Abstract Body: Sympathetic activation during exercise or emotional stress enhances cardiac performance and is thought to promote electrical coupling between cardiomyocytes, which strengthens myocardial contraction. Electrical coupling in the heart is mediated by gap junction channels (GJCs) formed by connexin proteins, located at the intercalated discs between cardiomyocytes. Connexin 43 (Cx43) is the predominant connexin in the ventricular myocardium and is essential for action potential propagation. However, the mechanisms regulating Cx43 function during sympathetic activation remain poorly understood.
Recent studies show that β-adrenergic stimulation, which mimics sympathetic drive, induces nitric oxide (NO) production and S-nitrosylation of cardiac proteins, including Cx43. We found that Cx43 at the intercalated discs becomes highly S-nitrosylated following β-adrenergic stimulation, and that NO enhances Cx43 mediated coupling in a heterologous expression system. Based on these findings, we hypothesize that S-nitrosylation of Cx43 enhances electrical coupling between cardiomyocytes and contributes to increased cardiac contractility.
To test this hypothesis, we identified the Cx43 S-nitrosylation site and created a knockin mouse in which cysteine 271 is replaced by serine (C271S), then characterized these mice using telemetric electrocardiography (ECG), ex vivo optical mapping, and echocardiography. We found that mice harboring the C271S mutation displayed normal Cx43 expression and subcellular localization, but undetectable Cx43 S-nitrosylation. Under basal conditions, C271S mice exhibit no differences from WT mice in arrhythmogenesis or ECG intervals, but β-adrenergic stimulation with isoproterenol reveals QRS prolongation. Importantly, after isoproterenol treatment, C271S hearts fail to augment conduction velocity and contractility to the same degree as WT hearts. Together, these findings indicate that Cx43 S-nitrosylation is an important sympathetic regulatory modification that enhances GJC coupling and cardiac function.