Water Channel Aquaporins with a Special Focus on the Kidney

Aquaporins are membrane water channel proteins through which water permeates the lipid bilayer. So far 13 aquaporin isoforms, aquaporin-0 (AQP0) to AQP12, have been identified in mammals. They are classified into 3 subgroups: 1) aquaporins that selectively transfer water, 2) aquaglyceroporins that transfer some small solutes as well as water, and 3) unorthodox aquaporins. They are widely distributed in most water-handling organs, such as kidney, and play important roles in water transfer. For example, segment-specific expression of AQP1, AQP2, AQP3, AQP4, AQP6, AQP7, and AQP11 in the renal tubular epithelium enables water reabsorption to produce concentrated urine. Aquaporins are usually localized to the plasma membrane. Some isoforms, such as AQP2, are present in the intracellular membrane compartment. AQP2 in collecting duct cells translocates between intracellular vesicles and the cell surface via a membrane trafficking mechanism. Mutations of AQP2 result in congenital nephrogenic diabetes insipidus. This review summarizes the distribution and physiological importance of aquaporins, with a special focus on the kidney.