Connexin expression systems: to what extent do they reflect the situation in the animal?

Connexin43 signaling contributes to spontaneous apoptosis in cultures of primary hepatocytes

Primary hepatocyte cultures suffer from the progressive occurrence of dedifferentiation followed by spontaneous apoptosis. This is associated with modifications in the expression of connexins (Cxs), which are the building stones of hemichannels that in turn form gap junctions between neighboring cells. Specifically, a shift is observed from the adult hepatocellular Cx32 species toward the fetal Cx43 isoform. The current study was set up to investigate the role of Cx43 in spontaneous apoptosis taking place in primary hepatocyte cultures. For this purpose, freshly isolated adult rat hepatocytes were cultivated in conventional conditions for 4 days with daily monitoring of Cx expression, Cx localization, and gap junction channel and hemichannel functionality. Gap junction activity was low shortly after isolation, whereas the inverse was observed for hemichannel functionality. Both channel types displayed high activity near the end stages of the cultivation period. The Cx32-to-Cx43 switch became progressively manifested at the translational level. At the transcriptional level, a fivefold decrease in Cx32 messenger RNA abundance and a twofold increase in Cx43 expression were noticed within the first 24 h of cultivation. Throughout the cultivation period, Cx32 was mainly located at the plasma membrane surface, whereas Cx43 immunostaining was more diffuse. Application of three Cx43 inhibitors resulted in the downregulation of both hemichannel functionality and gap junction activity. This was paralleled by decreased expression and activity of caspase 3 as well as by reduced expression of Bid. Collectively, these data show that Cx43 signaling actively contributes to the occurrence of spontaneous apoptosis in cultures of primary hepatocytes.

Involvement of cell junctions in hepatocyte culture functionality

In liver, like in other multicellular systems, the establishment of cellular contacts is a prerequisite for normal functioning. In particular, well-defined cell junctions between hepatocytes, including adherens junctions, desmosomes, tight junctions, and gap junctions, are known to play key roles in the performance of liver-specific functionality. In a first part of this review article, we summarize the current knowledge concerning cell junctions and their roles in hepatic (patho)physiology. In a second part, we discuss their relevance in liver-based in vitro modeling, thereby highlighting the use of primary hepatocyte cultures as suitable in vitro models for preclinical pharmaco-toxicological testing. We further describe the actual strategies to regain and maintain cell junctions in these in vitro systems over the long-term.

Trichostatin a enhances gap junctional intercellular communication in primary cultures of adult rat hepatocytes

The effects of histone deacetylase inhibitor Trichostatin A (TSA) on connexin (Cx) expression and gap junctional intercellular communication (GJIC) were investigated in primary cultures of adult rat hepatocytes. GJIC was monitored by using the scrape-loading/dye transfer method. Immunoblotting and immunocytochemistry were used to investigate Cx protein levels and localization. Cx gene expression was studied by means of quantitative reverse transcriptase-polymerase chain reaction. TSA increased Cx32 protein levels and affected negatively the Cx26 protein levels. The latter was preferentially located in the cytosol of cultured cells. TSA also promoted the appearance of Cx43 in the nuclear compartment of primary cultured hepatocytes. Overall, this resulted in enhanced GJIC activity. It is important to note that the time of onset of TSA treatment was crucial for the extent of its outcome and that the effects of TSA on Cx protein levels occurred independently of transcriptional changes. TSA differentially affects Cx proteins in primary rat hepatocyte cultures, suggesting distinct regulation and/or distinct roles of the different Cx species in the control of hepatic homeostasis. TSA enhances GJIC between primary cultured rat hepatocytes, an interesting finding supporting its use to further optimize liver-based in vitro models for pharmacotoxicological purposes.

Sharing signals: connecting lung epithelial cells with gap junction channels

Gap junction channels enable the direct flow of signaling molecules and metabolites between cells. Alveolar epithelial cells show great variability in the expression of gap junction proteins (connexins) as a function of cell phenotype and cell state. Differential connexin expression and control by alveolar epithelial cells have the potential to enable these cells to regulate the extent of intercellular coupling in response to cell stress and to regulate surfactant secretion. However, defining the precise signals transmitted through gap junction channels and the cross talk between gap junctions and other signaling pathways has proven difficult. Insights from what is known about roles for gap junctions in other systems in the context of the connexin expression pattern by lung cells can be used to predict potential roles for gap junctional communication between alveolar epithelial cells.

v-Src phosphorylation of connexin 43 on Tyr247 and Tyr265 disrupts gap junctional communication

The mechanism by which v-Src disrupts connexin (Cx)43 intercellular gap junctional communication (GJC) is not clear. In this study, we determined that Tyr247 (Y247) and the previously identified Tyr265 (Y265) site of Cx43 were the primary phosphorylation targets for activated Src in vitro. We established an in vivo experimental system by stably expressing v-Src and wild-type (wt) Cx43, or Y247F, Y265F, or Y247F/Y265F Cx43 mutants in a Cx43 knockout mouse cell line. Wt and mutant Cx43 localized to the plasma membrane in the absence or presence of v-Src. When coexpressed with v-Src, the Y247F, Y265F, and Y247F/Y265F Cx43 mutants exhibited significantly reduced levels of tyrosine phosphorylation compared with wt Cx43, indicating that Y247 and Y265 were phosphorylation targets of v-Src in vivo. Most importantly, GJC established by the Y247F, Y265F, and Y247F/Y265F Cx43 mutants was resistant to disruption by v-Src. Furthermore, we did not find evidence for a role for mitogen-activated protein kinase in mediating the disruption of GJC by v-Src. We conclude that phosphorylation on Y247 and Y265 of Cx43 is responsible for disrupting GJC in these mammalian cells expressing v-Src.

Propagation of intercellular calcium waves in C6 glioma cells transfected with connexins 43 or 32

In this study, gap junction-deficient C6 glioma cells, transfected with either connexin 43 (Cx43) or 32 (Cx32), have been used to evaluate the ability of these connexins to pass intercellular Ca2+ waves. Ca2+ waves, observed with fluorescence imaging using fura-2 or fluo-3, were initiated by mechanical stimulation in the presence of a supra-perfusion of the extracellular fluid or by the non-contact technique of flash photolysis of intracellular caged-IP3. Following manual mechanical stimulation, the parental C6 glioma cells and cells expressing Cx43 and Cx32 gap junctions all propagated intercellular Ca2+ waves. Ca2+ waves in cells expressing Cx43 traveled approximately twice the distance as compared to waves in cells expressing Cx32 or parental cells. The cells expressing Cx43 were also about twice as sensitive to ATP as cells expressing Cx32. In the presence of a supra-perfusion of extracellular fluid, the Ca2+ waves in parental cells were almost abolished while the mechanically induced Ca2+ waves in the cells expressing Cx43 and Cx32 propagate similar but limited distances of several cells in a direction opposite to the fluid flow. The photolytic release of IP3, but not Ca2+, in cells expressing Cx43 or Cx32 resulted in the propagation of Ca2+ waves that traveled distances similar to those observed in the presence of supra-perfusion. Parental C6 glioma cells did not initiate intercellular Ca2+ waves when stimulated by photolysis. From these studies we conclude that (1) both Cx43 and Cx32 based gap junctions are permeable to IP3 and can serve to communicate Ca2+ waves, (2) that Ca2+ wave propagation via gap junctions was dependent on the diffusion of IP3 but not Ca2+, (3) that an extracellular messenger capable of communicating waves is released from only the stimulated cell, and (4) that simultaneous intracellular and extracellular signaling can occur to enhance the propagation of intercellular Ca2+ waves.

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Multiple connexins form gap junction channels in rat basilar artery smooth muscle cells

Three connexins, Cx43, Cx40, and Cx37, have been found by protein or mRNA analysis to be prominent in mammalian blood vessels, but electrophysiological characterization of gap junction channels in freshly isolated vascular smooth muscle cells (SMCs) has not previously been reported. We used a dual-perforated patch-clamp technique to study gap junction conductances in SMC pairs from rat basilar arteries. Macroscopic junctional conductance (Gj) measured in 98 cell pairs with either Cs+ or K+ ranged between 0.68 and 24.8 nS. In weakly coupled cells (Gj<5 nS), single-channel currents were readily resolved without pharmacological uncoupling agents, allowing identification of 4 major unitary conductances. Two of these conductances, 80 to 120 pS and 150 to 200 pS, corresponded to the major conductance states for homotypic channels formed from Cx43 or Cx40, which we confirmed were present in smooth muscle by immunofluorescence analysis. Two other conductances, 220 to 280 pS and >300 pS, were identified that have not been previously reported in vascular SMCs. Macroscopic recordings revealed currents that deactivated incompletely over a broad range of transjunctional potentials. In about half of the pairs, we identified macroscopic as well as single-channel currents that exhibited marked voltage asymmetry, consistent with nonhomotypic, ie, either heterotypic or heteromeric channels. Our data indicate that basilar artery SMCs are coupled in vivo in a richly complex manner, involving Cx43, Cx40, and other large-conductance channels, and that a significant number of couplings involve putative nonhomotypic channels.

Molecular cloning and functional expression of the mouse gap junction gene connexin-57 in human HeLa cells

A new mouse connexin gene has been isolated that codes for a connexin protein of 505 amino acid residues. Based on the predicted molecular mass of 57.115 kDa, it has been designated connexin-57. Similar to most other mouse connexin genes, the coding region of connexin-57 is not interrupted by introns and exists in the mouse genome as a single-copy gene. Within the connexin family, this new gene shows highest sequence identity to porcine connexin-60 in the alpha group of connexins. The connexin-57 gene was mapped to a position on mouse chromosome 4, 30 centimorgans proximal to a cluster of previously mapped connexin genes. Low levels of connexin-57 mRNA were detected in skin, heart, kidney, testis, ovary, intestine, and in the mouse embryo after 8 days post coitum, but expression was not detected in brain, sciatic nerve or liver. In order to analyze gene function, the connexin-57 coding region was expressed by transfection in human HeLa cells, where it restored homotypic intercellular transfer of microinjected neurobiotin. Heterotypic transfer was observed between HeLa connexin-57 transfectants and HeLa cells, expressing murine connexin-43, -37, or -30.3. Double whole-cell voltage clamp analyses revealed that HeLa-connexin-57 transfectants expressed about 10 times more channels than parental HeLa cells. Voltage gating by transjunctional and transmembrane voltages as well as unitary conductance ( approximately 27 picosiemens) were different from intrinsic connexin channels in parental HeLa cells.

Individual gap junction plaques contain multiple connexins in arterial endothelium

Gap-junctional intercellular communication in endothelial cells is implicated in the coordination of growth, migration, and vasomotor responses. Up to 3 connexin types, connexin40 (Cx40), Cx37, and Cx43 may be expressed in vascular endothelium according to vascular site, species, and physiological conditions. To establish how these connexins are organized at the level of the individual endothelial gap junction, we used affinity-purified connexin-specific antibodies raised in 3 different species to permit double and triple immunolabeling in combination with confocal and electron microscopy. Using HeLa cells transfected with Cx37 and Cx40 for characterization, the anti-Cx37 antibody (raised in rabbit) and the anti-Cx40 antibody (raised in guinea pig) were shown to recognize single bands of 37 and 40 kDa, respectively, on Western blots and to give prominent punctate labeling at the cell borders, specifically in the corresponding transfectant. By applying these antibodies together with mouse monoclonal anti-Cx43 for double and triple immunofluorescence labeling at confocal microscopy, rat aortic and pulmonary arterial endothelia were found to express all 3 connexin types, whereas coronary artery endothelium expressed Cx40 and Cx37 but lacked Cx43. High-resolution en face confocal viewing of the aortic endothelium after double labeling demonstrated frequent colocalization of connexins, with distinct variation in the expression pattern within a given cell, where it made contact with different neighbors. Triple immunogold labeling at the electron-microscopic level revealed that aortic endothelial gap junctions commonly contain all 3 connexin types. This represents the first definitive demonstration of any cell type in vivo expressing 3 different connexins organized within the same gap-junctional plaque.

Gap junctions in normal and neoplastic mammary gland

Progress in the characterization of gap junctions and their constituent connexin sub-units is leading to a greater understanding of the structure, function, and regulation of this cell-cell communication channel. Although much of the experimental evidence generated to date is correlative, recent work utilizing reverse genetic approaches to manipulate connexin gene function has provided direct evidence that intercellular communication via gap junctions plays key roles in development, cellular differentiation, and organogenesis. Pathogenic mutations in human connexin genes have now been identified. Furthermore, a considerable body of experimental evidence correlates a loss of junctional communication with progression to a malignant phenotype. Although the cell biology of the mammary gland has been extensively studied, the role(s) of gap junctions in the development, differentiation, and maintenance of this tissue are unknown. Gap junctions were first reported in the mammary gland following freeze-fracture and electron microscopic analyses. The development of anti-connexin antibodies and the cloning of individual connexin isoforms have enabled this work to be extended, but there are contradictory reports in the temporal expression patterns of these proteins within mammary epithelium. In addition, a recent report in this Journal has implied by immunocytochemistry that there is up-regulation of connexin protein in some human breast tumours, a novel observation which may be inconsistent with the proposed tumour suppressor role for gap junctions.

Rapid determination of gap junction formation using HeLa cells microinjected with cDNAs encoding wild-type and chimeric connexins

A procedure for rapidly determining the functionality of gap junctions constructed of recombinant connexins in communication-deficient HeLa cells is described. Nuclear microinjection of cDNA encoding wild-type connexins (Cx) 26, 32, 43, and a range of connexin-aequorin (Cx-Aeq) chimerase resulted in generation of gap junction intercellular communication channels. Expression of recombinant protein was detected in > 95% of cells 18-72 h following nuclear microinjection, and the functionality of the channels generated was determined according to their ability to transfer the fluorescent dye tracers Lucifer yellow and propidium iodide. The dye transfer results obtained correlated closely with other published studies using stably transfected cells and yet are obtained as rapidly as 18 h following microinjection of cDNA. Expression of a truncated form of Cx43 (Cx43 delta 244) by this new method indicated diminished intercellular transfer of both dyes and supports a channel-gating mechanism that postulates interaction between the carboxyl tail and the intracellular loop.

Restricted expression of the gap junctional protein connexin 43 in the arterial system of the rat

Connexin 43 (Cx43) has been reported to be expressed in vascular smooth muscle cells and endothelial cells. Evidence for possible variations in Cx43 distribution within different parts of the vascular system is limited. We have therefore investigated the expression of Cx43 in the endothelia and media of 11 vessels of different size and function in the rat, using immunofluorescence and confocal laser scanning microscopy. The results showed that punctate Cx43 staining was abundant in the endothelia and media of all of the 5 elastic arteries examined. In the media, the amount of Cx43 staining decreased as the size of the elastic arteries became smaller. In the 6 muscular arteries examined, 2 different patterns of Cx43 staining were observed. In the first type, Cx43 expression was high in the endothelium but virtually absent from the media. Mesenteric resistance, hepatic and tail arteries were examples. In the second type, Cx43 staining was absent from both the media and the endothelia. The coronary, basilar, and middle cerebral arteries showed this appearance. The results suggest that expression of Cx43 is largely restricted to elastic arteries in the arterial system of the rat. The lack of immunodetectable Cx43 from the media of all muscular arteries examined, and from the endothelia of some of these arteries, raises the possibility of significant differences in the form of expression of Cx43 in these vessels or the presence of other connexins.

Regulation of vascular connexin43 gene expression by mechanical loads

Vascular tissues respond to changes in the mechanical forces imposed on them with changes in vasomotor tone in the short term and with structural remodeling in the long term. Since these responses involve intercellular communication, we have investigated regulation of the gap junction proteins, connexin26 (Cx26), connexin37 (Cx37), connexin40 (Cx40), and connexin43 (Cx43), by mechanical loads. Results were compared with parallel experiments on c-fos and GAPDH. Twenty percent stretch of cultured vascular smooth muscle cells caused a 3-fold increase in Cx43 mRNA levels by 2 hours. Cx26 was expressed at low levels but failed to respond to stretch, and Cx37 and Cx40 were not detected. c-fos mRNA levels increased after 30 minutes of stretch, whereas GAPDH mRNA did not change. Protein levels of Cx43 increased by 4 hours and remained elevated for 16 hours. Nuclear run-on experiments confirmed that Cx43 and c-fos were transcriptionally regulated by stretch. New protein synthesis was not a requirement for the stretch-induced rise in Cx43 expression, since mRNA levels were unaffected by treatment with cycloheximide. To examine transcriptional control of Cx43, stretched and unstretched vascular smooth muscle cells were transfected with a variety of promoter-reporter gene constructs. Cx43 sequences extending from within exon 1 (+162) to -1686 in the 5'-flanking region were coupled to the chloramphenicol acetyl transferase reporter gene. Deletions from the 5' end of these sequences differentially regulated reporter gene expression and indicated multiple potential regulatory sites. In particular, a putative activator protein-1 site at the -42 to -48 region was required for basal reporter activity. None of the promoter constructs revealed stretch sensitivity, indicating that the site of transcriptional control by stretch lies outside the -1686 to +162 region. Finally, Cx43 mRNA levels were assessed in cultured endothelial cells subjected to laminar shear stress of 15 dynes/cm2. Cx43 mRNA levels increased by approximately 4-fold at 1 hour and remained elevated for the duration of shear force. In conclusion, both mechanical strain and fluid shear stress caused increased expression of the gap junction protein Cx43.

Assembly of chimeric connexin-aequorin proteins into functional gap junction channels. Reporting intracellular and plasma membrane calcium environments

Chimeric proteins comprising connexins 26, 32, and 43 and aequorin, a chemiluminescent calcium indicator, were made by fusing the amino terminus of aequorin to the carboxyl terminus of connexins. The retention of function by the chimeric partners was investigated. Connexin 32-aequorin and connexin 43-aequorin retained chemiluminescent activity whereas that of connexin 26-aequorin was negligible. Immunofluorescent staining of COS-7 cells expressing the chimerae showed they were targeted to the plasma membrane. Gap junction intercellular channel formation by the chimerae alone and in combination with wild-type connexins was investigated. Stable HeLa cells expressing connexin 43-aequorin were functional, as demonstrated by Lucifer yellow transfer. Paris of Xenopus oocytes expressing connexin 43-aequorin were electrophysiologically coupled, but those expressing chimeric connexin 26 or 32 showed no detectable levels of coupling. The formation of heteromeric channels constructed of chimeric connexin 32 or connexin 43 and the respective wild-type connexins was inferred from the novel voltage gating properties of the junctional conductance. The results show that the preservation of function by each partner of the chimeric protein is dictated mainly by the nature of the connexin, especially the length of the cytoplasmic carboxyl-terminal domain. The aequorin partner of the connexin 43 chimera reported calcium levels in COS-7 cells in at least two different calcium environments.

Drosophila Shaking-B protein forms gap junctions in paired Xenopus oocytes

In most multicellular organisms direct cell-cell communication is mediated by the intercellular channels of gap junctions. These channels allow the exchange of ions and molecules that are believed to be essential for cell signalling during development and in some differentiated tissues. Proteins called connexins, which are products of a multigene family, are the structural components of vertebrate gap junctions. Surprisingly, molecular homologues of the connexins have not been described in any invertebrate. A separate gene family, which includes the Drosophila genes shaking-B and l(1)ogre, and the Caenorhabditis elegans genes unc-7 and eat-5, encodes transmembrane proteins with a predicted structure similar to that of the connexins. shaking-B and eat-5 are required for the formation of functional gap junctions. To test directly whether Shaking-B is a channel protein, we expressed it in paired Xenopus oocytes. Here we show that Shaking-B localizes to the membrane, and that its presence induces the formation of functional intercellular channels. To our knowledge, this is the first structural component of an invertebrate gap junction to be characterized.