A new exon in the 5' untranslated region of the connexin32 gene

Structure and Functions of Gap Junctions and Their Constituent Connexins in the Mammalian CNS

Numerous data obtained in the last 20 years indicate that all parts of the mature central nervous system, from the retina and olfactory bulb to the spinal cord and brain, contain cells connected by gap junctions (GJs). The morphological basis of the GJs is a group of joined membrane hemichannels called connexons, the subunit of each connexon is the protein connexin. In the central nervous system, connexins show specificity and certain types of them are expressed either in neurons or in glial cells. Connexins and GJs of neurons, combining certain types of inhibitory hippocampal and neocortical neuronal ensembles, provide synchronization of local impulse and rhythmic activity, thalamocortical conduction, control of excitatory connections, which reflects their important role in the processes of perception, concentration of attention and consolidation of memory, both on the cellular and at the system level. Connexins of glial cells are ubiquitously expressed in the brain, and the GJs formed by them provide molecular signaling and metabolic cooperation and play a certain role in the processes of neuronal migration during brain development, myelination, tissue homeostasis, and apoptosis. At the same time, mutations in the genes of glial connexins, as well as a deficiency of these proteins, are associated with such diseases as congenital neuropathies, hearing loss, skin diseases, and brain tumors. This review summarizes the existing data of numerous molecular, electrophysiological, pharmacological, and morphological studies aimed at progress in the study of the physiological and pathophysiological significance of glial and neuronal connexins and GJs for the central nervous system.

Structure, Regulation and Function of Gap Junctions in Liver

Gap junctions are a specialized group of cell-to-cell junctions that mediate direct intercellular communication between cells. They arise from the interaction of two hemichannels of adjacent cells, which in turn are composed of six connexin proteins. In liver, gap junctions are predominantly found in hepatocytes and play critical roles in virtually all phases of the hepatic life cycle, including cell growth, differentiation, liver-specific functionality and cell death. Liver gap junctions are directed through a broad variety of mechanisms ranging from epigenetic control of connexin expression to post-translational regulation of gap junction activity. This paper reviews established and novel aspects regarding the architecture, control and functional relevance of liver gap junctions.

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.

Tandem alternative splicing of zebrafish connexin45.6

Early studies suggested that most connexin genes share a relatively simple structure with a single intron of variable length interrupting the 5' untranslated region (UTR). Here we report that zebrafish cx45.6 shows six isoforms of alternative 5'UTRs which are generated from multiple promoter usage and alternative pre-mRNA splicing. Interestingly, cx45.6 undergoes tandem alternative splicing, which produces transcripts only differing by 3 nucleotides. This is the first study that has demonstrated tandem alternative pre-mRNA splicing in the connexin gene family. Expression patterns of cx45.6 alternative transcripts were demonstrated by real-time RT-PCR during zebrafish embryonic development and in adult tissues. The complexity of 5'UTR diversity suggests complicated regulatory mechanisms for cx45.6 gene expression at both transcriptional and post-transcriptional levels, and we propose that tandem alternative splicing in cx45.6 5'UTRs could play a role in translational control. These results lay groundwork for further investigations on the regulation and function of cx45.6 gene expression.

Gap junctional intercellular communication as a target for liver toxicity and carcinogenicity

Direct communication between hepatocytes, mediated by gap junctions, constitutes a major regulatory platform in the control of liver homeostasis, ranging from hepatocellular proliferation to hepatocyte cell death. Inherent to this pivotal task, gap junction functionality is frequently disrupted upon impairment of the homeostatic balance, as occurs during liver toxicity and carcinogenicity. In the present paper, the deleterious effects of a number of chemical and biological toxic compounds on hepatic gap junctions are discussed, including environmental pollutants, biological toxins, organic solvents, pesticides, pharmaceuticals, peroxides, metals and phthalates. Particular attention is paid to the molecular mechanisms that underlie the abrogation of gap junction functionality. Since hepatic gap junctions are specifically targeted by tumor promoters and epigenetic carcinogens, both in vivo and in vitro, inhibition of gap junction functionality is considered as a suitable indicator for the detection of nongenotoxic hepatocarcinogenicity.

Post-transcriptional regulation of gene expression by alternative 5′-untranslated regions in carcinogenesis

Post-transcriptional regulation, via 5′-UTRs (5′-untranslated regions), plays an important role in the control of eukaryotic gene expression. Recent analyses of the mammalian transcriptome suggest that most of the genes express multiple alternative 5′-UTRs and inappropriate expression of these regions has been shown to contribute to the development of carcinogenesis. The present review will focus on the complex post-transcriptional regulation of ERβ (oestrogen receptor β) expression. In particular, results from our laboratory suggest that the expression of alternative 5′-UTRs plays a key role in determining the level of ERβ protein expression. We have also shown that these alternative ERβ 5′-UTRs have a tissue-specific distribution and are differentially expressed between various normal and tumour tissues. Our results also suggest that alternative 5′-UTRs can influence downstream splicing events, thereby perhaps affecting ERβ function. These results suggest that alternative 5′-UTRs may have an overall influence on ER activity and this may have important implications for our understanding of cancer biology and treatment.

Biology and pathobiology of gap junctional channels in hepatocytes

The present review provides the state of the art of the current knowledge concerning gap junctional channels and their roles in liver functioning. In the first part, we summarize some relevant biochemical properties of hepatic gap junctional channels, including their structure and regulation. In the second part, we discuss the involvement of gap junctional channels in the occurrence of liver cell growth, liver cell differentiation, and liver cell death. We further exemplify their relevance in hepatic pathophysiology. Finally, a number of directions for future liver gap junctional channel research are proposed, and the up-regulation of gap junctional channel activity as a novel strategy in (liver) cancer therapy is illustrated.

Mechanisms underlying p53 regulation of PIK3CA transcription in ovarian surface epithelium and in ovarian cancer

Inactivation of the transcription factor and tumor suppressor p53, and overexpression or mutational activation of PIK3CA, which encodes the p110alpha catalytic subunit of phosphatidylinositol-3-kinase (PI3K), are two of the most common deleterious genomic changes in cancer, including in ovarian carcinomas. We investigated molecular mechanisms underlying interactions between these two mediators and their possible roles in ovarian tumorigenesis. We identified two alternate PIK3CA promoters and showed direct binding of and transcriptional inhibition by p53 to one of these promoters. Conditional suppression of functional p53 increased p110alpha transcripts, protein levels and PI3K activity in immortalized, non-tumorigenic ovarian surface epithelial (OSE) cells, the precursors of ovarian carcinoma. Conversely, overexpression of p53 by adenoviral infection and activation of p53 by gamma-irradiation both diminished p110alpha protein levels in normal OSE and ovarian cancer cells. The demonstration that p53 binds directly to the PIK3CA promoter and inhibits its activity identifies a novel mechanism whereby these two mediators regulate cellular functions, and whereby inactivation of p53 and subsequent upregulation of PIK3CA might contribute to the pathophysiology of ovarian cancer.

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.

Genomic organization and alternative transcripts of the human Connexin40 gene

The human Cx40 gene (NT_004434.5) was sorted out from the GenBank database and as a result of a BLAST homology search, two ESTs (BE784549 from a human lung database, and BE732411 from a human placenta database) overlapping with the coding exon 2 sequence and upstream regions of the gene were identified. These ESTs correspond to two transcripts 1A and 1B, which diverge from each other in their 5' regions. The transcript 1A corresponds to the only transcript previously identified for the mouse and rat Cx40 genes; whereas the transcript 1B is a new transcript. The human Cx40 gene therefore comprises three exons: exon 1A (100 bp), exon 1B (132 bp) and coding exon 2, with the exons 1A and 1B at 14 and 1.3 kb of the exon 2, respectively. The expression of these transcripts is cell-type specific. Transcript 1A is expressed in endothelial cells. Its expression was demonstrated in human umbilical vein endothelial cells (HUVEC). Transcript 1B is expressed in placental cytotrophoblasts. Its expression was demonstrated in malignant trophoblastic cells, BeWo, JAR and JEG-3, and purified cytotrophoblasts from human first trimester placental tissues. Interestingly, both transcripts 1A and 1B are expressed in the right atrial appendages (RAA), although the cell-type expression of the two transcripts in this particular tissue has not yet been determined. Both transcripts were found to be expressed in the various heart regions investigated, where transcript 1B was found to always occur rarely in comparison with transcript 1A. Transcripts 1A and 1B are both more abundant in the atria than in the ventricles. Luciferase reporter gene assays demonstrated that two genomic regions containing the exons 1A and 1B induced a cell-type specific expression. The 1.2 kb sequence, containing the exon 1A, induced an increase of the luciferase activity in HUVEC; whereas the 1.9 kb sequence, containing the exon 1B, induces an increase of expression of the luciferase activity in BeWo cells. The DNA sequence upstream of the exon 1A contains SP1 binding sites, but no TATA- or CAAT-box; whereas the region upstream of the exon 1B is preceded by three CAAT-boxes. Thus, in contrast to the mouse and rat Cx40 genes, the human Cx40 gene organized in three exons and generates two transcripts, which are cell-type specific.

Translational regulation of human neuronal nitric-oxide synthase by an alternatively spliced 5'-untranslated region leader exon

Expression of the neuronal nitric-oxide synthase (nNOS) mRNA is subject to complex cell-specific transcriptional regulation, which is mediated by alternative promoters. Unexpectedly, we identified a 89-nucleotide alternatively spliced exon located in the 5'-untranslated region between exon 1 variants and a common exon 2 that contains the translational initiation codon. Alternative splicing events that do not affect the open reading frame are distinctly uncommon in mammals; therefore, we assessed its functional relevance. Transient transfection of reporter RNAs performed in a variety of cell types revealed that this alternatively spliced exon acts as a potent translational repressor. Stably transfected cell lines confirmed that the alternatively spliced exon inhibited translation of the native nNOS open reading frame. Reverse transcription-PCR and RNase protection assays indicated that nNOS mRNAs containing this exon are common and expressed in both a promoter-specific and tissue-restricted fashion. Mutational analysis identified the functional cis-element within this novel exon, and a secondary structure prediction revealed that it forms a putative stem-loop. RNA electrophoretic mobility shift assay techniques revealed that a specific cytoplasmic RNA-binding complex interacts with this motif. Hence, a unique splicing event within a 5'-untranslated region is demonstrated to introduce a translational control element. This represents a newer model for the translational control of a mammalian mRNA.

Positive regulation of connexin32 transcription by hepatocyte nuclear factor-1alpha

Connexin32 (Cx32) encodes the predominant gap junction protein expressed by hepatocytes. We investigated the transcriptional control of Cx32 in expressing and nonexpressing rat liver cell lines and hypothesized that a putative hepatocyte nuclear factor-1 (HNF-1) binding site (centered at mp -187) in the liver-active, P1 promoter is essential for transcription of Cx32. HNF-1alpha was expressed by Cx32-expressing rat liver cell lines and bound the promoter at the -187 site, but was not expressed by non-Cx32-expressing hepatic lines. Stable transfection of non-Cx32-expressing WB-F344 rat liver epithelial cells with HNF-1alpha stimulated a transfected Cx32 promoter element (mp -244 to -33), binding of HNF-1alpha to the -187 site, and expression of endogenous Cx32. Site-directed mutagenesis of this HNF-1 binding site abolished HNF-1alpha binding and proximal promoter activity. Hepatic Cx32 expression was also significantly decreased in HNF-1alpha(-/-) mice. These data indicate that HNF-1alpha is a positive regulator of Cx32 expression in hepatic cells.

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.

Structural determinants of BRCA1 translational regulation

The BRCA1 gene is involved in sporadic breast and ovarian cancer mainly through reduced expression. BRCA1 mRNAs containing different leader sequences show different patterns of expression. In a normal mammary gland mRNA with a shorter leader sequence, 5'-UTRa is expressed only, whereas in breast cancer tissue mRNA with a longer leader, 5'-UTRb is expressed also. We show that the translation efficiency of transcripts containing 5'-UTRb is 10 times lower than those containing 5'-UTRa. The structures of 5'-UTRa and 5'-UTRb were determined by chemical and enzymatic probing aided by a new method developed for monitoring the number of co-existing stable conformers. Specific factors responsible for reduced translation of mRNA containing 5'-UTRb were determined using a variety of transcripts with mutations in the leader sequence. These factors include a stable secondary structure formed by truncated Alu element and upstream AUG codons. The novel mechanism by which BRCA1 may be involved in sporadic breast and ovarian cancer is proposed. It is based on the expression patterns of BRCA1 mRNAs and differences in their translatability. According to this mechanism the deregulation of the BRCA1 transcription in cancer, resulting in a higher proportion of translationally inhibited transcripts containing 5'-UTRb, contributes to the decrease in the BRCA1 protein observed in sporadic breast and ovarian cancers.

Morphologic and molecular evidence for gap junctions and connexin 43 and 45 expression in annulus fibrosus cells from the human intervertebral disc

Data are presented which provide evidence for gap junction formation and connexin (Cx) 43 and 45 gene expression in human intervertebral disc cells in vivo and in vitro. These findings in cells from the annulus are important in conjunction with the well-recognized loss of disc cells during aging and disc degeneration. As a result of this loss of cells, cell-cell communication, which we propose is an important, but as yet poorly understood, mechanism which links and coordinates cellular function throughout the entire population of disc cells, is also disrupted. These studies provide additional information on the fundamental cell biology of the disc cell and provide an additional framework for understanding aging, degeneration and potential repair of the human disc.

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.

Genomic organization and chromosomal localization of the mouse Connexin36 (mCx36) gene

Connexin36 (Cx36) is a new connexin that was recently cloned in mouse, rat and human. It is highly expressed in neurons of the CNS. To gain insight into the transcriptional regulation of this gene, we have cloned the genomic region containing the entire mCx36 gene and sequenced about 7.6kb around the coding region. The computer analysis of this sequence was helpful in defining putative regulative sequences. Using both 5'-RACE and RNAse protection assay, we have mapped the transcription starting site commonly used in both adult olfactory bulb and brain, in position -479 from the ATG. By 3'-RACE, we defined the polyadenylation site used that is located 1436nt downstream the stop codon. The expected transcript is 2875nt long and is consistent with the 2.9kb transcript found in the same tissues by Northern blot. Finally, we have mapped mCx36 on chromosome 2 in the position F3 in a region that is synthenic to human chromosome 15q14, where the human Cx36 gene has been recently mapped.

Expression of the gene for mitoribosomal protein S12 is controlled in human cells at the levels of transcription, RNA splicing, and translation

The human gene RPMS12 encodes a protein similar to bacterial ribosomal protein S12 and is proposed to represent the human mitochondrial orthologue. RPMS12 reporter gene expression in cultured human cells supports the idea that the gene product is mitochondrial and is localized to the inner membrane. Human cells contain at least four structurally distinct RPMS12 mRNAs that differ in their 5'-untranslated region (5'-UTR) as a result of alternate splicing and of 5' end heterogeneity. All of them encode the same polypeptide. The full 5'-UTR contains two types of sequence element implicated elsewhere in translational regulation as follows: a short upstream open reading frame and an oligopyrimidine tract similar to that found at the 5' end of mRNAs encoding other growth-regulated proteins, including those of cytosolic ribosomes. The fully spliced (short) mRNA is the predominant form in all cell types studied and is translationally down-regulated in cultured cells in response to serum starvation, even though it lacks both of the putative translational regulatory elements. By contrast, other splice variants containing one or both of these elements are not translationally regulated by growth status but are translated poorly in both growing and non-growing cells. Reporter analysis identified a 26-nucleotide tract of the 5'-UTR of the short mRNA that is essential for translational down-regulation in growth-inhibited cells. Such experiments also confirmed that the 5'-UTR of the longer mRNA variants contains negative regulatory elements for translation. Tissue representation of RPMS12 mRNA is highly variable, following a typical mitochondrial pattern, but the relative levels of the different splice variants are similar in different tissues. These findings indicate a complex, multilevel regulation of RPMS12 gene expression in response to signals mediating growth, tissue specialization, and probably metabolic needs.