Severe Cardiac Hypertrophy and Depressed Contractility in Transgenic Mice with Co-Overexpression of Calsequestrin and SERCA2a

Abstract

Cardiac-specific overexpression of calsequestrin (CSQ) is associated with hypertrophy, depressed contractility, impaired SR Ca2+ release and signs of heart failure. The Ca2+ATPase (SERCA2a) represents the major determinant of myocardial contractility. Its stable or transient expression results in increased contractile parameters and higher Ca2+ transients. Hence, we tested whether the additional expression of SERCA2a in CSQ-overexpressing hearts results in beneficial functional effects. For this purpose, we generated double transgenic mice overexpressing both CSQ and SERCA2a (TGCxS) by cross-breeding of single transgenic mouse lines (TGCSQ and TGSER). SERCA2a protein expression was increased by 1.4-fold in both TGSER and TGCxS compared to wild-type (WT) hearts. Consistently, at submaximal free Ca2+ levels (30 nmoL/L), 45Ca2+ uptake reached 12%, 25%, 14% and 30% of maximal Ca2+ levels in WT, TGSER, TGCSQ and TGCxS, respectively (n=5, P<0.05). CSQ was overexpressed by 10-fold in both TGCSQ and TGCxS compared to WT. The expression of triadin was decreased by 53% in both TGCSQ and TGCxS compared to WT (P<0.05). The ryanodine receptor expression was reduced by 44%, 33% and 86% in TGCSQ, TGSER and TGCxS, respectively, compared to WT (P<0.05). In addition, co-overexpression of CSQ and SERCA2a was associated with severe cardiac hypertrophy. We observed a heart weight to body weight ratio of 4.4±0.1, 4.9±0.1, 8.9±0.3 and 12.5±1.0 mg/g in WT, TGSER, TGCSQ and TGCxS, respectively (n=7, P<0.05). Stained tissue sections revealed an enhanced interstitial fibrosis in ventricles of both TGCSQ and TGCxS. Functional analysis was performed on isolated working heart preparations. Contractility (+dP/dt) was depressed in TGCxS (1378±144 mmHg/s) compared to WT, TGSER and TGCSQ (2461±119, 3644±161 and 1879±105 mmHg/s, respectively, n=6, P<0.05). We conclude that the additional overexpression of SERCA2a failed to rescue the impaired cardiac phenotype of CSQ transgenic mice.