Pseudomonas aeruginosa exoenzyme S, a bifunctional type-III secreted cytotoxin

Pseudomonas aeruginosa Cytotoxins: Mechanisms of Cytotoxicity and Impact on Inflammatory Responses

Pseudomonas aeruginosa is one of the most virulent opportunistic Gram-negative bacterial pathogens in humans. It causes many acute and chronic infections with morbidity and mortality rates as high as 40%. P. aeruginosa owes its pathogenic versatility to a large arsenal of cell-associated and secreted virulence factors which enable this pathogen to colonize various niches within hosts and protect it from host innate immune defenses. Induction of cytotoxicity in target host cells is a major virulence strategy for P. aeruginosa during the course of infection. P. aeruginosa has invested heavily in this strategy, as manifested by a plethora of cytotoxins that can induce various forms of cell death in target host cells. In this review, we provide an in-depth review of P. aeruginosa cytotoxins based on their mechanisms of cytotoxicity and the possible consequences of their cytotoxicity on host immune responses.

Pseudomonas aeruginosa Alters Critical Lung Epithelial Cell Functions through Activation of ADAM17

Severe epithelial dysfunction is one major hallmark throughout the pathophysiological progress of bacterial pneumonia. Junctional and cellular adhesion molecules (e.g., JAMA-A, ICAM-1), cytokines (e.g., TNFα), and growth factors (e.g., TGFα), controlling proper lung barrier function and leukocyte recruitment, are proteolytically cleaved and released into the extracellular space through a disintegrin and metalloproteinase (ADAM) 17. In cell-based assays, we could show that the protein expression, maturation, and activation of ADAM17 is upregulated upon infection of lung epithelial cells with Pseudomonas aeruginosa and Exotoxin A (ExoA), without any impact of infection by Streptococcus pneumoniae. The characterization of released extracellular vesicles/exosomes and the comparison to heat-inactivated bacteria revealed that this increase occurred in a cell-associated and toxin-dependent manner. Pharmacological targeting and gene silencing of ADAM17 showed that its activation during infection with Pseudomonas aeruginosa was critical for the cleavage of junctional adhesion molecule A (JAM-A) and epithelial cell survival, both modulating barrier integrity, epithelial regeneration, leukocyte adhesion and transepithelial migration. Thus, site-specific targeting of ADAM17 or blockage of the activating toxins may constitute a novel anti-infective therapeutic option in Pseudomonas aeruginosa lung infection preventing severe epithelial and organ dysfunctions and stimulating future translational studies.

The Pseudomonas aeruginosa Type III Secretion System Exoenzyme Effector ExoU Induces Mitochondrial Damage in a Murine Bone Marrow-Derived Macrophage Infection Model

Pseudomonas aeruginosa is a Gram-negative, opportunistic pathogen that causes nosocomial pneumonia, urinary tract infections, and bacteremia. A hallmark of P. aeruginosa pathogenesis is disruption of host cell function by the type III secretion system (T3SS) and its cognate exoenzyme effectors. The T3SS effector ExoU is phospholipase A2 (PLA2) that targets the host cell plasmalemmal membrane to induce cytolysis and is an important virulence factor that mediates immune avoidance. In addition, ExoU has been shown to subvert the host inflammatory response in a noncytolytic manner. In primary bone marrow-derived macrophages (BMDMs), P. aeruginosa infection is sensed by the nucleotide-binding domain containing leucine-rich repeats-like receptor 4 (NLRC4) inflammasome, which triggers caspase-1 activation and inflammation. ExoU transiently inhibits NLRC4 inflammasome-mediated activation of caspase-1 and its downstream target, interleukin 1β (IL-1β), to suppress activation of inflammation. In the present study, we sought to identify additional noncytolytic virulence functions for ExoU and discovered an unexpected association between ExoU, host mitochondria, and NLRC4. We show that infection of BMDMs with P. aeruginosa strains expressing ExoU elicited mitochondrial oxidative stress. In addition, mitochondria and mitochondrion-associated membrane fractions enriched from infected cells exhibited evidence of autophagy activation, indicative of damage. The observation that ExoU elicited mitochondrial stress and damage suggested that ExoU may also associate with mitochondria during infection. Indeed, ExoU phospholipase A2 enzymatic activity was present in enriched mitochondria and mitochondrion-associated membrane fractions isolated from P. aeruginosa-infected BMDMs. Intriguingly, enriched mitochondria and mitochondrion-associated membrane fractions isolated from infected Nlrc4 homozygous knockout BMDMs displayed significantly lower levels of ExoU enzyme activity, suggesting that NLRC4 plays a role in the ExoU-mitochondrion association. These observations prompted us to assay enriched mitochondria and mitochondrion-associated membrane fractions for NLRC4, caspase-1, and IL-1β. NLRC4 and pro-caspase-1 were detected in enriched mitochondria and mitochondrion-associated membrane fractions isolated from noninfected BMDMs, and active caspase-1 and active IL-1β were detected in response to P. aeruginosa infection. Interestingly, ExoU inhibited mitochondrion-associated caspase-1 and IL-1β activation. The implications of ExoU-mediated effects on mitochondria and the NLRC4 inflammasome during P. aeruginosa infection are discussed.

ExoU Induces Lung Endothelial Cell Damage and Activates Pro-Inflammatory Caspase-1 during Pseudomonas aeruginosa Infection

The Gram-negative, opportunistic pathogen Pseudomonas aeruginosa utilizes a type III secretion system to inject exoenzyme effectors into a target host cell. Of the four best-studied exoenzymes, ExoU causes rapid cell damage and death. ExoU is a phospholipase A₂ (PLA₂) that hydrolyses host cell membranes, and P. aeruginosa strains expressing ExoU are associated with poor outcomes in critically ill patients with pneumonia. While the effects of ExoU on lung epithelial and immune cells are well studied, a role for ExoU in disrupting lung endothelial cell function has only recently emerged. Lung endothelial cells maintain a barrier to fluid and protein flux into tissue and airspaces and regulate inflammation. Herein, we describe a pulmonary microvascular endothelial cell (PMVEC) culture infection model to examine the effects of ExoU. Using characterized P. aeruginosa strains and primary clinical isolates, we show that strains expressing ExoU disrupt PMVEC barrier function by causing substantial PMVEC damage and lysis, in a PLA₂-dependent manner. In addition, we show that strains expressing ExoU activate the pro-inflammatory caspase-1, in a PLA₂-dependent manner. Considering the important roles for mitochondria and oxidative stress in regulating inflammatory responses, we next examined the effects of ExoU on reactive oxygen species production. Infection of PMVECs with P. aeruginosa strains expressing ExoU triggered a robust oxidative stress compared to strains expressing other exoenzyme effectors. We also provide evidence that, intriguingly, ExoU PLA₂ activity was detectable in mitochondria and mitochondria-associated membrane fractions isolated from P. aeruginosa-infected PMVECs. Interestingly, ExoU-mediated activation of caspase-1 was partially inhibited by reactive oxygen species scavengers. Together, these data suggest ExoU exerts pleiotropic effects on PMVEC function during P. aeruginosa infection that may inhibit endothelial barrier and inflammatory functions.

The Role of Pseudomonas aeruginosa Virulence Factors in Cytoskeletal Dysregulation and Lung Barrier Dysfunction

Pseudomonas (P.) aeruginosa is an opportunistic pathogen that causes serious infections and hospital-acquired pneumonia in immunocompromised patients. P. aeruginosa accounts for up to 20% of all cases of hospital-acquired pneumonia, with an attributable mortality rate of ~30-40%. The poor clinical outcome of P. aeruginosa-induced pneumonia is ascribed to its ability to disrupt lung barrier integrity, leading to the development of lung edema and bacteremia. Airway epithelial and endothelial cells are important architecture blocks that protect the lung from invading pathogens. P. aeruginosa produces a number of virulence factors that can modulate barrier function, directly or indirectly, through exploiting cytoskeleton networks and intercellular junctional complexes in eukaryotic cells. This review summarizes the current knowledge on P. aeruginosa virulence factors, their effects on the regulation of the cytoskeletal network and associated components, and molecular mechanisms regulating barrier function in airway epithelial and endothelial cells. A better understanding of these processes will help to lay the foundation for new therapeutic approaches against P. aeruginosa-induced pneumonia.

NAD+-targeting by bacteria: an emerging weapon in pathogenesis

Nicotinamide adenine dinucleotide (NAD+) is a major cofactor in redox reactions in all lifeforms. A stable level of NAD+ is vital to ensure cellular homeostasis. Some pathogens can modulate NAD+ metabolism to their advantage and even utilize or cleave NAD+ from the host using specialized effectors known as ADP-ribosyltransferase toxins and NADases, leading to energy store depletion, immune evasion, or even cell death. This review explores recent advances in the field of bacterial NAD+-targeting toxins, highlighting the relevance of NAD+ modulation as an emerging pathogenesis strategy. In addition, we discuss the role of specific NAD+-targeting toxins in niche colonization and bacterial lifestyle as components of Toxin/Antitoxin systems and key players in inter-bacterial competition. Understanding the mechanisms of toxicity, regulation, and secretion of these toxins will provide interesting leads in the search for new antimicrobial treatments in the fight against infectious diseases.

Occurrence, antimicrobial susceptibility, and pathogenic factors of Pseudomonas aeruginosa in canine clinical samples

Background and Aim:

Pseudomonas aeruginosa is a relevant opportunistic and difficult to treat pathogen due to its widespread environmental diffusion, intrinsic resistance to many classes of antimicrobials, high ability to acquire additional resistance mechanisms, and wide range of pathogenic factors. The present study aimed to investigate the prevalence of P. aeruginosa in canine clinical samples, the antimicrobial susceptibility against antipseudomonal antibiotics, and the presence of extracellular pathogenic factors of the isolates, as well as their ability to produce biofilm.

Materials and Methods:

Overall, 300 clinical specimens from dogs with pyoderma or abscesses (n=58), otitis (n=59), and suspected bladder infection (n=183) were analyzed by standard bacteriological methods. P. aeruginosa isolates were tested for their antimicrobial susceptibility by disk and gradient diffusion methods to determine the minimum inhibitory concentrations. The ability of the isolates to produce biofilm was investigated by a microtiter plate assay, while virulence genes coding for elastase (lasB), exotoxin A (toxA), alkaline protease (aprA), hemolytic phospholipase C (plcH), and exoenzyme S (ExoS) were detected by polymerase chain reaction method.

Results:

A total of 24 isolates of P. aeruginosa were found in clinical specimens (urine n=3, skin/soft tissue n=6, and ear canal n=15). No resistance was found to ceftazidime, gentamicin, aztreonam, and imipenem (IMI), while low levels of resistance were found to enrofloxacin (ENR) (4.2%) and piperacillin-tazobactam (8.3%). However, 41.7% and 29.2% of the isolates showed intermediate susceptibility to ENR and IMI, respectively. Disk and gradient diffusion methods showed high concordance. The majority of the isolates revealed a weak (33.3%) or intermediate (45.8%) ability to form biofilm, while the strong biofilm producers (20.8%) derived exclusively from the ear canal samples. All isolates (100%) were positive for lasB, aprA, and plcH genes, while exoS and toxA were amplified in 21 (87.5%) and 22 (91.7%) isolates, respectively.

Conclusion:

In the present study, P. aeruginosa isolates from canine clinical samples were characterized by low levels of antimicrobial resistance against antipseudomonal drugs. However, the high presence of isolates with intermediate susceptibility for some categories of antibiotics, including carbapenems which are not authorized for veterinary use, could represent an early warning signal. Moreover, the presence of isolates with strong ability to produce biofilm represents a challenge for the interpretation of the antimicrobial susceptibility profile. In addition, the high prevalence of the extracellular pathogenic factors was indicative of the potential virulence of the isolates.

A systematic review and meta-analysis on Exo-toxins prevalence in hospital acquired Pseudomonas aeruginosa isolates

INTRODUCTION

Pseudomonas aeruginosa (PA) is an opportunistic pathogen that produces widespread and often overwhelming infections. Among different virulence factors, toxins are important bacterial agent which increases PA pathogenesis especially in immunocompromised patients. The aim of this meta-analysis was to determine the prevalence of exotoxin production in PA isolates in the world. Also according to the importance of drug resistance in isolates with more pathogenicity this estimation was conducted in resistant isolates.

METHODS

A systematic search was conducted in international database like PubMed, Scopus, Web of Science and Embase up to December 2018. Joanna Briggs Institute Checklist was used to evaluate the quality assessment of studies. Random effect model was applied to pool the prevalence data. Stata 13 software was used to analyze the data.

RESULTS

Total of 58 eligible studies that fulfilled the inclusion criteria of the study were selected for qualitative synthesis. Among exotoxins; the highest prevalence was related to exoT (0.83 (CI95%: 0.64-0.96)). Lowest prevalence rate was seen in exoU with estimated prevalence 0.32 (CI95%: 0.24-0.41). In Carbapenem resistance isolates exoA and exoT had the highest prevalence (1.00 (CI95%: 0.98-1.00)).

CONCLUSION

This first meta-analysis on PA isolates with toxin potency indicated high prevalence of exotoxin production in clinical isolates of PA which is an alarming point as a clinical aspect. It was found that the ExoT has the most prevalence rate among toxins. The results of simultaneous evaluation of exotoxins and antimicrobial resistance can develop treatment policies against PA infections in hospitals and hospitalized patients.

The Prevalence of Exoenzyme S Gene in Multidrug-Sensitive and Multidrug-Resistant Pseudomonas aeruginosa Clinical Strains

Pseudomonas aeruginosa rods are one of the most commonly isolated microorganisms from clinical specimens, usually responsible for nosocomial infections. Antibiotic-resistant P. aeruginosa strains may present reduced expression of virulence factors. This fact may be caused by appropriate genome management to adapt to changing conditions of the hospital environment. Virulence factors genes may be replaced by those crucial to survive, like antimicrobial resistance genes. The aim of this study was to evaluate, using PCR, the occurrence of exoenzyme S-coding gene (exoS) in two distinct groups of P. aeruginosa strains: 83 multidrug-sensitive (MDS) and 65 multidrug-resistant (MDR) isolates. ExoS gene was noted in 72 (48.7%) of the examined strains: 44 (53.0%) MDS and 28 (43.1%) MDR. The observed differences were not statistically significant (p = 0.1505). P. aeruginosa strains virulence is rather determined by the expression regulation of the possessed genes than the difference in genes frequency amongst strains with different antimicrobial susceptibility patterns.

Infectious endophthalmitis leading to evisceration: spectrum of bacterial and fungal pathogens and antibacterial susceptibility profile

Purpose

To describe the spectrum of bacterial and fungal pathogens in cases of endophthalmitis requiring evisceration and report their antimicrobial susceptibilities.

Methods

Retrospective, consecutive, and descriptive case series of endophthalmitis that underwent evisceration from January 2004 to December 2017. Vitreous samples from all patients had been investigated for bacteria and fungus using institutional protocol. Bacterial isolates were identified using analytical profile index (API) system until 2010 and Vitek-2 compact system (bioMérieux, France), thereafter. The susceptibility of bacterial isolates to a variety of antibiotics was determined by the Kirby-Bauer disk-diffusion method.

Results

Of 791 cases reviewed, culture positivity was reported in 388 cases (48.92%). Commonest clinical setting of endophthalmitis necessitating evisceration was post-microbial keratitis (58%), followed by post-trauma and post-cataract surgery (14–15%). The commonest isolate was Streptococcus pneumoniae, seen in 68 samples overall (17.52%). One hundred and eighty-three isolates (47.16%) were gram-positive, 86 (22.16%) were gram-negative, and fungi constituted 137 (35.3%) isolates. Streptococcus pneumoniae was the commonest gram-positive bacterial isolate seen in 68/183 samples (37.15%). Among gram-negative organisms, the commonest was Pseudomonas aeruginosa seen in 47/86 (54.65%). Aspergillus spp. formed the commonest fungal isolate, 58/137 (42.33%). The susceptibility of the gram-positive bacteria was highest with vancomycin, 136/147 (92.51%) and for gram-negative bacteria was seen best with imipenem 24/29 (82.75%). Susceptibility to ceftazidime was 31/61 (50.81%) in 31/61.

Conclusion

Endophthalmitis due to Pneumococci, Aspergillus, and Pseudomonas can be very fulminant and progress to require evisceration in spite of prompt and appropriate treatment.

Post-translational modification of ESKAPE pathogens as a potential target in drug discovery

ESKAPE pathogens are gaining clinical importance owing to their high pervasiveness and increasing resistance to various antimicrobials. These bacteria have several post-translational modifications (PTMs) that destabilize or divert host cell pathways. Prevalent PTMs of ESKAPE pathogens include addition of chemical groups (acetylation, phosphorylation, methylation and hydroxylation) or complex molecules (AMPylation, ADP-ribosylation, glycosylation and isoprenylation), covalently linked small proteins [ubiquitylation, ubiquitin-like proteins (UBL) conjugation and small ubiquitin-like modifier (SUMO)] or modification of amino acid side-chains (eliminylation and deamidation). Therefore, the understanding of different bacterial PTMs and host proteins manipulated by these PTMs provides better insight into host-pathogen interaction and will also help to develop new antibacterial agents against ESKAPE pathogens.

Presence of exoY, exoS, exoU and exoT genes, antibiotic resistance and biofilm production among Pseudomonas aeruginosa isolates in Northwest Iran

Background:Pseudomonas aeruginosa, as Gram-negative rod bacilli, has an important role in human infection. In the present study we aimed to investigate the presence of exo genes and biofilm production among Pseudomonas aeruginosa isolates in Northwest Iran.

Material and methods: 160 isolates of P. aeruginosa were collected and identified by biochemical tests and were characterized for antibiotic resistance. Biofilm production was evaluated by microtiter plate assay and the presence of exo genes was evaluated by allele-specific PCR (polymerase chain reaction). Chi-square test was used for statistical analysis.

Results: The most effective antibiotics against isolates were colistin and polymyxin B. 87% of the isolates were biofilm producers of which 69% were strongly biofilm producers. 55% of the isolates carried exoY, 52% of the isolates carried exoU, and 26.3% and 5% carried exoS and exoT, respectively.

Conclusion: Our findings showed different distribution of exo genes in clinical isolates of P. aeruginosa in Northwest Iran. ExoS and exoU were more prevalent in non-biofilm producers and exoY was more prevalent in biofilm producer isolates. These results might indicate the importance of exoY in biofilm production of Pseudomonas aeruginosa.

Pseudomonas aeruginosa ExoT Induces Atypical Anoikis Apoptosis in Target Host Cells by Transforming Crk Adaptor Protein into a Cytotoxin

Previously, we demonstrated that Pseudomonas aeruginosa ExoT induces potent apoptosis in host epithelial cells in a manner that primarily depends on its ADP-ribosyltransferase domain (ADPRT) activity. However, the mechanism underlying ExoT/ADPRT-induced apoptosis remains undetermined. We now report that ExoT/ADPRT disrupts focal adhesion sites, activates p38β and JNK, and interferes with integrin-mediated survival signaling; causing atypical anoikis. We show that ExoT/ADPRT-induced anoikis is mediated by the Crk adaptor protein. We found that Crk-/- knockout cells are significantly more resistant to ExoT-induced apoptosis, while Crk-/- cells complemented with Crk are rendered sensitive to ExoT-induced apoptosis. Moreover, a dominant negative (DN) mutant form of Crk phenocopies ExoT-induced apoptosis both kinetically and mechanistically. Crk is generally believed to be a component of focal adhesion (FA) and its role in cellular survival remains controversial in that it has been found to be either pro-survival or pro-apoptosis. Our data demonstrate that although Crk is recruited to FA sites, its function is likely not required for FA assembly or for survival per se. However, when modified by ExoT or by mutagenesis, it can be transformed into a cytotoxin that induces anoikis by disrupting FA sites and interfering with integrin survival signaling. To our knowledge, this is the first example whereby a bacterial toxin exerts its cytotoxicity by subverting the function of an innocuous host cellular protein and turning it against the host cell.

The molecular mechanism of acute lung injury caused by Pseudomonas aeruginosa: from bacterial pathogenesis to host response

Pseudomonas aeruginosa is the most common gram-negative pathogen causing pneumonia in immunocompromised patients. Acute lung injury induced by bacterial exoproducts is associated with a poor outcome in P. aeruginosa pneumonia. The major pathogenic toxins among the exoproducts of P. aeruginosa and the mechanism by which they cause acute lung injury have been investigated: exoenzyme S and co-regulated toxins were found to contribute to acute lung injury. P. aeruginosa secretes these toxins through the recently defined type III secretion system (TTSS), by which gram-negative bacteria directly translocate toxins into the cytosol of target eukaryotic cells. TTSS comprises the secretion apparatus (termed the injectisome), translocators, secreted toxins, and regulatory components. In the P. aeruginosa genome, a pathogenic gene cluster, the exoenzyme S regulon, encodes genes underlying the regulation, secretion, and translocation of TTSS. Four type III secretory toxins, namely ExoS, ExoT, ExoU, and ExoY, have been identified in P. aeruginosa. ExoS is a 49-kDa form of exoenzyme S, a bifunctional toxin that exerts ADP-ribosyltransferase and GTPase-activating protein (GAP) activity to disrupt endocytosis, the actin cytoskeleton, and cell proliferation. ExoT, a 53-kDa form of exoenzyme S with 75% sequence homology to ExoS, also exerts GAP activity to interfere with cell morphology and motility. ExoY is a nucleotidal cyclase that increases the intracellular levels of cyclic adenosine and guanosine monophosphates, resulting in edema formation. ExoU, which exhibits phospholipase A2 activity activated by host cell ubiquitination after translocation, is a major pathogenic cytotoxin that causes alveolar epithelial injury and macrophage necrosis. Approximately 20% of clinical isolates also secrete ExoU, a gene encoded within an insertional pathogenic gene cluster named P. aeruginosa pathogenicity island-2. The ExoU secretory phenotype is associated with a poor clinical outcome in P. aeruginosa pneumonia. Blockade of translocation by TTSS or inhibition of the enzymatic activity of translocated toxins has the potential to decrease acute lung injury and improve clinical outcome.

Burkholderia cenocepacia disrupts host cell actin cytoskeleton by inactivating Rac and Cdc42

Burkholderia cenocepacia, a member of the Burkholderia cepacia complex, is an opportunistic pathogen that causes devastating infections in patients with cystic fibrosis. The ability of B.  cenocepacia to survive within host cells could contribute significantly to its virulence in immunocompromised patients. In this study, we explored the mechanisms that enable B. cenocepacia to survive inside macrophages. We found that B. cenocepacia disrupts the actin cytoskeleton of infected macrophages, drastically altering their morphology. Submembranous actin undergoes depolymerization, leading to cell retraction. The bacteria perturb actin architecture by inactivating Rho family GTPases, particularly Rac1 and Cdc42. GTPase inactivation follows internalization of viable B. cenocepacia and compromises phagocyte function: macropinocytosis and phagocytosis are markedly inhibited, likely impairing the microbicidal and antigen-presenting capability of infected macrophages. The type VI secretion system is essential for the bacteria to elicit these changes. This is the first report demonstrating inactivation of Rho family GTPases by a member of the B.  cepacia complex.

The ADP-ribosylation domain of Pseudomonas aeruginosa ExoS is required for membrane bleb niche formation and bacterial survival within epithelial cells

Pseudomonas aeruginosa can establish a niche within the plasma membrane of epithelial cells (bleb niches) within which bacteria can survive, replicate, and swim at speeds detectable by real-time phase-contrast imaging. This novel virulence strategy is dependent on the bacterial type three secretion system (T3SS), since mutants lacking the T3SS needle or known T3SS effectors localize to perinuclear vacuoles and fail to replicate. Here, we determined which of the three effectors (ExoS, ExoT, or ExoY) were required for bleb niche formation and intracellular replication. PAO1 strains with mutations in exoS, exoT, exoY, or combinations thereof were compared to wild-type and complemented strains. P. aeruginosa exoS mutants, but not exoT or exoY mutants, lost the capacity for bleb niche formation and intracellular replication. Complementation with exoS rescued both phenotypes, either in the background of an exoS mutant or in a mutant lacking all three known effectors. Complementation with activity domain mutants of exoS revealed that the ADP-ribosyltransferase (ADP-r) activity of ExoS, but not the Rho-GAP activity nor the membrane localization domain (MLD) of ExoS, was required to elicit this phenotype. Membrane bleb niches that contained P. aeruginosa also bound annexin V-enhanced green fluorescent protein (EGFP), a marker of early apoptosis. These data show that P. aeruginosa bleb niches and intracellular survival involve ExoS ADP-r activity and implicate a connection between bleb niche formation and the known role(s) of ExoS-mediated apoptosis and/or Rab GTPase inactivation.

The membrane localization domain is required for intracellular localization and autoregulation of YopE in Yersinia pseudotuberculosis

Recent work has shown that a domain of YopE of Yersinia pseudotuberculosis ranging from amino acids 54 to 75 (R. Krall, Y. Zhang, and J. T. Barbieri, J. Biol. Chem. 279:2747-2753, 2004) is required for proper localization of YopE after ectopic expression in eukaryotic cells. This domain, called the membrane localization domain (MLD), has not been extensively studied in Yersinia. Therefore, an in cis MLD deletion mutant of YopE was created in Y. pseudotuberculosis. The mutant was found to secrete and translocate YopE at wild-type levels. However, the mutant was defective in the autoregulation of YopE expression after the infection of HeLa cells. Although the mutant translocated YopE at wild-type levels, it showed a delayed HeLa cell cytotoxicity. This delay was not caused by a change in GTPase activating protein (GAP) activity, since the mutant showed wild-type YopE GAP activity toward Rac1 and RhoA. The MLD mutant displayed a changed intracellular localization pattern of YopE in HeLa cells after infection, and the YopEDeltaMLD protein was found to be dispersed within the whole cell, including the nucleus. In contrast, wild-type YopE was found to localize to the perinuclear region of the cell and was not found in the nucleus. In addition, the yopEDeltaMLD mutant was avirulent. Our results suggest that YopE must target proteins other than RhoA and Rac1 and that the MLD is required for the proper targeting and hence virulence of YopE during infection. Our results raise the question whether YopE is a regulatory protein or a "true" virulence effector protein.

Common themes in the design and function of bacterial effectors

Central to the biology of many pathogenic bacteria are a number of specialized machines, known as type III, type IV, or type VI protein secretion systems. These machines have specifically evolved to deliver bacterial effector proteins into host cells with the capacity to modulate a variety of cellular functions. The identification of the biochemical activities of many effector proteins, coupled with a better understanding of their potential contribution to pathogenesis, has revealed common themes in the evolutionary design and function of these remarkable bacterial proteins.

Modulation of host cell endocytosis by the type III cytotoxin, Pseudomonas ExoS

Pseudomonas aeruginosa ExoS is a bifunctional type III cytotoxin that possesses Rho GTPase-activating protein (RhoGAP) and ADP-ribosyltransferase (ADPr) activities. In the current study, the RhoGAP and ADPr activities of ExoS were tested for the ability to disrupt mammalian epithelial cell physiology. RhoGAP, but not ADPr, inhibited internalization/phagocytosis of bacteria, while ADPr, but not RhoGAP, inhibited vesicle trafficking, both general fluid-phase uptake and EGF-activated EGF receptor (EGFR) degradation. In ADPr-intoxicated cells, upon EGF activation, EGFR co-localized with clathrin-coated vesicles (CCV), which did not mature into Rab5-positive early endosomes. Constitutively, active Rab5 recruited EGFR from CCV to early endosomes. Consistent with the inhibition of Rab5 function by ADPr, several Rab proteins including Rab5 and 9, but not Rab4, were ADP ribosylated by ExoS. Thus, the two enzymatic activities of ExoS have different effects on epithelial cells with RhoGAP inhibiting bacterial internalization and ADPr interfering with CCV maturation. The ability ADPr to inhibit mammalian vesicle trafficking provides a new mechanism for bacterial toxin-mediated virulence.

The type III toxins of Pseudomonas aeruginosa disrupt epithelial barrier function

The type III secreted toxins of Pseudomonas aeruginosa are important virulence factors associated with clinically important infection. However, their effects on bacterial invasion across mucosal surfaces have not been well characterized. One of the most commonly expressed toxins, ExoS, has two domains that are predicted to affect cytoskeletal integrity, including a GTPase-activating protein (GAP) domain, which targets Rho, a major regulator of actin polymerization; and an ADP-ribosylating domain that affects the ERM proteins, which link the plasma membrane to the actin cytoskeleton. The activities of these toxins, and ExoS specifically, on the permeability properties of polarized airway epithelial cells with intact tight junctions were examined. Strains expressing type III toxins altered the distribution of the tight junction proteins ZO-1 and occludin and were able to transmigrate across polarized airway epithelial monolayers, in contrast to DeltaSTY mutants. These effects on epithelial permeability were associated with the ADP-ribosylating domain of ExoS, as bacteria expressing plasmids lacking expression of the ExoS GAP activity nonetheless increased the permeation of fluorescent dextrans, as well as bacteria, across polarized airway epithelial cells. Treatment of epithelial cells with cytochalasin D depolymerized actin filaments and increased permeation across the monolayers but did not eliminate the differential effects of wild-type and toxin-negative mutants on the epithelial cells, suggesting that additional epithelial targets are involved. Confocal imaging studies demonstrated that ZO-1, occludin, and ezrin undergo substantial redistribution in human airway cells intoxicated by ExoS, -T, and -Y. These studies support the hypothesis that type III toxins enhance P. aeruginosa's invasive capabilities by interacting with multiple eukaryotic cytoskeletal components.

Plasma membrane localization affects the RhoGAP specificity ofPseudomonasExoS

Pseudomonas aeruginosa ExoS (453 amino acids) is a bifunctional type III cytotoxin, comprising a Rho GTPase-activating protein domain (RhoGAP), and a 14-3-3 dependent ADP-ribosyltransferase domain. In addition, ExoS contains a membrane localization domain (termed MLD, residues 51-77) which localizes and traffics ExoS within intoxicated host cells. While membrane localization has been shown to be essential for ExoS to ADP-ribosylate Ras, the relationship between intracellular localization and expression of RhoGAP activity has not been addressed. In this study, loss of MLD function was observed to abolish expression of ExoS RhoGAP activity in HeLa cells. One mutation within the MLD (R56, R63, D70 mutated to N, RRD-->N) diminished plasma membrane localization and altered the cell rounding phenotype elicited by ExoS RhoGAP. In addition, cell rounding caused by ExoS-MLD(RRD-->N) was reversed by dominant active Rac1, but not dominant active Cdc42, indicating a switch in ExoS RhoGAP substrate specificity. Mutation of the C-terminal polybasic region abolished the ability of dominant active Rac1 to protect HeLa cells from expression of the RhoGAP activity of ExoS-MLD(RRD-->N). This study shows the importance of membrane localization in the targeting of Rho GTPases by ExoS RhoGAP.

Intracellular trafficking of Pseudomonas ExoS, a type III cytotoxin

Pseudomonas aeruginosa ExoS is a bifunctional type III cytotoxin that disrupts Ras- and Rho-signaling pathways in mammalian cells. A hydrophobic region (residues 51-77, termed the membrane localization domain) targets ExoS to the plasma membrane (PM) and late endosomes of host cells. In the current study, metabolic inhibitors and dominant-negative proteins that disrupt known vesicle-trafficking pathways were used to define the intracellular trafficking of ExoS. Release of ExoS from PM was independent of dynamin and ADP ribosylation factor 6 but inhibited by methyl-beta-cyclodextrin, a cholesterol-depleting reagent, and perinuclear localization of ExoS was disrupted by nocodazole. p50 dynamitin, a dynein inhibitor partially disrupted perinuclear localization of ExoS. Methyl-beta-cyclodextrin and nocodazole inhibited the ability of type-III-delivered ExoS to ADP-ribosylated Golgi/endoplasmic reticulum-resident Ras. Methyl-beta-cyclodextrin also relocated ExoS from the perinuclear region to the PM, indicating that ExoS can cycle through anterograde as well as through retrograde trafficking pathways. These findings show that ExoS endocytosis is cholesterol dependent, and it utilizes host microtubules, for intracellular trafficking. Understanding how type III cytotoxins enter and traffic within mammalian cells may identify new targets for therapeutic intervention of gram-negative bacterial pathogens.

Intracellular localization of type III-delivered Pseudomonas ExoS with endosome vesicles

ExoS (453 amino acids) is a bi-functional type III cytotoxin produced by Pseudomonas aeruginosa. Residues 96-219 include the Rho GTPase-activating protein (RhoGAP) domain, and residues 234-453 include the 14-3-3-dependent ADP-ribosyltransferase domain. Earlier studies also identified an N-terminal domain (termed the membrane localization domain) that comprises residues 51-77 and includes a novel leucine-rich motif that targets ExoS to the perinuclear region of cultured cells. There is limited information on how ExoS or other type III cytotoxins enter and target intracellular host proteins. Type III-delivered ExoS localized to both plasma membrane and perinuclear region, whereas ExoS(DeltaMLD) was localized to the cytosol. Plasma membrane localization of ExoS was transient and had a half-life of approximately 20 min. Type III-delivered ExoS co-immunoprecipitated 14-3-3 proteins and Rab9, Rab6, and Rab5. Immunofluorescence experiments showed that ExoS colocalized with Rab9, Rab6, and Rab5. Fluorescent energy transfer was detected between ExoS and 14-3-3 proteins but not between ExoS and Rabs proteins. Together, these results indicate that type III-delivered ExoS localizes on the host endosomes and utilizes multiple pathways to traffic from the plasma membrane to the perinuclear region of intoxicated host cells.

Gene therapy and wound healing

Wound repair involves the sequential interaction of various cell types, extracellular matrix molecules, and soluble mediators. During the past 10 years, much new information on signals controlling wound cell behavior has emerged. This knowledge has led to a number of novel therapeutic strategies. In particular, the local delivery of pluripotent growth factor molecules to the injured tissue has been intensively investigated over the past decade. Limited success of clinical trails indicates that a crucial aspect of the growth factor wound healing strategy is the effective delivery of these polypeptides to the wound site. A molecular approach in which genetically modified cells synthesize and deliver the desired growth factor in regulated fashion has been used to overcome the limitations associated with the (topical) application of recombinant growth factor proteins. We have summarized the molecular and cellular basis of repair mechanisms and their failure, and we give an overview of techniques and studies applied to gene transfer in tissue repair.

Effects of the type III secreted pseudomonal toxin ExoS in the yeast Saccharomyces cerevisiae

Pseudomonas aeruginosa secretes a number of toxins by a type III system, and these are important in virulence. One of them, ExoS, is a bifunctional toxin, with a GTPase-activating protein domain, as well as ADP ribosyltransferase (ADPRT) activity. These two domains have numerous potential cellular targets, but the overall mechanism of ExoS action remains unclear. The effects of ExoS in a simple eukaryotic system, the yeast Saccharomyces cerevisiae, using a tetracycline-regulated expression system were studied. This system allowed controlled expression of ExoS in yeast, which was not possible using a galactose-induced system. ExoS was found to be an extremely potent inhibitor of yeast growth, and to be largely dependent on the activity of its ADPRT domain. ExoS produced a dramatic alteration in actin distribution, with the appearance of large aggregates of cortical actin, and thickened disorganized cables, entirely dependent on the ADPRT domain. This phenotype is suggestive of actin stabilization, which was verified by showing that the cortical aggregates of actin induced by ExoS were resistant to treatment with latrunculin A, an agent that prevents actin polymerization. ExoS increased the numbers of mating projections produced following growth arrest with mating pheromone, and prevented subsequent DNA replication, an effect that is again dependent on the ADPRT domain. Following pheromone removal, ExoS produced altered development of the mating projections, which became elongated with a swollen bud-like tip. These results suggest alternative pathways for ExoS action in eukaryotic cells that may result from activation of small GTPases, and this yeast expression system is well suited to explore these pathways.

Eukaryotic localization, activation and ubiquitinylation of a bacterial type III secreted toxin

Type III secretion is a widespread method whereby Gram-negative bacteria introduce toxins into eukaryotic cells. These toxins mimic or subvert a normal cellular process by interacting with a specific target, although how toxins reach their site of action is unclear. We set out to investigate the intracellular localization of a type III toxin of Pseudomonas aeruginosa called ExoU, which has phospholipase activity and requires a eukaryotic factor for activity. We found that ExoU is localized to the plasma membrane and undergoes modification within the cell by addition of two ubiquitin molecules at lysine-178. A region of five amino acids at position 679-683 near the C-terminus of the ExoU protein controls both membrane localization and ubiquitinylation. Site-directed mutagenesis identified a tryptophan at position 681 as crucial for these effects. We found that the same region at position 679-683 was also required for cell toxicity produced by ExoU as well as in vitro phospholipase activity. Localization of the phospholipase ExoU to the plasma membrane is thus required for activation and allows efficient utilization of adjacent substrate phospholipids.

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.

A leucine-rich motif targets Pseudomonas aeruginosa ExoS within mammalian cells

Type III cytotoxins contribute to the ability of bacterial pathogens to subvert the host innate immune system. ExoS (453 amino acids) is a bifunctional type III cytotoxin produced by Pseudomonas aeruginosa. Residues 96 to 232 comprise a Rho GTPase activating protein domain, while residues 233 to 453 comprise a 14-3-3-dependent ADP-ribosyltransferase domain. An N-terminal domain (termed the membrane localization domain [MLD]) targets ExoS to the Golgi-endoplasmic reticulum (Golgi-ER) of mammalian cells. This study identifies an amino acid motif that is responsible for the membrane binding properties of the MLD. Deletion mapping showed that the MLD included a symmetrical leucine-rich motif within residues 51 to 77 of ExoS. The terminal dileucines and internal leucines and an isoleucine within the MLD, but not charged or other hydrophobic residues, targeted a reporter protein to the Golgi-ER region of HeLa cells. Mutations of the leucines within the MLD did not affect type III secretion or translocation into HeLa cells but limited the ability of ExoS to ADP-ribosylate Ras GTPases. Mutations of charged residues within the MLD did not affect type III secretion, delivery into HeLa cells, or the ability of ExoS to ADP-ribosylate Ras GTPases. The organization of the leucines within the MLD of ExoS is different from that of previously described leucine-rich motifs but is present in several other bacterial proteins. This implies a role for intracellular targeting in the efficient targeting of mammalian cells by type III cytotoxins.

ADP-ribosylation of membrane proteins: unveiling the secrets of a crucial regulatory mechanism in mammalian cells

Many bacterial toxins kill animal cells by adenosine diphosphate (ADP)-ribosylating intracellular target proteins. Mammalian cells express toxin-related cell surface ADP-ribosyltransferases (ARTs) that transfer ADP-ribose from nicotinamide adenine dinucleotide (NAD) onto arginine residues of other membrane proteins. The association of these glycosylphosphatidylinositol (GPI)-anchored ectoenzymes with glycolipid rafts focuses them onto components of the signal transduction machinery. Exposing murine T cells to NAD, the ART substrate, induces a cascade of reactions that culminates in cell death by apoptosis. This mechanism, dubbed 'NAD-induced cell death' or NICD, is initiated when ART2 ADP-ribosylates the cytolytic P2X7 purinergic receptor, inducing formation of a cation channel, opening of a nonselective pore, shedding of CD62L from the cell surface, exposure of phosphatidylserine on the outer leaflet of the plasma membrane, breakdown of the mitochondrial membrane potential, and DNA-fragmentation. The ART substrate NAD is produced in large amounts inside the cell and can be released from damaged cells during inflammation and tissue injury. In the extracellular environment, the signaling function of NAD is terminated by NAD-degrading ectoenzymes such as CD38. We propose that ART2-catalyzed ADP-ribosylation of P2X7 represents the paradigm of a regulatory mechanism by which ART-expressing cells can sense and respond to the release of NAD from damaged cells.

ADP-ribosylation of integrin alpha7 modulates the binding of integrin alpha7beta1 to laminin

The extracellular domain of integrin alpha7 is ADP-ribosylated by an arginine-specific ecto-ADP-ribosyltransferase after adding exogenous NAD+ to intact C2C12 skeletal muscle cells. The effect of ADP-ribosylation on the structure or function of integrin alpha7beta1 has not been explored. In the present study, we show that ADP-ribosylation of integrin alpha7 takes place exclusively in differentiated myotubes and that this post-translational modification modulates the affinity of alpha7beta1 dimer for its ligand, laminin. ADP-ribosylation in the 37-kDa 'stalk' region of alpha7 that takes place at micromolar NAD+ concentrations increases the binding of the alpha7beta1 dimer to laminin. Increased in vitro binding of integrin alpha7beta1 to laminin after ADP-ribosylation of the 37-kDa fragment of alpha7 requires the presence of Mn2+ and it is not observed in the presence of Mg2+. In contrast, ADP-ribosylation of the 63-kDa N-terminal region comprising the ligand-binding site of alpha7 that occurs at approx. 100 microM NAD+ inhibits the binding of integrin alpha7beta1 to laminin. Furthermore, incubation of C2C12 myotubes with NAD+ increases the expression of an epitope on integrin beta1 subunit recognized by monoclonal antibody 9EG7. We discuss our results based on the current models of integrin activation. We also hypothesize that ADP-ribosylation may represent a mechanism of regulation of integrin alpha7beta1 function in myofibres in vivo when the continuity of the membrane is compromised and NAD+ is available as a substrate for ecto-ADP-ribosylation.

Role of the type III secreted exoenzymes S, T, and Y in systemic spread of Pseudomonas aeruginosa PAO1 in vivo

Pseudomonas aeruginosa uses a dedicated type III secretion system to deliver toxins directly into the cytoplasm of host cells. While progress has been made in elucidating the function of type III-secreted toxins in vitro, the in vivo functions of the type III-secreted exoenzymes are less well understood, particularly for the sequenced strain PAO1. Therefore, we have systematically deleted the genes for the three known type III effector molecules (exoS, exoT, and exoY) in P. aeruginosa PAO1 and assayed the effect of the deletions, both singly and in combination, on cytotoxicity in vitro and in vivo. We found that the type III secretion system acts differently on different cell types, causing an exoST-dependent rounding of a lung epithelial-like cell line in contrast to causing an exoSTY-independent but translocase (popB)-dependent lysis of a macrophage cell line. We utilized an in vivo competitive infection model to test each of our mutants, examining replication in the lung and spread to secondary sites such as the blood and spleen. Type III mutants inoculated intranasally exhibited only a minor defect in replication and survival in the lung, but popB and exoSTY triple mutants were profoundly defective in their ability to spread systemically. Intravenous injection of the mutants indicated that the type III secretion machinery is required for survival in the blood. Furthermore, our findings suggest that the effector-independent popB-dependent cytotoxicity that we and others have observed in vitro in macrophage cell lines may not be of great importance in vivo.

Gene transfer in tissue repair: status, challenges and future directions

Wound repair involves a complex interaction of various cell types, extracellular matrix molecules and soluble mediators. Details on signals controlling wound cell activities are beginning to emerge. In recent years this knowledge has been applied to a number of therapeutic strategies in soft tissue repair. Key challenges include re-adjusting the adult repair process in order to augment diseased healing processes, and providing the basis for a regenerative rather than a reparative wound environment. In particular, the local delivery of pluripotent growth factor molecules to the injured tissue has been intensively investigated over the past decade. Limited success of clinical trials indicates that an important aspect of the growth factor wound-healing paradigm is the effective delivery of these polypeptides to the wound site. A molecular genetic approach in which genetically modified cells synthesise and deliver the desired growth factor in a time-regulated manner is a powerful means to overcome the limitations associated with the (topical) application of recombinant growth factor proteins. This article summarises repair mechanisms and their failure, and gives an overview of techniques and studies applied to gene transfer in tissue repair. It also provides perspectives on potential targets for gene transfer technology.

Pseudomonas aeruginosa causes acute lung injury via the catalytic activity of the patatin-like phospholipase domain of ExoU

OBJECTIVE

Acute lung injury in Pseudomonas aeruginosa pneumonia depends primarily on ExoU toxin being delivered directly into the eukaryotic cell cytosol through the type III secretion system. The amino-acid sequence of ExoU has a potato patatin-like phospholipase domain, similar to the sequence of mammalian Ca-independent phospholipase A2. We examined whether the acute lung injury caused by cytotoxic P. aeruginosa was dependent on the patatin-like phospholipase domain of ExoU.

DESIGN

Laboratory investigation using an established mouse model for P. aeruginosa pneumonia with quantitative measurements of acute lung injury and mortality.

SETTING

University experimental research laboratory.

SUBJECTS

Balb/c mice.

INTERVENTIONS

First, a site-directional mutation was introduced in the predicted catalytically active site of the patatin-like phospholipase domain of recombinant ExoU protein. The effect of the mutation on the catalytic activity of ExoU was tested by the in vitro lysophospholipase A assay. Second, the same site-directional mutation was introduced into the exoU gene of P. aeruginosa PA103. Mice were intratracheally infected with either a wild-type P. aeruginosa strain PA103 or an isogenic mutant containing the mutation in exoU. Acute epithelial lung injury, lung edema, bacteremia, and mortality were evaluated quantitatively.

MEASUREMENTS AND MAIN RESULTS

Recombinant ExoU had lysophospholipase A activity. Site-directional mutations in the predicted catalytic site of ExoU caused a loss of the lysophospholipase A activity. Whereas the airspace instillation of PA103 caused acute lung injury and death of the infected mice, the airspace instillation of isogenic mutants secreting catalytically inactive ExoU were noncytotoxic and did not cause acute lung injury or death of the infected mice.

CONCLUSION

Virulent P. aeruginosa causes acute lung injury and death by the cytotoxic activity derived from the patatin-like phospholipase domain of ExoU.

Cytokeratin 18 interacts with the enteropathogenic Escherichia coli secreted protein F (EspF) and is redistributed after infection

Enteropathogenic Escherichia coli (EPEC) pathogenesis requires the delivery of effector proteins into host cytosol by a type III secretion system. The effector protein EspF, while critical for disruption of epithelial barrier function through alteration of tight junctions, is not required for bacterial viability or attachment. Yeast two-hybrid analyses revealed host intermediate filament (IF) protein cytokeratin 18 (CK18) as an interacting partner of EspF. This was confirmed by co-immunoprecipitation of extracts from EPEC-infected epithelial cells. EPEC infection increased detergent-soluble CK18 amounts without significantly altering CK18 expression. The adaptor protein 14-3-3 binds to CK18 and modulates its solubility. EPEC infection promoted CK18/14-3-3 interactions, corresponding to the increase of CK18 in the soluble fractions. 14-3-3 also co-immunoprecipitated with EspF, suggesting that CK18, 14-3-3 and EspF may form a complex in infected cells. The 14-3-3zeta isoform was co-immunoprecipitated with CK18, suggesting the involvement of specific signalling pathways. Immunofluorescence studies revealed a dramatic alteration in the architecture of the IF network in EPEC-infected epithelial cells. IF fragmentation, evident at 2 h post infection, progressed to a collapse of this network at later time points. The secretion mutant (DeltaescN) failed to alter CK18 solubility and IF morphology, while deletion of espF partially impaired the ability of EPEC to induce CK18 modifications. These results suggest that modifications in 14-3-3 interactions and IF network, modulated by type III secreted proteins, may be an important step in EPEC pathogenesis.

ExoS Rho GTPase-activating protein activity stimulates reorganization of the actin cytoskeleton through Rho GTPase guanine nucleotide disassociation inhibitor

ExoS is a bifunctional Type III cytotoxin of Pseudomonas aeruginosa with N-terminal Rho GTPase-activating protein (RhoGAP) and C-terminal ADP-ribosyltransferase domains. Although the ExoS RhoGAP inactivates Cdc42, Rac, and RhoA in vivo, the relationship between ExoS RhoGAP and the eukaryotic regulators of Rho GTPases is not clear. The present study investigated the roles of Rho GTPase guanine nucleotide disassociation inhibitor (RhoGDI) in the reorganization of actin cytoskeleton mediated by ExoS RhoGAP. A green fluorescent protein-RhoGDI fusion protein was engineered and found to elicit actin reorganization through the inactivation of Rho GTPases. Green fluorescent protein-RhoGDI and ExoS RhoGAP cooperatively stimulated actin reorganization and translocation of Cdc42 from membrane to cytosol, and a RhoGDI mutant, RhoGDI(I177D), that is defective in extracting Rho GTPases off the membrane inhibited the actions of RhoGDI and ExoS RhoGAP on the translocation of Cdc42 from membrane to cytosol. A human RhoGDI small interfering RNA was transfected into HeLa cells to knock down 90% of the endogenous RhoGDI expression. HeLa cells with knockdown RhoGDI were resistant to the reorganization of the actin cytoskeleton elicited by type III-delivered ExoS RhoGAP. This indicates that ExoS RhoGAP and RhoGDI function in series to inactivate Rho GTPases, in which RhoGDI extracting GDP-bound Rho GTPases off the membrane and sequestering them in cytosol is the rate-limiting step in Rho GTPase inactivation. A eukaryotic GTPase-activating protein, p50RhoGAP, showed a similar cooperativity with RhoGDI on actin reorganization, suggesting that ExoS RhoGAP functions as a molecular mimic of eukaryotic RhoGAPs to inactivate Rho GTPases through RhoGDI.

Frontal and stealth attack strategies in microbial pathogenesis

Interactions between microbes and human hosts can range from a benign, even symbiotic collaboration to a competition that may turn fatal--resulting in death of the host, the microbe or both. Despite advances that have been made over the past decades in understanding microbial pathogens, more people worldwide still die every year from infectious disease than from any other cause. This highlights the relevance of continuing to probe the mechanisms used by microorganisms to cause disease, and emphasizes the need for new model systems to advance our understanding of host-pathogen interactions.

The YopD translocator of Yersinia pseudotuberculosis is a multifunctional protein comprised of discrete domains

To establish an infection, Yersinia pseudotuberculosis utilizes a plasmid-encoded type III translocon to microinject several anti-host Yop effectors into the cytosol of target eukaryotic cells. YopD has been implicated in several key steps during Yop effector translocation, including maintenance of yop regulatory control and pore formation in the target cell membrane through which effectors traverse. These functions are mediated, in part, by an interaction with the cognate chaperone, LcrH. To gain insight into the complex molecular mechanisms of YopD function, we performed a systematic mutagenesis study to search for discrete functional domains. We highlighted amino acids beyond the first three N-terminal residues that are dispensable for YopD secretion and confirmed that an interaction between YopD and LcrH is essential for maintenance of yop regulatory control. In addition, discrete domains within YopD that are essential for both pore formation and translocation of Yop effectors were identified. Significantly, other domains were found to be important for effector microinjection but not for pore formation. Therefore, YopD is clearly essential for several discrete steps during efficient Yop effector translocation. Recognition of this modular YopD domain structure provides important insights into the function of YopD.

Ezrin/radixin/moesin proteins are high affinity targets for ADP-ribosylation by Pseudomonas aeruginosa ExoS

Pseudomonas aeruginosa ExoS is a bifunctional type III-secreted cytotoxin. The N terminus (amino acids 96-233) encodes a GTPase-activating protein activity, whereas the C terminus (amino acids 234-453) encodes a factor-activating ExoS-dependent ADP-ribosyltransferase activity. The GTPase-activating protein activity inactivates the Rho GTPases Rho, Rac, and Cdc42 in cultured cells and in vitro, whereas the ADP-ribosylation by ExoS is poly-substrate-specific and includes Ras as an early target for ADP-ribosylation. Infection of HeLa cells with P. aeruginosa producing a GTPase-activating protein-deficient form of ExoS rounded cells, indicating the ADP-ribosyltransferase domain alone is sufficient to elicit cytoskeletal changes. Examination of substrates modified by type III-delivered ExoS identified a 70-kDa protein as an early and predominant target for ADP-ribosylation. Matrix-assisted laser desorption ionization mass spectroscopy identified this protein as moesin, a member of the ezrin/radixin/moesin (ERM) family of proteins. ExoS ADP-ribosylated recombinant moesin at a linear velocity that was 5-fold faster and with a K(m) that was 2 orders of magnitude lower than Ras. Moesin homologs ezrin and radixin were also ADP-ribosylated, indicating the ERMs collectively represent high affinity targets of ExoS. Type III delivered ExoS ADP-ribosylated moesin and ezrin (and/or radixin) in cultured HeLa cells. The ERM proteins contribute to cytoskeleton dynamics, and the ability of ExoS to ADP-ribosylate the ERM proteins links ADP-ribosylation with the cytoskeletal changes associated with ExoS intoxication.

Lysophospholipase A activity of Pseudomonas aeruginosa type III secretory toxin ExoU

Acute lung injury in Pseudomonas aeruginosa pneumonia depends primarily on ExoU that is delivered directly into the eukaryotic cell via the type III secretion system. Recent studies demonstrated that ExoU has lipase activity, and that the cytotoxicity of ExoU is dependent on its patatin-like phospholipase domain. We investigated the phospholipase A (PLA) activity of ExoU. ExoU, but not non-catalytic ExoU-S142A, preincubated with the BEAS-2B cell lysate showed a weak increase of Ca(2+)-independent PLA(2) activity. When activated ExoU was mixed with secretory type PLA(2), more phospholipase activity was observed, suggesting that ExoU has lysophospholipase A (lysoPLA) activity. A significant increase in lysoPLA activity was also observed. Glycerol enhanced this activity and inhibitors of iPLA(2) suppressed ExoU's lysoPLA activity. Our results suggest that ExoU has a potent lysoPLA activity that requires the presence of the catalytically active site Ser(142) with an unknown eukaryotic cell factor(s) for its activation.

Dissection of homologous translocon operons reveals a distinct role for YopD in type III secretion by Yersinia pseudotuberculosis

The homologous pcrGVHpopBD and lcrGVHyopBD translocase operons of Pseudomonas aeruginosa and pathogenic Yersinia spp., respectively, are responsible for the translocation of anti-host effectors into the cytosol of infected eukaryotic cells. In Yersinia, this operon is also required for yop-regulatory control. To probe for key molecular interactions during the infection process, the functional interchangeability of popB/yopB and popD/yopD was investigated. Secretion of PopB produced in trans in a deltayopB null mutant of Yersinia was only observed when co-produced with its native chaperone PcrH, but this was sufficient to complement the yopB translocation defect. The Yersinia deltayopD null mutant synthesized and secreted PopD even in the absence of native PcrH, yet this did not restore YopD-dependent yop-regulatory control or effector translocation. Thus, this suggests that key residues in YopD, which are not conserved in PopD, are essential for functional Yersinia type III secretion.

State of the art: why do the lungs of patients with cystic fibrosis become infected and why can't they clear the infection?

Cystic Fibrosis (CF) lung disease, which is characterized by airway obstruction, chronic bacterial infection, and an excessive inflammatory response, is responsible for most of the morbidity and mortality. Early in life, CF patients become infected with a limited spectrum of bacteria, especially P. aeruginosa. New data now indicate that decreased depth of periciliary fluid and abnormal hydration of mucus, which impede mucociliary clearance, contribute to initial infection. Diminished production of the antibacterial molecule nitric oxide, increased bacterial binding sites (e.g., asialo GM-1) on CF airway epithelial cells, and adaptations made by the bacteria to the airway microenvironment, including the production of virulence factors and the ability to organize into a biofilm, contribute to susceptibility to initial bacterial infection. Once the patient is infected, an overzealous inflammatory response in the CF lung likely contributes to the host's inability to eradicate infection. In response to increased IL-8 and leukotriene B₄ production, neutrophils infiltrate the lung where they release mediators, such as elastase, that further inhibit host defenses, cripple opsonophagocytosis, impair mucociliary clearance, and damage airway wall architecture. The combination of these events favors the persistence of bacteria in the airway. Until a cure is discovered, further investigations into therapies that relieve obstruction, control infection, and attenuate inflammation offer the best hope of limiting damage to host tissues and prolonging survival.

Common infection strategies of plant and animal pathogenic bacteria

Gram-negative bacterial pathogens use common strategies to invade and colonize plant and animal hosts. In many species, pathogenicity depends on a highly conserved type-III protein secretion system that delivers effector proteins into the eukaryotic cell. Effector proteins modulate a variety of host cellular pathways, such as rearrangements of the cytoskeleton and defense responses. The specific set of effectors varies in different bacterial species, but recent studies have revealed structural and functional parallels between some effector proteins from plant and animal pathogenic bacteria. These findings suggest that bacterial pathogens target similar pathways in plant and animal host cells.

The mechanism of action of the Pseudomonas aeruginosa-encoded type III cytotoxin, ExoU

Pseudomonas aeruginosa delivers the toxin ExoU to eukaryotic cells via a type III secretion system. Intoxication with ExoU is associated with lung injury, bacterial dissemination and sepsis in animal model and human infections. To search for ExoU targets in a genetically tractable system, we used controlled expression of the toxin in Saccharomyces cerevisiae. ExoU was cytotoxic for yeast and caused a vacuolar fragmentation phenotype. Inhibitors of human calcium-independent (iPLA(2)) and cytosolic phospholipase A(2) (cPLA(2)) lipase activity reduce the cytotoxicity of ExoU. The catalytic domains of patatin, iPLA(2) and cPLA(2) align or are similar to ExoU sequences. Site-specific mutagenesis of predicted catalytic residues (ExoUS142A or ExoUD344A) eliminated toxicity. ExoU expression in yeast resulted in an accumulation of free palmitic acid, changes in the phospholipid profiles and reduction of radiolabeled neutral lipids. ExoUS142A and ExoUD344A expressed in yeast failed to release palmitic acid. Recombinant ExoU demonstrated lipase activity in vitro, but only in the presence of a yeast extract. From these data we conclude that ExoU is a lipase that requires activation or modification by eukaryotic factors.

c-Jun NH2-terminal kinase-mediated signaling is essential for Pseudomonas aeruginosa ExoS-induced apoptosis

As an opportunistic bacterial pathogen, Pseudomonas aeruginosa mainly affects immunocompromised individuals as well as patients with cystic fibrosis. In a previous study, we showed that ExoS of P. aeruginosa, when injected into host cells through a type III secretion apparatus, functions as an effector molecule to trigger apoptosis in various tissue culture cells. Here, we show that injection of the ExoS into HeLa cells activates c-Jun NH(2)-terminal kinase (JNK) phosphorylation while shutting down ERK1/2 and p38 phosphorylation. Inhibiting JNK activation by expression of a dominant negative JNK1 or with a specific JNK inhibitor abolishes ExoS-triggered apoptosis, demonstrating the requirement for JNK-mediated signaling. Following JNK phosphorylation, cytochrome c is released into the cytosol, leading to the activation of caspase 9 and eventually caspase 3. Although c-Jun phosphorylation is also observed as a result of JNK activation, ongoing host protein synthesis is not essential for the apoptotic induction, suggesting that c-Jun- or other AP-1-driven activation of gene expression is dispensable in this process. Therefore, ExoS has opposing effects on different cellular pathways that regulate apoptosis: it shuts down host cell survival signal pathways by inhibiting ERK1/2 and p38 activation, and it activates proapoptotic pathways through activation of JNK1/2 leading ultimately to cytochrome c release and activation of caspases. These results highlight the modulation of host cell signaling by the type III secretion system during interaction between P. aeruginosa and host cells.

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.

ExsD is a negative regulator of the Pseudomonas aeruginosa type III secretion regulon

Expression of the Pseudomonas aeruginosa type III secretion system is induced by contact with eukaryotic cells, serum or low Ca2+ concentrations. We report that ExsD, a unique protein, is a negative regulator of the type III regulon. Localization studies indicate that ExsD is not secreted by P. aeruginosa. To determine the role of exsD, a non-polar deletion was returned to the chromosome by allelic exchange. The delta exsD mutant is competent for type III secretion and translocation of the ExoU cytotoxin to eukaryotic host cells. To examine the effect of ExsD on transcription, lacZ transcriptional reporter fusions were integrated into the chromosome. Promoters controlling transcription of genes encoding the type III secretory, regulatory and effector proteins demonstrated significant derepression in the delta exsD background. Expression of ExsD from a multicopy plasmid completely repressed transcription of the regulon. Although a mutant in pscC, encoding a structural component of the type III translocase, is repressed for expression of the regulon, a delta exsD, pscC:: omega double mutant is derepressed. Bacterial two-hybrid data indicate that ExsD binds the transcriptional activator of the regulon, ExsA. We conclude that ExsD is a negative regulator and propose that ExsD functions as an ExsA antiactivator to regulate transcription of the regulon.

The family of toxin-related ecto-ADP-ribosyltransferases in humans and the mouse

ADP-ribosyltransferases including toxins secreted by Vibrio cholera, Pseudomonas aerurginosa, and other pathogenic bacteria inactivate the function of human target proteins by attaching ADP-ribose onto a critical amino acid residue. Cross-species polymerase chain reaction (PCR) and database mining identified the orthologs of these ADP-ribosylating toxins in humans and the mouse. The human genome contains four functional toxin-related ADP-ribosyltransferase genes (ARTs) and two related intron-containing pseudogenes; the mouse has six functional orthologs. The human and mouse ART genes map to chromosomal regions with conserved linkage synteny. The individual ART genes reveal highly restricted expression patterns, which are largely conserved in humans and the mouse. We confirmed the predicted extracellular location of the ART proteins by expressing recombinant ARTs in insect cells. Two human and four mouse ARTs contain the active site motif (R-S-EXE) typical of arginine-specific ADP-ribosyltransferases and exhibit the predicted enzyme activities. Two other human ARTs and their murine orthologues deviate in the active site motif and lack detectable enzyme activity. Conceivably, these ARTs may have acquired a new specificity or function. The position-sensitive iterative database search program PSI-BLAST connected the mammalian ARTs with most known bacterial ADP-ribosylating toxins. In contrast, no related open reading frames occur in the four completed genomes of lower eucaryotes (yeast, worm, fly, and mustard weed). Interestingly, these organisms also lack genes for ADP-ribosylhydrolases, the enzymes that reverse protein ADP-ribosylation. This suggests that the two enzyme families that catalyze reversible mono-ADP-ribosylation either were lost from the genomes of these nonchordata eucaryotes or were subject to horizontal gene transfer between kingdoms.