Tropomyosin 3 increases striated muscle isoform diversity.

Tropomyosin (TM), a component of the thin filament of the sarcomere, is encoded by a four-member multigene family: alpha-TM, beta-TM, TPM 3, and TPM 4. The alpha-TM, beta-TM, and TPM 3 genes each utilize an alternative splicing mechanism to encode a striated muscle isoform. Although the alpha-TM and beta-TM striated muscle isoforms are well characterized, relatively little is known about the TPM 3 isoform. We cloned and sequenced the murine TPM 3 cDNA and found that it exhibits a 93% nucleotide homology and 99% amino acid homology to the human sequence. Results show that, unlike humans, TPM 3 is not expressed in any developmental stage of murine hearts. TPM 3 message is expressed in slow-twitch skeletal muscle but is not found in representative fast-twitch musculature. The soleus, a representative slow-twitch muscle, expresses transcript levels of 65% beta-TM, 15% alpha-TM, and 20% TPM 3, but the TPM 3 protein accounts for approximately 31% of the total striated tropomyosin in slow-twitch muscle. In fast-twitch muscle, alpha-TM comprises 71% of the total striated muscle TM protein, and beta-TM comprises 29%. The results demonstrate that a translational mechanism regulates the production of the TM proteins, with beta-TM message not being efficiently translated. The unique distribution pattern of TPM 3 adds to the diversity of the tropomyosin family and strongly suggests functional significance for the different striated muscle TM isoforms.

Mouse model of a familial hypertrophic cardiomyopathy mutation in alpha-tropomyosin manifests cardiac dysfunction.

To investigate the functional consequences of a tropomyosin (TM) mutation associated with familial hypertrophic cardiomyopathy (FHC), we generated transgenic mice that express mutant alpha-TM in the adult heart. The missense mutation, which results in the substitution of asparagine for aspartic acid at amino acid position 175, occurs in a troponin T binding region of TM. S1 nuclease mapping and Western blot analyses demonstrate that increased expression of the alpha-TM 175 transgene in different lines causes a concomitant decrease in levels of endogenous alpha-TM mRNA and protein expression. In vivo physiological analyses show a severe impairment of both contractility and relaxation in hearts of the FHC mice, with a significant change in left ventricular fractional shortening. Myofilaments that contain alpha-TM 175 demonstrate an increased activation of the thin filament through enhanced Ca2+ sensitivity of steady-state force. Histological analyses show patchy areas of mild ventricular myocyte disorganization and hypertrophy, with occasional thrombi formation in the left atria. Thus, the FHC alpha-TM transgenic mouse can serve as a model system for the examination of pathological and physiological alterations imparted through aberrant TM isoforms.