Insulin-like growth factor-1 receptor in mature osteoblasts is required for periosteal bone formation induced by reloading.

Skeletal loading and unloading has a pronounced impact on bone remodeling, a process also regulated by insulin-like growth factor 1 (IGF-1) signaling. Skeletal unloading leads to resistance to the anabolic effect of IGF-1, while reloading after unloading restores responsiveness to IGF-1. However, a direct study of the importance of IGF-1 signaling in the skeletal response to mechanical loading remains to be tested. In this study, we assessed the skeletal response of osteoblast-specific deficient ( ) mice to unloading and reloading. The mice were hindlimb unloaded for 14 days and then reloaded for 16 days. mice displayed smaller cortical bone and diminished periosteal and endosteal bone formation at baseline. Periosteal and endosteal bone formation decreased with unloading in mice. However, the recovery of periosteal bone formation with reloading was completely inhibited in mice, although reloading-induced endosteal bone formation was not hampered. These changes in bone formation resulted in the abolishment of the expected increase in total cross-sectional area with reloading in mice compared to the control mice. These results suggest that the Igf-1r in mature osteoblasts has a critical role in periosteal bone formation in the skeletal response to mechanical loading.