AVPR2

V2R is the class A GPCR that translates circulating vasopressin (AVP) into renal water reabsorption - making it the master switch for urine concentration and fluid homeostasis. It also drives hemostatic release of von Willebrand factor and factor VIII from vascular endothelium. Loss-of-function mutations cause X-linked nephrogenic diabetes insipidus (NDI); gain-of-function variants cause nephrogenic syndrome of inappropriate antidiuresis (NSIAD). Approved antagonists (tolvaptan) and agonists (desmopressin) make this one of the most drug-validated peptide-adjacent targets in nephrology and hemostasis.

AVPR2 sits at Xq28, 3 exons, 371 amino acids, ~40 kDa. The receptor presents a canonical seven-transmembrane rhodopsin fold with a DRY motif at TM3/ICL2 and palmitoylation at Cys341/342. AVP binds the orthosteric pocket with Kd ~1 nM via a disulfide-stabilized cyclic conformation, engaging Gln96 (TM2), Lys116/Gln119 (TM3), and Phe287/288 (TM6). Activation outwardly displaces TM6 by 10–13 Å, opening the Gs docking crevice → adenylyl cyclase (AC6 isoform) → cAMP → PKA → phosphorylation of aquaporin-2 (AQP2) at Ser256/Ser269 → AQP2 insertion into apical membrane of collecting duct principal cells. GRK-mediated β-arrestin recruitment drives desensitization and clathrin-mediated internalization. In endothelium, the same Gs/cAMP axis triggers Weibel-Palade body exocytosis, releasing vWF and factor VIII into circulation. Over 300 loss-of-function AVPR2 mutations are catalogued; R137C/L gain-of-function mutations cause constitutive signaling and NSIAD.

Desmopressin (dDAVP) - N-terminal deamination plus D-Arg8 - is the approved peptide-class V2R agonist, with >1000-fold selectivity over V1a/V1b and a 1.5–2.5 hour half-life. Clinical indications: central diabetes insipidus, nocturnal enuresis, mild hemophilia A, and type 1 von Willebrand disease. Tolvaptan and conivaptan are approved non-peptide V2R antagonists for hyponatremia (SIADH, heart failure, cirrhosis) and ADPKD (tolvaptan). Pharmacoperones - pharmacological chaperones such as SR49059 - rescue ER-retained NDI mutants by stabilizing partially folded receptor, restoring up to 50% cell-surface expression in vitro; clinical translation remains early. For peptide research, the tractable recipes are: dDAVP analogs with further modifications for prolonged half-life or enhanced selectivity; cyclic AVP scaffolds with SAR-informed substitutions at positions 2–4 to probe subtype selectivity; and competitive fragment-based antagonists as research tools or starting points for NDI pharmacoperone development. The endothelial vWF/FVIII release axis is also an underexplored angle for hemostatic peptides.