PRRX1 + MSCs Enhance Mandibular Regeneration during Distraction Osteogenesis

Bone defect (BD) caused by trauma, infection, congenital defects, or neoplasia is a major cause of physical limitation. Distraction osteogenesis (DO) is a highly effective procedure for bone regeneration, while the concrete mechanism remains unknown. In this study, canine DO and BD models of the mandible were established. The results of micro–computed tomography and histological staining revealed that DO led to an increased mineralized volume fraction and robust new bone formation; in contrast, BD demonstrated incomplete bone union. Mesenchymal stem cells (MSCs) from DO and BD calluses were isolated and identified. Compared with BD-MSCs, DO-MSCs were found to have a stronger osteogenic capability. Single-cell RNA sequencing analysis was further performed to comprehensively define cell differences between mandibular DO and BD calluses. Twenty-six clusters of cells representing 6 major cell populations were identified, including paired related homeobox 1–expressing MSCs ( PRRX1 + MSCs), endothelial cells (ECs), T cells, B cells, neutrophils, and macrophages. Interestingly, 2 subpopulations in PRRX1 + MSCs in the DO group were found to express the marker of neural crest cells (NCCs) and were associated with the process of epithelial–mesenchymal transition. The immunofluorescence assay was performed to further corroborate these results in vivo and in vitro, experimentally validating that continuous distraction maintained the PRRX1 + MSCs in an embryonic-like state. Finally, we used CRISPR/Cas9 to knock out (KO) PRRX1 in the context of DO, which significantly blunted the capability of jawbone regeneration, resulting in a diminished NCC-like program and reduction of new bone volume. In addition, the ability of osteogenesis, cell migration, and proliferation in cultured PRRX1 KO MSCs was inhibited. Taken together, this study provides a novel, comprehensive atlas of the cell fates in the context of DO regeneration, and PRRX1 + MSCs act essential roles.