TRPV2 is Crucial for the Development of Excitation-Contraction Coupling in Neonatal Cardiomyocytes

We have previously reported that TRP vanilloid family type 2 channel (TRPV2) is a key molecule in stretch-induced Ca2+ response of cardiomyocytes. In this study, we aimed to understand the roles of TRPV2 in the development of excitation-contraction (E-C) coupling of cardiomyocytes and structural organization of intercalated discs between neighboring myocytes. We examined the mechanical stimulation-induced Ca2+ response in neonatal cardiomyocytes isolated from normal (control) and TRPV2-deficient mice. In neonatal cardiomyocytes isolated from control mice, hypotonicity- and stretch-induced Ca2+ responses were observed within 12 hours of culture after enzymatic isolation. These responses were decreased by tranilast, an inhibitor of TRPV2, suggesting involvement of TRPV2 channel. At this stage of the cultured cardiomyocytes, robust expression of TRPV2 was observed. After 48 hours, formation of intercalated discs and reorganization of myofibrils gradually progressed. After 72 hours, autogenic Ca2+ oscillation and beating were observed, indicating appropriate Ca2+-handling for E-C coupling. In contrast, TRPV2-deficient myocytes showed no Ca2+ response to the hypotonicity- and stretch-stimulation. These myocytes neither formed intercalated discs between neighbouring cells nor highly organized sarcomeric assembly of actin. These myocytes showed aberrant intracytoplasmic accumulation of connexin 43, internally defused N-cadherin, and reduced expression of Na2+/Ca2+ exchanger 1, NCX1. Even after 72 hours, Ca2+ handling for E-C coupling was not observed in TRPV2-deficient myocytes. Their beating was spontaneously elicited, but weak and asynchronous. Taken together, TRPV2 has a pivotal role in mechanical stimulation-induced Ca2+ response, and in structural and functional development of cardiomyocytes that enable E-C coupling and synchronous contraction with neighboring cells.