The Primary Cilium is a Self-Adaptable, Integrating Nexus for Mechanical Stimuli and Cell Signaling

Abstract

Mechanosensation is critical for cells to maintain homeostasis and in devastating diseases, including atherosclerosis, osteoporosis and cancer. Although several cellular mechanosensing structures have been described, none have been shown to adapt to mechanical stimuli or be regulated in non-excitable cells. Primary cilia are ubiquitous chemo-mechanical sensors and function as mechanosensors in several tissues, including kidney, liver, cartilage, bone, and the embryonic node, deflecting in response to mechanical stimuli. Several groups have shown cilia length adapts in response to mechanical stimuli while others have shown relatively small changes in length can affect cilia deflection and downstream load-induced changes in gene and protein expression. Collectively, this suggests cilia mechanosensitivity may be modulated. Here, we show that both mechanical and chemical mechanisms can alter ciliary rigidity. We exposed mouse inner medullary collecting duct cells transfected with a live-cell marker for primary cilia to flow. Cilium bending behavior was captured with high-speed confocal microscopy and modeled as a beam anchored by a torsional spring. We found exposure to flow stiffened the cilium up to 4-fold (n=12), deflecting less in response to subsequent exposures to flow. We hypothesized that modifying tubulin may mimic this stiffening. Post-translational modifications of tubulin, such as acetylation, have been shown to stiffen microtubules. Interestingly, acetylation can increase with mechanical stimuli. Using a potent pharmaceutical agent and a siRNA knockdown to alter acetylation, we showed that through acetylation the cell can biochemically regulate ciliary stiffness up to 4-fold (n=5/group). We further showed that this altered stiffness directly affects the sensitivity of the cell to mechanical signals, resulting in a 2-fold change in gene expression (n=5/group). We demonstrated, for the first time, a potential mechanism through which the cell can regulate its mechanosensing apparatus.