Platelet-activating factor inhibits ionotropic GABA receptor activity in cultured hippocampal neurons

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 receptor affects food intake and body weight

"Let's Move!" is a comprehensive initiative, launched by the First Lady, Michelle Obama, dedicates to solving problems of obesity, which is growing in child. The life behaviors do affect obesity; however, the mechanistic insight in molecular level is still not clear. In this study, by continually monitoring mouse body weight under chow and high fat western diets as well as metabolic, physical activity and food intake behaviors assessed in a CLAMS Comprehensive Lab Animal Monitoring System, we demonstrated that the platelet-activating factor receptor (PTAFR) contributes to modification of life behaviors. PTAFR does not affect metabolism of ingested dietary fat and carbohydrate in young animals; however, Ptafr ablation dramatically increased weight gain without affecting adipose tissue accumulation. Ptafr^-/- mice possess new habits that increased food intake and decreased movement. Our studies suggest that regulation of PTAFR activity may be a novel strategy to control obesity in children or young adults.

Platelet-activating factor-induced synaptic facilitation is associated with increased calcium/calmodulin-dependent protein kinase II, protein kinase C and extracellular signal-regulated kinase activities in the rat hippocampal CA1 region

Platelet-activating factor (PAF) is an important inflammatory lipid mediator affecting neural plasticity. In the present study, we demonstrated how PAF affects synaptic efficacy through activation of protein kinases in the rat hippocampal CA1 region. In cultured hippocampal neurons, 10 to 1000 nM PAF stimulated autophosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII) and phosphorylation of synapsin I and myristoylated alanine-rich protein kinase C substrate (MARCKS). In hippocampal CA1 slices, field excitatory postsynaptic potentials (fEPSPs) induced by stimulation of the Schaffer collateral/commissural pathways were significantly increased 10-50 min after exposure to 100 to 1000 nM PAF. Immunoblotting analysis showed that 100 nM PAF treatment for 10 or 50 min significantly and persistently increased CaMKII autophosphorylation in the hippocampal CA1 region. Increased protein kinase Calpha (PKCalpha) autophosphorylation was also seen at the same time point after PAF exposure. By contrast, extracellular signal-regulated kinase (ERK) phosphorylation was slightly but significantly increased at 10 min after PAF exposure. Consistent with increased CaMKII autophosphorylation, AMPA-type glutamate receptor subunit 1 (GluR1) (Ser-831) phosphorylation as a CaMKII postsynaptic substrate significantly increased after 10 or 50 min of treatment, whereas synapsin I (Ser-603) phosphorylation as a presynaptic substrate increased at 10 min in the hippocampal CA1 region. Phosphorylation of MARCKS (Ser-152/156) and NMDA receptor subunit 1 (NR1) (Ser-896) as PKCalpha substrates also significantly increased after 10 min but had not further increased by 50 min in the CA1 region. Increased of fEPSPs induced by PAF treatment completely and/or partly inhibited by KN93 and/or U0126 treatment. These results suggest that PAF induces synaptic facilitation through activation of CaMKII, PKC and ERK in the hippocampal CA1 region.

Metallothionein-III provides neuronal protection through activation of nuclear factor-kappaB via the TrkA/phosphatidylinositol-3 kinase/Akt signaling pathway

Metallothionein (MT)-III is associated with resistance to neuronal injury. However, the underlying mechanism for its effects is unclear. The present study investigated the mechanisms of MT-III protection of neuronal cells from hypoxia or DNA damage-induced cell death. MT-III reduced the hydrogen peroxide- or DNA damage-induced effects on neuronal cells, including the cell death, the activation of caspase-3 and -9, and the release of mitochondrial cytochrome c to the cytoplasm in a dose-dependent manner. MT-III also increased the activation of Akt, the phosphorylation and degradation of IkappaB, the nuclear translocation/accumulation and the transcriptional activity of nuclear factor-kappaB (NF-kappaB) in neuronal cells in a dose-dependent manner. The MT-III-induced antiapoptotic effects and increase in NF-kappaB activity were blocked by specific inhibitors of TrkA, phosphatidylinositol-3 kinase (PI3K), Akt, or NF-kappaB, indicating that MT-III provides neuronal protection by activating NF-kappaB through the TrkA/PI3K/Akt signaling pathway.

Proteomic analysis of activity-dependent synaptic plasticity in hippocampal neurons

Following long-term treatment with bicuculline and tetrodotoxin (TTX) aimed at modifying synaptic activity in cultured neurons, we used a proteomic approach to identify the associated changes in protein expression. The neurons were left untreated, or treated with bicuculline or TTX, and fractionated by means of differential detergent extraction, after which the proteins in each fraction were separated by means of two-dimensional (2D) gel electrophoresis, and 57 proteins of interest were identified by mass spectrometry. The proteins that showed altered expression and/or post-translational modifications include proteins or enzymes involved in regulating cell and protein metabolism, the cytoskeleton, or mitochondrial activity. These results suggest that extensive alterations in neuronal protein expression take place as a result of increased or decreased synaptic activity.

PGE2 glycerol ester, a COX-2 oxidative metabolite of 2-arachidonoyl glycerol, modulates inhibitory synaptic transmission in mouse hippocampal neurons

The oxygenation of endogenous cannabinoids (eCBs) 2-arachidonoyl glycerol (2-AG) and arachidonoyl ethanolamide by cyclooxygenase-2 (COX-2) produces novel types of prostanoids: prostaglandin glycerol esters (PG-Gs) and prostaglandin ethanolamides (PG-EAs). However, the physiological function of COX-2 oxidative metabolites of eCBs is still unclear. Here we demonstrate that PGE2-G, a COX-2 oxidative metabolite of 2-AG, induced a concentration-dependent increase in the frequency ofminiature inhibitory postsynaptic currents (mIPSCs) in primary cultured hippocampal neurons, an effect opposite to that of 2-AG. This increase was not inhibited by SR141716, a CB1 receptor antagonist, but was attenuated by an IP3 or MAPK inhibitor. In addition, we also examined the effects of other prostanoids derived from COX-2 oxygenation of eCBs on mIPSCs. PGD2-G, PGF2alpha-G and PGD2-EA, but not PGE2-EA or PGF2alpha-EA, also increased the frequency of mIPSCs. The eCB-derived prostanoid-induced responses appeared to be different from those of corresponding arachidonic acid-derived prostanoids, implying that these effects are not mediated via known prostanoid receptors. We further discovered that the inhibition of COX-2 activity reduced inhibitory synaptic activity and augmented depolarization-induced suppression of inhibition (DSI), whereas the enhancement of COX-2 augmented the synaptic transmission and abolished DSI. Our results, which show that COX-2 oxidative metabolites of eCBs exert opposite effects to their parent molecules on inhibitory synaptic transmission, suggest that alterations in COX-2 activity will have significant impact on endocannabinoid signalling in hippocampal synaptic activity.

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.

Postsynaptically synthesized prostaglandin E2 (PGE2) modulates hippocampal synaptic transmission via a presynaptic PGE2 EP2 receptor

Increasing evidence suggests that cyclooxygenase-2 (COX-2) is involved in synaptic transmission and plasticity, and prostaglandin E2 (PGE2) is a key molecule in COX-2-meduated synaptic modification. However, the precise mechanisms, in particular, which subtypes of PGE2 receptors (EPs) mediate the PGE2-induced synaptic response, are not clear. Recently, we demonstrated that EPs are expressed heterogeneously in the hippocampus, and EP2/4 are mainly expressed in presynaptic terminals. Here, we report that PGE2 increased synaptic stimulus-evoked amplitudes of EPSPs in hippocampal slices and frequency of miniature EPSCs (mEPSCs) in hippocampal neurons in culture. These actions were mimicked by an EP2 agonist and attenuated by protein kinase A inhibitors. Decrease of EP2 expression through silencing the EP2 gene eliminated PGE2-induced increase of the frequency of mEPSCs. COX-2 and microsomal PGE synthase-1 (mPGES-1) and mPGES-2 are present in postsynaptic dendritic spines, because they are colocalized with PSD-95 (postsynaptic density-95), a postsynaptic marker. In addition, the frequency of mEPSCs was enhanced in neurons pretreated with interleukin-1beta or lipopolysaccharide, which elevated expression of COX-2 and mPGES-1 and produced PGE2, and this enhancement was inhibited by a COX-2 inhibitor that inhibited production of PGE2. Our results suggest that PGE2 synthesized by postsynaptically localized COX-2 functions as a retrograde messenger in hippocampal synaptic signaling via a presynaptic EP2 receptor.

Variations in genes regulating neuronal migration predict reduced prefrontal cognition in schizophrenia and bipolar subjects from mediterranean Spain: A preliminary study

Both neural development and prefrontal cortex function are known to be abnormal in schizophrenia and bipolar disorder. In order to test the hypothesis that these features may be related with genes that regulate neuronal migration, we analyzed two genomic regions: the lissencephaly critical region (chromosome 17p) encompassing the LIS1 gene and which is involved in human lissencephaly; and the genes related to the platelet-activating-factor, functionally related to LIS1, in 52 schizophrenic patients, 36 bipolar I patients and 65 normal control subjects. In addition, all patients and the 25 control subjects completed a neuropsychological battery. Thirteen (14.8%) patients showed genetic variations in either two markers related with lissencephaly or in the platelet-activating-factor receptor gene. These patients performed significantly worse in the Wisconsin Card Sorting Test-Perseverative Errors in comparison with patients with no lissencephaly critical region/platelet-activating-factor receptor variations. The presence of lissencephaly critical region/platelet-activating-factor receptor variations was parametrically related to perseverative errors, and this accounted for 17% of the variance (P = 0.0001). Finally, logistic regression showed that poor Wisconsin Card Sorting Test-Perseverative Errors performance was the only predictor of belonging to the positive lissencephaly critical region/platelet-activating-factor receptor group. These preliminary findings suggest that the variations in genes involved in neuronal migration predict the severity of the prefrontal cognitive deficits in both disorders.

Lipid signaling in neural plasticity, brain repair, and neuroprotection

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

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

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

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.

Could platelet activating factor play a role in developmental dyslexia?

Post-mortem studies by Galaburda and colleagues on the brains of developmental dyslexics found characteristic neuronal abnormalities: ectopias, microgyria, and fewer large-soma cells in sensory thalamus. An association between dyslexia and immune dysfunction has also been proposed. We describe a mechanism which may explain these observations. Platelet-activating factor (PAF) is a pro-inflammatory lipid implicated in neurological disorders. We propose that PAF may also be involved in dyslexia.

Attenuated LTP in hippocampal dentate gyrus neurons of mice deficient in the PAF receptor

Platelet-activating factor (PAF), a bioactive lipid (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) derived from phospholipase A(2) and other pathways, has been implicated in neural plasticity and memory formation. Long-term potentiation (LTP) can be induced by the application of PAF and blocked by a PAF receptor (PAF-R) inhibitor in the hippocampal CA1 and dentate gyrus. To further investigate the role of PAF in synaptic plasticity, we compared LTP in dentate granule cells from hippocampal slices of adult mice deficient in the PAF-R and their age-matched wild-type littermates. Whole cell patch-clamp recordings were made in the current-clamp mode. LTP in the perforant path was induced by a high-frequency stimulation (HFS) and defined as >20% increase above baseline of the amplitude of excitatory postsynaptic potentials (EPSPs) from 26 to 30 min after HFS. HFS-induced enhancement of the EPSP amplitude was attenuated in cells from the PAF-R-deficient mice (163 +/- 14%, mean +/- SE; n = 32) when compared with that in wild-type mice (219 +/- 17%, n = 32). The incidence of LTP induction was also lower in the cells from the deficient mice (72%, 23 of 32 cells) than in the wild-type mice (91%, 29 of 32 cells). Using paired-pulse facilitation as a synaptic pathway discrimination, it appeared that there were differences in LTP magnitudes in the lateral perforant path but not in the medial perforant path between the two groups. BN52021 (5 microM), a PAF synaptosomal receptor antagonist, reduced LTP in the lateral path in the wild-type mice. However, neither BN52021, nor BN50730 (5 microM), a microsomal PAF-R antagonist, reduced LTP in the lateral perforant path in the receptor-deficient mice. These data provide evidence that PAF-R-deficient mice are a useful model to study LTP in the dentate gyrus and support the notion that PAF actively participates in hippocampal synaptic plasticity.