Cross-tolerance to otherwise lethal N-methyl-D-aspartate and oxygen-glucose deprivation in preconditioned cortical cultures

Neuroprotection against supra-lethal 'stroke in a dish' insults by an anti-excitotoxic receptor antagonist cocktail

The goal of this study was to identify cocktails of drugs able to protect cultured rodent cortical neurons against increasing durations of oxygen-glucose deprivation (OGD). As expected, a cocktail composed of an NMDA and AMPA receptor antagonists and a voltage gated Ca²⁺ channel blocker (MK-801, CNQX and nifedipine, respectively) provided complete neuroprotection against mild OGD. Increasingly longer durations of OGD necessitated increasing the doses of MK-801 and CNQX, until these cocktails ultimately failed to provide neuroprotection against supra-lethal OGD, even at maximal drug concentrations. Surprisingly, supplementation of any of these cocktails with blockers of TRPM7 channels for increasing OGD durations was not neuroprotective, unless these blockers possessed the ability to inhibit NMDA receptors. Supplementation of the maximally effective cocktail with other NMDA receptor antagonists augmented neuroprotection, suggesting insufficient NMDAR blockade by MK-801. Substitution of MK-801 in cocktails with high concentrations of a glycine site NMDA receptor antagonist caused the greatest improvements in neuroprotection, with the more potent SM-31900 superior to L689,560. Substitution of CQNX in cocktails with AMPA receptor antagonists at high concentrations also improved neuroprotection, particularly with the combination of SYM 2206 and NBQX. The most neuroprotective cocktail was thus composed of SM-31900, SYM2206, NBQX, nifedipine and the antioxidant trolox. Thus, the cumulative properties of antagonist potency and concentration in a cocktail dictate neuroprotective efficacy. The central target of supra-lethal OGD is excitotoxicity, which must be blocked to the greatest extent possible to minimize ion influx.

Resveratrol alleviates oxygen/glucose deprivation/reoxygenation‑induced neuronal damage through induction of mitophagy

Resveratrol confers neuroprotective effects in cerebral ischemia; however, the involvement of mitophagy in the neuroprotective function of resveratrol remains unclear. The aim of the present study was to investigate whether resveratrol exerts neuroprotective effects on primary cortical neurons subjected to oxygen/glucose deprivation/reoxygenation (OGD/R) via modulating mitophagy. The data demonstrated that resveratrol at 1–10 µM during reoxygenation improved cell viability and suppressed apoptosis following OGD/R in a concentration-dependent manner. Moreover, resveratrol alleviated OGD/R-induced loss of mitochondrial membrane potential and excessive oxidative stress. Confocal imaging of LC3 and TOM20 antibody-labeled mitochondria, as well as western blot analysis, demonstrated that mitophagy was further enhanced following resveratrol treatment. In addition, resveratrol was revealed to stimulate the phosphatase and tensin homolog-induced kinase 1/Parkin pathway. Mitophagy inhibition then inhibited the protective effects of resveratrol. These results indicated that resveratrol exerts its protective effects against OGD/R damage, at least in part, by promoting mitophagy.

Resveratrol inhibits oxygen-glucose deprivation-induced MMP-3 expression and cell apoptosis in primary cortical cells via the NF-κB pathway

Resveratrol (Res) or trans-3,4',5-trihydroxystilbene, has been proven to exert neuroprotective effects in cerebral ischemia. The aim of the present study was to investigate whether Res has neuroprotective effects in primary cortical neurons subjected to transient oxygen-glucose deprivation (OGD) via inhibiting the expression of the gene encoding stromelysin-1, also known as matrix metalloproteinase-3 (MMP-3), and via inhibiting cell apoptosis. Primary cortical cells were exposed to OGD, followed by reoxygenation to induce transient ischemia. Res (50 µM) was added into the culture medium during transient ischemia in the presence or absence of the nuclear factor (NF)-κB inhibitor pyrrolidine dithiocarbamate (PDTC; 10 µM) or 500 µM of the nitric oxide (NO) donor NOC-18. Cell viability was assessed using the tetrazolium reduction (MTT) assay. Cell apoptosis was evaluated by flow cytometry. MMP-3 expression was analyzed by western blot and reverse transcription-polymerase chain reaction (RT-PCR), while the levels of inducible NO synthase (iNOS), NF-κB, caspase-3, cleaved caspase-3, B-cell lymphoma 2 (Bcl-2) and Bcl-2-associated X protein (Bax) were assayed by western blot. NO was detected using a spectrophotometric method. We found that the cellular viability was significantly reduced by transient OGD and that this effect was reversed by Res treatment. In addition, OGD was shown to induce cell apoptosis, the expression of Bax and the activation of caspase-3, and inhibit the expression of Bcl-2, and these effects were also reversed by Res treatment. Res treatment significantly reduced the level of MMP-3 that was induced by transient OGD, via inhibition of NF-κB expression. In addition, Res inhibited iNOS expression and NO synthesis that were induced by OGD. MMP-3 expression induced by NO was attenuated by Res treatment and was partially restored by exogenous NO using NOC-18. Taken together, these findings indicate that OGD induces apoptosis through canonical apoptosis signaling and by modulating the expression of MMP-3; Res can reverse the OGD-induced MMP-3 expression and cell apoptosis via the NF-κB-iNOS/NO pathway. Therefore, Res may be a promising agent for the treatment of neuronal injury associated with stroke.

Resveratrol protects primary cortical neuron cultures from transient oxygen-glucose deprivation by inhibiting MMP-9

It was recently shown that resveratrol exerts neuroprotective effects against cerebral ischemia in mice. The aim of the present study was to further confirm these effects in in vitro primary cortical neuron cultures with transient oxygen-glucose deprivation (OGD), and to investigate whether these effects are due to the inhibition of matrix metalloproteinase-9 (MMP-9) and of cell apoptosis. Neuronal primary cultures of cerebral cortex were prepared from BALB/c mice embryos (13-15 days). Cells from 14- to 16-day cultures were subjected to OGD for 3 h, followed by 21 h of reoxygenation to simulate transient ischemia. Different doses of resveratrol were added into the culture medium during the simulation of transient ischemia. The effect of the extracellular signal-regulated kinase (ERK) inhibitor U0126 was studied by adding U0126 (5 µg/µl, 4 µl) into the culture medium during transient ischemia; as a control, we used treatment of cells with 50 µM of resveratrol. Cell viability was investigated using the 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide (MTT) reduction assay. Cell apoptosis was assessed by flow cytometry. The effects of resveratrol on the expression of MMP-9 were analyzed by western blotting and reverse transcription-polymerase chain reaction (RT-PCR), while the levels of ERK, phosphorylated (p)-ERK, cleaved caspase-3, Bax and Bcl-2 were measured by western blotting. The results of the MTT assay showed that cell viability is significantly reduced by transient OGD. OGD induced cell apoptosis, the expression of Bax and the activation of caspase-3 and ERK, inhibited the expression of Bcl-2 and increased the expression of MMP-9, while these effects were reversed by treatment with resveratrol. The therapeutic efficacy of resveratrol was shown to be dose-dependent, with the most suitable dose range determined at 50-100 µM. Treatment with U0126 inhibited MMP-9 and Bax expression and caspase-3 activation, while it further promoted the expression of the anti-apoptotic molecule Bcl-2, suggesting that resveratrol inhibits MMP-9 expression and cell apoptosis by attenuating the activation of ERK1/2. In conclusion, OGD can induce apoptosis through canonical apoptotic signals and by regulating the expression of MMP-9; the anti-apoptotic activity of resveratrol and its inhibitory effect on MMP-9 expression contribute in the reduced activation of ERK.

The nuclear calcium signaling target, activating transcription factor 3 (ATF3), protects against dendrotoxicity and facilitates the recovery of synaptic transmission after an excitotoxic insult

The focal swellings of dendrites ("dendritic beading") are an early morphological hallmark of neuronal injury and dendrotoxicity. They are associated with a variety of pathological conditions, including brain ischemia, and cause an acute disruption of synaptic transmission and neuronal network function, which contribute to subsequent neuronal death. Here, we show that increased synaptic activity prior to excitotoxic injury protects, in a transcription-dependent manner, against dendritic beading. Expression of activating transcription factor 3 (ATF3), a nuclear calcium-regulated gene and member of the core gene program for acquired neuroprotection, can protect against dendritic beading. Conversely, knockdown of ATF3 exacerbates dendritic beading. Assessment of neuronal network functions using microelectrode array recordings revealed that hippocampal neurons expressing ATF3 were able to regain their ability for functional synaptic transmission and to participate in coherent neuronal network activity within 48 h after exposure to toxic concentrations of NMDA. Thus, in addition to attenuating cell death, synaptic activity and expression of ATF3 render hippocampal neurons more resistant to acute dendrotoxicity and loss of synapses. Dendroprotection can enhance recovery of neuronal network functions after excitotoxic insults.

Quantitative imaging of microtubule alteration as an early marker of axonal degeneration after ischemia in neurons

Neuronal death can be preceded by progressive dysfunction of axons. Several pathological conditions such as ischemia can disrupt the neuronal cytoskeleton. Microtubules are basic structural components of the neuronal cytoskeleton that regulate axonal transport and neuronal function. Up-to-date, high-resolution observation of microtubules in living neuronal cells is usually accomplished using fluorescent-based microscopy techniques. However, this needs exogenous fluorescence markers to produce the required contrast. This is an invasive procedure that may interfere with the microtubule dynamics. In this work, we show, for the first time to our knowledge, that by using the endogenous (label-free) contrast provided by second harmonic generation (SHG) microscopy, it is possible to identify early molecular changes occurring in the microtubules of living neurons under ischemic conditions. This is done by measuring the intensity modulation of the SHG signal as a function of the angular rotation of the incident linearly polarized excitation light (technique referred to as PSHG). Our experiments were performed in microtubules from healthy control cultured cortical neurons and were compared to those upon application of several periods of oxygen and glucose deprivation (up to 120 min) causing ischemia. After 120-min oxygen and glucose deprivation, a change in the SHG response to the polarization was measured. Then, by using a three-dimensional PSHG biophysical model, we correlated this finding with the structural changes occurring in the microtubules under oxygen and glucose deprivation. To our knowledge, this is the first study performed in living neuronal cells that is based on direct imaging of axons and that provides the means of identifying the early symptoms of ischemia. Live observation of this process might bring new insights into understanding the dynamics and the mechanisms underlying neuronal degeneration or mechanisms of protection or regeneration.

NMDA receptor modulation by the neuropeptide apelin: implications for excitotoxic injury

Excitotoxic neuronal damage via over-activation of the NMDA receptor has been implicated in many neurodegenerative diseases. In vitro modeling of excitotoxic injury has shown that activation of G-protein coupled receptors (GPCRs) counteracts such injury through modulation of neuronal pro-survival pathways and/or NMDA receptor signaling. We have previously demonstrated that the GPCR APJ and its endogenous neuropeptide ligand apelin can protect neurons against excitotoxicity, but the mechanism(s) of this neuroprotection remain incompletely understood. We hypothesized that apelin can promote neuronal survival by activating pro-survival signaling as well as inhibiting NMDA receptor-mediated excitotoxic signaling cascades. Our results demonstrate that (i) apelin activates pro-survival signaling via inositol trisphosphate (IP(3) ), protein kinase C (PKC), mitogen-activated protein kinase kinase 1/2 (MEK1/2), and extracellular signal-regulated kinase-1/2 (ERK1/2) to protect against excitotoxicity, and (ii) apelin inhibits excitotoxic signaling by attenuating NMDA receptor and calpain activity, and by modulating NMDA receptor subunit NR2B phosphorylation at serine 1480. These studies delineate a novel apelinergic signaling pathway that concurrently promotes survival and limits NMDA receptor-mediated injury to protect neurons against excitotoxicity. Defining apelin-mediated neuroprotection advances our understanding of neuroprotective pathways and will potentially improve our ability to develop therapeutics for excitotoxicity-associated neurodegenerative disorders.

Preconditioning-induced ischemic tolerance: a window into endogenous gearing for cerebroprotection

Ischemic tolerance defines transient resistance to lethal ischemia gained by a prior sublethal noxious stimulus (i.e., preconditioning). This adaptive response is thought to be an evolutionarily conserved defense mechanism, observed in a wide variety of species. Preconditioning confers ischemic tolerance if not in all, in most organ systems, including the heart, kidney, liver, and small intestine. Since the first landmark experimental demonstration of ischemic tolerance in the gerbil brain in early 1990's, basic scientific knowledge on the mechanisms of cerebral ischemic tolerance increased substantially. Various noxious stimuli can precondition the brain, presumably through a common mechanism, genomic reprogramming. Ischemic tolerance occurs in two temporally distinct windows. Early tolerance can be achieved within minutes, but wanes also rapidly, within hours. Delayed tolerance develops in hours and lasts for days. The main mechanism involved in early tolerance is adaptation of membrane receptors, whereas gene activation with subsequent de novo protein synthesis dominates delayed tolerance. Ischemic preconditioning is associated with robust cerebroprotection in animals. In humans, transient ischemic attacks may be the clinical correlate of preconditioning leading to ischemic tolerance. Mimicking the mechanisms of this unique endogenous protection process is therefore a potential strategy for stroke prevention. Perhaps new remedies for stroke are very close, right in our cells.

Resveratrol preconditioning induces cellular stress proteins and is mediated via NMDA and estrogen receptors

Resveratrol pretreatment has been shown to provide neuroprotection in models of cerebral ischemia. This phenomenon, commonly termed preconditioning, promotes ischemic tolerance and may involve mild activation of endoplasmic reticulum stress pathways in the affected tissue. Systemic injection of resveratrol (2 x 10(-3), 2 x 10(-4), 1 x 10(-4) mg/kg) 30 min prior to a 4 h period of right middle cerebral artery occlusion significantly reduced infarct area in the insular region of rat prefrontal cortex. This affect was blocked when resveratrol treatment was combined with a non-selective estrogen receptor antagonist, or preceded by intracortical injection of an NMDA receptor antagonist. The neuroprotective effect of resveratrol was associated with reduced renal sympathetic nerve activity as well as induction of resident endoplasmic reticulum chaperone proteins, glucose-regulated proteins 78 and 94. The calcium-sensitive chaperone heat shock protein 70 and the cysteine protease m calpain did not respond to resveratrol pretreatment. However, a significant induction of heat shock protein 70 was observed in the contralateral cortex of resveratrol pretreated rats following 4 h of right middle cerebral artery occlusion. These data suggest that resveratrol preconditioning promotes ischemic tolerance in the short term, in part via effects mediated through activation of estrogen and NMDA receptors, as well as through mild activation of cellular stress proteins.

Cellular and molecular neurobiology of brain preconditioning

The tolerant brain which is a consequence of adaptation to repeated nonlethal insults is accompanied by the upregulation of protective mechanisms and the downregulation of prodegenerative pathways. During the past 20 years, evidence has accumulated to suggest that protective mechanisms include increased production of chaperones, trophic factors, and other antiapoptotic proteins. In contrast, preconditioning can cause substantial dampening of the organism's metabolic state and decreased expression of proapoptotic proteins. Recent microarray analyses have also helped to document a role of several molecular pathways in the induction of the brain refractory state. The present review highlights some of these findings and suggests that a better understanding of these mechanisms will inform treatment of a number of neuropsychiatric disorders.

Elevated synaptic activity preconditions neurons against an in vitro model of ischemia

Tolerance to otherwise lethal cerebral ischemia in vivo or to oxygen-glucose deprivation (OGD) in vitro can be induced by prior transient exposure to N-methyl-D-aspartic acid (NMDA): preconditioning in this manner activates extrasynaptic and synaptic NMDA receptors and can require bringing neurons to the "brink of death." We considered if this stressful requirement could be minimized by the stimulation of primarily synaptic NMDA receptors. Subjecting cultured cortical neurons to prolonged elevations in electrical activity induced tolerance to OGD. Specifically, exposing cultures to a K(+)-channel blocker, 4-aminopyridine (20-2500 microm), and a GABA(A) receptor antagonist, bicuculline (50 microm) (4-AP/bic), for 1-2 days resulted in potent tolerance to normally lethal OGD applied up to 3 days later. Preconditioning induced phosphorylation of ERK1/2 and CREB which, along with Ca(2+) spiking and OGD tolerance, was eliminated by tetrodotoxin. Antagonists of NMDA receptors or L-type voltage-gated Ca(2+) channels (L-VGCCs) applied during preconditioning decreased Ca(2+) spiking, phosphorylation of ERK1/2 and CREB, and OGD tolerance more effectively when combined, particularly at the lowest 4-AP concentration. Inhibiting ERK1/2 or Ca(2+)/calmodulin-dependent protein kinases (CaMKs) also reduced Ca(2+) spiking and OGD tolerance. Preconditioning resulted in altered neuronal excitability for up to 3 days following 4-AP/bic washout, based on field potential recordings obtained from neurons cultured on 64-channel multielectrode arrays. Taken together, the data are consistent with action potential-driven co-activation of primarily synaptic NMDA receptors and L-VGCCs, resulting in parallel phosphorylation of ERK1/2 and CREB and involvement of CaMKs, culminating in a potent, prolonged but reversible, OGD-tolerant phenotype.

Ischemic tolerance following low dose NMDA involves modulation of cellular stress proteins

Mild NMDA receptor activation is correlated with neuroprotection in models of cerebral ischemia. Neuroprotection with NMDA manifests as a form of ischemic tolerance and involves the induction of cellular stress systems sensitive to disturbances in cellular calcium homeostasis. Unilateral micro-injection of 10, 160 and 320 microM NMDA into the prefrontal cortex of a rat 30 min prior to permanent occlusion of the middle cerebral artery (MCAO) significantly reduced the area of infarct observed after 4 h of ischemia. The highest dose of NMDA (320 microM) prevented the propagation of ischemic damage through a direct toxicity on neuronal tissue adjacent to the injection site as demonstrated in thionin-stained sections. As a result, the degree of ischemia-induced damage was similar to that measured in rats pretreated with the low dose of NMDA (10 microM). Expression of heat shock protein (HSP) 70 and glucose-regulated protein (GRP) 94 in cortical samples taken from the region of infarct following MCAO was significantly reduced in rats pretreated with 10 microM NMDA compared to saline-injected control rats and rats pretreated with higher doses of NMDA. Furthermore, 10 microM NMDA did not appear to influence expression of m-calpain or GRP78, however, higher doses of NMDA did significantly induce expression of both proteins as assessed by Western blotting. In summary, our data demonstrate an in vivo rodent model of ischemic tolerance in which 30 min of neuronal preconditioning with 10 microM NMDA confers protection against a 4 h period of MCAO-induced ischemia. This effect may involve modulation of cellular stress signals, in particular HSP70 and GRP94.

Preconditioning with NMDA protects against toxicity of 3-nitropropionic acid or glutamate in cultured cerebellar granule neurons

A brief sub-lethal ischaemic stimulus has been reported to protect against subsequent ischaemic damage in vivo, and in vitro following periods of hypoxia or oxygen-glucose deprivation (OGD). Preconditioning against neurotoxic stimuli has been linked to N-methyl-d-aspartate (NMDA) receptors, since receptor blockade prevents the protection afforded by OGD, and low doses of NMDA treatment are capable of preconditioning. The current study demonstrated that NMDA preconditioning also protects against 3-nitropropionic acid (3-NPA), a generator of both excitotoxic and oxidative damage, in addition to glutamate. Cerebellar granule neuronal (CGN) cultures prepared from 8-day neonatal Sprague-Dawley rats were maintained for 8 days prior to NMDA stimulation for 6h. At 9 days in vitro (DIV), preconditioned and control cultures were subjected to a toxic insult (1 microM-10 mM glutamate or 1 microM-10 mM 3-NPA). Neuronal viability was assessed by use of a fluorescein diacetate assay. Protection was effective with 100 microM NMDA preconditioning for 6 h against 1-100 microM glutamate, and also against 1-500 microM 3-NPA. The study demonstrates that NMDA preconditioning can be beneficial against excitotoxic treatments, even when these are potentially complicated by associated oxidative damage and metabolic compromise, as is the case for 3-NPA.

Reduced calcium-dependent mitochondrial damage underlies the reduced vulnerability of excitotoxicity-tolerant hippocampal neurons

In central neurons, over-stimulation of NMDA receptors leads to excessive mitochondrial calcium accumulation and damage, which is a critical step in excitotoxic death. This raises the possibility that low susceptibility to calcium overload-induced mitochondrial damage might characterize excitotoxicity-resistant neurons. In this study, we have exploited two complementary models of preconditioning-induced excitotoxicity resistance to demonstrate reduced calcium-dependent mitochondrial damage in NMDA-tolerant hippocampal neurons. We have further identified adaptations in mitochondrial calcium handling that account for enhanced mitochondrial integrity. In both models, enhanced tolerance was associated with improved preservation of mitochondrial membrane potential and structure. In the first model, which exhibited modest neuroprotection, mitochondria-dependent calcium deregulation was delayed, even though cytosolic and mitochondrial calcium loads were quantitatively unchanged, indicating that enhanced mitochondrial calcium capacity accounts for reduced injury. In contrast, the second model, which exhibited strong neuroprotection, displayed further delayed calcium deregulation and reduced mitochondrial damage because downregulation of NMDA receptor surface expression depressed calcium loading. Reducing calcium entry also modified the chemical composition of the calcium-buffering precipitates that form in calcium-loaded mitochondria. It thus appears that reduced mitochondrial calcium loading is a major factor underlying the robust neuroprotection seen in highly tolerant cells.

Moderate growth restriction: deleterious and protective effects on white matter damage

The role for growth restriction in the multifactorial pathophysiology of developing white-matter damage remains debated. We studied rat pups with prenatal growth restriction (GR) induced by unilateral ligation of the uterine artery. Pups with severe GR exhibited white-matter damage that persisted to adulthood [Olivier, P., Baud, O., Evrard, P., Gressens, P.,Verney, C., 2005. Prenatal ischemia and white matter damage in rats. J. Neuropathol. Exp. Neurol. 64, 998-1006]. Moderate GR was associated with diffuse white-matter lesions, microglial activation, and astrogliosis. Loss of pre-oligodendrocytes on postnatal day 7 was followed by a delay in myelination. Following a cortical excitotoxic insult on postnatal day 5, the size of the induced white-matter lesion was smaller in pups with moderate GR and larger in pups with severe GR, compared to normal pups. The increased pre-oligodendrocyte proliferation seen in the white matter of pups with moderate GR subjected to this "double-hit" injury may constitute a heretofore-undescribed neuroprotective mechanism of immature white matter.

Involvement of nitric oxide synthase and ROS-mediated activation of L-type voltage-gated Ca2+ channels in NMDA-induced DPYSL3 degradation

Dihydropyrimidinase-like 3 (DPYSL3), a member of TUC (TOAD-64/Ulip/CRMP), is believed to play a role in neuronal differentiation, axonal outgrowth and possibly in neuronal regeneration. Recently, we have shown that in primary cortical neurons (PCN) NMDA and oxidative stress (H(2)O(2)) caused a calpain-dependent cleavage of DPYSL3 (62 kDa) resulting in the appearance of a lower molecular weight form (60 kDa) of DPYSL3. Our preliminary results had shown that antioxidants significantly reduced NMDA-induced DPYSL3 degradation, indicating involvement of ROS in calpain activation. The aim of this study was to investigate the possible involvement of NOS in NMDA-induced DPYSL3 degradation. We found that NOS inhibitor (L-NAME) significantly prevented NMDA-induced ROS formation, as well as intracellular Ca(2+) increase [Ca(2+)](i), DPYSL3 degradation and cell death. Further, exposure of PCN to NO donor (SNP) resulted in significant [Ca(2+)](i) increase, ROS generation and probable calpain-mediated DPYSL3 truncation. The NMDA- and oxidative stress (ROS)-induced DPYSL3 truncation was totally dependent on extracellular [Ca(2+)](i). While NMDA-induced DPYSL3 truncation was blocked by both NMDA receptor antagonist (MK801) [Kowara, R., Chen, Q., Milliken, M., Chakravarthy, B., 2005. Calpain-mediated degradation of dihydropyrimidinase-like 3 protein (DPYSL3) in response to NMDA and H(2)O(2) toxicity. J. Neurochem. 95 (2), 466-474] and L-VGCC (nimodipine) inhibitors, H(2)O(2)-induced increase in [Ca(2+)](i), ROS generation and DPYSL3 truncation was blocked only by nimodipine. These results indicate that changes in Ca(2+) homeostasis resulting from ROS-dependent activation of L-VGCC are sufficient to induce probable calpain-mediated DPYSL3 truncation and demonstrate for the first time the role of ROS in the mechanism leading to glutamate-induced calpain activation and DPYSL3 protein degradation. The probable calpain-mediated DPYSL3 truncation may have significant impact on its interaction with actin and its assembly, and in turn on growth cone integrity.

Preconditioning-induced activation of ERK5 is dependent on moderate Ca2+ influx via NMDA receptors and contributes to ischemic tolerance in the hippocampal CA1 region of rats

Accumulating evidence implicates activation (phosphorylation) of mitogen-activated protein kinases (MAPK) during nonlethal ischemic preconditioning in the protection of hippocampal CA1 neuron against subsequent ischemic events. In this paper, we undertook to identify the role of extracellular signal regulated kinase (ERK) 5 in cerebral ischemic preconditioning (CIP). Three minutes of ischemia was induced as preconditioning stimulus. Three days later, 6 min of ischemia was induced. The levels of ERK5 protein expression and its activation were detected with or without the CIP in hippocampal CA1 and the dentate gyrus (DG) regions. Our results showed that ERK5 was activated selectively in hippocampal CA1 region with, but not without, the ischemic preconditioning. Notably, during the later phase of reperfusion, the rise in ERK5 activation was strong and persistent with a peak occurring at the third day. The activation peak was effectively prevented and ERK5 protein expression was significantly decreased by intracerebroventricular infusion of ERK5 antisense oligonucleotide (every 24 h for 3 days before the preconditioning), but not by sense oligonucleotide or vehicle. Subsequently, the CA1 neuronal loss was largely elevated. Moreover, both MK801 (10 microM), an antagonist of NMDA receptor, and EGTA (100 mM, but neither 50 nor 150 mM), an extracellular Ca2+ chelator, not only effectively inhibited the ERK5 activation but also markedly abolished CIP-induced survival of the CA1 neurons. These results suggested that activation of the ERK5 pathway by CIP was at least partly dependent on moderate Ca2+ influx via NMDA receptor, which might contribute to ischemic tolerance in hippocampal CA1 region of rats.

Ischaemic preconditioning: therapeutic implications for stroke?

Ischaemic preconditioning (IPC), also known as ischaemic tolerance (IT), is a phenomenon whereby tissue is exposed to a brief, sublethal period of ischaemia, which activates endogenous protective mechanisms, thereby reducing cellular injury that may be caused by subsequent lethal ischaemic events. The first description of this phenomenon was in the heart, which was reported by Murry and co-workers in 1986. Subsequent studies demonstrated IPC in lung, kidney and liver tissue, whereas more recent studies have concentrated on the brain. The cellular mechanisms underlying the beneficial effects of IPC remain largely unknown. This phenomenon, which has been demonstrated by using various injury paradigms in both cultured neurons and animal brain tissue, may be utilised to identify and characterise therapeutic targets for small-molecule, antibody, or protein intervention. This review will examine the experimental evidence demonstrating the phenomenon termed IPC in models of cerebral ischaemia, the cellular mechanisms that may be involved and the therapeutic implications of these findings.

Calpain-mediated truncation of dihydropyrimidinase-like 3 protein (DPYSL3) in response to NMDA and H2O2 toxicity

Dihydropyrimidinase-like protein 3 (DPYSL3), a member of TUC (TOAD-64/Ulip/CRMP), is believed to play a role in neuronal differentiation, axonal outgrowth and, possibly, neuronal regeneration. In primary cortical cultures, glutamate (NMDA) excitotoxicity and oxidative stress (H2O2) caused the cleavage of DPYSL3, resulting in the appearance of a doublet of 62 kDa and 60 kDa. Pre-treatment of cell cultures with calpain inhibitors, but not caspase 3 inhibitor, before exposure to NMDA or H2O2 completely blocked the appearance of the doublet, suggesting calpain-mediated truncation. Furthermore, in vitro digestion of DPYSL3 in cell lysate with purified calpain revealed a cleavage product identical to that observed in NMDA- and H2O2-treated cells, and its appearance was blocked by calpain inhibitors. Analysis of the DPYSL3 protein sequence revealed a possible cleavage site for calpain (Val-Arg-Ser) on the C-terminus of DPYSL3. Collectively, these studies demonstrate for the first time that DPYSL3 is a calpain substrate. The physiological relevance of the truncated DPYSL3 protein remains to be determined.

The dual role of prostaglandin E2 in excitotoxicity and preconditioning-induced neuroprotection

Cyclooxygenase-2 is harmful in models of cerebral ischemia yet plays a protective role in preconditioning-induced ischemic tolerance in the heart. This study examined the mechanisms underlying cyclooxygenase-2-mediated neurotoxicity and preconditioning-induced neuroprotection in an in vitro model of cerebral ischemia. Inhibition of cyclooxygenase-2 protects cortical neuronal cultures from death induced by oxygen-glucose deprivation and reduces oxygen-glucose deprivation-induced increases in intracellular Ca(2+) ([Ca(2+)](i)). In the present study, we determined if prostaglandin E(2) (PGE(2)) is responsible for this cyclooxygenase-2-mediated effect. Rat cortical cultures expressed mRNA for the prostanoid EP(1)-EP(4) receptors. PGE(2) reversed the attenuation in [Ca(2+)](i) and the protection offered by cyclooxygenase-2 inhibition during oxygen-glucose deprivation. These effects likely occur via activation of the prostanoid EP(1) receptor since blocking this receptor during oxygen-glucose deprivation reduced [Ca(2+)](i) and neurotoxicity. Next, we considered if the moderate activation of this pathway, by preconditioning cultures with sub-lethal oxygen-glucose deprivation, influenced the development of tolerance to an otherwise lethal oxygen-glucose deprivation insult, 48 h later. Inhibition of cyclooxygenase-2 during oxygen-glucose deprivation-preconditioning abolished preconditioning-induced protection. Furthermore, cultures were rendered tolerant to oxygen-glucose deprivation by the transient exposure to exogenous PGE(2) 24 h prior to the insult, indicating that this product of the cyclooxygenase-2 pathway is sufficient to induce ischemic tolerance. This study shows that cyclooxygenase-2 and PGE(2) are involved in both oxygen-glucose deprivation-induced neurotoxicity and preconditioning-induced neuroprotection. While neurotoxic in the context of lethal oxygen-glucose deprivation, the moderate activation of this signalling pathway confers ischemic tolerance.

Amphiphilic β-sheet cobra cardiotoxin targets mitochondria and disrupts its network

Recent advance in understanding the role of toxin proteins in controlling cell death has revealed that pro-apoptotic viral proteins targeting mitochondria contain amphiphilic alpha-helices with pore-forming properties. Herein, we describe that the pore-forming amphiphilic beta-sheet cardiotoxins (or cytotoxins, CTXs) from Taiwan cobra (Naja atra) also target mitochondrial membrane after internalization and act synergistically with CTX-induced cytosolic calcium increase to disrupt mitochondria network. It is suggested that CTX-induced fragmentation of mitochondria play a role in controlling CTX-induced necrosis of myocytes and cause severe tissue necrosis in the victims.

Protection by cholesterol-extracting cyclodextrins: a role for N-methyl-d-aspartate receptor redistribution

Cyclodextrins (CDs) are cyclic oligosaccharides composed of a lipophilic central cavity and a hydrophilic outer surface. Some CDs are capable of extracting cholesterol from cell membranes and can affect function of receptors and proteins localized in cholesterol-rich membrane domains. In this report, we demonstrate the neuroprotective activity of some CD derivatives against oxygen-glucose deprivation (OGD), N-methyl-D-aspartic acid (NMDA) and glutamate in cortical neuronal cultures. Although all CDs complexed with NMDA or glutamate, only beta-, methylated beta- and sulfated beta-CDs displayed neuroprotective activity and lowered cellular cholesterol. Only CDs that lowered cholesterol levels redistributed the NMDA receptor NR2B subunit, PSD-95 (postsynaptic density protein 95 kDa) and neuronal nitric oxide synthase (nNOS) from Triton X-100 insoluble membrane domains to soluble fractions. Cholesterol repletion counteracted the ability of methylated beta-CD to protect against NMDA toxicity, and reversed NR2B, PSD-95 and nNOS localization to Triton X-100 insoluble membrane fraction. Surprisingly, neuroprotective CDs had minimal effect on NMDA receptor-mediated increases in intracellular Ca(2+) concentration ([Ca(2+)](i)), but did suppress OGD-induced increases in [Ca(2+)](i). beta-CD, but not Mbeta-CD, also caused a slight block of NMDA-induced currents, suggesting a minor contribution to neuroprotection by direct action on NMDA receptors. Taken together, data suggest that cholesterol extraction from detergent-resistant microdomains affects NMDA receptor subunit distribution and signal propagation, resulting in neuroprotection of cortical neuronal cultures against ischemic and excitotoxic insults. Since cholesterol-rich membrane domains exist in neuronal postsynaptic densities, these results imply that synaptic NMDA receptor subpopulations underlie excitotoxicity, which can be targeted by CDs without affecting overall neuronal Ca(2+) levels.

On the role of Ca2+ in cerebral ischemic preconditioning

Cerebral ischemic preconditioning (IPC) represents a potent endogenous method of inducing tolerance to otherwise lethal ischemia, both in in vivo and in vitro models. Investigation into the mechanism of this phenomenon has yet again transformed the way that neuroscientists view Ca2+. Generally viewed as an agent of neuronal death, particularly within an excitotoxic setting of cerebral ischemia, Ca2+ is now regarded as a key mediator of IPC. Classification of the role of Ca2+ in IPC defies simple description, but seems to possess a stimulatory role during the tolerance-inducing ischemia and an inhibitory or modulatory role during or following the second normally lethal ischemia.

Opposing effects of cyclooxygenase-2 selective inhibitors on oxygen-glucose deprivation-induced neurotoxicity

Cyclooxygenase-2 inhibitors protect against excitotoxicity in vitro yet provide conflicting results in in vivo models of ischemia. To bridge the gap in understanding the discrepancies among these studies, the effects of different cyclooxygenase-2 inhibitors were studied in an in vitro model of ischemia. Oxygen-glucose deprivation (OGD) induced cyclooxygenase-2 protein expression in neuronal cortical cultures. Cyclooxygenase-2 inhibitors exhibited opposing effects on neuronal death induced by OGD. The acidic sulfonamides, N-(2-cyclohexyloxy-4-nitrophenyl) methanesulfonamide (NS-398) and N-(4-nitro-2-phenoxyphenyl)-methanesulfonamide (nimesulide), aggravated neuronal death by enhancing OGD-induced increases in extracellular glutamate and intracellular Ca2+ levels. In contrast, 1-[(4-methylsulfonyl)phenyl]-3-tri-fluoromethyl-5-(4-fluorophenyl)pyrazole (SC-58125) dose-dependently protected cultures against OGD by suppressing increases in extracellular glutamate and intracellular Ca2+ levels. The NS-398-induced aggravation of neuronal death was lost if the inhibitor was added only following the OGD. The timing of inhibitor application also determined its effects on N-methyl-D-aspartate (NMDA)-induced excitoxicity. NS-398 was protective when added both during and post-NMDA exposure, but not if NS-398 was also applied for 60 min prior to the insult. In contrast, SC-58125 afforded protection against NMDA in the presence or absence of a pre-incubation period. This study demonstrates that certain cyclooxygenase-2 inhibitors have opposing effects on neuronal survival depending on the timing of application and the nature of the insult. These results may account for the discrepancies among previous studies which used different inhibitors and different models of neurotoxicity.

Preconditioning of cortical neurons by oxygen-glucose deprivation: tolerance induction through abbreviated neurotoxic signaling

Transient exposure of rat cortical cultures to nonlethal oxygen-glucose deprivation (OGD preconditioning) induces tolerance to otherwise lethal oxygen-glucose deprivation (OGD) or N-methyl-D-aspartate 24 h later. This study evaluates the role of cytosolic and mitochondrial Ca2+-dependent cellular signaling. Mechanistic findings are placed in context with other models of ischemic preconditioning or known neurotoxic pathways within cortical neurons. Tolerance to otherwise lethal OGD is suppressed by performing OGD preconditioning in the presence of the broad-scope catalytic antioxidants Mn(III)tetra(4-carboxyphenyl)porphyrin (MnTBAP) or Zn(II)tetra(4-carboxyphenyl)porphyrin [Zn(II)TBAP], but not by a less active analog, Mn(III)tetra(4-sulfonatophenyl)porphyrin, or a potent superoxide scavenger, Mn(III)tetra(N-ethyl-2-pyridyl)porphyrin chloride. Inhibitors of adenosine A1 receptors, nitric oxide synthase, mitogen-activated protein kinase, and poly(ADP-ribose) polymerase fail to suppress OGD preconditioning despite possible links with reactive oxygen species in other models of ischemic preconditioning. Preconditioning is suppressed by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), which has been ascribed elsewhere to inhibition of superoxide transport to the cytosol through mitochondrial anion channels. However, although it induces mitochondrial Ca2+ uptake, neuronal preconditioning is largely insensitive to mitochondrial uncoupling with carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone or 2,4-dinitrophenol. Un-couplers will prevent production of mitochondrial reactive oxygen species, implying nonmitochondrial targets by MnTBAP, Zn(II)TBAP, and DIDS. Emphasizing the importance of an increase in cytosolic Ca2+ during preconditioning, a Ca2+/calmodulin-dependent protein kinase II inhibitor, KN-62, suppresses development of subsequent tolerance. Summarizing, only those cellular transduction pathways that have the potential to be neurotoxic may be activated by preconditioning in cortical neurons. Finally, a marked decrease in extracellular glutamate is observed during otherwise lethal OGD in preconditioned cultures, suggesting that this end effector may represent a point of convergence across different preconditioning models.

Molecular mechanisms of cerebral ischemia-induced neuronal death

The mode of neuronal death caused by cerebral ischemia and reperfusion appears on the continuum between the poles of catastrophic necrosis and apoptosis: ischemic neurons exhibit many biochemical hallmarks of apoptosis but remain cytologically necrotic. The position on this continuum may be modulated by the severity of the ischemic insult. The ischemia-induced neuronal death is an active process (energy dependent) and is the result of activation of cascades of detrimental biochemical events that include perturbion of calcium homeostasis leading to increased excitotoxicity, malfunction of endoplasmic reticulum and mitochondria, elevation of oxidative stress causing DNA damage, alteration in proapoptotic gene expression, and activation of the effector cysteine proteases (caspases) and endonucleases leading to the final degradation of the genome. In spite of strong evidence showing that brain infarction can be reduced by inhibiting any one of the above biochemical events, such as targeting excitotoxicity, up-regulation of an antiapoptotic gene, or inhibition of a down-stream effector caspase, it is becoming clear that targeting a single gene or factor is not sufficient for stroke therapeutics. An effective neuroprotective therapy is likely to be a cocktail aimed at all of the above detrimental events evoked by cerebral ischemia and the success of such therapeutic intervention relies upon the complete elucidation of pathways and mechanisms of the cerebral ischemia-induced active neuronal death.

Protection of cortical neurons against oxygen-glucose deprivation and N-methyl-D-aspartate by DIDS and SITS

The Cl(-) channel blockers, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) or 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) dose-dependently protected against oxygen-glucose deprivation in cultured rat cortical neurons. DIDS or SITS attenuated oxygen-glucose deprivation-induced increases in extracellular glutamate concentrations and intracellular Ca(2+). DIDS or SITS provided moderate protection against N-methyl-D-aspartate (NMDA) toxicity and decreased NMDA receptor-mediated increases in intracellular Ca(2+). Whole-cell NMDA receptor currents were attenuated 39+/-2% and 21+/-3% by 1 mM DIDS and SITS, respectively. Other Cl(-) channel blockers as equipotent as DIDS and SITS did not decrease oxygen-glucose deprivation- or NMDA-mediated neuronal Ca(2+) influx or toxicity. Neurotoxicity by exogenous glutamate was not prevented by SITS and was exacerbated by DIDS. Reductions in oxygen-glucose deprivation-induced increases in intracellular Ca(2+) levels underlie neuroprotection by DIDS and SITS. This was a reflection of lower extracellular [glutamate], direct inhibition of Ca(2+) influx through postsynaptic NMDA receptors, and possibly through other protective properties associated with DIDS and SITS.