Novel KChIP2 isoforms increase functional diversity of transient outward potassium currents

Cardiomyocyte-specific loss of plasma membrane calcium ATPase 1 impacts cardiac rhythm and is associated with ventricular repolarisation dysfunction

Plasma membrane calcium ATPase 1 (PMCA1, Atp2b1) is emerging as a key contributor to cardiac physiology, involved in calcium handling and myocardial signalling. In addition, genome wide association studies have associated PMCA1 in several areas of cardiovascular disease including hypertension and myocardial infarction. Here, we investigated the role of PMCA1 in basal cardiac function and heart rhythm stability. Cardiac structure, heart rhythm and arrhythmia susceptibility were assessed in a cardiomyocyte-specific PMCA1 deletion (PMCA1^CKO) mouse model. PMCA1^CKO mice developed abnormal heart rhythms related to ventricular repolarisation dysfunction and displayed an increased susceptibility to ventricular arrhythmias. We further assessed the levels of cardiac ion channels using qPCR and found a downregulation of the voltage-dependent potassium channels, K_v4.2, with a corresponding reduction in the transient outward potassium current which underlies ventricular repolarisation in the murine heart. The changes in heart rhythm were found to occur in the absence of any structural cardiomyopathy. To further assess the molecular changes occurring in PMCA1^CKO hearts, we performed proteomic analysis. Functional characterisation of differentially expressed proteins suggested changes in pathways related to metabolism, protein-binding, and pathways associated cardiac function including β-adrenergic signalling. Together, these data suggest an important role for PMCA1 in basal cardiac function in relation to heart rhythm control, with reduced cardiac PMCA1 expression resulting in an increased risk of arrhythmia development.

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.

SCN1Bβ mutations that affect their association with Kv4.3 underlie early repolarization syndrome

Background

Abnormal cardiac ion channels current, including transient outward potassium current (I_to), is associated with early repolarization syndrome (ERS). Previous studies showed that mutations in SCN1Bβ both to increase the I_to current and to decrease the sodium current. Yet its role in ERS remains unknown.

Objective

To determine the role of mutations in the SCN1Bβ subunits in ERS.

Methods

We screened for mutations in the SCN1B genes from four families with ERS. Wild‐type and mutant SCN1Bβ genes were co‐expressed with wild‐type KCND3 in human embryonic kidney cells (HEK293). Whole‐cell patch‐clamp technique and co‐immunoprecipitation were used to study the electrophysiological properties and explore the underlying mechanisms.

Results

S248R and R250T mutations in SCN1Bβ were detected in 4 families’ probands. Neither S248R nor R250T mutation had significant influence on the sodium channel current density (I_Na) when co‐expressed with SCN5A/WT. Co‐expression of KCND3/WT and SCN1Bβ/S248R or SCN1Bβ/R250T increased the transient outward potassium current I_to by 27.44% and 199.89%, respectively (P < 0.05 and P < 0.01, respectively) when compared with SCN1Bβ/WT. Electrophysiological properties showed that S248R and R250T mutations decreased the steady‐state inactivation and recovery from inactivation of I_to channel. Co‐immunoprecipitation study demonstrated an increased association between SCN1Bβ mutations and Kv4.3 compared with SCN1Bβ/WT (P < 0.05 and P < 0.01, respectively).

Conclusion

The S248R and R250T mutations of SCN1Bβ gene caused gain‐of‐function of I_to by associated with Kv4.3, which maybe underlie the ERS phenotype of the probands.

Regulation of Kv4.3 and hERG potassium channels by KChIP2 isoforms and DPP6 and response to the dual K+ channel activator NS3623

Transient outward potassium current (I_to) contributes to early repolarization of many mammalian cardiac action potentials, including human, whilst the rapid delayed rectifier K⁺ current (I_Kr) contributes to later repolarization. Fast I_to channels can be produced from the Shal family KCNDE gene product Kv4.3s, although accessory subunits including KChIP2.x and DPP6 are also needed to produce a near physiological I_to. In this study, the effect of KChIP2.1 & KChIP2.2 (also known as KChIP2b and KChIP2c respectively), alone or in conjunction with the accessory subunit DPP6, on both Kv4.3 and hERG were evaluated. A dual I_to and I_Kr activator, NS3623, has been recently proposed to be beneficial in heart failure and the action of NS3623 on the two channels was also investigated. Whole-cell patch-clamp experiments were performed at 33 ± 1 °C on HEK293 cells expressing Kv4.3 or hERG in the absence or presence of these accessory subunits. Kv4.3 current magnitude was augmented by co-expression with either KChIP2.2 or KChIP2.1 and KChIP2/DPP6 with KChIP2.1 producing a greater effect than KChIP2.2. Adding DPP6 removed the difference in Kv4.3 augmentation between KChIP2.1 and KChIP2.2. The inactivation rate and recovery from inactivation were also altered by KChIP2 isoform co-expression. In contrast, hERG (Kv11.1) current was not altered by co-expression with KChIP2.1, KChIP2.2 or DPP6. NS3623 increased Kv4.3 amplitude to a similar extent with and without accessory subunit co-expression, however KChIP2 isoforms modulated the compound’s effect on inactivation time course. The agonist effect of NS3623 on hERG channels was not affected by KChIP2.1, KChIP2.2 or DPP6 co-expression.

Modulation of human Kv4.3/KChIP2 channel inactivation kinetics by cytoplasmic Ca2

The transient outward current (I _to) in the human heart is mediated by Kv4.3 channels complexed with Kv channel interacting protein (KChIP) 2, a cytoplasmic Ca²⁺-binding EF-hand protein known to modulate Kv4.3 inactivation gating upon heterologous co-expression. We studied Kv4.3 channels co-expressed with wild-type (wt) or EF-hand-mutated (ΔEF) KChIP2 in human embryonic kidney (HEK) 293 cells. Co-expression took place in the absence or presence of BAPTA-AM, and macroscopic currents were recorded in the whole-cell patch-clamp configuration with different free Ca²⁺ concentrations in the patch-pipette. Our data indicate that Ca²⁺ is not necessary for Kv4.3/KChIP2 complex formation. The Kv4.3/KChIP2-mediated current decay was faster and the recovery of Kv4.3/KChIP2 channels from inactivation slower with 50 μM Ca²⁺ than with BAPTA (nominal Ca²⁺-free) in the patch-pipette. The apparent Ca²⁺-mediated slowing of recovery kinetics was still observed when EF-hand 4 of KChIP2 was mutated (ΔEF4) but not when EF-hand 2 (ΔEF2) was mutated, and turned into a Ca²⁺-mediated acceleration of recovery kinetics when EF-hand 3 (ΔEF3) was mutated. In the presence of the Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93 cytoplasmic Ca²⁺ (50 μM) induced an acceleration of Kv4.3/KChIP2 recovery kinetics, which was still observed when EF-hand 2 was mutated (ΔEF2) but not when EF-hand 3 (ΔEF3) or EF-hand 4 (ΔEF4) was mutated. Our results support the notion that binding of Ca²⁺ to KChIP2 EF-hands can acutely modulate Kv4.3/KChIP2 channel inactivation gating, but the Ca²⁺-dependent gating modulation depends on CaMKII action. Our findings speak for an acute modulation of I _to kinetics and frequency-dependent I _to availability in cardiomyocytes under conditions with elevated Ca²⁺ levels and CaMKII activity.

KChIP2 genotype dependence of transient outward current (Ito) properties in cardiomyocytes isolated from male and female mice

The transient outward current (I_to) in cardiomyocytes is largely mediated by Kv4 channels associated with Kv Channel Interacting Protein 2 (KChIP2). A knockout model has documented the critical role of KChIP2 in I_to expression. The present study was conducted to characterize in both sexes the dependence of I_to properties, including current magnitude, inactivation kinetics, recovery from inactivation and voltage dependence of inactivation, on the number of functional KChIP2 alleles. For this purpose we performed whole-cell patch-clamp experiments on isolated left ventricular cardiomyocytes from male and female mice which had different KChIP2 genotypes; i.e., wild-type (KChIP2^+/+), heterozygous knockout (KChIP2^+/-) or complete knockout of KChIP2 (KChIP2^-/-). We found in both sexes a KChIP2 gene dosage effect (i.e., a proportionality between number of alleles and phenotype) on I_to magnitude, however, concerning other I_to properties, KChIP2^+/- resembled KChIP2^+/+. Only in the total absence of KChIP2 (KChIP2^-/-) we observed a slowing of I_to kinetics, a slowing of recovery from inactivation and a negative shift of a portion of the voltage dependence of inactivation. In a minor fraction of KChIP2^-/- myocytes I_to was completely lost. The distinct KChIP2 genotype dependences of I_to magnitude and inactivation kinetics, respectively, seen in cardiomyocytes were reproduced with two-electrode voltage-clamp experiments on Xenopus oocytes expressing Kv4.2 and different amounts of KChIP2. Our results corroborate the critical role of KChIP2 in controlling I_to properties. They demonstrate that the Kv4.2/KChIP2 interaction in cardiomyocytes is highly dynamic, with a clear KChIP2 gene dosage effect on Kv4 channel surface expression but not on inactivation gating.

Molecular Basis of Functional Myocardial Potassium Channel Diversity

Multiple types of voltage-gated K(+) and non-voltage-gated K(+) currents have been distinguished in mammalian cardiac myocytes based on differences in time-dependent and voltage-dependent properties and pharmacologic sensitivities. Many of the genes encoding voltage-gated K(+) (Kv) and non-voltage-gated K(+) (Kir and K2P) channel pore-forming and accessory subunits are expressed in the heart, and a variety of approaches have been, and continue to be, used to define the molecular determinants of native cardiac K(+) channels and to explore the molecular mechanisms controlling the diversity, regulation, and remodeling of these channels in the normal and diseased myocardium.

Different KChIPs compete for heteromultimeric assembly with pore-forming Kv4 subunits

Auxiliary Kv channel-interacting proteins 1-4 (KChIPs1-4) coassemble with pore-forming Kv4 α-subunits to form channel complexes underlying somatodendritic subthreshold A-type current that regulates neuronal excitability. It has been hypothesized that different KChIPs can competitively bind to Kv4 α-subunit to form variable channel complexes that can exhibit distinct biophysical properties for modulation of neural function. In this study, we use single-molecule subunit counting by total internal reflection fluorescence microscopy in combinations with electrophysiology and biochemistry to investigate whether different isoforms of auxiliary KChIPs, KChIP4a, and KChIP4bl, can compete for binding of Kv4.3 to coassemble heteromultimeric channel complexes for modulation of channel function. To count the number of photobleaching steps solely from cell membrane, we take advantage of a membrane tethered k-ras-CAAX peptide that anchors cytosolic KChIP4 proteins to the surface for reduction of background noise. Single-molecule subunit counting reveals that the number of KChIP4 isoforms in Kv4.3-KChIP4 complexes can vary depending on the KChIP4 expression level. Increasing the amount of KChIP4bl gradually reduces bleaching steps of KChIP4a isoform proteins, and vice versa. Further analysis of channel gating kinetics from different Kv4-KChIP4 subunit compositions confirms that both KChIP4a and KChIP4bl can modulate the channel complex function upon coassembly. Taken together, our findings show that auxiliary KChIPs can heteroassemble with Kv4 in a competitive manner to form heteromultimeric Kv4-KChIP4 channel complexes that are biophysically distinct and regulated under physiological or pathological conditions.

Gain-of-function mutation in TASK-4 channels and severe cardiac conduction disorder

Analyzing a patient with progressive and severe cardiac conduction disorder combined with idiopathic ventricular fibrillation (IVF), we identified a splice site mutation in the sodium channel gene SCN5A. Due to the severe phenotype, we performed whole-exome sequencing (WES) and identified an additional mutation in the KCNK17 gene encoding the K_2P potassium channel TASK-4. The heterozygous change (c.262G>A) resulted in the p.Gly88Arg mutation in the first extracellular pore loop. Mutant TASK-4 channels generated threefold increased currents, while surface expression was unchanged, indicating enhanced conductivity. When co-expressed with wild-type channels, the gain-of-function by G88R was conferred in a dominant-active manner. We demonstrate that KCNK17 is strongly expressed in human Purkinje cells and that overexpression of G88R leads to a hyperpolarization and strong slowing of the upstroke velocity of spontaneously beating HL-1 cells. Thus, we propose that a gain-of-function by TASK-4 in the conduction system might aggravate slowed conductivity by the loss of sodium channel function. Moreover, WES supports a second hit-hypothesis in severe arrhythmia cases and identified KCNK17 as a novel arrhythmia gene.

Cellular mechanism of premature ventricular contraction-induced cardiomyopathy

BACKGROUND

Frequent premature ventricular contractions (PVCs) are associated with increased risk of sudden cardiac death and can cause secondary cardiomyopathy.

OBJECTIVE

We sought to determine the mechanism(s) responsible for prolonged refractory period and left ventricular (LV) dysfunction demonstrated in our canine model of PVC-induced cardiomyopathy.

METHODS

Single myocytes were isolated from LV free wall of PVC and control canines and used for patch-clamp recording, intracellular Ca(2+) measurements, and immunocytochemistry/confocal microscopy. LV tissues adjacent to the area of myocyte isolation were used for the immunoblot quantification of protein expression.

RESULTS

In the PVC group, LV ejection fraction decreased from 57.6% ± 1.5% to 30.4% ± 3.1% after ≥4 months of ventricular bigeminy. Compared to control myocytes, PVC myocytes had decreased densities of both outward (transient outward current [Ito] and inward rectifier current [IK1]) and inward (L-type Ca current [ICaL]) currents, but no consistent changes in rapid or slow delayed rectifier currents. The reduction in Ito, IK1, and ICaL was accompanied by decreased protein levels of their channel subunits. The extent of reduction in Ito, IK1, and ICaL varied among PVC myocytes, creating marked heterogeneity in action potential configurations and durations. PVC myocytes showed impaired Ca-induced Ca release from the sarcoplasmic reticulum (SR), without increase in SR Ca leak or decrease in SR Ca store. This was accompanied by a decrease in dyad scaffolding protein, junctophilin-2, and loss of Cav1.2 registry with Ca-releasing channels (ryanodine receptor 2).

CONCLUSION

PVCs increase dispersion of action potential configuration/duration, a risk factor for sudden cardiac death, because of the heterogeneous reduction in Ito, IK1, and ICaL. The excitation-contraction coupling is impaired because of the decrease in ICaL and Cav1.2 misalignment with respect to ryanodine receptor 2.

Auxiliary KChIP4a suppresses A-type K+ current through endoplasmic reticulum (ER) retention and promoting closed-state inactivation of Kv4 channels

In the brain and heart, auxiliary Kv channel-interacting proteins (KChIPs) co-assemble with pore-forming Kv4 α-subunits to form a native K(+) channel complex and regulate the expression and gating properties of Kv4 currents. Among the KChIP1-4 members, KChIP4a exhibits a unique N terminus that is known to suppress Kv4 function, but the underlying mechanism of Kv4 inhibition remains unknown. Using a combination of confocal imaging, surface biotinylation, and electrophysiological recordings, we identified a novel endoplasmic reticulum (ER) retention motif, consisting of six hydrophobic and aliphatic residues, 12-17 (LIVIVL), within the KChIP4a N-terminal KID, that functions to reduce surface expression of Kv4-KChIP complexes. This ER retention capacity is transferable and depends on its flanking location. In addition, adjacent to the ER retention motif, the residues 19-21 (VKL motif) directly promote closed-state inactivation of Kv4.3, thus leading to an inhibition of channel current. Taken together, our findings demonstrate that KChIP4a suppresses A-type Kv4 current via ER retention and enhancement of Kv4 closed-state inactivation.

Hippocampal A-type current and Kv4.2 channel modulation by the sulfonylurea compound NS5806

We examined the effects of the sulfonylurea compound NS5806 on neuronal A-type channel function. Using whole-cell patch-clamp we studied the effects of NS5806 on the somatodendritic A-type current (I(SA)) in cultured hippocampal neurons and the currents mediated by Kv4.2 channels coexpressed with different auxiliary β-subunits, including both Kv channel interacting proteins (KChIPs) and dipeptidyl aminopeptidase-related proteins (DPPs), in HEK 293 cells. The amplitude of the I(SA) component in hippocampal neurons was reduced in the presence of 20 μM NS5806. I(SA) decay kinetics were slowed and the recovery kinetics accelerated, but the voltage dependence of steady-state inactivation was shifted to more negative potentials by NS5806. The peak amplitudes of currents mediated by ternary Kv4.2 channel complexes, associated with DPP6-S (short splice-variant) and either KChIP2, KChIP3 or KChIP4, were potentiated and their macroscopic inactivation slowed by NS5806, whereas the currents mediated by binary Kv4.2 channels, associated only with DPP6-S, were suppressed, and the NS5806-mediated slowing of macroscopic inactivation was less pronounced. Neither potentiation nor suppression and no effect on current decay kinetics in the presence of NS5806 were observed for Kv4.2 channels associated with KChIP3 and the N-type inactivation-conferring DPP6a splice-variant. For all recombinant channel complexes, NS5806 slowed the recovery from inactivation and shifted the voltage dependence of steady-state inactivation to more negative potentials. Our results demonstrate the activity of NS5806 on native I(SA) and possible molecular correlates in the form of recombinant Kv4.2 channels complexed with different KChIPs and DPPs, and they shed some light on the mechanism of NS5806 action.

Subcellular localization and transcription regulatory potency of KCNIP/Calsenilin/DREAM/KChIP proteins in cultured primary cortical neurons do not provide support for their role in CRE-dependent gene expression

KCNIP3/KChIP3 (voltage-dependent K+ channel interacting protein 3), alias Calsenilin and downstream regulatory element antagonist modulator (DREAM), is a multifunctional protein that modulates A-type potassium channels, affects processing of amyloid precursor protein and regulates transcription. KCNIP3 has been described to negatively influence the activity of CREB (cAMP/Ca(2+)-response element binding protein), an essential factor in neuronal activity-dependent gene expression regulation. However, reports on intracellular localization of KCNIP3 in neurons are diverse and necessitate additional analyses of distribution of KCNIPs in cells to clarify the potential of KCNIP3 to fulfill its functions in different cell compartments. Here, we examined localization of the entire family of highly similar KCNIP proteins in neuronal cells and show that over-expressed isoforms of KCNIP1/KChIP1, KCNIP2/KChIP2, KCNIP3/KChIP3, and KCNIP4/KChIP4 had varied, yet partially overlapping subcellular localization. In addition, although some of the over-expressed KCNIP isoforms localized to the nucleus, endogenous KCNIPs were not detected in nuclei of rat primary cortical neurons. Moreover, we analyzed the role of KCNIP proteins in cAMP/Ca(2+)-response element (CRE)-dependent transcription by luciferase reporter assay and electrophoretic mobility shift assay and report that our results do not support the role for KCNIPs, including DREAM/Calsenilin/KChIP3, in modulation of CREB-mediated transcription in neurons.

K(V)4.3 N-terminal deletion mutant Δ2-39: effects on inactivation and recovery characteristics in both the absence and presence of KChIP2b

Gating transitions in the K(V)4.3 N-terminal deletion mutant Δ2-39 were characterized in the absence and presence of KChIP2b. We particularly focused on gating characteristics of macroscopic (open state) versus closed state inactivation (CSI) and recovery. In the absence of KChIP2b Δ2-39 did not significantly alter the steady-state activation "a(4)" relationship or general CSI characteristics, but it did slow the kinetics of deactivation, macroscopic inactivation, and macroscopic recovery. Recovery kinetics (for both WT K(V)4.3 and Δ2-39) were complicated and displayed sigmoidicity, a process which was enhanced by Δ2-39. Deletion of the proximal N-terminal domain therefore appeared to specifically slow mechanisms involved in regulating gating transitions occurring after the channel open state(s) had been reached. In the presence of KChIP2b Δ2-39 recovery kinetics (from both macroscopic and CSI) were accelerated, with an apparent reduction in initial sigmoidicity. Hyperpolarizing shifts in both "a(4)" and isochronal inactivation "i" were also produced. KChIP2b-mediated remodeling of K(V)4.3 gating transitions was therefore not obligatorily dependent upon an intact N-terminus. To account for these effects we propose that KChIP2 regulatory domains exist in K(V)4.3 a subunit regions outside of the proximal N-terminal. In addition to regulating macroscopic inactivation, we also propose that the K(V)4.3 N-terminus may act as a novel regulator of deactivation-recovery coupling.

Interdependent roles for accessory KChIP2, KChIP3, and KChIP4 subunits in the generation of Kv4-encoded IA channels in cortical pyramidal neurons

The rapidly activating and inactivating voltage-dependent outward K(+) (Kv) current, I(A), is widely expressed in central and peripheral neurons. I(A) has long been recognized to play important roles in determining neuronal firing properties and regulating neuronal excitability. Previous work demonstrated that Kv4.2 and Kv4.3 α-subunits are the primary determinants of I(A) in mouse cortical pyramidal neurons. Accumulating evidence indicates that native neuronal Kv4 channels function in macromolecular protein complexes that contain accessory subunits and other regulatory molecules. The K(+) channel interacting proteins (KChIPs) are among the identified Kv4 channel accessory subunits and are thought to be important for the formation and functioning of neuronal Kv4 channel complexes. Molecular genetic, biochemical, and electrophysiological approaches were exploited in the experiments described here to examine directly the roles of KChIPs in the generation of functional Kv4-encoded I(A) channels. These combined experiments revealed that KChIP2, KChIP3, and KChIP4 are robustly expressed in adult mouse posterior (visual) cortex and that all three proteins coimmunoprecipitate with Kv4.2. In addition, in cortical pyramidal neurons from mice lacking KChIP3 (KChIP3(-/-)), mean I(A) densities were reduced modestly, whereas in mean I(A) densities in KChIP2(-/-) and WT neurons were not significantly different. Interestingly, in both KChIP3(-/-) and KChIP2(-/-) cortices, the expression levels of the other KChIPs (KChIP2 and 4 or KChIP3 and 4, respectively) were increased. In neurons expressing constructs to mediate simultaneous RNA interference-induced reductions in the expression of KChIP2, 3, and 4, I(A) densities were markedly reduced and Kv current remodeling was evident.

KChIP2 attenuates cardiac hypertrophy through regulation of Ito and intracellular calcium signaling

Recent evidence shows that the auxiliary subunit KChIP2, which assembles with pore-forming Kv4-subunits, represents a new potential regulator of the cardiac calcium-independent transient outward potassium current (I(to)) density. In hypertrophy and heart failure, KChIP2 expression has been found to be significantly decreased. Our aim was to examine the role of KChIP2 in cardiac hypertrophy and the effect of restoring its expression on electrical remodeling and cardiac mechanical function using a combination of molecular, biochemical and gene targeting approaches. KChIP2 overexpression through gene transfer of Ad.KChIP2 in neonatal cardiomyocytes resulted in a significant increase in I(to)-channel forming Kv4.2 and Kv4.3 protein levels. In vivo gene transfer of KChIP2 in aortic banded adult rats showed that, compared to sham-operated or Ad.beta-gal-transduced hearts, KChIP2 significantly attenuated the developed left ventricular hypertrophy, robustly increased I(to) densities, shortened action potential duration, and significantly altered myocyte mechanics by shortening contraction amplitudes and maximal rates of contraction and relaxation velocities and decreasing Ca(2+) transients. Interestingly, blocking I(to) with 4-aminopyridine in KChIP2-overexpressing adult cardiomyocytes significantly increased the Ca(2+) transients to control levels. One-day-old rat pups intracardially transduced with KChIP2 for two months then subjected to aortic banding for 6-8 weeks (to induce hypertrophy) showed similar echocardiographic, electrical and mechanical remodeling parameters. In addition, in cultured adult cardiomyocytes, KChIP2 overexpression increased the expression of Ca(2+)-ATPase (SERCA2a) and sodium calcium exchanger but had no effect on ryanodine receptor 2 or phospholamban expression. In neonatal myocytes, KChIP2 notably reversed Ang II-induced hypertrophic changes in protein synthesis and MAP-kinase activation. It also significantly decreased calcineurin expression, NFATc1 expression and nuclear translocation and its downstream target, MCiP1.4. Altogether, these data show that KChIP2 can attenuate cardiac hypertrophy possibly through modulation of intracellular calcium concentration and calcineurin/NFAT pathway.

The aromatic cluster in KCHIP1b affects Kv4 inactivation gating

The KChIP1b splice variant has been shown to induce slow recovery from inactivation for Kv4.2 whereas KChIP1a enhanced the recovery. Both splice variants differ only by the insertion of the exon1b, rich in aromatic residues (5/11). We analysed in detail the modifications of Kv4.2 gating induced by the KChIP1b splice variant and the role for the aromatic cluster in KChIP1b in inducing these changes. By substituting alanine for the aromatic residues individually or in combination, we could convert the KChIP1b recovery behaviour into that of KChIP1a. The replacement of one or two aromatic residues resulted in a partial restitution of the KChIP1a recovery behaviour. When three aromatic residues were replaced in the exon1b, the recovery from inactivation was fast with time constants that were similar to those obtained with KChIP1a. Moreover, similar findings were observed for closed state inactivation and for the voltage dependence of inactivation. Thus, reduction of the side chain bulkiness in exon1b resulted in the conversion of the KChIP1b phenotype into the KChIP1a phenotype. These results indicate that the aromatic cluster in exon1b modulates the transitions towards and from the closed inactivated states and the steady state distribution over the respective states.

Total chemical synthesis and biophysical characterization of the minimal isoform of the KChIP2 potassium channel regulatory subunit

The potassium channel accessory subunit KChIP2 associates with Kv4.2 channels in the cardiac myocyte and is involved in the regulation of the transient outward current (I(to)) during the early phase of repolarization of the action potential. As a first step to biophysically probe the mechanism of KChIP2, we have chemically synthesized its minimal isoform, KChIP2d, using Boc chemistry solid phase peptide synthesis in conjunction with native chemical ligation. The synthetic KChIP2d protein is primarily alpha-helical as predicted and becomes more structured upon binding calcium as assessed by (1)H-NMR and CD spectroscopy. Synthetic KChIP2d is in a monomer-dimer equilibrium in solution, and there is evidence for two monomer binding sites on an N-terminal peptide of Kv4.2. Planned future studies include the incorporation of fluorescent and spin labeled probes in KChIP2d to yield structural information in parallel with electrophysiologic studies to elucidate KChIP2d's mechanism of action.

Neuronal calcium sensor proteins: generating diversity in neuronal Ca2+ signalling

In neurons, intracellular calcium signals have crucial roles in activating neurotransmitter release and in triggering alterations in neuronal function. Calmodulin has been widely studied as a Ca(2+) sensor that has several defined roles in neuronal Ca(2+) signalling, but members of the neuronal calcium sensor protein family have also begun to emerge as key components in a number of regulatory pathways and have increased the diversity of neuronal Ca(2+) signalling pathways. The differing properties of these proteins allow them to have discrete, non-redundant functions.

Spatial distributions of Kv4 channels and KChip2 isoforms in the murine heart based on laser capture microdissection

OBJECTIVE

Regional differences in repolarizing K(+) current densities and expression levels of their molecular components are important for coordinating the pattern of electrical excitation and repolarization of the heart. The small size of hearts from mice may obscure these interventricular and/or transmural expression differences of K(+) channels. We have examined this possibility in adult mouse ventricle using a technology that provides very high spatial resolution of tissue collection.

METHODS

Conventional manual dissection and laser capture microdissection (LCM) were utilized to dissect tissue from distinct ventricular regions. RNA was isolated from epicardial, mid-myocardial and endocardial layers of both the right and left ventricles. Real-time RT-PCR was used to quantify the transcript expression in these different regions.

RESULTS

LCM revealed significant interventricular and transmural gradients for both Kv4.2 and the alpha-subunit of KChIP2. The expression profile of a second K(+) channel transcript, Kir2.1, which is responsible for the inwardly rectifying K(+) current I(k1), showed no interventricular or transmural gradients and therefore served as a negative control.

CONCLUSIONS

Our findings are in contrast to previous reports of a relatively uniform left ventricular transmural pattern of expression of Kv4.2, Kv4.3 and KChIP2 in adult mouse heart, which appear to be different than that in larger mammals. Specifically, our results demonstrate significant epi- to endocardial differences in the patterns of expression of both Kv4.2 and KChIP2.

Three-dimensional structure of the KChIP1-Kv4.3 T1 complex reveals a cross-shaped octamer

Brain I(A) and cardiac I(to) currents arise from complexes containing Kv4 voltage-gated potassium channels and cytoplasmic calcium-sensor proteins (KChIPs). Here, we present X-ray crystallographic and small-angle X-ray scattering data that show that the KChIP1-Kv4.3 N-terminal cytoplasmic domain complex is a cross-shaped octamer bearing two principal interaction sites. Site 1 comprises interactions between a unique Kv4 channel N-terminal hydrophobic segment and a hydrophobic pocket formed by displacement of the KChIP H10 helix. Site 2 comprises interactions between a T1 assembly domain loop and the KChIP H2 helix. Functional and biochemical studies indicate that site 1 influences channel trafficking, whereas site 2 affects channel gating, and that calcium binding is intimately linked to KChIP folding and complex formation. Together, the data resolve how Kv4 channels and KChIPs interact and provide a framework for understanding how KChIPs modulate Kv4 function.

DPP10 is an inactivation modulatory protein of Kv4.3 and Kv1.4

Voltage-gated K+ channels exist in vivo as multiprotein complexes made up of pore-forming and ancillary subunits. To further our understanding of the role of a dipeptidyl peptidase-related ancillary subunit, DPP10, we expressed it with Kv4.3 and Kv1.4, two channels responsible for fast-inactivating K+ currents. Previously, DPP10 has been shown to effect Kv4 channels. However, Kv1.4, when expressed with DPP10, showed many of the same effects as Kv4.3, such as faster time to peak current and negative shifts in the half-inactivation potential of steady-state activation and inactivation. The exception was recovery from inactivation, which is slowed by DPP10. DPP10 expressed with Kv4.3 caused negative shifts in both steady-state activation and inactivation of Kv4.3, but no significant shifts were detected when DPP10 was expressed with Kv4.3 + KChIP2b (Kv channel interacting protein). DPP10 and KChIP2b had different effects on closed-state inactivation. At −60 mV, KChIP2b nearly abolishes closed-state inactivation in Kv4.3, whereas it developed to a much greater extent in the presence of DPP10. Finally, expression of a DPP10 mutant consisting of its transmembrane and cytoplasmic 58 amino acids resulted in effects on Kv4.3 gating that were nearly identical to those of wild-type DPP10. These data show that DPP10 and KChIP2b both modulate Kv4.3 inactivation but that their primary effects are on different inactivation states. Thus DPP10 may be a general modulator of voltage-gated K+ channel inactivation; understanding its mechanism of action may lead to deeper understanding of the inactivation of a broad range of K+ channels.

A potassium channel (Kv4) cloned from the heart of the tunicate Ciona intestinalis and its modulation by a KChIP subunit

Voltage-gated ion channels of the Kv4 subfamily produce A-type currents whose properties are tuned by accessory subunits termed KChIPs, which are a family of Ca2+ sensor proteins. By modifying expression levels and the intrinsic biophysical properties of Kv4 channels, KChIPs modulate the excitability properties of neurons and myocytes. We studied how a Kv4 channel from a tunicate, the first branching clade of the chordates, is modulated by endogenous KChIP subunits. BLAST searches in the genome of Ciona intestinalis identified a single Kv4 gene and a single KChIP gene, implying that the diversification of both genes occurred during early vertebrate evolution, since the corresponding mammalian gene families are formed by several paralogues. In this study we describe the cloning and characterization of a tunicate Kv4 channel, CionaKv4, and a tunicate KChIP subunit, CionaKChIP. We demonstrate that CionaKChIP strongly modulates CionaKv4 by producing larger currents that inactivate more slowly than in the absence of the KChIP subunit. Furthermore, CionaKChIP shifted the midpoints of activation and inactivation and slowed deactivation and recovery from inactivation of CionaKv4. Modulation by CionaKChIP requires the presence of the intact N terminus of CionaKv4 because, except for a minor effect on inactivation, CionaKChIP did not modulate CionaKv4 channels that lacked amino acids 2-32. In summary, our results suggest that modulation of Kv4 channels by KChIP subunits is an ancient mechanism for modulating electrical excitability.

A-type K+ current of dopamine and GABA neurons in the ventral tegmental area

A-type K(+) current (I(A)) is a rapidly inactivating voltage-dependent potassium current which can regulate the frequency of action potential (AP) generation. Increased firing frequency of ventral tegmental area (VTA) neurons is associated with the reinforcing effects of some drugs of abuse like nicotine and ethanol. In the present study, we classified dopamine (DA) and GABA VTA neurons, and investigated I(A) properties and the physiological role of I(A) in these neurons using conventional whole cell current- and voltage-clamp recording. DA VTA neurons had a mean firing frequency of 3.5 Hz with a long AP duration. GABA VTA neurons had a mean firing frequency of 16.7 Hz with a short AP duration. For I(A) properties, the voltage-dependence of steady-state I(A) activation and inactivation was similar in DA and GABA VTA neurons. I(A) inactivation was significantly faster and became faster at positive voltages in GABA neurons than DA neurons. Recovery from inactivation was significantly faster in DA neurons than GABA neurons. I(A) current density at full recovery was significantly larger in DA neurons than GABA neurons. In DA and GABA VTA neurons, latency to the first AP after the recovery from membrane hyperpolarization (repolarization latency) was measured. Longer repolarization latency was accompanied by larger I(A) current density in DA VTA neurons, compared with GABA VTA neurons. We suggest that I(A) contributes more to the regulation of AP generation in DA VTA neurons than in GABA VTA neurons.

Transient outward potassium current, 'Ito', phenotypes in the mammalian left ventricle: underlying molecular, cellular and biophysical mechanisms

At least two functionally distinct transient outward K(+) current (I(to)) phenotypes can exist across the free wall of the left ventricle (LV). Based upon their voltage-dependent kinetics of recovery from inactivation, these two phenotypes are designated 'I(to,fast)' (recovery time constants on the order of tens of milliseconds) and 'I(to,slow)' (recovery time constants on the order of thousands of milliseconds). Depending upon species, either I(to,fast), I(to,slow) or both current phenotypes may be expressed in the LV free wall. The expression gradients of these two I(to) phenotypes across the LV free wall are typically heterogeneous and, depending upon species, may consist of functional phenotypic gradients of both I(to,fast) and I(to,slow) and/or density gradients of either phenotype. We review the present evidence (molecular, biophysical, electrophysiological and pharmacological) for Kv4.2/4.3 alpha subunits underlying LV I(to,fast) and Kv1.4 alpha subunits underlying LV I(to,slow) and speculate upon the potential roles of each of these currents in determining frequency-dependent action potential characteristics of LV subepicardial versus subendocardial myocytes in different species. We also review the possible functional implications of (i) ancillary subunits that regulate Kv1.4 and Kv4.2/4.3 (Kvbeta subunits, DPPs), (ii) KChIP2 isoforms, (iii) spider toxin-mediated block of Kv4.2/4.3 (Heteropoda toxins, phrixotoxins), and (iv) potential mechanisms of modulation of I(to,fast) and I(to,slow) by cellular redox state, [Ca(2)(+)](i) and kinase-mediated phosphorylation. I(to) phenotypic activation and state-dependent gating models and molecular structure-function relationships are also discussed.

Structure, alternative splicing, and expression of the human and mouse KCNIP gene family

Potassium channel-interacting proteins (KCNIPs, also named KChIPs) modulate A-type potassium channels and favor their surface expression. In addition, KCNIPs have been shown to interact with presenilins and also to function as transcriptional repressors. Here we describe the structures and alternative splicing of the human and mouse KCNIP genes, including novel splice variants for KCNIP1, KCNIP3, and KCNIP4, and show the expression of different KCNIP mRNAs in various mouse and human tissues and brain regions by RT-PCR. Furthermore, we describe the expression of KCNIP1, KCNIP2, KCNIP3, and KCNIP4 mRNAs in the adult mouse brain with in situ hybridization and show that all KCNIP mRNAs were expressed in the neurons of the mouse brain with specific patterns for each KCNIP. Our results show that alternatively spliced KCNIP mRNAs are expressed differentially and could contribute to the diversity of functions of the KCNIP proteins.

Molecular physiology of cardiac repolarization

The heart is a rhythmic electromechanical pump, the functioning of which depends on action potential generation and propagation, followed by relaxation and a period of refractoriness until the next impulse is generated. Myocardial action potentials reflect the sequential activation and inactivation of inward (Na(+) and Ca(2+)) and outward (K(+)) current carrying ion channels. In different regions of the heart, action potential waveforms are distinct, owing to differences in Na(+), Ca(2+), and K(+) channel expression, and these differences contribute to the normal, unidirectional propagation of activity and to the generation of normal cardiac rhythms. Changes in channel functioning, resulting from inherited or acquired disease, affect action potential repolarization and can lead to the generation of life-threatening arrhythmias. There is, therefore, considerable interest in understanding the mechanisms that control cardiac repolarization and rhythm generation. Electrophysiological studies have detailed the properties of the Na(+), Ca(2+), and K(+) currents that generate cardiac action potentials, and molecular cloning has revealed a large number of pore forming (alpha) and accessory (beta, delta, and gamma) subunits thought to contribute to the formation of these channels. Considerable progress has been made in defining the functional roles of the various channels and in identifying the alpha-subunits encoding these channels. Much less is known, however, about the functioning of channel accessory subunits and/or posttranslational processing of the channel proteins. It has also become clear that cardiac ion channels function as components of macromolecular complexes, comprising the alpha-subunits, one or more accessory subunit, and a variety of other regulatory proteins. In addition, these macromolecular channel protein complexes appear to interact with the actin cytoskeleton and/or the extracellular matrix, suggesting important functional links between channel complexes, as well as between cardiac structure and electrical functioning. Important areas of future research will be the identification of (all of) the molecular components of functional cardiac ion channels and delineation of the molecular mechanisms involved in regulating the expression and the functioning of these channels in the normal and the diseased myocardium.

Molecular mechanisms of regulation of fast-inactivating voltage-dependent transient outward K+ current in mouse heart by cell volume changes

The K(v)4.2/4.3 channels are the primary subunits that contribute to the fast-inactivating, voltage-dependent transient outward K(+) current (I(to,fast)) in the heart. I(to,fast) is the critical determinant of the early repolarization of the cardiac action potential and plays an important role in the adaptive remodelling of cardiac myocytes, which usually causes cell volume changes, during myocardial ischaemia, hypertrophy and heart failure. It is not known, however, whether I(to,fast) is regulated by cell volume changes. In this study we investigated the molecular mechanism for cell volume regulation of I(to,fast) in native mouse left ventricular myocytes. Hyposmotic cell swelling caused a marked increase in densities of the peak I(to,fast) and a significant shortening in phase 1 repolarization of the action potential duration. The voltage-dependent gating properties of I(to,fast) were, however, not altered by changes in cell volume. In the presence of either protein kinase C (PKC) activator (12,13-dibutyrate) or phosphatase inhibitors (calyculin A and okadaic acid), hyposmotic cell swelling failed to further up-regulate I(to,fast). When expressed in NIH/3T3 cells, both K(v)4.2 and K(v)4.3 channels were also strongly regulated by cell volume in the same voltage-independent but PKC- and phosphatase-dependent manner as seen in I(to,fast) in the native cardiac myocytes. We conclude that K(v)4.2/4.3 channels in the heart are regulated by cell volume through a phosphorylation/dephosphorylation pathway mediated by PKC and serine/threonine phosphatase(s). These findings suggest a novel role of K(v)4.2/4.3 channels in the adaptive electrical and structural remodelling of cardiac myocytes in response to myocardial hypertrophy, ischaemia and reperfusion.

KChIPs and Kv4 alpha subunits as integral components of A-type potassium channels in mammalian brain

Voltage-gated potassium (Kv) channels from the Kv4, or Shal-related, gene family underlie a major component of the A-type potassium current in mammalian central neurons. We recently identified a family of calcium-binding proteins, termed KChIPs (Kv channel interacting proteins), that bind to the cytoplasmic N termini of Kv4 family alpha subunits and modulate their surface density, inactivation kinetics, and rate of recovery from inactivation (An et al., 2000). Here, we used single and double-label immunohistochemistry, together with circumscribed lesions and coimmunoprecipitation analyses, to examine the regional and subcellular distribution of KChIPs1-4 and Kv4 family alpha subunits in adult rat brain. Immunohistochemical staining using KChIP-specific monoclonal antibodies revealed that the KChIP polypeptides are concentrated in neuronal somata and dendrites where their cellular and subcellular distribution overlaps, in an isoform-specific manner, with that of Kv4.2 and Kv4.3. For example, immunoreactivity for KChIP1 and Kv4.3 is concentrated in the somata and dendrites of hippocampal, striatal, and neocortical interneurons. Immunoreactivity for KChIP2, KChIP4, and Kv4.2 is concentrated in the apical and basal dendrites of hippocampal and neocortical pyramidal cells. Double-label immunofluorescence labeling revealed that throughout the forebrain, KChIP2 and KChIP4 are frequently colocalized with Kv4.2, whereas in cortical, hippocampal, and striatal interneurons, KChIP1 is frequently colocalized with Kv4.3. Coimmunoprecipitation analyses confirmed that all KChIPs coassociate with Kv4 alpha subunits in brain membranes, indicating that KChIPs 1-4 are integral components of native A-type Kv channel complexes and are likely to play a major role as modulators of somatodendritic excitability.

Regulation of cardiac shal-related potassium channel Kv 4.3 by serum- and glucocorticoid-inducible kinase isoforms in Xenopus oocytes

The human cardiac transient outward potassium current I(to) is formed by co-assembly of voltage-dependent K(+) channel (Kv 4.3) pore-forming alpha-subunits with differently spliced K channel interacting protein (KChIP) accessory proteins. I(to) is of considerable importance for the normal course of the cardiac ventricular action potential. The present study was performed to determine whether isoforms of the serum- and glucocorticoid-inducible kinase (SGK) family influence Kv 4.3/KChIP2b channel activity in the Xenopus laevis heterologous expression system. Co-expression of SGK1, but not of SGK2 or SGK3, increased Kv 4.3/KChIP2b channel currents. The up-regulation of the current was not due to changes in the activation curve or changes of channel inactivation. The currents in oocytes expressing Kv 4.3 alone were smaller than those in Kv 4.3/KChIP2b expressing oocytes, but were still stimulated by SGK1. The effect of wild-type SGK1 was mimicked by constitutively active SGK1 (SGK1 S422D) but not by an inactive mutant (SGK1 K127N). The current amplitude increase mediated by SGK1 was not dependent on NEDD4.2 or RAB5, nor did it reflect increased cell surface expression. In conclusion, SGK1 stimulates Kv 4.3 potassium channels expressed in Xenopus oocytes by a novel mechanism distinct from the known NEDD4.2-dependent pathway.