DEVELOPMENTAL REGULATION OF TROPONIN I ISOFORM GENES IN STRIATED MUSCLES OF TRANSGENIC MICE

摘要

The differentiation and diversification of striated muscle is a complex process involving numerous temporal and spatial alterations in the pattern of contractile protein isoform gene expression. In order to gain insight into the regulation of contractile protein isoform changes during skeletal and cardiac muscle formation, the expression of a transgene comprising a chloramphenicol acetyltransferase (CAT) reporter gene linked with sequences from -4200 to +12 of the human slow skeletal troponin I (TnI(s)) gene, and all three endogenous mouse troponin I (TnI) isoform genes, was investigated in embryonic, neonatal, and postnatal mice. The -4200TnI(s)CAT transgene was properly activated in the limb and trunk skeletal muscle primordia and the early embryonic atrium and ventricle of the heart. Along with the endogenous mouse TnI(s) gene, expression of the CAT transgene began to segregate into the presumptive slow-twitch myofibers at late fetal stages and expression declined in the neonatal and postnatal heart except for the conductive tissues, in which expression persisted into adulthood. However, expression of the CAT transgene during development did not completely follow the endogenous mouse TnI(s) gene. The expression of the CAT transgene was aberrantly low in the embryonic cardiac outflow tract and the ventricles of the fetal heart. In addition to its expression in striated muscles, the transgene was expressed aberrantly in the primordial axial skeleton. We conclude that the upstream sequences from the human TnI(s) gene contain sufficient regulatory information to confer appropriate transgene expression during the early differentiation of skeletal muscles and during the establishment of fiber type upon the maturation of myofibers. However, additional regulatory elements are likely to be required for correct temporal and spatial regulation in the heart and semitic mesoderm during development. In vitro DNA transfection of cultured skeletal and cardiac muscle cells identified a cell type-specific enhancer element within the first intron of the TnI(s) gene whose absence in the transgene may account for the aberrant expression observed in vivo. In addition, we provide the first evidence that the fast-twitch skeletal muscle isoform of troponin I, TnI(f), is transiently expressed during early cardiac muscle development. (C) 1995 Academic Press, Inc. [References: 62]