A Mechanism of Mechanotransduction Mediated by the Primary Cilium

Mechanotransduction is a process by which cells sense and convert mechanical loads into biochemical signals and transcriptional changes. This process is particularly critical in bone, a metabolically active tissue that continously remodels and adapts to mechanical loads in its local environment. Osteocytes are the most prevalent bone cell type and are responsible for coordinating skeletal adaptation. Recently, the loss of primary cilia, nonmotile antenna-like cellular structures, has been attributed to causing defects in skeletal development and loading-induced bone formation. While primary cilia have been implicated in osteocyte mechanotransduction, the molecular mechanism associated with this process is not understood. In this thesis, we demonstrate that the osteocyte primary cilium forms a microdomain that mediates osteogenic responses to mechanical loads. In the first study, we build a genetically encoded primary cilium-localized calcium biosensor and characterize ciliary calcium mobilization in response to mechanical loading with unprecedented sensitivity. Next, we apply similar techniques to monitor levels of another second messenger, cyclic AMP (cAMP), and are the first to demonstrate that the primary cilium segregates ciliary cAMP from the cytosol. In the third study, we link loading-induced bone formation in vivo to adenylyl cyclase 6 enzyme function, a component of the primary cilium-mediated mechanotransduction mechanism. Collectively, this thesis elucidates how osteocyte primary cilia convert mechanical stimuli into osteogenic responses at the molecular and tissue levels and characterizes the primary cilium as a microdomain that serves as a biochemical and mechanical signaling nexus. Improvements in our understanding of primary cilia-regulated mechanotransduction will advance research efforts in the bone, tissue engineering, and mechanobiology communities