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Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Aquaporin 2 (AQP2) trafficking is regulated by phosphorylation and dephosphorylation of serine residues in the AQP2 COOH terminus. Vasopressin (VP) binding to its receptor (V2R) leads to a cascade of events that result in phosphorylation of serine 256 (S256), S264, and S269, but dephosphorylation of S261. To identify which phosphatase is responsible for VP-induced S261 dephosphorylation, we pretreated cells with different phosphatase inhibitors before VP stimulation. Sanguinarine, a specific protein phosphatase (PP) 2C inhibitor, but not inhibitors of PP1, PP2A (okadaic acid), or PP2B (cyclosporine), abolished VP-induced S261 dephosphorylation. However, sanguinarine and VP significantly increased phosphorylation of ERK, a kinase that can phosphorylate S261; inhibition of ERK by PD98059 partially decreased baseline S261 phosphorylation. These data support a role of ERK in S261 phosphorylation but suggest that, upon VP treatment, increased phosphatase activity overcomes the increase in ERK activity, resulting in overall dephosphorylation of S261. We also found that sanguinarine abolished VP-induced S261 dephosphorylation in cells expressing mutated AQP2 S256A, suggesting that the phosphorylation state of S261 is independent of S256. Sanguinarine alone did not induce AQP2 membrane trafficking, nor did it inhibit VP-induced AQP2 membrane accumulation in cells and kidney tissues, suggesting that S261 does not play an observable role in acute AQP2 membrane accumulation. In conclusion, PP2C activity is required for S261 AQP2 dephosphorylation upon VP stimulation, which occurs independently of S256 phosphorylation. Understanding the pathways involved in modulating PP2C will help elucidate the role of S261 in cellular events involving AQP2., (Copyright © 2017 the American Physiological Society.)

Nephrogenic diabetes insipidus (NDI) is caused by impairment of vasopressin (VP) receptor type 2 signaling. Because potential therapies for NDI that target the canonical VP/cAMP/protein kinase A pathway have so far proven ineffective, alternative strategies for modulating aquaporin 2 (AQP2) trafficking have been sought. Successful identification of compounds by our high-throughput chemical screening assay prompted us to determine whether EGF receptor (EGFR) inhibitors stimulate AQP2 trafficking and reduce urine output. Erlotinib, a selective EGFR inhibitor, enhanced AQP2 apical membrane expression in collecting duct principal cells and reduced urine volume by 45% after 5 days of treatment in mice with lithium-induced NDI. Similar to VP, erlotinib increased exocytosis and decreased endocytosis in LLC-PK1 cells, resulting in a significant increase in AQP2 membrane accumulation. Erlotinib increased phosphorylation of AQP2 at Ser-256 and Ser-269 and decreased phosphorylation at Ser-261 in a dose-dependent manner. However, unlike VP, the effect of erlotinib was independent of cAMP, cGMP, and protein kinase A. Conversely, EGF reduced VP-induced AQP2 Ser-256 phosphorylation, suggesting crosstalk between VP and EGF in AQP2 trafficking and a role of EGF in water homeostasis. These results reveal a novel pathway that contributes to the regulation of AQP2-mediated water reabsorption and suggest new potential therapeutic strategies for NDI treatment., (Copyright © 2016 by the American Society of Nephrology.)