A protein kinase a-independent pathway controlling aquaporin 2 trafficking as a possible cause for the syndrome of inappropriate antidiuresis associated with polycystic kidney disease 1 haploinsufficiency

Renal water reabsorption is controlled by vasopressin (AVP) which binds to V2 receptors resulting in PKA activation, phosphorylation of AQP2 at serine 256 (pS256) and translocation to the plasma membrane. Besides S256, AVP causes dephosphorylation of S261. Recent studies showed that cyclin-dependent kinases can phosphorylate S261 AQP2 peptides in vitro. In an attempt to investigate the possible role of cdks on AQP2 phosphorylation, we identified a new PKA-independent pathway regulating AQP2 trafficking. In ex-vivo kidney slices and MDCK-AQP2 cells, R-roscovitine, a specific cdks inhibitor, increased pS256 and decreased pS261. The changes in AQP2 phosphorylation were paralleled by an increase in cell surface AQP2 expression and osmotic water permeability in the absence of forskolin stimulation. Of note, R-roscovitine didn’t alter cAMP-dependent PKA activity. Because phosphorylation results from the balance between kinase and phosphatase activity, we evaluated the possible contribution of protein phosphatases PP1, PP2A and PP2B. Of these, R-roscovitine treatment specifically reduced PP2A protein expression and activity in MDCK cells. Interestingly, in PKD1+/- mice displaying a syndrome of inappropriate antidiuresis with high level of pS256 despite unchanged AVP and cAMP, we found a reduced PP2A expression and activity and reduced pS261. Similarly to what previously found in PKD1+/- mice, R-roscovitine treatment caused a significant decrease in intracellular calcium in MDCK cells. Our data indicate that a reduced activity of PP2A, secondary to reduced intracellular Ca2+ levels, promotes AQP2 trafficking independently of the AVP-PKA axis. This pathway may be relevant for explaining pathological states characterized by inappropriate AVP secretion and positive water balance.