Direct interaction with Hoxd proteins reverses Gli3-repressor function to promote digit formation downstream of Shh

Abstract

Sonic hedgehog (Shh) signaling regulates both digit number and identity,but how different distinct digit types (identities) are specified remains unclear. Shh regulates digit formation largely by preventing cleavage of the Gli3 transcription factor to a repressor form that shuts off expression of Shh target genes. The functionally redundant 5′Hoxd genes regulate digit pattern downstream of Shh and Gli3, through as yet unknown targets. Enforced expression of any of several 5′Hoxd genes causes polydactyly of different distinct digit types with posterior transformations in a Gli3(+) background, whereas, in Gli3 null limbs,polydactylous digits are all similar, short and dysmorphic, even though endogenous 5′Hoxd genes are broadly misexpressed. We show that Hoxd12 interacts genetically and physically with Gli3, and can convert the Gli3 repressor into an activator of Shh target genes. Several 5′Hoxd genes,expressed differentially across the limb bud, interact physically with Gli3. We propose that a varying [Gli3]:[total Hoxd] ratio across the limb bud leads to differential activation of Gli3 target genes and contributes to the regulation of digit pattern. The resulting altered balance between `effective'Gli3 activating and repressing functions may also serve to extend the Shh activity gradient spatially or temporally.

Molecular genetic cascades for external genitalia formation: An emerging organogenesis program

“…Searching for the Hox downstream genes utilizing target chromosome isolation is in progress (Morgan et al, 2003). These strategies include Hox interaction partner assays during limb formation (Suzuki et al, 2003b;Knosp et al, 2004;Chen et al, 2004).…”

Anteroposterior patterning in the limb and digit specification: Contribution of mouse genetics

“…It must also depend on the properties of the protein encoded by the 5Ј-most genes, because expression of Hoxd1 to Hoxd9, which normally takes place over the whole limb mesenchyme at early stages, does not lead to Shh induction anteriorly (Zakany et al, 2004). Tarchini and Duboule (2006) have proposed that the Gli3R protein is the repressor for the 5Ј-located genes, because inactivation of Gli3 leads to expression of these Hoxd genes in the anterior part of the limb bud te Welscher et al, 2002b) and, in addition, GLI3 can directly interact with HOXD proteins (Chen et al, 2004). This is a very interesting hypothesis, because it provides a hierarchical link between the Hand2/Gli3 expression properties and the Hoxd patterning activities.…”

Genetic interaction of Gli3 and Alx4 during limb development

“…Ectopic proximal Hoxd13 together with reduced Hoxd11 expression causes severe reduction of both zeugopod skeletal elements in limbs of Ul mutants embryos (Herault et al, 1997;Peichel et al, 1997). Moreover, Chen et al (2004) have established that the Gli3 repressor interacts directly with Hoxd proteins to promote digit specification. Therefore, it is possible to assume that the specific genetic interaction of Gli3 with Alx4 impacts on limb bud morphogenesis at the level of Hox protein regulation and function rather than their expression.…”