Association of human protein-tyrosine phosphatase kappa with members of the armadillo family

Histamine induces tyrosine phosphorylation of endothelial cell-to-cell adherens junctions

Endothelial adherens junctions (AJ) promote intercellular adhesion and may contribute to the control of vascular permeability. These structures are formed by a transmembrane and cell-specific adhesive protein, vascular endothelial (VE)-cadherin, which is linked by its cytoplasmic tail to intracellular proteins called catenins (alpha-catenin, beta-catenin, and plakoglobin) and to the actin cytoskeleton. Little is known about the functional regulation of AJ in endothelial cells. In this study, we analyzed the effect of histamine on AJ organization in cultured endothelial cells. We first observed that histamine induced detectable intercellular gaps only in loosely-confluent cells, whereas this effect was strongly reduced or absent in long-confluent cultures. Despite this difference, in vitro permeability was augmented by histamine in both conditions. In resting conditions, tyrosine phosphorylation of AJ components and permeability values were higher in recently-confluent cells as compared with long-confluent cells. Histamine did not affect the phosphorylation state of AJ in recently-confluent cells but strongly increased this parameter in long-confluent cultures. In addition, in long-confluent cells, histamine caused dissociation of VE-cadherin from the actin cytoskeleton measured by a decrease of the amount of the molecule in the detergent-insoluble fraction of the cell extracts. Dibutyryl cAMP was able to prevent the effect of histamine on both tyrosine phosphorylation of AJ components and on endothelial permeability. The effect of histamine was specific for VE-cadherin because the phosphorylation state of neural (N)-cadherin, the other major endothelial cadherin, was unchanged by this agent. Hence AJ components are a target of histamine activation cascade; we suggest that induction of tyrosine phosphorylation of VE-cadherin and catenins contributes to the histamine effect on permeability, even in absence of frank intercellular gaps and cell retraction.

Proteolytic cleavage of beta-catenin by caspases: an in vitro analysis

Cleavage of structural proteins by caspases has been associated with the severe morphological changes occurring during the apoptotic process. One of the proteins regulating the connection of the actin filament with cadherins in a cell-cell adhesion complex is beta-catenin. During apoptosis, both an N-terminal and a small C-terminal part are removed from beta-catenin. Removal of the N-terminal part may result in a disconnection of the actin filament from a cadherin cell-cell adhesion complex. We demonstrate that caspase-8, -3 and -6 directly proteolyse beta-catenin in vitro. However, the beta-catenin cleavage products generated by caspase-8 were different from those generated by caspase-3 or caspase-6. Caspase-1, -2, -4/11 and -7 did not or only very inefficiently cleave beta-catenin. These data suggest that activation of procaspase-3, -6 or -8 by different stimuli in the cell might result in a differential proteolysis of beta-catenin.

The Receptor Protein Tyrosine Phosphatase, PTP-RO, Is Upregulated During Megakaryocyte Differentiation and Is Associated With the c-Kit Receptor

We have recently isolated a cDNA encoding a novel human receptor-type tyrosine phosphatase, termed PTP-RO (for a protein tyrosine phosphatase receptor omicron), from 5-fluorouracil–treated murine bone marrow cells. PTP-RO is a human homologue of murine PTPλ and is related to the homotypically adhering κ and μ receptor-type tyrosine phosphatases. PTP-RO is expressed in human megakaryocytic cell lines, primary bone marrow megakaryocytes, and stem cells. PTP-RO mRNA and protein expression are upregulated upon phorbol 12-myristate 13-acetate (PMA) treatment of the megakaryocytic cell lines CMS, CMK, and Dami. To elucidate the function of PTP-RO in megakaryocytic cells and its potential involvement in the stem cell factor (SCF)/c-Kit receptor pathway, COS-7 and 293 cells were cotransfected with the cDNAs of both the c-Kit tyrosine kinase receptor and PTP-RO. PTP-RO was found to be associated with the c-Kit receptor in these transfected cells and the SCF/Kit ligand induced a rapid tyrosine phosphorylation of PTP-RO. Interestingly, these transfected cells demonstrated a decrease in their proliferative response to the SCF/Kit ligand. In addition, we assessed the association of PTP-RO with c-Kit in vivo. The results demonstrated that PTP-RO associates with c-Kit but not with the tyrosine kinase receptor FGF-R and that PTP-RO is tyrosine-phosphorylated after SCF stimulation of Mo7e and CMK cells. Antisense oligonucleotides directed against PTP-RO mRNA sequences significantly inhibited megakaryocyte progenitor proliferation. Therefore, these data show that the novel tyrosine kinase phosphatase PTP-RO is involved in megakaryocytopoiesis and that its function is mediated by the SCF/c-Kit pathway.

The Receptor Protein Tyrosine Phosphatase, PTP-RO, Is Upregulated During Megakaryocyte Differentiation and Is Associated With the c-Kit Receptor

We have recently isolated a cDNA encoding a novel human receptor-type tyrosine phosphatase, termed PTP-RO (for a protein tyrosine phosphatase receptor omicron), from 5-fluorouracil–treated murine bone marrow cells. PTP-RO is a human homologue of murine PTPλ and is related to the homotypically adhering κ and μ receptor-type tyrosine phosphatases. PTP-RO is expressed in human megakaryocytic cell lines, primary bone marrow megakaryocytes, and stem cells. PTP-RO mRNA and protein expression are upregulated upon phorbol 12-myristate 13-acetate (PMA) treatment of the megakaryocytic cell lines CMS, CMK, and Dami. To elucidate the function of PTP-RO in megakaryocytic cells and its potential involvement in the stem cell factor (SCF)/c-Kit receptor pathway, COS-7 and 293 cells were cotransfected with the cDNAs of both the c-Kit tyrosine kinase receptor and PTP-RO. PTP-RO was found to be associated with the c-Kit receptor in these transfected cells and the SCF/Kit ligand induced a rapid tyrosine phosphorylation of PTP-RO. Interestingly, these transfected cells demonstrated a decrease in their proliferative response to the SCF/Kit ligand. In addition, we assessed the association of PTP-RO with c-Kit in vivo. The results demonstrated that PTP-RO associates with c-Kit but not with the tyrosine kinase receptor FGF-R and that PTP-RO is tyrosine-phosphorylated after SCF stimulation of Mo7e and CMK cells. Antisense oligonucleotides directed against PTP-RO mRNA sequences significantly inhibited megakaryocyte progenitor proliferation. Therefore, these data show that the novel tyrosine kinase phosphatase PTP-RO is involved in megakaryocytopoiesis and that its function is mediated by the SCF/c-Kit pathway.

Transcriptional regulation of a receptor protein tyrosine phosphatase gene hPTP-J by PKC-mediated signaling pathways in Jurkat and Molt-4 T lymphoma cells

The recently cloned type II receptor protein tyrosine phosphatase (RPTP) gene hPTP-J is a new member of the MAM (meprin, A5, PTPmicro) domain subfamily. We previously reported that hPTP-J mRNA was detected significantly in Jurkat T lymphoma cells and its expression was completely down-regulated by phorbol myristate acetate (PMA). In this study, we investigated what signaling pathways/molecules are involved in the transcriptional regulation of hPTP-J expression in Jurkat and Molt-4 T cell lines. The hPTP-J transcription was transiently up-regulated 20 min after the addition of PMA (20 ng/ml) to the Jurkat culture, followed by the complete down-regulation in 8 h after PMA addition. The transient up-regulation and the complete down-regulation induced by PMA was blocked by a PKC-specific inhibitor, GF109203X, suggesting that the regulatory effect of PMA on the hPTP-J transcription depends on protein kinase C activation. hPTP-J transcription was down-regulated not only by PMA but also by several signaling modulators including 1-oleoyl-2-acetylglycerol, forskolin, orthovanadate, manumycin and okadaic acid. Therefore, several signaling molecules such as protein tyrosine phosphatases, PP2A/CaMKIV and Ras are required for hPTP-J transcription in Jurkat and Molt-4 cells.

Protein-tyrosine phosphatases in development

One of the most important mechanisms of eukaryotic signalling is protein phosphorylation on tyrosine residues, which plays a pivotal role in development by regulating cell proliferation, differentiation and migration. Cellular phosphotyrosine (P.Tyr) levels are regulated by the antagonistic activities of the protein-tyrosine kinases (PTKs) and protein-tyrosine phosphatases (PTPs). We have good insight into the function of PTKs at the molecular level and into the role of PTK-mediated signalling in development. Intuitively, PTPs and PTKs are equally important in development. Over the past decade, much emphasis has been placed on elucidation of the function of PTPs, which has led to good insights into the mechanism of PTP-mediated dephosphorylation. Although still relatively little is known about the role of PTPs in cell signalling and development, evidence is now emerging that several PTPs are crucial for proper development. Here I will introduce PTP-mediated signalling and discuss recent findings regarding the function of PTPs in development.

A soluble version of the receptor-like protein tyrosine phosphatase kappa stimulates neurite outgrowth via a Grb2/MEK1-dependent signaling cascade

Receptor-like protein tyrosine phosphatase kappa (RPTPkappa) is expressed in the nervous system in a manner consistent with a role in axonal growth and guidance. The extracellular domain of RPTPkappa shares structural features with cell adhesion molecules and can support homophilic adhesion. In the present study we produced a soluble Fc-chimeric protein containing the full extracellular domain of RPTPkappa. Following affinity capture, the RPTPkappa-Fc was shown to promote the aggregation of Covasphere beads, confirming its homophilic binding activity. When added to cultures of cerebellar neurons as a soluble molecule, the RPTPkappa chimera stimulated neurite outgrowth. The neurite outgrowth response was substantially inhibited by a cell-permeable peptide inhibitor of Grb2 and by PD 098059, a drug that has been used to inhibit MEK1 activation in a wide range of cell types. These results demonstrate that RPTPkappa can stimulate neurite outgrowth and provide evidence that this might involve the coupling of Grb2 to a MAPK signal transduction cascade.

Thrombospondin-1 induces tyrosine phosphorylation of adherens junction proteins and regulates an endothelial paracellular pathway

Thrombospondin-1 (TSP) induces endothelial cell (EC) actin reorganization and focal adhesion disassembly and influences multiple EC functions. To determine whether TSP might regulate EC-EC interactions, we studied the effect of exogenous TSP on the movement of albumin across postconfluent EC monolayers. TSP increased transendothelial albumin flux in a dose-dependent manner at concentrations >/=1 microg/ml (2.2 nM). Increases in albumin flux were observed as early as 1 h after exposure to 30 microg/ml (71 nM) TSP. Inhibition of tyrosine kinases with herbimycin A or genistein protected against the TSP-induced barrier dysfunction by >80% and >50%, respectively. TSP-exposed monolayers exhibited actin reorganization and intercellular gap formation, whereas pretreatment with herbimycin A protected against this effect. Increased staining of phosphotyrosine-containing proteins was observed in plaque-like structures and at the intercellular boundaries of TSP-treated cells. In the presence of protein tyrosine phosphatase inhibition, TSP induced dose- and time-dependent increments in levels of phosphotyrosine-containing proteins; these TSP dose and time requirements were compatible with those defined for EC barrier dysfunction. Phosphoproteins that were identified include the adherens junction proteins focal adhesion kinase, paxillin, gamma-catenin, and p120(Cas). These combined data indicate that TSP can modulate endothelial barrier function, in part, through tyrosine phosphorylation of EC proteins.

Expression of a truncated receptor protein tyrosine phosphatase kappa in the brain of an adult transgenic mouse

Receptor protein tyrosine phosphatases (RPTPs) comprise a family of proteins that feature intracellular phosphatase domains and an ectodomain with putative ligand-binding motifs. Several RPTPs are expressed in the brain, including RPTP-kappa which participates in homophilic cell-cell interactions in vitro [Y.-P. Jiang, H. Wang, P. D'Eustachio, J.M. Musacchio, J. Schlessinger, J. Sap, Cloning and characterization of R-PTP-kappa, a new member of the receptor protein tyrosine phosphatase family with a proteolytically cleaved cellular adhesion molecule-like extracellular region, Mol. Cell. Biol. 13 (1993) 2942-2951; J. Sap, Y.-P. Jiang, D. Friedlander, M. Grumet, J. Schlessinger, Receptor tyrosine phosphatase R-PTP-kappa mediates homophilic binding, Mol. Cell. Biol. 14 (1994) 1-9]. The homology of RPTP-kappa's ectodomain to neural cell adhesion molecules indicates potential roles in developmental processes such as axonal growth and target recognition, as has been demonstrated for certain Drosophila RPTPs. The brain distribution of RPTP-kappa-expressing cells has not been determined, however. In a gene-trap mouse model with a beta-gal+neo (beta-geo) insertion in the endogenous RPTP-kappa gene, the consequent loss of RPTP-kappa's enzymatic activity does not produce any obvious phenotypic defects [W.C. Skarnes, J.E. Moss, S.M. Hurtley, R.S.P. Beddington, Capturing genes encoding membrane and secreted proteins important for mouse development, Proc. Natl. Acad. Sci. U.S.A. 92 (1995) 6592-6596]. Nevertheless, since the transgene's expression is driven by the endogenous RPTP-kappa promoter, distribution of the truncated RPTP-kappa/beta-geo fusion protein should reflect the regional and cellular expression of wild-type RPTP-kappa, and thus may identify sites where RPTP-kappa is important. Towards that goal, we have used this mouse model to map the distribution of the truncated RPTP-kappa/beta-geo fusion protein in the adult mouse brain using beta-galactosidase as a marker enzyme. Visualization of the beta-galactosidase activity revealed a non-random pattern of expression, and identified cells throughout the CNS that display RPTP-kappa promoter activity. Several neural systems highly expressed the transgene-most notably cortical, olfactory, hippocampal, hypothalamic, amygdaloid and visual structures. These well-characterized brain regions may provide a basis for future studies of RPTP-kappa function.

Phosphorylation and free pool of beta-catenin are regulated by tyrosine kinases and tyrosine phosphatases during epithelial cell migration

Cell migration requires precise control, which is altered or lost when tumor cells become invasive and metastatic. Although the integrity of cell-cell contacts, such as adherens junctions, is essential for the maintenance of functional epithelia, they need to be rapidly disassembled during migration. The transmembrane cell adhesion protein E-cadherin and the cytoplasmic catenins are molecular elements of these structures. Here we demonstrate that epithelial cell migration is accompanied by tyrosine phosphorylation of beta-catenin and an increase of its free cytoplasmic pool. We show further that the protein-tyrosine phosphatase LAR (leukocyte common antigen related) colocalizes with the cadherin-catenin complex in epithelial cells and associates with beta-catenin and plakoglobin. Interestingly, ectopic expression of protein-tyrosine phosphatase (PTP) LAR inhibits epithelial cell migration by preventing phosphorylation and the increase in the free pool of beta-catenin; moreover, it inhibits tumor formation in nude mice. These data support a function for PTP LAR in the regulation of epithelial cell-cell contacts at adherens junctions as well as in the control of beta-catenin signaling functions. Thus PTP-LAR appears to play an important role in the maintenance of epithelial integrity, and a loss of its regulatory function may contribute to malignant progression and metastasis.

Expression of receptor tyrosine phosphatases during development of the retinotectal projection of the chick

Receptor tyrosine kinases and receptor protein tyrosine phosphatases (RPTPs) appear to coordinate many aspects of neural development, including axon growth and guidance. Here, we focus on the possible roles of RPTPs in the developing avian retinotectal system. Using both in situ hybridization analysis and immunohistochemistry, we show for the first time that five RPTP genes--CRYPalpha, CRYP-2, PTPmu, PTPgamma, and PTPalpha--have different but overlapping expression patterns throughout the retina and the tectum. PTPalpha is restricted to Muller glia cells and radial glia of the tectum, indicating a possible function in controlling neuronal migration. PTPgamma expression is restricted to amacrine neurons. CRYPalpha and CRYP-2 mRNAs in contrast are expressed throughout the retinal ganglion cell layer from where axons grow out to their tectal targets. PTPmu is expressed in a subset of these ganglion cells. CRYPalpha, CRYP-2, and PTPmu proteins are also localized in growth cones of retinal ganglion cell axons and are present in defined laminae of the tectum. Thus, the spatial and temporal expression of three distinct RPTP subtypes--CRYPalpha, CRYP-2, and PTPmu--are consistent with the possibility of their involvement in axon growth and guidance of the retinotectal projection.

PTPmu regulates N-cadherin-dependent neurite outgrowth

Cell adhesion is critical to the establishment of proper connections in the nervous system. Some receptor-type protein tyrosine phosphatases (RPTPs) have adhesion molecule-like extracellular segments with intracellular tyrosine phosphatase domains that may transduce signals in response to adhesion. PTPmu is a RPTP that mediates cell aggregation and is expressed at high levels in the nervous system. In this study, we demonstrate that PTPmu promotes neurite outgrowth of retinal ganglion cells when used as a culture substrate. In addition, PTPmu was found in a complex with N-cadherin in retinal cells. To determine the physiological significance of the association between PTPmu and N-cadherin, the expression level and enzymatic activity of PTPmu were perturbed in retinal explant cultures. Downregulation of PTPmu expression through antisense techniques resulted in a significant decrease in neurite outgrowth on an N-cadherin substrate, whereas there was no effect on laminin or L1-dependent neurite outgrowth. The overexpression of a catalytically inactive form of PTPmu significantly decreased neurite outgrowth on N-cadherin. These data indicate that PTPmu specifically regulates signals required for neurites to extend on an N-cadherin substrate, implicating reversible tyrosine phosphorylation in the control of N-cadherin function. Together, these results suggest that PTPmu plays a dual role in the regulation of neurite outgrowth.

Tissue-specific expression of beta-catenin in normal mesenchyme and uveal melanomas and its effect on invasiveness

This paper is the first in a series aimed at understanding the role of beta-catenin in epithelial-mesenchymal transformation (EMT) and acquisition of mesenchymal invasive motility. Here, we compare the expression of this and related molecules in the two major tissue phenotypes, epithelial and mesenchymal, the latter including normal avian and mammalian fibroblasts and malignant human uveal melanoma cells. Previously, it was proposed that src initiates EMT by tyrosine phosphorylation of the cadherin/catenin complex resulting in a negative effect on epithelial gene expression. On the contrary, we found that although beta-catenin becomes diffuse in the cytoplasm during embryonic EMT, the cytoplasmic beta-catenin of the embryonic and adult mesenchymal cells we examined is not tyrosine phosphorylated. Pervanadate experiments indicate that cytoplasmic PTPases maintain this dephosphorylation. GSK-3beta is present, but little or no APC occurs in normal and neoplastic mesenchymal cells. The function of the nonphosphorylated cytoplasmic beta-catenin in mesenchyme may be related to invasive motility. Indeed, in order to invade extracellular matrix, transitional (Mel 252) melanoma cells transform from an epithelial to a mesenchymal phenotype with increased cytoplasmic beta-catenin. Moreover, antisense beta-catenin and plakoglobin ODNs inhibit Mel 252 and corneal fibroblast invasion of collagen. All fibroblastic, transitional, and spindle melanoma cells contain nuclear as well as cytoplasmic beta-catenin, but they are not significantly more invasive than normal fibroblasts that contain only cytoplasmic beta-catenin.

Cytokine-induced transcription of protein-tyrosine-phosphatases in human astrocytoma cells

Interleukin-1 (IL-1) and Tumor Necrosis Factor-a (TNFalpha) are potent mediators of inflammatory reactions in the brain. Although much is known about the effects of IL-1 on expression of secretory proteins, few studies have addressed the question of a selective, IL-1-dependent expression of genes involved in neuromodulatory effects of inflammation. Protein-tyrosine-phosphatases (PTP's) have been shown to regulate signal transduction and adhesion processes in the developing nervous system. They are candidates for inflammation-induced neuromodulation. Therefore, we investigated if IL-1 regulates expression of PTP's. We applied a DNA-fingerprinting method based on the PCR-amplification of conserved domains of gene families and observed IL-1-dependent induction of two PTP's, cytoplasmic PTPvarepsilon and receptor-PTPgamma, RPTPgamma, in human U373-MG astrocytoma cells. Using Northern blot analysis, we confirmed this result and also show that in addition to IL-1, TNFalpha but not IL-6 induces the transcription of cytoplasmic PTPvarepsilon and RPTPgamma in human astrocytoma cells. Given the important role for PTP's in neuromodulatory aspects such as axonal guidance and neurite outgrowth, cytokine-induced induction of PTP's may play an important pathenogenic role in the development of chronic inflammatory diseases in the brain.

Modification of the E-cadherin-catenin complex in mitotic Madin-Darby canine kidney epithelial cells

One of the hallmarks of polarized epithelial cells undergoing mitosis is their rounded morphology. This phenotype correlates with a reduced cell-substratum adhesion, apparently caused by a modulation of integrin function. However, it is still unclear whether the cadherin-mediated cell-cell adhesion is affected as well. To address this question, the cadherin complex was analyzed in different cell cycle stages of Madin-Darby canine kidney cells. By immunofluorescence, mitotic Madin-Darby canine kidney cells showed an increased staining of E-cadherin and the catenins (alpha-catenin, beta-catenin, plakoglobin, p120(ctn)) in the cytosol, suggesting a reorganization of the cadherin-catenin complex during mitosis. Biochemical analysis revealed that the overall amount of these components, as well as the proportion of the complex associated with the actin cytoskeleton, remained unchanged in mitotic cells. However, we found evidence for an internalization of E-cadherin during mitosis. In addition, the cadherin-catenin complex was analyzed for mitosis-specific changes in phosphorylation. We report a decrease in the tyrosine phosphorylation of beta-catenin, plakoglobin, and p120(ctn) during mitosis. Moreover, we observed a mitosis-specific Ser/Thr-phosphorylation of p120(ctn), as detected by the MPM-2 antibody. Hence, the cadherin/catenin complex is a target for different posttranslational modifications during mitosis, which may also have a profound impact on cadherin-mediated cell-cell adhesion.

The nonreceptor protein tyrosine phosphatase PTP1B binds to the cytoplasmic domain of N-cadherin and regulates the cadherin-actin linkage

Cadherin-mediated adhesion depends on the association of its cytoplasmic domain with the actin-containing cytoskeleton. This interaction is mediated by a group of cytoplasmic proteins: alpha-and beta- or gamma- catenin. Phosphorylation of beta-catenin on tyrosine residues plays a role in controlling this association and, therefore, cadherin function. Previous work from our laboratory suggested that a nonreceptor protein tyrosine phosphatase, bound to the cytoplasmic domain of N-cadherin, is responsible for removing tyrosine-bound phosphate residues from beta-catenin, thus maintaining the cadherin-actin connection (). Here we report the molecular cloning of the cadherin-associated tyrosine phosphatase and identify it as PTP1B. To definitively establish a causal relationship between the function of cadherin-bound PTP1B and cadherin-mediated adhesion, we tested the effect of expressing a catalytically inactive form of PTP1B in L cells constitutively expressing N-cadherin. We find that expression of the catalytically inactive PTP1B results in reduced cadherin-mediated adhesion. Furthermore, cadherin is uncoupled from its association with actin, and beta-catenin shows increased phosphorylation on tyrosine residues when compared with parental cells or cells transfected with the wild-type PTP1B. Both the transfected wild-type and the mutant PTP1B are found associated with N-cadherin, and recombinant mutant PTP1B binds to N-cadherin in vitro, indicating that the catalytically inactive form acts as a dominant negative, displacing endogenous PTP1B, and rendering cadherin nonfunctional. Our results demonstrate a role for PTP1B in regulating cadherin-mediated cell adhesion.

Protein tyrosine phosphatases: mechanisms of catalysis and regulation

Recent structural information suggests that the HC(X)5R active-site motif defines three distinct evolutionary families of phosphatases that employ a common catalytic mechanism. In two instances, regulation of phosphatase activity employs autoinhibitory mechanisms involving either intermolecular or intramolecular interactions, whereby inhibition is mediated by sterically blocking the active-site cleft.

Desmosomes: intercellular adhesive junctions specialized for attachment of intermediate filaments

Cell-cell adhesion is thought to play important roles in development, in tissue morphogenesis, and in the regulation of cell migration and proliferation. Desmosomes are adhesive intercellular junctions that anchor the intermediate filament network to the plasma membrane. By functioning both as an adhesive complex and as a cell-surface attachment site for intermediate filaments, desmosomes integrate the intermediate filament cytoskeleton between cells and play an important role in maintaining tissue integrity. Recent observations indicate that tissue integrity is severely compromised in autoimmune and genetic diseases in which the function of desmosomal molecules is impaired. In addition, the structure and function of many of the desmosomal molecules have been determined, and a number of the molecular interactions between desmosomal proteins have now been elucidated. Finally, the molecular constituents of desmosomes and other adhesive complexes are now known to function not only in cell adhesion, but also in the transduction of intracellular signals that regulate cell behavior.

Evidence that dorsal-ventral differences in gap junctional communication in the early Xenopus embryo are generated by beta-catenin independent of cell adhesion effects

Gap junctional communication (GJC) is regulated in the early Xenopus embryo and quantitative differences in junctional communication correlate with the specification of the dorsal-ventral axis. To address the mechanism that is responsible for regulating this differential communication, we investigated the function of beta-catenin during the formation of the dorsal-ventral axis in Xenopus embryos by blocking its synthesis with antisense oligodeoxynucleotides. This method has previously been shown to reduce the level of beta-catenin in the early embryo, prior to zygotic transcription, and to inhibit the formation of the dorsal axis (Heasman et al., 1994, Cell 79, 791-803). We show here that antisense inhibition of beta-catenin synthesis also reduces GJC among cells in the dorsal hemisphere of 32-cell embryos to levels similar to those observed among ventral cells. Full-length beta-catenin mRNA can restore elevated levels of dorsal GJC when injected into beta-catenin-deficient oocytes, demonstrating the specificity of the beta-catenin depletion with the antisense oligonucleotides. Thus, endogenous beta-catenin is required for the observed differential GJC. This regulation of GJC is the earliest known action of the dorsal regulator, beta-catenin, in Xenopus development. Two lines of evidence, presented here, indicate that beta-catenin acts within the cytoplasm to regulate GJC, rather than through an effect on cell adhesion. First, when EP-cadherin is overexpressed and increased adhesion is observed, embryos display both a ventralized phenotype and reduced dye transfer. Second, a truncated form of beta-catenin (i.e., the ARM region), that lacks the cadherin-binding domain, restores dorsal GJC to beta-catenin-depleted embryos. Thus, beta-catenin appears to regulate GJC independent of its role in cell-cell adhesion, by acting within the cytoplasm through a signaling mechanism.

Vascular endothelial growth factor induces VE-cadherin tyrosine phosphorylation in endothelial cells

Interendothelial junctions play an important role in the regulation of endothelial functions, such as vasculogenesis, angiogenesis, and vascular permeability. In this paper we show that vascular endothelial growth factor (VEGF), a potent inducer of new blood vessels and vascular permeability in vivo, stimulated the migration of endothelial cells after artificial monolayer wounding and induced an increase in paracellular permeability of human umbilical vein endothelial cells (HUVECs). Furthermore, VEGF increased phosphotyrosine labeling at cell-cell contacts. Biochemical analyses revealed a strong induction of VEGF-receptor-2 (flk-1/KDR) tyrosine-autophosphorylation by VEGF which was maximal after 5 minutes and was followed by receptor downregulation. 15 minutes to 1 hour after VEGF stimulation the endothelial adherens junction components VE-cadherin, beta-catenin, plakoglobin, and p120 were maximally phosphorylated on tyrosine, while alpha-catenin was not modified. PECAM-1/CD31, another cell-cell junctional adhesive molecule, was tyrosine phosphorylated with similar kinetics in response to VEGF. In contrast, activation of VEGF-receptor-1 (Flt-1) by its specific ligand placenta growth factor (PlGF) had no effect on the tyrosine phosphorylation of cadherins and catenins. Despite the rapid and transient receptor activation and the subsequent tyrosine phosphorylation of adherens junction proteins the cadherin complex remained stable and associated with junctions. Our results demonstrate that the endothelial adherens junction is a downstream target of VEGFR-2 signaling and suggest that tyrosine phosphorylation of its components may be involved in the the loosening of cell-cell contacts in established vessels to modulate transendothelial permeability and to allow sprouting and cell migration during angiogenesis.

Regulation of mammary differentiation by extracellular matrix involves protein-tyrosine phosphatases

Extracellular matrix and growth factors cooperate to regulate signaling pathways and gene transcription in adherent cells. However, the mechanism of extracellular matrix signaling is poorly defined. In mammary gland, the expression of milk protein genes is controlled by cross-talk between signals derived from the basement membrane protein, laminin, and the lactogenic hormone, prolactin. Signals from basement membrane are transduced by beta1 integrins and are required for prolactin to activate DNA binding of the milk protein gene transcription factor, Stat5. Here we show that basement membrane is necessary for tyrosine phosphorylation of the prolactin receptor and thus directly affects cytokine signaling and differentiation at the level of the plasma membrane. Prolactin does not induce tyrosine phosphorylation of its receptor, Jak2, or Stat5 in nondifferentiated breast epithelia cultured on collagen I, and we show that this is due to a vanadate-sensitive activity that inhibits the prolactin pathway. We suggest that protein-tyrosine phosphatases are novel targets for regulation by extracellular matrix and in mammary cells represent an additional control to the requirement of integrins for milk protein production.

Dynamic interaction of PTPmu with multiple cadherins in vivo

There is a growing body of evidence to implicate reversible tyrosine phosphorylation as an important mechanism in the control of the adhesive function of cadherins. We previously demonstrated that the receptor protein tyrosine phosphatase PTPmu associates with the cadherin-catenin complex in various tissues and cells and, therefore, may be a component of such a regulatory mechanism (Brady-Kalnay, S. M., D.L. Rimm, and N.K. Tonks. 1995. J. Cell Biol. 130:977- 986). In this study, we present further characterization of this interaction using a variety of systems. We observed that PTPmu interacted with N-cadherin, E-cadherin, and cadherin-4 (also called R-cadherin) in extracts of rat lung. We observed a direct interaction between PTPmu and E-cadherin after coexpression in Sf9 cells. In WC5 cells, which express a temperature-sensitive mutant form of v-Src, the complex between PTPmu and E-cadherin was dynamic, and conditions that resulted in tyrosine phosphorylation of E-cadherin were associated with dissociation of PTPmu from the complex. Furthermore, we have demonstrated that the COOH-terminal 38 residues of the cytoplasmic segment of E-cadherin was required for association with PTPmu in WC5 cells. Zondag et al. (Zondag, G., W. Moolenaar, and M. Gebbink. 1996. J. Cell Biol. 134: 1513-1517) have asserted that the association we observed between PTPmu and the cadherin-catenin complex in immunoprecipitates of the phosphatase arises from nonspecific cross-reactivity between BK2, our antibody to PTPmu, and cadherins. In this study we have confirmed our initial observation and demonstrated the presence of cadherin in immunoprecipitates of PTPmu obtained with three antibodies that recognize distinct epitopes in the phosphatase. In addition, we have demonstrated directly that the anti-PTPmu antibody BK2 that we used initially did not cross-react with cadherin. Our data reinforce the observation of an interaction between PTPmu and E-cadherin in vitro and in vivo, further emphasizing the potential importance of reversible tyrosine phosphorylation in regulating cadherin function.

Protein tyrosine phosphatases in the developing nervous system

Protein tyrosine phosphatases (PTPs) constitute a diverse family of intracellular and transmembrane proteins. Expression data and recent genetic analyses indicate that many PTPs play important roles in different aspects of nervous system development. Although PTP mechanisms are still poorly understood, current data suggest considerable complexity in these signaling pathways.

Altered cell adhesion activity by pervanadate due to the dissociation of alpha-catenin from the E-cadherin.catenin complex

Leukemia cells (K562) that grow as non-adhesive single cells and have no endogenous cadherin were transfected with an E-cadherin expression vector, and cell clones stably expressing E-cadherin on their surface were established. The expression of E-cadherin induced the up-regulation of catenins, and E-cadherin became associated with catenins. The transfected cells grew as floating aggregates. Cell aggregation was Ca2+-dependent and was inhibited by E-cadherin antibodies. The aggregates dissociated into single cells on the addition of pervanadate. Pervanadate caused a dramatic augmentation of the phosphorylation of E-cadherin, beta-catenin, and gamma-catenin (plakoglobin), but alpha-catenin was not detectably phosphorylated. After pervanadate treatment, beta-catenin and gamma-catenin migrated more slowly on gel electrophoresis, suggesting changes in their conformations due to eventual changes in their phosphorylation levels. In the treated cells, a significant amount of alpha-catenin was dissociated from the E-cadherin.catenin complex. Aggregates of cells expressing an E-cadherin chimeric molecule covalently linked with alpha-catenin were not dissociated on pervanadate treatment, supporting the idea that the dissociation of alpha-catenin from the complex underlies the observed E-cadherin dysfunction.

Genetic analysis of protein tyrosine phosphatases

Genetic analysis has enhanced our understanding of the biological roles of many protein tyrosine kinases (PTKs). More recently, studies utilizing both spontaneous mutants and mutants induced by homologous recombination techniques have begun to yield key insights into the role of specific protein tyrosine phosphatases (PTPs) and to suggest how PTKs and PTPs interact. Specific PTPs in Saccharomyces cerevesiae and Schizomyces pombe regulate MAP kinase pathways. Several Drosophila receptor PTPs control axonal targeting pathways, whereas the non-receptor PTP Corkscrew (Csw), plays an essential positive signaling role in multiple developmental pathways directed by receptor PTKs. The vertebrate homolog of Csw, SHP-2, also is required for growth factor signaling and normal development. Finally, very recent studies of other mammalian PTPs suggest that they have critical roles in processes as diverse as hematopoiesis and liver and pituitary development.

Differential expression of MAM-subfamily protein tyrosine phosphatases during mouse development

The MAM-subfamily of type II transmembrane protein tyrosine phosphatases (PTPases) currently comprises the enzymes PTPkappa, PTPmu and PCP2. In an effort to elucidate the individual physiological roles of these closely related proteins we performed a detailed analysis of their mRNA transcript distributions at different stages of mouse embryogenesis and postnatal brain development. Our in situ hybridization studies revealed distinct and complementary expression patterns of PTPkappa, PTPmu and PCP2 transcripts. Based on our results and previous reports we discuss MAM-PTPases as a new class of morphoregulatory molecules.

Receptor-like protein tyrosine phosphatases: alike and yet so different

Reversible phosphorylation on tyrosine residues is an extremely rapid and powerful posttranslational modification that is used in signalling pathways for the regulation of cell growth and differentiation. Over the past several years an impressive number of receptor-like protein tyrosine phosphatase (RPTPase) family members have been identified by molecular cloning, and undoubtedly many more will follow. This review provides an overview of the molecular data that are available for the currently identified RPTPases and discusses their possible biological implications.

The crystal structure of domain 1 of receptor protein-tyrosine phosphatase mu

Receptor-like protein-tyrosine phosphatases (RPTPs) play important roles in regulating intracellular processes. We have been investigating the regulation and function of RPTPmu, a receptor-like PTP related to the Ig superfamily of cell adhesion molecules. Recently, the crystal structure of a dimer of the membrane proximal domain of RPTPalpha (RPTPalpha D1) was described (Bilwes, A. M., den Hertog, J., Hunter, T., and Noel J. P. (1996) Nature 382, 555-559). Within this crystal structure, the catalytic site of each subunit of the dimer is sterically blocked by the insertion of the N-terminal helix-turn-helix segment of the dyad-related monomer. It was proposed that dimerization would lead to inhibition of catalytic activity and may provide a paradigm for the regulation of the RPTP family. We have determined the crystal structure, to 2.3 A resolution, of RPTPmu D1, which shares 46% sequence identity with that of RPTPalpha D1. Although the tertiary structures of RPTPalpha D1 and RPTPmu D1 are very similar, with a root mean square deviation between equivalent Calpha atoms of 1.1 A, the quaternary structures of these two proteins are different. Neither the catalytic site nor the N-terminal helix-turn-helix segment of RPTPmu D1 participates in protein-protein interactions. The catalytic site of RPTPmu D1 is unhindered and adopts an open conformation similar to that of the cytosolic PTP, PTP1B (Barford, D., Flint, A. J., and Tonks, N. K. (1994) Science 263, 1397-1404). We propose that dimerization-induced modulation of RPTP activity may not be a general feature of this family of enzymes.

Interendothelial junctions: structure, signalling and functional roles

Endothelial cell-cell adhesive junctions are formed by transmembrane adhesive proteins linked to a complex cytoskeletal network. These structures are important not only for maintaining adhesion between endothelial cells and, as a consequence, for the control of vascular permeability, but also for intracellular signalling properties. The establishment of intercellular junctions might affect the endothelial functional phenotype by the downregulation or upregulation of endothelial-specific activities.

Tyrosine phosphorylation and cadherin/catenin function

Cadherin-mediated cell-cell adhesion is perturbed in protein tyrosine kinase (PTK)-transformed cells. While cadherins themselves appear to be poor PTK substrates, their cytoplasmic binding partners, the Arm catenins, are excellent PTK substrates and therefore good candidates for mediating PTK-induced changes in cadherin behavior. These proteins, p120ctn, beta-catenin and plakoglobin, bind to the cytoplasmic region of classical cadherins and function to modulate adhesion and/or bridge cadherins to the actin cytoskeleton. In addition, as demonstrated recently for beta-catenin, these proteins also have crucial signaling roles that may or may not be related to their effects on cell-cell adhesion. Tyrosine phosphorylation of cadherin complexes is well documented and widely believed to modulate cell adhesiveness. The data to date, however, is largely correlative and the mechanism of action remains unresolved. In this review, we discuss the current literature and suggest models whereby tyrosine phosphorylation of Arm catenins contribute to regulation or perturbation of cadherin function.

Cadherins, catenins and APC protein: interplay between cytoskeletal complexes and signaling pathways

Cadherins play important roles in cell-cell adhesion during tissue differentiation. Cadherins are linked to the actin cytoskeleton by catenins (beta-catenin/armadillo, plakoglobin, and alpha-catenin). Recent results show that beta-catenin also binds to another cytoskeletal complex containing the adenomatous polyposis coli protein and microtubules, and interacts with several signaling pathways that include tyrosine kinases and phosphatases and Wnt/Wingless. Interplay between these cytoskeletal complexes and signaling pathways may regulate morphogenesis.

Receptor protein tyrosine phosphatases: involvement in cell-cell interaction and signaling

Receptor protein tyrosine phosphatases (RPTPs) represent a relatively new family of cell-surface receptors consisting of a variable, putative ligand-binding ectodomain followed by a single transmembrane segment and one or two intracellular catalytic domains. The RPTPs are thought to transduce extracellular signals by dephosphorylating tyrosine-phosphorylated intracellular substrates. As such, they are the enzymatic counterparts of the well studied receptor tyrosine kinases. However, little is known about the signaling mechanisms and biological functions of the RPTPs. Recent studies show that the extracellular domain of certain RPTPs can mediate either homophilic or heterophilic interactions and suggest a role in cadherin-mediated cell-cell adhesion, possibly via an action on catenins. This review will focus on the role of RPTPs in cell-cell interaction and the possible biological implications.

PSTPIP: a tyrosine phosphorylated cleavage furrow-associated protein that is a substrate for a PEST tyrosine phosphatase

We have investigated proteins which interact with the PEST-type protein tyrosine phosphatase, PTP hematopoietic stem cell fraction (HSCF), using the yeast two-hybrid system. This resulted in the identification of proline, serine, threonine phosphatase interacting protein (PSTPIP), a novel member of the actin- associated protein family that is homologous to Schizosaccharomyces pombe CDC15p, a phosphorylated protein involved with the assembly of the actin ring in the cytokinetic cleavage furrow. The binding of PTP HSCF to PSTPIP was induced by a novel interaction between the putative coiled-coil region of PSTPIP and the COOH-terminal, proline-rich region of the phosphatase. PSTPIP is tyrosine phosphorylated both endogenously and in v-Src transfected COS cells, and cotransfection of dominant-negative PTP HSCF results in hyperphosphorylation of PSTPIP. This dominant-negative effect is dependent upon the inclusion of the COOH-terminal, proline-rich PSTPIP-binding region of the phosphatase. Confocal microscopy analysis of endogenous PSTPIP revealed colocalization with the cortical actin cytoskeleton, lamellipodia, and actin-rich cytokinetic cleavage furrow. Overexpression of PSTPIP in 3T3 cells resulted in the formation of extended filopodia, consistent with a role for this protein in actin reorganization. Finally, overexpression of mammalian PSTPIP in exponentially growing S. pombe results in a dominant-negative inhibition of cytokinesis. PSTPIP is therefore a novel actin-associated protein, potentially involved with cytokinesis, whose tyrosine phosphorylation is regulated by PTP HSCF.

Cellular redistribution of protein tyrosine phosphatases LAR and PTPsigma by inducible proteolytic processing

Most receptor-like protein tyrosine phosphatases (PTPases) display a high degree of homology with cell adhesion molecules in their extracellular domains. We studied the functional significance of processing for the receptor-like PTPases LAR and PTPsigma. PTPsigma biosynthesis and intracellular processing resembled that of the related PTPase LAR and was expressed on the cell surface as a two-subunit complex. Both LAR and PTPsigma underwent further proteolytical processing upon treatment of cells with either calcium ionophore A23187 or phorbol ester TPA. Induction of LAR processing by TPA in 293 cells did require overexpression of PKCalpha. Induced proteolysis resulted in shedding of the extracellular domains of both PTPases. This was in agreement with the identification of a specific PTPsigma cleavage site between amino acids Pro821 and Ile822. Confocal microscopy studies identified adherens junctions and desmosomes as the preferential subcellular localization for both PTPases matching that of plakoglobin. Consistent with this observation, we found direct association of plakoglobin and beta-catenin with the intracellular domain of LAR in vitro. Taken together, these data suggested an involvement of LAR and PTPsigma in the regulation of cell contacts in concert with cell adhesion molecules of the cadherin/catenin family. After processing and shedding of the extracellular domain, the catalytically active intracellular portions of both PTPases were internalized and redistributed away from the sites of cell-cell contact, suggesting a mechanism that regulates the activity and target specificity of these PTPases. Calcium withdrawal, which led to cell contact disruption, also resulted in internalization but was not associated with prior proteolytic cleavage and shedding of the extracellular domain. We conclude that the subcellular localization of LAR and PTPsigma is regulated by at least two independent mechanisms, one of which requires the presence of their extracellular domains and one of which involves the presence of intact cell-cell contacts.

Protein tyrosine phosphatases in signal transduction

Protein-tyrosyl phosphorylation, regulated by protein tyrosine kinases and protein tyrosine phosphatases (PTPs), is a key cellular control mechanism. Until recently, little was known about PTPs. However, the past two years have witnessed an explosion of information about PTP structure, regulation and function. Crystal structures of several PTPs have provided insights into enzymatic mechanisms and regulation and suggested the design of 'substrate-trapping' mutants. Candidate homophilic and heterophilic ligands for transmembrane PTPs have been identified, and roles for transmembrane PTPs in regulating cell-cell interactions have been suggested. Finally, progress has been made in understanding signaling by Src homology 2 domain containing PTPs and PTPs controlling yeast osmoregulatory pathways.

A novel protein-tyrosine phosphatase related to the homotypically adhering kappa and mu receptors

Here we describe a novel member of the receptor-like protein-tyrosine phosphatases (PTPs) termed PTP lambda, which is homologous to the homotypically adherent PTPs kappa and mu. Murine PTP lambda contains MAM, IgG, fibronectin type III, and dual phosphatase domains. As has been demonstrated for PTPs kappa and mu, PTP lambda mediates homotypic adhesion in vitro, and PTP lambda is associated with beta catenin in kidney epithelial cells. The extracellular domain of PTP lambda is proteolytically processed in cell culture as well as in vivo. Northern blot analysis reveals that PTP lambda is expressed throughout embryonic development and is predominately found in adult brain, lung, and kidney. In situ hybridization to 15.5-day old rat embryos reveals that PTP lambda is expressed in a variety of embryonic neuronal sites as well as in the esophagus, lung bronchiolar epithelium, kidney glomerular epithelium, olfactory epithelium, and various cartilagenous sites. Analysis of neonatal brain demonstrates expression in cells of the hippocampus, cortex, and the substantia nigra. Finally, immunohistochemical analysis reveals expression of this PTP on specific neurons of the spinal cord as well as on isolated cortical neurons.