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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.

Rationale: The SCN10A gene encodes the neuronal sodium channel isoform Na(V)1.8. Several recent genome-wide association studies have linked SCN10A to PR interval and QRS duration, strongly suggesting an as-yet unknown role for Na(V)1.8 in cardiac electrophysiology., Objective: To demonstrate the functional presence of SCN10A/Nav1.8 in intracardiac neurons of the mouse heart., Methods and Results: Immunohistochemistry on mouse tissue sections showed intense Na(V)1.8 labeling in dorsal root ganglia and intracardiac ganglia and only modest Na(V)1.8 expression within the myocardium. Immunocytochemistry further revealed substantial Na(V)1.8 staining in isolated neurons from murine intracardiac ganglia but no Na(V)1.8 expression in isolated ventricular myocytes. Patch-clamp studies demonstrated that the Na(V)1.8 blocker A-803467 (0.5-2 μmol/L) had no effect on either mean sodium current (I(Na)) density or I(Na) gating kinetics in isolated myocytes but significantly reduced I(Na) density in intracardiac neurons. Furthermore, A-803467 accelerated the slow component of current decay and shifted voltage dependence of inactivation toward more negative voltages, as expected for blockade of Na(V)1.8-based I(Na). In line with these findings, A-803467 did not affect cardiomyocyte action potential upstroke velocity but markedly reduced action potential firing frequency in intracardiac neurons, confirming a functional role for Na(V)1.8 in cardiac neural activity., Conclusions: Our findings demonstrate the functional presence of SCN10A/Na(V)1.8 in intracardiac neurons, indicating a novel role for this neuronal sodium channel in regulation of cardiac electric activity.

Background: Pluripotent stem cells (PSCs) offer a new paradigm for modeling genetic cardiac diseases, but it is unclear whether mouse and human PSCs can truly model both gain- and loss-of-function genetic disorders affecting the Na(+) current (I(Na)) because of the immaturity of the PSC-derived cardiomyocytes. To address this issue, we generated multiple PSC lines containing a Na(+) channel mutation causing a cardiac Na(+) channel overlap syndrome., Method and Results: Induced PSC (iPSC) lines were generated from mice carrying the Scn5a(1798insD/+) (Scn5a-het) mutation. These mouse iPSCs, along with wild-type mouse iPSCs, were compared with the targeted mouse embryonic stem cell line used to generate the mutant mice and with the wild-type mouse embryonic stem cell line. Patch-clamp experiments showed that the Scn5a-het cardiomyocytes had a significant decrease in I(Na) density and a larger persistent I(Na) compared with Scn5a-wt cardiomyocytes. Action potential measurements showed a reduced upstroke velocity and longer action potential duration in Scn5a-het myocytes. These characteristics recapitulated findings from primary cardiomyocytes isolated directly from adult Scn5a-het mice. Finally, iPSCs were generated from a patient with the equivalent SCN5A(1795insD/+) mutation. Patch-clamp measurements on the derivative cardiomyocytes revealed changes similar to those in the mouse PSC-derived cardiomyocytes., Conclusion: Here, we demonstrate that both embryonic stem cell- and iPSC-derived cardiomyocytes can recapitulate the characteristics of a combined gain- and loss-of-function Na(+) channel mutation and that the electrophysiological immaturity of PSC-derived cardiomyocytes does not preclude their use as an accurate model for cardiac Na(+) channel disease.

Cardiac voltage-gated sodium channels are transmembrane proteins located in the cell membrane of cardiomyocytes. Influx of sodium ions through these ion channels is responsible for the initial fast upstroke of the cardiac action potential. This inward sodium current thus triggers the initiation and propagation of action potentials throughout the myocardium and consequently plays a central role in excitability of myocardial cells and proper conduction of the electrical impulse within the heart. The importance of sodium channels for normal cardiac electrical activity is emphasized by the occurrence of potentially lethal arrhythmias in the setting of inherited and acquired sodium channel disease. During common pathological conditions such as myocardial ischemia and heart failure, altered sodium channel function causes conduction disturbances and ventricular arrhythmias. In addition, sodium channel dysfunction caused by mutations in the SCN5A gene, encoding the major sodium channel in heart, is associated with a number of arrhythmia syndromes. Here, we provide an overview of the structure and function of the cardiac sodium channel, the clinical and biophysical characteristics of inherited and acquired sodium channel dysfunction, and the (limited) therapeutic options for the treatment of cardiac sodium channel disease., (© 2010 Blackwell Publishing Ltd.)

Aims: Treatment with the anticancer drug taxol (TXL), which polymerizes the cytoskeleton protein tubulin, may evoke cardiac arrhythmias based on reduced human cardiac sodium channel (Na(v)1.5) function. Therefore, we investigated whether enhanced tubulin polymerization by TXL affects Na(v)1.5 function and expression and whether these effects are beta1-subunit-mediated., Methods and Results: Human embryonic kidney (HEK293) cells, transfected with SCN5A cDNA alone (Na(v)1.5) or together with SCN1B cDNA (Na(v)1.5 + beta1), and neonatal rat cardiomyocytes (NRCs) were incubated in the presence and in the absence of 100 microM TXL. Sodium current (I(Na)) characteristics were studied using patch-clamp techniques. Na(v)1.5 membrane expression was determined by immunocytochemistry and confocal microscopy. Pre-treatment with TXL reduced peak I(Na) amplitude nearly two-fold in both Na(v)1.5 and Na(v)1.5 + beta1, as well as in NRCs, compared with untreated cells. Accordingly, HEK293 cells and NRCs stained with anti-Na(v)1.5 antibody revealed a reduced membrane-labelling intensity in the TXL-treated groups. In addition, TXL accelerated I(Na) decay of Na(v)1.5 + beta1, whereas I(Na) decay of Na(v)1.5 remained unaltered. Finally, TXL reduced the fraction of channels that slow inactivated from 31% to 18%, and increased the time constant of slow inactivation by two-fold in Na(v)1.5. Conversely, slow inactivation properties of Na(v)1.5 + beta1 were unchanged by TXL., Conclusion: Enhanced tubulin polymerization reduces sarcolemmal Na(v)1.5 expression and I(Na) amplitude in a beta1-subunit-independent fashion and causes I(Na) fast and slow inactivation impairment in a beta1-subunit-dependent way. These changes may underlie conduction-slowing-dependent cardiac arrhythmias under conditions of enhanced tubulin polymerization, e.g. TXL treatment and heart failure.

Cardiac sodium channel dysfunction caused by mutations in the SCN5A gene is associated with a number of relatively uncommon arrhythmia syndromes, including long-QT syndrome type 3 (LQT3), Brugada syndrome, conduction disease, sinus node dysfunction, and atrial standstill, which potentially lead to fatal arrhythmias in relatively young individuals. Although these various arrhythmia syndromes were originally considered separate entities, recent evidence indicates more overlap in clinical presentation and biophysical defects of associated mutant channels than previously appreciated. Various SCN5A mutations are now known to present with mixed phenotypes, a presentation that has become known as "overlap syndrome of cardiac sodium channelopathy." In many cases, multiple biophysical defects of single SCN5A mutations are suspected to underlie the overlapping clinical manifestations. Here, we provide an overview of current knowledge on SCN5A mutations associated with sodium channel overlap syndromes and discuss a possible role for modifiers in determining disease expressivity in the individual patient.

Background: Patients carrying the cardiac sodium channel (SCN5A) mutation 1795insD show sudden nocturnal death and signs of multiple arrhythmia syndromes including bradycardia, conduction delay, QT prolongation, and right precordial ST-elevation. We investigated the electrophysiological characteristics of a transgenic model of the murine equivalent mutation 1798insD., Methods and Results: On 24-hour continuous telemetry and surface ECG recordings, Scn5a(1798insD/+) heterozygous mice showed significantly lower heart rates, more bradycardic episodes (pauses > or = 500 ms), and increased PQ interval, QRS duration, and QTc interval compared with wild-type mice. The sodium channel blocker flecainide induced marked sinus bradycardia and/or sinus arrest in the majority of Scn5a(1798insD/+) mice, but not in wild-type mice. Epicardial mapping using a multielectrode grid on excised, Langendorff-perfused hearts showed preferential conduction slowing in the right ventricle of Scn5a(1798insD/+) hearts. On whole-cell patch-clamp analysis, ventricular myocytes isolated from Scn5a(1798insD/+) hearts displayed action potential prolongation, a 39% reduction in peak sodium current density and a similar reduction in action potential upstroke velocity. Scn5a(1798insD/+) myocytes displayed a slower time course of sodium current decay without significant differences in voltage-dependence of activation and steady-state inactivation, slow inactivation, or recovery from inactivation. Furthermore, Scn5a(1798insD/+) myocytes showed a larger tetrodotoxin-sensitive persistent inward current compared with wild-type myocytes., Conclusions: Mice carrying the murine equivalent of the SCN5A-1795insD mutation display bradycardia, right ventricular conduction slowing, and QT prolongation, similar to the human phenotype. These results demonstrate that the presence of a single SCN5A mutation is indeed sufficient to cause an overlap syndrome of cardiac sodium channel disease.

Objectives: We have tested whether a genotype-phenotype relationship exists in Brugada syndrome (BS) by trying to distinguish BS patients with (carriers) and those without (non-carriers) a mutation in the gene encoding the cardiac sodium channel (SCN5A) using clinical parameters., Background: Brugada syndrome is an inherited cardiac disease characterized by a varying degree of ST-segment elevation in the right precordial leads and (non)specific conduction disorders. In a minority of patients, SCN5A mutations can be found. Genetic heterogeneity has been demonstrated, but other causally related genes await identification. If a genotype-phenotype relationship exists, this might facilitate screening., Methods: In a multi-center study, we have collected data on demographics, clinical history, family history, electrocardiogram (ECG) parameters, His to ventricle interval (HV), and ECG parameters after pharmacologic challenge with I(Na) blocking drugs for BS patients with (n = 23), or those without (n = 54), an identified SCN5A mutation., Results: No differences were found in demographics, clinical history, or family history. Carriers had a significantly longer PQ interval on the baseline ECG and a significantly longer HV time. A PQ interval of > or =210 ms and an HV interval > or =60 ms seem to be predictive for the presence of an SCN5A mutation. After I(Na) blocking drugs, carriers had significantly longer PQ and QRS intervals and more increase in QRS duration., Conclusions: We observed significantly longer conduction intervals on baseline ECG in patients with established SCN5A mutations (PQ and HV interval and, upon class I drugs, more QRS increase). These results concur with the observed loss of function of mutated BS-related sodium channels. Brugada syndrome patients with, and those without, an SCN5A mutation can be differentiated by phenotypical differences.

A sodium current (INa) reduction occurs in the setting of many acquired and inherited conditions and is associated with cardiac conduction slowing and increased arrhythmia risks. The sodium channel blocker mexiletine has been shown to restore the trafficking of mutant sodium channels to the membrane. However, these studies were mostly performed in heterologous expression systems using high mexiletine concentrations. Moreover, the chronic effects on INa in a non-diseased cardiomyocyte environment remain unknown. In this paper, we investigated the chronic and acute effects of a therapeutic dose of mexiletine on INa and the action potential (AP) characteristics in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) of a healthy individual. Control hiPSC-CMs were incubated for 48 h with 10 µM mexiletine or vehicle. Following the wash-out of mexiletine, patch clamp analysis and immunocytochemistry experiments were performed. The incubation of hiPSC-CMs for 48 h with mexiletine (followed by wash-out) induced a significant increase in peak INa of ~75%, without any significant change in the voltage dependence of (in)activation. This was accompanied by a significant increase in AP upstroke velocity, without changes in other AP parameters. The immunocytochemistry experiments showed a significant increase in membrane Nav1.5 fluorescence following a 48 h incubation with mexiletine. The acute re-exposure of hiPSC-CMs to 10 µM mexiletine resulted in a small but significant increase in AP duration, without changes in AP upstroke velocity, peak INa density, or the INa voltage dependence of (in)activation. Importantly, the increase in the peak INa density and resulting AP upstroke velocity induced by chronic mexiletine incubation was not counteracted by the acute re-administration of the drug. In conclusion, the chronic administration of a clinically relevant concentration of mexiletine increases INa density in non-diseased hiPSC-CMs, likely by enhancing the membrane trafficking of sodium channels. Our findings identify mexiletine as a potential therapeutic strategy to enhance and/or restore INa and cardiac conduction. [ABSTRACT FROM AUTHOR]

Aims The microtubule (MT) network plays a major role in the transport of the cardiac sodium channel Nav1.5 to the membrane, where the latter associates with interacting proteins such as dystrophin. Alterations in MT dynamics are known to impact on ion channel trafficking. Duchenne muscular dystrophy (DMD), caused by dystrophin deficiency, is associated with an increase in MT detyrosination, decreased sodium current (I Na), and arrhythmias. Parthenolide (PTL), a compound that decreases MT detyrosination, has shown beneficial effects on cardiac function in DMD. We here investigated its impact on I Na and Nav1.5 subcellular distribution. Methods and results Ventricular cardiomyocytes (CMs) from wild-type (WT) and mdx (DMD) mice were incubated with either 10 µM PTL, 20 µM EpoY, or dimethylsulfoxide (DMSO) for 3–5 h, followed by patch-clamp analysis to assess I Na and action potential (AP) characteristics in addition to immunofluorescence and stochastic optical reconstruction microscopy (STORM) to investigate MT detyrosination and Nav1.5 cluster size and density, respectively. In accordance with previous studies, we observed increased MT detyrosination, decreased I Na and reduced AP upstroke velocity (V max) in mdx CMs compared to WT. PTL decreased MT detyrosination and significantly increased I Na magnitude (without affecting I Na gating properties) and AP V max in mdx CMs, but had no effect in WT CMs. Moreover, STORM analysis showed that in mdx CMs, Nav1.5 clusters were decreased not only in the grooves of the lateral membrane (LM; where dystrophin is localized) but also at the LM crests. PTL restored Nav1.5 clusters at the LM crests (but not at the grooves), indicating a dystrophin-independent trafficking route to this subcellular domain. Interestingly, Nav1.5 cluster density was also reduced at the intercalated disc (ID) region of mdx CMs, which was restored to WT levels by PTL. Treatment of mdx CMs with EpoY, a specific MT detyrosination inhibitor, also increased I Na density, while decreasing the amount of detyrosinated MTs, confirming a direct mechanistic link. Conclusion Attenuating MT detyrosination in mdx CMs restored I Na and enhanced Nav1.5 localization at the LM crest and ID. Hence, the reduced whole-cell I Na density characteristic of mdx CMs is not only the consequence of the lack of dystrophin within the LM grooves but is also due to reduced Nav1.5 at the LM crest and ID secondary to increased baseline MT detyrosination. Overall, our findings identify MT detyrosination as a potential therapeutic target for modulating I Na and subcellular Nav1.5 distribution in pathophysiological conditions. [ABSTRACT FROM AUTHOR]

The SCN5A-1795insD founder variant is a unique SCN5A gene variant found in a large Dutch pedigree that first came to attention in the late 1950s. To date, this is still one of the largest and best described SCN5A founder families worldwide. It was the first time that a single pathogenic variant in SCN5A proved to be sufficient to cause a sodium channel overlap syndrome. Affected family members displayed features of Brugada syndrome, cardiac conduction disease and long QT syndrome type 3, thus encompassing features of both loss and gain of sodium channel function. This brief summary takes us past 70 years of clinical experience and over 2 decades of research. It is remarkable to what extent researchers and clinicians have managed to gain understanding of this complex phenotype in a relatively short time. Extensive clinical, genetic, electrophysiological and molecular studies have provided fundamental insights into SCN5A and the cardiac sodium channel Nav1.5. [ABSTRACT FROM AUTHOR]

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'. [ABSTRACT FROM AUTHOR]

The cardiac sodium channel NaV1.5 is an essential modulator of cardiac excitability, with decreased NaV1.5 levels at the plasma membrane and consequent reduction in sodium current (I Na) leading to potentially lethal cardiac arrhythmias. NaV1.5 is distributed in a specific pattern at the plasma membrane of cardiomyocytes, with localization at the crests, grooves, and T-tubules of the lateral membrane and particularly high levels at the intercalated disc region. NaV1.5 forms a large macromolecular complex with and is regulated by interacting proteins, some of which are specifically localized at either the lateral membrane or intercalated disc. One of the NaV1.5 trafficking routes is via microtubules (MTs), which are regulated by MT plus-end tracking proteins (+TIPs). In our search for mechanisms involved in targeted delivery of NaV1.5, we here provide an overview of previously demonstrated interactions between NaV1.5 interacting proteins and +TIPs, which potentially (in)directly impact on NaV1.5 trafficking. Strikingly, +TIPs interact extensively with several intercalated disc- and lateral membrane-specific NaV1.5 interacting proteins. Recent work indicates that this interplay of +TIPs and NaV1.5 interacting proteins mediates the targeted delivery of NaV1.5 at specific cardiomyocyte subcellular domains, while also being potentially relevant for the trafficking of other ion channels. These observations are especially relevant for diseases associated with loss of NaV1.5 specifically at the lateral membrane (such as Duchenne muscular dystrophy), or at the intercalated disc (for example, arrhythmogenic cardiomyopathy), and open up potential avenues for development of new anti-arrhythmic therapies. [ABSTRACT FROM AUTHOR]

Aim: The voltage‐gated sodium channel NaV1.5, encoded by SCN5A, is essential for cardiac excitability and ensures proper electrical conduction. Early embryonic death has been observed in several murine models carrying homozygous Scn5amutations. We investigated when sodium current (INa) becomes functionally relevant in the murine embryonic heart and how Scn5a/NaV1.5 dysfunction impacts on cardiac development. Methods: Involvement of NaV1.5‐generated INa in murine cardiac electrical function was assessed by optical mapping in wild type (WT) embryos (embryonic day (E)9.5 and E10.5) in the absence and presence of the sodium channel blocker tetrodotoxin (30 µmol/L). INa was assessed by patch‐clamp analysis in cardiomyocytes isolated from WT embryos (E9.5‐17.5). In addition, cardiac morphology and electrical function was assessed in Scn5a‐1798insD−/− embryos (E9.5‐10.5) and their WT littermates. Results: In WT embryos, tetrodotoxin did not affect cardiac activation at E9.5, but slowed activation at E10.5. Accordingly, patch‐clamp measurements revealed that INa was virtually absent at E9.5 but robustly present at E10.5. Scn5a‐1798insD−/− embryos died in utero around E10.5, displaying severely affected cardiac activation and morphology. Strikingly, altered ventricular activation was observed in Scn5a‐1798insD−/− E9.5 embryos before the onset of INa, in addition to reduced cardiac tissue volume compared to WT littermates. Conclusion: We here demonstrate that NaV1.5 is involved in cardiac electrical function from E10.5 onwards. Scn5a‐1798insD−/− embryos displayed cardiac structural abnormalities at E9.5, indicating that NaV1.5 dysfunction impacts on embryonic cardiac development in a non‐electrogenic manner. These findings are potentially relevant for understanding structural defects observed in relation to NaV1.5 dysfunction. [ABSTRACT FROM AUTHOR]

The cardiac sodium channel NaV1.5, encoded by the SCN5A gene, is responsible for the fast upstroke of the action potential. Mutations in SCN5A may cause sodium channel dysfunction by decreasing peak sodium current, which slows conduction and facilitates reentry-based arrhythmias, and by enhancing late sodium current, which prolongs the action potential and sets the stage for early afterdepolarization and arrhythmias. Yet, some NaV1.5-related disorders, in particular structural abnormalities, cannot be directly or solely explained on the basis of defective NaV1.5 expression or biophysics. An emerging concept that may explain the large disease spectrum associated with SCN5A mutations centres around the multifunctionality of the NaV1.5 complex. In this alternative view, alterations in NaV1.5 affect processes that are independent of its canonical ion-conducting role. We here propose a novel classification of NaV1.5 (dys)function, categorized into (i) direct ionic effects of sodium influx through NaV1.5 on membrane potential and consequent action potential generation, (ii) indirect ionic effects of sodium influx on intracellular homeostasis and signalling, and (iii) non-ionic effects of NaV1.5, independent of sodium influx, through interactions with macromolecular complexes within the different microdomains of the cardiomyocyte. These indirect ionic and non-ionic processes may, acting alone or in concert, contribute significantly to arrhythmogenesis. Hence, further exploration of these multifunctional effects of NaV1.5 is essential for the development of novel preventive and therapeutic strategies. [ABSTRACT FROM AUTHOR]

Inheritable cardiac disorders, which may be associated with cardiomyopathic changes, are often associated with increased risk of sudden death in the young. Early linkage analysis studies in Mendelian forms of these diseases, such as hypertrophic cardiomyopathy and long-QT syndrome, uncovered large-effect genetic variants that contribute to the phenotype. In more recent years, through genotype-phenotype studies and methodological advances in genetics, it has become evident that most inheritable cardiac disorders are not monogenic but, rather, have a complex genetic basis wherein multiple genetic variants contribute (oligogenic or polygenic inheritance). Conversely, studies on genes underlying these disorders uncovered pleiotropic effects, with a single gene affecting multiple and apparently unrelated phenotypes. In this review, we explore these 2 phenomena: on the one hand, the evidence that variants in multiple genes converge to generate one clinical phenotype, and, on the other, the evidence that variants in one gene can lead to apparently unrelated phenotypes. Although multiple conditions are addressed to illustrate these concepts, the experience obtained in the study of long-QT syndrome, Brugada syndrome, and arrhythmogenic cardiomyopathy, and in the study of functions related to SCN5A (the gene coding for the α-subunit of the most abundant sodium channel in the heart) and PKP2 (the gene coding for the desmosomal protein plakophilin-2), as well, is discussed in more detail. [ABSTRACT FROM AUTHOR]

Aims Selective inhibition of cardiac late sodium current (INaL) is an emerging target in the treatment of ventricular arrhythmias. We investigated the electrophysiological effects of GS-458967 (GS967), a potent, selective inhibitor of INaL, in an overlap syndrome model of both gain and loss of sodium channel function, comprising cardiomyocytes derived from both human SCN5A-1795insD+/- induced pluripotent stem cells (hiPSC-CMs) and mice carrying the homologous mutation Scn5a-1798ins+/-. Methods and results On patch-clamp analysis, GS967 (300 nmol/l) reduced INaL and action potential (AP) duration in isolated ventricular myocytes from wild type and Scn5a-1798ins+/- mice, as well as in SCN5A-1795ins+/- hiPSC-CMs. GS967 did not affect the amplitude of peak INa, but slowed its recovery, and caused a negative shift in voltage-dependence of INa inactivation. GS967 reduced AP upstroke velocity in Scn5a-1798ins+/- myocytes and SCN5A-1795ins+/- hiPSC-CMs. However, the same concentration of GS967 did not affect conduction velocity in Scn5a-1798ins+/- mouse isolated hearts, as assessed by epicardial mapping. GS967 decreased the amplitude of delayed after depolarizations and prevented triggered activity in mouse Scn5a-1798ins+/- cardiomyocytes. Conclusion The INNaL inhibitor GS967 decreases repolarization abnormalities and has anti-arrhythmic effects in the absence of deleterious effects on cardiac conduction. Thus, selective inhibition of INaL constitutes a promising pharmacological treatment of cardiac channelopathies associated with enhanced INaL. Our findings furthermore implement hiPSC-CMs as a valuable tool for assessment of novel pharmacological approaches in inherited sodium channelopathies. [ABSTRACT FROM AUTHOR]

Highlights: [•] Acquired and inherited dysfunction of cardiac sodium channels is associated with arrhythmia. [•] Peak sodium current inhibition may be pro-arrhythmic in ischemic and structurally abnormal hearts. [•] Late sodium current inhibition has anti-arrhythmic potential in heart failure, cardiomyopathy and LQT3. [•] Unraveling sodium channel complexity and diversity may identify novel therapeutic targets. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The article presents a study on the differences in sodium channels that regulate the severity of conduction disease. Conduction slowing of electric impulse that causes heartbeat may bring lethal cardiac arrhythmias. The study revealed that mutations in sodium channel, voltage-gated, Type V, alpha subunit are associated with familial arrhythmia syndromes. The study found that sodium channel subunit &x03b2; may represent a genetic modifier of conduction and cardiac sodium channel disease.

Dysfunction of the cardiac sodium channel Nav1.5 (encoded by the SCN5A gene) is associated with arrhythmias and sudden cardiac death. SCN5A mutations associated with long QT syndrome type 3 (LQT3) lead to enhanced late sodium current and consequent action potential (AP) prolongation. Internalization and degradation of Nav1.5 is regulated by ubiquitylation, a post-translational mechanism that involves binding of the ubiquitin ligase Nedd4-2 to a proline-proline-serine-tyrosine sequence of Nav1.5, designated the PY-motif. We investigated the biophysical properties of the LQT3-associated SCN5A-p.Y1977N mutation located in the Nav1.5 PY-motif, both in HEK293 cells as well as in newly generated mice harboring the mouse homolog mutation Scn5a-p.Y1981N. We found that in HEK293 cells, the SCN5A-p.Y1977N mutation abolished the interaction between Nav1.5 and Nedd4-2, suppressed PY-motif-dependent ubiquitylation of Nav1.5, and consequently abrogated Nedd4-2 induced sodium current (INa) decrease. Nevertheless, homozygous mice harboring the Scn5a-p.Y1981N mutation showed no electrophysiological alterations nor changes in AP or (late) INa properties, questioning the in vivo relevance of the PY-motif. Our findings suggest the presence of compensatory mechanisms, with additional, as yet unknown, factors likely required to reduce the "ubiquitylation reserve" of Nav1.5. Future identification of such modulatory factors may identify potential triggers for arrhythmias and sudden cardiac death in the setting of LQT3 mutations. [ABSTRACT FROM AUTHOR]

Background--Sodium channel Nav1.5 underlies cardiac excitability and conduction. The last 3 residues of Nav1.5 (Ser-Ile-Val) constitute a PDZ domain--binding motif that interacts with PDZ proteins such as syntrophins and SAP97 at different locations within the cardiomyocyte, thus defining distinct pools of Nav1.5 multiprotein complexes. Here, we explored the in vivo and clinical impact of this motif through characterization of mutant mice and genetic screening of patients. Methods and Results--To investigate in vivo the regulatory role of this motif, we generated knock-in mice lacking the SIV domain (ΔSIV). ΔSIV mice displayed reduced Nav1.5 expression and sodium current (INa), specifically at the lateral myocyte membrane, whereas Nav1.5 expression and INa at the intercalated disks were unaffected. Optical mapping of ΔSIV hearts revealed that ventricular conduction velocity was preferentially decreased in the transversal direction to myocardial fiber orientation, leading to increased anisotropy of ventricular conduction. Internalization of wild-type and ΔSIV channels was unchanged in HEK293 cells. However, the proteasome inhibitor MG132 rescued ΔSIV INa, suggesting that the SIV motif is important for regulation of Nav1.5 degradation. A missense mutation within the SIV motif (p.V2016M) was identified in a patient with Brugada syndrome. The mutation decreased Nav1.5 cell surface expression and INa when expressed in HEK293 cells. Conclusions--Our results demonstrate the in vivo significance of the PDZ domain--binding motif in the correct expression of Nav1.5 at the lateral cardiomyocyte membrane and underline the functional role of lateral Nav1.5 in ventricular conduction. Furthermore, we reveal a clinical relevance of the SIV motif in cardiac disease. [ABSTRACT FROM AUTHOR]