Are primary cilia mechanical sensors?

The primary cilium is a single organelle protruding from the cell surface. Given this antenna-like structure, its function has been assumed to be sensing of the surrounding environment. Specifically, the main assumption has been that calcium-permeable ion channels within the cilium sense and respond to mechanical forces — a hypothesis that has been used to explain a large range of biological responses, from control of left–right axis determination during embryonic development to adult progression of polycystic kidney disease and some cancers. David Clapham and colleagues, however, present evidence that refutes this hypothesis. They measured Ca²⁺ signals in primary cilia following a mechanical force in mice engineered to express a sensor protein that fluoresces in response to Ca²⁺. They find no evidence of force-driven Ca²⁺ influx in primary cilia and conclude that if mechanosensation originates in primary cilia, it is not via calcium signalling.