Opposing roles for caspase and calpain death proteases in L-glutamate-induced oxidative neurotoxicity

Rhinacanthin-C but Not -D Extracted from Rhinacanthus nasutus (L.) Kurz Offers Neuroprotection via ERK, CHOP, and LC3B Pathways

Neurodegenerative diseases present an increasing problem as the world’s population ages; thus, the discovery of new drugs that prevent diseases such as Alzheimer’s, and Parkinson’s diseases are vital. In this study, Rhinacanthin-C and -D were isolated from Rhinacanthus nasustus, using ethyl acetate, followed by chromatography to isolate Rhinacanthin-C and -D. Both compounds were confirmed using NMR and ultra-performance-LCMS. Using glutamate toxicity in HT-22 cells, we measured cell viability and apoptosis, ROS build-up, and investigated signaling pathways. We show that Rhinacanthin-C and 2-hydroxy-1,4-naphthoquinone have neuroprotective effects against glutamate-induced apoptosis in HT-22 cells. Furthermore, we see that Rhinacanthin-C resulted in autophagy inhibition and increased ER stress. In contrast, low concentrations of Rhinacanthin-C and 2-hydroxy-1,4-naphthoquinone prevented ER stress and CHOP expression. All concentrations of Rhinacanthin-C prevented ROS production and ERK1/2 phosphorylation. We conclude that, while autophagy is present in HT-22 cells subjected to glutamate toxicity, its inhibition is not necessary for cryoprotection.

The neuroprotection of deproteinized calf blood extractives injection against Alzheimer's disease via regulation of Nrf-2 signaling

Alzheimer’s disease (AD) is characterized by cognitive decline due to the accumulation of extracellular β-amyloid (Aβ) plaques and neurofibrillary tangles in the brain, which impair glutamate (Glu) metabolism. Deproteinized Calf Blood Extractive Injection (DCBEI) is a biopharmaceutical that contains 17 types of amino acids and 5 types of nucleotides. In this study, we found that DCBEI pretreatment reduced L-Glu-dependent neuroexcitation toxicity by maintaining normal mitochondrial function in HT22 cells. DCBEI treatment also reduced the expression of pro-apoptosis proteins and increased the expression of anti-apoptosis proteins. Furthermore, DCBEI attenuated AD-like behaviors (detected via the Morris water maze test) in B6C3-Tg (APPswePSEN1dE9)/Nju double transgenic (APP/PS1) mice; this effect was associated with a reduction in the amount of Aβ and neurofibrillary tangle deposition and the concomitant reduction of phospho-Tau in the hippocampus. Metabonomic profiling revealed that DCBEI regulated the level of neurotransmitters in the hippocampus of APP/PS1 mice. Label-free proteomics revealed that DCBEI regulated the expression of Nrf-2 and its downstream targets, as well as the levels of phospho-protein kinase B and mitogen-activated protein kinase. Together, these data show that DCBEI can ameliorate AD symptoms by upregulating Nrf2-mediated antioxidative pathways and thus preventing mitochondrial apoptosis.

Using the Oxytosis/Ferroptosis Pathway to Understand and Treat Age-Associated Neurodegenerative Diseases

Oxytosis was first described over 30 years ago in nerve cells as a non-excitotoxic pathway for glutamate-induced cell death. The key steps of oxytosis, including glutathione depletion, lipoxygenase activation, reactive oxygen species accumulation, and calcium influx, were identified using a combination of chemical and genetic tools. A pathway with the same characteristics as oxytosis was identified in transformed fibroblasts in 2012 and named ferroptosis. Importantly, the pathophysiological changes seen in oxytosis and ferroptosis are also observed in multiple neurodegenerative diseases as well as in the aging brain. This led to the hypothesis that this pathway could be used as a screening tool to identify novel drug candidates for the treatment of multiple age-associated neurological disorders, including Alzheimer's disease (AD). Using this approach, we have identified several AD drug candidates, one of which is now in clinical trials, as well as new target pathways for AD.

The molecular mechanisms of MLKL-dependent and MLKL-independent necrosis

Necrosis, a type of unwanted and passive cell demise, usually occurs under the excessive external stress and is considered to be unregulated. However, under some special conditions such as caspase inhibition, necrosis is regulable in a well-orchestrated way. The term 'regulated necrosis' has been proposed to describe such programed necrosis. Recently, several forms of necrosis, including necroptosis, pyroptosis, ferroptosis, parthanatos, oxytosis, NETosis, and Na+/K+-ATPase-mediated necrosis, have been identified, and some crucial regulators governing regulated necrosis have also been discovered. Mixed lineage kinase domain-like pseudokinase (MLKL), a core regulator in necroptosis, acts as an executioner in response to ligands of death receptor family. Its activation requires the receptor-interacting protein kinases, RIP1 and RIP3. However, MLKL is only involved in necroptosis, i.e. MLKL is dispensable for necrosis. Therefore, this review is aimed at summarizing the molecular mechanisms of MLKL-dependent and MLKL-independent necrosis.

N-acetylcysteine reduces glutamate-induced cytotoxicity to fibroblasts of rat supraspinatus tendons

Purpose: Neuronal theory regarding rotator cuff degeneration has developed from the findings that glutamate, an amino acid and an excitatory neurotransmitter, is present in increased concentrations in tendon tissues with tendinopathy and that glutamate induces cell death in fibroblasts of origin in rat supraspinatus tendon. The purpose of the current study was to determine whether N-acetylcysteine (NAC) has cytoprotective effects against glutamate-induced fibroblast death. Materials and Methods: Primary cultured fibroblasts were obtained from rat supraspinatus tendons. Varying concentrations of glutamate (0.5, 1, 5, and 10 mM) and of NAC (0.5, 1, 2, and 5 mM) were used for evaluation of cytotoxicity. Cell viability, cell cycles, types of cell death, intracellular ROS production, expressions of caspase-3/7, and Ca²⁺ influx were evaluated. Results: Glutamate significantly induced cell death, apoptosis, and Ca²⁺ influx and significantly increased caspase-3/7 activity and intracellular ROS production (p < 0.001). NAC significantly reduced the glutamate-induced cell death, apoptosis, Ca²⁺ influx, caspase-3/7 activity, and intracellular ROS production (p < 0.001). Conclusions: The glutamate-induced cytotoxic effects can be reduced by NAC, an antioxidant, through the reduction of intracellular oxidative stress and/or Ca²⁺ influx.

Oxytosis/Ferroptosis-(Re-) Emerging Roles for Oxidative Stress-Dependent Non-apoptotic Cell Death in Diseases of the Central Nervous System

Although nerve cell death is the hallmark of many neurological diseases, the processes underlying this death are still poorly defined. However, there is a general consensus that neuronal cell death predominantly proceeds by regulated processes. Almost 30 years ago, a cell death pathway eventually named oxytosis was described in neuronal cells that involved glutathione depletion, reactive oxygen species production, lipoxygenase activation, and calcium influx. More recently, a cell death pathway that involved many of the same steps was described in tumor cells and termed ferroptosis due to a dependence on iron. Since then there has been a great deal of discussion in the literature about whether these are two distinct pathways or cell type- and insult-dependent variations on the same pathway. In this review, we compare and contrast in detail the commonalities and distinctions between the two pathways concluding that the molecular pathways involved in the regulation of ferroptosis and oxytosis are highly similar if not identical. Thus, we suggest that oxytosis and ferroptosis should be regarded as two names for the same cell death pathway. In addition, we describe the potential physiological relevance of oxytosis/ferroptosis in multiple neurological diseases.

Neuroprotective Effects of Kinetin Against Glutamate-Induced Oxidative Cytotoxicity in HT22 Cells: Involvement of Nrf2 and Heme Oxygenase-1

Oxidative stress is considered as one of key factors related to Alzheimer's disease (AD), while kinetin (KT) has been reported to exert anti-oxidative activities as well as neuroprotective effects both in vivo and in vitro. Thus, in this study, the neuroprotective effects of KT against glutamate-induced oxidative toxicity in HT22 cells were investigated. To evaluate the anti-oxidative capabilities of KT itself, several anti-oxidative assays in vitro were conducted. To evaluate the neuroprotective effects of KT, the levels of intracellular reactive oxygen species (ROS) and calcium influx, mitochondrial membrane potential (MMP), and cell death were measured by flow cytometry. Nuclear translocation of apoptosis inducing factor (AIF) and content of intracellular ATP were also determined. In addition, the phosphorylation levels of apoptosis signal-regulating kinase 1 (ASK-1), c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinases (p38) were evaluated as well. Besides, nuclear translocation of nuclear factor-E2-related factor 2 (Nrf2) and the expression of heme oxygenase-1 (HO-1) were also examined to reveal underlying mechanisms. Results showed that KT rescued cell death, and suppressed the accumulation of intracellular ROS and the increase of intracellular calcium influx. In addition, KT maintained normal function of mitochondria and inhibited the phosphorylation of ASK-1, JNK, and p38. KT also promoted nuclear translocation of Nrf2 and enhanced the expression of HO-1 both at protein and mRNA level. Importantly, blockage of Nrf2 almost completely abolished the neuroprotective effects of KT, while blockage of HO-1 expression partly neutralized its neuroprotective effects. Our results indicated that KT can protect HT22 cells from glutamate-induced cell death by activating Nrf2 pathway and inducing expression of HO-1, suggesting KT might be a drug candidate for treatment of AD and other neurodegenerative disorders related to oxidative stress.

The role of Ca2+ in cell death caused by oxidative glutamate toxicity and ferroptosis

Ca²⁺ ions play a fundamental role in cell death mediated by oxidative glutamate toxicity or oxytosis, a form of programmed cell death similar and possibly identical to other forms of cell death like ferroptosis. Ca²⁺ influx from the extracellular space occurs late in a cascade characterized by depletion of the intracellular antioxidant glutathione, increases in cytosolic reactive oxygen species and mitochondrial dysfunction. Here, we aim to compare oxidative glutamate toxicity with ferroptosis, address the signaling pathways that culminate in Ca²⁺ influx and cell death and discuss the proteins that mediate this. Recent evidence hints toward a role of the machinery responsible for store-operated Ca²⁺ entry (SOCE), which refills the endoplasmic reticulum (ER) after receptor-mediated ER Ca²⁺ release or other forms of store depletion. Pharmacological inhibition of SOCE or transcriptional downregulation of proteins involved in SOCE like the ER Ca²⁺ sensor STIM1, the plasma membrane Ca²⁺ channels Orai1 and TRPC1 and the linking protein Homer protects against oxidative glutamate toxicity and direct oxidative stress caused by hydrogen peroxide or 1-methyl-4-phenylpyridinium (MPP+) injury, a cellular model of Parkinson's disease. This suggests that SOCE inhibition might have some potential therapeutic effects in human disease associated with oxidative stress like neurodegenerative disorders.

Neuroprotective effects of 2,3,5,4'-tetrahydoxystilbene-2-O-β-D-glucoside from Polygonum multiflorum against glutamate-induced oxidative toxicity in HT22 cells

ETHNOPHARMACOLOGICAL RELEVANCE

Since ancient times, Polygonum multiflorum Thunb. has been used to treat premature grey hair, dizziness, and blurred vision in East Asia. A major bioactive constituent of this medicinal herb, 2,3,5,4'-tetrahydoxystilbene-2-O-β-D-glucoside (THSG), has antioxidant activity and exerts beneficial effects on cognition and memory.

AIM OF THE STUDY

The purpose of the current study was to determine if THSG affects hippocampal neuronal cell death and mitochondrial function following exposure to oxidative stress.

MATERIALS AND METHODS

HT22 hippocampal cells with or without THSG pretreatment were exposed to glutamate, and the effects on cell viability and expression of molecules related to apoptotic cell death were examined using biochemical techniques, flow cytometry, western immunoblotting, and real-time polymerase chain reaction.

RESULTS

Pretreatment with THSG significantly attenuated glutamate-induced loss of cell viability and release of lactate dehydrogenase as well as apoptotic cell death. THSG inhibited generation of reactive oxygen species (ROS), expression of heme oxygenase-1, and activation of caspase-3 and calpain-1 proteases, all of which were increased by glutamate. THSG inhibited glutamate-induced disruption of mitochondrial membrane potential (MMP) and voltage-dependent anion channel-1. It also regulated the ratio of Bax to Bcl-2.

CONCLUSIONS

These results indicate that THSG has a marked neuroprotective effect against glutamate-induced hippocampal damage by decreasing ROS production and stabilizing MMP. These findings suggest the potential of THSG as a new therapeutic agent for the treatment of cognitive disorders.

Neuroprotective effects of α-iso-cubebenol on glutamate-induced neurotoxicity

α-Iso-cubebenol is a natural compound isolated from Schisandra chinensis, and is reported to have beneficial bioactivity including anti-inflammatory and anti-tumor activities. Glutamate-induced oxidative neuronal damage has been implicated in a variety of neurodegenerative disorders. Here we investigated the mechanisms of α-iso-cubebenol protection of mouse hippocampus-derived neuronal cells (HT22 cells) from apoptotic cell death induced by the major excitatory neurotransmitter, glutamate. Pretreatment with α-iso-cubebenol markedly attenuated glutamate-induced loss of cell viability and release of lactate dehydrogenase), in a dose-dependent manner. α-Iso-cubebenol significantly reduced glutamate-induced intracellular reactive oxygen species and calcium accumulation. Strikingly, α-iso-cubebenol inhibited glutamate-induced mitochondrial depolarization, which releases apoptosis-inducing factor from mitochondria. α-Iso-cubebenol also suppressed glutamate-induced phosphorylation of extracellular-signal-regulated kinases. Furthermore, α-iso-cubebenol induced CREB phosphorylation and Nrf-2 nuclear accumulation and increased the promoter activity of ARE and CREB in HT22 cells. α-Iso-cubebenol also upregulated the expression of phase-II detoxifying/antioxidant enzymes such as HO-1 and NQO1. Subsequent studies revealed that the inhibitory effects of α-iso-cubebenol on glutamate-induced apoptosis were abolished by small interfering RNA-mediated knockdown of CREB and Nrf-2. These findings suggest that α-iso-cubebenol prevents excitotoxin-induced oxidative damage to neurons by inhibiting apoptotic cell death, and might be a potential preventive or therapeutic agent for neurodegenerative disorders.

A single fraction from Uncaria sinensis exerts neuroprotective effects against glutamate-induced neurotoxicity in primary cultured cortical neurons

We identified a neuroprotective single fraction among 62 ones of hexane extract from Uncaria sinensis (JGH43IA) and investigated its effects and mechanisms in primary cortical neurons. Pretreatment with JGH43IA showed a significantly increase cell viability in a dose-dependent manner with a decrease in the lactate dehydrogenase release. When we performed morphological assay and flow cytometry to determination of the type of cell death, pretreatment with JGH43IA showed a significant reduction of glutamate-induced apoptotic cell death. Then we explored the downstream signaling pathways of N-methyl-D-aspartate receptor (NMDAR) with calpain activation to elucidate possible pathways of neuroprotection by JGH43IA. Pretreatment with JGH43IA exhibited a significant attenuation of NMDAR GluN2B subunit activation and a decrease in active form of calpain 1 leading to subsequent cleavage of striatal-enriched protein tyrosine phosphatase (STEP). In addition, pretreatment with JGH43IA showed a marked increase of cAMP responsive element binding protein. These results suggest that JGH43IA may have neuroprotective effects through down-regulation of NMDAR GluN2B subunit and calpain 1 activation, and subsequent alleviation of STEP cleavage. This single fraction from U. sinensis might be a useful therapeutic agent for brain disorder associated with glutamate injury.

Kaempferol attenuates the glutamate-induced oxidative stress in mouse-derived hippocampal neuronal HT22 cells

It is thought that the neuronal cell loss caused by oxidative stress is the primary mechanism underlying the pathogenesis of several neurodegenerative disorders. Glutamate is an endogenous neurotransmitter, but at high concentrations it can act as a neurotoxicant by increasing the intracellular levels of reactive oxygen species (ROS). Therefore, the development of factors that can attenuate glutamate-induced oxidative stress in neuronal cells is a good strategy by which new drugs could be discovered that may treat or prevent neurodegenerative diseases. Here, the neuroprotective effects of kaempferol (KF) isolated from the stems of butterbur (Petasites japonicus) were examined in glutamate-treated hippocampal neuronal cells (HT22). The administration of KF (25 μM) resulted in a significant increase in cell viability (105.18 ± 7.48%) compared with the control (100.00 ± 3.05%), while glutamate (5 mM) reduced cell viability by 39.94 ± 1.61%. The glutamate-induced calcium (Ca(2+)) influx (1.93 ± 0.08-fold) was significantly reduced by 0.89 ± 0.02-fold following the administration of 25 μM KF. Additionally, when HT22 cells were stressed with excessive glutamate, there was a 3.70 ± 0.01-fold increase in intracellular ROS generation, even though this was effectively attenuated by KF (25 μM, 0.72 ± 0.01-fold). The protective effects of KF in HT22 cells were later confirmed using a lactate dehydrogenase (LDH) assay and a FITC-annexin V/propidium iodide double staining procedure. These findings also revealed that the neuroprotective effects of KF are a result of the regulation of the expression levels of proteins, such as Bcl-2, Bid, apoptosis-inducing factor (AIF), and mitogen-activated protein kinase (MAPK). This is the first report to investigate the neuroprotective influence of KF in glutamate-treated HT22 cells. These data demonstrate that KF may be a useful candidate for pharmacological therapies that can prevent and treat neurodegenerative diseases such as Alzheimer's disease (AD).

Neuroprotective effects of Polygonum multiflorum extract against glutamate-induced oxidative toxicity in HT22 hippocampal cells

ETHNOPHARMACOLOGICAL RELEVANCE

Dried roots of Polygonum multiflorum have traditionally been used in the retarding of aging process in East Asian countries and its extracts exhibit anti-oxidative activities.

MATERIALS AND METHODS

Neuroprotective effects of ethyl acetate extract from Polygonum multiflorum (EEPM) were investigated against glutamate-induced oxidative cell death in HT22 hippocampal cells. Cell viability, cytotoxicity, morphological, flow cytometry, and Western blot assays were performed in order to observe alterations of neuronal cell survival or death related pathways.

RESULTS

Pretreatment with EEPM resulted in significantly decreased glutamate-induced neurotoxicity and also resulted in drastically inhibited glutamate-induced apoptotic and necrotic neuronal death. To elucidate possible pathways of neuroprotection by EEPM, we explored the activation of mitogen activated protein kinases (MAPKs), phosphatidylinositol-3-kinase, and cAMP responsive element binding protein (CREB). Treatment with glutamate alone led to activation of extracellular regulated kinase (ERK), Jun N-terminal kinase, and p38 during the late phase after glutamate exposure, but pretreatment with EEPM resulted in significantly attenuated activation of these proteins. Pretreatment with EEPM resulted in increased activation of CREB. The specific inhibitors of ERK and p38, PD98059 and SB203580, abrogated the neuroprotective effects of EEPM. When we evaluated calpain I and striatal-enriched protein tyrosine phosphatase (STEP), active form of calpain I was significantly increased after glutamate exposure, and, along with this, active form of STEP showed a decrease. Pretreatment with EEPM resulted in significant recovery of pro-calpain I and active form of STEP caused by glutamate. Co-treatment with calpain inhibitor ALLN and EEPM had a synergistic effect on neuronal death and contributed to blockade of activation of both ERK and p38 with increased activation of CREB.

CONCLUSIONS

These results suggest that Polygonum multiflorum extract may have neuroprotective effects through both alleviation of ERK and p38 activation with increased activation of CREB under oxidative stress and has potential as a therapeutic intervention for treatment of oxidative neuronal death.

The cystine/glutamate antiporter system x(c)(-) in health and disease: from molecular mechanisms to novel therapeutic opportunities

The antiporter system x(c)(-) imports the amino acid cystine, the oxidized form of cysteine, into cells with a 1:1 counter-transport of glutamate. It is composed of a light chain, xCT, and a heavy chain, 4F2 heavy chain (4F2hc), and, thus, belongs to the family of heterodimeric amino acid transporters. Cysteine is the rate-limiting substrate for the important antioxidant glutathione (GSH) and, along with cystine, it also forms a key redox couple on its own. Glutamate is a major neurotransmitter in the central nervous system (CNS). By phylogenetic analysis, we show that system x(c)(-) is a rather evolutionarily new amino acid transport system. In addition, we summarize the current knowledge regarding the molecular mechanisms that regulate system x(c)(-), including the transcriptional regulation of the xCT light chain, posttranscriptional mechanisms, and pharmacological inhibitors of system x(c)(-). Moreover, the roles of system x(c)(-) in regulating GSH levels, the redox state of the extracellular cystine/cysteine redox couple, and extracellular glutamate levels are discussed. In vitro, glutamate-mediated system x(c)(-) inhibition leads to neuronal cell death, a paradigm called oxidative glutamate toxicity, which has successfully been used to identify neuroprotective compounds. In vivo, xCT has a rather restricted expression pattern with the highest levels in the CNS and parts of the immune system. System x(c)(-) is also present in the eye. Moreover, an elevated expression of xCT has been reported in cancer. We highlight the diverse roles of system x(c)(-) in the regulation of the immune response, in various aspects of cancer and in the eye and the CNS.

Effects of inhibitors on the synergistic interaction between calpain and caspase-3 during post-mortem aging of chicken meat

Calpain has been considered to be the most important protease involved in tenderization during the conversion of muscle into meat. However, recent evidence suggests the possible involvement of the key apoptosis protease, caspase, on post-mortem tenderization. This study used inhibitors of calpain and caspase-3 to treat chicken muscle immediately after slaughter and followed the changes in caspase-3 and calpain activities together with their expression during 5 days of aging. Addition of calpain inhibitors to the system resulted in significantly higher caspase-3 activities (p < 0.01) during storage. Western blot analysis of pro-caspase-3 and α-spectrin cleavage of the 120 kDa peptide (SBDP 120) showed that the addition of calpain inhibitors resulted in the formation of higher amounts of the active form of caspase-3 compared with the control (p < 0.01). Inclusion of inhibitors of caspase-3 led to lower calpain activities (p < 0.01) and dramatically reduced the expression of calpain-1 and calpain-2 (p < 0.01). Concomitantly, this inhibition resulted in greater calpastatin expression compared with the control (p < 0.01). The findings of this investigation show that calpain prevented the activation of caspase-3, whereas caspase-3 appeared to enhance the calpain activity during post-mortem aging through inhibition of calpastatin. It is therefore suggested that there is a relationship between caspase-3 and calpain which contributes to the tenderizing process during the conversion of muscle tissue into meat.

Asiatic acid, a pentacyclic triterpene in Centella asiatica, attenuates glutamate-induced cognitive deficits in mice and apoptosis in SH-SY5Y cells

AIM

To investigate whether asiatic acid (AA), a pentacyclic triterpene in Centella asiatica, exerted neuroprotective effects in vitro and in vivo, and to determine the underlying mechanisms.

METHODS

Human neuroblastoma SH-SY5Y cells were used for in vitro study. Cell viability was determined with the MTT assay. Hoechst 33342 staining and flow cytometry were used to examine the apoptosis. The mitochondrial membrane potential (MMP) and reactive oxygen species (ROS) were measured using fluorescent dye. PGC-1α and Sirt1 levels were examined using Western blotting. Neonatal mice were given monosodium glutamate (2.5 mg/g) subcutaneously at the neck from postnatal day (PD) 7 to 13, and orally administered with AA on PD 14 daily for 30 d. The learning and memory of the mice were evaluated with the Morris water maze test. HE staining was used to analyze the pyramidal layer structure in the CA1 and CA3 regions.

RESULTS

Pretreatment of SH-SY5Y cells with AA (0.1-100 nmol/L) attenuated toxicity induced by 10 mmol/L glutamate in a concentration-dependent manner. AA 10 nmol/L significantly decreased apoptotic cell death and reduced reactive oxygen species (ROS), stabilized the mitochondrial membrane potential (MMP), and promoted the expression of PGC-1α and Sirt1. In the mice models, oral administration of AA (100 mg/kg) significantly attenuated cognitive deficits in the Morris water maze test, and restored lipid peroxidation and glutathione and the activity of SOD in the hippocampus and cortex to the control levels. AA (50 and 100 mg/kg) also attenuated neuronal damage of the pyramidal layer in the CA1 and CA3 regions.

CONCLUSION

AA attenuates glutamate-induced cognitive deficits of mice and protects SH-SY5Y cells against glutamate-induced apoptosis in vitro.

Contribution of a mitochondrial pathway to excitotoxic neuronal necrosis

It is traditionally thought that excitotoxic necrosis is a passive mechanism that does not require the activation of a cell death program. In this study, we examined the contribution of the cytochrome c-dependent mitochondrial death pathway to excitotoxic neuronal necrosis, induced by exposing cultured cortical neurons to 1 mM glutamate for 6 hr and blocked by the NMDA antagonist, dizocilpine. Glutamate treatment induced early cytochrome c release, followed by activation of caspase-9 and caspase-3. Preincubation with the caspase-9 inhibitor z-LEHD-fmk, the caspase-3 inhibitor z-DEVD-fmk, or the specific pan-caspase inhibitor Q-VD-oph decreased the percentage of propidium iodide-positive neurons (52.5% +/- 3.1%, 39.4% +/- 3.5%, 44.6% +/- 3%, respectively, vs. 65% +/- 3% in glutamate + vehicle). EM studies showed mitochondrial release of cytochrome c in neurons in the early stages of necrosis and cleaved caspase-3 immunoreactivity in morphologically necrotic neurons. These results suggest that an active mechanism contributes to the demise of a subpopulation of excitotoxic necrotic neurons.

Activation of autophagy during glutamate-induced HT22 cell death

Recent evidence suggests that autophagy plays a role in oxidative injury-induced cell death. Here we examined whether glutamate-mediated oxidative toxicity induces autophagy in murine hippocampal HT22 cells and if autophagy induction affects the molecular events associated with cell death. Markers for autophagy induction including LC3 conversion, suppression of mTOR pathway, and GFP-LC3 dot formation were enhanced by glutamate treatment. By contrast, autophagy inhibition blocked glutamate-induced LC3 conversion and consequently reduced cell death. Activation of ERK1/2, a hallmark of glutamate-induced cytotoxicity, was also decreased by autophagy inhibition. Interestingly, autophagy inhibition also affected the expression of chaperones including Hsp60 and Hsp70, which are differentially regulated during HT22 cell death. Conversely, knock-down of Hsp60 greatly decreased LC3 conversion. Together these results suggest that glutamate-induced cytotoxicity involves autophagic cell death and chaperones may play a role in this process.