The Mohawk homeobox gene represents a marker and osteo-inhibitory factor in cranial suture stem cells [Mkx siRNA]

The regeneration of craniofacial bones of the mammalian skeleton necessitates the action of both intrinsic and extrinsic inductive factors from multiple cell types, which function in a hierarchical and temporal fashion to control the differentiation of osteogenic progenitors. Single-cell transcriptomics of developing mouse cranial suture recently identified a suture mesenchymal progenitor population with tendon- or ligament-associated gene expression profile previously uncharacterized. Here, we developed a Mohawk homeobox (MkxCG;R26RtdT) reporter mouse, finding that this teno-ligamentous gene identifies a cranial suture resident cell population within the adult mouse that gives rise to calvarial osteoblasts and osteocytes overtime during homeostatic conditions. Single cell RNA-Sequencing (scRNA-Seq) demonstrated that Mkx+ suture cells demonstrate a stem-like phenotype with expression of teno-ligamentous genes. Bone injury with Mkx+ cell ablation showed delayed bone healing. Remarkably, Mkx gene played a critical role as an osteo-inhibitory factor in cranial suture cells, as knockdown or knockout resulted in increased osteogenic differentiation. Furthermore, in vivo local deletion of Mkx in Mkx floxed mice resulted in robustly increased calvarial defect repair. Finally, we observed that mechanical stretch dynamically regulates Mkx expression in turn regulating calvarial cell osteogenesis. Overall, we identify Mkx+ cells within the suture mesenchyme as a progenitor cell population for adult craniofacial bones required for bone repair and Mkx itself as mechanical stretch responsive gene which functions to prevent osteogenic differentiation within the stem cell niche.