Chapter 11 - Renal Cilia Structure, Function, and Physiology

Abstract

The primary or immotile cilium is a nearly ubiquitous microtubule-based structure. It was once considered a vestigial remnant; however, recent advances have revealed that the cilium functions as a critical sensory and signaling center allowing cells to respond efficiently to external environmental cues. Seminal discoveries demonstrating that defects in cilia of mammals cause abnormal left–right body axis specification and cystic kidney disease have ignited strong research and clinical interest into the roles of the cilium. Defects in ciliary proteins are now known to be the cause of a group of disorders that are collectively called the ciliopathies . Patients with ciliopathies can present with a wide spectrum of disease phenotypes. These include cysts in the kidney, liver and pancreas, hydrocephalus, obesity, blindness, anosmia, cognitive deficits, ataxia, left-right body axis abnormalities, polydactyly and other bone malformations, deafness, and sterility. The molecular and cellular pathogenesis responsible for these phenotypes is beginning to emerge. This is occurring through research using model organisms such as Chlamydomonas and C. elegans , through identification of mutations in human patients with ciliary associated phenotypes, and through classical in vitro cell and biochemical studies. Currently, mutations in more than 45 genes have been identified in human ciliopathy patients; however, in most cases the genetic defect responsible for the phenotype is unknown. The ciliopathy phenotypes are thought to be a consequence of abnormal regulation of the diverse collection of receptors, channels, and other signaling machinery that are specifically enriched in the cilium. Importantly, these include the polycystin proteins, mutations in which cause polycystic kidney disease, one of the most common genetic diseases in humans.