The primary cilium encourages osteogenic behavior in periosteal osteochondroprogenitors and osteocytes during juvenile skeletal development and adult bone adaptation

Primary cilia are sensory organelles that facilitate early skeletal development, as well as maintenance and adaptation of bone later in life. These solitary, immotile organelles are known to be involved in cell differentiation, proliferation, and mechanotransduction, a process by which cells sense and covert external physical stimuli into intracellular biochemical signals. Bone is a metabolically active tissue that continuously recruits osteogenic precursors and relies on osteocytes, the sensory cells of bone, to coordinate skeletal maintenance. Overall bone quality is dependent on the integrity of the initial structure formed, as well as this organ’s ability to adapt to physical loads. Proper differentiation and controlled proliferation of osteogenic progenitors are critical to the initial formation of the skeleton, while osteocyte mechanotransduction is essential for adaptation of developed bone. These phenomena rely on primary cilia, but little is known about the origin of osteogenic precursors and the ciliary mechanisms that promote osteogenesis. In this thesis, we first characterize an osteochondroprogenitor (OCP) population that rapidly and extensively populates skeletal tissues during juvenile skeletal development (Chapter 2). We also demonstrate that the primary cilium is critical for these cells to differentiate and contribute to skeletogenesis. We then show this OCP population is required for adult bone adaptation and is mechanoresponsive (Chapter 3). Again, we demonstrate that primary cilia are necessary for these OCPs to sense physical stimuli and differentiate into active bone-forming cells. Finally, we identify a novel link between ciliary calcium and cAMP dynamics in the osteocyte primary cilium (Chapter 4). Specifically, we show that a calcium channel (TRPV4) and adenylyl cyclases, which produce cAMP, bind calcium to mediate calcium entry and cAMP production, respectively, and these phenomena are critical to fluid flow-induced osteogenesis. Collectively, our results demonstrate that an easily extracted progenitor population is pre-programmed towards an osteogenic fate and extensively contributes to bone generation through primary cilium-mediated mechanisms at multiple stages of life. Furthermore, we identified ciliary proteins that are potentially unique to the osteocyte and can be manipulated to encourage osteogenesis by tuning calcium/ cAMP dynamics. For these reasons, we propose that this OCP population and their primary cilia, as well as osteocyte ciliary proteins that coordinate calcium/ cAMP dynamics, are attractive therapeutic targets to encourage bone regeneration.