Water resorption in kidney cells is induced by vasopressin signalling through the cAMP/PKA axis, which increases membrane permeability by trafficking of the aquaporin-2 (AQP2) water channel from recycling endosomes to the plasma membrane. However, this response is attenuated in the presence of a high intraluminal Ca2+ concentration, which activates the Ca2+-sensing receptor (CaSR). Here (p. ), Giovanna Valenti and colleagues shed light on the mechanism by which CaSR activation counteracts AQP2 trafficking to the plasma membrane. Overexpression of CaSR in HEK-293 cells resulted in an increase in PKA activity, AQP2 phosphorylation at serine residue 256 (AQP2-pS256) and osmotic water permeability. Mimicking vasopressin by activating the cAMP-producing enzyme adenylate cyclase with forskolin increased levels of AQP2-pS256 as well as water permeability. By contrast, allosteric activation of CaSR decreased both basal and forskolin-induced levels of AQP2-pS265, and attenuated the increase in osmotic pressure in response to stimulation with forskolin. Interestingly, cells treated with a cell-permeable cAMP analogue were insensitive to CaSR signalling, indicating that CaSR affects AQP2-pS265 levels through inhibition of adenylate cyclase. Finally, the authors confirmed their results in ex-vivo experiments using rat kidney slices, mouse collecting-duct tubules and duct-tubule membrane vesicles. This study clearly demonstrates that activation of CaSR signalling inhibits thevasopressin-induced AQP2 trafficking to the plasma membrane, primarily by suppressing AQP2-pS256, thereby impairing the osmotic membrane water permeability.
