Rho protein regulates tight junctions and perijunctional actin organization in polarized epithelia

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.

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.

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.

Regulation of the intestinal epithelial paracellular barrier

Paracellular transport of orally-administered drugs, the passage of molecules between adjacent intestinal epithelial cells, is impeded by a range of structural and functional features found in the intestine. An increased knowledge of the mechanisms that govern the paracellular barrier will enable the pharmaceutical scientist to design novel and rational formulations and delivery platforms that will improve the oral bioavailability of therapeutic molecules, particularly proteins and peptides, which would be taken-up by the paracellular pathway.

Effects of cytotoxic necrotizing factor 1 and lethal toxin on actin cytoskeleton and VE-cadherin localization in human endothelial cell monolayers

Integrity of the vascular endothelium is largely dependent on endothelial cell shape and establishment of intercellular junctions. Certain pathogenic bacterial toxins alter the cytoskeletal architecture of intoxicated cells by modulating the GTPase activity of p21 Rho family proteins. In the present study we have analyzed the effect of Rho-directed toxins on the actin cytoskeleton and monolayer integrity of endothelial cells. We report here that Escherichia coli cytotoxic necrotizing factor 1 (CNF1) activates Rho in human umbilical vein endothelial cells (HUVEC). In confluent monolayers, CNF1 treatment induces prominent stress fiber formation without significantly modifying peripheral localization of VE-cadherin, a specific marker of vascular endothelial cell adherens junctions. Further, Rho activation with CNF1 blocks thrombin-induced redistribution of VE-cadherin staining and gap formation in HUVEC monolayers. Inhibition of Rho by prolonged treatment of cells with C3 exoenzyme (Clostridium botulinum) eliminates actin stress fibers without disrupting the continuity of VE-cadherin staining, indicating that Rho-dependent stress fibers are not required for maintaining this adhesion receptor at sites of intercellular contact. Lethal toxin (Clostridium sordellii), an inhibitor of Rac as well as Ras and Rap, potently disrupts the actin microfilament system and monolayer integrity in HUVEC cultures.

Transmembrane proteins in the tight junction barrier

Three types of transmembrane proteins have been identified within the tight junction, but it remains to be determined how they provide the molecular basis for regulating the paracellular permeability for water, solutes, and immune cells. Several of these proteins localize specifically within the continuous cell-to-cell contacts of the tight junction. One of these, occludin, is a cell adhesion molecule that has been demonstrated to influence ion and solute permeability. The claudins are a family of four-membrane spanning proteins; unexpectedly, other members of this family have already been characterized without recognizing their relationship to tight junctions. Junction adhesion molecule, the most recently identified tight junction component, is a member of the Ig superfamily and influences the paracellular transmigration of immune cells. A plaque of cytoplasmic proteins under the junction may be responsible for scaffolding the transmembrane proteins, creating a link to the perijunctional actin cytoskeleton and transducing regulatory signals that control the paracellular barrier.

Rho controls actin cytoskeletal assembly in renal epithelial cells during ATP depletion and recovery

Actin cytoskeletal disruption is a hallmark of ischemic injury and ATP depletion in a number of cell types, including renal epithelial cells. We manipulated Rho GTPase signaling by transfection and microinjection in LLC-PK proximal tubule epithelial cells and observed actin cytoskeletal organization following ATP depletion or recovery by confocal microscopy and quantitative image analysis. ATP depletion resulted in disruption of stress fibers, cortical F-actin, and apical actin bundles. Constitutively active RhoV14 prevented disruption of stress fibers and cortical F-actin during ATP depletion and enhanced the rate of stress fiber reassembly during recovery. Conversely, the Rho inhibitor C3 or dominant negative RhoN19 prevented recovery of F-actin assemblies upon repletion. Actin bundles in the apical microvilli and cytosolic F-actin were not affected by Rho signaling. Assembly of vinculin and paxillin into focal adhesions was disrupted by ATP depletion, and constitutively active RhoV14, although protecting stress fibers from disassembly, did not prevent dispersion of vinculin and paxillin, resulting in uncoupling of stress fiber and focal adhesion assembly. We propose that ATP depletion causes Rho inactivation during ischemia and that recovery of normal cellular architecture and function requires Rho.

Interferon-gamma decreases barrier function in T84 cells by reducing ZO-1 levels and disrupting apical actin

The effects of interferon-gamma (IFN-gamma) on tight junctions in T84 human intestinal epithelial cells were investigated. Treatment of T84 cells with IFN-gamma caused a dose- and time-dependent increase in monolayer permeability as assessed by transepithelial electrical resistance measurements. Examination of specific proteins associated with tight junctions by immunoblotting and confocal microscopy revealed changes in the expression levels and localization of some of these proteins after exposure of the cells to IFN-gamma. Specifically, IFN-gamma treatment resulted in an almost total loss of zonula occludens (ZO)-1, whereas the levels of ZO-2 and occludin showed relatively modest decreases compared with untreated cells. Loss of ZO-1 was associated with the altered localization of ZO-2 and occludin. In IFN-gamma-treated cells, ZO-2 and occludin were diffusely distributed, whereas, in control cells, they, along with ZO-1, were predominantly localized to the tight junctions. Analysis of ZO-1 protein and RNA by pulse chase and RT-PCR, respectively, showed an increase in protein turnover, a decrease in protein synthesis, and a reduction in RNA levels following IFN-gamma treatment. In contrast to ZO-1, ZO-2 and occludin did not show any major changes in these parameters. In addition, the organization of actin in the apical and tight junction regions, but not in the mid- or basal regions, of the cells was also perturbed by IFN-gamma treatment of cells. Time-course analysis of IFN-gamma-induced alterations in ZO-1 expression and apical actin perturbation indicated that these two effects were intimately linked and could not be dissociated. These results suggest that IFN-gamma affects barrier function in T84 cells by decreasing the levels of ZO-1 and perturbing apical actin organization, which leads to a disorganization of the tight junction and an increase in paracellular permeability.

Role of tyrosine phosphorylation in the reassembly of occludin and other tight junction proteins

After the simulation of anoxia by ATP depletion of MDCK cell monolayers with metabolic inhibitors, the tight junction (TJ) is known to become structurally perturbed, leading to loss of the permeability barrier. Peripheral TJ proteins such as zonula occludens 1 (ZO-1), ZO-2, and cingulin become extremely insoluble and associate into large macromolecular complexes (T. Tsukamoto and S. K. Nigam. J. Biol. Chem. 272: 16133-16139, 1997). For up to 3 h, this process is reversible by ATP repletion. We now show that the reassembly process depends on tyrosine phosphorylation. Recovery of transepithelial electrical resistance in ATP-replete monolayers was markedly inhibited by the tyrosine kinase inhibitor, genistein. Indirect immunofluorescence revealed a decrease in staining of occludin, a membrane component of the TJ, in the region of the TJ after ATP depletion, which reversed after ATP repletion; this reversal process was inhibited by genistein. Examination of the Triton X-100 solubilities of occludin and several nonmembrane TJ proteins revealed a shift of occludin and nonmembrane TJ proteins into an insoluble pool following ATP depletion. These changes reversed after ATP repletion, and the movement of insoluble occludin, ZO-1, and ZO-2 back into the soluble pool was again via a genistein-sensitive mechanism. Rate-zonal centrifugation analyses of detergent-soluble TJ proteins showed a reversible increase in higher density fractions following ATP depletion-repletion, although this change was not affected by genistein. In 32P-labeled cells, dephosphorylation of all studied TJ proteins was observed during ATP depletion, followed by rephosphorylation during ATP repletion; rephosphorylation of occludin was inhibited by genistein. Furthermore, during the ATP repletion phase, tyrosine phosphorylation of Triton X-100-insoluble occludin, which is localized at the junction, as well as ZO-2, p130/ZO-3 (though not ZO-1), and other proteins was evident; this tyrosine phosphorylation was completely inhibited by genistein. This indicates that tyrosine kinase activity is necessary for TJ reassembly during ATP repletion and suggests an important role for the tyrosine phosphorylation of occludin, ZO-2, p130/ZO-3, and possibly other proteins in the processes involved in TJ (re)formation.

Ethanol modulation of intestinal epithelial tight junction barrier

Previous studies have shown that high concentrations of ethanol (>/=40%) cause functional damage of the gastrointestinal epithelial barrier by direct cytotoxic effect on the epithelial cells. The effects of lower noncytotoxic doses of ethanol on epithelial barrier function are unknown. A major function of gastrointestinal epithelial cells is to provide a barrier against the hostile substances in the gastrointestinal lumen. The apicolaterally located tight junctions (TJs) form a paracellular seal between the lateral membranes of adjacent cells and act as a paracellular barrier. In this study, we investigated the effects of lower doses of ethanol on intestinal epithelial TJ barrier function using filter-grown Caco-2 intestinal epithelial monolayers. The Caco-2 TJ barrier function was assessed by measuring epithelial resistance or paracellular permeability of the filter-grown monolayers. Ethanol (0, 1, 2.5, 5, 7.5, and 10%) produced a dose-related drop in Caco-2 epithelial resistance and increase in paracellular permeability. Ethanol also produced a progressive disruption of TJ protein (ZO-1) with separation of ZO-1 proteins from the cellular junctions and formation of large gaps between the adjacent cells. Ethanol, at the doses used (</=10%), did not cause cytotoxicity (lactate dehydrogenase release) to the Caco-2 cells. Ethanol produced a disassembly and displacement of perijunctional actin and myosin filaments from the perijunctional areas. On ethanol removal, actin and myosin filaments rapidly reassembled at the cellular borders. Ethanol stimulated the Caco-2 myosin light chain kinase (MLCK) activity but did not affect the MLCK protein levels. Specific MLCK inhibitor ML-7 inhibited both ethanol increases in MLCK activity and TJ permeability without affecting the MLCK protein levels. Consistent with these findings, metabolic inhibitors sodium azide and 2,4-dinitrophenol significantly prevented ethanol-induced increase in Caco-2 TJ permeability, whereas cycloheximide or actinomycin D had no effect. The results of this study indicate that ethanol at low noncytotoxic doses causes a functional and structural opening of the Caco-2 intestinal epithelial TJ barrier by activating MLCK.

A mutant form of the rho protein can restore stress fibers and adhesion plaques in v-src transformed fibroblasts

The organization of polymerized actin in the mammalian cell is regulated by several members of the rho family. Three rho proteins, cdc42, rac and rho act in a cascade to organize the intracellular actin cytoskeleton. Rho proteins are involved in the formation of actin stress fibers and adhesion plaques in fibroblasts. During transformation of mammalian cells by oncogenes the cytoskeleton is rearranged and stress fibers and adhesion plaques are disintegrated. In this paper we investigate the function of the rho protein in RR1022 rat fibroblasts transformed by the Rous sarcoma virus. Two activated mutants of the rho protein, rho G14V and rho Q63L, and a dominant negative mutant, rho N1171, were stably transfected into RR1022 cells. The resulting cell lines were analysed for the organization of polymerized actin and adhesion plaques. Cells expressing rho Q63L, but not rho wt, rho G14V or rho N1171, showed an altered morphology. These cells displayed a flat, fibroblast like shape when compared with the RR1022 ancestor cells. Immunofluorescence analyses revealed that actin stress fibers and adhesion plaques were rearranged in these cells. We conclude from these data that an active rho protein can restore elements of the actin cytoskeleton in transformed cells by overriding the tyrosine kinase phosphorylation induced by the pp60(v-src).

Rho GTPases in development

It is becoming increasingly clear that the complex family of Rho-related GTPases and their associated regulators and targets are essential mediators of a variety of morphogenetic events required for normal development of multicellular organisms. It is worth noting that the results obtained thus far indicate that the Rho family proteins are largely associated with the regulation of morphogenesis, as opposed to other essential developmental processes such as cell proliferation and cell fate determination. Accumulating evidence also suggests that the role of these proteins and their associated signaling pathways in morphogenesis is in many, but not necessarily all, cases related to their ability to affect the organization of the actin cytoskeleton. Thus, these in vivo observations have served to corroborate similar findings in numerous cultured cell studies. As described, the power of genetics, particularly in Drosophila and C. elegans, has been critical to the recent identification and functional characterization of several Rho family signaling components. Moreover, evidence suggests that the highly evolutionarily conserved structures of many of these proteins translate into conservation of function as well. Thus, it will be possible, in many cases, to extrapolate the findings in the simple systems described herein to higher eukaryotes, including humans. Expanding use of these genetic model systems to dissect Rho-mediated signaling pathways in vivo will undoubtedly lead to a flood of new insights into the organization and function of these pathways in the coming years, especially in development. As the C. elegans genome sequencing effort nears completion and with the Drosophila genome project well underway, the identification of novel relevant genes will proceed with even greater speed. In addition, the rapidly expanding use of mouse knockout strategies, combined with recent developments in the associated knockout technology, will also contribute greatly to the investigation of mammalain Rho signaling pathways and their roles in development.

Lymphocyte Migration Through Brain Endothelial Cell Monolayers Involves Signaling Through Endothelial ICAM-1 Via a Rho-Dependent Pathway

Lymphocyte extravasation into the brain is mediated largely by the Ig superfamily molecule ICAM-1. Several lines of evidence indicate that at the tight vascular barriers of the central nervous system (CNS), endothelial cell (EC) ICAM-1 not only acts as a docking molecule for circulating lymphocytes, but is also involved in transducing signals to the EC. In this paper, we examine the signaling pathways in brain EC following Ab ligation of endothelial ICAM-1, which mimics adhesion of lymphocytes to CNS endothelia. ICAM-1 cross-linking results in a reorganization of the endothelial actin cytoskeleton to form stress fibers and activation of the small guanosine triphosphate (GTP)-binding protein Rho. ICAM-1-stimulated tyrosine phosphorylation of the actin-associated molecule cortactin and ICAM-1-mediated, Ag/IL-2-stimulated T lymphocyte migration through EC monolayers were inhibited following pretreatment of EC with cytochalasin D. Pretreatment of EC with C3 transferase, a specific inhibitor of Rho proteins, significantly inhibited the transmonolayer migration of T lymphocytes, endothelial Rho-GTP loading, and endothelial actin reorganization, without affecting either lymphocyte adhesion to EC or cortactin phosphorylation. These data show that brain vascular EC are actively involved in facilitating T lymphocyte migration through the tight blood-brain barrier of the CNS and that this process involves ICAM-1-stimulated rearrangement of the endothelial actin cytoskeleton and functional EC Rho proteins.

Glucocorticoid down-regulation of fascin protein expression is required for the steroid-induced formation of tight junctions and cell-cell interactions in rat mammary epithelial tumor cells

Glucocorticoid hormones, which are physiological regulators of mammary epithelium development, induce the formation of tight junctions in rat Con8 mammary epithelial tumor cells. We have discovered that, as part of this process, the synthetic glucocorticoid dexamethasone strongly and reversibly down-regulated the expression of fascin, an actin-bundling protein that also interacts with the adherens junction component beta-catenin. Ectopic constitutive expression of full-length mouse fascin containing a Myc epitope tag (Myc-fascin) in Con8 cells inhibited the dexamethasone stimulation of transepithelial electrical resistance, disrupted the induced localization of the tight junction protein occludin and the adherens junction protein beta-catenin to the cell periphery, and prevented the rearrangement of the actin cytoskeleton. Ectopic expression of either the carboxyl-terminal 213 amino acids of fascin, which includes the actin and beta-catenin-binding sites, or the amino-terminal 313 amino acids of fascin failed to disrupt the glucocorticoid induction of tight junction formation. Mammary tumor cells expressing the full-length Myc-fascin remained generally glucocorticoid responsive and displayed no changes in the levels or protein-protein interactions of junctional proteins or the amount of cytoskeletal associated actin filaments. However, a cell aggregation assay demonstrated that the expression of Myc-fascin abrogated the dexamethasone induction of cell-cell adhesion. Our results implicate the down-regulation of fascin as a key intermediate step that directly links glucocorticoid receptor signaling to the coordinate control of junctional complex formation and cell-cell interactions in mammary tumor epithelial cells.

Disruption of circumferential actin filament causes disappearance of occludin from the cell borders of rat hepatocytes in primary culture without distinct changes of tight junction strands

We investigated the relationship of actin filament organization to occludin and tight junction strands in primary cultured rat hepatocytes using an actin depolymerizing agent, mycalolide B. In control cultures, well-developed circumferential actin filaments and occludin immunoreactivity were observed on the most subapical plasma membrane of the cells, and tight junction strands formed well-developed networks in freeze-fracture replicas. In hepatocytes treated with 3 microM mycalolide B for 6 h, circumferential actin filaments and occludin immunoreactivity disappeared from the cell borders. However, there were no marked abnormalities of tight junction strands in freeze fracture replicas. Similar results were obtained from cells cultured in medium with 0.05 mM Ca2+ for 6 h. The close association of occludin with actin and the existence of intact tight junction strands that are virtually free of both occludin and actin suggest a physiological role of occludin, but not the other proteins forming the tight junction strands, in the linkage between actin cytoskeleton and tight junction.

The actin cytoskeleton reorganization induced by Rac1 requires the production of superoxide

The small GTPase rac1 controls actin redistribution to membrane ruffles in fibroblasts and other cell types, as well as the activation of the NADPH oxidase in phagocytes. We explored the possibility that these two processes could be related. We used a replication-deficient adenoviral vector to overexpress the constitutively active form of rac1, racV12, in human and mouse aortic endothelial cells. We show here that, in addition to membrane ruffle formation, racV12 induced an increase in the total amount of F-actin within endothelial cells. Concurrently, racV12-overexpressing cells produced significantly higher amounts of free radicals, as detected by the fluorescent probe 5-(and-6)-chloromethyl-2',7'-dichloro-dihydrofluorescein diacetate, than cells infected with a control virus encoding the bacterial beta-galactosidase (Ad-betaGal). To assess the specific role of superoxide in racV12-induced actin reorganization, we co-expressed the human enzyme Cu,Zn-superoxide dismutase (SOD), by means of another adenoviral vector construct. Overexpressed SOD reduced the concentration of superoxide detected in Ad-racV12-transfected cells and reversed the effects of Ad-racV12 on the content of filamentous actin. MnTMPyP, an SOD mimetic, as well as the antioxidant N-acetyl cysteine, had similar effects, in that they reduced not only the free radicals production, but also ruffle formation and the concentration of F-actin within racV12-overexpressing endothelial cells. Our data support the hypothesis that superoxide is one of the important mediators acting downstream of rac1 on the pathway of actin cytoskeleton remodeling in endothelial cells.

The tight junction protein ZO-1 establishes a link between the transmembrane protein occludin and the actin cytoskeleton

The tight junction protein ZO-1 belongs to a family of multidomain proteins known as the membrane-associated guanylate kinase homologs (MAGUKs). ZO-1 has been demonstrated to interact with the transmembrane protein occludin, a second tight junction-specific MAGUK, ZO-2, and F-actin, although the nature and functional significance of these interactions is poorly understood. To further elucidate the role of ZO-1 within the epithelial tight junction, we have introduced epitope-tagged fragments of ZO-1 into cultured MDCK cells and identified domains critical for the interaction with ZO-2, occludin, and F-actin. A combination of in vitro and in vivo binding assays indicate that both ZO-2 and occludin interact with specific domains within the N-terminal (MAGUK-like) half of ZO-1, whereas the unique proline-rich C-terminal half of ZO-1 cosediments with F-actin. Consistent with these observations, we found that a construct encoding the N-terminal half of ZO-1 is specifically associated with tight junctions, whereas the unique C-terminal half of ZO-1 is distributed over the entire lateral surface of the plasma membrane and other actin-rich structures. In addition, we have identified a 244-amino acid domain within the N-terminal half of ZO-1, which is required for the stable incorporation of ZO-1 into the junctional complex of polarized MDCK cells. These observations suggest that one functional role of ZO-1 is to organize components of the tight junction and link them to the cortical actin cytoskeleton.

Activation of Rho GTPases by Escherichia coli cytotoxic necrotizing factor 1 increases intestinal permeability in Caco-2 cells

The cytotoxic necrotizing factor 1 (CNF1) activates Rho GTPases by deamidation of glutamine-63 and thereby induces redistribution of the actin cytoskeleton and formation of stress fibers. Here, we have studied the effects of CNF1 on the transepithelial resistance of Caco-2 cells, a human intestinal epithelial cell line, in comparison with the Rho-inactivating toxin B of Clostridium difficile. Whereas toxin B decreased the transepithelial resistance of Caco-2 cells by about 80% after 4 h, CNF1 reduced it by about 40%. Significant changes of the transepithelial resistance induced by CNF1 were detected after 3 h of incubation. Half-maximal effects were observed with 10 and 41 ng of CNF1 and toxin B per ml, respectively. Flux measurement revealed no CNF1-induced increase of fluorescein isothiocyanate-dextran permeation within the first 4 h of incubation and a 2.9-fold increase after 24 h of incubation. In contrast, toxin B induced a 28-fold increase of permeation after 24 h. As detected by rhodamine-phalloidin staining, CNF1 increased polymerization of F actin at focal contacts of adjacent cells and induced formation of stress fibers. The data indicate that not only depolymerization but also polymerization of actin and subsequent reorganization of the actin cytoskeleton alter the barrier function of intestinal tight junctions.

The C terminus of E1A regulates tumor progression and epithelial cell differentiation

The E1A gene of adenovirus has been considered both a dominant oncogene and a tumor suppressor. It has been reported to induce epithelial cell but to prevent myoblast differentiation. E1A enables oncogenes that are unable to transform primary cells on their own to do so, yet suppresses tumor progression toward invasion and metastasis. To try to reconcile the seemingly, conflicting E1A phenotypes, we examined the expression of epithelial cell specific and characterizing proteins in immortalized or tumorigenically transformed primary epithelial cells expressing wild-type E1A or a C-terminal mutant that has lost tumor suppressive abilities. All the cell types continued to express cytokeratin. Epithelial cell morphology, social behavior, and growth characteristics were retained by cells expressing wild-type E1A, even in the presence of an activated ras oncogene. Mutant E1A-expressing cells were less well differentiated even in the absence of ras. They were specifically defective in cell-cell junctional complexes, such as tight and adherens junctions and desmosomes. There was also a preference for those actin structures prominent in fibroblasts: stress fibers and filopodia, while in the wild-type E1A expressing cells, cortical actin and circumferential actin filaments were dominant. Thus the E1A-mutant-expressing cells were already predisposed to a more advanced tumor stage even when they were only immortalized and not transformed. The results suggest the possibility that the C terminus of E1A may be involved in regulating epithelial mesenchymal transitions, which have previously been linked to tumor progression.

Rho GTPase signaling regulates tight junction assembly and protects tight junctions during ATP depletion

Tight junctions control paracellular permeability and cell polarity. Rho GTPase regulates tight junction assembly, and ATP depletion of Madin-Darby canine kidney (MDCK) cells (an in vitro model of renal ischemia) disrupts tight junctions. The relationship between Rho GTPase signaling and ATP depletion was examined. Rho inhibition resulted in decreased localization of zonula occludens-1 (ZO-1) and occludin at cell junctions; conversely, constitutive Rho signaling caused an accumulation of ZO-1 and occludin at cell junctions. Inhibiting Rho before ATP depletion resulted in more extensive loss of junctional components between transfected cells than control junctions, whereas cells expressing activated Rho better maintained junctions during ATP depletion than control cells. ATP depletion and Rho signaling altered phosphorylation signaling mechanisms. ZO-1 and occludin exhibited rapid decreases in phosphoamino acid content following ATP depletion, which was restored on recovery. Expression of Rho mutant proteins in MDCK cells also altered levels of occludin serine/threonine phosphorylation, indicating that occludin is a target for Rho signaling. We conclude that Rho GTPase signaling induces posttranslational effects on tight junction components. Our data also demonstrate that activating Rho signaling protects tight junctions from damage during ATP depletion.

Structural and functional regulation of tight junctions by RhoA and Rac1 small GTPases

Tight junctions (TJ) govern ion and solute diffusion through the paracellular space (gate function), and restrict mixing of membrane proteins and lipids between membrane domains (fence function) of polarized epithelial cells. We examined roles of the RhoA and Rac1 GTPases in regulating TJ structure and function in MDCK cells using the tetracycline repressible transactivator to regulate RhoAV14, RhoAN19, Rac1V12, and Rac1N17 expression. Both constitutively active and dominant negative RhoA or Rac1 perturbed TJ gate function (transepithelial electrical resistance, tracer diffusion) in a dose-dependent and reversible manner. Freeze-fracture EM and immunofluoresence microscopy revealed abnormal TJ strand morphology and protein (occludin, ZO-1) localization in RhoAV14 and Rac1V12 cells. However, TJ strand morphology and protein localization appeared normal in RhoAN19 and Rac1N17 cells. All mutant GTPases disrupted the fence function of the TJ (interdomain diffusion of a fluorescent lipid), but targeting and organization of a membrane protein in the apical membrane were unaffected. Expression levels and protein complexes of occludin and ZO-1 appeared normal in all mutant cells, although ZO-1 was more readily solubilized from RhoAV14-expressing cells with Triton X-100. These results show that RhoA and Rac1 regulate gate and fence functions of the TJ, and play a role in the spatial organization of TJ proteins at the apex of the lateral membrane.

Effects of regulated expression of mutant RhoA and Rac1 small GTPases on the development of epithelial (MDCK) cell polarity

MDCK cells expressing RhoA or Rac1 mutants under control of the tetracycline repressible transactivator were used to examine short-term effects of known amounts of each mutant before, during, or after development of cell polarity. At low cell density, Rac1V12 cells had a flattened morphology and intact cell-cell contacts, whereas Rac1N17 cells were tightly compacted. Abnormal intracellular aggregates formed between Rac1N17, F-actin, and E-cadherin in these nonpolarized cells. At all subsequent stages of polarity development, Rac1N17 and Rac1V12 colocalized with E-cadherin and F-actin in an unusual beaded pattern at lateral membranes. In polarized cells, intracellular aggregates formed with Rac1V12, F-actin, and an apical membrane protein (GP135). At low cell density, RhoAV14 and RhoAN19 were localized in the cytoplasm, and cells were generally flattened and more fibroblastic than epithelial in morphology. In polarized RhoAV14 cells, F-actin was diffuse at lateral membranes and prominent in stress fibers on the basal membrane. GP135 was abnormally localized to the lateral membrane and in intracellular aggregates, but E-cadherin distribution appeared normal. In RhoAN19 cells, F-actin, E-cadherin, and GP135 distributions were similar to those in controls. Expression of either RhoAV14 or RhoAN19 in Rac1V12 cells disrupted Rac1V12 distribution and caused cells to adopt the more fibroblastic, RhoA mutant phenotype. We suggest that Rac1 and RhoA are involved in the transition of epithelial cells from a fibroblastic to a polarized structure and function by direct and indirect regulation of actin and actin-associated membrane protein organizations.

A novel human cytomegalovirus glycoprotein, gpUS9, which promotes cell-to-cell spread in polarized epithelial cells, colocalizes with the cytoskeletal proteins E-cadherin and F-actin

Processes by which human herpesviruses penetrate and are released from polarized epithelial cells, which have distinct apical and basolateral membrane domains differing in protein and lipid content, are poorly understood. We recently reported that human cytomegalovirus (CMV) mutants with deletions of the gene US9 formed wild-type plaques in cultures of human fibroblasts but were impaired in the capacity for cell-to-cell spread in polarized human retinal pigment epithelial cells. Unlike the glycoproteins that are required for infection, the protein encoded by CMV US9 plays an accessory role by promoting dissemination of virus across cell-cell junctions of polarized epithelial cells. To identify the product and investigate its specialized functions, we selected Madine-Darby canine kidney II (MDCK) epithelial cells that constitutively express CMV US9 or, as a control, US8. The gene products, designated gpUS9 and gpUS8, were glycosylated proteins of comparable molecular masses but differed considerably in intracellular distribution and solubility. Immunofluorescence laser scanning confocal microscopy indicated that, like gpUS8, gpUS9 was present in the endoplasmic reticulum and Golgi compartments of nonpolarized cells. In polarized epithelial cells, gpUS9 also accumulated along lateral membranes, colocalizing with cadherin and actin, and was insoluble in Triton X-100, a property shared with proteins that associate with the cytoskeleton. We hypothesize that gpUS9 may enhance the dissemination of CMV in infected epithelial tissues by associating with the cytoskeletal matrix.

Regulation of TNF-alpha-induced reorganization of the actin cytoskeleton and cell-cell junctions by Rho, Rac, and Cdc42 in human endothelial cells

We have investigated the role of the small guanosine-trisphosphate (GTP)-binding proteins, Rho, Rac, and Cdc42, in the early responses of human umbilical vein endothelial cells (HUVECs) to TNF-alpha (tumor necrosis factor-alpha). Quiescent confluent HUVECs incubated with TNF-alpha for 5-30 min showed an increased formation of membrane ruffles, filopodia, and actin stress fibres followed by cell retraction and formation of intercellular gaps. This process was accompanied by the dispersion of cadherin-5 from intercellular junctions. TNF-alpha also induced a transient increase in polymerized F-actin, as determined both by measuring G-actin content and by quantifying fluorescent emission from fluorescein isothiocyanate (FITC)-phalloidin-labelled F-actin. Microinjection of cells with activated RhoA protein led to an increase in polymerized actin, formation of stress fibres, cell retraction as well as dispersion of cadherin-5. The proteins Cdc42 and Rac induced qualitatively similar effects to Rho, although not as dramatic and in addition induced formation of filopodia and lamellipodia. Microinjection of cells with a Rho inhibitor, C3 transferase, prevented gap formation caused by TNF-alpha. Similar effects were observed in cells microinjected with the dominant inhibitory proteins N17Cdc42 and N17Rac1. Cell retraction and gap formation were also prevented by inhibitors of myosin light chain kinase (MLCK). Our data suggest that Cdc42, Rac, and Rho are activated in a hierarchical cascade following stimulation with TNF-alpha leading to actomyosin-mediated cell retraction and formation of intercellular gaps.

Intestinal epithelial hyperpermeability. Mechanisms and relevance to disease

Pathologic increases in intestinal permeability to hydrophilic macromolecules has been identified in a number of clinical conditions. The significance of gut barrier dysfunction as a clinical issue remains to be delineated, although it seems likely that alterations in intestinal epithelial permeability play a causative role in a number of conditions ranging from inflammatory bowel disease to the development of complications after cardiopulmonary bypass. It is unlikely that any one mechanism can account for all cases of intestinal hyperpermeability. Rather, it is more probable that myriad factors or combinations of factors, including mesenteric ischemia and cytokine-induced phenomena, lead to alterations in permeability in different clinical entities. Nevertheless, from a purely mechanistic standpoint, some common themes, notably the role of ATP depletion, increases in [Ca2+]i, and cytoskeletal derangements in enterocytes, have emerged as being particularly important.

Escherichia coli cytotoxic necrotizing factor 1 effaces microvilli and decreases transmigration of polymorphonuclear leukocytes in intestinal T84 epithelial cell monolayers

Cytotoxic necrotizing factor type 1 (CNF1), a 110-kDa toxin-like protein from pathogenic Escherichia coli strains, induces an actin cytoskeleton reorganization consisting of the formation of prominent stress fibers by permanent activation of the small GTP-binding protein Rho. Since p21Rho regulates tight-junction permeability and perijunctional actin reorganization in epithelial intestinal cells (A. Nusrat, M. Giry, J. R. Turner, S. P. Colgan, C. A. Parkos, E. Lemichez, P. Boquet, and J. L. Madara, Proc. Natl. Acad. Sci. USA 92:10629-10633, 1995), we used polarized T84 epithelial intestinal cell monolayers to examine whether CNF1 could affect microvillus structure, transepithelial resistance, and polymorphonuclear leukocyte (PMN) transmigration. Incubation of T84 cells with CNF1 did not influence transepithelial resistance, suggesting that barrier function and surface polarity were not affected by the toxin. However, CNF1 effaced intestinal cell microvilli and induced a strong decrease of PMN transepithelial migration in either the luminal-to-basolateral or the basolateral-to-luminal direction. CNF1 could thus be a virulence factor exhibiting a new type of combined activity consisting of effacing of microvilli and occlusion of the epithelial barrier to PMNs. Attenuated transepithelial migration of PMNs could result in the enhanced growth and protection of luminal bacteria.

Regulation of the movement of solutes across tight junctions

The intercellular tight junction is the rate-limiting barrier in the paracellular pathway for permeation by ions and larger solutes. A variety of widely used electrical and flux approaches are used in the analyses of solute permeation through this pathway; however, each has limitations in practice. It is now clear that solute permeation across tight junctions is dynamically regulated by intracellular events with a common effector mechanism apparently tied to the cytoskeleton. These pathways, which regulate tight junction solute permeability, are targets that produce epithelial barrier dysfunction in a variety of disease states. However, regulation of solute permeation across the junctional barrier may also represent a potential means to improve bioavailability of orally administered bioactive solutes.

Interactions between bacterial toxins and intestinal cells

Bacterial toxins which act on intestinal cells display a great diversity of size, structure and mode of action. Some toxins interact with the cell by transducing a signal across the membrane leading to stimulation of intracellular second messenger (E. coli heat stable enterotoxin), others form pores (C. perfringens enterotoxin, ...) permitting the leakage of cellular components and cell lysis. The most sophisticated toxins comprise at least two functional domains or components, one being a binding domain permitting the internalization into the cell of an enzymatic domain which modifies an intracellular target. The enzymatic modification (ADP-ribosylation, UDP-glucosylation, glycohydrolysis, proteolysis, ...) of a specific target (heterotrimeric G-protein, small G-protein, monomeric actin, ribosomal RNA, ...) alters the cell physiology (increase of ions and water secretion, cytoskeleton rearrangement, protein synthesis inhibition, apoptosis, ...) and tissue organization (modification of barrier permeability, necrosis, ...). The study of bacterial toxins leads to the understanding of the interactions between pathogenic bacteria and their hosts and constitutes also a new approach in cell biology, by facilitating the exploration of certain regulatory pathways such as that controlling actin polymerization.

Guanine nucleotide-dependent translocation of RhoA from cytosol to high affinity membrane binding sites in human erythrocytes

The translocation of the small GTP-binding protein Rho from the cytosolic to membrane-bound form is an early step in many cellular signal-transduction events, but little is known regarding the mechanism of Rho association with the plasma membrane. We have used membranes from human erythrocytes to uncover a novel class of integral membrane components involved in the Rho-membrane association. Membranes of human erythrocytes contain several proteins of the Ras superfamily. Using specific antibodies and C3 exoenzyme of Clostridium botulinum we have identified one of them as RhoA. This protein was detected in both cytosol and membrane fractions of hypotonically lysed erythrocytes. We found that cytosolic Rho bound specifically to the cytoplasmic surface of the erythrocyte membrane and that the translocation of Rho to the membrane was absolutely dependent on the prior incubation of the cytosol with guanosine 5'--gamma-thio-triphosphate (1-50 microM) at low Mg2+ concentration. Rho binding sites could not be extracted from the membrane using conditions that extracted all other peripheral proteins and were unaffected by heat treatment and protease digestion. Rho binding was saturable, with a Kd in the range 1-5.0 nM, and the number of binding sites was estimated to be approx. (1-2) x 10(3) sites per cell. This is the first report of Rho binding to integral membrane components. The identity of these components may reveal novel aspects of the mechanism by which Rho exerts its multiple biochemical effects.

Platelet-derived lysophosphatidic acid decreases endothelial permeability in vitro

We previously reported that platelets release a soluble factor that decreases the solute permeability of cultured bovine aortic endothelial monolayers. This factor was characterized as heat stable, tryspsin sensitive, and not serotonin, adenosine, ADP, or ATP [F. R. Haselton and J. S. Alexander. Am. J. Physiol. 263 (Lung Cell Mol. Physiol. 7): L670-L678, 1992]. We now report its identity as lysophosphatidic acid (LPA). Endothelial permeability decreases rapidly, reversibly, and repeatedly when exposed to platelet supernatants. Continuous exposure produces a sustained decrease in permeability. Methanol extracts of platelet supernatants also decrease endothelial permeability. Treatment of methanol extracts of platelet supernatants with phospholipase B or alkaline phosphatase, which modify the structure of LPA, abolishes the permeability-decreasing activity. However, activity is unaffected by treatment with phospholipase A2. This pattern of enzyme inactivation is consistent with the structure of LPA. Furthermore, synthetic 1-oleoyl-LPA rapidly and significantly decreases endothelial permeability in a concentration-dependent manner. Platelet activation does not appear to be required to produce activity in supernatants from platelet isolations, since P-selectin expression is not increased and thromboxane B2 is < 14 pg/6,000 platelets. Our data show that platelets release a methanol-extractable compound with an enzyme degradation profile consistent with LPA, which decreases the permeability of endothelial monolayers in vitro. In vivo, LPA derived from platelets may be an important mediator of the transport barrier formed by the vascular endothelium.

Overexpression of the rhoC gene correlates with progression of ductal adenocarcinoma of the pancreas

It has been reported that the rho genes, which consist of a ras-related small GTPase protein family, regulate cytoskeletal structures and have the potential to transform cultured cells. To investigate the biological relevance of the rho genes in pancreatic carcinogenesis, we examined expressions of the rhoA, B and C genes by polymerase chain reaction after reverse transcription (RT-PCR) in 33 cases of ductal adenocarcinoma of the pancreas. In addition, mutations of the K-ras, rhoA, B and C genes were studied in the same series of tumour tissues to correlate with rho gene expressions. The expression levels of the rhoC gene were significantly higher in tumours than in non-malignant portions (P < 0.001). Metastatic lesions overexpressed the rhoC gene compared with primary tumours (P < 0.05). Carcinoma tissues with perineural invasion and lymph node metastasis exhibited significantly higher expressions of the rhoC gene than tumours without these manifestations (P < 0.001 and P < 0.05 respectively). Overexpression of the rhoC gene significantly correlated with poorer prognosis of patients with pancreatic adenocarcinoma (P < 0.05). In contrast, the expression levels of the rhoA and B genes showed no significant relationship with clinicopathological findings. Mutation was not found either in the rhoA, B or C gene sequences examined. K-ras gene mutation, detected in 27 out of 33 (81.8%) cases, did not affect the expression levels in any of the rho genes. These suggest that elevated expression of the rhoC gene may be involved in the progression of pancreatic carcinoma independent of K-ras gene activation.

Molecular structure and assembly of the tight junction

Polarized epithelial cells separate two extremely different cellular milieus. The tight junction (TJ) is the most apical component of the junctional complex and serves as the permeability barrier between these environments. The tight junctional complex appears to be a dynamic and regulated structure. Some of its protein components have been identified and include the transmembrane protein occludin. Nontransmembrane proteins on the cytosolic leaflet including ZO-1, ZO-2, cingulin, 7H6, and several unidentified phosphoproteins are also believed to be part of the TJ. Interactions of some of these proteins with the actin cytoskeleton are a major determinant of TJ structure and may also play a role in the regulation of TJ assembly. Recent progress using the "calcium switch" and the "ATP depletion-repletion" model of TJ formation offers new insight regarding how these structures form. TJ biogenesis appears to be regulated, in part, by classic signal transduction pathways involving heterotrimeric G proteins, release of intracellular Ca2+, and activation of protein kinase C. Although many of the details of the signaling pathways have yet to be defined, these observations may provide insight into how TJs form during tubular development. Furthermore, it may be possible to suggest potential therapeutic targets for intervention in a variety of diseases (e.g., ischemia, toxic injury to the kidney and other epithelial tissue) where TJ integrity has been compromised and reassembly is required.

Planar polarization of Drosophila and vertebrate epithelia

Our understanding of the actin and microtubule rearrangements that generate planar polarity in Drosophila and in vertebrate epithelia has been extended by recent discoveries. Three different Rho family proteins have been shown to mediate polarization in the wing and the eye of Drosophila. In vertebrates, the importance of myosin VIIa has been uncovered by mutations that cause defects in planar polarization in the ear. Advances in our understanding of the Frizzled pathway, which coordinates planar polarization in Drosophila, are moving the field closer to understanding the links between signal transduction and polarized cytoskeletal reorganization.

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

The Rho small G protein family, consisting of the Rho, Rac, and Cdc42 subfamilies, regulates various cell functions, such as cell shape change, cell motility, and cytokinesis, through reorganization of the actin cytoskeleton. We show here that the Rac and Rho subfamilies furthermore regulate cell-cell adhesion. We prepared MDCK cell lines stably expressing each of dominant active mutants of RhoA (sMDCK-RhoDA), Rac1 (sMDCK-RacDA), and Cdc42 (sMDCK-Cdc42DA) and dominant negative mutants of Rac1 (sMDCK-RacDN) and Cdc42 (sMDCK-Cdc42DN) and analyzed cell adhesion in these cell lines. The actin filaments at the cell-cell adhesion sites markedly increased in sMDCK-RacDA cells, whereas they apparently decreased in sMDCK-RacDN cells, compared with those in wild-type MDCK cells. Both E-cadherin and beta-catenin, adherens junctional proteins, at the cell-cell adhesion sites also increased in sMDCK-RacDA cells, whereas both of them decreased in sMDCK-RacDN cells. The detergent solubility assay indicated that the amount of detergent-insoluble E-cadherin increased in sMDCK-RacDA cells, whereas it slightly decreased in sMDCK-RacDN cells, compared with that in wild-type MDCK cells. In sMDCK-RhoDA, -Cdc42DA, and -Cdc42DN cells, neither of these proteins at the cell-cell adhesion sites was apparently affected. ZO-1, a tight junctional protein, was not apparently affected in any of the transformant cell lines. Electron microscopic analysis revealed that sMDCK-RacDA cells tightly made contact with each other throughout the lateral membranes, whereas wild-type MDCK and sMDCK-RacDN cells tightly and linearly made contact at the apical area of the lateral membranes. These results suggest that the Rac subfamily regulates the formation of the cadherin-based cell- cell adhesion. Microinjection of C3 into wild-type MDCK cells inhibited the formation of both the cadherin-based cell-cell adhesion and the tight junction, but microinjection of C3 into sMDCK-RacDA cells showed little effect on the localization of the actin filaments and E-cadherin at the cell-cell adhesion sites. These results suggest that the Rho subfamily is necessary for the formation of both the cadherin-based cell- cell adhesion and the tight junction, but not essential for the Rac subfamily-regulated, cadherin-based cell- cell adhesion.

Rho-stimulated contractility contributes to the fibroblastic phenotype of Ras-transformed epithelial cells

Oncogenic transformation of cells alters their morphology, cytoskeletal organization, and adhesive interactions. When the mammary epithelial cell line MCF10A is transformed by activated H-Ras, the cells display a mesenchymal/fibroblastic morphology with decreased cell-cell junctions but increased focal adhesions and stress fibers. We have investigated whether the transformed phenotype is due to Rho activation. The Ras-transformed MCF10A cells have elevated levels of myosin light chain phosphorylation and are more contractile than their normal counterparts, consistent with the activation of Rho. Furthermore, inhibitors of contractility restore a more normal epithelial phenotype to the Ras-transformed MCF10A cells. However, inhibiting Rho by microinjection of C3 exotransferase or dominant negative RhoA only partially restores the normal phenotype, in that it fails to restore normal junctional organization. This result prompted us to examine the effect that inhibiting Rho would have on the junctions of normal MCF10A cells. We have found that inhibiting Rho by C3 microinjection leads to a disruption of E-cadherin cytoskeletal links in adherens junctions and blocks the assembly of new adherens junctions. The introduction of constitutively active Rho into normal MCF10A cells did not mimic the Ras-transformed phenotype. Thus, these results lead us to conclude that some, but not all, characteristics of Ras-transformed epithelial cells are due to activated Rho. Whereas Rho is needed for the assembly of adherens junctions, high levels of activated Rho in Ras-transformed cells contribute to their altered cytoskeletal organization. However, additional events triggered by Ras must also be required for the disruption of adherens junctions and the full development of the transformed epithelial phenotype.

Cell adhesion and migration. I. Neutrophil adhesive interactions with intestinal epithelium

In many inflammatory conditions of the gastrointestinal tract, disease activity and patient symptoms correlate with the histological finding of neutrophil (PMN) migration across the epithelium. PMN interactions with intestinal epithelium can influence epithelial functions ranging from barrier maintenance to electrolyte secretion. Additionally, PMN recruitment to the epithelium can be modulated by epithelial interactions with luminal enteric pathogens. Adhesive interactions between PMN and intestinal epithelial cells have been shown to be distinct from interactions of PMN with endothelia. In particular, PMN transepithelial migration is modulated by a distinct array of cytokines including interferon-gamma and interleukin-4 and requires the PMN beta2-integrin CD11b/CD18 but is independent of CD11a/CD18, selectins, and intercellular adhesion molecule 1. Additionally, an integral membrane protein termed CD47 has recently been shown to play an important role in PMN transepithelial migration at point(s) subsequent to initial adhesive interactions. This article provides a brief overview of PMN interactions with epithelia and their functional consequences in relation to inflammatory disease.

Physiological regulation of epithelial tight junctions is associated with myosin light-chain phosphorylation

Tight junctions serve as the rate-limiting barrier to passive movement of hydrophilic solutes across intestinal epithelia. After activation of Na+-glucose cotransport, the permeability of intestinal tight junctions is increased. Because previous analyses of this physiological tight junction regulation have been restricted to intact mucosae, dissection of the mechanisms underlying this process has been limited. To characterize this process, we have developed a reductionist model consisting of Caco-2 intestinal epithelial cells transfected with the intestinal Na+-glucose cotransporter, SGLT1. Monolayers of SGLT1 transfectants demonstrate physiological Na+-glucose cotransport. Activation of SGLT1 results in a 22 +/- 5% fall in transepithelial resistance (TER) (P < 0.001). Similarly, inactivation of SGLT1 by addition of phloridzin increases TER by 24 +/- 2% (P < 0.001). The increased tight junction permeability is size selective, with increased flux of small nutrient-sized molecules, e.g., mannitol, but not of larger molecules, e.g., inulin. SGLT1-dependent increases in tight junction permeability are inhibited by myosin light-chain kinase inhibitors (20 microM ML-7 or 40 microM ML-9), suggesting that myosin regulatory light-chain (MLC) phosphorylation is involved in tight junction regulation. Analysis of MLC phosphorylation showed a 2.08-fold increase after activation of SGLT1 (P < 0.01), which was inhibited by ML-9 (P < 0.01). Thus monolayers incubated with glucose and myosin light-chain kinase inhibitors are comparable to monolayers incubated with phloridzin. ML-9 also inhibits SGLT1-mediated tight junction regulation in small intestinal mucosa (P < 0.01). These data demonstrate that epithelial cells are the mediators of physiological tight junction regulation subsequent to SGLT1 activation. The intimate relationship between tight junction regulation and MLC phosphorylation suggests that a critical step in regulation of epithelial tight junction permeability may be myosin ATPase-mediated contraction of the perijunctional actomyosin ring and subsequent physical tension on the tight junction.

Rho proteins play a critical role in cell migration during the early phase of mucosal restitution

In the intestine, several growth factors stimulate migration of epithelial cells, contributing to the maintenance of tissue integrity. The Ras-like GTPase Rho regulates a signal transduction pathway linking growth factor receptors to the formation of actin stress fibers and focal adhesions, presumed to be important for motility. Using an in vitro wound-induced migration assay, we have examined the role of Rho GTPases in the migration of IEC-6 and Caco-2 cells, and provide evidence that the Rho GTPases play an essential role in the initial phase of mucosal wound healing. Treatment of the cells with Clostridium difficile toxins A and B, inhibitors of the Rho family GTPases inhibited migration in a dose-dependent fashion. Microinjection of the inhibitory exchange factor Rho-guanine nucleotide dissociation inhibitor (GDI), or Clostridium botulinum C3 ADP-ribosyl transferase (C3) toxin, a Rho-ADP-ribosylating exoenzyme, potently inhibited migration. Microinjection of RhoT19N, a dominant negative form of RhoA, or in vitro ADP-ribosylated RhoA impaired the ability of cells to migrate. Rho-GDI and C3 exoenzyme also inhibited EGF-induced migration of IEC-6 cells. These results demonstrate that Rho is required for endogenous and EGF-induced migration of small intestinal crypt cells, and that Rho proteins are essential elements of a mechanism by which growth factors induce cell migration to restitute mucosal integrity.

The small GTPases Rho and Rac are required for the establishment of cadherin-dependent cell-cell contacts

Cadherins are calcium-dependent cell-cell adhesion molecules that require the interaction of the cytoplasmic tail with the actin cytoskeleton for adhesive activity. Because of the functional relationship between cadherin receptors and actin filament organization, we investigated whether members of the Rho family of small GTPases are necessary for cadherin adhesion. In fibroblasts, the Rho family members Rho and Rac regulate actin polymerization to produce stress fibers and lamellipodia, respectively. In epithelial cells, we demonstrate that Rho and Rac are required for the establishment of cadherin-mediated cell-cell adhesion and the actin reorganization necessary to stabilize the receptors at sites of intercellular junctions. Blocking endogenous Rho or Rac selectively removed cadherin complexes from junctions induced for up to 3 h, while desmosomes were not perturbed. In addition, withdrawal of cadherins from intercellular junctions temporally precedes the removal of CD44 and integrins, other microfilament-associated receptors. Our data showed that the concerted action of Rho and Rac modulate the establishment of cadherin adhesion: a constitutively active form of Rac was not sufficient to stabilize cadherindependent cell-cell contacts when endogenous Rho was inhibited. Upon induction of calcium-dependent intercellular adhesion, there was a rapid accumulation of actin at sites of cell-cell contacts, which was prevented by blocking cadherin function, Rho or Rac activity. However, if cadherin complexes are clustered by specific antibodies attached to beads, actin recruitment to the receptors was perturbed by inhibiting Rac but not Rho. Our results provide new insights into the role of the small GTPases in the cadherin-dependent cell- cell contact formation and the remodelling of actin filaments in epithelial cells.

Cloning and characterization of Ras-GRF2, a novel guanine nucleotide exchange factor for Ras

Conversion of Ras proteins into an activated GTP-bound state able to bind effector proteins is catalyzed by specific guanine nucleotide exchange factors in response to a large number of extracellular stimuli. Here we report the isolation of mouse cDNAs encoding Ras-GRF2, a multidomain 135-kDa protein containing a COOH-terminal Cdc25-related domain that stimulates release of GDP from Ras but not other GTPases in vitro. Ras-GRF2 bound specifically to immobilized Ras lacking bound nucleotides, suggesting stabilization of the nucleotide-free form of Ras as a mechanism of catalyzing nucleotide exchange. The NH2-terminal region of Ras-GRF2 is predicted to contain features common to various signaling proteins including two pleckstrin homology domains and a Dbl homology region. Ras-GRF2 also contains an IQ motif which was required for its apparent constitutive association with calmodulin in epithelial cells ectopically expressing Ras-GRF2. Transient expression of Ras-GRF2 in kidney epithelial cells stimulated GTP binding by Ras and potentiated calcium ionophore-induced activation of mitogen-activated protein kinase (ERK1) dependent upon the IQ motif. Calcium influx caused Ras-GRF2 subcellular localization to change from cytosolic to peripheral, suggesting a possible mechanism for controlling Ras-GRF2 interactions with Ras at the plasma membrane. Epithelial cells overexpressing Ras-GRF2 are morphologically transformed and grow in a disorganized manner with minimal intercellular contacts. Northern analysis indicated a 9-kb GRF2 transcript in brain and lung, where p135 Ras-GRF2 is known to be expressed, and RNAs of 12 kb and 2.2 kb were detected in several tissues. Thus, Ras-GRF2 proteins with different domain structures may be widely expressed and couple diverse extracellular signals to Ras activation.