Identification of proximal and distal regulatory elements of the rat connexin32 gene

Connexins: Synthesis, Post-Translational Modifications, and Trafficking in Health and Disease

Connexins are tetraspan transmembrane proteins that form gap junctions and facilitate direct intercellular communication, a critical feature for the development, function, and homeostasis of tissues and organs. In addition, a growing number of gap junction-independent functions are being ascribed to these proteins. The connexin gene family is under extensive regulation at the transcriptional and post-transcriptional level, and undergoes numerous modifications at the protein level, including phosphorylation, which ultimately affects their trafficking, stability, and function. Here, we summarize these key regulatory events, with emphasis on how these affect connexin multifunctionality in health and disease.

Regulation of connexin expression by transcription factors and epigenetic mechanisms

Gap junctions are specialized cell-cell junctions that directly link the cytoplasm of neighboring cells. They mediate the direct transfer of metabolites and ions from one cell to another. Discoveries of human genetic disorders due to mutations in gap junction protein (connexin [Cx]) genes and experimental data on connexin knockout mice provide direct evidence that gap junctional intercellular communication is essential for tissue functions and organ development, and that its dysfunction causes diseases. Connexin-related signaling also involves extracellular signaling (hemichannels) and non-channel intracellular signaling. Thus far, 21 human genes and 20 mouse genes for connexins have been identified. Each connexin shows tissue- or cell-type-specific expression, and most organs and many cell types express more than one connexin. Connexin expression can be regulated at many of the steps in the pathway from DNA to RNA to protein. In recent years, it has become clear that epigenetic processes are also essentially involved in connexin gene expression. In this review, we summarize recent knowledge on regulation of connexin expression by transcription factors and epigenetic mechanisms including histone modifications, DNA methylation, and microRNA. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

Regulation of connexin expression

Gap junctions contain cell-cell communicating channels that consist of multimeric proteins called connexins and mediate the exchange of low-molecular-weight metabolites and ions between contacting cells. Gap junctional communication has long been hypothesized to play a crucial role in the maintenance of homeostasis, morphogenesis, cell differentiation, and growth control in multicellular organisms. The recent discovery that human genetic disorders are associated with mutations in connexin genes and experimental data on connexin knockout mice have provided direct evidence that gap junctional communication is essential for tissue functions and organ development. Thus far, 21 human genes and 20 mouse genes for connexins have been identified. Each connexin shows tissue- or cell-type-specific expression, and most organs and many cell types express more than one connexin. Cell coupling via gap junctions is dependent on the specific pattern of connexin gene expression. This pattern of gene expression is altered during development and in several pathological conditions resulting in changes of cell coupling. Connexin expression can be regulated at many of the steps in the pathway from DNA to RNA to protein. However, transcriptional control is one of the most important points. In this review, we summarize recent knowledge on transcriptional regulation of connexin genes by describing the structure of connexin genes and transcriptional factors that regulate connexin expression.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Identification of functional regulatory regions of the connexin32 gene promoter

Connexin32 (Cx32) is the predominant gap junction protein expressed in adult rat hepatocytes. This study investigated transcriptional regulation of the rat Cx32 gene in MH(1)C(1) rat hepatoma cells using transient expression assays in conjunction with promoter mutagenesis and 5' nested deletion analysis. Site-directed mutagenesis of the -736 and -187 hepatocyte nuclear factor-1 (HNF-1) sites, the -196 and -116 Sp1 sites, and the -729 and -329 Yin Yang 1 (YY1) sites all significantly reduced promoter activity. We have defined the contribution of each individual site to promoter activity in the intact cell. A novel upstream region of the Cx32 promoter (-1042 to -758) was cloned and shown to contain negative regulatory elements. The transcription factors HNF-1 and Sp1 have important functional roles in the transcriptional regulation of basal and cell-specific Cx32 expression. The multifunctional transcription factor YY1 is also implicated.

Heterogeneous expression of connexin 36 in the olfactory epithelium and glomerular layer of the olfactory bulb

Gap junctions regulate a variety of cell functions by directly connecting two cells through intercellular channels. Connexins are gap junction channel-forming protein subunits. In this study, we studied the expression of connexin 36 (Cx36) in the olfactory epithelium and olfactory bulb of adult mice. In situ hybridization revealed that mRNA for Cx36 was expressed in the olfactory sensory epithelium, main olfactory bulb and accessory olfactory bulb. Expression of mRNA encoding Cx36 was observed in the olfactory epithelium mainly in ventral and lateral regions of the turbinates. Immunohistochemical determination of Cx36 protein expression showed sparse punctuate staining in the olfactory epithelial layer. Intense Cx36-like immunostaining was found in the olfactory nerve bundles underlying the olfactory epithelium and in the olfactory nerve layer and glomerular layer of the olfactory bulb. Mapping of the intensity of Cx36-like immunofluorescence in glomeruli throughout the main olfactory bulb indicated a heterogeneous distribution. A set of approximately 50 glomeruli located in the anterior and posterior limits of the olfactory bulb was more intensely labeled than other glomeruli. There was intense immunofluorescence signal in the glomerular layer of the accessory olfactory bulb and in the vomeronasal nerve. beta-Galactosidase distribution in the olfactory epithelium and olfactory bulb in Cx36 knockout mice (Deans et al. [2001] Neuron 31:477-485) supported the findings with immunofluorescence. Cx36-like immunofluorescence was absent in the olfactory nerve bundles in Cx36 knockout mice. The immunolocalization of Cx36 to the olfactory and vomeronasal nerves, and a subset of olfactory glomeruli suggest a functional role for Cx36 in odor coding.

Role of connexin32 and beta-catenin in tumor promotion in mouse liver

Tumor promoters are nonmutagenic chemicals that increase the probability of cancer by accelerating the clonal expansion of cells transformed during tumor initiation. The molecular mechanisms underlying this process are only partly understood but interference with signaling pathways regulating cell division and/or cell death is likely to be important. Ras- and beta-Catenin-dependent signaling is important for both of these processes and ras and beta-catenin genes are known mutational targets in mouse hepatocarcinogenesis. About 80% of liver tumors generated in mice by a promotional regimen including phenobarbital (PB) as tumor promoter and N-nitrosodiethylamine (DEN) as initiator showed beta-catenin mutations whereas Ha-ras mutations were not detected. By contrast, tumors from mice treated with DEN alone showed a approximately 30% Ha-ras mutation prevalence but no beta-catenin mutations. This result suggests that PB-mediated promotion in mouse liver consists in a positive selection for hepatocytes harboring mutations in beta-catenin. The gap junction protein connexin 32 (Cx32) was also found to be involved in tumor promotion by PB because Cx32 gene knockout mice were almost entirely resistent to the promotional effects of the barbiturate. The link between beta-catenin-signaling and Cx32-dependent gap junctional intercellular communication, if existent, remains obscure.

Expression of the mouse gap junction gene Gjb3 is regulated by distinct mechanisms in embryonic stem cells and keratinocytes

Connexins are the protein subunits of gap junction channels and are expressed in a highly regulated temporal and spatial pattern in embryonic development and adult life, with most cell types expressing more than one isoform. Connexin31 (Cx31) is encoded by the gene Gjb3 and expressed throughout mouse development n a complex pattern; in adult mice it becomes restricted to the granular layer of epidermis, testis, and placenta. In placenta, lack of Cx31 leads to transient dysmorphogenesis affecting embryonic survival. Here we have analyzed the structure of mouse Gjb3 as well as its transcriptional regulation by transient transfection of reporter gene constructs in HM1 mouse embryonic stem cells and a mouse keratinocytederived cell line, Hel37, as model systems for early development and skin, respectively. Like most connexin genes, Gjb3 is composed of two exons, the second of which contains the whole coding region and is separated from the first exon by an intron of 2.3 kb. Expression in keratinocytes is regulated by a basal promoter extending to 561 bp upstream of exon 1 in conjunction with a regulatory region between upstream positions 561 and 841. In contrast, expression of Gjb3 in embryonic stem cells depended on the basal promoter together with the intron. The enhancing effect of the intron was found only in embryonic stem cells and depended on its native position and the integrity of the splice sites. Thus, expression of Gjb3 in keratinocytes and embryonic stem cells is regulated by different cis-regulatory elements and differs in its requirements for the intron in situ.

Liver cell-specific transcriptional regulation of connexin32

Gap junctional intercellular communication facilitates liver homeostasis and growth control in the liver. The major gap junction protein expressed by hepatocytes is connexin32 (Cx32) and non-parenchymal hepatic cells do not express this gene. We investigated the regulation of Cx32 transcription by trans-activating factors in liver cells. Transient transfection assays using deletions of the rat Cx32 promoter (nt -753 to -33) linked to the luciferase gene were performed in MH1C1 rat hepatoma cells that express endogenous Cx32 compared with WB-F344 rat liver epithelial cells that do not. The basal promoter element was located within nt -134 to -33 and was 1.4-fold more active in MH1C1 cells than WB-F344 cells whereas the entire promoter fragment (nt -754 to -33) was four-fold more active in MH1C1 cells. Specific nuclear protein-DNA complexes that bound to Sp1 consensus sites within the basal promoter were formed using nuclear extracts from both types of cells. Additional promoter sequences increased promoter activity more strongly in MH1C1 cells than WB-F344 cells and this was correlated with the binding of hepatocyte nuclear factor-1 (HNF-1) to two HNF-1 consensus sites centered at -187 and -736. Expression of HNF-1 and binding to these elements was only observed with MH1C1 cells. Other specific protein-DNA complexes were formed, however, that included YY-1- and NF-1-containing complexes, but these were not related to promoter activity. Dexamethasone increased Cx32 promoter activity and expression in MH1C1 cells, but had little effect in WB-F344 cells and did not alter protein-DNA complex formation. These data suggest that Sp1 is responsible for Cx32 promoter basal activity, that HNF-1 determines the cell-specific expression of Cx32, and that dexamethasone increases Cx32 expression through other mechanisms.

Regulation of connexin32 and connexin43 gene expression by DNA methylation in rat liver cells

Gap junction proteins (connexins) are expressed in a cell-specific manner and expression is often reduced in neoplastic cells. We investigated the mechanisms of connexin32 (Cx32) and connexin43 (Cx43) expression in hepatic cells using MH1C1 rat hepatoma cells and freshly isolated, adult rat hepatocytes that express Cx32 but not Cx43 and WB-F344 rat liver epithelial cells that express Cx43 but not Cx32. Southern blotting after DNA restriction with MspI and HpaII indicated that two MspI/HpaII restriction sites in the Cx32 promoter (positions -147 and -847) were methylated in WB-F344 cells, but not in MH1C1 cells or hepatocytes. In contrast, an MspI/HpaII restriction site in the Cx43 promoter (position -38) was methylated in MH1C1 cells, but not in WB-F344 cells or hepatocytes. Transient transfection of the cell lines with connexin promoter-luciferase constructs indicated that the Cx32 promoter was 7-fold more active in MH1C1 cells and the Cx43 promoter was 5-fold more active in WB-F344 cells. These results suggest that transcription of Cx32 and Cx43 in hepatic cells is controlled by promoter methylation and by cell-specific transcription factors. Similar mechanisms may be involved in the reduced expression of these genes frequently observed in neoplastic cells.

Genomic organization of the rat connexin40 gene: identical transcription start sites in heart and lung

OBJECTIVES

The gap junction protein connexin(Cx)40 is developmentally and tissue-specifically expressed. How Cx40 expression is regulated is unknown. We therefore set out to characterize the 5'-untranslated end of both the Cx40 gene and mRNA from different tissues and ages and to identify the Cx40 promoter region.

METHODS

The PCR method 5'-RACE was used to amplify the 5'-end of rat Cx40 mRNAs. Genomic rat Cx40 clones were isolated from a lambda EMBL3 library. The promoter sequence was isolated by long distance PCR. The transcription start site was identified by primer extension and RNase protection assays.

RESULTS

Comparison of Cx40 genomic DNA and mRNA sequences revealed that the Cx40 gene contains a small untranslated exon, exon I, which is separated from the coding sequences by an intron of at least 5.5 Kb. The untranslated 5'-end of Cx40 mRNA sequences from adult rat lung, neonatal and adult rat heart and the rat aortic smooth muscle cell line A7r5 were identical. While the same transcription start site was found for the Cx40 mRNAs from different tissues and ages, and amount of Cx40 mRNA differed between tissues as follows: A7r5 cells > neonatal lung > adult lung > or = neonatal atrium > neonatal ventricle; Cx40 mRNA from adult atrium and ventricle was not readily detected by primer extension and RNase protection analyses. The genomic sequence upstream of the transcription start site contains multiple consensus binding sites for transcription factors putatively responsible for spatio-temporal control of Cx40 gene expression.

CONCLUSIONS

Similar to other connexin genes, the Cx40 gene contains two exons. The same exon I sequence is present in all tissues and developmental stages examined and the relative amounts of Cx40 mRNA in these compare well with published data. Together our data suggest that tissue-specific and developmentally regulated expression of the Cx40 gene is controlled within the same promoter region by mechanisms that have yet to be detailed.

Connections with connexins: the molecular basis of direct intercellular signaling

Adjacent cells share ions, second messengers and small metabolites through intercellular channels which are present in gap junctions. This type of intercellular communication permits coordinated cellular activity, a critical feature for organ homeostasis during development and adult life of multicellular organisms. Intercellular channels are structurally more complex than other ion channels, because a complete cell-to-cell channel spans two plasma membranes and results from the association of two half channels, or connexons, contributed separately by each of the two participating cells. Each connexon, in turn, is a multimeric assembly of protein subunits. The structural proteins comprising these channels, collectively called connexins, are members of a highly related multigene family consisting of at least 13 members. Since the cloning of the first connexin in 1986, considerable progress has been made in our understanding of the complex molecular switches that control the formation and permeability of intercellular channels. Analysis of the mechanisms of channel assembly has revealed the selectivity of inter-connexin interactions and uncovered novel characteristics of the channel permeability and gating behavior. Structure/function studies have begun to provide a molecular understanding of the significance of connexin diversity and demonstrated the unique regulation of connexins by tyrosine kinases and oncogenes. Finally, mutations in two connexin genes have been linked to human diseases. The development of more specific approaches (dominant negative mutants, knockouts, transgenes) to study the functional role of connexins in organ homeostasis is providing a new perception about the significance of connexin diversity and the regulation of intercellular communication.

Effect of tumor-promoting and anti-promoting chemicals on the viability and junctional coupling of human HeLa cells transfected with DNAs coding for various murine connexin proteins

Gap-junctional intercellular communication is thought to be essential for maintaining cellular homeostasis and growth control. Its perturbation entails toxicological implications and it has been correlated with the in vivo tumor-promoting potential of chemicals. Little is known about the mechanism(s) responsible for the tumor promoters interference with the cellular coupling. Moreover, nongenotoxic carcinogens, as well as connexins (gap-junctional protein subunits), are known to be organ-/tissue-specific; this implies that the effect of different agents should be evaluated on their specific target, that is, connexin. To investigate the role of different connexins in regulating gap-junctional gating and to compare the properties of homotypic junctional channels, we evaluated the effects of tissue-specific tumor promoters and anti-promoters on the viability and intercellular coupling (dye-transfer) of HeLa cells stably transfected with cDNAs coding for connexin(cx)43, cx40, cx26 and cx32. The results demonstrate that the transfectants possess individual junctional permeabilities, differentially affected by the chemicals, they also show different sensitivities to the cytotoxic effect of the compounds. These findings confirm that connexin diversity may be responsible for the different gating properties of gap-junctional channels, being also suggestive for their separate functions and independent regulatory mechanisms.

Use of alternate promoters for tissue-specific expression of the gene coding for connexin32

The promoter of rat connexin32 (Cx32), the gap junction protein found in liver, was studied in transgenic mice. Cx32 transgenes, containing 2.5-kb of sequence upstream from the promoter, exon I, the entire 6.1-kb intron and the beginning of the coding sequence linked to the gene encoding luciferase (Luc), were found to be expressed in mouse in the same tissue-specific manner as previously reported for Cx32. Another construct lacking the promoter, but retaining 1.8 kb from the 3' end of the intron, was found to be expressed specifically in the nervous system. This result suggested that a second promoter, different from that used in liver, functions in nervous tissue. The use of this promoter in normal rats was corroborated by sequence analysis of reverse-transcribed PCR products obtained from rat nervous tissue RNA. The second promoter drives the synthesis of a second Cx32 mRNA species that is processed to remove a small 345-bp intron that shares its acceptor splice site with the large intron. This finding could have implications for the genetic basis of the X-linked form of Charcot-Marie-Tooth disease (CMT-X) in those patients that do not exhibit mutations in the Cx32-coding region.

Basal promoter of the rat connexin 32 gene: identification and characterization of an essential element and its DNA-binding protein

The connexin 32 (Cx32) gene, a member of a multigene family, is expressed preferentially in the liver. The basal promoter complex of the rat Cx32 gene was previously localized to a 146-bp region (map positions [mp] -179 to -34) immediately upstream of the first exon. To investigate the biochemical factors contributing to the basal promoter activity, nuclear protein-DNA complexes within this region (mp -177 to -106) were investigated by using a DNA mobility shift assay. Three DNA-protein binding activities, termed Cx32-B1, Cx32-B2, and Cx32-B3, were identified with nuclear protein extracts from hepatoma cell lines, HuH7 and FAO-1. However, only Cx32-B2 binding activity was detected in nuclear protein extract from normal rat liver tissue. This activity was significantly more abundant in rat liver tissue than in hepatoma cell lines and tissues from various other organs. By using methylation interference footprinting, the Cx32-B2 complex was localized to the region between mp -152 and -127 and a DNA probe containing this region bound to a 60-kDa protein in rat liver nuclear extracts. Mutation of two nucleotides in the Cx32-B2 binding site abrogated the formation of the Cx32-B2 protein-DNA complex and significantly reduced the transcriptional activity of the Cx32 promoter. These results indicate that the Cx32-B2 complex is an essential component of the rat Cx32 basal promoter and is likely a major factor in the preferential expression of this gene in the liver.