Diversity of vertebrate nicotinic acetylcholine receptors

Neonicotinoid insecticides in non-target organisms: Occurrence, exposure, toxicity, and human health risks

Pesticides have consistently portrayed a crucial role in the history of modern agricultural production. Neonicotinoid insecticides are classified as the fourth generation of pesticides, following organophosphorus, pyrethroids, and carbamates. Due to their broad-spectrum insecticidal activity, unique neurotoxic mode of action, and versatility of application methods, neonicotinoids have been widely used worldwide since their introduction. Recent studies have shown that neonicotinoids are frequently detected in a variety of food and environmental media around the world, posing considerable safety risks to human health and ecosystems, and therefore have become an emerging contaminant. However, the toxic effects and exposure risks of neonicotinoids to non-target organisms, including humans, have not received sufficient attention. Therefore, based on previous studies, this critical review concisely evaluates the occurrence and exposure levels of neonicotinoids in the environment and the associated risks to human health. The toxic effects of neonicotinoids on non-target organisms are systematically reviewed, including the aspects of acute toxicity, reproductive development, nervous system, immune function, genetics, and others. The potential toxic mechanism of these pesticides is discussed. The existing knowledge gaps are identified, and future prospects for neonicotinoids are proposed to provide scientific guidance for the safe and rational use of neonicotinoids and future research directions.

The Role of α7-Nicotinic Acetylcholine Receptors in the Pathophysiology and Treatment of Parkinson's Disease

The α7 nicotinic acetylcholine receptor (α7-nAChR) is a pivotal regulator of neurotransmission, neuroprotection, and immune modulation in the central nervous system. This review explores its structural and functional attributes, highlighting its therapeutic potential in neurodegenerative disorders, particularly Parkinson's disease (PD). α7-nAChRs mediate synaptic plasticity, modulate inflammatory responses, and influence dopamine release, positioning them as a promising pharmacological target. Positive allosteric modulators (PAMs) enhance α7-nAChR activity mainly by reducing desensitization, offering a superior therapeutic approach compared with direct agonists. Emerging preclinical studies suggest that α7-nAChR activation mitigates dopaminergic neurodegeneration, improves L-dopa-induced dyskinesia, and reduces neuroinflammation. Despite promising findings, clinical trials have yielded mixed results, necessitating further research into optimizing α7-targeted therapies. This review underscores the significance of α7-nAChRs in PD pathophysiology and highlights future directions for their translational potential in neuroprotection and symptomatic relief.

The stoichiometry of the α4β2 neuronal nicotinic acetylcholine receptors determines the pharmacological properties of the neonicotinoids, and recently introduced butenolide and sulfoximine

Although neonicotinoids were considered safe for mammals for many decades, recent research has proven that these insecticides can alter cholinergic functions by interacting with neuronal nicotinic acetylcholine (ACh) receptors (nAChRs). One such receptor is the heteromeric α4β2 nAChR, which exists under two different stoichiometries: high sensitivity and low sensitivity α4β2 nAChRs. To replace these insecticides, new classes of insecticides have been developed, such as, sulfoximine, sulfoxaflor, and the butenolide, flupyradifurone. In this study, we injected Xenopus laevis oocytes with 1:10 and 10:1 α4:β2 subunit RNA ratios, in order to express the high (α4)2(β2)3 and low sensitivity (α4)3(β2)2 nAChRs. Using the two-electrode voltage-clamp technique, we found that the low sensitivity (α4)3(β2)2 nAChRs were activated by all tested insecticides, whereas the high sensitivity (α4)2(β2)3 nAChR was only activated by ACh. Imidacloprid, sulfoxaflor and flupyradifurone confirmed their agonist effects by reducing the responses to the ACh EC80 concentrations, for both low (α4)3(β2)2 and high sensitivity (α4)2(β2)3 stoichiometries. Clothianidin only inhibited ACh responses of the low sensitivity (α4)3(β2)2 stoichiometry. Mutation E226P in the α4 subunit of the low sensitivity (α4)3(β2)2 receptors inhibits the agonist potency of imidacloprid and flupyradifurone, whereas mutation L273T (in the β2 subunit) in the high sensitivity (α4)2(β2)3 nAChR leads to activation by all insecticides. Major agonist effects were found with the double mutation of the E226P in the α4 subunit, and the L273T in the β2 subunit of the high sensitivity (α4)2(β2)3 stoichiometry.

Alcohol-induced liver injury is mediated via α4-containing nicotinic acetylcholine receptors expressed in hepatocytes

BACKGROUND

Our previous study demonstrated that alcohol induced the expression of the α4 subunit of nicotinic acetylcholine receptors (nAChRs) in the livers of wild type mice (WT), and that whole-body α4 nAChR knockout mice (α4KO) showed protection against alcohol-induced steatosis, inflammation, and injury. Based on these findings, we hypothesized that hepatocyte-specific α4 nAChRs may directly contribute to the detrimental effects of alcohol on the liver.

METHODS

Hepatocyte-specific α4 knockout mice (α4HepKO) were generated, and the absence of α4 nAChR was confirmed through PCR of genomic DNA. Female WT and α4HepKO mice were exposed to alcohol in the NIAAA chronic + binge model. After 10 days on the Lieber-DeCarli liquid diet containing 5% (vol/vol) alcohol or isocaloric maltose-dextrin, the mice were gavaged with a single dose of alcohol or isocaloric maltose-dextrin. The mice were euthanized 9 h later and their organs harvested. Additionally, hepatocytes were isolated from WT, α4HepKO, α4floxed, and α4KO mice and exposed to 80 mM alcohol in vitro for 24 h. Steatosis, inflammation, and cell injury were assessed in both liver and isolated hepatocytes.

RESULTS

In WT mice, alcohol exposure resulted in hepatic steatosis, inflammation, and injury as evidenced by increased liver triglycerides, neutrophil infiltration, and serum concentrations of liver enzymes. All of these responses were markedly lower in α4HepKO mice. mRNA expression of genes involved in lipogenesis (Srebf1, Fasn, and Dgat2) and inflammation (TNFα, Cxcl5, Cxcl1, and Serpine1) were increased in the livers of WT mice exposed to alcohol in vivo and in WT hepatocytes exposed to alcohol in vitro. These changes were not observed in liver or hepatocytes from mice lacking α4 nAChRs.

CONCLUSIONS

α4 nAChRs expressed in hepatocytes mediate alcohol-associated hepatoxicity. Therefore, the development of therapeutic strategies targeting hepatocyte α4-containing nAChRs could help reduce the burden of ALD.

Sulfonium Moieties as Ammonium Bioisosteres: Novel Ligands for the Alpha7 Nicotinic Acetylcholine Receptor

In the pressing quest of novel treatments for chronic pain, α7 nAChR silent agonists show efficacy as anti-inflammatory modulators and represent a promising strategy. Recent findings reveal that a sulfonium ion can replace the quaternary ammonium nitrogen as an alternative pharmacophore for nAChR silent activation. This study reports the design, synthesis, and electrophysiological evaluation of a new series of sulfonium-based derivatives inspired by the archetypal silent agonist NS6740. Our findings identify NSS-9 as a novel sulfonium α7 silent agonist that effectively alleviates inflammatory pain in a mouse model, highlighting it as a lead compound for further optimization. These results provide insights into the potential of the sulfonium group as a chemotype interacting with the α7 binding site, making it a valuable scaffold for novel α7 silent agonists. Additionally, sulfonium compounds were tested on α9 nAChR, also involved in the cholinergic anti-inflammatory system, identifying one partial agonist and two antagonists.

A Pharmacophore for Drugs Targeting the α4α4 Binding Site of the (α4)3(β2)2 Nicotinic Acetylcholine Receptor

Neuronal nicotinic acetylcholine receptors (nAChRs) have an established role in pain pathways and devastating neurodegenerative diseases; however, few drugs have been successfully developed to target them. The most abundant nAChR in the brain, the α4β2 nAChR, is assembled from five subunits in a 3α:2β stoichiometry-(α4)3(β2)2. This receptor contains a unique agonist-binding site at the α4α4 interface in addition to two classical agonist-binding sites at α4β2 interfaces. Most known agonists target both α4α4 and α4β2 sites, however, a few compounds with selectivity for the α4α4 site have been identified. These α4α4 selective compounds have a modulator-like effect akin to benzodiazepines in the γ-aminobutyric acid type A receptor, which is desirable from a drug development perspective. The two most well characterised α4α4 selective compounds are CMPI and NS9283. Both are structurally very different from classical agonists, and it is puzzling how they occupy the same binding site. In the search for a common pharmacophore, we conducted extensive molecular dynamics simulations with both classical agonists and site-selective non-classical compounds. Analyses of the simulations revealed that the α4α4 binding site contains a unique pocket not found in the α4β2 binding site. CMPI and NS9283 were observed to bind in this pocket, thereby explaining why they are selective for the α4α4 binding site. The proposed binding mode featured a closed-loop C conformation, which is strongly correlated with agonism in nAChRs and explained key site-directed mutagenesis data for both compounds. Based on this binding mode, we proposed a pharmacophore for drugs targeting the α4α4 binding site. The proposed pharmacophore captures the essence of the original model, that is, nicotinic agonists act as a bridge between protein subunits. The pharmacophore model we propose is unique to the α4α4 binding site and provides a template for developing new site-selective therapeutic agents.

Nicotinic acetylcholine receptors in the brain

The nicotinic acetylcholine receptor (nAChR) is the archetypal neurotransmitter receptor within the superfamily of pentameric ligand-gated ion channels (pLGICs). Typically, it mediates fast synaptic transmission in response to its endogenous ligand, acetylcholine, and can also intervene in slower signaling mechanisms via intracellular metabolic cascades in association with G-protein-coupled receptors. This review covers the structural and functional aspects of the different neuronal nAChR subtypes and their cellular and anatomic distribution in the brain. The significant progress in our knowledge on the topic derives from the successful combination of biochemical, neuroanatomic, pharmacologic, and cell biology approaches, complemented by site-directed mutagenesis, single-channel electrophysiology, and structural biophysical studies. This multipronged approach provides a comprehensive description of nAChR in health and disease, offering improved chances of success in tackling neurologic and neuropsychiatric diseases involving phenotypic alterations of nAChRs, particularly in neurodegenerative diseases.

S-(+)-mecamylamine increases the firing rate of serotonin neurons and diminishes depressive-like behaviors in an animal model of stress

Mecamylamine, a noncompetitive blocker of nicotinic acetylcholine receptors (nAChRs), is the racemic mixture of two stereoisomers: S-(+)-mecamylamine (S-mec) and R-(-)-mecamylamine (R-mec), with distinct interactions with α4β2 nAChRs. It has been shown that mecamylamine increases glutamate release and excites serotonergic (5-HT) neurons in the dorsal raphe nucleus (DRN). In this study, we separately evaluated the effects of S-mec and R-mec on 5-HT neuron excitability. S-mec (3 μM) increased firing frequency by 40 %, while R-mec (3 μM) raised it by only 22 %. S-mec acts as a positive allosteric modulator on high-sensitivity (HS) α4β2 nAChRs at glutamate terminals, enhancing spontaneous excitatory postsynaptic currents (sEPSCs) in 5-HT neurons. Conversely, R-mec decreased sEPSCs by blocking HS α4β2 nAChRs and reduced GABA-mediated inhibitory currents (sIPSCs) by blocking α7 nAChRs at GABAergic terminals. These mechanisms make S-mec more effective than R-mec in enhancing 5-HT neuron firing. Moreover, combining S-mec with TC-2559, a selective agonist of HS α4β2 nAChRs, increased firing frequency by 65 %, exceeding the effect of S-mec alone. To validate these findings, we evaluated the antidepressant effects of S-mec (1 mg/kg) combined with TC-2559 or RJR-2403, another α4β2 nAChR agonist. This combination successfully reduced depression-like behaviors, suggesting a potential treatment strategy for patients resistant to conventional antidepressants.

Pharmacology and molecular modeling studies of sulfoxaflor, flupyradifurone and neonicotinoids on the human neuronal α7 nicotinic acetylcholine receptor

We conducted electrophysiological and molecular docking studies using a heterologous expression system (Xenopus oocytes) to compare the effects of four neonicotinoids (acetamiprid, imidacloprid, clothianidin and thiamethoxam), one sulfoximine, (sulfoxaflor), and one butenolide (flupyradifurone), on human α7 neuronal nicotinic acetylcholine receptors (nAChRs). All neonicotinoids (except thiamethoxam), as well as the recently introduced nAChR competitive modulators, flupyradifurone and sulfoxaflor, appear to be weaker agonists than acetylcholine. Two mutations in loop C (E211N and E211P) and one mutation in loop D (Q79K), known to be involved in the binding properties of neonicotinoids were introduced to the α7 wild type. Interestingly, the acetylcholine and nicotine-evoked activation was not modified in human α7 mutated receptors, but the net charge was enhanced for clothianidin and imidacloprid, respectively. Flupyradifurone responses strongly increased under the Q79K mutation. The molecular docking investigations demonstrated that the orientations and interactions of the ligands considered were in accordance with those observed experimentally. Specifically, the charged fragments of acetylcholine and nicotine, used as reference ligands, and their neonicotinoid homologs were found to be surrounded by aromatic residues, with key interactions with Trp171 and Y210. Furthermore, the molecular docking investigations predicted the water-mediated interaction between the carbonyl oxygen of acetylcholine and the Nsp2 nitrogen of the pyridine ring for nicotine (as well as for the majority of the corresponding neonicotinoid fragments) and main chain NH of L141. The docking scores, extending over a significant range of 6 kcal/mol, showed that most neonicotinoids were poorly stabilized in the α7 nAChR compared to acetylcholine, except sulfoxaflor.

The α3β4 nAChR tissue distribution identified by fluorescent α-conotoxin [D11A]LvIA

α3β4, a vital subtype of neuronal nicotinic acetylcholine receptors (nAChRs), is widely distributed in the brain, ganglia, and adrenal glands, associated with addiction and neurological diseases. However, the lack of specific imaging tools for α3β4 nAChRs has hindered the investigation of their tissue distribution and functions. [D11A]LvIA, a peptide derived from marine cone snails, demonstrates high affinity and potency for α3β4 nAChRs, making it a valuable pharmacological tool for studying this receptor subtype. In this study, three fluorescent conjugates of [D11A]LvIA were synthesized using 6-TAMRA-SE (R), Cy3-NHS-ester (Cy3), and BODIPY-FL NHS ester (BDP) dyes. The electrophysiological activities were assessed in Xenopus laevis oocytes by two-electrodes voltage clamp (TEVC). [D11A]LvIA-Cy3 and [D11A]LvIA-BDP show improved selectivity and affinity, with IC50 values of 512.70 nM and 343.50 nM, respectively, and [D11A]LvIA-Cy3 exhibits better stability in cerebrospinal fluid (CSF). Utilizing [D11A]LvIA-Cy3, we successfully visualized the distribution of α3β4 nAChRs in rat trigeminal ganglia, retina, adrenal glands, and various brain regions. This novel fluorescent peptide provides a significant pharmacological tool for the exploration and visualization in-situ distribution of α3β4 nAChRs in different tissues and also assists in clarifying the function.

Nicotine-induced CHRNA5 activation modulates CES1 expression, impacting head and neck squamous cell carcinoma recurrence and metastasis via MEK/ERK pathway

The mucosal epithelium of the head and neck region (including the oral cavity, nasal cavity, pharynx, nasopharynx, and larynx) is the primary site exposed to tobacco smoke, and its presence of nicotinic acetylcholine receptors (nAChRs) has been observed in the mucosal epithelial cells of this area. It remains unclear whether HNSC cells can migrate and invade through nAChR signaling. A model of HNSC cells exposed to nicotine is established. Cell proliferation following nicotine exposure is assessed using the CCK-8 assay, while migration and invasion are evaluated through wound healing and Transwell assays. The effects of CHRNA5 knockdown and overexpression are also investigated. Immunofluorescence staining is used to analyze CHRNA5 expression and localization, and clonogenic assays are performed to measure colony proliferation after CHRNA5 knockdown and overexpression. The interaction between CHRNA5 and CES1 is examined using molecular docking, co-immunoprecipitation, and immunofluorescence. Differentially expressed genes are subjected to pathway enrichment analysis, and MEK/ERK protein expression and phosphorylation are validated via western blot. Tumor formation assays are performed in nude mice using sh-CHRNA5 Cal27 cells, followed by western blot and immunohistochemical staining. Additionally, laryngeal and hypopharyngeal cancer tissues are analyzed through immunohistochemistry. Nicotine significantly enhanced the proliferation, migration, and invasion capabilities of head and neck tumor cells, including Cal27, Fadu, HN6, and Tu686 cells, through the expression of CHRNA5. Knockdown of CHRNA5 can reduce cell migration, invasion, and proliferation, whereas nicotine exposure can reverse this trend. Additionally, the mRNA and protein expression of CES1 decreases with the knockdown of CHRNA5, indicating a regulatory relationship between the two. Transcriptomics revealed that the knockdown of CHRNA5 is associated with the MEK/ERK signaling pathway. Further cellular- and tissue-level evidence confirmed that the levels of p-MEK/MEK, p-ERK/ERK, and CES1 decreased following knockdown of CHRNA5, a trend that nicotine can reverse. Nicotine promotes the proliferation, migration, and invasion of HNSC by upregulating CHRNA5 expression. Knockdown of CHRNA5 reduces these effects, which can be reversed by nicotine. Nicotine exposure activates CHRNA5, regulating CES1 expression via the MEK/ERK pathway, contributing to the recurrence and metastasis of head and neck squamous carcinoma.

Protein networking: nicotinic acetylcholine receptors and their protein-protein-associations

Nicotinic acetylcholine receptors (nAChR) are complex transmembrane proteins involved in neurotransmission in the nervous system and at the neuromuscular junction. nAChR disorders may lead to severe, potentially fatal pathophysiological states. To date, the receptor has been the focus of basic and applied research to provide novel therapeutic interventions. Since most studies have investigated only the nAChR itself, it is necessary to consider the receptor as part of its protein network to understand or elucidate-specific pathways. On its way through the secretory pathway, the receptor interacts with several chaperones and proteins. This review takes a closer look at these molecular interactions and focuses especially on endoplasmic reticulum biogenesis, secretory pathway sorting, Golgi maturation, plasma membrane presentation, retrograde internalization, and recycling. Additional knowledge regarding the nAChR protein network may lead to a more detailed comprehension of the fundamental pathomechanisms of diseases or may lead to the discovery of novel therapeutic drug targets.

Explorations of Agonist Selectivity for the α9* nAChR with Novel Substituted Carbamoyl/Amido/Heteroaryl Dialkylpiperazinium Salts and Their Therapeutic Implications in Pain and Inflammation

There is an urgent need for nonopioid treatments for chronic and neuropathic pain to provide effective alternatives amid the escalating opioid crisis. This study introduces novel compounds targeting the α9 nicotinic acetylcholine receptor (nAChR) subunit, which is crucial for pain regulation, inflammation, and inner ear functions. Specifically, it identifies novel substituted carbamoyl/amido/heteroaryl dialkylpiperazinium iodides as potent agonists selective for human α9 and α9α10 over α7 nAChRs, particularly compounds 3f, 3h, and 3j. Compound 3h (GAT2711) demonstrated a 230 nM potency as a full agonist at α9 nAChRs, being 340-fold selective over α7. Compound 3c was 10-fold selective for α9α10 over α9 nAChR. Compounds 2, 3f, and 3h inhibited ATP-induced interleukin-1β release in THP-1 cells. The analgesic activity of 3h was fully retained in α7 knockout mice, suggesting that analgesic effects were potentially mediated through α9* nAChRs. Our findings provide a blueprint for developing α9*-specific therapeutics for pain.

Postsynaptic receptors regulate presynaptic transmitter stability through transsynaptic bridges

Stable matching of neurotransmitters with their receptors is fundamental to synapse function and reliable communication in neural circuits. Presynaptic neurotransmitters regulate the stabilization of postsynaptic transmitter receptors. Whether postsynaptic receptors regulate stabilization of presynaptic transmitters has received less attention. Here, we show that blockade of endogenous postsynaptic acetylcholine receptors (AChR) at the neuromuscular junction destabilizes the cholinergic phenotype in motor neurons and stabilizes an earlier, developmentally transient glutamatergic phenotype. Further, expression of exogenous postsynaptic gamma-aminobutyric acid type A receptors (GABAA receptors) in muscle cells stabilizes an earlier, developmentally transient GABAergic motor neuron phenotype. Both AChR and GABAA receptors are linked to presynaptic neurons through transsynaptic bridges. Knockdown of specific components of these transsynaptic bridges prevents stabilization of the cholinergic or GABAergic phenotypes. Bidirectional communication can enforce a match between transmitter and receptor and ensure the fidelity of synaptic transmission. Our findings suggest a potential role of dysfunctional transmitter receptors in neurological disorders that involve the loss of the presynaptic transmitter.

Recombinant cellular model system for human muscle-type nicotinic acetylcholine receptor α12β1δε

The human muscle-type nicotinic acetylcholine receptor α12β1δε (nAChR) is a complex transmembrane receptor needed for drug screening for disorders like congenital myasthenic syndromes and multiple pterygium syndrome. Until today, most models are still using the nAChR from Torpedo californica electric ray. A simple reproducible cellular system expressing functional human muscle-type nAChR is still missing. This study addressed this issue and further tested the hypothesis that different chaperones, both biological and chemical, and posttranslational modification supporting substances as well as hypothermic incubation are able to increase the nAChR yield. Therefore, Gibson cloning was used to generate transfer plasmids carrying the sequence of nAChR or chosen biological chaperones to support the nAChR folding in the cellular host. Viral transduction was used for stable integration of these transgenes in Chinese hamster ovary cells (CHO). Proteins were detected with Western blot, in-cell and on-cell Western, and the function of the receptor with voltage clamp analysis. We show that the internalization of nAChR into plasma membranes was sufficient for detection and function. Additional transgenic overexpression of biological chaperones did result in a reduced nAChR expression. Chemical chaperones, posttranslational modification supporting substances, and hypothermic conditions are well-suited supporting applications to increase the protein levels of different subunits. This study presents a stable and functional cell line that expresses human muscle-type nAChR and yields can be further increased using the chemical chaperone nicotine without affecting cell viability. The simplified access to this model system should enable numerous applications beyond drug development. Graphical abstract created with http://biorender.com.

The many enigmas of nicotine

This review discusses the diverse effects of nicotine on the various nicotinic acetylcholine receptors of the central and peripheral nervous system and how those effects may promote the usage and addiction to tobacco products.

Effects of e-vapour and high-fat diet on the immunohistochemical staining of nicotinic acetylcholine receptors, apoptosis, microglia and astrocytes in the adult male mouse hippocampus

The use of e-cigarettes/e-vapour, and the consumption of a high-fat diet (HFD), are two popular lifestyle choices associated with alterations in the hippocampus. This study, using a mouse model, investigated the effects of exposure to e-vapour (± nicotine) and HFD (43% fat) consumption, on the expression of nicotinic acetylcholine receptor (nAChR) subunits α3, α4, α7 and β2, apoptosis markers caspase-3 and TUNEL, microglial marker Iba-1, and astrocyte marker GFAP, in hippocampal subregions of dentate gyrus (DG) and cornu ammonis (CA) 1-3. The major findings included: (1) HFD alone had minimal effect with no consistent pattern or interaction between the markers, (2) E-vapour (± nicotine) predominantly affected the CA2 subregion, decreasing α7 and β2 nAChR subunits and Iba-1, (3) Nicotine e-vapour increased TUNEL across all subregions, and (4) HFD, in the presence of nicotine-free e-vapour, decreased caspase-3 and increased TUNEL across all regions, and decreased Iba-1 in the CA subregions, while HFD and nicotine-containing e-vapour, subregion specifically affected the α3, α4 and α7 nAChR subunits, with a protective effect against change in GFAP in the DG and Iba-1 in the CA1 and CA3. These findings highlight that e-vapour itself alters nAChRs, particularly in the CA2 subregion, associated with a decrease in neuroinflammatory response (Iba-1) across the whole hippocampus, and the addition of nicotine increases cell apoptosis across the whole hippocampus. HFD alone was not detrimental in our model, but in the presence of nicotine-free e-vapour, it differentially affected apoptosis, while the addition of nicotine increased nAChR subunits.

Discovery of , a Novel Brain-Penetrant α6-Containing Nicotinic Receptor Antagonist for the Modulation of Motor Dysfunction

Nicotinic acetylcholine receptor (nAChR) α6 subunit RNA expression is relatively restricted to midbrain regions and is located presynaptically on dopaminergic neurons projecting to the striatum. This subunit modulates dopamine neurotransmission and may have therapeutic potential in movement disorders. We aimed to develop potent and selective α6-containing nAChR antagonists to explore modulation of dopamine release and regulation of motor function in vivo. High-throughput screening (HTS) identified novel α6-containing nAChR antagonists and led to the development of CVN417. This molecule blocks α6-containing nAChR activity in recombinant cells and reduces firing frequency of noradrenergic neurons in the rodent locus coeruleus. CVN417 modulated phasic dopaminergic neurotransmission in an impulse-dependent manner. In a rodent model of resting tremor, CVN417 attenuated this behavioral phenotype. These data suggest that selective antagonism of α6-containing nAChR, with molecules such as CVN417, may have therapeutic utility in treating the movement dysfunctions observed in conditions such as Parkinson's disease.

Contribution of the α5 nAChR Subunit and α5SNP to Nicotine-Induced Proliferation and Migration of Human Cancer Cells

Nicotine in tobacco is known to induce tumor-promoting effects and cause chemotherapy resistance through the activation of nicotinic acetylcholine receptors (nAChRs). Many studies have associated the α5 nicotinic receptor subunit (α5), and a specific polymorphism in this subunit, with (i) nicotine administration, (ii) nicotine dependence, and (iii) lung cancer. The α5 gene CHRNA5 mRNA is upregulated in several types of cancer, including lung, prostate, colorectal, and stomach cancer, and cancer severity is correlated with smoking. In this study, we investigate the contribution of α5 in the nicotine-induced cancer hallmark functions proliferation and migration, in breast, colon, and prostate cancer cells. Nine human cell lines from different origins were used to determine nAChR subunit expression levels. Then, selected breast (MCF7), colon (SW480), and prostate (DU145) cancer cell lines were used to investigate the nicotine-induced effects mediated by α5. Using pharmacological and siRNA-based experiments, we show that α5 is essential for nicotine-induced proliferation and migration. Additionally, upon downregulation of α5, nicotine-promoted expression of EMT markers and immune regulatory proteins was impaired. Moreover, the α5 polymorphism D398N (α5SNP) caused a basal increase in proliferation and migration in the DU145 cell line, and the effect was mediated through G-protein signaling. Taken together, our results indicate that nicotine-induced cancer cell proliferation and migration are mediated via α5, adding to the characterization of α5 as a putative therapeutical target.

Structure-Activity Relationships of Alanine Scan Mutants αO-Conotoxins GeXIVA[1,2] and GeXIVA[1,4]

αO-Conotoxin GeXIVA is a selective α9α10 nicotinic acetylcholine receptor (nAChR) inhibitor displaying two disulfide bonds that can form three isomers. The bead (GeXIVA[1,2]) and ribbon (GeXIVA[1,4]) isomers possess the highest activity on rat and human α9α10 nAChRs. However, the molecular mechanism by which they inhibit the α9α10 nAChR is unknown. Here, an alanine scan of GeXIVA was used to elucidate key interactions between the peptides and the α9α10 nAChR. The majority of GeXIVA[1,2] analogues preserved affinity at α9α10 nAChR, but [R17A]GeXIVA[1,2] enhanced selectivity on the α9α10 nAChR. The I23A replacement of GeXIVA[1,4] increased activity at both rat and human α9α10 nAChRs by 10-fold. Surprisingly, these results do not support the molecular model of an interaction in the orthosteric binding site proposed previously, but rather may involve allosteric coupling with the voltage-sensitive domain of the α9α10 nAChR. These results could help to guide further development of GeXIVA analogues as analgesics.

Levodopa-Induced Dyskinesias in Parkinson's Disease: An Overview on Pathophysiology, Clinical Manifestations, Therapy Management Strategies and Future Directions

Since its first introduction, levodopa has become the cornerstone for the treatment of Parkinson's disease and remains the leading therapeutic choice for motor control therapy so far. Unfortunately, the subsequent appearance of abnormal involuntary movements, known as dyskinesias, is a frequent drawback. Despite the deep knowledge of this complication, in terms of clinical phenomenology and the temporal relationship during a levodopa regimen, less is clear about the pathophysiological mechanisms underpinning it. As the disease progresses, specific oscillatory activities of both motor cortical and basal ganglia neurons and variation in levodopa metabolism, in terms of the dopamine receptor stimulation pattern and turnover rate, underlie dyskinesia onset. This review aims to provide a global overview on levodopa-induced dyskinesias, focusing on pathophysiology, clinical manifestations, therapy management strategies and future directions.

Dose-dependent effects of GAT107, a novel allosteric agonist-positive allosteric modulator (ago-PAM) for the α7 nicotinic cholinergic receptor: a BOLD phMRI and connectivity study on awake rats

Background

Alpha 7 nicotinic acetylcholine receptor (α7nAChR) agonists have been developed to treat schizophrenia but failed in clinical trials due to rapid desensitization. GAT107, a type 2 allosteric agonist-positive allosteric modulator (ago-PAM) to the α7 nAChR was designed to activate the α7 nAChR while reducing desensitization. We hypothesized GAT107 would alter the activity of thalamocortical neural circuitry associated with cognition, emotion, and sensory perception.

Methods

The present study used pharmacological magnetic resonance imaging (phMRI) to evaluate the dose-dependent effect of GAT107 on brain activity in awake male rats. Rats were given a vehicle or one of three different doses of GAT107 (1, 3, and 10 mg/kg) during a 35 min scanning session. Changes in BOLD signal and resting state functional connectivity were evaluated and analyzed using a rat 3D MRI atlas with 173 brain areas.

Results

GAT107 presented with an inverted-U dose response curve with the 3 mg/kg dose having the greatest effect on the positive BOLD volume of activation. The primary somatosensory cortex, prefrontal cortex, thalamus, and basal ganglia, particularly areas with efferent connections from the midbrain dopaminergic system were activated as compared to vehicle. The hippocampus, hypothalamus, amygdala, brainstem, and cerebellum showed little activation. Forty-five min post treatment with GAT107, data for resting state functional connectivity were acquired and showed a global decrease in connectivity as compared to vehicle.

Discussion

GAT107 activated specific brain regions involved in cognitive control, motivation, and sensory perception using a BOLD provocation imaging protocol. However, when analyzed for resting state functional connectivity there was an inexplicable, general decrease in connectivity across all brain areas.

Expression patterns of prosaposin and neurotransmitter-related molecules in the chick paratympanic organ

The paratympanic organ (PTO) is a small sense organ in the middle ear of birds that contains hair cells similar to those found in vestibuloauditory organs and receives afferent fibers from the geniculate ganglion. To consider the histochemical similarities between the PTO and vestibular hair cells, we examined the expression patterns of representative molecules in vestibular hair cells, including prosaposin, G protein-coupled receptor (GPR) 37 and GPR37L1 as prosaposin receptors, vesicular glutamate transporter (vGluT) 2 and vGluT3, nicotinic acetylcholine receptor subunit α9 (nAChRα9), and glutamic acid decarboxylase (GAD) 65 and GAD67, in the postnatal day 0 chick PTO and geniculate ganglion by in situ hybridization. Prosaposin mRNA was observed in PTO hair cells, supporting cells, and geniculate ganglion cells. vGluT3 mRNA was observed in PTO hair cells, whereas vGluT2 was observed in a small number of ganglion cells. nAChRα9 mRNA was observed in a small number of PTO hair cells. The results suggest that the histochemical character of PTO hair cells is more similar to that of vestibular hair cells than that of auditory hair cells in chicks.

Human α6β4 Nicotinic Acetylcholine Receptor: Heterologous Expression and Agonist Behavior Provide Insights into the Immediate Binding Site

Study of α6β4 nicotinic acetylcholine receptors (nAChRs) as a pharmacological target has recently gained interest because of their involvement in analgesia, control of catecholamine secretion, dopaminergic pathways, and aversive pathways. However, an extensive characterization of the human α6β4 nAChRs has been vitiated by technical difficulties resulting in poor receptor expression. In 2020, Knowland and collaborators identified BARP (β-anchoring and regulatory protein), a previously known voltage-gated calcium channel suppressor, as a novel human α6β4 chaperone. Here, we establish that co-expression of human BARP with human α6β4 in Xenopus oocytes, resulted in the functional expression of human α6β4 receptors with acetylcholine-elicited currents that allow an in-depth characterization of the receptor using two electrode voltage-clamp electrophysiology together with diverse agonists and receptor mutations. We report: 1) an extended pharmacological characterization of the receptor, and 2) key residues for agonist-activity located in or near the first shell of the binding pocket. SIGNIFICANCE STATEMENT: The human α6β4 nicotinic acetylcholine receptor has attained increased interest because of its involvement in diverse physiological processes and diseases. Although recognized as a pharmacological target, development of specific agonists has been hampered by limited knowledge of its structural characteristics and by challenges in expressing the receptor. By including the chaperone β-anchoring and regulatory protein for enhanced expression and employing different ligands, we have studied the pharmacology of α6β4, providing insight into receptor residues and structural requirements for ligands important to consider for agonist-induced activation.

Review: Nicotinic acetylcholine receptors to regulate important brain activity-what occurs at the molecular level?

Herein, we briefly review the role of nicotinic acetylcholine receptors in regulating important brain activity by controlled release of acetylcholine from subcortical neuron groups, focusing on a microscopic viewpoint and considering the nonlinear dynamics of biological macromolecules associated with neuron activity and how they give rise to advanced brain functions of brain.

Synthesis and Antagonist Activity of Methyllycaconitine Analogues on Human α7 Nicotinic Acetylcholine Receptors

Methyllycaconitine (MLA), 1, is a naturally occurring norditerpenoid alkaloid that is a highly potent (IC₅₀ = 2 nM) selective antagonist of α7 nicotinic acetylcholine receptors (nAChRs). Several structural factors affect its activity such as the neopentyl ester side-chain and the piperidine ring N-side-chain. The synthesis of simplified AE-bicyclic analogues 14-21 possessing different ester and nitrogen side-chains was achieved in three steps. The antagonist effects of synthetic analogues were examined on human α7 nAChRs and compared to that of MLA 1. The most efficacious analogue (16) reduced α7 nAChR agonist responses [1 nM acetylcholine (ACh)] to 53.2 ± 1.9% compared to 3.4 ± 0.2% for MLA 1. This demonstrates that simpler analogues of MLA 1 possess antagonist effects on human α7 nAChRs but also indicates that further optimization may be possible to achieve antagonist activity comparable to that of MLA 1.

Nicotinic acetylcholine receptors in neurological and psychiatric diseases

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that are widely distributed both pre- and post-synaptically in the mammalian brain. By modulating cation flux across cell membranes, neuronal nAChRs regulate neuronal excitability and the release of a variety of neurotransmitters to influence multiple physiologic and behavioral processes including synaptic plasticity, motor function, attention, learning and memory. Abnormalities of neuronal nAChRs have been implicated in the pathophysiology of neurologic disorders including Alzheimer's disease, Parkinson's disease, epilepsy, and Tourette´s syndrome, as well as psychiatric disorders including schizophrenia, depression, and anxiety. The potential role of nAChRs in a particular illness may be indicated by alterations in the expression of nAChRs in relevant brain regions, genetic variability in the genes encoding for nAChR subunit proteins, and/or clinical or preclinical observations where specific ligands showed a therapeutic effect. Over the past 25 years, extensive preclinical and some early clinical evidence suggested that ligands at nAChRs might have therapeutic potential for neurologic and psychiatric disorders. However, to date the only approved indications for nAChR ligands are smoking cessation and the treatment of dry eye disease. It has been argued that progress in nAChR drug discovery has been limited by translational gaps between the preclinical models and the human disease as well as unresolved questions regarding the pharmacological goal (i.e., agonism, antagonism or receptor desensitization) depending on the disease.

The role of nicotinic receptors in alcohol consumption

The use of alcohol causes significant morbidity and mortality across the globe. Alcohol use disorder (AUD) is defined by the excessive use of this drug despite a negative impact on the individual's life. While there are currently medications available to treat AUD, they have limited efficacy and several side effects. As such, it is essential to continue to look for novel therapeutics. One target for novel therapeutics is nicotinic acetylcholine receptors (nAChRs). Here we systematically review the literature on the involvement of nAChRs in alcohol consumption. Data from both genetic and pharmacology studies provide evidence that nAChRs modulate alcohol intake. Interestingly, pharmacological modulation of all nAChR subtypes examined can decrease alcohol consumption. The reviewed literature demonstrates that nAChRs should continue to be investigated as novel therapeutics for AUD.

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

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

The α9α10 acetylcholine receptor: a non-neuronal nicotinic receptor

Within the superfamily of pentameric ligand-gated ion channels, cholinergic nicotinic receptors (nAChRs) were classically identified to mediate synaptic transmission in the nervous system and the neuromuscular junction. The α9 and α10 nAChR subunits were the last ones to be identified. Surprisingly, they do not fall into the dichotomic neuronal/muscle classification of nAChRs. They assemble into heteropentamers with a well-established function as canonical ion channels in inner ear hair cells, where they mediate central nervous system control of auditory and vestibular sensory processing. The present review includes expression, pharmacological, structure-function, molecular evolution and pathophysiological studies, that define receptors composed from α9 and α10 subunits as distant and distinct members within the nAChR family. Thus, although α9 and α10 were initially included within the neuronal subdivision of nAChR subunits, they form a distinct clade within the phylogeny of nAChRs. Following the classification of nAChR subunits based on their main synaptic site of action, α9 and α10 should receive a name in their own right.

Nicotinic acetylcholine receptors and epilepsy

Despite recent advances in understanding the causes of epilepsy, especially the genetic, comprehending the biological mechanisms that lead to the epileptic phenotype remains difficult. A paradigmatic case is constituted by the epilepsies caused by altered neuronal nicotinic acetylcholine receptors (nAChRs), which exert complex physiological functions in mature as well as developing brain. The ascending cholinergic projections exert potent control of forebrain excitability, and wide evidence implicates nAChR dysregulation as both cause and effect of epileptiform activity. First, tonic-clonic seizures are triggered by administration of high doses of nicotinic agonists, whereas non-convulsive doses have kindling effects. Second, sleep-related epilepsy can be caused by mutations on genes encoding nAChR subunits widely expressed in the forebrain (CHRNA4, CHRNB2, CHRNA2). Third, in animal models of acquired epilepsy, complex time-dependent alterations in cholinergic innervation are observed following repeated seizures. Heteromeric nAChRs are central players in epileptogenesis. Evidence is wide for autosomal dominant sleep-related hypermotor epilepsy (ADSHE). Studies of ADSHE-linked nAChR subunits in expression systems suggest that the epileptogenic process is promoted by overactive receptors. Investigation in animal models of ADSHE indicates that expression of mutant nAChRs can lead to lifelong hyperexcitability by altering i) the function of GABAergic populations in the mature neocortex and thalamus, ii) synaptic architecture during synaptogenesis. Understanding the balance of the epileptogenic effects in adult and developing networks is essential to plan rational therapy at different ages. Combining this knowledge with a deeper understanding of the functional and pharmacological properties of individual mutations will advance precision and personalized medicine in nAChR-dependent epilepsy.

Selective α3β4 Nicotinic Acetylcholine Receptor Ligand as a Potential Tracer for Drug Addiction

α₃β₄ Nicotinic acetylcholine receptor (nAChR) has been recognized as an emerging biomarker for the early detection of drug addiction. Herein, α₃β₄ nAChR ligands were designed and synthesized to improve the binding affinity and selectivity of two lead compounds, (S)-QND8 and (S)-T2, for the development of an α₃β₄ nAChR tracer. The structural modification was achieved by retaining the key features and expanding the molecular structure with a benzyloxy group to increase the lipophilicity for blood-brain barrier penetration and to extend the ligand-receptor interaction. The preserved key features are a fluorine atom for radiotracer development and a p-hydroxyl motif for ligand-receptor binding affinity. Four (R)- and (S)-quinuclidine-triazole (AK1-AK4) were synthesized and the binding affinity, together with selectivity to α₃β₄ nAChR subtype, were determined by competitive radioligand binding assay using [³H]epibatidine as a radioligand. Among all modified compounds, AK3 showed the highest binding affinity and selectivity to α₃β₄ nAChR with a K_i value of 3.18 nM, comparable to (S)-QND8 and (S)-T2 and 3069-fold higher affinity to α₃β₄ nAChR in comparison to α₇ nAChR. The α₃β₄ nAChR selectivity of AK3 was considerably higher than those of (S)-QND8 (11.8-fold) and (S)-T2 (294-fold). AK3 was shown to be a promising α₃β₄ nAChR tracer for further development as a radiotracer for drug addiction.

Discovery, Characterization, and Engineering of LvIC, an α4/4-Conotoxin That Selectively Blocks Rat α6/α3β4 Nicotinic Acetylcholine Receptors

α6β4 nicotinic acetylcholine receptors (nAChRs) are expressed in the central and peripheral nervous systems, but their functions are not fully understood, largely because of a lack of specific ligands. Here, we characterized a novel α-conotoxin, LvIC, and designed a series of analogues to probe structure-activity relationships at the α6β4 nAChR. The potency and selectivity of these conotoxins were tested using two-electrode voltage-clamp recording on nAChR subtypes expressed in Xenopus laevis oocytes. One of the analogues, [D1G,ΔQ14]LvIC, potently blocked α6/α3β4 nAChRs (α6/α3 is a chimera) with an IC₅₀ of 19 nM, with minimal activity at other nAChR subtypes, including the structurally similar α6/α3β2β3 and α3β4 subtypes. Using NMR, molecular docking, and receptor mutation, structure-activity relationships of [D1G,ΔQ14]LvIC at the α6/α3β4 nAChR were defined. It is a potent and specific antagonist of α6β4 nAChRs that could potentially serve as a novel molecular probe to explore α6β4 nAChR-related neurophysiological and pharmacological functions.

Insights Into the Differential Desensitization of α4β2 Nicotinic Acetylcholine Receptor Isoforms Obtained With Positive Allosteric Modulation of Mutant Receptors

The development of highly efficacious positive allosteric modulators (PAMs) of α7 nicotinic acetylcholine receptors (nAChR) has proven useful in defining the ligand dependence of the conformational dynamics of α7 receptors. No such effective modulators are known to exist for the α4β2 nAChR of the brain, limiting our ability to understand the importance of desensitization for the activity profile of specific ligands. In this study, we used mutant β2 subunits that allowed the use of the α7 PAM 3a,4,5,9b-tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[c]quinoline-8-sulfonamide (TQS) to probe the desensitizing effects of nicotinic ligands on the two forms of α4β2 receptors; high sensitivity (HS) (two α4 and three β2 subunits) and low sensitivity (LS) (three α4 and two β2 subunits). A total of 28 different ligands of 8 different categories, based on activity and selectivity, were tested for their ability to induce TQS-sensitive desensitization of HS and LS α4β2 receptors. Results confirm that HS α4β2 receptor responses are strongly limited by desensitization, by at least an order of magnitude more so than the responses of LS receptors. The activation of α4β2 receptors by the smoking-cessation drugs cytisine and varenicline is strongly limited by desensitization, as is the activation of LS receptors by the HS-selective agonists 6-[5-[(2S)-2-Azetidinylmethoxy]-3-pyridinyl]-5-hexyn-1-ol dihydrochloride and 4-(5-ethoxy-3-pyridinyl)-N-methyl-(3E)-3-buten-1-amine difumarate. The evaluation of drugs previously identified as α7-selective agonists revealed varying patterns of α4β2 cross-desensitization that were predictive of the effects of these drugs on the activation of wild-type α4β2 receptors by acetylcholine, supporting the utility of TQS-sensitive receptors for the development of focused therapeutics. SIGNIFICANCE STATEMENT: To varying degrees, ligands regulate the balance of active and desensitized states of the two forms of the primary nAChR subtypes in brain. Using mutant beta subunits, an allosteric modulator can reverse ligand-induced desensitization, revealing the differential desensitization of the receptors by specific ligands. This study shows that drugs believed to be selective for therapeutic targets may cross-desensitize other targets and that, within a class of drugs, improved specificity can be achieved by using agents that reduce such cross-desensitization.

When "No-Smoking" is not enough: Hypoxia and nicotine acetylcholine receptor signaling may drive lung adenocarcinoma progression in never-smokers

The question of how lung cancer progresses in never-smokers remains largely unanswered. In our analysis of data from 1727 lung cancer patients, we observed a difference of only 47 days in the overall survival between lung adenocarcinoma patients who were smokers vis-a-vis never-smokers - the disease has a poor prognosis irrespective of the smoking status, or gender. We have investigated the possible collaboration between the nAChR and hypoxia signaling pathway to explicate a mechanism of disease progression in never-smokers using patient-derived tumor cells. We found a previously unidentified increase in both acetylcholine and nAChR-α7 levels in non-small cell lung cancer cells in hypoxia. A similar increase in ubiquitously expressed nAChR-α7 transcripts was also observed in other cancer lines and primary tumor tissues. A direct binding of HIF-1α with the hypoxia-response element (HRE) present at -48 position preceding the transcriptional start site in nAChR-α7 promoter region was established. Crucially, the increased acetylcholine levels in hypoxia drove a feedback loop via modulation of PI3K/AKT pathway to stabilize HIF-1α in hypoxia. Further, hypoxia-mediated metastasis and induction of HIF-1α in these cells was significantly reversed by bungarotoxin, an antagonist of nAChR-α7. The nAChR-AKT-HIF network needs to be further investigated to conclusively prove its mechanism and to explore its therapeutic potential. Our study gives a plausible explanation for the equally worse prognosis of lung adenocarcinoma in never-smokers wherein the nAChR signaling is enhanced in hypoxia by acetylcholine in the absence of nicotine.

Combinations of Beauveria bassiana and spinetoram for the management of four important stored-product pests: laboratory and field trials

The current study examines the efficacy of the semi-synthetic insecticide spinetoram and entomopathogenic fungi Beauveria bassiana (Balsamo-Crivelli) Vuillemin (Hypocreales: Cordycipitaceae) as wheat protectants against the lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae), the red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae), the granary weevil, Sitophilus granarius (L.) (Coleoptera: Curculionidae), and the khapra beetle, Trogoderma granarium Everts (Coleoptera: Dermestidae), under laboratory and field trials. One dose of B. bassiana, i.e., 1 × 107 conidia/kg wheat, two doses of spinetoram, i.e., spine1: 0.05 ppm (mg/kg wheat), spine2: 0.1 ppm, and their combinations (Bb + spine1, Bb + spine2) were evaluated at 20, 25, and 30 °C. All treatments provided significantly higher mortality at 30 °C compared with the other two temperatures. Maximum mortality levels were observed in the treatments where B. bassiana was combined with the higher dose of spinetoram (0.1 ppm). All treatments reduced progeny production in comparison with the control groups. Maximum progeny reduction was observed at 30 °C, on wheat treated with the Bb + spine2 combination. The combination Bb + spine2 also provided elevated mortality rates in both laboratory and field persistence trials, but at 180 days caused moderate mortality to all tested insect species. Concerning progeny, at laboratory persistence trials, the combination Bb + spine2 exhibited the lowest offspring emergence to all tested species compared to the other treatments and control. Overall, our study showed that R. dominica was the most susceptible species followed by S. granarius, T. castaneum, and T. granarium. Our findings revealed that the combination of B. bassiana and spinetoram may be a useful tool for efficient and advanced integrated pest management strategies for long storage periods under multiple temperatures.

Detection of differential selection pressure and functional-specific sites in subunits of vertebrate neuronal nicotinic acetylcholine receptors

The nicotinic acetylcholine receptors (nAChR) are made of subunits evolved from a common ancestor. Despite the similarity in their sequences and structures, the properties of these subunits vary significantly. Thus, identifying the evolution features and function-related sites specific to each subunit is essential for understanding the characteristics of the subunits and the receptors assembled by them. In this study, we examined the sequence features of the nine neuronal nAChRs subunits from representative vertebrate species. Analysis revealed that all the subunits were subject to strong purifying selection in evolution, and each was under a unique pattern of selection pressures. At the same time, the functional constraints were not uniform within each subunit, with different domains in the molecule being subject to different selection pressures. We also detected potential positive selection events in the subunits or subunit clusters, and identified the sites might be associated with the function specificity of each subunit. Furthermore, positive selection at some domains might contribute to the diversity of subunit function; for example, the β9 strand might be related to the agonist specificity of α subunit in heteromeric receptor and β4-β5 linker could be involved in Ca²⁺ permeability. Subunits α7, α4 and β2 subunits possess a strong adaptability in vertebrates. Our results highlighted the importance of tracking functional differentiation in protein sequence underlying functional properties of nAChRs. In summary, our work may provide clues on understanding the diversity and the function specificity of the nAChR subunits, as well as the receptors co-assembled by them.Communicated by Ramaswamy H. Sarma.

RC-4BC cells express nicotinic and muscarinic acetylcholine receptors

Acetylcholine is one of the most important endogenous neurotransmitters in a range of organisms spanning different animal phyla. Within pituitary gland it acts as autocrine and paracrine signal. In a current study we assessed expression profile of the different subunits of nicotinic as well as muscarinic acetylcholine receptors in RC-4BC cells, which are derived from rat pituitary gland tumor. Our findings indicate that β2, δ, and M2 subunits are expressed by the cells with the lowest Ct values compared to other tested subunits. The detected Ct values were 26.6±0.16, 27.95±0.5, and 28.8±0.25 for β2, δ, and M2 subunits, respectively.

Conus regius-Derived Conotoxins: Novel Therapeutic Opportunities from a Marine Organism

Conus regius is a marine venomous mollusk of the Conus genus that captures its prey by injecting a rich cocktail of bioactive disulfide bond rich peptides called conotoxins. These peptides selectively target a broad range of ion channels, membrane receptors, transporters, and enzymes, making them valuable pharmacological tools and potential drug leads. C. regius-derived conotoxins are particularly attractive due to their marked potency and selectivity against specific nicotinic acetylcholine receptor subtypes, whose signalling is involved in pain, cognitive disorders, drug addiction, and cancer. However, the species-specific differences in sensitivity and the low stability and bioavailability of these conotoxins limit their clinical development as novel therapeutic agents for these disorders. Here, we give an overview of the main pharmacological features of the C. regius-derived conotoxins described so far, focusing on the molecular mechanisms underlying their potential therapeutic effects. Additionally, we describe adoptable chemical engineering solutions to improve their pharmacological properties for future potential clinical translation.

Fetal blockade of nicotinic acetylcholine transmission causes autism-like impairment of biological motion preference in the neonatal chick

Several environmental chemicals are suspected risk factors for autism spectrum disorder (ASD), including valproic acid (VPA) and pesticides acting on nicotinic acetylcholine receptors (nAChRs), if administered during pregnancy. However, their target processes in fetal neuro-development are unknown. We report that the injection of VPA into the fetus impaired imprinting to an artificial object in neonatal chicks, while a predisposed preference for biological motion (BM) remained intact. Blockade of nAChRs acted oppositely, sparing imprinting and impairing BM preference. Beside ketamine and tubocurarine, significant effects of imidacloprid (a neonicotinoid insecticide) appeared at a dose ≤1 ppm. In accord with the behavioral dissociations, VPA enhanced histone acetylation in the primary cell culture of fetal telencephalon, whereas ketamine did not. VPA reduced the brain weight and the ratio of NeuN-positive cells (matured neurons) in the telencephalon of hatchlings, whereas ketamine/tubocurarine did not. Despite the distinct underlying mechanisms, both VPA and nAChR blockade similarly impaired imprinting to biological image composed of point-light animations. Furthermore, both impairments were abolished by postnatal bumetanide treatment, suggesting a common pathology underlying the social attachment malformation. Neurotransmission via nAChR is thus critical for the early social bond formation, which is hindered by ambient neonicotinoids through impaired visual predispositions for animate objects.

Smoking-mediated nicotinic acetylcholine receptors (nAChRs) for predicting outcomes for head and neck squamous cell carcinomas

Background

As a human tumor disease, head and neck squamous cell carcinoma (HNSCC) is associated with a high mortality rate worldwide. Nicotinic acetylcholine receptors (nAChRs) are transmembrane receptor proteins and exert their biological effects following activation by nicotine. We aimed to construct a prognostic signature based on the expression of nAChRs among smokers with HNSCC.

Methods

The transcriptome profile of nAChRs was obtained from The Cancer Genome Atlas (TCGA). Following the integration of survival information, univariate Cox regression and least absolute shrinkage and selection operator (LASSO) analyses were performed to screen the prognosis-related nAChRs and construct a prognostic signature. Kaplan–Meier (KM), receiver operating characteristic (ROC), principal component analysis (PCA), and independent prognostic analysis were utilized to verify the predictive power of the nAChR-associated prognostic signature. The expression of α5 nAChR in clinical samples was verified by quantitative reverse transcriptase PCR.

Results

Subunits α2, α5, α9, and β4 were related to the prognosis. The prognostic signature comprised the expression of subunits α5, α9, and β4. The nAChR-associated signature showed high sensitivity and specificity for prognostic prediction and was an independent factor for overall survival. Based on the clinical variables and expression of nAChRs, a nomogram was constructed for predicting the outcomes of HNSCC patients who were smokers in the clinical settings. In clinical specimens, α5 nAChR showed high expression in HNSCC tissues, especially among smokers.

Conclusions

The nAChR-associated signature constructed in this study may provide a better system for the classification of HNSCC patients and facilitate personalized treatment according to their smoking habits.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-10161-x.

LiGIoNs: A computational method for the detection and classification of ligand-gated ion channels

Ligand-Gated Ion Channels (LGICs) is one of the largest groups of transmembrane proteins. Due to their major role in synaptic transmission, both in the nervous system and the somatic neuromuscular junction, LGICs present attractive therapeutic targets. During the last few years, several computational methods for the detection of LGICs have been developed. These methods are based on machine learning approaches utilizing features extracted solely from the amino acid composition. Here we report the development of LiGIoNs, a profile Hidden Markov Model (pHMM) method for the prediction and ligand-based classification of LGICs. The method consists of a library of 10 pHMMs, one per LGIC subfamily, built from the alignment of representative LGIC sequences. In addition, 14 Pfam pHMMs are used to further annotate and classify unknown protein sequences into one of the 10 LGIC subfamilies. Evaluation of the method showed that it outperforms existing methods in the detection of LGICs. On top of that, LiGIoNs is the only currently available method that classifies LGICs into subfamilies. The method is available online at http://bioinformatics.biol.uoa.gr/ligions/.

Fluorescently Labeled α-Conotoxin TxID, a New Probe for α3β4 Neuronal Nicotinic Acetylcholine Receptors

Neuronal nicotinic acetylcholine receptors (nAChRs) are important ion channel membrane proteins that are widely distributed in the central nervous system (CNS) and peripheral nervous system (PNS). As an important member, α3β4 nAChRs are related to pain sensation in PNS and nicotine addiction in CNS. However, research related to the α3β4 nAChRs is greatly limited by the lack of subtype-selective pharmacological tools. The α-conotoxin (α-CTx) TxID from the marine cone snail, Conus textile, is a selective α3β4 nAChR antagonist with relatively high potency. In this study, a fluorescent dye (5-TAMRA SE) was used to label TxID on the N-terminus of α-CTx TxID, and pure TxID-F (fluorescent analogue of TxID) was obtained by HPLC. At the same time, the potency and selectivity of TxID-F were detected by high-performance liquid chromatography (HPLC). Additionally, the potency and selectivity of TxID-F were determined by using a two-electrode voltage-clamp technique on various nAChRs expressed in the Xenopus oocyte expression system. The results obtained by electrophysiology showed that TxID-F maintained the same order of potency (IC50 73 nM) as the native toxin (IC50 25 nM) for the α3β4 nAChR subtype. In addition, the results of fluorescent spectroscopy and circular dichroism showed TxID-F has the same fluorescence as 5-TAMRA SE, as well as similar profiles as TxID. The results of flow cytometry showed that the histogram shifted significantly to the right for the RAW264.7 cells expressing α3β4-containing nAChRs stained with TxID-F and confirmed by live cell imaging. The study of fluorescent-labeled α-CTx TxID provides a rich pharmacological tool to explore the structure-function relationship, distribution, and ligand-binding domain of α3β4 nAChR subtype in the future.

Single-Disulfide Conopeptide Czon1107, an Allosteric Antagonist of the Human α3β4 Nicotinic Acetylcholine Receptor

Conopeptides are peptides in the venom of marine cone snails that are used for capturing prey or as a defense against predators. A new cysteine-poor conopeptide, Czon1107, has exhibited non-competitive inhibition with an undefined allosteric mechanism in the human (h) α3β4 nicotinic acetylcholine receptors (nAChRs). In this study, the binding mode of Czon1107 to hα3β4 nAChR was investigated using molecular dynamics simulations coupled with mutagenesis studies of the peptide and electrophysiology studies on heterologous hα3β4 nAChRs. Overall, this study clarifies the structure–activity relationship of Czon1107 and hα3β4 nAChR and provides an important experimental and theoretical basis for the development of new peptide drugs.

Structural Insights into the Role of β3 nAChR Subunit in the Activation of Nicotinic Receptors

The β3 subunit of nicotinic acetylcholine receptors (nAChRs) participates in heteropentameric assemblies with some α and other β neuronal subunits forming a plethora of various subtypes, differing in their electrophysiological and pharmacological properties. While β3 has for several years been considered an accessory subunit without direct participation in the formation of functional binding sites, recent electrophysiology data have disputed this notion and indicated the presence of a functional (+) side on the extracellular domain (ECD) of β3. In this study, we present the 2.4 Å resolution crystal structure of the monomeric β3 ECD, which revealed rather distinctive loop C features as compared to those of α nAChR subunits, leading to intramolecular stereochemical hindrance of the binding site cavity. Vigorous molecular dynamics simulations in the context of full length pentameric β3-containing nAChRs, while not excluding the possibility of a β3 (+) binding site, demonstrate that this site cannot efficiently accommodate the agonist nicotine. From the structural perspective, our results endorse the accessory rather than functional role of the β3 nAChR subunit, in accordance with earlier functional studies on β3-containing nAChRs.

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

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

Stoichiometry-selective modulation of α4β2 nicotinic ACh receptors by divalent cations

BACKGROUND AND PURPOSE

α4β2 nicotinic ACh receptors (nAChRs) comprise the most abundant class of nAChRs in the nervous system. They assemble in two stoichiometric forms, each exhibiting distinct functional and pharmacological signatures. However, whether one or both forms are modulated by calcium or magnesium has not been established.

EXPERIMENTAL APPROACH

To assess the functional consequences of calcium and magnesium, each stoichiometric form was expressed in clonal mammalian fibroblasts and single-channel currents were recorded in the presence of a range of ACh concentrations.

KEY RESULTS

In the absence of divalent cations, each stoichiometric form exhibits high unitary conductance and simple gating kinetics composed of solitary channel openings or short bursts of openings. However, in the presence of calcium and magnesium, the conductance and gating kinetics change in a stoichiometry-dependent manner. Calcium and magnesium reduce the conductance of both stoichiometric forms, with each cation producing an equivalent reduction, but the reduction is greater for the (α4)₂ (β2)₃ form. Moreover, divalent cations promote efficient channel opening of the (α4)₃ (β2)₂ stoichiometry, while minimally affecting the (α4)₂ (β2)₃ stoichiometry. For the (α4)₃ (β2)₂ stoichiometry, at high but not low ACh concentrations, calcium in synergy with magnesium promote clustering of channel openings into episodes of many openings in quick succession.

CONCLUSION AND IMPLICATIONS

Modulation of the α4β2 nAChR by divalent cations depends on the ACh concentration, the type of cation and the subunit stoichiometry. The functional consequences of modulation are expected to depend on the regional distributions of the stoichiometric forms and synaptic versus extrasynaptic locations of the receptors.

Inflammation Regulation via an Agonist and Antagonists of α7 Nicotinic Acetylcholine Receptors in RAW264.7 Macrophages

The α7 nicotinic acetylcholine receptor (nAChR) is widely distributed in the central and peripheral nervous systems and is closely related to a variety of nervous system diseases and inflammatory responses. The α7 nAChR subtype plays a vital role in the cholinergic anti-inflammatory pathway. In vivo, ACh released from nerve endings stimulates α7 nAChR on macrophages to regulate the NF-κB and JAK2/STAT3 signaling pathways, thereby inhibiting the production and release of downstream proinflammatory cytokines and chemokines. Despite a considerable level of recent research on α7 nAChR-mediated immune responses, much is still unknown. In this study, we used an agonist (PNU282987) and antagonists (MLA and α-conotoxin [A10L]PnIA) of α7 nAChR as pharmacological tools to identify the molecular mechanism of the α7 nAChR-mediated cholinergic anti-inflammatory pathway in RAW264.7 mouse macrophages. The results of quantitative PCR, ELISAs, and transcriptome analysis were combined to clarify the function of α7 nAChR regulation in the inflammatory response. Our findings indicate that the agonist PNU282987 significantly reduced the expression of the IL-6 gene and protein in inflammatory macrophages to attenuate the inflammatory response, but the antagonists MLA and α-conotoxin [A10L]PnIA had the opposite effects. Neither the agonist nor antagonists of α7 nAChR changed the expression level of the α7 nAChR subunit gene; they only regulated receptor function. This study provides a reference and scientific basis for the discovery of novel α7 nAChR agonists and their anti-inflammatory applications in the future.

Ligand-gated ion channels as targets of neuroactive insecticides

The Cys-loop superfamily of ligand-gated ion channels (Cys-loop receptors) is one of the most ubiquitous ion channel families in vertebrates and invertebrates. Despite their ubiquity, they are targeted by several classes of pesticides, including neonicotinoids, phenylpyrazols, and macrolides such as ivermectins. The current commercialized compounds have high target site selectivity, which contributes to the safety of insecticide use. Structural analyses have accelerated progress in this field; notably, the X-ray crystal structures of acetylcholine binding protein and glutamate-gated Cl channels revealed the details of the molecular interactions between insecticides and their targets. Recently, the functional expression of the insect nicotinic acetylcholine receptor (nAChR) has been described, and detailed evaluations using the insect nAChR have emerged. This review discusses the basic concepts and the current insights into the molecular mechanisms of neuroactive insecticides targeting the ligand-gated ion channels, particularly Cys-loop receptors, and presents insights into target-based selectivity, resistance, and future drug design.