Abstract

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 mu opioid receptor (MOP). In HEK cells stably expressing a Flag-tagged version of human MOP, we compared the signals triggered by the non-internalizing and internalizing MOP agonists morphine and DAMGO, respectively. We studied three major and well-described components of MOP signaling: receptor internalization, G protein coupling, and ERK1/2 activation. Our results show that morphine and DAMGO display different profiles of receptor internalization and a similar ability to trigger the phosphorylation of ERK1/2. Our SPR analyses revealed that morphine and DAMGO evoke similar SPR signatures and that Gαi, cAMP-dependent pathways, and ERK1/2 have key roles in morphine- and DAMGO-mediated signaling. Most interestingly, we found that the so-called MOP neutral antagonists CTOP, naloxone, and naltrexone behave like partial agonists. Even more intriguing, BRET experiments indicate that CTAP 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 altogether, 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 (GPCRs) and complement the conventional approaches by revealing cellular responses that would have been undetectable otherwise.