Constitutive activation of sonic hedgehog signaling in the chicken mutant talpid(2): Shh-independent outgrowth and polarizing activity

We have examined the developmental properties of the polydactylous chicken mutant, talpid(2). Ptc, Gli1, Bmp2, Hoxd13, and Fgf4 are expressed througho ut the anteroposterior axis of the mutant limb bud, despite normal Shh expr ession. The expression of Gli3, Ihh, and Dhh appears to be normal, suggesti ng that the Shh signaling pathway is constitutively active in talpid(2) mut ants. We show that preaxial talpid(2) limb bud mesoderm has polarizing acti vity in the absence of detectable Shh mRNA. When the postaxial talpid(2) li mb bud (including all Shh-expressing cells) is removed, the preaxial cells reform a normal-shaped talpid(2) limb bud (regulate). However, a Shh-expres sing region (zone of polarizing activity) does not reform; nevertheless Fgf 4 expression in the apical ectodermal ridge is maintained. Such reformed ta lpid(2) limb buds develop complete talpid(2) limbs. After similar treatment , normal limb buds downregulate Fgf4, the preaxial cells do not regulate, a nd a truncated anteroposterior deficient limb forms. In talpid(2) limbs, di stal outgrowth is independent of Shh and correlates with Fgf4, but not Fgf8 , expression by the apical ectodermal ridge. We propose a model for talpid( 2) in which leaky activation of the Shh signaling pathway occurs in the abs ence of Shh ligand. (C) 1999 Academic Press.