Effects of differential expression of the 49-kilodalton exoenzyme S by Pseudomonas aeruginosa on cultured eukaryotic cells

The ADP-ribosyltransferase domain of the effector protein ExoS inhibits phagocytosis of Pseudomonas aeruginosa during pneumonia

Pseudomonas aeruginosa is a Gram-negative pathogen commonly associated with nosocomial infections such as hospital-acquired pneumonia. It uses a type III secretion system to deliver effector proteins directly into the cytosol of host cells. Type III secretion in P. aeruginosa has been linked to severe disease and worse clinical outcomes in animal and human studies. The majority of P. aeruginosa strains secrete ExoS, a bifunctional toxin with GTPase-activating protein and ADP-ribosyltransferase activities. Numerous in vitro studies have investigated the targets and cellular effects of ExoS, linking both its enzymatic activities with inhibition of bacterial internalization. However, little is known about how this toxin facilitates the progression of infection in vivo. In this study, we used a mouse model to investigate the role of ExoS in inhibiting phagocytosis during pneumonia. We first confirmed previous findings that the ADP-ribosyltransferase activity of ExoS, but not the GTPase-activating protein activity, was responsible for bacterial persistence and decreased host survival in this model. We then used two distinct assays to demonstrate that ExoS inhibited phagocytosis during pneumonia. In contrast to the findings of several in vitro studies, this in vivo inhibition was also dependent on the ADP-ribosyltransferase activity, but not the GTPase-activating protein activity, of ExoS. These results demonstrate for the first time the antiphagocytic function of ExoS in the context of an actual infection and indicate that blocking the ADP-ribosyltransferase activity of ExoS may have potential therapeutic benefit.

Pseudomonas aeruginosa is a major cause of hospital-acquired infections. To cause severe disease, this bacterium uses a type III secretion system that delivers four effector proteins, ExoS, ExoT, ExoU, and ExoY, into host cells. The majority of P. aeruginosa strains secrete ExoS, a bifunctional toxin with GTPase-activating protein and ADP-ribosyltransferase activities. In cell culture models, both enzymatic activities have been associated with decreased bacterial internalization. However, our study is the first to examine a role for ExoS in blocking phagocytosis in an animal model. We report that ExoS does inhibit phagocytosis during pneumonia. The ADP-ribosyltransferase activity, but not the GTPase-activating protein activity, of ExoS is necessary for this effect. Our findings highlight the ability of P. aeruginosa to manipulate the inflammatory response during pneumonia to facilitate bacterial survival.

Pathogenic potential and genetic diversity of environmental and clinical isolates of Pseudomonas aeruginosa

The aim of this study was to investigate the occurrence of virulence genes among clinical and environmental isolates of Pseudomonas aeruginosa and to establish their genetic relationships by Enterobacterial Repetitive Intergenic Consensus PCR (ERIC-PCR). A total of 60 P. aeruginosa isolates from environmental and clinical sources were studied. Of these, 20 bacterial isolates were from soil, 20 from water, and 20 from patients with cystic fibrosis. Analysis of ERIC-PCR demonstrated that the isolates of P. aeruginosa showed a considerable genetic variability, regardless of their habitat. Numerous virulence genes were detected in both clinical and environmental isolates, reinforcing the possible pathogenic potential of soil and water isolates. The results showed that the environmental P. aeruginosa has all the apparatus needed to cause disease in humans and animals.

PcrV antibody-antibiotic combination improves survival in Pseudomonas aeruginosa-infected mice

The type III secretion system (TTSS) of Pseudomonas aeruginosa, associated with acute infection, facilitates the direct injection of cytotoxins into the host cell cytoplasm. Mab166, a murine monoclonal antibody against PcrV, a protein located at the tip of the injectisome, has demonstrated efficacy against P. aeruginosa infection, resulting in reduced lung injury and increased survival in murine models of infection. We hypothesised that the administration of Mab166 in combination with an antibiotic would further improve the survival of P. aeruginosa-infected mice. A murine model of P. aeruginosa acute infection, three clinically relevant antibiotics (ciprofloxacin, tobramycin and ceftazidime) and the Mab166 antibody were used for this study. Consistently, compared to other treatment groups (antibiotic or antibody administered in isolation), the combination of Mab166 and antibiotic significantly improved the survival of mice infected with three times the lethal dose (LD(90)) of the highly cytotoxic ExoU-secreting strain, PA103. This synergistic effect was primarily due to enhanced bactericidal effect and protection against lung injury, which prevented bacterial dissemination to other organs. Hence, the combination of Mab166 with antibiotic administration provides a new, more effective strategy against P. aeruginosa airway infection, especially when large numbers of highly virulent strains are present.

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.

Life at the front: dissecting bacterial-host interactions at the ocular surface

The ocular surface usually looks quiet, presenting a general impression of biological inactivity. Yet, the ability of the cornea to maintain health while continually exposed to environmental insults, and in the relative absence of immune strategies afforded by other body sites, reflects its complexity. Because it is critical for transparency and, therefore, our survival, the fine structure of the cornea has likely provided the driving force for the evolution of what appears to be a truly remarkable system. While several molecules are now known to participate, we are only beginning to obtain the knowledge to fully explain the mechanisms involved in corneal resistance to infection. Full explanation will require a better understanding of the interplay between microbes and various components of the ocular surface, and of the critical factors determining health as the usual outcome. To understand infectious disease, we need to consider how the scenario changes in conditions associated with susceptibility. What we learn in the process could yield a wealth of potential therapies for a wide variety of diseases of the eye and of other sites.

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.

Exoenzyme S of Pseudomonas aeruginosa is not able to induce apoptosis when cells express activated proteins, such as Ras or protein kinase B/Akt

Intracellular targeting of the Pseudomonas aeruginosa toxins, such as exoenzyme S (ExoS), cause cell death, as well as morphological and physiological changes in various tissue culture cells and animal models. In this report we have investigated the mechanism behind ExoS-mediated cell death. In order to address this issue, we have used cell lines expressing activated forms of various components of the Ras signalling pathway in order to evaluate the importance of the Ras pathway for viability and survival upon ExoS infection. Here we show that activated Ras is able to protect cells against cell death, regardless of whether it has been ADP-ribosylated by ExoS. Further, an activated form of protein kinase B (PKB)/Akt also leads to decreased level of cell death in response to ExoS infection, indicating that an important ExoS survival target is located upstream of Raf-1 and PKB/Akt. Moreover, we show that ExoS infection inhibits phosphorylation of FOXO3a, and induces caspase-3 activity, which are hallmarks for induction of cell death. In conclusion, we suggest that Ras proteins are an important cellular target for the P. aeruginosa toxin ExoS, which induces cell death during pathogenesis as a means of defending the bacterium against eukaryotic phagocytosis.

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.

Metabolic regulation of type III secretion gene expression in Pseudomonas aeruginosa

Type III secretion-mediated cytotoxicity is one of the key virulence mechanisms of the opportunistic pathogen Pseudomonas aeruginosa. Prior data from several laboratories have established that metabolism is a key factor in the regulation of type III secretion gene expression in P. aeruginosa. Here we use a fluorescence-activated cell sorter (FACS)-based approach to investigate expression of type III secretion genes at a single-cell level. The data demonstrate that the metabolic state regulates the percentage of cells that are able to induce type III secretion gene expression under inducing conditions. We also present evidence that this regulation is the result of an effect of the growth conditions on the ability of P. aeruginosa to assemble a functional type III secretion apparatus. Preliminary data suggest that the metabolite that controls type III secretion gene expression is derived from acetyl-CoA and that this regulation may, in part, be mediated by changes in the intracellular concentration of cyclic-AMP.

Examination of the coordinate effects of Pseudomonas aeruginosa ExoS on Rac1

Exoenzyme S (ExoS) is a bifunctional toxin directly translocated into eukaryotic cells by the Pseudomonas aeruginosa type III secretory (TTS) process. The amino-terminal GTPase-activating (GAP) activity and the carboxy-terminal ADP-ribosyltransferase (ADPRT) activity of ExoS have been found to target but exert opposite effects on the same low-molecular-weight G protein, Rac1. ExoS ADP-ribosylation of Rac1 is cell line dependent. In HT-29 human epithelial cells, where Rac1 is ADP-ribosylated by TTS-ExoS, Rac1 was activated and relocalized to the membrane fraction. Arg66 and Arg68 within the GTPase-binding region of Rac1 were identified as preferred sites of ExoS ADP-ribosylation. The modification of these residues by ExoS would be predicted to interfere with Rac1 inactivation and explain the increase in active Rac1 caused by ExoS ADPRT activity. Using ExoS-GAP and ADPRT mutants to examine the coordinate effects of the two domains on Rac1 function, limited effects of ExoS-GAP on Rac1 inactivation were evident in HT-29 cells. In J774A.1 macrophages, where Rac1 was not ADP-ribosylated, ExoS caused a decrease in the levels of active Rac1, and this decrease was linked to ExoS-GAP. Using immunofluorescence staining of Rac1 to understand the cellular basis for the targeting of ExoS ADPRT activity to Rac1, an inverse relationship was observed between Rac1 plasma membrane localization and Rac1 ADP-ribosylation. The results obtained from these studies have allowed the development of a model to explain the differential targeting and coordinate effects of ExoS GAP and ADPRT activity on Rac1 within the host cell.

Type III secretion phenotypes of Pseudomonas aeruginosa strains change during infection of individuals with cystic fibrosis

Pseudomonas aeruginosa is a frequent cause of respiratory exacerbations in individuals with cystic fibrosis. An important virulence determinant of this pathogen is its type III protein secretion system. In this study, the type III secretion properties of 435 P. aeruginosa respiratory isolates from 56 chronically infected individuals with cystic fibrosis were investigated. Although it had been previously reported that 75 to 90% of P. aeruginosa isolates from patients with hospital-acquired pneumonia secreted type III proteins, only 12% of isolates from cystic fibrosis patients did so, with nearly all of these isolates secreting ExoS and ExoT but not ExoU. Despite the low overall prevalence of type III protein-secreting isolates, at least one secreting isolate was cultured from one-third of cystic fibrosis patients. Interestingly, the fraction of cystic fibrosis patient isolates capable of secreting type III proteins decreased with duration of infection. Although 90% of isolates from the environment, the presumed reservoir for the majority of P. aeruginosa strains that infect patients with cystic fibrosis, secreted type III proteins, only 49% of isolates from newly infected children, 18% of isolates from chronically infected children, and 4% of isolates from chronically infected adults with cystic fibrosis secreted these proteins. Within individual patients, isolates of clonal origin differed in their secretion phenotypes, indicating that as strains persisted in cystic fibrosis patient airways, their type III protein secretion properties changed. Together, these findings indicate that following infection of cystic fibrosis patient airways, P. aeruginosa strains gradually change from a type III protein secretion-positive phenotype to a secretion-negative phenotype.

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.

Characterization of Pseudomonas aeruginosa exoenzyme S as a bifunctional enzyme in J774A.1 macrophages

Pseudomonas aeruginosa exoenzyme S (ExoS) is a type III secretion (TTS) effector, which includes both a GTPase-activating protein (GAP) activity toward the Rho family of low-molecular-weight G (LMWG) proteins and an ADP-ribosyltransferase (ADPRT) activity that targets LMWG proteins in the Ras, Rab, and Rho families. The coordinate function of both activities of ExoS in J774A.1 macrophages was assessed by using P. aeruginosa strains expressing and translocating wild-type ExoS or ExoS defective in GAP and/or ADPRT activity. Distinct and coordinated functions were identified for both domains. The GAP activity was required for the antiphagocytic effect of ExoS and was linked to interference of lamellopodium and membrane ruffle formation. Alternatively, the ADPRT activity of ExoS altered cellular adherence and morphology and was linked to effects on filopodium formation. The cellular mechanism of ExoS GAP activity included an inactivation of Rac1 function, as determined in p21-activated kinase 1-glutathione S-transferase (GST) pull-down assays. The ADPRT activity of ExoS targeted Ras and RalA but not Rab or Rho proteins, and Ral binding protein 1-GST pull-down assays identified an effect of ExoS ADPRT activity on RalA activation. The results from these studies confirm the bifunctional nature of ExoS activity within macrophages when translocated by TTS.

Use of the Galleria mellonella caterpillar as a model host to study the role of the type III secretion system in Pseudomonas aeruginosa pathogenesis

Nonvertebrate model hosts represent valuable tools for the study of host-pathogen interactions because they facilitate the identification of bacterial virulence factors and allow the discovery of novel components involved in host innate immune responses. In this report, we determined that the greater wax moth caterpillar Galleria mellonella is a convenient nonmammalian model host for study of the role of the type III secretion system (TTSS) in Pseudomonas aeruginosa pathogenesis. Based on the observation that a mutation in the TTSS pscD gene of P. aeruginosa strain PA14 resulted in a highly attenuated virulence phenotype in G. mellonella, we examined the roles of the four known effector proteins of P. aeruginosa (ExoS, ExoT, ExoU, and ExoY) in wax moth killing. We determined that in P. aeruginosa strain PA14, only ExoT and ExoU play a significant role in G. mellonella killing. Strain PA14 lacks the coding sequence for the ExoS effector protein and does not seem to express ExoY. Moreover, using Delta exoU Delta exoY, Delta exoT Delta exoY, and Delta exoT Delta exoU double mutants, we determined that individual translocation of either ExoT or ExoU is sufficient to obtain nearly wild-type levels of G. mellonella killing. On the other hand, data obtained with a Delta exoT Delta exoU Delta exoY triple mutant and a Delta pscD mutant suggested that additional, as-yet-unidentified P. aeruginosa components of type III secretion are involved in virulence in G. mellonella. A high level of correlation between the results obtained in the G. mellonella model and the results of cytopathology assays performed with a mammalian tissue culture system validated the use of G. mellonella for the study of the P. aeruginosa TTSS.

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.

Auto-ADP-ribosylation of Pseudomonas aeruginosa ExoS

Pseudomonas aeruginosa Exoenzyme S (ExoS) is a bifunctional type-III cytotoxin. The N terminus possesses a Rho GTPase-activating protein (GAP) activity, whereas the C terminus comprises an ADP-ribosyltransferase domain. We investigated whether the ADP-ribosyltransferase activity of ExoS influences its GAP activity. Although the ADP-ribosyltransferase activity of ExoS is dependent upon FAS, a 14-3-3 family protein, factor-activating ExoS (FAS) had no influence on the activity of the GAP domain of ExoS (ExoS-GAP). In the presence of NAD and FAS, the GAP activity of full-length ExoS was reduced about 10-fold, whereas NAD and FAS did not affect the activity of the ExoS-GAP fragment. Using [(32)P]NAD, ExoS-GAP was identified as a substrate of the ADP-ribosyltransferase activity of ExoS. Site-directed mutagenesis revealed that auto-ADP-ribosylation of Arg-146 of ExoS was crucial for inhibition of GAP activity in vitro. To reveal the auto-ADP-ribosylation of ExoS in intact cells, tetanolysin was used to produce pores in the plasma membrane of Chinese hamster ovary (CHO) cells to allow the intracellular entry of [(32)P]NAD, the substrate for ADP-ribosylation. After a 3-h infection of CHO cells with Pseudomonas aeruginosa, proteins of 50 and 25 kDa were preferentially ADP-ribosylated. The 50-kDa protein was determined to be auto-ADP-ribosylated ExoS, whereas the 25-kDa protein appeared to represent a group of proteins that included Ras.

Type III protein secretion is associated with poor clinical outcomes in patients with ventilator-associated pneumonia caused by Pseudomonas aeruginosa

OBJECTIVE

Pseudomonas aeruginosa is a frequent cause of ventilator-associated pneumonia. Recent evidence suggests that production of type III secretion proteins is correlated with increased pathogenicity in both cellular and animal models of infection. The objective of this study was to determine whether this system contributes to disease severity in humans with ventilator-associated pneumonia.

DESIGN

Retrospective pilot cohort study.

SETTING

University hospital.

PATIENTS

Thirty-five mechanically ventilated patients with bronchoscopically confirmed ventilator-associated pneumonia caused by P. aeruginosa.

MEASUREMENTS AND MAIN RESULTS

Ventilator-associated pneumonia was categorized as severe (patients died or had a recurrence of their pneumonia despite appropriate antibiotic therapy) or mild (patients uneventfully recovered from their pneumonia). The type III secretion genotypes and phenotypes of isolates cultured from the patients with ventilator-associated pneumonia were determined. Whereas every examined isolate harbored type III secretion genes, only 27 (77%) were capable of secreting detectable amounts of type III proteins in vitro. Twenty-two (81%) of the patients infected with these 27 isolates had severe disease. Of the eight isolates that did not secrete type III proteins, only three (38%) were cultured from patients with severe disease. Thus, infection with a type-III-secreting isolate correlated with severe disease (p < .05). In vitro assays indicated that ExoU, the type III effector protein most closely linked to mortality in animal models, was secreted in detectable amounts in vitro by 10 (29%) of the 35 examined isolates. Nine (90%) of these 10 isolates were cultured from patients with severe disease (p < .05 when compared with the nonsecreting isolates). In contrast, ExoS was secreted by 16 (46%) of the 35 examined isolates. Twelve (75%) of these 16 isolates were cultured from patients with severe disease (p = .14 when compared with the nonsecreting isolates).

CONCLUSIONS

In patients with ventilator-associated pneumonia, type-III-secreting isolates were associated with worse clinical outcomes, suggesting that this secretion system plays an important role in human disease. Our findings support the hypothesis that antibodies targeted against these proteins may be useful as adjunctive therapy in intubated patients with P. aeruginosa colonization or infection.

Eukaryotic cell determination of ExoS ADP-ribosyltransferase substrate specificity

Exoenzyme S (ExoS) ADP-ribosylates multiple low-molecular-mass G- (LMMG-) proteins in vitro. Identification of the in vivo substrate specificity of ExoS has been hindered by its bacterial contact delivery into eukaryotic cells and difficulties in identifying ADP-ribosylated proteins within cells. Two-dimensional electrophoresis comparisons of substrate modifications by ExoS in vitro to that following bacterial translocation into HT-29 epithelial cells identified Ras, Ral, and Rab proteins and Rac1 as in vivo substrates of ExoS ADPRT activity. Cellular fractionation studies identified a relationship between membrane association and efficiency of substrate modification. Moreover, Rac and Cdc42 relocalized to the membrane in response to ExoS. Comparisons of substrate modification to time of exposure to ExoS identified a progression of substrate modification, with Ras, RalA, and Rab5 modified first, followed by Rab8 and 11, then Rab7 and Rac1. The data support that intrinsic properties of LMMG-proteins and their subcellular localization are determinants of bacterially translocated ExoS substrate selectivity.

Prevalence of type III secretion genes in clinical and environmental isolates of Pseudomonas aeruginosa

The type III secretion system of Pseudomonas aeruginosa transports four known effector proteins: ExoS, ExoT, ExoU and ExoY. However, the prevalence of the type III secretion system genes or the effector-encoding genes in clinical and environmental isolates of P. aeruginosa has not been well studied. Southern hybridization analyses and PCR were performed on over 100 P. aeruginosa isolates to determine the distribution of these genes. Clinical isolates were obtained from urine, endotracheal, blood and wound specimens, from the sputum of cystic fibrosis (CF) patients, and from non-hospital environmental sites. The popB gene was used as a marker for the presence of the large chromosomal locus encoding the type III secretion machinery proteins. Each isolate contained the popB gene, indicating that at least a portion of this large chromosomal locus was present in all isolates. Likewise, each isolate contained exoT-like sequences. In contrast, the exoS, exoU and exoY genes were variable traits. Overall, 72% of examined isolates contained the exoS gene, 28% contained the exoU gene, and 89% contained the exoY gene. Interestingly, an inverse correlation was noted between the presence of the exoS and exoU genes in that all isolates except two contained either exoS or exoU but not both. No significant difference in exoS, exoU or exoY prevalence was observed between clinical and environmental isolates or between isolates cultured from different disease sites except for CF respiratory isolates. CF isolates harboured the exoU gene less frequently and the exoS gene more frequently than did isolates from some of the other sites of infection, including the respiratory tract of patients without CF. These results suggest that the P. aeruginosa type III secretion system is present in nearly all clinical and environmental isolates but that individual isolates and populations of isolates from distinct disease sites differ in their effector genotypes. The ubiquity of type III secretion genes in clinical isolates is consistent with an important role for this system in human disease.

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.

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.

Expression of ExsA in trans confers type III secretion system-dependent cytotoxicity on noncytotoxic Pseudomonas aeruginosa cystic fibrosis isolates

Twelve Pseudomonas aeruginosa cystic fibrosis isolates that are not able to exert a type III secretion system (TTSS)-dependent cytotoxicity towards phagocytes have been further studied. The strains, although possessing TTSS genes and exsA, which encodes a positive regulator of the TTSS regulon, showed no transcriptional activation of the exsCBA regulatory operon. The expression of exsA in trans restored the in vitro secretion of TTSS proteins and ex vivo cytotoxicity.

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.

Cell death of human polymorphonuclear neutrophils induced by a Pseudomonas aeruginosa cystic fibrosis isolate requires a functional type III secretion system

With a coincubation model incorporating Pseudomonas aeruginosa and human polymorphonuclear neutrophils (PMNs), a cystic fibrosis (CF) P. aeruginosa isolate has been shown to resist the bactericidal action of PMNs and to induce their cellular death. An isogenic mutant of this CF isolate in which the type III secretion system was rendered nonfunctional was unable to induce cellular death of PMNs.

Pseudomonas aeruginosa induces type-III-secretion-mediated apoptosis of macrophages and epithelial cells

Pseudomonas aeruginosa is a gram-negative opportunistic pathogen that is cytotoxic towards a variety of eukaryotic cells. To investigate the effect of this bacterium on macrophages, we infected J774A.1 cells and primary bone-marrow-derived murine macrophages with the P. aeruginosa strain PA103 in vitro. PA103 caused type-III-secretion-dependent killing of macrophages within 2 h of infection. Only a portion of the killing required the putative cytotoxin ExoU. By three criteria, terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling assays, cytoplasmic nucleosome assays, and Hoechst staining, the ExoU-independent but type-III-secretion-dependent killing exhibited features of apoptosis. Extracellular bacteria were capable of inducing apoptosis, and some laboratory and clinical isolates of P. aeruginosa induced significantly higher levels of this form of cell death than others. Interestingly, HeLa cells but not Madin-Darby canine kidney cells were susceptible to type-III-secretion-mediated apoptosis under the conditions of these assays. These findings are consistent with a model in which the P. aeruginosa type III secretion system transports at least two factors that kill macrophages: ExoU, which causes necrosis, and a second, as yet unidentified, effector protein, which induces apoptosis. Such killing may contribute to the ability of this organism to persist and disseminate within infected patients.

RecombinantPseudomonasexoenzyme S and exoenzyme S fromPseudomonas aeruginosaDG1 share the ability to stimulate T lymphocyte proliferation

Exoenzyme S from P. aeruginosa DG1 and recombinant exoenzyme S derived from strain 388 have distinct characteristics, which has led to a controversy about their homology and their pathophysiologic consequences. We have been investigating the ability of exoenzyme S to activate T lymphocytes, and therefore performed studies to determine whether exoenzyme S from P. aeruginosa DG1 and recombinant exoenzyme S derived from strain 388 and expressed in Pseudomonas aeruginosa PA103 or in E. coli BL21(DE3), could induce T lymphocyte activation and proliferation. Both preparations were able to activate T cells and induce lymphocyte proliferation at similar levels as measured by flow cytometry of surface-activation markers and DNA synthesis, respectively. Further, a monoclonal antibody raised against exoenzyme S from strain DG1 partially neutralized T cell activation induced by recombinant exoenzyme S and bound to it in an immunoblot suggesting that the epitope responsible for T cell activation is shared by exoenzyme S from strain DG1 and recombinant exoenzyme S. These data suggest that the two different preparations of exoenzyme S, despite biochemical differences, share the characteristic that is responsible for T lymphocyte activation.Key words: exoenzyme S, Pseudomonas aeruginosa, T lymphocyte, cystic fibrosis.

Pseudomonas aeruginosa exoenzyme S disrupts Ras-mediated signal transduction by inhibiting guanine nucleotide exchange factor-catalyzed nucleotide exchange

Pseudomonas aeruginosa exoenzyme S double ADP-ribosylates Ras at Arg(41) and Arg(128). Since Arg(41) is adjacent to the switch 1 region of Ras, ADP-ribosylation could interfere with Ras-mediated signal transduction via several mechanisms, including interaction with Raf, or guanine nucleotide exchange factor-stimulated or intrinsic nucleotide exchange. Initial experiments showed that ADP-ribosylated Ras (ADP-r-Ras) and unmodified Ras (Ras) interacted with Raf with equal efficiencies, indicating that ADP-ribosylation did not interfere with Ras-Raf interactions. While ADP-r-Ras and Ras possessed equivalent intrinsic nucleotide exchange rates, guanine nucleotide exchange factor (Cdc25) stimulated the nucleotide exchange of ADP-r-Ras at a 3-fold slower rate than Ras. ADP-r-Ras did not affect the nucleotide exchange of Ras, indicating that the ADP-ribosylation of Ras was not a dominant negative phenotype. Ras-R41K and ADP-r-Ras R41K possessed similar exchange rates as Ras, indicating that ADP-ribosylation at Arg(128) did not inhibit Cdc25-stimulated nucleotide exchange. Consistent with the slower nucleotide exchange rate of ADP-r-Ras as compared with Ras, ADP-r-Ras bound its guanine nucleotide exchange factor (Cdc25) less efficiently than Ras in direct binding experiments. Together, these data indicate that ADP-ribosylation of Ras at Arg(41) disrupts Ras-Cdc25 interactions, which inhibits the rate-limiting step in Ras signal transduction, the activation of Ras by its guanine nucleotide exchange factor.

Differential sensitivity of human epithelial cells to Pseudomonas aeruginosa exoenzyme S

Exoenzyme S (ExoS) is an ADP-ribosyltransferase produced and directly translocated into eukaryotic cells by the opportunistic pathogen Pseudomonas aeruginosa. Model systems that allow bacterial translocation of ExoS have found ExoS to have multiple effects on eukaryotic cell function, affecting DNA synthesis, actin cytoskeletal structure, and cell matrix adherence. To understand mechanisms underlying differences observed in cell sensitivities to ExoS, we examined the effects of bacterially translocated ExoS on multiple human epithelial cell lines. Of the cell lines examined, confluent normal kidney (NK) epithelial cells were most resistant to ExoS, while tumor-derived cell lines were highly sensitive to ExoS. Analysis of the mechanisms of resistance indicated that cell association as well as an intrinsic resistance to morphological alterations were associated with increased resistance to ExoS. Conversely, increased sensitivity to ExoS appeared to be linked to epithelial cell growth, with tumor cells capable of undergoing non-contact-inhibited, anchorage-independent growth all being sensitive to ExoS, and NK cells becoming sensitive to ExoS when subconfluent and growing. Consistent with the possibility that growth-related, actin-based structures are involved in sensitivity to ExoS, scanning electron microscopy revealed cellular extensions from sensitive, growing cells to bacteria, which were not readily evident in resistant cells. In all studies, the severity of effects of ExoS on cell function directly correlated with the degree of Ras modification, indicating that sensitivity to ExoS in some manner related to the efficiency of ExoS translocation and its ADP-ribosylation of Ras. Our results suggest that factors expressed by growing epithelial cells are required for the bacterial contact-dependent translocation of ExoS; as normal epithelial cells differentiate into polarized confluent monolayers, expression of these factors is altered, and cells in turn become more resistant to the effects of ExoS.

Molecular basis of the interaction of Salmonella with the intestinal mucosa

Salmonella is one of the most extensively characterized bacterial pathogens and is a leading cause of bacterial gastroenteritis. Despite this, we are only just beginning to understand at a molecular level how Salmonella interacts with its mammalian hosts to cause disease. Studies during the past decade on the genetic basis of virulence of Salmonella have significantly advanced our understanding of the molecular basis of the host-pathogen interaction, yet many questions remain. In this review, we focus on the interaction of enterocolitis-causing salmonellae with the intestinal mucosa, since this is the initiating step for most infections caused by Salmonella. Animal and in vitro cell culture models for the interaction of these bacteria with the intestinal epithelium are reviewed, along with the bacterial genes that are thought to affect this interaction. Lastly, recent studies on the response of epithelial cells to Salmonella infection and how this might promote diarrhea are discussed.

Interruption of multiple cellular processes in HT-29 epithelial cells by Pseudomonas aeruginosa exoenzyme S

Exoenzyme S (ExoS), an ADP-ribosylating enzyme produced by the opportunistic pathogen Pseudomonas aeruginosa, is directly translocated into eukaryotic cells by bacterial contact. Within the cell, ExoS ADP-ribosylates the cell signaling protein Ras and causes inhibition of DNA synthesis and alterations in cytoskeletal structure. To further understand the interrelationship of the different cellular effects of ExoS, functional analyses were performed on HT-29 epithelial cells after exposure to ExoS-producing P. aeruginosa 388 and the non-ExoS-producing strain 388DeltaS. Two different mechanisms of morphological alteration were identified: (i) a more-transient and less-severe cell rounding caused by the non-ExoS-producing strain 388DeltaS and (ii) a more-severe, long-term cell rounding caused by ExoS-producing strain 388. Long-term effects of ExoS on cell morphology occurred in conjunction with ExoS-mediated inhibition of DNA synthesis and the ADP-ribosylation of Ras. ExoS was also found to cause alterations in HT-29 cell function, leading to the loss of cell adhesion and microvillus effacement. Nonadherent ExoS-treated cells remained viable but had a high proportion of modified Ras. While microvillus effacement was detected in both 388- and 388DeltaS-treated cells, effacement was more prevalent and rapid in cells exposed to strain 388. We conclude from these studies that ExoS can have multiple effects on epithelial cell function, with more severe cellular alterations associated with the enzymatic modification of Ras. The finding that ExoS had greater effects on cell growth and adherence than on cell viability suggests that ExoS may contribute to the P. aeruginosa infectious process by rendering cells nonfunctional.

ADP-ribosylation of oncogenic Ras proteins by pseudomonas aeruginosa exoenzyme S in vivo

The exoenzyme S (ExoS)-producing Pseudomonas aeruginosa strain, 388, and corresponding ExoS knock-out strain, 388deltaexoS, were used in a bacterial and mammalian co-culture system as a model for the contact-dependent delivery of ExoS into host cells. Examination of DNA synthesis and Ras ADP-ribosylation in tumour cell lines expressing normal and mutant Ras revealed a decrease in DNA synthesis concomitant with ADP-ribosylation of Ras proteins after exposure to ExoS-producing bacteria, but not after exposure to non-ExoS-producing bacteria. Examination of normal H-Ras, K-Ras and N-Ras by two-dimensional electrophoresis after exposure to bacteria revealed differences in the degree of ADP-ribosylation by ExoS, with H-Ras being modified most extensively. ADP-ribosylation of oncogenic forms of Ras was examined in vivo using cancer lines expressing mutant forms of H-, N- or K-Ras. The mutant Ras proteins were modified in a manner qualitatively similar to their normal counterparts. Using Ras/Raf-1 co-immunoprecipitation after co-culture, it was found that exposure to ExoS-producing bacteria caused a decrease in the amount of Raf-1 associated with EGF-activated Ras and oncogenic Ras. The results from this study indicate that ExoS ADP-ribosylates both normal and mutant Ras proteins in vivo and inhibits signalling through Ras.

Macrophages and epithelial cells respond differently to the Pseudomonas aeruginosa type III secretion system

The multiple effects of Pseudomonas aeruginosa type III secretion have largely been attributed to variations in cytotoxin expression between strains. Here we show that the target cell type is also important. While lung epithelial cells showed significant changes in morphology but not viability when infected with P. aeruginosa, macrophages were efficiently killed by P. aeruginosa. Both responses were dependent on the type III secretion system.

Pseudomonas aeruginosa exoenzyme S, a double ADP-ribosyltransferase, resembles vertebrate mono-ADP-ribosyltransferases

Previous data indicated that Pseudomonas aeruginosa exoenzyme S (ExoS) ADP-ribosylated Ras at multiple sites. One site appeared to be Arg41, but the second site could not be localized. In this study, the sites of ADP-ribosylation of c-Ha-Ras by ExoS were directly determined. Under saturating conditions, ExoS ADP-ribosylated Ras to a stoichiometry of 2 mol of ADP-ribose incorporated per mol of Ras. Nucleotide occupancy did not influence the stoichiometry or velocity of ADP-ribosylation of Ras by ExoS. Edman degradation and mass spectrometry of V8 protease generated peptides of ADP-ribosylated Ras identified the sites of ADP-ribosylation to be Arg41 and Arg128. ExoS ADP-ribosylated the double mutant, RasR41K,R128K, to a stoichiometry of 1 mol of ADP-ribose incorporated per mol of Ras, which indicated that Ras possessed an alternative site of ADP-ribosylation. The alternative site of ADP-ribosylation on Ras was identified as Arg135, which was on the same alpha-helix as Arg128. Arg41 and Arg128 are located within two different secondary structure motifs, beta-sheet and alpha-helix, respectively, and are spatially separated within the three-dimensional structure of Ras. The fact that ExoS could ADP-ribosylate a target protein at multiple sites, along with earlier observations that ExoS could ADP-ribosylate numerous target proteins, were properties that have been attributed to several vertebrate ADP-ribosyltransferases. This prompted a detailed alignment study which showed that the catalytic domain of ExoS possessed considerably more primary amino acid homology with the vertebrate mono-ADP-ribosyltransferases than the bacterial ADP-ribosyltransferases. These data are consistent with the hypothesis that ExoS may represent an evolutionary link between bacterial and vertebrate mono-ADP-ribosyltransferases.

Biological effects of Pseudomonas aeruginosa type III-secreted proteins on CHO cells

A strain of Pseudomonas aeruginosa that fails to express known type III-secreted effector proteins was constructed as an expression host. Individual effectors were expressed in trans, and their biological effects on CHO cells were assessed in an acute cellular infection model. Intoxication with ExoS, ExoT, or ExoY resulted in alterations in cell morphology. As shown in previous genetic studies, ExoU expression was linked to acute cytotoxicity.

The amino-terminal domain of Pseudomonas aeruginosa ExoS disrupts actin filaments via small-molecular-weight GTP-binding proteins

Pseudomonas aeruginosa delivers exoenzyme S (ExoS) into the intracellular compartment of eukaryotic cells via a type III secretion pathway. Intracellular delivery of ExoS is cytotoxic for eukaryotic cells and has been shown to ADP-ribosylate Ras in vivo and uncouple a Ras-mediated signal transduction pathway. Functional mapping has localized the FAS-dependent ADP-ribosyltransferase domain to the carboxyl-terminus of ExoS. A transient transfection system was used to examine cellular responses to the amino-terminal 234 amino acids of ExoS (DeltaC234). Intracellular expression of DeltaC234 elicited the rounding of Chinese hamster ovary (CHO) cells and the disruption of actin filaments in a dose-dependent manner. Expression of DeltaC234 did not inhibit the expression of two independent reporter proteins, GFP and luciferase, or induce trypan blue uptake, which indicated that expression of DeltaC234 was not cytotoxic to CHO cells. Carboxyl-terminal deletion proteins of DeltaC234 were less efficient in the elicitation of CHO cell rounding than DeltaC234. Cytoskeleton rearrangement elicited by DeltaC234 was blocked and reversed by the addition of cytotoxic necrotizing factor 1 (CNF-1). CNF-1 catalyses the deamidation of Gln-63 of members of the Rho subfamily of small-molecular-weight GTP-binding proteins, resulting in protein activation. This implies a role for small-molecular-weight GTP-binding proteins in the disruption of actin by DeltaC234. Together, these data identify ExoS as a cytotoxin that possesses two functional domains. Intracellular expression of the amino-terminal domain of ExoS elicits the disruption of actin, while expression of the carboxyl-terminal domain of ExoS possesses FAS-dependent ADP-ribosyltransferase activity and is cytotoxic to eukaryotic cells.

Regulation of ExoS production and secretion by Pseudomonas aeruginosa in response to tissue culture conditions

This study was initiated to characterize the regulation and secretion of ExoS by Pseudomonas aeruginosa during contact with eukaryotic cells. The production of ExoS was monitored by a sensitive ADP-ribosyltransferase activity assay, and specific activities were calculated for supernatant and cell-associated fractions. Time course analysis indicated that ExoS was produced after a lag period, suggesting that induction of the regulon is necessary for the expression of detectable amounts of enzyme activity. Under tissue culture growth conditions, ExoS was induced when P. aeruginosa was in contact with Chinese hamster ovary (CHO) cells or after growth in tissue culture medium with serum. The serum induction of ExoS appeared to result in generalized type III secretion, while induction by contact with CHO cells appeared to result in polarized type III secretion. Mutants in the type III secretory system that express a null phenotype for ExoS production in bacteriological medium produced but did not secrete the enzyme when P. aeruginosa was grown under inducing conditions in tissue culture medium. These results suggest that both induction and secretion of ExoS may differ when the bacteria are exposed to different growth environments. The putative type III translocation proteins and secretion apparatus of P. aeruginosa were required for translocation of bacterial factors that mediate changes in CHO cell morphology during infection.

Identification and characterization of SpcU, a chaperone required for efficient secretion of the ExoU cytotoxin

In recent studies, we have shown that Pseudomonas aeruginosa strains that are acutely cytotoxic in vitro damage the lung epithelium in vivo. Genetic analysis indicated that the factor responsible for acute cytotoxicity was controlled by ExsA and therefore was part of the exoenzyme S regulon. The specific virulence determinant responsible for epithelial damage in vivo and cytotoxicity in vitro was subsequently mapped to the exoU locus. The present studies are focused on a genetic characterization of the exoU locus. Northern blot analyses and complementation experiments indicated that a region downstream of exoU was expressed and that the expression of this region corresponded to increased ExoU secretion. DNA sequence analysis of a region downstream of exoU identified several potential coding regions. One of these open reading frames, SpcU (specific Pseudomonas chaperone for ExoU), encoded a small 15-kDa acidic protein (137 amino acids [pI 4.4]) that possessed a leucine-rich motif associated with the Syc family of cytosolic chaperones for the Yersinia Yops. T7 expression analysis and nickel chromatography of histidine-tagged proteins indicated that ExoU and SpcU associated as a noncovalent complex when coexpressed in Escherichia coli. The association of ExoU and SpcU required amino acids 3 to 123 of ExoU. In P. aeruginosa, ExoU and SpcU are coordinately expressed as an operon that is controlled at the transcriptional level by ExsA.

Intracellular expression of the ADP-ribosyltransferase domain of Pseudomonas exoenzyme S is cytotoxic to eukaryotic cells

Exoenzyme S of Pseudomonas aeruginosa is an ADP-ribosyltransferase, which is secreted via a type III-dependent secretion mechanism and has been demonstrated to exert cytotoxic effects on eukaryotic cells. Alignment studies predict that the amino-terminus of exoenzyme S has limited primary amino acid homology with the YopE cytotoxin of Yersinia, while biochemical studies have localized the FAS-dependent ADP-ribosyltransferase activity to the carboxyl-terminus. Thus, exoenzyme S could interfere with host cell physiology via several independent mechanisms. The goal of this study was to define the role of the ADP-ribosyltransferase domain in the modulation of eukaryotic cell physiology. The carboxyl-terminal 222 amino acids of exoenzyme S, which represent the FAS-dependent ADP-ribosyltransferase domain (termed deltaN222), and a point mutant, deltaN222-E381A, which possesses a 2000-fold reduction in the capacity to ADP-ribosylate, were transiently expressed in eukaryotic cells under the control of the immediate early CMV promoter. Lysates from cells transfected with deltaN222 expressed ADP-ribosyltransferase activity. Co-transfection of deltaN222, but not deltaN222-E381A, resulted in a decrease in the steady-state levels of two reporter proteins, green fluorescent protein and luciferase, in both CHO and Vero cells. In addition, transfection with deltaN222 resulted in a greater percentage of cells staining with trypan blue than when cells were transfected with either deltaN222-E381A or control plasmid. Together, these data indicate that expression of the ADP-ribosyltransferase domain of exoenzyme S is cytotoxic to eukaryotic cells.

Pseudomonas aeruginosa exoenzyme S is a mitogen but not a superantigen for human T lymphocytes

Virtually all cystic fibrosis (CF) patients become infected with Pseudomonas aeruginosa, and once the infection is established, the organism is rarely cleared. One of the P. aeruginosa virulence factors, exoenzyme S, has been shown to correlate with increased morbidity and mortality both in rat models of chronic pulmonary inflammation and in human CF patients. It has previously been shown that exoenzyme S is a potent stimulus for the proliferation of T cells in greater than 95% of adults, which could contribute to the pathogenesis of CF. The goal of this study was to determine the mechanism of T-cell stimulation by exoenzyme S in an effort to shed light on the immune response and contribute to understanding its role in P. aeruginosa pathogenesis. The current studies demonstrate that exoenzyme S stimulates naive T cells, since fetal blood lymphocytes proliferated and adult lymphocytes that expressed CD45RA proliferated. The percentage of T cells activated by exoenzyme S after a 4-h culture (as measured by CD69 surface expression) was intermediate in magnitude compared to levels induced by a panel of superantigens and mitogens. To determine the mechanism of activation, the requirement for accessory cells was investigated. The proliferative response to exoenzyme S was dependent on the presence of accessory cells but was not blocked by an anti-DR antibody. Exoenzyme S activated both CD4(+) and CD8(+) T cells, but CD4(+) T cells were preferentially activated. The Vbeta repertoire of donor T cells showed no preferential activation or preferential expansion after stimulation by exoenzyme S, suggesting that it is not a superantigen. Taken together, our data suggest that exoenzyme S is a T-cell mitogen but not a superantigen. Activation of a large percentage of T lymphocytes by exoenzyme S may produce a lymphocyte-mediated inflammatory response that should be considered in the pathogenesis of CF.

Modification of Ras in eukaryotic cells by Pseudomonas aeruginosa exoenzyme S

Genetic and functional data suggest that Pseudomonas aeruginosa exoenzyme S (ExoS), an ADP-ribosyltransferase, is translocated into eukaryotic cells by a bacterial type III secretory mechanism activated by contact between bacteria and host cells. Although purified ExoS is not toxic to eukaryotic cells, ExoS-producing bacteria cause reduced proliferation and viability, possibly mediated by bacterially translocated ExoS. To investigate the activity of translocated ExoS, we examined in vivo modification of Ras, a preferred in vitro substrate. The ExoS-producing strain P. aeruginosa 388 and an isogenic mutant strain, 388DeltaexoS, which fails to produce ExoS, were cocultured with HT29 colon carcinoma cells. Ras was found to be ADP-ribosylated during coculture with 388 but not with 388DeltaexoS, and Ras modification by 388 corresponded with reduction in HT29 cell DNA synthesis. Active translocation by bacteria was found to be required, since exogenous ExoS, alone or in the presence of 388DeltaexoS, was unable to modify intracellular Ras. Other ExoS-producing strains caused modification of Ras, indicating that this is not a strain-specific event. ADP-ribosylation of Rap1, an additional Ras family substrate for ExoS in vitro, was not detectable in vivo under conditions sufficient for Ras modification, suggesting possible ExoS substrate preference among Ras-related proteins. These results confirm that intracellular Ras is modified by bacterially translocated ExoS and that the inhibition of target cell proliferation correlates with the efficiency of Ras modification.

Identification of type III secreted products of the Pseudomonas aeruginosa exoenzyme S regulon

Extracellular protein profiles from wild-type and regulatory or secretory isogenic mutants of the Pseudomonas aeruginosa exoenzyme S regulon were compared to identify proteins coordinately secreted with ExoS. Data from amino-terminal sequence analysis of purified extracellular proteins were combined with data from nucleotide sequence analysis of loci linked to exoenzyme S production. We report the identification of P. aeruginosa homologs to proteins of Yersinia spp. that function as regulators of the low calcium response, regulators of secretion, and mediators of the type III translocation mechanism.

Ecto-ADP-ribosyltransferase activity of Pseudomonas aeruginosa exoenzyme S

Pseudomonas aeruginosa produces two ADP-ribosyltransferases, exotoxin A and exoenzyme S (ExoS). Although the physiological target protein remains to be defined, ExoS has been shown to ADP-ribosylate several eukaryotic proteins in vitro, including vimentin and members of the family of low-molecular-weight GTP-binding proteins. Recently, ExoS ADP-ribosyltransferase activity has been detected in the pleural fluid of rabbits infected with P. aeruginosa. This observation prompted an examination of the potential for ExoS to function as an ecto-ADP-ribosyltransferase. We have observed that ExoS preferentially ADP-ribosylated two extracellular serum proteins with molecular masses of 150 and 27 kDa. The ADP-ribosylation of these serum proteins by ExoS was stimulated by, but not dependent upon, exogenous FAS (for factor activating exoenzyme S), which indicated that serum contained endogenous FAS activity. Biochemical analysis showed that the 150-kDa ADP-ribosylated protein was immunoglobulin of the immunoglobulin G (IgG) and IgA classes. Subtyping showed that ExoS preferentially ADP-ribosylated human IgG3 and that ADP-ribosylation occurred within its Fc region. The 27-kDa protein ADP-ribosylated by ExoS was determined to be apolipoprotein A1. These data demonstrate ecto-ADP-ribosyltransferase activity by ExoS. This may extend the potential physiological consequences of ExoS during infection by P. aeruginosa beyond the implicated type III secretion-mediated intracellular delivery of ExoS into sensitive eukaryotic cells.

Type III secretion systems: machines to deliver bacterial proteins into eukaryotic cells?

Type III secretion systems in certain bacterial pathogens are induced upon contact with host cells and directly deliver virulence proteins into the host cell cytosol. The increasing number of Gram-negative bacterial pathogens discovered to encode type III secretion systems raises interesting questions. Are type III systems generic machines that deliver virulence proteins into host cells? Is contact with host cells a common regulatory cue for type III systems?