Mediation of neuronal apoptosis by Kv2.1-encoded potassium channels

Role of potassium channels in Abeta(1-40)-activated apoptotic pathway in cultured cortical neurons

Potassium channel dysfunction has been implicated in Alzheimer's disease (AD). In the present study, by using potassium channel blocker tetraethylammonium (TEA), we investigated the relationship between the enhancement of potassium currents and the alteration of apoptotic cascade in the neuronal apoptotic model induced by beta-amyloid peptide 1-40(Abeta(1-40)). Cortical neurons exposed to Abeta(1-40) 5 muM developed a specific increase in the delayed rectifier potassium current (I(K)), but not the transient outward potassium currents (I(A)), before the appearance of neuronal apoptosis. Abeta(1-40) induced various apoptotic features such as chromatin condensation, a decrease in the amount of Bcl-2 protein, an increase in the amount of Bax protein, cytochrome c release from mitochondria, and caspase-3 activation. Potassium channel blocker 5 mM TEA attenuated Abeta(1-40)-induced neuronal death and prevented the alterations of all above mentioned apoptotic indicators. The study indicates that I(K) enhancement might play an important role in certain form of programmed cell death induced by beta-amyloid peptide (Abeta). Increased potassium channel activity might trigger the activation of apoptosis cascade in Abeta(1-40)-treated rat cortical neurons.

cAMP/protein kinase A signalling pathway protects against neuronal apoptosis and is associated with modulation of Kv2.1 in cerebellar granule cells

Previously, we have reported that apoptosis of cerebellar granular neurons induced by incubation in 5 mm K(+) and serum-free medium (LK-S) was associated with an increase in the delayed rectifier K(+) current (I(K)). Here, we show that I(K) associated with apoptotic neurons is mainly encoded by a Kv2.1 subunit. Silencing Kv2.1 expression by small interfering RNA reduces I(K) and increases neuron viability. Forskolin is able to decrease the I(K) amplitude recording from neurons of both the LK-S and control group, and prevents apoptosis of granule cells that are induced by LK-S. Dibutyryl cAMP mimicks the effect of forskolin on the modulation of I(K) and, accordingly, the inhibitor of protein kinase A, H-89, aborts the neuron-protective effect induced by forskolin. Whereas the expression of Kv2.1 was silenced by Kv2.1 small interfering RNA, the inhibition of forskolin on the current amplitude was significantly reduced. Quantitative RT-PCR and whole-cell recording revealed that the expression of Kv2.1 was elevated in the apoptotic neurons, and forskolin significantly depressed the expression of Kv2.1. We conclude that the protection against apoptosis via the protein kinase A pathway is associated with a double modulation on I(K) channel properties and its expression of alpha-subunit that is mainly encoded by the Kv2.1 gene.

Non-conducting functions of voltage-gated ion channels

Various studies, mostly in the past 5 years, have demonstrated that, in addition to their well-described function in regulating electrical excitability, voltage-dependent ion channels participate in intracellular signalling pathways. Channels can directly activate enzymes linked to cellular signalling pathways, serve as cell adhesion molecules or components of the cytoskeleton, and their activity can alter the expression of specific genes. Here, I review these findings and discuss the extent to which the molecular mechanisms of such signalling are understood.

A vital role for voltage-dependent potassium channels in dopamine transporter-mediated 6-hydroxydopamine neurotoxicity

6-Hydroxydopamine (6-OHDA), a neurotoxic substrate of the dopamine transporter (DAT), is widely used in Parkinson's disease models. However, the molecular mechanisms underlying 6-OHDA's selectivity for dopamine neurons and the injurious sequelae that it triggers are not well understood. We tested whether ectopic expression of DAT induces sensitivity to 6-OHDA in non-dopaminergic rat cortical neurons and evaluated the contribution of voltage-dependent potassium channel (Kv)-dependent apoptosis to the toxicity of this compound in rat cortical and midbrain dopamine neurons. Cortical neurons expressing DAT accumulated dopamine and were highly vulnerable to 6-OHDA. Pharmacological inhibition of DAT completely blocked this toxicity. We also observed a p38-dependent Kv current surge in DAT-expressing cortical neurons exposed to 6-OHDA, and p38 antagonists and Kv channel blockers were neuroprotective in this model. Thus, DAT-mediated uptake of 6-OHDA recruited the oxidant-induced Kv channel dependent cell death pathway present in cortical neurons. Finally, we report that 6-OHDA also increased Kv currents in cultured midbrain dopamine neurons and this toxicity was blocked with Kv channel antagonists. We conclude that native DAT expression accounts for the dopamine neuron specific toxicity of 6-OHDA. Following uptake, 6-OHDA triggers the oxidant-associated Kv channel-dependent cell death pathway that is conserved in non-dopaminergic cortical neurons and midbrain dopamine neurons.

Voltage-gated k+ channel block by catechol derivatives: defining nonselective and selective pharmacophores

High-throughput screening led to the identification of a 3-norbornyl derivative of catechol called 48F10 (3-bicyclo[2.2.1]hept-2-yl-benzene-1,2-diol) as a Kv2.1 K(+) channel inhibitor. By virtue of the involvement of Kv2.1 channels in programmed cell death, 48F10 prevents apoptosis in cortical neurons and enterocytes. This uncharged compound acts with an apparent affinity of 1 muM at the tetraethylammonium (TEA) site at the external mouth of the Kv2.1 channel but is ineffective on Kv1.5. Here we investigated the basis of this selectivity with structure-activity studies. We find that catechol (1,2-benzenediol), unlike 48F10, inhibits Kv2.1 currents with a Hill coefficient of 2 and slows channel activation. Furthermore, this inhibition, which requires millimolar concentrations, is unaffected by external TEA or by mutation of the external tyrosine implicated in channel block by TEA and 48F10. In addition, catechol does not distinguish between Kv2.1 and Kv1.5. Thus, catechol acts at conserved sites that are distinct from 48F10. We also tested 11 catechol derivatives based on hydrocarbon adducts including norbornyl substructures, a 48F10 isomer, and a 48F10 diastereomer. These compounds are more potent than catechol, but none replicated the marked selectivity of 48F10 for Kv2.1 over Kv1.5. We conclude that the targeting of 48F10 to the TEA site at the external mouth of the Kv2.1 pore and away from other sites involved in nonselective Kv channel block by catechol requires the norbornyl group in a unique position and orientation on the catechol ring.

Apoptotic surface delivery of K+ channels

Apoptosis in cortical neurons requires efflux of cytoplasmic potassium mediated by a surge in Kv2.1 channel activity. Pharmacological blockade or molecular disruption of these channels in neurons prevents apoptotic cell death, while ectopic expression of Kv2.1 channels promotes apoptosis in non-neuronal cells. Here, we use a cysteine-containing mutant of Kv2.1 and a thiol-reactive covalent inhibitor to demonstrate that the increase in K+ current during apoptosis is due to de novo insertion of functional channels into the plasma membrane. Biotinylation experiments confirmed the delivery of additional Kv2.1 protein to the cell surface following an apoptotic stimulus. Finally, expression of botulinum neurotoxins that cleave syntaxin and synaptosome-associated protein of 25 kDa (SNAP-25) blocked upregulation of surface Kv2.1 channels in cortical neurons, suggesting that target soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins support proapoptotic delivery of K+ channels. These data indicate that trafficking of Kv2.1 channels to the plasma membrane causes the apoptotic surge in K+ current.

Obligatory role of ASK1 in the apoptotic surge of K+ currents

Apoptosis signal-regulating kinase 1 (ASK1) is a critical component of mitogen-activated protein kinase signaling pathways leading to cell death in response to cytokines and cellular stress. We use a dominant-negative (DN) form of ASK1 to show that this enzyme is necessary for the delayed surge in neuronal K+ channel activity, a required step in apoptosis. Furthermore, expression of ASK1 DN also suppresses the apoptotic increase in Kv2.1 currents transiently expressed in Chinese hamster ovary cells. Finally, over-expression of thioredoxin, an inhibitory binding partner of ASK1, is sufficient to halt the apoptotic current surge in neurons. Thus, ASK1 is an obligatory component of the pro-apoptotic modulation of K+ channels.

Endogenous KCNE subunits govern Kv2.1 K+ channel activation kinetics in Xenopus oocyte studies

Kv2.1 is a voltage-gated potassium (Kv) channel that generates delayed rectifier currents in mammalian heart and brain. The biophysical properties of Kv2.1 and other ion channels have been characterized by functional expression in heterologous systems, and most commonly in Xenopus laevis oocytes. A number of previous oocyte-based studies of mammalian potassium channels have revealed expression-level-dependent changes in channel properties, leading to the suggestion that endogenous oocyte factors regulate channel gating. Here, we show that endogenous oocyte potassium channel KCNE ancillary subunits xMinK and xMiRP2 slow the activation of oocyte-expressed mammalian Kv2.1 channels two-to-fourfold. This produces a sigmoidal relationship between Kv2.1 current density and activation rate in oocyte-based two-electrode voltage clamp studies. The effect of endogenous xMiRP2 and xMinK on Kv2.1 activation is diluted at high Kv2.1 expression levels, or by RNAi knockdown of either endogenous subunit. RNAi knockdown of both xMiRP2 and xMinK eliminates the correlation between Kv2.1 expression level and activation kinetics. The data demonstrate a molecular basis for expression-level-dependent changes in Kv channel gating observed in heterologous expression studies.

Attenuation of apoptosis in enterocytes by blockade of potassium channels

Apoptosis plays an important role in maintaining the balance between proliferation and cell loss in the intestinal epithelium. Apoptosis rates may increase in intestinal pathologies such as inflammatory bowel disease and necrotizing enterocolitis, suggesting pharmacological prevention of apoptosis as a therapy for these conditions. Here, we explore the feasibility of this approach using the rat epithelial cell line IEC-6 as a model. On the basis of the known role of K+ efflux in apoptosis in various cell types, we hypothesized that K+ efflux is essential for apoptosis in enterocytes and that pharmacological blockade of this efflux would inhibit apoptosis. By probing intracellular [K+] with the K+-sensitive fluorescent dye and measuring the efflux of 86Rb+, we found that apoptosis-inducing treatment with the proteasome inhibitor MG-132 leads to a twofold increase in K+ efflux from IEC-6 cells. Blockade of K+ efflux with tetraethylammonium, 4-aminopyridine, stromatoxin, chromanol 293B, and the recently described K+ channel inhibitor 48F10 prevents DNA fragmentation, caspase activation, release of cytochrome c from mitochondria, and loss of mitochondrial membrane potential. Thus K+ efflux occurs early in the apoptotic program and is required for the execution of later events. Apoptotic K+ efflux critically depends on activation of p38 MAPK. These results demonstrate for the first time the requirement of K+ channel-mediated K+ efflux for progression of apoptosis in enterocytes and suggest the use of K+ channel blockers to prevent apoptotic cell loss occurring in intestinal pathologies.

Dopamine modulation of two delayed rectifier potassium currents in a small neural network

Delayed rectifier potassium currents [I(K(V))] generate sustained, noninactivating outward currents with characteristic fast rates of activation and deactivation and play important roles in shaping spike frequency. The pyloric motor network in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus, is made up of one interneuron and 13 motor neurons of five different classes. Dopamine (DA) increases the firing frequencies of the anterior burster (AB), pyloric (PY), lateral pyloric (LP), and inferior cardiac (IC) neurons and decreases the firing frequencies of the pyloric dilator (PD) and ventricular dilator (VD) neurons. In all six types of pyloric neurons, I(K(V)) is small with respect to other K(+) currents. It is made up of at least two TEA-sensitive components that show differential sensitivity to 4-aminopyridine and quinidine, and have differing thresholds of activation. One saturable component is activated at potentials above -25 mV, whereas the second component appears at more depolarized voltages and does not saturate at voltage steps up to +45 mV. The magnitude of the components varies among cell types but also shows considerable variation within a single type. A subset of PY neurons shows a marked enhancement in spike frequency with DA; DA evokes a pronounced reversible increase in I(K(V)) conductance of < or = 30% in the PY neurons studied, and on average significantly increases both components of I(K(V)). The AB neuron also shows a reversible 20% increase in the steady state I(K(V)). DA had no effect on I(K(V)) in PD, LP, VD, and IC neurons. The physiological roles of these currents and their modulation by DA are discussed.

Protective effects of TASK-3 (KCNK9) and related 2P K channels during cellular stress

Tandem pore domain (or 2P) K channels form a recently isolated family of channels that are responsible for background K currents in excitable tissues. Previous studies have indicated that 2P K channel activity produces membrane hyperpolarization, which may offer protection from cellular insults. To study the effect of these channels in neuroprotection, we overexpressed pH-sensitive 2P K channels by transfecting the partially transformed C8 cell line with these channels. Tandem pore weak inward rectifier K channel (TWIK)-related acid-sensitive K channel 3 (TASK-3, KCNK9) as well as other pH sensitive 2P K channels (TASK-1 and TASK-2) enhanced cell viability by inhibiting the activation of intracellular apoptosis pathways. To explore the cellular basis for this protection in a more complex cellular environment, we infected cultured hippocampal slices with Sindbis virus constructs containing the coding sequences of these channels. Expression of TASK-3 throughout the hippocampal structure afforded neurons within the dentate and CA1 regions significant protection from an oxygen-glucose deprivation (OGD) injury. Neuroprotection within TASK-3 expressing slices was also enhanced by incubation with isoflurane. These results confirm a protective physiologic capability of TASK-3 and related 2P K channels, and suggest agents that enhance their activity, such as volatile anesthetics may intensify these protective effects.

Zinc accumulation after target loss: an early event in retrograde degeneration of thalamic neurons

Accumulation of cytoplasmic zinc is linked with a cascade of events leading to neuronal death. In many in vivo models of zinc-induced cell death, toxic concentrations of synaptically released zinc enter vulnerable neurons via neurotransmitter- or voltage-gated ion channels. In vitro studies demonstrate, in addition, that zinc can be liberated from intracellular stores following oxidative stress and contribute to cell death processes, including apoptosis. Here we describe accumulation of intracellular zinc in an in vivo model of cell death in the absence of presynaptic zinc release. We focused on the lateral geniculate nucleus (LGN) because LGN neurons undergo apoptosis when separated from their target, the primary visual cortex (V1), and the LGN is mostly devoid of zinc-containing presynaptic terminals. Infant and adult rats and adult mice received unilateral ablation of V1, either by aspiration or kainate injection. One to 14 days later, brain sections were stained with selenium autometallography or fluorescently labeled to localize zinc, or stained immunochemically for activated caspase-3. V1 lesions led to zinc accumulation in LGN neurons in infant and adult subjects. Zinc-containing neurons were evident 1-3 days after aspiration lesions, depending on age, but not until 14 days after kainate injection. Zinc accumulation was followed rapidly by immunostaining for activated caspase-3. Our data indicate that like neurotrauma and excitotoxicity, target deprivation leads to accumulation of zinc in apoptotic neurons. Moreover, zinc accumulation in vivo can occur in the absence of presynaptic zinc release. Together these findings suggest that accumulation of intracellular zinc is a ubiquitous component of the cell death cascade in neurons.

Kv2.1: a voltage-gated k+ channel critical to dynamic control of neuronal excitability

Neurons use a variety of mechanisms to dynamically control their own signaling capabilities. Regulation of voltage-dependent K+ channel localization and function has long been recognized as a major mechanism to achieve dynamic regulation of intrinsic neuronal excitability in a number of mammalian and non-mammalian neurons. Our recent evidence, together with compelling data from other laboratories, suggests that in mammalian neurons the Kv2.1 channel may play an especially prominent role in determining intrinsic neuronal excitability. Kv2.1 is widely expressed in brain and composes the majority of delayed rectifier K+ current in pyramidal neurons in cortex and hippocampus, and is also widely expressed in interneurons. Dynamic modulation of Kv2.1 localization and function by a mechanism involving activity-dependent Kv2.1 dephosphorylation dramatically impacts intrinsic excitability of neurons. Here we review previous studies of Kv2.1 localization and function in neurons, and summarize recent work regarding dynamic regulation of these characteristics. We also discuss possible roles of the Kv2.1 channel in neuronal and network excitability.

KCC2 expression in immature rat cortical neurons is sufficient to switch the polarity of GABA responses

During brain development, GABA and glycine switch from being depolarizing to being hyperpolarizing neurotransmitters. This conversion results from a gradual decrease in the chloride electrochemical equilibrium potential (ECl) of developing neurons, which correlates to an increase in the expression or activity of the potassium chloride cotransporter, KCC2. However, evidence as to whether KCC2 expression is sufficient, in and of itself, to induce this switch is lacking. In order to address this question, we used a gain-of-function approach by over-expressing human KCC2 (hKCC2) in immature cortical neurons, before endogenous up-regulation of KCC2. We found that premature expression of hKCC2 produced a substantial negative shift in the GABA reversal potential and decreased or abolished GABA-elicited calcium responses in cultured neurons. We conclude that KCC2 expression is not only necessary but is also sufficient for ending the depolarizing period of GABA in developing cortical neurons.

Increase of delayed rectifier potassium currents in large aspiny neurons in the neostriatum following transient forebrain ischemia

Large aspiny (LA) neurons in the neostriatum are resistant to cerebral ischemia whereas spiny neurons are highly vulnerable to the same insult. Excitotoxicity has been implicated as the major cause of neuronal damage after ischemia. Voltage-dependent potassium currents play important roles in controlling neuronal excitability and therefore influence the ischemic outcome. To reveal the ionic mechanisms underlying the ischemia-resistance, the delayed rectifier potassium currents (Ik) in LA neurons were studied before and at different intervals after transient forebrain ischemia using brain slices and acute dissociation preparations. The current density of Ik increased significantly 24 h after ischemia and returned to control levels 72 h following reperfusion. Among currents contributing to Ik, the margatoxin-sensitive currents increased 24 h after ischemia while the KCNQ/M current remained unchanged after ischemia. Activation of protein kinase A (PKA) down-regulated Ik in both control and ischemic LA neurons, whereas inhibition of PKA only up-regulated Ik and margatoxin-sensitive currents 72 h after ischemia, indicating an active PKA regulation on Ik at this time. Protein tyrosine kinases had a tonic inhibition on Ik to a similar extent before and after ischemia. Compared with that of control neurons, the spike width was significantly shortened 24 h after ischemia due to facilitated repolarization, which could be reversed by blocking margatoxin-sensitive currents. The increase of Ik in LA neurons might be one of the protective mechanisms against ischemic insult.

Characterization of chromanol 293B-induced block of the delayed-rectifier K+ current in heart-derived H9c2 cells

The effects of chromanol 293B on ion currents in rat embryonic heart-derived H9c2 cells were investigated in this study. Chromanol 293B suppressed the amplitude of delayed rectified K+ current (I(K)) in a concentration-dependent manner. The IC50 value for chromanol 293B-induced inhibition of I(K) was 8 microM. The I(K) present in these cells, the electrical properties of which resembled those for the Kv2.1-related K+ current, was sensitive to inhibition by quinidine or dendrotoxin, yet not by pandinotoxin-Kalpha, E-4031 or apamin. Chromanol 293B reduced the activation time constant of I(K) and the effective gating charge of this channel. However, little or no modification in the steady-state inactivation of I(K) in response to long-lasting conditioning pulses could be demonstrated in the presence of chromanol 293B. These results clearly demonstrate that chromanol 293B can effectively interact with the K+ channel functionally expressed in H9c2 myoblasts. The chromanol 293B-induced inhibition of these channels could primarily be attributed to open channel block.

Galantamine blocks cloned Kv2.1, but not Kv1.5 potassium channels

Galantamine is a cholinesterase inhibitor (AChEI) currently used in treatment of Alzheimer's disease (AD). In the present study, the effects of galantamine on currents of cloned Kv2.1 and Kv1.5 potassium channels were investigated by using patch-clamp whole cell recording techniques. Kv2.1 and Kv1.5 were stably expressed in HEK293 cells. Galantamine blocked Kv2.1 current in a concentration-dependent manner. When depolarizing from -50 to +40 mV, the IC50 of galantamine for inhibition of Kv2.1 was 5.6 microM. Galantamine 10 microM shifted the activation curve of Kv2.1 to negative potential by 4.0 mV. At the same concentration, galantamine shifted the inactivation curve to negative potential by 25.2 mV. While Kv1.5 was not sensitive to galantamine, Kv1.5 current was not changed by galantamine at concentration of 10 microM. Our data suggest that galantamine potently blocks Kv2.1, but not Kv1.5 channels.

Differential regulation of voltage-gated K+ channels by oxidized and reduced pyridine nucleotide coenzymes

The activity of the voltage-sensitive K+ (Kv) channels varies as a function of the intracellular redox state and metabolism, and several Kv channels act as oxygen sensors. However, the mechanisms underlying the metabolic and redox regulation of these channels remain unclear. In this study we investigated the regulation of Kv channels by pyridine nucleotides. Heterologous expression of Kvalpha1.5 in COS-7 cells led to the appearance of noninactivating currents. Inclusion of 0.1-1 mM NAD+ or 0.03-0.5 mM NADP+ in the internal solution of the patch pipette did not affect Kv currents. However, 0.5 and 1 mM NAD+ and 0.1 and 0.5 mM NADP+ prevented inactivation of Kv currents in cells transfected with Kvalpha1.5 and Kvbeta1.3 and shifted the voltage dependence of activation to depolarized potentials. The Kvbeta-dependent inactivation of Kvalpha currents was also decreased by internal pipette perfusion of the cell with 1 mM NAD+. The Kvalpha1.5-Kvbeta1.3 currents were unaffected by the internal application of 0.1 mM NADPH or 0.1 or 1 mM NADH. Excised inside-out patches from cells expressing Kvalpha1.5-Kvbeta1.3 showed transient single-channel activity. The mean open time and the open probability of these currents were increased by the inclusion of 1 mM NAD+ in the perfusate. These results suggest that NAD(P)+ prevents Kvbeta-mediated inactivation of Kv currents and provide a novel mechanism by which pyridine nucleotides could regulate specific K+ currents as a function of the cellular redox state [NAD(P)H-to-NAD(P)+ ratio].

Dichloroacetate prevents and reverses pulmonary hypertension by inducing pulmonary artery smooth muscle cell apoptosis

The pulmonary arteries (PA) in pulmonary arterial hypertension (PAH) are constricted and remodeled;. They have suppressed apoptosis, partly attributable to suppression of the bone morphogenetic protein axis and selective downregulation of PA smooth muscle cell (PASMC) voltage-gated K+ channels, including Kv1.5. The Kv downregulation-induced increase in [K+]i, tonically inhibits caspases, further suppressing apoptosis. Mitochondria control apoptosis and produce activated oxygen species like H2O2, which regulate vascular tone by activating K+ channels, but their role in PAH is unknown. We show that dichloroacetate (DCA), a metabolic modulator that increases mitochondrial oxidative phosphorylation, prevents and reverses established monocrotaline-induced PAH (MCT-PAH), significantly improving mortality. Compared with MCT-PAH, DCA-treated rats (80 mg/kg per day in drinking water on day 14 after MCT, studied on day 21) have decreased pulmonary, but not systemic, vascular resistance (63% decrease, P<0.002), PA medial thickness (28% decrease, P<0.0001), and right ventricular hypertrophy (34% decrease, P<0.001). DCA is similarly effective when given at day 1 or day 21 after MCT (studied day 28) but has no effect on normal rats. DCA depolarizes MCT-PAH PASMC mitochondria and causes release of H2O2 and cytochrome c, inducing a 10-fold increase in apoptosis within the PA media (TUNEL and caspase 3 activity) and decreasing proliferation (proliferating-cell nuclear antigen and BrdU assays). Immunoblots, immunohistochemistry, laser-captured microdissection-quantitative reverse-transcription polymerase chain reaction and patch-clamping show that DCA reverses the Kv1.5 downregulation in resistance PAs. In summary, DCA reverses PA remodeling by increasing the mitochondria-dependent apoptosis/proliferation ratio and upregulating Kv1.5 in the media. We identify mitochondria-dependent apoptosis as a potential target for therapy and DCA as an effective and selective treatment for PAH.

A molecular technique for detecting the liberation of intracellular zinc in cultured neurons

We have previously reported that oxidative stimuli liberate Zn(2+) from metalloproteins, a phenomenon that can trigger neuronal cell death. Excessive intracellular Zn(2+) in many cell types triggers the expression of genes that encode metal binding proteins, such as metallothionein, via the activation and nuclear translocation of metal response element (MRE)-binding transcription factor-1 (MTF-1). Cd(2+) strongly induces nuclear translocation of MTF-1 in non-neuronal cells, but it does so by displacing Zn(2+) from its metal binding sites within the cell and increasing the intracellular concentration of this ion. Here, we describe the use of MRE-driven expression of a luciferase reporter gene as a sensitive molecular assay for detecting increases in intracellular zinc concentrations. MRE transactivation was induced in primary cortical neurons upon brief exposure to Zn(2+) or Cd(2+). Enhanced MRE transactivation was observed upon co-exposure of neurons to Cd(2+) together with NMDA, as this metal can permeate through the receptor channel. Luciferase expression was observed regardless of whether or not neurons had been co-transfected with an MTF-1-containing plasmid, suggesting the presence of an endogenous MTF-1-like protein. Indeed, RT-PCR revealed that MTF-1 mRNA is present in neurons. In contrast, MTF-1 deficient dko7 cells were only observed to have MRE transactivation when co-transfected with MTF-1. Our results indicate that Cd(2+) can effectively induce transactivation of MRE in neurons by liberating Zn(2+) from its intracellular binding sites.

Regulation of ion channel localization and phosphorylation by neuronal activity

Voltage-dependent Kv2.1 K(+) channels, which mediate delayed rectifier Kv currents (I(K)), are expressed in large clusters on the somata and dendrites of principal pyramidal neurons, where they regulate neuronal excitability. Here we report activity-dependent changes in the localization and biophysical properties of Kv2.1. In the kainate model of continuous seizures in rat, we find a loss of Kv2.1 clustering in pyramidal neurons in vivo. Biochemical analysis of Kv2.1 in the brains of these rats shows a marked dephosphorylation of Kv2.1. In cultured rat hippocampal pyramidal neurons, glutamate stimulation rapidly causes dephosphorylation of Kv2.1, translocation of Kv2.1 from clusters to a more uniform localization, and a shift in the voltage-dependent activation of I(K). An influx of Ca(2+) leading to calcineurin activation is both necessary and sufficient for these effects. Our finding that neuronal activity modifies the phosphorylation state, localization and function of Kv2.1 suggests an important link between excitatory neurotransmission and the intrinsic excitability of pyramidal neurons.

Crosstalk between nitric oxide and zinc pathways to neuronal cell death involving mitochondrial dysfunction and p38-activated K+ channels

Nitric oxide (NO) and zinc (Zn2+) are implicated in the pathogenesis of cerebral ischemia and neurodegenerative diseases. However, their relationship and the molecular mechanism of their neurotoxic effects remain unclear. Here we show that addition of exogenous NO or NMDA (to increase endogenous NO) leads to peroxynitrite (ONOO-) formation and consequent Zn2+ release from intracellular stores in cerebrocortical neurons. Free Zn2+ in turn induces respiratory block, mitochondrial permeability transition (mPT), cytochrome c release, generation of reactive oxygen species (ROS), and p38 MAP kinase activation. This pathway leads to caspase-independent K+ efflux with cell volume loss and apoptotic-like death. Moreover, Zn2+ chelators, ROS scavengers, Bcl-xL, dominant-interfering p38, or K+ channel blockers all attenuate NO-induced K+ efflux, cell volume loss, and neuronal apoptosis. Thus, these data establish a new form of crosstalk between NO and Zn2+ apoptotic signal transduction pathways that may contribute to neurodegeneration.

Novel Neuroprotective K+Channel Inhibitor Identified by High-Throughput Screening in Yeast

Discovery of K+ channel modulators is limited by low-throughput capacity of existing K+ channel assays. To enable high-throughput screening for novel pharmacological modulators of K+ channels, we developed an assay based on growth of yeast that functionally expresses mammalian Kir2.1 channels. Screening of 10,000 small molecules from a combinatorial chemical library yielded 42 potential Kir2.1 inhibitors. One compound, 3-bicyclo[2.2.1]hept-2-yl-benzene-1,2-diol, was confirmed to inhibit K+ channels in patch-clamp measurements in mammalian cells with EC50 values of 60 and 1 microM for Kir2.1 and Kv2.1 channels, respectively. Inhibition of Kv2.1 channels decreased in the presence of the external pore blocker tetraethylammonium (TEA) and depended on a residue required for extracellular TEA action, suggesting that the identified compound targets the external mouth of the channel. Furthermore, at the nontoxic concentration of 3 microM, the identified compound completely abolished in vitro neuronal apoptosis mediated by Kv2.1 channels. Therefore, yeast-based screening has identified a novel uncharged neuroprotective mammalian K+ channel inhibitor.

Messenger RNA and protein expression analysis of voltage-gated potassium channels in the brain of A?25-35-treated rats

Potassium channel dysfunction has been implicated in Alzheimer's disease. In the present study, the expression of voltage-gated potassium channel (Kv) subunits in rat brain was measured after a single intracerebroventricular injection of beta-amyloid peptide 25-35 (Abeta(25-35)). After injection of Abeta, the spatial memory of rats was significantly impaired in the Morris water maze. Expression of five main Kv channel subunits (Kv1.5, Kv2.1, Kv1.4, Kv4.2, and Kv4.3) in mRNA level was assessed by using reverse transcription-polymerase chain reaction (RT-PCR). The mRNA levels of Kv2.1 and Kv1.4 were increased by 72% and 67%, respectively, in hippocampus, and Kv4.2 mRNA was increased by 58% in cortex. No other significant mRNA expression changes were found in Abeta-treated rats. The protein expression of Kv2.1, Kv1.4, and Kv4.2 was detected by using Western blotting. Kv2.1 and Kv1.4 protein levels were increased by 48% and 50%, respectively, in hippocampus of Abeta-treated rats, and Kv4.2 protein was increased by 42% in cerebral cortex. This study indicates that the expression up-regulation of Kv1.4, Kv2.1, and Kv4.2 in Abeta-induced cognitive impairment might play an important role in the pathogenesis of Alzheimer's disease.