Progress in nicotinic receptor structural biology

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.

Dissociation of Nicotine from Acetylcholine-Binding Protein under Terahertz Waves Radiation

The binding of nicotine (NCT) to acetylcholine-binding protein (AChBP) plays an important role in synaptic transmission and neurotransmitter regulation. However, effectively regulating their binding or dissociation processes remains a challenging problem. In this study, we employed all-atom molecular dynamics (MD) simulations to systematically investigate the impact of external terahertz (THz) waves on the binding kinetics between AChBP and NCT. We first identified the key residues (i.e., W143) and the key interactions (i.e., hydrogen bonding and cation-π interaction) in AChBP-NCT binding without THz waves. We then investigated the binding and dissociation of charged NCT with AChBP at three different frequencies (i.e., 13.02, 21.44, 42.55 THz). Importantly, the predominant vibrational modes at 13.02 THz can drive the rotation of the pentagonal ring on NCT. This leads to the disruption of hydrogen bonds between NCT and W143 and a reduced likelihood of forming cation-π interactions, resulting in the dissociation of NCT from AChBP. Additionally, we further investigated the influence of electric field intensities on the dissociation kinetics and found that when the electric field intensity exceeds a critical value (∼0.60 V/nm), the probability of ligand dissociation gradually rises as the intensity increases. In general, this study contributes to a better understanding of the effects of THz waves on protein-ligand interactions, which might also shed some light on potential applications in nicotine addiction treatment and therapeutic strategies for neurodegenerative diseases.

Molecular Recognition Properties of Nicotinic Ligands Determining Selectivity Between Insect and Mammalian Receptors

This investigation defines the roles of various amino acids, neighboring key conserved amino acids in loops C and D of the nicotinic acetylcholine (ACh) receptor (nAChR), in the selective molecular recognition of nicotinic ligands with diverse pharmacophores using Aplysia californica ACh binding protein Y55W (Ac-AChBP) mutants (+Q57R; + Q57R+S189 V; + Q57R+S189E; + Q57T; + Q57T+S189 V; + Q57T+S189E) and Lymnaea stagnalis AChBP (Ls-AChBP) mutants (Q55T; Q55T+S186E; Q55R) as insect and mammalian nAChR structural surrogates, respectively. N-nitro/cyanoimine insecticides show high affinity to four Ac-AChBPs containing Arg57 or Thr57 and Ser189 or Val189, except for those with Glu189. Pyrazinoyl compound selectively interacts with the three Ac-AChBPs containing Arg57 and Ser189, Val189, or Glu189. Cationic ligands prefer three Ac-AChBPs with Thr57 and Ser189, Val189, or Glu189 and two Ls-AChBPs providing Thr55 ± Glu186 over the four Ac- and Ls-AChBPs with Arg57/55. Accordingly, loop C contributes to N-nitro/cyanoimine insecticide action, and loop D controls the affinity of the pyrazinoyl or cationic ligand.

Structural insights into the molecular effects of the anthelmintics monepantel and betaine on the Caenorhabditis elegans acetylcholine receptor ACR-23

Anthelmintics are drugs used for controlling pathogenic helminths in animals and plants. The natural compound betaine and the recently developed synthetic compound monepantel are both anthelmintics that target the acetylcholine receptor ACR-23 and its homologs in nematodes. Here, we present cryo-electron microscopy structures of ACR-23 in apo, betaine-bound, and betaine- and monepantel-bound states. We show that ACR-23 forms a homo-pentameric channel, similar to some other pentameric ligand-gated ion channels (pLGICs). While betaine molecules are bound to the classical neurotransmitter sites in the inter-subunit interfaces in the extracellular domain, monepantel molecules are bound to allosteric sites formed in the inter-subunit interfaces in the transmembrane domain of the receptor. Although the pore remains closed in betaine-bound state, monepantel binding results in an open channel by wedging into the cleft between the transmembrane domains of two neighboring subunits, which causes dilation of the ion conduction pore. By combining structural analyses with site-directed mutagenesis, electrophysiology and in vivo locomotion assays, we provide insights into the mechanism of action of the anthelmintics monepantel and betaine.

The Nicotinic Acetylcholine Receptor and Its Pentameric Homologs: Toward an Allosteric Mechanism of Signal Transduction at the Atomic Level

The nicotinic acetylcholine receptor has served, since its biochemical identification in the 1970s, as a model of an allosteric ligand-gated ion channel mediating signal transition at the synapse. In recent years, the application of X-ray crystallography and high-resolution cryo-electron microscopy, together with molecular dynamic simulations of nicotinic receptors and homologs, have opened a new era in the understanding of channel gating by the neurotransmitter. They reveal, at atomic resolution, the diversity and flexibility of the multiple ligand-binding sites, including recently discovered allosteric modulatory sites distinct from the neurotransmitter orthosteric site, and the conformational dynamics of the activation process as a molecular switch linking these multiple sites. The model emerging from these studies paves the way for a new pharmacology based, first, upon the occurrence of an original mode of indirect allosteric modulation, distinct from a steric competition for a single and rigid binding site, and second, the design of drugs that specifically interact with privileged conformations of the receptor such as agonists, antagonists, and desensitizers. Research on nicotinic receptors is still at the forefront of understanding the mode of action of drugs on the nervous system.

α7 nicotinic acetylcholine receptor agonist attenuates allergen-induced immediate nasal response in murine model of allergic rhinitis

The expression of nicotinic acetylcholine receptor (nAChR) subunits on various immune cells suggests their involvement in allergic rhinitis. However, how exactly they contribute to this pathogenesis is not yet confirmed. Our present study examined the therapeutic potential of GTS-21, an α7 nAChR agonist, for treating allergic rhinitis by employing its mouse models. GTS-21 treatment reduced allergen-induced immediate nasal response in ovalbumin (OVA)-sensitized model. However, nasal hyperresponsiveness or eosinophil infiltration elicited in either the OVA-sensitized or T helper 2 cell-transplanted model was not affected by GTS-21. GTS-21 did not alter allergen-induced passive cutaneous anaphylaxis response in anti-dinitrophenyl IgE-sensitized mice. This evidence implies GTS-21's potential to alleviate allergic rhinitis without perturbing T cells or mast cells.

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.

Pathogenic genes implicated in sleep-related hypermotor epilepsy: a research progress update

Sleep-related hypermotor epilepsy (SHE) is a focal epilepsy syndrome characterized by a variable age of onset and heterogeneous etiology. Current literature suggests a prevalence rate of approximately 1.8 per 100,000 persons. The discovery of additional pathogenic genes associated with SHE in recent years has significantly expanded the knowledge and understanding of its pathophysiological mechanisms. Identified SHE pathogenic genes include those related to neuronal ligand- and ion-gated channels (CHRNA4, CHRNB2, CHRNA2, GABRG2, and KCNT1), genes upstream of the mammalian target of rapamycin complex 1 signal transduction pathway (DEPDC5, NPRL2, NPRL3, TSC1, and TSC2), and other genes (CRH, CaBP4, STX1B, and PRIMA1). These genes encode proteins associated with ion channels, neurotransmitter receptors, cell signal transduction, and synaptic transmission. Mutations in these genes can result in the dysregulation of encoded cellular functional proteins and downstream neuronal dysfunction, ultimately leading to epileptic seizures. However, the associations between most genes and the SHE phenotype remain unclear. This article presents a literature review on the research progress of SHE-related pathogenic genes to contribute evidence to genotype-phenotype correlations in SHE and establish the necessary theoretical basis for future SHE treatments.

The complexities of ligand/receptor interactions: Exploring the role of molecular vibrations and quantum tunnelling

Molecular vibrations and quantum tunneling may link ligand binding to the function of pharmacological receptors. The well-established lock-and-key model explains a ligand's binding and recognition by a receptor; however, a general mechanism by which receptors translate binding into activation, inactivation, or modulation remains elusive. The Vibration Theory of Olfaction was proposed in the 1930s to explain this subset of receptor-mediated phenomena by correlating odorant molecular vibrations to smell, but a mechanism was lacking. In the 1990s, inelastic electron tunneling was proposed as a plausible mechanism for translating molecular vibration to odorant physiology. More recently, studies of ligands' vibrational spectra and the use of deuterated ligand analogs have provided helpful information to study this admittedly controversial hypothesis in metabotropic receptors other than olfactory receptors. In the present work, based in part on published experiments from our laboratory using planarians as an experimental organism, I will present a rationale and possible experimental approach for extending this idea to ligand-gated ion channels.

Molecular docking analysis of chlorpyrifos at the human α7-nAChR and its potential relationship with neurocytoxicity in SH-SY5Y cells

The organophosphate insecticide chlorpyrifos (CPF), an acetylcholinesterase inhibitor, has raised serious concerns about human safety. Apart from inducing synaptic acetylcholine accumulation, CPF could also act at nicotinic acetylcholine receptors, like the α7-isoform (α7-nAChR), which could potentially be harmful to developing brains. Our aims were to use molecular docking to assess the binding interactions between CPF and α7-nAChR through, to test the neurocytotoxic and oxidative effects of very low concentrations of CPF on SH-SY5Y cells, and to hypothesize about the potential mediation of α7-nAChR. Docking analysis showed a significant binding affinity of CPH for the E fragment of the α7-nAChR (ΔGibbs: -5.63 to -6.85 Kcal/mol). According to the MTT- and Trypan Blue-based viability assays, commercial CPF showed concentration- and time-dependent neurotoxic effects at a concentration range (2.5-20 µM), ten-folds lower than those reported to have crucial effects for sheer CPF. A rise of the production of radical oxygen species (ROS) was seen at even lower concentrations (1-2.5 µM) of CPF after 24h. Notably, our docking analysis supports the antagonistic actions of CPF on α7-nAChR that were recently published. In conclusion, while α7-nAChR is responsible for neuronal survival and neurodevelopmental processes, its activity may also mediate the neurotoxicity of CPF.

The Cyclic Imine Core Common to the Marine Macrocyclic Toxins Is Sufficient to Dictate Nicotinic Acetylcholine Receptor Antagonism

Macrocyclic imine phycotoxins are an emerging class of chemical compounds associated with harmful algal blooms and shellfish toxicity. Earlier binding and electrophysiology experiments on nAChR subtypes and their soluble AChBP surrogates evidenced common trends for substantial antagonism, binding affinities, and receptor-subtype selectivity. Earlier, complementary crystal structures of AChBP complexes showed that common determinants within the binding nest at each subunit interface confer high-affinity toxin binding, while distinctive determinants from the flexible loop C, and either capping the nest or extending toward peripheral subsites, dictate broad versus narrow receptor subtype selectivity. From these data, small spiroimine enantiomers mimicking the functional core motif of phycotoxins were chemically synthesized and characterized. Voltage-clamp analyses involving three nAChR subtypes revealed preserved antagonism for both enantiomers, despite lower subtype specificity and binding affinities associated with faster reversibility compared with their macrocyclic relatives. Binding and structural analyses involving two AChBPs pointed to modest affinities and positional variability of the spiroimines, along with a range of AChBP loop-C conformations denoting a prevalence of antagonistic properties. These data highlight the major contribution of the spiroimine core to binding within the nAChR nest and confirm the need for an extended interaction network as established by the macrocyclic toxins to define high affinities and marked subtype specificity. This study identifies a minimal set of functional pharmacophores and binding determinants as templates for designing new antagonists targeting disease-associated nAChR subtypes.

Structural mechanisms of α7 nicotinic receptor allosteric modulation and activation

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

Molecular Docking Analysis at the Human α7-nAChR and Proliferative and Evoked-Calcium Changes in SH-SY5Y Cells by Imidacloprid and Acetamiprid Insecticides

Acetamiprid (ACE) and Imidacloprid (IMI) are widely-used neonicotinoid insecticides (NNIs) with functional activity at human acetylcholine nicotinic receptors and, therefore, with putative toxic effects. The objective of this study was the evaluation of the interactions between NNIs and α7-nAChR, as this receptor keeps intracellular Ca2+ ([Ca2+]i) to an optimum for an adequate neuronal functioning. Possible interactions between NNIs and the cryo-EM structure of the human α-7 nAChR were identified by molecular docking. Additionally, NNI effects were analyzed in neuroblastoma SH-SY5Y cells, as they naturally express α-7 nAChRs. Functional studies included proliferative/cytotoxic effects (MTT test) in undifferentiated SH-SY-5Y cells and indirect measurements of [Ca2+]i transients in retinoic acid-differentiated SH-SY-5Y cells loaded with Fluo-4 AM. Docking analysis showed that the binding of IMI and ACE occurred at the same aromatic cage that the specific α-7 nAChR agonist EVP-6124. IMI showed a better docking strength than ACE. According to the MTT assays, low doses (10-50 µM) of IMI better than ACE stimulated neuroblastoma cell proliferation. At higher doses (250-500 µM), IMI also prevailed over ACE and dose-dependently triggered more abrupt fluorescence changes due to [Ca2+]i mobilization in differentiated SH-SY5Y neurons. Indeed, only IMI blunted nicotine-evoked intracellular fluorescence stimulation (i.e., nicotine cross-desensitization). Summarizing, IMI demonstrated a superior docking strength and more robust cellular responses compared to ACE, which were likely associated with a stronger activity at α-7nAChRs. Through the interaction with α-7nAChRs, IMI would demonstrate its high neurotoxic potential for humans. More research is needed for investigating the proliferative effects of IMI in neuroblastoma cells.

Dual mutations in the whitefly nicotinic acetylcholine receptor β1 subunit confer target-site resistance to multiple neonicotinoid insecticides

Neonicotinoid insecticides, which target insect nicotinic acetylcholine receptors (nAChRs), have been widely and intensively used to control the whitefly, Bemisia tabaci, a highly damaging, globally distributed, crop pest. This has inevitably led to the emergence of populations with resistance to neonicotinoids. However, to date, there have been no reports of target-site resistance involving mutation of B. tabaci nAChR genes. Here we characterize the nAChR subunit gene family of B. tabaci and identify dual mutations (A58T&R79E) in one of these genes (BTβ1) that confer resistance to multiple neonicotinoids. Transgenic D. melanogaster, where the native nAChR Dβ1 was replaced with BTβ1A58T&R79E, were significantly more resistant to neonicotinoids than flies where Dβ1 were replaced with the wildtype BTβ1 sequence, demonstrating the causal role of the mutations in resistance. The two mutations identified in this study replace two amino acids that are highly conserved in >200 insect species. Three-dimensional modelling suggests a molecular mechanism for this resistance, whereby A58T forms a hydrogen bond with the R79E side chain, which positions its negatively-charged carboxylate group to electrostatically repulse a neonicotinoid at the orthosteric site. Together these findings describe the first case of target-site resistance to neonicotinoids in B. tabaci and provide insight into the molecular determinants of neonicotinoid binding and selectivity.

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.

Current Progress on Central Cholinergic Receptors as Therapeutic Targets for Alzheimer's Disease

Acetylcholine (ACh) is ubiquitously present in the nervous system and has been involved in the regulation of various brain functions. By modulating synaptic transmission and promoting synaptic plasticity, particularly in the hippocampus and cortex, ACh plays a pivotal role in the regulation of learning and memory. These procognitive actions of ACh are mediated by the neuronal muscarinic and nicotinic cholinergic receptors. The impairment of cholinergic transmission leads to cognitive decline associated with aging and dementia. Therefore, the cholinergic system has been of prime focus when concerned with Alzheimer's disease (AD), the most common cause of dementia. In AD, the extensive destruction of cholinergic neurons occurs by amyloid-β plaques and tau protein-rich neurofibrillary tangles. Amyloid-β also blocks cholinergic receptors and obstructs neuronal signaling. This makes the central cholinergic system an important target for the development of drugs for AD. In fact, centrally acting cholinesterase inhibitors like donepezil and rivastigmine are approved for the treatment of AD, although the outcome is not satisfactory. Therefore, identification of specific subtypes of cholinergic receptors involved in the pathogenesis of AD is essential to develop future drugs. Also, the identification of endogenous rescue mechanisms to the cholinergic system can pave the way for new drug development. In this article, we discussed the neuroanatomy of the central cholinergic system. Further, various subtypes of muscarinic and nicotinic receptors involved in the cognition and pathophysiology of AD are described in detail. The article also reviewed primary neurotransmitters that regulate cognitive processes by modulating basal forebrain cholinergic projection neurons.

Nicotinic acetylcholine receptors in cancer: Limitations and prospects

Nicotinic acetylcholine receptors (nAChRs) have long been considered to solely mediate neurotransmission. However, their widespread distribution in the human body suggests a more diverse physiological role. Additionally, the expression of nAChRs is increased in certain cancers, such as lung cancer, and has been associated with cell proliferation, epithelial-to-mesenchymal cell transition, angiogenesis and apoptosis prevention. Several compounds that interact with these receptors have been identified as potential therapeutic agents. They have been tested as drugs for treating nicotine addiction, alcoholism, depression, pain and Alzheimer's disease. This review focuses on nAChR-mediated signalling in cancer, presenting opportunities for the development of innovative nAChR-based anticancer drugs. It displays the differences in expression of each nAChR subunit between normal and cancer cells for selected cancer types, highlighting their possible involvement in specific cases. Antagonists of nAChRs that could complement existing cancer therapies are summarised and critically discussed. We hope that this review will stimulate further research on the role of nAChRs in cancer potentially leading to innovative cancer therapies.

Identification of the native Torpedo californica nicotinic acetylcholine receptor's glycan composition after a multi-step sequential purification method using MALDI-ToF MS

The Cys-loop pentameric ligand-gated ion channels comprise a dynamic group of proteins that have been extensively studied for decades, yielding a wealth of findings at both the structural and functional levels. The nicotinic acetylcholine receptor (nAChR) is no exception, as it is part of this large protein family involved in proper organismal function. Our efforts have successfully produced a highly pure nAChR in detergent complex (nAChR-DC), enabling more robust studies to be conducted on it, including beginning to experiment with high-throughput crystallization. Our homogeneous product has been identified and extensively characterized with 100% identity using Nano Lc MS/MS and MALDI ToF/ToF for each nAChR subunit. Additionally, the N-linked glycans in the Torpedo californica-nAChR (Tc-nAChR) subunits have been identified. To study this, the Tc-nAChR subunits were digested with PNGase F and the released glycans were analyzed by MALDI-ToF. The MS results showed the presence of high-mannose N-glycan in all native Tc-nAChR subunits. Specifically, the oligommanose population Man8-9GlcNac2 with peaks at m/z 1742 and 1904 ([M + Na]+ ions) were observed.

Alchemical Free Energy Calculations on Membrane-Associated Proteins

Membrane proteins have diverse functions within cells and are well-established drug targets. The advances in membrane protein structural biology have revealed drug and lipid binding sites on membrane proteins, while computational methods such as molecular simulations can resolve the thermodynamic basis of these interactions. Particularly, alchemical free energy calculations have shown promise in the calculation of reliable and reproducible binding free energies of protein-ligand and protein-lipid complexes in membrane-associated systems. In this review, we present an overview of representative alchemical free energy studies on G-protein-coupled receptors, ion channels, transporters as well as protein-lipid interactions, with emphasis on best practices and critical aspects of running these simulations. Additionally, we analyze challenges and successes when running alchemical free energy calculations on membrane-associated proteins. Finally, we highlight the value of alchemical free energy calculations calculations in drug discovery and their applicability in the pharmaceutical industry.

Discovery of a Novel Class of Benzimidazole-Based Nicotinic Acetylcholine Receptor Modulators: Positive and Negative Modulation Arising from Overlapping Allosteric Sites

Here, we present the discovery of a novel class of benzimidazole-based allosteric modulators of nicotinic acetylcholine receptors (nAChRs). The modulators were developed based on a compound (1) exhibiting positive modulatory activity at α4β2 nAChR in a compound library screening by functional characterization of 100 analogues of 1 at nAChRs. Two distinct series of positive and negative allosteric modulators (PAMs and NAMs, respectively) comprising benzimidazole as a shared structural moiety emerged from this SAR study. The PAMs mediated weak modulation of α4β2 and α6β2β3, whereas the NAMs exhibited essentially equipotent inhibition of α4β2, α6β2β3, α6β4β3, and α3β4 nAChRs, with analogue 9j [2-(2,4-dichlorophenoxy)-1,3-dimethyl-1-H-benzo[d]imidazole-3-ium] displaying high-nanomolar and low-micromolar IC50 values at the β2- and β4-containing receptor subtypes, respectively. We propose that the PAMs and NAMs act through overlapping sites in the nAChR, and these findings thus underline the heterogenous modes of modulation that can arise from a shared allosteric site in the receptor.

Advances in small molecule selective ligands for heteromeric nicotinic acetylcholine receptors

The study of nicotinic acetylcholine receptors (nAChRs) has significantly progressed in the last decade, due to a) the improved techniques available for structural studies; b) the identification of ligands interacting at orthosteric and allosteric recognition sites on the nAChR proteins, able to tune channel conformational states; c) the better functional characterization of receptor subtypes/subunits and their therapeutic potential; d) the availability of novel pharmacological agents able to activate or block nicotinic-mediated cholinergic responses with subtype or stoichiometry selectivity. The copious literature on nAChRs is related to the pharmacological profile of new, promising subtype selective derivatives as well as the encouraging preclinical and early clinical evaluation of known ligands. However, recently approved therapeutic derivatives are still missing, and examples of ligands discontinued in advanced CNS clinical trials include drug candidates acting at both neuronal homomeric and heteromeric receptors. In this review, we have selected heteromeric nAChRs as the target and comment on literature reports of the past five years dealing with the discovery of new small molecule ligands or the advanced pharmacological/preclinical investigation of more promising compounds. The results obtained with bifunctional nicotinic ligands and a light-activated ligand as well as the applications of promising radiopharmaceuticals for heteromeric subtypes are also discussed.

Agonist efficiency links binding and gating in a nicotinic receptor

Receptors signal by switching between resting (C) and active (O) shapes ('gating') under the influence of agonists. The receptor's maximum response depends on the difference in agonist binding energy, O minus C. In nicotinic receptors, efficiency (η) represents the fraction of agonist binding energy applied to a local rearrangement (an induced fit) that initiates gating. In this receptor, free energy changes in gating and binding can be interchanged by the conversion factor η. Efficiencies estimated from concentration-response curves (23 agonists, 53 mutations) sort into five discrete classes (%): 0.56 (17), 0.51(32), 0.45(13), 0.41(26), and 0.31(12), implying that there are 5 C versus O binding site structural pairs. Within each class efficacy and affinity are corelated linearly, but multiple classes hide this relationship. η unites agonist binding with receptor gating and calibrates one link in a chain of coupled domain rearrangements that comprises the allosteric transition of the protein.

Structural interplay of anesthetics and paralytics on muscle nicotinic receptors

General anesthetics and neuromuscular blockers are used together during surgery to stabilize patients in an unconscious state. Anesthetics act mainly by potentiating inhibitory ion channels and inhibiting excitatory ion channels, with the net effect of dampening nervous system excitability. Neuromuscular blockers act by antagonizing nicotinic acetylcholine receptors at the motor endplate; these excitatory ligand-gated ion channels are also inhibited by general anesthetics. The mechanisms by which anesthetics and neuromuscular blockers inhibit nicotinic receptors are poorly understood but underlie safe and effective surgeries. Here we took a direct structural approach to define how a commonly used anesthetic and two neuromuscular blockers act on a muscle-type nicotinic receptor. We discover that the intravenous anesthetic etomidate binds at an intrasubunit site in the transmembrane domain and stabilizes a non-conducting, desensitized-like state of the channel. The depolarizing neuromuscular blocker succinylcholine also stabilizes a desensitized channel but does so through binding to the classical neurotransmitter site. Rocuronium binds in this same neurotransmitter site but locks the receptor in a resting, non-conducting state. Together, this study reveals a structural mechanism for how general anesthetics work on excitatory nicotinic receptors and further rationalizes clinical observations in how general anesthetics and neuromuscular blockers interact.

Knockdown of the Nicotinic Acetylcholine Receptor β1 Subunit Decreases the Susceptibility to Five Neonicotinoid Insecticides in Whitefly (Bemisia tabaci)

The sweet potato whitefly, Bemisia tabaci, (Gennadius) (Hemiptera:Aleyrodidae) is a global pest of crops. Neonicotinoids are efficient insecticides used for control of this pest. Insecticidal targets of neonicotinoids are insect nicotinic acetylcholine receptors (nAChRs). Here, we characterized and cloned the full length of the nAChR β1 subunit (BTβ1) in B. tabaci and confirmed the consistency of BTβ1 in B. tabaci MEAM1 and MED. Expression levels of BTβ1 in different developmental stages and body parts of adults were investigated and compared in B. tabaci MED. dsRNA was prepared to knock down BTβ1 in adult B. tabaci and significantly decreases the susceptibility to five neonicotinoid insecticides, including imidacloprid, clothianidin, thiacloprid, nitenpyram, and dinotefuran. This study indicated BTβ1 as a notable site influencing the susceptibility of B. tabaci to neonicotinoids.

Autoimmunity to Neuronal Nicotinic Acetylcholine Receptors

Nicotinic acetylcholine receptors (nAChRs) are widely expressed in many and diverse cell types, participating in various functions of cells, tissues and systems. In this review, we focus on the autoimmunity against neuronal nAChRs, the specific autoantibodies and their mechanisms of pathological action in selected autoimmune diseases. We summarize the current relevant knowledge from human diseases as well as from experimental models of autoimmune neurological disorders related to antibodies against neuronal nAChR subunits. Despite the well-studied high immunogenicity of the muscle nAChRs where autoantibodies are the main pathogen of myasthenia gravis, autoimmunity to neuronal nAChRs seems infrequent, except for the autoantibodies to the ganglionic receptor, the α3 subunit containing nAChR (α3-nAChR), which are detected and are likely pathogenic in Autoimmune Autonomic Ganglionopathy (AAG). We describe the detection, presence and function of these antibodies and especially the recent development of a cell-based assay (CBA) which, contrary to until recently available assays, is highly specific for AAG. Rare reports of autoantibodies to the other neuronal nAChR subtypes include a few cases of antibodies to α7 and/or α4β2 nAChRs in Rasmussen encephalitis, schizophrenia, autoimmune meningoencephalomyelitis, and in some myasthenia gravis patients with concurrent CNS symptoms. Neuronal-type nAChRs are also present in several non-excitable tissues, however the presence and possible role of antibodies against them needs further verification. It is likely that the future development of more sensitive and disease-specific assays would reveal that neuronal nAChR autoantibodies are much more frequent and may explain the mechanisms of some seronegative autoimmune diseases.

Loop2 Size Modification Reveals Significant Impacts on the Potency of α-Conotoxin TxID

α4/6-conotoxin TxID, which was identified from Conus textile, simultaneously blocks rat (r) α3β4 and rα6/α3β4 nicotinic acetylcholine receptors (nAChRs) with IC₅₀ values of 3.6 nM and 33.9 nM, respectively. In order to identify the effects of loop2 size on the potency of TxID, alanine (Ala) insertion and truncation mutants were designed and synthesized in this study. An electrophysiological assay was used to evaluate the activity of TxID and its loop2-modified mutants. The results showed that the inhibition of 4/7-subfamily mutants [+9A]TxID, [+10A]TxID, [+14A]TxID, and all the 4/5-subfamily mutants against rα3β4 and rα6/α3β4 nAChRs decreased. Overall, ala-insertion or truncation of the 9th, 10th, and 11th amino acid results in a loss of inhibition and the truncation of loop2 has more obvious impacts on its functions. Our findings have strengthened the understanding of α-conotoxin, provided guidance for further modifications, and offered a perspective for future studies on the molecular mechanism of the interaction between α-conotoxins and nAChRs.

Silent agonists for α7 nicotinic acetylcholine receptors

We discuss models for the activation and desensitization of α7 nicotinic acetylcholine receptors (nAChRs) and the effects of efficacious type II positive allosteric modulators (PAMs) that destabilize α7 desensitized states. Type II PAMs such as PNU-120596 can be used to distinguish inactive compounds from silent agonists, compounds that produce little or no channel activation but stabilize the non-conducting conformations associated with desensitization. We discuss the effects of α7 nAChRs in cells of the immune system and their roles in modulating inflammation and pain through what has come to be known as the cholinergic anti-inflammatory system (CAS). Cells controlling CAS do not generate ion channel currents but rather respond to α7 drugs by modulating intracellular signaling pathways analogous to the effects of metabotropic receptors. Metabotropic signaling by α7 receptors appears to be mediated by receptors in nonconducting conformations and can be accomplished by silent agonists. We discuss electrophysiological structure-activity relationships for α7 silent agonists and their use in cell-based and in vivo assays for CAS regulation. We discuss the strongly desensitizing partial agonist GTS-21 and its effectiveness in modulation of CAS. We also review the properties of the silent agonist NS6740, which is remarkably effective at maintaining α7 receptors in PAM-sensitive desensitized states. Most silent agonists bind to sites overlapping those for orthosteric agonists, but some appear to bind to allosteric sites. Finally, we discuss α9^⁎ nAChRs and their potential role in CAS, and ligands that will be useful in defining and distinguishing the specific roles of α7 and α9 in CAS.

Signaling of nicotinic acetylcholine receptors in mononuclear phagocytes

Nicotinic acetylcholine receptors are not only expressed by the nervous system and at the neuro-muscular junction but also by mononuclear phagocytes, which belong to the innate immune system. Mononuclear phagocyte is an umbrella term for monocytes, macrophages, and dendritic cells. These cells play pivotal roles in host defense against infection but also in numerous often debilitating diseases that are characterized by exuberant inflammation. Nicotinic acetylcholine receptors of the neuronal type dominate in these cells, and their stimulation is mainly associated with anti-inflammatory effects. Although the cholinergic modulation of mononuclear phagocytes is of eminent clinical relevance for the prevention and treatment of inflammatory diseases and neuropathic pain, we are only beginning to understand the underlying mechanisms on the molecular level. The purpose of this review is to report and critically discuss the current knowledge on signal transduction mechanisms elicited by nicotinic acetylcholine receptors in mononuclear phagocytes.

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.

When is a hydrophobic gate not a hydrophobic gate?

The flux of ions through a channel is most commonly regulated by changes that result in steric occlusion of its pore. However, ion permeation can also be prevented by formation of a desolvation barrier created by hydrophobic residues that line the pore. As a result of relatively minor structural changes, confined hydrophobic regions in channels may undergo transitions between wet and dry states to gate the pore closed without physical constriction of the permeation pathway. This concept is referred to as hydrophobic gating, and many examples of this process have been demonstrated. However, the term is also now being used in a much broader context that often deviates from its original meaning. In this Viewpoint, we explore the formal definition of a hydrophobic gate, discuss examples of this process compared with other gating mechanisms that simply exploit hydrophobic residues and/or lipids in steric closure of the pore, and describe the best practice for identification of a hydrophobic gate.

Delineating the activity of the potent nicotinic acetylcholine receptor agonists (+)-anatoxin-a and (−)-hosieine-A

The affinity and thermodynamic parameters for the interactions of two naturally occurring neurotoxins, (+)-anatoxin-a and (−)-hosieine-A, with acetylcholine-binding protein were investigated using a fluorescence-quenching assay and isothermal titration calorimetry. The crystal structures of their complexes with acetylcholine-binding protein from Aplysia californica (AcAChBP) were determined and reveal details of molecular recognition in the orthosteric binding site. Comparisons treating AcAChBP as a surrogate for human α4β2 and α7 nicotinic acetylcholine receptors (nAChRs) suggest that the molecular features involved in ligand recognition and affinity for the protein targets are conserved. The ligands exploit interactions with similar residues as the archetypal nAChR agonist nicotine, but with greater affinity. (−)-Hosieine-A in particular has a high affinity for AcAChBP driven by a favorable entropic contribution to binding. The ligand affinities help to rationalize the potent biological activity of these alkaloids. The structural data, together with comparisons with related molecules, suggest that there may be opportunities to extend the hosieine-A scaffold to incorporate new interactions with the complementary side of the orthosteric binding site. Such a strategy may guide the design of new entities to target human α4β2 nAChR that may have therapeutic benefit.

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.

Nicotinic receptors modulate antitumor therapy response in triple negative breast cancer cells

BACKGROUND

Triple negative breast cancer is more aggressive than other breast cancer subtypes and constitutes a public health problem worldwide since it has high morbidity and mortality due to the lack of defined therapeutic targets. Resistance to chemotherapy complicates the course of patients’ treatment. Several authors have highlighted the participation of nicotinic acetylcholine receptors (nAChR) in the modulation of conventional chemotherapy treatment in cancers of the airways. However, in breast cancer, less is known about the effect of nAChR activation by nicotine on chemotherapy treatment in smoking patients.

AIM

To investigate the effect of nicotine on paclitaxel treatment and the signaling pathways involved in human breast MDA-MB-231 tumor cells.

METHODS

Cells were treated with paclitaxel alone or in combination with nicotine, administered for one or three 48-h cycles. The effect of the addition of nicotine (at a concentration similar to that found in passive smokers’ blood) on the treatment with paclitaxel (at a therapeutic concentration) was determined using the 3-(4,5 dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. The signaling mediators involved in this effect were determined using selective inhibitors. We also investigated nAChR expression, and ATP “binding cassette” G2 drug transporter (ABCG2) expression and its modulation by the different treatments with Western blot. The effect of the treatments on apoptosis induction was determined by flow cytometry using annexin-V and 7AAD markers.

RESULTS

Our results confirmed that treatment with paclitaxel reduced MDA-MB-231 cell viability in a concentration-dependent manner and that the presence of nicotine reversed the cytotoxic effect induced by paclitaxel by involving the expression of functional α7 and α9 nAChRs in these cells. The action of nicotine on paclitaxel treatment was linked to modulation of the protein kinase C, mitogen-activated protein kinase, extracellular signal-regulated kinase, and NF-κB signaling pathways, and to an up-regulation of ABCG2 protein expression. We also detected that nicotine significantly reduced the increase in cell apoptosis induced by paclitaxel treatment. Moreover, the presence of nicotine reduced the efficacy of paclitaxel treatment administered in three cycles to MDA-MB-231 tumor cells.

CONCLUSION

Our findings point to nAChRs as responsible for the decrease in the chemotherapeutic effect of paclitaxel in triple negative tumors. Thus, nAChRs should be considered as targets in smoking patients.

Strychnine and its mono- and dimeric analogues: a pharmaco-chemical perspective

Covering: up to November 2021Since its isolation in 1818, strychnine has attracted the attention of a plethora of chemists and pharmacologists who have established its structure, developed total syntheses, and examined its complex pharmacology. While numerous reviews on structure elucidation and total synthesis of strychnine are available, reports on structure-activity relationships (SARs) of this fascinating alkaloid are rare. In this review, we present and discuss structures, synthetic approaches, metabolic transformations, and the diverse pharmacological actions of strychnine and its mono- and dimeric analogues. Particular attention is given to its SARs at glycine receptors (GlyRs) in light of recently published high-resolution structures of strychnine-GlyR complexes. Other pharmacological actions of strychnine and its derivatives, such as their antagonistic properties at nicotinic acetylcholine receptors (nAChRs), allosteric modulation of muscarinic acetylcholine receptors as well as anti-cancer and anti-plasmodial effects are also critically reviewed, and possible future developments in the field are discussed.

Structural mechanism of muscle nicotinic receptor desensitization and block by curare

Binding of the neurotransmitter acetylcholine to its receptors on muscle fibers depolarizes the membrane and thereby triggers muscle contraction. We sought to understand at the level of three-dimensional structure how agonists and antagonists alter nicotinic acetylcholine receptor conformation. We used the muscle-type receptor from the Torpedo ray to first define the structure of the receptor in a resting, activatable state. We then determined the receptor structure bound to the agonist carbachol, which stabilizes an asymmetric, closed channel desensitized state. We find conformational changes in a peripheral membrane helix are tied to recovery from desensitization. To probe mechanisms of antagonism, we obtained receptor structures with the active component of curare, a poison arrow toxin and precursor to modern muscle relaxants. d-Tubocurarine stabilizes the receptor in a desensitized-like state in the presence and absence of agonist. These findings define the transitions between resting and desensitized states and reveal divergent means by which antagonists block channel activity of the muscle-type nicotinic receptor.

Conformational transitions and ligand-binding to a muscle-type nicotinic acetylcholine receptor

Fast synaptic communication requires receptors that respond to the presence of neurotransmitter by opening an ion channel across the post-synaptic membrane. The muscle-type nicotinic acetylcholine receptor from the electric fish, Torpedo, is the prototypic ligand-gated ion channel, yet the structural changes underlying channel activation remain undefined. Here we use cryo-EM to solve apo and agonist-bound structures of the Torpedo nicotinic receptor embedded in a lipid nanodisc. Using both a direct biochemical assay to define the conformational landscape and molecular dynamics simulations to assay flux through the pore, we correlate structures with functional states and elucidate the motions that lead to pore activation of a heteromeric nicotinic receptor. We highlight an underappreciated role for the complementary subunit in channel gating, establish the structural basis for the differential agonist affinities of α/δ versus α /γ sites, and explain why nicotine is less potent at muscle nicotinic receptors compared to neuronal ones.

α7 nicotinic acetylcholine receptors in the hippocampal circuit: taming complexity

Cholinergic innervation of the hippocampus uses the neurotransmitter acetylcholine (ACh) to coordinate neuronal circuit activity while simultaneously influencing the function of non-neuronal cell types. The α7 nicotinic ACh receptor (nAChR) subtype is highly expressed throughout the hippocampus, has the highest calcium permeability compared with other subtypes of nAChRs, and is of high therapeutic interest due to its association with a variety of neurological disorders and neurodegenerative diseases. In this review, we synthesize research describing α7 nAChR properties, function, and relationship to cognitive dysfunction within the hippocampal circuit and highlight approaches to help improve therapeutic development.

Antagonistic Mechanism of α-Conotoxin BuIA toward the Human α3β2 Nicotinic Acetylcholine Receptor

Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels that are abundantly expressed in the central and peripheral nervous systems, playing an important role in mediating neurotransmitter release and inter-synaptic signaling. Dysfunctional nAChRs are associated with neurological disorders, and studying the structure and function of nAChRs is essential for development of drugs or strategies for treatment of related diseases. α-Conotoxins are selective antagonists of the nAChR and are an important class of drug leads. So far, the antagonistic mechanism of α-conotoxins toward the nAChRs is still unclear. In this study, we built an α3β2 nAChR homology model and investigated its conformational transition mechanism upon binding with a highly potent inhibitor, α-conotoxin BuIA, through μs molecular dynamic simulations and site-directed mutagenesis studies. The results suggested that the α3β2 nAChR underwent global conformational transitions and was stabilized into a closed state with three hydrophobic gates present in the transmembrane domain by BuIA. Finally, the probable antagonistic mechanism of BuIA was proposed. Overall, the closed-state model of the α3β2 nAChR bound with BuIA is not only essential for understanding the antagonistic mechanism of α-conotoxins but also particularly valuable for development of therapeutic inhibitors in future.

Towards the Idea of Molecular Brains

How can single cells without nervous systems perform complex behaviours such as habituation, associative learning and decision making, which are considered the hallmark of animals with a brain? Are there molecular systems that underlie cognitive properties equivalent to those of the brain? This review follows the development of the idea of molecular brains from Darwin’s “root brain hypothesis”, through bacterial chemotaxis, to the recent discovery of neuron-like r-protein networks in the ribosome. By combining a structural biology view with a Bayesian brain approach, this review explores the evolutionary labyrinth of information processing systems across scales. Ribosomal protein networks open a window into what were probably the earliest signalling systems to emerge before the radiation of the three kingdoms. While ribosomal networks are characterised by long-lasting interactions between their protein nodes, cell signalling networks are essentially based on transient interactions. As a corollary, while signals propagated in persistent networks may be ephemeral, networks whose interactions are transient constrain signals diffusing into the cytoplasm to be durable in time, such as post-translational modifications of proteins or second messenger synthesis. The duration and nature of the signals, in turn, implies different mechanisms for the integration of multiple signals and decision making. Evolution then reinvented networks with persistent interactions with the development of nervous systems in metazoans. Ribosomal protein networks and simple nervous systems display architectural and functional analogies whose comparison could suggest scale invariance in information processing. At the molecular level, the significant complexification of eukaryotic ribosomal protein networks is associated with a burst in the acquisition of new conserved aromatic amino acids. Knowing that aromatic residues play a critical role in allosteric receptors and channels, this observation suggests a general role of π systems and their interactions with charged amino acids in multiple signal integration and information processing. We think that these findings may provide the molecular basis for designing future computers with organic processors.

Constructing and Tuning Excitatory Cholinergic Synapses: The Multifaceted Functions of Nicotinic Acetylcholine Receptors in Drosophila Neural Development and Physiology

Nicotinic acetylcholine receptors (nAchRs) are widely distributed within the nervous system across most animal species. Besides their well-established roles in mammalian neuromuscular junctions, studies using invertebrate models have also proven fruitful in revealing the function of nAchRs in the central nervous system. During the earlier years, both in vitro and animal studies had helped clarify the basic molecular features of the members of the Drosophila nAchR gene family and illustrated their utility as targets for insecticides. Later, increasingly sophisticated techniques have illuminated how nAchRs mediate excitatory neurotransmission in the Drosophila brain and play an integral part in neural development and synaptic plasticity, as well as cognitive processes such as learning and memory. This review is intended to provide an updated survey of Drosophila nAchR subunits, focusing on their molecular diversity and unique contributions to physiology and plasticity of the fly neural circuitry. We will also highlight promising new avenues for nAchR research that will likely contribute to better understanding of central cholinergic neurotransmission in both Drosophila and other organisms.

Nicotinic receptors: From protein allostery to computational neuropharmacology

We propose an extension and further development of the Monod-Wyman-Changeux model for allosteric transitions of regulatory proteins to brain communications and specifically to neurotransmitters receptors, with the nicotinic acetylcholine receptor (nAChR) as a model of ligand-gated ion channels. The present development offers an expression of the change of the gating isomerization constant caused by pharmacological ligand binding in terms of its value in the absence of ligands and several "modulation factors", which vary with orthosteric ligand binding (agonists/antagonists), allosteric ligand binding (positive allosteric modulators/negative allosteric modulators) and receptor desensitization. The new - explicit - formulation of such "modulation factors", provides expressions for the pharmacological attributes of potency, efficacy, and selectivity for the modulatory ligands (including endogenous neurotransmitters) in terms of their binding affinity for the active, resting, and desensitized states of the receptor. The current formulation provides ways to design neuroactive compounds with a controlled pharmacological profile, opening the field of computational neuro-pharmacology.

Nicotinic acetylcholine receptor redux: Discovery of accessories opens therapeutic vistas

The neurotransmitter acetylcholine (ACh) acts in part through a family of nicotinic ACh receptors (nAChRs), which mediate diverse physiological processes including muscle contraction, neurotransmission, and sensory transduction. Pharmacologically, nAChRs are responsible for tobacco addiction and are targeted by medicines for hypertension and dementia. Nicotinic AChRs were the first ion channels to be isolated. Recent studies have identified molecules that control nAChR biogenesis, trafficking, and function. These nAChR accessories include protein and chemical chaperones as well as auxiliary subunits. Whereas some factors act on many nAChRs, others are receptor specific. Discovery of these regulatory mechanisms is transforming nAChR research in cells and tissues ranging from central neurons to spinal ganglia to cochlear hair cells. Nicotinic AChR-specific accessories also enable drug discovery on high-confidence targets for psychiatric, neurological, and auditory disorders.

Multiple Sources of Cholinergic Input to the Superior Olivary Complex

The superior olivary complex (SOC) is a major computation center in the brainstem auditory system. Despite previous reports of high expression levels of cholinergic receptors in the SOC, few studies have addressed the functional role of acetylcholine in the region. The source of the cholinergic innervation is unknown for all but one of the nuclei of the SOC, limiting our understanding of cholinergic modulation. The medial nucleus of the trapezoid body, a key inhibitory link in monaural and binaural circuits, receives cholinergic input from other SOC nuclei and also from the pontomesencephalic tegmentum. Here, we investigate whether these same regions are sources of cholinergic input to other SOC nuclei. We also investigate whether individual cholinergic cells can send collateral projections bilaterally (i.e., into both SOCs), as has been shown at other levels of the subcortical auditory system. We injected retrograde tract tracers into the SOC in gerbils, then identified retrogradely-labeled cells that were also immunolabeled for choline acetyltransferase, a marker for cholinergic cells. We found that both the SOC and the pontomesencephalic tegmentum (PMT) send cholinergic projections into the SOC, and these projections appear to innervate all major SOC nuclei. We also observed a small cholinergic projection into the SOC from the lateral paragigantocellular nucleus of the reticular formation. These various sources likely serve different functions; e.g., the PMT has been associated with things such as arousal and sensory gating whereas the SOC may provide feedback more closely tuned to specific auditory stimuli. Further, individual cholinergic neurons in each of these regions can send branching projections into both SOCs. Such projections present an opportunity for cholinergic modulation to be coordinated across the auditory brainstem.

Elephants in the Dark: Insights and Incongruities in Pentameric Ligand-gated Ion Channel Models

The superfamily of pentameric ligand-gated ion channels (pLGICs) comprises key players in electrochemical signal transduction across evolution, including historic model systems for receptor allostery and targets for drug development. Accordingly, structural studies of these channels have steadily increased, and now approach 250 depositions in the protein data bank. This review contextualizes currently available structures in the pLGIC family, focusing on morphology, ligand binding, and gating in three model subfamilies: the prokaryotic channel GLIC, the cation-selective nicotinic acetylcholine receptor, and the anion-selective glycine receptor. Common themes include the challenging process of capturing and annotating channels in distinct functional states; partially conserved gating mechanisms, including remodeling at the extracellular/transmembrane-domain interface; and diversity beyond the protein level, arising from posttranslational modifications, ligands, lipids, and signaling partners. Interpreting pLGIC structures can be compared to describing an elephant in the dark, relying on touch alone to comprehend the many parts of a monumental beast: each structure represents a snapshot in time under specific experimental conditions, which must be integrated with further structure, function, and simulations data to build a comprehensive model, and understand how one channel may fundamentally differ from another.

Cholinergic boutons are closely associated with excitatory cells and four subtypes of inhibitory cells in the inferior colliculus

Acetylcholine (ACh) is a neuromodulator that has been implicated in multiple roles across the brain, including the central auditory system, where it sets neuronal excitability and gain and affects plasticity. In the cerebral cortex, subtypes of GABAergic interneurons are modulated by ACh in a subtype-specific manner. Subtypes of GABAergic neurons have also begun to be described in the inferior colliculus (IC), a midbrain hub of the auditory system. Here, we used male and female mice (Mus musculus) that express fluorescent protein in cholinergic cells, axons, and boutons to look at the association between ACh and four subtypes of GABAergic IC cells that differ in their associations with extracellular markers, their soma sizes, and their distribution within the IC. We found that most IC cells, including excitatory and inhibitory cells, have cholinergic boutons closely associated with their somas and proximal dendrites. We also found that similar proportions of each of four subtypes of GABAergic cells are closely associated with cholinergic boutons. Whether the different types of GABAergic cells in the IC are differentially regulated remains unclear, as the response of cells to ACh is dependent on which types of ACh receptors are present. Additionally, this study confirms the presence of these four subtypes of GABAergic cells in the mouse IC, as they had previously been identified only in guinea pigs. These results suggest that cholinergic projections to the IC modulate auditory processing via direct effects on a multitude of inhibitory circuits.

Potency- and Selectivity-Enhancing Mutations of Conotoxins for Nicotinic Acetylcholine Receptors Can Be Predicted Using Accurate Free-Energy Calculations

Nicotinic acetylcholine receptor (nAChR) subtypes are key drug targets, but it is challenging to pharmacologically differentiate between them because of their highly similar sequence identities. Furthermore, α-conotoxins (α-CTXs) are naturally selective and competitive antagonists for nAChRs and hold great potential for treating nAChR disorders. Identifying selectivity-enhancing mutations is the chief aim of most α-CTX mutagenesis studies, although doing so with traditional docking methods is difficult due to the lack of α-CTX/nAChR crystal structures. Here, we use homology modeling to predict the structures of α-CTXs bound to two nearly identical nAChR subtypes, α3β2 and α3β4, and use free-energy perturbation (FEP) to re-predict the relative potency and selectivity of α-CTX mutants at these subtypes. First, we use three available crystal structures of the nAChR homologue, acetylcholine-binding protein (AChBP), and re-predict the relative affinities of twenty point mutations made to the α-CTXs LvIA, LsIA, and GIC, with an overall root mean square error (RMSE) of 1.08 ± 0.15 kcal/mol and an R2 of 0.62, equivalent to experimental uncertainty. We then use AChBP as a template for α3β2 and α3β4 nAChR homology models bound to the α-CTX LvIA and re-predict the potencies of eleven point mutations at both subtypes, with an overall RMSE of 0.85 ± 0.08 kcal/mol and an R2 of 0.49. This is significantly better than the widely used molecular mechanics-generalized born/surface area (MM-GB/SA) method, which gives an RMSE of 1.96 ± 0.24 kcal/mol and an R2 of 0.06 on the same test set. Next, we demonstrate that FEP accurately classifies α3β2 nAChR selective LvIA mutants while MM-GB/SA does not. Finally, we use FEP to perform an exhaustive amino acid mutational scan of LvIA and predict fifty-two mutations of LvIA to have greater than 100X selectivity for the α3β2 nAChR. Our results demonstrate the FEP is well-suited to accurately predict potency- and selectivity-enhancing mutations of α-CTXs for nAChRs and to identify alternative strategies for developing selective α-CTXs.

Venom-Derived Neurotoxins Targeting Nicotinic Acetylcholine Receptors

Acetylcholine was the first neurotransmitter described. The receptors targeted by acetylcholine are found within organisms spanning different phyla and position themselves as very attractive targets for predation, as well as for defense. Venoms of snakes within the Elapidae family, as well as those of marine snails within the Conus genus, are particularly rich in proteins and peptides that target nicotinic acetylcholine receptors (nAChRs). Such compounds are invaluable tools for research seeking to understand the structure and function of the cholinergic system. Proteins and peptides of venomous origin targeting nAChR demonstrate high affinity and good selectivity. This review aims at providing an overview of the toxins targeting nAChRs found within venoms of different animals, as well as their activities and the structural determinants important for receptor binding.