Isolation and characterization of the human gene encoding Ito: further diversity by alternative mRNA splicing

Reversible cardiac disease features in an inducible CUG repeat RNA-expressing mouse model of myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is caused by a CTG repeat expansion in the DMPK gene. Expression of pathogenic expanded CUG repeat (CUGexp) RNA causes multisystemic disease by perturbing the functions of RNA-binding proteins, resulting in expression of fetal protein isoforms in adult tissues. Cardiac involvement affects 50% of individuals with DM1 and causes 25% of disease-related deaths. We developed a transgenic mouse model for tetracycline-inducible and heart-specific expression of human DMPK mRNA containing 960 CUG repeats. CUGexp RNA is expressed in atria and ventricles and induced mice exhibit electrophysiological and molecular features of DM1 disease, including cardiac conduction delays, supraventricular arrhythmias, nuclear RNA foci with Muscleblind protein colocalization, and alternative splicing defects. Importantly, these phenotypes were rescued upon loss of CUGexp RNA expression. Transcriptome analysis revealed gene expression and alternative splicing changes in ion transport genes that are associated with inherited cardiac conduction diseases, including a subset of genes involved in calcium handling. Consistent with RNA-Seq results, calcium-handling defects were identified in atrial cardiomyocytes isolated from mice expressing CUGexp RNA. These results identify potential tissue-specific mechanisms contributing to cardiac pathogenesis in DM1 and demonstrate the utility of reversible phenotypes in our model to facilitate development of targeted therapeutic approaches.

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.

Aberrant Expression of a Non-muscle RBFOX2 Isoform Triggers Cardiac Conduction Defects in Myotonic Dystrophy

Myotonic dystrophy type 1 (DM1) is a multisystemic genetic disorder caused by the CTG repeat expansion in the 3'-untranslated region of DMPK gene. Heart dysfunctions occur in ∼80% of DM1 patients and are the second leading cause of DM1-related deaths. Herein, we report that upregulation of a non-muscle splice isoform of RNA-binding protein RBFOX2 in DM1 heart tissue-due to altered splicing factor and microRNA activities-induces cardiac conduction defects in DM1 individuals. Mice engineered to express the non-muscle RBFOX240 isoform in heart via tetracycline-inducible transgenesis, or CRISPR/Cas9-mediated genome editing, reproduced DM1-related cardiac conduction delay and spontaneous episodes of arrhythmia. Further, by integrating RNA binding with cardiac transcriptome datasets from DM1 patients and mice expressing the non-muscle RBFOX2 isoform, we identified RBFOX240-driven splicing defects in voltage-gated sodium and potassium channels, which alter their electrophysiological properties. Thus, our results uncover a trans-dominant role for an aberrantly expressed RBFOX240 isoform in DM1 cardiac pathogenesis.

β Subunits Control the Effects of Human Kv4.3 Potassium Channel Phosphorylation

The transient outward K⁺ current, I_to, activates early in the cardiac myocyte action potential, to begin repolarization. Human I_to is generated primarily by two Kv4.3 potassium channel α subunit splice variants (Kv4.3L and Kv4.3S) that diverge only by a C-terminal, membrane-proximal, 19-residue stretch unique to Kv4.3L. Protein kinase C (PKC) phosphorylation of threonine 504 within the Kv4.3L-specific 19-residues mediates α-adrenergic inhibition of I_to in human heart. Kv4.3 is regulated in human heart by various β subunits, including cytosolic KChIP2b and transmembrane KCNEs, yet their impact on the functional effects of human Kv4.3 phosphorylation has not been reported. Here, this gap in knowledge was addressed using human Kv4.3 splice variants, T504 mutants, and human β subunits. Subunits were co-expressed in Xenopus laevis oocytes and analyzed by two-electrode voltage-clamp, using phorbol 12-myristate 13-acetate (PMA) to stimulate PKC. Unexpectedly, KChIP2b removed the inhibitory effect of PKC on Kv4.3L (but not Kv4.3L threonine phosphorylation by PKC per-se), while co-expression with KCNE2, but not KCNE4, restored PKC-dependent inhibition of Kv4.3L-KChIP2b to quantitatively resemble previously reported effects of α-adrenergic modulation of human ventricular I_to. In addition, PKC accelerated recovery from inactivation of Kv4.3L-KChIP2b channels and, interestingly, of both Kv4.3L and Kv4.3S alone. Thus, β subunits regulate the response of human Kv4.3 to PKC phosphorylation and provide a potential mechanism for modifying the response of I_to to α-adrenergic regulation in vivo.

β Subunits Functionally Differentiate Human Kv4.3 Potassium Channel Splice Variants

The human ventricular cardiomyocyte transient outward K⁺ current (I_to) mediates the initial phase of myocyte repolarization and its disruption is implicated in Brugada Syndrome and heart failure (HF). Human cardiac I_to is generated primarily by two Kv4.3 splice variants (Kv4.3L and Kv4.3S, diverging only by a C-terminal, S6-proximal, 19-residue stretch unique to Kv4.3L), which are differentially remodeled in HF, but considered functionally alike at baseline. Kv4.3 is regulated in human heart by β subunits including KChIP2b and KCNEs, but their effects were previously assumed to be Kv4.3 isoform-independent. Here, this assumption was tested experimentally using two-electrode voltage-clamp analysis of human subunits co-expressed in Xenopus laevis oocytes. Unexpectedly, Kv4.3L-KChIP2b channels exhibited up to 8-fold lower current augmentation, 40% slower inactivation, and 5 mV-shifted steady-state inactivation compared to Kv4.3S-KChIP2b. A synthetic peptide mimicking the 19-residue stretch diminished these differences, reinforcing the importance of this segment in mediating Kv4.3 regulation by KChIP2b. KCNE subunits induced further functional divergence, including a 7-fold increase in Kv4.3S-KCNE4-KChIP2b current compared to Kv4.3L-KCNE4-KChIP2b. The discovery of β-subunit-dependent functional divergence in human Kv4.3 splice variants suggests a C-terminal signaling hub is crucial to governing β-subunit effects upon Kv4.3, and demonstrates the potential significance of differential Kv4.3 gene-splicing and β subunit expression in myocyte physiology and pathobiology.

Kvβ1.1 (AKR6A8) senses pyridine nucleotide changes in the mouse heart and modulates cardiac electrical activity

The present study investigates the physiological role of Kvβ1 subunit for sensing pyridine nucleotide (NADH/NAD+) changes in the heart. We used Kvβ1.1 knockout (KO) or wild-type (WT) mice and established that Kvβ1.1 preferentially binds with Kv4.2 and senses the pyridine nucleotide changes in the heart. The cellular action potential duration (APD) obtained from WT cardiomyocytes showed longer APDs with lactate perfusion, which increases intracellular NADH levels, while the APDs remained unaltered in the Kvβ1.1 KO. Ex vivo monophasic action potentials showed a similar response, in which the APDs were prolonged in WT mouse hearts with lactate perfusion; however, the Kvβ1.1 KO mouse hearts did not show APD changes upon lactate perfusion. COS-7 cells coexpressing Kv4.2 and Kvβ1.1 were used for whole cell patch-clamp recordings to evaluate changes caused by NADH (lactate). These data reveal that Kvβ1.1 is required in the mediated inactivation of Kv4.2 currents, when NADH (lactate) levels are increased. In vivo, isoproterenol infusion led to increased NADH in the heart along with QTc prolongation in wild-type mice; regardless of the approach, our data show that Kvβ1.1 recognizes NADH changes and modulates Kv4.2 currents affecting AP and QTc durations. Overall, this study uses multiple levels of investigation, including the heterologous overexpression system, cardiomyocyte, ex vivo, and ECG, and clearly depicts that Kvβ1.1 is an obligatory sensor of NADH/NAD changes in vivo, with a physiological role in the heart.NEW & NOTEWORTHY Cardiac electrical activity is mediated by ion channels, and Kv4.2 plays a significant role, along with its binding partner, the Kvβ1.1 subunit. In the present study, we identify Kvβ1.1 as a sensor of pyridine nucleotide changes and as a modulator of Kv4.2 gating, action potential duration, and ECG in the mouse heart.

Cardiovascular Action of Insulin in Health and Disease: Endothelial L-Arginine Transport and Cardiac Voltage-Dependent Potassium Channels

Impairment of insulin signaling on diabetes mellitus has been related to cardiovascular dysfunction, heart failure, and sudden death. In human endothelium, cationic amino acid transporter 1 (hCAT-1) is related to the synthesis of nitric oxide (NO) and insulin has a vascular effect in endothelial cells through a signaling pathway that involves increases in hCAT-1 expression and L-arginine transport. This mechanism is disrupted in diabetes, a phenomenon potentiated by excessive accumulation of reactive oxygen species (ROS), which contribute to lower availability of NO and endothelial dysfunction. On the other hand, electrical remodeling in cardiomyocytes is considered a key factor in heart failure progression associated to diabetes mellitus. This generates a challenge to understand the specific role of insulin and the pathways involved in cardiac function. Studies on isolated mammalian cardiomyocytes have shown prolongated action potential in ventricular repolarization phase that produces a long QT interval, which is well explained by attenuation in the repolarizing potassium currents in cardiac ventricles. Impaired insulin signaling causes specific changes in these currents, such a decrease amplitude of the transient outward K(+) (Ito) and the ultra-rapid delayed rectifier (IKur) currents where, together, a reduction of mRNA and protein expression levels of α-subunits (Ito, fast; Kv 4.2 and IKs; Kv 1.5) or β-subunits (KChIP2 and MiRP) of K(+) channels involved in these currents in a MAPK mediated pathway process have been described. These results support the hypothesis that lack of insulin signaling can produce an abnormal repolarization in cardiomyocytes. Furthermore, the arrhythmogenic potential due to reduced Ito current can contribute to an increase in the incidence of sudden death in heart failure. This review aims to show, based on pathophysiological models, the regulatory function that would have insulin in vascular system and in cardiac electrophysiology.

Effects of neferine on Kv4.3 channels expressed in HEK293 cells and ex vivo electrophysiology of rabbit hearts

AIM

Neferine is an isoquinoline alkaloid isolated from seed embryos of Nelumbo nucifera (Gaertn), which has a variety of biological activities. In this study we examined the effects of neferine on Kv4.3 channels, a major contributor to the transient outward current (I(to)) in rabbit heart, and on ex vivo electrophysiology of rabbit hearts.

METHODS

Whole-cell Kv4.3 currents were recorded in HEK293 cells expressing human cardiac Kv4.3 channels using patch-clamp technique. Arterially perfused wedges of rabbit left ventricles (LV) were prepared, and transmembrane action potentials were simultaneously recorded from epicardial (Epi) and endocardial (Endo) sites with floating microelectrodes together with transmural electrocardiography (ECG).

RESULTS

Neferine (0.1-100 μmol/L) dose-dependently and reversibly inhibited Kv4.3 currents (the IC50 value was 8.437 μmol/L, and the maximal inhibition at 100 μmol/L was 44.12%). Neferine (10 μmol/L) caused a positive shift of the steady-state activation curve of Kv4.3 currents, and a negative shift of the steady-state inactivation curve. Furthermore, neferine (10 μmol/L) accelerated the inactivation but not the activation of Kv4.3 currents, and markedly slowed the recovery of Kv4.3 currents from inactivation. Neferine-induced blocking of Kv4.3 currents was frequency-dependent. In arterially perfused wedges of rabbit LV, neferine (1, 3, and 10 μmol/L) dose-dependently prolonged the QT intervals and action potential durations (APD) at both Epi and Endo sites, and caused dramatic increase of APD10 at Epi sites.

CONCLUSION

Neferine inhibits Kv4.3 channels likely by blocking the open state and inactivating state channels, which contributes to neferine-induced dramatic increase of APD10 at Epi sites of rabbit heart.

Ion Channels in the Heart

Optimal cardiac function depends on proper timing of excitation and contraction in various regions of the heart, as well as on appropriate heart rate. This is accomplished via specialized electrical properties of various components of the system, including the sinoatrial node, atria, atrioventricular node, His-Purkinje system, and ventricles. Here we review the major regionally determined electrical properties of these cardiac regions and present the available data regarding the molecular and ionic bases of regional cardiac function and dysfunction. Understanding these differences is of fundamental importance for the investigation of arrhythmia mechanisms and pharmacotherapy.

Cardiac potassium channel subtypes: new roles in repolarization and arrhythmia

About 10 distinct potassium channels in the heart are involved in shaping the action potential. Some of the K+ channels are primarily responsible for early repolarization, whereas others drive late repolarization and still others are open throughout the cardiac cycle. Three main K+ channels drive the late repolarization of the ventricle with some redundancy, and in atria this repolarization reserve is supplemented by the fairly atrial-specific KV1.5, Kir3, KCa, and K2P channels. The role of the latter two subtypes in atria is currently being clarified, and several findings indicate that they could constitute targets for new pharmacological treatment of atrial fibrillation. The interplay between the different K+ channel subtypes in both atria and ventricle is dynamic, and a significant up- and downregulation occurs in disease states such as atrial fibrillation or heart failure. The underlying posttranscriptional and posttranslational remodeling of the individual K+ channels changes their activity and significance relative to each other, and they must be viewed together to understand their role in keeping a stable heart rhythm, also under menacing conditions like attacks of reentry arrhythmia.

Mutations in KCND3 cause spinocerebellar ataxia type 22

OBJECTIVE

To identify the causative gene in spinocerebellar ataxia (SCA) 22, an autosomal dominant cerebellar ataxia mapped to chromosome 1p21-q23.

METHODS

We previously characterized a large Chinese family with progressive ataxia designated SCA22, which overlaps with the locus of SCA19. The disease locus in a French family and an Ashkenazi Jewish American family was also mapped to this region. Members from all 3 families were enrolled. Whole exome sequencing was performed to identify candidate mutations, which were narrowed by linkage analysis and confirmed by Sanger sequencing and cosegregation analyses. Mutational analyses were also performed in 105 Chinese and 55 Japanese families with cerebellar ataxia. Mutant gene products were examined in a heterologous expression system to address the changes in protein localization and electrophysiological functions.

RESULTS

We identified heterozygous mutations in the voltage-gated potassium channel Kv4.3-encoding gene KCND3: an in-frame 3-nucleotide deletion c.679_681delTTC p.F227del in both the Chinese and French pedigrees, and a missense mutation c.1034G>T p.G345V in the Ashkenazi Jewish family. Direct sequencing of KCND3 further identified 3 mutations, c.1034G>T p.G345V, c.1013T>C p.V338E, and c.1130C>T p.T377M, in 3 Japanese kindreds. Immunofluorescence analyses revealed that the mutant p.F227del Kv4.3 subunits were retained in the cytoplasm, consistent with the lack of A-type K(+) channel conductance in whole cell patch-clamp recordings.

INTERPRETATION

Our data identify the cause of SCA19/22 in patients of diverse ethnic origins as mutations in KCND3. These findings further emphasize the important role of ion channels as key regulators of neuronal excitability in the pathogenesis of cerebellar degeneration.

CREB critically regulates action potential shape and duration in the adult mouse ventricle

The cAMP response element binding protein (CREB) belongs to the CREB/cAMP response element binding modulator/activating transcription factor 1 family of cAMP-dependent transcription factors mediating a regulation of gene transcription in response to cAMP. Chronic stimulation of β-adrenergic receptors and the cAMP-dependent signal transduction pathway by elevated plasma catecholamines play a central role in the pathogenesis of heart failure. Ion channel remodeling, particularly a decreased transient outward current (I(to)), and subsequent action potential (AP) prolongation are hallmarks of the failing heart. Here, we studied the role of CREB for ion channel regulation in mice with a cardiomyocyte-specific knockout of CREB (CREB KO). APs of CREB KO cardiomyocytes were prolonged with increased AP duration at 50 and 70% repolarization and accompanied by a by 51% reduction of I(to) peak amplitude as detected in voltage-clamp measurements. We observed a 29% reduction of Kcnd2/Kv4.2 mRNA in CREB KO cardiomyocytes mice while the other I(to)-related channel subunits Kv4.3 and KChIP2 were not different between groups. Accordingly, Kv4.2 protein was reduced by 37% in CREB KO. However, we were not able to detect a direct regulation of Kv4.2 by CREB. The I(to)-dependent AP prolongation went along with an increase of I(Na) and a decrease of I(Ca,L) associated with an upregulation of Scn8a/Nav1.6 and downregulation of Cacna1c/Cav1.2 mRNA in CREB KO cardiomyocytes. Our results from mice with cardiomyocyte-specific inactivation of CREB definitively indicate that CREB critically regulates the AP shape and duration in the mouse ventricle, which might have an impact on ion channel remodeling in situations of altered cAMP-dependent signaling like heart failure.

Remodelling of human atrial K+ currents but not ion channel expression by chronic β-blockade

Chronic β-adrenoceptor antagonist (β-blocker) treatment in patients is associated with a potentially anti-arrhythmic prolongation of the atrial action potential duration (APD), which may involve remodelling of repolarising K(+) currents. The aim of this study was to investigate the effects of chronic β-blockade on transient outward, sustained and inward rectifier K(+) currents (I(TO), I(KSUS) and I(K1)) in human atrial myocytes and on the expression of underlying ion channel subunits. Ion currents were recorded from human right atrial isolated myocytes using the whole-cell-patch clamp technique. Tissue mRNA and protein levels were measured using real time RT-PCR and Western blotting. Chronic β-blockade was associated with a 41% reduction in I(TO) density: 9.3 ± 0.8 (30 myocytes, 15 patients) vs 15.7 ± 1.1 pA/pF (32, 14), p < 0.05; without affecting its voltage-, time- or rate dependence. I(K1) was reduced by 34% at -120 mV (p < 0.05). Neither I(KSUS), nor its increase by acute β-stimulation with isoprenaline, was affected by chronic β-blockade. Mathematical modelling suggested that the combination of I(TO)- and I(K1)-decrease could result in a 28% increase in APD(90). Chronic β-blockade did not alter mRNA or protein expression of the I(TO) pore-forming subunit, Kv4.3, or mRNA expression of the accessory subunits KChIP2, KChAP, Kvβ1, Kvβ2 or frequenin. There was no reduction in mRNA expression of Kir2.1 or TWIK to account for the reduction in I(K1). A reduction in atrial I(TO) and I(K1) associated with chronic β-blocker treatment in patients may contribute to the associated action potential prolongation, and this cannot be explained by a reduction in expression of associated ion channel subunits.

Homeostatic regulation of electrical excitability in physiological cardiac hypertrophy

Pathological biomechanical stresses cause cardiac hypertrophy, which is associated with QT prolongation and arrhythmias. Previous studies have demonstrated that repolarizing K(+) current densities are decreased in pressure overload-induced left ventricular hypertrophy, resulting in action potential and QT prolongation. Cardiac hypertrophy also occurs with exercise training, but this physiological hypertrophy is not associated with electrical abnormalities or increased arrhythmia risk, suggesting that repolarizing K(+) currents are upregulated, in parallel with the increase in myocyte size, to maintain normal cardiac function. To explore this hypothesis directly, electrophysiological recordings were obtained from ventricular myocytes isolated from two mouse models of physiological hypertrophy, one produced by swim-training of wild-type mice and the other by cardiac-specific expression of constitutively active phosphoinositide-3-kinase-p110α (caPI3Kα). Whole-cell voltage-clamp recordings revealed that repolarizing K(+) current amplitudes were higher in ventricular myocytes isolated from swim-trained and caPI3Kα, compared with wild-type, animals. The increases in K(+) current amplitudes paralleled the observed cellular hypertrophy, resulting in normalized or increased K(+) current densities. Electrocardiographic parameters, including QT intervals, as well as ventricular action potential waveforms in swim-trained animals/myocytes were indistinguishable from controls, demonstrating preserved electrical function. Additional experiments revealed that inward Ca(2+) current amplitudes/densities were also increased in caPI3Kα, compared with WT, left ventricular myocytes. The expression of transcripts encoding K(+), Ca(2+) and other ion channel subunits was increased in swim-trained and caPI3Kα ventricles, in parallel with the increase in myocyte size and with the global increases in total cellular RNA expression. In contrast to pathological hypertrophy, therefore, the functional expression of repolarizing K(+) (and depolarizing Ca(2+)) channels is increased with physiological hypertrophy, reflecting upregulation of the underlying ion channel subunit transcripts and resulting in increased current amplitudes and the normalization of current densities and action potential waveforms. Taken together, these results suggest that activation of PI3Kα signalling preserves normal myocardial electrical functioning and could be protective against the increased risk of arrhythmias and sudden death that are prevalent in pathological cardiac hypertrophy.

Determinants of CREB degradation and KChIP2 gene transcription in cardiac memory

BACKGROUND

Left ventricular pacing (LVP) to induce cardiac memory (CM) in dogs results in a decreased transient outward K current (I(to)) and reduced mRNA and protein of the I(to) channel accessory subunit, KChIP2. The KChIP2 decrease is attributed to a decrease in its transcription factor, cyclic adenosine monophosphate response element binding protein (CREB).

OBJECTIVE

This study sought to determine the mechanisms responsible for the CREB decrease that is initiated by LVP.

METHODS

CM was quantified as T-wave vector displacement in 18 LVP dogs. In 5 dogs, angiotensin II receptor blocker, saralasin, was infused before and during pacing. In 3 dogs, proteasomal inhibitor, lactacystin, was injected into the left anterior descending artery before LVP. Epicardial biopsy samples were taken before and after LVP. Neonatal rat cardiomyocytes (NRCM) were incubated with H(2)O(2) (50 micromol/l) for 1 hour with or without lactacystin.

RESULTS

LVP significantly displaced the T-wave vector and was associated with increased lipid peroxidation and increased tissue angiotensin II levels. Saralasin prevented T-vector displacement and lipid peroxidation. CREB was significantly decreased after 2 hours of LVP and was comparably decreased in H(2)O(2)-treated NRCM. Lactacystin inhibited the CREB decrease in LVP dogs and H(2)O(2)-treated NRCM. LVP and H(2)O(2) both induced CREB ubiquitination, and the H(2)O(2)-induced CREB decrease was prevented by knocking down ubiquitin.

CONCLUSION

LVP initiates myocardial angiotensin II production and reactive oxygen species synthesis, leading to CREB ubiquitination and its proteasomal degradation. This sequence of events would explain the pacing-induced reduction in KChIP2, and contribute to altered repolarization and the T-wave changes of cardiac memory.

Molecular determinants of cardiac transient outward potassium current (I(to)) expression and regulation

Rapidly activating and inactivating cardiac transient outward K(+) currents, I(to), are expressed in most mammalian cardiomyocytes, and contribute importantly to the early phase of action potential repolarization and to plateau potentials. The rapidly recovering (I(t)(o,f)) and slowly recovering (I(t)(o,s)) components are differentially expressed in the myocardium, contributing to regional heterogeneities in action potential waveforms. Consistent with the marked differences in biophysical properties, distinct pore-forming (alpha) subunits underlie the two I(t)(o) components: Kv4.3/Kv4.2 subunits encode I(t)(o,f), whereas Kv1.4 encodes I(t)(o,s), channels. It has also become increasingly clear that cardiac I(t)(o) channels function as components of macromolecular protein complexes, comprising (four) Kvalpha subunits and a variety of accessory subunits and regulatory proteins that influence channel expression, biophysical properties and interactions with the actin cytoskeleton, and contribute to the generation of normal cardiac rhythms. Derangements in the expression or the regulation of I(t)(o) channels in inherited or acquired cardiac diseases would be expected to increase the risk of potentially life-threatening cardiac arrhythmias. Indeed, a recently identified Brugada syndrome mutation in KCNE3 (MiRP2) has been suggested to result in increased I(t)(o,f) densities. Continued focus in this area seems certain to provide new and fundamentally important insights into the molecular determinants of functional I(t)(o) channels and into the molecular mechanisms involved in the dynamic regulation of I(t)(o) channel functioning in the normal and diseased myocardium.

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

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

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.

Effects of diltiazem and nifedipine on transient outward and ultra-rapid delayed rectifier potassium currents in human atrial myocytes

1. It is unknown whether the widely used L-type Ca(2+) channel antagonists diltiazem and nifedipine would block the repolarization K(+) currents, transient outward current (I(to1)) and ultra-rapid delayed rectifier K(+) current (I(Kur)), in human atrium. The present study was to determine the effects of diltiazem and nifedipine on I(to1) and I(Kur) in human atrial myocytes with whole-cell patch-clamp technique. 2. It was found that diltiazem substantially inhibited I(to1) in a concentration-dependent manner, with an IC(50) of 29.2+/-2.4 microM, and nifedipine showed a similar effect (IC(50)=26.8+/-2.1 muM). The two drugs had no effect on voltage-dependent kinetics of the current; however, they accelerated I(to1) inactivation significantly, suggesting an open channel block. 3. In addition, diltiazem and nifedipine suppressed I(Kur) in a concentration-dependent manner (at +50 mV, IC(50)=11.2+/-0.9 and 8.2+/-0.8 microM, respectively). These results indicate that the Ca(2+) channel blockers diltiazem and nifedipine substantially inhibit I(to1) and I(Kur) in human atrial myocytes.

Molecular correlates of altered expression of potassium currents in failing rabbit myocardium

Action potential (AP) prolongation is a hallmark of failing myocardium. Functional downregulation of K currents is a prominent feature of cells isolated from failing ventricles. The detailed changes in K current expression differ depending on the species, the region of the heart, and the mechanism of induction of heart failure. We used complementary approaches to study K current downregulation in pacing tachycardia-induced heart failure in the rabbit. The AP duration (APD) at 90% repolarization was significantly longer in cells isolated from failing hearts compared with controls (539 +/- 162 failing vs. 394 +/- 114 control, P < 0.05). The major K currents in the rabbit heart, inward rectifier potassium current (I(K1)), transient outward (I(to)), and delayed rectifier current (I(K)) were functionally downregulated in cells isolated from failing ventricles. The mRNA levels of Kv4.2, Kv1.4, KChIP2, and Kir2.1 were significantly downregulated, whereas the Kv4.3, Erg, KvLQT1, and minK were unaltered in the failing ventricles compared with the control left ventricles. Significant downregulation in the long splice variant of Kv4.3, but not in the total Kv4.3, Kv4.2, and KChIP2 immunoreactive protein, was observed in cells isolated from the failing ventricle with no change in Kv1.4, KvLQT1, and in Kir2.1 immunoreactive protein levels. Multiple cellular and molecular mechanisms underlie the downregulation of K currents in the failing rabbit ventricle.

Structure and function of Kv4-family transient potassium channels

Shal-type (Kv4.x) K(+) channels are expressed in a variety of tissue, with particularly high levels in the brain and heart. These channels are the primary subunits that contribute to transient, voltage-dependent K(+) currents in the nervous system (A currents) and the heart (transient outward current). Recent studies have revealed an enormous degree of complexity in the regulation of these channels. In this review, we describe the surprisingly large number of ancillary subunits and scaffolding proteins that can interact with the primary subunits, resulting in alterations in channel trafficking and kinetic properties. Furthermore, we discuss posttranslational modification of Kv4.x channel function with an emphasis on the role of kinase modulation of these channels in regulating membrane properties. This concept is especially intriguing as Kv4.2 channels may integrate a variety of intracellular signaling cascades into a coordinated output that dynamically modulates membrane excitability. Finally, the pathophysiology that may arise from dysregulation of these channels is also reviewed.

Phenotypic differences in transient outward K+ current of human and canine ventricular myocytes: insights into molecular composition of ventricular Ito

The Ca(2+)-independent transient outward K(+) current (I(to)) plays an important electrophysiological role in normal and diseased hearts. However, its contribution to ventricular repolarization remains controversial because of differences in its phenotypic expression and function across species. The dog, a frequently used model of human cardiac disease, exhibits altered functional expression of I(to). To better understand the relevance of electrical remodeling in dogs to humans, we studied the phenotypic differences in ventricular I(to) of both species with electrophysiological, pharmacological, and protein-chemical techniques. Several notable distinctions were elucidated, including slower current decay, more rapid recovery from inactivation, and a depolarizing shift of steady-state inactivation in human vs. canine I(to). Whereas recovery from inactivation of human I(to) followed a monoexponential time course, canine I(to) recovered with biexponential kinetics. Pharmacological sensitivity to flecainide was markedly greater in human than canine I(to), and exposure to oxidative stress did not alter the inactivation kinetics of I(to) in either species. Western blot analysis revealed immunoreactive bands specific for Kv4.3, Kv1.4, and Kv channel-interacting protein (KChIP)2 in dog and human, but with notable differences in band sizes across species. We report for the first time major variations in phenotypic properties of human and canine ventricular I(to) despite the presence of the same subunit proteins in both species. These data suggest that differences in electrophysiological and pharmacological properties of I(to) between humans and dogs are not caused by differential expression of the K channel subunit genes thought to encode I(to), but rather may arise from differences in molecular structure and/or posttranslational modification of these subunits.

Dihydropyridine Ca2+ channel antagonists and agonists block Kv4.2, Kv4.3 and Kv1.4 K+ channels expressed in HEK293 cells

(1) We have determined the molecular basis of nicardipine-induced block of cardiac transient outward K(+) currents (I(to)). Inhibition of I(to) was studied using cloned voltage-dependent K(+) channels (Kv) channels, rat Kv4.3L, Kv4.2, and Kv1.4, expressed in human embryonic kidney cell line 293 (HEK293) cells. (2) Application of the dihydropyridine Ca(2+) channel antagonist, nicardipine, accelerated the inactivation rate and reduced the peak amplitude of Kv4.3L currents in a concentration-dependent manner (IC(50): 0.42 micro M). The dihydropyridine (DHP) Ca(2+) channel agonist, Bay K 8644, also blocked this K(+) current (IC(50): 1.74 micro M). (3) Nicardipine (1 micro M) slightly, but significantly, shifted the voltage dependence of activation and steady-state inactivation to more negative potentials, and also slowed markedly the recovery from inactivation of Kv4.3L currents. (4) Coexpression of K(+) channel-interacting protein 2 (KChIP2) significantly slowed the inactivation of Kv4.3L currents as expected. However, the features of DHP-induced block of K(+) current were not substantially altered. (5) Nicardipine exhibited similar block of Kv1.4 and Kv4.2 channels stably expressed in HEK293 cells; IC(50)'s were 0.80 and 0.62 micro M, respectively. (6) Thus, at submicromolar concentrations, DHP Ca(2+) antagonist and agonist inhibit Kv4.3L and have similar inhibiting effects on other components of cardiac I(to), Kv4.2 and Kv1.4.

Mechanism of alpha-adrenergic regulation of expressed hKv4.3 currents

The transient outward potassium current (I(to)) is an important repolarizing current in the mammalian heart. I(to) is regulated by adrenergic stimulation; however, the effect of agonists on this current, and consequently the action potential duration and profile, is variable. An important source of the variability is the difference in the channel genes that underlie I(to). There are two subfamilies of candidate genes that are likely to encode I(to) in the mammalian heart: Kv4 and Kv1.4; the predominance of either gene is a function of the species, stage of development, and region of the heart. The existence of different isoforms of the Kv4 family (principally Kv4.2 or Kv4.3) further complicates the effect of alpha-adrenergic modulation of cardiac I(to). In the human ventricle, hKv4.3 is the predominant gene underlying I(to). Two splice variants of human Kv4.3 (hKv4.3) are present in the human ventricle; the longer splice variant contains a 19-amino acid insert in the COOH-terminus with a consensus protein kinase C (PKC) site. We used heterologous expression of hKv4.3 splice variants and studies of human ventricular myocytes to demonstrate that alpha-adrenergic modulation of I(to) occurs through a PKC signaling pathway and that only the long splice variant (hKv4.3-L) is modulated via this pathway. Only a single hKv4.3-L monomer in the tetrameric I(to) channel is required to confer sensitivity to phenylephrine (PE). Mutation of the PKC site in hKv4.3-L eliminates alpha-adrenergic modulation of the hKv4.3-encoded current. The similar, albeit less robust, modulation of human ventricular I(to) by PE suggests that hKv4.3-L is expressed in a functional form in the human heart.

Kinetic analysis of open- and closed-state inactivation transitions in human Kv4.2 A-type potassium channels

1. We studied the gating kinetics of Kv4.2 channels, the molecular substrate of neuronal somatodendritic A-type currents. For this purpose wild-type and mutant channels were transiently expressed in the human embryonic kidney (HEK) 293 cell line and currents were measured in the whole-cell patch-clamp configuration. 2. Kv4.2 channels inactivated from pre-open closed state(s) with a mean time constant of 959 ms at -50 mV. This closed-state inactivation was not affected by a deletion of the Kv4.2 N-terminus (Delta2-40). 3. Kv4.2 currents at +40 mV inactivated with triple-exponential kinetics. A fast component (tau = 11 ms) accounted for 73 %, an intermediate component (tau = 50 ms) for 23 % and a slow component (tau = 668 ms) for 4 % of the total decay. 4. Both the fast and the intermediate components of inactivation were slowed by a deletion of the Kv4.2 N-terminus (tau = 35 and 111 ms) and accounted for 33 and 56 %, respectively, of the total decay. The slow component was moderately accelerated by the truncation (tau = 346 ms) and accounted for 11 % of the total Kv4.2 current inactivation. 5. Recovery from open-state inactivation and recovery from closed-state inactivation occurred with similar kinetics in a strongly voltage-dependent manner. Neither recovery reaction was affected by the N-terminal truncation. 6. Kv4.2 Delta2-40 channels displayed slowed deactivation kinetics, suggesting that the N-terminal truncation leads to a stabilization of the open state. 7. Simulations with an allosteric model of inactivation, supported by the experimental data, suggested that, in response to membrane depolarization, Kv4.2 channels accumulate in the closed-inactivated state(s), from which they directly recover, bypassing the open state.

Molecular basis of functional voltage-gated K+ channel diversity in the mammalian myocardium

In the mammalian heart, Ca2+-independent, depolarization-activated potassium (K+) currents contribute importantly to shaping the waveforms of action potentials, and several distinct types of voltage-gated K+ currents that subserve this role have been characterized. In most cardiac cells, transient outward currents, Ito,f and/or Ito,s, and several components of delayed reactivation, including IKr, IKs, IKur and IK,slow, are expressed. Nevertheless, there are species, as well as cell-type and regional, differences in the expression patterns of these currents, and these differences are manifested as variations in action potential waveforms. A large number of voltage-gated K+ channel pore-forming (alpha) and accessory (beta, minK, MiRP) subunits have been cloned from or shown to be expressed in heart, and a variety of experimental approaches are being exploited in vitro and in vivo to define the relationship(s) between these subunits and functional voltage-gated cardiac K+ channels. Considerable progress has been made in defining these relationships recently, and it is now clear that distinct molecular entities underlie the various electrophysiologically distinct repolarizing K+ currents (i.e. Ito,f, Ito,s, IKr, IKs, IKur, IK,slow, etc.) in myocyardial cells.