Effectiveness and mechanistic effects of green mamba related peptide MQ232 in an orthologous mouse model of autosomal dominant polycystic kidney disease.

Tolvaptan, a vasopressin V2 receptor (V2R) inverse agonist, is the only treatment specifically approved for autosomal dominant polycystic kidney disease (ADPKD). In addition to inhibiting cAMP synthesis, it has other effects on V2R signaling. Our goal was to mechanistically characterize the effects of MQ232, an optimized peptide of the green mamba snake and a new V2R inverse agonist, in Pkd1RC/RC mice. Pkd1RC/RC mice were treated with MQ232 (0.5, 2, 4, or 8 nmols/kg/hour via minipump), tolvaptan (0.2% in chow), or vehicle from four to 16 weeks of age. MRI measurements of kidney volume were used for randomization and endpoint, urine was collected at 12 and 16 weeks, and mice were sacrificed at 16 weeks for blood and tissue collections. MQ232 and tolvaptan equally attenuated PKD. In male mice, increasing doses of MQ232 were associated with greater reductions in kidney cAMP, aquaporin-2 expression and phosphorylation, and with higher urine outputs, but equally ameliorated PKD; there was no correlation between kidney cAMP or urine output with disease attenuation. Phosphorylated/total ratios for Src, ERK, CRAF, Akt, RPS6, and Stat3 were, in general, lower in the treated mice, but this was less or reversed with increasing MQ232 doses. Analysis of previous studies with tolvaptan showed similar dose-dependent effects. This is consistent with these compounds being dual efficacy V2R ligands with inverse agonistic activity on Gαs/cAMP and, at high doses, activating β-arrestin-dependent or -independent Src and ERK signaling. Both compounds downregulated PAPPA. MQ232 inhibited IGF1Rβ phosphorylation, whereas tolvaptan at the dose/schedule used in the study did not. Vasopressin binding to V2R activates many cystogenic pathways: Gαs/cAMP, β-arrestin-Src and β-arrestin-ERK, and PAPPA and IGF1R signaling. Activation of cAMP-independent signaling may limit the benefit from MQ232 and tolvaptan when administered at high doses, questioning the rationale of titrating these compounds to maximally tolerated doses to treat ADPKD. Improved understanding of V2R signaling in ADPKD will lead to better therapies.