Putative roles of cilia in polycystic kidney disease

The last 10 years has witnessed an explosion in research into roles of cilia in cystic renal disease. Cilia are membrane-enclosed finger-like projections from the cell, usually on the apical surface or facing into a lumen, duct or airway. Ten years ago, the major recognised functions related to classical "9+2" cilia in the respiratory and reproductive tracts, where co-ordinated beating clears secretions and assists fertilisation respectively. Primary cilia, which have a "9+0" arrangement lacking the central microtubules, were anatomical curiosities but several lines of evidence have implicated them in both true polycystic kidney disease and other cystic renal conditions: ranging from the homology between Caenorhabditis elegans proteins expressed on sensory cilia to mammalian polycystic kidney disease (PKD) 1 and 2 proteins, through the discovery that orpk cystic mice have structurally abnormal cilia to numerous recent studies wherein expression of nearly all cyst-associated proteins has been reported in the cilia or its basal body. Functional studies implicate primary cilia in mechanosensation, photoreception and chemosensation but it is the first of these which appears most important in polycystic kidney disease: in the simplest model, fluid flow across the apical surface of renal cells bends the cilia and induces calcium influx, and this is perturbed in polycystic kidney disease. Downstream effects include changes in cell differentiation and polarity. Pathways such as hedgehog and Wnt signalling may also be regulated by cilia. These data support important roles for cilia in the pathogenesis of cystic kidney diseases but one must not forget that the classic polycystic kidney disease proteins are expressed in several other locations where they may have equally important roles, such as in cell-cell and cell-matrix interactions, whilst it is not just aberrant cilia signalling that can lead to de-differentiation, loss of polarity and other characteristic features of polycystic kidney disease. Understanding how cilia fit into the other aspects of polycystic kidney disease biology is the challenge for the next decade. This article is part of a Special Issue entitled: Polycystic Kidney Disease.