Mechanism of negative μ-opioid receptor modulation by sodium ions

Negative allosteric modulation of G-protein coupled receptors (GPCRs) by Na⁺ ions was first described in the 1970s for opioid receptors (ORs) and has subsequently been detected for most class A GPCRs. In high-resolution structures of inactive-state class A GPCRs, a Na⁺ ion binds to a conserved pocket near residue D2.50, whereas active-state structures of GPCRs are incompatible with Na⁺ binding. Correspondingly, Na⁺ diminishes agonist affinity, stabilizes the receptors in the inactive state, and reduces basal signaling. We applied a mutual-information based analysis to μs-timescale biomolecular simulations of the μ-opioid receptor (μ-OR). Our results reveal that Na⁺ binding is coupled to a water wire linking the Na⁺ binding site with the agonist binding pocket and to rearrangements in polar networks propagating conformational changes to the agonist and G-protein binding sites. These findings provide a new mechanistic link between the presence of the ion, altered agonist affinity, receptor deactivation, and lowered basal signaling levels.