Clustering of Cx43 cell-to-cell channels into gap junction plaques: regulation by cAMP and microfilaments

Time-course for in vitro development of basement membrane, gap junctions, and repair by adult endothelial cells seeded on precoated ePTFE

OBJECTIVES

To establish some functional characteristics after in vitro endothelialisation of precoated ePTFE grafts.

MATERIALS AND METHODS

Saphenous vein endothelial cells were confluently seeded on human serum-precoated ePTFE. Collagen I-precoated ePTFE served as control. Time-courses for development of laminin, collagen IV and connexin43 was followed-up with immunofluorescence. Endothelial migration and proliferation was studied with cresyl violet staining and tritiated thymidine-labelling.

RESULTS

Six hours post-seeding, basement membrane components were visualised subendothelially as thin fibrils. After 24 h, there was a distinct fibril network, which did not seem to be further enhanced. Connexin43 was detected 6 h post-seeding, and the number of intercellular connections appeared to be similar up to day 7. The time-courses were similar on serum- and collagen I-precoated ePTFE. Endothelialisation of a 4 mm wide gap showed migrating fronts largely composed of proliferating cells. The process was completed within 10 days on collagen I-precoated ePTFE, and 15 days on serum-precoated ePTFE.

CONCLUSIONS

These findings suggest human endothelial cells, seeded on precoated ePTFE, maintain some differentiated functions seen in vivo. This is of importance for cell retention, functionality and endothelial repair of in vitro endothelialised ePTFE-grafts after implantation in man.

Visualization and functional blocking of gap junction hemichannels (connexons) with antibodies against external loop domains in astrocytes

Astrocytes constitute a metabolically and electrically coupled syncytium which is essential for the regulation of ionic homeostasis of the interstitial brain fluid and for coordinating responses to neuronal activities. The structural avenues that couple individual astrocytes are provided by gap junctions which consist of transmembraneous channels that bridge the extracellular space. Each gap junction channel is composed of two hemichannels (connexons) that dock to each other via their extracellular loops. Here we report on the characterization of external loop antibodies that visualize hemichannels in specific plasma membranes of vital astrocytes. Hemichannels were found to be concentrated on cytoplasmic processes and filopodia of subconfluent cells. Specificity of hemichannel-binding was evaluated by blockage of Lucifer Yellow (LY) dye-transfer and Ca2+ transmission, as well as LY uptake under calcium-deficient conditions. Our data indicate that hemichannels are accessible from the extracytoplasmic side either for direct visualization or functional manipulation. The availability of such a probe will allow in vivo experiments which require selective and/or temporal blocking of gap junctions in animal models.

Down-regulation of gap junctional intercellular communication between osteoblastic MC3T3-E1 cells by basic fibroblast growth factor and a phorbol ester (12-O-tetradecanoylphorbol-13-acetate)

To address the relation between osteoblast growth and cell-to-cell communication, we examined the effects of basic fibroblast growth factor (bFGF) and 12-O-tetradecanoylphorbol-13-acetate (TPA), both potent stimulators of osteoblastic proliferation, on gap junctional intercellular communication between osteoblastic MC3T3-E1 cells. The level of intercellular communication was estimated by a photobleaching method. TPA inhibited the degree of intercellular communication in two different time-dependent manners. The early (< 1 h) inhibition by TPA was consistent with an increase in the phosphorylation of connexin 43 (Cx43). The later inhibition was caused by reduction in the total amount of Cx43 on the plasma membrane, due to the decrease in the level of Cx43 transcripts. These qualitative and quantitative modulations by TPA were inhibited by a selective inhibitor of protein kinase C, GF109203X. bFGF also attenuated the gap junctional intercellular communication. However, short exposure (< 5 h) to bFGF did not affect the communication. The fact that the growth factor immediately stimulated the phosphorylation of Cx43 indicates that the phosphorylation site(s) affected by bFGF was not involved in the inhibition of communication. The decrease in the intercellular communication level was detected by the longer exposure (> 8 h) to bFGF and paralleled the decline in the Cx-mRNA level. This inhibitory effect of bFGF was abolished by the addition of a tyrosine kinase inhibitor, herbimycin A. Thus, gap junctional intercellular communication between osteoblasts was down-regulated by osteoblastic mitogens through different mechanisms of the modulation of Cx43.

An inhibition of gap-junctional communication by cadherins

The action of Ca(2+)-dependent cell-cell adhesion molecules (cadherins) on cell-to-cell channel-mediated intercellular communication was investigated in mouse L and rat Morris hepatoma cells. These cells fail to adhere to one another in aggregation assays and thus seem to lack cell adhesion molecules. Expression of exogenous cadherin induced strong cell-cell adhesion in both cell types, but had opposite effects of communication, causing inhibition in L cells and improvement in hepatoma cells. Both cells express the connexin43 cell-to-cell channel protein. By western blot we found no cadherin-specific changes in connexin43 protein in either cell type, but connexin43 gap junctional plaque staining, i.e. connexin43 localization to cell-cell junctions, was inhibited in L cells and facilitated in hepatoma cells. In addition we found that the inhibitory effect is largely abolished by blockers of glycosylation. Cadherin-cadherin interactions are known to trigger cell type-specific intracellular signal cascades resulting in diverse end effects, and gap junctional communication/plaque formation seems a further example of such cell type-specificity.

Dynamics of connexins, E-cadherin and alpha-catenin on cell membranes during gap junction formation

We examined the dynamics of connexins, E-cadherin and alpha-catenin during gap-junction disassembly and assembly in regeneration hepatocytes by immunofluorescence microscopy, and immunogold-electron microscopy using SDS-digested freeze-replicas. The present findings suggest that during the disappearance of gap junctions most of the gap junction plaques are broken up into smaller aggregates, and then the gap junction proteins may be removed from the cell membrane, but some of the connexons or connexins remain dispersed in the plane of membrane as pure morphologically indistinguishable intramembrane proteins. Double-immunogold electron microscopy using a polyclonal antibody for connexins and a monoclonal antibody for E-cadherin or alpha-catenin revealed co-localization of these molecules at cell-to-cell contact sites during the reappearance of gap junction plaques. This implies that, at least in regenerating hepatocytes, the cadherin-catenin complex-mediated cell-to-cell contact sites act as foci for gap junction formation. In addition, connexin-immunoreactivity was also observed along tight junctional strands, suggesting that the gap junction may also form along the tight junctions.

Gap-junction disassembly and connexin 43 dephosphorylation induced by 18 beta-glycyrrhetinic acid

Gap-junction channels connect the interiors of adjacent cells and can be arranged into aggregates or plaques consisting of hundreds to thousands of channel particles. The mechanism of channel aggregation into plaques and whether plaques can disaggregate are not known. Many carcinogenic and tumor-promoting chemicals have been identified that inhibit cell-cell gap-junctional coupling. Here, we provide morphological evidence that 18 beta-glycyrrhetinic acid (18 beta-GA), a saponin isolated from licorice root that is an inhibitor of gap-junctional communication, caused the disassembly of gap-junction plaques in WB-F344 rat liver epithelial cells. This effect was dose (5-40 microM) and time dependent (1-4 h treatment). Gap-junction channels in WB-F344 cells are comprised of connexin 43 (Cx43), and the protein is phosphorylated to a species known as Cx43-P2 coincident with the assembly of channels into plaques. Consistent with this, the disassembly of plaques induced by 18 beta-GA was correlated with decreases in Cx43-P2 levels and increases in nonphosphorylated Cx43. Biochemical evidence indicated that these changes in the P2 and NP forms of Cx43 represented 18 beta-GA-induced dephosphorylation of Cx43-P2 and not its degradation or the inhibition of Cx43-NP phosphorylation. Okadaic acid and calyculin A, which are inhibitors of type 1 and type 2A protein phosphatases, prevented the dephosphorylation of Cx43, suggesting that one or both of these phosphatases were involved in Cx43 dephosphorylation. These data indicate that 18 beta-GA causes type 1 or type 2A protein phosphatase-mediated Cx43 dephosphorylation coincident with the disassembly of gap-junction plaques.

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.