Regulation of cell-cell adhesion by rac and rho small G proteins in MDCK cells

Regulation of the cytoskeleton and cell adhesion by the Rho family GTPases in mammalian cells

Members of the Rho family of small Ras-like GTPases--including RhoA, -B, and -C, Rac1 and -2, and Cdc42--exhibit guanine nucleotide-binding activity and function as molecular switches, cycling between an inactive GDP-bound state and an active GTP-bound state. The Rho family GTPases participate in regulation of the actin cytoskeleton and cell adhesion through specific targets. Identification and characterization of these targets have begun to clarify how the Rho family GTPases act to regulate cytoskeletal structure and cell-cell and cell-substratum contacts in mammalian cells. The Rho family GTPases are also involved in regulation of smooth muscle contraction, cell morphology, cell motility, neurite retraction, and cytokinesis. However, the molecular mechanisms by which the Rho family GTPases participate in the regulation of such processes are not well established.

Identification of a novel beta-catenin-interacting protein

Cadherin is a well-known cell-cell adhesion molecule, and it binds to beta-catenin, which in turn binds to alpha-catenin. However, little is known about the regulatory mechanism underlying the cadherin-mediated cell-cell adhesion. Here we purified two novel beta-catenin-interacting proteins with molecular masses of 180 kDa (p180) and 150 kDa (p150) from bovine brain cytosol by using glutathione S-transferase (GST)-beta-catenin affinity column chromatography. Mass spectral analysis revealed p180 to be identical to KIAA0313 which has a putative Rap1 guanine nucleotide exchange factor (GEF) domain and p150 to be the same as KIAA0705 which has a high degree of sequence similarity to the synaptic scaffolding molecule (S-SCAM), which binds beta-catenin and KIAA0313 in the yeast two-hybrid system and overlay assay, respectively (Ide et al., Biochem. Biophys. Res. Commun. 256, 456-461, 1999; Ohtsuka et al., Biochem. Biophys. Res. Commun. 265, 38-44, 1999). beta-Catenin was coimmunoprecipitated with KIAA0313 in Madin-Darby canine kidney II (MDCKII) cells, bovine brain cytosol, and EL cells. KIAA0313 and beta-catenin were partly colocalized at sites of cell-cell contact in MDCKII cells. Taken together, our data suggest that KIAA0313 associates with beta-catenin through KIAA0705 in vivo at sites of cell-cell contact.

Integrin-associated protein/CD47 regulates motile activity in human B-cell lines through CDC42

Cell migration requires a dynamic interaction between the cell, its substrate, and the cytoskeleton-associated motile apparatus. Integrin-associated protein (IAP)/CD47 is a 50-kd cell surface protein that is physically associated with β3 integrins and that modulates the functions of β3 integrins in various cells. However, in B-lymphocytes that express β1 integrins but few β3 integrins, the roles of IAP/CD47 remain to be determined. Cross-linking of IAP/CD47 by the immobilized anti-IAP/CD47 monoclonal antibody (mAb) B6H12, but not 2D3, produced signals to promote polarization with lamellipodia, a characteristic morphology during leukocyte migration, in pre-B and mature B-cell lines (BALL, Nalm6, ONHL-1, Daudi), but not in myeloma cell lines (RPMI8226, OPM-2). In the presence of the immobilized fibronectin (FN), soluble B6H12 could increase the rate of the polarization and activate migratory activity of BALL cells to FN in a transwell filter assay. Furthermore, the dominant-negative form of CDC42 completely blocked B6H12-induced morphologic and functional changes without inhibiting phorbol 12-myristate 13-acetate–induced spreading on FN in BALL cells, whereas the dominant-negative form of Rac1 inhibited all these changes. These findings demonstrate that in B-lymphocytes, IAP/CD47 may transduce the signals to activate the migratory activity, in which CDC42 may be specifically involved, and that IAP/CD47 shows synergistic effect with 4β1 on B-cell migration. These findings would provide new insight into the role of IAP/CD47 on B-cell function.

Feedback interactions between cell-cell adherens junctions and cytoskeletal dynamics in newt lung epithelial cells

To test how cell-cell contacts regulate microtubule (MT) and actin cytoskeletal dynamics, we examined dynamics in cells that were contacted on all sides with neighboring cells in an epithelial cell sheet that was undergoing migration as a wound-healing response. Dynamics were recorded using time-lapse digital fluorescence microscopy of microinjected, labeled tubulin and actin. In fully contacted cells, most MT plus ends were quiescent; exhibiting only brief excursions of growth and shortening and spending 87.4% of their time in pause. This contrasts MTs in the lamella of migrating cells at the noncontacted leading edge of the sheet in which MTs exhibit dynamic instability. In the contacted rear and side edges of these migrating cells, a majority of MTs were also quiescent, indicating that cell-cell contacts may locally regulate MT dynamics. Using photoactivation of fluorescence techniques to mark MTs, we found that MTs in fully contacted cells did not undergo retrograde flow toward the cell center, such as occurs at the leading edge of motile cells. Time-lapse fluorescent speckle microscopy of fluorescently labeled actin in fully contacted cells revealed that actin did not flow rearward as occurs in the leading edge lamella of migrating cells. To determine if MTs were required for the maintenance of cell-cell contacts, cells were treated with nocodazole to inhibit MTs. After 1-2 h in either 10 microM or 100 nM nocodazole, breakage of cell-cell contacts occurred, indicating that MT growth is required for maintenance of cell-cell contacts. Analysis of fixed cells indicated that during nocodazole treatment, actin became reduced in adherens junctions, and junction proteins alpha- and beta-catenin were lost from adherens junctions as cell-cell contacts were broken. These results indicate that a MT plus end capping protein is regulated by cell-cell contact, and in turn, that MT growth regulates the maintenance of adherens junctions contacts in epithelia.

Importance of spatial activation of Cdc42 and rac small G proteins by frabin for microspike formation in MDCK cells

BACKGROUND

Frabin is an actin filament (F-actin)-binding protein that shows GDP/GTP exchange activity for Cdc42 small G protein (Cdc42). Frabin furthermore induces indirect activation of Rac small G protein (Rac) in intact cells. We have recently shown that in nonepithelial cells, frabin induces the formation of both filopodia- and lamellipodia-like processes through the activation of Cdc42 and Rac, respectively. In epithelial cells such as MDCK cells, Cdc42 and Rac regulate cell-cell adherens junctions (AJs) via the accumulation of F-actin and E-cadherin, although neither Cdc42 nor Rac induces the formation of filopodia or lamellipodia. In this study, we have examined the effects of frabin on the reorganization of the actin cytoskeleton in MDCK cells.

RESULTS

Frabin induces the formation of microspikes at the basal area of the lateral membranes through the activation of Cdc42 and Rac in MDCK cells, although a dominant active mutant of Cdc42 or Rac alone, or both, did not induce the formation of microspikes. Furthermore, frabin weakly increased the accumulation of F-actin and E-cadherin at cell-cell AJs and the formation of stress fibres through the activation of Cdc42 and Rac, under conditions where the dominant active mutant of Cdc42 or Rac markedly showed these effects. The Cdc42- and Rac-induced formation of stress fibres was dependent on the activation of Rho small G protein.

CONCLUSION

These results indicate that the frabin-dependent spatial activation of Cdc42 and Rac is important for the formation of microspikes.

Integrin-associated protein/CD47 regulates motile activity in human B-cell lines through CDC42

Cell migration requires a dynamic interaction between the cell, its substrate, and the cytoskeleton-associated motile apparatus. Integrin-associated protein (IAP)/CD47 is a 50-kd cell surface protein that is physically associated with β3 integrins and that modulates the functions of β3 integrins in various cells. However, in B-lymphocytes that express β1 integrins but few β3 integrins, the roles of IAP/CD47 remain to be determined. Cross-linking of IAP/CD47 by the immobilized anti-IAP/CD47 monoclonal antibody (mAb) B6H12, but not 2D3, produced signals to promote polarization with lamellipodia, a characteristic morphology during leukocyte migration, in pre-B and mature B-cell lines (BALL, Nalm6, ONHL-1, Daudi), but not in myeloma cell lines (RPMI8226, OPM-2). In the presence of the immobilized fibronectin (FN), soluble B6H12 could increase the rate of the polarization and activate migratory activity of BALL cells to FN in a transwell filter assay. Furthermore, the dominant-negative form of CDC42 completely blocked B6H12-induced morphologic and functional changes without inhibiting phorbol 12-myristate 13-acetate–induced spreading on FN in BALL cells, whereas the dominant-negative form of Rac1 inhibited all these changes. These findings demonstrate that in B-lymphocytes, IAP/CD47 may transduce the signals to activate the migratory activity, in which CDC42 may be specifically involved, and that IAP/CD47 shows synergistic effect with 4β1 on B-cell migration. These findings would provide new insight into the role of IAP/CD47 on B-cell function.

Rac1 mutations produce aberrant epithelial differentiation in the developing and adult mouse small intestine

The mouse small intestinal epithelium undergoes continuous renewal throughout life. Previous studies suggest that differentiation of this epithelium is regulated by instructions that are received as cells migrate along crypt-villus units. The nature of the instructions and their intracellular processing remain largely undefined. In this report, we have used genetic mosaic analysis to examine the role of Rac1 GTPase-mediated signaling in controlling differentiation. A constitutively active mutation (Rac1Leu61) or a dominant negative mutation (Rac1Asn17) was expressed in the 129/Sv embryonic stem cell-derived component of the small intestine of C57Bl/6-ROSA26<->129/Sv mice. Rac1Leu61 induces precocious differentiation of members of the Paneth cell and enterocytic lineages in the proliferative compartment of the fetal gut, without suppressing cell division. Forced expression of the dominant negative mutation inhibits epithelial differentiation, without affecting cell division, and slows enterocytic migration along crypt-villus units. The effects produced by Rac1Leu61 or Rac1Asn17 in the 129/Sv epithelium do not spread to adjacent normal C57Bl/6 epithelial cells. These results provide in vivo evidence that Rac1 is involved in the import and intracellular processing of signals that control differentiation of a mammalian epithelium.

Rho family proteins in cell adhesion and cell migration

Cell migration and the regulation of cadherin-mediated homotypic cell-cell interactions are critical events during development, morphogenesis and wound healing. Aberrations in signalling pathways involved in the regulation of cell migration and cadherin-mediated cell-cell adhesion contribute to tumour invasion and metastasis. The rho family proteins, including cdc42, rac1 and rhoA, regulate signalling pathways that mediate the distinct actin cytoskeleton changes required for both cellular motility and cell-cell adhesion. Recent studies indicate that rac directly influences rho activity at the GTPase level and that the reciprocal balance between rac and rho activity can determine epithelial or mesenchymal cell morphology and migratory behaviour of epithelial (tumour) cells.

Oncogenic Ras downregulates Rac activity, which leads to increased Rho activity and epithelial-mesenchymal transition

Proteins of the Rho family regulate cytoskeletal rearrangements in response to receptor stimulation and are involved in the establishment and maintenance of epithelial cell morphology. We recently showed that Rac is able to downregulate Rho activity and that the reciprocal balance between Rac and Rho activity is a major determinant of cellular morphology and motility in NIH3T3 fibroblasts. Using biochemical pull-down assays, we analyzed the effect of transient and sustained oncogenic Ras signaling on the activation state of Rac and Rho in epithelial MDCK cells. In contrast to the activation of Rac by growth factor-induced Ras signaling, we found that sustained signaling by oncogenic RasV12 permanently downregulates Rac activity, which leads to upregulation of Rho activity and epithelial-mesenchymal transition. Oncogenic Ras decreases Rac activity through sustained Raf/MAP kinase signaling, which causes transcriptional downregulation of Tiam1, an activator of Rac in epithelial cells. Reconstitution of Rac activity by expression of Tiam1 or RacV12 leads to downregulation of Rho activity and restores an epithelial phenotype in mesenchymal RasV12- or RafCAAX-transformed cells. The present data reveal a novel mechanism by which oncogenic Ras is able to interfere with the balance between Rac and Rho activity to achieve morphological transformation of epithelial cells.

Different membrane targeting of prostaglandin EP3 receptor isoforms dependent on their carboxy-terminal tail structures

Mouse prostaglandin EP3 receptor consists of three isoforms, EP3alpha, beta and gamma, with different carboxy-terminal tails. To assess the role of their carboxy-terminal tails in membrane targeting, we examined subcellular localization of myc-tagged EP3 isoforms expressed in MDCK cells. Two isoforms, EP3alpha and EP3beta, were localized in the intracellular compartment but not in the plasma membrane, while the EP3gamma isoform was found in the lateral plasma membrane and in part in the intracellular compartment. Mutant EP3 receptor lacking the carboxy-terminal tail was localized in the intracellular compartment but not in the plasma membrane. Thus, EP3 isoforms differ in subcellular targeting, and the carboxy-terminal tails play an important role in determination of the membrane targeting of EP3 receptor.

Activation of cdc42, rac, PAK, and rho-kinase in response to hepatocyte growth factor differentially regulates epithelial cell colony spreading and dissociation

Hepatocyte growth factor (HGF), the ligand for the Met receptor tyrosine kinase, is a potent modulator of epithelial-mesenchymal transition and dispersal of epithelial cells, processes that play crucial roles in tumor development, invasion, and metastasis. Little is known about the Met-dependent proximal signals that regulate these events. We show that HGF stimulation of epithelial cells leads to activation of the Rho GTPases, Cdc42 and Rac, concomitant with the formation of filopodia and lamellipodia. Notably, HGF-dependent activation of Rac but not Cdc42 is dependent on phosphatidylinositol 3-kinase. Moreover, HGF-induced lamellipodia formation and cell spreading require phosphatidylinositol 3-kinase and are inhibited by dominant negative Cdc42 or Rac. HGF induces activation of the Cdc42/Rac-regulated p21-activated kinase (PAK) and c-Jun N-terminal kinase, and translocation of Rac, PAK, and Rho-dependent Rho-kinase to membrane ruffles. Use of dominant negative and activated mutants reveals an essential role for PAK but not Rho-kinase in HGF-induced epithelial cell spreading, whereas Rho-kinase activity is required for the formation of focal adhesions and stress fibers in response to HGF. We conclude that PAK and Rho-kinase play opposing roles in epithelial-mesenchymal transition induced by HGF, and provide new insight regarding the role of Cdc42 in these events.

The p120 catenin family: complex roles in adhesion, signaling and cancer

p120 catenin (p120) is the prototypic member of a growing subfamily of Armadillo-domain proteins found at cell-cell junctions and in nuclei. In contrast to the functions of the classical catenins (alpha-catenin, beta-catenin, and gamma-catenin/plakoglobin), which have been studied extensively, the first clues to p120's biological function have only recently emerged, and its role remains controversial. Nonetheless, it is now clear that p120 affects cell-cell adhesion through its interaction with the highly conserved juxtamembrane domain of classical cadherins, and is likely to have additional roles in the nucleus. Here, we summarize the data on the potential involvement of p120 both in promotion of and in prevension of adhesion, and propose models that attempt to reconcile some of the disparities in the literature. We also discuss the structural relationships and functions of several known p120 family members, as well as the potential roles of p120 in signaling and cancer.

Inhibiting cadherin function by dominant mutant E-cadherin expression increases the extent of tight junction assembly

Previous studies have shown that induction of cadherin-mediated cell-cell adhesion leads to tight junction formation, and that blocking cadherin-mediated cell-cell adhesion inhibits tight junction assembly. Here we report analysis of tight junction assembly in MDCK cells overexpressing a mutant E-cadherin protein that lacks an adhesive extracellular domain (T151 cells). Mutant E-cadherin overexpression caused a dramatic reduction in endogenous cadherin levels. Despite this, tight junction assembly was extensive. The number of tight junction strands observed by freeze-fracture electron microscopy significantly increased in T151 cells compared to that in control cells. Our data indicate that the hierarchical regulation of junctional complex assembly is not absolute, and that inhibition of cadherin function has both positive and negative effects on tight junction assembly.

Bacterial toxins inhibiting or activating small GTP-binding proteins

Amino acids located on the switch 1 or switch 2 domains of small GTPases of the Ras and Rho family are targets of several bacterial toxins. Exoenzyme C3 from Clostridium botulinum ADP-ribosylates specifically Rho at R43 and prevents the recruitment of Rho on the cell membrane. This blocks the downstream effects of the Rho GTPase. However, exoenzyme C3 is not a toxin, and chimeric proteins fusing C3 with the B moiety of either diphtheria toxin or Pseudomonas aeruginosa exotoxin A have been produced to intoxicate cells with low concentration of C3. C. difficile toxin B modifies by glucosylation Rho on T37 and Rac and Cdc42 on T35. Glucosylation of Rho, Rac, and Cdc42 blocks the binding of these GTPases on their downstream effectors. C. sordellii lethal toxin modifies Ras, Rap, and Rac on T35 by glucosylation. Cytotoxic necrotizing factor 1 (CNF1), from uropathogenic Escherichia coli strains, deamidates Q63 of Rho into E63, thereby blocking the intrinsic or GAP-mediated GTPase of Rho. This allows permanent activation of Rho. Thus, Rho GTPases are targets for three different toxin activities. Molecular mechanisms of these toxins are discussed.

Restoration of tight junction structure and barrier function by down-regulation of the mitogen-activated protein kinase pathway in ras-transformed Madin-Darby canine kidney cells

In the Madin-Darby canine kidney epithelial cell line, the proteins occludin and ZO-1 are structural components of the tight junctions that seal the paracellular spaces between the cells and contribute to the epithelial barrier function. In Ras-transformed Madin-Darby canine kidney cells, occludin, claudin-1, and ZO-1 were absent from cell-cell contacts but were present in the cytoplasm, and the adherens junction protein E-cadherin was weakly expressed. After treatment of the Ras-transformed cells with the mitogen-activated protein kinase kinase (MEK1) inhibitor PD98059, which blocks the activation of mitogen-activated protein kinase (MAPK), occludin, claudin-1, and ZO-1 were recruited to the cell membrane, tight junctions were assembled, and E-cadherin protein expression was induced. Although it is generally believed that E-cadherin-mediated cell-cell adhesion is required for tight junction assembly, the recruitment of occludin to the cell-cell contact area and the restoration of epithelial cell morphology preceded the appearance of E-cadherin at cell-cell contacts. Both electron microscopy and a fourfold increase in the transepithelial electrical resistance indicated the formation of functional tight junctions after MEK1 inhibition. Moreover, inhibition of MAPK activity stabilized occludin and ZO-1 by differentially increasing their half-lives. We also found that during the process of tight junction assembly after MEK1 inhibition, tyrosine phosphorylation of occludin and ZO-1, but not claudin-1, increased significantly. Our study demonstrates that down-regulation of the MAPK signaling pathway causes the restoration of epithelial cell morphology and the assembly of tight junctions in Ras-transformed epithelial cells and that tyrosine phosphorylation of occludin and ZO-1 may play a role in some aspects of tight junction formation.

Localization of AtROP4 and AtROP6 and interaction with the guanine nucleotide dissociation inhibitor AtRhoGDI1 from Arabidopsis

The small GTPases of the Rho family play a key role in actin cytoskeletal organization. In plants, a novel Rho subfamily, called ROP (Rho of plants), has been found. In Arabidopsis, 12 ROP GTPases have been identified which differ mainly at their C-termini. To test the localization of two members of this subfamily (AtROP4 and AtROP6), we have generated translational fusions with the green fluorescent protein (GFP). Microscopic analysis of transiently transfected BY2 cells revealed a predominant localization of AtROP4 in the perinuclear region, while AtROP6 was localized almost exclusively to the plasma membrane. Swapping of the AtROP4 and AtROP6 C-termini produced a change in localization. As RhoGDIs are known to bind to the C-terminus of GTPases of the Rho family, we searched for Arabidopsis RhoGDI genes. We identified the AtRhoGDI1 gene and mapped it to chromosome 3. AtRhoGDI1 encodes a 22.5 kDa protein which contains highly conserved amino acids in the isoprene binding pocket and exhibits 29% to 37% similarity to known mammalian RhoGDI homologues. The AtRhoGDI1 gene was expressed in all tissues studied. Using the yeast two-hybrid system, we showed specific interaction of AtRhoGDI1 with both AtROP4 and AtROP6 as well as with their GTP-locked mutants, but not with a GTPase of the RAB family. Recombinant GST-AtRhoGDI1 could bind GFP-AtROP4 from transgenic tobacco BY2 cell extracts, confirming the interaction observed with the two-hybrid system.

In vivo dynamics of the F-actin-binding protein neurabin-II

Neurabin-II (spinophilin) is a ubiquitously expressed F-actin-binding protein containing an N-terminal actin-binding domain, a PDZ (PSD95/discs large/ZO-1) domain and a C-terminal domain predicted to form a coiled-coil structure. We have stably expressed a green fluorescent protein (GFP)-tagged version of neurabin-II in PC12 cells, and characterized the in vivo dynamics of this actin-binding protein using confocal fluorescence microscopy. We show that GFP-neurabin-II localizes to actin filaments, especially at cortical sites and areas underlying sites of active membrane remodelling. GFP-neurabin-II labels only a subset of F-actin within these cells, as indicated by rhodamine-phalloidin staining. Both actin filaments and small, highly motile structures within the cell body are seen. Photobleaching experiments show that GFP-neurabin-II also exhibits highly dynamic behaviour when bound to actin filaments. Latrunculin B treatment results in rapid relocalization of GFP-neurabin-II to the cytosol, whereas cytochalasin D treatment causes the collapse of GFP-neurabin-II fluorescence to intensely fluorescent foci of F-actin within the cell body. This collapse is reversed on cytochalasin D removal, recovery from which is greatly accelerated by stimulation of cells with epidermal growth factor (EGF). Furthermore, we show that this EGF-induced relocalization of GFP-neurabin-II is dependent on the activity of the small GTPase Rac1 but not the activity of ADP-ribosylation factor 6.

Gastric hyperplasia in mice lacking the putative Cdc42 effector IQGAP1

Human IQGAP1 is a widely expressed 190-kDa Cdc42-, Rac1-, and calmodulin-binding protein that interacts with F-actin in vivo and that can cross-link F-actin microfilaments in vitro. Recent results have implicated IQGAP1 as a component of pathways via which Cdc42 or Rac1 modulates cadherin-based cell adhesion (S. Kuroda et al., Science 281:832-835, 1998), whereas yeast IQGAP-related proteins have been found to play essential roles during cytokinesis. To identify critical in vivo functions of IQGAP1, we generated deficient mice by gene targeting. We demonstrate that IQGAP1 null mutants arise at normal frequency and show no obvious defects during development or for most of their adult life. Loss of IQGAP1 also does not affect tumor development or tumor progression, but mutant mice exhibit a significant (P < 0.0001) increase in late-onset gastric hyperplasia relative to wild-type animals of the same genetic background. While we cannot exclude that functional redundancy with IQGAP2 contributes to the lack of developmental phenotypes, the restricted expression pattern of IQGAP2 is not obviously altered in adult IQGAP1 mutant mice. Thus, IQGAP1 does not serve any essential nonredundant functions during murine development but may serve to maintain the integrity of the gastric mucosa in older animals.

Localization of membrane-associated guanylate kinase (MAGI)-1/BAI-associated protein (BAP) 1 at tight junctions of epithelial cells

Membrane-associated guanylate kinase (MAGI)-1/BAI-associated protein (BAP) 1 and Synapse-associated protein (SAP) 97/human Discs-large tumor suppressor gene (hDLG) are ubiquitous isoforms of synaptic scaffolding molecule (S-SCAM) and Postsynaptic density (PSD)-95/SAP90, both of which are implicated in the structures of synapses, respectively. SAP97/hDLG is localized at epithelial junctions and may function as a scaffolding protein, but the subcellular localization or the function of MAGI-1/BAP1 has not been clarified. In intestinal epithelial cells, MAGI-1/BAP1 was localized at tight junctions, whereas SAP97/hDLG was localized diffusely at cell - cell junctions. In Madine Darby canine kidney (MDCK) cells, MAGI-1/BAP1 was colocalized with ZO-1, whereas SAP97/hDLG was colocalized with E-cadherin. In MDCK cells, dominant active and negative mutants of Rac1 small G protein changed the amounts of SAP97/hDLG at cell - cell junctions, but not that of MAGI-1/BAP1. When MDCK cells were switched to a low Ca2+ medium, E-cadherin disappeared from the plasma membrane, and cells were dissociated. The phorbol 12-myristate 13-acetate-treatment after the low Ca2+ switch induced a tight junction-like structure. MAGI-1/BAP1 was recruited with ZO-1 to this structure, but SAP97/hDLG or E-cadherin was not. These findings suggest that MAGI-1/BAP1 is a component of tight junctions of epithelial cells, and that its role is different from that of SAP97/hDLG.

Cell adhesion and Rho small GTPases

The Rho small GTPases, Cdc42, Rac1 and Rho, are implicated in regulation of integrin-mediated cell-substratum adhesion and cadherin-mediated cell-cell adhesion. Identification and characterization of effectors of these GTPases have provided insights into their modes of action. Rho-kinase, an effector of Rho, regulates integrin-mediated cell-substratum adhesion (focal adhesion) by regulating the phosphorylation state of myosin light chain (MLC): it directly phosphorylates MLC and also inactivates myosin phosphatase. IQGAP1, an effector of Cdc42 and Rac1, regulates cadherin-mediated cell-cell adhesion by interacting with (beta)-catenin and dissociating (alpha)-catenin from the cadherin-catenins complex. Activated Cdc42 and Rac1 inhibit IQGAP1, thereby stabilizing the cadherin-catenins complex. Cdc42/Rac1 and IQGAP1 thus appear to constitute a switch that regulates cadherin-mediated cell-cell adhesion.

Induction of cell scattering by expression of beta1 integrins in beta1-deficient epithelial cells requires activation of members of the rho family of GTPases and downregulation of cadherin and catenin function

Adhesion receptors, which connect cells to each other and to the surrounding extracellular matrix (ECM), play a crucial role in the control of tissue structure and of morphogenesis. In this work, we have studied how intercellular adhesion molecules and beta1 integrins influence each other using two different beta1-null cell lines, epithelial GE11 and fibroblast-like GD25 cells. Expression of beta1A or the cytoplasmic splice variant beta1D, induced the disruption of intercellular adherens junctions and cell scattering in both GE11 and GD25 cells. In GE11 cells, the morphological change correlated with the redistribution of zonula occluden (ZO)-1 from tight junctions to adherens junctions at high cell confluency. In addition, the expression of beta1 integrins caused a dramatic reorganization of the actin cytoskeleton and of focal contacts. Interaction of beta1 integrins with their respective ligands was required for a complete morphological transition towards the spindle-shaped fibroblast-like phenotype. The expression of an interleukin-2 receptor (IL2R)-beta1A chimera and its incorporation into focal adhesions also induced the disruption of cadherin-based adhesions and the reorganization of ECM-cell contacts, but failed to promote cell migration on fibronectin, in contrast to full-length beta1A. This indicates that the disruption of cell-cell adhesion is not simply the consequence of the stimulated cell migration. Expression of beta1 integrins in GE11 cells resulted in a decrease in cadherin and alpha-catenin protein levels accompanied by their redistribution from the cytoskeleton-associated fraction to the detergent-soluble fraction. Regulation of alpha-catenin protein levels by beta1 integrins is likely to play a role in the morphological transition, since overexpression of alpha-catenin in GE11 cells before beta1 prevented the disruption of intercellular adhesions and cell scattering. In addition, using biochemical activity assays for Rho-like GTPases, we show that the expression of beta1A, beta1D, or IL2R-beta1A in GE11 or GD25 cells triggers activation of both RhoA and Rac1, but not of Cdc42. Moreover, dominant negative Rac1 (N17Rac1) inhibited the disruption of cell-cell adhesions when expressed before beta1. However, all three GTPases might be involved in the morphological transition, since expression of either N19RhoA, N17Rac1, or N17Cdc42 reversed cell scattering and partially restored cadherin-based adhesions in GE11-beta1A cells. Our results indicate that beta1 integrins regulate the polarity and motility of epithelial cells by the induction of intracellular molecular events involving a downregulation of alpha-catenin function and the activation of the Rho-like G proteins Rac1 and RhoA.

Modulation of endocytic traffic in polarized Madin-Darby canine kidney cells by the small GTPase RhoA

Efficient postendocytic membrane traffic in polarized epithelial cells is thought to be regulated in part by the actin cytoskeleton. RhoA modulates assemblies of actin in the cell, and it has been shown to regulate pinocytosis and phagocytosis; however, its effects on postendocytic traffic are largely unexplored. To this end, we expressed wild-type RhoA (RhoAWT), dominant active RhoA (RhoAV14), and dominant inactive RhoA (RhoAN19) in Madin-Darby canine kidney (MDCK) cells expressing the polymeric immunoglobulin receptor. RhoAV14 expression stimulated the rate of apical and basolateral endocytosis, whereas RhoAN19 expression decreased the rate from both membrane domains. Polarized basolateral recycling of transferrin was disrupted in RhoAV14-expressing cells as a result of increased ligand release at the apical pole of the cell. Degradation of basolaterally internalized epidermal growth factor was slowed in RhoAV14-expressing cells. Although apical recycling of immunoglobulin A (IgA) was largely unaffected in cells expressing RhoAV14, transcytosis of basolaterally internalized IgA was severely impaired. Morphological and biochemical analyses demonstrated that a large proportion of IgA internalized from the basolateral pole of RhoAV14-expressing cells remained within basolateral early endosomes and was slow to exit these compartments. RhoAN19 and RhoAWT expression had little effect on these postendocytic pathways. These results indicate that in polarized MDCK cells activated RhoA may modulate endocytosis from both membrane domains and postendocytic traffic at the basolateral pole of the cell.

Identification and characterisation of human Junctional Adhesion Molecule (JAM)

It is widely believed that migrating immune cells utilise the intercellular junctions as routes of passage, and in doing so cause the transient disruption of junctional structures. Thus there is much interest in the molecules that have been identified at cell-cell contact points and their potential involvement in the control of leukocyte diapedesis. In this report we describe the human orthologue to Junctional Adhesion Molecule (JAM), a recently identified member of the immunoglobulin superfamily expressed at intercellular junctions (Martin-Padura et al., 1998). The human protein shares a highly conserved structure and sequence with the murine protein. However it is distinct in that it is constitutively expressed on circulating neutrophils, monocytes, platelets and lymphocyte subsets. This broad expression pattern is similar to another IgSF molecule, CD31, expressed at intercellular junctions, and may indicate further complexities in the control of leukocyte/ endothelial interactions.

Involvement of the small GTPases XRhoA and XRnd1 in cell adhesion and head formation in early Xenopus development

The Rho family of small GTPases regulates a variety of cellular functions, including the dynamics of the actin cytoskeleton, cell adhesion, transcription, cell growth and membrane trafficking. We have isolated the first Xenopus homologs of the Rho-like GTPases RhoA and Rnd1 and examined their potential roles in early Xenopus development. We found that Xenopus Rnd1 (XRnd1) is expressed in tissues undergoing extensive morphogenetic changes, such as marginal zone cells involuting through the blastopore, somitogenic mesoderm during somite formation and neural crest cells. XRnd1 also causes a severe loss of cell adhesion in overexpression experiments. These data and the expression pattern suggest that XRnd1 regulates morphogenetic movements by modulating cell adhesion in early embryos. Xenopus RhoA (XRhoA) is a potential XRnd1 antagonist, since overexpression of XRhoA increases cell adhesion in the embryo and reverses the disruption of cell adhesion caused by XRnd1. In addition to the potential roles of XRnd1 and XRhoA in the regulation of cell adhesion, we find a role for XRhoA in axis formation. When coinjected with dominant-negative BMP receptor (tBR) in the ventral side of the embryo, XRhoA causes the formation of head structures resembling the phenotype seen after coinjection of wnt inhibitors with dominant-negative BMP receptor. Since dominant-negative XRhoA is able to reduce the formation of head structures, we propose that XRhoA activity is essential for head formation. Thus, XRhoA may have a dual role in the embryo by regulating cell adhesion properties and pattern formation.

Rho-like GTPases: their role in epithelial cell-cell adhesion and invasion

The family members of small Rho-like GTPases, RhoA, Rac1 and Cdc42Hs, are regulators of diverse cellular signalling pathways, including cytoskeletal organisation, transcription and cell-cycle progression. Recent research has given insight into the complex regulation of cell-cell adhesion and migratory responses of epithelial cells. The Rho-like GTPases RhoA, Rac1 and Cdc42Hs as major determinants of cytoskeletal organisation have been identified as key regulators of epithelial architecture, as well as of cell migration. These findings highlight the complex regulation and cross-talk of GTPase-dependent signalling pathways arising from cell-cell and cell-matrix interactions. The molecular mechanism of how Rho-like GTPases couple to molecules mediating either cell-cell adhesion or cell migration will be of particular interest to understand the invasive phenotype of epithelial tumours.

Rac downregulates Rho activity: reciprocal balance between both GTPases determines cellular morphology and migratory behavior

Using biochemical assays to determine the activation state of Rho-like GTPases, we show that the guanine nucleotide exchange factor Tiam1 functions as a specific activator of Rac but not Cdc42 or Rho in NIH3T3 fibroblasts. Activation of Rac by Tiam1 induces an epithelial-like morphology with functional cadherin-based adhesions and inhibits migration of fibroblasts. This epithelial phenotype is characterized by Rac-mediated effects on Rho activity. Transient PDGF-induced as well as sustained Rac activation by Tiam1 or V12Rac downregulate Rho activity. We found that Cdc42 also downregulates Rho activity. Neither V14Rho or N19Rho affects Rac activity, suggesting unidirectional signaling from Rac towards Rho. Downregulation of Rho activity occurs independently of Rac- induced cytoskeletal changes and cell spreading. Moreover, Rac effector mutants that are defective in mediating cytoskeleton changes or Jun kinase activation both downregulate Rho activity, suggesting that neither of these Rac signaling pathways are involved in the regulation of Rho. Restoration of Rho activity in Tiam1-expressing cells by expression of V14Rho results in reversion of the epithelioid phenotype towards a migratory, fibroblastoid morphology. We conclude that Rac signaling is able to antagonize Rho activity directly at the GTPase level, and that the reciprocal balance between Rac and Rho activity determines cellular morphology and migratory behavior in NIH3T3 fibroblasts.

Coendocytosis of cadherin and c-Met coupled to disruption of cell-cell adhesion in MDCK cells--regulation by Rho, Rac and Rab small G proteins

Both E-cadherin, a cell-cell adhesion molecule, and c-Met, the hepatocyte growth factor (HGF)/scatter factor (SF) receptor, were colocalized at cell-cell adhesion sites of MDCK cells. HGF/SF or a phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), induced disruption of cell-cell adhesion, which was accompanied by endocytosis of both E-cadherin and c-Met. Reduction of medium Ca2+ to a micromolar range showed the same effects. Re-increase in medium Ca2+ to a millimolar range formed cell-cell adhesion, which was accompanied by exocytosis of E-cadherin and c-Met, followed by their re-colocalization at the cell-cell adhesion sites. These results suggest that E-cadherin and c-Met are colocalized at cell-cell adhesion sites and undergo co-endo-exocytosis. We have previously shown that TPA does not induce disruption of cell-cell adhesion and subsequent scattering of MDCK cells stably expressing a dominant active mutant of RhoA or Rac1 small G protein or a dominant negative mutant of Rab5 small G protein. In these cell lines, the HGF- or TPA-induced coendocytosis of E-cadherin and c-Met was inhibited, but the coendocytosis of E-cadherin and c-Met in response to reduction of medium Ca2+ was not affected. Wortmannin, an inhibitor of phosphoinositide (PI) 3-kinase, inhibited the HGF-induced disruption of cell-cell junction and endocytosis of E-cadherin and c-Met, but not the TPA-induced ones. These results suggest that disruption of cell-cell adhesion is involved in the HGF- or TPA-induced coendocytosis of E-cadherin and c-Met in MDCK cells, and that the Rho and Rab family members indirectly regulate this coendocytosis. In addition, coendocytosis of E-cadherin and c-Met in response to HGF is partly mediated by PI 3-kinase. The cross-talk between cell-cell and cell-matrix adherens junctions is discussed.

Cloning of a novel human Rac1b splice variant with increased expression in colorectal tumors

Rac1 is a member of the Ras superfamily of small GTPases involved in signal transduction pathways that induce the formation of lamellipodia, stimulate cell proliferation and activate the JNK/SAPK protein kinase cascade. Here we describe that amplification by RT-PCR of the entire Rac1 coding sequence from a series of human adult and fetal tissues revealed beside the expected Rac1 cDNA, a variant product which contained additional 57 nucleotides between codons 75 and 76. This variant resulted in an in-frame insertion of 19 new amino acids immediately behind the switch II region, including two potential threonine phosphorylation sites for casein kinase II and protein kinase C. Primers designed within and downstream of the inserted nucleotide sequence allowed isolation of a genomic clone with intronic consensus sequences demonstrating that the insertion corresponds to a novel, yet undescribed exon 3b. This Rac1 splice variant, designated Rac1b, was predominantly identified in skin and epithelial tissues from the intestinal tract. Most notably, the expression of rac1b versus rac1 was found to be elevated in colorectal tumors at various stages of neoplastic progression, as compared to their respective adjacent tissues. We suggest that the 19 amino acid-insertion following the switch II region may create a novel effector binding site in rac1b, and thus participate in signaling pathways related to the normal or neoplastic growth of the intestinal mucosa.

Requirement for Ras and phosphatidylinositol 3-kinase signaling uncouples the glucocorticoid-induced junctional organization and transepithelial electrical resistance in mammary tumor cells

In Con8 rat mammary epithelial tumor cells, the synthetic glucocorticoid dexamethasone stimulates the remodeling of the apical junction (tight and adherens junctions) and the transepithelial electrical resistance (TER), which reflects tight junction sealing. Indirect immunofluorescence revealed that dexamethasone induced the recruitment of endogenous Ras and the p85 regulatory subunit of phosphatidylinositol (PI) 3-kinase to regions of cell-cell contact, concurrently with the stimulation of TER. Expression of dominant-negative RasN17 abolished the dexamethasone stimulation in TER, whereas, dexamethasone induced the reorganization of tight junction and adherens junction proteins, ZO-1 and beta-catenin, as well as F-actin, to precise regions of cell-cell contact in a Ras-independent manner. Confocal microscopy revealed that RasN17 and the p85 regulatory subunit of PI 3-kinase co-localized with ZO-1 and F-actin at the tight junction and adherens junction, respectively. Treatment with either of the PI 3-kinase inhibitors, wortmannin or LY294002, or the MEK inhibitor PD 098059, which prevents MAPK signaling, attenuated the dexamethasone stimulation of TER without affecting apical junction remodeling. Similar to dominant-negative RasN17, disruption of both Ras effector pathways using a combination of inhibitors abolished the glucocorticoid stimulation of TER. Thus, the glucocorticoiddependent remodeling of the apical junction and tight junction sealing can be uncoupled by their dependence on Ras and/or PI 3-kinase-dependent pathways, implicating a new role for Ras and PI 3-kinase cell signaling events in the steroid control of cell-cell interactions.

Rac regulates the stability of the adherens junction and its components, thus affecting epithelial cell differentiation and transformation

We have previously reported that activated rac (V12rac) can bring about hypertransformation of ras-transformed epithelial cells, which can be suppressed by the dominant negative form of rac (N17rac). Starting with primary epithelial cells, a series of cell lines expressing wild type (WT) or mutated forms of ras or rac were generated and analysed for their adhesive function and expression and association of adherens junction (AJ) proteins. Normal, primary epithelial cells were self-adhesive and expressed AJs that were very stable. The expression of constitutively active ras resulted in a decrease in, but not loss of, cell-cell adhesion, with concomitantly decreased stability of AJ components. This was extremely exacerbated by the co-expression of constitutively activate rac, but was suppressed by dominant negative rac, which resulted in increased cell-cell adhesion and extremely stable AJs. alpha-catenin also failed to associate with E-cadherin-beta-catenin complexes in cells expressing V12rac. Expression of V12rac resulted in the loss of epithelial morphology. The extent of transformation of each cell type corresponded to the stability of the respective AJ complexes. Thus, rac seems to be involved in regulating the stability of AJs, which promote epithelial cell differentiation, and consequently, modulating tumor progression.

Vascular endothelial cell adherens junction assembly and morphogenesis induced by sphingosine-1-phosphate

Vascular endothelial cells undergo morphogenesis into capillary networks in response to angiogenic factors. We show here that sphingosine-1-phosphate (SPP), a platelet-derived bioactive lipid, activates the EDG-1 and -3 subtypes of G protein-coupled receptors on endothelial cells to regulate angiogenesis. SPP induces the Gi/mitogen-activated protein kinase/cell survival pathway and the small GTPase Rho- and Raccoupled adherens junction assembly. Both EDG-1-and EDG-3-regulated signaling pathways are required for endothelial cell morphogenesis into capillary-like networks. Indeed, SPP synergized with polypeptide angiogenic growth factors in the formation of mature neovessels in vivo. These data define SPP as a novel regulator of angiogenesis.

Cupidin, an isoform of Homer/Vesl, interacts with the actin cytoskeleton and activated rho family small GTPases and is expressed in developing mouse cerebellar granule cells

A developmentally regulated Homer/Vesl isoform, Cupidin (Homer 2a/Vesl-2Delta11), was isolated from postnatal mouse cerebellum using a fluorescent differential display strategy. The strongest expression of Cupidin was detected in the cerebellar granule cells at approximately postnatal day 7. Cupidin was enriched in the postsynaptic density fraction, and its immunoreactivity was concentrated at glomeruli of the inner granular layer when active synaptogenesis occurred. Cupidin protein could be divided into two functional domains: the N-terminal portion, which was highly conserved among Homer/Vesl family proteins, and the C-terminal portion, which consisted of a putative coiled-coil structure, including several leucine zipper motifs. The N-terminal fragment of Cupidin, which was able to associate with metabotropic glutamate receptor 1 (mGluR1), also interacted with F-actin in vitro. In keeping with this, F-actin immunocytochemically colocalized with Cupidin in cultured cerebellar granule cells, and a Cupidin-mGluR1-actin complex was immunoprecipitated from crude cerebellar lysates using an anti-Cupidin antibody. On the other hand, the C-terminal portion of Cupidin bound to Cdc42, a member of Rho family small GTPases, in a GTP-dependent manner in vitro, and Cupidin functionally interacted with activated-Cdc42 in a heterologous expression system. Together, our findings indicate that Cupidin may serve as a postsynaptic scaffold protein that links mGluR signaling with actin cytoskeleton and Rho family proteins, perhaps during the dynamic phase of morphological changes that occur during synapse formation in cerebellar granule cells.

Regulation of cadherin-mediated cell-cell adhesion by the Rho family GTPases

Reports in the past two years have shown that Cdc42, Rac1, and Rho - belonging to the Rho small GTPase family - participate in the regulation of cadherin-mediated cell-cell adhesion. IQGAP1, an effector of Cdc42 and Rac1, interacts with cadherin and beta-catenin and induces the dissociation of alpha-catenin from the cadherin-catenins complex leading to disruption of cell-cell adhesion: activated Cdc42 and Rac1 counteract the effect of IQGAP1. Thus, Cdc42 and Rac1 appear to regulate cadherin-mediated cell-cell adhesion acting through IQGAP1.

Similar and differential behaviour between the nectin-afadin-ponsin and cadherin-catenin systems during the formation and disruption of the polarized junctional alignment in epithelial cells

BACKGROUND

We have recently identified a novel cell-cell adhesion system, named NAP system, which is localized at cadherin-based cell-cell adherens junctions (AJs). The NAP system is composed of at least nectin, afadin and ponsin. Nectin is an immunoglobulin-like cell adhesion molecule. Afadin is an actin filament-binding protein which associates nectin with the actin cytoskeleton. Ponsin is an afadin-binding protein which furthermore binds to vinculin and provides a possible linkage of nectin-afadin to cadherin-catenin through vinculin. We compared here the behaviour of the NAP and cadherin-catenin systems during the formation and disruption of the polarized junctional alignment in epithelial cells.

RESULTS

At the early stage of the formation of the polarized junctional alignment in MTD-1 A cells, primordial spot-like junctions were formed at the tips of thin cellular protrusions radiating from adjacent cells. Nectin, afadin, ponsin, cadherin and catenin were simultaneously recruited to these junctions. As the cell polarization proceeded, the spot-like junctions were gradually fused to form belt-like AJs where all these proteins were concentrated. The disruption of cell-cell AJs in MDCK cells by culturing at a low Ca2+ concentration caused rapid endocytosis of cadherin, but not that of nectin or afadin. Addition of 12-O-tetradecanoylphorbol-13-acetate to the cells formed a tight junction-like structure where nectin and afadin, but not cadherin, accumulated.

CONCLUSION

These results indicate that the NAP and cadherin-catenin systems show similar and differential behaviour during the formation and disruption of the polarized junctional alignment in epithelial cells.

Cupidin, an isoform of Homer/Vesl, interacts with the actin cytoskeleton and activated rho family small GTPases and is expressed in developing mouse cerebellar granule cells

A developmentally regulated Homer/Vesl isoform, Cupidin (Homer 2a/Vesl-2Delta11), was isolated from postnatal mouse cerebellum using a fluorescent differential display strategy. The strongest expression of Cupidin was detected in the cerebellar granule cells at approximately postnatal day 7. Cupidin was enriched in the postsynaptic density fraction, and its immunoreactivity was concentrated at glomeruli of the inner granular layer when active synaptogenesis occurred. Cupidin protein could be divided into two functional domains: the N-terminal portion, which was highly conserved among Homer/Vesl family proteins, and the C-terminal portion, which consisted of a putative coiled-coil structure, including several leucine zipper motifs. The N-terminal fragment of Cupidin, which was able to associate with metabotropic glutamate receptor 1 (mGluR1), also interacted with F-actin in vitro. In keeping with this, F-actin immunocytochemically colocalized with Cupidin in cultured cerebellar granule cells, and a Cupidin-mGluR1-actin complex was immunoprecipitated from crude cerebellar lysates using an anti-Cupidin antibody. On the other hand, the C-terminal portion of Cupidin bound to Cdc42, a member of Rho family small GTPases, in a GTP-dependent manner in vitro, and Cupidin functionally interacted with activated-Cdc42 in a heterologous expression system. Together, our findings indicate that Cupidin may serve as a postsynaptic scaffold protein that links mGluR signaling with actin cytoskeleton and Rho family proteins, perhaps during the dynamic phase of morphological changes that occur during synapse formation in cerebellar granule cells.

Translocation of MARCKS and reorganization of the cytoskeleton by PMA correlates with the ion selectivity, the confluence, and transformation state of kidney epithelial cell lines

The role of protein kinase C (PKC) in the regulation of the cytoskeleton of epithelial cells with tightly sealed contacts, poor contacts, and without contacts were investigated by incubating them with a protein kinase C activator phorbol myristoyl acetate (PMA). The morphology and organization of the membrane skeleton and stress fibers as well as the localization of an actin-bundling PKC substrate MARCKS in confluent MDCK cells originating from the distal tubulus of dog kidney, LLC-PK1 cells originating from the proximal tubulus of pig kidney, src-transformed MDCK cells, epidermoid carcinoma A431 cells, and MDCK cells grown in low calcium medium (LC medium) in low density were visualized with phase contrast and immunofluorescence microscopy. Four different responses to the PMA-treatment in actin-based structures of cultured epithelial cells were observed: 1) disintegration of the membrane skeleton in confluent MDCK cells; 2) depolymerization of the stress fibers in confluent MDCK and LLC-PK1 cells; 3) formation of the membrane skeleton in A431 cells, and 4) formation of the stress fibers and membrane skeleton in LC-MDCK cells. Thus, it seems that in fully confluent tightly sealed epithelium, activation of PKC has a deleterious effect on actin-based structures, whereas in cells without contacts or loose contacts, activation of PKC by PMA results in improvement of actin-based cytoskeletal structures. The main difference between the two kidney cell lines used is their selectivity to ion transport: the monolayer of LLC-PK1 cells is anion selective and MDCK cells cation selective. We propose a model where alterations in the ionic milieu within the MDCK cells by means of cation channels affect the disintegration of the membrane skeleton. The distribution of MARCKS followed the distribution of fodrin in both cell lines upon PMA-treatment, suggesting that phosphorylation of MARCKS by PKC may contribute in the regulation of the integrity of the membrane skeleton.

The Borgs, a new family of Cdc42 and TC10 GTPase-interacting proteins

The Rho family of GTPases plays key roles in the regulation of cell motility and morphogenesis. They also regulate protein kinase cascades, gene expression, and cell cycle progression. This multiplicity of roles requires that the Rho GTPases interact with a wide variety of downstream effector proteins. An understanding of their functions at a molecular level therefore requires the identification of the entire set of such effectors. Towards this end, we performed a two-hybrid screen using the TC10 GTPase as bait and identified a family of putative effector proteins related to MSE55, a murine stromal and epithelial cell protein of 55 kDa. We have named this family the Borg (binder of Rho GTPases) proteins. Complete open reading frames have been obtained for Borg1 through Borg3. We renamed MSE55 as Borg5. Borg1, Borg2, Borg4, and Borg5 bind both TC10 and Cdc42 in a GTP-dependent manner. Surprisingly, Borg3 bound only to Cdc42. An intact CRIB (Cdc42, Rac interactive binding) domain was required for binding. No interaction of the Borgs with Rac1 or RhoA was detectable. Three-hemagglutinin epitope (HA(3))-tagged Borg3 protein was mostly cytosolic when expressed ectopically in NIH 3T3 cells, with some accumulation in membrane ruffles. The phenotype induced by Borg3 was reminiscent of that caused by an inhibition of Rho function and was reversed by overexpression of Rho. Surprisingly, it was independent of the ability to bind Cdc42. Borg3 also inhibited Jun kinase activity by a mechanism that was independent of Cdc42 binding. HA(3)-Borg3 expression caused substantial delays in the spreading of cells on fibronectin surfaces after replating, and the spread cells lacked stress fibers. We propose that the Borg proteins function as negative regulators of Rho GTPase signaling.

Rab5 induces Rac-independent lamellipodia formation and cell migration

Rab5 is a regulatory GTPase of vesicle docking and fusion that is involved in receptor-mediated endocytosis and pinocytosis. Introduction of active Rab5 in cells stimulates the rate of endocytosis and vesicle fusion, resulting in the formation of large endocytic vesicles, whereas dominant negative Rab5 inhibits vesicle fusion. Here we show that introduction of active Rab5 in fibroblasts also induced reorganization of the actin cytoskeleton but not of microtubule filaments, resulting in prominent lamellipodia formation. The Rab5-induced lamellipodia formation did not require activation of PI3-K or the GTPases Ras, Rac, Cdc42, or Rho, which are all strongly implicated in cytoskeletal reorganization. Furthermore, lamellipodia formation by insulin, Ras, or Rac was not affected by expression of dominant negative Rab5. In addition, cells expressing active Rab5 displayed a dramatic stimulation of cell migration, with the lamellipodia serving as the leading edge. Both lamellipodia formation and cell migration were dependent on actin polymerization but not on microtubules. These results demonstrate that Rab5 induces lamellipodia formation and cell migration and that the Rab5-induced lamellipodia formation occurs by a novel mechanism independent of, and distinct from, PI3-K, Ras, or Rho-family GTPases. Thus, Rab5 can control not only endocytosis but also actin cytoskeleton reorganization and cell migration, which provides strong support for an intricate relationship between these processes.

Mobility and cytoskeletal interactions of cell adhesion receptors

Clustering of cell adhesion receptors and their interactions with the cytoskeleton are key events in the formation and function of cell adhesion structures. On the free cell surface, cadherin molecules interact with the cytoskeleton/membrane skeleton by being bound or corralled, and such interactions are greatly enhanced by the formation of cadherin oligomers. Corralled cadherin molecules undergo hop diffusion from one compartment to an adjacent one (membrane skeleton fence model), which prompts the initial formation of small adhesion clusters at cell-cell contact sites, but larger-scale assemblies of cadherin and actin filaments might require a further co-ordinated recruitment of these molecules.

Progressive impairment of kidneys and reproductive organs in mice lacking Rho GDIalpha

The Rho small G protein family members regulate various actin cytoskeleton-dependent cell functions. The Rho GDI (GDP dissociation inhibitor) family, consisting of Rho GDIalpha, -beta, and -gamma, is a regulator that keeps the Rho family members in the cytosol as the GDP-bound inactive form and translocates the GDP-bound form from the membranes to the cytosol after the GTP-bound form accomplishes their functions. Rho GDIalpha is ubiquitously expressed in mouse tissues and shows GDI activity on all the Rho family members in vitro. We have generated mice lacking Rho GDIalpha by homologous recombination to clarify its in vivo function. Rho GDIalpha -/- mice showed several abnormal phenotypes. Firstly, Rho GDIalpha -/- mice were initially viable but developed massive proteinuria mimicking nephrotic syndrome, leading to death due to renal failure within a year. Histologically, degeneration of tubular epithelial cells and dilatation of distal and collecting tubules were readily detected in the kidneys. Secondly, Rho GDIalpha -/- male mice were infertile and showed impaired spermatogenesis with vacuolar degeneration of seminiferous tubules in their testes. Thirdly, Rho GDIalpha -/- embryos derived from Rho GDIalpha -/- female mice were defective in the postimplantation development. In addition, these morphological and functional abnormalities showed age-dependent progression. These results suggest that the signaling pathways of the Rho family members regulated by Rho GDIalpha play important roles in maintaining the structure and physiological function of at least kidneys and reproductive systems in adult mice.

Cdc42 and Rac1 regulate the interaction of IQGAP1 with beta-catenin

IQGAP1, a target of Cdc42 and Rac1 small GTPases, directly interacts with beta-catenin and negatively regulates E-cadherin-mediated cell-cell adhesion by dissociating alpha-catenin from the cadherin-catenin complex in vivo (Kuroda, S., Fukata, M., Nakagawa, M., Fujii, K., Nakamura, T., Ookubo, T., Izawa, I., Nagase, T., Nomura, N., Tani, H., Shoji, I., Matsuura, Y., Yonehara, S., and Kaibuchi, K. (1998) Science 281, 832-835). Here we investigated how Cdc42 and Rac1 regulate the IQGAP1 function. IQGAP1 interacted with the amino-terminal region (amino acids 1-183) of beta-catenin, which contains the alpha-catenin-binding domain. IQGAP1 dissociated alpha-catenin from the beta-catenin-alpha-catenin complex in a dose-dependent manner in vitro. Guanosine 5'-(3-O-thio)triphosphate (GTPgammaS).glutathione S-transferase (GST)-Cdc42 and GTPgammaS. GST-Rac1 inhibited the binding of IQGAP1 to beta-catenin in a dose-dependent manner in vitro, whereas neither GDP.GST-Cdc42, GDP. GST-Rac1, nor GTPgammaS.GST-RhoA did. The coexpression of dominant active Cdc42 with IQGAP1 suppressed the dissociation of alpha-catenin from the cadherin-catenin complex induced by the overexpression of IQGAP1 in L cells expressing E-cadherin (EL cells). Consistent with this, the overexpression of either dominant negative Cdc42 or Rac1 resulted in the reduction of E-cadherin-mediated cell adhesive activity in EL cells. These results indicate that Cdc42 and Rac1 negatively regulate the IQGAP1 function by inhibiting the interaction of IQGAP1 with beta-catenin, leading to stabilization of the cadherin-catenin complex.

Regulated CD44 cleavage under the control of protein kinase C, calcium influx, and the Rho family of small G proteins

CD44 is a cell surface receptor for several extracellular matrix components and is implicated in tumor cell invasion and metastasis. Our previous studies have shown that CD44 expressed in cancer cells is proteolytically cleaved at the extracellular domain through membrane-associated metalloproteases and that CD44 cleavage plays a critical role in CD44-mediated tumor cell migration (Okamoto, I., Kawano, Y., Tsuiki, H., Sasaki, J., Nakao, M., Matsumoto, M., Suga, M., Ando, M., Nakajima, M., and Saya, H. (1999) Oncogene 18, 1435-1446). In the present study, we first demonstrate rapid degradation of the membrane-tethered CD44 cleavage product through intracellular proteolytic pathways, and it occurs only after CD44 extracellular cleavage. To address the mechanisms regulating CD44 cleavage at the extracellular domain, we show that 12-O-tetradecanoylphorbol 13-acetate (TPA) and the calcium ionophore ionomycin rapidly enhance metalloprotease-mediated CD44 cleavage in U251MG cells via protein kinase C-dependent and -independent pathways, respectively, suggesting the existence of multiple distinct pathways for regulation of CD44 cleavage. Concomitant with TPA-induced CD44 cleavage, TPA treatment induces redistribution of CD44 and ERM proteins (ezrin, radixin, and moesin) to newly generated membrane ruffling areas. Treatment with lysophosphatidic acid, which is known to activate the Rho-dependent pathway, inhibits TPA-induced CD44 redistribution and CD44 cleavage. Furthermore, overexpression of Rac dominant active mutants results in the redistribution of CD44 to the Rac-induced ruffling areas and the enhancement of CD44 cleavage. These results suggest that the Rho family proteins play a role in regulation of CD44 distribution and cleavage.

Rho-like GTPases: their role in epithelial cell-cell adhesion and invasion

The family members of small Rho-like GTPases, RhoA, Rac1 and Cdc42Hs, are regulators of diverse cellular signalling pathways, including cytoskeletal organisation, transcription and cell-cycle progression. Recent research has given insight into the complex regulation of cell-cell adhesion and migratory responses of epithelial cells. The Rho-like GTPases RhoA, Rac1 and Cdc42Hs as major determinants of cytoskeletal organisation have been identified as key regulators of epithelial architecture, as well as of cell migration. These findings highlight the complex regulation and cross-talk of GTPase-dependent signalling pathways arising from cell-cell and cell-matrix interactions. The molecular mechanism of how Rho-like GTPases couple to molecules mediating either cell-cell adhesion or cell migration will be of particular interest to understand the invasive phenotype of epithelial tumours.

Cdc42, Rac1, and their effector IQGAP1 as molecular switches for cadherin-mediated cell-cell adhesion

Cell-cell adhesion is a dynamic process in various cellular and developmental situations. Cadherins, well-known Ca(2+)-dependent adhesion molecules, are thought to play a major role in the regulation of cell-cell adhesion. However, the molecular mechanism underlying the rearrangement of cadherin-mediated cell-cell adhesion is largely unknown. Cdc42 and Rac1, belonging to the Rho small GTPase family, have recently been shown to be involved in the regulation of cell-cell adhesion. In addition, IQGAP1, an effector for Cdc42 and Rac1, has been shown to regulate the cadherin function through interaction with beta-catenin, a molecule associated with cadherin. In this review, we will summarize the mode of action of Cdc42 and Rac1 as well as IQGAP1 as molecular switches for the cadherin function, and then discuss physiological processes in which the Cdc42/Rac1/IQGAP1 system may be involved.

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.

Regulation of E-cadherin: does hypoxia initiate the metastatic cascade?

The ability of tumours to metastasis is regarded as one of the hallmarks of malignancy. The process through which tumours evolve to achieve this has been termed the metastatic cascade. This cascade has been the subject of much investigation over many years. One of the vital events identified by these investigations is the reduction of adhesion between tumour cells facilitating invasion of the surrounding tissues and vascular channels, ultimately leading to the development of a distant metastasis. E-cadherin and its associated catenin complex have been identified as key molecules in cell adhesion. This review looks at the structure and interaction of the E-cadherin-catenin complex and the factors that appear to regulate E-cadherin expression and thus cell adhesion. From the data gathered, it has become possible to propose the hypothesis that the development of tumour hypoxia is the initiating factor that sets the tumour on the road to metastasis.

Distinct actions and cooperative roles of ROCK and mDia in Rho small G protein-induced reorganization of the actin cytoskeleton in Madin-Darby canine kidney cells

Rho, a member of the Rho small G protein family, regulates the formation of stress fibers and focal adhesions in various types of cultured cells. We investigated here the actions of ROCK and mDia, both of which have been identified to be putative downstream target molecules of Rho, in Madin-Darby canine kidney cells. The dominant active mutant of RhoA induced the formation of parallel stress fibers and focal adhesions, whereas the dominant active mutant of ROCK induced the formation of stellate stress fibers and focal adhesions, and the dominant active mutant of mDia induced the weak formation of parallel stress fibers without affecting the formation of focal adhesions. In the presence of C3 ADP-ribosyltransferase for Rho, the dominant active mutant of ROCK induced the formation of stellate stress fibers and focal adhesions, whereas the dominant active mutant of mDia induced only the diffuse localization of actin filaments. These results indicate that ROCK and mDia show distinct actions in reorganization of the actin cytoskeleton. The dominant negative mutant of either ROCK or mDia inhibited the formation of stress fibers and focal adhesions, indicating that both ROCK and mDia are necessary for the formation of stress fibers and focal adhesions. Moreover, inactivation and reactivation of both ROCK and mDia were necessary for the 12-O-tetradecanoylphorbol-13-acetate-induced disassembly and reassembly, respectively, of stress fibers and focal adhesions. The morphologies of stress fibers and focal adhesions in the cells expressing both the dominant active mutants of ROCK and mDia were not identical to those induced by the dominant active mutant of Rho. These results indicate that at least ROCK and mDia cooperatively act as downstream target molecules of Rho in the Rho-induced reorganization of the actin cytoskeleton.

Opposite regulation of transepithelial electrical resistance and paracellular permeability by Rho in Madin-Darby canine kidney cells

Small GTPase Rho has been thought to be important for the formation and the maintenance of tight junction in epithelial cells, but the role of Rho in the regulation of barrier function of tight junction is not well understood. We here examined whether Rho was involved in the barrier function of tight junction in Madin-Darby canine kidney (MDCK) cells. The activation of prostaglandin EP3beta receptor, coupled to a Rho activation pathway, induced the increase in transepithelial electrical resistance (TER) but the increase in paracellular flux of mannitol in the preformed monolayer of the MDCK cells expressing the EP3beta receptor. This effect of the EP3 receptor was mimicked by the expression of constitutively active RhoA but not by active Rac1 in MDCK cells, using an isopropyl-beta-D-thiogalactoside-inducible expression system. On the other hand, the activation of EP3beta receptor suppressed the elevation of TER and the decrease in paracellular mannitol flux during Ca(2+) switch-induced tight junction formation, whereas the expression of active RhoA or Rac1 did not apparently affect the TER development in the Ca(2+) switch. These results demonstrate that the EP3 receptor and active RhoA regulate permeabilities of ionic and nonionic molecules in opposite directions in the preformed monolayer, and the EP3 receptor suppresses the elevation of TER during the tight junction formation.

Involvement of Cdc42 small G protein in cell-cell adhesion, migration and morphology of MDCK cells

The Rho small G protein family consists of the Rho, Rac, and Cdc42 subfamilies and regulates various cell functions through reorganization of the actin cytoskeleton. We previously showed that the Rho subfamily regulates the formation of stress fibers and focal adhesions whereas the Rac subfamily regulates the E-cadherin-based cell-cell adhesion in MDCK cells. We studied here the function of the Cdc42 subfamily, consisting of two members, Cdc42Hs and G25k, in cell adhesion, migration, and morphology of MDCK cells. For this purpose, we made and used MDCK cell lines stably expressing each of dominant active mutants of Cdc42Hs (sMDCK-Cdc42HsDA) and G25K (sMDCK-G25KDA). Actin filaments at the cell-cell adhesion sites increased in both sMDCK-Cdc42HsDA and -G25KDA cells. Both E-cadherin and beta-catenin, adherens junctional proteins, at the cell-cell adhesion sites also increased in both sMDCK-Cdc42HsDA and -G25KDA cells. Electron microscopic analysis revealed that sMDCK-Cdc42HsDA cells tightly contacted with each other throughout the lateral membranes. Moreover, both the HGF- and TPA-induced disruption of the cadherin-based cell-cell adhesion and the subsequent cell migration were inhibited in both sMDCK-Cdc42HsDA and -G25KDA cells. Co-expression of the dominant negative mutant of Rac1, a member of the Rac subfamily, with the dominant active mutant of Cdc42Hs did not inhibit the increased accumulation of actin filaments at the cell-cell adhesion sites. These results suggest that the Cdc42 subfamily is involved in the cadherin-based cell-cell adhesion in a manner independent of the Rac subfamily. Furthermore, the cells were frequently enveloped by the large multinuclear cells in both sMDCK-Cdc42HsDA and -G25KDA cells. Video microscopic analysis revealed that the cells were engulfed by the large cells during cytokinesis.