YAP regulates transcriptional programs for layer-specific periosteal expansion during fracture repair.

Bone fracture repair initiates by periosteal expansion. The periosteum is a bilayered tissue composed of inner cambium and outer fibrous layers. Typically quiescent, periosteal progenitor cells proliferate upon fracture; however, the underlying transcriptional mechanisms remain unclear. Here, we show that deletion of the transcriptional regulators, yes-associated protein (YAP) and transcriptional coactivator with PDZ binding motif (TAZ), from Osterix-expressing cells, which reside in the cambium, impairs periosteal expansion. YAP activation increases chromatin accessibility, preferentially at TEA domain transcription factor (TEAD) binding sites, and regulates both cell-intrinsic and cell-extrinsic cellular functions. We identify bone morphogenetic protein 4 (Bmp4) as a YAP-TEAD target gene expressed in the cambium. In YAP/TAZ knockout mice, BMP4 delivery increased periosteal expansion through matrix accumulation and fibrous layer cell proliferation. Conversely, in wild-type mice, BMP4 delivery increased osteogenic activity and angiogenesis. Together, these data identify YAP-mediated transcriptional programs that promote layer-specific periosteal expansion.