Enhanced Myocardial Contractility and Increased Ca ²⁺ Transport Function in Transgenic Hearts Expressing the Fast-Twitch Skeletal Muscle Sarcoplasmic Reticulum Ca ²⁺ -ATPase

Abstract

In this study, we investigated whether the fast-twitch skeletal muscle sarco(endo)plasmic reticulum Ca2+ transport pump (SERCA1a) can functionally substitute the cardiac SERCA2a isoform and how its overexpression affects cardiac contractility. For this purpose, we generated transgenic (TG) mice that specifically overexpress SERCA1a in the heart, using the cardiac-specific alpha-myosin heavy chain promoter. Ectopic expression of SERCA1a resulted in a 2.5-fold increase in the amount of total SERCA protein. At the same time, the level of the endogenous SERCA2a protein was decreased by 50%, whereas the level of other muscle proteins, including calsequestrin, phospholamban, actin, and tropomyosin, remained unchanged. The steady-state level of SERCA phosphoenzyme intermediate was increased 2.5-fold, and the maximal velocity of Ca2+ uptake was increased 1.7-fold in TG hearts, demonstrating that the overexpressed protein is functional. Although the basal cytosolic calcium signal was decreased by 38% in TG cardiomyocytes, the amplitude of cytosolic calcium signal was increased by 71.8%. The rate of calcium resequestration was also increased in TG myocytes, which was reflected by a 51.6% decrease in the normalized time to 80% decay of calcium signal. This resulted in considerably increased peak rates of myocyte shortening and relengthening (50.0% and 66.6%, respectively). Cardiac functional analysis using isolated work-performing heart preparations revealed significantly faster rates of contraction and relaxation in TG hearts (41.9% and 39.5%, respectively). The time to peak pressure and the time to half-relaxation were shorter (29.1% and 32.7%, respectively). In conclusion, our study demonstrates that the SERCA1a pump can functionally substitute endogenous SERCA2a, and its overexpression significantly enhances Ca2+ transport and contractile function of the myocardium. These results also demonstrate that the SERCA pump level is a critical determinant of cardiac contractility.

Kinetics of cardiac muscle contraction and relaxation are linked and determined by properties of the cardiac sarcomere

“…We did not find any differences in mice with enhanced Ca 2ϩ cycling [i.e., overexpressing sarco(endo)plasmic reticulum Ca 2ϩ -ATPase (SERCA)1a] (19,25), reduced Ca 2ϩ cycling (SERC2a heterozygous knockout mice) (19,31), or various groups of control mice (36). Likewise, in mdx mice, which suffer from muscular dystrophy due to a lack of dystrophin, or in a more severe model where the compensatory utrophin is also lacking (13,21), this contraction-relaxation balance is completely maintained.…”

Altered dose response to β-agonists in SERCA1a-expressing hearts ex vivo and in vivo

“…To test whether SERCA1a can functionally substitute for SERCA2a and increase cardiac contractility, we have developed a transgenic (TG) mouse model expressing SERCA1a in the heart (16,19,23). In this model, SERCA1a expression resulted in a 2.5-fold increase in total SERCA pump protein level, but the expression of the endogenous SERCA2a isoform was reduced to ϳ50% in TG hearts.…”

Sarcoplasmic reticulum and calcium cycling targeting by gene therapy

“…35,36 It should be noted that the overexpression of SERCA2a at the protein level could only be achieved at B1.5. Homozygous null (SERCA2 À/À ) mice die early in development whereas heterozygous (SERCA2 +/À ) mice have been developed with a 65% decrease in SERCA2 protein levels, and although no cardiac pathology was found, when stressed by pressure overload, they developed HF much more quickly than wild-type control mice.…”