Misexpression of Fgf-4 in the chick limb inhibits myogenesis by down-regulating Frek expression

Skeletal muscle development involves an initial period of myoblast replication followed by a phase in which some myoblasts continue to proliferate while others undergo terminal differentiation. The latter process involves the permanent cessation of DNA synthesis, activation of muscle-specific gene expression, and fusion of single cells to generate multinucleated muscle fibres. The in vivo signals regulating the progression through all these steps remain unknown. Fibroblast growth factors (Fgfs) and Fgf receptors comprise a large family whose members have been shown to play multiple roles in the development of skeletal muscle in vitro. Exogenously applied Fgfs are able to stimulate proliferation and suppress myogenic differentiation in cell culture. We sought to determine the role played by Fgf-4 during limb myogenesis in vivo. Fgf-4 transcripts are located at both extremities of myotubes whereas the mRNAs of one of the Fgf receptors, Frek, are detected in mononucleated proliferating myoblasts surrounding the multinucleated fibres. Overexpression of mouse Fgf-4 (mFgf-4) using a replication-competent retrovirus, RCAS, leads to a down-regulation of muscle markers followed by an inhibition of terminal differentiation in limb muscles. Using quail/chick transplantations we were able to follow the muscle cells and found a dramatic decrease in their number after exposure to mFgf-4. Interestingly ectopic mFgf-4 down-regulates Frek transcripts in limb muscle areas. We conclude that overexpression of mFgf-4 inhibits myoblast proliferation, probably by down-regulating Frek mRNAs. This suggests a role for Fgf-4, located at the extremities of the myotubes, where it could be responsible for the absence of Frek mRNA in the muscle fibre.