RT Journal Article SR Electronic T1 Label-Free Monitoring of μ-Opioid Receptor–Mediated Signaling JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP 138 OP 149 DO 10.1124/mol.114.093450 VO 86 IS 2 A1 Philippe Bourassa A1 Hanieh Bagheri Tudashki A1 Graciela Pineyro A1 Michel Grandbois A1 Louis Gendron YR 2014 UL http://molpharm.aspetjournals.org/content/86/2/138.abstract AB In this study, we used a combination of traditional signaling investigation approaches, bioluminescence resonance energy transfer (BRET) biosensors, and the label-free approach surface plasmon resonance (SPR) spectroscopy to monitor the signaling cascades of the μ-opioid receptor (MOP). In human embryonic kidney cells stably expressing a Flag-tagged version of human MOP, we compared the signals triggered by the noninternalizing and internalizing MOP agonists morphine and DAMGO (Tyr-d-Ala-Gly-N-methyl-Phe-Gly-ol), respectively. We studied three major and well described components of MOP signaling: receptor internalization, G protein coupling, and activation of extracellular signal-regulated kinase ERK1/ERK2. Our results show that morphine and DAMGO display different profiles of receptor internalization and a similar ability to trigger the phosphorylation of ERK1/ERK2. Our SPR analyses revealed that morphine and DAMGO evoke similar SPR signatures and that Gαi, cAMP-dependent pathways, and ERK1/ERK2 have key roles in morphine- and DAMGO-mediated signaling. Most interestingly, we found that the so-called MOP neutral antagonists CTOP (d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH2), naloxone, and naltrexone behave like partial agonists. Even more intriguing, BRET experiments indicate that CTAP (d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2) induces similar conformational changes as naltrexone at the Gαi-βγ interface, whereas it appears as an inverse agonist based on its SPR response thus indicating distinct signaling mechanisms for the two ligands. Taken together, our results support the usefulness of label-free methods such as SPR to study whole-cell responses and signaling cascades triggered by G protein–coupled receptors and complement the conventional approaches by revealing cellular responses that would have been otherwise undetectable.