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

Inhibitory Effects of Clofilium on Membrane Currents Associated with Ca2+ Channels, NMDA Receptor Channels and Na+, K+-ATPase in Cortical Neurons

The class III antiarrhythmic agent 4-chloro-N,N-diethyl-N-heptyl-benzene butanaminium (clofilium) is known as a K+ channel open-channel blocker and has either anti- or proapoptotic property due to undefined mechanisms. Based on the evidence that neuronal viability is largely, sometimes critically, affected by voltage- and ligand-gated Ca2+ channels and the Na+, K+-ATPase, we tested the hypothesis that clofilium might additionally act on Ca2+ permeable ion channels and the Na+, K+-ATPase. Membrane currents associated with activities of voltage-gated Ca2+ channels, N-methyl-D-aspartate (NMDA) receptor channels and Na+, K+-ATPase were recorded using whole-cell recordings in cultured murine cortical neurons. Clofilium (0.1-100 micromol/l) inhibited high voltage-activated Ca2+ currents in concentration- and use-dependent manners. Clofilium acted as a potent antagonist of NMDA receptor channels, preferably blocked the NMDA steady-state current at a low concentration (0.1 micromol/l). At concentrations of >100 micromol/l, clofilium blocked both peak and steady-state NMDA currents in a voltage-independent manner. Clofilium also inhibited the Na+, K+-ATPase current with an IC50 of 7.5 micromol/l. Our data suggest that the pharmacological action of clofilium is far more complex than recognized before; the multiple actions of clofilium on membrane conductance may explain its diverse effects on cellular events and cell viability.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Role of Na+/Ca2+ exchange in regulating cytosolic Ca2+ in cultured human pulmonary artery smooth muscle cells

A rise in cytosolic Ca2+ concentration ([Ca2+]cyt) in pulmonary artery smooth muscle cells (PASMC) is an important stimulus for cell contraction, migration, and proliferation. Depletion of intracellular Ca2+ stores opens store-operated Ca2+ channels (SOC) and causes Ca2+ entry. Transient receptor potential (TRP) cation channels that are permeable to Na+ and Ca2+ are believed to form functional SOC. Because sarcolemmal Na+/Ca2+ exchanger has also been implicated in regulating [Ca2+]cyt, this study was designed to test the hypothesis that the Na+/Ca2+ exchanger (NCX) in cultured human PASMC is functionally involved in regulating [Ca2+]cyt by contributing to store depletion-mediated Ca2+ entry. RT-PCR and Western blot analyses revealed mRNA and protein expression for NCX1 and NCKX3 in cultured human PASMC. Removal of extracellular Na+, which switches the Na+/Ca2+ exchanger from the forward (Ca2+ exit) to reverse (Ca2+ entry) mode, significantly increased [Ca2+]cyt, whereas inhibition of the Na+/Ca2+ exchanger with KB-R7943 (10 microM) markedly attenuated the increase in [Ca2+]cyt via the reverse mode of Na+/Ca2+ exchange. Store depletion also induced a rise in [Ca2+]cyt via the reverse mode of Na+/Ca2+ exchange. Removal of extracellular Na+ or inhibition of the Na+/Ca2+ exchanger with KB-R7943 attenuated the store depletion-mediated Ca2+ entry. Furthermore, treatment of human PASMC with KB-R7943 also inhibited cell proliferation in the presence of serum and growth factors. These results suggest that NCX is functionally expressed in cultured human PASMC, that Ca2+ entry via the reverse mode of Na+/Ca2+ exchange contributes to store depletion-mediated increase in [Ca2+]cyt, and that blockade of the Na+/Ca2+ exchanger in its reverse mode may serve as a potential therapeutic approach for treatment of pulmonary hypertension.

Effects of aluminum chloride on sodium current, transient outward potassium current and delayed rectifier potassium current in acutely isolated rat hippocampal CA1 neurons

The effects of aluminum chloride (AlCl3) on sodium current (INa), the transient outward potassium (IA) and delayed rectifier potassium currents (IK) in hippocampal CA1 neurons of rats were studied using the whole cell patch-clamp technique. AlCl3 decreased INa, IA, and IK in a partly reversible, dose and voltage-dependent manner. AlCl3 prolonged the time to peak of INa, and increased the inactivation time constants of INa and IA . In addition, 1000 microM AlCl3 shifted the voltage dependence of steady-state activation of INa, IA and IK toward positive potential, and the voltage dependence of steady-state inactivation of INa, IA toward negative potential. These results imply that AlCl3 could affect the activation and inactivation courses of sodium current and potassium current of rat hippocampal CA1 neurons, which may contribute to damage of the central nervous system by aluminum.

A novel toxicity-based assay for the identification of modulators of voltage-gated Na+ channels

Voltage-gated Na+ channels are promising drug targets. Screening of large numbers of putative modulators, however, can be demanding and expensive. In this study, a simple, cheap, and robust assay to test the pharmacological modulation of Na+ channel function is presented. The assay makes use of the fact that the intracellular accumulation of Na+ ions can be cytotoxic. The toxicity of the Na+ channel activator veratridine in the presence of an inhibitor of the Na+/K+ ATPase (ouabain) in a Nav1.2a (rat brain IIA alpha) expressing cell line is assessed. Na+ channel blockers should reduce toxicity in this model. CHO cells which recombinantly expressed rat Nav1.2a subunits were seeded in 96-well plates, and cell survival was tested after 24 h incubation in medium containing veratridine and ouabain in the presence or absence of Na+ channel blockers. Propidium iodide fluorescence was used as toxicity readout. Veratridine (100 microM) or ouabain alone (500 microM) were not toxic to the cells. In the presence of 500 microM ouabain, however, veratridine induced halfmaximal cell death with an EC50 value of 15.1 +/- 2.3 microM. Ouabain's EC50 was 215.3 +/- 16.7 microM (with 30 microM veratridine). The effects of a number of Na+ channel blockers were tested and compared with their Na+ channel blocking activity measured in voltage-clamp experiments. Blockers from various chemical classes reduced toxicity half maximally with IC50 values ranging from 11.7 +/- 1.4 nM (tetrodotoxin) to 280.5 +/- 48.0 microM (lamotrigine). There was a linear relationship between the log IC50 values obtained by the two methods (slope: 1.1 +/- 0.08; correlation coefficient: 0.93). In summary, these data show that this novel toxicity assay is well suited to test Na+ channel blockers.

Kainic acid does not affect CA3 hippocampal region pyramidal cells in hypothyroid rats

Thyroid hormones exert a crucial role on trophic events of the central nervous system during development, adulthood, and ageing. The deficiency of thyroid hormones could also produce a deficiency in neurotransmission in the hippocampal region. Kainic acid (KA) has become an important tool for studying functions related to excitatory amino acid transmission in mammals. Its neurotoxic effects on the pyramidal neurons of the CA3 hippocampal region are well known. We have examined the neurotoxicity of KA on these cells in hypothyroid rats. The hypothyroid state was induced by administration of methimazole. After 4 weeks of treatment, KA was administered once intraperitoneally at doses of 0, 1, 2.5, and 5mg/kg to the hypothyroid group, and 0 and 5mg/kg to the euthyroid group. In the euthyroid group, KA reduced the neuronal density in the CA3 hippocampal region, and in the hypothyroid rats with no administration of KA, the neuronal density of the CA3 hippocampal region is reduced also. Administering KA in hypothyroid rats did not reduce the number of CA3 pyramidal cells.

Effect of gamma-hydroxybutyric acid on tissue Na+,K- ATPase levels after experimental head trauma

BACKGROUND

A failure of the Na(+),K(+)-ATPase activity (which is essential for ion flux across the cell membranes) occurs in many pathological conditions and may lead to cell dysfunction or even cell death. By altering the concentration of specific opioid peptides, gamma-hydroxybutyric acid (GHB) may change ion flux across cell membranes and produce the 'channel arrest' which we assumed will inhibit the failure of Na+,K(+)-ATPase activity and therefore lead to energy conservation and cell protection. Therefore we planned this study to see the effects of GHB at two different doses on Na(+),K(+)-ATPase activity in an experimental head trauma model.

METHODS

Forty New Zealand rabbits were divided equally into four groups: group I was the sham-operated group, group II (untreated group), group III received head trauma and intravenous (i.v.) 500 mg/kg GHB and group IV received head trauma and i.v. 50 mg/kg GHB. Head trauma was delivered by performing a craniectomy over the right hemisphere and dropping a weight of 10 g from a height of 80 cm. The non-traumatized (left) side was named as 'a' and the traumatized (right) side as 'b'. One hour after the trauma in groups II and III and craniotomy in group I, brain cortices were resected from both sides and in group I only from the right side was the tissue Na-K-ATPase activity determined.

RESULTS

The mean +/- SD of Na(+),K(+)-ATPase levels of each group are as follows: group I - 5.97 +/- 0.55; group IIa - 3.90 +/- 1.08; group IIb - 3.58 +/- 0.90; group IIIa - 5.53 +/- 0.60; group IIIb - 5.33 +/- 0.88; group IVa - 5.05 +/- 0.72; group IVb - 4.93 +/- 0.67. The Na(+),K(+)-ATPase levels of group IIa, IIb, IVa and IVb were significantly different from group S (P < 0.05). There were also significant differences between group IIa and groups IIIa and IVa; group IIb and groups IIIb and IVb (P < 0.05).

CONCLUSIONS

We conclude that GHB is effective in suppressing the decrease in Na(+),K(+)-ATPase levels in brain tissue at two different dose schedules after head trauma.

Assessment of apoptosis in epidermal lamellar cells in clinically normal horses and those with laminitis

OBJECTIVE

To determine and compare the number, type, location, and distribution of apoptotic epidermal cells in the laminae of clinically normal horses and horses with laminitis.

SAMPLE POPULATION

Formalin-fixed samples of digital lamellar tissue from 47 horses (including clinically normal horses [controls; n = 7], horses with acute [4] and chronic [7] naturally acquired laminitis, and horses with black walnut extract-induced [11] or carbohydrate overload-induced [18] laminitis).

PROCEDURE

Blocks of paraffin-embedded lamellar tissues were stained for DNA fragmentation with the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) technique. Differential immunohistochemical staining for caspases 3 and 14 were used to confirm apoptosis.

RESULTS

The number of TUNEL-positive epidermal cells per 0.1 mm of primary laminae was significantly greater in the acute laminitis group than in the other groups. In the acute laminitis group, there were 17 and 1,025 times as many TUNEL-positive basal layer cells and keratinocytes, respectively, compared with the control group. Apoptosis of TUNEL-positive basal layer cells was confirmed by results of caspase 3 immunohistochemical staining. The TUNEL-positive keratinocytes did not stain for caspases 3 or 14.

CONCLUSIONS AND CLINICAL RELEVANCE

The large number of apoptotic basal layer cells detected in the lamellar tissue of horses with acute naturally acquired laminitis suggests that apoptosis may be important in the development of acute laminitis. The role of the large number of TUNEL-positive keratinocytes detected in the interface of primary and secondary epidermal laminae of horses with acute laminitis remains to be elucidated.

Inhibition of Na+, K+-ATPase activity in rat striatum by the metabolites accumulated in Lesch-Nyhan disease

In the present study, we investigated the in vitro effect of hypoxanthine, xanthine and uric acid, metabolites accumulating in tissue of patients with Lesch-Nyhan disease, on Na(+), K(+)-ATPase activity in striatum of neonate rats. Results showed that all compounds significantly inhibited Na(+), K(+)-ATPase activity. We also studied the kinetics of the inhibition of Na(+), K(+)-ATPase activity caused by hypoxanthine. The apparent K(m) and V(max) of Na(+), K(+)-ATPase activity for ATP as the substrate and hypoxanthine as the inhibitor were 0.97 mM and 0.69 nmol inorganic phosphate (Pi) released per min per mg of protein, respectively. K(i)-value was 1.9 microM, and the inhibition was of the non-competitive type. We also observed that the inhibitory effects of hypoxanthine, xanthine and uric acid probably occur through the same mechanism, suggesting a common binding site for these oxypurines on Na(+), K(+)-ATPase. Therefore, it is conceivable that inhibition of brain Na(+), K(+)-ATPase activity may be involved at least in part in the neuronal dysfunction characteristic of patients with Lesch-Nyhan disease.

Activation of K+ channels: an essential pathway in programmed cell death

Cell apoptosis and proliferation are two counterparts in sharing the responsibility for maintaining normal tissue homeostasis. In recent years, the process of the programmed cell death has gained much interest because of its influence on malignant cell growth and other pathological states. Apoptosis is characterized by a distinct series of morphological and biochemical changes that result in cell shrinkage, DNA breakdown, and, ultimately, phagocytic death. Diverse external and internal stimuli trigger apoptosis, and enhanced K+ efflux has been shown to be an essential mediator of not only early apoptotic cell shrinkage, but also of downstream caspase activation and DNA fragmentation. The goal of this review is to discuss the role(s) played by K+ transport or flux across the plasma membrane in the regulation of the apoptotic volume decrease and apoptosis. Attention has also been paid to the role of inner mitochondrial membrane ion transport in the regulation of mitochondrial permeability and apoptosis. We provide specific examples of how deregulation of the apoptotic process contributes to pulmonary arterial medial hypertrophy, a major pathological feature in patients with pulmonary arterial hypertension. Finally, we discuss the targeting of K+ channels as a potential therapeutic tool in modulating apoptosis to maintain the balance between cell proliferation and cell death that is essential to the normal development and function of an organism.

Effects of berberine on potassium currents in acutely isolated CA1 pyramidal neurons of rat hippocampus

The effects of berberine, an isoquinoline alkaloid with antiarrhythmic action, on voltage-dependent potassium currents were studied in acutely isolated CA1 pyramidal neurons of rat hippocampus by using the whole-cell patch-clamp techniques. Berberine blocked transient outward potassium current (IA) and delayed rectifier potassium current (IK) in a concentration-dependent manner with EC50 of 22.94+/-4.96 microM and 10.86+/-1.06 microM, Emax of 67.47+/-4.00% and 67.14+/-1.79%, n of 0.77+/-0.08 and 0.96+/-0.07, respectively. Berberine 30 microM shifted the steady-state activation curve and inactivation curve of IA to more negative potentials, but mainly affected the inactivation kinetics. Berberine 30 microM positively shifted the steady-state activation curve of IK. These results suggested that blockades on K+ currents by berberine are preferential for IK, and contribute to its protective action against ischemic brain damage.

A proteomic study of resistance to deoxycholate-induced apoptosis

The development of apoptosis resistance appears to be an important factor in colon carcinogenesis. To gain an understanding of the molecular pathways altered during the development of apoptosis resistance, we selected three cell lines for resistance to induction of apoptosis by deoxycholate, an important etiologic agent in colon cancer. We then evaluated gene expression levels for 825 proteins in these resistant lines, compared with a parallel control line not subject to selection. Eighty-two proteins were identified as either over-expressed or under-expressed in at least two of the resistant lines, compared with the control. Thirty-five of the 82 proteins (43%) proved to have a known role in apoptosis. Of these 35 proteins, 21 were over-expressed and 14 were under-expressed. Of those that were over-expressed 18 of 21 (86%) are anti-apoptotic in some circumstances, of those that were under-expressed 11 of 14 (79%) are pro-apoptotic in some circumstances. This finding suggests that apoptosis resistance during selection among cultured cells, and possibly in the colon during progression to cancer, may arise by constitutive over-expression of multiple anti-apoptotic proteins and under-expression of multiple pro-apoptotic proteins. The major functional groups in which altered expression levels were found are post-translational modification (19 proteins), cell structure (cytoskeleton, microtubule, actin, etc.) (17 proteins), regulatory processes (11 proteins) and DNA repair and cell cycle checkpoint mechanisms (10 proteins). Our findings, overall, bear on mechanisms by which apoptosis resistance arises during progression to colon cancer and suggest potential targets for cancer treatment. In addition, assays of normal-appearing mucosa of colon cancer patients, for over- or under-expression of genes found to be altered in our resistant cell lines, may allow identification of early biomarkers of colon cancer risk.

Ouabain exerts biphasic effects on connexin functionality and expression in vascular smooth muscle cells

1. We have compared the effects of ouabain on the maintenance of gap junctional communication in rat aortic A7r5 smooth muscle cells, monkey COS-1 fibroblasts and human HeLa epithelial cells. 2. Ouabain (1 mM) interrupted dye coupling between confluent A7r5 cells within approximately 1 h, and high concentrations of ouabain were similarly required to reduce coupling between COS-1 cells selected to express the rat alpha1 Na+/K+-ATPase subunit, which is ouabain resistant. By contrast, low concentrations of ouabain (1-10 microM) attenuated dye transfer in wild-type COS-1 and HeLa cells, whose endogenous alpha1 subunits possess relatively high affinity for the glycoside (Ki approximately 0.3 vs approximately 100 microM) Ouabain-induced reductions in dye transfer therefore correlated with the ability of the glycoside to bind to the Na+/K+-ATPase isoenzymes expressed in these different cell lines. 3. No consistent relationship between inhibition of intercellular dye transfer and secondary changes in [Ca2+]i or pHi could be identified following incubation with ouabain. 4. In separate experiments, the effects of ouabain on real-time trafficking of connexin (Cx) protein were monitored by time-lapse microscopy of A7r5 cells transfected to express a fluorescent Cx43-green fluorescent protein (GFP) and the ability of the glycoside to modulate endogenous expression of Cx40 and Cx43 evaluated in A7r5 cells by immunochemical and Western blot analysis. 5. Ouabain (1 mM) depressed vesicular trafficking of Cx43-GFP after approximately 1 h, and caused a time-dependent loss of endogenous Cx40 and Cx43 protein that was first evident at 2 h and almost complete after 4 h. These effects of ouabain on Cx expression were reversed 90 min following washout of the glycoside. 6. We conclude that ouabain exerts biphasic effects on intercellular communication that involve an initial decrease in gap junctional permeability followed by a global reduction in the expression of Cx protein. Further studies are necessary to establish to what extent these actions of ouabain reflect inversion of the normal [Na+]i/[K+]i ratio and/or conversion of the Na+/K+-ATPase into a general signal transducer that regulates downstream protein synthesis.

Activation of voltage-sensitive sodium channels during oxygen deprivation leads to apoptotic neuronal death

Sodium (Na(+)) entry into neurons during hypoxia is known to be associated with cell death. However, it is not clear whether Na(+) entry causes cell death and by what mechanisms this increased Na(+) entry induces death. In this study we used cultures of rat neocortical neurons to show that an increase in intracellular sodium (Na(i)(+)) through voltage-sensitive sodium channels (VSSCs), during hypoxia contributes to apoptosis. Hypoxia increased Na(i)(+) and induced neuronal apoptosis, as assessed by electron microscopy, annexin V staining, and terminal UDP nick end labeling staining. Reducing Na(+) entry with the VSSC blocker, tetrodotoxin (TTX), attenuated apoptotic neuronal death via a reduction in caspase-3 activation. Since the attenuation of apoptosis by TTX during hypoxia suggested that the activation of VSSCs and Na(+) entry are crucial events in hypoxia-induced cell death, we also determined whether the activation of VSSCs per se could lead to apoptosis under resting conditions. Increasing Na(+) entry with the VSSC activator veratridine also induced neuronal apoptosis and caspase-3 activation. These data indicate that a) Na(+) entry via VSSCs during hypoxia leads to apoptotic cell death which is mediated, in part, by caspase-3 and b) activation of VSSCs during oxygen deprivation is a major event by which hypoxia induces cell death.

Inhibition of alpha-ketoglutarate dehydrogenase complex promotes cytochrome c release from mitochondria, caspase-3 activation, and necrotic cell death

Mitochondrial dysfunction has been implicated in cell death in many neurodegenerative diseases. Diminished activity of the alpha-ketoglutarate dehydrogenase complex (KGDHC), a key and arguably rate-limiting enzyme of the Krebs cycle, occurs in these disorders and may underlie decreased brain metabolism. The present studies used alpha-keto-beta-methyl-n-valeric acid (KMV), a structural analogue of alpha-ketoglutarate, to inhibit KGDHC activity to test effects of reduced KGDHC on mitochondrial function and cell death cascades in PC12 cells. KMV decreased in situ KGDHC activity by 52 +/- 7% (1 hr) or 65 +/- 4% (2 hr). Under the same conditions, KMV did not alter the mitochondrial membrane potential (MMP), as assessed with a method that detects changes as small as 5%. KMV also did not alter production of reactive oxygen species (ROS). However, KMV increased lactate dehydrogenase (LDH) release from cells by 100 +/- 4.7%, promoted translocation of mitochondrial cytochrome c to the cytosol, and activated caspase-3. Inhibition of the mitochondrial permeability transition pore (MPTP) by cyclosporin A (CsA) partially blocked this KMV-induced change in cytochrome c (-40%) and LDH (-15%) release, and prevented necrotic cell death. Thus, impairment of this key mitochondrial enzyme in PC12 cells may lead to cytochrome c release and caspase-3 activation by partial opening of the MPTP before the loss of mitochondrial membrane potentials.

Ouabain exerts biphasic effects on connexin functionality and expression in vascular smooth muscle cells

1. We have compared the effects of ouabain on the maintenance of gap junctional communication in rat aortic A7r5 smooth muscle cells, monkey COS-1 fibroblasts and human HeLa epithelial cells. 2. Ouabain (1 mM) interrupted dye coupling between confluent A7r5 cells within approximately 1 h, and high concentrations of ouabain were similarly required to reduce coupling between COS-1 cells selected to express the rat alpha1 Na+/K+-ATPase subunit, which is ouabain resistant. By contrast, low concentrations of ouabain (1-10 microM) attenuated dye transfer in wild-type COS-1 and HeLa cells, whose endogenous alpha1 subunits possess relatively high affinity for the glycoside (Ki approximately 0.3 vs approximately 100 microM) Ouabain-induced reductions in dye transfer therefore correlated with the ability of the glycoside to bind to the Na+/K+-ATPase isoenzymes expressed in these different cell lines. 3. No consistent relationship between inhibition of intercellular dye transfer and secondary changes in [Ca2+]i or pHi could be identified following incubation with ouabain. 4. In separate experiments, the effects of ouabain on real-time trafficking of connexin protein were monitored by time-lapse microscopy of A7r5 cells transfected to express a fluorescent Cx43-green fluorescent protein (GFP) and the ability of the glycoside to modulate endogenous expression of connexins (Cx) 40 and 43 evaluated in A7r5 cells by immunochemical and Western blot analysis. 5. Ouabain (1 mM) depressed vesicular trafficking of Cx43-GFP after approximately 1 h, and caused a time-dependent loss of endogenous Cx40 and Cx43 protein that was first evident at 2 h and almost complete after 4 h. These effects of ouabain on Cx expression were reversed approximately 90 min following washout of the glycoside. 6. We conclude that ouabain exerts biphasic effects on the intercellular communication that involve an initial decrease in gap junctional permeability followed by a global reduction in the expression of Cx protein. Further studies are necessary to establish to what extent these actions of ouabain reflect inversion of the normal [Na+]i/[K+]i ratio and/or conversion of the Na+/K+-ATPase into a general signal transducer that regulates downstream protein synthesis.

Investigation of the neuronal death mode induced by glutamate treatment in serum-, antioxidant-free primary cultured cortical neurons

Glutamate, the principal excitatory amino acid neurotransmitter in the central nervous system, is also a potential toxin leading to excitotoxicity when it is over-accumulated in extracellular space. Glutamate excitotoxicity has been implicated in many neurological disorders. To date, the death mode of neurons induced by glutamate remains ambiguous and is widely debated. To further clarify the neuronal death mode after glutamate treatment, we investigated this issue in mouse embryonic cortical neurons that were cultured in a serum-, antioxidant-free condition. Our study shows that exposure to low concentration of glutamate (30 microM) in a delayed time course induces neuronal death in a prevailing form of apoptosis which is accompanied by caspases activation and oligonucleosomal DNA fragmentation. Transient exposure to high concentration of glutamate (300 microM) induces the concurrence of necrosis and apoptosis in individual neurons characterized by necrotic cytoplasm-membrane features and apoptotic nuclear morphology which is not accompanied by caspases activation and oligonucleosomal DNA fragmentation.

Neuronal necrosis inhibition by insulin through protein kinase C activation

In the serum-free culture of rat embryonic neurons, most neurons rapidly died by necrosis, which was revealed by propidium iodide (PI)-positive staining as early as 3 h after the start of culture and by marked membrane disruption and mitochondrial swelling in transmission electron microscopic (TEM) analysis. However, neither nuclear condensation/fragmentation stained with Hoechst 33342 nor activated caspase-3-like immunoreactivity was observed. In the serum-deprived culture, on the other hand, neurons showed apoptotic features, such as caspase-3 activation and nuclear damages in TEM analysis. Insulin at relatively higher concentrations, up to 100 microg/ml, ameliorated the rapid decrease in survival activity measured with 2-(2-methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium, monosodium salt WST-8 assay and PI staining in the serum-free culture, despite the fact that brain-derived neurotrophic factor and insulin-like growth factor-I had no survival effect even at concentrations up to 100 microg/ml. Insulin-induced survival effects were abolished by the protein kinase C (PKC) inhibitor calphostin C but not by the phosphatidyl inositol-3-OH-kinase inhibitor wortmannin or the mitogen-activated protein kinase inhibitors PD98059 or U0126. Insulin significantly stimulated the PKC activity in cell lysates and suppressed the mitochondrial swelling and membrane disruption in TEM analysis in a calphostin C-reversible manner. All of these findings suggest that insulin inhibited the neuronal necrosis resistant to known neurotrophic factors under the serum-free culture through PKC mechanisms.

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.

Detection of early tissue injury in vivo using fluorescent cell death markers in rat dentate gyrus

Fluorescent markers of cell death offer superior selectivity and sensitivity, although their applicability in detecting tissue injury under in vivo conditions remains uncertain. Here we examined whether ethidium bromide and Hoechst 33342, two widely used markers for cell necrosis and apoptosis, can be used in vivo to detect different types of cell death induced by Na,K-ATPase inhibition. Microinfusion of fluorescent markers and ouabain was made unilaterally into adult rat dentate gyrus. It was found that, at different time points post-injury, dentate cells that were exposed to ouabain but not to vehicle control showed marked loss of membrane integrity and exhibited nuclear condensation, as revealed by ethidium bromide and Hoechst 33342 staining, respectively. However, this pattern of cell death was not associated with DNA fragmentation and formation of apoptotic bodies, suggesting involvement of atypical cell apoptosis.

Mechanisms of ouabain toxicity

The suggested involvement of ouabain in hypertension raised the need for a better understanding of its cellular action, but the mechanisms of ouabain toxicity are only now being uncovered. In the present study, we show that reduced glutathione (GSH) protected ouabain-sensitive (OS) cells from ouabain-induced toxicity and that the inhibition of GSH synthesis by D, L-buthionine-(S,R)-sulfoximine (BSO) sensitized ouabain-resistant (OR) cells. We could not observe formation of *OH or H2O2, but there was an increase in O2*-only in OS cells. Unexpectedly, an increased number of OR cells depolarized after treatment with ouabain, and BSO blocked this depolarization. Moreover, GSH increased ouabain-induced depolarization in OS cells. A sustained increase in tyrosine phosphorylation (P-Tyr) and Ras expression was observed after treatment of OS cells, and GSH prevented it. Conversely, BSO induced P-Tyr and Ras expression in ouabain-treated OR cells. The results obtained have three major implications: There is no direct correlation between membrane depolarization and ouabain-induced cell death; ouabain toxicity is not directly related to its classical action as a Na+, K+-ATPase inhibitor but seems to be associated to signal transduction, and GSH plays a major role in preventing ouabain-induced cell death.

Apoptosis in the adult striatum after transient forebrain ischemia and the effects of ischemic severity

The mechanisms of neuronal injury after cerebral ischemia have been under active investigation. The medium-size neurons in the dorsal striatum die within 24 h after transient cerebral ischemia. Using electron microscopy, the present study examined the nature of neuronal death in the striatum of adult rats following transient forebrain ischemia and tested the hypothesis that the ischemic severity might influence the nature of cell death. After severe ischemia (approximately 21 min ischemic depolarization), most neurons in the dorsal striatum died with swollen organelles and small irregular chromatin clumps resembling necrosis. The tissue damage in the dorsomedial striatum was less severe than that in the dorsolateral striatum and approximately 5% of the neurons in this region died with large chromatin clumps and relatively intact organelles resembling apoptosis. Some neurons displayed a mixture of necrotic- and apoptotic-like appearance. In contrast, the neurons with large somata only exhibited mild ultrastructural changes. After moderate ischemia (approximately 15 min ischemic depolarization), the tissue damage was less severe and the process of necrosis was temporally prolonged compared with that after severe ischemia. The apoptotic-like neuronal death was observed not only in the dorsomedial (approximately 6%) but also in the dorsolateral striatum (approximately 7%). The neurons in the striatum showed transient reversible changes after mild ischemia (approximately 10 min ischemic depolarization). The present study demonstrates that both apoptosis and necrosis occur in the adult striatum following transient forebrain ischemia and apoptosis occurs in the regions with less severe ischemia. These results suggest that ischemic severity might be one of the contributing factors to necrosis or apoptosis following transient global ischemia.

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.

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.

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.

Inhibition of Na+,K+-ATPase by ouabain triggers epithelial cell death independently of inversion of the [Na+]i/[K+]i ratio

Treatment with ouabain led to massive death of principal cells from collecting ducts (C7-MDCK), indicated by cell swelling, loss of mitochondrial function, an irregular pattern of DNA degradation, and insensitivity to pan-caspase inhibitor. Equimolar substitution of extracellular Na(+) by K(+) or choline(+) sharply attenuated the effect of ouabain on intracellular Na(+) and K(+) content but did not protect the cells from death in the presence of ouabain. In contrast to ouabain, inhibition of the Na(+)/K(+) pump in K(+)-free medium increased Na(+)(i) content but did not affect cell survival. In control and K(+)-free medium, ouabain triggered half-maximal cell death at concentrations of approximately 0.5 and 0.05 microM, respectively, which was consistent with elevation of Na(+)/K(+) pump sensitivity to ouabain in K(+)-depleted medium. Our results show for the first time that the death of ouabain-treated renal epithelial cells is independent of the inhibition of Na(+)/K(+) pump-mediated ion fluxes and the [Na(+)](i)]/[K(+)](i) ratio.

In vivo excitotoxicity induced by ouabain, a Na+/K+-ATPase inhibitor

The susceptibility of immature rat brain to neurotoxicity of N-methyl-D-aspartate (NMDA) has provided a widely used paradigm to study excitotoxicity relevant to acute neurodegenerative diseases such as cerebral ischemia. In this study, excitotoxicity was induced via injection of ouabain (1 mM/0.5 microL), a Na+/K+ -ATPase-inhibitor, into neonatal rat brain and compared with NMDA injection. The aim of the study was to induce excitotoxicity secondary to cellular membrane depolarization, thereby more closely mimicking the pathophysiologic processes of ischemia-induced brain injury where NMDA-receptor overstimulation by glutamate follows, not precedes, membrane depolarization. Na+/K+ -ATPase-inhibition caused an acute, 40% +/- 8% decrease of the apparent diffusion coefficient (ADC) of water, as measured using diffusion-weighted magnetic resonance imaging (MRI), and resulted in infarctlike lesions as measured using T2-weighted MRI and histology up to 2 weeks later. Localized one- and two-dimensional 1H-magnetic resonance spectroscopy (MRS) demonstrated that the early excitotoxic diffusion changes were not accompanied by an overall metabolic disturbance. Furthermore, 31P-MRS demonstrated that energy depletion is not a prerequisite for ADC decrease or excitotoxic cell death. Treatment with the NMDA-antagonist MK-801 (1 mg/kg) attenuated the volume of tissue exhibiting a decreased ADC (P < 0.005), demonstrating that the ouabain-induced injury is indeed excitotoxic in nature. The authors argue that, compared with NMDA-injection, ouabain-induced excitotoxicity elicits more appropriate glutamate-receptor overstimulation and is better suited to detect relevant neuroprotection in that it is more sensitive to attenuation of synaptic glutamate levels.

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.

Regulation of the frontocortical sodium pump by Na+ in Alzheimer's disease: difference from the age-matched control but similarity to the rat model

The Na+ and K+ dependence of the frontocortical Na,K-ATPase in Alzheimer's disease (AD) was compared with that in human control (Co) and rat AD model. In AD, the relationship between the Na/K ratio and the Na,K-ATPase activity showed noticeable left-shift with three-fold increase in the enzyme affinity for Na+ (K(0.5)=10 and 30 mM in AD and Co, respectively). The Na+ dependence of the enzyme in AD showed two different Hill coefficients (n(H)), 1.1 and 0.3, whereas the Co value of n(H) was higher (1.4). The rat AD model generated by ibotenic acid revealed a Na+ dependence similar to AD. The K+ dependence of the Na,K-ATPase showed no significant difference in AD and Co. Compared with Co, AD produced a shift in the break of the Na,K-ATPase Arrhenius plot, suggesting remarkable alterations in the enzyme lipid environment. Our findings support the hypothesis that dysfunction of the Na,K-ATPase in AD is provoked by altered Na+ dependence of the enzyme. An impairment of the pump functionality might serve as an early mechanism of AD that should be interrupted by selective pharmacological agents.

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.