Molecular cloning, tissue distribution, and hormonal control in the ovary of Cx41 mRNA, a novel Xenopus connexin gene transcript

Connexins: a myriad of functions extending beyond assembly of gap junction channels

Connexins constitute a large family of trans-membrane proteins that allow intercellular communication and the transfer of ions and small signaling molecules between cells. Recent studies have revealed complex translational and post-translational mechanisms that regulate connexin synthesis, maturation, membrane transport and degradation that in turn modulate gap junction intercellular communication. With the growing myriad of connexin interacting proteins, including cytoskeletal elements, junctional proteins, and enzymes, gap junctions are now perceived, not only as channels between neighboring cells, but as signaling complexes that regulate cell function and transformation. Connexins have also been shown to form functional hemichannels and have roles altogether independent of channel functions, where they exert their effects on proliferation and other aspects of life and death of the cell through mostly-undefined mechanisms. This review provides an updated overview of current knowledge of connexins and their interacting proteins, and it describes connexin modulation in disease and tumorigenesis.

Implication of gap junction coupling in amphibian vitellogenin uptake

The aim of the present study was to investigate the physiological role and the expression pattern of heterologous gap junctions during Xenopus laevis vitellogenesis. Dye transfer experiments showed that there are functional gap junctions at the oocyte/follicle cell interface during the vitellogenic process and that octanol uncouples this intercellular communication. The incubation of vitellogenic oocytes in the presence of biotinylated bovine serum albumin (b-BSA) or fluorescein dextran (FDX), showed that oocytes develop stratum of newly formed yolk platelets. In octanol-treated follicles no sign of nascent yolk sphere formation was observed. Thus, experiments in which gap junctions were downregulated with octanol showed that coupled gap junctions are required for endocytic activity. RT-PCR analysis showed that the expression of connexin 43 (Cx43) was first evident at stage II of oogenesis and increased during the subsequent vitellogenic stages (III, IV and V), which would indicate that this Cx is related to the process that regulates yolk uptake. No expression changes were detected for Cx31 and Cx38 during vitellogenesis. Based on our results, we propose that direct gap junctional communication is a requirement for endocytic activity, as without the appropriate signal from surrounding epithelial cells X. laevis oocytes were unable to endocytose VTG.

Cloning and functional characterization of a novel connexin expressed in somites of Xenopus laevis

Connexin-containing gap junctions play an essential role in vertebrate development. More than 20 connexin isoforms have been identified in mammals. However, the number identified in Xenopus trails with only six isoforms described. Here, identification of a new connexin isoform from Xenopus laevis is described. Connexin40.4 was found by screening expressed sequence tag databases and carrying out polymerase chain reaction on genomic DNA. This new connexin has limited amino acid identity with mammalian (<50%) connexins, but conservation is higher (approximately 62%) with fish. During Xenopus laevis development, connexin40.4 was first expressed after the mid-blastula transition. There was prominent expression in the presomitic paraxial mesoderm and later in the developing somites. In adult frogs, expression was detected in kidney and stomach as well as in brain, heart, and skeletal muscle. Ectopic expression of connexin40.4 in HEK293 cells, resulted in formation of gap junction like structures at the cell interfaces. Similar ectopic expression in neural N2A cells resulted in functional electrical coupling, displaying mild, asymmetric voltage dependence. We thus cloned a novel connexin from Xenopus laevis, strongly expressed in developing somites, with no apparent orthologue in mammals.

Xenopus connexins: how frogs bridge the gap

Animal species use specialized cell-to-cell channels, called gap junctions, to allow for a direct exchange of ions and small metabolites between their cells' cytoplasm. In invertebrates, gap junctions are formed by innexins, while vertebrates use connexin (Cx) proteins as gap-junction-building blocks. Recently, innexin homologs have been found in vertebrates and named pannexins. From progress in the different genome projects, it has become evident that every class of vertebrates uses their own unique set of Cxs to build their gap junctions. Here, we review all known Xenopus Cxs with respect to their expression, regulation, and function. We compare Xenopus Cxs with those of zebrafish and mouse, and provide evidence for the existence of several additional, non-identified, amphibian Cxs. Finally, we identify two new Xenopus pannexins by screening EST libraries.

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.

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.

Early embryonic expression of ion channels and pumps in chick and Xenopus development

An extensive body of literature implicates endogenous ion currents and standing voltage potential differences in the control of events during embryonic morphogenesis. Although the expression of ion channel and pump genes, which are responsible for ion flux, has been investigated in detail in nervous tissues, little data are available on the distribution and function of specific channels and pumps in early embryogenesis. To provide a necessary basis for the molecular understanding of the role of ion flux in development, we surveyed the expression of ion channel and pump mRNAs, as well as other genes that help to regulate membrane potential. Analysis in two species, chick and Xenopus, shows that several ion channel and pump mRNAs are present in specific and dynamic expression patterns in early embryos, well before the appearance of neurons. Examination of the distribution of maternal mRNAs reveals complex spatiotemporal subcellular localization patterns of transcripts in early blastomeres in Xenopus. Taken together, these data are consistent with an important role for ion flux in early embryonic morphogenesis; this survey characterizes candidate genes and provides information on likely embryonic contexts for their function, setting the stage for functional studies of the morphogenetic roles of ion transport.

Connexin43 expression during Xenopus development

The spatio-temporal expression of connexin43 in Xenopus laevis embryos was studied by in situ hybridization. Cx43 expression is first detected at stage 25 in the developing eye. In stage 32, expression was found in the margin of the lens placode, the cement gland, notochord, and in stage 37 in the branchial arches. Early limb buds show strong expression of Cx43 distally while later on expression is confined to sites of precartilage condensation.

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.

Molecular cloning and hormonal control in the ovary of connexin 31.5 mRNA and correlation with the appearance of oocyte maturational competence in red seabream

Gap junctions are aggregates of intercellular channels, composed of the protein connexin (Cx), between adjacent cells. This study examined whether, in the ovary of the red seabream Pagrus major, the connexin gene essential for the production of RNA and protein during the acquisition of oocyte maturational competence is active. Mixed primers for this reaction were designed on the basis of the high sequence homology of selected regions of known connexin genes. Polymerase-chain-reaction-amplified cDNA fragments generated by 3′ and 5′ rapid amplication of cDNA ends were combined to generate full-length cDNA sequences. The resulting 2400 base pair cDNA had an open reading frame encoding a polypeptide containing 275 amino acid residues (31493 Da; Cx31.5). Hydropathicity analysis of the predicted amino acid sequence indicated that red seabream Cx31.5 has four major hydrophobic regions and four major hydrophilic regions indicative of a topology similar to that of known connexins. Typical connexin consensus sequences were also observed in the first and second extracellular loops. During the acquisition of oocyte maturational competence, red seabream Cx31.5 mRNA transcription levels increased after treatment with gonadotropin-II. It is therefore proposed that expression of Cx31.5 contributes to the acquisition of oocyte maturational competence in this species.

Hormonal regulation and cellular distribution of connexin 32.2 and connexin 32.7 RNAs in the ovary of Atlantic croaker

The in vitro effects of human chorionic gonadotropin (hCG) on ovarian connexin (Cx) 32.2 and 32.7 RNA levels and ovarian follicle maturation were assessed, and the cellular distribution of Cx transcripts in the ovary was determined. hCG caused a concentration-dependent induction of Cx32.2 RNA, which peaked coincidentally with the appearance of morphological indices of oocyte maturational competence (OMC). Cx32.2 RNA levels declined thereafter in all treatment groups, although this decline was not accompanied by the onset of germinal vesicle breakdown (GVBD) at the lowest hCG concentration used. The levels of Cx32.7 RNA initially declined and subsequently increased to preincubation values after hCG treatment, but these changes were not dependent on hCG concentration. In a separate experiment, the decline in Cx32.7 RNA occurred in the presence or absence of hCG and was prevented by low (physiological) concentrations of estradiol-17beta (E2) or by protein kinase C (PKC) inhibitor, but was enhanced in the presence of high E2 concentrations or of PKC activator. These changes in Cx32. 7 RNA abundance were not associated with any indices of oocyte maturation. In situ hybridization of tissue sections showed the presence of Cx32.2 and Cx32.7 RNA in somatic cells of the ovarian follicle but not in oocytes. Cx32.2 RNA seemed to be present in granulosa and thecal cells, but the assay resolution was insufficient to reliably determine the distribution of Cx32.7 transcript by somatic cell type. In view of earlier findings that Cx32.2-based (but not Cx32.7-based) connexons can form functional homotypic channels, these results indicate that Cx32.2 gene expression in granulosa cells is sufficient for the formation of homologous gap junctions (GJ). Northern blot of RNA extracts from ovulated eggs, which are free of follicle cells, showed the presence of relatively low levels of both Cx RNAs. Thus, it is possible that Cx32.2 is present in oocytes and that it participates in heterologous (homotypic) GJ formation between the oocyte and the granulosa cells. In conclusion, Cx32.2 RNA levels in somatic cells of the ovarian follicle correlated positively with morphological indices of OMC acquisition, but subsequently declined during GVBD. These changes in Cx32.2 RNA may function in the regulation of GJ contacts during follicular maturation.

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.

Developmental and protein kinase-dependent regulation of ovarian connexin mRNA and oocyte maturational competence in Atlantic croaker

The acquisition of oocyte maturational competence (OMC) in ovarian follicles of Atlantic croaker is associated with increased gap junction (GJ) contacts and increased levels of ovarian connexin (Cx) 32.2 mRNA. However, the developmental control of ovarian Cx gene expression and the mechanisms of OMC acquisition are unknown. Ovarian Cx32.2 and Cx32.7 mRNA levels were determined in fish with gonadosomatic indices (GSI; gonad weight-to-body weight ratio) ranging from 0.1 to 13%. The mRNA level for both Cx increased from a low level in previtellogenic ovaries (GSI, <1%) to a peak level during the midstage of ovarian growth (GSI, 6-7%). Levels of Cx32.2 mRNA, but not Cx32.7 mRNA, declined markedly during late ovarian vitellogenic growth (GSI, 7-13%), and increased again upon stimulation of OMC by human chorionic gonadotropin (hCG). These changes in ovarian Cx32.2 mRNA seem to parallel previously reported changes in the incidence of oocyte-granulosa cell GJ during follicular growth and early maturation. In vitro treatment with hCG and protein kinase A (PKA) activators (dbcAMP and forskolin) induced ovarian Cx32.2 mRNA levels and OMC. The effects of hCG were blocked by PKA inhibitors (H89, H7). Protein kinase C (PKC) inhibitors (GF 109207X) had little effect on hCG-induced Cx32.2 mRNA or OMC, whereas PKC activators (PMA) blocked both events. There was no association between changes in Cx32.7 mRNA levels and OMC status in these experiments. In conclusion, changes in Cx32.2 gene expression seem to be involved in the regulation of oocyte-granulosa cell GJ during growth and differentiation of the croaker ovarian follicle. Also, the stimulation of OMC and Cx32.2 mRNA levels by hCG is mediated by PKA-dependent pathways and antagonized by PKC-dependent mechanisms.

Genetic diseases and gene knockouts reveal diverse connexin functions

Intercellular channels present in gap junctions allow cells to share small molecules and thus coordinate a wide range of behaviors. Remarkably, although junctions provide similar functions in all multicellular organisms, vertebrates and invertebrates use unrelated gene families to encode these channels. The recent identification of the invertebrate innexin family opens up powerful genetic systems to studies of intercellular communication. At the same time, new information on the physiological roles of vertebrate connexins has emerged from genetic studies. Mutations in connexin genes underlie a variety of human diseases, including deafness, demyelinating neuropathies, and lens cataracts. In addition, gene targeting of connexins in mice has provided new insights into connexin function and the significance of connexin diversity.

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.