ßarrestin1-biased agonism at human δ-opioid receptor by peptidic and alkaloid ligands

Abstract We have previously reported on the differential regulation of the human δ-opioid receptor (hDOR) by alkaloid (etorphine) and peptidic (DPDPE and deltorphin I) ligands, in terms of both receptor desensitization and post-endocytic sorting. Since sarrestins are well known to regulate G protein-coupled receptors (GPCRs) signaling and trafficking, we therefore investigated the role of sarrestin1 (the only isoform expressed in our cellular model) in the context of the hDOR. We established clonal cell lines of SK-N-BE cells over-expressing sarrestin1, its dominant negative mutant (sarrestin1 319–418 ), and shRNA directed against endogenous sarrestin1. Interestingly, both binding and confocal microscopy approaches demonstrated that sarrestin1 is required for hDOR endocytosis only when activated by etorphine. Conversely, functional experiments revealed that sarrestin1 is exclusively involved in hDOR desensitization promoted by the peptides. Taken together, these results provide substantial evidence for a sarrestin1-biased agonism at hDOR, where sarrestin1 is differentially involved during receptor desensitization and endocytosis depending on the ligand.