Regulating osteogenic differentiation of bone marrow mesenchymal stem cells by mTORC1 signaling pathway inhibits bone defect repair in mice.

Bone defects caused by traumatic injuries and orthopedic diseases have emerged as the most common challenges in contemporary orthopedics, characterized by treatment difficulty, long treatment time, and high economic costs. This study aims to demonstrate that the activated mTOR pathway in mesenchymal stromal cells regulates the bone repair process. Initially, the mammalian target of rapamycin (mTOR) pathway-activated mouse model was constructed by specifically knocking down the tuberous sclerosis complex 1 (TSC1) molecule in bone marrow mesenchymal stem cells (BMMSCs). Then, the differences in bone repair between transgenic mice and littermate control mice in a single-layer cortical bone defect model were evaluated by histological, immunohistochemical, and micro-CT analyses. Further, the effects of the mTOR pathway on the osteoinductive differentiation ability of BMMSCs and its mechanism were mainly verified by cellular osteogenic staining and Western blotting experiments. The activated mTORC1 in mesenchymal stromal cells during bone defect repair in mice inhibited not only the healing rate of bone but also their ability to differentiate into osteoblasts, resulting in a decrease in the number of osteoblasts. The ability of mTORC1 in mesenchymal stromal cells to regulate osteoblastic differentiation might be related to the NOTCH pathway. The activated mTOR pathway during bone defect repair could inhibit the osteogenic ability of BMMSCs and hinder the bone healing process. Accordingly, regulating the activation of the mTOR pathway might promote the repair of bone defects.