Abstract
Hedgehog signaling plays crucial roles in the development of calvarial bone, relying on the activation of Gli transcription factors. However, the molecular mechanism of the role of regulated Gli protein level in osteogenic specification of mesenchyme still remains elusive. Here, we show by conditionally inactivating Suppressor of Fused (Sufu), a critical repressor of Hedgehog signaling, in Wnt1-Cre-mediated cranial neural crest (CNC) or Dermo1-Cre-mediated mesodermal lineages that Sufu restraint of Hedgehog activity level is critical for differentiation of preosteogenic mesenchyme. Ablation of Sufu results in failure of calvarial bone formation, including CNC-derived bones and mesoderm-derived bones, depending on the Cre line being used. Although mesenchymal cells populate to frontonasal destinations, where they are then condensed, Sufu deletion significantly inhibits the proliferation of osteoprogenitor cells, and these cells no longer differentiate into osteoblasts. We show that there is suppression of Runx2 and Osterix, the osteogenic regulators, in calvarial mesenchyme in the Sufu mutant. We show that down-regulation of several genes upstream to Runx2 and Osterix is manifested within the calvarial primordia, including Bmp2 and its downstream genes Msx1/2 and Dlx5 By contrast, we find that Gli1, the Hedgehog activity readout gene, is excessively activated in mesenchyme. Deletion of Sufu in CNC leads to a discernible decrease in the repressive Gli3 form and an increase in the full-length Gli2. Finally, we demonstrate that simultaneous deletion of Gli2 and Sufu in CNC completely restores calvarial bone formation, suggesting that a sustained level of Hedgehog activity is critical in specification of the osteogenic mesenchymal cells.