Function and distribution of EspG2, a type III secretion system effector of enteropathogenic Escherichia coli

Enteropathogenic E. coli infection co-elicits lysosomal exocytosis and lytic host cell death

IMPORTANCE

Enteropathogenic Escherichia coli (EPEC) infection is a significant cause of gastroenteritis, mainly in children. Therefore, studying the mechanisms of EPEC infection is an important research theme. EPEC modulates its host cell life by injecting via a type III secretion machinery cell death modulating effector proteins. For instance, while EspF and Map promote mitochondrial cell death, EspZ antagonizes cell death. We show that these effectors also control lysosomal exocytosis, i.e., the trafficking of lysosomes to the host cell plasma membrane. Interestingly, the capacity of these effectors to induce or protect against cell death correlates completely with their ability to induce LE, suggesting that the two processes are interconnected. Modulating host cell death is critical for establishing bacterial attachment to the host and subsequent dissemination. Therefore, exploring the modes of LE involvement in host cell death is crucial for elucidating the mechanisms underlying EPEC infection and disease.

Atypical enteropathogenic E. coli are associated with disease activity in ulcerative colitis

With increasing urbanization and industrialization, the prevalence of inflammatory bowel diseases (IBDs) has steadily been rising over the past two decades. IBD involves flares of gastrointestinal (GI) inflammation accompanied by microbiota perturbations. However, microbial mechanisms that trigger such flares remain elusive. Here, we analyzed the association of the emerging pathogen atypical enteropathogenic E. coli (aEPEC) with IBD disease activity. The presence of diarrheagenic E. coli was assessed in stool samples from 630 IBD patients and 234 age- and sex-matched controls without GI symptoms. Microbiota was analyzed with 16S ribosomal RNA gene amplicon sequencing, and 57 clinical aEPEC isolates were subjected to whole-genome sequencing and in vitro pathogenicity experiments including biofilm formation, epithelial barrier function and the ability to induce pro-inflammatory signaling. The presence of aEPEC correlated with laboratory, clinical and endoscopic disease activity in ulcerative colitis (UC), as well as microbiota dysbiosis. In vitro, aEPEC strains induce epithelial p21-activated kinases, disrupt the epithelial barrier and display potent biofilm formation. The effector proteins espV and espG2 distinguish aEPEC cultured from UC and Crohn's disease patients, respectively. EspV-positive aEPEC harbor more virulence factors and have a higher pro-inflammatory potential, which is counteracted by 5-ASA. aEPEC may tip a fragile immune-microbiota homeostasis and thereby contribute to flares in UC. aEPEC isolates from UC patients display properties to disrupt the epithelial barrier and to induce pro-inflammatory signaling in vitro.

Comparative genomic analysis provides insight into the phylogeny and virulence of atypical enteropathogenic Escherichia coli strains from Brazil

Background

Atypical enteropathogenic Escherichia coli (aEPEC) are one of the most frequent intestinal E. coli pathotypes isolated from diarrheal patients in Brazil. Isolates of aEPEC contain the locus of enterocyte effacement, but lack the genes of the bundle-forming pilus of typical EPEC, and the Shiga toxin of enterohemorrhagic E. coli (EHEC). The objective of this study was to evaluate the phylogeny and the gene content of Brazilian aEPEC genomes compared to a global aEPEC collection.

Methodology

Single nucleotide polymorphism (SNP)-based phylogenomic analysis was used to compare 106 sequenced Brazilian aEPEC with 221 aEPEC obtained from other geographic origins. Additionally, Large-Scale BLAST Score Ratio was used to determine the shared versus unique gene content of the aEPEC studied.

Principal Findings

Phylogenomic analysis demonstrated the 106 Brazilian aEPEC were present in phylogroups B1 (47.2%, 50/106), B2 (23.6%, 25/106), A (22.6%, 24/106), and E (6.6%, 7/106). Identification of EPEC and EHEC phylogenomic lineages demonstrated that 42.5% (45/106) of the Brazilian aEPEC were in four of the previously defined lineages: EPEC10 (17.9%, 19/106), EPEC9 (10.4%, 11/106), EHEC2 (7.5%, 8/106) and EPEC7 (6.6%, 7/106). Interestingly, an additional 28.3% (30/106) of the Brazilian aEPEC were identified in five novel lineages: EPEC11 (14.2%, 15/106), EPEC12 (4.7%, 5/106), EPEC13 (1.9%, 2/106), EPEC14 (5.7%, 6/106) and EPEC15 (1.9%, 2/106). We identified 246 genes that were more frequent among the aEPEC isolates from Brazil compared to the global aEPEC collection, including espG2, espT and espC (P<0.001). Moreover, the nleF gene was more frequently identified among Brazilian aEPEC isolates obtained from diarrheagenic patients when compared to healthy subjects (69.7% vs 41.2%, P<0.05).

Conclusion

The current study demonstrates significant genomic diversity among aEPEC from Brazil, with the identification of Brazilian aEPEC isolates to five novel EPEC lineages. The greater prevalence of some virulence genes among Brazilian aEPEC genomes could be important to the specific virulence strategies used by aEPEC in Brazil to cause diarrheal disease.

Atypical EPEC (aEPEC) is one of the most frequent diarrheagenic Escherichia coli pathotypes isolated from patients in Brazil and is associated with diarrheal outbreaks. This study is the first to sequence the genomes of a collection of aEPEC isolates from a South American country, Brazil, and compare their phylogenetic relationships and gene content with a global collection of aEPEC. This approach identified Brazilian aEPEC genomes in previously characterized EPEC/EHEC phylogenomic lineages and resulted in the identification of five novel EPEC phylogenomic lineages, designated EPEC11 to EPEC15. We also observed that virulence genes, such as espG2, espT and espC were more frequently identified among the Brazilian aEPEC genomes, demonstrating potential differences in the virulence repertoire of this pathogen in Brazil.

Pathogenic Escherichia coli Hijacks GTPase-Activated p21-Activated Kinase for Actin Pedestal Formation

Enteropathogenic Escherichia coli and enterohemorrhagic E. coli (EPEC and EHEC, respectively) are extracellular pathogens that reorganize the host cell cytoskeleton to form "actin pedestals" beneath the tightly adherent bacteria, a critical step in pathogenesis. EPEC and EHEC inject effector proteins that manipulate host cell signaling cascades to trigger pedestal assembly. One such effector, EspG, has been reported to bind and activate p21-activated kinase (PAK), a key cytoskeletal regulator, but the function of this interaction and whether it impacts pedestal assembly are unknown. Here, we demonstrate that deletion of espG significantly impairs pedestal formation and attachment by both EPEC and EHEC. This role of EspG is shown to be dependent on its interaction with PAK. Unexpectedly, EspG was able to subvert PAK only in the presence of Rho family small GTPases, which function to both concentrate PAK at the membrane and stimulate PAK activation. Our findings reveal a novel mechanism by which EspG hijacks PAK and sustains its active state to drive bacterial attachment to host cells.IMPORTANCE Enteropathogenic E. coli and enterohemorrhagic E. coli (EPEC and EHEC, respectively) remain a significant global health problem. Both EPEC and EHEC initiate infection by attaching to cells in the host intestine, triggering the formation of actin-rich "pedestal" structures directly beneath the adherent pathogen. These bacteria inject their own receptor into host cells, which upon binding to a protein on the pathogen surface triggers pedestal formation. Multiple other proteins are also delivered into the cells of the host intestine, but how they contribute to disease is often less clear. Here, we show how one of these injected proteins, EspG, hijacks a host signaling pathway for pedestal production. This provides new insights into this essential early stage in EPEC and EHEC disease.

The Ex Vivo Colon Organ Culture and Its Use in Antimicrobial Host Defense Studies

The intestine displays an architecture of repetitive crypt structures consisting of different types of epithelial cells, lamina propia containing immune cells, and stroma. All of these heterogeneous cells contribute to intestinal homeostasis and participate in antimicrobial host defense. Therefore, identifying a surrogate model for studying immune response and antimicrobial activity of the intestine in an in vitro setting is extremely challenging. In vitro studies using immortalized intestinal epithelial cell lines or even primary crypt organoid culture do not represent the exact physiology of normal intestine and its microenvironment. Here, we discuss a method of culturing mouse colon tissue in a culture dish and how this ex vivo organ culture system can be implemented in studies related to antimicrobial host defense responses. In representative experiments, we showed that colons in organ culture express antimicrobial peptides in response to exogenous IL-1β and IL-18. Further, the antimicrobial effector molecules produced by the colon tissues in the organ culture efficiently kill Escherichia coli in vitro. This approach, therefore, can be utilized to dissect the role of pathogen- and danger-associated molecular patterns and their cellular receptors in regulating intestinal innate immune responses and antimicrobial host defense responses.

Enteropathogenic E. coli effectors EspG1/G2 disrupt microtubules, contribute to tight junction perturbation and inhibit restoration

Enteropathogenic Escherichia coli (EPEC) uses a type 3 secretion system to transfer effector proteins into the host intestinal epithelial cell. Several effector molecules contribute to tight junction disruption including EspG1 and its homologue EspG2 via a mechanism thought to involve microtubule destruction. The aim of this study was to investigate the contribution of EspG-mediated microtubule disruption to TJ perturbation. We demonstrate that wild type EPEC infection disassembles microtubules and induces the progressive movement of occludin away from the membrane and into the cytosol. Deletion of espG1/G2 attenuates both of these phenotypes. In addition, EPEC infection impedes barrier recovery from calcium switch, suggesting that inhibition of TJ restoration, not merely disruption, prolongs barrier loss. TJs recover more rapidly following infection with ΔespG1/G2 than with wild type EPEC, demonstrating that EspG1/G2 perpetuate barrier loss. Although EspG regulates ADP-ribosylation factor (ARF) and p21-activated kinase (PAK), these activities are not necessary for microtubule destruction or perturbation of TJ structure and function. These data strongly support a role for EspG1/G2 and its associated effects on microtubules in delaying the recovery of damaged tight junctions caused by EPEC infection.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

Enteropathogenic E. coli effectors EspG1/G2 disrupt tight junctions: new roles and mechanisms

Enteropathogenic E. coli (EPEC) infection is a major cause of infantile diarrhea in the developing world. Using a type-three secretion system, bacterial effector proteins are transferred to the host cell cytosol where they affect multiple physiological functions, ultimately leading to diarrheal disease. Disruption of intestinal epithelial cell tight junctions is a major consequence of EPEC infection and is mediated by multiple effector proteins, among them EspG1 and its homologue EspG2. EspG1/G2 contribute to loss of barrier function via an undefined mechanism that may be linked to their disruption of microtubule networks. Recently new investigations have identified additional roles for EspG. Sequestration of active ADP-ribosylating factor (ARF) proteins and promotion of p21-activated kinase (PAK) activity as well as inhibition of Golgi-mediated protein secretion have all been linked to EspG. In this review, we examine the functions of EspG1/G2 and discuss potential mechanisms of EspG-mediated tight junction disruption.

SadA, a trimeric autotransporter from Salmonella enterica serovar Typhimurium, can promote biofilm formation and provides limited protection against infection

Salmonella enterica is a major cause of morbidity worldwide and mortality in children and immunocompromised individuals in sub-Saharan Africa. Outer membrane proteins of Salmonella are of significance because they are at the interface between the pathogen and the host, they can contribute to adherence, colonization, and virulence, and they are frequently targets of antibody-mediated immunity. In this study, the properties of SadA, a purported trimeric autotransporter adhesin of Salmonella enterica serovar Typhimurium, were examined. We demonstrated that SadA is exposed on the Salmonella cell surface in vitro and in vivo during infection of mice. Expression of SadA resulted in cell aggregation, biofilm formation, and increased adhesion to human intestinal Caco-2 epithelial cells. Immunization of mice with folded, full-length, purified SadA elicited an IgG response which provided limited protection against bacterial challenge. When anti-SadA IgG titers were enhanced by administering alum-precipitated protein, a modest additional protection was afforded. Therefore, despite SadA having pleiotropic functions, it is not a dominant, protective antigen for antibody-mediated protection against Salmonella.

EspG of enteropathogenic and enterohemorrhagic E. coli binds the Golgi matrix protein GM130 and disrupts the Golgi structure and function

The enteric pathogens enteropathogenic Escherichia coli (EPEC), enterohaemorrhagic E. coli (EHEC) and Shigella flexneri all translocate at least one effector protein of the EspG protein family into host cells via a type III secretion system (T3SS). The EspG family comprises EspG, EspG2 and VirA. From a Y2H screen, we identified the Golgi matrix protein GM130 as a potential binding partner of EspG. We confirmed EspG:GM130 protein interaction by affinity co-purification. In co-immunoprecipitation experiments EspG was co-precipitated with GM130 while both GM130 and tubulins were co-precipitated with EspG. When expressed ectopically in HeLa cells, the EspG protein family all localized to the Golgi and induced fragmentation of the Golgi apparatus. All EspG family proteins were also able to disrupt protein secretion to a greater extent than the T3SS effector NleA/EspI, which has previously been shown to localize to the Golgi and interact with SEC24 to disrupt COPII vesicle formation. We hypothesize that EspG:GM130 interaction disrupts protein secretion either through direct disruption of GM130 function or through recruitment of other EspG interacting proteins to the Golgi.

Structural and functional studies indicate that the EPEC effector, EspG, directly binds p21-activated kinase

Bacterial pathogens secrete effectors into their hosts that subvert host defenses and redirect host processes. EspG is a type three secretion effector with a disputed function that is found in enteropathogenic Escherichia coli. Here we show that EspG is structurally similar to VirA, a Shigella virulence factor; EspG has a large, conserved pocket on its surface; EspG binds directly to the amino-terminal inhibitory domain of human p21-activated kinase (PAK); and mutations to conserved residues in the surface pocket disrupt the interaction with PAK.

Novel fold of VirA, a type III secretion system effector protein from Shigella flexneri

VirA, a secreted effector protein from Shigella sp., has been shown to be necessary for its virulence. It was also reported that VirA might be related to papain-like cysteine proteases and cleave alpha-tubulin, thus facilitating intracellular spreading. We have now determined the crystal structure of VirA at 3.0 A resolution. The shape of the molecule resembles the letter "V," with the residues in the N-terminal third of the 45-kDa molecule (some of which are disordered) forming one clearly identifiable domain, and the remainder of the molecule completing the V-like structure. The fold of VirA is unique and does not resemble that of any known protein, including papain, although its N-terminal domain is topologically similar to cysteine protease inhibitors such as stefin B. Analysis of the sequence conservation between VirA and its Escherichia coli homologs EspG and EspG2 did not result in identification of any putative protease-like active site, leaving open a possibility that the biological function of VirA in Shigella virulence may not involve direct proteolytic activity.

Structural and functional studies indicate that Shigella VirA is not a protease and does not directly destabilize microtubules

VirA, an essential virulence factor in Shigella disease pathogenesis, is involved in the uptake, motility, and cell-to-cell spread of Shigella organisms within the human host. These functions have been attributed to a VirA protease activity and a mechanism of microtubule destruction via tubulin degradation [Yoshida, S., et al. (2006) Science 314, 985-989]. We report functional and crystallographic data indicating a novel VirA structure that lacks these activities but highlights the homology to the EspG virulence factor of pathogenic Escherichia coli.

Host-pathogen interplay and the evolution of bacterial effectors

Many bacterial pathogens require a type III secretion system (T3SS) and suite of type III secreted effectors (T3SEs) to successfully colonize their hosts, extract nutrients and consequently cause disease. T3SEs, in particular, are key components of the bacterial arsenal, as they function directly inside the host to disrupt or suppress critical components of the defence network. The development of host defence and surveillance systems imposes intense selective pressures on these bacterial virulence factors, resulting in a host-pathogen co-evolutionary arms race. This arms race leaves its genetic signature in the pattern and structure of natural genetic variation found in T3SEs, thereby permitting us to infer the specific evolutionary processes and pressures driving these interactions. In this review, we summarize our current knowledge of T3SS-mediated host-pathogen co-evolution. We examine the evolution of the T3SS and the T3SEs that traverse it, in both plant and animal pathosystems, and discuss the processes that maintain these important pathogenicity determinants within pathogen populations. We go on to examine the possible origins of T3SEs, the mechanisms that give rise to new T3SEs and the processes that underlie their evolution.