Appropriate orthopedic force led to bone remodeling of mandibular condyle, while overloaded orthopedic force (OOF) induced condylar bone absorption. Bone absorption is ascribed to the imbalanced activities between osteoclasts (OCs) and osteoblasts (OBs), mechanism of which remains unclear. This study aimed to observe the condylar changes induced by OOF by mandible advancement appliance and to further investigate the role of mammalian target of Rapamycin (mTOR) and RANKL/OPG in osteoclastic differentiation of stem cells in vivo and in vitro. In vivo, the results of micro-CT analysis indicated that condylar bone resorption was induced by OOF through mandibular advancement appliance for 2 weeks and worsened time dependently. Morphologically, cartilage thickness was reduced, subchondral cortical bone line appeared not continuous, and subchondral bone exhibited irregular-shaped and owned uneven surface. The bone mineral density (BMD), bone volume/tissue volume (BV/TV), trabecular number (Tb.N), and trabecular thickness (Tb.Th) were decreased accomplished with the increased trabecular separation (Tb.Sp) determined by micro-CT. In addition, based on immunofluorescent labeling, OOF activated both OCs and OBs, but osteoclastogenesis prevailed over osteogenesis. The mTOR activation and ratio of RANKL/OPG in OBs were elevated by OOF. In vitro, the results of western blot and polymerase chain reaction (PCR) consistently suggested that the mTOR and RANKL/OPG ratio were upregulated by overloaded mechanical stretch. Pretreatment with mTOR inhibitor, rapamycin, could attenuate the activation of mTOR and the secretion of RANKL in OBs. Interestingly, based on the Trap staining, the supernatant of OBs exposed to OOF could promote osteoclastic differentiation of mesenchymal stem cells (MSCs), while its role was weakened by inhibition of mTOR in OBs. Collectively, OOF induced condylar bone absorption; in the process, osteoclastogenesis was prominent than osteogenesis. The activation of mTOR and secretion of RANKL/OPG were enhanced by OOF and were involved in promoting MSCs differentiating into OCs.

中文翻译：

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超载矫形力通过刺激大鼠间充质干细胞通过 mTOR 调节成骨细胞中的 RANKL/OPG 分泌分化为破骨细胞，从而诱导髁突软骨下骨吸收


适当的矫形力导致下颌骨髁突骨重塑，而过载矫形力（OOF）则导致髁突骨吸收。骨吸收归因于破骨细胞（OC）和成骨细胞（OB）之间的活性不平衡，其机制尚不清楚。本研究旨在观察下颌前移矫治器OOF引起的髁突变化，并进一步研究哺乳动物雷帕霉素靶蛋白（ mTOR ）和RANKL/OPG在体内外干细胞破骨细胞分化中的作用。在体内，显微CT分析结果表明，OOF通过下颌前移矫治器2周诱导髁突骨吸收，并且随着时间的推移而恶化。形态上软骨厚度减少，软骨下骨皮质线不连续，软骨下骨形状不规则，表面凹凸不平。骨矿物质密度 (BMD)、骨体积/组织体积 (BV/TV)、小梁数量 (Tb.N) 和小梁厚度 (Tb.Th) 随着小梁间距 (Tb.Sp) 的增加而降低，具体如下：显微CT。此外，基于免疫荧光标记，OOF同时激活OC和OB，但破骨细胞生成优于成骨。 OOF 提高了 OB 中mTOR 的激活和RANKL/OPG的比率。在体外，蛋白质印迹和聚合酶链反应 (PCR) 的结果一致表明，超负荷的机械拉伸会上调mTOR和RANKL/OPG比率。用mTOR抑制剂雷帕霉素预处理可以减弱 OB 中mTOR的激活和RANKL的分泌。 有趣的是，基于Trap染色，暴露于OOF的OB上清液可以促进间充质干细胞（MSC）的破骨细胞分化，但其作用因OB中mTOR的抑制而减弱。总的来说，OOF 诱导髁骨吸收；在此过程中，破骨细胞生成比成骨作用更为突出。 OOF增强了mTOR的激活和RANKL/OPG的分泌，并参与促进MSCs分化为OCs。