Interaction between neurone and microglia mediated by platelet-activating factor

In Vitro Blood-Brain Barrier Models for Neuroinfectious Diseases: A Narrative Review

The blood-brain barrier (BBB) is a complex, dynamic, and adaptable barrier between the peripheral blood system and the central nervous system. While this barrier protects the brain and spinal cord from inflammation and infection, it prevents most drugs from reaching the brain tissue. With the expanding interest in the pathophysiology of BBB, the development of in vitro BBB models has dramatically evolved. However, due to the lack of a standard model, a range of experimental protocols, BBB-phenotype markers, and permeability flux markers was utilized to construct in vitro BBB models. Several neuroinfectious diseases are associated with BBB dysfunction. To conduct neuroinfectious disease research effectively, there stems a need to design representative in vitro human BBB models that mimic the BBB's functional and molecular properties. The highest necessity is for an in vitro standardised BBB model that accurately represents all the complexities of an intact brain barrier. Thus, this in-depth review aims to describe the optimization and validation parameters for building BBB models and to discuss previous research on neuroinfectious diseases that have utilized in vitro BBB models. The findings in this review may serve as a basis for more efficient optimisation, validation, and maintenance of a structurally- and functionally intact BBB model, particularly for future studies on neuroinfectious diseases.

Targeted deletion of PAC1 receptors in retinal neurons enhances neuron loss and axonopathy in a model of multiple sclerosis and optic neuritis

Chronic inflammation drives synaptic loss in multiple sclerosis (MS) and is also commonly observed in other neurodegenerative diseases. Clinically approved treatments for MS provide symptomatic relief but fail to halt neurodegeneration and neurological decline. Studies in animal disease models have demonstrated that the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP, ADCYAP1) exhibits anti-inflammatory, neuroprotective and regenerative properties. Anti-inflammatory actions appear to be mediated primarily by two receptors, VPAC1 and VPAC2, which also bind vasoactive intestinal peptide (VIP). Pharmacological experiments indicate that another receptor, PAC1 (ADCYAP1R1), which is highly selective for PACAP, provides protection to neurons, although genetic evidence and other mechanistic information is lacking. To determine if PAC1 receptors protect neurons in a cell-autonomous manner, we used adeno-associated virus (AAV2) to deliver Cre recombinase to the retina of mice harboring floxed PAC1 alleles. Mice were then subjected to chronic experimental autoimmune encephalomyelitis (EAE), a disease model that recapitulates major clinical and pathological features of MS and associated optic neuritis. Unexpectedly, deletion of PAC1 in naïve mice resulted in a deficit of retinal ganglionic neurons (RGNs) and their dendrites, suggesting a homeostatic role of PAC1. Moreover, deletion of PAC1 resulted in increased EAE-induced loss of a subpopulation of RGNs purported to be vulnerable in animal models of glaucoma. Increased axonal pathology and increased secondary presence of microglia/macrophages was also prominently seen in the optic nerve. These findings demonstrate that neuronal PAC1 receptors play a homeostatic role in protecting RGNs and directly protects neurons and their axons against neuroinflammatory challenge. SIGNIFICANCE STATEMENT: Chronic inflammation is a major component of neurodegenerative diseases and plays a central role in multiple sclerosis (MS). Current treatments for MS do not prevent neurodegeneration and/or neurological decline. The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) has been shown to have anti-inflammatory, neuroprotective and regenerative properties but the cell type- and receptor-specific mechanisms are not clear. To test whether the protective effects of PACAP are direct on the PAC1 receptor subtype on neurons, we delete PAC1 receptors from neurons and investigate neuropathologigical changes in an animal model of MS. The findings demonstrate that PAC1 receptors on neurons play a homeostatic role in maintaining neuron health and can directly protect neurons and their axons during neuroinflammatory disease.

Potential neurotoxic activity of diverse molecules released by microglia

Microglia are the professional immune cells of the brain, which support numerous physiological processes. One of the defensive functions provided by microglia involves secretion of cytotoxins aimed at destroying invading pathogens. It is also recognized that the adverse activation of microglia in diseased brains may lead to secretion of cytotoxic molecules, which could be damaging to the surrounding cells, including neurons. Several of these toxins, such as reactive oxygen and nitrogen species, L-glutamate, and quinolinic acid, are widely recognized and well-studied. This review is focused on a structurally diverse group of less-established microglia neurotoxins, which were selected by applying the two criteria that these molecules 1) can be released by microglia, and 2) have the potential to be directly harmful to neurons. The following 11 molecules are discussed in detail: amyloid beta peptides (Aβ); cathepsin (Cat)B and CatD; C-X-C motif chemokine ligand (CXCL)10 and CXCL12 (5-67); high mobility group box (HMGB)1; lymphotoxin (LT)-α; matrix metalloproteinase (MMP)-2 and MMP-9; platelet-activating factor (PAF); and prolyl endopeptidase (PEP). Molecular mechanisms of their release by microglia and neurotoxicity, as well as available evidence implicating their involvement in human neuropathologies are summarized. Further studies on several of the above molecules are warranted to confirm either their microglial origin in the brain or direct neurotoxic effects. In addition, investigations into the differential secretion patterns of neurotoxins by microglia in response to diverse stimuli are required. This research could identify novel therapeutic targets for neurological disorders involving adverse microglial activation.

New Insights Into the Pathologic Roles of the Platelet-Activating Factor System

Described almost 50 years ago, the glycerophosphocholine lipid mediator Platelet-activating factor (PAF) has been implicated in many pathologic processes. Indeed, elevated levels of PAF can be measured in response to almost every type of pathology involving inflammation and cell damage/death. In this review, we provide evidence for PAF involvement in pathologic processes, with focus on cancer, the nervous system, and in photobiology. Importantly, recent insights into how PAF can generate and travel via bioactive extracellular vesicles such as microvesicle particles (MVP) are presented. What appears to be emerging from diverse pathologies in different organ systems is a common theme where pro-oxidative stressors generate oxidized glycerophosphocholines with PAF agonistic effects, which then trigger more enzymatic PAF synthesis via the PAF receptor. A downstream consequence of PAF receptor activation is the generation and release of MVP which provide a mechanism to transmit PAF as well as other bioactive agents. The knowledge gaps which when addressed could result in novel therapeutic strategies are also discussed. Taken together, an enhanced understanding of the PAF family of lipid mediators is essential in our improved comprehension of the relationship amongst the diverse cutaneous, cancerous, neurologic and systemic pathologic processes.

Longitudinal transcriptomic analysis of altered pathways in a CHMP2Bintron5-based model of ALS-FTD

Amyotrophic lateral sclerosis and frontotemporal dementia are two neurodegenerative diseases with currently no cure. These two diseases share a clinical continuum with overlapping genetic causes. Mutations in the CHMP2B gene are found in patients with ALS, FTD and ALS-FTD. To highlight deregulated mechanisms occurring in ALS-FTD linked to the CHMP2B gene, we performed a whole transcriptomic study on lumbar spinal cord from CHMP2B^intron5 mice, a model that develops progressive motor alterations associated with dementia symptoms reminiscent of both ALS and FTD. To gain insight into the transcriptomic changes taking place during disease progression this study was performed at three stages: asymptomatic, symptomatic and end stage. We showed that before appearance of motor symptoms, the major disrupted mechanisms were linked with the immune system/inflammatory response and lipid metabolism. These processes were progressively replaced by alterations of neuronal electric activity as motor symptoms appeared, alterations that could lead to motor neuron dysfunction. To investigate overlapping alterations in gene expression between two ALS-causing genes, we then compared the transcriptome of symptomatic CHMP2B^intron5 mice with the one of symptomatic SOD1^G86R mice and found the same families deregulated providing further insights into common underlying dysfunction of biological pathways, disrupted or disturbed in ALS. Altogether, this study provides a database to explore potential new candidate genes involved in the CHMP2B^intron5-based pathogenesis of ALS, and provides molecular clues to further understand the functional consequences that diseased neurons expressing CHMP2B mutant may have on their neighbor cells.

Comparing the role of Ginkgolide B and Ginkgolide K on cultured astrocytes exposed to oxygen‑glucose deprivation

Ginkgolide B (GB) and ginkgolide K (GK) are two main active monomers of ginkgolides that present a unique group of diterpenes found naturally in the leaves of the Ginkgo biloba tree. Astrocytes are the most abundant cell type within the central nervous system (CNS) and serve essential roles in maintaining healthy brain function. The present study compared the biological effects of GB and GK on astrocytes exposed to oxygen-glucose deprivation (OGD). The results demonstrated that GB and GK exhibit many different actions. The level of the platelet-activating factor (PAF) was elevated on astrocytes exposed to OGD, and inhibited by GB and GK treatment. Although GB and GK inhibited the expression of p-NF-κB/p65, GK exerted stronger anti-inflammatory and antioxidant effects on astrocytes exposed to OGD than GB by inhibiting interleukin (IL)-6 and tumor necrosis factor-α, and inducing IL-10 and the nuclear factor-erythroid 2-related factor 2/HO-1 signaling pathway. When compared with GB treatment, GK treatment maintained high levels of phosphoinositide 3-kinase/phosphorylated-protein kinase B expression, and induced a marked upregulation of Wnt family member 1 and brain derived neurotrophic factor, indicating that GK, as a natural plant compound, may have more attractive prospects for clinical application in the treatment of neurological disorders than GB.

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.

Platelet Activating Factor (PAF) Receptor Deletion or Antagonism Attenuates Severe HSV-1 Meningoencephalitis

Herpes simplex virus type 1 (HSV-1) is a human pathogen that may cause severe encephalitis. The exacerbated immune response against the virus contributes to the disease severity and death. Platelet activating factor (PAF) is a mediator capable of inducing increase in vascular permeability, production of cytokines on endothelial cells and leukocytes. We aimed to investigate the activation of PAF receptor (PAFR) and its contribution to the severity of the inflammatory response in the brain following HSV-1 infection. C57BL/6 wild-type (WT) and PAFR deficient (PAFR^-/-) mice were inoculated intracranially with 10⁴ plaque-forming units (PFU) of HSV-1. Visualization of leukocyte recruitment was performed using intravital microscopy. Cells infiltration in the brain tissue were analyzed by flow cytometry. Brain was removed for chemokine assessment by ELISA and for histopathological analysis. The pharmacological inhibition by the PAFR antagonist UK-74,505 was also analyzed. In PAFR^-/- mice, there was delayed lethality but no difference in viral load. Histopathological analysis of infected PAFR^-/- mice showed that brain lesions were less severe when compared to their WT counterparts. Moreover, PAFR^-/- mice showed less TCD4⁺, TCD8⁺ and macrophages in brain tissue. This reduction of the presence of leukocytes in parenchyma may be mechanistically explained by a decrease in leukocytes rolling and adhesion. PAFR^-/- mice also presented a reduction of the chemokine CXCL9 in the brain. In addition, by antagonizing PAFR, survival of C57BL/6 infected mice increased. Altogether, our data suggest that PAFR plays a role in the pathogenesis of experimental HSV-1 meningoencephalitis, and its blockade prevents severe disease manifestation.

Platelet Activating Factor Enhances Synaptic Vesicle Exocytosis Via PKC, Elevated Intracellular Calcium, and Modulation of Synapsin 1 Dynamics and Phosphorylation

Platelet activating factor (PAF) is an inflammatory phospholipid signaling molecule implicated in synaptic plasticity, learning and memory and neurotoxicity during neuroinflammation. However, little is known about the intracellular mechanisms mediating PAF’s physiological or pathological effects on synaptic facilitation. We show here that PAF receptors are localized at the synapse. Using fluorescent reporters of presynaptic activity we show that a non-hydrolysable analog of PAF (cPAF) enhances synaptic vesicle release from individual presynaptic boutons by increasing the size or release of the readily releasable pool and the exocytosis rate of the total recycling pool. cPAF also activates previously silent boutons resulting in vesicle release from a larger number of terminals. The underlying mechanism involves elevated calcium within presynaptic boutons and protein kinase C activation. Furthermore, cPAF increases synapsin I phosphorylation at sites 1 and 3, and increases dispersion of synapsin I from the presynaptic compartment during stimulation, freeing synaptic vesicles for subsequent release. These findings provide a conceptual framework for how PAF, regardless of its cellular origin, can modulate synapses during normal and pathologic synaptic activity.

Sequential expression of cyclooxygenase-2, glutamate receptor-2, and platelet activating factor receptor in rat hippocampal neurons after fluid percussion injury

Traumatic brain injury causes gene expression changes in different brain regions. Occurrence and development of traumatic brain injury are closely related, involving expression of three factors, namely cyclooxygenase-2, glutamate receptor-2, and platelet activating factor receptor. However, little is known about the correlation of these three factors and brain neuronal injury. In this study, primary cultured rat hippocampal neurons were subjected to fluid percussion injury according to Scott's method, with some modifications. RT-PCR and semi-quantitative immunocytochemical staining was used to measure the expression levels of cyclooxygenase-2, glutamate receptor-2, and platelet activating factor receptor. Our results found that cyclooxygenase-2 expression were firstly increased post-injury, and then decreased. Both mRNA and protein expression levels reached peaks at 8 and 12 hours post-injury, respectively. Similar sequential changes in glutamate receptor 2 were observed, with highest levels mRNA and protein expression at 8 and 12 hours post-injury respectively. On the contrary, the expressions of platelet activating factor receptor were firstly decreased post-injury, and then increased. Both mRNA and protein expression levels reached the lowest levels at 8 and 12 hours post-injury, respectively. Totally, our findings suggest that these three factors are involved in occurrence and development of hippocampal neuronal injury.

Microglia during development and aging

Microglia are critical nervous system-specific cells influencing brain development, maintenance of the neural environment, response to injury, and repair. They contribute to neuronal proliferation and differentiation, pruning of dying neurons, synaptic remodeling and clearance of debris and aberrant proteins. Colonization of the brain occurs during gestation with an expansion following birth with localization stimulated by programmed neuronal death, synaptic pruning, and axonal degeneration. Changes in microglia phenotype relate to cellular processes including specific neurotransmitter, pattern recognition, or immune-related receptor activation. Upon activation, microglia cells have the capacity to release a number of substances, e.g., cytokines, chemokines, nitric oxide, and reactive oxygen species, which could be detrimental or beneficial to the surrounding cells. With aging, microglia shift their morphology and may display diminished capacity for normal functions related to migration, clearance, and the ability to shift from a pro-inflammatory to an anti-inflammatory state to regulate injury and repair. This shift in microglia potentially contributes to increased susceptibility and neurodegeneration as a function of age. In the current review, information is provided on the colonization of the brain by microglia, the expression of various pattern recognition receptors to regulate migration and phagocytosis, and the shift in related functions that occur in normal aging.

HIV Associated Neurocognitive Disorders

Human immunodeficiency virus type 1 is associated with the development of neurocognitive disorders in many infected individuals, including a broad spectrum of motor impairments and cognitive deficits. Despite extensive research, the pathogenesis of HIV-associated neurocognitive disorders (HAND) is still not clear. This review provides a comprehensive view of HAND, including HIV neuroinvasion, HAND diagnosis and different level of disturbances, influence of highly-active antiretroviral therapy to HIV-associated dementia (HAD), possible pathogenesis of HAD, etc. Together, this review will give a thorough and clear understanding of HAND, especially HAD, which will be vital for future research, diagnosis and treatment.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

Ginkgo biloba extract in Alzheimer's disease: from action mechanisms to medical practice

Standardized extract from the leaves of the Ginkgo biloba tree, labeled EGb761, is one of the most popular herbal supplements. Numerous preclinical studies have shown the neuroprotective effects of EGb761 and support the notion that it may be effective in the treatment and prevention of neurodegenerative disorders such as Alzheimer’s disease (AD). Despite the preclinical promise, the clinical efficacy of this drug remains elusive. In this review, possible mechanisms underlying neuroprotective actions of EGb761 are described in detail, together with a brief discussion of the problem of studying this herb clinically to verify its efficacy in the treatment and prevention of AD. Moreover, various parameters e.g., the dosage and the permeability of the blood brain barrier (BBB), impacting the outcome of the clinical effectiveness of the extract are also discussed. Overall, the findings summarized in this review suggest that, a better understanding of the neuroprotective mechanisms of EGb761 may contribute to better understanding of the effectiveness and complexity of this herb and may also be helpful for design of therapeutic strategies in future clinical practice. Therefore, in future clinical studies, different factors that could interfere with the effect of EGb761 should be considered.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Protection of PMS777, a new AChE inhibitor with PAF antagonism, against amyloid-beta-induced neuronal apoptosis and neuroinflammation

Amyloid-beta (Abeta) plays a central role in the neuroinflammation and cholinergic neuronal apoptosis in Alzheimer's disease, and thus has been considered as a main determinant of this disease. In the previous study, we reported that PMS777, a novel bis-interacting ligand for acetylcholinesterase (AChE) inhibition and platelet-activating factor (PAF) receptor antagonism, could significantly attenuate PAF-induced neurotoxicity. Continuing our efforts, we further investigated the protective effect of PMS777 on Abeta-induced neuronal apoptosis in vitro and neuroinflammation in vivo. PMS777 (1-100 microM) was found to inhibit Abeta-induced human neuroblastoma SH-SY5Y cell apoptosis in a concentration-dependent manner. Concurrently, PMS777 increased ratio of bcl-2 to bax mRNA, and inhibited both mRNA expression and activity of caspase-3 in SH-SY5Y cells after the exposure with Abeta. In vivo experimental study demonstrated that PMS777 could attenuate Abeta-induced microglial and astrocytic activation in the rat hippocampus after systemic administration. These results suggest that PMS777 potently protects against Abeta-induced neuronal apoptosis and neuroinflammation, and warrants further investigations in connection with its potential value in the treatment of Alzheimer's disease.

Platelet-activating factor production in the spinal cord of experimental allergic encephalomyelitis mice via the group IVA cytosolic phospholipase A2-lyso-PAFAT axis

Platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) plays a critical role in inflammatory disorders including experimental allergic encephalomyelitis (EAE), an animal model for multiple sclerosis (MS). Although PAF accumulation in the spinal cord (SC) of EAE mice and cerebrospinal fluid of MS patients has been reported, little is known about the metabolic processing of PAF in these diseases. In this study, we demonstrate that the activities of phospholipase A(2) (PLA(2)) and acetyl-CoA:lyso-PAF acetyltransferase (LysoPAFAT) are elevated in the SC of EAE mice on a C57BL/6 genetic background compared with those of naive mice and correlate with disease severity. Correspondingly, levels of groups IVA, IVB, and IVF cytosolic PLA(2)s, group V secretory PLA(2), and LysoPAFAT transcripts are up-regulated in the SC of EAE mice. PAF acetylhydrolase activity is unchanged during the disease course. In addition, we show that LysoPAFAT mRNA and protein are predominantly expressed in microglia. Considering the substrate specificity and involvement of PAF production, group IVA cytosolic PLA(2) is likely to be responsible for the increased PLA(2) activity. These data suggest that PAF accumulation in the SC of EAE mice is profoundly dependent on the group IVA cytosolic PLA(2)/LysoPAFAT axis present in the infiltrating macrophages and activated microglia.

Neuroprotection of microglia conditioned media from apoptotic death induced by staurosporine and glutamate in cultures of rat cerebellar granule cells

Microglia, the immune cells of the mammalian CNS, have often been indicated as dangerous effector cells for their activation in response to traumatic CNS injuries or immunological stimuli and for their involvement in many chronic neurodegenerative diseases. Recently, several in vitro and in vivo studies have emphasized that microglial activity is essential in promoting neuronal survival. We have tested the efficacy of media directly conditioned by microglia or conditioned by microglia after having been exposed to apoptotic neurons, towards neuroprotection of rat cerebellar granule cells (CGCs) challenged with staurosporine or glutamate. Apoptotic death of CGC caused by staurosporine, as well as by a mild excitotoxic stimulus delivered through sub-chronic glutamate treatment, was significantly counteracted by microglia conditioned media. On the other hand, an acute excitotoxic insult delivered through a short pulse of glutamate exposure in the absence of magnesium and resulting in a mix of apoptotic and necrotic death was only marginally counteracted by microglia conditioned media. The present results extend the available information regarding the neuroprotective role of microglia and support the usefulness of employing the culture approach for perspective identification of neuroprotective factors released by these cells. Furthermore, the use of media previously exposed to apoptotic neurons to elicit the neuroprotective response of microglia, indicate the feasibility to re-create also in the isolated culture conditions, at least some of the elements at the basis of neuron/microglia cross-talk.

Synergism in the repression of COX-2- and TNFalpha-induction in platelet activating factor-stressed human neural cells

Platelet activating factor (PAF; beta-acetyl-gamma-O-hexadecyl-l-alpha-phosphatidylcholine) triggers a rapid pro-inflammatory gene expression program in primary cultures of human neural (HN) cells. Two genes and gene products consistently induced after PAF treatment are the cytosoluble prostaglandin synthase cycloooxygenase-2 (COX-2) and the pro-apoptotic tumor necrosis factor alpha (TNFalpha). Both of these mediators are associated with the activation of inflammatory signaling, neural cell dysfunction, apoptosis and brain cell death, and both have been found to be up-regulated after brain injury in vivo. In this study we investigated the effects of the non-halogenated synthetic glucocorticoid budesonide epimer R (BUDeR), the novel PAF antagonist LAU-0901, and the electron spin trap and free radical scavenger phenyl butyl nitrone (PBN), upon early COX-2 and TNFalpha gene activation and prostaglandin E(2) (PGE(2)) release in PAF-stressed primary HN cells. The data indicate that these three biochemically unrelated classes of inflammatory repressors act synergistically in modulating PAF-induced up-regulation of COX-2, TNFalpha, and PGE(2) by quenching oxidative stress or inflammatory signaling, resulting in increased HN cell survival. These, or analogous classes of compounds, may be useful in the design of more effective combinatorial pharmacotherapeutic strategies in the treatment of complex neuro-inflammatory disorders.

Cholesterol synthesis inhibitors protect against platelet-activating factor-induced neuronal damage

BACKGROUND

Platelet-activating factor (PAF) is implicated in the neuronal damage that accompanies ischemia, prion disease and Alzheimer's disease (AD). Since some epidemiological studies demonstrate that statins, drugs that reduce cholesterol synthesis, have a beneficial effect on mild AD, we examined the effects of two cholesterol synthesis inhibitors on neuronal responses to PAF.

METHODS

Primary cortical neurons were treated with cholesterol synthesis inhibitors (simvastatin or squalestatin) prior to incubation with different neurotoxins. The effects of these drugs on neuronal cholesterol levels and neuronal survival were measured. Immunoblots were used to determine the effects of simvastatin or squalestatin on the distribution of the PAF receptor and an enzyme linked immunoassay was used to quantify the amounts of PAF receptor.

RESULTS

PAF killed primary neurons in a dose-dependent manner. Pre-treatment with simvastatin or squalestatin reduced neuronal cholesterol and increased the survival of PAF-treated neurons. Neuronal survival was increased 50% by 100 nM simvastatin, or 20 nM squalestatin. The addition of mevalonate restored cholesterol levels, and reversed the protective effect of simvastatin. Simvastatin or squalestatin did not affect the amounts of the PAF receptor but did cause it to disperse from within lipid rafts.

CONCLUSION

Treatment of neurons with cholesterol synthesis inhibitors including simvastatin and squalestatin protected neurons against PAF. Treatment caused a percentage of the PAF receptors to disperse from cholesterol-sensitive domains. These results raise the possibility that the effects of statins on neurodegenerative disease are, at least in part, due to desensitisation of neurons to PAF.

Study of PMS777, a new type of acetylcholinesterase inhibitor, in human HepG2 cells. Comparison with tacrine and galanthamine on oxidative stress and mitochondrial impairment

Acetylcholinesterase inhibitors are commonly used as cognitive enhancers for dementia in aged people. Among them, tacrine (THA) but not galanthamine, was shown to exhibit hepatotoxicity which reduces its clinical use. PMS777, both a PAF antagonist and a new potent acetylcholinesterase inhibitor was recently demonstrated to reverse scopolamine-induced amnesia in mice without toxicity. In the present study, the effects of THA, galanthamine and PMS777 were compared in HepG2 cells on the oxidative parameters involved in the reported hepatotoxicity of THA. THA (> or = 10 microM) induced an oxidative stress as shown by elevated ROS and MDA production and by a decrease in GSH level. Moreover, mitochondrial membrane potential and redox status were decreased. At low concentrations (< or =10 microM), there was no significant disturbance. None of the oxidative stress markers was affected by PMS777 up to the maximum concentration tested and it is suggested that PMS777 is not cytotoxic for HepG2 cells. Galanthamine was also without cytotoxicity. Our results suggest that the toxic effect of THA above 10 microM may be caused by drug-induced mitochondrial energization impairment and destabilisation of membrane phospholipids associated with an oxidative stress. In contrast by preventing these dysfunctions, PMS777 could be safer than THA.

Platelet-activating factor antagonists protect amyloid-β damaged neurons from microglia-mediated death

Neurons treated with sub-lethal concentrations of amyloid-beta1-42 developed phenotypic changes and selectively bound a CD14-IgG chimera; in co-cultures, microglia recognised and killed these amyloid-beta1-42 -damaged neurons. Pre-treatment with the platelet-activating factor (PAF) antagonists (Hexa-PAF, CV6209 or ginkgolide B) reduced CD14-IgG binding to amyloid-beta1-42 -damaged neurons, and the presence of PAF antagonists in co-cultures increased neuronal survival in a dose-dependant manner. PAF antagonists also protected neurons treated with HuPrP82-146, a peptide found in prion diseases. Second messenger studies demonstrated that the addition of PAF mimicked some of the effects of amyloid-beta1-42 on neurons. PAF-damaged neurons bound CD14-IgG, and PAF-damaged neurons were killed by microglia in a CD14-dependent process. Neuronal death was inversely related to both the concentration of PAF, and the number of microglia added. The effects of PAF were reduced by an antagonist of the prostanoid D receptor (BWA868C) indicating that neuronal damage induced by PAF is partly mediated by prostaglandins. These observations are compatible with the hypothesis that sub-lethal concentrations of amyloid-beta1-42 stimulate a cascade of second messengers including PAF and the prostaglandins. At nanomolar concentrations PAF induces a change in neuronal phenotype that activates microglia via the CD14 molecule, these activated microglia then kill the amyloid-beta1-42 damaged neurons.

Synaptic activity becomes excitotoxic in neurons exposed to elevated levels of platelet-activating factor

Neurologic impairment in HIV-1-associated dementia (HAD) and other neuroinflammatory diseases correlates with injury to dendrites and synapses, but how such injury occurs is not known. We hypothesized that neuroinflammation makes dendrites susceptible to excitotoxic injury following synaptic activity. We report that platelet-activating factor, an inflammatory phospholipid that mediates synaptic plasticity and neurotoxicity and is dramatically elevated in the brain during HAD, promotes dendrite injury following elevated synaptic activity and can replicate HIV-1-associated dendritic pathology. In hippocampal slices exposed to a stable platelet-activating factor analogue, tetanic stimulation that normally induces long-term synaptic potentiation instead promoted development of calcium- and caspase-dependent dendritic beading. Chemical preconditioning with diazoxide, a mitochondrial ATP-sensitive potassium channel agonist, prevented dendritic beading and restored long-term potentiation. In contrast to models invoking excessive glutamate release, these results suggest that physiologic synaptic activity may trigger excitotoxic dendritic injury during chronic neuroinflammation. Furthermore, preconditioning may represent a novel therapeutic strategy for preventing excitotoxic injury while preserving physiologic plasticity.

Platelet activating factor and its metabolite promote sleep in rabbits

Platelet activating factor (PAF) is a key inflammatory mediator. PAF and its receptor are found in brain and PAF affects or is affected by the production of sleep promoting cytokines such as interleukin-1. PAF also interacts with several other sleep-regulatory substances such as nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, nitric oxide, prostaglandins, and prolactin. We thus hypothesized that PAF would increase sleep. In these experiments, each rabbit received an injection of 25 microl of 2% DMSO to obtain control values, and on a separate day received PAF or lyso-PAF, a metabolite of PAF. Ten, 100 and 500 nmol for each substance was injected intracerebroventricularly. Both PAF and lyso-PAF enhanced non-rapid eye movement (NREM) sleep but not REM sleep. Lyso-PAF, but not PAF, induced hyperthermia. Results are consistent with the hypothesis that the brain cytokine network is involved in physiological sleep regulation.

Serial analysis of gene expression in the hippocampus of patients with mesial temporal lobe epilepsy

Hippocampal sclerosis constitutes the most frequent neuropathological finding in patients with medically intractable mesial temporal lobe epilepsy. Serial analysis of gene expression was used to get a global view of the gene profile in human hippocampus in control condition and in epileptic condition associated with hippocampal sclerosis. Libraries were generated from control hippocampus, obtained by rapid autopsy, and from hippocampal surgical specimens of patients with mesial temporal lobe epilepsy and the classical pattern of hippocampal sclerosis. More than 50,000 tags were analyzed (28,282, control hippocampus; 25,953, hippocampal sclerosis) resulting in 9206 (control hippocampus) and 9599 (hippocampal sclerosis) unique tags (genes), each representing a specific mRNA transcript. Comparison of the two libraries resulted in the identification of 143 transcripts that were differentially expressed. These genes belong to a variety of functional classes, including basic metabolism, transcription regulation, protein synthesis and degradation, signal transduction, structural proteins, regeneration and synaptic plasticity and genes of unknown identity of function. The database generated by this study provides an extensive inventory of genes expressed in human control hippocampus, identifies new high-abundant genes associated with altered hippocampal morphology in patients with mesial temporal lobe epilepsy and serves as a reference for future studies aimed at detecting hippocampal transcriptional responses under various pathological conditions.

Induction of angiogenesis by platelet-activating factor in the rat striatum

Angiogenesis, the formation of new blood vessels, has been studied following injection of platelet-activating factor into the rat striatum. Immunohistochemical analysis for laminin showed that platelet-activating factor induced significant neovascularization in the striatum. A dose-dependent loss of striatal neurons accompanied this angiogenic activity. The platelet-activating factor-induced neovascularization and neurodegeneration were attenuated by the platelet-activating factor receptor antagonist, BN52021, or the inducible nitric oxide synthase inhibitor, aminoguanidine. Additionally, these agents significantly reduced levels of the angiogenic agent vascular endothelial growth factor, which was markedly enhanced in platelet-activating factor-injected striatum. Double immunofluorescence staining demonstrated that expression of vascular endothelial growth factor and inducible nitric oxide synthase were predominantly localized to microglia, suggesting that inflammatory responses mediated by activated microglia could contribute to platelet-activating factor-induced angiogenesis and neuronal loss.

Interplay among platelet-activating factor, oxidative stress, and group I metabotropic glutamate receptors modulates neuronal survival

Platelet-activating factor (PAF) is a potent phospholipid messenger in the nervous system that participates in synaptic plasticity and in pathologic processes, including neurodegeneration. Oxidative stress plays important roles in neuronal cell death. To define the interaction between PAF and oxidative radicals in neuronal death, we studied the effects of PAF in the presence of oxidative radicals in primary neurons in culture. Exogenous PAF (50 microM) caused PAF receptor-independent injury to neurons. A nonneurotoxic PAF concentration (500 nM) potentiated neuronal death caused by hydrogen peroxide as determined by lactate dehydrogenase (LDH) assay, Hoechst staining, and TUNEL analysis, but it did not potentiate neuronal death caused by menadione, a superoxide donor, or by the nitric oxide donors 3-morpholino-sydnonimine (SIN-1) and sodium nitroprusside (SNP). This potentiation of the hydrogen peroxide effect was selectively blocked by a PAF membrane-receptor antagonist, BN52021 (5 microM). The neurotoxic effect of PAF and hydrogen peroxide was also completely blocked by ebselen and partially decreased by pretreatment with (S)-3,5-dihydroxyphenylglycine (DHPG), a group I metabotropic glutamate receptor (mGluR) agonist. This study suggests that PAF-receptor antagonists may be useful for neuroprotection. A similar effect might also be obtained with group I mGluR agonists, probably by way of a different underlying mechanism.

Development of tactile allodynia and thermal hyperalgesia by intrathecally administered platelet-activating factor in mice

Platelet-activating factor (PAF) is a potent inflammatory lipid mediator in peripheral tissues. However, its role in mediation of nociception in central nervous system is unknown. In the present study, whether PAF plays some role in pain transduction in the spinal cord was studied in mice. Intrathecal injection of PAF induced tactile pain, tactile allodynia at as low as 10 fg to 1 pg with a peak response at 100 fg, while lyso-PAF was without effect in the range of doses. Tactile allodynia induced by PAF was blocked by a PAF receptor antagonists, TCV-309, WEB 2086 and BN 50739. The expression of PAF receptor mRNA by RT-PCR was observed in DRG and spinal cord in mice. ATP P2X receptor antagonists, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid and 2',3'-O-(2,4,6-trinitrophenyl)adenosine 5-triphosphate, NMDA receptor antagonist, MK 801 and nitric oxide synthetase inhibitor, 7-nitroindazole blocked the PAF-induced tactile allodynia. PAF-induced tactile allodynia and thermal hyperalgesia disappeared in neonatally capsaicin-treated adult mice, while tactile allodynia but not thermal hyperalgesia induced by intrathecally injected alpha,beta-methylene ATP, a P2X receptor agonist, was capsaicin-insensitive. The present study demonstrated that PAF is a potent inducer of tactile allodynia and thermal hyperalgesia at the level of the spinal cord. PAF-evoked tactile allodynia is suggested to be mediated by ATP and the following NMDA and NO cascade through capsaicin-sensitive fiber, different from exogenously injected alpha,beta-methylene ATP which is insensitive to capsaicin treatment.

Phospholipase A2 Inhibitors or Platelet-activating Factor Antagonists Prevent Prion Replication

A key feature of prion diseases is the conversion of the cellular prion protein (PrP(C)) into disease-related isoforms (PrP(Sc)), the deposition of which is thought to lead to neurodegeneration. In this study a pharmacological approach was used to determine the metabolic pathways involved in the formation of protease-resistant PrP (PrP(res)) in three prion-infected cell lines (ScN2a, SMB, and ScGT1 cells). Daily treatment of these cells with phospholipase A(2) (PLA(2)) inhibitors for 7 days prevented the accumulation of PrP(res). Glucocorticoids with anti-PLA(2) activity also prevented the formation of PrP(res) and reduced the infectivity of SMB cells. Treatment with platelet-activating factor (PAF) antagonists also reduced the PrP(res) content of cells, while the addition of PAF reversed the inhibitory effect of PLA(2) inhibitors on PrP(res) formation. ScGT1 cells treated with PLA(2) inhibitors or PAF antagonists for 7 days remained clear of detectable (PrPres) when grown in control medium for a further 12 weeks. Treatment of non-infected cells with PLA(2) inhibitors or PAF antagonists reduced PrP(C) levels suggesting that limiting cellular PrP(C) may restrict prion formation in infected cells. These data indicate a pivotal role for PLA(2) and PAF in controlling PrP(res) formation and identify them as potential therapeutic agents.

Involvement of the NMDA receptor/nitric oxide signal pathway in platelet-activating factor-induced neurotoxicity

Platelet-activating factor (PAF), a bioactive phospholipid implicated in neuronal excitotoxic death, augments the presynaptic release of glutamate. Excessive activation of postsynaptic glutamate receptors and subsequent downstream signals leads to excitotoxicity. The present study proposed that the NMDA receptor/nitric oxide (NO) signal pathway might be involved in PAF-induced neurotoxicity. After the cultured neurons were exposed to PAF for 24 h the percentage of neuronal death increased in a dose-dependent manner. The PAF effects were significantly prevented not only by BN52021, a PAF antagonist, but also by MK-801, an NMDA antagonist, and L-NAME, an NO synthase (NOS) inhibitor. Moreover, the increases in NOS activity and neuronal NOS expression induced by chronic exposure of the cultured neurons to PAF were dramatically blocked by BN52021 and MK-801, respectively. Our findings suggest that the NMDA receptor/NO signaling pathway might contribute to the pathological mechanism of cell death triggered via PAF receptor activation.

Ginkgolide B inhibits the neurotoxicity of prions or amyloid-beta1-42

BACKGROUND: Neuronal loss in Alzheimer's or prion diseases is preceded by the accumulation of fibrillar aggregates of toxic proteins (amyloid-beta1-42 or the prion protein). Since some epidemiological studies have demonstrated that the EGb 761 extract, from the leaves of the Ginkgo biloba tree, has a beneficial effect on Alzheimer's disease, the effect of some of the major components of the EGb 761 extract on neuronal responses to amyloid-beta1-42, or to a synthetic miniprion (sPrP106), were investigated. METHODS: Components of the EGb 761 extract were tested in 2 models of neurodegeneration. SH-SY5Y neuroblastoma cells were pre-treated with ginkgolides A or B, quercetin or myricetin, and incubated with amyloid-beta1-42, sPrP106, or other neurotoxins. After 24 hours neuronal survival and the production of prostaglandin E2 that is closely associated with neuronal death was measured. In primary cortical neurons apoptosis (caspase-3) in response to amyloid-beta1-42 or sPrP106 was measured, and in co-cultures the effects of the ginkgolides on the killing of amyloid-beta1-42 or sPrP106 damaged neurons by microglia was tested. RESULTS: Neurons treated with ginkgolides A or B were resistant to amyloid-beta1-42 or sPrP106. Ginkgolide-treated cells were also resistant to platelet activating factor or arachidonic acid, but remained susceptible to hydrogen peroxide or staurosporine. The ginkgolides reduced the production of prostaglandin E2 in response to amyloid-beta1-42 or sPrP106. In primary cortical neurons, the ginkgolides reduced caspase-3 responses to amyloid-beta1-42 or sPrP106, and in co-culture studies the ginkgolides reduced the killing of amyloid-beta1-42 or sPrP106 damaged neurons by microglia. CONCLUSION: Nanomolar concentrations of the ginkgolides protect neurons against the otherwise toxic effects of amyloid-beta1-42 or sPrP106. The ginkgolides also prevented the neurotoxicity of platelet activating factor and reduced the production of prostaglandin E2 in response to platelet activating factor, amyloid-beta1-42 or sPrP106. These results are compatible with prior reports that ginkgolides inhibit platelet-activating factor, and that platelet-activating factor antagonists block the toxicity of amyloid-beta1-42 or sPrP106. The results presented here suggest that platelet-activating factor antagonists such as the ginkgolides may be relevant treatments for prion or Alzheimer's diseases.

The role of platelet activating factor in prion and amyloid-β neurotoxicity

In the prion diseases, neurodegeneration is preceded by the accumulation of the disease-associated isoform of the prion protein (PrP). In the present study, neurones treated with three different phospholipase A2 inhibitors were resistant to the toxic effects of PrP peptides or a synthetic miniprion (sPrP106). Phospholipase A2 inhibitors also protected neurones against a toxic peptide found in Alzheimer's disease (amyloid-beta1-42). Further studies showed that neurones pre-treated with platelet activating factor (PAF) antagonists were equally resistant to PrP peptides or amyloid-beta1-42. Moreover, both phospholipase A2 inhibitors and PAF antagonists reduced the activation of caspase-3, a marker of apoptosis, and the production of prostaglandin E2 that is closely associated with neuronal death in prion or Alzheimer's diseases.

A potentially critical role of phospholipases in central nervous system ischemic, traumatic, and neurodegenerative disorders

Phospholipases are a diverse group of enzymes whose activation may be responsible for the development of injury following insult to the brain. Amongst the numerous isoforms of phospholipase proteins expressed in mammals are 19 different phospholipase A2's (PLA2s), classified functionally as either secretory, calcium dependent, or calcium independent, 11 isozymes belonging to three structural groups of PLC, and 3 PLD gene products. Many of these phospholipases have been identified in selected brain regions. Under normal conditions, these enzymes regulate the turnover of free fatty acids (FFAs) in membrane phospholipids affecting membrane stability, fluidity, and transport processes. The measurement of free fatty acids thus provides a convenient method to follow phospholipase activity and their regulation. Phospholipase activity is also responsible for the generation of an extensive list of intracellular messengers including arachidonic acid metabolites. Phospholipases are regulated by many factors including selective phosphorylation, intracellular calcium and pH. However, under abnormal conditions, excessive phospholipase activation, along with a decreased ability to resynthesize membrane phospholipids, can lead to the generation of free radicals, excitotoxicity, mitochondrial dysfunction, and apoptosis/necrosis. This review evaluates the critical contribution of the various phospholipases to brain injury following ischemia and trauma and in neurodegenerative diseases.

Involvement of platelet-activating factor and LIS1 in neuronal migration

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active lipid mediator. We have previously shown the expression of PAF receptor in neurons and microglia. PAF is produced in the brain from its precursor, and degraded by the enzyme PAF acetylhydrolase. LIS1 is a regulatory subunit of PAF acetylhydrolase, and is identical to a gene whose deletion causes the human neuronal migration disorder, type I lissencephaly. Indeed, Lis1 mutant mice display defects in neuronal migration and layering in vivo, and also in cerebellar granule cell migration in vitro. However, the roles of PAF and the PAF receptor in the neuronal migration remain to be determined. Here, we show that PAF receptor-deficient mice exhibited histological abnormalities in the embryonic cerebellum. PAF receptor-deficient cerebellar granule neurons migrated more slowly in vitro than wild-type neurons, consistent with the observation that a PAF receptor antagonist reduced the migration of wild-type neurons in vitro. Synergistic reduction of neuronal migration was observed in a double mutant of PAF receptor and LIS1. Unexpectedly, PAF affected the migration of PAF receptor-deficient neurons, suggesting a receptor-independent pathway for PAF action. The PAF receptor-independent response to PAF was abolished in granule neurons derived from the double mutant mice. Thus, our results suggest that the migration of cerebellar granule cells is regulated by PAF through receptor-dependent and receptor-independent pathways, and that LIS1 is a pivotal molecule that links PAF action and neuronal cell migration both in vivo and in vitro.

Putative role of proteolysis and inflammatory response in the toxicity of nerve and blister chemical warfare agents: implications for multi-threat medical countermeasures

Despite the contrasts in chemistry and toxicity, for blister and nerve chemical warfare agents there may be some analogous proteolytic and inflammatory mediators and pathological pathways that can be pharmacological targets for a single-drug multi-threat medical countermeasure. The dermal-epidermal separation caused by proteases and bullous diseases compared with that observed following exposure to the blister agent sulfur mustard (2,2'-dichlorodiethyl sulfide) has fostered the hypothesis that sulfur mustard vesication involves proteolysis and inflammation. In conjunction with the paramount toxicological event of cholinergic crisis that causes acute toxicity and precipitates neuronal degeneration, both anaphylactoid reactions and pathological proteolytic activity have been reported in nerve-agent-intoxicated animals. Two classes of drugs already have demonstrated multi-threat activity for both nerve and blister agents. Serine protease inhibitors can prolong the survival of animals intoxicated with the nerve agent soman and can also protect against vesication caused by the blister agent sulfur mustard. Poly (ADP-ribose) polymerase (PARP) inhibitors can reduce both soman-induced neuronal degeneration and sulfur-mustard-induced epidermal necrosis. Protease and PARP inhibitors, like many of the other countermeasures for blister and nerve agents, have potent primary or secondary anti-inflammatory pharmacology. Accordingly, we hypothesize that drugs with anti-inflammatory actions against either nerve or blister agent might also display multi-threat efficacy for the inflammatory pathogenesis of both classes of chemical warfare agent.

Selective inflammatory stimulations enhance release of microglial response factor (MRF)-1 from cultured microglia

The mrf-1 gene has been isolated from microglia exposed to cultured cerebellar granule neurons undergoing apoptosis. We have shown that mrf-1 is upregulated in response to neuronal death and degeneration both in vitro and in vivo. However, the exact role of MRF-1 remains unknown. Here we show that MRF-1 is released from cultured rat microglia, and its release is greatly enhanced under inflammatory conditions. When microglia were treated with ATP, the amount of MRF-1 that was released increased 10-fold compared to the basal level of release. Enhanced MRF-1 release was induced within 10 min and peaked within 1 h; after approximately 4 h, the MRF-1 release had returned to normal. MRF-1 release was stimulated by 2-methyl-thio-ATP (five-fold) and a P2X(7) selective agonist, 2'- and 3'-O-(4-benzoylbenzoyl)-ATP (ten-fold). Moreover, the ATP-stimulated MRF-1 release was inhibited by a P2X(7) selective antagonist, oxidized ATP (oATP), and also under a Ca(2+)-free condition. These results indicate that the effects of ATP are dependent on Ca(2+) influx through P2X(7) receptors. MRF-1 release was enhanced by Ca(2+)-ionophore A23187 (sixfold), thapsigargin (threefold); however, it was not enhanced by glutamate or lipopolysaccharide. Moreover, a platelet-activating factor enhanced microglial MRF-1 release in a dose-dependent manner. We also showed that a conditioned medium from cerebellar granule neurons undergoing apoptosis markedly increased MRF-1 release from microglia; that effect was significantly inhibited by oATP. These results indicate that selective inflammatory stimulations, including ATP and PAF, enhance MRF-1 release from microglia through a Ca(2+)-dependent mechanism and suggest that MRF-1 may play a role in cell-cell interactions under inflammatory conditions.

Microglia in human immunodeficiency virus-associated neurodegeneration

Infection with the human immunodeficiency virus (HIV) is associated with a syndrome of cognitive and motor abnormalities that may develop in the absence of opportunistic infections. Neurons are not productively infected by HIV. Thus, one hypothesis to explain the pathophysiology of HIV-associated dementia (HAD) suggests that signals released from other infected cell types in the CNS secondarily lead to neuronal injury. Microglia are the predominant resident CNS cell type productively infected by HIV-1. Neurologic dysfunction in HAD appears to be a consequence of microglial infection and activation. Several neurotoxic immunomodulatory factors are released from infected and activated microglia, leading to altered neuronal function, synaptic and dendritic degeneration, and eventual neuronal apoptosis. This review summarizes findings from clinical/pathological studies, animal models, and in vitro models of HAD. Most of these studies support the hypothesis that altered microglial physiology is the nidus for a cascade of events leading to neuronal dysfunction and death. Several molecular mediators of neuronal injury in HAD that emanate from microglia have been identified, and strategies for altering the impact of these neurotoxins are discussed.

Platelet-activating factor increases prostaglandin E(2) release from astrocyte-enriched cortical cell cultures

The phospholipid mediator platelet-activating factor (PAF) increased the release of prostaglandin E(2) (PGE(2)) from astrocyte-enriched cortical cell cultures in a concentration- and time-dependent manner. The nonhydrolyzable PAF analog methylcarbamyl-PAF (mc-PAF), the PAF intermediate lyso-PAF, and arachidonic acid (AA) also produced this effect. In contrast, phosphatidlycholine (PC) and lyso-PC, lipids that are structurally similar to PAF and lyso-PAF, had no effect on PGE(2) production, suggesting that PAF-induced PGE(2) release is not the consequence of nonspecific phospholipid-induced membrane perturbation. Antagonism of intracellular PAF binding sites completely abolished the ability of mc-PAF and lyso-PAF to mobilize PGE(2,) and attenuated the AA effect. Antagonism of the G-protein-coupled PAF receptor in plasma membranes had no significant effect on mc-PAF, lyso-PAF or AA-induced PGE(2) release. Based on the present findings, we hypothesize that intracellular PAF is a physiologic stimulus of PGE(2) production in astrocytes.

Platelet-activating factor receptor

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid mediator. Although PAF was named for its potential to induce platelet aggregation, intense investigations have elucidated potent biological actions of PAF in a broad range of cell types and tissues. PAF acts by binding to a unique G-protein-coupled seven transmembrane receptor, and activates multiple intracellular signaling pathways. In the last decade, we have identified the PAF receptor structures, intracellular signaling mechanisms, and genomic organizations. Recently, we found a single nucleotide polymorphism of the human PAF receptor (A224D) with an allele frequency of 7.8% in Japanese. Cells expressing this receptor exhibited the reduced cellular signaling, although the binding parameters remain unchanged. We have established two different types of genetically altered mice, i.e. PAF receptor-overexpressing mouse and PAF receptor-deficient mouse. These mutant mice provide a novel and specific approach for identifying the pathophysiological and physiological functions of PAF in vivo. This review focuses on phenotypes of these mutant mice and summarizes the previous reports regarding PAF and PAF receptor.

Platelet-activating factor induces permeability transition and cytochrome c release in isolated brain mitochondria

Platelet-activating factor (PAF), a potent bioactive phospholipid implicated in neuronal excitotoxic death, was assessed as a mediator of brain mitochondrial dysfunction. Carbamyl PAF, a non-hydrolyzable PAF analog, added to neurons in culture resulted in decreased mitochondrial membrane potential (DeltaPsi(M)) as measured by the DeltaPsi(M)-sensitive fluorophore 5,5', 6,6'-tetrachloro-1, 1', 3,3'-tetraethylethylbenzimidazolo-carbocyanide iodide (JC-1). To investigate whether PAF has a direct effect on the mitochondria, the mediator was added to rat brain mitochondria preparations and an increase in the permeability of the mitochondrial membrane, termed permeability transition (PT), and cytochrome c release were measured. We report that PAF causes both dose-dependent PT and cytochrome c release from isolated mitochondria. Furthermore, the selective PAF antagonist tetrahydro-4,7,8,10 methyl-1 (chloro-2 phenyl)-6 (methoxy-4 phenyl-carbamoyl)-9 pyrido [4',3'-4,5] thieno [3,2-f] triazolo-1,2,4 [4,3-a] diazepine-1,4 (BN50730), which has affinity for intracellular binding sites, and the peripheral benzodiazepine receptor ligands 7-chloro-5- [4'-chlorophenyl]-1,3-dihydro-1-methyl-2H-1,4-benzodiazepin-2-one (Ro5-4864) and 1-(-2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK11195), inhibit PAF induction of PT and cytochrome c release. These results suggest that PAF excitotoxicity involves, at least in part, alterations of the mitochondrial membrane.

Lack of evidence for associations between plasma platelet-activating factor acetylhydrolase deficiency and schizophrenia

Platelet-activating factor (PAF) is a potent phospholipid mediator that plays various roles in neuronal function and brain development. It is involved in NMDA receptor function. Release and degradation of PAF is controlled by intracellular and plasma PAF-acetylhydrolase (PAFAH). The plasma PAFAH gene (PLA2G7) is located on chromosome 6p. A previous study showed weak associations of the Ile198Thr and Val379Ala polymorphisms of this gene with schizophrenia that did not reach statistical significance after correction for multiple comparisons. Another study showed that a functional alteration of the enzyme with these two polymorphisms is likely, but the magnitude may be modest. Approximately 4% of the Japanese population lack plasma PAFAH because of a loss-of-function mutation (Val279Phe) in the PAFAH gene. Thus, the Val279Phe mutation is useful for examining whether a causal relation exists between PAFAH function and schizophrenia. We looked for an association between the Val279Phe mutation and schizophrenia in 191 Japanese patients with schizophrenia and in 188 Japanese controls. Similar genotypic and allelic distributions were observed in the two groups. These observations indicate that functional differences in the plasma form of PAFAH do not play a substantial role in the etiology of schizophrenia. However, the present study leaves open the possibility that other isoforms are involved.

Cross-talk between neurons and glia: highlights on soluble factors

The development of the nervous system is guided by a balanced action between intrinsic factors represented by the genetic program and epigenetic factors characterized by cell-cell interactions which neural cells might perform throughout nervous system morphogenesis. Highly relevant among them are neuron-glia interactions. Several soluble factors secreted by either glial or neuronal cells have been implicated in the mutual influence these cells exert on each other. In this review, we will focus our attention on recent advances in the understanding of the role of glial and neuronal trophic factors in nervous system development. We will argue that the functional architecture of the brain depends on an intimate neuron-glia partnership.

Roles of cytosolic phospholipase A(2) and platelet-activating factor receptor in the Ca-induced biosynthesis of PAF

Casein-elicited peritoneal exudate cells (PEC), mainly consisted of neutrophils, were collected from platelet-activating factor receptor-knock-out (PAFR-KO), cytosolic phospholipase A(2) knock-out (cPLA(2)-KO), and wild-type (WT) mice. After stimulation of PEC with calcium ionophore A 23187, PAF levels were measured by radio-ligand binding assay using receptor-rich membrane fraction prepared from the PAF receptor transgenic mice. We found that the level of PAF production by PEC was not different between WT and PAFR-KO mice. On the other hand, cPLA(2)-KO mice were deficient in the PAF production. These results provide the direct evidence while cPLA(2) is essential in the production of PAF, PAF receptor deficiency has little effect on the PAF production.