Signal relay by BMP antagonism controls the SHH/FGF4 feedback loop in vertebrate limb buds

Outgrowth and patterning of the vertebrate limb are controlled by reciproca l interactions between the posterior mesenchyme (polarizing region) and a s pecialized ectodermal structure, the apical ectodermal ridge (AER)(1). Soni c hedgehog (SHH) signalling by the polarizing region modulates fibroblast g rowth factor (FGF)4 signalling by the posterior AER, which in turn maintain s the polarizing region (SHH/FGF4 feedback loop)(2,3). Here we report that the secreted bone-morphogenetic-protein (BMP) antagonist Gremlin(4) relays the SHH signal from the polarizing region to the AER. Mesenchymal Gremlin e xpression is lost in limb buds of mouse embryos homozygous for the limb def ormity (Id) mutation, which disrupts establishment of the SHH/FGF4 feedback loop(5-7). Grafting Gremlin-expressing cells into Id mutant limb buds resc ues Fgf4 expression and restores the SHH/FGF4 feedback loop. Analysis of Sh h-null mutant embryos(8,9) reveals that SHH signalling is required for main tenance of Gremlin and Formin (the gene disrupted by the ld mutations)(10,1 1). In contrast, Formin, Gremlin and Fgf4 activation are independent of SHH signalling. This study uncovers the cascade by which the SHH signal is rel ayed from the posterior mesenchyme to the AER and establishes that Formin-d ependent activation of the BMP antagonist Gremlin is sufficient to induce F gf4 and establish the SHH/FGF4 feedback loop.