Bone is a dynamic tissue that adapts to meet the physical demands of its external environment. Reduced mechanical loading (disuse) from spaceflight, bed rest, aging, and injury all lead to a decrease bone density and strength, while physical activity is known to increase bone size and density. Thus, mechanotransduction, the process by which cells convert external mechanical forces into biochemical responses, is important to maintaining adult bone health and homeostasis.

An important part of the mechanosensing process, which is less well understood, is the molecular mechanism(s) by which bone cells sense mechanical loads and initiate intracellular signaling cascades in response to mechanical loading. Primary cilia are solitary, rigid structures that extend from cell body into extracellular space. They have been shown to be critical in development and regulation of cell proliferation. In addition, recent studies have implicated the primary cilium as a sensor of fluid flow in the embryonic node, kidney, liver, and bone cells in vitro. The two main projects of this dissertation focus on broadening the current understanding of the role of primary cilia as extracellular sensors in bone mechanotransduction.

A previous study in our lab has shown that cultured osteoblasts and osteocytes respond to dynamic fluid flow with primary cilium-dependent increases in osteogenic gene expression, however, these flow-induced primary cilium-dependent responses were not dependent on intracellular Ca²⁺, a ubiquitous second messenger that is known to be important in the response of bone cells to mechanical stimuli. In the first study of this dissertation, we sought to find the early response mechanism involved in these flowinduced primary cilium-dependent osteogenic responses. We found that dynamic fluid flow induces a primary cilium-dependent decrease in cyclic adenosine monophosphate (cAMP), and we took the first steps to elucidate the mechanism by which primary ciliumdependent cAMP decrease occurs, including determining that the primary ciliumdependent flow-induced decrease in cAMP is mediate by adenylyl cyclase 6, which is localized to the primary cilium.

The second study of this dissertation addresses the physiological relevance of primary cilia in mechanotransduction in bone. In this study, we generated an osteoblast- and osteocyte-specific knockout of Kif3a, a kinesin II subunit critical to primary cilia formation and function. These Kif3a conditional knockout mice exhibited no abnormal skeletal morphology, however they did exhibit decreased responsiveness to mechanical loading compared to controls, as measured by relative mineral apposition rate (rMAR) and relative bone formation rate (rBFR/BS). Taken together, these data indicate that primary cilia play an important role in osteocyte mechanosensing and mechanotransduction in bone. Elucidation of primary cilium-dependent mechanism(s) may be important for developing effective therapeutics for treatment of disuse bone loss.