3D structure of a serotonin receptor

Communication between neurons involves surface receptors, which can transform a chemical signal from the outside into a flux of ions across the cell membrane, i.e. an electrical signal. The new 3D structure of one of these receptors, the serotonin type 3 receptor, reveals its anatomy in detail and provides a snapshot of the full ion pathway.

HFSP Long-Term Fellow Hugues Nury and colleagues
authored on Sun, 02 November 2014

The serotonin type 3 receptor (5-HT3 receptor) is found in the brain and in the nervous system of the gut. It belongs to a large family called “Cys-loop receptors”, the members of which all perform signal transduction: they sense the presence of a chemical (called neurotransmitter: serotonin, glycine, acetylcholine or GABA) and very rapidly convert this signal into a flux of ions, i.e. electricity. Together they play a central role in many cognitive processes, and as a consequence they have a major role in pharmacology: nicotine, benzodiazepines (such as Valium), general anesthetics, many anti-emetics (such as Ondansetron) all target Cys-loop receptors. The anti-emetics are potent inhibitors of the 5-HT3 receptor, widely used to fight the adverse effect of chemotherapies.

Obtaining the crystallographic structure of the mouse 5-HT3 receptor was a long quest. The first obstacle was quantity and the researchers used mammalian cell lines to express large amounts (milligrams) of the receptor. Crystals were obtained after purification but their quality was terrible. The second obstacle was homogeneity. After enzymatic treatments to remove the carbohydrates and the flexible parts of the protein, the receptor was more homogeneous but the quality of crystals was still insufficient. Small single-chain antibodies, derived from a library obtained after the immunization of a llama with the 5-HT3 receptor, were screened for their capacity to tightly bind and stabilize the isolated receptor. Finally, the complex between a 5-HT3 receptor and five antibodies produced crystals of good quality.

The shape of the receptor resembles that of a rocket inserted in the membrane with its conical nose inside the cell. The 5 antibodies are attached, like fins on the rocket, to the extracellular part. The receptor is composed of five identical subunits, which are symmetrically arranged around the central ion channel. The ions that cross the membrane go through extracellular and intracellular vestibules separated by a narrow pore, which contains the amino-acids of the protein that act as the gate of the channel and the ones that make it selective for cations. The 3D structure depicts the atomic organization of the binding site; it also sheds a new light on the intracellular domain where the exit door for ions is located and where important amino-acids play a key role in defining the conductance of the channel.

This work was carried out in collaboration by researchers in the laboratory of Horst Vogel at the EPFL (Switzerland) where Hugues Nury was a HFSP fellow, the CNRS (France), the University of Basel and the Paul Scherrer Institute (Switzerland), the startup company Theranyx (France).

Reference

X-ray structure of the mouse serotonin 5-HT3 receptor. Ghérici Hassaine, Cédric Deluz, Luigino Grasso, Romain Wyss, Menno B. Tol,   Ruud Hovius, Alexandra Graff, Henning Stahlberg, Takashi Tomizaki, Aline Desmyter, Christophe Moreau, Xiao-Dan Li, Frédéric Poitevin, Horst Vogel & Hugues Nury. Nature 2014, 512:276-281 doi:10.1038/nature13552

Pubmed link