Loss of FGF receptor 1 signaling reduces skeletal muscle mass and disruptsmyofiber organization in the developing limb

The identities of extracellular growth factors that regulate skeletal muscl e development in vivo are largely unknown. We asked if FGFs, which act as r epressors of myogenesis in culture, play a similar role in vivo by ectopica lly expressing in the developing limb a truncated FGF receptor 1 (dnFGFR1) that acts as a dominant negative mutant. Hind limbs and the adjacent somite s of Hamburger and Hamilton (HH) stage 17 chickens were infected with a rep lication-competent RCAS virus encoding dnFGFR1. By ED5, the virus had sprea d extensively within the limb and the adjacent somites with little rostral or caudal expansion of the infection along the axial midline. Viral infecti on and mutant receptor expression were coincident as revealed by the distri bution of a viral coat protein and an HA epitope tag present on the carboxy terminus of dnFGFR1. Within 48 h following injection of dnFGFR1, we could detect no obvious changes in skeletal muscle precursor cell migration into the hind limb as compared to control limbs infected with an empty RCAN viru s. However, by 3 days following infection of RCAS-dnFGFR1 virus, the level of skeletal muscle-specific myosin heavy chain was decreased and the expres sion pattern altered, suggesting disruption of skeletal muscle development. Two Striking muscular phenotypes were observed in dnFGFR1-expressing limbs , including an average loss of 30% in skeletal muscle wet weight and a 50% decrease in myofiber density. At all ages examined the loss of skeletal mus cle mass was accompanied by a loss of myoblasts and an unexpected concomita nt loss of fibroblasts. Consistent with these observations, explants of inf ected cells revealed a reduction in the number of myonuclei in myotubes. Al though the myofiber density per unit area was decreased over 50% compared t o controls there were no detectable effects on myofiber diameter. The loss in myofiber density was, however, accompanied by an increase in the space s urrounding individual myofibers and a generalized loss of myofiber integrit y. It is noteworthy that long-bone development was unaffected by RCAS-dnFGF R1 infection, suggesting that FGFR2 and FGFR3 signaling was not disrupted. Our data provide conclusive evidence that FGFR1 signaling is necessary to m aintain myoblast number and plays a role in myofiber organization, (C) 2000 Academic Press.