Published online before print September 13, 2001, doi: 10.1161/hh2101.098466
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© 2001 American Heart Association, Inc.
Integrative Physiology |
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
From the Laboratories of Physiology (M.V.H., P.V., F.W.) and Cardiology (G.A., K.S.) and the Center for Transgene Technology and Gene Therapy (S.H., M.D., D.C., P.C.), Flanders Interuniversity Institute for Biotechnology, Katholieke Universiteit Leuven, Belgium; GenomatixUSA (T.R.), Cincinnati, Ohio; Department of Physiology and Cell Biology (M.P.), Ohio State University College of Medicine and Health Sciences, Columbus, Ohio; and Département de Physiologie (B.A., J.L.), Université Catholique de Louvain, Brussels, Belgium.
Correspondence to Dr Frank Wuytack, Laboratory of Physiology, Department of Medicine, KU Leuven, Campus Gasthuisberg, Herestraat 49, B-3000, Leuven, Belgium. E-mail frank.wuytack{at}med.kuleuven.ac.be
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 specificity is required for normal cardiac function. Gene targeting in mice successfully 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 expression 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 
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 activity and developed a mild compensatory concentric cardiac hypertrophy with impaired cardiac contractility and relaxation, but preserved ß-adrenergic response. Ca2+ uptake levels in SERCA2a-/- cardiac homogenates were reduced by 50%. In isolated cells, relaxation and Ca2+ removal by the SR were significantly reduced. Comparison of our data with those obtained in mice expressing similar cardiac levels of SERCA2a instead of SERCA2b indicate the importance of the muscle-specific SERCA2a isoform for normal cardiac development and for the cardiac contraction-relaxation cycle.
Key Words: Ca2+-transporting ATPase • alternative splicing • gene targeting • cardiovascular abnormalities • cardiomegaly
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