Abstract
By combining electrophysiological and computational approaches we have examined a series of positive allosteric modulators (PAMs) acting on the human α7 nicotinic acetylcholine receptor (nAChR). Electrophysiological studies have focused on three α7-selective PAMs (A-867744, TBS-516, and TQS) that display similar effects on wild-type α7 nAChRs. In addition to potentiating agonist-evoked responses, all three compounds reduce receptor desensitization and, consequently, are classed as type II PAMs. Despite having similar effects on wild-type receptors, A-867744 was found to have profoundly differing effects on mutated receptors compared with TBS-516 and TQS, a finding that is consistent with previous studies indicating that A-867744 may have a different mechanism of action compare with other α7-selective type II PAMs. Due to evidence that these PAMs bind within the α7 nAChR transmembrane region, we generated and validated new structural models of α7. Importantly, we have corrected a previously identified error in the transmembrane region of the original cryo–electron microscopy Torpedo model; the only pentameric ligand-gated ion channel imaged in a native lipid membrane. Real-space refinement was used to generate closed and open conformations on which the α7 models were based. Consensus docking with an extended series of PAMs with chemical similarity to A-867744, TBS-516, and TQS suggests that all bind to a broadly similar intersubunit transmembrane site. However, differences in the predicted binding of A-867744, compared with TBS-516 and TQS, may help to explain the distinct functional effects of A-867744. Thus, our revised structural models may provide a useful tool for interpreting functional effects of PAMs.
Highlights
Nicotinic acetylcholine receptors are members of a family of pentameric ligand-gated ion channels that includes receptors for 5-hydroxytrptamine, g-aminobutyric acid and glycine (Lester et al, 2004)
Extensive efforts have been aimed at the generation of positive allosteric modulator (PAM) that are selective for homomeric a7 nicotinic acetylcholine receptor (nAChR) (Faghih et al, 2008; Malysz et al, 2009b), a receptor
As expected, the orthosteric antagonist MLA caused complete displacement of [3H]-a-BTX (Fig. 2). These findings are consistent with previous studies demonstrating that A-867744 does not displace [3H]-MLA from a7 nAChRs (Malysz et al, 2009a) and provide additional support for the conclusion that A-867744 does not bind to the orthosteric site
Summary
Nicotinic acetylcholine receptors (nAChRs) are members of a family of pentameric ligand-gated ion channels (pLGICs) that includes receptors for 5-hydroxytrptamine, g-aminobutyric acid and glycine (Lester et al, 2004). There has been considerable interest in the identification of positive allosteric modulators (PAMs) of nAChRs (Bertrand and Gopalakrishnan, 2007; Williams et al, 2011; Chatzidaki and Millar, 2015). Supported by a Wellcome Trust PhD Studentship to J.N. Agonist activation of a7 nAChRs is sensitive to modulation by a variety of allosteric ligands (Chatzidaki and Millar, 2015). PAMs acting on a7 nAChRs are classified as either type I or type II PAMs, reflecting their differing effects upon agonist-induced desensitization. Whereas type I PAMs have little or no effect on desensitization, type II PAMs cause a reduction in receptor desensitization, as well as potentiating peak agonist responses (Bertrand and Gopalakrishnan, 2007)
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