GPCRs: Adrenergic receptor gets bent out of shape by morphine
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Morphine inhibits adrenergic receptor signaling by inducing a trans-conformational switch in the α2A-adrenergic receptor (α2A-AR) when in a complex with the morphine receptor, MOR.
G protein-coupled receptors (GPCRs) are classically thought to signal as monomers. However, recent studies have indicated that GPCRs can also exist as constitutive homo- and hetero-oligomers, which have been shown to modulate signaling to G proteins. In Nature Chemical Biology, Jean-Pierre Vilardaga, Martin Lohse and colleagues now explore the molecular mechanisms that mediate GPCR heterodimer cross-talk and identify direct trans-inhibition as a mechanism for receptor inactivation. They reveal that a complex consisting of norepinephrine (NE)-bound α2A-adrenergic receptor (α2A-AR) and μ-opioid receptor (MOR) undergoes a conformational switch upon binding of the MOR agonist morphine. This conformational change decreases both inhibitory G protein (Gi) and MAP kinase activation.
The authors used fluorescence resonance energy transfer (FRET) to verify a previously reported agonist-independent interaction between α2A-AR and MOR. They also employed a recently developed FRET technique variation that detects real time intramolecular GPCR conformation changes in live cells. Cyan fluorescent protein (CFP) was fused to the C terminus of α2A-AR, and a tetracysteine motif — which can be targeted by fluorescein arsernical hairpin binder (FlAsH) — was inserted into the intracellular domain of this receptor. The FRET signal from HEK293 cells transfected with MOR and the modified adrenergic receptor demonstrated an NE-induced conformational change in α2A-AR that is indicative of the receptor's active state. Treatment of the cells with both NE and morphine triggered a further rapid and reversible conformational switch in α2A-AR.
To determine whether effector signaling properties could be influenced by the MOR–α2A-AR trans-conformation effect, Vilardaga and colleagues investigated the modulation of Gi activation. They measured FRET changes between yellow fluorescent protein (YFP)-tagged Gαi and CFP-tagged Gγ2 in live cells stimulated with both GPCR ligands. Indeed, morphine specifically suppressed NE-induced Gi activation. Further downstream, morphine impaired NE-induced ERK1/2 phosphorylation in a concentration-dependent manner. Thus, activation of the opioid receptor protomer hinders adrenergic receptor-mediated cell signaling.
These findings depict a novel mechanism of rapid GPCR inactivation: the direct isomerization of the α2A-AR protomer by its activated binding partner, MOR, results in inhibition of adrenergic receptor mediated signaling. It will be interesting to test whether this mechanism applies to other GPCR oligomers and it remains to be seen whether this mode of MOR cross-talk can mediate the analgesic effects of morphine.
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Reference
Vilardaga, J-P., Nikolaev, V. O., Lorenz, K., Ferrandon, S., Zhuang, Z. & Lohse, M. J. Conformational cross-talk between α2A-adrenergic and μ-opioid receptors controls cell signaling. Nature Chemical Biology4, 126-131 (2008) | Article | PubMed |
