Neuroregulatory and neuropathological actions of the ether-phospholipid platelet-activating factor

Platelet-activating factor receptor (PAFR) regulates neuronal maturation and synaptic transmission during postnatal retinal development

Introduction

Platelet-activating factor (PAF), PAF receptor (PAFR), and PAF- synthesis/degradation systems are involved in essential CNS processes such as neuroblast proliferation, differentiation, migration, and synaptic modulation. The retina is an important central nervous system (CNS) tissue for visual information processing. During retinal development, the balance between Retinal Progenitor Cell (RPC) proliferation and differentiation is crucial for proper cell determination and retinogenesis. Despite its importance in retinal development, the effects of PAFR deletion on RPC dynamics are still unknown.

Methods

We compared PAFR knockout mice (PAFR-/-) retinal postnatal development proliferation and differentiation aspects with control animals. Electrophysiological responses were analyzed by electroretinography (ERG).

Results and discussion

In this study, we demonstrate that PAFR-/- mice increased proliferation during postnatal retinogenesis and altered the expression of specific differentiation markers. The retinas of postnatal PAFR-/- animals decreased neuronal differentiation and synaptic transmission markers, leading to differential responses to light stimuli measured by ERG. Our findings suggest that PAFR signaling plays a critical role in regulating postnatal RPC cell differentiation dynamics during retinal development, cell organization, and neuronal circuitry formation.

Screening for bilayer-active and likely cytotoxic molecules reveals bilayer-mediated regulation of cell function

A perennial problem encountered when using small molecules (drugs) to manipulate cell or protein function is to assess whether observed changes in function result from specific interactions with a desired target or from less specific off-target mechanisms. This is important in laboratory research as well as in drug development, where the goal is to identify molecules that are unlikely to be successful therapeutics early in the process, thereby avoiding costly mistakes. We pursued this challenge from the perspective that many bioactive molecules (drugs) are amphiphiles that alter lipid bilayer elastic properties, which may cause indiscriminate changes in membrane protein (and cell) function and, in turn, cytotoxicity. Such drug-induced changes in bilayer properties can be quantified as changes in the monomer↔dimer equilibrium for bilayer-spanning gramicidin channels. Using this approach, we tested whether molecules in the Pathogen Box (a library of 400 drugs and drug-like molecules with confirmed activity against tropical diseases released by Medicines for Malaria Venture to encourage the development of therapies for neglected tropical diseases) are bilayer modifiers. 32% of the molecules in the Pathogen Box were bilayer modifiers, defined as molecules that at 10 µM shifted the monomer↔dimer equilibrium toward the conducting dimers by at least 50%. Correlation analysis of the molecules' reported HepG2 cell cytotoxicity to bilayer-modifying potency, quantified as the shift in the gramicidin monomer↔dimer equilibrium, revealed that molecules producing <25% change in the equilibrium had significantly lower probability of being cytotoxic than molecules producing >50% change. Neither cytotoxicity nor bilayer-modifying potency (quantified as the shift in the gramicidin monomer↔dimer equilibrium) was well predicted by conventional physico-chemical descriptors (hydrophobicity, polar surface area, etc.). We conclude that drug-induced changes in lipid bilayer properties are robust predictors of the likelihood of membrane-mediated off-target effects, including cytotoxicity.

Interneuron Heterotopia in the Lis1 Mutant Mouse Cortex Underlies a Structural and Functional Schizophrenia-Like Phenotype

LIS1 is one of the principal genes related to Type I lissencephaly, a severe human brain malformation characterized by an abnormal neuronal migration in the cortex during embryonic development. This is clinically associated with epilepsy and cerebral palsy in severe cases, as well as a predisposition to developing mental disorders, in cases with a mild phenotype. Although genetic variations in the LIS1 gene have been associated with the development of schizophrenia, little is known about the underlying neurobiological mechanisms. We have studied how the Lis1 gene might cause deficits associated with the pathophysiology of schizophrenia using the Lis1/sLis1 murine model, which involves the deletion of the first coding exon of the Lis1 gene. Homozygous mice are not viable, but heterozygous animals present abnormal neuronal morphology, cortical dysplasia, and enhanced cortical excitability. We have observed reduced number of cells expressing GABA-synthesizing enzyme glutamic acid decarboxylase 67 (GAD67) in the hippocampus and the anterior cingulate area, as well as fewer parvalbumin-expressing cells in the anterior cingulate cortex in Lis1/sLis1 mutants compared to control mice. The cFOS protein expression (indicative of neuronal activity) in Lis1/sLis1 mice was higher in the medial prefrontal (mPFC), perirhinal (PERI), entorhinal (ENT), ectorhinal (ECT) cortices, and hippocampus compared to control mice. Our results suggest that deleting the first coding exon of the Lis1 gene might cause cortical anomalies associated with the pathophysiology of schizophrenia.

Platelet activating factor in the eye: Physiological roles, diseases and future perspectives

Platelet Activating Factor (PAF) is a known phospholipid mediator of inflammation. Since its first description in 1972, it has emerged as a key regulator of vital cellular signaling functions, as proliferation, cell adhesion, and apoptosis. Evidence suggests that interactions between PAF and its receptor (PAFR) play a critical role in nervous system tissues, including the retina. The retina is a very important constituent of the visual system, along with the cornea, sclera, choroid, iris, and ciliary body, that acts synergistically to provide vision and to maintain optical homeostasis. There is evidence that PAF may regulate a wide range of physiological functions in the visual system tissues, such as eye development, inflammation, epithelial wound healing, and synapsis. Due to their multiple functions, PAF and PAFR also have important pathological and clinical implications in ocular disorders such as Choroidal Neovascularization (CNV), Age Macular Degeneration, (AMD), Diabetic Retinopathy (DR), transplant responses, and pharmacological interactions. Studies with PAFR antagonists have shown promising results such as inhibition of neovascularization and chloroquine-induced retinopathies, as well as reducing inflammation and retinal cell death. Due to the importance of PAFR signaling in the visual system and ophthalmology research, this review aims to provide a general overview of current and future perspectives about PAF in eye biology.

Roles of endogenous ether lipids and associated PUFAs in the regulation of ion channels and their relevance for disease

Ether lipids (ELs) are lipids characterized by the presence of either an ether linkage (alkyl lipids) or a vinyl ether linkage [i.e., plasmalogens (Pls)] at the sn1 position of the glycerol backbone, and they are enriched in PUFAs at the sn2 position. In this review, we highlight that ELs have various biological functions, act as a reservoir for second messengers (such as PUFAs) and have roles in many diseases. Some of the biological effects of ELs may be associated with their ability to regulate ion channels that control excitation-contraction/secretion/mobility coupling and therefore cell physiology. These channels are embedded in lipid membranes, and lipids can regulate their activities directly or indirectly as second messengers or by incorporating into membranes. Interestingly, ELs and EL-derived PUFAs have been reported to play a key role in several pathologies, including neurological disorders, cardiovascular diseases, and cancers. Investigations leading to a better understanding of their mechanisms of action in pathologies have opened a new field in cancer research. In summary, newly identified lipid regulators of ion channels, such as ELs and PUFAs, may represent valuable targets to improve disease diagnosis and advance the development of new therapeutic strategies for managing a range of diseases and conditions.

Effects of Platelet-Activating Factor on Brain Microvascular Endothelial Cells

Platelet-activating factor (PAF) is a potent phospholipid mediator that exerts various pathophysiological effects by interacting with a G protein-coupled receptor. PAF has been reported to increase the permeability of the blood-brain barrier (BBB) via incompletely characterized mechanisms. We investigated the effect of PAF on rat brain microvascular endothelial cells (RBMVEC), a critical component of the BBB. PAF produced a dose-dependent increase in cytosolic Ca²⁺ concentration; the effect was prevented by the PAF receptor antagonist, WEB2086. The effect of PAF on cytosolic Ca²⁺ was abolished in Ca²⁺-free saline or in the presence of L-type voltage-gated Ca²⁺ channel inhibitor, nifedipine, indicating that Ca²⁺ influx is critical for PAF-induced increase in cytosolic Ca²⁺. PAF produced RBMVEC depolarization; the effect was inhibited by WEB2086. In cells loaded with [(4-amino-5-methylamino-2',7'-difluoro-fluorescein)diacetate] (DAF-FM), a nitric oxide (NO)-sensitive fluorescent dye, PAF increased the NO level; the effect was prevented by WEB2086, nifedipine or by l-NAME, an inhibitor of NO synthase. Immunocytochemistry studies indicate that PAF reduced the immunostaining of ZO-1, a tight junction-associated protein, increased F-actin fibers, and produced intercellular gaps. PAF produced a decrease in RBMVEC monolayer electrical resistance assessed with Electric Cell-Substrate Impedance Sensing (ECIS), indicative of a disruption of endothelial barrier function. In vivo studies indicate that PAF increased the BBB permeability, assessed with sodium fluorescein and Evans Blue methods, via PAF receptor-dependent mechanisms, consequent to Ca²⁺ influx and increased NO levels. Our studies reveal that PAF alters the BBB permeability by multiple mechanisms, which may be relevant for central nervous system (CNS) inflammatory disorders.

From peroxisomal disorders to common neurodegenerative diseases - the role of ether phospholipids in the nervous system

The emerging diverse roles of ether (phospho)lipids in nervous system development and function in health and disease are currently attracting growing interest. Plasmalogens, a subgroup of ether lipids, are important membrane components involved in vesicle fusion and membrane raft composition. They store polyunsaturated fatty acids and may serve as antioxidants. Ether lipid metabolites act as precursors for the formation of glycosyl-phosphatidyl-inositol anchors; others, like platelet-activating factor, are implicated in signaling functions. Consolidating the available information, we attempt to provide molecular explanations for the dramatic neurological phenotype in ether lipid-deficient human patients and mice by linking individual functional properties of ether lipids with pathological features. Furthermore, recent publications have identified altered ether lipid levels in the context of many acquired neurological disorders including Alzheimer's disease (AD) and autism. Finally, current efforts to restore ether lipids in peroxisomal disorders as well as AD are critically reviewed.

Current Understanding of Platelet-Activating Factor Signaling in Central Nervous System Diseases

Platelet-activating factor (PAF) is a bioactive lipid mediator which serves as a reciprocal messenger between the immune and nervous systems. PAF, a pluripotent inflammatory mediator, is extensively expressed in many cells and tissues and has either beneficial or detrimental effects on the progress of inflammation-related neuropathology. Its wide distribution and various biological functions initiate a cascade of physiological or pathophysiological responses during development or diseases. Current evidence indicates that excess PAF accumulation in CNS diseases exacerbates the inflammatory response and pathological consequences, while application of PAF inhibitors or PAFR antagonists by blocking this signaling pathway significantly reduces inflammation, protects cells, and improves the recovery of neural functions. In this review, we integrate the current findings of PAF signaling in CNS diseases and elucidate topics less appreciated but important on the role of PAF signaling in neurological diseases. We propose that the precise use of PAF inhibitors or PAFR antagonists that target the specific neural cells during the appropriate temporal window may constitute a potential therapy for CNS diseases.

Intracellular PAF-Acetylhydrolase Type I

Platelet-activating factor (PAF) is a phospholipid mediator whose synthesis and degradation depend on specific sets of enzymes. PAF-acetylhydrolase (PAF-AH) hydrolyzes the acetyl moiety of PAF at its sn-2 position and thereby inactivates it. PAF-AH Ib, originally identified in brain, exists in the cytoplasm of many (probably all) types of mammalian cells and tissues. PAF-AH Ib consists of three subunits (α1, α2, and β), in which the α subunits provide the catalytic activity. The finding that the β subunit is the product of the causative gene for Miller-Dieker lissencephaly led to extensive analyses of PAF-AH Ib subunits in the field of cell biology and neurobiology. More than 20 molecules are known to bind to PAF-AH Ib subunits, and PAF-AH Ib has been implicated in neuronal development, neuronal functions, Alzheimer's disease, bipolar disorder, cancer, spermatogenesis, and tolerance to hypoxia. However, in almost all of these cases, how the catalytic activity is involved and the identity of the most important substrate of this enzyme are unclear. In this chapter, the structure and functions of PAF-AH Ib and its subunit proteins are summarized and their contributions to human diseases are discussed.

Biomimetic choline-like graphene oxide composites for neurite sprouting and outgrowth

Neurodegenerative diseases or acute injuries of the nervous system always lead to neuron loss and neurite damage. Thus, the development of effective methods to repair these damaged neurons is necessary. The construction of biomimetic materials with specific physicochemical properties is a promising solution to induce neurite sprouting and guide the regenerating nerve. Herein, we present a simple method for constructing biomimetic graphene oxide (GO) composites by covalently bonding an acetylcholine-like unit (dimethylaminoethyl methacrylate, DMAEMA) or phosphorylcholine-like unit (2-methacryloyloxyethyl phosphorylcholine, MPC) onto GO surfaces to enhance neurite sprouting and outgrowth. The resulting GO composites were characterized by Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, UV-vis spectrometry, scanning electron microscopy, and contact angle analyses. Primary rat hippocampal neurons were used to investigate nerve cell adhesion, spreading, and proliferation on these biomimetic GO composites. GO-DMAEMA and GO-MPC composites provide the desired biomimetic properties for superior biocompatibility without affecting cell viability. At 2 to 7 days after cell seeding was performed, the number of neurites and average neurite length on GO-DMAEMA and GO-MPC composites were significantly enhanced compared with the control GO. In addition, analysis of growth-associate protein-43 (GAP-43) by Western blot showed that GAP-43 expression was greatly improved in biomimetic GO composite groups compared to GO groups, which might promote neurite sprouting and outgrowth. All the results demonstrate the potential of DMAEMA- and MPC-modified GO composites as biomimetic materials for neural interfacing and provide basic information for future biomedical applications of graphene oxide.

Developmental dynamics of PAFAH1B subunits during mouse brain development

Platelet-activating factor (PAF) mediates an array of biological processes in the mammalian central nervous system as a bioactive lipid messenger in synaptic function and dysfunction (plasticity, memory, and neurodegeneration). The intracellular enzyme that deacetylates the PAF (PAFAH1B) is composed of a tetramer of two catalytic subunits, ALPHA1 (PAFAH1B3) and ALPHA2 (PAFAH1B2), and a regulatory dimer of LIS1 (PAFAH1B1). We have investigated the mouse PAFAH1B subunit genes during brain development in normal mice and in mice with a hypomorphic allele for Lis1 (Lis1/sLis1; Cahana et al. [2001] Proc Natl Acad Sci U S A 98:6429-6434). We have analyzed quantitatively (by means of real-time polymerase chain reaction) and qualitatively (by in situ hybridization techniques) the amounts and expression patterns of their transcription in developing and postnatal brain, focusing mainly on differences in two laminated encephalic regions, the forebrain (telencephalon) and hindbrain (cerebellum) separately. The results revealed significant differences in cDNA content between these two brain subdivisions but, more importantly, between the LIS1 complex subunits. In addition, we found significant spatial differences in gene expression patterns. Comparison of results obtained with Lis1/sLis1 analysis also revealed significant temporal and spatial differences in Alpha1 and Lis1 expression levels. Thus, small changes in the amount of the Lis1 gene may differentially regulate expression of Alpha1 and Alpha2, depending on the brain region, which suggests different roles for each LIS1 complex subunit during neural differentiation and neural migration.

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.

PAF-AH Catalytic Subunits Modulate the Wnt Pathway in Developing GABAergic Neurons

Platelet-activating factor acetylhydrolase 1B (PAF-AH) inactivates the potent phospholipid platelet-activating factor (PAF) and is composed of two catalytic subunits (α1 and α2) and a dimeric regulatory subunit, LIS1. The function of the catalytic subunits in brain development remains unknown. Here we examined their effects on proliferation in the ganglionic eminences and tangential migration. In α1 and α2 catalytic subunits knockout mice we noticed an increase in the size of the ganglionic eminences resulting from increased proliferation of GABAergic neurons. Our results indicate that the catalytic subunits act as negative regulators of the Wnt signaling pathway. Overexpression of each of the PAF-AH catalytic subunits reduced the amount of nuclear beta-catenin and provoked a shift of this protein from the nucleus to the cytoplasm. In the double mutant mice, Wnt signaling increased in the ganglionic eminences and in the dorsal part of the cerebral cortex. In situ hybridization revealed increased and expanded expression of a downstream target of the Wnt pathway (Cyclin D1), and of upstream Wnt components (Tcf4, Tcf3 and Wnt7B). Furthermore, the interneurons in the cerebral cortex were more numerous and in a more advanced position. Transplantation assays revealed a non-cell autonomous component to this phenotype, which may be explained in part by increased and expanded expression of Sdf1 and Netrin-1. Our findings strongly suggest that PAF-AH catalytic subunits modulate the Wnt pathway in restricted areas of the developing cerebral cortex. We hypothesize that modulation of the Wnt pathway is the evolutionary conserved activity of the PAF-AH catalytic subunits.

Protein expression profiling during chick retinal maturation: a proteomics-based approach

Background

The underlying pathways that drive retinal neurogenesis and synaptogenesis are still relatively poorly understood. Protein expression analysis can provide direct insight into these complex developmental processes. The aim of this study was therefore to employ proteomic analysis to study the developing chick retina throughout embryonic (E) development commencing at day 12 through 13, 17, 19 and post-hatch (P) 1 and 33 days.

Results

2D proteomic and mass spectrometric analysis detected an average of 1514 spots per gel with 15 spots demonstrating either modulation or constitutive expression identified via MS. Proteins identified included alpha and beta-tubulin, alpha enolase, B-creatine kinase, gamma-actin, platelet-activating factor (PAF), PREDICTED: similar to TGF-beta interacting protein 1, capping protein (actin filament muscle Z line), nucleophosmin 1 (NPM1), dimethylarginine dimethylaminohydrolase, triosphoaphate isomerase, DJ1, stathmin, fatty acid binding protein 7 (FABP7/B-FABP), beta-synuclein and enhancer of rudimentary homologue.

Conclusion

This study builds upon previous proteomic investigations of retinal development and represents the addition of a unique data set to those previously reported. Based on reported bioactivity some of the identified proteins are most likely to be important to normal retinal development in the chick. Continued analysis of the dynamic protein populations present at the early stages and throughout retinal development will increase our understanding of the molecular events underpinning retinogenesis.

Role of secretory phospholipase a(2) in CNS inflammation: implications in traumatic spinal cord injury

Secretory phospholipases A(2) (sPLA(2)s) are a subfamily 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 (PAF). The hydrolysis of membrane phospholipids by PLA(2) is a rate-limiting step for generation of eicosanoids and PAF. To date, more than 10 isozymes of sPLA(2) have been found in the mammalian central nervous system (CNS). Under physiological conditions, sPLA(2)s are involved in diverse cellular responses, including host defense, phospholipid digestion and metabolism. However, under pathological situations, increased sPLA(2) activity and excessive production of free fatty acids and their metabolites may lead to inflammation, loss of membrane integrity, oxidative stress, and subsequent tissue injury. Emerging evidence suggests that sPLA(2) plays a role in the secondary injury process after traumatic or ischemic injuries in the brain and spinal cord. Importantly, sPLA(2) may act as a convergence molecule that mediates multiple key mechanisms involved in the secondary injury since it can be induced by multiple toxic factors such as inflammatory cytokines, free radicals, and excitatory amino acids, and its activation and metabolites can exacerbate the secondary injury. Blocking sPLA(2) action may represent a novel and efficient strategy to block multiple injury pathways associated with the CNS secondary injury. This review outlines the current knowledge of sPLA(2) in the CNS with emphasis placed on the possible roles of sPLA(2) in mediating CNS injuries, particularly the traumatic and ischemic injuries in the brain and spinal cord.

Lissencephaly 1 linking to multiple diseases: mental retardation, neurodegeneration, schizophrenia, male sterility, and more

Lissencephaly 1 (LIS1) was the first gene implicated in the pathogenesis of type-1 lissencephaly. More than a decade of research by multiple laboratories has revealed that LIS1 is a key node protein, which participates in several pathways, including association with the molecular motor cytoplasmic dynein, the reelin signaling pathway, and the platelet-activating factor pathway. Mutations in LIS1-interacting proteins, either in human, or in mouse models has suggested that LIS1 might play a role in the pathogenesis of numerous diseases such as male sterility, schizophrenia, neuronal degeneration, and viral infections.

Identification and quantitation of changes in the platelet activating factor family of glycerophospholipids over the course of neuronal differentiation by high-performance liquid chromatography electrospray ionization tandem mass spectrometry

Glycerophospholipids are important structural lipids in membranes with changes associated with progressive neurodegenerative disorders such as Alzheimer disease. Synthesis of the platelet activating factor (PAF) glycerophospholipid subclass is implicated in the control of neuronal differentiation and death. In this article, we combine nanoflow HPLC and mass spectrometry to screen, identify, and quantitate changes in glycerophospholipid subspecies, specifically PAF family members, over the course of neuronal differentiation. Furthermore, precursor ion scans for fragments characteristic of PAF phosphocholine family members and the standard additions of PAF subspecies were combined to perform absolute quantitation of PAF lipids in undifferentiated and differentiated PC12 cells. Surprisingly, a marked asymmetry was detected in the two predominant PAF species (C16:0, C18:0) over the course of differentiation. These results describe a new technique for the sensitive analysis of lipids combining nanoflow HPLC, ESI-MS, and precursor ion scan. Limits of detection of as little as 2 pg of PAF and LPC were obtained, and analysis of the lipidome of as little as 70,000 cells was performed on this system. Furthermore, application to the PC12 model identified a quantifiable difference between PAF molecular species produced over the course of neuronal differentiation.

PMS777, A Bis-interacting Ligand for PAF Receptor Antagonism and AChE Inhibition, Attenuates PAF-induced Neurocytotoxicity in SH-SY5Y Cells

(1) HIV-1 and viral proteins-evoked chronic brain inflammation, which is characterized by microglial activation, is the pivotal neuropathogenesis of HIV-1-associated dementia (HAD). Platelet-activating factor (PAF), mainly released from activated microglia and acts as a high potent inflammatory mediator and a neurotoxin, is indicated to be a principle initiator of neuroinflammation, neuronal dysfunction, and apoptosis related to HAD. Thus, bis-interacting ligands of acetylcholinesterase (AChE) inhibition and PAF receptor antagonism would be of great interest in the therapeutic potential of HAD not only for improvement of cognitive performance, but also for disease-modifying. (2). We have previously reported that a novel tetrahydrofuran-derived bis-interacting ligand PMS777 had satisfying potencies for PAF receptor blockade and AChE inhibition, and markedly improved cholinergic dysfunction-induced cognitive impairment in mice. Continuing with our research, we further investigated the neuroprotective activities of PMS777 on PAF-triggered neuronal injury in human neuroblastoma SH-SY5Y cells. (3) The bis-interacting ligand PMS777 (10 muM) obviously alleviated PAF-induced cell apoptosis in SH-SY5Y cells. Pretreatment with PMS777 also markedly inhibited intracellular Ca(2+) overload, down-regulation of anti-apoptotic bcl-2 mRNA, stimulation of pro-apoptotic bax mRNA expression and activation of caspase-3 pathway. Also, PMS777 could fine-tune pro-inflammatory cyclooxygenase-2 (cox-2) mRNA expression in PAF-treated cells. (4) These results suggest that PMS777 possesses a neuroprotective profile via anti-apoptotic/inflammatory signaling and warrant further investigations in connection with the potential value of this compound in HAD treatment.

Lipid signaling: sleep, synaptic plasticity, and neuroprotection

Increasing evidence indicates that bioactive lipids participate in the regulation of synaptic function and dysfunction. We have demonstrated that signaling mediated by platelet-activating factor (PAF) and cyclooxygenase (COX)-2-synthesized PGE2 is involved in synaptic plasticity, memory, and neuronal protection [Clark GD, Happel LT, Zorumski CF, Bazan NG. Enhancement of hippocampal excitatory synaptic transmission by platelet-activating factor. Neuron 1992; 9:1211; Kato K, Clark GD, Bazan NG, Zorumski CF. Platelet-activating factor as a potential retrograde messenger in CA1 hippocampal long-term potentiation. Nature 1994; 367:175; Izquierdo I, Fin C, Schmitz PK, et al. Memory enhancement by intrahippocampal, intraamygdala or intraentorhinal infusion of platelet-activating factor measured in an inhibitory avoidance. Proc Natl Acad Sci USA 1995; 92:5047; Chen C, Magee CJ, Bazan NG. Cyclooxygenase-2 regulates prostaglandin E2 signaling in hippocampal long-term synaptic plasticity. J Neurophysiol 2002; 87:2851]. Recently, we found that prolonged continuous wakefulness (primarily rapid eye movement (REM)-sleep deprivation, SD) causes impairments in hippocampal long-term synaptic plasticity and hippocampus-dependent memory formation [McDermott CM, LaHoste GJ, Chen C, Musto A, Bazan NG, Magee JC. Sleep deprivation causes behavioral, synaptic, and membrane excitability alterations in hippocampal neurons. J Neurosci 2003; 23:9687]. To explore the mechanisms underlying SD-induced impairments, we have studied several bioactive lipids in the hippocampus following SD. It appears that SD causes increases in prostaglandin D2 (PGD2) and 2-arachidonylglycerol (2-AG), and a decrease in PGE2, suggesting that these lipid messengers participate in memory consolidation during REM sleep. We have also explored the formation of endogenous neuroprotective lipids. Toward this aim, we have used ischemia-reperfusion damage and LC-PDA-ESI-MS-MS-based lipidomic analysis and identified docosanoids derived from synaptic phospholipid-enriched docosahexaenoic acid. Some of the docosanoids exert potent neuroprotective bioactivity [Marcheselli VL, Hong S, Lukiw WJ, et al. Novel docosanoids inhibit brain ischemia-reperfusion-mediated leukocyte infiltration and pro-inflammatory gene expression. J Biol Chem 2003; 278:43807; Mukherjee PK, Marcheselli VL, Serhan CN, Bazan, NG. Neuroprotectin D1: A docosahexaenoic acid-derived docosatriene protects human retinal pigment epithelial cells from oxidative stress. Proc Nat Acad Sci USA 2004; 101:8491). Taken together, these observations that signaling lipids participate in synaptic plasticity, cognition, and survival indicate that lipid signaling is closely associated with several functions (e.g; learning and memory, sleep, and experimental stroke) and pathologic events. Alterations in endogenous signaling lipids or their receptors resulting from drug abuse lead to changes in synaptic circuitry and induce profound effects on these important functions. In the present article, we will briefly review bioactive lipids involved in sleep, synaptic transmission and plasticity, and neuroprotection, focusing mainly on our experimental studies and how these signaling molecules are related to functions and implicated in some neurologic disorders.

Lipid signaling in neural plasticity, brain repair, and neuroprotection

The extensive networking of the cells of the nervous system results in large cell membrane surface areas. We now know that neuronal membranes contain phospholipid pools that are the 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 neuronal injury or neuroprotection. Prostaglandins are synthesized as a result of cyclooxygenase activity. In the first step of the arachidonic acid cascade, the short-lived precursor, prostaglandin H2, is synthesized. Additional steps in the cascade result in the synthesis of an array of prostaglandins, which participate in numerous physiological and neurological processes. Our laboratory recently reported that the membrane polyunsaturated fatty acid, docosahexaenoic acid, is the precursor of oxygenation products now known as the docosanoids, some of which are powerful counter-proinflammatory mediators. The mediator 10,17S-docosatriene (neuroprotectin D1, NPD1) counteracts leukocyte infiltration, NF-kappa activation, and proinflammatory gene expression in brain ischemia-reperfusion and is an apoptostatic mediator, potently counteracting oxidative stress-triggered apoptotic DNA damage in retinal pigment epithelial cells. NPD1 also upregulates the anti-apoptotic proteins Bcl-2 and Bcl-xL and decreases pro-apoptotic Bax and Bad expression. Another biologically active messenger derived from membrane phospholipids in response to synaptic activity is platelet-activating factor (PAF). The tight regulation of the balance between synthesis (via phospholipases) and degradation (via acetylhydrolases) of PAF modulates the functions of this lipid messenger. Under pathological conditions, this balance is tipped, and PAF becomes a proinflammatory mediator and neurotoxic agent. The newly discovered docosahexaenoic acid signaling pathways, as well as other lipid messengers related to synaptic activation, may lead to the clarification of clinical issues relevant to stroke, age-related macular degeneration, spinal cord injury, Alzheimer's disease, and other diseases that include neuroinflammatory components.

Platelet-activating factor enhances the expression of nerve growth factor in normal human astrocytes under hypoxia

Nerve growth factor (NGF) is required for the survival of neurons. We have addressed the effect of platelet-activating factor (PAF), one of the mediators of ischemic injury of the brain, on NGF expression in astrocytes. Normal human astrocytes in culture were stimulated with PAF, and levels of NGF mRNA and protein were analyzed by reverse transcription-polymerase chain reaction (RT-PCR), real-time quantitative PCR and enzyme-linked immunosorbent assay (ELISA). PAF increased the expressions of NGF mRNA and protein in astrocytes in time- and concentration-dependent manners. After 48-h stimulation, 10 nmol/L PAF increased the levels of NGF protein in astrocyte-conditioned medium by 1.4-fold. The PAF-induced stimulation of NGF expression was further enhanced (2.1-fold of the control) in the cells under hypoxic culture condition. BN52021 (Ginkgolide B), an antagonist for PAF binding sites, suppressed the effect of PAF. We conclude that PAF enhances NGF gene expression in human astrocytes, and the PAF-induced increase in the expression of NGF under hypoxia may benefit the protection of the nervous tissue by promoting neuronal survival.

Calcium channel blockers in cerebral ischaemia

Ischaemic stroke usually results from the obstruction of a major cerebral vessel which leads to a decrease in cerebral blood flow, and a subsequent reduction in ATP. This energy loss leads to impaired cellular function due to reduced ATP-dependent processes and a disruption in ionic gradients across membranes. Under these conditions, there is a significant efflux of K+ from cells producing cellular depolarisation and the movement of extracellular calcium into cells through calcium channels. It is this increase in intracellular calcium that leads to the 'calcium toxicity' that has been associated with cerebral ischaemia. Increased intracellular calcium triggers the break-down of phospholipids, proteins and nucleic acids. This is activated by calcium-dependent phospholipases, proteases and endonucleases, and contributes to structural and functional damage of the cell membrane, which compromises cell function and facilitates cell death. Calcium channel blockers are used routinely to treat cardiovascular disease and hypertension. Although some experimental studies over the last decade suggest efficacy/benefit in the treatment of experimental ischaemic stroke, clinical data do not bear this out. This article discusses the role of voltage-operated calcium channel blockers in stroke, and reviews much of the available experimental and clinical data.

Anti-apoptotic Actions of the Platelet-activating Factor Acetylhydrolase I α2 Catalytic Subunit

Platelet-activating factor (PAF) is an important mediator of cell loss following diverse pathophysiological challenges, but the manner in which PAF transduces death is not clear. Both PAF receptor-dependent and -independent pathways are implicated. In this study, we show that extracellular PAF can be internalized through PAF receptor-independent mechanisms and can initiate caspase-3-dependent apoptosis when cytosolic concentrations are elevated by approximately 15 pM/cell for 60 min. Reducing cytosolic PAF to less than 10 pM/cell terminates apoptotic signaling. By pharmacological inhibition of PAF acetylhydrolase I and II (PAF-AH) activity and down-regulation of PAF-AH I catalytic subunits by RNA interference, we show that the PAF receptor-independent death pathway is regulated by PAF-AH I and, to a lesser extent, by PAF-AH II. Moreover, the anti-apoptotic actions of PAF-AH I are subunit-specific. PAF-AH I alpha1 regulates intracellular PAF concentrations under normal physiological conditions, but expression is not sufficient to reduce an acute rise in intracellular PAF levels. PAF-AH I alpha2 expression is induced when cells are deprived of serum or exposed to apoptogenic PAF concentrations limiting the duration of pathological cytosolic PAF accumulation. To block PAF receptor-independent death pathway, we screened a panel of PAF antagonists (CV-3988, CV-6209, BN 52021, and FR 49175). BN 52021 and FR 49175 accelerated PAF hydrolysis and inhibited PAF-mediated caspase 3 activation. Both antagonists act indirectly to promote PAF-AH I alpha2 homodimer activity by reducing PAF-AH I alpha1 expression. These findings identify PAF-AH I alpha2 as a potent anti-apoptotic protein and describe a new means of pharmacologically targeting PAF-AH I to inhibit PAF-mediated cell death.

Platelet-activating factor receptor and respiratory and metabolic responses to hypoxia and hypercapnia

Activation of the platelet-activating factor receptor (PAFR) regulates neural transmission. A PAFR blocker reduced the peak hypoxic (pHVR) but not hypercapnic ventilatory (HCVR) responses in rats [Am. J. Physiol. 275 (1998) R604]. To further examine the role of PAFR in respiratory control, genotype-verified PAFR -/- and PAFR +/+ adult male mice underwent hypoxic and hypercapnic challenges. HCVR was similar in the two groups (p-NS). However, pHVR was significantly reduced in PAFR -/- mice (38 +/- 13% baseline [S.D.]) compared to PAFR +/+ mice (78 +/- 16% baseline; P < 0.001, ANOVA), with reduced tidal volume recruitments during pHVR. In addition, hypoxic ventilatory depression was attenuated in PAFR -/- mice (P < 0.01), and was primarily due to attenuation of the time-dependent decreases in oxygen consumption during sustained hypoxia (P < 0.01). Thus, PAFR expression/function modulates components of the acute ventilatory and metabolic adaptations to hypoxia but not to hypercapnia. Imbalances in PAFR activity may lead to maladaptive regulation of the tightly controlled metabolic-ventilatory relationships during hypoxia.

Platelet-activating factor receptor modulates respiratory adaptation to long-term intermittent hypoxia in mice

During hypoxia, release of platelet-activating factor (PAF) and activation of its cognate receptor (PAFR) regulate neural transmission and are required for full expression of peak hypoxic ventilatory response (pHVR) but not hypercapnic ventilatory response. However, it is unclear whether PAFR underlie components of long-term ventilatory adaptations to hypoxia. To examine this issue, adult male PAFR(+/+) and PAFR(-/-) mice were exposed to intermittent hypoxia (IH) consisting of 90 s 21% O(2) and 90 s 10% O(2) for 30 days, and normoxic and hypoxic ventilatory patterns were assessed using whole body plethysmography. Starting at day 14 of IH, normoxic ventilation in PAFR(-/-) was reduced significantly compared with PAFR(+/+) mice (P < 0.001), the latter exhibiting a prominent long-term ventilatory facilitation (LTVF). However, IH-exposed PAFR(-/-) mice had markedly enhanced pHVR and hypoxic ventilatory decline that became similar to those of IH-exposed PAFR(+/+) mice. Thus we postulate that PAFR expression and/or function underlies critical components of IH-induced LTVF but does not play a role in the potentiation of the hypoxic ventilatory response after IH exposures.

Synaptic lipid signaling: significance of polyunsaturated fatty acids and platelet-activating factor

Neuronal cellular and intracellular membranes are rich in specialized phospholipids that are reservoirs of lipid messengers released by specific phospholipases and stimulated by neurotransmitters, neurotrophic factors, cytokines, membrane depolarization, ion channel activation, etc. Secretory phospholipases A2 may be both intercellular messengers and generators of lipid messengers. The highly networked nervous system includes cells (e.g., astrocytes, oligodendrocytes, microglial cells, endothelial microvascular cells) that extensively interact with neurons; several lipid messengers participate in these interactions. This review highlights modulation of postsynaptic membrane excitability and long-term synaptic plasticity by cyclooxygenase-2-generated prostaglandin E2, arachidonoyldiacylcylglycerol, and arachidonic acid-containing endocannabinoids. The peroxidation of docosahexaenoic acid (DHA), a critical component of excitable membranes in brain and retina, is promoted by oxidative stress. DHA is also the precursor of enzyme-derived, neuroprotective docosanoids. The phospholipid platelet-activating factor is a retrograde messenger of long-term potentiation, a modulator of glutamate release, and an upregulator of memory formation. Lipid messengers modulate signaling cascades and contribute to cellular differentiation, function, protection, and repair in the nervous system. Lipidomic neurobiology will advance our knowledge of the brain, spinal cord, retina, and peripheral nerve function and diseases that affect them, and new discoveries on networks of signaling in health and disease will likely lead to novel therapeutic interventions.

Inactivation of a testis-specific Lis1 transcript in mice prevents spermatid differentiation and causes male infertility

Lis1 protein is the non-catalytic component of platelet-activating factor acetylhydrolase 1b (PAF-AH 1B) and associated with microtubular structures. Hemizygous mutations of the LIS1 gene cause type I lissencephaly, a brain abnormality with developmental defects of neuronal migration. Lis1 is also expressed in testis, but its function there has not been determined. We have generated a mouse mutant (LIS1GT/GT) by gene trap integration leading to selective disruption of a Lis1 splicing variant in testis. Homozygous mutant males are infertile with no other apparent phenotype. We demonstrate that Lis1 is predominantly expressed in spermatids, and spermiogenesis is blocked when Lis1 is absent. Mutant spermatids fail to form correct acrosomes and nuclei appear distorted in size and shape. The tissue architecture in mutant testis appears severely disturbed displaying collapsed seminiferous tubules, mislocated germ cells, and increased apoptosis. These results provide evidence for an essential and hitherto uncharacterized role of the Lis1 protein in spermatogenesis, particularly in the differentiation of spermatids into spermatozoa.

Meningitis and encephalitis in children. An update

Over the course of the past decade, much has changed on the landscape of meningitis and encephalitis in children. West Nile virus has emerged in the United States as a new etiologic pathogen causing encephalitis. Human herpesvirus-6 has been identified as a cause of encephalitis and febrile seizures. Lymphocytic choriomeningitis virus has been identified as an underrecognized neuroteratogen. The emergence of penicillin-resistant Streptococcus pneumoniae has complicated the treatment of bacterial meningitis, whereas the Haemophilus influenzae vaccine has fundamentally altered the disease's epidemiology. The recognition that much of the neuropathologic change induced by bacterial meningitis is inflammation mediated has paved the way to the demonstration that dexamethasone can substantially improve the outcome of bacterial meningitis in children. Although much progress has been made toward understanding, treating, and preventing these important infections, much remains to be learned.

Antagonists of the Platelet-activating Factor Receptor Block Long-term Potentiation in Hippocampal Slices

The effects of antagonists and agonists of the platelet-activating factor (PAF) receptor on the formation of long-term potentiation (LTP) were examined in slices of rat hippocampus. The antagonist rans-BTD (rans-2,5-bis-(3,4,5-trimethoxyphenyl)-1,3-dioxolane) at concentrations of 8 - 16 microM blocked LTP in field CA1 while the same concentration of a stereo isomer (cis-BTD) with low affinity for PAF receptors was without effect. CV3988, an antagonist structurally related to PAF, also attenuated LTP. The blockade of LTP by trans-BTD was partially reversed by simultaneous application of the non-metabolizable receptor agonist carbamyl-PAF. Trans-BTD did not change the following physiological measures: (i) paired-pulse facilitation, (ii) responses occurring during the short bursts given to induce LTP, (iii) N-methyl-d-aspartate receptor-mediated responses, and (iv) potentiation measured during the first minute after high-frequency stimulation. It thus appears that trans-BTD interferes with LTP at some step after induction and initial expression. These results suggest that activation of PAF receptors contributes to the stabilization of LTP, possibly via an effect on intracellular calcium levels.

Platelet-activating factor induces cell death in cultured astrocytes and oligodendrocytes: involvement of caspase-3

The biologically active lipid metabolite, platelet-activating factor (PAF), is thought to contribute to inflammatory processes and tissue damage in a variety of central nervous system (CNS) injuries. In previous studies, we found that after contusion spinal cord injury, treatment with a PAF antagonist led to significantly increased white matter tissue sparing as well as decreased mRNA levels for pro-inflammatory cytokines. Some studies suggest that PAF can also have toxic effects on neurons in vitro. Few studies, however, have examined the effects of PAF on glial cells of the CNS. In the present study, the potential for PAF to act as a toxin to cultured astrocytes was examined. Also investigated were the effects of PAF on oligodendrocytes at two different stages of development. Treatment with 0.02-2 microM PAF for 72 h resulted in significant levels of cell death in both cell types (P < 0.05), an effect that was blocked by the PAF receptor antagonists, WEB 2170 and BN 52021. To investigate PAF-induced glial cell death further, we looked for activation of the enzyme, caspase-3, which can be indicative of apoptosis. Immunocytochemistry demonstrated that PAF at all concentrations caused activation of caspase-3 at 24, 48, and 72 h after treatment in both cell types. Caspase-3-dependent cell death was further confirmed using knockout mice (-/-) deficient in the caspase-3 gene. Toxicity was lost when astrocytes (-/-) were exposed to 0.02-2 microM PAF (P < 0.01). Oligodendrocytes (-/-) were not susceptible to toxicity at 2 microM PAF (P < 0.001). The results demonstrate that the pro-inflammatory molecule, PAF, induces cell death in cultured CNS glial cells and that this effect is, in part, dependent on caspase-3 activation.

LIS1-no more no less

LIS1 is one of the genes that has a principle role in brain development since hemizygote mutations in LIS1 result in a severe brain malformation known as lissencephaly ('smooth brain'). LIS1 is a WD repeat protein and is known to be involved in several protein complexes that are likely to play a functional role in brain development. We discuss here the brain developmental phenotype observed in mice heterozygote for an N-terminal truncated LIS1 protein in view of known LIS1 protein interactions.

PAF antagonist treatment reduces pro-inflammatory cytokine mRNA after spinal cord injury

Platelet-activating factor (PAF) is a pro-inflammatory molecule which contributes to secondary damage after spinal cord injury (SCI). To test if PAF contributes to cytokine induction following SCI, female Long-Evans rats were pretreated with the PAF antagonist WEB 2170 prior to receiving a contusion injury at spinal cord level T10 using the NYU impactor. RNase protection assay (RPA) analysis revealed that IL-1alpha mRNA peaked at I h post-injury while IL-1beta and IL-6 mRNA levels were higher and peaked at 6 h.TNF-alpha mRNA was almost undetectable. All mRNA levels approached baseline by 24 h. Treatment with WEB 2170 (1 mg/kg, i.p.) 15 min prior to injury significantly decreased mRNA levels for all three cytokines at 6 h post-injury, but not at I h post-injury. These results demonstrate a role for PAF in proinflammatory cytokine induction after SCI.

Evidence for the involvement of cyclooxygenase activity in the development of cocaine sensitization

Phospholipase A2 (PLA(2)) activation generates the release of arachidonic acid (AA) and platelet-activating factor (PAF), two compounds which may be involved in neuroplasticity. In previous studies, we found that PLA(2) activation is involved in the development of stimulant sensitization. In the present study, we have examined the roles of AA and PAF in the development of stimulant sensitization using agonists and antagonists selective for PAF receptors or the induction of various AA cascade-mediated eicosanoids. Sprague-Dawley rats were treated for 5 days with cocaine (30 mg/kg) or D-amphetamine (1 mg/kg) preceded 15 min earlier by various antagonists, and then tested following a 10-day withdrawal period for cocaine (15 mg/kg) or D-amphetamine (0.5 mg/kg)-induced locomotion. Consistent with our earlier work, pretreatment with the PLA(2) inhibitor quinacrine (25 mg/kg) blocked the development of cocaine and amphetamine sensitization. The lipoxygenase (LOX) inhibitors nordihydroguaiaretic acid (NDGA) (5-10 mg/kg) and MK-886 (1 mg/kg) had no effect on cocaine sensitization. The PAF receptor antagonist WEB 2086 (5-10 mg/kg) reduced the development of cocaine sensitization. The cyclooxygenase (COX) inhibitors indomethacin (1-2 mg/kg), piroxicam (0.5-1 mg/kg), 6-methoxy-2-napthylacetic acid (6-MNA; 0.5-1 mg/kg), and NS-398 (0.5-1 mg/kg) blocked the development of cocaine sensitization. The COX inhibitors indomethacin (2 mg/kg) and 6-MNA (1 mg/kg) also reduced the development of amphetamine sensitization. Rats were administered bilateral intraventral tegmental area (VTA) injections of D-amphetamine (5 microg/side) or saline coadministered with indomethacin (0.5 microg/side) or vehicle three times over 5 days and were then tested after a 10-day withdrawal for D-amphetamine (0.5 mg/kg ip)-induced locomotion. Intra-VTA amphetamine induced a robust form of amphetamine sensitization, which was blocked by coadministration of indomethacin. Unilateral intra-VTA injections of PAF (1 microg) did not significantly alter cocaine (15 mg/kg ip)-induced locomotion when tested after a 3-day withdrawal. These findings suggest that COX, and possibly PAF, activity is involved in the development of stimulant sensitization. Neuroanatomical studies demonstrate that this may occur at the level of the VTA.

Production of platelet-activating factor by neuronal cells in the rat brain with cold injury

The production and localization of platelet-activating factor (PAF) in the brain following focal brain injury were examined. Immunofluorescent staining was used to detect PAF in the rat brain with cold-induced local brain injury. After cold injury, immediate-early PAF staining was observed within the cold lesion followed later by immunoreactivity in the ipsilateral white matter. PAF immunoreactivity was also clearly seen both in cortical neurons adjacent to the cold lesion and in the ipsilateral hippocampus which showed delayed neuronal degeneration. The data suggest that PAF synthesis occurs in the neuronal cells in the perilesional area and hippocampus as well as within the cold lesion site during the early stages of cold-induced brain injury. PAF expression may contribute to the onset and progression of further brain damage, such as delayed axotomy and delayed neuronal loss.

Roles for platelet-activating factor and *NO-derived oxidants causing neutrophil adherence after CO poisoning

Studies were conducted with rats to investigate whether platelet activating factor (PAF) and nitric oxide (*NO)-derived oxidants played roles in the initial adherence of neutrophils to vasculature in the brain after carbon monoxide (CO) poisoning. Before CO poisoning, rats were treated with the competitive PAF receptor antagonist WEB-2170 or with the peroxynitrite scavenger selenomethionine. Both agents caused significantly lower concentrations of myeloperoxidase in the brain after poisoning, indicating fewer sequestered neutrophils. Similarly, both agents reduced the concentration of nitrotyrosine, indicating less oxidative stress due to *NO-derived oxidants. There were no alterations in whole brain homogenate PAF concentration measured by immunoassay and bioassay, nor were there changes in phosphatidylcholine concentration. Immunohistochemical imaging showed PAF to be more heavily localized within perivascular zones after CO poisoning. Neutrophils colocalized with both PAF and nitrotyrosine in brains of rats killed immediately after CO poisoning. We conclude that qualitative changes in brain PAF are responsible for neutrophil adherence immediately after CO poisoning and that activated neutrophils trigger the initial rise in brain nitrotyrosine. Persistent PAF-mediated neutrophil adherence required production of *NO-derived oxidants because when oxidants were scavenged, neutrophil adherence was not maintained.

Synaptic plasticity in the medial vestibular nuclei: role of glutamate receptors and retrograde messengers in rat brainstem slices

The analysis of cellular-molecular events mediating synaptic plasticity within vestibular nuclei is an attempt to explain the mechanisms underlying vestibular plasticity phenomena. The present review is meant to illustrate the main results, obtained in vitro, on the mechanisms underlying long-term changes in synaptic strength within the medial vestibular nuclei. The synaptic plasticity phenomena taking place at the level of vestibular nuclei could be useful for adapting and consolidating the efficacy of vestibular neuron responsiveness to environmental requirements, as during visuo-vestibular recalibration and vestibular compensation. Following a general introduction on the most salient features of vestibular compensation and visuo-vestibular adaptation, which are two plastic events involving neuronal circuitry within the medial vestibular nuclei, the second and third sections describe the results from rat brainstem slice studies, demonstrating the possibility to induce long-term potentiation and depression in the medial vestibular nuclei, following high frequency stimulation of the primary vestibular afferents. In particular the mechanisms sustaining the induction and expression of vestibular long-term potentiation and depression, such as the role of various glutamate receptors and retrograde messengers have been described. The relevant role of the interaction between the platelet-activating factor, acting as a retrograde messenger, and the presynaptic metabotropic glutamate receptors, in determining the full expression of vestibular long-term potentiation is also underlined. In addition, the mechanisms involved in vestibular long-term potentiation have been compared with those leading to long-term potentiation in the hippocampus to emphasize the most significant differences emerging from vestibular studies. The fourth part, describes recent results demonstrating the essential role of nitric oxide, another retrograde messenger, in the induction of vestibular potentiation. Finally the fifth part suggests the possible functional significance of different action times of the two retrograde messengers and metabotropic glutamate receptors, which are involved in mediating the presynaptic mechanism sustaining vestibular long-term potentiation.

Differential interaction of platelet-activating factor and NMDA receptor function in hippocampal and dorsal striatal memory processes

The interaction between platelet activating factor (PAF) and NMDA receptor function in hippocampal and dorsal striatal memory processes was examined. In both a hidden and a visible platform water maze task, peripheral post-training injection of MK-801 (0.05 mg/kg) impaired memory. Post-training intrahippocampal infusions of PAF (1.0 microg/0.5 microl) enhanced memory in the hidden platform task, while intradorsal striatal infusion of PAF (1.0 microg/0.5 microl) enhanced memory in the visible platform task. The memory impairing effects of post-training injection of MK-801 was blocked by concurrent intrahippocampal infusion of PAF. In contrast, post-training injection of MK-801 blocked the memory enhancing effects of concurrent intradorsal striatal infusion of PAF. The results suggest that (1) the memory enhancing effects of intracerebral PAF infusion involve an interaction with NMDA receptor function, and (2) the nature of this interaction may represent a differential mechanism mediating the distinct roles of the hippocampus and dorsal striatum in cognitive memory and stimulus-response habit formation, respectively.

Cytidine (5') diphosphocholine-induced decrease in cerebral platelet activating factor is due to inactivation of its synthesizing enzyme cholinephosphotransferase in aged rats

Levels of cerebral platelet activating factor (PAF) result from the balance between the activities of its synthesizing enzyme dithiothreitol (DTT)- insensitive cholinephosphotransferase and its degradative enzyme PAF acetylhydrolase. Cerebral fractions of aged rats (19 months) displayed higher levels of PAF acetylhydrolase isoenzymes (P<0.05; n=4), unaltered levels of DTT-insensitive cholinephosphotransferase and lower PAF levels than young animals (2 months). Cerebral fractions of aged rats treated with cytidine (5') diphosphocholine displayed lower DTT-insensitive cholinephosphotransferase (55% after 8 days of treatment with 350 mg/kg per day, P<0.05; n=4), unaltered levels of PAF acetylhydrolase and lower PAF levels than untreated control animals. Thus our data would indicate that decrease of cerebral PAF may be attributed to an activation of PAF acetylhydrolase in ageing, and to an inactivation of DTT-insensitive cholinephosphotransferase in cytidine (5') diphosphocholine treated animals.

Reduction of hypoxic-ischemic brain swelling in the neonatal rat with PAF antagonist WEB 2170: lack of long-term protection

Platelet activating factor (PAF) is an inflammatory lipid mediator released by ischemic brain. Our objectives were to use an inhibitor of PAF that does not readily cross the blood-brain barrier, WEB 2170, to study the role of intravascular PAF on brain swelling and subsequent brain atrophy in a neonatal rat model of hypoxic-ischemic brain injury. We injured the right cerebral hemisphere of 7-d-old rats by ligating the right common carotid artery and exposing the rats to 8% oxygen for 2.25 h. Forty-two rats received saline or the PAF antagonist WEB 2170, 1 h before hypoxia. We found that WEB 2170 pretreatment reduced swelling by 64% (p = 0.003). In contrast, treatment immediately after hypoxic-ischemic injury did not reduce swelling. In two additional experiments involving 103 rats, we found that pretreatment or repeated doses of PAF antagonist before and after hypoxic-ischemic injury did not reduce atrophy. We also found that the brain-penetrating PAF antagonist, BN 52021, did not prevent atrophy in our Wistar rat model. In conclusion, we were unable to reduce long-term brain injury with either PAF antagonist. WEB 2170 pretreatment reduced brain swelling by 64% without reducing atrophy. This suggests that although brain swelling may accompany cerebral infarction, it does not contribute to the pathogenesis of infarction and subsequent atrophy in the neonatal rat. The ability to reduce early postischemic brain swelling without reducing atrophy may be particularly unique to the immature animal with a compliant skull.

Neuronal platelet-activating factor receptor signal transduction involves a pertussis toxin-sensitive G-protein

In most nonneural systems, platelet-activating factor (PAF) receptor effects are mediated by G-proteins that are often pertussis toxin-sensitive. The activation of pertussis toxin-sensitive G-proteins linked to PAF receptors results in the mobilization of intracellular calcium, at least in part, through the second messenger inositol triphosphate. We have sought to determine if a pertussis toxin-sensitive G-protein is involved in the PAF receptor-mediated phenomena of growth cone collapse and of synaptic enhancement in primary neuronal culture. Using infrared differential interference contrast microscopy and patch-clamp recording techniques, pertussis toxin, but not the inactive B oligomer of the toxin, was found to block both the growth cone collapse and the enhanced synaptic release of excitatory transmitter induced by a nonhydrolyzable PAF receptor agonist, making it likely that Go, Gq, or Gi is the G-protein transducer of PAF receptors in primary neurons. We believe that PAF acts directly on neuronal receptors, which are linked to pertussis toxin-sensitive G-proteins, on the tips of developing neurites, and on presynaptic nerve terminals, leading to growth cone collapse and enhanced synaptic release of transmitter.

Properties and regulation of microsomal PAF-synthesizing enzymes in rat brain cortex

Platelet-activating factor (PAF) is a phospholipid mediator of long-term potentiation, synaptic plasticity and memory formation as well as of the development of brain damage. In brain, PAF is synthesized by two distinct pathways but their relative contribution to its productions, in various physiological and pathological conditions, is not established. We have further investigated on the properties of the two enzymes that catalyze the last step of the de novo or remodeling pathways in rat brain microsomes, PAF-synthesizing phosphocholinetransferase (PAF-PCT) and lysoPAF acetyltransferase (lysoPAF-AT), respectively. The latter enzyme is fully active at microM Ca2+ concentration, inhibited by MgATP and activated by phosphorylation. Because the reversibility of the reaction catalyzed by PAF-PCT, its direction depends on the ratio [CDP-choline]/[CMP], which is related to the energy charge of the cell. These and other properties indicate that the de novo pathway should mainly contribute to PAF synthesis for maintaining its basal levels under physiological conditions. The remodeling pathway should be more involved in the production of PAF during ischemia. During reperfusion, the overproduction of PAF should be the result of the concomitant activation of both pathways.

Potential role for Ginkgo biloba extract in the treatment of glaucoma

Glaucoma is becoming recognized as a condition for which not only elevated intraocular pressure, but also non-pressure-dependent risk factors are responsible. New avenues of treatment into which investigations are being initiated include agents which could possibly improve blood flow to the eye and neuroprotective drugs. Only calcium channel blockers are presently available for such treatment in glaucoma, and these have not been widely adopted, in contrast to clinical trials involving a number of neuroprotectants in other neurologic disorders. Ginkgo biloba extract is freely available and has several biological actions which combine to make it a potentially important agent in the treatment of glaucoma: improvement of central and peripheral blood flow, reduction of vasospasm, reduction of serum viscosity, antioxidant activity, platelet activating factor inhibitory activity, inhibition of apoptosis, and inhibition of excitotoxicity. The effect of Ginkgo biloba extract as a potential antiglaucoma therapy deserves intensive scrutiny.

Analysis of lissencephaly-causing LIS1 mutations

Mutations in the LIS1 gene may result in severe abnormalities of brain cortical layering known as lissencephaly. Most lissencephaly-causing LIS1 mutations are deletions that encompass the entire gene, therefore the mechanism of the disease is regarded as haploinsufficiency. So far, 13 different intragenic mutations have been reported: one point mutation, H149R; deletion of exon 9, which results in deleted acids Delta301-334; deletion of exon 4, which results in deleted amino acids Delta40-64; 10 mutations resulting in truncated proteins and one predicted to result in extra amino acids. We studied the consequences of the point mutation, deletion mutation and one of the reported truncations. In order to study LIS1 structure function, we introduced an additional point mutation and other truncations in different regions of the protein. The consequences of these mutations to protein folding were studied by gel filtration, sucrose density gradient centrifugation and measuring resistance to trypsin cleavage. On the basis of our results, we suggest that all truncation mutations and lissencephaly-causing point mutations or internal deletion result in a reduction in the amount of correctly folded LIS1 protein.

Lis1, the Drosophila homolog of a human lissencephaly disease gene, is required for germline cell division and oocyte differentiation

Lissencephaly is a severe congenital brain malformation resulting from incomplete neuronal migration. One causal gene, LIS1, is homologous to nudF, a gene required for nuclear migration in A. nidulans. We have characterized the Drosophila homolog of LIS1 (Lis1) and show that Lis1 is essential for fly development. Analysis of ovarian Lis1 mutant clones demonstrates that Lis1 is required in the germline for synchronized germline cell division, fusome integrity and oocyte differentiation. Abnormal packaging of the cysts was observed in Lis1 mutant clones. Our results indicate that LIS1 is important for cell division and differentiation and the function of the membrane cytoskeleton. They support the notion that LIS1 functions with the dynein complex to regulate nuclear migration or cell migration.

The unfolding story of two lissencephaly genes and brain development

Formation of our highly structured human brain involves a cascade of events, including differentiation, fate determination, and migration of neural precursors. In humans, unlike many other organisms, the cerebral cortex is the largest component of the brain. As in other mammals, the human cerebral cortex is located on the surface of the telencephalon and generally consists of six layers that are formed in an orderly fashion. During neuronal development, newly born neurons, moving in a radial direction, must migrate through previously formed layers to reach their proper cortical position. This is one of several neuronal migration routes that takes place in the developing brain; other modes of migration are tangential. Abnormal neuronal migration may in turn result in abnormal development of the cortical layers and deleterious consequences, such as Lissencephaly. Lissencephaly, a severe brain malformation, can be caused by mutations in one of two known genes: LIS1 and doublecortin (DCX). Recent in vitro and in vivo studies, report on possible functions for these gene products.

Genes that regulate neuronal migration in the cerebral cortex

Malformations of cortical development are increasingly recognized as causes of mental retardation and epilepsy. However, little is known about the molecular and biochemical signals that control the proliferation, migration, and organization of the cells involved in normal cerebral cortical development. Analysis of genes required for cortical development will help elucidate the pathogenesis of some epilepsies. In humans, two striking examples of abnormal cortical development, with varying degrees of epilepsy and mental retardation, are 'double cortex' and lissencephaly. Double cortex (DC), also known as subcortical band heterotopia, shows an abnormal band of neurons in the white matter underlying a relatively normal cortex. In pedigrees, DC often occurs in females, whereas affected males show more severe lissencephaly (XLIS), i.e. an abnormally thick cortex with decreased or absent surface convolutions. We and others have identified a novel brain specific gene, doublecortin, that is mutated in Double Cortex/X-linked lissencephaly (DC/XLIS) patients. Although the cellular function of doublecortin (DCX) is unknown, sequence analysis reveals a cytoplasmic protein with potential MAP kinase phosphorylation sites, as well as a site that is likely to be phosphorylated by c-Abl, suggesting that doublecortin functions as an intracellular signaling molecule critical for the migration of developing neurons. Interestingly, the scrambler mouse mutant demonstrates abnormal lamination with some similarity to lissencephaly and reflects a mutation in the murine homolog of the Drosophila disabled gene, mdab1, which binds c-Abl. Although a direct interaction between doublecortin and mDab1 has not been demonstrated, it is plausible that these two proteins may be part of a common signaling pathway. Therefore, abnormalities in signal transduction may be an underlying mechanism for the neuronal migration defects in DC/XLIS and the scrambler mouse, but further research is necessary to determine how such abnormalities give rise to cortical malformations and epilepsy.

Glutamate receptor signaling interplay modulates stress-sensitive mitogen-activated protein kinases and neuronal cell death

Glutamate receptors modulate multiple signaling pathways, several of which involve mitogen-activated protein (MAP) kinases, with subsequent physiological or pathological consequences. Here we report that stimulation of the N-methyl-D-aspartate (NMDA) receptor, using platelet-activating factor (PAF) as a messenger, activates MAP kinases, including c-Jun NH2-terminal kinase, p38, and extracellular signal-regulated kinase, in primary cultures of hippocampal neurons. Activation of the metabotropic glutamate receptor (mGluR) blocks this NMDA-signaling through PAF and MAP kinases, and the resultant cell death. Recombinant PAF-acetylhydrolase degrades PAF generated by NMDA-receptor activation; the hetrazepine BN50730 (an intracellular PAF receptor antagonist) also inhibits both NMDA-stimulated MAP kinases and neuronal cell death. The finding that the NMDA receptor-PAF-MAP kinase signaling pathway is attenuated by mGluR activation highlights the exquisite interplay between glutamate receptors in the decision making process between neuronal survival and death.

Modulation of hypoxic ventilatory response by systemic platelet-activating factor receptor antagonist in the rat

Platelet activating factor (PAF) has recently emerged as an important modulator of neuronal excitability by enhancing synaptic glutamate release. Since PAF receptors (PAFR) are ubiquitously distributed in the brain, we hypothesized that PAF may play a role in respiratory control. To examine this issue, hypoxic (10% O2 for 15 min, n = 14) and hypercapnic (5% CO2 for 30 min, n = 6) challenges were performed in chronically-instrumented, unrestrained adult rats following administration of the pre-synaptic PAFR antagonist BN52021 (i.p. 20 mg/kg in 0.5 ml) or vehicle (Veh). In normoxia, BN52021 elicited VT decreases and corresponding f increases such that minute ventilation (VE) was unaffected. During hypercapnia, peak VE increased similarly after both treatments (103+/-18% in BN52021 vs. 94+/-19% in Veh, p-NS). In contrast, significant reductions in the peak hypoxic VE response occurred after BN52021 (42+/-10% vs. 104+/-18% in Veh, P<0.002). BN52021 increased normoxic arterial blood pressure and decreased heart rate. However, hypoxia-induced chronotropic responses were attenuated and depressor responses were enhanced by BN52021. We further examined protein kinase C (PKC) translocation patterns during acute hypoxia after systemic BN52021 administration. Activation of PKC beta and delta was blocked by BN52021, PKC gamma was attenuated, with no effects on PKC alpha, epsilon, theta, iota, mu, and zeta. We conclude that systemic administration of a PAFR antagonist attenuates cardioventilatory recruitment to hypoxia and selectively attenuates activation of PKC in the rat brainstem. We speculate that enhanced regional PAF production and release during hypoxic conditions may contribute important excitatory inputs and signal transduction pathways within neuronal structures underlying cardiovascular and respiratory control.