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 addition to mitogenic effects, fibroblast growth factors (FGF) inhibit skeletal muscle differentiation and the expression of members of a family of muscle-specific regulatory genes including MyoD and myogenin. We examined the possible coexpression of bFGF and myogenin by tandem in situ hybridization (detecting mRNA) and immunocytochemistry studies (detecting 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 myogenin and bFGF transcripts in similar regions of adjacent sections of focal degeneration and repair, particularly near recent segmental fiber damage. Using immunocytochemistry and in situ hybridization concurrently on the same sections, bFGF protein and myogenin mRNA were colocalized in both muscle precursors and new myotubes. The in vivo results were confirmed in vitro using primary explant cultures of mdx muscle. Approximately one-half of mononuclear cells in vivo were myogenic by the criterion of myogenin mRNA expression. Both myogenin and bFGF mRNAs were also colocalized with bFGF protein, indicating endogenous expression of bFGF in a subpopulation of myogenic cells. Small numbers of myogenic mononuclear cells were differentiated, as determined by the presence of developmental myosin heavy chain protein (DevMHC). These cells and new myotubes also colocalized myogenin, DevMHC, and bFGF. Since bFGF and myogenin are colocalized in mpec and myotubes in vivo and in vitro, endogenous expression of bFGF is not mutually exclusive of myogenic regulatory gene expression, either before or after differentiation of the skeletal muscle phenotype. Such features of coexpression suggest an important and complex role for bFGF in muscle regeneration in vivo.