Endochondral ossification occurs when an initial cartilage template is replaced by co-invading and mutually supportive osteoblast precursors and blood vessels. Osteoblast precursors, in the outer bone collar, coordinate their mobilization into the cartilaginous limb rudiment alongside blood vessel invasion to form the primary ossification center. But how the bone precursor cells mobilize, and direct angiogenesis and subsequent endochondral ossification is unclear. In endochondral bone formation, cellular decisions must integrate both mechanical and morphogenic cues to direct proper transcriptional programs. In fetal development, when the fetus is mechanically constrained (e.g., arthrogryposis caused by a low volume of amniotic fluid), improper mechanical cues can result in skeletal developmental dysplasias. Similarly, in fracture repair, which follows developmental programs, mechanical cues direct the mode of ossification and successful repair. Yet, the how skeletal progenitors integrate mechanical and morphogenic cues to mediate transcription is unclear. The transcriptional regulators Yes-associated protein (YAP) and Transcriptional co-activator with PDZ-binding domain (TAZ) are mechanosensitive, mediate angiogenic gene expression, mediate cell mobilization and their deletion from osteoblast precursors and their progeny is perinatal lethal. Thus, YAP and TAZ are crucially positioned to integrate mechanical and morphogenic cues to mediate endochondral bone development.

The goal of this thesis was to define the cellular and mechanical roles of YAP and TAZ in fetal development. Here, we conditionally deleted YAP and TAZ from Osx-expressing cells during fetal development. First, we determined the roles of YAP and TAZ at the tissue level in utero. YAP/TAZ deletion impaired osteogenesis in the primary ossficiation center, hypertrophic cartilage remodeling, and the mobilization of osteoblast precursors into the primary ossification center. Second, we determined the role of YAP and TAZ at the single cell level in utero. We performed extensive single cell RNA sequencing on fetal forelimbs. YAP/TAZ deletion decreased canonical osteogenic genes, such as Bglap, Bglap2, and Spp1, as well as increasing MMP production. Additionally, YAP/TAZ deletion decreased the expression of Cxc/12, which was only uniquely expressed by the vessel-associated osteoblast precursors. YAP/TAZ deletion impaired the cell/cell communication between vessel-associated osteoblast precursors and bone resident endothelial subtypes, through Cxcl12-signaling and extracellular matrix interactions. Lastly, we defined the roles of YAP/TAZ in the mechanoregulation of bone tissue development ex utero. Impairment of YAP/TAZ function both genetic and pharmacological reduced load-induced osteogenesis. Taken together, YAP/TAZ integrate mechanical and morphogenic cues to direct cartilage remodeling, vessel morphogenesis, osteoblast precursor mobilization, and load-induced bone formation. Understanding the transcriptional control and integration of the developmental programs required for fetal development and fracture repair will enable the development of new and potentially less invasive mechanotherapeutic strategies.