Connexin43 mRNA contains a functional internal ribosome entry site

Acute downregulation of Cx43 alters P2Y receptor expression levels in mouse spinal cord astrocytes

Propagation of intercellular calcium waves (ICW) between astrocytes depends on the diffusion of signaling molecules through gap junction channels and diffusion through the extracellular space of neuroactive substances acting on plasmalemmal receptors. The relative contributions of these two pathways vary in different brain regions and under certain pathological conditions. We have previously shown that in wild-type spinal cord astrocytes, ICW are primarily gap junction-dependent, but that deletion of the main gap junction protein (Cx43) by homologous recombination results in a switch in mode of ICW propagation to a purinoceptor-dependent mechanism. Such a compensatory mechanism for ICW propagation was related to changes in the pharmacological profile of P2Y receptors, from an adenine-sensitive P2Y(1), in wild-type, to a uridine-sensitive P2U receptor subtype, in Cx43 knockout (KO) astrocytes. Using oligonucleotide antisense to Cx43 mRNA for acute downregulation of connexin43 expression levels, we provide evidence for the molecular nature of such compensatory mechanism. Pharmacological studies and Western blot analysis indicate that there is a reciprocal regulation of P2Y(1) and P2Y(4) expression levels, such that downregulation of Cx43 leads to decreased expression of the adenine-sensitive P2Y(1) receptor and increased expression of the uridine-sensitive P2Y(4) receptor. This change in functional expression of the P2Y receptor subtype population in acutely downregulated Cx43 was paralleled by changes in the mode of ICW propagation, similar to that previously observed for Cx43 KO spinal cord astrocytes. On the basis of these results, we propose that Cx43 regulates both modes of ICW by altering P2Y receptor subtype expression in addition to providing intercellular coupling.

Unexpected induction of the human connexin 43 promoter by the ras signaling pathway is mediated by a novel putative promoter sequence

Connexin 43 (Cx43) is essential for survival and is tightly regulated at the transcriptional and post-transcriptional levels. A number of previous studies have demonstrated altered expression in malignant tissues, and in the presence of carcinogenic factors. We examined the effect of protooncogenes of Cx43 expression, and found no effect on Cx43 promoter activity in cells transformed with Src or erbB2. On the other hand, we identified and characterized a novel sequence that mediates Cx43 promoter regulation in cell lines engineered to overexpress H-Ras. Compared with wild-type NIH3T3 cells, both Cx43 mRNA and protein levels are increased in NIH3T3-Ras cells. The H-Ras+ cells also have enhanced Cx43 promoter activation, which is inhibited by the MEK1 inhibitor 2'-amino-3'-methoxyflavone (PD98059), suggesting that Ras-mediated Cx43 overexpression is via the mitogen activated protein kinase kinase/extracellular signal-regulated pathway. Deletion analysis of the Cx43 promoter revealed a 200-bp region downstream of the Cx43 transcription start site as the minimal sequence essential for the Ras-mediated Cx43 up-regulation. Using this 200-base pair fragment in electrophoretic mobility shift assays, we identified one main protein complex that binds efficiently and is more abundant in nuclear extracts from NIH3T3-Ras and MCF7-Ras cells compared with their matched controls. This complex selectively recognizes a consensus sequence, AGTTCAATCA, located at positions +149 to +158 of the Cx43 promoter. Supershift assays identified the 90-kDa heat shock protein (HSP90) and c-Myc as constituents of this DNA-binding complex. Treatment of cells with the HSP90 inhibitor geldanamycin resulted in repression of the Cx43 promoter activity, and inhibits binding of the complex to the Cx43 promoter. Coimmunoprecipitation studies confirmed the interaction between endogenous HSP90 and c-Myc. This study provides evidence that the transcriptional up-regulation of Cx43 by Ras-Raf-MAPK is mediated via the interaction of a novel Cx43 promoter element with a protein complex that contains both HSP90 and c-Myc.

Regulated expression of the X. tropicalis connexin43 promoter

The spatio-temporal expression pattern of the connexin43 gene during Xenopus development has been described (Van der Heyden et al. 2001). To further investigate the regulation and function of connexin43 (Cx43) in amphibians, we have isolated the gene from Xenopus tropicalis (Xt) and determined its structure. The X. tropicalis Cx43 gene displays the typical two exon-one intron connexin configuration, where the first exon is non-coding. The predicted amino acid sequence of the XtCx43 protein is highly homologous to that of X. laevis, chicken and mammals. Expression of XtCx43 cDNA in N2A cells results in gap-junction plaque formation. Promoter activity of a 3.5 kb upstream region of the X. tropicalis Cx43 gene, including exon 1, mimics endogenous timing of expression after injection of reporter constructs in X. laevis embryos.

Gap junctions, homeostasis, and injury

Gap junctions (Gj) play an important role in the communication between cells of many tissues. They are composed of channels that permit the passage of ions and low molecular weight metabolites between adjacent cells, without exposure to the extracellular environment. These pathways are formed by the interaction between two hemichannels on the surface of opposing cells. These hemichannels are formed by the association of six identical subunits, named connexins (Cx), which are integral membrane proteins. Cell coupling via Gj is dependent on the specific pattern of Cx gene expression. This pattern of gene expression is altered during several pathological conditions resulting in changes of cell coupling. The regulation of Cx gene expression is affected at different levels from transcription to post translational processes during injury. In addition, Gj cellular communication is regulated by gating mechanisms. The alteration of Gj communication during injury could be rationalized by two opposite theories. One hypothesis proposes that the alteration of Gj communication attenuates the spread of toxic metabolites from the injured area to healthy organ regions. The alternative proposition is that a reduction of cellular communication reduces the loss of important cellular metabolisms, such as ATP and glucose.

Sp1 and Sp3 activate the rat connexin40 proximal promoter

The rat gap junction protein connexin40 (rCx40) has a characteristic developmental and regional expression pattern, for which the exact regulatory mechanisms are not known. To identify the molecular factors controlling Cx40 expression, its proximal promoter was characterized. The proximal rCx40 promoter is the most conserved noncoding region within the Cx40-gene known thus far and contains five potential binding sites for Sp-family transcription factors. The binding of both Sp1 and Sp3 to each of these DNA elements was demonstrated by EMSA. Luciferase assays of the natural rCx40 proximal promoter or mutated derivatives in Cx40-expressing (NCM, primary rat neonatal cardiomyocytes and A7r5, rat smooth muscle embryonic thoracic aorta cells) and -nonexpressing cells (N2A, mouse neuroblastoma cells) revealed that all sites are contributing to basal promoter activity. Trans-activation assays in Drosophila Schneider line 2 cells demonstrated that Sp1 and Sp3 activate the rCx40 proximal promoter in a dose-dependent and additive manner.

Effect of angiotensin II and ethanol on the expression of connexin 43 in WB rat liver epithelial cells

The turnover of connexin 43 (Cx43) is very rapid in many cells and involves both the lysosomal and proteasomal protease pathways. Here we show that Ca(2+)-mobilizing agonists such as angiotensin II (Ang II) can up-regulate the expression of Cx43 in WB rat liver epithelial cells. Vasopressin had the same effect in A7R5 smooth-muscle cells. The effect of Ang II was not prevented by pretreatment with proteasomal or lysosomal inhibitors and was associated with an enhanced biosynthesis of Cx43 as measured by metabolic labelling experiments. The accumulation of Cx43 occurred in intracellular compartments and at the cell surface, as determined by confocal immunofluorescence studies and by immunoblotting of fractions soluble and insoluble in Triton X-100. Chronic treatment of WB cells with ethanol inhibited Cx43 expression; this was associated with decreased biosynthesis of Cx43. Neither treatment with Ang II nor treatment with ethanol altered the levels of Cx43 mRNA. Incubation of WB cells with Ang II did not alter gap-junctional communication as judged by a dye-coupling assay. However, treatment with ethanol markedly decreased gap-junctional communication and this effect was diminished in Ang-II-treated cells, demonstrating that gap-junctional communication is linked to the level of Cx43 expression. We conclude that Cx43 biosynthesis is regulated by Ca(2+)-mobilizing agonists and ethanol in WB cells. The changes in Cx43 expression might have a role in modifying the conduction of metabolites and second messengers between cells.

A new alternatively spliced transcript of the mouse connexin32 gene is expressed in embryonic stem cells, oocytes, and liver

The rodent gap junction protein connexin32 (Cx32) is highly expressed in hepatocytes, less abundantly in Schwann cells and oligodendrocytes, and at low levels in the early mouse embryo. In both hepatocytes and Schwann cells, Cx32 expression is directed by alternative promoter regions (P1 and P2) which activate differently spliced transcript isoforms. Here we describe a third Cx32 transcript isoform expressed in embryonic cells, oocytes, and liver. Using competitive polymerase chain reaction, we have found that this new Cx32 transcript containing exon 1A is 200-fold less abundant in liver than the Cx32 isoform with exon 1. In mouse oocytes, the exon 1A-containing Cx32 transcript is exclusively expressed. Immunoblot analyses revealed no Cx32 protein expression in embryonic stem cells, whereas it has previously been demonstrated in oocytes. When the putative Cx32 promoter region upstream of exon 1A was cloned before the lacZ reporter gene, transient transfection yielded weak expression in embryonic stem cells. Our results suggest that the exon 1A-containing Cx32 isoform is likely to be inherited as an oogenetic product but not translated during early embryogenesis.

Mouse connexin 45: genomic cloning and exon usage

Connexin 45 is a gap junction protein that is prominent in early embryos and is widely expressed in many mature cell types. To elucidate its gene structure, expression, and regulation, we isolated mouse Cx45 genomic clones. Alignment of the genomic DNA and cDNA sequences revealed the presence of three exons and two introns. The first two exons contained only 5' untranslated sequences, while exon 3 contained the remaining 5' UTR, the entire coding region, and the 3' UTR. An RT-PCR with exon-specific primers was utilized to examine exon usage in F9 mouse embryonal carcinoma cells and adult mouse tissues. In all samples, PCR products amplified using exon 2/exon 3 or exon 3/exon 3 primer pairs were much more abundant than products produced using exon 1/exon 2 or exon 1/exon 3 primer pairs, suggesting that Cx45 mRNAs containing exon 1 were relatively rare compared with mRNAs containing the other exons. Rapid amplification of cDNA ends (5'-RACE) was performed using antisense primers from within exon 3 and template RNA prepared from F9 cells or from adult mouse kidney. We obtained multiple RACE products from both templates, including products that contained all three exons and were spliced identically to the cDNA. However, clones were also isolated (from kidney) that began within the region previously identified as intron 1 and continued upstream with a sequence identical to the cDNA, including splicing to exon 3. These results show that mouse Cx45 has a gene structure that differs from that of previously studied connexins and allows the production of heterogeneous Cx45 mRNAs with differing 5' UTRs. These differences might contribute to regulation of Cx45 protein levels by modulating mRNA stability or translational efficiency.

Cloning and analysis of the untranslated regions of the Xenopus laevis Connexin30 mRNA

The full-length 5' and 3' untranslated regions (UTRs) of Xenopus laevis Connexin30 (Cx30) mRNA were cloned and sequenced. The Cx30 messenger contains a 148 nt 5' UTR and a 480 nt 3' UTR. Four different constructs were made to enable the analysis of the role of the Cx30 UTRs in translation efficiency and in protein localization in the early Xenopus embryo. Transcripts encoded the Green Fluorescent Protein (GFP) reporter and contained the 5' and 3' UTR of either Cx30 or globin. In vivo analyses after injection of the transcripts into one cell-stage Xenopus embryos showed that the Cx30 3' UTR enables very efficient translation. The 5' UTR was slightly inhibitory compared with the globin 5' UTR. The localization of the produced GFP was analyzed. GFP was ubiquitously expressed in all parts of the embryo. Based on this observation we conclude that neither the 5' UTR nor the 3' UTR confers specific localization of the translation of the Cx30 mRNA in the embryo.

Analysis of a Charcot-Marie-Tooth disease mutation reveals an essential internal ribosome entry site element in the connexin-32 gene

A mutation located in the 5'-untranslated region (5'-UTR) of the nerve-specific connexin-32 mRNA, previously found in a family with Charcot-Marie-Tooth disease (CMTX), was analyzed for its effect on the expression of a reporter gene (luciferase) in transgenic mice and in transfected cells. Whereas both mutant and wild-type genes appeared to be transcribed and spliced efficiently, no luciferase was detected from the mutant in either system, suggesting that the mutation affects translation of the mRNA. When the 5'-UTR of nerve-specific connexin-32 mRNA was inserted between the two genes of a bicistronic vector and transfected into various cell lines, expression of the second gene was significantly increased. Because the mutant did not facilitate translation of the second gene in the bicistronic mRNA system, this result suggested that the CMTX mutation abolished function of an internal ribosome entry site (IRES) in the 5'-UTR of the wild-type connexin-32 mRNA. The CMTX phenotype of the mutant 5'-UTR further suggested that the wild-type IRES was essential for the translation of the connexin-32 mRNA in nerve cells. In addition, other sequence elements of the connexin-32 IRES were characterized by mutation analysis. A mutation in either of the first two elements investigated showed loss of IRES function, whereas mutation of a third element showed gain of function.

Mature olfactory receptor neurons express connexin 43

The expression of the gap junction subunit connexin 43 was studied in the olfactory epithelium of adult mice. In agreement with conclusions from previous immunohistochemical studies, we observed expression of mRNA encoding for connexin 43 in layers of the epithelium containing nuclei belonging to sustentacular cells. However, we also observed expression of connexin 43 mRNA in the layers containing nuclei belonging to mature olfactory receptor neurons (ORNs), immature ORNs, and basal cells. Connexin 43 mRNA expression was low in dorsomedial regions of the nasal cavity but higher ventrally. This differential regional distribution was consistent with expression in a transgenic mouse of a LacZ reporter gene driven by the proximal 6.5 kb of the connexin 43 promoter. LacZ was expressed in cells colabeled with antibody against olfactory marker protein (OMP), corroborating that mature ORNs express connexin 43. LacZ staining also was observed in sustentacular and basal cells and in immature ORNs. Double-label studies with antibodies against connexin 43 and OMP and expression of connexin 43 in the epithelium of bulbectomized mice were also consistent with expression of connexin 43 in mature ORNs. This is the first report of expression of a connexin subunit in mature ORNs. Our findings of connexin subunits in mature ORNs raise the novel possibility that gap junctions may play a fundamental role in information processing in the olfactory epithelium.

Translational control of the Xenopus laevis connexin-41 5'-untranslated region by three upstream open reading frames

The Xenopus laevis Connexin-41 (Cx41) mRNA contains three upstream open reading frames (uORFs) in the 5'-untranslated region (UTR). We analyzed the translation efficiency of constructs containing the Cx41 5'-UTR linked to the green fluorescent protein reporter after injection of transcripts into one-cell stage Xenopus embryos. The translational efficiency of the wild-type Cx41 5'-UTR was only 2% compared with that of the beta-globin 5'-UTR. Mutation of each of the three uAUGs into AAG codons enhanced translation 82-, 9-, and 4-fold compared with the wild-type Cx41 5'-UTR. Based on these increased translation efficiencies, the percentages of ribosomes that recognized the uAUGs were calculated. Only 0.03% of the ribosomes that entered at the cap structure scanned the entire 5'-UTR and translated the main ORF. The results indicate that all uAUGs are recognized by the majority of the scanning ribosomes and that the three uAUGs strongly modulate translation efficiency in Xenopus laevis embryos. Based on these data, a model of ribosomal flow along the mRNA is postulated. We conclude that the three uORFs may play an important role in the regulation of Cx41 expression.