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BACKGROUND: Most smokers attempting to quit will quickly relapse to tobacco use even when treated with the most efficacious smoking cessation agents currently available. This highlights the need to develop effective new smoking cessation medications. Evidence suggests that positive allosteric modulators (PAM) and other enhancers of nicotinic acetylcholine receptor (nAChR) signaling could have therapeutic utility as smoking cessation agents. METHODS: 3-[3-(3-pyridyl)-1,2,4-oxadiazol-5-yl]benzonitrile (NS9283) was used as a starting point for medical chemistry efforts to develop novel small molecule enhancers of α4β2* nAChR stoichiometries containing a low-affinity agonist binding site at the interface of α4/α4 and α4/α5 subunits. RESULTS: The NS9283 derivative SR9883 enhanced the effect of nicotine on α4β2* nAChR stoichiometries containing low-affinity agonist binding sites, with EC50 values from 0.2-0.4 μM. SR9883 had no effect on α3β2* or α3β4* nAChRs. SR9883 was bioavailable after intravenous (1 mg kg-1) and oral (10-20 mg kg-1) administration and penetrated into the brain. When administered alone, SR9883 (5-10 mg kg-1) had no effect on locomotor activity or intracranial self-stimulation (ICSS) thresholds in mice. When co-administered with nicotine, SR9883 enhanced locomotor suppression and elevations of ICSS thresholds induced by nicotine. SR9883 (5 and 10 mg kg-1) decreased responding for intravenous nicotine infusions (0.03 mg kg-1 per infusion) but had no effect on responding for food rewards in rats. CONCLUSIONS: These data suggest that SR9883 is useful for investigating behavioral processes regulated by certain α4β2* nAChR stoichiometries. SR9883 and related compounds with favorable drug-like physiochemical and pharmacological properties hold promise as novel treatments of tobacco use disorder.

Background: Most smokers attempting to quit will quickly relapse to tobacco use even when treated with the most efficacious smoking cessation agents currently available. This highlights the need to develop effective new smoking cessation medications. Evidence suggests that positive allosteric modulators (PAM) and other enhancers of nicotinic acetylcholine receptor (nAChR) signaling could have therapeutic utility as smoking cessation agents. Methods: 3-[3-(3-pyridyl)-1,2,4-oxadiazol-5-yl]benzonitrile (NS9283) was used as a starting point for medical chemistry efforts to develop novel small molecule enhancers of a4ß2* nAChR stoichiometries containing a low-affinity agonist binding site at the interface of a4/a4 and a4/a5 subunits. Results: The NS9283 derivative SR9883 enhanced the effect of nicotine on a4ß2* nAChR stoichiometries containing low-affinity agonist binding sites, with EC50 values from 0.2-0.4 µM. SR9883 had no effect on a3ß2* or a3ß4* nAChRs. SR9883 was bioavailable after intravenous (1 mg kg-1) and oral (10-20 mg kg-1) administration and penetrated into the brain. When administered alone, SR9883 (5-10 mg kg-1) had no effect on locomotor activity or intracranial selfstimulation (ICSS) thresholds in mice. When co-administered with nicotine, SR9883 enhanced locomotor suppression and elevations of ICSS thresholds induced by nicotine. SR9883 (5 and 10 mg kg-1) decreased responding for intravenous nicotine infusions (0.03 mg kg-1 per infusion) but had no effect on responding for food rewards in rats. Conclusions: These data suggest that SR9883 is useful for investigating behavioral processes regulated by certain a4ß2* nAChR stoichiometries. SR9883 and related compounds with favorable drug-like physiochemical and pharmacological properties hold promise as novel treatments of tobacco use disorder. [ABSTRACT FROM AUTHOR]

We characterized the functional and molecular properties of nicotinic acetylcholine receptors (AChRs) expressed by IMR-32, a human neuroblastoma cell line, and compared them to human [alpha]3 AChRs expressed in stably transfected human embryonic kidney (HEK) cells. IMR-32 cells, like neurons of autonomic ganglia, have been shown to express [alpha]3, [alpha]5, [alpha]7, [beta]2, and [beta]4 AChR subunits. From these subunits, several types of [alpha]3 AChRs as well as homomeric [alpha]7 AChRs could be formed. However, as we show, the properties of functional AChRs in these cells overwhelmingly reflect [alpha]3[beta]4 AChRs. [alpha]7 AChR function was not detected, yet we estimate that there are 70% as many surface [alpha]7 AChRs in IMR-32 when compared with [alpha]3 AChRs. Agonist potencies ([EC.sub.50] values) followed the rank order of 1,1-dimethyl-4-phenylpiperazinium (DMPP; 16[+ or -]1 [micro]M) > nicotine (Nic; 48 [+ or -] 7 [micro]M) [less than or equal to] cytisine (Cyt; 57 [+ or -] 3 [micro]M) = acetylcholine (ACh; 59 [+ or -] 6 [micro]M). All agonists exhibited efficacies of at least 80% relative to ACh. The currents showed strong inward rectification and desensitized at a rate of 3 [s.sup.-1] (300 [micro]M ACh; -60 mV). Assays that used mAbs confirmed the predominance of [alpha]3- and [beta]4-containing AChRs in IMR-32 cells. Although 18% of total [alpha]3 AChRs contained [beta]2 subunits, no [beta]2 subunit was detected on the cell surface. Chronic Nic incubation increased the amount of total, but not surface [alpha]3[beta]2 AChRs in IMR-32 cells. Nic incubation and reduced culture temperature increased total and surface AChRs in [alpha]3[beta]2 transfected HEK cells. Characterization of various [alpha]3 AChRs expressed in HEK cell lines revealed that the functional properties of the [alpha]3[beta]4 cell line best matched those found for IMR-32 cells. The rank order of agonist potencies ([EC.sub.50] values) for this line was DMPP (14 [+ or -] 1 [micro]M) = Cyt (18 [+ or -] 1 [micro]M) > Nic (56 [+ or -] 15 [micro]M > ACh (79 [+ or -] 8 [micro]M). The efficacies of both Cyt and DMPP were ~80% when compared with ACh and the desensitization rate was 2 [s.sup.-1]. These data show that even with the potential to express several human nicotinic AChR subtypes, the functional properties of AChRs expressed by IMR-32 are completely attributable to [alpha]3[beta]4 AChRs. KEY WORDS: nicotinic receptor * autonomic * patch clamp

Positive allosteric modulators (PAMs) of nicotinic acetylcholine receptors (nAChR) are important therapeutic candidates as well as valuable research tools. We identified a novel type II PAM, (R)-7-bromo-N-(piperidin-3-yl)benzo[b]thiophene-2- carboxamide (Br-PBTC), which both increases activation and reactivates desensitized nAChRs. This compound increases acetylcholine- evoked responses ofα2* andα4* nAChRs but is without effect on α3* or α6* nAChRs (* indicates the presence of other nAChR subunits). Br-BPTC acts from the C-terminal extracellular sequences ofα4 subunits, which is also a PAM site for steroid hormone estrogens such as 17β-estradiol. Br-PBTC is much more potent than estrogens. Like 17β-estradiol, the non-steroid Br-PBTC only requires one α4 subunit to potentiate nAChR function, and its potentiation is stronger with more α4 subunits. This feature enables Br-BPTC to potentiate activation of (α4β2)(α6β2)β3 but not (α6β2)2β3 nAChRs. Therefore, this compound is potentially useful in vivo for determining functions of different α6* nAChR subtypes. Besides activation, Br-BPTC affects desensitization of nAChRs induced by sustained exposure to agonists. After minutes of exposure to agonists, Br-PBTC reactivated short term desensitized nAChRs that have at least two α4 subunits but not those with only one. Threeα4 subunits were required for Br-BPTC to reactivate long term desensitized nAChRs. These data suggest that higherPAM occupancy promotes channel opening more efficiently and overcomes short and long term desensitization. This C-terminal extracellular domain could be a target for developing subtype or state-selective drugs for nAChRs. [ABSTRACT FROM AUTHOR]

Neuronal nicotinic acetylcholine receptors containing α4, β2, and sometimes other subunits (α4β2* nAChRs) regulate addictive and other behavioral effects of nicotine. These nAChRs exist in several stoichiometries, typically with two high affinity acetylcholine (ACh) binding sites at the interface of α4 and β2 subunits and a fifth accessory subunit. A third low affinity ACh binding site is formed when this accessory subunit is α4 but not if it is β2. Agonists selective for the accessory ACh site, such as 3-[3-(3-pyridyl)-1,2,4-oxadiazol-5-yl]benzonitrile (NS9283), cannot alone activate a nAChR but can facilitate more efficient activation in combination with agonists at the canonical α4β2 sites. We therefore suggest categorizing agonists according to their site selectivity. NS9283 binds to the accessory ACh binding site; thus it is termed an accessory site-selective agonist. We expressed (α4β2)2 concatamers in Xenopus oocytes with free accessory subunits to obtain defined nAChR stoichiometries and α4/accessory subunit interfaces. We show that α2, α3, α4, and α6 accessory subunits can form binding sites for ACh and NS9283 at interfaces with α4 subunits, but β2 and β4 accessory subunits cannot. To permit selective blockage of the accessory site, α4 threonine 126 located on the minus side of α4 that contributes to the accessory site, but not the α4β2 sites, was mutated to cysteine. Alkylation of this cysteine with a thioreactive reagent blocked activity of ACh and NS9283 at the accessory site. Accessory agonist binding sites are promising drug targets. [ABSTRACT FROM AUTHOR]

Human (α6β2)(α4β2)β3 nicotinic acetylcholine receptors (AChRs) are essential for addiction to nicotine and a target for drug development for smoking cessation. Expressing this complex AChR is difficult, but has been achieved using subunit concatamers. In order to determine what limits expression of α6* AChRs and to efficiently express α6* AChRs using free subunits, we investigated expression of the simpler (α6β2)2β3 AChR. The concatameric form of this AChR assembles well, but is transported to the cell surface inefficiently. Various chimeras of α6 with the closely related α3 subunit increased expression efficiency with free subunits and produced pharmacologically equivalent functional AChRs. A chimera in which the large cytoplasmic domain of α6 was replaced with that of α3 increased assembly with β2 subunits and transport of AChRs to the oocyte surface. Another chimera replacing the unique methionine 211 of α6 with leucine found at this position in transmembrane domain 1 of α3 and other α subunits increased assembly of mature subunits containing β3 subunits within oocytes. Combining both α3 sequences in an α6 chimera increased expression of functional (α6β2)2β3 AChRs to 12-fold more than with concatamers. This is pragmatically useful, and provides insights on features of α6 subunit structure that limit its expression in transfected cells. [ABSTRACT FROM AUTHOR]

Objective Myasthenia gravis (MG) and its animal model, experimental autoimmune myasthenia gravis (EAMG), are antibody (Ab)-mediated autoimmune diseases, in which autoantibodies bind to and cause loss of muscle nicotinic acetylcholine receptors (AChRs) at the neuromuscular junction. To develop a specific immunotherapy of MG, we treated rats with ongoing EAMG by intraperitoneal injection of bacterially-expressed human muscle AChR constructs. Methods Rats with ongoing EAMG received intraperitoneal treatment with the constructs weekly for 5 weeks beginning after the acute phase. Autoantibody concentration, subclassification, and specificity were analyzed to address the underlying therapeutic mechanism. Results EAMG was specifically suppressed by diverting autoantibody production away from pathologically relevant specificities directed at epitopes on the extracellular surface of muscle AChRs toward pathologically irrelevant epitopes on the cytoplasmic domain. A mixture of subunit cytoplasmic domains was more effective than a mixture containing both extracellular and cytoplasmic domains or than only the extracellular domain of α1 subunits. Interpretation Therapy using only cytoplasmic domains, which lack pathologically relevant epitopes, avoids the potential liability of boosting the pathological response. Use of a mixture of bacterially-expressed human muscle AChR cytoplasmic domains for antigen-specific immunosuppression of myasthenia gravis has the potential to be specific, robust, and safe. ANN NEUROL 2010;67:441-451 [ABSTRACT FROM AUTHOR]

We prepared concatamers of α4 and β2 subunits for human nicotinic acetylcholine receptors (AChRs), in which the C terminus of α4 was linked to the N terminus of β2, or vice versa, via a tripeptide sequence repeated 6 or 12 times, and expressed them in Xenopus oocytes. Linkage did not substantially alter channel amplitude or channel open-duration. Linkage at the C terminus of α4 prevented AChR potentiation by 17-β-estradiol by disruption of its binding site. Assembly of AChRs from concatamers was less efficient, but function was much more efficient than that of unlinked subunits. With both linked and free subunits, greater ACh-induced currents per surface AChR were observed with the (α4)[sub3](β2)[sub2] stoichiometry than with the (α4)[sub2](β2)[sub3] stoichiometry. The (α4)[sub3](β2)[sub2] stoichiometry exhibited much lower ACh sensitivity. When concatamers were expressed alone, dipentameric AChRs were formed in which the (α4)[sub2](β2)[sub3] pentamer was linked to the (α4)[sub3](β2)[sub2] pentamer. Dipentamers were selectively expressed on the cell surface, whereas most monopentamers with dangling subunits were retained intracellularly. Coexpression of concatamers with monomeric β2, β4, or α4 subunits resulted in monopentamers, the stoichiometry of which was determined by the free subunit added. Linkage between the C terminus of β2 and the N terminus of α4 favored formation of ACh-binding sites within the concatamer, whereas linkage between the C terminus of α4 and the N terminus of β2 favored formation of ACh-binding sites between concatamers. These protein-engineering studies provide insight into the structure and function of α4β2 AChRs, emphasizing the functional differences between α4β2 AChRs of different stoichiometries. [ABSTRACT FROM AUTHOR]

Mechanisms that regulate early events in the biogenesis of the alpha7 nicotinic acetylcholine receptor (alpha7 AChR) are not well understood. Data presented here show that single amino acid mutations in the cytoplasmic loop of the alpha7 AChR, between position 335 and 343, abolish or attenuate expression of mature pentameric alpha7 AChRs in both human embryonic kidney tsA201 (HEK) and neuronal SH-SY5Y cells. Although the number of mature alpha7 AChRs is increased significantly in the presence of the chaperone protein resistant to inhibitors of cholineesterase-3 in HEK cells, sucrose gradient sedimentation reveals that the vast majority of alpha7 subunits are aggregated or improperly assembled. Transfection of alpha7 AChRs in SH-SY5Y cells, which endogenously express the alpha7 AChR, results in a much larger fraction of subunits assembled into mature AChRs. Thus, efficient assembly of alpha7 AChRs is influenced by several regions of the large cytoplasmic domain, as well perhaps by other parts of its structure, and requires as yet unknown factors not required by other AChR subtypes.

Neuronal nicotinic acetylcholine receptors (nAChRs) both mediate direct cholinergic synaptic transmission and modulate synaptic transmission by other neurotransmitters. Novel ligands are needed as probes to discriminate among structurally related nAChR subtypes. Alpha-conotoxin MII, a selective ligand that discriminates among a variety of nAChR subtypes, fails to discriminate well between some subtypes containing the closely related alpha3 and alpha6 subunits. Structure-function analysis of alpha-conotoxin MII was performed in an attempt to generate analogs with preference for alpha6-containing [alpha6(*) (asterisks indicate the possible presence of additional subunits)] nAChRs. Alanine substitution resulted in several analogs with decreased activity at alpha3(*) versus alpha6(*) nAChRs heterologously expressed in Xenopus laevis oocytes. From the initial analogs, a series of mutations with two alanine substitutions was synthesized. Substitution at His9 and Leu15 (MII[H9A;L15A]) resulted in a 29-fold lower IC(50) at alpha6beta4 versus alpha3beta4 nAChRs. The peptide had a 590-fold lower IC(50) for alpha6/alpha3beta2 versus alpha3beta2 and a 2020-fold lower IC(50) for alpha6/alpha3beta2beta3 versus alpha3beta2 nAChRs. MII[H9A;L15A] had little or no activity at alpha2beta2, alpha2beta4, alpha3beta4, alpha4beta2, alpha4beta4, and alpha7 nAChRs. Functional block by MII[H9A;L15A] of rat alpha6/alpha3beta2beta3 nAChRs (IC(50) = 2.4 nM) correlated well with the inhibition constant of MII[H9A;L15A] for [(125)I]alpha-conotoxin MII binding to putative alpha6beta2(*) nAChRs in mouse brain homogenates (K(i) = 3.3 nM). Thus, structure-function analysis of alpha-conotoxin MII enabled the creation of novel selective antagonists for discriminating among nAChRs containing alpha3 and alpha6 subunits.