Separable effects of human Kvbeta1.2 N- and C-termini on inactivation and expression of human Kv1.4

Kv Channel Ancillary Subunits: Where Do We Go from Here?

Voltage-gated potassium (Kv) channels each comprise four pore-forming α-subunits that orchestrate essential duties such as voltage sensing and K+ selectivity and conductance. In vivo, however, Kv channels also incorporate regulatory subunits-some Kv channel specific, others more general modifiers of protein folding, trafficking, and function. Understanding all the above is essential for a complete picture of the role of Kv channels in physiology and disease.

Intellectual Disability and Potassium Channelopathies: A Systematic Review

Intellectual disability (ID) manifests prior to adulthood as severe limitations to intellectual function and adaptive behavior. The role of potassium channelopathies in ID is poorly understood. Therefore, we aimed to evaluate the relationship between ID and potassium channelopathies. We hypothesized that potassium channelopathies are strongly associated with ID initiation, and that both gain- and loss-of-function mutations lead to ID. This systematic review explores the burden of potassium channelopathies, possible mechanisms, advancements using animal models, therapies, and existing gaps. The literature search encompassed both PubMed and Embase up to October 2019. A total of 75 articles describing 338 cases were included in this review. Nineteen channelopathies were identified, affecting the following genes: KCNMA1, KCNN3, KCNT1, KCNT2, KCNJ10, KCNJ6, KCNJ11, KCNA2, KCNA4, KCND3, KCNH1, KCNQ2, KCNAB1, KCNQ3, KCNQ5, KCNC1, KCNB1, KCNC3, and KCTD3. Twelve of these genes presented both gain- and loss-of-function properties, three displayed gain-of-function only, three exhibited loss-of-function only, and one had unknown function. How gain- and loss-of-function mutations can both lead to ID remains largely unknown. We identified only a few animal studies that focused on the mechanisms of ID in relation to potassium channelopathies and some of the few available therapeutic options (channel openers or blockers) appear to offer limited efficacy. In conclusion, potassium channelopathies contribute to the initiation of ID in several instances and this review provides a comprehensive overview of which molecular players are involved in some of the most prominent disease phenotypes.

The secret life of ion channels: Kv1.3 potassium channels and proliferation

Kv1.3 channels are involved in the switch to proliferation of normally quiescent cells, being implicated in the control of cell cycle in many different cell types and in many different ways. They modulate membrane potential controlling K⁺ fluxes, sense changes in potential, and interact with many signaling molecules through their intracellular domains. From a mechanistic point of view, we can describe the role of Kv1.3 channels in proliferation with at least three different models. In the "membrane potential model," membrane hyperpolarization resulting from Kv1.3 activation provides the driving force for Ca²⁺ influx required to activate Ca²⁺-dependent transcription. This model explains most of the data obtained from several cells from the immune system. In the "voltage sensor model," Kv1.3 channels serve mainly as sensors that transduce electrical signals into biochemical cascades, independently of their effect on membrane potential. Kv1.3-dependent proliferation of vascular smooth muscle cells (VSMCs) could fit this model. Finally, in the "channelosome balance model," the master switch determining proliferation may be related to the control of the Kv1.3 to Kv1.5 ratio, as described in glial cells and also in VSMCs. Since the three mechanisms cannot function independently, these models are obviously not exclusive. Nevertheless, they could be exploited differentially in different cells and tissues. This large functional flexibility of Kv1.3 channels surely gives a new perspective on their functions beyond their elementary role as ion channels, although a conclusive picture of the mechanisms involved in Kv1.3 signaling to proliferation is yet to be reached.

Synaptotagmin I delays the fast inactivation of Kv1.4 channel through interaction with its N-terminus

Background

The voltage-gated potassium channel Kv1.4 is an important A-type potassium channel and modulates the excitability of neurons in central nervous system. Analysis of the interaction between Kv1.4 and its interacting proteins is helpful to elucidate the function and mechanism of the channel.

Results

In the present research, synaptotagmin I was for the first time demonstrated to be an interacting protein of Kv1.4 and its interaction with Kv1.4 channel did not require the mediation of other synaptic proteins. Using patch-clamp technique, synaptotagmin I was found to delay the inactivation of Kv1.4 in HEK293T cells in a Ca²⁺-dependent manner, and this interaction was proven to have specificity. Mutagenesis experiments indicated that synaptotagmin I interacted with the N-terminus of Kv1.4 and thus delayed its N-type fast inactivation.

Conclusion

These data suggest that synaptotagmin I is an interacting protein of Kv1.4 channel and, as a negative modulator, may play an important role in regulating neuronal excitability and synaptic efficacy.

Identification of voltage-gated K(+) channel beta 2 (Kvβ2) subunit as a novel interaction partner of the pain transducer Transient Receptor Potential Vanilloid 1 channel (TRPV1)

The Transient Receptor Potential Vanilloid 1 (TRPV1, vanilloid receptor 1) ion channel plays a key role in the perception of thermal and inflammatory pain, however, its molecular environment in dorsal root ganglia (DRG) is largely unexplored. Utilizing a panel of sequence-directed antibodies against TRPV1 protein and mouse DRG membranes, the channel complex from mouse DRG was detergent-solubilized, isolated by immunoprecipitation and subsequently analyzed by mass spectrometry. A number of potential TRPV1 interaction partners were identified, among them cytoskeletal proteins, signal transduction molecules, and established ion channel subunits. Based on stringent specificity criteria, the voltage-gated K(+) channel beta 2 subunit (Kvβ2), an accessory subunit of voltage-gated K(+) channels, was identified of being associated with native TRPV1 channels. Reverse co-immunoprecipitation and antibody co-staining experiments confirmed TRPV1/Kvβ2 association. Biotinylation assays in the presence of Kvβ2 demonstrated increased cell surface expression levels of TRPV1, while patch-clamp experiments resulted in a significant increase of TRPV1 sensitivity to capsaicin. Our work shows, for the first time, the association of a Kvβ subunit with TRPV1 channels, and suggests that such interaction may play a role in TRPV1 channel trafficking to the plasma membrane.

Potentiation of the Kv1 family K(+) channel by cortisone analogues

The Kv1 family voltage-dependent K(+) channels are essential for termination of action potentials in neurons and myocytes. These channels form a stable complex with their beta subunits (Kvβ), some of which inhibit channel activity. Cortisone potentiates Kv1 channel by binding to Kvβ and promoting its dissociation from the channel, but its half-maximum effective concentration is ∼46 μM. To identify corticosteroids that are more efficient than cortisone, we examined 25 cortisone analogues and found that fluticasone propionate potentiates channel current with a half-maximum effective concentration (EC(50)) of 37 ± 1.1 nM. Further studies showed that fluticasone propionate potentiates channel current by inducing dissociation of Kvβ, and docking of fluticasone propionate into the cortisone binding site reveals potential interactions that enhance the EC(50) value. Thus, fluticasone propionate provides a starting point for rational design of more efficient small-molecule compounds that increase Kv1 activity and affect the integrity of the Kv1-Kvβ complex.

Cortisone dissociates the Shaker family K+ channels from their beta subunits

The Shaker family voltage-dependent potassium channels (Kv1) are expressed in a wide variety of cells and are essential for cellular excitability. In humans, loss-of-function mutations of Kv1 channels lead to hyperexcitability and are directly linked to episodic ataxia and atrial fibrillation. All Kv1 channels assemble with beta subunits (Kv betas), and certain Kv betas, for example Kv beta 1, have an N-terminal segment that closes the channel by the N-type inactivation mechanism. In principle, dissociation of Kv beta 1, although never reported, should eliminate inactivation and thus potentiate Kv1 current. We found that cortisone increases rat Kv1 channel activity by binding to Kv beta 1. A crystal structure of the Kv beta-cortisone complex was solved to 1.82-A resolution and revealed novel cortisone binding sites. Further studies demonstrated that cortisone promotes dissociation of Kv beta. The new mode of channel modulation may be explored by native or synthetic ligands to fine-tune cellular excitability.

Modification of K+ channel-drug interactions by ancillary subunits

Reconciling ion channel alpha-subunit expression with native ionic currents and their pharmacological sensitivity in target organs has proved difficult. In native tissue, many K(+) channel alpha-subunits co-assemble with ancillary subunits, which can profoundly affect physiological parameters including gating kinetics and pharmacological interactions. In this review, we examine the link between voltage-gated potassium ion channel pharmacology and the biophysics of ancillary subunits. We propose that ancillary subunits can modify the interaction between pore blockers and ion channels by three distinct mechanisms: changes in (1) binding site accessibility; (2) orientation of pore-lining residues; (3) the ability of the channel to undergo post-binding conformational changes. Each of these subunit-induced changes has implications for gating, drug affinity and use dependence of their respective channel complexes. A single subunit may modulate its associated alpha-subunit by more than one of these mechanisms. Voltage-gated potassium channels are the site of action of many therapeutic drugs. In addition, potassium channels interact with drugs whose primary target is another channel, e.g. the calcium channel blocker nifedipine, the sodium channel blocker quinidine, etc. Even when K(+) channel block is the intended mode of action, block of related channels in non-target organs, e.g. the heart, can result in major and potentially lethal side-effects. Understanding factors that determine specificity, use dependence and other properties of K(+) channel drug binding are therefore of vital clinical importance. Ancillary subunits play a key role in determining these properties in native tissue, and so understanding channel-subunit interactions is vital to understanding clinical pharmacology.

Voltage-gated potassium channels: regulation by accessory subunits

Voltage-gated potassium channels regulate cell membrane potential and excitability in neurons and other cell types. A precise control of neuronal action potential patterns underlies the basic functioning of the central and peripheral nervous system. This control relies on the adaptability of potassium channel activities. The functional diversity of potassium currents, however, far exceeds the considerable molecular diversity of this class of genes. Potassium current diversity contributes to the specificity of neuronal firing patterns and may be achieved by regulated transcription, RNA splicing, and posttranslational modifications. Another mechanism for regulation of potassium channel activity is through association with interacting proteins and accessory subunits. Here the authors highlight recent work that addresses this growing area of exploration and discuss areas of future investigation.

Modulation of voltage-dependent Shaker family potassium channels by an aldo-keto reductase

The beta subunit (Kvbeta) of the Shaker family voltage-dependent potassium channels (Kv1) is a cytosolic protein that forms a permanent complex with the channel. Sequence and structural conservation indicates that Kvbeta resembles an aldo-keto reductase (AKR), an enzyme that catalyzes a redox reaction using an NADPH cofactor. A putative AKR in complex with a Kv channel has led to the hypothesis that intracellular redox potential may dynamically influence the excitability of a cell through Kvbeta. Since the AKR function of Kvbeta has never been demonstrated, a direct functional coupling between the two has not been established. We report here the identification of Kvbeta substrates and the demonstration that Kvbeta is a functional AKR. We have also found that channel function is modulated when the Kvbeta-bound NADPH is oxidized. Further studies of the enzymatic properties of Kvbeta seem to favor the role of Kvbeta as a redox sensor. These results suggest that Kvbeta may couple the excitability of the cell to its metabolic state and present a new avenue of research that may lead to understanding of the physiological functions of Kvbeta.

Protein-protein interactions of a Kv? subunit in the cochlea

Accessory subunits associated with voltage-gated potassium (Kv) channels can influence the biophysical properties and promote the surface expression of channel-forming alpha-subunits. Previously, we cloned several alpha-subunits and a beta-subunit from a cDNA library of the chicken cochlea. In the present study, we raised an antibody against the N-terminus of chicken Kvbeta1.1 (cKvbeta1.1) and characterized the Kvbeta-related polypeptide in cochlear tissues and heterologous cells. The anti-cKvbeta1.1 antibody recognizes a 45-kDa polypeptide in chick cochlear extracts as well as in Chinese hamster ovary (CHO) cells transfected with cKvbeta1.1. The accessory subunit was localized to the ganglion cells of the chick cochlea using immunohistochemistry and in situ hybridization. Coimmunoprecipitation studies show that Kvbeta1.1 interacts with Shaker channel members Kvalpha1.2 and 1.3, both of which colocalize with beta to the cochlear ganglion cells. Additionally, coimmunoprecipitation studies show that Kvalpha1.2 and 1.3 interact with each other, suggesting that these ion channels are formed by heteromultimers. In comparison, Kvbeta did not coprecipitate with a member of the Shal subfamily. The presence of Kvbeta in the cochlea suggests that this subunit contributes to the modulation of auditory signals in the ganglion cells, presumably by regulating properties of inactivation as well as surface expression of Kvalpha channels.

Calnexin regulates mammalian Kv1 channel trafficking

Voltage-gated Kv1 channels are key factors regulating excitability in the mammalian central nervous system. Diverse posttranslational regulatory mechanisms operate to determine the density, subunit composition, and localization of Kv1 channel complexes in the neuronal plasma membrane. In this study, we investigated the role of the endoplasmic reticulum chaperone calnexin in the intracellular trafficking of Kv1 channels. We found that coexpressing calnexin with the Kv1.2alpha subunit in transfected mammalian COS-1 cells produced a dramatic dose-dependent increase in cell surface Kv1.2 channel complexes. In calnexin-transfected COS-1 cells, the proportion of Kv1.2 channels with mature N-linked oligosaccharide chains was comparable to that observed in neurons. In contrast, calnexin coexpression exerted no effects on trafficking of the intracellularly retained Kv1.1 or Kv1.6alpha subunits. We also found that calnexin and auxiliary Kvbeta2 subunit coexpression was epistatic, suggesting that they share a common pathway for promoting Kv1.2 channel surface expression. These results provide yet another component in the elaborate repertoire of determinants regulating the density of Kv1 channels in the plasma membrane.

Differential modulation of Kv1 channel-mediated currents by co-expression of Kvbeta3 subunit in a mammalian cell-line

The effect of Kvbeta3 subunit co-expression on currents mediated by the Shaker-related channels Kv1.1 to Kv1.6 in Chinese hamster ovary (CHO) cells was studied with patch-clamp techniques. In the presence of Kvbeta3, differences in the voltage dependence of activation for Kv1.1, Kv1.3 and Kv1.6 were detected, but not for Kv1.2- and Kv1.4-mediated currents. Co-expression of Kvbeta3 did not cause a significant increase in current density for any of the tested channels. In contrast to previous studies in Xenopus oocyte expression system, Kvbeta3 confered a rapid inactivation to all except Kv1.3 channels. Also, Kv1.6 channels that possess an N-type inactivation prevention (NIP) domain for Kvbeta1.1, inactivated rapidly when co-expressed with Kvbeta3. Onset and recovery kinetics of channel inactivation distinctly differed for the various Kv1alpha/Kvbeta3 subunit combinations investigated in this study. The results indicate that the choice of expression system may critically determine Kvbeta3 inactivating activity. This suggests that the presence of an inactivating domain and a receptor in a channel pore, although necessary, may not be sufficient for an effective rapid N-type inactivation of Kv1 channels in heterologous expression systems.

Effects of derivatives of sulfur dioxide on transient outward potassium currents in acutely isolated hippocampal neurons

The effect of SO2 derivatives, a common air pollutant and exists in vivo as an equilibrium between bisulfate and sulfite, on transient outward currents (TOCS) in hippocampal neurons were studied using the whole cell configuration of patch-clamp technique. TOCS that preliminary included a fast inactivating (A-current or IA) and a slow inactivating (D-current or ID ) current, were isolated based on the kinetics and pharmacological properties in the presence of 50 mM TEA. The results showed that SO2 derivatives reversibly increased the amplitudes of TOCS in a concentration dependent and voltage dependent. Half-increase dose on TOCS was 25 microM. In vivo, SO2 derivatives shifted the steady-state inactivation curve of TOCS in the depolarizing direction but had little effect on the activation curve. Half-maximal inactivation voltage of TOCS was -69.6+/-1.0 mV before and -56.8+/-0.4 mV after application of 10 microM SO2 derivatives. SO2 derivatives increased the maximal conductance and delayed the inactivation process of TOCS. These results suggest that SO2 derivatives would increase the excitability of hippocampal neurons.

Kv beta subunit oxidoreductase activity and Kv1 potassium channel trafficking

Voltage-gated Kv1 potassium channels consist of pore-forming alpha subunits and cytoplasmic Kv beta subunits. The latter play diverse roles in modulating the gating, stability, and trafficking of Kv1 channels. The crystallographic structure of the Kv beta2 subunit revealed surprising structural homology with aldo-keto reductases, including a triosephosphate isomerase barrel structure, conservation of key catalytic residues, and a bound NADP(+) cofactor (Gulbis, J. M., Mann, S., and MacKinnon, R. (1999) Cell 90, 943-952). Each Kv1-associated Kv beta subunit (Kv beta 1.1, Kv beta 1.2, Kv beta 2, and Kv beta 3) shares striking amino acid conservation in key catalytic and cofactor binding residues. Here, by a combination of structural modeling and biochemical and cell biological analyses of structure-based mutations, we investigate the potential role for putative Kv beta subunit enzymatic activity in the trafficking of Kv1 channels. We found that all Kv beta subunits promote cell surface expression of coexpressed Kv1.2 alpha subunits in transfected COS-1 cells. Kv beta1.1 and Kv beta 2 point mutants lacking a key catalytic tyrosine residue found in the active site of all aldo-keto reductases have wild-type trafficking characteristics. However, mutations in residues within the NADP(+) binding pocket eliminated effects on Kv1.2 trafficking. In cultured hippocampal neurons, Kv beta subunit coexpression led to axonal targeting of Kv1.2, recapitulating the Kv1.2 localization observed in many brain neurons. Similar to the trafficking results in COS-1 cells, mutations within the cofactor binding pocket reduced axonal targeting of Kv1.2, whereas those in the catalytic tyrosine did not. Together, these data suggest that NADP(+) binding and/or the integrity of the binding pocket structure, but not catalytic activity, of Kv beta subunits is required for intracellular trafficking of Kv1 channel complexes in mammalian cells and for axonal targeting in neurons.

KChAP/Kvbeta1.2 interactions and their effects on cardiac Kv channel expression

KChAP and voltage-dependent K+ (Kv) beta-subunits are two different types of cytoplasmic proteins that interact with Kv channels. KChAP acts as a chaperone for Kv2.1 and Kv4.3 channels. It also binds to Kv1.x channels but, with the exception of Kv1.3, does not increase Kv1.x currents. Kvbeta-subunits are assembled with Kv1.x channels; they exhibit "chaperone-like" behavior and change gating properties. In addition, KChAP and Kvbeta-subunits interact with each other. Here we examine the consequences of this interaction on Kv currents in Xenopus oocytes injected with different combinations of cRNAs, including Kvbeta1.2, KChAP, and either Kv1.4, Kv1.5, Kv2.1, or Kv4.3. We found that KChAP attenuated the depression of Kv1.5 currents produced by Kvbeta1.2, and Kvbeta1.2 eliminated the increase of Kv2.1 and Kv4.3 currents produced by KChAP. Both KChAP and Kvbeta1.2 are expressed in cardiomyocytes, where Kv1.5 and Kv2.1 produce sustained outward currents and Kv4.3 and Kv1.4 generate transient outward currents. Because they interact, either KChAP or Kvbeta1.2 may alter both sustained and transient cardiac Kv currents. The interaction of these two different classes of modulatory proteins may constitute a novel mechanism for regulating cardiac K+ currents.

Mutations in the Kv beta 2 binding site for NADPH and their effects on Kv1.4

Kv beta 2 enhances the rate of inactivation and level of expression of Kv1.4 currents. The crystal structure of Kv beta 2 binds NADP(+), and it has been suggested that Kv beta 2 is an oxidoreductase enzyme (). To investigate how this function might relate to channel modulation, we made point mutations in Kv beta 2 in either the NADPH docking or putative catalytic sites. Using the yeast two-hybrid system, we found that these mutations did not disrupt the interaction of Kv beta 2 with Kv alpha 1 channels. To characterize the Kv beta 2 mutants functionally, we coinjected wild-type or mutant Kv beta 2 cRNAs and Kv1.4 cRNA in Xenopus laevis oocytes. Kv beta 2 increased both the amplitude and rate of inactivation of Kv1.4 currents. The cellular content of Kv1.4 protein was unchanged on Western blot, but the amount in the plasmalemma was increased. Mutations in either the orientation or putative catalytic sites for NADPH abolished the expression-enhancing effect on Kv1.4 current. Western blots showed that both types of mutation reduced Kv1.4 protein. Like the wild-type Kv beta 2, both types of mutation increased the rate of inactivation of Kv1.4, confirming the physical association of mutant Kv beta 2 subunits with Kv1.4. Thus, mutations that should interfere with NADPH function uncouple the expression-enhancing effect of Kv beta 2 on Kv1.4 currents from its effect on the rate of inactivation. These results suggest that the binding of NADPH and the putative oxidoreductase activity of Kv beta 2 may play a role in the processing of Kv1.4.

KChAP as a chaperone for specific K(+) channels

The concept of chaperones for K(+) channels is new. Recently, we discovered a novel molecular chaperone, KChAP, which increased total Kv2.1 protein and functional channels in Xenopus oocytes through a transient interaction with the Kv2.1 amino terminus. Here we report that KChAP is a chaperone for Kv1.3 and Kv4.3. KChAP increased the amplitude of Kv1.3 and Kv4.3 currents without affecting kinetics or voltage dependence, but had no such effect on Kv1.1, 1.2, 1.4, 1.5, 1.6, and 3.1 or Kir2.2, HERG, or KvLQT1. Although KChAP belongs to a family of proteins that interact with transcription factors, upregulation of channel currents was not blocked by the transcription inhibitor actinomycin D. A 98-amino acid fragment of KChAP binds to the channel and is indistinguishable from KChAP in its enhancement of Kv4.3 current and protein levels. Using a KChAP antibody, we have coimmunoprecipitated KChAP with Kv2.1 and Kv4.3 from heart. We propose that KChAP is a chaperone for specific Kv channels and may have this function in cardiomyocytes where Kv4.3 produces the transient outward current, I(to).

PSD-95 and SAP97 exhibit distinct mechanisms for regulating K(+) channel surface expression and clustering

Mechanisms of ion channel clustering by cytoplasmic membrane-associated guanylate kinases such as postsynaptic density 95 (PSD-95) and synapse-associated protein 97 (SAP97) are poorly understood. Here, we investigated the interaction of PSD-95 and SAP97 with voltage-gated or Kv K(+) channels. Using Kv channels with different surface expression properties, we found that clustering by PSD-95 depended on channel cell surface expression. Moreover, PSD-95-induced clusters of Kv1 K(+) channels were present on the cell surface. This was most dramatically demonstrated for Kv1.2 K(+) channels, where surface expression and clustering by PSD-95 were coincidentally promoted by coexpression with cytoplasmic Kvbeta subunits. Consistent with a mechanism of plasma membrane channel-PSD-95 binding, coexpression with PSD-95 did not affect the intrinsic surface expression characteristics of the different Kv channels. In contrast, the interaction of Kv1 channels with SAP97 was independent of Kv1 surface expression, occurred intracellularly, and prevented further biosynthetic trafficking of Kv1 channels. As such, SAP97 binding caused an intracellular accumulation of each Kv1 channel tested, through the accretion of SAP97 channel clusters in large (3-5 microm) ER-derived intracellular membrane vesicles. Together, these data show that ion channel clustering by PSD-95 and SAP97 occurs by distinct mechanisms, and suggests that these channel-clustering proteins may play diverse roles in regulating the abundance and distribution of channels at synapses and other neuronal membrane specializations.