Role of small GTPases and alphavbeta5 integrin in Pseudomonas aeruginosa-induced increase in lung endothelial permeability

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.

Estrogen Alleviates Sex-Dependent Differences in Lung Bacterial Clearance and Mortality Secondary to Bacterial Pneumonia after Traumatic Brain Injury

Traumatic brain injury (TBI) is the leading cause of injury-related death and disability in patients under the age of 46 years. Survivors of the initial injury often endure systemic complications such as pulmonary infection, and Pseudomonas aeruginosa is one of the most common causes of nosocomial pneumonia in intensive care units. Female patients are less likely to develop secondary pneumonia after TBI, and pre-clinical studies have revealed a salutary role for estrogen after trauma. Therefore, we hypothesized that female mice would experience less mortality after post-TBI pneumonia with P. aeruginosa. We employed a mouse model of TBI followed by P. aeruginosa pneumonia. Male mice had greater mortality and impaired lung bacterial clearance after post-TBI pneumonia compared with female mice. This was confirmed as a difference in sex hormones, as oophorectomized wild-type mice had mortality and lung bacterial clearance similar to male mice. There were differences in tumor necrosis factor-α secretion in male and female alveolar macrophages after P. aeruginosa infection. Finally, injection of male or oophorectomized wild-type female mice with estrogen restored lung bacterial clearance and prevented mortality. Our model of TBI followed by P. aeruginosa pneumonia is among the first to reveal sex dimorphism in secondary, long-term TBI complications.

α-Tocopherol Attenuates the Severity of Pseudomonas aeruginosa-induced Pneumonia

Pseudomonas aeruginosa is a lethal pathogen that causes high mortality and morbidity in immunocompromised and critically ill patients. The type III secretion system (T3SS) of P. aeruginosa mediates many of the adverse effects of infection with this pathogen, including increased lung permeability in a Toll-like receptor 4/RhoA/PAI-1 (plasminogen activator inhibitor-1)-dependent manner. α-Tocopherol has antiinflammatory properties that may make it a useful adjunct in treatment of this moribund infection. We measured transendothelial and transepithelial resistance, RhoA and PAI-1 activation, stress fiber formation, P. aeruginosa T3SS exoenzyme (ExoY) intoxication into host cells, and survival in a murine model of pneumonia in the presence of P. aeruginosa and pretreatment with α-tocopherol. We found that α-tocopherol alleviated P. aeruginosa-mediated alveolar endothelial and epithelial paracellular permeability by inhibiting RhoA, in part, via PAI-1 activation, and increased survival in a mouse model of P. aeruginosa pneumonia. Furthermore, we found that α-tocopherol decreased the activation of RhoA and PAI-1 by blocking the injection of T3SS exoenzymes into alveolar epithelial cells. P. aeruginosa is becoming increasingly antibiotic resistant. We provide evidence that α-tocopherol could be a useful therapeutic agent for individuals who are susceptible to infection with P. aeruginosa, such as those who are immunocompromised or critically ill.

Impact of Bacterial Toxins in the Lungs

Bacterial toxins play a key role in the pathogenesis of lung disease. Based on their structural and functional properties, they employ various strategies to modulate lung barrier function and to impair host defense in order to promote infection. Although in general, these toxins target common cellular signaling pathways and host compartments, toxin- and cell-specific effects have also been reported. Toxins can affect resident pulmonary cells involved in alveolar fluid clearance (AFC) and barrier function through impairing vectorial Na⁺ transport and through cytoskeletal collapse, as such, destroying cell-cell adhesions. The resulting loss of alveolar-capillary barrier integrity and fluid clearance capacity will induce capillary leak and foster edema formation, which will in turn impair gas exchange and endanger the survival of the host. Toxins modulate or neutralize protective host cell mechanisms of both the innate and adaptive immunity response during chronic infection. In particular, toxins can either recruit or kill central players of the lung's innate immune responses to pathogenic attacks, i.e., alveolar macrophages (AMs) and neutrophils. Pulmonary disorders resulting from these toxin actions include, e.g., acute lung injury (ALI), the acute respiratory syndrome (ARDS), and severe pneumonia. When acute infection converts to persistence, i.e., colonization and chronic infection, lung diseases, such as bronchitis, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF) can arise. The aim of this review is to discuss the impact of bacterial toxins in the lungs and the resulting outcomes for pathogenesis, their roles in promoting bacterial dissemination, and bacterial survival in disease progression.

Neuronal Wiskott-Aldrich syndrome protein regulates Pseudomonas aeruginosa-induced lung vascular permeability through the modulation of actin cytoskeletal dynamics

Pulmonary edema associated with increased vascular permeability is a severe complication of Pseudomonas (P.) aeruginosa-induced acute lung injury. The mechanisms underlying P aeruginosa-induced vascular permeability are not well understood. In the present study, we investigated the role of neuronal Wiskott Aldrich syndrome protein (N-WASP) in modulating P aeruginosa-induced vascular permeability. Using lung microvascular endothelial and alveolar epithelial cells, we demonstrated that N-WASP downregulation attenuated P aeruginosa-induced actin stress fiber formation and prevented paracellular permeability. P aeruginosa-induced dissociation between VE-cadherin and β-catenin, but increased association between N-WASP and VE-cadherin, suggesting a role for N-WASP in promoting P aeruginosa-induced adherens junction rupture. P aeruginosa increased N-WASP-Y256 phosphorylation, which required the activation of Rho GTPase and focal adhesion kinase. Increased N-WASP-Y256 phosphorylation promotes N-WASP and integrin αVβ6 association as well as TGF-β-mediated permeability across alveolar epithelial cells. Inhibition of N-WASP-Y256 phosphorylation by N-WASP-Y256F overexpression blocked N-WASP effects in P aeruginosa-induced actin stress fiber formation and increased paracellular permeability. In vivo, N-WASP knockdown attenuated the development of pulmonary edema and improved survival in a mouse model of P aeruginosa pneumonia. Together, our data demonstrate that N-WASP plays an essential role in P aeruginosa-induced vascular permeability and pulmonary edema through the modulation of actin cytoskeleton dynamics.

The extreme C terminus of the Pseudomonas aeruginosa effector ExoY is crucial for binding to its eukaryotic activator, F-actin

Bacterial nucleotidyl cyclase toxins are potent virulence factors that upon entry into eukaryotic cells are stimulated by endogenous cofactors to catalyze the production of large amounts of 3'5'-cyclic nucleoside monophosphates. The activity of the effector ExoY from Pseudomonas aeruginosa is stimulated by the filamentous form of actin (F-actin). Utilizing yeast phenotype analysis, site-directed mutagenesis, functional biochemical assays, and confocal microscopy, we demonstrate that the last nine amino acids of the C terminus of ExoY are crucial for the interaction with F-actin and, consequently, for ExoY's enzymatic activity in vitro and toxicity in a yeast model. We observed that isolated C-terminal sequences of P. aeruginosa ExoY that had been fused to a carrier protein bind to F-actin and that synthetic peptides corresponding to the extreme ExoY C terminus inhibit ExoY enzymatic activity in vitro and compete with the full-length enzyme for F-actin binding. Interestingly, we noted that various P. aeruginosa isolates of the PA14 family, including highly virulent strains, harbor ExoY variants with a mutation altering the C terminus of this effector. We found that these naturally occurring ExoY variants display drastically reduced enzymatic activity and toxicity. Our findings shed light on the molecular basis of the ExoY-F-actin interaction, revealing that the extreme C terminus of ExoY is critical for binding to F-actin in target cells and that some P. aeruginosa isolates carry C-terminally mutated, low-activity ExoY variants.

Interaction with an endothelial lumen increases neutrophil lifetime and motility in response to P aeruginosa

Neutrophil infiltration into tissues is essential for host defense and pathogen clearance. Although many of the signaling pathways involved in the transendothelial migration of neutrophils are known, the role of the endothelium in regulating neutrophil behavior in response to infection within interstitial tissues remains unclear. Here we developed a microscale 3-dimensional (3D) model that incorporates an endothelial lumen, a 3D extracellular matrix, and an intact bacterial source to model the host microenvironment. Using this system, we show that an endothelial lumen significantly increased neutrophil migration toward a source of Pseudomonas aeruginosa Surprisingly, we found neutrophils, which were thought to be short-lived cells in vitro, migrate for up to 24 hours in 3D in the presence of an endothelial lumen and bacteria. In addition, we found that endothelial cells secrete inflammatory mediators induced by the presence of P aeruginosa, including granulocyte-macrophage colony-stimulating factor (GM-CSF), a known promoter of neutrophil survival, and interleukin (IL)-6, a proinflammatory cytokine. We found that pretreatment of neutrophils with a blocking antibody against the IL-6 receptor significantly reduced neutrophil migration to P aeruginosa but did not alter neutrophil lifetime, indicating that secreted IL-6 is an important signal between endothelial cells and neutrophils that mediates migration. Taken together, these findings demonstrate an important role for endothelial paracrine signaling in neutrophil migration and survival.

Pseudomonas aeruginosa Takes a Multi-Target Approach to Achieve Junction Breach

Pseudomonas aeruginosa is an opportunistic pathogen which uses a number of strategies to cross epithelial and endothelial barriers at cell–cell junctions. In this review, we describe how the coordinated actions of P. aeruginosa's virulence factors trigger various molecular mechanisms to disarm the junctional gate responsible for tissue integrity.

T3SS effector ExoY reduces inflammasome-related responses by suppressing bacterial motility and delaying activation of NF-κB and caspase-1

Type III-secreted effectors are essential for modulating host immune responses during the pathogenesis of Pseudomonas aeruginosa infections. Little is known about the impact of one of the effectors, ExoY, on inflammasome activation, which results in IL-1β production and pyroptotic cell death. In this study, we found that transcriptional expression of Il-1β was induced to a lesser extent in response to an exoY-harboring strain than to a deleted mutant. This suppressive effect of ExoY was verified by complementation assay as well as by direct translocation of exoY into host cells. In addition to the production of IL-1β, pyroptotic cell death was also diminished in response to an exoY-harboring strain. These inflammasome responses were mediated by the adenylate cyclase activity of ExoY, which plays a role in delaying the activation of NF-κB and caspase-1, a key component of inflammasome-mediated responses. Moreover, the negative effects of ExoY on these responses were in part conferred by the suppression of bacterial motility, which could reduce the degree of bacterial contact with cells. Together, these results demonstrate that the adenylate cyclase activity of P. aeruginosa ExoY can reduce inflammasome-related responses by influencing both the host and the bacterium itself by delaying the activation of inflammatory pathways and suppressing bacterial motility.

Identification of miRNA biomarkers of pneumonia using RNA-sequencing and bioinformatics analysis

Pneumonia is a lower respiratory tract infection that causes dramatic mortality worldwide. The present study aimed to investigate the pathogenesis of pneumonia and identify microRNA (miRNA) biomarkers as candidates for targeted therapy. RNA from the peripheral blood plasma of participants with pneumonia (severe, n=9; non-severe, n=9) and controls (n=9) was isolated and paired-end sequencing was performed on an Illumina HiSeq4000 system. Following the processing of raw reads, the sequences were aligned against the Genome Reference Consortium human genome assembly 38 reference genome using Bowtie2 software. Reads per kilobase of transcript per million mapped read values were obtained and the limma software package was used to identify differentially expressed miRNAs (DE-miRs). Then, DE-miR targets were predicted and subjected to enrichment analysis. In addition, a protein-protein interaction (PPI) network of the predicted targets was constructed. This analysis identified 11 key DE-miRs in pneumonia samples, including 6 upregulated miRNAs (including hsa-miR-34a and hsa-miR-455) and 5 downregulated miRNAs (including hsa-let-7f-1). All DE-miRs kept their upregulation/downregulation pattern in the control, non-severe pneumonia and severe pneumonia samples. Predicted target genes of DE-miRs in the subjects with non-severe pneumonia vs. the control and the subjects with severe pneumonia vs. the non-severe pneumonia group were markedly enriched in the adherens junction and Wnt signaling pathways. KALRN, Ras homolog family member A (RHOA), β-catenin (CTNNB1), RNA polymerase II subunit K (POLR2K) and amyloid precursor protein (APP) were determined to encode crucial proteins in the PPI network constructed. KALRN was predicted to be a target of hsa-mir-200b, while RHOA, CTNNB1, POLR2K and APP were predicted targets of hsa-let-7f-1. The results of the present study demonstrated that hsa-let-7f-1 may serve a role in the development of cancer and the Notch signaling pathway. Conversely, hsa-miR-455 may be an inhibitor of pneumonia pathogenesis. Furthermore, hsa-miR-200b might promote pneumonia via targeting KALRN.

Bacteria and endothelial cells: a toxic relationship

Pathogenic bacteria use the bloodstream as a highway for getting around the body, and thus have to find ways to enter and exit through the endothelium. Many bacteria approach this problem by producing toxins that can breach the endothelial barrier through diverse creative mechanisms, including directly killing endothelial cells (ECs), weakening the cytoskeleton within ECs, and breaking the junctions between ECs. Toxins can also modulate the immune response by influencing endothelial biology, and can modulate endothelial function by influencing the response of leukocytes. Understanding these interactions, in both the in vitro and in vivo contexts, is of critical importance for designing new therapies for sepsis and other severe bacterial diseases.

Actin activates Pseudomonas aeruginosa ExoY nucleotidyl cyclase toxin and ExoY-like effector domains from MARTX toxins

The nucleotidyl cyclase toxin ExoY is one of the virulence factors injected by the Pseudomonas aeruginosa type III secretion system into host cells. Inside cells, it is activated by an unknown eukaryotic cofactor to synthesize various cyclic nucleotide monophosphates. ExoY-like adenylate cyclases are also found in Multifunctional-Autoprocessing Repeats-in-ToXin (MARTX) toxins produced by various Gram-negative pathogens. Here we demonstrate that filamentous actin (F-actin) is the hitherto unknown cofactor of ExoY. Association with F-actin stimulates ExoY activity more than 10,000 fold in vitro and results in stabilization of actin filaments. ExoY is recruited to actin filaments in transfected cells and alters F-actin turnover. Actin also activates an ExoY-like adenylate cyclase MARTX effector domain from Vibrio nigripulchritudo. Finally, using a yeast genetic screen, we identify actin mutants that no longer activate ExoY. Our results thus reveal a new sub-group within the class II adenylyl cyclase family, namely actin-activated nucleotidyl cyclase (AA-NC) toxins.

Neuronal Wiskott-Aldrich syndrome protein regulates TGF-β1-mediated lung vascular permeability

TGF-β1 induces an increase in paracellular permeability and actin stress fiber formation in lung microvascular endothelial and alveolar epithelial cells via small Rho GTPase. The molecular mechanism involved is not fully understood. Neuronal Wiskott-Aldrich syndrome protein (N-WASP) has an essential role in actin structure dynamics. We hypothesized that N-WASP plays a critical role in these TGF-β1-induced responses. In these cell monolayers, we demonstrated that N-WASP down-regulation by short hairpin RNA prevented TGF-β1-mediated disruption of the cortical actin structure, actin stress filament formation, and increased permeability. Furthermore, N-WASP down-regulation blocked TGF-β1 activation mediated by IL-1β in alveolar epithelial cells, which requires actin stress fiber formation. Control short hairpin RNA had no effect on these TGF-β1-induced responses. TGF-β1-induced phosphorylation of Y256 of N-WASP via activation of small Rho GTPase and focal adhesion kinase mediates TGF-β1-induced paracellular permeability and actin cytoskeleton dynamics. In vivo, compared with controls, N-WASP down-regulation increases survival and prevents lung edema in mice induced by bleomycin exposure-a lung injury model in which TGF-β1 plays a critical role. Our data indicate that N-WASP plays a crucial role in the development of TGF-β1-mediated acute lung injury by promoting pulmonary edema via regulation of actin cytoskeleton dynamics.-Wagener, B. M., Hu, M., Zheng, A., Zhao, X., Che, P., Brandon, A., Anjum, N., Snapper, S., Creighton, J., Guan, J.-L., Han, Q., Cai, G.-Q., Han, X., Pittet, J.-F., Ding, Q. Neuronal Wiskott-Aldrich syndrome protein regulates TGF-β1-mediated lung vascular permeability.

Possible mechanisms of Pseudomonas aeruginosa-associated lung disease

Pseudomonas aeruginosa is an opportunistic bacterium causing lung injury in immunocompromised patients correlated with high morbidity and mortality. Many bacteria, including P. aeruginosa, use extracellular signals to synchronize group behaviors, a process known as quorum sensing (QS). In the P. aeruginosa complex QS system controls expression of over 300 genes, including many involved in host colonization and disease. P. aeruginosa infection elicits a complex immune response due to a large number of immunogenic factors present in the bacteria or released during infection. Here, we focused on the mechanisms by which P. aeruginosa triggers lung injury and inflammation, debating the possible ways that P. aeruginosa evades the host immune system, which leads to immune suppression and resistance.

SH2 domain-containing protein tyrosine phosphatase 2 and focal adhesion kinase protein interactions regulate pulmonary endothelium barrier function

Enhanced protein tyrosine phosphorylation is associated with changes in vascular permeability through formation and dissolution of adherens junctions and regulation of stress fiber formation. Inhibition of the protein tyrosine phosphorylase SH2 domain-containing protein tyrosine phosphatase 2 (SHP2) increases tyrosine phosphorylation of vascular endothelial cadherin and β-catenin, resulting in disruption of the endothelial monolayer and edema formation in the pulmonary endothelium. Vascular permeability is a hallmark of acute lung injury (ALI); thus, enhanced SHP2 activity offers potential therapeutic value for the pulmonary vasculature in diseases such as ALI, but this has not been characterized. To assess whether SHP2 activity mediates protection against edema in the endothelium, we assessed the effect of molecular activation of SHP2 on lung endothelial barrier function in response to the edemagenic agents LPS and thrombin. Both LPS and thrombin reduced SHP2 activity, correlated with decreased focal adhesion kinase (FAK) phosphorylation (Y(397) and Y(925)) and diminished SHP2 protein-protein associations with FAK. Overexpression of constitutively active SHP2 (SHP2(D61A)) enhanced baseline endothelial monolayer resistance and completely blocked LPS- and thrombin-induced permeability in vitro and significantly blunted pulmonary edema formation induced by either endotoxin (LPS) or Pseudomonas aeruginosa exposure in vivo. Chemical inhibition of FAK decreased SHP2 protein-protein interactions with FAK concomitant with increased permeability; however, overexpression of SHP2(D61A) rescued the endothelium and maintained FAK activity and FAK-SHP2 protein interactions. Our data suggest that SHP2 activation offers the pulmonary endothelium protection against barrier permeability mediators downstream of the FAK signaling pathway. We postulate that further studies into the promotion of SHP2 activation in the pulmonary endothelium may offer a therapeutic approach for patients suffering from ALI.

The Role of ExoS in Dissemination of Pseudomonas aeruginosa during Pneumonia

Hospital-acquired pneumonia is associated with high rates of morbidity and mortality, and dissemination to the bloodstream is a recognized risk factor for particularly poor outcomes. Yet the mechanism by which bacteria in the lungs gain access to the bloodstream remains poorly understood. In this study, we used a mouse model of Pseudomonas aeruginosa pneumonia to examine this mechanism. P. aeruginosa uses a type III secretion system to deliver effector proteins such as ExoS directly into the cytosol of eukaryotic cells. ExoS, a bi-functional GTPase activating protein (GAP) and ADP-ribosyltransferase (ADPRT), inhibits phagocytosis during pneumonia but has also been linked to a higher incidence of dissemination to the bloodstream. We used a novel imaging methodology to identify ExoS intoxicated cells during pneumonia and found that ExoS is injected into not only leukocytes but also epithelial cells. Phagocytic cells, primarily neutrophils, were targeted for injection with ExoS early during infection, but type I pneumocytes became increasingly injected at later time points. Interestingly, injection of these pneumocytes did not occur randomly but rather in discrete regions, which we designate ““fields of cell injection” (FOCI). These FOCI increased in size as the infection progressed and contained dead type I pneumocytes. Both of these phenotypes were attenuated in infections caused by bacteria secreting ADPRT-deficient ExoS, indicating that FOCI growth and type I pneumocyte death were dependent on the ADPRT activity of ExoS. During the course of infection, increased FOCI size was associated with enhanced disruption of the pulmonary-vascular barrier and increased bacterial dissemination into the blood, both of which were also dependent on the ADPRT activity of ExoS. We conclude that the ADPRT activity of ExoS acts upon type I pneumocytes to disrupt the pulmonary-vascular barrier during P. aeruginosa pneumonia, leading to bacterial dissemination.

Dissemination to the bloodstream is a poor prognostic sign in patients with hospital-acquired pneumonia, yet the mechanism by which this occurs is poorly understood. To begin to address this issue, we have used a mouse model of P. aeruginosa pneumonia to study the mechanism by which the type-III-secreted effector protein ExoS enhances bacterial dissemination. We show that intoxication of type I pneumocytes by ExoS leads to cell death and disruption of the pulmonary-vascular barrier, allowing bacterial dissemination into the bloodstream. These effects required the ADP-ribosyltransferase activity of ExoS, as strains secreting an ExoS variant lacking this activity demonstrated reduced type I pneumocytes death and pulmonary-vascular breakdown. This study indicates that inhibitors of the ADP-ribosyltransferase activity of ExoS could serve as novel therapeutics for the prevention of bacteremic pneumonia.

Pharmacological activation of Rap1 antagonizes the endothelial barrier disruption induced by exotoxins ExoS and ExoT of Pseudomonas aeruginosa

Most clinical strains of Pseudomonas aeruginosa, a leading agent of nosocomial infections, are multiresistant to antibiotherapy. Because of the paucity of new available antibiotics, the investigation of strategies aimed at limiting the action of its major virulence factors has gained much interest. The type 3 secretion system of P. aeruginosa and its effectors are known to be major determinants of toxicity and are required for bacterial dissemination in the host. Bacterial transmigration across the vascular wall is considered to be an important step in the infectious process. Using human endothelial primary cells, we demonstrate that forskolin (FSK), a drug inducing cyclic AMP (cAMP) elevation in eukaryotic cells, strikingly reduced the cell retraction provoked by two type 3 toxins, ExoS and ExoT, found in the majority of clinical strains. Conversely, cytotoxicity of a strain carrying the type 3 effector ExoU was unaffected by FSK. In addition, FSK altered the capacity of two ExoS/ExoT strains to transmigrate across cell monolayers. In agreement with these findings, other drugs and a cytokine inducing the increase of cAMP intracellular levels have also protected cells from retraction. cAMP is an activator of both protein kinase A and EPAC, a GTPase exchange factor of Rap1. Using activators or inhibitors of either pathway, we show that the beneficial effect of FSK is exerted by the activation of the EPAC/Rap1 axis, suggesting that its protective effect is mediated by reinforcing cell-cell and cell-substrate adhesion.

Postexposure aerosolized heparin reduces lung injury in chlorine-exposed mice

Chlorine (Cl2) is a highly reactive oxidant gas that, when inhaled, may cause acute lung injury culminating in death from respiratory failure. In this study, we tested the hypothesis that exposure of mice to Cl2 causes intra-alveolar and systemic activation of the coagulation cascade that plays an important role in development of lung injury. C57Bl/6 mice were exposed to Cl2 (400 for 30 min or 600 ppm for 45 min) in environmental chambers and then returned to room air for 1 or 6 h. Native coagulation (NATEM) parameters such as blood clotting time and clot formation time were measured in whole blood by the viscoelastic technique. D-dimers and thrombin-anti-thrombin complexes were measured in both plasma and bronchoalveolar lavage fluid (BALF) by ELISA. Our results indicate that mice exposed to Cl2 gas had significantly increased clotting time, clot formation time, and D-dimers compared with controls. The thrombin-anti-thrombin complexes were also increased in the BALF of Cl2 exposed animals. To test whether increased coagulation contributed to the development of acute lung injury, mice exposed to Cl2 and returned to room air were treated with aerosolized heparin or vehicle for 20 min. Aerosolized heparin significantly reduced protein levels and the number of inflammatory cells in the BALF at 6 h postexposure. These findings highlight the importance of coagulation abnormities in the development of Cl2-induced lung injury.

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.

Heat shock protein 90 inhibitors prevent LPS-induced endothelial barrier dysfunction by disrupting RhoA signaling

Permeability of the endothelial monolayer is increased when exposed to the bacterial endotoxin LPS. Our previous studies have shown that heat shock protein (Hsp) 90 inhibitors protect and restore LPS-mediated hyperpermeability in bovine pulmonary arterial endothelial cells. In this study, we assessed the effect of Hsp90 inhibition against LPS-mediated hyperpermeability in cultured human lung microvascular endothelial cells (HLMVECs) and delineated the underlying molecular mechanisms. We demonstrate that Hsp90 inhibition is critical in the early phase, to prevent LPS-mediated hyperpermeability, and also in the later phase, to restore LPS-mediated hyperpermeability in HLMVECs. Because RhoA is a well known mediator of endothelial hyperpermeability, we investigated the effect of Hsp90 inhibition on LPS-mediated RhoA signaling. RhoA nitration and activity were increased by LPS in HLMVECs and suppressed when pretreated with the Hsp90 inhibitor, 17-allylamino-17 demethoxy-geldanamycin (17-AAG). In addition, inhibition of Rho kinase, a downstream effector of RhoA, protected HLMVECs from LPS-mediated hyperpermeability and abolished LPS-induced myosin light chain (MLC) phosphorylation, a target of Rho kinase. In agreement with these findings, 17-AAG or dominant-negative RhoA attenuated LPS-induced MLC phosphorylation. MLC phosphorylation induced by constitutively active RhoA was also suppressed by 17-AAG, suggesting a role for Hsp90 downstream of RhoA. Inhibition of Src family kinases also suppressed RhoA activity and MLC phosphorylation. Together, these data indicate that Hsp90 inhibition prevents and repairs LPS-induced lung endothelial barrier dysfunction by suppressing Src-mediated RhoA activity and signaling.

Mechanisms of phagocytosis and host clearance of Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic bacterial pathogen responsible for a high incidence of acute and chronic pulmonary infection. These infections are particularly prevalent in patients with chronic obstructive pulmonary disease and cystic fibrosis: much of the morbidity and pathophysiology associated with these diseases is due to a hypersusceptibility to bacterial infection. Innate immunity, primarily through inflammatory cytokine production, cellular recruitment, and phagocytic clearance by neutrophils and macrophages, is the key to endogenous control of P. aeruginosa infection. In this review, we highlight recent advances toward understanding the innate immune response to P. aeruginosa, with a focus on the role of phagocytes in control of P. aeruginosa infection. Specifically, we summarize the cellular and molecular mechanisms of phagocytic recognition and uptake of P. aeruginosa, and how current animal models of P. aeruginosa infection reflect clinical observations in the context of phagocytic clearance of the bacteria. Several notable phenotypic changes to the bacteria are consistently observed during chronic pulmonary infections, including changes to mucoidy and flagellar motility, that likely enable or reflect their ability to persist. These traits are likewise examined in the context of how the bacteria avoid phagocytic clearance, inflammation, and sterilizing immunity.

In the absence of effector proteins, the Pseudomonas aeruginosa type three secretion system needle tip complex contributes to lung injury and systemic inflammatory responses

Herein we describe a pathogenic role for the Pseudomonas aeruginosa type three secretion system (T3SS) needle tip complex protein, PcrV, in causing lung endothelial injury. We first established a model in which P. aeruginosa wild type strain PA103 caused pneumonia-induced sepsis and distal organ dysfunction. Interestingly, a PA103 derivative strain lacking its two known secreted effectors, ExoU and ExoT [denoted PA103 (ΔU/ΔT)], also caused sepsis and modest distal organ injury whereas an isogenic PA103 strain lacking the T3SS needle tip complex assembly protein [denoted PA103 (ΔPcrV)] did not. PA103 (ΔU/ΔT) infection caused neutrophil influx into the lung parenchyma, lung endothelial injury, and distal organ injury (reminiscent of sepsis). In contrast, PA103 (ΔPcrV) infection caused nominal neutrophil infiltration and lung endothelial injury, but no distal organ injury. We further examined pathogenic mechanisms of the T3SS needle tip complex using cultured rat pulmonary microvascular endothelial cells (PMVECs) and revealed a two-phase, temporal nature of infection. At 5-hours post-inoculation (early phase infection), PA103 (ΔU/ΔT) elicited PMVEC barrier disruption via perturbation of the actin cytoskeleton and did so in a cell death-independent manner. Conversely, PA103 (ΔPcrV) infection did not elicit early phase PMVEC barrier disruption. At 24-hours post-inoculation (late phase infection), PA103 (ΔU/ΔT) induced PMVEC damage and death that displayed an apoptotic component. Although PA103 (ΔPcrV) infection induced late phase PMVEC damage and death, it did so to an attenuated extent. The PA103 (ΔU/ΔT) and PA103 (ΔPcrV) mutants grew at similar rates and were able to adhere equally to PMVECs post-inoculation indicating that the observed differences in damage and barrier disruption are likely attributable to T3SS needle tip complex-mediated pathogenic differences post host cell attachment. Together, these infection data suggest that the T3SS needle tip complex and/or another undefined secreted effector(s) are important determinants of P. aeruginosa pneumonia-induced lung endothelial barrier disruption.

Interleukin-8 regulates endothelial permeability by down-regulation of tight junction but not dependent on integrins induced focal adhesions

Interleukin-8 (IL-8) is a common inflammatory factor, which involves in various non-specific pathological processes of inflammation. It has been found that increased endothelial permeability accompanied with high expression of IL-8 at site of injured endothelium and atherosclerotic plaque at early stages, suggesting that IL-8 participated in regulating endothelial permeability in the developing processes of vascular disease. The purpose of this study is to investigate the regulation effects of IL-8 on the vascular endothelial permeability, and the mRNA and protein expression of tight junction components (i.e., ZO-1, Claudin-5 and Occludin). Endothelial cells were stimulated by IL-8 with the dose of 50, 100 and 200 ng/mL, and duration of 2, 4, 6, 8h, respectively. The mRNA and protein expression level of tight junction components with IL-8 under different concentration and duration was examined by RT-PCR and Western blot, respectively. Meanwhile, the integrins induced focal adhesions event with IL-8 stimulation was also investigated. The results showed that IL-8 regulated the permeability of endothelium by down-regulation of tight junction in a dose- and time-dependence manner, but was not by integrins induced focal adhesions. This finding reveals the molecular mechanism in the increase of endothelial cell permeability induced by IL-8, which is expected to provide a new idea as a therapeutic target in vascular diseases.

PAI-1 is an essential component of the pulmonary host response during Pseudomonas aeruginosa pneumonia in mice

RATIONALE

Elevated plasma and bronchoalveolar lavage fluid plasminogen activator inhibitor 1 (PAI-1) levels are associated with adverse clinical outcome in patients with pneumonia caused by Pseudomonas aeruginosa. However, whether PAI-1 plays a pathogenic role in the breakdown of the alveolar-capillary barrier caused by P aeruginosa is unknown.

OBJECTIVES

The role of PAI-1 in pulmonary host defence and survival during P aeruginosa pneumonia in mice was tested. The in vitro mechanisms by which P aeruginosa causes PAI-1 gene and protein expression in lung endothelial and epithelial cells were also examined.

METHODS AND RESULTS

PAI-1 null and wild-type mice that were pretreated with the PAI-1 inhibitor Tiplaxtinin had a significantly lower increase in lung vascular permeability than wild-type littermates after the airspace instillation of 1×10(7) colony-forming units (CFU) of P aeruginosa bacteria. Furthermore, P aeruginosa in vitro induced the expression of the PAI-1 gene and protein in a TLR4/p38/RhoA/NF-κB (Toll-like receptor 4/p38/RhoA/nuclear factor-κB) manner in lung endothelial and alveolar epithelial cells. However, in vivo disruption of PAI-1 signalling was associated with higher mortality at 24 h (p<0.03) and higher bacterial burden in the lungs secondary to decreased neutrophil migration into the distal airspace in response to P aeruginosa.

CONCLUSIONS

The results indicate that PAI-1 is a critical mediator that controls the development of the early lung inflammation that is required for the activation of the later innate immune response necessary for the eradication of P aeruginosa from the distal airspaces of the lung.

Cytoprotective-selective activated protein C attenuates Pseudomonas aeruginosa-induced lung injury in mice

Inhibition of the small GTPase RhoA attenuates the development of pulmonary edema and restores positive alveolar fluid clearance in a murine model of Pseudomonas aeruginosa pneumonia. Activated protein C (aPC) blocks the development of an unfavorably low ratio of small GTPase Rac1/RhoA activity in lung endothelium through endothelial protein C receptor (EPCR)/protease-activated receptor-1 (PAR-1)-dependent signaling mechanisms that include transactivating the sphingosine-1-phosphate (S1P) pathway. However, whether aPC's cytoprotective effects can attenuate the development of pulmonary edema and death associated with P. aeruginosa pneumonia in mice remains unknown. Thus, we determined whether the normalization of a depressed ratio of activated Rac1/RhoA by aPC would attenuate the P. aeruginosa-mediated increase in protein permeability across lung endothelial and alveolar epithelial barriers. Pretreatment with aPC significantly reduced P. aeruginosa-induced increases in paracellular permeability across pulmonary endothelial cell and alveolar epithelial monolayers via an inhibition of RhoA activation and a promotion of Rac1 activation that required the EPCR-PAR-1 and S1P pathways. Furthermore, pretreatment with aPC attenuated the development of pulmonary edema in a murine model of P. aeruginosa pneumonia. Finally, a cytoprotective-selective aPC mutant, aPC-5A, which lacks most of aPC's anticoagulant activity, reproduced the protective effect of wild-type aPC by attenuating the development of pulmonary edema and decreasing mortality in a murine model of P. aeruginosa pneumonia. Taken together, these results demonstrate a critical role for the cytoprotective activities of aPC in attenuating P. aeruginosa-induced lung vascular permeability and mortality, suggesting that cytoprotective-selective aPC-5A with diminished bleeding risks could attenuate the lung damage caused by P. aeruginosa in critically ill patients.

Cellular responses of A549 alveolar epithelial cells to serially collected Pseudomonas aeruginosa from cystic fibrosis patients at different stages of pulmonary infection

Pseudomonas aeruginosa is the major cause of chronic pulmonary disease in cystic fibrosis (CF) patients. During chronic infection, P. aeruginosa lose certain virulence factors, transform into a mucoid phenotype, and develop antibiotic resistance. We hypothesized that these genetic and phenotypic alterations of P. aeruginosa affect the airway epithelial responses. A549 cells were infected with 27 well-characterized isolates of P. aeruginosa from CF patients obtained during longitudinal observation, or with P. aeruginosa mutant strains lacking flagella, pili, lipopolysaccharide, or pyocyanin. Pseudomonas aeruginosa isolates from the early stages of the infection exhibited high adherence to A549 cells, were readily internalized, and able to induce reactive oxygen species (ROS) production, apoptosis of infected cells, and the release of granulocyte macrophage colony-stimulating factor. Late P. aeruginosa isolates collected from patients with chronic lung infection were shown to have reduced adherence to and internalization into A549 cells compared with bacteria from patients with intermittent P. aeruginosa colonization, and induced lower production of ROS and apoptosis, but caused high proinflammatory cytokine and adhesion molecule expression. Our findings suggest that despite the loss of virulence factors during the adaptation process in the CF lung by late P. aeruginosa strains, they retain high proinflammatory abilities that likely contribute to the disease pathogenesis.

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.