Predominant expression of platelet-activating factor receptor in the rat brain microglia

Underlying Mechanism of Lysosomal Membrane Permeabilization in CNS Injury: A Literature Review

Lysosomes play a crucial role in various intracellular pathways as their final destination. Various stressors, whether mild or severe, can induce lysosomal membrane permeabilization (LMP), resulting in the release of lysosomal enzymes into the cytoplasm. LMP not only plays a pivotal role in various cellular events but also significantly contributes to programmed cell death (PCD). Previous research has demonstrated the participation of LMP in central nervous system (CNS) injuries, including traumatic brain injury (TBI), spinal cord injury (SCI), subarachnoid hemorrhage (SAH), and hypoxic-ischemic encephalopathy (HIE). However, the mechanisms underlying LMP in CNS injuries are poorly understood. The occurrence of LMP leads to the activation of inflammatory pathways, increased levels of oxidative stress, and PCD. Herein, we present a comprehensive overview of the latest findings regarding LMP and highlight its functions in cellular events and PCDs (lysosome-dependent cell death, apoptosis, pyroptosis, ferroptosis, and autophagy). In addition, we consolidate the most recent insights into LMP in CNS injury by summarizing and exploring the latest advances. We also review potential therapeutic strategies that aim to preserve LMP or inhibit the release of enzymes from lysosomes to alleviate the consequences of LMP in CNS injury. A better understanding of the role that LMP plays in CNS injury may facilitate the development of strategic treatment options for CNS injury.

The role of intracellular calcium-store-mediated calcium signals in in vivo sensor and effector functions of microglia

Under physiological conditions microglia, the immune sentinels of the brain, constantly monitor their microenvironment. In the case of danger, damage or cell/tissue dyshomeostasis, they react with changes in process motility, polarization, directed process movement, morphology and gene expression profile; release pro- and anti-inflammatory mediators; proliferate; and clean brain parenchyma by means of phagocytosis. Based on recent transcriptomic and in vivo Ca2+ imaging data, we argue that the local cell/tissue dyshomeostasis is sensed by microglia via intracellular Ca2+ signals, many of which are mediated by Ca2+ release from the intracellular Ca2+ stores. These signals encode the strength, duration and spatiotemporal pattern of the stimulus and, at the same time, relay this information further to trigger the respective Ca2+ -dependent effector pathways. We also point to the fact that microglial Ca2+ signalling is sexually dimorphic and undergoes profound changes across the organism's lifespan. Interestingly, the first changes in microglial Ca2+ signalling are visible already in 9- to 11-month-old mice, roughly corresponding to 40-year-old humans.

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.

Arachidonic Acid Derivatives and Neuroinflammation

Neuroinflammation is characterized by dysregulated inflammatory responses localized within the brain and spinal cord. Neuroinflammation plays a pivotal role in the onset of several neurodegenerative disorders and is considered a typical feature of these disorders. Microglia perform primary immune surveillance and macrophage-like activities within the central nervous system. Activated microglia are predominant players in the central nervous system response to damage related to stroke, trauma, and infection. Moreover, microglial activation per se leads to a proinflammatory response and oxidative stress. During the release of cytokines and chemokines, cyclooxygenases and phospholipase A2 are stimulated. Elevated levels of these compounds play a significant role in immune cell recruitment into the brain. Cyclic phospholipase A2 plays a fundamental role in the production of prostaglandins by releasing arachidonic acid. In turn, arachidonic acid is biotransformed through different routes into several mediators that are endowed with pivotal roles in the regulation of inflammatory processes. Some experimental models of neuroinflammation exhibit an increase in cyclic phospholipase A2, leukotrienes, and prostaglandins such as prostaglandin E2, prostaglandin D2, or prostacyclin. However, findings on the role of the prostacyclin receptors have revealed that their signalling suppresses Th2-mediated inflammatory responses. In addition, other in vitro evidence suggests that prostaglandin E2 may inhibit the production of some inflammatory cytokines, attenuating inflammatory events such as mast cell degranulation or inflammatory leukotriene production. Based on these conflicting experimental data, the role of arachidonic acid derivatives in neuroinflammation remains a challenging issue.

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.

Platelet-Activating Factor Deteriorates Lysophosphatidylcholine-Induced Demyelination Via Its Receptor-Dependent and -Independent Effects

Accumulating evidence suggests that platelet-activating factor (PAF) increases the inflammatory response in demyelinating diseases such as multiple sclerosis. However, PAF receptor (PAFR) antagonists do not show therapeutic efficacy for MS, and its underlying mechanisms remain poorly understood. In the present study, we investigated the effects of PAF on an ex vivo demyelination cerebellar model following lysophosphatidylcholine (LPC, 0.5 mg/mL) application using wild-type and PAFR conventional knockout (PAFR-KO) mice. Demyelination was induced in cerebellar slices that were cultured with LPC for 18 h. Exogenous PAF (1 μM) acting on cerebellar slices alone did not cause demyelination but increased the severity of LPC-induced demyelination in both wild-type and PAFR-KO mice. LPC inhibited the expression of PAF-AH, MBP, TNF-α, and TGF-β1 but facilitated the expression of IL-1β and IL-6 in wild-type preparations. Of note, exogenous PAF stimulated microglial activation in both wild-type and PAFR-KO mice. The subsequent inflammatory cytokines TNFα, IL-1β, and IL-6 as well as the anti-inflammatory cytokine TGF-β1 demonstrated a diverse transcriptional profile with or without LPC treatment. PAF promoted TNF-α expression and suppressed TGF-β1 expression indiscriminately in wild-type and knockout slices; however, transcription of IL-1β and IL-6 was not significantly affected in both slices. The syntheses of IL-1β and IL-6 were significantly increased in LPC-induced demyelination preparations without PAF but showed a redundancy in PAF-treated wild-type and knockout slices. These data suggest that PAF can play a detrimental role in LPC-induced demyelination probably due to a redundant response of PAFR-dependent and PAFR-independent effects on inflammatory cytokines.

YY-1224, a terpene trilactone-strengthened Ginkgo biloba, attenuates neurodegenerative changes induced by β-amyloid (1-42) or double transgenic overexpression of APP and PS1 via inhibition of cyclooxygenase-2

BACKGROUND

Ginkgo biloba has been reported to possess free radical-scavenging antioxidant activity and anti-inflammatory properties. In our pilot study, YY-1224, a terpene trilactone-strengthened extract of G. biloba, showed anti-inflammatory, neurotrophic, and antioxidant effects.

RESULTS

We investigated the pharmacological potential of YY-1224 in β-amyloid (Aβ) (1-42)-induced memory impairment using cyclooxygenase-2 (COX-2) knockout (-/-) and APPswe/PS1dE9 transgenic (APP/PS1 Tg) mice. Repeated treatment with YY-1224 significantly attenuated Aβ (1-42)-induced memory impairment in COX-2 (+/+) mice, but not in COX-2 (-/-) mice. YY-1224 significantly attenuated Aβ (1-42)-induced upregulation of platelet-activating factor (PAF) receptor gene expression, reactive oxygen species, and pro-inflammatory factors. In addition, YY-1224 significantly inhibited Aβ (1-42)-induced downregulation of PAF-acetylhydrolase-1 (PAF-AH-1) and peroxisome proliferator-activated receptor γ (PPARγ) gene expression. These changes were more pronounced in COX-2 (+/+) mice than in COX-2 (-/-) mice. YY-1224 significantly attenuated learning impairment, Aβ deposition, and pro-inflammatory microglial activation in APP/PS1 Tg mice, whereas it significantly enhanced PAF-AH and PPARγ expression. A preferential COX-2 inhibitor, meloxicam, did not affect the pharmacological activity by YY-1224, suggesting that the COX-2 gene is a critical mediator of the neuroprotective effects of YY-1224. The protective activity of YY-1224 appeared to be more efficacious than a standard G. biloba extract (Gb) against Aβ insult.

CONCLUSIONS

Our results suggest that the protective effects of YY-1224 against Aβ toxicity may be associated with its PAF antagonistic- and PPARγ agonistic-potential as well as inhibition of the Aβ-mediated pro-inflammatory switch of microglia phenotypes through suppression of COX-2 expression.

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.

Dysfunctional epileptic neuronal circuits and dysmorphic dendritic spines are mitigated by platelet-activating factor receptor antagonism

Temporal lobe epilepsy or limbic epilepsy lacks effective therapies due to a void in understanding the cellular and molecular mechanisms that set in motion aberrant neuronal network formations during the course of limbic epileptogenesis (LE). Here we show in in vivo rodent models of LE that the phospholipid mediator platelet-activating factor (PAF) increases in LE and that PAF receptor (PAF-r) ablation mitigates its progression. Synthetic PAF-r antagonists, when administered intraperitoneally in LE, re-establish hippocampal dendritic spine density and prevent formation of dysmorphic dendritic spines. Concomitantly, hippocampal interictal spikes, aberrant oscillations, and neuronal hyper-excitability, evaluated 15–16 weeks after LE using multi-array silicon probe electrodes implanted in the dorsal hippocampus, are reduced in PAF-r antagonist-treated mice. We suggest that over-activation of PAF-r signaling induces aberrant neuronal plasticity in LE and leads to chronic dysfunctional neuronal circuitry that mediates epilepsy.

Increased arachidonic acid-containing phosphatidylcholine is associated with reactive microglia and astrocytes in the spinal cord after peripheral nerve injury

Peripheral nerve injury (PNI) triggers cellular and molecular changes in the spinal cord. However, little is known about how the polyunsaturated fatty acid-containing phosphatidylcholines (PUFA-PCs) are regulated in the spinal cord after PNI and the association of PUFA-PCs with the non-neuronal cells within in the central nervous system (CNS). In this study, we found that arachidonic acid-containing phosphatidylcholine (AA-PC), [PC(16:0/20:4)+K](+), was significantly increased in the ipsilateral ventral and dorsal horns of the spinal cord after sciatic nerve transection, and the increased expression of [PC(16:0/20:4)+K](+) spatiotemporally resembled the increase of reactive microglia and the astrocytes. From the lipidomics point of view, we conclude that [PC(16:0/20:4)+K](+) could be the main phospholipid in the spinal cord influenced by PNI, and the regulation of specific phospholipid molecule in the CNS after PNI is associated with the reactive microglia and astrocytes.

Glucosamine suppresses platelet-activating factor-induced activation of microglia through inhibition of store-operated calcium influx

Microglia activation and subsequent release of inflammatory mediators are implicated in the pathophysiology of neurodegenerative diseases. Platelet-activating factor (PAF), a potent lipid mediator synthesized by microglia, is known to stimulate microglia functional responses. In this study, we determined that endogenous PAF exert autocrine effects on microglia activation, as well as the underlying mechanism involved. We also investigated the effect of D-glucosamine (GlcN) on PAF-induced cellular activation in human HMO6 microglial cells. PAF induced sustained intracellular Ca(2+) ([Ca(2+)]i) increase through store-operated Ca(2+) channels (SOC) and reactive oxygen species (ROS) generation. PAF also induced pro-inflammatory markers through NFκB/COX-2 signaling. GlcN significantly inhibited PAF-induced Ca(2+) influx and ROS generation without significant cytotoxicity. GlcN downregulated excessive expression of pro-inflammatory markers and promoted filopodia formation through NFκB/COX-2 inhibition in PAF-stimulated HMO6 cells. Taken together, these data suggest that GlcN may offer substantial therapeutic potential for treating inflammatory and neurodegenerative diseases accompanied by microglial activation.

Platelet-activating factor attenuation of long-term potentiation in rat hippocampal slices via protein tyrosine kinase signaling

It is well established that HIV-1-infected mononuclear phagocytes release platelet activating factor (PAF) and elevated levels of PAF have been detected in blood and in the cerebrospinal fluid (CSF) of acquired immunodeficiency syndrome (AIDS) patients with HIV-associated neurocognitive disorders (HAND). It is our hypothesis that the elevated levels of PAF alter long-term potentiation (LTP) in the hippocampus, leading to neurocognitive dysfunction. To test this hypothesis, we studied the effects of PAF on LTP in the CA1 region of rat hippocampal slices. Our results showed incubation of hippocampal slices with PAF attenuated LTP. The PAF-mediated attenuation was blocked by ginkgolide B, a PAF receptor antagonist, suggesting PAF attenuation of LTP via PAF receptors. Application of lyso-PAF, an inactive PAF analog, had no apparent effect on LTP. Further investigation revealed an involvement of tyrosine kinase in PAF attenuation of LTP, which was demonstrated by lavendustin A (a specific protein tyrosine kinase inhibitor) blockage of PAF attenuation of LTP. As LTP is widely considered as the cellular and synaptic basis for learning and memory, the attenuation of LTP by PAF may contribute at least in part to the HAND pathogenesis.

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.

Bacterial exploitation of phosphorylcholine mimicry suppresses inflammation to promote airway infection

Regulation of neutrophil activity is critical for immune evasion among extracellular pathogens, yet the mechanisms by which many bacteria disrupt phagocyte function remain unclear. Here, we have shown that the respiratory pathogen Streptococcus pneumoniae disables neutrophils by exploiting molecular mimicry to degrade platelet-activating factor (PAF), a host-derived inflammatory phospholipid. Using mass spectrometry and murine upper airway infection models, we demonstrated that phosphorylcholine (ChoP) moieties that are shared by PAF and the bacterial cell wall allow S. pneumoniae to leverage a ChoP-remodeling enzyme (Pce) to remove PAF from the airway. S. pneumoniae-mediated PAF deprivation impaired viability, activation, and bactericidal capacity among responding neutrophils. In the absence of Pce, neutrophils rapidly cleared S. pneumoniae from the airway and impeded invasive disease and transmission between mice. Abrogation of PAF signaling rendered Pce dispensable for S. pneumoniae persistence, reinforcing that this enzyme deprives neutrophils of essential PAF-mediated stimulation. Accordingly, exogenous activation of neutrophils overwhelmed Pce-mediated phagocyte disruption. Haemophilus influenzae also uses an enzyme, GlpQ, to hydrolyze ChoP and subvert PAF function, suggesting that mimicry-driven immune evasion is a common paradigm among respiratory pathogens. These results identify a mechanism by which shared molecular structures enable microbial enzymes to subvert host lipid signaling, suppress inflammation, and ensure bacterial persistence at the mucosa.

Attenuated Reactive Gliosis and Enhanced Functional Recovery Following Spinal Cord Injury in Null Mutant Mice of Platelet-Activating Factor Receptor

Platelet-activating factor (PAF) is a unique phosphoglycerine that mediates the biological functions of both immune and nervous systems. Excessive PAF plays an important role in neural injury via its specific receptor (PAFR). In this study, we hypothesized that PAF signaling activates reactive gliosis after spinal cord injury (SCI), and blocking the PAF pathway would modify the glia scar formation and promote functional recovery. PAF microinjected into the normal wild-type spinal cord induced a dose-dependent activation of microglia and astrocytes. In the SCI mice, PAFR null mutant mice showed a better functional recovery in grip and rotarod performances than wild-type mice. Although both microglia and astrocytes were activated after SCI in wild-type and PAFR null mutant mice, expressions of IL-6, vimentin, nestin, and GFAP were not significantly elevated in PAFR null mutants. Disruption of PAF signaling inhibited the expressions of proteoglycan CS56 and neurocan (CSPG3). Intriguingly, compared to the wild-type SCI mice, less axonal retraction/dieback at 7 dpi but more NFH-labeled axons at 28 dpi was found in the area adjacent to the epicenter in PAFR null mutant SCI mice. Moreover, treatment with PAFR antagonist Ginkgolide B (GB) at the chronic phase rather than acute phase enhanced the functional recovery in the wild-type SCI mice. These findings suggest that PAF signaling participates in reactive gliosis after SCI, and blocking of this signaling enhances functional recovery and to some extent may promote axon regrowth.

Calcium ion influx in microglial cells: physiological and therapeutic significance

Microglial cells, the immunocompetent cells of the central nervous system (CNS), exhibit a resting phenotype under healthy conditions. In response to injury, however, they transform into an activated state, which is a hallmark feature of many CNS diseases. Factors or agents released from the neurons, blood vessels, and/or astrocytes could activate these cells, leading to their functional and structural modifications. Microglial cells are well equipped to sense environmental changes within the brain under both physiological and pathological conditions. Entry of calcium ions (Ca(2+)) plays a critical role in the process of microglial transformation; several channels and receptors have been identified on the surface of microglial cells. These include store-operated channel, Orai1, and its sensor protein, stromal interaction molecule 1 (STIM1), in microglial cells, and their functions are modulated under pathological stimulations. Transient receptor potential (TRP) channels and voltage- and ligand-gated channels (ionotropic and metabotropic receptors) are also responsible for Ca(2+) influx into the microglial cells. An elevation of intracellular Ca(2+) concentration subsequently regulates microglial cell functions by activating a diverse array of Ca(2+)-sensitive signaling cascades. Perturbed Ca(2+) homeostasis contributes to the progression of a number of CNS disorders. Thus, regulation of Ca(2+) entry into microglial cells could be a pharmacological target for several CNS-related pathological conditions. This Review addresses the recent insights into microglial cell Ca(2+) influx mechanisms, their roles in the regulation of functions, and alterations of Ca(2+) entry in specific CNS disorders.

Up-regulation of platelet-activating factor synthases and its receptor in spinal cord contribute to development of neuropathic pain following peripheral nerve injury

Background

Platelet-activating factor (PAF; 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a lipid mediator derived from cell membrane. It has been reported that PAF is involved in various pathological conditions, such as spinal cord injury, multiple sclerosis, neuropathic pain and intrathecal administration of PAF leads to tactile allodynia. However, the expression of PAF synthases and its receptor in the spinal cord following peripheral nerve injury is unknown.

Methods

Using the rat spared nerve injury (SNI) model, we investigated the expression of PAF synthases (LPCAT1 and 2) and PAF receptor (PAFr) mRNAs in the spinal cord. Reverse transcription polymerase chain reaction (RT-PCR) and double-labeling analysis of in situ hybridization histochemistry (ISHH) with immunohistochemistry (IHC) were employed for the analyses. Pain behaviors were also examined with PAFr antagonist (WEB2086).

Results

RT-PCR showed that LPCAT2 mRNA was increased in the ipsilateral spinal cord after injury, but not LPCAT1 mRNA. Double-labeling of ISHH with IHC revealed that LPCAT1 and 2 mRNAs were constitutively expressed by a subset of neurons, and LPCAT2 mRNA was increased in spinal microglia after nerve injury. RT-PCR showed that PAFr mRNA was dramatically increased in the ipsilateral spinal cord after nerve injury. Double-labeling analysis of ISHH with IHC revealed that after injury PAFr mRNA was predominantly colocalized with microglia in the spinal cord. Continuous intrathecal administration of the PAFr antagonist suppressed mechanical allodynia following peripheral nerve injury. Delayed administration of a PAFr antagonist did not reverse the mechanical allodynia.

Conclusions

Our data show the histological localization of PAF synthases and its receptor in the spinal cord following peripheral nerve injury, and suggest that PAF/PAFr signaling in the spinal cord acts in an autocrine or paracrine manner among the activated microglia and neurons, thus contributing to development of neuropathic pain.

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.

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.

Dual roles of brain serine hydrolase KIAA1363 in ether lipid metabolism and organophosphate detoxification

Serine hydrolase KIAA1363 is an acetyl monoalkylglycerol ether (AcMAGE) hydrolase involved in tumor cell invasiveness. It is also an organophosphate (OP) insecticide-detoxifying enzyme. The key to understanding these dual properties was the use of KIAA1363 +/+ (wildtype) and -/- (gene deficient) mice to define the role of this enzyme in brain and other tissues and its effectiveness in vivo in reducing OP toxicity. KIAA1363 was the primary AcMAGE hydrolase in brain, lung, heart and kidney and was highly sensitive to inactivation by chlorpyrifos oxon (CPO) (IC50 2 nM) [the bioactivated metabolite of the major insecticide chlorpyrifos (CPF)]. Although there was no difference in hydrolysis product monoalkylglycerol ether (MAGE) levels in +/+ and -/- mouse brains in vivo, isopropyl dodecylfluorophosphonate (30 mg/kg) and CPF (100 mg/kg) resulted in 23-51% decrease in brain MAGE levels consistent with inhibition of AcMAGE hydrolase activity. On incubating +/+ and -/- brain membranes with AcMAGE and cytidine-5'-diphosphocholine, the absence of KIAA1363 activity dramatically increased de novo formation of platelet-activating factor (PAF) and lyso-PAF, signifying that metabolically-stabilized AcMAGE can be converted to this bioactive lipid in brain. On considering detoxification, KIAA1363 -/- mice were significantly more sensitive than +/+ mice to ip-administered CPF (100 mg/kg) and parathion (10 mg/kg) with increased tremoring and mortality that correlated for CPF with greater brain acetylcholinesterase inhibition. Docking AcMAGE and CPO in a KIAA1363 active site model showed similar positioning of their acetyl and trichloropyridinyl moieties, respectively. This study establishes the relevance of KIAA1363 in ether lipid metabolism and OP detoxification.

Lipopolysaccharide-induced down-regulation of Ca2+ release-activated Ca2+ currents (I CRAC) but not Ca2+-activated TRPM4-like currents (I CAN) in cultured mouse microglial cells

Microglia are the main immunocompetent cells of the mammalian central nervous system (CNS). Activation of cultured microglial cells and subsequent release of nitric oxide and cytokines critically depends on intracellular calcium levels. Since microglia undergo dramatic morphological, biochemical and electrophysiological changes in response to pathological events in the CNS, we investigated temporal changes in expression levels of ion channels involved in cellular calcium homeostasis in mouse cortical microglial cells in culture. Specifically, we assessed the inward and delayed outward rectifier potassium currents (I IRK and I DRK), calcium (Ca2+) release-activated Ca2+ currents (I CRAC) and Ca2+-activated TRPM4-like currents (I CAN) in non-activated microglia and cells that were activated by exposure to lipopolysaccharide (LPS) between 3 and 48 h. Unstimulated microglial cells, subcultured from an astrocyte coculture, typically exhibited a ramified, rod-shaped morphology. During the first 3 days of culture cell size and shape were maintained, but the percentage of cells showing prominent I IRK went up and those expressing I DRK went down. Cells retaining I DRK exhibited smaller amplitudes, whereas those of I IRK and I CRAC were not affected. However, after 24 h of exposure to 1 microg ml(-1) LPS, most cells showed an amoeboid ('fried egg'-shaped) morphology with a 62% increase in cell capacitance. At that point in time, only 14% of the cells revealed I IRK and 3% had I DRK exclusively, whereas the majority of cells expressed both currents. The amplitudes of I CRAC and I IRK progressively decreased after stimulation, whereas I DRK transiently reached a maximum after 6 h of LPS exposure and then returned to pre-stimulation expression levels. Cultured microglia also revealed TRPM4-like, Ca2+-activated non-selective currents (I CAN) with an EC50 of 1.2 microm [Ca2+]i. The expression levels of this current did not change significantly during and after 24 h of LPS exposure. We propose that LPS-induced down-regulation of I IRK and I CRAC will reduce the cell's capacity to produce significant calcium influx upon receptor activation and result in decreased sensitivity to exogenous stimulation. In this scenario, I CAN expression would remain constant, although its activity would automatically be reduced due to the diminished calcium influx capacity of the cell.

Platelet activating factor-induced neuronal apoptosis is initiated independently of its G-protein coupled PAF receptor and is inhibited by the benzoate orsellinic acid

The bioactive lipid mediator platelet activating factor (PAF) is recognized as a key effecter of neuronal apoptosis, yet it is not clear whether its G-protein coupled receptor (PAFR) initiates or prevents PAF neurotoxicity. Using PAFR-/- and congenic wild-type mice, we show that PAF triggers caspase-3/7 activity and neuronal death in PAFR-/- but not PAFR+/+ cerebellar granule neurons. Restoring receptor expression by recombinant adenoviral infection protected cells from PAF challenge. Neuronal death was not mediated by nitric oxide or N-methyl-d-aspartate receptor signaling given that N-nitro-l-arginine methyl ester and MK-801 did not inhibit PAF-induced neuronal loss in PAFR-/- neurons. To intervene in PAFR-independent neurotoxicity, the anti-apoptotic actions of three structurally distinct PAF antagonists were compared to a panel of plant and fungal benzoic acid derivatives. We found that the PAF antagonist BN 52021 but not FR 49175 or CV 3988 inhibited PAFR-independent neurotoxicity. Orsellinic acid, a fungal-derived benzoic acid, blocked PAF-mediated neuronal apoptosis without affecting PAFR-mediated neuroprotection. These findings demonstrate that PAF can transduce apoptotic death in primary neurons independently of its G-protein coupled receptor, that PAFR activation is neuroprotective, and that orsellinic acid effectively attenuates PAFR-independent neuronal apoptosis.

Down-regulation of platelet-activating factor receptor gene expression during focal reversible cerebral ischemia in rats

Platelet-activating factor (PAF) is an endogenous potent phospholipid mediator in stroke and related to the post-ischemic brain damage. The aim of this study was to investigate the regulation and mechanisms of PAF receptor gene expression in the perifocal regions of cerebral infarction after middle cerebral artery occlusion/reperfusion. Sixty mature Wistar rats were randomly divided into 12 groups: sham-operated control group, simple ischemia 90 min group, 6, 12, 18 h, 1 day (1 d), 2, 3, 4, 5, 6, 7 d reperfusion groups. After the right middle cerebral artery occluded, the rats were suffered from ischemia for 90 min, and then reperfusion was allowed for different time courses. Reverse transcription-polymerase chain reaction (RT-PCR) and radioimmunoassay were applied to evaluate the PAF receptor messenger RNA (mRNA) expression and PAF levels in the perifocal regions of cerebral infarction respectively. RT-PCR analysis revealed that PAF receptor mRNA was 0.95 +/- 0.15 in control group. However, following ischemia-reperfusion, the levels of PAF receptor mRNA progressively decreased until 2 d of reperfusion (0.54 +/- 0.10), then returned to control group's levels gradually. Compared with the control group's (582 +/- 72 pg/g wet weight), the PAF concentrations of simple ischemic and 6, 12, 18 h, 1, 2 d reperfusion group were significantly higher than that of any other groups. These results indicate that PAF receptor gene expression may be subject to down-regulation in the perifocal regions of cerebral infarction after cerebral ischemia-reperfusion and relative to the increase of endogenous PAF concentrations.

Functional role of calcium signals for microglial function

In this review we summarize mechanisms of Ca(2+) signaling in microglial cells and the impact of Ca(2+) signaling and Ca(2+) levels on microglial function. So far, Ca(2+) signaling has been only characterized in cultured microglia and thus these data refer rather to activated microglia as observed in pathology when compared with the resting form found under physiological conditions. Purinergic receptors are the most prominently expressed ligand-gated Ca(2+)-permeable channels in microglia and control several microglial functions such as cytokine release in a Ca(2+)-dependent fashion. A large variety of metabotropic receptors are linked to Ca(2+) release from intracellular stores. Depletion of these intracellular stores triggers a capacitative Ca(2+) entry. While microglia are already in an activated state in culture, they can be further activated, for example, by exposure to bacterial endotoxin. This activation leads to a chronic increase of [Ca(2+)](i) and this Ca(2+) increase is a prerequisite for the release of nitric oxide and cytokines. Moreover, several factors (TNFalpha, IL-1beta, and IFN-gamma) regulate resting [Ca(2+)](i) levels.

Role of the pro-inflammatory cytokines TNF-alpha and IL-1beta in HIV-associated dementia

Human immunodeficiency virus-1 (HIV-1)-infected and immune-activated macrophages and microglia secrete neurotoxins. Two of these neurotoxins are the pro-inflammatory cytokines tumour necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta), which are thought to play a major role in inducing neuronal death. Both TNF-alpha and IL-1beta increase the permeability of the blood-brain barrier, through which subsequently HIV-infected monocytes can enter the brain. They both induce over-stimulation of the NMDA-receptor via several pathways, resulting in a lethal neuronal increase in Ca(2+) levels. Additionally, TNF-alpha co-operates with several other proinflammatory mediators to enhance their toxic effects. Although most research has focused on the neurotoxic effects of TNF-alpha and IL-1beta in HAD, there is also evidence that these cytokines can be neuroprotective. In this paper the effect of TNF-alpha and IL-1beta on neuronal life and death in HAD is discussed.

Inhibition of platelet-activating factor receptors in hippocampal plasma membranes attenuates the inflammatory nociceptive response in rats

Evidence suggests that platelet-activating factor (PAF) is a mediator in inflammatory-based pain. Using the biphasic formalin model in rats, we recently demonstrated that PAF antagonists which were selective for either intracellular or plasma membrane PAF receptors decreased the late-phase of the nociceptive response. Inasmuch as both of the PAF antagonists previously used were administered systemically, and reportedly are able to cross the blood-brain barrier, the anatomic locations at which PAF affects pain processing remained to be elucidated. Since PAF is required for hippocampal-dependent memory consolidation, and since the hippocampus has been shown to mediate the late-phase of formalin-induced nociception, the present study investigated the effects on nociception of administration of PAF antagonists within the hippocampus, and of using agents specific for either plasma membrane (BN 52021) or intracellular (BN 50730) PAF binding sites. Intrahippocampal injections of BN 52021 decreased the late-phase of the nociceptive response in a concentration-dependent manner. In contrast, intrahippocampal administration of BN 50730 had no effect on inflammatory nociception. These findings suggest that hippocampal plasma membrane PAF receptors, but not intracellular PAF binding sites, mediate tonic inflammatory pain processing in rats.

Mitochondrial ligand inhibits store-operated calcium influx and COX-2 production in human microglia

We used calcium sensitive fluorescence microscopy to investigate the actions of PK11195, a ligand for the mitochondrial peripheral benzodiazepine receptor (PBR), to modulate Ca2+ influx through store-operated channels (SOC) in human microglia. PK11195 effectively blocked SOC-mediated Ca2+ influx induced by platelet-activating factor (PAF) in a dose-dependent manner (IC50 of 9 microM). A prolonged SOC-mediated Ca2+ entry was also induced using the sarcoplasmic endoreticulum Ca2+-ATPase (SERCA) inhibitor cyclopiazonic acid (CPA) to deplete intracellular endoplasmic reticulum (ER) stores; a single concentration of PK11195 (at 20 microM) reduced SOC-mediated Ca2+ influx by 78%. RT-PCR and immunocytochemical analysis results showed PK11195 also inhibited the expression and production of cyclooxygenase-2 (COX-2) triggered by PAF stimulation. These results suggest that activation of the PBR in mitochondria is linked to reduced entry of Ca2+ through plasmalemmal SOC and subsequent modulation of cellular functions in human microglia.

Acute Subdural Hematoma: New Model Delineation and Effects of Coagulation Inhibitors

OBJECTIVE

To develop a highly reproducible rat model and behavioral tests for acute subdural hematoma (ASDH) and to investigate the role of intravascular coagulation and thrombin in the pathogenesis of brain injury in this model.

METHODS

A new method was implemented to inject 200 microl of autologous blood subdurally in rats. Immunohistochemistry was used to investigate intravascular fibrin deposition and thrombin levels in the cortex underlying the ASDH. Effectiveness of systemic heparin, argatroban, or ginkgolide B treatment was determined by histological lesion volume, number of occluded microvessels, and neurological deficits. Neurological deficits were monitored for 7 days after ASDH by use of forelimb placing, forelimb use asymmetry, and corner turn tests.

RESULTS

Consistent brain damage and sensorimotor deficits were observed in all animals with ASDH. Histological analysis demonstrated occluded microvessels and enlarged perivascular spaces in the underlying cortex starting 1 hour after hematoma induction. Fibrin and thrombin immunoreactivity were increased in the lesioned cortical parenchyma at 4 and 24 hours. However, no intravascular fibrin deposition was detected. Heparin induced hemorrhagic transformation in the cortical lesion and did not attenuate microvessel occlusion. Argatroban and ginkgolide B did not induce hemorrhage but failed to improve microvessel occlusion, lesion volume, and neurological deficits.

CONCLUSION

Intravascular coagulation and thrombin are not the major mediators of brain damage after ASDH. The model and behavioral tests presented in this study can be used to investigate other putative mechanisms of injury and to test future therapeutic interventions in ASDH.

Physiology of microglial cells

Microglial cells in culture and in situ express a defined pattern of K(+) channels, which is distinct from that of other glial cells and neurons. This pattern undergoes defined changes with microglial activation. As expected for a cell with immunological properties, microglia express a variety of cytokine and chemokine receptors, which are linked to the mobilization of Ca(2+) (cytosolic free calcium) from internal stores. Microglial cells also have the capacity to respond to neuronal activity: they express receptors for the major excitatory receptor glutamate and the main inhibitory receptor GABA (gamma-amino butyric acid). By expressing purinergic receptors, microglia can sense astrocyte activity in the form of Ca(2+) waves. Activation of transmitter receptors can affect cytokine release which is a potential means as to how brain activity can affect immune function.

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.

Platelet-activating factor enhancement of calcium influx and interleukin-6 expression, but not production, in human microglia

Calcium-sensitive fluorescence microscopy and molecular biology analysis have been used to study the effects of platelet-activating factor (PAF) on intracellular calcium [Ca²⁺]_i and IL-6 expression in human microglia. PAF (applied acutely at 100 nM) elicited a biphasic response in [Ca²⁺]_i consisting of an initial rapid increase of [Ca²⁺]_i due to release from internal stores, followed by a sustained influx. The latter phase of the [Ca²⁺]_i increase was blocked by SKF96365, a non-selective store-operated channel (SOC) inhibitor. RT-PCR analysis showed PAF treatment of microglia induced expression of the pro-inflammatory cytokine IL-6 in a time-dependent manner which was blocked in the presence of SKF96365. However, ELISA assay showed no production of IL-6 was elicited at any time point (1–24 h) for microglial exposures to PAF. These findings suggest that PAF stimulation of human microglia induces expression, but not production, of IL-6 and that SOC-mediated [Ca²⁺]_i influx contributes to the enhanced expression of the cytokine.

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.

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.

Microglial Calcium Responses to Platelet-Activating Factor are Inhibited by Analogue CAS 99103-16-9 and Dihydropyridine PCA 4248 but Not by Ginkgolide A

Calcium signals evoked in N9 microglial cells were monitored using the calcium indicator dye Fluo-4 in a fluorescence imaging plate reader. Platelet activating factor in the range 100 nM to 20 microM elicited graded calcium responses. The analogue CAS 99103-16-9 inhibited the evoked calcium rise with an apparent KB of 1.3 +/- 0.4 microM. The dihydropyridine PCA 4248 inhibited the evoked calcium rise with an apparent KB of 1.2 +/- 0.2 microM. Ginkgolide A at concentrations up to 18 microM had no effect on the evoked calcium rise. While CAS 99103-16-9 and PCA 4248 appear to be simple competitive inhibitors of platelet-activating factor responses, the efficacy of ginkgolide in more complex pharmacological situations may result from an action at a site other than the platelet-activating factor receptor.

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.

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.

Reciprocal interactions between microglia and neurons: from survival to neuropathology

Microglia represent a major cellular component of the brain, where they constitute a widely distributed network of immunoprotective cells. During the last decades, it has become clear that the functions traditionally ascribed to microglia, i.e. to dispose of dead cells and debris and to mediate brain inflammatory states, are only a fraction of a much wider repertoire of functions spanning from brain development to aging and neuropathology. The aim of the present survey is to critically discuss some of these functions, focusing in particular on the reciprocal microglia-neuron interactions and on the complex signaling systems subserving them. We consider first some of the functional interactions dealing with invasion, proliferation and migration of microglia as well as with the establishment of the initial blueprint of neural circuits in the developing brain. The signals related to the suppression of immunological properties of microglia by neurons in the healthy brain, and the derangement from this physiological equilibrium in aging and diseases, are then examined. Finally, we make a closer examination of the reciprocal signaling between damaged neurons and microglia and, on these bases, we propose that microglial activation, consequent to neuronal injury, is primarily aimed at neuroprotection. The loss of specific communication between damaged neurons and microglia is viewed as responsible for the turning of microglia to a hyperactivated state, which allows them to escape neuronal control and to give rise to persistent inflammation, resulting in exacerbation of neuropathology. The data surveyed here point at microglial-neuron interactions as the basis of a complex network of signals conveying messages with high information content and regulating the most important aspects of brain function. This network shares similar features with some fundamental principles governing the activity of brain circuits: it is provided with memory and it continuously evolves in relation to the flow of time and information.

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.

Calcium signaling in microglial cells

Receptor-mediated Ca(2+) signals are a common signal transduction mechanism in all living cells, including microglia. Recent years have brought major advances in our understanding of microglial Ca(2+) signaling. More than 20 receptor/ligand interactions leading to Ca(2+) signals in microglia have been described so far, and it seems that this is just the beginning. The literature has grown rapidly during the past few years, especially in regard to chemokine and ATP/UTP receptor signaling. This article presents a brief overview of the basics of Ca(2+) signaling, reviews the current literature on microglial Ca(2+) signaling, and discusses the current challenges and possible future directions of this emerging field.