Nicotinic receptors in human brain: topography and pathology

Cholinergic system in schizophrenia: A systematic review and meta-analysis

BACKGROUND/OBJECTIVES

Studies have shown widespread alterations in different components of the cholinergic system in schizophrenia, but to date the evidence has not been systematically reviewed and summarized. Here, we systematically review imaging and post-mortem studies on the central cholinergic system in schizophrenia/schizoaffective disorder.

SUBJECTS/METHODS

Searches were performed in Embase and Medline. Study designs included cross-sectional case control studies comparing individuals with schizophrenia/schizoaffective disorder to control population. Risk of bias was assessed with the NIH/NHLBI tool for Quality Assessment of Case-Control Studies. The current study followed the PRISMA 2020 guidelines (PROSPERO: CRD42023402126).

RESULTS

A total of 3259 studies were screened and 61 met eligibility criteria for the systematic review, including 8 in vivo neuroimaging and 53 post-mortem studies. About 74% of these studies described significant alterations, most often reductions in either muscarinic or nicotinic receptor levels in schizophrenia. We also conducted 3 meta-analyses showing reductions in M1/M4 muscarinic receptors in the striatum (g = -0.809, k = 3, n = 108), hippocampus (g = -0.872, k = 3, n = 84), and fronto-cingulate cortex (g = -0.438, k = 4, n = 295). Six neuroimaging studies reported associations with clinical symptom severity measures, and four investigations with cognitive dysfunction.

CONCLUSIONS

Our review demonstrates a widespread decrease in muscarinic and nicotinic receptor levels in schizophrenia, evident in both neuroimaging and post-mortem studies. Our meta-analyses show large to moderate effects for the reductions in M1/M4 muscarinic receptors in the striatum, hippocampus, and fronto-cingulate cortex. Limitations and future directions for the field are discussed.

The effects of different acetylcholinesterase inhibitors on EEG patterns in patients with Alzheimer's disease: A systematic review

OBJECTIVE

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and the most common type of dementia. The early diagnosis of AD is an important factor for the control of AD progression. Electroencephalography (EEG) can be used for early diagnosis of AD. Acetylcholinesterase inhibitors (AChEIs) are also used for the amelioration of AD symptoms. In this systematic review, we reviewed the effect of different AChEIs including donepezil, rivastigmine, tacrine, physostigmine, and galantamine on EEG patterns in patients with AD.

METHODS

PubMed electronic database was searched and 122 articles were found. After removal of unrelated articles, 24 articles were selected for the present study.

RESULTS

AChEIs can decrease beta, theta, and delta frequency bands in patients with AD. However, conflicting results were found for alpha band. Some studies have shown increased alpha frequency, while others have shown decreased alpha frequency following treatment with AChEIs. The only difference was the type of drug.

CONCLUSIONS

We found that studies reporting the decreased alpha frequency used donepezil and galantamine, while studies reporting the increased alpha frequency used rivastigmine and tacrine. It was suggested that future studies should focus on the effect of different AChEIs on EEG bands, especially alpha frequency in patients with AD, to compare their effects and find the reason for their different influence on EEG patterns. Also, differences between the effects of AChEIs on oligodendrocyte differentiation and myelination may be another important factor. This is the first article investigating the effect of different AChEIs on EEG patterns in patients with AD.

The role of astrocytic α7 nicotinic acetylcholine receptors in Alzheimer disease

The ongoing search for therapeutic interventions in Alzheimer disease (AD) has highlighted the complexity of this condition and the need for additional biomarkers, beyond amyloid-β (Aβ) and tau, to improve clinical assessment. Astrocytes are brain cells that control metabolic and redox homeostasis, among other functions, and are emerging as an important focus of AD research owing to their swift response to brain pathology in the initial stages of the disease. Reactive astrogliosis - the morphological, molecular and functional transformation of astrocytes during disease - has been implicated in AD progression, and the definition of new astrocytic biomarkers could help to deepen our understanding of reactive astrogliosis along the AD continuum. As we highlight in this Review, one promising biomarker candidate is the astrocytic α7 nicotinic acetylcholine receptor (α7nAChR), upregulation of which correlates with Aβ pathology in the brain of individuals with AD. We revisit the past two decades of research into astrocytic α7nAChRs to shed light on their roles in the context of AD pathology and biomarkers. We discuss the involvement of astrocytic α7nAChRs in the instigation and potentiation of early Aβ pathology and explore their potential as a target for future reactive astrocyte-based therapeutics and imaging biomarkers in AD.

Nicotinic acetylcholine receptor subtype expression, function, and pharmacology: Therapeutic potential of α-conotoxins

The pentameric nicotinic acetylcholine receptors (nAChRs) are typically classed as muscle- or neuronal-type, however, the latter has also been reported in non-neuronal cells. Given their broad distribution, nAChRs mediate numerous physiological and pathological processes including synaptic transmission, presynaptic modulation of transmitter release, neuropathic pain, inflammation, and cancer. There are 17 different nAChR subunits and combinations of these subunits produce subtypes with diverse pharmacological properties. The expression and role of some nAChR subtypes have been extensively deciphered with the aid of knock-out models. Many nAChR subtypes expressed in heterologous systems are selectively targeted by the disulfide-rich α-conotoxins. α-Conotoxins are small peptides isolated from the venom of cone snails, and a number of them have potential pharmaceutical value.

PET Imaging of Cholinergic Neurotransmission in Neurodegenerative Disorders

As a neuromodulator, the neurotransmitter acetylcholine plays an important role in cognitive, mood, locomotor, sleep/wake, and olfactory functions. In the pathophysiology of most neurodegenerative diseases, such as Alzheimer disease (AD) or Lewy body disorder (LBD), cholinergic receptors, transporters, or enzymes are involved and relevant as imaging targets. The aim of this review is to summarize current knowledge on PET imaging of cholinergic neurotransmission in neurodegenerative diseases. For PET imaging of presynaptic vesicular acetylcholine transporters (VAChT), (-)-¹⁸F-fluoroethoxybenzovesamicol (¹⁸F-FEOBV) was the first PET ligand that could be successfully translated to clinical application. Since then, the number of ¹⁸F-FEOBV PET investigations on patients with AD or LBD has grown rapidly and provided novel, important findings concerning the pathophysiology of AD and LBD. Regarding the α4β2 nicotinic acetylcholine receptors (nAChRs), various second-generation PET ligands, such as ¹⁸F-nifene, ¹⁸F-AZAN, ¹⁸F-XTRA, (-)-¹⁸F-flubatine, and (+)-¹⁸F-flubatine, were developed and successfully translated to human application. In neurodegenerative diseases such as AD and LBD, PET imaging of α4β2 nAChRs is of special value for monitoring disease progression and drugs directed to α4β2 nAChRs. For PET of α7 nAChR, ¹⁸F-ASEM and ¹¹C-MeQAA were successfully applied in mild cognitive impairment and AD, respectively. The highest potential for α7 nAChR PET is seen in staging, in evaluating disease progression, and in therapy monitoring. PET of selective muscarinic acetylcholine receptors (mAChRs) is still in an early stage, as the development of subtype-selective radioligands is complicated. Promising radioligands to image mAChR subtypes M1 (¹¹C-LSN3172176), M2 (¹⁸F-FP-TZTP), and M4 (¹¹C-MK-6884) were developed and successfully translated to humans. PET imaging of mAChRs is relevant for the assessment and monitoring of therapies in AD and LBD. PET of acetylcholine esterase activity has been investigated since the 1990s. Many PET studies with ¹¹C-PMP and ¹¹C-MP4A demonstrated cortical cholinergic dysfunction in dementia associated with AD and LBD. Recent studies indicated a solid relationship between subcortical and cortical cholinergic dysfunction and noncognitive dysfunctions such as balance and gait in LBD. Taken together, PET of distinct components of cholinergic neurotransmission is of great interest for diagnosis, disease monitoring, and therapy monitoring and to gain insight into the pathophysiology of different neurodegenerative disorders.

Therapeutic Strategies Targeting Amyloid-β Receptors and Transporters in Alzheimer's Disease

Alzheimer's disease (AD) is a chronic neurodegenerative disease that has been recognized as one of the most intractable medical problems with heavy social and economic costs. Amyloid-β (Aβ) has been identified as a major factor that participates in AD progression through its neurotoxic effects. The major mechanism of Aβ-induced neurotoxicity is by interacting with membrane receptors and subsequent triggering of aberrant cellular signaling. Besides, Aβ transporters also plays an important role by affecting Aβ homeostasis. Thus, these Aβ receptors and transporters are potential targets for the development of AD therapies. Here, we summarize the reported therapeutic strategies targeting Aβ receptors and transporters to provide a molecular basis for future rational design of anti-AD agents.

iPSC model of CHRFAM7A effect on α7 nicotinic acetylcholine receptor function in the human context

The α7 nicotinic acetylcholine receptor (α7nAChR) has been a promising target for diseases affecting cognition and higher cortical functions; however, the effect observed in animal models failed to translate into human clinical trials identifying a translational gap. CHRFAM7A is a human-specific fusion gene with properties that enable incorporation into the α7nAChR and, being human specific, CHRFAM7A effect was not accounted for in preclinical studies. We hypothesized that CHRFAM7A may account for this translational gap and understanding its function may offer novel insights when exploring α7nAChR as a drug target. CHRFAM7A is present in different copy number variations (CNV) in the human genome with high frequency. To study the functional consequences of the presence of the CHRFAM7A, two induced pluripotent stem cell (iPSC) lines (0 copy and 1 copy direct) were developed. The 0 copy line was rescued with CHRFAM7A transfection to control for genetic heterogeneity. As readouts for genotype-phenotype correlation, α7nAChR synaptic transmission and amyloid beta 1-42 (Aβ1-42) uptake were tested. Synaptic transmission in the presence of CHRFAM7A demonstrated that PNU-modulated desensitization of α7nAChR currents increased as a function of CHRFAM7A dosage. CHRFAM7A mitigated the dose response of Aβ1-42 uptake suggesting a protective effect beyond physiological concentrations. Furthermore, in the presence of CHRFAM7A Aβ1-42 uptake activated neuronal interleukin 1β (IL-1β) and tumor necrosis factor α (TNF-α) without activating the canonical inflammasome pathway. Lead optimization may identify more potent molecules when the screen has a model harboring CHRFAM7A. Incorporating pharmacogenetics into clinical trials may enhance signals in efficacy measures.

Alpha7 acetylcholine receptor autoantibodies are rare in sera of patients diagnosed with schizophrenia or bipolar disorder

The α7 acetylcholine receptor (AChR) has been linked with the onset of psychotic symptoms and we hypothesized therefore that it might also be an autoimmune target. Here, we describe a new radioimmunoassay (RIA) using iodine 125-labelled α-bungarotoxin and membrane extract from transfected HEK293 cells expressing human α7 AChR. This RIA was used to analyze sera pertaining to a cohort of 711 subjects, comprising 368 patients diagnosed with schizophrenia spectrum disorders, 140 with bipolar disorder, 58 individuals diagnosed of other mental disorders, and 118 healthy comparison subjects. We identified one patient whose serum tested positive although with very low levels (0.2 nM) for α7 AChR-specific antibodies by RIA. Three out of 711 sera contained antibodies against iodine 125-labelled α-bungarotoxin, because they precipitated with it in the absence of α7 AChR. This first evidence suggests that autoantibodies against α7 AChR are absent or very rare in these clinical groups.

Acetylcholinesterase and Butyrylcholinesterase – Important Enzymes of Human Body

The serine hydrolases and proteases are a ubiquitous group of enzymes that is fundamental to many critical lifefunctions. Human tissues have two distinct cholinesterase activities: acetylcholinesterase and butyrylcholinesterase. Acetylcholinesterase functions in the transmission of nerve impulses, whereas the physiological function of butyrylcholinesterase remains unknown. Acetylcholinesterase is one of the crucial enzymes in the central and peripheral nerve system. Organophosphates and carbamates are potent inhibitors of serine hydrolases and well suited probes for investigating the chemical reaction mechanism of the inhibition. Understanding the enzyme’s chemistry is essential in preventing and/or treating organophosphate and carbamate poisoning as well as designing new medicaments for cholinergic-related diseases like as Alzheimer’s disease.

Cyto- and receptor architectonic mapping of the human brain

Mapping of the human brain is more than the generation of an atlas-based parcellation of brain regions using histologic or histochemical criteria. It is the attempt to provide a topographically informed model of the structural and functional organization of the brain. To achieve this goal a multimodal atlas of the detailed microscopic and neurochemical structure of the brain must be registered to a stereotaxic reference space or brain, which also serves as reference for topographic assignment of functional data, e.g., functional magnet resonance imaging, electroencephalography, or magnetoencephalography, as well as metabolic imaging, e.g., positron emission tomography. Although classic maps remain pioneering steps, they do not match recent concepts of the functional organization in many regions, and suffer from methodic drawbacks. This chapter provides a summary of the recent status of human brain mapping, which is based on multimodal approaches integrating results of quantitative cyto- and receptor architectonic studies with focus on the cerebral cortex in a widely used reference brain. Descriptions of the methods for observer-independent and statistically testable cytoarchitectonic parcellations, quantitative multireceptor mapping, and registration to the reference brain, including the concept of probability maps and a toolbox for using the maps in functional neuroimaging studies, are provided.

18F-XTRA PET for Enhanced Imaging of the Extrathalamic α4β2 Nicotinic Acetylcholine Receptor

Reduced density of the α4β2 nicotinic acetylcholine receptor (α4β2-nAChR) in the cortex and hippocampus of the human brain has been reported in aging and patients with neurodegenerative disease. This study assessed the pharmacokinetic behavior of ¹⁸F-(-)-JHU86428 (¹⁸F-XTRA), a new radiotracer for in vivo PET imaging of the α4β2-nAChR, particularly in extrathalamic regions of interest in which the α4β2-nAChR is less densely expressed than in thalamus. ¹⁸F-XTRA was also used to evaluate the α4β2-nAChR in the hippocampus in human aging. Methods: Seventeen healthy nonsmoker adults (11 men, 6 women; age, 30-82 y) underwent PET neuroimaging over 90 or 180 min in a high-resolution research tomograph after bolus injection of ¹⁸F-XTRA. Methods to quantify binding of ¹⁸F-XTRA to the α4β2-nAChR in the human brain were compared, and the relationship between age and binding in the hippocampus was tested. Results: ¹⁸F-XTRA rapidly entered the brain, and time-activity curves peaked within 10 min after injection for extrathalamic regions and at approximately 70 min in the thalamus. The 2-tissue-compartment model (2TCM) predicted the regional time-activity curves better than the 1-tissue-compartment model, and total distribution volume (V_T) was well identified by the 2TCM in all ROIs. V_T values estimated using Logan analysis with metabolite-corrected arterial input were highly correlated with those from the 2TCM in all regions, and values from 90-min scan duration were on average within 5% of those values from 180 min of data. Parametric images of V_T were consistent with the known distribution of the α4β2-nAChR across the brain. Finally, an inverse correlation between V_T in the hippocampus and age was observed. Conclusion: Our results extend support for use of ¹⁸F-XTRA with 90 min of emission scanning in quantitative human neuroimaging of the extrathalamic α4β2-nAChR, including in studies of aging.

The distribution of the alpha7 nicotinic acetylcholine receptor in healthy aging: An in vivo positron emission tomography study with [18F]ASEM

Altered function of the alpha7 nicotinic acetylcholine receptor (α7-nAChR) is implicated in several neuropsychiatric diseases. Nevertheless, studies of the human cerebral α7-nAChR even in healthy aging are limited in number and to postmortem tissue.

METHODS

The distribution of the cerebral α7-nAChR was estimated in nine brain regions in 25 healthy volunteers (ages 21-86 years; median 57 years, interquartile range 52 years) using [18F]ASEM with positron emission tomography (PET) imaging. Regional total distribution volume (VT) measurements were calculated using the Logan method from each subject's 90 min dynamic PET data and their metabolite-corrected plasma input function. Spearman's rank or Pearson's correlation analysis was used depending on the normality of the data. Correlation between age and regional 1) volume relative to intracranial volume (volume ratio) and 2) [18F]ASEM VT was tested. Correlation between regional volume ratio and [18F]ASEM VT was also evaluated. Finally, the relationship between [18F]ASEM VT and neuropsychological measures was investigated in a subpopulation of 15 elderly healthy participants (those 50 years of age and older). Bonferroni correction for multiple comparisons was applied to statistical analyses.

RESULTS

A negative correlation between tissue volume ratio and age was observed in six of the nine brain regions including striatum and five cortical (temporal, occipital, cingulate, frontal, or parietal) regions. A positive correlation between [18F]ASEM VT and age was observed in all nine brain regions of interest (ROIs). There was no correlation between [18F]ASEM VT and volume ratio in any ROI after controlling for age. Regional [18F]ASEM VT and neuropsychological performance on each of eight representative subtests were not correlated among the well-performing subpopulation of elderly healthy participants.

CONCLUSIONS

Our results suggest an increase in cerebral α7-nAChR distribution over the course of healthy aging that should be tested in future longitudinal studies. The preservation of the α7-nAChR in the aging human brain supports the development of therapeutic agents that target this receptor for use in the elderly. Further study of the relationship between α7-nAChR availability and cognitive impairment over aging is needed.

Impact of ageing on postsynaptic neuronal nicotinic neurotransmission in auditory thalamus

KEY POINTS

Neuronal nicotinic acetylcholine receptors (nAChRs) play a fundamental role in the attentional circuitry throughout the mammalian CNS. In the present study, we report a novel finding that ageing negatively impacts nAChR efficacy in auditory thalamus, and this is probably the result of a loss of nAChR density (Bmax ) and changes in the subunit composition of nAChRs. Our data support the hypothesis that age-related maladaptive changes involving nAChRs within thalamocortical circuits partially underpin the difficulty that elderly adults experience with respect to attending to speech and other salient acoustic signals.

ABSTRACT

The flow of auditory information through the medial geniculate body (MGB) is regulated, in part, by cholinergic projections from the pontomesencephalic tegmentum. The functional significance of these projections is not fully established, although they have been strongly implicated in the allocation of auditory attention. Using in vitro slice recordings, we have analysed postsynaptic function and pharmacology of neuronal nicotinic ACh receptors (nAChRs) in young adult and the aged rat MGB. We find that ACh produces significant excitatory postsynaptic actions on young MGB neurons, probably mediated by β2-containing heteromeric nAChRs. Radioligand binding studies show a significant age-related loss of heteromeric nAChR receptor number, which supports patch clamp data showing an age-related loss in ACh efficacy in evoking postsynaptic responses. Use of the β2-selective nAChR antagonist, dihydro-β-erythroidine, suggests that loss of cholinergic efficacy may also be the result of an age-related subunit switch from high affinity β2-containing nAChRs to low affinity β4-containing nAChRs, in addition to the loss of total nAChR number. This age-related nAChR dysfunction may partially underpin the attentional deficits that contribute to the loss of speech understanding in the elderly.

Single nucleotide polymorphisms and genotypes of transient receptor potential ion channel and acetylcholine receptor genes from isolated B lymphocytes in myalgic encephalomyelitis/chronic fatigue syndrome patients

Objective

The pathomechanism of chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) is unknown; however, a small subgroup of patients has shown muscarinic antibody positivity and reduced symptom presentation following anti-CD20 intervention. Given the important roles of calcium (Ca²⁺) and acetylcholine (ACh) signalling in B cell activation and potential antibody development, we aimed to identify relevant single nucleotide polymorphisms (SNPs) and genotypes in isolated B cells from CFS/ME patients.

Methods

A total of 11 CFS/ME patients (aged 31.82 ± 5.50 years) and 11 non-fatigued controls (aged 33.91 ± 5.06 years) were included. Flow cytometric protocols were used to determine B cell purity, followed by SNP and genotype analysis for 21 mammalian TRP ion channel genes and nine mammalian ACh receptor genes. SNP association and genotyping analysis were performed using ANOVA and PLINK analysis software.

Results

Seventy-eight SNPs were identified in nicotinic and muscarinic acetylcholine receptor genes in the CFS/ME group, of which 35 were in mAChM3. The remaining SNPs were identified in nAChR delta (n = 12), nAChR alpha 9 (n = 5), TRPV2 (n = 7), TRPM3 (n = 4), TRPM4 (n = 1) mAChRM3 2 (n = 2), and mAChRM5 (n = 3) genes. Nine genotypes were identified from SNPs in TRPM3 (n = 1), TRPC6 (n = 1), mAChRM3 (n = 2), nAChR alpha 4 (n = 1), and nAChR beta 1 (n = 4) genes, and were located in introns and 3′ untranslated regions. Odds ratios for these specific genotypes ranged between 7.11 and 26.67 for CFS/ME compared with the non-fatigued control group.

Conclusion

This preliminary investigation identified a number of SNPs and genotypes in genes encoding TRP ion channels and AChRs from B cells in patients with CFS/ME. These may be involved in B cell functional changes, and suggest a role for Ca²⁺ dysregulation in AChR and TRP ion channel signalling in the pathomechanism of CFS/ME.

A critical review of neonicotinoid insecticides for developmental neurotoxicity

A comprehensive review of published and previously unpublished studies was performed to evaluate the neonicotinoid insecticides for evidence of developmental neurotoxicity (DNT). These insecticides have favorable safety profiles, due to their preferential affinity for nicotinic receptor (nAChR) subtypes in insects, poor penetration of the mammalian blood-brain barrier, and low application rates. Nevertheless, examination of this issue is warranted, due to their insecticidal mode of action and potential exposure with agricultural and residential uses. This review identified in vitro, in vivo, and epidemiology studies in the literature and studies performed in rats in accordance with GLP standards and EPA guidelines with imidacloprid, acetamiprid, thiacloprid, clothianidin, thiamethoxam, and dinotefuran, which are all the neonicotinoids currently registered in major markets. For the guideline-based studies, treatment was administered via the diet or gavage to primiparous female rats at three dose levels, plus a vehicle control (≥20/dose level), from gestation day 0 or 6 to lactation day 21. F1 males and females were evaluated using measures of motor activity, acoustic startle response, cognition, brain morphometry, and neuropathology. The principal effects in F1 animals were associated with decreased body weight (delayed sexual maturation, decreased brain weight, and morphometric measurements) and acute toxicity (decreased activity during exposure) at high doses, without neuropathology or impaired cognition. No common effects were identified among the neonicotinoids that were consistent with DNT or the neurodevelopmental effects associated with nicotine. Findings at high doses were associated with evidence of systemic toxicity, which indicates that these insecticides do not selectively affect the developing nervous system.

Therapeutic Potential of α7 Nicotinic Acetylcholine Receptors

Progress in the fields of neuroscience and molecular biology has identified the forebrain cholinergic system as being important in many higher order brain functions. Further analysis of the genes encoding the nicotinic acetylcholine receptors (nAChRs) has highlighted, in particular, the role of α7 nAChRs in these higher order brain functions as evidenced by their peculiar physiologic and pharmacological properties. As this receptor has gained the attention of scientists from academia and industry, our knowledge of its roles in various brain and bodily functions has increased immensely. We have also seen the development of small molecules that have further refined our understanding of the roles of α7 nAChRs, and these molecules have begun to be tested in clinical trials for several indications. Although a large body of data has confirmed a role of α7 nAChRs in cognition, the translation of small molecules affecting α7 nAChRs into therapeutics has to date only progressed to the stage of testing in clinical trials. Notably, however, most recent human genetic and biochemical studies are further underscoring the crucial role of α7 nAChRs and associated genes in multiple organ systems and disease states. The aim of this review is to discuss our current knowledge of α7 nAChRs and their relevance as a target in specific functional systems and disease states.

Neuronal α7 Nicotinic Receptors as a Target for the Treatment of Schizophrenia

Schizophrenia is a lifelong disease, the burden of which is often underestimated. Characterized by positive (e.g., hallucinations) and negative (e.g., avolition, amotivation) symptoms, schizophrenia is also accompanied with profound impairments in cognitive function that progress throughout the development of the disease. Although treatment with antipsychotic medications can effectively dampen some of the positive symptoms, these medications largely fail to reverse cognitive deficits or to mitigate negative symptoms. With a worldwide prevalence of approximately 1%, schizophrenia remains a large unmet medical need that stands to benefit greatly from (1) continued research to better understand the biological underpinnings of the disease and (2) the targeted development of novel therapeutics to improve the lives of those affected individuals. Improvements in our understanding of the neuronal networks associated with schizophrenia as well as progress in identifying genetic risk factors and environmental conditions that may predispose individuals to developing the disease are advancing new strategies to study and treat it. Herein, we review the evidence that supports the role of α7 nicotinic acetylcholine receptors in the central nervous system and why these receptors constitute a promising target to treat some of the prominent symptoms of schizophrenia.

Development of [(18)F]ASEM, a specific radiotracer for quantification of the α7-nAChR with positron-emission tomography

The alpha-7 subtype of the nicotinic acetylcholine receptor (α7-nAChR) is fundamental to physiology; it mediates various brain functions and represents an important target for drug discovery. Exploration of the brain nicotinic acetylcholine receptors (nAChRs) using positron-emission tomography (PET) will make it possible to better understand the important role of this receptor and to study its involvement in schizophrenia, bipolar disorder, Alzheimer's and Parkinson's diseases, drug dependence, inflammation and many other disorders and simplify the development of nicotinic drugs for treatment of these disorders. Until recently, PET imaging of α7-nAChRs has been impeded by the absence of good radiotracers. This review describes various endeavors to develop α7-nAChR PET tracers by several research groups including the author's group. Most initial PET tracers for imaging α7-nAChRs did not exhibit suitable imaging properties due to their low specific binding. Newly discovered [(18)F]ASEM is the first highly specific α7-nAChR radioligand and in 2014 it was translated to human PET imaging.

The separate and combined effects of monoamine oxidase A inhibition and nicotine on the mismatch negativity event related potential

The mismatch negativity (MMN) auditory event-related potential (ERP) has been extensively studied as a potential biomarker for abnormal auditory processing in schizophrenia (SZ), a population which exhibits abnormally high smoking rates. The relationship between nicotinic activation and cognition in SZ may be related to underlying nicotinic and NMDA receptor dysfunction within the disease. However, transient cognitive improvements via smoking in patients may also result from monoamine oxidase (MAO) inhibition, achieved through tobacco smoke. In 24 healthy non-smoking males, we investigated the separate and combined effects of nicotine and MAO-A inhibition via moclobemide (75mg) on the optimal-5 variation of the MMN paradigm. No significant drug effects were observed in our total sample, however, stratification of individuals into low (N=12) and high (N=12) baseline MMN amplitude groups revealed increases in duration MMN amplitude relative to placebo by nicotine, as well as moclobemide, but not after the combination of the two. Because previous research has shown there was no effect of monoamine modulation on MMN, this study shows an unexpected effect of moclobemide on duration MMN.

Human brain imaging of α7 nAChR with [(18)F]ASEM: a new PET radiotracer for neuropsychiatry and determination of drug occupancy

PURPOSE

Using the α7-nAChR radiotracer, [(18)F]ASEM, we present the first successful human positron emission tomography (PET) studies. Rodent occupancy with three clinically employed α7-nAChR drugs confirms the specificity of the radiotracer.

PROCEDURES

Five healthy male subjects were imaged for 90 min following IV [(18)F]ASEM. Two subjects were scanned for the second time (test/retest; TRV). Mouse biodistribution of [(18)F]ASEM was carried out in CD1 mice injected with using human equivalent doses of DMXB-A, EVP-6124, and varenicline to block specific binding.

RESULTS

[(18)F]ASEM readily entered the brain and peaked at 15 min post-injection with reversible kinetics and a peak %SUV of about 400 %. The regional human brain distribution of [(18)F]ASEM matched previous in vitro data and baboon PET results. The precuneus, parietal, occipital, cingulate cortexes, putamen, and thalamus showed high values of distribution volume (>20 ml/ml) and binding potentials >1 with TRV averaged 10.8 ± 5.1 %. In mouse distribution studies, there was significant dose-dependent blockade in the mouse brain with DMXB-A as well as the other two α7-nAChR drugs.

CONCLUSIONS

The characteristics of [(18)F]ASEM are consistent with the ability to quantify α7-nAChR in the human brain. [(18)F]ASEM is suitable for imaging neuropsychiatric disorders and target engagement (receptor occupancy) of potential α7-nAChR drugs.

LC3 overexpression reduces Aβ neurotoxicity through increasing α7nAchR expression and autophagic activity in neurons and mice

Autophagy is an intracellular degradation pathway with dynamic interactions for eliminating damaged organelles and protein aggregates by lysosomal digestion. The EGFP-conjugated microtubule-associated protein 1 light chain 3 (EGFP-LC3) serves to monitor autophagic process. Extracellular β-amyloid peptide accumulation is reported as a major cause in Alzheimer's disease (AD) pathogenesis; large numbers of autophagic vacuoles accumulate in patients' brains. We previously demonstrated that extracellular Aβ (eAβ) induces strong autophagic response and α7nAChR acts as a carrier to bind with eAβ; which further inhibits Aβ-induced neurotoxicity via autophagic degradation. In the present study, we overexpressed LC3 in both neuroblastoma cells (SH-SY5Y/pEGFP-LC3) and mice (TgEGFP-LC3) to assess the effect of LC3 overexpression on Aβ neurotoxicity. SH-SY5Y/pEGFP-LC3 cells and primary cortical neuron cultures derived from E17 (embryonic day 17) TgEGFP-LC3 mice showed not only better resistance against Aβ neurotoxicity but also higher α7nAChR expression and autophagic activity than control. Administration of α-bungarotoxin (α-BTX) to block α7nAChR antagonized the neuroprotective action of SH-SY5Y/pECGF-LC3 cells, suggesting that eAβ binding with α7nAChR is an important step in Aβ detoxification. LC3 overexpression thus exerts neuroprotection through increasing α7nAChR expression for eAβ binding and further enhancing autophagic activity for Aβ clearance in vitro and in vivo.

Molecular imaging of neuropsychiatric symptoms in Alzheimer's and Parkinson's disease

Neuropsychiatric symptoms (NPS) are very common in neurodegenerative diseases and are a major contributor to disability and caregiver burden. There is accumulating evidence that NPS may be a prodrome and/or a "risk factor" of neurodegenerative diseases. The medications used to treat these symptoms in younger patients are not very effective in patients with neurodegenerative disease and may have serious side effects. An understanding of the neurobiology of NPS is critical for the development of more effective intervention strategies. Targeting these symptoms may also have implications for prevention of cognitive or motor decline. Molecular brain imaging represents a bridge between basic and clinical observations and provides many opportunities for translation from animal models and human post-mortem studies to in vivo human studies. Molecular brain imaging studies in Alzheimer's disease (AD) and Parkinson's disease (PD) are reviewed with a primary focus on positron emission tomography studies of NPS. Future directions for the field of molecular imaging in AD and PD to understand the neurobiology of NPS will be discussed.

Neural nAChRs PET imaging probes

There have been a number of attempts to study PET radioligands for imaging nicotinic acetylcholine receptors (nAChRs) in the human brain, and the most successful tracers found are radioligands for imaging α4β2-nAChRs, which is the main cerebral nAChRs subtype. C-Nicotine and 2-[F]FA have been applied in many studies in humans. However, neither is entirely ideal and efforts have been made to develop radioligands with optimized imaging properties. Only a few reports have been published on radioligands for α7-nAChRs imaging, another important cerebral nAChRs subtype. This paper will review the development of PET radioligands for imaging cerebral nAChRs.

Activation of α4β2*/α6β2* nicotinic receptors alleviates anxiety during nicotine withdrawal without upregulating nicotinic receptors

Although nicotine mediates its effects through several nicotinic acetylcholine receptor (nAChR) subtypes, it remains to be determined which nAChR subtypes directly mediate heightened anxiety during withdrawal. Relative success in abstinence has been found with the nAChR partial agonist varenicline (Chantix; Pfizer, Groton, CT); however, treatment with this drug fails to alleviate anxiety in individuals during nicotine withdrawal. Therefore, it is hypothesized that success can be found by the repurposing of other nAChR partial agonists for cessation therapies that target anxiety. It is noteworthy that the selective partial agonists for α4β2, ABT-089 [2-methyl-3-[2(S)-pyrrolidinylmethoxy]pyridine], and α7, ABT-107 [5-(6-[(3R)-1-azabicyclo[2.2.2]oct-3-yloxy] pyridazin-3-yl)-1H-indole] (AbbVie, North Chicago, IL), have not been evaluated as possible therapeutics for nicotine cessation. Therefore, we examined the effect of ABT-089 and ABT-107 on anxiety during withdrawal from nicotine in the novelty-induced hypophagia (NIH) paradigm. We found that short-term administration of ABT-089 and ABT-107 alleviate anxiety-like behavior during withdrawal from nicotine while long-term administration of ABT-089 but not ABT-107 reduces anxiety-like behavior during withdrawal. After behavioral testing, brains were harvested and β2-containing nAChRs were measured using [(3)H]epibaditine. ABT-089 and ABT-107 do not upregulate nAChRs, which is in contrast to the upregulation of nAChRs observed after nicotine. Furthermore, ABT-089 is anxiogenic in nicotine naive animals, suggesting that the effects on anxiety are specifically related to the nicotine-dependent state. Together, these studies identify additional nAChR partial agonists that may aid in the rational development of smoking cessation aids.

Diverse strategies targeting α7 homomeric and α6β2* heteromeric nicotinic acetylcholine receptors for smoking cessation

Preclinical studies suggest that a diversity of nicotinic acetylcholine receptors (nAChRs) with different sensitivities to nicotine may contribute to tobacco addiction. Using rodent intravenous nicotine self-administration as a preclinical model with good predictive validity for therapeutic efficacy for tobacco cessation, investigators have identified heteromeric α6β2* and homomeric α7 nAChRs as promising novel therapeutic targets to promote smoking abstinence (*denotes possible assembly with other subunits). The data suggest that diverse strategies that target these subclasses of nAChRs, namely inhibition of α6β2* nAChRs and stimulation of α7 nAChRs, will support tobacco cessation. α6β2* nAChRs, members of the high-affinity family of β2* nAChRs, function similarly to α4β2* nAChRs, the primary target of the FDA-approved drug varenicline, but have a much more selective neuroanatomical pattern of expression in catecholaminergic nuclei. Although activation of β2* nAChRs facilitates nicotine self-administration, stimulation of α7 nAChRs appears to negatively modulate both nicotine reinforcement and β2* nAChR function in the mesolimbic dopamine system. Although challenges and caveats must be considered in the development of therapeutics that target these nAChR subpopulations, an accumulation of data suggests that α7 nAChR agonists, partial agonists, or positive allosteric modulators and α6β2* nAChR antagonists, partial agonists, or negative allosteric modulators may prove to be effective therapeutics for tobacco cessation.

Using rodents to model schizophrenia and substance use comorbidity

Schizophrenia and substance use disorders (SUD) often occur together, yet it is unclear why this is the case or how best to manage dual diagnosis. Rodent models are well suited to study how genes and environment interact to impact neurodevelopment, brain function and behaviors relevant to dual diagnosis. Indeed a variety of rodent models for schizophrenia display behavioral and physiological features relevant to SUD including: neurodevelopmental models, models of a rare variant (Disc1), to models of common variants (neurexin, dysbindin and neuregulin), and models of various gene-drug interactions. Thus it may be worthwhile to probe models of schizophrenia for insights relevant to SUD and dual diagnosis. However, future studies on dual diagnosis should involve characterization beyond measuring locomotor responses to self-administration tasks, include drug classes other than psychostimulants, and dissect the neuroadaptations that underlie risk for dual diagnosis.

Brain volumes and neuropsychological performance are related to current smoking and alcoholism history

BACKGROUND

Dual dependence on alcohol and nicotine is common, with many reports suggesting that more than 80% of alcoholics also smoke cigarettes. Even after cessation of alcohol consumption, many recovering alcoholics continue to smoke. In this exploratory study, we examined how current smoking and a history of alcoholism interacted in relation to brain volumes and neuropsychological performance.

METHODS

Participants were 14 abstinent long-term alcoholics (seven current smokers and seven nonsmokers), and 13 nonalcoholics (six current smokers and seven nonsmokers). The groups were equivalent in age, gender, education, and intelligence quotient. Two multiecho magnetization-prepared rapid acquisition with gradient echo (MP-RAGE) scans were collected for all participants using a 3T magnetic resonance imaging scanner with a 32 channel head coil. Brain volumes for each gray and white matter region of interest were derived using FreeSurfer. Participants completed a battery of neuropsychological tests measuring intelligence quotient, memory, executive functions, personality variables, and affect.

RESULTS

COMPARED TO NONSMOKING NONALCOHOLICS, ALCOHOLICS WHO SMOKE (THE COMORBID GROUP) HAD VOLUMETRIC ABNORMALITIES IN: pre- and para-central frontal cortical areas and rostral middle frontal white matter; parahippocampal and temporal pole regions; the amygdala; the pallidum; the ventral diencephalic region; and the lateral ventricle. The comorbid group performed worse than nonsmoking nonalcoholics on tests of executive functioning and on visually-based memory tests. History of alcoholism was associated with higher neuroticism scores among smokers, and current smoking was associated with higher sensation seeking scores and lower extraversion scores among nonalcoholics.

CONCLUSION

Results from this exploratory study support and extend prior reports showing that alcoholism and smoking, alone and in combination, are associated with structural brain abnormalities and poorer performance on neuropsychological tests. Therefore, it is important to consider smoking status in alcoholism studies and vice versa.

Enhancement of nicotinic receptors alleviates cytotoxicity in neurological disease models

The common pathological mechanisms among the spectrum of neurodegenerative diseases are supposed to be shared. Multiple lines of evidence, from molecular and cellular to epidemiological, have implicated nicotinic transmission in the pathology of the two most common neurodegenerative disorders, namely Alzheimer's disease (AD) and Parkinson's disease (PD). In this review article we present evidence of nicotinic acetylcholine receptor (nAChR)-mediated protection against neurotoxicity induced by β amyloid (Aβ), glutamate, rotenone, and 6-hydroxydopamine (6-OHDA) and the signal transduction involved in this mechanism. Our studies have clarified that survival signal transduction, the α7 nAChR/Src family/PI3K/AKT pathway and subsequent upregulation of Bcl-2 and Bcl-x, would lead to neuroprotection. In addition to the PI3K/AKT pathway, two other survival pathways, JAK2/STAT3 and MEK/ERK, are proposed by other groups. In rotenone- and 6-OHDA-induced PD models, nAChR-mediated neuroprotection was also observed, and the effect was blocked not only by α7 but also by α4β2 nAChR antagonists. We also document that nAChR stimulation blocks glutamate neurotoxicity in spinal cord motor neurons. These findings suggest that nAChR-mediated neuroprotection is achieved through subtypes of nAChRs and common signal cascades. An early diagnosis and protective therapy with nAChR stimulation could be effective in delaying the progression of neurodegenerative diseases such as AD, PD and amyotrophic lateral sclerosis.

Alpha7 neuronal nicotinic receptors as a drug target in schizophrenia

INTRODUCTION

Schizophrenia is a profoundly debilitating disease that represents not only an individual, but a societal problem. Once characterized solely by the hyperactivity of the dopaminergic system, therapies directed to dampen dopaminergic neurotransmission were developed. However, these drugs do not address the significant impairments in cognition and the negative symptoms of the disease, and it is now apparent that disequilibrium of many neurotransmitter systems is involved. Despite enormous efforts, minimal progress has been made toward the development of safer, more effective therapies to date.

AREAS COVERED

The high preponderance of smoking in schizophrenics suggests that nicotine may provide symptomatic improvement, which has led to investigation for selective molecules targeted to individual nicotinic receptor (nAChR) subtypes. Of special interest is activation of the homomeric α7nAChR, which is widely distributed in the brain and has been implicated in the pathophysiology of schizophrenia through numerous approaches.

EXPERT OPINION

Preclinical and clinical data suggest that neuronal α7nAChRs play an important role in cognitive functions. Moreover, some, but not all, early clinical trials conducted with α7nAChR agonists show cognitive benefits in schizophrenics. These encouraging results suggest that development of compounds targeting α7nAChRs will represent a valuable tool to mitigate symptoms associated with schizophrenia, and open new strategies for better pharmacological treatment of these patients.

Cerebellar nicotinic cholinergic receptors are intrinsic to the cerebellum: implications for diverse functional roles

Although recent studies have delineated the specific nicotinic subtypes present in the mammalian cerebellum, very little is known about their location or function within the cerebellum. This is of increased interest since nicotinic receptors (nAChRs) in the cerebellum have recently been implicated in the pathology of autism spectrum disorders. To begin to better understand the roles of these heteromeric nAChRs in the cerebellar circuitry and their therapeutic potential as targets for drug development, we used various chemical and stereotaxic lesion models in conjunction with slice electrophysiology to examine how specific heteromeric nAChR subtypes may influence the surrounding cerebellar circuitry. Using subunit-specific immunoprecipitation of radiolabeled nAChRs in the cerebella following N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride, p-chloroamphetamine, and pendunculotomy lesions, we show that most, if not all, cerebellar nicotinic receptors are present in cells within the cerebellum itself and not in extracerebellar afferents. Furthermore, we demonstrate that the β4-containing, but not the β2-containing, nAChRs intrinsic to the cerebellum can regulate inhibitory synaptic efficacy at two major classes of cerebellar neurons. These tandem findings suggest that nAChRs may present a potential drug target for disorders involving the cerebellum.

Lifetime cigarette smoking is associated with striatal volume measures

Nicotine, the primary addictive component of tobacco, affects the mammalian brain. Smokers' brains have smaller cortical grey matter volumes and/or lower densities compared with non-smokers'. Differences in subcortical structures like the striatum are however, less clear. A high concentration of nicotinic receptors makes the striatum a potential target for nicotine. In addition, striatal nuclei are essential components of the reward/reinforcement pathway involved in addiction. The aim of this study was to explore the relationship between striatal nuclei (caudate, putamen and nucleus accumbens area) volumes and lifetime smoking in a large community-based sample of 'young-old' individuals. Brain volumes were measured using a semi-automated method in 315 participants aged 64-70 years who were selected from a larger randomly sampled cohort and who consented to a magnetic resonance imaging scan. Multiple regression analysis was used to assess the relationship between striatal volumes and cigarette smoking measures while controlling for age, sex, intracranial and total brain volumes and general physical and mental health measures. Greater lifetime use of cigarettes (measured in pack-years) was associated with smaller left nucleus accumbens area volume (P = 0.018) and larger left putamen volume (P = 0.025). Greater putaminal volume was also associated with a lower age at smoking initiation (P = 0.004). In this generally healthy cohort, lifetime use of cigarettes is significantly associated with striatal volume measures. These changes could indicate predisposing factors for nicotine addiction, or an effect of chronic nicotine exposure or a combination of both.

Lipophilicity as a determinant of binding of procaine analogs to rat α3β4 nicotinic acetylcholine receptor

Nicotinic acetylcholine receptors (nAChRs) have been studied in detail with regard to their interaction with therapeutic and drug addiction-related compounds. Using a structure-activity approach, we have examined the relationship among the molecular features of a set of eight para-R-substituted N,N-[(dimethylamino)ethyl] benzoate hydrochlorides, structurally related to procaine and their affinity for the α(3)β(4) nAChR heterologously expressed in KXα3β4R2 cells. Affinity values (log[1/IC50]) of these compounds for the α(3)β(4) nAChR were determined by their competition with [(3)H]TCP binding. Log(1/IC50) values were analyzed considering different hydrophobic and electronic parameters and those related to molar refractivity. These have been experimentally determined or were taken from published literature. In accordance with literature observations, the generated cross-validated quantitative structure-activity relationship (QSAR) equations indicated a significant contribution of hydrophobic term to binding affinity of procaine analogs to the receptor and predicted affinity values for several local anesthetics (LAs) sets taken from the literature. The predicted values by using the QSAR model correlated well with the published values both for neuronal and for electroplaque nAChRs. Our work also reveals the general structure features of LAs that are important for interaction with nAChRs as well as the structural modifications that could be made to enhance binding affinity.

Alpha7 nicotinic acetylcholine receptors modulate motivation to self-administer nicotine: implications for smoking and schizophrenia

Individuals diagnosed with schizophrenia have an exceptionally high risk for tobacco dependence. Postmortem studies show that these individuals have significant reductions in α7 nicotinic acetylcholine receptors (nAChRs) in several brain areas. Decreased α7-mediated function might not only be linked to schizophrenia but also to increased tobacco consumption. The purpose of this study was to determine whether pharmacological blockade of α7 nAChRs would increase motivation of rats to intravenously self-administer nicotine (NIC) during a progressive ratio schedule of reinforcement (PR). Before PR, rats received local infusions of 0, 10, or 20 pmol of a selective α7 nAChR antagonist, α-conotoxin ArIB [V11L,V16D] (ArIB) into the nucleus accumbens (NAc) shell or the anterior cingulate cortex, brain areas that contribute to motivation for drug reward. We additionally sought to determine whether local infusion of 0, 10, or 40 nmol of a selective α7 nAChR agonist, PNU 282987, into these brain areas would decrease motivation for NIC use. Infusion of ArIB into the NAc shell and anterior cingulate cortex resulted in a significant increase in active lever pressing, breakpoints, and NIC intake, suggesting that a decrease in α7 nAChR function increases motivation to work for NIC. In contrast, PNU 282987 infusion resulted in reductions in these measures when administered into the NAc shell, but had no effect after administration into the anterior cingulate cortex. These data identify reduction of α7 nAChR function as a potential mechanism for elevated tobacco use in schizophrenia and also identify activation of α7 nAChRs as a potential strategy for tobacco cessation therapy.

Assessment of α7 nicotinic acetylcholine receptor availability in juvenile pig brain with [¹⁸F]NS10743

PURPOSE

To conduct a quantitative PET assessment of the specific binding sites in the brain of juvenile pigs for [(18)F]NS10743, a novel diazabicyclononane derivative targeting α7 nicotinic acetylcholine receptors (α7 nAChRs).

METHODS

Dynamic PET recordings were made in isoflurane-anaesthetized juvenile pigs during 120 min after administration of [(18)F]NS10743 under baseline conditions (n = 3) and after blocking of the α7 nAChR with NS6740 (3 mg·kg(-1) bolus + 1 mg·kg(-1)·h(-1) continuous infusion; n = 3). Arterial plasma samples were collected for determining the input function of the unmetabolized tracer. Kinetic analysis of regional brain time-radioactivity curves was performed, and parametric maps were calculated relative to arterial input.

RESULTS

Plasma [(18)F]NS10743 passed readily into the brain, with peak uptake occurring in α7 nAChR-expressing brain regions such as the colliculi, thalamus, temporal lobe and hippocampus. The highest SUV(max) was approximately 2.3, whereas the lowest uptake was in the olfactory bulb (SUV(max) 1.53 ± 0.32). Administration of NS6740 significantly decreased [(18)F]NS10743 binding late in the emission recording throughout the brain, except in the olfactory bulb, which was therefore chosen as reference region for calculation of BP(ND). The baseline BP(ND) ranged from 0.39 ± 0.08 in the cerebellum to 0.76 ± 0.07 in the temporal lobe. Pretreatment and constant infusion with NS6740 significantly reduced the BP(ND) in regions with high [(18)F]NS10743 binding (temporal lobe -29%, p = 0.01; midbrain: -35%, p = 0.02), without significantly altering the BP(ND) in low binding regions (cerebellum: -16%, p = 0.2).

CONCLUSION

This study confirms the potential of [(18)F]NS10743 as a target-specific radiotracer for the molecular imaging of central α7 nAChRs by PET.

Inhibition mechanism of rat α₃β₄ nicotinic acetylcholine receptor by the Alzheimer therapeutic tacrine

Nicotinic acetylcholine receptors (nAChRs) were studied in detail in the past regarding their interaction with therapeutic and drug addiction related compounds. Using fast kinetic whole-cell recording, we have now studied effects of tacrine, an agent used clinically to treat Alzheimer's disease, on currents elicited by activation of rat α(3)β(4) nAChR heterologously expressed in KXα3β4R2 cells. Characterization of receptor activation by nicotine used as agonist revealed a K(d) of 23 ± 0.2 μM and 4.3 ± 1.3 for the channel opening equilibrium constant, Φ(-1). Experiments were performed to investigate whether tacrine is able to activate the α(3)β(4) nAChR. Tacrine did not activate whole-cell currents in KXα3β4R2 cells but inhibited receptor activity at submicromolar concentration. Dose-response curves obtained with increasing agonist or inhibitor concentration revealed competitive inhibition of nAChRs by tacrine, with an apparent inhibition constant, K(I), of 0.8 μM. The increase of Φ(-1) in the presence of tacrine suggests that the drug stabilizes a nonconducting open channel form of the receptor. Binding studies with TCP and MK-801 ruled out tacrine binding to common allosteric sites of the receptor. Our study suggests a novel mechanism for action of tacrine on nAChRs besides inhibition of acetylcholine esterase.

Function of partially duplicated human α77 nicotinic receptor subunit CHRFAM7A gene: potential implications for the cholinergic anti-inflammatory response

The neuronal α7 nicotinic receptor subunit gene (CHRNA7) is partially duplicated in the human genome forming a hybrid gene (CHRFAM7A) with the novel FAM7A gene. The hybrid gene transcript, dupα7, has been identified in brain, immune cells, and the HL-60 cell line, although its translation and function are still unknown. In this study, dupα7 cDNA has been cloned and expressed in GH4C1 cells and Xenopus oocytes to study the pattern and functional role of the expressed protein. Our results reveal that dupα7 transcript was natively translated in HL-60 cells and heterologously expressed in GH4C1 cells and oocytes. Injection of dupα7 mRNA into oocytes failed to generate functional receptors, but when co-injected with α7 mRNA at α7/dupα7 ratios of 5:1, 2:1, 1:1, 1:5, and 1:10, it reduced the nicotine-elicited α7 current generated in control oocytes (α7 alone) by 26, 53, 75, 93, and 94%, respectively. This effect is mainly due to a reduction in the number of functional α7 receptors reaching the oocyte membrane, as deduced from α-bungarotoxin binding and fluorescent confocal assays. Two additional findings open the possibility that the dominant negative effect of dupα7 on α7 receptor activity observed in vitro could be extrapolated to in vivo situations. (i) Compared with α7 mRNA, basal dupα7 mRNA levels are substantial in human cerebral cortex and higher in macrophages. (ii) dupα7 mRNA levels in macrophages are down-regulated by IL-1β, LPS, and nicotine. Thus, dupα7 could modulate α7 receptor-mediated synaptic transmission and cholinergic anti-inflammatory response.

NMR structure of the transmembrane domain of the n-acetylcholine receptor beta2 subunit

Nicotinic acetylcholine receptors (nAChRs) are involved in fast synaptic transmission in the central and peripheral nervous system. Among the many different types of subunits in nAChRs, the beta2 subunit often combines with the alpha4 subunit to form alpha4beta2 pentameric channels, the most abundant subtype of nAChRs in the brain. Besides computational predictions, there is limited experimental data available on the structure of the beta2 subunit. Using high-resolution NMR spectroscopy, we solved the structure of the entire transmembrane domain (TM1234) of the beta2 subunit. We found that TM1234 formed a four-helix bundle in the absence of the extracellular and intracellular domains. The structure exhibited many similarities to those previously determined for the Torpedo nAChR and the bacterial ion channel GLIC. We also assessed the influence of the fourth transmembrane helix (TM4) on the rest of the domain. Although secondary structures and tertiary arrangements were similar, the addition of TM4 caused dramatic changes in TM3 dynamics and subtle changes in TM1 and TM2. Taken together, this study suggests that the structures of the transmembrane domains of these proteins are largely shaped by determinants inherent in their sequence, but their dynamics may be sensitive to modulation by tertiary and quaternary contacts.