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Structural studies of mammalian Cys-loop receptors

Periodic Reporting for period 4 - PentaBrain (Structural studies of mammalian Cys-loop receptors)

Reporting period: 2019-12-01 to 2021-11-30

The transmission of information from one neuron to the next is mediated by a large set of neurotransmitter receptors. Among those, the project aimed at understanding better one class of receptors named pentameric ligand-gated ion channels. Why ? Because they participate in many physiological functions (voluntary motion, memory, sleep, anxiety, reward, pain, ...) are involved in numerous pathologies (epilepsy, Parkinson and Alzheimer diseases, ...) and are the targets of a legion of psycho-active and therapeutic compounds (including nicotine, antidepressants, antiemetics, anesthetics, ...). How ? By looking at the molecular structure of those receptors to decipher how drugs bind to them and how this binding eventually yield a signalling output.
In terms of methods, we have optimised ways of obtaining large quantities of pure and stable receptors, which enables their imaging by cryo-electron microscopy. We also participated in making that imaging easier when the receptors are embedded into a tiny discs made of lipids.
In terms of applications, we have shown how serotonin receptors activate and how a panel of antiemetics (including tropisetron and palonosetron) or an antidepressant (vortioxetine) inhibit them. We have also shown how agonists, and in particular nicotine, binds to the muscle-type acetylcholine receptor. We have identified that agonist-bound receptors (both the serotonin and the acetylcholine ones) can exist in intermediate states. We have described how the receptors block or allow the passage of ions (the signalling) through a gate, using a mechanism of wetting/dewetting.
Taken together, the project has shed light onto the molecular mechanism of action and the structural pharmacology of pentameric-ligand channels and has helped the development of techniques for their study.
During the project, we have explored the molecular working of several pentameric ligand-gated ion channels

A large emphasis was put on the serotonin 5-HT3 receptor. Using electron microscopy, we have obtained four structures of the serotonin 5-HT3 receptor. With an inhibited state, a putative pre-active state, and two putative distinct open states, these structures have extended our understanding of the gating mechanisms. They have also provided unprecedented details of serotonin binding and of orthosteric and allosteric site geometries. Those key results were published in an article lead by Dr. Polovinkin, the first PhD student of the team, in the journal Nature in 2018. We have also provided a more exhaustive description of the binding of anti-emetics to the serotonin receptor, using a combination of electron microscopy and molecular dynamics.

We have also studied the acetylcholine receptor that sits at the neuromuscular junction, using the homologous receptor from the Torpedo fish. The Torpedo fish receptor is of key historical importance : it was the first receptor ever discovered. We have shown how agonists, and in particular nicotine at high concentration, bind to and activate the acetylcholine receptor. We have described a possible intermediate state of the receptor in the presence of an agonist, a result that questions the assumption that agonist-bound receptors rather exist in a state called desensitised, and is therefore of broad relevance to the research field.

Those results were reported in scientific publications, and presented at European and international conferences by the team members.
When the project was initiated, there were only few structures of mammalian pentameric ligand-gated channels, in most cases obtained by crystallography. At the end of the project, the panorama of the research field has changed a lot : we among others participated in enriching the structural knowledge with quite many new structures, in most cases obtained by electron microscopy. In turns those structures enriched our mechanistic and pharmacological knowledge. One of the contribution of the team that goes beyond the state of the art have been to openly question the assignment of structures to physiological states, in particular to the open and desensitised states. Indeed, the challenge of matching structures to states without ambiguity transcends our sole results on serotonin and acetylcholine receptors and pertains to the whole field of pentameric ligand-gated channels structural biology.