Mechanosensation, the ability for cells to sense and respond to physical cues, is a ubiquitous process among living organisms and its dysfunction can lead to devastating diseases, including atherosclerosis, osteoporosis, and cancer. The primary cilium is a solitary, immotile organelle that projects from the surface of virtually every cell in the human body and can function as a mechanosensor across diverse biological contexts, deflecting in response to fluid flow, pressure, touch, and vibration. It can detect urinary flow rate in the kidney, monitor bile flow in the liver, and distinguish the direction of nodal flow in embryos. In this thesis, we examined the interplay of biology and mechanics in the context of this multifunctional sensory organelle from the tissue to subcellular scale.

In the first part of this work, we examined the cilium at the tissue level. Primary cilia are just beginning to be appreciated in bone with studies recently reporting loss of cilia results in defects in skeletal development and adaptation. We disrupted primary cilia in osteocytes, the principal mechanosensing cells in bone, and demonstrated that loss of primary cilia in osteocytes impairs load-induced bone formation. Over the course of our work with primary cilia, we also identified the need for more standardized imaging approaches to the cilium and presented an improvement to distinguishing proteins within the cilium from the rest of the cell.

In the later part of this work, we examined the primary cilium at the subcellular level. While deflection is integral to the cilium’s mechanosensory function, it remains poorly understood and characterized. Using a novel combination of experimental and computational techniques to capture and determine the mechanical properties of the cilium, we demonstrated cilium deflection can be mechanically and chemically modulated. We revealed a mechanism, acetylation, through which this mechanosensor can adapt and regulate overall cellular mechanosensing. By modifying our combined experimental and computational approach, we analyzed cilium deflection in vivo for the first time.

Collectively, this work uncovers new insights across biological scales in the primary cilium as an extracellular nexus integrating mechanical stimuli and cellular signaling. Understanding the mechanisms driving cilium mechanosensing has broad reaching implications and unlocks the cilium’s potential as a therapeutic target to treat impaired cellular mechanosensing critical to a multitude of diseases.