title: Role of Phosphorylation Sites in Desensitization of µ-Opioid Receptor.
abstract: Phosphorylation of residues in the C-terminal tail of the µ-opioid receptor (MOPr) is thought to be a key step in desensitization and internalization. Phosphorylation of C-terminal S/T residues is required for internalization (Just et al., 2013), but its role in desensitization is unknown. This study examined the influence of C-terminal phosphorylation sites on rapid desensitization of MOPr. Wild-type MOPr, a 3S/T-A mutant (S363A, T370A, S375A) that maintains internalization, 6S/T-A (S363A, T370A, S375A, T376A, T379A, T383A) and 11S/T-A (all C-terminal S/T residues mutated) mutants not internalized by MOPr agonists were stably expressed in AtT20 cells. Perforated patch-clamp recordings of MOPr-mediated activation of G-protein-activated inwardly rectifying potassium channel (Kir3.X) (GIRK) conductance by submaximal concentrations of Met(5)-enkephalin (ME) and somatostatin (SST; coupling to native SST receptor [SSTR]) were used to examine desensitization induced by exposure to ME and morphine for 5 minutes at 37°C. The rates of ME- and morphine-induced desensitization did not correlate with phosphorylation using phosphorylation site-specific antibodies. ME-induced MOPr desensitization and resensitization did not differ from wild-type for 3S/T-A and 6S/T-A but was abolished in 11S/T-A. Morphine-induced desensitization was unaffected in all three mutants, as was heterologous desensitization of SSTR. Morphine-induced desensitization (but not ME) was reduced by protein kinase C inhibition in wild-type MOPr and abolished in the 11S/T-A mutant, as was heterologous desensitization. These findings establish that MOPr desensitization can occur independently of S/T phosphorylation and internalization; however, C-terminal phosphorylation is necessary for some forms of desensitization because mutation of all C-terminal sites (11S/T-A) abolishes desensitization induced by ME.
authors.affiliation: Discipline of Pharmacology, University of Sydney, New South Wales, Sydney, Australia (A.Y., S.Si., Y.-P.D., M.J.C.); and Institute of Pharmacology and Toxicology, Jena University Hospital, Friedrich Schiller University Jena, Jena, Germany (E.M., S.Sc.).
authors.forename: Arsalan
authors.initials: A
authors.lastname: Yousuf
authors.affiliation: Discipline of Pharmacology, University of Sydney, New South Wales, Sydney, Australia (A.Y., S.Si., Y.-P.D., M.J.C.); and Institute of Pharmacology and Toxicology, Jena University Hospital, Friedrich Schiller University Jena, Jena, Germany (E.M., S.Sc.).
authors.forename: Elke
authors.initials: E
authors.lastname: Miess
authors.affiliation: Discipline of Pharmacology, University of Sydney, New South Wales, Sydney, Australia (A.Y., S.Si., Y.-P.D., M.J.C.); and Institute of Pharmacology and Toxicology, Jena University Hospital, Friedrich Schiller University Jena, Jena, Germany (E.M., S.Sc.).
authors.forename: Setareh
authors.initials: S
authors.lastname: Sianati
authors.affiliation: Discipline of Pharmacology, University of Sydney, New South Wales, Sydney, Australia (A.Y., S.Si., Y.-P.D., M.J.C.); and Institute of Pharmacology and Toxicology, Jena University Hospital, Friedrich Schiller University Jena, Jena, Germany (E.M., S.Sc.).
authors.forename: Yan-Ping
authors.initials: YP
authors.lastname: Du
authors.affiliation: Discipline of Pharmacology, University of Sydney, New South Wales, Sydney, Australia (A.Y., S.Si., Y.-P.D., M.J.C.); and Institute of Pharmacology and Toxicology, Jena University Hospital, Friedrich Schiller University Jena, Jena, Germany (E.M., S.Sc.).
authors.forename: Stefan
authors.initials: S
authors.lastname: Schulz
authors.affiliation: Discipline of Pharmacology, University of Sydney, New South Wales, Sydney, Australia (A.Y., S.Si., Y.-P.D., M.J.C.); and Institute of Pharmacology and Toxicology, Jena University Hospital, Friedrich Schiller University Jena, Jena, Germany (E.M., S.Sc.) mac.christie@sydney.edu.au.
authors.forename: MacDonald J
authors.initials: MJ
authors.lastname: Christie
mesh_terms: D001665:Binding Sites / Q000502:physiology; D002460:Cell Line; D006801:Humans; D010766:Phosphorylation / Q000502:physiology; D017450:Receptors, Opioid, mu / Q000235:genetics* / Q000378:metabolism*
chemical_list: D017450:Receptors, Opioid, mu