Characterization of vasopressin V2 receptor mutants in nephrogenic diabetes insipidus in a polarized cell model

X-linked nephrogenic diabetes insipidus (NDI) is caused by mutations in the gene encoding the vasopressin V2 receptor (V2R). For the development of a tailored therapy for NDI, knowledge of the cellular fate of V2R mutants is needed. It would be useful when this fate could be predicted from the location and type of mutation. To identify similarities and differences in localization, maturation, stability, and degradation of COOH-terminal GFP-tagged V2R mutants, we stably expressed nine mutants in polarized Madin-Darby canine kidney cells. The mutants V2R-L44P, -Delta 62-64, -I130F, -S167T, - S167L, and -V206D were mainly expressed in the endoplasmic reticulum ( ER) as immature proteins. These mutants had relatively short half-lives due to proteasomal degradation, except for V2R-Delta 62-64. In contrast, V2R-R113W, -G201D, and -T204N were expressed in the ER and in the basolateral membrane as immature, high-mannose glycosylated, and mature complex-glycosylated proteins. The immature forms of V2R-R113W and -T204N, but not V2R-G201D, were rapidly degraded. The mature forms varied extensively in their stability and were degraded by only lysosomes (V2R-T204N and wild-type V2R) or lysosomes and proteasomes (V2R-G201D, -R113W). These data reveal that most missense V2R mutations lead to retention in the ER and suggest that mutations that likely distort a transmembrane domain or introduce a charged amino acid close to it make a V2R mutant more prone to ER retention. Because six of the mutants tested showed significant increases in intracellular cAMP levels on transient expression in COS cells, activation of these six receptors following rescue of cell-surface expression might provide a cure for NDI patients.