Robustness, or the ability of a system to buffer against environmental and genetic variations, is often conferred by feedback mechanisms. In the vertebrate limb, fibroblast growth factor (FGF) signaling from the apical ectodermal ridge (AER), Sonic Hedgehog (Shh) signaling from the zone of polarizing activity, and bone morphogenetic protein (BMP) signaling cooperate to direct limb outgrowth and pattern features such as the digits. FGF signaling activates Shh signaling, which inhibits BMP signaling through the BMP antagonist Gremlin1 (GREM1), whereas BMP signaling restricts the range of FGF signaling, thus creating a SHH-GREM1-FGF feedback loop. A second feedback loop is established by the activation of Grem1 expression by BMP signaling. Bénazet et al. investigated the mechanism of these feedback loops and determined that GREM1 primarily antagonizes BMP4 but may also antagonize other BMP ligands in the limb. Expression of the BMP-regulated gene Msx2 increased and expression of Shh and Fgf decreased in the limb buds of Grem1 loss-of-function mutants. Through genetic manipulations and bead implantation experiments, the authors determined that BMP4 signaling initiated Grem1 expression within 2 hours but that the SHH-GREM1-FGF feedback loop required about 12 hours. Signaling dynamics data were used to build a mathematical model that identified GREM1 as the critical node in the limb patterning network and predicted an early requirement for BMP4 in limb bud development. By removing BMP4 functions at different stages of development, the authors found that BMP4 was indeed required early in limb bud development to modulate AER formation and initiate Grem1 expression. The model suggests that these interconnected feedback loops are self-regulating and may thus lend robustness to the vertebrate limb patterning system.
J.-D. Bénazet, M. Bischofberger, E. Tiecke, A. Gonçalves, J. F. Martin, A. Zuniga, F. Naef, R. Zeller, A self-regulatory system of interlinked signaling feedback loops controls mouse limb patterning. Science 323, 1050–1053 (2009). [Abstract] [Full Text]