Label-Free Monitoring ofμ-Opioid Receptor–Mediated Signaling

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 m-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-DAla-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 Ga i , 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-NH 2 ), naloxone, and naltrexone behave like partial agonists. Even more intriguing, BRET experiments indicate that CTAP (D-Phe-CysTyr-D-Trp-Arg-Thr-Pen-Thr-NH 2 ) induces similar conformational changes as naltrexone at the Ga i -bg 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 labelfree 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.

Label-free cell signaling pathway deconvolution of angiotensin type 1 receptor reveals time-resolved G-protein activity and distinct AngII and AngIIIIV responses

“…Although some reports have described the capacity of AT1R to couple to Gi, to our knowledge coupling to Gs has not been found. Therefore, we only assessed the contributions of the Gi pathway using the inhibitor of Gi activation, PTX, an approached used previously by our group in a study of mu-opioid receptor signalling by SPR [27]. As shown in Fig.…”

“…Surface Plasmon Resonance (SPR) was previously validated by our group as detection method to monitor live cells responses to various GPCRs stimulations [26,27]. In our previous work, using confocal microscopy and SPR-based imaging to monitor actin cytoskeleton remodelling and atomic force microscopy to quantify cell contraction, we have shown that early events of the AT1R-dependent responses are associated with cell body contraction whereas the later phase of the response involves actin mobilization and extensive spreading of the cell [26].…”