Aquaporin Water Channels in the Kidney

Aquaporins (AQPs) are water channel proteins of cellular membranes serving in the permeation of water across the membrane. AQP families are found virtually in all types of life ranging from bacteria to plant and animal cells. In mammals, at least 13 isoforms of AQPs have been identified. They are classified into three subtypes: classical aquaporins, aquaglyceroporins, and superaquaporins. These AQPs are differentially expressed in a wide variety of cells and tissues in the body, and play important roles in water metabolism. In the kidney, at least 6 isoforms of AQPs, namely AQP1, AQP2, AQP3, AQP4, AQP6, and AQP7, are reported to be expressed. Water transfer occurs mainly in the proximal tubules and collecting ducts in the kidney. In the proximal tubules, AQP1 and AQP7 are expressed, among which AQP1 plays a major role in water reabsorption. In the collecting ducts, AQP2, AQP3, AQP4, and AQP6 are expressed. AQP3 and AQP4 are localized at the basolateral membrane. AQP2 is stored in the cytoplasmic vesicles and is translocated to the apical plasma membrane in response to antidiuretic hormone. Mutations of AQP2 lead to either loss of channel function or mistrafficking and result in nephrogenic diabetes insipidus, the inability to concentrate urine.