Potential state-selective hydrogen bond formation can modulate activation and desensitization of the α7 nicotinic acetylcholine receptor

Impact of modulation of the α7 nicotinic acetylcholine receptor on nicotine reward in the mouse conditioned place preference test

RATIONALE

The α7 nicotinic acetylcholine receptor (nAChR) has been implicated as a target in modulating nicotine reward. However, the effect of pharmacological agents that have been shown to alter the channel properties of the α7 nAChR is not well understood in nicotine reward.

OBJECTIVES

This study aimed to investigate the impact of α7 nAChR pharmacological modulation on nicotine conditioned place preference (CPP) in mice by using positive allosteric modulators (PAMs) and a silent agonist.

METHODS

The effect of the orthosteric α7 nAChR full agonist PNU282987 (1.3 and 9 mg/kg, s.c.), Type I α7 PAM NS1738 (1 and 10 mg/kg; i.p.), the Type II α7 PAM PNU120596 (0.3, 1, and 3 mg/kg, i.p.), and the α7 silent agonist NS6740 (1 and 3 mg/kg, i.p) on nicotine CPP was measured in mice. Mice were conditioned with either saline or nicotine (0.5 mg/kg) for 3 days in the CPP paradigm.

RESULTS

The α7 full orthosteric agonist PNU282987 and the Type II α7 nAChR PAM PNU120596 reduced nicotine CPP, while the silent agonist NS6740 and Type I PAM NS1738 had no effect. The effects of PNU282987 and PNU120596 did not have an effect on morphine CPP.

CONCLUSIONS

Taken together, our results suggest that modulation of the α7 nAChR can play important roles in nicotine CPP in mice. In addition, the Type II α7 nAChR PAM PNU120596 attenuated nicotine reward suggesting that endogenous acetylcholine/choline tone is sufficient to reduce nicotine CPP. These findings highlight a beneficial effect of using α7 nAChR PAMs in nicotine reward.

Discovery of an intrasubunit nicotinic acetylcholine receptor-binding site for the positive allosteric modulator Br-PBTC

Nicotinic acetylcholine receptor (nAChR) ligands that lack agonist activity but enhance activation in the presence of an agonist are called positive allosteric modulators (PAMs). nAChR PAMs have therapeutic potential for the treatment of nicotine addiction and several neuropsychiatric disorders. PAMs need to be selectively targeted toward certain nAChR subtypes to tap this potential. We previously discovered a novel PAM, (R)-7-bromo-N-(piperidin-3-yl)benzo[b]thiophene-2-carboxamide (Br-PBTC), which selectively potentiates the opening of α4β2*, α2β2*, α2β4*, and (α4β4)₂α4 nAChRs and reactivates some of these subtypes when desensitized (* indicates the presence of other subunits). We located the Br-PBTC-binding site through mutagenesis and docking in α4. The amino acids Glu-282 and Phe-286 near the extracellular domain on the third transmembrane helix were found to be crucial for Br-PBTC's PAM effect. E282Q abolishes Br-PBTC potentiation. Using (α4^E282Qβ2)₂α5 nAChRs, we discovered that the trifluoromethylated derivatives of Br-PBTC can potentiate channel opening of α5-containing nAChRs. Mutating Tyr-430 in the α5 M4 domain changed α5-selectivity among Br-PBTC derivatives. There are two kinds of α4 subunits in α4β2 nAChRs. Primary α4 forms an agonist-binding site with another β2 subunit. Accessory α4 forms an agonist-binding site with another α4 subunit. The pharmacological effect of Br-PBTC depends both on its own and agonists' occupancy of primary and accessory α4 subunits. Br-PBTC reactivates desensitized (α4β2)₂α4 nAChRs. Its full efficacy requires intact Br-PBTC sites in at least one accessory and one primary α4 subunit. PAM potency increases with higher occupancy of the agonist sites. Br-PBTC and its derivatives should prove useful as α subunit-selective nAChR PAMs.

Analysis of the stability and affinity of BlaR-CTD protein to β-lactam antibiotics based on docking and mutagenesis studies

Owing to the thermal instability and low affinity of BlaR-CTD to some β-lactams, the receptor assay based on BlaR-CTD is limited in the detection of abundant variety of drugs and the result is often unstable. In this study, the three-dimensional structure of BlaR-CTD from Bacillus licheniformis ATCC14580 was constructed by homologous modeling based on the crystal structure of BlaR-CTD from B. licheniformis 749/I, and the binding sites of this protein to 40 β-lactams were also obtained by molecular docking. To improve the stability and affinity of the protein, 23 mutant proteins were designed based on docking and homologous alignment results as well as by inserting disulfide bond and building the salt bridge. The mutation was rationality evaluated by SIFT and PloyPhen2 software. The heterologous expressed and purified mutant proteins were then subjected to the activity and stability assay. It was shown that among all mutant proteins, I188K/S19C/G24C, A138E/R50C/Q147C and S190Y/E183C/I188K respectively exhibited a higher affinity to 33, 22 and 21 β-lactams than the wild-type protein, while I188K/S19C/G24C exhibited the best stability. This may due to that the conformation of the active site in mutant protein I188K/S19C/G24C changed, and the random coli in the surface of protein activity increased. Our study suggests a possible structure-function relationship on the stability and affinity of BlaR-CTD, which provides new insights into protein rational design study and lays a solid foundation for establishing the receptor-based screening assay for the detection of β-lactam residues.

Persistent activation of α7 nicotinic ACh receptors associated with stable induction of different desensitized states

BACKGROUND AND PURPOSE

GAT107 ((3aR,4S,9bS)-4-(4-bromo-phenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta-[c]quinoline-8-sulfonamide) is a positive allosteric modulator (PAM) and agonist of α7 nicotinic acetylcholine receptors (nAChRs)that can cause a prolonged period of primed potentiation of acetylcholine responses after drug washout. NS6740 is a silent agonist of α7 nAChRs that has little or no efficacy for activating the ion channel but induces stable desensitization states, some of which can be converted into channel-active states by PAMs. Although GAT107 and NS6740 appear to stably induce different non-conducting states, both agents are effective treatment for inflammation and inflammatory pain models. We sought to better understand how both of these drugs that have opposite effects on channel activation could regulate signal transduction.

EXPERIMENTAL APPROACH

Voltage-clamp experiments were conducted with α7 nAChRs expressed in Xenopus oocytes.

KEY RESULTS

Long-lived sensitivity to a PAM or to an agonist was produced by NS6740 or GAT107 respectively. With sequential applications, these two drugs induced varying levels of persistent activation, which is a unique condition for a receptor that is known for rapid desensitization. The non-conducting states induced by NS6740 or GAT107 differ in their sensitivity to an α7 nAChR-selective antagonist and in how effectively they promote current.

CONCLUSIONS & IMPLICATIONS

Our data suggest that the persistent currents represent a dynamic interconversion between different stable desensitized states and the PAM-inducible conducting states. However, the similarity of NS6740 and GAT107 effects on inflammation and pain suggests that the different stable non-conducting states have common activity on signal transduction.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc.

Kinetic properties and open probability of α7 nicotinic acetylcholine receptors

The alpha7 nicotinic acetylcholine receptor (nAChR) has some peculiar kinetic properties. From the literature of α7 nAChR-mediated currents we concluded that experimentally measured kinetic properties reflected properties of the solution exchange system, rather than genuine kinetic properties of the receptors. We also concluded that all experimentally measured EC50 values for agonists must inherently be inaccurate. The aim of this study was to assess the undistorted kinetic properties of α7 nAChRs, and to construct an improved kinetic model, which can also serve as a basis of modeling the effect of the positive allosteric modulator PNU-120596, as it is described in the accompanying paper. Agonist-evoked currents were recorded from GH4C1 cells stably transfected with pCEP4/rat α7 nAChR using patch-clamp and fast solution exchange. We used two approaches to circumvent the problem of insufficient solution exchange rate: extrapolation and kinetic modeling. First, using different solution exchange rates we recorded evoked currents, and extrapolated their amplitude and kinetics to instantaneous solution exchange. Second, we constructed a kinetic model that reproduced concentration-dependence and solution exchange rate-dependence of receptors, and then we simulated receptor behavior at experimentally unattainably fast solution exchange. We also determined open probabilities during choline-evoked unmodulated and modulated currents using nonstationary fluctuation analysis. The peak open probability of 10 mM choline-evoked currents was 0.033 ± 0.006, while in the presence of choline (10 mM) and PNU-120596 (10 μM), it was increased to 0.599 ± 0.058. Our kinetic model could adequately reproduce low open probability, fast kinetics, fast recovery and solution exchange rate-dependent kinetics.

An outline of desensitization in pentameric ligand-gated ion channel receptors

Pentameric ligand-gated ion channel (pLGIC) receptors exhibit desensitization, the progressive reduction in ionic flux in the prolonged presence of agonist. Despite its pathophysiological importance and the fact that it was first described over half a century ago, surprisingly little is known about the structural basis of desensitization in this receptor family. Here, we explain how desensitization is defined using functional criteria. We then review recent progress into reconciling the structural and functional basis of this phenomenon. The extracellular-transmembrane domain interface is a key locus. Activation is well known to involve conformational changes at this interface, and several lines of evidence suggest that desensitization involves a distinct conformational change here that is incompatible with activation. However, major questions remain unresolved, including the structural basis of the desensitization-induced agonist affinity increase and the mechanism of pore closure during desensitization.

Point-to-point ligand-receptor interactions across the subunit interface modulate the induction and stabilization of conformational states of alpha7 nAChR by benzylidene anabaseines

The homomeric α7 nicotinic acetylcholine receptor is a well-studied therapeutic target, though its characteristically rapid desensitization complicates the development of drugs with specific agonist effects. Moreover, some experimental compounds such as GTS-21 (2,4diMeOBA), a derivative of the α7-selective partial agonist benzylidene anabaseine (BA), produce a prolonged residual desensitization (RD) in which the receptor remains non-activatable long after the drug has been removed from extracellular solution. In contrast, the desensitization caused by GTS-21's dihydroxy metabolite (2,4diOHBA) is relatively short-lived. RD is hypothetically due to stable binding of the ligand to the receptor in its desensitized state. We can attribute the reduction in RD to a single BA hydroxyl group on the 4' benzylidene position. Computational prediction derived from homology modeling showed the serine36 (S36) residue of α7 as a reasonable candidate for point-to-point interaction between BA compounds and the receptor. Through evaluating the activity of BA and simple derivatives on wild-type and mutant α7 receptors, it was observed that the drug-receptor pairs which were capable of hydrogen bonding at residue 36 exhibited significantly less stable desensitization. Further experiments involving the type II positive allosteric modulator (PAM) PNU-120596 showed that the various BA compounds' preference to induce either a PAM-sensitive (D(s)) or PAM-insensitive (D(i)) desensitized state is concentration dependent and suggested that both states are destabilized by S36 H-bonding. These results indicate that the fine-tuning of agonists for specific interaction with S36 can facilitate the development of therapeutics with targeted effects on ion channel desensitization properties and conformational state stability.