Direct Pore Binding as a Mechanism for Isoflurane Inhibition of the Pentameric Ligand-gated Ion Channel ELIC.

Qiang Chen,Yan Xu,Marta M Wells,Monica N Kinde,Aina E Cohen,Palaniappa Arjunan,Pei Tang

doi:10.1038/srep13833

Abstract

Pentameric ligand-gated ion channels (pLGICs) are targets of general anesthetics, but molecular mechanisms underlying anesthetic action remain debatable. We found that ELIC, a pLGIC from Erwinia chrysanthemi, can be functionally inhibited by isoflurane and other anesthetics. Structures of ELIC co-crystallized with isoflurane in the absence or presence of an agonist revealed double isoflurane occupancies inside the pore near T237(6′) and A244(13′). A pore-radius contraction near the extracellular entrance was observed upon isoflurane binding. Electrophysiology measurements with a single-point mutation at position 6′ or 13′ support the notion that binding at these sites renders isoflurane inhibition. Molecular dynamics simulations suggested that isoflurane binding was more stable in the resting than in a desensitized pore conformation. This study presents compelling evidence for a direct pore-binding mechanism of isoflurane inhibition, which has a general implication for inhibitory action of general anesthetics on pLGICs.

Highlights

General anesthetics are administered to approximately 256 million people each year[1]
In the transmembrane domain (TMD) of Pentameric ligand-gated ion channels (pLGICs), diverse intra- and inter-subunit allosteric sites for general anesthetic binding have been identified through various approaches, including NMR spectroscopy[15,16,17], photoaffinity labeling[11,18,19,20,21], crystallography[10,22,23], and molecular dynamics (MD) simulations[12,13,14,24,25]
This study provides solid structural data to support the significance of direct pore binding as a mechanism of functional inhibition of pLGICs by general anesthetics

Summary

Introduction

General anesthetics are administered to approximately 256 million people each year[1]. For the pore in the TMD, previous site-directed mutagenesis, electrophysiology measurements, and photoaffinity labeling suggested an open-channel blocking mechanism for functional inhibitions by anesthetics[26,27,28,29,30], but available structural data were not sufficient to support such a mechanism[10]. In addition to the previously proposed open-channel blocking action[26,27,28], the direct pore-binding mechanism suggested by our high-resolution structural data contains another component of anesthetic inhibition, which is achieved by stabilizing the resting channel conformation. Similar to isoflurane, several other representative general anesthetics, including halothane, sevoflurane, propofol, thiopental, and etomidate, inhibit ELIC and do so as effectively on ELIC as on nAChRs. This study provides solid structural data to support the significance of direct pore binding as a mechanism of functional inhibition of pLGICs by general anesthetics

Methods

Results

Conclusion