Aquaporin-2 (AQP2): its intracellular compartment and trafficking.

Aquaporins (AQPs) are membrane proteins serving in the transfer of water and small solutes across cellular membranes. At least 7 isoforms, namely AQP1, AQP2, AQP3, AQP4, AQP6, AQP7 and AQP11 are expressed in the kidney. Among them, AQP2 plays a pivotal role in the concentration of urine. AQP2 is expressed in the principal cells of the collecting ducts. It is localized in the intracellular compartment and is translocated to the cell surface upon anti-diuretic hormone stimulation. Analyses in cultured cells expressing AQP2 have provided clues to the trafficking of AQP2. AQP2 resides in the subapical vesicles, some of which are Rab11-positive, suggesting their close relationship with apical recycling endosomes. Upon stimulation with forskolin, AQP2 is translocated to the cell surface. After washout, AQP2 is endocytosed to early endosomes and then transferred to the apical storage compartment. Some of AQP2 is excreted as exosomes. Actin cytoskeleton plays important roles in the trafficking of AQP2. Analyses of molecules found in AQP2-containing vesicles will shed light on the mechanism of AQP2 translocation.