Mitochondrial and extramitochondrial apoptotic signaling pathways in cerebrocortical neurons

Methamphetamine induces neuronal apoptosis via cross-talks between endoplasmic reticulum and mitochondria-dependent death cascades

Methamphetamine (METH) is an illicit drug that causes neurodegenerative effects in humans. In rodents, METH induces apoptosis of striatal glutamic acid decarboxylase (GAD) -containing neurons. This paper provides evidence that METH-induced cell death occurs consequent to interactions of ER stress and mitochondrial death pathways. Specifically, injections of METH are followed by an almost immediate activation of proteases calpain and caspase-12, events consistent with drug-induced ER stress. Involvement of ER stress was further supported by observations of increases in the expression of GRP78/BiP and CHOP. Participation of the mitochondrial pathway was demonstrated by the transition of AIF, smac/DIABLO, and cytochrome c from mitochondrial into cytoplasmic fractions. These changes occur before the apoptosome-associated pro-caspase-9 cleavage. Effector caspases-3 and -6, but not -7, were cleaved with the initial time of caspase-3 activation occurring before caspase 9 cleavage; this suggests possible earlier cleavage of caspase-3 by caspase-12. These events preceded proteolysis of the caspase substrates DFF-45, lamin A, and PARP in nuclear fractions. These findings indicate that METH causes neuronal apoptosis in part via cross-talks between ER- and mitochondria-generated processes, which cause activation of both caspase-dependent and -independent pathways.

Activated protein C prevents neuronal apoptosis via protease activated receptors 1 and 3

Activated protein C (APC), a serine protease with anticoagulant and anti-inflammatory activities, exerts direct cytoprotective effects on endothelium via endothelial protein C receptor-dependent activation of protease activated receptor 1 (PAR1). Here, we report that APC protects mouse cortical neurons from two divergent inducers of apoptosis, N-methyl-D-aspartate (NMDA) and staurosporine. APC blocked several steps in NMDA-induced apoptosis downstream to nitric oxide, i.e., caspase-3 activation, nuclear translocation of apoptosis-inducing factor (AIF), and induction of p53, and prevented staurosporine-induced apoptosis by blocking caspase-8 activation upstream of caspase-3 activation and AIF nuclear translocation. Intracerebral APC infusion dose dependently reduced NMDA excitotoxicity in mice. By using different anti-PARs antibodies and mice with single PAR1, PAR3, or PAR4 deletion, we demonstrated that direct neuronal protective effects of APC in vitro and in vivo require PAR1 and PAR3. Thus, PAR1 and PAR3 mediate anti-apoptotic signaling by APC in neurons, which may suggest novel treatments for neurodegenerative disorders.

HOXA5-induced apoptosis in breast cancer cells is mediated by caspases 2 and 8

HOXA5 is a transcriptional factor whose expression is lost in more than 60% of breast carcinomas. Our previous work demonstrated that the overexpression of HOXA5 in MCF7 cells resulted in cell death through a p53-dependent apoptotic pathway. To determine whether p53-independent apoptotic pathways are involved in HOXA5-induced cell death, we engineered a p53-mutant breast cancer cell line, Hs578T, to inducibly express HOXA5. Induction of HOXA5 expression led to cell death with features typical of apoptosis within 24 h, and the expression levels of mutant p53 and its target genes either decreased or remained unchanged. To decipher apoptotic pathways, the HOXA5-expressing cells were treated with a variety of apoptotic inhibitors. Besides a general caspase inhibitor, caspase 2- and 8-specific inhibitors largely abolished HOXA5-induced apoptosis, whereas caspase 1-, 3-, 6-, and 9-specific inhibitors had no significant effects. Western blot analysis further confirmed that caspases 2 and 8 were activated after the induction of HOXA5 expression. Further, several small interfering RNAs which specifically silenced caspase 2 and caspase 8 expression significantly blocked HOXA5-induced apoptosis. HOXA5 expression could also sensitize cells to tumor necrosis factor alpha-induced apoptosis by at least 100-fold. These results indicate that expression of HOXA5 can induce apoptosis through an apoptotic mechanism mediated by caspases 2 and 8.

Mitochondrial control of neuron death and its role in neurodegenerative disorders

Genetic or functional mitochondrial alterations can result in the initiation of cell death programs that are believed to contribute to cell death in diabetes, ageing and neurodegenerative disorders. Mitochondria are being considered the main link between cellular stress signals activated during acute and chronic nerve cell injury, and the execution of nerve cell death. This second function of mitochondria is regulated by several families of proteins that can trigger an increase in permeability of the outer and/or inner mitochondrial membrane. One example of this is the formation of the mitochondrial permeability transition pore (MPTP). This process can trigger the release of cell death-inducing factors from mitochondria, as well as a dissipation of the mitochondrial transmembrane potential, depletion of ATP, and increased free radical formation. Among the factors released from mitochondria are cytochrome c, the apoptosis inductor factor (AIF), and caspases. We review the role of the MPTP in diverse physiological and pathological processes, including neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis (ALS). The design of drugs that could interfere with the functions of the MPTP could allow novel therapeutic approaches for the treatment of acute and chronic nerve cell injury.

Neuronal gelsolin prevents apoptosis by enhancing actin depolymerization

Gelsolin (gsn), an actin-severing protein, protects neurons from excitotoxic cell death via inactivation of membranous Ca(2+) channels. Its role during apoptotic cell death, however, has remained unclear. Using several models of neuronal cell death, we demonstrate that endogenous gelsolin has anti-apoptotic properties that correlate to its dynamic actions on the cytoskeleton. We show that neurons lacking gelsolin (gsn(-/-)) have enhanced apoptosis following exposure to staurosporine, thapsigargin, or the cholinergic toxin ethylcholine aziridinium (AF64A). AF64A-induced loss of mitochondrial membrane potential and activation of caspase-3 was specifically enhanced in gsn(-/-) neurons and could be reversed by pharmacological inhibition of mitochondrial permeability transition. Moreover, increased caspase-3 activation and cell death in AF64A-treated gsn(-/-) neurons were completely reversed by pharmacological depolymerization of actin filaments and further enhanced by their stabilization. In conclusion, actin remodeling by endogenous gelsolin or analogues protects neurons from apoptosis mediated by mitochondria and caspase-3.

Role of mitochondria in neuron apoptosis during ischemia-reperfusion injury

To investigate the role of mitochondria in neuronal apoptosis, ischemia-reperfusion mediated neuronal cell injury model was established by depriving of glucose, serum and oxygen in media. DNA fragmentation, cell viability, cytochrome C releasing, caspase3 activity and mitochondrial transmembrane potential were observed after N2a cells suffered the insults. The results showed that N2a cells in ischemic territory exhibited survival damage, classical cell apoptosis change, DNA ladder and activation of caspase3. Apoptosis-related alterations in mitochondrial functions, including release of cytochrome C and depression of mitochondrial transmembrane potential (deltapsim) were testified in N2a cells after mimic ischemia-reperfusion. Moreover, activation of caspase3 occurred following the release of cytochrome C. However, the inhibitor of caspase3, Ac-DEVD-CHO, couldn't completely rescue N2a cells from apoptosis. Administration of cyclosporine A, an inhibitor of mitochondria permeability transition pore only partly inhibited caspase3 activity and reduced DNA damage. Interestingly, treatment of Z-IETD-FMK, an inhibitor of caspase8 could completely reverse DNA fragmentation, but can't completely inhibit caspase3 activity. It was concluded that there were caspase3 dependent and independent cellular apoptosis pathways in N2a cells suffering ischemia-reperfusion insults. Mitochondria dysfunction may early trigger apoptosis and amplify apoptosis signal.

Angiotensin II attenuates chemical hypoxia-induced caspase-3 activation in primary cortical neuronal cultures

In this study we determined whether caspase-3 is required in mouse cortical neurons for sodium azide-mediated apoptosis. Primary cortical neuronal cultures were treated with a cell permeable caspase-3 inhibitor, DEVD (1 nM-100 fM), prior to sodium azide-induced hypoxia. Treatment with the caspase-3 inhibitor resulted in a dose-dependent decrease in apoptosis, suggesting that sodium azide-induced apoptosis is mediated through a caspase-3 dependent pathway. Levels of cytochrome-c release and caspase-3 cleavage were assayed by Western analysis. Cytochrome-c release and caspase-3 cleavage were observed at 5 h (85.3+/-5.8%) and 8 h (53.4+/-14.9%), respectively. We have previously reported that angiotensin II, acting through the AT(2) receptor subtype, protects cultured mouse cortical neurons from sodium azide-induced apoptosis. We also examined whether the protective effect of angiotensin II is mediated through modulation of caspase-3. Pre-treatment of cells with angiotensin II and the AT(1) receptor antagonist, losartan, reduced levels of sodium azide-induced caspase-3 cleavage by 95.0+/-4.0%. Cells pre-treated with the AT(2) receptor antagonist, PD123319 showed a smaller reduction of caspase-3 cleavage (53.8+/-3.4%). Our findings indicate that sodium azide-induced apoptosis is caspase-3 dependent and that angiotensin II protects cortical neurons from chemical-induced apoptosis by reducing caspase-3 cleavage.

Mitochondrial fission in apoptosis, neurodegeneration and aging

A decline in mitochondrial function is well recognized in neurodegenerative diseases and aging, and is thought to play a causal role in their biology. Unfortunately, the molecular basis underlying this detrimental loss in mitochondrial function remains mysterious. Interestingly, mitochondria undergo frequent fission and fusion. This process is regulated by molecular machinery that has been highly conserved during evolution, including dynamin-related GTPases that manifest opposing effects. A balance between mitochondrial fission and fusion events is required for normal mitochondrial and cellular function. Emerging evidence indicates that mitochondria undergo rapid and extensive fission at an early stage during apoptosis. A clue that these new findings are of significance for the pathogenesis of neurodegenerative disease is provided by the observation that OPA-1, a dynamin-related GTPase regulating mitochondrial fusion, is mutated in humans with dominant optic atrophy, which is characterized by degeneration of retinal ganglion cells and childhood blindness. Loss of function of OPA-1, analogous to deficiency of its yeast homologue, Mgm1p, is expected to lead to mitochondrial fission, loss of mitochondrial DNA, respiratory deficits and an increase in reactive oxygen species. Here we review the molecular mediators controlling mitochondrial fission and fusion, and how death effector molecules may hijack this ancient machinery. A shift in the rate of mitochondrial fission or fusion may provide a new mechanistic explanation for the mitochondrial dysfunction in neurodegenerative diseases and normal aging, and may offer a new target for therapeutic intervention.

Nitric-oxide-induced depolarization of neuronal mitochondria: implications for neuronal cell death

Nitric oxide (NO(*)) has known toxic effects on central nervous system neurons. This study characterized the effect of NO(*) on mitochondrial membrane changes by exploring the relationship among NO(*), excitatory receptor activation, and the induction of peroxynitrite, a highly toxic NO(*) reactant, to neuronal injury. Cultured rat cortical neurons were exposed to the NO(*) generator, diethylenetriamine/nitric oxide adduct, and were examined for signs of cell death, mitochondrial membrane potential changes (Deltapsi(m)), and the induction of a mitochondrial permeability transition (MPT). Neurons were also examined for nitrotyrosine (NT) immunoreactivity, a marker of reactive nitrogen species (RNS) formation. Neurons exposed to NO(*) or to N-methyl-D-aspartate (NMDA) exhibited similar rapid depolarization of mitochondria, which was prevented by an NMDA receptor antagonist. Electrophysiological studies demonstrated NO(*) potentiation of NMDA-induced NMDA receptor currents. NO(*) and NMDA-treated neurons had evidence of mitochondrial-specific NT immunoreactivity that was prevented by a SOD/catalase mimetic (EUK-134). EUK-134 treatment reduced both NO(*) and NMDA-induced NT formation and neuronal cell death. EUK-134 did not prevent NO-induced Deltapsi(m) but partially prevented NMDA-induced Deltapsi(m) loss. Although NO(*) and NMDA both induced MPT and MPT inhibitors prevented NO-induced Deltapsi(m), they did not result in significant neuroprotection, in contrast to treatment designed to decrease peroxynitrite formation. These data suggest that NO-induced NMDA receptor activation is closely linked to intramitochondrial NO-peroxynitrite/RNS formation and thereby acts as a major mediator of neuronal cell death.

The effects of oxidative stress on mitochondrial transmembrane potential in retinal ganglion cells

Retinal ganglion cells (RGCs) are central neurons that undergo apoptosis after axonal injury. As the relationship between mitochondrial and oxidative signaling of apoptosis in neuronal systems is unclear, we sought to achieve a better understanding of the interplay of these two pathways by investigating the effect of direct oxidative stress on mitochondrial membrane potential in cultured RGCs, as measured with the dual-emission probe JC-1. Treatment with hydrogen peroxide caused RGC mitochondrial depolarization. Several pharmacological treatments were used to define the mechanism. Whereas cycloheximide, tris(2-carboxyethyl)phosphine, and cyclosporin A were unable to prevent the depolarization, bongkrekic acid significantly reduced the severity of the depolarization. This suggests that the hydrogen peroxide-induced depolarization may act through mitochondrial permeability transition pore opening independent of thiol oxidation, and may be preventable under certain conditions.

Molecular mechanisms of calcium-dependent neurodegeneration in excitotoxicity

Excitotoxicity contributes to neuronal degeneration in many acute CNS diseases, including ischemia, trauma, and epilepsy, and may also play a role in chronic diseases, such as amyotrophic lateral sclerosis (ALS). Key mediators of excitotoxic damage are Ca ions (Ca(2+)), which under physiological conditions govern a multitude of cellular processes, including cell growth, differentiation, and synaptic activity. Consequently, homeostatic mechanisms exist to maintain a low intracellular Ca(2+) ion concentration so that Ca(2+) signals remain spatially and temporally localized. This permits multiple independent Ca-mediated signaling pathways to occur in the same cell. In excitotoxicity, excessive synaptic release of glutamate can lead to the disregulation of Ca(2+) homeostasis. Glutamate activates postsynaptic receptors, including the ionotropic N-methyl-D-aspartate (NMDA), 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) proprionate (AMPA), and kainate receptors. Upon their activation, these open their associated ion channel to allow the influx of Ca(2+) and Na(+) ions. Although physiological elevations in intracellular Ca(2+) are salient to normal cell functioning, the excessive influx of Ca(2+) together with any Ca(2+) release from intracellular compartments can overwhelm Ca(2+)-regulatory mechanisms and lead to cell death. Although Ca(2+) disregulation is paramount to neurodegeneration, the exact mechanism by which Ca(2+) ions actually mediate excitotoxicity is less clear. One hypothesis outlined in this review suggests that Ca(2+)-dependent neurotoxicity occurs following the activation of distinct signaling cascades downstream from key points of Ca(2+) entry at synapses, and that triggers of these cascades are physically co-localized with specific glutamate receptors. Thus, we summarize the importance of Ca(2+) regulation in mammalian neurons and the excitotoxicity hypothesis, and focus on the molecular determinants of glutamate receptor-mediated excitotoxic mechanisms.

Ca(2+) signalling in mitochondria: mechanism and role in physiology and pathology

Over recent years, a renewed interest in mitochondria in the field of Ca(2+) signalling has highlighted their central role in regulating important physiological and pathological events in animal cells. Mitochondria take up calcium through an uptake pathway that, due to its low-Ca(2+) affinity, demands high local calcium concentrations to work. In different cell systems high-Ca(2+) concentration microdomains are generated, upon cell stimulation, in proximity of either plasma membrane or sarco/endoplasmic reticulum Ca(2+) channels. Mitochondrial Ca(2+) accumulation has a dual role, an universal one, which consists in satisfying energy demands by increasing the ATP production through the activation of mitochondrial enzymes, and a cell type specific one, which, through the modulation of the spatio-temporal dynamics of calcium signals, contributes to modulate specific cell functions. Recent work has revealed the central role of mitochondria dysfunction in determining both necrotic and apoptotic cell death. Evidence is also accumulating that suggests that alterations in mitochondrial function may act as predisposing factors in the pathogenesis of a number of neurodegenerative disorders. These include inherited disorders of the mitochondrial genome in which a defect in mitochondrial calcium accumulation has been shown to correlate with a defect in ATP production, thus suggesting a possible involvement of mitochondrial Ca(2+) dysfunction also for this group of diseases. This review analyses recent developments in the area of mitochondrial Ca(2+) signalling and attempts to summarise cell physiology and cell pathology aspects of the mitochondrial Ca(2+) transport machinery.

Interactions between mitochondrial bioenergetics and cytoplasmic calcium in cultured cerebellar granule cells

The mitochondrion has moved to the center stage in the drama of the life and death of the neuron. The mitochondrial membrane potential controls the ability of the organelle to generate ATP, generate reactive oxygen species and sequester Ca(2+) entering the cell. Each of these processes interact, and their deconvolution is far from trivial. The cultured cerebellar granule cell provides a model in which knowledge gained from studies on isolated mitochondria can be applied to study the role played by the organelles in the maintenance of Ca(2+) homeostasis in the cell under resting, stimulated and pathophysiological conditions. In particular, mitochondria play a complex role in the response of the neuron to excitotoxic stimulation of NMDA and AMPA-kainate selective glutamate receptors. One goal of research in this area is to provide clues as to possible ways in which modulators of mitochondrial function may be used as neuroprotective agents, since mitochondrial Ca(2+) accumulation seems to play a key role in glutamate excitotoxicity.

Staurosporine-induced apoptosis in human cornea epithelial cells in vitro

BACKGROUND

Apoptosis is a an important process in corneal development, homeostasis, and disease. This study was performed to determine for the first time basic temporal apoptotic features of SV-40 immortalized human corneal epithelial (HCE) cells. Additionally, we introduce a sensitive analysis of confocal microscopic images to measure the kinetics of staurosporine (STS) induced phosphatidylserine (PS) membrane translocation and early nuclear morphological changes.

METHODS

HCE cells were cultured in the presence of STS to induce apoptosis. Caspase-3 activity was measured with the fluorogenic substrate z-DEVD-rhodamine 110. We determined mitochondrial viability with a 4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzenedisulfonate reduction assay, and chromatin degradation with a fluorometric method using 4,6-diamidino-2-phenylindole (DAPI). Membrane translocation of PS and nuclear alterations were assessed by quantitative fluorescence microscopy. Image processing routines were written in interactive data language (IDL).

RESULTS

Nuclear alterations like hyperchromicity, pyknosis, and active chromatin reorganization evolved instantly after STS induction. They were followed by PS translocation, DNA fragmentation, mitochondrial breakdown, and caspase-3 activation, which were detected between approximately 90 min and 4 h.

CONCLUSIONS

Morphological and texture sensitive descriptors proved to be highly susceptible for the quantification of early apoptotic nuclear characteristics in HCE cells. We propose this method to be considered for the detection of subtle nuclear reorganization in cellular studies.

In vivo application of mitochondrial pore inhibitors blocks the induction of apoptosis in axotomized neonatal facial motoneurons

Axotomy induces apoptosis in motoneurons of neonatal rodents. To identify the key players in motoneuron apoptosis, we assessed the progression of apoptosis at 4 h intervals following facial motoneuron axotomy. The mitochondrial release of cytochrome c, caspase-3 activation and nuclear condensation were first observed in the motoneuron cell bodies 16 h postaxotomy. In vivo application of inhibitors of the mitochondrial permeability transition pore, Bongkrekic acid and cyclosporin A prevented cytochrome c release as well as caspase-3 activation and attenuated motoneuron apoptosis. Similarly, in vivo application of RU360, an inhibitor of the mitochondrial calcium uniporter, also protected axotomized motoneurons from apoptosis. Taken together, our results show that cytochrome c release and subsequent caspase-3 activation are critical events that precipitate the apoptotic death of axotomized neonatal motoneurons in vivo. In addition, these results provide evidence that application of mitochondrial pore inhibitors in vivo can block the induction of apoptosis following motoneuron axotomy.

Exogenous erythropoietin protects against dorsal root ganglion apoptosis and pain following peripheral nerve injury

Erythropoietin (Epo) has been shown to have potent anti-apoptotic activity in central nervous system neurons in animal models of ischaemic injury. Recently, Epo and its receptor (EpoR) have been identified in the peripheral nervous system [Campana & Myers (2001), FASEB J., 15, 1804-1806]. Herein, we demonstrate that in painful neuropathy caused by L5 spinal nerve crush (SNC), therapy with recombinant human Epo (rhEpo) reduced dorsal root ganglion (DRG) apoptosis and pain behaviours. Quantification of both DRG neurons and satellite cells revealed that vehicle-treated, crush-injured DRGs had 35.5 +/- 8.3% apoptotic neurons and 23.5 +/- 2.36% satellite cells compared with 7.5 +/- 6.3% apoptotic neurons and 6.4 +/- 3.94% satellite cells in rhEpo-treated, crush-injured DRGs (P < 0.05). While rhEpo-treated animals were not initially protected from mechanical allodynia associated with L5 SNC, rhEpo did significantly improve recovery rates compared to vehicle-treated animals (P < 0.01). Systemic rhEpo therapy increased JAK2 phosphorylation, a key anti-apoptotic signalling molecule for Epo-induced neuroprotection, in DRGs after crush. Dual immunofluorescence demonstrated Epo-induced JAK2-p was associated with both neuronal and glial cells. JAK2-p was associated with NF200-positive large neurons and with smaller neurons. This population of small neurons did not colocalize with IB4, a marker of nonpeptidergic, glial derived growth factor-responsive neurons. The findings link anti-apoptosis activities of Epo/EpoR/JAK2 in DRG neurons capable of inducing protracted pain states with reductions in pain behaviours, and therefore support a role for Epo therapy in the treatment of neuropathic pain.

Involvement of mitochondrial K+ release and cellular efflux in ischemic and apoptotic neuronal death

We measured and manipulated intracellular potassium (K+) fluxes in cultured hippocampal neurons in an effort to understand the involvement of K+ in neuronal death under conditions of ischemia and exposure to apoptotic stimuli. Measurements of the intracellular K+ concentration using the fluorescent probe 1,3-benzenedicarboxylic acid, 4,4'-[1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diylbis(5-methoxy-6,2-benzofurandiyl)]bis-, tetrakis [(acetyloxy) methyl] ester (PBFI) revealed that exposure of neurons to cyanide (chemical hypoxia), glutamate (excitotoxic insult) or staurosporine (apoptotic stimulus) results in efflux of K+ and cell death. Treatment of neurons with 5-hydroxydecanoate (5HD), an inhibitor of mitochondrial K+ channels, reduced K+ fluxes in neurons exposed to each insult and increased the resistance of the cells to death. K+ efflux was attenuated, levels of oxyradicals were decreased, mitochondrial membrane potential was stabilized and release of cytochrome c from mitochondria was attenuated in neurons treated with 5HD. K+ was rapidly released into the cytosol from mitochondria when neurons were exposed to the K+ channel opener, diazoxide, or to the mitochondrial uncoupler, carbonyl cyanide 4(trifluoromethoxy)phenylhydrazone (FCCP), demonstrating that the intramitochondrial K+ concentration is greater than the cytosolic K+ concentration. The release of K+ from mitochondria was followed by efflux through plasma membrane K+ channels. In vivo studies showed that 5HD reduces ischemic brain damage without affecting cerebral blood flow in a mouse model of focal ischemic stroke. These findings suggest that intracellular K+ fluxes play a key role in modulating neuronal oxyradical production and cell survival under ischemic conditions, and that agents that modify K+ fluxes may have therapeutic benefit in stroke and related neurodegenerative conditions.

Traumatic brain injury in infants and children: mechanisms of secondary damage and treatment in the intensive care unit

Unfortunately no specific pharmacologic therapies are available for the treatment of TBI in patients. Current investigation of contemporary therapies for the treatment of TBI consists of recycling of previously tested therapies in the era of contemporary neurointensive care. These therapies include hypothermia, decompressive craniectomy, osmotherapy, and controlled hyperventilation. It is hoped that more detailed knowledge regarding the dominant pathophysiologic mechanisms associated with TBI-excitotoxicity, CBF dysregulation, oxidative stress, and programmed cell death-will catapult an efficacious intervention from the laboratory bench to the bedside. This intervention may be a potent agent targeting a single dominant pathway, a broad-spectrum intervention such as hypothermia, or, more likely, a combination of therapies. Meanwhile, practitioners must offer meticulous supportive neurointensive care using clinically proven therapies aimed at minimizing cerebral swelling for the management of pediatric patients who are victims of TBI.

Possible role for memantine in protecting retinal ganglion cells from glaucomatous damage

Glaucoma is a neurodegenerative disease typified by progressive loss of retinal ganglion cells (RGCs). Mild excitotoxicity has been implicated as one of the factors contributing to RGC death during the glaucomatous process. This type of excitotoxic cell death is due, at least in part, to somewhat excessive activation of N-methyl-D-aspartate (NMDA)-type glutamate receptors. NMDA-receptor activity, however, is also essential for normal neuronal function. This means that potential neuroprotective agents that block virtually all NMDA-receptor activity will have unacceptable clinical side effects. Studies in our laboratory have shown that the adamantane derivative, memantine, blocks only excessive NMDA-receptor activity without disrupting normal activity. Past clinical use has demonstrated that memantine is safe, and it has recently been approved in Europe for the treatment of Alzheimer's disease and vascular dementia. Clinical studies of the safety and efficacy of memantine in glaucoma are currently underway. A series of second-generation memantine derivatives called nitro-memantines are currently in development and may prove to have even greater neuroprotective properties than does memantine.

Ceramide-induced neuronal apoptosis is associated with dephosphorylation of Akt, BAD, FKHR, GSK-3beta, and induction of the mitochondrial-dependent intrinsic caspase pathway

Neuronal apoptosis has been implicated as an important mechanism of cell death in acute and chronic neurodegenerative disorders. Ceramide is a product of sphingolipid metabolism which induces neuronal apoptosis in culture, and ceramide levels increase in neurons during various conditions associated with cell death. In this study we investigate the mechanism of ceramide-induced apoptosis in primary cortical neuronal cells. We show that ceramide treatment initiates a cascade of biochemical alterations associated with cell death: earliest signal transduction changes involve Akt dephosphorylation and inactivation followed by dephosphorylation of proapoptotic regulators such as BAD (proapoptotic Bcl-2 family member), Forkhead family transcription factors, glycogen synthase kinase 3-beta, mitochondrial depolarization and permeabilization, release of cytochrome c into the cytosol, and caspase-3 activation. Bongkrekic acid, an agent that inhibits mitochondrial depolarization, significantly reduces ceramide-induced cell death and correlated caspase-3 activation. Together, these data demonstrate the importance of the mitochondrial-dependent intrinsic pathway of caspase activation for ceramide-induced neuronal apoptosis.

NMDA receptor-mediated extracellular adenosine accumulation in rat forebrain neurons in culture is associated with inhibition of adenosine kinase

The effect of N-methyl-d-aspartate (NMDA) on regulation of extracellular adenosine was investigated in rat forebrain neurons in culture. NMDA evoked accumulation of extracellular adenosine with an EC50 value of 4.8 +/- 1.2 microM. The effect of NMDA was blocked by (+)-5-methyl-10,11-dihydro-5H-dibenzo [a, d] cyclohepten-5,10-imine hydrogen maleate indicating that NMDA receptor activation was involved. The NMDA effect was also blocked by chelation of extracellular Ca2+ indicating that influx of calcium was required. The nitric oxide-cyclic GMP signalling pathway was not involved, as nitric oxide synthase inhibitors were unable to block, and cGMP analogs were unable to mimic, the effect of NMDA. The source for extracellular adenosine was likely to be intracellular adenosine as the ecto-5'-nucleotidase inhibitor alpha beta-methylene-ADP was unable to block the effect of NMDA. One possible cause of intracellular adenosine accumulation might be NMDA receptor-mediated inhibition of mitochondrial function and ATP hydrolysis. We found that NMDA caused a concentration dependent depletion of intracellular ATP with an EC50 value of 21 +/- 8 microM. NMDA also caused a significant decrease in adenosine kinase activity, assayed by two different methods. Consistent with the hypothesis that inhibition of adenosine kinase is sufficient to cause an increase in extracellular adenosine, inhibition of adenosine kinase by 5'-iodotubercidin resulted in elevation of extracellular adenosine. However, in the presence of a concentration of 5'-iodotubercidin that inhibited over 90% of adenosine kinase activity, exposure to NMDA still caused adenosine accumulation. These studies suggest that several possible mechanisms are likely to be involved in NMDA-evoked extracellular adenosine accumulation.

BAD and Bcl-2 regulation are early events linking neuronal endoplasmic reticulum stress to mitochondria-mediated apoptosis

The mechanism of endoplasmic reticulum (ER)-mediated apoptosis in neurons was examined. Using primary cortical neurons, we show that nordihydroguaiaretic acid (NDGA) and brefeldin A (BFA), two ER stressors, induce early ER stress as shown by Western blotting of the eukaryotic initiation factor-2alpha (eIF2alpha), an ER stress marker. This event was associated with an enhancement of neuronal apoptosis as demonstrated by the time-dependent increase in caspase-3 activity and by nuclear fragmentation. The study of the apoptotic signaling showed the translocation of cytochrome c from the mitochondrial matrix to the cytosol. Further evaluation of the apoptotic process revealed that NDGA and BFA induced a rapid dephosphorylation of BAD and decrease expression of Bcl-2. Altogether, our results indicate that neuronal ER stress is associated with an apoptotic cascade involving the mitochondria.

Acute and chronic alterations in calcium homeostasis in 3-nitropropionic acid-treated human NT2-N neurons

3-Nitropropionic acid (3-NP), an irreversible inhibitor of succinate dehydrogenase, induced ATP depletion and both necrosis and apoptosis in human NT2-N neurons. Necrosis occurred predominantly within the first two days, and increased in a dose-dependent fashion with the concentration of 3-NP, whereas apoptosis was observed after 24 h or later at a similar rate in 0.1 mM and 5 mM 3-NP. We focused our efforts on intracellular calcium homeostasis during the first 48 h in 1 mM 3-NP, a period during which 10% of the neurons died by necrosis and 3% by apoptosis. All NT2-N neurons showed a stereotyped [Ca(2+)](i) rise, from 48+/-2 to 140+/-12 nM (mean +/-S.E.M.), during the first 2 h in 3-NP. Despite severe ATP depletion, however, [Ca(2+)](i) remained above 100 nM in only 17% and 25% of the NT2-N neurons after 24 and 48 h in 3-NP, respectively, indicating that most neurons were able to recover from acute [Ca(2+)](i) rise, and suggesting that chronic [Ca(2+)](i) dysregulation is a better indicator of subsequent necrosis. Blockade of N-methyl-D-aspartate-glutamate receptor by MK-801 substantially ameliorated 3-NP-induced ATP depletion, subsequent chronic [Ca(2+)](i) elevation, and survival. Moreover, xestospongin C, an inhibitor of endoplasmic reticulum Ca(2+) release, enhanced the capacity of NT2-N neurons to maintain [Ca(2+)](i) homeostasis and resist necrosis while subjected to sustained energy deprivation. As far as we know, this report is the first to employ human neurons to study the pathophysiology of 3-NP neurotoxicity.

NCX-4016, a nitric oxide-releasing aspirin, protects endothelial cells against apoptosis by modulating mitochondrial function

We investigated the effect of a nitric oxide (NO)-releasing derivative of aspirin (NCX-4016) on a mitochondria-dependent model of apoptosis in human umbilical endothelial cells (HUVEC). Exposure of HUVEC to staurosporine caused a progressive fall in mitochondrial membrane potential (DeltaPsi(m)) and apoptosis. Exposure to an NO donor, (z)-1-[2-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NO), caused an early (1-3h) hyperpolarization of DeltaPsi(m) and reduction of apoptosis that was followed (at 4-8 h) by an accelerated collapse of DeltaPsi(m) and cell death. In contrast, treatment with NCX-4016, but not with aspirin or a non-NO-releasing analogue of NCX-4016, protected HUVEC against the apoptotic actions of staurosporine for up to 8 h. Confocal microscopy demonstrated that although NCX-4016 released NO in subcellular compartments, DETA-NO caused a generalized increase in cytosolic fluorescence. Exposure to DETA-NO resulted in a rapid and profound inhibition of cell respiration (78.3 +/- 6.4%), whereas NCX-4016 caused a less pronounced reduction in oxygen consumption (43.5 +/- 5.3%). Staurosporine caused a time-dependent activation of proapoptotic caspases. NCX-4016 prevented this activation, whereas DETA-NO failed to inhibit caspase activity. In contrast to DETA-NO, NCX-4016 did not increase mitochondrial oxidative stress. These data demonstrated that NCX-4016 conveys NO directly inside endothelial cells and modulates mitochondrial function.

Huperzine A attenuates apoptosis and mitochondria-dependent caspase-3 in rat cortical neurons

The neuroprotection of huperzine A against apoptosis was investigated. In cultures of rat primary cortical neurons, neuronal apoptosis was induced by serum deprivation for 24 h, which was accompanied by enhanced caspase-3 activity and the release of cytochrome c into the cytosol from mitochondria. Pretreating the neurons for 2 h with huperzine A (0.1-10 microM) improved neuronal survival. Huperzine A at a concentration of 1 microM significantly attenuated apoptosis by inhibiting the mitochondria-caspase pathway directly and indirectly.

Mitochondria, Ca2+ and neurodegenerative disease

Mitochondria play a central role in cell biology not only as producers of ATP, but also in the sequestration of Ca(2+) and the generation of free radicals. They are also repositories of several proteins which regulate apoptosis. Perturbations in the normal functions of mitochondria will inevitably disturb cell function, may sensitise cells to neurotoxic insults and may initiate cell death. Neuronal Ca(2+) overload, such as follows excessive stimulation of Ca(2+) permeant excitatory amino acid receptors, can cause cell death. Recent evidence suggests that the accumulation of Ca(2+) into mitochondria during episodes of cellular Ca(2+) overload initiates a cascade of events that culminate in cell death. Cell death appears to require not only mitochondrial Ca(2+) overload, but rather a combination of raised intramitochondrial Ca(2+) concentration with increased production of nitric oxide and possibly other oxyradical species. Cell death may proceed through either necrotic or apoptotic mechanisms, depending on the rate of consumption and depletion of ATP. Evidence is also accumulating to suggest that more subtle alterations in mitochondrial function may serve as predisposing factors in the pathogenesis of a number of neurodegenerative disorders.

Direct cleavage of AMPA receptor subunit GluR1 and suppression of AMPA currents by caspase-3: implications for synaptic plasticity and excitotoxic neuronal death

Cysteine proteases of the caspase family play central roles in excecuting the cell death process in neurons during development of the nervous system and in neurodegenerative disorders. Recent findings suggest that caspases may also play roles in modulating neuronal plasticity in the absence of cell death. We previously reported that caspases can be activated in dendrites and synapses in response to activation of glutamate receptors. In the present study we demonstrate that the GluR1 subunit of the AMPA subtype of glutamate receptor is directly cleaved by caspase-3, and provide evidence that the cleavage of this subunit modulates neuronal excitability in ways that suggest important roles for caspases in regulating synaptic plasticity and cell survival. Whole-cell patch-clamp recordings in cultured rat hippocampal neurons showed that caspase activation in response to apoptotic stimuli selectively decreases AMPA channel activity without decreasing NMDA channel activity. Perfusion of neurons with recombinant caspase-3 resulted in a decreased AMPA current, demonstrating that caspase-3 activity is sufficient to suppress neuronal responses to glutamate. Exposure of radiolabeled GluR1 to recombinant caspase-3 resulted in cleavage of GluR1, demonstrating that this glutamate receptor protein is a direct substrate of this caspase. Our findings suggest roles for caspases in the modulation of neuronal excitability in physiological settings, and also identify a mechanism whereby caspases ensure that neurons die by apoptosis rather than excitotoxic necrosis in developmental and pathological settings.

Mechanisms of hypoxic neurodegeneration in the developing brain

Asphyxia and other insults to the developing brain are responsible for several human neurodevelopmental disorders. The pattern of neonatal brain injury differs from that seen in the adult nervous system, and there are wide differences in regional vulnerability. Recent evidence suggests that two events that contribute to this pattern of selective vulnerability are developmental changes in excitatory glutamate-containing neurotransmitter circuits and the propensity for immature neurons to die by apoptosis rather than necrosis. Developmental up-regulation of NMDA receptors with enhanced function and increased expression of caspase-3 at critical periods in development are linked to these mechanisms. Although these molecular changes enhance the developing brain's capacity for plasticity by helping to prune redundant synapses and neurons, they can become "Achilles heels" in the face of a brain energy crisis.

Mitochondrial permeability transition can be directly monitored in living neurons

Mitochondria have been suggested as key players in apoptotic cell death of neurons and many other tissues, since the release of proapoptotic molecules from mitochondria is implicated in caspase activation. As a potential release mechanism, the occurrence of a large pore opening in the inner membrane (mitochondrial permeability transition pore, PTP) has been proposed, but has not yet been observed directly in neurons. We investigated whether the calcein/Co2+-quenching technique introduced by Petronilli et al. [Biofactors 8 (1998) 263], which allows direct observation of PTP opening, can be applied to neurons. Exposure of calcein-loaded neurons to Co2+ ions resulted in the fading of diffuse cytoplasmic calcein fluorescence, with organelle-restricted fluorescent spots remaining. These spots were colocalized with mitochondrially-entrapped tetramethylrhodamineethylester (TMRE) fluorescence and corresponded to colocalization of calcein and TMRE fluorescence in digitonin-permeabilized neurons. Importantly, extensive neuronal calcium loading, which is assumed to induce PTP opening, resulted in significant fading of mitochondrial fluorescence, suggesting the occurrence of permeability transition. This fluorescence decrease could be completely prevented by the PTP blocker cyclosporin A.

Caspase cascades in human immunodeficiency virus-associated neurodegeneration

Many patients infected with human immunodeficiency virus-1 (HIV-1) develop a syndrome of neurologic deterioration known as HIV-associated dementia (HAD). Neurons are not productively infected by HIV-1; thus, the mechanism of HIV-induced neuronal injury remains incompletely understood. Several investigators have observed evidence of neuronal injury, including dendritic degeneration, and apoptosis in CNS tissue from patients with HAD. Caspase enzymes, proteases associated with the process of apoptosis, are synthesized as inactive proenzymes and are activated in a proteolytic cascade after exposure to apoptotic signals. Here we demonstrate that HAD is associated with active caspase-3-like immunoreactivity that is localized to the soma and dendrites of neurons in affected regions of the human brain. Additionally, the cascade of caspase activation was studied using an in vitro model of HIV-induced neuronal apoptosis. Increased caspase-3 proteolytic activity and mitochondrial release of cytochrome c were observed in cerebrocortical cultures exposed to the HIV coat protein gp120. Specific inhibitors of both the Fas/tumor necrosis factor-alpha/death receptor pathway and the mitochondrial caspase pathway prevented gp120-induced neuronal apoptosis. Caspase inhibition also prevented the dendrite degeneration observed in vivo in transgenic mice with CNS expression of HIV/gp120. These findings suggest that pharmacologic interventions aimed at the caspase enzyme pathways may be beneficial for the prevention or treatment of HAD.

Exogenous phospholipids specifically affect transmembrane potential of brain mitochondria and cytochrome C release

Release of cytochrome c, a decrease of membrane potential (Deltapsi(m)), and a reduction of cardiolipin (CL) of rat brain mitochondria occurred upon incubation in the absence of respiratory substrates. Since CL is critical for mitochondrial functioning, CL enrichment of mitochondria was achieved by fusion with CL liposomes. Fusion was triggered by potassium phosphate at concentrations producing mitochondrial permeability transition pore opening but not cytochrome c release, which was observed only at >10 mm. Cyclosporin A inhibited phosphate-induced CL fusion, whereas Pronase pretreatment of mitochondria abolished it, suggesting that mitochondrial permeability transition pore and protein(s) are involved in the fusion process. Phosphate-dependent fusion was enhanced in respiratory state 3 and influenced by phospholipid classes in the order CL > phosphatidylglycerol (PG) > phosphatidylserine. The probe 10-nonylacridine orange indicated that fused CL had migrated to the inner mitochondrial membrane. In state 3, CL enrichment of mitochondria resulted in a pH decrease in the intermembrane space. Cytofluorimetric analysis of mitochondria stained with 3,3'-diexyloxacarbocyanine iodide and 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzymidazolylcarbocyanine iodide showed Deltapsi(m) increase upon fusion with CL or PG. In contrast, phosphatidylserine fusion required Deltapsi(m) consumption, suggesting that Deltapsi(m) is the driving force in mitochondrial phospholipid importation. Moreover, enrichment with CL and PG brought the low energy mitochondrial population to high Deltapsi(m) values and prevented phosphate-dependent cytochrome c release.

Activation of mitochondrial ATP-dependent potassium channels protects neurons against ischemia-induced death by a mechanism involving suppression of Bax translocation and cytochrome c release

Neurons express a variety of plasma-membrane potassium channels that play important roles in regulating neuronal excitability and synaptic transmission, but also contain mitochondrial ATP-sensitive potassium channels, the functions of which are unknown. Studies of cardiac cells suggest that similar mitochondrial ATP-sensitive potassium channels are involved in the process of ischemic preconditioning, suggesting a role in regulating cell survival. The authors report that mice given diazoxide, an activator of mitochondrial ATP-sensitive potassium channels, exhibited a large (60% to 70%) decrease in cortical infarct size after permanent occlusion of the middle cerebral artery. Diazoxide decreases neuronal apoptosis and increases astrocyte survival and activation in the penumbral region of the ischemic cortex. The neuroprotective effect of diazoxide is abolished by 5-hydroxydecanoate, a selective antagonist of mitochondrial ATP-sensitive potassium channels. Studies of cultured hippocampal neurons reveal that diazoxide depolarizes mitochondria, prevents cytochrome c release, and protects cells against death induced by staurosporine and chemical hypoxia. Diazoxide increased the levels of Bcl2 and inhibited the association of Bax with mitochondria in neurons exposed to an apoptotic insult, suggesting that activation of mitochondrial ATP-sensitive potassium channels may stabilize mitochondrial function by differentially modulating proapoptotic and antiapoptotic proteins. Collectively, the data suggest that mitochondrial ATP-sensitive potassium channels play a key role in modulating neuronal survival under ischemic conditions, and identify agents that activate mitochondrial ATP-sensitive potassium channels as potential therapeutics for stroke and related neurodegenerative conditions.

Kainate receptor-mediated apoptosis in primary cultures of cerebellar granule cells is attenuated by mitogen-activated protein and cyclin-dependent kinase inhibitors

1. Previous studies have suggested that neuronal apoptosis is the result of an abortive attempt to re-enter the cell cycle, and more recently the cyclin-dependent (CDKs) and the mitogen-activated protein (MAP) kinases, two superfamilies of kinases that influence and control cell cycle progression, have been implicated in neuronal apoptosis. 2. Here, to examine whether CDK/MAPK related pathways are involved in excitotoxicity, we studied the actions of various kinase inhibitors on apoptosis induced by the ionotropic glutamate (Glu) receptor agonist, kainate (KA), in primary cultures of murine cerebellar granule cells (CGCs). 3. KA-mediated neurotoxicity was concentration-dependent, as determined by a cell viability assay monitoring the reduction of 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide (MTT), and largely apoptotic in nature, as shown by morphological examination and labelling of DNA fragmentation in situ using terminal deoxynucleotidyl transferase (TdT)-mediated dUTP digoxigenin nick-end labelling (TUNEL). 4. KA-mediated neurotoxicity and apoptosis was completely attenuated by the mixed CDK and MAP kinase inhibitor, olomoucine, in a concentration-dependent manner (50 - 600 microM), and partially by roscovitine (1 - 100 microM), a more selective CDK inihibitor. 5. The p38 MAP kinase inhibitor, SB203580 (1 - 100 microM), partially attenuated KA receptor-mediated apoptosis, as did the MAP kinase kinase inhibitors PD98509 (1 - 100 microM) and U0126 (1 - 100 microM). 6. These findings provide new evidence for a complex network of interacting pathways involving CDK/MAPK that control apoptosis downstream of KA receptor activation in excitotoxic neuronal cell death.

Dominant-interfering forms of MEF2 generated by caspase cleavage contribute to NMDA-induced neuronal apoptosis

Myocyte enhancer factor-2 (MEF2) transcription factors are activated by p38 mitogen-activated protein kinase during neuronal and myogenic differentiation. Recent work has shown that stimulation of this pathway is antiapoptotic during development but proapoptotic in mature neurons exposed to excitotoxic or other stress. We now report that excitotoxic (N-methyl-D-aspartate) insults to mature cerebrocortical neurons activate caspase-3, -7, in turn cleaving MEF2A, C, and D isoforms. MEF2 cleavage fragments containing a truncated transactivation domain but preserved DNA-binding domain block MEF2 transcriptional activity via dominant interference. Transfection of constitutively active MEF2 (MEF2C-CA) rescues MEF2 transcriptional activity after N-methyl-D-aspartate insult and prevents neuronal apoptosis. Conversely, dominant-interfering MEF2 abrogates neuroprotection by MEF2C-CA. These results define a pathway to excitotoxic neuronal stress/apoptosis via caspase-catalyzed cleavage of MEF2. Additionally, we show that similar MEF2 cleavage fragments are generated in vivo during focal stroke damage. Hence, this pathway appears to have pathophysiological relevance in vivo.

Increased sensitivity to N-methyl-D-aspartate receptor-mediated excitotoxicity in a mouse model of Huntington's disease

Previous work suggests N-methyl-D-aspartate receptor (NMDAR) activation may be involved in degeneration of medium-sized spiny striatal neurons in Huntington's disease (HD). Here we show that these neurons are more vulnerable to NMDAR-mediated death in a YAC transgenic FVB/N mouse model of HD expressing full-length mutant huntingtin, compared with wild-type FVB/N mice. Excitotoxic death of these neurons was increased after intrastriatal injection of quinolinate in vivo, and after NMDA but not AMPA exposure in culture. NMDA-induced cell death was abolished by an NR2B subtype-specific antagonist. In contrast, NMDAR-mediated death of cerebellar granule neurons was not enhanced, consistent with cell-type and NMDAR subtype specificity. Moreover, increased NMDA-evoked current amplitude and caspase-3 activity were observed in transgenic striatal neurons. Our data support a role for NR2B-subtype NMDAR activation as a trigger for selective neuronal degeneration in HD.

Excitotoxicity in neonatal hypoxia

Hypoxic-ischemic encephalopathy (HIE) in neonates is a disorder of excessive neuronal excitation that includes seizures, abnormal EEG activity, and delayed failure of oxidative metabolism with elevated levels of lactic acid in the brain. Evidence from experimental models and clinical investigation indicates that HIE is triggered by a profound disruption in the function of glutamate synapses so that re-uptake of glutamate from the synapse is impaired and post-synaptic membranes containing glutamate receptors are depolarized. Severe hypoxemia preferentially depolarizes neuronal membranes, while ischemia probably has greater impact on the activity of glial glutamate re-uptake. Together, severe hypoxia and ischemia trigger a delayed cascade of events that may result in cell death by necrosis and/or apoptosis. Apoptosis is far more prominent in the neonate than in the adult and activation of cysteine proteases such as caspase-3 is a very important pathway in excitotoxic neonatal injury. Understanding the complex molecular networks triggered by an excitotoxic insult in the neonate provides insight into patterns of selective neuronal vulnerability and potential therapeutic strategies.

Gas1 is induced during and participates in excitotoxic neuronal death

We have performed differential screening to identify genes participating in NMDA-induced neuronal death. The gas1 (growth arrest-specific gene 1) gene, whose product is known to inhibit cell cycle progression, was induced in cultured corticohippocampal neurons committed to die after a brief exposure to NMDA. Overexpression of Gas1 in cultured hippocampal neurons and in human neuroblastoma NB69 cells produced a marked reduction in the number of viable cells. Furthermore, gas1 antisense oligodeoxynucleotide or antisense mRNA protected hippocampal neurons or NB69 cells from neuronal death. Importantly, Gas1-induced neuronal death was attenuated by coexpression of the human Bcl-2 protein or the baculoviral caspase inhibitor OpIAP2. While Gas1 does not directly interact with Bcl-2, OpIAP2 coimmunoprecipitates with Gas1. In addition, induction of gas1 also occurred in rat brain in two models of excitotoxicity: delayed neuronal death after intraperitoneal kainate injection and neuronal death in hippocampal slices after ischemia. These results indicate that Gas1 is induced by activation of glutamate receptors and is part of the gene expression program directing neuronal death after mild excitotoxic insults.

Kainic acid-induced neuronal cell death in cerebellar granule cells is not prevented by caspase inhibitors

1. We examined the role of non-NMDA receptors in kainic acid (KA)-induced apoptosis in cultures of rat cerebellar granule cells (CGCs). KA (1 - 500 microM) induced cell death in a concentration-dependent manner, which was prevented by NBQX and GYKI 52466, non-NMDA receptor antagonists. Moreover, AMPA blocked KA-induced excitotoxicity, through desensitization of AMPA receptors. 2. Similarly, KA raised the intracellular calcium concentration of CGCs, which was inhibited by NBQX and GYKI 52466. Again, AMPA (100 microM) abolished the KA (100 microM)-induced increase in intracellular calcium concentration. 3. KA-induced cell death in CGCs had apoptotic features, which were determined morphologically, by DNA fragmentation, and by expression of the prostate apoptosis response-4 protein (Par-4). 5. KA (500 microM) slightly (18%) increased caspase-3 activity, which was strongly enhanced by colchicine (1 microM), an apoptotic stimulus. However, neither Z-VAD.fmk, a pan-caspase inhibitor, nor the more specific caspase-3 inhibitor, Ac-DEVD-CHO, prevented KA-induced cell death or apoptosis. In contrast, both drugs inhibited colchicine-induced apoptosis. 5. The calpain inhibitor ALLN had no effect on KA or colchicine-induced neurotoxicity. 6. Our findings indicate that colchicine-induced apoptosis in CGCs is mediated by caspase-3 activation, unlike KA-induced apoptosis.

Role of glutathione depletion and reactive oxygen species generation in apoptotic signaling in a human B lymphoma cell line

The primary objective of this study was to determine the sequence of biochemical signaling events that occur after modulation of the cellular redox state in the B cell lymphoma line, PW, with emphasis on the role of mitochondrial signaling. L-Buthionine sulphoximine (BSO), which inhibits gamma glutamyl cysteine synthetase (gammaGCS), was used to modulate the cellular redox status. The sequence and role of mitochondrial events and downstream apoptotic signals and mediators was studied. After BSO treatment, there was an early decline in cellular glutathione (GSH), followed by an increase in reactive oxygen species (ROS) production, which induced a variety of apoptotic signals (detectable at different time points) in the absence of any external apoptotic stimuli. The sequence of biochemical events accompanying apoptosis included a 95% decrease in total GSH and a partial (25%) preservation of mitochondrial GSH, without a significant increase in ROS production at 24h. Early activation and nuclear translocation of the nuclear factor kappa B subunit Rel A was observed at approximately 3h after BSO treatment. Cytochrome c release into the cytosol was also seen after 24h of BSO treatment. p53 protein expression was unchanged after redox modulation for up to 72 h, and p21waf1 independent loss of cellular proliferation was observed. Surprisingly, a truncated form of p53 was expressed in a time-dependent manner, beginning at 24h after BSO incubation. Irreversible commitment to apoptosis occurred between 48 and 72 h after BSO treatment when mitochondrial GSH was depleted, and there was an increase in ROS production. Procaspase 3 protein levels showed a time-dependent reduction following incubation with BSO, notably after 48 h, that corresponded with increasing ROS levels. At 96 h, caspase 3 cleavage products were detectable. The pan-caspase inhibitor zVADfmk, partially blocked the induction of apoptosis at 48 h, and was ineffective after 72 h. PW cells could be rescued from apoptosis by removing them from BSO after up to 48, but not 72 h incubation with BSO. Mitochondrial transmembrane potential (DeltaPsi(m)) remained intact in most of the cells during the 72 h observation period, indicating that DeltaPsi(m) dissipation is not an early signal for the induction of redox dependent apoptosis in PW cells. These data suggest that a decrease in GSH alone can act as a potent early activator of apoptotic signaling. Increased ROS production following mitochondrial GSH depletion, represents a crucial event, which irreversibly commits PW cells to apoptosis.

The adenine nucleotide translocator in apoptosis

Alteration of mitochondrial membrane permeability is a central mechanism leading invariably to cell death, which results, at least in part, from the opening of the permeability transition pore complex (PTPC). Indeed, extended PTPC opening is sufficient to trigger an increase in mitochondrial membrane permeability and apoptosis. Among the various PTPC components, the adenine nucleotide translocator (ANT) appears to act as a bi-functional protein which, on the one hand, contributes to a crucial step of aerobic energy metabolism, the ADP/ATP translocation, and on the other hand, can be converted into a pro-apoptotic pore under the control of onco- and anti-oncoproteins from the Bax/Bcl-2 family. In this review, we will discuss recent advances in the cooperation between ANT and Bax/Bcl-2 family members, the multiplicity of agents affecting ANT pore function and the putative role of ANT isoforms in apoptosis control.

Neuronal survival and cell death signaling pathways

Neuronal viability is maintained through a complex interacting network of signaling pathways that can be perturbed in response to a multitude of cellular stresses. A shift in the balance of signaling pathways after stress or in response to pathology can have drastic consequences for the function or the fate of a neuron. There is significant evidence that acutely injured and degenerating neurons may die by an active mechanism of cell death. This process involves the activation of discrete signaling pathways that ultimately compromise mitochondrial structure, energy metabolism and nuclear integrity. In this review we examine recent evidence pertaining to the presence and activation of anti- and pro-cell death regulatory pathways in nervous system injury and degeneration.

The mechanism of mitochondrial membrane potential retention following release of cytochrome c in apoptotic GT1-7 neural cells

The relationship is investigated between mitochondrial membrane potential (Delta Psi(M)), respiration and cytochrome c (cyt c) release in single neural bcl-2 transfected cells (GT1-7bcl-2) or GT1-7puro cells during apoptosis induced by staurosporine (STS). Bcl-2 inhibited the mitochondrial release of cyt c and apoptosis. Three different cell responses to STS were identified in GT1-7puro cells: (i) neither Delta Psi(M) nor cyt c were significantly affected; (ii) a decrease in Delta Psi(M) was accompanied by a complete release of cyt c; or (iii) cyt c release occurred independently of a loss of Delta Psi(M). The endogenous inner membrane proton leak of the in situ mitochondria, monitored by respiration in the presence of oligomycin, was increased by STS by 92% in puro cells, but by only 23% in bcl-2 cells. STS decreased respiratory capacity, in the presence of protonophore, by 31% in puro cells and by 20% in bcl-2 cells. In the absence of STS, oligomycin hyperpolarized mitochondria within both puro and bcl-2-transfected cells, indicating that the organelles were net generators of ATP. However after 15 h exposure to STS oligomycin rapidly collapsed residual mitochondrial polarization in the puro cells, indicating that Delta Psi(M) had been maintained by ATP synthase reversal. bcl-2 cells in contrast, maintained Delta Psi(M) until protonophore was added. These results indicate that the maintenance of Delta Psi(M) following release of cyt c may be a consequence of ATP synthase reversal and cytoplasmic ATP hydrolysis in STS-treated GT1-7 cells.

Acute spinal cord injury, part I: pathophysiologic mechanisms

Spinal cord injury (SCI) is a devastating and common neurologic disorder that has profound influences on modern society from physical, psychosocial, and socioeconomic perspectives. Accordingly, the present decade has been labeled the Decade of the Spine to emphasize the importance of SCI and other spinal disorders. Spinal cord injury may be divided into both primary and secondary mechanisms of injury. The primary injury, in large part, determines a given patient's neurologic grade on admission and thereby is the strongest prognostic indicator. However, secondary mechanisms of injury can exacerbate damage and limit restorative processes, and hence, contribute to overall morbidity and mortality. A burgeoning body of evidence has facilitated our understanding of these secondary mechanisms of injury that are amenable to pharmacological interventions, unlike the primary injury itself. Secondary mechanisms of injury encompass an array of perturbances and include neurogenic shock, vascular insults such as hemorrhage and ischemia-reperfusion, excitotoxicity, calcium-mediated secondary injury and fluid-electrolyte disturbances, immunologic injury, apoptosis, disturbances in mitochondrion function, and other miscellaneous processes. Comprehension of secondary mechanisms of injury serves as a basis for the development and application of targeted pharmacological strategies to confer neuroprotection and restoration while mitigating ongoing neural injury. The first article in this series will comprehensively review the pathophysiology of SCI while emphasizing those mechanisms for which pharmacologic therapy has been developed, and the second article reviews the pharmacologic interventions for SCI.

Ca(2+)-induced inhibition of apoptosis in human SH-SY5Y neuroblastoma cells: degradation of apoptotic protease activating factor-1 (APAF-1)

During apoptotic and excitotoxic neuron death, challenged mitochondria release the pro-apoptotic factor cytochrome c. In the cytosol, cytochrome c is capable of binding to the apoptotic protease-activating factor-1 (APAF-1). This complex activates procaspase-9 in the presence of dATP, resulting in caspase-mediated execution of apoptotic neuron death. Many forms of Ca(2+)-mediated neuron death, however, do not lead to prominent activation of the caspase cascade despite significant release of cytochrome c from mitochondria. We demonstrate that elevation of cytosolic Ca(2+) induced prominent degradation of APAF-1 in human SH-SY5Y neuroblastoma cells and in a neuronal cell-free apoptosis system. Loss of APAF-1 correlated with a reduced ability of cytochrome c to activate caspase-3-like proteases. Ca(2+) induced the activation of calpains, monitored by the cleavage of full-length alpha-spectrin into a calpain-specific 150-kDa breakdown product. However, pharmacological inhibition of calpain activity indicated that APAF-1 degradation also occurred via calpain-independent pathways. Our data suggest that Ca(2+) inhibits caspase activation during Ca(2+)-mediated neuron death by triggering the degradation of the cytochrome c-binding protein APAF-1.

Synergistic protective effect of caspase inhibitors and bFGF against brain injury induced by transient focal ischaemia

We tested the hypothesis that combined use of trophic factors and caspase inhibitors increases brain resistance to ischaemia in mice. Intracerebroventricular administration of bFGF (>10 ng) 30 min after MCA occlusion decreased infarct size and neurological deficit in a dose-dependent manner following 2 h ischemia and reperfusion (20 h). Combined administration of the subthreshold doses of bFGF (3 ng) and caspase inhibitors (z-VAD.FMK, 27 ng or z-DEVD.FMK, 80 mg) reduced infarct volume by 60%, and reduced neurological deficit. Treatment with a subthreshold dose of bFGF (3 ng) extended the therapeutic window for z-DEVD.FMK (480 ng) from 1 to 3 h after reperfusion. Caspase-3 activity in the ischaemic brain was increased 30 min and 2 h after reperfusion but, was significantly reduced in bFGF-treated animals by 29 and 16%, respectively. Caspase-3 activity was not reduced by a direct bFGF effect because addition of bFGF (10 nM - 2 microM) did not decrease recombinant caspase-3 activity, in vitro. Our data show that combining caspase inhibitors and bFGF lengthens the treatment window for the second treatment, plus lowers the dosage requirements for neuroprotection. These findings are important because low doses of caspase inhibitors or bFGF reduce the possibility of side effects plus extend the short treatment window for ischaemic stroke.

Spontaneous changes in mitochondrial membrane potential in cultured neurons

Using the mitochondrial membrane potential (DeltaPsi(m))-sensitive fluorescent dyes 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolocarbocyanine iodide (JC-1) and tetramethylrhodamine methyl ester (TMRM), we have observed spontaneous changes in the DeltaPsi(m) of cultured forebrain neurons. These fluctuations in DeltaPsi(m) appear to represent partial, transient depolarizations of individual mitochondria. The frequency of these DeltaPsi(m) fluctuations can be significantly lowered by exposure to a photo-induced oxidant burden, an ATP synthase inhibitor, or a glutamate-induced sodium load, without changing overall JC-1 fluorescence intensity. These spontaneous fluctuations in JC-1 signal were not inhibited by altering plasma membrane activity with tetrodotoxin or MK-801 or by blocking the mitochondrial permeability transition pore (PTP) with cyclosporin A. Neurons loaded with TMRM showed similar, low-amplitude, spontaneous fluctuations in DeltaPsi(m). We hypothesize that these DeltaPsi(m) fluctuations are dependent on the proper functioning of the mitochondria and reflect mitochondria alternating between the active and inactive states of oxidative phosphorylation.