An inhibition of gap-junctional communication by cadherins

E-cadherin and β-catenin adhesion proteins correlate positively with connexins in colorectal cancer

The majority of solid cancers present with qualitative and quantitative aberrations of adhesion proteins, including E-cadherin and β-catenin, and connexin (Cx) gap junction proteins, which is consistent with alterations in the expression and location of such proteins in neoplastic cells. Since there are no data on the correlation between adhesion proteins and Cxs in human colorectal cancer (CRC), the aim of the present study was to evaluate the expression and correlation between these proteins. Tissue specimens were obtained from 151 cases of surgically removed colorectal adenocarcinomas. The samples were examined by immunohistochemistry with the use of antibodies against E-cadherin, β-catenin and the three Cxs: Cx26, Cx32 and Cx43. The aberrant expression of the studied adhesion proteins (primarily cytoplasmic for E-cadherin and cytoplasmic and/or nuclear for β-catenin) was observed, whereas only a minority of cases revealed normal membranous distribution of the labeling. The present study is the first in the literature to reveal a correlation between the expression of E-cadherin and β-catenin and the examined Cxs in CRC in humans. The positive correlation between the Cxs, particularly Cx26 and Cx32, and the adhesive proteins occurred in patients without lymph node metastases and in the moderately differentiated tumors (G2). Such a dependency was not observed in the analysis of the correlation between Cx43 and E-cadherin. However, a positive correlation between these proteins was observed in patients with lymph nodes metastases. Additionally, a link between the expression of these adhesion proteins was observed. The present study indicates, for the first time, that the expression of adhesion proteins, E-cadherin and β-catenin, is closely associated with the expression of three studied Cxs in CRC, and that this correlation may improve an understanding of the carcinogenic process in this cancer.

Connexins, gap junctions and tissue invasion

Formation of metastases negatively impacts the survival prognosis of cancer patients. Globally, if the various steps involved in their formation are relatively well identified, the molecular mechanisms responsible for the emergence of invasive cancer cells are still incompletely resolved. Elucidating what are the mechanisms that allow cancer cells to evade from the tumor is a crucial point since it is the first step of the metastatic potential of a solid tumor. In order to be invasive, cancer cells have to undergo transformations such as down-regulation of cell-cell adhesions, modification of cell-matrix adhesions and acquisition of proteolytic properties. These transformations are accompanied by the capacity to "activate" stromal cells, which may favor the motility of the invasive cells through the extracellular matrix. Since modulation of gap junctional intercellular communication is known to be involved in cancer, we were interested to consider whether these different transformations necessary for the acquisition of invasive phenotype are related with gap junctions and their structural proteins, the connexins. In this review, emerging roles of connexins and gap junctions in the process of tissue invasion are proposed.

Neuroependymal denudation is in progress in full-term human foetal spina bifida aperta

In human spina bifida aperta (SBA), cerebral pathogenesis [hydrocephalus, Sylvius aqueduct (SA) stenosis and heterotopias] is poorly understood. In animal models, loss of ventricular lining (ependymal denudation) causes SA stenosis and hydrocephalus. We aimed to investigate whether ependymal denudation also takes place in human foetal SBA. Considering that ependymal denudation would be related to alterations in junction proteins, sections through SA of five SBA and six control foetuses (gestational ages ranged between 37 and 40 weeks) were immunostained for markers of ependyma (caveolin 1, βIV-tubulin, S100), junction proteins (N-cadherin, connexin-43, neural cell adhesion molecule (NCAM), blood vessels (Glut-1) and astrocytes [glial fibrillary acidic protein (GFAP)]. In control foetuses, ependymal denudation was absent. In SBA foetuses different stages of ependymal denudation were observed: (i) intact ependyma/neuroepithelium; (ii) imminent ependymal denudation (with abnormal subcellular location of junction proteins); (iii) ependymal denudation (with protrusion of neuropile into SA, formation of rosettes and macrophage invasion); (iv) astroglial reaction. It is suggested that abnormalities in the formation of gap and adherent junctions result in defective ependymal coupling, desynchronized ciliary beating and ependymal denudation, leading to hydrocephalus. The presence of various stages of ependymal denudation within the same full-term SBA foetuses suggests continuation of the process after birth.

Assembly of connexin43 into gap junctions is regulated differentially by E-cadherin and N-cadherin in rat liver epithelial cells

Cadherins have been thought to facilitate the assembly of connexins (Cxs) into gap junctions (GJs) by enhancing cell-cell contact, however the molecular mechanisms involved in this process have remained unexplored. We examined the assembly of GJs composed of Cx43 in isogenic clones derived from immortalized and nontransformed rat liver epithelial cells that expressed either epithelial cadherin (E-Cad), which curbs the malignant behavior of tumor cells, or neuronal cadherin (N-Cad), which augments the invasive and motile behavior of tumor cells. We found that N-cad expression attenuated the assembly of Cx43 into GJs, whereas E-Cad expression facilitated the assembly. The expression of N-Cad inhibited GJ assembly by causing endocytosis of Cx43 via a nonclathrin-dependent pathway. Knock down of N-Cad by ShRNA restored GJ assembly. When both cadherins were simultaneously expressed in the same cell type, GJ assembly and disassembly occurred concurrently. Our findings demonstrate that E-Cad and N-Cad have opposite effects on the assembly of Cx43 into GJs in rat liver epithelial cells. These findings imply that GJ assembly and disassembly are the down-stream targets of the signaling initiated by E-Cad and N-Cad, respectively, and may provide one possible explanation for the disparate role played by these cadherins in regulating cell motility and invasion during tumor progression and invasion.

DE-Cadherin, a Core Component of the Adherens Junction Complex Modifies Subcellular Localization of theDrosophilaGap Junction Protein Innexin2

The Drosophila innexin multigene family of gap junction encoding proteins consists of eight family members whose function in epithelial morphogenesis is mostly unknown. We have recently shown that innexin2 plays a crucial role in the organization of embryonic epithelia. Innexin2 protein accumulates in the epidermis in the apico-lateral membrane domain and colocalizes with core proteins of adherens junctions, such as DE-cadherin and Armadillo, the ss -catenin homolog. Innexin2 localization is altered in both armadillo and DE-cadherin mutants Biochemical interaction studies point to a direct interaction of DE-cadherin and Armadillo with innexin2 suggesting a close link between gap junction and adherens junction biogenesis. We have used the Drosophila Schneider cell tissue culture system to further study the interaction of innexin2 with DE-cadherin. Our results provide evidence that DE-cadherin may be a key component to control trafficking, and localization of Innexin2 to the plasma membrane.

Reciprocal influence of connexins and apical junction proteins on their expressions and functions

Membranes of adjacent cells form intercellular junctional complexes to mechanically anchor neighbour cells (anchoring junctions), to seal the paracellular space and to prevent diffusion of integral proteins within the plasma membrane (tight junctions) and to allow cell-to-cell diffusion of small ions and molecules (gap junctions). These different types of specialised plasma membrane microdomains, sharing common adaptor molecules, particularly zonula occludens proteins, frequently present intermingled relationships where the different proteins co-assemble into macromolecular complexes and their expressions are co-ordinately regulated. Proteins forming gap junction channels (connexins, particularly) and proteins fulfilling cell attachment or forming tight junction strands mutually influence expression and functions of one another.

Dual effect of cell-cell contact disruption on cytosolic calcium and insulin secretion

Cell-to-cell interactions play an important role in insulin secretion. Compared with intact islets, dispersed pancreatic beta-cells show increased basal and decreased glucose-stimulated insulin secretion. In this study, we used mouse MIN6B1 cells to investigate the mechanisms that control insulin secretion when cells are in contact with each other or not. RNAi-mediated silencing of the adhesion molecule E-cadherin in confluent cells reduced glucose-stimulated secretion to the levels observed in isolated cells but had no impact on basal secretion. Dispersed cells presented high cytosolic Ca(2+) activity, depolymerized cytoskeleton and ERK1/2 activation in low glucose conditions. Both the increased basal secretion and the spontaneous Ca(2+) activity were corrected by transient removal of Ca(2+) or prolonged incubation of cells in low glucose, a procedure that restored the ability of dispersed cells to respond to glucose (11-fold stimulation). In conclusion, we show that dispersed pancreatic beta-cells can respond robustly to glucose once their elevated basal secretion has been corrected. The increased basal insulin secretion of dispersed cells is due to spontaneous Ca(2+) transients that activate downstream Ca(2+) effectors, whereas engagement of cell adhesion molecules including E-cadherin contributes to the greater secretory response to glucose seen in cells with normal intercellular contacts.

Gap junctional complexes: From partners to functions

Gap junctions (GJ), specialised membrane structures that mediate cell-to-cell communication in almost all animal tissues, are composed of intercellular channel-forming integral membrane proteins termed connexins (Cxs), innexins or pannexins. The activity of these channels is closely regulated, particularly by intramolecular modifications as phosphorylation of proteins, via the formation of multiprotein complexes where pore-forming subunits bind to auxiliary channel subunits and associate with scaffolding proteins that play essential roles in channel localization and activity. Scaffolding proteins link signalling enzymes, substrates, and potential effectors (such as channels) into multiprotein signalling complexes that may be anchored to the cytoskeleton. Protein-protein interactions play essential roles in channel localization and activity and, besides their cell-to-cell channel-forming functions, gap junctional proteins now appear involved in different cellular functions (e.g. transcriptional and cytoskeletal regulation). The present review summarizes the recent progress regarding the proteins capable of interacting with junctional proteins and their functional importance.

Connexin43 repression following epithelium-to-mesenchyme transition in embryonal carcinoma cells requires Snail1 transcription factor

Embryonic stem (ES) cells and embryonal carcinoma (EC) cells express high amounts of functional connexin43 (Cx43). During mesoderm formation and subsequent cardiac differentiation, Cx43 is initially down-regulated but is up-regulated again as the emerging cardiomyocytes mature. In this study, we investigated the regulation of Cx43 expression during early phases of differentiation in F9 and P19 EC cells. We found a striking inverse correlation between the expression of Cx43 and that of the transcriptional repressor Snail1. No clear relationship was found with Smad-interacting-protein1 (SIP1), another transcription factor inducing epithelium-to-mesenchyme transition (EMT). Promoter-reporter assays indicated Cx43 repression at the promoter level by ectopically expressed Snail1. To establish whether the Cx43 down-regulation depends on endogenous Snail1, MES-1 cells, differentiated derivatives of P19 EC, were stably transfected by an siRNA construct silencing Snail1 expression. This resulted in a mesenchyme-to-epithelium transition, which was accompanied by increased levels of Cx43 mRNA and protein and enhanced metabolic and electrical coupling. We conclude that Snail1-mediated EMT results in a Cx43 repression.

Ablation of gap junctional communication in hepatocytes of transgenic mice does not lead to disrupted cellular homeostasis or increased spontaneous tumourigenesis

Gap junctions between murine hepatocytes are composed of two subunit proteins, connexin26 (Cx26) and connexin32 (Cx32). Previously, we found increased formation of chemically induced liver tumours but no increase in spontaneous development of preneoplastic hepatic foci in mice that lacked Cx32 and expressed decreased amounts of Cx26. In order to clarify this tumour-suppressive effect and to overcome embryonic lethality of constitutive Cx26-deficient mice, cell type-specific targeting of the Cx26 gene was performed. Mice with loxP-flanked Cx26 coding DNA were crossed with mice expressing the Cre recombinase exclusively in hepatocytes. Progeny mice lacking Cx26 in the liver were viable and fertile with no obvious signs of phenotypic alterations. To generate mice that totally lack gap junctional intercellular coupling, these mice were crossed with constitutive Cx32-deficient mice. We found no increase in spontaneously induced liver tumour formation in Cx26 and double deficient Cx26/Cx32 mice. Occasionally, double deficient livers exhibited morphological alterations, like amyloidosis, and a slightly increased basal proliferation rate of hepatocytes. Although the absence of gap junction channels led to altered expression of adhesion-related proteins like E-cadherin and actin, microarray analyses of total liver transcripts yielded only few differences between Cx26-deficient and double deficient livers compared to control samples. Our results suggest that total lack of gap junctional communication due to hepatocytic ablation of Cx26 and Cx32 does not drastically alter basal hepatocytic function and does not lead to increased spontaneous liver tumour formation.

Expression of connexins 26 and 43 in canine hyperplastic and neoplastic mammary glands

Gap junctions are the only communicating junctions found in animal tissues and are composed of proteins known as connexins. Alterations in connexin expression have been associated with oncogenesis; reported studies in rodent and human mammary glands, which normally express connexins 26 and 43, confirm these alterations in malignancies. Mammary neoplasms represent the second most frequent neoplasm in dogs, and since there are no reports on the study of connexins in canine mammary glands, the present study investigated the expression of connexins 26 and 43 in normal, hyperplastic, and neoplastic mammary glands of this species, to verify if altered patterns of connexin staining are related to higher cell proliferation and malignant phenotypes. A total of 4 normal, 8 hyperplastic mammary glands, 9 benign, and 51 malignant mammary gland neoplasms were submitted for the immunostaining of connexins 26 and 43, E-cadherin, and proliferating cell nuclear antigen (PCNA). Normal, hyperplastic, and benign neoplastic mammary glands showed a punctate pattern for connexin 26 and 43 staining and an intercellular E-cadherin staining. Malignant neoplasms, especially the most aggressive cases with high cell proliferation rates, presented either fewer gap junction spots on the cell membranes or increased cytoplasmic immunostaining. Malignant tumors also expressed a less intense immunostaining of E-cadherin; the expression of this adhesion molecule is important for the transportation of connexins to cell membranes and in forming communicating gap junctions. Deficient expression of E-cadherin could be related to the aberrant connexin localization and may contribute to the malignant phenotype. In conclusion, the expression and distribution of connexins and E-cadherin are inversely correlated to cell proliferation in malignant mammary neoplasms of dogs and may well be related to their more aggressive histologic type and biologic behavior.

Connexin43 Associated with an N-cadherin-containing Multiprotein Complex Is Required for Gap Junction Formation in NIH3T3 Cells

Previous studies have indicated an intimate linkage between gap junction and adherens junction formation. It was suggested this could reflect the close membrane-membrane apposition required for junction formation. In NIH3T3 cells, we observed the colocalization of connexin43 (Cx43alpha1) gap junction protein with N-cadherin, p120, and other N-cadherin-associated proteins at regions of cell-cell contact. We also found that Cx43alpha1, N-cadherin, and N-cadherin-associated proteins were coimmunoprecipitated by antibodies to either Cx43alpha1, N-cadherin, or various N-cadherin-associated proteins. These findings suggest that Cx43alpha1 and N-cadherin are coassembled in a multiprotein complex containing various N-cadherin-associated proteins. Studies using siRNA knockdown indicated that cell surface expression of Cx43alpha1 required N-cadherin, and conversely, N-cadherin cell surface expression required Cx43alpha1. Pulse-chase labeling and cell surface biotinylation experiments indicated that in the absence of N-cadherin, Cx43alpha1 cell surface trafficking is blocked. Surprisingly, siRNA knockdown of p120, an N-cadherin-associated protein known to modulate cell surface turnover of N-cadherin, reduced N-cadherin cell surface expression without altering Cx43alpha1 expression. These observations suggest that in contrast to the coregulated cell surface trafficking of Cx43alpha1 and N-cadherin, N-cadherin turnover at the cell surface may be regulated independently of Cx43alpha1. Functional studies showed gap junctional communication is reduced and cell motility inhibited with N-cadherin or Cx43alpha1 knockdown, consistent with the observed loss of both gap junction and cadherin contacts with either knockdown. Overall, these studies indicate that the intracellular coassembly of connexin and cadherin is required for gap junction and adherens junction formation, a process that likely underlies the intimate association between gap junction and adherens junction formation.

The differential adhesion hypothesis: a direct evaluation

The differential adhesion hypothesis (DAH), advanced in the 1960s, proposed that the liquid-like tissue-spreading and cell segregation phenomena of development arise from tissue surface tensions that in turn arise from differences in intercellular adhesiveness. Our earlier measurements of liquid-like cell aggregate surface tensions have shown that, without exception, a cell aggregate of lower surface tension tends to envelop one of higher surface tension to which it adheres. We here measure the surface tensions of L cell aggregates transfected to express N-, P- or E-cadherin in varied, measured amounts. We report that in these aggregates, in which cadherins are essentially the only cell-cell adhesion molecules, the aggregate surface tensions are a direct, linear function of cadherin expression level. Taken together with our earlier results, the conclusion follows that the liquid-like morphogenetic cell and tissue rearrangements of cell sorting, tissue spreading and segregation represent self-assembly processes guided by the diminution of adhesive-free energy as cells tend to maximize their mutual binding. This conclusion relates to the physics governing these morphogenetic phenomena and applies independently of issues such as the specificities of intercellular adhesives.

Diversity in protein–protein interactions of connexins: emerging roles

Gap junctions, specialised membrane structures that mediate cell-to-cell communication in almost all tissues, are composed of channel-forming integral membrane proteins termed connexins. The activity of these intercellular channels is closely regulated, particularly by intramolecular modifications as phosphorylations of proteins by protein kinases, which appear to regulate the gap junction at several levels, including assembly of channels in the plasma membrane, connexin turnover as well as directly affecting the opening and closure ("gating") of channels. The regulation of membrane channels by protein phosphorylation/dephosphorylation processes commonly requires the formation of a multiprotein complex, where pore-forming subunits bind to auxiliary proteins (e.g. scaffolding proteins, catalytic and regulatory subunits), that play essential roles in channel localisation and activity, linking signalling enzymes, substrates and effectors into a structure frequently anchored to the cytoskeleton. The present review summarises the up-to-date progress regarding the proteins capable of interacting or at least of co-localising with connexins and their functional importance.

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.

Neonatal hypothyroidism alters the localization of gap junctional protein connexin 43 in the testis and messenger RNA levels in the epididymis of the rat

The objectives of this study were to determine the effects of propylthiouracil (PTU)-induced neonatal hypothyroidism on the gap junctional protein Cx43 in rat testis and epididymis. PTU (0.02%) was administered via lactation from birth to Day 30, and the rats were sampled at 14, 18, 22, 26, 30, and 91 days of age. Testicular Cx43 was localized along the plasma membranes and cytoplasm of Sertoli cells until Day 22. At Day 30, the immunostaining was localized exclusively along the plasma membrane of Sertoli cells. In PTU-treated rats, Cx43 did not localize to the plasma membrane and was still cytoplasmic at 30 days of age. Occludin was present in tubules of treated rats, but was not localized to the blood-testis barrier in 30-day-old rats, as in controls. There were no differences in Cx43 immunostaining in the adult testis. In the proximal epididymis (initial segment, caput, corpus), Cx43 mRNA levels were lower in PTU-treated rats at 14, 18, and 22 days of age, but no differences were observed in the distal (cauda) epididymis at these ages. In 22- and 30-day-old rats, Cx43 was localized along the plasma membrane between principal and basal cells throughout the epididymis. In PTU-treated rats, Cx43 was not detectable in initial segment, caput, or corpus epididymidis. In the cauda epididymidis, however, Cx43 immunostaining in PTU-treated rats was similar to controls. These data suggest that thyroid hormones regulate Cx43-dependent gap junctional communication in the testis and epididymis.

Impaired trafficking of connexins in androgen-independent human prostate cancer cell lines and its mitigation by alpha-catenin

Gap junctions, composed of connexins, provide a pathway of direct intercellular communication for the diffusion of small molecules between cells. Evidence suggests that connexins act as tumor suppressors. We showed previously that expression of connexin-43 and connexin-32 in an indolent prostate cancer cell line, LNCaP, resulted in gap junction formation and growth inhibition. To elucidate the role of connexins in the progression of prostate cancer from a hormone-dependent to -independent state, we introduced connexin-43 and connexin-32 into an invasive, androgen-independent cell line, PC-3. Expression of these proteins in PC-3 cells resulted in intracellular accumulation. Western blot analysis revealed a lack of Triton-insoluble, plaque-assembled connexins. In contrast to LNCaP cells, connexins could not be cell surface-biotinylated and did not reside in the cell surface derived endocytic vesicles, in PC-3 cells, suggesting impaired trafficking to the cell surface. Intracellular accumulation of connexins was observed in several androgen-independent prostate cancer cell lines. Transient expression of alpha-catenin facilitated the trafficking of both connexins to the cell surface and induced gap junction assembly. Our results suggest that impaired trafficking, and not the inability to form gap junctions, is the major cause of communication deficiency in human prostate cancer cell lines.

Expression of gap junction genes connexin32 and connexin43 mRNAs and proteins, and their role in hepatocarcinogenesis

AIM

To investigate the relationship between hepatocarcinogenesis and the expression of connexin32 (cx32), connexin43 (cx43) mRNAs and proteins in vitro.

METHODS

Gap junction genes cx32 and cx43 mRNA in hepatocellular carcinoma cell lines HHCC, SMMC-7721 and normal liver cell line QZG were detected by in situ hybridization (ISH) with digoxin-labeled cx32, and cx43 cDNA probes. Expression of Cx32 and Cx43 proteins in the cell lines was revealed by indirect immuno-fluorescence and flow cytometry (FCM).

RESULTS

Blue positive hybridization signals of cx32 and cx43 mRNAs detected by ISH with cx32 and cx43 cDNA probes respectively were located in cytoplasm of cells of HHCC, SMMC-7721 and QZG. No significant difference of either cx32 mRNA or cx43 mRNA was tested among HHCC, SMMC-7721 and QZG (P=2.673, HHCC vs QZG; P=1.375, SMMC-7721 vs QZG). FCM assay showed that the positive rates of Cx32 protein in HHCC, SMMC-7721 and QZG were 0.7%, 1.7% and 99.0%, and the positive rates of Cx43 protein in HHCC, SMMC-7721 and QZG were 7.3%, 26.5% and 99.1% respectively. Significant differences of both Cx32 and Cx43 protein expression existed between hepatocellular carcinoma cell lines and normal liver cell line (P=0.0069, HHCC vs QZG; P=0.0087, SMMC-7721 vs QZG). Moreover, the fluorescent intensities of Cx32 and Cx43 proteins in HHCC, SMMC-7721 were lower than that in QZG.

CONCLUSIONS

Hepatocellular carcinoma cell lines HHCC and SMMC-7721 exhibited lower positive rates and fluorescent intensities of Cx32, Cx43 proteins compared with that of normal liver cell line QZG. It is suggested that lower expression of both Cx32 and Cx43 proteins in hepatocellular carcinoma cells could play pivotal roles in the hepatocarcinogenesis. Besides, genetic defects of cx32 and cx43 in post-translational processing should be considered.

Matrix-bound fibroblasts regulate angiogenesis by modulation of VE-cadherin

BACKGROUND

Vascular endothelial cadherin (VE-cadherin/cadherin-5) has previously been described as playing a specific role in angiogenesis due to its localisation at areas of intercellular contact, where it functions in maintenance of tubular architecture. Matrix-bound fibroblasts have been show to produce a number of factors that are important in inducing angiogenesis and to promote tubule-formation by human endothelial cells, an indicator of angiogenic potential.

RESULTS

Tubule formation stimulated by fibroblast-derived growth factors can be prevented by the addition of monoclonal antibody to VE-cadherin. In addition, fibroblasts-derived growth factors are able to modulate the expression and hence the regulation of this endothelial cell specific cadherin.

CONCLUSIONS

The change in VE-cadherin expression of human endothelial cells by fibroblast-derived growth factors may contribute to the regulation of angiogenesis.

Cadherins and catenins, Wnts and SOXs: embryonic patterning in Xenopus

Wnt signaling plays a critical role in a wide range of developmental and oncogenic processes. Altered gene regulation by the canonical Wnt signaling pathway involves the cytoplasmic stabilization of beta-catenin, a protein critical to the assembly of cadherin-based cell-cell adherence junctions. In addition to binding to cadherins, beta-catenin also interacts with transcription factors of the TCF-subfamily of HMG box proteins and regulates their activity. The Xenopus embryo has proven to be a particularly powerful experimental system in which to study the role of Wnt signaling components in development and differentiation. We review this literature, focusing on the role of Wnt signaling and interacting components in establishing patterns within the early embryo.

Differential modulation of cadherin-mediated cell-cell adhesion by platelet endothelial cell adhesion molecule-1 isoforms through activation of extracellular regulated kinases

The role of platelet endothelial cell adhesion molecule-1 (PECAM-1) in endothelial cell-cell interactions and its contribution to cadherin-mediated cell adhesion are poorly understood. Such studies have been difficult because all known endothelial cells express PECAM-1. We have used Madin-Darby canine kidney (MDCK) cells as a model system in which to evaluate the role of PECAM-1 isoforms that differ in their cytoplasmic domains in cell-cell interactions. MDCK cells lack endogenous PECAM-1 but form cell-cell junctions similar to those of endothelial cells, in which PECAM-1 is concentrated. MDCK cells were transfected with two isoforms of murine PECAM-1, Delta15 and Delta14&15, the predominant isoforms expressed in vivo. Expression of the Delta15 isoform resulted in apparent dedifferentiation of MDCK cells concomitant with the loss of adherens junctions, down-regulation of E-cadherin, alpha- and beta-catenin expression, and sustained activation of extracellular regulated kinases. The Delta15 isoform was not concentrated at cell-cell contacts. In contrast, the Delta14&15 isoform localized to sites of cell-cell contact and had no effect on MDCK cell morphology, cadherin/catenin expression, or extracellular regulated kinase activity. Thus, the presence of exon 14 in the cytoplasmic domain of PECAM-1 has dramatic effects on the ability of cells to maintain adherens junctions and an epithelial phenotype. Therefore, changes in the expression of exon 14 containing PECAM-1 isoforms, which we have observed during development, may have profound functional consequences.

Cadherin repertoire determines partner-specific gap junctional communication during melanoma progression

Reduced gap junction activity has long been implicated in tumorigenesis. To elucidate the potential role of intercellular communication in melanoma development, we examined gap junctional capability of melanocytic cells from various stages of tumor progression in coculture models using dye transfer assays. Normal melanocytes coupled with keratinocytes by gap junctional formation, whereas melanoma cells did not. Instead, melanoma cells communicated among themselves and with fibroblasts. This switch in communication partners coincided with a shift from E-cadherin to N-cadherin expression during melanoma development. Forced expression of E-cadherin by adenoviral gene transfer in N-cadherin-expressing melanoma cells restored gap junctional compatibility with keratinocytes. Our data suggest that (1) melanocyte transformation is associated with loss of the pre-existing gap junctional activity with keratinocytes but a concomitant gain of communication with a newly juxtaposed cell type, the fibroblasts, (2) the specificity of gap junctional formation during melanoma development is determined by the cadherin profile on the melanocytic cells and (3) the overall gap junctional activity of melanocytic cells is not reduced with transformation.

Inhibition of endothelial cell migration, intercellular communication, and vascular tube formation by thromboxane A(2)

The eicosanoid thromboxane A(2) (TXA(2)) is released by activated platelets, monocytes, and the vessel wall and interacts with high affinity receptors expressed in several tissues including endothelium. Whether TXA(2) might alter endothelial migration and tube formation, two determinants of angiogenesis, is unknown. Thus, we investigated the effect of the TXA(2) mimetic [1S-(1alpha, 2beta(5Z),3alpha(1E,3R), 4alpha]-7-[3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)-7-o xab icyclo- [2.2.1]heptan-2-yl]-5'-heptenoic acid (IBOP) on human endothelial cell (HEC) migration and angiogenesis in vitro. IBOP stimulation inhibited HEC migration by 50% and in vitro capillary formation by 75%. These effects of IBOP were time- and concentration-dependent with an IC(50) of 25 nM. IBOP did not affect integrin expression or cytoskeletal morphology of HEC. Since gap junction-mediated intercellular communication increases in migrating HEC, we determined whether IBOP might inhibit coupling or connexin expression in HEC. IBOP reduced the passage of microinjected dyes between HEC by 50%, and the effects of IBOP on migration and tube formation were mimicked by the gap junction inhibitor 18beta-glycyrrhetinic acid (1 microM) with a similar time course and efficacy. IBOP (24 h) did not affect the expression or phosphorylation of connexin 43 in whole HEC lysates. Immunohistologic examination of HEC suggested that IBOP may impair functional coupling by altering the cellular distribution of gap junctions, leading to increased connexin 43 internalization. Thus, this finding that TXA(2) mimetics can prevent HEC migration and tube formation, possibly by impairing intercellular communication, suggests that antagonizing TXA(2) signaling might enhance vascularization of ischemic tissue.

Deficient epithelial-fibroblast heterocellular gap junction communication can be overcome by co-culture with an intermediate cell type but not by E-cadherin transgene expression

Epithelial, fibroblast and intermediate cell lines were employed to examine the mechanism(s) essential for heterocellular gap junction intercellular communication in vitro. These cell lines were characterized extensively for cell type based on morphology, intermediate cytoskeletal proteins, cell adhesion molecules and their associated proteins, tight junction proteins as well as functional differentiation. All cell types expressed connexin43 and were dye-coupled in homocellular culture. Epithelial and intermediate cells or fibroblasts and intermediate cells readily assembled heterocellular connexin43-positive gap junction plaques when co-cultured, while gap junction plaques in mixed cultures of epithelial cells and fibroblasts were rare. Dye microinjection studies were used to show that there was little gap junction intercellular communication between epithelial cells and fibroblasts. However, intermediate cells were able to communicate with epithelial cells and, to a lesser extent, fibroblasts and could transfer dye to both epithelial cells and fibroblasts when all three cell types were cultured together. Fibroblasts that were stably transfected with a cDNA encoding E-cadherin had a greater tendency to aggregate and exhibited a more epithelial-like phenotype but heterocellular gap junction intercellular communication with epithelial cells, which endogenously express E-cadherin, was not enhanced. These results suggest that mutual expression of E-cadherin is insufficient to stimulate gap junction formation between epithelial cells and fibroblasts. Moreover, our results also demonstrate that communication gaps between epithelial cells and fibroblasts can be bridged by intermediate cells, a process that may be important in mammary gland development, growth, differentiation and cancer.

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.

The S100 family of multipurpose calcium-binding proteins

Antibodies reactive with S100 protein are useful markers in a diagnostic immunohistochemistry laboratory dealing with cutaneous tumors. However, S100 protein is not a single protein but instead a group of S100 proteins with diverse functions. S100 proteins constitute a family of acidic calcium-binding proteins that are important in intracellular calcium metabolism. Recent evidence that some S100 proteins are secreted makes it likely that they are also involved in cell-cell interactions. The exploration of the status of the different members of the S100 family may yield not only diagnostic clues but also relevant functional information about the cells. Considerable recent progress has been made in our understanding of S100 proteins. This review surveys some of these findings that may be either directly or indirectly relevant to cutaneous pathology.

Transplacental uptake of glucose is decreased in embryonic lethal connexin26-deficient mice

Mice that harbor a targeted homozygous defect in the gene coding for the gap junctional protein connexin26 died in utero during the transient phase from early to midgestation. From day 10 post coitum onwards, development of homozygous embryos was retarded, which led to death around day 11 post coitum. Except for growth retardation, no gross morphological alterations were detected between homozygous connexin26-defective embryos and wild-type littermates. At day 9 postcoitum, when chorioallantoic placenta started to function, connexin26 was weakly expressed in the yolk sac epithelium, between syncytiotrophoblasts I and II in the labyrinth region of the placenta, and in the skin of the embryo. At day 10 post coitum, expression of connexin26 in the placenta was much stronger than at the other locations. To analyze involvement of connexin26 in the placental transfer of nutrients, we have measured embryonic uptake of the nonmetabolizable glucose analogue 3-O-[14C]methylglucose, injected into the maternal tail vein. At day 10 post coitum, viable, homozygous connexin26-defective embryos accumulated only approximately 40% of the radioactivity measured in wild-type and heterozygous littermates of the same size. We conclude that the uptake of glucose, and presumably other nutrients as well, from maternal blood into connexin26-deficient mouse embryos was severely impaired and apparently not sufficient to support the rapid organogenesis during midgestation. Our results suggest that connexin26 gap junction channels likely fulfill an essential role in the transfer of maternal nutrients and embryonic waste products between syncytiotrophoblast I and II in the labyrinth layer of the mouse placenta.