Insights into Distinct Signaling Profiles of the Μor Activated by Diverse Agonists

Drugs targeting the mu-opioid receptor (mu OR) are the most effective analgesics available but are also associated with fatal respiratory depression through a pathway that remains unclear. Here we investigated the mechanistic basis of action of lofentanil (LFT) and mitragynine pseudoindoxyl (MP), two mu OR agonists with different safety profiles. LFT, one of the most lethal opioids, and MP, a kratom plant derivative with reduced respiratory depression in animal studies, exhibited markedly different efficacy profiles for G protein subtype activation and beta-arrestin recruitment. Cryo-EM structures of mu OR-Gi1 complex with MP (2.5 angstrom) and LFT (3.2 angstrom) revealed that the two ligands engage distinct subpockets, and molecular dynamics simulations showed additional differences in the binding site that promote distinct active-state conformations on the intracellular side of the receptor where G proteins and beta-arrestins bind. These observations highlight how drugs engaging different parts of the mu OR orthosteric pocket can lead to distinct signaling outcomes.