Replacement of the muscle-specific sarcoplasmic reticulum Ca2+-ATPase isoform SERCA2a by the nonmuscle SERCA2b homologue causes mild concentric hypertrophy and impairs contraction-relaxation of the heart

The cardiac sarco(endo)plasmic reticulum Ca2+-ATPase gene (ATP2A2) encodes the following two different protein isoforms: SERCA2a (muscle-specific) and SERCA2b (ubiquitous). We have investigated whether this isoform specificit y is required for normal cardiac function. Gene targeting in mice successfu lly disrupted the splicing mechanism responsible for generating the SERCA2a isoform. Homozygous SERCA2a(-/-) mice displayed a complete loss of SERCA2a mRNA and protein resulting in a switch to the SERCA2b isoform. The express ion of SERCA2b mRNA and protein in hearts of SERCA2a(-/-) mice corresponded to only 50% of wild-type SERCA2 levels. Cardiac phospholamban mRNA levels were unaltered in SERCA2a(-/-) mice, but total phospholamban protein levels increased 2-fold. The transgenic phenotype was characterized by a approxim ate to 20% increase in embryonic and neonatal mortality (early phenotype), with histopathologic evidence of major cardiac malformations. Adult SERCA2a (-/-) animals (adult phenotype) showed a reduced spontaneous nocturnal acti vity and developed a mild compensatory concentric cardiac hypertrophy with impaired cardiac contractility and relaxation. but preserved beta -adrenerg ic response. Ca2+ uptake levels in SERCA2a(-/-) cardiac homogenates were re duced by approximate to 50%. In isolated cells, relaxation and Ca2+ removal by the SR were significantly reduced. Comparison of our data with those ob tained in mice expressing similar cardiac levels of SERCA2a instead of SERC A2b indicate the importance of the muscle-specific SERCA2a isoform for norm al cardiac development and for the cardiac contraction-relaxation cycle.