In cardiomyocytes, altered release of Ca²⁺ from intracellular stores are known to affect the susceptibility of ventricular arrhythmias associated with cardiac remodeling. Upon catecholamine stimulation nicotinic acid adenine dinucleotide phosphate (NAADP) activates Two-pore channels (TPC) at lysosomal/sarcoplasmic reticular junctions which subsequently contribue to the development of ventricular arrhythmias and pathological cardiac remodeling [1-3]. We identified novel Organellar Ca²⁺ Regulator proteins, called OCaR1, OCaR2 and OCaR3 contributing to Ca²⁺ release from acidic organelles and aimed to determine their implications in pathological cardiac remodeling. 

Methods: We generated global and cardiomyocyte-specific OCaR1- and OCaR2-deficient mice (OCaR^KO and OCaR^CM-KO) and analyzed Ca²⁺ signaling in isolated adult cardiomyocytes under resting and pacing conditions using several agonists/antagonists to address NAADP-sensitive Ca²⁺ stores including NAADP, Isoproterenol, Ned-19 and TPC-modulators, among others. We induced cardiac remodeling by chronic Isoproterenol (ISO) and Angiotensin II infusion. Cardiac contractility was determined by pressure-volume loop and echocardiography measurements. Telemetric ECGs were recorded in conscious freely moving mice. Whole transcriptome sequencing was done by NGS and we analyzed changes in gene expression during cardiac remodeling. 

Results: OCaR1 and OCaR2 transcripts were detected in RNA from adult heart as well as in adult mouse cardiomyocytes isolated by laser capture microdissection. In resting cardiomyocytes from OCaR2^KO or OCaR2^CM-KO mice, ISO and NAADP evoked spontaneous intracellular Ca²⁺ oscillations that were not observed in cells from corresponding control mice or from OCaR1^KO mice. Exaggerated ISO-evoked Ca²⁺ transients in OCaR2^CM-KO were blocked by the NAADP- and TPC antagonists. In beating OCaR2^CM-KO myocytes, ISO (100nM) stimulation led to an increase of spontaneous diastolic Ca²⁺ transients by more than 50%. Upon chronic ISO treatment for 7 days, OCaR2^CM-KO mice developed increased cardiac hypertrophy accompanied by significantly different gene regulation. In contrast, P/V loop analysis revealed no differences in cardiac contractility parameters in OCaR2^CM-KO mice, neither under basal condition nor upon ISO stimulation. However, ECG telemetry uncovered that the overstimulation of the b-adrenergic axis resulted in polymorphic ventricular tachycardia leading to sudden cardiac death within 20-40 min that was not observed in any of the control animals. Additionally, infarction via ischemia/reperfusion led to an increased occurrence of ISO-triggered salvos accompanied by decreased cardiac function and dilatation of the left ventricle in OCaR2^CM-KO.

Conclusion: We identified OCaR2 proteins in cardiomyocytes as key signaling molecules in the development of catecholamine induced Ca²⁺ oscillations arising from the lysosomal-SR junction downstream. Inactivation of OCaR2 proteins in murine cardiomyocytes had no impact on basal cardiac contractility or on the inotropic response to catecholamines. In contrast, OCaR2 proteins are critical determinants of neurohumoral-induced cardiac hypertrophy as well as of the development of fatal catecholaminergic ventricular tachycardia and cardiac dysfunction.

References:  
1. Nebel et al., J Biol Chem. 2013 May 31;288(22):16017-30
2. Capel et al., J Biol Chem. 2015 Dec 11;290(50):30087-98
3. Simon et al., 10.1161/CIRCULATIONAHA.120.046761