Modest Reductions of Cardiac Calsequestrin Increase Sarcoplasmic Reticulum Ca 2+ Leak Independent of Luminal Ca 2+ and Trigger Ventricular Arrhythmias in Mice

Abstract

Cardiac calsequestrin–null mice ( Casq2 −/− ) display catecholaminergic ventricular tachycardia akin to humans with CASQ2 mutations. However, the specific contribution of Casq2 deficiency to the arrhythmia phenotype is difficult to assess because Casq2 −/− mice also show significant reductions in the sarcoplasmic reticulum (SR) proteins junctin and triadin-1 and increased SR volume. Furthermore, it remains unknown whether Casq2 regulates SR Ca 2+ release directly or indirectly by buffering SR luminal Ca 2+ . To address both questions, we examined heterozygous ( Casq2 +/− ) mice, which have a 25% reduction in Casq2 but no significant decrease in other SR proteins. Casq2 +/− mice (n=35) challenged with isoproterenol displayed 3-fold higher rates of ventricular ectopy than Casq2 +/+ mice (n=31; P <0.05). Programmed stimulation induced significantly more ventricular tachycardia in Casq2 +/− mice than in Casq2 +/+ mice. Field-stimulated Ca 2+ transients, cell shortening, L-type Ca 2+ current, and SR volume were not significantly different in Casq2 +/− and Casq2 +/+ myocytes. However, in the presence of isoproterenol, SR Ca 2+ leak was significantly increased in Casq2 +/− myocytes ( Casq2 +/− 0.18±0.02 F ratio versus Casq2 +/+ 0.11±0.01 F ratio , n=57, 60; P <0.01), resulting in a significantly higher rate of spontaneous SR Ca 2+ releases and triggered beats. SR luminal Ca 2+ measured using Mag-Fura-2 was not altered by Casq2 reduction. As a result, the relationship between SR Ca 2+ leak and SR luminal Ca 2+ was significantly different between Casq2 +/− and Casq2 +/+ myocytes ( P <0.01). Thus, even modest reductions in Casq2 increase SR Ca 2+ leak and cause ventricular tachycardia susceptibility under stress. The underlying mechanism is likely the direct regulation of SR Ca 2+ release channels by Casq2 rather than altered luminal Ca 2+ .