Tuberous Sclerosis Complex 1-Targeted Depletion of Macrophages Promotes Osteogenesis by Modulating Secretion of Oncostatin M in Inflammatory Stage of Bone Healing

In bone healing, earlier bone formation benefits bone repair. The first process of repair following bone injury involves the interaction between macrophage polarization and osteogenic activation of osteoblast linage cells, but radical differences between the contributions of classically-activated M1 macrophages and alternatively-activated M2 macrophages to osteogenesis remain obscure. To test our hypothesis that M1 macrophages promote bone healing, we generated transgenic mice with myeloid lineage-specific Tsc1 deletion (TSC1KO) to investigate the functional roles of M1 macrophages in the process of healing bone defects. We demonstrated that constitutive activation of mammalian target of rapamycin complex 1 (mTORC1) regulated macrophage polarization during bone healing. By creating cortical defects as a model of bone repair in the TSC1KO mice and their littermates, we surprisingly found that osteogenic responses in the bone defect region, where repair occurred by intramembranous ossification (IO), in TSC1KO mice were promoted due to the enhanced M1-polarized macrophages. Additionally, the suggestion that Oncostatin M (OSM) secreted by M1-polarized macrophages but not M2 macrophages likely functioned as a paracrine factor in this repair process was verified by its induction of osteoblastic differentiation and matrix mineralization. Interestingly, the expression level of the OSM receptor (OSMR) was continually increased in osteoblast linage cells. Finally, this mechanistic target of OSM activated the signaling transduction system of JAK/STAT/RUNX2 in MSCs to stimulate recruitment of osteoblast lineage cells and activate IO. These results indicate that TSC1-targeted depletion of macrophages promotes bone healing by inducing secretion of OSM. These collective findings highlight that regulation of M1 macrophage polarization is a novel basis for improvement of bone regeneration and that regulation of macrophage polarization should be a potential therapeutic strategy to treat defects in the repair phase of bone healing.