Various functions of ClC-type Cl- channels.

Cellular functions of Cl- channels are poorly understood, in contrast to well-established roles of cation channels. Recently, important achievements in Cl- channel research have been sequentially reported, including cloning of many Cl- channel cDNAs, linkage of gene abnormalities to human inherited disorders, analysis of knock-out mouse phenotype, analysis of crystal structure, and regulation by protein-protein interaction. Intracellular membrane Cl- channels are important for acidification of intracellular vesicles: ClC-5 functions for re-absorption of low-molecular-weight proteins in renal proximal tubule, and ClC-7 for absorption of bone matrix by osteoclasts. Abnormal functions of these channels result in Dent's disease characterized by proteinuria and kidney stones and by osteopetrosis, respectively. Plasma membrane Cl- channels, ClC-K1, ClC-K2, and ClC-3B, are expressed predominantly in epithelial cells and are important for uni-directional Cl- transport across the epithelia. Abnormalities of these channels are also related to human diseases: abnormal ClC-K1 to diabetes insipidus and abnormal ClC-K2 to Bartter's syndrome.