Molecular and cellular biology of dynamics and mechanisms of myofibrillogenesis

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1.Order of appearance of myofibrillar proteins in embryonic chicken somites : Immunofluorescence microscopy revealed that in chicken cervical somites the proteins constituting myofibrils are not synthesized simultaneously and the turning-on of the gene expression of the adult isoforms was not dependent on the completion of the appearance of the major myofibrillar proteins. In situ hybridization showed that cardiac type troponin C mRNA appeared one stage (about 3 hours) earlier than its protein.2.Troponin and dystrophin in regenerating muscle : In regenerating fibers of the slow (anterior latissimus dorsi) and fast (posterior latissimus dorsi) muscles of adult chickens, embryonic troponin was expressed earlier than dystrophin and changed to its adult isoform later than the stage when dystrophin exhibited the same appearance as that in normal muscle fibers. Further, although regenerating fibers in denervated muscle switched on the expression of troponin similarly to those in innervated m … More uscle, many of them again reverted to an embryonic state. However, no difference was found in the dystrophin expression between regenerating fibers with nerves intact and those with nerves resected. Thus, it appears that the expression of troponin and dystrophin genes in regulated independently during regeneration and nerves are required for the full development of the former protein while unnecessary for the latter.3.Protein dynamics during myofibrillogenesis : Immunogold electron microscopy of cardiomyocytes microinjected with biotin-actin and -myosin showed that gold labeling was first found at the A band and the A-I junctional level of proximal myofibrils, respectively. Thus, polymerization of actin and myosin seems to occur in association with myosin filaments and with A-I junctional regions of myofibrils, respectively. Fluorescence recovery (FR) study of rhodamine-labeled muscle and nonmuscle actins microinjected into cultured chicken cardiomyocytes and fibroblasts showed that stress fibers had faster FR than myofibrils ; in myofibrils FR of muscle actin was faster than that of nonmuscle actin, and nascent myofibrils displayd different FR ; and stress fibers had similar FR at both proximal and terminal portions with both muscle and nonmuscle actins. Less