Aquaporins of plasma membranes of epithelial cells

The early 90s have brought us a discovery of a new class of membrane proteins--aquaporins with a function of transmembrane water channels. Being genetically closed proteins aquaporins are members of a large family of channel-forming proteins called MIPs (major intrinsic proteins). All aquaporins, except AQP4, are mercury-sensitive. Many aquaporins have been cloned and identified. Polyclonal antibodies grown against some of them promoted numerous studies of aquaporin localization and distribution in animal and plant tissues. Up to the present, 10 and 2 aquaporins have been described in mammalian and amphibian epithelial tissues, respectively. One of described aquaporins, AQP2, whose localization is confined to kidney collecting duct principal cells, has been found to be a hormone-depending water channel. The insertion of apical vesicles bearing AQP2 was shown to be regulated by vasopressin, meanwhile all other aquaporins are inserted into the plasma membrane constitutively. There is a vast evidence showing that the integrity of microtubules is necessary for both pathways of aquaporin insertion. AQP2 is important for normal kidney functioning and AQP2 mutations cause water-balance disorders. On the contrary, the AQP1 mutations are not accompanied by any evident clinical pathology. This review is focused on a discussion of the data so far available on aquaporin distribution in different animal tissues.