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

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 GO 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 GO, 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 GO 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 GO repressor into an activator of Shh target genes. Several 5'Hoxd genes, expressed differentially across the limb bud, interact physically with GO. We propose that a varying [Gli3]:[total Hoxd] ratio across the limb bud leads to differential activation of GO target genes and contributes to the regulation of digit pattern. The resulting altered balance between 'effective' GO activating and repressing functions may also serve to extend the Shh activity gradient spatially or temporally.