NMDA receptor-mediated K+ efflux and neuronal apoptosis

Uncoupling cell shrinkage from apoptosis reveals that Na+ influx is required for volume loss during programmed cell death

Cell shrinkage, or the loss of cell volume, is a ubiquitous characteristic of programmed cell death that is observed in all examples of apoptosis, independent of the death stimulus. This decrease in cell volume occurs in synchrony with other classical features of apoptosis. The molecular basis for cell shrinkage during apoptosis involves fluxes of intracellular ions including K+, Na+, and Cl-. Here we show for the first time that these ion fluxes, but not cell shrinkage, are necessary for apoptosis. Using sodium-substituted medium during anti-Fas treatment of Jurkat cells, we observed cellular swelling, a property normally associated with necrosis, in contrast to the typical cell shrinkage. Surprisingly, these swollen cells displayed all of the other classical features of apoptosis, including chromatin condensation, externalization of phosphatidylserine, caspase activity, poly(ADP)-ribose polymerase cleavage, and internucleosomal DNA degradation. These swollen cells had a marked decrease in intracellular potassium, and subsequent inhibition of this potassium loss completely blocked apoptosis. Reintroduction of sodium ions in cell cultures reversed this cellular swelling, resulting in a dramatic loss of cell volume and the characteristic apoptotic morphology. Additionally, inhibition of sodium influx using a sodium channel blocker saxitoxin completely prevented the onset of anti-Fas-induced apoptosis in Jurkat cells. These findings suggest that sodium influx can control not only changes in cell size but also the activation of apoptosis, whereas potassium ion loss controls the progression of the cell death process. Therefore cell shrinkage can be separated from other features of apoptosis.

Na+, K+-ATPase: the new face of an old player in pathogenesis and apoptotic/hybrid cell death

The Na(+), K(+)-ATPase is a ubiquitous membrane transport protein in mammalian cells, responsible for establishing and maintaining high K(+) and low Na(+) in the cytoplasm required for normal resting membrane potentials and various cellular activities. The ionic homeostasis maintained by the Na(+), K(+)-ATPase is also critical for cell growth, differentiation, and cell survival. Although the toxic effects of blocking the Na(+), K(+)-ATPase by ouabain and other selective inhibitors have been known for years, the mechanism of action remained unclear. Recent progress in two areas has significantly advanced our understanding of the role and mechanism of Na(+), K(+)-ATPase in cell death. Along with increased recognition of apoptosis in a wide range of disease states, Na(+), K(+)-ATPase deficiency has been identified as a contributor to apoptosis and pathogenesis. More importantly, accumulating evidence now endorses a close relationship between ionic homeostasis and apoptosis, namely the regulation of apoptosis by K(+) homeostasis. Since Na(+), K(+)-ATPase is the primary system for K(+) uptake, dysfunction of the transport enzyme and resultant disruption of ionic homeostasis have been re-evaluated for their critical roles in apoptosis and apoptosis-related diseases. In this review, instead of giving a detailed description of the structure and regulation of Na(+), K(+)-ATPase, the author will focus on the most recent evidence indicating the unique role of Na(+), K(+)-ATPase in cell death, including apoptosis and the newly recognized "hybrid death" of concurrent apoptosis and necrosis in the same cells. It is also hoped that discussion of some seemingly conflicting reports will inspire further debate and benefit future investigation in this important research field.

Ca2+-independent, but voltage- and activity-dependent regulation of the NMDA receptor outward K+ current in mouse cortical neurons

To test the novel hypothesis that the K+ efflux mediated by NMDA receptors might be regulated differently than the influx of Ca2+ and Na+ through the same receptor channels, NMDA receptor whole-cell currents carried concurrently or individually by Ca2+, Na+ and K+ were analysed in cultured mouse cortical neurons. In contrast to the NMDA inward current carried by Ca2+ and Na+, the NMDA receptor outward K+ current or NMDA-K current, recorded either in the presence or absence of extracellular Ca2+ and Na+, and at different or the same membrane potentials, showed much less sensitivity to alterations in intracellular Ca2+ concentration and underwent little rundown. In line with a selective regulation of the NMDA receptor K+ permeability, the ratio of the NMDA inward/outward currents decreased, and the reversal potential of composite NMDA currents recorded in physiological solutions shifted by -8.5 mV after repeated activation of NMDA receptors. Moreover, a depolarizing pre-pulse of a few seconds or a burst of brief depolarizing pulses selectively augmented the subsequent NMDA-K current, but not the NMDA inward current. On the other hand, a hyperpolarizing pre-pulse showed the opposite effect of reducing the NMDA-K current. The voltage- and activity-dependent regulation of the NMDA-K current did not require the existence of extracellular Ca2+ or Ca2+ influx; it was, however, affected by the duration of the pre-pulse and was subject to a time-dependent decay. The burst of excitatory activity revealed a lasting upregulation of the NMDA-K current even 5 s after termination of the pre-pulses. Our data reveal a selective regulation of the NMDA receptor K+ permeability and represent a novel model of voltage- and excitatory activity-dependent plasticity at the receptor level.

Stimulation of Kv1.3 potassium channels by death receptors during apoptosis in Jurkat T lymphocytes

The loss of intracellular potassium is a pivotal step in the induction of apoptosis but the mechanisms underlying this response are poorly understood. Here we report caspase-dependent stimulation of potassium channels by the Fas receptor in a human Jurkat T cell line. Receptor activation with Fas ligand for 30 min increased the amplitude of voltage-activated potassium currents 2-fold on average. This produces a sustained outward current, approximately 10 pA, at physiological membrane potentials during Fas ligand-induced apoptosis. Both basal and Fas ligand-induced currents were blocked completely by toxins that selectively inhibit Kv1.3 potassium channels. Kv1.3 stimulation required the expression of Fas-associated death domain protein and activation of caspase 8, but did not require activation of caspase 3 or protein synthesis. Furthermore, Kv1.3 stimulation by Fas ligand was prevented by chronic stimulation of protein kinase C with 20 nm phorbol 12-myristate 13-acetate during Fas ligand treatment, which also blocks apoptosis. Thus, Fas ligand increases Kv1.3 channel activity through the same canonical apoptotic signaling cascade that is required for potassium efflux, cell shrinkage, and apoptosis.

K+-dependent cerebellar granule neuron apoptosis. Role of task leak K+ channels

Rat mature cerebellar granule, unlike hippocampal neurons, die by apoptosis when cultured in a medium containing a physiological concentration of K+ but survive under high external K+ concentrations. Cell death in physiological K+ parallels the developmental expression of the TASK-1 and TASK-3 subunits that encode the pH-sensitive standing outward K+ current IKso. Genetic transfer of the TASK subunits in hippocampal neurons, lacking IKso, induces cell death, while their genetic inactivation protects cerebellar granule neurons. Neuronal death of cultured rat granule neurons is also prevented by conditions that specifically reduce K+ efflux through the TASK-3 channels such as extracellular acidosis and ruthenium red. TASK leak K+ channels thus play an important role in K+-dependent apoptosis of cerebellar granule neurons in culture.

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

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

Regulation and critical role of potassium homeostasis in apoptosis

Programmed cell death or apoptosis is broadly responsible for the normal homeostatic removal of cells and has been increasingly implicated in mediating pathological cell loss in many disease states. As the molecular mechanisms of apoptosis have been extensively investigated a critical role for ionic homeostasis in apoptosis has been recently endorsed. In contrast to the ionic mechanism of necrosis that involves Ca(2+) influx and intracellular Ca(2+) accumulation, compelling evidence now indicates that excessive K(+) efflux and intracellular K(+) depletion are key early steps in apoptosis. Physiological concentration of intracellular K(+) acts as a repressor of apoptotic effectors. A huge loss of cellular K(+), likely a common event in apoptosis of many cell types, may serve as a disaster signal allowing the execution of the suicide program by activating key events in the apoptotic cascade including caspase cleavage, cytochrome c release, and endonuclease activation. The pro-apoptotic disruption of K(+) homeostasis can be mediated by over-activated K(+) channels or ionotropic glutamate receptor channels, and most likely, accompanied by reduced K(+) uptake due to dysfunction of Na(+), K(+)-ATPase. Recent studies indicate that, in addition to the K(+) channels in the plasma membrane, mitochondrial K(+) channels and K(+) homeostasis also play important roles in apoptosis. Investigations on the K(+) regulation of apoptosis have provided a more comprehensive understanding of the apoptotic mechanism and may afford novel therapeutic strategies for apoptosis-related diseases.

Signaling from synapse to nucleus: the logic behind the mechanisms

Signaling from synapse to nucleus is vital for activity-dependent control of neuronal gene expression and represents a sophisticated form of neural computation. The nature of specific signal initiators, nuclear translocators and effectors has become increasingly clear, and supports the idea that the nucleus is able to make sense of a surprising amount of fast synaptic information through intricate biochemical mechanisms. Information transfer to the nucleus can be conveyed by physical translocation of messengers at various stages within the multiple signal transduction cascades that are set in motion by a Ca(2+) rise near the surface membrane. The key role of synapse-to-nucleus signaling in circadian rhythms, long-term memory, and neuronal survival sheds light on the logical underpinning of these signaling mechanisms.

Anti-epileptic drugs as possible neuroprotectants in cerebral ischemia

Many similarities exist between cerebral ischemia and epilepsy regarding brain-damaging and auto-protective mechanisms that are activated following the injurious insult. Therefore, drugs that are effective in minimizing seizure-induced brain damage may also be useful in minimizing ischemic injury. Use of such drugs in stroke victims may have important clinical and financial advantages. Therefore, the authors conducted a Medline search of studies involving the use of anti-epileptic drugs (AEDs) as possible neuroprotectants and summarize the data. Most AEDs have been tested in animal models of focal or global ischemia and some were already tested in humans, for a possible neuroprotective effect. The existing data is rather scant and insufficient but it appears that only drugs that have multiple mechanisms of action seem to have some potential in conferring a degree of neuroprotection that could be clinically applicable to stroke patients. In conclusion, some of the newer AEDs show promise as possible neuroprotectants in the setup of acute ischemic stroke but more studies are needed before clinical trials in humans could be undertaken.

Apoptotic insults impair Na+, K+-ATPase activity as a mechanism of neuronal death mediated by concurrent ATP deficiency and oxidant stress

The Na+, K+-ATPase (Na+, K+-pump) plays critical roles in maintaining ion homeostasis. Blocking the Na+, K+-pump may lead to apoptosis. By contrast, whether an apoptotic insult may affect the Na+, K+-pump activity is largely undefined. In cultured cortical neurons, the Na+, K+-pump activity measured as a membrane current Ipump was time-dependently suppressed by apoptotic insults including serum deprivation, staurosporine, and C2-ceramide, concomitant with depletion of intracellular ATP and production of reactive oxygen species. Signifying a putative relationship among these events, Ipump was highly sensitive to changes in ATP and reactive oxygen species levels. Moreover, the apoptosis-associated Na+, K+-pump failure and serum deprivation-induced neuronal death were antagonized by pyruvate and succinate in ATP- and reactive-oxygen-species-dependent manners. We suggest that failure of the Na+, K+-pump as a result of a combination of energy deficiency and production of reactive oxygen species is a common event in the apoptotic cascade; preserving the pump activity provides a neuroprotective strategy in certain pathological conditions.

Molecular mechanisms of cerebral ischemia-induced neuronal death

The mode of neuronal death caused by cerebral ischemia and reperfusion appears on the continuum between the poles of catastrophic necrosis and apoptosis: ischemic neurons exhibit many biochemical hallmarks of apoptosis but remain cytologically necrotic. The position on this continuum may be modulated by the severity of the ischemic insult. The ischemia-induced neuronal death is an active process (energy dependent) and is the result of activation of cascades of detrimental biochemical events that include perturbion of calcium homeostasis leading to increased excitotoxicity, malfunction of endoplasmic reticulum and mitochondria, elevation of oxidative stress causing DNA damage, alteration in proapoptotic gene expression, and activation of the effector cysteine proteases (caspases) and endonucleases leading to the final degradation of the genome. In spite of strong evidence showing that brain infarction can be reduced by inhibiting any one of the above biochemical events, such as targeting excitotoxicity, up-regulation of an antiapoptotic gene, or inhibition of a down-stream effector caspase, it is becoming clear that targeting a single gene or factor is not sufficient for stroke therapeutics. An effective neuroprotective therapy is likely to be a cocktail aimed at all of the above detrimental events evoked by cerebral ischemia and the success of such therapeutic intervention relies upon the complete elucidation of pathways and mechanisms of the cerebral ischemia-induced active neuronal death.

Block of Na+,K+-ATPase and induction of hybrid death by 4-aminopyridine in cultured cortical neurons

K(+) channel blockers such as 4-aminopyridine (4-AP) can be toxic to neurons; the cellular mechanism underlying the toxicity, however, is obscure. In cultured mouse cortical neurons, we tested the hypothesis that the toxic effect of 4-AP might result from inhibiting the Na(+),K(+)-ATPase (Na(+),K(+)-pump) and thereafter induction of a hybrid death of concomitant apoptosis and necrosis. The Na(+),K(+)-pump activity, monitored as whole-cell membrane currents, was markedly blocked by 4-AP in concentration- and voltage-dependent manners in low millimolar ranges. At similar concentrations, 4-AP induced a neuronal death sensitive to attenuation by the caspase inhibitor Z-VAD-FMK (Z-Val-Ala-Asp(OMe)-fluoromethyl ketone) or Ca(2+) chelator BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester). Electron microscopy confirmed hybrid ultrastructural features of coexisting apoptotic and necrotic components in same cells. We suggest that 4-AP is a potent antagonist of the Na(+),K(+)-ATPase and an inducer of the hybrid death of central neurons.

Temporal alterations in voltage gated Ca2+ channel immunoreactivities in the gerbil hippocampus following ischemic insults

In the present study, temporal changes of voltage-gated Ca(2+) channel (VGCC) immunoreactivities were evaluated in the gerbil hippocampus following ischemia. P/Q-type VGCC immunoreactivity was elevated in the hippocampus in the 3 h post-ischemic group. In the 30 min post-ischemic group, N-type VGCC immunoreactivity began to increase only in the CA1 region. L-type (alpha1C) VGCC immunoreactivity was significantly increased in the 12 h post-ischemic group. L-type (alpha1D) VGCC immunoreactivity began to increase in the CA1 region in the 30 min post-ischemic group and peaked in the 12 h post-ischemic group. These findings suggest that the altered VGCC immunoreactivities following ischemia may play an important role in the ischemic neuronal injury.

Alterations of kynurenic acid content in the retina in response to retinal ganglion cell damage

The present study is the first to examine the modulation of retinal kynurenic acid (KYNA) content in response to N-methyl-D-aspartate (NMDA)-induced cell death in adult rat retinal ganglion cells (RGC). Adult Brown Norway rats were intravitreally injected with NMDA or PBS. Surviving RGC were retrogradely labeled with fluorogold and counted in wholemounts of retinas 2, 7 and 14 days after injection. Retinal KYNA content was measured by HPLC at the same time points. RGC numbers decreased significantly 2, 7 and 14 days after NMDA injection if compared to control retinas. KYNA concentration increased significantly two days after NMDA-injection. However, 7 and 14 days after injection retinal KYNA content was found markedly decreased in NMDA-treated eyes as compared to controls. It is conceivable that KYNA deficiency is causally related to the pathology of excitotoxic retinal diseases.

Searching for mechanisms of N-methyl-D-aspartate-induced glutathione efflux in organotypic hippocampal cultures

N-Methyl-D-aspartate (NMDA)-receptor stimulation evoked a selective and partly delayed elevated efflux of glutathione, phosphoethanolamine, and taurine from organotypic rat hippocampus slice cultures. The protein kinase inhibitors H9 and staurosporine had no effect on the efflux. The phospholipase A2 inhibitors quinacrine and 4-bromophenacyl bromide, as well as arachidonic acid, a product of phospholipase A2 activity, did not affect the stimulated efflux. Polymyxin B, an antimicrobal agent that inhibits protein kinase C, and quinacrine in high concentration (500 microM), blocked efflux completely. The stimulated efflux after but not during NMDA incubation was attenuated by a calmodulin antagonist (W7) and an anion transport inhibitor (DNDS). Omission of calcium increased the spontaneous efflux with no or small additional effects by NMDA. In conclusion, NMDA receptor stimulation cause an increased selective efflux of glutathione, phosphoethanolamine and taurine in organotypic cultures of rat hippocampus. The efflux may partly be regulated by calmodulin and DNDS sensitive channels.

Estradiol enhances the neurotoxicity of glutamate in GT1-7 cells through an estrogen receptor-dependent mechanism

Glutamate plays an important role in neuroendocrine regulation of reproduction through acting on the N-methyl-D-asparate receptor (NMDAR) in the preoptic area (POA). However, a larger dose of glutamate is neurotoxic. Estradiol (E2) increases the responsiveness of neurons to glutamate through activation and/or expression of NMDAR. In order to investigate whether estradiol modulates the neurotoxic effect of glutamate on the neurons through estrogen receptor (ER), immortalized GT1-7 cells, which simultaneously express ER and NMDAR were used. Tamoxifen and ICI 182,780, ER antagonist, were used to investigate whether the ER is involved in the effect of estradiol on glutamate-induced neurotoxicity. MK-801, a NMDAR antagonist, was used to confirm the enhancement of NMDAR-mediated neurotoxicity by estradiol. Neurotoxicity was evaluated by cell viability and LDH efflux. Cell death was observed by flow cytometry and DNA fragmentation. The results showed that: (1) estradiol (10 nM, incubated for 3 days) significantly enhanced the glutamate-induced neuronal death; (2) the percentages of necrosis and apoptosis were elevated after glutamate treatment, and estradiol significantly enhanced the glutamate-induced cell death; (3) glutamate-induced DNA fragmentation was enhanced by E2-pretreatment; (4) the induction of cell death and increase of LDH efflux after glutamate treatment were also enhanced by E2-pretreatment; (5) both the tamoxifen and ICI 182,780 abolished the estradiol-enhanced NMDAR expression and neurotoxicity of glutamate; (6) higher dose of MK-801 (2 microM) was needed in E2-pretreated cells than in non-E2-pretreated group to block the glutamate-induced neurotoxicity. These results suggested that pretreatment of estradiol might enhance the expression of NMDAR and subsequent glutamate-induced neurotoxicity on the GT1-7 cells through an ER-dependent manner.

Role for chloride but not potassium channels in apoptosis in primary rat cortical cultures

Recent evidence suggests a predominant role for potassium (K) efflux in apoptotic cell death yet there exists controversy as to the exact nature of this involvement of K. In the present study we tested the anti-apoptotic efficacy of K channel blockers, tetraethylammonium Cl (TEA), and high extracellular K, the sodium (Na) channel blocker, tetrodotoxin (TTX) and the Cl channel blocker, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid, (SITS) against staurosporine-induced apoptosis in primary rat cortical cultures. Surprisingly, we failed to observe anti-apoptotic effects with TEA, high K or TTX. We did, however, observe significant dose-dependent inhibition of apoptosis with SITS. In conclusion we demonstrate no role for K or Na in neuronal apoptosis, but rather an important role for a SITS-sensitive mechanism such as Cl.

Slight impairment of Na+,K+-ATPase synergistically aggravates ceramide- and beta-amyloid-induced apoptosis in cortical neurons

Dysfunction of the Na(+),K(+)-ATPase (Na(+),K(+)-pump), due to reduced energy supply or increased endogenous ouabain-like inhibitors, likely occurs under pathological conditions in the central nervous system. In cultured mouse cortical neurons, we examined the hypothesis that a mild non-toxic inhibition of the Na(+),K(+)-ATPase could synergistically sensitize the vulnerability of neurons to normally non-lethal apoptotic signals. Ouabain at a low concentration of 0.1 microM slightly lessened the Na(+),K(+)-pump activity measured as an ouabain-sensitive current, yet did not affect K(+) homeostasis and viability of cortical neurons. Co-exposure to 0.1 microM ouabain plus non-lethal C(2)-ceramide (5 microM) or beta-amyloid 1-42 (5 microM), however, induced marked intracellular K(+) loss, caspase-3 cleavage, DNA laddering, and synergistically triggered neuronal death. The caspase inhibitor Z-Val-Ala-Asp(OMe)-fluoromethyl ketone (Z-VAD-FMK) predominantly blocked the caspase activation and neuronal death. These results suggest that slight impairment of Na(+),K(+)-pump activity may amplify the disruption of K(+) homeostasis in the presence of a non-lethal apoptotic insult, leading to activation of apoptotic cascade and substantial neuronal injury.

Cytochrome c activates K+ channels before inducing apoptosis

Cell shrinkage is an early prerequisite for apoptosis. The apoptotic volume decrease is due primarily to loss of cytoplasmic ions. Increased outward K+ currents have indeed been implicated in the early stage of apoptosis in many cell types. We found that cytoplasmic dialysis of cytochrome c (cyt-c), a mitochondria-dependent apoptotic inducer, increases K+ currents before inducing nuclear condensation and breakage in pulmonary vascular smooth muscle cells. The cyt-c-mediated increase in K+ currents took place rapidly and was not affected by treatment with a specific inhibitor of caspase-9. Cytoplasmic dialysis of recombinant (active) caspase-9 negligibly affected the K+ currents. Furthermore, treatment of the cells with staurosporine (ST), an apoptosis inducer that mediates translocation of cyt-c from mitochondria to the cytosol, also increased K+ currents, caused cell shrinkage, and induced apoptosis (determined by apoptotic nuclear morphology and TdT-UTP nick end labeling assay). The staurosporine-induced increase in K+ currents concurred to the volume decrease but preceded the activation of apoptosis (nuclear condensation and breakage). These results suggest that the cyt-c-induced activation of K+ channels and the resultant K+ loss play an important role in initiating the apoptotic volume decrease when cells undergo apoptosis.

Cell volume control and signal transduction in apoptosis

Apoptosis is a physiological form of death in which cells turn-on an intrinsic genetic program that eventually leads to their destruction in a highly regulated manner. This process renders elimination of "unwanted cells" in the body, and accounts for cellular turnover and homeostasis of tissues in multicellular organisms. Consequently, an imbalance in the apoptotic rate in a particular tissue can lead to profound effects in the whole organism. Exposure of cells to apoptotic stimuli induces a rapid loss of cell volume (apoptotic volume decrease) that plays a pivotal role in the decision of a cell to undergo apoptosis. Interestingly, the apoptotic volume decrease is driven by changes in ionic fluxes across the plasma membrane that promote a decrease in the intracellular ions that ultimately also leads to a reduction in intracellular ionic strength. Despite an intensive research effort however, the cellular and molecular mechanisms that trigger changes in cell volume during apoptosis remain poorly understood. Nevertheless, this apoptotic volume decrease has been shown to be a necessary component of the apoptotic cascade and an important point of modulation for the entire cell death process. In this review, we will focus on the importance of the apoptotic volume decrease in the context of signaling and modulation of programmed cell death.

Cannabinoids in the treatment of glaucoma

The leading cause of irreversible blindness is glaucoma, a disease normally characterized by the development of ocular hypertension and consequent damage to the optic nerve at its point of retinal attachment. This results in a narrowing of the visual field, and eventually results in blindness. A number of drugs are available to lower intraocular pressure (IOP), but, occasionally, they are ineffective or have intolerable side-effects for some patients and can lose efficacy with chronic administration. The smoking of marijuana has decreased IOP in glaucoma patients. Cannabinoid drugs, therefore, are thought to have significant potential for pharmaceutical development. However, as the mechanism surrounding their effect on IOP initially was thought to involve the CNS, issues of psychoactivity hindered progress. The discovery of ocular cannabinoid receptors implied an explanation for the induction of hypotension by topical cannabinoid applications, and has stimulated a new phase of ophthalmic cannabinoid research. Featured within these investigations is the possibility that at least some cannabinoids may ameliorate optic neuronal damage through suppression of N-methyl-D-aspartate receptor hyperexcitability, stimulation of neural microcirculation, and the suppression of both apoptosis and damaging free radical reactions, among other mechanisms. Separation of therapeutic actions from side-effects now seems possible through a diverse array of novel chemical, pharmacological, and formulation strategies.

K+ currents generated by NMDA receptor activation in rat hippocampal pyramidal neurons

Long lasting outward currents mediated by Ca2+-activated K+ channels can be induced by Ca2+ influx through N-methyl-D-aspartate (NMDA)-receptor channels in voltage-clamped hippocampal pyramidal neurons. Using specific inhibitors, we have attempted to identify the channels that underlie these outward currents. At a holding potential of -50 mV, applications of 1 mM NMDA to the soma of cultured hippocampal pyramidal neurons induced the expected inward currents. In 44% of cells tested, these were followed by outward currents (average amplitude 60 +/- 7 pA) that peaked 2.5 s after the initiation of the inward NMDA currents and decayed with a time constant of 1.4 s. In 43% of those cells exhibiting an outward current, SK channel inhibitors, UCL 1848 (100 nM) and apamin (100 nM) abolished the outward current. In the remainder of the cells, the outward currents were either insensitive or only partly inhibited (44 +/- 4%) by 100 nM UCL 1848. In these cells, the outward currents were reduced by the slow afterhyperpolarization (sAHP) inhibitors, muscarine (3 microM; 43 +/- 9%), UCL 1880 (3 microM; 34 +/- 10%), and UCL 2027 (3 microM; 57 +/- 6%). Neither the BK channel inhibitor, charybdotoxin (100 nM), nor the Na+/K+ ATPase inhibitor, ouabain (100 microM), reduced these outward currents. Irrespective of the pharmacology, the time course of the outward current did not differ. Interestingly, no correlation was observed between the presence of a slow apamin-insensitive afterhyperpolarization and an outward current insensitive to SK channel blockers following NMDA-receptor activation. It is concluded that an NMDA-mediated rise in [Ca2+]i can result in the activation of apamin-sensitive SK channels and of the channels that underlie the sAHP. The activation of these channels may, however, depend on their location relative to NMDA receptors as well as on the spatial Ca2+ buffering within individual neurons.

Cerebral ischemia and trauma-different etiologies yet similar mechanisms: neuroprotective opportunities

Cerebral ischemia leads to brain damage caused by pathogenetic mechanisms that are also activated by neurotrauma. These mechanisms include among others excitotoxicity, over production of free radicals, inflammation and apoptosis. Furthermore, cerebral ischemia and trauma both trigger similar auto-protective mechanisms including the production of heat shock proteins, anti-inflammatory cytokines and endogenous antioxidants. Neuroprotective therapy aims at minimizing the activation of toxic pathways and at enhancing the activity of endogenous neuroprotective mechanisms. The similarities in the damage-producing and endogenous auto-protective mechanisms may imply that neuroprotective compounds found to be active against one of these conditions may indeed be also protective in the other. This review summarizes the pathogenetic events of ischemic and traumatic brain injury and reviews the neuroprotective strategies employed thus far in each of these conditions with a special emphasize on their clinical relevance and on future directions in the field of neuronal protection.

Chloride channels and hepatocellular function: prospects for molecular identification

Hepatocytes possess chloride channels at the plasma membrane and in multiple intracellular compartments. These channels are required for cell volume regulation and acidification of intracellular organelles. Evidence also supports a role of chloride channels in modulation of apoptosis and cell growth. Swelling- and Ca(2+)-activated chloride channels have been identified in hepatocyte plasma membranes, and chloride channels have been observed in the membranes of lysosomes, endosomes, Golgi, endoplasmic reticulum, mitochondria, and the nucleus. This review summarizes the functions of these channels and discusses the specific channel molecules they may represent. Chloride channel molecules shown to be expressed in hepatocytes include members of the ClC channel family (ClC-2, ClC-3, ClC-5, and ClC-7), members of the newly identified CLIC family of intracellular chloride channels (CLIC-1 and CLIC-4), the mitochondrial voltage-dependent anion channel, and a newly identified intracellular channel, MCLC (Mid-1 related chloride channel). Current understanding does not include a molecular identification of most of the observed channel functions, but details of the molecular properties of these channel molecules should allow future identification and further understanding of chloride channel function in hepatocytes.

Neuroprotective effects of lithium in cultured cells and animal models of diseases

Lithium, the major drug used to treat manic depressive illness, robustly protects cultured rat brain neurons from glutamate excitotoxicity mediated by N-methyl-D-aspartate (NMDA) receptors. The lithium neuroprotection against glutamate excitotoxiciy is long-lasting, requires long-term pretreatment and occurs at therapeutic concentrations of this drug. The neuroprotective mcchanisms involve inactivation of NMDA receptors, decreased expression of pro-apoptotic proteins, p53 and Bax, enhanced expression of the cytoprotective protein, Bcl-2, and activation of the cell survival kinase, Akt. In addition, lithium pretreatment suppresses glutamate-induced loss of the activities of Akt, cyclic AMP-response element binding protein (CREB), c-Jun - N-terminal kinase (JNK) and p38 kinase. Lithium also reduces brain damage in animal models of neurodegenerative diseases in which excitotoxicity has been implicated. In the rat model of stroke using middle cerebral artery occlusion, lithium markedly reduces neurologic deficits and decreases brain infarct volume even when administered after the onset of ischemia. In a rat Huntington's disease model, lithium significantly reduces brain lesions resulting from intrastriatal infusion of quinolinic acid, an excitotoxin. Our results suggest that lithium might have utility in the treatment of neurodegenerative disorders in addition to its common use for the treatment of bipolar depressive patients.

Flupirtine blocks apoptosis in batten patient lymphoblasts and in human postmitotic CLN3- and CLN2-deficient neurons

Multiple gene defects cause Batten disease. Accelerated apoptosis accounts for neurodegeneration in the late infantile and juvenile forms that are due to defects in the CLN3 and CLN2 genes. Extensive neuronal death is seen in CLN2- and CLN3-deficient human brain as well as in CLN6-deficient sheep brain and retina. When neurons in late infantile and juvenile brain survive, they manage to do so by upregulating the neuroprotective molecule Bcl-2. The CLN3 gene has antiapoptotic properties at the molecular level. We show that the CLN2 gene is neuroprotective: it enhances growth of NT2 cells and maintains survival of human postmitotic hNT neurons. Conversely, blocking CLN3 or CLN2 expression in hNT neurons with adenoviral antisense-CLN3 or antisense-CLN2-AAV2 constructs causes apoptosis. The drug flupirtine is a triaminopyridine derivative that acts as a nonopioid analgesic. Flupirtine upregulates Bcl-2, increases glutathione levels, activates an inwardly rectifying potassium channel, and delays loss of intermitochondrial membrane calcium retention capacity. We show that flupirtine aborts etoposide-induced apoptosis in CLN1-, CLN2-, CLN3-, and CLN6-deficient as well as normal lymphoblasts. Flupirtine also prevents the death of CLN3- and CLN2-deficient postmitotic hNT neurons at the mitochondrial level. We show that a mechanism of neuroprotection exerted by flupirtine involves complete functional antagonism of N-methyl-D-aspartate or N-methyl-D-aspartate-induced neuronal apoptosis. Flupirtine may be useful as a drug capable of halting the progression of neurodegenerative diseases caused by dysregulated apoptosis.

Alterations of single potassium channel activity in CA1 pyramidal neurons after transient forebrain ischemia

Selective neuronal injury in the CA1 zone of hippocampus following transient cerebral ischemia has been well documented. Extracellular potassium concentration markedly increases during ischemia/hypoxia. Accumulating evidence has indicated that the outward potassium currents, including delayed rectifier potassium current, not only influence membrane excitability but also mediate apoptosis. It has been shown that the amplitude of delayed rectifier potassium current in CA1 neurons significantly increased after cerebral ischemia. To elucidate the mechanisms underlying the changes of potassium currents following ischemia, single potassium channel activities of rat CA1 neurons were compared before and after transient forebrain ischemia. Using cell-attached configuration, depolarizing voltage steps activated outward single channel events. The channel properties, the kinetics and pharmacology of these events resemble the delayed rectifier potassium current. After ischemia, the unitary amplitude of single channels significantly increased, the open probability, mean open time and open time constant also significantly increased while the conductance remained unchanged. These data indicate that the increase of single channel activity is responsible, at least in part, for the increase of delayed rectifier potassium current in CA1 neurons after cerebral ischemia.

Bupivacaine, but not tetracaine, protects against the in vitro ischemic insult of rat hippocampal CA1 neurons

Neuroprotective actions of local anesthetics, bupivacaine and tetracaine, against the irreversible membrane dysfunction induced by in vitro ischemia were investigated. Intracellular recordings were made from hippocampal CA1 neurons in rat brain slice preparations. Oxygen and glucose deprivation (in vitro ischemia) produced a rapid depolarization after approximately 5 min of exposure. When oxygen and glucose were reintroduced, the membrane depolarized further and reached at 0 mV: the membrane showed no functional recovery (irreversible membrane dysfunction). Pretreatment with tetracaine or bupivacaine significantly prolonged the latency of rapid depolarization. Bupivacaine, but not tetracaine, restored the membrane potential after the reintroduction of oxygen and glucose. Tetracaine and bupivacaine depressed both field postsynaptic potentials and presynaptic volleys. The drugs also reduced the dV/dt of Ca(2+)-dependent spikes and the rapid rise of [Ca(2+)](i) induced by in vitro ischemia. Compared with tetracaine, bupivacaine markedly suppressed the resting K(+) conductance and the ATP-sensitive and Ca(2+)-dependent K(+) conductances. Moreover, in the presence of tetraethylammonium (TEA), a majority of CA1 neurons impaled with Cs acetate-filled electrodes showed complete or partial recovery of the membrane potential after reintroducing oxygen and glucose. These results suggest that the neuroprotective action of bupivacaine is mainly due to the suppression of the K(+) conductances.

Ionic mechanism of ouabain-induced concurrent apoptosis and necrosis in individual cultured cortical neurons

Energy deficiency and dysfunction of the Na+, K+-ATPase are common consequences of many pathological insults. The nature and mechanism of cell injury induced by impaired Na+, K+-ATPase, however, are not well defined. We used cultured cortical neurons to examine the hypothesis that blocking the Na+, K+-ATPase induces apoptosis by depleting cellular K+ and, concurrently, induces necrotic injury in the same cells by increasing intracellular Ca2+ and Na+. The Na+, K+-ATPase inhibitor ouabain induced concentration-dependent neuronal death. Ouabain triggered transient neuronal cell swelling followed by cell shrinkage, accompanied by intracellular Ca2+ and Na+ increase, K+ decrease, cytochrome c release, caspase-3 activation, and DNA laddering. Electron microscopy revealed the coexistence of ultrastructural features of both apoptosis and necrosis in individual cells. The caspase inhibitor Z-Val-Ala-Asp(OMe)-fluoromethyl ketone (Z-VAD-FMK) blocked >50% of ouabain-induced neuronal death. Potassium channel blockers or high K+ medium, but not Ca2+ channel blockade, prevented cytochrome c release, caspase activation, and DNA damage. Blocking of K+, Ca2+, or Na+ channels or high K+ medium each attenuated the ouabain-induced cell death; combined inhibition of K+ channels and Ca2+ or Na+ channels resulted in additional protection. Moreover, coapplication of Z-VAD-FMK and nifedipine produced virtually complete neuroprotection. These results suggest that the neuronal death associated with Na+, K+-pump failure consists of concurrent apoptotic and necrotic components, mediated by intracellular depletion of K+ and accumulation of Ca2+ and Na+, respectively. The ouabain-induced hybrid death may represent a distinct form of cell death related to the brain injury of inadequate energy supply and disrupted ion homeostasis.

Role of K(+) efflux in apoptosis induced by AMPA and kainate in mouse cortical neurons

Activation of ionotropic glutamate receptors can induce neuronal apoptosis in vitro and in vivo. We showed previously that activation of the N-methyl-D-aspartic acid (NMDA) subtype of glutamate receptors in a low Ca(2+) and low Na(+) condition induced apoptotic neuronal death, and that the K(+) efflux via NMDA receptor channels was likely a key event in NMDA-induced apoptosis. Since non-NMDA receptors, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) and kainate receptors, are also permeable to K(+), we tested the hypothesis that stimulating K(+) efflux via non-NMDA receptor channels could induce apoptosis in cultured cortical neurons. Using a Ca(2+)-free and Na(+)-free external solution, application of kainate revealed outward membrane currents carried by K(+) efflux. In a low Ca(2+)/low Na(+) medium, a 5-h exposure to 50-500 microM AMPA in the presence of the NMDA receptor antagonist MK801 induced dose-dependent neuronal death 24 h after the onset of the insult, accompanied by intracellular K(+) reduction and caspase-3 activation. The AMPA-induced cell death was attenuated by the caspase inhibitor Z-Val-Ala-Asp(OMe)-fluoromethyl ketone (Z-VAD-FMK) and by the protein synthesis inhibitor cycloheximide. Reducing K(+) efflux by raising extracellular K(+) concentration from 5 to 25 mM attenuated AMPA-triggered cell death, the Ca(2+) channel antagonist nifedipine showed no effect on the AMPA toxicity. Kainate induced similar neuronal death sensitive to attenuation by Z-VAD-FMK or elevated extracellular K(+).We suggest that the non-NMDA receptor-mediated K(+) efflux may participate in apoptotic process and that blocking excessive K(+) efflux mediated by NMDA and non-NMDA receptors may selectively prevent neuronal apoptosis under certain pathological conditions.

Prolonged survival and decreased abnormal movements in transgenic model of Huntington disease, with administration of the transglutaminase inhibitor cystamine

An expanded polyglutamine domain in huntingtin underlies the pathogenic events in Huntington disease (HD), characterized by chorea, dementia and severe weight loss, culminating in death. Transglutaminase (TGase) may be critical in the pathogenesis, via cross-linking huntingtin. Administration of the TGase competitive inhibitor, cystamine, to transgenic mice expressing exon 1 of huntingtin containing an expanded polyglutamine repeat, altered the course of their HD-like disease. Cystamine given intraperitoneally entered brain where it inhibited TGase activity. When treatment began after the appearance of abnormal movements, cystamine extended survival, reduced associated tremor and abnormal movements and ameliorated weight loss. Treatment did not influence the appearance or frequency of neuronal nuclear inclusions. Unexpectedly, cystamine treatment increased transcription of one of the two genes shown to be neuroprotective for polyglutamine toxicity in Drosophila, dnaj (also known as HDJ1 and Hsp40 in humans and mice, respectively). Inhibition of TGase provides a new treatment strategy for HD and other polyglutamine diseases.

Lithium suppresses excitotoxicity-induced striatal lesions in a rat model of Huntington's disease

Huntington's disease is a progressive, inherited neurodegenerative disorder characterized by the loss of subsets of neurons primarily in the striatum. In this study, we assessed the neuroprotective effect of lithium against striatal lesion formation in a rat model of Huntington's disease in which quinolinic acid was unilaterally infused into the striatum. For this purpose, we used a dopamine receptor autoradiography and glutamic acid decarboxylase mRNA in situ hybridization analysis, methods previously shown to be adequate for quantitative analysis of the excitotoxin-induced striatal lesion size. Here we demonstrated that subcutaneous injections of LiCl for 16 days prior to quinolinic acid infusion considerably reduced the size of quinolinic acid-induced striatal lesion. Furthermore, these lithium pre-treatments also decreased the number of striatal neurons labeled with the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling assay. Immunohistochemistry and western blotting demonstrated that lithium-elicited neuroprotection was associated with an increase in Bcl-2 protein levels. Our results raise the possibility that lithium may be considered as a neuroprotective agent in treatment of neurodegenerative diseases such as Huntington's disease.

Mononuclear phagocyte biophysiology influences brain transendothelial and tissue migration: implication for HIV-1-associated dementia

Mononuclear phagocyte (MP) brain migration influence neuronal damage during HIV-1-associated dementia (HAD). We demonstrate that potassium channels, expressed in human monocyte-derived macrophages (MDM), are vital for MP movement through Boyden chemotactic chambers, an artificial blood-brain barrier and organotypic hippocampal brain slices. MDM migration is inhibited by voltage-and calcium-activated potassium channel blockers that include charybodotoxin, margatoxin, agatoxin and apamin. This is observed both in uninfected and HIV-1-infected MP. The results suggest that potassium channels affect MDM brain migration through altering cell volume and shape. Such mechanisms likely affect MP-induced neuronal destruction during HAD.

Simultaneous intracellular calcium and sodium flux imaging in human vanilloid receptor 1 (VR1)-transfected human embryonic kidney cells: a method to resolve ionic dependence of VR1-mediated cell death

The vanilloid receptor 1 (VR1) is a ligand-gated, nonselective cation channel important for the sensory processing of painful stimuli. Activation of VR1 leads to increases in intracellular concentrations of calcium and sodium. Prolonged activation of VR1 in mammalian expression systems leads to cell death. The mechanism of VR1-mediated toxicity may have relevance to pathophysiological processes that can occur in neurons. Therefore, we have evaluated the relative contributions of intracellular calcium and sodium changes to VR1-mediated toxicity in human embryonic kidney 293 cells stably transfected with the human VR1 channel. The data demonstrate that VR1 receptor agonists capsaicin and resiniferatoxin lead to a sustained increase in intracellular calcium and sodium in a concentration-dependent manner, followed by cell death. Pretreatment with VR1 receptor antagonists capsazepine or ruthenium red block both the calcium and sodium responses to agonists, and block agonist-induced cell death in a concentration-dependent manner. However, addition of antagonists several minutes after agonists selectively reverses the agonist-induced increase in intracellular calcium, but does not reverse the elevated intracellular sodium concentration. Nonetheless, antagonists retain protective efficacy against capsaicin toxicity when added several minutes after capsaicin, conditions in which the cells still manifest elevated intracellular sodium, but not elevated intracellular calcium. In addition, a transient VR1-mediated increase in intracellular calcium that returns to baseline within minutes, induced by a rapid drop in pH, from pH 7.5 to pH 6.3, also does not lead to cell death. Collectively, these data demonstrate that the most important intracellular ionic change for mediating VR1-dependent toxicity is a sustained increase of calcium.

Nitric oxide induces apoptosis by activating K+ channels in pulmonary vascular smooth muscle cells

Nitric oxide (NO) is an endogenous endothelium-derived relaxing factor that regulates vascular smooth muscle cell proliferation and apoptosis. This study investigated underlying mechanisms involved in NO-induced apoptosis in human and rat pulmonary artery smooth muscle cells (PASMC). Exposure of PASMC to NO, which was derived from the NO donor S-nitroso-N-acetyl-penicillamine, increased the percentage of cells undergoing apoptosis. Increasing extracellular K+ concentration to 40 mM or blocking K+ channels with 1 mM tetraethylammonia (TEA), 100 nM iberiotoxin (IBTX), and 5 mM 4-aminopyridine (4-AP) significantly inhibited the NO-induced apoptosis. In single PASMC, NO reversibly increased K+ currents through the large-conductance Ca(2+)-activated K+ (K(Ca)) channels, whereas TEA and IBTX markedly decreased the K(Ca) currents. In the presence of TEA, NO also increased K+ currents through voltage-gated K+ (K(v)) channels, whereas 4-AP significantly decreased the K(v) currents. Opening of K(Ca) channels with 0.3 mM dehydroepiandrosterone increased K(Ca) currents, induced apoptosis, and further enhanced the NO-mediated apoptosis. Furthermore, NO depolarized the mitochondrial membrane potential. These observations indicate that NO induces PASMC apoptosis by activating K(Ca) and K(v) channels in the plasma membrane. The resulting increase in K+ efflux leads to cytosolic K+ loss and eventual apoptosis volume decrease and apoptosis. NO-induced apoptosis may also be related to mitochondrial membrane depolarization in PASMC.

Necrotic volume increase and the early physiology of necrosis

Whether a lethally injured mammalian cell undergoes necrosis or apoptosis may be determined by the early activation of specific ion channels at the cell surface. Apoptosis requires K+ and Cl- efflux, which leads to cell shrinking, an active phenomenon termed apoptotic volume decrease (AVD). In contrast, necrosis has been shown to require Na+ influx through membrane carriers and more recently through stress-activated non-selective cation channels (NSCCs). These ubiquitous channels are kept dormant in viable cells but become activated upon exposure to free-radicals. The ensuing Na+ influx leads to cell swelling, an active response that may be termed necrotic volume increase (NVI). This review focuses on how AVD and NVI become conflicting forces at the beginning of cell injury, on the events that determine irreversibility and in particular, on the ion fluxes that decide whether a cell is to die by necrosis or by apoptosis.

Anandamide activates vanilloid receptor 1 (VR1) at acidic pH in dorsal root ganglia neurons and cells ectopically expressing VR1

The vanilloid receptor type 1 (VR1) is a heat-activated ionophore preferentially expressed in nociceptive neurons of trigeminal and dorsal root ganglia (DRG). VR1, which binds and is activated by capsaicin and other vanilloid compounds, was noted to interact with the endocannabinoid anandamide (ANA) and certain inflammatory metabolites of arachidonic acid in a pH-dependent manner. At pH < or = 6.5 ANA induced (45)Ca(2+) uptake either in primary cultures of DRG neurons or cells ectopically expressing C-terminally tagged recombinant forms of VR1 with an EC(50) = approximately 10 microm at pH 5.5. Capsazepine, a potent antagonist of vanilloids, inhibited ANA-induced Ca(2+) transport in both cell systems. Vanilloids displaced [(3)H]ANA in VR1-expressing cells, suggesting competition for binding to VR1. Ratiometric determination of intracellular free calcium and confocal imaging of the VR1-green fluorescent fusion protein revealed that, at low pH (< or =6.5), ANA could induce an elevation of intracellular free Ca(2+) and consequent intracellular membrane changes in DRG neurons or transfected cells expressing VR1. These actions of ANA were similar to the effects determined previously for vanilloids. The ligand-induced changes in Ca(2+) at pH < or = 6.5 are consistent with the idea that ANA and other eicosanoids act as endogenous ligands of VR1 in a conditional fashion in vivo. The pH dependence suggests that tissue acidification in inflammation, ischemia, or traumatic injury can sensitize VR1 to eicosanoids and transduce pain from the periphery.

Cisplatin-induced apoptosis of mesothelioma cells is affected by potassium ion flux modulator amphotericin B and bumetanide

Chemotherapeutic anti-cancer drugs induce cell death by the process of apoptosis. Efflux of potassium ions (K(+)) is necessary for cell volume reduction during apoptosis and increased inward pumping of K(+) thus counteracts apoptosis. Potassium flux modulation could therefore interact with apoptosis and affect the efficiency of cancer chemotherapeutics. We explored if the K(+) efflux stimulator amphotericin B, with or without the Na(+), K(+), 2Cl(-)-cotransport (K(+) influx) blocker bumetanide, could affect cisplatin- and carboplatin-induced apoptosis and cytotoxicity in the pulmonary mesothelioma cell line (P31). Apoptosis was determined by quantifying free nucleosomes and caspase-3 activity, and cytotoxicity was determined by clone formation and a fluorometric assay. The pan-caspase enzyme inhibitor Boc-D-FMK was used to further determine the role of caspase activity in K(+)-flux-modulated cisplatin-/carboplatin-induced apoptosis and cytotoxicity. Amphotericin B (3.2 micromol/L) combined with bumetanide (100 micromol/L) potentiated cisplatin-induced free nucleosome and caspase-3 activity. The combination of the K(+) modulators did not, however, increase cisplatin cytotoxicity. The caspase inhibitor Boc-D-FMK, but unexpectedly also bumetanide, markedly reduced cisplatin cytotoxicity and annihilated the augmented cytotoxicity of cisplatin in the presence of amphotericin B. Carboplatin cytotoxicity was reduced by bumetanide, but not affected by amphotericin B. Carboplatin and carboplatin/bumetanide cytotoxicity was further reduced by Boc-D-FMK. We conclude that the ability of cisplatin, and to a lesser extent carboplatin, to induce apoptosis is indeed influenced by cellular potassium flux modulators. We suggest that K(+) ionophores such as amphotericin B, and K(+) influx blockers such as bumetanide, alone or in combination, should be further evaluated for their potential clinical usefulness in influencing tumor cell apoptosis induced by cisplatin and other cancer chemotherapeutics.

Ion homeostasis and apoptosis

Alterations in the transmembrane gradients of several physiological ions may influence programmed cell death. In particular, recent data suggest that increases in intracellular calcium may either promote or inhibit apoptosis, depending on the level, timing and location, whereas loss of intracellular potassium promotes apoptosis.

Bcl-2 decreases voltage-gated K+ channel activity and enhances survival in vascular smooth muscle cells

Cell shrinkage is an incipient hallmark of apoptosis in a variety of cell types. The apoptotic volume decrease has been demonstrated to attribute, in part, to K+ efflux; blockade of plasmalemmal K+ channels inhibits the apoptotic volume decrease and attenuates apoptosis. Using combined approaches of gene transfection, single-cell PCR, patch clamp, and fluorescence microscopy, we examined whether overexpression of Bcl-2, an anti-apoptotic oncoprotein, inhibits apoptosis in pulmonary artery smooth muscle cells (PASMC) by diminishing the activity of voltage-gated K+ (Kv) channels. A human bcl-2 gene was infected into primary cultured rat PASMC using an adenoviral vector. Overexpression of Bcl-2 significantly decreased the amplitude and current density of Kv currents (I(Kv)). In contrast, the apoptosis inducer staurosporine (ST) enhanced I(Kv). In bcl-2-infected cells, however, the ST-induced increase in I(Kv) was completely abolished, and the ST-induced apoptosis was significantly inhibited compared with cells infected with an empty adenovirus (-bcl-2). Blockade of Kv channels in control cells (-bcl-2) by 4-aminopyridine also inhibited the ST-induced increase in I(Kv) and apoptosis. Furthermore, overexpression of Bcl-2 accelerated the inactivation of I(Kv) and downregulated the mRNA expression of the pore-forming Kv channel alpha-subunits (Kv1.1, Kv1.5, and Kv2.1). These results suggest that inhibition of Kv channel activity may serve as an additional mechanism involved in the Bcl-2-mediated anti-apoptotic effect on vascular smooth muscle cells.

Is caspase-3 inhibition a valid therapeutic strategy in cerebral ischemia?

Neurodegenerative diseases are characterized by progressive impairment of brain function as a consequence of ongoing neuronal cell death. Apoptotic mechanisms have been implicated in this process and a major involvement of caspase-3, a typical pro-apoptotic executioner protease, has been claimed. In this review, the role of caspase-3 in neuronal cell loss in animal models of stroke is discussed and critically evaluated. In summary, it is concluded that the biochemical evidence favoring caspase-3 as a therapeutic target in cerebral ischemia is not convincing, and the development of selective caspase-3 inhibitors for the treatment of human stroke must be viewed as high risk.

Glutamate neurotoxicity, oxidative stress and mitochondria

The excitatory neurotransmitter glutamate plays a major role in determining certain neurological disorders. This situation, referred to as 'glutamate neurotoxicity' (GNT), is characterized by an increasing damage of cell components, including mitochondria, leading to cell death. In the death process, reactive oxygen species (ROS) are generated. The present study describes the state of art in the field of GNT with a special emphasis on the oxidative stress and mitochondria. In particular, we report how ROS are generated and how they affect mitochondrial function in GNT. The relationship between ROS generation and cytochrome c release is described in detail, with the released cytochrome c playing a role in the cell defense mechanism against neurotoxicity.

Chronotropic Effect of Angiotensin II via Type 2 Receptors in Rat Brain Neurons

Previously, we determined that angiotensin II (Ang II) elicits an Ang II type 2 (AT2) receptor–mediated increase of neuronal delayed rectifier K+( I KV) current in neuronal cultures from newborn rat hypothalamus and brain stem. This requires generation of lipoxygenase (LO) metabolites of arachidonic acid (AA) and activation of serine/threonine phosphatase type 2A (PP-2A). Enhancement of I KV results in a decrease in net inward current during the action potential (AP) upstroke as well as shortening of the refractory period, which may lead to alterations in neuronal firing rate. Thus, in the present study, we used whole-cell current clamp recording methods to investigate the AT2 receptor–mediated effects of Ang II on the firing rate of cultured neurons from the hypothalamus and brain stem. At room temperature, these neurons exhibited spontaneous APs with an amplitude of 77.72 ± 2.7 mV ( n = 20) and they fired at a frequency of 0.8 ± 0.1 Hz ( n = 11). Most cells had a prolonged early after-depolarization that followed an initial fully developed AP. Superfusion of Ang II (100 nM) plus losartan (LOS, 1 μM) to block Ang II type 1 receptors elicited a significant chronotropic effect that was reversed by the AT2 receptor inhibitor PD 123,319 (1 μM). LOS alone had no effect on any of the parameters measured. The chronotropic effect of Ang II was reversed by the general LO inhibitor 5,8,11,14-eicosatetraynoic acid (10 μM) or by the selective PP-2A inhibitor okadaic acid (1 nM) and was mimicked by the 12-LO metabolite of AA 12-(S)-hydroxy-(5Z, 8Z, 10E, 14Z)-eicosatetraynoic acid. These data indicate that Ang II elicits an AT2 receptor–mediated increase in neuronal firing rate, an effect that involves generation of LO metabolites of AA and activation of PP-2A.

Ligand-induced dynamic membrane changes and cell deletion conferred by vanilloid receptor 1

The real time dynamics of vanilloid-induced cytotoxicity and the specific deletion of nociceptive neurons expressing the wild-type vanilloid receptor (VR1) were investigated. VR1 was C-terminally tagged with either the 27-kDa enhanced green fluorescent protein (eGFP) or a 12-amino acid epsilon-epitope. Upon exposure to resiniferatoxin, VR1eGFP- or VR1epsilon-expressing cells exhibited pharmacological responses similar to those of cells expressing the untagged VR1. Within seconds of vanilloid exposure, the intracellular free calcium ([Ca(2+)](i)) was elevated in cells expressing VR1. A functional pool of VR1 also was localized to the endoplasmic reticulum that, in the absence of extracellular calcium, also was capable of releasing calcium upon agonist treatment. Confocal imaging disclosed that resiniferatoxin treatment induced vesiculation of the mitochondria and the endoplasmic reticulum ( approximately 1 min), nuclear membrane disruption (5-10 min), and cell lysis (1-2 h). Nociceptive primary sensory neurons endogenously express VR1, and resiniferatoxin treatment induced a sudden increase in [Ca(2+)](i) and mitochondrial disruption which was cell-selective, as glia and non-VR1-expressing neurons were unaffected. Early hallmarks of cytotoxicity were followed by specific deletion of VR1-expressing cells. These data demonstrate that vanilloids disrupt vital organelles within the cell body and, if administered to sensory ganglia, may be employed to rapidly and selectively delete nociceptive neurons.

Activation of K+ channels induces apoptosis in vascular smooth muscle cells

Intracellular K+ plays an important role in controlling the cytoplasmic ion homeostasis for maintaining cell volume and inhibiting apoptotic enzymes in the cytosol and nucleus. Cytoplasmic K+ concentration is mainly regulated by K+ uptake via Na+-K+-ATPase and K+ efflux through K+ channels in the plasma membrane. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), a protonophore that dissipates the H+ gradient across the inner membrane of mitochondria, induces apoptosis in many cell types. In rat and human pulmonary artery smooth muscle cells (PASMC), FCCP opened the large-conductance, voltage- and Ca2+-sensitive KK+ (maxi-K) channels, increased K+ currents through maxi-K channels [I(K(Ca))], and induced apoptosis. Tetraethylammonia (1 mM) and iberiotoxin (100 nM) decreased I(K(Ca)) by blocking the sarcolemmal maxi-K channels and inhibited the FCCP-induced apoptosis in PASMC cultured in media containing serum and growth factors. Furthermore, inhibition of K+ efflux by raising extracellular K+ concentration from 5 to 40 mM also attenuated PASMC apoptosis induced by FCCP and the K+ ionophore valinomycin. These results suggest that FCCP-mediated apoptosis in PASMC is partially due to an increase of maxi-K channel activity. The resultant K+ loss through opened maxi-K channels may serve as a trigger for cell shrinkage and caspase activation, which are major characteristics of apoptosis in pulmonary vascular smooth muscle cells.

Pro-apoptotic function of calsenilin/DREAM/KChIP3

Apoptotic cell death and increased production of amyloid b peptide (Ab) are pathological features of Alzheimer's disease (AD), although the exact contribution of apoptosis to the pathogenesis of the disease remains unclear. Here we describe a novel pro-apoptotic function of calsenilin/DREAM/KChIP3. By antisense oligonucleotide-induced inhibition of calsenilin/DREAM/KChIP3 synthesis, apoptosis induced by Fas, Ca2+-ionophore, or thapsigargin is attenuated. Conversely, calsenilin/DREAM/KChIP3 expression induced the morphological and biochemical features of apoptosis, including cell shrinkage, DNA laddering, and caspase activation. Calsenilin/DREAM/KChIP3-induced apoptosis was suppressed by caspase inhibitor Z-VAD and by Bcl-XL, and was potentiated by increasing cytosolic Ca2+, expression of Swedish amyloid precursor protein mutant (APPSW) or presenilin 2 (PS2), but not by a PS2 deletion lacking its C-terminus (PS2/411stop). In addition, calsenilin/DREAM/KChIP3 expression increased Ab42 production in cells expressing APPsw, which was potentiated by PS2, but not by PS2/411stop, which suggests a role for apoptosis-associated Ab42 production of calsenilin/DREAM/KChIP3.

Plasma membrane depolarization without repolarization is an early molecular event in anti-Fas-induced apoptosis

The movement of intracellular monovalent cations has previously been shown to play a critical role in events leading to the characteristics associated with apoptosis. A loss of intracellular potassium and sodium occurs during apoptotic cell shrinkage establishing an intracellular environment favorable for nuclease activity and caspase activation. We have now investigated the potential movement of monovalent ions in Jurkat cells that occur prior to cell shrinkage following the induction of apoptosis. A rapid increase in intracellular sodium occurs early after apoptotic stimuli suggesting that the normal negative plasma membrane potential may change during cell death. We report here that diverse apoptotic stimuli caused a rapid cellular depolarization of Jurkat T-cells that occurs prior to and after cell shrinkage. In addition to the early increase in intracellular Na(+), (86)Rb(+) studies reveal a rapid inhibition of K(+) uptake in response to anti-Fas. These effects on Na(+) and K(+) ions were accounted for by the inactivation of the Na(+)/K(+)-ATPase protein and its activity. Furthermore, ouabain, a cardiac glycoside inhibitor of the Na(+)/K(+)-ATPase, potentiated anti-Fas-induced apoptosis. Finally, activation of an anti-apoptotic signal, i.e. protein kinase C, prevented both cellular depolarization in response to anti-Fas and all downstream characteristics associated with apoptosis. Thus cellular depolarization is an important early event in anti-Fas-induced apoptosis, and the inability of cells to repolarize via inhibition of the Na(+)/K(+)-ATPase is a likely regulatory component of the death process.