In Vivo Effects of Osteoblast-Specific Overexpression of Gα (s) and Gα (q/11) on Bone Formation and Bone Remodeling During Fracture Healing

Although the regenerative potential of bone is high, fracture healing is sometimes compromised resulting in fracture non-union. Successful prevention and treatment of non-union rely on accurate prognosis of fracture and treatment assignment but are often impeded by a high degree of heterogeneity in fracture pathogenesis and treatment response. Burgeoning evidence that indicates differential levels of G proteins among healthy populations offers a plausible explanation, since G protein signaling plays regulatory roles in bone development and remodeling. We previously demonstrated the in vivo effects of G protein level variation on bone development by characterizing the skeletal phenotypes of two transgenic mouse lines that overexpress normal Gα (s) (G(s)-Tg) or Gα (11) (G(11)-Tg) in osteoblast lineage cells under the control of the 3.6-kb Col1a1 promoter. G(s)-Tg mice showed high bone mass but diminished bone quality due to increased formation of woven bone and cortical porosity. Conversely, G(11)-Tg mice displayed a low bone mass phenotype with reduced bone strength primarily due to increased bone resorption. It still remains unclear, however, how variation in G protein signaling in osteoblasts affects the rate and quality of bone repair. To characterize bone healing in Gs-Tg and G11-Tg mice, we induced a stabilized transverse osteotomy in the tibia of wild-type, G(s)-Tg or G(11)-Tg at 8 weeks of age and assessed the progression of bone healing at the tissue and molecular levels by Micro-CT, histomorphometry, and quantitative gene expression analysis at 1, 2, 3, and 4 weeks post-fracture. The effects of increased G protein signaling during repair on Wnt and Hh signaling, the two signaling pathways essential in osteogenesis, were also examined by assessing mRNA levels of key components of each pathway. Persistent upregulation of Wnt and Hh signaling in G(s)-Tg fractures led to enhanced callus mineralization and new woven bone formation at the expense of cartilaginous callus formation and defective bone remodeling. G(11)-Tg fractures, in contrast, showed minimal changes in Wnt and Hh pathways and consequently led to only subtle changes in the healing response with a transient increase in callus mineralization followed by its rapid resorption. Our results indicate that G protein signaling in osteoblasts regulates bone formation and remodeling during fracture repair in part by tightly controlling the strength and duration of Wnt and Hh signaling pathways.