Motor or sensor: a new aspect of primary cilia function.

Cilia are hair-like structures that protrude from the surface of the cell and are evolutionary well conserved. The characteristic feature of cilia is their motility and, in ciliated epithelia such as the trachea, their principal function is to transport materials along the cell surface. Each epithelial cell has many cilia on its surface. As well as this multiple form of motile cilia seen in the epithelium, there are primary cilia, also known as a monocilium because each cell has only one cilium. These types of cilia are thought to be non-motile because they lack a central pair of microtubules, are anomalous and have no function. However, recent studies have shown that primary cilia are involved in both developmental and pathological processes, including the establishment of left-right asymmetry and polycystic kidney disease. During development, cells in the node rotate their primary cilia to produce an extracellular current that is essential for the determination of left-right asymmetry of the body. In the kidney, primary cilia act as mechanosensors to detect fluid flow. Without such cilia, the kidney develops multiple cysts that eventually destroy kidney function. Furthermore, studies have identified a variety of proteins that are localized in the cilia and their diverse roles in various ciliary functions. These studies suggest the diversity of primary cilia. To elucidate how ciliary proteins interact and perform their functions in primary cilia will help us understand both their function and their diversity.