Differential sensitivity of human epithelial cells to Pseudomonas aeruginosa exoenzyme S

Evidence for intracellular Pseudomonas aeruginosa

Pseudomonas aeruginosa is a significant cause of global morbidity and mortality. Although it is often regarded as an extracellular pathogen toward human cells, numerous investigations report its ability to survive and replicate within host cells, and additional studies demonstrate specific mechanisms enabling it to adopt an intracellular lifestyle. This ability of P. aeruginosa remains less well-investigated than that of other intracellular bacteria, although it is currently gaining attention. If intracellular bacteria are not killed after entering host cells, they may instead receive protection from immune recognition and experience reduced exposure to antibiotic therapy, among additional potential advantages shared with other facultative intracellular pathogens. For this review, we compiled studies that observe intracellular P. aeruginosa across strains, cell types, and experimental systems in vitro, as well as contextualize these findings with the few studies that report similar observations in vivo. We also seek to address key findings that drove the perception that P. aeruginosa remains extracellular in order to reconcile what is currently understood about intracellular pathogenesis and highlight open questions regarding its contribution to disease.

Exotoxin S secreted by internalized Pseudomonas aeruginosa delays lytic host cell death

The Pseudomonas aeruginosa toxin ExoS, secreted by the type III secretion system (T3SS), supports intracellular persistence via its ADP-ribosyltransferase (ADPr) activity. For epithelial cells, this involves inhibiting vacuole acidification, promoting vacuolar escape, countering autophagy, and niche construction in the cytoplasm and within plasma membrane blebs. Paradoxically, ExoS and other P. aeruginosa T3SS effectors can also have antiphagocytic and cytotoxic activities. Here, we sought to reconcile these apparently contradictory activities of ExoS by studying the relationships between intracellular persistence and host epithelial cell death. Methods involved quantitative imaging and the use of antibiotics that vary in host cell membrane permeability to selectively kill intracellular and extracellular populations after invasion. Results showed that intracellular P. aeruginosa mutants lacking T3SS effector toxins could kill (permeabilize) cells when extracellular bacteria were eliminated. Surprisingly, wild-type strain PAO1 (encoding ExoS, ExoT and ExoY) caused cell death more slowly, the time extended from 5.2 to 9.5 h for corneal epithelial cells and from 10.2 to 13.0 h for HeLa cells. Use of specific mutants/complementation and controls for initial invasion showed that ExoS ADPr activity delayed cell death. Triggering T3SS expression only after bacteria invaded cells using rhamnose-induction in T3SS mutants rescued the ExoS-dependent intracellular phenotype, showing that injected effectors from extracellular bacteria were not required. The ADPr activity of ExoS was further found to support internalization by countering the antiphagocytic activity of both the ExoS and ExoT RhoGAP domains. Together, these results show two additional roles for ExoS ADPr activity in supporting the intracellular lifestyle of P. aeruginosa; suppression of host cell death to preserve a replicative niche and inhibition of T3SS effector antiphagocytic activities to allow invasion. These findings add to the growing body of evidence that ExoS-encoding (invasive) P. aeruginosa strains can be facultative intracellular pathogens, and that intracellularly secreted T3SS effectors contribute to pathogenesis.

While the ADPr domain of the T3SS effector ExoS plays multiple roles in the intracellular lifestyle of P. aeruginosa, ExoS can also be cytotoxic and/or antiphagocytic. Here, we show that when P. aeruginosa enters the cytosol of epithelial cells, cell death is triggered independently of T3SS effector toxins, but ExoS ADPr activity delays this to enable continued intracellular survival and replication. Using rhamnose induction to express the T3SS only after invasion restored this ExoS-dependent phenotype, showing that intracellularly secreted effectors can enable intracellular pathogenesis. ExoS ADPr activity also countered antiphagocytic activity of ExoS and ExoT RhoGAP domains. These results show two additional roles for ExoS ADPr activity in promoting internalization of P. aeruginosa and protecting the intracellular niche, continuing to challenge the notions that P. aeruginosa is exclusively an extracellular pathogen, that it needs to inject T3SS effectors across plasma membranes, and that ExoS is necessarily cytotoxic to host cells.

Metastatic MTLn3 and non-metastatic MTC adenocarcinoma cells can be differentiated by Pseudomonas aeruginosa

Cancer patients are known to be highly susceptible to Pseudomonas aeruginosa (Pa) infection, but it remains unknown whether alterations at the tumor cell level can contribute to infection. This study explored how cellular changes associated with tumor metastasis influence Pa infection using highly metastatic MTLn3 cells and non-metastatic MTC cells as cell culture models. MTLn3 cells were found to be more sensitive to Pa infection than MTC cells based on increased translocation of the type III secretion effector, ExoS, into MTLn3 cells. Subsequent studies found that higher levels of ExoS translocation into MTLn3 cells related to Pa entry and secretion of ExoS within MTLn3 cells, rather than conventional ExoS translocation by external Pa. ExoS includes both Rho GTPase activating protein (GAP) and ADP-ribosyltransferase (ADPRT) enzyme activities, and differences in MTLn3 and MTC cell responsiveness to ExoS were found to relate to the targeting of ExoS-GAP activity to Rho GTPases. MTLn3 cell migration is mediated by RhoA activation at the leading edge, and inhibition of RhoA activity decreased ExoS translocation into MTLn3 cells to levels similar to those of MTC cells. The ability of Pa to be internalized and transfer ExoS more efficiently in association with Rho activation during tumor metastasis confirms that alterations in cell migration that occur in conjunction with tumor metastasis contribute to Pa infection in cancer patients. This study also raises the possibility that Pa might serve as a biological tool for dissecting or detecting cellular alterations associated with tumor metastasis.

Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria

Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.

Domain analyses reveal that Chlamydia trachomatis CT694 protein belongs to the membrane-localized family of type III effector proteins

The Chlamydia trachomatis type three-secreted effector protein CT694 is expressed during late-cycle development yet is secreted by infectious particles during the invasion process. We have previously described the presence of at least two functional domains within CT694. CT694 was found to interact with the human protein Ahnak through a C-terminal domain and affect formation of host-cell actin stress fibers. Immunolocalization analyses of ectopically expressed pEGFP-CT694 also revealed plasma membrane localization for CT694 that was independent of Ahnak binding. Here we provide evidence that CT694 contains multiple functional domains. Plasma membrane localization and CT694-induced alterations in host cell morphology are dependent on an N-terminal domain. We demonstrate that membrane association of CT694 is dependent on a domain resembling a membrane localization domain (MLD) found in anti-host proteins from Yersinia, Pseudomonas, and Salmonella spp. This domain is necessary and sufficient for localization and morphology changes but is not required for Ahnak binding. Further, the CT694 MLD is able to complement ExoS ΔMLD when ectopically expressed. Taken together, our data indicate that CT694 is a multidomain protein with the potential to modulate multiple host cell processes.

Examining the role of actin-plasma membrane association in Pseudomonas aeruginosa infection and type III secretion translocation in migratory T24 epithelial cells

The opportunistic pathogen Pseudomonas aeruginosa targets wounded epithelial barriers, but the cellular alteration that increases susceptibility to P. aeruginosa infection remains unclear. This study examined how cell migration contributes to the establishment of P. aeruginosa infections using (i) highly migratory T24 epithelial cells as a cell culture model, (ii) mutations in the type III secretion (T3S) effector ExoS to manipulate P. aeruginosa infection, and (iii) high-resolution immunofluorescent microscopy to monitor ExoS translocation. ExoS includes both GTPase-activating (GAP) and ADP-ribosyltransferase (ADPRT) activities, and P. aeruginosa cells expressing wild-type ExoS preferentially bound to the leading edge of T24 cells, where ExoS altered leading-edge architecture and actin anchoring in conjunction with interrupting T3S translocation. Inactivation of ExoS GAP activity allowed P. aeruginosa to be internalized and secrete ExoS within T24 cells, but as with wild-type ExoS, translocation was limited in association with disruption of actin anchoring. Inactivation of ExoS ADPRT activity resulted in significantly enhanced T3S translocation by P. aeruginosa cells that remained extracellular and in conjunction with maintenance of actin-plasma membrane association. Infection with P. aeruginosa expressing ExoS lacking both GAP and ADPRT activities resulted in the highest level of T3S translocation, and this occurred in conjunction with the entry and alignment of P. aeruginosa and ExoS along actin filaments. Collectively, in using ExoS mutants to modulate and visualize T3S translocation, we were able to (i) confirm effector secretion by internalized P. aeruginosa, (ii) differentiate the mechanisms underlying the effects of ExoS GAP and ADPRT activities on P. aeruginosa internalization and T3S translocation, (iii) confirm that ExoS ADPRT activity targeted a cellular substrate that interrupted T3S translocation, (iv) visualize the ability of P. aeruginosa and ExoS to align with actin filaments, and (v) demonstrate an association between actin anchoring at the leading edge of T24 cells and the establishment of P. aeruginosa infection. Our studies also highlight the contribution of ExoS to the opportunistic nature of P. aeruginosa infection through its ability to exert cytotoxic effects that interrupt T3S translocation and P. aeruginosa internalization, which in turn limit the P. aeruginosa infectious process.

Injection of Pseudomonas aeruginosa Exo toxins into host cells can be modulated by host factors at the level of translocon assembly and/or activity

Pseudomonas aeruginosa type III secretion apparatus exports and translocates four exotoxins into the cytoplasm of the host cell. The translocation requires two hydrophobic bacterial proteins, PopB and PopD, that are found associated with host cell membranes following infection. In this work we examined the influence of host cell elements on exotoxin translocation efficiency. We developed a quantitative flow cytometry based assay of translocation that used protein fusions between either ExoS or ExoY and the ß-lactamase reporter enzyme. In parallel, association of translocon proteins with host plasma membranes was evaluated by immunodetection of PopB/D following sucrose gradient fractionation of membranes. A pro-myelocytic cell line (HL-60) and a pro-monocytic cell line (U937) were found resistant to toxin injection even though PopB/D associated with host cell plasma membranes. Differentiation of these cells to either macrophage- or neutrophil-like cell lines resulted in injection-sensitive phenotype without significantly changing the level of membrane-inserted translocon proteins. As previous in vitro studies have indicated that the lysis of liposomes by PopB and PopD requires both cholesterol and phosphatidyl-serine, we first examined the role of cholesterol in translocation efficiency. Treatment of sensitive HL-60 cells with methyl-ß-cyclodextrine, a cholesterol-depleting agent, resulted in a diminished injection of ExoS-Bla. Moreover, the PopB translocator was found in the membrane fraction, obtained from sucrose-gradient purifications, containing the lipid-raft marker flotillin. Examination of components of signalling pathways influencing the toxin injection was further assayed through a pharmacological approach. A systematic detection of translocon proteins within host membranes showed that, in addition to membrane composition, some general signalling pathways involved in actin polymerization may be critical for the formation of a functional pore. In conclusion, we provide new insights in regulation of translocation process and suggest possible cross-talks between eukaryotic cell and the pathogen at the level of exotoxin translocation.

Role of host cell polarity and leading edge properties in Pseudomonas type III secretion

Type III secretion (T3S) functions in establishing infections in a large number of Gram-negative bacteria, yet little is known about how host cell properties might function in this process. We used the opportunistic pathogen Pseudomonas aeruginosa and the ability to alter host cell sensitivity to Pseudomonas T3S to explore this problem. HT-29 epithelial cells were used to study cellular changes associated with loss of T3S sensitivity, which could be induced by treatment with methyl-beta-cyclodextrin or perfringolysin O. HL-60 promyelocytic cells are innately resistant to Pseudomonas T3S and were used to study cellular changes occurring in response to induction of T3S sensitivity, which occurred following treatment with phorbol esters. Using both cell models, a positive correlation was observed between eukaryotic cell adherence to tissue culture wells and T3S sensitivity. In examining the type of adhesion process linked to T3S sensitivity in HT-29 cells, a hierarchical order of protein involvement was identified that paralleled the architecture of leading edge (LE) focal complexes. Conversely, in HL-60 cells, induction of T3S sensitivity coincided with the onset of LE properties and the development of actin-rich projections associated with polarized cell migration. When LE architecture was examined by immunofluorescent staining for actin, Rac1, IQ-motif-containing GTPase-activating protein 1 (IQGAP1) and phosphatidylinositol 3 kinase (PI3 kinase), intact LE structure was found to closely correlate with host cell sensitivity to P. aeruginosa T3S. Our model for host cell involvement in Pseudomonas T3S proposes that cortical actin polymerization at the LE alters membrane properties to favour T3S translocon function and the establishment of infections, which is consistent with Pseudomonas infections targeting wounded epithelial barriers undergoing cell migration.

ExoS controls the cell contact-mediated switch to effector secretion in Pseudomonas aeruginosa

Type III secretion is used by many gram-negative bacterial pathogens to directly deliver protein toxins (effectors) into targeted host cells. In all cases, secretion of effectors is triggered by host cell contact, although the mechanism is unclear. In Pseudomonas aeruginosa, expression of all type III secretion-related genes is up-regulated when secretion is triggered. We were able to visualize this process using a green fluorescent protein reporter system and to use it to monitor the ability of bacteria to trigger effector secretion on cell contact. Surprisingly, the action of one of the major type III secreted effectors, ExoS, prevented triggering of type III secretion by bacteria that subsequently attached to cells, suggesting that triggering of secretion is feedback regulated. Evidence is presented that translocation (secretion of effectors across the host cell plasma membrane) of ExoS is indeed self-regulated and that this inhibition of translocation can be achieved by either of its two enzymatic activities. The translocator proteins PopB, PopD, and PcrV are secreted via the type III secretion system and are required for pore formation and translocation of effectors across the host cell plasma membrane. Here we present data that secretion of translocators is in fact not controlled by calcium, implying that triggering of effector secretion on cell contact represents a switch in secretion specificity, rather than a triggering of secretion per se. The requirement for a host cell cofactor to control effector secretion may help explain the recently observed phenomenon of target cell specificity in both the Yersinia and P. aeruginosa type III secretion systems.

A genotypic and phenotypic comparison of type III secretion profiles of Pseudomonas aeruginosa cystic fibrosis and bacteremia isolates

The type III secretion system (TTSS) of Pseudomonas aeruginosa enables delivery of a number of toxins involved in the disruption of eukaryotic epithelial surfaces. Whilst the ability to secrete ExoS facilitates invasion and internalization, the secretion of ExoU mediates acute cytotoxicity. In order to determine any association with the ability to secrete these toxins with the nature and severity of human infection, the TTSS genotypes and phenotypes of 163 clinical isolates were determined by multiplex PCR and Western blotting. An exoS+/exoU- genotype was associated with chronic infection in patients with cystic fibrosis whilst an exoS-/exoU+ genotype was associated with strains isolated from blood. Secretion of the ExoU protein was more commonly seen in isolates obtained from blood, suggesting this ability may be important in the development of acute invasive infection. Detection of TTSS toxins in clinical material may be useful in targeting antimicrobial therapy or identifying individuals infected with aggressive strains of P. aeruginosa.

Pseudomonas aeruginosa infection of airway epithelial cells modulates expression of Kruppel-like factors 2 and 6 via RsmA-mediated regulation of type III exoenzymes S and Y

Pseudomonas aeruginosa is an important opportunistic pathogen which is capable of causing both acute and chronic infections in immunocompromised patients. Successful adaptation of the bacterium to its host environment relies on the ability of the organism to tightly regulate gene expression. RsmA, a small RNA-binding protein, controls the expression of a large number of virulence-related genes in P. aeruginosa, including those encoding the type III secretion system and associated effector proteins, with important consequences for epithelial cell morphology and cytotoxicity. In order to examine the influence of RsmA-regulated functions in the pathogen on gene expression in the host, we compared global expression profiles of airway epithelial cells in response to infection with P. aeruginosa PAO1 and an rsmA mutant. The RsmA-dependent response of host cells was characterized by significant changes in the global transcriptional pattern, including the increased expression of two Kruppel-like factors, KLF2 and KLF6. This increased expression was mediated by specific type III effector proteins. ExoS was required for the enhanced expression of KLF2, whereas both ExoS and ExoY were required for the enhanced expression of KLF6. Neither ExoT nor ExoU influenced the expression of the transcription factors. Additionally, the increased gene expression of KLF2 and KLF6 was associated with ExoS-mediated cytotoxicity. Therefore, this study identifies for the first time the human transcription factors KLF2 and KLF6 as targets of the P. aeruginosa type III exoenzymes S and Y, with potential importance in host cell death.

Interactions between effector proteins of the Pseudomonas aeruginosa type III secretion system do not significantly affect several measures of disease severity in mammals

The effector proteins of the type III secretion systems of many bacterial pathogens act in a coordinated manner to subvert host cells and facilitate the development and progression of disease. It is unclear whether interactions between the type-III-secreted proteins of Pseudomonas aeruginosa result in similar effects on the disease process. We have previously characterized the contributions to pathogenesis of the type-III-secreted proteins ExoS, ExoT and ExoU when secreted individually. In this study, we extend our prior work to determine whether these proteins have greater than expected effects on virulence when secreted in combination. In vitro cytotoxicity and anti-internalization activities were not enhanced when effector proteins were secreted in combinations rather than alone. Likewise in a mouse model of pneumonia, bacterial burden in the lungs, dissemination and mortality attributable to ExoS, ExoT and ExoU were not synergistically increased when combinations of these effector proteins were secreted. Because of the absence of an appreciable synergistic increase in virulence when multiple effector proteins were secreted in combination, we conclude that any cooperation between ExoS, ExoT and ExoU does not translate into a synergistically significant enhancement of disease severity as measured by these assays.

Characterization of an ExoS Type III translocation-resistant cell line

Pseudomonas aeruginosa ExoS is a type III-secreted type III-secreted, bifunctional protein that causes diverse effects on eukaryotic cell function. The coculture of P. aeruginosa strains expressing ExoS with HL-60 myeloid cells revealed the cell line to be resistant to the toxic effects of ExoS. Differentiation of HL-60 cells with phorbol 12-myristate 13-acetate (TPA) rendered the cell line sensitive to ExoS. To understand the cellular basis for the alteration in sensitivity, undifferentiated and TPA-differentiated HL-60 cells were compared for differences in bacterial adherence, type III secretion induction, and ExoS translocation. These comparisons found that ExoS was translocated more efficiently in TPA-differentiated HL-60 cells than in undifferentiated cells. The studies support the ability of eukaryotic cells to influence P. aeruginosa TTS at the level of membrane translocation.

Relative contributions of Pseudomonas aeruginosa ExoU, ExoS, and ExoT to virulence in the lung

Pseudomonas aeruginosa uses a type III secretion system to promote development of severe disease, particularly in patients with impaired immune defenses. While the biochemical and enzymatic functions of ExoU, ExoS, and ExoT, three effector proteins secreted by this system, are well defined, the relative roles of each protein in the pathogenesis of acute infections is not clearly understood. Since ExoU and ExoS are usually not secreted by the same strain, it has been difficult to directly compare the effects of these proteins during infection. In the work described here, several isogenic mutants of a bacterial strain that naturally secretes ExoU, ExoS, and ExoT were generated to carefully evaluate the relative contribution of each effector protein to pathogenesis in a mouse model of acute pneumonia. Measurements of mortality, bacterial persistence in the lung, and dissemination indicated that secretion of ExoU had the greatest impact on virulence while secretion of ExoS had an intermediate effect and ExoT had a minor effect. It is of note that these results conclusively show for the first time that ExoS is a virulence factor. Infection with isogenic mutants secreting wild-type ExoS, ExoS defective in GTPase-activating protein (GAP) activity, or ExoS defective in ADP-ribosyltransferase activity demonstrated that the virulence of ExoS was largely dependent on its ADP-ribosyltransferase activity. The GAP activity of this protein had only a minor effect in vivo. The relative virulence associated with each of these type III effector proteins may have important prognostic implications for patients infected with P. aeruginosa.

Cell line differences in bacterially translocated ExoS ADP-ribosyltransferase substrate specificity

Exoenzyme S (ExoS) is an ADP-ribosyltransferase (ADPRT) directly translocated into eukaryotic cells by the type III secretory (TTS) process of Pseudomonas aeruginosa. Comparisons of the functional effects of ExoS on human epithelial and murine fibroblastic cells showed that human epithelial cells exhibited an overall increased sensitivity to the effects of bacterially translocated ExoS on cell proliferation, morphology and re-adherence. ExoS was also found to ADP-ribosylate a greater number of low-molecular-mass G (LMMG) proteins in human epithelial cells, as compared to murine fibroblasts. Examination of the cellular mechanism for differences in ExoS ADPRT substrate modification found that the more restricted pattern of substrate modification in murine fibroblasts was not linked to the efficiency of bacterial adherence nor to the efficiency of ExoS internalization by the TTS process. In exploring the cellular nature of patterns of substrate modification, more extensive substrate modification was detected in human and simian cell lines, while rodent cell lines, including rat, mouse and hamster lines, consistently exhibited the more limited pattern of LMMG protein ADP-ribosylation. Patterns of substrate modification were not altered by cellular transformation and occurred independently of cell type. These studies suggest that eukaryotic cell properties, as recognized through studies of cells of different animal origins, affect the substrate targeting of ExoS ADPRT activity, and that this in turn can influence the severity of effects of ExoS on host-cell function.

Intracellular localization modulates targeting of ExoS, a type III cytotoxin, to eukaryotic signalling proteins

ExoS is a bifunctional type III cytotoxin produced by Pseudomonas aeruginosa. Residues 96-232 comprise the Rho GTPase activating protein (Rho GAP) domain, whereas residues 233-453 comprise the 14-3-3-dependent ADP-ribosyltransferase domain. Earlier studies showed that the N-terminus targeted ExoS to intracellular membranes within eukaryotic cells. This N-terminal targeting region is now characterized for cellular and biological contributions to intoxications by ExoS. An ExoS(1-107)-green fluorescent protein (GFP) fusion protein co-localized with alpha-mannosidase, which indicated that the fusion protein localized near the Golgi. Residues 51-72 of ExoS (termed the membrane localization domain, MLD) were necessary and sufficient for membrane localization within eukaryotic cells. Deletion of the MLD did not inhibit type III secretion of ExoS from P. aeruginosa or type III delivery of ExoS into eukaryotic cells. Type III-delivered ExoS(DeltaMLD) localized within the cytosol of eukaryotic cells, whereas type III-delivered ExoS was membrane associated. Although type III-delivered ExoS(DeltaMLD) stimulated the reorganization of the actin cytoskeleton (a Rho GAP activity), it did not ADP-ribosylate Ras. Type III-delivered ExoS(DeltaMLD) and ExoS showed similar capacities for eliciting a cytotoxic response in CHO cells, which uncoupled the ADP-ribosylation of Ras from the cytotoxicity elicited by ExoS.

Exoenzyme S shows selective ADP-ribosylation and GTPase-activating protein (GAP) activities towards small GTPases in vivo

Intracellular targeting of the Pseudomonas aeruginosa toxins exoenzyme S (ExoS) and exoenzyme T (ExoT) initially results in disruption of the actin microfilament structure of eukaryotic cells. ExoS and ExoT are bifunctional cytotoxins, with N-terminal GTPase-activating protein (GAP) and C-terminal ADP-ribosyltransferase activities. We show that ExoS can modify multiple GTPases of the Ras superfamily in vivo. In contrast, ExoT shows no ADP-ribosylation activity towards any of the GTPases tested in vivo. We further examined ExoS targets in vivo and observed that ExoS modulates the activity of several of these small GTP-binding proteins, such as Ras, Rap1, Rap2, Ral, Rac1, RhoA and Cdc42. We suggest that ExoS is the major ADP-ribosyltransferase protein modulating small GTPase function encoded by P. aeruginosa. Furthermore, we show that the GAP activity of ExoS abrogates the activation of RhoA, Cdc42 and Rap1.

Independent and coordinate effects of ADP-ribosyltransferase and GTPase-activating activities of exoenzyme S on HT-29 epithelial cell function

Type III-mediated translocation of exoenzyme S (ExoS) into HT-29 epithelial cells by Pseudomonas aeruginosa causes complex alterations in cell function, including inhibition of DNA synthesis, altered cytoskeletal structure, loss of readherence, microvillus effacement, and interruption of signal transduction. ExoS is a bifunctional protein having both GTPase-activating (GAP) and ADP-ribosyltransferase (ADPRT) functional domains. Comparisons of alterations in HT-29 cell function caused by P. aeruginosa strains that translocate ExoS having GAP or ADPRT mutations allowed the independent and coordinate functions of the two activities to be assessed. An E381A ADPRT mutation revealed that ExoS ADPRT activity was required for effects of ExoS on DNA synthesis and long-term cell rounding. Conversely, the R146A GAP mutation appeared to have little impact on the cellular effects of ExoS. While transient cell rounding was detected following exposure to the E381A mutant, this rounding was eliminated by an E379A-E381A ADPRT double mutation, implying that residual ADPRT activity, rather than GAP activity, was effecting transient cell rounding by the E381A mutant. To explore this possibility, E381A and R146A-E381A mutants were examined for their ability to ADP-ribosylate Ras in vitro or in vivo. While no ADP-ribosylation of Ras was detected by either mutant in vitro, both mutants were able to modify Ras when translocated by the bacteria, with the R146A-E381A mutant causing more efficient modification than the E381A mutant, in association with increased inhibition of DNA synthesis. Comparisons of Ras ADP-ribosylation by wild-type and E381A mutant ExoS by two-dimensional electrophoresis found the former to ADP-ribosylate Ras at two sites, while the latter modified Ras only once. These studies draw attention to the key role of ExoS ADPRT activity in causing the effects of bacterially translocated ExoS on DNA synthesis and cell rounding. In addition, the studies provide insight into the enhancement of ExoS ADPRT activity within the eukaryotic cell microenvironment and into possible modulatory roles that the GAP and ADPRT domains might have on the function of each other.

ADP-ribosylation of Rab5 by ExoS of Pseudomonas aeruginosa affects endocytosis

Pseudomonas aeruginosa exoenzyme S (ExoS) is an ADP-ribosyltransferase that modifies low-molecular-weight GTPases. Here we studied the effect of Rab5 ADP-ribosylation by ExoS on its cellular function, i.e., regulation of early endocytic events. Coculture of CHO cells with P. aeruginosa induced a marked decrease in horseradish peroxidase (HRP) uptake compared to noninfected cells, while coculture with a P. aeruginosa mutant strain that fails to produce ExoS did not lead to any change in HRP uptake. Microinjection of recombinant ExoS into Xenopus oocytes induced strong inhibition of basal HRP uptake by oocytes. Moreover, coinjection of recombinant ExoS with Rab5 abolished the typical stimulation of HRP uptake obtained after GTPase microinjection. Cytosols prepared from injected oocytes were used in an endosome-endosome fusion assay. Cytosol from ExoS-microinjected oocytes was ineffective in promoting endosome-endosome fusion. However, in these conditions, the addition of Rab5 to the assay led to fusion recovery. Finally, we found that the interaction of Rab5 with EEA1 was markedly diminished after Rab5 ADP-ribosylation by ExoS.

Comparison of the exoS gene and protein expression in soil and clinical isolates of Pseudomonas aeruginosa

Exoenzyme S (ExoS) is translocated into eukaryotic cells by the type III secretory process and has been hypothesized to function in conjunction with other virulence factors in the pathogenesis of Pseudomonas aeruginosa. To gain further understanding of how ExoS might contribute to P. aeruginosa survival and virulence, ExoS expression and the structural gene sequence were determined in P. aeruginosa soil isolates and compared with ExoS of clinical isolates. Significantly higher levels of ExoS ADP-ribosyltransferase (ADPRT) activity were detected in culture supernatants of soil isolates compared to those of clinical isolates. The higher levels of ADPRT activity of soil isolates reflected both the increased production of ExoS and the production of ExoS having a higher specific activity. ExoS structural gene sequence comparisons found the gene to be highly conserved among soil and clinical isolates, with the greatest number of nonsynonymous substitutions occurring within the region of ExoS encoding GAP function. The lack of amino acid changes in the ADPRT region in association with a higher specific activity implies that other factors produced by P. aeruginosa or residues outside the ADPRT region are affecting ExoS ADPRT activity. The data are consistent with ExoS being integral to P. aeruginosa survival in the soil and suggest that, in the transition of P. aeruginosa from the soil to certain clinical settings, the loss of ExoS expression is favored.

Triggering the ExoS regulon of Pseudomonas aeruginosa: A GFP-reporter analysis of exoenzyme (Exo) S, ExoT and ExoU synthesis

The ExoS regulon of Pseudomonas aeruginosa encodes diverse type III secreted effector proteins which have been shown to exert cytotoxic effects in cell culture experiments. However, little information exists about the environmental conditions and stimuli for upregulation of the ExoS regulon. Translational reporter fusion proteins of exoenzyme (Exo) S, ExoT and ExoU, as well as the type II secreted exotoxin A (ETA) to the green fluorescent protein (GFP), were constructed in order to compare exoprotein production under diverse growth conditions. Reporter protein activity was recorded by FACS-analysis and by conventional and confocal laser scanning microscopy. Low ion concentration induced co-ordinated upregulation of ExoS, ExoT and ExoU with a maximum effect at 37 degrees C. A dose-dependent upregulation was seen with human serum or increasing NaCl concentrations. A type III secretion-negative pcrD mutant of P. aeruginosa showed a weak ExoS response to environmental stimuli, compared with the parental strain, suggesting a negative regulatory mechanism. Co-culture with the mammalian cell lines J774A.1 or HeLa led to rapid upregulation of ExoS, ExoT and ExoU synthesis. These data suggest that the ExoS regulon of P. aeruginosa can be triggered by a variety of environmental signals as well as by cell contact with eukaryotic cells.

Pseudomonas aeruginosa mediated apoptosis requires the ADP-ribosylating activity of exoS

Pseudomonas aeruginosa is an opportunistic bacterial pathogen that primarily infects immunocompromised individuals and patients with cystic fibrosis. Using a tissue culture system, invasive strains of P. aeruginosa were discovered to induce apoptosis at high frequency in HeLa and other epithelial and fibroblast cell lines. This apoptotic phenotype in the infected cells was determined by several criteria including (i) visual changes in cell morphology, (ii) induction of chromatin condensation and nuclear marginalization, (iii) the presence of a high percentage of cells with subG1 DNA content, and (iv) activation of caspase-3 activity. Induction of the type III secretion machinery, but not invasion of P. aeruginosa is required for induction of apoptosis. The apoptosis phenotype is independent of the cytoskeletal rearrangements that occur in the host cell early after infection. Mutants in P. aeruginosa exoS fail to induce apoptosis and complementation with wild-type exoS restored the apoptosis-inducing capacity, demonstrating that ExoS is the effector molecule. Analysis of exoS activity mutants shows that the ADP-ribosylating capacity of ExoS is essential for inducing the apoptotic pathway.

Pseudomonas aeruginosa cystic fibrosis isolates induce rapid, type III secretion-dependent, but ExoU-independent, oncosis of macrophages and polymorphonuclear neutrophils

Pseudomonas aeruginosa, an opportunistic pathogen responsible most notably for severe infections in cystic fibrosis (CF) patients, utilizes the type III secretion system for eukaryotic cell intoxication. The CF clinical isolate CHA shows toxicity towards human polymorphonuclear neutrophils (PMNs) which is dependent on the type III secretion system but independent of the cytotoxin ExoU. In the present study, the cytotoxicity of this strain toward human and murine macrophages was demonstrated. In low-multiplicity infections (multiplicity of infection, 10), approximately 40% of the cells die within 60 min. Analysis of CHA-infected cells by transmission electron microscopy, DNA fragmentation assay, and Hoechst staining revealed the hallmarks of oncosis: cellular and nuclear swelling, disintegration of the plasma membrane, and absence of DNA fragmentation. A panel of 29 P. aeruginosa CF isolates was screened for type III system genotype, protein secretion profile, and cytotoxicity toward PMNs and macrophages. This study showed that six CF isolates were able to induce rapid ExoU-independent oncosis on phagocyte cells.