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Pentameric ligand-gated ion channels (pLGIC) catalyze the selective transfer of ions across the cell membrane in response to a specific neurotransmitter. A variety of chemically diverse molecules, including the Alzheimer's drug memantine, block ion conduction at vertebrate pLGICs by plugging the channel pore. We show that memantine has similar potency in ELIC, a prokaryotic pLGIC, when it contains an F16'S pore mutation. X-ray crystal structures, using both memantine and its derivative, Br-memantine, reveal that the ligand is localized at the extracellular entryway of the channel pore, and the pore is in a more closed conformation than wild-type ELIC in both the presence and absence of memantine. However, using voltage clamp fluorometry we observe fluorescence changes in opposite directions during channel activation and pore block, revealing an additional conformational transition not apparent from the crystal structures. These results have important implications for drugs such as memantine, which block channel pores. publisher: Elsevier articletitle: The Prokaryote Ligand-Gated Ion Channel ELIC Captured in a Pore Blocker-Bound Conformation by the Alzheimer’s Disease Drug Memantine journaltitle: Structure articlelink: http://dx.doi.org/10.1016/j.str.2014.07.013 content_type: article copyright: Copyright © 2014 Elsevier Ltd. All rights reserved. ispartof: Structure vol:22 issue:10 pages:1399-1407 ispartof: location:United States status: published

The Erwinia ligand-gated ion channel (ELIC) is a bacterial homologue of vertebrate Cys-loop ligand-gated ion channels. It is activated by GABA, and this property, combined with its structural similarity to GABAA and other Cys-loop receptors, makes it potentially an excellent model to probe their structure and function. Here we characterise the pharmacological profile of ELIC, examining the effects of compounds that could activate or inhibit the receptor. We confirm that a range of amino acids and classic GABAA receptor agonists do not elicit responses in ELIC, and we show the receptor can be at least partially activated by 5-aminovaleric acid and γ-hydroxybutyric acid, which are weak agonists. A range of GABAA receptor non-competitive antagonists inhibit GABA-elicited ELIC responses including α-endosulfan (IC50 = 17 μM), dieldrin (IC50 = 66 μM), and picrotoxinin (IC50 = 96 μM) which were the most potent. Docking suggested possible interactions at the 2′ and 6′ pore-lining residues, and mutagenesis of these residues supports this hypothesis for α-endosulfan. A selection of compounds that act at Cys-loop and other receptors also showed some efficacy at blocking ELIC responses, but most were of low potency (IC50 > 100 μM). Overall our data show that a number of compounds can inhibit ELIC, but it has limited pharmacological similarity to GLIC and to Cys-loop receptors., Highlights ► ELIC is structurally and functionally similar to Cys-loop receptors. ► ELIC can be activated by GABA but not other GABAA receptor agonists. ► ELIC responses are blocked by some compounds that block the channel of GABA-activated and other Cys-loop receptors. ► The potency of the compounds that block ELIC is lower than in Cys-loop receptors and in GLIC. ► ELIC pharmacology is distinct from that of related receptors.

International audience; Chronic pain is a highly prevalent and poorly managed human health problem. We used microarray-based expression genomics in 25 inbred mouse strains to identify dorsal root ganglion (DRG)-expressed genetic contributors to mechanical allodynia, a prominent symptom of chronic pain. We identified expression levels of Chrna6, which encodes the α6 subunit of the nicotinic acetylcholine receptor (nAChR), as highly associated with allodynia. We confirmed the importance of α6* (α6-containing) nAChRs by analyzing both gain- and loss-of-function mutants. We find that mechanical allodynia associated with neuropathic and inflammatory injuries is significantly altered in α6* mutants, and that α6* but not α4* nicotinic receptors are absolutely required for peripheral and/or spinal nicotine analgesia. Furthermore, we show that Chrna6's role in analgesia is at least partially due to direct interaction and cross-inhibition of α6* nAChRs with P2X2/3 receptors in DRG nociceptors. Finally, we establish the relevance of our results to humans by the observation of genetic association in patients suffering from chronic postsurgical and temporomandibular pain.

Pentameric ligand-gated ion channels (pLGICs) catalyze the selective transfer of ions across the cell membrane in response to a specific neurotransmitter. A variety of chemically diverse molecules block ion conduction by plugging the channel pore. Understanding the structural basis of pore block is important, as some of these molecules, including the antipsychotic chlorpromazine, the local anaesthetic lidocaine and the Alzheimer's disease drug memantine exert their therapeutic effects by specifically blocking these and other receptors. In this study, we report X-ray crystal structures of a prokaryote model ion channel ELIC containing a pore mutation F16'S, which is inhibited by pore blockers with affinities similar to human pLGICs. Structures in complex with memantine and its brominated derivative, Br-memantine, reveal that these pore blockers bind at the extracellular entryway of the channel pore, and cause a structural change, stabilizing the pore-lining helices in a conformation that is more closed than in the unbound structure. In addition, we observe that memantine binds at the agonist binding site, where it can act as a competitive antagonist. We further investigated the conformational dynamics of the pore domain by using voltage clamp fluorometry, which demonstrated that the pore helix orientation changes in opposite directions during pore block and channel activation, consistent with the crystal structures. These data are the first to reveal a conformational change in the pore initiated by a channel blocker, and have important implications for the action of drugs such as memantine which bind to channel pores.

The Gloeobacter ligand-gated ion channel, GLIC, has up to 28% sequence identity with eukaryotic Cys-loop receptors, and many key residues are conserved, especially in the 2nd trans-membrane pore lining region, M2. The M2 region is responsible for ion selectivity, ion flux, and binding a wide range of non-competitive inhibitors (Alqazzaz et al., 2011). The aim of this work was to investigate the GLIC M2 region, especially His235 (or 11'), which has been proposed as essential in proton sensing linked to channel opening (Wang et al., 2012), and residues that are conserved across the Cys-loop receptor family. To probe the roles of specific amino acids in GLIC M2 trans-membrane domain, we substituted M2 residues lining the ion pore and M2 residues facing M3 trans-membrane domains. We generated over 40 mutations at various positions including those at Glu 222(-2'), Thr 226 (2'), Ser 230 (6'), Leu 232 (8'), Ile 233 (9'), Ala 234 (10'), Ile 236 (12'), Ala 237 (13') and Phe 238 (14') using site-directed mutagenesis, and tested their function using two-electrode voltage clamp as previously described (Alqazzaz et al., 2011). Our results showed that Ser6', Ile9' and His11' residues are very sensitive to subsitution with all substituents resulting in non-functional receptors. We conclude that His 11' has a role in channel opening/closing, and the residues Thr226, Ser230, and Ile233 also play an important role in receptor function.Alqazzaz M, Thompson AJ, Price KL, Breitinger HG, Lummis SC (2011). Cys-loop receptor channel blockers also block GLIC. Biophysical journal 101(12): 2912-2918.Wang HL, Cheng X, Sine SM (2012). Intramembrane proton binding site linked to activation of bacterial pentameric ion channel. The Journal of biological chemistry 287(9): 6482-6489.

Several neuronal cell types co-express nicotinic acetylcholine receptors (nAChRs) and P2X receptors (P2XRs); and previous experiments from several laboratories show that these two ligand-gated channel subtypes interact with each other, both functionally and physically. We extended these studies to interactions between alpha6 subunit-containing (alpha6∗) nAChRs and P2X2 or P2X3 receptors (P2X2Rs, P2X3Rs). In Xenopus oocytes, alpha6 subunits mutated in the M2 region co-expressed with beta4 subunits displayed ACh-induced currents. P2X2 or P2X3 receptors expressed in the same oocytes displayed ATP-induced currents. When ACh and ATP were applied simultaneously, the total current was less than the sum of the individual currents_the conventional definition of cross-inhibition_for alpha6beta4-P2X2 oocytes. Several combinations of fluorescent protein (FP)-fused alpha6, beta4, P2X2, and P2X3 subunits were expressed in cultured mouse cortical cells and Neuro2a cells. In Forster resonance energy transfer (FRET) measurements using fluorescent lifetime imaging microscopy (FLIM) and normalized FRET (NFRET), we detected physical contact between alpha6beta2 nAChRs and P2X2Rs, between alpha6beta4 nAChRs and P2X2Rs, and between alpha6beta4 nAChRs and P2X3Rs. Thus, two lines of evidence show interactions between alpha6∗ nAChRs and P2XRs.Support: Beckman fellowship to CIR, NIH (NRSA to CIR, DA-19375, NS034407), Wellcome Trust, Yousef Jameel Award.