Elimination of rapid potassium channel inactivation by phosphorylation of the inactivation gate

Characteristics of A-type voltage-gated K+ currents expressed on sour-sensing type III taste receptor cells in mice

Sour taste is detected by type III taste receptor cells that generate membrane depolarization with action potentials in response to HCl applied to the apical membranes. The shape of action potentials in type III cells exhibits larger afterhyperpolarization due to activation of transient A-type voltage-gated K+ currents. Although action potentials play an important role in neurotransmitter release, the electrophysiological features of A-type K+ currents in taste buds remain unclear. Here, we examined the electrophysiological properties of A-type K+ currents in mouse fungiform taste bud cells using in-situ whole-cell patch clamping. Type III cells were identified with SNAP-25 immunoreactivity and/or electrophysiological features of voltage-gated currents. Type III cells expressed A-type K+ currents which were completely inhibited by 10 mM TEA, whereas IP3R3-immunoreactive type II cells did not. The half-maximal activation and steady-state inactivation of A-type K+ currents were 17.9 ± 4.5 (n = 17) and - 11.0 ± 5.7 (n = 17) mV, respectively, which are similar to the features of Kv3.3 and Kv3.4 channels (transient and high voltage-activated K+ channels). The recovery from inactivation was well fitted with a double exponential equation; the fast and slow time constants were 6.4 ± 0.6 ms and 0.76 ± 0.26 s (n = 6), respectively. RT-PCR experiments suggest that Kv3.3 and Kv3.4 mRNAs were detected at the taste bud level, but not at single-cell levels. As the phosphorylation of Kv3.3 and Kv3.4 channels generally leads to the modulation of cell excitability, neuromodulator-mediated A-type K+ channel phosphorylation likely affects the signal transduction of taste.

The binding and mechanism of a positive allosteric modulator of Kv3 channels

Small-molecule modulators of diverse voltage-gated K+ (Kv) channels may help treat a wide range of neurological disorders. However, developing effective modulators requires understanding of their mechanism of action. We apply an orthogonal approach to elucidate the mechanism of action of an imidazolidinedione derivative (AUT5), a highly selective positive allosteric modulator of Kv3.1 and Kv3.2 channels. AUT5 modulation involves positive cooperativity and preferential stabilization of the open state. The cryo-EM structure of the Kv3.1/AUT5 complex at a resolution of 2.5 Å reveals four equivalent AUT5 binding sites at the extracellular inter-subunit interface between the voltage-sensing and pore domains of the channel's tetrameric assembly. Furthermore, we show that the unique extracellular turret regions of Kv3.1 and Kv3.2 essentially govern the selective positive modulation by AUT5. High-resolution apo and bound structures of Kv3.1 demonstrate how AUT5 binding promotes turret rearrangements and interactions with the voltage-sensing domain to favor the open conformation.

The Phosphorylation of Kv1.3: A Modulatory Mechanism for a Multifunctional Ion Channel

The voltage-gated potassium channel Kv1.3 plays a pivotal role in a myriad of biological processes, including cell proliferation, differentiation, and apoptosis. Kv1.3 undergoes fine-tuned regulation, and its altered expression or function correlates with tumorigenesis and cancer progression. Moreover, posttranslational modifications (PTMs), such as phosphorylation, have evolved as rapid switch-like moieties that tightly modulate channel activity. In addition, kinases are promising targets in anticancer therapies. The diverse serine/threonine and tyrosine kinases function on Kv1.3 and the effects of its phosphorylation vary depending on multiple factors. For instance, Kv1.3 regulatory subunits (KCNE4 and Kvβ) can be phosphorylated, increasing the complexity of channel modulation. Scaffold proteins allow the Kv1.3 channelosome and kinase to form protein complexes, thereby favoring the attachment of phosphate groups. This review compiles the network triggers and signaling pathways that culminate in Kv1.3 phosphorylation. Alterations to Kv1.3 expression and its phosphorylation are detailed, emphasizing the importance of this channel as an anticancer target. Overall, further research on Kv1.3 kinase-dependent effects should be addressed to develop effective antineoplastic drugs while minimizing side effects. This promising field encourages basic cancer research while inspiring new therapy development.

Tunable Action Potential Repolarization Governed by Kv3.4 Channels in Dorsal Root Ganglion Neurons

The Kv3.4 channel regulates action potential (AP) repolarization in nociceptors and excitatory synaptic transmission in the spinal cord. We hypothesize that this is a tunable role governed by protein kinase-C-dependent phosphorylation of the Kv3.4 cytoplasmic N-terminal inactivation domain (NTID) at four nonequivalent sites. However, there is a paucity of causation evidence linking the phosphorylation status of Kv3.4 to the properties of the AP. To establish this link, we used adeno-associated viral vectors to specifically manipulate the expression and the effective phosphorylation status of Kv3.4 in cultured dorsal root ganglion (DRG) neurons from mixed-sex rat embryos at embryonic day 18. These vectors encoded GFP (background control), wild-type (WT) Kv3.4, phosphonull (PN) Kv3.4 mutant (PN = S[8,9,15,21]A), phosphomimic (PM) Kv3.4 mutant (PM = S[8,9,15,21]D), and a Kv3.4 nonconducting dominant-negative (DN) pore mutant (DN = W429F). Following viral infection of the DRG neurons, we evaluated transduction efficiency and Kv3.4 expression and function via fluorescence microscopy and patch clamping. All functional Kv3.4 constructs induced current overexpression with similar voltage dependence of activation. However, whereas Kv3.4-WT and Kv3.4-PN induced fast transient currents, the Kv3.4-PM induced currents exhibiting impaired inactivation. In contrast, the Kv3.4-DN abolished the endogenous Kv3.4 current. Consequently, Kv3.4-DN and Kv3.4-PM produced APs with the longest and shortest durations, respectively, whereas Kv3.4-WT and Kv3.4-PN produced intermediate results. Moreover, the AP widths and maximum rates of AP repolarization from these groups are negatively correlated. We conclude that the expression and effective phosphorylation status of the Kv3.4 NTID confer a tunable mechanism of AP repolarization, which may provide exquisite regulation of pain signaling in DRG neurons.SIGNIFICANCE STATEMENT The AP is an all-or-none millisecond-long electrical impulse that encodes information in the frequency and patterns of repetitive firing. However, signaling may also depend on the plasticity and diversity of the AP waveform. For instance, the shape and duration of the AP may regulate nociceptive synaptic transmission between a primary sensory afferent to a secondary neuron in the spinal cord. Here, we used mutants of the Kv3.4 voltage-gated potassium channel to manipulate its expression and effective phosphorylation status in dorsal root ganglion neurons and directly show how the expression and malleable inactivation properties of Kv3.4 govern the AP duration and repolarization rate. These results elucidate a mechanism of neural AP plasticity that may regulate pain signaling.

PKC regulation of ion channels: The involvement of PIP2

Ion channels are integral membrane proteins whose gating has been increasingly shown to depend on the presence of the low-abundance membrane phospholipid, phosphatidylinositol (4,5) bisphosphate. The expression and function of ion channels is tightly regulated via protein phosphorylation by specific kinases, including various PKC isoforms. Several channels have further been shown to be regulated by PKC through altered surface expression, probability of channel opening, shifts in voltage dependence of their activation, or changes in inactivation or desensitization. In this review, we survey the impact of phosphorylation of various ion channels by PKC isoforms and examine the dependence of phosphorylated ion channels on phosphatidylinositol (4,5) bisphosphate as a mechanistic endpoint to control channel gating.

PKCε associates with the Kv3.4 channel to promote its expression in a kinase activity-dependent manner

The voltage-gated potassium channel Kv3.4 is a crucial regulator of nociceptive signaling in the dorsal root ganglion (DRG) and the dorsal horn of the spinal cord. Moreover, Kv3.4 dysfunction has been linked to neuropathic pain. Although kinases and phosphatases can directly modulate Kv3.4 gating, the signaling mechanisms regulating the expression and stability of the Kv3.4 protein are generally unknown. We explored a potential role of PKCε and found an unexpected interaction that has a positive effect on Kv3.4 expression. Co-immunoprecipitation studies revealed a physical association between PKCε and Kv3.4 in both heterologous cells and rat DRG neurons. Furthermore, in contrast to the wild-type and constitutively active forms of PKCε, expression of a catalytically inactive form of the enzyme inhibits Kv3.4 expression and membrane localization through a dominant negative effect. Co-expression of Kv3.4 with the wild-type, constitutively active, or catalytically inactive forms of PKCε had no significant effects on Kv3.4 gating. These results suggest that a novel physical interaction of the Kv3.4 channel with functional PKCε primarily determines its stability and localization in DRG neurons. This interaction is akin to those of previously identified accessory ion channel proteins, which could be significant in neural tissues where Kv3.4 regulates electrical signaling.

Inhibition of overexpressed Kv3.4 augments HPV in endotoxemic mice

Background

Hypoxic pulmonary vasoconstriction (HPV) is a reaction of the pulmonary vasculature upon hypoxia, diverting blood flow into ventilated areas to preserve oxygenation. It is impaired in endotoxemia or ARDS. Voltage gated potassium channels have been shown to play a key role in the regulation of HPV. The aim of the study was to identify a voltage gated potassium channel involved in dysregulated HPV during endotoxemia.

Methods

Lungs of male C57BL/6 mice with and without endotoxemia (n = 6 ea. group) were analyzed for Kv3.4 gene and protein expression. HPV was examined in isolated perfused lungs of mice with and without endotoxemia and with and without selective Kv3.4 blocker BDS-I (n = 7 ea. group). Pulmonary artery pressure (PAP) and pressure-flow curves were measured during normoxic (FiO₂ 0.21) and hypoxic (FiO₂ 0.01) ventilation. HPV was quantified as the increase in perfusion pressure in response to hypoxia in percent of baseline perfusion pressure (ΔPAP) in the presence and absence of BDS-I.

Results

Kv3.4 gene (3.2 ± 0.5-fold, p < 0.05) and protein (1.5 ± 0.1-fold p < 0.05) expression levels were increased in endotoxemic mouse lungs. Endotoxemia reduced HPV (∆PAP control: 121.2 ± 8.7% vs. LPS 19.5 ± 8.0%, means ± SEM) while inhibition of Kv3.4 with 50 nM BDS-I augmented HPV in endotoxemic but not in control lungs (∆PAP control BDS-I: 116.6 ± 16.0% vs. LPS BDS-I 84.4 ± 18.2%, means ± SEM).

Conclusions

Kv3.4 gene and protein expressions are increased in endotoxemic mouse lungs. Selective inhibition of Kv3.4 augments HPV in lungs of endotoxemic mice, but not in lungs of control mice.

Impact of intracellular hemin on N-type inactivation of voltage-gated K+ channels

N-type inactivation of voltage-gated K⁺ channels is conferred by the N-terminal “ball” domains of select pore-forming α subunits or of auxiliary β subunits, and influences electrical cellular excitability. Here, we show that hemin impairs inactivation of K⁺ channels formed by Kv3.4 α subunits as well as that induced by the subunits Kvβ1.1, Kvβ1.2, and Kvβ3.1 when coexpressed with α subunits of the Kv1 subfamily. In Kvβ1.1, hemin interacts with cysteine and histidine residues in the N terminus (C7 and H10) with high affinity (EC₅₀ 100 nM). Similarly, rapid inactivation of Kv4.2 channels induced by the dipeptidyl peptidase-like protein DPP6a is also sensitive to hemin, and the DPP6a mutation C13S eliminates this dependence. The results suggest a common mechanism for a dynamic regulation of Kv channel inactivation by heme/hemin in N-terminal ball domains of Kv α and auxiliary β subunits. Free intracellular heme therefore has the potential to regulate cellular excitability via modulation of Kv channel inactivation.

Transient Potassium Channels: Therapeutic Targets for Brain Disorders

Transient potassium current channels (I_A channels), which are expressed in most brain areas, have a central role in modulating feedforward and feedback inhibition along the dendroaxonic axis. Loss of the modulatory channels is tightly associated with a number of brain diseases such as Alzheimer’s disease, epilepsy, fragile X syndrome (FXS), Parkinson’s disease, chronic pain, tinnitus, and ataxia. However, the functional significance of I_A channels in these diseases has so far been underestimated. In this review, we discuss the distribution and function of I_A channels. Particularly, we posit that downregulation of I_A channels results in neuronal (mostly dendritic) hyperexcitability accompanied by the imbalanced excitation and inhibition ratio in the brain’s networks, eventually causing the brain diseases. Finally, we propose a potential therapeutic target: the enhanced action of I_A channels to counteract Ca²⁺-permeable channels including NMDA receptors could be harnessed to restore dendritic excitability, leading to a balanced neuronal state.

A-Type KV Channels in Dorsal Root Ganglion Neurons: Diversity, Function, and Dysfunction

A-type voltage-gated potassium (Kv) channels are major regulators of neuronal excitability that have been mainly characterized in the central nervous system. By contrast, there is a paucity of knowledge about the molecular physiology of these Kv channels in the peripheral nervous system, including highly specialized and heterogenous dorsal root ganglion (DRG) neurons. Although all A-type Kv channels display pore-forming subunits with similar structural properties and fast inactivation, their voltage-, and time-dependent properties and modulation are significantly different. These differences ultimately determine distinct physiological roles of diverse A-type Kv channels, and how their dysfunction might contribute to neurological disorders. The importance of A-type Kv channels in DRG neurons is highlighted by recent studies that have linked their dysfunction to persistent pain sensitization. Here, we review the molecular neurophysiology of A-type Kv channels with an emphasis on those that have been identified and investigated in DRG nociceptors (Kv1.4, Kv3.4, and Kv4s). Also, we discuss evidence implicating these Kv channels in neuropathic pain resulting from injury, and present a perspective of outstanding challenges that must be tackled in order to discover novel treatments for intractable pain disorders.

Regulation of Nociceptive Glutamatergic Signaling by Presynaptic Kv3.4 Channels in the Rat Spinal Dorsal Horn

Presynaptic voltage-gated K⁺ (Kv) channels in dorsal root ganglion (DRG) neurons are thought to regulate nociceptive synaptic transmission in the spinal dorsal horn. However, the Kv channel subtypes responsible for this critical role have not been identified. The Kv3.4 channel is particularly important because it is robustly expressed in DRG nociceptors, where it regulates action potential (AP) duration. Furthermore, Kv3.4 dysfunction is implicated in the pathophysiology of neuropathic pain in multiple pain models. We hypothesized that, through their ability to modulate AP repolarization, Kv3.4 channels in DRG nociceptors help to regulate nociceptive synaptic transmission. To test this hypothesis, we investigated Kv3.4 immunoreactivity (IR) in the rat cervical superficial dorsal horn (sDH) in both sexes and implemented an intact spinal cord preparation to investigate glutamatergic synaptic currents from second order neurons in the sDH under conditions that selectively inhibit the Kv3.4 current. We found presynaptic Kv3.4 IR in peptidergic and nonpeptidergic nociceptive fibers of the sDH. The Kv3.4 channel is hypersensitive to 4-aminopyridine and tetraethylammonium (TEA). Accordingly, 50 μm 4-aminopyridine and 500 μm TEA significantly prolong the AP, slow the maximum rate of repolarization in small-diameter DRG neurons, and potentiate monosynaptic excitatory postsynaptic currents (EPSCs) in dorsal horn laminae I and II through a presynaptic mechanism. In contrast, highly specific inhibitors of BK, Kv7, and Kv1 channels are less effective modulators of the AP and have little to no effect on EPSCs. The results strongly suggest that presynaptic Kv3.4 channels are major regulators of nociceptive synaptic transmission in the spinal cord.SIGNIFICANCE STATEMENT Intractable neuropathic pain can result from disease or traumatic injury and many studies have been conducted to determine the underlying pathophysiological changes. Voltage-gated ion channels, including the K⁺ channel Kv3.4, are dysregulated in multiple pain models. Kv3.4 channels are ubiquitously expressed in the dorsal root ganglion (DRG), where they are major regulators of DRG excitability. However, little is known about the ionic mechanisms that regulate nociceptive synaptic transmission at the level of the first synapse in the spinal cord, which is critical to pain transmission in both intact and pathological states. Here, we show that Kv3.4 channels have a significant impact on glutamatergic synaptic transmission in the dorsal horn, further illuminating its potential as a molecular pain therapeutic target.

Kv3 Channels: Enablers of Rapid Firing, Neurotransmitter Release, and Neuronal Endurance

The intrinsic electrical characteristics of different types of neurons are shaped by the K+ channels they express. From among the more than 70 different K+ channel genes expressed in neurons, Kv3 family voltage-dependent K+ channels are uniquely associated with the ability of certain neurons to fire action potentials and to release neurotransmitter at high rates of up to 1,000 Hz. In general, the four Kv3 channels Kv3.1-Kv3.4 share the property of activating and deactivating rapidly at potentials more positive than other channels. Each Kv3 channel gene can generate multiple protein isoforms, which contribute to the high-frequency firing of neurons such as auditory brain stem neurons, fast-spiking GABAergic interneurons, and Purkinje cells of the cerebellum, and to regulation of neurotransmitter release at the terminals of many neurons. The different Kv3 channels have unique expression patterns and biophysical properties and are regulated in different ways by protein kinases. In this review, we cover the function, localization, and modulation of Kv3 channels and describe how levels and properties of the channels are altered by changes in ongoing neuronal activity. We also cover how the protein-protein interaction of these channels with other proteins affects neuronal functions, and how mutations or abnormal regulation of Kv3 channels are associated with neurological disorders such as ataxias, epilepsies, schizophrenia, and Alzheimer's disease.

Nongenomic Glucocorticoid Suppression of a Postsynaptic Potassium Current via Emergent Autocrine Endocannabinoid Signaling in Hypothalamic Neuroendocrine Cells following Chronic Dehydration

Glucocorticoids rapidly stimulate endocannabinoid synthesis and modulation of synaptic transmission in hypothalamic neuroendocrine cells via a nongenomic signaling mechanism. The endocannabinoid actions are synapse-constrained by astrocyte restriction of extracellular spatial domains. Exogenous cannabinoids have been shown to modulate postsynaptic potassium currents, including the A-type potassium current (I_A), in different cell types. The activity of magnocellular neuroendocrine cells is shaped by a prominent I_A. We tested for a rapid glucocorticoid modulation of the postsynaptic I_K and I_A in magnocellular neuroendocrine cells of the hypothalamic paraventricular nucleus (PVN) using whole-cell recordings in rat brain slices. Application of the synthetic glucocorticoid dexamethasone (Dex) had no rapid effect on the I_K or I_A amplitude, voltage dependence, or kinetics in magnocellular neurons in slices from untreated rats. In magnocellular neurons from salt-loaded rats, however, Dex application caused a rapid suppression of the I_A and a depolarizing shift in I_A voltage dependence. Exogenously applied endocannabinoids mimicked the rapid Dex modulation of the I_A, and CB1 receptor antagonists and agonists blocked and occluded the Dex-induced changes in the I_A, respectively, suggesting an endocannabinoid dependence of the rapid glucocorticoid effect. Preincubation of control slices in a gliotoxin resulted in the partial recapitulation of the glucocorticoid-induced rapid suppression of the I_A. These findings demonstrate a glucocorticoid suppression of the postsynaptic I_A in PVN magnocellular neurons via an autocrine endocannabinoid-dependent mechanism following chronic dehydration, and suggest a possible role for astrocytes in the control of the autocrine endocannabinoid actions.

Calcineurin Dysregulation Underlies Spinal Cord Injury-Induced K+ Channel Dysfunction in DRG Neurons

Dysfunction of the fast-inactivating Kv3.4 potassium current in dorsal root ganglion (DRG) neurons contributes to the hyperexcitability associated with persistent pain induced by spinal cord injury (SCI). However, the underlying mechanism is not known. In light of our previous work demonstrating modulation of the Kv3.4 channel by phosphorylation, we investigated the role of the phosphatase calcineurin (CaN) using electrophysiological, molecular, and imaging approaches in adult female Sprague Dawley rats. Pharmacological inhibition of CaN in small-diameter DRG neurons slowed repolarization of the somatic action potential (AP) and attenuated the Kv3.4 current. Attenuated Kv3.4 currents also exhibited slowed inactivation. We observed similar effects on the recombinant Kv3.4 channel heterologously expressed in Chinese hamster ovary cells, supporting our findings in DRG neurons. Elucidating the molecular basis of these effects, mutation of four previously characterized serines within the Kv3.4 N-terminal inactivation domain eliminated the effects of CaN inhibition on the Kv3.4 current. SCI similarly induced concurrent Kv3.4 current attenuation and slowing of inactivation. Although there was little change in CaN expression and localization after injury, SCI induced upregulation of the native regulator of CaN 1 (RCAN1) in the DRG at the transcript and protein levels. Consistent with CaN inhibition resulting from RCAN1 upregulation, overexpression of RCAN1 in naive DRG neurons recapitulated the effects of pharmacological CaN inhibition on the Kv3.4 current and the AP. Overall, these results demonstrate a novel regulatory pathway that links CaN, RCAN1, and Kv3.4 in DRG neurons. Dysregulation of this pathway might underlie a peripheral mechanism of pain sensitization induced by SCI.SIGNIFICANCE STATEMENT Pain sensitization associated with spinal cord injury (SCI) involves poorly understood maladaptive modulation of neuronal excitability. Although central mechanisms have received significant attention, recent studies have identified peripheral nerve hyperexcitability as a driver of persistent pain signaling after SCI. However, the ion channels and signaling molecules responsible for this change in primary sensory neuron excitability are still not well defined. To address this problem, this study used complementary electrophysiological and molecular methods to determine how Kv3.4, a voltage-gated K⁺ channel robustly expressed in dorsal root ganglion neurons, becomes dysfunctional upon calcineurin (CaN) inhibition. The results strongly suggest that CaN inhibition underlies SCI-induced dysfunction of Kv3.4 and the associated excitability changes through upregulation of the native regulator of CaN 1 (RCAN1).

K+ Channel Kv3.4 Is Essential for Axon Growth by Limiting the Influx of Ca2+ into Growth Cones

Membrane excitability in the axonal growth cones of embryonic neurons influences axon growth. Voltage-gated K⁺ (Kv) channels are key factors in controlling membrane excitability, but whether they regulate axon growth remains unclear. Here, we report that Kv3.4 is expressed in the axonal growth cones of embryonic spinal commissural neurons, motoneurons, dorsal root ganglion neurons, retinal ganglion cells, and callosal projection neurons during axon growth. Our in vitro (cultured dorsal spinal neurons of chick embryos) and in vivo (developing chick spinal commissural axons and rat callosal axons) findings demonstrate that knockdown of Kv3.4 by a specific shRNA impedes axon initiation, elongation, pathfinding, and fasciculation. In cultured dorsal spinal neurons, blockade of Kv3.4 by blood depressing substance II suppresses axon growth via an increase in the amplitude and frequency of Ca²⁺ influx through T-type and L-type Ca²⁺ channels. Electrophysiological results show that Kv3.4, the major Kv channel in the axonal growth cones of embryonic dorsal spinal neurons, is activated at more hyperpolarized potentials and inactivated more slowly than it is in postnatal and adult neurons. The opening of Kv3.4 channels effectively reduces growth cone membrane excitability, thereby limiting excessive Ca²⁺ influx at subthreshold potentials or during Ca²⁺-dependent action potentials. Furthermore, excessive Ca²⁺ influx induced by an optogenetic approach also inhibits axon growth. Our findings suggest that Kv3.4 reduces growth cone membrane excitability and maintains [Ca²⁺]_i at an optimal concentration for normal axon growth.SIGNIFICANCE STATEMENT Accumulating evidence supports the idea that impairments in axon growth contribute to many clinical disorders, such as autism spectrum disorders, corpus callosum agenesis, Joubert syndrome, Kallmann syndrome, and horizontal gaze palsy with progressive scoliosis. Membrane excitability in the growth cone, which is mainly controlled by voltage-gated Ca²⁺ (Cav) and K⁺ (Kv) channels, modulates axon growth. The role of Cav channels during axon growth is well understood, but it is unclear whether Kv channels control axon outgrowth by regulating Ca²⁺ influx. This report shows that Kv3.4, which is transiently expressed in the axonal growth cones of many types of embryonic neurons, acts to reduce excessive Ca²⁺ influx through Cav channels and thus permits normal axon outgrowth.

TNF-R1 and FADD mediate UVB-Induced activation of K+ channels in corneal epithelial cells

The goal of this study was to elucidate the role of Fas, TNF-R1, FADD and cytochrome c in UVB-induced K+ channel activation, an early step in UVB-induced apoptosis, in human corneal limbal epithelial (HCLE) cells. HCLE cells were treated with Fas, TNF-R1 or FADD siRNA and exposed to 80 or 150 mJ/cm2 UVB. K+ channel activation and loss of intracellular K+ were measured using whole-cell patch-clamp recording and ion chromatography, respectively. Cytochrome c was measured with an ELISA kit. Cells in which Fas was knocked down exhibited identical UVB-induced K+ channel activation and loss of intracellular K+ to control cells. Cells in which TNF-R1 or FADD were knocked down demonstrated reduced K+ channel activation and decreased loss of intracellular K+ following UVB, relative to control cells. Application of TNF-α, the natural ligand of TNF-R1, to HCLE cells induced K+ channel activation and loss of intracellular K+. Cytochrome c was translocated to the cytosol by 2 h after exposure to 150 mJ/cm2 UVB. However, there was no release by 10 min post-UVB. The data suggest that UVB activates TNF-R1, which in turn may activate K+ channels via FADD. This conclusion is supported by the observation that TNF-α also causes loss of intracellular K+. This signaling pathway appears to be integral to UVB-induced K+ efflux, since knockdown of TNF-R1 or FADD inhibits the UVB-induced K+ efflux. The lack of rapid cytochrome c translocation indicates cytochrome c does not play a role in UVB-induced K+ channel activation.

Kv3.1 uses a timely resurgent K(+) current to secure action potential repolarization

High-frequency action potential (AP) transmission is essential for rapid information processing in the central nervous system. Voltage-dependent K_v3 channels play an important role in this process thanks to their high activation threshold and fast closure kinetics, which reduce the neuron's refractory period. However, premature K_v3 channel closure leads to incomplete membrane repolarization, preventing sustainable AP propagation. Here, we demonstrate that K_v3.1b channels solve this problem by producing resurgent K⁺ currents during repolarization, thus ensuring enough repolarizing power to terminate each AP. Unlike previously described resurgent Na⁺ and K⁺ currents, K_v3.1b's resurgent current does not originate from recovery of channel block or inactivation but results from a unique combination of steep voltage-dependent gating kinetics and ultra-fast voltage-sensor relaxation. These distinct properties are readily transferrable onto an orthologue K_v channel by transplanting the voltage-sensor's S3–S4 loop, providing molecular insights into the mechanism by which K_v3 channels contribute to high-frequency AP transmission.

K_v3 potassium channels have an important role in the repolarization of action potentials in fast-spiking neurons. Here, the authors use electrophysiology and modelling to report on an interesting mechanism that might explain their gating behaviour.

Ca(2+)/calmodulin regulates Kvβ1.1-mediated inactivation of voltage-gated K(+) channels

A-type K⁺ channels open on membrane depolarization and undergo subsequent rapid inactivation such that they are ideally suited for fine-tuning the electrical signaling in neurons and muscle cells. Channel inactivation mostly follows the so-called ball-and-chain mechanism, in which the N-terminal structures of either the K⁺ channel’s α or β subunits occlude the channel pore entry facing the cytosol. Inactivation of Kv1.1 and Kv1.4 channels induced by Kvβ1.1 subunits is profoundly decelerated in response to a rise in the intracellular Ca²⁺ concentration, thus making the affected channel complexes negative feedback regulators to limit neuronal overexcitation. With electrophysiological and biochemical experiments we show that the Ca²⁺ dependence is gained by binding of calmodulin to the “chain” segment of Kvβ1.1 thereby compromising the mobility of the inactivation particle. Furthermore, inactivation regulation via Ca²⁺/calmodulin does not interfere with the β subunit’s enzymatic activity as an NADPH-dependent oxidoreductase, thus rendering the Kvβ1.1 subunit a multifunctional receptor that integrates cytosolic signals to be transduced to altered electrical cellular activity.

Kv3.4 channel function and dysfunction in nociceptors

Recently, we reported the isolation of the Kv3.4 current in dorsal root ganglion (DRG) neurons and described dysregulation of this current in a spinal cord injury (SCI) model of chronic pain. These studies strongly suggest that rat Kv3.4 channels are major regulators of excitability in DRG neurons from pups and adult females, where they help determine action potential (AP) repolarization and spiking properties. Here, we characterized the Kv3.4 current in rat DRG neurons from adult males and show that it transfers 40–70% of the total repolarizing charge during the AP across all ages and sexes. Following SCI, we also found remodeling of the repolarizing currents during the AP. In the light of these studies, homomeric Kv3.4 channels expressed in DRG nociceptors are emerging novel targets that may help develop new approaches to treat neuropathic pain.

Dysregulation of Kv3.4 channels in dorsal root ganglia following spinal cord injury

Spinal cord injury (SCI) patients develop chronic pain involving poorly understood central and peripheral mechanisms. Because dysregulation of the voltage-gated Kv3.4 channel has been implicated in the hyperexcitable state of dorsal root ganglion (DRG) neurons following direct injury of sensory nerves, we asked whether such a dysregulation also plays a role in SCI. Kv3.4 channels are expressed in DRG neurons, where they help regulate action potential (AP) repolarization in a manner that depends on the modulation of inactivation by protein kinase C (PKC)-dependent phosphorylation of the channel's inactivation domain. Here, we report that, 2 weeks after cervical hemicontusion SCI, injured rats exhibit contralateral hypersensitivity to stimuli accompanied by accentuated repetitive spiking in putative DRG nociceptors. Also in these neurons at 1 week after laminectomy and SCI, Kv3.4 channel inactivation is impaired compared with naive nonsurgical controls. At 2-6 weeks after laminectomy, however, Kv3.4 channel inactivation returns to naive levels. Conversely, Kv3.4 currents at 2-6 weeks post-SCI are downregulated and remain slow-inactivating. Immunohistochemistry indicated that downregulation mainly resulted from decreased surface expression of the Kv3.4 channel, as whole-DRG-protein and single-cell mRNA transcript levels did not change. Furthermore, consistent with Kv3.4 channel dysregulation, PKC activation failed to shorten the AP duration of small-diameter DRG neurons. Finally, re-expressing synthetic Kv3.4 currents under dynamic clamp conditions dampened repetitive spiking in the neurons from SCI rats. These results suggest a novel peripheral mechanism of post-SCI pain sensitization implicating Kv3.4 channel dysregulation and potential Kv3.4-based therapeutic interventions.

The phosphoinositide sensitivity of the K(v) channel family

Recently, we screened several KV channels for possible dependence on plasma membrane phosphatidylinositol 4,5-bisphosphate (PI(4,5)P 2). The channels were expressed in tsA-201 cells and the PI(4,5)P 2 was depleted by several manipulations in whole-cell experiments with parallel measurements of channel activity. In contrast to reports on excised-patches using Xenopus laevis oocytes, we found only KV 7, but none of the other tested KV channels, to be strongly dependent on PI(4,5)P 2. We now have extended our study to KV 1.2 channels, a KV channel we had not previously tested, because a new published study on excised patches showed regulation of the voltage-dependence of activation by PI(4,5)P 2. In full agreement with those published results, we found a reduction of current amplitude by ~20% after depletion of PI(4,5)P 2 and a small left shift in the activation curve of KV 1.2 channels. We also found a small reduction of KV 11.1 (hERG) currents that was not accompanied by a gating shift. In conclusion, our whole-cell methods yield a PI(4,5)P 2-dependence of KV 1.2 currents in tsA-201 cells that is comparable to findings from excised patches of Xenopus laevis oocytes. We discuss possible physiological rationales for PI(4,5)P 2 sensitivity of some ion channels and insensitivity of others.

The amplitude and inactivation properties of the delayed potassium currents are regulated by protein kinase activity in hair cells of the frog semicircular canals

In hair cells dissected from the frog crista ampullaris, the combination of a calcium-dependent (IKCa) and a purely voltage-dependent component (IKV) gives rise to the delayed potassium current complex (IKD). These currents have been recently reported to display slow depolarization-induced inactivation and biphasic inactivation removal by hyperpolarization. The amplitude and inactivation kinetics of both IKCa and IKV are drastically modulated by a previously unrecognized mechanism of protein phosphorylation (sensitive to kinase inhibitors H89 and KT5823), which does not interfere with the transient potassium current (IA) or the calcium current (ICa). IKD amplitude was stable in cells patched with pipettes containing 8 mM ATP or under perforated-patch; under these conditions, a 10 min treatment with 10 µM H89 or 1–10 µM KT5823 reduced IKD amplitude by a mean of 67% at +40 mV. Similarly affected was the isolated IKV component (ICa blocked with Cd²⁺). Thus, a large potassium conductance can be activated by depolarization, but it is made available to the cell to a variable extent that depends on membrane potential and protein kinase activity. The total gKD ranged 4.6–44.0 nS in control cells, according to the level of steady-state inactivation, and was reduced to 1.4–2.7 nS after protein kinase inhibition. When sinusoidal membrane potential changes in the −70/−10 mV range were applied, to mimic receptor response to hair bundle deflection, IKD proved the main current dynamically activated and the only one regulated by PK: H89 decreased the total outward charge during each cycle by 60%. Phosphorylation appears to control both the amount of IKCa and IKV conductance activated by depolarization and the fraction thereof which can be rescued by removal of inactivation. The balance between the depolarizing transduction current and the repolarizing potassium current, and eventually the transmitter release at the cytoneural junction, are therefore modulated by a phosphorylation-mediated process.

The delayed rectifier potassium conductance in the sarcolemma and the transverse tubular system membranes of mammalian skeletal muscle fibers

A two-microelectrode voltage clamp and optical measurements of membrane potential changes at the transverse tubular system (TTS) were used to characterize delayed rectifier K currents (IK(V)) in murine muscle fibers stained with the potentiometric dye di-8-ANEPPS. In intact fibers, IK(V) displays the canonical hallmarks of K(V) channels: voltage-dependent delayed activation and decay in time. The voltage dependence of the peak conductance (gK(V)) was only accounted for by double Boltzmann fits, suggesting at least two channel contributions to IK(V). Osmotically treated fibers showed significant disconnection of the TTS and displayed smaller IK(V), but with similar voltage dependence and time decays to intact fibers. This suggests that inactivation may be responsible for most of the decay in IK(V) records. A two-channel model that faithfully simulates IK(V) records in osmotically treated fibers comprises a low threshold and steeply voltage-dependent channel (channel A), which contributes ∼31% of gK(V), and a more abundant high threshold channel (channel B), with shallower voltage dependence. Significant expression of the IK(V)1.4 and IK(V)3.4 channels was demonstrated by immunoblotting. Rectangular depolarizing pulses elicited step-like di-8-ANEPPS transients in intact fibers rendered electrically passive. In contrast, activation of IK(V) resulted in time- and voltage-dependent attenuations in optical transients that coincided in time with the peaks of IK(V) records. Normalized peak attenuations showed the same voltage dependence as peak IK(V) plots. A radial cable model including channels A and B and K diffusion in the TTS was used to simulate IK(V) and average TTS voltage changes. Model predictions and experimental data were compared to determine what fraction of gK(V) in the TTS accounted simultaneously for the electrical and optical data. Best predictions suggest that K(V) channels are approximately equally distributed in the sarcolemma and TTS membranes; under these conditions, >70% of IK(V) arises from the TTS.

Regulation of voltage-gated potassium channels by PI(4,5)P2

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) regulates activities of numerous ion channels including inwardly rectifying potassium (K_ir) channels, KCNQ, TRP, and voltage-gated calcium channels. Several studies suggest that voltage-gated potassium (K_V) channels might be regulated by PI(4,5)P₂. Wide expression of K_V channels in different cells suggests that such regulation could have broad physiological consequences. To study regulation of K_V channels by PI(4,5)P₂, we have coexpressed several of them in tsA-201 cells with a G protein–coupled receptor (M₁R), a voltage-sensitive lipid 5-phosphatase (Dr-VSP), or an engineered fusion protein carrying both lipid 4-phosphatase and 5-phosphatase activity (pseudojanin). These tools deplete PI(4,5)P₂ with application of muscarinic agonists, depolarization, or rapamycin, respectively. PI(4,5)P₂ at the plasma membrane was monitored by Förster resonance energy transfer (FRET) from PH probes of PLCδ1 simultaneously with whole-cell recordings. Activation of Dr-VSP or recruitment of pseudojanin inhibited K_V7.1, K_V7.2/7.3, and K_ir2.1 channel current by 90–95%. Activation of M₁R inhibited K_V7.2/7.3 current similarly. With these tools, we tested for potential PI(4,5)P₂ regulation of activity of K_V1.1/K_Vβ1.1, K_V1.3, K_V1.4, and K_V1.5/K_Vβ1.3, K_V2.1, K_V3.4, K_V4.2, K_V4.3 (with different KChIPs and DPP6-s), and hERG/KCNE2. Interestingly, we found a substantial removal of inactivation for K_V1.1/K_Vβ1.1 and K_V3.4, resulting in up-regulation of current density upon activation of M₁R but no changes in activity upon activating only VSP or pseudojanin. The other channels tested except possibly hERG showed no alteration in activity in any of the assays we used. In conclusion, a depletion of PI(4,5)P₂ at the plasma membrane by enzymes does not seem to influence activity of most tested K_V channels, whereas it does strongly inhibit members of the K_V7 and K_ir families.

Cytoplasmic domains and voltage-dependent potassium channel gating

The basic architecture of the voltage-dependent K⁺ channels (Kv channels) corresponds to a transmembrane protein core in which the permeation pore, the voltage-sensing components and the gating machinery (cytoplasmic facing gate and sensor–gate coupler) reside. Usually, large protein tails are attached to this core, hanging toward the inside of the cell. These cytoplasmic regions are essential for normal channel function and, due to their accessibility to the cytoplasmic environment, constitute obvious targets for cell-physiological control of channel behavior. Here we review the present knowledge about the molecular organization of these intracellular channel regions and their role in both setting and controlling Kv voltage-dependent gating properties. This includes the influence that they exert on Kv rapid/N-type inactivation and on activation/deactivation gating of Shaker-like and eag-type Kv channels. Some illustrative examples about the relevance of these cytoplasmic domains determining the possibilities for modulation of Kv channel gating by cellular components are also considered.

Modulation of Kv3.4 channel N-type inactivation by protein kinase C shapes the action potential in dorsal root ganglion neurons

Fast inactivation of heterologously expressed Kv3.4 channels is dramatically slowed upon phosphorylation of the channel's N-terminal (N-type) inactivation gate by protein kinase C (PKC). However, the presence and physiological importance of this exquisite modulation in excitable tissues were unknown. Here, we employed minimally invasive cell-attached patch-clamping, single-cell qPCR and specific siRNAs to unambiguously demonstrate that fast-inactivating Kv3.4 channels underlie a robust high voltage-activated A-type K(+) current (I(AHV)) in nociceptive dorsal root ganglion neurons from 7-day-old rats. We also show that PKC activation with phorbol 12,13-dibutyrate (PDBu) causes a 4-fold slowing of Kv3.4 channel inactivation and, consequently, accelerates the repolarization of the action potential (AP) by 22%, which shortens the AP duration by 14%. G-protein coupled receptor (GPCR) agonists eliminate I(AHV) fast inactivation in a membrane-delimited manner, suggesting a Kv3.4 channel signalling complex. Preincubation of the neurons with the PKC inhibitor bisindolylmaleimide II inhibits the effect of GPCR agonists and PDBu. Furthermore, activation of PKC via GPCR agonists recapitulates the effects of PDBu on the AP. Finally, transfection of the neurons with Kv3.4 siRNA prolongs the AP by 25% and abolishes the GPCR agonist-induced acceleration of the AP repolarization. These results show that Kv3.4 channels help shape the repolarization of the nociceptor AP, and that modulation of Kv3.4 channel N-type inactivation by PKC regulates AP repolarization and duration. We propose that the dramatic modulation of I(AHV) fast inactivation by PKC represents a novel mechanism of neural plasticity with potentially significant implications in the transition from acute to chronic pain.

A novel N-terminal motif of dipeptidyl peptidase-like proteins produces rapid inactivation of KV4.2 channels by a pore-blocking mechanism

The somatodendritic subthreshold A-type K(+) current in neurons (I(SA)) depends on its kinetic and voltage-dependent properties to regulate membrane excitability, action potential repetitive firing, and signal integration. Key functional properties of the K(V)4 channel complex underlying I(SA) are determined by dipeptidyl peptidase-like proteins known as dipeptidyl peptidase 6 (DPP6) and dipeptidyl peptidase 10 (DPP10). Among the multiple known DPP10 isoforms with alternative N-terminal sequences, DPP10a confers exceptionally fast inactivation to K(V)4.2 channels. To elucidate the molecular basis of this fast inactivation, we investigated the structure-function relationship of the DPP10a N-terminal region and its interaction with the K(V)4.2 channel. Here, we show that DPP10a shares a conserved N-terminal sequence (MNQTA) with DPP6a (aka DPP6-E), which also induces fast inactivation. Deletion of the NQTA sequence in DPP10a eliminates this dramatic fast inactivation, and perfusion of MNQTA peptide to the cytoplasmic face of inside-out patches inhibits the K(V)4.2 current. DPP10a-induced fast inactivation exhibits competitive interactions with internally applied tetraethylammonium (TEA), and elevating the external K(+) concentration accelerates recovery from DPP10a-mediated fast inactivation. These results suggest that fast inactivation induced by DPP10a or DPP6a is mediated by a common N-terminal inactivation motif via a pore-blocking mechanism. This mechanism may offer an attractive target for novel pharmacological interventions directed at impairing I(SA) inactivation and reducing neuronal excitability.

Homeostatic plasticity of striatal neurons intrinsic excitability following dopamine depletion

The striatum is the major input structure of basal ganglia and is involved in adaptive control of behaviour through the selection of relevant informations. Dopaminergic neurons that innervate striatum die in Parkinson disease, leading to inefficient adaptive behaviour. Neuronal activity of striatal medium spiny neurons (MSN) is modulated by dopamine receptors. Although dopamine signalling had received substantial attention, consequences of dopamine depletion on MSN intrinsic excitability remain unclear. Here we show, by performing perforated patch clamp recordings on brain slices, that dopamine depletion leads to an increase in MSN intrinsic excitability through the decrease of an inactivating A-type potassium current, I(A). Despite the large decrease in their excitatory synaptic inputs determined by the decreased dendritic spines density and the increase in minimal current to evoke the first EPSP, this increase in intrinsic excitability resulted in an enhanced responsiveness to their remaining synapses, allowing them to fire similarly or more efficiently following input stimulation than in control condition. Therefore, this increase in intrinsic excitability through the regulation of I(A) represents a form of homeostatic plasticity allowing neurons to compensate for perturbations in synaptic transmission and to promote stability in firing. The present observations show that this homeostatic ability to maintain firing rates within functional range also occurs in pathological conditions, allowing stabilizing neural computation within affected neuronal networks.

Excitatory roles of protein kinase C in striatal cholinergic interneurons

Protein kinase C (PKC) plays critical roles in neuronal activity and is widely expressed in striatal neurons. However, it is not clear how PKC activation regulates the excitability of striatal cholinergic interneurons. In the present study, we found that PKC activation significantly inhibited A-type potassium current (I(A)), but had no effect on delayed rectifier potassium currents. Consistently, application of PKC activator caused an increase of firing in response to depolarizing currents in cholinergic interneurons, which was persistent in the presence of both excitatory and inhibitory neurotransmission blockers. These excitatory effects of PKC could be partially mimicked and occluded by blockade of I(A) with potassium channel blocker 4-aminopyridine. In addition, immunostaining demonstrated that PKCalpha, but not PKCgamma and PKCepsilon, was expressed in cholinergic interneurons. Furthermore, activation of group I metabotropic glutamate receptors (mGluRs) led to an inhibition of I(A) through a PKC-dependent pathway. These data indicate that activation of PKC, most likely PKCalpha, increases the neuronal excitability of striatal cholinergic interneurons by down-regulating I(A). Group I mGluR-mediated I(A) inhibition might be important for the glutamatergic regulation of cholinergic tone in the neostriatum.

N type rapid inactivation in human Kv1.4 channels: functional role of a putative C-terminal helix

Voltage gated potassium channels are tetrameric membrane proteins, which have a central role in cellular excitability. Human Kv1.4 channels open on membrane depolarization and inactivate rapidly by a 'ball and chain' mechanism whose molecular determinants have been mapped to the cytoplasmic N terminus of the channel. Here we show that the other terminal end of the channel also plays a role in channel inactivation. Swapping the C-terminal residues of hKv1.4 with those from two non-inactivating channels (hKv1.1 and hKv1.2) affects the rates of inactivation, as well as the recovery of the channel from the inactivated state. Secondary structure predictions of the hKv1.4 sequence reveal a helical structure at its distal C-terminal. Complete removal or partial disruption of this helical region results in channels with remarkably slowed inactivation kinetics. The ionic selectivity and voltage-dependence of channel opening were similar to hKv1.4, indicative of an unperturbed channel pore. These results demonstrate that fast inactivation is modulated by structural elements in the C-terminus, suggesting that the process involves the concerted action of the N- and C-termini.

Kv4.2 is a locus for PKC and ERK/MAPK cross-talk

Transient outward K+ currents are particularly important for the regulation of membrane excitability of neurons and repolarization of action potentials in cardiac myocytes. These currents are modulated by PKC (protein kinase C) activation, and the K+- channel subunit Kv4.2 is a major contributor to these currents. Furthermore, the current recorded from Kv4.2 channels expressed in oocytes is reduced by PKC activation. The mechanism underlying PKC regulation of Kv4.2 currents is unknown. In the present study, we determined that PKC directly phosphorylates the Kv4.2 channel protein. In vitro phosphorylation of the intracellular N- and C-termini of Kv4.2 GST (glutathione transferase) tagged fusion protein revealed that the C-terminal of Kv4.2 was phosphorylated by PKC, whereas the N-terminal was not. Amino acid mapping and site-directed mutagenesis revealed that the phosphorylated residues on the Kv4.2 C-terminal were Ser447 and Ser537. A phospho-site-specific antibody showed that phosphorylation at the Ser537 site was increased in the hippocampus in response to PKC activation. Surface biotinylation experiments revealed that mutation to alanine of both Ser447 and Ser537 in order to block phosphorylation at both of the PKC sites increased surface expression compared with wild-type Kv4.2. Electrophysiological recordings of the wild-type and both the alanine and aspartate mutant Kv4.2 channels expressed with KChIP3 (Kv4 channel-interacting protein 3) revealed no significant difference in the half-activation or half-inactivation voltage of the channel. Interestingly, Ser537 lies within a possible ERK (extracellular-signal-regulated kinase)/MAPK (mitogen-activated protein kinase) recognition (docking) domain in the Kv4.2 C-terminal sequence. We found that phosphorylation of Kv4.2 by PKC enhanced ERK phosphorylation of the channel in vitro. These findings suggest the possibility that Kv4.2 is a locus for PKC and ERK cross-talk.

Modulation of transient and persistent inward currents by activation of protein kinase C in spinal ventral neurons of the neonatal rat

Neuronal excitability can be regulated through modulation of voltage threshold (Vth). Previous studies suggested that this modulation could be mediated by modulation of transient sodium currents (I(T)) and/or persistent inward current (PIC). Modulation of I(T) and PIC through activation of protein kinase C (PKC) has previously been described as a mechanism controlling neuronal excitability. We investigated modulation of I(T) and PIC by PKC in neonatal rat spinal ventral neurons. In whole cell voltage clamp, activation of PKC by application of 1-oleoyl-2-acetyl-sn-glycerol (OAG, 10-30 microM) resulted in 1) a reduction of I(T) amplitude by 33% accompanied an increase in half-width and a decrease in the maximal rise and decay rates of the I(T); 2) a reduction of PIC amplitude by 49%, with a depolarization of PIC onset by 4.5 mV. Activation of PKC caused varied effects on Vth for eliciting I(T), with an unchanged Vth or depolarized Vth being the most common effects. In current-clamp recordings, PKC activation produced a small but significant depolarization (2.0 mV) of Vth for action potential generation with an increase in half-width and a decrease in amplitude and the maximal rise and decay rates of action potentials. Inclusion of PKCI19-36 (10-30 microM), a PKC inhibitor, in the recording pipette could block the OAG effects on I(T) and PIC. The ability of serotonin to hyperpolarize Vth was not altered by PKC activation or inhibition. This study demonstrates that activation of PKC decreases the excitability of spinal ventral neurons and that Vth can be modulated by multiple mechanisms.

Protein kinase C modulates inactivation of Kv3.3 channels

Modulation of some Kv3 family potassium channels by protein kinase C (PKC) regulates their amplitude and kinetics and adjusts firing patterns of auditory neurons in response to stimulation. Nevertheless, little is known about the modulation of Kv3.3, a channel that is widely expressed throughout the nervous system and is the dominant Kv3 family member in auditory brainstem. We have cloned the cDNA for the Kv3.3 channel from mouse brain and have expressed it in a mammalian cell line and in Xenopus oocytes to characterize its biophysical properties and modulation by PKC. Kv3.3 currents activate at positive voltages and undergo inactivation with time constants of 150-250 ms. Activators of PKC increased current amplitude and removed inactivation of Kv3.3 currents, and a specific PKC pseudosubstrate inhibitor peptide prevented the effects of the activators. Elimination of the first 78 amino acids of the N terminus of Kv3.3 produced noninactivating currents suggesting that PKC modulates N-type inactivation, potentially by phosphorylation of sites in this region. To identify potential phosphorylation sites, we investigated the response of channels in which serines in this N-terminal domain were subjected to mutagenesis. Our results suggest that serines at positions 3 and 9 are potential PKC phosphorylation sites. Computer simulations of model neurons suggest that phosphorylation of Kv3.3 by PKC may allow neurons to maintain action potential height during stimulation at high frequencies, and may therefore contribute to stimulus-induced changes in the intrinsic excitability of neurons such as those of the auditory brainstem.

Potassium channel phosphorylation in excitable cells: providing dynamic functional variability to a diverse family of ion channels

Phosphorylation of potassium channels affects their function and plays a major role in regulating cell physiology. Here, we review previous studies of potassium channel phosphorylation, focusing first on studies employing site-directed mutagenesis of recombinant channels expressed in heterologous cells. We then discuss recent mass spectrometric-based approaches to identify and quantify phosphorylation at specific sites on native and recombinant potassium channels, and newly developed mass spectrometric-based techniques that may prove beneficial to future studies of potassium channel phosphorylation, its regulation, and its mechanism of channel modulation.

A new mode of regulation of N-type inactivation in a Caenorhabditis elegans voltage-gated potassium channel

N-type inactivation in voltage-gated K+ (Kv) channels is a widespread means to modulate neuronal excitability and signaling. Here we have shown a novel mechanism of N-type inactivation in a Caenorhabditis elegans Kv channel. The N-terminal sequence of KVS-1 contains a domain of 22 amino acids that resembles the inactivation ball in A-type channels, which is preceded by a domain of eighteen amino acids. Wild type KVS-1 currents can be described as A-type; however, their kinetics are significantly (approximately 5-fold) slower. When the putative inactivation ball is deleted, the current becomes non-inactivating. Inactivation is restored in non-inactivating channels by diffusion of the missing inactivation domain in the cytoplasm. Deletion of the domain in front of the ball speeds inactivation kinetics approximately 5-fold. We conclude that KVS-1 is the first example of a novel type of Kv channel simultaneously possessing an N-inactivating ball preceded by an N inactivation regulatory domain (NIRD) that acts to slow down inactivation through steric mechanisms.

Bidirectional activity-dependent regulation of neuronal ion channel phosphorylation

Activity-dependent dephosphorylation of neuronal Kv2.1 channels yields hyperpolarizing shifts in their voltage-dependent activation and homoeostatic suppression of neuronal excitability. We recently identified 16 phosphorylation sites that modulate Kv2.1 function. Here, we show that in mammalian neurons, compared with other regulated sites, such as serine (S)563, phosphorylation at S603 is supersensitive to calcineurin-mediated dephosphorylation in response to kainate-induced seizures in vivo, and brief glutamate stimulation of cultured hippocampal neurons. In vitro calcineurin digestion shows that supersensitivity of S603 dephosphorylation is an inherent property of Kv2.1. Conversely, suppression of neuronal activity by anesthetic in vivo causes hyperphosphorylation at S603 but not S563. Distinct regulation of individual phosphorylation sites allows for graded and bidirectional homeostatic regulation of Kv2.1 function. S603 phosphorylation represents a sensitive bidirectional biosensor of neuronal activity.

The concerted contribution of the S4-S5 linker and the S6 segment to the modulation of a Kv channel by 1-alkanols

Gating of voltage-gated K(+) channels (K(v) channels) depends on the electromechanical coupling between the voltage sensor and activation gate. The main activation gate of K(v) channels involves the COOH-terminal section of the S6 segment (S6-b) and the S4-S5 linker at the intracellular mouth of the pore. In this study, we have expanded our earlier work to probe the concerted contribution of these regions to the putative amphipathic 1-alkanol site in the Shaw2 K(+) channel. In the S4-S5 linker, we found a direct energetic correlation between alpha-helical propensity and the inhibition of the Shaw2 channel by 1-butanol. Spectroscopic structural analyses of the S4-S5 linker supported this correlation. Furthermore, the analysis of chimeric Shaw2 and K(v)3.4 channels that exchanged their corresponding S4-S5 linkers showed that the potentiation induced by 1-butanol depends on the combination of a single mutation in the S6 PVPV motif (PVAV) and the presence of the Shaw2 S4-S5 linker. Then, using tandem-heterodimer subunits, we determined that this potentiation also depends on the number of S4-S5 linkers and PVAV mutations in the K(v) channel tetramer. Consistent with the critical contribution of the Shaw2 S4-S5 linker, the equivalent PVAV mutation in certain mammalian K(v) channels with divergent S4-S5 linkers conferred weak potentiation by 1-butanol. Overall, these results suggest that 1-alkanol action in Shaw2 channels depends on interactions involving the S4-S5 linker and the S6-b segment. Therefore, we propose that amphiphilic general anesthetic agents such as 1-alkanols may modulate gating of the Shaw2 K(+) channel by an interaction with its activation gate.

Opposite regulation of Slick and Slack K+ channels by neuromodulators

Slick (Slo2.1) and Slack (Slo2.2) are two novel members of the mammalian Slo potassium channel gene family that may contribute to the resting potentials of cells and control their basal level of excitability. Slo2 channels have sensors that couple channel activity to the intracellular concentrations of Na+ and Cl- ions (Yuan et al., 2003). We now report that activity of both Slo2 channels is controlled by neuromodulators through Galphaq-protein coupled receptors (GqPCRs) (the M1 muscarinic receptor and the mGluR1 metabotropic glutamate receptor). Experiments coexpressing channels and receptors in Xenopus oocytes show that Slo2.1 and Slo2.2 channels are modulated in opposite ways: Slo2.1 is strongly inhibited, whereas Slo2.2 currents are strongly activated through GqPCR stimulation. Differential regulation involves protein kinase C (PKC); application of the PKC activator PMA, to cells expressing channels but not receptors, inhibits Slo2.1 whole-cell currents and increases Slo2.2 currents. Synthesis of a chimera showed that the distal carboxyl region of Slo2.1 controls the sensitivity of Slo2.1 to PMA. Slo2 channels have widespread expression in brain (Bhattacharjee et al., 2002, 2005). Using immunocytochemical techniques, we show coexpression of Slo2 channels with the GqPCRs in cortical and hippocampal brain sections and in cultured hippocampal neurons. The differential control of these novel channels by neurotransmitters may elicit long-lasting increases or decreases in neuronal excitability and, because of their widespread distribution, may provide a mechanism to activate or repress electrical activity in many systems of the brain.

Functional properties and oxidative modulation of A-type K currents in hippocampal granule cells of control and chronically epileptic rats

A-type K+ channels are crucial determinants of neuronal firing. For example, reducing the amplitude of A-type currents (I(A)) increases seizure susceptibility. We have therefore examined the functional and molecular properties of I(A) in dentate granule neurons following pilocarpine-induced status epilepticus (SE). We found that the levels of various A-type channel subunit mRNAs are unaltered following SE. Furthermore, current density and biophysical properties of I(A) recorded in outside-out and cell-attached patches from dentate granule cells are not modified by SE. However, I(A) in both control and epileptic rats was powerfully regulated by the cellular redox state. I(A) was recorded in outside-out patches with the recording pipette containing either reduced (GSH) or oxidized (GSSG) glutathione. In both control and epileptic rats, the presence of GSSG caused a similar, marked acceleration of recovery from inactivation. Additionally, GSSG produced a small but significant reduction of I(A) amplitudes only in control rats. The inactivation time course of I(A) during depolarizing voltage steps was not modified by GSH or GSSG. Cell-attached recordings, in which the intracellular milieu is conserved, revealed a slow time course of recovery more comparable to that with GSH. In summary, epileptic activity does not produce chronic changes in the molecular and functional properties of the somatic I(A) of dentate granule cells. However, I(A) is powerfully modulated by oxidation in both control and epileptic rats. This finding suggests that the availability of I(A) may be strongly regulated by changes in the GSH/GSSG ratio occurring during prolonged seizure activity or hypoxia.

Phosphorylation and protonation of neighboring MiRP2 sites: function and pathophysiology of MiRP2-Kv3.4 potassium channels in periodic paralysis

MinK-related peptide 2 (MiRP2) and Kv3.4 subunits assemble in skeletal muscle to create subthreshold, voltage-gated potassium channels. MiRP2 acts on Kv3.4 to shift the voltage dependence of activation, speed recovery from inactivation, suppress cumulative inactivation and increase unitary conductance. We previously found an R83H missense mutation in MiRP2 that segregated with periodic paralysis in two families and diminished the effects of MiRP2 on Kv3.4. Here we show that MiRP2 has a single, functional PKC phosphorylation site at serine 82 and that normal MiRP2-Kv3.4 function requires phosphorylation of the site. The R83H variant does not prevent PKC phosphorylation of neighboring S82; rather, the change shifts the voltage dependence of activation and endows MiRP2-Kv3.4 channels with sensitivity to changes in intracellular pH across the physiological range. Thus, current passed by single R83H channels decreases as internal pH is lowered (pK(a) approximately 7.3, consistent with histidine protonation) whereas wild-type channels are largely insensitive. These findings identify a key regulatory domain in MiRP2 and suggest a mechanistic link between acidosis and episodes of periodic paralysis.

ERK/MAPK regulates the Kv4.2 potassium channel by direct phosphorylation of the pore-forming subunit

Kv4.2 is the primary pore-forming subunit encoding A-type currents in many neurons throughout the nervous system, and it also contributes to the transient outward currents of cardiac myocytes. A-type currents in the dendrites of hippocampal CA1 pyramidal neurons are regulated by activation of ERK/MAPK, and Kv4.2 is the likely pore-forming subunit of that current. We showed previously that Kv4.2 is directly phosphorylated at three sites by ERK/MAPK (T602, T607, and S616). In this study we determined whether direct phosphorylation of Kv4.2 by ERK/MAPK is responsible for the regulation of the A-type current observed in neurons. We made site-directed mutants, changing the phosphosite serine (S) or threonine (T) to aspartate (D) to mimic phosphorylation. We found that the T607D mutation mimicked the electrophysiological changes elicited by ERK/MAPK activation in neurons: a rightward shift of the activation curve and an overall reduction in current compared with wild type (WT). Surprisingly, the S616D mutation caused the opposite effect, a leftward shift in the activation voltage. K+ channel-interacting protein (KChIP)3 ancillary subunit coexpression with Kv4.2 was necessary for the T607D effect, as the T607D mutant when expressed in the absence of KChIP3 was not different from WT Kv4.2. These data suggest that direct phosphorylation of Kv4.2 at T607 is involved in the dynamic regulation of the channel function by ERK/MAPK and an interaction of the primary subunit with KChIP is also necessary for this effect. Overall these studies provide new insights into the structure-function relationships for MAPK regulation of membrane ion channels.

Acoustic environment determines phosphorylation state of the Kv3.1 potassium channel in auditory neurons

Sound localization by auditory brainstem nuclei relies on the detection of microsecond interaural differences in action potentials that encode sound volume and timing. Neurons in these nuclei express high amounts of the Kv3.1 potassium channel, which allows them to fire at high frequencies with short-duration action potentials. Using computational modeling, we show that high amounts of Kv3.1 current decrease the timing accuracy of action potentials but enable neurons to follow high-frequency stimuli. The Kv3.1b channel is regulated by protein kinase C (PKC), which decreases current amplitude. Here we show that in a quiet environment, Kv3.1b is basally phosphorylated in rat brainstem neurons but is rapidly dephosphorylated in response to high-frequency auditory or synaptic stimulation. Dephosphorylation of the channel produced an increase in Kv3.1 current, facilitating high-frequency spiking. Our results indicate that the intrinsic electrical properties of auditory neurons are rapidly modified to adjust to the ambient acoustic environment.

Molecular physiology and modulation of somatodendritic A-type potassium channels

The somatodendritic subthreshold A-type K+ current (ISA) in nerve cells is a critical component of the ensemble of voltage-gated ionic currents that determine somatodendritic signal integration. The underlying K+ channel belongs to the Shal subfamily of voltage-gated K+ channels. Most Shal channels across the animal kingdom share a high degree of structural conservation, operate in the subthreshold range of membrane potentials, and exhibit relatively fast inactivation and recovery from inactivation. Mammalian Shal K+ channels (Kv4) undergo preferential closed-state inactivation with features that are generally inconsistent with the classical mechanisms of inactivation typical of Shaker K+ channels. Here, we review (1) the physiological and genetic properties of ISA, 2 the molecular mechanisms of Kv4 inactivation and its remodeling by a family of soluble calcium-binding proteins (KChIPs) and a membrane-bound dipeptidase-like protein (DPPX), and (3) the modulation of Kv4 channels by protein phosphorylation.

The MinK-related peptides

Voltage-gated potassium (Kv) channels mediate rapid, selective diffusion of K+ ions through the plasma membrane, controlling cell excitability, secretion and signal transduction. KCNE genes encode a family of single transmembrane domain proteins called MinK-related peptides (MiRPs) that function as ancillary or beta subunits of Kv channels. When co-expressed in heterologous systems, MiRPs confer changes in Kv channel conductance, gating kinetics and pharmacology, and are fundamental to recapitulation of the properties of some native currents. Inherited mutations in KCNE genes are associated with diseases of cardiac and skeletal muscle, and the inner ear. This article reviews our current understanding of MiRPs--their functional roles, the mechanisms underlying their association with Kv alpha subunits, their patterns of native expression and emerging evidence of the potential roles of MiRPs in the brain. The ubiquity of MiRP expression and their promiscuous association with Kv alpha subunits suggest a prominent role for MiRPs in channel dependent systems.

Association of potassium channel Kv3.4 subunits with pre- and post-synaptic structures in brainstem and spinal cord

Voltage-gated K+ channels (Kv) are divided into eight subfamilies (Kv1-8) and play a major role in determining the excitability of neurones. Members of the Kv3 subfamily are highly abundant in the CNS, with each Kv3 gene (Kv3.1-Kv3.4) exhibiting a unique pattern of expression, although single neurones can express more than one subtype. Of the Kv3 subunits relatively little is known of the Kv3.4 subunit distribution in the nervous system, particularly in the brainstem and spinal cord of the rat. We performed immunohistochemistry to determine both the cellular and sub-cellular distribution of the Kv3.4 subunit in these areas. Kv3.4 subunit immunoreactivity (Kv3.4-IR) was widespread, with dense, punctate staining in many regions including the intermediolateral cell column (IML) and the dorsal vagal nucleus (DVN), nucleus ambiguus (NA) and nucleus tractus solitarius (NTS). In the ventral horn a presynaptic location was confirmed by co-localization of Kv3.4-IR with the synaptic vesicle protein, SV2 and also with the glutamate vesicle markers vesicular glutamate transporter (VGluT) 1, VGluT2 or the glycine transporter GlyT2, suggesting a role for the channel in both excitatory and inhibitory neurotransmission. Electron microscopy confirmed a presynaptic terminal location of Kv3.4-IR in the VH, IML, DVN, NA and NTS. Interestingly however, patches of Kv3.4-IR were also revealed postsynaptically in dendritic and somatic structures throughout these areas. This staining was striking due to its localization at synaptic junctions at terminals with morphological features consistent with excitatory functions, suggesting an association with the postsynaptic density. Therefore the pre and postsynaptic localization of Kv3.4-IR suggests a role both in the control of transmitter release and in regulating neuronal excitability.

Disinactivation of N-type inactivation of voltage-gated K channels by an erbstatin analogue

In some A-type voltage-gated K channels, rapid inactivation is achieved through the binding of an N-terminal domain of the pore-forming alpha-subunit or an associated beta-subunit to a cytoplasmic acceptor located at or near the channel pore using the ball-and-chain machinery (1-5). This inactivation involving the N terminus is known as N-type inactivation. Here, we describe an erbstatin (Erb) analogue as a small molecule inhibitor of the N-type inactivation in channels of Kv1.4 and Kv1.1+Kvbeta1. We show that this inhibition of inactivation (designated as "disinactivation") is potent and selective for N-type inactivation in heterologous cells (Chinese hamster ovary and Xenopus oocytes) expressing these A-type channels. In Chinese hamster ovary cells, Erb increased the inactivation time constant of Kv1.4 from 86.5 +/- 9.5 to 150 +/- 10 ms (n = 6, p < 0.0 1). Similarly, Erb increased the inactivation time constant of Kv1.1+Kvbeta1 from 10 +/- 0.9 to 49.3 +/- 7 ms (n = 7, p < 0.01). The EC(50) for disinactivating Kv1.1+Kvbeta1 was 10.4 +/- 0.9 microm (n = 2-9). Erb had no effect upon another A-channel, Kv4.3, which does not utilize the ball-and-chain mechanism. The mechanism of Erb-induced disinactivation was also investigated. Neither cysteine oxidation nor tyrosine kinase inhibition was involved. The results demonstrate that Erb can be used as a base structure to identify potent, selective small molecule inhibitors of intracellular protein-protein interactions, and that these disinactivators may offer another therapeutic approach to the treatment of seizure disorders.

Oscillatory burst discharge generated through conditional backpropagation of dendritic spikes

Gamma frequencies of burst discharge (>40 Hz) have become recognized in select cortical and non-cortical regions as being important in feature extraction, neural synchrony and oscillatory discharge. Pyramidal cells of the electrosensory lateral line lobe (ELL) of Apteronotus leptorhynchus generate burst discharge in relation to specific features of sensory input in vivo that resemble those recognized as gamma frequency discharge when examined in vitro. We have shown that these bursts are generated by an entirely novel mechanism termed conditional backpropagation that involves an intermittent failure of dendritic Na+ spike conduction. Conditional backpropagation arises from a frequency-dependent broadening of dendritic spikes during repetitive discharge, and a mismatch between the refractory periods of somatic and dendritic spikes. A high threshold class of K+ channel, AptKv3.3, is expressed at high levels and distributed over the entire soma-dendritic axis of pyramidal cells. AptKv3.3 channels are shown to contribute to the repolarization of both somatic and dendritic spikes, with pharmacological blockade of dendritic Kv3 channels revealing an important role in controlling the threshold for burst discharge. The entire process of conditional back-propagation and burst output is successfully simulated using a new compartmental model of pyramidal cells that incorporates a cumulative inactivation of dendritic K+ channels during repetitive discharge. This work is important in demonstrating how the success of spike backpropagation can control the output of a principle sensory neuron, and how this process is regulated by the distribution and properties of voltage-dependent K+ channels.

Regulation of K+ channel activities in plants: from physiological to molecular aspects

Plant voltage-gated channels belonging to the Shaker family participate in sustained K+ transport processes at the cell and whole plant levels, such as K+ uptake from the soil solution, long-distance K+ transport in the xylem and phloem, and K+ fluxes in guard cells during stomatal movements. The attention here is focused on the regulation of these transport systems by protein-protein interactions. Clues to the identity of the regulatory mechanisms have been provided by electrophysiological approaches in planta or in heterologous systems, and through analogies with their animal counterparts. It has been shown that, like their animal homologues, plant voltage-gated channels can assemble as homo- or heterotetramers associating polypeptides encoded by different Shaker genes, and that they can bind auxiliary subunits homologous to those identified in mammals. Furthermore, several regulatory processes (involving, for example, protein kinases and phosphatases, G proteins, 14-3-3s, or syntaxins) might be common to plant and animal Shakers. However, the molecular identification of plant channel partners is still at its beginning. This paper reviews current knowledge on plant K+ channel regulation at the physiological and molecular levels, in the light of the corresponding knowledge in animal cells, and discusses perspectives for the deciphering of regulatory networks in the future.