This neurochemistry study (2022) presents structures of the serotonin receptor 5-HT2RA bound to psilocin, LSD, serotonin and the non-hallucinogenic analogue lisuride. The researchers were then able to design arrestin-biased ligands that displayed antidepressant-like activity in mice without hallucinogenic effects. The research presented here provides a foundation for the design of safe and effective non-hallucinogenic psychedelic analogues.
“Drugs that target the human serotonin 2A receptor (5-HT2AR) are used to treat neuropsychiatric diseases; however, many have hallucinogenic effects, hampering their use. Here, we present structures of 5-HT2AR complexed with the psychedelic drugs psilocin (the active metabolite of psilocybin) and D-lysergic acid diethylamide (LSD), as well as the endogenous neurotransmitter serotonin and the non-hallucinogenic psychedelic analogue lisuride. Serotonin and psilocin display a second binding mode in addition to the canonical mode, which enabled the design of the psychedelic IHCH-7113 (a substructure of antipsychotic lumateperone) and several 5-HT2AR β-arrestin–biased agonists that displayed antidepressant-like activity in mice but without hallucinogenic effects. The 5-HT2AR complex structures presented herein and the resulting insights provide a solid foundation for the structure-based design of safe and effective non-hallucinogenic psychedelic analogues with therapeutic effects.”
Authors: Dongmei Cao, Jing Yu, Huan Wang, Zhipu Luo, Xinyu Liu, Licong He, Jianzhong Qi, Luyu Fan, Lingjie Tang, Zhangcheng Chen, Jingsong Li, Jianjun Cheng & Sheng Wang
“Psychedelic drugs such as lysergic acid diethylamide (LSD) and mushroom-derived psilocybin exert their effects by binding the serotonin 2A receptor (5-HT2AR). These drugs also have antidepressant effects, but the hallucinations they cause complicate their use as therapeutics. Cao et al. present structures of 5-HT2AR bound to psychedelic drugs, the endogenous ligand serotonin, and the non-hallucinogenic drug lisuride. The structures reveal ligand-receptor interactions that cause a bias toward arrestin recruitment. Based on these insights, the authors designed arrestin-biased ligands that displayed antidepressant-like activity in mice without hallucination effects. Arrestin recruitment alone is insufficient for antidepressant effects, but the low G-protein signalling of the arrestin-biased ligands appears to allow antidepressant effects without causing hallucination. —VV”
Serotonin, or 5-HT, is a neurotransmitter that modulates many human behavioral processes. Psychedelic drugs, such as LSD and psilocybin, have shown promise for addressing many neuropsychiatric diseases.
Previous studies have shown that oleamide, an endogenous fatty acid amide, can potentiate human serotonin 2A receptor (5-HT2AR) – mediated signaling, suggesting that abnormal sensitivity of 5-HT2AR to fatty acids may be responsible for some aspects of psychiatric disorders.
Lipid activation of 5-HT2AR
Based on modeling and site-directed mutagenesis studies, serotonin and psilocin are predicted to bind to serotonin receptors in a similar manner to ergoline ligands. The conformations of two ergoline-bound 5-HT2AR structures are reminiscent of structures of serotonin bound to 5-HT1AR and 5-HT1DR.
All of our 5-HT2AR complex structures showed clear density maps occupying the previously identified side-extended pocket (SEP), and the glycerol groups of monoolein were inserted into the SEP in the serotonin-, psilocin-, LSD-, and lisuride-bound 5-HT2AR structures.
We found that monoolein directly binds to the serotonin and psilocin receptors and induces G protein partial agonism without detectable b-arrestin2 activity. The 5-HT2AR selective antagonist MDL100907 can block monoolein’s G protein partial agonism.
We found that oleamide, oleylethanolamide, and 2-oleoyl glycerol activated 5-HT2AR – mediated G protein signaling, but oleic acid and oleyl-La-lysophosphatidic acid did not.
A glycine residue at position 5.42 in 5-HT2AR allows the SEP to extend from the OBP, whereas other aminergic receptors have an alanine, serine, cysteine, or threonine and the side chain blocks the cavity. Lipids do not induce robust G protein signaling at 5-HT2BRor 5-HT2CR.
Our crystal structure of serotonin and psilocin bound to 5-HT2AR shows that the indole core of both ligands fits into a narrow cleft previously described as the EBP (20, 28). Mutagenesis of many contact residues to alanine reduced the affinity of serotonin and psilocin binding to 5-HT2AR.
The authors suggest that serotonin and psilocin occupy two different positions in the 5-HT2A receptor, the OBP and EBP, and that these positions might differentially affect receptor function. They also suggest that the L3627.35 residue in the EBP affects ligand bias.
We previously reported that the EBP in 5-HT2AR is different in ligand recognition, and that this results in divergent effects on ligand bias at 5-HT2BR, especially for b-arrestin signaling. We hypothesized that the serotonin and psilocin binding pose in the EBP would modulate b-arrestin signaling.
To understand the role of the EBP in 5-HT2AR – mediated b-arrestin signaling, the x-ray structures of the 5-HT2AR – lisuride and 5-HT2AR – LSD complexes were solved to a resolution of 2.6 . The overall structure of 5-HT2AR-bound LSD is similar to the previously reported structure.
We have previously shown that LSD’s diethylamide contacts TM3 and TM7 within the EBP and that this recognition is stereoselective. Interestingly, lisuride is an agonist of 5-HT2AR but not 5-HT2BR.
The two ethyl groups of LSD contact residue Y3707.43 in the EBP, which only interacts with one ethyl group of lisuride. This results in an auxiliary mechanism of 5-HT2AR – mediated b-arrestin signaling activation.
Structure-oriented synthesis of arrestin-biased compounds
We posited that targeting the EBP in 5-HT2AR could enhance b-arrestin recruitment, facilitating identification of b-arrestin – biased ligands. We identified three rigid moieties that could target the EBP and mimic the second binding pose of serotonin and psilocin, without engaging the bottom hydrophobic deep binding pocket that is responsible for antagonist activity.
Three molecules were synthesized that were able to bind to the EBP: IHCH-7113, IHCH-7117, and IHCH-7125. IHCH-7113 showed a weak preference for b-arrestin2 association over Gq signaling.
The tetracyclic scaffold of IHCH-7113 occupied the EBP in 5-HT2AR and alanine mutagenesis of the contacting residues decreased lumateperone’s affinity. This may explain the antagonist activity of lumateperone at 5-HT2AR.
We synthesized two analogs of lumateperone with different lengths of the linker group and phenyl substitutions to test the hypothesis that the 4-fluorophenyl group determines its antagonist activity at 5-HT2AR. These analogs were modest 5-HT2AR agonists and were arrestin-biased by factors of 6.70 and 12.76, respectively.
We solved the crystal structures of IHCH-7086 bound to 5-HT2AR, which showed b-arrestin – mediated signaling without detectable Gq activity. The 2-methoxyphenyl moiety of IHCH-7086 does not interact with the conserved TM5 serine, which potentially explains the nondetectable G protein activity of IHCH-7086 at 5-HT2AR.
Effects of 5HT2AR-mediated signaling on hallucination and antidepressant-like behavior
Research has shown that the mouse head twitch response (HTR) strongly correlates with the production of psychedelic-induced hallucinations in humans. However, lissuride lacks comparable psychoactive properties in humans and also fails to induce the HTR in mice.
We studied psychedelics, lisuride, and 5-HT2AR arrestin-bias ligands for their ability to induce HTR in mice. We found that IHCH-7113 produced HTR at doses as low as 0.125 mg/kg, and that this effect was abolished by the 5-HT2AR selective antagonist MDL100907.
We used heat maps to analyze the transduction efficiency of hallucinogenic psychedelics and their nonhallucinogenic analogs in G protein signaling and b-arrestin association at 5-HT2AR. The Y3707.43W mutation significantly reduced LSD’s transduction efficiency for both b-arrestin recruitment and G protein signaling.
Hallucinogens like psilocybin and LSD have been described to have potential therapeutic effects for depression (2). Lisuride, IHCH-7079, and IHCH-7086 were also shown to have antidepressant-like effects in vivo, and their effect was abolished by the 5-HT2AR selective antagonist MDL100907. C57BL/6J mice treated with corticosterone for 21 days showed an increase in immobility. IHCH-7079 and IHCH-7086 reduced immobility, and the effects were abolished by MDL100907, suggesting that their low efficacy may be sufficient for antidepressant effects.
Six new 5-HT2AR crystal structures have been reported, which reveal the structural basis of the lipid activation on 5-HT2AR and a second binding mode for serotonin and psilocin. This enables structure-based design of b-arrestin – biased ligands for treatment of neuropsychiatric diseases.