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

Novel role for Na,K-ATPase in phosphatidylinositol 3-kinase signaling and suppression of cell motility

The Na,K-ATPase, consisting of alpha- and beta-subunits, regulates intracellular ion homeostasis. Recent studies have demonstrated that Na,K-ATPase also regulates epithelial cell tight junction structure and functions. Consistent with an important role in the regulation of epithelial cell structure, both Na,K-ATPase enzyme activity and subunit levels are altered in carcinoma. Previously, we have shown that repletion of Na,K-ATPase beta1-subunit (Na,K-beta) in highly motile Moloney sarcoma virus-transformed Madin-Darby canine kidney (MSV-MDCK) cells suppressed their motility. However, until now, the mechanism by which Na,K-beta reduces cell motility remained elusive. Here, we demonstrate that Na,K-beta localizes to lamellipodia and suppresses cell motility by a novel signaling mechanism involving a cross-talk between Na,K-ATPase alpha1-subunit (Na,K-alpha) and Na,K-beta with proteins involved in phosphatidylinositol 3-kinase (PI3-kinase) signaling pathway. We show that Na,K-alpha associates with the regulatory subunit of PI3-kinase and Na,K-beta binds to annexin II. These molecular interactions locally activate PI3-kinase at the lamellipodia and suppress cell motility in MSV-MDCK cells, independent of Na,K-ATPase ion transport activity. Thus, these results demonstrate a new role for Na,K-ATPase in regulating carcinoma cell motility.

Rac-WAVE2 signaling is involved in the invasive and metastatic phenotypes of murine melanoma cells

WAVEs (WASP-family verprolin-homologous proteins) regulate the actin cytoskeleton through activation of Arp2/3 complex. As cell motility is regulated by actin cytoskeleton rearrangement and is required for tumor invasion and metastasis, blocking actin polymerization may be an effective strategy to prevent tumor dissemination. We show that WAVEs, especially WAVE2, are essential for invasion and metastasis of melanoma cells. Malignant B16F10 mouse melanoma cells expressed more WAVE1 and WAVE2 proteins and showed higher Rac activity than B16 parental cells, which are neither invasive nor metastatic. The effect of WAVE2 silencing by RNA interference (RNAi) on the highly invasive nature of B16F10 cells was more dramatic than that of WAVE1 RNAi. Membrane ruffling, cell motility, invasion into the extracellular matrix, and pulmonary metastasis of B16F10 cells were suppressed by WAVE2 RNAi. WAVE2 RNAi also had a profound effect on invasion induced by a constitutively active form of Rac (RacCA). In addition, ectopic expression of both RacCA and WAVE2 in B16 cells resulted in further increase in the invasiveness than that observed in B16 cells expressing only RacCA. Thus, WAVE2 acts as the primary effector downstream of Rac to achieve invasion and metastasis, suggesting that suppression of WAVE2 activity holds a promise for preventing cancer invasion and metastasis.

Cell adhesion receptors, tyrosine kinases and actin modulators: a complex three-way circuitry

The interaction of cells with surrounding matrix and neighbouring cells governs many aspects of cell behaviour. Aside from transmitting signals from the external environment, adhesion receptors also receive signals from the cell interior. Here we review the interrelationship between adhesion receptors, tyrosine kinases (both growth factor receptor and non-receptor) and modulators of the actin cytoskeletal network. Deregulation of many aspects of these signalling pathways in cancer highlights the need for a better understanding of the complexities involved.

Force measurements in E-cadherin-mediated cell doublets reveal rapid adhesion strengthened by actin cytoskeleton remodeling through Rac and Cdc42

We have used a modified, dual pipette assay to quantify the strength of cadherin-dependent cell–cell adhesion. The force required to separate E-cadherin–expressing paired cells in suspension was measured as an index of intercellular adhesion. Separation force depended on the homophilic interaction of functional cadherins at the cell surface, increasing with the duration of contact and with cadherin levels. Severing the link between cadherin and the actin cytoskeleton or disrupting actin polymerization did not affect initiation of cadherin-mediated adhesion, but prevented it from developing and becoming stronger over time. Rac and Cdc42, the Rho-like small GTPases, were activated when E-cadherin–expressing cells formed aggregates in suspension. Overproduction of the dominant negative form of Rac or Cdc42 permitted initial E-cadherin–based adhesion but affected its later development; the dominant active forms prevented cell adhesion outright. Our findings highlight the crucial roles played by Rac, Cdc42, and actin cytoskeleton dynamics in the development and regulation of strong cell adhesion, defined in terms of mechanical forces.

Molecular mechanisms of membrane polarity in renal epithelial cells

Exciting discoveries in the last decade have cast light onto the fundamental mechanisms that underlie polarized trafficking in epithelial cells. It is now clear that epithelial cell membrane asymmetry is achieved by a combination of intracellular sorting operations, vectorial delivery mechanisms and plasmalemma-specific fusion and retention processes. Several well-defined signals that specify polarized segregation, sorting, or retention processes have, now, been described in a number of proteins. The intracellular machineries that decode and act on these signals are beginning to be described. In addition, the nature of the molecules that associate with intracellular trafficking vesicles to coordinate polarized delivery, tethering, docking, and fusion are also becoming understood. Combined with direct visualization of polarized sorting processes with new technologies in live-cell fluorescent microscopy, new and surprising insights into these once-elusive trafficking processes are emerging. Here we provide a review of these recent advances within an historically relevant context.

Mesenchymal-epithelial transition during somitic segmentation is regulated by differential roles of Cdc42 and Rac1

Mesenchymal-epithelial transitions (MET) are crucial for vertebrate organogenesis. The roles of Rho family GTPases in such processes during actual development remain largely unknown. By electroporating genes into chick presomitic mesenchymal cells, we demonstrate that Cdc42 and Rac1 play important and different roles in the MET that generates the vertebrate somites. Presomitic mesenchymal cells, which normally contribute to both the epithelial and mesenchymal populations of the somite, were hyperepithelialized when Cdc42 signaling was blocked. Conversely, cells taking up genes that elevate Cdc42 levels remained mesenchymal. Thus, Cdc42 activity levels appear critical for the binary decision that defines the epithelial and mesenchymal somitic compartments. Proper levels of Rac1 are necessary for somitic epithelialization, since cells with activated or inhibited Rac1 failed to undergo correct epithelialization. Furthermore, Rac1 appears to be required for Paraxis to act as an epithelialization-promoting transcription factor during somitogenesis.

Early enterocytic differentiation of HT-29 cells: biochemical changes and strength increases of adherens junctions

We have characterized the modulation of cell-cell adhesion and the structure of adherens junctions in the human colon adenocarcinoma HT-29 cell line that differentiates into enterocytes after glucose substitution for galactose in the medium. We demonstrate that differentiated cells (HT-29 Gal) rapidly established E-cadherin-mediated interactions in aggregation assays. This effect is not due to an increase in E-cadherin expression during this early stage of cell differentiation, but rather results from the maturation of preexisting adherens junctions. These junctions are characterized by the redistribution of E-cadherin to the basolateral membrane and its co-localization with the actin cytoskeleton. Subcellular fractionation studies indicate that actin-associated E-cadherins bind beta-catenin and p120ctn. Furthermore, the p120ctn/E-cadherin association is upregulated. These data reveal a cooperative interaction between p120ctn and E-cadherin that corresponds to mature functional adherens junctions able to initiate tight cell-cell adhesion required for epithelium architecture and further affirm the gatekeeper role of p120ctn.

Involvement of Cdc42 and Rac small G proteins in invadopodia formation of RPMI7951 cells

BACKGROUND

Invadopodia are membrane protrusions into the extracellular matrix by aggressive tumour cells. These structures are associated with sites of matrix degradation and invasiveness of malignant tumour cells in an in vitro fibronectin degradation/invasion assay. The Rho family small G proteins, consisting of the Rho, Rac and Cdc42 subfamilies, are implicated in various cell functions, such as cell shape change, adhesion, and motility, through reorganization of the actin cytoskeleton. We studied the roles of the Rho family small G proteins in invadopodia formation.

RESULTS

We first demonstrated that invadopodia of RPMI7951 human melanoma cells extended into the matrix substratum on a vertical view using a laser scanning confocal microscope system. We confirmed that invadopodia were rich in actin filaments (F-actin) and visualized clearly with F-actin staining on a vertical view as well as on a horizontal view. We then studied the roles of Rho, Rac, and Cdc42 in invasiveness of the same cell line. In the in vitro fibronectin degradation/invasion assay, a dominant active mutant of Cdc42 enhanced dot-like degradation, whereas a dominant active mutant of Rac enhanced diffuse-type degradation. Furthermore, frabin, a GDP/GTP exchange protein for Cdc42 with F-actin-binding activity, enhanced both dot-like and diffuse-type degradation. However, a dominant active mutant of Rho did not affect the fibronectin degradation. Moreover, inhibition of phosphatidylinositol-3 kinase (PI3K) disrupted the Rac and Cdc42-dependent actin structures and blocked the fibronectin degradation.

CONCLUSION

These results suggest that Cdc42 and Rac play important roles in fibronectin degradation and invasiveness in a coordinate manner through the frabin-Cdc42/Rac-PI3K signalling pathway.

Endocytosis of E-cadherin regulated by Rac and Cdc42 small G proteins through IQGAP1 and actin filaments

E-cadherin is a key cell–cell adhesion molecule at adherens junctions (AJs) and undergoes endocytosis when AJs are disrupted by the action of extracellular signals. To elucidate the mechanism of this endocytosis, we developed here a new cell-free assay system for this reaction using the AJ-enriched fraction from rat liver. We found here that non-trans-interacting, but not trans-interacting, E-cadherin underwent endocytosis in a clathrin-dependent manner. The endocytosis of trans-interacting E-cadherin was inhibited by Rac and Cdc42 small G proteins, which were activated by trans-interacting E-cadherin or trans-interacting nectins, which are known to induce the formation of AJs in cooperation with E-cadherin. This inhibition was mediated by reorganization of the actin cytoskeleton by Rac and Cdc42 through IQGAP1, an actin filament-binding protein and a downstream target of Rac and Cdc42. These results indicate the important role of the Rac/Cdc42-IQGAP1 system in the dynamic organization and maintenance of the E-cadherin–based AJs.

Roles played by a subset of integrin signaling molecules in cadherin-based cell-cell adhesion

Integrins can intercommunicate with cadherins. Here, we examined their possible relationship by use of small interfering RNA–mediated protein knockdown in HeLa cells. We found that a subset of integrin signaling molecules, namely Fak and paxillin, but not p130 Crk-associated substrate or proline-rich tyrosine kinase 2, participate in processes regulating N-cadherin–based cell–cell adhesion. Paxillin was found to be required primarily for the recruitment of Fak to robust focal adhesions. Our results suggest that at least some signals involving Fak are linked to a mechanism down-regulating Rac1 activity at the cell periphery, which appears to be important for the formation of N-cadherin–based adhesions in motile cells. Our analyses simultaneously exemplified the essential role of Fak in the maintenance of cell–cell adhesions in collective cell migration, a type of migration occurring in embryonic development and carcinoma invasion.

JAM4 enhances hepatocyte growth factor-mediated branching and scattering of Madin-Darby canine kidney cells

Junctional adhesion molecule (JAM) 4 is a member of immunoglobulin superfamily that interacts with MAGI-1, a membrane-associated guanylate kinase protein at tight junctions in epithelial cells. We prepared Madin-Darby canine kidney II (MDCK) cells expressing JAM4 (MDCK-JAM4) and compared them with wild MDCK cells. The treatment of hepatocyte growth factor (HGF) induced more prominent branching and scattering in MDCK-JAM4 cells. Subsequently we attempted to identify signalling pathways modified by JAM4. The over-expression of JAM4 induced the formation of protrusions in COS-7 cells. Although those protrusions were different from typical lamellipodia, the dominant negative mutant of Rac suppressed them. The pull-down assay using CDC42 and Rac interactive binding domain of PAK also supports that Rac is activated in COS-7 cells expressing JAM4. Taken together, JAM4 itself activates Rac and may augment Rac activation by HGF, resulting in the enhancement of branching and scattering.

The RacGEF Tiam1 inhibits migration and invasion of metastatic melanoma via a novel adhesive mechanism

Rho-like GTPases such as RhoA, Rac1 and Cdc42 are key regulators of actin-dependent cell functions including cell morphology, adhesion and migration. Tiam1 (T lymphoma invasion and metastasis 1), a guanine nucleotide exchange factor that activates Rac, is an important regulator of cell shape and invasiveness in epithelial cells and fibroblasts. Overexpression of Tiam1 in metastatic melanoma cells converted the constitutive mesenchymal phenotype into an epithelial-like phenotype. This included the induction of stringent cell-cell contacts mediated by the Ig-like receptor ALCAM (activated leukocyte cell adhesion molecule) and actin redistribution to cell-cell junctions. This phenotypic switch was dependent on increased Rac but not Rho activity, and on the redistribution and adhesive function of ALCAM, whereas cadherins were not involved. Although cell proliferation was significantly enhanced, the gain of cell-cell junctions strongly counteracted cell motility and invasion as shown for two- and three-dimensional collagen assays as well as invasion into human skin reconstructs. The reverse transition from mesenchymal invasive to a resident epithelial-like phenotype implicates a role for Tiam1/Rac signaling in the control of cell-cell contacts through a novel ALCAM-mediated mechanism.

p21-activated kinase regulates endothelial permeability through modulation of contractility

Endothelial cells lining the vasculature have close cell-cell associations that maintain separation of the blood fluid compartment from surrounding tissues. Permeability is regulated by a variety of growth factors and cytokines and plays a role in numerous physiological and pathological processes. We examined a potential role for the p21-activated kinase (PAK) in the regulation of vascular permeability. In both bovine aortic and human umbilical vein endothelial cells, PAK is phosphorylated on Ser141 during the activation downstream of Rac, and the phosphorylated subfraction translocates to endothelial cell-cell junctions in response to serum, VEGF, bFGF, TNFalpha, histamine, and thrombin. Blocking PAK activation or translocation prevents the increase in permeability across the cell monolayer in response to these factors. Permeability correlates with myosin phosphorylation, formation of actin stress fibers, and the appearance of paracellular pores. Inhibition of myosin phosphorylation blocks the increase in permeability. These data suggest that PAK is a central regulator of endothelial permeability induced by multiple growth factors and cytokines via an effect on cell contractility. PAK may therefore be a suitable drug target for the treatment of pathological conditions where vascular leak is a contributing factor, such as ischemia and inflammation.

Activation of Cdc42 by trans interactions of the cell adhesion molecules nectins through c-Src and Cdc42-GEF FRG

Nectins, Ca²⁺-independent immunoglobulin-like cell–cell adhesion molecules, initiate cell–cell adhesion by their trans interactions and recruit cadherins to cooperatively form adherens junctions (AJs). In addition, the trans interactions of nectins induce the activation of Cdc42 and Rac small G proteins, which increases the velocity of the formation of AJs. We examined here how nectins induce the activation of Cdc42 in MDCK epithelial cells and L fibroblasts. Nectins recruited and activated c-Src at the nectin-based cell–cell adhesion sites. FRG, a GDP/GTP exchange factor specific for Cdc42, was then recruited there, tyrosine phosphorylated by c-Src, and activated, causing an increase in the GTP-bound active form of Cdc42. Inhibition of the nectin-induced activation of c-Src suppressed the nectin-induced activation of FRG and Cdc42. Inhibition of the nectin-induced activation of FRG or depletion of FRG by RNA interference suppressed the nectin-induced activation of Cdc42. These results indicate that nectins induce the activation of Cdc42 through c-Src and FRG locally at the nectin-based cell–cell adhesion sites.

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.

Matrix-specific activation of Src and Rho initiates capillary morphogenesis of endothelial cells

Interstitial collagen I stimulates microvascular endothelial cells to form solid cords that imitate precapillary structures found during angiogenesis. Time-lapse microscopy identified cell retraction and disruption of cell-cell contacts as early critical steps in collagen I-induced capillary morphogenesis. These early stages paralleled collagen I activation of Src kinase and GTPase Rho through beta1 integrins. The Src inhibitor PP2, dominant-negative Src, and Rho inhibitor exoenzyme C3 transferase each inhibited collagen I induction of actin stress fibers that mediate cell retraction and each inhibited capillary morphogenesis. Collagen I also disrupted VE-cadherin from intercellular junctions through a Src-dependent mechanism; both the Src inhibitor PP2 and dominant-negative Src preserved VE-cadherin localization to regions of cell-cell contact. An active Src mutant disrupted VE-cadherin and cell-cell contacts similarly to collagen I. In sharp contrast, laminin-1 did not induce capillary morphogenesis, and laminin-1 did not induce activation of Src or Rho. Rather, laminin-1 induced persistent activation of the GTPase Rac. Thus, these studies identify activation of Src and Rho as key mechanisms by which collagen I provokes capillary morphogenesis of microvascular endothelial cells, and they define marked differences between the functions of collagen I and laminin-1 in regulating endothelial cell morphogenesis.

RhoA/ROCK and Cdc42 regulate cell-cell contact and N-cadherin protein level during neurodetermination of P19 embryonal stem cells

RhoGTPases regulate actin-based signaling cascades and cellular contacts. In neurogenesis, their action modulates cell migration, neuritogenesis, and synaptogenesis. Murine P19 embryonal stem cells differentiate to neurons upon aggregation in the presence of retinoic acid, and we previously showed that RhoA and Cdc42 RhoGTPases are sequentially up-regulated during neuroinduction, suggesting a role at this very early developmental stage. In this work, incubation of differentiating P19 cells with C3 toxin resulted in decreased aggregate cohesion and cadherin protein level. In contrast, C3 effects were not observed in cells overexpressing recombinant dominant active RhoA. On the other hand, C3 did not affect cadherin in uninduced cells and their postmitotic neuronal derivatives, respectively expressing E- and N-cadherin. RhoA is thus influential on cell aggregation and cadherin expression during a sensitive time window that corresponds to the switch of E- to N-cadherin. Cell treatment with Y27632 inhibitor of Rho-associated-kinase ROCK, or advanced overexpression of Cdc42 by gene transfer of a constitutively active form of the protein reproduced C3 effects. RhoA-antisense RNA also reduced cadherin level and the size of cell aggregates, and increased the generation of fibroblast-like cells relative to neurons following neuroinduction. Colchicin, a microtubule disrupter, but not cytochalasin B actin poison, importantly decreased cadherin in neurodifferentiating cells. Overall, our results indicate that the RhoA/ROCK pathway regulates cadherin protein level and cell-cell interactions during neurodetermination, with an impact on the efficiency of the process. The effect on cadherin seems to involve microtubules. The importance of correct timing of RhoA and Cdc42 functional expression in neurogenesis is also raised.

Rho localization in cells and tissues

Rho family small GTPases regulate cytoskeletal organization. Although their spatiotemporal activities appear to be important for cellular morphogenesis, there has been little characterization of the localization of Rho family GTPases in cells and tissues. Here we show precise localization of Rho subfamily proteins in mammalian cultured cells and tissues through evaluation of anti-Rho antibodies and fixation protocols. Although Rho is not a structural protein but functions as a switching molecule, it often localizes at several distinct domains or structures of cells. In cultured epithelial cells, Rho was highly accumulated at lateral membranes. However, in fibroblastic cells, Rho appeared to be distributed evenly in the cytoplasm. Rho concentration at the cleavage furrow at cytokinesis was generally observed. In A431 cells, Rho translocation from the cytoplasm to elongating microvilli at the apical membrane within 30 s after EGF stimulation was clearly demonstrated. Also, Myc- or GFP-tagged RhoA did not always reflect the localization of endogenous Rho, indicating a drawback of protein-tagging methods for localization research. In mouse tissues, Rho localization differed depending on cell type, probably reflecting the functional differences of each cell type.

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.

Rab13 regulates PKA signaling during tight junction assembly

The GTPase Rab13 regulates the assembly of functional epithelial tight junctions (TJs) through a yet unknown mechanism. Here, we show that expression of the GTP-bound form of Rab13 inhibits PKA-dependent phosphorylation and TJ recruitment of the vasodilator-stimulated phosphoprotein, an actin remodelling protein. We demonstrate that Rab13GTP directly binds to PKA and inhibits its activity. Interestingly, activation of PKA abrogates the inhibitory effect of Rab13 on the recruitment of vasodilator-stimulated phosphoprotein, ZO-1, and claudin1 to cell–cell junctions. Rab13 is, therefore, the first GTPase that controls PKA activity and provides an unexpected link between PKA signaling and the dynamics of TJ assembly.

Src SH3/2 domain-mediated peripheral accumulation of Src and phospho-myosin is linked to deregulation of E-cadherin and the epithelial-mesenchymal transition

Elevated Src kinase in epithelial cancer cells induces adhesion changes that are associated with a mesenchymal-like state. We recently showed that Src induces dynamic integrin adhesions in KM12C colon cancer cells, whereas E-cadherin-dependent cell-cell contacts become disorganized. This promotes a fibroblastic-like morphology and expression of the mesenchymal marker vimentin. Furthermore, Src-induced deregulation of E-cadherin, and the associated mesenchymal transition, is dependent on integrin signaling (Avizienyte et al., Nat. Cell Biol. 2002, 4, 632-638), although the nature of downstream signals that mediate these Src- and integrin-dependent effects are unknown. Here we show that the SH2 and SH3 domains of Src mediate peripheral accumulation of phospho-myosin, leading to integrin adhesion complex assembly, whereas loss of SH2 or SH3 function restores normal regulation of E-cadherin and inhibits vimentin expression. Inhibitors of MEK, ROCK, or MLCK also suppress peripheral accumulation of phospho-myosin and Src-induced formation of integrin-dependent adhesions, whereas at the same time restoring E-cadherin redistribution to regions of cell-cell contact. Our data therefore implicate peripheral phospho-myosin activity as a point of convergence for upstream signals that regulate integrin- and E-cadherin-mediated adhesions. This further implicates spatially regulated contractile force as a determinant of epithelial cell plasticity, particularly in cancer cells that can switch between epithelial and mesenchymal-like states.

RhoA, Rac1, and Cdc42 exert distinct effects on epithelial barrier via selective structural and biochemical modulation of junctional proteins and F-actin

Epithelial intercellular junctions regulate cell-cell contact and mucosal barrier function. Both tight junctions (TJs) and adherens junctions (AJs) are regulated in part by their affiliation with the F-actin cytoskeleton. The cytoskeleton in turn is influenced by Rho family small GTPases such as RhoA, Rac1, and Cdc42, all of which constitute eukaryotic targets for several pathogenic organisms. With a tetracycline-repressible system to achieve regulated expression in Madin-Darby canine kidney (MDCK) epithelial cells, we used dominant-negative (DN) and constitutively active (CA) forms of RhoA, Rac1, and Cdc42 as tools to evaluate the precise contribution of each GTPase to epithelial structure and barrier function. All mutant GTPases induced time-dependent disruptions in epithelial gate function and distinct morphological alterations in apical and basal F-actin pools. TJ proteins occludin, ZO-1, claudin-1, claudin-2, and junctional adhesion molecule (JAM)-1 were dramatically redistributed in the presence of CA RhoA or CA Cdc42, whereas only claudins-1 and -2 were redistributed in response to CA Rac1. DN Rac1 expression also induced selective redistribution of claudins-1 and -2 in addition to JAM-1, whereas DN Cdc42 influenced only claudin-2 and DN RhoA had no effect. AJ protein localization was unaffected by any mutant GTPase, but DN Rac1 induced a reduction in E-cadherin detergent solubility. All CA GTPases increased the detergent solubility of claudins-1 and -2, but CA RhoA alone reduced claudin-2 and ZO-1 partitioning to detergent-insoluble membrane rafts. We conclude that Rho family GTPases regulate epithelial intercellular junctions via distinct morphological and biochemical mechanisms and that perturbations in barrier function reflect any imbalance in active/resting GTPase levels rather than simply loss or gain of GTPase activity.

The transcription factor Snail downregulates the tight junction components independently of E-cadherin downregulation

Snail, a transcriptional repressor of E-cadherin expression, is involved in epithelial-mesenchymal transitions during development. We demonstrate that Snail activity is not restricted to E-cadherin downregulation. Expression of tight junction proteins, including claudin-1, occludin and ZO-1, was downregulated in MDCK cells exogenously expressing Snail protein. Although occludin mRNA levels were downregulated by Snail expression, the transcription of claudin-1 and ZO-1 were unaffected. Reporter assays using the claudin-1 promoter region revealed that promoter activity was not affected by Snail overexpression. Decreased synthesis of claudin-1 protein was observed, however, suggesting that Snail may act in translation initiation. Snail expression also altered the splicing pattern of p120. The levels of mRNA encoding the epithelial variant decreased, while the fibroblastic mRNA form increased. Although ectopic E-cadherin expression resulted in a downregulation of Snail-induced fibronectin expression, fibroblastic morphology was affected only minimally; the expression of tight junctional proteins remained at low levels. These results indicate that Snail is involved in both the direct transcriptional repression of genes, such as E-cadherin and occludin, and post-transcriptional events, including downregulation of claudin-1. These data support the idea that Snail is a transcription factor possessing pleiotropic activities.

Positive role of IQGAP1, an effector of Rac1, in actin-meshwork formation at sites of cell-cell contact

The small guanosine triphosphatase Rac1 is activated by E-cadherin-mediated cell-cell adhesion and is required for the accumulation of actin filaments, E-cadherin, and beta-catenin at sites of cell-cell contact. However, the modes of activation and action of Rac1 remain to be clarified. We here found that suppression of IQGAP1, an actin-binding protein and an effector of Rac1, by small interfering RNA apparently reduced the accumulation of actin filaments, E-cadherin, and beta-catenin at sites of cell-cell contact in Madin-Darby canine kidney II epithelial cells under the conditions in which knockdown of Rac1 reduced them. Knockdown of Rac1 did not affect the localization of these junctional components in cells expressing a constitutively active IQGAP1 mutant defective in Rac1/Cdc42 binding. Knockdown of either Rac1 or IQGAP1 accelerated the 12-O-tetradecanoylphorbol-13-acetate-induced cell-cell dissociation. The basal Rac1 activity, which was maintained by E-cadherin-mediated cell-cell adhesion, was inhibited in the IQGAP1-knocked down cells, whereas the Rac1 activity was increased in the cells overexpressing IQGAP1. Together, these results indicate that Rac1 enhances the accumulation of actin filaments, E-cadherin, and beta-catenin by acting on IQGAP1 and suggest that there exists a positive feedback loop comprised of "E-cadherin-mediated cell-cell adhesion --> Rac1 activation --> actin-meshwork formation by IQGAP1 --> increasing E-cadherin-mediated cell-cell adhesion."

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.

Novel kelch-like protein, KLEIP, is involved in actin assembly at cell-cell contact sites of Madin-Darby canine kidney cells

Dynamic rearrangements of cell-cell adhesion underlie a diverse range of physiological processes, but their precise molecular mechanisms are still obscure. Thus, identification of novel players that are involved in cell-cell adhesion would be important. We isolated a human kelch-related protein, Kelch-like ECT2 interacting protein (KLEIP), which contains the broad-complex, tramtrack, bric-a-brac (BTB)/poxvirus, zinc finger (POZ) motif and six-tandem kelch repeats. KLEIP interacted with F-actin and was concentrated at cell-cell contact sites of Madin-Darby canine kidney cells, where it colocalized with F-actin. Interestingly, this localization took place transiently during the induction of cell-cell contact and was not seen at mature junctions. KLEIP recruitment and actin assembly were induced around E-cadherin-coated beads placed on cell surfaces. The actin depolymerizing agent cytochalasin B inhibited this KLEIP recruitment around E-cadherin-coated beads. Moreover, constitutively active Rac1 enhanced the recruitment of KLEIP as well as F-actin to the adhesion sites. These observations strongly suggest that KLEIP is localized on actin filaments at the contact sites. We also found that N-terminal half of KLEIP, which lacks the actin-binding site and contains the sufficient sequence for the localization at the cell-cell contact sites, inhibited constitutively active Rac1-induced actin assembly at the contact sites. We propose that KLEIP is involved in Rac1-induced actin organization during cell-cell contact in Madin-Darby canine kidney cells.

Tumor-related Alternatively Spliced Rac1b Is Not Regulated by Rho-GDP Dissociation Inhibitors and Exhibits Selective Downstream Signaling

Rac1 is a member of the Rho family of small GTPases, which control signaling pathways that regulate actin cytoskeletal dynamics and gene transcription. Rac1 is activated by guanine nucleotide exchange factors and inactivated by GTPase-activating proteins. In addition, Rho-GDP dissociation inhibitors (Rho-GDIs) can inhibit Rac1 by sequestering it in the cytoplasm. We have found previously that colorectal tumors express an alternatively spliced variant, Rac1b, containing 19 additional amino acids following the switch II region. Here we characterized the regulation and downstream signaling of Rac1b. Although little Rac1b protein is expressed in cells, the amount of activated Rac1b protein often exceeds that of activated Rac1, suggesting that Rac1b contributes significantly to the downstream signaling of Rac in cells. The regulation of both Rac1 and Rac1b activities is dependent on guanine nucleotide exchange factors and GTPase-activating proteins, but the difference in their activation is mainly determined by the inability of Rac1b to interact with Rho-GDI. As a consequence, most Rac1b remains bound to the plasma membrane and is not sequestered by Rho-GDI in the cytoplasm. Unlike Rac1, activated Rac1b is unable to induce lamellipodia formation and is unable to bind and activate p21-activated protein kinase nor activate the downstream protein kinase JNK. However, both Rac1 and Rac1b are able to activate NFkappaB to the same extent. These data suggest that alternative splicing of Rac1 leads to a highly active Rac variant that differs in regulation and downstream signaling.

SRC-induced disintegration of adherens junctions of madin-darby canine kidney cells is dependent on endocytosis of cadherin and antagonized by Tiam-1

The effects of Src tyrosine kinase activation in subconfluent temperature sensitive (ts)-Src-transformed Madin-Darby canine kidney (MDCK) cells were analyzed by shifting them from nonpermissive (40.5 degrees C) to permissive (35 degrees C) temperature. Already, in 15 minutes, adherens junction components were released from the lateral walls and accumulated to basal surfaces. Simultaneously, membranous actin staining vanished, actin bundles appeared at the basal surface, and the cells flattened. The only component phosphorylated and translocated after the shift to 35 degrees C was p120ctn. The epithelial-mesenchymal transition could be inhibited by a specific inhibitor of Src kinase, PP2, or by inhibiting endocytosis. Therefore, Src activation was responsible for the transition, but not because of phosphorylation of adherens junction components but by way of activation of endocytic machinery and RhoGTPase. Expression of an RacGEF, Tiam-1 (T-lymphoma invasion and metastasis gene 1), prevented flattening of Src-transformed MDCK cells at 35 degrees C and resulted in accumulation of cadherin to lateral membranes. In the case where the Src-MDCK cells were cultivated at 35 degrees C and shifted for short time periods to 40.5 degrees C, cadherin rapidly returned to lateral membranes, whereas actin and p120ctn followed hours afterward. This further supports the view that cadherin internalization is the primary target of Src kinase. We also looked at the cell morphology and distribution of cadherin and Tiam-1 in cells grown in three-dimensional gels composed of collagen and laminin or in Matrigel. At nonpermissive temperature, both Src-MDCK and Tiam-1-transfected Src-MDCK cells exhibited nonpolarized morphology in collagen I, a loose cluster in the mixture of collagen I and laminin, and a differentiated cyst in Matrigel. In growth factor-depleted Matrigel, the Src-MDCK cells grew in nondifferentiated clusters, whereas Tiam-1-transfected cells went to apoptosis. The differentiated phenotype of both cell lines could be rescued by Matrigel-conditioned medium, platelet-derived growth factor, or cholera toxin. Concomitantly, both cadherin and Tiam-1 were recruited to lateral membranes. Therefore, cadherin and Tiam-1 seem to be the key players in the differentiation process of MDCK cells.

Involvement of Nectin-activated Cdc42 Small G Protein in Organization of Adherens and Tight Junctions in Madin-Darby Canine Kidney Cells

Nectins, Ca2+-independent immunoglobulin-like cell-cell adhesion molecules, trans-interact and form cell-cell adhesion, which increases the velocities of the formation of the E-cadherin-based adherens junctions (AJs) and the claudin-based tight junctions (TJs) in Madin-Darby canine kidney (MDCK) cells. The trans-interactions of nectins furthermore induce activation of Cdc42 and Rac small G proteins, but the roles of these small G proteins activated in this way remain unknown. We examined here the role and the mode of action of Cdc42 in the organization of AJs and TJs in MDCK cells. We first made the NWASP-Cdc42 and Rac interactive binding (CRIB) domain, an inhibitor of activated Cdc42, fused to the Ki-Ras CAAX motif (NWASP-CRIB-CAAX; where A is aliphatic amino acid), which was targeted to the cell-cell adhesion sites. We then found that overexpression of NWASP-CRIB-CAAX reduced the velocities of the formation of AJs and TJs. Conversely, overexpression of a constitutively active mutant of Cdc42 (V12Cdc42) increased their velocities, and the inhibitory effect of NWASP-CRIB-CAAX was suppressed by co-expression with V12Cdc42. The inhibitory effect of NWASP-CRIB-CAAX on the formation of AJs and TJs was suppressed by co-expression of nectin-1 of which trans-interaction activated endogenous Cdc42. Moreover, the formation of the claudin-based TJs required a greater amount of activated Cdc42 than that of the E-cadherin-based AJs. These results indicate that the Cdc42 activated by the trans-interactions of nectins is involved in the organization of AJs and TJs in different mechanisms in MDCK cells.

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.

CD151 regulates epithelial cell-cell adhesion through PKC- and Cdc42-dependent actin cytoskeletal reorganization

CD151, a member of the tetraspanin family proteins, tightly associates with integrin α3β1 and localizes at basolateral surfaces of epithelial cells. We found that overexpression of CD151 in A431 cells accelerated intercellular adhesion, whereas treatment of cells with anti-CD151 mAb perturbed the integrity of cortical actin filaments and cell polarity. E-Cadherin puncta formation, indicative of filopodia-based adhesion zipper formation, as well as E-cadherin anchorage to detergent-insoluble cytoskeletal matrix, was enhanced in CD151-overexpressing cells. Levels of GTP-bound Cdc42 and Rac were also elevated in CD151-overexpressing cells, suggesting the role of CD151 in E-cadherin–mediated cell–cell adhesion as a modulator of actin cytoskeletal reorganization. Consistent with this possibility, engagement of CD151 by the substrate-adsorbed anti-CD151 mAb induced prominent Cdc42-dependent filopodial extension, which along with E-cadherin puncta formation, was strongly inhibited by calphostin C, a protein kinase C (PKC) inhibitor. Together, these results indicate that CD151 is involved in epithelial cell–cell adhesion as a modulator of PKC- and Cdc42-dependent actin cytoskeletal reorganization.

The C-terminal domain of Rac1 contains two motifs that control targeting and signaling specificity

Rho-like GTPases control a wide range of cellular functions such as integrin- and cadherin-mediated adhesion, cell motility, and gene expression. The hypervariable C-terminal domain of these GTPases has been implicated in membrane association and effector binding. We found that cell-permeable peptides, encoding the C termini of Rac1, Rac2, RhoA, and Cdc42, interfere with GTPase signaling in a specific fashion in a variety of cellular models. Pull-down assays showed that the C terminus of Rac1 does not associate to either RhoGDI or to Pak. In contrast, the C terminus of Rac1 (but not Rac2 or Cdc42) binds to phosphatidylinositol 4,5-phosphate kinase (PIP5K) via amino acids 185-187 (RKR). Moreover, Rac1 associates to the adapter protein Crk via the N-terminal Src homology 3 (SH3) domain of Crk and the proline-rich stretch in the Rac1 C terminus. These differential interactions mediate Rac1 localization, as well as Rac1 signaling, toward membrane ruffling, cell-cell adhesion, and migration. These data show that the C-terminal, hypervariable domain of Rac1 encodes two distinct binding motifs for signaling proteins and regulates intracellular targeting and differential signaling in a unique and non-redundant fashion.

Cadherin-mediated cellular signaling

Recent cadherin studies focusing on cellular signaling have shown that several pathways are activated by cadherin-mediated cell-cell contact. Cadherin-mediated contacts activate Rho family GTPases, regulate the availability of beta-catenin to participate in Wnt signaling, and function in receptor tyrosine kinase signaling. Although different classical cadherins bind to the same cytosolic proteins via their cytoplasmic tails, one message that is clear from the recent literature is that downstream signals emanating from cadherin-mediated contacts are both cadherin-specific and cell-context-specific.

The proteinase-activated receptor-2 mediates phagocytosis in a Rho-dependent manner in human keratinocytes

Recent work shows that the G-protein-coupled receptor proteinase activated receptor-2 activates signals that stimulate melanosome uptake in keratinocytes in vivo and in vitro. The Rho family of GTP-binding proteins is involved in cytoskeletal remodeling during phagocytosis. We show that proteinase-activated receptor-2 mediated phagocytosis in human keratinocytes is Rho dependent and that proteinase-activated receptor-2 signals to activate Rho. In contrast, Rho activity did not affect either proteinase-activated receptor-2 activity or mRNA and protein levels. We explored the signaling mechanisms of proteinase-activated receptor-2 mediated Rho activation in human keratinocytes and show that activation of proteinase-activated receptor-2, either through specific proteinase-activated receptor-2 activating peptides or through trypsinization, elevates cAMP in keratinocytes. Proteinase-activated receptor-2 mediated Rho activation was pertussis toxin insensitive and independent of the protein kinase A signaling pathway. These data are the first to show that proteinase-activated receptor-2 mediated phagocytosis is Rho dependent and that proteinase-activated receptor-2 signals to Rho and cAMP in keratinocytes. Because phagocytosis of melanosomes is recognized as an important mechanism for melanosome transfer to keratinocytes, these results suggest that Rho is a critical signaling intermediate in melanosome uptake in keratinocytes.

VCAM-1-mediated Rac signaling controls endothelial cell-cell contacts and leukocyte transmigration

Leukocyte adhesion is mediated totally and transendothelial migration partially by heterotypic interactions between the beta1- and beta2-integrins on the leukocytes and their ligands, Ig-like cell adhesion molecules (Ig-CAM), VCAM-1, and ICAM-1, on the endothelium. Both integrins and Ig-CAMs are known to have signaling capacities. In this study we analyzed the role of VCAM-1-mediated signaling in the control of endothelial cell-cell adhesion and leukocyte transendothelial migration. Antibody-mediated cross-linking of VCAM-1 on IL-1beta-activated primary human umbilical vein endothelial cells (pHUVEC) induced actin stress fiber formation, contractility, and intercellular gaps. The effects induced by VCAM-1 cross-linking were inhibited by C3 toxin, indicating that the small GTPase p21Rho is involved. In addition, the effects of VCAM-1 were accompanied by activation of Rac, which we recently showed induce intercellular gaps in pHUVEC in a Rho-dependent fashion. With the use of a cell-permeable peptide inhibitor, it was shown that Rac signaling is required for VCAM-1-mediated loss of cell-cell adhesion. Furthermore, VCAM-1-mediated signaling toward cell-cell junctions was accompanied by, and dependent on, Rac-mediated production of reactive oxygen species and activation of p38 MAPK. In addition, it was found that inhibition of Rac-mediated signaling blocks transendothelial migration of monocytic U937 cells. Together, these data indicate that VCAM-1-induced, Rac-dependent signaling plays a key role in the modulation of vascular-endothelial cadherin-mediated endothelial cell-cell adhesion and leukocyte extravasation.

Differential regulation of junctional complex assembly in renal epithelial cell lines

Several signaling pathways that regulate tight junction and adherens junction assembly are being characterized. Calpeptin activates stress fiber assembly in fibroblasts by inhibiting SH2-containing phosphatase-2 (SHP-2), thereby activating Rho-GTPase signaling. Here, we have examined the effects of calpeptin on stress fiber and junctional complex assembly in Madin-Darby canine kidney (MDCK) and LLC-PK epithelial cells. Calpeptin induced disassembly of stress fibers and inhibition of Rho GTPase activity in MDCK cells. Interestingly, calpeptin augmented stress fiber formation in LLC-PK epithelial cells. Calpeptin treatment of MDCK cells resulted in a displacement of zonula occludens-1 (ZO-1) and occludin from cell-cell junctions and a loss of phosphotyrosine on ZO-1 and ZO-2, without any detectable effect on tight junction permeability. Surprisingly, calpeptin increased paracellular permeability in LLC-PK cells even though it did not affect tight junction assembly. Calpeptin also modulated adherens junction assembly in MDCK cells but not in LLC-PK cells. Calpeptin treatment of MDCK cells induced redistribution of E-cadherin and beta-catenin from intercellular junctions and reduced the association of p120ctn with the E-cadherin/catenin complex. Together, our studies demonstrate that calpeptin differentially regulates stress fiber and junctional complex assembly in MDCK and LLC-PK epithelial cells, indicating that these pathways may be regulated in a cell line-specific manner.

Cdc42 plays a critical role in assembly of sarcomere units in series of cardiac myocytes

Cardiomyocyte hypertrophy is observed in various cardiovascular diseases and causes heart failure. We here examined the role of small GTP-binding proteins of Rho family in phenylephrine (PE)-or leukocyte inhibitory factor (LIF)-induced hypertrophic morphogenesis of cultured neonatal rat cardiomyocytes. Both LIF and PE increased cell size of cardiomyocytes. LIF induced an increase in the length/width ratio of cardiomyocytes, while PE did not change the ratio. Adenoviral gene transfer of constitutively active mutants of Cdc42 increased the length/width ratio of cardiomyocytes and dominant negative mutants of Cdc42 conversely inhibited LIF-induced cell-elongation, while mutants of RhoA and Rac1 did not affect the length/width ratio of cardiomyocytes. These results suggest that Cdc42, but not RhoA and Rac1, is involved in LIF-induced sarcomere assembly in series in cardiomyocytes.

Sertoli-germ cell adherens junction dynamics in the testis are regulated by RhoB GTPase via the ROCK/LIMK signaling pathway

During spermatogenesis, cell-cell actin-based adherens junctions (AJs), such as ectoplasmic specializations (ESs), between Sertoli and germ cells undergo extensive restructuring in the seminiferous epithelium to facilitate germ cell movement across the epithelium. Although the mechanism(s) that regulates AJ dynamics in the testis is virtually unknown, Rho GTPases have been implicated in the regulation of these events in other epithelia. Studies have shown that the in vitro assembly of the Sertoli-germ cell AJs but not of the Sertoli cell tight junctions (TJs) is associated with a transient but significant induction of RhoB. Immunohistochemistry has shown that the localization of RhoB in the seminiferous epithelium is stage specific, being lowest in stages VII-VIII prior to spermiation, and displays cell-specific association during the epithelial cycle. Throughout the cycle, RhoB was localized near the site of basal and apical ESs but was restricted to the periphery of the nuclei in elongating (but not elongated) spermatids, spermatocytes, and Sertoli cells. However, RhoB was not detected near the site of apical ESs at stages VII-VIII. Furthermore, disruption of AJs in Sertoli-germ cell cocultures either by hypotonic treatment or by treatment with 1-(2,4-dichlorobenzyl)-indazole-3-carbohydrazide (AF-2364) also induced RhoB expression. When adult rats were treated with AF-2364 to perturb Sertoli-germ cell AJs in vivo, a approximately 4-fold induction in RhoB in the testis, but not in kidney and brain, was detected within 1 h, at least approximately 1-4 days before germ cell loss from the epithelium could be detected by histological analysis. The signaling pathway(s) by which AF-2364 perturbed the Sertoli-germ cell AJs apparently began with an initial activation of integrin, which in turn activated RhoB, ROCK1, (Rho-associated protein kinase 1, also called ROKbeta), LIMK1 (LIM kinase 1, also called lin-11 isl-1 mec3 kinase 1), and cofilin but not p140mDia and profilin via phosphorylation. Immunoprecipitation and immunoblots revealed that the induction of LIMK1 was mediated via an increase in its phospho-Ser but not phospho-Tyr content. Furthermore, Y-27632 ([(R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexane-carboxamide, 2HCl]), a specific ROCK inhibitor, could effectively delay the AF-2364-induced germ cell loss from the seminiferous epithelium in vivo, illustrating that the integrin/RhoB/ROCK/LIMK pathway indeed plays a crucial role in the regulation of Sertoli-germ cell AJ dynamics. The fact that the RhoB pathway in the kidney and brain was not activated suggests that AF-2364 exerts its effects primarily at the testis-specific ES multiprotein complex structures between Sertoli cells and spermatids. In summary, this report illustrates that Sertoli germ cell AJ dynamics are regulated, at least in part, via the integrin/ROCK/LIMK/cofilin signaling pathway.

Molecular mechanisms of epithelial morphogenesis

Epithelial morphogenesis comprises the various processes by which epithelia contribute to organ formation and body shape. These complex and diverse events play a central role in animal development and regeneration. Recently, the characterization of some of the molecular mechanisms involved in epithelial morphogenesis has provided an abundance of new information on the role and regulation of the cytoskeleton, cell-cell adhesion, and cell-matrix adhesion in these processes. In this review, we discuss our current understanding of the molecular mechanisms driving cell shape changes, cell intercalation, fusion of epithelia, ingression, egression, and cell migration. Our discussion is mostly focused on results from Drosophila and mammalian tissue culture but also draws on the insights gained from other organisms.

Modulation of Rac1 and ARF6 activation during epithelial cell scattering

Epithelial cell scattering encompasses the dissolution of intercellular junctions, cell-cell dissociation, cell spreading, and motility. The Rac1 and ARF6 GTPases have been shown to regulate one or more of these aforementioned processes. In fact, activated Rac1 has been shown to promote cell-cell adhesion as well as to enhance cell motility, leading to conflicting reports on the effect of Rac1 activation on epithelial cell motility. In this study, we have examined the activation profiles of endogenous Rac1 and ARF6 during the sequential stages of epithelial cell scattering. Using Madin-Darby canine kidney cells treated with hepatocyte growth factor/scatter factor or cell lines stably expressing activated v-Src, we show that Rac1 and ARF6 exhibit distinct activation profiles during cell scattering. We have found that an initial ARF6-dependent decrease in the levels of Rac1-GTP is necessary to induce cell-cell dissociation. This is followed by a steady increase in Rac1 and ARF6 activation and cell migration. In sum, this study documents the progression of ARF6 and Rac1 activities during epithelial cell scattering.

Ezrin regulates E-cadherin-dependent adherens junction assembly through Rac1 activation

Ezrin, a membrane cytoskeleton linker, is involved in cellular functions, including epithelial cell morphogenesis and adhesion. A mutant form of ezrin, ezrin T567D, maintains the protein in an open conformation, which when expressed in Madin-Darby canine kidney cells causes extensive formation of lamellipodia and altered cell-cell contacts at low cell density. Furthermore, these cells do not form tubules when grown in a collagen type I matrix. While measuring the activity of Rho family GTPases, we found that Rac1, but not RhoA or Cdc 42, is activated in ezrin T567D-expressing cells, compared with cells expressing wild-type ezrin. Together with Rac1 activation, we observed an accumulation of E-cadherin in intracellular compartments and a concomitant decrease in the level of E-cadherin present at the plasma membrane. This effect could be reversed with a dominant negative form of Rac1, N17Rac1. We show that after a calcium switch, the delivery of E-cadherin from an internalized pool to the plasma membrane is greatly delayed in ezrin T567D-producing cells. In confluent cells, ezrin T567D production decreases the rate of E-cadherin internalization. Our results identify a new role for ezrin in cell adhesion through the activation of the GTPase Rac1 and the trafficking of E-cadherin to the plasma membrane.

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.

Cdc42 and Rac small G proteins activated by trans-interactions of nectins are involved in activation of c-Jun N-terminal kinase, but not in association of nectins and cadherin to form adherens junctions, in fibroblasts

BACKGROUND

Nectins are Ca2+-independent immunoglobulin-like cell-cell adhesion molecules which associate with cadherins to form adherens junctions (AJs) in epithelial cells and fibroblasts. Nectin-1 and -3 are members of the nectin family which most strongly trans-interact, causing cell-cell adhesion. The trans-interaction between nectin-1 and -3 induces the activation of both Cdc42 and Rac small G proteins in epithelial cells. We studied the roles of Cdc42 and Rac activated in this way in L fibroblasts stably expressing both nectin-1 and E-cadherin (nectin-1-EL cells).

RESULTS

The trans-interaction between nectin-1 and -3 induced the activation of Cdc42 and Rac in nectin-1-EL cells. Cdc42, and presumably Rac, activated in this way, induced the activation of c-Jun N-terminal kinase (JNK), but not p38 mitogen-activated protein (MAP) kinase or extracellular signal-regulated kinase (ERK). Cdc42 or Rac was not essential for the association of nectin-1 and E-cadherin to form AJs. Reorganization of the actin cytoskeleton was not required for the association of nectin-1 and E-cadherin.

CONCLUSION

These results indicate that Cdc42 and Rac activated by the trans-interaction of nectins selectively induce the activation of JNK, but are not essential for the association of nectins and cadherin to form AJs in fibroblasts.

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.

Endothelial barrier strengthening by activation of focal adhesion kinase

Endothelial cell barrier (EC) properties regulate blood tissue fluid flux. To determine the role of endothelial-matrix interactions in barrier regulation, we induced cell shrinkage by exposing confluent endothelial monolayers to hyperosmolarity. The dominant effect of a 15-min hyperosmolar exposure was an increase in the trans-endothelial electrical resistance, indicating the induction of barrier strengthening. Hyperosmolar exposure also increased activity of focal adhesion kinase and E-cadherin accumulation at the cell periphery. Concomitantly, the density of actin filaments increased markedly. In EC monolayers stably expressing constitutively active or dominant negative isoforms of Rac1, the actin response to hyperosmolar exposure was enhanced or blocked, respectively, although the response in trans-endothelial resistance was unaffected, indicating that the endothelial barrier enhancement occurred independently of actin. However, in monolayers expressing a kinase-deficient mutant of focal adhesion kinase, the hyperosmolarity-induced increases in activity of focal adhesion and peripheral E-cadherin enhancement were blocked and the induced increase of electrical resistance was markedly blunted. These findings indicate that in EC exposed to hyperosmolar challenge, the involvement of focal adhesion kinase was critical in establishing barrier strengthening.

Involvement of nectin in the localization of IQGAP1 at the cell-cell adhesion sites through the actin cytoskeleton in Madin-Darby canine kidney cells

IQGAP1, a putative downstream target of the Rho family small G proteins, Cdc42 and Rac, localizes at adherens junctions (AJs) in epithelial cells. It has been suggested that IQGAP1 localizes at AJs through its binding to beta-catenin, and negatively regulates the E-cadherin-mediated cell-cell adhesion. Nectin is a Ca(2+)-independent, immunoglobulin-like cell-cell adhesion molecule that localizes at AJs. Nectin is associated with E-cadherin through their respective cytoplasmic tail-binding proteins, afadin and catenins, and involved in the formation of AJs cooperatively with E-cadherin. Here we investigated a role of nectin in the localization of IQGAP1 at AJs. Ca(2+) chelation from the medium causes disruption of the E-cadherin-mediated cell-cell adhesion, but not the nectin-based cell-cell adhesion, in Madin-Darby canine kidney (MDCK) cells. IQGAP1 remained at the residual nectin-based cell-cell adhesion sites where the E-cadherin immunofluorescence signal disappeared. Restoration of Ca(2+) in the medium causes re-accumulation of E-cadherin to the residual nectin-based cell-cell adhesion sites to re-form AJs. Nectin inhibitors inhibit this re-accumulation of E-cadherin to re-form AJs by impairing the nectin-based cell-cell adhesion. The nectin inhibitors also reduced the localization of IQGAP1 at the cell-cell adhesion sites. When MDCK cells were incubated with microbeads coated with the extracellular fragment of nectin that interacts with cellular nectin, IQGAP1 also accumulated at the bead-MDCK cell contact sites. The accumulation of IQGAP1 at the cell-cell adhesion sites was inhibited by actin filament-disrupting agents, latrunculin A and cytochalasin D. These results indicate that nectin is involved in the localization of IQGAP1 at AJs through the actin cytoskeleton.

Rac promotes epithelial cell rearrangement during tracheal tubulogenesis in Drosophila

Cell rearrangement, accompanied by the rapid assembly and disassembly of cadherin-mediated cell adhesions, plays essential roles in epithelial morphogenesis. Various in vitro and cell culture studies on the small GTPase Rac have suggested it to be a key regulator of cell adhesion, but this notion needs to be verified in the context of embryonic development. We used the tracheal system of Drosophila to investigate the function of Rac in the epithelial cell rearrangement, with a special attention to its role in regulating epithelial cadherin activity. We found that a reduced Rac activity led to an expansion of cell junctions in the embryonic epidermis and tracheal epithelia, which was accompanied by an increase in the amount of Drosophila E-Cadherin-Catenin complexes by a post-transcriptional mechanism. Reduced Rac activity inhibited dynamic epithelial cell rearrangement. Hyperactivation of Rac, on the other hand, inhibited assembly of newly synthesized E-Cadherin into cell junctions and caused loss of tracheal cell adhesion, resulting in cell detachment from the epithelia. Thus, in the context of Drosophila tracheal development, Rac activity must be maintained at a level necessary to balance the assembly and disassembly of E-Cadherin at cell junctions. Together with its role in cell motility, Rac regulates plasticity of cell adhesion and thus ensures smooth remodeling of epithelial sheets into tubules.

Glucocorticoid down-regulation of RhoA is required for the steroid-induced organization of the junctional complex and tight junction formation in rat mammary epithelial tumor cells

In Con8 mammary epithelial tumor cells, we have documented previously that the synthetic glucocorticoid dexamethasone induces the reorganization of the tight junction and adherens junction (apical junction) and stimulates the monolayer transepithelial electrical resistance (TER), which is a reliable in vitro measurement of tight junction sealing. Western blots demonstrated that dexamethasone treatment down-regulated the level of the RhoA small GTPase prior to the stimulation of the monolayer TER. To test the role of RhoA in the steroid regulation of apical junction dynamics functionally, RhoA levels were altered in Con8 cells by transfection of either constitutively active (RhoA.V14) or dominant negative (RhoA.DN19) forms of RhoA. Ectopic expression of constitutively active RhoA disrupted the dexamethasone-stimulated localization of zonula occludens-1 and beta-catenin to sites of cell-cell contact, inhibited tight junction sealing, and prevented the complete formation of the F-actin ring structure at the apical side of the cell monolayer. In a complementary manner, dominant negative RhoA caused a precocious organization of the tight junction, adherens junction, and the F-actin rings in the absence of steroid, whereas the monolayer TER remained glucocorticoid-responsive. Taken together, our results demonstrate that the glucocorticoid down-regulation of RhoA is a required step in the steroid signaling pathway which controls the organization of the apical junctional complex and the actin cytoskeleton in mammary epithelial cells.

Identification of a tight junction-associated guanine nucleotide exchange factor that activates Rho and regulates paracellular permeability

Rho family GTPases are important regulators of epithelial tight junctions (TJs); however, little is known about how the GTPases themselves are controlled during TJ assembly and function. We have identified and cloned a canine guanine nucleotide exchange factor (GEF) of the Dbl family of proto-oncogenes that activates Rho and associates with TJs. Based on sequence similarity searches and immunological and functional data, this protein is the canine homologue of human GEF-H1 and mouse Lfc, two previously identified Rho-specific exchange factors known to associate with microtubules in nonpolarized cells. In agreement with these observations, immunofluorescence of proliferating MDCK cells revealed that the endogenous canine GEF-H1/Lfc associates with mitotic spindles. Functional analysis based on overexpression and RNA interference in polarized MDCK cells revealed that this exchange factor for Rho regulates paracellular permeability of small hydrophilic tracers. Although overexpression resulted in increased size-selective paracellular permeability, such cell lines exhibited a normal overall morphology and formed fully assembled TJs as determined by measuring transepithelial resistance and by immunofluorescence and freeze-fracture analysis. These data indicate that GEF-H1/Lfc is a component of TJs and functions in the regulation of epithelial permeability.

Intestinal epithelial responses to enteric pathogens: effects on the tight junction barrier, ion transport, and inflammation

The effects of pathogenic organisms on host intestinal epithelial cells are vast. Innumerable signalling pathways are triggered leading ultimately to drastic changes in physiological functions. Here, the ways in which enteric bacterial pathogens utilise and impact on the three major physiological functions of the intestinal epithelium are discussed: alterations in the structure and function of the tight junction barrier, induction of fluid and electrolyte secretion, and activation of the inflammatory cascade. This field of investigation, which was virtually non-existent a decade ago, has now exploded, thus rapidly expanding our understanding of bacterial pathogenesis. Through increased delineation of the ways in which microbes alter host physiology, we simultaneous gain insight into the normal regulatory mechanisms of the intestinal epithelium.