Constitutive activation of sonic hedgehog signaling in the chicken mutant talpid2: Shh-independent outgrowth and polarizing activity

Abstract

We have examined the developmental properties of the polydactylous chicken mutant, talpid2. Ptc, Gli1, Bmp2, Hoxd13, and Fgf4 are expressed throughout the anteroposterior axis of the mutant limb bud, despite normal Shh expression. The expression of Gli3, Ihh, and Dhh appears to be normal, suggesting that the Shh signaling pathway is constitutively active in talpid2 mutants. We show that preaxial talpid2 limb bud mesoderm has polarizing activity in the absence of detectable Shh mRNA. When the postaxial talpid2 limb bud (including all Shh-expressing cells) is removed, the preaxial cells reform a normal-shaped talpid2 limb bud (regulate). However, a Shh-expressing region (zone of polarizing activity) does not reform; nevertheless Fgf4 expression in the apical ectodermal ridge is maintained. Such reformed talpid2 limb buds develop complete talpid2 limbs. After similar treatment, normal limb buds downregulate Fgf4, the preaxial cells do not regulate, and a truncated anteroposterior deficient limb forms. In talpid2 limbs, distal outgrowth is independent of Shh and correlates with Fgf4, but not Fgf8, expression by the apical ectodermal ridge. We propose a model for talpid2 in which leaky activation of the Shh signaling pathway occurs in the absence of Shh ligand.