Orphan nuclear receptor Nur77 affects cardiomyocyte calcium homeostasis and adverse cardiac remodelling

Distinct stressors may induce heart failure. As compensation, β-adrenergic stimulation enhances myocardial contractility by elevating cardiomyocyte intracellular Ca2+ ([Ca2+]i). However, chronic β-adrenergic stimulation promotes adverse cardiac remodelling. Cardiac expression of nuclear receptor Nur77 is enhanced by β-adrenergic stimulation, but its role in cardiac remodelling is still unclear. We show high and rapid Nur77 upregulation in cardiomyocytes stimulated with β-adrenergic agonist isoproterenol. Nur77 knockdown in culture resulted in hypertrophic cardiomyocytes. Ventricular cardiomyocytes from Nur77-deficient (Nur77-KO) mice exhibited elevated diastolic and systolic [Ca2+]i and prolonged action potentials compared to wild type (WT). In vivo, these differences resulted in larger cardiomyocytes, increased expression of hypertrophic genes

Intercalated disc abnormalities, reduced Na+ current density, and conduction slowing in desmoglein-2 mutant mice prior to cardiomyopathic changes

Mutations in genes encoding desmosomal proteins have been implicated in the pathogenesis of arrhythmogenic right ventricular cardiomyopathy (ARVC). However, the consequences of these mutations in early disease stages are unknown. We investigated whether mutation-induced intercalated disc remodelling impacts on electrophysiological properties before the onset of cell death and replacement fibrosis. Transgenic mice with cardiac overexpression of mutant Desmoglein2 (Dsg2) Dsg2-N271S (Tg-NS/L) were studied before and after the onset of cell death and replacement fibrosis. Mice with cardiac overexpression of wild-type Dsg2 and wild-type mice served as controls. Assessment by electron microscopy established that intercellular space widening at the desmosomes/adherens junctions occurred in Tg-NS/L mice before the onset of necrosis and fibrosis. At this stage, epicardial mapping in Langendorff-perfused hearts demonstrated prolonged ventricular activation time, reduced longitudinal and transversal conduction velocities, and increased arrhythmia inducibility. A reduced action potential (AP) upstroke velocity due to a lower Na current density was also observed at this stage of the disease. Furthermore, co-immunoprecipitation demonstrated an in vivo interaction between Dsg2 and the Na channel protein Na(V)1.5. Intercellular space widening at the level of the intercalated disc (desmosomes/adherens junctions) and a concomitant reduction in AP upstroke velocity as a consequence of lower Na current density lead to slowed conduction and increased arrhythmia susceptibility at disease stages preceding the onset of necrosis and replacement fibrosis. The demonstration of an in vivo interaction between Dsg2 and Na(V)1.5 provides a molecular pathway for the observed electrical disturbances during the early ARVC stage

Functional Na(V)1.8 Channels in Intracardiac Neurons The Link Between SCN10A and Cardiac Electrophysiology

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 mu mol/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. (Circ Res. 2012;111:333-343.

Genetic variation in GNB5 causes bradycardia by augmenting the cholinergic response via increased acetylcholine-activated potassium current (I K,ACh)

Mutations in GNB5, encoding the G-protein β5 subunit (Gβ5), have recently been linked to a multisystem disorder that includes severe bradycardia. Here, we investigated the mechanism underlying bradycardia caused by the recessive p.S81L Gβ5 variant. Using CRISPR/Cas9-based targeting, we generated an isogenic series of human induced pluripotent stem cell (hiPSC) lines that were either wild type, heterozygous or homozygous for the GNB5 p.S81L variant. These were differentiated into cardiomyocytes (hiPSC-CMs) that robustly expressed the acetylcholine-activated potassium channel [I(KACh); also known as I K,ACh]. Baseline electrophysiological properties of the lines did not differ. Upon application of carbachol (CCh), homozygous p.S81L hiPSC-CMs displayed an increased acetylcholine-activated potassium current (I K,ACh) density and a more pronounced decrease of spontaneous activity as compared to wild-type and heterozygous p.S81L hiPSC-CMs, explaining the bradycardia in homozygous carriers. Application of the specific I(KACh) blocker XEN-R0703 resulted in near-complete reversal of the phenotype. Our results provide mechanistic insights and proof of principle for potential therapy in patients carrying GNB5 mutations

Genetic variation in GNB5 causes bradycardia by augmenting the cholinergic response via increased acetylcholine-activated potassium current (IK,ACh)

Mutations in GNB5, encoding the G-protein β5 subunit (Gβ5), have recently been linked to a multisystem disorder that includes severe bradycardia. Here, we investigated the mechanism underlying bradycardia caused by the recessive p.S81L Gβ5 variant. Using CRISPR/Cas9-based targeting, we generated an isogenic series of human induced pluripotent stem cell (hiPSC) lines that were either wild type, heterozygous or homozygous for the GNB5 p.S81L variant. These were differentiated into cardiomyocytes (hiPSC-CMs) that robustly expressed the acetylcholine-activated potassium channel [I(KACh); also known as IK,ACh]. Baseline electrophysiological properties of the lines did not differ. Upon application of carbachol (CCh), homozygous p.S81L hiPSC-CMs displayed an increased acetylcholine-activated potassium current (IK,ACh) density and a more pronounced decrease of spontaneous activity as compared to wild-type and heterozygous p.S81L hiPSC-CMs, explaining the bradycardia in homozygous carriers. Application of the specific I(KACh) blocker XEN-R0703 resulted in near-complete reversal of the phenotype. Our results provide mechanistic insights and proof of principle for potential therapy in patients carrying GNB5 mutations.This article has an associated First Person interview with the first author of the paper

Functional modulation of atrio-ventricular conduction by enhanced late sodium current and calcium-dependent mechanisms in Scn5a1798insD/+ mice

SCN5A mutations are associated with arrhythmia syndromes, including Brugada syndrome, long QT syndrome type 3 (LQT3), and cardiac conduction disease. Long QT syndrome type 3 patients display atrio-ventricular (AV) conduction slowing which may contribute to arrhythmogenesis. We here investigated the as yet unknown underlying mechanisms

Charakterisierung von Früh- und Spätmanifestationen bei Narkolepsie

Einleitung: Die Erstmanifestation der Narkolepsie erfolgt im 2. und 4. Lebensjahrzehnt. Klinische und polysomnographische prospektive Daten beider Manifestationsgruppen wurden hinsichtlich ihrer Phänotypen verglichen. Methoden: Daten von 136 Patienten (63 Männer, 73 Frauen, Alter 13-78) wurden mittels Treysaer Narkolepsie-Fragebogen (51 Fragen zu Demographie, Symptomen und Ko-Morbidität), Polysomnographie (PSG), Multiplem Schlaf Latenz Test (MSLT, beide n=90), Epworth Sleepiness Scale (ESS, n=84) evaluiert, statistische Auswertung erfolgte mittels Mann-Whitney-U-Test. Ergebnisse: Das mittlere Erstmanifestationalter betrug in Gruppe 1 16,6 Jahre (n=106) in Gruppe 2 40,1 Jahre (n=31). Der Trennwert der Erstmanifestationen liegt bei 30 Jahren. Bei Frauen treten erste Symptome signifikant früher auf als bei Männern (p=0,015). In beiden Gruppen ist Tagesschläfrigkeit überwiegend erstes Symptom. Die durchschnittliche Latenz zwischen erstem Symptom und Schlafattacken beträgt 1,5 Jahre in Gruppe1, 0,6 Jahre in Gruppe2. Die Latenz zwischen erstem Symptom und Kataplexien beträgt 6,3 Jahre in Gruppe1, 1,7 Jahre in Gruppe2. Patienten der Gruppe2 erkranken signifikant häufiger an Schlafapnoe, Diabetes und Bluthochdruck. In beiden Gruppen ist die Komorbidität mit kardiovaskulären Erkrankungen und anderen Schlafstörungen höher als in der Bevölkerung. In Gruppe1 ist die durchschnittliche Einschlaflatenz im MSLT kürzer (p=0,007) sowie die Anzahl der Sleep-onset-REM-Perioden höher (p=0,043) als in Gruppe2. In der PSG war Wake after sleep onset (p=0,045) in Gruppe1 kürzer als in Gruppe2. Keine signifikanten Unterschiede fanden sich bei ESS-Score, Total sleep time, Einschlaf- und REM-Latenz. Schlussfolgerung: Patienten mit Früh- bzw. Spätmanifestation unterscheiden sich deutlich in klinischen und polysomnographischen Aspekten. Die Ergebnisse beweisen die Existenz von zwei unterschiedlichen Phänotypen der Narkolepsie. Alle narkoleptischen Symptome beginnen bei Patienten mit Spätmanifestation früher