Suppression of programmed neuronal death by a thapsigargin-induced Ca2+ influx

An evolutionarily conserved coronin-dependent pathway defines cell population size

Maintenance of cell population size is fundamental to the proper functioning of multicellular organisms. Here, we describe a cell-intrinsic cell density–sensing pathway that enabled T cells to reach and maintain an appropriate population size. This pathway operated “kin-to-kin” or between identical or similar T cell populations occupying a niche within a tissue or organ, such as the lymph nodes, spleen, and blood. We showed that this pathway depended on the cell density–dependent abundance of the evolutionarily conserved protein coronin 1, which coordinated prosurvival signaling with the inhibition of cell death until the cell population reached threshold densities. At or above threshold densities, coronin 1 expression peaked and remained stable, thereby resulting in the initiation of apoptosis through kin-to-kin intercellular signaling to return the cell population to the appropriate cell density. This cell population size-controlling pathway was conserved from amoeba to humans, thus providing evidence for the existence of a coronin-regulated, evolutionarily conserved mechanism by which cells are informed of and coordinate their relative population size.

Mechanism of the lifespan extension induced by submaximal SERCA inhibition in C. elegans

We have reported recently that submaximal inhibition of the Sarco Endoplasmic Reticulum Ca²⁺ ATPase (SERCA) produces an increase in the lifespan of C. elegans worms. We have explored here the mechanism of this increased survival by studying the effect of SERCA inhibition in several mutants of signaling pathways related to longevity. Our data show that the mechanism of the effect is unrelated with the insulin signaling pathway or the sirtuin activity, because SERCA inhibitors increased lifespan similarly in mutants of these pathways. However, the effect required functional mitochondria and both the AMP kinase and TOR pathways, as the SERCA inhibitors were ineffective in the corresponding mutants. The same effects were obtained after reducing SERCA expression with submaximal RNAi treatment. The SERCA inhibitors did not induce ER-stress at the concentrations used, and their effect was not modified by inactivation of the OP50 bacterial food. Altogether, our data suggest that the effect may be due to a reduced ER-mitochondria Ca²⁺ transfer acting via AMPK activation and mTOR inhibition to promote survival.

Alpha-synuclein aggregates activate calcium pump SERCA leading to calcium dysregulation

Aggregation of α‐synuclein is a hallmark of Parkinson's disease and dementia with Lewy bodies. We here investigate the relationship between cytosolic Ca²⁺ and α‐synuclein aggregation. Analyses of cell lines and primary culture models of α‐synuclein cytopathology reveal an early phase with reduced cytosolic Ca²⁺ levels followed by a later Ca²⁺ increase. Aggregated but not monomeric α‐synuclein binds to and activates SERCA in vitro, and proximity ligation assays confirm this interaction in cells. The SERCA inhibitor cyclopiazonic acid (CPA) normalises both the initial reduction and the later increase in cytosolic Ca²⁺. CPA protects the cells against α‐synuclein‐aggregate stress and improves viability in cell models and in Caenorhabditis elegans in vivo. Proximity ligation assays also reveal an increased interaction between α‐synuclein aggregates and SERCA in human brains affected by dementia with Lewy bodies. We conclude that α‐synuclein aggregates bind SERCA and stimulate its activity. Reducing SERCA activity is neuroprotective, indicating that SERCA and down‐stream processes may be therapeutic targets for treating α‐synucleinopathies.

The Role of Mitochondria in the Activation/Maintenance of SOCE: The Contribution of Mitochondrial Ca2+ Uptake, Mitochondrial Motility, and Location to Store-Operated Ca2+ Entry

In most cell types, the depletion of internal Ca²⁺ stores triggers the activation of Ca²⁺ entry. This crucial phenomenon is known since the 1980s and referred to as store-operated Ca²⁺ entry (SOCE). With the discoveries of the stromal-interacting molecules (STIMs) and the Ca²⁺-permeable Orai channels as the long-awaited molecular constituents of SOCE, the role of mitochondria in controlling the activity of this particular Ca²⁺ entry pathway is kind of buried in oblivion. However, the capability of mitochondria to locally sequester Ca²⁺ at sites of Ca²⁺ release and entry was initially supposed to rule SOCE by facilitating the Ca²⁺ depletion of the endoplasmic reticulum and removing entering Ca²⁺ from the Ca²⁺-inhibitable channels, respectively. Moreover, the central role of these organelles in controlling the cellular energy metabolism has been linked to the activity of SOCE. Nevertheless, the exact molecular mechanisms by which mitochondria actually determine SOCE are still pretty obscure. In this essay we describe the complexity of the mitochondrial Ca²⁺ uptake machinery and its regulation, molecular components, and properties, which open new ways for scrutinizing the contribution of mitochondria to SOCE. Moreover, data concerning the variability of the morphology and cellular distribution of mitochondria as putative determinants of SOCE activation, maintenance, and termination are summarized.

Voltage-gated potassium channels at the crossroads of neuronal function, ischemic tolerance, and neurodegeneration

Voltage-gated potassium (Kv) channels are widely expressed in the central and peripheral nervous system and are crucial mediators of neuronal excitability. Importantly, these channels also actively participate in cellular and molecular signaling pathways that regulate the life and death of neurons. Injury-mediated increased K(+) efflux through Kv2.1 channels promotes neuronal apoptosis, contributing to widespread neuronal loss in neurodegenerative disorders such as Alzheimer's disease and stroke. In contrast, some forms of neuronal activity can dramatically alter Kv2.1 channel phosphorylation levels and influence their localization. These changes are normally accompanied by modifications in channel voltage dependence, which may be neuroprotective within the context of ischemic injury. Kv1 and Kv7 channel dysfunction leads to neuronal hyperexcitability that critically contributes to the pathophysiology of human clinical disorders such as episodic ataxia and epilepsy. This review summarizes the neurotoxic, neuroprotective, and neuroregulatory roles of Kv channels and highlights the consequences of Kv channel dysfunction on neuronal physiology. The studies described in this review thus underscore the importance of normal Kv channel function in neurons and emphasize the therapeutic potential of targeting Kv channels in the treatment of a wide range of neurological diseases.

Mitochondrial Ca2+ uptake and not mitochondrial motility is required for STIM1-Orai1-dependent store-operated Ca2+ entry

Store-operated Ca2+ entry (SOCE) is established by formation of subplasmalemmal clusters of the endoplasmic reticulum (ER) protein, stromal interacting molecule 1 (STIM1) upon ER Ca2+ depletion. Thereby, STIM1 couples to plasma membrane channels such as Orai1. Thus, a close proximity of ER domains to the plasma membrane is a prerequisite for SOCE activation, challenging the concept of local Ca2+ buffering by mitochondria as being essential for SOCE. This study assesses the impact of mitochondrial Ca2+ handling and motility on STIM1–Orai1-dependent SOCE. High-resolution microscopy showed only 10% of subplasmalemmal STIM1 clusters to be colocalized with mitochondria. Impairments of mitochondrial Ca2+ handling by inhibition of mitochondrial Na+-Ca2+ exchanger (NCXmito) or depolarization only partially suppressed Ca2+ entry in cells overexpressing STIM1-Orai1. However, SOCE was completely abolished when both NCXmito was inhibited and the inner mitochondrial membrane was depolarized, in STIM1- and Orai1-overexpressing cells. Immobilization of mitochondria by expression of mAKAP-RFP-CAAX, a construct that physically links mitochondria to the plasma membrane, affected the Ca2+ handling of the organelles but not the activity of SOCE. Our observations indicate that mitochondrial Ca2+ uptake, including reversal of NCXmito, is fundamental for STIM1–Orai1-dependent SOCE, whereas the proximity of mitochondria to STIM1-Orai1 SOCE units and their motility is not required.

NSAIDs, Mitochondria and Calcium Signaling: Special Focus on Aspirin/Salicylates

Aspirin (acetylsalicylic acid) is a well-known nonsteroidal anti-inflammatory drug (NSAID) that has long been used as an anti-pyretic and analgesic drug. Recently, much attention has been paid to the chemopreventive and apoptosis-inducing effects of NSAIDs in cancer cells. These effects have been thought to be primarily attributed to the inhibition of cyclooxygenase activity and prostaglandin synthesis. However, recent studies have demonstrated unequivocally that certain NSAIDs, including aspirin and its metabolite salicylic acid, exert their anti-inflammatory and chemopreventive effects independently of cyclooxygenase activity and prostaglandin synthesis inhibition. It is becoming increasingly evident that two potential common targets of NSAIDs are mitochondria and the Ca²⁺ signaling pathway. In this review, we provide an overview of the current knowledge regarding the roles of mitochondria and Ca²⁺ in the apoptosis-inducing effects as well as some side effects of aspirin, salicylates and other NSAIDs, and introducing the emerging role of L-type Ca²⁺ channels, a new Ca²⁺ entry pathway in non-excitable cells that is up-regulated in human cancer cells.

L-type Ca2+ channels: a new player in the regulation of Ca2+ signaling, cell activation and cell survival in immune cells

Ca(2+) is a highly versatile intracellular second messenger in many cell types, and regulates many complicated cellular processes, including cell activation, proliferation and apoptosis. Influx of Ca(2+) from the extracellular fluid is required for sustained elevation of the cytosolic Ca(2+) concentration and full activation of Ca(2+)-dependent processes. It is widely accepted that Ca(2+) release-activated Ca(2+) channels are the major routes of Ca(2+) influx in electrically non-excitable cells, including hematopoietic cells, whereas voltage-gated Ca(2+) channels such as L-type Ca(2+) channels (LTCCs) serve as the principal routes of Ca(2+) entry into electrically excitable cells such as neurons and myocytes. However, recent pharmacological and molecular genetic studies have revealed the existence of functional LTCCs and/or LTCC-like channels in a variety of immune cells including mast cells. In this article, we review recent advances in our understanding of Ca(2+) signaling in immune cells with a special interest in mast cells. We highlight roles for LTCCs in antigen receptor-mediated mast cell activation and survival.

Ca v 1.2 L-type Ca2+ channel protects mast cells against activation-induced cell death by preventing mitochondrial integrity disruption

In non-excitable cells, store-operated Ca(2+) channels (SOCs) are the principal routes of Ca(2+) entry. Recently, store-independent Ca(2+) channels which are pharmacologically and/or immunologically similar to L-type Ca(2+) channels (LTCCs) have been shown to exist in various hematopoietic cells, including T cells, B cells and neutrophils. We previously reported that mast cells express LTCCs which regulate mast cell effector responses in a distinct manner from SOCs. In the present study, we examined the possible role for LTCCs in mast cell survival. Both RBL-2H3 mast cells and bone marrow-derived mast cells underwent considerable apoptosis after treatment with thapsigargin (Tg) but not stimulation through the high-affinity IgE receptor (Fc epsilon RI). The LTCC-selective antagonists such as nifedipine greatly augmented Fc epsilon RI-mediated apoptosis, while the LTCC-selective agonist (S)-BayK8644 blocked Tg-induced apoptosis. The modulation of apoptosis was accompanied by altered mitochondrial integrity, as measured with the mitochondrial membrane potential, cytochrome c release and caspase-3/7 activation. Fc epsilon RI stimulation induced mitochondrial Ca(2+) ([Ca(2+)](m)) entry through both SOCs and LTCCs, while Tg evoked [Ca(2+)](m) entry through LTCCs but not SOCs. The LTCC-selective antagonists blocked [Ca(2+)](m) entry, whereas (S)-BayK8644 augmented Tg-induced [Ca(2+)](m) entry. Moreover, blockade of the expression of the alpha(1C) subunit of Ca(v)1.2 LTCC using small-interfering RNA strongly augmented Fc epsilon RI-mediated apoptosis, mitochondrial integrity, and mitochondrial Ca(2+) collapse, and abolished the protective effects of (S)-BayK8644 against Tg-induced apoptosis. These findings suggest that Ca(v)1.2 LTCC protects mast cells against activation-induced cell death by preventing mitochondrial integrity disruption.

Melatonin antagonizes apoptosis via receptor interaction in U937 monocytic cells

Among the non-neurological functions of melatonin, much attention is being directed to the ability of melatonin to modulate the immune system, whose cells possess melatonin-specific receptors and biosynthetic enzymes. Melatonin controls cell behaviour by eliciting specific signal transduction actions after its interaction with plasma membrane receptors (MT(1), MT(2)); additionally, melatonin potently neutralizes free radicals. Melatonin regulates immune cell loss by antagonizing apoptosis. A major unsolved question is whether this is due to receptor involvement, or to radical scavenging considering that apoptosis is often dependent on oxidative alterations. Here, we provide evidence that on U937 monocytic cells, apoptosis is antagonized by melatonin by receptor interaction rather than by radical scavenging. First, melatonin and a set of synthetic analogues prevented apoptosis in a manner that is proportional to their affinity for plasma membrane receptors but not to their antioxidant ability. Secondly, melatonin's antiapoptotic effect required key signal transduction events including G protein, phospholipase C and Ca(2+) influx and, more important, it is sensitive to the specific melatonin receptor antagonist luzindole.

Carboxyamido-triazole modulates retinal pigment epithelial and choroidal endothelial cell attachment, migration, proliferation, and MMP-2 secretion of choroidal endothelial cells

PURPOSE

To determine the effect of the calcium signaling modulating drug carboxyamido-triazole (CAI) on substeps of exudative age-related macular degeneration (AMD) in vitro.

MATERIALS AND METHODS

Zymography and ELISA determined the effect of CAI on MMP-2 production of choroidal endothelial cells (CECs) stimulated by bFGF and VEGF. The effects of CAI on attachment of retinal pigment endothelial (RPE) cells/CECs onto fibronectin, laminin, collagen IV, and migration toward fibronectin were investigated. Proliferation induced by serum and bFGF (10 microg/ml) with and without CAI (0.1-10 microM) was measured by cell counting and 3H-uptake. Viability and apoptosis of the exposed cells was assessed by an MTT and an apoptosis assay.

RESULTS

CAI inhibited serum- and bFGF-induced proliferation, cell attachment onto fibronectin and collagen IV, but only CEC attachment onto laminin. Inhibition of MMP-2 production was observed (10 microM CAI). CAI reduced the cellular viability by apoptosis induction.

CONCLUSIONS

CAI inhibits substeps of exudative macular degeneration and may be of value for the treatment of the disease.

Store-Operated Calcium Channels

In electrically nonexcitable cells, Ca2+influx is essential for regulating a host of kinetically distinct processes involving exocytosis, enzyme control, gene regulation, cell growth and proliferation, and apoptosis. The major Ca2+entry pathway in these cells is the store-operated one, in which the emptying of intracellular Ca2+stores activates Ca2+influx (store-operated Ca2+entry, or capacitative Ca2+entry). Several biophysically distinct store-operated currents have been reported, but the best characterized is the Ca2+release-activated Ca2+current, ICRAC. Although it was initially considered to function only in nonexcitable cells, growing evidence now points towards a central role for ICRAC-like currents in excitable cells too. In spite of intense research, the signal that relays the store Ca2+content to CRAC channels in the plasma membrane, as well as the molecular identity of the Ca2+sensor within the stores, remains elusive. Resolution of these issues would be greatly helped by the identification of the CRAC channel gene. In some systems, evidence suggests that store-operated channels might be related to TRP homologs, although no consensus has yet been reached. Better understood are mechanisms that inactivate store-operated entry and hence control the overall duration of Ca2+entry. Recent work has revealed a central role for mitochondria in the regulation of ICRAC, and this is particularly prominent under physiological conditions. ICRACtherefore represents a dynamic interplay between endoplasmic reticulum, mitochondria, and plasma membrane. In this review, we describe the key electrophysiological features of ICRACand other store-operated Ca2+currents and how they are regulated, and we consider recent advances that have shed insight into the molecular mechanisms involved in this ubiquitous and vital Ca2+entry pathway.

Clinical perspectives: neuroprotection lessons from hypoxia-tolerant organisms

An effective treatment for brain ischemia is a pressing medical need. Research on brain ischemia has largely focused on understanding the mechanisms of neuron death as a way of identifying targets for therapy. An attractive alternative approach is to identify the survival strategies of hypoxia-tolerant neurons. The adaptation of vertebrate neurons to hypoxia occurs in at least three major ways: (1) as a constitutive property of neurons in anoxia-tolerant turtles and fish, (2) as a property of intra-uterine and early post-natal mammalian development, and (3) as part of a slower, chronic process, as in acclimitization to high altitude. Research on hypoxia-tolerant neurons has already revised several earlier concepts, including the role of calcium in cell death and survival, and the value of N-methyl-d-aspartate (NMDA) receptor antagonism. A broad and fundamental understanding of how neurons adapt to hypoxia is likely to help guide efforts to find new treatments for brain hypoxia and ischemia.

Moderate increases in intracellular calcium activate neuroprotective signals in hippocampal neurons

Although large increases in neuronal intracellular calcium concentrations ([Ca(2+)](i)) are lethal, moderate increases in [Ca(2+)](i) of 50-200 nM may induce immediate or long-term tolerance of ischemia or other stresses. In neurons in rat hippocampal slice cultures, we determined the relationship between [Ca(2+)](i), cell death, and Ca(2+)-dependent neuroprotective signals before and after a 45 min period of oxygen and glucose deprivation (OGD). Thirty minutes before OGD, [Ca(2+)](i) was increased in CA1 neurons by 40-200 nM with 1 nM-1 microM of a Ca(2+)-selective ionophore (calcimycin or ionomycin-"Ca(2+) preconditioning"). Ca(2+) preconditioning greatly reduced cell death in CA1, CA3 and dentate during the following 7 days, even though [Ca(2+)](i) was similar (approximately 2 microM) in preconditioned and control neurons 1 h after the OGD. When pre-OGD [Ca(2+)](i) was lowered to 25 nM (10 nM ionophore in Ca(2+)-free medium) or increased to 8 microM (10 microM ionophore), more than 90% of neurons died. Increased levels of the anti-apoptotic protein protein kinase B (Akt) and the MAP kinase ERK (p42/44) were present in preconditioned slices after OGD. Reducing Ca(2+) influx, inhibiting calmodulin, and preventing Akt or MAP kinase p42/44 upregulation prevented Ca(2+) preconditioning, supporting a specific role for Ca(2+) in the neuroprotective process. Further, in continuously oxygenated cultured hippocampal/cortical neurons, preconditioning for 30 min with 10 nM ionomycin reduced cell death following a 4 microM increase in [Ca(2+)](i) elicited by 1 microM ionomycin. Thus, a zone of moderately increased [Ca(2+)](i) before a potentially lethal insult promotes cell survival, uncoupling subsequent large increases in [Ca(2+)](i) from initiating cell death processes.

Inhibition of apoptosis in pulmonary endothelial cells by altered pH, mitochondrial function, and ATP supply

We investigated the effect of altered extracellular pH, mitochondrial function, and ATP content on development of apoptosis in human pulmonary artery endothelial cells after treatment with staurosporine (STS). STS produced a concentration- and time-dependent increase in caspase-3 activity in pH 7.4 medium that reached a peak at 6 h. The increase in caspase activity was associated with significant DNA fragmentation. Fluorescent imaging of treated monolayers in pH 7.4 medium with Hoechst-33342-propidium iodide demonstrated a large percentage of apoptotic cells ( approximately 40%) with no evidence of necrosis. Caspase activity, DNA fragmentation, and percentage of apoptotic cells were reduced after STS treatment in acidic media (pH 7.0 and 6.6). The Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM inhibited STS-induced apoptosis, whereas the rise in intracellular Ca2+concentration in STS-treated cells in pH 7.4 medium was reduced in pH 7.0 medium. These results suggest that one mechanism for inhibitory effects of acidosis may be a pH-induced alteration in Ca2+ signaling. Treatment with STS in the presence of oligomycin (10 microM), an inhibitor of the mitochondrial F(0)F(1)-ATPase, in glucose-free media abolished caspase activation and DNA fragmentation in association with severe ATP depletion ( approximately 2% of control cells). Imaging demonstrated a change in the mode of cell death from apoptosis to necrosis under these conditions. This change was linked to the level of ATP depletion, because STS treatment in the absence of glucose or the presence of oligomycin in media with glucose still leads to apoptosis in the presence of only moderate ATP depletion. These results demonstrate that pH, mitochondrial function, and ATP supply are important variables that regulate STS-induced apoptosis in human pulmonary artery endothelial cells.

Interference of activated factor VII in apoptosis of erytholeukemic K562 cells

Coagulation factor VIIa (FVIIa) is a key protease initiating the coagulation cascade in the presence of its receptor, tissue factor (TF). FVIIa elicits several cellular responses, probably involving other receptors(s) than TF. This study investigates the implication of recombinant FVIIa on the apoptosis of K562 erythroleukemia cells. These cells undergo apoptosis when induced to differentiate towards the erythroid lineage by hemin. They do not express TF, but can be transfected to do so. FVIIa treatment significantly reduced the degree of hemin-induced apoptosis in K562 cells, but not in TF+ derived transfectants. Induction of apoptosis by hemin also elicited decrease in intracellular Ca2+ concentration ([Ca2+]i), but FVIIa restored this [Ca2+]i close to that of non-treated cells. These results suggest that FVIIa acts via a TF-independent pathway to counteract apoptosis by a mechanism involving its Gla domain and linked to the maintenance of Ca2+ homeostasis in K562 cells.

Effects of radiation on Ca2+signaling in salivary epithelial cell lines transfected with Bcl-2 and Bcl-XL

The effects of radiation on the Ca2+ signaling system in HSY cells transfected with the Bcl-2 or Bcl-XL gene were studied. Bcl-2 overexpression did not alter carbachol (CCh)-elicited initial increase in cytosolic free Ca2+ concentrations ([Ca2+]i), but Bcl-XL overexpression dramatically reduced this response. Exposure to 10 Gy gamma-ray did not alter basal [Ca2+]i. By contrast, the CCh-stimulated initial [Ca2+]i increase was reduced at 0.5 and 4 h post-irradiation in all cell types and remained decreased at 24 h in wild-type and control-transfected cells, but recovered in Bcl-2- and Bcl-XL-transfectants. The formation of inositol 1,4,5-trisphosphate (IP3) in response to CCh at 4-h post-irradiation was decreased in wild-type and control-transfected cells, but not in Bcl-2 and Bcl-XL transfectants. The capacity of the IP3-sensitive Ca2+ store was significantly reduced by radiation in all cells except Bcl-XL transfectants. Ca2+ influx after stimulation with CCh was suppressed by exposure to radiation in wild-type and control-transfected cells, but not in Bcl-2- and Bcl-XL-transfectants. However, radiation enhanced Ca2+ influx activated by thapsigargin in all cell types. These results suggest that 1) radiation diminishes IP3 formation and Ca2+ release in response to CCh, but potentiates the store-operated Ca2+ influx; and 2) overexpression of Bcl-2 or Bcl-XL partially protects cells from radiation-induced inhibition of Ca2+ signaling.

Distinct phosphorylation patterns underlie Akt activation by different survival factors in neurons

The survival of cultured cerebellar granule neurons can be maintained by depolarizing levels of potassium (high K(+), HK), insulin-like growth factor (IGF-1), cyclic AMP or lithium. We examined the possibility that the signaling pathways activated by these different factors converge and that Akt might represent such a point of convergence. Consistent with this possibility, we find that Akt is phosphorylated and activated by all four survival factors. The pattern of Akt phosphorylation induced by the four survival factors, however, shows differences. While IGF-1 induces phosphorylation of Akt at both Ser473 and Thr308, HK and cyclic AMP stimulate phosphorylation at Thr308 only. Lithium increases phosphorylation at Ser473 but not at Thr308. Our results are consistent with the possibility that Akt is a central component of different survival-promoting pathways in granule neurons. The different phosphorylation patterns, however, point to a previously unappreciated complexity in the regulation of Akt activity in neurons. Finally, we provide evidence indicating that SGK, a kinase that is structurally related to Akt, is also activated by the four survival factors.

Sindbis virus-induced neuronal death is both necrotic and apoptotic and is ameliorated by N-methyl-D-aspartate receptor antagonists

Virus infection of neurons leads to different outcomes ranging from latent and noncytolytic infection to cell death. Viruses kill neurons directly by inducing either apoptosis or necrosis or indirectly as a result of the host immune response. Sindbis virus (SV) is an alphavirus that induces apoptotic cell death both in vitro and in vivo. However, apoptotic changes are not always evident in neurons induced to die by alphavirus infection. Time lapse imaging revealed that SV-infected primary cortical neurons exhibited both apoptotic and necrotic morphological features and that uninfected neurons in the cultures also died. Antagonists of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors protected neurons from SV-induced death without affecting virus replication or SV-induced apoptotic cell death. These results provide evidence that SV infection activates neurotoxic pathways that result in aberrant NMDA receptor stimulation and damage to infected and uninfected neurons.

First and second messenger role of calcium. Survival versus apoptosis in serum-free cultured granulosa explants

In order to elucidate the causal relationship between increased intracellular free calcium ([Ca2+]i) and induction of apoptosis, serum-free cultured granulosa cell (GC) explants were subjected to various experimental protocols interfering with normal Ca2+ homeostasis. Modulation of apoptotic indices was calculated on DAPI-stained GC explants. In some experiments [Ca2+]i was measured with the Ca2+ probe fura-2 in combination with epifluorescence microscopy. Buffering of [Ca2+]i with BAPTA-AM resulted in inhibition of apoptosis, while increasing extracellular Ca2+ (otherwise called [Ca2+]e load) resulted in a biphasic response characterized by an initial inhibitory effect on apoptosis followed by a delayed phase of increased apoptosis that became apparent at 4 h after withdrawal of the [Ca2+]e load. The initial inhibitory effect of the [Ca2+]e load on apoptosis was dependent on the concentration of the load (range 2-50 mM), was augmented when the [Ca2+]e load was applied in the presence of the Ca2+ channel blocker methoxyverapamil, and was mimicked by applying Mg2+ and Gd3+, two Ca(2+)-receptor agonists. These observations point towards the involvement of an extracellular Ca(2+)-sensing receptor (CaR). Measurements of [Ca2+]i showed that the ion was increased just after [Ca2+]e load, followed by recovery that was complete at 2 h after the load. Collectively these data suggest that a [Ca2+]e load initiates apoptosis, becoming manifest 4 h later, by the provoked [Ca2+]i increase, and this effect is preceded by an apoptosis-inhibiting phase presumably involving CaR activation. We conclude that Ca2+ may act as a first (extracellular) messenger promoting cell survival and as a second (intracellular) messenger activating the cell death pathway.

Age-related resistance to alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid-induced hippocampal lesion

This study compares the effects of acute alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) administration in the hippocampus in adult (3 months) and middle-aged (15 months) rats at 15 days postinjection. Injection of 1 and 2.7 mM AMPA produced dose-dependent neurodegeneration, assessed by Nissl staining; a glial reaction shown by glial fibrillary acidic protein immunocytochemistry; and calcification, revealed by alizarin red staining. Furthermore, at both doses, these alterations were significantly greater in 3-month-old rats. Finally, at AMPA 2.7 mM, no significant changes in the density of hippocampal parvalbumin- or calbindin-immunoreactive neurons or in choline acetyltransferase, glutamate uptake, or GABA uptake activities were found in 15-month-old animals, whereas significant reductions in parvalbumin (-76%) and calbindin (-32%) immunostaining and in GABA uptake (-27%) were observed in 3-month-old animals compared to the respective sham-operated or control animals. In conclusion, this study clearly demonstrates that in rats the vulnerability of hippocampal neurons and the glial and calcification reactions to AMPA-induced injury decreased with age between 3 and 15 months. Our results also indicate that hippocampal cholinergic, glutamatergic, and GABAergic systems show an adaptive response to excitotoxic damage in both adult and middle-aged animals.

Intraischemic but not postischemic hypothermia prevents non-selective hippocampal downregulation of AMPA and NMDA receptor gene expression after global ischemia

Hypothermia may afford histological neuroprotection induced by ischemia by preventing aberrant Ca2+ influx through NMDA (N-methyl-D-aspartic acid) or Ca2+-permeable AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid) receptors. Expression of hippocampal GluR1A, GluR2B, GluR3C and NMDAR1 (NR1) subunits was investigated by in situ hybridization at 1 and 7 days after 10-min transient global ischemia in the presence and absence of intraischemic or postischemic brain hypothermia (30 degrees C). At 1 day, normothermic ischemia markedly suppressed the expression of GluR1A, GluR2B, and GluR3C receptor mRNAs to a similar degree in the vulnerable CA1. Less vulnerable CA3a-c subregions were also acutely downregulated. NR1 mRNA expression was reduced in CA1 but to a lesser extent than AMPA mRNAs. At 7 days after normothermic ischemia, a time of marked CA1 cell loss, all three AMPA transcripts were nearly absent in CA1 while a percentage (33.9+/-7.2%) of NR1 mRNA remained. Intraischemic hypothermia fully blocked the damage and non-selective mRNA downregulations at 1 and 7 days. By contrast, postischemic hypothermia postponed neurodegeneration but only partially rescued the expression of AMPA and NR1 mRNAs at 7 days and not at 1 day after the insult. Therefore, hippocampal AMPA receptor mRNAs decline at a relatively similar rate after normothermic global ischemia and cellular neuroprotection by intraischemic hypothermia occurred independently of altered subunit composition of AMPA receptors. Since decreases persist within resistant neurons under the postischemic condition, AMPA receptor-mediated Ca2+ currents probably do not contribute to selective vulnerability.

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.

Mitochondrial Ca(2+)homeostasis in the regulation of apoptotic and necrotic cell deaths

Using distinct models of apoptosis and necrosis, we have investigated the effect of mitochondrial Ca(2+)(Ca(m)) homeostasis in the regulation of cell death in neuroblastoma cells as well as cardiac myocytes. The steady state level of Ca(m)was determined as the FCCP-releasable Ca(2+). Culturing cells with low concentration of extracellular Ca(2+)(Ca(o)) or with EGTA triggered an early reduction in both the Ca(m)store and the membrane potential (DeltaPsi(m)). This was followed by the detection of cytochrome c release, caspase activation, and apoptosis. Inhibitors of the mitochondrial permeability transition pore such as cyclosporin A and Bcl-2 blocked the release of Ca(m)and inhibited apoptosis. In contrast, mitochondrial Ca(2+)overload resulted in necrotic cell death. Culturing cells in the presence of excess Ca(o)led to increased Ca(m)load together with a decrease of DeltaPsi(m)that reached maximum at 1 h, with necrosis occurring at 2 h. While the decline of Ca(m)and DeltaPsi(m)was a coupled reaction for apoptosis, this relationship was uncoupled during necrosis. Clonazepam, a relatively specific inhibitor of the mitochondrial Na/Ca exchanger, was able to protect the cells from necrosis by reducing Ca(m)overload. Importantly, combination of clonazepam and cyclosporin showed a cooperative effect in further reducing the Ca(m)overload and abolished cell death. The data imply the participation of Ca(m)homeostasis in the regulation of apoptosis and necrosis.

Neural activity and survival in the developing nervous system

Recent evidence suggests that blockade of normal excitation in the immature nervous system may have profound effects on neuronal survival during the period of natural cell death. Cell loss following depression of electrical activity in the central nervous system (CNS) may explain the neuropsychiatric deficits in humans exposed to alcohol or other CNS depressants during development. Thus, understanding the role of electrical activity in the survival of young neurons is an important goal of modern basic and clinical neuroscience. Here we review the evidence from in vivo and in vitro model systems that electrical activity participates in promoting neuronal survival. We discuss the potential role of moderate elevations of intracellular calcium in promoting survival, and we address the possible ways in which activity and conventional trophic factors may interact.

Distribution of glutamate and preproenkephalin messenger RNAs following transient focal cerebral ischemia

Middle cerebral artery occlusion may result in increased activation of N-methyl-D-aspartate- or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type receptors by glutamate and lead to neuronal cell death. To characterize molecular events that precede cell death following transient focal ischemia, in situ hybridization histochemistry was used to measure levels of glutamate receptor subunit 1 (GluR1), GluR2, GluR3, N-methyl-D-aspartate receptor subunit 1 (NR1) and preproenkephalin messenger RNAs in adult rats at various recirculation times (1.5, 3 and 24 h) following a 90-min period of middle cerebral artery occlusion. At 1.5 and 3 h recirculation, autoradiography showed pronounced but differential decreases in AMPA, NR1 and preproenkephalin messenger RNA expression throughout the infarcted ipsilateral striatum. Non-uniform patterns of in situ hybridization grains emerged such that many striatal neurons were depleted of AMPA and preproenkephalin messenger RNAs, while others retained control levels. In cortical regions destined to undergo infarction, GluR2 and NR1 messenger RNAs were preferentially reduced relative to the contralateral side (to 75+/-8.5% and 66+/-4.5%, respectively); GluR1, GluR3 and preproenkephalin messenger RNAs were unaltered. At 24 h recirculation, depletion of striatal and cortical messenger RNAs became less selective. GluR3 and preproenkephalin messenger RNAs were up-regulated in ipsilateral spared regions of the striatum, and GluR1 and GluR2 messenger RNAs increased bilaterally in the cingulate cortex and in selective nuclei of the amygdala. Histological cell death or neurodegeneration was not detected in areas of reduced glutamate and preproenkephalin messenger RNA expression in either the ipsilateral striatum or cortex before 24 h. These findings suggest that complex and long-lasting decreases in messenger RNA expression occur prior to significant cell loss in regions destined to undergo infarction. Increased formation of Ca2+-permeable AMPA receptor assemblies may occur in "unspared" and "spared" regions via different mechanisms and contribute to alterations in post-ischemic synaptic activity. The possibility arises that there may be altered relationships between glutamatergic and enkephalin synapses, since the dorsolateral striatum, where preproenkephalin messenger RNA expression is acutely reduced, receives innervation by the affected ipsilateral cortical region.

Annexin V counteracts apoptosis while inducing Ca(2+) influx in human lymphocytic T cells

We have previously shown that when annexin V is present during the execution of a cell death program, apoptosis is delayed. This is reflected by the inhibition of DNA cleavage and of the release of apoptotic membrane particles, and by reduction of the proteolytic processing of caspase-3. Here, we have studied the mechanism(s) through which annexin V counteracts apoptosis in the human CEM T cell line. The degree of apoptosis inhibition was associated with an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)). Reduction of the extracellular Ca(2+) concentration by EGTA abolished the anti-apoptotic effect, suggesting that annexin V favors Ca(2+) influx and that Ca(2+) acts as an inhibitor rather than an activator of apoptosis in CEM T cells. The effects on apoptosis and [Ca(2+)](i) of several modified annexins with different electrophysiological properties indicate that the N-terminal domain of annexin V is necessary for the Ca(2+)-dependent anti-apoptotic action of annexin V. These results suggest that annexin V regulates membrane Ca(2+) permeability and is protective against apoptosis by increasing [Ca(2+)](i) in CEM T cells.

Abnormal Ca(2+) regulation in oligodendrocytes from the dysmyelinating jimpy mouse

Jimpy (jp) is a point mutation in the gene on the X chromosome which codes for the major myelin proteolipid protein. Most oligodendrocytes (OLs) in the jp mouse undergo cell death at the time when they should be actively myelinating. Loss of mature OLs results in severe CNS dysmyelination. Dying jp OLs have the morphology of apoptotic cells but it is not clear how the mutation activates biochemical pathways which lead to programmed death of OLs in jp CNS. There is compelling evidence from a number of systems that high levels of intracellular Ca(2+) ([Ca2+]i) can activate downstream processes which result in both apoptotic and necrotic cell death. To determine whether [Ca2+](i) dysregulation might be involved in the death of jp OLs, we used ratiometric imaging to determine levels of [Ca2+](i) in OLs cultured from jp and normal CNS and in immortalized cell lines derived from jp and normal OLs. Immortalized jp OLs and OLs isolated directly from jp brain both showed a similar elevation in [Ca2+](i) ranging from 60% to 150% over control values. A higher baseline [Ca2+](i) in jp OLs might increase their vulnerability to other insults due to abnormal protein processing or changes in signaling pathways which act as a final trigger for cell death.

Differential coupling of G-protein-linked receptors to Ca2+ mobilization through inositol(1,4,5)trisphosphate or ryanodine receptors in cerebellar granule cells in primary culture

Rat cerebellar granule cells in primary culture possess muscarinic, metabotropic glutamatergic, histaminergic and alpha-adrenergic receptors which couple to phosphoinositide-specific phospholipase C. We have determined the ability of these receptors to elevate inositol(1,4,5)trisphosphate and to release intracellular calcium, in order to establish the correlation between these two responses. In resting cerebellar granule cells, only the muscarinic agonist carbachol evoked significant increases in both inositol(1,4, 5)trisphosphate and cytoplasmic free Ca2+. Mild depolarization (20 mM KCl) enhanced inositol(1,4,5)trisphosphate elevation by carbachol and histamine, but not by noradrenaline or the metabotropic glutamate agonist 1S,3R ACPD. In contrast, Ca2+-release responses were modified differently by 20 mM KCl-depolarization: the responses to carbachol, histamine and 1S,3R ACPD, but not the responses to noradrenaline, were markedly enhanced. The contribution of ryanodine-sensitive Ca2+-release channels (ryanodine receptors) to the calcium release signal in depolarized cells was determined. Ryanodine (10 microM) inhibited most effectively the cytoplasmic Ca2+ elevation evoked by 1S,3R ACPD (> 90%), while Ca2+ release upon stimulation by carbachol and histamine was only inhibited by approximately 60% and remained larger than in the absence of KCl. Our data are consistent with a specific coupling between metabotropic glutamate receptors and ryanodine-sensitive Ca2+-release channels which may not require generation of inositol(1, 4,5)trisphosphate.

Ca2+ influx in resting rat sensory neurones that regulates and is regulated by ryanodine-sensitive Ca2+ stores

1. Store-operated, voltage-independent Ca2+ channels are activated by depletion of intracellular Ca2+ stores and mediate Ca2+ influx into non-excitable cells at resting membrane potential. We used microfluorimetry, patch-clamp and Mn2+-quench techniques to explore the possibility that a similar mechanism exists in rat dorsal root ganglion (DRG) neurones in primary culture. 2. Following caffeine-induced depletion, ryanodine-sensitive Ca2+ stores refilled with Ca2+ at resting membrane potential. The refilling process required extracellular Ca2+, was blocked by 2 mM Ni2+, and was facilitated by membrane hyperpolarization from -55 to -80 mV, indicating a key role for Ca2+ influx. This influx of Ca2+ was not affected by the voltage-operated Ca2+ channel (VOCC) antagonists nicardipine (10 microM), nimodipine (10 microM) or omega-grammotoxin SIA (1 microM). 3. When ryanodine-sensitive Ca2+ stores were depleted in Ca2+-free media, a return to 2 mM external Ca2+ resulted in a pronounced [Ca2+]i overshoot, indicating an increased permeability to Ca2+. Depletion of Ca2+ stores also produced a 2-fold increase in the rate of Mn2+ influx. The [Ca2+]i overshoot and Mn2+ entry were both inhibited by Ni2+, but not by VOCC antagonists. 4. Caffeine induced periodic Ca2+ release from, and reuptake into, ryanodine-sensitive stores. The [Ca2+]i oscillations were arrested by removal of extracellular Ca2+ or by addition of Ni2+, but they were not affected by VOCC antagonists. Hyperpolarization increased the frequency of this rhythmic activity. 5. These data suggest the presence of a Ca2+ entry pathway in mammalian sensory neurones that is distinct from VOCCs and is regulated by ryanodine-sensitive Ca2+ stores. This pathway participates in refilling intracellular Ca2+ stores and maintaining [Ca2+]i oscillations and thus controls the balance between intra- and extracellular Ca2+ reservoirs in resting DRG neurones.

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.

Glutamate inhibits GABA excitatory activity in developing neurons

In contrast to the mature brain, in which GABA is the major inhibitory neurotransmitter, in the developing brain GABA can be excitatory, leading to depolarization, increased cytoplasmic calcium, and action potentials. We find in developing hypothalamic neurons that glutamate can inhibit the excitatory actions of GABA, as revealed with fura-2 digital imaging and whole-cell recording in cultures and brain slices. Several mechanisms for the inhibitory role of glutamate were identified. Glutamate reduced the amplitude of the cytoplasmic calcium rise evoked by GABA, in part by activation of group II metabotropic glutamate receptors (mGluRs). Presynaptically, activation of the group III mGluRs caused a striking inhibition of GABA release in early stages of synapse formation. Similar inhibitory actions of the group III mGluR agonist L-AP4 on depolarizing GABA activity were found in developing hypothalamic, cortical, and spinal cord neurons in vitro, suggesting this may be a widespread mechanism of inhibition in neurons throughout the developing brain. Antagonists of group III mGluRs increased GABA activity, suggesting an ongoing spontaneous glutamate-mediated inhibition of excitatory GABA actions in developing neurons. Northern blots revealed that many mGluRs were expressed early in brain development, including times of synaptogenesis. Together these data suggest that in developing neurons glutamate can inhibit the excitatory actions of GABA at both presynaptic and postsynaptic sites, and this may be one set of mechanisms whereby the actions of two excitatory transmitters, GABA and glutamate, do not lead to runaway excitation in the developing brain. In addition to its independent excitatory role that has been the subject of much attention, our data suggest that glutamate may also play an inhibitory role in modulating the calcium-elevating actions of GABA that may affect neuronal migration, synapse formation, neurite outgrowth, and growth cone guidance during early brain development.

Neurotransmitters, KCl and antioxidants rescue striatal neurons from apoptotic cell death in culture

Striatal neurons grown in low density culture on serum-free media and in the absence of glia die within 3 days of plating. In this study, we sought to determine the mechanism of cell death (e.g., apoptosis) and whether trophic influences, such as, growth factors, neurotransmitters, antioxidants or KCl-mediated depolarization could improve their survival. We found that striatal neurons grown in this manner die via apoptosis unless treated with one of several different rescuing agents. One way to prevent the death of most striatal neurons was continual treatment with 5-20 microM dopamine (DA) or other monoamines. Although the survival effect of DA was mimicked by the specific D1 receptor agonist, SKF38393, no D1 or D2 receptor antagonists blocked the effect. As with DA, chronic depolarization with KCl (12-39 mM) or treatment with antioxidants, such as the vitamin E analog, Trolox (10-10-500 microM), or the hormone, melatonin (10-10-500 microM) also rescued striatal neurons from impending cell death. Surprisingly, growth factors, such as BDNF, bFGF, GDNF, NGF, NT3 and EGF, demonstrated no ability to rescue striatal neurons in this model, suggesting that death was not solely caused by the absence of essential trophic factors. We conclude that a variety of agents, but not growth factors, can prevent the demise of striatal neurons, presumably by neutralizing damage at one or more steps in the death cascade.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Selective reduction of GluR2 protein in adult hippocampal CA3 neurons following status epilepticus but prior to cell loss

Kainic acid (KA) induces status epilepticus and delayed neurodegeneration of CA3 hippocampal neurons. Downregulation of glutamate receptor 2 (GluR2) subunit mRNA [the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) subunit that limits Ca2+ permeability] is thought to a play role in this neurodegeneration, possibly by increased formation of Ca2+ permeable AMPA receptors. The present study examined early hippocampal decreases in GluR2 mRNA and protein following kainate-induced status epilepticus and correlated expression changes with the appearance of dead or dying cells by several histological procedures. At 12 h, in situ hybridization followed by emulsion dipping showed nonuniform decreases in GluR2 mRNA hybridization grains overlying morphologically healthy-appearing CA3 neurons. GluR1 and N-methyl-D-aspartate receptor mRNAs were unchanged. At 12-16 h, when little argyrophilia or cells with some features of apoptosis were detected by silver impregnation or electron microscopy, single immunohistochemistry with GluR2 and GluR2/3 subunit-specific antibodies demonstrated a pattern of decreased GluR2 receptor protein within CA3 neurons that appeared to predict a pattern of damage, similar to the mRNA observations. Double immunolabeling showed that GluR2 immunofluorescence was depleted and that GluR1 immunofluorescence was sustained in clusters of the same CA3 neurons. Quantitation of Western blots showed increased GluR1:GluR2 ratios in CA3 but not in CA1 or dentate gyrus subfields. Findings indicate that the GluR1:GluR2 protein ratio is increased in a population of CA3 neurons prior to significant cell loss. Data are consistent with the "GluR2 hypothesis" that reduced expression of GluR2 subunits will increase formation of AMPA receptors permeable to Ca2+ and predict vulnerability to a particular subset of pyramidal neurons following status epilepticus.

Mechanisms of spontaneous cytosolic Ca2+ transients in differentiated human neuronal cells

We have studied Ca2+ homeostasis in a unique model of human neurons, the NT2N cell, which differentiates from a human teratocarcinoma cell line, NTera2/C1.D1 by retinoic acid treatment. When perifused with Krebs-HEPES buffer containing 2.5 mM CaCl2, fura-2 loaded NT2N cells produced spontaneous cytosolic Ca2+ oscillations, or Ca2+ transients. These cytosolic Ca2+ transients were not blocked by antagonists of glutamate (6-cyano-7-nitroquinoxaline-2,3-dione and D(-)-2-amino-5-phosphonopentanoic acid) or muscarinic (atropine) receptors. Omission of extracellular Ca2+ completely abolished Ca2+ oscillations and decreased the average Ca2+ level from 106 +/- 14 nM to 59 +/- 8 nM. Addition of the L-type Ca2+ channel blocker nifedipine (1 or 10 microM) or of the N-type inhibitor omega-conotoxin GVIA (5 microM) significantly, although incompletely, suppressed Ca2+ oscillations, while omega-conotoxin MVIIC (5 microM), a selective antagonist of P- and Q-channels, had no effect. Ni2+, at 100 microM, a concentration selective for T-type channels, did not inhibit Ca2+ transients. Non-specific blockage of Ca2+ channels by higher concentrations of Ni2+ (2-5 mM) or Co2+ (1 mM) abolished Ca2+ oscillations completely. The endoplasmic reticulum Ca2+-ATPase inhibitor, thapsigargin (1 microM), slightly decreased Ca2+ oscillation frequency, and induced a small transitory increase in the average cytosolic Ca2+ concentration. The mRNAs of L- (alpha1D subunit) and N-type (alpha1B subunit) Ca2+ channel were present in NT2N cells, while that of a T-type Ca2+ channel (alpha1-subunit) was not present in the NT2N cells as shown by reverse transcription-polymerase chain reaction. In conclusion, NT2N neuronal cells generate cytosolic Ca2+ oscillations mainly by influx of extracellular Ca2+ through multiple channels, which include L- and N-type channels, and do not require activation of glutamate or muscarinic receptors.

Protection From Apoptosis by Steel Factor But Not Interleukin-3 Is Reversed Through Blockade of Calcium Influx

Steel factor (SLF), the ligand for the c-Kit receptor, protects hemopoietic progenitors and mast cells from apoptosis. We show here that protection of 32D-Kit cells or mast cells from apoptosis by SLF is abrogated through concurrent inhibition of Ca2+influx. In contrast, cell survival promoted by interleukin-3 is not affected by Ca2+ influx blockers. In the presence of blockers, increasing stimulation by SLF leads to greater levels of cell death in the population, indicating that it is the combination of activation by SLF with concurrent blockade of Ca2+ influx that results in apoptosis. The p815 mastocytoma, which expresses a mutated, constitutively active c-kit receptor, dies apoptotically in the presence of Ca2+ influx blockers alone. Ionomycin protects cells from SLF plus blocker-induced apoptosis, confirming specificity for Ca2+ ion blockade in cell death induction. Overexpression of bcl-2, which protects 32D-Kit cells from factor withdrawal, does not protect cells from apoptosis by SLF plus blocker. In contrast, caspase inhibitors YVAD-CHO, DEVD-FMK, and Boc-Asp-FMK protect cells from SLF plus blocker-induced death. These observations highlight the importance of SLF-stimulated Ca2+ influx in the protection of cells from apoptosis and demonstrate a new mechanism for inducing bcl-2 insensitive, caspase-dependent apoptosis through the combination of SLF stimulation with Ca2+ influx blockade.

Protection From Apoptosis by Steel Factor But Not Interleukin-3 Is Reversed Through Blockade of Calcium Influx

Steel factor (SLF), the ligand for the c-Kit receptor, protects hemopoietic progenitors and mast cells from apoptosis. We show here that protection of 32D-Kit cells or mast cells from apoptosis by SLF is abrogated through concurrent inhibition of Ca2+influx. In contrast, cell survival promoted by interleukin-3 is not affected by Ca2+ influx blockers. In the presence of blockers, increasing stimulation by SLF leads to greater levels of cell death in the population, indicating that it is the combination of activation by SLF with concurrent blockade of Ca2+ influx that results in apoptosis. The p815 mastocytoma, which expresses a mutated, constitutively active c-kit receptor, dies apoptotically in the presence of Ca2+ influx blockers alone. Ionomycin protects cells from SLF plus blocker-induced apoptosis, confirming specificity for Ca2+ ion blockade in cell death induction. Overexpression of bcl-2, which protects 32D-Kit cells from factor withdrawal, does not protect cells from apoptosis by SLF plus blocker. In contrast, caspase inhibitors YVAD-CHO, DEVD-FMK, and Boc-Asp-FMK protect cells from SLF plus blocker-induced death. These observations highlight the importance of SLF-stimulated Ca2+ influx in the protection of cells from apoptosis and demonstrate a new mechanism for inducing bcl-2 insensitive, caspase-dependent apoptosis through the combination of SLF stimulation with Ca2+ influx blockade.

Mediation of neuronal apoptosis by enhancement of outward potassium current

Apoptosis of mouse neocortical neurons induced by serum deprivation or by staurosporine was associated with an early enhancement of delayed rectifier (IK) current and loss of total intracellular K+. This IK augmentation was not seen in neurons undergoing excitotoxic necrosis or in older neurons resistant to staurosporine-induced apoptosis. Attenuating outward K+ current with tetraethylammonium or elevated extracellular K+, but not blockers of Ca2+, Cl-, or other K+ channels, reduced apoptosis, even if associated increases in intracellular Ca2+ concentration were prevented. Furthermore, exposure to the K+ ionophore valinomycin or the K+-channel opener cromakalim induced apoptosis. Enhanced K+ efflux may mediate certain forms of neuronal apoptosis.

Overexpression of glyceraldehyde-3-phosphate dehydrogenase is involved in low K+-induced apoptosis but not necrosis of cultured cerebellar granule cells

We have reported that overexpression of glyceraldehyde-3-phosphate dehydrogenase (GAPDH; EC 1.2.1.12) is involved in age-induced apoptosis of the cultured cerebellar granule cells that grow in a depolarizing concentration (25 mM) of KCI. The present study was undertaken to investigate whether GAPDH overexpression also occurs and participates in apoptosis of the cerebellar granule cells that result from switching the culturing conditions from high (25 mM) to low (5 mM) concentrations of KCl. We found that exposure of granule cells to low potassium (K+) for 24 hr induces not only apoptosis but also necrotic damage. The latter is supported by the morphological observations that a subpopulation of neurons showed cell swelling, extensive cytoplasmic vacuolization, damaged mitochondria, and apparently intact nuclei. Treatments with two antisense but not sense oligodeoxyribonucleotides directed against GAPDH attenuated low K+-induced neuronal death by approximately 50%. Morphological inspection revealed that GAPDH antisense oligonucleotides preferentially blocked low K+-induced apoptosis with little or no effect on necrotic damage. Similar to antisense oligonucleotides, actinomycin-D partially inhibited low K+-induced death of granule cells with a predominant effect on apoptosis. In contrast, cycloheximide almost completely blocked low K+-induced neuronal death and seemed to prevent both apoptotic and necrotic damage. The levels of GAPDH mRNA and protein were markedly increased in a time-dependent manner after low K+ exposure. The overexpression of GAPDH mRNA and protein was completely blocked by cycloheximide, actinomycin-D, and its antisense but not sense oligonucleotides. Taken together, these results lend credence to the view that exposure of cerebellar granule cells to low K+ induces both apoptosis and necrosis and that only the apoptotic component involves overexpression of GAPDH.