Abstract 1247: Heart Function is Transiently Sustained Despite a Near Loss of SERCA2 Protein in Cardiomyocyte-specific Serca2null Mice
The SERCA2 Ca2+ ATPase is of central importance for refilling of the sarcoplasmic reticulum (SR) Ca2+ store and cardiac contractility. Reduced SERCA2 function is associated with heart failure. We hypothesized that loss of SERCA2 would result in immediate severe myocardial contractile dysfunction and death. Transgenic mice were generated with a Cre-loxP strategy in which tamoxifen induces Serca2 (Atp2a2) gene excision in the cardiomyocytes (SERCA2KO) of adult mice. In SERCA2KO mice, SERCA2 protein was rapidly reduced in left ventricular myocardium with a half-life < 3 days. After 4 weeks, SERCA2 protein was reduced to < 5% of control values. In isolated cardiomyocytes, SERCA2a, SERCA2b, SERCA1 and SERCA3 proteins were not detectable. Strikingly, SERCA2KO mice did not present clinical signs of circulatory failure at 4 weeks. Fractional shortening was preserved, and cardiac output was reduced to 80% of control values. The left atrial diameter, lung weight and left ventricular end-diastolic pressure (LVEDP) were slightly increased in SERCA2KO mice compared with controls, and the maximal rates of pressure development and decline in the left ventricle were affected with a prolongation of the ventricular relaxation time. After seven weeks, SERCA2KO mice developed severe congestive heart failure with dilated chambers, elevated LVEDP and pronounced increases in lung and atrial weights. Cardiac output was reduced to 70% of control values. There were no indications of major cardiomyocyte disarray in the myocardium at the 4 or 7 week timepoints. The abundance of Na+,Ca2+ exchanger, L-type Ca2+ channel 1c and alpha2delta1 subunit proteins and Pmca1 mRNA were all increased at 4 and 7 weeks. The expression of calsequestrin protein and Ryr2 mRNA were unchanged. L-type Ca2+ channel alpha2delta1 subunit and PMCA1 expression were further enhanced at 7 weeks in SERCA2KO mice. Thus, cardiac function is supported in SERCA2KO mice for several weeks despite the near absence of SERCA2 protein. Alterations in the expression of Ca2+ transporting proteins suggest that Ca2+ transients are generated over the plasma membrane rather than the SR. However, the adaptations induced by loss of SERCA2 are not sufficient for long-term support of heart function in adult mice.