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, talpid(2). 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 talpid(2) mutants. We show that preaxial talpid(2) limb bud mesoderm has polarizing activity in the absence of detectable Shh mRNA. When the postaxial talpid(2) limb bud (including all Shh-expressing cells) is removed, the preaxial cells reform a normal-shaped talpid(2) 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 talpid(2) limb buds develop complete talpid(2) limbs. After similar treatment, normal limb buds downregulate Fgf4, the preaxial cells do not regulate, and a truncated anteroposterior deficient limb forms. In talpid(2) 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 talpid(2) in which leaky activation of the Shh signaling pathway occurs in the absence of Shh ligand.

Replicated anterior zeugopod (raz): a polydactylous mouse mutant with lowered Shh signaling in the limb bud

“…Support for the concept of a role for Gli3 repressor/activator distribution within the limb bud as a mediator of AP patterning can also be found in the studies of talpid chick mutants that lead to an autopod with 7-10 similar digits with no recognizable AP polarity (Hinchliffe and Ede, 1967;Abbott et al, 1959). In contrast to our predictions for the raz/raz limb bud, the talpid 2 mutant has a very high level of Gli3 activator throughout the limb bud correlated with digits that all have posterior character (Caruccio et al, 1999). Conversely, the talpid 3 mutant would be predicted to have a uniform Gli3 repressor/activator ratio throughout the limb bud, as Gli3 expression is expanded into the posterior mesenchyme and Ptch expression is low and uniform throughout the limb (Lewis et al, 1999), thereby permitting Shh to diffuse throughout the limb.…”

Expression and regulation of eHAND during limb development

“…Thus, the limbs of the ta 2 mutant do not express detectable levels of eHAND. These limbs are known to have the Shh pathway constitutively activated all along the anteroposterior axis, and consequently, all the limb mesoderm is posteriorized (Caruccio et al, 1999;Wang et al, 2000). Ozd is a recently characterized chick mutant that lacks Shh activity specifically at limb level (Ros et al, 2003).…”

“…Talpid 2 (ta 2 ) is a homozygous recessive lethal mutation in the chick characterized by polysyndactyly of the limbs Fallon, 1991, 1992). The molecular analysis of the ta 2 mutant limb indicates a constitutive activation of the Shh pathway all along the anteroposterior axis of the mesoderm (Caruccio et al, 1999). Thus, we hypothesized that eHAND expression should be reduced or absent in the ta 2 limbs.…”

Estimating the spatiotemporal pattern of volumetric growth rate from fate maps in chick limb development

“…FGFs from AER and SHH from ZPA are known to promote the cell proliferation (Caruccio et al, 1999;Lu et al, 2005;Reiter and Solursh, 1982;Moon and Capecchi, 2000;Boulet et al, 2004). Switching of the volumetric growth pattern from the spatially uniform one to the distally biased one may reflects the timing after which such proliferationpromoting factors work effectively.…”

“…Imbalance of these cellular responses leads to an improper spatiotemporal pattern of volumetric growth rate, which results in morphological anomaly of a limb. For example, in the mutant in which sonic hedgehog (SHH) signaling is constitutively activated or fibroblast growth factors (FGFs) are overexpressed at the apical ectodermal ridge (AER), the limb bud shape becomes wide or balloon-like because of the modified regulation of cell proliferation in the mesenchyme (Caruccio et al, 1999;Lu et al, 2005). In addition, a mutant mouse that lacks the central portion of the AER has a bifurcated limb shape (Niederreither et al, 2002).…”