Modulation of SRCa2+release by the triadin-to-calsequestrin ratio in ventricular myocytes

Abstract

Calsequestrin (CSQ) is a Ca2+storage protein that interacts with triadin (TRN), the ryanodine receptor (RyR), and junctin (JUN) to form a macromolecular tetrameric Ca2+signaling complex in the cardiac junctional sarcoplasmic reticulum (SR). Heart-specific overexpression of CSQ in transgenic mice (TGCSQ) was associated with heart failure, attenuation of SR Ca2+release, and downregulation of associated junctional SR proteins, e.g., TRN. Hence, we tested whether co-overexpression of CSQ and TRN in mouse hearts (TGCxT) could be beneficial for impaired intracellular Ca2+signaling and contractile function. Indeed, the depressed intracellular Ca2+concentration ([Ca]i) peak amplitude in TGCSQwas normalized by co-overexpression in TGCxTmyocytes. This effect was associated with changes in the expression of cardiac Ca2+regulatory proteins. For example, the protein level of the L-type Ca2+channel Cav1.2 was higher in TGCxTcompared with TGCSQ. Sarco(endo)plasmic reticulum Ca2+-ATPase 2a (SERCA2a) expression was reduced in TGCxTcompared with TGCSQ, whereas JUN expression and [3H]ryanodine binding were lower in both TGCxTand TGCSQcompared with wild-type hearts. As a result of these expressional changes, the SR Ca2+load was higher in both TGCxTand TGCSQmyocytes. In contrast to the improved cellular Ca2+, transient co-overexpression of CSQ and TRN resulted in a reduced survival rate, an increased cardiac fibrosis, and a decreased basal contractility in catheterized mice, working heart preparations, and isolated myocytes. Echocardiographic and hemodynamic measurements revealed a depressed cardiac performance after isoproterenol application in TGCxTcompared with TGCSQ. Our results suggest that co-overexpression of CSQ and TRN led to a normalization of the SR Ca2+release compared with TGCSQmice but a depressed contractile function and survival rate probably due to cardiac fibrosis, a lower SERCA2a expression, and a blunted response to β-adrenergic stimulation. Thus the TRN-to-CSQ ratio is a critical modulator of the SR Ca2+signaling.