Molecular and Physiological Effects of -Tropomyosin Ablation in the Mouse

From the Department of Molecular Genetics, Biochemistry, and Microbiology (P.R., D.F.W.) and the Department of Pathology and Laboratory Medicine (G.B.), University of Cincinnati (Ohio) Medical Center; the Department of Medical Physiology (M.M.), Texas A&M University Health Science Center, College Station; the Division of Molecular Cardiovascular Biology (T.H.), Children's Hospital Medical Center, Cincinnati, Ohio; and the Department of Physiology and Biophysics (B.M.W., C.E., R.J.S.), University of Illinois at Chicago.

Correspondence to Dr David F. Wieczorek, Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati Medical Center, Cincinnati, OH 45267-0524. E-mail wieczodf{at}ucbeh.san.uc.edu

Abstract—Tropomyosin (TM) is an integral component of the thin filament in muscle fibers and is involved in regulating actin-myosin interactions. TM is encoded by a family of four alternatively spliced genes that display highly conserved nucleotide and amino acid sequences. To assess the functional and developmental significance of -TM, the murine -TM gene was disrupted by homologous recombination. Homozygous -TM null mice are embryonic lethal, dying between 8 and 11.5 days post coitum. Mice that are heterozygous for -TM are viable and reproduce normally. Heterozygous knockout mouse hearts show a 50% reduction in cardiac muscle -TM mRNA, with no compensatory increase in transcript levels by striated muscle ß-TM or TM-30 isoforms. Surprisingly, this reduction in -TM mRNA levels in heterozygous mice is not reflected at the protein level, where normal amounts of striated muscle -TM protein are produced and integrated in the myofibril. Quantification of -TM mRNA bound in polysomal fractions reveals that both wild-type and heterozygous knockout animals have similar levels. These data suggest that a change in steady-state level of -TM mRNA does not affect the relative amount of mRNA translated and amount of protein synthesized. Physiological analyses of myocardial and myofilament function show no differences between heterozygous -TM mice and control mice. The present study suggests that translational regulation plays a major role in the control of TM expression.

Key Words: tropomyosin • knockout mouse • translational regulation

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