Influence of the M3-M4 intracellular domain upon nicotinic acetylcholine receptor assembly, targeting and function

Clinical, molecular, physiologic, and therapeutic feature of patients with CHRNA4 and CHRNB2 deficiency: A systematic review

The α4β2 nAChRs are crucial ion channels that control neurotransmitter release and play a role in various physiologic and pathologic processes. CHRNA4 encodes the α4-nAChRs, while CHRNB2 encodes the β2-nAChRs. Recent studies have found different variants of α4β2-nAChRs in individuals with conditions such as AD, ADHD, ALS, PD, and brain abnormalities. We conducted a scoping review following a six-stage methodology structure and adhering to PRISMA guidelines. We systematically reviewed articles using relevant keywords up to October 2, 2023. In this summary, we cover the clinical symptoms reported, the genes and protein structure of CHRNA4 and CHRNB2, mutations in these genes, inheritance patterns, the functional impact of mutations and polymorphisms in CHRNA4 and CHRNB2, and the epidemiology of these diseases. Recent research indicates that nAChRs may play a significant role in neurodegenerative disorders, possibly impacting neuronal function through yet undiscovered regulatory pathways. Studying how nAChRs interact with disease-related aggregates in neurodegenerative conditions may lead to new treatment options for these disorders.

Auxiliary protein and chaperone regulation of neuronal nicotinic receptor subtype expression and function

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of pentameric, ligand-gated ion channels that are located on the surface of neurons and non-neuronal cells and have multiple physiological and pathophysiological functions. In order to reach the cell surface, many nAChR subtypes require the help of chaperone and/or auxiliary/accessory proteins for their assembly, trafficking, pharmacological modulation, and normal functioning in vivo. The use of powerful genome-wide cDNA screening has led to the identification and characterisation of the molecules and mechanisms that participate in the assembly and trafficking of receptor subtypes, including chaperone and auxiliary or accessory proteins. The aim of this review is to describe the latest findings concerning nAChR chaperones and auxiliary proteins and pharmacological chaperones, and how some of them control receptor biogenesis or regulate channel activation and pharmacology. Some auxiliary proteins are subtype selective, some regulate various subtypes, and some not only modulate nAChRs but also target other receptors and signalling pathways. We also discuss how changes in auxiliary proteins may be involved in nAChR dysfunctions.

Two residues determine nicotinic acetylcholine receptor requirement for RIC-3

Nicotinic acetylcholine receptors (N-AChRs) mediate fast synaptic signaling and are members of the pentameric ligand-gated ion channel (pLGIC) family. They rely on a network of accessory proteins in vivo for correct formation and transport to the cell surface. Resistance to cholinesterase 3 (RIC-3) is an endoplasmic reticulum protein that physically interacts with nascent pLGIC subunits and promotes their oligomerization. It is not known why some N-AChRs require RIC-3 in heterologous expression systems, whereas others do not. Previously we reported that the ACR-16 N-AChR from the parasitic nematode Dracunculus medinensis does not require RIC-3 in Xenopus laevis oocytes. This is unusual because all other nematode ACR-16, like the closely related Ascaris suum ACR-16, require RIC-3. Their high sequence similarity limits the number of amino acids that may be responsible, and the goal of this study was to identify them. A series of chimeras and point mutations between A. suum and D. medinensis ACR-16, followed by functional characterization with electrophysiology, identified two residues that account for a majority of the receptor requirement for RIC-3. ACR-16 with R/K159 in the cys-loop and I504 in the C-terminal tail did not require RIC-3 for functional expression. Mutating either of these to R/K159E or I504T, residues found in other nematode ACR-16, conferred a RIC-3 requirement. Our results agree with previous studies showing that these regions interact and are involved in receptor synthesis. Although it is currently unclear what precise mechanism they regulate, these residues may be critical during specific subunit folding and/or assembly cascades that RIC-3 may promote.

Binding motif for RIC-3 chaperon protein in serotonin type 3A receptors

Serotonin or 5-hydroxytryptamine type 3 (5-HT3) receptors belong to the family of pentameric ligand-gated ion channels (pLGICs) that are therapeutic targets for psychiatric disorders and neurological diseases. Due to structural conservation and significant sequence similarities of pLGICs' extracellular and transmembrane domains, clinical trials for drug candidates targeting these two domains have been hampered by off-subunit modulation. With the present study, we explore the interaction interface of the 5-HT3A subunit intracellular domain (ICD) with the resistance to inhibitors of choline esterase (RIC-3) protein. Previously, we have shown that RIC-3 interacts with the L1-MX segment of the ICD fused to maltose-binding protein. In the present study, synthetic L1-MX-based peptides and Ala-scanning identify positions W347, R349, and L353 as critical for binding to RIC-3. Complementary studies using full-length 5-HT3A subunits confirm that the identified Ala substitutions reduce the RIC-3-mediated modulation of functional surface expression. Additionally, we find and characterize a duplication of the binding motif, DWLR…VLDR, present in both the MX-helix and the transition between the ICD MA-helix and transmembrane segment M4. Analogous Ala substitutions at W447, R449, and L454 disrupt MAM4-peptide RIC-3 interactions and reduce modulation of functional surface expression. In summary, we identify the binding motif for RIC-3 in 5-HT3A subunits at two locations in the ICD, one in the MX-helix and one at the MAM4-helix transition.

Speculation on How RIC-3 and Other Chaperones Facilitate α7 Nicotinic Receptor Folding and Assembly

The process of how multimeric transmembrane proteins fold and assemble in the endoplasmic reticulum is not well understood. The alpha7 nicotinic receptor (α7 nAChR) is a good model for multimeric protein assembly since it has at least two independent and specialized chaperones: Resistance to Inhibitors of Cholinesterase 3 (RIC-3) and Nicotinic Acetylcholine Receptor Regulator (NACHO). Recent cryo-EM and NMR data revealed structural features of α7 nAChRs. A ser-ala-pro (SAP) motif precedes a structurally important but unique "latch" helix in α7 nAChRs. A sampling of α7 sequences suggests the SAP motif is conserved from C. elegans to humans, but the latch sequence is only conserved in vertebrates. How RIC-3 and NACHO facilitate receptor subunits folding into their final pentameric configuration is not known. The artificial intelligence program AlphaFold2 recently predicted structures for NACHO and RIC-3. NACHO is highly conserved in sequence and structure across species, but RIC-3 is not. This review ponders how different intrinsically disordered RIC-3 isoforms from C. elegans to humans interact with α7 nAChR subunits despite having little sequence homology across RIC-3 species. Two models from the literature about how RIC-3 assists α7 nAChR assembly are evaluated considering recent structural information about the receptor and its chaperones.

Structures of highly flexible intracellular domain of human α7 nicotinic acetylcholine receptor

The intracellular domain (ICD) of Cys-loop receptors mediates diverse functions. To date, no structure of a full-length ICD is available due to challenges stemming from its dynamic nature. Here, combining nuclear magnetic resonance (NMR) and electron spin resonance experiments with Rosetta computations, we determine full-length ICD structures of the human α7 nicotinic acetylcholine receptor in a resting state. We show that ~57% of the ICD residues are in highly flexible regions, primarily in a large loop (loop L) with the most mobile segment spanning ~50 Å from the central channel axis. Loop L is anchored onto the MA helix and virtually forms two smaller loops, thereby increasing its stability. Previously known motifs for cytoplasmic binding, regulation, and signaling are found in both the helices and disordered flexible regions, supporting the essential role of the ICD conformational plasticity in orchestrating a broad range of biological processes.

The intracellular domain (ICD) of Cys-loop receptors mediates many of their functions, but no complete structure of a Cys-loop receptor ICD is available to date. Here, the authors combine NMR and ESR spectroscopy to determine the full-length ICD structures of the human α7 nicotinic acetylcholine receptor (α7nAChR).

Sleep-related hypermotor epilepsy associated mutations uncover important kinetic roles of α4β2- nicotinic acetylcholine receptor intracellular structures

Sleep-related hypermotor epilepsy (SHE) is a group of seizure disorders prominently associated with mutations in nicotinic acetylcholine receptors (nAChR). The most prevalent central nervous system nAChR subtype contains α4 and β2 subunits, in two ratios. (α4β2)₂β2-nAChR have high agonist sensitivity (HS-isoform), whereas (α4β2)₂α4-nAChR agonist responses exhibit a small high-sensitivity, and a predominant low-sensitivity, phase of function (LS-isoform). Multiple non-synonymous mutations in the second and third transmembrane domains of α4 and β2 subunits are associated with SHE. We recently demonstrated that two additional, SHE-associated, missense mutations in the major cytoplasmic loops of these subunits [α4(R336H) and β2(V337G)] cause increased macroscopic function-per receptor. Here, we use single-channel patch-clamp electrophysiology to show that these mutations influence single-channel amplitudes and open- and closed-state kinetics. Pure populations of HS- or LS-isoform α4β2-nAChR were expressed by injecting either 1:10 or 30:1 α4:β2 cRNA ratios, respectively, into Xenopus laevis oocytes. Functional properties of the resulting mutant α4β2-nAChR isoforms were compared to their wildtype counterparts. α4(R336H) subunit incorporation minimally affected single-channel amplitudes, whereas β2(V337G) subunit incorporation reduced them significantly in both isoforms. However, for both mutant subunits, increased function-per-receptor was predominantly caused by altered single channel kinetics. The α4(R336H) mutation primarily destabilizes desensitized states between openings. By contrast, the β2(V337G) mutation principally stabilizes receptor open states. The use of naturally-occurring and physiologically-impactful mutations has allowed us to define valuable new insights regarding the functional roles of nAChR intracellular domains. Further mechanistic context is provided by intracellular-domain structures recently published for other members of the Cys-loop receptor superfamily (α3β4-nAChR and 5-HT_3AR).

The intracellular domain of homomeric glycine receptors modulates agonist efficacy

Like other pentameric ligand-gated channels, glycine receptors (GlyRs) contain long intracellular domains (ICDs) between transmembrane helices 3 and 4. Structurally characterized GlyRs are generally engineered to have a very short ICD. We show here that for one such construct, zebrafish GlyREM, the agonists glycine, β-alanine, taurine, and GABA have high efficacy and produce maximum single-channel open probabilities greater than 0.9. In contrast, for full-length human α1 GlyR, taurine and GABA were clearly partial agonists, with maximum open probabilities of 0.46 and 0.09, respectively. We found that the elevated open probabilities in GlyREM are not due to the limited sequence differences between the human and zebrafish orthologs, but rather to replacement of the native ICD with a short tripeptide ICD. Consistent with this interpretation, shortening the ICD in the human GlyR increased the maximum open probability produced by taurine and GABA to 0.90 and 0.70, respectively, but further engineering it to resemble GlyREM (by introducing the zebrafish transmembrane helix 4 and C terminus) had no effect. Furthermore, reinstating the native ICD to GlyREM converted taurine and GABA to partial agonists, with maximum open probabilities of 0.66 and 0.40, respectively. Structural comparison of transmembrane helices 3 and 4 in short- and long-ICD GlyR subunits revealed that ICD shortening does not distort the orientation of these helices within each subunit. This suggests that the effects of shortening the ICD stem from removing a modulatory effect of the native ICD on GlyR gating, revealing a new role for the ICD in pentameric ligand-gated channels.

Comment on "On the Functional Annotation of Open-Channel Structures in the Glycine Receptor"

Recently, we reported the simulation of a stable open state of the glycine receptor. Central to the stability of the simulations was the behavior of the highly conserved leucine residues at the 9' gate, which were found to rotate into adjacent pockets, thus providing a structural rationale for decades of biochemical observations. In contrast, a previously reported model from Cerdan et al. (2018) resembled a more collapsed state. However, in support of their model, they draw attention to the agreement between calculated and experimental conductance measurements and argue that our model tends to overestimate ion flow. Here, we argue that there are many pitfalls with this approach and that the apparent agreement most likely reflects a fortuitous cancellation of errors. The computed values are highly sensitive to very small changes in model parameters. It is also likely that polarization effects will be very significant, and these have not yet been considered.

In Silico Modeling of the α7 Nicotinic Acetylcholine Receptor: New Pharmacological Challenges Associated with Multiple Modes of Signaling

The α7 nicotinic acetylcholine receptor is a homopentameric ion-channel of the Cys-loop superfamily characterized by its low probability of opening, high calcium permeability, and rapid desensitization. The α7 receptor has been targeted for the treatment of the cognitive symptoms of schizophrenia, depression, and Alzheimer's disease, but it is also involved in inflammatory modulation as a part of the cholinergic anti-inflammatory pathway. Despite its functional importance, in silico studies of the α7 receptor cannot produce a general model explaining the structural features of receptor activation, nor predict the mode of action for various ligand classes. Two particular problems in modeling the α7 nAChR are the absence of a high-resolution structure and the presence of five potentially nonequivalent orthosteric ligand binding sites. There is wide variability regarding the templates used for homology modeling, types of ligands investigated, simulation methods, and simulation times. However, a systematic survey focusing on the methodological similarities and differences in modeling α7 has not been done. In this work, we make a critical analysis of the modeling literature of α7 nAChR by comparing the findings of computational studies with each other and with experimental studies under the main topics of structural studies, ligand binding studies, and comparisons with other nAChR. In light of our findings, we also summarize current problems in the field and make suggestions for future studies concerning modeling of the α7 receptor.

Triple arginines as molecular determinants for pentameric assembly of the intracellular domain of 5-HT3A receptors

Serotonin type 3A receptors are homopentameric ligand-gated ion channels that are thought to assemble via interactions involving the subunits’ extracellular and transmembrane domains. Pandhare et al. reveal that channel assembly is also determined by three arginine residues in the receptor’s intracellular domain.

Serotonin type 3 receptors (5-HT₃Rs) are cation-conducting pentameric ligand-gated ion channels and members of the Cys-loop superfamily in eukaryotes. 5-HT₃Rs are found in the peripheral and central nervous system, and they are targets for drugs used to treat anxiety, drug dependence, and schizophrenia, as well as chemotherapy-induced and postoperative nausea and emesis. Decades of research of Cys-loop receptors have identified motifs in both the extracellular and transmembrane domains that mediate pentameric assembly. Those efforts have largely ignored the most diverse domain of these channels, the intracellular domain (ICD). Here we identify molecular determinants within the ICD of serotonin type 3A (5-HT_3A) subunits for pentameric assembly by first identifying the segments contributing to pentamerization using deletion constructs of, and finally by making defined amino acid substitutions within, an isolated soluble ICD. Our work provides direct experimental evidence for the contribution of three intracellular arginines, previously implicated in governing the low conductance of 5-HT_3ARs, in structural features such as pentameric assembly.

A Review on the Receptor-ligand Molecular Interactions in the Nicotinic Receptor Signaling Systems

Nicotine is regarded as the main active addictive ingredient in tobacco products driving continued tobacco abuse behavior (smoking) to the addiction behavior, whereas nicotinic acetylcholine receptors (nAChR) is the crucial effective apparatus or molecular effector of nicotine and acetylcholine and other similar ligands. Many nAChR subunits have been revealed to bind to either neurotransmitters or exogenous ligands, such as nicotine and acetylcholine, being involved in the nicotinic receptor signal transduction. Therefore, the nicotinic receptor signalling molecules and the receptor-ligand molecular interactions between nAChRs and their ligands are universally regarded as crucial mediators of cellular functions and drug targets in medical treatment and clinical diagnosis. Given numerous endeavours have been made in defining the roles of nAChRs in response to nicotine and other addictive drugs, this review focuses on studies and reports in recent years on the receptor-ligand interactions between nAChR receptors and ligands, including lipid-nAChR and protein-nAChR molecular interactions, relevant signal transduction pathways and their molecular mechanisms in the nicotinic receptor signalling systems. All the references were carefully retrieved from the PubMed database by searching key words "nicotine", "acetylcholine", "nicotinic acetylcholine receptor(s)", "nAChR*", "protein and nAChR", "lipid and nAChR", "smok*" and "tobacco". All the relevant referred papers and reports retrieved were fully reviewed for manual inspection. This effort intend to get a quick insight and understanding of the nicotinic receptor signalling and their molecular interactions mechanisms. Understanding the cellular receptor-ligand interactions and molecular mechanisms between nAChRs and ligands will lead to a better translational and therapeutic operations and outcomes for the prevention and treatment of nicotine addiction and other chronic drug addictions in the brain's reward circuitry.

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

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

Serine residues in the α4 nicotinic acetylcholine receptor subunit regulate surface α4β2* receptor expression and clustering

BACKGROUND AND PURPOSE

Chronic nicotine exposure upregulates α4β2^* nicotinic acetylcholine receptors (nAChRs) in the brain. The goal of this study was to examine the role of three serine residues in the large cytoplasmic loop of the α4 subunit on α4β2^* upregulation in neurons.

EXPERIMENTAL APPROACH

Serine residues S336, S470 and S530 in mouse α4 were mutated to alanine and then re-expressed in primary neurons from cortex, hippocampus and subcortex of α4 KO mice. Mutant and wild type α4 expressing neurons were treated with nicotine (0.1, 1 and 10 μM) and assessed for α4β2^* upregulation.

KEY RESULTS

α4β2^* nAChRs expressing S336A or S470A mutants were deficient at cell surface upregulation in both subcortex and hippocampal neurons. S530A α4β2^* mutants exhibited aberrant surface upregulation in subcortical neurons. None of the mutants affected surface upregulation in cortical neurons or upregulation of total α4β2^* binding sites in any region. Further, dense domains or clusters of α4β2^* nAChRs were observed in the neuronal surface. The impact of nicotine exposure on the intensity, area, and density of these clusters was dependent upon individual mutations.

CONCLUSIONS AND IMPLICATIONS

Effects of α4 nAChR mutants on surface upregulation varied among brain regions, suggesting that the cellular mechanism of α4β2^* upregulation is complex and involves cellular identity. We also report for the first time that α4β2^* nAChRs form clusters on the neuronal surface and that nicotine treatment alters the characteristics of the clusters in an α4 mutant-dependent manner. This finding adds a previously unknown layer of complexity to the effects of nicotine on α4β2^* expression and function.

Interaction of the α7-nicotinic subunit with its human-specific duplicated dupα7 isoform in mammalian cells: Relevance in human inflammatory responses

The α7 nicotinic receptor subunit and its partially duplicated human-specific dupα7 isoform are coexpressed in neuronal and non-neuronal cells. In these cells, α7 subunits form homopentameric α7 nicotinic acetylcholine receptors (α7-nAChRs) implicated in numerous pathologies. In immune cells, α7-nAChRs are essential for vagal control of inflammatory response in sepsis. Recent studies show that the dupα7 subunit is a dominant-negative regulator of α7-nAChR activity in Xenopus oocytes. However, its biological significance in mammalian cells, particularly immune cells, remains unexplored, as the duplicated form is indistinguishable from the original subunit in standard tests. Here, using immunocytochemistry, confocal microscopy, coimmunoprecipitation, FRET, flow cytometry, and ELISA, we addressed this challenge in GH4C1 rat pituitary cells and RAW264.7 murine macrophages transfected with epitope- and fluorescent protein-tagged α7 or dupα7. We used quantitative RT-PCR of dupα7 gene expression levels in peripheral blood mononuclear cells (PBMCs) from patients with sepsis to analyze its relationship with PBMC α7 mRNA levels and with serum concentrations of inflammatory markers. We found that a physical interaction between dupα7 and α7 subunits in both cell lines generates heteromeric nAChRs that remain mainly trapped in the endoplasmic reticulum. The dupα7 sequestration of α7 subunits reduced membrane expression of functional α7-nAChRs, attenuating their anti-inflammatory capacity in lipopolysaccharide-stimulated macrophages. Moreover, the PBMC's dupα7 levels correlated inversely with their α7 levels and directly with the magnitude of the patients' inflammatory state. These results indicate that dupα7 probably reduces human vagal anti-inflammatory responses and suggest its involvement in other α7-nAChR-mediated pathophysiological processes.

Emerging Molecular Mechanisms of Signal Transduction in Pentameric Ligand-Gated Ion Channels

Nicotinic acetylcholine, serotonin type 3, γ-amminobutyric acid type A, and glycine receptors are major players of human neuronal communication. They belong to the family of pentameric ligand-gated ion channels, sharing a highly conserved modular 3D structure. Recently, high-resolution structures of both open- and closed-pore conformations have been solved for a bacterial, an invertebrate, and a vertebrate receptor in this family. These data suggest that a common gating mechanism occurs, coupling neurotransmitter binding to pore opening, but they also pinpoint significant differences among subtypes. In this Review, we summarize the structural and functional data in light of these gating models and speculate about their mechanistic consequences on ion permeation, pathological mutations, as well as functional regulation by orthosteric and allosteric effectors.

A Transcriptomic Survey of Ion Channel-Based Conotoxins in the Chinese Tubular Cone Snail (Conus betulinus)

Conotoxins in the venom of cone snails (Conus spp.) are a mixture of active peptides that work as blockers, agonists, antagonists, or inactivators of various ion channels. Recently we reported a high-throughput method to identify 215 conotoxin transcripts from the Chinese tubular cone snail, C. betulinus. Here, based on the previous datasets of four transcriptomes from three venom ducts and one venom bulb, we explored ion channel-based conotoxins and predicted their related ion channel receptors. Homologous analysis was also performed for the most abundant ion channel protein, voltage-gated potassium (Kv; with Kv1.1 as the representative), and the most studied ion channel receptor, nicotinic acetylcholine receptor (nAChR; with α2-nAChR as the representative), in different animals. Our transcriptomic survey demonstrated that ion channel-based conotoxins and related ion channel proteins/receptors transcribe differentially between the venom duct and the venom bulb. In addition, we observed that putative κ-conotoxins were the most common conotoxins with the highest transcription levels in the examined C. betulinus. Furthermore, Kv1.1 and α2-nAChR were conserved in their functional domains of deduced protein sequences, suggesting similar effects of conotoxins via the ion channels in various species, including human beings. In a word, our present work suggests a high-throughput way to develop conotoxins as potential drugs for treatment of ion channel-associated human diseases.

Understanding structure-function relationships of the human neuronal acetylcholine receptor: insights from the first crystal structures of neuronal subunits

Nicotinic ACh receptors (nAChRs) are the best studied members of the superfamily of pentameric ligand-gated ion channels (pLGICs). Neuronal nAChRs regulate neuronal excitability and neurotransmitter release in the nervous system and form either homo- or hetero-pentameric complexes with various combinations of the 11 neuronal nAChR subunits (α2-7, α9, α10 and β2-4) known to exist in humans. In addition to their wide distribution in the nervous system, neuronal nAChRs have been also found in immune cells and many peripheral tissues. These nAChRs are important drug targets for neurological and neuropsychiatric diseases (e.g. Alzheimer's, schizophrenia) and substance addiction (e.g. nicotine), as well as in a variety of diseases such as chronic pain, auditory disorders and some cancers. To decipher the functional mechanisms of human nAChRs and develop efficient and specific therapeutic drugs, elucidation of their high-resolution structures is needed. Recent studies, including the X-ray crystal structures of the near-intact α4β2 nAChR and of the ligand-binding domains of the α9 and α2 subunits, have advanced our knowledge on the detailed structure of the ligand-binding sites formed between the same and different subunits and revealed many other functionally important interactions. The aim of this review is to highlight some of the structural and functional findings of these studies and to compare them with recent breakthrough findings on other pLGIC members and earlier data from their homologous ACh-binding proteins.

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.

Distinctive effects of nicotinic receptor intracellular-loop mutations associated with nocturnal frontal lobe epilepsy

Previously characterized nicotinic acetylcholine receptor (nAChR) autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE)-associated mutations are found in α2, α4 and β2 subunit transmembrane (TM) domains. They predominantly increase ACh potency and, for β2-subunit mutants, increase macroscopic currents. Two recently-identified mutations, α4(R336H) and β2(V337G), located in the intracellular cytoplasmic loop (C2) have been associated with non-familial NFLE. Effects of these mutations on α4β2-nAChR function and expression were studied for the first time, using two-electrode voltage clamp recordings in Xenopus laevis oocytes. Biased-ratio preparations elucidated the mutations' effects at alternate isoforms: high-sensitivity [HS; (α4)2(β2)3] or low-sensitivity [LS; (α4)3(β2)2] via 1:10 or 30:1 [α4:β2] cRNA injection ratios, respectively. An unbiased (1:1 [α4:β2] cRNA) injection ratio was also used to study potential shifts in isoform expression. α4(R336H)-containing receptors showed significant increases in maximal ACh-induced currents (Imax) in all preparations (140% increase compared to wild type control). β2(V337G)-containing receptors significantly increased Imax in the LS-favoring preparation (20% increase compared to control). Expression of either mutation consistently produced enrichment of HS-isoform expression in all preparations. α4β2-nAChR harboring either NFLE mutant subunit showed unchanged ACh, sazetidine-A, nicotine, cytisine and mecamylamine potency. However, both mutant subunits enhanced partial agonist efficacies in the LS-biased preparation. Using β2-subunit-specific [(125)I]mAb 295 immunolabeling, nAChR cell-surface expression was determined. Antibody binding studies revealed that the β2(V337G) mutation tended to reduce cell-surface expression, and function per receptor was significantly increased by either NFLE mutant subunit in HS-favoring preparations. These findings identify both common and differing features between TM- and C2-domain AD/NFLE-associated mutations. As we discuss, the shared features may be particularly salient to AD/NFLE etiology.

In vivo functional analysis of the Drosophila melanogaster nicotinic acetylcholine receptor Dα6 using the insecticide spinosad

The vinegar fly, Drosophila melanogaster, has been used to identify and manipulate insecticide resistance genes. The advancement of genome engineering technology and the increasing availability of pest genome sequences has increased the predictive and diagnostic capacity of the Drosophila model. The Drosophila model can be extended to investigate the basic biology of the interaction between insecticides and the proteins they target. Recently we have developed an in vivo system that permits the expression and study of key insecticide targets, the nicotinic acetylcholine receptors (nAChRs), in controlled genetic backgrounds. Here this system is used to study the interaction between the insecticide spinosad and a nAChR subunit, Dα6. Reciprocal chimeric subunits were created from Dα6 and Dα7, a subunit that does not respond to spinosad. Using the in vivo system, the Dα6/Dα7 chimeric subunits were tested for their capacity to respond to spinosad. Only the subunits containing the C-terminal region of Dα6 were able to respond to spinosad, thus confirming the importance this region for spinosad binding. A new incompletely dominant, spinosad resistance mechanism that may evolve in pest species is also examined. First generated using chemical mutagenesis, the Dα6(P146S) mutation was recreated using the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 system, the first use of this technology to introduce a resistant mutation into a controlled genetic background. Both alleles present with the same incompletely dominant, spinosad resistance phenotype, proving the P146S replacement to be the causal mutation. The proximity of the P146S mutation to the conserved Cys-loop indicates that it may impair the gating of the receptor. The results of this study enhance the understanding of nAChR structure:function relationships.

Neuronal Nicotinic Acetylcholine Receptor Structure and Function and Response to Nicotine

Nicotinic acetylcholine receptors (nAChRs) belong to the "Cys-loop" superfamily of ligand-gated ion channels that includes GABAA, glycine, and serotonin (5-HT3) receptors. There are 16 homologous mammalian nAChR subunits encoded by a multigene family. These subunits combine to form many different nAChR subtypes with various expression patterns, diverse functional properties, and differing pharmacological characteristics. Because cholinergic innervation is pervasive and nAChR expression is extremely broad, practically every area of the brain is impinged upon by nicotinic mechanisms. This review briefly examines the structural and functional properties of the receptor/channel complex itself. The review also summarizes activation and desensitization of nAChRs by the low nicotine concentrations obtained from tobacco. Knowledge of the three-dimensional structure and the structural characteristics of channel gating has reached an advanced stage. Likewise, the basic functional properties of the channel also are reasonably well understood. It is these receptor/channel properties that underlie the participation of nAChRs in nearly every anatomical region of the mammalian brain.

Heterologous expression and nonsense suppression provide insights into agonist behavior at α6β2 nicotinic acetylcholine receptors

The α6-containing subtypes of the nicotinic acetylcholine receptor (nAChR) are localized to presynaptic terminals of the dopaminergic pathways of the central nervous system. Selective ligands for these nAChRs are potentially useful in both Parkinson's disease and addiction. For these and other goals, it is important to distinguish the binding behavior of agonists at the α6-β2 binding site versus other subtypes. To study this problem, we apply nonsense suppression-based non-canonical amino acid mutagenesis. We report a combination of four mutations in α6β2 that yield high-level heterologous expression in Xenopus oocytes. By varying mRNA injection ratios, two populations were observed with unique characteristics, likely due to differing stoichiometries. Responses to nine known nAChR agonists were analyzed at the receptor, and their corresponding EC50 values and efficacies are reported. The system is compatible with nonsense suppression, allowing structure-function studies between Trp149 - a conserved residue on loop B found to make a cation-π interaction at several nAChR subtypes - and several agonists. These studies reveal that acetylcholine forms a strong cation-π interaction with the conserved tryptophan, while nicotine and TC299423 do not, suggesting a unique pharmacology for the α6β2 nAChR.

Functional evaluation of key interactions evident in the structure of the eukaryotic Cys-loop receptor GluCl

The publication of the first high-resolution crystal structure of a eukaryotic Cys-loop receptor, GluClα, has provided valuable structural information on this important class of ligand-gated ion channels (LGIC). However, limited functional data exist for the GluCl receptors. Before applying the structural insights from GluCl to mammalian Cys-loop receptors such as nicotinic acetylcholine and GABA receptors, it is important to ensure that established functional features of mammalian Cys-loop receptors are present in the more distantly related GluCl receptors. Here, we seek to identify ligand-binding interactions that are generally associated with Cys-loop receptors, including the frequently observed cation−π interaction. Our studies were performed on the highly homologous GluClβ receptor, because GluClα is not activated by glutamate in Xenopus laevis oocytes. Mutagenesis of the signal peptide and pore lining helix was performed to enhance functional expression and sensitivity to applied ligand, respectively. Conventional and unnatural amino acid mutagenesis indicate a strong cation−π interaction between Y206 and the protonated amine of glutamate, as well as other important ionic and hydrogen bond interactions between the ligand and the binding site, consistent with the crystal structure.

Expression of cloned α6* nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (AChRs) are ACh-gated ion channels formed from five homologous subunits in subtypes defined by their subunit composition and stoichiometry. Some subtypes readily produce functional AChRs in Xenopus oocytes and transfected cell lines. α6β2β3* AChRs (subtypes formed from these subunits and perhaps others) are not easily expressed. This may be because the types of neurons in which they are expressed (typically dopaminergic neurons) have unique chaperones for assembling α6β2β3* AChRs, especially in the presence of the other AChR subtypes. Because these relatively minor brain AChR subtypes are of major importance in addiction to nicotine, it is important for drug development as well as investigation of their functional properties to be able to efficiently express human α6β2β3* AChRs. We review the issues and progress in expressing α6* AChRs. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Molecular cloning and characterization of a glycine-like receptor gene from the cattle tick Rhipicephalus (Boophilus) microplus (Acari: Ixodidae)

The cattle tick Rhipicephalus (Boophilus) microplus is the most economically important ectoparasite affecting the cattle industry in tropical and subtropical areas around the world. The principal method of tick control has relied mainly on the use of chemical acaricides, including ivermectin; however, cattle tick populations resistant to ivermectin have recently been reported in Brazil, Mexico, and Uruguay. Currently, the molecular basis for ivermectin susceptibility and resistance are not well understood in R. microplus. This prompted us to search for potential molecular targets for ivermectin. Here, we report the cloning and molecular characterization of a R. microplus glycine-like receptor (RmGlyR) gene. The characterized mRNA encodes for a 464-amino acid polypeptide, which contains features common to ligand-gated ion channels, such as a large N-terminal extracellular domain, four transmembrane domains, a large intracellular loop and a short C-terminal extracellular domain. The deduced amino acid sequence showed around 30% identity to GlyRs from some invertebrate and vertebrate organisms. The polypeptide also contains the PAR motif, which is important for forming anion channels, and a conserved glycine residue at the third transmembrane domain, which is essential for high ivermectin sensitivity. PCR analyses showed that RmGlyR is expressed at egg, larval and adult developmental stages. Our findings suggest that the deduced receptor is an additional molecular target to ivermectin and it might be involved in ivermectin resistance in R. microplus.

Efficient expression of functional (α6β2)2β3 AChRs in Xenopus oocytes from free subunits using slightly modified α6 subunits

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.

The duplicated α7 subunits assemble and form functional nicotinic receptors with the full-length α7

The α7 nicotinic acetylcholine receptor gene (CHRNA7) is linked to schizophrenia. A partial duplication of CHRNA7 (CHRFAM7A) is found in humans on 15q13-14. Exon 6 of CHRFAM7A harbors a 2-bp deletion polymorphism, CHRFAM7AΔ2bp, which is also associated with schizophrenia. To understand the effects of the duplicated subunits on α7 receptors, we fused α7, dupα7, and dupΔα7 subunits with various fluorescent proteins. The duplicated subunits co-localized with full-length α7 subunits in mouse neuroblastoma cells (Neuro2a) as well as rat hippocampal neurons. We investigated the interaction between the duplicated subunits and full-length α7 by measuring Förster resonance energy transfer using donor recovery after photobleaching and fluorescence lifetime imaging microscopy. The results revealed that the duplicated proteins co-assemble with α7. In electrophysiological studies, Leu at the 9'-position in the M2 membrane-spanning segment was replaced with Cys in dupα7 or dupΔα7, and constructs were co-transfected with full-length α7 in Neuro2a cells. Exposure to ethylammonium methanethiosulfonate inhibited acetylcholine-induced currents, showing that the assembled functional nicotinic acetylcholine receptors (nAChRs) included the duplicated subunit. Incorporation of dupα7 and dupΔα7 subunits modestly changes the sensitivity of receptors to choline and varenicline. Thus, the duplicated proteins are assembled and transported to the cell membrane together with full-length α7 subunits and alter the function of the nAChRs. The characterization of dupα7 and dupΔα7 as well as their influence on α7 nAChRs may help explain the pathophysiology of schizophrenia and may suggest therapeutic strategies.

Are nicotinic acetylcholine receptors coupled to G proteins?

It was, until recently, accepted that the two classes of acetylcholine (ACh) receptors are distinct in an important sense: muscarinic ACh receptors signal via heterotrimeric GTP binding proteins (G proteins), whereas nicotinic ACh receptors (nAChRs) open to allow flux of Na+, Ca2+, and K+ ions into the cell after activation. Here we present evidence of direct coupling between G proteins and nAChRs in neurons. Based on proteomic, biophysical, and functional evidence, we hypothesize that binding to G proteins modulates the activity and signaling of nAChRs in cells. It is important to note that while this hypothesis is new for the nAChR, it is consistent with known interactions between G proteins and structurally related ligand-gated ion channels. Therefore, it underscores an evolutionarily conserved metabotropic mechanism of G protein signaling via nAChR channels.

Merging old and new perspectives on nicotinic acetylcholine receptors

This review covers history underlying the discovery of the molecular mediators of nicotine's effects in the brain and the diversity of the nicotinic acetylcholine receptor (nAChR) subtypes. Models are presented for both their structure and their function as mediators of signal transduction, with special consideration of the differences between the two main subtypes: heteromeric receptors, which are specialized for rapid electrochemical signal transduction, and homomeric α7 receptors, which have come to be implicated in both ionotropic and metabotropic signaling. This review presents perspectives on the pharmacology and therapeutic targeting of nAChRs for the treatment of nicotine dependence or disease.

Atomistic insights into human Cys-loop receptors by solution NMR

Cys-loop receptors are pentameric ligand-gated ion channels (pLGICs) mediating fast neurotransmission in the central and peripheral nervous systems. They are important targets for many currently used clinical drugs, such as general anesthetics, and for allosteric modulators with potential therapeutic applications. Here, we provide an overview of advances in the use of solution NMR in structural and dynamic characterization of ion channels, particularly human Cys-loop receptors. We present challenges to overcome and realistic solutions for achieving high-resolution structural information for this family of receptors. We discuss how subtle structural differences among homologous channels define unique channel pharmacological properties and advocate the necessity to determine high-resolution structures for individual receptor subtypes. Finally, we describe drug binding to the TMDs of Cys-loop receptors identified by solution NMR and the associated dynamics changes relevant to channel functions.

Pharmacological chaperoning of nAChRs: a therapeutic target for Parkinson's disease

Chronic exposure to nicotine results in an upregulation of neuronal nicotinic acetylcholine receptors (nAChRs) at the cellular plasma membrane. nAChR upregulation occurs via nicotine-mediated pharmacological receptor chaperoning and is thought to contribute to the addictive properties of tobacco as well as relapse following smoking cessation. At the subcellular level, pharmacological chaperoning by nicotine and nicotinic ligands causes profound changes in the structure and function of the endoplasmic reticulum (ER), ER exit sites, the Golgi apparatus and secretory vesicles of cells. Chaperoning-induced changes in cell physiology exert an overall inhibitory effect on the ER stress/unfolded protein response. Cell autonomous factors such as the repertoire of nAChR subtypes expressed by neurons and the pharmacological properties of nicotinic ligands (full or partial agonist versus competitive antagonist) govern the efficiency of receptor chaperoning and upregulation. Together, these findings are beginning to pave the way for developing pharmacological chaperones to treat Parkinson's disease and nicotine addiction.

Rare human nicotinic acetylcholine receptor α4 subunit (CHRNA4) variants affect expression and function of high-affinity nicotinic acetylcholine receptors

Nicotine, the primary psychoactive component in tobacco smoke, produces its behavioral effects through interactions with neuronal nicotinic acetylcholine receptors (nAChRs). α4β2 nAChRs are the most abundant in mammalian brain, and converging evidence shows that this subtype mediates the rewarding and reinforcing effects of nicotine. A number of rare variants in the CHRNA4 gene that encode the α4 nAChR subunit have been identified in human subjects and appear to be underrepresented in a cohort of smokers. We compared three of these variants (α4R336C, α4P451L, and α4R487Q) to the common variant to determine their effects on α4β2 nAChR pharmacology. We examined [(3)H]epibatidine binding, interacting proteins, and phosphorylation of the α4 nAChR subunit with liquid chromatography and tandem mass spectrometry (LC-MS/MS) in HEK 293 cells and voltage-clamp electrophysiology in Xenopus laevis oocytes. We observed significant effects of the α4 variants on nAChR expression, subcellular distribution, and sensitivity to nicotine-induced receptor upregulation. Proteomic analysis of immunopurified α4β2 nAChRs incorporating the rare variants identified considerable differences in the intracellular interactomes due to these single amino acid substitutions. Electrophysiological characterization in X. laevis oocytes revealed alterations in the functional parameters of activation by nAChR agonists conferred by these α4 rare variants, as well as shifts in receptor function after incubation with nicotine. Taken together, these experiments suggest that genetic variation at CHRNA4 alters the assembly and expression of human α4β2 nAChRs, resulting in receptors that are more sensitive to nicotine exposure than those assembled with the common α4 variant. The changes in nAChR pharmacology could contribute to differences in responses to smoked nicotine in individuals harboring these rare variants.

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.

Mutagenic analysis of the intracellular portals of the human 5-HT3A receptor

Structural models of Cys-loop receptors based on homology with the Torpedo marmorata nicotinic acetylcholine receptor infer the existence of cytoplasmic portals within the conduction pathway framed by helical amphipathic regions (termed membrane-associated (MA) helices) of adjacent intracellular M3-M4 loops. Consistent with these models, two arginine residues (Arg(436) and Arg(440)) within the MA helix of 5-hydroxytryptamine type 3A (5-HT3A) receptors act singularly as rate-limiting determinants of single-channel conductance (γ). However, there is little conservation in primary amino acid sequences across the cytoplasmic loops of Cys-loop receptors, limiting confidence in the fidelity of this particular aspect of the 5-HT3A receptor model. We probed the majority of residues within the MA helix of the human 5-HT3A subunit using alanine- and arginine-scanning mutagenesis and the substituted cysteine accessibility method to determine their relative influences upon γ. Numerous residues, prominently those at the 435, 436, 439, and 440 positions, were found to markedly influence γ. This approach yielded a functional map of the 5-HT3A receptor portals, which agrees well with the homology model.

The structural mechanism of the Cys-loop receptor desensitization

The cys-loop receptors are neurotransmitter-operated ion channels, which mediate fast synaptic transmission for communication between neurons. However, prolonged exposure to the neurotransmitter drives the receptor to a desensitization state, which plays an important role in shaping synaptic transmission. Much progress has been made through more than half a century's research since Katz and Thesleff first descried desensitization for muscle nicotinic acetylcholine receptor. In this review, we summarized recent research developments of receptor desensitization. Now, it has been identified that many parts of the receptor, such as the pore domain (including the hinge in the M2-M3 linker), the binding domain, the coupling region, and the intracellular domain, are all involved in the cys-loop receptor desensitization and that uncoupling between the amino-terminal domain and channel lining domain seems to play a central role in desensitization. This uncoupling is mainly governed by the balance between coupling strength and relative tightness of gating machinery and influenced by other parts of the receptor. Agonist binding induces conformational change to overcome the gating barrier to open the channel through the stressed coupling region, which is subsequently broken, causing receptor desensitization. With rapid advancement in structural biology of membrane receptors, final validation of this mechanism is expected to occur in the near future when the high-resolution structure of the desensitized state is available.

Length and amino acid sequence of peptides substituted for the 5-HT3A receptor M3M4 loop may affect channel expression and desensitization

5-HT3A receptors are pentameric neurotransmitter-gated ion channels in the Cys-loop receptor family. Each subunit contains an extracellular domain, four transmembrane segments (M1, M2, M3, M4) and a 115 residue intracellular loop between M3 and M4. In contrast, the M3M4 loop in prokaryotic homologues is <15 residues. To investigate the limits of M3M4 loop length and composition on channel function we replaced the 5-HT3A M3M4 loop with two to seven alanine residues (5-HT3A-A_{n = 2–7}). Mutants were expressed in Xenopus laevis oocytes and characterized using two electrode voltage clamp recording. All mutants were functional. The 5-HT EC₅₀'s were at most 5-fold greater than wild-type (WT). The desensitization rate differed significantly among the mutants. Desensitization rates for 5-HT3A-A₂, 5-HT3A-A₄, 5-HT3A-A₆, and 5-HT3A-A₇ were similar to WT. In contrast, 5-HT3A-A₃ and 5-HT3A-A₅ had desensitization rates at least an order of magnitude faster than WT. The one Ala loop construct, 5-HT3A-A₁, entered a non-functional state from which it did not recover after the first 5-HT application. These results suggest that the large M3M4 loop of eukaryotic Cys-loop channels is not required for receptor assembly or function. However, loop length and amino acid composition can effect channel expression and desensitization. We infer that the cytoplasmic ends of the M3 and M4 segments may undergo conformational changes during channel gating and desensitization and/or the loop may influence the position and mobility of these segments as they undergo gating-induced conformational changes. Altering structure or conformational mobility of the cytoplasmic ends of M3 and M4 may be the basis by which phosphorylation or protein binding to the cytoplasmic loop alters channel function.

5-HT3 receptor ion size selectivity is a property of the transmembrane channel, not the cytoplasmic vestibule portals

5-HT3A receptors select among permeant ions based on size and charge. The membrane-associated (MA) helix lines the portals into the channel’s cytoplasmic vestibule in the 4-Å resolution structure of the homologous acetylcholine receptor. 5-HT3A MA helix residues are important determinants of single-channel conductance. It is unknown whether the portals into the cytoplasmic vestibule also determine the size selectivity of permeant ions. We sought to determine whether the portals form the size selectivity filter. Recently, we showed that channels functioned when the entire 5-HT3A M3–M4 loop was replaced by the heptapeptide M3–M4 loop sequence from GLIC, a bacterial Cys-loop neurotransmitter gated ion channel homologue from Gloebacter violaceus. We used homomeric 5-HT3A receptors with either a wild-type (WT) M3–M4 loop or the chimeric heptapeptide (5-HT3A–glvM3M4) loop, i.e., with or without portals. In Na⁺-containing buffer, the WT receptor current–voltage relationship was inwardly rectifying. In contrast, the 5-HT3A–glvM3M4 construct had a negative slope conductance region at voltages less than −80 mV. Glutamine substitution for the heptapeptide M3–M4 loop arginine eliminated the negative slope conductance region. We measured the relative permeabilities and conductances of a series of inorganic and organic cations ranging from 0.9 to 4.5 Å in radius (Li⁺, Na⁺, ammonium, methylammonium, ethanolammonium, 2-methylethanolammonium, dimethylammonium, diethanolammonium, tetramethylammonium, choline, tris [hydroxymethyl] aminomethane, and N-methyl-d-glucamine). Both constructs had measurable conductances with Li⁺, ammonium, and methylammonium (size range of 0.9–1.8-Å radius). Many of the organic cations >2.4 Å acted as competitive antagonists complicating measurement of conductance ratios. Analysis of the permeability ratios by excluded volume theory indicates that the minimal pore radius for 5-HT3A and 5-HT3–glvM3M4 receptors was similar, ∼5 Å. We infer that the 5-HT3A size selectivity filter is located in the transmembrane channel and not in the portals into the cytoplasmic vestibule. Thus, the determinants of size selectivity and conductance are located in physically distinct regions of the channel protein.

Chaperoning α7 neuronal nicotinic acetylcholine receptors

The α7 subtype of nicotinic acetylcholine receptors (AChRs) is one of the most abundant members of the Cys-loop family of receptors present in the central nervous system. It participates in various physiological processes and has received much attention as a potential therapeutic target for a variety of pathologies. The importance of understanding the mechanisms controlling AChR assembly and cell-surface delivery lies in the fact that these two processes are key to determining the functional pool of receptors actively engaged in synaptic transmission. Here we review recent studies showing that RIC-3, a protein originally identified in the worm Caenorhabditis elegans, modulates the expression of α7 AChRs in a subtype-specific manner. Potentiation of AChR expression by post-transcriptional events is also critically assessed.

Nicotine up-regulates alpha4beta2 nicotinic receptors and ER exit sites via stoichiometry-dependent chaperoning

The up-regulation of α4β2* nicotinic acetylcholine receptors (nAChRs) by chronic nicotine is a cell-delimited process and may be necessary and sufficient for the initial events of nicotine dependence. Clinical literature documents an inverse relationship between a person’s history of tobacco use and his or her susceptibility to Parkinson’s disease; this may also result from up-regulation. This study visualizes and quantifies the subcellular mechanisms involved in nicotine-induced nAChR up-regulation by using transfected fluorescent protein (FP)-tagged α4 nAChR subunits and an FP-tagged Sec24D endoplasmic reticulum (ER) exit site marker. Total internal reflection fluorescence microscopy shows that nicotine (0.1 µM for 48 h) up-regulates α4β2 nAChRs at the plasma membrane (PM), despite increasing the fraction of α4β2 nAChRs that remain in near-PM ER. Pixel-resolved normalized Förster resonance energy transfer microscopy between α4-FP subunits shows that nicotine stabilizes the (α4)₂(β2)₃ stoichiometry before the nAChRs reach the trans-Golgi apparatus. Nicotine also induces the formation of additional ER exit sites (ERES). To aid in the mechanistic analysis of these phenomena, we generated a β2_{enhanced-ER-export} mutant subunit that mimics two regions of the β4 subunit sequence: the presence of an ER export motif and the absence of an ER retention/retrieval motif. The α4β2_{enhanced-ER-export} nAChR resembles nicotine-exposed nAChRs with regard to stoichiometry, intracellular mobility, ERES enhancement, and PM localization. Nicotine produces only small additional PM up-regulation of α4β2_{enhanced-ER-export} receptors. The experimental data are simulated with a model incorporating two mechanisms: (1) nicotine acts as a stabilizing pharmacological chaperone for nascent α4β2 nAChRs in the ER, eventually increasing PM receptors despite a bottleneck(s) in ER export; and (2) removal of the bottleneck (e.g., by expression of the β2_{enhanced-ER-export} subunit) is sufficient to increase PM nAChR numbers, even without nicotine. The data also suggest that pharmacological chaperoning of nAChRs by nicotine can alter the physiology of ER processes.

Mutant human β4 subunit identified in amyotrophic lateral sclerosis patients impairs nicotinic receptor function

Recently identified mutations in the genes encoding the neuronal nicotinic ACh receptor (nAChR) subunits in patients affected by sporadic amyotrophic lateral sclerosis (sALS) may represent a factor which enhances disease susceptibility, in particular in association with ambient causes such as cigarette smoking. In this work, we characterize the functional properties of nAChRs containing the β4R349C subunit, the mutation most frequently encountered in sALS patients. The mutation was coexpressed with wild-type α3 or α4 subunits or with mutant α4R487Q subunit, which has been detected in one patient together with β4R349C mutation. None of the functional parameters examined showed differences between α4β4 and α4R487Qβ4 nAChRs. By contrast, β4R349C mutation, independent of the companion α subunit, caused the reduction in potency of both ACh and nicotine, decreased the density of whole-cell current evoked by maximal transmitter concentrations, and altered the kinetics of ACh-evoked whole-cell currents. These data confirm that sALS-associated mutations in nicotinic subunits may markedly perturb cholinergic transmission in individuals bearing the mutations.

Transcripts of the nicotinic acetylcholine receptor subunit gene Pxylα6 with premature stop codons are associated with spinosad resistance in diamondback moth, Plutella xylostella

The cDNA sequence of the α6 nicotinic acetylcholine receptor subunit of diamondback moth (Plutella xylostella) was cloned and sequenced. Transcripts were similar between the spinosad-susceptible G88 and Wapio strains. All transcripts from the spinosad-resistant Pearl-Sel strain contained premature stop codons, and most transcripts have not been previously reported. None of these truncated transcripts were seen in the spinosad-susceptible strains. Proteins made from these transcripts would likely have no, or greatly altered, receptor function. An F(2) backcross and spinosad bioassay showed that all spinosad bioassay survivors produced truncated α6 transcripts. Thus, it appears that spinosad resistance in diamondback moth is due to a mutation(s) that results in no functional Pxylα6 being produced.

Identification of a domain which affects kinetics and antagonistic potency of clozapine at 5-HT3 receptors

The widely used atypical antipsychotic clozapine is a potent competitive antagonist at 5-HT₃ receptors which may contribute to its unique psychopharmacological profile. Clozapine binds to 5-HT₃ receptors of various species. However, the structural requirements of the respective binding site for clozapine remain to be determined. Differences in the primary sequences within the 5-HT_3A receptor gene in schizophrenic patients may result in an alteration of the antipsychotic potency and/or the side effect profile of clozapine. To determine these structural requirements we constructed chimeras with different 5-HT_3A receptor sequences of murine and human origin and expressed these mutants in human embryonic kidney (HEK) 293 cells. Clozapine antagonises recombinant mouse 5-HT_3A receptors with higher potency compared to recombinant human 5-HT_3A receptors. 5-HT activation curves and clozapine inhibition curves yielded the parameters EC₅₀ and IC₅₀ for all receptors tested in the range of 0.6–2.7 µM and 1.5–83.3 nM, respectively. The use of the Cheng-Prusoff equation to calculate the dissociation constant K_b values for clozapine revealed that an extracellular sequence (length 86 aa) close to the transmembrane domain M1 strongly determines the binding affinity of clozapine. K_b values of clozapine were significantly lower (0.3–1.1 nM) for receptors containing the murine sequence and higher when compared with receptors containing the respective human sequence (5.8–13.4 nM). Thus, individual differences in the primary sequence of 5-HT₃ receptors may be crucial for the antipsychotic potency and/or the side effect profile of clozapine.

Rare missense variants of neuronal nicotinic acetylcholine receptor altering receptor function are associated with sporadic amyotrophic lateral sclerosis

Sporadic amyotrophic lateral sclerosis (SALS) is a motor neuron degenerative disease of unknown etiology. Current thinking on SALS is that multiple genetic and environmental factors contribute to disease liability. Since neuronal acetylcholine receptors (nAChRs) are part of the glutamatergic pathway, we searched for sequence variants in CHRNA3, CHRNA4 and CHRNB4 genes, encoding neuronal nicotinic AChR subunits, in 245 SALS patients and in 450 controls. We characterized missense variants by in vitro mutagenesis, cell transfection and electrophysiology. Sequencing the regions encoding the intracellular loop of AChRs subunits disclosed 15 missense variants (6.1%) in 14 patients compared with only six variants (1.3%) in controls (P = 0.001; OR 4.48, 95% CI 1.7-11.8). The frequency of variants in exons encoding extracellular and transmembrane domains and in intronic regions did not differ. NAChRs formed by mutant alpha3 and alpha4 and wild-type (WT) beta4 subunits exhibited altered affinity for nicotine (Nic), reduced use-dependent rundown of Nic-activated currents (I(Nic)) and reduced desensitization leading to sustained intracellular Ca(2+) concentration, in comparison with WT-nAChR. The cellular loop has a crucial importance for receptor trafficking and regulating ion channel properties. Missense variants in this domain are significantly over-represented in SALS patients and alter functional properties of nAChR in vitro, resulting in increased Ca(2+) entry into the cells. We suggest that these gain-of-function variants might contribute to disease liability in a subset of SALS because Ca(2+) signals mediate nAChR's neuromodulatory effects, including regulation of glutamate release and control of cell survival.

Aplysia cys-loop glutamate-gated chloride channels reveal convergent evolution of ligand specificity

Among the members of the superfamily of cys-loop ligand-gated ion channels (LGICs) are receptors distinguished by the presence of two cys-loops in the ligand-binding domain, for example, the glycine receptor. Such receptors have thus far been cloned only from vertebrates and from ecdysozoa (arthropods and nematodes). We have now cloned and expressed two 2-cys-loop receptors from Aplysia californica, a lophotrocozoan, and have shown that they form homomeric glutamate receptors. We have also built up a database including the two receptors cloned here, previously cloned vertebrate and ecdysozoan 2-cys-loop receptors taken from GenBank, and the same type of receptors obtained by a search of recently cloned genomes, including two non-vertebrate chordates, an echinoderm, a crustacean, an annelid, and another mollusk. We subjected these receptors to phylogenetic analysis, alone and in combination with GABA-A receptors from the same phyla and from a recently cloned cnidarian. The phylogenetic analysis revealed the presence of two independent clades of glutamate receptors: one from lophotrocozoa and other from ecdysozoa, and suggests that the ancestors of the current 2-cys-loop receptor types diverged from the GABA-A receptors and from each other before the bilateria-cnidaria split. Finally, combining the results from the phylogenetic analysis with those obtained from an analysis of the 2-cys-loop receptors in light of recently published hypotheses concerning the glycine binding pocket, we predict that glycine receptors are not exclusively a vertebrate-receptor type.

A review of experimental techniques used for the heterologous expression of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop family of neurotransmitter-gated ion channels, a family that also includes receptors for gamma-aminobutyric acid, glycine and 5-hydroxytryptamine. In humans, nAChRs have been implicated in several neurological and psychiatric disorders and are major targets for pharmaceutical drug discovery. In addition, nAChRs are important targets for neuroactive pesticides in insects and in other invertebrates. Historically, nAChRs have been one of the most intensively studied families of neurotransmitter receptors. They were the first neurotransmitter receptors to be biochemically purified and the first to be characterized by molecular cloning and heterologous expression. Although much has been learnt from studies of native nAChRs, the expression of recombinant nAChRs has provided dramatic advances in the characterization of these important receptors. This review will provide a brief history of the characterization of nAChRs by heterologous expression. It will focus, in particular, upon studies of recombinant nAChRs, work that has been conducted by many hundreds of scientists during a period of almost 30 years since the molecular cloning of nAChR subunits in the early 1980s.