Chinese Academy of Sciences scholar Cell published an article showing how fentanyl and morphine act on μ opioid receptors

Pain, especially chronic pain, is a common neurological phenomenon
.
The most common chronic pains include low back pain, arthritis pain, migraines, and cancer pain, all of which seriously affect people's physical and mental health
.
Opioids, such as morphine and fentanyl, are currently the most widely used powerful
painkillers.
They exert analgesic effects
by acting on G protein-coupled receptors (opioid receptors).

Most opioids used clinically are μ opioid receptor (μ OR) agonists
.
Opioid use can cause many side effects, including addiction, respiratory depression, and constipation
.
It has previously been reported that the analgesic effect of opioids is mediated by the G protein signaling pathway, while side effects are caused
by the astatin signaling pathway of μOR.
However, the lack of molecular understanding of the G protein priority signaling mechanism of μOR greatly hinders the rational design and discovery of G protein biased μOR agonists, which may be safer for the treatment of pain
.

A research team led by Xu Huaqiang/Zhuang Youwen, Xie Xin and Wang Mingwei from the Shanghai Institute of Materia Medica, Chinese Academy of Sciences recently published a study in the journal Cell reporting and analyzing the high-resolution cryo-electron microscopy (cryo-EM) structure of opioid analgesics such as fentanyl, morphine and oleoidine to activate μOR in humans, revealing for the first time the recognition
and activation mechanism of fentanyl and morphine-induced μOR.

For the first time, researchers obtained the three-dimensional structure
of human μOR bound to equilibrium agonists such as fentanyl, morphine, and peptide tyr-d-ala-gly-n-me-phy-gly-ol (DAMGO).
These agonists have G protein and blockin signaling activity
.
The researchers also bound
μOR to G protein-preference agonists such as TRV130, SR17018, and PZM21.

Then, through functional analysis and molecular dynamics simulation at the cellular level, the signaling characteristics
of μOR under different signaling agonist activation were analyzed.
The results showed that fentanyl occupies an extra binding bag
on the outside of the μOR cells around TM2 and TM3 compared to morphine.
The fentanyl aniline ring side chain forms a π-π hydrophobic interaction
with residues W295 and Y328.
The longer fentanyl interacts with μOR, the higher the potency of fentanyl than morphine
.

Based on the structure of fentanyl-bound μOR, the researchers further explored the structure-activity relationship
between fentanyl and its derivatives and μOR through molecular docking and mutagenesis.
They found that the efficacy of fentanyl and fentanyl analogues was highly correlated
with varying degrees of interaction between ligands and μOR residues (D149, Y150, W135, and W320).

Intensive structural analysis and molecular dynamics simulations showed that G protein-biased agonists such as PZM21 tend to bind
to the TM2/3 side of the μOR ligand binding capsule.
In contrast, equilibrium agonists such as fentanyl and DAMGO interact more extensively and equilibrium with the μOR transmembrane domain, resulting in μOR having a more compact intracellular lumen
.
This condition favors the inhibin coupling of μOR, thus explaining the molecular determinants
of μOR inhibitor activity.

Fentanyl and its analogues are the main cause of
the "opioid crisis.
" However, how they bind and activate μOR remains a mystery
.
This study gives for the first time the structure of fentanyl binding to μOR and reveals the specific pattern
of fentanyl binding to morphine.
It provides insight
into SAR for fentanyl and its analogues.
Through the combined application of multiple functional analyses and molecular dynamics simulations, molecular understanding
of μOR bias and ligand selectivity is increased.
This study deepens the understanding of the regulatory mechanisms of μOR signaling and may contribute to the development of next-generation opioid analgesics
with fewer side effects.

Article link: https://doi.
org/10.
1016/j.
cell.
2022.
09.
041

Molecular recognition of morphine and fentanyl by the human μ-opioid receptor