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Membrane lipids are potent modulators of the nicotinic acetylcholine receptor (nAChR) from Torpedo. Lipids influence nAChR function by both conformational selection and kinetic mechanisms, stabilizing varying proportions of activatable versus non-activatable conformations, as well as influencing the transitions between these conformational states. Of note, some membranes stabilize an electrically silent uncoupled conformation that binds agonist but does not undergo agonist-induced conformational transitions. The uncoupled nAChR, however, does transition to activatable conformations in relatively thick lipid bilayers, such as those found in lipid rafts. In this review, we discuss current understanding of lipid-nAChR interactions in the context of increasingly available high resolution structural and functional data. These data highlight different sites of lipid action, including the lipid-exposed M4 transmembrane α-helix. Current evidence suggests that lipids alter nAChR function by modulating interactions between M4 and the adjacent transmembrane α-helices, M1 and M3. These interactions have also been implicated in both the folding and trafficking of nAChRs to the cell surface. We review current mechanistic understanding of lipid-nAChR interactions, and highlight potential biological roles for lipid-nAChR interactions in modulating the synaptic response. This article is part of a Special Issue entitled: Lipid-protein interactions.

Summary The gating of pentameric ligand gated ion channels is sensitive to a variety of allosteric modulators that act on structures that are peripheral to those involved in the allosteric pathway leading from the agonist site to the channel gate. One such structure, the lipid-exposed transmembrane α-helix, M4, is the target of lipids, neurosteroids, and disease-causing mutations. Here we show that M4 interactions with the adjacent transmembrane α-helices, M1 and M3, modulate pLGIC function. Enhanced M4 interactions promote, while ineffective interactions reduce channel function. The interface chemistry governs the intrinsic strength of M4-M1/M3 inter-helical interactions, both influencing channel gating and imparting distinct susceptibilities to the potentiating effects of a lipid-facing M4 congenital myasthenic syndrome mutation. Through aromatic substitutions, functional studies, and molecular dynamics simulations, we elucidate a mechanism by which M4 modulates channel function.

Background:The role of the outermost M4 transmembrane ␣-helix in prokaryotic pentameric ligand-gated ion channel (pLGIC) function is unknown. Results: Interactions between the M4 C terminus and both the adjacent M3 ␣-helix and the ␤6-␤7 loop are essential in one, but not another prokaryotic pLGIC. Conclusion: M4 contributes differently to maturation/function. Significance: Variations in M4 may contribute to subunit-specific functional differences.

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With the long-term goal of using a chimeric approach to dissect the distinct lipid sensitivities and thermal stabilities of the pentameric ligand-gated ion channels (pLGIC), GLIC and ELIC, we constructed chimeras by cross-combining their extracellular (ECD) and transmembrane (TMD) domains. As expected, the chimera formed between GLIC-ECD and ELIC-TMD (GE) responded to protons, the agonist for GLIC, but not cysteamine, the agonist for ELIC, although GE exhibited a 25-fold decrease in proton-sensitivity relative to wild type. The chimera formed between ELIC-ECD and the GLIC-TMD (EG) was usually toxic, unless it contained a pore-lining Ile9'Ala gain-of-function mutation. No significant improvements in expression/toxicity were observed with extensive loop substitutions at the ECD/TMD interface. Surprisingly, oocytes expressing EG-I9'A responded to both the ELIC agonist, cysteamine and the GLIC agonist, protons - the latter at pH values ≤4.0. The cysteamine- and proton-induced currents in EG-I9'A were inhibited by the GLIC TMD pore blocker, amantadine. The cysteamine-induced response of EG-I9'A was also inhibited by protons at pH values down to 4.5, but potentiated at lower pH values. Proton-induced gating at low pH was not abolished by mutation of an intramembrane histidine residue previously implicated in GLIC TMD function. We show that the TMD plays a major role governing the thermal stability of a pLGIC, and identify three distinct mechanisms by which agonists and protons influence the gating of the EG chimera. A structural basis for the impaired function of GE is suggested.

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then tested the GLIC-ELIC chimera with crotonic acid and picrotoxin. Crotonic acid inhibits GLIC with an IC50 of 110mM; our data indicate it binds to the extracellular domain. Picrotoxin (IC50¼2.6mM) blocks the GLIC pore(4); it likely cannot access the ELIC pore(5), but may bind to the extracellular domain (IC50¼96mM;6). These compounds were less potent than expected in the chimera (IC50>300mM). Overall the data suggest that domain specific effects may not be accurately reproduced in complex chimeras with intercommunicating domains, such as an orthosteric binding site and a pore in ligand-gated ion channels.