Not just glue: cell-cell junctions as cellular signaling centers

Claudin proteins, outside-in signaling, and carcinogenesis

Environment affects an individual's development and disease risk which then suggest that the environmental cues must have ways of reaching to the cellular nuclei to orchestrate desired genetic changes. Polarized and differentiated epithelial cells join together by cell-cell adhesions to create a protective sheet which separates body's internal milieu from its environment, albeit in highly regulated manner. Among these cell-cell adhesions, a key role of tight junction, the apical cell-cell adhesion, in maintaining epithelial cell polarity and differentiation is well recognized. Moreover, significant changes in expression and cellular distribution of claudin proteins, integral component of the tight junction, characterize pathophysiological changes including neoplastic growth and progression. Studies have further confirmed existence of complex claudin-based interactomes and demonstrated that changes in such protein partnering can influence barrier integrity and communication between a cell and its environment to produce undesired outcome. Cell signaling is the process by which cells respond to their environment to make dynamic decisions to live, grow and proliferate, or die. Thus, pivotal role of the deregulated tight junction structure/function in influencing cellular signaling cascades to alter cellular phenotype can be envisaged, however, is not well understood. Needless to mention that advanced knowledge in this area can help improve therapeutic considerations and preventive measures. Here, we discuss potential role of the tight junction in the regulation of "outside-in" signaling to regulate cancer growth, with specific focus upon the claudin family of proteins.

ICAM-2 regulates vascular permeability and N-cadherin localization through ezrin-radixin-moesin (ERM) proteins and Rac-1 signalling

Background

Endothelial junctions control functions such as permeability, angiogenesis and contact inhibition. VE-Cadherin (VECad) is essential for the maintenance of intercellular contacts. In confluent endothelial monolayers, N-Cadherin (NCad) is mostly expressed on the apical and basal membrane, but in the absence of VECad it localizes at junctions. Both cadherins are required for vascular development. The intercellular adhesion molecule (ICAM)-2, also localized at endothelial junctions, is involved in leukocyte recruitment and angiogenesis.

Results

In human umbilical vein endothelial cells (HUVEC), both VECad and NCad were found at nascent cell contacts of sub-confluent monolayers, but only VECad localized at the mature junctions of confluent monolayers. Inhibition of ICAM-2 expression by siRNA caused the appearance of small gaps at the junctions and a decrease in NCad junctional staining in sub-confluent monolayers. Endothelioma lines derived from WT or ICAM-2-deficient mice (IC2neg) lacked VECad and failed to form junctions, with loss of contact inhibition. Re-expression of full-length ICAM-2 (IC2 FL) in IC2neg cells restored contact inhibition through recruitment of NCad at the junctions. Mutant ICAM-2 lacking the binding site for ERM proteins (IC2 ΔERM) or the cytoplasmic tail (IC2 ΔTAIL) failed to restore junctions. ICAM-2-dependent Rac-1 activation was also decreased in these mutant cell lines. Barrier function, measured in vitro via transendothelial electrical resistance, was decreased in IC2neg cells, both in resting conditions and after thrombin stimulation. This was dependent on ICAM-2 signalling to the small GTPase Rac-1, since transendothelial electrical resistance of IC2neg cells was restored by constitutively active Rac-1. In vivo, thrombin-induced extravasation of FITC-labeled albumin measured by intravital fluorescence microscopy in the mouse cremaster muscle showed that permeability was increased in ICAM-2-deficient mice compared to controls.

Conclusions

These results indicate that ICAM-2 regulates endothelial barrier function and permeability through a pathway involving N-Cadherin, ERMs and Rac-1.

Septate junctions are required for ommatidial integrity and blood-eye barrier function in Drosophila

The anatomical organization of the Drosophila ommatidia is achieved by specification and contextual placement of photoreceptors, cone and pigment cells. The photoreceptors must be sealed from high ionic concentrations of the hemolymph by a barrier to allow phototransduction. In vertebrates, a blood-retinal barrier (BRB) is established by tight junctions (TJs) present in the retinal pigment epithelium and endothelial membrane of the retinal vessels. In Drosophila ommatidia, the junctional organization and barrier formation is poorly understood. Here we report that septate junctions (SJs), the vertebrate analogs of TJs, are present in the adult ommatidia and are formed between and among the cone and pigment cells. We show that the localization of Neurexin IV (Nrx IV), a SJ-specific protein, coincides with the location of SJs in the cone and pigment cells. Somatic mosaic analysis of nrx IV null mutants shows that loss of Nrx IV leads to defects in ommatidial morphology and integrity. nrx IV hypomorphic allelic combinations generated viable adults with defective SJs and displayed a compromised blood-eye barrier (BEB) function. These findings establish that SJs are essential for ommatidial integrity and in creating a BEB around the ion and light sensitive photoreceptors. Our studies may provide clues towards understanding the vertebrate BEB formation and function.

Enzymatic Cross-Linking versus Radical Polymerization in the Preparation of Gelatin PolyHIPEs and Their Performance as Scaffolds in the Culture of Hepatocytes

Highly open porous biodegradable scaffolds, based on gelatin A3, were fabricated with the aim of using them for tissue-engineering applications. The fabrication process is based on an emulsion-templating technique. In the preparation of gelatin scaffolds two different cross-linking procedures were adopted: (I) radical polymerization of the methacrylate functionalities, previously introduced onto the gelatin chains and (II) formation of isopeptide bridges among the gelatin chains promoted by the enzyme microbial transglutaminase. The method of cross-linking exerts a pronounced effect on the morphology of the porous biomaterials: radical polymerization of methacrylated gelatin allowed the production of scaffolds with a better defined porous structure, while the enzymatically cross-linked scaffolds were characterized by a thinner skeletal framework. A suitable sample of each kind of the differently cross-linked porous biomaterials was tested for the culture of hepatocytes. The scaffold obtained by radical polymerization possessed a morphology characterized by relatively large voids and interconnects, and as a consequence, it was more suitable for hepatocytes colonization. On the other hand, the enzymatically cross-linked scaffold resulted in less cytotoxicity and the cultured hepatocytes expressed a better differentiated phenotype, as evidenced by a greater expression and more correct localization of key adhesion proteins.

Development of a model for microphysiological simulations: small nodes of ranvier from peripheral nerves of mice reconstructed by electron tomography

The node of Ranvier is a complex structure found along myelinated nerves of vertebrate animals. Specific membrane, cytoskeletal, junctional, extracellular matrix proteins and organelles interact to maintain and regulate associated ion movements between spaces in the nodal complex, potentially influencing response variation during repetitive activations or metabolic stress. Understanding and building high resolution three dimensional (3D) structures of the node of Ranvier, including localization of specific macromolecules, is crucial to a better understanding of the relationship between its structure and function and the macromolecular basis for impaired conduction in disease. Using serial section electron tomographic methods, we have constructed accurate 3D models of the nodal complex from mouse spinal roots with resolution better than 7.5 nm. These reconstructed volumes contain 75-80% of the thickness of the nodal region. We also directly imaged the glial axonal junctions that serve to anchor the terminal loops of the myelin lamellae to the axolemma. We created a model of an intact node of Ranvier by truncating the volume at its midpoint in Z, duplicating the remaining volume and then merging the new half volume with mirror symmetry about the Z-axis. We added to this model the distribution and number of Na+ channels on this reconstruction using tools associated with the MCell simulation program environment. The model created provides accurate structural descriptions of the membrane compartments, external spaces, and formed structures enabling more realistic simulations of the role of the node in modulation of impulse propagation than have been conducted on myelinated nerve previously.

Comparison of molecular mechanisms mediating cell contact phenomena in model developmental systems: an exploration of universality

Are there universal molecular mechanisms associated with cell contact phenomena during metazoan ontogenesis? Comparison of adhesion systems in disparate model systems indicates the existence of unifying principles. Requirements for multicellularity are (a) the construction of three-dimensional structures involving a crucial balance between adhesiveness and motility; and (b) the establishment of integration at molecular, cellular, tissue, and organismal levels of organization. Mechanisms for (i) cell-cell and cell-substrate adhesion, (ii) cell movement, (iii) cell-cell communication, (iv) cellular responses, (v) regulation of these processes, and (vi) their integration with patterning, growth, and other developmental processes are all crucial to metazoan development, and must have been present for the emergence and radiation of Metazoa. The principal unifying themes of this review are the dynamics and regulation of cell contact phenomena. Our knowledge of the dynamic molecular mechanisms underlying cell contact phenomena remains fragmentary. Here we examine the molecular bases of cell contact phenomena using extant model developmental systems (representing a wide range of phyla) including the simplest i.e. sponges, and the eukaryotic protist Dictyostelium discoideum, the more complex Drosophila melanogaster, and vertebrate systems. We discuss cell contact phenomena in a broad developmental context. The molecular language of cell contact phenomena is complex; it involves a plethora of structurally and functionally diverse molecules, and diverse modes of intermolecular interactions mediated by protein and/or carbohydrate moieties. Reasons for this are presumably the necessity for a high degree of specificity of intermolecular interactions, the requirement for a multitude of different signals, and the apparent requirement for an increasingly large repertoire of cell contact molecules in more complex developmental systems, such as the developing vertebrate nervous system. However, comparison of molecular models for dynamic adhesion in sponges and in vertebrates indicates that, in spite of significant differences in the details of the way specific cell-cell adhesion is mediated, similar principles are involved in the mechanisms employed by members of disparate phyla. Universal requirements are likely to include (a) rapidly reversible intermolecular interactions; (b) low-affinity intermolecular interactions with fast on-off rates; (c) the compounding of multiple intermolecular interactions; (d) associated regulatory signalling systems. The apparent widespread employment of molecular mechanisms involving cadherin-like cell adhesion molecules suggests the fundamental importance of cadherin function during development, particularly in epithelial morphogenesis, cell sorting, and segregation of cells.

Drosophila contactin, a homolog of vertebrate contactin, is required for septate junction organization and paracellular barrier function

Septate junctions (SJs) in epithelial and neuronal cells play an important role in the formation and maintenance of charge and size selective barriers. They form the basis for the ensheathment of nerve fibers in Drosophila and for the attachment of myelin loops to axonal surface in vertebrates. The cell-adhesion molecules NRX IV/Caspr/Paranodin (NCP1),contactin and Neurofascin-155 (NF-155) are all present at the vertebrate axo-glial SJs. Mutational analyses have shown that vertebrate NCP1 and its Drosophila homolog, Neurexin IV (NRX IV) are required for the formation of SJs. In this study, we report the genetic, molecular and biochemical characterization of the Drosophila homolog of vertebrate contactin, CONT. Ultrastructural and dye-exclusion analyses of Contmutant embryos show that CONT is required for organization of SJs and paracellular barrier function. We show that CONT, Neuroglian (NRG)(Drosophila homolog of NF-155) and NRX IV are interdependent for their SJ localization and these proteins form a tripartite complex. Hence, our data provide evidence that the organization of SJs is dependent on the interactions between these highly conserved cell-adhesion molecules.

Shh signaling within the dental epithelium is necessary for cell proliferation, growth and polarization

Sonic hedgehog (Shh), a member of the mammalian Hedgehog (Hh) family, plays a key role during embryogenesis and organogenesis. Tooth development, odontogenesis, is governed by sequential and reciprocal epithelial-mesenchymal interactions. Genetic removal of Shh activity from the dental epithelium, the sole source of Shh during tooth development, alters tooth growth and cytological organization within both the dental epithelium and mesenchyme of the tooth. In this model it is not clear which aspects of the phenotype are the result of the direct action of Shh on a target tissue and which are indirect effects due to deficiencies in reciprocal signalings between the epithelial and mesenchymal components. To distinguish between these two alternatives and extend our understanding of Shh's actions in odontogenesis, we have used the Cre-loxP system to remove Smoothened (Smo) activity in the dental epithelium. Smo, a seven-pass membrane protein is essential for the transduction of all Hh signals. Hence, removal of Smo activity from the dental epithelium should block Shh signaling within dental epithelial derivatives while preserving normal mesenchymal signaling. Here we show that Shh-dependent interactions occur within the dental epithelium itself. The dental mesenchyme develops normally up until birth. In contrast, dental epithelial derivatives show altered proliferation, growth, differentiation and polarization. Our approach uncovers roles for Shh in controlling epithelial cell size, organelle development and polarization. Furthermore, we provide evidence that Shh signaling between ameloblasts and the overlying stratum intermedium may involve subcellular localization of Patched 2 and Gli1 mRNAs, both of which are targets of Shh signaling in these cells.

Induction of the IFN-gamma gene and protein is linked to the establishment of cell polarity in a porcine epithelial cell line

We report here original properties of a porcine trophectoderm cell line, TBA B4-3, that developed a polarized phenotype with high transepithelial electrical resistance (TER) values and functional tight junctions (TJs) when grown on a microporous membrane. We found that treatment of polarized TBA B4-3 cells with a strong protein kinase C (PKC) agonist, phorbol 12-myristate-13-acetate (PMA), induced 3-4 days later a transient interferon-gamma (IFN-gamma) mRNA expression and vectorial IFN-gamma protein secretion toward the apical side of the monolayer. Exposure of TBA B4-3 cells to PMA first resulted in a rapid and profound disorganization of the monolayer structure mainly characterized by the appearance of multilayered polyp-like foci structures, a strong decrease of the TER, and a increase of permeability correlated with changes in the organization and localization of the TJ-associated proteins (ZO-1 and occludin) and filamentous actin (f-actin). After PMA removal, spontaneous return to the initial polarized monolayer state occurred, characterized by TER rising to prestimulation values, TJ protein relocalization, and multilayered cell structures fading. This return was strictly correlated with transient IFN-gamma gene induction. Our report represents the first example of an inducible expression of IFN-gamma by a polarized epithelial cell. After PMA treatment, the close correlation between establishment of cell polarity and IFN-gamma gene expression suggests a link between these phenomena. This also suggests a novel biological mechanism by which transient and reversible disorganization of a polarized monolayer of epithelial cells could trigger regulated expression of a cytokine gene by these cells.

Control of growth and organ size in Drosophila

Transplantation experiments have shown that developing metazoan organs carry intrinsic information about their size and shape. Organ and body size are also sensitive to extrinsic cues provided by the environment, such as the availability of nutrients. The genetic and molecular pathways that contribute to animal size and shape are numerous, yet how they cooperate to control growth is mysterious. The recent identification and characterization of several mutations affecting growth in Drosophila melanogaster promises to provide insights. Many of these mutations affect the extrinsic control of animal size; others affect the organ-intrinsic control of pattern and size. In this review, we summarize the characteristics of some of these mutations and their roles in growth and size control. In addition, we speculate about possible connections between the extrinsic and intrinsic pathways controlling growth and pattern.

E-, N- and P-cadherin, and alpha-, beta- and gamma-catenin protein expression in normal, hyperplastic and carcinomatous human prostate

The expression of E-, N- and P-cadherin, alpha-, beta- and gamma-catenin, and actin was studied by immunohistochemistry, ELISA, and Western blot analysis in normal prostates, and in the prostates of men with benign prostatic hyperplasia and men with prostatic carcinoma, in order to evaluate their possible role in the pathogenesis of these diseases. Present results reveal that the immunophenotype of hyperplastic prostates differs from those of both normal and carcinomatous prostates in the intracellular distribution (observed by immunohistochemistry) and the intensity (measured by ELISA) of immunoreactions to cadherins, catenins, and actin. Hyperplastic prostates differ form normal prostates in the weaker immunoreaction to the three cadherin types, the two catenins, and actin, as well as in the intracellular distribution of P-cadherin, beta- and gamma-catenin, and actin. Differences between benign prostatic hyperplasia and prostatic carcinoma are less marked because hyperplastic prostates differ from carcinomatous prostates only in the weaker immunoreactions to P-cadherin, and alpha-catenin. The most remarkable findings in this study were: (1) alpha-catenin production was elevated in prostatic carcinoma in comparison with benign prostatic hyperplasia and normal prostate; and (2) P-cadherin expression in benign prostatic hyperplasia is reduced with regard to those of normal and carcinomatous prostates. It may be concluded that a decreased immunoreaction to cadherins, catenins, and actin, as well as changes in the intracellular distribution of actin in prostatic cells are not necessarily suggestive of malignancy, because these alterations are also present in BPH, and thus, the loss of cadherin-catenin-mediated adhesion alone is not sufficient to establish an invasive phenotype.

Mice devoid of fer protein-tyrosine kinase activity are viable and fertile but display reduced cortactin phosphorylation

The ubiquitous Fer protein-tyrosine kinase has been proposed to regulate diverse processes such as cell growth, cell adhesion, and neurite outgrowth. To gain insight into the biological function of Fer, we have targeted the fer locus with a kinase-inactivating missense mutation (fer(D743R)). Mice homozygous for this mutation develop normally, have no overt phenotypic differences from wild-type mice, and are fertile. Since these mice lack both Fer and the testis-specific FerT kinase activities, these proteins are clearly not essential for development and survival. No differences were observed in overall cellularity of bone marrow, spleen, or thymus in the absence of Fer activity. While most platelet-derived growth factor (PDGF)-induced tyrosine phosphorylation was unchanged in fer(D743R) homozygous embryonic fibroblasts, cortactin phosphorylation was reduced. However, Fer kinase activity was not required for PDGF-induced Stat3, p120(ctn), or epidermal growth factor (EGF)-induced beta-catenin phosphorylation. Also, no defects were observed in changes to the actin cytoskeleton, adherens junctions, or focal adhesions in PDGF- or EGF-stimulated fer(D743R) homozygous embryonic fibroblasts. Therefore, Fer likely serves a redundant role in regulating cell growth, cell adhesion, retinal development, and spermatogenesis but is required for efficient phosphorylation of cortactin.

Localization of proteins to the apico-lateral junctions of Drosophila epithelia

We have examined the distribution of proteins in the apico-lateral cell junctions in Drosophila imaginal discs. The subcellular distribution of these proteins in normal and mutant proliferating cells was analyzed with marker antibodies and confocal microscopy. Antibodies to phosphotyrosine (PY), Armadillo (Arm) and Drosophila E-cadherin (DE-cad) as well as FITC phalloidin marking filamentous actin, labeled the site of the adherens junction, whereas antibodies to Discs large (DIg), Fasciclin III (FasIII) and Coracle (Cor) labeled the more basal septate junction. The junctional proteins labeled by these antibodies underwent specific changes in distribution during the cell cycle. We have previously shown that a loss-of-function dlg mutation, which causes neoplastic imaginal disc overgrowth, leads to loss of the septate junctions and the formation of what appear to be ectopic adherens junctions [Woods et al., 1996]. We therefore extended this study to examine the effects of mutations in other genes that also cause imaginal disc overgrowth. Based on staining with PY and DIg antibodies, the apico-lateral junctional complexes appeared normal in tissue from the hyperplastic overgrowth mutants fat, dco, gd and wts. However, imaginal disc tissue from the neoplastic overgrowth mutants dlg and lgl showed abnormal distribution of the junctional markers including a complete loss of apico-basal polarity in loss-of-function dlg mutations. These results support the idea that some of the proteins of apico-lateral junctions are required both for apico-basal cell polarity and for the signalling mechanisms controlling cell proliferation, whereas others are required more specifically in cell-cell signalling.

Expanded functions in the apical cell domain to regulate the growth rate of imaginal discs

The Drosophila expanded (ex) gene encodes a product (Ex) that shares homology with the Protein 4.1 family of proteins, many of which are enriched at specific lateral cell junctions and the apical cellular domain. Ex colocalizes with actin in the apical domain of imaginal disc epithelial cells, where it partially overlaps the distribution of phosphotyrosine (PY)-containing proteins. This suggests that Ex is present in or associated with adherens junctions. Genetic studies show that Ex is necessary for proper regulation of final cell number in adult wings and for the formation of eyes, distal leg, and distal antennal segments. We have generated mitotic clones that lack Ex using the twin spot technique, and demonstrated that the primary function of Ex is to regulate cell proliferation. Overexpressing Ex protein results in a decrease in final cell number in wings, suggesting a direct relationship between Ex function and proliferation rate.

The tight junction protein ZO-1 and an interacting transcription factor regulate ErbB-2 expression

Epithelial tight junctions regulate paracellular diffusion and restrict the intermixing of apical and basolateral plasma membrane components. We now identify a Y-box transcription factor, ZONAB (ZO-1-associated nucleic acid-binding protein), that binds to the SH3 domain of ZO-1, a submembrane protein of tight junctions. ZONAB localizes to the nucleus and at tight junctions, and binds to sequences of specific promoters containing an inverted CCAAT box. In reporter assays, ZONAB and ZO-1 functionally interact in the regulation of the ErbB-2 promoter in a cell density-dependent manner. In stably transfected overexpressing cells, ZO-1 and ZONAB control expression of endogenous ErbB-2 and function in the regulation of paracellular permeability. These data indicate that tight junctions directly participate in the control of gene expression and suggest that they function in the regulation of epithelial cell differentiation.

Characterization of the gene encoding pinin/DRS/memA and evidence for its potential tumor suppressor function

Several cell adhesion-related proteins have been shown to act as tumor-suppressors (TS) in the neoplastic progression of epithelial-derived tumors. Pinin/DRS/memA was first identified in our laboratory and it was shown to be a cell adhesion-related molecule. Our previous study demonstrated that restoration of pinin expression in transformed cells not only positively influenced cellular adhesive properties but also reversed the transformed phenotype to more epithelial-like. Here, we show by FISH analysis that the gene locus for pinin is within 14q13. The alignment of the pinin gene with STS markers localized the gene to the previously identified TS locus D14S75-D14S288. Northern analyses revealed diminished pinin mRNA in renal cell carcinomas (RCC) and certain cancer cell lines. Immunohistochemical examination of tumor samples demonstrated absent or greatly reduced pinin in transitional cell carcinoma (TCC) and RCC tumors. TCC-derived J82 cells as well as EcR-293 cells transfected with full-length pinin cDNA demonstrated inhibition of anchorage-independent growth of cells in soft agar. Furthermore, methylation analyses revealed that aberrant methylation of pinin CpG islands was correlated with decreased/absent pinin expression in a subset of tumor tissues. These data lend significant support to the hypothesis that pinin/DRS/memA may act as a tumor suppressor in certain types of cancers.

Erythroblastic Synartesis: An Auto-immune Dyserythropoiesis

Erythroblastic synartesis is a rare form of acquired dyserythropoiesis, first described by Breton-Gorius et al in 1973. This syndrome is characterized by the presence of septate-like membrane junctions and “glove finger” invaginations between erythroblasts, which are very tightly linked together. This phenomenon, responsible for ineffective erythropoiesis, leads to an isolated severe anemia with reticulocytopenia. In the following report, we describe 3 new cases of erythroblastic synartesis associated with dysimmunity and monoclonal gammapathy. In all cases, the diagnosis was suggested by characteristic morphological appearance of bone marrow smears, and further confirmed by electron microscopy. Ultrastructural examination of abnormal erythroblast clusters showed that these cells were closely approximated with characteristic intercellular membrane junctions. The pathogenesis of the dyserythropoiesis was modeled in vitro using crossed erythroblast cultures and immunoelectron microscopy: when cultured in the presence of autologous serum, the erythroblasts from the patients displayed synartesis, whereas these disappeared when cultured in normal serum. Moreover, synartesis of normal erythroblasts were induced by the patient IgG fraction. Immunogold labeling showed that the monoclonal IgG were detected in, and restricted to, the synartesis. A discrete monoclonal plasmacytosis was also found in the patient bone marrow. The adhesion receptor CD36 appeared to be concentrated in the junctions, suggesting that it might be involved in the synartesis. These experiments indicated that a monoclonal serum immunoglobulin (IgG in the present cases) directed at erythroblast membrane antigen was responsible for the erythroblast abnormalities. Specific therapy of the underlying lymphoproliferation was followed by complete remission of the anemia in these cases.

Relationship between Na(+),K(+)-ATPase and cell attachment

A prolonged ouabain blockade of the Na(+),K(+)-ATPase detaches cells from each other and from the substrate. This suggests the existence of a link between pump (P) and attachment (A). In the present work, we report that MDCK-W cells treated with ouabain increase tyrosine phosphorylation and content of active MAP kinase, redistribute molecules involved in cell attachment (occludin, ZO-1, desmoplakin, cytokeratin, alpha-actinin, vinculin and actin), and detach. Genistein and UO126, inhibitors of protein tyrosine kinase and of MAP kinase kinase, respectively, block this detachment. The content of P190(Rho-GAP), a GTPase activating protein of the Rho small G-protein subfamily, is increased by ouabain, suggesting that both the Rho/Rac and MAPK pathways are involved. Another clone of MDCK cells whose Na(+),K(+)-ATPase has a negligible affinity for the drug, show none of the effects described for MDCK-W and remain attached. Ma104 cells, a line that has a high affinity for ouabain and stops pumping, fail to modify phosphorylation, as well as the pattern of distribution of attaching molecules, and remain in the monolayer. Taken together, these results suggest that there is a mechanism (P-->A) that transduces a blockade of the pump in a detachment of the cell from neighbors and substrate, in which Ma104 cells are faulty.

Erythroblastic Synartesis: An Auto-immune Dyserythropoiesis

Erythroblastic synartesis is a rare form of acquired dyserythropoiesis, first described by Breton-Gorius et al in 1973. This syndrome is characterized by the presence of septate-like membrane junctions and “glove finger” invaginations between erythroblasts, which are very tightly linked together. This phenomenon, responsible for ineffective erythropoiesis, leads to an isolated severe anemia with reticulocytopenia. In the following report, we describe 3 new cases of erythroblastic synartesis associated with dysimmunity and monoclonal gammapathy. In all cases, the diagnosis was suggested by characteristic morphological appearance of bone marrow smears, and further confirmed by electron microscopy. Ultrastructural examination of abnormal erythroblast clusters showed that these cells were closely approximated with characteristic intercellular membrane junctions. The pathogenesis of the dyserythropoiesis was modeled in vitro using crossed erythroblast cultures and immunoelectron microscopy: when cultured in the presence of autologous serum, the erythroblasts from the patients displayed synartesis, whereas these disappeared when cultured in normal serum. Moreover, synartesis of normal erythroblasts were induced by the patient IgG fraction. Immunogold labeling showed that the monoclonal IgG were detected in, and restricted to, the synartesis. A discrete monoclonal plasmacytosis was also found in the patient bone marrow. The adhesion receptor CD36 appeared to be concentrated in the junctions, suggesting that it might be involved in the synartesis. These experiments indicated that a monoclonal serum immunoglobulin (IgG in the present cases) directed at erythroblast membrane antigen was responsible for the erythroblast abnormalities. Specific therapy of the underlying lymphoproliferation was followed by complete remission of the anemia in these cases.

A recessive mutation leading to vertebral ankylosis in zebrafish is associated with amino acid alterations in the homologue of the human membrane-associated guanylate kinase DLG3

We describe the characterization of the zebrafish homologue of the human gene DLG3. The zebrafish dlg3 gene encodes a membrane-associated guanylate kinase containing a single PDZ domain. This gene was cloned using a gene-trap construct inserted in the gene's first intron. The insertion co-segregates with a viable mutation called humpback (hmp), which leads to formation of ankylotic vertebrae in adult fishes. Insertion and mutation have both been mapped to chromosome 12, in a segment which is syntenic with region p12 to q12 of human chromosome 17. The hmp mutant phenotype, however, appears to be due to two point mutations in the guanylate kinase domain rather than to the transgene insertion itself. The results of this study are discussed in the light of the possible function of the guanylate kinase domain.

Ubiquitin-dependent degradation of IkappaBalpha is mediated by a ubiquitin ligase Skp1/Cul 1/F-box protein FWD1

Activation of the transcription factor nuclear factor kappa B (NF-kappaB) is controlled by proteolysis of its inhibitory subunit (IkappaB) via the ubiquitin-proteasome pathway. Signal-induced phosphorylation of IkappaBalpha by a large multisubunit complex containing IkappaB kinases is a prerequisite for ubiquitination. Here, we show that FWD1 (a mouse homologue of Slimb/betaTrCP), a member of the F-box/WD40-repeat proteins, is associated specifically with IkappaBalpha only when IkappaBalpha is phosphorylated. The introduction of FWD1 into cells significantly promotes ubiquitination and degradation of IkappaBalpha in concert with IkappaB kinases, resulting in nuclear translocation of NF-kappaB. In addition, FWD1 strikingly evoked the ubiquitination of IkappaBalpha in the in vitro system. In contrast, a dominant-negative form of FWD1 inhibits the ubiquitination, leading to stabilization of IkappaBalpha. These results suggest that the substrate-specific degradation of IkappaBalpha is mediated by a Skp1/Cull 1/F-box protein (SCF) FWD1 ubiquitin-ligase complex and that FWD1 serves as an intracellular receptor for phosphorylated IkappaBalpha. Skp1/Cullin/F-box protein FWD1 might play a critical role in transcriptional regulation of NF-kappaB through control of IkappaB protein stability.

The interaction of intestinal epithelial cells and intraepithelial lymphocytes in host defense

Intestinal intraepithelial lymphocytes (i-IEL) are located at the basolateral surfaces of intestinal epithelial cells (i-EC) and play important roles in the homeostasis of intestinal microenvironment. i-IEL comprise unique T cell populations including CD4-CD8alphaalpha+ T cells expressing T cell receptor (TCR)alphabeta or TCRgammadelta and CD4+ CD8alphaalpha+ T cells expressing TCR alphabeta. We show here that CD4+ CD8alphaalpha+ i-IEL belongs to Th1 type T cells capable of responding to self-MHC class I on i-EC and that a significant fraction of i-IEL expressed Fas ligand (Fas-L) and induced apoptosis in the i-EC via Fas-dependent pathway. i-IEL may recognize and eliminate the effete i-EC for homeostatic regulation of intestinal epithelia. The interaction of i-EC and i-IEL through E-cadherin/alphaEbeta7 integrin is important for homing and maintenance of i-IEL in intestine. Listeria monocytogenes are also known to interact with E-cadherin on i-EC and invade into the epithelial cells. Invasion of L. monocytogenes into i-EC activated NFkappa-B and subsequently up-regulated the expression of IL-15 gene, which has a NFkappa-B binding site at the promoter region. i-IEL, especially gammadelta T cells, were significantly activated to produce Th1 type cytokines at the early stage after oral infection with L. monocytogenes in mice and rats. The activation of i-IEL coincided with a peak response of IL-15 production by i-EC after infection. Taken together, mutual interaction of i-IEL and i-EC may be important not only for homeostatic regulation but also host defense against microbial infection in intestine.

Interleukin-15 may be responsible for early activation of intestinal intraepithelial lymphocytes after oral infection with Listeria monocytogenes in rats

Exogenous interleukin-15 (IL-15) stimulates intestinal intraepithelial lymphocytes (i-IEL) from mice to proliferate and produce gamma interferon (IFN-gamma) in vitro. To determine whether endogenous IL-15 is involved in activation of i-IEL during intestinal infection, we examined IL-15 synthesis by intestinal epithelial cells (i-EC) after infection with Listeria monocytogenes in rats. In in vitro experiments, invasion of L. monocytogenes into IEC-6 cells, a rat small intestine epithelial cell line, evidently induced IL-15 mRNA expression coincident with nuclear factor kappaB (NF-kappaB) activation, which is essential for IL-15 gene expression. IL-15 synthesis was detected in rat i-EC on day 1 after an oral inoculation of L. monocytogenes in vivo. The numbers of T-cell receptor (TCR) gamma delta+ T cells, NKR.P1(+) cells, and CD3(+) CD8(+) alpha alpha cells in i-IEL were significantly increased on day 1 after oral infection. The i-IEL from infected rats produced larger amounts of IFN-gamma upon stimulation with immobilized anti-TCR gamma delta or anti-NKR.P1 monoclonal antibodies. These results suggest that IL-15 produced by i-EC may stimulate significant fractions of i-IEL to produce IFN-gamma at an early phase of oral infection with L. monocytogenes.

CNK, a RAF-binding multidomain protein required for RAS signaling

Kinase suppressor of ras (ksr) is required for efficient signal transmission within the RAS/MAPK cascade. A screen for mutations that modify a ksr-dependent phenotype identified a novel gene, connector enhancer of ksr (cnk), that functions upstream or in parallel to RAF in the RAS pathway. cnk encodes a protein containing several protein-protein interaction domains, suggesting that it brings different signaling molecules together. CNK is required in multiple receptor tyrosine kinase pathways where it appears to be a tyrosine phosphorylation target. Finally, CNK physically interacts with RAF and appears to localize to cell-cell contact regions. Together, these findings suggest that CNK is a novel component of a RAS-dependent signaling pathway that regulates RAF function and/or targets RAF to a specific subcellular compartment upon RAS activation.

Involvement of the tyrosine kinase fer in cell adhesion

The Fer protein belongs to the fes/fps family of nontransmembrane receptor tyrosine kinases. Lack of success in attempts to establish a permanent cell line overexpressing it at significant levels suggested a strong negative selection against too much Fer protein and pointed to a critical cellular function for Fer. Using a tetracycline-regulatable expression system, overexpression of Fer in embryonic fibroblasts was shown to evoke a massive rounding up, and the subsequent detachment of the cells from the substratum, which eventually led to cell death. Induction of Fer expression coincided with increased complex formation between Fer and the cadherin/src-associated substrate p120(cas) and elevated tyrosine phosphorylation of p120(cas). beta-Catenin also exhibited clearly increased phosphotyrosine levels, and Fer and beta-catenin were found to be in complex. Significantly, although the levels of alpha-catenin, beta-catenin, and E-cadherin were unaffected by Fer overexpression, decreased amounts of alpha-catenin and beta-catenin were coimmunoprecipitated with E-cadherin, demonstrating a dissolution of adherens junction complexes. A concomitant decrease in levels of phosphotyrosine in the focal adhesion-associated protein p130 was also observed. Together, these results provide a mechanism for explaining the phenotype of cells overexpressing Fer and indicate that the Fer tyrosine kinase has a function in the regulation of cell-cell adhesion.

Fibroblast-like cells from tendons differ from skin fibroblasts in their ability to form three-dimensional structures in vitro

Tendon samples cultured in vitro produced cells (TC) with fibroblast-like morphology and confluence occurred within 5 weeks. Histological staining demonstrated proteoglycan and collagen secretion by TC. Immunohistochemical staining revealed type I collagen but no type III. Assay of total collagen demonstrated a rapid increase in synthesis with time in culture. Cultures allowed to become 'superconfluent' spontaneously formed three-dimensional structures after about 4 weeks, which became macroscopic, tendon-like structures (TLS). Cells within TLS seemed under cell-generated tension. Haematoxylin and eosin staining of sections of tendon, of TLS and of TC cultures demonstrated similarities in morphology. These studies were performed using human and rabbit cells and findings were similar for the two species, but with some differences in cell metabolism. Skin fibroblasts were also cultured as a comparison.

Physical connections between feeder cells and recipient normal mammary epithelial cells

Cells of the LA7 rat mammary tumor line stimulate proliferation of normal mouse mammary epithelial cells in culture when in direct physical contact with them. We examined junctional connections between these LA7 feeders and the recipient mouse mammary epithelial cells in order to study the role these junctions may play in growth signaling. Tight junctions and desmosomes between LA7 feeders and mouse mammary cells were detected by immunocytochemistry. These junctions connected every cell of either type with each of its neighbors. Adherens junctions, although evident between mouse mammary cells, could not be detected between LA7 and mouse mammary cells or between LA7 cells themselves. However, E-cadherin, the transmembrane protein of adherens junctions, was present in LA7 cell lysates. beta-catenin, which normally binds cadherins, was detected at the borders of LA7 cells. Presence of gap junctions between LA7 and mouse mammary cells was determined by traverse of lucifer yellow from an injected LA7 cell to surrounding mouse mammary epithelial cells. The experiments thus indicate that three types of intercellular junctions occur between cells active in direct cell-cell-stimulated proliferation signaling.

Direct association of presenilin-1 with beta-catenin

Families bearing mutations in the presenilin-1 (PSI) gene develop Alzheimer's disease (AD). However, the mechanism through which PS1 causes AD is unclear. The co-immunoprecipitation with PS1 in transfected COS-7 cells indicates that PSI directly interacts with endogenous beta-catenin, and the interaction requires residues 322450 of PSI and 445-676 of beta-catenin. Both proteins are co-localized in the endoplasmic reticulum. Over-expression of PS1 reduces the level of cytoplasmic beta-catenin, and inhibits beta-catenin-T cell factor-regulated transcription. These results indicate that PSI plays a role as inhibitor of the beta-catenin signal, which may be connected with the AD dysfunction.

E-cadherin and alpha-, beta-, and gamma-catenin protein expression in relation to metastasis in human breast carcinoma

In the metastatic process, various cell-cell adhesion molecules seem to play an important role. E-cadherin, a transmembrane protein with an extracellular and an intracellular domain, is one of the key players involved in cell-cell adhesion. The function of E-cadherin in preventing metastasis in tumour development is believed to be dependent on intracellular catenins. In a previous study, the expression of E-cadherin was examined in a series of human breast carcinomas. In that study, down-regulation of E-cadherin failed to correlate with lymph node and/or distant metastasis. In the present study, the expression of alpha-, beta-, and gamma-catenins has been examined in a subset of the same tumours in order to evaluate their possible role in breast cancer metastasis. Tumour tissues from 90 primary breast carcinomas were immunostained for alpha-, beta-, and gamma-catenins. Reduced or absent immunoreactivity in the tumour tissue was seen in 63 (70.0 per cent) for alpha-catenin, in 50 (55.6 per cent) for beta-catenin, and in 50 (55.6 per cent) for gamma-catenin. Reduced expression of each of the catenins alone failed to correlate to metastasis. However, when all of the four proteins (E-cadherin, alpha-catenin, beta-catenin, and gamma-catenin) were analysed as one group, a significant association was seen between reduction in immunoreactivity of at least one of these four proteins and the presence of metastases. These results indicate that if one of these proteins is down-regulated, the function of the others in suppressing metastasis is altered. A significant association was seen between lobular invasive tumours and beta-catenin expression.

Tight junctions

Tight junctions are the most apical intercellular junctions of epithelial and endothelial cells and create a regulatable semipermeable diffusion barrier between individual cells. On a cellular level, they form an intramembrane diffusion fence that restricts the intermixing of apical and basolateral membrane components. In addition to these well defined functions, more recent evidence suggests that tight junctions are also involved in basic cellular processes like the regulation of cell growth and differentiation.

Reduced gene expression of E-cadherin and associated catenins in human cervical carcinoma cell lines

Cell-cell adhesion mediated by E-cadherin is often lost or disturbed in human carcinomas. For regular adhesive function, E-cadherin has to form complexes with peripheral cytoplasmic catenins which are multifunctional proteins that are also involved in signal transduction and growth regulation. We have analyzed the expression levels of the genes encoding alpha-catenin, beta-catenin and plakoglobin in correlation to the E-cadherin expression levels in cell lines derived from human cervical carcinomas. Reduced mRNA and protein levels were detected for plakoglobin, whereas alpha- and beta-catenin showed only reduced protein (but not mRNA) levels. The alterations in catenin gene expression were often associated with absent or reduced E-cadherin. The findings indicate that a reduction of catenin gene expression may contribute to the development of cervical carcinomas.

Putative mechanism through which N-cadherin-mediated cell contact maintains calcium homeostasis and thereby prevents ovarian cells from undergoing apoptosis

To date most of the studies involving the maintenance of ovarian cell viability have focused on the endocrine, paracrine, and autocrine factors that inhibit these cells from undergoing programmed cell death or apoptosis. Recently, studies have demonstrated that cell contact also prevents ovarian cells from dying via an apoptotic mechanism. In this commentary, the role that homophilic binding of the cell adhesion molecule, N-cadherin, plays in maintaining ovarian cell viability is presented. These studies showed that N-cadherin homophilic binding (1) is part of the mechanism through which cell contact maintains cell viability, (2) results in the activation (i.e. tyrosine phosphorylation) of the fibroblast growth factor (FGF) receptor, and (3) prevents a sustained elevation in intracellular free calcium ([Ca2+]i) which triggers apoptosis. These studies also revealed that hepatocyte growth factor (HGF), also known as scatter factor (SF), disrupts cell contact, which leads to a sustained increase in [Ca2+]i levels and ultimately to cell death. Based on these studies, this commentary presents a putative mechanism that relates the cellular and molecular mechanism through which basic FGF, N-cadherin, and HGF/SF interact to regulate [Ca2+]i levels and ultimately ovarian cell survival.

Characterization of a cDNA encoding a novel band 4.1-like protein in zebrafish

Membrane skeleton protein 4.1 and other members of a family of proteins that link the cytoskeleton to the plasma membrane may play an integral role in cell communication during development. The polymerase chain reaction and degenerate oligodeoxynucleotide primers to consensus sequences in the putative membrane-binding domain of the protein 4.1 superfamily were used to isolate cDNAs encoding members of the zebrafish protein 4.1 family. Zebrafish stage- and tissue-specific first strand cDNA was used in the PCR. After the reaction, amplicons of the predicted size were sequenced to confirm their relationship to the protein 4.1 superfamily. One cDNA, with a high degree of similarity to a mouse novel band 4.1-like cDNA, was used to probe a zebrafish adult brain library. A 2.4-kb cDNA was isolated and found to encode a 619 amino acid polypeptide homologous to mouse novel band 4.1-like protein 4. Zebrafish nbl4 mRNA is maternally supplied and is expressed throughout embryogenesis. In adults, nbl4 is found in the ovary, eye, heart, and brain, but not in gut or skeletal muscle. When synthetic nbl4 mRNA is translated in vitro it binds calmodulin in a calcium-dependent manner. These data indicate that zebrafish nbl4 is a maternal transcript owing to its presence before the midblastula transition, and it is present later on in specific adult structures. The ability to bind calmodulin would suggest that the function of nbl4 protein may be potentially regulated via a calcium-calmodulin dependent mechanism.

Increased beta-catenin protein and somatic APC mutations in sporadic aggressive fibromatoses (desmoid tumors)

Sporadic aggressive fibromatosis (also called desmoid tumor) is a monoclonal proliferation of spindle (fibrocyte-like) cells that is locally invasive but does not metastasize. A similarity to abdominal fibromatoses (desmoids) in familial adenomatous polyposis and a cytogenetic study showing partial deletion of 5q in a subset of aggressive fibromatoses suggests that the adenomatous polyposis coli (APC) gene plays a role in its pathogenesis. APC helps regulate the cellular level of beta-catenin, which is a downstream mediator in Wnt (Wingless) signaling. beta-Catenin has a nuclear function (binds transcription factors) and a cell membrane function (is a component of epithelial cell adherens junctions). Six cases of aggressive fibromatosis of the extremities from patients without familial adenomatous polyposis, or a family history of colon cancer, were studied. Immunohistochemistry, using carboxy and amino terminus antibodies to APC, and DNA sequencing showed that three of the six contained an APC-truncating mutation, whereas normal tissues did not contain a mutation. Western blot and Northern dot blot showed that all six tumors had a higher level of beta-catenin protein than surrounding normal tissues, despite containing similar levels of beta-catenin mRNA. Immunohistochemistry localized beta-catenin throughout the cell in tumor tissues, although it localized more to the periphery in cells from normal tissues. Reverse transcription polymerase chain reaction showed that the tumors expressed N-cadherin but not E-cadherin (a pattern of expression of proteins making up adherens junctions similar to fibrocytes), suggesting that the specific adherens junctions present in epithelial cells are not necessary for beta-catenin function. Increased beta-catenin may cause the growth advantage of cells in this tumor through a nuclear mechanism. The increased protein level, relative to the RNA level, suggests that beta-catenin is degraded at a lower rate compared with normal tissues. In some cases, this is caused by a somatic mutation resulting in a truncated APC protein.

Differential effects of EGF and amphiregulin on adhesion molecule expression and migration of colon carcinoma cells

Epidermal growth factor (EGF) is a potent morphogen affecting cell shape and motility through regulation of adhesive interactions. We have characterized the morphological effects of EGF on GP2d and GP5d colon carcinoma cell lines and have compared the ability of the heparin-binding EGF receptor ligand amphiregulin (AR) to elicit the same effects. EGF induced a marked epithelial-mesenchymal transition in both cell lines. This effect was evident at 7 pM EGF and was associated with a reduction in cellular adherens junctions and diminished cell-cell contact; it was also associated with an increase in expression of alpha2-integrin as well as enhanced adhesion to the substratum and cell spreading. These changes in adhesion molecule expression were accompanied by enhanced migration on collagen. Blockade of cell growth with mitomycin C did not prevent the EGF-induced morphological change, showing that the mitogenic and morphogenic responses of the GP cells were separable. The phosphatidyl inositol (PI) 3-kinase inhibitor wortmannin inhibited basal proliferation but had no effect on the EGF-induced morphological change, further suggesting that the PI 3-kinase pathway was not involved in the morphogenic response of these cells. Amphiregulin stimulated proliferation of both cell lines, but could only elicit a modest morphological change if used at considerably higher doses or if growth was blocked with mitomycin C. In cells treated with 55 nM AR, alpha2-integrin expression was slightly increased; however, unlike the EGF case, adherens junctions remained intact. These differences in the ability of EGF and amphiregulin to affect cellular adhesion and migration may be significant factors influencing normal and tumor cell behavior.

beta-catenin is a target for the ubiquitin-proteasome pathway

beta-catenin is a central component of the cadherin cell adhesion complex and plays an essential role in the Wingless/Wnt signaling pathway. In the current model of this pathway, the amount of beta-catenin (or its invertebrate homolog Armadillo) is tightly regulated and its steady-state level outside the cadherin-catenin complex is low in the absence of Wingless/Wnt signal. Here we show that the ubiquitin-dependent proteolysis system is involved in the regulation of beta-catenin turnover. beta-catenin, but not E-cadherin, p120(cas) or alpha-catenin, becomes stabilized when proteasome-mediated proteolysis is inhibited and this leads to the accumulation of multi-ubiquitinated forms of beta-catenin. Mutagenesis experiments demonstrate that substitution of the serine residues in the glycogen synthase kinase 3beta (GSK3beta) phosphorylation consensus motif of beta-catenin inhibits ubiquitination and results in stabilization of the protein. This motif in beta-catenin resembles a motif in IkappaB (inhibitor of NFkappaB) which is required for the phosphorylation-dependent degradation of IkappaB via the ubiquitin-proteasome pathway. We show that ubiquitination of beta-catenin is greatly reduced in Wnt-expressing cells, providing the first evidence that the ubiquitin-proteasome degradation pathway may act downstream of GSK3beta in the regulation of beta-catenin.

The SH3 domain of the tight junction protein ZO-1 binds to a serine protein kinase that phosphorylates a region C-terminal to this domain

ZO-1 is a tight junction phosphoprotein partially homologous to a tumor suppressor in Drosophila. The homologous region contains an SH3 domain with an unidentified function. Using fusion proteins containing the SH3 domain and various N- and C-terminal sequences, we tested for association of a kinase with this protein domain in extracts of MDCK cells. We show that the SH3 domain of ZO-1 binds a serine protein kinase that phosphorylates a region immediately C-terminal to the SH3 domain. This kinase associates specifically with the SH3 domain of ZO-1 and appears to be also associated with junctional complexes extracted from MDCK cells.

Evidence for regulated dimerization of cell-cell adhesion molecule (C-CAM) in epithelial cells

C-CAM is a Ca(2+)-independent cell adhesion molecule (CAM) belonging to the immunoglobulin superfamily. Addition of chemical cross-linkers to isolated rat liver plasma membranes, intact epithelial cells and purified preparations of C-CAM stabilized one major C-CAM-containing product whose apparent molecular mass was approximately twice that of the C-CAM monomer. The failure to detect additional proteins after cleavage of the cross-linked species demonstrated that C-CAM exists as non-covalently linked dimers both in solution and on the cell surface. Dimerization occurred to the same extent in adherent monolayers and in single cell populations, indicating that dimer formation was the result of cis-interactions within the membranes of individual cells. Using isoform-specific anti-peptide antibodies, both C-CAM1 and C-CAM2 were found to be involved in dimerization, forming predominantly homo-dimeric species. Both calmodulin and Ca2+ ionophore modulated the level of dimer formation, suggesting a role for regulated self-association in the functional activity of C-CAM.

Drosophila alpha-catenin and E-cadherin bind to distinct regions of Drosophila Armadillo

Adherens junctions are multiprotein complexes mediating cell-cell adhesion and communication. They are organized around a transmembrane cadherin, which binds a set of cytoplasmic proteins required for adhesion and to link the complex to the actin cytoskeleton. Three components of Drosophila adherens junctions, analogous to those in vertebrates, have been identified: Armadillo (homolog of beta-catenin), Drosophila E-cadherin (DE-cadherin), and alpha-catenin. We carried out the first analysis of the interactions between these proteins using in vitro binding assays, the yeast two-hybrid system, and in vivo assays. We identified a 76-amino acid region of Armadillo that is necessary and sufficient for binding alpha-catenin and found that the N-terminal 258 amino acids of alpha-catenin interact with Armadillo. A large region of Armadillo, spanning six central Armadillo repeats, is required for DE-cadherin binding, whereas only 41 amino acids of the DE-cadherin cytoplasmic tail are sufficient for Armadillo binding. Our data complement and extend results obtained in studies of vertebrate adherens junctions, providing a foundation for understanding how junctional proteins assemble and a basis for interpreting existing mutations and creating new ones.

A Drosophila neurexin is required for septate junction and blood-nerve barrier formation and function

Septate and tight junctions are thought to seal neighboring cells together and to function as barriers between epithelial cells. We have characterized a novel member of the neurexin family, Neurexin IV (NRX), which is localized to septate junctions (SJs) of epithelial and glial cells. NRX is a transmembrane protein with a cytoplasmic domain homologous to glycophorin C, a protein required for anchoring protein 4.1 in the red blood cell. Absence of NRX results in mislocalization of Coracle, a Drosophila protein 4.1 homolog, at SJs and causes dorsal closure defects similar to those observed in coracle mutants. nrx mutant embryos are paralyzed, and electrophysiological studies indicate that the lack of NRX in glial-glial SJs causes a breakdown of the blood-brain barrier. Electron microscopy demonstrates that nrx mutants lack the ladder-like intercellular septa characteristic of pleated SJs (pSJs). These studies identify NRX as the first transmembrane protein of SJ and demonstrate a requirement for NRX in the formation of septate-junction septa and intercellular barriers.

Differential targeting of T- and N-cadherin in polarized epithelial cells

To test whether glycosyl phosphatidylinositol-linked T-cadherin is a component of cell junctions like classical cadherins, we have examined its distribution and targeting in polarized epithelial cells. In vivo, T-cadherin was detected on the apical cell surface of the chick intestinal epithelium. In cultures of transfected Madin-Darby canine kidney cells, T-cadherin was also expressed apically, whereas classical N-cadherin resided basolaterally. Both cadherins were directly targeted to their respective membrane domains. Mutant proteins were expressed in Madin-Darby canine kidney cells to identify the regions responsible for differential cadherin localization. NDeltacyt, an N-cadherin cytoplasmic domain deletion mutant, was stably distributed basolaterally. This mutant was transported to both the apical and basolateral membrane compartments, followed by preferential removal from the apical surface. T-NDeltacyt, a T-cadherin mutant with the N-cadherin cytoplasmic domain deletion, was localized basolaterally, whereas N-TGPI, a GPI-anchored N-cadherin mutant, resided at the apical domain. The T-cadherin carboxyl-terminal 76 amino acids contain the apical targeting signal and include the signal for GPI anchor attachment. Basolateral localization of N-cadherin is achieved through targeting signals in the cytoplasmic domain. Thus, GPI-linked T-cadherin is not a component of cell junctions, consistent with a function as a recognition rather than a cell adhesion molecule.

Ultrastructural localization of Shaker-related potassium channel subunits and synapse-associated protein 90 to septate-like junctions in rat cerebellar Pinceaux

The Pinceau is a paintbrush-like network of cerebellar basket cell axon branchlets embracing the initial segment of the Purkinje cell axon. Its electrical activity contributes to the control of the cerebellar cortical output through the Purkinje cell axon by generating an inhibitory field effect. In addition to the structural features of the Pinceau, its repertoire of voltage-gated ion channels is likely to be an important aspect of this function. Therefore, we investigated the fine structural distribution of voltage-activated potassium (Kv1.1, Kv1.2, Kv3.4) and sodium channel proteins in the Pinceau. The ultrastructural localization of potassium channel subunits was compared to the distribution of synapse-associated protein 90 (SAP90), a protein capable to induce in vitro clustering of Kv1 proteins. With an improved preembedding technique including ultrasmall gold particles, silver enhancement and gold toning, we could show that antibodies recognizing Kv1.1, Kv1.2 and SAP90 are predominantly localized to septate-like junctions, which connect the basket cell axonal branchlets. Kv3.4 immunoreactivity is not concentrated in junctional regions but uniformly distributed over the Pinceau and the pericellular basket surrounding the Purkinje cell soma. In contrast, voltage-activated sodium channels were not detected in the Pinceau, but localized to the Purkinje cell axon initial segment. The results suggest that Kv1.1 and Kv1.2 form heterooligomeric delayed rectifier type Kv channels, being colocalized to septate-like junctions by interaction with SAP90.

The role of segment polarity genes during early oogenesis in Drosophila

In the Drosophila ovary, hedgehog (hh) signaling from cells near the apical tip of the germarium stimulates the proliferation and specification of somatic cells in region 2 of the germarium, 2–5 cells away from the hh-expressing cells (A. J. Forbes, H. Lin, P. Ingham and A. Spradling (1996) Development 122, 1125–1135). This report examines the role during early oogenesis of several genes that are known to function in hh-mediated signaling during embryonic and larval development (P. Ingham (1995) Current Opin. Genetics Dev. 5, 528–534). As in imaginal discs, engrailed (en) is co-expressed with hh in the germarium, while patched (ptc) and cubitus interruptus (ci) are expressed in somatic cells throughout the germarium and in developing egg chambers, with ptc expression being elevated within 10 cell diameters of the source of the hh signal. Moreover, the somatic cell overproliferation caused by ectopic hh expression is accompanied by elevated levels of ptc and is phenocopied in ptc- somatic clones. These analyses suggest that ptc and ci are components of the hh signaling pathway in the germarium. However, unlike embryos and imaginal discs, neither wingless (wg) nor decapentaplegic (dpp) appear to mediate the ovarian hh signal. wg is expressed in ‘cap cells,’ a subset of hh-expressing cells located adjacent to germ-line stem cells, but is unaffected by ectopic hh expression. Nor does the ectopic expression of wg or dpp mimic the effect of ectopic hh expression. We propose that Hh diffuses from apical cells, including cap cells, and regulates the proliferation of nearby ovarian somatic cells by antagonizing the negative effects of ptc on ci activity in these cells, thereby allowing the transcription of ci-dependent genes, including ptc itself.

Association between a transmembrane protein tyrosine phosphatase and the cadherin-catenin complex

Cadherins are calcium-dependent cell adhesion molecules that play fundamental roles in embryonic development, tissue morphogenesis, and cancer. A prerequisite for their function is association with the actin cytoskeleton via the catenins. Tyrosine phosphorylation of beta-catenin, which correlates with a reduction in cadherin-dependent cell adhesion, may provide cells with a mechanism to regulate cadherin activity. Here we report that beta-catenin immune precipitates from PC12 cells contain tyrosine phosphatase activity which dephosphorylates beta-catenin in vitro. In addition, we show that a member of the leukocyte antigen-related protein (LAR)-related transmembrane tyrosine phosphatase family (LAR-PTP) associates with the cadherin-catenin complex. This association required the amino-terminal domain of beta-catenin but does not require the armadillo repeats, which mediate association with cadherins. The interaction also is detected in PC9 cells, which lack alpha-catenin. Thus, the association is not mediated by alpha-catenin or by cadherins. Interestingly, LAR-PTPs are phosphorylated on tyrosine in a TrkA-dependent manner, and their association with the cadherin-catenin complex is reduced in cells treated with NGF. We propose that changes in tyrosine phosphorylation of beta-catenin mediated by TrkA and LAR-PTPs control cadherin adhesive function during processes such as neurite outgrowth.

An in vivo structure-function study of armadillo, the beta-catenin homologue, reveals both separate and overlapping regions of the protein required for cell adhesion and for wingless signaling

Armadillo, the Drosophila homologue of vertebrate beta-catenin, plays a pivotal role both in Wingless signaling and in assembly of adherens junctions. We performed the first in vivo structure-function study of an adherens junction protein, by generating and examining a series of Armadillo mutants in the context of the entire animal. We tested each mutant by assaying its biological function, its ability to bind proteins that normally associate with Armadillo in adherens junctions, its cellular localization, and its pattern of phosphorylation. We mapped the binding sites for DE-cadherin and alpha-catenin. Although these bind to Armadillo independently of each other, binding of each is required for the function of adherens junctions. We identified two separate regions of Armadillo critical for Wingless signaling. We demonstrated that endogenous Armadillo accumulates in the nucleus and provide evidence that it may act there in transducing Wingless signal. We found that the Arm repeats, which make up the central two-thirds of Armadillo, differ among themselves in their functional importance in different processes. Finally, we demonstrated that Armadillo's roles in adherens junctions and Wingless signaling are independent. We discuss the potential biochemical role of Armadillo in each process.

armadillo, bazooka, and stardust are critical for early stages in formation of the zonula adherens and maintenance of the polarized blastoderm epithelium in Drosophila

Cellularization of the Drosophila embryo results in the formation of a cell monolayer with many characteristics of a polarized epithelium. We have used antibodies specific to cellular junctions and nascent plasma membranes to study the formation of the zonula adherens (ZA) in relation to the establishment of basolateral membrane polarity. The same approach was then used as a test system to identify X-linked zygotically active genes required for ZA formation. We show that ZA formation begins during cellularization and that the basolateral membrane domain is established at mid-gastrulation. By creating deficiencies for defined regions of the X chromosome, we have identified genes that are required for the formation of the ZA and the generation of basolateral membrane polarity. We show that embryos mutant for both stardust (sdt) and bazooka (baz) fail to form a ZA. In addition to the failure to establish the ZA, the formation of the monolayered epithelium is disrupted after cellularization, resulting in formation of a multilayered cell sheet by mid-gastrulation. SEM analysis of mutant embryos revealed a conversion of cells exhibiting epithelial characteristics into cells exhibiting mesenchymal characteristics. To investigate how mutations that affect an integral component of the ZA itself influence ZA formation, we examined embryos with reduced maternal and zygotic supply of wild-type Arm protein. These embryos, like embryos mutant for both sdt and baz, exhibit an early disruption of ZA formation. These results suggest that early stages in the assembly of the ZA are critical for the stability of the polarized blastoderm epithelium.