[Aggrephores and aquaporins]

Abstract

The high water permeability of some specialized cells is accounted for by the presence of specific water channels in their plasma membranes. These channels, that play an important role in water homeostasis are now known as aquaporins. Mammalian kidney, in particular contains several forms of aquaporins. There are constitutive water channels (aquaporin 1), which are involved in the bulk of water reabsorption in the kidney. Aquaporin 1 is also found in red blood cells, some endothelial cells and several other tissues. Other water channels (aquaporin 2) are involved in the fine tuning of the final osmolarity of urine in mammalian kidney or amphibian urinary bladder. The latter are regulated by a membrane shuttle mechanism of vesicles called aggrephores, that contain the water channels. This mechanism is under hormonal control. Upon stimulation, aggrephores rapidly fuse with the apical plasma membrane of target epithelial cells, which increases their water permeability. After hormone withdrawal, aggrephores are internalized by endocytosis. Finally some kidney water channels (aquaporin 3) are localized in the basolateral plasma membrane of the same cells, where they contribute to the high transepithelial water permeability. Five classes of aquaporins are now known--they all belong to the same family of transmembrane proteins. Unlike previous members of the family, aquaporin 5, that was cloned most recently, is involved in the water secretion that occurs in lacrimal and salivary glands.