Nicotinic acetylcholine receptor transmembrane mutations convert ivermectin from a positive to a negative allosteric modulator

Ivermectin increases striatal cholinergic activity to facilitate dopamine terminal function

Ivermectin (IVM) is a commonly prescribed antiparasitic treatment with pharmacological effects on invertebrate glutamate ion channels resulting in paralysis and death of invertebrates. However, it can also act as a modulator of some vertebrate ion channels and has shown promise in facilitating L-DOPA treatment in preclinical models of Parkinson's disease. The pharmacological effects of IVM on dopamine terminal function were tested, focusing on the role of two of IVM's potential targets: purinergic P2X4 and nicotinic acetylcholine receptors. Ivermectin enhanced electrochemical detection of dorsal striatum dopamine release. Although striatal P2X4 receptors were observed, IVM effects on dopamine release were not blocked by P2X4 receptor inactivation. In contrast, IVM attenuated nicotine effects on dopamine release, and antagonizing nicotinic receptors prevented IVM effects on dopamine release. IVM also enhanced striatal cholinergic interneuron firing. L-DOPA enhances dopamine release by increasing vesicular content. L-DOPA and IVM co-application further enhanced release but resulted in a reduction in the ratio between high and low frequency stimulations, suggesting that IVM is enhancing release largely through changes in terminal excitability and not vesicular content. Thus, IVM is increasing striatal dopamine release through enhanced cholinergic activity on dopamine terminals.

Symmetrical Bispyridinium Compounds Act as Open Channel Blockers of Cation-Selective Ion Channels

Current treatments against organophosphate poisoning (OPP) do not directly address effects mediated by the overstimulation of nicotinic acetylcholine receptors (nAChR). Non-oxime bispyridinium compounds (BPC) promote acetylcholine esterase-independent recovery of organophosphate-induced paralysis. Here, we test the hypothesis that they act by positive modulatory action on nAChRs. Using two-electrode voltage clamp analysis in combination with mutagenesis and molecular docking analysis, the potency and molecular mode of action of a series of nine BPCs was investigated on human α7 and muscle-type nAChRs expressed in Xenopus laevis oocytes. The investigated BPCs inhibited α7 and/or muscle-type nAChRs with IC50 values in the high nanomolar to high micromolar range. Further analysis of the most potent analogues revealed a noncompetitive, voltage-dependent inhibition. Co-application with the α7-selective positive allosteric modulator PNU120596 and generation of α7/5HT3 receptor chimeras excluded direct interaction with the PNU120596 binding site and binding to the extracellular domain of the α7 nAChR, suggesting that they act as open channel blockers (OCBs). Molecular docking supported by mutagenesis localized the BPC binding area in the outer channel vestibule between the extracellular and transmembrane domains. Analysis of BPC action on other cation-selective channels suggests a rather nonspecific inhibition of pentameric cation channels. BPCs have been shown to ameliorate organophosphate-induced paralysis in vitro and in vivo. Our data support molecular action as OCBs at α7 and muscle-type nAChRs and suggest that their positive physiological effects are more complex than anticipated and require further investigation.

Structural mechanisms of α7 nicotinic receptor allosteric modulation and activation

The α7 nicotinic acetylcholine receptor is a pentameric ligand-gated ion channel that plays an important role in cholinergic signaling throughout the nervous system. Its unique physiological characteristics and implications in neurological disorders and inflammation make it a promising but challenging therapeutic target. Positive allosteric modulators overcome limitations of traditional α7 agonists, but their potentiation mechanisms remain unclear. Here, we present high-resolution structures of α7-modulator complexes, revealing partially overlapping binding sites but varying conformational states. Structure-guided functional and computational tests suggest that differences in modulator activity arise from the stable rotation of a channel gating residue out of the pore. We extend the study using a time-resolved cryoelectron microscopy (cryo-EM) approach to reveal asymmetric state transitions for this homomeric channel and also find that a modulator with allosteric agonist activity exploits a distinct channel-gating mechanism. These results define mechanisms of α7 allosteric modulation and activation with implications across the pentameric receptor superfamily.

Side Groups Convert the α7 Nicotinic Receptor Agonist Ether Quinuclidine into a Type I Positive Allosteric Modulator

The quinuclidine scaffold has been extensively used for the development of nicotinic acetylcholine receptor (nAChR) agonists, with hydrophobic substituents at position 3 of the quinuclidine framework providing selectivity for α7 nAChRs. In this study, six new ligands (4-9) containing a 3-(pyridin-3-yloxy)quinuclidine moiety (ether quinuclidine) were synthesized to gain a better understanding of the structural-functional properties of ether quinuclidines. To evaluate the pharmacological activity of these ligands, two-electrode voltage-clamp and single-channel recordings were performed. Only ligand 4 activated α7 nAChR. Ligands 5 and 7 had no effects on α7 nAChR, but ligands 6, 8, and 9 potentiated the currents evoked by ACh. Ligand 6 was the most potent and efficacious of the potentiating ligands, with an estimated EC50 for potentiation of 12.6 ± 3.32 μM and a maximal potentiation of EC20 ACh responses of 850 ± 120%. Ligand 6 increased the maximal ACh responses without changing the kinetics of the current responses. At the single-channel level, the potentiation exerted by ligand 6 was evidenced in the low micromolar concentration range by the appearance of prolonged bursts of channel openings. Furthermore, computational studies revealed the preference of ligand 6 for an intersubunit site in the transmembrane domain and highlighted some putative key interactions that explain the different profiles of the synthesized ligands. Notably, Met276 in the 15' position of the transmembrane domain 2 almost abolished the effects of ligand 6 when mutated to Leu. We conclude that ligand 6 is a novel type I positive allosteric modulator (PAM-I) of α7 nAChR.

Potentiation and allosteric agonist activation of α7 nicotinic acetylcholine receptors: binding sites and hypotheses

Considerable progress has been made in recent years towards the identification and characterisation of novel subtype-selective modulators of nicotinic acetylcholine receptors (nAChRs). In particular, this has focussed on modulators of α7 nAChRs, a nAChR subtype that has been identified as a target for drug discovery in connection with a range of potential therapeutic applications. This review focusses upon α7-selective modulators that bind to receptor sites other than the extracellular 'orthosteric' agonist binding site for the endogenous agonist acetylcholine (ACh). Such compounds include those that are able to potentiate responses evoked by orthosteric agonists such as ACh (positive allosteric modulators; PAMs) and those that are able to activate α7 nAChRs by direct allosteric activation in the absence of an orthosteric agonist (allosteric agonists or 'ago-PAMs'). There has been considerable debate about the mechanism of action of α7-selective PAMs and allosteric agonists, much of which has centred around identifying the location of their binding sites on α7 nAChRs. Based on a variety of experimental evidence, including recent structural data, there is now clear evidence indicating that at least some α7-selective PAMs bind to an inter-subunit site located in the transmembrane domain. In contrast, there are differing hypotheses about the site or sites at which allosteric agonists bind to α7 nAChRs. It will be argued that the available evidence supports the conclusion that direct allosteric activation by allosteric agonists/ago-PAMs occurs via the same inter-subunit transmembrane site that has been identified for several α7-selective PAMs.

Structural Elucidation of Ivermectin Binding to α7nAChR and the Induced Channel Desensitization

The α7 nicotinic acetylcholine receptor (α7nAChR) mediates signaling in the central nervous system and cholinergic anti-inflammatory pathways. Ivermectin is a positive allosteric modulator of a full-length α7nAChR and an agonist of the α7nAChR construct containing transmembrane (TMD) and intracellular (ICD) domains, but structural insights of the binding have not previously been determined. Here, combining nuclear magnetic resonance as a primary experimental tool with Rosetta comparative modeling and molecular dynamics simulations, we have revealed details of ivermectin binding to the α7nAChR TMD + ICD and corresponding structural changes in an ivermectin-induced desensitized state. Ivermectin binding was stabilized predominantly by hydrophobic interactions from interfacial residues between adjacent subunits near the extracellular end of the TMD, where the inter-subunit gap was substantially expanded in comparison to the apo structure. The ion-permeation pathway showed a profile distinctly different from the resting-state profile but similar to profiles of desensitized α7nAChR. The ICD also exhibited structural changes, including reorientation of the MX and h3 helices relative to the channel axis. The resulting structures of the α7nAChR TMD + ICD in complex with ivermectin provide opportunities for discovering new modulators of therapeutic potential and exploring the structural basis of cytoplasmic signaling under different α7nAChR functional states.

A new family of glutamate-gated chloride channels in parasitic sea louse Caligus rogercresseyi: A subunit refractory to activation by ivermectin is dominant in heteromeric assemblies

Sea louse ectoparasitosis is a major threat to fish aquaculture. Avermectins such as ivermectin and emamectin have been effectively used against sea louse infestation, but the emergence of resistance has limited their use. A better understanding of the molecular targets of avermectins is essential to the development of novel treatment strategies or new, more effective drugs. Avermectins are known to act by inhibiting neurotransmission through allosteric activation of glutamate-gated chloride channels (GluCls). We have investigated the GluCl subunit present in Caligus rogercresseyi, a sea louse affecting aquaculture in the Southern hemisphere. We identify four new subunits, CrGluCl-B to CrGluCl-E, and characterise them functionally. CrGluCl-A (previously reported as CrGluClα), CrGluCl-B and CrGluCl-C all function as glutamate channel receptors with different sensitivities to the agonist, but in contrast to subunit -A and -C, CrGluCl-B is not activated by ivermectin but is rather antagonised by the drug. CrGluCl-D channel appears active in the absence of any stimulation by glutamate or ivermectin and CrGluCl-E does not exhibit any activity. Notably, the expression of CrGluCl-B with either -A or -C subunits gives rise to receptors unresponsive to ivermectin and showing altered response to glutamate, suggesting that coexpression has led to the preferential formation of heteromers to which the presence of CrGluCl-B confers the property of ivermectin-activation refractoriness. Furthermore, there was evidence for heteromer formation with novel properties only when coexpressing pairs E/C and D/B CrGluCl subtypes. Site-directed mutagenesis shows that three transmembrane domain residues contribute to the lack of activation by ivermectin, most crucially Gln 15' in M2, with mutation Q15'T (the residue present in ivermectin-activated subunits A and C) conferring ivermectin activation to CrGluCl-B. The differential response to avermectin of these Caligus rogercresseyi GluClsubunits, which are highly conserved in the Northern hemisphere sea louse Lepeophtheirus salmonis, could have an influence on the response of these parasites to treatment with macrocyclic lactones. They could serve as molecular markers to assess susceptibility to existing treatments and might be useful molecular targets in the search for novel antiparasitic drugs.

Phenotypic Profiling of Macrocyclic Lactones on Parasitic Schistosoma Flatworms

Macrocyclic lactones are front-line therapies for parasitic roundworm infections; however, there are no comprehensive studies on the activity of this drug class against parasitic flatworms. Ivermectin is well known to be inactive against flatworms. However, the structure-activity relationship of macrocyclic lactones may vary across phyla, and it is entirely possible other members of this drug class do in fact show antiparasitic activity on flatworms. For example, there are several reports hinting at the anti-schistosomal activity of doramectin and moxidectin. To explore this class further, we developed an automated imaging assay combined with measurement of lactate levels from worm media. This assay was applied to the screening of 21 macrocyclic lactones (avermectins, milbemycins, and others such as spinosyns) against adult schistosomes. These in vitro assays identified several macrocyclic lactones (emamectin, milbemycin oxime, and the moxidectin metabolite 23-ketonemadectin) that caused contractile paralysis and lack of lactate production. Several of these were also active against miracidia, which infect the snail intermediate host. Hits prioritized from these in vitro assays were administered to mice harboring patent schistosome infections. However, no reduction in worm burden was observed. Nevertheless, these data show the utility of a multiplexed in vitro screening platform to quantitatively assess drug action and exclude inactive compounds from a chemical series before proceeding to in vivo studies. While the prototypical macrocyclic lactone ivermectin displays minimal activity against adult Schistosoma mansoni, this family of compounds does contain schistocidal compounds which may serve as a starting point for development of new anti-flatworm chemotherapies.

Modulation of P2X4 receptor activity by ivermectin and 5-BDBD has no effect on the development of ARPKD in PCK rats

Autosomal recessive polycystic kidney disease (ARPKD) is an inherited pathology caused mainly by mutations of the polycystic kidney and hepatic disease 1 (PKHD1) gene, which usually leads to end-stage renal disease. Previous studies suggested that the P2X purinoreceptor 4 (P2X4 R) may play an important role in the progression of ARPKD. To test this hypothesis, we assessed the chronic effects of ivermectin (P2X4 R allosteric modulator) and 5-BDBD (P2X4 R antagonist) on the development of ARPKD in PCK/CrljCrl-Pkhd1pck/CRL (PCK) rats. Our data indicated that activation of ATP-mediated P2X4 R signaling with ivermectin for 6 weeks in high dose (50 mg/L; water supplementation) decreased the total body weight of PCK rats while the heart and kidney weight remained unaffected. Smaller doses of ivermectin (0.5 or 5 mg/L, 6 weeks) or the inhibition of P2X4 R signaling with 5-BDBD (18 mg/kg/day, food supplement for 8 weeks) showed no effect on electrolyte balance or the basic physiological parameters. Furthermore, cystic index analysis for kidneys and liver revealed no effect of smaller doses of ivermectin (0.5 or 5 mg/L) and 5-BDBD on the cyst development of PCK rats. We observed a slight increase in the cystic liver index on high ivermectin dose, possibly due to the cytotoxicity of the drug. In conclusion, this study revealed that pharmacological modulation of P2X4 R by ivermectin or 5-BDBD does not affect the development of ARPKD in PCK rats, which may provide insights for future studies on investigating the therapeutic potential of adenosine triphosphate (ATP)-P2 signaling in PKD diseases.

P-glycoprotein inhibition affects ivermectin-induced behavioural alterations in fed and fasted zebrafish (Danio rerio)

The role of the blood-brain barrier ATP-binding cassette protein transporter P-glycoprotein (P-gp) in protecting zebrafish (Danio rerio) from the central nervous system neurotoxicant ivermectin (IVM, 22,23-dihydroavermectin B1a + 22,23-dihydroavermectin B1b) was examined in the absence and presence of the competitive inhibitor cyclosporin A (CsA). Zebrafish injected intraperitoneally with 1, 2, 5, or 10 µmol/kg IVM exhibited mortality 30 min following administration at the highest dose. At sublethal doses > 1 µmol/kg, IVM altered the swimming performance, exploratory behaviour, motor coordination, escape response and olfactory response in exposed fish. When fish were exposed to IVM in the presence of CsA, alterations in swimming and behaviours increased significantly and at the highest IVM/CsA ratio resulted in a complete lack of exploratory and olfactory behaviours. In separate experiments, fish were either fed or fasted, and the effects of IVM and CsA administration were examined. The effects of IVM administration and the exacerbated effects seen with CsA co-administration were not affected by fasting. This study provides evidence that P-gp provides a protective role in the BBB of fish against environmental neurotoxicants. The results also show that P-gp activity is maintained even under conditions of food deprivation, suggesting that this chemical defence system is prioritized over other energy expenditures during diet limitation.

Stoichiometry-Selective Antagonism of α4β2 Nicotinic Acetylcholine Receptors by Fluoroquinolone Antibiotics

Quinolone antibiotics disrupt bacterial DNA synthesis by interacting with DNA gyrase and topoisomerase IV. However, in addition, they have been shown to act as inhibitors of pentameric ligand-gated ion channels such as GABA_A receptors and the α7 nicotinic acetylcholine receptor (nAChR). In the present study, we have examined the effects of quinolone antibiotics on the human α4β2 nAChR, an important subtype that is widely expressed in the central nervous system. A key feature of α4β2 nAChRs is their ability to coassemble into two distinct stoichiometries, (α4)₂(β2)₃ and (α4)₃(β2)₂, which results in differing affinities for acetylcholine. The effects of nine quinolone antibiotics were examined on both stoichiometries of the α4β2 receptor by two-electrode voltage-clamp recording. All compounds exhibited significant inhibition of α4β2 nAChRs. However, all of the fluoroquinolone antibiotics examined (ciprofloxacin, enoxacin, enrofloxacin, difloxacin, norfloxacin, pefloxacin, and sparfloxacin) were significantly more potent inhibitors of (α4)₂(β2)₃ nAChRs than of (α4)₃(β2)₂ nAChRs. This stoichiometry-selective effect was most pronounced with pefloxacin, which inhibited (α4)₂(β2)₃ nAChRs with an IC₅₀ of 26.4 ± 3.4 μM but displayed no significant inhibition of (α4)₃(β2)₂ nAChRs. In contrast, two nonfluorinated quinolone antibiotics (cinoxacin and oxolinic acid) exhibited no selectivity in their inhibition of the two stoichiometries of α4β2. Computational docking studies suggest that pefloxacin interacts selectively with an allosteric transmembrane site at the β2(+)/β2(−) subunit interface, which is consistent with its selective inhibition of (α4)₂(β2)₃. These findings concerning the antagonist effects of fluoroquinolones provide further evidence that differences in the subunit stoichiometry of heteromeric nAChRs can result in substantial differences in pharmacological properties.

Therapeutic Targeting of α7 Nicotinic Acetylcholine Receptors

The α7-type nicotinic acetylcholine receptor is one of the most unique and interesting of all the members of the cys-loop superfamily of ligand-gated ion channels. Since it was first identified initially as a binding site for α-bungarotoxin in mammalian brain and later as a functional homomeric receptor with relatively high calcium permeability, it has been pursued as a potential therapeutic target for numerous indications, from Alzheimer disease to asthma. In this review, we discuss the history and state of the art for targeting α7 receptors, beginning with subtype-selective agonists and the basic pharmacophore for the selective activation of α7 receptors. A key feature of α7 receptors is their rapid desensitization by standard "orthosteric" agonist, and we discuss insights into the conformational landscape of α7 receptors that has been gained by the development of ligands binding to allosteric sites. Some of these sites are targeted by positive allosteric modulators that have a wide range of effects on the activation profile of the receptors. Other sites are targeted by direct allosteric agonist or antagonists. We include a perspective on the potential importance of α7 receptors for metabotropic as well as ionotropic signaling. We outline the challenges that exist for future development of drugs to target this important receptor and approaches that may be considered to address those challenges. SIGNIFICANCE STATEMENT: The α7-type nicotinic acetylcholine receptor (nAChR) is acknowledged as a potentially important therapeutic target with functional properties associated with both ionotropic and metabotropic signaling. The functional properties of α7 nAChR can be regulated in diverse ways with the variety of orthosteric and allosteric ligands described in this review.

Computational analysis of non-competitive antagonist arylguanidine-α7 nAChR complexes

meta-Chlorophenylguanidine (1) is a non-competitive α7 nicotinic acetylcholine receptor (nAChR) antagonist. Here we examined the hydrogen bond donor role of the anilinic N₁-H on the inhibitory effect of 1 by preparing its N₁-CH₃ counterpart 2. Analog 2 was found to be at least as potent as 1 as a non-competitive α7 nAChR antagonist in a patch-clamp assay. To establish a structural basis for the mode of interaction of guanidines 1 and 2, we generated 100 homology models of the hα7 nAChR. This was followed by Connolly surface (SYBYL-X2.1) and blind docking (AutoDock 4.1) studies to identify eight possible binding pockets, two of which were supported by empirical data and employed in our docking studies. The optimized model-ligand complexes were analyzed using a Hydropathic INTeractions (HINT) analysis in order to compare and contrast different binding pockets and modes. We identified a potential allosteric binding site and distinct rotameric binding modes for 1 and 2 at α7 nAChRs. These differences in the binding orientations minimized the importance of an anilinic NH function for the antagonist activity at nACh receptors.

Differing Activity Profiles of the Stereoisomers of 2,3,5,6TMP-TQS, a Putative Silent Allosteric Modulator of α7 nAChR

Many synthetic compounds to which we attribute specific activities are produced as racemic mixtures of stereoisomers, and it may be that all the desired activity comes from a single enantiomer. We have previously shown this to be the case with the α7 nicotinic acetylcholine receptor positive allosteric modulator (PAM) 3a,4,5,9b-Tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[c]quinoline-8-sulfonamide (TQS) and the α7 ago-PAM 4BP-TQS. Cis-trans-4-(2,3,5,6-tetramethylphenyl)-3a,4,5,9b-te-trahydro-3H-cyclopenta[c]quinoline-8-sulfonamide (2,3,5,6TMP-TQS), previously published as a "silent allosteric modulator" and an antagonist of α7 allosteric activation, shares the same scaffold with three chiral centers as the aforementioned compounds. We isolated the enantiomers of 2,3,5,6TMP-TQS and determined that the (-) isomer was a significantly better antagonist than the (+) isomer of the allosteric activation of both wild-type α7 and the nonorthosterically activatible C190A α7 mutant by the ago-PAM GAT107 (the active isomer of 4BP-TQS). In contrast, (+)2,3,5,6TMP-TQS proved to be an α7 PAM. (-)2,3,5,6TMP-TQS was shown to antagonize the allosteric activation of α7 by the structurally unrelated ago-PAM B-973B as well as the allosteric activation of the TQS-sensitive α4β2L15'M mutant. In silico docking of 2,3,5,6TMP-TQS in the putative allosteric activation binding site suggested a specific interaction of the (-) enantiomer with α7T106, and allosteric activation of α7T106 mutants was not inhibited by (-)2,3,5,6TMP-TQS, confirming the importance of this interaction and supporting the model of the allosteric binding site. Comparisons and contrasts between 2,3,5,6TMP-TQS isomers and active and inactive enantiomers of other TQS-related compounds identify the orientation of the cyclopentenyl ring to the plane of the core quinoline to be a crucial determinate of PAM activity. SIGNIFICANCE STATEMENT: Many synthetic ligands are in use as racemic preparations. We show that one enantiomer of the TQS analog Cis-trans-4-(2,3,5,6-tetramethylphenyl)-3a,4,5,9b-te-trahydro-3H-cyclopenta[c]quinoline-8-sulfonamide, originally reported to lack activity when used as a racemic preparation, is an α7 nicotinic acetylcholine receptor positive allosteric modulator (PAM). The other enantiomer is not a PAM, but it is an effective allosteric antagonist. In silico studies and structural comparisons identify essential elements of both the allosteric ligands and receptor binding sites important for these allosteric activities.

Design, Synthesis, and Functional Evaluation of a Novel Series of Phosphonate-Functionalized 1,2,3-Triazoles as Positive Allosteric Modulators of α7 Nicotinic Acetylcholine Receptors

The α7 nicotinic acetylcholine receptor is a pentameric ligand-gated ion channel widely distributed in the central nervous system, mainly in the hippocampus and cortex. The enhancement of its activity by positive allosteric modulators (PAMs) is a promising therapeutic strategy for cognitive deficits and neurodegenerative disorders. With the aim of developing novel scaffolds with PAM activity, we designed and synthesized a series of phosphonate-functionalized 1,4-disubstituted 1,2,3-triazoles using supported copper nanoparticles as the cycloaddition reaction catalyst and evaluated their activity on α7 receptors by single-channel and whole-cell recordings. We identified several triazole derivatives that displayed PAM activity, with the compound functionalized with the methyl phosphonate group being the most efficacious one. At the macroscopic level, α7 potentiation was evidenced as an increase of the maximal currents elicited by acetylcholine with minimal effects on desensitization, recapitulating the actions of type I PAMs. At the single-channel level, the active compounds did not affect channel amplitude but significantly increased the duration of channel openings and activation episodes. By using chimeric and mutant α7 receptors, we demonstrated that the new α7 PAMs share transmembrane structural determinants of potentiation with other chemically nonrelated PAMs. To gain further insight into the chemical basis of potentiation, we applied structure-activity relationship strategies involving modification of the chain length, inversion of substituent positions in the triazole ring, and changes in the aromatic nucleus. Our findings revealed that the phosphonate-functionalized 1,4-disubstituted 1,2,3-triazole is a novel pharmacophore for the development of therapeutic agents for neurological and neurodegenerative disorders associated with cholinergic dysfunction.

Flavonoids as positive allosteric modulators of α7 nicotinic receptors

The use of positive allosteric modulators (PAM) of α7 nicotinic receptors is a promising therapy for neurodegenerative, inflammatory and cognitive disorders. Flavonoids are polyphenolic compounds showing neuroprotective, anti-inflammatory and pro-cognitive actions. Besides their well-known antioxidant activity, flavonoids trigger intracellular pathways and interact with receptors, including α7. To reveal how the beneficial actions of flavonoids are linked to α7 function, we evaluated the effects of three representative flavonoids -genistein, quercetin and the neoflavonoid 5,7-dihydroxy-4-phenylcoumarin- on whole-cell and single-channel currents. All flavonoids increase the maximal currents elicited by acetylcholine with minimal effects on desensitization and do not reactivate desensitized receptors, a behaviour consistent with type I PAMs. At the single-channel level, they increase the duration of the open state and produce activation in long-duration episodes with a rank order of efficacy of genistein > quercetin ≥ neoflavonoid. By using mutant and chimeric α7 receptors, we demonstrated that flavonoids share transmembrane structural determinants with other PAMs. The α7-PAM activity of flavonoids results in decreased cell levels of reactive oxygen species. Thus, allosteric potentiation of α7 may be an additional mechanism underlying neuroprotective actions of flavonoids, which may be used as scaffolds for designing new therapeutic agents.

Insights Into Nicotinic Receptor Signaling in Nicotine Addiction: Implications for Prevention and Treatment

BACKGROUND

Nicotinic acetylcholine receptors (nAChRs) belong to the Cys-loop ligandgated ion-channel (LGIC) superfamily, which also includes the GABA, glycine, and serotonin receptors. Many nAChR subunits have been identified and shown to be involved in signal transduction on binding to them of either the neurotransmitter acetylcholine or exogenous ligands such as nicotine. The nAChRs are pentameric assemblies of homologous subunits surrounding a central pore that gates cation flux, and they are expressed at neuromuscular junctions throughout the nervous system.

METHODS AND RESULTS

Because different nAChR subunits assemble into a variety of pharmacologically distinct receptor subtypes, and different nAChRs are implicated in various physiological functions and pathophysiological conditions, nAChRs represent potential molecular targets for drug addiction and medical therapeutic research. This review intends to provide insights into recent advances in nAChR signaling, considering the subtypes and subunits of nAChRs and their roles in nicotinic cholinergic systems, including structure, diversity, functional allosteric modulation, targeted knockout mutations, and rare variations of specific subunits, and the potency and functional effects of mutations by focusing on their effects on nicotine addiction (NA) and smoking cessation (SC). Furthermore, we review the possible mechanisms of action of nAChRs in NA and SC based on our current knowledge.

CONCLUSION

Understanding these cellular and molecular mechanisms will lead to better translational and therapeutic operations and outcomes for the prevention and treatment of NA and other drug addictions, as well as chronic diseases, such as Alzheimer's and Parkinson's. Finally, we put forward some suggestions and recommendations for therapy and treatment of NA and other chronic diseases.

Preclinical evaluation of avermectins as novel therapeutic agents for alcohol use disorders

The deleterious effects of alcohol use disorders (AUDs) on human health have been documented worldwide. The enormous socioeconomic burden coupled with lack of efficacious pharmacotherapies underlies the need for improved treatment strategies. At present, there is a growing body of preclinical evidence that demonstrates the potential of avermectins [ivermectin (IVM), selamectin (SEL), abamectin (ABM), and moxidectin (MOX)] in treatment of AUDs. Avermectins are derived by fermentation of soil micro-organism, Streptomyces avermitilis, and have been extensively used for treatment of parasitic infections. From the mechanistic standpoint, avermectins are positive modulators of purinergic P2X4 receptors (P2X4Rs). P2X4Rs belong to P2X superfamily of cation-permeable ion channels gated by adenosine 5'-triphosphate (ATP). Building evidence has implicated a role for P2X4Rs in regulation of ethanol intake and that ethanol can inhibit ATP-gated currents in P2X4Rs. Investigations using recombinant cell models and animal models of alcohol drinking have reported that IVM, ABM, and MOX, but not SEL, were able to antagonize the inhibitory effects of ethanol on P2X4Rs in vitro and reduce ethanol intake in vivo. Furthermore, IVM was shown to reduce ethanol consumption via P2X4R potentiation in vivo, supporting the involvement of P2X4Rs in IVM's anti-alcohol effects and that P2X4Rs can be used as a platform for developing novel anti-alcohol compounds. Taken together, these findings support the utility of avermectins as a novel class of drug candidates for treatment of AUDs.

Nicotinic acetylcholine receptors at the single-channel level

Over the past four decades, the patch clamp technique and nicotinic ACh (nACh) receptors have established an enduring partnership. Like all good partnerships, each partner has proven significant in its own right, while their union has spurred innumerable advances in life science research. A member and prototype of the superfamily of pentameric ligand-gated ion channels, the nACh receptor is a chemo-electric transducer, binding ACh released from nerves and rapidly opening its channel to cation flow to elicit cellular excitation. A subject of a Nobel Prize in Physiology or Medicine, the patch clamp technique provides unprecedented resolution of currents through single ion channels in their native cellular environments. Here, focusing on muscle and α7 nACh receptors, we describe the extraordinary contribution of the patch clamp technique towards understanding how they activate in response to neurotransmitter, how subtle structural and mechanistic differences among nACh receptor subtypes translate into significant physiological differences, and how nACh receptors are being exploited as therapeutic drug targets.

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/.

Solution NMR Studies of Anesthetic Interactions with Ion Channels

NMR spectroscopy is one of the major tools to provide atomic resolution protein structural information. It has been used to elucidate the molecular details of interactions between anesthetics and ion channels, to identify anesthetic binding sites, and to characterize channel dynamics and changes introduced by anesthetics. In this chapter, we present solution NMR methods essential for investigating interactions between ion channels and general anesthetics, including both volatile and intravenous anesthetics. Case studies are provided with a focus on pentameric ligand-gated ion channels and the voltage-gated sodium channel NaChBac.

An allosteric link connecting the lipid-protein interface to the gating of the nicotinic acetylcholine receptor

The mechanisms underlying lipid-sensing by membrane proteins is of considerable biological importance. A unifying mechanistic question is how a change in structure at the lipid-protein interface is translated through the transmembrane domain to influence structures critical to protein function. Gating of the nicotinic acetylcholine receptor (nAChR) is sensitive to its lipid environment. To understand how changes at the lipid-protein interface influence gating, we examined how a mutation at position 418 on the lipid-facing surface of the outer most M4 transmembrane α-helix alters the energetic couplings between M4 and the remainder of the transmembrane domain. Human muscle nAChR is sensitive to mutations at position 418, with the Cys-to-Trp mutation resulting in a 16-fold potentiation in function that leads to a congenital myasthenic syndrome. Energetic coupling between M4 and the Cys-loop, a key structure implicated in gating, do not change with C418W. Instead, Trp418 and an adjacent residue couple energetically with residues on the M1 transmembrane α-helix, leading to a reorientation of M1 that stabilizes the open state. We thus identify an allosteric link connecting the lipid-protein interface of the nAChR to altered channel function.

Molecular interactions of type I and type II positive allosteric modulators with the human α7 nicotinic acetylcholine receptor: an in silico study

The binding site locations and structural components for type I and type II positive allosteric modulators (PAMs) of the α7 nicotinic acetylcholine receptor (nAChR) have not been fully characterized yet. In this regard, homology models of the human α7 nAChR and hα7/m5-HT_3A chimera, built using the crystal structure of the serotonin type 3A receptor (5-ΗΤ_3ΑR), were used for molecular docking and molecular dynamics simulations to study the molecular interactions of selected type I (5-hydroxyindol, NS-1738, and LY-2087101) and type II (PNU-120596, PAM-2, and TBS-516) PAMs. The docking results indicate: (1) a site located in the extracellular domain (ECD) for type I PAMs such as NS-1738 and LY-2087101, but not for 5-HI; (2) an overlapping site in the ECD-transmembrane domain (TMD) junction for all studied PAMs. Additional docking results on the hα7/m5-HT_3A chimera supported experimental results indicating that the ECD site might be relevant for type I PAM activity; and (3) two TMD sites, an intrasubunit site that recognizes type II PAMs, and an intersubunit pocket with high specificity for 5-HI (type I PAM). The in silico α7TSLMF mutant results support the view that M1-Ser223 and M3-Ile281 are key residues for the interaction of PAM-2 and PNU-120596 with the intrasubunit cavity. Our in silico results are in agreement with experimental data showing that the intrasubunit cavity is relevant for the activity of type II PAMs, and suggest that the ECD-TMD junction and intersubunit sites could be significant for the activity of type I PAMs.

An allosteric binding site of the α7 nicotinic acetylcholine receptor revealed in a humanized acetylcholine-binding protein

Nicotinic acetylcholine receptors (nAChRs) belong to the family of pentameric ligand-gated ion channels and mediate fast excitatory transmission in the central and peripheral nervous systems. Among the different existing receptor subtypes, the homomeric α7 nAChR has attracted considerable attention because of its possible implication in several neurological and psychiatric disorders, including cognitive decline associated with Alzheimer's disease or schizophrenia. Allosteric modulators of ligand-gated ion channels are of particular interest as therapeutic agents, as they modulate receptor activity without affecting normal fluctuations of synaptic neurotransmitter release. Here, we used X-ray crystallography and surface plasmon resonance spectroscopy of α7-acetylcholine-binding protein (AChBP), a humanized chimera of a snail AChBP, which has 71% sequence similarity with the extracellular ligand-binding domain of the human α7 nAChR, to investigate the structural determinants of allosteric modulation. We extended previous observations that an allosteric site located in the vestibule of the receptor offers an attractive target for receptor modulation. We introduced seven additional humanizing mutations in the vestibule-located binding site of AChBP to improve its suitability as a model for studying allosteric binding. Using a fragment-based screening approach, we uncovered an allosteric binding site located near the β8-β9 loop, which critically contributes to coupling ligand binding to channel opening in human α7 nAChR. This work expands our understanding of the topology of allosteric binding sites in AChBP and, by extrapolation, in the human α7 nAChR as determined by electrophysiology measurements. Our insights pave the way for drug design strategies targeting nAChRs involved in ion channel-mediated disorders.

Molecular function of α7 nicotinic receptors as drug targets

Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels involved in many physiological and pathological processes. In vertebrates, there are seventeen different nAChR subunits that combine to yield a variety of receptors with different pharmacology, function, and localization. The homomeric α7 receptor is one of the most abundant nAChRs in the nervous system and it is also present in non-neuronal cells. It plays important roles in cognition, memory, pain, neuroprotection, and inflammation. Its diverse physiological actions and associated disorders have made of α7 an attractive novel target for drug modulation. Potentiation of the α7 receptor has emerged as a novel therapeutic strategy for several neurological diseases, such as Alzheimer's and Parkinson's diseases, and inflammatory disorders. In contrast, increased α7 activity has been associated with cancer cell proliferation. The presence of different drug target sites offers a great potential for α7 modulation in different pathological contexts. In particular, compounds that target allosteric sites offer significant advantages over orthosteric agonists due to higher selectivity and a broader spectrum of degrees and mechanisms of modulation. Heterologous expression of α7, together with chaperone proteins, combined with patch clamp recordings have provided important advances in our knowledge of the molecular basis of α7 responses and their potential modulation for pathological processes. This review gives a synthetic view of α7 and its molecular function, focusing on how its unique activation and desensitization features can be modified by pharmacological agents. This fundamental information offers insights into therapeutic strategies.

Ivermectin and its target molecules: shared and unique modulation mechanisms of ion channels and receptors by ivermectin

Ivermectin (IVM) is an antiparasitic drug that is used worldwide and rescues hundreds of millions of people from onchocerciasis and lymphatic filariasis. It was discovered by Satoshi Ōmura and William C. Campbell, to whom the 2015 Nobel Prize in Physiology or Medicine was awarded. It kills parasites by activating glutamate-gated Cl^- channels, and it also targets several ligand-gated ion channels and receptors, including Cys-loop receptors, P2X₄ receptors and fernesoid X receptors. Recently, we found that IVM also activates a novel target, the G-protein-gated inwardly rectifying K⁺ channel, and also identified the structural determinant for the activation. In this review, we aim to provide an update and summary of recent progress in the identification of IVM targets, as well as their modulation mechanisms, through molecular structures, chimeras and site-directed mutagenesis, and molecular docking and modelling studies.

Neonicotinoid insecticides differently modulate acetycholine-induced currents on mammalian α7 nicotinic acetylcholine receptors

BACKGROUND AND PURPOSE

Neonicotinoid insecticides are described as poor agonists of mammalian nicotinic ACh receptors. In this paper, we show that their effects on mammalian nicotinic receptors differ between compounds.

EXPERIMENTAL APPROACH

Two-electrode voltage-clamp electrophysiology was used to characterize the pharmacology of three neonicotinoid insecticides on nicotinic α7 receptors expressed in Xenopus oocytes. Single and combined application of clothianidin, acetamiprid and thiamethoxam were tested.

RESULTS

Two neonicotinoid insecticides, clothianidin and acetamiprid, were partial agonists of mammalian neuronal α7 nicotinic receptors, whereas another neonicotinoid insecticide, thiamethoxam, which is converted to clothianidin in insect and plant tissues, had no effect. Pretreatment with clothianidin and acetamiprid (10 μM) ACh significantly enhanced the subsequent currents evoked by ACh (100 μM ) whereas pretreatment with thiamethoxam (10 μM) reduced ACh-induced current amplitudes.A combination of the three neonicotinoids decreased the ACh-evoked currents.

CONCLUSIONS AND IMPLICATIONS

The present findings suggest that neonicotinoid insecticides differ markedly in their direct effects on mammalian α7 nicotinic ACh receptors and can also modulate ACh-induced currents. Furthermore, our data indicate a previously unknown modulation of mammalian α7 nicotinic receptors by a combination of clothianidin, acetamiprid and thiamethoxam.

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.

Splice Variants of pH-Sensitive Chloride Channel Identify a Key Determinant of Ivermectin Sensitivity in the Larvae of the Silkworm Bombyx mori

The pH-sensitive chloride channels (pHCls) are broadly expressed in insects, but little is known about their physiologic role, diversity, and sensitivity to insecticides acting on relevant chloride channels. Here we have sequenced 50 transcripts of the pHCl-1 gene from the brain, third thoracic ganglion (T3G), and midgut of larvae of silkworm Bombyx mori It was found that >50 variants were expressed with distinct splicing in the T3G compared with the brain and midgut. Of the variants detected, variant 9, which was expressed most abundantly in the larvae, was reconstituted in Xenopus laevis oocytes to characterize its pH and ivermectin sensitivity. Variant 9 formed a functional pHCl with half-maximal activation at a pH of 7.87, and was activated by ivermectin irrespective of the extracellular pH. This was in contrast to variant 1, which was activated more profoundly at acidic rather than basic pH. To identify a key determinant for such differential ivermectin sensitivity, different amino acids in variants 1 and 9 were swapped, and the effects of the mutations on ivermectin sensitivity were investigated. The V275S mutation of variant 1 enhanced ivermectin sensitivity, whereas the S275V mutation of variant 9 caused a reduction in sensitivity. In homology models of the Bombyx pHCls, Val275 of variant 1 interacted more strongly with Ala273 than Ser275 of variant 9 at the channel gate. This interaction is likely to prevent ivermectin-induced opening of the channel, accounting, at least partially, for the differential macrolide action on the two variants.

Crystal Structures of Human GlyRα3 Bound to Ivermectin

Ivermectin acts as a positive allosteric modulator of several Cys-loop receptors including the glutamate-gated chloride channels (GluCls), γ-aminobutyric acid receptors (GABA_ARs), glycine receptors (GlyRs), and neuronal α7-nicotinic receptors (α7 nAChRs). The crystal structure of Caenorhabditis elegans GluCl complexed with ivermectin revealed the details of its ivermectin binding site. Although the electron microscopy structure of zebrafish GlyRα1 complexed with ivermectin demonstrated a similar binding orientation, detailed structural information on the ivermectin binding and pore opening for Cys-loop receptors in vertebrates has been elusive. Here we present the crystal structures of human GlyRα3 in complex with ivermectin at 2.85 and 3.08 Å resolution. Our structures allow us to explore in detail the molecular recognition of ivermectin by GlyRs, GABA_ARs, and α7 nAChRs. Comparisons with previous structures reveal how the ivermectin binding expands the ion channel pore. Our results hold promise in structure-based design of GlyR modulators for the treatment of neuropathic pain.

Mutational Analysis at Intersubunit Interfaces of an Anionic Glutamate Receptor Reveals a Key Interaction Important for Channel Gating by Ivermectin

The broad-spectrum anthelmintic drug ivermectin (IVM) activates and stabilizes an open-channel conformation of invertebrate chloride-selective glutamate receptors (GluClRs), thereby causing a continuous inflow of chloride ions and sustained membrane hyperpolarization. These effects suppress nervous impulses and vital physiological processes in parasitic nematodes. The GluClRs are pentamers. Homopentameric receptors assembled from the Caenorhabditis elegans (C. elegans) GluClα (GLC-1) subunit can inherently respond to IVM but not to glutamate (the neurotransmitter). In contrast, heteromeric GluClα/β (GLC-1/GLC-2) assemblies respond to both ligands, independently of each other. Glutamate and IVM bind at the interface between adjacent subunits, far away from each other; glutamate in the extracellular ligand-binding domain, and IVM in the ion-channel pore periphery. To understand the importance of putative intersubunit contacts located outside the glutamate and IVM binding sites, we introduced mutations at intersubunit interfaces, between these two binding-site types. Then, we determined the effect of these mutations on the activation of the heteromeric mutant receptors by glutamate and IVM. Amongst these mutations, we characterized an α-subunit point mutation located close to the putative IVM-binding pocket, in the extracellular end of the first transmembrane helix (M1). This mutation (αF276A) moderately reduced the sensitivity of the heteromeric GluClαF276A/βWT receptor to glutamate, and slightly decreased the receptor subunits’ cooperativity in response to glutamate. In contrast, the αF276A mutation drastically reduced the sensitivity of the receptor to IVM and significantly increased the receptor subunits’ cooperativity in response to IVM. We suggest that this mutation reduces the efficacy of channel gating, and impairs the integrity of the IVM-binding pocket, likely by disrupting important interactions between the tip of M1 and the M2-M3 loop of an adjacent subunit. We hypothesize that this physical contact between M1 and the M2-M3 loop tunes the relative orientation of the ion-channel transmembrane helices M1, M2 and M3 to optimize pore opening. Interestingly, pre-exposure of the GluClαF276A/βWT mutant receptor to subthreshold IVM concentration recovered the receptor sensitivity to glutamate. We infer that IVM likely retained its positive modulation activity by constraining the transmembrane helices in a preopen orientation sensitive to glutamate, with no need for the aforementioned disrupted interactions between M1 and the M2-M3 loop.

Orthosteric and allosteric potentiation of heteromeric neuronal nicotinic acetylcholine receptors

Heteromeric nicotinic ACh receptors (nAChRs) were thought to have two orthodox agonist-binding sites at two α/β subunit interfaces. Highly selective ligands are hard to develop by targeting orthodox agonist sites because of high sequence similarity of this binding pocket among different subunits. Recently, unorthodox ACh-binding sites have been discovered at some α/α and β/α subunit interfaces, such as α4/α4, α5/α4 and β3/α4. Targeting unorthodox sites may yield subtype-selective ligands, such as those for (α4β2)₂ α5, (α4β2)₂ β3 and (α6β2)₂ β3 nAChRs. The unorthodox sites have unique pharmacology. Agonist binding at one unorthodox site is not sufficient to activate nAChRs, but it increases activation from the orthodox sites. NS9283, a selective agonist for the unorthodox α4/α4 site, was initially thought to be a positive allosteric modulator (PAM). NS9283 activates nAChRs with three engineered α4/α4 sites. PAMs, on the other hand, act at allosteric sites where ACh cannot bind. Known PAM sites include the ACh-homologous non-canonical site (e.g. morantel at β/α), the C-terminus (e.g. Br-PBTC and 17β-estradiol), a transmembrane domain (e.g. LY2087101) or extracellular and transmembrane domain interfaces (e.g. NS206). Some of these PAMs, such as Br-PBTC and 17β-estradiol, require only one subunit to potentiate activation of nAChRs. In this review, we will discuss differences between activation from orthosteric and allosteric sites, their selective ligands and clinical implications. These studies have advanced understanding of the structure, assembly and pharmacology of heteromeric neuronal nAChRs.

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.

Trapping of ivermectin by a pentameric ligand-gated ion channel upon open-to-closed isomerization

Ivermectin (IVM) is a broad-spectrum anthelmintic drug used to treat human parasitic diseases like river blindness and lymphatic filariasis. By activating invertebrate pentameric glutamate-gated chloride channels (GluCl receptors; GluClRs), IVM induces sustained chloride influx and long-lasting membrane hyperpolarization that inhibit neural excitation in nematodes. Although IVM activates the C. elegans heteromeric GluClα/β receptor, it cannot activate a homomeric receptor composed of the C. elegans GluClβ subunits. To understand this incapability, we generated a homopentameric α7-GluClβ chimeric receptor that consists of an extracellular ligand-binding domain of an α7 nicotinic acetylcholine receptor known to be potentiated by IVM, and a chloride-selective channel domain assembled from GluClβ subunits. Application of IVM prior to acetylcholine inhibited the responses of the chimeric α7-GluClβR. Adding IVM to activated α7-GluClβRs, considerably accelerated the decline of ACh-elicited currents and stabilized the receptors in a non-conducting state. Determination of IVM association and dissociation rate constants and recovery experiments suggest that, following initial IVM binding to open α7-GluClβRs, the drug induces a conformational change and locks the ion channel in a closed state for a long duration. We further found that IVM also inhibits the activation by glutamate of a homomeric receptor assembled from the C. elegans full-length GluClβ subunits.

B-973, a novel piperazine positive allosteric modulator of the α7 nicotinic acetylcholine receptor

The alpha7 (α7) nicotinic acetylcholine receptor is a therapeutic target for cognitive disorders. Here we describe 3-(3,4-difluorophenyl)-N-(1-(6-(4-(pyridin-2-yl)piperazin-1-yl)pyrazin-2-yl)ethyl)propanamide (B-973), a novel piperazine-containing molecule that acts as a positive allosteric modulator of the α7 receptor. We characterize the action of B-973 on the α7 receptor using electrophysiology and radioligand binding. At 0.1mM acetylcholine, 1μM B-973 potentiated peak acetylcholine-induced currents 6-fold relative to maximal acetylcholine (3mM) and slowed channel desensitization, resulting in a 6900-fold increase in charge transfer. The EC₅₀ of B-973 was approximately 0.3μM at acetylcholine concentrations ranging from 0.03 to 3mM. At a concentration of 1μM, B-973 shifted the acetylcholine EC₅₀ of peak currents from 0.30mM in control to 0.007mM. B-973 slowed channel deactivation upon acetylcholine removal (τ=50s) and increased the affinity of the α7 agonist [³H]A-585539. In the absence of exogenously added acetylcholine, application of B-973 at concentrations >1μM induced large methyllycaconitine-sensitive currents, suggesting B-973 can function as an Ago-PAM at high concentrations. B-973 will be a useful probe for investigating the biological consequences of increasing α7 receptor activity through allosteric modulation.

Pharmacological profile of Ascaris suum ACR-16, a new homomeric nicotinic acetylcholine receptor widely distributed in Ascaris tissues

Background and Purpose

Control of nematode parasite infections relies largely on anthelmintic drugs, several of which act on nicotinic ACh receptors (nAChRs), and there are concerns about the development of resistance. There is an urgent need for development of new compounds to overcome resistance and novel anthelmintic drug targets. We describe the functional expression and pharmacological characterization of a homomeric nAChR, ACR‐16, from a nematode parasite.

Experimental Approach

Using RT‐PCR, molecular cloning and two‐electrode voltage clamp electrophysiology, we localized acr‐16 mRNA in Ascaris suum (Asu) and then cloned and expressed acr‐16 cRNA in Xenopus oocytes. Sensitivity of these receptors to cholinergic anthelmintics and a range of nicotinic agonists was tested.

Key Results

Amino acid sequence comparison with vertebrate nAChR subunits revealed ACR‐16 to be most closely related to α7 receptors, but with some striking distinctions. acr‐16 mRNA was recovered from Asu somatic muscle, pharynx, ovijector, head and intestine. In electrophysiological experiments, the existing cholinergic anthelmintic agonists (morantel, levamisole, methyridine, thenium, bephenium, tribendimidine and pyrantel) did not activate Asu‐ACR‐16 (except for a small response to oxantel). Other nAChR agonists: nicotine, ACh, cytisine, 3‐bromocytisine and epibatidine, produced robust current responses which desensitized at a rate varying with the agonists. Unlike α7, Asu‐ACR‐16 was insensitive to α‐bungarotoxin and did not respond to genistein or other α7 positive allosteric modulators. Asu‐ACR‐16 had lower calcium permeability than α7 receptors.

Conclusions and Implications

We suggest that ACR‐16 has diverse tissue‐dependent functions in nematode parasites and is a suitable drug target for development of novel anthelmintic compounds.

Functional characterization of ivermectin binding sites in α1β2γ2L GABA(A) receptors

GABA_A receptors (GABA_ARs) are the major inhibitory neurotransmitter receptors in the brain and are therapeutic targets for many indications including sedation, anesthesia and anxiolysis. There is, however, considerable scope for the development of new therapeutics with improved beneficial effects and reduced side-effect profiles. The anthelminthic drug, ivermectin, activates the GABA_AR although its binding site is not known. The molecular site of action of ivermectin has, however, been defined by crystallography in the homologous glutamate-gated chloride channel. Resolving the molecular mechanisms of ivermectin binding to α1β2γ2L GABA_ARs may provide insights into the design of improved therapeutics. Given that ivermectin binds to subunit interfaces, we sought to define (1) which subunit interface sites it binds to, (2) whether these sites are equivalent in terms of ivermectin sensitivity or efficacy, and (3) how many must be occupied for maximal efficacy. Our approach involved precluding ivermectin from binding to particular interfaces by introducing bulky M3 domain 36′F sidechains to the “+” side of those interfaces. We thereby demonstrated that ivermectin produces irreversible channel activation only when it binds to the single γ2L-β2 interface site. When it binds to α1-β2 sites it elicits potentiation of GABA-gated currents but has no irreversible activating effect. Ivermectin cannot bind to the β2-α1 interface site due to its endogenous bulky 36′ methionine. Replacing this with an alanine creates a functional site at this interface, but surprisingly it is inhibitory. Molecular docking simulations reveal that the γ2L-β2 interface forms more contacts with ivermectin than the other interfaces, possibly explaining why ivermectin appears to bind irreversibly at this interface. This study demonstrates unexpectedly stark pharmacological differences among GABA_AR ivermectin binding sites.

Allosteric modulation of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are receptors for the neurotransmitter acetylcholine and are members of the 'Cys-loop' family of pentameric ligand-gated ion channels (LGICs). Acetylcholine binds in the receptor extracellular domain at the interface between two subunits and research has identified a large number of nAChR-selective ligands, including agonists and competitive antagonists, that bind at the same site as acetylcholine (commonly referred to as the orthosteric binding site). In addition, more recent research has identified ligands that are able to modulate nAChR function by binding to sites that are distinct from the binding site for acetylcholine, including sites located in the transmembrane domain. These include positive allosteric modulators (PAMs), negative allosteric modulators (NAMs), silent allosteric modulators (SAMs) and compounds that are able to activate nAChRs via an allosteric binding site (allosteric agonists). Our aim in this article is to review important aspects of the pharmacological diversity of nAChR allosteric modulators and to describe recent evidence aimed at identifying binding sites for allosteric modulators on nAChRs.

Allosteric modulation of ligand gated ion channels by ivermectin

Ivermectin acts as a positive allosteric regulator of several ligand-gated channels including the glutamate-gated chloride channel (GluCl), gamma aminobutyric acid type-A receptor, glycine receptor, neuronal alpha7-nicotinic receptor and purinergic P2X4 receptor. In most of the ivermectin-sensitive channels, the effects of ivermectin include the potentiation of agonist-induced currents at low concentrations and channel opening at higher concentrations. Based on mutagenesis, electrophysiological recordings and functional analysis of chimeras between ivermectin-sensitive and ivermectin-insensitive receptors, it has been concluded that ivermectin acts by insertion between transmembrane helices. The three-dimensional structure of C. elegans GluCl complexed with ivermectin has revealed the details of the ivermectin-binding site, however, no generic motif of amino acids could accurately predict ivermectin binding site for other ligand gated channels. Here, we will review what is currently known about ivermectin binding and modulation of Cys-loop receptor family of ligand-gated ion channels and what are the critical structural determinants underlying potentiation of the P2X4 receptor channel.

ELIC-α7 Nicotinic acetylcholine receptor (α7nAChR) chimeras reveal a prominent role of the extracellular-transmembrane domain interface in allosteric modulation

The native α7 nicotinic acetylcholine receptor (α7nAChR) is a homopentameric ligand-gated ion channel mediating fast synaptic transmission and is of pharmaceutical interest for treatment of numerous disorders. The transmembrane domain (TMD) of α7nAChR has been identified as a target for positive allosteric modulators (PAMs), but it is unclear whether modulation occurs through changes entirely within the TMD or changes involving both the TMD and the extracellular domain (ECD)-TMD interface. In this study, we constructed multiple chimeras using the TMD of human α7nAChR and the ECD of a prokaryotic homolog, ELIC, which is not sensitive to these modulators, and for which a high resolution structure has been solved. Functional ELIC-α7nAChR (EA) chimeras were obtained when their ECD-TMD interfaces were modified to resemble either the ELIC interface (EAELIC) or α7nAChR interface (EAα7). Both EAα7 and EAELIC show similar activation response and desensitization characteristics, but only EAα7 retained the unique pharmacology of α7nAChR evoked by PAMs, including potentiation by ivermectin, PNU-120596, and TQS, as well as activation by 4BP-TQS. This study suggests that PAM modulation through the TMD has a more stringent requirement at the ECD-TMD interface than agonist activation.

Caenorhabditis elegans neuromuscular junction: GABA receptors and ivermectin action

The prevalence of human and animal helminth infections remains staggeringly high, thus urging the need for concerted efforts towards this area of research. GABA receptors, encoded by the unc-49 gene, mediate body muscle inhibition in Caenorhabditis elegans and parasitic nematodes and are targets of anthelmintic drugs. Thus, the characterization of nematode GABA receptors provides a foundation for rational anti-parasitic drug design. We therefore explored UNC-49 channels from C. elegans muscle cultured cells of the first larval stage at the electrophysiological and behavioral levels. Whole-cell recordings reveal that GABA, muscimol and the anthelmintic piperazine elicit macroscopic currents from UNC-49 receptors that decay in their sustained presence, indicating full desensitization. Single-channel recordings show that all drugs elicit openings of ∼2.5 pA (+100 mV), which appear either as brief isolated events or in short bursts. The comparison of the lowest concentration required for detectable channel opening, the frequency of openings and the amplitude of macroscopic currents suggest that piperazine is the least efficacious of the three drugs. Macroscopic and single-channel GABA-activated currents are profoundly and apparently irreversibly inhibited by ivermectin. To gain further insight into ivermectin action at C. elegans muscle, we analyzed its effect on single-channel activity of the levamisol-sensitive nicotinic receptor (L-AChR), the excitatory receptor involved in neuromuscular transmission. Ivermectin produces a profound inhibition of the frequency of channel opening without significant changes in channel properties. By revealing that ivermectin inhibits C. elegans muscle GABA and L-AChR receptors, our study adds two receptors to the already known ivermectin targets, thus contributing to the elucidation of its pleiotropic effects. Behavioral assays in worms show that ivermectin potentiates piperazine-induced paralysis, thus suggesting that their combination is a good strategy to overcome the increasing resistance of parasites, an issue of global concern for human and animal health.

NMR structures of the human α7 nAChR transmembrane domain and associated anesthetic binding sites

The α7 nicotinic acetylcholine receptor (nAChR), assembled as homomeric pentameric ligand-gated ion channels, is one of the most abundant nAChR subtypes in the brain. Despite its importance in memory, learning and cognition, no structure has been determined for the α7 nAChR TM domain, a target for allosteric modulators. Using solution state NMR, we determined the structure of the human α7 nAChR TM domain (PDB ID: 2MAW) and demonstrated that the α7 TM domain formed functional channels in Xenopus oocytes. We identified the associated binding sites for the anesthetics halothane and ketamine; the former cannot sensitively inhibit α7 function, but the latter can. The α7 TM domain folds into the expected four-helical bundle motif, but the intra-subunit cavity at the extracellular end of the α7 TM domain is smaller than the equivalent cavity in the α4β2 nAChRs (PDB IDs: 2LLY; 2LM2). Neither drug binds to the extracellular end of the α7 TM domain, but two halothane molecules or one ketamine molecule binds to the intracellular end of the α7 TM domain. Halothane and ketamine binding sites are partially overlapped. Ketamine, but not halothane, perturbed the α7 channel-gate residue L9'. Furthermore, halothane did not induce profound dynamics changes in the α7 channel as observed in α4β2. The study offers a novel high-resolution structure for the human α7 nAChR TM domain that is invaluable for developing α7-specific therapeutics. It also provides evidence to support the hypothesis: only when anesthetic binding perturbs the channel pore or alters the channel motion, can binding generate functional consequences.

Plant toxins that affect nicotinic acetylcholine receptors: a review

Plants produce a wide variety of chemical compounds termed secondary metabolites that are not involved in basic metabolism, photosynthesis, or reproduction. These compounds are used as flavors, fragrances, insecticides, dyes, hallucinogens, nutritional supplements, poisons, and pharmaceutical agents. However, in some cases these secondary metabolites found in poisonous plants perturb biological systems. Ingestion of toxins from poisonous plants by grazing livestock often results in large economic losses to the livestock industry. The chemical structures of these compounds are diverse and range from simple, low molecular weight toxins such as oxalate in halogeton to the highly complex norditerpene alkaloids in larkspurs. While the negative effects of plant toxins on people and the impact of plant toxins on livestock producers have been widely publicized, the diversity of these toxins and their potential as new pharmaceutical agents for the treatment of diseases in people and animals has also received widespread interest. Scientists are actively screening plants from all regions of the world for bioactivity and potential pharmaceuticals for the treatment or prevention of many diseases. In this review, we focus the discussion to those plant toxins extensively studied at the USDA Poisonous Plant Research Laboratory that affect the nicotinic acetylcholine receptors including species of Delphinium (Larkspurs), Lupinus (Lupines), Conium (poison hemlock), and Nicotiana (tobaccos).

Propofol binding to the resting state of the gloeobacter violaceus ligand-gated ion channel (GLIC) induces structural changes in the inter- and intrasubunit transmembrane domain (TMD) cavities

General anesthetics exert many of their CNS actions by binding to and modulating membrane-embedded pentameric ligand-gated ion channels (pLGICs). The structural mechanisms underlying how anesthetics modulate pLGIC function remain largely unknown. GLIC, a prokaryotic pLGIC homologue, is inhibited by general anesthetics, suggesting anesthetics stabilize a closed channel state, but in anesthetic-bound GLIC crystal structures the channel appears open. Here, using functional GLIC channels expressed in oocytes, we examined whether propofol induces structural rearrangements in the GLIC transmembrane domain (TMD). Residues in the GLIC TMD that frame intrasubunit and intersubunit water-accessible cavities were individually mutated to cysteine. We measured and compared the rates of modification of the introduced cysteines by sulfhydryl-reactive reagents in the absence and presence of propofol. Propofol slowed the rate of modification of L240C (intersubunit) and increased the rate of modification of T254C (intrasubunit), indicating that propofol binding induces structural rearrangements in these cavities that alter the local environment near these residues. Propofol acceleration of T254C modification suggests that in the resting state propofol does not bind in the TMD intrasubunit cavity as observed in the crystal structure of GLIC with bound propofol (Nury, H., Van Renterghem, C., Weng, Y., Tran, A., Baaden, M., Dufresne, V., Changeux, J. P., Sonner, J. M., Delarue, M., and Corringer, P. J. (2011) Nature 469, 428-431). In silico docking using a GLIC closed channel homology model suggests propofol binds to intersubunit sites in the TMD in the resting state. Propofol-induced motions in the intersubunit cavity were distinct from motions associated with channel activation, indicating propofol stabilizes a novel closed state.

Pharmacological insights into the role of P2X4 receptors in behavioural regulation: lessons from ivermectin

Purinergic ionotropic P2X receptors are a family of cation-permeable channels that bind extracellular adenosine 5'-triphosphate. In particular, convergent lines of evidence have recently highlighted P2X(4) receptors as a potentially critical target in the regulation of multiple nervous and behavioural functions, including pain, neuroendocrine regulation and hippocampal plasticity. Nevertheless, the role of the P2X(4) receptor in behavioural organization remains poorly investigated. To study the effects of P2X(4) activation, we tested the acute effects of its potent positive allosteric modulator ivermectin (IVM, 2.5-10 mg/kg i.p.) on a broad set of paradigms capturing complementary aspects of perceptual, emotional and cognitive regulation in mice. In a novel open field, IVM did not induce significant changes in locomotor activity, but increased the time spent in the peripheral zone. In contrast, IVM produced anxiolytic-like effects in the elevated plus maze and marble burying tasks, as well as depression-like behaviours in the tail-suspension and forced swim tests. The agent induced no significant behavioural changes in the conditioned place preference test and in the novel object recognition task. Finally, the drug induced a dose-dependent decrease in sensorimotor gating, as assessed by pre-pulse inhibition (PPI) of the acoustic startle reflex. In P2X(4) knockout mice, the effects of IVM in the open field and elevated plus maze were similar to those observed in wild type mice; conversely, the drug significantly increased startle amplitude and failed to reduce PPI. Taken together, these results suggest that P2X(4) receptors may play a role in the regulation of sensorimotor gating.

Derquantel and abamectin: effects and interactions on isolated tissues of Ascaris suum

Startect(®) is a novel anthelmintic combination of derquantel and abamectin. It is hypothesized that derquantel and abamectin interact pharmacologically. We investigated the effects of derquantel, abamectin and their combination on somatic muscle nicotinic acetylcholine receptors and pharyngeal muscle glutamate gated chloride receptor channels of Ascaris suum. We used muscle-strips to test the effects of abamectin, derquantel, and abamectin+derquantel together on the contraction responses to different concentrations of acetylcholine. We found that abamectin reduced the response to acetylcholine, as did derquantel. In combination (abamectin+derquantel), inhibition of the higher acetylcholine concentration response was greater than the predicted additive effect. A two-micropipette current-clamp technique was used to study electrophysiological effects of the anthelmintics on: (1) acetylcholine responses in somatic muscle and; (2) on l-glutamate responses in pharyngeal preparations. On somatic muscle, derquantel (0.1-30μM) produced a potent (IC50 0.22, CI 0.18-0.28μM) reversible antagonism of acetylcholine depolarizations. Abamectin (0.3μM) produced a slow onset inhibition of acetylcholine depolarizations. We compared effects of abamectin and derquantel on muscle preparations pretreated for 30min with these drugs. The effect of the combination was significantly greater than the predicted additive effect of both drugs at higher acetylcholine concentrations. On the pharynx, application of derquantel produced no significant effect by itself or on responses to abamectin and l-glutamate. Abamectin increased the input conductance of the pharynx (EC50 0.42, CI 0.13-1.36μM). Our study demonstrates that abamectin and derquantel interact at nicotinic acetylcholine receptors on the somatic muscle and suggested synergism can occur.

A nicotinic acetylcholine receptor transmembrane point mutation (G275E) associated with resistance to spinosad in Frankliniella occidentalis

High levels of resistance to spinosad, a macrocyclic lactone insecticide, have been reported previously in western flower thrips, Frankliniella occidentalis, an economically important insect pest of vegetables, fruit and ornamental crops. We have cloned the nicotinic acetylcholine receptor (nAChR) α6 subunit from F. occidentalis (Foα6) and compared the nucleotide sequence of Foα6 from susceptible and spinosad-resistant insect populations (MLFOM and R1S respectively). A single nucleotide change has been identified in Foα6, resulting in the replacement of a glycine (G) residue in susceptible insects with a glutamic acid (E) in resistant insects. The resistance-associated mutation (G275E) is predicted to lie at the top of the third α-helical transmembrane domain of Foα6. Although there is no direct evidence identifying the location of the spinosad binding site, the analogous amino acid in the C. elegans glutamate-gated chloride channel lies in close proximity (4.4 Å) to the known binding site of ivermectin, another macrocyclic lactone pesticide. The functional consequences of the resistance-associated mutation have been examined in the human nAChR α7 subunit. Introduction of an analogous (A272E) mutation in α7 abolishes the modulatory effects of spinosad whilst having no significant effect upon activation by acetylcholine, consistent with spinosad having an allosteric mechanism of action.

Positive allosteric modulation by ivermectin of human but not murine P2X7 receptors

BACKGROUND AND PURPOSE

In mammalian cells, the anti-parasitic drug ivermectin is known as a positive allosteric modulator of the ATP-activated ion channel P2X4 and is used to discriminate between P2X4- and P2X7-mediated cellular responses. In this paper we provide evidence that the reported isoform selectivity of ivermectin is a species-specific phenomenon.

EXPERIMENTAL APPROACH

Complementary electrophysiological and fluorometric methods were applied to evaluate the effect of ivermectin on recombinantly expressed and on native P2X7 receptors. A biophysical characterization of ionic currents and of the pore dilation properties is provided.

KEY RESULTS

Unexpectedly, ivermectin potentiated currents in human monocyte-derived macrophages that endogenously express hP2X7 receptors. Likewise, currents and [Ca(2+) ](i) influx through recombinant human (hP2X7) receptors were potently enhanced by ivermectin at submaximal or saturating ATP concentrations. Since intracellular ivermectin did not mimic or prevent its activity when applied to the bath solution, the binding site of ivermectin on hP2X7 receptors appears to be accessible from the extracellular side. In contrast to currents through P2X4 receptors, ivermectin did not cause a delay in hP2X7 current decay upon ATP removal. Interestingly, NMDG(+) permeability and Yo-Pro-1 uptake were not affected by ivermectin. On rat or mouse P2X7 receptors, ivermectin was only poorly effective, suggesting a species-specific mode of action.

CONCLUSIONS AND IMPLICATIONS

The data indicate a previously unrecognized species-specific modulation of human P2X7 receptors by ivermectin that should be considered when using this cell-biological tool in human cells and tissues.

Multiple cholinergic signaling pathways in pituitary gonadotrophs

Acetylcholine (ACh) has been established as a paracrine factor in the anterior pituitary gland, but the receptors mediating ACh action and the cell types bearing these receptors have not been identified. Our results showed that the expression of the nicotinic subunits mRNAs followed the order β2 > β1 = α9 > α4 in cultured rat pituitary cells. The expression of the subunits in immortalized LβT2 mouse gonadotrophs followed the order β2 > α4 = α1. M4 > M3 muscarinic receptor mRNA were also identified in pituitary and LβT2 cells. The treatment of cultured pituitary cells with GnRH down-regulated the expression of α9 and α4 mRNAs, without affecting the expression of M3 and M4 receptor mRNAs, and ACh did not alter the expression of GnRH receptor mRNA. We also performed double immunostaining to show the expression of β2-subunit and M4 receptor proteins in gonadotrophs. Functional nicotinic channels capable of generating an inward current, facilitation of electrical activity, and Ca(2+) influx were identified in single gonadotrophs and LβT2 cells. In both cell types, the M3 receptor-mediated, phospholipase C-dependent Ca(2+) mobilization activated an outward apamin-sensitive K(+) current and caused hyperpolarization. The activation of M4 receptors by ACh inhibited cAMP production and GnRH-induced LH release in a pertussis toxin-sensitive manner. We concluded that multiple cholinergic receptors are expressed in gonadotrophs and that the main secretory action of ACh is inhibitory through M4 receptor-mediated down-regulation of cAMP production. The expression of nicotinic receptors in vitro compensates for the lack of regular GnRH stimulation of gonadotrophs.

α7 nicotinic acetylcholine receptors and their role in cognition

The precise role of nicotinic acetylcholine receptors (nAChRs) in central cognitive processes still remains incompletely understood almost 150 years after its initial discovery. Central nAChRs are activated by acetylcholine, which functions in the extracellular space as a nonsynaptic messenger. Recently, a novel concept in the nAChR mode of operation has been described as a fast-type nonsynaptic transmission. In this review, we attempt to summarise the experimental findings that support the role of one of the most distributed receptor subtypes, the α7 nAChRs, and particularly focus on its procognitive effects following receptor activation. The basic characteristics of α7 nAChRs are discussed, from receptor homology to cellular-level functions. Synaptic plasticity is often implicated with α7 nAChRs on the basis of several diverse studies. Here, we provide a summary of the plastic features of the α7 receptor subtype and its role in higher level cognitive function. Finally, recent clinical evidence is reviewed, which demonstrates with increasing confidence the promise α7 nAChRs as a molecular target in future pharmacotherapy to prevent cognitive decline in various types of dementia, specifically, via the development of positive allosteric modulator compounds.