Differential regulation of neuronal nicotinic acetylcholine receptor subunit gene promoters by Brn-3 POU family transcription factors

Functional characterization of SNPs in CHRNA3/B4 intergenic region associated with drug behaviors

The cluster of human neuronal nicotinic receptor genes (CHRNA5/A3/B4) (15q25.1) has been associated with a variety of smoking and drug-related behaviors, as well as risk for lung cancer. CHRNA3/B4 intergenic single nucleotide polymorphisms (SNPs) rs1948 and rs8023462 have been associated with early initiation of alcohol and tobacco use, and rs6495309 has been associated with nicotine dependence and risk for lung cancer. An in vitro luciferase expression assay was used to determine whether these SNPs and surrounding sequences contribute to differences in gene expression using cell lines either expressing proteins characteristic of neuronal tissue or derived from lung cancers. Electrophoretic mobility shift assays (EMSAs) were performed to investigate whether nuclear proteins from these cell lines bind SNP alleles differentially. Results from expression assays were dependent on cell culture type and haplotype. EMSAs indicated that rs8023462 and rs6495309 bind nuclear proteins in an allele-specific way. Additionally, GATA transcription factors appeared to bind rs8023462 only when the minor/risk allele was present. Much work has been done to describe the rat Chrnb4/a3 intergenic region, but few studies have examined the human intergenic region effects on expression; therefore, these studies greatly aid human genetic research as it relates to observed nicotine phenotypes, lung cancer risk and potential underlying genetic mechanisms. Data from these experiments support the hypothesis that SNPs associated with human addiction-related phenotypes and lung cancer risk can affect gene expression, and are potential therapeutic targets. Additionally, this is the first evidence that rs8023462 interacts with GATA transcription factors to influence gene expression.

From smoking to lung cancer: the CHRNA5/A3/B4 connection

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that modulate key physiological processes ranging from neurotransmission to cancer signaling. These receptors are activated by the neurotransmitter, acetylcholine, and the tobacco alkaloid, nicotine. Recently, the gene cluster encoding the alpha3, alpha5 and beta4 nAChR subunits received heightened interest after a succession of linkage analyses and association studies identified multiple single-nucleotide polymorphisms in these genes that are associated with an increased risk for nicotine dependence and lung cancer. It is not clear whether the risk for lung cancer is direct or an effect of nicotine dependence, as evidence for both scenarios exist. In this study, we summarize the body of work implicating nAChRs in the pathogenesis of lung cancer, with special focus on the clustered nAChR subunits and their emerging role in this disease state.

The nicotinic acetylcholine receptor CHRNA5/A3/B4 gene cluster: dual role in nicotine addiction and lung cancer

More than 1 billion people around the world smoke, with 10 million cigarettes sold every minute. Cigarettes contain thousands of harmful chemicals including the psychoactive compound, nicotine. Nicotine addiction is initiated by the binding of nicotine to nicotinic acetylcholine receptors, ligand-gated cation channels activated by the endogenous neurotransmitter, acetylcholine. These receptors serve as prototypes for all ligand-gated ion channels and have been extensively studied in an attempt to elucidate their role in nicotine addiction. Many of these studies have focused on heteromeric nicotinic acetylcholine receptors containing α4 and β2 subunits and homomeric nicotinic acetylcholine receptors containing the α7 subunit, two of the most abundant subtypes expressed in the brain. Recently however, a series of linkage analyses, candidate-gene analyses and genome-wide association studies have brought attention to three other members of the nicotinic acetylcholine receptor family: the α5, α3 and β4 subunits. The genes encoding these subunits lie in a genomic cluster that contains variants associated with increased risk for several diseases including nicotine dependence and lung cancer. The underlying mechanisms for these associations have not yet been elucidated but decades of research on the nicotinic receptor gene family as well as emerging data provide insight on how these receptors may function in pathological states. Here, we review this body of work, focusing on the clustered nicotinic acetylcholine receptor genes and evaluating their role in nicotine addiction and lung cancer.

Transcriptional Repression by a Conserved Intronic Sequence in the Nicotinic Receptor α3 Subunit Gene

The genes encoding the nicotinic acetylcholine receptor alpha3, alpha5, and beta4 subunits are genomically clustered. These genes are co-expressed in a variety of cells in the peripheral and central nervous systems. Their gene products assemble in a number of stoichiometries to generate several nicotinic receptor subtypes that have distinct pharmacological and physiological properties. Signaling through these receptors is critical for a variety of fundamental biological processes. Despite their importance, the transcriptional mechanisms underlying their coordinated expression remain to be completely elucidated. By using a bioinformatics approach, we identified a highly conserved intronic sequence within the fifth intron of the alpha3 subunit gene. Reporter gene analysis demonstrated that this sequence, termed "alpha3 intron 5," inhibits the transcriptional activities of the alpha3 and beta4 subunit gene promoters. This repressive activity is position- and orientation-independent. Importantly, repression occurs in a cell type-specific manner, being present in cells that do not express the receptor genes or expresses them at very low levels. Electrophoretic mobility shift assays demonstrate that nuclear proteins specifically interact with alpha3 intron 5 at two distinct sites. We propose that this intronic repressor element is important for the restricted expression patterns of the nicotinic receptor alpha3 and beta4 subunit genes.

Coordinated regulation of gene expression by Brn3a in developing sensory ganglia

Mice lacking the POU-domain transcription factor Brn3a exhibit marked defects in sensory axon growth and abnormal sensory apoptosis. We have determined the regulatory targets of Brn3a in the developing trigeminal ganglion using microarray analysis of Brn3a mutant mice. These results show that Brn3 mediates the coordinated expression of neurotransmitter systems, ion channels, structural components of axons and inter- and intracellular signaling systems. Loss of Brn3a also results in the ectopic expression of transcription factors normally detected in earlier developmental stages and in other areas of the nervous system. Target gene expression is normal in heterozygous mice, consistent with prior work showing that autoregulation by Brn3a results in gene dosage compensation. Detailed examination of the expression of several of these downstream genes reveals that the regulatory role of Brn3a in the trigeminal ganglion appears to be conserved in more posterior sensory ganglia but not in the CNS neurons that express this factor.

Abnormal chemosensory jump 6 is a positive transcriptional regulator of the cholinergic gene locus in Drosophila olfactory neurons

Cholinergic neurons acquire their neurotransmitter phenotype, in part, by expressing the cholinergic gene locus. Previous studies have indicated that the 5' flanking DNA of the locus contains both positive and negative regulatory elements important for expression in different subsets of cholinergic neurons in Drosophila and other animals. Approximately 300 bases of proximal 5' flanking DNA control expression in Drosophila CNS neurons essential for viability, whereas more distal regulatory elements are important for expression in PNS sensory neurons. In this study we identify the POU domain transcription factor abnormal chemosensory jump 6 (Acj6) as a necessary positive transcriptional regulator for cholinergic locus expression in primary olfactory neurons. Choline acetyltransferase enzyme activity, protein levels, mRNA, and a fluorescent cholinergic reporter gene are all decreased in olfactory neurons of acj6 mutants. Decreased cholinergic expression was observed in both adults and larvae. The presence of a specific Acj6 binding site has been identified in the cholinergic locus 5' flanking DNA, suggesting that Acj6 may play a direct role in specifying the cholinergic neurotransmitter phenotype of most olfactory neurons. Transgenic expression of two different isoforms of Acj6 restricted to olfactory neurons indicates that additional trans factors may be required for cholinergic locus expression. Transgenic expression in all cholinergic neurons, however, results in lethality when a POU IV box element is absent but is essentially benign when present, indicating the importance of this motif in specifying different functional roles for Acj6.

Regulation of central neuron synaptic targeting by the Drosophila POU protein, Acj6

Mutations in the Drosophila class IV POU domain gene, abnormal chemosensory jump 6 (acj6), have previously been shown to cause physiological deficits in odor sensitivity. However, loss of Acj6 function also has a severe detrimental effect upon coordinated larval and adult movement that cannot be explained by the simple loss in odorant detection. In addition to olfactory sensory neurons, Acj6 is expressed in a distinct subset of postmitotic interneurons in the central nervous system from late embryonic to adult stages. In the larval and adult brain, Acj6 is highly expressed in central brain, optic and antennal lobe neurons. Loss of Acj6 function in larval optic lobe neurons results in disorganized retinal axon targeting and synapse selection. Furthermore, the lamina neurons themselves exhibit disorganized synaptic arbors in the medulla of acj6 mutant pupal brains, suggesting that Acj6 may play a role in regulating synaptic connections or structure. To further test this hypothesis, we misexpressed two Acj6 isoforms in motor neurons where they are not normally found. The two Acj6 isoforms are produced from alternatively spliced acj6 transcripts, resulting in significant structural differences in the amino-terminal POU IV box. Acj6 misexpression caused marked alterations at the neuromuscular junction, with contrasting effects upon nerve terminal branching and synapse formation associated with specific Acj6 isoforms. Our results suggest that the class IV POU domain factor, Acj6, may play an important role in regulating synaptic target selection by central neurons and that the amino-terminal POU IV box is important for regulation of Acj6 activity.

Interactions between regulatory proteins that bind to the nicotinic receptor beta4 subunit gene promoter

The genes encoding the alpha3, alpha5 and beta4 subunits of nicotinic acetylcholine receptors are tightly clustered within the genome. As these three subunits constitute the predominant acetylcholine receptor subtype expressed in the peripheral nervous system, their genomic proximity suggests a regulatory mechanism ensuring their coordinate expression. We previously identified two transcriptional regulatory elements within the beta4 promoter. One of these elements, a CT box, interacts with the regulatory factors heterogeneous nuclear ribonucleoprotein K and Puralpha. Another element, a CA box, interacts with Sp1 and Sp3. The binding site for a fifth factor, Sox10, overlaps the CT and CA boxes. As the CT and CA boxes are adjacent, we postulated that the proteins that bind to the elements interact. Here we report that the CT box-binding factors interact with each other as do the CA box-binding factors. However, there are no direct associations between the two pairs of proteins. Interestingly though, Sox10 directly interacts with all four proteins, suggesting a central role in beta4 gene expression for this member of the Sox family of regulatory factors.

Cell type-specific activation of neuronal nicotinic acetylcholine receptor subunit genes by Sox10

The regulatory factor Sox10 is expressed in neural crest derivatives during development as well as in the adult CNS and peripheral nervous system. Mutations of the human Sox10 gene have been identified in patients with Waardenburg-Hirschsprung syndrome that is characterized by defects in neural crest development. Previous studies suggested that Sox10 might function as an important transcriptional regulator of neural crest development. No natural target genes of Sox10 have yet been identified. Although human Sox10 activates a synthetic promoter consisting of a TATA box and multiple Sox consensus sequences, no transcriptional activity of the rat Sox10 homolog has been detected. Here we report that the neuronal nicotinic acetylcholine receptor beta4 and alpha3 subunit gene promoters are transactivated by rat Sox10 in a cell type-specific manner. The alpha3 and beta4 subunits, in combination with the alpha5 subunit, make up the predominant nicotinic receptor subtype expressed in the peripheral nervous system. Transfections using Sox10 mutants indicate that the C-terminal region is dispensable for its ability to activate the beta4 and alpha3 promoters. Rat Sox10 was originally identified as an accessory protein of the POU domain protein Tst-1/Oct6/SCIP in glial cells. Tst-1/Oct6/SCIP was shown previously to activate the alpha3 promoter. We now demonstrate that it can transactivate the beta4 promoter as well. However, we were unable to detect any synergistic effects of Sox10 and Tst-1/Oct6/SCIP on beta4 or alpha3 promoter activity. Finally, we present data suggesting that recombinant Sox10 protein can directly interact with a previously characterized regulatory region of the beta4 gene.

Cell type-specific activation of neuronal nicotinic acetylcholine receptor subunit genes by Sox10

The regulatory factor Sox10 is expressed in neural crest derivatives during development as well as in the adult CNS and peripheral nervous system. Mutations of the human Sox10 gene have been identified in patients with Waardenburg-Hirschsprung syndrome that is characterized by defects in neural crest development. Previous studies suggested that Sox10 might function as an important transcriptional regulator of neural crest development. No natural target genes of Sox10 have yet been identified. Although human Sox10 activates a synthetic promoter consisting of a TATA box and multiple Sox consensus sequences, no transcriptional activity of the rat Sox10 homolog has been detected. Here we report that the neuronal nicotinic acetylcholine receptor beta4 and alpha3 subunit gene promoters are transactivated by rat Sox10 in a cell type-specific manner. The alpha3 and beta4 subunits, in combination with the alpha5 subunit, make up the predominant nicotinic receptor subtype expressed in the peripheral nervous system. Transfections using Sox10 mutants indicate that the C-terminal region is dispensable for its ability to activate the beta4 and alpha3 promoters. Rat Sox10 was originally identified as an accessory protein of the POU domain protein Tst-1/Oct6/SCIP in glial cells. Tst-1/Oct6/SCIP was shown previously to activate the alpha3 promoter. We now demonstrate that it can transactivate the beta4 promoter as well. However, we were unable to detect any synergistic effects of Sox10 and Tst-1/Oct6/SCIP on beta4 or alpha3 promoter activity. Finally, we present data suggesting that recombinant Sox10 protein can directly interact with a previously characterized regulatory region of the beta4 gene.

Evidence that POU factor Brn-3B regulates expression of Pax-6 in neuroretina cells

The Pax-6 gene encodes a transcriptional master regulator involved in the development of the eye. The quail Pax-6 gene is expressed in the neuroretina from two promoters, P0 and P1, and is regulated by an intragenic neuroretina-specific enhancer (EP enhancer). The activity of this enhancer is restricted to the P0 promoter, which is activated at the onset of neuronal differentiation. In this article, we show that the POU domain transcription factor Brn-3b, which is expressed in various regions of the brain including retina and sensory neurons, is one of the factors interacting with the EP enhancer. Brn-3b strongly activates the EP enhancer in neuroretina cells but not in other cell types. Interestingly, this activation appears to be specific for Brn-3b, as the closely related POU factors Brn-3a and Brn-3c do not show activation of the EP enhancer. Our results identify the Pax-6 gene as a new potential downstream effector of the POU transcription factor Brn-3b.

Regulation of ion channel expression in neural cells by hormones and growth factors

Voltage-and ligand-gated ion channels are key players in synaptic transmission and neuron-glia communication in the nervous system. Expression of these proteins can be regulated at several levels (transcriptional, translational, or posttranslational) and by multiple extracellular factors in the developing and mature nervous system. A wide variety of hormones and growth factors have been identified as important in neural cell differentiation, which is a complex process involving the acquisition of cell-type-specific ion channel phenotypes. Much literature has already accumulated describing the structural and functional characteristics of ion channels, but relatively little is known about the factors that influence their synthesis and cell surface expression, although this area has generated considerable interest in the context of neural cell development. This article reviews several examples of regulated expression of these channels by cellular factors, namely peptide growth factors and steroid hormones, and discusses, where applicable, current understanding of molecular mechanisms underlying such regulation of voltage-and neurotransmitter-gated ion channels.

Auditory hair cell precursors immortalized from the mammalian inner ear

Mammalian auditory hair cells are few in number, experimentally inaccessible, and do not proliferate postnatally or in vitro. Immortal cell lines with the potential to differentiate into auditory hair cells would substantially facilitate auditory research, drug development, and the isolation of critical molecules involved in hair cell biology. We have established two conditionally immortal cell lines that express at least five characteristic hair cell markers. These markers are the transcription factor Brn3.1, the alpha 9 subunit of the acetylcholine receptor, the stereociliary protein fimbrin and the myosins VI and VIIA. These hair cell precursors permit functional studies of cochlear genes and in the longer term they will provide the means to explore therapeutic methods of stimulating auditory hair cell regeneration.

Differential effects of heterogeneous nuclear ribonucleoprotein K on Sp1- and Sp3-mediated transcriptional activation of a neuronal nicotinic acetylcholine receptor promoter

The neuronal nicotinic acetylcholine receptor gene family consists of 11 members, alpha2-alpha9 and beta2-beta4. Three of the genes, those encoding the alpha3, alpha5, and beta4 subunits, are clustered tightly within the genome. These three subunits constitute the predominant acetylcholine receptor subtype expressed in the peripheral nervous system. The genomic proximity of the three genes suggests a regulatory mechanism ensuring their coordinate expression. However, it is likely that gene-specific regulatory mechanisms are also functioning because the expression patterns of the three genes, although similar, are not identical. Previously we identified regulatory elements within the beta4 promoter region and demonstrated that these elements interact specifically with nuclear proteins. One of these elements, E1, interacts with the regulatory factor Puralpha as well as three other unidentified DNA-binding proteins with molecular masses of 31, 65, and 114 kDa. Another element, E2, interacts with Sp1 and Sp3. Because E1 and E2 are immediately adjacent to one another, we postulated that the proteins that bind to the elements interact to regulate beta4 gene expression. Here we report the identification of the 65-kDa E1-binding protein as heterogeneous nuclear ribonucleoprotein K and demonstrate that it affects the transactivation of beta4 promoter activity by Sp1 and Sp3 differentially.

Sp1 and Sp3 regulate expression of the neuronal nicotinic acetylcholine receptor beta4 subunit gene

Neuronal nicotinic acetylcholine receptors play important roles in signal transduction within the nervous system. The receptors exist in a variety of functionally distinct subtypes that are determined by their subunit structures. The subunits are encoded by 11 genes, alpha2-alpha9 and beta2-beta4. Three of the genes, alpha3, alpha5, and beta4, are tightly clustered, and their encoded proteins make up the predominant receptor subtype in the peripheral nervous system. The tight linkage of the genes suggests there may be a common regulatory mechanism underlying their expression. However, although their expression patterns significantly overlap, they are not identical, indicating that independent regulatory mechanisms must also exist. Our studies have focused upon the gene encoding the beta4 subunit for which we have identified several transcriptional regulatory elements. One of these elements, E2, specifically interacts with the general transcription factor Sp1. Here we show that another member of the Sp family of factors, Sp3, can specifically interact with E2 whereas two other members, Sp2 and Sp4, cannot. Co-transfection experiments indicate that Sp3 can transactivate a beta4 promoter/reporter gene construct and, furthermore, that Sp1 and Sp3 can transactivate the beta4 reporter construct synergistically. The transactivation is dependent upon an intact E2 and may involve direct interactions between Sp1 and Sp3.

Codistribution of nicotinic acetylcholine receptor subunit alpha3 and beta4 mRNAs during rat brain development

We have used in situ hybridization to characterize the ontogeny of alpha3 and beta4 nicotinic acetylcholine receptor (nAChR) subunit mRNA expression in rat brain. Transcripts for both subunits were detected in embryonic brain, although overlapping expression of alpha3 mRNA was only evident in areas of strong beta4 mRNA expression, including the medial habenula, locus coeruleus, the cerebellar primordium, and several motor and sensory brainstem nuclei. During the perinatal period, the independent expression of alpha3 mRNA declined, and greater correspondence in the temporal and spatial expression of alpha3 and beta4 subunit mRNAs emerged. In general, beta4 mRNA expression preceded that of alpha3 mRNA by 1 to 2 days. Overlapping expression patterns were transiently detected in the caudate putamen, basal forebrain, frontal and visual cortices, and in the CA3 field of hippocampus. Codistribution that lasted throughout development and into adulthood was noted in a number of brain areas, including the retrosplenial cortex, subiculum, medial habenula, interpeduncular nucleus, locus coeruleus, and brainstem motor nuclei. In many of these regions, alpha5 subunit mRNA was also expressed. Colocalization of alpha3 and beta4 mRNAs with choline acetyltransferase mRNA was detected in cholinergic neurons of the brainstem motor nuclei, nucleus ambiguus, dorsal motor nucleus of the vagus, motor trigeminal nucleus, and facial nucleus, but not in most forebrain cholinergic cells. The extensive correspondence in temporal and spatial distribution of alpha3 and beta4 mRNAs throughout postnatal brain development suggests that these subunits may be coordinately regulated and may form functional neuronal nAChRs with significant developmental roles.

Human neuronal nicotinic receptors

Nicotine is a very widely used drug of abuse, which exerts a number of neurovegetative, behavioural and psychological effects by interacting with neuronal nicotinic acetylcholine receptors (NAChRs). These receptors are distributed widely in human brain and ganglia, and form a family of ACh-gated ion channels of different subtypes, each of which has a specific pharmacology and physiology. As human NAChRs have been implicated in a number of human central nervous system disorders (including the neurodegenerative Alzheimer's disease, schizophrenia and epilepsy), they are suitable potential targets for rational drug therapy. Much of our current knowledge about the structure and function of NAChRs comes from studies carried out in other species, such as rodents and chicks, and information concerning human nicotinic receptors is still incomplete and scattered in the literature. Nevertheless, it is already evident that there are a number of differences in the anatomical distribution, physiology, pharmacology, and expression regulation of certain subtypes between the nicotinic systems of humans and other species. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining the molecular basis of their functional diversity viewed mainly from pharmacological and biochemical perspectives. It will also summarize our current knowledge concerning the structure and function of the NAChRs expressed by other species, and the newly discovered drugs used to classify their numerous subtypes. Finally, the role of NAChRs in behaviour and pathology will be considered.

Inhibition of neuronal process outgrowth and neuronal specific gene activation by the Brn-3b transcription factor

The differentiation of the ND7 neuronal cell line to a nondividing phenotype bearing numerous neurite processes is accompanied by a dramatic increase in the levels of the activating POU family transcription factor Brn-3a and a corresponding fall in the levels of the closely related inhibitory factor Brn-3b. We have previously shown that the artificial overexpression of Brn-3a in these cells can induce neurite outgrowth and the activation of genes encoding synaptic vesicle proteins in the absence of a differentiation-inducing stimulus. Here we show that overexpression of Brn-3b can reduce process outgrowth and synaptic vesicle gene expression following exposure to a stimulus which would normally induce differentiation. These inhibitory effects are abolished by altering a single amino acid in the POU homeodomain of Brn-3b to its equivalent in Brn-3a. The converse mutation in Brn-3a allows it to inhibit process outgrowth in response to a differentiation-inducing stimulus. Hence a single amino acid difference results in these closely related factors having opposite effects and allows the balance between them to regulate differentiation.