Enteropathogenic Escherichia coli subverts phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate upon epithelial cell infection

Bacterial exploitation of host cell signaling

As a way of enhancing infections, bacterial pathogens often alter host cell signaling pathways. Here, we describe recent work that highlights a new phosphatase from an intestinal and wound-invading pathogen that manipulates host cell phosphoinositide circuits. Despite the active-site homology between bacterial inositol phosphatases and mammalian phosphatases, sequence differences between them suggest that the development of specific inhibitors may be feasible.

The bacterial intimins and invasins: a large and novel family of secreted proteins

Background

Gram-negative bacteria have developed a limited repertoire of solutions for secreting proteins from the cytoplasmic compartment to the exterior of the cell. Amongst the spectrum of secreted proteins are the intimins and invasins (the Int/Inv family; TC# 1.B.54) which are characterized by an N-terminal β-barrel domain and a C-terminal surface localized passenger domain. Despite the important role played by members of this family in diseases mediated by several species of the Enterobacteriaceae, there has been little appreciation for the distribution and diversity of these proteins amongst Gram-negative bacteria. Furthermore, there is little understanding of the molecular events governing secretion of these proteins to the extracellular milieu.

Principal Findings

In silico approaches were used to analyze the domain organization and diversity of members of this secretion family. Proteins belonging to this family are predominantly associated with organisms from the γ-proteobacteria. Whilst proteins from the Chlamydia, γ-, β- and ε-proteobacteria possess β-barrel domains and passenger domains of various sizes, Int/Inv proteins from the α-proteobacteria, cyanobacteria and chlorobi possess only the predicted β-barrel domains. Phylogenetic analyses revealed that with few exceptions these proteins cluster according to organismal type, indicating that divergence occurred contemporaneously with speciation, and that horizontal transfer was limited. Clustering patterns of the β-barrel domains correlate well with those of the full-length proteins although the passenger domains do so with much less consistency. The modular subdomain design of the passenger domains suggests that subdomain duplication and deletion have occurred with high frequency over evolutionary time. However, all repeated subdomains are found in tandem, suggesting that subdomain shuffling occurred rarely if at all. Topological predictions for the β-barrel domains are presented.

Conclusion

Based on our in silico analyses we present a model for the biogenesis of these proteins. This study is the first of its kind to describe this unusual family of bacterial adhesins.

Role for ADP ribosylation factor 1 in the regulation of hepatitis C virus replication

We hypothesized that ADP-ribosylation factor 1 (Arf1) plays an important role in the biogenesis and maintenance of infectious hepatitis C virus (HCV). Huh7.5 cells, in which HCV replicates and produces infectious viral particles, were exposed to brefeldin A or golgicide A, pharmacological inhibitors of Arf1 activation. Treatment with these agents caused a reduction in viral RNA levels, the accumulation of infectious particles within the cells, and a reduction in the levels of these particles in the extracellular medium. Fluorescence analyses showed that the viral nonstructural (NS) proteins NS5A and NS3, but not the viral structural protein core, shifted their localization from speckle-like structures in untreated cells to the rims of lipid droplets (LDs) in treated cells. Using pulldown assays, we showed that ectopic overexpression of NS5A in Huh7 cells reduces the levels of GTP-Arf1. Downregulation of Arf1 expression by small interfering RNA (siRNA) decreased both the levels of HCV RNA and the production of infectious viral particles and altered the localization of NS5A to the peripheries of LDs. Together, our data provide novel insights into the role of Arf1 in the regulation of viral RNA replication and the production of infectious HCV.

Vascular endothelial growth factor receptor 1 contributes to Escherichia coli K1 invasion of human brain microvascular endothelial cells through the phosphatidylinositol 3-kinase/Akt signaling pathway

Escherichia coli is the most common Gram-negative organism causing neonatal meningitis. Previous studies demonstrated that E. coli K1 invasion of brain microvascular endothelial cells (BMEC) is required for penetration into the central nervous system, but the microbe-host interactions that are involved in this process remain incompletely understood. Here we report the involvement of vascular endothelial growth factor receptor 1 (VEGFR1) expressed on human brain microvascular endothelial cells (HBMEC) in E. coli K1 invasion of HBMEC. Our results showed that treatment of confluent HBMEC with pan-VEGFR inhibitors significantly inhibited E. coli K1 invasion of HBMEC. Immunofluorescence results indicated the colocalization of VEGFR1 with E. coli K1 during bacterial invasion of HBMEC. The E. coli-induced actin cytoskeleton rearrangements in HBMEC were blocked by VEGFR inhibitors but not by VEGFR2-specific inhibitors. The small interfering RNA (siRNA) knockdown of VEGFR1 in HBMEC significantly attenuated E. coli invasion and the concomitant actin filament rearrangement. Furthermore, we found an increased association of VEGFR1 with the p85 subunit of phosphatidylinositol 3-kinase (PI3K) in HBMEC infected with E. coli K1 and that E. coli K1-triggered Akt activation in HBMEC was blocked by VEGFR1 siRNA and VEGFR inhibitors. Taken together, our results demonstrate that VEGFR1 contributes to E. coli K1 invasion of HBMEC via recruitment of the PI3K/Akt signaling pathway.

Enterohemorrhagic E. coli requires N-WASP for efficient type III translocation but not for EspFU-mediated actin pedestal formation

Upon infection of mammalian cells, enterohemorrhagic E. coli (EHEC) O157:H7 utilizes a type III secretion system to translocate the effectors Tir and EspF_U (aka TccP) that trigger the formation of F-actin-rich ‘pedestals’ beneath bound bacteria. EspF_U is localized to the plasma membrane by Tir and binds the nucleation-promoting factor N-WASP, which in turn activates the Arp2/3 actin assembly complex. Although N-WASP has been shown to be required for EHEC pedestal formation, the precise steps in the process that it influences have not been determined. We found that N-WASP and actin assembly promote EHEC-mediated translocation of Tir and EspF_U into mammalian host cells. When we utilized the related pathogen enteropathogenic E. coli to enhance type III translocation of EHEC Tir and EspF_U, we found surprisingly that actin pedestals were generated on N-WASP-deficient cells. Similar to pedestal formation on wild type cells, Tir and EspF_U were the only bacterial effectors required for pedestal formation, and the EspF_U sequences required to interact with N-WASP were found to also be essential to stimulate this alternate actin assembly pathway. In the absence of N-WASP, the Arp2/3 complex was both recruited to sites of bacterial attachment and required for actin assembly. Our results indicate that actin assembly facilitates type III translocation, and reveal that EspF_U, presumably by recruiting an alternate host factor that can signal to the Arp2/3 complex, exhibits remarkable versatility in its strategies for stimulating actin polymerization.

The food-borne pathogen enterohemorrhagic E. coli (EHEC) O157:H7 can cause severe diarrhoea and life-threatening systemic illnesses. During infection, EHEC attaches to cells lining the human intestine and injects Tir and EspF_U, two bacterial molecules that alter the host cell actin cytoskeleton and stimulate the formation of “pedestals” just beneath bound bacteria. Pedestal formation promotes colonization during the later stages of infection. N-WASP, a host protein known to regulate actin assembly in mammalian cells, was previously shown to be manipulated by Tir and EspF_U to stimulate actin assembly, and to be required for EHEC to generate actin pedestals. Surprisingly, we show here that N-WASP promotes the efficient delivery of Tir and EspF_U into mammalian cells, and that when we utilized a related E. coli to enhance type III delivery of Tir and EspF_U, actin pedestals assembled even in its absence. Thus, EHEC stimulates at least two pathways of actin assembly to generate pedestals, one mediated by N-WASP and one by an unidentified alternate factor. This flexibility likely reflects an important function of pedestal formation by EHEC, and study of the underlying mechanisms may provide new insights into the pathogenesis of infection as well as the regulation of the actin cytoskeleton of mammalian cells.

Manipulation of host membrane machinery by bacterial pathogens

Subversion of host membrane machinery is important for the uptake, survival, and replication of bacterial pathogens. Understanding how pathogens manipulate host membrane transport pathways provides mechanistic insight into how infection occurs and is also revealing new information on biochemical processes involved in the functioning of eukaryotic cells. In this review we discuss several of the canonical host pathways targeted by bacterial pathogens and emerging areas of investigation in this exciting field.

Cytoskeleton-modulating effectors of enteropathogenic and enterohaemorrhagic Escherichia coli: Tir, EspFU and actin pedestal assembly

A variety of microbes manipulate the cytoskeleton of mammalian cells to promote their internalization, motility and/or spread. Among such bacteria, enteropathogenic Escherichia coli and enterohemorrhagic Escherichia coli are closely related pathogens that adhere to human intestinal cells and reorganize the underlying actin cytoskeleton into 'pedestals'. The assembly of pedestals is likely to be an important step in colonization, and is triggered by the E. coli virulence factors translocated intimin receptor and E. coli secreted protein F in prophage U, which modulate multiple host signaling cascades that lead to actin polymerization. In recent years, these bacterial effectors have been exploited as powerful experimental tools for investigating actin cytoskeletal and membrane dynamics, and several studies have significantly advanced our understanding of the regulation of actin assembly in mammalian cells and the potential role of pedestal formation in pathogenesis.

Phosphoinositides influence pathogen surfing: EPEC rights the SHIP

Tir, a translocated effector protein from enteropathogenic E. coli (EPEC), contains two phosphotyrosines that initiate cellular signaling cascades, leading to localized actin polymerization into pedestals. A recent study now shows that two additional tyrosines within Tir recruit the inositol phosphatase SHIP2 to generate a PI(3,4)P2-enriched membrane platform that stabilizes pedestal assembly.

Regulation of the actin cytoskeleton-plasma membrane interplay by phosphoinositides

The plasma membrane and the underlying cortical actin cytoskeleton undergo continuous dynamic interplay that is responsible for many essential aspects of cell physiology. Polymerization of actin filaments against cellular membranes provides the force for a number of cellular processes such as migration, morphogenesis, and endocytosis. Plasma membrane phosphoinositides (especially phosphatidylinositol bis- and trisphosphates) play a central role in regulating the organization and dynamics of the actin cytoskeleton by acting as platforms for protein recruitment, by triggering signaling cascades, and by directly regulating the activities of actin-binding proteins. Furthermore, a number of actin-associated proteins, such as BAR domain proteins, are capable of directly deforming phosphoinositide-rich membranes to induce plasma membrane protrusions or invaginations. Recent studies have also provided evidence that the actin cytoskeleton-plasma membrane interactions are misregulated in a number of pathological conditions such as cancer and during pathogen invasion. Here, we summarize the wealth of knowledge on how the cortical actin cytoskeleton is regulated by phosphoinositides during various cell biological processes. We also discuss the mechanisms by which interplay between actin dynamics and certain membrane deforming proteins regulate the morphology of the plasma membrane.

Detergent-resistant microdomains mediate activation of host cell signaling in response to attaching-effacing bacteria

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 causes outbreaks of bloody diarrhea and the hemolytic-uremic syndrome. EHEC intimately adheres to epithelial cells, effaces microvilli and induces attaching-effacing (AE) lesions. Detergent-resistant microdomains (lipid rafts) serve as membrane platforms for the recruitment of signaling complexes to mediate host responses to infection. The aim of this study was to define the role of lipid rafts in activating signal transduction pathways in response to AE bacterial pathogens. Epithelial cell monolayers were infected with EHEC (MOI 100:1, 3 h, 37 degrees C) and lipid rafts isolated by buoyant density ultracentrifugation. Phosphoinositide 3-kinase (PI3K) localization to lipid rafts was confirmed using PI3K and anti-caveolin-1 antibodies. Mice with cholesterol storage disease Niemann-Pick, type C were used as in vivo models to confirm the role of lipid rafts in mediating signaling response to AE organisms. In contrast to uninfected cells, PI3K was recruited to lipid rafts in response to EHEC infection. Metabolically active bacteria and cells with intact cholesterol-rich microdomains were necessary for the recruitment of second messengers to lipid rafts. Recruitment of PI3K to lipid rafts was independent of the intimin (eaeA) gene, type III secretion system, and production of Shiga-like toxins. Colonization of NPC(-/-) colonic mucosa by Citrobacter rodentium and AE lesion formation were both delayed, compared with wild-type mice infected with the murine-specific AE bacterial pathogen. C. rodentium-infected NPC(-/-) mice had reduced colonic epithelial hyperplasia (64+/-8.251 vs 112+/-2.958 microm; P<0.05) and decreased secretion of IFN-gamma (17.6+/-17.6 vs 71+/-26.3 pg/ml, P<0.001). Lipid rafts mediate host cell signal transduction responses to AE bacterial infections both in vitro and in vivo. These findings advance the current understanding of microbial-eukaryotic cell interactions in response to enteric pathogens that hijack signaling responses mediated through lipid rafts.

E. coli secreted protein F promotes EPEC invasion of intestinal epithelial cells via an SNX9-dependent mechanism

Enteropathogenic Escherichia coli (EPEC) infection requires the injection of effector proteins into intestinal epithelial cells (IECs) via type 3 secretion. Type 3-secreted effectors can interfere with IEC signalling pathways via specific protein-protein interactions. For example, E. coli secreted protein F (EspF) binds sorting nexin 9 (SNX9), an endocytic regulator, resulting in tubulation of the plasma membrane. Our aim was to determine the mechanism of EspF/SNX9-induced membrane tubulation. Point mutation of the SNX9 lipid binding domains or truncation of the EspF SNX9 binding domains significantly inhibited tubulation, as did inhibition of clathrin coated pit (CCP) assembly. Although characterized as non-invasive, EPEC are known to invade IECs in vitro and in vivo. Indeed, we found significant invasion of Caco-2 cells by EPEC, which, like tubulation, was blocked by pharmacological inhibition of CCPs. Interestingly, however, inhibition of dynamin activity did not prevent tubulation or EPEC invasion, which is in contrast to Salmonella invasion, which requires dynamin activity. Our data also indicate that EPEC invasion is dependent on EspF and its interaction with SNX9. Together, these findings suggest that EspF promotes EPEC invasion of IECs by harnessing the membrane-deforming activity of SNX9.