Cilium-generated signaling and cilia-related disorders

Biologists have long known that humans experience their environment through cilia. Light, odorant, and sound perception depend on these microtubule-filled, complex organelles present on cells in primary sensory tissues. Recently, discoveries on the mechanism of assembly of cilia (flagella) in the lowly, biflagellated, eucaryotic green alga Chlamydomonas have triggered a renaissance of interest in the organelles along with a recognition of their key sensory roles in nonsensory tissues. Chlamydomonas researchers uncovered an entirely new set of cellular machinery essential for transporting the protein components of cilia and flagella in all ciliated/flagellated eukaryotic cells between their site of synthesis in the cell body and their site of assembly at the tip of the flagellum (intraflagellar transport: IFT). Prompted by the surprising observations that disruption of IFT genes in mice led to polycystic kidney disease (PKD) and that PKD proteins are present on the sensory cilia of Caenorhabditis elegans, researchers have made a direct connection between PKD and cilia. At least five (and possibly all) of the seven identified human genes disrupted in PKD and a related disorder nephronophthisis encode proteins expressed in the primary cilia that project into the lumen from the epithelial cells that line renal tubules. Moreover, the renal cilia are flow sensors and at least two of the PKD genes encode ciliary transmembrane proteins essential for mechanosensation. Although their roles have not yet been as clearly identified, cilia also are at the center of a rare human disorder, Bardet-Biedl syndrome (BBS), in which patients exhibit phenotypes of common human diseases, including obesity and increased incidence of hypertension and diabetes. Five of the eight known BBS genes encode basal body or cilia proteins in mice or humans, and homologues of two of the remaining genes are present in basal bodies/cilia of model organisms. Here we briefly describe the biology of cilia and flagella, we outline how studies on model organisms have led to our current understanding of the roles of these organelles and their proteins in health and disease, and we highlight the notion that the primary cilia present on cells throughout the body, even those on brain neurons, may be essential for as yet undiscovered cilium-generated signaling functions.