A Human Polymorphism in CHRNA5 Is Linked to Relapse to Nicotine Seeking in Transgenic Rats

CHRNA5 links chandelier cells to severity of amyloid pathology in aging and Alzheimer's disease

Changes in high-affinity nicotinic acetylcholine receptors are intricately connected to neuropathology in Alzheimer's Disease (AD). Protective and cognitive-enhancing roles for the nicotinic α5 subunit have been identified, but this gene has not been closely examined in the context of human aging and dementia. Therefore, we investigate the nicotinic α5 gene CHRNA5 and the impact of relevant single nucleotide polymorphisms (SNPs) in prefrontal cortex from 922 individuals with matched genotypic and post-mortem RNA sequencing in the Religious Orders Study and Memory and Aging Project (ROS/MAP). We find that a genotype robustly linked to increased expression of CHRNA5 (rs1979905A2) predicts significantly reduced cortical β-amyloid load. Intriguingly, co-expression analysis suggests CHRNA5 has a distinct cellular expression profile compared to other nicotinic receptor genes. Consistent with this prediction, single nucleus RNA sequencing from 22 individuals reveals CHRNA5 expression is disproportionately elevated in chandelier neurons, a distinct subtype of inhibitory neuron known for its role in excitatory/inhibitory (E/I) balance. We show that chandelier neurons are enriched in amyloid-binding proteins compared to basket cells, the other major subtype of PVALB-positive interneurons. Consistent with the hypothesis that nicotinic receptors in chandelier cells normally protect against β-amyloid, cell-type proportion analysis from 549 individuals reveals these neurons show amyloid-associated vulnerability only in individuals with impaired function/trafficking of nicotinic α5-containing receptors due to homozygosity of the missense CHRNA5 SNP (rs16969968A2). Taken together, these findings suggest that CHRNA5 and its nicotinic α5 subunit exert a neuroprotective role in aging and Alzheimer's disease centered on chandelier interneurons.

CHRNA5 gene variation affects the response of VTA dopaminergic neurons during chronic nicotine exposure and withdrawal

Nicotine is the principal psychoactive component in tobacco that drives addiction through its action on neuronal nicotinic acetylcholine receptors (nAChR). The nicotinic receptor gene CHRNA5, which encodes the α5 subunit, is associated with nicotine use and dependence. In humans, the CHRNA5 missense variant rs16969968 (G > A) is associated with increased risk for nicotine dependence and other smoking-related phenotypes. In rodents, α5-containing nAChRs in dopamine (DA) neurons within the ventral tegmental area (VTA) powerfully modulate nicotine reward and reinforcement. Although the neuroadaptations caused by long-term nicotine exposure are being actively delineated at both the synaptic and behavioral levels, the contribution of α5-containing nAChRs to the cellular adaptations associated with long-term nicotine exposure remain largely unknown. To gain insight into the mechanisms behind the influence of α5-containing nAChRs and the rs16969968 polymorphism on nicotine use and dependence, we used electrophysiological approaches to examine changes in nAChR function arising in VTA neurons during chronic nicotine exposure and multiple stages of nicotine withdrawal. Our results demonstrate that CHRNA5 mutation leads to profound changes in VTA nAChR function at baseline, during chronic nicotine exposure, and during short-term and prolonged withdrawal. Whereas nAChR function was suppressed in DA neurons from WT mice undergoing withdrawal relative to drug-naïve or nicotine-drinking mice, α5-null mice exhibited an increase in nAChR function during nicotine exposure that persisted throughout 5-10 weeks of withdrawal. Re-expressing the hypofunctional rs16969968 CHRNA5 variant in α5-null VTA DA neurons did not rescue the phenotype, with α5-SNP neurons displaying a similar increased response to ACh during nicotine exposure and early stages of withdrawal. These results demonstrate the importance of VTA α5-nAChRs in the response to nicotine and implicate them in the time course of withdrawal.

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

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

Impact of the gut microbiome on nicotine's motivational effects and glial cells in the ventral tegmental area in male mice

A link between gut dysbiosis and the pathogenesis of brain disorders has been identified. A role for gut bacteria in drug reward and addiction has been suggested but very few studies have investigated their impact on brain and behavioral responses to addictive drugs so far. In particular, their influence on nicotine's addiction-like processes remains unknown. In addition, evidence shows that glial cells shape the neuronal activity of the mesolimbic system but their regulation, within this system, by the gut microbiome is not established. We demonstrate that a lack of gut microbiota in male mice potentiates the nicotine-induced activation of sub-regions of the mesolimbic system. We further show that gut microbiota depletion enhances the response to nicotine of dopaminergic neurons of the posterior ventral tegmental area (pVTA), and alters nicotine's rewarding and aversive effects in an intra-VTA self-administration procedure. These effects were not associated with gross behavioral alterations and the nicotine withdrawal syndrome was not impacted. We further show that depletion of the gut microbiome modulates the glial cells of the mesolimbic system. Notably, it increases the number of astrocytes selectively in the pVTA, and the expression of postsynaptic density protein 95 in both VTA sub-regions, without altering the density of the astrocytic glutamatergic transporter GLT1. Finally, we identify several sub-populations of microglia in the VTA that differ between its anterior and posterior sub-parts, and show that they are re-organized in conditions of gut microbiota depletion. The present study paves the way for refining our understanding of the pathophysiology of nicotine addiction.

Hedgehog-interacting protein acts in the habenula to regulate nicotine intake

Hedgehog-interacting protein (HHIP) sequesters Hedgehog ligands to repress Smoothened (SMO)-mediated recruitment of the GLI family of transcription factors. Allelic variation in HHIP confers risk of chronic obstructive pulmonary disease and other smoking-related lung diseases, but underlying mechanisms are unclear. Using single-cell and cell-type-specific translational profiling, we show that HHIP expression is highly enriched in medial habenula (MHb) neurons, particularly MHb cholinergic neurons that regulate aversive behavioral responses to nicotine. HHIP deficiency dysregulated the expression of genes involved in cholinergic signaling in the MHb and disrupted the function of nicotinic acetylcholine receptors (nAChRs) through a PTCH-1/cholesterol-dependent mechanism. Further, CRISPR/Cas9-mediated genomic cleavage of the Hhip gene in MHb neurons enhanced the motivational properties of nicotine in mice. These findings suggest that HHIP influences vulnerability to smoking-related lung diseases in part by regulating the actions of nicotine on habenular aversion circuits.

A common SNP in Chrna5 enhances morphine reward in female mice

The single nucleotide polymorphism (SNP) D398N (rs16969968) in CHRNA5, the gene encoding the α5 subunit of the nicotinic acetylcholine receptors (nAChR), has been associated with both nicotine and opiate dependence in human populations. Expression of this SNP on presynaptic VTA dopaminergic (DA) neurons is known to cause a reduction in calcium signaling, leading to alterations in transmitter signaling and altered responses to drugs of abuse. To examine the impact of the Chrna5 SNP on opiate reward and underlying dopaminergic mechanisms, mice harboring two copies of the risk-associated allele (Chrna5 A/A) at a location equivalent to human rs16969968 were generated via CRISPR/cas9 genome editing. We sought to determine whether Chrna5 A/A mice show differences in sensitivity to rewarding properties of morphine using the conditioned place preference paradigm. When mice were tested two weeks after conditioning, female Chrna5 A/A mice showed significantly enhanced preference for the morphine-paired chamber relative to WT females, suggesting that this genotype may enhance opioid reward specifically in females. In contrast, Chrna5 genotype had no effect on locomotor sensitization in male or female mice. Relative to WT females, peak amplitude of ACh-gated currents recorded from VTA DA neurons in Chrna5 A/A females was potentiated 1 day after conditioning with morphine. Increased FOS expression was also observed in Chrna5 A/A mice relative to WT mice following exposure to the morphine CPP chamber. We propose that impaired α5 nAChR subunit function alters DA neuron response following repeated morphine exposures, and that this early cellular response could contribute to enhanced opiate reward two weeks after conditioning.

Relapse-like behavior and nAChR sensitization following intermittent access nicotine self-administration

Many tobacco smokers consume nicotine intermittently, but the underlying mechanisms and neurobiological changes associated with intermittent nicotine intake are unclear. Understanding intermittent nicotine intake is a high priority, as it could promote therapeutic strategies to attenuate tobacco consumption. We examined nicotine intake behavior and neurobiological changes in male rats that were trained to self-administer nicotine during brief (5 min) trials interspersed with longer (15 min) drug-free periods. Rats readily adapted to intermittent access (IntA) SA following acquisition on a continuous access (ContA) schedule. Probabilistic analysis of IntA nicotine SA suggested reduced nicotine loading behavior compared to ContA, and nicotine pharmacokinetic modeling revealed that rats taking nicotine intermittently may have increased intake to maintain blood levels of nicotine that are comparable to ContA SA. After IntA nicotine SA, rats exhibited an increase in unreinforced responses for nicotine-associated cues (incubation of craving) and specific alterations in the striatal proteome after 7 days without nicotine. IntA nicotine SA also induced nAChR functional upregulation in the interpeduncular nucleus (IPN), and it enhanced nicotine binding in the brain as determined via [¹¹C]nicotine positron emission tomography. Reducing the saliency of the cue conditions during the 5 min access periods attenuated nicotine intake, but incubation of craving was preserved. Together, these results indicate that IntA conditions promote nicotine SA and nicotine seeking after a nicotine-free period.

Identification of the Risk Genes Associated With Vulnerability to Addiction: Major Findings From Transgenic Animals

Understanding risk factors for substance use disorders (SUD) can facilitate medication development for SUD treatment. While a rich literature exists discussing environmental factors that influence SUD, fewer articles have focused on genetic factors that convey vulnerability to drug use. Methods to identify SUD risk genes include Genome-Wide Association Studies (GWAS) and transgenic approaches. GWAS have identified hundreds of gene variants or single nucleotide polymorphisms (SNPs). However, few genes identified by GWAS have been verified by clinical or preclinical studies. In contrast, significant progress has been made in transgenic approaches to identify risk genes for SUD. In this article, we review recent progress in identifying candidate genes contributing to drug use and addiction using transgenic approaches. A central hypothesis is if a particular gene variant (e.g., resulting in reduction or deletion of a protein) is associated with increases in drug self-administration or relapse to drug seeking, this gene variant may be considered a risk factor for drug use and addiction. Accordingly, we identified several candidate genes such as those that encode dopamine D₂ and D₃ receptors, mGluR₂, M₄ muscarinic acetylcholine receptors, and α₅ nicotinic acetylcholine receptors, which appear to meet the risk-gene criteria when their expression is decreased. Here, we describe the role of these receptors in drug reward and addiction, and then summarize major findings from the gene-knockout mice or rats in animal models of addiction. Lastly, we briefly discuss future research directions in identifying addiction-related risk genes and in risk gene-based medication development for the treatment of addiction.

Neurobiological Mechanisms of Nicotine Reward and Aversion

Neuronal nicotinic acetylcholine receptors (nAChRs) regulate the rewarding actions of nicotine contained in tobacco that establish and maintain the smoking habit. nAChRs also regulate the aversive properties of nicotine, sensitivity to which decreases tobacco use and protects against tobacco use disorder. These opposing behavioral actions of nicotine reflect nAChR expression in brain reward and aversion circuits. nAChRs containing α4 and β2 subunits are responsible for the high-affinity nicotine binding sites in the brain and are densely expressed by reward-relevant neurons, most notably dopaminergic, GABAergic, and glutamatergic neurons in the ventral tegmental area. High-affinity nAChRs can incorporate additional subunits, including β3, α6, or α5 subunits, with the resulting nAChR subtypes playing discrete and dissociable roles in the stimulatory actions of nicotine on brain dopamine transmission. nAChRs in brain dopamine circuits also participate in aversive reactions to nicotine and the negative affective state experienced during nicotine withdrawal. nAChRs containing α3 and β4 subunits are responsible for the low-affinity nicotine binding sites in the brain and are enriched in brain sites involved in aversion, including the medial habenula, interpeduncular nucleus, and nucleus of the solitary tract, brain sites in which α5 nAChR subunits are also expressed. These aversion-related brain sites regulate nicotine avoidance behaviors, and genetic variation that modifies the function of nAChRs in these sites increases vulnerability to tobacco dependence and smoking-related diseases. Here, we review the molecular, cellular, and circuit-level mechanisms through which nicotine elicits reward and aversion and the adaptations in these processes that drive the development of nicotine dependence. SIGNIFICANCE STATEMENT: Tobacco use disorder in the form of habitual cigarette smoking or regular use of other tobacco-related products is a major cause of death and disease worldwide. This article reviews the actions of nicotine in the brain that contribute to tobacco use disorder.

Cholinergic modulation of hierarchical inhibitory control over cortical resting state dynamics: Local circuit modeling of schizophrenia-related hypofrontality

Nicotinic acetylcholine receptors (nAChRs) modulate the cholinergic drive to a hierarchy of inhibitory neurons in the superficial layers of the PFC, critical to cognitive processes. It has been shown that genetic deletions of the various types of nAChRs impact the properties of ultra-slow transitions between high and low PFC activity states in mice during quiet wakefulness. The impact characteristics depend on specific interneuron populations expressing the manipulated receptor subtype. In addition, recent data indicate that a genetic mutation of the α5 nAChR subunit, located on vasoactive intestinal polypeptide (VIP) inhibitory neurons, the rs16969968 single nucleotide polymorphism (α5 SNP), plays a key role in the hypofrontality observed in schizophrenia patients carrying the SNP. Data also indicate that chronic nicotine application to α5 SNP mice relieves the hypofrontality. We developed a computational model to show that the activity patterns recorded in the genetically modified mice can be explained by changes in the dynamics of the local PFC circuit. Notably, our model shows that these altered PFC circuit dynamics are due to changes in the stability structure of the activity states. We identify how this stability structure is differentially modulated by cholinergic inputs to the parvalbumin (PV), somatostatin (SOM) or the VIP inhibitory populations. Our model uncovers that a change in amplitude, but not duration of the high activity states can account for the lowered pyramidal (PYR) population firing rates recorded in α5 SNP mice. We demonstrate how nicotine-induced desensitization and upregulation of the β2 nAChRs located on SOM interneurons, as opposed to the activation of α5 nAChRs located on VIP interneurons, is sufficient to explain the nicotine-induced activity normalization in α5 SNP mice. The model further implies that subsequent nicotine withdrawal may exacerbate the hypofrontality over and beyond one caused by the SNP mutation.

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Prefrontal cortex shows ultra-slow alterations between low and high activity states at rest. This activity is characteristically decreased in schizophrenia patients.

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Model identifies local circuit origin of hypofrontality associated with schizophrenia and a5 nicotinic receptor malfunction.

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Decrease in PFC VIP-interneuron excitability drives decrease in high-activity-state stability and overall hypofrontality.

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Model shows desensitization/upregulation of SOM-expressed β2-NAChRs drive nicotine-induced renormalization of PFC activity.

An innate contribution of human nicotinic receptor polymorphisms to COPD-like lesions

Chronic Obstructive Pulmonary Disease is a generally smoking-linked major cause of morbidity and mortality. Genome-wide Association Studies identified a locus including a non-synonymous single nucleotide polymorphism in CHRNA5, rs16969968, encoding the nicotinic acetylcholine receptor α5 subunit, predisposing to both smoking and Chronic Obstructive Pulmonary Disease. Here we report that nasal polyps from rs16969968 non-smoking carriers exhibit airway epithelium remodeling and inflammation. These hallmarks of Chronic Obstructive Pulmonary Disease occur spontaneously in mice expressing human rs16969968. They are significantly amplified after exposure to porcine pancreatic elastase, an emphysema model, and to oxidative stress with a polymorphism-dependent alteration of lung function. Targeted rs16969968 expression in epithelial cells leads to airway remodeling in vivo, increased proliferation and production of pro-inflammatory cytokines through decreased calcium entry and increased adenylyl-cyclase activity. We show that rs16969968 directly contributes to Chronic Obstructive Pulmonary Disease-like lesions, sensitizing the lung to the action of oxidative stress and injury, and represents a therapeutic target.

Human polymorphisms in nicotinic acetylcholine receptor genes have been linked to both smoking and lung diseases like Chronic Obstructive Pulmonary Disease (COPD) or lung cancer. Here the authors identify a direct role for a human coding polymorphism in COPD-like lesions independent of smoke or nicotine exposure.

Addiction-related neuroadaptations following chronic nicotine exposure

The addiction-relevant molecular, cellular, and behavioral actions of nicotine are derived from its stimulatory effects on neuronal nicotinic acetylcholine receptors (nAChRs) in the central nervous system. nAChRs expressed by dopamine-containing neurons in the ventral midbrain, most notably in the ventral tegmental area (VTA), contribute to the reward-enhancing properties of nicotine that motivate the use of tobacco products. nAChRs are also expressed by neurons in brain circuits that regulate aversion. In particular, nAChRs expressed by neurons in the medial habenula (mHb) and the interpeduncular nucleus (IPn) to which the mHb almost exclusively projects regulate the "set-point" for nicotine aversion and control nicotine intake. Different nAChR subtypes are expressed in brain reward and aversion circuits and nicotine intake is titrated to maximally engage reward-enhancing nAChRs while minimizing the recruitment of aversion-promoting nAChRs. With repeated exposure to nicotine, reward- and aversion-related nAChRs and the brain circuits in which they are expressed undergo adaptations that influence whether tobacco use will transition from occasional to habitual. Genetic variation that influences the sensitivity of addiction-relevant brain circuits to the actions of nicotine also influence the propensity to develop habitual tobacco use. Here, we review some of the key advances in our understanding of the mechanisms by which nicotine acts on brain reward and aversion circuits and the adaptations that occur in these circuits that may drive addiction to nicotine-containing tobacco products.

Linking the CHRNA5 SNP to drug abuse liability: From circuitry to cellular mechanisms

Genetics are known to be a significant risk factor for drug abuse. In human populations, the single nucleotide polymorphism (SNP) D398N in the gene CHRNA5 has been associated with addiction to nicotine, opioids, cocaine, and alcohol. In this paper, we review findings from studies in humans, rodent models, and cell lines and provide evidence that collectively suggests that the Chrna5 SNP broadly influences the response to drugs of abuse in a manner that is not substance-specific. This finding has important implications for our understanding of the role of the cholinergic system in reward and addiction vulnerability. This article is part of the special issue on 'Vulnerabilities to Substance Abuse.'

Concatemers to re-investigate the role of α5 in α4β2 nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are pentameric ion channels expressed in the central nervous systems. nAChRs containing the α4, β2 and α5 subunits are specifically involved in addictive processes, but their functional architecture is poorly understood due to the intricacy of assembly of these subunits. Here we constrained the subunit assembly by designing fully concatenated human α4β2 and α4β2α5 receptors and characterized their properties by two-electrodes voltage-clamp electrophysiology in Xenopus oocytes. We found that α5-containing nAChRs are irreversibly blocked by methanethiosulfonate (MTS) reagents through a covalent reaction with a cysteine present only in α5. MTS-block experiments establish that the concatemers are expressed in intact form at the oocyte surface, but that reconstitution of nAChRs from loose subunits show inefficient and highly variable assembly of α5 with α4 and β2. Mutational analysis shows that the concatemers assemble both in clockwise and anticlockwise orientations, and that α5 does not contribute to ACh binding from its principal (+) site. Reinvestigation of suspected α5-ligands such as galantamine show no specific effect on α5-containing concatemers. Analysis of the α5-D398N mutation that is linked to smoking and lung cancer shows no significant effect on the electrophysiological function, suggesting that its effect might arise from alteration of other cellular processes. The concatemeric strategy provides a well-characterized platform for mechanistic analysis and screening of human α5-specific ligands.

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.

Alterations in nicotinic receptor alpha5 subunit gene differentially impact early and later stages of cocaine addiction: a translational study in transgenic rats and patients

Cocaine addiction is a chronic and relapsing disorder with an important genetic component. Human candidate gene association studies showed that the single nucleotide polymorphism (SNP) rs16969968 in the α5 subunit (α5SNP) of nicotinic acetylcholine receptors (nAChRs), previously associated with increased tobacco dependence, was linked to a lower prevalence of cocaine use disorder (CUD). Three additional SNPs in the α5 subunit, previously shown to modify α5 mRNA levels, were also associated with CUD, suggesting an important role of the subunit in this pathology. To investigate the link between this subunit and CUD, we submitted rats knockout for the α5 subunit gene (α5KO), or carrying the α5SNP, to cocaine self-administration (SA) and showed that the acquisition of cocaine-SA was impaired in α5SNP rats while α5KO rats exhibited enhanced cocaine-induced relapse associated with altered neuronal activity in the nucleus accumbens. In addition, we observed in a human cohort of patients with CUD that the α5SNP was associated with a slower transition from first cocaine use to CUD. We also identified a novel SNP in the β4 nAChR subunit, part of the same gene cluster in the human genome and potentially altering CHRNA5 expression, associated with shorter time to relapse to cocaine use in patients. In conclusion, the α5SNP is protective against CUD by influencing early stages of cocaine exposure while CHRNA5 expression levels may represent a biomarker for the risk to relapse to cocaine use. Drugs modulating α5 containing nAChR activity may thus represent a novel therapeutic strategy against CUD.

Genetic susceptibility to nicotine addiction: Advances and shortcomings in our understanding of the CHRNA5/A3/B4 gene cluster contribution

Over the last decade, robust human genetic findings have been instrumental in elucidating the heritable basis of nicotine addiction (NA). They highlight coding and synonymous polymorphisms in a cluster on chromosome 15, encompassing the CHRNA5, CHRNA3 and CHRNB4 genes, coding for three subunits of the nicotinic acetylcholine receptor (nAChR). They have inspired an important number of preclinical studies, and will hopefully lead to the definition of novel drug targets for treating NA. Here, we review these candidate gene and genome-wide association studies (GWAS) and their direct implication in human brain function and NA-related phenotypes. We continue with a description of preclinical work in transgenic rodents that has led to a mechanistic understanding of several of the genetic hits. We also highlight important issues with regards to CHRNA3 and CHRNB4 where we are still lacking a dissection of their role in NA, including even in preclinical models. We further emphasize the use of human induced pluripotent stem cell-derived models for the analysis of synonymous and intronic variants on a human genomic background. Finally, we indicate potential avenues to further our understanding of the role of this human genetic variation. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Association of TERT, OGG1, and CHRNA5 Polymorphisms and the Predisposition to Lung Cancer in Eastern Algeria

Lung cancer remains the most common cancer in the world. The genetic polymorphisms (rs2853669 in TERT, rs1052133 in OGG1, and rs16969968 in CHRNA5 genes) were shown to be strongly associated with the risk of lung cancer. Our study's aim is to elucidate whether these polymorphisms predispose Eastern Algerian population to non-small-cell lung cancer (NSCLC). To date, no study has considered this association in the Algerian population. This study included 211 healthy individuals and 144 NSCLC cases. Genotyping was performed using TaqMan probes and Sanger sequencing, and the data were analyzed using multivariate logistic regression adjusted for covariates. The minor allele frequencies (MAFs) of TERT rs2853669, CHRNA5 rs16969968, and OGG1 rs1052133 polymorphisms in controls were C: 20%, A: 31%, and G: 29%, respectively. Of the three polymorphisms, none shows a significant association, but stratified analysis rs16969968 showed that persons carrying the AA genotype are significantly associated with adenocarcinoma risk (pAdj = 0.03, ORAdj = 2.55). Smokers with an AA allele have a larger risk of lung cancer than smokers with GG or GA genotype (pAdj = 0.03, ORAdj = 3.91), which is not the case of nonsmokers. Our study suggests that CHRNA5 rs16969968 polymorphism is associated with a significant increase of lung adenocarcinoma risk and with a nicotinic addiction.

The nicotinic receptor alpha5 coding polymorphism rs16969968 as a major target in disease: Functional dissection and remaining challenges

Nicotinic acetylcholine receptors (nAChRs) are major signalling molecules in the central and peripheral nervous system. Over the last decade, they have been linked to a number of major human psychiatric and neurological conditions, like smoking, schizophrenia, Alzheimer's disease and many others. Human Genome-Wide Association Studies (GWAS) have robustly identified genetic alterations at a locus of chromosome 15q to several of these diseases. In this review, we discuss a major coding polymorphism in the alpha5 subunit, referred to as α5SNP, and its functional dissection in vitro and in vivo. Its presence at high frequency in many human populations lends itself to pharmaceutical intervention in the context of 'positive allosteric modulators' (PAMs). We will present the prospects of this novel treatment, and the remaining challenges to identify suitable molecules.

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.

Neural circuits and nicotinic acetylcholine receptors mediate the cholinergic regulation of midbrain dopaminergic neurons and nicotine dependence

Midbrain dopaminergic (DA) neurons are governed by an endogenous cholinergic system, originated in the mesopontine nuclei. Nicotine hijacks nicotinic acetylcholine receptors (nAChRs) and interferes with physiological function of the cholinergic system. In this review, we describe the anatomical organization of the cholinergic system and the key nAChR subtypes mediating cholinergic regulation of DA transmission and nicotine reward and dependence, in an effort to identify potential targets for smoking intervention. Cholinergic modulation of midbrain DA systems relies on topographic organization of mesopontine cholinergic projections, and activation of nAChRs in midbrain DA neurons. Previous studies have revealed that α4, α6, and β2 subunit-containing nAChRs expressed in midbrain DA neurons and their terminals in the striatum regulate firings of midbrain DA neurons and activity-dependent dopamine release in the striatum. These nAChRs undergo modification upon chronic nicotine exposure. Clinical investigation has demonstrated that partial agonists of these receptors elevate the success rate of smoking cessation relative to placebo. However, further investigations are required to refine the drug targets to mitigate unpleasant side-effects.

Translational Molecular Approaches in Substance Abuse Research

Excessive abuse of psychoactive substances is one of the leading contributors to morbidity and mortality worldwide. In this book chapter, we review translational research strategies that are applied in the pursuit of new and more effective therapeutics for substance use disorder (SUD). The complex, multidimensional nature of psychiatric disorders like SUD presents difficult challenges to investigators. While animal models are critical for outlining the mechanistic relationships between defined behaviors and genetic and/or molecular changes, the heterogeneous pathophysiology of brain diseases is uniquely human, necessitating the use of human studies and translational research schemes. Translational research describes a cross-species approach in which findings from human patient-based data can be used to guide molecular genetic investigations in preclinical animal models in order to delineate the mechanisms of reward circuitry changes in the addicted state. Results from animal studies can then inform clinical investigations toward the development of novel treatments for SUD. Here we describe the strategies that are used to identify and functionally validate genetic variants in the human genome which may contribute to increased risk for SUD, starting from early candidate gene approaches to more recent genome-wide association studies. We will next examine studies aimed at understanding how transcriptional and epigenetic dysregulation in SUD can persistently alter cellular function in the disease state. In our discussion, we then focus on examples from the literature illustrating molecular genetic methodologies that have been applied to studies of different substances of abuse - from alcohol and nicotine to stimulants and opioids - in order to exemplify how these approaches can both delineate the underlying molecular systems driving drug addiction and provide insights into the genetic basis of SUD.

A Taxon-Specific and High-Throughput Method for Measuring Ligand Binding to Nicotinic Acetylcholine Receptors

The binding of compounds to nicotinic acetylcholine receptors is of great interest in biomedical research. However, progress in this area is hampered by the lack of a high-throughput, cost-effective, and taxonomically flexible platform. Current methods are low-throughput, consume large quantities of sample, or are taxonomically limited in which targets can be tested. We describe a novel assay which utilizes a label-free bio-layer interferometry technology, in combination with adapted mimotope peptides, in order to measure ligand binding to the orthosteric site of nicotinic acetylcholine receptor alpha-subunits of diverse organisms. We validated the method by testing the evolutionary patterns of a generalist feeding species (Acanthophis antarcticus), a fish specialist species (Aipysurus laevis), and a snake specialist species (Ophiophagus hannah) for comparative binding to the orthosteric site of fish, amphibian, lizard, snake, bird, marsupial, and rodent alpha-1 nicotinic acetylcholine receptors. Binding patterns corresponded with diet, with the Acanthophis antarcticus not showing bias towards any particular lineage, while Aipysurus laevis showed selectivity for fish, and Ophiophagus hannah a selectivity for snake. To validate the biodiscovery potential of this method, we screened Acanthophis antarcticus and Tropidolaemus wagleri venom for binding to human alpha-1, alpha-2, alpha-3, alpha-4, alpha-5, alpha-6, alpha-7, alpha-9, and alpha-10. While A. antarcticus was broadly potent, T. wagleri showed very strong but selective binding, specifically to the alpha-1 target which would be evolutionarily selected for, as well as the alpha-5 target which is of major interest for drug design and development. Thus, we have shown that our novel method is broadly applicable for studies including evolutionary patterns of venom diversification, predicting potential neurotoxic effects in human envenomed patients, and searches for novel ligands of interest for laboratory tools and in drug design and development.

Integrative Transcriptomics Reveals Sexually Dimorphic Control of the Cholinergic/Neurokine Interface in Schizophrenia and Bipolar Disorder

RNA sequencing analyses are often limited to identifying lowest p value transcripts, which does not address polygenic phenomena. To overcome this limitation, we developed an integrative approach that combines large-scale transcriptomic meta-analysis of patient brain tissues with single-cell sequencing data of CNS neurons, short RNA sequencing of human male- and female-originating cell lines, and connectomics of transcription factor and microRNA interactions with perturbed transcripts. We used this pipeline to analyze cortical transcripts of schizophrenia and bipolar disorder patients. Although these pathologies show massive transcriptional parallels, their clinically well-known sexual dimorphisms remain unexplained. Our method reveals the differences between afflicted men and women and identifies disease-affected pathways of cholinergic transmission and gp130-family neurokine controllers of immune function interlinked by microRNAs. This approach may open additional perspectives for seeking biomarkers and therapeutic targets in other transmitter systems and diseases.

Profound alteration in reward processing due to a human polymorphism in CHRNA5: a role in alcohol dependence and feeding behavior

Human genetic variation in the nicotinic receptor gene cluster CHRNA5/A3/B4, in particular the non-synonymous and frequent CHRNA5 variant rs16969968 (α5SNP), has an important consequence on smoking behavior in humans. A number of genetic association studies have additionally implicated the CHRNA5 gene in addictions to other drugs, and also body mass index (BMI). Here, we model the α5SNP, in a transgenic rat line, and establish its role in alcohol dependence, and feeding behavior. Rats expressing the α5SNP consume more alcohol, and exhibit increased relapse to alcohol seeking after abstinence. This high-relapsing phenotype is reflected in altered activity in the insula, linked to interoception, as established using c-Fos immunostaining. Similarly, relapse to food seeking is increased in the transgenic group, while a nicotine treatment reduces relapse in both transgenic and control rats. These findings point to a general role of this human polymorphism in reward processing, and multiple addictions other than smoking. This could pave the way for the use of medication targeting the nicotinic receptor in the treatment of alcohol use and eating disorders, and comorbid conditions in smokers.

Chronic Nicotine Exposure Alters the Neurophysiology of Habenulo-Interpeduncular Circuitry

Antagonism of nicotinic acetylcholine receptors (nAChRs) in the medial habenula (MHb) or interpeduncular nucleus (IPN) triggers withdrawal-like behaviors in mice chronically exposed to nicotine, implying that nicotine dependence involves the sensitization of nicotinic signaling. Identification of receptor and/or neurophysiological mechanisms underlying this sensitization is important, as it could promote novel therapeutic strategies to reduce tobacco use. Using an approach involving photoactivatable nicotine, we previously demonstrated that chronic nicotine (cNIC) potently enhances nAChR function in dendrites of MHb neurons. However, whether cNIC modulates downstream components of the habenulo-interpeduncular (Hb-IP) circuit is unknown. In this study, cNIC-mediated changes to Hb-IP nAChR function were examined in mouse (male and female) brain slices using molecular, electrophysiological, and optical techniques. cNIC enhanced action potential firing and modified spike waveform characteristics in MHb neurons. Nicotine uncaging revealed nAChR functional enhancement by cNIC on proximal axonal membranes. Similarly, nAChR-driven glutamate release from MHb axons was enhanced by cNIC. In IPN, the target structure of MHb axons, neuronal morphology, and nAChR expression is complex, with stronger nAChR function in the rostral subnucleus [rostral IPN (IPR)]. As in MHb, cNIC induced strong upregulation of nAChR function in IPN neurons. This, coupled with cNIC-enhanced nicotine-stimulated glutamate release, was associated with stronger depolarization responses to brief (1 ms) nicotine uncaging adjacent to IPR neurons. Together, these results indicate that chronic exposure to nicotine dramatically alters nicotinic cholinergic signaling and cell excitability in Hb-IP circuits, a key pathway involved in nicotine dependence.SIGNIFICANCE STATEMENT This study uncovers several neuropharmacological alterations following chronic exposure to nicotine in a key brain circuit involved in nicotine dependence. These results suggest that smokers or regular users of electronic nicotine delivery systems (i.e., "e-cigarettes") likely undergo sensitization of cholinergic circuitry in the Hb-IP system. Reducing the activity of Hb-IP nAChRs, either volitionally during smoking cessation or inadvertently via receptor desensitization during nicotine intake, may be a key trigger of withdrawal in nicotine dependence. Escalation of nicotine intake in smokers, or tolerance, may involve stimulation of these sensitized cholinergic pathways. Smoking cessation therapeutics are only marginally effective, and by identifying cellular/receptor mechanisms of nicotine dependence, our results take a step toward improved therapeutic approaches for this disorder.

The role of the nAChR subunits α5, β2, and β4 on synaptic transmission in the mouse superior cervical ganglion

Our previous immunoprecipitation analysis of nicotinic acetylcholine receptors (nAChRs) in the mouse superior cervical ganglion (SCG) revealed that approximately 55%, 24%, and 21% of receptors are comprised of α3β4, α3β4α5, and α3β4β2 subunits, respectively. Moreover, mice lacking β4 subunits do not express α5-containing receptors but still express a small number of α3β2 receptors. Here, we investigated how synaptic transmission is affected in the SCG of α5β4-KO and α5β2-KO mice. Using an ex vivo SCG preparation, we stimulated the preganglionic cervical sympathetic trunk and measured compound action potentials (CAPs) in the postganglionic internal carotid nerve. We found that CAP amplitude was unaffected in α5β4-KO and α5β2-KO ganglia, whereas the stimulation threshold for eliciting CAPs was significantly higher in α5β4-KO ganglia. Moreover, intracellular recordings in SCG neurons revealed no difference in EPSP amplitude. We also found that the ganglionic blocking agent hexamethonium was the most potent in α5β4-KO ganglia (IC50 : 22.1 μmol/L), followed by α5β2-KO (IC50 : 126.7 μmol/L) and WT ganglia (IC50 : 389.2 μmol/L). Based on these data, we estimated an IC50 of 568.6 μmol/L for a receptor population consisting solely of α3β4α5 receptors; and we estimated that α3β4α5 receptors comprise 72% of nAChRs expressed in the mouse SCG. Similarly, by measuring the effects of hexamethonium on ACh-induced currents in cultured SCG neurons, we found that α3β4α5 receptors comprise 63% of nAChRs. Thus, in contrast to our results obtained using immunoprecipitation, these data indicate that the majority of receptors at the cell surface of SCG neurons consist of α3β4α5.

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.