TY - JOUR T1 - The Mu-Opioid Receptor Variant N190K is Unresponsive to Peptide Agonists Yet Can be Rescued by Small Molecule Drugs JF - Molecular Pharmacology JO - Mol Pharmacol DO - 10.1124/mol.110.064188 SP - mol.110.064188 AU - Jean-Philippe Fortin AU - Lei Ci AU - Jonathan C. Schroeder AU - Carmit Goldstein AU - Maria Claudia Montefusco AU - Inga Peter AU - Steven E. Reis AU - Gordon S. Huggins AU - Martin Beinborn AU - Alan S. Kopin Y1 - 2010/08/11 UR - http://molpharm.aspetjournals.org/content/early/2010/08/11/mol.110.064188.abstract N2 - The mu-opioid receptor (MOR) plays an important role in modulating analgesia, feeding behavior and a range of autonomic functions. In the current study, we investigated the degree to which thirteen naturally-occurring missense mutations affect the pharmacological properties of the human MOR. Following expression of each receptor in HEK293 cells, signaling (Gαi/o-mediated) induced by peptide agonists was assessed utilizing luciferase reporter gene assays. Multiple mutants (S66F, S147C, R260H, R265C, R265H and S268P) show a significant reduction in agonist potency. At the N190K variant, agonist-mediated signaling was essentially absent. ELISA, microscopic analysis and radioligand binding assays revealed that this mutant shows markedly reduced cell surface expression, whereas all other receptor variants were expressed at normal levels. Surface expression of the N190K variant could be increased by incubation with the alkaloid agonist buprenorphine, or with either of the structurally related MOR antagonists, naltrexone or naloxone. Surprisingly, both putative antagonists, despite being inactive at the wild type MOR, triggered a concentration-dependent increase in N190K receptor mediated signaling. In contrast, peptidic ligands failed to promote expression or rescue function of the N190K mutant. Subsequent analysis of the N190K variant in an ethnically diverse cohort identified this isoform in a subgroup of African Americans. Taken together, our studies reveal that the N190K mutation leads to severe functional alterations and, in parallel, changes the response to established MOR ligands. The extent to which this mutation results in physiological changes or affects drug sensitivity in selected populations (e.g. those with chronic pain or addiction) remains to be investigated. ER -