Regulated binding of PTP1B-like phosphatase to N-cadherin: control of cadherin-mediated adhesion by dephosphorylation of beta-catenin

Essential tyrosine residues for interaction of the non-receptor protein-tyrosine phosphatase PTP1B with N-cadherin

Expression of a dominant-negative, catalytically inactive form of the nonreceptor protein-tyrosine phosphatase PTP1B in L-cells constitutively expressing N-cadherin results in loss of N-cadherin-mediated cell-cell adhesion. PTP1B interacts directly with the cytoplasmic domain of N-cadherin, and this association is regulated by phosphorylation of tyrosine residues in PTP1B. The following three tyrosine residues in PTP1B are potential substrates for tyrosine kinases: Tyr-66, Tyr-152, and Tyr-153. To determine the tyrosine residue(s) that are crucial for the cadherin-PTP1B interaction we used site-directed mutagenesis to create catalytically inactive PTP1B constructs bearing additional single, double, or triple mutations in which tyrosine was substituted by phenylalanine. Mutation Y152F eliminates binding to N-cadherin in vitro, whereas mutations Y66F and Y153F do not. Overexpression of the catalytically inactive PTP1B with the Y152F mutation in L-cells constitutively expressing N-cadherin has no effect on N-cadherin-mediated adhesion, and immunoprecipitation reveals that the mutant Y152F PTP1B does not associate with N-cadherin in situ. Furthermore, among cells overexpressing the Y152F mutant endogenous PTP1B associates with N-cadherin and is tyrosine-phosphorylated.

Tyrosine phosphorylation of p120(ctn) in v-Src transfected L cells depends on its association with E-cadherin and reduces adhesion activity

Cadherins are transmembrane glycoproteins involved in Ca2+-dependent cell-cell adhesion. Using L cells expressing one of three functional E-cadherin constructs, the wild-type, a chimeric molecule with alpha-catenin (EalphaC), and a tail-less one, we determined the effect of v-Src expression on E-cadherin-mediated adhesion. The aggregation of L cells expressing the wild-type or EalphaC chimeric protein, which both interact with p120(ctn), was reduced by v-Src expression, whereas that of L cells expressing the tail-less E-cadherin was not affected by the expression. Tyrosine phosphorylation of p120(ctn) was observed in v-Src-transformed L cells expressing the wild-type or EalphaC chimeric protein, but not in ones expressing the tail-less E-cadherin. Thus, tyrosine phosphorylation of p120(ctn) depends on the complex formation with E-cadherin and the resulting membrane localization. Constitutive phosphorylation of p120(ctn) on serine and threonine residues also depends on the complex formation and membrane localization. Coexpression of the p120(ctn) protein with an N-terminal deletion, which eliminates some potential tyrosine phosphorylation sites, or the protein with a single amino acid substitution (tyrosine at 217 to phenylalanine) resulted in an increase in the aggregation of v-Src-transformed EL and EalphaCL cells. These results indicate that tyrosine phosphorylation of p120(ctn) is involved in the v-Src modulation of E-cadherin-mediated cell adhesion.

PTP1B regulates neurite extension mediated by cell-cell and cell-matrix adhesion molecules

N-cadherin and beta1-integrin adhesion and signaling play important roles in growth cone adhesion and guidance. Each of these adhesion receptor systems is composed of multiprotein complexes, and both adhesion and downstream signaling events are regulated through the interaction of protein tyrosine kinases and phosphatases with many of the proteins that make up these complex systems. Work from our laboratory reported that the nonreceptor protein tyrosine phosphatase PTP1B is localized to adherens junctions and focal adhesion complexes and regulates both N-cadherin- and beta1-integrin-mediated adhesion. PTP1B appears to modulate integrin-mediated adhesion through regulation of src activation and cadherin-mediated adhesion through dephosphorylation of beta-catenin. We have continued these studies and report that PTP1B is localized to the tips of growing neurites and that introduction of a noncatalytic mutant of PTP1B into PC12 cells results in inhibition of N-cadherin- and beta1-integrin-mediated neurite outgrowth but is without effect on neurite outgrowth on poly-L-lysine. Moreover, suppressing the level of PTP1B in primary embryonic chick neural retina cells using antisense oligonucleotides also inhibits N-cadherin- and beta1-integrin-mediated neurite outgrowth. Neither of these techniques reduces the levels of expression of either adhesion receptor. We conclude that PTP1B is a regulatory component of the molecular complex required for both N-cadherin and beta1-integrin-mediated axon growth.

Identification of protein-tyrosine phosphatase 1B as the major tyrosine phosphatase activity capable of dephosphorylating and activating c-Src in several human breast cancer cell lines

c-Src tyrosine kinase activity is elevated in several types of human cancer, and this has been attributed to elevated c-Src expression levels, increased c-Src specific activity, and activating mutations in c-Src. We have found a number of human breast cancer cell lines with elevated c-Src specific activity that also possess elevated phosphatase activity directed against the carboxyl-terminal negative regulatory domain of Src family kinases. To identify this phosphatase, cell extracts from MDA-MB-435S cells were chromatographed and the fractions were assayed for phosphatase activity. Four peaks of phosphatase activity directed against the nonspecific substrate poly(Glu/Tyr) were detected. One peak also dephosphorylated a peptide modeled against the c-Src carboxyl-terminal negative regulatory domain and intact human c-Src. Immunoblotting and immunodepletion experiments identified the phosphatase as protein-tyrosine phosphatase 1B (PTP1B). Examination of several human breast cancer cell lines with increased c-Src activity showed elevated levels of PTP1B protein relative to normal control breast cells. In vitro c-Src reactivation experiments confirmed the ability of PTP1B to dephosphorylate and activate c-Src. In vivo overexpression of PTP1B in 293 cells caused a 2-fold increase of endogenous c-Src kinase activity. Our findings indicate that PTP1B is the primary protein-tyrosine phosphatase capable of dephosphorylating c-Src in several human breast cancer cell lines and suggests a regulatory role for PTP1B in the control of c-Src kinase activity.

Cadherins: molecular codes for axon guidance and synapse formation

The formation of the myriad of neuronal connections within the vertebrate nervous system relies on expression of molecular tags that match extending axon populations with synaptic target sites. Recent work suggests that cadherins, a group of calcium-dependent cell adhesion molecules, are candidates to serve such a role. The diversity of the cadherin family in the nervous system allows for a multitude of interactions to specify neuronal connections. Specific cadherin types demarcate subpopulations of developing axons that interconnect within neuronal circuits. Expression of different cadherin species at select synapse populations raises exciting prospects for this molecule class in controlling adhesive interactions during synaptogenesis and plasticity. Regulation of cadherin-mediated adhesive strength is an attractive mechanism to explain the different cadherin functions in axon growth and at synapses.

Cell volume-dependent phosphorylation of proteins of the cortical cytoskeleton and cell-cell contact sites. The role of Fyn and FER kinases

Cell volume affects diverse functions including cytoskeletal organization, but the underlying signaling pathways remained undefined. We have shown previously that shrinkage induces Fyn-dependent tyrosine phosphorylation of the cortical actin-binding protein, cortactin. Because FER kinase was implicated in the direct phosphorylation of cortactin, we investigated the osmotic responsiveness of FER and its relationship to Fyn and cortactin. Shrinkage increased FER activity and tyrosine phosphorylation. These effects were abolished by the Src family inhibitor PP2 and strongly mitigated in Fyn-deficient but not in Src-deficient cells. FER overexpression caused cortactin phosphorylation that was further enhanced by hypertonicity. Exchange of tyrosine residues 421, 466, and 482 for phenylalanine prevented cortactin phosphorylation by hypertonicity and strongly decreased it upon FER overexpression, suggesting that FER targets primarily the same osmo-sensitive tyrosines. Because constituents of the cell-cell contacts are substrates of Fyn and FER, we investigated the effect of shrinkage on the adherens junctions. Hypertonicity provoked Fyn-dependent tyrosine phosphorylation in beta-catenin, alpha-catenin, and p120(Cas) and caused the dissociation of beta-catenin from the contacts. This process was delayed in Fyn-deficient or PP2-treated cells. Thus, FER is a volume-sensitive kinase downstream from Fyn, and the Fyn/FER pathway may contribute to the cell size-dependent reorganization of the cytoskeleton and the cell-cell contacts.

Cellular interactions and signaling in cartilage development

The long bones of the developing skeleton, such as those of the limb, arise from the process of endochondral ossification, where cartilage serves as the initial anlage element and is later replaced by bone. One of the earliest events of embryonic limb development is cellular condensation, whereby pre-cartilage mesenchymal cells aggregate as a result of specific cell-cell interactions, a requisite step in the chondrogenic pathway. In this review an extensive examination of historical and recent literature pertaining to limb development and mesenchymal condensation has been undertaken. Topics reviewed include limb initiation and axial induction, mesenchymal condensation and its regulation by various adhesion molecules, and regulation of chondrocyte differentiation and limb patterning. The complexity of limb development is exemplified by the involvement of multiple growth factors and morphogens such as Wnts, transforming growth factor-beta and fibroblast growth factors, as well as condensation events mediated by both cell-cell (neural cadherin and neural cell adhesion molecule) and cell-matrix adhesion (fibronectin, proteoglycans and collagens), as well as numerous intracellular signaling pathways transduced by integrins, mitogen activated protein kinases, protein kinase C, lipid metabolites and cyclic adenosine monophosphate. Furthermore, information pertaining to limb patterning and the functional importance of Hox genes and various other signaling molecules such as radical fringe, engrailed, Sox-9, and the Hedgehog family is reviewed. The exquisite three-dimensional structure of the vertebrate limb represents the culmination of these highly orchestrated and strictly regulated events. Understanding the development of cartilage should provide insights into mechanisms underlying the biology of both normal and pathologic (e.g. osteoarthritis) adult cartilage.

Cell-cell adhesion-mediated tyrosine phosphorylation of nectin-2delta, an immunoglobulin-like cell adhesion molecule at adherens junctions

We have recently found a novel functional unit of cell-cell adhesion at cadherin-based adherens junctions, consisting of at least nectin, an immunoglobulin-like cell adhesion molecule, and afadin, an actin filament-binding protein which connects nectin to the actin cytoskeleton. Among the members of the nectin family, we have found here that nectin-2delta is tyrosine-phosphorylated in response to cell-cell adhesion. Expression of E-cadherin induced tyrosine phosphorylation of nectin-2delta, while disruption of cell-cell adhesion by an anti-E-cadherin antibody reduced the tyrosine phosphorylation of nectin-2delta. An inhibitor specific for Src family kinase or expression of Csk reduced tyrosine phosphorylation of nectin-2delta. In addition, Src kinase tyrosine phosphorylates the recombinant cytoplasmic region of nectin-2delta in vitro. The major tyrosine phosphorylation site of nectin-2delta was Tyr505 in the cytoplasmic region, because the mutant nectin-2delta, of which Tyr505 was replaced by Phe, showed a loss of tyrosine phosphorylation in vivo and in vitro. These results, together with our recent observations, indicate that the cadherin-catenin system and the nectin-afadin system are closely connected to each other. The cadherin-mediated cell-cell adhesion system may link to the activation of a Src family kinase, that is, at least in part, responsible for the tyrosine phosphorylation of the cytoplasmic region of nectin-2delta. Oncogene (2000) 19, 4022 - 4028.

Cadherins in the central nervous system

The central nervous system (CNS) is divided into diverse embryological and functional compartments. The early embryonic CNS consists of a series of transverse subdivisions (neuromeres) and longitudinal domains. These embryonic subdivisions represent histogenetic fields in which neurons are born and aggregate in distinct cell groups (brain nuclei and layers). Different subsets of these aggregates become selectively connected by nerve fiber tracts and, finally, by synapses, thus forming the neural circuits of the functional systems in the CNS. Recent work has shown that 30 or more members of the cadherin family of morphoregulatory molecules are differentially expressed in the developing and mature brain at almost all stages of development. In a regionally specific fashion, most cadherins studied to date are expressed by the embryonic subdivisions of the early embryonic brain, by developing brain nuclei, cortical layers and regions, and by fiber tracts, neural circuits and synapses. Each cadherin shows a unique expression pattern that is distinct from that of other cadherins. Experimental evidence suggests that cadherins contribute to CNS regionalization, morphogenesis and fiber tract formation, possibly by conferring preferentially homotypic adhesiveness (or other types of interactions) between the diverse structural elements of the CNS. Cadherin-mediated adhesive specificity may thus provide a molecular code for early embryonic CNS regionalization as well as for the development and maintenance of functional structures in the CNS, from embryonic subdivisions to brain nuclei, cortical layers and neural circuits, down to the level of individual synapses.

Emerging issues in receptor protein tyrosine phosphatase function: lifting fog or simply shifting?

Transmembrane (receptor) tyrosine phosphatases are intimately involved in responses to cell-cell and cell-matrix contact. Several important issues regarding the targets and regulation of this protein family are now emerging. For example, these phosphatases exhibit complex interactions with signaling pathways involving SRC family kinases, which result from their ability to control phosphorylation of both activating and inhibitory sites in these kinases and possibly also their substrates. Similarly, integrin signaling illustrates how phosphorylation of a single protein, or the activity of a pathway, can be controlled by multiple tyrosine phosphatases, attesting to the intricate integration of these enzymes in cellular regulation. Lastly, we are starting to appreciate the roles of intracellular topology, tyrosine phosphorylation and oligomerization among the many mechanisms regulating tyrosine phosphatase activity.

Evidence that tyrosine phosphorylation regulates N-cadherin turnover during retinal development

N-cadherin, a member of the cadherin family of calcium-dependent cell adhesion molecules, mediates adhesive and signaling interactions between cells during development. N-Cadherin undergoes dynamic spatiotemporal changes in expression which correlate with morphogenetic movements of cells during organogenesis and histogenesis. We have previously shown that N-cadherin expression during development is regulated by several mechanisms, including mRNA expression, cytokine modulation, and proteolytically mediated turnover, yielding the NCAD90 protein. The present study was directed at determining the extent to which N-cadherin in primary embryonic cells is the target of endogenous kinases and phosphatases, as well as the effects of modulation of these enzymes on NCAD90 expression. The results of phosphoamino acid analyses, peptide mapping, and measurements of N-cadherin and NCAD90 expression in embryonic tissues indicate that N-cadherin is indeed the target of endogenous kinase and phosphatase action, and that modulation of different classes of these enzymes can result in either stimulation or inhibition of NCAD90 production. These results provide a mechanistic explanation for observations that cadherin function is downregulated following expression of exogenously introduced viral tyrosine kinases and provide a function for the tyrosine phosphatases recently found in association with cadherins. The results indicate that N-cadherin expression during retinal development is possibly regulated in part by modulation of its phosphorylation state, the balance of which may determine whether N-cadherin remains stably expressed or is targeted for proteolytically mediated turnover to produce NCAD90.

Lymphocyte migration through monolayers of endothelial cell lines involves VCAM-1 signaling via endothelial cell NADPH oxidase

Lymphocytes migrate from the blood across endothelial cells to reach foreign substances sequestered in peripheral lymphoid organs and inflammatory sites. To study intracellular signaling in endothelial cells during lymphocyte migration, we used murine endothelial cell lines that promote lymphocyte migration and constitutively express VCAM-1. The maximum rate of resting splenic lymphocyte migration across monolayers of the endothelial cells occurred at 0-24 h. This migration was inhibited by anti-VCAM-1 or anti-alpha4 integrin, suggesting that VCAM-1 adhesion was required for migration. To determine whether signals within the endothelial cells were required for migration, irreversible inhibitors of signal transduction molecules were used to pretreat the endothelial cell lines. Inhibitors of NADPH oxidase activity (diphenyleneiodonium and apocynin) blocked migration >65% without affecting adhesion. Because NADPH oxidase catalyzes the production of reactive oxygen species (ROS), we examined whether ROS were required for migration. Scavengers of ROS inhibited migration without affecting adhesion. Furthermore, VCAM-1 ligand binding stimulated NADPH oxidase-dependent production of ROS by the endothelial cells lines and primary endothelial cell cultures. Finally, VCAM-1 ligand binding induced an apocynin-inhibitable actin restructuring in the endothelial cell lines at the location of the lymphocyte or anti-VCAM-1-coated bead, suggesting that an NADPH oxidase-dependent endothelial cell shape change was required for lymphocyte migration. In summary, VCAM-1 signaled the activation of endothelial cell NADPH oxidase, which was required for lymphocyte migration. This suggests that endothelial cells are not only a scaffold for lymphocyte adhesion, but play an active role in promoting lymphocyte migration.

The nonreceptor tyrosine kinase fer mediates cross-talk between N-cadherin and beta1-integrins

Cadherins and integrins must function in a coordinated manner to effectively mediate the cellular interactions essential for development. We hypothesized that exchange of proteins associated with their cytoplasmic domains may play a role in coordinating function. To test this idea, we used Trojan peptides to introduce into cells and tissues peptide sequences designed to compete for the interaction of specific effectors with the cytoplasmic domain of N-cadherin, and assayed their effect on cadherin- and integrin-mediated adhesion and neurite outgrowth. We show that a peptide mimicking the juxtamembrane (JMP) region of the cytoplasmic domain of N-cadherin results in inhibition of N-cadherin and beta1-integrin function. The effect of JMP on beta1-integrin function depends on the expression of N-cadherin and is independent of transcription or translation. Treatment of cells with JMP results in the release of the nonreceptor tyrosine kinase Fer from the cadherin complex and its accumulation in the integrin complex. A peptide that mimics the first coiled-coil domain of Fer prevents Fer accumulation in the integrin complex and reverses the inhibitory effect of JMP. These findings suggest a new mechanism through which N-cadherin and beta1-integrins are coordinately regulated: loss of an effector from the cytoplasmic domain of N-cadherin and gain of that effector by the beta1-integrin complex.

Coordinate regulation of cadherin and integrin function by the chondroitin sulfate proteoglycan neurocan

N-cadherin and beta1-integrins play decisive roles in morphogenesis and neurite extension and are often present on the same cell. Therefore, the function of these two types of adhesion systems must be coordinated in time and space to achieve the appropriate cell and tissue organization. We now show that interaction of the chondroitin sulfate proteoglycan neurocan with its GalNAcPTase receptor coordinately inhibits both N-cadherin- and beta1-integrin-mediated adhesion and neurite outgrowth. Furthermore, the inhibitory activity is localized to an NH(2)-terminal fragment of neurocan containing an Ig loop and an HA-binding domain. The effect of neurocan on beta1-integrin function is dependent on a signal originating from the cadherin cytoplasmic domain, possibly mediated by the nonreceptor protein tyrosine kinase Fer, indicating that cadherin and integrin engage in direct cross-talk. In the developing chick, neural retina neurocan is present in the inner plexiform layer from day 7 on, and the GalNAcPTase receptor becomes restricted to the inner nuclear layer and the ganglion cell layer (as well as the fiber layer), the two forming a sandwich. These data suggest that the coordinate inhibition of cadherin and integrin function on interaction of neurocan with its receptor may prevent cell and neurite migration across boundaries.

Development of the visual system of the chick. I. Cell differentiation and histogenesis

This review summarizes present knowledge on the embryonic development of the avian visual projections, based on the domestic chick as a model system. The reductionist goal to understand formation and function of complex neuroanatomical systems on a causal level requires a synthesis of classic developmental biology with recent advances on the molecular mechanisms of cell differentiation and histogenesis. It is the purpose of this article. We are discussing the processes underlying patterning of the anterior neural tube, when the retina and optic tectum are specified and their axial polarity is determined. Then the development of these structures is described from the molecular to the anatomical level. Following sections deal with the establishment of secondary visual connections, and the developmental interactions between compartments of the retinotectal system. Using this latter pathway, from the retina to the optic tectum, many investigations aimed at mechanisms of axonal pathfinding and connectivity have accumulated a vast body of research, which will be covered by a following review.

Pleiotrophin signals increased tyrosine phosphorylation of beta beta-catenin through inactivation of the intrinsic catalytic activity of the receptor-type protein tyrosine phosphatase beta/zeta

Pleiotrophin (PTN) is a platelet-derived growth factor-inducible, 18-kDa heparin-binding cytokine that signals diverse phenotypes in normal and deregulated cellular growth and differentiation. To seek the mechanisms of PTN signaling, we studied the interactions of PTN with the receptor protein tyrosine phosphatase (RPTP) beta/zeta in U373-MG cells. Our results suggest that PTN is a natural ligand for RPTP beta/zeta. PTN signals through "ligand-dependent receptor inactivation" of RPTP beta/zeta and disrupts its normal roles in the regulation of steady-state tyrosine phosphorylation of downstream signaling molecules. We have found that PTN binds to and functionally inactivates the catalytic activity of RPTP beta/zeta. We also have found that an active site-containing domain of RPTP beta/zeta both binds beta-catenin and functionally reduces its levels of tyrosine phosphorylation when added to lysates of pervanidate-treated cells. In contrast, an (inactivating) active-site mutant of RPTP beta/zeta also binds beta-catenin but fails to reduce tyrosine phosphorylation of beta-catenin. Finally, in parallel to its ability to inactivate endogenous RPTP beta/zeta, PTN sharply increases tyrosine phosphorylation of beta-catenin in PTN-treated cells. The results suggest that in unstimulated cells, RPTP beta/zeta is intrinsically active and functions as an important regulator in the reciprocal control of the steady-state tyrosine phosphorylation levels of beta-catenin by tyrosine kinases and phosphatases. The results also suggest that RPTP beta/zeta is a functional receptor for PTN; PTN signals through ligand-dependent receptor inactivation of RPTP beta/zeta to increase levels of tyrosine phosphorylation of beta-catenin to initiate downstream signaling. PTN is the first natural ligand identified for any of the RPTP family; its identification provides a unique tool to pursue the novel signaling pathway activated by PTN and the relationship of PTN signaling with other pathways regulating beta-catenin.

N-cadherin influences migration of oligodendrocytes on astrocyte monolayers

Oligodendrocyte cell migration is required for the development of the nervous system and the repopulation of demyelinated lesions in the adult central nervous system. We have investigated the role of the calcium-dependent adhesion molecules, the cadherins, in oligodendrocyte-astrocyte interaction and oligodendrocyte progenitor migration. Immunostaining demonstrated the expression of N-cadherin on the surfaces of both oligodendrocytes and astrocytes, and oligodendrocyte-like cells adhered to and spread on N-cadherin substrates. The blocking of cadherin function by antisera or specific peptides reduced adhesion of oligodendroglia to astrocyte monolayers, diminished contact time between oligodendrocyte processes and individual astrocytes, and significantly increased the migration of oligodendrocyte-like cells on astrocyte monolayers. Furthermore, a soluble cadherin molecule without adhesive properties increased oligodendroglial proliferation on various extracellular matrix substrates. These data suggest that cadherins are at least partially responsible for the poor migration-promoting properties of astrocytes and that decreasing cell-cell adhesion might effect repopulation of demyelinated multiple sclerosis lesions by oligodendrocyte progenitors.

The catalytic activity of the Src family kinases is required to disrupt cadherin-dependent cell-cell contacts

Despite the importance of epithelial cell contacts in determining cell behavior, we still lack a detailed understanding of the assembly and disassembly of intercellular contacts. Here we examined the role of the catalytic activity of the Src family kinases at epithelial cell contacts in vitro. Like E- and P-cadherin, Ca(2+) treatment of normal and tumor-derived human keratinocytes resulted in c-Yes (and c-Src and Fyn), as well as their putative substrate p120(CTN), being recruited to cell-cell contacts. A tyrosine kinase inhibitor with selectivity against the Src family kinases, PD162531, and a dominant-inhibitory c-Src protein that interferes with the catalytic function of the endogenous Src kinases induced cell-cell contact and E-cadherin redistribution, even in low Ca(2+), which does not normally support stable cell-cell adhesion. Time-lapse microscopy demonstrated that Src kinase inhibition induced stabilization of transiently formed intercellular contacts in low Ca(2+). Furthermore, a combination of E- and P-cadherin-specific antibodies suppressed cell-cell contact, indicating cadherin involvement. As a consequence of contact stabilization, normal cells were unable to dissociate from an epithelial sheet formed at high density and repair a wound in vitro, although individual cells were still motile. Thus, cadherin-dependent contacts can be stabilized both by high Ca(2+) and by inhibiting Src activity in low (0.03 mM) Ca(2+) in vitro.

Regulation of signaling by protein-tyrosine phosphatases: potential roles in the nervous system

During neuronal development, cells respond to a variety of environmental cues through cell surface receptors that are coupled to a signaling transduction machinery based on protein tyrosine phosphorylation and dephosphorylation. Receptor and non-receptor tyrosine kinases have received a great deal of attention; however, in the last few years, receptor (plasma membrane associated) and non-receptor protein-tyrosine phosphatases (PTPs) have also been shown to play important roles in development of the nervous system. In many cases PTPs have provocative distribution patterns or have been shown to be associated with specific cell adhesion and growth factor receptors. Additionally, altering PTP expression levels or activity impairs neuronal behavior. In this review we outline what is currently known about the role of PTPs in development, differentiation and neuronal physiology.

The juxtamembrane domain of cadherin regulates integrin-mediated adhesion and neurite outgrowth

Axons are guided along their trajectories during development by many different systems of adhesion, attraction, and repulsion. Thus, many distinct, and potentially competing, receptor systems respond to environmental cues, and the information must be coordinated inside the growth cone to ensure that extension follows the appropriate path. In this brief review we bring together two studies, each of which has defined different aspects of a pathway through which N-cadherin regulates beta1-integrin function allowing for coordinated responses to environmental cues during neurite extension. First we review progress in defining the binding to cells and the subsequent effects on adhesion and neurite outgrowth of the chondroitin sulfate proteoglycan, neurocan. Neurocan binds to a cell surface glycosyltransferase associated with N-cadherin (but not integrin), initiating a signal which results in loss of cadherin and integrin-function-suggesting that these two adhesion receptor systems engage in cross-talk, allowing coordinate regulation. Second, we review the use of "Trojan" peptides, peptides which mimic specific sequences in the cytoplasmic domain of N-cadherin attached to a cell permeation sequence, to reveal protein-protein interactions critical to cadherin-integrin cross-talk. One peptide mimicking a 20 amino acid sequence in the juxtamembrane region of N-cadherin has the same effect as neurocan, blocking both cadherin- and integrin-mediated adhesion and neurite outgrowth. Both neurocan and the peptide cause the release of the non-receptor tyrosine kinase Fer from the cadherin complex and its binding to the integrin complex. These data define an epigenetic pathway through which environmental cues are capable of coordinately regulating the activity of two developmentally important adhesion systems.

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.

Distinct spatial and temporal distributions of aggrecan and versican in the embryonic chick heart

Although chondroitin sulfate proteoglycans (CSPGs) are major components of the embryonic extracellular matrix, little attention has been paid to specific CSPGs in early heart development, in part because appropriate antibodies were not available. Therefore we prepared specific polyclonal antibodies against chicken aggrecan, versican, neurocan, and phosphacan. Western blotting and immunohistochemical studies revealed the presence of aggrecan and versican in stages 12-21 chicken embryo hearts in distinctive spatial and temporal patterns. Because this is the first demonstration of aggrecan in heart tissue, we further used RT-PCR to confirm that aggrecan is expressed in the heart and in situ hybridization to confirm the pattern of expression determined using antibodies. Versican is found in the myocardium and the myocardial basement membrane. In contrast, aggrecan is specifically colocalized with several groups of migrating cells including endocardial cushion tissue cells, epicardial cells, a mesenchymal cell population in the outflow tract that may be of neural crest origin, and a mesenchymal cell population in the inflow tract. The combined observations indicate that versican and aggrecan are expressed in unique patterns and suggest that they play very different roles in development.

ATP depletion increases tyrosine phosphorylation of beta-catenin and plakoglobin in renal tubular cells

This study examines the hypothesis that the loss of integrity of the junctional complex induced by ATP depletion is related to alterations in tyrosine phosphorylation of the adherens junction proteins beta-catenin and plakoglobin. ATP depletion of cultured mouse proximal tubular (MPT) cells induces a marked increase in tyrosine phosphorylation of both beta-catenin and plakoglobin. The tyrosine phosphatase inhibitor vanadate has the same effect in ATP-replete (control) monolayers, whereas genistein, a tyrosine kinase inhibitor, reduces phosphorylation of both proteins in ATP-replete monolayers and prevents the hyperphosphorylation of these proteins with ATP depletion. This study also demonstrates that the fall in the transepithelial resistance of MPT monolayers induced by ATP depletion can be reproduced by treatment of ATP-replete monolayers with vanadate, whereas genistein substantially ameliorates the fall in transepithelial resistance induced by ATP depletion. Also, using immunofluorescence microscopy it was demonstrated that ATP depletion results in a marked diminution of E-cadherin staining in the basolateral membrane of MPT cells. Vanadate mimics this effect of ATP depletion, whereas genistein ameliorates the reduction in the intensity of E-cadherin staining induced by ATP depletion. Because it is has been well established that hyperphosphorylation of the catenins leads to dissociation of the adherens junction and to dysfunction of the junctional complex, it is proposed that the increase in tyrosine phosphorylation of catenins observed in MPT cells during ATP depletion contributes to the loss of function of the junctional complex associated with sublethal injury.

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.

Controversies at the cytoplasmic face of the cadherin-based adhesion complex

Cadherin-mediated cell-cell interactions are modulated by protein interactions at the cytoplasmic face of the membrane. Recent work has shown that phosphorylation of both p120(ctn) and beta-catenin affects their interaction with cadherins and the molecular connections to the cytoskeleton. The cytoskeletal connections most probably include interactions between alpha-catenin, and/or alpha-actinin, vinculin, ZO-1, actin and possibly spectrin.

Dissociation of PTPase levels from their modulation of insulin receptor signal transduction

Protein tyrosine phosphatases have been implicated in the regulation of receptor tyrosine kinase signalling pathways, including that of the insulin receptor. Here, cell density-dependent changes in PTPase expression have been exploited to investigate the relationship between cellular PTPase levels and the insulin receptor signal transduction pathway. Increasing cell density (20%, 50%, and >90%) in the rat McA-RH7777 hepatoma cell line resulted in increased protein expression of the receptor-like PTPase LAR (14-fold), and the nonreceptor PTPases PTP1B (11-fold) and SHP2 (10-fold). Each of these PTPases has previously been implicated in regulating insulin receptor signal transduction. Despite these marked increases, maximum insulin receptor autophosphorylation as well as receptor expression actually increased 2-fold. MAP kinase also increased approximately 2-fold as a function of cell density and paralleled increases in expression levels. Neither sensitivity nor maximum responsiveness to insulin were decreased at increasing cell densities as assessed by activation of PI 3-kinase. Duration of response was also unimpaired. These results suggest that expression levels of relevant PTPases are not the primary determinant in their modulation of insulin receptor kinase activity. Restricted accessibility at the molecular level or involvement of accessory proteins may be more critical parameters.

PECAM-1 (CD31) functions as a reservoir for and a modulator of tyrosine-phosphorylated beta-catenin

Catenins function as regulators of cellular signaling events in addition to their previously documented roles in adherens junction formation and function. Evidence to date suggests that beta and gamma catenins can act as signaling molecules, bind transcriptional factors and translocate to the nucleus. Beta- and gamma-catenin are also major substrates for protein tyrosine kinases, and tyrosine phosphorylation of junctional proteins is correlated with decreased adhesiveness. One way in which catenin functions are modulated is by dynamic incorporation into junctional complexes which controls, in part, the cytoplasmic levels of catenins. Here we show that: (1) vascular endothelial growth factor (VEGF) induces beta-catenin tyrosine phosphorylation in a time-, and dose-dependent manner and that VEGF receptors co-localize to areas of endothelial cell-cell contact in vitro and in vivo. (2) Platelet-endothelial cell adhesion molecule (PECAM)-1 can function as a reservoir for, and modulator of, tyrosine phosphorylated beta-catenin. (3) PECAM-1 can prevent beta-catenin nuclear translocation in transfected SW480 colon carcinoma cells. We suggest that PECAM-1 may play a role in modulating beta-catenin tyrosine phosphorylation levels, localization and signaling and by doing so, functions as an important modulator of the endothelium.

Dynamics of E-cadherin and gamma-catenin complexes during dedifferentiation of polarized MDCK cells

BACKGROUND

E-cadherin mediated cell-cell adhesion and hepatocyte growth factor (HGF) are important for renal epithelial morphogenesis. We previously showed that HGF dedifferentiates previously well polarized Madin-Darby canine kidney (MDCK) cell monolayers grown on filters. The regulation of E-cadherin during epithelial dedifferentiation is not known. We hypothesized that E-cadherin mediated cell-cell adhesion is modulated during HGF induced dedifferentiation of MDCK cell monolayers.

METHODS

We analyzed E-cadherin/gamma-catenin interaction and distribution during epithelial dedifferentiation in vitro using a model of polarized MDCK cell monolayers treated with HGF.

RESULTS

Surface immunoprecipitation experiments showed that HGF increased the amount of cell surface E-cadherin associated with gamma-catenin. Biochemical and morphological examination of the TX-100 solubility of junctional E-cadherin and gamma-catenin in control and HGF treated cells showed an increase in solubility of only E-cadherin during loss of cell polarity. Metabolic labeling of control and HGF treated cells showed that HGF stimulated the synthetic rate of E-cadherin and gamma-catenin molecules. Inulin flux across MDCK cell monolayers increases with HGF treatment.

CONCLUSION

These data provide evidence for both the dissociation of E-cadherin molecules from the actin cytoskeleton and an increase in the total number of E-cadherin/gamma-catenin complexes on the cell surface during HGF-induced dedifferentiation of polarized renal epithelium. These data support the hypothesis that E-cadherin function is inhibited by a mechanism of detachment from the actin based cytoskeleton during HGF induced dedifferentiation of polarized renal epithelia.

Regulation of DLG localization at synapses by CaMKII-dependent phosphorylation

Discs large (DLG) mediates the clustering of synaptic molecules. Here we demonstrate that synaptic localization of DLG itself is regulated by CaMKII. We show that DLG and CaMKII colocalize at synapses and exist in the same protein complex. Constitutively activated CaMKII phenocopied structural abnormalities of dlg mutant synapses and dramatically increased extrajunctional DLG. Decreased CaMKII activity caused opposite alterations. In vitro, CaMKII phosphorylated a DLG fragment with a stoichiometry close to one. Moreover, expression of site-directed dlg mutants that blocked or mimicked phosphorylation had effects similar to those observed upon inhibiting or constitutively activating CaMKII. We propose that CaMKII-dependent DLG phosphorylation regulates the association of DLG with the synaptic complex during development and plasticity, thus providing a link between synaptic activity and structure.

Post-translational modifications of ICA512, a receptor tyrosine phosphatase-like protein of secretory granules

The autoantigen of type I diabetes ICA512 is a receptor tyrosine phosphatase-like protein enriched in the secretory granule membranes of neurons and peptide secreting endocrine cells. While the function of ICA512 remains unknown, it is thought to link regulated neuropeptide and peptide hormone secretion with signal transduction pathways involving tyrosine phosphorylation/dephosphorylation. To characterize further its biochemical properties, we conducted studies in the bovine pituitary, an abundant source of native ICA512, as well as in fibroblasts transfected with various human ICA512 cDNA constructs. Based on these studies we have established that the signal peptide of ICA512 encompasses residues 1-34 and that the ectodomain of ICA512 undergoes multiple post-translation modifications, including N-glycosylation. Newly synthesized ICA512 appears first as a pro-protein of 110 kDa that is then converted by post-translational modifications into a 130-kDa species. Cleavage of pro-ICA512 at a consensus for furin-like convertases generates a 60-66-kDa ICA512 transmembrane fragment (amino acids 449-979). Such processing ICA512 is not restricted to neuroendocrine cells, as it can also occur in transfected fibroblasts. Finally, the predicted N-terminal fragment of ICA512 resulting from this cleavage (amino acids 35-448) or parts thereof are present in the neurosecretosomes of posterior pituitary, raising the possibility that they may be secreted upon exocytosis of secretory granules.

Small GTPases and regulation of cadherin dependent cell-cell adhesion

Cadherins belong to a superfamily of cell-cell adhesion receptors that bind to the same type of molecules (homotypic interaction) in a calcium dependent manner. Different members of the family are found in a wide variety of cell types and cadherin adhesive function plays a role in cell fate, segregation, and differentiation, which ensures the higher order of organisation found in many tissues. This review will focus on the role that cadherin adhesiveness plays in the differentiation of epithelial cells, and how cadherin function can be regulated by proteins of the small GTPase family. In the text, readers are referred to recent reviews and other chapters covering important topics that are not discussed here because of space limitation.

Targeted deficiency or cytosolic truncation of the VE-cadherin gene in mice impairs VEGF-mediated endothelial survival and angiogenesis

Vascular endothelial cadherin, VE-cadherin, mediates adhesion between endothelial cells and may affect vascular morphogenesis via intracellular signaling, but the nature of these signals remains unknown. Here, targeted inactivation (VEC-/-) or truncation of the beta-catenin-binding cytosolic domain (VECdeltaC/deltaC) of the VE-cadherin gene was found not to affect assembly of endothelial cells in vascular plexi, but to impair their subsequent remodeling and maturation, causing lethality at 9.5 days of gestation. Deficiency or truncation of VE-cadherin induced endothelial apoptosis and abolished transmission of the endothelial survival signal by VEGF-A to Akt kinase and Bcl2 via reduced complex formation with VEGF receptor-2, beta-catenin, and phosphoinositide 3 (PI3)-kinase. Thus, VE-cadherin/ beta-catenin signaling controls endothelial survival.

The E-cadherin/epidermal growth factor receptor interaction: a hypothesis of reciprocal and reversible control of intercellular adhesion and cell proliferation

The E-cadherin/catenin complex is a calcium-dependent cell-cell adhesion molecule, whose function is critical to the integrity of the adherens junction and which plays a role in the establishment and maintenance of normal epithelial morphology and differentiation. Loss of E-cadherin-mediated adhesion appears to be a fundamental aspect of the neoplastic phenotype which in some cases appears to be mediated by post-translational modifications (i.e. tyrosine phosphorylation) of its interacting proteins, the catenins which link E-cadherin to the actin cytoskeleton. There is increasing experimental evidence to suggest that epidermal growth factor receptor tyrosine phosphorylation may lead to the inactivation of the E-cadherin/catenin complex in cancer cells through its interaction with beta- or gamma-catenin in the cytoskeleton. Modulation of epidermal growth factor receptor activity by pharmacological agents has the potential to regulate a variety of cellular processes including adhesion, differentiation, and proliferation.

The cell biology of the blood-brain barrier

The blood-brain barrier (BBB) is formed by brain capillary endothelial cells (ECs). In the late embryonic and early postnatal period, these cells respond to inducing factors found in the brain environment by adopting a set of defined characteristics, including high-electrical-resistance tight junctions. Although the factors have not been identified definitively, a great deal of information about brain ECs has been obtained, especially recently. This review concentrates on a cell biological analysis of the BBB, with an emphasis on regulation of the specialized intercellular junctions. The development of these junctions seems to depend on two primary processes: the appearance of high levels of the tight junction protein occludin and intracellular signaling processes that control the state of phosphorylation of junctional proteins. Recent studies have revealed that the BBB can be modulated in an ongoing way to respond to environmental stimuli.

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.

The E-cadherin/epidermal growth factor receptor interaction: a hypothesis of reciprocal and reversible control of intercellular adhesion and cell proliferation

The E-cadherin/catenin complex is a calcium-dependent cell-cell adhesion molecule, whose function is critical to the integrity of the adherens junction and which plays a role in the establishment and maintenance of normal epithelial morphology and differentiation. Loss of E-cadherin-mediated adhesion appears to be a fundamental aspect of the neoplastic phenotype which in some cases appears to be mediated by post-translational modifications (i.e. tyrosine phosphorylation) of its interacting proteins, the catenins which link E-cadherin to the actin cytoskeleton. There is increasing experimental evidence to suggest that epidermal growth factor receptor tyrosine phosphorylation may lead to the inactivation of the E-cadherin/catenin complex in cancer cells through its interaction with beta- or gamma-catenin in the cytoskeleton. Modulation of epidermal growth factor receptor activity by pharmacological agents has the potential to regulate a variety of cellular processes including adhesion, differentiation, and proliferation.

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.

Impaired integrin-mediated adhesion and signaling in fibroblasts expressing a dominant-negative mutant PTP1B

To investigate the role of nonreceptor protein tyrosine phosphatase 1B (PTP1B) in beta1-integrin- mediated adhesion and signaling, we transfected mouse L cells with normal and catalytically inactive forms of the phosphatase. Parental cells and cells expressing the wild-type or mutant PTP1B were assayed for (a) adhesion, (b) spreading, (c) presence of focal adhesions and stress fibers, and (d) tyrosine phosphorylation. Parental cells and cells expressing wild-type PTP1B show similar morphology, are able to attach and spread on fibronectin, and form focal adhesions and stress fibers. In contrast, cells expressing the inactive PTP1B have a spindle-shaped morphology, reduced adhesion and spreading on fibronectin, and almost a complete absence of focal adhesions and stress fibers. Attachment to fibronectin induces tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin in parental cells and cells transfected with the wild-type PTP1B, while in cells transfected with the mutant PTP1B, such induction is not observed. Additionally, in cells expressing the mutant PTP1B, tyrosine phosphorylation of Src is enhanced and activity is reduced. Lysophosphatidic acid temporarily reverses the effects of the mutant PTP1B, suggesting the existence of a signaling pathway triggering focal adhesion assembly that bypasses the need for active PTP1B. PTP1B coimmunoprecipitates with beta1-integrin from nonionic detergent extracts and colocalizes with vinculin and the ends of actin stress fibers in focal adhesions. Our data suggest that PTP1B is a critical regulatory component of integrin signaling pathways, which is essential for adhesion, spreading, and formation of focal adhesions.

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.

The counteradhesive protein SPARC regulates an endothelial paracellular pathway through protein tyrosine phosphorylation

SPARC (Secreted Protein Acidic and Rich in Cysteine) regulates the transendothelial flux of macromolecules through a paracellular pathway. We now have demonstrated that SPARC-induced increments in albumin flux across postconfluent endothelial cell (EC) monolayers are mediated, in part, through protein tyrosine phosphorylation. SPARC increased tyrosine phosphorylation of EC proteins up to 12-fold within 1 h. The phosphotyrosine-containing proteins were immunolocalized to the intercellular boundaries. Two substrates for SPARC-induced tyrosine phosphorylation were identified as beta-catenin and paxillin. Inhibition of tyrosine kinases with herbimycin A or genistein reversed the barrier dysfunction induced by SPARC by 71% and 49%, respectively. Herbimycin A also protected against SPARC-induced intercellular gap formation. In contrast, inhibition of tyrosine phosphatases with sodium orthovanadate or phenylarsine oxide enhanced the loss of barrier function associated with SPARC treatment by 120% and 88%, respectively. These data indicate that SPARC influences EC-EC interactions through a tyrosine phosphorylation-dependent signaling pathway.

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.

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.

H2O2 and genistein differentially modulate protein tyrosine phosphorylation, endothelial morphology, and monolayer barrier function

The effects of hydrogen peroxide (H2O2) and the protein tyrosine kinase (PTK) inhibitor, genistein, to modulate protein tyrosine phosphorylation (PTP) and endothelial barrier function were examined in bovine pulmonary artery endothelial cell (EC) monolayers. H2O2 stimulated a concentration (100-800 microM) and time-dependent increase in the phosphotyrosine (PY) content of multiple (56-72, 93-97, 113-142, and 161-183 kDa) EC proteins. A size-selected solute permeability assay of EC monolayer barrier function showed that (200 microM) H2O2 elevated EC monolayer permeability to large solutes. This effect was associated with paracellular hole formation and a loss of beta-catenin immunostaining at these sites. In contrast, genistein (100 microM, 1 h) reduced basal PY protein content and reorganized F-actin to beta-catenin containing cell-cell junctions, enhancing endothelial monolayer barrier function. In addition, genistein prevented the H2O2-induced increases in tyrosine phosphorylation, monolayer permeability, and paracellular hole formation. These data suggest that H2O2 and genistein differentially regulate PTP, endothelial morphology, and monolayer barrier function.

E-Cadherin-dependent growth suppression is mediated by the cyclin-dependent kinase inhibitor p27(KIP1)

Recent studies have demonstrated the importance of E-cadherin, a homophilic cell-cell adhesion molecule, in contact inhibition of growth of normal epithelial cells. Many tumor cells also maintain strong intercellular adhesion, and are growth-inhibited by cell- cell contact, especially when grown in three-dimensional culture. To determine if E-cadherin could mediate contact-dependent growth inhibition of nonadherent EMT/6 mouse mammary carcinoma cells that lack E-cadherin, we transfected these cells with an exogenous E-cadherin expression vector. E-cadherin expression in EMT/6 cells resulted in tighter adhesion of multicellular spheroids and a reduced proliferative fraction in three-dimensional culture. In addition to increased cell-cell adhesion, E-cadherin expression also resulted in dephosphorylation of the retinoblastoma protein, an increase in the level of the cyclin-dependent kinase inhibitor p27(kip1) and a late reduction in cyclin D1 protein. Tightly adherent spheroids also showed increased levels of p27 bound to the cyclin E-cdk2 complex, and a reduction in cyclin E-cdk2 activity. Exposure to E-cadherin-neutralizing antibodies in three-dimensional culture simultaneously prevented adhesion and stimulated proliferation of E-cadherin transfectants as well as a panel of human colon, breast, and lung carcinoma cell lines that express functional E-cadherin. To test the importance of p27 in E-cadherin-dependent growth inhibition, we engineered E-cadherin-positive cells to express inducible p27. By forcing expression of p27 levels similar to those observed in aggregated cells, the stimulatory effect of E-cadherin-neutralizing antibodies on proliferation could be inhibited. This study demonstrates that E-cadherin, classically described as an invasion suppressor, is also a major growth suppressor, and its ability to inhibit proliferation involves upregulation of the cyclin-dependent kinase inhibitor p27.

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.

Distribution of a murine protein tyrosine phosphatase BL-beta-galactosidase fusion protein suggests a role in neurite outgrowth

We have generated a gene trap insertion into the protein tyrosine phosphatase-BL (PTP-BL) locus, which produces a fusion of the N-terminal half of PTP-BL with beta-galactosidase. During development, beta-galactosidase activity was seen in all epithelial cells: strong staining was observed in the stomach and kidney epithelium, the ependymal layer of the central nervous system, and the surface ectoderm. Particularly prominent beta-galactosidase activity was seen in the peripheral nervous system, which correlated with neurite outgrowth. In epithelial cells, staining was seen in the apical portion of the cells. In nerves, beta-galactosidase activity was associated with growth cones as well as with Schwann cells. This suggests that the amino-terminal portion of PTP-BL contains sequences sufficient to target the fusion protein to specific subcellular compartments. In situ hybridization with a PTP-BL probe demonstrated that all tissues in which beta-galactosidase activity was seen were genuine sites of expression of the PTP-BL gene, although differences in the stability of the PTP-BL protein and the PTP-BL-beta-galactosidase fusion protein may exist. The distribution of beta-galactosidase activity in the peripheral nervous system, together with the structure of the wild-type protein, suggests that this phosphatase may have a role in regulation of the cytoskeleton during the development of the peripheral nervous system.

Increased tyrosine phosphorylation causes redistribution of adherens junction and tight junction proteins and perturbs paracellular barrier function in MDCK epithelia

Polarized monolayers of strain II Madin-Darby canine kidney cells (MDCK II) were treated with vanadate/H2O2, known inhibitors of protein tyrosine phosphatase activity. Vanadate/H2O2 treatment resulted in a rapid increase in paracellular permeability as revealed by decreased transepithelial resistance and increased permeability to inulin. These alterations in epithelial barrier function coincided with increased phosphotyrosine immunofluorescence in the vicinity of intercellular junctions and with redistribution of F-actin, the adherens junction protein E-cadherin and the tight junction protein ZO-1. The effects of vanadate/H2O2 on intercellular junction permeability and protein distribution were completely blocked by the specific protein tyrosine kinase (PTK) inhibitor tyrphostin 25 and partially inhibited by the alternative PTK inhibitor genistein. The relative potency of these two inhibitors in blocking the effects of vanadate/H2O2 on intercellular junctions correlated with their abilities to inhibit tyrosine phosphorylation. The potent ser/thr protein kinase inhibitor staurosporine had only a small influence on the vanadate/H2O2-induced increase in paracellular permeability and did not affect the observed redistribution of intercellular junction proteins or phosphotyrosine immunofluorescence. The relative potencies of these distinct protein kinase inhibitors in reversing the effects of vanadate/H2O2 indicate that these effects are directly related to tyrosine phosphorylation. In conclusion, our data provide evidence that enhanced tyrosine phosphorylation of intercellular junction proteins in MDCK epithelia increases paracellular permeability and can also induce prominent reorganization of the junctional complex.