Apoptotic surface delivery of K+ channels

Non-conducting functions of potassium channels in cancer and neurological disease

Cancer and neurodegenerative disease, albeit fundamental differences, share some common pathogenic mechanisms. Accordingly, both conditions are associated with aberrant cell proliferation and migration. Here, we review the causative role played by potassium (K+) channels, a fundamental class of proteins, in cancer and neurodegenerative disease. The concept that emerges from the review of the literature is that K+ channels can promote the development and progression of cancerous and neurodegenerative pathologies by dysregulating cell proliferation and migration. K+ channels appear to control these cellular functions in ways that not necessarily depend on their conducting properties and that involve the ability to directly or indirectly engage growth and survival signaling pathways. As cancer and neurodegenerative disease represent global health concerns, identifying commonalities may help understand the molecular basis for those devastating conditions and may facilitate the design of new drugs or the repurposing of existing drugs.

Antagonistic roles of Ras-MAPK and Akt signaling in integrin-K+ channel complex-mediated cellular apoptosis

Complexes formed with α5-integrins and the voltage-gated potassium (K⁺ ) channel KCNB1 (Kv2.1), known as IKCs, transduce the electrical activity at the plasma membrane into biochemical events that impinge on cytoskeletal remodeling, cell differentiation, and migration. However, when cells are subject to stress of oxidative nature IKCs turn toxic and cause inflammation and death. Here, biochemical, pharmacological, and cell viability evidence demonstrates that in response to oxidative insults, IKCs activate an apoptotic Mitogen-activated protein kinase/extracellular signal-regulated kinase (Ras-MAPK) signaling pathway. Simultaneously, wild-type (WT) KCNB1 channels sequester protein kinase B (Akt) causing dephosphorylation of BCL2-associated agonist of cell death (BAD), a major sentinel of apoptosis progression. In contrast, IKCs formed with C73A KCNB1 variant that does not induce apoptosis (IKC_C73A ), do not sequester Akt and thus are able to engage cell survival mechanisms. Taken together, these data suggest that apoptotic and survival forces co-exist in IKCs. Integrins send death signals through Ras-MAPK and KCNB1 channels simultaneously sabotage survival mechanisms. Thus, the combined action of integrins and KCNB1 channels advances life or death.

Voltage-Gated Potassium Channels as Regulators of Cell Death

Ion channels allow the flux of specific ions across biological membranes, thereby determining ion homeostasis within the cells. Voltage-gated potassium-selective ion channels crucially contribute to the setting of the plasma membrane potential, to volume regulation and to the physiologically relevant modulation of intracellular potassium concentration. In turn, these factors affect cell cycle progression, proliferation and apoptosis. The present review summarizes our current knowledge about the involvement of various voltage-gated channels of the Kv family in the above processes and discusses the possibility of their pharmacological targeting in the context of cancer with special emphasis on Kv1.1, Kv1.3, Kv1.5, Kv2.1, Kv10.1, and Kv11.1.

Ions, the Movement of Water and the Apoptotic Volume Decrease

The movement of water across the cell membrane is a natural biological process that occurs during growth, cell division, and cell death. Many cells are known to regulate changes in their cell volume through inherent compensatory regulatory mechanisms. Cells can sense an increase or decrease in their cell volume, and compensate through mechanisms known as a regulatory volume increase (RVI) or decrease (RVD) response, respectively. The transport of sodium, potassium along with other ions and osmolytes allows the movement of water in and out of the cell. These compensatory volume regulatory mechanisms maintain a cell at near constant volume. A hallmark of the physiological cell death process known as apoptosis is the loss of cell volume or cell shrinkage. This loss of cell volume is in stark contrast to what occurs during the accidental cell death process known as necrosis. During necrosis, cells swell or gain water, eventually resulting in cell lysis. Thus, whether a cell gains or loses water after injury is a defining feature of the specific mode of cell death. Cell shrinkage or the loss of cell volume during apoptosis has been termed apoptotic volume decrease or AVD. Over the years, this distinguishing feature of apoptosis has been largely ignored and thought to be a passive occurrence or simply a consequence of the cell death process. However, studies on AVD have defined an underlying movement of ions that result in not only the loss of cell volume, but also the activation and execution of the apoptotic process. This review explores the role ions play in controlling not only the movement of water, but the regulation of apoptosis. We will focus on what is known about specific ion channels and transporters identified to be involved in AVD, and how the movement of ions and water change the intracellular environment leading to stages of cell shrinkage and associated apoptotic characteristics. Finally, we will discuss these concepts as they apply to different cell types such as neurons, cardiomyocytes, and corneal epithelial cells.

Lessons from Recent Advances in Ischemic Stroke Management and Targeting Kv2.1 for Neuroprotection

Achieving neuroprotection in ischemic stroke patients has been a multidecade medical challenge. Numerous clinical trials were discontinued in futility and many were terminated in response to deleterious treatment effects. Recently, however, several positive reports have generated the much-needed excitement surrounding stroke therapy. In this review, we describe the clinical studies that significantly expanded the time window of eligibility for patients to receive mechanical endovascular thrombectomy. We further summarize the results available thus far for nerinetide, a promising neuroprotective agent for stroke treatment. Lastly, we reflect upon aspects of these impactful trials in our own studies targeting the Kv2.1-mediated cell death pathway in neurons for neuroprotection. We argue that recent changes in the clinical landscape should be adapted by preclinical research in order to continue progressing toward the development of efficacious neuroprotective therapies for ischemic stroke.

The Redox Biology of Excitotoxic Processes: The NMDA Receptor, TOPA Quinone, and the Oxidative Liberation of Intracellular Zinc

This special issue of Frontiers in Neuroscience-Neurodegeneration celebrates the 50th anniversary of John Olney's seminal work introducing the concept of excitotoxicity as a mechanism for neuronal cell death. Since that time, fundamental research on the pathophysiological activation of glutamate receptors has played a central role in our understanding of excitotoxic cellular signaling pathways, leading to the discovery of many potential therapeutic targets in the treatment of acute or chronic/progressive neurodegenerative disorders. Importantly, excitotoxic signaling processes have been found repeatedly to be closely intertwined with oxidative cellular cascades. With this in mind, this review looks back at long-standing collaborative efforts by the authors linking cellular redox status and glutamate neurotoxicity, focusing first on the discovery of the redox modulatory site of the N-methyl-D-aspartate (NMDA) receptor, followed by the study of the oxidative conversion of 3,4-dihydroxyphenylalanine (DOPA) to the non-NMDA receptor agonist and neurotoxin 2,4,5-trihydroxyphenylalanine (TOPA) quinone. Finally, we summarize our work linking oxidative injury to the liberation of zinc from intracellular metal binding proteins, leading to the uncovering of a signaling mechanism connecting excitotoxicity with zinc-activated cell death-signaling cascades.

Targeted disruption of Kv2.1-VAPA association provides neuroprotection against ischemic stroke in mice by declustering Kv2.1 channels

Kv2.1 channels mediate cell death-enabling loss of cytosolic potassium in neurons following plasma membrane insertion at somatodendritic clusters. Overexpression of the carboxyl terminus (CT) of the cognate channel Kv2.2 is neuroprotective by disrupting Kv2.1 surface clusters. Here, we define a seven-amino acid declustering domain within Kv2.2 CT (DP-2) and demonstrate its neuroprotective efficacy in a murine ischemia-reperfusion model. TAT-DP-2, a membrane-permeable derivative, induces Kv2.1 surface cluster dispersal, prevents post-injurious pro-apoptotic potassium current enhancement, and is neuroprotective in vitro by disrupting the association of Kv2.1 with VAPA. TAT-DP-2 also induces Kv2.1 cluster dispersal in vivo in mice, reducing infarct size and improving long-term neurological function following stroke. We suggest that TAT-DP-2 induces Kv2.1 declustering by disrupting Kv2.1-VAPA association and scaffolding sites required for the membrane insertion of Kv2.1 channels following injury. We present the first evidence of targeted disruption of Kv2.1-VAPA association as a neuroprotective strategy following brain ischemia.

The versatile Kv channels in the nervous system: actions beyond action potentials

Voltage-gated K+ (Kv) channel opening repolarizes excitable cells by allowing K+ efflux. Over the last two decades, multiple Kv functions in the nervous system have been found to be unrelated to or beyond the immediate control of excitability, such as shaping action potential contours or regulation of inter-spike frequency. These functions include neuronal exocytosis and neurite formation, neuronal cell death, regulation of astrocyte Ca2+, glial cell and glioma proliferation. Some of these functions have been shown to be independent of K+ conduction, that is, they suggest the non-canonical functions of Kv channels. In this review, we focus on neuronal or glial plasmalemmal Kv channel functions which are unrelated to shaping action potentials or immediate control of excitability. Similar functions in other cell types will be discussed to some extent in appropriate contexts.

Effects of curcumin complexes on MDA‑MB‑231 breast cancer cell proliferation

Curcumin displays anticancer properties; however, some issues with the drug delivery mode limit its therapeutic use. Although reformulation and derivatization of curcumin have improved its bioavailability, curcumin derivatives may not retain the same anticancer properties as the parent compound. The present study investigated the anticancer properties of two curcumin complexes, the iron‑curcumin [Fe(Cur)3] and boron‑curcumin [B(Cur)2] complexes, in the MDA‑MB‑231 breast cancer cell line. The cellular localization of curcumin, B(Cur)2 and Fe(Cur)3 was determined by fluorescence microscopy. Cell proliferation, migration and invasion were also analysed. Furthermore, apoptosis‑associated proteins were detected by using a proteome profiler array, and ion channel gene expression was analysed by reverse transcription‑quantitative PCR. The results demonstrated that the three compounds were localized in the perinuclear and cytoplasmic regions of the cell, and displayed cytotoxicity with IC50 values of 25, 35 and 8 µM for curcumin, B(Cur)2 and Fe(Cur)3, respectively. In addition, the three compounds inhibited cell invasion, whereas only curcumin and B(Cur)2 inhibited cell migration. Furthermore, cell exposure to curcumin resulted in an increase in the relative expression of the two key proapoptotic proteins, cytochrome c and cleaved caspase‑3, as well as the antiapoptotic protein haem oxygenase‑1. In addition, curcumin increased the expression levels of the voltage‑gated potassium channels Kv2.1 and Kv3.2. Similarly, the expression levels of the chloride channel bestrophin‑1 and the calcium channel coding gene calcium voltage‑gated channel auxiliary subunit γ4 were increased following exposure to curcumin. Taken together, these results indicated that Fe(Cur)3 and B(Cur)2 may display similar anticancer properties as curcumin, suggesting that chemical complexation may be considered as a strategy for improving the potency of curcumin in the treatment of breast cancer.

Kv2 channels create endoplasmic reticulum / plasma membrane junctions: a brief history of Kv2 channel subcellular localization

The potassium channels Kv2.1 and Kv2.2 are widely expressed throughout the mammalian brain. Kv2.1 provides the majority of delayed rectifying current in rat hippocampus while both channels are differentially expressed in cortex. Particularly unusual is their neuronal surface localization pattern: while half the channel population is freely-diffusive on the plasma membrane as expected from the generalized Singer & Nicolson fluid mosaic model, the other half localizes into micron-sized clusters on the soma, dendrites, and axon initial segment. These clusters contain hundreds of channels, which for Kv2.1, are largely non-conducting. Competing theories of the mechanism underlying Kv2.1 clustering have included static tethering to being corralled by an actin fence. Now, recent work has demonstrated channel clustering is due to formation of endoplasmic reticulum/plasma membrane (ER/PM) junctions through interaction with ER-resident VAMP-associated proteins (VAPs). Interaction between surface Kv2 channels and ER VAPs groups channels together in clusters. ER/PM junctions play important roles in inter-organelle communication: they regulate ion flux, are involved in lipid transfer, and are sites of endo- and exocytosis. Kv2-induced ER/PM junctions are regulated through phosphorylation of the channel C-terminus which in turn regulates VAP binding, providing a rapid means to create or dismantle these microdomains. In addition, insults such as hypoxia or ischemia disrupt this interaction resulting in ER/PM junction disassembly. Kv2 channels are the only known plasma membrane protein to form regulated, injury sensitive junctions in this manner. Furthermore, it is likely that concentrated VAPs at these microdomains sequester additional interactors whose functions are not yet fully understood.

Defining the Kv2.1-syntaxin molecular interaction identifies a first-in-class small molecule neuroprotectant

The neuronal cell death-promoting loss of cytoplasmic K+ following injury is mediated by an increase in Kv2.1 potassium channels in the plasma membrane. This phenomenon relies on Kv2.1 binding to syntaxin 1A via 9 amino acids within the channel intrinsically disordered C terminus. Preventing this interaction with a cell and blood-brain barrier-permeant peptide is neuroprotective in an in vivo stroke model. Here a rational approach was applied to define the key molecular interactions between syntaxin and Kv2.1, some of which are shared with mammalian uncoordinated-18 (munc18). Armed with this information, we found a small molecule Kv2.1-syntaxin-binding inhibitor (cpd5) that improves cortical neuron survival by suppressing SNARE-dependent enhancement of Kv2.1-mediated currents following excitotoxic injury. We validated that cpd5 selectively displaces Kv2.1-syntaxin-binding peptides from syntaxin and, at higher concentrations, munc18, but without affecting either synaptic or neuronal intrinsic properties in brain tissue slices at neuroprotective concentrations. Collectively, our findings provide insight into the role of syntaxin in neuronal cell death and validate an important target for neuroprotection.

Molecular Neuroprotection Induced by Zinc-Dependent Expression of Hepatitis C-Derived Protein NS5A Targeting Kv2.1 Potassium Channels

We present the design of an innovative molecular neuroprotective strategy and provide proof-of-concept for its implementation, relying on the injury-mediated activation of an ectopic gene construct. As oxidative injury leads to the intracellular liberation of zinc, we hypothesize that tapping onto the zinc-activated metal regulatory element (MRE) transcription factor 1 system to drive expression of the Kv2.1-targeted hepatitis C protein NS5A (hepatitis C nonstructural protein 5A) will provide neuroprotection by preventing cell death-enabling cellular potassium loss in rat cortical neurons in vitro. Indeed, using biochemical and morphologic assays, we demonstrate rapid expression of MRE-driven products in neurons. Further, we report that MRE-driven NS5A expression, induced by a slowly evolving excitotoxic stimulus, functionally blocks injurious, enhanced Kv2.1 potassium whole-cell currents and improves neuronal viability. We suggest this form of "on-demand" neuroprotection could provide the basis for a tenable therapeutic strategy to prevent neuronal cell death in neurodegeneration.

Defective trafficking of Kv2.1 channels in MPTP-induced nigrostriatal degeneration

Intracellular protein trafficking is tightly regulated, and improper trafficking might be the fundamental provocateur for human diseases including neurodegeneration. In neurons, protein trafficking to and from the plasma membrane affects synaptic plasticity. Voltage-gated potassium channel 2.1 (Kv2.1) is a predominant delayed rectifier potassium (K⁺ ) current, and electrical activity patterns of dopamine (DA) neurons within the substantia nigra are generated and modulated by the orchestrated function of different ion channels. The pathological hallmark of Parkinson's disease (PD) is the progressive loss of these DA neurons, resulting in the degeneration of striatal dopaminergic terminals. However, whether trafficking of Kv2.1 channels contributes to PD remains unclear. In this study, we demonstrated that MPTP/MPP⁺ increases the surface expression of the Kv2.1 channel and causes nigrostriatal degeneration by using a subchronic MPTP mouse model. The inhibition of the Kv2.1 channel by using a specific blocker, guangxitoxin-1E, protected nigrostriatal projections against MPTP/MPP⁺ insult and thus facilitated the recovery of motor coordination. These findings highlight the importance of trafficking of Kv2.1 channels in the pathogenesis of PD.

Zinc as a Neuromodulator in the Central Nervous System with a Focus on the Olfactory Bulb

The olfactory bulb (OB) is central to the sense of smell, as it is the site of the first synaptic relay involved in the processing of odor information. Odor sensations are first transduced by olfactory sensory neurons (OSNs) before being transmitted, by way of the OB, to higher olfactory centers that mediate olfactory discrimination and perception. Zinc is a common trace element, and it is highly concentrated in the synaptic vesicles of subsets of glutamatergic neurons in some brain regions including the hippocampus and OB. In addition, zinc is contained in the synaptic vesicles of some glycinergic and GABAergic neurons. Thus, zinc released from synaptic vesicles is available to modulate synaptic transmission mediated by excitatory (e.g., N-methyl-D aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)) and inhibitory (e.g., gamma-aminobutyric acid (GABA), glycine) amino acid receptors. Furthermore, extracellular zinc can alter the excitability of neurons through effects on a variety of voltage-gated ion channels. Consistent with the notion that zinc acts as a regulator of neuronal activity, we and others have shown zinc modulation (inhibition and/or potentiation) of amino acid receptors and voltage-gated ion channels expressed by OB neurons. This review summarizes the locations and release of vesicular zinc in the central nervous system (CNS), including in the OB. It also summarizes the effects of zinc on various amino acid receptors and ion channels involved in regulating synaptic transmission and neuronal excitability, with a special emphasis on the actions of zinc as a neuromodulator in the OB. An understanding of how neuroactive substances such as zinc modulate receptors and ion channels expressed by OB neurons will increase our understanding of the roles that synaptic circuits in the OB play in odor information processing and transmission.

Zn(2+) -induced Ca(2+) release via ryanodine receptors triggers calcineurin-dependent redistribution of cortical neuronal Kv2.1 K(+) channels

KEY POINTS

Increases in intracellular Zn(2+) concentrations are an early, necessary signal for the modulation of Kv2.1 K(+) channel localization and physiological function. Intracellular Zn(2+) -mediated Kv2.1 channel modulation is dependent on calcineurin, a Ca(2+) -activated phosphatase. We show that intracellular Zn(2+) induces a significant increase in ryanodine receptor-dependent cytosolic Ca(2+) transients, which leads to a calcineurin-dependent redistribution of Kv2.1 channels from pre-existing membrane clusters to diffuse localization. As such, the link between Zn(2+) and Ca(2+) signalling in this Kv2.1 modulatory pathway is established. We observe that a sublethal ischaemic preconditioning insult also leads to Kv2.1 redistribution in a ryanodine receptor-dependent fashion. We suggest that Zn(2+) may be an early and ubiquitous signalling molecule mediating Ca(2+) release from the cortical endoplasmic reticulum via ryanodine receptor activation.

ABSTRACT

Sublethal injurious stimuli in neurons induce transient increases in free intracellular Zn(2+) that are associated with regulating adaptive responses to subsequent lethal injury, including alterations in the function and localization of the delayed-rectifier potassium channel, Kv2.1. However, the link between intracellular Zn(2+) signalling and the observed changes in Kv2.1 remain undefined. In the present study, utilizing exogenous Zn(2+) treatment, along with a selective Zn(2+) ionophore, we show that transient elevations in intracellular Zn(2+) concentrations are sufficient to induce calcineurin-dependent Kv2.1 channel dispersal in rat cortical neurons in vitro, which is accompanied by a relatively small but significant hyperpolarizing shift in the voltage-gated activation kinetics of the channel. Critically, using a molecularly encoded calcium sensor, we found that the calcineurin-dependent changes in Kv2.1 probably occur as a result of Zn(2+) -induced cytosolic Ca(2+) release via activation of neuronal ryanodine receptors. Finally, we couple this mechanism with an established model for in vitro ischaemic preconditioning and show that Kv2.1 channel modulation in this process is also ryanodine receptor-sensitive. Our results strongly suggest that intracellular Zn(2+) -initiated signalling may represent an early and possibly widespread component of Ca(2+) -dependent processes in neurons.

Targeting a Potassium Channel/Syntaxin Interaction Ameliorates Cell Death in Ischemic Stroke

The voltage-gated K⁺ channel Kv2.1 has been intimately linked with neuronal apoptosis. After ischemic, oxidative, or inflammatory insults, Kv2.1 mediates a pronounced, delayed enhancement of K⁺ efflux, generating an optimal intracellular environment for caspase and nuclease activity, key components of programmed cell death. This apoptosis-enabling mechanism is initiated via Zn²⁺-dependent dual phosphorylation of Kv2.1, increasing the interaction between the channel's intracellular C-terminus domain and the SNARE (soluble N-ethylmaleimide-sensitive factor activating protein receptor) protein syntaxin 1A. Subsequently, an upregulation of de novo channel insertion into the plasma membrane leads to the critical enhancement of K⁺ efflux in damaged neurons. Here, we investigated whether a strategy designed to interfere with the cell death-facilitating properties of Kv2.1, specifically its interaction with syntaxin 1A, could lead to neuroprotection following ischemic injury in vivo The minimal syntaxin 1A-binding sequence of Kv2.1 C terminus (C1aB) was first identified via a far-Western peptide screen and used to create a protherapeutic product by conjugating C1aB to a cell-penetrating domain. The resulting peptide (TAT-C1aB) suppressed enhanced whole-cell K⁺ currents produced by a mutated form of Kv2.1 mimicking apoptosis in a mammalian expression system, and protected cortical neurons from slow excitotoxic injury in vitro, without influencing NMDA-induced intracellular calcium responses. Importantly, intraperitoneal administration of TAT-C1aB in mice following transient middle cerebral artery occlusion significantly reduced ischemic stroke damage and improved neurological outcome. These results provide strong evidence that targeting the proapoptotic function of Kv2.1 is an effective and highly promising neuroprotective strategy.SIGNIFICANCE STATEMENT Kv2.1 is a critical regulator of apoptosis in central neurons. It has not been determined, however, whether the cell death-enabling function of this K⁺ channel can be selectively targeted to improve neuronal survival following injury in vivo The experiments presented here demonstrate that the cell death-specific role of Kv2.1 can be uniquely modulated to provide neuroprotection in an animal model of acute ischemic stroke. We thus reveal a novel therapeutic strategy for neurological disorders that are accompanied by Kv2.1-facilitated forms of cell death.

Disruption of KV2.1 somato-dendritic clusters prevents the apoptogenic increase of potassium currents

As the predominant mediator of the delayed rectifier current, K_V2.1 is an important regulator of neuronal excitability. K_V2.1, however, also plays a well-established role in apoptotic cell death. Apoptogenic stimuli induce syntaxin-dependent trafficking of K_V2.1, resulting in an augmented delayed rectifier current that acts as a conduit for K⁺ efflux required for pro-apoptotic protease/nuclease activation. Recent evidence suggests that K_V2.1 somato-dendritic clusters regulate the formation of endoplasmic reticulum-plasma membrane junctions that function as scaffolding sites for plasma membrane trafficking of ion channels, including K_V2.1. However, it is unknown whether K_V2.1 somato-dendritic clusters are required for apoptogenic trafficking of K_V2.1. By overexpression of a protein derived from the C-terminus of the cognate channel K_V2.2 (K_V2.2CT), we induced calcineurin-independent disruption of K_V2.1 somato-dendritic clusters in rat cortical neurons, without altering the electrophysiological properties of the channel. We observed that K_V2.2CT-expressing neurons are less susceptible to oxidative stress-induced cell death. Critically, expression of K_V2.2CT effectively blocked the increased current density of the delayed rectifier current associated with oxidative injury, supporting a vital role of K_V2.1-somato-dendritic clusters in apoptogenic increases in K_V2.1-mediated currents.

The anticonvulsant retigabine suppresses neuronal KV2-mediated currents

Enhancement of neuronal M-currents, generated through K_V7.2-K_V7.5 channels, has gained much interest for its potential in developing treatments for hyperexcitability-related disorders such as epilepsy. Retigabine, a K_V7 channel opener, has proven to be an effective anticonvulsant and has recently also gained attention due to its neuroprotective properties. In the present study, we found that the auxiliary KCNE2 subunit reduced the K_V7.2-K_V7.3 retigabine sensitivity approximately 5-fold. In addition, using both mammalian expression systems and cultured hippocampal neurons we determined that low μM retigabine concentrations had ‘off-target’ effects on K_V2.1 channels which have recently been implicated in apoptosis. Clinical retigabine concentrations (0.3–3 μM) inhibited K_V2.1 channel function upon prolonged exposure. The suppression of the K_V2.1 conductance was only partially reversible. Our results identified K_V2.1 as a new molecular target for retigabine, thus giving a potential explanation for retigabine’s neuroprotective properties.

Dysregulation of a potassium channel, THIK-1, targeted by caspase-8 accelerates cell shrinkage

Activation of caspases is crucial for the execution of apoptosis. Although the caspase cascade associated with activation of the initiator caspase-8 (CASP8) has been investigated in molecular and biochemical detail, the physiological role of CASP8 is not fully understood. Here, we identified a two-pore domain potassium channel, tandem-pore domain halothane-inhibited K⁺ channel 1 (THIK-1), as a novel CASP8 substrate. The intracellular region of THIK-1 was cleaved by CASP8 in apoptotic cells. Overexpression of THIK-1, but not its mutant lacking the CASP8-target sequence in the intracellular portion, accelerated cell shrinkage in response to apoptotic stimuli. In contrast, knockdown of endogenous THIK-1 by RNA interference resulted in delayed shrinkage and potassium efflux. Furthermore, a truncated THIK-1 mutant lacking the intracellular region, which mimics the form cleaved by CASP8, led to a decrease of cell volume of cultured cells without apoptotic stimulation and excessively promoted irregular development of Xenopus embryos. Taken together, these results indicate that THIK-1 is involved in the acceleration of cell shrinkage. Thus, we have demonstrated a novel physiological role of CASP8: creating a cascade that advances the cell to the next stage in the apoptotic process.

Oxidation of K(+) Channels in Aging and Neurodegeneration

Reversible regulation of proteins by reactive oxygen species (ROS) is an important mechanism of neuronal plasticity. In particular, ROS have been shown to act as modulatory molecules of ion channels-which are key to neuronal excitability-in several physiological processes. However ROS are also fundamental contributors to aging vulnerability. When the level of excess ROS increases in the cell during aging, DNA is damaged, proteins are oxidized, lipids are degraded and more ROS are produced, all culminating in significant cell injury. From this arose the idea that oxidation of ion channels by ROS is one of the culprits for neuronal aging. Aging-dependent oxidative modification of voltage-gated potassium (K(+)) channels was initially demonstrated in the nematode Caenorhabditis elegans and more recently in the mammalian brain. Specifically, oxidation of the delayed rectifier KCNB1 (Kv2.1) and of Ca(2+)- and voltage sensitive K(+) channels have been established suggesting that their redox sensitivity contributes to altered excitability, progression of healthy aging and of neurodegenerative disease. Here I discuss the implications that oxidation of K(+) channels by ROS may have for normal aging, as well as for neurodegenerative disease.

Calstabin 2: An important regulator for learning and memory in mice

Calstabin2, also named FK506 binding protein 12.6 (FKBP12.6), is a subunit of ryanodine receptor subtype 2 (RyR2) macromolecular complex, which is an intracellular calcium channel and abundant in the brain. Previous studies identified a role of leaky neuronal RyR2 in posttraumatic stress disorder (PTSD). However, the functional role of Calstabin2 in the cognitive function remains unclear. Herein, we used a mouse model of genetic deletion of Calstabin2 to investigate the function of Calstabin2 in cognitive dysfunction. We found that Calstabin2 knockout (KO) mice showed significantly reduced performance in Morris Water Maze (MWM), long-term memory (LTM) contextual fear testing, and rotarod test when compared to wild type (WT) littermates. Indeed, genetic deletion of Calstabin2 reduced long-term potentiation (LTP) at the hippocampal CA3-CA1 connection, increased membrane excitability, and induced RyR2 leak. Finally, we demonstrated that the increase in cytoplasmic calcium activated Ca(2+) dependent potassium currents and led to neuronal apoptosis in KO hippocampal neurons. Thus, these results suggest that neuronal RyR2 Ca(2+) leak due to Calstabin2 deletion contributes to learning deficiency and memory impairment.

Critical role of Casein kinase 2 in hepatitis C NS5A-mediated inhibition of Kv2.1 K(+) channel function

Inhibiting injury-induced increases in outward K(+) currents is sufficient to block cell death in cortical neuronal injury models. It is now known that apoptosis is facilitated in hepatocytes by the same K(+) channel as in cortical neurons, namely, the delayed rectifier K(+) channel Kv2.1. The hepatitis C virus (HCV) protein NS5A prevents the apoptosis-enabling loss of intracellular potassium by inhibiting Kv2.1 function and thus blocking hepatocyte cell death. Critically, neurons expressing NS5A1b (from HCV genotype 1b), but not NS5A1a, can be protected from lethal injurious stimuli via a block of Kv2.1-mediated potassium currents. Here, we identify a key component unique to NS5A1b, which is necessary for restricting Kv2.1 currents and establishing neuroprotection. By comparing the sequence differences between NS5A1b and 1a we identify putative casein kinase 2 (CK2) phosphorylation regions unique to the 1b genotype. We show that selective inhibition of CK2 in cortical neurons results in loss of NS5A1b's ability to depress outward potassium currents, and, surprisingly, potentiates currents in non-NS5A-expressing cells. As such, our results suggest that NS5A1b-mediated inhibition of Kv2.1 function is critically dependent on its phosphorylation status at genotypic-specific CK2-directed residues. Importantly, inhibiting NS5A viral replicative function with the novel HCV drug Ledipasvir does not impair the ability of this protein to block Kv2.1 function. This suggests that the modulation of NS5A function by CK2 may be a component of HCV unique to the regulation of apoptosis.

Concerted Trafficking Regulation of Kv2.1 and KATP Channels by Leptin in Pancreatic β-Cells

In pancreatic β-cells, voltage-gated potassium 2.1 (Kv2.1) channels are the dominant delayed rectifier potassium channels responsible for action potential repolarization. Here, we report that leptin, a hormone secreted by adipocytes known to inhibit insulin secretion, causes a transient increase in surface expression of Kv2.1 channels in rodent and human β-cells. The effect of leptin on Kv2.1 surface expression is mediated by the AMP-activated protein kinase (AMPK). Activation of AMPK mimics whereas inhibition of AMPK occludes the effect of leptin. Inhibition of Ca(2+)/calmodulin-dependent protein kinase kinase β, a known upstream kinase of AMPK, also blocks the effect of leptin. In addition, the cAMP-dependent protein kinase (PKA) is involved in Kv2.1 channel trafficking regulation. Inhibition of PKA prevents leptin or AMPK activators from increasing Kv2.1 channel density, whereas stimulation of PKA is sufficient to promote Kv2.1 channel surface expression. The increased Kv2.1 surface expression by leptin is dependent on actin depolymerization, and pharmacologically induced actin depolymerization is sufficient to enhance Kv2.1 surface expression. The signaling and cellular mechanisms underlying Kv2.1 channel trafficking regulation by leptin mirror those reported recently for ATP-sensitive potassium (KATP) channels, which are critical for coupling glucose stimulation with membrane depolarization. We show that the leptin-induced increase in surface KATP channels results in more hyperpolarized membrane potentials than control cells at stimulating glucose concentrations, and the increase in Kv2.1 channels leads to a more rapid repolarization of membrane potential in cells firing action potentials. This study supports a model in which leptin exerts concerted trafficking regulation of KATP and Kv2.1 channels to coordinately inhibit insulin secretion.

Regulation of Pro-Apoptotic Phosphorylation of Kv2.1 K+ Channels

Caspase activity during apoptosis is inhibited by physiological concentrations of intracellular K⁺. To enable apoptosis in injured cortical and hippocampal neurons, cellular loss of this cation is facilitated by the insertion of Kv2.1 K⁺ channels into the plasma membrane via a Zn²⁺/CaMKII/SNARE-dependent process. Pro-apoptotic membrane insertion of Kv2.1 requires the dual phosphorylation of the channel by Src and p38 at cytoplasmic N- and C-terminal residues Y124 and S800, respectively. In this study, we investigate if these phosphorylation sites are mutually co-regulated, and whether putative N- and C-terminal interactions, possibly enabled by Kv2.1 intracellular cysteine residues C73 and C710, influence the phosphorylation process itself. Studies were performed with recombinant wild type and mutant Kv2.1 expressed in Chinese hamster ovary (CHO) cells. Using immunoprecipitated Kv2.1 protein and phospho-specific antibodies, we found that an intact Y124 is required for p38 phosphorylation of S800, and, importantly, that Src phosphorylation of Y124 facilitates the action of the p38 at the S800 residue. Moreover, the actions of Src on Kv2.1 are substantially decreased in the non-phosphorylatable S800A channel mutant. We also observed that mutations of either C73 or C710 residues decreased the p38 phosphorylation at S800 without influencing the actions of Src on tyrosine phosphorylation of Kv2.1. Surprisingly, however, apoptotic K⁺ currents were suppressed only in cells expressing the Kv2.1(C73A) mutant but not in those transfected with Kv2.1(C710A), suggesting a possible structural alteration in the C-terminal mutant that facilitates membrane insertion. These results show that intracellular N-terminal domains critically regulate phosphorylation of the C-terminal of Kv2.1, and vice versa, suggesting possible new avenues for modifying the apoptotic insertion of these channels during neurodegenerative processes.

Diverse mechanisms underlying the regulation of ion channels by carbon monoxide

Carbon monoxide (CO) is firmly established as an important, physiological signalling molecule as well as a potent toxin. Through its ability to bind metal-containing proteins, it is known to interfere with a number of intracellular signalling pathways, and such actions can account for its physiological and pathological effects. In particular, CO can modulate the intracellular production of reactive oxygen species, NO and cGMP levels, as well as regulate MAPK signalling. In this review, we consider ion channels as more recently discovered effectors of CO signalling. CO is now known to regulate a growing number of different ion channel types, and detailed studies of the underlying mechanisms of action are revealing unexpected findings. For example, there are clear areas of contention surrounding its ability to increase the activity of high conductance, Ca(2+) -sensitive K(+) channels. More recent studies have revealed the ability of CO to inhibit T-type Ca(2+) channels and have unveiled a novel signalling pathway underlying tonic regulation of this channel. It is clear that the investigation of ion channels as effectors of CO signalling is in its infancy, and much more work is required to fully understand both the physiological and the toxic actions of this gas. Only then can its emerging use as a therapeutic tool be fully and safely exploited.

Oxidative modulation of K+ channels in the central nervous system in neurodegenerative diseases and aging

SIGNIFICANCE

Oxidative stress and damage are well-established components of neurodegenerative diseases, contributing to neuronal death during disease progression. Here, we consider key K(+) channels as target proteins that can undergo oxidative modulation, describe what is understood about how this influences disease progression, and consider regulation of these channels by gasotransmitters as a means of cellular protection.

RECENT ADVANCES

Oxidative regulation of the delayed rectifier Kv2.1 and the Ca(2+)- and voltage-sensitive BK channel are established, but recent studies contest how their redox sensitivity contributes to altered excitability, progression of neurodegenerative diseases, and healthy aging.

CRITICAL ISSUES

Both Kv2.1 and BK channels have recently been established as target proteins for regulation by the gasotransmitters carbon monoxide and hydrogen sulfide. Establishing the molecular basis of such regulation, and exactly how this influences excitability and vulnerability to apoptotic cell death will determine whether such regulation can be exploited for therapeutic benefit.

FUTURE DIRECTIONS

Developing a more comprehensive picture of the oxidative modulation of K(+) channels (and, indeed, other ion channels) within the central nervous system in health and disease will enable us to better understand processes associated with healthy aging as well as distinct processes underlying progression of neurodegenerative diseases. Advances in the growing understanding of how gasotransmitters can regulate ion channels, including redox-sensitive K(+) channels, are a research priority for this field, and will establish their usefulness in design of future approaches for the treatment of such diseases.

Heme Oxygenase-1 Influences Apoptosis via CO-mediated Inhibition of K+ Channels

Hypoxic/ischemic episodes can trigger oxidative stress-mediated loss of central neurons via apoptosis, and low pO2 is also a feature of the tumor microenvironment, where cancer cells are particularly resistant to apoptosis. In the CNS, ischemic insult increases expression of the CO-generating enzyme heme oxygenase-1 (HO-1), which is commonly constitutively active in cancer cells. It has been proposed that apoptosis can be regulated by the trafficking and activity of K(+) channels, particularly Kv2.1. We have explored the idea that HO-1 may influence apoptosis via regulation of Kv2.1. Overexpression of Kv2.1 in HEK293 cells increased their vulnerability to oxidant-induced apoptosis. CO (applied as the donor CORM-2) protected cells against apoptosis and inhibited Kv2.1 channels. Similarly in hippocampal neurones, CO selectively inhibited Kv2.1 and protected neurones against oxidant-induced apoptosis. In medulloblastoma sections we identified constitutive expression of HO-1 and Kv2.1, and in the medulloblastoma-derived cell line DAOY, hypoxic HO-1 induction or exposure to CO protected cells against apoptosis, and also selectively inhibited Kv2.1 channels expressed in these cells. These studies are consistent with a central role for Kv2.1 in apoptosis in both central neurones and cancer cells. They also suggest that HO-1 expression can strongly influence apoptosis via CO-mediated regulation of Kv2.1 activity.

Heme oxygenase-1 protects against Alzheimer's amyloid-β(1-42)-induced toxicity via carbon monoxide production

Heme oxygenase-1 (HO-1), an inducible enzyme up-regulated in Alzheimer's disease, catabolises heme to biliverdin, Fe²⁺ and carbon monoxide (CO). CO can protect neurones from oxidative stress-induced apoptosis by inhibiting Kv2.1 channels, which mediates cellular K⁺ efflux as an early step in the apoptotic cascade. Since apoptosis contributes to the neuronal loss associated with amyloid β peptide (Aβ) toxicity in AD, we investigated the protective effects of HO-1 and CO against Aβ_1-42 toxicity in SH-SY5Y cells, employing cells stably transfected with empty vector or expressing the cellular prion protein, PrP^c, and rat primary hippocampal neurons. Aβ_1-42 (containing protofibrils) caused a concentration-dependent decrease in cell viability, attributable at least in part to induction of apoptosis, with the PrP^c-expressing cells showing greater susceptibility to Aβ_1-42 toxicity. Pharmacological induction or genetic over-expression of HO-1 significantly ameliorated the effects of Aβ_1-42. The CO-donor CORM-2 protected cells against Aβ_1-42 toxicity in a concentration-dependent manner. Electrophysiological studies revealed no differences in the outward current pre- and post-Aβ_1-42 treatment suggesting that K⁺ channel activity is unaffected in these cells. Instead, Aβ toxicity was reduced by the L-type Ca²⁺ channel blocker nifedipine, and by the CaMKKII inhibitor, STO-609. Aβ also activated the downstream kinase, AMP-dependent protein kinase (AMPK). CO prevented this activation of AMPK. Our findings indicate that HO-1 protects against Aβ toxicity via production of CO. Protection does not arise from inhibition of apoptosis-associated K⁺ efflux, but rather by inhibition of AMPK activation, which has been recently implicated in the toxic effects of Aβ. These data provide a novel, beneficial effect of CO which adds to its growing potential as a therapeutic agent.

Mechanisms of activation of voltage-gated potassium channels

Voltage-gated potassium ion channels (Kv) play an important role in a variety of cellular processes, including the functioning of excitable cells, regulation of apoptosis, cell growth and differentiation, the release of neurotransmitters and hormones, maintenance of cardiac activity, etc. Failure in the functioning of Kv channels leads to severe genetic disorders and the development of tumors, including malignant ones. Understanding the mechanisms underlying Kv channels functioning is a key factor in determining the cause of the diseases associated with mutations in the channels, and in the search for new drugs. The mechanism of activation of the channels is a topic of ongoing debate, and a consensus on the issue has not yet been reached. This review discusses the key stages in studying the mechanisms of functioning of Kv channels and describes the basic models of their activation known to date.

Syntaxin-binding domain of Kv2.1 is essential for the expression of apoptotic K+ currents

Intracellular signalling cascades triggered by oxidative injury can lead to upregulation of Kv2.1 K(+) channels at the plasma membrane of dying neurons. Membrane incorporation of new channels is necessary for enhanced K(+) efflux and a consequent reduction of intracellular K(+) that facilitates apoptosis. We showed previously that the observed increase in K(+) currents is a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated process, and that the SNARE protein syntaxin binds directly to Kv2.1 channels. In the present study, we tested whether disrupting the interaction of Kv2.1 and syntaxin promoted the survival of cortical neurons following injury. Syntaxin is known to bind to Kv2.1 in a domain comprising amino acids 411-522 of the channel's cytoplasmic C terminus (C1a). Here we show that this domain is required for the apoptotic K(+) current enhancement. Moreover, expression of an isolated, Kv2.1-derived C1a peptide is sufficient to suppress the injury-induced increase in currents by interfering with Kv2.1/syntaxin binding. By subdividing the C1a peptide, we were able to localize the syntaxin binding site on Kv2.1 to the most plasma membrane-distal residues of C1a. Importantly, expression of this peptide segment in neurons prevented the apoptotic K(+) current enhancement and cell death following an oxidative insult, without greatly impairing baseline K(+) currents or normal electrical profiles of neurons. These results establish that binding of syntaxin to Kv2.1 is crucial for the manifestation of oxidant-induced apoptosis, and thereby reveal a potential new direction for therapeutic intervention in the treatment of neurodegenerative disorders.

Oxidation of KCNB1 K(+) channels in central nervous system and beyond

KCNB1, a voltage-gated potassium (K(+)) channel that conducts a major delayed rectifier current in the brain, pancreas and cardiovascular system is a key player in apoptotic programs associated with oxidative stress. As a result, this protein represents a bona fide drug target for limiting the toxic effects of oxygen radicals. Until recently the consensus view was that reactive oxygen species trigger a pro-apoptotic surge in KCNB1 current via phosphorylation and SNARE-dependent incorporation of KCNB1 channels into the plasma membrane. However, new evidence shows that KCNB1 can be modified by oxidants and that oxidized KCNB1 channels can directly activate pro-apoptotic signaling pathways. Hence, a more articulated picture of the pro-apoptotic role of KCNB1 is emerging in which the protein induces cell's death through distinct molecular mechanisms and activation of multiple pathways. In this review article we discuss the diverse functional, toxic and protective roles that KCNB1 channels play in the major organs where they are expressed.

Cyclin e1 regulates Kv2.1 channel phosphorylation and localization in neuronal ischemia

Kv2.1 is a major delayed rectifying K(+) channel normally localized to highly phosphorylated somatodendritic clusters in neurons. Excitatory stimuli induce calcineurin-dependent dephosphorylation and dispersal of Kv2.1 clusters, with a concomitant hyperpolarizing shift in the channel's activation kinetics. We showed previously that sublethal ischemia, which renders neurons transiently resistant to excitotoxic cell death, can also induce Zn(2+)-dependent changes in Kv2.1 localization and activation kinetics, suggesting that activity-dependent modifications of Kv2.1 may contribute to cellular adaptive responses to injury. Recently, cyclin-dependent kinase 5 (Cdk5) was shown to phosphorylate Kv2.1, with pharmacological Cdk5 inhibition being sufficient to decluster channels. In another study, cyclin E1 was found to restrict neuronal Cdk5 kinase activity. We show here that cyclin E1 regulates Kv2.1 cellular localization via inhibition of Cdk5 activity. Expression of cyclin E1 in human embryonic kidney cells prevents Cdk5-mediated phosphorylation of Kv2.1, and cyclin E1 overexpression in rat cortical neurons triggers dispersal of Kv2.1 channel clusters. Sublethal ischemia in neurons induces calcineurin-dependent upregulation of cyclin E1 protein expression and cyclin E1-dependent Kv2.1 channel declustering. Importantly, overexpression of cyclin E1 in neurons is sufficient to reduce excitotoxic cell death. These results support a novel role for neuronal cyclin E1 in regulating the phosphorylation status and localization of Kv2.1 channels, a likely component of signaling cascades leading to ischemic preconditioning.

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

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

Plasma gelsolin protects HIV-1 gp120-induced neuronal injury via voltage-gated K+ channel Kv2.1

Plasma gelsolin (pGSN), a secreted form of gelsolin, is constitutively expressed throughout the central nervous system (CNS). The neurons, astrocytes and oligodendrocytes are the major sources of pGSN in the CNS. It has been shown that levels of pGSN in the cerebrospinal fluid (CSF) are decreased in several neurological conditions including HIV-1-associated neurocognitive disorders (HAND). Although there is no direct evidence that a decreased level of pGSN in CSF is causally related to the pathogenesis of neurological disorders, neural cells, if lacking pGSN, are more vulnerable to cell death. To understand how GSN levels relate to neuronal injury in HAND, we studied the effects of pGSN on HIV-1 gp120-activated outward K+ currents in primary rat cortical neuronal cultures. Incubation of rat cortical neurons with gp120 enhanced the outward K+ currents induced by voltage steps and resulted in neuronal apoptosis. Treatment with pGSN suppressed the gp120-induced increase of delayed rectifier current (IK) and reduced vulnerability to gp120-induced neuronal apoptosis. Application of Guangxitoxin-1E (GxTx), a Kv2.1 specific channel inhibitor, inhibited gp120 enhancement of IK and associated neuronal apoptosis, similar effects to pGSN. Western blot and PCR analysis revealed gp120 exposure to up-regulate Kv2.1 channel expression, which was also inhibited by treatment with pGSN. Taken together, these results indicate pGSN protects neurons by suppressing gp120 enhancement of IK through Kv2.1 channels and reduction of pGSN in HIV-1-infected brain may contribute to HIV-1-associated neuropathy.

Curcumin protects microglia and primary rat cortical neurons against HIV-1 gp120-mediated inflammation and apoptosis

Curcumin is a molecule found in turmeric root that has anti-inflammatory, antioxidant, and anti-tumor properties and has been widely used as both an herbal drug and a food additive to treat or prevent neurodegenerative diseases. To explore whether curcumin is able to ameliorate HIV-1-associated neurotoxicity, we treated a murine microglial cell line (N9) and primary rat cortical neurons with curcumin in the presence or absence of neurotoxic HIV-1 gp120 (V3 loop) protein. We found that HIV-1 gp120 profoundly induced N9 cells to produce reactive oxygen species (ROS), tumor necrosis factor-α (TNF-α) and monocyte chemoattractant protein-1 (MCP-1). HIV-1 gp120 also induced apoptosis of primary rat cortical neurons. Curcumin exerted a powerful inhibitory effect against HIV-1 gp120-induced neuronal damage, reducing the production of ROS, TNF-α and MCP-1 by N9 cells and inhibiting apoptosis of primary rat cortical neurons. Curcumin may exert its biological activities through inhibition of the delayed rectification and transient outward potassium (K(+)) current, as curcumin effectively reduced HIV-1 gp120-mediated elevation of the delayed rectification and transient outward K(+) channel current in neurons. We conclude that HIV-1 gp120 increases ROS, TNF-α and MCP-1 production in microglia, and induces cortical neuron apoptosis by affecting the delayed rectification and transient outward K(+) channel current. Curcumin reduces production of ROS and inflammatory mediators in HIV-1-gp120-stimulated microglia, and protects cortical neurons against HIV-1-mediated apoptosis, most likely through inhibition of HIV-1 gp120-induced elevation of the delayed rectification and transient outward K(+) current.

Convergent Ca2+ and Zn2+ signaling regulates apoptotic Kv2.1 K+ currents

A simultaneous increase in cytosolic Zn(2+) and Ca(2+) accompanies the initiation of neuronal cell death signaling cascades. However, the molecular convergence points of cellular processes activated by these cations are poorly understood. Here, we show that Ca(2+)-dependent activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is required for a cell death-enabling process previously shown to also depend on Zn(2+). We have reported that oxidant-induced intraneuronal Zn(2+) liberation triggers a syntaxin-dependent incorporation of Kv2.1 voltage-gated potassium channels into the plasma membrane. This channel insertion can be detected as a marked enhancement of delayed rectifier K(+) currents in voltage clamp measurements observed at least 3 h following a short exposure to an apoptogenic stimulus. This current increase is the process responsible for the cytoplasmic loss of K(+) that enables protease and nuclease activation during apoptosis. In the present study, we demonstrate that an oxidative stimulus also promotes intracellular Ca(2+) release and activation of CaMKII, which, in turn, modulates the ability of syntaxin to interact with Kv2.1. Pharmacological or molecular inhibition of CaMKII prevents the K(+) current enhancement observed following oxidative injury and, importantly, significantly increases neuronal viability. These findings reveal a previously unrecognized cooperative convergence of Ca(2+)- and Zn(2+)-mediated injurious signaling pathways, providing a potentially unique target for therapeutic intervention in neurodegenerative conditions associated with oxidative stress.

Molecular mechanisms underlying the apoptotic effect of KCNB1 K+ channel oxidation

Potassium (K(+)) channels are targets of reactive oxygen species in the aging nervous system. KCNB1 (formerly Kv2.1), a voltage-gated K(+) channel abundantly expressed in the cortex and hippocampus, is oxidized in the brains of aging mice and of the triple transgenic 3xTg-AD mouse model of Alzheimer's disease. KCNB1 oxidation acts to enhance apoptosis in mammalian cell lines, whereas a KCNB1 variant resistant to oxidative modification, C73A-KCNB1, is cytoprotective. Here we investigated the molecular mechanisms through which oxidized KCNB1 channels promote apoptosis. Biochemical evidence showed that oxidized KCNB1 channels, which form oligomers held together by disulfide bridges involving Cys-73, accumulated in the plasma membrane as a result of defective endocytosis. In contrast, C73A-mutant channels, which do not oligomerize, were normally internalized. KCNB1 channels localize in lipid rafts, and their internalization was dynamin 2-dependent. Accordingly, cholesterol supplementation reduced apoptosis promoted by oxidation of KCNB1. In contrast, cholesterol depletion exacerbated apoptotic death in a KCNB1-independent fashion. Inhibition of raft-associating c-Src tyrosine kinase and downstream JNK kinase by pharmacological and molecular means suppressed the pro-apoptotic effect of KCNB1 oxidation. Together, these data suggest that the accumulation of KCNB1 oligomers in the membrane disrupts planar lipid raft integrity and causes apoptosis via activating the c-Src/JNK signaling pathway.

Trafficking mechanisms underlying neuronal voltage-gated ion channel localization at the axon initial segment

Voltage-gated ion channels are diverse and fundamental determinants of neuronal intrinsic excitability. Voltage-gated K(+) (Kv) and Na(+) (Nav) channels play complex yet fundamentally important roles in determining intrinsic excitability. The Kv and Nav channels located at the axon initial segment (AIS) play a unique and especially important role in generating neuronal output in the form of anterograde axonal and backpropagating action potentials. Aberrant intrinsic excitability in individual neurons within networks contributes to synchronous neuronal activity leading to seizures. Mutations in ion channel genes give rise to a variety of seizure-related "channelopathies," and many of the ion channel subunits associated with epilepsy mutations are localized at the AIS, making this a hotspot for epileptogenesis. Here we review the cellular mechanisms that underlie the trafficking of Kv and Nav channels found at the AIS, and how Kv and Nav channel mutations associated with epilepsy can alter these processes.

Regulation of neuronal proapoptotic potassium currents by the hepatitis C virus nonstructural protein 5A

Apoptosis-enabling neuronal potassium efflux is mediated by an enhancement of K+ currents. In cortical neurons, increased currents are triggered by dual phosphorylation of Kv2.1 by Src and p38 at channel residues Y124 and S800. It was recently shown that a K+ current surge is also present in hepatocytes undergoing apoptosis, and that the hepatitis C virus (HCV) nonstructural protein 5A (NS5A) could inhibit Kv2.1-mediated currents and block cell death. Here, we show that NS5A1b (from HCV genotype 1b) expression in rat neurons depresses delayed rectifier potassium currents, limits the magnitude of the K+ current surge following exposure to activated microglia, and is neuroprotective. In a non-neuronal recombinant expression system, cells expressing Kv2.1 mutated at residue Y124, but not S800 mutants, are insensitive to NS5A1b-mediated current inhibition. Accordingly, NS5A1b coexpression prevents phosphorylation of wild-type Kv2.1 by Src at Y124, but is unable to inhibit p38 phosphorylation of the channel at S800. The actions of the viral protein are genotype-selective, as NS5A1a does not depress neuronal potassium currents nor inhibit Src phosphorylation of Kv2.1. Our results indicate that NS5A1b limits K+ currents following injury, leading to increased neuronal viability. NS5A1b may thus serve as a model for a new generation of neuroprotective agents.

Kv2.1 cell surface clusters are insertion platforms for ion channel delivery to the plasma membrane

Voltage-gated K+ (Kv) channels regulate membrane potential in many cell types. Although the channel surface density and location must be well controlled, little is known about Kv channel delivery and retrieval on the cell surface. The Kv2.1 channel localizes to micron-sized clusters in neurons and transfected human embryonic kidney (HEK) cells, where it is nonconducting. Because Kv2.1 is postulated to be involved in soluble N-ethylmaleimide–sensitive factor attachment protein receptor–mediated membrane fusion, we examined the hypothesis that these surface clusters are specialized platforms involved in membrane protein trafficking. Total internal reflection–based fluorescence recovery after photobleaching studies and quantum dot imaging of single Kv2.1 channels revealed that Kv2.1-containing vesicles deliver cargo at the Kv2.1 surface clusters in both transfected HEK cells and hippocampal neurons. More than 85% of cytoplasmic and recycling Kv2.1 channels was delivered to the cell surface at the cluster perimeter in both cell types. At least 85% of recycling Kv1.4, which, unlike Kv2.1, has a homogeneous surface distribution, is also delivered here. Actin depolymerization resulted in Kv2.1 exocytosis at cluster-free surface membrane. These results indicate that one nonconducting function of Kv2.1 is to form microdomains involved in membrane protein trafficking. This study is the first to identify stable cell surface platforms involved in ion channel trafficking.

SNARE-dependent upregulation of potassium chloride co-transporter 2 activity after metabotropic zinc receptor activation in rat cortical neurons in vitro

The major outward chloride transporter in neurons is the potassium chloride co-transporter 2 (KCC2), critical for maintaining an inhibitory reversal potential for GABA(A) receptor channels. In a recent study, we showed that Zn(2+) regulates GABA(A) reversal potentials in the hippocampus by enhancing the activity of KCC2 through an increase in its surface expression. Zn(2+) initiates this process by activating the Gq-coupled metabotropic Zn(2+) receptor/G protein-linked receptor 39 (mZnR/GPR39). Here, we first demonstrated that mZnR/GPR39 is functional in cortical neurons in culture, and then tested the hypothesis that the increase in KCC2 activity is mediated through a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-dependent process. We established the presence of functional mZnR in rat cultured cortical neurons by loading cells with a Ca(2+) indicator and exposing cells to Zn(2+), which triggered consistent Ca(2+) responses that were blocked by the Gq antagonist YM-254890, but not by the metabotropic glutamate receptor antagonist (RS)-α-methyl-4-carboxyphenylglycine (MCPG). Importantly, Zn(2+) treatment under these conditions did not increase the intracellular concentrations of Zn(2+) itself. We then measured KCC2 activity by monitoring both the rate and relative amount of furosemide-sensitive NH(4)(+) influx through the co-transporter using an intracellular pH-sensitive fluorescent indicator. We observed that Zn(2+) pretreatment induced a Ca(2+)-dependent increase in KCC2 activity. The effects of Zn(2+) on KCC2 activity were also observed in wild-type mouse cortical neurons in culture, but not in neurons obtained from mZnR/GPR39(-/-) mice, suggesting that Zn(2+) acts through mZnR/GPR39 activation to upregulate KCC2 activity. We next transfected rat cortical neurons with a plasmid encoding botulinum toxin C1 (Botox C1), which cleaves the SNARE proteins syntaxin 1 and synaptosomal-associated protein 25 (SNAP-25). Basal KCC2 activity was similar in both transfected and non-transfected neurons. Non-transfected cells, or cells transfected with marker vector alone, showed a Zn(2+)-dependent increase in KCC2 activity. In contrast, KCC2 activity in neurons expressing Botox C1 was unchanged by Zn(2+). These results suggest that SNARE proteins are necessary for the increased activity of KCC2 after Zn(2+) stimulation of mZnR/GPR39.

The neurophysiology and pathology of brain zinc

Our understanding of the roles played by zinc in the physiological and pathological functioning of the brain is rapidly expanding. The increased availability of genetically modified animal models, selective zinc-sensitive fluorescent probes, and novel chelators is producing a remarkable body of exciting new data that clearly establishes this metal ion as a key modulator of intracellular and intercellular neuronal signaling. In this Mini-Symposium, we will review and discuss the most recent findings that link zinc to synaptic function as well as the injurious effects of zinc dyshomeostasis within the context of neuronal death associated with major human neurological disorders, including stroke, epilepsy, and Alzheimer's disease.

Redox regulation of intracellular zinc: molecular signaling in the life and death of neurons

Zn(2+) has emerged as a major regulator of neuronal physiology, as well as an important signaling agent in neural injury. The intracellular concentration of this metal is tightly regulated through the actions of Zn(2+) transporters and the thiol-rich metal binding protein metallothionein, closely linking the redox status of the cell to cellular availability of Zn(2+). Accordingly, oxidative and nitrosative stress during ischemic injury leads to an accumulation of neuronal free Zn(2+) and the activation of several downstream cell death processes. While this Zn(2+) rise is an established signaling event in neuronal cell death, recent evidence suggests that a transient, sublethal accumulation of free Zn(2+) can also play a critical role in neuroprotective pathways activated during ischemic preconditioning. Thus, redox-sensitive proteins, like metallothioneins, may play a critical role in determining neuronal cell fate by regulating the localization and concentration of intracellular free Zn(2+).

Pancreatic β-cell prosurvival effects of the incretin hormones involve post-translational modification of Kv2.1 delayed rectifier channels

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are the major incretin hormones that exert insulinotropic and anti-apoptotic actions on pancreatic β-cells. Insulinotropic actions of the incretins involve modulation of voltage-gated potassium (Kv) channels. In multiple cell types, Kv channel activity has been implicated in cell volume changes accompanying initiation of the apoptotic program. Focusing on Kv2.1, we examined whether regulation of Kv channels in β-cells contributes to the prosurvival effects of incretins. Overexpression of Kv2.1 in INS-1 β-cells potentiated apoptosis in response to mitochondrial and ER stress and, conversely, co-stimulation with GIP/GLP-1 uncoupled this potentiation, suppressing apoptosis. In parallel, incretins promoted phosphorylation and acetylation of Kv2.1 via pathways involving protein kinase A (PKA)/mitogen- and stress-activated kinase-1 (MSK-1) and histone acetyltransferase (HAT)/histone deacetylase (HDAC). Further studies demonstrated that acetylation of Kv2.1 was mediated by incretin actions on nuclear/cytoplasmic shuttling of CREB binding protein (CBP) and its interaction with Kv2.1. Regulation of β-cell survival by GIP and GLP-1 therefore involves post-translational modifications (PTMs) of Kv channels by PKA/MSK-1 and HAT/HDAC. This appears to be the first demonstration of modulation of delayed rectifier Kv channels contributing to the β-cell prosurvival effects of incretins and of 7-transmembrane G protein-coupled receptor (GPCR)-stimulated export of a nuclear lysine acetyltransferase that regulates cell surface ion channel function.

Carbon monoxide protects against oxidant-induced apoptosis via inhibition of Kv2.1

Oxidative stress induces neuronal apoptosis and is implicated in cerebral ischemia, head trauma, and age-related neurodegenerative diseases. An early step in this process is the loss of intracellular K(+) via K(+) channels, and evidence indicates that K(v)2.1 is of particular importance in this regard, being rapidly inserted into the plasma membrane in response to apoptotic stimuli. An additional feature of neuronal oxidative stress is the up-regulation of the inducible enzyme heme oxygenase-1 (HO-1), which catabolizes heme to generate biliverdin, Fe(2+), and carbon monoxide (CO). CO provides neuronal protection against stresses such as stroke and excitotoxicity, although the underlying mechanisms are not yet elucidated. Here, we demonstrate that CO reversibly inhibits K(v)2.1. Channel inhibition by CO involves reactive oxygen species and protein kinase G activity. Overexpression of K(v)2.1 in HEK293 cells increases their vulnerability to oxidant-induced apoptosis, and this is reversed by CO. In hippocampal neurons, CO selectively inhibits K(v)2.1, reverses the dramatic oxidant-induced increase in K(+) current density, and provides marked protection against oxidant-induced apoptosis. Our results provide a novel mechanism to account for the neuroprotective effects of CO against oxidative apoptosis, which has potential for therapeutic exploitation to provide neuronal protection in situations of oxidative stress.

Contribution of the delayed-rectifier potassium channel Kv2.1 to acute spinal cord injury in rats

Recent studies have reported that delayed-rectifier Kv channels regulate apoptosis in the nervous system. Herein, we investigated changes in the expression of the delayed-rectifier Kv channels Kv1.2, Kv2.1, and Kv3.1 after acute spinal cord injury (SCI) in rats. We performed RT-PCR analysis and found an increase in the level of Kv2.1 mRNA after SCI but no significant changes in the levels of Kv1.2 and Kv3.1 mRNA. Western blot analysis revealed that Kv2.1 protein levels rapidly decreased and then dramatically increased from 1 day, whereas Kv3.1b protein levels gradually and sharply decreased at 5 days. Kv1.2 protein levels did not change significantly. In addition, Kv2.1 clusters were disrupted in the plasma membranes of motor neurons after SCI. Interestingly, the expressional changes and translocation of Kv2.1 were consistent with the apoptotic changes on day 1. Therefore, these results suggest that Kv2.1 channels probably contribute to neuronal cell responses to SCI.