Targeting Msx2 as a brake in the fusion fate of osteoclasts and an anabolic therapy in pre-clinical models of osteoporosis.

Highly conserved homeobox genes are closely related to bone formation during embryogenesis, while their role in adult bone resorption remains unclear. In this study, we found that the homeobox gene MSX2 actively participates bone metabolism. Myeloid-specific Msx2 deficiency safeguards bone mass under physiological and pathological conditions. Loss of Msx2 acts as a "brake" in the fusion fate of osteoclasts, resulting in a larger population of pre-osteoclasts. Pre-osteoclasts secrete platelet-derived growth factor-BB (PDGF-BB), which promotes angiogenesis-mediated bone formation. Mechanistically, MSX2 directly binds to the vital osteoclastogenic transcription factor PU.1 and protects it from FBXW7-mediated ubiquitination degradation. Msx2 and Fbxw7 double knockout mitigated the protective effect of MSX2 deficiency on bone mass. Finally, we identified a natural compound, morusinol, that specifically destroys the combination of MSX2 and PU.1, promoting PU.1 degradation and attenuating ovariectomy-induced bone loss. Overall, our results demonstrate that targeting Msx2 is a promising anabolic therapy for osteoporosis.