Calcium movement in ischemia-tolerant hippocampal CA1 neurons after transient forebrain ischemia in gerbils

Molecular Mechanisms of Neuroprotection after the Intermittent Exposures of Hypercapnic Hypoxia

The review introduces the stages of formation and experimental confirmation of the hypothesis regarding the mutual potentiation of neuroprotective effects of hypoxia and hypercapnia during their combined influence (hypercapnic hypoxia). The main focus is on the mechanisms and signaling pathways involved in the formation of ischemic tolerance in the brain during intermittent hypercapnic hypoxia. Importantly, the combined effect of hypoxia and hypercapnia exerts a more pronounced neuroprotective effect compared to their separate application. Some signaling systems are associated with the predominance of the hypoxic stimulus (HIF-1α, A1 receptors), while others (NF-κB, antioxidant activity, inhibition of apoptosis, maintenance of selective blood-brain barrier permeability) are mainly modulated by hypercapnia. Most of the molecular and cellular mechanisms involved in the formation of brain tolerance to ischemia are due to the contribution of both excess carbon dioxide and oxygen deficiency (ATP-dependent potassium channels, chaperones, endoplasmic reticulum stress, mitochondrial metabolism reprogramming). Overall, experimental studies indicate the dominance of hypercapnia in the neuroprotective effect of its combined action with hypoxia. Recent clinical studies have demonstrated the effectiveness of hypercapnic-hypoxic training in the treatment of childhood cerebral palsy and diabetic polyneuropathy in children. Combining hypercapnic hypoxia with pharmacological modulators of neuro/cardio/cytoprotection signaling pathways is likely to be promising for translating experimental research into clinical medicine.

Protective effects of ischemic preconditioning against neuronal apoptosis and dendritic injury in the hippocampus are age-dependent

Ischemic preconditioning with non-lethal ischemia can be protective against lethal forebrain ischemia. We hypothesized that aging may aggravate ischemic susceptibility and reduce brain plasticity against preconditioning. Magnetic resonance diffusion tensor imaging (DTI) is a sensitive tool to detect brain integrity and white matter architecture. This study used DTI and histopathology to investigate the effect of aging on ischemic preconditioning. In this study, adult and middle-aged male Mongolian gerbils were subjected to non-lethal 5-min forebrain ischemia (ischemic preconditioning) or sham-operation, followed by 3 days of reperfusion, and then lethal 15-min forebrain ischemia. A 9.4-Tesla MR imaging system was used to study DTI indices, namely fractional anisotropy (FA), mean diffusivity (MD), and intervoxel coherence (IC) in the hippocampal CA1 and dentate gyrus (DG) areas. In situ expressions of microtubule-associated protein 2 (MAP2, dendritic marker protein) and apoptosis were also examined. The 5-min ischemia did not cause dendritic and neuronal injury and any significant change in DTI indices and MAP2 in adult and middle-aged gerbils. The 15-min ischemia-induced significant delayed neuronal apoptosis and early dendritic injury evidenced by DTI and MAP2 studies in both CA1 and DG areas with more severe injury in middle-aged gerbils than adult gerbils. Ischemic preconditioning could improve neuronal apoptosis in CA1 area and dendritic integrity in both CA1 and DG areas with better improvement in adult gerbils than middle-aged gerbils. This study thus suggests an age-dependent protective effect of ischemic preconditioning against both neuronal apoptosis and dendritic injury in hippocampus after forebrain ischemia.

Sodium/calcium exchanger as main effector of endogenous neuroprotection elicited by ischemic tolerance

The ischemic tolerance (IT) paradigm represents a fundamental cell response to certain types or injury able to render an organ more "tolerant" to a subsequent, stronger, insult. During the 16th century, the toxicologist Paracelsus described for the first time the possibility that a noxious event might determine a state of tolerance. This finding was summarized in one of his most important mentions: "The dose makes the poison". In more recent years, ischemic tolerance in the brain was first described in 1991, when it was demonstrated by Kirino and collaborators that two minutes of subthreshold brain ischemia in gerbils produced tolerance against global brain ischemia. Based on the time in which the conditioning stimulus is applied, it is possible to define preconditioning, perconditioning and postconditioning, when the subthreshold insult is applied before, during or after the ischemic event, respectively. Furthermore, depending on the temporal delay from the ischemic event, two different modalities are distinguished: rapid or delayed preconditioning and postconditioning. Finally, the circumstance in which the conditioning stimulus is applied on an organ distant from the brain is referred as remote conditioning. Over the years the "conditioning" paradigm has been applied to several brain disorders and a number of molecular mechanisms taking part to these protective processes have been described. The mechanisms are usually classified in three distinct categories identified as triggers, mediators and effectors. As concerns the putative effectors, it has been hypothesized that brain cells appear to have the ability to adapt to hypoxia by reducing their energy demand through modulation of ion channels and transporters, which delays anoxic depolarization. The purpose of the present review is to summarize the role played by plasmamembrane proteins able to control ionic homeostasis in mediating protection elicited by brain conditioning, particular attention will be deserved to the role played by Na⁺/Ca²⁺ exchanger.

Reperfusion Changes After Stroke and Practical Approaches for Neuroprotection

Reperfusion is the first line of care in a growing number of eligible acute ischemic stroke patients. Early reperfusion with thrombolytic drugs and endovascular mechanical devices is associated with improved outcome and lower mortality rates compared with natural history. Reperfusion is not without risk, however, and may result in reperfusion injury, which manifests in hemorrhagic transformation, brain edema, infarct progression, and neurologic worsening. In this article, the functional and structural changes and underlying molecular mechanisms of ischemia and reperfusion are reviewed. The pathways that lead to reperfusion injury and novel neuroprotective strategies with endogenous properties are discussed.

Evidence for a role of plasma membrane calcium pumps in neurodegenerative disease: Recent developments

Plasma membrane Ca2+ ATPases (PMCAs) are a major system for calcium extrusion from all cells. Different PMCA isoforms and splice variants are involved in the precise temporal and spatial handling of Ca2+ signals and the re-establishment of resting Ca2+ levels in the nervous system. Lack or inappropriate expression of specific PMCAs leads to characteristic neuronal phenotypes, which may be reciprocally exacerbated by genetic predisposition through alleles in other genes that modify PMCA interactions, regulation, and function. PMCA dysfunction is often poorly compensated in neurons and may lead to changes in synaptic transmission, altered excitability and, with long-term calcium overload, eventual cell death. Decrease and functional decline of PMCAs are hallmarks of neurodegeneration during aging, and mutations in specific PMCAs are responsible for neuronal dysfunction and accelerated neurodegeneration in many sensory and cognitive diseases.

Spatiotemporal expression of osteopontin in the striatum of rats subjected to the mitochondrial toxin 3-nitropropionic acid correlates with microcalcification

Our aim was to elucidate whether osteopontin (OPN) is involved in the onset of mineralisation and progression of extracellular calcification in striatal lesions due to mitochondrial toxin 3-nitropropionic acid exposure. OPN expression had two different patterns when observed using light microscopy. It was either localised to the Golgi complex in brain macrophages or had a small granular pattern scattered in the affected striatum. OPN labelling tended to increase in number and size over a 2-week period following the lesion. Ultrastructural investigations revealed that OPN is initially localised to degenerating mitochondria within distal dendrites, which were then progressively surrounded by profuse OPN on days 7–14. Electron probe microanalysis of OPN-positive and calcium-fixated neurites indicated that OPN accumulates selectively on the surfaces of degenerating calcifying dendrites, possibly via interactions between OPN and calcium. In addition, 3-dimensional reconstruction of OPN-positive neurites revealed that they are in direct contact with larger OPN-negative degenerating dendrites rather than with fragmented cell debris. Our overall results indicate that OPN expression is likely to correlate with the spatiotemporal progression of calcification in the affected striatum, and raise the possibility that OPN may play an important role in the initiation and progression of microcalcification in response to brain insults.

Spatiotemporal Progression of Microcalcification in the Hippocampal CA1 Region following Transient Forebrain Ischemia in Rats: An Ultrastructural Study

Calcification in areas of neuronal degeneration is a common finding in several neuropathological disorders including ischemic insults. Here, we performed a detailed examination of the onset and spatiotemporal profile of calcification in the CA1 region of the hippocampus, where neuronal death has been observed after transient forebrain ischemia. Histopathological examinations showed very little alizarin red staining in the CA1 pyramidal cell layer until day 28 after reperfusion, while prominent alizarin red staining was detected in CA1 dendritic subfields, particularly in the stratum radiatum, by 14 days after reperfusion. Electron microscopy using the osmium/potassium dichromate method and electron probe microanalysis revealed selective calcium deposits within the mitochondria of degenerating dendrites at as early as 7 days after reperfusion, with subsequent complete mineralization occurring throughout the dendrites, which then coalesced to form larger mineral conglomerates with the adjacent calcifying neurites by 14 days after reperfusion. Large calcifying deposits were frequently observed at 28 days after reperfusion, when they were closely associated with or completely engulfed by astrocytes. In contrast, no prominent calcification was observed in the somata of CA1 pyramidal neurons showing the characteristic features of necrotic cell death after ischemia, although what appeared to be calcified mitochondria were noted in some degenerated neurons that became dark and condensed. Thus, our data indicate that intrahippocampal calcification after ischemic insults initially occurs within the mitochondria of degenerating dendrites, which leads to the extensive calcification that is associated with ischemic injuries. These findings suggest that in degenerating neurons, the calcified mitochondria in the dendrites, rather than in the somata, may serve as the nidus for further calcium precipitation in the ischemic hippocampus.

Increased Expression of Osteopontin in the Degenerating Striatum of Rats Treated with Mitochondrial Toxin 3-Nitropropionic Acid: A Light and Electron Microscopy Study

The mycotoxin 3-nitropropionic acid (3NP) is an irreversible inhibitor that induces neuronal damage by inhibiting mitochondrial complex II. Neurodegeneration induced by 3NP, which is preferentially induced in the striatum, is caused by an excess influx and accumulation of calcium in mitochondria. Osteopontin (OPN) is a glycosylated phosphoprotein and plays a role in the regulation of calcium precipitation in the injured brain. The present study was designed to examine whether induction of OPN protein is implicated in the pathogenesis of 3NP-induced striatal neurodegeneration. We observed overlapping regional expression of OPN, the neurodegeneration marker Fluoro-Jade B, and the microglial marker ionized calcium-binding adaptor molecule 1 (Iba1) in the 3NP-lesioned striatum. OPN expression was closely associated with the mitochondrial marker NADH dehydrogenase (ubiquinone) flavoprotein 2 in the damaged striatum. In addition, immunoelectron microscopy demonstrated that OPN protein was specifically localized to the inner membrane and matrix of the mitochondria in degenerating striatal neurons, and cell fragments containing OPN-labeled mitochondria were also present within activated brain macrophages. Thus, our study revealed that OPN expression is associated with mitochondrial dysfunction produced by 3NP-induced alteration of mitochondrial calcium homeostasis, suggesting that OPN is involved in the pathogenesis of striatal degeneration by 3NP administration.

Ionic homeostasis in brain conditioning

Most of the current focus on developing neuroprotective therapies is aimed at preventing neuronal death. However, these approaches have not been successful despite many years of clinical trials mainly because the numerous side effects observed in humans and absent in animals used at preclinical level. Recently, the research in this field aims to overcome this problem by developing strategies which induce, mimic, or boost endogenous protective responses and thus do not interfere with physiological neurotransmission. Preconditioning is a protective strategy in which a subliminal stimulus is applied before a subsequent harmful stimulus, thus inducing a state of tolerance in which the injury inflicted by the challenge is mitigated. Tolerance may be observed in ischemia, seizure, and infection. Since it requires protein synthesis, it confers delayed and temporary neuroprotection, taking hours to develop, with a pick at 1–3 days. A new promising approach for neuroprotection derives from post-conditioning, in which neuroprotection is achieved by a modified reperfusion subsequent to a prolonged ischemic episode. Many pathways have been proposed as plausible mechanisms to explain the neuroprotection offered by preconditioning and post-conditioning. Although the mechanisms through which these two endogenous protective strategies exert their effects are not yet fully understood, recent evidence highlights that the maintenance of ionic homeostasis plays a key role in propagating these neuroprotective phenomena. The present article will review the role of protein transporters and ionic channels involved in the control of ionic homeostasis in the neuroprotective effect of ischemic preconditioning and post-conditioning in adult brain, with particular regards to the Na⁺/Ca2⁺ exchangers (NCX), the plasma membrane Ca2⁺-ATPase (PMCA), the Na⁺/H⁺ exchange (NHE), the Na⁺/K⁺/2Cl⁻ cotransport (NKCC) and the acid-sensing cation channels (ASIC). Ischemic stroke is the third leading cause of death and disability. Up until now, all clinical trials testing potential stroke neuroprotectants failed. For this reason attention of researchers has been focusing on the identification of brain endogenous neuroprotective mechanisms activated after cerebral ischemia. In this context, ischemic preconditioning and ischemic post-conditioning represent two neuroprotecive strategies to investigate in order to identify new molecular target to reduce the ischemic damage.

Ultrastructural investigation of microcalcification and the role of oxygen-glucose deprivation in cultured rat hippocampal slices

Intracellular calcium accumulation is associated with cell death in several neuropathological disorders including brain ischemia, but the exact mechanisms of calcification need to be clarified. We used organotypic hippocampal slice culture - cultures subjected to oxygen-glucose deprivation (OGD) mimicking the in vivo situation to investigate the events underlying ectopic calcification. Alizarin red staining indicating calcium deposition was observed in the cornu ammonis (CA)1 and dentate gyrus regions in control hippocampal slices despite no specific labeling for cell death markers. Electron microscopy using the osmium/potassium dichromate method revealed scattered degenerated cells throughout the normally appearing CA1 region. They contained electron-dense precipitates within mitochondria, and electron probe microanalysis confirmed that they were calcifying mitochondria. Selective calcium deposition was noted within, but not beyond, mitochondria in these mineralized cells. They showed ultrastructural features of non-necrotic, non-apoptotic cell death and retained their compact ultrastructure, even after the majority of mitochondria were calcified. Unexpectedly, no intracellular calcification was noted in necrotic CA1 pyramidal cells after OGD, and there was no progression of calcification in OGD-lesioned slices. In addition, mineralized cells in both control and OGD-lesioned slices were closely associated with or completely engulfed by astrocytes but not microglia. These astrocytes were laden with heterogeneous cytoplasmic inclusions that appeared to be related with their phagocytic activity. These data demonstrate that microcalcification specifically associated with mitochondria might lead to a novel type of cell death and suggest that astrocytes may be involved in the phagocytosis of these mineralized cells and possibly in the regulation of ectopic calcification.

Force spectroscopy measurements show that cortical neurons exposed to excitotoxic agonists stiffen before showing evidence of bleb damage

In ischemic and traumatic brain injury, hyperactivated glutamate (N-methyl-D-aspartic acid, NMDA) and sodium (Nav) channels trigger excitotoxic neuron death. Na⁺, Ca⁺⁺ and H₂O influx into affected neurons elicits swelling (increased cell volume) and pathological blebbing (disassociation of the plasma membrane’s bilayer from its spectrin-actomyosin matrix). Though usually conflated in injured tissue, cell swelling and blebbing are distinct processes. Around an injury core, salvageable neurons could be mildly swollen without yet having suffered the bleb-type membrane damage that, by rendering channels leaky and pumps dysfunctional, exacerbates the excitotoxic positive feedback spiral. Recognizing when neuronal inflation signifies non-lethal osmotic swelling versus blebbing should further efforts to salvage injury-penumbra neurons. To assess whether the mechanical properties of osmotically-swollen versus excitotoxically-blebbing neurons might be cytomechanically distinguishable, we measured cortical neuron elasticity (gauged via atomic force microscopy (AFM)-based force spectroscopy) upon brief exposure to hypotonicity or to excitotoxic agonists (glutamate and Nav channel activators, NMDA and veratridine). Though unperturbed by solution exchange per se, elasticity increased abruptly with hypotonicity, with NMDA and with veratridine. Neurons then invariably softened towards or below the pre-treatment level, sometimes starting before the washout. The initial channel-mediated stiffening bespeaks an abrupt elevation of hydrostatic pressure linked to NMDA or Nav channel-mediated ion/H₂O fluxes, together with increased [Ca⁺⁺]_int-mediated submembrane actomyosin contractility. The subsequent softening to below-control levels is consistent with the onset of a lethal level of bleb damage. These findings indicate that dissection/identification of molecular events during the excitotoxic transition from stiff/swollen to soft/blebbing is warranted and should be feasible.

Sustained expression of osteopontin is closely associated with calcium deposits in the rat hippocampus after transient forebrain ischemia

The present study was designed to evaluate the extent and topography of osteopontin (OPN) protein expression in the rat hippocampus 4 to 12 weeks following transient forebrain ischemia, and to compare OPN expression patterns with those of calcium deposits and astroglial and microglial reactions. Two patterns of OPN staining were recognized by light microscopy: 1) a diffuse pattern of tiny granular deposits throughout the CA1 region at 4 weeks after ischemia and 2) non-diffuse ovoid to round deposits, which formed conglomerates in the CA1 pyramidal cell layer over the chronic interval of 8 to 12 weeks. Immunogold-silver electron microscopy and electron probe microanalysis demonstrated that OPN deposits were indeed diverse types of calcium deposits, which were clearly delineated by profuse silver grains indicative of OPN expression. Intracellular OPN deposits were frequently observed within reactive astrocytes and neurons 4 weeks after ischemia but rarely at later times. By contrast, extracellular OPN deposits progressively increased in size and appeared to be gradually phagocytized by microglia or brain macrophages and some astrocytes over 8 to 12 weeks. These data indicate an interaction between OPN and calcium in the hippocampus in the chronic period after ischemia, suggesting that OPN binding to calcium deposits may be involved in scavenging mechanisms.

Overlapping distribution of osteopontin and calcium in the ischemic core of rat brain after transient focal ischemia

Osteopontin (OPN), an adhesive glycoprotein, has recently been proposed to act as an opsonin that facilitates phagocytosis of neuronal debris by macrophages in the ischemic brain. The present study was designed to elucidate the process whereby OPN binds to neuronal cell debris in a rat model of ischemic stroke. Significant co-localization of the OPN protein and calcium deposits in the ischemic core were observed by combining alizarin red staining and OPN immunohistochemistry. In addition, electron microscopy (EM) using the osmium/potassium dichromate method revealed that electron-dense precipitates, typical of calcium deposits, were localized mainly along the periphery of putative degenerating neurites. This topical pattern of calcium precipitates resembled the distribution of OPN as detected by immunogold-silver EM. Combining immunogold-silver EM and electron probe microanalysis further demonstrated that the OPN protein was localized at the periphery of cell debris or degenerating neurites, corresponding with locally higher concentrations of calcium and phosphorus, and that the relative magnitude of OPN accumulation was comparable to that of calcium and phosphorus. These data suggest that calcium precipitation provides a matrix for the binding of the OPN protein within the debris or degenerating neurites induced by ischemic injury. Therefore, OPN binding to calcium deposits may be involved in phagocytosis of such debris, and may participate in the regulation of ectopic calcification in the ischemic brain.

Ischemic preconditioning: postischemic structural changes in the brain

Ischemic brain damage can be prevented or at least significantly reduced when there is a preceding brief ischemic period that does not exceed the threshold for tissue damage--a phenomenon termed "ischemic preconditioning" (ischemic PC). Experimental PC in rodents is now considered to be a model for transient ischemic attacks in humans, and there is increasing hope for translating the knowledge of underlying mechanisms in the animal models into the clinic to enhance endogenous neuroprotective mechanisms in patients with stroke. However, although PC was originally defined as a subtoxic stimulus without any morphologic damage, there is a growing body of evidence from studies using sensitive techniques that postischemic structural alterations of brain tissue manifest not only after ischemia with prior PC but also after the PC stimulus itself. Furthermore, it has become evident over time that the primary shortcomings of many experimental studies on PC are the short observation intervals. The few studies with extended postischemic survival periods done to date provide clear evidence of considerable structural changes and even cell death, which may only be postponed by PC. Therefore, further studies are needed to elucidate structural long-term changes after PC and to validate the persistence of the neuroprotective effects.

Molecular physiology of preconditioning-induced brain tolerance to ischemia

Ischemic tolerance describes the adaptive biological response of cells and organs that is initiated by preconditioning (i.e., exposure to stressor of mild severity) and the associated period during which their resistance to ischemia is markedly increased. This topic is attracting much attention because preconditioning-induced ischemic tolerance is an effective experimental probe to understand how the brain protects itself. This review is focused on the molecular and related functional changes that are associated with, and may contribute to, brain ischemic tolerance. When the tolerant brain is subjected to ischemia, the resulting insult severity (i.e., residual blood flow, disruption of cellular transmembrane gradients) appears to be the same as in the naive brain, but the ensuing lesion is substantially reduced. This suggests that the adaptive changes in the tolerant brain may be primarily directed against postischemic and delayed processes that contribute to ischemic damage, but adaptive changes that are beneficial during the subsequent test insult cannot be ruled out. It has become clear that multiple effectors contribute to ischemic tolerance, including: 1) activation of fundamental cellular defense mechanisms such as antioxidant systems, heat shock proteins, and cell death/survival determinants; 2) responses at tissue level, especially reduced inflammatory responsiveness; and 3) a shift of the neuronal excitatory/inhibitory balance toward inhibition. Accordingly, an improved knowledge of preconditioning/ischemic tolerance should help us to identify neuroprotective strategies that are similar in nature to combination therapy, hence potentially capable of suppressing the multiple, parallel pathophysiological events that cause ischemic brain damage.

Constitutively active calcineurin mediates delayed neuronal death through Fas-ligand expression via activation of NFAT and FKHR transcriptional activities in mouse brain ischemia

We recently demonstrated that a constitutively active form of calcineurin (CaN) is generated by calpain-mediated limited proteolysis following brain ischemia. The calpain-induced CaN activation mediated delayed neuronal death through translocation of nuclear factor of activated T-cells (NFAT) into nuclei after brain ischemia. We also previously demonstrated that activation of forkhead in rhabdomyosarcoma (FKHR), a forkhead transcription factor and substrate of protein kinase-B (Akt), mediated ischemia-induced neuronal death through Fas-ligand expression in gerbil hippocampus. FKHR activation occurred through decreased Akt activity and concomitant dephosphorylation mediated by undefined phosphatases. In this study, we show that phosphorylated Ser-256 of FKHR is dephosphorylated by constitutively active CaN and that in turn FKHR forms a complex with CaN that is translocated into nuclei after brain ischemia. After nuclear translocation of NFAT and FKHR, both NFAT and FKHR stimulated expression of Fas-ligand by binding to its promoter region. Consistent with activation of the Fas-ligand promoter by FKHR dephosphorylation, Fas-ligand expression increased 2 days after ischemia/reperfusion, and treatment with the CaN inhibitor FK506 inhibited that expression. These results suggest that FKHR is a downstream target of CaN and that constitutively active CaN mediates delayed neuronal death through Fas-ligand expression via up-regulation of both NFAT and FKHR transcriptional activity in brain ischemia.

Generation of constitutively active calcineurin by calpain contributes to delayed neuronal death following mouse brain ischemia

Calpain, a Ca(2+)-dependent cysteine protease, in vitro converts calcineurin (CaN) to constitutively active forms of 45 kDa and 48 kDa by cleaving the autoinhibitory domain of the 60 kDa subunit. In a mouse middle cerebral artery occlusion (MCAO) model, calpain converted the CaN A subunit to the constitutively active form with 48 kDa in vivo. We also confirmed increased Ca(2+)/CaM-independent CaN activity in brain extracts. The generation of constitutively active and Ca(2+)/CaM-independent activity of CaN peaked 2 h after reperfusion in brain extracts. Increased constitutively active CaN activity was associated with dephosphorylation of dopamine-regulated phosphoprotein-32 in the brain. Generation of constitutively active CaN was accompanied by translocation of nuclear factor of activated T-cells (NFAT) into nuclei of hippocampal CA1 pyramidal neurons. In addition, a novel calmodulin antagonist, DY-9760e, blocked the generation of constitutively active CaN by calpain, thereby inhibiting NFAT nuclear translocation. Together with previous studies indicating that NFAT plays a critical role in apoptosis, we propose that calpain-induced CaN activation in part mediates delayed neuronal death in brain ischemia.

Hypoxic preconditioning and tolerance via hypoxia inducible factor (HIF) 1alpha-linked induction of P450 2C11 epoxygenase in astrocytes

The brain's adaptive response to ischemic preconditioning (IPC) is mediated in part via hypoxia inducible factor (HIF)-responsive genes. We previously showed that IPC induces cytochrome P450 2C11 expression in the brain, associated with protection from stroke. Cytochrome P450 2C11 is an arachidonic acid (AA) epoxygenase expressed in astrocytes, which metabolizes AA to epoxyeicosatrienoic acids (EETs). We tested the hypotheses that hypoxic preconditioning (HPC) induces 2C11 expression in astrocytes via HIF-1alpha, and that the P450 epoxygenase pathway contributes to enhanced astrocyte tolerance to ischemia-like injury induced by oxygen-glucose deprivation (OGD). Primary cultured astrocytes were incubated under normoxic or hypoxic conditions for 1, 3, 6, 24, or 48 h, and protein levels of P450 2C11 and HIF-1alpha were measured by Western blotting. Additionally, 2C11 mRNA was measured by Northern blotting, and binding of HIF-1alpha to 2C11 promoter was evaluated using electrophoretic mobility shift assay (EMSA) with 2C11 promoter DNA containing putative HIF-binding sites. Levels of 2C11 mRNA and protein were significantly increased starting at 3 and 6 h of hypoxia, respectively. The increase in 2C11 expression was preceded by an increase in HIF-1alpha protein at 1 h of hypoxia, and EMSA showed a specific and direct interaction between 2C11 promoter DNA and HIF-1alpha in nuclear extracts from astrocytes. HPC and EETs reduced astrocyte cell death, and P450 epoxygenase inhibition prevented protection by HPC. We conclude that HPC induces tolerance in astrocytes, at least in part, via HIF-1alpha-linked upregulation of P450 2C11.

Protective preconditioning by transient global ischemia in the rat: components of delayed injury progression and lasting protection distinguished by comparisons of depolarization thresholds for cell loss at long survival times

Robust ischemic preconditioning has been shown in rodent brain, but there are concerns regarding the persistence of neuron protection. This issue was examined in rat hippocampus following 4-vessel occlusion (4-VO) ischemia, using DC shifts characteristic of ischemic depolarization to reproducibly define insult severity. Preconditioning ischemia producing 2 to 3.5 mins depolarization was followed at intervals of 2, 5, or 7 days by test insults of varied duration, after which CA1 counts were obtained at 1, 2, 4, or 12 weeks. Neuron loss in naive animals increased with depolarization time longer than 4 mins regardless of postischemic survival interval. Preconditioning 2, 5, or 7 days before test insults prolonged the injury threshold evaluated at 1 week survival to 15, 9, or 6 mins, respectively, showing robust protection and a rapid decay of the protected state. However, by 2 weeks survival after preconditioning at a 2-day interval, the injury threshold dramatically regressed from 15 to 9 mins. Thereafter protection remained relatively stable through 1 month, but slight progression of neuron injury was evident at 3 months. Inflammatory responses were seen in both naive and preconditioned hippocampi throughout this interval, appropriate to the extent of neuron injury. These studies show distinct components of transient and lasting protection after ischemic preconditioning. Finally, it was found that ischemic depolarization was delayed by approximately 1 min in optimally preconditioned rat hippocampus, in contrast to previous results in the gerbil, identifying one specific mechanism by which insult severity is reduced in this model.

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.

Upregulation of GluR2 decreases intracellular Ca2+ following ischemia in developing gerbils

Developing animals are known to be resistant to cerebral ischemia. To investigate the mechanisms by which developing animals exhibit ischemic resistance, we examined the changes in intracellular calcium ([Ca2+]i) after oxygen-glucose deprivation (OGD) using hippocampal slices from gerbils. We found that increases of [Ca2+]i in hippocampal CA1 neurons is significantly less after OGD in developing gerbils than in adults. Western blot analysis of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid) receptors (AMPARs) showed that GluR2 expression, but not that of the other AMPARs is significantly higher in developing gerbils than in adults. Expression of the anti-apoptotic proteins such as HSP70, Bcl-XL, and plasma membrane Ca2+-ATPase type1 (PMCA1) are not higher in the developing gerbils than in adults. These results suggest that the higher expression of GluR2 is important for the smaller increases in [Ca2+]i and enhanced resistance to ischemia-induced neuronal damage in developing animals.

Time course and cellular distribution of hsp27 and hsp72 stress protein expression in a quantitative gerbil model of ischemic injury and tolerance: thresholds for hsp72 induction and hilar lesioning in the context of ischemic preconditioning

The distribution and time course of expression of the heat shock/stress proteins, hsp27 and hsp72, were evaluated in a highly controlled gerbil model of ischemic injury and tolerance induction, in which the duration of ischemic depolarization in each hippocampus provides a precise quantitative index of insult severity. Gerbils were subjected to brief priming insults (2- to 3.5-minute depolarization) that produce optimal preconditioning, to severe test insults (6- to 8.5-minute depolarization) that produce complete CA1 neuron loss in naive animals, or to combined insults administered 1 week apart, after which almost complete tolerance to CA1 neuron injury is observed. Immunoreactivities of hsp27, hsp72, glial fibrillary acidic protein and microtubule-associated protein 2 (MAP2) were evaluated in animals perfused at defined intervals after the final insult in each treatment group, using a variation of established antigen-retrieval procedures that significantly improves detection of many proteins in vibratome brain sections. Hsp72 was detected in CA1 neurons of some hippocampi 2 to 4 days after preconditioning, but this was only seen after the longest priming depolarizations, whereas shorter insults that still induced optimal tolerance failed to induce hsp72. Hsp72 was induced after test insults in preconditioned hippocampi, but at a higher depolarization threshold than observed for naive animals. An astrocytic localization of hsp27 was observed in regions of neuron injury, as indicated by reduced MAP2 immunoreactivity, and was primarily restricted to dentate hilus after preconditioning insults. These results establish that limited hilar lesions are characteristic of optimal preconditioning, whereas prior neuronal expression of either hsp72 or hsp27 is not required for ischemic tolerance.

Induced hippocampal neuron protection in an optimized gerbil ischemia model: insult thresholds for tolerance induction and altered gene expression defined by ischemic depolarization

Preconditioning of hippocampal CA1 neurons was evaluated in a gerbil model of transient global ischemia using extracellular recording of DC potential shifts characteristic of ischemic depolarization to precisely define the duration of both priming and test insults. Brief ischemia resulting in depolarizations of 2.5 to 3.5 minutes consistently induced maximal tolerance (95% protection) against subsequent challenges 2 days later with an approximate doubling of the insult duration required for complete CA1 neuron loss from 6 to 12 minutes depolarization when evaluated 1 week after the test insult. Significant protection persisted at 2 months survival, although the apparent injury threshold regressed to approximately 8 minutes, indicating delayed progression of injury after longer test insults. In situ hybridization was used to evaluate depolarization thresholds for induction of mRNAs encoding the 70 kDa heat shock/stress protein, hsp72, as well as several immediate-early genes (c-fos, c-jun, junB, and junD). Immediate-early genes were prominently expressed after short insults inducing tolerance, whereas appreciable hsp72 induction only occurred after insults approaching the threshold for neuron injury. These results establish an ischemic preconditioning model with the predictability needed for mechanistic studies and demonstrate that prior transcriptional activation of the postischemic heat shock response is not required for expression of delayed tolerance.

Effect of mutant alpha-synuclein on dopamine homeostasis in a new human mesencephalic cell line

Mutations in alpha-synuclein have been linked to rare, autosomal dominant forms of Parkinson's disease. Despite its ubiquitous expression, mutant alpha-synuclein primarily leads to the loss of dopamine-producing neurons in the substantia nigra. alpha-Synuclein is a presynaptic nerve terminal protein of unknown function, although several studies suggest it is important for synaptic plasticity and maintenance. The present study utilized a new human mesencephalic cell line, MESC2.10, to study the effect of A53T mutant alpha-synuclein on dopamine homeostasis. In addition to expressing markers of mature dopamine neurons, differentiated MESC2.10 cells are electrically active, produce dopamine, and express wild-type human alpha-synuclein. Lentivirus-induced overexpression of A53T mutant alpha-synuclein in differentiated MESC2.10 cells resulted in down-regulation of the vesicular dopamine transporter (VMAT2), decreased potassium-induced and increased amphetamine-induced dopamine release, enhanced cytoplasmic dopamine immunofluorescence, and increased intracellular levels of superoxide. These results suggest that mutant alpha-synuclein leads to an impairment in vesicular dopamine storage and consequent accumulation of dopamine in the cytosol, a pathogenic mechanism that underlies the toxicity of the psychostimulant amphetamine and the parkinsonian neurotoxin 1-methyl-4-phenylpyridinium. Interestingly, cells expressing A53T mutant alpha-synuclein were resistant to amphetamine-induced toxicity. Because extravesicular, cytoplasmic dopamine can be easily oxidized into reactive oxygen species and other toxic metabolites, mutations in alpha-synuclein might lead to Parkinson's disease by triggering protracted, low grade dopamine toxicity resulting in terminal degeneration and ultimately cell death.

Neuroprotection and P450 2C11 upregulation after experimental transient ischemic attack

BACKGROUND AND PURPOSE

Transient ischemic attack (TIA) is a risk factor for stroke. However, TIA may also serve as a preconditioning stimulus, reducing damage from subsequent stroke. We tested the hypothesis that experimental TIA induces expression of P450 2C11, an arachidonic acid epoxygenase that produces vasodilator epoxyeicosatrienoic acids, leading to increased tissue perfusion and reduced stroke damage.

METHODS

Wistar rats underwent three 10-minute middle cerebral artery occlusions (TIA) or sham surgery. Three days later, animals were subjected to 2-hour middle cerebral artery occlusion and 24 hours of reperfusion. Brains were stained with 2,3,5-triphenyltetrazolium chloride for infarct size measurement or processed for quantification of P450 2C11 mRNA and protein with the use of RNase protection assay and Western blotting. Regional cerebral blood flow (CBF) at the end of 2-hour ischemia was measured in separate groups of rats with iodoantipyrine autoradiography.

RESULTS

Cerebral infarct was reduced by >50% in TIA- versus sham-preconditioned animals. 2C11 mRNA and protein were increased in ipsilateral hemisphere by 3 days after TIA but not sham surgery. Induction of 2C11 by TIA was also evident in ipsilateral hemisphere at 24 hours after 2-hour middle cerebral artery occlusion and 24 hours of reperfusion. End-ischemic regional CBF was not different between TIA- and sham-pretreated groups.

CONCLUSIONS

We conclude that experimental TIA induces ischemic tolerance by a mechanism temporally linked to upregulation of P450 2C11. Enzyme induction does not attenuate ischemic severity by amplifying end-ischemic CBF.

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.

Extracellular signal-regulated kinase and c-Jun N-terminal protein kinase in ischemic tolerance

The alterations and involvement of extracellular signal-regulated kinase (ERK) and c-Jun N-terminal protein kinase (JNK) activation were examined in the hippocampal CA1 region in a rat model of global brain ischemic tolerance. Western blotting study showed that ERK activation (diphosphorylation) level was decreased (3.75-, 0.56-, and 0.23-fold vs sham control) and JNK activation level was increased (3.82-, 4.63-, and 5.30-fold vs sham control) 3 days after more severe ischemic insults (6 min, 8 min, and 10 min of ischemia, respectively). These alterations were significantly prevented by pretreatment with preconditioning ischemia, which also provided neuronal protection against ischemic injury. Inhibition of ERK activation after preconditioning ischemia by PD98059, a specific ERK kinase inhibitor, significantly prevented the inhibitory effects of preconditioning ischemia on both JNK activation and ischemic injury. The results suggest that ERK activation after preconditioning ischemia may result in the prevention of JNK activation and thus be involved in the protective responses in ischemic tolerance in hippocampal CA1 region.

Resistance to NMDA toxicity correlates with appearance of nuclear inclusions, behavioural deficits and changes in calcium homeostasis in mice transgenic for exon 1 of the huntington gene

Transgenic Huntington's disease (HD) mice, expressing exon 1 of the human HD gene (lines R6/1 and R6/2), are totally resistant to striatal lesions caused by the NMDA receptor agonist quinolinic acid (QA). Here we show that this resistance develops gradually over time in both R6/1 and R6/2 mice, and that it occurred earlier in R6/2 (CAG-155) than in R6/1 (CAG-115) mice. The development of the resistance coincided with the appearance of nuclear inclusions and with the onset of motor deficits. In the HD mice, hippocampal neurons were also resistant to QA, especially in the CA1 region. Importantly, there was no change in susceptibility to QA in transgenic mice with a normal CAG repeat (CAG-18). R6/1 mice were also resistant to NMDA-, but not to AMPA-induced striatal damage. Interestingly, QA-induced current and calcium influx in striatal R6/2 neurons were not decreased. However, R6/2 neurons had a better capacity to handle cytoplasmic calcium ([Ca2+]c) overload following QA and could avoid [Ca2+]c deregulation and cell lysis. In addition, basal [Ca2+]c levels were increased five-fold in striatal R6/2 neurons. This might cause an adaptation of R6 neurons to excitotoxic stress resulting in an up-regulation of defense mechanisms, including an increased capacity to handle [Ca2+]c overload. However, the increased level of basal [Ca2+]c in the HD mice might also disturb intracellular signalling in striatal neurons and thereby cause neuronal dysfunction and behavioural deficits.

Ischemic pre-conditioning affects the subcellular distribution of protein kinase C and calcium/calmodulin-dependent protein kinase II in the gerbil hippocampal CA1 neurons

Brief ischemic episode, which in itself is not lethal, confers tolerance to subsequent ischemic insults. Since intracellular signal transduction system has been implicated in ischemic cell death, we studied the effect of pre-conditioning on the changes in the subcellular distribution of protein kinase Cgamma (PKCgamma) as well as CaM kinase II (CaMKII). Gerbils were pre-conditioned by a sublethal 2 min cerebral ischemia 24 h prior to lethal 5 min ischemia. The pre-conditioning generally downregulated PKCgamma and CaMKII in the CA1 hippocampus. Especially at the starting point of the second lethal ischemia, the cytosolic PKCgamma level was about 40% lower in the pre-conditioned group. Also, the crude synaptosomal CaMKII level at 24 h reperfusion following the second ischemia was significantly lower in the pre-conditioned group, showing enhanced recovery of CaMKII translocation. Present results suggest that ischemic pre-conditioning may downregulate calcium-mediated cell signaling system, enhancing normalization of calcium homeostasis, perturbed by the second ischemia of lethal duration.

Cerebral protection by hypoxic preconditioning in a murine model of focal ischemia-reperfusion

Sublethal periods of hypoxia or ischemia can induce adaptive mechanisms to protect against subsequent lethal ischemic insults in a process known as ischemic preconditioning. In the present study, we developed a murine model of cerebral preconditioning using several common strains of adult mice. Animals were exposed to sublethal hypoxia (11% oxygen for 2 h) 48 h prior to a 90 min period of transient focal middle cerebral artery occlusion, induced by an intraluminal filament; injury was assessed 24 h later by TTC staining. Infarct volume in hypoxia-preconditioned animals was reduced 46%, 58%, and 64% in C57Bl/6, 129SvEv, and Swiss-Webster ND4 mice relative to their respective untreated controls. This non-invasive murine model of ischemic tolerance should be useful for elucidating the molecular basis of this protection using transgenic and knockout mice.

Protective and regenerative response endogenously induced in the ischemic brain

Neuronal cells are highly vulnerable to ischemic insult. Because adult neurons are highly differentiated and cannot self-propagate, loss of neurons often results in functional deficits in mammalian brains. However, it has recently been shown that neurons and neuronal circuits exhibit protective and regenerative responses in a rodent model of experimental ischemia. At first, neurons respond by producing several protective proteins such as heat shock proteins (HSPs) after sublethal ischemia and then acquire tolerance against a subsequent ischemic insult (ischemic tolerance). Once neurons suffer irreversible injury, two repair processes, neurogenesis and synaptogenesis, are endogenously induced. Neuronal stem and (or) progenitor cells can proliferate in two brain areas in adult animals: the subventricular zone and the subgranular zone in the dentate gyrus. After ischemic insult, these stem (progenitor) cells proliferate and differentiate into neurons in the dentate gyrus of the hippocampus. Reactive synaptogenesis has been also observed in the injured brain following a period of long-term infarction, but it is unclear if it can compensate for disconnected circuits. Understanding the molecular mechanism underlying these protective and regenerative responses will be important in developing a new strategy for aimed at the augmentation of resistance against ischemic insult and the replacement of injured neurons and neuronal circuits.Key words: ischemic tolerance, neurogenesis, synaptogenesis.

Release of cytochrome c from mitochondria to cytosol in gerbil hippocampal CA1 neurons after transient forebrain ischemia

We examined cytosolic cytochrome c in gerbil hippocampal CA1 and CA3 regions after induction of 5-min ischemia by immunoblotting. In the CA1 region, cytochrome c was detected in the cytosolic fraction from 1 to 6 h after ischemia by Western blotting, while it was not detected in the CA3 region. Following intraventricular administration of cyclosporin A (CsA), detectable cytosolic cytochrome c was dramatically decreased, and about 80% of CA1 neurons survived after ischemia. The present studies demonstrate that cytochrome c is translocated from mitochondria to the cytosol in the early stage of delayed neuronal cell death, and suggest the involvement of the mitochondrial permeability transition.

Arachidonic acid and leukotriene C4: role in transient cerebral ischemia of gerbils

Accumulation of arachidonic acid (AA) is greatest in brain regions most sensitive to transient ischemia. Free AA released after ischemia is either: 1) reincorporated into the membrane phospholipids, or 2) oxidized during reperfusion by lipoxygenases and cyclooxygenases, producing leukotrienes (LT), prostaglandins, thromboxanes and oxygen radicals. AA, its metabolite LTC4 and lipid peroxides (generated during AA metabolism) have been implicated in the blood-brain barrier (BBB) dysfunction, edema and neuronal death after ischemia/reperfusion. This report describes the time course of AA release, LTC4 accumulation and association with the physiological outcome during transient cerebral ischemia of gerbils. Significant amount of AA was detected immediately after 10 min ischemia (0 min reperfusion) which returned to sham levels within 30 min reperfusion. A later release of AA occurred after 1 d. LTC4 levels were elevated at 0-6 h and 1 d after ischemia. Increased lipid peroxidation due to AA metabolism was observed between 2-6 h. BBB dysfunction occurred at 6 h. Significant edema developed at 1 and 2 d after ischemia and reached maximum at 3 d. Ischemia resulted in approximately 80% neuronal death in the CA1 hippocampal region. Pretreatment with a 5-lipoxygenase inhibitor, AA861 resulted in significant attenuation of LTC4 levels (Baskaya et al. 1996. J. Neurosurg. 85: 112-116) and CA1 neuronal death. Accumulation of AA and LTC4, together with highly reactive oxygen radicals and lipid peroxides, may alter membrane permeability, resulting in BBB dysfunction, edema and ultimately to neuronal death.

Ischemic cell death in brain neurons

This review is directed at understanding how neuronal death occurs in two distinct insults, global ischemia and focal ischemia. These are the two principal rodent models for human disease. Cell death occurs by a necrotic pathway characterized by either ischemic/homogenizing cell change or edematous cell change. Death also occurs via an apoptotic-like pathway that is characterized, minimally, by DNA laddering and a dependence on caspase activity and, optimally, by those properties, additional characteristic protein and phospholipid changes, and morphological attributes of apoptosis. Death may also occur by autophagocytosis. The cell death process has four major stages. The first, the induction stage, includes several changes initiated by ischemia and reperfusion that are very likely to play major roles in cell death. These include inhibition (and subsequent reactivation) of electron transport, decreased ATP, decreased pH, increased cell Ca(2+), release of glutamate, increased arachidonic acid, and also gene activation leading to cytokine synthesis, synthesis of enzymes involved in free radical production, and accumulation of leukocytes. These changes lead to the activation of five damaging events, termed perpetrators. These are the damaging actions of free radicals and their product peroxynitrite, the actions of the Ca(2+)-dependent protease calpain, the activity of phospholipases, the activity of poly-ADPribose polymerase (PARP), and the activation of the apoptotic pathway. The second stage of cell death involves the long-term changes in macromolecules or key metabolites that are caused by the perpetrators. The third stage of cell death involves long-term damaging effects of these macromolecular and metabolite changes, and of some of the induction processes, on critical cell functions and structures that lead to the defined end stages of cell damage. These targeted functions and structures include the plasmalemma, the mitochondria, the cytoskeleton, protein synthesis, and kinase activities. The fourth stage is the progression to the morphological and biochemical end stages of cell death. Of these four stages, the last two are the least well understood. Quite little is known of how the perpetrators affect the structures and functions and whether and how each of these changes contribute to cell death. According to this description, the key step in ischemic cell death is adequate activation of the perpetrators, and thus a major unifying thread of the review is a consideration of how the changes occurring during and after ischemia, including gene activation and synthesis of new proteins, conspire to produce damaging levels of free radicals and peroxynitrite, to activate calpain and other Ca(2+)-driven processes that are damaging, and to initiate the apoptotic process. Although it is not fully established for all cases, the major driving force for the necrotic cell death process, and very possibly the other processes, appears to be the generation of free radicals and peroxynitrite. Effects of a large number of damaging changes can be explained on the basis of their ability to generate free radicals in early or late stages of damage. Several important issues are defined for future study. These include determining the triggers for apoptosis and autophagocytosis and establishing greater confidence in most of the cellular changes that are hypothesized to be involved in cell death. A very important outstanding issue is identifying the critical functional and structural changes caused by the perpetrators of cell death. These changes are responsible for cell death, and their identity and mechanisms of action are almost completely unknown.

Activation of the extracellular signal-regulated protein kinase cascade in the hippocampal CA1 region in a rat model of global cerebral ischemic preconditioning

A short period of sublethal preconditioning ischemia (3 min) followed by two days of reperfusion provides almost complete protection against ischemic cell death induced by a second (9 min) lethal ischemic episode. Here, we have investigated the extracellular signal-regulated protein kinase kinase and extracellular signal-regulated protein kinase, two kinases known to activate gene transcription and to be of importance for cell survival, after sublethal preconditioning ischemia in the rat hippocampal CA1 region. The activation levels of these two kinases were also studied after a second ischemic episode both in preconditioned and nonconditioned brains. An increased phosphorylation of the extracellular signal-regulated protein kinase kinase was found in neuronal cell bodies, particularly in the nucleus, 30 min, 4 h and two days of reperfusion after preconditioning ischemia. Two days after preconditioning ischemia both extracellular signal-regulated protein kinase kinase and extracellular signal-regulated protein kinase were markedly phosphorylated. During the early reperfusion period (30 min) after the second ischemic insult the phosphorylation levels of these two kinases were increased in both nonconditioned and preconditioned brains. In the late reperfusion time (one day), the phosphorylation levels of the extracellular signal-regulated protein kinase kinase and extracellular signal-regulated protein kinase were decreased in preconditioned brains, but remained elevated in nonconditioned brains. We conclude that phosphorylation of the extracellular signal-regulated protein kinase kinase and extracellular signal-regulated protein kinase after sublethal ischemia correlates with the neuroprotection induced by preconditioning, possibly by transcriptional activation of neuroprotective genes. Also, preconditioning enhances normalization of the disturbed cell signaling through the extracellular signal-regulated protein kinase cascade induced by lethal ischemia.

Changes in protein tyrosine phosphorylation in the rat brain after cerebral ischemia in a model of ischemic tolerance

A brief period of sublethal cerebral ischemia, followed by several days of recovery, renders the brain resistant to a subsequent lethal ischemic insult, a phenomenon termed ischemic preconditioning or tolerance. Ischemic tolerance was established in the rat two-vessel occlusion model of ischemia, induced by occlusion of both carotid arteries in combination with hypotension. Ischemic preconditioning (3 minutes) provided maximal neuroprotection when induced 2 days prior to a lethal ischemic insult of 9-minute duration. Neuroprotection persisted for at least 8 weeks. Since neurotransmission has been implicated in ischemic cell death, the effect of ischemic preconditioning on tyrosine phosphorylation of proteins and on the levels of glutamate receptor subunits in hippocampus and neocortex was studied. Regional levels of tyrosine phosphorylation of proteins in general and the N-methyl-D-aspartate receptor subunit NR2 in particular are markedly enhanced after ischemia in nonconditioned brains, in both the synaptosomal fraction and the whole-tissue homogenate of rat neocortex and hippocampus, but recover to control levels only in the preconditioned brain. Ischemic preconditioning selectively induces a decrease in the levels of the NR2A and NR2B subunits and a modest decrease in the levels of NR1 subunit proteins in the synaptosomal fraction of the neocortex but not hippocampus after the second lethal ischemia. It was concluded that ischemic preconditioning prevents a persistent change in cell signaling as evidenced by the tyrosine phosphorylation of proteins after the second lethal ischemic insult, which may abrogate the activation of detrimental cellular processes leading to cell death.

Increased intracellular Ca2+ concentration in the hippocampal CA1 area during global ischemia and reperfusion in the rat: a possible cause of delayed neuronal death

The crucial role of free cytosolic Ca2+ in ischemic neuronal damage has been studied in recent years. In the present report, changes in the intracellular Ca2+ concentration in the hippocampal CA1 area during transient global ischemia and reperfusion were measured using in vivo Ca2+ fluorometry with fura-2 in the four-vessel occlusion and reperfusion model in halothane-anesthetized rats. Marked changes were seen during 10-min global ischemia, with the intracellular Ca2+ concentration increasing gradually following application of the ischemic insult and rapidly about 2 min after the beginning of ischemia, and continuing to increase until reperfusion. On reperfusion, the intracellular Ca2+ concentration began to decrease and returned to the pre-ischemic level within 15 min. Induction of severe global ischemia was confirmed by the complete suppression of synaptic activity and the decrease in hippocampal temperature in the CA1 area. After seven days, CA1 pyramidal cell loss was observed histopathologically in the same rats which had undergone measurement of the intracellular Ca2+ concentration changes. In the present study, a temporal profile of the free cytosolic Ca2+ dynamics during ischemic and early post-ischemic period was determined in vivo. The results demonstrate that the intracellular Ca2+ concentration in the hippocampal CA1 area is transiently and markedly increased during a brief ischemia-inducing delayed neuronal death, implying that Ca2+ overload during cerebral ischemia is a possible cause of the delayed cell death of CA1 pyramidal neurons.

Competing processes of cell death and recovery of function following ischemic preconditioning

The goal of the present study was to determine the neuroprotective efficacy of ischemic preconditioning using behavioral, electrophysiological and histological endpoints at various time points up to 90 days postischemia. Gerbils were exposed to a brief, non-injurious episode of forebrain ischemia (1.5 min) on each of 2 consecutive days. Three days following this preconditioning procedure, the animals received a 5 min occlusion. Other animals underwent sham surgery or a 5 min occlusion without preconditioning. Ischemic preconditioning appeared to provide striking histological protection at both rostral (approximately 80% and approximately 67% of sham) and posterior levels of hippocampus (approximately 94% and approximately 78% of sham) at 3 and 10 days survival, respectively. However, in spite of the near normal number of CA1 neurons, animals displayed marked impairments in an open field test of habituation as well as reduced dendritic field potentials in the CA1 area. Additionally, in ischemic animals the basal and apical dendritic regions of CA1 were nearly devoid of the cytoskeletal protein microtubule associated protein 2 (MAP2). Staining levels of MAP2 in preconditioned and sham animals were similar. With increasing survival time, open field behavior as well as CA1 field potential amplitude recovered. Nonetheless, CA1 cell death in ischemic preconditioned animals continued over the 90-day survival period (P<0.05, vs. sham levels). Ischemic preconditioning provides a significant degree of neuroprotection characterized by a complex interplay of protracted cell death and neuroplasticity (recovery of function). These competing processes are best elucidated using a combination of functional and histological endpoints as well as multiple and extended survival times (i.e., greater than 7-10 days).

Histopathologic clues to the pathways of neuronal death following ischemia/hypoxia

This review describes histopathologic observations made with both light and electron microscopy using both conventional staining techniques and histochemistry. Several conditions are analyzed: Ischemic cell change; delayed neuronal death; selective vulnerability. The histopathologic support for the calcium hypothesis and for the excitotoxic hypothesis explaining neuronal death is also reviewed. The findings lead to several suggestions relevant to attempts at developing interventional therapies administered after the onset of ischemia/hypoxia. (1) Except in gerbils, delayed neuronal death and more rapid neuronal death appear to be on the same continuum of cellular events. The lag between ischemia and either onset or termination of these shared events depends upon the severity and/or duration of ischemia/hypoxia. We still do not know whether the "delay," when it occurs, is a delay between ischemia and initiation of the lethal sequence or is, instead, a delay between an immediate initiation of the sequence and its lethal termination. (2) Selective vulnerability (e.g., of CA1 sector in hippocampus) is only relative. The changes are again those of ischemic cell change and are identical to the changes seen elsewhere in more severe ischemia. (3) There is histopathologic support for both the calcium hypothesis and for the cytotoxic hypothesis. Indeed, there is histopathologic support linking the two hypotheses and linking these mechanisms to the appearance of ischemic cell change. However, the histopathologic data are surprisingly sparse. The role of either hypothesis in explaining neuronal death in all areas of brain, in all types of ischemic insult, and at all times following such an insult remains to be established. (3) Apoptosis may be an important mode of neuronal death following ischemia. It differs from acute ischemic cell change; nevertheless, both calcium overload and/or excitotoxic neurotransmitters may trigger apoptosis. (4) Third cell change has been described: Eosinophilic neurons that are not shrunken and whose nuclei are not pyknotic but contain clumped chromatin. The pathogenesis and fate of these neurons remains uncertain. It is possible that they represent early apoptotic neurons. Adequate assessment of apoptosis and its relationship (to both these neurons and to neurons displaying classical ischemic cell change) may depend upon dual staining with conventional aniline dyes and with histochemical techniques designed to detect intranuclear fragments of DNA.

Neuroprotective nitric oxide synthase inhibitor reduces intracellular calcium accumulation following transient global ischemia in the gerbil

By observing the ultrastructural intracellular Ca2+ distribution with Ca(2+)-oxalate-pyroantimonate method, we examined whether the protective mechanism of the nitric oxide (NO) synthase inhibitor, N omega-nitro-L-arginine (LNNA), involves change of the intracellular Ca2+ movement in delayed neuronal death (DND) in gerbil hippocampal CA1 neurons following 5-min forebrain ischemia. In the group intraventricularly administered 5.0 mg/ml LNNA, 15 min after reperfusion the intracellular Ca2+ deposits and the mitochondrial Ca2+ uptake index increased to levels similar to those in the control group administered only artificial cerebro-spinal fluid, but by 180 min after reperfusion they had returned to the preischemic level. By 15 min after reperfusion Ca2+ deposits in the endoplasmic reticulum (ER) had almost disappeared in both groups, but at 180 min of reperfusion, the ER in only the LNNA group showed Ca2+ deposits. It is suggested that the neuronal toxicity of NO involves the dysfunction of the intracellular Ca2+ transport system including the mitochondria and ER.