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 Ca2+ release directly or indirectly by buffering SR luminal Ca2+. 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 Ca2+ transients, cell shortening, L-type Ca2+ current, and SR volume were not significantly different in Casq2+/- and Casq2+/+ myocytes. However, in the presence of isoproterenol, SR Ca2+ 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 Ca2+ releases and triggered beats. SR luminal Ca2+ measured using Mag-Fura-2 was not altered by Casq2 reduction. As a result, the relationship between SR Ca2+ leak and SR luminal Ca2+ was significantly different between Casq2+/- and Casq2+/+ myocytes (P<0.01). Thus, even modest reductions in Casq2 increase SR Ca2+ leak and cause ventricular tachycardia susceptibility under stress. The underlying mechanism is likely the direct regulation of SR Ca2+ release channels by Casq2 rather than altered luminal Ca2+.