Dynamic control of mTORC1 facilitates bone healing in mice

Bone healing requires well-orchestrated sequential actions of osteoblasts and osteoclasts. Previous studies have demonstrated that the mechanistic target of rapamycin complex 1 (mTORC1) plays a critical role in the metabolism of osteoblasts and osteoclasts. However, the role of mTORC1 in bone healing remains unclear. Here, we showed that a dynamic change in mTORC1 activity during the process was essential for proper healing and can be harnessed therapeutically for treatment of bone fractures. Low mTORC1 activity induced by osteoblastic Raptor knockout or rapamycin treatment promoted osteoblast-mediated osteogenesis, thus leading to better bone formation and shorter bone union time. Rapamycin treatment in vitro also revealed that low mTORC1 activity enhanced osteoblast differentiation and maturation. However, rapamycin treatment affected the recruitment of osteoclasts to new bone sites, thus resulting in delayed callus absorption in bone marrow cavity. Mechanistically, decreased mTORC1 activity inhibited the recruitment of osteoclast progenitor cells to healing sites through a decrease in osteoblastic expression of monocyte chemoattractant protein-1, thus inhibiting osteoclast-mediated remodeling. Therefore, normal mTORC1 activity was necessary for bone remodeling stage. Furthermore, through the use of sustained-release materials at the bone defect, we confirmed that localized application of rapamycin in early stages accelerated bone healing without affecting bone remodeling. Together, these findings revealed that the activity of mTORC1 continually changed during bone healing, and staged rapamycin treatment could be used to promote bone healing.