PKA-Independent Vasopressin Signaling in Renal Collecting Duct

Vasopressin regulates renal water excretion by binding to the Gs-coupled vasopressin receptor (V2R) in collecting duct cells, resulting in cyclic AMP-dependent increases in epithelial water permeability through regulation of the aquaporin-2 (AQP2) water channel. Our prior studies showed that CRISPR-mediated deletion of protein kinase A (PKA) in cultured mpkCCD cells largely eliminates these regulatory events. These PKA-null cells provide a means of identifying PKA-independent signaling downstream from the V2 receptor. We carried out large-scale quantitative protein mass spectrometry (SILAC) to identify PKA-independent phosphorylation changes in response to V2R-selective vasopressin analog, dDAVP. The results show that V2R-mediated vasopressin signaling is predominantly, but not entirely, PKA-dependent. Target motif analysis of the phosphopeptides increased in response to dDAVP in PKA-null cells indicates that the vasopressin activates of one or more members of the AMPK/SNF1 subfamily of basophilic protein kinases. Among the upregulated phosphorylation sites were three known targets of SNF1-subfamily kinases, namely Lipe (S559), Crtc1 (S151) and Arhgef2 (S151). One of the phosphorylation sites that increased in occupancy in PKA-null cells was Ser256 of AQP2, a site critical for vasopressin-mediated trafficking of AQP2 to the cell surface. Beyond this, PKA-independent active site phosphorylation changes were also seen for protein kinases Stk39 (SPAK) and Prkci (Protein kinase C iota). Cyclic AMP levels were ~10-fold higher in PKA-null than in PKA-intact cells in the presence of phosphodiesterase inhibitor IBMX, consistent with a marked acceleration of cAMP production in PKA-null cells. The findings are indicative of substantial PKA-independent signaling downstream from the Gs-coupled V2 receptor.