ansfe

PLA1

PLA2

rase

DAG

Free fatty acid 2-acyl-lysophospholipid

1-acyl-lysophospholipid

Lysophospholipase Lysophospholipase COX LOX X EPO

Glycerophosphodiester

CTP

Phosphodiesterase PGs, LTs, TXs, LXs, EETs

Phosphobase

CDP-base CTP: cytidylyltransferase

Base Kinase

Phosphatase

Base

Fig. 3.2  Acylation/deacylation cycle for the incorporation and release of arachidonic acid in the brain. Agonist (A); Receptor (R); arachidonic acid (ARA); arachidonyl-Coenzyme A (ARA-CoA); phospholipase A2 (PLA2); phospholipase A1 (PLA1); prostaglandins (PGs); Leukotrienes (LTs); thromboxanes (TXs); lipoxins (LXs); epoxyeicosatetraenoic acids (EETs); diacylglycerol (DAG); cyclooxygenase (COX); lipoxygenase (LOX); epoxygenase (EPOX); and acyl-CoA hydrolase

MgCl2 results in the formation of [1-14C]arachidonyl-CoA. The omission of ATP or CoA results in a 98% decrease in enzymic activity, indicating the absolute requirement of ATP and CoA for the acyl-CoA synthetase reaction (Reddy et al., 1984; Reddy and Bazan, 1984). The addition of unlabeled ARA and DHA inhibits the enzymic activity with Ki values of 31 mM for both fatty acids. Based on various kinetic parameters and effect of inhibitors, it is suggested that a single acyl-CoA synthetase may be associated with the activation of long-chain fatty acids. In contrast, other investigators have reported the occurrence of several isoforms of longchain acyl-CoA synthetase, namely ACSL1, ACSL2, ACSL3, ACSL4, ACSL5, and ACSL6 in brain tissue (Marszalek et al., 2005; Van Horn et al., 2005). Among above isoforms, ACSL3 and ACSL6 are the predominant ACSL isoforms in brain. These isoforms have been cloned from rat brain. In a direct competition assay with palmitic acid, all the polyunsaturated fatty acids competitively retard the activation of palmitic acid. Upregulation of ACSL results in rapid synthesis of arachidonyl-CoA and docosahexaenoyl-CoA in brain. Intracellular concentrations of acyl-CoAs are controlled by the balance among acyl-CoA synthesizing enzymes, acyl-CoA utilizing enzymes (acyl-CoA: lysophospholipid acyltransferase), and fatty acid metabolizing enzymes (acyl-CoA hydrolases) (Farooqui et al., 2000a; Corkey et al., 2000).

3.2  Deacylation/Reacylation Cycle (Land’s Cycle) and Its Importance

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The intracellular levels of free unbound acyl-CoA esters are tightly regulated by feedback inhibition of the acyl-CoA synthetase, and are buffered by specific acylCoA binding proteins. In addition to being intermediates in fatty acid metabolism in brain, acyl-CoAs also modulate ion fluxes, vesicle trafficking, protein phosphorylation, long-term potentiation in the hippocampus (Zhang et al., 2000), synaptic vesicle formation (Schmidt et  al., 1999), and gene expression (Faergeman and Knudsen, 1997).

3.2.2 Acyl-CoA: Lyso-phospholipid Acyltransferase in the Brain The transfer of acyl group from acyl-CoA to lysophospholipid acceptor is catalyzed by acyl-CoA: lysophospholipid acyltransferase, which is located in microsomal fraction and can be solubilized from bovine brain microsomes using Miranol (Baker and Chang, 1981; Deka et al., 1986). Three hundred fold purification of acyl-CoA: lysophospholipid acyltransferase from bovine brain microsomes is achieved by DEAE-cellulose, Blue-2-agarose, and Matrex green chromatographies with an overall recovery of 6.0% (Deka et al., 1986). The purified enzyme moves as a single protein band (molecular mass of 43 kDa) on SDS-PAGE. The purified enzyme is specific for lyso-PtdCho. Enzyme shows less specificity toward the acyl-CoA derivatives. Among various acyl-CoAs, enzyme shows preference for arachidonylCoA. High concentrations of arachidonoyl-CoA inhibit the enzymic activity. Differences in the rate of acylation of different lysophospholipid subclasses of glycerophospholipids by human brain acyl-CoA: lysophospholipid acyltransferase have also been reported (Ross and Kish, 1994). The rate with lysophosphatidylinositol (lysoPtdIns) is highest, followed by lysoPtdCho and LysoPtdSer. In contrast, acylation of lysoPtdEtn is barely detectable. Based on various kinetic and metabolic studies, lysoPtdCho acyltransferase may compete with acyl-CoA hydrolase and lysophospholipase for acyl-CoA and lysoPtdCho, respectively (Ross and Kish, 1994). At least two families of lysophospholipid acyltransferases have been identified in neural and non-neural tissues. Acyl-CoA lysocardiolipin acyltransferase 1 (ALCAT1) is ­initially identified as a microsomal lysocardiolipin acyltransferase and acyl-CoA: lysophosphatidylethanolamine acyltransferase 2 (LPEAT2) is predominantly expressed in brain (Cao et al., 2004, 2008). ALCAT1 plays an important role in cardiolipin remodeling. In addition, ALCAT1 possesses acyltransferase activities toward lysophosphatidylinositol (LPI) and lysophosphatidylglycerol (LPG) (Cao et al., 2008). Human embryonic kidney 293 (HEK293) cells overexpressing human ALCAT1 display significant increase in LPI acyltransferase (LPIAT) and LPG acyltransferase (LPGAT) activities with several fatty acyl-CoAs. The affinity of ALCAT1 toward LPI and LPG depends on fatty acyl-CoA (Zhao et al., 2009). The reduction in the expression of ALCAT1 in Hela cells significantly lowers 6 LPIAT and LPGAT activities, but has no effect on ALCAT1 activity. Based on structural and functional studies, it is suggested that critical amino acids D168 and L169 within ALCAT1 may play an important role in lysophospholipid substrate binding

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3  Metabolism, Roles, and Involvement of Lyso-glycerophospholipids…

(Zhao et al., 2009). LPEAT shows significant acyl-CoA-dependent ­acyltransferase activity toward 1-O-alkenyl-lysophosphatidylethanolamine, ­lysophosphatidylgly cerol, 1-O-alkyl-lysophosphatidylcholine, lysophosphatidylserine, and lysophosphatidylcholine, but lacks appreciable acylating activity toward glycerol 3-phosphate, lysophosphatidic acid, lysophosphatidylinositol, and diacylglycerol, indicating that multiple but selective roles of LPEAT2 as an enzyme are involved in phospholipid remodeling. Although LPEAT2 recognizes a broad range of medium and long-chain fatty acyl-CoA, its activity is not affected by Ca2+. When overexpressed in mammalian cells, LPEAT2 is localized to the endoplasmic reticulum. Treatment with LPEAT siRNA in HEK293T cells significantly reduces LPEAT and 1-alkenyl-LPEAT activities, but produces no effect on other lysophospholipid acylating activities (Cao et al., 2008). Inhibition of LPCAT retards TNF-a expression and secretion by preventing the translocation of TLR4 into membrane lipid raft domains in MM6 cells (Jackson et al., 2008a), suggesting that LPCAT may regulate the assembly of a functional LPS receptor complex in the membrane by controlling membrane PtdCho composition. Indeed, upregulation of LPCAT activity, by cytokines, such as IFN-a, increases cellular responses to LPS by stimulating the assembly of the LPS receptor (Jackson et al., 2008a). Collective evidence suggests that ALCAT1 and LPEAT2 play an important role in the biosynthesis of cardiolipin and ethanolamine-­containing phospholipids in neural and non-neural tissues. AcylCoA: lysophospholipid acyltransferase-­mediated remodeling of glycerophospholipids may be involved in maintaining membrane asymmetry and diversity, which modulates membrane fluidity and curvature. Presence of multiple forms of acylCoA: lysophospholipid acyltransferases and their genes encoding these activities explain the occurrence of more than 100 glycerophospholipids molecular species in brain and other mammalian tissues (Yamashita et al., 1997; Cao et al., 2008).

3.2.3 Phospholipases A2 (PLA2) in the Brain PLA2s constitute a superfamily of enzymes that catalyze the hydrolysis of unsaturated fatty acids from the sn-2 position of glycerol moiety of neural membrane phospholipids. These enzymes are classified into two groups: (a) intracellular PLA2 and (b) extracellular PLA2. The intracellular PLA2 are further divided into (a) cytosolic PLA2 (cPLA2), (b) calcium-independent PLA2 (iPLA2), and (c) plasmalogen-selective PLA2 (PlsEtn-PLA2), whereas extracellular PLA2 includes secretory PLA2 (sPLA2) and PAF acetylhydrolases (Hirabayashi et  al., 2004; Ohto et  al., 2005; Kolko et al., 2006; Farooqui and Horrocks, 2007; Ong et al., 2010). cPLA2 (mol. mass 85 kDa) is present in the cytosol, and is activated by mM concentrations of Ca2+ and protein serine/threonine kinases. Calcium is not required for the catalytic activity of cPLA2, but it is important for the interaction of this group of enzymes with the phospholipid membrane (Farooqui et al., 1997). iPLA2 is also localized in the cytosol (mol. mass 80 kDa). Calcium is not required for its activity. Although

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79

the precise function of the iPLA2 is not known, it may be associated with tissue developments and hormone secretion. sPLA2 (mol mass 18–20  kDa) contains ­multiple disulfide (S-S) bonds. It is stimulated by mM concentrations of Ca2+, and its function is primarily extracellular. PAF acetylhydrolase inactivates PAF and hydrolyzes oxidized phospholipids to yield lysophospholipids (Farooqui and Horrocks, 2007). Molecular biology and cloning studies have resulted in isolation and ­characterization of many paralogs, splice variants, and multiple forms of cPLA2, iPLA2, and sPLA2 in neural and non-neural tissues. cPLA2 paralogs include cPLA2a, cPLA2b, cPLA2g, cPLA2d, cPLA2e, and cPLA2z. iPLA2 splice variants include iPLA2a, iPLA2b, and iPLA2g. Multiple forms of sPLA2 include sPLA2- IB, sPLA2IIA, sPLA2-IIC, sPLA2-IID, sPLA2-IIE, sPLA2-IIF, sPLA2-III, sPLA2-V, sPLA2-X, sPLA2-XIIA, and sPLA2-XIIB (Hirabayashi et al., 2004; Ohto et al., 2005; Kolko et al., 2006; Farooqui and Horrocks, 2007; Ong et al., 2010). Paralogs, splice variants, and multiple forms of PLA2 superfamily differ from each other in enzymic properties, tissue distribution, cellular and subcellular localizations, and role in various physiological and pathophysiological conditions (Farooqui and Horrocks, 2007; Ong et al., 2010). In addition to hydrolyzing fatty acids from the sn-2 position of glycerol moiety of neural membrane phospholipids, PLA2 isoforms also have other biochemical activities. Thus, in lungs acidic iPLA2 activity has nonselenium glutathione peroxidase (NSGPx) activity (Chen et  al., 2000) and cPLA2 from several sources also shows lysophospholipase and 7-hydroxycoumarin esterase activities (Leslie, 1991; Murakami et  al., 1997). Among paralogs of cPLA2, cPLA2g (mol. mass 61 kDa) has 30% sequence identity with its ortholog, cPLA2a. cPLA2g not only contains a potential prenylation motif at its C terminus, but also displays PLA2, lysophospholipase, coenzyme A (CoA)-independent transacylation and lysophospholipid dismutase (LPLase/transacylase) activities. This observation indicates that cPLA2 paralogs are involved in fatty acid remodeling of phospholipids as well as in the clearance of toxic lysophospholipids in cells (Yamashita et al., 2005). Collectively these studies suggest that members of PLA2 superfamily constitute a complex signal transduction network that not only maintains cross-talk among excitatory amino acid, dopamine, retinoid, cannabinoid, cytokine, and growth factor receptors through the generation and modulation of lipid mediators, but also contribute to enzymic and nonenzymic transacylation reaction. This cross-talk among above receptors and tight regulation of all PLA2 isoforms is essential for maintaining normal neural cell function. Upregulation of various PLA2 isoforms and splice variants is associated with the degradation of neural membrane phospholipids and generation of arachidonic acid and docosahexaenoic-derived lipid metabolites that have been implicated in fundamental neural cell functions including neurotransmitter release, LTP and LTD induction, the auditory startle reflex and sensorimotor gating and vacuous chewing movements nociception, neuroinflammation, oxidative stress, and neurodegeneration (Farooqui, 2009). Collective evidence suggests that members of PLA2 superfamily are involved in the synthesis of lipid mediators that have been implicated in fundamental cellular responses, including growth, differentiation, adhesion, migration, secretion, and apoptosis (Farooqui and Horrocks, 2007; Farooqui, 2009; Ong et al., 2010).

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3  Metabolism, Roles, and Involvement of Lyso-glycerophospholipids…

3.2.4 Long-Chain Acyl-CoA Hydrolase or Thioesterases in the Brain Long-chain acyl-CoA hydrolases or thioesterases catalyze the hydrolysis of CoA esters to free coenzyme A (CoASH) and carboxylic acids (Hunt and Alexson, 2002). Among various brain regions, pons, medulla, and midbrain have higher activity of long-chain acyl-CoA hydrolase than in the cerebral cortex and caudate nucleus (Yamada et al., 1996). At the cellular level, long-chain acyl-CoA hydrolase is localized to neurons (Yamada et al., 1996). In addition to various nuclei, some neuronal cells, such as mitral cells in the olfactory bulb, pyramidal cells in the cerebral ­cortex, and Purkinje cells in the cerebellum, are also immunostained with antilong-chain acyl-CoA hydrolase antibody (Yamada et al., 1996). At the subcellular level, longchain acyl-CoA hydrolases are localized in endoplasmic reticulum, cytosol, mitochondria, and peroxisomes. In developing brain, microsomal long-chain acyl-CoA hydrolase activity decreases sharply during the first 20 days after birth, whereas the acyl-CoA synthetase activity remains high indicating that long-chain acyl-CoA are needed for the synthesis of neural membrane phospholipids in neural membranes during the intense period of myelination. Brain long-chain acyl-CoA hydrolase has been purified and characterized from the rat and human brain cytosols (Lin et al., 1984; Yamada et al., 1996, 1999). The native enzyme has molecular mass of 104 kDa. It is made up of subunits of 36 kDa. The enzyme shows high activity with long-chain acylCoAs. With palmitoyl-CoA as substrate, the rat brain enzyme shows maximal velocity of 262 nmol/min/mg and Km of 5.7 mM and human brain shows maximal velocity of 295 nmol/min/mg and Km of 6.4 mM. It also hydrolyzes other long-chain acylCoAs (C8–18). Rat brain long-chain acyl-CoA hydrolase shows very high activity toward arachidonoyl-CoA (ARA-CoA). Arachidonic acid (ARA) and not its CoA ester is the substrate for enzymes in the production of prostaglandins, leukotrienes, and thromboxanes. Rat brain long-chain acyl-CoA hydrolase is inhibited by p-chloromercuribenzoate, lysophospholipids, palmitoyl carnitine, and bovine serum albumin. Metal ions such as Mn2+, Mg2+, and Ca2+ also inhibit enzymic activity (Lin et al., 1984; Yamada et al., 1996; Broustas and Hajra, 1995). Rat brain long-chain acyl-CoA hydrolase is inactivated by diethyl pyrocarbonate, an active center histidine-reacting agent. There is a lot of similarity in amino acid sequence of rat and human brain longchain acyl-CoA hydrolase. The human brain long-chain acyl-CoA hydrolase cDNA encodes a 338-amino acid sequence, which is 95% identical to that of a rat homolog. The human brain long-chain acyl-CoA hydrolase gene spans about 130 kb and comprised nine exons, and was mapped to 1p36.2 on the cytogenetic ideogram. Recently several acyl-CoA hydrolases have been identified in liver peroxisomes, where they act as auxiliary enzymes in a- and b-oxidation in this organelle and are regulated by peroxisome proliferator-activated receptors (PPARs) (Hunt and Alexson, 2002). The physiological roles of long-chain acyl-CoA hydrolases remain elusive. These enzymes play a key role in regulating and maintaining normal levels of deacylation/reacylation cycle associated lipid metabolite such as acyl-CoA, CoASH, and free fatty acid levels. These metabolites are closely associated with the regulation potential for many physiological and pathological processes in the brain.

3.2  Deacylation/Reacylation Cycle (Land’s Cycle) and Its Importance O O R2

C

R2C

O

O C

H

O

C

H2C

R1

+

HO

O

C

H

H2 C

O

O

P

C

R3

O

O H2C

CH2CH2NH2

O

O

P

O

O

HO C

H

H2C

O

O

C

R1

O

CH2CH2NH2

H2 C

O

C O C

H

H 2C

O

O

+ O P

CH2CH2NH2

O

O H2C

81

R2

O

C

R3

O P

O

CH2CH2NH2

O

Fig. 3.3  CoA-dependent and CoA-independent transacylation reactions

3.2.5 CoA-independent Reacylation in the Brain Brain microsomes contain a CoA-independent transacylation system, which transfers fatty acids from diacyl glycerophospholipids to various glycerophospholipids in the absence of any cofactor (MacDonald and Sprecher, 1991; Yamashita et al., 1997). The only fatty acids transferred by this system are C20 and C22 polyunsaturated fatty acids. Diacyl-glycerophosphocholine is the most preferred substrate (Fig. 3.3). Human brain homogenates possess the ability to transfer fatty acids from lysoPtdCho to lysoPtdEtn but not to lysoPtdSer or lysoPtdIns (Ross and Kish, 1994). Recent studies indicate that purified recombinant cPLA2g catalyzes an acyltransferase reaction from one molecule of lyso-PtdCho to another, forming PtdCho. Lyso-PtdCho or lyso-PtdEtn can act as acyl donor and acceptor. No acyl group transfer is observed with lyso-PtdSer, lysoPtdIns, and lyso-PtdH. PtdCho and PtdEtn also act as weak acyl donors (Yamashita et al., 2009). Acyl group transfer is controlled by reaction conditions, such as addition of lyso-PtdH/ PtdH, pH above 8, and increase in temperature. These parameters can change the balance between lysophospholipase and transacylation activities, suggesting that lysophospholipase/transacylation activities of cPLA2g may be regulated by various factors. As lysophospholipids are known to accumulate in ischemic injury and induce alterations in brain function, the cPLA2g may have a protective role through clearance of lysophospholipids by its transacylation activity (Yamashita et al., 2009).

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3.3 Effects of Lysophospholipids on Neural Membrane Metabolism As stated above, due to their detergent-like properties high levels of lysophospholipids are potentially toxic to cells (Weltzien, 1979; Farooqui et al., 1997). The levels of lysophospholipids are strictly controlled. High levels of lysophospholipids under pathological situations perturb neural membrane structure, affect many membranebound enzymes related to signal transduction, and cause cell lysis (Farooqui and Horrocks, 2006, 2007).

3.3.1 Lysophosphatidylcholine (Lyso-PtdCho) Lyso-PtdCho is an amphiphilic and proinflammatory phospholipid, which is synthesized in brain through the action of PLA1 and PLA2 on PtdCho and is metabolized by lysophospholipases and acyltransferases. The incorporation of lyso-PtdCho in neural membranes perturbs the orderly packed glycerophospholipid molecules in the lipid bilayer. Lyso-PtdCho not only interacts directly with ion channels and neurotransmitter receptors, but also modulates their activity indirectly through the modulation of neural membrane fluidity (Farooqui and Horrocks, 2007) (Fig. 3.4). Injections of lyso-PtdCho into brain not only produce acute inflammatory demyelination at the injection site, but also promote blood–brain barrier breakdown, and interstitial edema around the injection site (Lovas et al., 2000; Ousman and David, 2000, 2001; Degaonkar et al., 2002, 2005). Similarly, the treatment of cerebellar slices with lyso-PtdCho markedly affects their integrity and also causes demyelination in  vitro (Birgbauer et  al., 2004). In spheroid cultures, repeated exposure to lyso-PtdCho for a week results not only in 30% loss of MBP protein concentration and sharp decrease in 2¢,3¢-cyclic nucleotide 3¢-phosphodiesterase activity with partial remyelination after a week of recovery (Vereyken et al., 2009). The number of dividing cells is increased after lyso-PtdCho exposure. Only oligodendrocytes undergo cell division. Microglia, astrocytes, and neurons are not affected by lyso-PtdCho exposure. This suggests that lyso-PtdCho toxicity is ­specific for myelin and oligodendrocytes. Addition of cholesterol and simvastatin to cell cultures reduces Lyso-PtdCho toxicity, suggesting that lyso-PtdCho acts through altering membrane composition (Vereyken et al., 2009). Thus, spheroid culture model for demyelination with potential for remyelination offers a good model not only for testing drugs, but also studying molecular mechanisms of remyelination (Vereyken et al., 2009). Injections of lyso-PtdCho in mice result in statistically significant higher concentrations of 5-hydroxytryptamine (5-HT) and Norepinephrine (NA) in the blood (Itokwa et al., 2007). As stated above, Lyso-PtdCho interacts with neurotransmitter, cannabinoid, and growth factor receptors (Fig. 3.5). Thus, lyso-PtdCho interferes

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Deramification of microglia Intensification of pain

Transportation of DHA

Lyso phosphatidylcholine

Modulation of receptor function

Modulation of enzyme activities

Modulation of ion channels

Modulation of exocytosis

Regulation of cytokine and Chemokine expression

Fig. 3.4  Roles of lyso-phosphatidylcholine in brain

Lyso-phosphatidylcholine

Modulation of heparin Binding receptor

Modulation of Cannabinoid receptors

Modulation of NMDA receptor

Modulation of PDGF Receptors Modulation of dopaminergic Receptors

Fig. 3.5  Modulation of various receptors by lyso-phosphatidylcholine

with dopaminergic neurotransmission and promotes the induction of bradykinesia in rats (Lee et al., 2005). Lyso-PtdCho may act as a ligand to its own receptors (Zhu et al., 2001). It is likely that modulation of dopamine receptor function by lyso-PtdCho may be due to an interaction between dopaminergic receptor and

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lyso-PtdCho receptor (Lee et al., 2004; Soga et al., 2005). The occurrence of G protein-coupled lyso-PtdCho receptor G2A has been recently reported in T cells, where G2A contributes to the recruitment of T cells at sites of inflammation (Ghasemlou et al., 2007; Osmers et al., 2009). Since demyelinating diseases can be associated with painful sensory phenomena such as tactile allodynia and hyperalgesia, studies have been performed of the effect of lyso-PtdCho in animal model of pain. Intracerebroventricular injections of lyso-PtdCho result in increased behavioral responses and allodynia in carrageenaninduced acute and chronic models of inflammatory pain (Vahidi et al., 2006), suggesting that generation of lyso-PtdCho may be closely associated with the modulation nociceptive processes in neurons (Ji et al., 2003). The administration of PLA2 inhibitors blocks behavioral responses and allodynia (Yeo et al., 2004). In contrast, injections of lyso-PtdEtn have no significant effect on behavioral responses. Lyso-PtdCho also inhibits N-methyl-d-aspartate (NMDA) receptor responses, both in nucleated patches taken from cultured neurons and in cells expressing recombinant NMDA receptors (Fig. 3.5). This inhibition is reversible, voltage independent, and stronger at nonsaturating doses of agonist. It is not linked to the charge on the polar head and is not mimicked by lyso-PtdH or PtdCho (Casado and Ascher, 1998), suggesting that the lyso-PtdCho effect requires insertion into the lipid bilayer. In brain, lyso-PtdCho also regulates exocytosis and inflammation by modulating membrane-bound enzymes (protein kinases, nitric oxide synthase and cyclooxygenase-2, CTP-phosphocholine cytidylyltransferases), receptors, and ion channels, the transcription of cytokines, chemokines, MMP, adhesion molecule, and platelet aggregation (Oishi et al., 1988; Yuan et al., 1996; Murugesan et al., 2003). Lyso-PtdCho promotes the transformation of ramified resting microglia consisting of a small cell body and long processes with secondary branching into the activated deramified and amoeboid microglia, which actively participate in the maintenance of brain immune function. Although the morphological transformation of ramified microglia into the amoeboid microglia is observed under a wide variety of in vitro as well as in vivo conditions (Raivich et al., 1999; Schilling et al., 2004a), the precise molecular mechanism of deramification in the microglia remains largely unknown. It is proposed that lyso-PtdCho promotes microglial activation through the modulation of P2X7R signaling (Takenouchi et  al., 2007). Thus, in a mouse microglial cell line (MG6) and primary microglial cell cultures, lyso-PtdCho induces the sustained increase in the intracellular Ca2+ (Cai2+) through P2X7 nucleotide receptor (P2X7R) channels activated by ATP or BzATP. This increase in (Cai2+) is blocked by P2X7R antagonists, brilliant blue G and oxidized ATP. G2A, a receptor for lysoPtdCho, is expressed in MG6 cells, but not involved in promoting the effect of lysoPtdCho on the P2X7R-mediated change in (Cai2+). Furthermore, lyso-­PtdCho increases the P2X7R-associated formation of membrane pores and the activation of p44/42 mitogen-activated protein kinase supporting the view that lyso-PtdCho may regulate microglial functions in the brain by enhancing the sensitivity of P2X7R (Takenouchi et al., 2007). The activated microglial cells migrate to area of injured brain tissue

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and they engulf and destroy microbes and cellular debris (Gehrmann et al., 1995; Farooqui, 2009). The role of microglia in brain may not always be ­beneficial. Uncontrolled microglial activation and the subsequent excessive ­neuroinflammation may contribute to a variety of the CNS pathologies including neurodegenerative diseases (Farooqui et al., 2007). Thus, activation of microglia has to be strictly regulated, and the apoptotic elimination of activated microglia is thought to be one mechanism of the microglial self-regulation (Farooqui, 2009). The deramification of microglial cells can be blocked by inhibition of nonselective cation channels and K+-Cl− cotransporters. Lyso-PtdCho not only stimulates cell motility, but facilitates the release of proinflammatory cytokines. Docosahexaenoic acid (DHA) is an essential fatty acid required for the normal function of several tissues, especially the brain (Farooqui, 2009). Lyso-PtdCho is a preferred carrier of DHA to the brain (Bernoud et  al., 1999; Picq et  al., 2010), although the pathways of the formation of DHA-containing lysophospholipids in plasma have not been delineated. It is recently proposed that endothelial phospholipase A1 may be responsible for the generation of DHA lysophospholipids in plasma (Chen and Subbaiah, 2007). Substrate specificities studies with deuterium-labeled phospholipids with different polar head groups, as well as DHA-enriched natural phospholipids indicate that recombinant endothelial phospholipase A1 shows the polar head group specificity in the order of PtdEtn > PtdCho > PtdSer > PtdH. Within the same phospholipid class, the recombinant endothelial phospholipase A1 shows preference for the species containing DHA at the sn-2 position, and is inactive in the hydrolysis of phospholipids containing an ether linkage. Since cells of blood–brain barrier secrete endothelial phospholipase A1, it is suggested that enzyme may play an important role in the delivery of DHA lysophospholipid carriers to the brain (Chen and Subbaiah, 2007). Collectively, studies mentioned above support the view that in brain, at low concentrations lyso-PtdCho may have multiple effects ranging from membrane fusion, exocytosis, signal transduction to gene expression, and DHA transport into the brain.

3.3.2 Lyso-phosphatidylethanolamine (Lyso-PtdEtn) Lyso-PtdEtn synthesis has been reported to occur in rat brain synaptosomes (Iwata et al., 1986). Lyso-PtdEtn synthesis is markedly stimulated by calcium ion. The addition of veratridine to synaptosomal preparation stimulates the accumulation of lysoPtdEtn. This observation suggests that sodium channels along with enhanced calcium influx may be closely associated with the enhanced synthesis of lyso-PtdEtn (Iwata et al., 1986). In SK-OV3 ovarian and OVCAR-3 ovarian cancer cells lyso-PtdEtnmediated increase in Cai2+ is inhibited by U-73122. Moreover, pertussis toxin (PTX) almost completely blocks the lyso-PtdEtn-mediated increase in Cai2+ indicating the involvement of PTX-sensitive G-proteins. In addition, lyso-PtdEtn also ­stimulates chemotactic migration and cellular invasion in SK-OV3 ovarian cancer cells. Based

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on detailed investigation, it is suggested that LPE stimulates a membrane bound receptor, different from well-known LPA receptors, resulting in chemotactic migration and cellular invasion in SK-OV3 ovarian cancer cells (Park et al., 2007).

3.3.3 Lyso-phosphatidylserine (Lyso-PtdSer) Very little is known about the metabolism of lyso-PtdSer in brain tissue, but ­significant information on lyso-PtdSer is available on mast cells, leukocytes, and platelets. The deacylation of PtdSer by PLA1 and PLA2 generates 2-acyl-1-lysosn-GroPSer and 1-acyl-2-lyso-sn-GroPSer, respectively (Bruni et al., 1982; Inoue et al., 1989; Nagai et al., 1999; Hosono et al., 2001; Aoki et al., 2002; Kim et al., 2008; Nakamura et al., 2010). The addition of lyso-PtdSer to leukocytes and mast cells results in its rapid incorporation. In the presence of phorbol esters, lyso-­ PtdSer is rapidly converted into PtdSer by an acyl-CoA: lyso-PtdSer acyltransferase (Mietto et al., 1987). Lyso-PtdSer is an autocoid and immunological regulator, which dramatically augments the degranulation of rat peritoneal mast cells (RPMCs) (Bruni et al., 1988; Sugo et al., 2006). This effect is mediated by a lysoPtdSer receptor. Lyso-PtdSer also induces a dose-dependent inhibition of forskolin-stimulated cAMP accumulation in human GPR34-expressing Chinese hamster ovary (CHO/hGPR34) cells. These cells do not respond to other structurally related phospholipids (Sugo et al., 2006). Similar concentrations of PtdSer induce RPMC degranulation as well as the activation of GPR34 suggesting that activation of GPR34 is closely associated with both these processes and GPR34 is a functional mast cell lyso-PtdSer receptor. It is well known that mast cells are located throughout the human body and exposure to allergen results in their stimulation via the immunoglobulin E (IgE) receptor (Fc(epsilon)RI) to release several proinflammatory mediators such as tumor necrosis factor-a (TNF-a), reactive oxygen and nitrogen species such as ROS, NO, proteases, and lipid-derived mediators (Ozben and Erdogan, 2008; Theoharides and Kalogeromitros, 2006). Lyso-PtdSer-mediated stimulation of mast cells is closely associated with the development of allergic inflammation through the release of cytokines (IL-b1, interleukin (LTE4)), which modulate vasodilatation, bronchoconstriction, cellular chemotaxis, and increase vascular permeability (Theoharides and Kalogeromitros, 2006). Lyso-PtdSer also increases intracellular calcium ions in L2071 mouse fibroblast cells. U-73122 blocks this increase, but pertussis toxin does not, suggesting that lyso-PtdSer stimulates calcium signaling through the activation of PLC coupled G-protein coupled receptor (Park et al., 2006). The lyso-PtdH receptor antagonist, VPC 32183, has no effect on lyso-PtdSer-mediated calcium mobilization, indicating that lyso-PtdSer binds to its own receptor. In L2071 cells, lyso-PtdSer interacts with extracellular signal-regulated protein kinase (ERK) and p38 kinase. Pertussis toxin blocks the activation of both kinases, indicating the association of pertussis toxin-sensitive G-proteins in the stimulatory process. In L2071 cells,

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PtdSer stimulates chemotactic migration. Pertussis toxin completely inhibits this process, indicating the involvement of pertussis toxin-sensitive Gi protein(s). This chemotaxis of L2071 cells induced by lyso-PtdSer is also dramatically retarded by LY294002, a potent and selective inhibitor of phosphoinositide 3-kinases and by PD98059, a selective noncompetitive inhibitor of the MAPK pathway. Collective evidence ­suggests that lyso-PtdSer stimulates at least two different signaling cascades: one involving a pertussis toxin-insensitive but phospholipase C-dependent intracellular calcium ion increase, and the other a pertussis toxinsensitive chemotactic migration mediated by phosphoinositide 3-kinase and ERK (Park et al., 2005, 2006). Studies on lyso-PtdSer and lyso-PtdH-mediated increase in intracellular calcium in mouse bone marrow-derived mast cells (BMMC), and rat C6 glioma and human HCT116 colon cancer cells indicate that increases in calcium due to LPS and LPA can be blocked by PTX, U-73122 and 2-APB. This indicates that both lipids stimulate calcium signaling via G proteins (Gi/o types), PLC activation, and subsequent InsP3 generation. It is also reported that Ki16425 completely blocks lyso-PtdSer-mediated Ca2+ response in all cell types, but that the effect of VPC32183 varies from complete inhibition in BMMC and C6 glioma cells to partial inhibition in HCT116 cells. It is concluded that lyso-PtdSer increases intracellular Ca2+ through Ki16425/VPC32183-sensitive G protein-coupled receptors (GPCR), G protein, PLC, and InsP3 in mouse BMMC, rat C6, and human HCT116 cells (Kim et al., 2008).

3.3.4 Lyso-phosphatidylinositol (Lyso-PtdIns) In neural membranes, two mechanisms generate lyso-PtdIns; one is associated with PLA1-mediated deacylation of PtdIns and the other involves the reverse reaction of lyso-PtdIns acyltransferase (Ueda et al., 1993; Yamashita et al., 2003). In synaptic membranes, lyso-PtdIns is catabolized by a lyso-PtdIns-specific PLC (Kobayashi et  al., 1996). Lyso-PtdIns interacts with GPR55 receptor, a novel ligand for the orphan G protein-coupled receptor 55, which promotes Rho-dependent signaling. Additional events downstream of GPR55 include activation of ERK-MAP kinase and Ca2+ release from stores, as well as the induction of a number of transcription factors (Pertwee, 2007; Nevalainen and Irving, 2010). Although GPR55 has very little sequence homology with the CB1 or CB2 cannabinoid receptors, it clearly interacts with certain cannabinoid ligands. However, the physiological significance of this signaling remains elusive. In PC12 cell lyso-PtdIns induces exocytosis, which depends not only on the cellular cholesterol content, but also on the integrity of lipid rafts in the PC12 cell membranes (Ma et al., 2010). Depletion of cholesterol by preincubating cells with the cholesterol chelator results in the attenuation of exocytosis. Lyso-PtdInsmediated exocytosis also depends on calcium ions. The depletion of calcium by

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thapsigargin treatment blocks the lyso-PtdIns-mediated exocytosis. Collective evidence suggests that lyso-PtdIns-mediated exocytosis depends on the integrity of the lipid raft and intracellular calcium (Ma et al., 2010). In non-neural cells, lysoPtdIns not only stimulates the degradation of phosphoinositide and mobilization of cytosolic Ca2+, but also enhances arachidonic acid release, indicating that it activates both PLC and PLA2 activities. The effects of lyso-PtdIns can be distinguished from those of the well-known mitogen lyso-PtdH, which modulates the signaling pathways differently. These results suggest that the mitogenic activity of lysoPtdIns may involve the activation of PLC and PLA2 and are relatively specific for ras-transformed cells (Falasca et  al., 1995). Addition of ionophore A23187 to H-Ras-transformed fibroblasts results in a 10-fold increase in the levels of this lysolipid in the extracellular medium (Falasca et al., 1998), suggesting the stimulation of PLA2 activity. In H-Ras-transformed fibroblasts, extracellular lyso-PtdIns breaks down rapidly into inositol 1:2-cyclic phosphate. It is proposed that the formation and release of lyso-PtdIns may function as an autocrine mechanism, which may contribute to the Ras-dependent stimulation of cell growth (Falasca et al., 1998). Collective evidence demonstrates that in non-neural tissues lyso-PtdIns not only performs autocrine function, but also contributes to CoA-dependent transacylation system, which modulates polyphosphoinositide homeostasis in rat liver microsomes (Yamashita et al., 2003).

3.3.5 Lyso-cardiolipin (Lyso-Ptd2Gro) Cardiolipin (diphosphatidyl glycerol) is almost exclusively located in the inner mitochondrial membrane, where it is synthesized from phosphatidylglycerol and cytidinediphosphate-diacylglycerol. After primary synthesis, the mature acyl chain composition of lyso-Ptd2Gro is achieved by at least two remodeling mechanisms (Houtkooper and Vaz, 2008; Paradies et al., 2009). This phospholipid is not only intimately associated with mitochondrial bioenergetic processes, but also plays active roles in apoptosis and mitochondrial membrane dynamics (Farooqui, 2009; Paradies et al., 2009). During apoptotic cell death, Ptd2Gro interacts with two major components of proapoptotic machinery – tBid and cytochrome c. The binding of Ptd2Gro to cytochrome c unfolds the protein and that the complex functions as a peroxidase, catalyzing Ptd2Gro peroxidation essential for the release of proapoptotic factors, whereas binding with tBid not only activates proapoptotic protein, Bak and Bax, but also orchestrates apoptosis, supporting changes in mitochondrial electeron transport (Tyurin et al., 2007). At the molecular level during apoptosis TNFR1/ Fas promotes the cleavage of cytosolic Bid to truncated Bid (tBid), which translocates to mitochondria, where it mediates the destabilization of the mitochondrial bioenergetic homeostasis. These changes produce mild uncoupling of mitochondrial state-4 respiration, associated with an inhibition of the adenosine diphosphate

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(ADP)-stimulated respiration and phosphorylation rate. tBid mediates the ­disruption of mitochondrial homeostasis, blocks the overexpression of Bcl-2 and Bcl-XL. In mitochondrial membranes, Ptd2Gro induces inhibits state-3 respiration by modulating the activity of the ADP/ATP translocator (Gonzalvez et al., 2005). In the cytosol, the released cytochrome c mediates the allosteric activation of apoptosis-protease activating factor 1, which is required for the proteolytic maturation of caspase-9 and caspase-3. Collectively, these studies demonstrate that interactions of tBid with lyso-Ptd2Gro ‘primes’ the mitochondrial outer membrane via segregation of lipid domains, facilitating membrane discontinuity and leakage of above-mentioned apoptogenic factors (Esposti et al., 2003; Garrido et al., 2006).

3.3.6 Lyso-ethanolamine and Choline Plasmalogens (Lyso-PlsEtn and Lyso-PlsCho) Hydrolysis of ethanolamine and choline plasmalogens (1-O-alk-1 eny-2-acyl-snglycero-3-phosphoethanolamine and 1-O-alk-1 eny-2-acyl-sn-glycero-3-phosphocholine) by plasmalogen-selective phospholipase A2 (PlsEtn-PLA2) generates lyso-PlsEtn and lyso-PlsCho (Farooqui and Horrocks, 2001; Farooqui, 2010b) (Fig. 3.6). In neural membranes, lyso-PlsEtn and lyso-PlsCho are either rapidly incorporated through reacylation to maintain normal levels of plasmalogens or hydrolyzed by an enzyme called lysoplasmalogenase into fatty aldehyde and sn-glycero-3-phosphoethanolamine or sn-glycero-3-phosphocholine (JurkowitzAlexander et al., 1989). Lysoplasmalogens can also be transformed into an acetylated platelet-activating factor analog either by a CoA-independent transacetylase activity or through acetyl transferase activity (Lee, 1998). Lysoplasmalogens are amphiphilic molecules, which due to their detergent-like properties regulate physi­cochemical properties and neural membrane integrity not only by interacting with membrane-bound structural proteins, but also by modulating activities of enzymes. Their interactions with neural membranes increase membrane ­fluidity, which not only affect permeability, but also modulate activities of various enzymes, such as cAMP-dependent protein kinase (PKA) and Na+, K+ATPase (Williams and Ford, 1997; Han and Gross, 1991). The activation of PKA by lysoplasmalogens suggests that lysoplasmalogenase may play an important role in turning off signal transduction processes involving PKA-mediated phosphorylation. At the plasma membrane level, synthesis of lyso-PlsEtn may induce alterations in permeability resulting in influx of external calcium, which not only facilitates the translocation and activation of downstream enzymes (cyclooxygenases and lipoxygenases) that convert ARA into eicosanoids (Phillis et  al., 2006). In addition, lyso-PlsEtn or lyso-PlsCho-mediated alterations in membrane permeability may allow influx of external calcium, which may influence activities of calcium-dependent enzymes. By altering the action potential

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1-Alkyl-GroPCho Lyso-platelet activating factor

1-Alkyl-2-acyl-GroPCho • Generates PAF analogs

AcylCoA:lysophospholipis acyltransferase

• Increases membrane fluidity • Activates PKA

O R O

H2C

O

R C O C

H

H2C

O

CH

CH R

O P O

CH2

CH2

C

CH2

H

O

O

se

o ys

R

O P O

CH2

CH2

NH2

O Lyso-ethanolamine plasmalogen

o al

m

as

L

OH

H2C

H2C

CH CH

ge

pl

H

H

na

Ethanolamine plasmalogen

HO C

O

PlsEtn-PLA2

NH2

O

H2C

H2C HO C

O R

CH2 C Aldehyde

H

O P O

CH2

CH2

NH2

O sn-Glycero-3-phosphoethanolamine

Fig. 3.6  Metabolism of plasmalogen and lysoplasmalogen in brain. Platelet activating factor (PAF); protein kinase A (PKA)

and inducing depolarization, high levels of lyso-PlsEtn or lyso-PlsCho in heart tissue alter the action potential and produce spontaneous contraction, which may induce and maintain arrhythmic activity in the heart muscle (Caldwell and Baumgarten, 1998).

3.3.7 Lyso-phosphatidic Acid (Lyso-PtdH) Lyso-phosphatidic acid (lyso-PtdH, 1-O-acyl-2-hydroxy-sn-glycerol-3-phosphate) is the simplest phospholipid that acts as a growth-factor. It is not only an intermediate in the de novo phospholipid biosynthesis, but also an extracellular lipid agonist. Lyso-PtdH is synthesized by the glycerophosphate acyltransferase-mediated acylation of sn-glycerol-3-phosphate by acyl CoA in the membranes of endoplasmic reticulum and mitochondria (Das and Hajra, 1989; Pagès et al., 2001). LysoPtdH is also synthesized by the action of PtdH-specific PLA2 on phosphatidic acid (PtdH) (Thomson and Clark, 1995). In addition, action of PLD on lyso-PtdCho generates lyso-PtdH (Pagès et al., 2001; Moolenaar et al., 2004). A plasma enzyme called autotaxin (ATX) is responsible for the most of lyso-PtdH synthesis in our

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bodies. ATX converts lysophospholipids such as lyso-PtdCho to Lyso-PtdH by its lysophospholipase D activity (Sugimoto et  al., 2006; Xie and Meier, 2004; Okudaira et al., 2010). Lyso-PtdH is catabolized by several enzymes. Thus lysoPtdH is converted to PtdH by a lyso-PtdH-acyltransferase (Pagès et al., 2001). This enzyme is localized in microsomes and plasma membranes. It is crucial for de novo synthesis of glycerophospholipids. Lyso-PtdH can also be converted into monoacylglycerol by lysophosphatidate phosphohydrolase (lysophosphatidic acid phosphatase, LPP) (Brindley et al., 2002). The dephosphorylation of lyso-PtdH is the major pathway that terminates signaling processes mediated by lyso-PtdH. Expression of LPPs on internal membranes regulates not only lyso-PtdH-mediated signaling, but also signaling associated with different lipid phosphate. It is likely that different LPPs may perform distinct roles, based on integrin binding, their locations, and their abilities to metabolize different lipid phosphates in  vivo (Brindley and Pilquil, 2009). In neural cells, lyso-PtdH produces neurochemical effects, including stimulation of the release of noradrenaline from cerebral cortical synaptosomes, inhibition of glutamate uptake by astrocytes, elevation of neuronal intracellular calcium, and stimulation of dopamine release from PC12 cells (Tigyi et al., 1996a, b; Holtsberg et  al., 1997; Ye et  al., 2002; Moolenaar et  al., 2004; Anliker and Chun, 2004). Lyso-PtdH retards b-adrenergic-mediated shape change and enhances forskolinmediated increase in cyclic AMP levels in glial cells (Koschel and Tas, 1993; Kreps et al., 1993). Lyso-PtdH contributes to the induction of growth cone collapse and neurite retraction in neuroblastoma, PC12, and primary neuronal cell cultures. Lyso-PtdH acts through the activation of the Rho/ROCK and the phosphatidylinositol 3-kinase/Akt pathways, preventing neuronal differentiation (Dottori et  al., 2008). Lyso-PtdH also induces the reversal of stellation in astrocytes (Ramakers and Moolenaar, 1998). Lyso-PtdH modulates tight-junction permeability of brainderived endothelial cells and its high concentration disrupts blood–brain barrier function (Tigyi et  al., 1995; Schulze et  al., 1997). It increases the migration of murine microglial cells through the activation of Ca2+-activated K+ currents (Schilling et  al., 2004b). Collective evidence indicates that lyso-PtdH acts as an intracellular messenger in a paracrine/autocrine manner (Xie et al., 2002). It modulates neurite retraction, cytoskeleton reorganization, neurogenesis, neural migration, myelination, apoptosis, and neurotransmitter release (Fukushima, 2004) (Fig. 3.7). Lyso-PtdH modulates above neurochemical response through six types of 7-transmembrane G-protein-coupled receptors called Lyso-PtdH receptors (Fig. 3.8). These receptors have an apparent mol mass of 38–40 kDa. Lyso-PtdH receptors are classified into at least six groups namely LPA1, LPA2, LPA3, LPA4, LPA5, and LPA6 (Okudaira et al., 2010; Chun, 1999; Fukushima et al., 2001). Based on their primary structures, Lyso-PtdH receptors are divided into two families. LPA1 (EDG2), LPA2 (EDG4), and LPA3 (EDG7) belong to the endothelial differentiation gene (EDG) family. These receptors share high sequence homology to S1P receptors and cannabinoid receptors (CB1 and CB2). In contrast, LPA4 (P2Y9/ GPR23), LPA5 (GPR94), and LPA6 (P2Y5) fall under the P2Y receptor family, whose members target nucleotides rather than lysophospholipids. The brain

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PAF PLA2

PtdCho

PLD

PAF receptor signaling

Lyso-PtdCho

Myelination

Neurite retraction

Lyso-PLD Cytoskeleton reorganization

PtdH phosphatase

DAG Kinase

DAG

Lyso-PtdH

Acyltranferase

DAG-lipase

Lyso-PtdH-R

Lyso-PtdH phosphatase

PtdH-specific PLA2 PtdH

2-AG

Differentiation

Neuritogenesis

Cannabinoid receptor signaling

Apoptosis

MAG-lipase

ARA COX-2

Eicosanoids

Neural cell migration Eicosanoid receptor signaling

Fig. 3.7  Metabolism and roles of lysophosphatidic acid in brain. Phosphatidylcholine (PtdCho); lysophosphatidylcholine (lyso-PtdCho); lysophosphatidic acid (lyso-PtdH); phosphatidic acid (PtdH); diacylglycerol (DAG); 2-arachidonylglycerol (2-AG); platelet activating factor (PAF); phospholipase A2 (PLA2); phospholipase D (PLD); and lyso-phospholipase D (lyso-PLD)

expresses relatively high levels of LPA4. In B103 neuroblastoma cell cultures, ­lyso-PtdH produces growth cone collapse and neurite retraction through G12/13RhoA-Rho-associated kinase (ROCK) activation (Lee et al., 2007) and in human embryonic stem cells LPA4 modulates inhibition of neurosphere formation and neuronal differentiation (Dottori et al., 2008). Emerging evidence suggests that the binding of lyso-PtdH to its receptors activates multiple signal transduction pathways, including those initiated by the small GTPases Ras, Rho, and Rac. Another important target of lyso-PtdH is the peroxisome proliferator-activated receptor gamma (PPARg), which not only plays an essential role in ­regulating lipid and glucose homeostasis and cell proliferation, but also modulating apoptosis and inflammation (Tigyi, 2010). These responses have a direct impact on human ­diseases, particularly neurological disorders, diabetes, atherosclerosis, and cancer. It is activated by unsaturated acyl species of lyso-PtdH (Tsukahara et al., 2006). Studies on LPA receptor knockout mice indicate that LPA signaling may be involved in neurological disorders, cardiovascular diseases, angiogenesis, reproduction, cancer progression, and neuropathic pain (Okudaira et al., 2010).

3.4  Lyso-phospholipids in Neurotraumatic and Neurodegenerative Diseases

A

Lyso-PtdH

NMD-AR

PtdCho PtdCho

PLA2

/L TX

D

-PL

yso

2+

Ca

A

G12/13

Gi

ARA + Lyso-PtdCho

PtdIns 3K

RHO

SOS

PtdIns-4,5-P2

MAG ARA

93

TIAM1

+

AKT

Rho kinase

RAS

+

PKC activation +

[Ca2+]i

RAC

Survival

ERK

Stress fiber formation Cell rounding

Eicosanoids

Cell spreading Migration

DNA synthesis Viability

Fig. 3.8  Lyso-PtdH-mediated signaling in brain. Agonist (A); N-methyl-D-aspartate (NMDA-R) Phosphatidylcholine (PtdCho); lyso-phosphatidylcholine (lyso-PtdCho); arachidonic acid (ARA); autotaxin (ATX); lyso-phospholipase D (lyso-PLD); phospholipase C (PLC); phosphatidylinositol 4,5-bisphosphate (PtdIns 4,5-P2); inositol 1,4,5-trisphosphate (InsP3); protein kinase C (PKC); diacylglycerol (DAG); monoacylglycerol (MAG); serine/threonine protein kinases (AKT); small GTPases (Rac and Rho); and salt-overly sensitive pathway (SOS)

3.4 Lyso-phospholipids in Neurotraumatic and Neurodegenerative Diseases Neurotraumatic diseases (stroke, spinal cord trauma, and traumatic head injury) are characterized by glutamate-mediated overstimulation of NMDA receptor, activation of PLA2 and PLC, and increased degradation of neural membrane phospholipids resulting in enhanced production of free fatty acids and lysophospholipids, which under pathological conditions are either converted into platelet activation factor or hydrolyzed by lysophospholipases (Farooqui and Horrocks, 2007; Farooqui et al., 2007). Under oxidative stress (anaerobic conditions) lysophospholipids are transformed into LO radicals, which abstract a hydrogen radical from another lysophospholipid molecule. The radical thus formed adds oxygen and decomposes into a 2-oxolysophospholipid and a HOO radical (protonated superoxide). HOO radical in turn abstracts hydrogen atoms from other molecules and produces H2O2 (Spiteller, 2010). In contrast, in neurodegenerative diseases (AD) glutamate is not

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released, but significantly lowers the expression of NR2A and NR2B transcripts in ­susceptible regions of AD brain is observed (Bi and Sze, 2002; Hynd et al., 2004). This process results in an overactivation of glutamate receptors in a tonic rather than a phasic manner (Parson et al., 2007). This continuous mild activation may lead to neuronal damage through the stimulation of Ca2+-dependent enzymes related to lipid, protein, and nucleic acid metabolism. Ca2+-dependent enzymes associated with ­glycerophospholipid, sphingolipid, and cholesterol metabolism modulate levels of glycerophospholipid-, sphingolipid-, and cholesterol-derived lipid mediators (Farooqui and Horrocks, 2007).

3.4.1 Lyso-phospholipids in Ischemic Injury Ischemic injury in the brain is accompanied by the stimulation of isoforms of PLA2, massive release of free fatty acids, and increase in levels of lysophosphatidylcholine, which inhibits CTP: phosphocholine cytidylyltransferase, an enzyme that modulates PtdCho synthesis (Adibhatla et al., 2004). In contrast to the membrane damaging effects of lysophospholipids (Blondeau et  al., 2002) in transient model of global ischemia, intravenous injections of lyso-PtdCho facilitate the survival of CA1 pyramidal neurons, even when the treatment is started 30 min after 15-min global ischemia. In contrast, lysophosphatidic acid produces no protection. Several mechanisms may be associated with neuroprotective effects of lyso-PtdCho. Lyso-PtdCho may either produce beneficial effects by modulating 2P-domain K+ channels (Blondeau et al., 2002), or may act through its anti-inflammatory effects. Treatment of RAW 264.7 cells with 2-docosahexaenoyl lyso-PtdCho decreases LPS-induced formation of nitric oxide (NO), tumor necrosis factor-a (TNF-a), or IL-6 by inhibiting LTC4 synthesis (Hung et  al., 2010). Furthermore, intravenous administration of 2-(17-hydroperoxydocosahexaneoyl)-lyso-PtdCho more effectively blocks zymosan A-induced plasma leakage than 2-docosahexaenoyl lyso-PtdCho. This observation suggests that 2-(17-hydroperoxydocosahexaneoyl)-lyso-PtdCho, a product from oxygenation of 2-docosahexaenoyl-lyso-PtdCho by 15-lipoxygenase (LOX), may be an active metabolite, intimately responsible for anti-inflammatory effects of 2-docosahexaenoyl-lyso-PtdCho. In addition, 2-docosahexaenoyl-lyso-PtdCho is more efficient than 1-docosahexaenoyl-lysoPC or docosahexaenoic acid (DHA) as substrate for 15-lipoxygenases such as soybean LOX-1, leukocyte 12/15-LOX, and human 15-LOX-2. Collectively, these studies suggest that 2-docosahexaenoyl-lysoPtdCho and its oxygenation products may exert anti-inflammatory action after oral administration (Hung et al., 2010). Although several isoforms of PLA2, including cPLA2, sPLA2, and circulating lipoprotein-associated phospholipase A2 (Lp-PLA2) or platelet-activating factor acetylhydrolase are activated in brain following ischemic injury, but Lp-PLA2 has emerged as a novel biomarker for cerebrovascular and cardiovascular diseases (Zalewski et al., 2009; Mannheim et al., 2008). Lp-PLA2 is a 45-kDa protein with

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95

441 amino acids that is distinct from other members of the phospholipase A2 family in that it is calcium independent. It is produced by inflammatory cells, co-travels with circulating low-density lipoprotein (LDL), and hydrolyzes phospholipids ­containing oxidatively fragmented residues at the sn-2 position (oxidized ­phospholipids; OxPLs) into proinflammatory lyso-PtdCho and oxidized nonesterified fatty acids in LDL. OxPLs accumulate in the artery wall and are closely associated with vascular inflammation and atherosclerosis involved in cerebrovascular and cardiovascular diseases. Plasma levels of OxPLs present on apolipoprotein B-100 particles (OxPL/apolipoprotein B) are not only involved in coronary artery, carotid, and peripheral arterial disease, but also strongly correlated with lipoprotein levels (Tsimikas et al., 2007). Selective inhibition of Lp-PLA2 with darapladib (an inhibitor under development and testing by GlaxoSmithKline plc) retards the development of advanced coronary atherosclerosis in diabetic and hypercholesterolemic swine (Wilensky et  al., 2008). Darapladib markedly inhibits plasma and lesion Lp-PLA2 activity and decreases the levels of lyso-PtdCho in the lesions. Detailed studies on coronary gene expression indicate that darapladib exerts a general antiinflammatory action by reducing the expression of 24 genes associated with macrophage and T lymphocyte functioning. Darapladib treatment not only produces a considerable decrease in plaque area, but also induces notable decrease in necrotic core area. Collective evidence suggests that selective inhibition of Lp-PLA2 retards the progression and formation of advanced coronary atherosclerotic lesions. In addition Darapladib also stabilizes these dangerous plaques by decreasing the size of the core and reducing the number of inflammatory-immune cells present within the plaque (Wilensky et al., 2008). Levels of lyso-PtdH are increased in cerebral ischemia (Sun et al., 1992; Tigyi et al., 1995). Aspirin taken for 1 month significantly lowers lyso-PtdH level in stroke patients (n = 142) (2.41 ± 1.03 mM/L) compared with that before taking acetylsalicylate (4.06 ± 1.03 mM/L (Li et  al., 2008)). There is a close relationship between increased plasma lyso-PtdH levels and platelet activation. As mentioned above, high levels of lyso-PtdH induce neurite retraction, a process associated with neurodegeneration. It is proposed that lyso-PtdH acts through Rho-mediated signaling to induce contraction- related cytoskeleton changes to reorganize astrocytic morphology (Tigyi et al., 1996a, b; Ramakers and Moolenaar, 1998).

3.4.2 Lyso-phospholipids in Alzheimer Disease AD is characterized by accumulation of aggregated Ab peptide and neurofibrillary tangles composed of hyperphosphorylated t protein. Experimental evidence indicates that Ab accumulation precedes and drives t aggregation (Oddo et al., 2003; Octave, 2005). In AD, increase in activities of PLA2 isoforms and lysophospholipases is accompanied by elevation in phosphodiesters, phosphomonoesters, fatty acids,

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prostaglandins, isoprostanes, 4-hydroxynonenals, and other lipid mediators (Table 8.2) (Farooqui and Horrocks, 2006, 2007). Very little is known about ­levels of ­lysophospholipids in Alzheimer disease (AD). Determination of lyso-PtdCho in CSF of AD patients and subjects with subjective memory complaints without dementia by tandem mass spectrometry indicate that lyso-PtdCho levels and the lyso-PtdCho/PtdCho ratio are significantly decreased in CSF of AD patients compared to controls. The decrease in lyso-PtdCho levels may be due to increase in lysophospholipase activity in AD patients (Farooqui et al., 1990). The lower lysoPtdCho/PtdCho ratio in CSF of patients with AD indicates that alterations in the metabolism of choline-containing phospholipids in the brain in AD may be closely associated with membrane alterations in AD (Mulder et  al., 2003). Physicochemical and pathological consequences of decrease in lyso-PtdCho/ PtdCho ratio may cause alterations in membrane fluidity and permeability, alterations in ion homeostasis, and induction of oxidative stress (Farooqui and Horrocks, 2007; Farooqui, 2009). Hyperphosphorylation and accumulation of tau in neurons (and glial cells) is one of the major pathologic hallmarks in AD and other tauopathies, such as Pick disease (PiD), progressive supranuclear palsy, corticobasal degeneration, and argyrophilic grain disease (Ferrer et  al., 2005). Hyperphosphorylation of tau is modulated by several kinases that phosphorylate specific sites of tau in vitro. Based on the effect of kinase inhibitors, involvement of glycogen synthase kinase-3 (GSK-3) has been proposed. Phosphorylation of GSK-3 at Ser9 inactivates GSK-3 in the majority of neurons with neurofibrillary tangles and dystrophic neurites of senile plaques of AD, and in Pick bodies. Although detailed investigations have not been performed, lyso-PtdH-mediated activation of GSK-3 occurs in the Rho pathway and may represent an important link between microtubule and microfilament dynamics in AD (Sayas et al., 1999, 2002).

3.4.3 Lyso-phospholipids in MS and EAE Multiple sclerosis (MS) is an autoimmune disease characterized by demyelination and axonal transaction resulting in reversible and irreversible neurological deficits. The pathophysiology of MS is illusive, but may involve multifactorial immunemediated demyelination (Steinman et al., 2002). Activities and expression of isoforms of PLA2 (cPLA2 and iPLA2 as well as sPLA2) are increased in different stages of experimental autoimmune encephalomyelitis (EAE), a commonly used animal model of MS in rodents. As stated above, PLA2 isozyme generated arachidonic acid via cyclooxygenases and lipoxygenases is metabolized into protaglandins and leukotrienes, which contribute to neuroinflammation (Calder, 2003). The other product of PLA2 isozyme catalyzed reaction, lyso-PtdCho, also facilitates neuroinflammation and demyelination by upregulating the expression of proinflammatory cytokines and chemokines and initiating the synthesis of platelet activating factor (Ousman and David, 2000, 2001).

References

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3.4.4 Lyso-phospholipids in NCL Infantile-onset Neuronal Ceroid Lipofuscinosis (INCL) is a severe pediatric neurodegenerative disorder caused by mutations in the gene encoding palmitoyl-protein thioesterase 1 (PPT1), an enzyme responsible for the removal of a palmitate posttranslational modification from an unknown set of substrate proteins (Zhang et al., 2007). PPT1-knockout (PPT1-KO) mice brain containing increased levels of lysoPtdCho caused the activation of cPLA2. Age-dependent increase in lyso-PtdCho levels in the PPT1-KO mouse brain positively correlates with elevated expression of the genes characteristically associated with phagocytes. It is proposed that increased synthesis of lyso-ptdCho in the PPT1-KO mouse brain is at least one of the mechanisms that promote phagocyte infiltration in the pathophysiology of INCL (Zhang et al., 2007).

3.5 Conclusion Action of PLA1 and PLA2 generates lysophospholipids in the brain. In addition, lysophospholipids are also generated through the transacylation reactions between phospholipids. In normal brain, lysophospholipids are not only involved in the modulation of receptor-mediated neurotransmitter (dopamine, glutamate, and acetylcholine) release and regulation of activities of enzymic activities (protein kinases, nitric oxide synthases, and cytidylyltransferases), but also in transport of docosahexaenoic acid through the blood–brain barrier, microglial cell activation and deramification, neuroinflammation, and modulation of exocytosis and in transcription of cytokines and chemokines. In neurotraumatic and neurodegenerative diseases, the accumulation of lysophospholipids promotes oxidative stress, demyelination, and neural cell injury. In neural membranes, their levels are controlled not only by their reacylation, but also through activities of lysophospholipases, as well as PLA2. In neural and non-neural cells, lyso-glycerophospholipids also induce morphological changes in neural membranes including membrane fusion and lysis. They also induce secretion of hormones, growth factors, and other lipid mediators that modulate the homeostasis and dynamics of membrane glycerophospholipids.

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Chapter 4

Platelet-Activating Factor in Brain: Its Metabolism, Roles, and Involvement in Neurological Disorders

4.1 Introduction Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a potent biologically active mediator that is involved in intracellular and extracellular communication. PAF belongs to a family of biologically active and structurally related alkyl phospholipids produced by neural and non-neural cells (neural cells, macrophages, platelets, endothelial cells, mast cells, and neutrophils). PAF not only induces platelet aggregation, but also enhances histamine and serotonin release resulting in vasodilation and enhancement in vascular permeability, increase in eosinophil and neutrophil motility, and degranulation producing edema, hyperemia, itching, and pain (Farooqui et  al., 2008). PAF is normally present in the mammalian tissues in picomolar concentrations and is found both in the cytosol and body fluids including blood plasma, cerebrospinal fluid, urine, and amniotic fluid (Lynch and Hensen, 1986). PAF-synthesizing enzymes (phospholipase A2 and acetyltransferase) are modulated by MAP kinase signaling pathways (Farooqui et  al., 2008) whereas PAF-hydrolyzing enzyme (PAF-acetyl hydrolase) is regulated by proinflammatory cytokines such as LPS, TNF-a, IL-1, IL-8, and IFN-a. Thus, levels of PAF are tightly regulated both by PAF synthesizing enzymes (plasmalogen-phospholipase A2 and acetyltransferase) and PAF hydrolyzing enzymes (PAF-acetylhydrolases) (Farooqui et al., 2008). In brain tissue, PAF may be associated with neural cell migration, gene expression, calcium mobilization, noniception, and long-term potentiation (Fig 4.1). PAF also modulates neural plasticity (Moriguchi et  al., 2010). In cultured hippocampal neurons, PAF increases autophosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII) and phosphorylation of synapsin I and myristoylated alanine-rich protein kinase C substrate (MARCKS). In hippocampal neurons, PAF exposure also increases protein kinase Ca (PKCa) autophosphorylation and extracellular signal-regulated kinase (ERK) phosphorylation. Collective evidence suggests that PAF induces synaptic facilitation through activation of CaMKII, PKC, and ERK in the hippocampal CA1 region (Moriguchi et al., 2010). Although enzymes associated with A.A. Farooqui, Lipid Mediators and Their Metabolism in the Brain, DOI 10.1007/978-1-4419-9940-5_4, © Springer Science+Business Media, LLC 2011

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Excitotoxicity Gene expression Calcium mobilization Platelet activating factor

BBB permeability Long-term

potentiation

Apoptosis Immune function

Fig. 4.1  Roles of platelet-activating factors in the brain

PAF synthesis have not been purified and fully characterized from brain tissue, many reports on PAF synthesis in mammalian brain have been published (Francescangeli et al., 2000; Goracci et al., 2009). In neural cells PAF is not stored, but synthesized either spontaneously or under appropriate stimulation. In neuronal and glial cell cultures, acetylcholine (ACh) dramatically stimulates PAF synthesis, and addition of cholinergic receptor antagonist, atropine, produces the inhibition of PAF synthesis (Sogos et al., 1990). PAF may be involved in interactions between astroglial cells and neurons. Astrocytic PAF mediates neurotrophic signaling from astrocytes to injured neurons. It is suggested that interplay between PAF and the neurotrophic receptor may be involved in regenerative processes in the brain tissue. In brain, physiological activity of PAF is not limited to its proinflammatory function, neurotoxicity, apoptosis, and blood–brain barrier permeability, but also associated with neurotrophic effects (Fig. 4.1). In non-neural cells, PAF is not only involved in neutrophil adhesion, chemotaxis, allergic reactions, and reproduction, but also in increased vascular permeability, vasodialation, and circulatory system disturbances (Chao and Olson, 1993; Honda et al., 2002; Bazan, 2003).

4.2 PAF Biosynthesis in the Brain Three different pathways are responsible for PAF synthesis in mammalian tissues (Honda et al., 2002; Snyder, 1995). De novo synthesis is a minor pathway responsible for the synthesis of basal levels of PAF for maintaining normal functions under physiological conditions. The remodeling pathway is a major pathway of PAF generation. High levels of PAF are synthesized from a specific subclass of PtdCho that

4.2  PAF Biosynthesis in the Brain

CH2

DHAP

HO

109

O-CH2-(CH2)n-CH3

CH

O

+ Acetyl-CoA

Acetyl-CoA acetyltransferase

O CH3 -C -O

O P OH O

CH2

CH2 CH CH2

O-CH2-(CH2)n-CH3 O O P OH O

1-O-alkyl-2-acetyl-sn-glycero-3-phosphate

1-O-alkyl-glycero-3-phosphate

Phosphohydrolase Pi

O CH3 -C - O

CH2 CH CH2

O-CH2-(CH2)n-CH3 O O P O - choline O

PAF

O

Cholinephosphotransferase CDP

CH3-C - O

O-CH2-(CH2)n-CH3

CH CH2

CDP-Choline

Cytidylytransferase

CH2

OH

1-O-alkyl-2-acetyl-sn-glycerol

Phosphocholine+ CTP

Fig. 4.2  De novo synthesis of platelet-activating factor in the brain

contains an ether bond, rather than an ester bond at the sn-1 position of the glycerol backbone (Snyder, 1995; Bazan, 2003) in response to glutamate release and stimulation in cytokine expression (tumor necrosis factor-a, interferon-g, and interleukin-1) under pathological conditions (Sogos et  al., 1990; Prescott et  al., 2000; Kunievsky and Yavin, 1994). Under oxidative stress, oxidative breakdown of PtdCho also results in generation of minor oxidized phospholipids, which can produce PAFlike activity in neural and non-neural cells. This minor pathway is called as oxidative fragmentation pathway of PAF synthesis.

4.2.1 De Novo Synthesis of PAF In resting state, neural cells synthesize by the de novo synthesis. Three enzymes namely 1-alkyl-2-lyso-sn-glycero-3-phosphate (alkyl-lyso-GP): acetyl-CoA acetyltransferase, 1-alkyl-2-acetyl-sn-glycero-3-phosphate phosphohydrolase, and dithiothreitol(DTT)-insensitive1-alkyl-2-acetyl-sn-glycerol:CDP-cholinephospho­transferase (Heller et al., 1991; Snyder, 1995) are associated with the de novo synthesis of PAF (Fig.  4.2). In this pathway, a 1-Alkyl-2-lyso-sn-glycero-3-phosphate (alkyllysoGP): acetyl-CoA acetyltransferase transforms 1-O-alkyl-glycero-3-phosphate into 1-O-alkyl-2-acetyl-sn-glycero-3-phosphate in the presence of acetyl-CoA (Lee et al., 1986; Baker and Chang, 1993). Specific activity of ­alkyl-lyso-GP:acetyl-CoA

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acetyltransferase in nuclear fraction of cerebral cortex is three times those of the microsomal fraction (Baker and Chang, 1996). This enzyme shows optimal activity between pH 8–9 and is inhibited by MgATP or oleoyl CoA. The alkyl-lyso-glycerol phosphate acetyl-CoA acetyltransferase is distinguished from the nuclear lyso-PAF acetyltransferase by a greater sensitivity to MgATP inhibition (Baker and Chang, 1996, 1998, 2000). Bovine serum albumin or the fatty acyl CoA synthetase inhibitor (Triacsin C) prevents MgATP inhibition. Addition of exogenous free fatty acids promotes the MgATP-induced inhibition. Oleoyl CoA, in the absence of MgATP, also inhibits alkyl-lyso-GP:acetyl-CoA acetyltransferase suggesting that MgATP stimulates the conversion of nuclear free fatty acids to fatty acyl CoA. Fatty acyl CoA may directly inhibit nuclear alkyl-lyso-GP:acetyl-CoA acetyltransferase (Baker and Chang, 2002). 1-O-alkyl-2-acetyl-sn-glycero-3-phosphate is then dephosphorylated into 1-O-alkyl-2-acetyl-sn-glycerol by 1-alkyl-2-acetyl-sn-glycero-3-phosphate phosphohydrolase (Lee et al., 1988). Very little information is available on brain 1-alkyl2-acetyl-sn-glycero-3-phosphate phosphohydrolase. This phosphohydrolase shows no notable substrate selectivities with regard to variations in alkyl chain length (C16:0 versus C18:0) at the sn-1 position or short-chain acyl groups (C2:0 to C6:0, with the exception of C3:0) at the sn-2 position of the glycerol moiety. In the brain, the enzymic activity of 1-alkyl-2-acetyl-sn-glycero-3-phosphate phosphohydrolase is 30- to 90-fold higher than alkyl-lyso-GP:acetyl-CoA acetyltransferase (Lee et al., 1988). 1-O-Alkyl-2-acetyl-sn-glycerol is converted into PAF by a DTT-insensitive microsomal 1-alkyl-2-acetyl-sn-glycerol: CDP-choline phosphotransferase. Although this enzyme occurs in brain, 1-alkyl-2-acetyl-sn-glycerol: CDP-choline phosphotransferase has not been purified and characterized from brain tissue. Availability of CDP-choline is the rate-limiting step in the PAF synthesis (Snyder, 1997; Gimenez and Aguilar, 2001). Therefore, factors that regulate cytidylyltransferase activity play an important role in modulating the levels of PAF in various tissues. For example, fatty acids (Blank et al., 1988; Vallari et al., 1990) and intracellular levels of CDP-choline (Lee et al., 1990) have been shown to upregulate PAF biosynthesis through the de  novo synthesis. Brain PAF-synthesizing 1-alkyl-2acetyl-sn-glycerol: CDP-choline phosphotransferase not only shows similar subcellular localization, Mg2+ requirement, and Ca2+ inhibitory properties, but also has similar thermal inactivation profiles as the kidney medulla enzyme (Goracci and Francescangeli, 1991; Francescangeli et al., 2000). It shows optimal activity at pH 8.0, and Km and Vmax values for 1-alkyl-2-acetylglycerol were 42  mM and 3.0  nmol/min/mg protein, respectively (Goracci and Francescangeli, 1991). Subcellular distribution studies in 15-day old rabbit cerebral cortices indicate that PAF-synthesizing cholinephosphotransferase shows highest specific activity in microsomal fraction. The microsomal fraction is subfractioned into rough and smooth microsomal fractions (Baker and Chang, 1993). Rough microsomal subfraction has higher specific activity than smooth microsomal fraction. PAFsynthesizing cholinephosphotransferase is inhibited by Triton X-100 (Francescangeli et al., 2000).

4.2  PAF Biosynthesis in the Brain

H2C

O CH3

(CH2)n CH2

C

O CH2

111

(CH2)n

CH3

PLA2

Ca

H2C

2+

HO

O C OH O H2C O P

O CH2CH2N(CH3)3

O

C OH

(CH2)n

CH3

O

O P

H2C Fatty acid

O

CH2

O

CH2CH2N(CH3)3

O

Acetyl-CoA Acetyltransferase

Acyltransferase

Acyl-CoA

Acetic acid

H 2C

O

CH2

HO C OH H 2C

(CH2)n

CH3 Acetylhydrolase

O

O P

O

O CH2CH2N(CH3)3

O

CH3

C

H2C O

C OH

H2C

Lyso-PAF

CH2 (CH2)n

O

CH3

O

O P

O CH2CH2N(CH3)3

O

PAF

Fig. 4.3  Remodeling pathway for the synthesis of platelet-activating factor in brain and inflammatory cells

4.2.2 Remodeling Pathway for PAF Synthesis The remodeling pathway involves a structural modification of preexisting etherlinked phospholipids that serve as structural components of membranes (Stafforini et  al., 1987; Snyder et  al., 1996). The remodeling pathway occurs primarily in inflammatory cells (Fig. 4.3). In this pathway, arachidonic acid is hydrolyzed from 1-O-alkyl-2-arachidonyl-sn-glycero-3-phosphocholine by cytosolic phospholipase A2 (cPLA2), resulting in the generation of 1-O-alkyl-2-lyso-sn-glycero-3phosphocholine (lyso-PAF) (Rubin et al., 2005). Lyso-PAF is acetylated by acetyCoA: 1-O-alkyl-2-lysophosphatidylcholine acetyltransferase to produce PAF. This enzyme is localized in microsomes. Acetyl CoA: lyso-PAF acetyltransferase has been purified and cloned from non-neural tissues (Shindou et al., 2007). The enzyme has molecular mass of 60 kDa. Addition of detergents to the reaction mixture results in total inactivation of the enzymic activity. Glycerol stabilizes the enzyme activity. DTT and mercaptoethanol significantly inhibit acetyl CoA: lyso-PAF acetyltransferase, indicating that there are disulfide bridges present in the enzyme. Tyrosine kinase inhibitors and calmodulin antagonists inhibit acetyl CoA: lyso-PAF acetyltransferase activity in H2O2-stimulated cells, suggesting that tyrosine kinase and calcium/calmodulin-dependent protein kinase may be associated with regulation of acetyltransferase activity (Tosaki et  al., 2007). Similarly, in human neutrophils,

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4  Platelet-Activating Factor in Brain: Its Metabolism, Roles, and Involvement…

PlsEtn

Receptor

A

PlsEtn-PLA2

Lyso-PlsEtn

ARA or DHA COX, LOX

PakCho

15-LOX PGs, LTs, Txs and LXs PlsEtn

Lyso-PAF Resolvins and neuroprotectins

PAF hydrolase

Lyso-PAF acetyltransferase PAF

Cellular response and modulation of neural cell function

Fig. 4.4  Conversion of plasmalogen into platelet-activating factor in neural and non-neural cells. Plasmalogens are hydrolyzed by PlsEtn-PLA2 into lyso-PlsEtn. Transacylation of lysoPlsEtn (lysoplasmalogen) with PakCho (alkylacyl-glycerol-3-phosphocholine) results in the production of lyso-PAF. Acetylation of lyso-PAF by acetyl-CoA: 1-alkyl-sn-glycero-3-phosphorylcholine 2-O-acetyltransferase results in synthesis of PAF

acetyltransferase activity is modulated by mitogen-activated protein kinases, namely the p38 kinase (Nixon et al., 1999). Thus non-neural acetyl CoA: lyso-PAF acetyltransferase activity is modulated by phosphorylation/dephosphorylation process. In the endothelial cells, acetyl CoA: lyso-PAF acetyltransferase is not only regulated by cytosolic Ca2+ and H+, but through the activation of protein kinases C and A (Heller et al., 1991). Collective evidence suggests that acetyl CoA: lyso-PAF acetyltransferase activity is regulated by phosphorylation/dephosphorylation processes. PAF can also be synthesized from plasmalogens. Thus, endothelin-mediated stimulation of brain PlsEtn-PLA2 decreases PlsEtn levels, and evokes the generation of PAF (Collado et al., 2003). The second step of PAF production requires the conversion of lyso-PAF to PAF by the enzyme acetyl CoA: lyso-PAF acetyltransferase (Fig. 4.4). Both of these enzymes are activated by post-translational phosphorylation (Prescott et al., 1990; Baker et al., 2002). The transfer of the acetyl group from PAF to lysoplasmalogen in a CoA-independent manner is catalyzed by PAF:lysoplasmalogen transacetylase in HL-60 cells, endothelial cells, and a variety of rat tissues (Karasawa et al., 1999; Lee et al., 1992). The purified enzyme catalyzes the transacetylation of the acetyl group not only from PAF to lysoplasmalogen forming plasmalogen analogs of PAF, but also to sphingosine producing N-acetylsphingosine (C2-ceramide) (Karasawa et al., 1999). In addition, this enzyme

4.2  PAF Biosynthesis in the Brain

ArachidonylPtdCho

Lyso-PtdEtn

113

PAF-R

1-O-Alkyl-2arachidonylPtdCho

PtdIns-4,5-P3 Gi

PtdIns3K

cPLA2 ARA

ERK

PtdIns-4,5-P2

+

Akt S473 P

Akt cPLA2 +

COX, LOX

PtdEtn

Lyso-PAF

Acetyltransferase

Eicosanoids

P38 MARK

Bcl-2 Cell survival

Cell death

PAF

Cellular response and modulation of neural cell function

Fig. 4.5  PAF receptor-mediated modulation of cellular responses. Lysophosphatidyethanolamine (lyso-PtdEtn); phosphatidylethanolamine (PtdEtn); lyso-platelet-activating factor (lyso-PAF); platelet-activating factor (PAF); lysophosphatidyethanolamine (lyso-PlsEtn); cytosolic phospholipase A2 (cPLA2); cyclooxygenase (COX); 15-lipoxygenase (15-LOX); arachidonic acid (AA); docosahexaenoic acid (DHA); extracellular-signal-regulated protein kinase (ERK); and p38 mitogen-activated protein kinase (p38 MAPK); G protein (Gi); plus sign indicates activation, and minus sign indicates inhibition

has PAF-acetylhydrolase activity in the absence of lipid acceptor molecules. Collective evidence suggests that PAF-dependent transacetylase is an enzyme that modifies the cellular functions of PAF through generation of other diverse lipid mediators (Karasawa et al., 1999). The addition of exogenous lysoplasmalogens or thrombin to endothelial cell cultures markedly stimulates the synthesis of PAF in a CoA-independent manner (McHowat et al., 2001). This suggests that plasmalogen-selective PLA2, an enzyme that hydrolyzes plasmalogen into lysoplasmalogen, may modulate PAF synthesis (Fig. 4.5). Based on these observations, it has been proposed that cross-talk (interplay) may occur between plasmalogen and PAF metabolism in vivo. This crosstalk may be involved in fine-tuning not only PAF-receptor-induced biological responses, but also generation and maintenance of levels of other lipid mediators such as arachidonic acid-derived eicosanoids and docosahexaenoic acid-derived docosanoids through the receptor-mediated plasmalogen degradation of plasmalogens (Farooqui and Horrocks, 2007). In non-neural cells, activation of the PAF receptor (PAF-R) modulates PtdIns 3-kinase signaling, resulting in the conversion of PtdIns 3,4,5- trisphosphate into PtdIns-4,5-bisphosphate by Akt, a process

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4  Platelet-Activating Factor in Brain: Its Metabolism, Roles, and Involvement…

closely associated with the activation of mitochondria and caspase-mediated apoptotic cell death (Fig. 4.5) (Lu et al., 2008). The effect of PAF-R activation is mimicked by LY-294002, an inhibitor of PtdIns 3-kinase. Conversion of PtdIns4,5-bisphosphate into PtdIns 3,4,5-trisphosphate results in the inhibition of mitochondrial depolarization inducing cell survival.

4.2.3 Oxidative Fragmentation Pathway for PAF Synthesis The third pathway for the synthesis of PAF involves oxidative fragmentation of PtdCho. Oxidation of 1-O-alkyl-2-arachidonyl-sn-glycerophosphocholine results in the generation of many species of 1-O-alkyl phospholipids with different short-chain substituents at the sn-2 position. These 1-O-alkyl phospholipids interact with PAF-R and mediate a variety of biological effects (Stafforini et al., 1996; Prescott et al., 2000). The fragmentation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine by ozonolysis generates 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine (Smiley et  al., 1991; Prescott et  al., 2000). This phospholipid stimulates human neutrophils at submicromolar concentrations, and its effects are blocked by specific PAF receptor antagonists (WEB2086, L659989, and CV3988) (Smiley et al., 1991). Similarly, oxidation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine with soybean lipoxygenase results in the production of 15-hydroperoxy derivatives, which are further oxidized to numerous fragmented phospholipids, some of which interact and activate polymorphonuclear leukocytes. The hydrolysis of these derivatives with PAF acetylhydrolase blocks their biologic activity. Based on these observations, the occurrence of oxidative fragmentation pathway of PAF analog production has been proposed in non-neural tissues.

4.3 Catabolism of PAF in the Brain PAF is hydrolyzed by an enzyme called PAF acetylhydrolase. This enzyme selectively removes short acyl chains (C2 to C9) from the sn-2 position of PAF (Fig. 4.6), and has no activity with acyl chains longer than C9. PAF structural analogs are also broken down by PAF-acetylhydrolase. These analogs competitively inhibit PAF-acetylhydrolase activity (Stafforini et  al., 1997). PAF receptor antagonists block PAF-acetylhydrolase activity. This is in contrast to cPLA2 and sPLA2 that require Ca2+. cPLA2 uses Ca2+ for binding to phospholipid substrate, which is located in neural membranes, whereas sPLA2 needs mM concentrations of Ca2+ for catalysis (Farooqui et  al., 2006). Besides short chain phospholipids, PAFacetylhydrolase also degrades short-chain diacylglycerols, triacylglycerols, and acetylated alkanols (Tselepis and Chapman, 2002; Min et  al., 2001). Unlike cPLA2 and sPLA2, PAF-acetyl hydrolase degrades PAF, its analogs, and shortchain oxidized phospholipids in a calcium-independent manner (Gelb et al., 2000).

4.3  Catabolism of PAF in the Brain

115

CH2OR

CH2OR O

PAF acetylhydrolase

CH3COCH

HOCH

CH3COH

O

O

(Lyso-PAF)

(PAF) CH2OR O C

+

H2COPOCH2CH2N(CH3)3

H2COPOCH2CH2N(CH3)3

CH3CO

O

O

O

H

H2COH

(1-Alkyl-2-acetyl-sn-glycerol)

Neutral lipid acetylhydrolase

(Acetate)

CH2OCH2CH2R O HOC

H

+

CH3COH

H2COH

(Alkylglycerol)

(Acetate)

Fig. 4.6  Hydrolysis of PAF and 1-alkyl-2-acetyl-sn-glycerol by PAF-acetylhydrolase

The structure of PAF-acetylhydrolase is similar to neutral lipases and serine esterases. PAF-acetylhydrolase contains a serine/aspartate/histidine catalytic triad whose linear orientation and spacing are consistent with the a/b hydrolase conformation of other lipases and esterases. At least three PAF-acetyl hydrolases occur in mammalian tissues. These enzymes have different physicochemical and kinetic properties, and are encoded by different genes. Two PAF-acetylhydrolases, Type I, and Type II, are intracellularly found in brain and other visceral organs, and the third PAF-acetylhydrolase is extracellular that is found in plasma (Arai et al., 2002; Tjoelker and Stafforini, 2000; Arai, 2002; Stafforini, 2009). Although Type I and Type II are serine-dependent enzymes, their ability to hydrolyze PAF is quite different. Type I PAF-acetylhydrolases are found in two forms (Ia and Ib isoforms) in brain tissue. Ia is a heterodimeric isoform composed of a1 and a2 subunits, whereas Ib isoform is composed of a1, a2, and b subunits, and is coupled with G protein. Ib isoform is abundantly found in brain tissue (Hattori et al., 1996). The majority of adult tissues, except the brain express only a2 subunit. Type I PAF-acetylhydrolases are expressed in macrophages and in tissues containing a high content of inflammatory cells (Derewenda and Derewenda, 1998; Arai, 2002; McMullen et al., 2000). Type II PAF-acetylhydrolase is a monomer of 40 kDa with 392 amino acid residues. PAF acetylhydrolases contain a Gly-X-Ser-X-Gly motif that is characteristic of lipases and serine esterases. The purified type I and II PAF acetylhydrolases show similar activity against PAF and oxidatively modified phosphatidylcholine, but do not hydrolyze phosphatidylcholine or phosphatidylethanolamine with two longchain acyl groups (Hattori et  al., 1993, 1996; Manya et  al., 1998, 1999). These enzymes differ from each other in primary sequence, tissue distribution and localization, subunit composition, and substrate specificities.

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4  Platelet-Activating Factor in Brain: Its Metabolism, Roles, and Involvement…

Plasma PAF-acetylhydrolase has been purified from several sources using multiple column chromatographic procedures. SDS-PAGE indicates that molecular mass varies from 43 to 63 depending on animal species (Karasawa et  al., 2003). Plasma PAF acetylhydrolase shares 43% homology with type II PAF acetylhydrolase. The binding of purified enzyme with concanavalin A suggests that plasma PAF acetylhydrolase is a glycoprotein. Determination of carbohydrates in purified enzyme indicates that it contains N-linked heterogenous sugar chains (9 kDa) containing sialic acid. Treatment of human plasma PAF-acetylhydrolase with peptide N-glycosidase F results in production of lower molecular mass peptide as indicated by the changes in mobility on the SDS-PAGE. This observation again supports N-glycosylated nature of human plasma PAF-acetylhydrolase. Intracellular type II and plasma PAF acetyl hydrolases have high affinity for esterified F2-isoprostanes, a series of prostaglandin-like compounds formed in vivo through nonenzymic peroxidation of arachidonic acid, but the rate of esterified F2-isoprostane hydrolysis is slower compared with the rate of hydrolysis of other substrates hydrolyzed by these enzymes (Stafforini et al., 2006; Stafforini, 2009). Accumulating evidence suggests that PAF-acetyl hydrolases are unique serine esterases that transform PAF into lysoPAF (Blank et  al., 1981). The levels of PAF-acetyl hydrolase are critical for the regulation of the circulatory PAF and its physiological and pathological activities (Tjoelker and Stafforini, 2000). Some forms of PAF-acetylhydrolases are specific for PAF while others act on variety of PAF analogs. Thus, PAF-acetylhydrolases act as general scavengers of glycerophospholipids species, which may accumulate inappropriately under pathological conditions (Tjoelker and Stafforini, 2000). In brain tissue, PAF and its metabolism are involved in neuroprotection, inhibition of inflammation, modulation of long-term potentiation, modulation of gene expression, and neuronal migration (Umemura et al., 2007; Tjoelker and Stafforini, 2000; Bate et al., 2004; Bazan, 2003; Grassi et al., 1998) (Fig. 4.1).

4.4 PAF Receptors in the Brain As mentioned above, PAF may be associated with neural cell migration, gene expression, calcium mobilization, noniception, and long-term potentiation in brain tissue (Fig.  4.1). PAF interacts with specific receptors called as PAF receptors (PAF-Rs). These receptors have been cloned and characterized from neural and non-neural tissues (Honda et al., 1991). PAF-Rs consist of seven transmembrane helices, which are coupled with G proteins such as Gao, Gai, Gbg, and Gaq (Honda et al., 1991; Clark et al., 2000). PAF-R mRNA has been detected in hypothalamus, medulla-pons, olfactory bulb, hippocampus, cerebral cortex, spinal cord, thalamus, and cerebellum in rat brain. In situ hybridization and Northern blotting studies indicate that mRNA for PAF-R is expressed in both neurons and glial cells (Bito et al., 1992; Marcheselli et al., 1990; Mori et al., 1996; Ishii et al., 1996). Stimulation of PAF-Rs results in activation of phospholipases (PLA2, PLC, and PLD), GTPases, and kinases (Table 4.1) (Maclennan et al., 1996; Mori et al., 1996;

4.4  PAF Receptors in the Brain

117

Table 4.1  PAF-mediated stimulation of enzymic activities in neural and non-neural cells Enzyme Effect Reference Phospholipase A2 Stimulation Izumi and Shimizu, 1995 Phospholipase C Stimulation Izumi and Shimizu, 1995 Phospholipase D Stimulation Izumi and Shimizu, 1995 Adenylate cyclase Inhibition Izumi and Shimizu, 1995 PtdIns-3-kinase Stimulation Izumi and Shimizu, 1995 MAP-kinase, protein tyrosine kinase Stimulation DeCoster et al., 1998 Stimulation Izumi and Shimizu, 1995 Cyclooxygenase-2 Stimulation Bazan et al., 1991 Metalloproteinase-9 Stimulation Ottino et al., 2005 Caspase-3 Stimulation Hostettler and Carlson, 2002

PAF-R

Extracellular

Remodeling pathway

1-O-alkyl-2-arachidonyl-snglycerophosphocholine Gi +

PAF

PtdIns kinase

MEKK

PKC

Raf-1

PKC

Eicosanoids

Intracellular

?

cPLA2

ARA

PM

PtdIns-4,5-P2 Gq

DAG + InsP3

Ca

2+

mobilization

MAPKK

MAPK

Cellular response

Fig. 4.7  PAF receptor-mediated modulation of kinases and gene expression in brain. Phosphatidylinositol 3-kinase (PtdIns kinase); protein kinase Cz (PKCz); mitogen-activated protein kinase kinase kinase (MEKK); protein kinase C (PKC); product of oncogene c-raf (raf-1). This protein functions in the MARK/ERK signal transduction; mitogen-activated protein kinase kinase (MARKK); mitogen-activated protein kinase (MARK); platelet activating factor (PAF); and platelet activating factor receptor (PAF-R)

Honda et al., 2002; Kornecki and Ehrlich, 1991; Clark et al., 2000). Activation of PLA2, PLC, and PLD liberates arachidonic acid, which is converted into eicosanoids through the action of cyclooxygenases and lipoxygenases (Fig. 4.7). PAF-mediated stimulation of polyphosphoinositide turnover generates diacylglycerol (DAG), and inositol 1,4,5-trisphosphate (InsP3) inducing protein kinase C activation and

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4  Platelet-Activating Factor in Brain: Its Metabolism, Roles, and Involvement…

elevation in intracellular calcium levels (Izumi and Shimizu, 1995; Ishii and Shimizu, 2000). PAF activates guanylate cyclase, which generates cGMP; and inhibits adenylate cyclase, which lowers cAMP levels. These processes modulate phosphorylation of many proteins through the activation of various kinases including MAP-kinases, phosphatidylinositol 3-kinase, and tyrosine kinases (Chao and Olson, 1993; Honda et al., 2002; Ishii and Shimizu, 2000). These enzymes are closely associated with modulation of gene expression in neuronal and glial cells (Bazan et al., 1994). In addition, PAF-R activation is not only involved in the elevation of calcium via calcium channel opening as well as from intracellular stores (Izumi and Shimizu, 1995; Ishii and Shimizu, 2000), but also in upregulation of nerve growth factor expression and generation of several lipid mediators, processes that are closely associated with neurodegeneration as well as neuroprotection in brain tissue.

4.5 PAF in Neurological and Visceral Disorders As stated above, PAF is not stored, but synthesized by activated neural cells (neurons, astrocytes, oligodendrocytes, and microglial cells) as well as non-neural cells (platelets, inflammatory and endothelial cells) on cellular demand by remodeling, de novo synthesis, and oxidative fragmentation pathways. De novo synthesis is not affected by the external stimuli. In response to neural injury, convulsions, and oxidative stress, the remodeling and oxidative fragmentation pathways are activated in neural and non-neural cells. Treatment of neural cells with neurotransmitters such as dopamine, acetylcholine, and glutamate results in PAF synthesis in a calciumdependent manner (Sogos et al., 1990). Microglia, which express functional PAF receptors to a high level, show a marked chemotactic response to PAF. Microglia derived from PAF-receptor-deficient mice do not show chemotactic response (Aihara et al., 2000). Thus, PAF functions as a key messenger in neuron-microglial interactions. On exposure to oxidative stress, neural and non-neural cells generate large quantities of oxidatively fragmented phospholipids or PAF-like lipids from neural and non-neural membrane phospholipids in brain regions, where the density of PAF receptors and PAF acetylhydrolase activity is very low. This may result in accumulation of PAF and PAF-like molecules, which may induce excitotoxic events and abnormal signal transduction processes, resulting in inflammation and oxidative stress associated with neural and non-neural diseases (Farooqui et al., 2007, 2008).

4.5.1 PAF in Neurological Disorders PAF and PAF-like molecules accumulate during ischemia and traumatic brain injury, (TBI) and spinal cord injury (SCI) (Farooqui et al., 2008) (Fig. 4.8) and PAF receptor antagonists provide neuroprotection in the gerbil brain ischemia/reperfusion model by decreasing glutamate-mediated polyunsaturated fatty acid release

4.5  PAF in Neurological and Visceral Disorders

119

Cyto

1-O-alkyl-2-arachidonylsn-phosphocholine

Degradation

ROS

IκK IκB-P

p65 p50

Ca2+

Remodeling pathway

ARA

Eicosanoids

+

cPLA2

Gi +

NMDA-R

Glu

Cytokine-R

+

Lyso-PAF

NF-κ B

Neuroinflammation

NF-κ B RE

Increased levels of PAF

TNF-α,IL-1β IL-6 & proinflammatory enzymes

Nucleus

Miller-Dieker lissencephaly

HIV

Stroke

TBI and SCI

Prion diseases

MS

Bacterial miningitis

Neurological diseases

Fig. 4.8  PAF receptor-mediated alterations in lipid mediators and association of platelet-activating factor with neural and non-neural diseases. Nuclear factor-kappa B (NF-kB); Nuclear factor-kappa B response element (NF-kB-RE); tumor necrosis factor-a (TNF-a); interleukin-b1 (IL-b1); cytosolic phospholipase A2 (cPLA2); reactive oxygen species (ROS); N-methyl-d-aspartate receptor (NMDA-R); arachidonic acid (ARA); traumatic brain injury (TBI); spinal cord injury (SCI); multiple sclerosis (MS); and human immunodeficiency (HIV)

and restoring cerebral blood flow (Panetta et al., 1987; Belayev et al., 2009). During ischemic injury and epileptic seizures, the rates of PAF synthesis and breakdown no longer maintain a modulated PAF pool size resulting in increased PAF levels due to remodeling pathway. Thus, PAF accumulates during ischemia-reperfusion injury, and is involved in the activation of platelets, neutrophils, and proinflammatory signaling. At higher concentrations, PAF not only becomes proinflammatory messenger, but also activates COX-2 expression as well as several early response genes that encode transcription factors (Squinto et al., 1989; Bazan et al., 1994). PAF receptor antagonists provide sustained neuroprotection that may be valuable in designing the stroke therapy. In TBI and SCI, most of the damage that occurs in brain and spinal cord tissues is due to secondary injury caused by glutamate, Ca2+ overload, and oxidative stress. High levels of PAF can be detected in the rat brain after cold-induced local brain injury (Tokutomi et al., 2001). Brain damage caused by head injury can be partially prevented by BN-52021, a PAF antagonist. PAF also contributes to the secondary damage, following SCI. It is proposed that excitotoxicity, oxidative stress, and

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4  Platelet-Activating Factor in Brain: Its Metabolism, Roles, and Involvement…

increased expression of cytokine induction may contribute to spinal cord trauma (Hostettler and Carlson, 2002). Treatment with PAF antagonist, WEB 2170, 15 min prior to injury significantly decreases mRNA levels for cytokines, and oxidative stress at 6 h post injury. These results suggest that PAF modulates the induction of proinflammatory cytokine after spinal cord trauma (Hostettler and Carlson, 2002). Increase in PAF levels is closely associated with neuroinflammation in epileptic seizures, Miller-Dieker lissencephaly, bacterial meningitis, multiple sclerosis, prion diseases, and HIV replication associated with AIDS dementia complex (Fig.  4.8) (Feuerstein, 1996; Farooqui et al., 2008). In addition, alterations in PAF have been reported to occur in Miller-Dieker lissencephaly, a neurodevelopmental disease characterized by thick cerebral cortex formed without its usual folds (Hattori et al., 1994; Walsh, 1998). As stated above, brain contains significant amounts of PAFacetylhydrolase isoform Ib, an oligomeric protein complex, which contains three types of subunits: two homologous (63% identity) 26 kDa catalytic subunits (a1 and a2) harboring PAF-acetylhydrolase activity, and the 45 kDa b-subunit (LIS1), a product of the causal gene for Miller-Dieker lissencephaly (McMullen et al., 2000). During fetal development, the preferentially expressed a1-subunit forms a homodimer, which binds to a homodimer of LIS1, whereas in adult organisms a1/a2 and a2/a2 dimers, also bound to dimeric LIS1, are the prevailing species. It is reported that a1 expression is restricted to actively migrating neurons in rats and that switching of catalytic subunits from the a1/a2 heterodimer to the a2/a2 homodimer takes place in these cells during brain development, suggesting that PAF acetylhydrolase plays a role(s) in neuronal migration (Manya et al., 1998). The consequences of this ‘switching’ may contribute to the pathogenesis of Miller-Dieker lissencephaly (McMullen et al., 2000). Expression of LIS1 in the dentate gyrus modulates PAF-acetyl hydrolase activity (Shmueli et al., 1999). Reduction in LIS1 protein levels found in lissencephaly patients may render animals more susceptible to seizures. Overexpression of LIS-1 has been reported to interfere with cell division, with alterations in chromosome attachment to the mitotic spindle and on the interaction of astral microtubules with the cell cortex. LIS1/b also plays an important role in the induction of nuclear movement and control of microtubule organization. Although it is suggested that mutations in LIS-1 may produce a lissencephalic phenotype either by interfering with the movement of neuronal nuclei within extending processes, or by interference with the division cycle of neuronal progenitor cells in the ventricular and subventricular zones of the developing nervous system, the role of PAF and PAF-acetylhydrolase in the pathophysiology still remains controversial (Vallee et al., 2000; Arai, 2002). In animal models of lissencephaly, PAF agonists and antagonists modulate migration of cerebral granule and hippocampal cells indicating that PAF and PAF-like molecules may act like neural cell migration stop signal (Adachi et al., 1997; Bix and Clark, 1998). Alzheimer disease (AD), the cause of one of the most common types of dementia, is a brain disorder characterized by the formation of two main protein aggregates: senile plaques and neurofibrillary tangles. It is accompanied by the production and accumulation of the Ab1-42 peptide (Ab), loss of synapses, dementia, and gradual cognitive decline. PAF has also been implicated in the neuronal damage in Alzheimer disease (AD) (Table 4.2). The mechanisms linking neural cell injury to

4.5  PAF in Neurological and Visceral Disorders Table 4.2  Involvement of PAF in neurological disorders Effect of PAF Neurological disorder PAF levels antagonist Ischemia Increased Beneficial Traumatic brain injury Increased Beneficial Spinal cord injury Increased Beneficial Alzheimer disease Increased Beneficial Prion diseases Increased Beneficial Multiple sclerosis Increased Beneficial HIV-mediated dementia Meningitis

Increased Increased

Beneficial Beneficial

Neuropathy Subarachnoid hemorrhage Migraine

Increased Increased Increased

Not known

121

Reference Belayev et al., 2009 Tokutomi et al., 2001 Hostettler and Carlson, 2002 McLamon et al., 2005 Bate et al., 2010 Edwards and Constantinescu, 2009; Kihara et al., 2008 Tsoupras et al., 2008 Maclennan et al., 1996; Townsend and Scheld, 1994 Noris et al., 1993 Hirashima et al., 1994 Sarchielli et al., 2004

PAF synthesis are fully understood. However, it is well known that microglia from AD patients have significantly higher (20%) basal Ca2+ [Ca2+]i relative to microglia from nondemented patients (McLamon et  al., 2005). In addition, microglia from AD patients show diminished amplitudes (reduction of 61%) of SOC-mediated Ca2+ entry induced by PAF and prolonged time courses (increase of 60%) of ATP responses with respect to microglia from nondemented patients (McLamon et al., 2005). These observations support the view that significant abnormalities are present in Ca2+-mediated signal transduction in microglia isolated from AD patients. It is likely that PAF receptor-mediated mobilization of calcium through calcium channels and intracellular stores may contribute to the stimulation of calcium-dependent enzymes, resulting in neural injury (Clark et al., 2000; Farooqui et al., 2008). Other mechanisms of PAF-mediated neurotoxicity may involve Ab. Treatment of hippocampal neurons with Ab reduces the amounts of synaptophysin, a presynaptic membrane protein that is essential for neurotransmission, indicating synapse damage. Pretreatment of cortical or hippocampal neuronal cultures with PAF antagonists, ginkgolides A or B protects against Ab-mediated loss of synaptophysin. This protective effect is observed with nanomolar concentrations of ginkgolides. It is suggested that PAF antagonists may protect against the synaptic damage and the cognitive loss seen during the early stages of AD (Bate et  al., 2008). Similarly, Ginkgo biloba extract EGb761 (quercetin and ginkgolide B) also protects against Ab-induced neurotoxicity by blocking Ab-induced cell apoptosis, ROS production, mitochondrial dysfunction and activation of c-jun N-terminal kinase (JNK), extracellular signal-regulated kinase 1/2 (ERK1/2), and Akt signaling pathways (Shi et al., 2009). Collectively, these studies suggest that PAF plays an important role in inflammation, oxidative stress, and neurodegeneration in AD and PAF antagonists protect against PAF-R-mediated neurotoxicity. Studies on the effect of PAF in 1-methyl-4-phenylpyridinium (MPP+), a toxin used for producing Parkinson disease-like neurochemical changes, in PC12 cells

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indicate that PAF modulates MPP+-mediated apoptotic cell death in concentrationdependent manner. Treatment with lower concentration (0.75 mM) of PAF significantly attenuates the MPP+-induced toxicity by increasing Bax levels, decreasing Bid and Bcl-2 levels, and altering mitochondrial membrane potential. These processes result in the release of cytochrome c and subsequent caspase-3 activation (Ha and Lee, 2010). The inhibitory effect of PAF does not involve nuclear factor-kappaB (NF-kB) activation. In contrast, PAF at the concentrations more than 2.5 mM are not only toxic, but produce additive effect on the MPP+ toxicity. It is suggested that PAF at low concentrations may not be toxic and may retard the MPP+ toxicity by suppressing the apoptosis-related protein activation. In contrast, higher concentrations of PAF may produce an additive toxic effect against the MPP+ toxicity by increasing apoptosis-related protein activation. Prion diseases are accompanied by the accumulation of abnormal isoforms of cellular prion protein (PrPC), motor dysfunctions, synaptic loss, dementia, spongiosis, astroglyosis, and neurodegeneration (Farooqui, 2010). Pretreatment of cell cultures with PLA2 inhibitors (AACOCF3, MAFP, and aristolochic acids) protects against PrP82-146-mediated synapse degeneration. Synapse degeneration is increased following the addition of a specific PLA2 activating peptide (PLAP) and the addition of PrP82-146. Activation of PLA2 is the first step in the generation of platelet-activating factor (PAF) and PAF receptor antagonists (ginkgolide B, HexaPAF, and CV6029) protect against synapse degeneration induced by PrP82-146 and PLAP (Bate et al. 2010). These results support the view that PrP82-146-mediated activation of cytoplasmic PLA2 may elevate levels of PAF and PGE2. The increase in PAF and PGE2 causes synapse degeneration. It is proposed that PLA2 inhibitors and PAF antagonists may cross the blood–brain barrier and protect against the synapse degeneration observed in prion diseases (Bate et al., 2010). PAF plays a critical role in experimental allergic encephalomyelitis (EAE), an animal model for multiple sclerosis (MS). Levels of PAF in the spinal cord of EAE mice and cerebrospinal fluid of MS patients are increased (Kihara et al., 2008). This increase in PAF is coupled with the increased activities of PLA2 and acetylCoA:lyso-PAF acetyltransferase (LysoPAFAT) in the SC of EAE mice compared with those of naive mice and correlate with disease severity (Kihara et al., 2008). Although levels of other isoforms of cPLA2, sPLA2, and LysoPAFAT transcripts are upregulated in the spinal cord of EAE mice, PAF acetylhydrolase activity is unchanged during the course of EAE. Collectively, these studies suggest that PAF accumulation in the spinal cord of EAE mice is profoundly dependent on the group cPLA2/LysoPAFAT activities present in the infiltrating macrophages and activated microglia (Kihara et al., 2008). PAF also plays an important role in the pathogenesis of several AIDS manifestations such as AIDS dementia complex, Kaposi’s sarcoma, and HIV-related nephropathy. PAF antagonists have been studied in these conditions with promising results (Tsoupras et  al., 2008). Increase in PAF levels is observed in the internal jugular venous blood of migraine patients. PAF levels are decreased at the end of the migraine attack, reaching levels significantly lower than those measured before migraine attack (Sarchielli et al., 2004). PAF levels are also increased in multiple

4.5  PAF in Neurological and Visceral Disorders Table 4.3  PAF levels in visceral diseases Visceral diseases PAF levels Asthma Increased Systemic lupus erythematosus Increased Juvenile rheumatoidarthritis Increased Acute myocardial infarction Increased Sepsis Increased Crohn disease Increased Ulcerative colitis Increased

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Reference Tsukioka et al., 1996 Tetta et al., 1990 Tselepis et al., 1999 Lupia et al., 2003 Mathiak et al., 1997 Hocke et al., 1999 Hocke et al., 1999

sclerosis, and viral and bacterial infections (Edwards and Constantinescu, 2009). Administration of PAF antagonists has beneficial effect in animal models of multiple sclerosis (Kihara et al., 2008).

4.5.2 PAF in Visceral Disorders PAF is closely associated with the pathogenesis of bronchial asthma (Table 4.3). It not only induces key pathogenic features of asthma, but also influences the activity of cells involved in immune-inflammatory process. Apart from its known potent ability to activate platelets, PAF administration can mimic some of the abnormalities observed in asthma, including bronchoconstriction, bronchial hyper responsiveness, and gas exchange impairment, which may be mediated by leukotrienes acting as secondary mediators of some PAF effects. PAF may therefore serve as a possible direct target for antiasthmatic drugs. Surprisingly, PAF receptor antagonists have not been useful in providing beneficial clinical effects. It is suggested that combining the effect of PAF antagonists with antagonists of other mediators associated with the pathogenesis of asthma may improve clinical efficacy (KasperskaZajac et al., 2008). In systemic lupus erythematosus (SLE), PAF levels are elevated not only in plasma during the most active phases of the disease (Tetta et al., 1990), but also in monocytes of SLE patients compared with inactive patients and controls (Bussolati et  al., 2000) . This is due to decrease in PAF-acetylhydrolase activity. Enhanced PAF may be responsible for the increase in immunoglobulin production by B lymphocytes in SLE patients (Smith and Shearer, 1994; Mazer et al., 1991). Juvenile rheumatoid arthritis (JRA) is the most common childhood chronic systemic autoimmune inflammatory disease characterized by progressive joint damage. Patients with active JRA have lower plasma PAF-acetylhydrolase activity and higher plasma triglyceride levels than age-matched control subjects (Tselepis et al., 1999). This may result in increased levels of PAF and inflammatory cytokines (Table  4.3). In addition, patients with active JRA show low levels of HDL2 and HDL3. These alterations suggest that active JRA is associated with partial loss of the anti-inflammatory activity of plasma Apo B- and Apo A-I-containing lipoproteins (Tselepis et al., 1999).

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Sepsis is a serious and complex clinical syndrome caused by an overly active host response to infection. PAF is associated with the pathology of endotoxic shock, and the blood PAF levels are increased during endotoxemia and that the administration of PAF antagonists in animals protects them from the deleterious effects of endotoxin (Giral et al., 1996). The clinical trials of PAF receptor antagonists have failed to demonstrate efficacy in diseases such as septic shock, asthma, and pancreatitis (Mathiak et al., 1997). Levels of PAF are also increased in stool of patients with ulcerative colitis and Crohn disease (Hocke et al., 1999). PAF antagonists may be used for the treatment of inflammatory bowel diseases.

4.5.3 PAF in Kainic Acid Neurotoxicity Kainic acid (KA) is a cyclic and nondegradable analog of glutamate. KA causes neuronal loss in specific striatal and hippocampal areas of the brain after intraventricular and intracerebral injections. KA-induced neurotoxicity and epileptogenesis is accompanied by increase in cPLA2 activity, elevation in arachidonic acid oxidation products, and alterations in localization of PAF-R mRNA. Although PAF-R mRNA is normally expressed by neurons and microglia in rat hippocampus, KA-mediated neurotoxicity restricts the expression of PAF-R mRNA to apoptotic neurons and glial cells (Bennett et al., 1998). PAF along with excitatory amino acids increase mitogen-activated protein (MAP) kinases in neurons and glial cells. Treatment of hippocampal neuronal cultures with PAF and KA indicate that MAP kinase activity is significantly increased (De Coster et al., 1998). Extracellular signalregulated kinase, c-Jun N-terminal kinase, and p38 kinases are also increased by kainate or PAF in hippocampal neurons and non-neural cells (Margues et al., 2002). The activation of kinases is blocked by the KA receptor antagonists CNQX and PAF receptor antagonist, BN 50730. The PAF receptor antagonist, BN 50730 also retards KA-induced activation. CNQX has no effect on PAF activation of the kinases, indicating that PAF is downstream of kainate activation (De Coster et al., 1998). These results indicate that PAF and KA activate similar kinase pathways. PAF acts downstream of the kainate receptor, and excessive stimulation of PAF receptors contribute to neurodegeneration in PAF-mediated neurotoxicity (De Coster et al., 1998).

4.6 Molecular Mechanism Associated with PAF-Mediated Neural Injury As stated above, PAF-Rs are coupled with the activation of PLA2, PLC, and PLD. Interactions of PAF with its receptors not only enhance the breakdown of PtdCho, PlsEtn, and PtdIns but also mobilizes calcium (Clark et  al., 2000; Honda et  al., 2002), resulting in generation of many lipid mediators including, arachidonic acid, generation of arachidonic acid-derived prostaglandins, leukotrienes thromboxanes lipoxins, liberation of diacylglycerol, and inositol 1,4,5-trisphosphate (InsP3).

4.7  Conclusion

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PAF- induced elevation in calcium is not only responsible for neuronal growth cone collapse (Rehder et  al., 1992), but also for the activation of guanylate cyclases, which produce cGMP. This second messenger is involved in phosphorylation of various proteins through the activation of various kinases including MAP-kinases, phosphatidylinositol 3-kinase, and tyrosine kinases (Chao and Olson, 1993; Honda et al., 2002). Excessive production of above lipid mediators and phosphorylation of neural protein may cause neural injury. In addition, PAF facilitates the generation of ROS, which interact with NFkB. In the nucleus NFkB mediates the transcription of many genes implicated in inflammatory and immune responses. These genes include COX-2, intracellular adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1), E-selectin, TNF-a, IL-1b, IL-6, sPLA2, inducible nitric oxide synthase (iNOS), and matrix metalloproteinases (MMPs). In neural membrane oxidized phospholipids, the length of the sn-2 carbon chain dictates affinity for PAF-R. Phospholipid species with sn-2 acetyl group show the highest affinity (Carolan and Casale, 1990; Ishii et al., 1997), whereas with higher carbon chain length show lower affinity. Substituting phosphoethanolamine for phosphocholine substantively reduces binding (O’Flaherty et al., 1994). The sn-1 ether linkage is not required for PAF agonist and PAF-R interactions, but does increase potency several hundred-fold (Prescott et al., 2000). The sn-1 carbon chain length and degree of unsaturation may modulate the activation of different PAF-R signaling pathways (Carolan and Casale, 1990). These PAF isoform-specific differences may explain observations that PAF-R expression augments chemotherapeutic cytotoxicity yet provide protection cells TNF-a, TRAIL, and 1-O-hexadecyl-2acetyl-sn-glycero-3-phosphocholine (C16-PAF)-induced toxicity (Li et al., 2003). It is recently shown that the length of the sn-1 carbon chain of PAF dictates the molecular mechanism of neurodegeneration in PAF-R−/−, PAF-R+/+ and null mice. Both C16-PAF and C18-PAF are toxic to neurons that lack PAF-R (PAF-R−/−), but this toxicity is initiated by different cell death pathways (Ryan et al., 2008). Exposure of PAF-R-deficient neurons to C16-PAF results in a signaling cascade through the activation of executioner caspase 7, whereas C18-PAF-mediated cell death is accompanied by caspase-independent pathway. PAF-mediated neurodegeneration is also modulated by PAF-R expression. Ectopic or endogenous PAF-R expression protects neurons from C16-PAF-mediated neurodegeneration, whereas PAF-R expression transforms C18-PAF-mediated neurodegeneration from a caspase-independent to a caspase-dependent signaling pathway. Collective evidence suggests that carbon chain length may dictate the molecular mechanism underlying the apoptotic cell death in cerebellar granule neurons.

4.7 Conclusion PAF is a phospholipid mediator released from activated neural and non-neural cells that promotes pathophysiologic inflammation. PAF interacts with specific receptors called as PAF receptors (PAF-Rs). These receptors have been cloned and characterized from neural and non-neural tissues. PAF-Rs consist of seven transmembrane

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helices, which are coupled with G proteins. Stimulation of PAF-Rs is linked with the activation of PLA2, PLC and PLD, GTPases, and kinases. Activation of various phospholipases liberates arachidonic acid, which is converted into eicosanoids through the action of cyclooxygenases and lipoxygenases. These processes are closely associated with PAF-mediated neural injury. Levels of PAF are modulated by the activities of PAF-synthesizing and PAF-degrading enzymes. Levels of PAF are markedly increased in neurological disorders (ischemia/reperfusion injury, traumatic brain injury, spinal cord injury, Alzheimer disease, and Miller-Dieker lissencephaly) and visceral diseases (asthma, systemic lupuserythematosus, juvenile rheumatoid arthritis, acute myocardial infarction, sepsis, and Crohn disease). Neither the cell types nor mechanism responsible for the synthesis of PAF and its target cells have been fully identified. In animal models of above neural and visceral diseases, administration of PAF antagonists result in beneficial effects, but clinical trials of PAF antagonists in humans have failed. Thus, more studies are required on the synthesis of better PAF antagonists and randomize clinical trials in humans.

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Lynch, J.M.. and Hensen, P.M. (1986). The intracellular retention of newly synthesized platelet-activating factor. J Immunol. 137: 2653–2661. Maclennan K. M., Smith P. F., and Darlington C. L. (1996). Platelet-activating factor in the CNS. Prog. Neurobiol. 50: 585–596. Manya H., Aoki J., Watanabe M., Adachi T., Asou H., Inoue Y., Arai H., and Inoue K. (1998). Switching of platelet-activating factor acetylhydrolase catalytic subunits in developing rat brain. J. Biol. Chem. 273:18567–18572. Manya H., Aoki J., kato H., Ishii J., Hino S., Arai H., and Inoue K. (1999). Biochemical characterization of various catalytic complexes of the brain platelet-activating factor acetylhydrolase. J.Biol. Chem. 274: 31827–31832. Marcheselli V. L., Rossowska M. J., Domingo M. T., Braquet P., and Bazan N. G. (1990). Distinct platelet-activating factor binding sites in synaptic endings and in intracellular membranes of rat cerebral cortex. J. Biol. Chem. 265:9140–9145. Margues S.A., Dy L.C., Southall M.D., Yi O., Smietana E., Kapur R., Margues M., Travers J.B. and Spandau D.F. (2002). The platelet-activating factor receptor activates the extracellular signal-regulated kinase mitogen-activated protein kinase and induces proliferation of epidermal cells through an epidermal growth factor-receptor-dependent pathway. J. Pharmacol. Exp. Ther. 300: 1026–1035. Mathiak G., Szewczyk D., Abdullah F., Ovadia P., and Rabinovici R. (1997). Platelet-activating factor (PAF) in experimental and clinical sepsis. Shock. 7:391–404. Mazer B.D., Sawami H., Franklin R., and Gelfand E.W. (1991). B-cell activation and regulation of immunoglobulin synthesis by platelet-activating factor. Neth. J. Med. 39: 244–253. McHowat J., Kell P. J., O’Neill H. B., and Creer M. H. (2001). Endothelial cell PAF synthesis following thrombin stimulation utilizes Ca2+-independent phospholipase A2. Biochemistry 40: 14921–14931. McLamon J.G., Choi H.B., Lue L.F., Walker D.G., and Kim S.U. (2005). Perturbations in calciummediated signal transduction in microglia from Alzheimer’s disease patients. J. Neurosci. Res. 81:426–435. McMullen T.W., Li J., Sheffield P.J., Aoki J., Martin T.W., Arai H., Inoue K., and Derewenda Z.S. (2000). The functional implications of the dimerization of the catalytic subunits of the mammalian brain platelet-activating factor acetylhydrolase (Ib). Protein Eng. 13: 865–871. Min J. H., Wilder C., Aoki J., Arai K., Inoue K., Paul L., and Gelb M. H. (2001). Platelet-activating factor acetylhydrolases: Broad substrate specificity and lipoprotein binding does not modulate the catalytic properties of the plasma enzyme. Biochemistry 40:4539–4549. Mori M., Aihara M., Kume K., Hamanoue M., Kohsaka S., and Shimizu T. (1996). Localization of platelet-activating factor receptor in the rat brain. Adv. Exp. Med. Biol. 407:357–363: 357–363. Moriguchi S., Shioda N., Yamamoto Y., and Fukunaga K. (2010). Platelet-activating factor-induced synaptic facilitation is associated with increased calcium/calmodulin-dependent protein kinase II, protein kinase C and extracellular signal-regulated kinase activities in the rat hippocampal CA1 region. Neuroscience 166:1158–1168. Nixon A. B., O’Flaherty J. T., Salyer J. K., and Wykle R. L. (1999). Acetyl-CoA:1-O-alkyl-2-lysosn-glycero-3-phosphocholine acetyltransferase is directly activated by p38 kinase. J. Biol. Chem. 274: 5469–5473. Noris M., Benigni A., Boccardo P., Gotti E., Benfenati E., Aiello S., Todeschini M., and Remuzzi G. (1993). Urinary excretion of platelet activating factor in patient with immune-mediated glomerulonephritis. Kidney Int. 43:426–429. O’Flaherty J. T., Tessner, T., Greene D., Redman J. R., and Wykle R. L. (1994). Comparison of 1-O-alkyl-, 1-O-alk-1’-enyl-, and 1-O-acyl-2-acetyl-sn- glycero-3-phosphoethanolamines and − 3-phosphocholines as agonists of the platelet-activating factor family. Biochim. Biophys. Acta. 1210: 209–216. Ottino P., He J., Axelrod T.W., Bazan H.E. (2005). PAF-induced furin and MT1-MMP expression is independent of MMP-2 activation in corneal myofibroblasts. Invest Ophthalmol Vis Sci. 46: 487–496.

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Panetta T., Marcheselli V.L., Braquet P., Spinnewyn B., and Bazan N.G. (1987). Effects of a platelet activating factor antagonist (BN 52021) on free fatty acids, diacylglycerols, polyphosphoinositides and blood flow in the gerbil brain: inhibition of ischemia-reperfusion induced cerebral injury. Biochem. Biophys. Res. Commun. 149:580–587. Prescott S. M., Zimmerman G. A., and McIntyre T. M. (1990). Platelet-activating factor. J. Biol. Chem. 265: 17381–17384. Prescott S. M., Zimmerman G. A., Stafforini D. M., and McIntyre T. M. (2000). Platelet-activating factor and related lipid mediators. Annu. Rev. Biochem. 69: 419–445. Ryan S.D., Harris C.S., Carswell C.L., Baenziger J.E., and Bennett S.A. (2008). Heterogeneity in the sn-1 carbon chain of platelet-activating factor glycerophospholipids determines pro- or anti-apoptotic signaling in primary neurons. J. Lipid Res. 49:2250–2258. Rehder V., Jensen J.R., and Kater S.B. (1992). The initial stages of neural regeneration are dependent upon intracellular calcium levels. Neuroscience. 51: 565–574. Rubin B. B., Downey G. P., Koh A., Degousee N., Ghomashchi F., Nallan L., Stefanski E., Harkin D. W., Sun C. X., Smart B. P., Lindsay T. F., Cherepanov V., Vachon E., Kelvin D., Sadilek M., Brown G. E., Yaffe M. B., Plumb J., Grinstein S., Glogauer M., and Gelb M. H. (2005). Cytosolic phospholipase A2-a is necessary for platelet-activating factor biosynthesis, efficient neutrophilmediated bacterial killing, and the innate immune response to pulmonary infection - cPLA2-a. Sarchielli P., Alberti A., Coppola F., Baldi A., Gallai B., Floridi A., Floridi A., Capocchi G., and Gallai V. (2004). Platelet-activating factor (PAF) in internal jugular venous blood of migraine without aura patients assessed during migraine attacks. Cephalalgia. 24: 623–630. Shi C., Zhao L., Zhu B., Li Q., Yew D.T., Yao Z., and Xu J. (2009). Dosage effects of EGb761 on hydrogen peroxide-induced cell death in SH-SY5Y cells. Chem. Biol. Interact 180:389–397. Shindou H., Hishikawa D., Nakanishiu H., Harayama T., Ishii S., Taguchi R., and Shimizu T. (2007). A single enzyme catalyzes both platelet-activating factor production and membrane biogenesis of inflammatory cells - Cloning and characterization of acetyl-CoA:lyso-PAF acetyltransferase. J. Biol. Chem. 282: 6532–6539. Shmueli O., Cahana A., and Reiner O. (1999). Platelet-activating factor (PAF) acetylhydrolase activity, LIS1 expression, and seizures. J Neurosci Res. 57:176–184. Smiley P. L., Stremler K. E., Prescott S. M., Zimmerman G. A., and McIntyre T. M. (1991). Oxidatively fragmented phosphatidylcholines activate human neutrophils through the receptor for platelet-activating factor. J. Biol. Chem. 266: 11104–11110. Smith C.S. and Shearer W.T. (1994). Activation of NF-kB and immunoglobulin expression in a human B cell line. Cell Immunol. 155:292–303. Snyder F. (1995). Platelet-activating factor: the biosynthetic and catabolic enzymes. Biochem. J. 305: 689–705. Snyder F. (1997). CDP-choline:alkylacetylglycerol cholinephosphotransferase catalyzes the final step in the de novo synthesis of platelet-activating factor. Biochim. Biophys. Acta 1348: 111–116. Snyder F., Fitzgerald V., and Blank M. L. (1996). Biosynthesis of platelet-activating factor and enzyme inhibitors. Adv. Exp. Med. Biol. 416: 5–10. Sogos V., Bussolino F., Pilia E., Torelli S., and Gremo F. (1990). Acetylcholine-induced production of platelet-activating factor by human fetal brain cells in culture. J. Neurosci. Res. 27: 706–711. Stafforini D. M., McIntyre T. M., Carter M. E., and Prescott S. M. (1987). Human plasma plateletactivating factor acetylhydrolase Association with lipoprotein particles and role in the degradation of platelet-activating factor. J. Biol. Chem. 262: 4215–4222. Stafforini D. M., Prescott S. M., Zimmerman G. A., and McIntyre T. M. (1996). Mammalian platelet-activating factor acetylhydrolases. Biochim. Biophys. Acta 1301: 161–173. Stafforini D.M., McIntyre T.M., Zimmerman G.A., and Prescott S.M. (1997). Platelet-activating factor acetylhydrolases. J. Biol. Chem. 272: 17895–17898. Stafforini D.M., Sheller J.R., Blackwell T.S., Sapirstein A., Yull F.E., McIntyre T.M., Bonventre J.V., Prescott S.M., and Roberts L.J. 2nd. (2006). Release of free F2-isoprostanes from esterified phospholipids is catalyzed by intracellular and plasma platelet-activating factor acetylhydrolases. J Biol Chem. 281:4616–4623.

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Stafforini D.M. (2009). Biology of platelet-activating factor acetylhydrolase (PAF-AH, lipoprotein associated phospholipase A2. Cardiovasc. Drugs Ther. 23:73–83. Squinto S.P., Block A.L., Braquet P., and Bazan N.G. (1989). Platelet-activating factor stimulates a fos/jun/AP-1 transcriptional signaling system in human neuroblastoma cells. J Neurosci Res. 24:558–566. Tetta C., Bussolino F., Modena V., Montrucchio G., Segoloni G., Pescarmona G., and Camussi G. (1990). Release of platelet-activating factor in systemic lupus erythematosus. Int Arch Allergy Appl Immunol. 91:244–256. Tjoelker L. W. and Stafforini D. M. (2000). Platelet-activating factor acetylhydrolases in health and disease. Biochim. Biophys. Acta 1488:102–123. Tokutomi T., Maruiwa H., Hirohata M., Miyagi T., and Shigemori M. (2001). Production of platelet-activating factor by neuronal cells in the rat brain with cold injury. Neurol Res. 23:605–611. Tosaki T., Sakamoto H., Kitahara J., Imai H., and Nakagawa Y. (2007). Enhancement of acetylCoA: 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase activity by hydrogen peroxide. Biol. Pharm. Bull. 30: 272–278. Townsend G.C., and Scheld W.M. (1994). Platelet-activating factor augments meningeal inflammation elicited by Haemophilus influenzae lipooligosaccharide in an animal model of meningitis. Infect Immun. 62:3739–3744. Tselepis A.D., Elisaf M., Besis S., Karabina S.A., Chapman M.J., and Siamopoulou A. (1999). Association of the inflammatory state in active juvenile rheumatoid arthritis with hypo-highdensity lipoproteinemia and reduced lipoprotein-associated platelet-activating factor acetylhydrolase activity. Arthritis. Rheum. 42: 373–383. Tselepis A. D. and Chapman M. J. (2002). Inflammation, bioactive lipids and atherosclerosis: potential roles of a lipoprotein-associated phospholipase A2, platelet activating factor-acetylhydrolase. Atherosclerosis Suppl. 3: 57–68. Tsoupras A.B., Chini M., Tsogas N., Fragopoulou E., Nomikos T., Lioni A., Mangafas N., Demopoulos C.A., Antonopoulou S., and Lazanas M.C. (2008). Anti-platelet-activating factor effects of highly active antiretroviral therapy (HAART): a new insight in the drug therapy of HIV infection? AIDS Res Hum Retroviruses. 24:1079–1086. Tsukioka K., Matsuzaki M., Nakamata M., Kayahara H., and Nakagawa T. (1996). Increased plasma level of platelet-activating factor (PAF) and decreased serum PAF acetylhydrolase (PAFAH) activity in adults with bronchial asthma. J. Investig. Allergol. Clin. Immunol. 6:22–29. Umemura K., Kato I., Hirashima Y., Ishii Y., Inoue T., Aoki J., Kono N., Oya T., Hayashi N., Hamada H., Endo S., Oda M., Arai H., Kinouchi H., and Hiraga K. (2007). Neuroprotective role of transgenic PAF-acetylhydrolase II in mouse models of focal cerebral ischemia. Stroke 38:1063–1068. Vallari D. S., Record M., and Snyder F. (1990). Conversion of alkylacetylglycerol to plateletactivating factor in HL-60 cells and subcellular localization of the mediator. Arch. Biochem. Biophys. 276: 538–545. Vallee R.B., Faulkner N.E., and Tai C.Y. (2000). The role of cytoplasmic dynein in the human brain developmental disease lissencephaly. Biochim. Biophys. Acta 1496:89–98. Walsh C.A. (1998). LISsen up! Nature Genet. 19:307–308.

Chapter 5

Cannabinoids in the Brain: Their Metabolism, Roles, and Involvement in Neurological Disorders

5.1 Introduction The cannabinoid system-mediated signaling involves specific G protein-coupled receptors (CB1, CB2, and CB3), exogenous (marijuana-derived cannabinoids), endogenous cannabinoids ligands (2-arachidonoylglycerol, anandamide, noladin, virodhamine, and N-arachidonyl-dopamine), and a machinery associated with the synthesis and degradation of endocannabinoid. The endocannabinoid system is associated with the critical phase for cerebral development, regulating the release and action of different neurotransmitters and influencing fundamental developmental processes such as neuronal cell proliferation, migration, differentiation, morphogenesis, and synaptogenesis (Harkany et  al., 2008). Among cannabinoid receptors, CB1 receptors are mainly expressed in the central and the peripheral nervous systems (nerve terminals, spinal dorsal horn, and periaqueductal gray), where they not only participate in the inhibition of neurotransmitter release, but also in induction and maintenance of synaptic efficacy (Kano et al., 2009). Among various brain regions, CB1 receptors are found at highest concentrations in the hippocampus, neocortex, basal ganglia, cerebellum, and anterior olfactory nucleus (Glass et al., 1997). Moderate receptor levels are also present in the basolateral amygdala, hypothalamus, and the periaqueductal gray matter of the midbrain (Glass et  al., 1997). CB2 receptors are abundantly detected in immune and glial cells and brainstem area of the brain where they are associated with the modulation of cytokine release (Pertwee and Ross, 2002; Ahluwalia et al., 2002; Pertwee, 2005; Van Sickle et al., 2005). Both CB1 and CB2 genes encode a seven-transmembrane-domain protein belonging to the Gai protein-coupled receptor family (Munro et al., 1993). CB1 and CB2 receptors have been cloned from several sources. Although based on electrophysiological evidence, the occurrence of a third receptor (“CB3” or Anandamide receptor) in brain and in endothelial tissues has been reported (Breivogel et al., 2001), but its cloning, expression, and characterization have not yet been accomplished. There is 44% similarity in amino acid sequence of CB1 and CB2 receptor proteins. CB1 and A.A. Farooqui, Lipid Mediators and Their Metabolism in the Brain, DOI 10.1007/978-1-4419-9940-5_5, © Springer Science+Business Media, LLC 2011

133

134

5  Cannabinoids in the Brain: Their Metabolism, Roles… O

O

O

CN

Me

NH

OMe O

OMe

N

I

O

N

OMe N

I

N Cl

LY 320135 (CB1)

O

O Me

NH

N

Cl

N

N

Cl

N

NH

Cl

N

Me

N 1

AM 630 (CB1)

O

N

3 5

O

AM281 (CB1)

Cl

Me

I

NH

N

N Cl

Me Me SR 144528 (CB2)

Cl

Cl SR 141716A (CB1)

AM 251 (CB1)

Fig. 5.1  Chemical structures of some cannabinoid receptor agonists

CB2 differ considerably in their amino acid sequence and tertiary structures. CB1 receptor has four clusters of potential cAMP-dependent protein kinase and Ca2+calmodulin-dependent protein kinase sites. These clusters are conserved across the human, rat, and mouse receptor proteins. There is a single potential protein kinase C site that is also conserved in these CB1 receptors. Although human CB2 receptor shares 81% amino acid identity with rat and mouse, unlike CB1 receptor there is no PKC site in CB2 receptors. Cannabinoid receptor agonists interact with CB1 and CB2 receptors and modulate a variety of neurochemical responses, such as analgesia, hypothermia, spontaneous activity, hypotension, modulation of Ca2+, inhibition of adenylate cyclase, and modulation of long-term potentiation and nociception (Beltramo, 2009; Varvel et al., 2009; Wolf et al., 2010). Considerable progress has been made on biology, pharmacology, and genetics of cannabinoid receptors. Thus, the genomic location of the human cannabinoid receptor gene (CB1) has been mapped to chromosome 6q14-q15 (Hoehe et al., 1991). The mouse CB1 and CB2 receptor genes are located in the proximal arm of chromosome 4 (Onaivi et  al., 2002). CB1 and CB2 receptor genes in rat have been mapped to chromosome 5. The bovine CB1 receptor gene has been mapped to chromosome 9q22 (Pfister-Genskow et al., 1997; Basavarajappa, 2007). These investigations on pharmacology of cannabinoid receptors have led to the development of CB1- and CB2-selective agonists and antagonists. Cannabinoid receptor synthetic agonists and antagonists include CP-55,940, WIN-55,212-2, SR141716A, AM251, AM281 and LY320135 (CB1selective), and SR144528 and AM630 (CB2-selective) (Figs.  5.1 and 5.2). The development of selective antagonists and the creation of genetically engineered

5.1  Introduction

135 O OH

N

OH

Me

O

N Me

Me

HO

O

CP 55940

WIN 55,212-2 11 Me 9

Me

O

N H

OH

OH

1

Me Me

3

O

1'

R-(+)-Methanandamide O

Palmitoyl-EA (CB2 only)

N H

OH

Fig. 5.2  Structures of some cannabinoid receptor antagonists

mice lacking CB1 (Ledent et al., 1999) or CB2 (Buckley et al., 2000) have enabled investigators to determine relative contribution of each cannabinoid receptor to neurochemical and pharmacological effects of cannabinoids. Thus, numerous compounds have been synthesized with the goal of separating psychotropic activity from analgesic/anti-inflammatory action. Among the most promising is 1¢,1¢-dimethylheptyl-THC-11-oic acid (ajulemic acid, AJA) (Fig. 5.1) (Burstein and Zurier, 2009). Oral administration of low-dose AJA suppresses joint inflammation and tissue injury in rats with adjuvant arthritis. Treatment of human cells with AJA not only reduces generation of interleukin (IL)-1b, but suppresses matrix metalloproteinases, and increases apoptosis of human T lymphocytes. In addition, AJA also reduces production of IL-6 by human monocyte-derived macrophages (Burstein and Zurier, 2009). Although in the adjuvant arthritis model, the occurrence of synovial inflammation has been reported in AJA-treated rats, it does not progress, and articular cartilage degradation and bone erosion do not occur as it happens in rats treated with placebo (Stebulis et al., 2008). Collective evidence suggests that resolution of inflammation takes place earlier in AJA-treated animals compared to placebo group. At the molecular level, addition of AJA to human synovial cells in  vitro not only increases the generation of PGJ2, an eicosanoid that facilitates resolution of inflammation, but also enhances the production of lipoxin A4, an eicosanoid which facilitates the resolution of inflammation (Stebulis et al., 2008; Burstein and Zurier, 2009).

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In the immune system, endocannabinoids are associated with several functions. They modulate T- and B-lymphocytes proliferation and apoptosis, macrophageinduced death of sensitized cells, inflammatory cytokine generation, immune cell activation by inflammatory stimuli, chemotaxis, and inflammatory cell migration (Klein, 2005). CB2 receptors mediate immunosuppressive effect of endocannabinoids in immune cells through the inhibition of the cAMP/protein kinaseA (PKA) pathway (by decreasing the expression of cAMP-responsive genes). Endocannabinoids also paticipate at the nuclear level, where they modulate phosphorylation of IkB that enhances the transcription of several apoptotic genes modulated not only by NF-kB, but also by peroxisome proliferator-activated receptor gamma (PPAR-g)-dependent inhibition of nuclear factor of activated T cells (NFAT). Endocannabinoids interfere with cell cycle through the activation of p21waf-1/ cip-1 and induction of G1/S phase arrest (Malfitano et al., 2006).

5.2 Cannabinoid Receptor-Mediated Signaling in the Brain CB1 receptor-mediated signal transduction occurs through interaction with Gi/o proteins to inhibit adenylyl cyclase, inhibit voltage-gated Ca2+ channels, activate inward rectifying K+ channels, activate mitogen-activated protein kinases (MAPK), focal adhesion kinase, phosphatidylinositol 3-kinase, and c-Jun N-terminal kinase, which regulate nuclear transcription factors (Howlett et al., 2010) (Fig. 5.3). CB1 activation evokes a transient Ca2+ elevation in a phospholipase C (PLC)-dependent manner through either Gi/o or Gq proteins (Sugiura et  al., 1997). CB1 receptors, like other GPCRs, interact with several other proteins including GPCR-associated sorting proteins, factor associated with neutral sphingomyelinase, other GPCRs (heterodimerization), and the novel cannabinoid receptor-interacting proteins (CRIP(1a/b)). These proteins play important roles in the regulation of intracellular trafficking, desensitization, downregulation, signal transduction, and constitutive activity of CB1 receptor (Smith et al., 2010). Activated and phosphorylated CB1 receptor also bind with b-arrestin molecules, which promotes the formation of signaling complexes and participates in the regulation of G-protein-coupled receptors (GPCR) signaling. As stated above, brain CB1 receptors are involved in regulating synaptic functions, memory, and motor learning. In addition, activation of CB1 receptor inhibits type 5-HT3 ion channels (Barann et al., 2002), modulates the production of nitric oxide (Howlett and Mukhopadhyay, 2000), alters sodium channel conductance (Nicholson et al., 2003) and activates the Na+/H+ exchanger (Bouaboula et al., 1999). The activation of CB2 receptor is associated with upregulation of the expression of key genes involved in lipid synthesis (e.g., PPAR transcription factors and some of their target genes) (Dobrosi et al., 2008). These processes involve CB2 receptorcoupled signaling, which involves the MAPK pathway. This signaling is not only blocked by specific agonists/antagonists of CB2 receptor, but also by RNA interference. Because cells with “silenced” CB2 have significantly suppressed basal lipid synthesis, it is suggested that human sebocytes (highly specialized and

5.3  Occurrence and Synthesis of Endocannabinoids in the Brain CB2 channel

+ +

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CREB

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ELK-1

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?

Krox-24

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NUCLEUS

Gene transcription

Fig. 5.3  CB1 receptor-mediated signaling. Adenylyl cyclase (AC); cAMP-dependent protein kinase (PKA); protein kinase C (PKC); phospholipase C (PLC); diacylglycerol (DAG); -130-Cas (CA); arachidonyl-phosphatidylcholine (ARA-PtdCho); cytosolic phospholipase A2 (cPLA2); arachidonic acid (ARA); reactive oxygen species (ROS); nuclear factor-kappa B (NF-kB); cAMP response element-binding (CREB); member of ETS oncogene family (ELK1); proto-oncogene (c-fos and c-Jun); Mitogen-activated protein kinase pathways mediated by ERK, JNK, and p38 protein kinases

sebum-producing epithelial cells) may utilize a paracrine–autocrine, endogenously active, CB2-mediated endocannabinoid signaling system for positively regulating lipid production and apoptosis (Dobrosi et al., 2008). CB2 receptors are found in microglial cells, not in astrocytes (Nunez et  al., 2004). They are upregulated in response to inflammation and chronic pain (Maresz et al., 2005). CB2 receptor activation represents a very promising therapeutic target in neural and visceral inflammatory states, where there is immune activation and motility dysfunction (Maresz et al., 2005).

5.3 Occurrence and Synthesis of Endocannabinoids in the Brain The natural ligands of CB1 and CB2 receptors are endogenous, lipid-like substances called endocannabinoids, which include arachidonoyl ethanolamide or anandamide and 2-arachidonoylglycerol (2-AG) (Sugiura, 2009). They mimic

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5  Cannabinoids in the Brain: Their Metabolism, Roles…

H2C

O C

O

OH

H2C

OH

H2C

OH

OH

N H

Anandamide

2-Arachidonyl glycerol H2 C O

O

CH

O

CH H2C

NH2

O

OH

Virodhamine

Noladin ether

OH OH

O OH N H

Docosatetraenoyl ethanolamide

O N H

N-Arachidonyldopamine

Fig. 5.4  Chemical structures of endocannabinoids

several pharmacological effects of d-9-tetrahydrocannabinol, the active principle of Cannabis sativa preparations like hashish and marijuana. 2-AG acts as a full agonist, whereas anandamide is a partial agonist for CB1 and CB2 receptors. In addition, other ARA containing endogenous molecules, such as virodhamine (O-arachidonyl-ethanolamine), noladin, and arachidonyldopamine have been reported to occur in brain (Fig.  5.4). The presence of N-palmitoylethanolamine, N-oleoylethanolamine, and N-stearoylethanolamine has also been reported in human, rat, and mouse brain where they retard the degradation of 2-AG and anandamide (Mechoulam et al., 1998, 2002). In the brain, endocannabinoids act as neuromodulators or retrograde messengers, which inhibit the release of various neurotransmitters. In the peripheral and neural tissues, endocannabinoids are associated with the modulation of proteins and nuclear factors involved in cell proliferation, differentiation, and apoptosis, as paracrine or autocrine mediators supporting the view that endocannabinoids play a role in the control of cell fate (Guzman et al., 2001). 2-AG and anandamide are short-lived mediators, which are synthesized through two distinct pathways (Fig. 5.5). Termination of 2-AG and anandamide-mediated signaling involves not only the transport of endocannabinoids from the extracellular to the intracellular space, but also intracellular degradation by either through their hydrolysis by MAG-lipase and fatty acid amide hydrolase, or oxidation catalyzed by cyclooxygenases/lipoxygenases (Di Marzo, 2006a; Farooqui, 2009a). Degradation of 2-AG and anandamide not only augments cellular uptake and termination of extracellular signaling, but also modulates the intracellular signaling events of these two endocannabinoids. Collective evidence suggests that endocannabinoids are neuromessengers, which are synthesized on demand and released from postsynaptic

5.3  Occurrence and Synthesis of Endocannabinoids in the Brain

139

-

1, 2-Arachidonyl-PtdCho

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PtdEtn

+

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2+

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1

3

PKA

NMDA-R

channel

+

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+

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2

cAMP

1-Lyso-2arachidonylPtdCho

1-Acyl-2-arachidonylglycerol

4 8 2-arachidonylPtdH

9

2 2-arachidonylglycerol

6

ARA + Glycerol

N-ArachidonylPtdEtn

5 Anandamide

7

ARA + Ethanolamine

Fig. 5.5  Pathway showing the generation of anandamide and 2-arachidonylglycerol in brain. Phosphatidylcholine (PtdCho); phosphatidylethanolamine (PtdEtn); protein kinase A (PKA); Cannabinoid receptor1 (CB1) N-methyl-D-aspartate receptor (NMDA-R); N-acyltransferase (1); phospholipase C (2); Adenylyl cyclase (3); diacylglycerol lipase (4); N-acylphosphatidylethanolamine-specific phospholipase D (5); monoacylglycerol lipase (6); and amidase (7). Activation of receptors coupled to the phosphatidylinositol-specific phospholipase C and diacylglycerol lipase pathway leads to increases in 2-AG production

neurons to restrain the release of classical neurotransmitters from presynaptic terminals. This retrograde signaling occurs at dopaminergic, GABAergic, and glutamatergic synapses and is associated with short and long-term synaptic plasticity involved in a variety of brain functions, including anxiety, fear, and mood (Gaetani et al., 2009; Farooqui, 2009a).

5.3.1 Metabolism of 2-Arachidonylglycerol in the Brain Transfer of ARA from sn-1 position of 1,2-arachidonyl-PtdCho to the N-position of PtdEtn generates 1-lyso-2-arachidonyl-PtdCho and N-arachidonyl-PtdEtn. This reaction is catalyzed by a Ca2+-dependent, membrane-associated N-acyltransferase (Fig.  5.5). 1-Lyso-arachidonyl-PtdCho is converted to 2-AG by PLC. Another pathway for the generation of 2-AG involves the hydrolysis of 1,2-arachidonyl-­ PtdCho by PLC, followed by the action of DAG-lipase on 1-acyl-2-arachidonylglycerol. 2-AG interacts with CB1 and CB2 receptors and act as neurotransmitter or neuromodulator in brain, immune and cardiovascular systems. Its levels in the brain are 170-fold higher than anandamide (Stella et al., 1997). As stated above, stimulation of both receptors inhibit cAMP formation via Gi/o proteins, and activate mitogen-activated-protein kinase (Childers and Breivogel, 1998). Unlike traditional neurotransmitters, ARA-derived endocannabinoids are not stored in vesicles.

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5  Cannabinoids in the Brain: Their Metabolism, Roles… PGE2-GE 15-HETE-GE 15 -L X O

COX-2

Induction of neurite outgrowth Activation of K+ channel

EPOX

-L 12

12-HETE-GE

MAG-lipase

2-Arachidonyl glycerol

O X

2-(14,15-EET)-GE

Full agonist for CB1 receptor Onset of dementia Notch mRNA2 Presenilin 2

Glycerol + Arachidonate

Fig. 5.6  Degradation and role of 2-arachidonylglycerol in brain. Cyclooxygenase-2 (COX-2); prostaglandin E2-glyceryl ester (PGE2-GE); 15-lipoxygenase (15-LOX); 15- hydroperoxyeicosatetraenoic acid glyceryl ester (15-HETE-GE); 12-lipoxygenase (12-LOX); 12- hydroperoxyeicosatetraenoic acid glyceryl ester (12-HETE-GE); epoxygenase (EPOX); glyceryl ester (2-(14,15-EET)-GE; and monoacylglycerol lipase (MAG-lipase)

They are generated from ARA-containing glycerophospholipids within the neural membranes. 2-AG is hydrolyzed by monoacylglycerol lipase (MAG-lipase) into arachidonic acid and glycerol. This enzyme is heterogeneously distributed in the rat brain with the highest levels observed in regions expressing CB1 receptors, such as the cortex, thalamus, hippocampus, and cerebellum (Dinh et  al., 2002, 2004). Inhibition of monoacylglycerol lipase by 4-nitrophenyl 4-(dibenzo[d][1,3]dioxol-5yl(hydroxy)methyl)piperidine-1-carboxylate (JZL184) reduces neuropathic pain through distinct receptor-mediated mechanisms and may present good targets for the development of analgesic drugs (Kinsey et al., 2009). Considering the involvement of endocannabinoids and cannabinoid receptors in numerous physiological processes monoacylglycerol lipase is now considered as a promising target for not only the treatment of pain and inflammatory disorders, but also cancer research. 2-AG is also metabolized by cyclooxygenase-2 (COX-2), which oxygenates 2-AG to generate prostaglandin glyceryl esters (PGE2-GE or PGE2-G) (Fig.  5.6) (Sang and Chen, 2006; Sang et al., 2007). PGE2-G mediates the mobilization of Ca2+ in RAW264.7 cells at pm to nm levels. The mobilization of Ca2+ is concomitant with a transient elevation of PtsInsP3 levels. Ca2+ mobilization can be blocked by the PtdInsP3 receptor antagonist, TMB-8 (Nirodi et al., 2004). Although depletion of extracellular Ca2+ reduces, it does not eliminate the response and is consistent with an initial release of Ca2+ from intracellular stores followed by capacitative entry. Depletion of Ca2+ from endoplasmic reticulum by pretreatment of cells with thapsigargin retards the PGE2-G response. Intraplantar administration of PGE2-G stimulates behavioral sensitivity to thermal and mechanical stimulation in a manner likely to be independent of cannabinoid and prostanoid receptors. These effects are only partially reversed by coadministration of PGE2-G with a cocktail of prostaglandin receptor antagonists. PGE2-G stimulates hippocampal glutamatergic transmission and induces neuronal death through caspase-3 activation. Stimulation of CB1 receptor elevates levels of PGE2-G and this elevation is not inhibited by SR141716, a CB1

5.3  Occurrence and Synthesis of Endocannabinoids in the Brain

141 GPR55

PGE2-EA 15-HETE-EA

5,6 8,9 11,12 14,15EETs-EA

15-LOX EPOX

COX-2

Notch mRNA1 PPARs

Anandamide

CB1

FAAH

CB2

12-LOX 12-HETE-EA

Ethanolamine + Arachidonate

TRPV1 Presenilin 1

Fig. 5.7  Degradation of role of anadamide in brain. Cyclooxygenase-2 (COX-2); prostaglandin E2-ethanolamide (PGE2-EA); 15-lipoxygenase (15-LOX); 15- hydroperoxyeicosatetraenoic acid ethanolamide (15-HETE-EA); 12-lipoxygenase (12-LOX); 12- hydroperoxyeicosatetraenoic acid ethanolamide (12-HETE-EA); epoxygenase (EPOX); ethanolamide (2-(14,15-EET)-EA; and monoacylglycerol lipase (MAG-lipase)

receptor antagonist. This observation suggests that multiple mechanisms may contribute to the increase in PGE2-G levels. NMDA receptor antagonists have been shown to inhibit PGE2-G-mediated neurotoxicity (Sang and Chen, 2006; Sang et al., 2007). 2-AG-derived prostaglandins do not bind or interact with known prostaglandin receptors. They probably exert their effects through a unique and previously uncharacterized receptor (Guindon and Hohmann, 2008, 2009). In a macrophage cell line, PGE2-G mediate their effect through their interactions with NF-kB in a concentration-dependent manner; high doses of PGE2-G increase and low doses of PGE2-G reduce the activation of NF-kB (Hu et al., 2008). It is proposed that endogenous COX-2-derived 2-AG metabolites enhance nociception through a mechanism that may involve NF-kB-mediated signal transduction. Their effect is opposite to that of ARA-derived prostaglandin on inhibitory synaptic transmission, and alterations in COX-2 activity may have significant impact on endocannabinoid signaling in hippocampal synaptic activity (Sang and Chen, 2006). Metabolism of 2-AG by 12-lipoxygenase (12-LOX) and 15-lipoxygenase (15LOX) generates 12-hydroperoxyeicosatetraenoic acid glyceryl ester (12-HETE-GE) and 15- hydroperoxyeicosatetraenoic acid glyceryl ester (15-HETE-GE), respectively (Moody et al., 2001; Kozak et al., 2002) (Fig. 5.6). These metabolites mediate their biological activities via established receptors, including the cannabinoid receptors, peroxisome proliferator-activated receptor (PPAR-a), and transient receptor potential vanilloid receptor (TRPV1) (Kozak et  al., 2002; Di Marzo, 2006a). In vitro, cytochrome P450 enzymes have also been implicated in metabolizing 2-AG (Snider et al., 2007). Oxidative metabolism of both of 2-AG by microsomal P450 epoxygenases produces a diverse array of oxygenated products (Figs. 5.6 and 5.7), including epoxides (epoxyeicosatetraenoic acids).

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5.3.2 Metabolism of Anandamide in the Brain N-arachidonoyl-phosphatidylethanolamine (N-arachidonyl-PtdEtn), the precursor for anandamide, is synthesized by the enzymic transfer of arachidonic acid in the sn-1 position of a phosphatidylcholine to the amide group of a phosphatidylethanolamine (Fig.  5.5). This reaction is catalyzed by a calcium-independent N-acyl-transferase (NAT) (Jin et al., 2009). N-arachidonyl-PtdEtn is transformed into anandamide by N-acylphosphatidyl-ethanolamine-specific PLD, a member of the metallob-lactamase family, which specifically hydrolyzes N-acylphosphatidylethanolamine among glycerophospholipids, and appears to be constitutively active (Di Marzo et al., 1996; Ueda et  al., 2005). Recombinant N-acylphosphatidylethanolamine PLD hydrolyzed various N-acylphosphatidylethanolamines, including the anandamide precursor N-arachidonoylphosphatidylethanolamine at similar rates, but is inactive with phosphatidylcholine and phosphatidylethanolamine (Okamoto et  al., 2004). Involvement of N-acylphosphatidyl-ethanolamine-specific PLD in the biosynthesis of ananadamide has been put in doubt because N-acylphosphatidyl-ethanolaminespecific PLD knockout mice do not show any deficit in anandamide synthesis suggesting that multiple enzymic mechanisms may contribute to the generation of anandamide (Leung et al., 2006). Fatty acid amide hydrolase, a postsynaptic membrane-bound enzyme that regulates levels of anandamide by hydrolyzing it into arachidonic acid and ethanolamine, terminates anandamide-mediated signaling in neurons and astrocytes (Di Marzo, 2006b). Like 2-AG, anandamide is also oxidized by COX-2, 12-LOX, and 15-LOX resulting in the generation of protstaglandin E2 ethanolamide (PGE2-EA), 12-HETE ethamolamide (12-HETE-EA) and 15-HETE ehanolamide (15-HETE-EA), respectively (Fig.  5.7). These metabolites are significantly more stable metabolically than free acid PGs, suggesting that COX-2 action on endocannabinoids may provide oxygenated lipids with sufficiently long half-lives to act as systemic mediators or prodrugs (Kozak et al., 2004). Cytochrome P450 enzymes have also been implicated in metabolizing anandamide (Snider et  al., 2007). Action of Epoxygenase on anandamide generates a diverse array of oxygenated products including epoxides (epoxyeicosatetraenoic acids) (Snider et al., 2007). These metabolites have been reported to interact with PPAR-a and mediate many neurochemical activities (Di Marzo, 2006a, b). Anandamide is also an endogenous agonist for the TRPV1 (Smart et al., 2000) and the gastrin-releasing peptide 55 receptor (Pertwee, 2005; Lauckner et al., 2008). TRPV1 receptors are expressed in nociceptive sensory neurons and can detect/ respond to noxious mechanical, thermal (heat), and chemical (capsaicin) stimuli (Piomelli, 2001; Caterina et al., 1997; Ross, 2003). The administration of anandamide directly into the hippocampus, increases rapid-eye-movement (REM) sleep in a dose-dependent manner during the dark but not during the light phase of the cycle. The increase of REMs can be blocked by the CB1 antagonist, AM251. This effect is specific for the hippocampus since anandamide administration in the surrounding cortex does not elicit any change in REMs. These results support the idea of a direct relationship between hippocampal activity and sleep mechanisms by means of anandamide (Rueda-Orozco et al., 2010).

5.4  Interplay Among Cannabinoid, Glutamate, and Dopamine Receptors…

143

5.4 Interplay Among Cannabinoid, Glutamate, and Dopamine Receptors in the Basal Ganglia It is becoming increasingly evident that coordinated interplay (cross-talk) among various receptors is essential for normal synaptic function. This interplay involves various neurotransmitters, neuropeptides, growth factors, and lipid mediators including eicosanoids, docosanoids, endocannabinoids, and platelet-activating factor and their receptors (Farooqui, 2009b). Thus, the presence of CB1 receptors in several GABAergic and glutamatergic synapses within the basal ganglia as well as the presence of TRPV1 receptors in nigrostriatal dopaminergic neurons enables endocannabinoids to directly modulate the release of GABA and glutamate. In addition, the preferential presynaptic location of these receptors makes it likely that endocannabinoids mainly control presynaptic events, such as the synthesis, release, or reuptake of glutamate and dopamine (Gerdeman and Fernández-Ruiz, 2008). This suggestion is supported by studies not only on the administration of cannabinoids combined with agonists or antagonists for GABA and glutamate receptors but also by electrophysiological and neurochemical analysis studies (Köfalvi et al., 2005). Based on these studies, it is proposed that an important function of cannabinoid signaling within the basal ganglia circuitry is to modulate GABAergic and glutamatergic signaling at synapses (Fig.  5.8) (Gerdeman and Fernández-Ruiz, 2008). Furthermore, anandamide has been shown to directly block certain presynaptic events in dopaminergic synapses within the striatum by acting through TRPV1 receptors located in nigrostriatal dopaminergic neurons (de Lago et al., 2004). Furthermore, dopamine transmission is also modulated through interactions with G protein/adenylyl cyclase Phospholipids

2-Arachidonylglycerol or anandamide

CB1-mediated alterations in GABAergic neurotransmission Decrease in GABA release Stimulation of GABA uptake

CB1-mediated alterations in glutamatergic neurotransmission Decrease in glutamate release

CB1or TRPV1-mediated alterations in dopaminergic neurotransmission Decrease in dopamine synthesis and release

Synaptic communication

Inhibition of motor activity

Fig. 5.8  Modulation of GABA-, glutamate-, and dopamine-neurotransmission by endocannabinoids

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signal transduction mechanisms shared by both CB1 and D1/D2 dopamine receptors (Di Marzo et al., 2000). Interactions and cross-talk between CB1 receptors and dopamine D2 receptors modulate synaptic activity within nigrostriatal neuronal networks. In addition, CB1 receptors are localized preferentially in the nigrostriatal circuitry, where their ability to modulate GABA-mediated neurotransmission coincides with the capacity of GABA- and dopamine-containing neurons to produce endocannabinoids (Köfalvi et al., 2005). In vitro studies indicate that at subsaturating agonist concentrations endocannabinoid and dopamine signaling promote the heterodimerization of CB1 receptors and D2 receptors (Kearn et al., 2005) and not only stimulate cAMP production, but also enhances ERK1/2 activity. If present in vivo, CB1–D2-receptor heterodimers may be involved in the control of motor behavior through phosphorylation of the dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) (Andersson et al., 2005). Phospho-DARPP-32-mediated Ca2+ influx is known to facilitate Ca2+-dependent endocannabinoid production and control of neuronal activity in the basal ganglia. Collective evidence suggests that interplay among endocannabinoids, glutamate, and dopamine signaling systems may influence neuronal development, modulate motor activity, and is necessary for the maintenance of normal synaptic function. Apparent heterogeneity of various receptors in brain warrants more studies on the organizational, spatial, and temporal constraints of endocannabinoid signaling at the cellular and neuronal network levels.

5.5 Endocannabinoids and Neurological Disorders As stated above, CB1 receptors are expressed in neurons, whereas some CB2 receptors are found mostly in microglia, which during injury and neurodegeneration become activated and overexpress cannabinoid receptors. The overexpression of cannabinoid receptors and alterations in endocannabinoid levels have been reported to occur in neurotraumatic diseases (ischemia, spinal cord trauma, and traumatic brain injury) and neurodegenerative diseases, such as Alzheimer disease (AD), multiple sclerosis (MS), amyotropic lateral sclerosis (ALS), Parkinson disease (PD), and Huntington chorea (HD). In addition, cannabis has been used for centuries for its pain-relieving properties. The main active ingredient of cannabis, 9-tetrahydrocannabinol, induces antinociceptive effects by binding to CB1 and CB2 receptors. In particular CB2 receptors have emerged as an interesting target for chronic pain treatment as demonstrated by several studies on inflammatory and neuropathic preclinal pain models (Anand et al., 2009; Beltramo, 2009). The molecular mechanisms involved in CB2 receptor-mediated analgesia are still controversial, but it is becoming increasingly evident that cannabinoids modulate pain either through their effect on inflammatory cells and/or on nociceptors and spinal cord relay centers. In addition, cannabinoids also produce muscle relaxation, immunosuppression, antiallergic effects, sedation, improvement of mood, stimulation of appetite, antiemesis, lowering of intraocular pressure, bronchodilation, and neuroprotective and antineoplastic effects. The immunomodulatory effects of endocannabinoids are

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mediated through CB2 receptors on glia and microglia. It is proposed that overexpression of CB1 and CB2 receptors and alterations in endocannabinoid levels may be initial attempt of brain tissue to counteract harmful effects of excitotoxicity and inflammation and provide a balanced metabolism during the pathogenesis of AD, MS, ALS, PD, and HD (Bisogno and Di Marzo, 2010). Collective evidence suggests that the endocannabinoid system protects neurons against glutamate excitotoxicity and neuroinflammation in both in vitro and in vivo models of neurotraumatic and neurodegenerative diseases (Bisogno and Di Marzo, 2010).

5.5.1 Endocannabinoids in Neurotraumatic Diseases Ischemic injury is accompanied by the appearance of CB2-expressing cells and accumulation of N-acylethanolamines at the lesion site in the rat brain (Berger et al., 2004; Ashton et al., 2007; Degn et al., 2007). Levels of 2-AG are markedly increased at 4  h, but returned to basal level 12  h after induction of ischemia (Degn et  al., 2007). No changes are observed in anandamide levels in this model of ischemic injury. It is suggested that N-acylethanolamines and 2-AG may be involved in regulation of neuroprotection during focal cerebral ischemia. In mouse cerebral cortex, focal cerebral ischemia greatly increases levels of palmitoylethanolamide, only moderately increases anandamide, and has no effect on 2-arachidonoylglycerol levels (Franklin et al., 2003). Palmitoylethanolamide potentiates anandamide-induced microglial cell migration, without affecting other steps of microglial activation, such as proliferation, particle engulfment, and nitric oxide production. This potentiation of microglial cell migration by palmitoylethanolamide is associated with the reduction in cAMP levels. It is proposed that palmitoylethanolamide acts through CB2-linked Gi/o-coupled receptors (Franklin et al., 2003). In a mouse model of ischemic stroke, CB2 agonist (JWH-133) reduces the infarct size measured 3 days after onset of ischemia. The CB2 activation not only decreases the number of neutrophils in the ischemic brain, but also reduces the activity of myeloperoxidase and inhibits adherence of neutrophils to brain endothelial cells (Murikinati et al., 2010). In addition, JWH-133 also interferes with the migration of neutrophils, which is mediated by the endogenous chemokine CXCL2 through activation of the MAP kinase p38. This effect on neutrophils may be responsible for the neuroprotection mediated by JWH-133. Thus, by activating p38 in neutrophils, CB2 agonists inhibit neutrophil recruitment to the brain and protect against ischemic brain injury. Collective evidence suggests that exogenous and endogenous cannabinoids provide neuroprotection in a variety of in vitro and in vivo models of ischemic injury through different mechanisms, including prevention of excitotoxicity by cannabinoid CB1-mediated inhibition of glutamatergic transmission, reduction of calcium influx, antioxidant activity, activation of the phosphatidylinositol 3-kinase/protein kinase B pathway, induction of phosphorylation of extracellular-regulated kinases and expression of transcription factors and neurotrophins, lowering of cerebrovasoconstriction, and induction of hypothermia (van der Stelt and Di Marzo, 2005). The release of endocannabinoids during ischemic injury may constitute a protective response.

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Spinal cord trauma is accompanied by mechanical insult to the spinal cord followed by secondary injury, which contributes to the spread of the damage and neurological deficits through the release of glutamate, influx of calcium, secretion of cytokines, generation of high levels of lipid mediators, and apoptotic cell death (Farooqui, 2010). During early stages of spinal cord trauma, levels of anandamide and palmitoylethanolamide are increased along with upregulation of the synthesizing enzyme, NAPE-phospholipase D and a downregulation of the degradative enzyme fatty acid amide hydrolase (FAAH) in spinal cord lesion (Garcia-Ovejero et  al., 2009). In delayed stages of spinal cord trauma, levels of 2-AG are increased along with a strong upregulation of the synthesizing enzyme diacylglycerol lipase-a that is expressed by neurons, astrocytes, and infiltrating immune cells. The monoacylglycerol lipase activity is also moderately increased in the lesion but only 7 days after spinal cord trauma. Administration of a single dose of 2-AG early after spinal cord trauma reduces lesion expansion (Arevalo-Martin et al., 2010). 2-AG treatment also preserves the white matter around the epicenter of the injury site by protecting myelin and reducing oligodendrocyte loss. In addition, 2-AG also prevents the myelin damage and delays oligodendrocyte loss induced at 10 mm from the epicenter. Interestingly, the early protective action of 2-AG is maintained 28  days after injury when the lesion size is still smaller, and the preservation of white matter is better in 2-AG-treated animals. Thus, generation of endogenous cannabinoid may be a protective mechanism for spinal cord trauma (Arevalo-Martin et al., 2010). Like spinal cord trauma, traumatic brain injury (TBI) is accompanied by the release of glutamate, calcium influx, upregulation of cytokines, accumulation of harmful mediators, and apoptotic cell death leading to brain damage and dysfunction (Farooqui, 2010). Neurochemical changes in TBI also lead to significant increase in 2-AG levels (Panikashvili et  al., 2001). Administration of synthetic 2-AG to mice after TBI reduces brain edema, decreases infarct volume, and reduces hippocampal cell death compared with controls resulting in better clinical recovery in a dose-dependent manner. Administration of 2-AG with 2-acyl-glycerols significantly enhances functional recovery. The beneficial effect of 2-AG can be attenuated by SR-141761A, an antagonist of the CB1 cannabinoid receptor (Panikashvili et  al., 2001). CB1 receptor knockout mice CB1−/− following TBI show minor spontaneous recovery after 24 h compared to wild-type (WT) mice. Administration of 2-AG in the CB1−/− mice does not improve neurological performance and edema formation (Panikashvili et al., 2005). In addition, 2-AG retards 3- to 4- fold increase in nuclear factor-kB (NF-kB) transactivation, after 24  h after TBI in the WT mice, while TBI has no effect on NF-kB in the CB1−/− mice (Panikashvili et al., 2005). Studies on the effect of anandamide on primary cell culture indicate that this endocannabinoid also binds to vanilloid (VR1) receptors and induces cell death. Intracerebroventricular administration of anandamide induces sustained cerebral edema, regional cell loss, and impairment in long-term cognitive function (Cernak et  al., 2004). These effects are mediated, in part, through interactions between anadamide and vanilloid receptor (VR1) as well as through calpain-dependent mechanisms, but not through CB1 receptors or caspases. These studies also indicate that

5.5  Endocannabinoids and Neurological Disorders

147

Neuroprotective effects of endocannabinoid

Mutiple sclerosis

HIV associated diseases

Parkinson disease

Amyotrophic lateral sclerosis

Alzheimer disease

Fig. 5.9  Neuroprotective effects of endocannabinoids in neurodegenerative diseases

central administration of anandamide also significantly upregulates genes involved in proinflammatory/microglial-related responses (Cernak et  al., 2004). Accumulating evidence suggests that endo- and exocannabinoids may produce both neuroprotective and neurotoxic effects in animal models of spinal cord trauma and traumatic brain injury.

5.5.2 Endocannabinoids in Neurodegenerative Disorders Cannabinoid-based drugs produce beneficial effects in basal ganglia disorders, including PD and HD (Fernandez-Ruiz, 2009) (Fig. 5.9). Beneficial effects include the alleviation of specific motor symptoms, namely choreic movements with cannabinoid receptor type 1 (CB1)/transient receptor potential vanilloid type 1 agonists in HD and bradykinesia with CB1 antagonists and tremor with CB1 agonists in PD. Several cannabinoid agonists have been used for the treatment of HD and PD in various animal models. Degeneration of dopaminergic neurons in animal model of PD is reduced by agonists of CB1, CB2, and non-CB1 or non-CB2 receptors – an effect may be associated with modulation of interactions between glial cells and neurons (LastresBecker et al., 2005). CB1 receptors, however, also exert detrimental effects on dopamine cell survival by potentiating the toxic effects of the TRPV1 agonist, capsaicin (Kim et al., 2005). It is thus likely that endocannabinoids such as anandamide, which activates both TRPV1 and CB1 receptors (Di Marzo, 2006a) might contribute to PD pathophysiology by favoring apoptosis of dopamine neurons. Collective evidence indicates that cannabinoids delay the progression of these diseases and protect neurons via (a) a reduction in neuroinflammation through the activation of CB2 receptors, which are located in glial cells, (b) a normalization of glutamate homeostasis then limiting excitotoxicity, and (c) an antioxidant effect exerted by cannabinoid receptorindependent mechanisms (Pazos et al., 2008; Garcia-Arencibia et al., 2009).

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In AD, Ab-mediated brain damage is accompanied by endocannabinoids released from neurons and glial cells resulting in elevation in Erk activity and induction of brain-derived neurotrophic factor (BDNF) and activation of CB1-mediated neuroprotective pathways (Huestis et al., 2001; Marsicano et al., 2003). In addition, endocannabinoids modulate the release of inflammatory mediators in microglia through cannabinoid CB2 receptors. It should be noted that activated microglial cells in AD express significant levels of cannabinoid CB2 receptors supporting the view that endocannabinoids may trigger CB2-dependent proinflammatory cytokine release and neuroinflammation, a process closely associated with the pathogenesis of AD (Maresz et al., 2005; Benito et al., 2003). Induction of endocannabinoid-mediated neuroinflammation in AD is also supported by high activity of FAAH in astrocytes surrounding neurite plaques. FAAH may produce ARA through the hydrolysis of AEA and 2-AG and eicosanoids derived from ARA may enhance neuroinflammation. Surprisingly intracerebroventricular administration of the synthetic cannabinoid (WIN55,212-2) to rats prevents Ab-induced microglial activation, cognitive impairment, and loss of neuronal markers (Fig. 5.9). Other synthetic cannabinoids (HU-210 and JWH-133) prevent Ab-mediated activation of cultured microglial cells, as judged by mitochondrial activity, cell morphology, and tumor necrosis factor-a release; these effects are independent of the antioxidant action of cannabinoid compounds and are also induced by a CB2-selective agonist (Table  5.1). Moreover, cannabinoids abrogate microglia-mediated neurotoxicity after Ab addition to rat cortical cocultures (Ramirez et al., 2005). It is proposed that cannabinoid receptors may be associated with the pathology of AD and that cannabinoids may exert beneficial effects on neurodegenerative process in AD (Ramirez et al., 2005). In G93A-SOD1 mutant mice (animal model ALS), administration of nonselective cannabinoid partial agonists prior to, or upon, symptom appearance minimally delays disease onset and prolongs survival through undefined mechanisms. In

Table 5.1  Therapeutic potentials of cannabinoids for neurological diseases Endocannabinoids/agonists/ Disease antagonists Reference MS Dexanabinol (HU-211) Cabranes et al., 2005; Croxford, 2003 HD CB1/vanilloid agonists Micale et al., 2007 AD HU-210, WIN55,212-2, and Micale et al., 2007; Rivers and Ashton, JWH-133, CB2 agonists 2010; Ramirez et al., 2005 ALS CB2 agonist (AM-1241) Shoemaker et al., 2007 PD CB1 antagonists; vanilloid Fernandez-Ruiz, 2009; Giuffrida and type 1 agonists McMahon, 2010 Ischemia Dexanabinol (HU-211) Croxford, 2003 TBI Dexanabinol (HU-211) Eljaschewitsch et al., 2006; Croxford, 2003 SCI 2-Arachidonylglycerol Arevalo-Martin et al., 2010 Depression AM251, AM404, HU210 Vinod and Hungund, 2006 HIV wasting disease Dexanabinol (HU-211) Croxford, 2003 Motor dysfunction CB1 and CB2 agonists Fernandez-Ruiz, 2009

5.6  Use of FAAH and MGL Inhibitors for Inflammatory Pain

149

G93A-SOD1 mutant mice ALS progression is accompanied by dramatic upregulation of CB2 receptor mRNA in spinal cords. More importantly, daily injections of the selective CB2 agonist AM-1241 started at the onset of ALS increases the survival interval after disease onset by 56%. It is suggested that CB2 agonists may slow motor neuron degeneration and preserve motor function, and represent a novel therapeutic modality for treatment of ALS (Fig. 5.9) (Shoemaker et al., 2007). Similarly, in hSOD1G93A mice genetic ablation of fatty acid amide hydrolase not only exerts robust anti-inflammatory and neuroprotective effects, but also causes a delay in the progression of ALS (Raman et al., 2004; Bilsland et al., 2006). CB2 receptormediated augmentation of endocannabinoids levels results in a neuroprotective effect in hSOD1G93A mice (Table 5.1). In contrast, activation of the CB1 receptor produces a negative influence on motor neuron survival (Bilsland et al., 2006). In experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis, decrease in cannabinoid signaling is accompanied by alterations in levels of anandamide and 2-arachidonoylglycerol in motor-related regions (striatum, midbrain) along with motor disturbances in the basal ganglia (Cabranes et al., 2005). Cannabinoid-based drugs, AM404, arvanil, and OMDM2 reduce the magnitude of the neurological impairment in EAE rats (Fig. 5.9), whereas VDM11 does not produce any effect. The beneficial effects of AM404 are reversed by blocking TRPV1 receptors with capsazepine, but not by blocking CB1 receptors with SR141716, thus indicating the involvement of endovanilloid mechanisms in these effects (Cabranes et al., 2005). Cannabinoids can alleviate tremor and spasticity in EAE, and clinical trials of the use of these compounds for these symptoms are in progress (Croxford, 2003). Studies on the levels of endocannabinoids (ECs) in cerebrospinal fluid samples from MS patients indicate that levels of AEA are increased in the CSF from relapsing MS patients, but levels of 2-AG are not affected (Centonze et al., 2007). Peripheral lymphocytes of these patients also higher levels of AEA, an effect associated with increased synthesis and reduction in degradation of this endocannabinoid. Increased synthesis, reduced degradation, and increased levels of AEA were also detected in the brains of EAE mice in the acute phase of the disease, possibly accounting for its antiexcitotoxic action in this disorder (Centonze et al., 2007). This observation is tempting to speculate that the AEA and 2-AG are differentially engaged in CNS inflammatory processes. Although AEA and 2-AG share many pharmacological actions, they may be associated with differential regulatory mechanisms involved in pathological events (Chevaleyre et al., 2006). Collectively, these studies support the view that in MS patients and in animal models of MS, stimulation of CB1 and of CB2 receptors produces beneficial effects against the inflammatory process (Huntley, 2006; Arévalo-Martín et al., 2003; Eljaschewitsch et al., 2006).

5.6 Use of FAAH and MGL Inhibitors for Inflammatory Pain Pharmacological inhibition of FAAH and monoacylglycerol lipase has been used as therapeutic target for pain in inflammatory pain models. Inhibition of monoacylglycerol lipase by JZL184 (Fig.  5.10) reduces capsaicin-mediated nocifensive

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a

N

N O

O

O

O

b

O

P

F

d

CH3

C O

NH2 O

OCH3

O

H N

O

c

HN O

O

f OH

e

OH O NH

O

NH

Fig. 5.10  Chemical structures of inhibitors of endocannabinoids metabolism used for the treatment of pain in neurological disorders. Fatty acid amide hydrolase inhibitor (OL135) (a); diacylglycerol lipase inhibitor (03841) (b); anandamide membrane transporter (AMT inhibitor) (UCM707) (c); JZL184 (d); and AMT inhibitor (e); and JZL 195 (f)

behavior and thermal hyperalgesia. It is suggested that this may be due to increasing accumulation of 2-AG. In contrast, inhibition of fatty acid amide hydrolase by URB597 (Fig. 5.10) blocks capsaicin-mediated mechanical allodynia. This may be caused by the accumulation of anandamide (Spradley et al., 2010). Combined treatment with inihibitors of monoacylglycerol lipase and fatty acid amide hydrolase produces additive effect due to the accumulation of anandamide and 2-AG. Based on these observations, it is proposed that specific inhibitors of monoacylglycerol lipase and fatty acid amide hydrolase may serve as attractive therapeutic targets for the treatment of pain in neurotraumatic and neurodegenerative diseases (Bari et al., 2006). Recently, JZL 195, a potent inhibitor of both FAAH and MAGL (IC50 = 2 and 4 nM, respectively) has been synthesized. Although it has no effect on neuropathy target esterase, ABHD6 and other brain serine hydrolases (Long et al., 2009), this compound blocks monoacylglycerol lipase and fatty acid amide hydrolase activities in time- and dose-dependent manner in vivo. The in vivo inhibitory actions of JZL 195 against fatty acid amide hydrolase and monoacylglycerol lipase are comparable to those of the selective inhibitors JZL 184 and PF-3845, respectively. Through its inhibitory actions, JZL 195 simultaneously augments brain levels of anandamide and 2-AG, producing antinociceptive, cataleptic, and hypomotility effects like

References

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those produced by direct CB1 agonists. Collective evidence suggests that activating the endogenous cannabinoid system by targeting fatty acid amide hydrolase and monoacylglycerol lipase through fatty acid amide hydrolase and monoacylglycerol lipase inhibitors may be a promising strategy to treat pain and inflammation. This strategy lacks untoward side effects typically associated with Cannabis sativa (Schlosburg et al., 2009).

5.7 Conclusion Endocannabinoids are lipid mediators that modulate brain function by interacting with cannabinoid receptors as well as the transient receptor potential vanilloid 1 (TRPV1) receptor. Anandamide and 2-AG are well-characterized retrograde messenger at synapses in terms of their production, mode of action, and degradation. These endocannabinoids mimic several pharmacological effects of the exogenous cannabinoid d9-tetrahydrocannabinol. The activity of endocannabinoids at their receptors is limited by cellular uptake through specific membrane transporters, followed by intracellular degradation by a fatty acid amide hydrolase or by a monoacylglycerol lipase. Although mechanisms for biosynthesis and degradation of anandamide and 2-AG are fairly well established, the manner by which these endocannabinoids accumulate in cells remains controversial. Many studies indicate that endocannabinoids are taken up into neural cells via a facilitated process. Other reports indicate that the uptake of endocannabinoids is facilitated not only by the diffusion, but also through endocytosis. It is proposed that endocannabinoid system modulates a broad range of physiological processes in the CNS and periphery, including pain and inflammation, mood and anxiety disorders, neurodegenerative disorders, cancer, epilepsy, stroke, hypertension, glaucoma, and obesity (Pacher et al., 2006).

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Chapter 6

Neurochemical Aspects of 4-Hydroxynonenal

6.1 Introduction 4-Hydroxynonenal (4-HNE) is a major end-product of peroxidation of membrane arachidonic acid (ARA). This nine carbon a, b-unsaturated aldehyde contains three functional groups, which often act in concert and help to explain its high reactivity (Poli and Schaur, 2000). Most importantly, there is a conjugated system of a C=C double bond and a C=O carbonyl group which provide a partial positive charge to carbon 3 due to the presence of mobile pi-electrons. This positive charge is further enhanced by the inductive effect of the hydroxy group at carbon 4. Therefore, 4-HNE is considered to be soft electrophiles and is prone to be attacked by nucleophiles, such as thiol or amino groups. This reaction occurs primarily at carbon 3 and secondarily at the carbonyl carbon 1. Amino acids that react with 4-HNE are cysteine, histidine, and lysine, proteins and peptides leading to the formation of stable Michael adducts with a hemiacetal structure (Schaur, 2003; Poli et al., 2006) (Fig. 6.1). Among the protein residues that react with 4-HNE, Cys exhibits the highest reactivity, followed by His and Lys (Petersen and Doorn, 2004), but Cys–HNE adducts are less stable than His–HNE adducts (Uchida, 2003). These chemical reactions proceed through reactions between the C1/4C double bond with a nucleophile (Cys, glutathione (GSH) and amine) via 1,2- and 1,4-Michael addition (Nadkarni and Sayre, 1995). The 1,2-Michael addition involves the reaction of a primary amine (Lys) with the a,bunsaturated carbonyl, resulting in the formation of a Schiff base at acidic pH. This step is reversible (Petersen and Doorn, 2004) (Fig. 6.1). The biological effects of 4-HNE are mostly modulated by its lipophilic properties. Although hydrophilic properties play some role in the reactivity of 4-HNE, their effect is less pronounced than lipophilic properties. Because of this reason 4- HNE tends to interact and concentrate in biomembrane rather than in the aqueous space of cells. In neural membranes, concentration of 4-HNE reaches as high as 50 mM. Compared to free radicals, 4-HNE is relatively stable and can diffuse within or even escape from the cell and attack targets far from the site of its generation. It exhibits great reactivity A.A. Farooqui, Lipid Mediators and Their Metabolism in the Brain, DOI 10.1007/978-1-4419-9940-5_6, © Springer Science+Business Media, LLC 2011

159

160

6  Neurochemical Aspects of 4-Hydroxynonenal OH

OH O

H3C

4-HNE

His

O H3C

Michael adduct

His

Lys

OH

OH

O O H3C

H3C

NH

Cyclic hemiacetal

His

Lys Lys OH

NH-Lys N-Lys

H 3C

NH Lys

O H3C NH-Lys

Schiff base Michael adductcrosslink formation

Fig. 6.1  Chemical structures of 4-HNE and its amino acid adducts. Lysine (Lys); and histi­dine (His)

with proteins, DNA, and phospholipids, generating a variety of intra and intermolecular covalent adducts. At the membrane level, proteins and amino lipids can be covalently modified by 4-HNE. This aldehyde also acts as bioactive lipid mediator that produces rapid cell death not only through the depletion of sulfhydryl groups, but also through alterations in calcium homeostasis (Farooqui, 2009).

6.2 Synthesis of 4-HNE in the Brain As mentioned above, 4-HNE is a degradation product of 9- and 13-hydroperoxides of n-6 fatty acids (arachidonic and linoleic acids) (Esterbauer et al., 1991). Studies on model system of 9-hydroperoxides of linoleic acid have indicated that b-cleavage of 9(S)-hydroperoxy-octadecadienoic acid (9(S)-HPODE) is decomposed through a three-step process involving Hock cleavage (Fig.  6.2) (Schneider et  al., 2001, 2004). The Hock cleavage splits C–C bond as well as O–O bond yielding nonenal and 9-oxo-nonaic acid. Peroxidation of nonenal in the 4-position results in a racemic 4-hydroperoxy-nonenal (4-HPNE), which can be generated either nonenzymically or through enzymic reactions catalyzed by hydroperoxide lyase and alkenal oxygenase. The hydroperoxy group of 4-HPNE is then reduced to produce 4-HNE. Similarly, 13(S)-HPODE is also decomposed first through peroxidation and then

6.3  Metabolism of 4-HNE in the Brain

161

9

9(S)-Hydroperoxyoctadecadienoic acid

HOOC

9(S)-HPODE

HOO Hock cleavage

Hyroperoxide lyase H

HOOC

9-Oxo-nonanic acid

O

nonenal O

-H

•

O O2-, Reduction

Alkenal oxygenase

OOH

O Reduction

OH

O

4-HNE

Fig. 6.2  Synthesis of 4-HNE from 9-hydroxy derivative of linoleic acid

by Hock cleavage producing 4-HNE (Schneider et al., 2001, 2004). Ferrous iron (Fe2+)- or vitamin C-mediated decomposition of lipid hydroperoxides also result in generation of 4-HNE (Gu et al., 2007; Lee et al., 2001). Recently, an alternative mechanism involving an intermolecular cross-linking of a peroxyl radical has also been proposed for the formation of 4-HNE. This mechanism involves the styrene– oxygen polymerization and depolymerization (Schneider et al., 2008).

6.3 Metabolism of 4-HNE in the Brain The primary enzymic pathways of 4-HNE detoxification in brain and liver involves aldehyde dehydrogenase, alcohol dehydrogenase, aldehyde reductase, and glutathione S-transferase (Gallagher and Gardner, 2002; Gardner et al., 2003). Among above enzyme systems, in brain glutathione S-transferase is a predominant mechanism for neuroprotection against 4-HNE-mediated neurotoxicity (Gardner et al., 2003). In vitro and in vivo studies indicate that 4-HNE undergoes many enzymic and nonenzymic reactions (Fig. 6.3), which are closely associated with the modulation of cellular functions. Oxidation of 4-HNE to 4-hydroxynon-2-enoic acid (4-HNA) is catalyzed by the mitochondrial aldehyde dehydrogenase, an NAD-dependent

162

6  Neurochemical Aspects of 4-Hydroxynonenal

4-Hydroxynonenal

C=C bond reduction

Michael addition

C=O group reduction

C=O group oxidation

Schiff-base formation

Epoxidation

Acetal and thio-acetal formation

Fig. 6.3  Metabolic fate of 4-HNE

enzyme, which contains three domains; two dinucleotide-binding domains and a small three-stranded b-sheet domain, which is involved in subunit interactions in this tetrameric enzyme (Mitchell and Petersen, 1987). 4-HNE is not only a substrate for mitochondrial aldehyde dehydrogenase, but also a potent inhibitor (Ki = 0.5 mM) of this enzyme, when acetaldehyde was used as a substrate (Mitchell and Petersen, 1991). In addition, reduction of 4-HNE is catalyzed by aldoketo reductase and/or alcohol dehydrogenases in a NADPH-dependent manner yielding 1,4-dihydroxy-2nonene (DHN) (Burczynski et al., 2001; Srivastava et al., 1999). The conjugation between 4-HNE and glutathione is catalyzed by glutathione-S-transferase resulting in formation of glutathione-4HNE conjugate. This enzyme catalyzes the conjugate formation 600 times faster than the spontaneous nonenzymic reaction (Alin et al., 1985). Glutathione-4-HNE conjugate is also utilized by aldose reductase, an NADPH-dependent enzyme that reduces glutathione-4-HNE conjugate glutathione1,4-dihydroxy-2-nonene (glutathione-DHN) (Esterbauer et  al., 1991). The reduction of C=C bond in 4-HNE is catalyzed by alkenal/one oxidoreductase yielding 4-hydroxynonenal acid (4-HAA). C=C double bond in 4-HNE undergoes Michael addition reaction with cysteine or glutathione (GSH), resulting in the formation of Michael adduct. This reaction is catalyzed by glutathione-S-transferase. Similarly, amino compounds such as lysine, ethanolamine, guanine, and the imidazole group of histidine also undergo Michael additions with 4- HNE (Zhou and Decker, 1999). Thus, brain tissue contains multiple enzymic pathway for the detoxification of 4-HNE. Although 4-HNE is reduced by aldehyde dehydrogenases and aldolase reductases, the majority of cellular 4-HNE is metabolized by glutathione-S-transferase reaction. Overexpression of glutathione-S-transferase causes marked reduction in 4-HNE levels (Sharma et al., 2004).

6.4  Effect of 4-HNE on Enzyme Activities

163

6.4 Effect of 4-HNE on Enzyme Activities At low concentrations (0.8–2.8  mM), 4-HNE affects many enzymes (Fig.  6.4) associated with cell growth, gene expression, long-term potentiation, inflammation, apoptosis, and blood–brain barrier permeability (Fig. 6.5). Accumulation of 4-HNE during the initial stages of oxidative injury can exacerbate the cellular oxidative damage cascade by eliciting further protein and oxidative injury not only by altering mitochondrial function and redox status and decreasing ROS clearance (Raza et al., 2008), but also by electrophilically attacking the nucleophilic sites on proteins (Esterbauer et al., 1991). Thus, treatment of PC12 cells with 4-HNE induces oxidative stress by compromising the mitochondrial redox metabolism (Raza et al., 2008). As stated above, 4-HNE treatment reduces glutathione pool and increases ROS, protein carbonylation, and apoptosis. These processes are not only accompanied by marked inhibition in the activities of the mitochondrial respiratory enzymes, cytochrome c oxidase and aconitase, but also by increase in nuclear translocation of NF-kB/p65 protein leading to upregulation of tumor necrosis factor alpha (TNF-a). Furthermore, in PC12 cells, 4-HNE treatment also results in the release of mitochondrial cytochrome c, activation of poly-(ADP-ribose) polymerase (PARP), DNA Glycerophospholipids

COX-2 (↑)

Arachidonic acid

PLC (↑)

Ca2+ATPase (↓)

PLD (↑)

Na+, K+ATPase (↓)

PKC (↑)

Adenylate cyclase (↑)

4-HNE

JNK (↑)

Cysteine Ligase (↑)

Aldolase Reductase (↑)

Caspase-3 (↑) iNOS (↑)

MAP-Kinase (↑)

Nrf2 translocation

G 6-P dehydrogenase (↓)

Nrf2/ARE

Detoxification

Fig. 6.4  Modulation of enzyme activities by 4-HNE (Upward arrow indicate stimulation and downward arrow indicate inhibition)

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6  Neurochemical Aspects of 4-Hydroxynonenal Cell growth Gene expression Protein turnover

4-Hydroxynonenal

BBB permeability Long-term potentiation Apoptosis Inflammation

Fig. 6.5  Modulation of neurochemical processes by 4-HNE

fragmentation, and decreased expression of antiapoptotic Bcl-2 proteins. In addition, 4-HNE modulates MAP kinases, PKC isoforms, cell-cycle regulators, receptor tyrosine kinases, and caspases. Collectively these studies suggest that 4-HNEinduced cytotoxicity may be associated, at least in part, with the altered mitochondrial redox, mitochondrial energy metabolism, membrane dynamics, and respiratory dysfunctions that ultimately leads to apoptotic cell death (Raza et al., 2008).

6.4.1 Modulation of Kinases by 4-HNE Mitogen-activated protein (MAP) kinases are proline-directed serine/threonine kinases that are activated by dual phosphorylation on threonine and tyrosine residues in response to a wide array of extracellular stimuli (Davis, 1999). Three distinct groups of MAP kinases, namely extracellular-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 have been identified in mammalian cells and are closely associated with signal transduction processes involved in transfer of signal from the cell surface to the nucleus. ERK plays a major role in cell proliferation and differentiation as well as survival processes mediated by various growth factors (Karin, 1995). Exposure of cultured microglial Ra2 cells with 4-HNE significantly increases expression levels of cPLA2. The activated form of cPLA2 is phosphorylated (p-cPLA2) at amino acid residue S(505) on immunoblots (Shibata et  al., 2010a). Pretreatment of Ra2 cells with the antioxidant N-acetylcysteine, the extracellular signal-regulated kinase (ERK) inhibitor PD98059 or the p38 mitogenactivated protein kinase (MAPK) inhibitor SB203580 blocks the 4-HNE-mediated increased expression of cPLA2 and p-cPLA2. Immunocytochemical analysis indicates that staining for p-cPLA2 in Ra2 cells is localized in the cytoplasm and more intense in the 4-HNE-stimulated group than in the vehicle group (Shibata et al., 2010a). These results support the view that 4-HNE upregulates and phosphorylates cPLA2 in microglia via the ERK and p38 MAPK pathways. In contrast, the stress-activated

6.4  Effect of 4-HNE on Enzyme Activities

165 A

NMDA-R

NMDA

cPLA2

PtdCho

Cell cycle arrest

R

2

+

Ca2+

4-HNE

PLC

+ +

ARA

Depletion of GSH

PtdIns-4,5-P

DAG

PKC +

+

MARK signaling pathway Oxidative stress P38, ERK, JNK

InsP3

Ca2+

Mitochondria

Alterations in Ca2+ homeostasis Cytochrome c

RNA binding factor (AUF) COX-2

c-foc, c-jun, AP-1

NF-kB

NUCLEUS

Apoptosis

Fig. 6.6  Modulation of signal transduction processes by 4-HNE. N-Methyl-d-aspartate receptor (NMDA-R); phosphatidylcholine (PtdCho); cytosolic phospholipase A2 (cPLA2); Receptor (R); phosphatidylinositol 4,5-bisphosphate (PtdIns 4,5-P2); inositol 1,4,5-trisphosphate (InsP3); phospholipase C (PLC); diacylglycerol (DAG); arachidonic acid (ARA); intracellular calcium (Ca2+); and protein kinase C (PKC); 4-hydroxynonenal (4-HNE); glutathione (GSH); mitogen-activated kinase (MARK); extracellular-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), p38 mitogenactivated Protein Kinase (p38) and nuclear factor-kappaB (NF-kB)

protein kinase (SAPK) or JNK (Kyriakis et al., 1994) and p38 mitogen-activated protein kinase (p38 MAP kinase) (Han et al., 1994) are associated with the coordinated activation of various inflammatory cytokines and environmental stressors (Raingeaud et al., 1995). JNK is closely associated with signal transduction pathway that modulates apoptosis (Chen et  al., 1996). 4-HNE stimulates tyrosine phosphorylation through the activation of c-Jun N-terminal kinase (JNK) and MAP-kinase (p38), which facilitate transfer of extracellular signals through signal transduction process into intracellular responses through phosphorylation cascades (Fig.  6.6) (Uchida, 2003; Leonarduzzi et  al., 2004). 4-HNE-mediated JNK activation promotes its translocation in the nucleus (Fig.  6.6) where JNK-dependent phosphorylation of c-Jun and the transcription factor activator protein (AP-1) binding take place (Camandola et al., 2000; Cheng et al., 2001). This leads to the transcription of a number of genes having AP-1 consensus sequences in their promoter regions. Treatment of PC12 cells with 4-HNE results in maximal activation of JNK within 15–30  min before returning to control level after 1  h post-treatment. In contrast, activities of extracellular signal-regulated kinase (ERK) and p38 MAP kinase remain unchanged from their basal levels (Song et al., 2001). SEK1, an upstream

166

6  Neurochemical Aspects of 4-Hydroxynonenal

kinase of JNK, is also activated (phosphorylated) within 5  min after 4-HNE treatment and remains activated for up to 60 min. Pretreatment of PC12 cells with 8-(4-chlorophenylthio)-cAMP, a survival promoting agent blocks both the 4-HNEmediated JNK activation and apoptotic cell death. Pretreatment of PC12 cells with SB203580, a specific inhibitor of p38 MAP kinase, has no effect on 4-HNE-mediated apoptotic cell death. Collective evidence suggests that 4-HNE mediates early activation of JNK and p38 proteins but downregulate the basal activity of ERK-1/2. In 3 T3 fibroblasts, overexpression of dominant negative c-Jun and JNK1 is blocked by 4-HNE-mediated apoptosis, which indicate a role for JNK-c-Jun/AP-1 pathway during apoptosis (Kutuk and Basaga, 2007).

6.4.2 Modulation of Caspases by 4-HNE Caspases are a family of at least 14 aspartate-specific cysteine proteases that are involved in the initiation and execution of apoptotic cell death and the proteolytic maturation of inflammatory cytokines such as IL-1b and IL-18 (Creagh et al., 2003; Cohen, 1997). Caspases are normally expressed as inactive proenzymes (zymogens) that become activated during apoptosis. In neural and non-neural cells, 4-HNE induces apoptosis through the release of cytochrome c from mitochondria and caspase-9 and caspase-3 activation (Raza et al., 2008). Detailed investigations on the involvement of 4-HNE in apoptotic cell death in HepG2 cells indicate that 4-HNE-mediated apoptotic signaling involves both the Fas-mediated extrinsic and the p53-mediated intrinsic pathways (Chaudhary et al., 2010). Fas-mediated death inducing signaling complex (DISC)-independent apoptosis pathway is associated with the activation of apoptosis signal-regulating kinase 1 (ASK1), JNK, and caspase-3. In contrast, p53-mediated apoptotic pathway in HepG2 cells involves the activation of Bax, p21, JNK, and caspase-3. Treatment of HepG2 cells to 4-HNE not only results in the activation of both Fas and Daxx, but also the export of Daxx from the nucleus to cytoplasm, where it binds to Fas and activates Fas-mediated apoptosis via the ASK1, which in turn activates the JNK pathway and that this pathway is independent of the Fas-FADD-caspase8 pathway operative in various cell types (Yang et al., 1997). Thus, Daxx is primarily a nuclear protein, which translocates from the nucleus to cytoplasm during stress and acts as a death receptor adaptor at the cell surface (Salomoni and Khelifi, 2006). Depletion of Daxx by siRNA blocks apoptotic cell death. 4-HNE-mediated translocation of Daxx is also accompanied by the activation and nuclear accumulation of heat shock factor 1(HSF1) and upregulation of heat shock protein Hsp70 (Sharma et  al., 2008; Chaudhary et  al., 2010) (Fig.  6.7). Although the molecular mechanism associated with Hsp70-mediated neuroprotection is not fully understood, it is proposed that phosphorylated Hsps interact with Daxx and prevent its interactions with both ASK1 and Fas resulting in prevention of Daxx-mediated apoptosis. These effects of 4-HNE in HepG2 cells can be attenuated by ectopic expression of hGSTA4-4, the isozyme of glutathione S-transferase with high affinity for 4-HNE. In the nucleus, HSF-1 also promotes the expression of hsp-40

6.4  Effect of 4-HNE on Enzyme Activities

167

4-HNE

FasL

PtdCho

cPLA2

Depletion of GSH

NMDA-R

NMDA

Fas-R

+

+

Alterations in Ca2+ homeostasis

Ca2+

ARA Hsp70 Hsp90 HSF1

-

MARK signaling pathway

Nrf2

Keap1

Nrf2 P

HSF1

P38, ERK, JNK

Cytochrome c

Client BAG3 Hsp70

Nrf2

Gene regulation

Client

Daxx

Client

BAG3

Hsp70

h2

c-fos, cjun, AP-1

Caspase-3

Apoptosis

HSF1

AOE

JNK

Hsp70

Hsp90

ARE

Mitochondria

4-HNE

Oxidative stress

S-S S-S

ASK1

HSF1

(BclxL, Mcl Degradation)

Hsp induction

NUCLEUS

Fig. 6.7  Hypothetical representation showing the involvement of 4-HNE in Fas-mediated apoptotic signaling and Nrf2-mediated antioxidant effects. N-Methyl-d-aspartate receptor (NMDA-R); phosphatidylcholine (PtdCho); cytosolic phospholipase A2 (cPLA2); Fas Receptor (Fas-R); Fas ligand (FasL); arachidonic acid (ARA); intracellular calcium (Ca2+); 4-hydroxynonenal (4-HNE); glutathione (GSH); mitogen-activated kinase (MARK); extracellular-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), p38 mitogen-activated Protein Kinase (p38) heat shock protein 70 (Hsp70); heat shock protein 90 (Hsp90) and heat shock transcription factor 1 (HSF1)

and hsp-70, which facilitate the recovery of cells from thermally and chemically induced damage by promoting the stabilization of Bcl-XL. This inhibits Bax translocation to the mitochondrion and the consequent stress-induced apoptosis. In addition, 4-HNE induces phosphorylation of Daxx at Ser668 and Ser671 and promotes its cytoplasmic export. These observations indicate that while 4-HNE induces toxicity through multiple mechanisms, in parallel it evokes signaling for defense mechanisms to self-regulate its toxicity and can simultaneously affect multiple signaling pathways through its interactions with membrane receptors and transcription factors/ repressors (Sharma et al., 2008; Chaudhary et al., 2010).

6.4.3 Modulation of Glutathione S-Transferases by 4-HNE Glutathione S-transferases or glutathione transferases (GSTs) are a functionally diverse family of soluble multifunctional enzymes that catalyze the conjugation of reduced glutathione to electrophilic centers on a wide variety of substrates including

168

6  Neurochemical Aspects of 4-Hydroxynonenal

HOO

HO

COOH

COOH

GSH GSTA1-1 A2-2

CH3

CH3

9-Hydroperoxy LNA

9-Hydroxy LN A

O O

O C

O

O

ROOH

O P

C

O ROOH

O

GSH O

O (H3C)3N+

C

GSTA1-1 A2-2

O O

PtdCho hydroperoxide H C

O

ROH

O P

C O

O (H3C)3N+

PtdCho hydrooxide HO

CH3

O

CH3

GSH

O

GSTA4-4

OH

4-HNE

ROH

GS GS-4-HNE

Fig. 6.8  Detoxification of lipid peroxidation products by GST. Glutathione S-transferase (GST); glutathione (GSH); Linoleic acid (LNA); and 4-hydroxynonenal (4-HNE)

xenobiotic and endogenous electrophiles including 4-HNE. Several forms of GSTs are known to occur in brain tissue. Although the GSTA1-1 isoform is regarded as a highly promiscuous enzyme, GSTA4-4 is distinguished by a striking specificity toward 4-HNE and related alkenal substrates (Hou et al., 2007). In addition to GST activity, these enzymes also have peroxidase and isomerase activities. They protect neural cells from H2O2-mediated cell death by blocking JNK activity (thus protecting cells against H2O2-induced cell death). GSTs not only reduce lipid hydroperoxides through their Se-independent glutathione peroxidase activity, but also detoxify lipid peroxidation end products, including 4-HNE (Sharma et al., 2004) (Fig. 6.8). Thus, GSTs are a major contributors to the eukaryotic cell’s defenses against oxidative stress. Oxidative stress-mediated elevation in 4-HNE levels is accompanied by transient induction of hGST5.8 and 76 kDa Ral-binding GTPase activating protein (RLIP76). A coordinated action of GSTs (GSTA4-4 and hGST5.8) promotes the conjugation of 4-HNE with GSH to form the conjugate (GS-HNE) and the transporter 76  kDa Ral-binding GTPase activating protein (RLIP76) facilitates ATPdependent transport of GS-HNE. This observation strongly supports the view that both proteins play an important role in the regulation of the intracellular levels of 4-HNE (Cheng et al., 2001; Yang et al., 2003). Cells with induced hGST5.8 and

6.4  Effect of 4-HNE on Enzyme Activities

169

RLIP76 transport GS-HNE at a several fold higher rate as compared to the controls which further confirms the role of these proteins in regulations of cellular concentrations of 4-HNE. Collective evidence suggests that conjugation to 4-HNE with GSH is the primary route of HNE detoxification followed by oxidative/reductive modifications and, ultimately, mercapturic acid formation (Alary et al., 1998). Induction of GSTs involves an antioxidant-responsive response element (ARE) and the transcription factor nuclear factor E2-related factor 2 (Nrf2), which is bound to the Kelch-like ECH associated protein 1 (Keap1) in the cytoplasm (Fig.  6.7). Antioxidants and phytochemicals (isothiocyanates, curcumin, caffeic acid phenethyl ester, and polyphenols) disrupt the Keap-Nrf2 complex, allowing Nrf2 to translocate to the nucleus and mediate expression of Phase II genes via interaction with ARE. These genes include genes for glutathione S-transferase (GSTA1 and GSTA4), NAD(P)H:quinone oxidoreductase 1, UDP-glucuronosyltransferase, g-glutamate cysteine ligase, and hemeoxygenase-1. Phosphorylation of Nrf2 by MARK cascade may also facilitate dissociation of Keap-Nrf2 complex, promoting translocation of Nrf2 to the nucleus GSTs. Since 4-HNE stimulates MARK cascade, it is likely 4-HNE may indirectly modulate Nrf2-mediated signaling in the nucleus (Jeong et al., 2006). Silencing Nrf2 by siRNA increases the cytotoxicity of 4-HNE, but not as effectively as silencing of HSF1 (see below). Silencing HSF1 expression promotes the activation of JNK proapoptotic signaling and selectively decreases expression of the antiapoptotic Bcl-2 family member Bcl-XL. Overexpression of Bcl-XL attenuates 4-HNE-induced apoptosis in HSF1-silenced cells. Based on these results, it is proposed that the activation of HSF1 and stabilization of Bcl-XL mediate a protective response that may control the molecular aspects of lipid peroxidation and oxidative stress-mediated cellular injury.

6.4.4 Modulation of ATPases by 4-HNE 4-HNE also interacts and inhibits a number of membrane-bound enzymes, including Na+, K+-ATPase, Mg2+-ATPase, and Ca2+-ATPase in neural and non-neural tissues. The Ki values for Na+, K+-ATPase, Mg2+-ATPase, and Ca2+-ATPase are 40, 91, and 12  mM, respectively. In synaptosomes, 4-HNE inhibits Na+, K+-ATPase, but has very little effect on synaptosomal Ca2+- and Mg2+-ATPase activities. On the basis of detailed kinetic analysis of the Na+, K+-ATPase activity, it is suggested that 4-HNEinduced effect is caused by the decreased affinity for the substrate. ATP completely protects the ATPase from the 4-HNE attack. Treatment of synaptosomes with 4-HNE reduces membrane lipid fluidity near the lipid/water interface, but has no effect in the interior of lipid bilayer. In addition, 4-HNE-mediated inhibition of the ATPase activity in synaptosomes is closely associated with membrane fluidity and dynamics (Kadoya et al., 2003). In addition, inhibition of Na+, K+-ATPase by 4-HNE results in the depolarization of neuronal membranes leading to the opening of NMDA receptor channels and the influx of additional calcium ions into the cell (Kadoya et al., 2003). This calcium entry can be very harmful for neuronal integrity.

170

6  Neurochemical Aspects of 4-Hydroxynonenal

In the inner mitochondrial membrane, 4-HNE blocks ADP and ATP transport (Picklo et  al., 1999). This event may play a substantial role in disruption of the energy-producing capacity of mitochondria. Beside this, 4-HNE also modulates activities of enzymes of lipid, protein, and nucleic acid metabolism, glucose and glutamate transporters (Mark et al., 1997; Blanc et al., 1998).

6.4.5 Modulation of Other Enzymes and Cell Cycle by 4-HNE In addition to above metabolic changes, 4-HNE stimulates basal and GTP-stimulated phospholipase C and adenylate cyclase activities. It inhibits the activity of ornithine decarboxylase (Rossi et al., 1993). In non-neural cells at low levels (i.e.  S1P1 > S1P4 = S1P5 in contrast to cells of oligodendroglial lineage where expression of S1P1 and S1P5

9.3  Sphingosine 1 Phosphate Receptors in the Brain

P

2+

ABC transporter

G

PLC

Sph K1

InsP3

DAG

MEK

S1P

ErK

+

PKC

Creb

NUCLEUS

PtdIns-4,5-P2

Ca

Sph P

S1P

S-1-P-R

+

Ca2+

Internal stores

Raf P

IL-1b

IL1b-R

TrkC

NT3

253

Protein phosphorylation

Transcription

Synaptic plasticity, cell proliferation, differentiation, and cell survival

Fig. 9.7  Hypothetical diagram showing interactions between neurotrophin 3 and S1P receptors. Neurotrophin3 (NT3); Raf, MEK, and ErK (serine/threonine-selective protein kinases); cAMP Response Element Binding (CREB); S1P receptor (S1P-R); phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P2); Inositol 1,4,5-trisphosphate (InsP3)phospholipase C (PLC); diacylglycerol (DAG); protein kinase C (PKC)

predominates (Jaillard et al., 2005). Although, terminal Schwan cells express S1P1 receptors, these receptors are absent from myelinating Schwann cells (Kobashi et al., 2006). It is proposed that S1P mediates its effect through the activation of Rac1 and RhoA and elevation in Schwann cells migration (Barber et al., 2004). Direct injection of S1P into the striatum of mice induces astrocyte proliferation and astrogliosis (Sorensen et al., 2003). In cultured astrocytes, S1P treatment results in the stimulation of multiple S1P receptors, which results in the activation of PLC and ERK that leads to astrocytic cell proliferation (Wu et al., 2008). All S1PRs have been cloned and accumulating evidence suggests that S1P/S1PR signaling not only plays an important role in neural development, regulation of neural stem cells, and glial migration, but also facilitates astrocyte proliferation, neuroprotection against apoptosis, modulation of neuronal excitability, and glutamatergic neurotransmission (Chun et al., 2000; Toman and Spiegel, 2002; Yamagata et al., 2003; Harada et al., 2004; Kajimoto et al., 2007; Kimura et al., 2007; Milstien et al., 2007; Ohuchi et al., 2008). S1P also produces direct intracellular effects, which are independent of S1PRs (Spiegel and Milstien, 2003). The molecular mechanisms of intracellular action of

254

9  Sphingosine and Sphingosine 1 Phosphate in the Brain

S1P are still elusive. S1P not only enhances Ca2+ mobilization, but also stimulates the ERK/MAPK pathway to promote cell growth and survival (Takabe et al., 2008). It may also activate the PI3K/Akt pathway to modulate the balance between pro- and anti-apoptotic Bcl-2 proteins and activator protein 1 and NF-kB, which promotes survival (Takabe et al., 2008).

9.4 Sphingosylphosphorylcholine in the Brain Sphingosylphosphorylcholine (SPC) is a metabolic product of sphingomyelin (Fig. 9.8). It occurs in plasma and is a constituent of lipoproteins. It is synthesized by the action of sphingomyelin deacylase on sphingomyelin (Nixon et al., 2008). SPC increases [Ca2+]i in rat brain preparations (Furuya et al., 1996), and stimulates sodium hydrogen antiporter mechanism in a pituitary cell line (Tornquist et  al., 1997). However, the functional significance of these observations is not clearly understood. SPC acts as an endogenous inhibitor of the ubiquitous Ca2+ sensor calmodulin (CaM). Using fluorescence stopped-flow technique, it is shown that both the peptide and SPC micelles bind to CaM in a rapid and reversible manner with comparable affinities, supporting the view that SPC is a competitive inhibitor of CaM-target peptide interactions. It not only disrupts the CaM-binding domain of O

OH O

P

choline

O

NH O

Sphingomyelin Sphingomyelin deacylase

O

OH O Sphingosylphosphorylcholine

NH2 Sphingosylphosphorylcholine

Modulation of Ca2+ homeostasis

Modulation of inflammation

Fig. 9.8  Generation and roles of sphingosylphosphorylcholine

P

choline

O

Modulation of enzyme activities

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ryanodine receptor type 1and PtdIns3P receptor type 1, but also the plasma membrane Ca2+ pump. Accumulating evidence suggests that SPC regulates signal transduction processes by inhibiting Ca2+/CaM-dependent enzymes, such as phosphodiesterase and calcineurin. It is also reported that SPC, along with S1P and lysoPtdH increases nerve growth factor synthesis in astrocytes (Furukawa et al., 2007), suggesting that SPC may play a role in the survival and functional maintenance of neural cells. These effects may be mediated through S1PRs, which are expressed in the CNS (MacLennan et al., 2001). In brain, SPC not only acts as a mitogen, but also as a proinflammatory mediator. In cultured human melanocytes, SPC inhibits melanin synthesis in a dose-dependent manner, and reduces the activity of tyrosinase, the rate-limiting melanogenic enzyme (Kim et al., 2006). In human melanocytes, SPC induces short-thick dendrites, but has no effect on tyrosinase activity in a cell-free system. Furthermore, SPC reduces melanin synthesis by downregulating microphthalmia-associated transcription factor (MITF) and ERK pathway involved in the melanogenic signaling cascade (Kim et al., 2006). Under physiological conditions, levels of SPC are very low in brain tissue, but under pathological conditions, such as subarachnoid hemorrhage (SAH), SPC levels are significantly increased in CSF of SAH patients (Kurokawa et al., 2009; Wirrig et al., 2010). It is suggested that SPC may be involved in the development of cerebral vasospasm. Although, the molecular mechanism of SPC-mediated cerebral vasospasm is not known, in rat cerebral arteries, SPC not only activates p38 mitogenactivated protein kinase (MAPK), but also increases levels of nuclear factor-kappaB (NF-kB) and CCAAT (cytidine-cytidine-adenosine-adenosine-thymidine)-enhancerbinding protein. Both transcription factors are activated by SPC in a p38MAPKdependent manner along with increased expression of MCP-1 in the cerebral arteries (Wirrig et al., 2010). Collective evidence suggests that SPC, but not S1P, acts as a proinflammatory mediator in cerebral arteries. This may contribute to inflammation observed after SAH and may be part of the initiating event in vasospasm (Kurokawa et al., 2009; Wirrig et al., 2010).

9.5 FTY720 and Its Neurochemical Effects FTY720 is an immunosuppressant and structural analog of sphingosine (Fig. 9.9), which is highly effective in inhibiting autoimmunity and graft rejection in animal models of allotransplantation in an autoimmune disease. It is phosphorylated in vivo by SphK2 (Mandala et  al., 2002), which are abundantly expressed in the brain (Blondeau et al., 2007; Bryan et al., 2008). Like endogenous S1P, the phosphorylated form of FTY720 (FTY720-P) activates S1PRs, mainly S1P1, in a dose-dependent manner. Binding of FTY720-P to the S1P1 receptor has been shown to result in its internalization and long-term downregulation of its expression (Oo et  al., 2007). FTY720 also inhibits ceramide synthases, decreasing cellular levels of dihydroceramides, ceramides, sphingosine, and S1P, but increasing levels of dihydrosphingosine and dihydrosphingosine 1-phosphate (DHS1P) (Berdyshev et  al., 2009).

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Drug

Target Inhibition of cPLA2

NH2

Effect Neuroprotection

Cannabinoid receptor 1

Neuroprotection

Stimulation of ERK1/2

Neuroprotection

OH OH Inhibition of ceramide synthase

Neuroprotection

FTY720 Inhibition of S1P lyase

ATP

Neuroprotection

Sphingosine Kinase 2 ADP

NH2

OH O

P O

OH OH

S1P1

Neuroprotection

FTY720-phosphate

Fig. 9.9  Phosphorylation FTY720 and target enzymes and receptor for FTY720 and FTY720phosphate. Cytosolic phospholipase A2 (cPLA2); ERK1/2 (serine/threonine-selective protein kinase); and S1P receptor (S1P1)

The FTY720-mediated modulation of sphingolipid de novo biosynthesis is similar to that of fumonisin B1, a classical inhibitor of ceramide synthases, but differs in the efficiency to inhibit biosynthesis of short-chain versus long-chain ceramides. In vitro kinetic studies indicate that FTY720 is a competitive inhibitor of ceramide synthase 2 toward dihydrosphingosine with an apparent Ki of 2.15 mM. FTY720-induces upregulation of DHS1P, which is mediated by sphingosine kinase (SphK) 1, but not SphK2, as confirmed by experiments using SphK1/2 silencing with small interfering RNA (Berdyshev et al., 2009). FTY720-phosphate is stable in the presence of active sphingosine-1-phosphate lyase, indicating that the lyase does not degrade FTY720. Conversely, FTY720 inhibits sphingosine-1-phosphate lyase activity in  vitro (Berdyshev et al., 2009). Administration of FTY720 in mice inhibits tissue sphingosine1-phosphate lyase activity within 12 h, whereas lyase gene and protein expression are not significantly affected by this drug (Bandhuvula et  al., 2005; Berdyshev et  al., 2009). In addition, FTY720 induces numerous effects on the immune system including inhibition of egress of both naive and activated CD4+, CD8+, and B lymphocytes from peripheral lymphoid organs and thymus, cysteinyl leukotriene-dependent T cell chemotaxis to lymph nodes, peripheral blood lymphopenia, egress of lymphocytes from the spleen, displacement of B cells from the marginal zone of the spleen, decreased b1 integrin expression on marginal zone B cells, homing of hematopoietic progenitor cells to the bone marrow, and decreased vascular permeability (Mandala et al., 2002; Sanchez et al., 2003; Kimura et al., 2004).

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In addition, FTY720 retards inflammation (Coelho et al., 2007; Payne et al., 2007) by inhibiting cPLA2 and reducing antigen-induced secretion of PGD2 and cysteinylleukotriene. Although FTY720 also reduces cPLA2-mediated arachidonic acid release in response to antigen, it has no effect on translocation of cPLA2 or ERK1/2 activation, suggesting that it does not interfere with FcepsilonRI-mediated events, leading to cPLA2 activation (Payne et al., 2007). Thus, FTY720 drastically inhibits recombinant cPLA2a activity, whereas FTY720-phosphate, sphingosine, or S1P have no effect. It is proposed that FTY720 reduces inflammation by blocking the production of inflammatory lipid mediators, like eicosanoids and platelet-activating factor (Ong et al., 2010). Treatment of oligodendrocyte progenitors with FTY720 produces activation of extracellular signal-regulated kinase 1/2 and Akt and protects cells from apoptosis, but arrests their differentiation (Coelho et al., 2007).

9.6 Sphingosine 1 Phosphate in Neurological Disorders In brain sphingolipid metabolism, sphingolipid-derived lipid mediators are associated with several signaling pathways that control neuronal survival, migration, and differentiation, responsiveness to trophic factors, synaptic stability, synaptic transmission, and neuron–glia interactions. These processes are involved in stabilizing of central and peripheral myelin. Many neurotraumatic and neurodegenerative diseases are accompanied by increase in expression of cytokine and enrichment of sphingolipid metabolisms due to upregulation of SMase activity (Farooqui, 2009). Thus, sphingolipid metabolism is deregulated in neurotraumatic and neurodegenerative diseases, leading not only to the expression of abnormal sphingolipid patterns and abnormal sphingolipid–protein interactions, but also increase in levels of ceramide, ceramide 1 phosphate, decrease in sphingosine, and sphingosine 1 phosphate, alterations in calcium homeostasis, and altered neural membrane organization (Farooqui et al., 2007; Farooqui, 2009). In addition, most neurodegenerative diseases also involve the activation of microglial cells along with neuroinflammation, which is supported by increased expression of proinflammatory cytokines, elevation in eicosanoids, increase in platelet-activating factor levels, and increase in nitric oxide production (Farooqui, 2009, 2010). Collective evidence suggests that abnormal sphingolipid metabolism and above neurochemical processes are closely associated with the pathogenesis of neurotraumatic and neurodegenerative diseases (Piccinini et al., 2010; Farooqui, 2009, 2010).

9.6.1 Sphingosine 1 Phosphate in Ischemia Stroke is accompanied by the interruption of blood flow to the brain due to the blockage of a cerebral artery. Neurochemically, stroke is characterized by the disruption of glucose and oxygen supply, oxidative stress, and neuroinflammation.

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Sphingosine 1 phosphate and FTY720

Traumatic brain injury

Alzheimer disease

Spinal cord injury

Ischemic injury

Multiple sclerosis

Fig. 9.10  Use of FTY720 for the treatment of neurotraumatic and neurodegenerative diseases

In addition, interactions between the brain and the peripheral immune system results in a biphasic immune response, which consists of an early activation of peripheral immune cells with massive production of proinflammatory cytokines followed by a systemic immunosuppression within days of cerebral ischemia leading to massive immune cell loss in spleen and thymus. These processes not only lead to the development of infarction, but also apoptotic and necrotic cell death. Cerebral ischemia upregulates the sphingomyelin-ceramide pathway, which involves calciumindependent c-Jun NH2-terminal kinase and serine/threonine phosphatase PP2A activation in hippocampal glia (Hasegawa et  al., 2010; Adibhatla et  al., 2008). Furthermore, S1P has neuroprotective effect in oxygen-glucose deprivation (OGD) model of ischemia in SH-SY5Y human neuroblastoma cells. Treatment of cell cultures with specific PKC epsilon (V1-2), retards neuroprotective effect of S1P on OGD/recovery-induced necrosis (Agudo-Lopez et al., 2010). FTY720 is an analog of S1P. It is known to induce depletion of circulating lymphocytes by preventing the egress of lymphocytes from the lymph nodes. At the molecular level, the depletion of circulating lymphocyte is due to a downregulation of the S1P1 (Brinkmann et al., 2004). Injections of FTY720 in a mouse model of ischemia decrease lesion size and improve neurological function (Fig. 9.10). FTY720-treated animals not only significantly retain Akt and extracellular signal-regulated kinase phosphorylation and Bcl-2 expression, but show decrease in the expression of caspase-3 and terminal deoxynucleotidyl transferase-mediated uridine 5¢-triphosphate-biotin nick endlabeling-positive neurons at 24 h after middle cerebral artery occlusion along with decrease in the numbers of infiltrating neutrophils, activated microglia/macrophages in the ischemic lesion, and reduction in immunohistochemical features of apoptotic cell death in the ischemic lesion (Czech et  al., 2009). Accumulating evidence

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suggests that abnormalities in sphingolipid metabolism may contribute to the pathogenesis of stroke, and FTY720 can be used for the treatment of ischemic injury (Fig. 9.10).

9.6.2 Sphingosine 1 Phosphate in Traumatic Brain and Spinal Cord Injuries Traumatic injury to brain and spinal cord (TBI and SCI) may lead to permanent disability in young adults. Sphingolipid signaling has been implicated in pathophysiology of TBI and SCI (Singh and Hall, 2008; Farooqui, 2010). Generation of S1P may represent a novel neuroprotective target to counteract the pathophysiology of acute brain and spinal cord injury in regard to apoptotic cell death mechanisms, mitochondrial dysfunction, lipid hydrolysis, and oxidative damage mechanisms (Singh and Hall, 2008). Furthermore, it is suggested that S1P agonist may increase the resistance of the brain tissue to injury by promoting neurotrophic activity, neurogenesis, and angiogenesis. In contrast, treatment with antagonists of S1P activity may cause proregenerative effects via promotion of neurite growth and inhibition of astrogliotic scarring (Singh and Hall, 2008). Many immune inhibitors including tacrolimus (FK506) have been reported to produce beneficial effects by inducing regeneration in brain and spinal cord following TBI and SCI (Zhang et al., 2009). The combination of FTY720 and tacrolimus has been reported to produce synergistic immunosuppresive effects in rat allograft models without causing critical adverse effects (Zhang et al., 2009).

9.6.3 Sphingosine 1 Phosphate in Alzheimer Disease Alzheimer disease (AD) is a progressive neurodegenerative disease and the most common cause of dementia in the aging population. Loss of synapse, defect in cholinergic system, accumulation of insoluble amyloid b (Ab), and neuronal degeneration are some of the physiological sequelae that are closely associated with the pathogenesis of this devastating disease (Farooqui, 2010). Sphingolipid metabolism is altered in AD (Haughey et al., 2010). Determination of sphingolipid metabolismderived lipid mediators in brain from AD patients and age-matched normal subjects indicate that increase in expression of acid SMase and ceramidase is accompanied with reduction in levels of sphingomyelin and elevation in ceramide and sphingosine (He et al., 2010). Levels of S1P are decreased. Importantly, there is significant correlation among the brain acid SMase activity, S1P levels, and levels of Ab and hyperphosphorylated t protein. Treatment of neuronal cell cultures with Ab oligomers also leads to the activation of acid SMase activity, elevation in ceramide levels, and induction of apoptotic cell death (He et al., 2010). Pretreatment of the

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neuronal culture with purified, recombinant ceramidase blocks the induction of Ab-mediated apoptosis. Based on these observations, it is proposed that acid activation of SMase ceramidase along with elevation in ceramide and sphingosine and reduction in S1P may be important neurochemical events involved in the pathogenesis of AD (He et al., 2010).

9.6.4 Sphingosine 1 Phosphate in Multiple Sclerosis Multiple sclerosis (MS) is an autoimmune, neurological disease of unknown etiology. It is a chronic inflammatory disease, which is characterized by demyelination in young adults. Recently, sphingolipid metabolism has been linked to the pathophysiology of MS and it is reported that oligodendrocyte apoptosis is one of the critical events followed by glial activation and infiltration of lymphocytes and macrophages (Jana and Pahan, 2010). Current therapeutic approaches to treat MS focus on the suppression of the immune system and on blockade of T cell blood– brain barrier transmigration into the brain parenchyma. S1P can be recognized by receptors located on T and B cells. Interaction of FTY720 phosphate with S1P1 not only antagonizes S1P1 on T cells, but also inhibits S1P/S1P1-dependent lymphocyte egress from secondary lymphoid organs (Chun and Hartung, 2010). FTY720 phosphate initially activates lymphocyte S1P1 via high-affinity receptor binding, leading to downregulation of S1P1. This downregulation of S1P1 prevents lymphocyte egress from lymphoid tissues, thereby reducing autoaggressive lymphocyte infiltration into the brain tissue. S1P receptors are also expressed by many CNS cell types. Prophylactic administration of FTY720 to animals with experimental autoimmune encephalitis (EAE), a model of MS, completely prevents development of EAE features, whereas therapeutic administration significantly reduces clinical severity of EAE (Fig. 9.10). MS pathology also involves axonal loss. The extent of axonal transection at the early stages of MS lesion formation correlates with inflammatory infiltration by macrophages and lymphocytes. Inflammatory mediators and toxic products that are secreted by activated macrophages include nitric oxide (Smith et  al., 2001), matrix metalloproteinases (Newman et  al., 2001), glutamate excitotoxicity (Smith et  al., 2000) and direct CD81 T-cell cytotoxic action (Medana et al., 2001). All these factors are capable of damaging axons. It is shown that FTY720 protects against further axon loss when administered in a clinically relevant manner, after the onset of clinical disease. It is likely that the decrease in the expression of iNOS and reduction in numbers of CD31 lymphocytes following therapeutic treatment may at least partly explain the ability of FTY720 significantly preventing further inflammationassociated axon loss. Therapeutic efficacy observed in animal studies has been substantiated in phase 2 and 3 trials involving patients with relapsing or relapsingremitting MS (Chun and Hartung, 2010). In phase 3 trials, FTY720 has not been proven superior to standard treatments. Thus, more studies are needed on the beneficial effects of FTY720 in MS patients.

9.7  Conclusion

261

9.6.5 Sphingosine 1 Phosphate in Kainic Acid Neurotoxicity Kainic acid (KA) is a cyclic and nondegradable analog of glutamate. Its systemic administration into adult rats induces excitotoxicity, a common mechanism of neuronal injury after head injury, stroke, and certain neurodegenerative diseases. Astrocytes respond to KA-mediated injury by proliferation and hypertrophy, induction of PLA2 isoforms, elevation in levels of prostaglandins and lipid peroxides, increase in caspase-3 activity, and increase in expression of glial fibrillary acidic protein (GFAP) (Farooqui et al., 2008). In addition, studies on temporal changes in sphingosine kinase 1 (SPHK1)/sphingosine 1 phosphate receptor 1(S1P1) in mouse hippocampus following KA-induced neurotoxicity indicate that lowest level of SPHK1 protein expression is found 2  h after KA treatment. Six hours after KA treatment, the expression of SPHK1 and S1P1 proteins steadily increases in the hippocampus. Immunohistochemical analysis indicates that SPHK1 and S1P1 are more immunoreactive in astrocytes within the hippocampus of KA-treated mice than in hippocampus of control mice. These results indicate that SPHK1/S1P1 signaling axis may play an important role in astrocytes proliferation during KA-induced excitotoxicity (Lee et al., 2010).

9.7 Conclusion S1P has emerged as a new class of potent bioactive lipid mediator which is associated with a variety of cellular processes, such as cell differentiation, apoptosis, and proliferation. Sphingomyelin and ceramide are precursor for sphingosine, which on phosphorylation by SphK yields S1P. An important feature of the sphingolipid metabolic pathway is the compartmentalization into endoplasmic reticulum, the Golgi apparatus, lysosome and plasma membrane, and this compartmentalization makes the transport of sphingolipids critical for proper neural cell functioning. In neural cells, S1P regulates differentiation, proliferation, inflammation, and cell migration. Several biological effectors have been shown to promote the synthesis of S1P, including growth factors, cytokines, and antigen and G-protein-coupled receptor agonists. Interest in neurochemical actions of S1P is focused recently on two distinct cellular actions of this lipid, namely, its function as an intracellular second messenger, capable of triggering calcium release from internal stores, and as an extracellular ligand activating specific G protein-coupled receptors. Inhibition of SphK stimulation strongly reduces or even blocks cellular events triggered by several proinflammatory agonists, such as receptor-stimulated DNA synthesis, Ca2+ mobilization, cytokine production, and apoptosis. Deregulation of sphingolipid metabolism occurs in neurotraumatic and neurodegenerative diseases. FTY720, a fingolimod, has been used for the treatment of ischemic and traumatic injuries in the brain and spinal cord. Moreover, blocking S1P1 inhibits lymphocyte egress from lymphatic organs. This drug can be used for the treatment MS in patients.

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Kihara A., Mitsutake S., Mizutani Y., and Igarashi Y. (2007). Metabolism and biological functions of two phosphorylated sphingolipids, sphingosin 1-phosphate and ceramide 1-phosphate. Prog Lipid Res 46:124–144. Kim D.S., Park S.H., Kwon S.B., Park E.S., Huh C.H., Youn S.W., and Park K.C. (2006). Sphingosylphosphorylcholine-induced ERK activation inhibits melanin synthesis in human melanocytes. Pigment Cell Res. 19:146–153. Kimura A., Ohmori T., Ohkawa R., Madoiwa S., Mimuro J., Murakami T., Kobayashi E., Hoshino Y., Yatomi Y., and Sakota Y. (2007). Essential roles of sphingosine 1-phoshate/S1P1 receptor axis in the migration of neural stem cells toward a site of spinal cord injury. Stem Cells 25:115–124. Kimura T., Boehmler A. M., Seitz G., Kuci S., Wiesner T., Brinkmann V., Kanz L., and Mohle R. (2004) The sphingosine 1-phosphate receptor agonist FTY720 supports CXCR4-dependent migration and bone marrow homing of human CD34+ progenitor cells. Blood 103, 4478–4486. Kobashi H., Yaoi T., Oda R., Okajima S., Fujiwara H., Kubo T., and Fushiki S. (2006). Lysophospholipid receptors are differentially expressed in rat terminal Schwann cells, as revealed by a single cell rt-PCR and in situ hybridization. Acta Histochem Cytochem. 39:55–60. Kobayashi N., Nishi T., Hirata T., Kihara A., Sano T., Igarashi Y., Yamaguchi A. (2006). Sphingosine 1-phosphate is released from the cytosol of rat platelets in a carrier mediated manner. J Lipid Res 47:614–621. Kumar S., Kahn M.A., Dinh L., and de Vellis J. (1998). NT-3-mediated TrkC receptor activation promotes proliferation and cell survival of rodent progenitor oligodendrocyte cells in vitro and in vivo. J Neurosci Res 54:754–65. Kurokawa T., Yumiya Y., Fujisawa H., Shirao S., Kashiwagi S., Sato M., Kishi H., Miwa S., Mogami K., Kato S., Akimura T., Soma M., Ogasawara K., Ogawa A., Kobayashi S., and Suzuki M. (2009). Elevated concentrations of sphingosylphosphorylcholine in cerebrospinal fluid after subarachnoid hemorrhage: a possible role as a spasmogen. J Clin Neurosci. 16:1064–1068. Lee D.H., Jeon B.T., Jeong E.A., Kim J.S., Cho Y.W., Kim H.J., Kang S.S., Cho G.J., Choi W.S., Roh G.S. (2010). Altered expression of sphingosine kinase 1 and sphingosine-1-phosphate receptor 1 in mouse hippocampus after kainic acid treatment. Biochem Biophys Res Commun. 393:476–480. Liu H., Toman R.E., Goparaju S.K., Maceyka M., Nava V.E., Sankala H., Payne S.G., Bektas M., Ishii I., Chun J., Milstien S., and Spiegel S. (2003). Sphingosine kinase type 2 is a putative BH3-only protein that induces apoptosis. J. Biol. Chem. 278 :40330–40336. Maceyka M., Sankala H., Hait N. C., Stunff H. L., Liu H., Toman R., Collier C., Zhang, M., Satin L. S.; Merrill A. H., Milstien S,, Jr., Spiegel S. (2005). SphK1 and SphK2, sphingosine kinase isoenzymes with opposing functions in sphingolipid metabolism. J. Biol. Chem. 280: 37118–37118. MacLennan A.J., Carney P.R., Zhu W.J., Chaves A.H., Garcia J., Grimes J.R., et al. (2001). An essential role for the H218/AGR16/Edg-5/LP(B2) sphingosine 1-phosphate receptor in neuronal excitability. Eur J Neurosci 14:203–209. Mandala S., Hajdu R., Bergstrom J., Quackenbush E., Xie J., Milligan J., Thornton R.,Shei G., Card D., Keohane C., Rosenbach M., Hale J., Lynch C.L., Rupprecht K., Parsons W., Rosen H. (2002). Alteration of Lymphocyte Trafficking by Sphingosine-1-Phosphate Receptor Agonists. Science 296:346–349. Medana I., Martinic M.A., Wekerle H., and Neumann H. (2001). Transection of major histocompatibility complex class I-induced neurites by cytotoxic T lymphocytes. Am J Pathol 159:809–815. Meyer zu Heringdorf D., Lass H., Kuchar I., Lipinski M., Alemany R., Rumenapp U., Jakobs K.H. (2001). Stimulation of intracellular sphingosine-1-phosphate production by G-protein-coupled sphingosine-1-phosphate receptors. Eur J Pharmacol. 414:145–154. Milstien S., Gude D., and Spiegel S. (2007). Sphingosine 1-phosphate in neural signalling and function. Acta Paediatr 96:40–43. Mizugishi K., Yamashita T., Olivera A., Miller G.F., Spiegel S., and Proia R.L. (2005). Essential role for sphingosine kinases in neural and vascular development. Mol Cell Biol. 25:11113–11121. Mullershausen F., Craveiro L.M., Shin Y., Cortes-Cros M., Bassilana F., Osinde M., Wishart W.L., Guerini D., Thallmair M., Schwab M.E., Sivasankaran R., Seuwen K., and Dev K.K. (2007).

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Singh I.N. and Hall E.D. (2008). Multifaceted roles of sphingosine-1-phosphate: how does this bioactive sphingolipid fit with acute neurological injury? J. Neurosci. Res. 86:1419–1433. Siskind L.J., Fluss S., Bui M., and Colombini M. (2005). Sphingosine forms channels in membranes that differ greatly from those formed by ceramide. J. Bioenerg. Biomembr. 37:227–236. Smith T., Groom A., Zhu B., Turski L. (2000). Autoimmune encephalomyelitis ameliorated by AMPA antagonists. Nat Med 6:62–66. Smith K.J., Kapoor R., and Felts P.A. (2001). Electrically active axons degenerate when exposed to nitric oxide. Ann Neurol 49:470–476. Sorensen S.D., Nicole O., Peavy R.D., Montoya L.M., Lee C.J., Murphy T.J., Traynelis S.F., Hepler J.R. (2003). Common signaling pathways link activation of murine PAR-1, LPA, and S1P receptors to proliferation of astrocytes. Mol. Pharmacol. 64:1199–1209. Spiegel S. and Milstien S. (2000). Sphingosine-1-phosphate: signaling inside and out. FEBS Lett. 476:55–57. Spiegel S., and Kolesnick R. (2002). Sphingosine 1-phosphate as a therapeutic agent. Leukemia. 16:1596–1602. Spiegel S., and Milstien S. (2003). Exogenous and intracellularly generated sphingosine 1-phosphate can regulate cellular processes by divergent pathways. Biochem Soc Trans. 31:1216–1219. Sukocheva O., Wadham C., Holmes A., Albanese N., Verrier E., Feng F., Bernal A., Derian C.K., Ullrich A., Vadas M.A., and Xia P. (2006). Estrogen transactivates EGFR via the sphingosine 1-phosphate receptor Edg-3: the role of sphingosine kinase-1. J Cell Biol 173:301–310. Taha T. A., Kitatani K., El-Alwani M., J. Bielawski J., Hannun Y.A., and Obeid L.M. (2006). Loss of sphingosine kinase-1 activates the intrinsic pathway of programmed cell death: modulation of sphingolipid levels and the induction of apoptosis. FASEB J. 20:482–484 . Takabe K., Paugh S.W., Milstien S., and Spiegel S. (2008). “Inside-out” signaling of sphingosine1-phosphate: therapeutic targets. Phamacol. Rev. 60:181–195. Toman R.E., and Spiegel S. (2002). Lysophospholipid receptors in the nervous system. Neurochem Res 27:619–627. Tornquist K., Woodside M., and Grinstein S. (1997). Sphingosylphosphorylcholine activates an amiloride-insensitive Na+–H+-exchange mechanism in GH4C1 cells. Eur J Biochem. 248:394–400. van Koppen C.J., Meyer zu Heringdorf D., Alemany R., Jakobs K.H. (2001). Sphingosine kinasemediated calcium signaling by muscarinic acetylcholine receptors. Life Sci 2001;68:2535–2540. Waeber C., Blondeau N., and Salomone S. (2004). Vascular sphingosine-1-phosphate S1P1 and S1P3 receptors. Drug News Perspect. 17:365–382. Whetzel A.M., Bolick D.T., and Hedrick C.C. (2009). Sphingosine-1-phosphate inhibits high glucose-mediated ERK1/2 action in endothelium through induction of MAP kinase phosphatase-3. Am J Physiol Cell Physiol. 296:C339–345. Wirrig C., Hunter I., Mathieson F.A., and Nixon G.F. (2010). Sphingosylphosphorylcholine is a proinflammatory mediator in cerebral arteries. J. Cereb. Blood Metab. 2010 June 16, Epub ahead of print. Wu Y.P., Mizugishi K., Bektas M., Sandhoff R., and Proia R.L. (2008). Sphingosine kinase 1/S1P receptor signaling axis controls glial proliferation in mice with Sandhoff disease. Hum Mol Genet. 17:2257–2264. Yamagata K., Tagami M., Torii Y., Takenaga F., Tsumagari S., Itoh .S, Yamori Y., and Nara Y. (2003). Sphingosine 1-phosphate induces the production of glial cell line-derived neurotrophic factor and cellular proliferation in astrocytes. Glia 41:199–206. Yatomi Y. (2008). Plasma sphingosine 1-phosphate metabolism and analysis. Biochim Biophys Acta. 1780:606–611. Zhang J., Zhang A., Sun Y., Cao X., and Zhang N. (2009). Treatment with immunosuppressants FTY720 and tacrolimus promotes functional recovery after spinal cord injury in rats. Tohoku J Exp Med. 219:295–302.

Chapter 10

Cholesterol and Hydroxycholesterol in the Brain

10.1 Introduction The brain has higher concentration of cholesterol compared to other body parts (25% of the body’s free cholesterol) (Dietschy and Turley, 2004). In brain, cholesterol resides in two pools: one major pool associated with the myelin sheaths playing an important role in propagation of the electrical signals along the axons and the other minor pool associated with the plasma membranes of astrocytes and neurons (Dietschy and Turley, 2004). The distribution of cholesterol in neural membranes is asymmetric across the plane of the membrane, with outer and innerleaflets containing 25% and 75% cholesterol, respectively. The asymmetric distribution of cholesterol along with asymmetric distribution of glycerophospholipids (PtdEtn in the exofacial layer and PtdCho in the inner leaflet) is necessary for optimal neural plasticity and synaptic transmission. The half-life of cholesterol in the brain is 170  days (Andersson et al., 1990). Cholesterol not only serves as a precursor for the synthesis of oxysterols, steroid hormones, vitamin D, and lipoproteins, but regulates activities of membrane-bound enzymes, receptors, and ion channels (Simons and Ikonen, 2000). Accumulating evidence suggests that neural membrane cholesterol not only provides neural plasticity and facilitates synaptic transmission, but also modulates endocytosis, and antigen expression (Fig. 10.1). Dynamic clustering of cholesterol along with sphingolipids results in the formation of specialized structures called microdomains or rafts (Farooqui, 2009). In neural membranes, raft formation occurs by self-association of sphingolipids via their long saturated hydrocarbon chains. Cholesterol condenses this packing by positioning between hydrocarbon chains below the large head groups of the sphingolipids (Farooqui, 2009). These interactions lead to the formation of a less fluid, liquid ordered phase, separate from a phosphatidylcholine-rich liquid-disordered phase (Simons and Ikonen, 2000). In neural and non-neural cells, the depletion of cholesterol induces autophagy, a process

A.A. Farooqui, Lipid Mediators and Their Metabolism in the Brain, DOI 10.1007/978-1-4419-9940-5_10, © Springer Science+Business Media, LLC 2011

267

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Roles of cholesterol in brain

Synaptogenesis

Raft formation

Neuroplasticity

Endocytosis Generation of neurosteroids

Fig. 10.1  Roles of cholesterol in the brain

by which cells digest their own components (Cheng et al., 2006; Clark et al., 2008). Autophagy is a homeostatic process for recycling of proteins and organelles that are increased during times of nutrient deprivation. The structure of cholesterol makes it susceptible to a variety of radical attacks due to the 5,6-double bond and the concomitant vinylic methylene group at C-7 in the B ring. In addition to C-17, cholesterol has isooctyl side chain, which undergoes enzymic oxidation largely by cytochromes P450-dependent oxygenases, but this site of oxidation is typically not a target for ROS relevant to cellular biochemistry (Smith, 1981). Brain contains cytochrome P450-dependent oxygenases, which convert cholesterol into 24(S)-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol (Fig.  10.2). It is becoming increasingly evident that the major mechanism of brain cholesterol elimination is the conversion of cholesterol into 24(S)-hydroxycholesterol by CYP46A1, a neuron-specific cytochrome P450. Cholesterol is also oxidized to cholesterol oxides and converted into cholesterol ester via acyl-CoA:cholesterol acyltransferase (ACAT) (Björkhem et  al., 1998). ACAT is an integral membrane protein, which is found in the endoplasmic reticulum (ER) (Chang et  al., 2009). This enzyme modulates the dynamic equilibrium between free and esterified cholesterol in the brain. Two isoforms, namely, ACAT1 and ACAT2 of ACAT have been identified in neural and non-neural tissues (Rudel et al., 2001). ACAT activity not only plays a role in the reesterification process of cholesterol (Zhang et  al., 2003), but also acts as a cholesterol sensor protein. Hydroxycholesterols (oxysterols) are agonists for nuclear receptors, such as LXR, which act as cholesterol sensors: when cellular hydroxysterols accumulate as a result of increasing concentrations of cholesterol, LXR induces the transcription of genes that protect cells from cholesterol overload. In addition, LXR are also involved in cholesterol homeostasis (Fig. 10.3).

10.2  Synthesis of Cholesterol and Hydroxycholesterols in the Brain

269

a

Cholesterol 25-hydroxylase

HO

CYP27A1 CH2OH

c

b OH

CYP46A1 HO

HO

d OH

HO

Fig. 10.2  Conversion of cholesterol into hydroxycholesterols. Cholesterol (a), 25-hydroxycholesterol (b). 27-hydroxycholesterol (c), and 24-hydroxycholesterol (d). CYP27 (sterol 27-hydroxylase), and CYP46A1 (24-cholesterol hydroxylase) Raft perturbation, alterations in membrane permeability Gene expression Cholesterol homeostasis

Hydroxycholesterols

Exocytosis Agonists for LXR Apoptosis Modulation of enzymic activities

Fig. 10.3  Roles of hydroxycholesterols in the brain

10.2 Synthesis of Cholesterol and Hydroxycholesterols in the Brain As stated above, cholesterol accounts for 20–25% of the total body cholesterol in fresh brain. Most brain cholesterol is found in the myelin sheets (70%) and the remaining 30% of brain cholesterol is divided between membranes of glial cells

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(20%) and neurons (10%). Cholesterol contents in brain are independent of dietary uptake or hepatic synthesis because cholesterol does not cross the blood–brain barrier (BBB) (Jurevics and Morell, 1995). Brain synthesizes it own cholesterol through de  novo synthesis and total cholesterol levels steadily increase between ages 20–65 years in both men and women. The highest rate of cholesterol synthesis in the first postnatal weeks in humans and rodents may be associated with glial cell proliferation, neurite outgrowth, microtubule stability, synaptogenesis, and myelination. After 65 years, cholesterol levels begin to decrease. Thus, cholesterol is an integral component of neural membranes, where it has been implicated in the assembly and maintenance of lipid rafts. In brain, cholesterol plays a crucial role in membrane organization, dynamics, function, and sorting (Simons and Ikonen, 2000). Cholesterol also plays an important role in modulating the function of many proteins in the brain. This modulation either involves direct binding of cholesterol with proteins or alterations in physical properties of neural membranes, such as fluidity, curvature, and stiffness. At the cellular level, cholesterol is synthesized by oligodendrocytes, astrocytes, and neurons, particularly during early development along with myelin synthesis. Plasma lipoproteins are unable to deliver cholesterol to the brain. The de  novo synthesis of cholesterol in the brain is an energy-consuming process comprising multiple intermediates and requiring many enzymes. Although biosynthesis mainly occurs in the endoplasmic reticulum, some cholesterol is also synthesized in peroxisomes (Kovacs et al., 2001). Acetyl-CoA and acetoacetyl-CoA are transformed into 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). HMG-CoA reductase converts HMG-CoA into mevalonate, which is phosphorylated to isopentenyl pyrophosphate (IPP) and other active isoprenoid units. These units condense to form a 10-carbon lipid GPP, 15-carbon lipid FPP, and the 20-carbon lipid GGPP. These reactions are catalyzed by farnesylpyrophosphate, and geranylgeranyl-pyrophosphate synthases. Formation of FPP is a branch point, where the synthesis of squalene starts. This reaction is catalyzed by squalene synthase. Two alternative pathways exist in the later stages of cholesterol synthesis. Squalene is converted to lanosterol, followed by desmosterol and cholesterol (Figs. 10.4 and 10.5). This is called as Bloch pathway, whilst the synthesis of cholesterol via the conversion of lanosterol to lathosterol, and conversion of lathosterol to 7-dehydrocholesterol is called as Kandutsch–Russell pathway (Lütjohann et al., 2002; Thelen et al., 2006) (Figs. 10.4 and 10.5). In addition to cholesterol, mevalonate is also metabolized to heme, ubiquinones, and vitamin D. Cholesterol contents of neural membranes are tightly regulated by several mechanisms that balance the level of cholesterol synthesis, uptake, and efflux (Farooqui et al., 2007). Accumulating evidence suggests that nuclear receptors, a superfamily of ligand-activated transcription factors provide an indispensable regulatory framework in modulating and controlling genes for cholesterol metabolism. The mechanism of transcriptional regulation by nuclear receptors, such as LXRs involves formation of heterodimers with RXRs, which is activated by 9-cis-retinoic acid (a vitamin A derivative). Upon heterodimerization, LXR/RXR binds to specific DNA sequences called LXR-responsive elements (LXREs) in the target genes (Fig. 10.6).

10.2  Synthesis of Cholesterol and Hydroxycholesterols in the Brain

271

Acetyl-CoA + Acetoacetyl-CoA 1 HMG-CoA Statins

2 Mevalonate

IPP 7-Dehydrocholesterol GPP

Kandutsch-Russell pathway

Dolichol

3 Cholesterol

CYP51

Lanosterol Bloch pathway

Desmosterol

Squalene

4

FPP 5 GGPP

Farnesylated Proteins (Ras)

Ubiquinone

NADPH oxidase

eNOS

6 Geranylgeranylated proteins (RhoA, RhoB, Rap1)

Fig. 10.4  Biosynthesis of cholesterol in the brain. HMG-CoA synthase (1), HMG-CoA reductase (2), FPP synthase (3), squalene synthase (4), farnesyltransferase (5), GGPP synthase (5), and gernylgeranyltransferase (6). Isopentenylpyrophosphate (IPP), geranylpyrophosphate (GPP), farnesylpyrophosphate (FPP), farnesyl protein transferase (FPTase), geranylgeranylpyrophosphate (GGPP), geranylgeranyl protein transferase (GGPTase), and endothelial nitric oxide synthase (eNOS)

Thus, LXR/RXR functions as a sensor of cellular cholesterol concentration and mediates cholesterol efflux by inducing the transcription of key cholesterol shuffling vehicles, namely, ATP-binding cassette transporter A1 (ABCA1) and ApoE. ABCA1 is a membrane protein that facilitates the formation of ApoE-cholesterolphospholipid complex and requires the hydrolysis of ATP for its activity, whereas ApoE is a cholesterol transport protein that plays a central role in the brain response to injury and neurodegeneration in mammalian species (Fig. 10.7). In addition, biosynthesis of cholesterol is regulated by insulin-induced genes (INSIGs) and sterol regulatory element binding proteins (SREBP), in particular SREBP-2 and controlled through feedback regulation by sterol, including cholesterol itself. SREBP2 controls the expression of enzymes involved in cholesterol biosynthesis, including the rate-limiting enzyme HMG-CoA reductase (HMGR) (Brown and Goldstein, 1997; DeBose-Boyd, 2008). In mammals, most growth and differentiation of the brain occurs in the first few weeks or years after birth, and the cholesterol required for such growth apparently comes exclusively from de novo synthesis. The highest rate of synthesis occurs during first postnatal weeks in humans and rodents (Jurevics and Morell, 1995; Jurevics et al., 1997). As the brain matures and cholesterol pools in the brain become constant, the rate of de novo synthesis of cholesterol in the brain

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a

b

HO

HO

c

d

HO

HO

e

HO

Fig. 10.5  Chemical structures of various intermediates of cholesterol biosynthesis. Lanosterol (a), lthosterol (b), 7-dehydrocholesterol (c), zymosterol (d), and desmosterol (e)

markedly declines. The Bloch pathway contributes more to cholesterol biosynthesis in young animals, whereas the Kandutsch–Russell pathway is more involved in aged animals (Thelen et al., 2006; Ong et al., 2010). The majority of brain cholesterol is present in myelin sheets and whilst there is a clear role of oligodendrocyte cholesterol in myelin synthesis, the respective roles of neurons and glial cells in cholesterol biosynthesis in the adult brain are less well understood (Pfrieger, 2003). As stated above, the accepted view is that in the adult nervous system, astrocytes are responsible for brain cholesterol, and synaptic plasticity is modulated by cholesterol homeostasis, which is controlled by cell export proteins and lipoproteins such as apolipoprotein E (apoE) (Fig. 10.7). The latter plays an important role in the translocation of cholesterol from astrocytes to neurons (Levi et al., 2005). Compared to glial cells, neurons show different profiles of cholesterol biosynthetic enzymes, post-squalene sterol precursors and cholesterol metabolites, and produce cholesterol less efficiently possibly due to low levels of lanosterol-converting enzymes (Pfrieger, 2003; Nieweg et al., 2009). Nevertheless, there have been suggestions that besides glial cells, adult neurons are also capable of cholesterol synthesis (Saito et al., 1987; Suzuki et al., 2007).

10.2  Synthesis of Cholesterol and Hydroxycholesterols in the Brain

273

A1 A2

R1

R1

cPLA2

PlsEtn-PLA2 Hydroxysterols (-) ER

ARA

DHA 15-LOX Docosanoids

-

PPAR RXR

(-)

5-LOX

Cholesterol

ROS

Eicosanoids

LXR RXR

NF-kB

Receptor dimerization Mature SREBP LXR RXR LXRE Posttranslational modification

PPAR RXR SRE mRNA for cholesterol synthesizing enzymes, Transcription & cholesterol efflux

NF-kB-RE

PPRE Fatty acid desaturase & elongase mRNA & transcription

mRNA for cytokines, proinflammatory Enzymes , & transcription

NUCLEUS

Fig. 10.6  Transcriptional regulation of cholesterol homeostasis regulating enzymes, fatty acid desaturases and elongases, and proinflammatory enzymes and cytokines in the brain. Agonists (A1 and A2), receptors (R1 and R2), plasmalogen (PlsEtn), phosphatidylcholine (PtdCho), plasmalogen-selective phospholipase A2 (PlsEtn-PLA2), cytosolic phospholipase A2 (cPLA2), 15-lipoxygenase (15-LOX), 5-lipoxygenase (5-LOX), arachidonic acid (ARA), liver X receptor (LXR), liver X receptor response element; retinoid receptor (RXR), sterol regulatory element binding protein (SREBP), peroxisome proliferator alpha (PPARa), sterol response element (SRE), nuclear factorkB (NF-kB), nuclear factor-kB response element (NF-kB-RE), endoplasmic reticulum (ER), and negative sign (−) indicate inhibition

Astrocytes synthesize and secrete two- to threefold more cholesterol than neurons, and secrete it as lipoprotein particles, which serve as cholesterol carriers (DeMattos et al., 2001; Vance et al., 2005). Although neurons synthesize enough cholesterol to survive and grow, the formation of numerous mature synapses during synaptogenesis demands additional amounts of cholesterol, which is provided by astrocytes (Fig. 10.7). Thus, the availability of cholesterol appears to limit synaptogenesis and development of mature synapse. This may explain the delayed onset of synaptogenesis after glia differentiation and neurobehavioral manifestations of defects in cholesterol or lipoprotein homeostasis (Mauch et  al., 2001; Pfrieger, 2003). Inhibition of cholesterol synthesis by inhibitor in hippocampal slices reduces synaptic plasticity. This observation supports the view that one of the major roles of cholesterol is to modulate neural membrane plasticity and that it can cause either a positive or negative effect depending upon membrane constituents. The enrichment of cholesterol in lipid rafts may be responsible for reduction in lateral mobility of raft components. Collective evidence

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+

+

de novo synthesis

Memory consolidation

CYP46

Cholesterol

ABCA1 ABCG1

Astrocyte

Nucleus 27-Hydroxycholesterol

O2

NADPH + H+ NADP+

24-Hydroxycholesterol

Neuron

Acetate

LXR

+ ApoE

ApoE

Cholesterol

27-Hydroxycholesterol

Brain

X BBB 24-Hydroxycholesterol

Cholesterol

27-Hydroxycholesterol

Blood Metabolism

Fig. 10.7  Interactions between cholesterol metabolism of neurons and astrocytes. CYP46 (24-cholesterol hydroxylase), liver X receptor (LXR), ATP-binding cassette A1 (ABCA1), ATPbinding cassette G1 (ABCG1), and blood–brain barrier (BBB)

suggests that supply of astrocyte-derived cholesterol is necessary not only for fine-tuning neural cholesterol dynamics for basic synapse function, but also for optimal neuroplasticity and behavior. In addition, cholinergic function, ionotropic and metabotropic receptor machinery, and neural oxidative stress reactions are also modulated by brain cholesterol homeostasis (Nelson and Alkon, 2005).

10.3  Degradation of Cholesterol in the Brain

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10.3 Degradation of Cholesterol in the Brain Four cytochrome P450-dependent hydroxylases, namely, CYPs 7A1, 27A1, 11A1, and 46A1 are known to occur in neural and non-neural tissues. These enzymes play key roles in cholesterol homeostasis through the elimination of excess cholesterol through blood–brain barrier into blood from where it is taken to liver by plasma lipoproteins. CYP46A1 enzyme is a microsomal enzyme, which is selectively expressed in hippocampal and cortical neurons, and is responsible for the majority of cholesterol turnover in the central nervous system (Ramirez et al., 2008). Substrate specificity of P450 46A1 is not limited to cholesterol. A number of other structurally related steroids are also hydroxylated by this enzyme. Hydroxysterols bind two different targets, liver X receptors (LXRs) and oxysterol-binding proteins (ORPs). Among three major naturally occurring hydroxysterols (27-hydroxycholesterol, 24(S)hydroxycholesterol, and 25-hydroxycholesterol), only 25-hydroxycholesterol is known to bind ORPs. The brain-specific 24(S)-hydroxycholesterol and the peripherally synthesized 27-hydroxycholesterol have been reported to be primarily ligands for LXR ligands (Russell, 2000; Fu et al., 2001). LXR ligands increase cholesterol efflux by inducing the expression of cholesterol-binding (ApoE and ApoD) proteins and cholesterol transporters belonging to the ATP-binding cassette (ABC) family (Zelcer and Tontonoz, 2006). Furthermore, LXRs produce their anti-inflammatory effects by antagonizing NF-kB-induced gene expression (Joseph et  al., 2003) (Fig. 10.8). The mechanism associated with the anti-NF-kB activity of LXRs involves the recruitment of sumoylated LXR to the NF-kB motif, which then retards the degradation of associated nuclear receptor corepressor (N-CoR) complexes (Ghisletti et al., 2007). In addition, LXR signaling also represses TLR4-induced expression of iNOS, COX-2, and IL-6 in murine macrophages. Disruption of the cholesterol 24-hydroxylase gene in the mouse reduces both cholesterol turnover and synthesis in the brain, but does not alter steady-state levels of cholesterol in the tissue. Mice deficient in 24-hydroxylase exhibit impaired learning and defective hippocampal long-term potentiation, suggesting that the metabolism of cholesterol by this enzyme is required for learning and memory formation (Ramirez et  al., 2008). Accumulating evidence suggests that neurons meet the majority of their cholesterol supply from astrocytes not only by ApoE-dependent transport, but also through the sterol transporters ATP-binding cassette A1 (ABCA1) and G1 (ABCG1), which are present in glial cells and under control of LXR. The other target of hydroxycholesterol is linked to APP processing. Oxysterol Binding Protein 1 (OSBP1) overexpression or silencing reduces or increases APP processing through the regulation of APP trafficking (Zerbinatti et al., 2008). Studies on the expression pattern of Cyp27A1 and other related genes in primary glial and neuronal cultures of rat brain indicate that primary astrocytes express different sterol hydroxylases and are able to uptake exogenous 27-hydroxycholesterol (Gilardi et al., 2009). Since astrocytes are associated with maintenance of homeostasis in the brain, it is hypothesized that impairment of CYP27 activity in astrocytes may alter their role from providing delivery of cholesterol to neurons to the release of signaling molecules (Gilardi et al., 2009).

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10  Cholesterol and Hydroxycholesterol in the Brain Cholesterol efflux

PtdCho

+

cPLA2

1 (-)

2+

Ca

sPLA2

Cholesterol hydroxylases

Hydroxycholesterols

Positive loop (+)

ROS

Eicosanoids

+

ApoE

-

Lyso-PtdCho + ARA

PAF

Cholesterol

ABCA1

NMDAR

Glu

(O2-, OH., H2O2)

+

+

IkB/NFkB

+

Oxidative stress Neuroinflammation

IkB

-

NF-kB-RE

Apoptosis

LXR COX-2 sPLA2 iNOS TNF-α IL-1β IL-6

RXR

Genes related to inflammation Suppression of Arc

Cholesterol efflux COX-2, iNOS IL-1β,& IL-6

PLTP1, ABCA1. ApoE, & LPL

Fig. 10.8  Interactions between hydroxycholesterols and PLA2-mediated inflammatory signaling. Phosphatidylcholine (PtdCho), cytosolic phospholipase A2 (PLA2), secretory phospholipase A2 (sPLA2), lysophosphatidylcholine (lyso-PtdCho) arachidonic acid (ARA), platelet-activating factor (PAF), reactive oxygen species (ROS), N-Methyl-d-aspartate receptor (NMDA-R), nuclear factorkB (NF-kB), nuclear factor-kB response element (NF-kB-RE), retinoic acid receptor (RXR), liver X receptor (LXR), cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), tumor necrosis factor-a (TNF-a), and interleukin1b (IL-1b) upward arrows indicate increase and downward arrows indicate decrease

In liver, CYP27A1 is a microsomal enzyme, which transforms cholesterol into 7-a hydroxycholesterol. This enzyme is also found in mitochondria, where it metabolizes cholesterol to 27-hydroxycholesterol. CYP27A1 is a polyfunctional enzyme that in addition to generating 27-hydroxycholesterol and 3b-hydroxy-5cholestenoic acid in extra-hepatic tissues also oxygenates bile acid intermediates in the liver and vitamin D3 in the kidney (Masumoto et al., 1988). 27-Hydroxycholesterol and 3b-hydroxy-5-cholestenoic acid are ligands for the nuclear receptors, LXRa and LXRb. These receptors activate the transcription of several genes involved in lipid metabolism (Song and Liao, 2000). Catalytic efficiencies of cholesterolmetabolizing P450-dependent hydroxylases vary significantly in brain, liver, lungs, and probably reflect physiological requirements of different organs for the rate of cholesterol turnover. Collective evidence suggests that cytochrome P450-dependent hydroxylases (CYP7A1, 27A1, 11A1, and 46A1) represent a unique system that

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facilitates elimination of cholesterol from different tissues to fit their needs (Pikuleva, 2006). 24(S)-hydroxycholesterol plays a pivotal role in promoting altered inflammatory signaling, apoptotic responses, and Alzheimer disease (AD)-type changes in brain (Schippling et  al., 2000; Michikawa, 2004; Vaya and Schipper, 2007) (See below). In addition, 24(S)-hydroxycholesterol stimulates the synthesis and secretion of ApoE, an effect which may be modulated by the ApoE allele (Abildayeva et al., 2006). ATP-Binding Cassette (ABC) transporters, particularly the A and G subfamilies play important roles in cholesterol efflux (Borst et al., 2000). ABCA1 is known to play a critical role in peripheral lipid transport by regulating cholesterol efflux from the plasma membrane to the lipid acceptor protein-AI (apoA-I). High expression of ABCA1 occurs in neurons of hypothalamus, thalamus, amygdala, cholinergic basal forebrain, and hippocampus in the adult rat brain (Koldamova et al., 2003). ABCA1 expression is known to be regulated by LXRs (Venkateswaran et al., 2000). In liver, cholesterol metabolite, 24(S)-hydroxycholesterol is efficiently converted into bile acids and excreted in bile in its sulfated and glucuronidated form (Pikuleva, 2006).

10.4 Roles of Hydroxycholesterols in the Brain As stated above, cholesterol homeostasis is maintained through the oxidation of cholesterol to several hydroxycholesterols and cholesterol oxidation products (COPs). Among hydroxyl-cholesterols and COPs, 24(S)-hydroxycholesterol is a major metabolite of cholesterol metabolism (Russell, 2000; He et al., 2006). The conversion of cholesterol to 24(S)-hydroxycholesterol is catalyzed by the cholesterol24-hydroxylase (CYP46), an enzyme, which is enriched in the striatum and the cortex. Knocking out cholesterol 24-hydroxylase in mice produces 40% reduction in de novo cholesterol synthesis in the brain (Lund et al., 1999). In the human adult, brain cholesterol turnover gives rise to a daily flux of about 7  mg of 24(S)hydroxycholesterol from the brain into the circulation, so that 24-hydroxycholesterol levels in plasma are taken as an index of brain cholesterol elimination (Lütjohann et  al., 1996). Although downregulation of cholesterol synthesis via the SREBP/ SCAP regulatory pathway is common for 24(S)-hydroxycholesterol synthesis, more variations exist with respect to other hydroxycholesterols, which may also act as ligands for the nuclear receptor LXRa. Because this receptor regulates the expression of cholesterol 7a-hydroxylase and ABC transporter proteins, hydroxycholesterols and their intermediate steroid metabolites modulate a number of biological processes, including differentiation, exocytosis, enzyme activities, and immune function (Schroepfer, 2000) (Figs. 10.3 and 10.8). In addition, hydroxycholesterols also modulate cholesterol trafficking, gene transcription, PtdCho synthesis, inflammation, and cognitive function. In addition to having a direct physical effect on membranes and modulation of Ca2+ signals, hydroxysterols also inhibit the phosphorylation of endothelial nitric oxide synthase (eNOS) and cPLA2 through LXRmediated mechanism (Fig. 10.8).

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10.4.1 Hydroxycholesterols in Neural Cell Differentiation Cellular differentiation is defined as a process by which a generic cell transforms into a specific type of cell in response to specific triggers from the body or the cell itself. This process results in the maturation of a less specialized cell into a more specialized cell type in order to perform a specific function without any alterations in the genetic material. In multicellular organisms differentiation occurs many times not only during the development, but also in adulthood (Sue O’Shea, 2002). Hydroxycholesterols modulate neural cell differentiation and cell death in various types of neural cell cultures. Thus, 22 (R)-hydroxycholesterol, a steroid intermediate in the pathway of pregnenolone formation from cholesterol, stimulates neurite outgrowth formation in rat pheochromocytoma cells (PC12 cells) and differentiated human Ntera2/D1 teratocarcinoma neurons. Although the mechanism associated with neuronal differentiation is not fully understood, it is proposed that 22 (R)hydroxycholesterols interact with LXRs through NF-kB-dependent mechanism and modulate neuronal differentiation in rat PC12 cells. No neurite outgrowth formation occurs without the participation of LXRs (Schmidt et al., 1999). In human Ntera2/ D1 teratocarcinoma precursor cells (NT2), 22(R)-hydroxycholesterol inhibits the proliferation, but induces differentiation in neurons and astrocytes to produce “neuron-like” or “astrocyte-like” cells (Yao et  al., 2007). 22 (R)-Hydroxycholesterolmediated differentiation of NT2 cells is accompanied by elevations in the expression of neurofilament protein NF200, the cytoskeletal proteins microtubule-associated protein type II (MAP2a and MAP2b), glial fibrillary acidic protein (GFAP) and glial cell line-derived neurotrophic factor receptor-a 2 (GFRa 2). These effects are produced only by 22(R)-hydroxycholesterol. 22(S)-Hydroxycholesterol, an enantiomer of 22(R)-hydroxycholesterol, does not produce differentiation and has no effect on the expression of above parameters indicating that these effects are stereospecific. Furthermore, other steroids have also failed to induce differentiation in NT2 cells (Yao et al., 2007). Similarly, 22 (R)-hydroxycholesterol also modulates neurosteroid formation in SH-SY5Y cells. This process is inhibited by the cytP450scc inhibitor, aminoglutethimide. Treatments of differentiated SH-SY5Y cells with the microtubule-depolymerizing drug colchicine and the actin microfilamentaltering agent cytochalasin D reduces pregnenolone synthesis without affecting cell viability. Since the addition of the cell-permeant cholesterol analog, 22(R)hydroxycholesterol promotes pregnenolone synthesis, it is proposed that modulation of progesterone synthesis by 22(R)-hydroxycholesterol may be associated with neuronal plasticity, cognitive function, and also neurodegenerative disorders (Guarneri et al., 2000). 22-Hydroxycholesterol also regulates cell division, ventral midbrain (VM) neurogenesis, and dopaminergic (DA) neuronal development (Sacchetti et al., 2009). The deletion of LXRa and LXRb slows cell cycle progression and VM neurogenesis, causing a decrease in DA neurons at birth. Activation of LXRa and LXRb with hydroxycholesterols increases the number of DA neurons in mouse embryonic stem cells (ESCs) and in wild-type, but not in LXRa and LXRb(−/−) VM progenitor cultures. Likewise, oxysterol treatment of human ESCs (hESCs)

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during DA differentiation upregulate neurogenesis and the number of mature DA neurons, while reducing proliferating progenitors (Sacchetti et al., 2009). In contrast, 25-hydroxycholesterol produces neurotoxicity in NGF-differentiated PC12 cells in dose- and time-dependent manner (Chang et al., 1998a). 25-Hydroxycholesterolmediated cell death can be prevented by treatment with vitamin E and methyl-betacyclodextrin (Chang et al., 1998b). This suggests that 25-hydroxycholesterol-mediated cell death may involve oxidative stress.

10.4.2 Hydroxycholesterols in Exocytosis Exocytosis is defined as a process by which intracellular vesicles containing soluble proteins, lipids, and neurotransmitters are released into the extracellular compartment. This process involves binding of vesicles with plasma membrane and discharge of vesicles out of the cell. Hydroxycholesterols (7-ketocholesterol, 24(S)-hydroxycholesterol, and cholesterol 5, 6 b-epoxide) have reported to induce exocytosis in PC12 cells (Ma et al., 2010). 7-Ketocholesterol-mediated exocytosis can be attenuated by pretreatment with methyl-beta-cyclodextrin (MbCD). Moreover, treatment of PC12 cells with thapsigargin, an agent that depletes intracellular calcium, or addition of lanthanum chloride, an agent that blocks calcium channels also attenuates 7-ketocholesterol-mediated exocytosis in PC12 cells. Fura-2 imaging studies indicate that treatment with 7- ketocholesterol results in rapid and sustained increases in intracellular calcium levels, and this effect can also be attenuated by MbCD, thapsigargin, or lanthanum chloride (Ma et al., 2010). Collectively, these results indicate that by 7-ketocholesterol-mediated neurotransmitter release depends on the integrity of cholesterol-rich lipid domains on PC12 membranes and a rise in intracellular calcium, either through release from internal stores or influx via calcium channels may be closely associated with exocytosis.

10.4.3 Hydroxycholesterols in Apoptosis Apoptosis is characterized by nuclear chromatin condensation, DNA fragmentation, cell shrinkage, and bleb and apoptotic body formation (Farooqui et  al., 2004). Plasma membrane and other subcellular organelles such as mitochondria and endoplasmic reticulum remain active during apoptosis. Neurochemically, apoptosis is characterized by the externalization of PtdSer from inner leaflet to the outer leaflet, cytochrome c release, caspase-3 activation, and DNA ladder formation. Apoptosis is a characteristic feature of the normal developmental process as well as a response of cells to stress or other environmental insults (Oppenheim, 1991). While the mechanisms leading to apoptotic cell death are not yet understood, several factors including expression of bcl-2 or bcl-xL genes and neurotrophic survival factors have been reported to prevent apoptotic cell death (Farooqui, 2009). Hydroxycholesterols are

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Table 10.1  Neurochemical effects of various hydroxycholesterols in brain Mediator Effect Reference Velázquez et al., 2006; 24-Hydroxycholesterol Modulation of cholesterol homeostasis, Ong et al., 2003 apoptosis, oxidative stress, and inflammation 25-Hydroxycholesterol Modulation of apoptosis, oxidative Kim et al., 2006; Ong stress, and inflammation et al., 2003 27-Hydroxycholesterol Elevation in Ab, suppression of Arc, Velázquez et al., 2006; apoptosis, oxidative stress Lizard et al., 2000; Mateos et al., 2009 22-Hydroxycholesterol Modulation of neurogenesis Sacchetti et al., 2009 7-Ketocholesterol Modulation of apoptosis, oxidative Chang et al., 1998a stress, and inflammation Ab beta amyloid, Arc activity-regulated cytoskeleton-associated protein

cytotoxic to human neuroblastoma cell line, SH-SY5Y, oligodendrocyte cell line (158N), and endothelial cells. In SH-SYSY and 158N cells, 24(S)-hydroxycholesterol and 25-hydroxycholesterol induce apoptotic cell death through the stimulation of caspase-3 (Kolsch et al., 2001; Trousson et al., 2009; Rantham Prabhakara et al., 2008) (Table 10.1). This enzyme causes proteolytic cleavage of a variety of enzymes (protein kinase C, cytosolic phospholipase A2, calcium-independent phospholipase A2, phospholipase C), cytoskeletal proteins (a-spectrin, b-spectrin, actin, vimentin, Bcl-2 family of apoptosis-related proteins), and DNA modulating enzymes (poly [ADPribose]polymerase) (Kolsch et al., 2001; Garcia et al., 1992). Other oxysterols, such as 25-hydroxycholesterol, 7b-hydroxycholesterol, and 7-ketocholesterol, also produce toxic effects on neural cell cultures (Chang et  al., 1998a; Ong et  al., 2003). 7b-Hydroxycholesterol, and 7-ketocholesterol modulate secretion of IL-1b, which is synthesized as an inactive 33 kDa propeptide. Proteolytic cleavage of this propeptide produces active IL-1b. The release of this cytokine facilitates apoptotic cell death. Studies on the effect of 7-ketocholesterol and 1-methyl-4-phenylpyridinium (MPP+) in differentiated PC12 cells indicate that 7-ketocholesterol significantly increases the MPP+-mediated nuclear damage, reduces the mitochondrial transmembrane potential, increases cytosolic cytochrome c, activates caspase-3, enhances reactive oxygen species production, and depletes reduced glutathione (Kim et al., 2006). N-Acetylcysteine, ascorbate, trolox, and calmodulin antagonists block the cytotoxic effect of MPP+ in the presence of 7-ketocholesterol indicating that 7-ketocholesterol produces a synergistic effect with MPP+ (Kim et al., 2006; Han et al., 2007). The molecular mechanism associated with hydroxyl- and ketocholesterolmediated toxic effect is not fully understood. However, 7-ketocholesterol triggers the stimulation of NADPH oxidase, generation of superoxide anions, loss of mitochondrial transmembrane potential (∆Ym), release of cytochrome c, and activation of caspase-3. These processes are closely associated with apoptotic cell death (Lizard et al., 2000). The presence of 24-hydroxycholesterol and 25-hydroxycholesterol in plasma is an indication of neurodegeneration in brain. Other cholesterol-derived metabolite, such as 7-oxocholesterol not only modulates Ca2+ signals, but also inhibits the phosphorylation of endothelial nitric oxide synthase and cPLA2

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(Millanvoye-Van Brussel et al., 2004). Collective evidence suggests that hydroxysterols have neurotoxic effects on neural cell cultures, and among them 24-hydroxycholesterol can be used as a marker for neurodegeneration (Rojo et al., 2006).

10.5 Cholesterol and Hydroxycholesterols in Neurological Disorders Intake of cholesterol enriched high fat diet not only causes BBB dysfunction in New Zealand white rabbits (Ghribi et al., 2006), but also results in cerebral amyloidosis in wild-type rabbits (Sparks et  al., 1994), suggesting that high fat cholesterolenriched diet may be a risk factor for neurodegenerative diseases, such as AD. Epidemiological and biochemical investigations also indicate that there is a link between cholesterol turnover and neurological diseases, and hypercholesterolemia per-se is an important risk factor for AD, PD, and HD (Wolozin, 2004; Huang et al., 2007; Wolozin et al., 2007). These diseases involve the accumulation of intracellular or extracellular cerebral deposits of misfolded protein called amyloidogenic proteins. Thus, accumulation of b-amyloid (Ab), a-synuclein, and huntingtin occurs in AD, PD, and HD respectively (Farooqui, 2010). Although molecular mechanisms by which amyloidogenic proteins produce neurodegeneration are not fully understood, marked increase in oxidative stress and neuroinflammation along with elevation in hydroxycholesterol levels and lipid mediators of phospholipid and sphingolipid metabolism have been reported to occur in AD, PD, and HD (Farooqui, 2010). In addition, increase in hydroxycholesterol levels also occurs in multiple sclerosis (MS), ischemic injury, and traumatic brain injury (TBI), Niemann-Pick disease (NPD), and cerebrotendinous xanthomatosis (CTX) (Fig. 10.9).

Traumatic brain injury

Hydroxycholesterols

Ischemia

Cerebrotendinous xanthomatosis

Niemann-Pick type C

Alzheimer disease

Multiple sclerosis Huntington disease

Fig. 10.9  Alterations in hydroxycholesterol in neurological disorders upward arrows indicate increase in hydroxycholesterol

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10.5.1 Cholesterol and Hydroxycholesterol in Alzheimer Disease It is well known that AD is characterized by progressive neuronal degeneration, gliosis, and accumulation of plaques and tangles, which are aggregates of amyloidb (Ab) peptides derived from proteolytic cleavages of amyloid precursor protein (APP), and hyperphosphorylated tau protein, respectively (Farooqui, 2010). APP is processed via a- or the b-secretase pathways (Thinakaran and Koo, 2008), resulting in the shedding of nearly the entire ectodomain and generation of membranetethered b-or a-C-terminal fragments, respectively. The b- and a-C-terminal fragments are subsequently cleaved within the transmembrane domain by g-secretase. Cholesterol directly interacts with APP and stimulates its insertion into phospholipid monolayers (Fantini and Yahi, 2010). It also interacts with Ab protofibrils. However, it remains controversial whether cholesterol enhances or reduces Ab polymerization. Since, the generation of Ab peptides through APP proteolysis occurs within lipid rafts and is sensitive to inhibitors of cholesterol biosynthesis, its association with cholesterol homeostasis in AD may not be simply through the regulation of Ab fibrillogenesis (Wolozin, 2004; Fantini and Yahi, 2010), but other parameters may also play an important role in the above process. Association of sterol homeostasis, particularly cholesterol metabolism, is currently the subject of intense interest in the pathogenesis of AD. In AD, increase in lipid peroxidation not only intensifies neuronal death, but also induces the synthesis of oxidized cholesterol (hydroxysterols) (Arca et al., 2007). These events directly or indirectly may promote APP processing in favor of insoluble b-amyloid deposition (Nelson and Alkon, 2005), tau hyperphosphorylation and NFT formation, mitochondrial insufficiency, and neuronal cell death (Melov et al., 2007). Thus, Ab and APP are known to oxidize cholesterol to form 7b-hydroxycholesterol that is a pro-apoptotic oxysterol with neurotoxicity at nanomolar concentrations. 7b-Hydroxycholesterol also blocks the secretion of soluble APP from cultured rat hippocampal H19-7/IGF-IR neuronal cells (Nelson and Alkon, 2005). Levels of 24(S)-hydroxycholesterol in plasma and cerebrospinal fluid (CSF) are significantly higher in AD and vascular demented patients at early stages of the disease compared to healthy subjects (Lutjohann and von Bergmann, 2003). Statins, the inhibitors of cholesterol biosynthesis, modulate APP processing and neuronal secretion of Ab along with de novo cholesterol synthesis. High-doses of simvastatin (80 mg/day) given to AD patients with hypercholesterolemia not only lead to a significant decrease in CSF concentrations of 24(S)-hydroxycholesterol, but also a significant decrease in Ab-levels in cerebrospinal fluid supports the view that 24-hydroxycholesterol may be associated with the pathogenesis of AD (Lutjohann and von Bergmann, 2003). 24(S)-Hydroxycholesterol exerts its effect on APP processing by increasing the a-secretase activity as well as the a/b-secretase activity ratio (Famer et al., 2007). In CSF from AD patients, levels of ApoE, tau, p-tau (hyperphosphorylated tau) are significantly increased along with 24(S)hydroxycholesterol compared to CSF from control subjects. It is stated that during neurodegenerative process cholesterol is converted into 24(S)-hydroxycholesterol. The release of 24(S)-hydroxycholesterol from neurons induces ApoE secretion by

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astrocytes, and both processes are related to the intensity of the neurodegenerative process and neuronal injury (Leoni et al., 2010). ApoE is also involved in the scavenging of tau from neurons. The direct correlations between ApoE, 24(S)hydroxycholesterol, and tau suggest that cholesterol metabolism may be involved in the generation of both tau and Ab, and ApoE is released by astrocytes in order to counteract this ongoing process (Leoni et al., 2010). Levels of 22(R)-hydroxycholesterol, a steroid intermediate in pregnenolone synthesis pathway are decreased in hippocampal and frontal cortex samples from AD patients compared to age-matched controls (Yao et al., 2002). 22(R)-Hydroxycholesterol has been shown to protect PC12 and differentiated human Ntera2/D1 teratocarcinoma (NT2N) neurons from Ab toxicity and death. The effect of 22(R)-hydroxycholesterol seems to be stereospecific because its enantiomer 22(S)-hydroxycholesterol fails to protect cultured cells from Ab-induced cell death. Moreover, the effect of 22(R)-hydroxycholesterol is specific for Abinduced cell death because it does not protect against glutamate-induced neurotoxicity. It is proposed that 22(R)-hydroxycholesterol offers a new means of neuroprotection against Ab toxicity. There is a significant flux of another oxysterol, 27-hydroxycholesterol from the circulation into the brain. Oxysterols also inhibit HMG-CoA reductase activity in astrocytes. 25-Hydroxycholesterol has a dual effect on cell proliferation: at higher concentrations it produces cell proliferation in a dose-dependent manner, while at lower concentrations it promotes neural cell survival (Velázquez et al., 2006). Cholesterol-enriched diets and hydroxycholesterol (27-hydroxycholesterol) increase Ab and phosphorylated tau levels in rodents. Although the mechanisms by which cholesterol and 27-hydroxycholesterol modulate the production of Ab and tau protein phosphorylation is not fully understood, but in transgenic mice for AD, leptin, an adipocytokine, has been reported to control and modulate Ab production and tau protein hyperphosphorylation. Recent studies indicate that feeding rabbits with a 2% cholesterol-enriched diet for 12 weeks decreases the levels of leptin by approximately 80%. Incubation of hippocampal organotypic slices with 27-hydroxycholesterol not only reduces leptin levels by approximately 30%, but also increases Ab(1–40) and Ab(1–42) levels to 1.5-fold and 3-fold, respectively along with elevation in phosphorylated tau protein levels. Treatment of hippocampal organotypic slices with leptin retards the 27-hydroxycholesterol-mediated increase in Ab and phosphorylated tau protein by decreasing the levels of BACE-1 and GSK-3b, respectively. Accumulating evidence suggests that cholesterolenriched diets and 27-hydroxycholesterol induce AD-like pathology by altering leptin signaling (Marwarha et al., 2010). Although the molecular mechanism associated with 27-hydroxycholesterol-mediated effect is not known, this metabolite is not only known to increase Ab production, caspase 12 activity, and gadd153 (also called CHOP) expression, but also decreases mitochondrial membrane potential, increases levels of the nuclear factor-kB (NF-kB) and heme-oxygenase 1 (HO-1) along with alterations in calcium homeostasis. Additionally, 27-hydroxycholesterol induces glutathione depletion, ROS generation, inflammation, and apoptotic-mediated cell death (Dasari et al., 2010). In addition, cholesterol-enriched diet turns on

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expression of genes associated with AD-like pathology in hippocampal and cortical area of mice brain (Mateos et al., 2009). This gene expression includes the expression of Activity-regulated cytoskeleton-associated protein (Arc). High cholesterol diet suppresses Arc in mice. This gene plays an important role in memory consolidation in rodents and its expression is reduced in autopsy brains obtained from AD patients (Ginsberg et al., 2000; Guzowski et al., 2000). The expression of Arc is regulated by NMDA receptor (Fig. 10.8). This glutamate receptor modulates excitotoxicity and alterations in its activities are closely associated with pathophysiology of AD (Farooqui, 2010). Treatment of hippocampal neuronal cultures with 27-hydroxycholesterol produces changes, which are similar to the in vivo effects of cholesterol on the expression of Arc and NMDA receptor. It is proposed that this may be one of the mechanism by which hypercholesterolemia may contribute to the pathogenesis of neurodegenerative diseases (Björkhem et  al., 2009). In addition, abnormalities in cholesterol metabolism correlate well with reports indicating that the possession of the apoE allele e4 is a strong risk factor for AD development, because there is a stepwise increase, as a function of alleles (e2 to e3 to e4), in serum total and low-density lipoprotein (LDL) cholesterol levels (Frikke-Schmidt et al., 2000). The finding that statin treatment decreases the prevalence of AD (Wolozin et al., 2000) supports the suggestion that reduction in serum total cholesterol level and LDL-cholesterol may result in decrease in cellular cholesterol level in the CNS, which in turn may retard the synthesis of amyloid b-protein (Ab) in neurons (Simons et al., 1998) and secretion of Ab from cells into the CSF (Fassbender et al., 2002). Collective evidence suggests that alterations in cholesterol and hydroxycholesterols levels may be associated with pathophysiology of AD.

10.5.2 Cholesterol and Hydroxycholesterol in Parkinson Disease Parkinson disease (PD) is a neurodegenerative disorders that is characterized by the gradual and selective loss of dopaminergic neurons in the substantia nigra pars compacta (Farooqui, 2010). Oxidative stress, inflammation, and a-synuclein overexpression are major risk factors for the development of PD and Lewy body dementia. Recent studies implicate cholesterol in the pathogenesis of PD, and it is shown that statins may lower the risk of PD (Johnson et al., 1999; Huang et al., 2007; Wolozin et al., 2007; Hu et al., 2008). Cholesterol and a-synuclein interact with each other in lipid rafts (Fortin et al., 2004) and that consumption of diet enriched in cholesterol increases the risk factor of developing PD. Studies on cell culture model and transgenic mice model of PD indicate that the cholesterol-depleting agent methyl-bcyclodextrin reduces the level of a-synuclein in membrane fractions (Bar-On et al., 2006). Moreover, metabolic inhibition of cholesterol biosynthesis with statins decreases the levels of a-synuclein accumulation in neuronal membranes, whereas cholesterol supplementation of cultured neurons enhances a-synuclein aggregation (Bar-On et al., 2008). These studies are consistent with recent reports indicating that lovastatin treatment of a-synuclein transgenic mice is accompanied by a marked

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reduction of a-synuclein aggregation and abrogation of neuronal pathology (Koob et al., 2010), supporting the view that treatment with statins may be beneficial for PD patients. It is also shown that levels of oxidized cholesterol metabolites are markedly increased in the brains of PD patients and oxidized cholesterol metabolites can enhance a-synuclein aggregation providing a mechanistic link between oxidative stress and development of PD (Bosco et al., 2006). Although the role of a-synuclein in brain remains elusive, recent studies indicate that a-synuclein may be associated with synaptic vesicle trafficking probably via lipid binding (Bar-On et al., 2006, 2008) and in neurotransmitter release. 27-Hydroxycholesterol increases the levels of a-synuclein and induces apoptosis, whereas 24-hydroxycholesterol increases the levels of tyrosine hydroxylase, the enzyme that catalyzes the synthesis of dopamine, reflective specificity of the actions of 27- versus 24-hydroxycholesterol (Rantham Prabhakara et al., 2008). The differential effects of 24- and 27-hydroxycholesterols on a-synuclein may be similar to the role of hydroxycholesterols in APP processing (Prasanthi et al., 2009; Marwarha et al., 2010) and on cholesterol storage in Niemann-Pick C1 disease (Frolov et al., 2003).

10.5.3 Cholesterol and Hydroxycholesterol in Huntington Disease Huntington Disease (HD) is a hereditary and progressive neurodegenerative disorder that is characterized by mid-life onset causing involuntary movements, cognitive, physical, and emotional deterioration, personality changes, dementia, and premature death. HD is characterized by neurodegeneration in the basal ganglia and cerebral cortex (Cepeda et al., 2001). The genetic defect in HD involves the presence of unstable CAG trinucleotide repeat in exon 1 of the HD gene, which encodes for a polyglutamine expansion near the N-terminal end of a large protein called huntingtin (a 348 kDa protein essential for embryogenesis). Insoluble aggregates containing huntingtin occur in cytosol and nuclei of HD patients, transgenic animal, and cell culture models of HD. Accumulation of mutant huntingtin along with transcriptional repression, proteasome impairment, oxidative injury, and mitochondrial dysfunction may lead to neurodegeneration in HD (Farooqui, 2010). Alterations in cholesterol metabolism occur in murine HD models and HD patients (Valenza et al., 2010; Leoni et al., 2008). The synthesis of cholesterol is reduced in multiple transgenic and knock-in HD rodent models. In animal model of HD, the cholesterol-dependent activities of neurons mainly rely on the transport of cholesterol from astrocytes on ApoE-containing particles. mRNA levels of cholesterol synthesizing enzymes and mRNA for cholesterol efflux transporters are severely reduced in primary HD astrocytes, along with impaired cellular production and secretion of ApoE (Valenza et al., 2010). Plasma 24(S)-hydroxycholesterol levels are significantly higher in controls than in HD patients at all disease stages (Leoni et al., 2008). In pre-HD, subject’s plasma levels of 24(S)-hydroxycholesterol have been reported to be similar to control subjects, and thus significantly higher than

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those of HD patients at any disease stage. Changes in 24(S)-hydroxycholesterol levels parallel the large decrease in caudate volumes, a process that occurs in HD stage 1 and is associated with neuronal loss (Leoni et  al., 2008). It is likely that dysregulation in cholesterol metabolism is linked to specific actions of the mutant huntingtin on sterol regulatory element-binding proteins leading to lower cholesterol levels in caudate areas of the brain. Neural cells expressing mutant huntingtin show increase in NMDA receptor density in cholesterol-enriched domains (del Toro et al., 2010). This increase in NMDA receptors contribute to increase in susceptibility to excitotoxic insults. Treatment of neural cells with simvastatin or b-cyclodextrin protects neural cells against NMDA-mediated excitotoxicity (del Toro et al., 2010). It is proposed that mutant huntingtin-mediated accumulation of cholesterol contributes to NMDA-mediated excitotoxicity. Emerging evidence suggests that dysregulation of cholesterol and sphingolipid metabolism may contribute to the pathogenesis of HD through increased sensitivity to excitotoxicity (del Toro et  al., 2010), and changes in 24(S)-hydroxycholesterol levels complement findings of MRI morphometry, which has become a valuable tool to follow neurodegenerative changes in the early stages of HD (Leoni et al., 2008).

10.5.4 Cholesterol and Hydroxycholesterols in Niemann-Pick Type C Niemann-Pick type C (NPC) disease is an autosomal recessive lysosomal storage disorder. At the subcellular level, NPC is characterized by improper trafficking of cholesterol, bis(monoacyl-glycerol)phosphate (BMP), and sphingolipids in lysosome-like storage organelles (LSOs), which become engorged with these lipids. Progressive neuronal loss, especially of cerebellar Purkinje cells, is a hallmark of NPC. Ballooned neurons, axonal abnormalities, and astroglyosis along with severe demyelination are pathological characteristics of NPC. It is suggested that the initial defect in cholesterol or sphingolipid trafficking causes eventual traffic jam in these LSOs resulting in not only retention of these lipids, but also transmembrane proteins, which transiently associate with the late endosomes in normal cells, on their way to other cellular destinations such as the trans-Golgi network (TGN) and plasma membrane (PM) (Mukherjee and Maxfield, 2004). Two cholesterol-binding proteins, NPC1 (a large transmembrane protein with 1278 amino acid residues) and NPC2 (an intra-lysosomal soluble protein comprising 132 amino acid residues) are known to occur in mammalian tissues. They act in concert to export lipoproteinderived cholesterol and sphingolipids from lysosomes to TGN and PM (Wiegand et al., 2003; Friedland et al., 2003). Thus, NPC1 plays a major role in the sorting of glycolipids as well as cholesterol within the late endosomes, whereas NPC2 primarily plays a role in the egress of cholesterol and, potentially, glycolipids from lysosomes. Mutations in either NPC1 or NPC2 are the cause of the NPC. The most common form of the disease (95% cases) is caused by mutation in NPC1gene and remaining cases result from mutation in the NPC2 gene. Mutation in NPC1 protein causes

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impairment of intracellular cholesterol trafficking and dysregulation of cholesterol biosynthesis. Disruption of cholesterol homeostasis also contributes to deregulation of autophagic activity and early-onset neuroinflammation, which may contribute to the pathogenesis of NPC disease. Cholesterol trafficking defects result in diminished production of cholesterol derivatives including neurosteroids, which normally help mediating brain development, growth, and differentiation (Mellon, 2007). NPC1 knockout mice show mitochondrial cholesterol accumulation, mitochondrial glutathione depletion, and release of apoptogenic proteins (Fernandez et al., 2009). NPC1 binds cholesterol and 25-hydroxycholesterol with higher affinity than 24(S)hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol (Infante et al., 2008). NPC2 binds a range of cholesterol-related molecules (cholesterol precursors, plant sterols, some oxysterols, cholesterol sulfate, cholesterol acetate, and 5-a-cholestan-3-one) (Liou et al., 2006). Immunoelectron microscopic study of the primate brain showed that NPC1 is localized primarily in astrocytic processes at the sides of synapses (Patel et  al., 1999), whilst NPC2 is observed mainly in small diameter dendrites or dendritic spines (Ong et al., 2004). Collectively, these studies suggest that alterations in hydroxycholesterol levels may contribute to the pathogenesis of NPC.

10.5.5 Cholesterol and Hydroxycholesterol in Multiple Sclerosis Multiple sclerosis is an inflammatory neurodegenerative condition that is characterized by the demyelination of axons in discrete regions of the brain and spinal cord and infiltration of activated macrophages into the brain parenchyma. The most accepted component of MS etiology is crossing of activated T-cells through the blood–brain barrier and initiation of an inflammatory response to myelin (Hartung and Rieckman, 1997). This is followed by scars or plaques formation in the damaged areas leading into faulty nerve conduction. Initial demyelination of CNS axons triggers catastrophic events, resulting in further immune activation and inflammatory injury to axons (Craner et al., 2004). Thus, activated macrophages have been shown to play an important role in the propagation of the disease (Trapp et al., 1998). MS starts as a relapsing-remitting disease (RRMS), and is followed by a progressive phase (SPMS). The progressive phase causes the greatest disability and has no effective therapy, but the processes that drive SPMS are mostly unknown. Determination of 25(S)-hydroxycholesterol levels in MS patients at different stages of the disease indicate that in the oldest groups of patients, the levels of 24(S)-hydroxycholesterol are significantly lower than in the controls, possibly reflecting loss of neuronal cells responsible for the synthesis and this decrease correlates with the disease burden evaluated by magnetic resonance imaging (MRI). Furthemore, there is a significant inverse relation between the expanded disability status scale-grade of the disease and the plasma cholesterol-related levels of 24(S)hydroxycholesterol. There is a tendency to increased plasma levels of 24(S)hydroxycholesterol in the younger patients. High levels of 24(S)-hydroxycholesterol

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have been reported to occur in MS patients in the third and fourth decades of life (Leoni et al., 2002; Teunissen et al., 2003, 2007; Leoni et al., 2003). More studies are needed not only in brain tissue from MS patients, but also in serum and CSF levels in various phases of MS.

10.5.6 Hydroxycholesterol in Traumatic Brain Injury Secondary phase of traumatic brain injury (TBI) is accompanied not only by alterations in cholesterol homeostasis, but also activation of microglial cells and astrocytes, and oligodendroglia leading to microgliosis, astrocytosis, and demyelination (Raghupathi, 2004; Farooqui, 2010). Inflammatory reactions, oxidative stress, and nitrosative stress are major components of secondary injury. Following TBI, activity of cholesterol 24S-hydroxylase (Cyp46) and levels of 24(S)-hydroxycholesterol are significantly increased in brain tissue (Cartagena et al., 2010). Alterations in levels of this cholesterol metabolite increase SREBP-1 mRNA and full-length protein but have no effect on levels of cleaved SREBP-1. In contrast, 24(S)-hydroxycholesterol decreases levels of LXRindependent SREBP-2 mRNA, full-length protein, and SREBP-2 active cleavage product. In neuroblastoma cells, 24(S)-hydroxycholesterol decreases HMG CoA reductase, squalene synthase, and FPP synthase activities as well as their mRNA levels but has no effect on mRNA of fatty acid synthesis genes acetyl CoA carboxylase or fatty acid synthase. Levels of fatty acid synthase mRNA are not altered by 24(S)-hydroxycholesterol treatment, but acetyl CoA carboxylase mRNA levels are significantly decreased, indicating that changes to transcription of cholesterol synthesis genes after TBI are related with the increase in Cyp-46 activity (Cartagena et al., 2010), but not with alterations in fatty acid synthesis genes, which are modulated by other mechanisms.

10.5.7 Cholesterol and Hydroxycholesterols in Cerebrotendinous Xanthomatosis Cerebrotendinous xanthomatosis (CTX) is a rare autosomal recessive disorder caused by mutations in the CYP27A1 gene coding for the enzyme sterol 27-hydroxylase, a mitochondrial P-450 enzyme with broad substrate specificity. The rates of hydroxylation of the sterols are: 7a-hydroxy-4-cholesten-3-one > 4-cholesten-3one > 7a-hydroxycholesterol > 24-hydroxy-4-cholesten-3-one > cholesterol > 25-hydroxy4-cholesten-3-one > 24-hydroxycholesterol > or = 25-hydroxycholesterol (Norlin et al., 2003). CTX is characterized by tendon xanthomas, diarrhea, cataracts, and progressive neurological dysfunction, including dementia, psychiatric disturbances, pyramidal and/or cerebellar signs, and seizures (Gallus et al., 2006). CTX patients develop severe premature atherosclerosis and elevated levels of cholestanol (a by-product of abnormal bile acid synthesis) despite normal serum levels of

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cholesterol. Based on this observation, it is suggested that sterol 27-hydroxylase is an anti-atherosclerotic enzyme. Sterol 27-Hydroxylase not only catalyzes bile acid hydroxylation (Russell, 2003) and activation of vitamin D (Usui et al., 1990), but also stimulates cholesterol efflux in cultured cells (Escher et al., 2003) through the formation of 27-hydroxycholesterol. This metabolite is a ligand for LXR receptor. CYP27 mediates LXR activation as a response to cholesterol loading, and this may be the dominant mechanism for induction of LXR-responsive genes in human macrophages. Sterol 27-hydroxylase may accordingly protect these cells from cholesterol overload by two concurrent but separate mechanisms. One mechanism (Björkhem et al., 1994) utilizes the increased solubility of 27-hydroxycholesterol and cholestenoic acid to excrete these compounds from the cell. The second mechanism operates via activation of LXR, stimulating ABCA1-mediated reverse cholesterol transport. Both of these pathways are impaired in CTX patients, lacking sterol 27-hydroxylase activity.

10.6 Cholesterol and Hydroxycholesterols in Kainic Acid Neurotoxicity Kainic acid (KA) is a cyclic and nondegradable analog of glutamate. Its injections produce loss of neurons in specific striatal and hippocampal areas of the brain (Farooqui et al., 2008). Neuronal axons and nerve terminals are more resistant to the destructive effects of KA than the cell soma. KA induces its effects by interacting with specific receptors, the KA receptors (KAR). KA administration in rodents has been used as an animal model to study molecular mechanism of neurodegeneration (Farooqui et  al., 2001). Intraventricular injections of KA are known to produce increase in cholesterol immunostaining with filipin in the CA1 region of rat hippocampus (Ong et  al., 2003). Studies on cholesterol biosynthetic pathway in KA-mediated lesions indicate that mRNA level of SREBP-2 is significantly reduced at both 1  day and 2  weeks post-kainate injection. Immunohistochemical analyses show significant reduction in SREBP-2 immunoreactivity in the KA-induced lesions. These studies show that there is high level of SREBP-2 expression in the normal hippocampus, and that KA-mediated neuronal injury results in a significant reduction of SREBP-2 expression in the damaged areas (Kim and Ong, 2009; Ong et al., 2010). KA injections also produce a significant decrease in HMG-CoA reductase. Lanosterol synthase and CYP51, which are responsible for conversion of squalene to lanosterol, and lanosterol to cholesterol, show significant reduction in mRNA expression at 1  week and 2  week post-KA injection, respectively. KA injections also produce increase in ABC1 mRNA expression in the hippocampus after 1 week and 2 weeks. The increase in ABCA1 mRNA is paralleled by an increase in ABCA1 protein expression, in reactive astrocytes in the degenerating hippocampus (Ong et al., 2010). The expression of ABCA1 regulated by LXRs (Venkateswaran et al., 2000) and their major ligands are oxysterols (Repa et al., 2007). Thus, the increase of ABCA1 after

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KA-mediated neurodegeneration may be due to increased oxysterols since oxysterols can activate liver X receptors which modulate ABCA1 expression (Cao et al., 2007). In parallel with the increase in ABCA1 and ABCG1, increase in expression of another enzyme involved in cholesterol export into the bloodstream, cholesterol 24 hydroxylase, and its product 24-hydroxycholesterol is found in the degenerating hippocampus after KA-induced lesions (He et al., 2006). Analysis of brain tissue by lipidomics indicates that there is a significant increase in cholesterol, 24-hydroxycholesterol, and 7-ketocholesterol in hippocampal region at 2 weeks after injury (Ong et al., 2003). As stated above, cholesterol oxidation products produce toxic effect on neuronal cultures through decrease in staining of the AMPA receptor subunit GluR1 in the CA field. The addition of glutathione to hippocampal cultures and slices blocks the decrease in GluR1 immunoreactivity. In organotypic slices from the hippocampus of rats injected with KA plus lovastatin show significantly lower levels of cholesterol, 24-hydroxycholesterol, and 7-ketocholesterol, compared to those that have been treated with KA alone, suggesting that lovastatin modulates the loss of hippocampal neurons after KA treatment. This observation suggests that KA-induced toxicity is mediated through cholesterol oxidation products (Park et  al., 2000; Ong et al., 2010).

10.7 Conclusion The brain is the richest source of cholesterol. Cholesterol contents of brain are independent of dietary cholesterol contents. BBB prevents the entry of cholesterol from the circulation into the brain. So that de  novo synthesis of cholesterol is responsible for almost all cholesterol present in the brain. Based on the above discussion, it is obvious that alterations in cholesterol composition of lipid rafts can significantly affect cell function. It is anticipated that as the neurochemical importance of cholesterol metabolism in neurodegeneration increases considerably, the methodology to detect subtle changes in lipid raft composition will become perfect. Cholesterol is oxidized to hydroxycholesterols by cytochrome P450-dependent hydroxylases. These enzymes play key roles in cholesterol homeostasis through the elimination of excess cholesterol from brain through blood–brain barrier into blood from where it is taken to liver by plasma lipoproteins. Among hydroxycholesterols, 24-hydroxycholesterol is the major metabolite. High levels of 24-hydroxycholesterol in serum are index for neurodegeneration. Thus levels of 24-hydroxycholesterol are significantly increased in AD, PD, NPC, and TBI. Marked alterations in cholesterol metabolism and increases in hydroxycholesterol occur during KA-mediated neurotoxicity. Alterations in hydroxycholesterols may play important roles in neural differentiation, cell cycle arrest, and apoptosis. At present, it is not known whether the generation of hydroxycholesterols is the initiating point in neurodegeneration (primary effect), or it is the end result of neurodegenerative process itself (secondary effect).

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Kim, J.H., and Ong, W.Y (2009). Localization of the transcription factor, sterol regulatory element binding protein-2 (SREBP-2) in the normal rat brain and changes after kainate-induced excitotoxic injury. J Chem Neuroanat 37:71–77. Koldamova R.P., Lefterov I.M., Ikonomovic M.D., Skoko J., Lefterov P.I., Isanski B.A., DeKosky S.T. and Lazo J.S. (2003). 22R-hydroxycholesterol and 9-cis-retinoic acid induce ATP-binding cassette transporter A1 expression and cholesterol efflux in brain cells and decrease amyloid beta secretion. J. Biol. Chem. 278:13244–13256. Kolsch H., Ludwig M., Lutjohann D., Rao M.L. (2001). Neurotoxicity of 24-hydroxycholesterol, an important cholesterol elimination product of the brain, may be prevented by vitamin E and estradiol-17beta. J Neural Transm. 108:475–488. Koob A.O., Ubhi K., Paulsson J.F., Kelly J., Rockenstein E., Mante M., Adame A., Masliah E. (2010). Lovastatin ameliorates alpha-synuclein accumulation and oxidation in transgenic mouse models of alpha-synucleinopathies. Exp Neurol. 221:267–274. Kovacs W.J., Faust P.L., Keller G.A., and Krisans S.K. (2001). Purification of brain peroxisomes and localization of 3-hydroxy-3-methylglutaryl coenzyme A reductase. Eur J Biochem. 268:4850–4859. Leoni V., Masterman T., Diczfalusy U., De Luca G., Hillert J., and Bjorkhem I. (2002). Changes in human plasma levels of the brain specific oxysterol 24S-hydroxycholesterol during progression of multiple sclerosis. Neurosci Lett 331:163. Leoni V, Masterman T, Patel P, Meaney S, Diczfalusy U, Bjorkhem I. (2003). Side chain oxidized oxysterols in cerebrospinal fluid and the integrity of blood–brain and blood–cerebrospinal fluid barriers. J Lipid Res 44:793–799. Leoni V., Mariotti C., Tabrizi S.J., Valenza M. Wild E.J., Henley S.M. Hobbs N.Z., Mandelli M.L., Grisoli M., Björkhem I., Cattaneo E., and Di Donato S. (2008). Plasma 24 S-hydroxycholesterol and caudate MRI in pre-manifest and early Huntington’s disease. Brain. 131:2851–2859. Leoni V., Solomon A., and Kivipelto M. (2010). Links between ApoE, brain cholesterol metabolism, tau and amyloid beta-peptide in patients with cognitive impairment. Biochem Soc Trans. 38:1021–1025. Levi O., Lütjohann D., Devir A., von Bergmann K., Hartmann T., and Michaelson D.M. (2005). Regulation of hippocampal cholesterol metabolism by apoE and environmental stimulation. J Neurochem 95:987–997. Liou H.L., Dixit S.S., Xu S., Tint G.S., Stock A.M., and Lobel P. (2006). NPC2, the protein deficient in Niemann-Pick C2 disease, consists of multiple glycoforms that bind a variety of sterols. J Biol Chem 281:36710–36723. Lizard G., Miguet C., Bessède G., Monier S., Gueldry S., Neel D., and Gambert P. (2000). Impairment with various antioxidants of the loss of mitochondrial transmembrane potential and of the cytosolic release of cytochrome c occuring during 7-ketocholesterol-induced apoptosis. Free Radic. Biol. Med. 28:743–753. Lund E.G., Guileyardo J.M., and Russel D.W. (1999). cDNA cloning of cholesterol 24-hydroxylase, a mediator of the cholesterol homeostasis in the brain. Proc Natl Acad Sci USA 96:7238–7243. Lütjohann D., Breuer O., Ahlborg G., Nennesmo I., Siden Å., Diczfalusy U., and Bjorkhem I. (1996). Cholesterol homeostasis in human brain: evidence for an age-dependent flux of 24S-hydroxycholesterol from the brain into the circulation. Proc Natl Acad Sci USA 93:9799–9804. Lütjohann D., Brzezinka A., Barth E., Abramowski D., Staufenbiel M., Bergmann K., Beyreuther K., Multhaup G. and Bayer T.A. (2002). Profile of cholesterol-related sterols in aged amyloid precursor protein transgenic mouse brain. J. Lipid Res. 43:1078–1085. Lutjohann D. and von Bergmann K. (2003). 24S-hydroxycholesterol: a marker of brain cholesterol metabolism. Pharmacopsychiatry 36 Suppl. 2:S102-S106. Ma M.T., Zhang J., Farooqui A.A., Chen P., and Ong W.Y. (2010). Effects of cholesterol oxidation products on exocytosis. Neurosci Lett. 476:36–41. Marwarha G., Dasari B., Prasanthi J.R., Schommer J., and Ghribi O. (2010). Leptin reduces the accumulation of Abeta and phosphorylated tau induced by 27-hydroxycholesterol in rabbit organotypic slices. J Alzheimers Dis. 19:1007–1019.

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Chapter 11

Perspective and Direction for Future Studies on Lipid Mediators

11.1 Introduction Neural membranes are composed of phospholipids, sphingolipids, cholesterol, and proteins. The distribution of lipids in two leaflets of lipid bilayer is asymmetric (Ikeda et al., 2006; Yamaji-Hasegawa and Tsujimoto, 2006). Asymmetric distribution of lipids is needed for structural integrity necessary for protein function. Sphingolipids and cholesterol interact with each other via hydrogen bonds, hydrophobic forces, and van der Waal interactions (Simons and Ikonen, 1997). In addition, the head groups of sphingolipids bind to each other through hydrophilic interactions. These interactions result in lateral association of sphingolipids and cholesterol. Phospholipid composition of outer and inner monolayer halves is quite different. Thus, ethanolamine and serine containing phospholipids are located in the cytofacial site of inner monolayer, whereas choline containing phospholipids are localized on the exofacial side of outer monolayer. Among phospholipids phosphatidylethanolamine (PtdEtn), plasmenylethanolamine (PlsEtn), and phosphatidylserine (PtdSer) are enriched in docosahexaenoic acid (DHA, 22:6n-3) at the sn-2 position of the glycerol moiety, whereas phosphatidylcholine (PtdCho), phosphatidylinositol (PtdIns), and phosphatidic acid (PtdH) contain arachidonic acid (ARA, 20:4n-6) (Farooqui et al., 2000; Tillman and Cascio, 2003). In neural membranes, the fate of each lipid class is closely linked to neural cell function. For example, cholesterol and sphingolipids are involved in formation of lipid microdomains or lipid rafts, which float within the membrane and acts as molecular sorting machines and platforms for signal transduction pathways (Zajchowski and Robbins, 2002; Fantini et al., 2002; Lucero and Robbins, 2004). Thus, lipid rafts act as a unique compartment of the plasma membrane, which not only ensures correct intracellular trafficking of proteins and lipids, such as protein–protein interactions by concentrating certain proteins in these microdomains, while excluding others, but also modulate signal transduction processes associated with neural cell functions. In addition, sphingolipids play an important role in inhibiting or stimulating the oligomerization of amyloidogenic proteins, which are closely associated with pathogenesis of neurodegenerative A.A. Farooqui, Lipid Mediators and Their Metabolism in the Brain, DOI 10.1007/978-1-4419-9940-5_11, © Springer Science+Business Media, LLC 2011

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diseases (Fantini and Yahi, 2010; Farooqui, 2010). Cholesterol has a dual role: regulation of protein–sphingolipid interactions through a fine tuning of sphingolipid conformation (indirect effect), and facilitation of pore (or channel) formation through direct binding to amyloidogenic proteins (Fantini and Yahi, 2010). Phospholipids, sphingolipids, and cholesterol provide neural membranes with integrity and are storage depots for lipid mediators. Interactions between phospholipids and sphingolipids are involved in maintaining lipid asymmetry, a dynamic process, which is necessary in maintaining normal neural membrane functions such as neuroplasticity and vesicular transport (Farooqui, 2009a). At the same time, local or global changes in lipid asymmetry are essential for cell cycle progression, apoptosis, and platelet coagulation. The disruption of asymmetry may also lead to apoptotic cell death. In neural membranes, the binding of transmembrane and peripheral proteins link lipid metabolism with phospholipid that is necessary for vertical positioning and tight integration of proteins in the lipid bilayer (Palsdottir and Hunte, 2004). The binding between phospholipids and proteins is not only stabilized by multiple noncovalent interactions between protein residues and phospholipid head groups, but also by hydrophobic tails of phospholipids (Palsdottir and Hunte, 2004; Farooqui and Horrocks, 2007). As stated above, neural membrane also contain sphingolipids (sphingomyelin, ceramide, cerebroside, sulfatide, and sphingosine) and cholesterol, which not only provide neural membrane with physicochemical characteristics (organization, fluidity, permeability, and structural integrity), but also modulate many neural cell functions, such as cell activation, differentiation, proliferation, survival, growth arrest, migration, and apoptosis. Collective evidence suggests that lipid signaling not only involves generation of lipid mediators, but also an orchestrated coupling among lipid metabolism, lipid organization, and the action of protein machines that execute appropriate downstream reactions. Remarkable enhancement of phospholipid, sphingolipid, and cholesterol metabolism has been reported to occur at plasma membrane and nuclear levels. Phospholipid, sphingolipid, and cholesterol metabolism in the nucleus may be as complex as that exists in the plasma membrane. However, an important feature of nuclear lipid metabolism pathways is their operational independence from metabolic pathways that are associated with plasma membrane and other subcellular organelles (Albi et al., 2006, 2008; Albi and Viola Magni, 2007; Ledeen and Wu, 2008; Martelli et  al., 2005; Farooqui and Horrocks, 2006; Farooqui, 2009a; Farooqui et al., 2010).

11.2 Lipid Mediators in the Brain Receptor-mediated catabolism of phospholipid-, sphingolipid-, and cholesterol by phospholipases, cyclooxygenases, lipoxygenases, acyltransferases, sphingomyelinases, and cytochrome P450 hydroxylases results in the generation of lipid mediators, which are lipophilic molecules that facilitate signal transduction processes, regulate cell–cell communication, neural cell proliferation, and differentiation along with control of molecular and cellular events associated with oxidative stress,

11.2  Lipid Mediators in the Brain

301 A R

Lipid mediator Synthesizing enzyme Neural cell survival

Neural cell differentiation

Lipid mediators

Neural cell migration

Neural cell proliferation

Immune responses

Oxidative stress

Inflammation

Gene expression

Mitogenesis

Apoptosis

Neural dysfunction

Fig. 11.1  Hypothetical diagram showing interactions between Agonist (A) and Receptor (R) and modulation of signal neural cell proliferation and differentiation, oxidative stress, inflammation, and gene expression by lipid mediators (Modified from Farooqui, 2009a)

inflammation, and gene expression in the brain (Fig.  11.1) (Farooqui, 2009a; Farooqui et  al., 2010). Phospholipid-derived lipid mediators are diacylglycerols (DAG), phosphatidylinositol 1,4,5-trisphosphate (Ins(1,4,5-P3), eicosanoids (prostaglandins, leukotrienes, thromboxanes, and lipoxins), docosanoids (D and E series resolvins, protectins, and neuroprotectins), endocannabinoids (2-arachidonylglycerol and anandamide), lysophospholipids, fatty acids, platelet-activating factor (PAF) and oxidized phospholipids. The sphingolipid-derived lipid mediators include ceramide, ceramide 1-phosphate, sphingosine, sphingosine 1-phosphate, and cholesterol-derived lipid mediators include 22-, 24-, 25-, 27-hydroxycholesterols (Fig.  11.2). The nonenzymic lipid mediators of glycerophospholipid metabolism are isoprostanes (isoP) and neuroprostanes (NeuroP), 4-hydroxynonenal (4-HNE), and 4-hydroxyhexenal (4-HHE) (Fam and Morrow, 2003; Phillis et  al., 2006; Farooqui and Horrocks, 2007; Bazan, 2005a, b; Serhan, 2005a, b, c). These lipid mediators are involved in controlling the duration and magnitude of acute inflammation, oxidative stress, as well as the return of the injury site to homeostasis in the process of catabasis (the decline of the disease state). Another function of lipid mediators and their signal transduction network is to convey the message of extracellular signals from the cell surface to the nucleus to induce a biological response at the gene level. The conveying process involves the transfer of signal from lipid mediator to nuclear pores (large proteinaceous assemblies) that provide the sole

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11  Perspective and Direction for Future Studies on Lipid Mediators

Lipid mediators

Precursors

24-Hydroxycholesterol Cholesterol

7-Ketocholesterol 25Hydroxyocholesterol Ceramide-1-phosphate

Lipids

Sphingolipids

Ceramide

Sphingosine1-phosphate

Sphingosine

DAG Eicosanoids Glycerophospholipids

Arachidonic acid

4-HNE ROS

Lysophosphatidic acid

Docosatrienes Resolvins

Docosahexaenoic acid

4-HHE Lysophospholipids

PAF

Fig. 11.2  Glycerophospholipid, sphingolipid, and cholesterol-derived lipid mediators in brain tissue. Diacylglycerol (DAG); prostaglandins (PG); leucotrienes (LT); thromboxane (TX); 4-hydroxynonenal (4-HNE); reactive oxygen species (ROS); 4-hydroxyhexanal (4-HHE); plateletactivating factor (PAF); 2-arachidonylglycerol (2-AG); N-arachidonoylethanolamine (AEA); N-arachidonyl-dopamine (NAD); and virodhamine (VDA). isoprostane (IsoP); neuroprostane (NeuroP); isoprostane (IsoP); lysophosphatidic acid (Lyso-PtdH) (Modified from Farooqui et al., 2010)

gateway for the exchange of material between cytoplasm and nucleus at the interphase (Fahrenkrog, 2006). Intracellular lipid mediator trafficking is a highly organized and dynamic process involving interactions between plasma membrane and nucleus-derived lipid mediators and proteins associated with intracellular signal transduction processes. Nuclear pore complexes support two modes of transport: (a) passive diffusion of ions, metabolites, and (b) intermediate-sized macromolecules. These processes facilitate receptor-mediated translocation of proteins, RNA, and ribonucleoprotein complexes. Faithful, continuous nuclear pore complex assembly is the key for maintaining normal physiological function and is closely tied to proper cell division. It is generally assumed that both modes of transport occur through a single diffusion channel located within the central pore of the nuclear pore complex. As such, the nuclear pore complex and nuclear transport play central roles in translocating signals from the cell membrane to the nucleus where they initiate biochemical and morphological changes related to a particular function (Fahrenkrog, 2006; Naim et al., 2007).

11.3  Interactions Among Phospholipid-, Sphingolipid-, and Cholesterol-Derived…

303

11.3 Interactions Among Phospholipid-, Sphingolipid-, and Cholesterol-Derived Lipid Mediators in the Brain In neural cells, metabolism of phospholipid, sphingolipid, and cholesterol is interrelated and interconnected. Many cellular stimuli (neurotransmitters, cytokines, and growth factors) modulate more than one enzyme of phospholipid, sphingolipid, and cholesterol metabolism at the same time. This phenomenon produces complexity in regulatory processes associated with phospholipid, sphingolipid, and cholesterol metabolism. Under normal conditions, homeostasis among enzymes of phospholipid, sphingolipid, cholesterol metabolism is based not only on optimal levels of lipid mediators and organization of signaling network, but also on the complexity and interconnectedness of their metabolism. Under pathological conditions, marked increases in levels of lipid mediators disturb the signaling networks, and result in loss of communication among phospholipid, sphingolipid, and cholesterol metabolism. This process not only threatens the integrity of neural cell lipid homeostasis, but also facilitates neural cell death (Farooqui et al., 2007a, b, c; Farooqui and Horrocks, 2007). In addition to above processes, the organization and compartmentalization of phospholipids, sphingolipids, and cholesterol in neural membranes of various subcellular fractions provides structural and functional integrity that facilitates the appropriate interactions with integral membrane proteins. An organized compartmentalization is needed for modulating regular cellular functions such as cell proliferation, differentiation, communication, and controlled adaptive responses through interactions among lipid mediators (Ivanova et al., 2004). However, under pathological conditions, high levels and intense interactions among phospholipid, sphingolipid, and cholesterol-derived lipid mediators produce harmful effects that facilitate apoptotic cell death (Farooqui, 2009a). At the molecular level, close interactions between the cPLA2-generated second messengers (ARA and its metabolites), and the SMase-generated second messenger (ceramide and ceramide 1-phosphate) occurs (Farooqui, 2009a; Vanags et al., 1997; Robinson et al., 1997; Malaplate-Armand et al., 2006; Farooqui et al., 2010) (Fig. 11.3). This process is closely associated with oxidative stress, inflammation, and apoptosis. PAF, a phospholipid-derived lipid mediator, also promote the activation of sphingomyelinase and generation of ceramide leading to the activation of scramblase with subsequent phosphatidylserine exposure, a process associated with apoptotic cell death (Lang et al., 2005). Interestingly, sphingolipid-derived lipid mediators, ceramide, ceramide 1 phosphate (C1P), sphingosine 1 phosphate (S1P) modulates activities of several enzymes associated with the synthesis of phospholipid-derived lipid mediators, such as arachidonic acid and its oxidative metabolites. Thus, ceramide upregulates cPLA2 and COX-2 expression and stimulates PGE2 synthesis (Farooqui, 2009a), a lipid mediator that produces neuroinflammation. Similarly, sphingosine 1-phosphate (S1P) upregulates COX-2 in a concentration- and time-dependent manner, but has no effect on COX-1 expression (Nodai et al., 2007). Suramin, an antagonist of S1P3 receptor, completely prevents S1P-mediated COX-2 expression. Thus, ceramide, ceramide 1-phosphate, sphingosine, and S1P stimulate enzymes associated

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11  Perspective and Direction for Future Studies on Lipid Mediators Glu

COX-2

+

Cholesterol

Cholesterol 24-hydroxylase

Lyso+ ARA PtdCho

PAF

+

+

SM

SMase

cPLA2

PtdCho

Glu

Ceramide 1

+

2

7-ketocholesterol

PGE2

3 Sphingosine-1-P

Inflammation

TNF-α IL-1β IL-6

Apoptosis

+

+

Nucleus

Gene transcription COX-2 sPLA2 iNOS

Proinflammatory cytokines

Proinflammatory enzymes

+

Oxidative stress

ROS

24-Hydroxycholesterol

Ceramide-1-P Sphingosine

ROS

PM

Mitochondrial dysfunction

Cytochrome c Cathepsin D Caspase cascade

Fig. 11.3  Hypothetical diagram showing interactions among phospholipid-, sphingolipid-, and cholesterol-derived lipid mediators. Phosphatidylcholine (PtdCho); sphingomyelin (SM); cytosolic phospholipase A2 (cPLA2); sphingomyelinase (SMase); arachidonic acid (ARA); reactive oxygen species (ROS); cyclooxygenase-2 (COX-2); reactive oxygen species (ROS); ceramide -1-kinase (1); ceramidase (2); sphingosine-1-kinase (3); ceramide-1-phosophate (ceramide-1-P); sphingosine-1-phosphate (sphingosine-1-P); and plasma membrane (PM). Proinflammatory genes include TNF-a; IL-1b; nitric oxide synthase; cyclooxygenase-2 (COX-2); sPLA2. Positive sign (+) indicates stimulation, upward arrow indicates increase, and downward arrow indicates decrease in levels of precursors and lipid mediators. Upward arrows indicate increase in levels, where as downward arrows indicate decrease

with the synthesis of lipid mediators-derived from neural membrane phospholipids (Pyne, 2004; Pettus et al., 2004; Nodai et al., 2007). In addition, in rat brain slices sphingomyelinase and ceramide decrease the levels of plasmalogens through the activation of PlsEtn-PLA2 (Latorre et al., 2003). The decrease in plasmalogens by sphingomyelinase or ceramide is inhibited by quinacrine, ganglioside, and bromoenol lactone, which are inhibitors of plasmalogen-selective PLA2 activity (Latorre et al., 2003; Farooqui and Horrocks, 2001). Treatment with caspase-3 inhibitor, acetyl-l-aspartyl-l-glutamyl-l-valyl-l-aspartyl-chloromethylketone (Ac-DEVDCMK), partially blocks the ceramide-induced stimulation of plasmalogen-selective PLA2 without altering sphingomyelinase-elicited ceramide accumulation (Latorre et al., 2003; Farooqui and Horrocks, 2004). In addition, S1P is metabolized to phosphoethanolamine and hexadecanol, which are prerequisite for the synthesis of phospholipids (Park et  al., 2006). Collective evidence suggests that interactions between sphingolipid- and phospholipid-derived lipid mediators are closely associated with cell proliferation, differentiation, apoptosis, migration, and cell survival

11.3  Interactions Among Phospholipid-, Sphingolipid-, and Cholesterol-Derived…

305

PLA2,PLC, & PLD COX, LOX, & EPOX Sphingomyelinases

Lipid mediator synthesizing enzymes

Ceramide & sphingosine kinases Acetyltransferases N-Acyltransferases Cytochrome P450-dependent hydroxylases

Fig. 11.4  Enzymes associated with the synthesis of lipid mediators. Phospholipase A2 (PLA2); phospholipase C (PLC); phospholipase D (PLD); Cyclooxygenase (COX); lipoxygenase (LOX); and epoxygenase (EPOX)

(Wymann and Schneiter, 2008; Hannun and Obeid, 2008). Thus, lipid mediator synthesizing enzymes (phospholipases A2, C, D, cyclooxygenases, 5-lipoxygenase, phosphoinositide 3-kinase, sphingomyelinases, ceramide kinases, sphingosine kinase and cholesterol hydroxylases) (Fig. 11.4) and their downstream targets constitute a complex lipid signaling network with multiple nodes of interactions and cross-talk among various lipid mediators. Alterations and imbalances in this network due to abnormalities in lipid mediator production may contribute to the pathogenesis of neural trauma, neurodegenerative, and neuropsychiatric diseases (Farooqui, 2009a; Wymann and Schneiter, 2008; Hannun and Obeid, 2008). Ceramide, the central core in sphingolipid metabolism is not only a precursor of complex sphingolipids, but is also involved in the regulation of signal transduction processes associated with apoptosis, cell growth, differentiation, senescence, inflammation, and neurodegenerative disorders (Farooqui et al., 2007a; Farooqui, 2009a). The presence of cholesterol in neural membrane enhances or inhibits the interactions between ceramide and amyloidogenic proteins (Ab, a-synuclein, and huntingtin). It is suggested that presence of cholesterol in neural membranes has a major impact on interaction between ceramide and amyloidogenic protein. In the case on non-hydroxyfatty acid (NFA) containing ceramide, cholesterol usually accelerates protein binding to sphingolipid due to hydrogen bonding (Fantini et al., 2002) and this also applies for amyloidogenic proteins (Yahi et  al., 2010). In contrast, ceramide containing a hydroxylated fatty acid (HFA), the OH group of cholesterol is excluded from the H-bond network and it cannot exert its conformational effect on the ceramide. Thus, cholesterol does not enhance but rather perturbs the organization of ceramide and can even retard ceramide binding to amyloidogenic proteins (Yahi et al., 2010). Thus, according to the distribution of HFAs versus NFAs in ceramide, an increase of membrane cholesterol may produce opposite effects on sphingolipid-mediated amyloidogenic protein binding and aggregation. These effects of

306

11  Perspective and Direction for Future Studies on Lipid Mediators

cholesterol are associated with fine tuning of ceramide conformation for interaction between amyloidogenic proteins and ceramide (Fantini and Yahi, 2010). In wild-type and sterol-resistant Chinese-hamster ovary (CHO) cells, oxysterols promote the translocation and activation of CTP: phosphocholine cytidylyltransferase a to nuclear membrane where it promote the synthesis of PtdCho (Gehrig et al., 2009). Increasing evidence suggests that oxysterols act as ligands of liver X receptors, transcription factors with key roles in lipid metabolism. Oxysterols not only bind to transcription factors, but also interact with the INSIG (insulin-induced gene) proteins. This suggests that oxysterols regulate the transport and maturation of sterol-regulatory element binding proteins (Olkkonen and Hynynen, 2009). Nothing is known about the interactions between hydroxycholesterols and sphingolipids in neural plasma membranes. However, in Chinese hamster ovary (CHO)-K1 cells, 25-hydroxycholesterol stimulates the synthesis of sphingomyelin (Ridgway, 1995; Lagace et al., 1995). The role of oxysterol-binding protein (OSBP), a high affinity receptor for 25-hydroxycholesterol, in activation of sphingomyelin synthesis has also been assessed by overexpression in CHO-K1 cells. When compared to mock transfected controls, three CHO-K1 clones overexpressing OSBP by 10- to 15-fold show a two- to threefold enhancement of [3H]serine incorporation into sphingomyelin when treated with 25-hydroxycholesterol. The closer examination of one of these clones (CHO-OSBP cells) produces a >8.5-fold stimulation of sphingomyelin synthesis with 25-hydroxycholesterol compared to 3.5-fold in controls, slightly higher basal levels of sphingomyelin synthesis, and a more rapid response to 25-hydroxycholesterol. [3H]Serine incorporation into phosphatidylserine, phosphatidylethanolamine, ceramide, or glucosylceramide is affected only by