Programmed cell death: its possible contribution to neurotoxicity mediated by calcium channel antagonists

Neuronal apoptosis after CNS injury: the roles of glutamate and calcium

While a role has been well established for excitotoxic necrosis in the pathogenesis of traumatic or ischemic damage to the CNS, accumulating evidence now suggests that apoptosis may also be a prominent contributor. In this review we focus on the role of glutamate and attendant intracellular calcium influx in triggering or modifying excitotoxic necrosis and apoptosis, raising the possibility that calcium influx may affect these two death pathways in opposite directions. Incorporating consideration of both pathways will probably be needed to develop the most effective neuroprotective treatments for CNS injury.

Activation of group I metabotropic glutamate receptors reduces neuronal apoptosis but increases necrotic cell death in vitro

Glutamate released during acute CNS insults acts at metabotropic glutamate receptors (mGluR), including group I mGluR. Blockade of group I mGluR during in vitro neuronal trauma provides neuroprotection, whereas activation exacerbates such injury. However, the effects of group I mGluR agonists or antagonists have been primarily studied in in vitro models characterized by necrotic cell death. We examined the role of group I mGluR in the modulation of neuronal injury induced during oxygen-glucose deprivation (OGD), a well-studied model of necrosis, and by application of two well established pro-apoptotic agents: staurosporine and etoposide. Inhibition of group I mGluR attenuated necrosis induced by OGD, whereas selective activation of group I mGluR exacerbated such injury. In contrast, activation of group I mGluR, including selective activation of mGluR5, significantly attenuated apoptotic cell death induced by both staurosporine and etoposide. This effect was completely reversed by co-application of a group I mGluR antagonist. Thus, group I mGluR appear to exhibit opposite effects on necrotic and apoptotic neuronal cell death. Our findings suggest that activation of mGluR1 exacerbates neuronal necrosis whereas both mGluR1 and mGluR5 play a role in attenuation of neuronal apoptosis.

Comparison of the LDH and MTT assays for quantifying cell death: validity for neuronal apoptosis?

Neuronal apoptosis induced in cortical cultures by exposure to serum deprivation, staurosporine, nifedipine, or C2-ceramide was assayed by lactate dehydrogenase (LDH) release or inhibition of 3-(4, 5-dimethylthiazol-2-yl)2,5-diphenyl-tetrazolium bromide (MTT) reduction. The protective effects of neurotrophin-4, Z-Val-Ala-Asp-fluoromethylketone (ZVAD), and cycloheximide against each insult were also assayed. The level of injury for each insult was similar whether determined by LDH release or inhibition of MTT reduction, but effects of anti-apoptotic agents were assay dependent. ZVAD and cycloheximide protected neurons from nifedipine-induced death, when assayed by LDH release, but not MTT reduction. In contrast, only cycloheximide attenuated C2-ceramide-induced LDH release, while ZVAD and cycloheximide actually enhanced the C2-ceramide induced inhibition of MTT reduction. Counting of trypan blue positive cells provided results consistent with values obtained using the LDH assay. These results indicate that both LDH release and MTT reduction accurately determine apoptotic death of neurons. However, the MTT assay does not always correctly quantify neuroprotective effects, this likely reflects differences in the point of the death pathway that the neuroprotective agents act. Therefore, while the MTT assay is of limited value in assessing the efficacy of neuroprotective strategies, it may provide information regarding whether specific anti-apoptotic agents act up or downstream of mitochondrial dysfunction.

Calcium channelopathies

Calcium is an important intracellular signaling molecule, and altered calcium channel function can cause widespread cellular changes. Genetic mutations in calcium channels that cause what appear to be trivial alterations of calcium currents in vitro can result in serious diseases in muscles and the nervous system. This article reviews calcium channelopathies in humans and mice.

N-Methyl-D-aspartate receptor blockade induces neuronal apoptosis in cortical culture

Whereas excessive activation of the NMDA receptor may contribute to ischemic neuronal injury, physiologic activation may promote neuronal survival under certain conditions. Consistently, it has recently been shown that NMDA antagonists induce apoptosis of central neurons in immature rats. In the present study, we have examined whether NMDA antagonists induce neuronal apoptosis also in a culture condition. Exposure of cortical cultures (DIV 10-13) to MK-801 (1-10 microM) for 48 h resulted in death of about 30-40% of neurons. Similar neuronal death was induced by exposure to other NMDA antagonists, D-AP5 and dextromethorphan. The neuronal death was dependent on the culture age; MK-801 induced much less neuronal death in younger (DIV 7) and older (DIV 16-19) cultures. The NMDA antagonist-induced neuronal death was accompanied by cell body shrinkage, nuclear fragmentation, and cleavage/activation of caspase-3. Furthermore, it was attenuated by cycloheximide and zVAD-fmk, indicating that the death occurred mainly by the apoptosis mechanism. As in several other apoptosis models, high-potassium medium blocked the NMDA antagonist-induced apoptosis, which was reversed by voltage-gated calcium channel blockers. The present results demonstrate that NMDA antagonists induce neuronal apoptosis in cortical culture, consistent with the findings obtained in immature rats. Since the activation of the voltage-gated calcium channels attenuated the NMDA antagonist-induced apoptosis, it may be another example of the "calcium set point hypothesis."

Beta-amyloid-induced apoptosis of cerebellar granule cells and cortical neurons: exacerbation by selective inhibition of group I metabotropic glutamate receptors

Administration of beta-amyloid fragment 25-35 (Abeta25-35) to cultured rat cerebellar granule cells (CGC) or cortical neurons caused cell death that was characterized by morphological and nuclear changes consistent with apoptosis. Inhibition of NMDA receptors produced a mild exacerbation of Abeta25-35 toxicity in cortical neurons; a similar effect was induced by AMPA/kainate receptor inhibition in CGC. Selective activation of group I metabotropic glutamate receptors (mGluR) by dihyroxyphenylglycine (DHPG) had no effect on Abeta25-35-induced apoptosis in either cell type, and was unaffected by blockade of ionotropic glutamate receptors. In contrast, selective inhibition of group I mGluR by (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) exacerbated Abeta toxicity in cortical neurons, whereas this treatment was without effect on CGC. However, AIDA significantly increased Abeta-induced apoptosis in CGC in the presence of either NMDA or AMPA/kainate receptor inhibition; blockade of both ionotropic glutamate receptor classes further increased the exacerbation of apoptosis following treatment with AIDA. These findings suggest that Abeta25-35-induced neuronal injury leads to activation of group I mGluR, which attenuates the resulting apoptosis.

The changing landscape of ischaemic brain injury mechanisms

Thrombolysis has become established as an acute treatment for human stroke. But despite multiple clinical trials, neuroprotective strategies have yet to be proved effective in humans. Here we discuss intrinsic tissue mechanisms of ischaemic brain injury, and present a perspective that broadening of therapeutic targeting beyond excitotoxicity and neuronal calcium overload will be desirable for developing the most effective neuroprotective therapies.

High sensitivity of immature GABAergic neurons to blockers of voltage-gated calcium channels

The involvement of voltage-gated calcium channels in the survival of immature CNS neurons was studied in aggregating brain cell cultures by examining cell type-specific effects of various channel blockers. Nifedipine (10 microM), a specific blocker of L-type calcium channels, caused a pronounced and irreversible decrease of glutamic acid decarboxylase activity, whereas the activity of choline acetyltransferase was significantly less affected. Flunarizine (1-10 microM, a relatively unspecific ion channel blocker) elicited similar effects, that were attenuated by NMDA. The glia-specific marker enzymes, glutamine synthetase and 2',3'-cyclic nucleotide 3'-phosphohydrolase, were affected only after treatment with high concentrations of nifedipine (50 microM) or NiCl2 (100 microM, shown to block T-type calcium channels). Nifedipine (50 microM), NiCl2 (100 microM), and flunarizine (5 microM) also caused a significant increase in the soluble nucleosome concentration, indicating increased apoptotic cell death. This effect was prevented by cycloheximide (1 microM). Furthermore, the combined treatment with calcicludine (10 nM, blocking L-type calcium channels) and funnel-web spider toxin-3.3 (100 nM, blocking T-type channels) also caused a significant increase in free nucleosomes as well as a decrease in glutamic acid decarboxylase activity. In contrast, cell viability was not affected by peptide blockers specific for N-, P-, and/or Q-type calcium channels. Highly differentiated cultures showed diminished susceptibility to nifedipine and flunarizine. The present data suggest that the survival of immature neurons, and particularly that of immature GABAergic neurons, requires the sustained entry of Ca2+ through voltage-gated calcium channels.

Calcium ionophores can induce either apoptosis or necrosis in cultured cortical neurons

Cultured cortical neurons exposed for 24 h to low concentrations of the Ca2+ ionophores, ionomycin (250 nM) or A-23187 (100 nM), underwent apoptosis, accompanied by early degeneration of neurites, cell body shrinkage, chromatin condensation and internucleosomal DNA fragmentation. This death could be blocked by protein synthesis inhibitors, as well as by the growth factors brain-derived neurotrophic factor or insulin-like growth factor I. If the ionomycin concentration was increased to 1-3 microM, then neurons underwent necrosis, accompanied by early cell body swelling without DNA laddering, or sensitivity to cycloheximide or growth factors. Calcium imaging with Fura-2 suggested a possible basis for the differential effects of low and high concentrations of ionomycin. At low concentrations, ionomycin induced greater increases in intracellular Ca2+ concentration in neurites than in neuronal cell bodies, whereas at high concentrations, ionomycin produced large increases in intracellular Ca2+ concentration in both neurites and cell bodies. We hypothesize that the ability of low concentrations of Ca2+ ionophores to raise intracellular Ca2+ concentration preferentially in neurites caused early neurite degeneration, leading to loss of growth factor availability to the cell body and consequent apoptosis, whereas high concentrations of ionophores produced global cellular Ca2+ overload and consequent necrosis.

Apoptosis in neurodegenerative diseases: the role of mitochondria

Nerve cell death is the central feature of the human neurodegenerative diseases. It has long been thought that nerve cell death in these disorders occurs by way of necrosis, a process characterized by massive transmembrane ion currents, compromise of mitochondrial ATP production, and the formation of high levels of reactive oxygen species combining to induce rapid disruption of organelles, cell swelling, and plasma membrane rupture with a secondary inflammatory response. Nuclear DNA is relatively preserved. Recent evidence now indicates that the process of apoptosis rather than necrosis primarily contributes to nerve cell death in neurodegeneration. This has opened up new avenues for understanding the pathogenesis of neurodegeneration and may lead to new and more effective therapeutic approaches to these diseases.

Effects of dotarizine and flunarizine on chromaffin cell viability and cytosolic Ca2+

Dotarizine (a novel piperazine derivative with antimigraine properties) and flunarizine (a Ca2+ channel antagonist) were compared concerning: first, their ability to cause chromaffin cell damage in vitro; second, the possible correlation of their octanol/water partition coefficients and those of another 28 compounds (i.e., Ca2+ channel antagonists, blockers of histamine H1 receptors, antimycotics, beta-adrenoceptor antagonists, neuroleptics), with their ability to cause cell damage; third, their capacity to protect the cells against the damaging effects of veratridine; and fourth, their capabilities to enhance the basal cytosolic Ca2+ concentration in fura-2-loaded single chromaffin cells, or to modify the pattern of [Ca2+]i oscillations elicited by veratridine. After 24-h exposure to 1-30 microM dotarizine, the viability of bovine adrenal chromaffin cells (measured under phase contrast or as lactate dehydrogenase, released into the medium) was similar to that of control, untreated cells; at 100 microM, 80% lactate dehydrogenase release was produced. At 1-3 microM flunarizine caused no cell damage; however 10 microM caused 20% lactate dehydrogenase release and 30 and 100 microM over 90% lactate dehydrogenase release. The time course of cell damage was considerably faster for flunarizine, in comparison to dotarizine. Out of 30 molecules tested (at 10 microM), having different octanol/water partition coefficients (log P), dotarizine was among the molecules causing no cell damage; flunarizine caused 20% cell loss, lidoflazine and verapamil over 50% cell loss, and penfluridol, draflazine, astemizole or nifedipine over 80% cell loss. No correlation was found between log P and cytotoxicity. Both dotarizine (10-30 microM) and flunarizine (3-10 microM) provided protection against veratridine-induced cell death; however, at 30 microM dotarizine afforded a pronounced protection while flunarizine enhanced the cytotoxic effects of veratridine. Dotarizine (30 microM) (but not flunarizine) caused a prompt transient elevation of the basal [Ca2+]i. Both compounds abolished the K+-induced increases of [Ca2+]i as well as the oscillations of [Ca2+]i induced by veratridine. The blocking effects of dotarizine were readily reversed after washout, while those of flunarizine were long-lasting. These differences might be relevant to the clinical use of dotarizine as an antimigraine drug.

Expression of calbindin-D28k in C6 glial cells stabilizes intracellular calcium levels and protects against apoptosis induced by calcium ionophore and amyloid beta-peptide

The calcium binding protein, calbindin-D28k is normally present in neurons. Recently we reported that brain injury and tumor necrosis factors (TNFs) induce calbindin-D28k in astrocytes. TNF-treated calbindin expressing astrocytes were resistant to acidosis and calcium ionophore toxicity, suggesting that calbindin may have a cytoprotective role in astrocytes in the injured brain (M.P. Mattson, B. Cheng, S.A. Baldwin, V.L. Smith-Swintosky, J. Keller, J. Geddes, Scheff, J.W., Christakos, S., Brain injury and tumor necrosis factors induce calbindin-D28k in astrocytes: evidence for a cytoprotective response, J. Neurosci. Res., 42 (1995) 257). In order to obtain direct evidence for a role of calbindin, using the eukaryotic expression vector pREP4, rat calbindin-D28k was stably expressed in C6 rat astocytoma glial cells. Cytotoxicity in response to calcium ionophore or amyloid beta-peptide (which accumulates in the brain in Alzheimer's disease and has been reported to be neurotoxic) was measured by MTT reduction in vector transfected cells and in calbindin transfected clones. Stably expressed calbindin resulted in increased cell survival in the presence of calcium ionophore (1-10 microM) or amyloid beta-peptide (10-100 microM). In addition, the calcium ionophore or amyloid beta-peptide mediated rise in intracellular calcium in vector transfected cells was significantly attenuated in calbindin expressing cells. Apoptotic cell death was detected by the Hoechst method in vector transfected C6 glial cells treated with calcium ionophore or beta-amyloid (34-36% apoptotic cells/culture). The number of apoptotic nuclei was significantly attenuated in similarly treated calbindin-D28k transfected clones (10-13% apoptotic cells/culture; p<0.01). Our results support the involvement of calcium fluxes in apoptosis and suggest that calbindin-D28k, by buffering calcium, can suppress death in apoptosis susceptible cells in the central nervous system.

Reduced voltage-dependent Ca2+ signaling in CA1 neurons after brief ischemia in gerbils

An initial overload of intracellular Ca2+ plays a critical role in the delayed death of hippocampal CA1 neurons that die a few days after transient ischemia. Without direct evidence, the prevailing hypothesis has been that Ca2+ overload may recur until cell death. Here, we report the first measurements of intracellular Ca2+ in living CA1 neurons within brain slices prepared 1, 2, and 3 days after transient (5 min) ischemia. With no sign of ongoing Ca2+ overload, voltage-dependent Ca2+ transients were actually reduced after 2-3 days of reperfusion. Resting Ca2+ levels and recovery rate after loading were similar to neurons receiving no ischemic insult. The tetrodotoxin-insensitive Ca spike, normally generated by these neurons, was absent at 2 days postischemia, as was a large fraction of Ca2+-dependent spike train adaptation. These surprising findings may lead to a new perspective on delayed neuronal death and intervention.

Single tottering mutations responsible for the neuropathic phenotype of the P-type calcium channel

Recent genetic and molecular biological analyses have revealed many forms of inherited channelopathies. Homozygous ataxic mice, tottering (tg) and leaner (tgla) mice, have mutations in the P/Q-type Ca2+ channel alpha1A subunit gene. Although their clinical phenotypes, histological changes, and locations of gene mutations are known, it remains unclear what phenotypes the mutant Ca2+ channels manifest, or whether the altered channel properties are the primary consequence of the mutations. To address these questions, we have characterized the electrophysiological properties of Ca2+ channels in cerebellar Purkinje cells, where the P-type is the dominant Ca2+ channel, dissociated from the normal, tg, and tgla mice, and compared them with the properties of the wild-type and mutant alpha1A channels recombinantly expressed with the alpha2 and beta subunits in baby hamster kidney cells. The most striking feature of Ca2+ channel currents of mutant Purkinje cells was a marked reduction in current density, being reduced to approximately 60 and approximately 40% of control in tg and tgla mice, respectively, without changes of cell size. The Ca2+ channel currents in the tg Purkinje cells showed a relative increase in non-inactivating component in voltage-dependent inactivation. Besides the same change, those of the tgla mice showed a more distinct change in voltage dependence of activation and inactivation, being shifted in the depolarizing direction by approximately 10 mV, with a broader voltage dependence of inactivation. In the recombinant expression system, the tg channel with a missense mutation (P601L) and one form of the two possible tgla aberrant splicing products, tgla (short) channel, showed a significant reduction in current density, while the other form of the tgla channels, tgla (long), had a current density comparable to the normal control. On the other hand, the shift in voltage dependence of activation and inactivation was observed only for the tgla (long) channel. Comparison of properties of the native and recombinant mutant channels suggests that single tottering mutations are directly responsible for the neuropathic phenotypes of reduction in current density and deviations in gating behavior, which lead to neuronal death and cerebellar atrophy.

Ca2+ channel antagonists and neuroprotection from cerebral ischemia

Stroke is the third leading cause of death and the main disabling neurologic disease. The finding in experimental studies that neuronal death does not occur immediately after ischemic injury has encouraged the development of neuroprotective agents. Various Ca2+ channel antagonists, that is, L-type-selective or non-selective derivatives from classical Ca2+ channel antagonists, have been examined for their ability of neuroprotection through improvement of cerebral blood circulation or inhibition of Ca2+ overload induced by excessive glutamate release. Although some of the antagonists showed efficient neuroprotection in animal models, systemic hypotension limited the utility of these drugs, and none of the compounds showed beneficial effects in treatments for acute ischemic stroke in clinical trials. Drugs other than Ca2+ channel antagonists developed on the basis of the glutamate-Ca2+ overload hypothesis were shown also to lack clinical benefit. Recently, some mechanisms have been proposed to interpret neuronal death in relation to hyperexcitability or apoptosis after ischemic insult. In these hypotheses, activation of the Ca2+ channel types selectively expressed in neuronal tissues is proposed as a critical step of the pathways toward neurodegeneration. Thus, it is increasingly recognized that developing highly selective compounds for neuronal Ca2+ channels is not only important for treatment of stroke but also for elucidation of mechanisms that underlie neurodegeneration.

Depletion of intracellular zinc induces protein synthesis-dependent neuronal apoptosis in mouse cortical culture

The central nervous system (CNS) contains a large amount of zinc; a substantial fraction of it is located inside synaptic vesicles of glutamatergic terminals in chelatable forms and released in a calcium-dependent manner with intense neuronal activity. Recently, it has been shown that excessive zinc influx can kill neurons in rats subjected to transient forebrain ischemia. On the other hand, severe depletion of zinc has been also reported to induced cell death in certain nonneuronal cells. Since decreases in tissue zinc have been associated with Alzheimer's disease (AD) and senile macular degeneration, we examined whether depletion of intracellular zinc with a zinc chelator can directly induce neuronal death in mouse cortical cultures. Exposure of cortical cultures to a cell-permeant zinc-chelator, N,N,N',N'-tetrakis (2-pyridylmethyl) ethylenediamine (TPEN, 0.5-3.0 microM) induced gradually developing neuronal degeneration accompanied by various features of apoptosis: cell body shrinkage, nuclear condensation and fragmentation, and internucleosomal DNA breakage. At higher concentrations, TPEN induced additional glial cell death. TPEN-induced cell death was completely blocked by coaddition of zinc. Addition of a protein synthesis inhibitor cycloheximide as well as a caspase inhibitor carbobenzoxy-valyl-alanyl-aspartyl-fluoromethyl ketone (zVAD-fmk) markedly attenuated TPEN-induced neuronal death. On the other hand, brain-derived neurotrophic factor (BDNF), insulin-like growth factor-1 (IGF-1), phorbol 12-myristate 13-acetate (PMA), high K+, or an antioxidant, trolox, did not show any protective effect. The present results demonstrated that depletion of intracellular zinc induces protein synthesis-dependent neuronal apoptosis in cortical culture. Combined with the findings that extracellular zinc may promote extracellular beta-amyloid (A beta) aggregation and that total tissue zinc is reduced in AD, present results suggest a possibility that redistribution of zinc from intracellular to extracellular space may synergistically contribute to neuronal apoptosis in AD.

Altered calcium channel currents in Purkinje cells of the neurological mutant mouse leaner

Mutations of the alpha1A calcium channel subunit have been shown to cause such human neurological diseases as familial hemiplegic migraine, episodic ataxia-2, and spinocerebellar ataxia 6 and also to cause the murine neurological phenotypes of tottering and leaner. The leaner phenotype is recessive and characterized by ataxia with cortical spike and wave discharges (similar to absence epilepsy in humans) and a gradual degeneration of cerebellar Purkinje and granule cells. The mutation responsible is a single-base substitution that produces truncation of the normal open reading frame beyond repeat IV and expression of a novel C-terminal sequence. Here, we have used whole-cell recordings to determine whether the leaner mutation alters calcium channel currents in cerebellar Purkinje cells, both because these cells are profoundly affected in leaner mice and because they normally express high levels of alpha1A. In Purkinje cells from normal mice, 82% of the whole-cell current was blocked by 100 nM omega-agatoxin-IVA. In Purkinje cells from homozygous leaner mice, this omega-agatoxin-IVA-sensitive current was 65% smaller than in control cells. Although attenuated, the omega-agatoxin-IVA-sensitive current in homozygous leaner cells had properties indistinguishable from that of normal Purkinje neurons. Additionally, the omega-agatoxin-IVA-insensitive current was unaffected in homozygous leaner mice. Thus, the leaner mutation selectively reduces P-type currents in Purkinje cells, and the alpha1A subunit and P-type current appear to be essential for normal cerebellar function.

Neuron degeneration induced by verapamil and attenuated by EGb761

Calcium channel blockers are used as neuroprotective agents, as glutamate antagonists. However, it has been found that calcium channel blockers may compromise neuronal survival after long-term exposure. To explore the mechanisms of the toxicity of calcium channel blockers on neurons, we studied the morphological characteristics and biochemical changes of cultured cortical neurons treated with verapamil, a calcium channel blocker. We now report that cerebral cortical cultures exposed to verapamil for 48 h undergo neuronal degeneration in both concentration-dependent and time-dependent fashion, possibly partially through the activation of apoptosis. On the other hand, it was found that Ginkgo biloba extract (EGb761) attenuated verapamil-induced neuronal injury, suggesting the possibility of using verapamil combined with EGb761 clinically. Furthermore, both B-50 immunoactivity (BIA) and the concentration of intracellular calcium in single neurons ([Ca2+]i) decreased after a 48-h exposure to verapamil, suggesting that the mechanisms of verapamil-induced degeneration may be associated with the disruption of intracellular calcium homeostasis and the inhibition of normal axonal elongation.

De novo mutation in CACNA1A caused acetazolamide-responsive episodic ataxia

With the recent report of mutations in the calcium channel gene CACNA1A in two families with episodic ataxia type 2, we investigated a patient with nonfamilial episodic vertigo and ataxia responsive to acetazolamide for similar mutations. Single-strand conformation polymorphism (SSCP) analysis of exon 23 identified an extra band in the patient that was not present in other relatives or in normal controls. Exon 23 of the patient showed a spontaneous C to T substitution at position 4410 resulting in an early stop codon. Patients with nonfamilial episodic ataxia may respond to acetazolamide and may have mutations in CACNA1A.

Muscimol-induced death of GABAergic neurons in rat brain aggregating cell cultures

During brain development, spontaneous neuronal activity has been shown to play a crucial role in the maturation of neuronal circuitries. Activity-related signals may cause selective neuronal cell death and/or rearrangement of neuronal connectivity. To study the effects of sustained inhibitory activity on developing inhibitory (GABAergic) neurons, three-dimensional primary cell cultures of fetal rat telencephalon were used. In relatively immature cultures, muscimol (10 microns), a GABAA receptor agonist, induced a transient increase in apoptotic cell death, as evidenced by a cycloheximide-sensitive increase of free nucleosomes and an increased frequency of DNA double strand breaks (TUNEL labeling). Furthermore, muscimol caused an irreversible reduction of glutamic acid decarboxylase activity, indicating a loss of GABAergic neurons. The muscimol-induced death of GABAergic neurons was attenuated by the GABAA receptor blockers bicuculline (100 microns) and picrotoxin (100 microns), by depolarizing potassium concentrations (30 mM KCl) and by the L-type calcium channel activator BAY K8644 (2 microns). As compared to the cholinergic marker (choline acetyltransferase activity), glutamic acid decarboxylase activity was significantly more affected by various agents known to inhibit neuronal activity, including tetrodotoxin (1 micron), flunarizine (5 microns), MK 801 (50 microns) and propofol (40 microns). The present results suggest that the survival of a subpopulation of immature GABAergic neurons is dependent on sustained neuronal activity and that these neurons may undergo apoptotic cell death in response to GABAA autoreceptor activation.

Progressive ataxia due to a missense mutation in a calcium-channel gene

We describe a family with severe progressive cerebellar ataxia involving the trunk, the extremities, and speech. The proband, who has prominent atrophy of the cerebellum, shown by magnetic resonance imaging, was confined to a wheelchair at the age of 44 years. Two sons have episodes of vertigo and ataxia that are not responsive to acetazolamide. Quantitative eye-movement testing showed a consistent pattern of abnormalities localizing to the cerebellum. Genotyping suggested linkage to chromosome 19p, and SSCP showed an aberrant migrating fragment in exon 6 of the calcium-channel gene CACNA1A, which cosegregated with the disease. Sequencing of exon 6 identified a G-->A transposition in one allele, at nucleotide 1152, resulting in a predicted glycine-to-arginine substitution at codon 293. The CAG-repeat expansion associated with spinocerebellar ataxia 6 was not present in any family members. This family is unique in having a non-CAG-repeat mutation that leads to severe progressive ataxia. Since a great deal is known about the function of calcium channels, we speculate on how this missense mutation leads to the combination of clinical symptoms and signs.

A hypothalamic neuronal cell line persistently infected with scrapie prions exhibits apoptosis

Neuronal death and vacuolation are characteristics of the CNS degeneration found in prion diseases. Relatively few cultured cell lines have been identified that can be persistently infected with scrapie prions, and none of these cells show cytopathologic changes reminiscent of prion neuropathology. The differentiated neuronal cell line GT1, established from gonadotropin hormone releasing-hormone neurons immortalized by genetically targeted tumorigenesis in transgenic mice (P. L. Mellon, JJ. Windle, P. C. Goldsmith, C. A. Padula, J. L. Roberts, and R. I. Weiner, Neuron 5:1-10, 1990), was examined for its ability to support prion formation. We found that GT1 cells could be persistently infected with mouse RML prions and that conditioned medium from infected cells could transfer prions to uninfected cells. In many but not all experiments, a subpopulation of cells showed reduced viability, morphological signs of neurodegeneration and vacuolation, and features of apoptosis. Subclones of GT1 cells that were stably transfected with the trk4 gene encoding the high-affinity nerve growth factor (NGF) receptor (GT1-trk) could also be persistently infected. NGF increased the viability of the scrapie-infected GT1-trk cells and reduced the morphological and biochemical signs of vacuolation and apoptosis. GT1 cells represent a novel system for studying the molecular mechanisms underlying prion infectivity and subsequent neurodegenerative changes.

Dissociation of effects of glutamate receptor antagonists on excitotoxic and hypoxic neuronal cell death in a novel rat cortical culture system

A novel in vitro cell culture model has been developed to investigate the mechanisms of delayed neuronal cell death following exposure to excitatory amino acids and hypoxia. Medium change damages cortical cells possibly leading to preselection of the neuronal population. This model allowed compounds to be administered in the absence of a medium change. In this system, the noncompetitive N-methyl-D-aspartate (NMDA) antagonist, MK-801, attenuated the neurotoxic effects of overnight exposure to glutamate and NMDA completely, and partially protected neurones exposed to alpha-amino-3-hydroxy-5-methyl-isoxazole-4-proprionate (AMPA). The non-NMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, CNQX, did not attenuate the effects of glutamate or NMDA but blocked the excitotoxic effects of AMPA completely. These results suggest partial involvement of NMDA receptor activation in AMPA-induced toxicity. By contrast, hypoxia-induced neuronal degeneration in this model was attenuated by either NMDA or non-NMDA antagonism, which confirms previous reports that the mechanisms of hypoxic and excitotoxic neurodegeneration in these in vitro models are not identical. A number of other compounds, which have been reported previously as neuroprotective in vitro and in vivo, including the calcium channel antagonists, SB 201823, flunarizine, and nifedipine, and the nitric oxide synthase inhibitor N-nitro-L-arginine methyl ester, L-NAME, demonstrated no significant neuroprotective effects in this in vitro system. In common with other in vitro models that include a change of medium, these data suggest that this system does not have predictive validity for the identification of novel neuroprotective agents in vivo.

Slowly triggered excitotoxicity occurs by necrosis in cortical cultures

This study examined the possibility that the excitotoxin-induced death of cultured cortical neurons might occur by apoptosis, specifically focusing on the slowly triggered death induced by low concentrations of excitotoxin. Cultured murine cortical neurons (days in vitro 10-12) were exposed continuously to N-methyl-D-aspartate (10-15 microM), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (3-100 microM) or kainate (30-60 microM) over 24 h. Within 2 h of exposure onset, neuronal cell body swelling was visible under phase-contrast optics. At this point, transmission electron microscopy revealed disruption of cell membranes and organelles, mitochondrial swelling and scattered chromatin condensation at the periphery of nuclei. By 8 h after exposure onset, many neurons were devoid of cytoplasmic structures, but nuclear membranes remained relatively intact. This excitotoxic degeneration was not blocked by the protein synthesis inhibitor, cycloheximide, or the growth factors, brain-derived neurotrophic factor or insulin-like growth factor-1, agents that did block serum deprivation-induced apoptosis death in other cultures. DNA agarose gel electrophoresis, however, revealed the transient occurrence of internucleosomal DNA fragmentation, appearing 4-8 h after exposure onset, but absent 24 h after exposure onset. The present results suggest that even slowly triggered excitotoxicity occurs by necrosis, and raise a cautionary note in interpreting internucleosomal DNA fragmentation in isolation as evidence for apoptosis.

Apoptosis in neurodegenerative disorders: potential for therapy by modifying gene transcription

Apoptotic, rather than necrotic, nerve cell death now appears as likely to underlie a number of common neurological conditions including stroke, Alzheimer's disease, Parkinson's disease, hereditary retinal dystrophies and Amyotrophic Lateral Sclerosis. Apoptotic neuronal death is a delayed, multistep process and therefore offers a therapeutic opportunity if one or more of these steps can be interrupted or reversed. Research is beginning to show how specific macromolecules play a role in determining the apoptotic death process. We are particularly interested in the critical nature of gradual mitochondrial failure in the apoptotic process and propose that a maintenance of mitochondrial function through the pharmacological modulation of gene expression offers an opportunity for the effective treatment of some types of neurological dysfunction. Our research into the development of small diffusible molecules that reduce apoptosis has grown from studies of the irreversible MAO-B inhibitor (-)-deprenyl. (-)-Deprenyl can reduce neuronal death independently of MAO-B inhibition even after neurons have sustained seemingly lethal damage. (-)-Deprenyl can also influence the process outgrowth of some glial and neuronal populations and can reduce the concentrations of oxidative radicals in damaged cells at concentrations too small to inhibit MAO. In accord with earlier work of others, we showed that (-)-deprenyl alters the expression of a number of mRNAs or of proteins in nerve and glial cells and that the alterations in gene expression/protein synthesis are the result of a selective action on transcription. The alterations in gene expression/protein synthesis are accompanied by a decrease in DNA fragmentation characteristic of apoptosis and the death of responsive cells. The onco-proteins Bcl-2 and Bax and the scavenger proteins Cu/Zn superoxide dismutase (SOD1) and Mn superoxide dismutase (SOD-2) are among the 40-50 proteins whose synthesis is altered by (-)-deprenyl. Since mitochondrial membrane potential correlates with mitochondrial ATP production, we have used confocal laser imaging techniques in living cells to show that the transcriptional changes induced by (-)-deprenyl result in a maintenance of mitochondrial membrane potential, a decrease in intramitochondrial calcium and a decrease in cytoplasmic oxidative radical levels. We therefore propose that (-)-deprenyl acts on gene expression to maintain mitochondrial function and decrease cytoplasmic oxidative radical levels and thereby reduces apoptosis. An understanding of the molecular steps by which (-)-deprenyl selectively alters transcription may lead to the development of new therapies for neurodegenerative diseases.

Autosomal dominant cerebellar ataxia (SCA6) associated with small polyglutamine expansions in the alpha 1A-voltage-dependent calcium channel

A polymorphic CAG repeat was identified in the human alpha 1A voltage-dependent calcium channel subunit. To test the hypothesis that expansion of this CAG repeat could be the cause of an inherited progressive ataxia, we genotyped a large number of unrelated controls and ataxia patients. Eight unrelated patients with late onset ataxia had alleles with larger repeat numbers (21-27) compared to the number of repeats (4-16) in 475 non-ataxia individuals. Analysis of the repeat length in families of the affected individuals revealed that the expansion segregated with the phenotype in every patient. We identified six isoforms of the human alpha 1A calcium channel subunit. The CAG repeat is within the open reading frame and is predicted to encode glutamine in three of the isoforms. We conclude that a small polyglutamine expansion in the human alpha 1A calcium channel is most likely the cause of a newly classified autosomal dominant spinocerebellar ataxia, SCA6.

Nimodipine improves kainic acid induced neurotoxicity in cerebellar granular cell culture: a double-blind dose-response study

The neuroprotective role of nimodipine was tested in kainic acid (50 and 100 microM) induced neurotoxicity in cerebellar granular cell cultures of 4 to 7 day-old rat pups. Nimodipine was applied in 50, 100 and 200 microM concentrations. Kainate, in either dose, induced cerebellar granular cell death in respect to controls and the results were statistically significant (P = 0.000 for both doses). However, kainic acid in 100 microM concentration led to higher rates of cell death than 50 microM (P = 0.017). The neuroprotective role of nimodipine in kainate induced neurotoxicity was dose dependent. Kainate toxicity in 50 microM concentration was blocked by 50 and 100 microM nimodipine concentrations (P = 0.006 and P = 0.002, respectively) while 200 microM nimodipine was found ineffective. The most effective nimodipine dose for 100 microM kainic acid neurotoxicity was 200 microM (P = 0.000) while 50 and 100 microM concentrations of nimodipine were found ineffective. In this study, we have proven the dose-dependent neuroprotective role of nimodipine in kainate induced neurotoxicity in cerebellar granular cell cultures of rat pups.

Absence epilepsy in tottering mutant mice is associated with calcium channel defects

Mutations at the mouse tottering (tg) locus cause a delayed-onset, recessive neurological disorder resulting in ataxia, motor seizures, and behavioral absence seizures resembling petit mal epilepsy in humans. A more severe allele, leaner (tg(la)), also shows a slow, selective degeneration of cerebellar neurons. By positional cloning, we have identified an alpha1A voltage-sensitive calcium channel gene that is mutated in tg and tg(la) mice. The alpha1A gene is widely expressed in the central nervous system with prominent, uniform expression in the cerebellum. alpha1A expression does not mirror the localized pattern of cerebellar degeneration observed in tg(la) mice, providing evidence for regional differences in biological function of alpha1A channels. These studies define the first mutations in a mammalian central nervous system-specific voltage-sensitive calcium channel and identify the first gene involved in absence epilepsy.

Preincubation with protein synthesis inhibitors protects cortical neurons against oxygen-glucose deprivation-induced death

Twenty-four hour exposure to cycloheximide produced a concentration-dependent reduction in protein synthesis in mouse cortical cell cultures. Unexpectedly, a 24 h pretreatment with cycloheximide exposure also reduced neuronal vulnerability to subsequent oxygen-glucose deprivation-induced injury, measured both acutely (cell swelling) or after one day (cell lysis). This neuroprotective effect was attenuated if the period of cycloheximide pretreatment was shortened to 8 h, and lost if the pretreatment was shortened to 1 h. A comparable neuroprotective effect was also induced by 24 h pretreatment with another protein synthesis inhibitor, emetine. The neuroprotection induced by pretreatment with cycloheximide or emetine was probably not attributable to reduction of apoptosis: (i) neuronal death under these conditions occurs by N-methyl-D-aspartate receptor-mediated excitotoxic necrosis, not apoptosis; (ii) the same cycloheximide pretreatment did not block staurosporine-induced apoptosis. Also unlikely as an explanation is reduction in postsynaptic vulnerability to excitotoxicity, as death induced by exogenous addition of N-methyl-D-aspartate, kainate, or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate was little affected by cycloheximide pretreatment. Rather, the protective effect of cycloheximide pretreatment was probably explained, at least in part, by marked reduction in the glutamate release induced by oxygen-glucose deprivation.

Neuronal cell death in the mammalian nervous system: the calmortin hypothesis

1. This review is concerned with the calcium-dependent mechanisms involved in neuronal cell death. To this end, it provides definitions of the major types of cell death and then describes what is known of their occurrence during development and degeneration of the mammalian nervous system. 2. An analysis is presented of the different sources and compartments of calcium in neurons and of how these are related to the known calcium-dependent enzymes whose excess activation will lead to cell death. 3. The review uses the relatively large amount of pertinent information now available for other cell types, especially thymocytes, to reveal our limited knowledge of how calcium controls neuronal cell death. 4. In the final section, consideration is given to the identification of those factors that may mitigate against the calcium-dependent pathways leading to neuronal degeneration.

Nitric oxide-mediated death of cultured neonatal retinal ganglion cells: neuroprotective properties of glutamate and chondroitin sulfate proteoglycan

The release of nitric oxide and stimulation of glutamate receptors by excitatory amino acids has been linked to neuronal degeneration and toxicity. In the rat retina approximately 60% of retinal ganglion cells (RGCs) die during the first postnatal week. In this study we examined the effects of nitric oxide synthase blockers and glutamate on the survival of neonatal RGCs in vitro over a 16 h assay period. Less than 10% of P1 RGCs survived in serum free defined media alone (control), however survival was increased, in a dose-dependent manner, when L-glutamate (10 microM-10 mM) was added to the media; a maximum of 70% of RGCs could be maintained with the addition of 5 mM glutamate. This effect was blocked by the NMDA and non-NMDA receptor blockers APV and DNQX and was age dependent; the survival of RGCs from P5 but not P7 rats was enhanced by the addition of glutamate even in high calcium concentrations (10 mM). When the nitric oxide synthase blockers L-NAME (5 mM) or haemoglobin (25 microM) were added to the culture media, up to 61% of P1 RGCs survived. The addition of the 480 kDa chondroitin sulfate proteoglycan (SCCP) previously shown to enhance RGC survival in vivo and in vitro, potentiated the action of glutamate and L-NAME and increased RGC survival to over 90% with almost all RGCs expressing a profusion of processes. These results suggest that the release of nitric oxide and glutamate by cells within the retina may contribute to the regulation of RGC numbers in vivo during development.

Blockade of glutamate receptors unmasks neuronal apoptosis after oxygen-glucose deprivation in vitro

Mouse cortical cell cultures exposed to transient oxygen-glucose deprivation developed marked acute cell body swelling followed by neurodegeneration, consistent with necrosis-type death. This death was not attenuated by the protein synthesis inhibitor, cycloheximide, but was attenuated by addition of the N-methyl-D-asparate antagonist, MK-801 (dizocilpine maleate), and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione. If the deprivation insult was extended to overcome the protective effect of glutamate antagonists, neuronal death resulted that was associated with cell body shrinkage and DNA fragmentation, and was attenuated by cycloheximide. These data suggest that oxygen-glucose deprivation can induce in cortical neurons both excitotoxic necrosis, and apoptosis dependent on new macromolecule synthesis.

The role of inducible transcription factors in apoptotic nerve cell death

Recent studies have shown that certain types of nerve cell death in the brain occur by an apoptotic mechanism. Researchers have demonstrated that moderate hypoxic-ischemic (HI) episodes and status epilepticus (SE) can cause DNA fragmentation as well as other morphological features of apoptosis in neurons destined to die, whereas more severe HI episodes lead to neuronal necrosis and infarction. Although somewhat controversial, some studies have demonstrated that protein synthesis inhibition prevents HI-and SE-induced nerve cell death in the brain, suggesting that apoptotic nerve cell death in the adult brain is de novo protein synthesis-dependent (i.e., programmed). The identity of the proteins involved in HI-and SE-induced apoptosis in the adult brain is unclear, although based upon studies in cell culture, a number of potential cell death and anti-apoptosis genes have been identified. In addition, a number of studies have demonstrated that inducible transcription factors (ITFs) are expressed for prolonged periods in neurons undergoing apoptotic death following HI and SE. These results suggest that prolonged expression of ITFs (in particular c-jun) may form part of the biological cascade that induces apoptosis in adult neurons. These various studies are critically discussed and in particular the role of inducible transcription factors in neuronal apoptosis is evaluated.

Potentiated necrosis of cultured cortical neurons by neurotrophins

The effects of neurotrophins on several forms of neuronal degeneration in murine cortical cell cultures were examined. Consistent with other studies, brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4/5 all attenuated the apoptotic death induced by serum deprivation or exposure to the calcium channel antagonist nimodipine. Unexpectedly, however, 24-hour pretreatment with these same neurotrophins markedly potentiated the necrotic death induced by exposure to oxygen-glucose deprivation or N-methyl-D-aspartate. Thus, certain neurotrophins may have opposing effects on different types of death in the same neurons.

Molecular approaches to amyotrophic lateral sclerosis

New discoveries are expanding our knowledge of mechanisms involved in amyotrophic lateral sclerosis (ALS) pathogenesis. Some recent advances in our understanding of motoneuron death in familial ALS (fALS) and sporadic ALS (sALS) are reviewed, with emphasis on molecular similarities that may further unite these phenotypically linked diseases.

Calbindin-D28k, calretinin, and recoverin immunoreactivities in developing chick pineal gland

Calbindin-D28k, calretinin, and recoverin, three intracellular calcium-binding proteins belonging to the troponin C/calmodulin superfamily, were immunohistochemically localized in chick pineal during development [from embryonic day 16 (E16) to postnatal day 14 (P14)]. At E18, only calretinin immunoreactivity could be detected in nuclei from follicular pinealocytes. With development, calretinin immunoreactivity expanded from nucleus to cytoplasm, and calretinin immuno-positive cell number increased. At P14 almost al pinealocytes were calretinin positive. Calbindin-D28k immunoreactivity was not detected before E20. During development, many follicular and parafollicular pinealocytes became strongly calbindin-D28k positive, reaching a peak both in intensity and in number at P7; thereafter their number decreased. In addition to pinealocytes, neuron-like cells appeared calbindin-D28k positive at E20 and calretinin positive at P7. Recoverin, a myristoylated protein isolated from vertebrate photoreceptor and which might participate in the inactivation of the phototransduction cascade, was transiently expressed in follicular and parafollicular pinealocytes from P1 to P14 with a maximal expression at P7. This transitory expression may coincide with a transitory light sensitivity period in chick pinealocytes, before complete maturity of the pineal gland.

Cytotoxicity of immunoglobulins from amyotrophic lateral sclerosis patients on a hybrid motoneuron cell line

Patients with amyotrophic lateral sclerosis possess antibodies (ALS IgGs) that bind to L-type skeletal muscle voltage-gated calcium channels (VGCCs) and inhibit L-type calcium current. To determine whether interaction of ALS IgGs with neuronal VGCCs might influence motoneuron survival, we used a motoneuron-neuroblastoma hybrid (VSC 4.1) cell line expressing binding sites for inhibitors of L-, N-, and P-type VGCCs. Using direct viable cell counts, quantitation of propidium iodide- and fluorescein diacetate-labeled cells, and lactate dehydrogenase release to assess cell survival, we document that ALS IgG kills 40-70% of cAMP-differentiated VSC 4.1 cells within 2 days. ALS IgG-mediated cytotoxicity is dependent on extracellular calcium and is prevented by peptide antagonists of N- or P-type VGCCs but not by dihydropyridine modulators of L-type VGCCs. Preincubating IgG with purified intact L-type VGCC or with isolated VGCC alpha 1 subunit also blocks ALS IgG-mediated cytotoxicity. These results suggest that ALS IgG may directly lead to motoneuron cell death by a mechanism requiring extracellular calcium and mediated by neuronal-type calcium channels.

Voltage gated calcium channel currents of rat dorsal root ganglion (DRG) cells are blocked by Al3+

The effects of the trivalent cation aluminum (Al3+) on voltage activated calcium channel currents were examined. Al3+ blocks sustained and transient components of voltage activated calcium channel currents of cultured rat dorsal root ganglion (DRG) cells. Currents were elicited by voltage jumps from -80 to 0 mV. The channel block was use dependent. Steady state blockade occurred after 1 to 5 min, when opening the channel every 10 s. There was little or no recovery after washing. Threshold concentration was about 20 microM Al3+ and total blockade (> 80%) was obtained at 200 microM Al3+; the IC50 was 83 microM and the Hill number was around 3. The degree of blockade was pH dependent, and increased with H+ concentration. The current-voltage relation frequently shifted to depolarised voltages after applying Al3+. The degree of the shift was a function of Al3+ concentration, but the magnitude differed from cell to cell. In the effective concentration range (< 200 microM Al3+) the effect was quite specific to voltage activated calcium channel currents. Voltage activated potassium and sodium channels were reduced less than 15% by 200 microM Al3+. We conclude that Al3+ is a potent and irreversible blocker of voltage activated calcium channel currents in mammalian neurons.

Apoptosis is induced by beta-amyloid in cultured central nervous system neurons

The molecular mechanism responsible for the neurodegeneration in Alzheimer disease is not known; however, accumulating evidence suggests that beta-amyloid peptide (A beta P) contributes to this degeneration. We now report that synthetic A beta Ps trigger the degeneration of cultured neurons through activation of an apoptotic pathway. Neurons treated with A beta Ps exhibit morphological and biochemical characteristics of apoptosis, including membrane blebbing, compaction of nuclear chromatin, and internucleosomal DNA fragmentation. Aurintricarboxylic acid, an inhibitor of nucleases, prevents DNA fragmentation and delays cell death. Our in vitro results suggest that apoptosis may play a role in the neuronal loss associated with Alzheimer disease.