Aquaporin-2 (AQP2) is a water channel protein located in the apical cell membranes of the kidney's collecting duct principal cells and in intracellular vesicles. It is encoded by the AQP2 gene and is the only aquaporin regulated by vasopressin. AQP2 plays a crucial role in urine concentration and body water homeostasis by facilitating water reabsorption from the primary urine into the bloodstream. Vasopressin binds to receptors on the cell surface, triggering a signaling pathway that moves AQP2-containing vesicles to fuse with the plasma membrane, thus increasing water permeability. This process is regulated both in the short term (minutes) through vesicle trafficking and in the long term (days) through changes in AQP2 gene expression. AQP2 is also regulated by food intake, with fasting reducing its expression independently of vasopressin. Dysfunction of AQP2, such as mutations in the AQP2 gene, can lead to nephrogenic diabetes insipidus, a condition characterized by excessive urination and dehydration.
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