English: Molecular pathways involved in AQP2-mediated water transport in the kidney. (A) Signalling cascades and molecular pathways involved in AQP2-mediated water transport in relation to vasopressin (AVP) and vasopressin receptor (AVPR2) in the principal cells of the collecting ducts 22, 33, 37, 115. The increased influx of water by AQP2 tetramer at the apical site requires a complex cascade of intracellular processes in concert with efflux of water by AQP3 and AQP4 tetramers at the basolateral membrane. The AVPR2 is composed of 7 membrane-spanning helices. Upon binding of AVP within the transmembrane helices II–IV, allosteric structural changes occur 78, 79, the G-alpha-s heterodimeric protein is stimulated, and activates the adenylyl cyclase. This step results in increased intracellular levels of cyclic adenosine monophosphate (cAMP), activation of protein kinase A (PKA), phosphorylation of AQP2 in intracellular vesicles at serine 256 and other residues in the AQP2 COOH terminal 49, 50 (see also Fig. 2), trafficking of endocytic vesicles to the apical plasma membrane, and fusion of AQP2-containing vesicles with the apical membrane. As stated in the text, PKA is also responsible for phosphorylation of the membrane-associated RhoA, association with GDI to form the inactive complex RhoA-GDI, a step facilitating AQP2 insertion into the plasma membrane during VP/PKA/cAMP-induced AQP2 translocation 62. The docking system for vesicles might include specific receptors in the collecting duct cells which are associated with certain membrane domains housing AQP2 (e.g. syntaxin-4). Abbreviation: PDEs, phosphodiesterases. See also 33, 37, 247, 248. (B) Proposed model of transcytotic trafficking of AQP2 from basolateral to apical membrane in principal cell of the collecting ducts. At least eight steps are involved: (1) Synthesis in the endoplasmic reticulum and transport to the trans-Golgi network; (2) rapid insertion of AQP2 into the basolateral membrane; (3) rapid internalization by clathrin-dependent endocytosis which is responsible for limited expression of basolateral AQP2. This step is blockable by low temperature (4°C); (5) AQP2 transcytosis to the perinuclear recycling compartment and the apical recycling endosomes via the microtubule-dependent mechanism. This step is inhibitable by colchicine; (7) exocytosis of AQP2 at the apical membrane; (8) recycling of AQP2 towards the apical recycling endosomes via the clathrin-dependent endocytosis. Thin dotted arrows show alternative pathways (?) of AQP2. Asterisks indicate where vasopressin (AVP) stimulus is inducing increased exocytosis and recycling of AQP2 with effect on transepithelial water flux (apical side) and cell migration, tubulogenesis, and likely transepithelial water flux (basolateral side). See also 69, 70.