Bone mass is sensitive to mechanical stresses such that under periods of reduced strain (i.e. spaceflight, bed rest, etc) bone density is reduced, while repetitive load bearing stresses (i.e. physical activity) on the skeleton lead to increased bone formation. The mechanism of this regulation is not clear, but is thought to involve mechanosensors that detect shear forces caused by fluid flow through the bone canaliculi in response to the compression of the bone. The osteocytes located in the bone matrix are thought to be the cells responsible for detecting this mechanical signal. The osteocyte has an elaborate communication system interconnecting with the osteoblasts that line the bone and mediate mineral deposition and with the osteoclasts that reabsorb bone. Thus, the osteocyte is ideally positioned to sense external stresses and elicit appropriate adaptive responses needed to remodel bone;however, the mechanism by which it mediates this sensory response is unknown. Based on data from ductal epithelium in the kidney, we predict that the mechanical stress placed on the Done will be perceived by a primary cilium located on the osteocyte. As in the kidney, the primary cilia would function as a mechanosensor detecting flow through deflection of the axoneme. We predict this will result in calcium entry through the cilium requiring the nonselective cation channel polycystin-2. As in the renal epithelium this would lead to the release of internal calcium stores. While the physiological consequence of the calcium signal in the kidney is unknown, in the case of bone we predict it will regulate pathways governing bone formation and reabsorption. It is these possibilities that we want to test in this Pilot and Feasibility study. Thus the goal of this application is to evaluate the importance of the primary cilium and polycystin-2 for normal bone homeostasis. We will first determine the position of the cilium on the osteocyte in relation to the canaliculi and evaluate whether the polycystin-2 channel is present on the cilia as seen in the tubules of the kidney. We will then disrupt cilia or polycystin-2 function in the osteocytes/osteoblast. Since congenic mutations that completely disrupt cilia formation or polycystin-2 function are not viable and die before bone development occurs, we will cross conditional floxed mutant alleles of Tg737 and Kif3a (cilia assembly genes) and polycystin-2 with transgenic mice expressing Cre recombinase from the osteocalcin (OC::Cre) promoter. We will also use an OC::Cre line where Cre activity will be tamoxifen inducible which will allow us to disrupt cilia/polycystin-2 function during development or in adults. The consequence of the loss of cilia and polycystin-2 function on bone development and maintenance will be assessed by microCT, DEXA, and by histomorphometry. Future research directions of this application would involve the isolation of osteocytes to evaluate their ability to respond to fluid flow in presence or absence of cilia and polycystin-2. In addition, we will assess the effect of cilia) dysfunction on pathways, such as IGF and NO, that are involved in the regulation of bone homeostasis. Overall, these analyses will provide novel insights into the mechanosensory role of the primary cilium on the osteocyte and how this cell regulates bone formation.
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