The phosphothreonine lyase activity of a bacterial type III effector family

Biological and biochemical characterization of anthrax lethal factor, a proteolytic inhibitor of MEK signaling pathways

The secretion of factors that block critical intracellular signaling pathways is a common strategy used by pathogenic bacteria for disabling host defenses and causing disease. Anthrax lethal toxin (LeTx) has been shown to cleave and inactivate mitogen-activated protein kinase (MAPK) kinases (MKKs or MEKs) and to inhibit MKK signaling. Cleavage of MKKs by LeTx prevents activation of their downstream substrates, the MAPKs. Because MAPK pathways regulate a variety of crucial cellular functions including proliferation, survival, differentiation, adhesion, and motility, LeTx has become a focus of study as an investigative tool as well as for the treatment and prevention of diseases due to malfunctions in MAPK signaling. This chapter describes methods for expressing and purifying the components of LeTx and focuses on techniques available for assessing its activity.

OspF directly attenuates the activity of extracellular signal-regulated kinase during invasion by Shigella flexneri in human dendritic cells

Shigella spp., Gram-negative pathogenic bacteria, deliver various effector molecules into the host cell cytoplasm through their type III secretion system to facilitate their invasive process and control the host innate immune responses. Although the function of these effectors is well characterized in epithelial cells during Shigella infection, it has not been elucidated in the dendritic cell (DC), a major antigen presenting cell playing an important role in the initiation of immune responses. In this study, we showed that an invasive Shigella strain (M90T), but not its non-invasive counterpart strain (BS176) induced apoptotic cell death in the human monocyte-derived DCs. Confocal microscopy using a lysosome-associated membrane protein 2 specific antibody demonstrated that the M90T escaped from phagosomes 2h post-DC invasion while BS176 remained in the phagosome. Furthermore, Shigella expressed outer Shigella protein F (OspF), one of the effector proteins that are released through type III secretion system during the invasion, at non-secretion state and further up-regulated OspF expression in the cytoplasm of DC during the invasion. Interestingly, in the host cell, OspF could directly bind to the extracellular signal-regulated kinase (Erk) 1/2 and dephosphorylate phospho-Erk. These results suggest that induction of OspF is enhanced during Shigella invasion of DCs and decreases the phosphorylation level of Erk1/2, which could be at least partially involved in the apoptotic death of DC, eventually resulting in the down-regulation of the host immune response.

Virulent Shigella flexneri subverts the host innate immune response through manipulation of antimicrobial peptide gene expression

Antimicrobial factors are efficient defense components of the innate immunity, playing a crucial role in the intestinal homeostasis and protection against pathogens. In this study, we report that upon infection of polarized human intestinal cells in vitro, virulent Shigella flexneri suppress transcription of several genes encoding antimicrobial cationic peptides, particularly the human β-defensin hBD-3, which we show to be especially active against S. flexneri. This is an example of targeted survival strategy. We also identify the MxiE bacterial regulator, which controls a regulon encompassing a set of virulence plasmid-encoded effectors injected into host cells and regulating innate signaling, as being responsible for this dedicated regulatory process. In vivo, in a model of human intestinal xenotransplant, we confirm at the transcriptional and translational level, the presence of a dedicated MxiE-dependent system allowing S. flexneri to suppress expression of antimicrobial cationic peptides and promoting its deeper progression toward intestinal crypts. We demonstrate that this system is also able to down-regulate additional innate immunity genes, such as the chemokine CCL20 gene, leading to compromised recruitment of dendritic cells to the lamina propria of infected tissues. Thus, S. flexneri has developed a dedicated strategy to weaken the innate immunity to manage its survival and colonization ability in the intestine.

Plant pathogenic bacterial type III effectors subdue host responses

Like animals, plants sense bacterial pathogens through surface-localized pattern recognition receptors (PRRs) and intracellular nucleotide-binding leucine-rich repeat proteins (NB-LRR) and trigger defense responses. Many plant-pathogenic bacteria secrete a large repertoire of effector proteins into host cells to modulate host responses, enabling successful infection and multiplication in plants. A number of these effector proteins target plant innate immunity signaling pathways, while others induce specific host genes to enhance plant susceptibility. Substantial progress has been made in the past two years concerning biochemical function of effectors and their host targets. These advances provide new insights into regulatory mechanisms of plant immunity and host-pathogen co-evolution.

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.

Saccharomyces boulardii interferes with Shigella pathogenesis by postinvasion signaling events

Saccharomyces boulardii is gaining in popularity as a treatment for a variety of diarrheal diseases as well as inflammatory bowel disease. This study was designed to examine the effect of this yeast on infection by Shigella flexneri, a highly infectious and human host-adapted enteric pathogen. We investigated key interactions between the bacteria and host cells in the presence of the yeast in addition to a number of host responses including proinflammatory events and markers. Although the presence of the yeast during infection did not alter the number of bacteria that was able to attach or invade human colon cancer-derived T-84 cells, it did positively impact the tight junction protein zonula occluden-2 and significantly increase the barrier integrity of model epithelia. The yeast also decreased ERK, JNK, and NF-kappaB activation in response to S. flexneri, events likely responsible for the observed reductions in IL-8 secretion and the transepithelial migration of polymorphonuclear leukocytes across T-84 monolayers. These results, suggesting that the yeast allowed for a dampened inflammatory response, were confirmed in vivo utilizing a highly relevant model of human fetal colonic tissue transplanted into scid mice. Furthermore, a cell-free S. boulardii culture supernatant was also capable of reducing IL-8 secretion by infected T-84 cells. These data suggest that although the use of S. boulardii during infection with S. flexneri may alleviate symptoms associated with the inflammatory response of the host, it would not prevent infection.

SpvC is a Salmonella effector with phosphothreonine lyase activity on host mitogen-activated protein kinases

SpvC is encoded by the Salmonella virulence plasmid. We have investigated the biochemical function of SpvC and the mechanism by which it is secreted by bacteria and translocated into infected macrophages. We constructed a strain carrying a deletion in spvC and showed that the strain is attenuated for systemic virulence in mice. SpvC can be secreted in vitro by either the SPI-1 or SPI-2 type III secretion systems. Cell biological and genetic experiments showed that translocation of the protein into the cytosol of macrophages by intracellular bacteria is dependent on the SPI-2 T3SS. Using antibodies specific to phospho-amino acids and mass spectrometry we demonstrate that SpvC has phosphothreonine lyase activity on full-length phospho-Erk (pErk) and a synthetic 13-amino-acid phospho-peptide containing the TXY motif. A Salmonella strain expressing spvC from a plasmid downregulated cytokine release from infected cells.

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.

Pathogen subversion of cell-intrinsic innate immunity

The mammalian immune system has evolved under continuous selective pressure from a wide range of microorganisms that colonize and replicate in animal hosts. A complex set of signaling networks initiate both innate and adaptive immunity in response to the diverse pathogens that mammalian hosts encounter. In response, viral and microbial pathogens have developed or acquired sophisticated mechanisms to avoid, counteract and subvert sensors, signaling networks and a range of effector functions that constitute the host immune response. This balance of host response and pathogen countermeasures contributes to chronic infection in highly adapted pathogens that have coevolved with their host. In this review we outline some of the themes that are beginning to emerge in the mechanisms by which pathogens subvert the early innate immune response.

Intercepting host MAPK signaling cascades by bacterial type III effectors

The evolutionarily conserved MAP kinase (MAPK) cascades play essential roles in plant and animal innate immunity. A recent explosion of research has uncovered a myriad of virulence strategies used by pathogenic bacteria to intercept MAPK signaling through diverse type III effectors injected into host cells. Here, we review the latest literature and discuss the various mechanisms that pathogenic bacteria use to manipulate host MAPK signaling cascades.

Pathogen virulence factors as molecular probes of basic plant cellular functions

To successfully colonize plants, pathogens have evolved a myriad of virulence factors that allow them to manipulate host cellular pathways in order to gain entry into, multiply and move within, and eventually exit the host for a new infection cycle. In the past few years, substantial progress has been made in characterizing the host targets of viral and bacterial virulence factors, providing unique insights into basic plant cellular processes such as gene silencing, vesicle trafficking, hormone signaling, and innate immunity. Identification of the host targets of additional pathogen virulence factors promises to continue shedding light on fundamental cellular mechanisms in plants, thus enhancing our understanding of plant signaling, metabolism, and cell biology.

The NleE/OspZ family of effector proteins is required for polymorphonuclear transepithelial migration, a characteristic shared by enteropathogenic Escherichia coli and Shigella flexneri infections

Enteropathogenic Escherichia coli (EPEC) and Shigella flexneri are human host-specific pathogens that infect intestinal epithelial cells. However, each bacterial species employs a different infection strategy within this environmental niche. EPEC attaches to the apical surface of small intestine enterocytes, causing microvillus effacement and rearrangement of the host cell cytoskeleton beneath adherent bacteria. In contrast, S. flexneri invades the large intestine epithelium at the basolateral membrane, replicates, and spreads cell to cell. Both EPEC and S. flexneri rely on type three secretion systems (T3SS) to secrete effectors into host cells, and both pathogens recruit polymorphonuclear leukocytes (PMNs) from the submucosa to the lumen of the intestine. In this report, we compared the virulence functions of the EPEC T3SS effector NleE and the homologous Shigella protein Orf212. We discovered that Orf212 was secreted by the S. flexneri T3SS and renamed this protein OspZ. Infection of polarized T84 intestinal epithelial cells with an ospZ deletion mutant of S. flexneri resulted in reduced PMN transepithelial migration compared to infection by the wild type. An nleE deletion mutant of EPEC showed a similar reduction of PMN migration. The ability to induce PMN migration was restored in both mutants when either ospZ or nleE was expressed from a plasmid. An infection of T84 cells with the delta ospZ mutant resulted in reduced extracellular signal-related kinase phosphorylation and NF-kappaB activation compared to infection with the wild type. Therefore, we conclude that OspZ and NleE have similar roles in the upstream induction of host signaling pathways required for PMN transepithelial migration in Shigella and EPEC infections.

Defense suppression by virulence effectors of bacterial phytopathogens

Phytopathogenic bacteria and plants are locked in molecular struggles that determine the outcome of an infection. Bacteria make effector molecules that can induce defenses if recognized by specific host resistance (R) proteins. In susceptible hosts, however, effectors frequently promote virulence by suppressing host defenses. Defense-inducing and defense-suppressing activities are often related, as virulence-associated host modifications can elicit R protein activation. Thus, understanding of how an effector elicits defenses can translate into understanding of how it promotes virulence and vice versa. To control host cell functions, such as defense gene expression and vesicle trafficking, effectors use various biochemical activities, including protein modification, transcriptional regulation, and hormone mimicry. Progress with individual effectors will lead to an integrated view of how the activities of a collection of effectors intersect with genetically variable host plants to regulate susceptibility and resistance.

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.