Mutual genetic antagonism involving GLI3 and dHAND prepatterns the vertebrate limb bud mesenchyme prior to SHH signaling

The bHLH transcription factor dHAND is required for establishment of SHH signaling by the limb bud organizer in posterior mesenchyme, a step crucial to development of vertebrate paired appendages. We show that the transcriptional repressor GLI3 restricts dHAND expression to posterior mesenchyme prior to activation of SHH signaling in mouse limb buds. dHAND, in turn, excludes anterior genes such as Gli3 and Alx4 from posterior mesenchyme. Furthermore, genetic interaction of GLI3 and dHAND directs establishment of the SHH/ FGF signaling feedback loop by restricting the BMP antagonist GREMLIN posteriorly. These interactions polarize the nascent limb bud mesenchyme prior to SHH signaling.