Abstract 796: Reduction of Cardiac Calsequestrin Increases Sarcoplasmic Reticulum Ca2+-Leak Independent of Luminal Ca2+and Triggers Ventricular Arrhythmias in Mice
Deletion of cardiac calsequestrin (Casq2) causes VT in mice akin to CASQ2 mutations. However, other sarcoplasmic reticulum (SR) proteins (triadin-1/junctin) are also reduced in homozygous (Casq2−/−) mice and may contribute to their arrhythmia phenotype. Furthermore, it is unknown if Casq2 regulates SR Ca2+-release directly or indirectly by buffering SR luminal Ca2+. To address both questions, we examined heterozygous (Casq2+/−) mice with a 25% reduction in Casq2, but no decrease in other SR proteins.
Results: 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 VT in Casq2+/− mice. Field-stimulated Ca2+ transients, cell shortening, L-type Ca2+ current and SR volume were not different between the 2 groups. However, in presence of isoproterenol, SR Ca2+ leak was significantly higher in Casq2+/− myocytes (Casq2+/− 0.18±0.02 Fratio vs Casq2 +/+ 0.11±0.01 Fratio, n=57, 60; p=0.01), resulting in a significantly higher rate of spontaneous SR Ca2+-releases/triggered beats (Casq2+/− 0.38±0.07 vs Casq2+/+ 0.12±0.06, n=47, 34; p=0.007). Luminal SR Ca2+ was not significantly different between the 2 groups. However, on plotting SR Ca2+ leak as a function of luminal SR Ca2+, leak remained significantly higher in Casq2+/− myocytes (figure⇓).
Conclusion: Even modest reductions in Casq2 increase SR Ca2+ leak and cause VT susceptibility under stress, which could explain increased risk for VT in CASQ2+/− mutations. The underlying mechanism is likely the direct modulation of SR Ca2+-release by Casq2 rather than decreased Ca2+ buffering in the SR lumen.