Biochemical basis of partial nephrogenic diabetes insipidus phenotypes

Biochemical properties of mutant type 2 vasopressin receptors (V2Rs) causing a partial phenotype of nephrogenic diabetes insipidus were investigated in transiently transfected HEK 293 cells. Cell surface expression of the V2R was not altered by substituting Asp85 in the second transmembrane region by Asn as determined by saturation binding assays. Although the affinity of the mutant V2R for arginine vasopressin (AVP) was reduced only 6-fold, the response of adenylyl cyclase activity to AVP revealed a 50-fold right shift in EC50 and a decreased maximum response for the mutant V2R. These data indicated that replacement of Asp85 by Asn affected coupling of the receptor to Gs, a conclusion substantiated by a 20-fold decrease in the calculated coupling efficiency of this receptor. The Gly201Asp mutation in the second extracellular loop, also found associated with an NDI partial phenotype, decreased cell surface expression of the V2R with minor reduction in ligand-binding affinity and coupling efficiency to Gs. A pronounced difference was observed for this mutant V2R between the stimulation of adenylyl cyclase activity promoted by AVP and the V2 vasopressin receptor agonist deamino[Cys1,D-Arg8]-vasopressin, suggesting an involvement of Gly201 in the selectivity of the receptor for different ligands. These data demonstrated that while decreased ligand-binding affinity and decreased coupling to Gs are responsible for the attenuation of response to ligand in the Asp85Asn mutant V2R, cell surface expression of the V2R is the major factor reducing cellular responses to ligand for the Gly201Asp mutant V2R.