Hedgehog signaling regulates bone homeostasis through orchestrating osteoclast differentiation and osteoclast-osteoblast coupling

Imbalance of bone homeostasis induces bone degenerative diseases such as osteoporosis. Hedgehog (Hh) signaling plays critical roles in regulating the development of limb and joint. However, its unique role in bone homeostasis remained largely unknown. Here, we found that canonical Hh signaling pathway was gradually augmented during osteoclast differentiation. Genetic inactivation of Hh signaling in osteoclasts, using Ctsk-Cre;Smo(f/f) conditional knockout mice, disrupted both osteoclast formation and subsequent osteoclast-osteoblast coupling. Concordantly, either Hh signaling inhibitors or Smo/Gli2 knockdown stunted in vitro osteoclast formation. Mechanistically, Hh signaling positively regulated osteoclast differentiation via transactivation of Traf6 and stabilization of TRAF6 protein. Then, we identified connective tissue growth factor (CTGF) as an Hh-regulatory bone formation-stimulating factor derived from osteoclasts, whose loss played a causative role in osteopenia seen in CKO mice. In line with this, recombinant CTGF exerted mitigating effects against ovariectomy induced bone loss, supporting a potential extension of local rCTGF treatment to osteoporotic diseases. Collectively, our findings firstly demonstrate that Hh signaling, which dictates osteoclast differentiation and osteoclast-osteoblast coupling by regulating TRAF6 and CTGF, is crucial for maintaining bone homeostasis, shedding mechanistic and therapeutic insights into the realm of osteoporosis.

Automated summary

The imbalance of bone homeostasis leads to bone degenerative diseases, such as osteoporosis. The Hedgehog (Hh) signaling pathway plays a crucial role in limb and joint development, but its specific role in bone homeostasis is not well understood. This study discovered that the Hh signaling pathway is gradually enhanced during osteoclast differentiation, and its inactivation affects osteoclast formation and osteoclast-osteoblast coupling. The study also revealed the positive regulation of osteoclast differentiation by Hh signaling through the transactivation of Traf6 and stabilization of TRAF6 protein. In addition, connective tissue growth factor (CTGF) was identified as a bone formation-stimulating factor regulated by Hh signaling and its loss contributed to osteopenia. This research provides mechanistic and therapeutic insights into osteoporosis by highlighting the importance of Hh signaling in maintaining bone homeostasis.