N-methyl-D-aspartate receptor blockade enhances neuronal apoptosis induced by serum deprivation

Homeostatic interplay between electrical activity and neuronal apoptosis in the developing neocortex

An intriguing feature of nervous system development in most animal species is that the initial number of generated neurons is higher than the number of neurons incorporated into mature circuits. A substantial portion of neurons is indeed eliminated via apoptosis during a short time window - in rodents the first two postnatal weeks. While it is well established that neurotrophic factors play a central role in controlling neuronal survival and apoptosis in the peripheral nervous system (PNS), the situation is less clear in the central nervous system (CNS). In postnatal rodent neocortex, the peak of apoptosis coincides with the occurrence of spontaneous, synchronous activity patterns. In this article, we review recent results that demonstrate the important role of electrical activity for neuronal survival in the neocortex, describe the role of Ca²⁺ and neurotrophic factors in translating electrical activity into pro-survival signals, and finally discuss the clinical impact of the tight relation between electrical activity and neuronal survival versus apoptosis.

Impacts of tissue-type plasminogen activator (tPA) on neuronal survival

Tissue-type plasminogen activator (tPA) a serine protease is constituted of five functional domains through which it interacts with different substrates, binding proteins, and receptors. In the last years, great interest has been given to the clinical relevance of targeting tPA in different diseases of the central nervous system, in particular stroke. Among its reported functions in the central nervous system, tPA displays both neurotrophic and neurotoxic effects. How can the protease mediate such opposite functions remain unclear but several hypotheses have been proposed. These include an influence of the degree of maturity and/or the type of neurons, of the level of tPA, of its origin (endogenous or exogenous) or of its form (single chain tPA versus two chain tPA). In this review, we will provide a synthetic snapshot of our current knowledge regarding the natural history of tPA and discuss how it sustains its pleiotropic functions with focus on excitotoxic/ischemic neuronal death and neuronal survival.

Serine racemase: a key player in apoptosis and necrosis

A fine balance between cell survival and cell death is required to sculpt the nervous system during development. However, an excess of cell death can occur following trauma, exposure to neurotoxins or alcohol, and some developmental and neurodegenerative diseases, such as Alzheimer's disease (AD). N-Methyl-D-aspartate receptors (NMDARs) support synaptic plasticity and survival of many neuronal populations whereas inappropriate activation may promote various forms of cell death, apoptosis, and necrosis representing the two extremes of a continuum of cell death processes both “in vitro” and “in vivo.” Hence, by identifying the switches controlling pro-survival vs. apoptosis and apoptosis vs. pro-excitotoxic outcome of NMDAR stimulation, NMDAR modulators could be developed that selectively block the cell death enhancing pro-survival signaling or synaptic plasticity mediated by NMDAR. Among these modulators, a role is emerging for the enzyme serine racemase (SR) that synthesizes D-serine, a key co-agonist with glutamate at NMDAR. This review summarizes the experimental evidence from “in vitro” neuronal cultures—with special emphasis on cerebellar granule neurons (CGNs)—and “in vivo” models of neurodegeneration, where the dual role of the SR/D-serine pathway as a master regulator of apoptosis and the apoptosis-necrosis shift will be discussed.

High t-PA release by neonate brain microvascular endothelial cells under glutamate exposure affects neuronal fate

Glutamate excitotoxicity is a consolidated hypothesis in neonatal brain injuries and tissue plasminogen activator (t-PA) participates in the processes through proteolytic and receptor mediated effects. In brain microvascular endothelial cell (nBMEC) cultures from neonates, t-PA content and release upon glutamate are higher than in adult (aBMECs) cultures. Owing to the variety of t-PA substrates and receptor targets, the study was aimed at determining the putative roles of endothelial t-PA in the neonatal brain parenchyma under glutamate challenge. Basal t-PA release was 4.4 fold higher in nBMECs vs aBMECs and glutamate was 20 fold more potent to allow Evans blue vascular permeability in neonate microvessels indicating that, under noxious glutamate (50 μM) exposure, high amounts of endothelial t-PA stores may be mobilized and may access the nervous parenchyma. Culture media from nBMECS or aBMECs challenged by excitotoxic glutamate were applied to neuron cultures at DIV 11. While media from adult cells did not evoke more LDH release in neuronal cultures that under glutamate alone, media from nBMECs enhanced 2.2 fold LDH release. This effect was not observed with media from t-PA(-/-) nBMECs and was inhibited by hr-PAI-1. In Cortical slices from 10 day-old mice, hrt-PA associated with glutamate evoked neuronal necrosis in deeper (more mature) layers, an effect reversed by NMDA receptor GluN1 amino-terminal domain antibody capable of inhibiting t-PA potentiation of the receptor. In superficial layers (less mature), hrt-PA alone inhibited apoptosis, an effect reversed by the EGF receptor antagonist AG1478. Applied to immature neurons in culture (DIV5), media from nBMEC rescued 85.1% of neurons from cell death induced by serum deprivation. In cortical slices, the anti-apoptotic effect of t-PA fitted with age dependent localization of less mature neurons. These data suggest that in the immature brain, propensity of vessels to release high amounts of t-PA may not only impact vascular integrity but may also influence neuronal fate, via regulation of apoptosis in immature cells and, as in adult by potentiating glutamate toxicity in mature neurons. The data point out putative implication of microvessels in glutamate neurotoxicity in the development, and justify research towards vessel oriented neuroprotection strategies in neonates.

Neuropathological sequelae of developmental exposure to antiepileptic and anesthetic drugs

Glutamate (Glu) and γ-aminobutyric acid (GABA) are major neurotransmitters in the mammalian brain which regulate brain development at molecular, cellular, and systems level. Sedative, anesthetic, and antiepileptic drugs (AEDs) interact with glutamate and GABA receptors to produce their desired effects. The question is posed whether such interference with glutamatergic and GABAergic neurotransmission may exert undesired, and perhaps even detrimental effects on human brain development. Preclinical research in rodents and non-human primates has provided extensive evidence that sedative, anesthetic, and AEDs can trigger suicide of neurons and oligodendroglia, suppress neurogenesis, and inhibit normal synapse development and sculpting. Behavioral correlates in rodents and non-human primates consist of long-lasting cognitive impairment. Retrospective clinical studies in humans exposed to anesthetics or AEDs in utero, during infancy or early childhood have delivered conflicting but concerning results in terms of a correlation between drug exposure and impaired neurodevelopmental outcomes. Prospective studies are currently ongoing. This review provides a short overview of the current state of knowledge on this topic.

NR2D-containing NMDA receptors mediate tissue plasminogen activator-promoted neuronal excitotoxicity

Although the molecular bases of its actions remain debated, tissue-type plasminogen activator (tPA) is a paradoxical brain protease, as it favours some learning/memory processes, but increases excitotoxic neuronal death. Here, we show that, in cultured cortical neurons, tPA selectively promotes NR2D-containing N-methyl-D-aspartate receptor (NMDAR)-dependent activation. We show that tPA-mediated signalling and neurotoxicity through the NMDAR are blocked by co-application of an NR2D antagonist (phenanthrene derivative (2S(*), 3R(*))-1-(phenanthrene-2-carbonyl)piperazine-2,3-dicarboxylic acid, PPDA) or knockdown of neuronal NR2D expression. In sharp contrast with cortical neurons, hippocampal neurons do not exhibit NR2D both in vitro and in vivo and are consequently resistant to tPA-promoted NMDAR-mediated neurotoxicity. Moreover, we have shown that activation of synaptic NMDAR prevents further tPA-dependent NMDAR-mediated neurotoxicity and sensitivity to PPDA. This study shows that the earlier described pro-neurotoxic effect of tPA is mediated by NR2D-containing NMDAR-dependent extracellular signal-regulated kinase activation, a deleterious effect prevented by synaptic pre-activation.

Lithium down-regulates tau in cultured cortical neurons: a possible mechanism of neuroprotection

In tauopathies such as Alzheimer's disease (AD), the moleccular mechanisms of tau protein agregation into neurofibrillary tangles (NFTs) and their contribution to neurodegeneration are not fully understood. Recent studies indirectly demonstrated that tau, regardless of its aggregation, might represent a key mediator of neurodegeneration, especially that induced by the amyloid (Abeta) pathology. Lithium is a medication for bipolar mood disorders. Its therapeutic mechanism of action remains unclear, in part because of the large number of biochemical effects attributed to lithium. Since lithium directly inhibits glycogen synthase kinase-3beta (GSK3beta), a key enzyme involved in tau phosphorylation, it was suggested that the therapeutic use of lithium could be expanded from mood disorders to neurodegenerative conditions. Lithium has been also reported to protect cultured neurons against Abeta toxicity, and to prevent NFTs accumulation and cognitive impairments in transgenic models of tauopathies. However, the exact mechanism of neuroprotection provided by lithium remains unknown. Here, we show that exposure of cultured cortical neurons to lithium decreased tau protein levels. This decrease was not linked to the activation of proteolytic processes including calpains, caspases and proteasome or to neuronal loss, but was rather associated with a reduction in tau mRNA levels. Moreover, prior exposure to lithium, at concentrations effective in reducing tau protein levels, markedly reduced pre-aggregated Abeta-induced neuronal apoptosis. Our findings raise the possibility that lithium could exert its neuroprotective effect against Abeta toxicity through the downregulation of tau proteins and that, at least, by acting at the level of tau mRNA.

Glutamate antagonists are neurotoxins for the developing brain

Neurotransmitters and neuromodulators are essential for normal nervous system development. Disturbances in the expression timetable or intensity of neurotransmitter signalling during critical periods of brain development can lead to permanent damage. Neuroactive drugs and environmental toxins interfere with neurotransmitter signalling and may thereby provide one mechanism underlying neurological abnormalities. Glutamate is the main excitatory neurotransmitter in the mammalian central nervous system and mediates neurotransmission across most excitatory synapses. In this article we review the timely expression of the excitatory neurotransmitter glutamate and its receptors during brain development, briefly review glutamate receptor antagonists and present clinical and experimental evidence describing their adverse effects in the developing brain.

Effect of NMDA on staurosporine-induced activation of caspase-3 and LDH release in mouse neocortical and hippocampal cells

To achieve a better understanding of developmentally regulated NMDA- and staurosporine-induced apoptotic processes, we investigated the concerted action of these agents on caspase-3 activity and LDH release in neocortical and hippocampal cell cultures at different stages in vitro (DIV). Hoechst 33342 and MAP-2 stainings were additionally employed to visualize apoptotic changes and cell damage. The vulnerability of neocortical cells to NMDA was more prominent at later culture stages, whereas hippocampal neurons were more susceptible to NMDA treatment at earlier stages. A persistent activation of caspase-3 by staurosporine was found at all experimental stages. Despite of certain differences in susceptibility to NMDA and staurosporine, both tissues responded to regulatory action of NMDA towards staurosporine-activated caspase-3 in a similar way. Combined treatment with NMDA and staurosporine resulted in a substantial increase in caspase-3 activity in neocortical and hippocampal neurons on 2 DIV. Additive effects were also observed in neocortical cultures on 12 DIV. In contrast, NMDA substantially inhibited staurosporine-induced caspase-3 activity on 7 DIV in neocortical and hippocampal cultures. Additionally, pro-apoptotic effects of 17beta-estradiol were attenuated by NMDA on 7 DIV. Changes in vulnerability to NMDA- and staurosporine-mediated activation of caspase-3 were not strictly related to LDH release. Our data revealed that NMDA can both enhance and inhibit the staurosporine-induced neuronal cell apoptosis. The pro-apoptotic effect of NMDA was exhibited at early and late culture stages, whereas the anti-apoptotic effect was transient occurring on 7 DIV only.

Activation of extracellular signal-regulated kinase 5 may play a neuroprotective role in hippocampal CA3/DG region after cerebral ischemia

Extracellular signal-regulated kinase 5 (ERK5), the newest member of the mitogen-activated protein (MAP) kinase family of proteins, is widely expressed in many tissues, including the brain. Here we investigated the activation and subcellular localization of ERK5 by immunoblotting and immunohistochemistry as well as its potential role following cerebral ischemia in rat hippocampus. Transient cerebral ischemia was induced by the four-vessel occlusion method in Sprague-Dawley rats. Our results first indicated that the strongly activated ERK5 immunoreactivity was seen in the CA3/DG region but not in the CA1 pyramidal cell of rat hippocampus following reperfusion. In cytosol extracts, ERK5 activation was rapidly increased, with a peak at 30 min, and then gradually decreased to basal level at 3 days of reperfusion. In nucleus extracts, both phospho-ERK5 and its protein expression were persistently enhanced during the later reperfusion period (from 6 hr to 3 days). To elucidate further the possible role of ERK5 activation and subcellular localization in ischemic insult, rats were intraperitoneally administrated with nifedipine (ND) and dextromethorphan (DM), inhibitors of two types of calcium channels, 20 min prior to ischemia. Our findings showed that ND or DM significantly reduced activated ERK5 immunoreactivity in the nucleus and that most of the CA3/DG neurons were lost 3 days later. Most importantly, intracerebroventricular infusion of ERK5 antisense oligonucleotides (AS; every 24 hr for 3 days before ischemia), but not sense oligonucleotides or vehicle, not only markedly decreased the level of ERK5 and p-ERK5 but also largely caused neuronal loss in the CA3/DG region at 3 days of reperfusion. Taken together, the results strongly suggest that ERK5 was selectively activated in the hippocampal CA3/DG region and subsequently translocated from the cytosol to the nucleus through activation of N-methyl-D-aspartate receptor and L-type voltage-gated calcium channel, which might act as an important survival signal in ischemia-induced neuronal cell damage of the CA3/DG region.

Application of a systems biology approach to developmental neurotoxicology

Systems biology can be applied to enhance the understanding of complex biological processes such as apoptosis in the developing brain. Systems biology, as applied to toxicology, provides a structure to arrange information in the form of a biological model. The approach allows for the subsequent and iterative perturbation of the initial model with the use of toxicants, and the comparison of the resulting data against the proposed biological model. It is postulated that the exposure of the developing rat to NMDA antagonists, e.g., ketamine or phencyclidine (PCP), causes a compensatory up-regulation of NMDA receptors, thereby making cells bearing these receptors more vulnerable to excitotoxic effects of endogenous glutamate. Although comprehensive gene expression/proteomic studies and mathematical modeling remain to be accomplished, a biological model has been established and perturbed in an iterative manner to allow confirmation of the biological pathway for NMDA antagonist-induced brain cell death in the developing rat.

Linking alterations in tau phosphorylation and cleavage during neuronal apoptosis

Neurofibrillary tangles (NFTs) are classic lesions of Alzheimer's disease. NFTs are bundles of abnormally phosphorylated tau, the paired helical filaments. The initiating mechanisms of NFTs and their role in neuronal loss are still unknown. Accumulating evidence supports a role for the activation of proteolytic enzymes, caspases, in neuronal death observed in brains of patients with Alzheimer's disease. Alterations in tau phosphorylation and tau cleavage by caspases have been previously reported in neuronal apoptosis. However, the links between the alterations in tau phosphorylation and its proteolytic cleavage have not yet been documented. Here, we show that, during staurosporine-induced neuronal apoptosis, tau first undergoes transient hyperphosphorylation, which is followed by dephosphorylation and cleavage. This cleavage generated a 10-kDa fragment in addition to the 17- and 50-kDa tau fragments previously reported. Prior tau dephosphorylation by a glycogen synthase kinase-3beta inhibitor, lithium, enhanced tau cleavage and sensitized neurons to staurosporine-induced apoptosis. Caspase inhibition prevented tau cleavage without reversing changes in tau phosphorylation linked to apoptosis. Furthermore, the microtubule depolymerizing agent, colchicine, induced tau dephosphorylation and caspase-independent tau cleavage and degradation. Both phenomena were blocked by inhibiting protein phosphatase 2A (PP2A) by okadaic acid. These experiments indicate that tau dephosphorylation precedes and is required for its cleavage and degradation. We propose that the absence of cleavage and degradation of hyperphosphorylated tau (due to PP2A inhibition) may lead to its accumulation in degenerating neurons. This mechanism may contribute to the aggregation of hyperphosphorylated tau into paired helical filaments in Alzheimer's disease where reduced PP2A activity has been reported.

Effect of isoflurane on neuronal apoptosis in rats subjected to focal cerebral ischemia

Although isoflurane can reduce ischemic neuronal injury after short postischemic recovery intervals, this neuroprotective efficacy is not sustained. Neuronal apoptosis can contribute to the gradual increase in infarct size after ischemia. This suggests that isoflurane, although capable of reducing early neuronal death, may not inhibit ischemia-induced apoptosis. We investigated the effects of isoflurane on markers of apoptosis in rats subjected to focal ischemia. Fasted Wistar-Kyoto rats were anesthetized with isoflurane and randomly allocated to awake (n = 40) or isoflurane (n = 40) groups. Animals in both groups were subjected to focal ischemia by filament occlusion of the middle cerebral artery for 70 min. Pericranial temperature was servo-controlled at 37 degrees C +/- 0.2 degrees C throughout the experiment. In the awake group, isoflurane was discontinued and the animals were allowed to awaken. In the isoflurane group, isoflurane anesthesia was maintained at 1.5 MAC (minimum alveolar anesthetic concentration). Animals were killed 7 h, 1 day, 4 days, or 7 days after reperfusion (n = 10/group/time point). The area of cerebral infarction was measured by image analysis in a hematoxylin and eosin stained section. In three adjacent sections, apoptotic neurons were identified by TUNEL staining and immunostaining for active caspase-9 and caspase-3. Infarct size was smaller in the isoflurane group than the awake group 7 h, 1 day, and 4 days after reperfusion (P < 0.05). However, this difference was absent 7 days after reperfusion. The number of apoptotic (TUNEL, caspase-3, and caspase-9 positive) cells 1 day after ischemia was significantly more in the awake versus isoflurane group. After a recovery period of 4 or 7 days, the number of apoptotic cells in the isoflurane group was more than in the awake group. After 7 days, the number of caspase-3 and -9 positive neurons was more in the isoflurane group (P < 0.05). The data indicate that isoflurane delays but does not prevent the development of cerebral infarction caused by ischemia. Isoflurane reduced the development of apoptosis early after ischemia but did not prevent it at later stages of postischemic recovery.

IMPLICATIONS

The effect of isoflurane on neuronal apoptosis was investigated in rats subjected to focal cerebral ischemia. In isoflurane-anesthetized animals, ischemia-induced apoptosis occurred during the later stages of postischemic recovery. Isoflurane did not inhibit postischemic neuronal apoptosis.

Effects of estrone on N-METHYL-d-aspartic acid- and staurosporine-induced changes in caspase-3-like protease activity and lactate dehydrogenase-release: time- and tissue-dependent effects in neuronal primary cultures

A growing body of evidence indicates that estrogens affect apoptotic processes in neuronal cells. However, their effects seem to depend on type of neuronal tissue, stage of development and apoptosis inducing factors. In the present study we compared effects of estrone (100 and 500 nM) on N-methyl-D-aspartic acid (NMDA) (1 mM)- and staurosporine (1 microM)-induced caspase-3-like activity and lactate dehydrogenase (LDH)-release in primary cultures of rat hippocampal and neocortical neurons. Fluorometric and colorimetric determination of enzyme activity was performed 6 h, 14 h, and 24 h after exposure to apoptotic agents. In the hippocampal cell cultures on 7 days in vitro (DIV), a time-dependent NMDA-induced activation of caspase-3-like proteases was accompanied by increased LDH-release. In neocortical cell cultures on 7 DIV NMDA did not affect caspase activity and decreased LDH-release. In neocortical cell cultures on 12 DIV NMDA inhibited spontaneous caspase activity, but was toxic to neurons after 24 h exposure suggesting that these cells underwent necrotic rather than apoptotic death. Estrone has attenuated both pro- and anti-apoptotic NMDA-induced changes in rat primary neuronal cultures acting independently of estrogen receptors, as detected with ICI 182, 780. In hippocampal neurons estrone antagonized not only the NMDA-induced caspase-3-like activity, but also NMDA-mediated LDH-release. However, in neocortical neurons estrone either attenuated NMDA-induced inhibition of caspase-3-like activity (12 DIV) or partly blocked NMDA-mediated decrease in LDH-release (7 DIV). In contrast to NMDA, staurosporine elevated caspase-3-like activity and LDH-release in a time-dependent manner in all used culture systems. Estrone inhibited pro-apoptotic effects of staurosporine in neocortical neurons, but only at later stage of development in vitro, which points to the protective role of estrogens during the brain tissue maturation. Since estrone triggered its effects via non-genomic mechanisms, it suggests that the other estradiol metabolites exhibiting low affinity to hormone receptors may be potent neuroprotective agents, which could retain the favorable and minimize the adverse side effects of estrogens.

Role of N-methyl-D-aspartate receptors in the neuroprotective activation of extracellular signal-regulated kinase 1/2 by cisplatin

Neurons are exposed to damaging stimuli that can trigger cell death and subsequently cause serious neurological disorders. Therefore, it is important to define defense mechanisms that can be activated in response to damage to reduce neuronal loss. Here we report that cisplatin (CPDD), a neurotoxic anticancer drug that damages DNA, triggered apoptosis and activated the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway in cultured rat cortical neurons. Inhibition of ERK1/2 activation using either pharmacological inhibitors or a dominant-negative mutant of the ERK1/2 activator, mitogen-activated protein kinase kinase 1, increased the toxicity of CPDD. Interestingly, N-methyl-d-aspartate (NMDA) receptor (NMDAR) antagonists reduced the ERK1/2 activation and exacerbated apoptosis in CPDD-treated neurons. Pre-treatment with CPDD increased ERK1/2 activation triggered by exogenous NMDA, suggesting that CPDD augmented NMDAR responsiveness. CPDD-enhanced response of NMDAR and CPDD-mediated ERK1/2 activation were both decreased by inhibition of poly(ADP-ribose) polymerase (PARP). Interestingly, PARP activation did not produce ATP depletion, suggesting involvement of a non-energetic mechanism in NMDAR regulation by PARP. Finally, CPDD toxicity was reduced by brain-derived neurotrophic factor, and this protection required ERK1/2. In summary, our data identify a novel compensatory circuit in central nervous system neurons that couples the DNA injury, through PARP and NMDAR, to the defensive ERK1/2 activation.

Corpus callosum and visual cortex of mice with deletion of the NMDA-NR1 receptor: I. Accelerated development of callosal projection neurons

Many pharmacological experiments show that the ionotropic receptor NMDA has both neurotrophic and neuroexcitotoxic effects. The neurotrophic function is manifested in many ways including acceleration of neuronal development, enhancement of neuronal migration, neuroprotection, blockage of apoptosis, prevention of aging and prematurity, as well as effects on synaptic plasticity and synaptogenesis. On the other hand, the neuroexcitotoxic function is manifested in its role in neurological and psychiatric diseases such as epilepsy, Parkinson's disease and schizophrenia. The present study explores the consequences of complete and partial absence of NMDA-NR1 receptors throughout development. Using DiI tracing in vitro, the development of corpus callosum projection neurons in transgenic mice with deletion of the NMDA-NR1 receptor was observed in visual cortex. Compared to littermate controls, the histogenesis and neuronal development of corpus callosum cells of origin was found to be accelerated in NR1-/- mice. That is, the corpus callosum projection neurons in NR1 knockout mice developed earlier and faster than in littermate heterozygous and wild-type mice. However, the corpus callosum projection neurons in NR1 heterozygous mice developed earlier and faster than in littermate wild-type mice. This suggests that NMDA-NR1 receptors are involved in sequencing and/or temporal regulation of neuronal development, and that there is a gene-dose effect. Studies from other laboratories suggest that the observed phenomenon of prematurity or accelerated development is a direct effect of altered expression of genes found in mice with deletion of the NMDA-NR1 receptor.

The estrogen receptor is not essential for all estrogen neuroprotection: new evidence from a new analog

We synthesized an estrogen analog, ZYC-5, lacking activity at the classical estrogen receptor and examined its neuroprotective potential against necrosis induced by N-methyl-d-aspartate (NMDA) and apoptosis/necrosis induced by the NMDA receptor antagonist (+)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP). ZYC-5 protected cortical neurons in a dose-dependent manner, and the neuroprotection was more robust than with 17beta-estradiol. The effect of ZYC-5 was not mediated by the classical estrogen receptor, because it was unaffected by the antagonists 4-hydroxytamoxifen and ICI 182,780. The ZYC-5 protection against excitotoxicity was not directly mediated through the NMDA receptor, because there was no effect of ZYC-5 on NMDA current or the intracellular calcium increase induced by NMDA. Results obtained with the free-radical-sensitive dye, dihydroethidium, suggested that the neuroprotection of ZYC-5 was partly related to its radical scavenging properties. Although some of estrogen's neuroprotective effects may depend upon the estrogen receptor, our results suggest the possibility of neuroprotection without hormonal side effects.

NMDA antagonists exacerbate neuronal death caused by proteasome inhibition in cultured cortical and striatal neurons

The proteasome is involved in multiple cellular processes including control of the cell cycle, apoptosis and intracellular signalling; loss of proteasome function has been postulated to participate in the pathogenesis of triplet repeat diseases. We examined the vulnerability of central neurons to proteasome inhibition and tested the ability of anti-excitotoxic and anti-apoptotic treatments to attenuate proteasome inhibition-induced neuronal death. Exposure of murine neocortical cultures to proteasome inhibitors (0.1-10 microm clasto-lactacystin beta-lactone or MG-132) for 48 h resulted in widespread neuronal death associated with a reduction in intracellular free calcium; higher inhibitor concentrations killed astrocytes. Cultured striatal neurons were more vulnerable than cortical neurons. Within each population, the NADPH diaphorase-positive neuronal subpopulation was more vulnerable than the general neuronal population. Enhancing calcium entry with S(-)BayK8644 or kainate, or blocking apoptosis with cycloheximide, actinomycin D or Z-VAD.FMK attenuated neuronal death, whereas, reducing calcium entry with NMDA antagonists or R(+)BayK8644 potentiated neuronal death. These findings suggest that proteasome inhibition can induce selective neuronal apoptosis associated with intracellular calcium starvation, and point to manipulation of intracellular calcium as a specific therapeutic strategy. In particular, concern is raised that glutamate receptor antagonists might exacerbate, rather than attenuate, proteasome inhibition-induced neuronal death.

Nitric oxide produced by non-motoneuron cells enhances rat embryonic motoneuron sensitivity to excitotoxins: comparison in mixed neuron/glia or purified cultures

The present study compares the sensitivity to chronic exposure to glutamate agonists of SMI-32-positive rat-derived embryonic motoneurons under both mixed neuron/glia and purified cultures. We found that in spite of a trophic role of glia on cultured motoneurons, SMI-32-positive cells are more sensitive to excitotoxicity in the presence of glia than in purified culture, very likely through nitric oxide released by non-neuronal cells. The rank order of potency for inducing toxicity after 48 h incubation was AMPA>kainate>NMDA, with EC(50): 0.43, 4.9 and 49 microM, respectively, in mixed neuron/glia culture and 14, 32 and 135 microM in purified cultures. The effect of NMDA was dose-dependently potentiated by glycine, with similar potency in the two culture conditions. The effect of agonists was completely antagonized by the specific antagonists CNQX, BNQX and MK801 in both culture conditions. Motoneurons were similarly immunoreactive to NR1 and GluR2 antibodies under both mixed neuron/glia and purified cultures, thus confirming the presence of the calcium-impermeant AMPA receptor subtypes and of the obligatory subunit for NMDA receptors. The effect of kainate in mixed neuron/glia culture was reduced by the addition of 40 microM N-nitro-L-arginine or L-NAME, which shifted the EC(50) to 9 microM. By contrast, L-NAME did not modify the effect of kainic acid in purified cultures. These results suggest that the release of nitric oxide by non-neuronal cells in culture enhances glutamate excitotoxicity in SMI-32-positive cells, and that direct activation of ionotropic glutamate receptors is not enough to explain the mechanism of chronic motoneuron degeneration occurring in vivo in amyotrophic lateral sclerosis (ALS).

Neuroprotective effects of extracellular glutamate are absent in hippocampal organotypic cultures treated with the amyloid peptide Abeta(25-35)

Hippocampal cells are particularly vulnerable in Alzheimer's disease but the cause of cell death is unknown. Amyloid toxicity has been implicated in hippocampal cell death, but its specific mechanisms are poorly understood. We used confocal microscopy to examine the effects of the amyloid peptide fragment 25-35 (Abeta(25-35)) on cell death in organotypic hippocampal slice cultures. Addition of glutamate to the culture medium significantly improved nerve cell survival in cultures subjected to consecutive medium exchanges. This effect was lost if cultures were treated with the amyloid peptide fragment Abeta(25-35) but not the inactive peptide 35-25. These data suggest that one of the mechanisms responsible for amyloid toxicity may be inhibition of the survival promoting effects of extracellular glutamate.

Glutamate-mediated neuroprotection against N-methyl-D-aspartate toxicity: a role for metabotropic glutamate receptors

We studied N-methyl-D-aspartate-induced cell death in organotypic hippocampal slices from seven-day-old Wistar rat pups cultured for 12-14 days in a medium containing no added glutamate. Propidium iodide fluorescence intensity was used as an indicator of cell death measured with the help of confocal microscopy. Exposure of slices for 2h to L-glutamate (1-500 microM) prior to the N-methyl-D-aspartate challenge significantly reduced N-methyl-D-aspartate-induced cell death. Glutamate at 10 and 500 microM concentrations was highly protective against N-methyl-D-aspartate-induced cell death, but was less protective at the 1 microM concentration. The protection was not blocked by the Na(+) channel blocker tetrodotoxin (1 microM), the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonopentanoic acid (20 microM) or the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). 1S, 3R-1-Aminocyclopentane-trans-1,3-dicarboxylic acid, an agonist at metabotropic glutamate receptor types 1, 2/3 and 5, was protective at 100 microM but not at 50 microM. In contrast, the ionotropic glutamate receptor agonist aspartate (250 microM) facilitated N-methyl-D-aspartate toxicity. Treatment of slices with the protein kinase C inhibitor staurosporine (0.2 microM) or antisense oligonucleotide (10nM, 72 h) that selectively inhibits metabotropic glutamate receptor type 5 synthesis significantly reduced glutamate protection. These results suggest that ambient glutamate may reduce nerve cell susceptibility to injury caused by excessive N-methyl-D-aspartate receptor activation by acting at metabotropic glutamate receptors linked to protein kinase C.