Developmental and muscle-specific regulation of avian fast skeletal troponin T isoform expression by mRNA splicing.

We have investigated the developmental regulation of the avian fast skeletal muscle troponin T (TnTf) gene of the Japanese quail. Sequence analysis of troponin T mRNA, cDNA clones, and a genomic DNA segment demonstrate that the avian, fast skeletal TnTf protein isoforms are produced from a single gene. This TnTf gene is expressed in skeletal muscle, but not in adult cardiac muscles or in non-muscle tissues. In addition to known TnT isoforms, three new isoforms of TnT are described. These isoforms arise by regulated alternative RNA splicing of exons in the 5' and 3' regions of TnTf transcripts. Alternative splicing of the 5' TnTf exons involves splicing of multiple exons in different combinations (i.e. not mutually exclusive), whereas 3' alternative splicing involves mutually exclusive splice choices between two exons (alpha or beta exons). S1 nuclease protection and primer extension analyses show that alternative splicing of both 5' and 3' exons is precisely regulated and coordinated in physiologically different striated muscles, which express distinct, restricted combinations of 5' and 3' alternatively spliced exons in mRNA transcripts. In contrast, different embryonic muscles and clonal embryonic myoblast cultures coexpress the 3' alternative splice choices. This indicates that alternative splicing of TnTf mRNAs is controlled in different adult muscles by specific trans factors, and not by the restricted expression of different spliced forms in different embryonic myoblast lineages. Comparison of TnTf isoform expression in quail and chicken flight muscle (Wilkinson, J. M., Moir, A. J., and Waterfield, M. D. (1984) Eur. J. Biochem. 143, 47-56) to TnTf isoforms of the rat (Breitbart, R. E., and Nadal-Ginard, B. (1986) J. Mol. Biol. 188, 313-324), and rabbit (Pearlstone, J. R., Carpenter, M. R., and Smillie, M. B. (1976) Proc. Natl. Acad. Sci. U. S. A. 73, 1902-1906) indicates that the avian gene contains an additional exon(s) not present in mammalian genes. The alternative exon sequences TnTf mRNAs expressed in anatomically distinct quail muscles can be correlated with sequences in TnTf protein isoforms in these chicken muscles. Thus, the regulated splicing of alternative exons in TnT transcripts, and not selective translation of stochastically spliced TnT mRNAs, regulates TnTf isoform expression in specific muscles.