Yanmin Zhang

Genetic Na+ channelopathies and sinus node dysfunction.

Voltage-gated Na+ channels are transmembrane proteins that produce the fast inward Na+ current responsible for the depolarization phase of the cardiac action potential. They play fundamental roles in the initiation, propagation, and maintenance of normal cardiac rhythm. Inherited mutations in SCN5A, the gene encoding the pore-forming alpha-subunit of the cardiac-type Na+ channel, result in a spectrum of disease entities termed Na+ channelopathies. These include multiple arrhythmic syndromes, such as the long QT syndrome type 3 (LQT3), Brugada syndrome (BrS), an inherited cardiac conduction defect (CCD), sudden infant death syndrome (SIDS) and sick sinus syndrome (SSS). To date, mutational analyses have revealed more than 200 distinct mutations in SCN5A, of which at least 20 mutations are associated with sinus node dysfunction including SSS. This review summarizes recent findings bearing upon: (i) the functional role of distinct voltage-gated Na+ currents in sino-atrial node pacemaker function; (ii) genetic Na+ channelopathy and its relationship to sinus node dysfunction.

TGF-β1-Mediated Fibrosis and Ion Channel Remodeling Are Key Mechanisms in Producing the Sinus Node Dysfunction Associated With SCN5A Deficiency and Aging

Background—Mutations in the cardiac Na+ channel gene (SCN5A) can adversely affect electric function in the heart, but effects can be age dependent. We explored the interacting effects of Scn5a disruption and aging on the pathogenesis of sinus node dysfunction in a heterozygous Scn5a knockout (Scn5a+/−) mouse model. Methods and Results—We compared functional, histological, and molecular features in young (3 to 4 month) and old (1 year) wild type and Scn5a+/− mice. Both Scn5a disruption and aging were associated with decreased heart rate variability, reduced sinoatrial node automaticity, and slowed sinoatrial conduction. They also led to increased collagen and fibroblast levels and upregulated transforming growth factor-&bgr;1 (TGF-&bgr;1) and vimentin transcripts, providing measures of fibrosis and reduced Nav1.5 expression. All these effects were most noticeable in old Scn5a+/− mice. Na+ channel inhibition by Nav1.5-E3 antibody directly increased TGF-&bgr;1 production in both cultured human cardiac myocytes and fibroblasts. Finally, aging was associated with downregulation of a wide range of ion channel and related transcripts and, again, was greatest in old Scn5a+/− mice. The quantitative results from these studies permitted computer simulations that successfully replicated the observed sinoatrial node phenotypes shown by the different experimental groups. Conclusions—These results implicate a tissue degeneration triggered by Nav1.5 deficiency manifesting as a TGF-&bgr;1-mediated fibrosis accompanied by electric remodeling in the sinus node dysfunction associated with Scn5a disruption or aging. The latter effects interact to produce the most severe phenotype in old Scn5a+/− mice. In demonstrating this, our findings suggest a novel regulatory role for Nav1.5 in cellular biological processes in addition to its electrophysiologic function.

Epac activation, altered calcium homeostasis and ventricular arrhythmogenesis in the murine heart

The recently described exchange protein directly activated by cAMP (Epac) has been implicated in distinct protein kinase A-independent cellular signalling pathways. We investigated the role of Epac activation in adrenergically mediated ventricular arrhythmogenesis. In contrast to observations in control conditions (nu2009=u200920), monophasic action potentials recorded in 2 of 10 intrinsically beating and 5 of 20 extrinsically paced Langendorff-perfused wild-type murine hearts perfused with the Epac activator 8-pCPT-2′-O-Me-cAMP (8-CPT, 1xa0μM) showed spontaneous triggered activity. Three of 20 such extrinsically paced hearts showed spontaneous ventricular tachycardia (VT). Programmed electrical stimulation provoked VT in 10 of 20 similarly treated hearts (Pu2009<u20090.001; nu2009=u200920). However, there were no statistically significant accompanying changes (Pu2009>u20090.05) in left ventricular epicardial (40.7u2009±u20091.2 versus 44.0u2009±u20091.7xa0ms; nu2009=u200910) or endocardial action potential durations (APD90; 51.8u2009±u20092.3 versus 51.9u2009±u20092.2xa0ms; nu2009=u200910), transmural (ΔAPD90) (11.1u2009±u20092.6 versus 7.9u2009±u20092.8xa0ms; nu2009=u200910) or apico-basal repolarisation gradients, ventricular effective refractory periods (29.1u2009±u20091.7 versus 31.2u2009±u20092.4xa0ms in control and 8-CPT-treated hearts, respectively; nu2009=u200910) and APD90 restitution characteristics. Nevertheless, fluorescence imaging of cytosolic Ca2+ levels demonstrated abnormal Ca2+ homeostasis in paced and resting isolated ventricular myocytes. Epac activation using isoproterenol in the presence of H-89 was also arrhythmogenic and similarly altered cellular Ca2+ homeostasis. Epac-dependent effects were reduced by Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibition with 1xa0μM KN-93. These findings associate VT in an intact cardiac preparation with altered cellular Ca2+ homeostasis and Epac activation for the first time, in the absence of altered repolarisation gradients previously implicated in reentrant arrhythmias through a mechanism dependent on CaMKII activity.

Physiological consequences of the P2328S mutation in the ryanodine receptor (RyR2) gene in genetically modified murine hearts

Aim:u2002 To explore the physiological consequences of the ryanodine receptor (RyR2)‐P2328S mutation associated with catecholaminergic polymorphic ventricular tachycardia (CPVT).

Atrial arrhythmia, triggering events and conduction abnormalities in isolated murine RyR2-P2328S hearts.

RyR2 mutations are associated with catecholaminergic polymorphic tachycardia, a condition characterized by ventricular and atrial arrhythmias. The present experiments investigate the atrial electrophysiology of homozygotic murine RyR2‐P2328S (RyR2S/S) hearts for ectopic triggering events and for conduction abnormalities that might provide a re‐entrant substrate.

The cardiac sodium channel mutation delQKP 1507-1509 is associated with the expanding phenotypic spectrum of LQT3, conduction disorder, dilated cardiomyopathy, and high incidence of youth sudden death.

Aim We report diverse phenotypic consequences of the delQKP-1507–1509 cardiac sodium channel mutation in three generations of a Chinese family. Methods and results Clinical and electrocardiographic (ECG), echocardiographic examination was followed by direct sequencing of SCN5A, KCNQ1, HERG, and LAMIN A/C to screen genomic DNA from blood samples. Of two mutation carriers, the proband was born with conduction disorders including second-degree atrioventricular (AV) block with prolonged QTc interval, additionally showing left anterior fascicular block (LAFB), incomplete right bundle-branch block (IRBBB), and intermittent third-degree AV block at 2 years, and clinical presentations of multiple syncope despite normal electroencephalograms at 8 years. Continuous ECG monitoring following presentation at 13 years revealed prolonged QTc and biphasic T-waves, multiple episodes of ventricular tachycardia, ventricular fibrillation, and torsades de pointes. Transthoracal echocardiography then revealed left ventricular dilatation and reduced systolic function. Another mutation carrier showed features of long QT syndrome type 3 (LQT3), LAFB, and dilated cardiomyopathy (DCM). Two additional subjects died suddenly at 13 and 33 years. Conclusion This data compliments and expands the spectrum of phenotypes resulting from this known gain-of-function mutation, including not only LQT3, cardiac conduction defects, and sudden death but also DCM, hitherto associated with loss-of-function mutations, for the first time.

Scn3b knockout mice exhibit abnormal sino‐atrial and cardiac conduction properties

Aim:u2002 In contrast to extensive reports on the roles of Nav1.5 α‐subunits, there have been few studies associating the β‐subunits with cardiac arrhythmogenesis. We investigated the sino‐atrial and conduction properties in the hearts of Scn3b−/− mice.

Increased right ventricular repolarization gradients promote arrhythmogenesis in a murine model of Brugada syndrome.

Repolarization Gradients in Brugada Syndrome.u2002u2002Introduction: Brugada syndrome (BrS) is associated with loss of Na+ channel function and increased risks of a ventricular tachycardia exacerbated by flecainide but reduced by quinidine. Previous studies in nongenetic models have implicated both altered conduction times and repolarization gradients in this arrhythmogenicity. We compared activation latencies and spatial differences in action potential recovery between different ventricular regions in a murine Scn5a+/− BrS model, and investigated the effect of flecainide and quinidine upon these.

In vivo studies of Scn5a+/− mice modeling Brugada syndrome demonstrate both conduction and repolarization abnormalities

Objectives We investigate the extent to which the electrocardiographic (ECG) properties of intact Scn5a+/− mice reproduce the corresponding clinical Brugada syndrome phenotype and use this model to investigate the role of conduction and repolarization abnormalities in the arrhythmogenic mechanism. Methods and Results The ECGs were obtained from anesthetized wild-type and Scn5a+/− mice, before and after administration of the known pro- and antiarrhythmic agents flecainide and quinidine. The ECG intervals were measured and their dispersions calculated. Scn5a+/− hearts showed ventricular arrhythmias, ST elevation, and conduction disorders including increased QT dispersion, accentuated by flecainide. Quinidine did not cause ventricular arrhythmias but exerted variable effects on ST segments and worsened conduction abnormalities. Conclusions The ECG features in an Scn5a+/− mouse establish it as a suitable model for Brugada syndrome and demonstrate abnormal conduction and repolarization phenomena. Altered QT dispersion, taken to indicate increased transmural repolarization gradients, may be useful in clinical risk stratification.

Conduction Slowing Contributes to Spontaneous Ventricular Arrhythmias in Intrinsically Active Murine RyR2-P2328S Hearts

Conduction Changes in RyR2‐P2328S Hearts.u2002Introduction: The familial condition catecholaminergic polymorphic ventricular tachycardia (CPVT) is characterized by episodic bidirectional ventricular tachycardia (BVT), polymorphic ventricular tachycardia (PVT), and ventricular fibrillation following adrenergic challenge. It is associated with mutations involving the cardiac ryanodine receptor (RyR2).