Functional selective oxytocin-derived agonists discriminate between individual G protein family subtypes.

We used a bioluminescence resonance energy transfer (BRET) biosensor to screen for functional selective ligands of the human oxytocin receptor (OTR). We demonstrated that Oxytocin (OT) promoted the direct engagement and activation of Gq and all the Gi/o subtypes at the OTR. Other peptidic analogues, chosen because of specific substitutions in key OT structural/functional residues, all showed biased activation of G protein subtypes. No ligand, except OT, activated GoA or GoB, and, with only one exception, all the peptides that activated Gq also activated Gi2 and Gi3, but not Gi1, GoA or GoB, indicating a strong bias towards these subunits. Two peptides (DNalOVT and atosiban) activated only Gi1 or Gi3, failed to recruit ?-arrestins and did not to induce receptor internalization, providing the first clear examples of ligands differentiating individual Gi/o family members. Both analogs inhibited cell proliferation, showing that a single Gi subtype-mediated pathway is sufficient to prompt this physiological response. These analogs represent unique tools for examining the contribution of Gi/o members in complex biological responses and open the way to the development of drugs with peculiar selectivity profiles. This is of particular relevance because OT has been shown to improve symptoms in neurodevelopmental and psychiatric disorders characterized by abnormal social behaviors such as autism. Functional selective ligands, activating a specific G protein signaling pathway, may possess a higher efficacy and specificity on OT-based therapeutics.