Evidence from cultured rat cortical neurons of differences in the mechanism of ischemic preconditioning of brain and heart

NMDA receptor-driven calcium influx promotes ischemic human cardiomyocyte apoptosis through a p38 MAPK-mediated mechanism

N-methyl-D-aspartate receptor (NMDAR) activity plays a key role in cerebral ischemia. Although NMDAR is also expressed in cardiomyocytes, little research has been performed on NMDAR activity in myocardial ischemia. Here, using an in vitro oxygen-glucose deprivation (OGD) cardiomyocyte model, we evaluated the effects of NMDAR activity upon calcium influx, viability, apoptosis, and investigated the roles of several key mitogen-activated protein kinases (MAPKs). Primary human neonatal cardiomyocytes were cultured under OGD conditions to mimic in vivo ischemic conditions. Enhancing NMDAR activity via NMDA significantly promoted calcium influx, decreased cell viability, increased apoptosis, and enhanced p38 MAPK phosphorylation in OGD cardiomyocytes (all P < 0.05). These effects were rescued by several calcium-channel blockers (ie, MK-801, La³⁺ , Gap26 peptide, 18β-glycyrrhetinic acid) but most potently rescued via the NMDAR-specific antagonist MK-801 or removal of extracellular free calcium (all P < 0.05). Knocking-down p38 MAPK activity by small-molecule inhibition or genetic methods significantly increased cell viability and reduced apoptosis (all P < 0.05). Enhancing p38 MAPK activity abolished MK-801's apoptosis-reducing effects in a p38 MAPK-dependent manner. In conclusion, NMDAR-driven calcium influx promotes apoptosis in ischemic human cardiomyocytes, an effect which can be attributed to enhanced p38 MAPK activity.

N-Methyl-D-Aspartate Receptor-Driven Calcium Influx Potentiates the Adverse Effects of Myocardial Ischemia-Reperfusion Injury Ex Vivo

BACKGROUND

Despite the adverse effects of N-methyl-D-aspartate receptor (NMDAR) activity in cardiomyocytes, no study has yet examined the effects of NMDAR activity under ex vivo ischemic-reperfusion (I/R) conditions. Therefore, our aim was to comprehensively evaluate the effects of NMDAR activity through an ex vivo myocardial I/R rat model.

METHODS

Isolated rat hearts were randomly segregated into 6 groups (n = 20 in each group): (1) an untreated control group; (2) a NMDA-treated control group; (3) an untreated I/R group; (4) an I/R+NMDA group treated with NMDA; (5) an I/R+NMDA+MK-801 group treated with NMDA and the NMDAR inhibitor MK-801; and (6) an I/R+NMDA+[Ca]-free group treated with NMDA and [Ca]-free buffer. The 4 I/R groups underwent 30 minutes of ischemia followed by 50 minutes of reperfusion. Left ventricular pressure signals were analyzed to assess cardiac performance. Myocardial intracellular calcium levels ([Ca]i) were assessed in isolated ventricular cardiomyocytes. Creatine kinase, creatine kinase isoenzyme MB, lactate dehydrogenase, cardiac troponin I, and cardiac troponin T were assayed from coronary effluents. TTC and TUNEL staining were used to measure generalized myocardial necrosis and apoptosis levels, respectively. Western blotting was applied to assess the phosphorylation of PKC-δ, PKC-ε, Akt, and extracellular signal-regulated kinase.

RESULTS

Enhanced NMDAR activity under control conditions had no significant effects on the foregoing variables. In contrast, enhanced NMDAR activity under I/R conditions produced significant increases in [Ca]i levels (∼1.2% increase), significant losses in left ventricular function (∼5.4% decrease), significant multi-fold increases in creatine kinase, creatine kinase isoenzyme MB, lactate dehydrogenase, cardiac troponin I, and cardiac troponin T, significant increases in generalized myocardial necrosis (∼36% increase) and apoptosis (∼150% increase), and significant multi-fold increases in PKC-δ, PKC-ε, Akt, and extracellular signal-regulated kinase phosphorylation (all P < 0.05). These adverse effects were rescued by the NMDAR inhibitor MK-801 or [Ca]-free buffer (all P < 0.05).

CONCLUSIONS

NMDAR-driven calcium influx potentiates the adverse effects of myocardial I/R injury ex vivo.

Brief low [Mg(2+)]o-induced Ca(2+) spikes inhibit subsequent prolonged exposure-induced excitotoxicity in cultured rat hippocampal neurons

Reducing [Mg²⁺]_o to 0.1 mM can evoke repetitive [Ca²⁺]_i spikes and seizure activity, which induces neuronal cell death in a process called excitotoxicity. We examined the issue of whether cultured rat hippocampal neurons preconditioned by a brief exposure to 0.1 mM [Mg²⁺]_o are rendered resistant to excitotoxicity induced by a subsequent prolonged exposure and whether Ca²⁺ spikes are involved in this process. Preconditioning by an exposure to 0.1 mM [Mg²⁺]_o for 5 min inhibited significantly subsequent 24 h exposure-induced cell death 24 h later (tolerance). Such tolerance was prevented by both the NMDA receptor antagonist D-AP5 and the L-type Ca²⁺ channel antagonist nimodipine, which blocked 0.1 mM [Mg²⁺]_o-induced [Ca²⁺]_i spikes. The AMPA receptor antagonist NBQX significantly inhibited both the tolerance and the [Ca²⁺]_i spikes. The intracellular Ca²⁺ chelator BAPTA-AM significantly prevented the tolerance. The nonspecific PKC inhibitor staurosporin inhibited the tolerance without affecting the [Ca²⁺]_i spikes. While Gö6976, a specific inhibitor of PKCα had no effect on the tolerance, both the PKCε translocation inhibitor and the PKCζ pseudosubstrate inhibitor significantly inhibited the tolerance without affecting the [Ca²⁺]_i spikes. Furthermore, JAK-2 inhibitor AG490, MAPK kinase inhibitor PD98059, and CaMKII inhibitor KN-62 inhibited the tolerance, but PI-3 kinase inhibitor LY294,002 did not. The protein synthesis inhibitor cycloheximide significantly inhibited the tolerance. Collectively, these results suggest that low [Mg²⁺]_o preconditioning induced excitotoxic tolerance was directly or indirectly mediated through the [Ca²⁺]_i spike-induced activation of PKCε and PKCξ, JAK-2, MAPK kinase, CaMKII and the de novo synthesis of proteins.

A critical review of mechanisms regulating remote preconditioning-induced brain protection

Remote preconditioning (rPC) is the phenomenon whereby brief organ ischemia evokes an endogenous response such that a different (remote) organ is protected against subsequent, normally injurious ischemia. Experiments show rPC to be effective at evoking cardioprotection against ischemic heart injury and, more recently, neuroprotection against brain ischemia. Such is the enthusiasm for rPC that human studies have been initiated. Clinical trials suggest rPC to be safe (phase II trial) and effective in reducing stroke incidence in a population with high stroke risk. However, despite the therapeutic potential of rPC, there is a large gap in knowledge regarding the effector mechanisms of rPC and how it might be orchestrated to improve outcome after stroke. Here we provide a critical review of mechanisms that are directly attributable to rPC-induced neuroprotection in preclinical trials of rPC.

Posttranslational modifications of lysine and evolving role in heart pathologies-recent developments

The alteration in proteome composition induced by environmental changes and various pathologies is accompanied by the modifications of proteins by specific cotranslational and PTMs. The type and site stoichiometry of PTMs can affect protein functions, alter cell signaling, and can have acute and chronic effects. The particular interest is drawn to those amino acid residues that can undergo several different PTMs. We hypothesize that these selected amino acid residues are biologically rare and act within the cell as molecular switches. There are, at least, 12 various lysine modifications currently known, several of them have been shown to be competitive and they influence the ability of a particular lysine to be modified by a different PTM. In this review, we discuss the PTMs that occur on lysine, specifically neddylation and sumoylation, and the proteomic approaches that can be applied for the identification and quantification of these PTMs. Of interest are the emerging roles for these modifications in heart disease and what can be inferred from work in other cell types and organs.

The role of protein kinase C epsilon in neural signal transduction and neurogenic diseases

Protein kinase C epsilon (PKC ɛ) is one of major isoforms in novel PKC family. Although it has been extensively characterized in the past decade, the role of PKC ɛ in neuron is still not well understood. Advances in molecular biology have now removed significant barriers to the direct investigation of PKC ɛ functions in vivo, and PKC ɛ has been increasingly implicated in the neural biological functions and associated neurogenic diseases. Recent studies have provided important insights into the influence of PKC ɛ on cortical processing at both the single cell level and network level. These studies provide compelling evidence that PKC ɛ could regulate distinct aspects of neural signal transduction and suggest that the coordinated actions of a number of molecular signals contribute to the specification and differentiation of PKC ɛ signal pathway in the developing brain.

Mechanisms and prospects of ischemic tolerance induced by cerebral preconditioning

In the brain, brief episodes of ischemia induce tolerance against a subsequent severe episode of ischemia. This phenomenon of endogenous neuroprotection is known as preconditioning-induced ischemic tolerance. The purpose of this review is to summarize the current state of knowledge about mechanisms and potential applications of cerebral preconditioning and ischemic tolerance. Articles related to the terms ischemic preconditioning and ischemic tolerance were systematically searched via MEDLINE/PubMed, and articles published in English related to the nervous system were selected and analyzed. The past two decades have provided interesting insights into the molecular mechanisms of this neuroprotective phenomenon. Although both rapid and delayed types of tolerance have been documented in experimental settings, the delayed type has been found to be more prominent in the case of neuronal ischemic tolerance. Many intracellular signaling pathways have been implicated regarding ischemic preconditioning. Most of these are associated with membrane receptors, kinase cascades, and transcription factors. Moreover, ischemic tolerance can be induced by exposing animals or cells to diverse types of endogenous and exogenous stimuli that are not necessarily hypoxic or ischemic in nature. These cross-tolerances raise the hope that, in the future, it will be possible to pharmacologically activate or mimic ischemic tolerance in the human brain. Another promising approach is remote preconditioning in which preconditioning of one organ or system leads to the protection of a different (remote) organ that is difficult to target, such as the brain. The preconditioning strategy and related interventions can confer neuroprotection in experimental ischemia, and, thus, have promise for practical applications in cases of vascular neurosurgery and endo-vascular therapy.

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.

The role of the ubiquitin proteasome system in ischemia and ischemic tolerance

Ubiquitin modification targets a protein for rapid degradation by the proteasome. However, polyubiquitination of proteins can result in multiple functions depending on the topology of the ubiquitin chain. Therefore, ubiquitin signaling offers a more complex and versatile biology compared with many other posttranslational modifications. One area of potential for the application of this knowledge is the field of ischemia-induced brain damage, as occurs following a stroke. The ubiquitin proteasome system may exert a dual role on neuronal outcome following ischemia. Harmful ischemia results in an overload of the ubiquitin proteasome system, and blocking the proteasome reduces brain infarction following ischemia. However, the rapid and selective degradation of proteins following brief ischemia results in endogenous protection against ischemia. Therefore, further understanding of the molecular signaling mechanisms that regulate the ubiquitin proteasome system may reveal novel therapeutic targets to reduce brain damage when ischemia is predicted or reduce the activation of the cell death mechanisms and the inflammatory response following stroke. The aim of this review is to discuss some of the recent advances in the understanding of protein ubiquitination and its implications for novel stroke therapies.

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.

epsilonPKC confers acute tolerance to cerebral ischemic reperfusion injury

In response to mild ischemic stress, the brain elicits endogenous survival mechanisms to protect cells against a subsequent lethal ischemic stress, referred to as ischemic tolerance. The molecular signals that mediate this protection are thought to involve the expression and activation of multiple kinases, including protein kinase C (PKC). Here we demonstrate that epsilonPKC mediates cerebral ischemic tolerance in vivo. Systemic delivery of psiepsilonRACK, an epsilonPKC-selective peptide activator, confers neuroprotection against a subsequent cerebral ischemic event when delivered immediately prior to stroke. In addition, activation of epsilonPKC by psiepsilonRACK treatment decreases vascular tone in vivo, as demonstrated by a reduction in microvascular cerebral blood flow. Here we demonstrate the role of acute and transient epsilonPKC in early cerebral tolerance in vivo and suggest that extra-parenchymal mechanisms, such as vasoconstriction, may contribute to the conferred protection.

Estrogen preconditioning protects the hippocampal CA1 against ischemia

Estrogen is neuroprotective against ischemia in both in vivo and in vitro injury models. Because of the promising preclinical data on neuroprotection, the Women's Estrogen for Stroke Trial was initiated. The outcomes from this trial were, however, unsuccessful and questions emerged about the safety of chronic estrogen treatment in women. In contrast to the chronic estrogen treatment strategy, the present study aims to investigate: (1) the neuroprotective efficacy of single estrogen pretreatment/preconditioning; and (2) the existence of a similarity between estrogen- and ischemic preconditioning-induced neuroprotection against cerebral ischemia. The efficacy of estrogen was tested in an in vitro model of cerebral ischemia using hippocampal organotypic slice culture system. The hippocampal organotypic slice cultures were generated from female neonatal (9-11 days old) Sprague-Dawley rats. The slices were exposed to estradiol-17beta (0.5, 1, 5 nM) for various durations (1, 2 or 4 h) 48 h prior to ischemia (40 min of oxygen-glucose deprivation). For ischemic preconditioning, slices were exposed to sublethal oxygen-glucose deprivation (15 min), 48 h prior to lethal oxygen-glucose deprivation. Quantification of cell death in hippocampal CA1 region was conducted by using propidium iodide fluorescence staining technique. Results demonstrated that estrogen preconditioning significantly protects the hippocampal CA1 region against ischemia (P<0.001) and mimicked ischemic preconditioning-induced neuroprotection. The propidium iodide fluorescence values of estrogen preconditioning, ischemic preconditioning and ischemia groups were 21+/-2 (mean+/-S.E.M.) (1 nM; 2 h; n=15), 18+/-2 (5 nM; 4 h; n=12), 32+/-3 (n=8), 65+/-3 (n=27), respectively. Further, estrogen preconditioning initiated a calcium-mediated signaling pathway leading to protection of CA1 neurons against ischemia. Future investigations in estrogen preconditioning may suggest new estrogen regimens that avoid potential side effects of chronic estrogen treatment for stroke patients.

Rapid degradation of Bim by the ubiquitin-proteasome pathway mediates short-term ischemic tolerance in cultured neurons

A previous exposure to a non-harmful ischemic insult (preconditioning) protects the brain against subsequent harmful ischemia (ischemic tolerance). In contrast to delayed gene-mediated ischemic tolerance, little is known about the molecular mechanisms that regulate rapid ischemic tolerance, which occurs within 1 h following preconditioning. Here we have investigated the degradation of the pro-apoptotic Bcl-2 family member Bim as a mechanism of rapid ischemic tolerance. Bim protein levels were reduced 1 h following preconditioning and occurred concurrent with an increase in Bim ubiquitination. Ubiquitinated proteins are degraded by the proteasome, and inhibition of the proteasome with MG132 (a proteasome inhibitor) prevented Bim degradation and blocked rapid ischemic tolerance. Inhibition of p42/p44 mitogen-activated protein kinase activation by U0126 reduced Bim ubiquitination and Bim degradation and blocked rapid ischemic tolerance. Finally, inhibition of Bim expression using antisense oligonucleotides also reduced cell death following ischemic challenge. Our results suggest that following preconditioning ischemia, Bim is rapidly degraded by the ubiquitin-proteasome system, resulting in rapid ischemic tolerance. This suggests that the rapid degradation of cell death-promoting proteins by the ubiquitin-proteasome pathway may represent a novel therapeutic strategy to reduce cell damage following neuropathological insults, e.g. stroke.

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.

The Role of Protein Kinase C in Cerebral Ischemic and Reperfusion Injury

Background and Purpose— Stroke is a leading cause of disability and death in the United States, yet limited therapeutic options exist. The need for novel neuroprotective agents has spurred efforts to understand the intracellular signaling pathways that mediate cellular response to stroke. Protein kinase C (PKC) plays a central role in mediating ischemic and reperfusion damage in multiple tissues, including the brain. However, because of conflicting reports, it remains unclear whether PKC is involved in cell survival signaling, or mediates detrimental processes.

Summary of Review— This review will examine the role of PKC activity in stroke. In particular, we will focus on more recent insights into the PKC isozyme-specific responses in neuronal preconditioning and in ischemia and reperfusion-induced stress.

Conclusion— Examination of PKC isozyme activities during stroke demonstrates the clinical promise of PKC isozyme-specific modulators for the treatment of cerebral ischemia.

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.

Short-term serum supplementation improves glucose-oxygen deprivation in primary cortical cultures grown under serum-free conditions

Brain ischemia can be studied in vitro by depriving primary neurons of oxygen and glucose by replacing oxygen with argon and glucose with its antimetabolite 2-deoxy-D-glucose. In this contribution, we explain how to construct a reliably functioning ischemia chamber and use it to study neuronal cell death in neuron-enriched fetal primary cortical cultures grown under serum-free conditions. We observed that these cultures exhibited a significant cell death even during exposure to oxygenated balanced salt solution used as control for oxygen-glucose deprivation. We show that addition of only 2% fetal calf serum 24 h prior, during, and after treatment almost abolished this undesirable cell loss and proportionally increased cell death induced by oxygen-glucose deprivation. Western blots and immunocytochemistry showed that these effects were mainly due to an increase in neuronal viability under control conditions accompanied by a limited glial proliferation independent of the treatment condition. Under these modified conditions, the cultures could also still be effectively preconditioned by a short-term oxygen-glucose deprivation. In summary, this modified protocol combines the advantages of serum-free neuronal culture, where potentially toxic antimitotic substances can be omitted, with a serum-mediated protection of neurons against unspecific factors and concomitant sensitization for oxygen-glucose deprivation.

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.

CREB-mediated Bcl-2 protein expression after ischemic preconditioning

Bcl-2 plays a pivotal role in the control of cell death and is upregulated by ischemic tolerance. Because Bcl-2 expression is regulated by the transcription factor cyclic AMP response element-binding protein (CREB), we investigated the role of CREB activation in two models of ischemic preconditioning: focal ischemic tolerance after middle cerebral artery occlusion (MCAO) and in vitro ischemic tolerance modeled by oxygen-glucose deprivation (OGD). After preconditioning ischemia (30 minutes MCAO or 30 minutes OGD), phosphorylation of CREB was increased, and there was an increased interaction between the bcl-2 cyclic AMP-responsive element (CRE) promoter and nuclear proteins after preconditioning ischemia in vivo and in vitro. Chromatin immunoprecipitation revealed an increased interaction between CREB-binding protein and the bcl-2 CRE rather than CREB, after preconditioning ischemia. Ischemic tolerance was blocked by a CRE decoy oligonucleotide, which also blocked Bcl-2 expression. The protein kinase A inhibitor H89, the calcium/calmodulin kinase inhibitor KN62, and the MEK inhibitor U0126 blocked ischemic tolerance, but not the phosphatidylinositol 3-kinase inhibitor LY294002. H89, KN62, and U0126 reduced CREB activation and Bcl-2 expression. Taken together, these data suggest that after ischemic preconditioning CREB activation regulates the expression of the prosurvival protein Bcl-2.

Oxygen-sensitive {delta}-opioid receptor-regulated survival and death signals: novel insights into neuronal preconditioning and protection

The detrimental effect of severe hypoxia (SH) on neurons can be mitigated by hypoxic preconditioning (HPC), but the molecular mechanisms involved remain unclear, and an understanding of these may provide novel solutions for hypoxic/ischemic disorders (e.g. stroke). Here, we show that the delta-opioid receptor (DOR), an oxygen-sensitive membrane protein, mediates the HPC protection through specific signaling pathways. Although SH caused a decrease in DOR expression and neuronal injury, HPC induced an increase in DOR mRNA and protein levels and reversed the reduction in levels of the endogenous DOR peptide, leucine enkephalin, normally seen during SH, thus protecting the neurons from SH insult. The HPC-induced protection could be blocked by DOR antagonists. The DOR-mediated HPC protection depended on an increase in ERK and Bcl 2 activity, which counteracted the SH-induced increase in p38 MAPK activities and cytochrome c release. The cross-talk between ERK and p38 MAPKs displays a "yinyang" antagonism under the control of the DOR-G protein-protein kinase C pathway. Our findings demonstrate a novel mechanism of HPC neuroprotection (i.e. the intracellular up-regulation of DOR-regulated survival signals).

Development of an ischemic tolerance model in a PC12 cell line

Although ischemic tolerance has been described in a variety of primary cell culture systems, no similar in vitro models have been reported with any cell line. A model of ischemic preconditioning in the rat pheochromocytoma PC12 cell line is described here. When compared to nonpreconditioned cells, preexposure of PC12 cells to 6 hours of oxygen and glucose deprivation (OGD) significantly increased cell viability after 15 hours of OGD 24 hours later. Flow cytometry analysis of cells labeled with specific markers for apoptosis, Annexin V, and Hoechst 33342, and of DNA content, revealed that apoptosis is involved in OGD-induced PC12 cell death and that preconditioning of the cells mainly counteracts the effect of apoptosis. Immunocytochemistry of caspase-3, a central executioner in the apoptotic process, further confirmed the activation of apoptotic pathways in OGD-induced PC12 cell death. This model may be useful to investigate the cellular mechanisms involved in neuronal transient tolerance following ischemia.

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.

Neuroprotective gene expression profiles in ischemic cortical cultures preconditioned with IGF-1 or bFGF

The mechanisms underlying growth factor preconditioning of neurons are only partially elucidated, and no studies have been conducted in this area using a gene profiling approach. We used cDNA microarrays to compare the transcriptional profiles of cells preconditioned either with insulin-like growth factor I (IGF-1) or basic fibroblast growth factor (bFGF), to identify differentially regulated genes that may function in growth factor signaling, response to oxygen-glucose deprivation (OGD), and most importantly, cell survival. Primary rat cortical cultures were treated with bFGF or IGF-1 for 2, 24, or 24 h followed by OGD for 90 min, and compared with cells that were subject to OGD without growth factor pretreatment. Although the majority of surveyed genes were unchanged in all experimental treatments, 175 genes (10% of the cDNAs on the chip) were found to be differentially regulated in at least one of the treatment conditions. Hierarchical clustering of these 175 genes was used to identify four expression clusters: IGF-1 regulated, bFGF regulated, OGD regulated, and putative neuroprotective genes. Further analysis using realtime RT-PCR confirmed that we had identified genes that are regulated by single growth factors, as well as several more that are co-regulated by both IGF-1 and bFGF. These genes can influence neuronal survival by affecting diverse pathways such as growth factor signal transduction (CD44, DTR, DUSP6, EPS8, IGFBP3), DNA repair and transcription (FOXJ1), metabolic homeostasis (RASA1, SHMT2), cytoskeletal stability (MSN, MAPT) and cholesterol biosynthesis (FDFT1, FDPS).

Cell-specific role for epsilon- and betaI-protein kinase C isozymes in protecting cortical neurons and astrocytes from ischemia-like injury

Activation of epsilon protein kinase C (epsilonPKC) has been shown to protect cardiac myocytes against ischemia and reperfusion injury. However, the role of PKC in ischemic brain injury is less well defined. Western blot analysis of murine neurons and astrocytes in primary culture demonstrated epsilon- and betaIPKC expression in both cell types. Activation of epsilonPKC increased in neuronal cultures in response to the ischemia-like insult of oxygen-glucose deprivation (OGD). Isozyme-specific peptide activators or inhibitors of PKC were applied at various times before, during and after the OGD period. Neuron-astrocyte mixed cultures pretreated with a selective epsilonPKC activator peptide showed a significant reduction in neuronal injury after OGD and reperfusion, compared to cultures pretreated with control peptide. The epsilonPKC activator peptide counteracted the increased damage induced by pretreatment with the epsilonPKC-selective inhibitor peptide in relatively pure neuronal cultures subjected to OGD. Neither epsilonPKC activator nor inhibitor peptides affected injury of neurons when applied after OGD onset. In contrast, the betaIPKC-selective inhibitor peptide increased injury in astrocyte cultures exposed to OGD at all application times tested. Our data demonstrate protection of neurons by selective activation of epsilonPKC but enhanced astrocyte cell death with selective inhibition of betaIPKC. Thus PKC isozymes exhibit cell type-specific effects on ischemia-like injury.

Epsilon protein kinase C mediated ischemic tolerance requires activation of the extracellular regulated kinase pathway in the organotypic hippocampal slice

Ischemic preconditioning (IPC) promotes brain tolerance against subsequent ischemic insults. Using the organotypic hippocampal slice culture, we conducted the present study to investigate (1) the role of adenosine A1 receptor (A1AR) activation in IPC induction, (2) whether epsilon protein kinase C (epsilonPKC) activation after IPC is mediated by the phosphoinositol pathway, and (3) whether epsilonPKC protection is mediated by the extracellular signal-regulated kinase (ERK) pathway. Our results demonstrate that activation of A1AR emulated IPC, whereas blockade of the A1AR during IPC diminished neuroprotection. The neuroprotection promoted by the A1AR was also reduced by the epsilonPKC antagonist. To determine whether epsilonPKC activation in IPC and A1AR preconditioning is mediated by activation of the phosphoinositol pathway, we incubated slices undergoing IPC or adenosine treatment with a phosphoinositol phospholipase C inhibitor. In both cases, preconditioning neuroprotection was significantly attenuated. To further characterize the subsequent signal transduction pathway that ensues after epsilonPKC activation, mitogen-activated protein kinase kinase was blocked during IPC and pharmacologic preconditioning (PPC) (with epsilonPKC, NMDA, or A1AR agonists). This treatment significantly attenuated IPC- and PPC-induced neuroprotection. In conclusion, we demonstrate that epsilonPKC activation after IPC/PPC is essential for neuroprotection against oxygen/glucose deprivation in organotypic slice cultures and that the ERK pathway is downstream to epsilonPKC.

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.

Ischemic preconditioning in the brain

PURPOSE OF REVIEW

Brain ischemia is responsible for significant morbidity and mortality associated with cardiovascular surgery, and is the end result of multiple disease states, including cardiac arrest, stroke, and traumatic brain injury. Despite significant resources dedicated to developing neuroprotective strategies, little progress has been made in this regard. Neuronal ischemic preconditioning is an endogenous neuroprotective strategy that provides sustained and robust ischemic tolerance. Identification of the mechanisms responsible for mediating the preconditioning response may offer novel therapeutic targets and further our understanding of the natural adaptations to brain injury.

RECENT FINDINGS

Recent research efforts have elucidated many intracellular signaling pathways that ultimately lead to ischemic tolerance after a preconditioning stimulus. Most of these are associated with glutamate receptor signal transduction, the intracellular kinases, and several transcription regulators. Microarray analysis has identified several gene families that warrant further investigation to identify novel candidates for neuroprotective therapies. These include genes involved in synaptic architecture and signal propagation, cell cycle and transcription regulators, and mediators of apoptosis such as the heat shock proteins and anti-apoptotic mitochondrial proteins.

SUMMARY

Neuronal ischemic preconditioning is an endogenous mechanism that leads to robust neuroprotection from ischemia. Identification of the upstream pathways that initiate preconditioning and candidate genes that mediate this phenomenon may offer novel therapeutic targets, with applicability to a variety of disease states and perioperative complications.

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.

Hypothermic preconditioning induces rapid tolerance to focal ischemic injury in the rat

Stressful, preconditioning stimuli can elicit rapid and delayed forms of tolerance to ischemic injury. The identification and characterization of preconditioning stimuli that are effective, but relatively benign, could enhance the clinical applicability of induced tolerance. This study examines the efficacy of brief hypothermia as a preconditioning stimulus for inducing rapid tolerance. Rats were administered hypothermic preconditioning or sham preconditioning and after an interval of 20-120 min were subjected to transient focal ischemia using a three-vessel occlusion model. The volume of cerebral infarction was measured 24 h or 7 days after ischemia. In other experiments, the depth or duration of the hypothermic stimulus was manipulated, or a protein synthesis inhibitor (anisomycin) was administered. Twenty minutes of hypothermia delivered 20 or 60 (but not 120) min prior to ischemia significantly reduces cerebral infarction. The magnitude of protection is enhanced with deeper levels of hypothermia, but is not affected by increasing the duration of the hypothermic stimulus. Treatment with a protein synthesis inhibitor does not block the induction of rapid tolerance. Hypothermic preconditioning elicits a rapid form of tolerance to focal ischemic injury. Unlike delayed tolerance induced by hypothermia, rapid tolerance is not dependent on either de novo protein synthesis or the duration of the preconditioning stimulus. These findings suggest that the mechanisms underlying rapid and delayed tolerance induced by hypothermia differ fundamentally. Brief hypothermia could provide a rapid means of inducing transient tissue protection in the context of predictable ischemic events.

Preconditioned resistance to oxygen-glucose deprivation-induced cortical neuronal death: alterations in vesicular GABA and glutamate release

Central neurons exposed to several types of sublethal stress, including ischemia, acquire resistance to injury induced by subsequent ischemic insults, a phenomenon called ischemic preconditioning. We modeled this phenomenon in vitro, utilizing exposure to 45 mM KCl to reduce the vulnerability of cultured murine cortical neurons to subsequent oxygen-glucose deprivation. Twenty-four hours after preconditioning, cultures exhibited enhanced depolarization-induced, tetanus toxin-sensitive GABA release and a modest decrease in glutamate release. Total cellular GABA levels were unaltered. Inhibition of GABA degradation with the GABA transaminase inhibitor (+/-)-gamma-vinyl GABA, or addition of low levels of GABA, muscimol, or chlormethiazole to the bathing medium, mimicked the neuroprotective effect of preconditioning against oxygen-glucose deprivation-induced death. However, neuronal death was enhanced by higher levels of these manipulations, as well as by prior selective destruction of GABAergic neurons by kainate. Finally, selective blockade of GABA(A) receptors during oxygen-glucose deprivation or removal of GABAergic neurons eliminated the neuroprotective effects of prior preconditioning. Taken together, these data predict that presynaptic alterations, specifically enhanced GABA release together with reduced glutamate release, may be important mediators of ischemic preconditioning, but suggest caution in regard to interventions aimed at increasing GABA(A) receptor activation.

Epsilon PKC is required for the induction of tolerance by ischemic and NMDA-mediated preconditioning in the organotypic hippocampal slice

Glutamate receptors and calcium have been implicated as triggering factors in the induction of tolerance by ischemic preconditioning (IPC) in the brain. However, little is known about the signal transduction pathway that ensues after the IPC induction pathway. The main goals of the present study were to determine whether NMDA induces preconditioning via a calcium pathway and promotes translocation of the protein kinase C epsilon (epsilonPKC) isozyme and whether this PKC isozyme is key in the IPC signal transduction pathway. We corroborate here that IPC and a sublethal dose of NMDA were neuroprotective, whereas blockade of NMDA receptors during IPC diminished IPC-induced neuroprotection. Calcium chelation blocked the protection afforded by both NMDA and ischemic preconditioning significantly, suggesting a significant role of calcium. Pharmacological preconditioning with the nonselective PKC isozyme activator phorbol myristate acetate could not emulate IPC, but blockade of PKC activation with chelerythrine during IPC blocked its neuroprotection. These results suggested that there might be a dual involvement of PKC isozymes during IPC. This was corroborated when neuroprotection was blocked when we inhibited epsilonPKC during IPC and NMDA preconditioning, and IPC neuroprotection was emulated with the activator of epsilonPKC. The possible correlation between NMDA, Ca2+, and epsilonPKC was found when we emulated IPC with the diacylglycerol analog oleoylacetyl glycerol, suggesting an indirect pathway by which Ca2+ could activate the calcium-insensitive epsilonPKC isozyme. These results demonstrated that the epsilonPKC isozyme played a key role in both IPC- and NMDA-induced tolerance.

Protein kinase C-epsilon is involved in the adenosine-activated signal transduction pathway conferring protection against ischemia-reperfusion injury in primary rat neuronal cultures

Adenosine activates a signal transduction pathway (STP) in the heart and the brain, conferring protection against ischemia-reperfusion insult. Activation of protein kinase C (PKC), probably mainly PKC-epsilon, has been demonstrated to be part of the heart STP, but its role in the neuronal pathway is less clear. Here, we provide proof for the participation of PKC-epsilon in the neuronal adenosine-activated STP. Primary rat neuronal cultures were exposed to chemical ischemia by iodoacetate, followed by reperfusion. The cultured neurons were protected against this insult by activation of the adenosine mechanism, by N6-(R)-phenylisopropyladenosine [R(-)-PIA], a specific A1 adenosine receptor agonist. Exposure of the cultures to bisindolylmaleimide I, a highly selective PKC inhibitor, abrogated the protection. The exposure of the cultures to R(-)-PIA was found to result in phosphorylation (activation) of PKC-epsilon. Furthermore, insertion into the cells of a specific peptide inhibitor of PKC-epsilon translocation (epsilonV1-2), also abrogated the protection conferred by R(-)-PIA. These results demonstrate that activation of PKC-epsilon is a vital step in the neuronal adenosine-activated STP.

Preconditioning and adenosine protect human proximal tubule cells in an in vitro model of ischemic injury

Renal ischemic reperfusion injury results in unacceptably high mortality and morbidity during the perioperative period. It has been recently demonstrated that ischemic preconditioning or adenosine receptor modulations attenuate renal ischemic reperfusion injury in vivo. An in vitro model of ischemic renal injury was used in cultured human proximal tubule (HK-2) cells to further elucidate the protective signaling cascades against renal ischemic reperfusion injury. ATP depletion preconditioning (1 h of antimycin A and 2-deoxyglucose treatment followed by 1 h of recovery), adenosine, an A(1) adenosine receptor selective agonist, or an A(2a) adenosine receptor selective agonist significantly attenuated subsequent severe ATP depletion injury of HK-2 cells. In contrast, an adenosine receptor antagonist failed to prevent protection induced by ATP depletion preconditioning. Cytoprotection by ATP depletion preconditioning or A(1) adenosine receptor activation was prevented by inhibitors of extracellular signal-regulated mitogen-activated kinases, protein kinase C, and tyrosine kinases. The A(1) and A(2a) adenosine receptor-mediated cytoprotection were also dependent on G(i/o) proteins and PKA activation, respectively. It is concluded that ATP depletion preconditioning and A(1) and A(2a) adenosine receptor activation protect HK-2 cells against severe ATP depletion injury via distinct signaling pathways.

Improvement in neuronal survival after ischemic preconditioning in hippocampal slice cultures

The main goals of the current study were to assess: (a) whether a sublethal ischemic insult could protect the CA1 subregion of the hippocampus in organotypic slices against a lethal ischemic insult; and (b) whether this protection is long lasting as determined with an accurate immunohistochemical neuronal marker, NeuN. Hippocampal slice cultures were grown for 12-14 days in vitro. Slices were exposed either to oxygen/glucose deprivation (OGD) for 45 min (ischemia), or OGD for 15 min (ischemic preconditioning), 48 h prior to 45 min OGD, or were untreated (sham). Cell death was estimated by propidium iodide fluorescence 1 day after OGD and by NeuN immunohistochemistry 7 days after OGD. Image analysis was employed to measure the relative optical density of the NeuN-signal in all groups. After ischemia, damaged neurons were shrunken or lost and NeuN immunoreactivity was reduced. Relative optical density of NeuN (ROD [NeuN]) was 0.193+/-0.015 in control (sham) (n=9). In slices that underwent ischemia, ROD [NeuN] declined to 0.108+/-0.018 (n=5) in CA1 (*P<0.05 ROD [NeuN] in preconditioned slice cultures was 0.190+/-0.037 (76% higher than the ischemia group). Similar results were found after measuring PI fluorescence. In the CA1 sub-region, PI fluorescence was about 13, 47 and 17% in the sham, ischemic and IPC groups, respectively. We suggest that the immunohistochemical approach validates the dye uptake method used in slice cultures and yields quantitative data specific for neurons. We also conclude that the organotypic hippocampal slice model is useful for studying delayed ischemic preconditioning that is maintained for hours or days after the preconditioning event.

Characteristics of hypothermic preconditioning influencing the induction of delayed ischemic tolerance

OBJECT

A brief period of hypothermia has recently been shown to induce delayed tolerance to ischemic brain injury. This form of tolerance is initiated several hours after hypothermic preconditioning (HPC) and persists for a few days. Hypothermia-induced tolerance could provide a means for limiting cellular injury during predictable periods of ischemia, such as those that occur during many surgical procedures. The purpose of this study was to characterize the parameters of HPC that regulate the induction of delayed tolerance.

METHODS

The general design of the experiments was to perform HPC or a sham procedure on adult Sprague-Dawley rats. Twenty-four hours later, the animals were subjected to a transient period of ischemia induced by a 1-hour period of three-vessel occlusion. Infarct volume was assessed 24 hours postischemia. In the first series of experiments, the depth of global (that is, whole-body) HPC was set at 25.5, 28.5, or 31.5 degrees C, and the duration of HPC was fixed at 20 minutes. In the second series of experiments, the duration of global HPC was set at 20, 60, 120, or 180 minutes, and the depth of HPC was set at 33 or 34.5 degrees C. In the third series of experiments, focal HPC was administered by selectively cooling the head to achieve a cortical temperature of 28.5 or 31.5 degrees C for 20 minutes, with the duration of HPC fixed at 20 minutes. The magnitude of tolerance induced by HPC was dependent on the depth and duration of the hypothermic stimulus. The parameters of hypothermia that are capable of inducing tolerance are similar to, or less severe than, those already in clinical use during intraoperative procedures. Focal cooling was as effective as global cooling for eliciting tolerance, indicating that it is possible to establish tolerance while limiting the potential complications of systemic hypothermia.

CONCLUSIONS

The results of these experiments indicate that HPC may provide an effective and safe means for limiting cellular injury resulting from predictable periods of central nervous system ischemia.

Activation of adenosine triphosphate-sensitive potassium channels confers protection against rotenone-induced cell death: therapeutic implications for Parkinson's disease

It is anticipated that further understanding of the protective mechanism induced by ischemic preconditioning will improve prognosis for patients of ischemic injury. It is not known whether preconditioning exerts beneficial actions in neurodegenerative diseases, in which ischemic injury plays a causative role. Here we show that transient activation of ATP-sensitive potassium channels, a trigger in ischemic preconditioning signaling, confers protection in PC12 cells and SH-SY5Y cells against neurotoxic effect of rotenone and MPTP, mitochondrial complex I inhibitors that have been implicated in the pathogenesis of Parkinson's disease. The degree of protection is in proportion to the bouts of exposure to an ATP-sensitive potassium channel opener, a feature reminiscent of ischemic tolerance in vivo. Protection is sensitive to a protein synthesis inhibitor, indicating the involvement of de novo protein synthesis in the protective processes. Pretreatment of PC12 cells with preconditioning stimuli FeSO(4) or xanthine/xanthine oxidase also confers protection against rotenone-induced cell death. Our results demonstrate for the first time the protective role of ATP-sensitive potassium channels in a dopaminergic neuronal cell line against rotenone-induced neurotoxicity and conceptually support the view that ischemic preconditioning-derived therapeutic strategies may have potential and feasibility in therapy for Parkinson's disease.

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

In vitro ischemic preconditioning induced by subjecting rat cortical cultures to nonlethal oxygen-glucose deprivation protects against a subsequent exposure to otherwise lethal oxygen-glucose deprivation. We provide evidence that attenuation of the postsynaptic N-methyl-D-aspartate (NMDA) receptor- and Ca(2+)-dependent neurotoxicity underlies oxygen-glucose deprivation tolerance. It is demonstrated that extended tolerance to otherwise lethal NMDA or oxygen-glucose deprivation can be induced by either of their sublethal forms of preconditioning. These four pathways are linked, since NMDA receptor blockade during preconditioning by oxygen-glucose deprivation eliminates tolerance. These results suggest that NMDA tolerance, induced by nonlethal activation of these receptors during oxygen-glucose deprivation preconditioning, underlies oxygen-glucose deprivation tolerance. Several neurotoxic downstream Ca(2+)-dependent signaling events specifically linked to NMDA receptor activation are attenuated during otherwise lethal oxygen-glucose deprivation in preconditioned cultures. Specifically, calpain activation, as well as degradation of microtubule-associated protein-2 and postsynaptic density-95, are attenuated 2 h following otherwise lethal NMDA treatment alone or oxygen-glucose deprivation in preconditioned cultures. Formation of microtubule-associated protein-2-labeled dendritic varicosities is also attenuated in preconditioned cultures within 1 h of lethal oxygen-glucose deprivation or NMDA application. Intracellular Ca(2+) levels, measured using the high- or low-affinity dyes Fluo-4 (K(d) approximately equal 345 nM) or Fluo-4FF (K(d) approximately equal 9.7 microM) respectively, are markedly attenuated during lethal oxygen-glucose deprivation in preconditioned cultures.Collectively, the results suggest the attenuation of the postsynaptic NMDA-mediated component of otherwise lethal oxygen-glucose deprivation through the suppression of Ca(2+)-dependent neurotoxic signaling, a mechanism that is initially induced by transient nonlethal activation of this receptor during ischemic preconditioning.

Cortical spreading depression transiently activates MAP kinases

Cortical spreading depression (CSD) has been shown to have neuroprotective effects when administered in advance of cerebral ischemia. The mechanism by which CSD induces its neuroprotective effect however remains to be elucidated. Since MAP kinases have been shown to impart neuroprotection in ischemic preconditioning paradigms, we attempted to determine the role CSD may have in the activation of MAPK. We show that CSD is capable of increasing the phosphorylation of ERK in a MEK-dependent manner. This phosphorylation is, however, transient, as phosphorylated ERK levels return to control levels 45 min after 2 h of CSD elicitation. Immunohistochemical analysis reveals that the phosphorylated form of ERK is located ubiquitously in cells of the CSD-treated cortex while CSD-elicited MEK phosphorylation resides solely in the nuclei. These data suggest that CSD may act via the MAP kinase pathways to mediate preconditioning.

Protein kinase C isoforms as therapeutic targets in nervous system disease states

Neuronal tissues display high levels of protein kinase C (PKC) activity and isoform expression. The activation of this enzymatic system is important in the control of short and long term brain functions (ion channel regulation, receptor modulation, neurotransmitter release, synaptic potentiation/depression, neuronal survival) that are related to diverse brain pathologies. This review will describe recent developments in PKC regulation and changes in levels, isoforms and activation in acute and chronic neurodegenerative pathologies as well as in affective and psychic disorders. The recent availability of isoform selective inhibitors and activators may help to understand better the relevance of PKC in central nervous system (CNS) physiology and pathology and to identify new and safer pharmacologic strategies to be tested in different disease states.

Attenuation of neurotoxicity in cortical cultures and hippocampal slices from E2F1 knockout mice

The E2F1 transcription factor modulates neuronal apoptosis induced by staurosporine, DNA damage and beta-amyloid. We demonstrate E2F1 involvement in neuronal death induced by the more physiological oxygen-glucose deprivation (OGD) in mouse cortical cultures and by anoxia in mouse hippocampal slices. E2F1(+/+) and (-/-) cultures were comparable, in that they contained similar neuronal densities, responded with similar increases in intracellular calcium concentration ([Ca(2+)]i) to glutamate receptor agonists, and showed similar NMDA receptor subunit mRNA expression levels for NR1, NR2A and NR2B. Despite these similarities, E2F1(-/-) cultures were significantly less susceptible to neuronal death than E2F1(+/+) cultures 24 and 48 h following 120-180 min of OGD. Furthermore, the absence of E2F1 significantly improved the ability of CA1 neurons in hippocampal slices to recover synaptic transmission following a transient anoxic insult in vitro. These results, along with our finding that E2F1 mRNA levels are significantly increased following OGD, support a role for E2F1 in the modulation of OGD- and anoxia-induced neuronal death. These findings are consistent with studies showing that overexpression of E2F1 in postmitotic neurons causes neuronal degeneration and the absence of E2F1 decreases infarct volume following cerebral ischemia.

2-deoxyglucose enhances epileptic tolerance evoked by transient incomplete brain ischemia in mice

The aim of the study was to assess the influence of chronic treatment with a non-metabolisable glucose analogue, 2-deoxyglucose (2-DG) at a 150 mg/kg dose on long-term epileptic tolerance (ET) evoked by 30 min bilateral carotid artery clamping (BCCA) in mice. The effects of protein synthesis inhibition with cycloheximide (CHX), given in three daily doses of 2.5 mg/kg starting either 1 day before (peri-insult regimen) or 1 day after the priming insult (post-insult regimen), on ET development was also studied. Seizures were induced 14 days after BCCA with 3.5 mg/kg of bicuculline; this dose (CD97) evokes convulsions in 97% of normal untreated mice. BCCA resulted in decreased mortality, prolonged latency to the onset of generalised convulsions and decreased overall seizure score. CHX given in the post-insult regimen did not influence, while the peri-insult regimen abolished, all signs of BCCA-evoked ET. 2-DG treatment of sham-operated animals resulted in a moderate but significant decrease in mortality rate and a tendency toward a lower seizure score. BCCA combined with 2-DG treatment resulted in a marked decrease in mortality rate, as well as reduction in all indicators of seizure susceptibility. CHX abolished the antiepileptic effects of BCCA alone, as well as BCCA combined with 2-DG, while it did not influence the 2-DG-related decrease in mortality. We conclude that the development of BCCA-induced epileptic tolerance, as well as unmasking antiepileptic effects of 2-DG by BCCA, is dependent on protein synthesis.

Transient NMDA receptor inactivation provides long-term protection to cultured cortical neurons from a variety of death signals

NMDA receptor antagonists, such as (+)-5-methyl-10, 11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801), potently block glutamate-induced neuronal death in myriad in vitro cell models and effectively attenuate ischemic damage in vivo. In this report, a novel role for MK-801 and other NMDA receptor antagonists in preconditioning neurons to withstand a wide range of subsequent lethal insults is described. A brief 30 min exposure to 0.1 microM MK-801, applied up to 96 hr before a "lethal" insult, protected primary cortical neurons from a diverse group of neurotoxic agents, including NMDA, beta-amyloid, staurosporine, etoposide, and oxygen-glucose deprivation. This neuroprotective preconditioning by MK-801 arose from transient NMDA receptor inactivation, because the noncompetitive NMDA receptor antagonists memantine and nylindin and the competitive antagonist AP-5 gave similar effects. MK-801 protection was dependent on new protein synthesis during the first 2 hr, but not from 2 to 5 hr, after MK-801 exposure. The MK-801 transient did not alter the ability of NMDA to trigger normally lethal [Ca(2+)](i) influx 48 hr later, but it did block early downstream signaling events coupled to NMDA neurotoxicity, including PKC inactivation and the activation of calpain. Moreover, MK-801 protected neurons from staurosporine-induced apoptosis, although caspase activation in these cells was unimpeded. It is likely that the stress associated with transient inactivation of NMDA receptors triggered a rapid compensatory survival response that provided long-term protection from a spectrum of insults, inducing apoptotic and nonapoptotic death. The possibility that MK-801 preconditioning blocks an event common to seemingly diverse death mechanisms suggests it will be an important tool for obtaining a clearer understanding of the salient molecular events at work in neuronal death and survival pathways.

Potentiation by histamine of synaptically mediated excitotoxicity in cultured hippocampal neurones: a possible role for mast cells

Excessive glutamatergic neurotransmission, particularly when mediated by the N:-methyl-D-aspartate (NMDA) subtype of glutamate receptor, is thought to underlie neuronal death in a number of neurological disorders. Histamine has been reported to potentiate NMDA receptor-mediated events under a variety of conditions. In the present study we have utilized primary hippocampal neurone cultures to investigate the effect of mast cell-derived, as well as exogenously applied, histamine on neurotoxicity evoked by excessive synaptic activity. Exposure of mature cultures for 15 min to an Mg(2+)-free/glycine-containing buffer to trigger synaptic transmission through NMDA receptors, caused a 30-35% neuronal loss over 24 h. When co-cultured with hippocampal neurones, activated mast cells increased excitotoxic injury to 60%, an effect that was abolished in the presence of histaminase. Similarly, addition of histamine during magnesium deprivation produced a concentration-dependent potentiation (+ 60%; EC(50) : 5 microM) of neuronal death which was inhibited by sodium channel blockers and NMDA receptor antagonists, although this effect did not involve known histamine receptors. The histamine effect was further potentiated by acidification of the culture medium. Cultures 'preconditioned' by sublethal (5 min) Mg(2+) deprivation exhibited less neuronal death than controls when exposed to a more severe insult. NMDA receptor activation and the extracellular regulated kinase cascade were required for preconditioning neuroprotection. The finding that histamine potentiates NMDA receptor-mediated excitotoxicity may have important implications for our understanding of conditions where enhanced glutamatergic neurotransmission is observed in conjunction with tissue acidification, such as cerebral ischaemia and epilepsy.

The role of opioid receptors in hypoxic preconditioning against seizures in brain

Preconditioning is defined as an adaptive mechanism produced by short periods of hypoxia/ischemia, resulting in protection against subsequent ischemic insult, and development of seizures. Results of the present study demonstrate that an episode of normobar hypoxia reduces the susceptibility to convulsions induced by pentylenetetrazol (PTZ) 30 min, 24 h, as well as 4 and 7 days later. Administration of morphine showed similar effects after 24 h. Naloxone, given before ischemic preconditioning, as well as morphine, blocked the development of the protection. Administration of D-Ala-Met-enkephalin-Gly-ol (DAMGO - a selective mu-opioid receptor agonist), as well as trans-3, 4-dichloro-N-methyl-N-[7-(1-pyrrolidinyl) cycloexilbenzeneacetamide ethane sulfonate] (U-69,593 - a selective kappa-opioid receptor agonist), mimicked the effects of hypoxic preconditioning (HPC). (-)-N-(Cyclopropylmethyl)-4,14-dimethoxymorphinan-6-one (cyprodime - a selective mu-opioid receptor antagonist, as well as nor-binaltorphimine dihydrochloride (nor-BNI - selective kappa-opioid receptors antagonist), given before HPC as well as before respective opioid receptor agonists, blocked the development of the protection. This study provides evidence that mu- and kappa-opioid receptors are involved in HPC against seizures in the brain.

Preconditioning-induced neuroprotection is mediated by reactive oxygen species

The current study was performed to determine the role of reactive oxygen species (ROS) in preconditioning against different forms of neuronal damage. Primary cultures of chick embryonic neurons were treated with either FeSO(4) (100 microM; 15 min) to generate hydroxyl radicals or xanthine/xanthinoxidase (10 microM/0.5 mU ml(-1); 15 min; =X/XO (pre)) to produce superoxide radicals. Both stimuli moderately enhanced ROS formation as measured by fluorescence microscopy. This preconditioning significantly protected the neurons against subsequent glutamate (1 mM)-induced excitotoxic damage, staurosporine (200 nM)-induced neuronal apoptosis and oxidative damage caused by exposure to xanthine/xanthinoxidase (500 microM/5 mU ml(-1); 1 h; =X/XO (dam)). The antioxidants vitamin E (10 microM) and 2-OH-estradiol (1 microM), present during the 15-min preconditioning period, completely abolished the protective effect of X/XO (pre). Furthermore, glutamate, staurosporine or X/XO (dam) markedly enhanced oxygen radical formation. Preceding preconditioning by mild ROS stimulation with X/XO (pre) or Fe(2+) reduced this oxygen radical burst. Again, the effect of X/XO (pre) could be blocked by coadministration of vitamin E or 2-OH-estradiol. However, the FeSO(4)-mediated preconditioning was not abolished by the radical scavengers. To address this phenomenon, the effect of vitamin E and 2-OH-estradiol on Fe(2+)- and X/XO (pre)-induced ROS formation kinetics within the 15 min of preconditioning was monitored. The moderate rise of intracellular ROS content during preconditioning was only reduced permanently by the antioxidants, when the neurons were treated with X/XO (pre), but not when Fe(2+) was used. Thus, an immediate and constant radical scavenging seems to be indispensable to abolish the ROS-induced neuronal preconditioning. The current results indicate that preconditioning by moderate ROS-stimulation protects cultured neurons against different damaging agents and prevents against the subsequent massive oxygen radical formation.

Activation of mitogen-activated protein kinases in gerbil hippocampus with ischemic tolerance induced by 3-nitropropionic acid

The present study investigated the activation of c-Jun NH2-terminal kinases (JNK), p38 mitogen-activated protein kinases (p38) and extracellular signal-regulated kinases (ERK) in the gerbil hippocampus by immunohistochemistry to clarify the role of these kinases in ischemic tolerance induced by3-NP. Intraperitoneal administration of 3-NP (3 or 10 mg/kg) caused the activation of JNK in CA1 subfield, which induced tolerance to subsequent ischemia and prevented delayed neuronal death (DND). As concerns p38 and ERK, no activation was induced by intoxication of 3-NP. Our results show the activation of JNK following chemical preconditioning with low dose of 3-NP is closely related to the acquisition of resistance to DND.