The Shh-independent activator function of the full-length Gli3 protein and its role in vertebrate limb digit patterning

Abstract

Anterior-posterior (A/P) limb patterning in vertebrates is determined by the counteraction between the Sonic Hedgehog (Shh) and the Gli3 transcription factor. Shh exerts its effect on Gli3 by regulating the full-length Gli3 protein processing to generate a Gli3 repressor gradient along the A/P axis of the limb. However, it is not clear whether the full-length Gli3 is an activator in vivo and plays any role in the limb patterning. Here we show that mouse limbs expressing only a Gli3 repressor form exhibit mild polysyndactyly and a partial loss of digit identity, while limbs expressing only a full-length Gli3 protein display severe polysyndactyly and a complete loss of digit identity. Interestingly, when the full-length Gli3 and the repressor are equally expressed in the limb, the digit patterning is overall normal except for an extra anterior digit. Furthermore, in the presence of one Gli3 wild type allele, a Gli3 mutant allele that expresses only the full-length form can rescue the Shh mutant digit phenotype to a great extent. The full-length Gli3 protein can also activate Shh target gene expression without Shh. Thus, our data indicate that the full-length Gli3 protein is an activator in vivo and that the ratio of the Gli3 activator to repressor, but neither the Gli3 repressor gradient nor the Gli3 activator/repressor ratio gradient, determines limb digit patterning.

Small Ubiquitin-like Modifier (SUMO) Modification Inhibits GLI2 Protein Transcriptional Activity in Vitro and in Vivo

“…In contrast, little is known about how GLI2 and GLI3 are activated by HH signaling. Our previous studies showed that both GLI2 and GLI3 mutants that are not phosphorylatable at the first four PKA sites can up-regulate SHH target genes in the absence of SHH in vivo, suggesting that PKA phosphorylation is at least partially responsible for their inactivation (16,22). In supporting this view, a recent study showed that GLI2 is activated in PKA mutant mice (23).…”

Ftm is a novel basal body protein of cilia involved in Shh signalling

“…Although our western blot analysis revealed equal levels of Gli3-83 in Ftm -/-and wild-type embryos, the ratio of Gli3-190 to Gli3-83 is changed due to an increased amount of . A recent study by Wang et al (Wang et al, 2007) supports this interpretation.…”

Gli3 coordinates three-dimensional patterning and growth of the tectum and cerebellum by integrating Shh and Fgf8 signaling

“…In the absence of Shh activity, Gli3 is processed into an N-terminal repressor form that suppresses Shh target genes (Hu et al, 2006;Wang et al, 2000). Shh signaling attenuates the level of Gli3R, and full-length Gli3 can act as a weak activator (Gli3A) when the concentration of Shh is high (Bai et al, 2004;Dai et al, 1999;Wang et al, 2007). Gli3, however, is also transcriptionally downregulated in cells receiving high levels of Shh.…”