Kappa but not delta or mu opioid receptors form homodimers at low membrane densities

Abstract

Opioid receptors (ORs) have been observed as homo-and heterodimers, but it is unclear if the dimers are stable under physiological conditions. Here we use three live-cell imaging approaches to assess dimerization of ORs at different expression levels. At high membrane densities, a split GFP assay reveals that OR dimerizes, while OR and OR stay monomeric. In contrast, singlemolecule imaging showed no OR dimers at low receptor densities. To reconcile our seemingly contradictory results, we used a high-density single-molecule assay to assess membrane protein interactions at densities 100x higher than conventional single-molecule imaging. We observe that OR is monomeric at low densities and forms dimers at densities that are considered physiological. In contrast, OR and OR stay monomeric even at the highest densities covered. The observation of long-lasting OR dimers but not higher order aggregates suggests that OR dimerization is a specific effect and not a result of increasing expression. IntroductionORs are G protein-coupled receptors (GPCRs) from class A with three genes coding for the OR, OR, and OR. Based on pharmacological profiles, more subtypes were proposed, which can be explained by the existence of splicing variants, posttranslational modifications and/or direct interactions between receptors. Dimerization of ORs has been covered in multiple studies, yet the conclusions were contradictory, as for many other GPCRs, likely due to the use of differing methodological approaches and experimental conditions [1].So far, the major techniques to assess dimerization of ORs were co-immunoprecipitation followed by Western blotting, and bioluminescence resonance energy transfer (BRET) [2][3][4][5][6]. Both are bulk techniques, where the signal is obtained from a large population of cells. Not only is the major part of the signal caused by a small, highly expressing fraction of the cells, but in addition, these few cells express the receptors at the highest density. Therefore, the signal mainly reflects the receptor's behavior at a membrane density that is far above the physiological range, but cannot capture the state of the receptors at low membrane densities, as they prevail in many cells in vivo.In this work, we quantitatively measured OR dimerization using different microscopy approaches that work at different densities. Our starting point was a split GFP fluorescence complementation assay in cells with expression levels of 50-100 / µm 2 . This assay gave us a first indication that the OR has a tendency to dimerize, while the OR and OR resembled the monomeric control, which was the PDGFR transmembrane domain (PDGFRTM). However, with conventional dualcolor single-molecule imaging at densities below 5 / µm 2 , we did not observe a significantly higher dimerization for OR than for the other ORs. Therefore, we turned to a recently developed technique called PhotoGate that allows tracking of single molecules in an originally crowded environment by controlling the density of fluorescent molecules in a re...

Induced pluripotent stem cells as a platform to understand patient‐specific responses to opioids and anaesthetics

“…Typical allosteric modulators can either enhance or diminish receptor signalling in response to an anaesthetic, but new drugs may act on both allosteric and orthosteric binding sites (Wisler, Rockman, & Lefkowitz, 2018). Receptor homodimers may also react differently to drugs than heterodimeric receptors (Cechova, Lan, Barthes, Jung, & Ulbrich, 2020). Finally, many of these receptors exhibit tissue‐specific expression and multiple splice variants (Barker et al, 1993; Knierim et al, 2019; Michel et al, 2014) (Figure 2).…”

“…Receptor homodimers may also react differently to drugs than heterodimeric receptors (Cechova, Lan, Barthes, Jung, & Ulbrich, 2020). Finally, many of these receptors exhibit tissue-specific expression and multiple splice variants (Barker et al, 1993;Knierim et al, 2019;Michel et al, 2014) (Figure 2).…”