Deamidation of RhoA glutamine 63 by the Escherichia coli CNF1 toxin requires a short sequence of the GTPase switch 2 domain

CNF1, a toxin produced by pathogenic Escherichia coli strains, deamidates the RhoA GTP-binding protein glutamine 63 and impairs RhoGAP-mediated GTP hydrolysis resulting in RhoA permanent activation. Using peptides derived from the RhoA sequence, we found that DTAGQEDYDRL (corresponding to RhoA 59-69 residues) was the minimum RhoA-derived peptide which could be deamidated in vitro by the CNF1 catalytic domain (CNF1-Cter). Site-directed mutagenesis outside the RhoA 59-69 sequence had no influence on glutamine 63 deamidation by CNF1-Cter. RhoA proteins with substitutions L57G, D65G, Y66G, or R70G were not affected in their ability to be deamidated by CNF1-Cter, whereas this was abolished by the R68G substitution. Arginine 68 is part of the DYDRL motif that is strictly conserved in Rho, Rac, and Cdc42 but not in other small GTP-binding proteins consistent with the observation that only Rho, Rac, and Cdc42 can be modified by CNF1.

The 5' region of cnf1 harbours a translational regulatory mechanism for CNF1 synthesis and encodes the cell-binding domain of the toxin

The Escherichia coli cytotoxic necrotizing factor 1 (CNF1) is organized into three functional domains: the N-terminal part containing the cell-binding domain, a putative central membrane-spanning region, and a C-terminal catalytic region. On the basis of competition assays between CNF1 and GST-recombinant proteins containing different N-terminal fragments, and point mutations, we restricted the binding region to the first 190 amino acids. Hydrophilic amino acids 53-75 are strictly necessary to cell receptor recognition. Using different cnf1-lacZ translational fusions, we demonstrated that the mRNA corresponding to the first 48 codons of cnf1 is involved in the translational regulation of CNF1 synthesis. This regulation consists of both a positive and a negative control. The positive control is exerted by codons 6-20, including a putative downstream box that enhances the translational expression of cnf1. The negative control depends on codons 45-48. In this region, an anti-Shine-Dalgarno sequence, highly homologous to the core of the internal complementary sequence already reported for growth rate-regulated metabolic genes, has been detected. To some extent, the inner structural organization of CNF1 would thus suggest the compiling of several functions in a single mRNA protein system.

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.

Neutrophil F-actin and myosin but not microtubules functionally regulate transepithelial migration induced by interleukin 8 across a cultured intestinal epithelial monolayer

The role of the polymorphonuclear leukocyte (PMN) cytoskeleton during the transmigration across colonic epithelial cells is not very well understood. In order to study the role of different components of the PMN cytoskeleton during transepithelial migration across a colonic epithelial cell monolayer (T84), PMN were preincubated with drugs affecting either the actin cytoskeleton (cytochalasin B, iota toxin of Clostridium perfringens, and phalloidin) or the microtubules (colchicine and taxol). The role of PMN myosin during transepithelial migration was investigated using the inhibitor 2,3-butanedione monoxime (BDM) and DC3B toxin. PMN intracellular Ca2+, during neutrophil adhesion and translocation across the epithelium, was assessed by the Ca2+ chelator 1, 2bis-(2-aminophenoxy)-ethane-N,N,N', N'-tetra-acetic acid tetrakis (acetoxymethyl) ester (BAPTA-AM). Transmigration of PMN was initiated by applying either interleukin-8 or formyl-met-leu-phe (fMLP). While colchicine and taxol preexposure did not influence PMN transepithelial migration, treatment with cytochalasin B, iota toxin, phalloidin, BDM, DC3B toxin and BAPTA-AM greatly diminished migration of PMN across T84 monolayers. Similarly, cell-cell contacts established between PMN and epithelial cells during the transmigration were diminished after treatment of PMN with iota toxin or cytochalasin B. These data show that the neutrophil actin cytokeleton and myosin, but not the microtubules, evoke a Ca2+ -dependent motility that facilitates migration across the colonic epithelial barrier.

G-protein-stimulated phospholipase D activity is inhibited by lethal toxin from Clostridium sordellii in HL-60 cells

Lethal toxin (LT) from Clostridium sordellii has been shown in HeLa cells to glucosylate and inactivate Ras and Rac and, hence, to disorganize the actin cytoskeleton. In the present work, we demonstrate that LT treatment provokes the same effects in HL-60 cells. We show that guanosine 5'-O-(3-thiotriphosphate)-stimulated phospholipase D (PLD) activity is inhibited in a time- and dose-dependent manner after an overnight treatment with LT. A similar dose response to the toxin was found when PLD activity was stimulated by phorbol 12-myristate 13-acetate via the protein kinase C pathway. The toxin effect on actin organization seemed unlikely to account directly for PLD inhibition as cytochalasin D and iota toxin from Clostridium perfringens E disorganize the actin cytoskeleton without modifying PLD activity. However, the enzyme inhibition and actin cytoskeleton disorganization could both be related to a major decrease observed in phosphatidylinositol 4,5-bisphosphate (PtdIns(4, 5)P2). Likely in a relationship with this decrease, recombinant ADP-ribosylation factor, RhoA, Rac, and RalA were not able to reconstitute PLD activity in LT-treated cells permeabilized and depleted of cytosol. Studies of phosphoinositide kinase activities did not allow us to attribute the decrease in PtdIns(4,5)P2 to inactivation of PtdIns4P 5-kinase. LT was also found to provoke a major inhibition in phosphatidylinositol 3-kinase that could not account for the inhibition of PLD activity because wortmannin, at doses that fully inhibit phosphatidylinositol 3-kinase, had no effect on the phospholipase activity. Among the three small G-proteins, Ras, Rac, and RalA, inactivated by LT and involved in PLD regulation, inactivation of Ral proteins appeared to be responsible for PLD inhibition as LT toxin (strain 9048) unable to glucosylate Ral proteins did not modify PLD activity. In HL-60 cells, LT treatment appeared also to modify cytosol components in relationship with PLD inhibition as a cytosol prepared from LT-treated cells was less efficient than one from control HL-60 cells in stimulating PLD activity. Phosphatidylinositol transfer proteins involved in the regulation of polyphosphoinositides and ADP-ribosylation factor, a major cytosolic PLD activator in HL-60 cells, were unchanged, whereas the level of cytosolic protein kinase Calpha was decreased after LT treatment. We conclude that in HL-60 cells, lethal toxin from C. sordellii, in inactivating small G-proteins involved in PLD regulation, provokes major modifications at the membrane and the cytosol levels that participate in the inhibition of PLD activity. Although Ral appeared to play an essential role in PLD activity, we discuss the role of other small G-proteins inactivated by LT in the different modifications observed in HL-60 cells.

Coxiella burnetii induces reorganization of the actin cytoskeleton in human monocytes

Coxiella burnetii, an obligate intracellular bacterium which survives in myeloid cells, causes Q fever in humans. We previously demonstrated that virulent C. burnetii organisms are poorly internalized by monocytes compared to avirulent variants. We hypothesized that a differential mobilization of the actin cytoskeleton may account for this distinct phagocytic behavior. Scanning electron microscopy demonstrated that virulent C. burnetii stimulated profound and polymorphic changes in the morphology of THP-1 monocytes, consisting of membrane protrusions and polarized projections. These changes were transient, requiring 5 min to reach their maximum extent and vanishing after 60 min of incubation. In contrast, avirulent variants of C. burnetii did not induce any significant changes in cell morphology. The distribution of filamentous actin (F-actin) was then studied with a specific probe, bodipy phallacidin. Virulent C. burnetii induced a profound and transient reorganization of F-actin, accompanied by an increase in the F-actin content of THP-1 cells. F-actin was colocalized with myosin in cell protrusions, suggesting that actin polymerization and the tension of actin-myosin filaments play a role in C. burnetii-induced morphological changes. In addition, contact between the cell and the bacterium seems to be necessary to induce cytoskeleton reorganization. Bacterial supernatants did not stimulate actin remodeling, and virulent C. burnetii organisms were found in close apposition with F-actin protrusions. The manipulation of the actin cytoskeleton by C. burnetii may therefore play a critical role in the internalization strategy of this bacterium.

Rho-dependent cell spreading activated by E.coli cytotoxic necrotizing factor 1 hinders apoptosis in epithelial cells

Cell-cell and cell-matrix interactions play a pivotal role in numerous cell functions including cell survival and death. In this work, we report evidence that the Rho-dependent cell spreading activated by a protein toxin from E. coli, the cytotoxic necrotizing factor 1 (CNF1), is capable of hindering apoptosis in HEp-2 cells. In addition to the promotion of cell spreading, CNF1 protects cells from the experimentally-induced rounding up and detachment and improves the ability of cells to adhere to each other and to the extracellular matrix by modulating the expression of proteins related to cell adhesion. In particular, the expression of integrins such as alpha 5, alpha 6 and alpha v, as well as of some heterotypic and homotypic adhesion-related proteins such as the Focal Adhesion Kinase, E-cadherin, alpha and beta catenins were significantly increased in cells exposed to CNF1. Our results suggest, however, that the promotion of Rho-dependent cell spreading is the key mechanism in protecting cells against apoptosis rather than cell adhesion per se. A toxin inducing cell spreading without activating Rho, such as Cytochalasin B, was in fact ineffective in favouring cell survival. These data are of relevance (i) for the understanding of the role of the actin-dependent and especially Rho-dependent cellular activities involved in apoptosis regulation and (ii) in providing some clues to understanding the mechanisms by which bacteria, by controlling cell fate, might exert their pathogenic activity.

Effect of cytotoxic necrotizing factor-1 on actin cytoskeleton in human monocytes: role in the regulation of integrin-dependent phagocytosis

Cytotoxic necrotizing factor-1 (CNF1) is isolated from pathogenic strains of Escherichia coli and catalyzes the activation of Rho GTPases by the deamidation of a glutamine residue. This toxin induces stress fiber formation, cell spreading, and membrane folding and promotes phagocytosis competence in epithelial cells. We show that CNF1 induces morphologic changes in monocytic cells: polarized-like shape in THP-1 cells, lamellipodia, and cell spreading in adherent monocytes. CNF1 also increased filamentous actin (F-actin) content in a time- and dose-dependent manner. In addition, the toxin profoundly reorganized the actin cytoskeleton: redistribution of F-actin in polarized deformations of THP-1 cells and disorganization of microfilament network in monocytes. We also studied the effects of CNF1 on phagocytosis. It markedly impaired the ingestion of unopsonized zymosan involving CR type 3. However, CNF1 had no effect on the uptake of iC3b-coated zymosan or IgG-mediated phagocytosis of SRBC. In addition, CNF1 induced clustering of CR3 and Fc gammaRII (CD32) but selectively impaired the colocalization of CR3 with F-actin. It is likely that CNF1-induced reorganization of actin cytoskeleton down-modulates integrin activation-dependent phagocytosis by preventing the codistribution of CR3 with F-actin. CNF1 may control some features of integrin-dependent phagocytosis in myeloid cells through its action on Rho GTP binding proteins and cytoskeletal organization.

Regulation of the actin cytoskeleton by thrombin in human endothelial cells: role of Rho proteins in endothelial barrier function

Endothelial barrier function is regulated at the cellular level by cytoskeletal-dependent anchoring and retracting forces. In the present study we have examined the signal transduction pathways underlying agonist-stimulated reorganization of the actin cytoskeleton in human umbilical vein endothelial cells. Receptor activation by thrombin, or the thrombin receptor (proteinase-activated receptor 1) agonist peptide, leads to an early increase in stress fiber formation followed by cortical actin accumulation and cell rounding. Selective inhibition of thrombin-stimulated signaling systems, including Gi/o (pertussis toxin sensitive), p42/p44, and p38 MAP kinase cascades, Src family kinases, PI-3 kinase, or S6 kinase pathways had no effect on the thrombin response. In contrast, staurosporine and KT5926, an inhibitor of myosin light chain kinase, effectively blocked thrombin-induced cell rounding and retraction. The contribution of Rho to these effects was analyzed by using bacterial toxins that either activate or inhibit the GTPase. Escherichia coli cytotoxic necrotizing factor 1, an activator of Rho, induced the appearance of dense actin cables across cells without perturbing monolayer integrity. Accordingly, lysophosphatidic acid, an activator of Rho-dependent stress fiber formation in fibroblasts, led to reorganization of polymerized actin into stress fibers but failed to induce cell rounding. Inhibition of Rho with Clostridium botulinum exoenzyme C3 fused to the B fragment of diphtheria toxin caused loss of stress fibers with only partial attenuation of thrombin-induced cell rounding. The implication of Rac and Cdc42 was analyzed in transient transfection experiments using either constitutively active (V12) or dominant-interfering (N17) mutants. Expression of RacV12 mimicked the effect of thrombin on cell rounding, and RacN17 blocked the response to thrombin, whereas Cdc42 mutants were without effect. These observations suggest that Rho is involved in the maintenance of endothelial barrier function and Rac participates in cytoskeletal remodeling by thrombin in human umbilical vein endothelial cells.

Bacterial toxins and the Rho GTP-binding protein: what microbes teach us about cell regulation

In the present review activities of two bacterial toxins, Clostridium botulinum exoenzyme C3 and Escherichia coli CNF1, both acting on the GTP-binding protein Rho are analyzed. Proteins belonging to the Rho family regulate the actin cytoskeleton and act as molecular switches in a number of signal transduction pathways. C3 and CNF1 have opposite effects on Rho thus representing useful tools for studies on cell division, cell differentiation and apoptosis.

Toxin-induced activation of Rho GTP-binding protein increases Bcl-2 expression and influences mitochondrial homeostasis

It is now well established that apoptosis plays a pivotal role in several physiological and pathological situations. Consequently, the mechanisms controlling the cell fate are currently the subject of intense investigation. In this work, we report that an Escherichia coli protein toxin (Cytotoxic Necrotizing Factor 1, CNF1) which activates the Rho GTP-binding protein and prevent apoptosis in epithelial cells was able to: (i) influence the mitochondrial homeostasis and (ii) modulate the expression of proteins belonging to the Bcl-2 family. In particular, the content of the antiapoptotic products Bcl-2 and Bcl-XL resulted to be increased in treated cells, whereas the expression of the proapoptotic protein Bax remained unaltered. CNF1 induces cell spreading via activation of Rho and cell spreading has been reported to promote cell survival. Cytochalasin B, which provokes most of the morphological changes typical of CNF1, including cell spreading, but without the involvement of Rho, was unable to counteract apoptosis. Altogether our results suggest a link between the Rho GTP-binding protein and the regulation of the mitochondrial homeostasis via an effect on the antiapoptotic proteins of the Bcl-2 family.

The small GTP-binding protein RacG regulates uroid formation in the protozoan parasite Entamoeba histolytica

Entamoeba histolytica is a protozoan parasite that invades human intestine leading to ulceration and destruction of tissue. Amoebic movement and phagocytosis of human cells is accompanied by characteristic changes in cell morphology. Amoebae become polarized, developing a frontal pseudopod and a well-defined rear zone of membrane accumulation designated the uroid. In motile eukaryotic cells, a phenomenon that contributes to movement is the capping of receptors at the cell surface. During the capping process, E. histolytica concentrates ligand-receptor complexes in the uroid. Interestingly, some of these surface receptors are involved in the survival of the parasite. While looking for regulators of capping and uroid formation, we identified RacG, an E. histolytica protein that is homologous to human Rac1. This protein belongs to the Rac subfamily of small GTPases implicated in interactions between the actin cytoskeleton and the membrane of mammalian cells. Cloning of the EhracG gene and analysis of the protein activity either in murine fibroblasts or in E. histolytica revealed that EhRacG induces a characteristic Rac phenotype. When expressed in amoebae, an EhRacG-V12 mutant protein not only deregulated cell polarity, but also caused a defect in cytokinesis. Analysis of the cytoskeleton in amoebae bearing this mutant revealed that F-actin concentrated at the periphery of the cell. In addition, the number and localization of uroids were modified. These results suggest a role for EhRacG in amoebic morphogenesis and cytokinesis.

Inhibition of Rho is required for cAMP-induced melanoma cell differentiation

Up-regulation of the cAMP pathway by forskolin or alpha-melanocyte stimulating hormone induces melanocyte and melanoma cell differentiation characterized by stimulation of melanin synthesis and dendrite development. Here we show that forskolin-induced dendricity is associated to a disassembly of actin stress fibers. Since Rho controls actin organization, we studied the role of this guanosine triphosphate (GTP)-binding protein in cAMP-induced dendrite formation. Clostridium botulinum C3 exotransferase, which inhibits Rho, mimicked the effect of forskolin in promoting dendricity and stress fiber disruption, while the Escherichia coli toxin cytotoxic necrotizing factor-1 (CNF-1), which activates Rho and the expression of a constitutively active Rho mutant, blocked forskolin-induced dendrite outgrowth. In addition, overexpression of a constitutively active form of the Rho target p160 Rho-kinase (P160(ROCK)) prevented the dendritogenic effects of cAMP. Our results suggest that inhibition of Rho and of its target p160(ROCK) are required events for cAMP-induced dendrite outgrowth in B16 cells. Furthermore, we present evidence that Rho is involved in the regulation of melanogenesis. Indeed, Rho inactivation enhanced the cAMP stimulation of tyrosinase gene transcription and protein expression, while Rho constitutive activation impaired these cAMP-induced effects. This reveals that, in addition to controlling dendricity, Rho also participates in the regulation of melanin synthesis by cAMP.

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.

In vitro selective elimination of HIV-infected cells from peripheral blood in AIDS patients by the immunotoxin DAB389CD4

OBJECTIVES

To study the antiviral efficacy of the recombinant immunotoxin DAB389CD4 against wild-type strains of HIV and to analyse its potential toxicity in non-infected peripheral blood mononuclear cells (PBMC).

DESIGN AND METHODS

PBMC from HIV-seropositive patients were cultured in the presence of DAB389CD4. After 30 days in culture, viral load was assessed by quantification of RNA levels in supernatants and HIV-specific polymerase chain reaction (PCR) was performed for measuring proviral DNA as an indicator of remaining virus in cells. To study the toxicity of DAB389CD4, PBMC from healthy donors were isolated and cell viability and lymphocyte proliferation were assessed after immunotoxin treatment.

RESULTS

DAB389CD4 presented a strong antiviral activity in five of the six primary isolates decreasing p24 production in cultures to undetectable levels and eliminating selectively HIV-infected cells as measured by HIV DNA-specific PCR. One viral isolate was resistant to DAB389CD4 treatment. The immunotoxin was active against both syncytial and non-syncytial HIV strains. DAB389CD4 was not toxic in non-infected PBMC as measured by different techniques: trypan blue exclusion, methyl thiazol tetrazolium oxidation, lymphocyte proliferation, and CD4 cell count.

CONCLUSIONS

DAB389CD4 showed a strong antiviral and specific activity against primary HIV isolates by killing selectively HIV-infected cells without affecting non-infected cells. This antiviral effect produced the eradication of HIV in cultures and indicated the potential use of this drug as a new therapeutic tool in combination with antiretroviral drugs. This immunotoxin would be especially interesting in the context of the marginal populations of HIV-infected cells remaining after successful antiviral treatment.

Toxins from anaerobic bacteria: specificity and molecular mechanisms of action

Major advances have been made in the past five years in the identification of cellular targets of toxins produced by anaerobic bacteria. These targets include the vesicular membrane docking and fusion apparatus, the actin cytoskeleton, the signal transduction machinery and the cell membrane. The recent discovery that large clostridial toxins (Clostridium difficile A and B toxins, C. sordellii lethal and hemorrhagic toxins, and alpha C. novyi toxin) are monoglucosyltransferases, together with the establishment of the perfringolysin crystal structure, has led to new insights in the field of toxins from anaerobic bacteria.

Cytotoxic necrotizing factor 1 from Escherichia coli: a toxin with a new intracellular activity for eukaryotic cells

Certain pathogenic Escherichia coli strains elaborate a toxin, the cytotoxic necrotizing factor type 1 (CNF1). CNF1 covalently and specifically modifies the p21 Rho GTP-binding protein in mammalian cells by deamidation of the p21 Rho glutamine 63. CNF1 modification of Rho leads to permanent activation of the GTP-binding protein by blocking intrinsic and RhoGAP GTPase activities. Rho activation by CNF1 induces reorganization of the actin cytoskeleton into large stress fibers and the multiplication of focal contact points. Deamidation is a new catalytic activity described for an intracellularly acting toxin.

Hinderance of apoptosis and phagocytic behaviour induced by Escherichia coli cytotoxic necrotizing factor 1: two related activities in epithelial cells

Several microbial factors are recognized able to modulate apoptosis. In this work, we analyze the activity of a toxin from pathogenic strains of Escherichia coli, the Cytotoxic Necrotizing Factor 1 (CNF1), which influences epithelial cell structure and function. This toxin activates the Rho GTP-binding protein leading to actin filament reorganization, membrane ruffling and multinucleation. Functionally, CNF1 induces a phagocytic activity in non-professional phagocytes. Surprisingly, we found that the increase of phagocytic activity induced by CNF1 corresponds to a decrease of apoptotic cell death. We therefore hypothesize that the two activities together might have a finalistic role in the pathogenesis of bacterial infection.

Functional analysis of four tetraspans, CD9, CD53, CD81, and CD82, suggests a common role in costimulation, cell adhesion, and migration: only CD9 upregulates HB-EGF activity

Molecules of the tetraspan superfamily are engaged in multimolecular complexes containing other proteins such as beta 1 integrins and MHC antigens. Although their functions are not clear, they have been suggested to play a role in cell adhesion and migration, signal transduction, and costimulation. We have in this paper directly compared the functional properties of four tetraspans, CD9, CD53, CD81, and CD82. mAbs to any of these molecules were able to deliver a costimulatory signal for CD3-mediated activation of the T cell line Jurkat. CD82 mAbs were the most efficient in triggering this effect. Moreover, engagement of CD9, CD81, and CD82 induced the homotypic aggregation of the megakaryocytic cell line HEL, and inhibited the migration of this cell line. Similar results were obtained with the preB cell line NALM-6 using the CD9 and CD81 mAbs. The CD81 mAb 5A6 produced the strongest effects. Therefore, the tetraspans are recognized by mAbs which produce similar effects on the same cell lines. This is consistent with the tetraspans being included in large molecular complexes and possibly forming a tetraspan network (the tetraspan web). We also demonstrate that the tetraspans are likely to keep specific functional properties inside this network. Indeed, we have demonstrated that the human CD9 is able, like the monkey molecule, to upregulate the activity of the transmembrane precursor of heparin-binding EGF as a receptor for the diphtheria toxin when cotransfected in murine LM cells. Neither CD81, nor CD82 had such activity. By using chimeric CD9/CD81 molecules we demonstrate that this activity requires the second half of CD9, which contains the large extracellular loop, the fourth transmembrane region, and the last short cytoplasmic domain.

Inhibition of RhoA translocation and calcium sensitization by in vivo ADP-ribosylation with the chimeric toxin DC3B

Pretreatment of intact rabbit portal vein smooth muscle with the chimeric toxin DC3B (10(-6) M, 48 h; ; ) ADP-ribosylated endogenous RhoA, including cytosolic RhoA complexed with rhoGDI, and inhibited the tonic phase of phenylephrine-induced contraction and the Ca2+-sensitization of force by phenylephrine, endothelin and guanosine triphosphate (GTP)gammaS, but did not inhibit Ca2+-sensitization by phorbol dibutyrate. DC3B also inhibited GTPgammaS-induced translocation of cytosolic RhoA () to the membrane fraction. In DC3B-treated muscles the small fraction of membrane-associated RhoA could be immunoprecipitated, even after exposure to GTPgammaS, which prevents immunoprecipitation of non-ADP-ribosylated RhoA. Dissociation of cytosolic RhoA-rhoGDI complexes with SDS restored the immunoprecipitability and ADP ribosylatability of RhoA, indicating that both the ADP-ribosylation site (Asn 41) and RhoA insert loop (Wei et al., 1997) are masked by rhoGDI and that the long axes of the two proteins are in parallel in the heterodimer. We conclude that RhoA plays a significant role in G-protein-, but not protein kinase C-mediated, Ca2+ sensitization and that ADP ribosylation inhibits in vivo the Ca2+-sensitizing effect of RhoA by interfering with its binding to a membrane-associated effector.

Clostridium perfringens epsilon-toxin acts on MDCK cells by forming a large membrane complex

Epsilon-toxin is produced by Clostridium perfringens types B and D and is responsible for a rapidly fatal enterotoxemia in animals, which is characterized by edema in several organs due to an increase in blood vessel permeability. The Madin-Darby canine kidney (MDCK) cell line has been found to be susceptible to epsilon-toxin (D. W. Payne, E. D. Williamson, H. Havard, N. Modi, and J. Brown, FEMS Microbiol. Lett. 116:161-168, 1994). Here we present evidence that epsilon-toxin cytotoxic activity is correlated with the formation of a large membrane complex (about 155 kDa) and efflux of intracellular K+ without entry of the toxin into the cytosol. Epsilon-toxin induced swelling, blebbing, and lysis of MDCK cells. Iodolabeled epsilon-toxin bound specifically to MDCK cell membranes at 4 and 37 labeled C and was associated with a large complex (about 155 kDa). The binding of epsilon-toxin to the cell surface was corroborated by immunofluorescence staining. The complex formed at 37 degrees C was more stable than that formed at 4 degrees C, since it was not dissociated by 5% sodium dodecyl sulfate and boiling.

Dissection of pathways implicated in integrin-mediated actin cytoskeleton assembly. Involvement of protein kinase C, Rho GTPase, and tyrosine phosphorylation

A panel of antibodies to the alphaIIbbeta3 integrin was used to promote adhesion of Chinese hamster ovary cells transfected with the alphaIIbbeta3 fibrinogen receptor. While some alphaIIbbeta3 antibodies were not able to induce p125 focal adhesion kinase (p125FAK) tyrosine phosphorylation, all the antibodies equally support cell adhesion but not spreading and assembly of actin stress fibers. Absence of stress fibers was also obtained by plating on antibodies directed to the hamster beta1 integrin. In contrast, cells plated on matrix proteins spread organizing actin stress fibers. Treatment with phorbol esters phorbol 12-myristate 13-acetate (PMA) induced cells to spread on antibodies-coated dishes but not to organize actin in stress fibers. The combination of PMA and cytotoxic necrotizing factor 1 (CNF1), a specific Rho activator, induced cell spreading and organization of stress fibers. PMA or the combination of PMA and CNF1 also increases tyrosine phosphorylation of p125FAK in response to antibodies that were otherwise unable to trigger this response. These data show that: 1) matrix proteins and antibodies differ in their ability to induce integrin-dependent actin cytoskeleton organization (while matrix induced stress fibers formation, antibodies did not); 2) p125FAK tyrosine phosphorylation is insufficient per se to trigger actin stress fibers formation since antibodies that activate p125FAK tyrosine phosphorylation did not lead to actin stress fibers assembly; and 3) the inability of anti-integrin antibodies to trigger stress fibers organization is overcome by concomitant activation of the protein kinase C (PKC) and Rho pathways; PKC activation leads to cell spreading and Rho activation is required to organize actin stress fibers.

Escherichia coli cytotoxic necrotizing factor 1 (CNF1), a toxin that activates the Rho GTPase

Cytotoxic necrotizing factor 1 (CNF1), a 110-kDa protein toxin from pathogenic Escherichia coli induces actin reorganization into stress fibers and retraction fibers in human epithelial cultured cells allowing them to spread. CNF1 is acting in the cytosol since microinjection of the toxin into HEp-2 cells mimics the effects of the externally applied CNF1. Incubation in vitro of CNF1 with recombinant small GTPases induces a modification of Rho (but not of Rac, Cdc42, Ras, or Rab6) as demonstrated by a discrete increase in the apparent molecular weight of the molecule. Preincubation of cells with CNF1 impairs the cytotoxic effects of Clostridium difficile toxin B, which inactivates Rho but not those of Clostridium sordellii LT toxin, which inhibits Ras and Rac. As shown for Rho-GTP, CNF1 activates, in a time- and dose-dependent manner, a cytoskeleton-associated phosphatidylinositol 4-phosphate 5-kinase. However, neither the phosphatidylinositol 4,5-bisphosphate (PIP2) nor the phosphatidylinositol 3,4-bisphosphate (PI 3,4-P2) or 3,4,5-trisphosphate (PIP3) cellular content were found increased in CNF1 treated HEp-2 cells. Cellular effects of CNF1 were not blocked by LY294002, a stable inhibitor of the phosphoinositide 3-kinase. Incubation of HEp-2 cells with CNF1 induces relocalization of myosin 2 in stress fibers but not in retraction fibers. Altogether, our data indicate that CNF1 is a toxin that selectively activates the Rho GTP-binding protein, thus inducing contractility and cell spreading.

Toxin-induced activation of the G protein p21 Rho by deamidation of glutamine

Pathogenic Escherichia coli are responsible for a variety of diseases, including diarrhoea, haemolytic uraemic syndrome, kidney infection, septicaemia, pneumonia and meningitis. Toxins called cytotoxic necrotizing factors (CNFs) are among the virulence factors produced by uropathogenic (CNF1) or enteropathogenic (CNF2) E. coli strains that cause diseases in humans and animals, respectively. CNFs induce an increase in the content of actin stress fibres and focal contacts in cultured cells. Effects of CNFs on the actin cytoskeleton correlated with a decrease in the electrophoretic mobility of the GTP-binding protein Rho and indirect evidence indicates that CNF1 might constitutively activate Rho. Here we show that CNF1 catalyses the deamidation of a glutamine residue at position 63 of Rho, turning it into glutamic acid, which inhibits both intrinsic GTP hydrolysis and that stimulated by its GTPase-activating protein (GAP). Thus, this deamidation of glutamine 63 by CNF1 leads to the constitutive activation of Rho, and induces the reorganization of actin stress fibres. To our knowledge, CNF1 is the first example of a bacterial toxin acting by deamidation of a specific target protein.