The cardiac sodium channel: gating function and molecular pharmacology

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.

Ion channels as biomarkers of altered myogenesis in myofiber precursors of Duchenne muscular dystrophy

Myogenesis is essential for skeletal muscle formation, growth, and regeneration and can be altered in Duchenne muscular dystrophy (DMD), an X-linked disorder due to the absence of the cytoskeletal protein dystrophin. Ion channels play a pivotal role in muscle differentiation and interact with the dystrophin complex. To investigate ion channel involvement in myogenesis in dystrophic settings, we performed electrophysiological characterization of two immortalized mouse cell lines, wild-type (WT) H2K-2B4 and the dystrophic (DYS) H2K-SF1, and measured gene expression of differentiation markers and ion channels. Inward and outward currents/density increased as differentiation progressed in both WT and DYS cells. However, day-11 DYS cells showed higher (27%) inward current density with an increased expression ratio of Scn5a/Scn4a and decreased (48%) barium-sensitive outward current compared to WT. Furthermore, day-11 DYS cells showed more positive resting membrane potential (+10 mV) and lower membrane capacitance (50%) compared to WT. DYS cells also had reduced Myog and Myf5 expression at days 6 and 11. Overall, ion channel profile and myogenesis appeared altered in DYS cells. These results are a first step in validating ion channels as potential drug targets to ameliorate muscle degeneration in DMD settings and as differentiation biomarkers in innovative platforms.

Precision Medicine and Cardiac Channelopathies: Human iPSCs Take the Lead

Sudden cardiac death (SCD) is one of the leading causes of death worldwide, usually involving young people. SCD remains a critical public health problem accounting for 185,000-450,000 deaths annually, representing around 7%-18% of all deaths globally. As per evidence, ∼2%-54% of sudden unexpected deaths in people under the age of 35 years fail to show evidence of structural cardiac abnormalities at autopsy, making ion channelopathies the probable causes in such cases. The most generally recognized cardiac ion channelopathies with genetic testing are long QT syndrome (LQTS), Brugada syndrome (BrS), short QT syndrome (SQTS), and catecholaminergic polymorphic ventricular tachycardia (CPVT). The substantial progress in understanding the genetics of ion channelopathies in the last 2 decades has obliged the early diagnosis and prevention of SCD to a certain extent. In this review, we analyze the critical challenges and recent advancements in the identification, risk stratification, and clinical management of potentially fatal cardiac ion channel disorders. We also emphasize the application of precision medicine (PM) and artificial intelligence (AI) for comprehending the underlying genetic mechanisms, especially the role of human induced pluripotent stem cell (iPSC) based platforms to unravel the primary refractory clinical problems associated with channelopathies.

SCN5A channelopathy: arrhythmia, cardiomyopathy, epilepsy and beyond

Influx of sodium ions through voltage-gated sodium channels in cardiomyocytes is essential for proper electrical conduction within the heart. Both acquired conditions associated with sodium channel dysfunction (myocardial ischaemia, heart failure) as well as inherited disorders secondary to mutations in the gene SCN5A encoding for the cardiac sodium channel Nav1.5 are associated with life-threatening arrhythmias. Research in the last decade has uncovered the complex nature of Nav1.5 distribution, function, in particular within distinct subcellular subdomains of cardiomyocytes. Nav1.5-based channels furthermore display previously unrecognized non-electrogenic actions and may impact on cardiac structural integrity, leading to cardiomyopathy. Moreover, SCN5A and Nav1.5 are expressed in cell types other than cardiomyocytes as well as various extracardiac tissues, where their functional role in, e.g. epilepsy, gastrointestinal motility, cancer and the innate immune response is increasingly investigated and recognized. This review provides an overview of these novel insights and how they deepen our mechanistic knowledge on SCN5A channelopathies and Nav1.5 (dys)function. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Subcellular diversity of Nav1.5 in cardiomyocytes: distinct functions, mechanisms and targets

In cardiomyocytes, the rapid depolarisation of the membrane potential is mediated by the α-subunit of the cardiac voltage-gated Na⁺ channel (Na_V 1.5), encoded by the gene SCN5A. This ion channel allows positively charged Na⁺ ions to enter the cardiomyocyte, resulting in the fast upstroke of the action potential and is therefore crucial for cardiac excitability and electrical propagation. This essential role is underscored by the fact that dysfunctional Na_V 1.5 is associated with high risk for arrhythmias and sudden cardiac death. However, development of therapeutic interventions regulating Na_V 1.5 has been limited due to the complexity of Na_V 1.5 structure and function and its diverse roles within the cardiomyocyte. In particular, research from the last decade has provided us with increased knowledge on the subcellular distribution of Na_V 1.5 as well as the proteins which it interacts with in distinct cardiomyocyte microdomains. We here review these insights, detailing the potential role of Na_V 1.5 within subcellular domains as well as its dysfunction in the setting of arrhythmia disorders. We furthermore provide an overview of current knowledge on the pathways involved in (microdomain-specific) trafficking of Na_V 1.5, and their potential as novel targets. Unravelling the complexity of Na_V 1.5 (dys)function may ultimately facilitate the development of therapeutic strategies aimed at preventing lethal arrhythmias. This is not only of importance for pathophysiological conditions where sodium current is specifically decreased within certain subcellular regions, such as in arrhythmogenic cardiomyopathy and Duchenne muscular dystrophy, but also for other acquired and inherited disorders associated with Na_V 1.5.

Increased atrial effectiveness of flecainide conferred by altered biophysical properties of sodium channels

Atrial fibrillation (AF) affects over 1% of the population and is a leading cause of stroke and heart failure in the elderly. A feared side effect of sodium channel blocker therapy, ventricular pro-arrhythmia, appears to be relatively rare in patients with AF. The biophysical reasons for this relative safety of sodium blockers are not known. Our data demonstrates intrinsic differences between atrial and ventricular cardiac voltage-gated sodium currents (INa), leading to reduced maximum upstroke velocity of action potential and slower conduction, in left atria compared to ventricle. Reduced atrial INa is only detected at physiological membrane potentials and is driven by alterations in sodium channel biophysical properties and not by NaV1.5 protein expression. Flecainide displayed greater inhibition of atrial INa, greater reduction of maximum upstroke velocity of action potential, and slowed conduction in atrial cells and tissue. Our work highlights differences in biophysical properties of sodium channels in left atria and ventricles and their response to flecainide. These differences can explain the relative safety of sodium channel blocker therapy in patients with atrial fibrillation.

Genomic and Non-Genomic Regulatory Mechanisms of the Cardiac Sodium Channel in Cardiac Arrhythmias

Nav1.5 is the predominant cardiac sodium channel subtype, encoded by the SCN5A gene, which is involved in the initiation and conduction of action potentials throughout the heart. Along its biosynthesis process, Nav1.5 undergoes strict genomic and non-genomic regulatory and quality control steps that allow only newly synthesized channels to reach their final membrane destination and carry out their electrophysiological role. These regulatory pathways are ensured by distinct interacting proteins that accompany the nascent Nav1.5 protein along with different subcellular organelles. Defects on a large number of these pathways have a tremendous impact on Nav1.5 functionality and are thus intimately linked to cardiac arrhythmias. In the present review, we provide current state-of-the-art information on the molecular events that regulate SCN5A/Nav1.5 and the cardiac channelopathies associated with defects in these pathways.

Dynamic Clamp in Electrophysiological Studies on Stem Cell-Derived Cardiomyocytes-Why and How?

ABSTRACT

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) are supposed to be a good human-based model, with virtually unlimited cell source, for studies on mechanisms underlying cardiac development and cardiac diseases, and for identification of drug targets. However, a major drawback of hPSC-CMs as a model system, especially for electrophysiological studies, is their depolarized state and associated spontaneous electrical activity. Various approaches are used to overcome this drawback, including the injection of "synthetic" inward rectifier potassium current (IK1), which is computed in real time, based on the recorded membrane potential ("dynamic clamp"). Such injection of an IK1-like current results in quiescent hPSC-CMs with a nondepolarized resting potential that show "adult-like" action potentials on stimulation, with functional availability of the most important ion channels involved in cardiac electrophysiology. These days, dynamic clamp has become a widely appreciated electrophysiological tool. However, setting up a dynamic clamp system can still be laborious and difficult, both because of the required hardware and the implementation of the dedicated software. In the present review, we first summarize the potential mechanisms underlying the depolarized state of hPSC-CMs and the functional consequences of this depolarized state. Next, we explain how an existing manual patch clamp setup can be extended with dynamic clamp. Finally, we shortly validate the extended setup with atrial-like and ventricular-like hPSC-CMs. We feel that dynamic clamp is a highly valuable tool in the field of cellular electrophysiological studies on hPSC-CMs and hope that our directions for setting up such dynamic clamp system may prove helpful.

A Molecularly Detailed NaV1.5 Model Reveals a New Class I Antiarrhythmic Target

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Antiarrhythmic therapies remain suboptimal due to our inability to predict how drug interactions with ion channels will affect the ability of the tissue to initiate and sustain an arrhythmia.

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We built a computational framework that allows for in silico design of precision-targeted therapeutic agents that simultaneously assesses antiarrhythmic markers of success and failure at multiple spatial and time scales. Using this framework, a novel in silico mexiletine “booster” was designed that may dramatically improve the efficacy of mexiletine in suppression of arrhythmia triggers.

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These results provide a roadmap for the design of novel molecular-based therapy to treat myriad arrhythmia syndromes, including ventricular tachycardia, heart failure arrhythmias, and inherited arrhythmia syndromes.

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In summary, computational modeling approaches to drug discovery represent a novel tool to design and test precision-targeted therapeutic agents. By exploiting nontraditional ion channel drug targets, an entirely new dimension can be added to the wide parameter space of traditional antiarrhythmic drugs to develop more precision-targeted and potent Class I therapeutic agents.

Antiarrhythmic treatment strategies remain suboptimal due to our inability to predict how drug interactions with ion channels will affect the ability of the tissues to initiate and sustain an arrhythmia. We built a multiscale molecular model of the Na⁺ channel domain III (domain III voltage-sensing domain) to highlight the molecular underpinnings responsible for mexiletine drug efficacy. This model predicts that a hyperpolarizing shift in the domain III voltage-sensing domain is critical for drug efficacy and may be leveraged to design more potent Class I molecules. The model was therefore used to design, in silico, a theoretical mexiletine booster that can dramatically rescue a mutant resistant to the potent antiarrhythmic effects of mexiletine. Our framework provides a strategy for in silico design of precision-targeted therapeutic agents that simultaneously assesses antiarrhythmic markers of success and failure at multiple spatial and time scales. This approach provides a roadmap for the design of novel molecular-based therapy to treat myriad arrhythmia syndromes, including ventricular tachycardia, heart failure arrhythmias, and inherited arrhythmia syndromes.

Simultaneous assessment of compound activity on cardiac Nav1.5 peak and late currents in an automated patch clamp platform

INTRODUCTION

High throughput in vitro profiling of the cardiac Nav1.5 peak sodium current (I_Na) is widely used in cardiac safety screening. However, there is no standardized high throughput method to measure late I_Na. This study assessed the pharmacological and biophysical properties of veratridine and ATX-II, as well as the channel mutation (Nav1.5-∆KPQ) on the late I_Na. We describe a method for simultaneous measurement of both peak and late I_Na.

METHODS

The planar patch clamp technique (QPatch) was applied to record the peak and late I_Na.

RESULTS

The Nav1.5-∆KPQ mutant produced a small late I_Na (41.9 ± 5.4 pA) not large enough to enable compound profiling. In contrast in wild type Nav1.5 expressing cells veratridine (100 μM) and ATX-II (100 nM) enhanced concentration-dependent increases in the late I_Na (maximum responses of 1162.2 ± 258.5 pA and 392.4 ± 71.3 pA, respectively). Veratridine inhibited, whereas, ATX-II had a minimal effect, on the peak I_Na and preserved the current-voltage curve. Peak and late I_Na inhibition was characterized for 25 clinical I_Na blockers in the presence of ATX-II. Compound IC₅₀ values for peak I_Na generated in the absence or presence of ATX-II correlated. The potency of the late I_Na block was found to be dependent on whether it was measured at the end of the depolarizing pulse or during the ramp.

DISCUSSION

In the presence of ATX-II, both peak and late I_Na could be assessed simultaneously. Late I_Na may be best assessed using the maximum response obtained during the ramp of the voltage protocol.

Subtype Specificity of β-Toxin Tf1a from Tityus fasciolatus in Voltage Gated Sodium Channels

Scorpion venoms are a complex mixture of components. Among them the most important are peptides, which presents the capacity to interact and modulate several ion channel subtypes, including voltage-gated sodium channels (Na_V). Screening the activity of scorpion toxins on different subtypes of Na_V reveals the scope of modulatory activity and, in most cases, low channel selectivity. Until now there are approximately 60 scorpion toxins experimentally assayed on Na_V channels. However, the molecular bases of interaction between scorpion toxins and Na_V channels are not fully elucidated. The activity description of new scorpion toxins is crucial to enhance the predictive strength of the structural–function correlations of these Na_V modulatory molecules. In the present work a new scorpion toxin (Tf1a) was purified from Tityus fasciolatus venom by RP-HPLC, and characterized using electrophysiological experiments on different types of voltage-gated sodium channels. Tf1a was able to modify the normal function of Na_V tested, showing to be a typical β-NaScTx. Tf1a also demonstrated an unusual capability to alter the kinetics of Na_V1.5.

Electrocardiogram changes and atrial arrhythmias in individuals carrying sodium channel SCN5A D1275N mutation

INTRODUCTION

The cardiac sodium channel SCN5A regulates atrioventricular and ventricular depolarization as well as cardiac conduction. Patients with cardiac electrical abnormalities have an increased risk of sudden cardiac death (SCD) and cardio-embolic stroke. Optimal management of cardiac disease includes the understanding of association between the causative mutations and the clinical phenotype. A 12-lead electrocardiogram (ECG) is an easy and inexpensive tool for finding risk patients.

MATERIALS AND METHODS

A blood sample for DNA extraction was obtained in a Finnish family with 43 members; systematic 12-lead ECG analysis was performed in 13 of the family members carrying an SCN5A D1275N mutation. Conduction defects and supraventricular arrhythmias, including atrial fibrillation/flutter, atrioventricular nodal re-entry tachycardia (AVNRT) and junctional rhythm were searched for.

RESULTS

Five (38%) mutation carriers had fascicular or bundle branch block, 10 had atrial arrhythmias; no ventricular arrhythmias were found. Notching of the R- and S waves - including initial QRS fragmentation - and prolonged S-wave upstroke were present in all the affected family members. Notably, four (31%) affected family members had a stroke before the age of 31 and two experienced premature death.

CONCLUSIONS

A 12-lead ECG can be used to predict arrhythmias in SCN5A D1275N mutation carriers. Key messages The 12-lead ECG may reveal cardiac abnormalities even before clinical symptoms occur. Specific ECG findings - initial QRS fragmentation, prolonged S-wave upstroke as well as supraventricular arrhythmias - were frequently encountered in all SCN5A D1257N mutation carriers. ECG follow-up is recommended for all SCN5A D1275N mutation carriers.

Prolonged Right Ventricular Ejection Delay in Brugada Syndrome Depends on the Type of SCN5A Variant - Electromechanical Coupling Through Tissue Velocity Imaging as a Bridge Between Genotyping and Phenotyping

BACKGROUND

Patients with Brugada syndrome (BrS) and a history of syncope or sustained ventricular arrhythmia have longer right ventricular ejection delays (RVEDs) than asymptomatic BrS patients. Different types ofSCN5Avariants leading to different reductions in sodium current (I_Na) may have different effects on conduction delay, and consequently on electromechanical coupling (i.e., RVED). Thus, we investigated the genotype-phenotype relationship by measuring RVED to establish whether BrS patients carrying more severeSCN5Avariants leading to premature protein truncation (T) and presumably 100%I_Nareduction have a longer RVED than patients carrying missense variants (M) with different degrees ofI_Nareduction.Methods and Results:There were 34 BrS patients (mean [±SD] age 43.3±12.9 years; 52.9% male) carrying anSCN5Avariant and 66 non-carriers in this cross-sectional study. Patients carrying aSCN5Avariant were divided into T-carriers (n=13) and M-carriers (n=21). Using tissue velocity imaging, RVED and left ventricular ejection delay (LVED) were measured as the time from QRS onset to the onset of the systolic ejection wave at the end of the isovolumetric contraction. T-carriers had longer RVEDs than M-carriers (139.3±15.1 vs. 124.8±11.9 ms, respectively; P=0.008) and non-carriers (127.7±17.3 ms, P=0.027). There were no differences in LVED among groups.

CONCLUSIONS

Using the simple, non-invasive echocardiographic parameter RVED revealed a more pronounced 'electromechanical' delay in BrS patients carrying T variants ofSCN5A.

In-Silico Modeling of the Functional Role of Reduced Sialylation in Sodium and Potassium Channel Gating of Mouse Ventricular Myocytes

Cardiac ion channels are highly glycosylated membrane proteins with up to 30% of the protein's mass containing glycans. Heart diseases often accompany individuals with congenital disorders of glycosylation (CDG). However, cardiac dysfunction among CDG patients is not yet fully understood. There is an urgent need to study how aberrant glycosylation impacts cardiac electrical signaling. Our previous works reported that congenitally reduced sialylation achieved through deletion of the sialyltransferase gene, ST3Gal4, leads to altered gating of voltage-gated Na⁺ and K⁺ channels ( and , respectively). However, linking the impact of reduced sialylation on ion channel gating to the action potential (AP) is difficult without performing computer experiments. Also, decomposing the sum of K⁺ currents is difficult because of complex structures and components of channels (e.g., , and ). In this study, we developed in-silico models to describe the functional role of reduced sialylation in both and gating and the AP using in vitro experimental data. Modeling results showed that reduced sialylation changes gating as follows: 1) The steady-state activation voltages of isoforms are shifted to a more depolarized potential. 2) Aberrant K⁺ currents ( and ) contribute to a prolonged AP duration, and altered Na⁺ current ( ) contributes to a shortened AP refractory period. This study contributes to a better understanding of the functional role of reduced sialylation in cardiac dysfunction that shows strong potential to provide new pharmaceutical targets for the treatment of CDG-related heart diseases.

Assessment of the Risk Factors and Outcomes for Postoperative Atrial Fibrillation Patients Undergoing Isolated Coronary Artery Bypass Grafting

BACKGROUND

Atrial fibrillation is the most common complication of cardiac surgery and is associated with significant morbidity and mortality. Recognizing patients at high risk for developing postoperative atrial fibrillation (POAF) may help identify those who could benefit from strategies to prevent POAF. This study was conducted to delineate outcomes and to assess risk factors for POAF among Taiwanese patients undergoing coronary artery bypass grafting (CABG).

METHODS

From January 2009 until February 2012, this prospective study included 266 consecutive patients admitted to our hospital with coronary artery disease. All patients underwent isolated CABG. Patients with preoperative permanent atrial fibrillation and concomitant surgery were excluded. Multiple risk factors associated with the incidence of POAF were collected and evaluated.

RESULTS

POAF occurred in 126 of 226 patients (47.37%). Univariate analysis revealed that significant risk factors for the condition were age, gender, diabetes, dyslipidemia, smoking, impaired renal function, impaired cardiac function, and increased serum electrolytes. Multivariate analysis showed dyslipidemia [hazard ratio (HR): 0.418; 95% confidence interval (Cl): 0.190-0.915, p = 0.029], impaired renal function as indicated by an estimated glomerular filtration rate < 60 mL/min/1.73 m(2) (HR: 3.174; 95% CI: 1.432-7.037, p = 0.004), and serum sodium (HR: 1.112; 95% Cl: 1.047-1.182, p = 0.001) prior to cardiopulmonary bypass as significant. Moreover, POAF was associated with lower 30-day, 1- and 3-year cumulative survival rates and higher early postoperative complications.

CONCLUSIONS

Patients with isolated CABG who were administered β-blockers, angiotensin converting enzyme inhibitor/angiotensin receptor blockers treatment, and lipid therapy before CABG were associated with reduced POAF, while those with impaired renal function and higher serum sodium before CABG predisposed POAF in a Taiwanese population.

KEY WORDS

Atrial fibrillation (AF); Coronary artery bypass grafting (CABG); Coronary artery disease (CAD); Postoperative atrial fibrillation (POAF).

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.

Parameterization for In-Silico Modeling of Ion Channel Interactions with Drugs

Since the first Hodgkin and Huxley ion channel model was described in the 1950s, there has been an explosion in mathematical models to describe ion channel function. As experimental data has become richer, models have concomitantly been improved to better represent ion channel kinetic processes, although these improvements have generally resulted in more model complexity and an increase in the number of parameters necessary to populate the models. Models have also been developed to explicitly model drug interactions with ion channels. Recent models of drug-channel interactions account for the discrete kinetics of drug interaction with distinct ion channel state conformations, as it has become clear that such interactions underlie complex emergent kinetics such as use-dependent block. Here, we describe an approach for developing a model for ion channel drug interactions. The method describes the process of extracting rate constants from experimental electrophysiological function data to use as initial conditions for the model parameters. We then describe implementation of a parameter optimization method to refine the model rate constants describing ion channel drug kinetics. The algorithm takes advantage of readily available parallel computing tools to speed up the optimization. Finally, we describe some potential applications of the platform including the potential for gaining fundamental mechanistic insights into ion channel function and applications to in silico drug screening and development.

Human iPS cell model of type 3 long QT syndrome recapitulates drug-based phenotype correction

Long QT syndrome is a potentially life-threatening disease characterized by delayed repolarization of cardiomyocytes, QT interval prolongation in the electrocardiogram, and a high risk for sudden cardiac death caused by ventricular arrhythmia. The genetic type 3 of this syndrome (LQT3) is caused by gain-of-function mutations in the SCN5A cardiac sodium channel gene which mediates the fast Na_v1.5 current during action potential initiation. Here, we report the analysis of LQT3 human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). These were generated from a patient with a heterozygous p.R1644H mutation in SCN5A known to interfere with fast channel inactivation. LQT3 hiPSC-CMs recapitulated pathognomonic electrophysiological features of the disease, such as an accelerated recovery from inactivation of sodium currents as well as action potential prolongation, especially at low stimulation rates. In addition, unlike previously described LQT3 hiPSC models, we observed a high incidence of early after depolarizations (EADs) which is a trigger mechanism for arrhythmia in LQT3. Administration of specific sodium channel inhibitors was found to shorten action and field potential durations specifically in LQT3 hiPSC-CMs and antagonized EADs in a dose-dependent manner. These findings were in full agreement with the pharmacological response profile of the underlying patient and of other patients from the same family. Thus, our data demonstrate the utility of patient-specific LQT3 hiPSCs for assessing pharmacological responses to putative drugs and for improving treatment efficacies.

SCN5A(K817E), a novel Brugada syndrome-associated mutation that alters the activation gating of NaV1.5 channel

BACKGROUND

Brugada syndrome (BrS) is an inherited lethal arrhythmic disorder characterized by syncope and sudden cardiac death from ventricular tachyarrhythmias. Here we identified a novel K817E mutation of SCN5A gene in a man with type 1 BrS electrocardiogram pattern using next-generation sequencing targeted for 73 cardiac disorder-related genes. SCN5A encodes the α-subunit of NaV1.5 voltage-gated Na(+) channel, and some of its mutations are linked to BrS. The proband had no mutation in any of the other arrhythmia-related genes sequenced.

OBJECTIVE

We investigated whether the K817E mutation causes a functional change of NaV1.5 channel responsible for the BrS phenotype.

METHODS

We compared the electrophysiological properties of the whole-cell currents mediated by wild-type and mutant channels heterologously expressed in human embryonic kidney 293 cells by using a voltage-clamp technique.

RESULTS

The K817E mutation reduced the Na(+) current density by 39.0%-91.4% at membrane potentials from -55 to -5 mV. This reduction resulted from a ~24-mV positive shift in the voltage dependence of activation. The mutation also decelerated recovery from both fast and intermediate inactivation, whereas it had little effect on the cell surface expression, single-channel conductance, voltage-dependence of fast inactivation, entry into intermediate inactivation, use-dependent loss of channel availability, or closed-state inactivation.

CONCLUSION

The K817E mutation of SCN5A gene leads to loss of function of NaV1.5 channel and may underlie the BrS phenotype of the proband.

The cardiac sodium channel gene SCN5A and its gene product NaV1.5: Role in physiology and pathophysiology

The gene SCN5A encodes the main cardiac sodium channel NaV1.5. This channel predominates the cardiac sodium current, INa, which underlies the fast upstroke of the cardiac action potential. As such, it plays a crucial role in cardiac electrophysiology. Over the last 60years a tremendous amount of knowledge regarding its function at the electrophysiological and molecular level has been acquired. Furthermore, genetic studies have shown that mutations in SCN5A are associated with multiple cardiac diseases (e.g. Brugada syndrome, Long QT syndrome, conduction disease and cardiomyopathy), while genetic variation in the general population has been associated with differences in cardiac conduction and risk of arrhythmia through genome wide association studies. In this review we aim to give an overview of the current knowledge (and the gaps therein) on SCN5A and NaV1.5.

Statistical Metamodeling and Sequential Design of Computer Experiments to Model Glyco-Altered Gating of Sodium Channels in Cardiac Myocytes

Glycan structures account for up to 35% of the mass of cardiac sodium ( Nav ) channels. To question whether and how reduced sialylation affects Nav activity and cardiac electrical signaling, we conducted a series of in vitro experiments on ventricular apex myocytes under two different glycosylation conditions, reduced protein sialylation (ST3Gal4(-/-)) and full glycosylation (control). Although aberrant electrical signaling is observed in reduced sialylation, realizing a better understanding of mechanistic details of pathological variations in INa and AP is difficult without performing in silico studies. However, computer model of Nav channels and cardiac myocytes involves greater levels of complexity, e.g., high-dimensional parameter space, nonlinear and nonconvex equations. Traditional linear and nonlinear optimization methods have encountered many difficulties for model calibration. This paper presents a new statistical metamodeling approach for efficient computer experiments and optimization of Nav models. First, we utilize a fractional factorial design to identify control variables from the large set of model parameters, thereby reducing the dimensionality of parametric space. Further, we develop the Gaussian process model as a surrogate of expensive and time-consuming computer models and then identify the next best design point that yields the maximal probability of improvement. This process iterates until convergence, and the performance is evaluated and validated with real-world experimental data. Experimental results show the proposed algorithm achieves superior performance in modeling the kinetics of Nav channels under a variety of glycosylation conditions. As a result, in silico models provide a better understanding of glyco-altered mechanistic details in state transitions and distributions of Nav channels. Notably, ST3Gal4(-/-) myocytes are shown to have higher probabilities accumulated in intermediate inactivation during the repolarization and yield a shorter refractory period than WTs. The proposed statistical design of computer experiments is generally extensible to many other disciplines that involve large-scale and computationally expensive models.

Characterization of maturation of neuronal voltage-gated sodium channels SCN1A and SCN8A in rat myocardium

Background

Sodium channels predominantly expressed in brain are expressed in myocardial tissue and play an important role in cardiac physiology. Alterations of sodium channels are known to result in neurological disease in infancy and childhood. It will be of interest to study the expression of brain-type sodium channels in the developing myocardium.

Methods

The expression of neuronal sodium channels (SCN1A, SCN8A) and the cardiac isoform SCN5A in the developing rat myocardium was studied by rtPCR, Western blot, and immunohistochemistry at different stages of antenatal and postnatal development.

Results

Significant changes of sodium channel expression during development were detected. Whereas SCN5A RNA increased to maximum levels on day 21 after birth, the highest SCN1A RNA levels were detected on day 1 to 7 after birth. SCN8A RNA was maximally expressed during embryonic development. At the protein level, the amount of SCN5A protein increased along with the RNA level. SCN1A protein level decreased after birth in contrast to RNA expression. Western blot could not detect SCN8A protein in the myocardium at any stage of development. Immunohistochemistry however proved the presence of SCN8A protein in the developing rat myocardium.

Conclusions

Heart- and brain-type sodium channels are differentially expressed during ontogenesis. The high expression level of SCN1A in the perinatal period and early infancy indicates its importance in preserving a regular cardiac rhythm in this early phase of life. Altered regulation of sodium channels might result in severe cardiac rhythm disturbances.

Cardiac sodium channel mutations: why so many phenotypes?

Mutations of the cardiac sodium channel (Nav1.5) can induce gain or loss of channel function. Gain-of-function mutations can cause long QT syndrome type 3 and possibly atrial fibrillation, whereas loss-of-function mutations are associated with a variety of phenotypes, such as Brugada syndrome, cardiac conduction disease, sick sinus syndrome, and possibly dilated cardiomyopathy. The phenotypes produced by Nav1.5 mutations vary according to the direct effect of the mutation on channel biophysics, but also with age, sex, body temperature, and between regions of the heart. This phenotypic variability makes genotype-phenotype correlations difficult. In this Perspectives article, we propose that phenotypic variability not ascribed to mutation-dependent changes in channel function might be the result of additional modifiers of channel behaviour, such as other genetic variation and alterations in transcription, RNA processing, translation, post-translational modifications, and protein degradation. Consideration of these modifiers might help to improve genotype-phenotype correlations and lead to new therapeutic strategies.

SKF-96365 strongly inhibits voltage-gated sodium current in rat ventricular myocytes

SKF-96365 (1-(beta-[3-(4-methoxy-phenyl) propoxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride) is a general TRPC channel antagonist commonly used to characterize the potential functions of TRPC channels in cardiovascular system. Recent reports showed that SKF-96365 induced a reduction in cardiac conduction. The present study investigates whether the reduced cardiac conduction caused by SKF-96365 is related to the blockade of voltage-gated sodium current (I Na) in rat ventricular myocytes using the whole-cell patch voltage-clamp technique. It was found that SKF-96365 inhibited I Na in rat ventricular myocytes in a concentration-dependent manner. The compound (1 μM) negatively shifted the potential of I Na availability by 9.5 mV, increased the closed-state inactivation of I Na, and slowed the recovery of I Na from inactivation. The inhibition of cardiac I Na by SKF-96365 was use-dependent and frequency-dependent, and the IC₅₀ was decreased from 1.36 μM at 0.5 Hz to 1.03, 0.81, 0.61, 0.56 μM at 1, 2, 5, 10 Hz, respectively. However, the selective TRPC3 antagonist Pyr3 decreased cardiac I Na by 8.5% at 10 μM with a weak use and frequency dependence. These results demonstrate that the TRPC channel antagonist SKF-96365 strongly blocks cardiac I Na in use-dependent and frequency-dependent manners. Caution should be taken for interpreting the alteration of cardiac electrical activity when SKF-96365 is used in native cells as a TRPC antagonist.

Reduced Penetrance and Variable Expression of SCN5A Mutations and the Importance of Co-inherited Genetic Variants: Case Report and Review of the Literature

Mutations in the SCN5A gene are responsible for multiple phenotypical presentations including Brugada syndrome, long QT syndrome, progressive familial heart block, sick sinus syndrome, dilated cardiomyopathy, lone atrial fibrillation and multiple overlap syndromes. These different phenotypic expressions of a mutation in a single gene can be explained by variable expression and reduced penetrance. One of the possible explanations of these phenomena is the co-inheritance of genetic variants. We describe a family where the individuals exhibit a compound heterozygosity in the SCN5A gene including a mutation (R1632H) and a new variant (M858L). Individuals with both the mutation and new variant present with a more severe phenotype including spontaneous atrial tachyarrhythmia at young age. We give an overview of the different phenotypes of "SCN5A disease" and discuss the importance of co-inherited genetic variants in the expression of SCN5A disease.

Pharmacogenetics of drug-induced arrhythmias

Abnormal functioning of cardiac ion channels can disrupt cardiac myocyte action potentials and thus cause potentially lethal cardiac arrhythmias. Ion channel dysfunction has been observed at all stages in channel ontogeny, from biogenesis to regulation, and arises from genetic or environmental factors, or both. Acquired arrhythmias - including those that are drug induced - are more common than solely inherited arrhythmias but, in some cases, also contain an identifiable genetic component. This interplay between the pharmacology and genetics - known as 'pharmacogenetics' - of cardiac ion channels and the systems that impact them presents both challenges and opportunities to academics, pharmaceutical companies and clinicians seeking to develop and utilize therapies for cardiac rhythm disorders. In this review, we discuss ion channel pharmacogenetics in the context of both causation and treatment of cardiac arrhythmias, focusing on the long QT syndromes.

Sodium channels, cardiac arrhythmia, and therapeutic strategy

Cardiac sodium channels are transmembrane proteins distributed in atrial and ventricular myocytes and Purkinje fibers. A large and rapid Na(+) influx through these channels initiates action potential and thus excitation-contraction coupling of cardiac cells. Cardiac sodium channel is composed of a pore-forming α-subunit and one or two accessory β-subunits. The cardiac α-subunit is encoded by gene SCN5A located on chromosome 3p21. There are four types of β-subunits identified so far, and β1 is the primary β-subunit in cardiac Na(+) channels. The gene responsible for β1 subunits is SCNB. The expression of β-subunits together with α subunits enhances the Na(+) current and modifies the channel activities. In addition, interactions of the cardiac Na(+) channel with other proteins may facilitate the channel activity and membrane expression of the channel. Over the past two decades, molecular genetic studies have identified the linkage of gene mutations of the Na(+) channel proteins and other regulatory proteins to many inherited arrhythmogenic diseases. The most common cardiac arrhythmogenic diseases associated with Na(+) channelopathies are long QT syndrome (LQT3) and Brugada syndromes (BrSs). This chapter intends to summarize the current understanding of the normal sodium-channel structure and function, the gene mutation-associated cardiac arrhythmias, and the current diagnosis and management of these diseases.

Localization of sodium channel subtypes in mouse ventricular myocytes using quantitative immunocytochemistry

Voltage-gated sodium channels are responsible for the rising phase of the action potential in cardiac muscle. Previously, both TTX-sensitive neuronal sodium channels (NaV1.1, NaV1.2, NaV1.3, NaV1.4 and NaV1.6) and the TTX-resistant cardiac sodium channel (NaV1.5) have been detected in cardiac myocytes, but relative levels of protein expression of the isoforms were not determined. Using a quantitative approach, we analyzed z-series of confocal microscopy images from individual mouse myocytes stained with either anti-NaV1.1, anti-NaV1.2, anti-NaV1.3, anti-NaV1.4, anti-NaV1.5, or anti-NaV1.6 antibodies and calculated the relative intensity of staining for these sodium channel isoforms. Our results indicate that the TTX-sensitive channels represented approximately 23% of the total channels, whereas the TTX-resistant NaV1.5 channel represented 77% of the total channel staining in mouse ventricular myocytes. These ratios are consistent with previous electrophysiological studies in mouse ventricular myocytes. NaV1.5 was located at the cell surface, with high density at the intercalated disc, but was absent from the transverse (t)-tubular system, suggesting that these channels support surface conduction and inter-myocyte transmission. Low-level cell surface staining of NaV1.4 and NaV1.6 channels suggest a minor role in surface excitation and conduction. Conversely, NaV1.1 and NaV1.3 channels are localized to the t-tubules and are likely to support t-tubular transmission of the action potential to the myocyte interior. This quantitative immunocytochemical approach for assessing sodium channel density and localization provides a more precise view of the relative importance and possible roles of these individual sodium channel protein isoforms in mouse ventricular myocytes and may be applicable to other species and cardiac tissue types.

Channelopathies from mutations in the cardiac sodium channel protein complex

The cardiac sodium current underlies excitability in heart, and inherited abnormalities of the proteins regulating and conducting this current cause inherited arrhythmia syndromes. This review focuses on inherited mutations in non-pore forming proteins of sodium channel complexes that cause cardiac arrhythmia, and the deduced mechanisms by which they affect function and dysfunction of the cardiac sodium current. Defining the structure and function of these complexes and how they are regulated will contribute to understanding the possible roles for this complex in normal and abnormal physiology and homeostasis. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Ranolazine for congenital and acquired late INa-linked arrhythmias: in silico pharmacological screening

RATIONALE

The antianginal ranolazine blocks the human ether-a-go-go-related gene-based current IKr at therapeutic concentrations and causes QT interval prolongation. Thus, ranolazine is contraindicated for patients with preexisting long-QT and those with repolarization abnormalities. However, with its preferential targeting of late INa (INaL), patients with disease resulting from increased INaL from inherited defects (eg, long-QT syndrome type 3 or disease-induced electric remodeling (eg, ischemic heart failure) might be exactly the ones to benefit most from the presumed antiarrhythmic properties of ranolazine.

OBJECTIVE

We developed a computational model to predict if therapeutic effects of pharmacological targeting of INaL by ranolazine prevailed over the off-target block of IKr in the setting of inherited long-QT syndrome type 3 and heart failure.

METHODS AND RESULTS

We developed computational models describing the kinetics and the interaction of ranolazine with cardiac Na(+) channels in the setting of normal physiology, long-QT syndrome type 3-linked ΔKPQ mutation, and heart failure. We then simulated clinically relevant concentrations of ranolazine and predicted the combined effects of Na(+) channel and IKr blockade by both the parent compound ranolazine and its active metabolites, which have shown potent blocking effects in the therapeutically relevant range. Our simulations suggest that ranolazine is effective at normalizing arrhythmia triggers in bradycardia-dependent arrhythmias in long-QT syndrome type 3 as well tachyarrhythmogenic triggers arising from heart failure-induced remodeling.

CONCLUSIONS

Our model predictions suggest that acute targeting of INaL with ranolazine may be an effective therapeutic strategy in diverse arrhythmia-provoking situations that arise from a common pathway of increased pathological INaL.

Cardiac sodium channelopathy associated with SCN5A mutations: electrophysiological, molecular and genetic aspects

Over the last two decades, an increasing number of SCN5A mutations have been described in patients with long QT syndrome type 3 (LQT3), Brugada syndrome, (progressive) conduction disease, sick sinus syndrome, atrial standstill, atrial fibrillation, dilated cardiomyopathy, and sudden infant death syndrome (SIDS). Combined genetic, electrophysiological and molecular studies have provided insight into the dysfunction and dysregulation of the cardiac sodium channel in the setting of SCN5A mutations identified in patients with these inherited arrhythmia syndromes. However, risk stratification and patient management is hindered by the reduced penetrance and variable disease expressivity in sodium channelopathies. Furthermore, various SCN5A-related arrhythmia syndromes are known to display mixed phenotypes known as cardiac sodium channel overlap syndromes. Determinants of variable disease expressivity, including genetic background and environmental factors, are suspected but still largely unknown. Moreover, it has become increasingly clear that sodium channel function and regulation is more complicated than previously assumed, and the sodium channel may play additional, as of yet unrecognized, roles in cardiac structure and function. Development of cardiac structural abnormalities secondary to SCN5A mutations has been reported, but the clinical relevance and underlying mechanisms are unclear. Increased insight into these issues would enable a major next step in research related to cardiac sodium channel disease, ultimately enabling improved diagnosis, risk stratification and treatment strategies.

A heterozygous deletion mutation in the cardiac sodium channel gene SCN5A with loss- and gain-of-function characteristics manifests as isolated conduction disease, without signs of Brugada or long QT syndrome

BACKGROUND

The SCN5A gene encodes for the α-subunit of the cardiac sodium channel NaV1.5, which is responsible for the rapid upstroke of the cardiac action potential. Mutations in this gene may lead to multiple life-threatening disorders of cardiac rhythm or are linked to structural cardiac defects. Here, we characterized a large family with a mutation in SCN5A presenting with an atrioventricular conduction disease and absence of Brugada syndrome.

METHOD AND RESULTS

In a large family with a high incidence of sudden cardiac deaths, a heterozygous SCN5A mutation (p.1493delK) with an autosomal dominant inheritance has been identified. Mutation carriers were devoid of any cardiac structural changes. Typical ECG findings were an increased P-wave duration, an AV-block I° and a prolonged QRS duration with an intraventricular conduction delay and no signs for Brugada syndrome. HEK293 cells transfected with 1493delK showed strongly (5-fold) reduced Na(+) currents with altered inactivation kinetics compared to wild-type channels. Immunocytochemical staining demonstrated strongly decreased expression of SCN5A 1493delK in the sarcolemma consistent with an intracellular trafficking defect and thereby a loss-of-function. In addition, SCN5A 1493delK channels that reached cell membrane showed gain-of-function aspects (slowing of the fast inactivation, reduction in the relative fraction of channels that fast inactivate, hastening of the recovery from inactivation).

CONCLUSION

In a large family, congregation of a heterozygous SCN5A gene mutation (p.1493delK) predisposes for conduction slowing without evidence for Brugada syndrome due to a predominantly trafficking defect that reduces Na(+) current and depolarization force.

Na⁺ transport in the normal and failing heart - remember the balance

In the heart, intracellular Na(+) concentration ([Na(+)]i) is a key modulator of Ca(2+) cycling, contractility and cardiac myocyte metabolism. Several Na(+) transporters are electrogenic, thus they both contribute to shaping the cardiac action potential and at the same time are affected by it. [Na(+)]i is controlled by the balance between Na(+) influx through various pathways, including the Na(+)/Ca(2+) exchanger and Na(+) channels, and Na(+) extrusion via the Na(+)/K(+)-ATPase. [Na(+)]i is elevated in HF due to a combination of increased entry through Na(+) channels and/or Na(+)/H(+) exchanger and reduced activity of the Na(+)/K(+)-ATPase. Here we review the major Na(+) transport pathways in cardiac myocytes and how they participate in regulating [Na(+)]i in normal and failing hearts. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes."

Redox control of cardiac excitability

Reactive oxygen species (ROS) have been associated with various human diseases, and considerable attention has been paid to investigate their physiological effects. Various ROS are synthesized in the mitochondria and accumulate in the cytoplasm if the cellular antioxidant defense mechanism fails. The critical balance of this ROS synthesis and antioxidant defense systems is termed the redox system of the cell. Various cardiovascular diseases have also been affected by redox to different degrees. ROS have been indicated as both detrimental and protective, via different cellular pathways, for cardiac myocyte functions, electrophysiology, and pharmacology. Mostly, the ROS functions depend on the type and amount of ROS synthesized. While the literature clearly indicates ROS effects on cardiac contractility, their effects on cardiac excitability are relatively under appreciated. Cardiac excitability depends on the functions of various cardiac sarcolemal or mitochondrial ion channels carrying various depolarizing or repolarizing currents that also maintain cellular ionic homeostasis. ROS alter the functions of these ion channels to various degrees to determine excitability by affecting the cellular resting potential and the morphology of the cardiac action potential. Thus, redox balance regulates cardiac excitability, and under pathological regulation, may alter action potential propagation to cause arrhythmia. Understanding how redox affects cellular excitability may lead to potential prophylaxis or treatment for various arrhythmias. This review will focus on the studies of redox and cardiac excitation.

Detecting drug-induced prolongation of the QRS complex: new insights for cardiac safety assessment

BACKGROUND

Drugs slowing the conduction of the cardiac action potential and prolonging QRS complex duration by blocking the sodium current (I(Na)) may carry pro-arrhythmic risks. Due to the frequency-dependent block of I(Na), this study assesses whether activity-related spontaneous increases in heart rate (HR) occurring during standard dog telemetry studies can be used to optimise the detection of class I antiarrhythmic-induced QRS prolongation.

METHODS

Telemetered dogs were orally dosed with quinidine (class Ia), mexiletine (class Ib) or flecainide (class Ic). QRS duration was determined standardly (5 beats averaged at rest) but also prior to and at the plateau of each acute increase in HR (3 beats averaged at steady state), and averaged over 1h period from 1h pre-dose to 5h post-dose.

RESULTS

Compared to time-matched vehicle, at rest, only quinidine and flecainide induced increases in QRS duration (E(max) 13% and 20% respectively, P<0.01-0.001) whereas mexiletine had no effect. Importantly, the increase in QRS duration was enhanced at peak HR with an additional effect of +0.7 ± 0.5 ms (quinidine, NS), +1.8 ± 0.8 ms (mexiletine, P<0.05) and +2.8 ± 0.8 ms (flecainide, P<0.01) (calculated as QRS at basal HR-QRS at high HR).

CONCLUSION

Electrocardiogram recordings during elevated HR, not considered during routine analysis optimised for detecting QT prolongation, can be used to sensitise the detection of QRS prolongation. This could prove useful when borderline QRS effects are detected. Analysing during acute increases in HR could also be useful for detecting drug-induced effects on other aspects of cardiac function.

The comparative sensitivity of three in vitro safety pharmacology models for the detection of lidocaine-induced cardiac effects

INTRODUCTION

In the current ICH S7B guideline, in vitro evaluation of proarrhythmic liability is limited to the risk of QT interval prolongation, whilst the effect of new chemical entities on cardiac conductivity is often overlooked. The aim of this work was to compare the effects of the sodium channel blocker, lidocaine in three in vitro safety pharmacology models: hNa(v)1.5 channel test, atrial action potential (AP) and Purkinje fiber AP and to identify the most sensitive model for detecting cardiac conduction slowing.

METHODS

Whole-cell patch-clamp methods were used to record the sodium current (I(Na)) encoded by hNa(v)1.5 in stably transfected HEK293 cells at ambient temperature. Transmembrane APs were recorded in rabbit Purkinje fibers and rabbit and guinea-pig left stimulated atria at physiological temperature. Parameters involved in depolarization or repolarization were reported.

RESULTS

Lidocaine (from 10 to 1000 μM) decreased the amplitude of I(Na) (IC(50): 256±37 μM) in a concentration-dependent manner. In the Purkinje fiber assay, lidocaine (10, 30 and 100 μM) had no effects on maximal upstroke velocity (Vmax), but shortened AP duration at 90% repolarization (APD(90)). At 30 and 100 μM, lidocaine also increased AP triangulation. In guinea-pig atria, lidocaine decreased Vmax starting from 30 μM and conduction velocity (CV) at 100 μM, but had no effects on other parameters. In rabbit atria, lidocaine decreased Vmax and CV at 100 μM without affecting APD(90). The effects of 100 μM lidocaine on Vmax and CV were more marked in rabbit than in guinea-pig atria.

CONCLUSION

Rabbit atria are more sensitive than rabbit Purkinje fibers or guinea-pig atria for detecting lidocaine-induced cardiac conduction slowing. These data suggest that isolated rabbit atria in addition to the hNa(v)1.5 channel assay could be relevant models to predict drug-induced conduction slowing.

In silico optimization of atrial fibrillation-selective sodium channel blocker pharmacodynamics

Atrial fibrillation (AF) is the most common type of clinical arrhythmia. Currently available anti-AF drugs are limited by only moderate efficacy and an unfavorable safety profile. Thus, there is a recognized need for improved antiarrhythmic agents with actions that are selective for the fibrillating atrium. State-dependent Na(+)-channel blockade potentially allows for the development of drugs with maximal actions on fibrillating atrial tissue and minimal actions on ventricular tissue at resting heart rates. In this study, we applied a mathematical model of state-dependent Na(+)-channel blocking (class I antiarrhythmic drug) action, along with mathematical models of canine atrial and ventricular cardiomyocyte action potentials, AF, and ventricular proarrhythmia, to determine the relationship between their pharmacodynamic properties and atrial-selectivity, AF-selectivity (atrial Na(+)-channel block at AF rates versus ventricular block at resting rates), AF-termination effectiveness, and ventricular proarrhythmic properties. We found that drugs that target inactivated channels are AF-selective, whereas drugs that target activated channels are not. The most AF-selective drugs were associated with minimal ventricular proarrhythmic potential and terminated AF in 33% of simulations; slightly fewer AF-selective agents achieved termination rates of 100% with low ventricular proarrhythmic potential. Our results define properties associated with AF-selective actions of class-I antiarrhythmic drugs and support the idea that it may be possible to develop class I antiarrhythmic agents with optimized pharmacodynamic properties for AF treatment.

Activation of protein kinase C alters the intracellular distribution and mobility of cardiac Na+ channels

Na(+) current derived from expression of the cardiac isoform SCN5A is reduced by receptor-mediated or direct activation of protein kinase C (PKC). Previous work has suggested a possible role for loss of Na(+) channels at the plasma membrane in this effect, but the results are controversial. In this study, we tested the hypothesis that PKC activation acutely modulates the intracellular distribution of SCN5A channels and that this effect can be visualized in living cells. In human embryonic kidney cells that stably expressed SCN5A with green fluorescent protein (GFP) fused to the channel COOH-terminus (SCN5A-GFP), Na(+) currents were suppressed by an exposure to PKC activation. Using confocal microscopy, colocalization of SCN5A-GFP channels with the plasma membrane under control and stimulated conditions was quantified. A separate population of SCN5A channels containing an extracellular epitope was immunolabeled to permit temporally stable labeling of the plasma membrane. Our results demonstrated that Na(+) channels were preferentially trafficked away from the plasma membrane by PKC activation, with a major contribution by Ca(2+)-sensitive or conventional PKC isoforms, whereas stimulation of protein kinase A (PKA) had the opposite effect. Removal of the conserved PKC site Ser(1503) or exposure to the NADPH oxidase inhibitor apocynin eliminated the PKC-mediated effect to alter channel trafficking, indicating that both channel phosphorylation and ROS were required. Experiments using fluorescence recovery after photobleaching demonstrated that both PKC and PKA also modified channel mobility in a manner consistent with the dynamics of channel distribution. These results demonstrate that the activation of protein kinases can acutely regulate the intracellular distribution and molecular mobility of cardiac Na(+) channels in living cells.

Cardiac Safety Implications of hNav1.5 Blockade and a Framework for Pre-Clinical Evaluation

The human cardiac sodium channel (hNav1.5, encoded by the SCN5A gene) is critical for action potential generation and propagation in the heart. Drug-induced sodium channel inhibition decreases the rate of cardiomyocyte depolarization and consequently conduction velocity and can have serious implications for cardiac safety. Genetic mutations in hNav1.5 have also been linked to a number of cardiac diseases. Therefore, off-target hNav1.5 inhibition may be considered a risk marker for a drug candidate. Given the potential safety implications for patients and the costs of late stage drug development, detection, and mitigation of hNav1.5 liabilities early in drug discovery and development becomes important. In this review, we describe a pre-clinical strategy to identify hNav1.5 liabilities that incorporates in vitro, in vivo, and in silico techniques and the application of this information in the integrated risk assessment at different stages of drug discovery and development.

Pathophysiology of the cardiac late Na current and its potential as a drug target

A pathological increase in the late component of the cardiac Na(+) current, I(NaL), has been linked to disease manifestation in inherited and acquired cardiac diseases including the long QT variant 3 (LQT3) syndrome and heart failure. Disruption in I(NaL) leads to action potential prolongation, disruption of normal cellular repolarization, development of arrhythmia triggers, and propensity to ventricular arrhythmia. Attempts to treat arrhythmogenic sequelae from inherited and acquired syndromes pharmacologically with common Na(+) channel blockers (e.g. flecainide, lidocaine, and amiodarone) have been largely unsuccessful. This is due to drug toxicity and the failure of most current drugs to discriminate between the peak current component, chiefly responsible for single cell excitability and propagation in coupled tissue, and the late component (I(NaL)) of the Na(+) current. Although small in magnitude as compared to the peak Na(+) current (~1-3%), I(NaL) alters action potential properties and increases Na(+) loading in cardiac cells. With the increasing recognition that multiple cardiac pathological conditions share phenotypic manifestations of I(NaL) upregulation, there has been renewed interest in specific pharmacological inhibition of I(Na). The novel antianginal agent ranolazine, which shows a marked selectivity for late versus peak Na(+) current, may represent a novel drug archetype for targeted reduction of I(NaL). This article aims to review common pathophysiological mechanisms leading to enhanced I(NaL) in LQT3 and heart failure as prototypical disease conditions. Also reviewed are promising therapeutic strategies tailored to alter the molecular mechanisms underlying I(Na) mediated arrhythmia triggers.

Toluene effects on oxidative stress in brain regions of young-adult, middle-age, and senescent Brown Norway rats

The influence of aging on susceptibility to environmental contaminants is not well understood. To extend knowledge in this area, we examined effects in rat brain of the volatile organic compound, toluene. The objective was to test whether oxidative stress (OS) plays a role in the adverse effects caused by toluene exposure, and if so, if effects are age-dependent. OS parameters were selected to measure the production of reactive oxygen species (NADPH Quinone oxidoreductase 1 (NQO1), NADH Ubiquinone reductase (UBIQ-RD)), antioxidant homeostasis (total antioxidant substances (TAS), superoxide dismutase (SOD), γ-glutamylcysteine synthetase (γ-GCS), glutathione transferase (GST), glutathione peroxidase (GPX), glutathione reductase (GRD)), and oxidative damage (total aconitase and protein carbonyls). In this study, Brown Norway rats (4, 12, and 24 months) were dosed orally with toluene (0, 0.65 or 1g/kg) in corn oil. Four hours later, frontal cortex, cerebellum, striatum, and hippocampus were dissected, quick frozen on dry ice, and stored at -80°C until analysis. Some parameters of OS were found to increase with age in select brain regions. Toluene exposure also resulted in increased OS in select brain regions. For example, an increase in NQO1 activity was seen in frontal cortex and cerebellum of 4 and 12 month old rats following toluene exposure, but only in the hippocampus of 24 month old rats. Similarly, age and toluene effects on glutathione enzymes were varied and brain-region specific. Markers of oxidative damage reflected changes in oxidative stress. Total aconitase activity was increased by toluene in frontal cortex and cerebellum at 12 and 24 months, respectively. Protein carbonyls in both brain regions and in all age groups were increased by toluene, but step-down analyses indicated toluene effects were statistically significant only in 12month old rats. These results indicate changes in OS parameters with age and toluene exposure resulted in oxidative damage in frontal cortex and cerebellum of 12 month old rats. Although increases in oxidative damage are associated with increases in horizontal motor activity in older rats, further research is warranted to determine if these changes in OS parameters are related to neurobehavioral and neurophysiological effects of toluene in animal models of aging.

Long-term modulation of Na+ and K+ channels by TGF-β1 in neonatal rat cardiac myocytes

Previous work shows that transforming growth factor-β1 (TGF-β1) promotes several heart alterations, including atrial fibrillation (AF). In this work, we hypothesized that these effects might be associated with a potential modulation of Na(+) and K(+) channels. Atrial myocytes were cultured 1-2 days under either control conditions, or the presence of TGF-β1. Subsequently, Na(+) (I(Na)) and K(+) (I(K)) currents were investigated under whole-cell patch-clamp conditions. Three K(+) currents were isolated: inward rectifier (I(Kin)), outward transitory (I(to)), and outward sustained (I(Ksus)). Interestingly, TGF-β1 decreased (50%) the densities of I(Kin) and I(Ksus) but not of I(to). In addition, the growth factor reduced by 80% the amount of I(Na) available at -80 mV. This effect was due to a significant reduction (30%) in the maximum I(Na) recruited at very negative potentials or I(max), as well as to an increased fraction of inactivated Na(+) channels. The latter effect was, in turn, associated to a -7 mV shift in V(1/2) of inactivation. TGF-β1 also reduced by 60% the maximum amount of intramembrane charge movement of Na(+) channels or Q(max), but did not affect the corresponding voltage dependence of activation. This suggests that TGF-β1 promotes loss of Na(+) channels from the plasma membrane. Moreover, TGF-β1 also reduced (50%) the expression of the principal subunit of Na(+) channels, as indicated by western blot analysis. Thus, TGF-β1 inhibits the expression of Na(+) channels, as well as the activity of K(+) channels that give rise to I(Ksus) and I(Kin). These results may contribute to explaining the previously observed proarrhythmic effects of TGF-β1.

Arrhythmia and right heart disease: from genetic basis to clinical practice

Historically, left ventricular cardiomyopathy and coronary heart disease have been regarded as the main causes of ventricular arrhythmia and sudden cardiac death. However, within last two decades, arrhythmias originating from the right ventricle have begun to attract the attention of the scientific world for a number of reasons. Ventricular arrhythmias originating from the right ventricle usually affect younger patients and can lead to sudden cardiac death. The pathophysiologic mechanism of these arrhythmias is not fully understood, which can leave room for a range of different interpretations. Moreover, the intriguing world of genetics is increasingly being drawn into the pathogenesis, diagnosis and prognosis of some of these arrhythmias. This review considers the pathogenesis, diagnosis and treatment of arrhythmogenic right ventricular cardiomyopathy or dysplasia (ARVD), Brugada syndrome, right ventricular outflow tract ventricular tachycardia, and arrhythmias in the right side of the heart due to congenital heart disease. In addition, because ventricular arrhythmias associated with right ventricular heart diseases such as Brugada syndrome and ARVD can explain up to 10-30% of sudden cardiac deaths in young adults in the general population and an even greater percentage in young athletes, this article contains a brief analysis of screening tests used before participation in sports, life-style modification, and treatment options for athletes affected by these conduction disorders.

Cardiac sodium channelopathies

Cardiac sodium channel are protein complexes that are expressed in the sarcolemma of cardiomyocytes to carry a large inward depolarizing current (INa) during phase 0 of the cardiac action potential. The importance of INa for normal cardiac electrical activity is reflected by the high incidence of arrhythmias in cardiac sodium channelopathies, i.e., arrhythmogenic diseases in patients with mutations in SCN5A, the gene responsible for the pore-forming ion-conducting alpha-subunit, or in genes that encode the ancillary beta-subunits or regulatory proteins of the cardiac sodium channel. While clinical and genetic studies have laid the foundation for our understanding of cardiac sodium channelopathies by establishing links between arrhythmogenic diseases and mutations in genes that encode various subunits of the cardiac sodium channel, biophysical studies (particularly in heterologous expression systems and transgenic mouse models) have provided insights into the mechanisms by which INa dysfunction causes disease in such channelopathies. It is now recognized that mutations that increase INa delay cardiac repolarization, prolong action potential duration, and cause long QT syndrome, while mutations that reduce INa decrease cardiac excitability, reduce electrical conduction velocity, and induce Brugada syndrome, progressive cardiac conduction disease, sick sinus syndrome, or combinations thereof. Recently, mutation-induced INa dysfunction was also linked to dilated cardiomyopathy, atrial fibrillation, and sudden infant death syndrome. This review describes the structure and function of the cardiac sodium channel and its various subunits, summarizes major cardiac sodium channelopathies and the current knowledge concerning their genetic background and underlying molecular mechanisms, and discusses recent advances in the discovery of mutation-specific therapies in the management of these channelopathies.