Regulation of cell-cell adhesion of MDCK cells by Cdc42 and Rac1 small GTPases

Epithelial polarization in the 3D matrix requires MST3 signaling to regulate ZO-1 position

Apical-basal cell polarity must be tightly controlled for epithelial cyst and tubule formation, and these are important functional units in various epithelial organs. Polarization is achieved through the coordination of several molecules that divide cells into an apical domain and a basolateral domain, which are separated from tight and adherens junctions. Cdc42 regulates cytoskeletal organization and the tight junction protein ZO-1 at the apical margin of epithelial cell junctions. MST kinases control organ size through the regulation of cell proliferation and cell polarity. For example, MST1 relays the Rap1 signal to induce cell polarity and adhesion of lymphocytes. Our previous study showed that MST3 was involved in E-cadherin regulation and migration in MCF7 cells. In vivo, MST3 knockout mice exhibited higher ENaC expression at the apical site of renal tubules, resulting in hypertension. However, it was not clear whether MST3 was involved in cell polarity. Here, control MDCK cells, HA-MST3 and HA-MST3 kinase-dead (HA-MST3-KD) overexpressing MDCK cells were cultured in collagen or Matrigel. We found that the cysts of HA-MST3 cells were fewer and smaller than those of control MDCK cells; ZO-1 was delayed to the apical site of cysts and in cell-cell contact in the Ca2+ switch assay. However, HA-MST3-KD cells exhibited multilumen cysts. Intensive F-actin stress fibers were observed in HA-MST3 cells with higher Cdc42 activity; in contrast, HA-MST3-KD cells had lower Cdc42 activity and weaker F-actin staining. In this study, we identified a new MST3 function in the establishment of cell polarity through Cdc42 regulation.

Changes in E-cadherin rigidity sensing regulate cell adhesion

Mechanical cues are sensed and transduced by cell adhesion complexes to regulate diverse cell behaviors. Extracellular matrix (ECM) rigidity sensing by integrin adhesions has been well studied, but rigidity sensing by cadherins during cell adhesion is largely unexplored. Using mechanically tunable polyacrylamide (PA) gels functionalized with the extracellular domain of E-cadherin (Ecad-Fc), we showed that E-cadherin-dependent epithelial cell adhesion was sensitive to changes in PA gel elastic modulus that produced striking differences in cell morphology, actin organization, and membrane dynamics. Traction force microscopy (TFM) revealed that cells produced the greatest tractions at the cell periphery, where distinct types of actin-based membrane protrusions formed. Cells responded to substrate rigidity by reorganizing the distribution and size of high-traction-stress regions at the cell periphery. Differences in adhesion and protrusion dynamics were mediated by balancing the activities of specific signaling molecules. Cell adhesion to a 30-kPa Ecad-Fc PA gel required Cdc42- and formin-dependent filopodia formation, whereas adhesion to a 60-kPa Ecad-Fc PA gel induced Arp2/3-dependent lamellipodial protrusions. A quantitative 3D cell-cell adhesion assay and live cell imaging of cell-cell contact formation revealed that inhibition of Cdc42, formin, and Arp2/3 activities blocked the initiation, but not the maintenance of established cell-cell adhesions. These results indicate that the same signaling molecules activated by E-cadherin rigidity sensing on PA gels contribute to actin organization and membrane dynamics during cell-cell adhesion. We hypothesize that a transition in the stiffness of E-cadherin homotypic interactions regulates actin and membrane dynamics during initial stages of cell-cell adhesion.

The interdependence of the Rho GTPases and apicobasal cell polarity

Signaling via the Rho GTPases provides crucial regulation of numerous cell polarization events, including apicobasal (AB) polarity, polarized cell migration, polarized cell division and neuronal polarity. Here we review the relationships between the Rho family GTPases and epithelial AB polarization events, focusing on the 3 best-characterized members: Rho, Rac and Cdc42. We discuss a multitude of processes that are important for AB polarization, including lumen formation, apical membrane specification, cell-cell junction assembly and maintenance, as well as tissue polarity. Our discussions aim to highlight the immensely complex regulatory mechanisms that encompass Rho GTPase signaling during AB polarization. More specifically, in this review we discuss several emerging common themes, that include: 1) the need for Rho GTPase activities to be carefully balanced in both a spatial and temporal manner through a multitude of mechanisms; 2) the existence of signaling feedback loops and crosstalk to create robust cellular responses; and 3) the frequent multifunctionality that exists among AB polarity regulators. Regarding this latter theme, we provide further discussion of the potential plasticity of the cell polarity machinery and as a result the possible implications for human disease.

Mutant B-RAF regulates a Rac-dependent cadherin switch in melanoma

The ability of cells to invade into the dermis is a critical event in the development of cutaneous melanoma and ultimately an indicator of poor prognosis. However, the molecular events surrounding the acquisition of this invasive phenotype remain incompletely understood. Mutations in B-RAF are frequent in melanoma and are known to regulate the invasive phenotype. In this study, we sought to determine the molecular mechanisms controlling melanoma invasion. We found that mutant B-RAF signaling regulates a cadherin switch. In melanoma cells expressing mutant B-RAF we observed high levels of N-cadherin and low levels of E-cadherin. Depletion of mutant B-RAF, by siRNA, caused a decrease in the levels of N-cadherin and an increase in the levels of E-cadherin. Mechanistically, we found that this cadherin switch required the activity of Rac1 and its GEF, Tiam1, both of which show suppressed activity in the presence of mutant B-RAF. Consistent with the work of others, we found that depletion of mutant B-RAF decreased the invasive capacity of the melanoma cells. However, simultaneous depletion of B-RAF and Rac or Tiam1 resulted in invasive capacity similar to that of control cells. Taken together, our results suggest that mutant B-RAF signaling downregulates Tiam1/Rac activity resulting in an increase in N-cadherin levels and a decrease in E-cadherin levels and ultimately enhanced invasion.

The effects of modifying RhoA and Rac1 activities on heterotypic contact inhibition of locomotion

Contact inhibition of locomotion (CIL) occurs when a cell ceases moving in the same direction following contact with another cell. Homotypic and heterotypic CIL occur between cells of the same and different types, respectively. Using Abercrombie's confronted explants assay we studied the effect of changing Rac1 or RhoA activities on heterotypic CIL between NIH3T3 and chicken heart fibroblasts. Both dominant active (L61) and dominant negative (N17) Rac1 expressed in NIH3T3 cells resulted in loss of heterotypic CIL. N17Rac1 expression caused RhoA activation. Increasing RhoA activity directly (V14RhoA) or indirectly (downregulation of N-cadherin or p120-catenin) also resulted in loss of CIL. High RhoA activity has been associated with tumour invasion and our results are consistent with loss of heterotypic CIL playing a role.

Rac1 acts in conjunction with Nedd4 and dishevelled-1 to promote maturation of cell-cell contacts

The Rho-GTPase Rac1 promotes actin polymerization and membrane protrusion that mediate initial contact and subsequent maturation of cell-cell junctions. Here we report that Rac1 associates with the ubiquitin-protein ligase neural precursor cell expressed developmentally down-regulated 4 (Nedd4). This interaction requires the hypervariable C-terminal domain of Rac1 and the WW domains of Nedd4. Activated Rac1 colocalises with endogenous Nedd4 at epithelial cell-cell contacts. Reduction of Nedd4 expression by shRNA results in reduced transepithelial electrical resistance (TER) and concomitant changes in the distribution of adherens and tight junction markers. Conversely, expression of Nedd4 promotes TER, suggesting that Nedd4 cooperates with Rac1 in the induction of junctional maturation. We found that Nedd4, but not Nedd4-2, mediates the ubiquitylation and degradation of the adapter protein dishevelled-1 (Dvl1), the expression of which negatively regulates cell-cell contact. Nedd4-mediated ubiquitylation requires its binding to the C-terminal domain of Dvl1, comprising the DEP domain, and targets an N-terminal lysine-rich region upstream of the Dvl1 DIX domain. We found that endogenous Rac1 colocalises with endogenous Dvl1 in intracellular puncta as well as on cell-cell junctions. Finally, activated Rac1 was found to stimulate Nedd4 activity, resulting in increased ubiquitylation of Dvl1. Together, these data reveal a novel Rac1-dependent signalling pathway that, through Nedd4-mediated ubiquitylation of Dvl1, stimulates the maturation of epithelial cell-cell contacts.

Rac1 and RhoA GTPases have antagonistic functions during N-cadherin-dependent cell-cell contact formation in C2C12 myoblasts

BACKGROUND INFORMATION

N-cadherin, a member of the Ca(2+)-dependent cell-cell adhesion molecule family, plays an essential role in the induction of the skeletal muscle differentiation programme. However, the molecular mechanisms which govern the formation of N-cadherin-dependent cell-cell contacts in myoblasts remain unexplored.

RESULTS

In the present study, we show that N-cadherin-dependent cell contact formation in myoblasts is defined by two stages. In the first phase, N-cadherin is highly mobile in the lamellipodia extensions between the contacting cells. The second stage corresponds to the formation of mature N-cadherin-dependent cell contacts, characterized by the immobilization of a pool of N-cadherin which appears to be clustered in the interdigitated membrane structures that are also membrane attachment sites for F-actin filaments. We also demonstrated that the formation of N-cadherin-dependent cell-cell contacts requires a co-ordinated and sequential activity of Rac1 and RhoA. Rac1 is involved in the first stage and facilitates N-cadherin-dependent cell-cell contact formation, but it is not absolutely required. Conversely, RhoA is necessary for N-cadherin-dependent cell contact formation, since, via ROCK (Rho-associated kinase) signalling and myosin 2 activation, it allows the stabilization of N-cadherin at the cell-cell contact sites.

CONCLUSIONS

We have shown that Rac1 and RhoA have opposite effects on N-cadherin-dependent cell-cell contact formation in C2C12 myoblasts and act sequentially to allow its formation.

Tissue organization by cadherin adhesion molecules: dynamic molecular and cellular mechanisms of morphogenetic regulation

This review addresses the cellular and molecular mechanisms of cadherin-based tissue morphogenesis. Tissue physiology is profoundly influenced by the distinctive organizations of cells in organs and tissues. In metazoa, adhesion receptors of the classical cadherin family play important roles in establishing and maintaining such tissue organization. Indeed, it is apparent that cadherins participate in a range of morphogenetic events that range from support of tissue integrity to dynamic cellular rearrangements. A comprehensive understanding of cadherin-based morphogenesis must then define the molecular and cellular mechanisms that support these distinct cadherin biologies. Here we focus on four key mechanistic elements: the molecular basis for adhesion through cadherin ectodomains, the regulation of cadherin expression at the cell surface, cooperation between cadherins and the actin cytoskeleton, and regulation by cell signaling. We discuss current progress and outline issues for further research in these fields.

Rapid suppression of activated Rac1 by cadherins and nectins during de novo cell-cell adhesion

Cell-cell adhesion in simple epithelia involves the engagement of E-cadherin and nectins, and the reorganization of the actin cytoskeleton and membrane dynamics by Rho GTPases, particularly Rac1. However, it remains unclear whether E-cadherin and nectins up-regulate, maintain or suppress Rac1 activity during cell-cell adhesion. Roles for Rho GTPases are complicated by cell spreading and integrin-based adhesions to the extracellular matrix that occur concurrently with cell-cell adhesion, and which also require Rho GTPases. Here, we designed a simple approach to examine Rac1 activity upon cell-cell adhesion by MDCK epithelial cells, without cell spreading or integrin-based adhesion. Upon initiation of cell-cell contact in 3-D cell aggregates, we observed an initial peak of Rac1 activity that rapidly decreased by ∼66% within 5 minutes, and further decreased to a low baseline level after 30 minutes. Inhibition of E-cadherin engagement with DECMA-1 Fab fragments or competitive binding of soluble E-cadherin, or nectin2alpha extracellular domain completely inhibited Rac1 activity. These results indicate that cadherins and nectins cooperate to induce and then rapidly suppress Rac1 activity during initial cell-cell adhesion, which may be important in inhibiting the migratory cell phenotype and allowing the establishment of initially weak cell-cell adhesions.

Faulty epithelial polarity genes and cancer

Epithelial architecture is formed in tissues and organs when groups of epithelial cells are organized into polarized structures. The epithelial function and integrity as well as signaling across the epithelial layer is orchestrated by apical junctional complexes (AJCs), which are landmarks for PAR/CRUMBS and lateral SCRIB polarity modules and by dynamic interactions of the cells with underlying basement membrane (BM). These highly organized epithelial architectures are demolished in cancer. In all advanced epithelial cancers, malignant cells have lost polarity and connections to the basement membrane and they have become proliferative, motile, and invasive. Clearly, loss of epithelial integrity associates with tumor progression but does it contribute to tumor development? Evidence from studies in Drosophila and recently also in vertebrate models have suggested that even the oncogene-driven enforced cell proliferation can be conditional, dependant on the influence of cell-cell or cell-microenvironment contacts. Therefore, loss of epithelial integrity may not only be an obligate consequence of unscheduled proliferation of malignant cells but instead, malignant epithelial cells may need to acquire capacity to break free from the constraints of integrity to freely and autonomously proliferate. We discuss how epithelial polarity complexes form and regulate epithelial integrity, highlighting the roles of enzymes Rho GTPases, aPKCs, PI3K, and type II transmembrane serine proteases (TTSPs). We also discuss relevance of these pathways to cancer in light of genetic alterations found in human cancers and review molecular pathways and potential pharmacological strategies to revert or selectively eradicate disorganized tumor epithelium.

Zebrafish teeth as a model for repetitive epithelial morphogenesis: dynamics of E-cadherin expression

BACKGROUND

The development of teeth is the result of interactions between competent mesenchyme and epithelium, both of which undergo extensive morphogenesis. The importance of cell adhesion molecules in morphogenesis has long been acknowledged but remarkably few studies have focused on the distribution and function of these molecules in tooth development.

RESULTS

We analyzed the expression pattern of an important epithelial cadherin, E-cadherin, during the formation of first-generation teeth as well as replacement teeth in the zebrafish, using in situ hybridization and whole mount immunostaining to reveal mRNA expression and protein distribution. E-cadherin was detected in every layer of the enamel organ during the different stages of tooth development, but there were slight differences between first-generation and replacement teeth in the strength and distribution of the signal. The dental papilla, which is derived from the mesenchyme, did not show any expression. Remarkably, the crypts surrounding the functional teeth showed an uneven distribution of E-cadherin throughout the pharyngeal region.

CONCLUSIONS

The slight differences between E-cadherin expression in zebrafish teeth and developing mouse and human teeth are discussed in the light of fundamental differences in structural and developmental features of the dentition between zebrafish and mammals. Importantly, the uninterrupted expression of E-cadherin indicates that down-regulation of E-cadherin is not required for formation of an epithelial tooth bud. Further research is needed to understand the role of other cell adhesion systems during the development of teeth and the formation of replacement teeth.

A vertebrate-specific Chp-PAK-PIX pathway maintains E-cadherin at adherens junctions during zebrafish epiboly

Background

In early vertebrate development, embryonic tissues modulate cell adhesiveness and acto-myosin contractility to correctly orchestrate the complex processes of gastrulation. E-cadherin (E-cadh) is the earliest expressed cadherin and is needed in the mesendodermal progenitors for efficient migration [1], [2]. Regulatory mechanisms involving directed E-cadh trafficking have been invoked downstream of Wnt11/5 signaling [3]. This non-canonical Wnt pathway regulates RhoA-ROK/DAAM1 to control the acto-myosin network. However, in this context nothing is known of the intracellular signals that participate in the correct localization of E-cadh, other than a need for Rab5c signaling [3].

Methodology/Principal Findings

By studying loss of Chp induced by morpholino-oligonucleotide injection in zebrafish, we find that the vertebrate atypical Rho-GTPase Chp is essential for the proper disposition of cells in the early embryo. The underlying defect is not leading edge F-actin assembly (prominent in the cells of the envelope layer), but rather the failure to localize E-cadh and β-catenin at the adherens junctions. Loss of Chp results in delayed epiboly that can be rescued by mRNA co-injection, and phenocopies zebrafish E-cadh mutants [4], [5]. This new signaling pathway involves activation of an effector kinase PAK, and involvement of the adaptor PAK-interacting exchange factor PIX. Loss of signaling by any of the three components results in similar underlying defects, which is most prominent in the epithelial-like envelope layer.

Conclusions/Significance

Our current study uncovers a developmental pathway involving Chp/PAK/PIX signaling, which helps co-ordinate E-cadh disposition to promote proper cell adhesiveness, and coordinate movements of the three major cell layers in epiboly. Our data shows that without Chp signaling, E-cadh shifts to intracellular vesicles rather than the adhesive contacts needed for directed cell movement. These events may mirror the requirement for PAK2 signaling essential for the proper formation of the blood-brain barrier [6], [7].

Glycogen synthase kinase 3 in the world of cell migration

Glycogen synthase kinase 3 (GSK3) is one of the few master switch kinases that regulate many aspects of cell functions. Recent studies on cell polarization and migration have shown that GSK3 is also essential for proper regulation of these processes. GSK3 influences cell migration as one of the regulators of the spatiotemporally controlled dynamics of the actin cytoskeleton, microtubules, and cell-to-matrix adhesions. In this mini-review, the effects of GSK3 on these three aspects of cell migration will be discussed.

Rac1 activation inhibits E-cadherin-mediated adherens junctions via binding to IQGAP1 in pancreatic carcinoma cells

Background

Monomeric GTPases of the Rho family control a variety of cellular functions including actin cytoskeleton organisation, cell migration and cell adhesion. Defects in these regulatory processes are involved in tumour progression and metastasis. The development of metastatic carcinoma is accompanied by deregulation of adherens junctions, which are composed of E-cadherin/β- and α-catenin complexes.

Results

Here, we show that the activity of the monomeric GTPase Rac1 contributes to inhibition of E-cadherin-mediated cell-cell adhesion in pancreatic carcinoma cells. Stable expression of constitutively active Rac1(V12) reduced the amount of E-cadherin on protein level in PANC-1 pancreatic carcinoma cells, whereas expression of dominant negative Rac1(N17) resulted in an increased amount of E-cadherin. Extraction of proteins associated with the actin cytoskeleton as well as coimmunoprecipitation analyses demonstrated markedly decreased amounts of E-cadherin/catenin complexes in Rac1(V12)-expressing cells, but increased amounts of functional E-cadherin/catenin complexes in cells expressing Rac1(N17). Cell aggregation and migration assays revealed, that cells containing less E-cadherin due to expression of Rac1(V12), exhibited reduced cell-cell adhesion and increased cell motility. The Rac/Cdc42 effector protein IQGAP1 has been implicated in regulating cell-cell adhesion. Coimmunoprecipitation studies showed a decrease in the association between IQGAP1 and β-catenin in Rac1(V12)-expressing PANC-1 cells and an association of IQGAP1 with Rac1(V12). Elevated association of IQGAP1 with the E-cadherin adhesion complex via β-catenin correlated with increased intercellular adhesion of PANC-1 cells.

Conclusion

These results indicate that active Rac1 destabilises E-cadherin-mediated cell-cell adhesion in pancreatic carcinoma cells by interacting with IQGAP1 which is associated with a disassembly of E-cadherin-mediated adherens junctions. Inhibition of Rac1 activity induced increased E-cadherin-mediated cellular adhesion.

Multifaceted role of Rho, Rac, Cdc42 and Ras in intercellular junctions, lessons from toxins

Tight junctions (TJs) and adherens junctions (AJs) are dynamic structures linked to the actin cytoskeleton, which control the paracellular permeability of epithelial and endothelial barriers. TJs and AJs are strictly regulated in a spatio-temporal manner by a complex signaling network, including Rho/Ras-GTPases, which have a pivotal role. Rho preferentially regulates TJs by controlling the contraction of apical acto-myosin filaments, whereas Rac/Cdc42 mainly coordinate the assembly-disassembly of AJ components. However, a subtle balance of Rho/Ras-GTPase activity and interplay between these molecules is required to maintain an optimal organization and function of TJs and AJs. Conversely, integrity of intercellular junctions generates signals through Rho-GTPases, which are involved in the regulation of multiple cellular processes. Rho/Ras-GTPases and the control of intercellular junctions are the target of various bacterial toxins responsible for severe diseases in man and animals, and are part of their mechanism of action. This review focuses on the regulation of TJs and AJs by Rho/Ras-GTPases through molecular approaches and bacterial toxins.

Anchoring junctions as drug targets: role in contraceptive development

In multicellular organisms, cell-cell interactions are mediated in part by cell junctions, which underlie tissue architecture. Throughout spermatogenesis, for instance, preleptotene leptotene spermatocytes residing in the basal compartment of the seminiferous epithelium must traverse the blood-testis barrier to enter the adluminal compartment for continued development. At the same time, germ cells must also remain attached to Sertoli cells, and numerous studies have reported extensive restructuring at the Sertoli-Sertoli and Sertoli-germ cell interface during germ cell movement across the seminiferous epithelium. Furthermore, the proteins and signaling cascades that regulate adhesion between testicular cells have been largely delineated. These findings have unveiled a number of potential "druggable" targets that can be used to induce premature release of germ cells from the seminiferous epithelium, resulting in transient infertility. Herein, we discuss a novel approach with the aim of developing a nonhormonal male contraceptive for future human use, one that involves perturbing adhesion between Sertoli and germ cells in the testis.

On The Biomedical Promise of Cell Penetrating Peptides: Limits Versus Prospects

The cell membrane poses a substantial hurdle to the use of pharmacologically active biomacromolecules that are not per se actively translocated into cells. An appealing approach to deliver such molecules involves tethering or complexing them with so-called cell penetrating peptides (CPPs) that are able to cross the plasma membrane of mammalian cells. The CPP approach is currently a major avenue in engineering delivery systems that are hoped to mediate the non-invasive import of problematic cargos into cells. The large number of different cargo molecules that have been efficiently delivered by CPPs ranges from small molecules to proteins and even liposomes and particles. With respect to the involved mechanism(s) there is increasing evidence for endocytosis as a major route of entry. Moreover, in terms of intracellular trafficking, current data argues for the transport to acidic early endosomal compartments with cytosolic release mediated via retrograde delivery through the Golgi apparatus and the endoplasmic reticulum. The focus of this review is to revisit the performance of cell penetrating peptides for drug delivery. To this aim we cover both accomplishments and failures and report on new prospects of the CPP approach. Besides a selection of successful case histories of CPPs we also review the limitations of CPP mediated translocation. In particular, we comment on the impact of (i) metabolic degradation, (ii) the cell line and cellular differentiation state dependent uptake of CPPs, as well as (iii) the regulation of their endocytic traffic by Rho-family GTPases. Further on, we aim at the identification of promising niches for CPP application in drug delivery. In this context, as inspired by current literature, we focus on three principal areas: (i) the delivery of antineoplastic agents, (ii) the delivery of CPPs as antimicrobials, and (iii) the potential of CPPs to target inflammatory tissues.

MEK/ERK regulates adherens junctions and migration through Rac1

Polyamine depletion with the ornithine decarboxylase inhibitor alpha-difluoromethyl ornithine (DFMO), prevents Rac1 activation causing the formation of a thick actin cortex at the cell periphery and inhibits migration of intestinal epithelial cells. In the present study, we demonstrate that MEK activation by EGF increased Rac1 activation, dissociation of intercellular contacts, and migration in both control and polyamine-depleted cells, while U0126, a specific inhibitor of MEK1, prevented disruption of junctions as well as EGF-induced Rac1 activation. Constitutively active MEK1 (CA-MEK) expression altered cell-cell contacts in control and polyamine depleted cells. The expression of constitutively active Rac1 (CA-Rac1) restored beta-catenin to the cell periphery and prevented the formation of actin cortex and caused the appearance of F-actin stress fibers in polyamine-depleted cells. Inhibition of Rac activation by NSC23766, a specific inhibitor of Tiam1, an upstream guanidine nucleotide exchange factor for Rac1, reproduced the beta-catenin localization and actin structure of polyamine-depleted cells. Tiam1 localized more extensively with beta-catenin at the cell periphery in CA-Rac1 cells compared to vector cells. Polyamine depletion decreased the expression of E-cadherin to a greater extent compared to beta-catenin. Subcellular fractionation further confirmed our immuno-localization and western blotting observations. These data suggest that EGF acting through MEK1/ERK to activate Rac1 regulates cell-cell contacts. Thus, decreased migration in polyamine depleted cells may be due to the inhibition of Tiam1 activation of Rac1 and the subsequent decreased expression of beta-catenin and E-cadherin leading to reduced cell-cell contacts.

Differentiation restricted endocytosis of cell penetrating peptides in MDCK cells corresponds with activities of Rho-GTPases

PURPOSE

Cellular entry of biomacromolecules is restricted by the barrier function of cell membranes. Tethering such molecules to cell penetrating peptides (CPPs) that can translocate cell membranes has opened new horizons in biomedical research. Here, we investigate the cellular internalization of hCT(9-32)-br, a human calcitonin derived branched CPP, and SAP, a gamma-zein related sequence.

METHODS

Internalization of fluorescence labelled CPPs was performed with both proliferating and confluent MDCK cells by means of confocal laser scanning microscopy (CLSM) and fluorescence activated cell sorting (FACS) using appropriate controls. Internalization was further elaborated in an inflammatory, IFN-gamma/TNF-alphaa induced confluent MDCK model mimicking inflammatory epithelial pathologies. Activities of active form Rho-GTPases (Rho-A and Rac-1) in proliferating and confluent MDCK cells were monitored by pull-down assay and Western blot analysis.

RESULTS

We observed marked endocytic uptake of the peptides into proliferating MDCK by a process suggesting both lipid rafts and clathrin-coated pits. In confluent MDCK, however, we noted a massive but compound-unspecific slow-down of endocytosis. This corresponded with a down-regulation of endocytosis by Rho-GTPases, previously identified to be intimately involved in endocytic traffic. In fact, we found endocytic internalization to relate with active Rho-A; vice versa, MDCK cell density, degree of cellular differentiation and endocytic slow-down were found to relate with active Rac-1. To our knowledge, this is the first study to cast light on the previously observed differentiation restricted internalization of CPPs into epithelial cell models. In the inflammatory IFN-gamma/TNF-alphaa induced confluent MDCK model mimicking inflammatory epithelial pathologies, CPP internalization was enhanced in a cytokine concentration-dependent way resulting in maximum enhancement rates of up to 90%. We suggest a cytokine induced redistribution of lipid rafts in confluent MDCK to cause this enhancement.

CONCLUSION

Our findings emphasize the significance of differentiated cell models in the study of CPP internalization and point towards inflammatory epithelial pathologies as potential niche for the application of CPPs for cellular delivery.

Rac-WAVE-mediated actin reorganization is required for organization and maintenance of cell-cell adhesion

During cadherin-dependent cell-cell adhesion, the actin cytoskeleton undergoes dynamic reorganization in epithelial cells. Rho-family small GTPases, which regulate actin dynamics, play pivotal roles in cadherin-dependent cell-cell adhesion; however, the precise molecular mechanisms that underlie cell-cell adhesion formation remain unclear. Here we show that Wiskott-Aldrich syndrome protein family verprolin-homologous protein (WAVE)-mediated reorganization of actin, downstream of Rac plays an important role in normal development of cadherin-dependent cell-cell adhesions in MDCK cells. Rac-induced development of cadherin-dependent adhesions required WAVE2-dependent actin reorganization. The process of cell-cell adhesion is divided into three steps: formation of new cell-cell contacts, stabilization of these new contacts and junction maturation. WAVE1 and WAVE2 were expressed in MDCK cells. The functions of WAVE1 and WAVE2 were redundant in this system but WAVE2 appeared to play a more significant role. During the first step, WAVE2-dependent lamellipodial protrusions facilitated formation of cell-cell contacts. During the second step, WAVE2 recruited actin filaments to new cell-cell contacts and stabilized newly formed cadherin clusters. During the third step, WAVE2-dependent actin reorganization was required for organization and maintenance of mature cell-cell adhesions. Thus, Rac-WAVE-dependent actin reorganization is not only involved in formation of cell-cell adhesions but is also required for their maintenance.

VCAM-1 inhibits TGFβ stimulated epithelial–mesenchymal transformation by modulating Rho activity and stabilizing intercellular adhesion in epicardial mesothelial cells

Regulation of epithelial-mesenchymal transformation (EMT) is of central importance both in normal development and in disease. During heart development, cells of the superficial epicardial mesothelium undergo EMT to give rise to precursor cells of the coronary vasculature and cardiac fibroblasts. Here we report that the alpha(4)beta(1) integrin ligand, VCAM-1, inhibits EMT of chick epicardial mesothelial cells stimulated by TGFbeta isoforms. We further investigated the molecular basis of this inhibition using cultured chick embryonic and rat adult epicardial mesothelial cells. We observed that VCAM-1 increased cortical actin filaments at intercellular junctions and reduced stress fibers across epicardial cells. VCAM-1 inhibited stress fiber formation by TGFbeta1, TGFbeta2, TGFbeta3 and lysophosphatidic acid and altered Rho activity stimulated by TGFbeta3. This was accompanied by an increase in tyrosine phosphorylation of p190RhoGAP. All three TGFbeta isoforms weakened intercellular adhesion, reduced membrane localization of beta-catenin and E-cadherin and stimulated epicardial EMT in chick hearts. Each of these effects was restricted by simultaneous VCAM-1 treatment. Our data support the hypothesis that VCAM-1 can alter epicardial EMT at two key points: it limits Rho-dependent events such as stress fiber formation and it maintains the association of beta-catenin and E-cadherin with the adherens junction.

A novel agent, methylophiopogonanone B, promotes Rho activation and tubulin depolymerization

Cytoskeletal reorganization, including reconstruction of actin fibers and microtubules, is essential for various biological processes, such as cell migration, proliferation and dendrite formation. We show here that methylophiopogonanone B (MOPB) induces cell morphological change via melanocyte dendrite retraction and stress fiber formation. Since members of the Rho family of small GTP-binding proteins act as master regulators of dendrite formation and actin cytoskeletal reorganization, and activated Rho promotes dendrite retraction and stress fiber formation, we studied the effects of MOPB on the small GTPases using normal human epidermal melanocytes and HeLa cells. In in vitro binding assay, MOPB significantly increased GTP-Rho, but not GTP-Rac or GTP-CDC42. Furthermore, a Rho inhibitor, a Rho kinase inhibitor and a small GTPase inhibitor each blocked MOPB-induced stress fiber formation. The effect of MOPB on actin reorganization was blocked in a Rho dominant negative mutant. These results suggest MOPB acts via the Rho signaling pathway, and it may directly or indirectly activate Rho. Quantitative Western blot analysis indicated that MOPB also induced microtubule destabilization and tubulin depolymerization. Thus, MOPB appears to induce Rho activation, resulting in actin cytoskeletal reorganization, including dendrite retraction and stress fiber formation.

Glycogen synthase kinase-3 acts upstream of ADP-ribosylation factor 6 and Rac1 to regulate epithelial cell migration

Cell sheet movement during epithelial wound closure is a complex process involving collective cell migration. We have found that glycogen synthase kinase-3 (GSK-3) activity is required for membrane protrusion and crawling of cells at the wound edge and those behind it in wounded Madin-Darby canine kidney (MDCK) epithelial cell monolayers. RNA interference-based silencing of GSK-3alpha and GSK-3beta expression also results in slowed cell sheet migration, with the effect being more pronounced with knockdown of GSK-3beta. Both GSK-3alpha and GSK-3beta are in activated states during the most active phase of cell migration. In addition to having a positive control or permissive, rather than negative, function in MDCK cell migration, GSK-3 appears to act upstream of the small GTPases ADP-ribosylation factor 6 (ARF6) and Rac1. Expression of constitutively active ARF6 restores a protrusive, migratory phenotype in cells treated with GSK-3 inhibitors. It does not, however, restore to normal levels the directional polarization of cells behind the wound edge toward the wound area, implying the existence of a separate ARF6-independent branch of the GSK-3 pathway that regulates proper wound-directed polarization of these cells. Finally, inhibition of GSK-3 also strongly reduces activation of Rac1 and cell scatter in response to hepatocyte growth factor/scatter factor, which triggers dispersal and migration of cells in monolayer culture as fibroblast-like individual cells, a mode of epithelial cell motility distinct from the collective migration of wound closure.

IQGAP1: a key regulator of adhesion and migration

The dynamic rearrangement of cell-cell adhesion is one of the major physiological events in tissue development and tumor metastasis. Polarized cell migration, another key event, is a tightly regulated process that occurs during tissue development, chemotaxis and wound healing. Rho-family small GTPases, especially Rac1 and Cdc42, play pivotal roles in these processes through one of their effectors, IQGAP1. Recent studies reveal that IQGAP1 regulates cadherin-mediated cell-cell adhesion both positively and negatively. It captures and stabilizes microtubules through the microtubule-binding protein CLIP-170 near the cell cortex, leading to establishment of polarized cell morphology and directional cell migration. Furthermore, Rac1 and Cdc42 link the adenomatous polyposis coli (APC) protein to actin filaments through IQGAP1 at the leading edge and thereby regulate polarization and directional migration.

PTHrP induces changes in cell cytoskeleton and E-cadherin and regulates Eph/Ephrin kinases and RhoGTPases in murine secondary trophoblast cells

The differentiation of murine trophoblast giant cells (TGCs) is well characterised at the molecular level and, to some extent, the cellular level. Currently, there is a rudimentary understanding about factors regulating the cellular differentiation of secondary TGCs. Using day 8.5 p.c.-ectoplacental cone (EPC) explant in serum-free culture, we have found parathyroid hormone-related protein (PTHrP) to regulate cellular changes during TGC differentiation. PTHrP greatly stimulated the formation and organisation of actin stress fibres and actin expression in trophoblast outgrowth. This coincided with changing cell shape into a flattened/fibroblastic morphology, suppression of E-cadherin expression, and increased cell spreading in culture. PTHrP also increased the nuclear staining of beta-catenin and, similar to activator protein-2gamma (AP-2gamma), showed microtubule-dependent nuclear localisation in vitro. These cellular and behavioural changes correlated with changes in the expression of RhoGTPases and in both expression and phosphorylation of Eph/Ephrin kinases. The effects of PTHrP on trophoblast cellular differentiation were abolished after blocking its action. In conclusion, PTHrP provides an excellent example of the extrinsic factors that, through their network of activities, plays an important role in cellular differentiation of secondary TGCs.

Emerging roles for p120-catenin in cell adhesion and cancer

Although originally identified as a Src substrate, p120-catenin (p120) is now known to regulate cell-cell adhesion through its interaction with the cytoplasmic tail of classical and type II cadherins. New evidence indicates that p120 regulates cadherin turnover at the cell surface, thereby controlling the amount of cadherin available for cell-cell adhesion. This function is necessary but not sufficient to promote strong adhesion, which is further controlled by signals acting on the amino-terminal p120 regulatory domain. p120 also modulates the activities of RhoA, Rac, and Cdc42, suggesting that along with other Src substrates, p120 regulates actin dynamics. Thus, p120 is a master regulator of cadherin abundance and activity, and likely participates in regulating the balance between adhesive and motile cellular phenotypes. This review summarizes recent progress in understanding mechanisms of p120 action, and discusses new implications with respect to roles for p120 in disease and cancer.

Membrane potential and endocytic activity control disintegration of cell-cell adhesion and cell fusion in vinculin-injected MDBK cells

Cell fusion occurs during fertilization and in the formation of organs such as muscles, placenta, and bones. We have developed an experimental model for epithelial cell fusion which permits analysis of the processes during junction disintegration and formation of polykaryons (Palovuori and Eskelinen [2000] Eur. J. Cell. Biol. 79: 961-974). In the present work, we analyzed the process in detail. Cell fusion was achieved by microinjecting into the cytoplasm of kidney epithelial Madin-Darby bovine kidney (MDBK) cells TAMRA-tagged vinculin, which incorporated into lateral membranes, focal adhesions and nucleus, and, prior fusion, induced internalization of actin, cadherin and plakoglobin to small clusters in cytoplasm. Injected vinculin was still visible at lateral membranes after removal of junctional proteins indicating that it was tightly associated and perturbed the cell-cell contact sites resulting in membrane fragmentation. Injection of active Rac together with vinculin induced accumulation of cadherin to the membranes, but did not affect vinculin-membrane association. However, it hampered cell fusion probably by supporting adherens junctions. In order to stop endocytosis, we lowered intracellular pH of vinculin-injected cells to 5.5 with the aid of nigericin in KCl buffer. In acidified cells, injected vinculin delineated lateral membranes as thick layers, cadherin remained in situ, and cell fusion was completely inhibited. Since this treatment also leads to cell depolarization, we checked the vinculin incorporation in a KCl solution containing nigericin at neutral pH. In these circumstances, both endogenous and injected vinculin delineated lateral membranes as very thin discontinuous layers, but still fusion was hampered most likely due to perturbation in the initial vinculin-membrane association. We suggest that vinculin might function as a sensor of the environment triggering cell fusion during development in circumstances where membrane potential and local and transient pH gradients play a role.

The Rac exchange factor Tiam1 is required for the establishment and maintenance of cadherin-based adhesions

Rho family proteins are essential for the formation of adherens junctions, which are required for the maintenance of epithelial integrity. Activated Rac and the Rac exchange factor Tiam1 have been shown to promote the formation of adherens junctions and the accompanying induction of an epithelioid phenotype in a number of cell lines. Here we show that Madin-Darby canine kidney II cells in which Tiam1 was down-regulated using short interfering RNA disassembled their cadherin-based adhesions and acquired a flattened, migratory, and mesenchymal morphology. In addition, the expression of E1A in mesenchymal V12Ras-transformed Madin-Darby canine kidney II cells led simultaneously to the up-regulation of the Tiam1 protein, the activation of Rac, the formation of cadherin-based adhesions, and reversion to an epithelial phenotype. This finding suggests that E1A induces an epithelial morphology through the up-regulation of Tiam1 and, thereby, the activation of Rac and the formation of cadherin-based adhesions. Indeed, we found that E1A is able to induce an epithelial-like morphology accompanied by the formation of cadherin-based adhesions only in wild-type but not in Tiam1-deficient primary mouse embryonic fibroblasts. These studies indicate that the Rac activator Tiam1 is essential for the formation as well as the maintenance of cadherin-based adhesions.

A novel role of nectins in inhibition of the E-cadherin-induced activation of Rac and formation of cell-cell adherens junctions

Nectins are Ca(2+)-independent immunoglobulin (Ig)-like cell-cell adhesion molecules. The trans-interactions of nectins recruit cadherins to the nectin-based cell-cell adhesion, resulting in formation of cell-cell adherens junctions (AJs) in epithelial cells and fibroblasts. The trans-interaction of E-cadherin induces activation of Rac small G protein, whereas the trans-interactions of nectins induce activation of not only Rac but also Cdc42 small G protein. We showed by the fluorescent resonance energy transfer (FRET) imaging that the trans-interaction of E-cadherin induced dynamic activation and inactivation of Rac, which led to dynamic formation and retraction of lamellipodia. Moreover, we found here that the nectins, which did not trans-interact with other nectins (non-trans-interacting nectins), inhibited the E-cadherin-induced activation of Rac and reduced the velocity of the formation of the E-cadherin-based cell-cell AJs. The inhibitory effect of non-trans-interacting nectins was suppressed by the activation of Cdc42 induced by the trans-interactions of nectins. These results indicate a novel role of nectins in regulation of the E-cadherin-induced activation of Rac and formation of cell-cell AJs.

MALS is a binding partner of IRSp53 at cell-cell contacts

Insulin receptor substrate p53 (IRSp53) is a key player in cytoskeletal dynamics, interacting with the actin modulators WAVE2 and Mena. Here, we identified a PDZ protein, MALS, as an IRSp53-interacting protein using a yeast two-hybrid screen. A pull-down assay showed that IRSp53 and MALS interact through the PDZ domain of MALS and the C-terminal PDZ-binding sequence of IRSp53. Their interaction in MDCK cells was also demonstrated by co-immunoprecipitation. Immunocytochemistry showed the colocalization of IRSp53 and MALS at cell-cell contacts. Cytochalasin D induced the redistribution of both proteins to the cytosol. Thus, MALS is a partner of IRSp53 anchoring the actin-based membrane cytoskeleton at cell-cell contacts.

Distinct Rho GTPase activities regulate epithelial cell localization of the adhesion molecule CEACAM1: involvement of the CEACAM1 transmembrane domain

CEACAM1 is an intercellular adhesion glycoprotein. As CEACAM1 plays an important role in epithelial cell signaling and functions, we have examined its localization in epithelial cells. We have observed that distribution at cell contacts is not always seen in these cells, suggesting that CEACAM1 localization might be regulated. In Swiss 3T3 cells, the targeting of CEACAM1 at cell-cell boundaries is regulated by the Rho GTPases. In the present study, we have used the MDCK epithelial cells to characterize the effects of the Rho GTPases and their effectors on CEACAM1 intercellular targeting. Activated Cdc42 and Rac1 or their downstream effector PAK1 targeted CEACAM1 to sites of cell-cell contacts. On the other hand, neither activated RhoA nor activated Rho kinase directed CEACAM1 to cell boundaries, resulting in a condensed distribution of CEACAM1 at the cell surface. Interestingly, inhibition of this pathway resulted in CEACAM1 intercellular localization suggesting that a tightly regulated balance of Rho GTPase activities is necessary to target CEACAM1 at cell-cell boundaries. In addition, using CEACAM1 mutants and chimeric fusion constructs containing domains of the colony-stimulating factor receptor, we have shown that the transmembrane domain of CEACAM1 is responsible for the Cdc42-induced targeting at cell-cell contacts.

NZO-3 expression causes global changes to actin cytoskeleton in Madin-Darby canine kidney cells: linking a tight junction protein to Rho GTPases

We previously demonstrated that exogenous expression of a truncated form of the tight junction protein ZO-3 affected junctional complex assembly and function. Current results indicate that this ZO-3 construct influences actin cytoskeleton dynamics more globally. We show that expression of the amino-terminal half of ZO-3 (NZO-3) in Madin-Darby canine kidney cells results in a decreased number of stress fibers and focal adhesions and causes an increased rate of cell migration in a wound healing assay. We also demonstrate that RhoA activity is reduced in NZO-3-expressing cells. We determined that ZO-3 interacts with p120 catenin and AF-6, proteins localized to the junctional complex and implicated in signaling pathways important for cytoskeleton regulation and cell motility. We also provide evidence that NZO-3 interacts directly with the C terminus of ZO-3, and we propose a model where altered interactions between ZO-3 and p120 catenin in NZO-3-expressing cells affect RhoA GTPase activity. This study reveals a potential link between ZO-3 and RhoA-related signaling events.

Cadherin engagement inhibits RhoA via p190RhoGAP

Cadherins are transmembrane receptors that mediate cell-cell adhesion in epithelial cells. A number of changes occur during cadherin-mediated junction formation, one of which is a rearrangement of the actin cytoskeleton. Key regulators of actin cytoskeletal dynamics in cells are the Rho family of GTPases. We have demonstrated in previous studies that cadherin signaling suppresses RhoA activity and activates Rac1. The signaling events downstream of cadherins that modulate the activity of Rho family proteins remain unknown. Here we have identified a pathway by which RhoA becomes inactivated by cadherins. To determine whether cadherins regulate RhoA through activation of a GTPase-activating protein (GAP) for RhoA, we used constitutively active RhoA to isolate activated GAPs. Using this assay, we have identified the RhoA-specific GAP, p190RhoGAP, downstream from engaged cadherins. We found that cadherin engagement induced tyrosine phosphorylation of p190RhoGAP and increased its binding to p120RasGAP. The increased precipitation of p190RhoGAP with 63LRhoA was blocked by addition of PP2 suggesting that Src family kinases are required downstream from cadherin signaling. The inhibition of RhoA activity by cadherins was antagonized by expression of a dominant negative p190RhoGAP. Taken together, these data demonstrate that p190RhoGAP activity is critical for RhoA inactivation by cadherins.

Direct cadherin-activated cell signaling: a view from the plasma membrane

Classical cadherin adhesion molecules are key determinants of cell recognition and tissue morphogenesis, with diverse effects on cell behavior. Recent developments indicate that classical cadherins are adhesion-activated signaling receptors. In particular, early-immediate Rac signaling is emerging as a mechanism to coordinate cadherin-actin integration at the plasma membrane.

Rac-1 and IQGAP are potential regulators of E-cadherin–catenin interactions during murine preimplantation development

Adherens junction formation is fundamental for compaction and trophectoderm differentiation during mammalian preimplantation development. We recently isolated an IQGAP-2 cDNA from a differential display-polymerase chain reaction screen of bovine preimplantation developmental stages. IQGAP-1 and -2 proteins mediate E-cadherin-based cell-to-cell adhesion through interactions with beta-catenin and the Rho GTPases, rac1 and cdc42. Our study demonstrates IQGAP-1,-2, rac-1 and cdc42 mRNAs are present throughout murine preimplantation development. IQGAP-1 and rac-1 protein distribution changes from predominantly plasma membrane associated to predominantly cytoplasmic as the embryo progresses through cleavage divisions and compaction to the blastocyst stage.

Regulation of Xenopus embryonic cell adhesion by the small GTPase, rac

TGF-beta family signalling pathways are important for germ layer formation and gastrulation in vertebrate embryos and have been studied extensively using embryos of Xenopus laevis. Activin causes changes in cell movements and cell adhesion in Xenopus animal caps and dispersed animal cap cells. Rho family GTPases, including rac, mediate growth factor-induced changes in the actin cytoskeleton, and consequently, in cell adhesion and motility, in a number of different cell types. Ectopic expression of mutant rac isoforms in Xenopus embryos was combined with animal cap adhesion assays and a biochemical assay for rac activity to investigate the role of rac in activin-induced changes in cell adhesion. The results indicate that (1) the perturbation of rac signalling disrupts embryonic cell-cell adhesion, (2) that rac activity is required for activin-induced changes in cell adhesive behavior on fibronectin, and (3) that activin increases endogenous rac activity in animal cap explants.

Site-specific alteration of actin assembly visualized in living renal epithelial cells during ATP depletion

Disruption of normal actin organization in renal tubular epithelial cells is an important element of renal injury induced by ischemia. Studies of fixed cells indicate that the cytoskeleton is disrupted by both ischemia and ATP depletion in a site-specific manner. However, few studies have examined these effects in living cells, and the relationship between the time course of ATP reduction and alteration of the cytoskeleton remains unclear. Here, time-lapse video images of cultured renal epithelial cells expressing an enhanced green fluorescent protein (EGFP)-actin fusion protein were obtained, and the kinetics of fluorescence actin distribution before and during ATP depletion is quantified and compared with measured ATP levels. This study found that assembly of lamellar actin is inhibited rapidly as cellular ATP levels are reduced, whereas disruption of actin in stress fibers is more gradual and persistent. Actin associated with focal adhesions is largely resistant to ATP depletion in these experiments, and, consistent with previous studies, particulate aggregates of actin were formed within the cytoplasm of ATP-depleted cells. Most surprisingly, time-lapse imaging of EGFP-actin distribution, quantitative fluorescence imaging of phalloidin-stained cells, and ultrastructural studies indicate that assembly of actin filaments occurs at sites of epithelial cell-cell attachment in ATP-depleted cells. This assembly is initiated early during ATP depletion and continues after ATP levels are maximally reduced. Assembly of actin at sites of cell-cell attachment may be an element of the pathology of injury induced by ischemia, or alternatively, could reflect the function of a protective mechanism. These studies directly demonstrate site-specific alteration of actin assembly in living epithelial cells during ATP depletion. The results also reveal that actin reorganization continues after ATP levels are maximally decreased and that epithelial cell-cell attachments are sites of actin assembly in ATP-depleted cells.

Extracellular matrix, junctional integrity and matrix metalloproteinase interactions in endothelial permeability regulation

Vascular endothelial permeability is maintained by the regulated apposition of adherens and tight junctional proteins whose organization is controlled by several pharmacological and physiological mediators. Endothelial permeability changes are associated with: (1) the spatial redistribution of surface cadherins and occludin, (2) stabilization of focal adhesive bonds and (3) the progressive activation of matrix metalloproteinases (MMPs). In response to peroxide, histamine and EDTA, endothelial cells sequester VE-cadherin and alter its cytoskeletal binding. Simultaneously, these mediators enhance focal adhesion to the substratum. Oxidants, cytokines and pharmacological mediators also trigger the activation of matrix metalloproteinases (MMPs) in a cytoskeleton and tyrosine phosphorylation dependent manner to degrade occludin, a well-characterized tight junction element. These related in vitro phenomena appear to co-operate during inflammation, to increase endothelial permeability, structurally stabilize cells while also remodelling cell junctions and substratum.

ERBIN associates with p0071, an armadillo protein, at cell-cell junctions of epithelial cells

BACKGROUND

ERBIN, an ErbB2 receptor-interacting protein, belongs to a recently described family of proteins termed the LAP [leucine-rich repeats and PSD-95/dLg-A/ZO-1 (PDZ) domains] family which has essential roles in establishment of cell polarity.

RESULTS

To identify new ERBIN-binding proteins, we screened a yeast two-hybrid library, using the carboxyl-terminal fragment of ERBIN containing PDZ domain as the bait, and we isolated p0071 (also called plakophilin-4) as an ERBIN-interacting protein. p0071 is a member of the p120 catenin family, which are defined as proteins with 10 armadillo repeats, and localizes along the cell-cell border. The ERBIN PDZ domain binds the COOH-terminus of p0071 containing the PDZ domain-binding sequence. Endogenous ERBIN was co-immunoprecipitated with p0071. In fully polarized Madin-Darby canine kidney (MDCK) cells, ERBIN co-localized largely with beta-catenin and partly with desmoplakin along the lateral plasma membrane domain. At these cell-cell contact regions, ERBIN co-localizes with p0071. Over-expression of the dominant active forms of Cdc42, Rac1 or RhoA, Rho family small GTPases, resulted in a marked accumulation of ERBIN at the cell-cell contacts of MDCK and HeLa cells.

CONCLUSION

These results show that ERBIN interacts in vivo with p0071 and that it may be involved in the organization of adherens junctions and the desmosomes of epithelia. In addition, we demonstrated that the subcellular localization of ERBIN might be regulated by Rho family small GTPases.

The Cdc42 and Rac1 GTPases are required for capillary lumen formation in three-dimensional extracellular matrices

Here we show a requirement for the Cdc42 and Rac1 GTPases in endothelial cell (EC) morphogenesis in three-dimensional extracellular matrices. Cdc42 and Rac1 specifically regulate EC intracellular vacuole and lumen formation in both collagen and fibrin matrices. Clostridium difficile toxin B(which blocks all three Rho GTPases) completely inhibited the ability of ECs to form both vacuoles and lumens, whereas C3 transferase, a selective inhibitor of Rho, did not. Expression of either dominant-negative (N17) or constitutively active (V12) Cdc42 using recombinant adenoviruses dramatically inhibited EC vacuole and lumen formation in both collagen and fibrin matrices. Both vacuole and lumen formation initiated in ECs expressing dominant-negative(N17) Rac1 but later collapsed, indicating a role for Rac1 during later stages of vessel development. Analysis of cultures using confocal microscopy revealed green fluorescent protein-V12Rac1, -Rac1 wild-type and -Cdc42 wild-type chimeric proteins targeted to intracellular vacuole membranes during the lumen formation process. Also, expression of the verprolin-cofilin-acidic domain of N-WASP, a downstream Cdc42 effector, in ECs completely interfered with vacuole and lumen formation. These results collectively reveal a novel role for Cdc42 and Rac1 in the process of EC vacuole and lumen formation in three-dimensional extracellular matrices.

K-Ras mediates cytokine-induced formation of E-cadherin-based adherens junctions during liver development

The E-cadherin-based adherens junction (AJ) is essential for organogenesis of epithelial tissues including the liver, although the regulatory mechanism of AJ formation during development remains unknown. Using a primary culture system of fetal hepatocytes in which oncostatin M (OSM) induces differentiation, we show here that OSM induces AJ formation by altering the subcellular localization of AJ components including E-cadherin and catenins. By retroviral expression of dominant-negative forms of signaling molecules, Ras was shown to be required for the OSM-induced AJ formation. Fetal hepatocytes derived from K-Ras knockout (K-Ras-/-) mice failed to form AJs in response to OSM, whereas AJ formation was induced normally by OSM in mutant hepatocytes lacking both H-Ras and N-Ras. Moreover, the defective phenotype of K-Ras-/- hepatocytes was restored by expression of K-Ras, but not by H-Ras and N-Ras. Finally, pull-down assays using the Ras-binding domain of Raf1 demonstrated that OSM directly activates K-Ras in fetal hepatocytes. These results indicate that K-Ras specifically mediates cytokine signaling for formation of AJs during liver development.

Rho-family GTPases in cadherin-mediated cell-cell adhesion

Cell-cell adhesions are rearranged dynamically during tissue development and tumour metastasis. Recently, Rho-family GTPases, including RhoA, Rac1 and Cdc42, have emerged as key regulators of cadherin-mediated cell-cell adhesion. Following the identification and characterization of regulators and effectors of Rho GTPases, signal transduction pathways from cadherin to Rho GTPases and, in turn, from Rho GTPases to cadherin, are beginning to be clarified.

Rac affects invasion of human renal cell carcinomas by up-regulating tissue inhibitor of metalloproteinases (TIMP)-1 and TIMP-2 expression

Rho-like GTPases, including Cdc42, Rac1, and RhoA, regulate distinct actin cytoskeleton changes required for adhesion, migration, and invasion of cells. Tiam1 specifically activates Rac, and earlier studies have demonstrated that Tiam1-Rac signaling affects migration and invasion in a cell type- and cell substrate-specific manner. In the present study, we examined the role of Tiam1-Rac signaling in migration and invasion of human renal cell carcinomas. Stable overexpression of Tiam1 or constitutively active V12-Rac1 in a human renal cell carcinoma cell line (clearCa-28) strongly inhibited cell migration by promoting E-cadherin-mediated cell-cell adhesion. Blocking E-cadherin-mediated adhesion by E-cadherin-specific HAV peptides allowed cells to migrate, but was not sufficient to antagonize Tiam1- and V12-Rac1-induced inhibition of Matrigel invasion, suggesting that Rac may influence invasion also through other mechanisms. Indeed, Tiam1-mediated Rac activation induced transcriptional up-regulation of tissue inhibitor of metalloproteinases-1 (TIMP-1) and post-transcriptional up-regulation of TIMP-2, whereas secretion and activity levels of their counterparts, matrix metalloproteinase-9 and matrix metalloproteinase-2, respectively, were not affected. Application of recombinant TIMP-1 and TIMP-2 proteins significantly inhibited invasion of mock-transfected clearCa-28 cells, supporting a role of TIMPs in Rac-mediated inhibition of invasion. To our knowledge, this is the first evidence that increased Rac signaling may inhibit invasion of epithelial tumor cells by up-regulation of TIMP-1 and TIMP-2.

Cadherin engagement regulates Rho family GTPases

The formation of cell-cell adherens junctions is a cadherin-mediated process associated with reorganization of the actin cytoskeleton. Because Rho family GTPases regulate actin dynamics, we investigated whether cadherin-mediated adhesion regulates the activity of RhoA, Rac1, and Cdc42. Confluent epithelial cells were found to have elevated Rac1 and Cdc42 activity but decreased RhoA activity when compared with low density cultures. Using a calcium switch method to manipulate junction assembly, we found that induction of cell-cell junctions increased Rac1 activity, and this was inhibited by E-cadherin function-blocking antibodies. Using the same calcium switch procedure, we found little effect on RhoA activity during the first hour of junction assembly. However, over several hours, RhoA activity significantly decreased. To determine whether these effects are mediated directly through cadherins or indirectly through engagement of other surface proteins downstream from junction assembly, we used a model system in which cadherin engagement is induced without cell-cell contact. For these experiments, Chinese hamster ovary cells expressing C-cadherin were plated on the extracellular domain of C-cadherin immobilized on tissue culture plates. Whereas direct cadherin engagement did not stimulate Cdc42 activity, it strongly inhibited RhoA activity but increased Rac1 activity. Deletion of the C-cadherin cytoplasmic domain abolished these effects.

Cdc42-dependent modulation of tight junctions and membrane protein traffic in polarized Madin-Darby canine kidney cells

Polarized epithelial cells maintain the asymmetric composition of their apical and basolateral membrane domains by at least two different processes. These include the regulated trafficking of macromolecules from the biosynthetic and endocytic pathway to the appropriate membrane domain and the ability of the tight junction to prevent free mixing of membrane domain-specific proteins and lipids. Cdc42, a Rho family GTPase, is known to govern cellular polarity and membrane traffic in several cell types. We examined whether this protein regulated tight junction function in Madin-Darby canine kidney cells and pathways that direct proteins to the apical and basolateral surface of these cells. We used Madin-Darby canine kidney cells that expressed dominant-active or dominant-negative mutants of Cdc42 under the control of a tetracycline-repressible system. Here we report that expression of dominant-active Cdc42V12 or dominant-negative Cdc42N17 altered tight junction function. Expression of Cdc42V12 slowed endocytic and biosynthetic traffic, and expression of Cdc42N17 slowed apical endocytosis and basolateral to apical transcytosis but stimulated biosynthetic traffic. These results indicate that Cdc42 may modulate multiple cellular pathways required for the maintenance of epithelial cell polarity.