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

The Role of MEF2 Transcription Factor Family in Neuronal Survival and Degeneration

The family of myocyte enhancer factor 2 (MEF2) transcription factors comprises four highly conserved members that play an important role in the nervous system. They appear in precisely defined time frames in the developing brain to turn on and turn off genes affecting growth, pruning and survival of neurons. MEF2s are known to dictate neuronal development, synaptic plasticity and restrict the number of synapses in the hippocampus, thus affecting learning and memory formation. In primary neurons, negative regulation of MEF2 activity by external stimuli or stress conditions is known to induce apoptosis, albeit the pro or antiapoptotic action of MEF2 depends on the neuronal maturation stage. By contrast, enhancement of MEF2 transcriptional activity protects neurons from apoptotic death both in vitro and in preclinical models of neurodegenerative diseases. A growing body of evidence places this transcription factor in the center of many neuropathologies associated with age-dependent neuronal dysfunctions or gradual but irreversible neuron loss. In this work, we discuss how the altered function of MEF2s during development and in adulthood affecting neuronal survival may be linked to neuropsychiatric disorders.

Synergistic Neuroprotection by a PAF Antagonist Plus a Docosanoid in Experimental Ischemic Stroke: Dose-Response and Therapeutic Window

OBJECTIVE

We tested the hypothesis that blocking pro-inflammatory platelet-activating factor receptor (PAFR) with LAU-0901 (LAU) plus administering a selected docosanoid, aspirin-triggered neuroprotectin D1 (AT-NPD1), which activates cell-survival pathways after middle cerebral artery occlusion (MCAo), would lead to neurological recovery. Dose-response and therapeutic window were investigated.

MATERIALS AND METHODS

Male SD rats were subjected to 2 hours of MCAo. Behavior testing (days 1-7) and ex vivo MRI on day 7 were conducted. In dose-response, rats were treated with LAU (45 and 60 mg/kg; IP), AT-NPD1 (111, 222, 333 µg/kg; IV), LAU+AT-NPD1 (LAU at 3 hours and AT-NPD1 at 3.15 hours) or vehicle. In the therapeutic window, vehicle, LAU (60 mg/kg), AT-NPD1 (222 µg/kg), and LAU+AT-NPD1 were administered at 3, 4, 5, and 6 hours after onset of MCAo.

RESULTS

LAU and AT-NPD1 treatments alone improved behavior by 40-42% and 20-30%, respectively, and LAU+AT-NPD1 by 40% compared to the vehicle group. T2-weighted imaging (T2WI) volumes were reduced with all doses of LAU and AT-NPD1 by 73-90% and 67-83% and LAU+AT-NPD1 by 94% compared to vehicle. In the therapeutic window, LAU+AT-NPD1, when administered at 3, 4, 5, and 6 hours, improved behavior by 50, 56, 33, and 26% and reduced T2WI volumes by 93, 90, 82, and 84% compared to vehicle.

CONCLUSIONS

We have shown here for the first time that LAU plus AT-NPD1 treatment affords high-grade neuroprotection in MCAo, equaling or exceeding that afforded by LAU or AT-NPD1 alone at considerably moderate doses. It has a broad therapeutic window extending to 6 hours after stroke onset.

Examining the role of astrogliosis and JNK signaling in post-traumatic epilepsy

OBJECTIVE

Post-traumatic epilepsy is a devastating complication of traumatic brain injury that has no targeted pharmacological therapy. Previous literature has explored the role of the c-Jun N-terminal kinase (JNK) pathway in epilepsy and the creation of epileptogenic foci by reactive astrogliosis; however, the relationship between reactive astrogliosis and the c-Jun N-terminal kinase signaling pathway in the development of post-traumatic epilepsy has not been thoroughly examined.

METHODS

Four experimental groups, consisting of c57/b16 male mice, were examined: (1) control, (2) traumatic brain injury of graded severity (mild, moderate, severe), (3) sub-convulsive kainic acid alone without traumatic brain injury (15 mg/kg i.p.), and (4) sub-convulsive kainic acid administered 72 h after moderate traumatic brain injury. Modified Racine scale from 1 to 72 h and total beam breaks at 72 h were used to assess seizure activity. Immunohistochemistry and western blot were utilized to examine astrogliosis (GFAP), microglia activation (IBA-1), and phosphorylated JNK in prefrontal cortex samples collected from the contracoup side at 72 h post-injury.

RESULTS

Astrogliosis, measured by GFAP, was increased after traumatic brain injury and increased commensurately based on the degree of injury. Mice with traumatic brain injury demonstrated a four-fold increase in phosphorylated JNK: p < 0.001. Sub-convulsive kainic acid administration did not increase seizure activity nor phosphorylation of JNK in mice without traumatic brain injury; however, sub-convulsive kainic acid administration in mice with moderate traumatic brain injury did increase phosphorylated JNK. Seizure activity was worse in mice, with traumatic brain injury, administered kainic acid than mice administered kainic acid.

CONCLUSIONS

Reactive astrocytes may have dysfunctional glutamate regulation causing an increase in phosphorylated JNK after kainic acid administration. Future studies exploring the effects of JNK inhibition on post-traumatic epilepsy are recommended.

Hormesis and Ginkgo biloba (GB): Numerous biological effects of GB are mediated via hormesis

Ginkgo biloba (GB) extracts have been shown to commonly induce biphasic dose responses in a range of cell types and endpoints (e.g., cochlea neural stem cells, cell viability, cell proliferation). The magnitude and width of the low dose stimulation of these biphasic dose responses are similar to those reported for hormetic dose responses. These hormetic dose responses occur within direct stimulatory responses as well as in preconditioning experimental protocols, displaying acquired resistance within an adaptive homeodynamic and temporal framework and repeated measurement protocols. The demonstrated GB dose responses further reflect the general occurrence of hormetic dose responses that consistently appear to be independent of the biological model, endpoint, inducing agent, and/or mechanism. These findings have important implications for consideration(s) of study designs involving dose selection, dose spacing, sample size, and statistical power. This illustrates and strengthens the need to characterize the low dose stimulatory response range and optimal dose in order to explore potential public health and clinical applications of plant-derived agents, such as GB.

Cell-permeable p38 MAP kinase protects adult hippocampal neurons from cell death

p38 mitogen-activated protein (MAP) kinase (p38) is a member of the MAP kinase family. Previous reports using p38 chemical inhibitors have suggested that its activation contributes to hippocampal neuronal cell death rather than cell survival. In this study, we used both a cell-permeable p38 protein containing the HIV protein transduction domain (PTD) and cultured adult hippocampal neurons, which were differentiated from cultured adult hippocampal neural stem/progenitor cells (NPCs), to evaluate the direct function of p38 on adult hippocampal neurons. Our immunocytochemical experiments demonstrated that wild-type cell-permeable p38 protein prevents cell death of adult hippocampal neurons induced by a low glucose condition. Our findings indicate that cell-permeable p38 protein may be useful in preventing the degeneration of higher brain function occurring through hippocampal neuronal cell death, and furthermore, that the maintenance of intracellular p38 levels could be another therapeutic target for neurodegenerative diseases such as Alzheimer's disease (AD).

JIP1-Mediated JNK Activation Negatively Regulates Synaptic Plasticity and Spatial Memory

The c-Jun N-terminal kinase (JNK) signal transduction pathway is implicated in learning and memory. Here, we examined the role of JNK activation mediated by the JNK-interacting protein 1 (JIP1) scaffold protein. We compared male wild-type mice with a mouse model harboring a point mutation in the Jip1 gene that selectively blocks JIP1-mediated JNK activation. These male mutant mice exhibited increased NMDAR currents, increased NMDAR-mediated gene expression, and a lower threshold for induction of hippocampal long-term potentiation. The JIP1 mutant mice also displayed improved hippocampus-dependent spatial memory and enhanced associative fear conditioning. These results were confirmed using a second JIP1 mutant mouse model that suppresses JNK activity. Together, these observations establish that JIP1-mediated JNK activation contributes to the regulation of hippocampus-dependent, NMDAR-mediated synaptic plasticity and learning.SIGNIFICANCE STATEMENT The results of this study demonstrate that c-Jun N-terminal kinase (JNK) activation induced by the JNK-interacting protein 1 (JIP1) scaffold protein negatively regulates the threshold for induction of long-term synaptic plasticity through the NMDA-type glutamate receptor. This change in plasticity threshold influences learning. Indeed, mice with defects in JIP1-mediated JNK activation display enhanced memory in hippocampus-dependent tasks, such as contextual fear conditioning and Morris water maze, indicating that JIP1-JNK constrains spatial memory. This study identifies JIP1-mediated JNK activation as a novel molecular pathway that negatively regulates NMDAR-dependent synaptic plasticity and memory.

JNK1 controls adult hippocampal neurogenesis and imposes cell-autonomous control of anxiety behaviour from the neurogenic niche

Promoting adult hippocampal neurogenesis is expected to induce neuroplastic changes that improve mood and alleviate anxiety. However, the underlying mechanisms remain largely unknown and the hypothesis itself is controversial. Here we show that mice lacking Jnk1, or c-Jun N-terminal kinase (JNK) inhibitor-treated mice, display increased neurogenesis in adult hippocampus characterized by enhanced cell proliferation and survival, and increased maturation in the ventral region. Correspondingly, anxiety behaviour is reduced in a battery of tests, except when neurogenesis is prevented by AraC treatment. Using engineered retroviruses, we show that exclusive inhibition of JNK in adult-born granule cells alleviates anxiety and reduces depressive-like behaviour. These data validate the neurogenesis hypothesis of anxiety. Moreover, they establish a causal role for JNK in the hippocampal neurogenic niche and anxiety behaviour, and advocate targeting of JNK as an avenue for novel therapies against affective disorders.

cJun N-terminal kinase (JNK) mediates cortico-striatal signaling in a model of Parkinson's disease

The cJun N-terminal kinase (JNK) signaling pathway has been extensively studied with regard to its involvement in neurodegenerative processes, but little is known about its functions in neurotransmission. In a mouse model of Parkinson's disease (PD), we show that the pharmacological activation of dopamine D1 receptors (D1R) produces a large increase in JNK phosphorylation. This effect is secondary to dopamine depletion, and is restricted to the striatal projection neurons that innervate directly the output structures of the basal ganglia (dSPN). Activation of JNK in dSPN relies on cAMP-induced phosphorylation of the dopamine- and cAMP-regulated phosphoprotein of 32kDa (DARPP-32), but does not require N-methyl-d-aspartate (NMDA) receptor transmission. Electrophysiological experiments on acute brain slices from PD mice show that inhibition of JNK signaling in dSPN prevents the increase in synaptic strength caused by activation of D1Rs. Together, our findings show that dopamine depletion confers to JNK the ability to mediate dopamine transmission, informing the future development of therapies for PD.

Evidence of Presynaptic Localization and Function of the c-Jun N-Terminal Kinase

The c-Jun N-terminal kinase (JNK) is part of a stress signalling pathway strongly activated by NMDA-stimulation and involved in synaptic plasticity. Many studies have been focused on the post-synaptic mechanism of JNK action, and less is known about JNK presynaptic localization and its physiological role at this site. Here we examined whether JNK is present at the presynaptic site and its activity after presynaptic NMDA receptors stimulation. By using N-SIM Structured Super Resolution Microscopy as well as biochemical approaches, we demonstrated that presynaptic fractions contained significant amount of JNK protein and its activated form. By means of modelling design, we found that JNK, via the JBD domain, acts as a physiological effector on T-SNARE proteins; then using biochemical approaches we demonstrated the interaction between Syntaxin-1-JNK, Syntaxin-2-JNK, and Snap25-JNK. In addition, taking advance of the specific JNK inhibitor peptide, D-JNKI1, we defined JNK action on the SNARE complex formation. Finally, electrophysiological recordings confirmed the role of JNK in the presynaptic modulation of vesicle release. These data suggest that JNK-dependent phosphorylation of T-SNARE proteins may have an important functional role in synaptic plasticity.

Pulmonary Proteome and Protein Networks in Response to the Herbicide Paraquat in Rats

Paraquat (PQ) has been one of the most widely used herbicides in the world. PQ, when ingested, is toxic to humans and may cause acute respiratory distress syndrome. To investigate molecular perturbation in lung tissues caused by PQ, Sprague Dawley male rats were fed with PQ at a dose of 25 mg/kg body weight for 20 times in four weeks. The effects of PQ on cellular processes and biological pathways were investigated by analyzing proteome in the lung tissues in comparison with the control. Among the detected proteins, 321 and 254 proteins were over-represented and under-represented, respectively, in the PQ-exposed rat lung tissues in comparison with the no PQ control. All over- and under-represented proteins were subjected to Ingenuity Pathway Analysis to create 25 biological networks and 38 pathways of interacting protein clusters. Over-represented proteins were involved in the C-jun-amino-terminal kinase pathway, caveolae-mediated endocytosis signaling, cardiovascular-cancer-respiratory pathway, regulation of clathrin-mediated endocytosis, non-small cell lung cancer signaling, pulmonary hypertension, glutamate receptor, immune response and angiogenesis. Under-represented proteins occurred in the p53 signaling pathway, mitogen-activated protein kinase signaling pathway, cartilage development and angiogenesis inhibition in the PQ-treated lungs. The results suggest that PQ may generate reactive oxygen species, impair the MAPK/p53 signaling pathway, activate angiogenesis and depress apoptosis in the lungs.

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

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

P38 MAPK inhibition protects against glutamate neurotoxicity and modifies NMDA and AMPA receptor subunit expression

NMDA and AMPA receptors are thought to be responsible for Ca(++) influx during glutamate-induced excitotoxicity and, therefore, hippocampal neuronal death. We assessed whether excitotoxicity induced by neonatal treatment with monosodium glutamate in rats at postnatal age of 1, 3, 5, and 7 modifies the hippocampal expression of the NMDAR subunit NR1 and the AMPAR subunits GluR1/GluR2 at postnatal days 8, 10, 12, and 14. We also assessed the involvement of MAPK signaling by using the p38 inhibitor SB203580. Our results showed that monosodium glutamate induces neuronal death and alters the expression of the subunits evaluated in the hippocampus at all ages studied, which could be prevented by SB203580 treatment.Furthermore, expression of the NRSF gene silencing factor also increased in response to excitotoxicity, suggesting a relationship in suppressing GluR2-expression, which was regulated by the p38-MAPK pathway inhibitor SB203580. This result suggests that selectively blocking the pro-death signaling pathway may reduce neuronal death in some neurodegenerative diseases in which these neurotoxic processes are present and produce major clinical benefits in the treatment of these pathologies.

Platelet-activating factor receptor antagonism targets neuroinflammation in experimental epilepsy

PURPOSE

Temporal lobe epilepsy is associated with the inflammatory process related to the basic mechanisms that lead to seizure susceptibility and brain damage. Platelet-activating factor (PAF), a potent, short-lived phospholipid mediator of inflammation, participates in physiologic signaling in the brain. However, after seizures, PAF accumulates in the brain and activates intracellular signaling related with inflammation-mediated excitotoxicity and hippocampal hyperexcitability. The objective of this study is to evaluate the effect of PAF antagonism on hippocampal hyperexcitability, seizure susceptibility, and neuroprotection using the kindling paradigm and pilocarpine-induced seizure damage models.

METHODS

The PAF antagonist, LAU-0901 (60 mg/kg, i.p.), or vehicle, was administrated each day of kindling or daily during the 4 weeks after status epilepticus (SE). We analyzed seizure severity, electrical activity, cellular damage, and inflammation in the hippocampi of both treated groups.

KEY FINDINGS

LAU-0901 limits the progression of kindling and attenuates seizure susceptibility 1 week after the kindling procedure. In addition, under the seizure-damage conditions studied here, we observed that LAU-0901 induces hippocampal neuroprotection and limits somatostatin interneuronal cell loss and inflammation.

SIGNIFICANCE

Our results indicate that modulation of PAF overactivity attenuates seizure susceptibility, hippocampal hyperexcitability, and neuroinflammation.

NR2B-NMDA receptor mediated modulation of the tyrosine phosphatase STEP regulates glutamate induced neuronal cell death

The present study examines the role of a neuron-specific tyrosine phosphatase (STEP, striatal-enriched tyrosine phosphatase) in excitotoxic cell death. Our findings demonstrate that p38 MAPK, a stress-activated kinase that is known to play a role in the etiology of excitotoxic cell death is a substrate of STEP. Glutamate-mediated NMDA receptor stimulation leads to rapid but transient activation of p38 MAPK, which is primarily dependent on NR2A-NMDA receptor activation. Conversely, activation of NR2B-NMDA receptors leads to dephosphorylation and subsequent activation of STEP, which in turn leads to inactivation of p38 MAPK. Thus, during transient NMDA receptor stimulation, increases in STEP activity appears to limit the duration of activation of p38 MAPK and improves neuronal survival. However, if NR2B-NMDA receptor stimulation is sustained, protective effects of STEP activation are lost, as these stimuli cause significant degradation of active STEP, leading to secondary activation of p38 MAPK. Consistent with this observation, a cell transducible TAT-STEP peptide that constitutively binds to p38 MAPK attenuated neuronal cell death caused by sustained NMDA receptor stimulation. The findings imply that the activation and levels of STEP are dependent on the duration and magnitude of NR2B-NMDA receptor stimulation and STEP serves as a modulator of NMDA receptor dependent neuronal injury, through its regulation of p38 MAPK.

Increased operant responding for ethanol in male C57BL/6J mice: specific regulation by the ERK1/2, but not JNK, MAP kinase pathway

RATIONALE

Extracellular signal-regulated protein kinase (ERK(1/2)) is a member of the mitogen-activated protein kinase (MAPK) signaling pathway and a key molecular target for ethanol (EtOH) and other drugs of abuse.

OBJECTIVE

The aim of the study was to assess the role of two MAPK pathways, ERK(1/2) and c-Jun N-terminal kinase (JNK), on the modulation of EtOH and sucrose self-administration.

MATERIALS AND METHODS

C57BL/6J mice were trained to lever press on a fixed-ratio 4 schedule with 9% EtOH/2% sucrose, or 2% sucrose, as the reinforcer. In experiments 1 and 2, mice were injected with the MEK(1/2) inhibitor SL 327 (0-100 mg/kg) and the JNK inhibitor AS 6012452 (0-56 mg/kg) prior to self-administration. In experiment 3, SL 327 (0-100 mg/kg) was administered prior to performance on a progressive ratio (PR) schedule of EtOH reinforcement. In experiment 4, SL 327 and AS 601245 were injected 2 h before a locomotor test.

RESULTS

SL 327 (30 mg/kg) significantly increased EtOH self-administration without affecting locomotion. Higher doses of SL 327 and AS 601245 reduced EtOH-reinforced responding and locomotor activity. Reductions of both ligands on sucrose self-administration were due to decreases in motor activity. SL 327 pretreatment had no effect on PR responding.

CONCLUSIONS

ERK(1/2) activity is more directly involved in modulating the reinforcing properties of EtOH than JNK activity due to its selective potentiation of EtOH-reinforced responding. The specificity of this effect to EtOH self-administration, rather than sucrose self-administration, suggests that the mechanism by which ERK(1/2) increases EtOH-reinforced responding does not generalize to all reinforcing solutions and is not due to increased motivation to consume EtOH.

LAU-0901, a novel platelet-activating factor antagonist, is highly neuroprotective in cerebral ischemia

Platelet-activating factor (PAF) is a bioactive phospholipid that accumulates during ischemia-reperfusion and is involved in the activation of platelets, neutrophils, and pro-inflammatory signaling. PAF has been suggested to enhance brain ischemia-reperfusion damage. LAU-0901, a novel PAF receptor antagonist, was examined in models of focal cerebral ischemia in rats and mice. Sprague-Dawley rats were anesthetized and received 2-hour middle cerebral artery occlusion (MCAo) by intraluminal suture. LAU-0901 (30, 60, 90 mg/kg; n=9-11) or vehicle (n=11) was administered i.p. at 2 h after onset of MCAo. The neurological status was evaluated at 60 min, and on days 1, 2, 3 and 7 after MCAo. In the dose-response study in mice, C57BL/6 mice were anesthetized and received 1 h MCAo by intraluminal suture. LAU-0901 (15, 30, 60 mg/kg; n=7-9) or vehicle (n=8) was given i.p. at 1 h after onset of MCAo. Local cerebral blood flow (LCBF) was measured at 1, 2, 4, and 6 h after MCAo in mice. LAU-0901 treated rats showed improved neurological score throughout the 7-day survival period. LAU-0901 treatment (30, 60 and 90 mg/kg) reduced total corrected infarct volume compared to vehicle rats by 76, 88 and 90%, respectively. Mice treated with LAU-0901 (30 and 60 mg/kg) reduced total infarction by 29% and 66%, respectively. LCBF was improved by treatment with LAU-0901 (30 mg/kg) by 77% of baseline at 6 h. In conclusion, we demonstrate for the first time that LAU-0901 improves behavioral scores, LCBF and reduces infarct volume after focal cerebral ischemia in rats and mice. Thus, this PAF receptor antagonist exhibits potent and sustained neuroprotection that may be of value for the design of stroke therapies.

Contrasting roles of neuronal Msk1 and Rsk2 in Bad phosphorylation and feedback regulation of Erk signalling

Activated extracellular-signal-regulated kinase (Erk) phosphorylates and activates downstream kinases including ribosomal S6 kinase 2 (Rsk2/RPS6KA3) and mitogen- and stress-activated kinase 1 (Msk1, RPS6KA5). Rsk2 plays an important role in neuronal plasticity, as patients with Coffin-Lowry syndrome, where Rsk2 is dysfunctional, have impaired cognitive function. However, the relative role of neuronal Rsk2 and Msk1 in activating proteins downstream of Erk is unclear. In PC12 cells and in cortical neurones, the calcium ionophore A23187-induced phosphorylation of Erk, Msk1, Rsk2 and also the Bcl-2-associated death protein (Bad), which protects against neurotoxicity. Specific knockdown of Msk1 with small interfering RNA reduced the ability of A23187 to induce Bad phosphorylation in both PC12 cells and cortical neurones. Conversely, specific knockdown of Rsk2 potentiated Bad phosphorylation following A23187 treatment, and also elevated Erk phosphorylation in both cell types. This indicates that Msk1 rather than Rsk2 mediates neuronal Bad phosphorylation following Ca(2+) influx and implicates Rsk2 in a negative-feedback regulation of Erk activity.

c-Jun N-terminal kinase activation responses induced by hippocampal kindling are mediated by reactive astrocytes

Hippocampal kindling, a model of mesial temporal lobe epilepsy, is developed through repetitive stimulation of the hippocampus and leads to increased after-discharges as measured by EEG and an enduring seizure-prone state. Synthesis of new proteins is thought to form the basis for sustained seizure-induced physiological and/or pathological changes in synaptic reorganization and apoptotic/necrotic neuronal death. Here we examined the effect of kindling on stimulus-induced c-Jun N-terminal kinase (JNK) and p38 phosphorylation, events postulated to lie upstream of seizure-induced changes in gene transcription. We found that stimulus-induced phosphorylation of JNK, but not of p38, is significantly enhanced in kindled animals compared with their naive counterparts in the CA1 subregion of the hippocampus. Immunofluorescent staining confirmed this region-specific pattern of JNK activation and revealed that reactive astrocytes mediate this effect. Astrocyte proliferation and hypertrophy, as well as upregulation of vimentin protein levels, common markers of astrogliosis, were present after 4 d of kindling. Moreover, this reactive astrogliosis was associated with neuronal death as visualized with Fluoro-jade B and anti-active caspase-3 staining. Stimulus-induced phosphorylation of the JNK substrate paxillin was enhanced in kindled animals, but not that of c-Jun. Moreover, a pan-antibody against MAPK/CDK (mitogen-activated protein kinases/cyclin-dependent kinase) substrates indicated the presence of phosphorylated proteins in cytosolic, membrane, and nuclear fractions. The consequence of these phosphorylation events is not completely understood, but these findings suggest a selective astrocytic signaling response to aberrant synaptic activity, signaling that may modulate kindling progression and/or neuronal death.

HIV-1 coreceptors CCR5 and CXCR4 both mediate neuronal cell death but CCR5 paradoxically can also contribute to protection

The chemokine receptors CCR5 and CXCR4 serve, in addition to CD4, as coreceptors for human immunodeficiency virus-1 (HIV-1), and infection with HIV-1 can cause dementia. In brain-derived cells, HIV-1 envelope glycoprotein gp120 initiates a signaling cascade that involves p38 mitogen-activated protein kinase and leads to neuronal cell death. Using mixed neuronal/glial cultures from rats and mice genetically deficient in one or both HIV coreceptors, we show here that CCR5, CXCR4 or both can mediate HIV/gp120 neurotoxicity depending on the viral strain. Paradoxically, we also found evidence for a CCR5-mediated neuroprotective pathway. We identify protein kinase Akt/PKB as an essential component of this pathway, which can be triggered by the CCR5 agonists macrophage inflammatory protein-1beta and regulated-and-normal-T-cell-expressed-and-secreted. Moreover, these CCR5 ligands prevent neuronal cell death induced by stromal cell-derived factor-1, a CXCR4 agonist. Both neurons and glia coexpress CXCR4 and CCR5. Ca2+ imaging experiments demonstrate that engagement of CCR5 prevents CXCR4-triggered increases in intracellular free Ca2+. This finding suggests that CCR5 ligands can protect neurons at least, in part, by modulating CXCR4-mediated toxicity through heterologous desensitization.

Cell death pathways differ in several mouse models with motoneurone disease: analysis of pure motoneurone populations at a presymptomatic age

To identify candidate genes that are responsible for motoneurone degeneration, we combined laser capture microdissection with microarray technology. We analysed gene expression in pure motoneurones from two mouse mutants that develop motoneurone degeneration, progressive motor neuronopathy and wobbler. At a presymptomatic age, there was a significant differential expression of a restricted number of genes (25 and 72 in progressive motor neuronopathy and wobbler respectively, of 22 600 transcripts screened). We compared these results to our previous analyses in the copper-zinc superoxide dismutase mutant mouse (SOD1(G93A)) in which we observed a de-regulation of 27 genes. Some of these genes were de-regulated uniquely in one mouse mutant and some have already been identified in cell death pathways implicated in amyotrophic lateral sclerosis and animal models of motoneurone degeneration (i.e. de-regulation of intermediate filaments, axonal transport, the ubiquitin-proteasome system and excitotoxicity). One gene, vimentin, was differentially up-regulated in all mouse mutants; this main candidate gene has been confirmed by in situ hybridization and immunohistochemistry to be expressed in motoneurones in all mouse mutants. Furthermore, vimentin expression correlated with the state of motoneurone degeneration. These results identify early molecular changes that may be involved in the pathogenesis of motoneurones leading to cell death and favour a complex multipathway induction of the disease; surprisingly, there was no important modification in cell death-associated genes. This is the first study to show a clear difference in the genes that are de-regulated at an early stage in three different mouse models of motoneurone disease.

Pro-apoptotic role of c-Jun in NMDA-induced neurotoxicity in the rat retina

We examined the role of c-Jun on N-methyl-D-aspartate (NMDA)-induced neurotoxicity in the rat retina. An increase in c-Jun mRNA, c-Jun protein and phosphorylated c-Jun (p-c-Jun) levels in the retina was detected 3 hr after intravitreal injection of NMDA (20 nmol). These levels peaked after 12 hr, and then returned to their control levels by 24 hr. c-Jun and p-c-Jun immunoreactivities were observed in the retinal ganglion cell layer (RGCL), especially in retinal ganglion cells (RGCs), and in the inner nuclear layer (INL) 12 hr after NMDA injection, and terminal deoxynucleotidyl transferase-mediated nick-end labeling (TUNEL)-positive cells were immunopositive for c-Jun and p-c-Jun. A c-Jun antisense oligodeoxynucleotide (AS ODN), which was simultaneously injected with NMDA, penetrated the cells in the RGCL and the INL, suppressed the NMDA-induced increase in c-Jun and p-c-Jun protein levels and reduced the number of TUNEL-positive cells in the RGCL 12 hr after the injection. The protective effect of c-Jun AS ODN on the NMDA-treated retina was also shown by the RGCL cell count and measurement of the IPL thickness, as well as by quantitative real-time PCR analysis of Thy-1 mRNA 7 days after the injection. These results suggest that c-Jun synthesis and phosphorylation participate in NMDA-induced neuronal cell death.

A signaling mechanism from G alpha q-protein-coupled metabotropic glutamate receptors to gene expression: role of the c-Jun N-terminal kinase pathway

Galphaq-protein-coupled group I metabotropic glutamate receptors (mGluRs) are densely expressed in brain neurons and are actively involved in various cellular activities. In this study, we investigated the role of group I mGluRs in regulating the c-Jun N-terminal kinase (JNK)/stress-activated protein kinase in cultured neurons. We found that selective activation of mGluR5 induced a rapid and transient phosphorylation of JNK. In a series of studies to determine the mechanisms, we found that the conventional mGluR5-associated signaling pathways (inositol-1,4,5-triphosphate-mediated Ca2+ release and activation of protein kinase C) were not involved in the mGluR5 regulation. Instead, ligand stimulation of mGluR5 caused a dynamic transactivation of the epidermal growth factor (EGF) receptor, which in turn triggered a downstream signaling pathway to upregulate JNK phosphorylation. Furthermore, the mGluR5-dependent JNK activation specifically activated c-Jun, but not activating transcription factor-2 or JunD, and increased activator protein-1 (AP-1)-mediated endogenous transcriptional activity. Together, we identified a novel mGluR5-to-nucleus communication through the EGF/JNK pathway, which functions to regulate AP-1-mediated transcription.

Glutamate activatesc-fos in glial cells via a novel mechanism involving the glutamate receptor subtype mGlu5 and the transcriptional repressor DREAM

Activation of c-fos in brain is related to coupling of neuronal activity to gene expression, but also to pathological conditions such as seizures or excitotoxicity-induced cell death. Glutamate activates c-fos in neurons through the calcium-dependent phosphorylation of CREB by ERK and/or CaMKIV kinase pathways downstream NMDA-receptors. In glial cells, however, the activation of c-fos by glutamate is poorly understood. Because glial cells actively modulate neuronal excitability and the brain's response to injury, we studied the mechanisms by which glutamate activates c-fos in rat cortical glial cells. Glutamate potently induced c-fos mRNA in a calcium-dependent manner, as demonstrated by using the calcium chelator BAPTA-AM. Glutamate-induced c-fos mRNA expression was not sensitive to inhibitors of ERK, p38(MAPK), or CaMK pathways, indicating that glial c-fos is activated by a distinct mechanism. Thapsigargin abolished the glutamate effect on c-fos mRNA, indicating ER calcium mobilization. Additionally, glutamate induction of c-fos mRNA was sensitive to the mGluR5 antagonist MPEP but not the NMDA-R antagonist MK-801. In luciferase reporter assays, DRE, which actively represses c-fos by binding the calcium-binding transcriptional repressor DREAM, was activated by glutamate, whereas SRE and CRE were not. Finally, glutamate caused the nuclear export of DREAM in astrocytes, and transfection of astrocytes with a mutant variant of DREAM that constitutively binds DNA inhibited glutamate-induced c-Fos expression. These findings are in sharp contrast to the mechanism described in neurons and suggest a novel pathway activated by glutamate in glial cells that employs mGluR5, ER calcium, and the derepression of c-fos at the DRE.

Experimental and potential future therapeutic approaches for HIV-1 associated dementia targeting receptors for chemokines, glutamate and erythropoietin

Severe and debilitating neurological problems that include behavioral abnormalities, motor dysfunction and frank dementia can occur after infection with the human immunodeficiency virus-1 (HIV-1). Infected peripheral immune-competent cells, in particular macrophages, infiltrate the central nervous system (CNS) and provoke a neuropathological response involving all cell types in the brain. HIV-1 infection results in activation of chemokine receptors, inflammatory mediators, extracellular matrix-degrading enzymes and glutamate receptor-mediated excitotoxicity, all of which can trigger numerous downstream signaling pathways that result in disruption of neuronal and glial function. Despite many major improvements in the control of viral infection in the periphery, a truly effective therapy for HIV-1 associated dementia is currently not available. This review will discuss experimental and potentially future therapeutic strategies based on recently uncovered pathologic mechanisms contributing to neuronal damage induced by HIV-1.

N-methyl-D-aspartate receptor subtype mediated bidirectional control of p38 mitogen-activated protein kinase

N-methyl-d-aspartate receptor (NMDAR) stimulation activates many downstream mechanisms involved in both cell survival and cell death. The manner in which the NMDAR regulates one of these pathways, the p38 mitogen-activated protein kinase (p38) pathway, is currently unknown. In the present study, we have defined a developmental-, concentration-, and time-dependent phosphorylation and subsequent dephosphorylation of p38. In cultured hippocampal neurons 7-8 days in vitro (DIV7-8), NMDAR stimulation leads to a concentration-dependent increase in p38 phosphorylation (phospho-p38). However, in more mature neurons (>DIV17) application of NMDA produces concentration-dependent effects, such that low concentrations result in sustained increases in phospho-p38 levels, and high concentrations dephosphorylate p38 within 5 min. Conantokin G, an antagonist of NR1/2A/2B and NR1/2B receptors, inhibits p38 phosphorylation, while NR1/2B-specific antagonists prevent the rapid dephosphorylation of p38 without affecting p38 activation. Furthermore, inhibition of calcineurin prevents the activation of p38, whereas inhibition of phosphoinositide 3-kinase (PI3K) prevents the rapid dephosphorylation of p38. Our results support the presence of subtype-dependent pathways regulating p38 activation and deactivation: one involves NR1/2A/2B receptors activating calcineurin and resulting in p38 phosphorylation, and the other utilizes NR1/2B receptors binding to and activating PI3K and leading to the dephosphorylation of p38 in a manner involving both NR1/2A/2B receptor activation and tyrosine phosphorylation of NR2B. The ability of NMDAR subtype-specific mechanisms to regulate p38 has implications for NMDAR-mediated synaptic plasticity, gene regulation, and excitotoxicity.

Pathogenic factors in vascular dementia and Alzheimer's disease. Multiple actions of heparin that probably are beneficial

The following areas are discussed in this review: atherogenesis; cerebrovascular factors; hypoperfusion; beta-amyloid production; beta-amyloid fibril formation; beta-sheets; metal cations; reactive oxygen species/free radicals; chronic inflammatory factors; endogenous plasma heparin; lipoprotein lipase; polyamines; protein kinase C; casein kinases; phospholipase A2; serine proteases; myeloperoxidase; cyclooxygenase 2; cysteine proteases; caspases; proprotein convertases; aspartic proteases; cyclin proteinases; thrombin; tau hyperphosphorylation; advanced glycosylation end products; activator protein 1; calcium; apolipoprotein E epsilon4; histamine; blood-brain barrier; glutamate; transglutaminase; insulin-like growth factor 1.

Stress-induced platelet-activating factor synthesis in human neutrophils

Platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine; PAF) is a potent inflammatory mediator produced by cells in response to physical or chemical stress. The mechanisms linking cell injury to PAF synthesis are unknown. We used liquid chromatography-tandem mass spectrometry to investigate stress-induced PAF synthesis in human neutrophils. PAF synthesis induced by extracellular pH 5.4 correlated with the activation of a stress-activated kinase, p38 mitogen-activated protein kinase (MAPK), and was blocked by the p38 MAPK inhibitor SB 203580. A key enzyme of PAF synthesis, acetyl-CoA:lysoPAF acetyltransferase, which we have previously shown is a target of p38 MAPK, was also activated in an SB 203580-sensitive fashion. Another MAPK pathway, extracellular signal-regulated kinase-1/2 (ERK-1/2), was also activated. Surprisingly, the pharmacological blockade of the ERK-1/2 pathway with PD 98059 did not block, but rather enhanced, PAF accumulation. Two unexpected actions of PD 98059 may underlie this phenomenon: an augmentation of stress-induced p38 MAPK phosphorylation and an inhibition of PAF catabolism. The latter effect did not appear to be due to a direct inhibition of PAF acetylhydrolase. Finally, similar results were obtained using another form of cellular stress, hypertonic sodium chloride. These data are consistent with a model in which stress-induced PAF accumulation is regulated positively by p38 MAPK and negatively by ERK-1/2. Such a model contrasts with the PAF accumulation induced by other forms of stimulation, which we and others have found is up-regulated by both p38 MAPK and ERK-1/2.

[Effects of ginkgolide on cerebral blood flow in dogs]

OBJECTIVE

To observe the effects of ginkgolide (GL) on the cerebral blood flow in dogs.

METHODS

Dogs anesthetized with sodium pentobarbital were randomly divided into 5 groups, with 4 dogs in each group. Ginkgolide of 4.86, 14.6 and 43.7 mg/kg and Xingling Granule of 0.22 g/kg were administered by gavage to the dogs in each of 4 groups. The dogs in the other group were administered with edible oil (1 ml/kg) as control group. The cerebral blood flow, systolic blood pressure, diastolic blood pressure, mean arterial blood pressure and electro-cardiogram of the dogs were measured at different times after the administration.

RESULTS

Ginkgolide of 4.86, 14.6 and 43.7 mg/kg had no obvious effects on the blood pressure and the heart rate. Ginkgolide of 14.6 and 43.7 mg/kg increased the cerebral blood flow 90 minutes after administration, and ginkgolide of 43.7 mg/kg obviously decreased the cerebral vascular resistance 150 minutes after administration.

CONCLUSION

Ginkgolide can increase the cerebral blood flow and decrease the cerebral vascular resistance, and it has no obvious effects on blood pressure and heart rate in dogs.

Quantification of sPLA2-induced early and late apoptosis changes in neuronal cell cultures using combined TUNEL and DAPI staining

The terminal deoxynucleotidyl transferase (TdT) dUTP nick end labeling (TUNEL) stain is in wide use for measuring apoptosis in neurons, as well as in other cell types. TUNEL may give false positive results due to variations in labeling technique as well as staining of cells that have undergone non-apoptotic DNA strand breaks. Therefore, in isolation, TUNEL is not a certain indicator of apoptosis. Recently, we have demonstrated the potent apoptotic effect of secreted phospholipase A2 from group III (sPLA2-III) on primary cortical neurons from rat. Here we describe a computer-assisted method for quantifying TUNEL-positive neurons after sPLA2-III induced apoptosis. Extent of TUNEL is normalized to total nuclear content using 4',6-diamidino-2-phenylindole (DAPI) staining. Furthermore, DAPI counterstaining allows for determination of a nuclear morphology indicator, based on nuclear size and roundness, which we call the nuclear area factor. We found that the nuclear area factor is an early indicator of cell death (significant after 4 h post treatment), while TUNEL staining is significant at later times (26 h). Thus, the independent staining techniques using TUNEL and DAPI complement each other, and with commercially available image analysis software, may be used to indicate early as well as delayed cell injury processes.

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.

Downregulation of COX-2 and JNK expression after induction of ischemic tolerance in the gerbil brain

The response of the inducible isoform of the prostaglandin H2 synthase (COX-2) and the c-Jun N-terminal kinase (JNK) in post-ischemic neuronal damage was assessed in a model of ischemic tolerance in Mongolian Gerbils. After a single 6-min bilateral carotid occlusion, histological damage was evident in the CA1 region of hippocampus, correlated with a high expression of JNK and COX-2 mRNA. However, in the group of animals with a 2-min ischemia and the tolerance group, in which a 2-min bilateral carotid occlusion was followed 3 days later by a 6-min ischemia, no hippocampal or cortical damage was detected. Accordingly, the JNK and COX-2 mRNA levels remained unaffected. We suggest that the level of JNK and COX-2 expression may determine the outcome as either post-ischemic cell death or tolerance.

Glutamate signaling to Ras-MAPK in striatal neurons: mechanisms for inducible gene expression and plasticity

Extracellular signals can regulate mitogen-activated protein kinase (MAPK) cascades through a receptor-mediated mechanism in postmitotic neurons of adult mammalian brain. Both ionotropic and metabotropic glutamate receptors (mGluRs) are found to possess such an ability in striatal neurons. NMDA and AMPA receptor signals seem to share a largely common route to MAPK phosphorylation which involves first activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) via Ca2+ influx, followed by subsequent induction of phosphoinositide 3-kinase (PI3-kinase). Through its lipid and protein kinase activity, active PI3-kinase may transduce signals to Ras-MAPK cascades via at least two distinct pathways. A novel, Ca(2+)-independent pathway is believed to mediate mGluR signals to Ras-MAPK activation. As an information superhighway between the surface membrane and the nucleus, Ras-MAPK cascades, through activating their specific nuclear transcription factor targets, are actively involved in the regulation of gene expression. Emerging evidence shows that MAPK-mediated genomic responses in striatal neurons to drug exposure contribute to the development of neuroplasticity related to addictive properties of drugs of abuse.

Neuroprotectin D1: a docosahexaenoic acid-derived docosatriene protects human retinal pigment epithelial cells from oxidative stress

Docosahexaenoic acid (DHA) is a lipid peroxidation target in oxidative injury to retinal pigment epithelium (RPE) and retina. Photoreceptor and synaptic membranes share the highest content of DHA of all cell membranes. This fatty acid is required for RPE functional integrity; however, it is not known whether specific mediators generated from DHA contribute to its biological significance. We used human ARPE-19 cells and demonstrated the synthesis of 10,17S-docosatriene [neuroprotectin D1 (NPD1)]. This synthesis was enhanced by the calcium ionophore A-23187, by IL-1beta, or by supplying DHA. Under these conditions, there is a time-dependent release of endogenous free DHA followed by NPD1 formation, suggesting that phospholipase A(2) releases the mediator's precursor. Added NPD1 potently counteracted H(2)O(2)/tumor necrosis factor alpha oxidative-stress-triggered apoptotic RPE DNA damage. NPD1 also up-regulated the antiapoptotic proteins Bcl-2 and Bcl-x(L) and decreased proapoptotic Bax and Bad expression. Moreover, NPD1 (50 nM) inhibited oxidative-stress-induced caspase-3 activation. NPD1 also inhibited IL-1beta-stimulated expression of cyclooxygenase 2 promoter transfected into ARPE-19 cells. Overall, NPD1 protected RPE cells from oxidative-stress-induced apoptosis, and we predict that it will similarly protect neurons. This lipid mediator therefore may indirectly contribute to photoreceptor cell survival as well. Because both RPE and photoreceptor cells die in retinal degenerations, our findings contribute to the understanding of retinal cell survival signaling and potentially to the development of new therapeutic strategies.

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

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

Platelet-activating factor activates mitogen-activated protein kinases, inhibits proliferation, induces differentiation and suppresses the malignant phenotype of human colon carcinoma cells

Recent studies suggest that the action of platelet-activating factor (PAF), a potent phospholipid modulator of allergic and inflammatory reactions, is diverse and functions as a modulator of a variety of physiological and pathological events in many cell types and tissues. Its role (if any) in modulating the proliferation, transformation and/or differentiation of epithelial colonic cells, however, is not known. In this study, we showed that PAF is biologically active in epithelial-derived human colon carcinoma cells with different phenotypic properties. These cells expressed the PAF receptor. PAF activated three prominent mitogen-activated protein kinase modules (ERK, p38MAPK and Jun N-terminal kinases) in these cells, inhibited proliferation and induced differentiation (measured by the induction of Waf1/p21 and the induction of the differentiation-related marker CEA). The net effect of PAF treatment was the suppression of malignant cell behavior (measured by anchorage-independent growth and cellular invasion). It is concluded that PAF is a modulator of proliferation and differentiation in human epithelial-derived colon carcinoma cells.

Metabotropic mGlu5 receptors regulate adenosine A2A receptor signaling

Dopamine, by activating dopamine D1-type receptors, and adenosine, by activating adenosine A(2A) receptors, stimulate phosphorylation of DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of M(r) 32,000) at Thr-34. In this study, we investigated the effect of metabotropic glutamate (mGlu) receptors on DARPP-32 phosphorylation at Thr-34 in neostriatal slices. A broad-spectrum mGlu receptor agonist, trans-ACPD, and a group I mGlu receptor agonist, DHPG, stimulated DARPP-32 phosphorylation at Thr-34. Studies with mGlu receptor antagonists revealed that the effects of trans-ACPD and DHPG were mediated through activation of mGlu5 receptors. The action of mGlu5 receptors required activation of adenosine A(2A) receptors by endogenous adenosine. Conversely, the action of adenosine A(2A) receptors required activation of mGlu5 receptors by endogenous glutamate. Coactivation of mGlu5 and adenosine A(2A) receptors by exogenous agonists synergistically increased DARPP-32 phosphorylation. mGlu5 receptors did not require activation of dopamine D1-type receptors by endogenous dopamine, nor did dopamine D1-type receptors require activation of mGlu5 receptors by endogenous glutamate. DHPG potentiated the effect of forskolin, but not that of 8-bromo-cAMP, and stimulated DARPP-32 phosphorylation in the presence of the phosphodiesterase inhibitor IBMX, suggesting that mGlu5 receptors stimulate the rate of cAMP formation coupled to adenosine A(2A) receptors. The action of mGlu5 receptors was attenuated by inhibitors of extracellular signal-regulated kinase, but not by inhibitors of phospholipase C, p38, casein kinase 1, or Cdk5. The results demonstrate that mGlu5 receptors potentiate adenosine A(2A)DARPP-32 signaling by stimulating the adenosine A(2A) receptor-mediated formation of cAMP in an extracellular signal-regulated kinase-dependent manner.

Platelet activating factor-induced apoptosis is inhibited by ectopic expression of the platelet activating factor G-protein coupled receptor

The pro-inflammatory lipid mediator platelet activating factor (PAF: 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) accumulates in ischemia, epilepsy, and human immunodeficiency virus-1-associated dementia and is implicated in neuronal loss. The present study was undertaken to establish a role for its G-protein coupled receptor in regulating neurotoxicity. PC12 cells do not express PAF receptor mRNA as demonstrated by northern analysis and RT-PCR. In the absence of the G-protein coupled receptor, PAF (0.1-1 micro m) triggered chromatin condensation, DNA strand breaks, oligonucleosomal fragmentation, and nuclear disintegration characteristic of apoptosis. Lyso-PAF (0.001-1 micro m), the immediate metabolite of PAF, did not elicit apoptotic death. Concentrations of PAF or lyso-PAF that exceeded critical micelle concentration had physicochemical effects on plasma membrane resulting in necrosis. Apoptosis but not necrosis was inhibited by the PAF antagonist BN52021 (1-100 micro m) but not CV3988 (0.2-20 micro m). Ectopic PAF receptor expression protected PC12 transfectants from ligand-induced apoptosis. PAF receptor-mediated protection was inhibited by CV3988 (1 micro m). These data provide empirical evidence that: (i) PAF can initiate apoptosis independently of its G-protein coupled receptor; (ii) PAF signaling initiated by its G-protein coupled receptor is cytoprotective to PC12 cells; (iii) the pro- and anti-apoptotic effects of PAF on PC12 cells can be pharmacologically distinguished using two different PAF antagonists.

What synaptic lipid signaling tells us about seizure-induced damage and epileptogenesis

Glutamate, the most abundant excitatory neurotransmitter in the mammalian CNS, plays a central role in many neuronal functions, such as long-term potentiation, which is necessary for learning and memory formation. The fast excitatory glutamate neurotransmission is mediated by ionotropic receptors that include AMPA/kainate and N-methyl-D-aspartate (NMDA) receptors, while the slow glutamate responses are mediated through its interaction with metabotropic receptors (mGluRs) coupled to G-proteins. During seizures, massive release of glutamate underlies excitotoxic neuronal damage as it triggers an overflow of calcium in postsynaptic neurons mediated by NMDA-gated channels. The early upstream postsynaptic events involve the activation of phospholipases, with the release of membrane-derived signaling molecules, such as free arachidonic acid (AA), eicosanoids, and platelet-activating factor (PAF). These bioactive lipids modulate the early neuronal responses to stimulation as they affect the activities of ion channels, receptors, and enzymes; and when released into the extracellular space, they can contribute to the modulation of presynaptic neurotransmitter release/re-uptake, and/or affect other neighboring neuronal/glial cells. The downstream postsynaptic events target the nucleus, leading to activation of gene-expression cascades. Syntheses of new proteins are the basis for seizure-induced sustained physiological and/or pathological changes that occur hours, days, or months later, such as synaptic reorganization and repair, and apoptotic/necrotic neuronal death. The intricate mesh of signaling pathways converging to the nucleus, and connecting upstream to downstream synaptic events, are at present the focus of many research efforts. We describe in this chapter how seizure-induced glutamate release activates the hydrolysis of membrane AA-phospholipids via phospholipase A2 (PLA2), PLC, and PLD, thus releasing bioactive lipids that, in turn, modulate neurotransmission. We discuss mechanisms through which lipid messengers, such as AA and PAF, may turn into injury mediators participating in seizure-induced brain damage.

Platelet-activating factor regulates chloride transport in colonic epithelial cell monolayers

Platelet-activating factor (PAF) has been implicated in the pathogenesis of gastrointestinal diseases such as necrotizing enterocolitis, Crohn's disease, and ulcerative colitis. However, neither the physiologic role of PAF in the intestine, nor the mechanisms by which PAF participates in the pathogenesis of disease are well understood. The aim of the present study was to determine the direct effect of PAF on intestinal epithelial cell ion transport, and to delineate the mechanisms of regulation. Ion transport was evaluated by measuring short circuit current (I(sc)) in HT29-CL19A cell monolayers using Ussing chambers. PAF receptor polarity was assessed using domain-selective biotinylation followed by immunoprecipitation and streptavidin blotting of intact epithelial monolayers. PAF (1-200 microM) stimulated I(sc) that followed the direction of a Cl(-) gradient and was specifically inhibited by the Cl(-) channel blockers glybenclamide, 2,2' iminodibenzoic acid and 4,4' diisothiocyanostilbene-2, 2' disulfonic acid, but was unaffected by the inhibition of prostaglandin synthesis with indomethacin. Stimulated I(sc) was only detected after apical addition of PAF, correlating with the results of biotinylation experiments indicating an exclusive apical polarity of the PAF receptor. PAF receptor antagonists CV6209 and octylonium bromide abolished PAF-stimulated I(sc). Thus, mucosal acting PAF directly and specifically stimulates ion transport via activation of an apical Cl(-) channel in intestinal epithelial cell monolayers independent of prostaglandin biosynthesis.

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

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

Kainate receptor-mediated apoptosis in primary cultures of cerebellar granule cells is attenuated by mitogen-activated protein and cyclin-dependent kinase inhibitors

1. Previous studies have suggested that neuronal apoptosis is the result of an abortive attempt to re-enter the cell cycle, and more recently the cyclin-dependent (CDKs) and the mitogen-activated protein (MAP) kinases, two superfamilies of kinases that influence and control cell cycle progression, have been implicated in neuronal apoptosis. 2. Here, to examine whether CDK/MAPK related pathways are involved in excitotoxicity, we studied the actions of various kinase inhibitors on apoptosis induced by the ionotropic glutamate (Glu) receptor agonist, kainate (KA), in primary cultures of murine cerebellar granule cells (CGCs). 3. KA-mediated neurotoxicity was concentration-dependent, as determined by a cell viability assay monitoring the reduction of 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide (MTT), and largely apoptotic in nature, as shown by morphological examination and labelling of DNA fragmentation in situ using terminal deoxynucleotidyl transferase (TdT)-mediated dUTP digoxigenin nick-end labelling (TUNEL). 4. KA-mediated neurotoxicity and apoptosis was completely attenuated by the mixed CDK and MAP kinase inhibitor, olomoucine, in a concentration-dependent manner (50 - 600 microM), and partially by roscovitine (1 - 100 microM), a more selective CDK inihibitor. 5. The p38 MAP kinase inhibitor, SB203580 (1 - 100 microM), partially attenuated KA receptor-mediated apoptosis, as did the MAP kinase kinase inhibitors PD98509 (1 - 100 microM) and U0126 (1 - 100 microM). 6. These findings provide new evidence for a complex network of interacting pathways involving CDK/MAPK that control apoptosis downstream of KA receptor activation in excitotoxic neuronal cell death.

Dominant-interfering forms of MEF2 generated by caspase cleavage contribute to NMDA-induced neuronal apoptosis

Myocyte enhancer factor-2 (MEF2) transcription factors are activated by p38 mitogen-activated protein kinase during neuronal and myogenic differentiation. Recent work has shown that stimulation of this pathway is antiapoptotic during development but proapoptotic in mature neurons exposed to excitotoxic or other stress. We now report that excitotoxic (N-methyl-D-aspartate) insults to mature cerebrocortical neurons activate caspase-3, -7, in turn cleaving MEF2A, C, and D isoforms. MEF2 cleavage fragments containing a truncated transactivation domain but preserved DNA-binding domain block MEF2 transcriptional activity via dominant interference. Transfection of constitutively active MEF2 (MEF2C-CA) rescues MEF2 transcriptional activity after N-methyl-D-aspartate insult and prevents neuronal apoptosis. Conversely, dominant-interfering MEF2 abrogates neuroprotection by MEF2C-CA. These results define a pathway to excitotoxic neuronal stress/apoptosis via caspase-catalyzed cleavage of MEF2. Additionally, we show that similar MEF2 cleavage fragments are generated in vivo during focal stroke damage. Hence, this pathway appears to have pathophysiological relevance in vivo.