Abstract 415: Sarcoplasmic Reticulum Volume Expansion in Response to Cardiac Calsequestrin Deletion Maintains Sarcoplasmic Calcium Storage and Contractile Function in Mice
Cardiac calsequestrin (Casq2) is the major Ca2+binding protein located in the junctional sarcoplasmic reticulum (SR) of cardiac muscle and thought to be the key Ca2+ storage protein required for SR Ca2+ release. Recently, CASQ2 mutations resulting in truncated CASQ2 have been associated with a syndrome of catecholaminergic polymorphic ventricular tachycardia in humans. However, homozygous mutation carriers presumably lacking functional Casq2 display surprisingly normal cardiac contractile function. To determine the mechanisms whereby hearts lacking Casq2 preserve contractile function, we generated Casq2 null (Casq2−/−) mice. Here, we report that Casq2−/− mice are viable and have normal contractile function by echocardiography (Fractional shortening: 38±2.9 vs. 38±3.0, n=6 each, p=n.s.). SR Ca2+release and fractional shortening was not significantly different between field-stimulated fura-2 loaded Casq2+/+and Casq2−/− myocytes. Total SR Ca2+ content measured by rapid caffeine application was decreased by only 14% (Caffeine transient [fluorescence ratio]: Casq2+/+ 1.42±0.067, n=28, vs. Casq2−/− 1.22±0.055, n=47, p<0.05). Immunoblot analysis demonstrated absence of Casq2 protein and profound decreases in the Casq2-binding proteins triadin-1 and junctin in Casq2−/− hearts. SR Ca2+release channel and SR uptake pump proteins were not altered. Up-regulation of skeletal calsequestrin or other SR Ca2+-binding proteins was not apparent in Casq2−/− hearts. Electron micrograph analysis demonstrated an empty junctional SR and a 50% increase of SR volume compared with Casq2+/+ myocytes (p<0.0001).
Conclusion: Unexpectedly, Casq2 is not required for cardiac Ca2+ storage and release, which appears to be maintained by a heretofore unreported expansion of SR volume.