Stimulation of protein kinase C inhibits bursting in disease-linked mutant human cardiac sodium channels

Biophysical mechanisms of myocardium sodium channelopathies

Genetic variants of gene SCN5A encoding the alpha-subunit of cardiac voltage-gated sodium channel Nav1.5 are associated with various diseases, including long QT syndrome (LQT3), Brugada syndrome (BrS1), and progressive cardiac conduction disease (PCCD). In the last decades, the great progress in understanding molecular and biophysical mechanisms of these diseases has been achieved. The LQT3 syndrome is associated with gain-of-function of sodium channels Nav1.5 due to impaired inactivation, enhanced activation, accelerated recovery from inactivation or the late current appearance. In contrast, BrS1 and PCCD are associated with the Nav1.5 loss-of-function, which in electrophysiological experiments can be manifested as reduced current density, enhanced fast or slow inactivation, impaired activation, or decelerated recovery from inactivation. Genetic variants associated with congenital arrhythmias can also disturb interactions of the Nav1.5 channel with different proteins or drugs and cause unexpected reactions to drug administration. Furthermore, mutations can affect post-translational modifications of the channels and their sensitivity to pH and temperature. Here we briefly review the current knowledge on biophysical mechanisms of LQT3, BrS1 and PCCD. We focus on limitations of studies that use heterologous expression systems and induced pluripotent stem cells (iPSC) derived cardiac myocytes and summarize our understanding of genotype-phenotype relations of SCN5A mutations.

Impacts of gene variants on drug effects-the foundation of genotype-guided pharmacologic therapy for long QT syndrome and short QT syndrome

The clinical significance of optimal pharmacotherapy for inherited arrhythmias such as short QT syndrome (SQTS) and long QT syndrome (LQTS) has been increasingly recognised. The advancement of gene technology has opened up new possibilities for identifying genetic variations and investigating the pathophysiological roles and mechanisms of genetic arrhythmias. Numerous variants in various genes have been proven to be causative in genetic arrhythmias. Studies have demonstrated that the effectiveness of certain drugs is specific to the patient or genotype, indicating the important role of gene-variants in drug response. This review aims to summarize the reported data on the impact of different gene-variants on drug response in SQTS and LQTS, as well as discuss the potential mechanisms by which gene-variants alter drug response. These findings may provide valuable information for future studies on the influence of gene variants on drug efficacy and the development of genotype-guided or precision treatment for these diseases.

Methylation determines the extracellular calcium sensitivity of the leak channel NALCN in hippocampal dentate granule cells

The sodium leak channel NALCN is a key player in establishing the resting membrane potential (RMP) in neurons and transduces changes in extracellular Ca²⁺ concentration ([Ca²⁺]_e) into increased neuronal excitability as the downstream effector of calcium-sensing receptor (CaSR). Gain-of-function mutations in the human NALCN gene cause encephalopathy and severe intellectual disability. Thus, understanding the regulatory mechanisms of NALCN is important for both basic and translational research. This study reveals a novel mechanism for NALCN regulation by arginine methylation. Hippocampal dentate granule cells in protein arginine methyltransferase 7 (PRMT7)-deficient mice display a depolarization of the RMP, decreased threshold currents, and increased excitability compared to wild-type neurons. Electrophysiological studies combined with molecular analysis indicate that enhanced NALCN activities contribute to hyperexcitability in PRMT7−/− neurons. PRMT7 depletion in HEK293T cells increases NALCN activity by shifting the dose-response curve of NALCN inhibition by [Ca²⁺]_e without affecting NALCN protein levels. In vitro methylation studies show that PRMT7 methylates a highly conserved Arg1653 of the NALCN gene located in the carboxy-terminal region that is implicated in CaSR-mediated regulation. A kinase-specific phosphorylation site prediction program shows that the adjacent Ser1652 is a potential phosphorylation site. Consistently, our data from site-specific mutants and PKC inhibitors suggest that Arg1653 methylation might modulate Ser1652 phosphorylation mediated by CaSR/PKC-delta, leading to [Ca²⁺]_e-mediated NALCN suppression. Collectively, these data suggest that PRMT7 deficiency decreases NALCN methylation at Arg1653, which, in turn, decreases CaSR/PKC-mediated Ser1652 phosphorylation, lifting NALCN inhibition, thereby enhancing neuronal excitability. Thus, PRMT7-mediated NALCN inhibition provides a potential target for the development of therapeutic tools for neurological diseases.

The addition of a methyl group to an arginine residue on the ion channel NALCN contributes to suppress the activity of this membrane protein and reduces neuronal excitability. Hana Cho, Jong-Sun Kang and colleagues at Sungkyunkwan University in South Korea found that neurons in the hippocampus of mice lacking an enzyme that mediates the transfer of methyl groups to proteins have increased NALCN activity and are more likely to fire an electrical signal. Furthermore, they showed that NALCN methylation facilitates the phosphorylation of an adjacent amino acid that prevents channel activation in response to extracellular calcium concentrations. These findings suggest that NALCN methylation has a key role in regulating the channel’s sensitivity to calcium. Moreover, they reveal a new mechanism for regulating neuronal excitability that could be targeted therapeutically to ameliorate diseases characterised by neuronal hyperexcitability.

Downregulation of adult and neonatal Nav1.5 in the dorsal root ganglia and axon of peripheral sensory neurons of rats with spared nerve injury

Previous studies demonstrated that Nav1.5 splice variants, including Nav1.5a and Nav1.5c, were expressed in dorsal root ganglia (DRG) neurons. However, since nine Nav1.5 isoforms have been identified, whether other Nav1.5 splice variants, especially the neonatal Nav1.5 splice variant, express in the DRG neurons is still unknown. In this study, we systematically investigated the expression of adult and neonatal Nav1.5 isoforms in the DRG neurons and axon of peripheral sensory neurons of rats with spared nerve injury (SNI) by RT-PCR, DNA sequencing, restriction enzyme digestion, immunohistochemistry and immunofluorescence methods. The results demonstrated that both adult and neonatal Nav1.5 isoforms were expressed in the DRG neurons, but their expression ratio was ~2.5:1. In SNI rat models, the expression of both adult and neonatal Nav1.5 decreased by approximately a half in both mRNA and protein levels. In contrast, the expression of protein kinase C (PKC)-γ, one of the negative modulators for sodium currents, increased by ~1-fold. Taken together, this study first confirmed the expression of both adult and neonatal Nav1.5 isoforms in the DRG neurons and axon of peripheral sensory neurons of rat, but their expression level decreased in pain models. The upregulation of PKC-γ may directly or indirectly downregulate the expression of Nav1.5 isoforms in SNI rat models, which may further involve in the pathological process of neuropathic pain.

Regulation of Cardiac Voltage-Gated Sodium Channel by Kinases: Roles of Protein Kinases A and C

In the heart, voltage-gated sodium (Na_v) channel (Na_v1.5) is defined by its pore-forming α-subunit and its auxiliary β-subunits, both of which are important for its critical contribution to the initiation and maintenance of the cardiac action potential (AP) that underlie normal heart rhythm. The physiological relevance of Na_v1.5 is further marked by the fact that inherited or congenital mutations in Na_v1.5 channel gene SCN5A lead to altered functional expression (including expression, trafficking, and current density), and are generally manifested in the form of distinct cardiac arrhythmic events, epilepsy, neuropathic pain, migraine, and neuromuscular disorders. However, despite significant advances in defining the pathophysiology of Na_v1.5, the molecular mechanisms that underlie its regulation and contribution to cardiac disorders are poorly understood. It is rapidly becoming evident that the functional expression (localization, trafficking and gating) of Na_v1.5 may be under modulation by post-translational modifications that are associated with phosphorylation. We review here the molecular basis of cardiac Na channel regulation by kinases (PKA and PKC) and the resulting functional consequences. Specifically, we discuss: (1) recent literature on the structural, molecular, and functional properties of cardiac Na_v1.5 channels; (2) how these properties may be altered by phosphorylation in disease states underlain by congenital mutations in Na_v1.5 channel and/or subunits such as long QT and Brugada syndromes. Our expectation is that understanding the roles of these distinct and complex phosphorylation processes on the functional expression of Na_v1.5 is likely to provide crucial mechanistic insights into Na channel associated arrhythmogenic events and will facilitate the development of novel therapeutic strategies.

Role of protein kinase C in metabolic regulation of the cardiac Na+ channel

BACKGROUND

The reduced form of nicotinamide adenine dinucleotide (NADH) increases in cardiomyopathy, activates protein kinase C (PKC), up-regulates mitochondrial reactive oxygen species (mitoROS), and down-regulates the cardiac Na⁺ channel (Na_V1.5).

OBJECTIVE

The purpose of this study was to determine how NADH signals down-regulation of Na_V1.5.

METHODS

Isolated mouse cardiomyocytes were used for patch-clamp recording and for monitoring mitoROS with MitoSOX Red. HEK293 cells were used for transient transfections. HEK293 cells stably expressing human Na_V1.5 were used for single channel recording, whole-cell patch-clamp recording, activity measurements of phospholipase C and phospholipase D (PLD), channel protein purification, and co-immunoprecipitation with PKC isoforms. HL-1 cells were used for mitochondria isolation.

RESULTS

NADH enhanced PLD activity (1.6- ± 0.1-fold, P <.01) and activated PKCδ. Activated PKCδ translocated to mitochondria and up-regulated mitoROS (2.8- ± 0.3-fold, P <.01) by enhancing the activities of mitochondrial complexes I, II, and IV (1.1- to 1.5-fold, P <.01). PKCδ also interacted with Na_V1.5 to down-regulate Na⁺ current (I_Na). Reduction in I_Na by activated PKCδ was prevented by antioxidants and by mutating the known PKC phosphorylation site S1503. At the single channel level, the mechanism of current reduction by PKC and recovery by protein kinase A was a change in single channel conductance.

CONCLUSION

NADH activated PKCδ by enhancing PLD activity. PKCδ modulated both mitoROS and Na_V1.5. PKCδ elevated mitoROS by enhancing mitochondrial oxidative phosphorylation complex activities. PKCδ-mediated channel phosphorylation and mitoROS were both required to down-regulate Na_V1.5 and alter single channel conductance.

Cardiac Sodium Channel Mutations: Why so Many Phenotypes?

The cardiac Na(+) channel (Nav1.5) conducts a depolarizing inward Na(+) current that is responsible for the generation of the upstroke Phase 0 of the action potential. In heart tissue, changes in Na(+) currents can affect conduction velocity and impulse propagation. The cardiac Nav1.5 is also involved in determination of the action potential duration, since some channels may reopen during the plateau phase, generating a persistent or late inward current. Mutations of cardiac Nav1.5 can induce gain or loss of channel function because of an increased late current or a decrease of peak current, respectively. Gain-of-function mutations cause Long QT syndrome type 3 and possibly atrial fibrillation, while loss-of-function channel mutations are associated with a wider variety of phenotypes, such as Brugada syndrome, cardiac conduction disease, dilated cardiomyopathy, and sick sinus node syndrome. The penetrance and phenotypes resulting from Nav1.5 mutations also vary with age, gender, body temperature, circadian rhythm, and between regions of the heart. This phenotypic variability makes it difficult to correlate genotype-phenotype. We propose that mutations are only one contributor to the phenotype and additional modifications on Nav1.5 lead to the phenotypic variability. Possible modifiers include other genetic variations and alterations in the life cycle of Nav1.5 such as gene transcription, RNA processing, translation, posttranslational modifications, trafficking, complex assembly, and degradation. In this chapter, we summarize potential modifiers of cardiac Nav1.5 that could help explain the clinically observed phenotypic variability. Consideration of these modifiers could help improve genotype-phenotype correlations and lead to new therapeutic strategies.

Actions of KMUP-1, a xanthine and piperazine derivative, on voltage-gated Na(+) and Ca(2+) -activated K(+) currents in GH3 pituitary tumour cells

BACKGROUND AND PURPOSE

7-[2-[4-(2-Chlorophenyl)piperazinyl]ethyl]-1,3-dimethylxanthine (KMUP-1) is a xanthine-based derivative. It has soluble GC activation and K(+) -channel opening activity. Effects of this compound on ion currents in pituitary GH3 cells were investigated in this study.

EXPERIMENTAL APPROACH

The aim of this study was to evaluate effects of KMUP-1 on the amplitude and gating of voltage-gated Na(+) current (INa ) in pituitary GH3 cells and in HEKT293T cells expressing SCN5A. Both the amplitude of Ca(2+) -activated K(+) current and the activity of large-conductance Ca(2+) -activated K(+) (BKCa ) channels were also studied.

KEY RESULTS

KMUP-1 depressed the transient and late components of INa with different potencies. The IC50 values required for its inhibitory effect on transient and late INa were 22.5 and 1.8 μM respectively. KMUP-1 (3 μM) shifted the steady-state inactivation of INa to a hyperpolarized potential by -10 mV, despite inability to alter the recovery of INa from inactivation. In cell-attached configuration, KMUP-1 applied to bath increased BKCa -channel activity; however, in inside-out patches, this compound applied to the intracellular surface had no effect on it. It prolonged the latency in the generation of action currents elicited by triangular voltage ramps. Additionally, KMUP-1 decreased the peak INa with a concomitant increase of current inactivation in HEKT293T cells expressing SCN5A.

CONCLUSIONS AND IMPLICATIONS

Apart from activating BKCa channels, KMUP-1 preferentially suppresses late INa . The effects of KUMP-1 on ion currents presented here constitute an underlying ionic mechanism of its actions.

Mitochondrial dysfunction causing cardiac sodium channel downregulation in cardiomyopathy

Cardiomyopathy is associated with cardiac Na(+) channel downregulation that may contribute to arrhythmias. Previously, we have shown that elevated intracellular NADH causes a decrease in cardiac Na(+) current (I(Na)) signaled by an increase in mitochondrial reactive oxygen species (ROS). In this study, we tested whether the NADH-mitochondria ROS pathway was involved in the reduction of I(Na) in a nonischemic cardiomyopathic model and correlated the findings with myopathic human hearts. Nonischemic cardiomyopathy was induced in C57BL/6 mice by hypertension after unilateral nephrectomy, deoxycorticosterone acetate (DOCA) pellet implantation, and salt water substitution. Sham operated mice were used as controls. After six weeks, heart tissue and ventricular myocytes isolated from mice were utilized for whole cell patch clamp recording, NADH/NAD(+) level measurements, and mitochondrial ROS monitoring with confocal microscopy. Human explanted hearts were studied using optical mapping. Compared to the sham mice, the arterial blood pressure was higher, the left ventricular volume was significantly enlarged (104.7±3.9 vs. 87.9±6.1 μL, P<0.05), and the ejection fraction was reduced (37.1±1.8% vs. 49.4±3.7%, P<0.05) in DOCA mice. Both the whole cell and cytosolic NADH level were increased (279±70% and 123±2% of sham, respectively, P<0.01), I(Na) was decreased (60±10% of sham, P<0.01), and mitochondrial ROS overproduction was observed (2.9±0.3-fold of sham, P<0.01) in heart tissue and myocytes of myopathic mice vs. sham. Treatment of myocytes with NAD(+) (500 μM), mitoTEMPO (10 μM), chelerythrine (50 μM), or forskolin (5 μM) restored I(Na) back to the level of sham. Injection of NAD(+) (100mg/kg) or mitoTEMPO (0.7 mg/kg) twice (at 24h and 1h before myocyte isolation) to animals also restored I(Na). All treatments simultaneously reduced mitochondrial ROS levels to that of controls. CD38 was found to transduce the extracellular NAD(+) signal. Correlating with the mouse model, failing human hearts showed a reduction in conduction velocity that improved with NAD(+). Nonischemic cardiomyopathy was associated with elevated NADH level, PKC activation, mitochondrial ROS overproduction, and a concomitant decrease in I(Na). Reducing mitochondrial ROS by application of NAD(+), mitoTEMPO, PKC inhibitors, or PKA activators, restored I(Na). NAD(+) improved conduction velocity in human myopathic hearts.

Distinct RGK GTPases differentially use α1- and auxiliary β-binding-dependent mechanisms to inhibit CaV1.2/CaV2.2 channels

Ca_V1/Ca_V2 channels, comprised of pore-forming α₁ and auxiliary (β,α₂δ) subunits, control diverse biological responses in excitable cells. Molecules blocking Ca_V1/Ca_V2 channel currents (I_Ca) profoundly regulate physiology and have many therapeutic applications. Rad/Rem/Rem2/Gem GTPases (RGKs) strongly inhibit Ca_V1/Ca_V2 channels. Understanding how RGKs block I_Ca is critical for insights into their physiological function, and may provide design principles for developing novel Ca_V1/Ca_V2 channel inhibitors. The RGK binding sites within Ca_V1/Ca_V2 channel complexes responsible for I_Ca inhibition are ambiguous, and it is unclear whether there are mechanistic differences among distinct RGKs. All RGKs bind β subunits, but it is unknown if and how this interaction contributes to I_Ca inhibition. We investigated the role of RGK/β interaction in Rem inhibition of recombinant Ca_V1.2 channels, using a mutated β (β_2aTM) selectively lacking RGK binding. Rem blocked β_2aTM-reconstituted channels (74% inhibition) less potently than channels containing wild-type β_2a (96% inhibition), suggesting the prevalence of both β-binding-dependent and independent modes of inhibition. Two mechanistic signatures of Rem inhibition of Ca_V1.2 channels (decreased channel surface density and open probability), but not a third (reduced maximal gating charge), depended on Rem binding to β. We identified a novel Rem binding site in Ca_V1.2 α_1C N-terminus that mediated β-binding-independent inhibition. The Ca_V2.2 α_1B subunit lacks the Rem binding site in the N-terminus and displays a solely β-binding-dependent form of channel inhibition. Finally, we discovered an unexpected functional dichotomy amongst distinct RGKs— while Rem and Rad use both β-binding-dependent and independent mechanisms, Gem and Rem2 use only a β-binding-dependent method to inhibit Ca_V1.2 channels. The results provide new mechanistic perspectives, and reveal unexpected variations in determinants, underlying inhibition of Ca_V1.2/Ca_V2.2 channels by distinct RGK GTPases.

The cardiac sodium channel is post-translationally modified by arginine methylation

The α subunit of the cardiac sodium channel (Na(v)1.5) is an essential protein in the initial depolarization phase of the cardiomyocyte action potential. Post-translational modifications such as phosphorylation are known to regulate Na(v)1.5 function. Here, we used a proteomic approach for the study of the post-translational modifications of Na(v)1.5 using tsA201 cells as a model system. We generated a stable cell line expressing Na(v)1.5, purified the sodium channel, and analyzed Na(v)1.5 by MALDI-TOF and LC-MS/MS. We report the identification of arginine methylation as a novel post-translational modification of Na(v)1.5. R513, R526, and R680, located in the linker between domains I and II in Na(v)1.5, were found in mono- or dimethylated states. The functional relevance of arginine methylation in Na(v)1.5 is underscored by the fact that R526H and R680H are known Na(v)1.5 mutations causing Brugada and long QT type 3 syndromes, respectively. Our work describes for the first time arginine methylation in the voltage-gated ion channel superfamily.

Alternative splicing of the cardiac sodium channel creates multiple variants of mutant T1620K channels

Alternative splicing creates several Na_v1.5 transcripts in the mammalian myocardium and in various other tissues including brain, dorsal root ganglia, breast cancer cells as well as neuronal stem cell lines. In total nine Na_v1.5 splice variants have been discovered. Four of them, namely Na_v1.5a, Na_v1.5c, Na_v1.5d, and Na_v1.5e, generate functional channels in heterologous expression systems. The significance of alternatively spliced transcripts for cardiac excitation, in particular their role in SCN5A channelopathies, is less well understood. In the present study, we systematically investigated electrophysiological properties of mutant T1620K channels in the background of all known functional Na_v1.5 splice variants in HEK293 cells. This mutation has been previously associated with two distinct cardiac excitation disorders: with long QT syndrome type 3 (LQT3) and isolated cardiac conduction disease (CCD). When investigating the effect of the T1620K mutation, we noticed similar channel defects in the background of hNa_v1.5, hNa_v1.5a, and hNa_v1.5c. In contrast, the hNa_v1.5d background produced differential effects: In the mutant channel, some gain-of-function features did not emerge, whereas loss-of-function became more pronounced. In case of hNa_v1.5e, the neonatal variant of hNa_v1.5, both the splice variant itself as well as the corresponding mutant channel showed electrophysiological properties that were distinct from the wild-type and mutant reference channels, hNa_v1.5 and T1620K, respectively. In conclusion, our data show that alternative splicing is a mechanism capable of generating a variety of functionally distinct wild-type and mutant hNa_v1.5 channels. Thus, the cellular splicing machinery is a potential player affecting genotype-phenotype correlations in SCN5A channelopathies.

Regulation of sodium channel activity by phosphorylation

Voltage-gated sodium channels carry the major inward current responsible for action potential depolarization in excitable cells as well as providing additional inward current that modulates overall excitability. Both their expression and function is under tight control of protein phosphorylation by specific kinases and phosphatases and this control is particular to each type of sodium channel. This article examines the impact and mechanism of phosphorylation for isoforms where it has been studied in detail in an attempt to delineate common features as well as differences.

Autonomic modulation and antiarrhythmic therapy in a model of long QT syndrome type 3

Aims

Clinical observations in patients with long QT syndrome carrying sodium channel mutations (LQT3) suggest that bradycardia caused by parasympathetic stimulation may provoke torsades de pointes (TdP). β-Adrenoceptor blockers appear less effective in LQT3 than in other forms of the disease.

Methods and results

We studied effects of autonomic modulation on arrhythmias in vivo and in vitro and quantified sympathetic innervation by autoradiography in heterozygous mice with a knock-in deletion (ΔKPQ) in the Scn5a gene coding for the cardiac sodium channel and increased late sodium current (LQT3 mice). Cholinergic stimulation by carbachol provoked bigemini and TdP in freely roaming LQT3 mice. No arrhythmias were provoked by physical stress, mental stress, isoproterenol, or atropine. In isolated, beating hearts, carbachol did not prolong action potentials per se, but caused bradycardia and rate-dependent action potential prolongation. The muscarinic inhibitor AFDX116 prevented effects of carbachol on heart rate and arrhythmias. β-Adrenoceptor stimulation suppressed arrhythmias, shortened rate-corrected action potential duration, increased rate, and minimized difference in late sodium current between genotypes. β-Adrenoceptor density was reduced in LQT3 hearts. Acute β-adrenoceptor blockade by esmolol, propranolol or chronic propranolol in vivo did not suppress arrhythmias. Chronic flecainide pre-treatment prevented arrhythmias (all P < 0.05).

Conclusion

Cholinergic stimulation provokes arrhythmias in this model of LQT3 by triggering bradycardia. β-Adrenoceptor density is reduced, and β-adrenoceptor blockade does not prevent arrhythmias. Sodium channel blockade and β-adrenoceptor stimulation suppress arrhythmias by shortening repolarization and minimizing difference in late sodium current.

Differential modulation of late sodium current by protein kinase A in R1623Q mutant of LQT3

AIMS

In the type 3 long QT syndrome (LQT3), shortening of the QT interval by overdrive pacing is used to prevent life-threatening arrhythmias. However, it is unclear whether accelerated heart rate induced by beta-adrenergic agents produces similar effects on the late sodium current (I(Na)) to those by overdrive pacing therapy. We analyzed the beta-adrenergic-like effects of protein kinase A and fluoride on I(Na) in R1623Q mutant channels.

MAIN METHODS

cDNA encoding either wild-type (WT) or R1623Q mutant of hNa(v)1.5 was stably transfected into HEK293 cells. I(Na) was recorded using a whole-cell patch-clamp technique at 23 degrees C.

KEY FINDINGS

In R1623Q channels, 2 mM pCPT-AMP and 120 mM fluoride significantly delayed macroscopic current decay and increased relative amplitude of the late I(Na) in a time-dependent manner. Modulations of peak I(Na) gating kinetics (activation, inactivation, recovery from inactivation) by fluoride were similar in WT and R1623Q channels. The effects of fluoride were almost completely abolished by concomitant dialysis with a protein kinase inhibitor. We also compared the effect of pacing with that of beta-adrenergic stimulation by analyzing the frequency-dependence of the late I(Na). Fluoride augmented frequency-dependent reduction of the late I(Na), which was due to preferential delay of recovery of late I(Na). However, the increase in late I(Na) by fluoride at steady-state was more potent than the frequency-dependent reduction of late I(Na).

SIGNIFICANCE

Different basic mechanisms participate in the QT interval shortening by pacing and beta-adrenergic stimulation in the LQT3.

SCN5A channelopathies--an update on mutations and mechanisms

Voltage-gated Na+ channels mediate the rapid upstroke of the action potential in excitable tissues. Na(v)1.5, encoded by the SCN5A gene, is the predominant isoform in the heart. Mutations in SCN5A are associated with distinct cardiac excitation disorders often resulting in life-threatening arrhythmias. This review outlines the currently known SCN5A mutations linked to three distinct cardiac rhythm disorders: long QT syndrome subtype 3 (LQT3), Brugada syndrome (BS), and cardiac conduction disease (CCD). Electrophysiological properties of the mutant channels are summarized and discussed in terms of Na+ channel structure-function relationships and regarding molecular mechanisms underlying the respective cardiac dysfunction. Possible reasons for less convincing genotype-phenotype correlations are suggested.

Knockin Animal Models of Inherited Arrhythmogenic Diseases: What Have We Learned From Them?

Mouse models are becoming an increasingly accepted method of studying human diseases. Knockin and knockout techniques have several advantages over traditional transgenic overexpression, and the versatility of the knockin mouse allows the study of both gain of function mutations through targeted mutagenesis, as well as the replacement of one gene by another functional gene. Here, we will review the methods available to generate knockin mice; provide an overview of the techniques used to study electrophysiology in the mice at the cellular, organ, and whole animal level; and highlight knockin mice that have implications for inherited arrhythmias. Specifically, we will focus on models that used knockin mice to clarify gene expression, identify similarities and differences between related genes, and model human arrhythmia syndromes. Our goal is to provide the reader with a general understanding of studies done on knockin mouse models of inherited arrhythmias as well as ideas for future directions.

Molecular basis of ranolazine block of LQT-3 mutant sodium channels: evidence for site of action

1 We studied the effects of ranolazine, an antianginal agent with promise as an antiarrhythmic drug, on wild-type (WT) and long QT syndrome variant 3 (LQT-3) mutant Na(+) channels expressed in human embryonic kidney (HEK) 293 cells and knock-in mouse cardiomyocytes and used site-directed mutagenesis to probe the site of action of the drug. 2 We find preferential ranolazine block of sustained vs peak Na(+) channel current for LQT-3 mutant (DeltaKPQ and Y1795C) channels (IC(50)=15 vs 135 microM) with similar results obtained in HEK 293 cells and knock-in myocytes. 3 Ranolazine block of both peak and sustained Na(+) channel current is significantly reduced by mutation (F1760A) of a single residue previously shown to contribute critically to the binding site for local anesthetic (LA) molecules in the Na(+) channel. 4 Ranolazine significantly decreases action potential duration (APD) at 50 and 90% repolarization by 23+/-5 and 27+/-3%, respectively, in DeltaKPQ mouse ventricular myocytes but has little effect on APD of WT myocytes. 5 Computational modeling of human cardiac myocyte electrical activity that incorporates our voltage-clamp data predicts marked ranolazine-induced APD shortening in cells expressing LQT-3 mutant channels. 6 Our results demonstrate for the first time the utility of ranolazine as a blocker of sustained Na(+) channel activity induced by inherited mutations that cause human disease and further, that these effects are very likely due to interactions of ranolazine with the receptor site for LA molecules in the sodium channel.

Computer simulation of wild-type and mutant human cardiac Na+ current

Long QT syndrome (LQTS) and Brugada syndrome (BrS) are inherited diseases predisposing to ventricular arrhythmias and sudden death. Genetic studies linked LQTS and BrS to mutations in genes encoding for cardiac ion channels. Recently, two novel missense mutations at the same codon in the gene encoding the cardiac Na+ channel (SCN5A) have been identified: Y1795C (causing the LQTS phenotype) and Y1795H (causing the BrS phenotype). Functional studies in HEK293 cells showed that both mutations alter the inactivation of Na+ current and cause a sustained Na+ current upon depolarisation. In this paper, a nine state Markov model was used to simulate the Na+ current in wild-type Na+ cardiac channel and the current alterations observed in Y1795C and Y1795H mutant channels. The model includes three distinct closed states, a conducting open state and five inactivation states (one fast-, two intermediate- and two closed-inactivation). Transition rates between these states were identified on the basis of previously published voltage-clamp experiments. The model was able to reproduce the experimental Na+ current in mutant channels just by altering the assignment of model parameters with respect to wild-type case. Parameter assignment was validated by performing action potential clamp experiments and comparing experimental and simulated I(Na) current. The Markov model was subsequently introduced in the Luo-Rudy model of ventricular myocyte to investigate "in silico" the consequences on the ventricular cell action potential of the two mutations. Coherently with their phenotypes, the Y1795C mutation prolongs the action potential, while the Y1795H mutation causes only negligible changes in action potential morphology.

Effect of ethanol on cardiac single sodium channel gating

Alcohol in modest and higher doses has the potential to induce cardiac arrhythmias. The most famous alcohol-related arrhythmia is the "holiday heart syndrome". Furthermore, there is a clear association between excessive alcohol consumption and the risk of sudden cardiac death. However, the acute effects of ethanol on arrhythmia induction are not well understood. The effect of ethanol on single cardiac sodium channels has not been studied yet. To elucidate the effect of ethanol on human cardiac sodium channels we performed a patch clamp study in HEK-293 cells overexpressing the human cardiac sodium channel. We used HEK-293 cells overexpressing the human cardiac sodium channel (Na(1.5)). Single channel gating was investigated by the cell-attached patch clamp technique. Sodium channel currents were elicited by depolarizing pulses from -120 to -20mV for a duration of 150ms. Single channel availability, open probability and peak average current were assessed baseline and after addition of ethanol in increasing concentrations (0.50 per thousand (10.9mM), 1.00 per thousand (21.7mM), 2.00 per thousand (43.5mM) and 4.00 per thousand (87.0mM)). We found a concentration-dependent reduction of open probability which was statistically significant at 2.00 per thousand ethanol (66.5+/-14% of control). At higher concentrations (4.00 per thousand) also availability decreased to 66.5+/-11.0% of control. This resulted in a significant decrease of peak average current at 2.00 per thousand and at 4.00 per thousand ethanol (61.8+/-7.4 and 53.0+/-8.2% of control). For the first time the present study demonstrates acute inhibitory effects of ethanol on single cardiac sodium channel gating and provides one potential mechanism for the well known clinical observation that ethanol triggers supraventricular and ventricular arrhythmias.

Action potential duration, rate of stimulation, and intracellular sodium

In the first section of this short review the change of the cardiac action potential (APD) with the rate of stimulation under physiological conditions is described and mechanistically analyzed. A fast phase of adaptation is mainly caused by changes in gating characteristics of ionic currents, and rapid modulation of the Na(+)/Ca(2+) exchanger. The slower phase is largely conditioned by incomplete recovery from inactivation of the late Na(+) current (late I(Na)) and changes in ion concentrations of [K(+)](e), [Na(+)](i), and [Ca(2+)](i), which cause secondary changes in the permeation and the gating of ion channels and flux through transporters. In a second section, an analysis is presented of the rate dependence of APD in pathological conditions and its importance in the genesis of arrhythmias in hypertrophy, heart failure, congenital, and acquired LQT syndromes is summarized. The role of the late I(Na), Na(+), and Ca(2+) overload is emphasized. Special attention is given to the paradoxical transient lengthening of APD in LQT3 syndrome for the sudden increase in rate in this setting. The third section consists of a short commentary on Na(+) and Ca(2+) overload and drugs which block the late I(Na).

Rapid protein kinase C-dependent reduction of rat skeletal muscle voltage-gated sodium channels by ciliary neurotrophic factor

The ciliary neurotrophic factor (CNTF), known to exert long-term myotrophic effects, has not yet been shown to induce a rapid biological response in skeletal muscles. The present in vitro study gives rise to the possibility that CNTF could affect the sodium channel activity implied in the triggering of muscle fibre contraction. Therefore, we investigated the effects of an external CNTF application on macroscopic sodium current (I(Na)) in rat native fast-twitch skeletal muscle (flexor digitorum brevis, FDB) by using a cell-attached patch-clamp technique. The I(Na) peak amplitude measured at a depolarizing pulse from -100 to -10 mV is rapidly reduced in a time- and dose-dependent manner by CNTF (0.01-20 ng ml(-1)). The maximal decrease is 25% after 10 min incubation in 2 ng ml(-1) CNTF. There was no alteration in activation or inactivation kinetics, or in activation curves constructed from current-voltage relationships in the presence of CNTF. In contrast, the relative I(Na) inhibition induced by CNTF is accompanied by a hyperpolarizing shift in the midpoint of the inactivation curves: -6 and -10 mV for the steady-state fast and slow inactivation, respectively. Furthermore, CNTF induces a 5 mV hyperpolarization of the resting membrane potential of the fibres. The effects of CNTF are similar to those of 1-oleoyl-2-acetyl-sn-glycerol (OAG), a protein kinase C (PKC) activator, when no effect is observed in the presence of chelerythrine, a PKC inhibitor. These results suggest that, in skeletal muscle, CNTF can rapidly decrease sodium currents by altering inactivation gating, probably through an intracellular PKC-dependent mechanism that could lead to decreased membrane excitability. The present study contributes to a better understanding of the physiological role of endogenous CNTF.

Regulatory actions of the A-kinase anchoring protein Yotiao on a heart potassium channel downstream of PKA phosphorylation

A-kinase anchoring proteins (AKAPs) are thought to be passive members of protein complexes that coordinate the association of cAMP-dependent protein kinase A (PKA) with cellular substrates to facilitate targeted PKA protein phosphorylation. I(Ks), the slow heart potassium current, is carried by the I(Ks) potassium channel, a substrate for PKA phosphorylation in response to sympathetic nerve stimulation, is a macromolecular complex that includes the KCNQ1 alpha subunit, the KCNE1 regulatory subunit, and the AKAP Yotiao. Disruption of this regulation by mutation in the long QT syndrome is associated with elevated risk of sudden death. Here, we have studied the effects of the AKAP Yotiao on the function of the I(Ks) channel that had been mutated to simulate channel phosphorylation, and we report direct AKAP-mediated alteration of channel function distinct from its role in the coordination of channel phosphorylation by PKA. These data reveal previously undescribed actions of Yotiao that occur subsequent to channel phosphorylation and provide evidence that this adaptor protein also may serve as an effector in regulating this important ion channel.