Uncoupling Crk signal transduction by Pseudomonas exoenzyme T

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.

14-3-3 Activated Bacterial Exotoxins AexT and ExoT Share Actin and the SH2 Domains of CRK Proteins as Targets for ADP-Ribosylation

Bacterial exotoxins with ADP-ribosyltransferase activity can be divided into distinct clades based on their domain organization. Exotoxins from several clades are known to modify actin at Arg177; but of the 14-3-3 dependent exotoxins only Aeromonas salmonicida exoenzyme T (AexT) has been reported to ADP-ribosylate actin. Given the extensive similarity among the 14-3-3 dependent exotoxins, we initiated a structural and biochemical comparison of these proteins. Structural modeling of AexT indicated a target binding site that shared homology with Pseudomonas aeruginosa Exoenzyme T (ExoT) but not with Exoenzyme S (ExoS). Biochemical analyses confirmed that the catalytic activities of both exotoxins were stimulated by agmatine, indicating that they ADP-ribosylate arginine residues in their targets. Side-by-side comparison of target protein modification showed that AexT had activity toward the SH2 domain of the Crk-like protein (CRKL), a known target for ExoT. We found that both AexT and ExoT ADP-ribosylated actin and in both cases, the modification compromised actin polymerization. Our results indicate that AexT and ExoT are functional homologs that affect cytoskeletal integrity via actin and signaling pathways to the cytoskeleton.

Bacterial Nucleotidyl Cyclases Activated by Calmodulin or Actin in Host Cells: Enzyme Specificities and Cytotoxicity Mechanisms Identified to Date

Many pathogens manipulate host cell cAMP signaling pathways to promote their survival and proliferation. Bacterial Exoenzyme Y (ExoY) toxins belong to a family of invasive, structurally-related bacterial nucleotidyl cyclases (NC). Inactive in bacteria, they use proteins that are uniquely and abundantly present in eukaryotic cells to become potent, unregulated NC enzymes in host cells. Other well-known members of the family include Bacillus anthracis Edema Factor (EF) and Bordetella pertussis CyaA. Once bound to their eukaryotic protein cofactor, they can catalyze supra-physiological levels of various cyclic nucleotide monophosphates in infected cells. Originally identified in Pseudomonas aeruginosa, ExoY-related NC toxins appear now to be more widely distributed among various γ- and β-proteobacteria. ExoY-like toxins represent atypical, poorly characterized members within the NC toxin family. While the NC catalytic domains of EF and CyaA toxins use both calmodulin as cofactor, their counterparts in ExoY-like members from pathogens of the genus Pseudomonas or Vibrio use actin as a potent cofactor, in either its monomeric or polymerized form. This is an original subversion of actin for cytoskeleton-targeting toxins. Here, we review recent advances on the different members of the NC toxin family to highlight their common and distinct functional characteristics at the molecular, cytotoxic and enzymatic levels, and important aspects that need further characterizations.

Pseudomonas aeruginosa ExoT induces G1 cell cycle arrest in melanoma cells

Recently, we demonstrated that Pseudomonas aeruginosa Exotoxin T (ExoT) employs two distinct mechanisms to induce potent apoptotic cytotoxicity in a variety of cancer cell lines. We further demonstrated that it can significantly reduce tumour growth in an animal model for melanoma. During these studies, we observed that melanoma cells that were transfected with ExoT failed to undergo mitosis, regardless of whether they eventually succumbed to ExoT-induced apoptosis or survived in ExoT's presence. In this report, we sought to investigate ExoT's antiproliferative activity in melanoma. We delivered ExoT into B16 melanoma cells by bacteria (to show necessity) and by transfection (to show sufficiency). Our data indicate that ExoT exerts a potent antiproliferative function in melanoma cells. We show that ExoT causes cell cycle arrest in G1 interphase in melanoma cells by dampening the G1/S checkpoint proteins. Our data demonstrate that both domains of ExoT; (the ADP-ribosyltransferase (ADPRT) domain and the GTPase activating protein (GAP) domain); contribute to ExoT-induced G1 cell cycle arrest in melanoma. Finally, we show that the ADPRT-induced G1 cell cycle arrest in melanoma cells likely involves the Crk adaptor protein. Our data reveal a novel virulence function for ExoT and further highlight the therapeutic potential of ExoT against cancer.

Exceptionally versatile – arginine in bacterial post-translational protein modifications

Post-translational modifications (PTM) are the evolutionary solution to challenge and extend the boundaries of genetically predetermined proteomic diversity. As PTMs are highly dynamic, they also hold an enormous regulatory potential. It is therefore not surprising that out of the 20 proteinogenic amino acids, 15 can be post-translationally modified. Even the relatively inert guanidino group of arginine is subject to a multitude of mostly enzyme mediated chemical changes. The resulting alterations can have a major influence on protein function. In this review, we will discuss how bacteria control their cellular processes and develop pathogenicity based on post-translational protein-arginine modifications.

Targeting ADP-ribosylation as an antimicrobial strategy

ADP-ribosylation (ADPr) is an ancient reversible modification of cellular macromolecules controlling major biological processes as diverse as DNA damage repair, transcriptional regulation, intracellular transport, immune and stress responses, cell survival and proliferation. Furthermore, enzymatic reactions of ADPr are central in the pathogenesis of many human diseases, including infectious conditions. By providing a review of ADPr signalling in bacterial systems, we highlight the relevance of this chemical modification in the pathogenesis of human diseases depending on host-pathogen interactions. The post-antibiotic era has raised the need to find alternative approaches to antibiotic administration, as major pathogens becoming resistant to antibiotics. An in-depth understanding of ADPr reactions provides the rationale for designing novel antimicrobial strategies for treatment of infectious diseases. In addition, the understanding of mechanisms of ADPr by bacterial virulence factors offers important hints to improve our knowledge on cellular processes regulated by eukaryotic homologous enzymes, which are often involved in the pathogenesis of human diseases.

14-3-3 proteins activate Pseudomonas exotoxins-S and -T by chaperoning a hydrophobic surface

Pseudomonas are a common cause of hospital-acquired infections that may be lethal. ADP-ribosyltransferase activities of Pseudomonas exotoxin-S and -T depend on 14-3-3 proteins inside the host cell. By binding in the 14-3-3 phosphopeptide binding groove, an amphipathic C-terminal helix of ExoS and ExoT has been thought to be crucial for their activation. However, crystal structures of the 14-3-3β:ExoS and -ExoT complexes presented here reveal an extensive hydrophobic interface that is sufficient for complex formation and toxin activation. We show that C-terminally truncated ExoS ADP-ribosyltransferase domain lacking the amphipathic binding motif is active when co-expressed with 14-3-3. Moreover, swapping the amphipathic C-terminus with a fragment from Vibrio Vis toxin creates a 14-3-3 independent toxin that ADP-ribosylates known ExoS targets. Finally, we show that 14-3-3 stabilizes ExoS against thermal aggregation. Together, this indicates that 14-3-3 proteins activate exotoxin ADP-ribosyltransferase domains by chaperoning their hydrophobic surfaces independently of the amphipathic C-terminal segment.

The cellular toxicity of Pseudomonas exotoxin-S and -T depends on their activation by 14-3-3 but the underlying molecular mechanism is not fully understood. Here, the authors show that a previously unrecognized 14-3-3:exotoxin binding interface is sufficient for complex formation and toxin activation.

Apoptosis and Compensatory Proliferation Signaling Are Coupled by CrkI-Containing Microvesicles

Apoptosis has been implicated in compensatory proliferation signaling (CPS), whereby dying cells induce proliferation in neighboring cells as a means to restore homeostasis. The nature of signaling between apoptotic cells and their neighboring cells remains largely unknown. Here we show that a fraction of apoptotic cells produce and release CrkI-containing microvesicles (distinct from exosomes and apoptotic bodies), which induce proliferation in neighboring cells upon contact. We provide visual evidence of CPS by videomicroscopy. We show that purified vesicles in vitro and in vivo are sufficient to stimulate proliferation in other cells. Our data demonstrate that CrkI inactivation by ExoT bacterial toxin or by mutagenesis blocks vesicle formation in apoptotic cells and inhibits CPS, thus uncoupling apoptosis from CPS. We further show that c-Jun amino-terminal kinase (JNK) plays a pivotal role in mediating vesicle-induced CPS in recipient cells. CPS could have important ramifications in diseases that involve apoptotic cell death.

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.

p130Cas/BCAR1 scaffold protein in tissue homeostasis and pathogenesis

BCAR1 (also known as p130Cas/BCAR1) is an adaptor protein that belongs to the CAS family of scaffold proteins. In the past years, increasing evidence has demonstrated the ability of p130Cas/BCAR1 to activate signaling originating from mechanical stimuli, cell-extracellular matrix (ECM) adhesion and growth factor stimulation cascades during normal development and disease in various biological models. In this review we will specifically discuss the more recent data on the contribution of p130Cas/BCAR1 in the regulation of tissue homeostasis and its potential implications in pathological conditions.

Sequential inactivation of Rho GTPases and Lim kinase by Pseudomonas aeruginosa toxins ExoS and ExoT leads to endothelial monolayer breakdown

Pseudomonas aeruginosa is a major human opportunistic pathogen and one of the most important causal agents of bacteremia. For non-blood-borne infection, bacterial dissemination requires the crossing of the vascular endothelium, the main barrier between blood and the surrounding tissues. Here, we investigated the effects of P. aeruginosa type 3 secretion effectors, namely ExoS, ExoT, and ExoY, on regulators of actin cytoskeleton dynamics in primary endothelial cells. ExoS and ExoT similarly affected the Lim kinase-cofilin pathway, thereby promoting actin filament severing. Cofilin activation was also observed in a mouse model of P. aeruginosa-induced acute pneumonia. Rho, Rac, and Cdc42 GTPases were sequentially inactivated, leading to inhibition of membrane ruffling, filopodia, and stress fiber collapse, and focal adhesion disruption. At the end of the process, ExoS and ExoT produced a dramatic retraction in all primary endothelial cell types tested and thus a rupture of the endothelial monolayer. ExoY alone had no effect in this context. Cell retraction could be counteracted by overexpression of actin cytoskeleton regulators. In addition, our data suggest that moesin is neither a direct exotoxin target nor an important player in this process. We conclude that any action leading to inhibition of actin filament breakdown will improve the barrier function of the endothelium during P. aeruginosa infection.

The Chlamydia trachomatis type III secretion chaperone Slc1 engages multiple early effectors, including TepP, a tyrosine-phosphorylated protein required for the recruitment of CrkI-II to nascent inclusions and innate immune signaling

Chlamydia trachomatis, the causative agent of trachoma and sexually transmitted infections, employs a type III secretion (T3S) system to deliver effector proteins into host epithelial cells to establish a replicative vacuole. Aside from the phosphoprotein TARP, a Chlamydia effector that promotes actin re-arrangements, very few factors mediating bacterial entry and early inclusion establishment have been characterized. Like many T3S effectors, TARP requires a chaperone (Slc1) for efficient translocation into host cells. In this study, we defined proteins that associate with Slc1 in invasive C. trachomatis elementary bodies (EB) by immunoprecipitation coupled with mass spectrometry. We identified Ct875, a new Slc1 client protein and T3S effector, which we renamed TepP (Translocated early phosphoprotein). We provide evidence that T3S effectors form large molecular weight complexes with Scl1 in vitro and that Slc1 enhances their T3S-dependent secretion in a heterologous Yersinia T3S system. We demonstrate that TepP is translocated early during bacterial entry into epithelial cells and is phosphorylated at tyrosine residues by host kinases. However, TepP phosphorylation occurs later than TARP, which together with the finding that Slc1 preferentially engages TARP in EBs leads us to postulate that these effectors are translocated into the host cell at different stages during C. trachomatis invasion. TepP co-immunoprecipitated with the scaffolding proteins CrkI-II during infection and Crk was recruited to EBs at entry sites where it remained associated with nascent inclusions. Importantly, C. trachomatis mutants lacking TepP failed to recruit CrkI-II to inclusions, providing genetic confirmation of a direct role for this effector in the recruitment of a host factor. Finally, endocervical epithelial cells infected with a tepP mutant showed altered expression of a subset of genes associated with innate immune responses. We propose a model wherein TepP acts downstream of TARP to recruit scaffolding proteins at entry sites to initiate and amplify signaling cascades important for the regulation of innate immune responses to Chlamydia.

Exoenzyme S ADP-ribosylates Rab5 effector sites to uncouple intracellular trafficking

Pseudomonas aeruginosa exoenzyme S (ExoS) ADP-ribosylates multiple eukaryotic targets to promote cytopathology and bacterial colonization. ADP-ribosylation of the small GTPase Rab5 has previously been shown to block fluid-phase endocytosis and trafficking of plasma membrane receptors to the early endosomes as well as inhibit phagocytosis of the bacterium. In this study, ExoS is shown to be capable of ADP-ribosylating 6 candidate arginine residues that are located in the effector binding region or in the C terminus of Rab5. Two Rab5 derivatives were engineered, which contained Arg→Ala mutations at four Arg residues within the effector binding region (EF) or two Arg residues within the C-terminal tail (TL). Expression of Rab5(TL) does not affect the ability of ExoS to modify intracellular trafficking, while expression of Rab5(EF) rescued the ability of ExoS to inhibit intracellular trafficking. ADP-ribosylation of effector arginines likely uncouples Rab5 signaling to downstream effectors. This is a different mechanism for inhibition than observed for the ADP-ribosylation of Ras by ExoS, where ADP-ribosylated Ras loses the ability to bind guanine nucleotide exchange factor (GEF). Other experiments showed that expression of dominant negative Rab5(Ser34Asn) does not inhibit ExoS trafficking to the perinuclear region of intoxicated cells. This study provides insight into a mechanism for how ExoS ADP-ribosylation of Rab5 inhibits Rab5 function.

Regulation of Rab5 function during phagocytosis of live Pseudomonas aeruginosa in macrophages

Pseudomonas aeruginosa, a Gram-negative opportunistic human pathogen, is a frequent cause of severe hospital-acquired infections. Effectors produced by the type III secretion system disrupt mammalian cell membrane trafficking and signaling and are integral to the establishment of P. aeruginosa infection. One of these effectors, ExoS, ADP-ribosylates several host cell proteins, including Ras and Rab GTPases. In this study, we demonstrated that Rab5 plays a critical role during early stages of P. aeruginosa invasion of J774-Eclone macrophages. We showed that live, but not heat-inactivated, P. aeruginosa inhibited phagocytosis and that this occurred in conjunction with downregulation of Rab5 activity. Inactivation of Rab5 was dependent on ExoS ADP-ribosyltransferase activity, and in J744-Eclone cells, ExoS ADP-ribosyltransferase activity caused a more severe inhibition of phagocytosis than ExoS Rho GTPase activity. Furthermore, we found that expression of Rin1, a Rab5 guanine exchange factor, but not Rabex5 and Rap6, partially reversed the inactivation of Rab5 during invasion of live P. aeruginosa. These studies provide evidence that live P. aeruginosa cells are able to influence their rate of phagocytosis in macrophages by directly regulating activation of Rab5.

p130Cas: a key signalling node in health and disease

p130Cas/breast cancer anti-oestrogen resistance 1 (BCAR1) is a member of the Cas (Crk-associated substrate) family of adaptor proteins, which have emerged as key signalling nodes capable of interactions with multiple proteins, with important regulatory roles in normal and pathological cell function. The Cas family of proteins is characterised by the presence of multiple conserved motifs for protein-protein interactions, and by extensive tyrosine and serine phosphorylations. Recent studies show that p130Cas contributes to migration, cell cycle control and apoptosis. p130Cas is essential during early embryogenesis, with a critical role in cardiovascular development. Furthermore, p130Cas has been reported to be involved in the development and progression of several human cancers. p130Cas is able to perform roles in multiple processes due to its capacity to regulate a diverse array of signalling pathways, transducing signals from growth factor receptor tyrosine kinases, non-receptor tyrosine kinases, and integrins. In this review we summarise the current understanding of the structure, function, and regulation of p130Cas, and discuss the importance of p130Cas in both physiological and pathophysiological settings, with a focus on the cardiovascular system and cancer.

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

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

Traversal of multilayered corneal epithelia by cytotoxic Pseudomonas aeruginosa requires the phospholipase domain of exoU

PURPOSE

Pseudomonas aeruginosa isolates from microbial keratitis are invasive or cytotoxic toward mammalian cells, depending on their type III secreted toxins. Cytotoxic strains express ExoU, a phospholipase that contributes to corneal virulence. This study determined whether the ExoU phospholipase domain is required for P. aeruginosa traversal of the human corneal epithelium.

METHODS

P. aeruginosa traversal of airlifted, multilayered, human corneal epithelial cells was quantified in vitro up to 8 hours after apical inoculation with ∼10⁶ cfu of strain PA14, or an isogenic exoU mutant (PA14ΔexoU). In addition, PA14ΔexoU or its triple effector mutant PA14ΔexoUΔexoTΔexoY, were complemented with exoU (pUCPexoU), phospholipase-inactive exoU (pUCPexoUD344A), or control plasmid (pUCP18). Transepithelial resistance (TER) was measured (by epithelial volt ohmmeter), and cytotoxicity was determined by trypan blue staining.

RESULTS

PA14 traversed more efficiently than its exoU mutant at 4, 6, and 8 hours after inoculation (100-, 20-, and 8-fold, respectively; P < 0.05), but not at 2 hours. Cells exposed to PA14 lost TER to baseline (P < 0.05). Controls confirmed PA14 cytotoxicity toward these corneal epithelial cells that was absent with exoU mutants. Epithelial traversal, cytotoxicity, and lost TER were restored for PA14ΔexoU, or PA14ΔexoUΔexoTΔexoY, by complementation with pUCPexoU, but not by complementation with pUCPexoUD344A.

CONCLUSIONS

Traversal of multilayered corneal epithelia in vitro by cytotoxic P. aeruginosa requires ExoU with an active phospholipase domain. Correlative loss of TER with traversal by wild-type, or exoU-complemented, bacteria suggests involvement of epithelial cell death and/or lost tight junction integrity. However, traversal by exoU mutants without reduced TER suggests that additional mechanisms are also operative.

The Tensin-3 protein, including its SH2 domain, is phosphorylated by Src and contributes to tumorigenesis and metastasis

In cell lines from advanced lung cancer, breast cancer, and melanoma, endogenous tensin-3 contributes to cell migration, anchorage-independent growth, and tumorigenesis. Although SH2 domains have not been reported previously to be phosphorylated, the tensin-3 SH2 domain is a physiologic substrate for Src. Tyrosines in the SH2 domain contribute to the biological activity of tensin-3, and phosphorylation of these tyrosines can regulate ligand binding. In a mouse breast cancer model, tensin-3 tyrosines are phosphorylated in a Src-associated manner in primary tumors, and experimental metastases induced by tumor-derived cell lines depend on endogenous tensin-3. Thus, tensin-3 is implicated as an oncoprotein regulated by Src and possessing an SH2 domain with a previously undescribed mechanism for the regulation of ligand binding.

The type III secretion system of Pseudomonas aeruginosa: infection by injection

The Gram-negative bacterium Pseudomonas aeruginosa uses a complex type III secretion apparatus to inject effector proteins into host cells. The configuration of this secretion machinery, the activities of the proteins that are injected by it and the consequences of this process for infection are now being elucidated. This Review summarizes our current knowledge of P. aeruginosa type III secretion, including the secretion and translocation machinery, the regulation of this machinery, and the associated chaperones and effector proteins. The features of this interesting secretion system have important implications for the pathogenesis of P. aeruginosa infections and for other type III secretion systems.

Crk and CrkL adaptor proteins: networks for physiological and pathological signaling

The Crk adaptor proteins (Crk and CrkL) constitute an integral part of a network of essential signal transduction pathways in humans and other organisms that act as major convergence points in tyrosine kinase signaling. Crk proteins integrate signals from a wide variety of sources, including growth factors, extracellular matrix molecules, bacterial pathogens, and apoptotic cells. Mounting evidence indicates that dysregulation of Crk proteins is associated with human diseases, including cancer and susceptibility to pathogen infections. Recent structural work has identified new and unusual insights into the regulation of Crk proteins, providing a rationale for how Crk can sense diverse signals and produce a myriad of biological responses.

Pseudomonas aeruginosa virulence and therapy: evolving translational strategies

OBJECTIVE

Although most reviews of Pseudomonas aeruginosa therapeutics focus on antibiotics currently in use or in the pipeline, we review evolving translational strategies aimed at using virulence factor antagonists as adjunctive therapies.

DATA SOURCE

Current literature regarding P. aeruginosa virulence determinants and approaches that target them, with an emphasis on type III secretion, quorum-sensing, biofilms, and flagella.

DATA EXTRACTION AND SYNTHESIS

P. aeruginosa remains one of the most important pathogens in nosocomial infections, with high associated morbidity and mortality. Its predilection to develop resistance to antibiotics and expression of multiple virulence factors contributes to the frequent ineffectiveness of current therapies. Among the many P. aeruginosa virulence determinants that impact infections, type III secretion, quorum sensing, biofilm formation, and flagella have been the focus on much recent investigation. Here we review how increased understanding of these important bacterial structures and processes has enabled the development of novel approaches to inhibit each. These promising translational strategies may lead to the development of adjunctive therapies capable of improving outcomes.

CONCLUSIONS

Adjuvant therapies directed against virulence factors have the potential to improve outcomes in P. aeruginosa infections.

Focal adhesion components are essential for mammalian cell cytokinesis

The final stages in mammalian cytokinesis are poorly understood. Previously, we reported that the ADP-ribosyltransferase activity of Pseudomonas aeruginosa type III secreted toxin ExoT inhibits late stages of cytokinesis. Given that Crk adaptor proteins are the major substrates of ExoT ADP-ribosyltransferase activity, we tested the involvement of Crk in cytokinesis. We report that the focal adhesion-associated proteins, Crk and paxillin are essential for completion of cytokinesis. When their function is absent, the cytoplasmic bridge fails to resolve and the daughter cells fuse to form a binucleated cell. During cytokinesis, Crk is required for syntaxin-2 recruitment to the midbody, while paxillin is required for both Crk and syntaxin-2 localization to this compartment. Our data demonstrate that the subcellular localization and the activity of RhoA and citron K, which are essential for early stages of cytokinesis, are not dependent on paxillin, Crk or syntaxin-2. These studies reveal a novel role for Crk and paxillin in cytokinesis and suggest that focal adhesion complex, as a unit, may partake in this fundamental cellular process.

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.

RNAi screen reveals an Abl kinase-dependent host cell pathway involved in Pseudomonas aeruginosa internalization

Internalization of the pathogenic bacterium Pseudomonas aeruginosa by non-phagocytic cells is promoted by rearrangements of the actin cytoskeleton, but the host pathways usurped by this bacterium are not clearly understood. We used RNAi-mediated gene inactivation of ∼80 genes known to regulate the actin cytoskeleton in Drosophila S2 cells to identify host molecules essential for entry of P. aeruginosa. This work revealed Abl tyrosine kinase, the adaptor protein Crk, the small GTPases Rac1 and Cdc42, and p21-activated kinase as components of a host signaling pathway that leads to internalization of P. aeruginosa. Using a variety of complementary approaches, we validated the role of this pathway in mammalian cells. Remarkably, ExoS and ExoT, type III secreted toxins of P. aeruginosa, target this pathway by interfering with GTPase function and, in the case of ExoT, by abrogating P. aeruginosa–induced Abl-dependent Crk phosphorylation. Altogether, this work reveals that P. aeruginosa utilizes the Abl pathway for entering host cells and reveals unexpected complexity by which the P. aeruginosa type III secretion system modulates this internalization pathway. Our results furthermore demonstrate the applicability of using RNAi screens to identify host signaling cascades usurped by microbial pathogens that may be potential targets for novel therapies directed against treatment of antibiotic-resistant infections.

Mortality from Pseudomonas aeruginosa infections, one of the leading causes of hospital acquired infections, approaches 40%, and multiple drug resistant infections are common and increasing. Internalization of P. aeruginosa by the host cell appears to play a fundamental role in the pathogenesis of this opportunistic bacterium, but the host cell factors involved in this process are incompletely understood. We used a targeted RNAi screen in Drosophila S2 cells to identify a subset of regulators of the host actin cytoskeleton that contribute to bacterial entry and confirmed their involvement in infection of mammalian cells. We found that P. aeruginosa can modulate this internalization pathway in a complex manner by injecting the bacterial toxins ExoS and ExoT into the host cell via its type III secretion system. The identified host cell molecules may serve as targets for novel drugs to treat infections resistant to conventional antibiotics and may be applicable to a wide range of pathogens.

Molecular mechanisms of the cytotoxicity of ADP-ribosylating toxins

Bacterial pathogens utilize toxins to modify or kill host cells. The bacterial ADP-ribosyltransferases are a family of protein toxins that covalently transfer the ADP-ribose portion of NAD to host proteins. Each bacterial ADP-ribosyltransferase toxin modifies a specific host protein(s) that yields a unique pathology. These toxins possess the capacity to enter a host cell or to use a bacterial Type III apparatus for delivery into the host cell. Advances in our understanding of bacterial toxin action parallel the development of biophysical and structural biology as well as our understanding of the mammalian cell. Bacterial toxins have been utilized as vaccines, as tools to dissect host cell physiology, and more recently for the development of novel therapies to treat human disease.

Enhanced sensitivity to cholera toxin in ADP-ribosylarginine hydrolase-deficient mice

Cholera toxin (CT) produced by Vibrio cholerae causes the devastating diarrhea of cholera by catalyzing the ADP-ribosylation of the alpha subunit of the intestinal Gs protein (Gsalpha), leading to characteristic water and electrolyte losses. Mammalian cells contain ADP-ribosyltransferases similar to CT and an ADP-ribosyl(arginine)protein hydrolase (ADPRH), which cleaves the ADP-ribose-(arginine)protein bond, regenerating native protein and completing an ADP-ribosylation cycle. We hypothesized that ADPRH might counteract intoxication by reversing the ADP-ribosylation of Gsalpha. Effects of intoxication on murine ADPRH-/- cells were greater than those on wild-type cells and were significantly reduced by overexpression of wild-type ADPRH in ADPRH-/- cells, as evidenced by both ADP-ribose-arginine content and Gsalpha modification. Similarly, intestinal loops in the ADPRH-/- mouse were more sensitive than their wild-type counterparts to toxin effects on fluid accumulation, Gsalpha modification, and ADP-ribosylarginine content. Thus, CT-catalyzed ADP-ribosylation of cell proteins can be counteracted by ADPRH, which could function as a modifier gene in disease. Further, our study demonstrates that enzymatic cross talk exists between bacterial toxin ADP-ribosyltransferases and host ADP-ribosylation cycles. In disease, toxin-catalyzed ADP-ribosylation overwhelms this potential host defense system, resulting in persistence of ADP-ribosylation and intoxication of the cell.

Further characterization of a type III secretion system (T3SS) and of a new effector protein from a clinical isolate of Aeromonas hydrophila--part I

A type III secretion system (T3SS)-associated cytotoxin, AexT, with ADP-ribosyltransferase activity and homology to Pseudomonas aeruginosa bifuncational toxins ExoT/S, was recently identified from a fish pathogen Aeromonas salmonicida. In this study, we reported the molecular characterization of an aexT-like toxin gene (designated as aexU) from a diarrheal isolate SSU of A. hydrophila. The aexU gene was 1539bp in length and encoded a protein of 512 amino acid (aa) residues. The NH(2)-terminus of AexU (aa residues 1-231) exhibited a 67% homology with the NH(2)-terminus of AexT from A. salmonicida. Importantly, its COOH-terminus (aa residues 232-512) had no homology with any known functional proteins in the database; however, the full-length AexU retained ADP-ribosyltransferase activity. The expression and subsequent secretion of AexU was T3SS dependent, as inactivation of the ascV gene that codes for an inner-membrane component of the T3SS channel from the wild-type (WT) bacterium, blocked translocation of AexU in HT-29 human colonic epithelial cells. We provided evidence that inactivation of acrV and axsE genes (homologs of lcrV and exsE in Yersinia species and P. aeruginosa, respectively) from A. hydrophila SSU, altered expression and/or secretion of AexU. We deleted an aexU gene from the WT, as well as from the DeltaaopB mutant, of A. hydrophila, generating a single knockout (DeltaaexU) and a double knockout mutant, DeltaaopB/DeltaaexU. Increased phagocytosis was observed in RAW264.7 murine macrophages infected with the DeltaaopB/DeltaaexU mutant, as compared to macrophages when infected with the parental DeltaaopB strain. Further, mice infected with the DeltaaexU mutant had a 60% survival rate, compared to animals infected with the WT or the DeltaaexU-complemented strain that caused 90-100% of the animals to die at a 2-3 LD(50s) dose. Immunization of mice with the recombinant AexU protected them from subsequent lethal challenge dose by the WT bacterium. Finally, we detected specific anti-AexU antibodies in the sera of mice that survived challenge by the WT bacterium, which may indicate that AexU plays an important role in the pathogenesis of Aeromonas infections.

The ubiquitin ligase Cbl-b limits Pseudomonas aeruginosa exotoxin T-mediated virulence

Pseudomonas aeruginosa, an important cause of opportunistic infections in humans, delivers bacterial cytotoxins by type III secretion directly into the host cell cytoplasm, resulting in disruption of host cell signaling and host innate immunity. However, little is known about the fate of the toxins themselves following injection into the host cytosol. Here, we show by both in vitro and in vivo studies that the host ubiquitin ligase Cbl-b interacts with the type III-secreted effector exotoxin T (ExoT) and plays a key role in vivo in limiting bacterial dissemination mediated by ExoT. We demonstrate that, following polyubiquitination, ExoT undergoes regulated proteasomal degradation in the host cell cytosol. ExoT interacts with the E3 ubiquitin ligase Cbl-b and Crk, the substrate for the ExoT ADP ribosyltransferase (ADPRT) domain. The efficiency of degradation is dependent upon the activity of the ADPRT domain. In mouse models of acute pneumonia and systemic infection, Cbl-b is specifically required to limit the dissemination of ExoT-producing bacteria whereas c-Cbl plays no detectable role. To the best of our knowledge, this represents the first identification of a mammalian gene product that is specifically required for in vivo resistance to disease mediated by a type III-secreted effector.

Pseudomonas aeruginosa type III-secreted toxin ExoT inhibits host-cell division by targeting cytokinesis at multiple steps

Pseudomonas aeruginosa is an opportunistic pathogen that requires preexisiting epithelial injury to cause acute infections. We report that P. aeruginosa inhibits mammalian cytokinesis in a type III secretion system and exotoxin T (ExoT)-dependent manner. ExoT is a bifunctional type III secretion system effector protein that contains an N-terminal GTPase-activating protein domain and a C-terminal ADP-ribosyl transferase domain. Each of its domains inhibits cytokinesis in a kinetically, morphologically, and mechanistically distinct manner. The GTPase-activating protein-mediated inhibition of cytokinesis occurs early, likely as a consequence of its inhibitory effect on RhoA. The ADP-ribosyl transferase domain inhibits late steps of cytokinesis by blocking syntaxin-2 localization to the midbody, an event essential for completion of cytokinesis. These findings provide an example of a bacterial pathogen targeting cytokinesis.