5-HT2C Receptor Structures Reveal the Structural Basis of GPCR Polypharmacology

Abstract

Drugs frequently require interactions with multiple targets-via a process known as polypharmacology-to achieve their therapeutic actions. Currently, drugs targeting several serotonin receptors, including the 5-HT receptor, are useful for treating obesity, drug abuse, and schizophrenia. The competing challenges of developing selective 5-HT receptor ligands or creating drugs with a defined polypharmacological profile, especially aimed at G protein-coupled receptors (GPCRs), remain extremely difficult. Here, we solved two structures of the 5-HT receptor in complex with the highly promiscuous agonist ergotamine and the 5-HT receptor-selective inverse agonist ritanserin at resolutions of 3.0 Å and 2.7 Å, respectively. We analyzed their respective binding poses to provide mechanistic insights into their receptor recognition and opposing pharmacological actions. This study investigates the structural basis of polypharmacology at canonical GPCRs and illustrates how understanding characteristic patterns of ligand-receptor interaction and activation may ultimately facilitate drug design at multiple GPCRs.

Structural determinants of 5-HT2B receptor activation and biased agonism

“…Although lisuride is a 5-HT 2B R antagonist, it lacks selectivity for 5-HT 2B R, as is commonly observed for many ergolines 19 . LY266097, however, is a purported selective 5-HT 2B R antagonist 31 that contains a distinct tetrahydro-beta-carboline pharmacophore, and determining the binding mode of LY266097 could illuminate novel structural determinants 5-HT 2B R selectivity.…”

“…1c, Table 1). Methylergonovine forms a salt bridge with D135 3.32 in the presumed orthosteric site, and the ergoline ring system forms an edge-to-face π- π stack with residues F340 6.51 and F341 6.52 in TM6—interactions commonly observed in aminergic 20,23 and 5-HT structures 13,17,19 . The binding mode of methylergonovine when compared with LSD and ergotamine bound to 5-HT 2B R reveals a subtly different positioning of the indole N (1)-H towards TM5 residues, with ERG the deepest toward A225 5.46 and LSD the shallowest toward the backbone of G221 5.42 (Supplementary Fig.…”

“…5g), a key residue implicated in conferring 5-HT 2 subtype-selectivity 19 . To determine if LY266097’s methyl substitution is partly responsible for Gq-partial agonism, we synthesized the des-methyl analog of LY266097 finding that it is nearly a full agonist (EC 50 = 20 nM Emax = 93%, Fig.…”

“…Although BRIL fusion 5-HT 2B R structures adopt the “active intermediate” state, more ‘active-like’ 5-HT 2B R structures will likely shed light on key trigger motifs involved in G protein versus β-arrestin2 recruitment. Indeed, current inactive state structures of 5-HT 2C 19 and 5-HT 1B 18 , which have both been co-crystallized with inverse agonists, implicate the conserved P-I-F trigger as motifs essential for inactivation and biased signaling.…”

“…To date, three 5-HT receptors have been solved by X-ray crystallography: the 5-HT 1B 17,18 , 5-HT 2B 12 , and 5-HT 2C 19 receptors, all in complex with the VHD-inducing antimigraine drug ERG. For the 5-HT 2B receptor, LSD and ERG structures are available and reveal that ergot ligands engage a presumed orthosteric binding pocket (OBP), which is likely shared with the endogenous ligand 5-HT 12,13,17 .…”