Mouse limbs expressing only the G1i3 repressor resemble those of Sonic hedgehog mutants

Anterioposterior vertebrate limb patterning is controlled by opposing action between Sonic Hedgehog (Shh) and the G1i3 transcriptional repressor. Unexpectedly, Gli3(Delta 699) mutant mice, which are thought to express only a G1i3 repressor and not the full-length activator, exhibit limb phenotypes inconsistent with those of Shh mutant mice. Therefore, it remains debatable whether Shh patterns the anterioposterior limb primarily by inhibiting generation of the G1i3 repressor. However, one caveat is that Gli3(Delta 699) may not be as potent as the natural form of Gli3 repressor because of the nature of the mutant allele. In the present study, we created a conditional Gli3 mutant allele that exclusively expresses G1i3 repressor in the presence of Cre recombinase. Using this mutant, we show that the phenotypes of mouse limbs expressing only the G1i3 repressor exhibit no or single digit, resembling those of Shh mutant limbs. Consistent with the limb phenotypes, the expression of genes dependent on Shh signaling is also inhibited in both mutants. This inhibition by the G1i3 repressor is independent of Shh. Thus, our study clarifies the current controversy and provides important genetic evidence to support the hypothesis that Shh patterns the anterioposterior limb primarily through the inhibition of Gli3 repressor formation. (C) 2013 Elsevier Inc. All rights reserved.