Gating of nicotinic ACh receptors: Latest insights into ligand binding and function

Laboratory of Neurobiology, National Institute of Environmental Health Sciences, Department of Health and Human Services, PO Box 12233, Research Triangle Park, NC 27709, USA.
The Journal of Physiology (Impact Factor: 5.04). 11/2009; 588(Pt 4):597-602. DOI: 10.1113/jphysiol.2009.182691
Source: PubMed


Nicotinic acetylcholine receptors (nAChRs) are in the superfamily of cys-loop receptors, and are widely expressed in the nervous system where they participate in a variety of physiological functions, including regulating excitability and neurotransmitter release, as well as neuromuscular contraction. Members of the cys-loop family of receptors, which also includes the molluscan ACh-binding protein (AChBP), a soluble protein that is analogous to the extracellular ligand-binding domain of the cys-loop receptors, are pentameric assemblies of five subunits, with each subunit arranged around a central pore. The binding of ACh to the extracellular interface between two subunits induces channel opening. With the recent 4 A resolution of the Torpedo nAChR, and the crystal structure of the AChBP, much has been learned about the structure of the ligand-binding domain and the channel pore, as well as major structural rearrangements that may confer channel opening, including a major rearrangement of the C-loop within the ligand binding pocket, and perhaps other regions including the F-loop (the beta8-beta9 linker), the beta1-beta2 linker and the cys-loop. Here I will review the latest findings from my lab aimed at a further understanding of the function of the neuronal nAChR channels (and in particular the role of desensitization), and our search for novel AChBP species that may lead to a further understanding of the function of the cys-loop receptor family.

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    • "Conversely, in the b2-containing receptor, desensitization can be decreased only by substitution with the b4 N-terminal domain (residues 1e212) (Bohler et al., 2001). For the coupling interface, the Cys-loop (Rayes et al., 2009), pre-M1 region (Yakel, 2010a), the N-terminus of M1 (Bianchi et al., 2001; Engblom et al., 2002; Lu and Huang, 1998; McCormack et al., 2010), and the M2-M3 linker (Yakel, 2010b) all contribute to channel gating and desensitization . Moreover, the intracellular loop, especially the MAS, also plays a role in the rate of the Cys-loop receptor desensitization . "
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    ABSTRACT: Ligand-gated ion channels play a role in mediating fast synaptic transmission for communication between neurons. However, the structural basis for the functional coupling of the binding and pore domains, resulting in channel opening, remains a topic of intense investigation. Here, a series of α7 nicotinic receptor mutants were constructed for expression in cultured mammalian cells, and their single-channel properties were examined using the patch-clamp technique combined with radio ligand binding and the fluorescence staining technique. We demonstrated that the replacement of the four pore-lining residues in the channel domain of the α7 nicotinic receptor with the hydrophilic residue serine prolongs the open-channel lifetime, although the conductance of these mutants decreases. At the coupling interface between the extracellular and transmembrane domains, when the VRW residues in the Cys-loop were substituted with the corresponding residues (i.e., IYN) in the 5-HT3A receptor, the single-channel activity elicited by acetylcholine is impaired. This effect occurred despite the expression of the mutant receptors on the cell surface and despite the fact that the apparent Kd values were much lower than those of the wild-type α7 receptor. When we further lowered the channel-gating barrier of this chimera to enhance the open-channel probability, the loss of function was rescued. Overall, we explored the microscopic mechanisms underlying the interplay between the channel domains and the coupling interface that affect the channel activity, and we generated an allosteric gating model for the α7 receptor. This model shows that the gating machinery and coupling assembly codetermine the single-channel gating kinetics. These results likely apply to all channels in the Cys-loop receptor family. Copyright © 2015. Published by Elsevier Ltd.
    Neuropharmacology 04/2015; 95. DOI:10.1016/j.neuropharm.2015.04.010 · 5.11 Impact Factor
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    • "Nicotine activation of α7*nAChR results in Ca2+ induced Ca2+ release (Figure 7, Step 2). α7*nAChR have relatively short open-times and a fast rate of desensitization at high agonist concentrations [50,51]. Even at the very low concentrations of agonist used here, the Ca2+ influx via α7*nAChR is unlikely to be sufficient to maintain intracellular Ca2+ signals for the observed time course. "
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    ABSTRACT: Presynaptic nicotinic acetylcholine receptors (nAChRs) have long been implicated in the modulation of CNS circuits. We previously reported that brief exposure to low concentrations of nicotine induced sustained potentiation of glutamatergic transmission at ventral hippocampal (vHipp)-striatal synapses. Here, we exploited nAChR subtype-selective antagonists and agonists and α7*nAChR knockout mutant mice (α7-/-) to elucidate the signaling mechanisms underlying nAChR-mediated modulation of synaptic transmission. Using a combination of micro-slices culture from WT and α7-/-mice, calcium imaging, and immuno-histochemical techniques, we found that nicotine elicits localized and oscillatory increases in intracellular Ca(2+) along vHipp axons that persists for up to 30 minutes. The sustained phase of the nicotine-induced Ca(2+) response was blocked by α-BgTx but not by DHβE and was mimicked by α7*nAChR agonists but not by non-α7*nAChR agonists. In vHipp slices from α7-/- mice, nicotine elicited only transient increases of axonal Ca(2+) signals and did not activate CaMKII. The sustained phase of the nicotine-induced Ca(2+) response required localized activation of CaMKII, phospholipase C, and IP3 receptor mediated Ca(2+)-induced Ca(2+) release (CICR). In conclusion, activation of presynaptic nAChRs by nicotine elicits Ca(2+) influx into the presynaptic axons, the sustained phase of the nicotine-induced Ca(2+) response requires that axonal α7*nAChR activate a downstream signaling network in the vHipp axons.
    PLoS ONE 12/2013; 8(12):e82719. DOI:10.1371/journal.pone.0082719 · 3.23 Impact Factor
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    • "Only 5 subunit does not binds ligand since one tyrosine residue is missing [12]. The nAChR sub-types may comprise binary (homomeric including the phylogenetically ancient 7 subunits, or heteromeric nAChR such as 42), ternary (such as 425) or quaternary (such as 3245) complexes [13]. When activated by acetylcholine , or other agonists such as nicotine, the nAChR allow intracellular influx of Ca 2+ determining several effects on pathways regulating cell proliferation, survival and motility [reviewed in 14]. "
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    ABSTRACT: Cigarette smoking is one of the major risk factors for COPD and COPD severity. In turn COPD is a major independent risk factor for lung cancer. Genome-wide association (GWA) studies both in lung cancer and COPD highlighted the same variants (SNPs) on chromosome 15q25 marking the gene cluster CHRNA3-CHRNB4-CHRNA5 for these smoking related diseases, showing a stimulating connection between this common genetic region and smoking behavior and smoking related illnesses. Different authors identified two candidate regions associated with age at smoking initiation in patients with COPD. The nicotinic acetylcholine receptor polymorphism (rs1051730) on chromosome 15q25 is associated with major tobacco-related diseases in the general population with additional increased risk of COPD as well as lung cancer. Moreover variants on the gene cluster CHRNA3-CHRNB4-CHRNA5 are associated with nicotine addiction antismoking therapy and antismoking therapy side-effects. These findings not only support the notion that variants can influence any therapy for smoking cessation, but offer rational bases to develop new drugs and new therapeutic strategies.
    Current Medicinal Chemistry 10/2012; 19(34). DOI:10.2174/092986712804143312 · 3.85 Impact Factor
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