Mohammed Rabeea Hazim, Anderton Helen, Brameld John Michael, Sweetman Dylan
School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough, United Kingdom.
PLoS One. 2017 Oct 3;12(10):e0185775. doi: 10.1371/journal.pone.0185775. eCollection 2017.
Limb muscles derive from pax3 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 pax3 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 pax3 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.
肢体肌肉起源于表达Pax3的前体细胞，这些细胞从轴下肌节迁移到发育中的肢体芽中。一旦到达那里，它们就开始分化并表达肌肉决定基因，如MyoD。这个过程受多种诱导或抑制信号的组合调节，包括Fgf18、视黄酸、HGF、Notch和胰岛素样生长因子（IGFs）。众所周知，IGFs会影响肌肉发育的后期阶段，并促进增殖和分化。我们以鸡胚胎为实验模型，研究了它们在早期肢体芽肌生成中的作用。将浸泡在纯化重组IGF-I、IGF-II或特定信号通路小分子抑制剂中的珠子移植到发育中的鸡胚肢体中，结果表明，IGF-I和IGF-II均能诱导早期肌生成标志物Pax3、MyoD以及肌细胞生成素的表达。IGF-I型受体抑制剂（鬼臼毒素，PPP）和MEK抑制剂（U0126）均能阻断它们对Pax3和MyoD表达的影响。PI3K抑制剂LY294002能阻断IGF-II对Pax3 mRNA的诱导，但不能阻断IGF-I的诱导，能阻断IGF-I对MyoD mRNA的诱导，但不能阻断IGF-II的诱导。此外，FGFR/VEGFR抑制剂SU5402能阻断两种IGFs对MyoD的诱导，但对Pax3的诱导没有影响，这表明FGF或VEGF信号在它们诱导MyoD的过程中发挥作用。原位杂交证实了这一点，显示肢体成肌细胞中已知的MyoD调节因子FGF18是由IGF-I诱导的。除了通过诱导肌细胞生成素表达对肌生成后期阶段产生众所周知的影响外，IGF-I和IGF-II均能在发育中的鸡胚中诱导Pax3和MyoD表达, 表明它们也调节肌生成的早期阶段。数据表明，IGFs对通过PI3K的IGF1R信号转导可能有略微不同的影响，并且它们对MyoD表达的刺激作用可能是间接的，可能是通过诱导FGF18表达。
