Tropomyosin expression and dynamics in developing avian embryonic muscles

The expression of striated muscle proteins occurs early in the developing embryo in the somites and forming heart. A major component of the assembling myofibrils is the actin-binding protein tropomyosin. In vertebrates, there are four genes for tropomyosin (TM), each of which can be alternatively spliced. TPM1 can generate at least 10 different isoforms including the striated muscle-specific TPM1alpha and TPM1kappa. We have undertaken a detailed study of the expression of various TM isoforms in 2-day-old (stage HH 10-12; 33 h) heart and somites, the progenitor of future skeletal muscles. Both TPM1alpha and TPM1kappa are expressed transiently in embryonic heart while TPM1alpha is expressed in somites. Both RT-PCR and in situ hybridization data suggest that TPM1kappa is expressed in embryonic heart whereas TPM1alpha is expressed in embryonic heart, and also in the branchial arch region of somites, and in the somites. Photobleaching studies of Yellow Fluorescent Protein-TPM1alpha and -TPM1kappa expressed in cultured avian cardiomyocytes revealed that the dynamics of the two probes was the same in both premyofibrils and in mature myofibrils. This was in sharp contrast to skeletal muscle cells in which the fluorescent proteins were more dynamic in premyofibrils. We speculate that the differences in the two muscles is due to the appearance of nebulin in the skeletal myocytes premyofibrils transform into mature myofibrils.