Mechanism of negative μ-opioid receptor modulation by sodium ions

The negative allosteric modulation of G-protein coupled receptors (GPCRs) by Na+ ions, known as the sodium effect, was first described in the 1970s for opioid receptors (ORs). Since then, it has been detected almost universally amongst class A GPCRs. High-resolution structures of class A GPCRs in the inactive state exhibit a Na+ ion bound to a conserved pocket near residue D2.50, whereas the active state structures of GPCRs are incompatible with Na+ binding. Correspondingly, Na+ ions diminish the affinity of receptor agonists, stabilize the receptors in the inactive state, and reduce basal signalling levels. Despite these observations, a detailed mechanistic explanation of how Na+ ions negatively modulate the receptor and inhibit activation has remained elusive. Here, we apply a mutual-information based analysis method to s-timescale all-atom molecular dynamics simulations of the -OR to decipher conformational changes within the protein matrix and protein-internal water molecules that are directly coupled to the binding of Na+. Our results reveal that Na+ binding is tightly coupled to a water wire that links the Na+ binding site with the agonist binding pocket. Furthermore, Na+ binding leads to rearrangements in polar protein networks that propagate conformational changes both to the agonist and the intracellular G-protein binding sites via conserved micro-switch motifs. Our findings provide a mechanistic link between the presence of the ion and altered agonist binding affinity, receptor deactivation and the depression of basal signalling.