IGFs, insulin, Shh, bFGF, and TGF-beta 1 interact synergistically to promote somite myogenesis in vitro

Studies from our group and others have shown that in vitro somite myogenesi s is regulated by neural tube and notochord factors including Wnt, Sonic he dgehog (Shh), and basic fibroblast growth factor (bFGF) together with trans forming growth factor-beta 1 (TGF-beta 1). In this study we report that ins ulin and insulin-like growth factors I and II (IGF-I and -II) also promote myogenesis in explant cultures containing single somites or somite-sized pi eces of segmental plate mesoderm from 2-day (stage 10-14) chicken embryos a nd that the combination of insulin/IGFs with bFGF plus TGF-beta 1 promotes even higher levels of myogenesis. We also found that Shh promotes myogenesi s in this in vitro system and that Shh interacts synergistically with insul in/IGFs to promote high levels of myogenesis. RT-PCR analysis detected insu lin, IGF-II, insulin receptor, and IGF receptor mRNAs in both the neural tu be and the somites, whereas IGF-I transcripts were detected in entire embry os but not in the neural tube or somites. Treatment of somite-neural tube c ocultures with anti-insulin, anti-IGF-II, anti-insulin receptor, or anti-IG F receptor blocking antibodies caused a significant decrease in myogenesis. These results are consistent with the hypothesis that systemic IGF-I as we ll as insulin and IGF-II secreted by the neural tube act as additional earl y myogenic signals during embryogenesis. Further studies indicate that insu lin, IGFs, bFGF, and Shh also stimulate somite cell proliferation and influ ence apoptosis. (C) 2000 Academic Press.