Complex subunit assembly of neuronal voltage-gated K+ channels. Basis for high-affinity toxin interactions and pharmacology

Snake Venom: A Promising Source of Neurotoxins Targeting Voltage-Gated Potassium Channels

The venom derived from various sources of snakes represents a vast collection of predominantly protein-based toxins that exhibit a wide range of biological actions, including but not limited to inflammation, pain, cytotoxicity, cardiotoxicity, and neurotoxicity. The venom of a particular snake species is composed of several toxins, while the venoms of around 600 venomous snake species collectively encompass a substantial reservoir of pharmacologically intriguing compounds. Despite extensive research efforts, a significant portion of snake venoms remains uncharacterized. Recent findings have demonstrated the potential application of neurotoxins derived from snake venom in selectively targeting voltage-gated potassium channels (Kv). These neurotoxins include BPTI-Kunitz polypeptides, PLA2 neurotoxins, CRISPs, SVSPs, and various others. This study provides a comprehensive analysis of the existing literature on the significance of Kv channels in various tissues, highlighting their crucial role as proteins susceptible to modulation by diverse snake venoms. These toxins have demonstrated potential as valuable pharmacological resources and research tools for investigating the structural and functional characteristics of Kv channels.

K V 1/D‐type potassium channels inhibit the excitability of bronchopulmonary vagal afferent nerves

Abstract

The K_V1/D‐type potassium current (I_D) is an important determinant of neuronal excitability. This study explored whether and how I_D channels regulate the activation of bronchopulmonary vagal afferent nerves. The single‐neuron RT‐PCR assay revealed that nearly all mouse bronchopulmonary nodose neurons expressed the transcripts of α‐dendrotoxin (α‐DTX)‐sensitive, I_D channel‐forming K_V1.1, K_V1.2 and/or K_V1.6 α‐subunits, with the expression of K_V1.6 being most prevalent. Patch‐clamp recordings showed that I_D, defined as the α‐DTX‐sensitive K⁺ current, activated at voltages slightly more negative than the resting membrane potential in lung‐specific nodose neurons and displayed little inactivation at subthreshold voltages. Inhibition of I_D channels by α‐DTX depolarized the lung‐specific nodose neurons and caused an increase in input resistance, decrease in rheobase, as well as increase in action potential number and firing frequency in response to suprathreshold current steps. Application of α‐DTX to the lungs via trachea in the mouse ex vivo vagally innervated trachea–lungs preparation led to action potential discharges in nearly half of bronchopulmonary nodose afferent nerve fibres, including nodose C‐fibres, as detected by the two‐photon microscopic Ca²⁺ imaging technique and extracellular electrophysiological recordings. In conclusion, I_D channels act as a critical brake on the activation of bronchopulmonary vagal afferent nerves by stabilizing the membrane potential, counterbalancing the subthreshold depolarization and promoting the adaptation of action potential firings. Down‐regulation of I_D channels, as occurs in various inflammatory diseases, may contribute to the enhanced C‐fibre activity in airway diseases that are associated with excessive coughing, dyspnoea, and reflex bronchospasm and secretions.

Key points

The α‐dendrotoxin (α‐DTX)‐sensitive D‐type K⁺ current (I_D) is an important determinant of neuronal excitability.

Nearly all bronchopulmonary nodose afferent neurons in the mouse express I_D and the transcripts of α‐DTX‐sensitive, I_D channel‐forming K_V1.1, K_V1.2 and/or K_V1.6 α‐subunits.

Inhibition of I_D channels by α‐DTX depolarizes the bronchopulmonary nodose neurons, reduces the minimal depolarizing current needed to evoke an action potential (AP) and increases AP number and AP firing frequency in response to suprathreshold stimulations.

Application of α‐DTX to the lungs ex vivo elicits AP discharges in about half of bronchopulmonary nodose C‐fibre terminals.

Our novel finding that I_D channels act as a critical brake on the activation of bronchopulmonary vagal afferent nerves suggests that their down‐regulation, as occurs in various inflammatory diseases, may contribute to the enhanced C‐fibre activity in airway inflammation associated with excessive respiratory symptoms.

Inhibitory effect of the selective serotonin reuptake inhibitor paroxetine on human Kv1.3 channels

Paroxetine is one of the most effective selective serotonin reuptake inhibitors used to treat depressive and panic disorders that reduce the viability of human T lymphocytes, in which Kv1.3 channels are highly expressed. We examined whether paroxetine could modulate human Kv1.3 channels acutely and directly with the aim of understanding the biophysical effects and the underlying mechanisms of the drug. Kv1.3 channel proteins were expressed in Xenopus oocytes. Paroxetine rapidly inhibited the steady-state current and peak current of these channels within 6 min in a concentration-dependent manner; IC₅₀s were 26.3 μM and 53.9 μM, respectively, and these effects were partially reversed by washout, which excluded the possibility of genomic regulation. At the same test voltage, paroxetine blockade of the steady-state currents was higher than that of the peak currents, and the inhibition of the steady-state current increased relative to the degree of depolarization. Paroxetine decreased the inactivation time constant in a concentration-dependent manner, but it did not affect the activation time constant, which resulted in the acceleration of intrinsic inactivation without changing ultrarapid activation. Blockade of Kv1.3 channels by paroxetine exhibited more rapid inhibition at higher activation frequencies showing the use-dependency of the blockade. Overall, these results show that paroxetine directly suppresses human Kv1.3 channels in an open state and accelerates the process of steady-state inactivation; thus, we have revealed a biophysical mechanism for possible acute immunosuppressive effects of paroxetine.

Marine Toxins Targeting Kv1 Channels: Pharmacological Tools and Therapeutic Scaffolds

Toxins from marine animals provide molecular tools for the study of many ion channels, including mammalian voltage-gated potassium channels of the Kv1 family. Selectivity profiling and molecular investigation of these toxins have contributed to the development of novel drug leads with therapeutic potential for the treatment of ion channel-related diseases or channelopathies. Here, we review specific peptide and small-molecule marine toxins modulating Kv1 channels and thus cover recent findings of bioactives found in the venoms of marine Gastropod (cone snails), Cnidarian (sea anemones), and small compounds from cyanobacteria. Furthermore, we discuss pivotal advancements at exploiting the interaction of κM-conotoxin RIIIJ and heteromeric Kv1.1/1.2 channels as prevalent neuronal Kv complex. RIIIJ's exquisite Kv1 subtype selectivity underpins a novel and facile functional classification of large-diameter dorsal root ganglion neurons. The vast potential of marine toxins warrants further collaborative efforts and high-throughput approaches aimed at the discovery and profiling of Kv1-targeted bioactives, which will greatly accelerate the development of a thorough molecular toolbox and much-needed therapeutics.

KV1.2 channel-specific blocker from Mesobuthus eupeus scorpion venom: Structural basis of selectivity

Scorpion venom is an unmatched source of selective high-affinity ligands of potassium channels. There is a high demand for such compounds to identify and manipulate the activity of particular channel isoforms. The objective of this study was to obtain and characterize a specific ligand of voltage-gated potassium channel K_V1.2. As a result, we report the remarkable selectivity of the peptide MeKTx11-1 (α-KTx 1.16) from Mesobuthus eupeus scorpion venom to this channel isoform. MeKTx11-1 is a high-affinity blocker of K_V1.2 (IC₅₀ ∼0.2 nM), while its activity against K_V1.1, K_V1.3, and K_V1.6 is 10 000, 330 and 45 000 fold lower, respectively, as measured using the voltage-clamp technique on mammalian channels expressed in Xenopus oocytes. Two substitutions, G9V and P37S, convert MeKTx11-1 to its natural analog MeKTx11-3 (α-KTx 1.17) having 15 times lower activity and reduced selectivity to K_V1.2. We produced MeKTx11-1 and MeKTx11-3 as well as their mutants MeKTx11-1(G9V) and MeKTx11-1(P37S) recombinantly and demonstrated that point mutations provide an intermediate effect on selectivity. Key structural elements that explain MeKTx11-1 specificity were identified by molecular modeling of the toxin-channel complexes. Confirming our molecular modeling predictions, site-directed transfer of these elements from the pore region of K_V1.2 to K_V1.3 resulted in the enhanced sensitivity of mutant K_V1.3 channels to MeKTx11-1. We conclude that MeKTx11-1 may be used as a selective tool in neurobiology.

Characterization and molecular basis for the block of Kv1.3 channels induced by carvedilol in HEK293 cells

Carvedilol is a non-selective β-adrenoreceptor antagonist and exhibits a wide range of biological activities. The voltage-gated K⁺ (Kv) channel is one of the target ion channels of this compound. The rapidly activating Kv1.3 channel is expressed in several different tissues and plays an important role in the regulation of physiological functions, including cell proliferation and apoptosis. However, little is known about the possible action of carvedilol on Kv1.3 currents. Using the whole-cell configuration of the patch-clamp technique, we have revealed that exposure to carvedilol produced a concentration-dependent blocking of Kv1.3 channels heterologously expressed in HEK293 cells, with an IC₅₀ value of 9.7 μM. This chemical decelerated the deactivation tail current of Kv1.3 currents, resulting in a tail crossover phenomenon. In addition, carvedilol generated a markedly hyperpolarizing shift (20 mV) of the inactivation curve, but failed to affect the activation curve. Mutagenesis experiments of Kv1.3 channels identified G427 and H451, two related sites of TEA block, as important residues for carvedilol-mediated blocking. The present results suggest that carvedilol acts directly on Kv1.3 currents by inducing closed- and open-channel block and helps to elucidate the mechanisms of action of this compound on Kv channels.

Shaker-related voltage-gated potassium channels Kv1 in human hippocampus

In this study, we investigated the tissue expression levels, alpha subunit composition and distribution of Shaker-related voltage-dependent potassium Kv1 channels in human hippocampus by combining western blotting experiments, toxin autoradiography, in vivo radioligand binding studies, immunoprecipitation and immunohistochemistry. Tissue expression of Kv1.1 and Kv1.2 α-subunits in human post-mortem brain tissue was confirmed in immunoblot analysis using a panel of specific monoclonal and polyclonal antibodies. Immunoprecipitation experiments using toxin-prelabeled Kv1 channels revealed that all toxin-sensitive Kv1 channels in human hippocampus contained either a Kv1.1 or Kv1.2 α-subunit with the majority being composed of Kv1.1/Kv1.2 heterotetramers. Receptor autoradiography suggested Kv1.1/Kv1.2 channel expression in the molecular layer of dentate gyrus. In accordance, immunohistochemical experiments also observed Kv1.1 and Kv1.2 α-subunits in the molecular layer of the dentate gyrus, in addition to the CA3 stratum lucidum and the CA1 stratum oriens. These findings indicate expression in axons and terminals of hippocampal pathways, namely the perforant path, the mossy fiber pathway and the Schaffer collaterals. Herein we present the first direct demonstration that Kv1.1 and Kv1.2 channel proteins are targeted to distinct compartments of the human hippocampal formation and that this expression pattern largely reflects their distribution profile in murine brain.

Cerebellar learning modulates surface expression of a voltage-gated ion channel in cerebellar cortex

Numerous experiments using ex vivo electrophysiology suggest that mammalian learning and memory involves regulation of voltage-gated ion channels in terms of changes in function. Yet, little is known about learning-related regulation of voltage-gated ion channels in terms of changes in expression. In two experiments, we examined changes in cell surface expression of the voltage-gated potassium channel alpha-subunit Kv1.2 in a discrete region of cerebellar cortex after eyeblink conditioning (EBC), a well-studied form of cerebellar-dependent learning. Kv1.2 in cerebellar cortex is expressed almost entirely in basket cells, primarily in the axon terminal pinceaux (PCX) region, and Purkinje cells, primarily in dendrites. Cell surface expression of Kv1.2 was measured using both multiphoton microscopy, which allowed measurement confined to the PCX region, and biotinylation/western blot, which measured total cell surface expression. In the first experiment, rats underwent three sessions of EBC, explicitly unpaired stimulus exposure, or context-only exposure and the results revealed a decrease in Kv1.2 cell surface expression in the unpaired group as measured with microscopy but no change as measured with western blot. In the second experiment, the same three training groups underwent only one half of a session of training, and the results revealed an increase in Kv1.2 cell surface expression in the unpaired group as measured with western blot but no change as measured with microscopy. In addition, rats in the EBC group that did not express conditioned responses (CRs) exhibited the same increase in Kv1.2 cell surface expression as the unpaired group. The overall pattern of results suggests that cell surface expression of Kv1.2 is changed with exposure to EBC stimuli in the absence, or prior to the emergence, of CRs.

Mechanism of functional interaction between potassium channel Kv1.3 and sodium channel NavBeta1 subunit

The voltage-gated potassium channel subfamily A member 3 (Kv1.3) dominantly expresses on T cells and neurons. Recently, the interaction between Kv1.3 and NavBeta1 subunits has been explored through ionic current measurements, but the molecular mechanism has not been elucidated yet. We explored the functional interaction between Kv1.3 and NavBeta1 through gating current measurements using the Cut-open Oocyte Voltage Clamp (COVC) technique. We showed that the N-terminal 1–52 sequence of hKv1.3 disrupts the channel expression on the Xenopus oocyte membrane, suggesting a potential role as regulator of hKv1.3 expression in neurons and lymphocytes. Our gating currents measurements showed that NavBeta1 interacts with the voltage sensing domain (VSD) of Kv1.3 through W172 in the transmembrane segment and modifies the gating operation. The comparison between G-V and Q-V with/without NavBeta1 indicates that NavBeta1 may strengthen the coupling between hKv1.3-VSD movement and pore opening, inducing the modification of kinetics in ionic activation and deactivation.

A Rational Design of a Selective Inhibitor for Kv1.1 Channels Prevalent in Demyelinated Nerves That Improves Their Impaired Axonal Conduction

K⁺ channels containing Kv1.1 α subunits, which become prevalent at internodes in demyelinated axons, may underlie their dysfunctional conduction akin to muscle weakness in multiple sclerosis. Small inhibitors were sought with selectivity for the culpable hyper-polarizing K⁺ currents. Modeling of interactions with the extracellular pore in a Kv1.1-deduced structure identified diaryldi(2-pyrrolyl)methane as a suitable scaffold with optimized alkyl ammonium side chains. The resultant synthesized candidate [2,2'-((5,5'(di-p-topyldiaryldi(2-pyrrolyl)methane)bis(2,2'carbonyl)bis(azanediyl)) diethaneamine·2HCl] (8) selectively blocked Kv1.1 channels (IC₅₀ ≈ 15 μM) recombinantly expressed in mammalian cells, induced a positive shift in the voltage dependency of K⁺ current activation, and slowed its kinetics. It preferentially inhibited channels containing two or more Kv1.1 subunits regardless of their positioning in concatenated tetramers. In slices of corpus callosum from mice subjected to a demyelination protocol, this novel inhibitor improved neuronal conduction, highlighting its potential for alleviating symptoms in multiple sclerosis.

Mutations underlying Episodic Ataxia type-1 antagonize Kv1.1 RNA editing

Adenosine-to-inosine RNA editing in transcripts encoding the voltage-gated potassium channel Kv1.1 converts an isoleucine to valine codon for amino acid 400, speeding channel recovery from inactivation. Numerous Kv1.1 mutations have been associated with the human disorder Episodic Ataxia Type-1 (EA1), characterized by stress-induced ataxia, myokymia, and increased prevalence of seizures. Three EA1 mutations, V404I, I407M, and V408A, are located within the RNA duplex structure required for RNA editing. Each mutation decreased RNA editing both in vitro and using an in vivo mouse model bearing the V408A allele. Editing of transcripts encoding mutant channels affects numerous biophysical properties including channel opening, closing, and inactivation. Thus EA1 symptoms could be influenced not only by the direct effects of the mutations on channel properties, but also by their influence on RNA editing. These studies provide the first evidence that mutations associated with human genetic disorders can affect cis-regulatory elements to alter RNA editing.

KV1 and KV3 Potassium Channels Identified at Presynaptic Terminals of the Corticostriatal Synapses in Rat

In the last years it has been increasingly clear that K_V-channel activity modulates neurotransmitter release. The subcellular localization and composition of potassium channels are crucial to understanding its influence on neurotransmitter release. To investigate the role of K_V in corticostriatal synapses modulation, we combined extracellular recording of population-spike and pharmacological blockage with specific and nonspecific blockers to identify several families of K_V channels. We induced paired-pulse facilitation (PPF) and studied the changes in paired-pulse ratio (PPR) before and after the addition of specific K_V blockers to determine whether particular K_V subtypes were located pre- or postsynaptically. Initially, the presence of K_V channels was tested by exposing brain slices to tetraethylammonium or 4-aminopyridine; in both cases we observed a decrease in PPR that was dose dependent. Further experiments with tityustoxin, margatoxin, hongotoxin, agitoxin, dendrotoxin, and BDS-I toxins all rendered a reduction in PPR. In contrast heteropodatoxin and phrixotoxin had no effect. Our results reveal that corticostriatal presynaptic K_V channels have a complex stoichiometry, including heterologous combinations K_V1.1, K_V1.2, K_V1.3, and K_V1.6 isoforms, as well as K_V3.4, but not K_V4 channels. The variety of K_V channels offers a wide spectrum of possibilities to regulate neurotransmitter release, providing fine-tuning mechanisms to modulate synaptic strength.

Intracerebellar infusion of the protein kinase M zeta (PKMζ) inhibitor zeta-inhibitory peptide (ZIP) disrupts eyeblink classical conditioning

Protein kinase M zeta (PKM-ζ), a constitutively active N-terminal truncated form of PKC-ζ, has long been implicated in a cellular correlate of learning, long-term potentiation (LTP). Inhibition of PKM-ζ with zeta-inhibitory peptide (ZIP) has been shown in many brain structures to disrupt maintenance of AMPA receptors, irreversibly disrupting numerous forms of learning and memory that have been maintained for weeks. Delay eyeblink conditioning (EBC) is an established model for the assessment of cerebellar learning; here, we show that PKC-ζ and PKM-ζ are highly expressed in the cerebellar cortex, with highest expression found in Purkinje cell (PC) nuclei. Despite being highly expressed in the cerebellar cortex, no studies have examined how regulation of cerebellar PKM-ζ may affect cerebellar-dependent learning and memory. Given its disruption of learning in other brain structures, we hypothesized that ZIP would also disrupt delay EBC. We have shown that infusion of ZIP into the lobulus simplex of the rat cerebellar cortex can indeed significantly disrupt delay EBC. (PsycINFO Database Record

Cerebellar secretin modulates eyeblink classical conditioning

We have previously shown that intracerebellar infusion of the neuropeptide secretin enhances the acquisition phase of eyeblink conditioning (EBC). Here, we sought to test whether endogenous secretin also regulates EBC and to test whether the effect of exogenous and endogenous secretin is specific to acquisition. In Experiment 1, rats received intracerebellar infusions of the secretin receptor antagonist 5-27 secretin or vehicle into the lobulus simplex of cerebellar cortex immediately prior to sessions 1-3 of acquisition. Antagonist-infused rats showed a reduction in the percentage of eyeblink CRs compared with vehicle-infused rats. In Experiment 2, rats received intracerebellar infusions of secretin or vehicle immediately prior to sessions 1-2 of extinction. Secretin did not significantly affect extinction performance. In Experiment 3, rats received intracerebellar infusions of 5-27 secretin or vehicle immediately prior to sessions 1-2 of extinction. The secretin antagonist did not significantly affect extinction performance. Together, our current and previous results indicate that both exogenous and endogenous cerebellar secretin modulate acquisition, but not extinction, of EBC. We have previously shown that (1) secretin reduces surface expression of the voltage-gated potassium channel α-subunit Kv1.2 in cerebellar cortex and (2) intracerebellar infusions of a Kv1.2 blocker enhance EBC acquisition, much like secretin. Kv1.2 is almost exclusively expressed in cerebellar cortex at basket cell-Purkinje cell pinceaus and Purkinje cell dendrites; we propose that EBC-induced secretin release from PCs modulates EBC acquisition by reducing surface expression of Kv1.2 at one or both of these sites.

Margatoxin is a non-selective inhibitor of human Kv1.3 K+ channels

Margatoxin (MgTx), an alpha-KTx scorpion toxin, is considered a selective inhibitor of the Kv1.3K + channel. This peptide is widely used in ion channel research; however, a comprehensive study of its selectivity with electrophysiological methods has not been published yet. The lack of selectivity might lead to undesired side effects upon therapeutic application or may lead to incorrect conclusion regarding the role of a particular ion channel in a physiological or pathophysiological response either in vitro or in vivo. Using the patch-clamp technique we characterized the selectivity profile of MgTx using L929 cells expressing mKv1.1 channels, human peripheral lymphocytes expressing Kv1.3 channels and transiently transfected tsA201 cells expressing hKv1.1, hKv1.2, hKv1.3, hKv1.4-IR, hKv1.5, hKv1.6, hKv1.7, rKv2.1, Shaker-IR, hERG, hKCa1.1, hKCa3.1 and hNav1.5 channels. MgTx is indeed a high affinity inhibitor of Kv1.3 (Kd = 11.7 pM) but is not selective, it inhibits the Kv1.2 channel with similar affinity (Kd = 6.4 pM) and Kv1.1 in the nanomolar range (Kd = 4.2 nM). Based on our comprehensive data MgTX has to be considered a non-selective Kv1.3 inhibitor, and thus, experiments aiming at elucidating the significance of Kv1.3 in in vitro or in vivo physiological responses have to be carefully evaluated.

Regulation of action potential delays via voltage-gated potassium Kv1.1 channels in dentate granule cells during hippocampal epilepsy

Action potential (AP) responses of dentate gyrus granule (DG) cells have to be tightly regulated to maintain hippocampal function. However, which ion channels control the response delay of DG cells is not known. In some neuron types, spike latency is influenced by a dendrotoxin (DTX)-sensitive delay current (ID) mediated by unidentified combinations of voltage-gated K(+) (Kv) channels of the Kv1 family Kv1.1-6. In DG cells, the ID has not been characterized and its molecular basis is unknown. The response phenotype of mature DG cells is usually considered homogenous but intrinsic plasticity likely occurs in particular in conditions of hyperexcitability, for example during temporal lobe epilepsy (TLE). In this study, we examined response delays of DG cells and underlying ion channel molecules by employing a combination of gramicidin-perforated patch-clamp recordings in acute brain slices and single-cell reverse transcriptase quantitative polymerase chain reaction (SC RT-qPCR) experiments. An in vivo mouse model of TLE consisting of intrahippocampal kainate (KA) injection was used to examine epilepsy-related plasticity. Response delays of DG cells were DTX-sensitive and strongly increased in KA-injected hippocampi; Kv1.1 mRNA was elevated 10-fold, and the response delays correlated with Kv1.1 mRNA abundance on the single cell level. Other Kv1 subunits did not show overt changes in mRNA levels. Kv1.1 immunolabeling was enhanced in KA DG cells. The biophysical properties of ID and a delay heterogeneity within the DG cell population was characterized. Using organotypic hippocampal slice cultures (OHCs), where KA incubation also induced ID upregulation, the homeostatic reversibility and neuroprotective potential for DG cells were tested. In summary, the AP timing of DG cells is effectively controlled via scaling of Kv1.1 subunit transcription. With this antiepileptic mechanism, DG cells delay their responses during hyperexcitation.

Pharmacological characteristics of Kv1.1- and Kv1.2-containing channels are influenced by the stoichiometry and positioning of their α subunits

Voltage-sensitive neuronal Kv1 channels composed of four α subunits and four associated auxiliary β subunits control neuronal excitability and neurotransmission. Limited information exists on the combinations of α subunit isoforms (i.e. Kv1.1-1.6) or their positions in the oligomers, and how these affect sensitivity to blockers. It is known that TEA (tetraethylammonium) inhibits Kv1.1 channels largely due to binding a critical tyrosine (Tyr379) in the pore, whereas Val381 at the equivalent location in Kv1.2 makes it insensitive. With the eventual aim of developing blockers for therapeutic purposes, Kv1.1 and 1.2 α subunit genes were concatenated to form combinations representing those in central neurons, followed by surface expression in HEK (human embryonic kidney)-293 cells as single-chain functional proteins. Patch-clamp recordings demonstrated the influences of the ratios and positioning of these α subunits on the biophysical and pharmacological properties of oligomeric K+ channels. Raising the ratio of Kv1.1 to Kv1.2 in Kv1.2-1.2-1.1-1.2 led to the resultant channels being more sensitive to TEA and also affected their biophysical parameters. Moreover, mutagenesis of one or more residues in the first Kv1.2 to resemble those in Kv1.1 increased TEA sensitivity only when it is adjacent to a Kv1.1 subunit, whereas placing a non-interactive subunit between these two diminished susceptibility. The findings of the present study support the possibility of α subunits being precisely arranged in Kv1 channels, rather than being randomly assembled. This is important in designing drugs with abilities to inhibit particular oligomeric Kv1 subtypes, with the goal of elevating neuronal excitability and improving neurotransmission in certain diseases.

Shaker-related potassium channels in the central medial nucleus of the thalamus are important molecular targets for arousal suppression by volatile general anesthetics

The molecular targets and neural circuits that underlie general anesthesia are not fully elucidated. Here, we directly demonstrate that Kv1-family (Shaker-related) delayed rectifier K(+) channels in the central medial thalamic nucleus (CMT) are important targets for volatile anesthetics. The modulation of Kv1 channels by volatiles is network specific as microinfusion of ShK, a potent inhibitor of Kv1.1, Kv1.3, and Kv1.6 channels, into the CMT awakened sevoflurane-anesthetized rodents. In heterologous expression systems, sevoflurane, isoflurane, and desflurane at subsurgical concentrations potentiated delayed rectifier Kv1 channels at low depolarizing potentials. In mouse thalamic brain slices, sevoflurane inhibited firing frequency and delayed the onset of action potentials in CMT neurons, and ShK-186, a Kv1.3-selective inhibitor, prevented these effects. Our findings demonstrate the exquisite sensitivity of delayed rectifier Kv1 channels to modulation by volatile anesthetics and highlight an arousal suppressing role of Kv1 channels in CMT neurons during the process of anesthesia.

Social networking among voltage-activated potassium channels

Voltage-activated potassium channels (Kv channels) are ubiquitously expressed proteins that subserve a wide range of cellular functions. From their birth in the endoplasmic reticulum, Kv channels assemble from multiple subunits in complex ways that determine where they live in the cell, their biophysical characteristics, and their role in enabling different kinds of cells to respond to specific environmental signals to generate appropriate functional responses. This chapter describes the types of protein-protein interactions among pore-forming channel subunits and their auxiliary protein partners, as well as posttranslational protein modifications that occur in various cell types. This complex oligomerization of channel subunits establishes precise cell type-specific Kv channel localization and function, which in turn drives a diverse range of cellular signal transduction mechanisms uniquely suited to the physiological contexts in which they are found.

Cellular mechanisms and behavioral consequences of Kv1.2 regulation in the rat cerebellum

The potassium channel Kv1.2 α-subunit is expressed in cerebellar Purkinje cell (PC) dendrites where its pharmacological inhibition increases excitability (Khavandgar et al., 2005). Kv1.2 is also expressed in cerebellar basket cell (BC) axon terminals (Sheng et al., 1994), where its blockade increases BC inhibition of PCs (Southan and Robertson, 1998a). Secretin receptors are also expressed both in PC dendrites and BC axon terminals (for review, see (Yuan et al., 2011). The effect of secretin on PC excitability is not yet known, but, like Kv1.2 inhibitors, secretin potently increases inhibitory input to PCs (Yung et al., 2001). This suggests secretin may act in part by suppressing Kv1.2. Receptor-mediated endocytosis is a mechanism of Kv1.2 suppression (Nesti et al., 2004). This process can be regulated by protein kinase A (PKA) (Connors et al., 2008). Since secretin receptors activate PKA (Wessels-Reiker et al., 1993), we tested the hypothesis that secretin regulates Kv1.2 trafficking in the cerebellum. Using cell-surface protein biotinylation of rat cerebellar slices, we found secretin decreased cell-surface Kv1.2 levels by modulating Kv1.2 endocytic trafficking. This effect was mimicked by activating adenylate cyclase (AC) with forskolin, and was blocked by pharmacological inhibitors of AC or PKA. Imaging studies identified the BC axon terminal and PC dendrites as loci of AC-dependent Kv1.2 trafficking. The physiological significance of secretin-regulated Kv1.2 endocytosis is supported by our finding that infusion into the cerebellar cortex of either the Kv1.2 inhibitor tityustoxin-Kα, or of the Kv1.2 regulator secretin, significantly enhances acquisition of eyeblink conditioning in rats.

Vm24, a natural immunosuppressive peptide, potently and selectively blocks Kv1.3 potassium channels of human T cells

Blockade of Kv1.3 K(+) channels in T cells is a promising therapeutic approach for the treatment of autoimmune diseases such as multiple sclerosis and type 1 diabetes mellitus. Vm24 (α-KTx 23.1) is a novel 36-residue Kv1.3-specific peptide isolated from the venom of the scorpion Vaejovis mexicanus smithi. Vm24 inhibits Kv1.3 channels of human lymphocytes with high affinity (K(d) = 2.9 pM) and exhibits >1500-fold selectivity over other ion channels assayed. It inhibits the proliferation and Ca(2+) signaling of human T cells in vitro and reduces delayed-type hypersensitivity reactions in rats in vivo. Our results indicate that Vm24 has exceptional pharmacological properties that make it an excellent candidate for treatment of certain autoimmune diseases.

Effect of synthetic aβ peptide oligomers and fluorinated solvents on Kv1.3 channel properties and membrane conductance

The impact of synthetic amyloid β (1–42) (Aβ_1–42) oligomers on biophysical properties of voltage-gated potassium channels Kv 1.3 and lipid bilayer membranes (BLMs) was quantified for protocols using hexafluoroisopropanol (HFIP) or sodium hydroxide (NaOH) as solvents prior to initiating the oligomer formation. Regardless of the solvent used Aβ_1–42 samples contained oligomers that reacted with the conformation-specific antibodies A11 and OC and had similar size distributions as determined by dynamic light scattering. Patch-clamp recordings of the potassium currents showed that synthetic Aβ_1–42 oligomers accelerate the activation and inactivation kinetics of Kv 1.3 current with no significant effect on current amplitude. In contrast to oligomeric samples, freshly prepared, presumably monomeric, Aβ_1–42 solutions had no effect on Kv 1.3 channel properties. Aβ_1–42 oligomers had no effect on the steady-state current (at −80 mV) recorded from Kv 1.3-expressing cells but increased the conductance of artificial BLMs in a dose-dependent fashion. Formation of amyloid channels, however, was not observed due to conditions of the experiments. To exclude the effects of HFIP (used to dissolve lyophilized Aβ_1–42 peptide), and trifluoroacetic acid (TFA) (used during Aβ_1–42 synthesis), we determined concentrations of these fluorinated compounds in the stock Aβ_1–42 solutions by ¹⁹F NMR. After extensive evaporation, the concentration of HFIP in the 100× stock Aβ_1–42 solutions was ∼1.7 μM. The concentration of residual TFA in the 70× stock Aβ_1–42 solutions was ∼20 μM. Even at the stock concentrations neither HFIP nor TFA alone had any effect on potassium currents or BLMs. The Aβ_1–42 oligomers prepared with HFIP as solvent, however, were more potent in the electrophysiological tests, suggesting that fluorinated compounds, such as HFIP or structurally-related inhalational anesthetics, may affect Aβ_1–42 aggregation and potentially enhance ability of oligomers to modulate voltage-gated ion channels and biological membrane properties.

Mature and precursor brain-derived neurotrophic factor have individual roles in the mouse olfactory bulb

Background

Sensory deprivation induces dramatic morphological and neurochemical changes in the olfactory bulb (OB) that are largely restricted to glomerular and granule layer interneurons. Mitral cells, pyramidal-like neurons, are resistant to sensory-deprivation-induced changes and are associated with the precursor to brain-derived neurotrophic factor (proBDNF); here, we investigate its unknown function in the adult mouse OB.

Principal Findings

As determined using brain-slice electrophysiology in a whole-cell configuration, brain-derived neurotrophic factor (BDNF), but not proBDNF, increased mitral cell excitability. BDNF increased mitral cell action potential firing frequency and decreased interspike interval in response to current injection. In a separate set of experiments, intranasal delivery of neurotrophic factors to awake, adult mice was performed to induce sustained interneuron neurochemical changes. ProBDNF, but not BDNF, increased activated-caspase 3 and reduced tyrosine hydroxylase immunoreactivity in OB glomerular interneurons. In a parallel set of experiments, short-term sensory deprivation produced by unilateral naris occlusion generated an identical phenotype.

Conclusions

Our results indicate that only mature BDNF increases mitral cell excitability whereas proBDNF remains ineffective. Our demonstration that proBDNF activates an apoptotic marker in vivo is the first for any proneurotrophin and establishes a role for proBDNF in a model of neuronal plasticity.

Different potassium channels are involved in relaxation of rat renal artery induced by P1075

The ATP-sensitive K(+) channels opener (K(ATP)CO), P1075 [N-cyano-N'-(1,1-dimethylpropyl)-N″-3-pyridylguanidine], has been shown to cause relaxation of various isolated animal and human blood vessels by opening of vascular smooth muscle ATP-sensitive K(+) (K(ATP)) channels. In addition to the well-known effect on the opening of K(ATP) channels, it has been reported that vasorelaxation induced by some of the K(ATP)COs includes some other K(+) channel subtypes. Given that there is still no information on other types of K(+) channels possibly involved in the mechanism of relaxation induced by P1075, this study was designed to examine the effects of P1075 on the rat renal artery with endothelium and with denuded endothelium and to define the contribution of different K(+) channel subtypes in the P1075 action on this blood vessel. Our results show that P1075 induced a concentration-dependent relaxation of rat renal artery rings pre-contracted by phenylephrine. Glibenclamide, a selective K(ATP) channels inhibitor, partly antagonized the relaxation of rat renal artery induced by P1075. Tetraethylammonium (TEA), a non-selective inhibitor of Ca(2+)-activated K(+) channels, as well as iberiotoxin, a most selective blocker of large-conductance Ca(2+) -activated K(+) (BK(Ca)) channels, did not abolish the effect of P1075 on rat renal artery. In contrast, a non-selective blocker of voltage-gated K(+) (K(V)) channels, 4-aminopyridine (4-AP), as well as margatoxin, a potent inhibitor of K(V)1.3 channels, caused partial inhibition of the P1075-induced relaxation of rat renal artery. In addition, in this study, P1075 relaxed contractions induced by 20 mM K(+) , but had no effect on contractions induced by 80 mM K(+). Our results showed that P1075 induced strong endothelium-independent relaxation of rat renal artery. It seems that K(ATP), 4-AP- and margatoxin-sensitive K(+) channels located in vascular smooth muscle mediated the relaxation of rat renal artery induced by P1075.

Early changes in cerebellar physiology accompany motor dysfunction in the polyglutamine disease spinocerebellar ataxia type 3

The relationship between cerebellar dysfunction, motor symptoms, and neuronal loss in the inherited ataxias, including the polyglutamine disease spinocerebellar ataxia type 3 (SCA3), remains poorly understood. We demonstrate that before neurodegeneration, Purkinje neurons in a mouse model of SCA3 exhibit increased intrinsic excitability resulting in depolarization block and the loss of the ability to sustain spontaneous repetitive firing. These alterations in intrinsic firing are associated with increased inactivation of voltage-activated potassium currents. Administration of an activator of calcium-activated potassium channels, SKA-31, partially corrects abnormal Purkinje cell firing and improves motor function in SCA3 mice. Finally, expression of the disease protein, ataxin-3, in transfected cells increases the inactivation of Kv3.1 channels and shifts the activation of Kv1.2 channels to more depolarized potentials. Our results suggest that in SCA3, early Purkinje neuron dysfunction is associated with altered physiology of voltage-activated potassium channels. We further suggest that the observed changes in Purkinje neuron physiology contribute to disease pathogenesis, underlie at least some motor symptoms, and represent a promising therapeutic target in SCA3.

rKv1.2 overexpression in the central medial thalamic area decreases caffeine-induced arousal

The voltage-gated potassium channel Kv1.2 belongs to the shaker-related family and has recently been implicated in the control of sleep profile on the basis of clinical and experimental evidence in rodents. To further investigate whether increasing Kv1.2 activity would promote sleep occurrence in rats, we developed an adeno-associated viral vector that induces overexpression of rat Kv1.2 protein. The viral vector was first evaluated in vitro for its ability to overexpress rat Kv1.2 protein and to produce functional currents in infected U2OS cells. Next, the adeno-associated Kv1.2 vector was injected stereotaxically into the central medial thalamic area of rats and overexpression of Kv1.2 was showed by in situ hybridization, ex vivo electrophysiology and immunohistochemistry. Finally, the functional effect of Kv1.2 overexpression on sleep facilitation was investigated using telemetry system under normal conditions and following administration of the arousing agent caffeine, during the light phase. While no differences in sleep profile were observed between the control and the treated animals under normal conditions, a decrease in the pro-arousal effect of caffeine was seen only in the animals injected with the adeno-associated virus-Kv1.2 vector. Overall, our data further support a role of the Kv1.2 channel in the control of sleep profile, particularly under conditions of sleep disturbance.

Role of Kv1 potassium channels in regulating dopamine release and presynaptic D2 receptor function

Dopamine (DA) release in the CNS is critical for motor control and motivated behaviors. Dysfunction of its regulation is thought to be implicated in drug abuse and in diseases such as schizophrenia and Parkinson's. Although various potassium channels located in the somatodendritic compartment of DA neurons such as G-protein-gated inward rectifying potassium channels (GIRK) have been shown to regulate cell firing and DA release, little is presently known about the role of potassium channels localized in the axon terminals of these neurons. Here we used fast-scan cyclic voltammetry to study electrically-evoked DA release in rat dorsal striatal brain slices. We find that although G-protein-gated inward rectifying (GIRK) and ATP-gated (K_ATP) potassium channels play only a minor role, voltage-gated potassium channels of the Kv1 family play a major role in regulating DA release. The use of Kv subtype-selective blockers confirmed a role for Kv1.2, 1.3 and 1.6, but not Kv1.1, 3.1, 3.2, 3.4 and 4.2. Interestingly, Kv1 blockers also reduced the ability of quinpirole, a D2 receptor agonist, to inhibit evoked DA overflow, thus suggesting that Kv1 channels also regulate presynaptic D2 receptor function. Our work identifies Kv1 potassium channels as key regulators of DA release in the striatum.

Function and mechanism of axonal targeting of voltage-sensitive potassium channels

Precise localization of various ion channels into proper subcellular compartments is crucial for neuronal excitability and synaptic transmission. Axonal K(+) channels that are activated by depolarization of the membrane potential participate in the repolarizing phase of the action potential, and hence regulate action potential firing patterns, which encode output signals. Moreover, some of these channels can directly control neurotransmitter release at axonal terminals by constraining local membrane excitability and limiting Ca(2+) influx. K(+) channels differ not only in biophysical and pharmacological properties, but in expression and subcellular distribution as well. Importantly, proper targeting of channel proteins is a prerequisite for electrical and chemical functions of axons. In this review, we first highlight recent studies that demonstrate different roles of axonal K(+) channels in the local regulation of axonal excitability. Next, we focus on research progress in identifying axonal targeting motifs and machinery of several different types of K(+) channels present in axons. Regulation of K(+) channel targeting and activity may underlie a novel form of neuronal plasticity. This research field can contribute to generating novel therapeutic strategies through manipulating neuronal excitability in treating neurological diseases, such as multiple sclerosis, neuropathic pain, and Alzheimer's disease.

Arrangement of Kv1 alpha subunits dictates sensitivity to tetraethylammonium

Shaker-related Kv1 channels contain four channel-forming α subunits. Subfamily member Kv1.1 often occurs oligomerized with Kv1.2 α subunits in synaptic membranes, and so information was sought on the influence of their positions within tetramers on the channels’ properties. Kv1.1 and 1.2 α genes were tandem linked in various arrangements, followed by expression as single-chain proteins in mammalian cells. As some concatenations reported previously seemed not to reliably position Kv1 subunits in their assemblies, the identity of expressed channels was methodically evaluated. Surface protein, isolated by biotinylation of intact transiently transfected HEK-293 cells, gave Kv1.1/1.2 reactivity on immunoblots with electrophoretic mobilities corresponding to full-length concatenated tetramers. There was no evidence of protein degradation, indicating that concatemers were delivered intact to the plasmalemma. Constructs with like genes adjacent (Kv1.1-1.1-1.2-1.2 or Kv1.2-1.2-1.1-1.1) yielded delayed-rectifying, voltage-dependent K⁺ currents with activation parameters and inactivation kinetics slightly different from the diagonally positioned genes (Kv1.1-1.2-1.1-1.2 or 1.2–1.1-1.2-1.1). Pore-blocking petidergic toxins, α dendrotoxin, agitoxin-1, tityustoxin-Kα, and kaliotoxin, were unable to distinguish between the adjacent and diagonal concatamers. Unprecedentedly, external application of the pore-blocker tetraethylammonium (TEA) differentially inhibited the adjacent versus diagonal subunit arrangements, with diagonal constructs having enhanced susceptibility. Concatenation did not directly alter the sensitivities of homomeric Kv1.1 or 1.2 channels to TEA or the toxins. TEA inhibition of currents generated by channels made up from dimers (Kv1.1-1.2 and/or Kv1.2-1.1) was similar to the adjacently arranged constructs. These collective findings indicate that assembly of α subunits can be directed by this optimized concatenation, and that subunit arrangement in heteromeric Kv channels affects TEA affinity.

Localization of Kv1.3 channels in presynaptic terminals of brainstem auditory neurons

Elimination of the Kv1.3 voltage-dependent potassium channel gene produces striking changes in the function of the olfactory bulb, raising the possibility that this channel also influences other sensory systems. We have examined the cellular and subcellular localization of Kv1.3 in the medial nucleus of the trapezoid body (MNTB) in the auditory brainstem, a nucleus in which neurons fire at high rates with high temporal precision. A clear gradient of Kv1.3 immunostaining along the lateral to medial tonotopic axis of the MNTB was detected. Highest levels were found in the lateral region of the MNTB, which corresponds to neurons that respond selectively to low-frequency auditory stimuli. Previous studies have demonstrated that MNTB neurons and their afferent inputs from the cochlear nucleus express three other members of the Kv1 family, Kv1.1, Kv1.2, and Kv1.6. Nevertheless, confocal microscopy of MNTB sections coimmunostained for Kv1.3 with these subunits revealed that the distribution of Kv1.3 differed significantly from other Kv1 family subunits. In particular, no axonal staining of Kv1.3 was detected, and most prominent labeling was in structures surrounding the somata of the principal neurons, suggesting specific localization to the large calyx of Held presynaptic endings that envelop the principal cells. The presence of Kv1.3 in presynaptic terminals was confirmed by coimmunolocalization with the synaptic markers synaptophysin, syntaxin, and synaptotagmin and by immunogold electron microscopy. Kv1.3 immunogold particles in the terminals were arrayed along the plasma membrane and on internal vesicular structures. To confirm these patterns of staining, we carried out immunolabeling on sections from Kv1.3(-/-) mice. No immunoreactivity could be detected in Kv1.3(-/-) mice either at the light level or in immunogold experiments. The finding of a tonotopic gradient in presynaptic terminals suggests that Kv1.3 may regulate neurotransmitter release differentially in neurons that respond to different frequencies of sound.

A new Kv1.2 channelopathy underlying cerebellar ataxia

A forward genetic screen of mice treated with the mutagen ENU identified a mutant mouse with chronic motor incoordination. This mutant, named Pingu (Pgu), carries a missense mutation, an I402T substitution in the S6 segment of the voltage-gated potassium channel Kcna2. The gene Kcna2 encodes the voltage-gated potassium channel α-subunit Kv1.2, which is abundantly expressed in the large axon terminals of basket cells that make powerful axo-somatic synapses onto Purkinje cells. Patch clamp recordings from cerebellar slices revealed an increased frequency and amplitude of spontaneous GABAergic inhibitory postsynaptic currents and reduced action potential firing frequency in Purkinje cells, suggesting that an increase in GABA release from basket cells is involved in the motor incoordination in Pgu mice. In line with immunochemical analyses showing a significant reduction in the expression of Kv1 channels in the basket cell terminals of Pgu mice, expression of homomeric and heteromeric channels containing the Kv1.2(I402T) α-subunit in cultured CHO cells revealed subtle changes in biophysical properties but a dramatic decrease in the amount of functional Kv1 channels. Pharmacological treatment with acetazolamide or transgenic complementation with wild-type Kcna2 cDNA partially rescued the motor incoordination in Pgu mice. These results suggest that independent of known mutations in Kcna1 encoding Kv1.1, Kcna2 mutations may be important molecular correlates underlying human cerebellar ataxic disease.

Voltage-gated delayed rectifier K v 1-subunits may serve as distinctive markers for enteroglial cells with different phenotypes in the murine ileum

Due to entangled results concerning K(v)1 subunit distribution in the gastrointestinal wall, we aimed to unravel the expression of the delayed rectifier potassium subunits K(v)1.1 and K(v)1.2 in the murine ileum. Presence and distribution of both subunits were determined in cryosections and whole-mount preparations of the ileum of three different murine strains by indirect immunofluorescence, and analysed by conventional fluorescence and confocal microscopy. Distribution of both subunits was similar in the ileum of the three strains. K(v)1.1 immunoreactivity (IR) was found in some S100-expressing enteroglial cells (EGC) located at the periphery of myenteric ganglia, in S100-positive EGC along interganglionic, intramuscular and vascular nerve fibres, and in S100-positive EGC of the submucous plexus. K(v)1.1 IR was also observed in some GFAP-expressing EGC at the periphery of myenteric ganglia, and in GFAP-positive EGC of submucous ganglia. K(v)1.2 IR was detected in some intramuscular S100-positive EGC, in almost all submucous S100-expressing EGC, and in a few GFAP-expressing EGC. K(v)1.2 IR was also expressed in a majority of enteric neurons. Coding of these neurons showed that all cholinergic and most nitrergic neurons express K(v)1.2. In conclusion, the results showed that K(v)1.1 and K(v)1.2 were predominantly expressed in distinct EGC phenotypes. K(v)1.2 was also observed in distinct neuron subpopulations. Our results support the active role of EGC with distinct phenotypes in intestinal functions, which is relevant in view of their modulating role on intestinal barrier and inflammatory responses.

Localization and targeting of voltage-dependent ion channels in mammalian central neurons

The intrinsic electrical properties and the synaptic input-output relationships of neurons are governed by the action of voltage-dependent ion channels. The localization of specific populations of ion channels with distinct functional properties at discrete sites in neurons dramatically impacts excitability and synaptic transmission. Molecular cloning studies have revealed a large family of genes encoding voltage-dependent ion channel principal and auxiliary subunits, most of which are expressed in mammalian central neurons. Much recent effort has focused on determining which of these subunits coassemble into native neuronal channel complexes, and the cellular and subcellular distributions of these complexes, as a crucial step in understanding the contribution of these channels to specific aspects of neuronal function. Here we review progress made on recent studies aimed to determine the cellular and subcellular distribution of specific ion channel subunits in mammalian brain neurons using in situ hybridization and immunohistochemistry. We also discuss the repertoire of ion channel subunits in specific neuronal compartments and implications for neuronal physiology. Finally, we discuss the emerging mechanisms for determining the discrete subcellular distributions observed for many neuronal ion channels.

Kv1.5 association modifies Kv1.3 traffic and membrane localization

Kv1.3 activity is determined by raft association. In addition to Kv1.3, leukocytes also express Kv1.5, and both channels control physiological responses. Because the oligomeric composition may modify the channel targeting to the membrane, we investigated heterotetrameric Kv1.3/Kv1.5 channel traffic and targeting in HEK cells. Kv1.3 and Kv1.5 generate multiple heterotetramers with differential surface expression according to the subunit composition. FRET analysis and pharmacology confirm the presence of functional hybrid channels. Raft association was evaluated by cholesterol depletion, caveolae colocalization, and lateral diffusion at the cell surface. Immunoprecipitation showed that both Kv1.3 and heteromeric channels associate with caveolar raft domains. However, homomeric Kv1.3 channels showed higher association with caveolin traffic. Moreover, FRAP analysis revealed higher mobility for hybrid Kv1.3/Kv1.5 than Kv1.3 homotetramers, suggesting that heteromers target to distinct surface microdomains. Studies with lipopolysaccharide-activated macrophages further supported that different physiological mechanisms govern Kv1.3 and Kv1.5 targeting to rafts. Our results implicate the traffic and localization of Kv1.3/Kv1.5 heteromers in the complex regulation of immune system cells.

Presence of HSV-1 immediate early genes and clonally expanded T-cells with a memory effector phenotype in human trigeminal ganglia

The latent persistence of herpes simplex virus type 1 (HSV-1) in human trigeminal ganglia (TG) is accompanied by a chronic CD8 T-cell infiltrate. The focus of the current work was to look for HSV-1 transcription activity as a potential trigger of the immune response and to characterize the immune cell infiltrates by this feature. We combined in situ hybridization, laser cutting microscopy, and single cell RT-PCR to demonstrate the expression of the HSV-1 immediate early (IE) genes ICP0 and ICP4 in human trigeminal neurons. Using CDR3 spectratyping, we showed that the infiltrating T-cells are clonally expanded, indicating an antigen-driven immune response. Moreover, the persisting CD8+ T-cells had features of the memory effector phenotype. The voltage-gated potassium channel Kv1.3, a marker of chronic activated memory effector cells, and the chemokines CCL5 and CXCL10 were expressed by a subpopulation of infiltrating cells. The corresponding chemokine receptors CCR5 and CXCR3 were co-expressed on virtually all CD8 T-cells. In addition, T-cells expressed granzymes and perforin. In contrast to animal models of HSV-1 latency, hardly any FoxP3-positive regulatory T-cells were detected in human TG. Thus, HSV-1 IE genes are expressed in human TG and the infiltrating T-cells bear several characteristics that suggest viral antigenic stimulation.

Structural consequences of Kcna1 gene deletion and transfer in the mouse hippocampus

PURPOSE

Mice lacking the Kv1.1 potassium channel alpha subunit encoded by the Kcna1 gene develop recurrent behavioral seizures early in life. We examined the neuropathological consequences of seizure activity in the Kv1.1(-/-) (knock-out) mouse, and explored the effects of injecting a viral vector carrying the deleted Kcna1 gene into hippocampal neurons.

METHODS

Morphological techniques were used to assess neuropathological patterns in hippocampus of Kv1.1(-/-) animals. Immunohistochemical and biochemical techniques were used to monitor ion channel expression in Kv1.1(-/-) brain. Both wild-type and knockout mice were injected (bilaterally into hippocampus) with an HSV1 amplicon vector that contained the rat Kcna1 subunit gene and/or the E. coli lacZ reporter gene. Vector-injected mice were examined to determine the extent of neuronal infection.

RESULTS

Video/EEG monitoring confirmed interictal abnormalities and seizure occurrence in Kv1.1(-/-) mice. Neuropathological assessment suggested that hippocampal damage (silver stain) and reorganization (Timm stain) occurred only after animals had exhibited severe prolonged seizures (status epilepticus). Ablation of Kcna1 did not result in compensatory changes in expression levels of other related ion channel subunits. Vector injection resulted in infection primarily of granule cells in hippocampus, but the number of infected neurons was quite variable across subjects. Kcna1 immunocytochemistry showed "ectopic" Kv1.1 alpha channel subunit expression.

CONCLUSIONS

Kcna1 deletion in mice results in a seizure disorder that resembles--electrographically and neuropathologically--the patterns seen in rodent models of temporal lobe epilepsy. HSV1 vector-mediated gene transfer into hippocampus yielded variable neuronal infection.

Concatemers of brain Kv1 channel alpha subunits that give similar K+ currents yield pharmacologically distinguishable heteromers

At least five subtypes of voltage-gated (Kv1) channels occur in neurons as tetrameric combinations of different alpha subunits. Their involvement in controlling cell excitability and synaptic transmission make them potential targets for neurotherapeutics. As a prerequisite for this, we established herein how the characteristics of hetero-oligomeric K(+) channels can be influenced by alpha subunit composition. Since the three most prevalent Kv1 subunits in brain are Kv1.2, 1.1 and 1.6, new Kv1.6-1.2 and Kv1.1-1.2 concatenated constructs in pIRES-EGFP were stably expressed in HEK cells and the biophysical plus pharmacological properties of their K(+) currents determined relative to those for the requisite homo-tetramers. These heteromers yielded delayed-rectifier type K(+) currents whose activation, deactivation and inactivation parameters are fairly similar although substituting Kv1.1 with Kv1.6 led to a small negative shift in the conductance-voltage relationship, a direction unexpected from the characteristics of the parental homo-tetramers. Changes resulting from swapping Kv1.6 for Kv1.1 in the concatemers were clearly discerned with two pharmacological agents, as measured by inhibition of the K(+) currents and Rb(+) efflux. alphaDendrotoxin and 4-aminopyridine gave a similar blockade of both hetero-tetramers, as expected. Most important for pharmacological dissection of channel subtypes, dendrotoxin(k) and tetraethylammonium readily distinguished the susceptible Kv1.1-1.2 containing oligomers from the resistant Kv1.6-1.2 channels. Moreover, the discriminating ability of dendrotoxin(k) was further confirmed by its far greater ability to displace (125)I-labelled alphadendrotoxin binding to Kv1.1-1.2 than Kv1.6-1.2 channels. Thus, due to the profiles of these two channel subtypes being found to differ, it seems that only multimers corresponding to those present in the nervous system provide meaningful targets for drug development.

Regulation of Kv1 channel trafficking by the mamba snake neurotoxin dendrotoxin K

Modulation of voltage-gated potassium (Kv) channel surface expression can profoundly affect neuronal excitability. Some, but not all, mammalian Shaker or Kv1 alpha subunits contain a dominant endoplasmic reticulum (ER) retention signal in their pore region, preventing surface expression of Kv1.1 homotetrameric channels and of heteromeric Kv1 channels containing more than one Kv1.1 subunit. The critical amino acid residues within this ER pore-region retention signal are also critical for high-affinity binding of snake dendrotoxins (DTX). This suggests that ER retention may be mediated by an ER protein with a domain structurally similar to that of DTX. One facet of such a model is that expression of soluble DTX in the ER lumen should compete for binding to the retention protein and allow for surface expression of retained Kv1.1. Here, we show that luminal DTX expression dramatically increased both the level of cell surface Kv1.1 immunofluorescence staining and the proportion of Kv1.1 with processed N-linked oligosaccharides. Electrophysiological analyses showed that luminal DTX expression led to significant increases in Kv1.1 currents. Together, these data showed that luminal DTX expression increases surface expression of functional Kv1.1 homotetrameric channels and support a model whereby a DTX-like ER protein regulates abundance of cell surface Kv1 channels.

Targeting effector memory T cells with the small molecule Kv1.3 blocker PAP-1 suppresses allergic contact dermatitis

The voltage-gated potassium channel Kv1.3 has been recently identified as a molecular target that allows for selective pharmacological suppression of effector memory T (T(EM)) cells without affecting the function of naïve and central memory T cells. We here investigated whether PAP-1, a small molecule Kv1.3 blocker (EC50=2 nM), could suppress allergic contact dermatitis (ACD). In a rat model of ACD, we first confirmed that the infiltrating cells in the elicitation phase are indeed CD8+ CD45RC- memory T cells with high Kv1.3 expression. In accordance with its selective effect on T(EM) cells, PAP-1 did not impair sensitization, but potently suppressed oxazolone-induced inflammation by inhibiting the infiltration of CD8+ T cells and reducing the production of the inflammatory cytokines IFN-gamma, IL-2, and IL-17 when administered intraperitoneally or orally during the elicitation phase. PAP-1 was equally effective when applied topically, demonstrating that it effectively penetrates skin. We further show that PAP-1 is not a sensitizer or an irritant and exhibits no toxicity in a 28-day toxicity study. Based on these results we propose that PAP-1 could potentially be developed into a drug for the topical treatment of inflammatory skin diseases such as psoriasis.

Involvement of Kv channel subtypes on GABA release in mechanically dissociated neurons from the rat substantia nigra

The seven members of Shaker-related K(+) channel family, which are known to regulate membrane excitability and transmitter release, have been identified in the CNS. It is known that the substantia nigra pars reticulata (SNr) receives GABAergic inputs mainly from the striatum and sends GABAergic outputs to the thalamus. An immunohistochemical study shows that the Kv1 family, particularly Kv1.4, is expressed in the SNr, while it is reported that Kv1.2 mRNA is detected in the striatal neurons. Therefore, which Kv channels may be involved in the release of GABA in the SNr remains still controversial. To address this issue, we tested the effects of various K(+) channel blockers on the synaptic transmission using mechanically dissociated SNr neurons known as "synaptic bouton preparation", that retained functional presynaptic nerve terminals which enable us to examine miniature inhibitory postsynaptic currents (mIPSCs) by conventional whole-cell patch clamp recordings. Based on the sensitivities of mIPSCs to the Kv channel blockers, we concluded that Kv channels, in particular Kv1.2 subunit play the most significant role in the release of GABA at the presynaptic terminals projecting to the SNr neurons.

A genetically modified mouse model probing the selective action of ifenprodil at the N-methyl-d-aspartate type 2B receptor

Selective antagonism of N-methyl-d-aspartate (NMDA) 2B subunit containing receptors has been suggested to have potential therapeutic application for multiple CNS disorders. The amino terminal NR2B residues 1 to 282 were found to be both necessary and sufficient for the binding and function of highly NR2B subunit specific antagonists like ifenprodil and CP-101,606. Using a genetic approach in mice, we successfully replaced the murine NR2B gene function by "knocking-in" (KI) a chimeric human NR2A/B cDNA containing the minimal domain abolishing ifenprodil binding into the endogenous NR2B locus. Patch-clamp recording from hippocampal cultures of the NR2B KI mice demonstrated that their NMDA receptors have reduced sensitivity to both ifenprodil and CP-101,606, as predicted, but also have a lower affinity for glycine. The NR2B KI mice exhibited normal locomotor activity making this ifenprodil-insensitive mouse model a valuable tool to test the specificity of NR2B selective antagonists in vivo.

Neuromyotonia and limbic encephalitis sera target mature Shaker-type K+ channels: subunit specificity correlates with clinical manifestations

Autoantibodies to Shaker-type (Kv1) K+ channels are now known to be associated with three syndromes. Peripheral nerve hyperexcitability is the chief manifestation of acquired neuromyotonia; the combination of neuromyotonia with autonomic and CNS involvement is called Morvan's syndrome (MoS); and CNS manifestations without peripheral involvement is called limbic encephalitis (LE). To determine the cellular basis of these clinical manifestations, we immunostained mouse neural tissues with sera from patients with neuromyotonia (n = 10), MoS (n = 2) or LE (n = 5), comparing with specific antibodies to relevant K+ channel subunits. Fourteen of 17 patients' sera were positive for Kv1.1, Kv1.2 or Kv1.6 antibodies by immunoprecipitation of 125I-alpha-dendrotoxin-labelled rabbit brain K+ channels. Most sera (11 out of 17) labelled juxtaparanodes of peripheral myelinated axons, co-localizing with Kv1.1 and Kv1.2. In the CNS, all sera tested (n = 12) co-localized with one or more areas of high Kv1.1, Kv1.2 or Kv1.6 channel expression: 10 out of 12 sera co-localized with Kv1.1 and Kv1.2 at spinal cord juxtaparanodes or cerebellar layers, while 3 out of 12 sera co-localized additionally (n = 2) or exclusively (n = 1) with Kv1.6 subunits in Purkinje cells, motor and hippocampal neurons. However, only sera from LE patients labelled the hippocampal areas that are enriched in excitatory, Kv1.1-positive axon terminals. All sera (17 out of 17) labelled one or more of these Kv1 subunits when expressed at the cell membrane of transfected HeLa cells, but not when they were retained in the endoplasmic reticulum. Again, LE sera labelled Kv1.1 subunits more prominently than did MoS or neuromyotonia sera, suggesting an association between higher Kv1.1 specificity and limbic manifestations. In contrast, neuromyotonia sera bound more strongly to Kv1.2 subunits than to Kv1.1 or Kv1.6. These studies support the hypothesis that antibodies to mature surface membrane-expressed Shaker-type K+ channels cause acquired neuromyotonia, MoS and LE, and suggest that future assays based on immunofluorescence of cells expressing individual Kv1 subunits will prove more sensitive than the immunoprecipitation assay. Although more than one type of antibody is often detectable in individual sera, higher affinity for certain subunits or subunit combinations may determine the range of clinical manifestations.

Expression and biophysical properties of Kv1 channels in supragranular neocortical pyramidal neurones

Potassium channels are extremely diverse regulators of neuronal excitability. As part of an investigation into how this molecular diversity is utilized by neurones, we examined the expression and biophysical properties of native Kv1 channels in layer II/III pyramidal neurones from somatosensory and motor cortex. Single-cell RT-PCR, immunocytochemistry, and whole cell recordings with specific peptide toxins revealed that individual pyramidal cells express multiple Kv1 alpha-subunits. The most abundant subunit mRNAs were Kv1.1 > 1.2 > 1.4 > 1.3. All of these subunits were localized to somatodendritic as well as axonal cell compartments. These data suggest variability in the subunit complexion of Kv1 channels in these cells. The alpha-dendrotoxin (alpha-DTX)-sensitive current activated more rapidly and at more negative potentials than the alpha-DTX-insensitive current, was first observed at voltages near action potential threshold, and was relatively insensitive to holding potential. The alpha-DTX-sensitive current comprised about 10% of outward current at steady-state, in response to steps from -70 mV. From -50 mV, this percentage increased to approximately 20%. All cells expressed an alpha-DTX-sensitive current with slow inactivation kinetics. In some cells a transient component was also present. Deactivation kinetics were voltage dependent, such that deactivation was slow at potentials traversed by interspike intervals during repetitive firing. Because of its kinetics and voltage dependence, the alpha-DTX-sensitive current should be most important at physiological resting potentials and in response to brief stimuli. Kv1 channels should also be important at voltages near threshold and corresponding to interspike intervals.

Kv1 channels selectively prevent dendritic hyperexcitability in rat Purkinje cells

Purkinje cells, the sole output of the cerebellar cortex, encode the timing signals required for motor coordination in their firing rate and activity pattern. Dendrites of Purkinje cells express a high density of P/Q-type voltage-gated calcium channels and fire dendritic calcium spikes. Here we show that dendritic subthreshold Kv1.2 subunit-containing Kv1 potassium channels prevent generation of random spontaneous calcium spikes. With Kv1 channels blocked, dendritic calcium spikes drive bursts of somatic sodium spikes and prevent the cell from faithfully encoding motor timing signals. The selective dendritic function of Kv1 channels in Purkinje cells allows them to effectively suppress dendritic hyperexcitability without hindering the generation of somatic action potentials. Further, we show that Kv1 channels also contribute to dendritic integration of parallel fibre synaptic input. Kv1 channels are often targeted to soma and axon and the data presented support a major dendritic function for these channels.

The voltage-gated potassium channel Kv1.3 is highly expressed on inflammatory infiltrates in multiple sclerosis brain

Multiple Sclerosis (MS) is characterized by central nervous system perivenular and parenchymal mononuclear cell infiltrates consisting of activated T cells and macrophages. We recently demonstrated that elevated expression of the voltage-gated potassium channel, Kv1.3, is a functional marker of activated effector memory T (T(EM)) cells in experimental allergic encephalomyelitis and in myelin-specific T cells derived from the peripheral blood of patients with MS. Herein, we show that Kv1.3 is highly expressed in postmortem MS brain inflammatory infiltrates. The expression pattern revealed not only Kv1.3(+) T cells in the perivenular infiltrate but also high expression in the parenchyma of demyelinated MS lesions and both normal appearing gray and white matter. These cells were uniformly chemokine receptor 7 negative (CCR7(-)), consistent with an effector memory phenotype. Using double-labeling immunohistochemistry and confocal microscopy, we demonstrated colocalization of Kv1.3 with CD3, CD4, CD8, and some CD68 cells. The expression patterns mirrored in vitro experiments showing polarization of Kv1.3 to the immunological synapse. Kv1.3 was expressed in low to moderate levels on CCR7(+) central memory T cells from cerebrospinal fluid, but, when these cells were stimulated in vitro, they rapidly became Kv1.3(high)/CCR7(-) T(EM), suggesting that a subset of cerebrospinal fluid cells existed in a primed state ready to become T(EM). These studies provide further rationale for the use of specific Kv1.3 antagonists in MS.

Biophysical and pharmacological properties of the voltage-gated potassium current of human pancreatic beta-cells

Voltage-gated potassium (Kv) currents of human pancreatic islet cells were studied by whole-cell patch clamp recording. On average, 75% of the cells tested were identified as beta-cells by single cell, post-recording RT-PCR for insulin mRNA. In most cells, the dominant Kv current was a delayed rectifier. The delayed rectifier activated at potentials above -20 mV and had a V(1/2) for activation of -5.3 mV. Onset of inactivation was slow for a major component (tau = 3.2 s at +20 mV) observed in all cells; a smaller component (tau = 0.30 s) with an amplitude of approximately 25% was seen in some cells. Recovery from inactivation had a tau of 2.5 s at -80 mV and steady-state inactivation had a V(1/2) of -39 mV. In 12% of cells (21/182) a low-threshold, transient Kv current (A-current) was present. The A-current activated at membrane potentials above -40 mV, inactivated with a time constant of 18.5 ms at -20 mV, and had a V(1/2) for steady-state inactivation of -52 mV. TEA inhibited total Kv current with an IC50 = 0.54 mm and PAC, a disubstituted cyclohexyl Kv channel inhibitor, inhibited with an IC50 = 0.57 microm. The total Kv current was insensitive to margatoxin (100 nm), agitoxin-2 (50 nm), kaliotoxin (50 nm) and ShK (50 nm). Hanatoxin (100 nm) inhibited total Kv current by 65% at +20 mV. Taken together, these data provide evidence of at least two distinct types of Kv channels in human pancreatic beta-cells and suggest that more than one type of Kv channel may be involved in the regulation of glucose-dependent insulin secretion.

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

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