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

Forty Years Since the Structural Elucidation of Platelet-Activating Factor (PAF): Historical, Current, and Future Research Perspectives

In the late 1960s, Barbaro and Zvaifler described a substance that caused antigen induced histamine release from rabbit platelets producing antibodies in passive cutaneous anaphylaxis. Henson described a ‘soluble factor’ released from leukocytes that induced vasoactive amine release in platelets. Later observations by Siraganuan and Osler observed the existence of a diluted substance that had the capacity to cause platelet activation. In 1972, the term platelet-activating factor (PAF) was coined by Benveniste, Henson, and Cochrane. The structure of PAF was later elucidated by Demopoulos, Pinckard, and Hanahan in 1979. These studies introduced the research world to PAF, which is now recognised as a potent phospholipid mediator. Since its introduction to the literature, research on PAF has grown due to interest in its vital cell signalling functions and more sinisterly its role as a pro-inflammatory molecule in several chronic diseases including cardiovascular disease and cancer. As it is forty years since the structural elucidation of PAF, the aim of this review is to provide a historical account of the discovery of PAF and to provide a general overview of current and future perspectives on PAF research in physiology and pathophysiology.

Novel mGluR5 positive allosteric modulator improves functional recovery, attenuates neurodegeneration, and alters microglial polarization after experimental traumatic brain injury

Traumatic brain injury (TBI) causes microglial activation and related neurotoxicity that contributes to chronic neurodegeneration and loss of neurological function. Selective activation of metabotropic glutamate receptor 5 (mGluR5) by the orthosteric agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), is neuroprotective in experimental models of TBI, and has potent anti-inflammatory effects in vitro. However, the therapeutic potential of CHPG is limited due to its relatively weak potency and brain permeability. Highly potent, selective and brain penetrant mGluR5 positive allosteric modulators (PAMs) have been developed and show promise as therapeutic agents. We evaluated the therapeutic potential of a novel mGluR5 PAM, VU0360172, after controlled cortical impact (CCI) in mice. Vehicle, VU0360172, or VU0360172 plus mGluR5 antagonist (MTEP), were administered systemically to CCI mice at 3 h post-injury; lesion volume, hippocampal neurodegeneration, microglial activation, and functional recovery were assessed through 28 days post-injury. Anti-inflammatory effects of VU0360172 were also examined in vitro using BV2 and primary microglia. VU0360172 treatment significantly reduced the lesion, attenuated hippocampal neurodegeneration, and improved motor function recovery after CCI. Effects were mediated by mGluR5 as co-administration of MTEP blocked the protective effects of VU0360172. VU0360172 significantly reduced CD68 and NOX2 expression in activated microglia in the cortex at 28 days post-injury, and also suppressed pro-inflammatory signaling pathways in BV2 and primary microglia. In addition, VU0360172 treatment shifted the balance between M1/M2 microglial activation states towards an M2 pro-repair phenotype. This study demonstrates that VU0360172 confers neuroprotection after experimental TBI, and suggests that mGluR5 PAMs may be promising therapeutic agents for head injury.

Platelet activating factors in depression and coronary artery disease: a potential biomarker related to inflammatory mechanisms and neurodegeneration

The persistence of a depressive episode in coronary artery disease (CAD) patients not only heightens the risk of acute ischemic events, but it is also associated with accelerated cognitive decline. Antidepressant interventions for depression in CAD have only modest effects and novel approaches are limited by a poor understanding of etiological mechanisms. This review proposes that the platelet activating factor (PAF) family of lipids might be associated with the persistence of a depressive episode and related neurodegenerative pathology in CAD due to their association with leading etiological mechanisms for depression in CAD such as inflammation, oxidative and nitrosative stress, vascular endothelial dysfunction, and platelet reactivity. The evidence implicating PAFs in CAD, vascular pathology, and neurodegenerative processes is also presented. We also propose future directions for the investigation of PAFs as mediators of persistent depression. In summary, PAFs are implicated in leading mechanisms associated with depression in CAD. PAFs may therefore be associated with the persistence of depression in CAD and related to neurodegenerative and cognitive sequelae.

Increased prion protein processing and expression of metabotropic glutamate receptor 1 in a mouse model of Alzheimer's disease

Prion protein (PrP(C) ), a glycosylphosphatidylinositol-anchored protein corrupted in prion diseases, has been shown recently to interact with group I metabotropic glutamate receptors (mGluRs). Moreover, both PrP(C) and mGluRs were proposed to function as putative receptors for β-amyloid in Alzheimer's disease. PrP(C) can be processed in neurons via α or β-cleavage to produce PrP(C) fragments that are neuroprotective or toxic, respectively. We found PrP(C) α-cleavage to be 2-3 times higher in the cortex of APPswe/PS1dE9 mice, a mouse model of Alzheimer's disease. A similar age-dependent increase was observed for PrP(C) β-cleavage. Moreover, we observed considerable age-dependent increase in cortical expression of mGluR1, but not mGluR5. Exposure of cortical neuronal cultures to β-amyloid oligomers upregulated mGluR1 and PrP(C) α-cleavage, while activation of group I mGluRs increased PrP(C) shedding from the membrane, likely due to increased levels of a disintegrin and metalloprotease10, a key disintegrin for PrP(C) shedding. Interestingly, a similar increase in a disintegrin and metalloprotease10 was detected in the cortex of 9-month-old APPswe/PS1dE9 animals. Our experiments reveal novel and complex processing of PrP(C) in connection with mGluR overexpression that seems to be triggered by β-amyloid peptides. Prion protein (PrP(C) ) and metabotropic glutamate receptors (mGluR) are implicated in Alzheimer's disease (AD). We found age-dependent increase in PrP(C) processing, ADAM10 and mGluR1 levels in AD mouse model. These changes could be reproduced in cultured cortical neurons treated with Aβ peptide. Our findings suggest that increased levels of Aβ can trigger compensatory responses that may affect neuronal toxicity.

Activation of mGluR5 and inhibition of NADPH oxidase improves functional recovery after traumatic brain injury

Abstract Traumatic brain injury (TBI) induces microglial activation, which can contribute to secondary tissue loss. Activation of mGluR5 reduces microglial activation and inhibits microglial-mediated neurodegeneration in vitro, and is neuroprotective in experimental models of CNS injury. In vitro studies also suggest that the beneficial effects of mGluR5 activation involve nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibition in activated microglia. We hypothesized that activation of mGluR5 by the selective agonist CHPG after TBI in mice is neuroprotective and that its therapeutic actions are mediated by NADPH oxidase inhibition. Vehicle, CHPG, or CHPG plus the mGluR5 antagonist (MPEP), were administered centrally, 30 minutes post-TBI, and functional recovery and lesion volume was assessed. CHPG significantly attenuated post-traumatic sensorimotor and cognitive deficits, and reduced lesion volumes; these effects were blocked by MPEP, thereby indicating neuroprotection involved selective activation of mGluR5. CHPG treatment also reduced NFκB activity and nitrite production in lipopolysaccharide-stimulated microglia and the protective effects of CHPG treatment were abrogated in NADPH oxidase deficient microglial cultures (gp91(phox-/-)). To address whether the neuroprotective effects of CHPG are mediated via the inhibition of NADPH oxidase, we administered the NADPH oxidase inhibitor apocynin with or without CHPG treatment after TBI. Both apocynin or CHPG treatment alone improved sensorimotor deficits and reduced lesion volumes when compared with vehicle-treated mice; however, the combined CHPG + apocynin treatment was not superior to CHPG alone. These data suggest that the neuroprotective effects of activating mGluR5 receptors after TBI are mediated, in part, via the inhibition of NADPH oxidase.

Ketamine-induced neuronal damage and altered N-methyl-D-aspartate receptor function in rat primary forebrain culture

Ketamine, a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, is frequently used in pediatric general anesthesia. Accumulating evidence from animal experiments has demonstrated that ketamine causes neuronal cell death during the brain growth spurt. To elucidate the underlying mechanisms associated with ketamine-induced neuronal toxicity and search for approaches or agents to prevent ketamine's adverse effects on the developing brain, a primary nerve cell culture system was utilized. Neurons harvested from the forebrain of newborn rats were maintained under normal control conditions or exposed to either ketamine (10 µM) or ketamine plus L-carnitine (an antioxidant; 1-100 µM) for 24h, followed by a 24-h withdrawal period. Ketamine exposure resulted in elevated NMDA receptor (NR1) expression, increased generation of reactive oxygen species (ROS) as indicated by higher levels of 8-oxoguanine production, and enhanced neuronal damage. Coadministration of L-carnitine significantly diminished ROS generation and provided near complete protection of neurons from ketamine-induced cell death. NMDA receptors regulate channels that are highly permeable to calcium, and calcium imaging data demonstrated that neurons exposed to ketamine had a significantly elevated amplitude of calcium influx and higher intracellular free calcium concentrations ([Ca(2+)]i) evoked by NMDA (50 µM), compared with control neurons. These findings suggest that prolonged ketamine exposure produces an increase in NMDA receptor expression (compensatory upregulation), which allows for a higher/toxic influx of calcium into neurons once ketamine is removed from the system, leading to elevated ROS generation and neuronal cell death. L-Carnitine appears to be a promising agent in preventing or reversing ketamine's toxic effects on neurons at an early developmental stage.

Metabotropic glutamate receptor-mediated signaling in neuroglia

Metabotropic glutamate (mGlu) receptors are G-protein-coupled receptors, which include eight subtypes that have been classified into three groups (I-III) based upon sequence homology, signal transduction mechanism and pharmacological profile. Although most studied with regard to neuronal function and modulation, mGlu receptors are also expressed by neuroglia-including astrocytes, microglia and oligodendrocytes. Activation of mGlu receptors on neuroglia under both physiologic and pathophysiologic conditions mediates numerous actions that are essential for intrinsic glial cell function, as well as for glial-neuronal interactions. Astrocyte mGlu receptors play important physiological roles in regulating neurotransmission and maintaining neuronal homeostasis. However, mGlu receptors on astrocytes and microglia also serve to modulate cell death and neurological function in a variety of pathophysiological conditions such as acute and chronic neurodegenerative disorders. The latter effects are complex and bi-directional, depending on which mGlu receptor sub-types are activated.

Agonist-induced internalization of mGluR1alpha is mediated by caveolin

Agonist-induced internalization of metabotropic glutamate receptors (mGluRs) plays an important role in neuronal signaling. Although internalization of mGluRs has been reported to be mediated by clathrin-dependent pathway, studies describing clathrin-independent pathways are emerging. Here, we report that agonist-induced internalization of mGluR1alpha is mediated by caveolin. We show that two caveolin-binding motifs of mGluR1alpha interact with caveolin1/2. Using cell surface-immunoprecipitation and total internal reflection fluorescence imaging, we found that agonist-induced internalization of mGluR1alpha is regulated by caveolin-binding motifs of the receptor in heterologous cells. Moreover, in the cerebellum, group I mGluR agonist dihydroxyphenylglycol increased the interaction of phosphorylated caveolin with mGluR1alpha. This interaction was blocked by methyl-beta-cyclodextrin, known to disrupt caveolin/caveolae-dependent signaling by cholesterol depletion. Methyl-beta-cyclodextrin also blocked the agonist-induced internalization of mGluR1alpha. Thus, these findings represent the evidence for agonist-induced internalization of mGluR1alpha via caveolin and suggest that caveolin might play a role in synaptic metaplasticity by regulating internalization of mGluR1alpha in the cerebellum.

Metabotropic glutamate receptors as targets for multipotential treatment of neurological disorders

Glutamate is a major excitatory neurotransmitter in the CNS that is involved in numerous cellular functions, including cell death and survival. Metabotropic glutamate receptors (mGluR) are G-protein coupled receptors that have been classified into three groups on the basis of signal transduction pathways and pharmacological profiles. Group I, II, and III mGluRs are found on cell types within and peripheral to the CNS, including neurons, microglia, astrocytes, oligodendrocytes, T- and B-cell lymphocytes, osteoblasts, hepatocytes, and endothelial cells, among others. These receptors have a number of effects on cells that can influence outcome after trauma, including reducing neuronal and oligodendroglial cell death, inflammation, and endothelial permeability. Thus, mGluRs are a promising multipotential therapeutic approach. Because the pathology of CNS trauma and neurodegeneration is multifactorial (including, for example, oxidative stress, mitochondrial breakdown, and inflammation), therapies that serve to modulate multiple pathophysiological pathways may prove more effective than those directed at a single target. This review examines the multipotential therapeutic utility of mGluR modulation in acute and chronic injury and neurodegeneration.

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.

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.

Calpain-Mediated mGluR1α Truncation: A Key Step in Excitotoxicity

Excitotoxicity mediated by glutamate receptors plays crucial roles in ischemia and other neurodegenerative diseases. Whereas overactivation of ionotropic glutamate receptors is neurotoxic, the role of metabotropic glutamate receptors (mGluRs), and especially mGluR1, remains equivocal. Here we report that activation of NMDA receptors results in calpain-mediated truncation of the C-terminal domain of mGluR1alpha at Ser(936). The truncated mGluR1alpha maintains its ability to increase cytosolic calcium while it no longer activates the neuroprotective PI(3)K-Akt signaling pathways. Full-length and truncated forms of mGluR1alpha play distinct roles in excitotoxic neuronal degeneration in cultured neurons. A fusion peptide derived from the calpain cleavage site of mGluR1alpha efficiently blocks NMDA-induced truncation of mGluR1alpha in primary neuronal cultures and exhibits neuroprotection against excitotoxicity both in vitro and in vivo. These findings shed light on the relationship between NMDA and mGluR1alpha and indicate the existence of a positive feedback regulation in excitotoxicity involving calpain and mGluR1alpha.

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.

Neuroprotective effects of ebselen are associated with the regulation of Bcl-2 and Bax proteins in cultured mouse cortical neurons

There is little information available on the mechanisms underlying the neuroprotective actions of the organoselenium compound ebselen. In this study, we sought to determine the relationship between alterations in the expression of Bcl-2 and Bax proteins and intracellular levels of calcium and the protective effects of ebselen with a concentration range of 0.01-20 microM against glutamate toxicity in cultured mouse cortical neurons. Pretreatment with ebselen at moderate doses (4-12 microM), but not at lower or higher doses, significantly improved glutamate-induced suppression of cell viability. Pretreatment with ebselen (8 microM) also prevented apoptotic alterations, completely reversed the suppression of Bcl-2 expression, and significantly inhibited Bax overexpression, but did not alter elevated intracellular concentrations of calcium induced by glutamate. Pre-, co-, and post-treatment with ebselen (8 microM) had similar potency in improving the decreased viability of glutamate-exposed cells. These results indicate that the neuroprotective effects of ebselen at low doses are associated with the regulation of Bcl-2 and Bax proteins but appear to be independent of glutamate-mediated elevation of intracellular calcium, suggesting that different mechanisms are involved in the actions of low and high dose regimens. Ebselen may be an effective agent used for early treatment of acute brain injuries.

Driving cellular plasticity and survival through the signal transduction pathways of metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) share a common molecular morphology with other G protein-linked receptors, but there expression throughout the mammalian nervous system places these receptors as essential mediators not only for the initial development of an organism, but also for the vital determination of a cell's fate during many disorders in the nervous system that include amyotrophic lateral sclerosis, Parkinson's disease, Alzheimer's disease, Huntington's disease, Multiple Sclerosis, epilepsy, trauma, and stroke. Given the ubiquitous distribution of these receptors, the mGluR system impacts upon neuronal, vascular, and glial cell function and is activated by a wide variety of stimuli that includes neurotransmitters, peptides, hormones, growth factors, ions, lipids, and light. Employing signal transduction pathways that can modulate both excitatory and inhibitory responses, the mGluR system drives a spectrum of cellular pathways that involve protein kinases, endonucleases, cellular acidity, energy metabolism, mitochondrial membrane potential, caspases, and specific mitogen-activated protein kinases. Ultimately these pathways can converge to regulate genomic DNA degradation, membrane phosphatidylserine (PS) residue exposure, and inflammatory microglial activation. As we continue to push the envelope for our understanding of this complex and critical family of metabotropic receptors, we should be able to reap enormous benefits for both clinical disease as well as our understanding of basic biology in the nervous system.

Heterogeneous expression and regulation of hippocampal prostaglandin E2 receptors

Although prostaglandin E2 (PGE2) has been shown to be critical to hippocampal synaptic signaling and neuronal survival, it is still not clear which subtypes of PGE2 receptors (EPs) are expressed and how these EPs are regulated in the hippocampus. To address these questions, the expression of the EPs was profiled in the hippocampus. Messenger RNAs and proteins of the four receptors, EP 1-4, were detected both in the hippocampus and in the neocortex. EP 2 and EP 3 appeared in greater abundance, whereas EP 1 and EP 4 were barely detectable. EP 1, EP 2 and EP 4 were mainly colocalized with synaptophysin, suggesting the presence of EP 1, EP 2, and EP 4 in presynaptic terminals. It appeared that interleukin-1 beta increased the expression of EP 2 and EP 4 mRNAs. A blockade of synaptic transmission with either tetrodotoxin or MK-801 plus 6,7-dinitroquinoxaline-2,3-dione (DNQX) for 6 hr increased EP 3 and EP 4 mRNA, whereas high K(+) (90 mM) or 4-aminopyridine enhanced EP 2 and EP 4. The EP 1 level did not change significantly under these conditions. The expressions of EP 2, EP 4, and EP 3 were further elevated or reduced in neurons treated with high K(+) for 24 hr. However, mRNA of EP 3 was down-regulated in neurons treated with tetrodotoxin or MK-801 plus DNQX for 24 hr. In addition, both EP 2 and EP 4 mRNAs were up-regulated within 4 hr after high-frequency stimulation associated with long-term potentiation induction in hippocampal slices. Our results indicate that the four EPs are heterogeneously expressed in the hippocampus, and their expression is differentially regulated by neuronal activities, suggesting that EPs may actively participate in hippocampal synaptic transmission and plasticity.