Neuronal-type alpha-bungarotoxin receptors and the alpha 5-nicotinic receptor subunit gene are expressed in neuronal and nonneuronal human cell lines

A historic perspective on the current progress in elucidation of the biologic significance of non-neuronal acetylcholine

The "5th International Symposium on Non-neuronal Acetylcholine: from bench to bedside" was held on September 27-29, 2019 in Hyatt Regency, Long Beach, CA, USA. Approximately 50 scientists from 11 countries over 6 continents participated in this meeting. The major topics included an overall biologic significance of non-neuronal acetylcholine (ACh) and the roles of the non-neuronal cholinergic systems in mucocutaneous, respiratory, digestive, immunologic, endocrine, cardiovascular, musculoskeletal and kidney diseases, and cancer. This meeting facilitated continued work to advance the fundamental science and translational aspects of the interdisciplinary studies on non-neuronal ACh. The progress made has opened a new chapter in the field of cholinergic pharmacology, and advanced our knowledge beyond regulation of individual cell- and tissue-types, defining a new paradigm of selective pharmacological regulation of vital function of practically all types of non-neuronal cells. It is now clear that the autocrine and paracrine control of non-neuronal cells by non-neuronal ACh is implemented through synergistic, additive, and reciprocal effects triggered by two different cholinergic receptor classes. Each biologic effect of ACh is determined by a unique combination of cholinergic receptors subtype expressed at each stage of cell development and differentiation. The plasticity of the non-neuronal cholinergic system helps adjust homeostasis to new environmental conditions.

Cholinergic Receptors and Addiction

Human behavior can be controlled by physical or psychological dependencies associated with addiction. One of the most insidious addictions in our society is the use of tobacco products which contain nicotine. This addiction can be associated with specific receptors in the brain that respond to the natural neurotransmitter acetylcholine. These nicotinic acetylcholine receptors (nAChR) are ligand-gated ion channels formed by the assembly of one or multiple types of nAChR receptor subunits. In this paper, we review the structure and diversity of nAChR subunits and our understanding for how different nAChR subtypes play specific roles in the phenomenon of nicotine addiction. We focus on receptors containing β2 and/or α6 subunits and the special significance of α5-containing receptors. These subtypes all have roles in regulating dopamine-mediated neurotransmission in the mesolimbic reward pathways of the brain. We also discuss the unique roles of homomeric α7 nAChR in behavioral responses to nicotine and how our knowledge of nAChR functional diversity may help guide pharmacotherapeutic approaches for treating nicotine addiction. While nicotine addiction is a truly global problem, the use of areca nut (betel) products is also a serious addiction associated with public health issues across most of South Asia, impacting as many as 600 million people. We discuss how cholinergic receptors of the brain are also involved with areca addiction and the unique challenges for dealing with addiction to this substance.

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.

The fifth subunit in α3β4 nicotinic receptor is more than an accessory subunit

The α3β4 subtype is the predominant neuronal nicotinic acetylcholine receptor present in the sensory and autonomic ganglia and in a subpopulation of brain neurons. This subtype can form pentameric receptors with either 2 or 3 β4 subunits that have different pharmacologic and functional properties. To further investigate the role of the fifth subunit, we coexpressed a dimeric construct coding for a single polypeptide containing the β4 and α3 subunit sequences, with different monomeric subunits. With this strategy, which allowed the formation of single populations of receptors with unique stoichiometry, we demonstrated with immunofluorescence and biochemical and functional assays that only the receptors with 3 β4 subunits are efficiently expressed at the plasma membrane. Moreover, the LFM export motif of β4 subunit in the fifth position exerts a unique function in the regulation of the intracellular trafficking of the receptors, their exposure at the cell surface, and consequently, their function, whereas the same export motif present in the β4 subunits forming the acetylcholine binding site is dispensable.-Crespi, A., Plutino, S., Sciaccaluga, M., Righi, M., Borgese, N., Fucile, S., Gotti, C., Colombo, S. F. The fifth subunit in α3β4 nicotinic receptor is more than an accessory subunit.

α9- and α7-containing receptors mediate the pro-proliferative effects of nicotine in the A549 adenocarcinoma cell line

BACKGROUND AND PURPOSE

Tobacco smoke contains many classes of carcinogens and although nicotine is unable to initiate tumourigenesis in humans and rodents, it promotes tumour growth and metastasis in lung tumours by acting on neuronal nicotinic ACh receptors (nAChRs). The aim of this study was to identify molecularly, biochemically and pharmacologically which nAChR subtypes are expressed and functionally activated by nicotine in lung cancer cell lines.

EXPERIMENTAL APPROACH

We used A549 and H1975 adenocarcinoma cell lines derived from lung tumours to test the in vitro effects of nicotine, and nAChR subtype-specific peptides and compounds.

KEY RESULTS

The two adenocarcinoma cell lines express distinctive nAChR subtypes, and this affects their nicotine-induced proliferation. In A549 cells, nAChRs containing the α7 or α9 subunits not only regulate nicotine-induced cell proliferation but also the activation of the Akt and ERK pathways. Blocking these nAChRs by means of subtype-specific peptides, or silencing their expression by means of subunit-specific siRNAs, abolishes nicotine-induced proliferation and signalling. Moreover, we found that the α7 antagonist MG624 also acts on α9-α10 nAChRs, blocks the effects of nicotine on A549 cells and has dose-dependent cytotoxic activity.

CONCLUSIONS AND IMPLICATIONS

These results highlight the pathophysiological role of α7- and α9-containing receptors in promoting non-small cell lung carcinoma cell growth and intracellular signalling and provide a framework for the development of new drugs that specifically target the receptors expressed in lung tumours.

LINKED ARTICLES

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

Reconstitution of synaptic Ion channels from rodent and human brain in Xenopus oocytes: a biochemical and electrophysiological characterization

Disruption in the expression and function of synaptic proteins, and ion channels in particular, is critical in the pathophysiology of human neuropsychiatric and neurodegenerative diseases. However, very little is known regarding the functional and pharmacological properties of native synaptic human ion channels, and their potential changes in pathological conditions. Recently, an electrophysiological technique has been enabled for studying the functional and pharmacological properties of ion channels present in crude membrane preparation obtained from post-mortem frozen brains. We here extend these studies by showing that human synaptic ion channels also can be studied in this way. Synaptosomes purified from different regions of rodent and human brain (control and Alzheimer's) were characterized biochemically for enrichment of synaptic proteins, and expression of ion channel subunits. The same synaptosomes were also reconstituted in Xenopus oocytes, in which the functional and pharmacological properties of the native synaptic ion channels were characterized using the voltage clamp technique. We show that we can detect GABA, (RS)-α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and NMDA receptors, and modulate them pharmacologically with selective agonists, antagonists, and allosteric modulators. Furthermore, changes in ion channel expression and function were detected in synaptic membranes from Alzheimer's brains. Our present results demonstrate the possibility to investigate synaptic ion channels from healthy and pathological brains. This method of synaptosomes preparation and injection into oocytes is a significant improvement over the earlier method. It opens the way to directly testing, on native ion channels, the effects of novel drugs aimed at modulating important classes of synaptic targets. Disruption in the expression and function of synaptic ion channels is critical in the pathophysiology of human neurodegenerative diseases. We here show that synaptosomes purified from rodent and human frozen brain (control and Alzheimer disease) can be studied both biochemically and functionally. This method opens the way to directly testing the effects of novel drugs on native ion channels.

Myasthenia gravis-like syndrome presenting as a component of the paraneoplastic syndrome of lung adenocarcinoma in a nonsmoker

Adenocarcinoma of the lung is the most common form of lung cancer in nonsmokers. It is commonly seen in the periphery of the lungs. Myasthenia gravis is generally associated with mediastinal malignancies and rarely associated with adenocarcinoma of the lung. We present a case of a 38-year-old male nonsmoker with rapidly progressive adenocarcinoma of the lung associated with myasthenia gravis, a patient whom expired within 27 days of hospital admission and diagnosis.

Similar activity of mecamylamine stereoisomers in vitro and in vivo

A previous characterization of mecamylamine stereoisomers using nicotinic acetylcholine receptors expressed in Xenopus oocytes revealed only small differences between the activity of the R and S forms of mecamylamine. However, that work was limited in the breadth of receptor subtypes tested, especially in regard to the discrimination of high and low sensitivity receptors, which differ in the ratios of alpha and beta subunits. We report new data using subunit concatamers, which produce uniform populations of high-sensitivity or low-sensitivity receptors, as well as alpha2, alpha5, and alpha6-containing receptors, which were not studied previously. Consistent with previous studies, we found that beta4-containing receptors were most sensitive to mecamylamine and that the IC50 values for the inhibition of net charge were lower than for inhibition of peak currents. No large differences were seen between the activities of the mecamylamine isomers. Additionally, a previously reported potentiation of high-sensitivity α4β2 receptors by S-mecamylamine could not be reproduced in the oocyte system, even with mutants that had greatly reduced sensitivity to mecamylamine inhibition or when the selective agonist TC-2559 was used. In vivo studies suggested that the R-isomer might be somewhat more potent than the S isomer at blocking CNS effects of nicotine. Although the potency difference was no more than a factor of two, it is consistent with lower LD50 estimates previously reported for the R isomer. Our results significantly extend knowledge of the nicotinic acetylcholine receptor activity profile of mecamylamine and support the hypothesis that these effects are not strongly stereoisomer selective.

Nicotine and lung cancer

Tobacco use in cancer patients is associated with increased cancer treatment failure and decreased survival. Nicotine is one of over 7,000 compounds in tobacco smoke and nicotine is the principal chemical associated with addiction. The purpose of this article is to review the tumor promoting activities of nicotine. Nicotine and its metabolites can promote tumor growth through increased proliferation, angiogenesis, migration, invasion, epithelial to mesenchymal transition, and stimulation of autocrine loops associated with tumor growth. Furthermore, nicotine can decrease the biologic effectiveness of conventional cancer treatments such as chemotherapy and radiotherapy. Common mechanisms appear to involve activation of nicotinic acetylcholine receptors and beta-adrenergic receptors leading to downstream activation of parallel signal transduction pathways that facilitate tumor progression and resistance to treatment. Data suggest that nicotine may be an important mechanism by which tobacco promotes tumor development, progression, and resistance to cancer treatment.

Tnfα, Cox2 and AdipoQ adipokine gene expression levels are modulated in murine adipose tissues by both nicotine and nACh receptors containing the β2 subunit

Studies have provided evidences for the effects of nicotine on adipose tissues, as well as in inflammatory response. We hypothesized that nicotine affects adipokine gene expression in adipose tissues via specific neuronal nicotinic acetylcholine receptors (nAChRs). First, we described the expression of multiple nAChR subunit genes in mouse white and brown adipose tissues (WAT and BAT), and detected differential expression in WAT and BAT (α2>α5>β2 and α2>β2>β4, respectively). Additionally, when nicotine was administered to wild-type mice, it significantly affected the expression of adipokine genes, such as Tnfα, AdipoQ, Haptoglobin and Mcp1 in WAT. Next, we demonstrated that in mice deficient for the β2 nAChR subunit (β2-/- mice), the expression levels of Cox2 and Ngfβ genes in WAT, and Leptin, Cox2, AdipoQ and Haptoglobin in BAT, were significantly altered. Furthermore, interactions between mouse β2 subunit and nicotine treatment affected the expression levels of the adipokine genes Tnfα, Cox2 and AdipoQ in WAT and of AdipoQ in BAT. Finally, analysis of a cellular model of cultured adipocytes demonstrated that application of nicotine after silencing of the β2 nAChR subunit significantly elevated the expression level of Cox2 gene. Together, our data suggest a molecular link between the β2 nACh receptor subunit and the expression levels of specific adipokines, which is also affected by nicotine.

Association of the CHRNA3 locus with lung cancer risk and prognosis in Chinese Han population

INTRODUCTION

Recent genome-wide association studies in Caucasians revealed association with lung cancer risk of single nucleotide polymorphisms (SNPs) in the locus containing two nicotine acetylcholine receptor CHRNA genes. However, the reported risk SNPs are extremely rare in Asians. This study sought to identify other variants on CHRNA3 associated with lung cancer susceptibility and to explore whether SNPs of CHRNA3 are of prognostic factors in patients with non-small cell lung cancer (NSCLC) in Chinese Han population.

METHODS

A case-control study of 529 cases and 567 controls was performed to study the association of three SNPs (rs3743076, rs3743078, and rs3743073) in CHRNA3 with lung cancer risk in Chinese Han population using logistic regression models. The relationship between CHRNA3 polymorphisms with overall survival among 122 patients with advanced stage (stage IIIb and IV) NSCLC were evaluated using Cox multiple model based on the International Association for the Study of Lung Cancer recommended tumor, node, metastasis new staging.

RESULTS

Patients with genotypes TG or GG for the novel SNP rs3743073 in CHRNA3 gene, compared with those with TT, showed an increased risk of lung cancer (adjusted odds ratio = 1.91; 95% confidence interval, 1.38-2.63; p = 9.67 x 10) and worst survival (adjusted hazard ratio = 2.35; 95% confidence interval, 1.05-5.26; p = 0.04) in patients with advanced stage NSCLC based on International Association for the Study of Lung Cancer recommended tumor, node, metastasis new staging.

CONCLUSIONS

These results suggest that the rs3743073 polymorphism in CHRNA3 is predictive for lung cancer risk and prognostic in advanced stage NSCLC in Chinese Han population.

Acetylcholine receptor pathway and lung cancer

Genome-wide association studies revealed chromosome regions 15q24-25 were associated with a higher risk for development of lung cancer. The 15q24-25 region encompasses the nicotinic acetylcholine receptor subunit genes (nAchR alpha3, alpha5, and beta4) that play a role in nicotine addiction. This review reports information of the acetylcholine receptor and lung cancer. In patients diagnosed with smoking-related lung cancer and who continue smoking, a negative correlation with lung cancer survival has been shown. The reduced treatment efficacy may well be explained by the pluriform effect of nicotine on tumor cell proliferation, apoptosis, epithelialmesenchymal transition, proinvasive, and angiogenic effects, which are reenforced by an autocrine/paracrine loop. Overall, there is no evidence that nicotine itself induces cancer, but nicotine promotes in vivo the growth of cancer cells and the proliferation of endothelial cells. This suggests that nicotine after initiation may contribute to the progression phase of cancer development. Continuation of smoking after lung cancer has been diagnosed should be discouraged because smoking may not only support tumor growth but also interferes with treatment.

alpha7-Nicotinic receptor antagonists at the beginning of a clinical era for NSCLC and Mesothelioma?

Of the human solid cancers, Non-Small Cell Lung Cancer (NSCLC) and Malignant Pleural Mesothelioma (MPM) display a natural history supporting the concept that they develop from multiple preneoplastic pathways. Recently, new evidence suggested that nicotinic Acetylcholine Receptors (nAChRs) play a significant role in lung cancer predisposition and natural history. This review is based on some translational research aimed at evaluating the potential therapeutic effect of nAChR antagonists on NSCLC and MPM. The background and rationale of this approach are based on the experimental observations that: (a) NSCLC and MPM cells express nAChRs and (b) the activation of these receptors by agonists, namely nicotine, inhibits apoptosis, whereas receptor antagonists have a pro-apoptotic effect.

Risk for nicotine dependence and lung cancer is conferred by mRNA expression levels and amino acid change in CHRNA5

Nicotine dependence risk and lung cancer risk are associated with variants in a region of chromosome 15 encompassing genes encoding the nicotinic receptor subunits CHRNA5, CHRNA3 and CHRNB4. To identify potential biological mechanisms that underlie this risk, we tested for cis-acting eQTLs for CHRNA5, CHRNA3 and CHRNB4 in human brain. Using gene expression and disease association studies, we provide evidence that both nicotine-dependence risk and lung cancer risk are influenced by functional variation in CHRNA5. We demonstrated that the risk allele of rs16969968 primarily occurs on the low mRNA expression allele of CHRNA5. The non-risk allele at rs16969968 occurs on both high and low expression alleles tagged by rs588765 within CHRNA5. When the non-risk allele occurs on the background of low mRNA expression of CHRNA5, the risk for nicotine dependence and lung cancer is significantly lower compared to those with the higher mRNA expression. Together, these variants identify three levels of risk associated with CHRNA5. We conclude that there are at least two distinct mechanisms conferring risk for nicotine dependence and lung cancer: altered receptor function caused by a D398N amino acid variant in CHRNA5 (rs16969968) and variability in CHRNA5 mRNA expression.

Role of non-neuronal nicotinic acetylcholine receptors in angiogenesis

Angiogenesis is a critical physiological process for cell survival and development. Endothelial cells, necessary for the course of angiogenesis, express several non-neuronal nicotinic acetylcholine receptors (AChRs). The most important functional non-neuronal AChRs are homomeric alpha7 AChRs and several heteromeric AChRs formed by a combination of alpha3, alpha5, beta2, and beta4 subunits, including alpha3beta4-containing AChRs. In endothelial cells, alpha7 AChR stimulation indirectly triggers the activation of the integrin alphavbeta3 receptor and an intracellular MAP kinase (ERK) pathway that mediates angiogenesis. Non-selective cholinergic agonists such as nicotine have been shown to induce angiogenesis, enhancing tumor progression. Moreover, alpha7 AChR selective antagonists such as alpha-bungarotoxin and methyllycaconitine as well as the non-specific antagonist mecamylamine have been shown to inhibit endothelial cell proliferation and ultimately blood vessel formation. Exploitation of such pharmacologic properties can lead to the discovery of new specific cholinergic antagonists as anti-cancer therapies. Conversely, the pro-angiogenic effect elicited by specific agonists can be used to treat diseases that respond to revascularization such as diabetic ischemia and atherosclerosis, as well as to accelerate wound healing. In this mini-review we discuss the pharmacological evidence supporting the importance of non-neuronal AChRs in angiogenesis. We also explore potential intracellular mechanisms by which alpha7 AChR activation mediates this vital cellular process.

Extremely low-frequency electromagnetic field (ELF-EMF) does not affect the expression of α3, α5 and α7 nicotinic receptor subunit genes in SH-SY5Y neuroblastoma cell line

Neuronal nicotinic acetylcholine receptors (nAChRs) are involved in a number of functional processes, including cognition, learning and memory, and alterations in their expression and/or activity have been implicated in various neurological disorders such as Alzheimer's disease (AD), Parkinson's disease and schizophrenia. Epidemiological studies have shown that exposure to electromagnetic fields (EMF) may contribute to the pathogenesis of neurodegenerative diseases such as Alzheimer's disease. Given the role of nAChRs in physiological and pathological conditions, we wondered whether an extremely low-frequency electromagnetic field (ELF-EMF) may affect the expression of the molecules involved in neurodegenerative processes. In order to investigate this possibility, we studied the expression of alpha3, alpha5 and alpha7 nicotinic subunits upon exposure of the SH-SY5Y human neuroblastoma cell line to a 50 Hz power-line magnetic field in a "blind trial" system; various magnetic flux densities and exposure times were applied. Our studies show that the expression of some relevant components of the cholinergic nicotinic system, which is one of the most affected neurotransmission systems in AD, did not undergo any change at molecular level by environmental exposure to ELF-EMF.

In vitro characterization of 6-[18F]fluoro-A-85380, a high-affinity ligand for alpha4beta2* nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are involved in tobacco dependence and several other neuropathologies (e.g., Alzheimer's disease, Parkinson's disease), as well as in attention, learning, and memory. Performing in vivo imaging of these receptors in humans holds great promise for understanding their role in these conditions. Recently, three radiohalogenated analogs of 3-(2(S)-azetidinylmethoxy)pyridine (A- 85380) were used successfully for the in vivo visualization of alpha4beta2* nicotinic receptors in the human brain with PET/SPECT. Herein, we present the results of the in vitro characterization of one of these radioligands, 6-[18F]fluoro-3-(2(S)-azetidinylmethoxy)-pyridine (6-[18F]fluoro-A-85380), which is a fluoro-analog of the potent nonopioid analgesic ABT-594. In human postmortem cortical tissue, 6-[18F]fluoro-A-85380 reversibly binds with high affinity to a single population of sites (Kd = 59 pM at 37 degrees C, Bmax = 0.7 pmol/g tissue). The binding is fully reversible and is characterized at 37 degrees C by T(1/2assoc) = 2.2 min (at a ligand concentration of 39 pM) and by T(1/2dissoc) = 3.6 min. 6-Fluoro-A-85380 exhibits clear selectivity for alpha4beta2* over the other major mammalian nicotinic receptor subtypes: alpha7, alpha3beta4, and muscle-type. These results suggest that 6-[18F]fluoro-A-85380 is a promising radioligand for in vivo imaging of brain alpha4beta2* nicotinic receptors.

Neuronal nicotinic receptors: from structure to pathology

Neuronal nicotinic receptors (NAChRs) form a heterogeneous family of ion channels that are differently expressed in many regions of the central nervous system (CNS) and peripheral nervous system. These different receptor subtypes, which have characteristic pharmacological and biophysical properties, have a pentameric structure consisting of the homomeric or heteromeric combination of 12 different subunits (alpha2-alpha10, beta2-beta4). By responding to the endogenous neurotransmitter acetylcholine, NAChRs contribute to a wide range of brain activities and influence a number of physiological functions. Furthermore, it is becoming evident that the perturbation of cholinergic nicotinic neurotransmission can lead to various diseases involving nAChR dysfunction during development, adulthood and ageing. In recent years, it has been discovered that NAChRs are present in a number of non-neuronal cells where they play a significant functional role and are the pathogenetic targets in several diseases. NAChRs are also the target of natural ligands and toxins including nicotine (Nic), the most widespread drug of abuse. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining their regional and cellular localisation and the molecular basis of their functional diversity mainly in pharmacological and biochemical terms. The recent availability of mice with the genetic ablation of single or double nicotinic subunits or point mutations have shed light on the role of nAChRs in major physiological functions, and we will here discuss recent data relating to their behavioural phenotypes. Finally, the role of NAChRs in disease will be considered in some details.

Long-term exposure to nicotine, via ras pathway, induces cyclin D1 to stimulate G1 cell cycle transition

Nicotine, a major component in tobacco, has been implicated as a potential factor that promotes the development of lung cancer. However, the molecular mechanism of its action is still unclear. In this study, we have shown that, via nicotinic acetylcholine receptors, persistent exposure of mouse epithelial cells to nicotine elicits Ras signaling and subsequent Raf/MAP kinase activity, accompanied by a significant increase in cyclin D1 promoter activity and its protein expression. AP-1 is required for activation of the cyclin D1 promoter. The induction of cyclin D1 expression and its promoter activity by nicotine is abolished by the suppression of Raf/MAP kinase signaling. Furthermore, upon nicotine treatment, the cells do not arrest in the G(1) phase of the cell cycle following serum starvation. The perturbation of the G(1) cell cycle checkpoint is caused by the deregulation of retinoblastoma/E2F activity. Therefore, our data indicated that by targeting the Ras pathway, long-term exposure to nicotine disrupts cell cycle restriction machinery and thus potentiates tumor development.

Developmental changes in the expression of nicotinic acetylcholine receptor alpha-subunits in the rat heart

Neuronal nicotinic acetylcholine receptors (nAChR) are ligand-gated ion channels that consist of various subunits. During ontogeny, muscular and neuronal nAChR undergo changes in the distribution and subunit composition in skeletal muscle and brain, respectively. Here, we have investigated the occurrence of the ligand-binding alpha-subunits of neuronal nAChR by means of reverse transcription/polymerase chain reaction and immunohistochemistry in the rat heart during prenatal and postnatal development and after capsaicin-induced sensory denervation. mRNAs coding for the alpha4, alpha5, alpha7 and alpha10 subunits were detected throughout all developmental stages. Messenger coding for the alpha2 subunit was first detectable at developmental stage E20; alpha3 subunit mRNA was expressed throughout all prenatal developmental stages, whereas it was restricted postnatally to the atria. mRNA for alpha6 was observed at E14-P8 but was absent thereafter. At no developmental stage could an unequivocal signal for alpha9 nAChR subunit mRNA be obtained. The expression pattern was unchanged by capsaicin treatment. Immunohistochemistry demonstrated alpha7 subunits on cardiac neurons, fibroblasts and cardiomyocytes and alpha2/4 subunits on cardiomyocytes with a postnatal redistribution to intercalated discs, as shown by cryo-immunoelectron microscopy. Our results indicate an additional non-neuronal expression of nAChR subunits in the rat heart that, as in skeletal muscle, precedes functional innervation and then undergoes changes in its distribution on the surface of cells.

Mice lacking neuronal nicotinic acetylcholine receptor β4-subunit and mice lacking both α5- and β4-subunits are highly resistant to nicotine-induced seizures

Nicotine, the main addictive component of tobacco, evokes a wide range of dose-dependent behaviors in rodents, and when administrated in high doses, it can induce clonic-tonic seizures. Nicotine acts through the nicotinic acetylcholine receptors (nAChRs). Mutations in the human α4- and the β2-nAChR subunit genes cause autosomal dominant nocturnal frontal lobe epilepsy. Using transgenic mice with mutations in nAChR subunits, it was demonstrated previously that the α4-, α5-, and α7-subunits are involved in nicotine-induced seizures. To examine the possibility that the β4-subunit is also involved in this phenotype, we tested mice with homozygous β4-subunit deficiency. The β4 null mice were remarkably resistant to nicotine-induced seizures compared with wild-type and α5 null mice. We also generated mice with double deficiency of both α5- and β4-nAChR subunits and demonstrated that they were more resistant to nicotine’s convulsant effect than either the α5 or the β4 single mutant mice. In addition, the single α5 mutants and the double α5β4-deficient mice exhibited a significantly shorter latency time to seizure than that of the wild-type mice. Our results thus show that β4-containing nAChRs have a crucial role in the pathogenesis of nicotine-induced seizures. Furthermore, by comparing multiple mutant mice with single and double subunit deficiency, we suggest that nicotinic receptors containing either α5- or β4-subunits are involved in nicotine-induced seizures and that receptors containing both subunits are likely to contribute to this phenomena as well. However, the α5-subunit, but not the β4-subunit, regulates the rate of response to high doses of nicotine.

Alpha7-nicotinic acetylcholine receptors affect growth regulation of human mesothelioma cells: role of mitogen-activated protein kinase pathway

This study presents data suggesting that both human mesothelioma (cell lines and human mesothelioma biopsies) and human normal mesothelial cells express receptors for acetylcholine and that stimulation of these receptors by nicotine prompted cell growth via activation of nicotinic cholinergic receptors. Thus, these data demonstrate that: (a) human mesothelioma cells and human biopsies of mesothelioma as well as of normal pleural mesothelial cells express functionally alpha-7 nicotinic acethlycholine receptors, evaluated by alpha-bungarotoxin-FITC binding, receptor binding assay, Western blot, and reverse transcription-PCR; (b) choline acetyltransferase immunostaining is present in mesothelioma cells; (c) mesothelioma cell growth is modulated by the cholinergic system in which agonists (i.e., nicotine) has a proliferative effect, and antagonists (i.e., curare) has an inhibitory effect, evaluated by cell cloning, DNA synthesis and cell cycle; (d) nicotine induces Ca(+2) influx, evaluated by [(45)Ca(2+)] uptake, and consequently activation of mitogen-activated protein kinase pathway (extracellular signal-regulated kinase and p90(RSK) phosphorylation), evaluated by Western blot; and (e) apoptosis mechanisms in mesothelioma cells are under the control of the cholinergic system (nicotine antiapoptotic via induction of nuclear factor-kappaB complexes and phosphorylation of Bad at Ser(112); curare proapoptotic via G(0)-G(1) arrest p21(waf-1) dependent but p53 independent). The involvement of the nonneuronal cholinergic system in mesothelioma appears reasonable and open up new therapeutic strategies.

Mouse strain-specific nicotinic acetylcholine receptor expression by inhibitory interneurons and astrocytes in the dorsal hippocampus

The response by individuals to nicotine is likely to reflect the interaction of this compound with target nAChRs. However, resolving how different genetic backgrounds contribute to unique mouse strain-specific responses to this compound remains an important and unresolved issue. To examine this question in detail, expression of the nicotine acetylcholine receptor (nAChR) subunits alpha3, alpha4, alpha5, alpha7, beta2, and beta4 was measured in the dorsal hippocampus using immunohistochemistry in mouse strains or lines BALB/c, C3H/J, C57BL/6, CBA/J, DBA/2, Long Sleep (LS), Short Sleep (SS), and CF1. The nAChRs in all mice colocalized with glutamic acid decarboxylase (GAD)-positive interneurons that were subclassified into at least four groups based on nAChR subunit heterogeneity. A notable difference between mouse strains was the expression of nAChRs by astrocyte subpopulations in CA1 subregions whose numbers vary inversely with nAChR-immunostained neurons. This novel relationship also correlated with published parameters of strain sensitivity to nicotine. Attempts to identify the origin of this significant difference in nAChR expression among strains included comparison of the entire nAChRalpha4 gene sequence. Although multiple polymorphisms were identified, including two that changed nAChRalpha4 amino acid coding, none of these clearly correlate with strain-related differences in cell type-specific nAChR expression. These findings suggest that mouse strain-specific behavioral and physiological responses to nicotine are likely to be a reflection of a complex interplay between genetic factors that shape differences in expression and cellular architecture of this modulatory neurotransmitter system in the mammalian nervous system.

Establishment and characterization of a cell line from smokeless tobacco associated oral squamous cell carcinoma

A cell line, AMOS-III has been established from the surgically resected specimen of an untreated primary human oral squamous cell carcinoma of the floor of mouth from a chronic smokeless tobacco consumer. Immunocytochemical analysis showed epithelial specific antigen, cytokeratins 5, 10, 13 and 16 and integrin alpha(6) markers in AMOS-III cells, confirming the epithelial lineage of the cell line. Analyses of morphology, ultrastructure, karyotype, anchorage independent growth and immunocytochemical properties of the cell line demonstrated the transformed phenotype of epithelial cells. AMOS-III cells have doubling time of 42-44 h. Giemsa-banding patterns of chromosomes confirmed the human origin of the AMOS-III cells. Molecular analysis of cancer-related gene products, p53 and p21(cip1/waf1) showed the presence of wild type p21(cip1/waf1) and truncated p53 proteins. The molecular mechanism underlying the action of retinoids in preventing the occurrence of second primary tumors in oral cancer patients remain to be clearly defined. Treatment of AMOS-III cells with all-trans retinoic acid (ATRA) at 10(-4) microM resulted in 81% cell death. ATRA treatment resulted in enhanced expression of p21(cip1/waf1), nuclear translocation of retinoic acid receptors and apoptotic cell death. Thus, this cell line provides an in vitro model for elucidating the mechanism involving p53 inactivation and p21(cip1/waf1) overexpression in smokeless tobacco-induced oral cancer. Furthermore, the ATRA responsiveness of the cell line underscores its potential utility in identifying the retinoid responsive molecular targets in oral cancer cells.

Functional Nicotinic Acetylcholine Receptors Are Expressed in B Lymphocyte-Derived Cell Lines

Nicotine has been shown to affect B lymphocyte immune response. In this study, we have explored the presence of nicotinic receptors in B lymphocyte-derived cell lines, myeloma X63-Ag8 and hybridoma 1D6. We found that myeloma expressed on average 10,170 +/- 1,100 [3H]epibatidine and 6,730 +/- 370 125I-alpha-bungarotoxin binding sites per cell, thus reflecting the presence of both homomeric and heteromeric nicotinic receptors. More specifically, the presence of alpha4- and alpha7-containing nicotinic receptor subunits was demonstrated in both myeloma and hybridoma cells with subunit-specific antibodies. It was significantly higher in dividing than in resting cells. Long-term exposure to nicotine, at physiological concentration found in smokers, resulted in up-regulation of both alpha4 and alpha7 subunits in hybridoma cells. Additionally, nicotine stimulated hybridoma cell proliferation, whereas it decreased antibody production. In contrast, alpha7-specific snake toxins inhibited cell proliferation but increased antibody production. It is concluded that myeloma and hybridoma cells express alpha4- and alpha7-containing nicotinic receptors, which participate in regulating cell proliferation and function.

Immunization with neuronal nicotinic acetylcholine receptor induces neurological autoimmune disease

Neuronal nicotinic AChRs (nAChRs) are implicated in the pathogenesis of diverse neurological disorders and in the regulation of small-cell lung carcinoma growth. Twelve subunits have been identified in vertebrates, and mutations of one are recognized in a rare form of human epilepsy. Mice with genetically manipulated neuronal nAChR subunits exhibit behavioral or autonomic phenotypes. Here, we report the first model of an acquired neuronal nAChR disorder and evidence for its pertinence to paraneoplastic neurological autoimmunity. Rabbits immunized once with recombinant alpha3 subunit (residues 1-205) develop profound gastrointestinal hypomotility, dilated pupils with impaired light response, and grossly distended bladders. As in patients with idiopathic and paraneoplastic autoimmune autonomic neuropathy, the severity parallels serum levels of ganglionic nAChR autoantibody. Failure of neurotransmission through abdominal sympathetic ganglia, with retention of neuronal viability, confirms that the disorder is a postsynaptic channelopathy. In addition, we found ganglionic nAChR protein in small-cell carcinoma lines, identifying this cancer as a potential initiator of ganglionic nAChR autoimmunity. The data support our hypothesis that immune responses driven by distinct neuronal nAChR subtypes expressed in small-cell carcinomas account for several lung cancer-related paraneoplastic disorders affecting cholinergic systems, including autoimmune autonomic neuropathy, seizures, dementia, and movement disorders.

Autonomic function in mice lacking alpha5 neuronal nicotinic acetylcholine receptor subunit

Neuronal acetylcholine nicotinic receptors (nAChR) are composed of 12 subunits (alpha2-10, beta2-4), of which alpha3, alpha5, alpha7, beta2 and beta4 subunits are known to exist in the autonomic nervous system (ANS). alpha5 subunits possess unique biophysical and pharmacological properties. The present study was undertaken to examine the functional role and pharmacological properties of the nAChR alpha5 subunits in the ANS using mice lacking alpha5 nAChR subunits (alpha5-/-). These mice grew to normal size showing no obvious physical or neurological deficit. They also showed normality in thermoregulation, pupil size and resting heart rate under physiological conditions. The heart rate and rectal temperature did not differ between alpha5-/- and wild-type mice during exposure to cold stress. An impairment of cardiac parasympathetic ganglionic transmission was observed during high frequency vagal stimulation, which caused cardiac arrest in all wild-type animals while alpha5-/- mice were more resistant. Deficiency of alpha5 subunits strikingly increased the sensitivity to a low concentration of hexamethonium, leading to a nearly complete blockade of bradycardia in response to vagal stimulation. Such a concentration of hexamethonium only slightly depressed the effects of vagal stimulation in control mice. Deficiency of alpha5 subunits significantly increased ileal contractile responses to cytisine and epibatidine. These results suggest that alpha5 subunits may affect the affinity and sensitivity of agonists and antagonists in the native receptors. Previous studies revealed that alpha5 subunits form functional receptors only in combination with other alpha and beta subunits. Thus, the data presented here imply that alpha5 subunits modulate the activity of nAChR in autonomic ganglia in vivo.

Nicotine modulates the effects of retinoids on growth inhibition and RAR beta expression in lung cancer cells

Epidemiological and animal studies have demonstrated that vitamin A and its natural and synthetic derivatives, retinoids, are effective agents in preventing the development of tobacco-associated cancers. Unfortunately, clinical trials of retinoids on cigarette smokers have shown lack of efficacy in preventing lung cancer. In our study, we investigated the effect of nicotine on the anti-cancer activity of all trans-retinoic acid (trans-RA) in human lung cancer cells. Our results demonstrated that nicotine could abrogate the growth inhibitory effect of trans-RA by suppressing its ability to induce the expression of RA receptor beta (RAR beta), a tumor suppressor. The inhibitory effect of nicotine was accompanied with induction of orphan receptor TR3. Inhibition of TR3 expression by overexpression of TR3 anti-sense RNA in H460 lung cancer cells strongly prevented the suppressive effect of nicotine on trans-RA activity. Treatment with nicotine or the cotransfection of TR3 expression vector inhibited the induction of RAR beta promoter activity by trans-RA in transient transfection assays. The inhibition of RAR beta promoter activity was due to the interaction of TR3 with orphan receptor COUP-TF, resulting in inhibition of COUP-TF DNA binding and transactivation on the RAR beta promoter. Furthermore, we found that nicotine failed to suppress the effect of a retinoid X receptor (RXR)-selective retinoid SR11237 on inducing both growth inhibition and RAR beta promoter activity, due to the ability of SR11237 to activate the RAR beta promoter through the RXR/TR3 heterodimer. Together, our results demonstrate that nicotine suppresses the growth inhibitory effects of trans-RA by inhibiting RAR beta expression through its induction of TR3 expression and suggest that RXR-selective retinoids may be more effective than classical retinoids for preventing and treating tobacco-associated cancers.

Expression of the alpha4 isoform of the nicotinic acetylcholine receptor in the fetal human cerebral cortex

Nicotinic acetylcholine receptors are likely to play an important role in neuronal migration during development. Furthermore, the alpha4 receptor subunit gene is related to a hereditary juvenile form of epilepsy. Only little information is available, however, on the expression of cerebrocortical nicotinic acetylcholine receptors during human fetal development. Using non-isotopic in situ hybridization and immunohistochemistry, we have studied the distribution of the alpha4 subunit of the nicotinic acetylcholine receptor mRNA and protein in the human frontal cortex at middle (17-24 weeks of gestation) and late (34-42 weeks of gestation) fetal stages. Both, alpha4 receptor mRNA and alpha4 receptor protein were observed beginning during week 17-18 of gestation. At this time of development, a few weakly labeled mRNA-containing cells were present mainly in the ventricular zone, the subplate and the cortical plate. A similar distribution pattern was found for the receptor protein. Around week 38 of gestation, the distribution in the cerebral cortex of alpha4 subunit-containing cells was similar to that of adult human cortices with the highest densities of labeled neurons found in layers II/III, followed by layers V and VI. Nicotinic acetylcholine receptor-containing neurons appear rather early in human fetal development. Given functional maturity, they may interact during cortical development with acetylcholine released from corticopetal fibers or other yet unknown sources subserving the process of neuronal migration and pathfinding.

Human bronchial epithelial and endothelial cells express alpha7 nicotinic acetylcholine receptors

The epithelial or endothelial cells that line the human bronchi and the aorta express nicotinic acetylcholine receptors (nAChRs) of alpha3 subtypes. We report here that human bronchial epithelial cells (BEC) and aortic endothelial cells (AEC) express also the nAChR alpha7 subunit, which forms functional nAChRs. Polymerase chain reaction and in situ hybridization experiments detected alpha7 subunit mRNA in cultured human BEC and AEC and in sections of rat trachea. The binding of radiolabeled alpha-bungarotoxin revealed a few thousand binding sites per cell in cultured human BEC and human and bovine AEC. Western blot and immunohistochemistry experiments demonstrated that cultured BEC and AEC express a protein(s) recognized by anti-alpha7 antibodies. Whole-cell patch-clamp studies of cultured human BEC demonstrated the presence of fast-desensitizing currents activated by choline and nicotine that were blocked reversibly by methyllycaconitine (1 nM) and irreversibly by alpha-bungarotoxin (100 nM), consistent with the expression of functional alpha7 nAChRs. In some cells, choline activated also slowly decaying currents, confirming previous reports that BEC express functional alpha3beta4 nAChRs. Exposure of cultured BEC to nicotine (1 microM) for 3 days up-regulated functional alpha7 and alpha3 nAChRs, as indicated by the increased number of cells responding to acetylcholine and choline, with both fast-desensitizing currents, which were blocked irreversibly by alpha-bungarotoxin, and with slowly desensitizing currents, which are alpha-bungarotoxin-insensitive currents. The presence of alpha7 nAChRs in BEC and AEC suggests that some toxic effects of tobacco smoke could be mediated through these nicotine-sensitive receptors.

Neuronal and extraneuronal expression and regulation of the human alpha5 nicotinic receptor subunit gene

The mRNA encoding the human alpha5 nicotinic subunit was detected in several structures of the nervous system but appeared to be mainly expressed in cerebellum, thalamus, and the autonomic ganglia. For the first time, the alpha5 transcript was also detected in several non-neuronal tissues, with maximal expressions being found throughout the gastrointestinal tract, thymus, and testis. Many other extraneuronal sites expressed alpha5, but there were also nonexpressing organs, such as the liver, spleen, and kidney. To understand the transcriptional mechanisms controlling such a diversified expression of alpha5 in neuronal and nonneuronal cells, we isolated the 5'-regulatory region of the human gene and characterized its properties. Here we identify the alpha5 core promoter and demonstrate that the DNA regions surrounding it contain elements (with positive or negative activities) that work in a tissue-specific fashion. In particular, the segment specifying the 5'-untranslated region in neuronal cells has most of the properties of an enhancer because it activates a heterologous promoter in a position- and orientation-independent fashion. We therefore conclude that the expression of alpha5 relies on a highly complex promoter that uses distinct regulatory elements to comply with the different functional and developmental requirements of the various tissues and organs.

Neuronal nicotinic receptors, important new players in brain function

Acetylcholine receptors are cationic channels whose opening is controlled by acetylcholine. They are key molecules in the cholinergic nicotinic transmission in a number of areas of the central and peripheral nervous system. Because of the structural complexity, given by the numerous subunits that forms these receptors, they have different pharmacological and biophysical properties. Here we give a brief account of the known and consolidated data regarding neuronal nicotinic receptors, as as an introduction to the articles reported in this issue, in order to allow readers who are not familiar with the field to place the detailed information in the right context.

Transcriptional regulation of the human alpha5 nicotinic receptor subunit gene in neuronal and non-neuronal tissues

The human alpha5 nicotinic receptor subunit gene appears to be expressed in several structures of the nervous system, but also in a number of non-neuronal tissues, with maximal expressions occurring in the entire gastrointestinal tract, thymus and testis. To understand whether specific transcriptional mechanisms are involved in the tissue-specific expression of the alpha5 subunit in neuronal and non-neuronal cells, we isolated the 5'-regulatory region of the human gene and characterized its functional properties. We demonstrate that specific DNA elements, with positive or negative activities depending on the cell type, are responsible for the diversified expression of the alpha5 subunit in different tissues. We therefore conclude that the expression of the alpha5 subunit relies on a highly complex promoter that uses distinct regulatory elements to comply with the different functional and developmental requirements of the various tissues and organs.

Neuronal nicotinic receptors in non-neuronal cells: new mediators of tobacco toxicity?

The nicotinic acetylcholine receptors are prototypic ionotropic receptors that mediate fast synaptic transmission. However, also non-excitable cells, and particularly the tegumental cells that line external and internal body surfaces, express acetylcholine receptors of neuronal type sensitive to nicotine. Bronchial epithelial cells, endothelial cells of blood vessels and skin keratinocytes express neuronal nicotinic receptors composed of alpha(3), alpha(5), beta(2) and beta(4) subunits, similar to those expressed in sympathetic ganglia, and neuronal nicotinic receptors composed of alpha(7) subunits. Neuronal nicotinic receptors in tegumental cells are involved in modulating cell shape and motility, and therefore in maintaining the integrity of the surfaces lined by those cells. Neuronal nicotinic receptors in non-neuronal tissues may modulate other functions, including cell proliferation and differentiation. Acetylcholine is synthesized, secreted and degraded by a variety of cells, including the tegumental cells that express neuronal nicotinic receptors. Thus, acetylcholine may function as a local "hormone" that is able to modulate cell functions that require fast adaptation to new conditions. The presence of neuronal nicotinic receptors sensitive to nicotine in tissues known to be involved in tobacco toxicity, like bronchi and blood vessels, raises the possibility that they mediate some of the toxic effects of smoking.

Nicotine, brain nicotinic receptors, and neuropsychiatric disorders

Neuronal nicotinic acetylcholine receptors (nAChRs) represent a large family of ligand-gated cation channels with diverse structures and properties. In contrast to the muscular nAChRs, the physiological functions of neuronal nAChRs are not well defined to date. Behavioral studies indicate that brain nAChRs participate in complex functions such as attention, memory, and cognition, whereas clinical data suggest their involvement in the pathogenesis of certain neuropsychiatric disorders (Alzheimer's and Parkinson's diseases, Tourette's syndrome, schizophrenia, depression, etc.). For the majority of these disorders, the use of nAChRs' agonists may represent either a prophylactic (especially for Alzheimer's and Parkinson's diseases) or a symptomatic treatment. The possible mechanisms underlying these beneficial effects as well as the characteristics and potential therapeutic use of new, subtype-selective nAChRs agonists are presented.

Decreased protein levels of nicotinic receptor subunits in the hippocampus and temporal cortex of patients with Alzheimer's disease

Deficits of cortical nicotinic acetylcholine receptors (nAChRs) have been observed in Alzheimer's disease (AD) by receptor binding assays. Little is known about the receptor subunit specificity influenced by AD, and it might be of importance for therapeutic strategies. In the present study, the protein levels of nAChR alpha3, alpha4, alpha7, and beta2 subunits were investigated using western blot analysis on postmortem brains of patients with AD and age-matched controls. The results showed that in human postmortem brain samples, bands with molecular masses of 52, 42, and 50 kDa were detected by anti-alpha4, anti-alpha7, and anti-beta2 antibodies, respectively. When anti-alpha3 antibody was used, one major band of 49 kDa and two minor bands of 70 and 38 kDa were detected. In AD patients, as compared with age-matched controls, the alpha4 subunit was reduced significantly by approximately 35 and 47% in the hippocampus and temporal cortex, respectively. A significant reduction of 25% in the alpha3 subunit was also observed in the hippocampus and a 29% reduction in the temporal cortex. For the alpha7 subunit, the protein level was reduced significantly by 36% in the hippocampus of AD patients, but no significant change was detected in the temporal cortex. In neither the hippocampus nor the temporal cortex was a significant difference observed in the beta2 subunit between AD patients and controls. These results reveal brain region-specific changes in the protein levels of the nAChR alpha3, alpha4, and alpha7 subunits in AD.

Expression of a neuronal nicotinic acetylcholine receptor in insect and mammalian host cell systems

Different mammalian and insect somatic host cell systems were tested in their ability to express, fold, and assemble alpha7-type neuronal acetylcholine receptor (AChR) both at the transcriptional and translational level. For this purpose we employed clonal cell lines derived from the neural crest, such as PC12 cells from a rat adrenal pheochromocytoma, and GH3 cells isolated from a rat pituitary tumor, as well as non-neuronal cells such as NIH-3T3 fibroblasts from embryonic NIH Swiss mouse and Sf9 cells from ovary tissue of the Spodoptera frugiperda butterfly. Total RNA, isolated from either transfected or non-transfected PC12, GH3 or 3T3 cells, or recombinant AcNPV-infected and mock-infected Sf9 cells was analyzed by Northern blot. PC12 cells, which endogenously express alpha7 AChR, and all its heterologous alpha7-transfectant clones, exhibited variable but generally high amounts of a single transcript. GH3 and NIH-3T3 transfectant clones and recombinant AcNPV-infected Sf9 cells expressed variable levels of alpha7-mRNA, with a single transcript that co-migrated with the 28S rat rRNA. Only the neural crest-derived cell lines appeared to functionally express the alpha7 AChR, as measured by their [125I]alpha-bungarotoxin binding ability. The results suggest that heterologous expression of alpha7 is regulated not at the transcriptional, but at the postranslational level and that not all host cell systems appear to express the cellular factors needed for the correct postranslational modifications leading to mature and functional alpha7 AChR. Furthermore, the results suggest that tightly controlled expression mechanisms have evolved in parallel with this ancient cholinergic sequence.

Single channel properties of human alpha3 AChRs: impact of beta2, beta4 and alpha5 subunits

1. We performed single channel analysis on human alpha3 acetylcholine receptors (AChRs) in Xenopus oocytes and native AChRs from the human neuroblastoma cell line IMR-32. alpha3 AChRs exhibit channel properties that reflect subunit composition. 2. alpha3beta2 AChR open times were 0.71 +/- 0.14 and 3.5 +/- 0.4 ms with a predominant conductance of 26 pS. alpha3beta4 AChRs had open times of 1.4 +/- 0.2 and 6.5 +/- 0.8 ms and a predominant conductance of 31 pS. Burst times were 0.82 +/- 0.12 and 5.3 +/- 0.7 ms for alpha3beta2 and 1.7 +/- 0.1 and 16 +/- 1 ms for alpha3beta4. Desensitization was faster for AChRs with the beta2 subunit than for those with the beta4 subunit. 3. One open time for alpha3alpha5beta2 AChRs (5.5 +/- 0.3 ms) was different from those of alpha3beta2 AChRs. For alpha3alpha5beta4 AChRs, an additional conductance, open time and burst time (36 pS, 22 +/- 3 ms and 43 +/- 4 ms, respectively) were different from those for alpha3beta4 AChRs. 4. alpha3 AChRs were inhibited by hexamethonium or mecamylamine. The rate constants for block of alpha3beta4 by hexamethonium and of alpha3beta2 by mecamylamine were 1.2 x 107 and 4.6 x 107 M-1 s-1, respectively. 5. AChRs from IMR-32 cells had a predominant conductance of 32 pS and open times of 1.5 +/- 0.3 and 9.6 +/- 1.2 ms. These properties were most similar to those of alpha3beta4 AChRs expressed in oocytes. Antibodies revealed that 5 +/- 2 % of IMR-32 alpha3 AChRs contained alpha5 subunits and 6 +/- 2 % contained beta2 subunits. IMR-32 alpha3 AChRs are primarily alpha3beta4 AChRs.

Chronic nicotine treatment up-regulates human alpha3 beta2 but not alpha3 beta4 acetylcholine receptors stably transfected in human embryonic kidney cells

Human nicotinic acetylcholine receptor (AChR) subtypes alpha3 beta2, alpha3 beta2 alpha5, alpha3 beta4, and alpha3 beta4 alpha5 were stably expressed in cells derived from the human embryonic kidney cell line 293. alpha3 beta4 AChRs were found in prominent 2-micrometer patches on the cell surface, whereas most alpha3 beta2 AChRs were more diffusely distributed. The functional properties of the alpha3 AChRs in tsA201 cells were characterized by whole cell patch clamp using both acetylcholine and nicotine as agonists. Nicotine was a partial agonist on alpha3 beta4 AChRs and nearly a full agonist on alpha3 beta2 alpha5 AChRs. Chronic exposure of cells expressing alpha3 beta2 AChRs or alpha3 beta2 alpha5 AChRs to nicotine or carbamylcholine increased their amount up to 24-fold but had no effect on the amount of alpha3beta4 or alpha3 beta4 alpha5 AChRs, i.e. the up-regulation of alpha3 AChRs depended on the presence of beta2 but not beta4 subunits in the AChRs. This was also found to be true of alpha3 AChRs in the human neuroblastoma SH-SY5Y. In the absence of nicotine, alpha3 beta2 AChRs were expressed at much lower levels than alpha3 beta4 AChRs, but in the presence of nicotine, the amount of alpha3 beta2 AChRs exceeded that of alpha3 beta4 AChRs. Up-regulation was seen for both total AChRs and surface AChRs. Up-regulated alpha3beta2 AChRs were functional. The nicotinic antagonists curare and dihydro-beta-erythroidine also up-regulated alpha3 beta2 AChRs, but only by 3-5-fold. The channel blocker mecamylamine did not cause up-regulation of alpha3 beta2 AChRs and inhibited up-regulation by nicotine. Our data suggest that up-regulation of alpha3 beta2 AChRs in these lines by nicotine results from both increased subunit assembly and decreased AChR turnover.

Nerve growth factor abrogates the tumorigenicity of human small cell lung cancer cell lines

Nerve growth factor (NGF) has antiproliferative and differentiating effects on adenomas of neuroendocrine origin. Cell lines derived from small-cell lung carcinoma (SCLC), a very aggressive neuroendocrine tumor, express NGF receptors. The role of NGF in the control of proliferation and progression of this carcinoma, however, has never been investigated. Chronic exposure of NCI-N-592 and GLC8 SCLC cell lines to NGF remarkably inhibited their proliferation rate both in vitro and in vivo, prevented their anchorage-independent clonal growth in soft agar, impaired their invasive capacity in vitro, and abolished their tumorigenic potential in nude mice. The proliferative response of SCLC cell lines to nicotine was also remarkably impaired by in vitro NGF treatment. Furthermore, NGF treatment activates in SCLC cell lines the expression and secretion of NGF. NGF thus reverts SCLC cell lines to a noninvasive, nontumorigenic phenotype that does not respond to nicotine and produces NGF.

Nicotine stimulates branching and expression of SP-A and SP-C mRNAs in embryonic mouse lung culture

Although the effects of maternal smoking on fetal growth and viability are overwhelmingly negative, there is a paradoxical enhancement of lung maturation as evidenced, in part, by a lower incidence of respiratory distress syndrome in infants of smoking mothers. Other epidemiologic and experimental evidence further support the view that a tobacco smoke constituent, possibly nicotine, affects the development of the lung in utero. We are studying the direct effects of nicotine on murine lung development using a serumless organ culture system. We have found that embryonic lungs explanted at 11 days gestation showed a 32% increase in branching after 4 days in culture in the presence of 1 microM nicotine and 7- to 15-fold increases in mRNAs encoding surfactant proteins A and C after 11 days. The effect of nicotine exposure on surfactant gene expression is apparently mediated by nicotinic acetylcholine receptors because it was blocked by D-tubocurarine. The nicotine-induced stimulation of surfactant gene expression could, in part, account for the effect of maternal smoking on the incidence of respiratory distress syndrome.

Nicotinic system involvement in Alzheimer's and Parkinson's diseases. Implications for therapeutics

Advances in our understanding of the structure, function and distribution of nicotinic acetylcholine receptors in the CNS have provided the impetus for new studies examining the role(s) that these receptors and associated processes may play in CNS functions. Further motivation has come from the realisation that such receptors must be involved in the maintenance of cigarette smoking, and from clues provided by studies of degenerative neurological diseases such as Alzheimer's disease and Parkinson's disease, in which the loss of nicotinic receptors has been described. Ongoing investigations of the molecular substructure of central nicotinic receptors and their pharmacology have begun to open up new possibilities for novel CNS therapeutics with nicotinic agents. Exploiting these possibilities will require understanding of the role(s) that these receptor systems play in human cognitive, behavioural, motor and sensory functioning. Clues from careful studies of human cognition are beginning to emerge and will provide direction for studies of potentially therapeutic novel nicotinic agents. Despite the promising results of acute studies, few long term studies with nicotine or nicotinic drugs have been performed in dementing disorders. Thus there is uncertainty as to whether long term nicotinic treatment will provide sustained cognitive benefit. It is even more uncertain whether such cognitive benefit will have a significant clinical impact on patients and their families. To maximise the potential benefit of long term treatment with nicotinic agonists (or other cholinergic drugs), we suggest that drug treatment should be combined with cognitive rehabilitation strategies. This will enable patients and/or their families to focus on the particular cognitive domains that may be improved.

Parallel detection of Na,K-ATPase alpha subunit isoforms by pan-specific monoclonal mAb 9A7

While emphasis has been placed upon those proteins which either mediate or respond to the rapid influx of calcium following depolarization, there has been little emphasis upon those proteins which aid in the reequilibration of the membrane potential. In an effort to identify presynaptic membrane proteins implicated in neurosecretion, monoclonal antibodies were screened against proteins which cosegregated with neuronal voltage-dependent calcium channels (VDCC) following immunoprecipitation. One monoclonal antibody (mAb 9A7) identified a 110-kDa protein. Micropeptide sequencing of (i) the mAb 9A7 immunoaffinity purified antigen and (ii) the 110-kDa protein present in the neuronal (N-type) VDCC preparation (McEnery et al., 1991, Proc. Natl. Acad. Sci. 88, 11095-11099) indicated identity with the alpha subunit(s) of the Na,K-ATPase. Further characterization by Western blotting, immunochemical localization, and immunoaffinity purification indicated that mAb 9A7 not only recognized the alpha3 isoform which is predominant in neuronal tissues but also identified the alpha1 and alpha2 isoforms. mAb 9A7 exhibited a wide cross-species reactivity and recognized human, rat, and mouse alpha subunit isoforms at an internal epitope. The pan-specificity of mAb 9A7 and the differential mobility of the alpha1 isoform relative to the alpha2 and alpha3 permitted parallel detection of multiple alpha isoforms. Western blot analysis of undifferentiated rat pheochromocytoma cell line (PC12) and human neuroblastoma (IMR32) cells indicated coexpression of the alpha1 and alpha3 isozymes. Upon differentiation of IMR32 cells by dibutrylyl-cAMP, a substantial increase in the alpha3 relative to the alpha1 isoform was observed. While the enrichment of total Na,K-ATPase may reflect the increased demand for ATP-dependent ion transport as IMR32 cells become more excitable, the specific increase in the alpha3 isoform suggests a unique role of this isoform during IMR32 cell differentiation.

Rabies virus infection of IMR-32 human neuroblastoma cells and effect of neurochemical and other agents

IMR-32 human neuroblastoma cells are a continuous nerve cell line expressing neuronal nicotinic acetylcholine receptors. These cells were found to be susceptible to infection by rabies virus (CVS strain). After infection, viral antigen accumulated in the cell body in puncta and larger masses and spread out into the processes until at 3-4 days the entire cell was filled with antigen and lysed. A variety of chemical agents including cholinergic agonists and antagonists were tested for ability to inhibit infection of IMR-32 cells in a fluorescent focus assay. Agents found to inhibit infection were antibodies against the viral glycoprotein, gangliosides, a synthetic peptide of the neurotoxin-binding site of Torpedo acetylcholine receptor alpha1 subunit, alpha-bungarotoxin, and lysosomotropic agents. All other agents tested including other cholinergic ligands and synthetic peptides were not effective. Except for lysosomotropic agents, the agents which inhibited infection also inhibited attachment of virus to the cell surface. These results indicate that IMR-32 cells are a useful model in studying the interaction of a neurotropic virus with human neurons. The ability of alpha-bungarotoxin to inhibit infection suggests that neuronal alpha-bungarotoxin-binding receptors might serve as central nervous system receptors for rabies virus.