The cardiac ryanodine receptor luminal Ca²⁺ sensor governs Ca²⁺ waves, ventricular tachyarrhythmias and cardiac hypertrophy in calsequestrin-null mice

CASQ2 (cardiac calsequestrin) is commonly believed to serve as the SR (sarcoplasmic reticulum) luminal Ca²⁺ sensor. Ablation of CASQ2 promotes SCWs (spontaneous Ca²⁺ waves) and CPVT (catecholaminergic polymorphic ventricular tachycardia) upon stress but not at rest. How SCWs and CPVT are triggered by stress in the absence of the CASQ2-based luminal Ca²⁺ sensor is an important unresolved question. In the present study, we assessed the role of the newly identified RyR2 (ryanodine receptor 2)-resident luminal Ca²⁺ sensor in determining SCW propensity, CPVT susceptibility and cardiac hypertrophy in Casq2-KO (knockout) mice. We crossbred Casq2-KO mice with RyR2 mutant (E4872Q^+/−) mice, which lack RyR2-resident SR luminal Ca²⁺ sensing, to generate animals with both deficiencies. Casq2^+/− and Casq2^−/− mice showed stress-induced VTs (ventricular tachyarrhythmias), whereas Casq2^+/−/E4872Q^+/− and Casq2^−/−/E4872Q^+/− mice displayed little or no stress-induced VTs. Confocal Ca²⁺ imaging revealed that Casq2^−/− hearts frequently exhibited SCWs after extracellular Ca²⁺ elevation or adrenergic stimulation, whereas Casq2^−/−/E4872Q^+/− hearts had few or no SCWs under the same conditions. Cardiac hypertrophy developed and CPVT susceptibility increased with age in Casq2^−/− mice, but not in Casq2^−/−/E4872Q^+/− mice. However, the amplitudes and dynamics of voltage-induced Ca²⁺ transients in Casq2^−/− and Casq2^−/−/E4872Q^+/− hearts were not significantly different. Our results indicate that SCWs, CPVT and hypertrophy in Casq2-null cardiac muscle are governed by the RyR2-resident luminal Ca²⁺ sensor. This implies that defects in CASQ2-based lumi-nal Ca²⁺ sensing can be overridden by the RyR2-resident luminal Ca²⁺ sensor. This makes this RyR2-resident sensor a promising molecular target for the treatment of Ca²⁺-mediated arrhythmias.