Shigella deliver an effector protein to trigger host microtubule destabilization, which promotes Rac1 activity and efficient bacterial internalization

Evidence for intracellular Pseudomonas aeruginosa

Pseudomonas aeruginosa is a significant cause of global morbidity and mortality. Although it is often regarded as an extracellular pathogen toward human cells, numerous investigations report its ability to survive and replicate within host cells, and additional studies demonstrate specific mechanisms enabling it to adopt an intracellular lifestyle. This ability of P. aeruginosa remains less well-investigated than that of other intracellular bacteria, although it is currently gaining attention. If intracellular bacteria are not killed after entering host cells, they may instead receive protection from immune recognition and experience reduced exposure to antibiotic therapy, among additional potential advantages shared with other facultative intracellular pathogens. For this review, we compiled studies that observe intracellular P. aeruginosa across strains, cell types, and experimental systems in vitro, as well as contextualize these findings with the few studies that report similar observations in vivo. We also seek to address key findings that drove the perception that P. aeruginosa remains extracellular in order to reconcile what is currently understood about intracellular pathogenesis and highlight open questions regarding its contribution to disease.

Microtubules provide force to promote membrane uncoating in vacuolar escape for a cyto-invasive bacterial pathogen

Intracellular bacterial pathogens gain entry to mammalian cells inside a vacuole derived from the host membrane. Some of them escape the bacteria-containing vacuole (BCV) and colonize the cytosol. Bacteria replicating within BCVs coopt the microtubule network to position it within infected cells, whereas the role of microtubules for cyto-invasive pathogens remains obscure. Here, we show that the microtubule motor cytoplasmic dynein-1 and specific activating adaptors are hijacked by the enterobacterium Shigella flexneri. These host proteins were found on infection-associated macropinosomes (IAMs) formed during Shigella internalization. We identified Rab8 and Rab13 as mediators of dynein recruitment and discovered that the Shigella effector protein IpaH7.8 promotes Rab13 retention on moving BCV membrane remnants, thereby facilitating membrane uncoating of the Shigella-containing vacuole. Moreover, the efficient unpeeling of BCV remnants contributes to a successful intercellular spread. Taken together, our work demonstrates how a bacterial pathogen subverts the intracellular transport machinery to secure a cytosolic niche.

HTRA1 rs11528744, BCRA1 rs9928736, and B3GLCT rs4381465 are associated with age-related macular degeneration in a Chinese population

Purpose: Age-related macular degeneration (AMD) is a leading cause of vision loss. A Previous study based on the co-localization analysis of the genome-wide association study (GWAS) and eQTL genetic signals have reported that single nucleotide polymorphisms (SNPs), including rs760975, rs11528744, rs3761159, rs7212510, rs6965458, rs7559693, rs56108400, rs28495773, rs9928736, rs11777697, rs4381465 are associated with AMD in Americans. The aim of this study was to investigate the association of these SNPs in a Han Chinese population.

Methods: There were 576 patients with wet AMD and 572 healthy controls collected in this study. All SNPs were genotyped by flight mass spectrum. Hardy–Weinberg equilibrium was applied to evaluate allele distributions for both AMD and control groups. The genotype and allele frequencies were evaluated using the χ2 tests. Odds ratio (OR) and 95% confidence intervals (95% CI) were calculated for the risk of genotype and allele.

Results: Three of the 11 SNPs (rs11528744 in HTRA1, rs9928736 in BCRA1 and rs4381465 in B3GLCT) were found to be significantly associated with AMD in the allelic model (corrected p = 0.001, OR = 1.391, 95%CI = 1.179–1.640 for rs11528744; corrected p = 0.004, OR = 0.695, 95%CI = 0.544–0.888 for rs9928736; corrected p = 0.002, OR = 0.614, 95%CI = 0.448–0.841 for rs4381465). There were no differences for the remaining eight SNPs between AMD cases and healthy controls.

Conclusion: Our results showed that HTRA1 rs11528744, BCRA1 rs9928736, and B3GLCT rs4381465 were associated with wet AMD, suggesting that HTRA1, BCRA1, and B3GLCT genes may be involved in the development of AMD.

NOD-Like Receptors: Guards of Cellular Homeostasis Perturbation during Infection

The innate immune system relies on families of pattern recognition receptors (PRRs) that detect distinct conserved molecular motifs from microbes to initiate antimicrobial responses. Activation of PRRs triggers a series of signaling cascades, leading to the release of pro-inflammatory cytokines, chemokines and antimicrobials, thereby contributing to the early host defense against microbes and regulating adaptive immunity. Additionally, PRRs can detect perturbation of cellular homeostasis caused by pathogens and fine-tune the immune responses. Among PRRs, nucleotide binding oligomerization domain (NOD)-like receptors (NLRs) have attracted particular interest in the context of cellular stress-induced inflammation during infection. Recently, mechanistic insights into the monitoring of cellular homeostasis perturbation by NLRs have been provided. We summarize the current knowledge about the disruption of cellular homeostasis by pathogens and focus on NLRs as innate immune sensors for its detection. We highlight the mechanisms employed by various pathogens to elicit cytoskeleton disruption, organelle stress as well as protein translation block, point out exemplary NLRs that guard cellular homeostasis during infection and introduce the concept of stress-associated molecular patterns (SAMPs). We postulate that integration of information about microbial patterns, danger signals, and SAMPs enables the innate immune system with adequate plasticity and precision in elaborating responses to microbes of variable virulence.

Using proteomics to identify host cell interaction partners for VgrG and IglJ

Francisella tularensis is a highly virulent intracellular bacterium and the causative agent of tularemia. The disease is characterized by the suboptimal innate immune response and consequently by the impaired adaptive immunity. The virulence of this pathogen depends on proteins encoded by a genomic island termed the Francisella Pathogenicity Island (FPI). However, the precise biological roles of most of the FPI-encoded proteins remain to be clarified. In this study, we employed stable isotope labeling by amino acids in cell culture (SILAC) in combination with affinity protein purification coupled with liquid chromatography–mass spectrometry to identify potential protein-effector binding pairs for two FPI virulence effectors IglJ and VgrG. Our results may indicate that while the IglJ protein interactions primarily affect mitochondria, the VgrG interactions affect phagosome and/or autophagosome biogenesis via targeting components of the host’s exocyst complex.

Staying out or Going in? The Interplay between Type 3 and Type 5 Secretion Systems in Adhesion and Invasion of Enterobacterial Pathogens

Enteric pathogens rely on a variety of toxins, adhesins and other virulence factors to cause infections. Some of the best studied pathogens belong to the Enterobacterales order; these include enteropathogenic and enterohemorrhagic Escherichia coli, Shigella spp., and the enteropathogenic Yersiniae. The pathogenesis of these organisms involves two different secretion systems, a type 3 secretion system (T3SS) and type 5 secretion systems (T5SSs). The T3SS forms a syringe-like structure spanning both bacterial membranes and the host cell plasma membrane that translocates toxic effector proteins into the cytoplasm of the host cell. T5SSs are also known as autotransporters, and they export part of their own polypeptide to the bacterial cell surface where it exerts its function, such as adhesion to host cell receptors. During infection with these enteropathogens, the T3SS and T5SS act in concert to bring about rearrangements of the host cell cytoskeleton, either to invade the cell, confer intracellular motility, evade phagocytosis or produce novel structures to shelter the bacteria. Thus, in these bacteria, not only the T3SS effectors but also T5SS proteins could be considered "cytoskeletoxins" that bring about profound alterations in host cell cytoskeletal dynamics and lead to pathogenic outcomes.

Recent insights into Shigella

PURPOSE OF REVIEW

Diarrhoea is a major global health problem, and recent studies have confirmed Shigella as a major contributor to this burden. Here, we review recent advances in Shigella research; focusing on their epidemiology, pathogenesis, antimicrobial resistance, and the role of the gut microbiome during infection.

RECENT FINDINGS

Enhanced epidemiological data, combined with new generation diagnostics, has highlighted a greater burden of Shigella disease than was previously estimated, which is not restricted to vulnerable populations in low-middle income countries. As we gain an ever more detailed insight into the orchestrated mechanisms that Shigella exploit to trigger infection, we can also begin to appreciate the complex role of the gut microbiome in preventing and inducing such infections. The use of genomics, in combination with epidemiological data and laboratory investigations, has unravelled the evolution and spread of various species. Such measures have identified resistance to antimicrobials as a key contributor to the success of specific clones.

SUMMARY

We need to apply novel findings towards sustainable approaches for treating and preventing Shigella infections. Vaccines and alternative treatments are under development and may offer an opportunity to reduce the burden of Shigella disease and restrict the mobility of antimicrobial resistant clones.

The role of microtubules and the dynein/dynactin motor complex of host cells in the biogenesis of the Coxiella burnetii-containing vacuole

Microtubules (Mts) are dynamic cytoskeleton structures that play a key role in vesicular transport. The Mts-mediated transport depends on motor proteins named kinesins and the dynein/dynactin motor complex. The Rab7 adapter protein FYCO1 controls the anterograde transport of the endocytic compartments through the interaction with the kinesin KIF5. Rab7 and its partner RILP induce the recruitment of dynein/dynactin to late endosomes regulating its retrograde transport to the perinuclear area to fuse with lysosomes. The late endosomal-lysosomal fusion is regulated by the HOPS complex through its interaction with RILP and the GTPase Arl8. Coxiella burnetii (Cb), the causative agent of Q fever, is an obligate intracellular pathogen, which generates a large compartment with autophagolysosomal characteristics named Cb-containing vacuole (CCV). The CCV forms through homotypic fusion between small non-replicative CCVs (nrCCV) and through heterotypic fusion with other compartments, such as endosomes and lysosomes. In this work, we characterise the role of Mts, motor proteins, RILP/Rab7 and Arl8 on the CCV biogenesis. The formation of the CCV was affected when either the dynamics and/or the acetylation state of Mts were modified. Similarly, the overexpression of the dynactin subunit non-functional mutants p150^Glued and RILP led to the formation of small nrCCVs. This phenomenon is not observed in cells overexpressing WT proteins, the motor KIF5 or its interacting protein FYCO1. The formation of the CCV was normal in infected cells that overexpressed Arl8 alone or together with hVps41 (a HOPS subunit) or in cells co-overexpressing hVps41 and RILP. The dominant negative mutant of Arl8 and the non-functional hVps41 inhibited the formation of the CCV. When the formation of CCV was affected, the bacterial multiplication diminished. Our results suggest that nrCCVs recruit the molecular machinery that regulate the Mts-dependent retrograde transport, Rab7/RILP and the dynein/dynactin system, as well as the tethering processes such as HOPS complex and Arl8 to finally originate the CCV where C. burnetii multiplies.

Shigella and Enteroinvasive Escherichia Coli

Shigella and enteroinvasive Escherichia coli (EIEC) are gram-negative bacteria responsible for bacillary dysentery (shigellosis) in humans, which is characterized by invasion and inflammatory destruction of the human colonic epithelium. Different EIEC and Shigella subgroups rose independently from commensal E. coli through patho-adaptive evolution that included loss of functional genes interfering with the virulence and/or with the intracellular lifestyle of the bacteria, as well as acquisition of genetic elements harboring virulence genes. Among the latter is the large virulence plasmid encoding for a type three secretion system (T3SS), which enables translocation of virulence proteins (effectors) from the bacterium directly into the host cell cytoplasm. These effectors enable the pathogen to subvert epithelial cell functions, promoting its own uptake, replication in the host cytosol, and dissemination to adjacent cells while concomitantly inhibiting pro-inflammatory cell death. Furthermore, T3SS effectors are directly involved in Shigella manipulation of immune cells causing their dysfunction and promoting cell death. In the current chapter, we first describe the evolution of the enteroinvasive pathovars and then summarize the overall knowledge concerning the pathogenesis of these bacteria, with a particular focus on Shigella flexneri. Subversion of host cell functions in the human gut, both epithelial and immune cells, by different virulence factors is especially highlighted.

Tracking elusive cargo: Illuminating spatio-temporal Type 3 effector protein dynamics using reporters

Type 3 secretion systems form an integral part of the arsenal of many pathogenic bacteria. These injection machines, together with their cargo of subversive effector proteins, are capable of manipulating the cellular environment of the host in order to ensure persistence of the pathogen. In order to fully appreciate the functions of Type 3 effectors, it is necessary to gain spatio‐temporal knowledge of each effector during the process of infection. A number of genetic modifications have been exploited in order to reveal effector protein secretion, translocation and subsequent activity, and localisation within host cells. In this review, we will discuss the many available approaches for tracking effector protein dynamics and discuss the challenges faced to improve the current technologies and gain a clearer picture of effector protein function.

How Do the Virulence Factors of Shigella Work Together to Cause Disease?

Shigella is the major cause of bacillary dysentery world-wide. It is divided into four species, named S. flexneri, S. sonnei, S. dysenteriae, and S. boydii, which are distinct genomically and in their ability to cause disease. Shigellosis, the clinical presentation of Shigella infection, is characterized by watery diarrhea, abdominal cramps, and fever. Shigella's ability to cause disease has been attributed to virulence factors, which are encoded on chromosomal pathogenicity islands and the virulence plasmid. However, information on these virulence factors is not often brought together to create a detailed picture of infection, and how this translates into shigellosis symptoms. Firstly, Shigella secretes virulence factors that induce severe inflammation and mediate enterotoxic effects on the colon, producing the classic watery diarrhea seen early in infection. Secondly, Shigella injects virulence effectors into epithelial cells via its Type III Secretion System to subvert the host cell structure and function. This allows invasion of epithelial cells, establishing a replicative niche, and causes erratic destruction of the colonic epithelium. Thirdly, Shigella produces effectors to down-regulate inflammation and the innate immune response. This promotes infection and limits the adaptive immune response, causing the host to remain partially susceptible to re-infection. Combinations of these virulence factors may contribute to the different symptoms and infection capabilities of the diverse Shigella species, in addition to distinct transmission patterns. Further investigation of the dominant species causing disease, using whole-genome sequencing and genotyping, will allow comparison and identification of crucial virulence factors and may contribute to the production of a pan-Shigella vaccine.

Manipulation of host membranes by the bacterial pathogens Listeria, Francisella, Shigella and Yersinia

Bacterial pathogens display an impressive arsenal of molecular mechanisms that allow survival in diverse host niches. Subversion of plasma membrane and cytoskeletal functions are common themes associated to infection by both extracellular and intracellular pathogens. Moreover, intracellular pathogens modify the structure/stability of their membrane-bound compartments and escape degradation from phagocytic or autophagic pathways. Here, we review the manipulation of host membranes by Listeria monocytogenes, Francisella tularensis, Shigella flexneri and Yersinia spp. These four bacterial model pathogens exemplify generalized strategies as well as specific features observed during bacterial infection processes.

The genomic signatures of Shigella evolution, adaptation and geographical spread

Shigella spp. are some of the key pathogens responsible for the global burden of diarrhoeal disease. These facultative intracellular bacteria belong to the family Enterobacteriaceae, together with other intestinal pathogens, such as Escherichia coli and Salmonella spp. The genus Shigella comprises four different species, each consisting of several serogroups, all of which show phenotypic similarity, including invasive pathogenicity. DNA sequencing suggests that this similarity results from the convergent evolution of different Shigella spp. founders. Here, we review the evolutionary relationships between Shigella spp. and E . coli, and we highlight how the genomic plasticity of these bacteria and their acquisition of a distinctive virulence plasmid have enabled the development of such highly specialized pathogens. Furthermore, we discuss the insights that genotyping and whole-genome sequencing have provided into the phylogenetics and intercontinental spread of Shigella spp.

Virulence Gene Regulation in Shigella

Shigella species are the causative agents of bacillary dysentery in humans, an invasive disease in which the bacteria enter the cells of the epithelial layer of the large intestine, causing extensive tissue damage and inflammation. They rely on a plasmid-encoded type III secretion system (TTSS) to cause disease; this system and its regulation have been investigated intensively at the molecular level for decades. The lessons learned have not only deepened our knowledge of Shigella biology but also informed in important ways our understanding of the mechanisms used by other pathogenic bacteria to cause disease and to control virulence gene expression. In addition, the Shigella story has played a central role in the development of our appreciation of the contribution of horizontal DNA transfer to pathogen evolution.A 30-kilobase-pair "Entry Region" of the 230-kb virulence plasmid lies at the heart of the Shigella pathogenesis system. Here are located the virB and mxiE regulatory genes and most of the structural genes involved in the expression of the TTSS and its effector proteins. Expression of the virulence genes occurs in response to an array of environmental signals, including temperature, osmolarity, and pH.At the top of the regulatory hierarchy and lying on the plasmid outside the Entry Region isvirF, encoding an AraC-like transcription factor.Virulence gene expression is also controlled by chromosomal genes,such as those encoding the nucleoid-associated proteins H-NS, IHF, and Fis, the two-component regulators OmpR/EnvZ and CpxR/CpxA, the anaerobic regulator Fnr, the iron-responsive regulator Fur, and the topoisomerases of the cell that modulate DNA supercoiling. Small regulatory RNAs,the RNA chaperone Hfq,and translational modulation also affect the expression of the virulence phenotypetranscriptionally and/orposttranscriptionally.

Selective protection of an ARF1-GTP signaling axis by a bacterial scaffold induces bidirectional trafficking arrest

Bidirectional vesicular transport between the endoplasmic reticulum (ER) and Golgi is mediated largely by ARF and Rab GTPases, which orchestrate vesicle fission and fusion, respectively. How their activities are coordinated in order to define the successive steps of the secretory pathway and preserve traffic directionality is not well understood in part due to the scarcity of molecular tools that simultaneously target ARF and Rab signaling. Here, we take advantage of the unique scaffolding properties of E. coli secreted protein G (EspG) to describe the critical role of ARF1/Rab1 spatiotemporal coordination in vesicular transport at the ER-Golgi intermediate compartment. Structural modeling and cellular studies show that EspG induces bidirectional traffic arrest by tethering vesicles through select ARF1-GTP/effector complexes and local inactivation of Rab1. The mechanistic insights presented here establish the effectiveness of a small bacterial catalytic scaffold for studying complex processes and reveal an alternative mechanism of immune regulation by an important human pathogen.

Recent advances in understanding enteric pathogenic Escherichia coli

Although Escherichia coli can be an innocuous resident of the gastrointestinal tract, it also has the pathogenic capacity to cause significant diarrheal and extraintestinal diseases. Pathogenic variants of E. coli (pathovars or pathotypes) cause much morbidity and mortality worldwide. Consequently, pathogenic E. coli is widely studied in humans, animals, food, and the environment. While there are many common features that these pathotypes employ to colonize the intestinal mucosa and cause disease, the course, onset, and complications vary significantly. Outbreaks are common in developed and developing countries, and they sometimes have fatal consequences. Many of these pathotypes are a major public health concern as they have low infectious doses and are transmitted through ubiquitous mediums, including food and water. The seriousness of pathogenic E. coli is exemplified by dedicated national and international surveillance programs that monitor and track outbreaks; unfortunately, this surveillance is often lacking in developing countries. While not all pathotypes carry the same public health profile, they all carry an enormous potential to cause disease and continue to present challenges to human health. This comprehensive review highlights recent advances in our understanding of the intestinal pathotypes of E. coli.

Actin cytoskeleton manipulation by effector proteins secreted by diarrheagenic Escherichia coli pathotypes

The actin cytoskeleton is a dynamic structure necessary for cell and tissue organization, including the maintenance of epithelial barriers. Disruption of the epithelial barrier coincides with alterations of the actin cytoskeleton in several disease states. These disruptions primarily affect the paracellular space, which is normally regulated by tight junctions. Thereby, the actin cytoskeleton is a common and recurring target of bacterial virulence factors. In order to manipulate the actin cytoskeleton, bacteria secrete and inject toxins and effectors to hijack the host cell machinery, which interferes with host-cell pathways and with a number of actin binding proteins. An interesting model to study actin manipulation by bacterial effectors is Escherichia coli since due to its genome plasticity it has acquired diverse genetic mobile elements, which allow having different E. coli varieties in one bacterial species. These E. coli pathotypes, including intracellular and extracellular bacteria, interact with epithelial cells, and their interactions depend on a specific combination of virulence factors. In this paper we focus on E. coli effectors that mimic host cell proteins to manipulate the actin cytoskeleton. The study of bacterial effector-cytoskeleton interaction will contribute not only to the comprehension of the molecular causes of infectious diseases but also to increase our knowledge of cell biology.

[Pathogenesis of Shigella: the study of bacteria-host interplay at the intestinal mucosal barriers]

Shigella are capable of invading and colonizing the intestinal epithelium, which results in strong inflammatory colitis. Shigella deliver a subset of effectors via the type III secretion system during infection into host cells. The delivered effectors mimic and usurp the host cellular functions, and modulate host cell signaling and immune response, thus playing pivotal roles in promoting the bacterial infection and circumventing host defense systems. This article overviews the characteristics of pathogenesis of Shigella, and highlights current topics related to the roles of the effectors in promoting bacterial infection.

Modulation of host microtubule dynamics by pathogenic bacteria

The eukaryotic cytoskeleton is a vulnerable target of many microbial pathogens during the course of infection. Rearrangements of host cytoskeleton benefit microbes in various stages of their infection cycle such as invasion, motility, and persistence. Bacterial pathogens deliver a number of effector proteins into host cells for modulating the dynamics of actin and microtubule cytoskeleton. Alteration of the actin cytoskeleton is generally achieved by bacterial effectors that target the small GTPases of the host. Modulation of microtubule dynamics involves direct interaction of effector proteins with the subunits of microtubules or recruiting cellular proteins that affect microtubule dynamics. This review will discuss effector proteins from animal and human bacterial pathogens that either destabilize or stabilize host micro-tubules to advance the infectious process. A compilation of these research findings will provide an overview of known and unknown strategies used by various bacterial effectors to modulate the host microtubule dynamics. The present review will undoubtedly help direct future research to determine the mechanisms of action of many bacterial effector proteins and contribute to understanding the survival strategies of diverse adherent and invasive bacterial pathogens.

Shigella flexneri T3SS effector IpaH4.5 modulates the host inflammatory response via interaction with NF-κB p65 protein

Shigella species possess a type III secretion system (T3SS), which is required for human infection and that delivers effector proteins into target host cells. Here, we show that the effector, IpaH4.5 dampens the pro-inflammatory cytokine response. In both the Sereny test and a murine lung infection model, the Shigella ΔipaH4.5 mutant strain caused more severe inflammatory responses and significantly induced higher pro-inflammatory cytokine levels (MIP-2 and TNF-α) in the lung homogenates of wild type-infected mice. Moreover, there was a threefold decrease in bacterial colonization of the mutant compared with the WT and ΔipaH4.5/ipaH4.5-rescued strains. Yeast two-hybrid screening showed that IpaH4.5 specifically interacts with the p65 subunit of NF-κB. Ten truncated versions of IpaH4.5 and p65 spanning different regions were constructed and expressed to further map the IpaH binding sites with p65. The results revealed thatthe p65 region spanning amino acids 1-190 of p65 interacted with the IpaH4.5/1-293 N-terminal region. In vitro, IpaH4.5 displayed ubiquitin ligase activity towards ubiquitin and p65. Furthermore, the transcriptional activity of NF-κB was shown to be inhibited by IpaH4.5 utilizing a dual-luciferase reporter gene detection system containing NF-κB promoter response elements. Thus, we conclude that the IpaH4.5 protein is an E3 ubiquitin ligase capable of directly regulating the host inflammatory response by inhibiting the NF-κB signalling pathway.

Calpain activation by the Shigella flexneri effector VirA regulates key steps in the formation and life of the bacterium's epithelial niche

The enteropathogen Shigella flexneri invades epithelial cells, leading to inflammation and tissue destruction. We report that Shigella infection of epithelial cells induces an early genotoxic stress, but the resulting p53 response and cell death are impaired due to the bacterium's ability to promote p53 degradation, mainly through calpain protease activation. Calpain activation is promoted by the Shigella virulence effector VirA and dependent on calcium flux and the depletion of the endogenous calpain inhibitor calpastatin. Further, although VirA-induced calpain activity is critical for regulating cytoskeletal events driving bacterial uptake, calpain activation ultimately leads to necrotic cell death, thereby restricting Shigella intracellular growth. Therefore, calpains work at multiple steps in regulating Shigella pathogenesis by disrupting the p53-dependent DNA repair response early during infection and regulating both formation and ultimate death of the Shigella epithelial replicative niche.

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.

Functional domains and motifs of bacterial type III effector proteins and their roles in infection

A key feature of the virulence of many bacterial pathogens is the ability to deliver effector proteins into eukaryotic cells via a dedicated type three secretion system (T3SS). Many bacterial pathogens, including species of Chlamydia, Xanthomonas, Pseudomonas, Ralstonia, Shigella, Salmonella, Escherichia and Yersinia, depend on the T3SS to cause disease. T3SS effectors constitute a large and diverse group of virulence proteins that mimic eukaryotic proteins in structure and function. A salient feature of bacterial effectors is their modular architecture, comprising domains or motifs that confer an array of subversive functions within the eukaryotic cell. These domains/motifs therefore represent a fascinating repertoire of molecular determinants with important roles during infection. This review provides a snapshot of our current understanding of bacterial effector domains and motifs where a defined role in infection has been demonstrated.

Unifying themes in microbial associations with animal and plant hosts described using the gene ontology

Microbes form intimate relationships with hosts (symbioses) that range from mutualism to parasitism. Common microbial mechanisms involved in a successful host association include adhesion, entry of the microbe or its effector proteins into the host cell, mitigation of host defenses, and nutrient acquisition. Genes associated with these microbial mechanisms are known for a broad range of symbioses, revealing both divergent and convergent strategies. Effective comparisons among these symbioses, however, are hampered by inconsistent descriptive terms in the literature for functionally similar genes. Bioinformatic approaches that use homology-based tools are limited to identifying functionally similar genes based on similarities in their sequences. An effective solution to these limitations is provided by the Gene Ontology (GO), which provides a standardized language to describe gene products from all organisms. The GO comprises three ontologies that enable one to describe the molecular function(s) of gene products, the biological processes to which they contribute, and their cellular locations. Beginning in 2004, the Plant-Associated Microbe Gene Ontology (PAMGO) interest group collaborated with the GO consortium to extend the GO to accommodate terms for describing gene products associated with microbe-host interactions. Currently, over 900 terms that describe biological processes common to diverse plant- and animal-associated microbes are incorporated into the GO database. Here we review some unifying themes common to diverse host-microbe associations and illustrate how the new GO terms facilitate a standardized description of the gene products involved. We also highlight areas where new terms need to be developed, an ongoing process that should involve the whole community.

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.

EseG, an effector of the type III secretion system of Edwardsiella tarda, triggers microtubule destabilization

Edwardsiella tarda is a Gram-negative enteric pathogen that causes hemorrhagic septicemia in fish and both gastrointestinal and extraintestinal infections in humans. A type III secretion system (T3SS) was recently shown to contribute to pathogenesis, since deletions of various T3SS genes increased the 50% lethal dose (LD(50)) by about 1 log unit in the blue gourami infection model. In this study, we report EseG as the first identified effector protein of T3SS. EseG shares partial homology with two Salmonella T3SS effectors (SseG and SseF) over a conserved domain (amino acid residues 142 to 192). The secretion of EseG is dependent on a functional T3SS and, in particular, requires the chaperone EscB. Experiments using TEM-1 β-lactamase as a fluorescence-based reporter showed that EseG was translocated into HeLa cells at 35°C. Fractionation of infected HeLa cells demonstrated that EseG was localized to the host membrane fraction after translocation. EseG is able to disassemble microtubule structures when overexpressed in mammalian cells. This phenotype may require a conserved motif of EseG (EseG(142-192)), since truncated versions of EseG devoid of this motif lose their ability to cause microtubule destabilization. By demonstrating the function of EseG, our study contributes to the understanding of E. tarda pathogenesis. Moreover, the approach established in this study to identify type III effectors can be used to identify and characterize more type III and possible type VI effectors in Edwardsiella.

The Shigella dysenteriae serotype 1 proteome, profiled in the host intestinal environment, reveals major metabolic modifications and increased expression of invasive proteins

Shigella dysenteriae serotype 1 (SD1) causes the most severe form of epidemic bacillary dysentery. We present the first comprehensive proteome analysis of this pathogen, profiling proteins from bacteria cultured in vitro and bacterial isolates from the large bowel of infected gnotobiotic piglets (in vivo). Overall, 1061 distinct gene products were identified. Differential display analysis revealed that SD1 cells switched to an anaerobic energy metabolism in vivo. High in vivo abundances of amino acid decarboxylases (GadB and AdiA) which enhance pH homeostasis in the cytoplasm and protein disaggregation chaperones (HdeA, HdeB and ClpB) were indicative of a coordinated bacterial survival response to acid stress. Several type III secretion system effectors were increased in abundance in vivo, including OspF, IpaC and IpaD. These proteins are implicated in invasion of colonocytes and subversion of the host immune response in S. flexneri. These observations likely reflect an adaptive response of SD1 to the hostile host environment. Seven proteins, among them the type III secretion system effectors OspC2 and IpaB, were detected as antigens in Western blots using piglet antisera. The outer membrane protein OmpA, the heat shock protein HtpG and OspC2 represent novel SD1 subunit vaccine candidates and drug targets.

Bacterial guanine nucleotide exchange factors SopE-like and WxxxE effectors

Subversion of Rho family small GTPases, which control actin dynamics, is a common infection strategy used by bacterial pathogens. In particular, Salmonella enterica serovar Typhimurium, Shigella flexneri, enteropathogenic Escherichia coli (EPEC), and enterohemorrhagic Escherichia coli (EHEC) translocate type III secretion system (T3SS) effector proteins to modulate the Rho GTPases RhoA, Cdc42, and Rac1, which trigger formation of stress fibers, filopodia, and lamellipodia/ruffles, respectively. The Salmonella effector SopE is a guanine nucleotide exchange factor (GEF) that activates Rac1 and Cdc42, which induce "the trigger mechanism of cell entry." Based on a conserved Trp-xxx-Glu motif, the T3SS effector proteins IpgB1 and IpgB2 of Shigella, SifA and SifB of Salmonella, and Map of EPEC and EHEC were grouped together into a WxxxE family; recent studies identified the T3SS EPEC and EHEC effectors EspM and EspT as new family members. Recent structural and functional studies have shown that representatives of the WxxxE effectors share with SopE a 3-D fold and GEF activity. In this minireview, we summarize contemporary findings related to the SopE and WxxxE GEFs in the context of their role in subverting general host cell signaling pathways and infection.

Tracking the dynamic interplay between bacterial and host factors during pathogen-induced vacuole rupture in real time

Escape into the host cell cytosol following invasion of mammalian cells is a common strategy used by invasive pathogens. This requires membrane rupture of the vesicular or vacuolar compartment formed around the bacteria after uptake into the host cell. The mechanism of pathogen-induced disassembly of the vacuolar membrane is poorly understood. We established a novel, robust and sensitive fluorescence microscopy method that tracks the precise time point of vacuole rupture upon uptake of Gram-negative bacteria. This revealed that the enteroinvasive pathogen Shigella flexneri escapes rapidly, in less than 10 min, from the vacuole. Our method demonstrated the recruitment of host factors, such as RhoA, to the bacterial entry site and their continued presence at the point of vacuole rupture. We found a novel host marker for ruptured vacuoles, galectin-3, which appears instantly in the proximity of bacteria after escape into the cytosol. Furthermore, we show that the Salmonella effector proteins, SifA and PipB2, stabilize the vacuole membrane inhibiting bacterial escape from the vacuole. Our novel approach to track vacuole rupture is ideally suited for high-content and high-throughput approaches to identify the molecular and cellular mechanisms of membrane rupture during invasion by pathogens such as viruses, bacteria and parasites.

A new paradigm of bacteria-gut interplay brought through the study of Shigella

Bacteria-gut epithelial interplay and the mucosal immune response are the most critical issues in determining the fate of bacterial infection and the severity of diseases. Shigella species (abbreviated here as Shigella), the causative agent of bacillary dysentery (shigellosis), are highly adapted human pathogens that are capable of invading and colonizing the intestinal epithelium, which results in severe inflammatory colitis. Shigella secrete a large and diverse number (more then 50) of effectors via the type III secretion system (TTSS) during infection, some of which are delivered into the surrounding bacterial space and some others into the host cell cytoplasm and nucleus. The delivered effectors mimic and usurp the host cellular functions, and modulate host cell signaling and immune response, thus playing pivotal roles in promoting bacterial infection and circumventing host defense systems. This article overviews the pathogenic characteristics of Shigella, and highlights current topics related to the bacterial infectious stratagem executed by the TTSS-secreted effectors. Though bacterial stratagems and the molecular mechanisms of infection vary greatly among pathogens, the current studies of Shigella provide a paradigm shift in bacterial pathogenesis.

Shigella type III secretion effectors: how, where, when, for what purposes?

Bacteria of Shigella spp., the causative agents of shigellosis in humans, possess a repertoire of approximately 25-30 effectors injected into host cells by a type III secretion apparatus (T3SA). The T3SA activity is activated upon contact of bacteria with cells and controls expression of some effectors. Recent structural and functional studies suggest that two different sets of effectors are involved in inducing actin cytoskeleton reorganization to promote entry of bacteria into epithelial cells and in modulating cell signaling pathways to dampen innate immune responses induced upon infection, respectively. Schematically, effectors involved in entry are produced independently of the T3SA activity, whereas effectors involved in controlling the cell responses are produced upon activation of the T3SA.

GEF-H1 mediated control of NOD1 dependent NF-kappaB activation by Shigella effectors

Shigella flexneri has evolved the ability to modify host cell function with intracellular active effectors to overcome the intestinal barrier. The detection of these microbial effectors and the initiation of innate immune responses are critical for rapid mucosal defense activation. The guanine nucleotide exchange factor H1 (GEF-H1) mediates RhoA activation required for cell invasion by the enteroinvasive pathogen Shigella flexneri. Surprisingly, GEF-H1 is requisite for NF-κB activation in response to Shigella infection. GEF-H1 interacts with NOD1 and is required for RIP2 dependent NF-κB activation by H-Ala-D-γGlu-DAP (γTriDAP). GEF-H1 is essential for NF-κB activation by the Shigella effectors IpgB2 and OspB, which were found to signal in a NOD1 and RhoA Kinase (ROCK) dependent manner. Our results demonstrate that GEF-H1 is a critical component of cellular defenses forming an intracellular sensing system with NOD1 for the detection of microbial effectors during cell invasion by pathogens.

Shigella is a bacterium that causes food poisoning and serious intestinal infections with diarrheal illness. Pathogens like Shigella utilize intracellular active effectors to overcome the intestinal barrier and invade the host. We demonstrate that intestinal epithelial cells can sense the disturbance of the tight junctional seal, which normally prevents access of microbes to the circulation. A signaling molecule, which is required for cell invasion by Shigella, also activates messengers that activate immune defenses. This pathway of intestinal pathogen detection is activated by Shigella products, which are injected into host cells by the pathogen and depends on intracellular microbial recognition receptors. The detection of altered cellular function by bacterial effectors may be important for the ability to rapidly respond to barrier disruption in the intestine with the attraction and activation of immune cells to defend against the intruders.

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.

The Yeast Saccharomyces cerevisiae: a versatile model system for the identification and characterization of bacterial virulence proteins

Microbial pathogens utilize complex secretion systems to deliver proteins into host cells. These effector proteins target and usurp host cell processes to promote infection and cause disease. While secretion systems are conserved, each pathogen delivers its own unique set of effectors. The identification and characterization of these effector proteins has been difficult, often limited by the lack of detectable signal sequences and functional redundancy. Model systems including yeast, worms, flies, and fish are being used to circumvent these issues. This technical review details the versatility and utility of yeast Saccharomyces cerevisiae as a system to identify and characterize bacterial effectors.

A functional genomic yeast screen to identify pathogenic bacterial proteins

Many bacterial pathogens promote infection and cause disease by directly injecting into host cells proteins that manipulate eukaryotic cellular processes. Identification of these translocated proteins is essential to understanding pathogenesis. Yet, their identification remains limited. This, in part, is due to their general sequence uniqueness, which confounds homology-based identification by comparative genomic methods. In addition, their absence often does not result in phenotypes in virulence assays limiting functional genetic screens. Translocated proteins have been observed to confer toxic phenotypes when expressed in the yeast Saccharomyces cerevisiae. This observation suggests that yeast growth inhibition can be used as an indicator of protein translocation in functional genomic screens. However, limited information is available regarding the behavior of non-translocated proteins in yeast. We developed a semi-automated quantitative assay to monitor the growth of hundreds of yeast strains in parallel. We observed that expression of half of the 19 Shigella translocated proteins tested but almost none of the 20 non-translocated Shigella proteins nor ∼1,000 Francisella tularensis proteins significantly inhibited yeast growth. Not only does this study establish that yeast growth inhibition is a sensitive and specific indicator of translocated proteins, but we also identified a new substrate of the Shigella type III secretion system (TTSS), IpaJ, previously missed by other experimental approaches. In those cases where the mechanisms of action of the translocated proteins are known, significant yeast growth inhibition correlated with the targeting of conserved cellular processes. By providing positive rather than negative indication of activity our assay complements existing approaches for identification of translocated proteins. In addition, because this assay only requires genomic DNA it is particularly valuable for studying pathogens that are difficult to genetically manipulate or dangerous to culture.

Many bacterial pathogens promote infection and ultimately cause disease, in part, through the actions of proteins that the bacteria directly inject into host cells. These proteins subvert host cell processes to favor survival of the pathogen. The identification of such proteins can be limited since many of the injected proteins lack homology with other virulence proteins and pathogens that no longer express the proteins are often unimpaired in conventional assays of pathogenesis. Many of these proteins target cellular processes conserved from mammals to yeast, and overexpression of these proteins in yeast results in growth inhibition. We have established a high throughput growth assay amenable to systematically screening open reading frames from bacterial pathogens for those that inhibit yeast growth. We observe that yeast growth inhibition is a sensitive and specific indicator of proteins that are injected into host cells. Expression of about half of the injected bacterial proteins but almost none of the bacteria-confined proteins results in yeast growth inhibition. Since this assay only requires genomic DNA it is particularly valuable for studying pathogens that are difficult to genetically manipulate or dangerous to grow in the laboratory.

Molecular pathogenesis of Shigella spp.: controlling host cell signaling, invasion, and death by type III secretion

Shigella spp. are gram-negative pathogenic bacteria that evolved from harmless enterobacterial relatives and may cause devastating diarrhea upon ingestion. Research performed over the last 25 years revealed that a type III secretion system (T3SS) encoded on a large plasmid is a key virulence factor of Shigella flexneri. The T3SS determines the interactions of S. flexneri with intestinal cells by consecutively translocating two sets of effector proteins into the target cells. Thus, S. flexneri controls invasion into EC, intra- and intercellular spread, macrophage cell death, as well as host inflammatory responses. Some of the translocated effector proteins show novel biochemical activities by which they intercept host cell signal transduction pathways. An understanding of the molecular mechanisms underlying Shigella pathogenesis will foster the development of a safe and efficient vaccine, which, in parallel with improved hygiene, should curb infections by this widespread pathogen.

Type III secretion systems and disease

Type III secretion systems (T3SSs) are complex bacterial structures that provide gram-negative pathogens with a unique virulence mechanism enabling them to inject bacterial effector proteins directly into the host cell cytoplasm, bypassing the extracellular milieu. Although the effector proteins vary among different T3SS pathogens, common pathogenic mechanisms emerge, including interference with the host cell cytoskeleton to promote attachment and invasion, interference with cellular trafficking processes, cytotoxicity and barrier dysfunction, and immune system subversion. The activity of the T3SSs correlates closely with infection progression and outcome, both in animal models and in human infection. Therefore, to facilitate patient care and improve outcomes, it is important to understand the T3SS-mediated virulence processes and to target T3SSs in therapeutic and prophylactic development efforts.

Interactions of bacterial effector proteins with host proteins

Pathogenic bacteria have evolved several clever survival strategies for manipulating host cell signaling pathways to establish beneficial replicative niches within the host. Recent literature has revealed novel mechanisms adopted by bacteria to manipulate host responses. For instance, host signaling pathways that were traditionally thought to be regulated by phosphorylation events have now been shown to be irreversibly blocked by bacterially-mediated acetylation, beta-elimination, and lytic modifications. This review highlights some of the common host proteins and signaling cascades targeted by such pathogens.

Rac interacts with Abi-1 and WAVE2 to promote an Arp2/3-dependent actin recruitment during chlamydial invasion

Chlamydiae are Gram-negative obligate intracellular pathogens to which access to an intracellular environment is fundamental to their development. Chlamydial attachment to host cells induces the activation of the Rac GTPase, which is required for the localization of WAVE2 at the sites of chlamydial entry. Co-immunoprecipitation experiments demonstrated that Chlamydia trachomatis infection promoted the interaction of Rac with WAVE2 and Abi-1, but not with IRSp53. siRNA depletion of WAVE2 and Abi-1 abrogated chlamydia-induced actin recruitment and significantly reduced the uptake of the pathogen by the depleted cells. Chlamydia invasion also requires the Arp2/3 complex as demonstrated by its localization to the sites of chlamydial attachment and the reduced efficiency of chlamydial invasion in cells overexpressing the VCA domain of the neural Wiskott-Aldrich syndrome protein. Thus, C. trachomatis activates Rac and promotes its interaction with WAVE2 and Abi-1 to activate the Arp2/3 complex resulting in the induction of actin cytoskeletal rearrangements that are required for invasion.

Shigella chromosomal IpaH proteins are secreted via the type III secretion system and act as effectors

Shigella possess 220 kb plasmid, and the major virulence determinants, called effectors, and the type III secretion system (TTSS) are exclusively encoded by the plasmid. The genome sequences of S. flexneri strains indicate that several ipaH family genes are located on both the plasmid and the chromosome, but whether their chromosomal IpaH cognates can be secreted from Shigella remains unknown. Here we report that S. flexneri strain, YSH6000 encodes seven ipaH cognate genes on the chromosome and that the IpaH proteins are secreted via the TTSS. The secretion kinetics of IpaH proteins by bacteria, however, showed delay compared with those of IpaB, IpaC and IpaD. Expression of the each mRNA of ipaH in Shigella was increased after bacterial entry into epithelial cells, and the IpaH proteins were secreted by intracellular bacteria. Although individual chromosomal ipaH deletion mutants showed no appreciable changes in the pathogenesis in a mouse pulmonary infection model, the DeltaipaH-null mutant, whose chromosome lacks all ipaH genes, was attenuated to mice lethality. Indeed, the histological examination for mouse lungs infected with the DeltaipaH-null showed a greater inflammatory response than induced by wild-type Shigella, suggesting that the chromosomal IpaH proteins act synergistically as effectors to modulate the host inflammatory responses.

Shigella IpgB1 promotes bacterial entry through the ELMO-Dock180 machinery

Shigella use a special mechanism to invade epithelial cells called 'the trigger mechanism of entry', which allows epithelial cells to trap several bacteria simultaneously. On contact, Shigella deliver effectors into epithelial cells through the type III secretion system. Here, we show that one of the effectors, IpgB1, has a pivotal role in producing membrane ruffles by exploiting the RhoG-ELMO-Dock180 pathway to stimulate Rac1 activity. Using pulldown assays, we identified engulfment and cell motility (ELMO) protein as the IpgB1 binding partner. IpgB1 colocalized with ELMO and Dock180 in membrane ruffles induced by Shigella. Shigella invasiveness and IpgB1-induced ruffles were less in ELMO- and Dock180-knockdown cells compared with wild-type cells. Membrane association of ELMO-Dock180 with ruffles were promoted when cells expressed an IpgB1-ELMO chimera, establishing that IpgB1 mimics the role of RhoG in producing membrane ruffles. Taken together, our findings show that IpgB1 mimicry is the key to invasion by Shigella.

OspF and OspC1 are Shigella flexneri type III secretion system effectors that are required for postinvasion aspects of virulence

Shigella flexneri is the causative agent of dysentery, and its pathogenesis is mediated by a type III secretion system (T3SS). S. flexneri secretes effector proteins into the eukaryotic cell via the T3SS, and these proteins usurp host cellular functions to the benefit of the bacteria. OspF and OspC1 are known to be secreted by S. flexneri, but their functions are unknown. We transformed S. flexneri with a plasmid that expresses a two-hemagglutinin tag (2HA) in frame with OspF or OspC1 and verified that these proteins are secreted in a T3SS-dependent manner. Immunofluorescence of HeLa cells infected with S. flexneri expressing OspF-2HA or OspC1-2HA revealed that both proteins localize in the nucleus and cytoplasm of host cells. To elucidate the function of these T3SS effectors, we constructed DeltaospF and DeltaospC1 deletion mutants by allelic exchange. We found that DeltaospF and DeltaospC1 mutants invade host cells and form plaques in confluent monolayers similar to wild-type S. flexneri. However, in the polymorphonuclear (PMN) cell migration assay, a decrease in neutrophil migration was observed for both mutants in comparison to the migration of wild-type bacteria. Moreover, infection of polarized T84 intestinal cells infected with DeltaospF and DeltaospC1 mutants resulted in decreased phosphorylation of extracellular signal-regulated kinase 1/2 in comparison to that of T84 cells infected with wild-type S. flexneri. To date, these are the first examples of T3SS effectors implicated in mitogen-activated protein kinase kinase/extracellular signal-regulated kinase pathway activation. Ultimately, OspF and OspC1 are essential for PMN transepithelial migration, a phenotype associated with increased inflammation and bacterial access to the submucosa, which are fundamental aspects of S. flexneri pathogenesis.

Microtubule-severing activity of Shigella is pivotal for intercellular spreading

Some pathogenic bacteria actually invade the cytoplasm of their target host cells. Invasive bacteria acquire the propulsive force to move by recruiting actin and inducing its polymerization. Here we show that Shigella movement within the cytoplasm was severely hindered by microtubules and that the bacteria destroyed surrounding microtubules by secreting VirA by means of the type III secretion system. Degradation of microtubules by VirA was dependent on its alpha-tubulin-specific cysteine protease-like activity. virA mutants did not move within the host cytoplasm and failed to move into adjacent cells.