THE FUNCTIONAL SIGNIFICANCE OF THE STRIATED ISOFORM OF TROPOMYOSIN 3 IN NORMAL AND PATHOLOGICAL STATES
Striated muscle tropomyosin, a thin filament protein of the sarcomere, has three isoforms: ?-TM, ?-TM and TPM 3. Although the ?-TM and ?-TM isoforms are well characterized, little was known about TPM 3. We cloned and sequenced the murine TPM 3 cDNA. It exhibits a 93% nucleotide homology and 99% amino acid homology to the human sequence. TPM 3 is not endogenously expressed in murine hearts; however, it is found in slow-twitch but not in fast-twitch musculature. Results suggest a molecular mechanism regulates the stability or production of TM RNAs and proteins, with ?-TM message either not as efficiently translated or as stable as ?-TM and TPM 3. In order to determine the functional significance of TPM 3, we generated six transgenic mouse lines that produce varying levels of TPM 3 message in the heart. TPM 3 protein accounts for 40-60% of the striated muscle tropomyosin. The TG mice have normal life spans and exhibit no gross morphological aberrations. Physiological assessment of TG TPM 3 mice reveals hyperdynamic effects on systolic and diastolic function, decreased calcium sensitivity in force generation and attenuation of length dependent calcium activation. This is the first work to demonstrate a correlation between ?-TM and TPM 3 content and cardiac performance. A mutant TPM 3 (Met8Arg), linked to nemaline myopathy, was overexpressed in transgenic mouse hearts. Results show that high levels of transgene incorporation are lethal, and only one line of mice survived. In the surviving line, mutant RNA is expressed at levels comparable to the wild-type TPM 3 overexpressers, with the protein accounting for ~40% of the total striated tropomyosin. The mutant mice have normal life spans, but some mice have a minor heart phenotype. Rods, typically seen in nemaline patients, are not found in any of the animals. Physiologically, there is a mild hyperdynamic effect that is intermediate to NTG mice and the wild-type TG mice. If mutant TPM 3 levels are increased through TGxTG matings, the situation is lethal. The mice die before 14 days of age postpartum, with their hearts exhibiting a dilated phenotype and have thrombi formation. Thus, this mutation is only tolerated at low levels in murine hearts.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:tropomyosin tpm3 nemaline muscle rods
Date of Publication:01/01/2001