TEMPORAL EXPRESSION OF REGULATORY AND STRUCTURAL MUSCLE PROTEINS DURING MYOGENESIS OF SATELLITE CELLS ON ISOLATED ADULT-RAT FIBERS

Myogenic precursors in adult skeletal muscle (satellite cells) are mit otically quiescent but can proliferate in response to a variety of str esses including muscle injury. To gain further understanding of adult myoblasts, we analyzed myogenesis of satellite cells on intact fibers isolated from adult rat muscle. In this culture model, satellite cells are maintained in their in situ position underneath the fiber basemen t membrane. In the present study patterns of satellite cell proliferat ion, expression of myogenic regulatory factor proteins, and expression of differentiation-specific, cytoskeletal proteins were determined, v ia immunohistochemistry of cultured fibers. The temporal appearance an d the numbers of cells positive for proliferating cell nuclear antigen (PCNA) or for MyoD were similar, suggesting that MyoD is present in d etectable amounts in proliferating but not quiescent satellite cells. Satellite cells positive for myogenin, cu-smooth muscle actin (alpha S Mactin), or developmental sarcomeric myosin (DEVmyosin) appeared follo wing the decline in PCNA and MyoD expression. However, expression of m yogenin and alpha SMactin was transient, while DEVmyosin expression wa s continuously maintained. Moreover, the number of DEVmyosin+ cells wa s only half of the number of myogenin+ or alpha SMactin+ cells-indicat ing, perhaps, that only 50% of the satellite cell descendants entered the phase of terminal differentiation. We further determined that the number of proliferating satellite cells can be modulated by basic FGF but the overall schedule of cell cycle entry, proliferation, different iation, and temporal expression of regulatory and structural proteins was unaffected. We thus conclude that satellite cells conform to a hig hly coordinated program when undergoing myogenesis at their native pos ition along the muscle fiber. (C) 1994 Academic Press, Inc.