Auxiliary subunits control biophysical properties and response to compound NS5806 of the Kv4 potassium channel complex

KV Channel-Interacting Proteins in the Neurological and Cardiovascular Systems: An Updated Review

K_V channel-interacting proteins (KChIP1-4) belong to a family of Ca²⁺-binding EF-hand proteins that are able to bind to the N-terminus of the K_V4 channel α-subunits. KChIPs are predominantly expressed in the brain and heart, where they contribute to the maintenance of the excitability of neurons and cardiomyocytes by modulating the fast inactivating-K_V4 currents. As the auxiliary subunit, KChIPs are critically involved in regulating the surface protein expression and gating properties of K_V4 channels. Mechanistically, KChIP1, KChIP2, and KChIP3 promote the translocation of K_V4 channels to the cell membrane, accelerate voltage-dependent activation, and slow the recovery rate of inactivation, which increases K_V4 currents. By contrast, KChIP4 suppresses K_V4 trafficking and eliminates the fast inactivation of K_V4 currents. In the heart, I_Ks, I_Ca,L, and I_Na can also be regulated by KChIPs. I_Ca,L and I_Na are positively regulated by KChIP2, whereas I_Ks is negatively regulated by KChIP2. Interestingly, KChIP3 is also known as downstream regulatory element antagonist modulator (DREAM) because it can bind directly to the downstream regulatory element (DRE) on the promoters of target genes that are implicated in the regulation of pain, memory, endocrine, immune, and inflammatory reactions. In addition, all the KChIPs can act as transcription factors to repress the expression of genes involved in circadian regulation. Altered expression of KChIPs has been implicated in the pathogenesis of several neurological and cardiovascular diseases. For example, KChIP2 is decreased in failing hearts, while loss of KChIP2 leads to increased susceptibility to arrhythmias. KChIP3 is increased in Alzheimer's disease and amyotrophic lateral sclerosis, but decreased in epilepsy and Huntington's disease. In the present review, we summarize the progress of recent studies regarding the structural properties, physiological functions, and pathological roles of KChIPs in both health and disease. We also summarize the small-molecule compounds that regulate the function of KChIPs. This review will provide an overview and update of the regulatory mechanism of the KChIP family and the progress of targeted drug research as a reference for researchers in related fields.

Modulation of KV4.3-KChIP2 Channels by IQM-266: Role of DPP6 and KCNE2

The transient outward potassium current (I_tof) is generated by the activation of K_V4 channels assembled with KChIP2 and other accessory subunits (DPP6 and KCNE2). To test the hypothesis that these subunits modify the channel pharmacology, we analyzed the electrophysiological effects of (3-(2-(3-phenoxyphenyl)acetamido)-2-naphthoic acid) (IQM-266), a new KChIP2 ligand, on the currents generated by K_V4.3/KChIP2, K_V4.3/KChIP2/DPP6 and K_V4.3/KChIP2/KCNE2 channels. CHO cells were transiently transfected with cDNAs codifying for different proteins (K_V4.3/KChIP2, K_V4.3/KChIP2/DPP6 or K_V4.3/KChIP2/KCNE2), and the potassium currents were recorded using the whole-cell patch-clamp technique. IQM-266 decreased the maximum peak of K_V4.3/KChIP2, K_V4.3/KChIP2/DPP6 and K_V4.3/KChIP2/KCNE2 currents, slowing their time course of inactivation in a concentration-, voltage-, time- and use-dependent manner. IQM-266 produced an increase in the charge in K_V4.3/KChIP2 channels that was intensified when DPP6 was present and abolished in the presence of KCNE2. IQM-266 induced an activation unblocking effect during the application of trains of pulses to cells expressing K_V4.3/KChIP2 and K_V4.3/KChIP2/KCNE2, but not in K_V4.3/KChIP2/DPP6 channels. Overall, all these results are consistent with a preferential IQM-266 binding to an active closed state of Kv4.3/KChIP2 and Kv4.3/KChIP2/KCNE2 channels, whereas in the presence of DPP6, IQM-266 binds preferentially to an inactivated state. In conclusion, DPP6 and KCNE2 modify the pharmacological response of K_V4.3/KChIP2 channels to IQM-266.

Inducing Ito,f and phase 1 repolarization of the cardiac action potential with a Kv4.3/KChIP2.1 bicistronic transgene

The fast transient outward potassium current (I_to,f) plays a key role in phase 1 repolarization of the human cardiac action potential (AP) and its reduction in heart failure (HF) contributes to the loss of contractility. Therefore, restoring I_to,f might be beneficial for treating HF. The coding sequence of a P2A peptide was cloned, in frame, between Kv4.3 and KChIP2.1 genes and ribosomal skipping was confirmed by Western blotting. Typical I_to,f properties with slowed inactivation and accelerated recovery from inactivation due to the association of KChIP2.1 with Kv4.3 was seen in transfected HEK293 cells. Both bicistronic components trafficked to the plasmamembrane and in adenovirus transduced rabbit cardiomyocytes both t-tubular and sarcolemmal construct labelling appeared. The resulting current was similar to I_to,f seen in human ventricular cardiomyocytes and was 50% blocked at ~0.8 mmol/l 4-aminopyridine and increased ~30% by 5 μmol/l NS5806 (an I_to,f agonist). Variation in the density of the expressed I_to,f, in rabbit cardiomyocytes recapitulated typical species-dependent variations in AP morphology. Simultaneous voltage recording and intracellular Ca²⁺ imaging showed that modification of phase 1 to a non-failing human phenotype improved the rate of rise and magnitude of the Ca²⁺ transient. I_to,f expression also reduced AP triangulation but did not affect I_Ca,L and I_Na magnitudes. This raises the possibility for a new gene-based therapeutic approach to HF based on selective phase 1 modification.

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Action potential phase 1 depends on fast transient outward current (I_to,f).

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Construction of a bicistronic transgene for Kv4.3 and KChIP2.1 with P2A separator

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Expressed bicistronic Kv4.3/KChIP2.1 proteins traffic to the cell surface membrane

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Viral transduction with Kv4.3/KChIP2.1 increases I_to,f in cardiomyocytes.

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Kv4.3/KChIP2.1 transgene expression increased AP phase 1 and EC coupling

SUMOylation of the Kv4.2 Ternary Complex Increases Surface Expression and Current Amplitude by Reducing Internalization in HEK 293 Cells

Kv4 α-subunits exist as ternary complexes (TC) with potassium channel interacting proteins (KChIP) and dipeptidyl peptidase-like proteins (DPLP); multiple ancillary proteins also interact with the α-subunits throughout the channel’s lifetime. Dynamic regulation of Kv4.2 protein interactions adapts the transient potassium current, IA, mediated by Kv4 α-subunits. Small ubiquitin-like modifier (SUMO) is an 11 kD peptide post-translationally added to lysine (K) residues to regulate protein–protein interactions. We previously demonstrated that when expressed in human embryonic kidney (HEK) cells, Kv4.2 can be SUMOylated at two K residues, K437 and K579. SUMOylation at K437 increased surface expression of electrically silent channels while SUMOylation at K579 reduced IA maximal conductance (Gmax) without altering surface expression. KChIP and DPLP subunits are known to modify the pattern of Kv4.2 post-translational decorations and/or their effects. In this study, co-expressing Kv4.2 with KChIP2a and DPP10c altered the effects of enhanced Kv4.2 SUMOylation. First, the effect of enhanced SUMOylation was the same for a TC containing either the wild-type Kv4.2 or the mutant K437R Kv4.2, suggesting that either the experimental manipulation no longer enhanced K437 SUMOylation or K437 SUMOylation no longer influenced Kv4.2 surface expression. Second, instead of decreasing IA Gmax, enhanced SUMOylation at K579 now produced a significant ∼37–70% increase in IA maximum conductance (Gmax) and a significant ∼30–50% increase in Kv4.2g surface expression that was accompanied by a 65% reduction in TC internalization. Blocking clathrin-mediated endocytosis (CME) in HEK cells expressing the Kv4.2 TC mimicked and occluded the effect of SUMO on IA Gmax; however, the amount of Kv4.2 associated with the major adaptor for constitutive CME, adaptor protein 2 (AP2), was not SUMO dependent. Thus, SUMOylation reduced Kv4.2 internalization by acting downstream of Kv4.2 recruitment into clathrin-coated pits. In sum, the two major findings of this study are: SUMOylation of Kv4.2 at K579 regulates TC internalization most likely by promoting channel recycling. Additionally, there is a reciprocity between Kv4.2 SUMOylation and the Kv4.2 interactome such that SUMOylation regulates the interactome and the interactome influences the pattern and effect of SUMOylation.

Control of Biophysical and Pharmacological Properties of Potassium Channels by Ancillary Subunits

Potassium channels facilitate and regulate physiological processes as diverse as electrical signaling, ion, solute and hormone secretion, fluid homeostasis, hearing, pain sensation, muscular contraction, and the heartbeat. Potassium channels are each formed by either a tetramer or dimer of pore-forming α subunits that co-assemble to create a multimer with a K⁺-selective pore that in most cases is capable of functioning as a discrete unit to pass K⁺ ions across the cell membrane. The reality in vivo, however, is that the potassium channel α subunit multimers co-assemble with ancillary subunits to serve specific physiological functions. The ancillary subunits impart specific physiological properties that are often required for a particular activity in vivo; in addition, ancillary subunit interaction often alters the pharmacology of the resultant complex. In this chapter the modes of action of ancillary subunits on K⁺ channel physiology and pharmacology are described and categorized into various mechanistic classes.

Pharmacological Approaches for the Modulation of the Potassium Channel KV4.x and KChIPs

Ion channels are macromolecular complexes present in the plasma membrane and intracellular organelles of cells. Dysfunction of ion channels results in a group of disorders named channelopathies, which represent an extraordinary challenge for study and treatment. In this review, we will focus on voltage-gated potassium channels (K_V), specifically on the K_V4-family. The activation of these channels generates outward currents operating at subthreshold membrane potentials as recorded from myocardial cells (I_TO, transient outward current) and from the somata of hippocampal neurons (I_SA). In the heart, K_V4 dysfunctions are related to Brugada syndrome, atrial fibrillation, hypertrophy, and heart failure. In hippocampus, K_V4.x channelopathies are linked to schizophrenia, epilepsy, and Alzheimer's disease. K_V4.x channels need to assemble with other accessory subunits (β) to fully reproduce the I_TO and I_SA currents. β Subunits affect channel gating and/or the traffic to the plasma membrane, and their dysfunctions may influence channel pharmacology. Among K_V4 regulatory subunits, this review aims to analyze the K_V4/KChIPs interaction and the effect of small molecule KChIP ligands in the A-type currents generated by the modulation of the K_V4/KChIP channel complex. Knowledge gained from structural and functional studies using activators or inhibitors of the potassium current mediated by K_V4/KChIPs will better help understand the underlying mechanism involving K_V4-mediated-channelopathies, establishing the foundations for drug discovery, and hence their treatments.

Auxiliary subunits control biophysical properties and response to compound NS5806 of the Kv4 potassium channel complex

Kv4 pore‐forming subunits co‐assemble with β‐subunits including KChIP2 and DPP6 and the resultant complexes conduct cardiac transient outward K⁺ current (I_to). Compound NS5806 has been shown to potentate I_to in canine cardiomyocytes; however, its effects on I_to in other species yet to be determined. We found that NS5806 inhibited native I_to in a concentration‐dependent manner (0.1~30 μM) in both mouse ventricular cardiomyocytes and human‐induced pluripotent stem cell‐derived cardiomyocytes (hiPSC‐CMs), but potentiated I_to in the canine cardiomyocytes. In HEK293 cells co‐transfected with cloned Kv4.3 (or Kv4.2) and β‐subunit KChIP2, NS5806 significantly increased the peak current amplitude and slowed the inactivation. In contrast, NS5806 suppressed the current and accelerated inactivation of the channels when cells were co‐transfected with Kv4.3 (or Kv4.2), KChIP2 and another β‐subunit, DPP6‐L (long isoform). Western blot analysis showed that DPP6‐L was dominantly expressed in both mouse ventricular myocardium and hiPSC‐CMs, while it was almost undetectable in canine ventricular myocardium. In addition, low level of DPP6‐S expression was found in canine heart, whereas levels of KChIP2 expression were comparable among all three species. siRNA knockdown of DPP6 antagonized the I_to inhibition by NS5806 in hiPSC‐CMs. Molecular docking simulation suggested that DPP6‐L may associate with KChIP2 subunits. Mutations of putative KChIP2‐interacting residues of DPP6‐L reversed the inhibitory effect of NS5806 into potentiation of the current. We conclude that a pharmacological modulator can elicit opposite regulatory effects on Kv4 channel complex among different species, depending on the presence of distinct β‐subunits. These findings provide novel insight into the molecular design and regulation of cardiac I_to. Since I_to is a potential therapeutic target for treatment of multiple cardiovascular diseases, our data will facilitate the development of new therapeutic I_to modulators.