COLOCALIZATION OF BFGF AND THE MYOGENIC REGULATORY GENE MYOGENIN IN DYSTROPHIC MDX-MUSCLE PRECURSORS AND YOUNG MYOTUBES IN-VIVO

Tissue culture studies using muscle cell lines suggest that in additio n to mitogenic effects, fibroblast growth factors (FGF) inhibit skelet al muscle differentiation and the expression of members of a family of muscle-specific regulatory genes including MyoD and myogenin. We exam ined the possible coexpression of bFGF and myogenin by tandem in situ hybridization (detecting mRNA) and immunocytochemistry studies (detect ing protein) to determine whether myogenic cells in vivo endogenously produce bFGF. Mdx mouse muscle, which shows characteristic dystrophic damage and regeneration, demonstrated mononuclear cells containing myo genin and bFGF transcripts in similar regions of adjacent sections of focal degeneration and repair, particularly near recent segmental fibe r damage. Using immunocytochemistry and in situ hybridization concurre ntly on the same sections, bFGF protein and myogenin mRNA were colocal ized in both muscle precursors and new myotubes. The in vivo results w ere confirmed in vitro using primary explant cultures of mdx muscle. A pproximately one-half of mononuclear cells in vivo were myogenic by th e criterion of myogenin mRNA expression. Both myogenin and bFGF mRNAs were also colocalized with bFGF protein, indicating endogenous express ion of bFGF in a subpopulation of myogenic cells. Small numbers of myo genic mononuclear cells were differentiated, as determined by the pres ence of developmental myosin heavy chain protein (DevMHC). These cells and new myotubes also colocalized myogenin, DevMHC, and bFGF. Since b FGF and myogenin are colocalized in mpc and myotubes in vivo and in vi tro, endogenous expression of bFGF is not mutually exclusive of myogen ic regulatory gene expression, either before or after differentiation of the skeletal muscle phenotype. Such features of coexpression sugges t an important and complex role for bFGF in muscle regeneration in viv o. (C) 1995 Academic Press, Inc.