Molecular and Physiological Effects of a-Tropomyosin Ablation in the Mouse

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 a-TM, the murine a-TM gene was disrupted by homologous recombination. Homozygous a-TM null mice are embryonic lethal, dying between 8 and 11.5 days post coitum. Mice that are heterozygous for a-TM are viable and reproduce normally. Heterozygous knockout mouse hearts show a 50% reduction in cardiac muscle a-TM mRNA, with no compensatory increase in transcript levels by striated muscle b-TM or TM-30 isoforms. Surprisingly, this reduction in a-TM mRNA levels in heterozygous mice is not reflected at the protein level, where normal amounts of striated muscle a-TM protein are produced and integrated in the myofibril. Quantification of a-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 a-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 a-TM mice and control mice. The present study suggests that translational regulation plays a major role in the control of TM expression. (Circ Res.