Isoform-specific mechanisms of α3β4*-nicotinic acetylcholine receptor modulation by the prototoxin lynx1

Scalable production of recombinant three-finger proteins: from inclusion bodies to high quality molecular probes

BACKGROUND

The three-finger proteins are a collection of disulfide bond rich proteins of great biomedical interests. Scalable recombinant expression and purification of bioactive three-finger proteins is quite difficult.

RESULTS

We introduce a working pipeline for expression, purification and validation of disulfide-bond rich three-finger proteins using E. coli as the expression host. With this pipeline, we have successfully obtained highly purified and bioactive recombinant α-Βungarotoxin, k-Bungarotoxin, Hannalgesin, Mambalgin-1, α-Cobratoxin, MTα, Slurp1, Pate B etc. Milligrams to hundreds of milligrams of recombinant three finger proteins were obtained within weeks in the lab. The recombinant proteins showed specificity in binding assay and six of them were crystallized and structurally validated using X-ray diffraction protein crystallography.

CONCLUSIONS

Our pipeline allows refolding and purifying recombinant three finger proteins under optimized conditions and can be scaled up for massive production of three finger proteins. As many three finger proteins have attractive therapeutic or research interests and due to the extremely high quality of the recombinant three finger proteins we obtained, our method provides a competitive alternative to either their native counterparts or chemically synthetic ones and should facilitate related research and applications.

Analogs of α-conotoxin PnIC selectively inhibit α7β2- over α7-only subtype nicotinic acetylcholine receptors via a novel allosteric mechanism

This study was undertaken to identify and characterize the first ligands capable of selectively identifying nicotinic acetylcholine receptors containing α7 and β2 subunits (α7β2-nAChR subtype). Basal forebrain cholinergic neurons express α7β2-nAChR. Here, they appear to mediate neuronal dysfunction induced by the elevated levels of oligomeric amyloid-β associated with early Alzheimer's disease. Additional work indicates that α7β2-nAChR are expressed across several further critically important cholinergic and GABAergic neuronal circuits within the central nervous system. Further studies, however, are significantly hindered by the inability of currently available ligands to distinguish heteromeric α7β2-nAChR from the closely related and more widespread homomeric α7-only-nAChR subtype. Functional screening using two-electrode voltage-clamp electrophysiology identified a family of α7β2-nAChR-selective analogs of α-conotoxin PnIC (α-CtxPnIC). A combined electrophysiology, functional kinetics, site-directed mutagenesis, and molecular dynamics approach was used to further characterize the α7β2-nAChR selectivity and site of action of these α-CtxPnIC analogs. We determined that α7β2-nAChR selectivity of α-CtxPnIC analogs arises from interactions at a site distinct from the orthosteric agonist-binding site shared between α7β2- and α7-only-nAChR. As numerous previously identified α-Ctx ligands are competitive antagonists of orthosteric agonist-binding sites, this study profoundly expands the scope of use of α-Ctx ligands (which have already provided important nAChR research and translational breakthroughs). More immediately, analogs of α-CtxPnIC promise to enable, for the first time, both comprehensive mapping of the distribution of α7β2-nAChR and detailed investigations of their physiological roles.

Lynx1 and the family of endogenous mammalian neurotoxin-like proteins and their roles in modulating nAChR function

The promise of nicotinic receptors as a therapeutic target has yet to be fully realized, despite solid data supporting their involvement in neurological and neuropsychiatric diseases. The reasons for this are likely complex and manifold, having to do with the widespread action of the cholinergic system and the biophysical mechanism of action of nicotinic receptors leading to fast desensitization and down-regulation. Conventional drug development strategies tend to focus on receptor subtype-specific action of candidate therapeutics, although the broad agonist, nicotine, is being explored in the clinic. The potential negative effects of nicotine make the search for alternate strategies warranted. Prototoxins are a promising yet little-explored avenue of nicotinic receptor drug development. Nicotinic receptors in the brain belong to a complex of proteins, including those that bind to the extracellular face of the receptor, as well as chaperones that bind the intracellular domain, etc. Lynx prototoxins have allosteric modularity effects on receptor function and number and have been implicated in complex in vivo processes such as neuroplasticity, learning, and memory. Their mechanism of action and binding specificity on sets of nAChR subtypes present intriguing possibilities for more efficacious and nuanced therapeutic targeting than nicotinic receptor subtypes alone. An allosteric drug may restrict its actions to physiologically relevant time points, which tend to be correlated with salient events which would be encoded into long-term memory storage. Rather than blanketing the brain with a steady and prolonged elevation of agonist, an allosteric nAChR compound could avoid side effects and loss of efficacy over time. This review details the potential strengths and challenges of prototoxin proteins as therapeutic targets, and some of the utility of such therapeutics based on the emerging understanding of cholinergic signaling in a growing number of complex neural processes.

A potential cost of evolving epibatidine resistance in poison frogs

BACKGROUND

Some dendrobatid poison frogs sequester the toxin epibatidine as a defense against predators. We previously identified an amino acid substitution (S108C) at a highly conserved site in a nicotinic acetylcholine receptor β2 subunit of dendrobatid frogs that decreases sensitivity to epibatidine in the brain-expressing α4β2 receptor. Introduction of S108C to the orthologous high-sensitivity human receptor similarly decreased sensitivity to epibatidine but also decreased sensitivity to acetylcholine, a potential cost if this were to occur in dendrobatids. This decrease in the acetylcholine sensitivity manifested as a biphasic acetylcholine concentration-response curve consistent with the addition of low-sensitivity receptors. Surprisingly, the addition of the β2 S108C into the α4β2 receptor of the dendrobatid Epipedobates anthonyi did not change acetylcholine sensitivity, appearing cost-free. We proposed that toxin-bearing dendrobatids may have additional amino acid substitutions protecting their receptors from alterations in acetylcholine sensitivity. To test this, in the current study, we compared the dendrobatid receptor to its homologs from two non-dendrobatid frogs.

RESULTS

The introduction of S108C into the α4β2 receptors of two non-dendrobatid frogs also does not affect acetylcholine sensitivity suggesting no additional dendrobatid-specific substitutions. However, S108C decreased the magnitude of neurotransmitter-induced currents in Epipedobates and the non-dendrobatid frogs. We confirmed that decreased current resulted from fewer receptors in the plasma membrane in Epipedobates using radiolabeled antibodies against the receptors. To test whether S108C alteration of acetylcholine sensitivity in the human receptor was due to (1) adding low-sensitivity binding sites by changing stoichiometry or (2) converting existing high- to low-sensitivity binding sites with no stoichiometric alteration, we made concatenated α4β2 receptors in stoichiometry with only high-sensitivity sites. S108C substitutions decreased maximal current and number of immunolabeled receptors but no longer altered acetylcholine sensitivity.

CONCLUSIONS

The most parsimonious explanation of our current and previous work is that the S108C substitution renders the β2 subunit less efficient in assembling/trafficking, thereby decreasing the number of receptors in the plasma membrane. Thus, while β2 S108C protects dendrobatids against sequestered epibatidine, it incurs a potential physiological cost of disrupted α4β2 receptor function.

Microglia-mediated synaptic pruning in the nucleus accumbens during adolescence: A preliminary study of the proteomic consequences and putative female-specific pruning target

Adolescence is a period of copious neural development, particularly in the 'reward' circuitry of the brain, and reward-related behavioral development, including social development. One neurodevelopmental mechanism that appears to be common across brain regions and developmental periods is the requirement for synaptic pruning to produce mature neural communication and circuits. We published that microglia-C3-mediated synaptic pruning also occurs in the nucleus accumbens (NAc) reward region during adolescence to mediate social development in male and female rats. However, both the adolescent stage in which microglial pruning occurred, and the synaptic pruning target, were sex specific. NAc pruning occurred between early and mid-adolescence in male rats to eliminate dopamine D1 receptors (D1rs), and between pre- and early adolescence in female rats (P20-30) to eliminate an unknown, non-D1r target. In this report, we sought to better understand the proteomic consequences of microglial pruning in the NAc, and what the female pruning target might be. To do this, we inhibited microglial pruning in the NAc during each sex's pruning period and collected tissue for mass spectrometry proteomic analysis and ELISA validation. We found that the proteomic consequences of inhibiting microglial pruning in the NAc were inversely proportional between the sexes, and a novel, female-specific pruning target may be Lynx1. Please note, if this preprint will be pushed further to publication it will not be by me (AMK), as I am leaving academia. So, I'm going to write more conversationally.

A potential cost of evolving epibatidine resistance in poison frogs

Background

Some poison arrow frogs sequester the toxin epibatidine as a defense against predators. We previously identified a single amino acid substitution (S108C) at a highly conserved site in a neuronal nicotinic acetylcholine receptor (nAChR) ß2 subunit that prevents epibatidine from binding to this receptor. When placed in a homologous mammalian nAChR this substitution minimized epibatidine binding but also perturbed acetylcholine binding, a clear cost. However, in the nAChRs of poison arrow frogs, this substitution appeared to have no detrimental effect on acetylcholine binding and, thus, appeared cost-free.

Results

The introduction of S108C into the α4β2 nAChRs of non-dendrobatid frogs also does not affect ACh sensitivity, when these receptors are expressed in Xenopus laevis oocytes. However, α4β2 nAChRs with C108 had a decreased magnitude of neurotransmitter-induced currents in all species tested ( Epipedobates anthonyi , non-dendrobatid frogs, as well as human), compared with α4β2 nAChRs with the conserved S108. Immunolabeling of frog or human α4β2 nAChRs in the plasma membrane using radiolabeled antibody against the β2 nAChR subunit shows that C108 significantly decreased the number of cell-surface α4β2 nAChRs, compared with S108.

Conclusions

While S108C protects these species against sequestered epibatidine, it incurs a potential physiological cost of disrupted α4β2 nAChR function. These results may explain the high conservation of a serine at this site in vertebrates, as well as provide an example of a tradeoff between beneficial and deleterious effects of an evolutionary change. They also provide important clues for future work on assembly and trafficking of this important neurotransmitter receptor.

Alkaloid ligands enable function of homomeric human α10 nicotinic acetylcholine receptors

In the nervous system, nicotinic acetylcholine receptors (nAChRs) rapidly transduce a chemical signal into one that is electrical via ligand-gated ion flux through the central channel of the receptor. However, some nAChR subunits are expressed by non-excitable cells where signal transduction apparently occurs through non-ionic mechanisms. One such nAChR subunit, α10, is present in a discreet subset of immune cells and has been implicated in pathologies including cancer, neuropathic pain, and chronic inflammation. Longstanding convention holds that human α10 subunits require co-assembly with α9 subunits for function. Here we assessed whether cholinergic ligands can enable or uncover ionic functions from homomeric α10 nAChRs. Xenopus laevis oocytes expressing human α10 subunits were exposed to a panel of ligands and examined for receptor activation using voltage-clamp electrophysiology. Functional expression of human α10 nAChRs was achieved by exposing the oocytes to the alkaloids strychnine, brucine, or methyllycaconitine. Furthermore, acute exposure to the alkaloid ligands significantly enhanced ionic responses. Acetylcholine-gated currents mediated by α10 nAChRs were potently inhibited by the snake toxins α-bungarotoxin and α-cobratoxin but not by α-conotoxins that target α9 and α9α10 nAChRs. Our findings indicate that human α10 homomers are expressed in oocytes and exposure to certain ligands can enable ionic functions. To our knowledge, this is the first demonstration that human α10 subunits can assemble as functional homomeric nAChRs. These findings have potential implications for receptor regulatory-mechanisms and will enable structural, functional, and further pharmacological characterization of human α10 nAChRs.

Expression and Roles of Lynx1, a Modulator of Cholinergic Transmission, in Skeletal Muscles and Neuromuscular Junctions in Mice

Lynx1 is a glycosylphosphatidylinositol (GPI)-linked protein shown to affect synaptic plasticity through modulation of nicotinic acetylcholine receptor (nAChR) subtypes in the brain. Because of this function and structural similarity to α-bungarotoxin, which binds muscle-specific nAChRs with high affinity, Lynx1 is a promising candidate for modulating nAChRs in skeletal muscles. However, little is known about the expression and roles of Lynx1 in skeletal muscles and neuromuscular junctions (NMJs). Here, we show that Lynx1 is expressed in skeletal muscles, increases during development, and concentrates at NMJs. We also demonstrate that Lynx1 interacts with muscle-specific nAChR subunits. Additionally, we present data indicating that Lynx1 deletion alters the response of skeletal muscles to cholinergic transmission and their contractile properties. Based on these findings, we asked if Lynx1 deletion affects developing and adult NMJs. Loss of Lynx1 had no effect on NMJs at postnatal day 9 (P9) and moderately increased their size at P21. Thus, Lynx1 plays a minor role in the structural development of NMJs. In 7- and 12-month-old mice lacking Lynx1, there is a marked increase in the incidence of NMJs with age- and disease-associated morphological alterations. The loss of Lynx1 also reduced the size of adult muscle fibers. Despite these effects, Lynx1 deletion did not alter the rate of NMJ reinnervation and stability following motor axon injury. These findings suggest that Lynx1 is not required during fast remodeling of the NMJ, as is the case during reformation following crushing of motor axons and development. Instead, these data indicate that the primary role of Lynx1 may be to maintain the structure and function of adult and aging NMJs.

Biophysical characterization of lynx-nicotinic receptor interactions using atomic force microscopy

Nicotinic acetylcholine receptors (nAChRs) are broadly expressed in the central and peripheral nervous systems, playing essential roles in cholinergic neurotransmission. The lynx family proteins, a subset of the Ly6/uPAR superfamily expressed in multiple brain regions, have been shown to bind to nAChRs and modulate their function via allosteric regulation. The binding interactions between lynx and nAChRs, however, have not been systematically quantified and compared. In this work, we characterized the interactions between lynx1 or lynx2 and α3β4- or α7-nAChRs using single-molecule atomic force microscopy (AFM). The AFM technique allows the quantification of the off-rate of lynx-nAChR binding and of the energetic barrier width between the bound state and transition state, providing a biophysical means to compare the selectivity of lynx proteins for nAChR subtypes. Results indicate that lynx1 has a marginal preference for α7- over α3β4-nAChRs. Strikingly, lynx2 exhibits a two order of magnitude stronger affinity for α3β4- compared to α7-nAChRs. Together, the AFM assay serves as a valuable tool for the biophysical characterization of lynx-nAChR binding affinities. Revealing the differential affinities of lynx proteins for nAChR subtypes will help elucidate how lynx regulates nAChR-dependent functions in the brain, including nicotine addiction and other critical pathways.

Differential Expression Patterns of Lynx Proteins and Involvement of Lynx1 in Prepulse Inhibition

Negative allosteric modulators, such as lynx1 and lynx2, directly interact with nicotinic acetylcholine receptors (nAChRs). The nAChRs are integral to cholinergic signaling in the brain and have been shown to mediate different aspects of cognitive function. Given the interaction between lynx proteins and these receptors, we examined whether these endogenous negative allosteric modulators are involved in cognitive behaviors associated with cholinergic function. We found both cell-specific and overlapping expression patterns of lynx1 and lynx2 mRNA in brain regions associated with cognition, learning, memory, and sensorimotor processing, including the prefrontal cortex (PFC), cingulate cortex, septum, hippocampus, amygdala, striatum, and pontine nuclei. Since lynx proteins are thought to play a role in conditioned associations and given the expression patterns across brain regions, we first assessed whether lynx knockout mice would differ in a cognitive flexibility task. We found no deficits in reversal learning in either the lynx1^–/– or lynx2^–/– knockout mice. Thereafter, sensorimotor gating was examined with the prepulse inhibition (PPI) assessment. Interestingly, we found that both male and female lynx1^–/– mice exhibited a deficit in the PPI behavioral response. Given the comparable expression of lynx2 in regions involved in sensorimotor gating, we then examined whether removal of the lynx2 protein would lead to similar behavioral effects. Unexpectedly, we found that while male lynx2^–/– mice exhibited a decrease in the baseline startle response, no differences were found in sensorimotor gating for either male or female lynx2^–/– mice. Taken together, these studies provide insight into the expression patterns of lynx1 and lynx2 across multiple brain regions and illustrate the modulatory effects of the lynx1 protein in sensorimotor gating.

Multidimensional Intersection of Nicotine, Gene Expression, and Behavior

The cholinergic system plays a crucial role in nervous system function with important effects on developmental processes, cognition, attention, motivation, reward, learning, and memory. Nicotine, the reinforcing component of tobacco and e-cigarettes, directly acts on the cholinergic system by targeting nicotinic acetylcholine receptors (nAChRs) in the brain. Activation of nAChRs leads to a multitude of immediate and long-lasting effects in specific cellular populations, thereby affecting the addictive properties of the drug. In addition to the direct actions of nicotine in binding to and opening nAChRs, the subsequent activation of circuits and downstream signaling cascades leads to a wide range of changes in gene expression, which can subsequently alter further behavioral expression. In this review, we provide an overview of the actions of nicotine that lead to changes in gene expression and further highlight evidence supporting how these changes can often be bidirectional, thereby inducing subsequent changes in behaviors associated with further drug intake.

Lynx1 prototoxins: critical accessory proteins of neuronal nicotinic acetylcholine receptors

Nicotinic receptors of the cholinergic system are ligand-gated ion channels, responding to the excitatory neurotransmitter, acetylcholine, and the addictive component of tobacco, nicotine. They help to transduce salient information in the environment by activating specific neural circuitry in normal and disease states. While nicotinic receptors are promising neurological and neuropsychiatric disorder targets, they have fallen out of favor after several late-stage clinical failures. Targeting the complex of the nicotinic receptor, including lynx1 accessory proteins, could be the key to unlocking the intractable nAChR for therapeutic development. Lynx1 binds to the extracellular face of the nAChR and acts as a critical modulator, suppressing memory, learning, and plasticity. Lynx1 removal in animal models leads to memory and plasticity enhancements, some of which have therapeutic relevance for neuropsychiatric and neurological disease. A review of the lynx1 accessory modulator and its role in modulating neuronal nAChRs will be discussed.

The α5 Nicotinic Acetylcholine Receptor Subunit Differentially Modulates α4β2* and α3β4* Receptors

Nicotine, the principal reinforcing compound in tobacco, acts in the brain by activating neuronal nicotinic acetylcholine receptors (nAChRs). This review summarizes our current knowledge regarding how the α5 accessory nAChR subunit, encoded by the CHRNA5 gene, differentially modulates α4β2^* and α3β4^* receptors at the cellular level. Genome-wide association studies have linked a gene cluster in chromosomal region 15q25 to increased susceptibility to nicotine addiction, lung cancer, chronic obstructive pulmonary disease, and peripheral arterial disease. Interestingly, this gene cluster contains a non-synonymous single-nucleotide polymorphism (SNP) in the human CHRNA5 gene, causing an aspartic acid (D) to asparagine (N) substitution at amino acid position 398 in the α5 nAChR subunit. Although other SNPs have been associated with tobacco smoking behavior, efforts have focused predominantly on the D398 and N398 variants in the α5 subunit. In recent years, significant progress has been made toward understanding the role that the α5 nAChR subunit—and the role of the D398 and N398 variants—plays on nAChR function at the cellular level. These insights stem primarily from a wide range of experimental models, including receptors expressed heterologously in Xenopus oocytes, various cell lines, and neurons derived from human induced pluripotent stem cells (iPSCs), as well as endogenous receptors in genetically engineered mice and—more recently—rats. Despite providing a wealth of available data, however, these studies have yielded conflicting results, and our understanding of the modulatory role that the α5 subunit plays remains incomplete. Here, we review these reports and the various techniques used for expression and analysis in order to examine how the α5 subunit modulates key functions in α4β2^* and α3β4^* receptors, including receptor trafficking, sensitivity, efficacy, and desensitization. In addition, we highlight the strikingly different role that the α5 subunit plays in Ca²⁺ signaling between α4β2^* and α3β4^* receptors, and we discuss whether the N398 α5 subunit variant can partially replace the D398 variant.

Implications of Oligomeric Amyloid-Beta (oAβ42) Signaling through α7β2-Nicotinic Acetylcholine Receptors (nAChRs) on Basal Forebrain Cholinergic Neuronal Intrinsic Excitability and Cognitive Decline

Neuronal and network-level hyperexcitability is commonly associated with increased levels of amyloid-β (Aβ) and contribute to cognitive deficits associated with Alzheimer's disease (AD). However, the mechanistic complexity underlying the selective loss of basal forebrain cholinergic neurons (BFCNs), a well-recognized characteristic of AD, remains poorly understood. In this study, we tested the hypothesis that the oligomeric form of amyloid-β (oAβ₄₂), interacting with α7-containing nicotinic acetylcholine receptor (nAChR) subtypes, leads to subnucleus-specific alterations in BFCN excitability and impaired cognition. We used single-channel electrophysiology to show that oAβ₄₂ activates both homomeric α7- and heteromeric α7β2-nAChR subtypes while preferentially enhancing α7β2-nAChR open-dwell times. Organotypic slice cultures were prepared from male and female ChAT-EGFP mice, and current-clamp recordings obtained from BFCNs chronically exposed to pathophysiologically relevant level of oAβ₄₂ showed enhanced neuronal intrinsic excitability and action potential firing rates. These resulted from a reduction in action potential afterhyperpolarization and alterations in the maximal rates of voltage change during spike depolarization and repolarization. These effects were observed in BFCNs from the medial septum diagonal band and horizontal diagonal band, but not the nucleus basalis. Last, aged male and female APP/PS1 transgenic mice, genetically null for the β2 nAChR subunit gene, showed improved spatial reference memory compared with APP/PS1 aged-matched littermates. Combined, these data provide a molecular mechanism supporting a role for α7β2-nAChR in mediating the effects of oAβ₄₂ on excitability of specific populations of cholinergic neurons and provide a framework for understanding the role of α7β2-nAChR in oAβ₄₂-induced cognitive decline.

Three-finger proteins from snakes and humans acting on nicotinic receptors: Old and new

The first toxin to give rise to the three-finger protein (TFP) family was α-bungarotoxin (α-Bgt) from Bungarus multicinctus krait venom. α-Bgt was crucial for research on nicotinic acetylcholine receptors (nAChRs), and in this Review article we focus on present data for snake venom TFPs and those of the Ly6/uPAR family from mammalians (membrane-bound Lynx1 and secreted SLURP-1) interacting with nAChRs. Recently isolated from Bungarus candidus venom, αδ-bungarotoxins differ from α-Bgt: they bind more reversibly and distinguish two binding sites in Torpedo californica nAChR. Naja kaouthia α-cobratoxin, classical blocker of nAChRs, was shown to inhibit certain GABA-A receptor subtypes, whereas α-cobratoxin dimer with 2 intermolecular disulfides has a novel type of 3D structure. Non-conventional toxin WTX has additional 5th disulfide not in the central loop, as α-Bgt, but in the N-terminal loop, like all Ly6/uPAR proteins, and inhibits α7 and Torpedo nAChRs. A water-soluble form of Lynx1, ws-Lynx1, was expressed in E. coli, its ¹ H-NMR structure and binding to several nAChRs determined. For SLURP-1, similar information was obtained with its recombinant analogue rSLURP-1. A common feature of ws-Lynx1, rSLURP-1, and WTX is their activity against nAChRs and muscarinic acetylcholine receptors. Synthetic SLURP-1, identical to the natural protein, demonstrated some differences from rSLURP-1 in distinguishing nAChR subtypes. The loop II fragment of the Lynx1 was synthesized having the same µM affinity for the Torpedo nAChR as ws-Lynx1. This review illustrates the productivity of parallel research of nAChR interactions with the two TFP groups.

Dynamics and Interactions of GPI-Linked lynx1 Protein with/without Nicotinic Acetylcholine Receptor in Membrane Bilayers

Nicotinic acetylcholine receptors (nAChRs) participate in diverse biological processes, such as mood, learning, and addiction. Glycosylphosphatidylinositol-linked lynx1 is an allosteric modulator of nAChR function, including shifts in agonist sensitivity, reduced desensitization, and slower recovery from desensitization. This modulation is thought to be achieved by lynx1's interaction with nAChR subunits, particularly at the α4:α4 interface. In this study, we used molecular modeling and simulation to study the structure, dynamics, and interactions of lynx1 when bound to nAChRs, as well as unbound, monomeric lynx1 in membranes. Though lynx1 structures are similar in both states, its dynamics is more restricted in the bound state than in the unbound one. When bound, interactions between lynx1 and nAChR are observed to be maintained throughout the simulations. Of particular note, lynx1 demonstrates prolonged interactions with the receptor C-loop in one of the nAChR α4 subunits, a region important for agonist binding and possibly the transition between open/closed states. During interactions with lynx1, an α4 C-loop tends to be restricted in either a closed or open state, whereas the C-loop state transitions are more evident when lynx1 is unbound. Interestingly, the conformational change of the C-loop is stochastic, suggesting that lynx1 can influence nAChR (critical for its multimodal action), for instance, by shifting its agonist sensitivity and recovery from desensitization.

Nicotinic Receptors Underlying Nicotine Dependence: Evidence from Transgenic Mouse Models

Nicotine underlies the reinforcing properties of tobacco cigarettes and e-cigarettes. After inhalation and absorption, nicotine binds to various nicotinic acetylcholine receptor (nAChR) subtypes localized on the pre- and postsynaptic membranes of cells, which subsequently leads to the modulation of cellular function and neurotransmitter signaling. In this chapter, we begin by briefly reviewing the current understanding of nicotine's actions on nAChRs and highlight considerations regarding nAChR subtype localization and pharmacodynamics. Thereafter, we discuss the seminal discoveries derived from genetically modified mouse models, which have greatly contributed to our understanding of nicotine's effects on the reward-related mesolimbic pathway and the aversion-related habenulo-interpeduncular pathway. Thereafter, emerging areas of research focusing on modulation of nAChR expression and/or function are considered. Taken together, these discoveries have provided a foundational understanding of various genetic, neurobiological, and behavioral factors underlying the motivation to use nicotine and related dependence processes, which are thereby advancing drug discovery efforts to promote long-term abstinence.

Isomerization of Asp7 in Beta-Amyloid Enhances Inhibition of the α7 Nicotinic Receptor and Promotes Neurotoxicity

Cholinergic dysfunction in Alzheimer’s disease (AD) can be mediated by the neuronal α7 nicotinic acetylcholine receptor (α7nAChR). Beta-amyloid peptide (Aβ) binds to the α7nAChR, disrupting the receptor’s function and causing neurotoxicity. In vivo not only Aβ but also its modified forms can drive AD pathogenesis. One of these forms, iso-Aβ (containing an isomerized Asp7 residue), shows an increased neurotoxicity in vitro and stimulates amyloidogenesis in vivo. We suggested that such effects of iso-Aβ are α7nAChR-dependent. Here, using calcium imaging and electrophysiology, we found that iso-Aβ is a more potent inhibitor of the α7nAChR-mediated calcium current than unmodified Aβ. However, Asp7 isomerization eliminated the ability of Aβ to decrease the α7nAChR levels. These data indicate differences in the interaction of the peptides with the α7nAChR, which we demonstrated using computer modeling. Neither Aβ nor iso-Aβ competed with ¹²⁵I-α-bungarotoxin for binding to the orthosteric site of the receptor, suggesting the allosteric binging mode of the peptides. Further we found that increased neurotoxicity of iso-Aβ was mediated by the α7nAChR. Thus, the isomerization of Asp7 enhances the inhibitory effect of Aβ on the functional activity of the α7nAChR, which may be an important factor in the disruption of the cholinergic system in AD.

Evolution and Medical Significance of LU Domain-Containing Proteins

Proteins containing Ly6/uPAR (LU) domains exhibit very diverse biological functions and have broad taxonomic distributions in eukaryotes. In general, they adopt a characteristic three-fingered folding topology with three long loops projecting from a disulfide-rich globular core. The majority of the members of this protein domain family contain only a single LU domain, which can be secreted, glycolipid anchored, or constitute the extracellular ligand binding domain of type-I membrane proteins. Nonetheless, a few proteins contain multiple LU domains, for example, the urokinase receptor uPAR, C4.4A, and Haldisin. In the current review, we will discuss evolutionary aspects of this protein domain family with special emphasis on variations in their consensus disulfide bond patterns. Furthermore, we will present selected cases where missense mutations in LU domain-containing proteins leads to dysfunctional proteins that are causally linked to genesis of human disease.

Water-soluble variant of human Lynx1 induces cell cycle arrest and apoptosis in lung cancer cells via modulation of α7 nicotinic acetylcholine receptors

Lynx1 is the first three-finger prototoxin found in the mammalian central nervous system. It is a GPI-anchored protein modulating nicotinic acetylcholine receptors (nAChRs) in the brain. Besides the brain, the Lynx1 protein was found in the lung and kidney. Endogenous Lynx1 controls the nicotine-induced up-regulation of the expression of α7 type nAChRs in lung adenocarcinoma A549 cells as well as the cell growth. Here, we analyzed the Lynx1 expression in the set of human epithelial cells. The Lynx1 expression both at the mRNA and protein level was detected in normal oral keratinocytes, and lung, colon, epidermal, and breast cancer cells, but not in embryonic kidney cells. Co-localization of Lynx1 with α7-nAChRs was revealed in a cell membrane for lung adenocarcinoma A549 and colon carcinoma HT-29 cells, but not for breast adenocarcinoma MCF-7 and epidermoid carcinoma A431 cells. The recombinant water-soluble variant of Lynx1 without a GPI-anchor (ws-Lynx1) inhibited the growth of A549 cells causing cell cycle arrest via modulation of α7-nAChRs and activation of different intracellular signaling cascades, including PKC/IP₃, MAP/ERK, p38, and JNK pathways. A549 cells treatment with ws-Lynx1 resulted in phosphorylation of the proapoptotic tumor suppressor protein p53 and different kinases participated in the regulation of gene transcription, cell growth, adhesion, and differentiation. Externalization of phosphatidylserine, an early apoptosis marker, observed by flow cytometry, confirmed the induction of apoptosis in A549 cells upon the ws-Lynx1 treatment. Our data revealed the ability of ws-Lynx1 to regulate homeostasis of epithelial cancer cells.

Lynx Prototoxins: Roles of Endogenous Mammalian Neurotoxin-Like Proteins in Modulating Nicotinic Acetylcholine Receptor Function to Influence Complex Biological Processes

The cholinergic system modulates many biological functions, due to the widespread distribution of cholinergic neuronal terminals, and the diffuse release of its neurotransmitter, acetylcholine. Several layers of regulation help to refine and control the scope of this excitatory neurotransmitter system. One such regulatory mechanism is imparted through endogenous toxin-like proteins, prototoxins, which largely control the function of nicotinic receptors of the cholinergic system. Prototoxins and neurotoxins share the distinct three finger toxin fold, highly effective as a receptor binding protein, and the former are expressed in the mammalian brain, immune system, epithelium, etc. Prototoxins and elapid snake neurotoxins appear to be related through gene duplication and divergence from a common ancestral gene. Protein modulators can provide a graded response of the cholinergic system, and within the brain, stabilize neural circuitry through direct interaction with nicotinic receptors. Understanding the roles of each prototoxin (e.g., lynx1, lynx2/lypd1, PSCA, SLURP1, SLURP2, Lypd6, lypd6b, lypdg6e, PATE-M, PATE-B, etc.), their binding specificity and unique expression profile, has the potential to uncover many fascinating cholinergic-dependent mechanisms in the brain. Each family member can provide a spatially restricted level of control over nAChR function based on its expression in the brain. Due to the difficulty in the pharmacological targeting of nicotinic receptors in the brain as a result of widespread expression patterns and similarities in receptor sequences, unique interfaces between prototoxin and nicotinic receptor could provide more specific targeting than nicotinic receptors alone. As such, this family is intriguing from a long-term therapeutic perspective.

Chronic Menthol Does Not Change Stoichiometry or Functional Plasma Membrane Levels of Mouse α3β4-Containing Nicotinic Acetylcholine Receptors

Heteromeric α3β4 nicotinic acetylcholine (ACh) receptors (nAChRs) are pentameric ligand-gated cation channels that include at least two α3 and two β4 subunits. They have functions in peripheral tissue and peripheral and central nervous systems. We examined the effects of chronic treatment with menthol, a major flavor additive in tobacco cigarettes and electronic nicotine delivery systems, on mouse α3β4 nAChRs transiently transfected into neuroblastoma-2a cells. Chronic menthol treatment at 500 nM, near the estimated menthol concentration in the brain following cigarette smoking, altered neither the [ACh]-response relationship nor Zn2+ sensitivity of ACh-evoked currents, suggesting that menthol does not change α3β4 nAChR subunit stoichiometry. Chronic menthol treatment failed to change the current density (peak current amplitude/cell capacitance) of 100 μM ACh-evoked currents. Chronic menthol treatment accelerated desensitization of 100 and 200 μM ACh-evoked currents. Chronic nicotine treatment (250 μM) decreased ACh-induced currents, and we found no additional effect of including chronic menthol. These data contrast with previously reported, marked effects of chronic menthol on β2* nAChRs studied in the same expression system. Mechanistically, the data support the emerging interpretation that both chronic menthol and chronic nicotine act on nAChRs in the early exocytotic pathway, and that this pathway does not present a rate-limiting step to the export of α3β4 nAChRs; these nAChRs include endoplasmic reticulum (ER) export motifs but not ER retention motifs. Previous reports show that smoking mentholated cigarettes enhances tobacco addiction; but our results show that this effect is unlikely to arise via menthol actions on α3β4 nAChRs.

Distinctive single-channel properties of α4β2-nicotinic acetylcholine receptor isoforms

Central nervous system nicotinic acetylcholine receptors (nAChR) are predominantly of the α4β2 subtype. Two isoforms exist, with high or low agonist sensitivity (HS-(α4β2)2β2- and LS-(α4β2)2α4-nAChR). Both isoforms exhibit similar macroscopic potency and efficacy values at low acetylcholine (ACh) concentrations, mediated by a common pair of high-affinity α4(+)/(-)β2 subunit binding interfaces. However LS-(α4β2)2α4-nAChR also respond to higher concentrations of ACh, acting at a third α4(+)/(-)α4 subunit interface. To probe isoform functional differences further, HS- and LS-α4β2-nAChR were expressed in Xenopus laevis oocytes and single-channel responses were assessed using cell-attached patch-clamp. In the presence of a low ACh concentration, both isoforms produce low-bursting function. HS-(α4β2)2β2-nAChR exhibit a single conductance state, whereas LS-(α4β2)2α4-nAChR display two distinctive conductance states. A higher ACh concentration did not preferentially recruit either conductance state, but did result in increased LS-(α4β2)2α4-nAChR bursting and reduced closed times. Introduction of an α4(+)/(-)α4-interface loss-of-function α4W182A mutation abolished these changes, confirming this site's role in mediating LS-(α4β2)2α4-nAChR responses. Small or large amplitude openings are highly-correlated within individual LS-(α4β2)2α4-nAChR bursts, suggesting that they arise from distinct intermediate states, each of which is stabilized by α4(+)/(-)α4 site ACh binding. These findings are consistent with α4(+)/(-)α4 subunit interface occupation resulting in allosteric potentiation of agonist actions at α4(+)/(-)β2 subunit interfaces, rather than independent induction of high conductance channel openings.

Molecular Mechanisms Associated with Nicotine Pharmacology and Dependence

Tobacco dependence is a leading cause of preventable disease and death worldwide. Nicotine, the main psychoactive component in tobacco cigarettes, has also been garnering increased popularity in its vaporized form, as derived from e-cigarette devices. Thus, an understanding of the molecular mechanisms underlying nicotine pharmacology and dependence is required to ascertain novel approaches to treat drug dependence. In this chapter, we review the field's current understanding of nicotine's actions in the brain, the neurocircuitry underlying drug dependence, factors that modulate the function of nicotinic acetylcholine receptors, and the role of specific genes in mitigating the vulnerability to develop nicotine dependence. In addition to nicotine's direct actions in the brain, other constituents in nicotine and tobacco products have also been found to alter drug use, and thus, evidence is provided to highlight this issue. Finally, currently available pharmacotherapeutic strategies are discussed, along with an outlook for future therapeutic directions to achieve to the goal of long-term nicotine cessation.

Augmenting the antinociceptive effects of nicotinic acetylcholine receptor activity through lynx1 modulation

Neuronal nicotinic acetylcholine receptors (nAChRs) of the cholinergic system have been linked to antinociception, and therefore could be an alternative target for pain alleviation. nAChR activity has been shown to be regulated by the nicotinic modulator, lynx1, which forms stable complexes with nAChRs and has a negative allosteric action on their function. The objective in this study was to investigate the contribution of lynx1 to nicotine-mediated antinociception. Lynx1 contribution was investigated by mRNA expression analysis and electrophysiological responses to nicotine in the dorsal raphe nucleus (DRN), a part of the pain signaling pathway. In vivo antinociception was investigated in a test of nociception, the hot-plate analgesia assay with behavioral pharmacology. Lynx1/α4β2 nAChR interactions were investigated using molecular dynamics computational modeling. Nicotine evoked responses in serotonergic and GABAergic neurons in the DRN are augmented in slices lacking lynx1 (lynx1KO). The antinociceptive effect of nicotine and epibatidine is enhanced in lynx1KO mice and blocked by mecamylamine and DHβE. Computer simulations predict preferential binding affinity of lynx1 to the α:α interface that exists in the stoichiometry of the low sensitivity (α4)₃(β2)₂ nAChRs. Taken together, these data point to a role of lynx1 in mediating pain signaling in the DRN through preferential affinity to the low sensitivity α4β2 nAChRs. This study suggests that lynx1 is a possible alternative avenue for nociceptive modulation outside of opioid-based strategies.

Molecular and cellular characterization of nicotinic acetylcholine receptor subtypes in the arcuate nucleus of the mouse hypothalamus

Nicotine, acting through nicotinic acetylcholine receptors (nAChRs), increases the firing rate of both orexigenic agouti-related peptide (AgRP) and anorexigenic pro-opiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARC), yet nicotine and other nAChR agonists decrease food intake in mice. Viral-mediated knockdown of the β4 nAChR subunit in all neuronal cell types in the ARC prevents the nicotinic agonist cytisine from decreasing food intake, but it is not known whether the β4 subunit is selectively expressed in anorexigenic neurons or how other nAChR subtypes are distributed in this nucleus. Using translating ribosome affinity purification (TRAP) on ARC tissue from mice with ribosomes tagged in either AgRP or POMC cells, we examined nAChR subunit mRNA levels using real-time PCR. Both AgRP and POMC cells express a comparable panel of nAChR subunits with differences in α7 mRNA levels and a trend for difference in α4 levels, but no differences in β4 expression. Immunoprecipitation of assembled nAChRs revealed that the β4 subunit forms assembled channels with α3, β2 and α4, but not other subunits found in the ARC. Finally, using cell type-selective, virally delivered small hairpin RNAs targeting either the β4 or α7 subunit, we examined the contribution of each subunit in either AgRP or POMC cells to the behavioural response to nicotine, refining the understanding of nicotinic regulation of this feeding circuit. These experiments identify a more complex set of nAChRs expressed in ARC than in other hypothalamic regions. Thus, the ARC appears to be a particular target of nicotinic modulation.

A tale of ligands big and small: an update on how pentameric ligand-gated ion channels interact with agonists and proteins

Pentameric ligand-gated ion channels (pLGICs, also known as Cys-loop receptors) are a large family of ion channels expressed in all Bilateria and in several groups of bacteria and archaea. They are activated by small-molecule neurotransmitters to mediate fast transmission at many central and peripheral nervous system synapses and are the target of several drugs and insecticides. Here we review recent advances in the field, focussing on new insights on the structure of the agonist-binding site and on newly discovered protein-protein interactions involving pLGICs.

Getting a Handle on Neuropharmacology by Targeting Receptor-Associated Proteins

Targeted therapy for neuropsychiatric disorders requires selective modulation of dysfunctional neuronal pathways. Receptors relevant to CNS disorders typically have associated proteins discretely expressed in specific neuronal pathways; these accessory proteins provide a new dimension for drug discovery. Recent studies show that targeting a TARP auxiliary subunit of AMPA receptors selectively modulates neuronal excitability in specific forebrain pathways relevant to epilepsy. Other medicinally important ion channels, gated by glutamate, γ-aminobutyric acid (GABA), and acetylcholine, also have associated proteins, which may be druggable. This emerging pharmacology of receptor-associated proteins provides a new approach for improving drug efficacy while mitigating side effects.

Deletion of lynx1 reduces the function of α6* nicotinic receptors

The α6 nicotinic acetylcholine receptor (nAChR) subunit is an attractive drug target for treating nicotine addiction because it is present at limited sites in the brain including the reward pathway. Lynx1 modulates several nAChR subtypes; lynx1-nAChR interaction sites could possibly provide drug targets. We found that dopaminergic cells from the substantia nigra pars compacta (SNc) express lynx1 mRNA transcripts and, as assessed by co-immunoprecipitation, α6 receptors form stable complexes with lynx1 protein, although co-transfection with lynx1 did not affect nicotine-induced currents from cell lines transfected with α6 and β2. To test whether lynx1 is important for the function of α6 nAChRs in vivo, we bred transgenic mice carrying a hypersensitive mutation in the α6 nAChR subunit (α6L9′S) with lynx1 knockout mice, providing a selective probe of the effects of lynx1 on α6* nAChRs. Lynx1 removal reduced the α6 component of nicotine-mediated rubidium efflux and dopamine (DA) release from synaptosomal preparations with no effect on numbers of α6β2 binding sites, indicating that lynx1 is functionally important for α6* nAChR activity. No effects of lynx1 removal were detected on nicotine-induced currents in slices from SNc, suggesting that lynx1 affects presynaptic α6* nAChR function more than somatic function. In the absence of agonist, lynx1 removal did not alter DA release in dorsal striatum as measured by fast scan cyclic voltammetry. Lynx1 removal affected some behaviors, including a novel-environment assay and nicotine-stimulated locomotion. Trends in 24-hour home-cage behavior were also suggestive of an effect of lynx1 removal. Conditioned place preference for nicotine was not affected by lynx1 removal. The results show that some functional and behavioral aspects of α6-nAChRs are modulated by lynx1.

Proteins and chemical chaperones involved in neuronal nicotinic receptor expression and function: an update

Neuronal nicotinic ACh receptors (nAChRs) are a family of ACh-gated cation channels, and their homeostasis or proteostasis is essential for the correct physiology of the central and peripheral nervous systems. The proteostasis network regulates the folding, assembly, degradation and trafficking of nAChRs in order to ensure their efficient and functional expression at the cell surface. However, as nAChRs are multi-subunit, multi-span, integral membrane proteins, the folding and assembly is a very inefficient process, and only a small proportion of subunits can form functional pentamers. Moreover, the efficiency of assembly and trafficking varies widely depending on the nAChR subtypes and the cell type in which they are expressed. A detailed understanding of the mechanisms that regulate the functional expression of nAChRs in neurons and non-neuronal cells is therefore important. The purpose of this short review is to describe more recent findings concerning the chaperone proteins and target-specific and target-nonspecific pharmacological chaperones that modulate the expression of nAChR subtypes, and the possible mechanisms that underlie the dynamic changes of cell surface nAChRs.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc.