Conjugated Linoleic Acid and Brain Metabolism: A Possible Anti-Neuroinflammatory Role Mediated by PPARα Activation

Fatty acids play a crucial role in the brain as specific receptor ligands and as precursors of bioactive metabolites. Conjugated linoleic acid (CLA), a group of positional and geometric isomers of linoleic acid (LA, 18:2 n-6) present in meat and dairy products of ruminants and synthesized endogenously in non-ruminants and humans, has been shown to possess different nutritional properties associated with health benefits. Its ability to bind to peroxisome proliferator-activated receptor (PPAR) α, a nuclear receptor key regulator of fatty acid metabolism and inflammatory responses, partly mediates these beneficial effects. CLA is incorporated and metabolized into brain tissue where induces the biosynthesis of endogenous PPARα ligands palmitoylethanolamide (PEA) and oleoylethanolamide (OEA), likely through a positive feedback mechanism where PPARα activation sustains its own cellular effects through ligand biosynthesis. In addition to PPARα, PEA and OEA may as well bind to other receptors such as TRPV1, further extending CLA own anti-neuroinflammatory actions. Future studies are needed to investigate whether dietary CLA may exert anti-inflammatory activity, particularly in the setting of neurodegenerative diseases and neuropsychiatric disorders with a neuroinflammatory basis.

Whole transcriptome expression of trigeminal ganglia compared to dorsal root ganglia in Rattus Norvegicus

The trigeminal ganglia (TG) subserving the head and the dorsal root ganglia (DRG) subserving the rest of the body are homologous handling sensory neurons. Differences exist, as a number of signaling substances cause headache but no pain in the rest of the body. To date, very few genes involved in this difference have been identified. We aim to reveal basal gene expression levels in TG and DRG and detect genes that are differentially expressed (DE) between TG and DRG. RNA-Sequencing from six naïve rats describes the whole transcriptome expression profiles of TG and DRG. Differential expression analysis was followed by pathway analysis to identify DE processes between TG and DRG. In total, 64 genes had higher and 55 genes had lower expressed levels in TG than DRG. Higher expressed genes, including S1pr5 and Gjc2, have been related to phospholipase activity. The lower expressed genes, including several Hox genes and Slc5a7, have been related to tyrosine and phenylalanine metabolism. Tissue-specific expression was identified for Gabra6 and Gabrd in TG, and for several Hox genes in DRG. Furthermore, genes that were known to be associated with headache/migraine were mostly moderately to highly expressed in one or both tissues. We present a comprehensive overview of the expression profiles of whole tissue comparison of TG and DRG. Further, we showed DE genes/pathways between TG and DRG, including several known migraine-associated genes. This study provides a basis for further pain-related studies using TG and DRG in rats.

Targeting brain and peripheral plasticity of the lipidome in acute kainic acid-induced epileptic seizures in mice via quantitative mass spectrometry

Epilepsy is a highly common chronic neurological disorder, manifested in many different types, affecting ~1% of the worldwide human population. The molecular mechanisms of epileptogenesis have not yet been clarified, and pharmacoresistance exhibited by 30-40% of epilepsy patients remains a major obstacle in medical care. Growing evidence indicates a role of lipid signalling pathways in epileptogenesis, thus lipid signals emerge as potential biomarkers for the onset and evolving course of the epileptic disorder, as well as potential therapeutic agents and targets. For this purpose, we applied a lipidomic strategy to unravel lipid alterations in brain regions, periphery tissues and plasma that are specific for acute epileptic seizures in mice at 1h after seizure induction by systemic kainic acid injection as compared to vehicle controls. Specifically, levels of (i) selected phospholipids and sphingomyelins, (ii) the endocannabinoids anandamide (AEA) and 2-arachidonoyl glycerol (2-AG), and the endocannabinoid-related compounds oleoylethanolamide (OEA) and palmitoylethanolamide (PEA), (iii) arachidonic acid (AA), (iv) selected eicosanoids, and (v) fatty acyl content of lipidome were determined in pulverized tissues from six brain regions of kainic acid induced epileptic seizure models and vehicle controls: hypothalamus, hippocampus, thalamus, striatum, cerebellum and cerebral cortex, and from peripheral organs, such as heart and lungs, and in plasma. Alterations in lipid levels after acute epileptic seizures as compared to non-seizure controls were found to be brain region- and periphery tissue-specific, including specific plasma lipid correlates, highlighting their value as marker candidates in translational research studies, and/or drug discovery and response monitoring.

Imaging mass spectrometry reveals loss of polyunsaturated cardiolipins in the cortical contusion, hippocampus, and thalamus after traumatic brain injury

Traumatic brain injury (TBI) leads to changes in ion fluxes, alterations in mitochondrial function, and increased generation of reactive oxygen species, resulting in secondary tissue damage. Mitochondria play important signaling roles in coordination of multiple metabolic platforms in addition to their well-known role in bioenergetics. Mitochondrial signaling strongly depends on cardiolipin (CL), a mitochondria-specific structurally unusual anionic phospholipid containing four fatty acyl chains. While our previous reports indicated that CL is selectively oxidized and presents itself as a target for the redox therapy following TBI, the topography of changes of CL in the injured brain remained to be defined. Here, we present a matrix-assisted laser desorption/ionization imaging study which reports regio-specific changes in CL, in a controlled cortical impact model of TBI in rats. Matrix-assisted laser desorption/ionization imaging revealed that TBI caused early decreases in CL in the contusional cortex, ipsilateral hippocampus, and thalamus with the most highly unsaturated CL species being most susceptible to loss. Phosphatidylinositol was the only other lipid species that exhibited a significant decrease, albeit to a lesser extent than CL. Signals for other lipids remained unchanged. This is the first study evaluating the spatial distribution of CL loss after acute brain injury. We propose that the CL loss may constitute an upstream mechanism for CL-driven signaling in different brain regions as an early response mechanism and may also underlie the bioenergetic changes that occur in hippocampal, cortical, and thalamic mitochondria after TBI.

Phospholipase A₂-Generated Lipid Mediators in the Brain: The Good, the Bad, and the Ugly

Phospholipase A2 (PLA2) generates arachidonic acid, docosahexaenoic acid, and lysophospholipids from neural membrane phospholipids. These metabolites have a variety of physiological effects by themselves and also are substrates for the synthesis of more potent lipid mediators such as eicosanoids, platelet activating factor, and 4-hydroxynonenal (4-HNE). At low concentrations, these mediators act as second messengers. They affect and modulate several cell functions, including signal transduction, gene expression, and cell proliferation, but at high concentrations, these lipid mediators cause neurotoxicity. Among the metabolites generated by PLA2, 4-HNE is the most cytotoxic metabolite and is associated with the apoptotic type of neural cell death. Levels of 4-HNE are markedly increased in neurological disorders such as Alzheimer disease, Parkinson disease, ischemia, spinal cord trauma, and head injury. The purpose of this review is to summarize and integrate the vast literature on metabolites generated by PLA2 for a wider audience. The authors hope that this discussion will jump-start more studies not only on the involvement of PLA2 in neurological disorders but also on the importance of PLA2-generated lipid mediators in physiological and pathological processes.

Auto-amplification system for prostaglandin F2α in bovine corpus luteum

The bovine corpus luteum (CL) is hypothesized to utilize a local auto-amplification system for prostaglandin (PG) F2α production. The objective of the present study was to determine if such a PGF2α auto-amplification system exists in the bovine CL, and if so, which factors regulate it. PGF2α significantly stimulated intra-luteal PGF2α production in all luteal phases, but did not affect PGE2 production. The stimulatory effect of exogenous PGF2α on CL PGF2α production was lower at the early luteal phase. Indomethacin, an inhibitor of prostaglandin-endoperoxide synthase (PTGS), significantly suppressed the PGF2α-stimulated PGF2α production by luteal tissue, indicating that the PGF2α in the medium was of luteal origin. Consistent with these secreted-PGF2α profiles, PGF2α receptor (PTGFR) protein expression was higher during the mid and late luteal phases than at early and developing luteal phases. Treatment of cultured bovine luteal cells obtained from the mid-luteal phase with PGF2α (1 µM) significantly increased the expressions of PTGS2, PGF synthase (PGFS), and carbonyl reductase1 (CBR1) at 24 hr post-treatment. Together, these results suggest the presence of a local auto-amplification system for PGF2α mediated by PTGS2, PGFS, and CBR1 in the bovine CL, which may play an important role in luteolysis.

Luteoprotective mechanisms of prostaglandin F2α stimulated by luteinizing hormone in the bovine corpus luteum

Luteinizing hormone (LH) regulates several ovarian functions. However, the luteoprotective mechanisms of LH involved in the maintenance of bovine corpus luteum (CL) function are not well understood. Since prostaglandin F2α (PGF), PGE2 and progesterone (P4) are well documented as antiapoptotic factors in the bovine CL, we hypothesized that LH protects the CL by stimulating the local production and action of PGF, PGE2 and P4. Cultured bovine luteal cells obtained at the mid-luteal stage (days 8–12 of the estrous cycle) were treated with LH (10 ng/ml), onapristone (OP: a specific P4 receptor antagonist, 100 μM) and indomethacin [INDO; a cyclooxygenase (COX) inhibitor, 100 μM] for 24 h. LH with and without OP significantly increased the mRNA and protein expressions of COX-2, PGF synthase and carbonyl reductase (P<0.05) but not the mRNA and protein expressions of COX-1 and PGE synthase in bovine luteal cells. In addition, these treatments significantly increased PGF and P4 production (P<0.05) but not PGE2 production. Luteal cell viability was significantly increased by LH alone (P<0.05), but LH-increased cell viability was reduced by LH in combination with INDO as well as OP (P<0.05). The overall results suggest that LH prevents luteal cell death by stimulating luteal PGF and P4 production and supports CL function during the luteal phase in cattle.

Carotenoids and Alzheimer's disease: an insight into therapeutic role of retinoids in animal models

Carotenoids play a pivotal role in prevention of many degenerative diseases mediated by oxidative stress including neurodegenerative diseases like Alzheimer's Disease (AD). The involvement of retinoids in physiology, AD pathology and their therapeutic role in vitro and in vivo has been extensively studied. This review focuses on the role of carotenoids like retinoic acid (RA), all trans retinoic acid (ATRA), lycopene and β-carotene in prevention of AD symptoms primarily through inhibition of amyloid beta (Aβ) formation, deposition and fibril formation either by reducing the levels of p35 or inhibiting corresponding enzymes. The role of antioxidant micronutrients in prevention or delaying of AD symptoms has been included. This study emphasizes the dietary supplementation of carotenoids to combat AD and warrants further studies on animal models to unravel their mechanism of neuroprotection.

Phospholipase A2 and its molecular mechanism after spinal cord injury

Phospholipases A(2) (PLA(2)s) are a diverse family of lipolytic enzymes which hydrolyze the acyl bond at the sn-2 position of glycerophospholipids to produce free fatty acids and lysophospholipids. These products are precursors of bioactive eicosanoids and platelet-activating factor which have been implicated in pathological states of numerous acute and chronic neurological disorders. To date, more than 27 isoforms of PLA(2) have been found in the mammalian system which can be classified into four major categories: secretory PLA(2), cytosolic PLA(2), Ca(2+)-independent PLA(2), and platelet-activating factor acetylhydrolases. Multiple isoforms of PLA(2) are found in the mammalian spinal cord. Under physiological conditions, PLA(2)s are involved in diverse cellular responses, including phospholipid digestion and metabolism, host defense, and signal transduction. However, under pathological situations, increased PLA(2) activity, excessive production of free fatty acids and their metabolites may lead to the loss of membrane integrity, inflammation, oxidative stress, and subsequent neuronal injury. There is emerging evidence that PLA(2) plays a key role in the secondary injury process after traumatic spinal cord injury. This review outlines the current knowledge of the PLA(2) in the spinal cord with an emphasis being placed on the possible roles of PLA(2) in mediating the secondary SCI.

All-trans retinoic acid as a novel therapeutic strategy for Alzheimer's disease

Retinoic acid, an essential factor derived from vitamin A, has been shown to have a variety of functions including roles as an antioxidant and in cellular differentiation. Since oxidative stress and dedifferentiation of neurons appear to be common pathological elements of a number of neurodegenerative disorders, we speculated that retinoic acid may offer therapeutic promise. In this vein, recent compelling evidence indicates a role of retinoic acid in cognitive activities and anti-amyloidogenic properties. Here, we review the actions of retinoic acid that indicate that it may have therapeutic properties ideally served for the treatment of neurodegenerative diseases such as Alzheimer's disease.

Studies on plasmalogen-selective phospholipase A2 in brain

Plasmalogen-selective phospholipase A(2) (PlsEtn-PLA(2)) has been purified from pig brain using multiple column chromatographic procedure. The purified enzyme migrates as a single band on polyacrylamide. It is stimulated by Triton X-100 and inhibited by sodium deoxycholate. Purified PlsEtn-PLA(2) is inhibited by iodoacetate, and this inhibition can be prevented by beta-meracaptoethanol. Treatment of neuronal cell cultures with kainic acid stimulates PlsEtn-PLA(2) activity in a dose-dependent manner, and this stimulation can be blocked by Ly294486, a selective kainic acid receptor antagonist. Activities of PlsEtn-PLA(2) are markedly increased in plasma membrane and synaptosomal plasma membrane fraction prepared from nucleus basalis and hippocampal region of brains from Alzheimer disease patients compared to age-matched controls. It is proposed that accumulation of ceramide and increased expression of cytokines may be responsible for the stimulation of PlsEtn-PLA(2) in Alzheimer disease.

Comparison of biochemical effects of statins and fish oil in brain: the battle of the titans

Neural membranes are composed of glycerophospholipids, sphingolipids, cholesterol and proteins. The distribution of these lipids within the neural membrane is not random but organized. Neural membranes contain lipid rafts or microdomains that are enriched in sphingolipids and cholesterol. These rafts act as platforms for the generation of glycerophospholipid-, sphingolipid-, and cholesterol-derived second messengers, lipid mediators that are necessary for normal cellular function. Glycerophospholipid-derived lipid mediators include eicosanoids, docosanoids, lipoxins, and platelet-activating factor. Sphingolipid-derived lipid mediators include ceramides, ceramide 1-phosphates, and sphingosine 1-phosphate. Cholesterol-derived lipid mediators include 24-hydroxycholesterol, 25-hydroxycholesterol, and 7-ketocholesterol. Abnormal signal transduction processes and enhanced production of lipid mediators cause oxidative stress and inflammation. These processes are closely associated with the pathogenesis of acute neural trauma (stroke, spinal cord injury, and head injury) and neurodegenerative diseases such as Alzheimer disease. Statins, the HMG-CoA reductase inhibitors, are effective lipid lowering agents that significantly reduce risk for cardiovascular and cerebrovascular diseases. Beneficial effects of statins in neurological diseases are due to their anti-excitotoxic, antioxidant, and anti-inflammatory properties. Fish oil omega-3 fatty acids, eicosapentaenoic acid and docosahexaenoic acid, have similar anti-excitotoxic, antioxidant and anti-inflammatory effects in brain tissue. Thus the lipid mediators, resolvins, protectins, and neuroprotectins, derived from eicosapentaenoic acid and docosahexaenoic acid retard neuroinflammation, oxidative stress, and apoptotic cell death in brain tissue. Like statins, ingredients of fish oil inhibit generation of beta-amyloid and provide protection from oxidative stress and inflammatory processes. Collective evidence suggests that antioxidant, anti-inflammatory, and anti-apoptotic properties of statins and fish oil contribute to the clinical efficacy of treating neurological disorders with statins and fish oil. We speculate that there is an overlap between neurochemical events associated with neural cell injury in stroke and neurodegenerative diseases. This commentary compares the neurochemical effects of statins with those of fish oil.

The roles of NADPH oxidase and phospholipases A2 in oxidative and inflammatory responses in neurodegenerative diseases

Reactive oxygen species (ROS) are produced in mammalian cells through enzymic and non-enzymic mechanisms. Although some ROS production pathways are needed for specific physiological functions, excessive production is detrimental and is regarded as the basis of numerous neurodegenerative diseases. Among enzymes producing superoxide anions, NADPH oxidase is widespread in mammalian cells and is an important source of ROS in mediating physiological and pathological processes in the cardiovascular and the CNS. ROS production is linked to the alteration of intracellular calcium homeostasis, activation of Ca(2+)-dependent enzymes, alteration of cytoskeletal proteins, and degradation of membrane glycerophospholipids. There is evolving evidence that ROS produced by NADPH oxidase regulate neuronal functions and degrade membrane phospholipids through activation of phospholipases A(2) (PLA(2)). This review is intended to cover recent studies describing ROS generation from NADPH oxidase in the CNS and its downstream activation of PLA(2), namely, the group IV cytosolic cPLA(2) and the group II secretory sPLA(2). A major focus is to elaborate the dual role of NADPH oxidase and PLA(2) in mediating the oxidative and inflammatory responses in neurodegenerative diseases, including cerebral ischemia and Alzheimer's disease. Elucidation of the signaling pathways linking NADPH oxidase with the multiple forms of PLA(2) will be important in understanding the oxidative and degradative mechanisms that underline neuronal damage and glial activation and will facilitate development of therapeutic intervention for prevention and treatment of these and other neurodegenerative diseases.

Modulation of inflammation in brain: a matter of fat

Neuroinflammation is a host defense mechanism associated with neutralization of an insult and restoration of normal structure and function of brain. Neuroinflammation is a hallmark of all major CNS diseases. The main mediators of neuroinflammation are microglial cells. These cells are activated during a CNS injury. Microglial cells initiate a rapid response that involves cell migration, proliferation, release of cytokines/chemokines and trophic and/or toxic effects. Cytokines/chemokines stimulate phospholipases A2 and cyclooxygenases. This results in breakdown of membrane glycerophospholipids with the release of arachidonic acid (AA) and docosahexaenoic acid (DHA). Oxidation of AA produces pro-inflammatory prostaglandins, leukotrienes, and thromboxanes. One of the lyso-glycerophospholipids, the other products of reactions catalyzed by phospholipase A2, is used for the synthesis of pro-inflammatory platelet-activating factor. These pro-inflammatory mediators intensify neuroinflammation. Lipoxin, an oxidized product of AA through 5-lipoxygenase, is involved in the resolution of inflammation and is anti-inflammatory. Docosahexaenoic acid is metabolized to resolvins and neuroprotectins. These lipid mediators inhibit the generation of prostaglandins, leukotrienes, and thromboxanes. Levels of prostaglandins, leukotrienes, and thromboxanes are markedly increased in acute neural trauma and neurodegenerative diseases. Docosahexaenoic acid and its lipid mediators prevent neuroinflammation by inhibiting transcription factor NFkappaB, preventing cytokine secretion, blocking the synthesis of prostaglandins, leukotrienes, and thromboxanes, and modulating leukocyte trafficking. Depending on its timing and magnitude in brain tissue, inflammation serves multiple purposes. It is involved in the protection of uninjured neurons and removal of degenerating neuronal debris and also in assisting repair and recovery processes. The dietary ratio of AA to DHA may affect neurodegeneration associated with acute neural trauma and neurodegenerative diseases. The dietary intake of docosahexaenoic acid offers the possibility of counter-balancing the harmful effects of high levels of AA-derived pro-inflammatory lipid mediators.

Inhibitors of brain phospholipase A2 activity: their neuropharmacological effects and therapeutic importance for the treatment of neurologic disorders

The phospholipase A(2) family includes secretory phospholipase A(2), cytosolic phospholipase A(2), plasmalogen-selective phospholipase A(2), and calcium-independent phospholipase A(2). It is generally thought that the release of arachidonic acid by cytosolic phospholipase A(2) is the rate-limiting step in the generation of eicosanoids and platelet activating factor. These lipid mediators play critical roles in the initiation and modulation of inflammation and oxidative stress. Neurological disorders, such as ischemia, spinal cord injury, Alzheimer's disease, multiple sclerosis, prion diseases, and epilepsy are characterized by inflammatory reactions, oxidative stress, altered phospholipid metabolism, accumulation of lipid peroxides, and increased phospholipase A(2) activity. Increased activities of phospholipases A(2) and generation of lipid mediators may be involved in oxidative stress and neuroinflammation associated with the above neurological disorders. Several phospholipase A(2) inhibitors have been recently discovered and used for the treatment of ischemia and other neurological diseases in cell culture and animal models. At this time very little is known about in vivo neurochemical effects, mechanism of action, or toxicity of phospholipase A(2) inhibitors in human or animal models of neurological disorders. In kainic acid-mediated neurotoxicity, the activities of phospholipase A(2) isoforms and their immunoreactivities are markedly increased and phospholipase A(2) inhibitors, quinacrine and chloroquine, arachidonyl trifluoromethyl ketone, bromoenol lactone, cytidine 5-diphosphoamines, and vitamin E, not only inhibit phospholipase A(2) activity and immunoreactivity but also prevent neurodegeneration, suggesting that phospholipase A(2) is involved in the neurodegenerative process. This also suggests that phospholipase A(2) inhibitors can be used as neuroprotectants and anti-inflammatory agents against neurodegenerative processes in neurodegenerative diseases.

Cyclooxygenases, lipoxygenases, and epoxygenases in CNS: Their role and involvement in neurological disorders

Three enzyme systems, cyclooxygenases that generate prostaglandins, lipoxygenases that form hydroxy derivatives and leukotrienes, and epoxygenases that give rise to epoxyeicosatrienoic products, metabolize arachidonic acid after its release from neural membrane phospholipids by the action of phospholipase A(2). Lysophospholipids, the other products of phospholipase A(2) reactions, are either reacylated or metabolized to platelet-activating factor. Under normal conditions, these metabolites play important roles in synaptic function, cerebral blood flow regulation, apoptosis, angiogenesis, and gene expression. Increased activities of cyclooxygenases, lipoxygenases, and epoxygenases under pathological situations such as ischemia, epilepsy, Alzheimer's disease, Parkinson disease, amyotrophic lateral sclerosis, and Creutzfeldt-Jakob disease produce neuroinflammation involving vasodilation and vasoconstriction, platelet aggregation, leukocyte chemotaxis and release of cytokines, and oxidative stress. These are closely associated with the neural cell injury which occurs in these neurological conditions. The metabolic products of docosahexaenoic acid, through these enzymes, generate a new class of lipid mediators, namely docosatrienes and resolvins. These metabolites antagonize the effect of metabolites derived from arachidonic acid. Recent studies provide insight into how these arachidonic acid metabolites interact with each other and other bioactive mediators such as platelet-activating factor, endocannabinoids, and docosatrienes under normal and pathological conditions. Here, we review present knowledge of the functions of cyclooxygenases, lipoxygenases, and epoxygenases in brain and their association with neurodegenerative diseases.

Increase in cholesterol and cholesterol oxidation products, and role of cholesterol oxidation products in kainate-induced neuronal injury

Little is known about changes in sterols, in particular cholesterol, and cholesterol oxidation products (COPs) in oxidative injury in neural tissues. We have therefore examined changes in cholesterol and COPs using a model of excitotoxic injury. Intracerebroventricular injections of kainate in rats resulted in an increase in immunoreactivity to cholesterol in the affected CA fields of the hippocampus. The increase was confirmed by increased filipin staining of cholesterol in adjacent sections from the same animals, and in hippocampal slice or neuronal cultures after kainate treatment. In neuronal cultures, addition of lovastatin, an inhibitor of cholesterol synthesis, attenuated the increased filipin staining after kainate treatment, indicating that the increase in cholesterol could involve increased cholesterol synthesis. Furthermore, gas chromatographic mass spectrometric (GC/MS) analysis of cholesterol and COPs in kainate-injected rat brain showed a marked increase in cholesterol and COPs including 7-ketocholesterol, 3 days after kainate treatment. The addition of some COPs, including 7-ketocholesterol and cholesterol epoxides to hippocampal slices resulted in neuronal injury as reflected by decreased staining of a neuronal marker in the affected CA fields. The ability of these COPs to produce neuronal injury was attenuated by glutathione, suggesting that oxidative mechanisms are involved in neuronal injury induced by these products. These results, together with GC/MS results that showed significant increase in 7-ketocholesterol at 3 days post-kainate injury suggest that 7-ketocholesterol may be a factor in aggravating oxidative damage to neurons, after the initial stages of kainate-induced neuronal injury.

Docosahexaenoic acid, fatty acid-interacting proteins, and neuronal function: breastmilk and fish are good for you

In contrast to other tissues, the nervous system is enriched in the polyunsaturated fatty acids (PUFAs): arachidonic acid (AA, 20:4 n-6) and docosahexaenoic acid (DHA, 22:6 n-3). Despite their abundance in the nervous system, AA and DHA cannot be synthesized de novo by mammals; they, or their precursors, must be ingested from dietary sources and transported to the brain. During late gestation and the early postnatal period, neurodevelopment is exceptionally rapid, and substantial amounts of PUFAs, especially DHA, are critical to ensure neurite outgrowth as well as proper brain and retina development. Here, we review the various functions of DHA in the nervous system, the proteins involved in its internalization and metabolism into phospholipids, and its relationship to several neurological disorders, including Alzheimer's disease and depression.

Intracerebroventricular injection of phospholipases A2 inhibitors modulates allodynia after facial carrageenan injection in mice

The present study was carried out, using inhibitors to secretory phospholipase A2 (sPLA2, 12-epi-scalaradial), cytosolic phospholipase A2 (cPLA2, AACOCF3), or calcium-independent phospholipase A2 (iPLA2, bromoenol lactone), to compare possible contributions of central nervous PLA2 isoforms to the development of allodynia after facial carrageenan injection in mice. C57BL/6J (B6) mice showed increased responses to facial stimulation using a von Frey hair (1 g force), at 8 h, 1 day, and 3 days after facial carrageenan injection. On the other hand, BALB/c mice did not show increased responses at any of the time points. In both B6 and BALB/c mice, intracerebroventricular injection of inhibitors to each of the three PLA2 isoforms significantly reduced responses to von Frey hair stimulation at 8 h and 1 day after facial carrageenan injection, but at 3 days after injection, only the sPLA2 inhibitor had an effect. Since BALB/c mice did not show increased responses after facial carrageenan injection, the reduction in responses actually indicates that there is loss of normal sensitivity to von Frey hair stimulation after intracerebroventricular injection of each of these inhibitors, in this strain of mice. The effects of PLA2 inhibitors are unlikely to be due simply to inhibition of arachidonic acid generation, since intracerebroventricular injection of arachidonic acid also had an anti-nociceptive effect. The above results support an important role of central nervous PLA2s in neurotransmission and pain transmission.

Brain phospholipases A2: a perspective on the history

The phospholipases A2 (PLA2) belong to a large family of enzymes involved in the generation of several second messengers that play an important role in signal transduction processes associated with normal brain function. The phospholipase A2 family includes secretory phospholipase A2, cytosolic phospholipase A2, calcium-independent phospholipase A2, plasmalogen-selective phospholipase A2 and many other enzymes with phospholipase A2 activity that have not been classified. Few attempts have been made purify and characterize the multiple forms of PLA2 and none have been fully characterized and cloned from brain tissue. A tight regulation of phospholipase A2 isozymes is necessary for maintaining physiological levels of free fatty acids including arachidonic acid and its metabolites in the various types of neural cells. Under normal conditions, phospholipase A2 isozymes may be involved in neurotransmitter release, long-term potentiation, growth and differentiation, and membrane repair. Under pathological conditions, high levels of lipid metabolites generated by phospholipase A2 are involved in neuroinflammation, oxidative stress, and neural cell injury.

Retinoic acid-mediated phospholipase A2 signaling in the nucleus

Retinoic acid modulates a wide variety of biological processes including proliferation, differentiation, and apoptosis. It interacts with specific receptors in the nucleus, the retinoic acid receptors (RARs). The molecular mechanism by which retinoic acid mediates cellular differentiation and growth suppression in neural cells remains unknown. However, retinoic acid-induced release of arachidonic acid and its metabolites may play an important role in cell proliferation, differentiation, and apoptosis. In brain tissue, arachidonic acid is mainly released by the action of phospholipase A2 (PLA2) and phospholipase C (PLC)/diacylglycerol lipase pathways. We have used the model of differentiation in LA-N-1 cells induced by retinoic acid. The treatment of LA-N-1 cells with retinoic acid produces an increase in phospholipase A2 activity in the nuclear fraction. The pan retinoic acid receptor antagonist, BMS493, can prevent this increase in phospholipase A2 activity. This suggests that retinoic acid-induced stimulation of phospholipase A2 activity is a retinoic acid receptor-mediated process. LA-N-1 cell nuclei also have phospholipase C and phospholipase D (PLD) activities that are stimulated by retinoic acid. Selective phospholipase C and phospholipase D inhibitors block the stimulation of phospholipase C and phospholipase D activities. Thus, both direct and indirect mechanisms of arachidonic acid release exist in LA-N-1 cell nuclei. Arachidonic acid and its metabolites markedly affect the neurite outgrowth and neurotransmitter release in cells of neuronal and glial origin. We propose that retinoic acid receptors coupled with phospholipases A2, C and D in the nuclear membrane play an important role in the redistribution of arachidonic acid in neuronal and non-nuclear neuronal membranes during differentiation and growth suppression. Abnormal retinoid metabolism may be involved in the downstream transcriptional regulation of phospholipase A2-mediated signal transduction in schizophrenia and Alzheimer disease (AD). The development of new retinoid analogs with diminished toxicity that can cross the blood-brain barrier without harm and can normalize phospholipase A2-mediated signaling will be important in developing pharmacological interventions for these neurological disorders.

Citicoline decreases phospholipase A2 stimulation and hydroxyl radical generation in transient cerebral ischemia

Neuroprotection by citicoline (CDP-choline) in transient cerebral ischemia has been demonstrated previously. Citicoline has undergone several Phase III clinical trials for stroke, and is being evaluated for treatment of Alzheimer's and Parkinson's diseases. Phospholipid degradation and generation of reactive oxygen species (ROS) are major factors causing neuronal injury in CNS trauma and neurodegenerative diseases. Oxidative metabolism of arachidonic acid (released by the action of phospholipases) contributes to ROS generation. We examined the effect of citicoline on phospholipase A(2) (PLA(2)) activity in relation to the attenuation of hydroxyl radical (OH.) generation after transient forebrain ischemia of gerbil. PLA(2) activity (requires mM Ca(2+)) increased significantly (P < 0.05) in both membrane (50.2 +/- 2.2 pmol/min/mg protein compared to sham 35.9 +/- 3.2) and mitochondrial fractions (77.0 +/- 1.2 pmol/min/mg protein compared to sham 33.9 +/- 1.2) after cerebral ischemia and 2 hr reperfusion in gerbil, which was significantly attenuated (P < 0.01) by citicoline (membrane, 39.9. +/- 2.2 and mitochondria, 41.9 +/- 3.2 pmol/min/mg protein). In vitro, citicoline and its components cytidine and choline had no effect on PLA(2) activity, and thus citicoline as such is not a PLA(2) inhibitor. Ischemia/reperfusion resulted in significant OH. generation (P < 0.01) and citicoline significantly (P < 0.01) attenuated their formation (expressed as 2,3-dihydroxybenzoic acid/salicylate ratio; ischemia/24 hr reperfusion, 6.30 +/- 0.23; sham, 2.56 +/- 0.27; ischemia/24 hr reperfusion + citicoline, 4.85 +/- 0.35). These results suggest that citicoline affects PLA(2) stimulation and decreases OH. generation after transient cerebral ischemia.

Secretory phospholipase A2 activity in the normal and kainate injected rat brain, and inhibition by a peptide derived from python serum

The present study aimed to elucidate sPLA(2) activity in the normal and kainate-lesioned hippocampus using selective inhibitors of sPLA(2). In normal rats the highest levels of sPLA(2) were observed in the hippocampus, pons, and medulla, followed by the cerebral neocortex and caudate nucleus. After intracerebroventricular kainate injections an increase in total PLA(2) activity was observed in the rat hippocampus. Using a selective sPLA(2) inhibitor 12-epi-scalaradial, sPLA(2) activity was found to be significantly increased by 2.5-fold on the side of the intracerebroventricular injection compared to the contralateral side. A peptide P-NT.II, derived from the amino acid sequence of "PLA(2)-inhibitory protein," discovered in the serum of the reticulated python, also showed potent sPLA(2) inhibitory activity in homogenates from the kainate-injected hippocampus. These results show that there is a high level of sPLA(2) activity in the normal hippocampus, pons, and medulla oblongata, and that the level increases further in the hippocampus after kainate-induced excitotoxic injury. The increased PLA(2) activity was inhibited by P-NT.II, indicating a potential use of this peptide as a PLA(2) inhibitory agent in the brain.

Citicoline mechanisms and clinical efficacy in cerebral ischemia

Citicoline, an intermediate in the biosynthesis of phosphatidylcholine (PtdCho), has shown beneficial effects in various CNS injury models and neurodegenerative diseases. PtdCho hydrolysis by phospholipase A(2) (PLA(2)) after cerebral ischemia and reperfusion yields arachidonic acid (ArAc) and lyso-PtdCho. ArAc oxidative metabolism results in formation of reactive oxygen species and lipid peroxides. Lyso-PtdCho could inhibit activity of cytidine triphosphate-phosphocholine cytidylyltransferase (the rate-limiting enzyme in PtdCho biosynthesis), resulting in impaired PtdCho synthesis. Citicoline significantly increased glutathione levels and attenuated release of ArAc and the loss of PtdCho, cardiolipin, and sphingomyelin following transient cerebral ischemia. These effects could be explained by an effect of citicoline on PLA(2). Based on these observations, a mechanism has been hypothesized. This Mini-Review summarizes recent experimental data on the effects of citicoline in cerebral ischemia and evaluates several factors that might have hindered efficacy of citicoline in stroke clinical trials in the United States. Clinical stroke trials of citicoline in Europe and Japan have demonstrated beneficial effects. U.S. trials shown only marginal effects, which might be due to the 24 hr time window, the dose and route of administration, and the stringency of the primary outcome parameters. Recent evaluation of U.S. clinical data suggests that reduction of infarct growth may be a more sensitive measure of the citicoline effect than improvement on the NIH Stroke Scale (NIHSS) by > or =7 points. The citicoline neuroprotective mechanism has not been clearly identified, and its potential in stroke treatment might still be fully recognized in the United States. The clinical efficacy of citicoline should be examined further in light of the recent phase III stroke clinical trials and experimental data for cerebral ischemia.

Differential effects of calcium-dependent and calcium-independent phospholipase A(2) inhibitors on kainate-induced neuronal injury in rat hippocampal slices

Brain tissue contains multiple forms of intracellular phospholipase A(2) (PLA(2)) activity that differ from each other in many ways including their response to specific inhibitors. The systemic administration of kainic acid to rats produces a marked increase in cPLA(2) activity in neurons and astrocytes. This is associated with increased lipid peroxidation as evidenced by accumulation of 4-hydroxynonenal (4-HNE) modified proteins. The present study describes the effect of specific inhibitors of Ca(2+)-dependent or Ca(2+)-independent PLA(2) on kainite-induced excitotoxic injury in rat hippocampal slices. Specific inhibitors of Ca(2+)-dependent PLA(2) prevented the decrease of a neuronal marker, GluR1, and increase in cPLA(2) and 4-HNE immunoreactivities in slices treated with kainate. This shows that cPLA(2) plays an important role in kainite-induced neurotoxicity and that cPLA(2) inhibitors can be used to protect hippocampal slices from damage induced by kainate.

Plasmalogens: workhorse lipids of membranes in normal and injured neurons and glia

Plasmalogens are unique glycerophospholipids because they have an enol ether double bond at the sn-1 position of the glycerol backbone. They are found in all mammalian tissues, with ethanolamine plasmalogens 10-fold higher than choline plasmalogens except in muscles. The enol ether double bond at the sn-1 position makes plasmalogens more susceptible to oxidative stress than the corresponding ester-bonded glycerophospholipids. Plasmalogens are not only structural membrane components and a reservoir for second messengers but may also be involved in membrane fusion, ion transport, and cholesterol efflux. Plasmalogens may also act as antioxidants, thus protecting cells from oxidative stress. Receptor-mediated degradation of plasmalogens by plasmalogen-selective phospholipase A2 results in the generation of arachidonic acid, eicosanoids, and platelet activating factor. Low levels of these metabolites have trophic effects, but at high concentration they are cytotoxic and may be involved in allergic response, inflammation, and trauma. Levels of plasmalogens are decreased in several neurological disorders including Alzheimer's disease, ischemia, and spinal cord trauma. This may be due to the stimulation of plasmalogen-selective phospholipase A2. A deficiency of plasmalogens in peroxisomal disorders and Niemann-Pick type C disease indicates that this deficiency may be due to the decreased activity of plasmalogen synthesizing enzymes that occur in peroxisomes.

Does CDP-choline modulate phospholipase activities after transient forebrain ischemia?

Ten min forebrain ischemia/1-day reperfusion resulted in significant decreases in total phosphatidylcholine (PtdCho), phosphatidylinositol (PtdIns), and cardiolipin in gerbil hippocampus. CDP-choline restored cardiolipin levels, arachidonic acid content of PtdCho, partially but significantly restored total PtdCho, and had no effect on PtdIns. These data suggest that CDP-choline prevented the activation of phospholipase A(2) (rather than inhibiting phospholipase A(2) activity) but did not affect activities of PtdCho-phospholipases C and/or D, or phosphoinositide-phospholipase C. CDP-choline also provided significant protection for hippocampal CA(1) neurons.