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Limb muscles derive from p ax3  expressing precursor cells that migrate from the hypaxial somite into the developing limb bud. Once there they begin to differentiate and express muscle determination genes such as  MyoD . This process is regulated by a combination of inductive or inhibitory signals including Fgf18, retinoic acid, HGF, Notch and IGFs. IGFs are well known to affect late stages of muscle development and to promote both proliferation and differentiation. We examined their roles in early stage limb bud myogenesis using chicken embryos as an experimental model. Grafting beads soaked in purified recombinant IGF-I, IGF-II or small molecule inhibitors of specific signaling pathways into developing chick embryo limbs showed that both IGF-I and IGF-II induce expression of the early stage myogenic markers p ax3  and  MyoD  as well as  myogenin . Their effects on  pax3  and  MyoD  expression were blocked by inhibitors of both the IGF type I receptor (picropodophyllotoxin, PPP) and MEK (U0126). The PI3K inhibitor LY294002 blocked IGF-II, but not IGF-I, induction of p ax3  mRNA as well as the IGF-I, but not IGF-II, induction of  MyoD  mRNA. In addition SU5402, an FGFR/ VEGFR inhibitor, blocked the induction of  MyoD  by both IGFs but had no effect on  pax3  induction, suggesting a role for FGF or VEGF signaling in their induction of  MyoD . This was confirmed by in situ hybridization showing that  FGF18 , a known regulator of  MyoD  in limb myoblasts, was induced by IGF-I. In addition to their well-known effects on later stages of myogenesis via their induction of  myogenin  expression, both IGF-I and IGF-II induced  pax3  and  MyoD  expression in developing chick embryos, indicating that they also regulate early stages of myogenesis. The data suggests that the IGFs may have slightly different effects on IGF1R signal transduction via PI3K and that their stimulatory effects on  MyoD  expression may be indirect, possibly via induction of  FGF18  expression.

Effects of insulin like growth factors on early embryonic chick limb myogenesis