Hedgehog signaling molecules in bone marrow cells at the initial stage of fracture repair

Ihh is a secreted protein expressed in chondrocytes in cartilaginous soft callus and thought to be involved in regulation of chondrogenic differentiation in fracture repair processes. However, gene expression and function of Ihh and its signaling molecules, Ptc and Smo, at the initial stage of fracture repair remain unknown. In the present study, we showed by RT-PCR of mouse rib fractures that the upregulation of Ihh mRNA occurred within hours after fracture, immediately followed by that of Ptc mRNA, and that both Ihh and Ptc mRNAs exhibited the time course similar to those of OP and OC mRNAs at the initial stage of fracture repair. The transcript level of Smo mRNA gradually increased within hours after fracture and was continuously maintained throughout the subsequent fracture repair processes. By in situ hybridization analysis, the transcripts of Ptc and Smo genes localized in bone marrow of unfractured ribs, and those of Ihh, Ptc, and Smo were expressed in the vicinity of the fracture site at 8 h after fracture. Furthermore, in adherent bone marrow cells in culture, mrIhh-N upregulated the gene expression of TGF-beta(1) as well as OPGL, a potent stimulator of osteoclastogenesis and osteoclast activity. These observations suggest that Ihh may play roles in the initial stage of fracture repair via TGF-beta(1) and OPGL.