Bacterial effector binding to ribosomal protein s3 subverts NF-kappaB function

Global proteomics reveals pathways of mesenchymal stem cells altered by Mycobacterium tuberculosis

Mycobacterium tuberculosis (M. tb) has a remarkable ability to persist inside host cells. Several studies showed that M. tb infects and survives inside bone marrow mesenchymal stem cells (BM-MSCs) escaping the host immune system. Here, we have identified various cellular pathways that are modulated in human BM-MSCs upon infection with virulent M. tb and the proteomic profile of these cells varies from that of avirulent M. tb infected cells. We found that virulent M. tb infection reshapes host pathways such as stem cell differentiation, alternative splicing, cytokine production, mitochondrial function etc., which might be modulated by M. tb to persist inside this unconventional niche of human BM-MSCs. Additionally, we observed that virulent M. tb infection suppresses various cellular processes. This study uncovers the differences in the host proteomic profiles resulting from the virulent versus avirulent M. tb infection that can pave the way to identify host-directed therapeutic targets for the treatment of tuberculosis.

The ever-increasing necessity of mass spectrometry in dissecting protein post-translational modifications catalyzed by bacterial effectors

Protein post-translational modifications (PTMs), such as ADP-ribosylation and phosphorylation, regulate multiple fundamental biological processes in cells. During bacterial infection, effector proteins are delivered into host cells through dedicated bacterial secretion systems and can modulate important cellular pathways by covalently modifying their host targets. These strategies enable intruding bacteria to subvert various host processes, thereby promoting their own survival and proliferation. Despite rapid expansion of our understanding of effector-mediated PTMs in host cells, analytical measurements of these molecular events still pose significant challenges in the study of host-pathogen interactions. Nevertheless, with major technical breakthroughs in the last two decades, mass spectrometry (MS) has evolved to be a valuable tool for detecting protein PTMs and mapping modification sites. Additionally, large-scale PTM profiling, facilitated by different enrichment strategies prior to MS analysis, allows high-throughput screening of host enzymatic substrates of bacterial effectors. In this review, we summarize the advances in the studies of two representative PTMs (i.e., ADP-ribosylation and phosphorylation) catalyzed by bacterial effectors during infection. Importantly, we will discuss the ever-increasing role of MS in understanding these molecular events and how the latest MS-based tools can aid in future studies of this booming area of pathogenic bacteria-host interactions.

RpS3 Is Required for Spermatogenesis of Drosophila melanogaster

Ribosomal proteins (RPs) constitute the ribosome, thus participating in the protein biosynthesis process. Emerging studies have suggested that many RPs exhibit different expression levels across various tissues and function in a context-dependent manner for animal development. Drosophila melanogaster RpS3 encodes the ribosomal protein S3, one component of the 40S subunit of ribosomes. We found that RpS3 is highly expressed in the reproductive organs of adult flies and its depletion in male germline cells led to severe defects in sperm production and male fertility. Immunofluorescence staining showed that RpS3 knockdown had little effect on early germ cell differentiation, but strongly disrupted the spermatid elongation and individualization processes. Furthermore, we observed abnormal morphology and activity of mitochondrial derivatives in the elongating spermatids of RpS3-knockdown testes, which could cause the failure of axoneme elongation. We also found that RpS3 RNAi inhibited the formation of the individualization complex that takes charge of disassociating the spermatid bundle. In addition, excessive apoptotic cells were detected in the RpS3-knockdown testes, possibly to clean the defective spermatids. Together, our data demonstrated that RpS3 plays an important role in regulating spermatid elongation and individualization processes and, therefore, is required for normal Drosophila spermatogenesis.

Bacterial effector kinases and strategies to identify their target host substrates

Post-translational modifications (PTMs) are critical in regulating protein function by altering chemical characteristics of proteins. Phosphorylation is an integral PTM, catalyzed by kinases and reversibly removed by phosphatases, that modulates many cellular processes in response to stimuli in all living organisms. Consequently, bacterial pathogens have evolved to secrete effectors capable of manipulating host phosphorylation pathways as a common infection strategy. Given the importance of protein phosphorylation in infection, recent advances in sequence and structural homology search have significantly expanded the discovery of a multitude of bacterial effectors with kinase activity in pathogenic bacteria. Although challenges exist due to complexity of phosphorylation networks in host cells and transient interactions between kinases and substrates, approaches are continuously being developed and applied to identify bacterial effector kinases and their host substrates. In this review, we illustrate the importance of exploiting phosphorylation in host cells by bacterial pathogens via the action of effector kinases and how these effector kinases contribute to virulence through the manipulation of diverse host signaling pathways. We also highlight recent developments in the identification of bacterial effector kinases and a variety of techniques to characterize kinase-substrate interactions in host cells. Identification of host substrates provides new insights for regulation of host signaling during microbial infection and may serve as foundation for developing interventions to treat infection by blocking the activity of secreted effector kinases.

Shewanella infection in humans: Epidemiology, clinical features and pathogenicity

The genus Shewanella consists of Gram-negative proteobacteria that are ubiquitously distributed in environment. As the members of this genus have rapidly increased within the past decade, several species have become emerging pathogens worldwide, attracting the attention of the medical community. These species are also associated with severe community- and hospital-acquired infections. Patients infected with Shewanella spp. had experiences of occupational or recreational exposure; meanwhile, the process of infection is complex and the pathogenicity is influenced by a variety of factors. Here, an exhaustive internet-based literature search was carried out in PUBMED using terms “Achromobacter putrefaciens,” “Pseudomonas putrefaciens,” “Alteromonas putrefaciens” and “Shewanella” to search literatures published between 1978 and June 2022. We provided a comprehensive review on the epidemiology, clinical features and pathogenicity of Shewanella, which will contribute a better understanding of its clinical aetiology, and facilitate the timely diagnosis and effective treatment of Shewanella infection for clinicians and public health professionals.

Targeting of microvillus protein Eps8 by the NleH effector kinases from enteropathogenic E. coli

Attaching and effacing (AE) lesion formation on enterocytes by enteropathogenic Escherichia coli (EPEC) requires the EPEC type III secretion system (T3SS). Two T3SS effectors injected into the host cell during infection are the atypical kinases, NleH1 and NleH2. However, the host targets of NleH1 and NleH2 kinase activity during infection have not been reported. Here phosphoproteomics identified Ser775 in the microvillus protein Eps8 as a bona fide target of NleH1 and NleH2 phosphorylation. Both kinases interacted with Eps8 through previously unrecognized, noncanonical "proline-rich" motifs, PxxDY, that bound the Src Homology 3 (SH3) domain of Eps8. Structural analysis of the Eps8 SH3 domain bound to a peptide containing one of the proline-rich motifs from NleH showed that the N-terminal part of the peptide adopts a type II polyproline helix, and its C-terminal "DY" segment makes multiple contacts with the SH3 domain. Ser775 phosphorylation by NleH1 or NleH2 hindered Eps8 bundling activity and drove dispersal of Eps8 from the AE lesion during EPEC infection. This finding suggested that NleH1 and NleH2 altered the cellular localization of Eps8 and the cytoskeletal composition of AE lesions during EPEC infection.

SIAH1-mediated RPS3 ubiquitination contributes to chemosensitivity in epithelial ovarian cancer

The E3 ligase SIAH1 is deregulated in human cancers and correlated with poor prognosis, but its contributions to chemoresistance in epithelial ovarian cancer (EOC) are not evident. Herein we found that SIAH1 was decreased in EOC tumour tissues and cell lines and negatively correlated with the RPS3 levels. SIAH1 overexpression suppressed tumour cell growth, colony formation, invasion, metastasis, and cisplatin resistance in vivo and in vitro. SIAH1 promoted RPS3 ubiquitination and degradation using the RING-finger domain, and these steps were required for RPS3 localization to the cytoplasm, which led to subsequent NF-κB inactivation and thereby conferred chemosensitivity. Moreover, ectopic expression of RPS3 or depletion of RPS3 ubiquitination mediated by SIAH1 via the K214R mutant significantly impaired cisplatin-induced tumour suppression in cells stably expressing SIAH1. Together, our findings reveal a tumour suppressor function of SIAH1 and provide evidence showing that the SIAH1-RPS3-NF-κB axis may act as an appealing strategy for tackling treatment resistance in EOC.

Escherichia coli 0157:H7 virulence factors and the ruminant reservoir

PURPOSE OF REVIEW

This review updates recent findings about Escherichia coli O157:H7 virulence factors and its bovine reservoir. This Shiga toxin (Stx)-producing E. coli belongs to the Enterohemorrhagic E. coli (EHEC) pathotype causing hemorrhagic colitis. Its low infectious dose makes it an efficient, severe, foodborne pathogen. Although EHEC remains in the intestine, Stx can translocate systemically and is cytotoxic to microvascular endothelial cells, especially in the kidney and brain. Disease can progress to life-threatening hemolytic uremic syndrome (HUS) with hemolytic anemia, acute kidney failure, and thrombocytopenia. Young children, the immunocompromised, and the elderly are at the highest risk for HUS. Healthy ruminants are the major reservoir of EHEC and cattle are the primary source of human exposure.

RECENT FINDINGS

Advances in understanding E. coli O157:H7 pathogenesis include molecular mechanisms of virulence, bacterial adherence, type three secretion effectors, intestinal microbiome, inflammation, and reservoir maintenance.

SUMMARY

Many aspects of E. coli O157:H7 disease remain unclear and include the role of the human and bovine intestinal microbiomes in infection. Therapeutic strategies involve controlling inflammatory responses and/or intestinal barrier function. Finally, elimination/reduction of E. coli O157:H7 in cattle using CRISPR-engineered conjugative bacterial plasmids and/or on-farm management likely hold solutions to reduce infections and increase food safety/security.

Effects of fecal stream deprivation on human intestinal barrier after loop ileostomy

BACKGROUND AND AIM

Intestinal homeostasis is closely associated with the normal intestinal luminal physiological environment. Temporary loop ileostomy changes the intestinal structure and diverts the fecal stream, thereby disturbing the intestinal environment. This study aimed to clarify the changing situation of the human intestinal mucosa barrier in the absence of a fecal stream after loop ileostomy.

METHODS

We obtained paired samples from the fed (fecal stream maintained) and unfed (no fecal stream) portions of the loop ileostomy and subjected these samples to RNA sequencing. We also determined transepithelial electrical resistance. The mucus layer thickness and content of MUC2, tight junction proteins, and common antimicrobial peptides in ileum mucosa were studied.

RESULTS

Transcriptome data revealed that genes associated with enhancing the intestinal barrier function of the unfed ileum were significantly decreased and genes associated with immune defense response were significantly increased. The transepithelial electrical resistance was lower and the mucus layer thickness was thinner in the unfed ileal mucosa than in the fed ileum. The MUC2, Occludin, and zonula occludens 1 content was lower in the unfed ileum than in the fed ileum. α-Defensin 5, α-defensin 6, and lysozyme content was higher in the unfed ileum than in the enterally fed ileum.

CONCLUSION

Intestinal barrier function is weakened after long-term fecal diversion, but antimicrobiota defense function is strengthened. Thus, the intestinal mucosa barrier adopts an alternative stable state during fecal diversion, which may explain the clinical paucity of cases of enterogenic infection caused by loop ileostomy.

PRRSV Induces HMGB1 Phosphorylation at Threonine-51 Residue to Enhance Its Secretion

Porcine reproductive and respiratory syndrome virus (PRRSV) induces secretion of high mobility group box 1 (HMGB1) to mediate inflammatory response that is involved in the pulmonary injury of infected pigs. Our previous study indicates that protein kinase C-delta (PKC-delta) is essential for HMGB1 secretion in PRRSV-infected cells. However, the underlying mechanism in HMGB1 secretion induced by PRRSV infection is still unclear. Here, we discovered that the phosphorylation level of HMGB1 in threonine residues increased in PRRSV-infected cells. A site-directed mutagenesis study showed that HMGB1 phosphorylation at threonine-51 was associated with HMGB1 secretion induced by PRRSV infection. Co-immunoprecipitation (co-IP) of HMGB1 failed to precipitate PKC-delta, but interestingly, mass spectrometry analysis of the HMGB1 co-IP product showed that PRRSV infection enhanced HMGB1 binding to ribosomal protein S3 (RPS3), which has various extra-ribosomal functions. The silencing of RPS3 by siRNA blocked HMGB1 secretion induced by PRRSV infection. Moreover, the phosphorylation of HMGB1 at threonine-51 was correlated with the interaction between HMGB1 and RPS3. In vivo, PRRSV infection also increased RPS3 levels and nuclear accumulation in pulmonary alveolar macrophages. These results demonstrate that PRRSV may induce HMGB1 phosphorylation at threonine-51 and increase its interaction with RPS3 to enhance HMGB1 secretion. This finding provides insights into the pathogenesis of PRRSV infection.

PurA facilitates Edwardsiella piscicida to escape NF-κB signaling activation

The host NF-κB signaling pathway plays critical role in defensing against bacterial infection. However, bacteria also evolve strategies to escape from host clearance. Edwardsiella piscicida is a threatening pathogen in aquaculture, while the molecular mechanism of E. piscicida in inhibiting NF-κB signaling remains largely unknown. Herein, using E. piscicida transposon insertion mutant library combined with a NF-κB luciferase reporter system, we identified forty-six genes of E. piscicida, which were involved in inhibiting the NF-κB signaling activation in vitro. Moreover, we further explored the top 10 significantly changed mutants through zebrafish larvae infection model and validated that six genes were involved in inhibiting NF-κB activation in vivo. Specifically, we identified the adenylosuccinate synthase mutated strain (ΔpurA) infection exhibited a robust activation of NF-κB signaling, along with higher expression of cxcl8a and cxcl8b to mediate the recruitment of neutrophils in vivo. Taken together, these results identified the key factors of E. piscicida in inhibiting NF-κB activation, which will contribute to better understanding the pathogenesis of this important pathogen.

Virulence-related O islands in enterohemorrhagic Escherichia coli O157:H7

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a principally foodborne pathogen linked to serious diseases, including bloody diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome. Comparative genomics analysis revealed that EHEC O157 contains 177 unique genomic islands, termed O islands, compared with the nonpathogenic E. coli K-12 laboratory strain. These O islands contribute largely to the pathogenicity of EHEC O157:H7 by providing numerous virulence factors, effectors, virulence regulatory proteins, and virulence regulatory sRNAs. The present review aimed to provide a comprehensive understanding of the research progress on the function of O islands, especially focusing on virulence-related O islands.

Pathophysiology of enteropathogenic Escherichia coli during a host infection

Enteropathogenic Escherichia coli (EPEC) is a major cause of infantile diarrhea in developing countries. However, sporadic outbreaks caused by this microorganism in developed countries are frequently reported recently. As an important zoonotic pathogen, EPEC is being monitored annually in several countries. Hallmark of EPEC infection is formation of attaching and effacing (A/E) lesions on the small intestine. To establish A/E lesions during a gastrointestinal tract (GIT) infeciton, EPEC must thrive in diverse GIT environments. A variety of stress responses by EPEC have been reported. These responses play significant roles in helping E. coli pass through GIT environments and establishing E. coli infection. Stringent response is one of those responses. It is mediated by guanosine tetraphosphate. Interestingly, previous studies have demonstrated that stringent response is a universal virulence regulatory mechanism present in many bacterial pathogens including EPEC. However, biological signficance of a bacterial stringent response in both EPEC and its interaction with the host during a GIT infection is unclear. It needs to be elucidated to broaden our insight to EPEC pathogenesis. In this review, diverse responses, including stringent response, of EPEC during a GIT infection are discussed to provide a new insight into EPEC pathophysiology in the GIT.

Binding specificity of type three secretion system effector NleH2 to multi-cargo chaperone CesT and their phosphorylation

Gram-negative pathogens like Enteropathogenic Escherichia coli (EPEC) utilize the type three secretion system (T3SS) to translocate various effector proteins that are needed to "hijack" the host system for pathogenic survival. Specialized T3SS chaperones inside bacterial cells stabilize these effector proteins and facilitate their translocation. CesT is a unique multi-cargo chaperone that interacts with and translocates ~10 different effector proteins. Here, we report the specific interaction between CesT and its key effector, NleH2, and explore the potential role of NleH2 as a kinase for CesT phosphorylation. First, we identified the chaperone-binding domain (CBD; 19-97aa) of NleH2, and mapped the specific interaction sites for both CesT and NleH2. The N- and C-terminal residues of the CBD interact with the dimeric interface of CesT. Further, we compared the CesT binding to NleH2, to that of another key effector Tir and with the global carbon regulator CsrA. Notably, the effectors have the binding regions at the β-sheet core and dimer interface of CesT, whereas the CsrA regulator interacts predominantly through the C-terminal region, which is found ~17 Å away from the effectors-binding sites. Next, we showed that NleH2 remains an active kinase even as a complex with CesT and is responsible for its autophosphorylation as well as phosphorylation of CesT at Tyr153. Collectively, our findings enhance the understanding of the role of multi-cargo chaperone CesT in orchestrating effector translocation through T3SS.

Identification of a Family of Vibrio Type III Secretion System Effectors That Contain a Conserved Serine/Threonine Kinase Domain

Vibrio parahaemolyticus is a marine Gram-negative bacterium that is a leading cause of seafood-borne gastroenteritis. Pandemic strains of V. parahaemolyticus rely on a specialized protein secretion machinery known as the type III secretion system 2 (T3SS2) to cause disease. The T3SS2 mediates the delivery of effector proteins into the cytosol of infected cells, where they subvert multiple cellular pathways. Here, we identify a new T3SS2 effector protein encoded by VPA1328 (VP_RS21530) in V. parahaemolyticus RIMD2210633. Bioinformatic analysis revealed that VPA1328 is part of a larger family of uncharacterized T3SS effector proteins with homology to the VopG effector protein in Vibrio cholerae AM-19226. These VopG-like proteins are found in many but not all T3SS2 gene clusters and are distributed among diverse Vibrio species, including V. parahaemolyticus, V. cholerae, V. mimicus, and V. diabolicus and also in Shewanella baltica. Structure-based prediction analyses uncovered the presence of a conserved C-terminal kinase domain in VopG orthologs, similar to the serine/threonine kinase domain found in the NleH family of T3SS effector proteins. However, in contrast to NleH effector proteins, in tissue culture-based infections, VopG did not impede host cell death or suppress interleukin 8 (IL-8) secretion, suggesting a yet undefined role for VopG during V. parahaemolyticus infection. Collectively, our work reveals that VopG effector proteins, a new family of likely serine/threonine kinases, is widely distributed in the T3SS2 effector armamentarium among marine bacteria. IMPORTANCE Vibrio parahaemolyticus is the leading bacterial cause of seafood-borne gastroenteritis worldwide. The pathogen relies on a type III secretion system to deliver a variety of effector proteins into the cytosol of infected cells to subvert cellular function. In this study, we identified a novel Vibrio parahaemolyticus effector protein that is similar to the VopG effector of Vibrio cholerae. VopG-like effectors were found in diverse Vibrio species and contain a conserved serine/threonine kinase domain that bears similarity to the kinase domain in the enterohemorrhagic Escherichia coli (EHEC) and Shigella NleH effectors that manipulate host cell survival pathways and host immune responses. Together our findings identify a new family of Vibrio effector proteins and highlight the role of horizontal gene transfer events among marine bacteria in shaping T3SS gene clusters.

Unraveling enterohemorrhagic Escherichia coli infection: the promising role of dietary compounds and probiotics in bacterial elimination and host innate immunity boosting

The innate immune system has developed sophisticated strategies to defense against infections. Host cells utilize the recognition machineries such as toll-like receptors and nucleotide binding and oligomerization domain-like receptors to identify the pathogens and alert immune system. However, some pathogens have developed tactics to evade host defenses, including manipulation of host inflammatory response, interference with cell death pathway, and highjack of phagocytosis signaling for a better survival and colonization in host. Enterohemorrhagic Escherichia coli (EHEC) is a notorious foodborne pathogen that causes severe tissue damages and gastrointestinal diseases, which has been reported to disturb host immune responses. Diverse bioactive compounds such as flavonoids, phenolic acids, alkaloids, saccharides, and terpenoids derived from food varieties and probiotics have been discovered and investigated for their capability of combating bacterial infections. Some of them serve as novel antimicrobial agents and act as immune boosters that harness host immune system. In this review, we will discuss how EHEC, specifically E. coli O157:H7, hijacks the host immune system and interferes with host signaling pathway; and highlight the promising role of food-derived bioactive compounds and probiotics in harnessing host innate immunity and eliminating E. coli O157:H7 infection with multiple strategies.

The Galleria mellonella-Enteropathogenic Escherichia coli Model System: Characterization of Pathogen Virulence and Insect Immune Responses

The use of Galleria mellonella (Linnaeus) (Lepidoptera: Pyralidae), an economical insect model, for the study of enteropathogenic Escherichia coli (Migula) (EPEC), a diarrheagenic human pathogen, has been demonstrated previously but remains poorly understood. The present study characterizes the Galleria-EPEC system extensively for future studies using this system. We found that EPEC causes disease in G. mellonella larvae when injected intrahemocoelically but not orally. Disease manifests as increased mortality, decreased survival time, delayed pupation, decreased pupal mass, increased pupal duration, and hemocytopenia. Disease symptoms are dose-dependent and can be used as metrics for measuring EPEC virulence in future studies. The type III secretion system was only partially responsible for EPEC virulence in G. mellonella while the majority of the virulence remains unknown in origin. EPEC elicits insect anti-bacterial immune responses including melanization, hemolymph coagulation, nodulation, and phagocytosis. The immune responses were unable to control EPEC replication in the early stage of infection (≤3 h post-injection). EPEC clearance from the hemocoel does not guarantee insect survival. Overall, this study provided insights into EPEC virulence and pathogenesis in G. mellonella and identified areas of future research using this system.

The T3SS Effector Protease NleC Is Active within Citrobacter rodentium

Whether type III secretion system (T3SS) effector proteins encoded by Gram-negative bacterial pathogens have intra-bacterial activities is an important and emerging area of investigation. Gram-negative bacteria interact with their mammalian hosts by using secretion systems to inject virulence proteins directly into infected host cells. Many of these injected protein effectors are enzymes that modify the structure and inhibit the function of mammalian proteins. The underlying dogma is that T3SS effectors are inactive until they are injected into host cells, where they then fold into their active conformations. We previously observed that the T3SS effectors NleB and SseK1 glycosylate Citrobacter rodentium and Salmonella enterica proteins, respectively, leading to enhanced resistance to environmental stress. Here, we sought to extend these studies to determine whether the T3SS effector protease NleC is also active within C. rodentium. To do this, we expressed the best-characterized mammalian substrate of NleC, the NF-κB p65 subunit in C. rodentium and monitored its proteolytic cleavage as a function of NleC activity. Intra-bacterial p65 cleavage was strictly dependent upon NleC. A p65 mutant lacking the known CE cleavage motif was resistant to NleC. Thus, we conclude that, in addition to NleB, NleC is also enzymatically active within C. rodentium.

Ribosomal protein S3 associates with the TFIIH complex and positively regulates nucleotide excision repair

In mammalian cells, the bulky DNA adducts caused by ultraviolet radiation are mainly repaired via the nucleotide excision repair (NER) pathway; some defects in this pathway lead to a genetic disorder known as xeroderma pigmentosum (XP). Ribosomal protein S3 (rpS3), a constituent of the 40S ribosomal subunit, is a multi-functional protein with various extra-ribosomal functions, including a role in the cellular stress response and DNA repair-related activities. We report that rpS3 associates with transcription factor IIH (TFIIH) via an interaction with the xeroderma pigmentosum complementation group D (XPD) protein and complements its function in the NER pathway. For optimal repair of UV-induced duplex DNA lesions, the strong helicase activity of the TFIIH complex is required for unwinding damaged DNA around the lesion. Here, we show that XP-D cells overexpressing rpS3 showed markedly increased resistance to UV radiation through XPD and rpS3 interaction. Additionally, the knockdown of rpS3 caused reduced NER efficiency in HeLa cells and the overexpression of rpS3 partially restored helicase activity of the TFIIH complex of XP-D cells in vitro. We also present data suggesting that rpS3 is involved in post-excision processing in NER, assisting TFIIH in expediting the repair process by increasing its turnover rate when DNA is damaged. We propose that rpS3 is an accessory protein of the NER pathway and its recruitment to the repair machinery augments repair efficiency upon UV damage by enhancing XPD helicase function and increasing its turnover rate.

Type III secretion system effectors form robust and flexible intracellular virulence networks

Infections with many Gram-negative pathogens, including Escherichia coli, Salmonella, Shigella, and Yersinia, rely on type III secretion system (T3SS) effectors. We hypothesized that while hijacking processes within mammalian cells, the effectors operate as a robust network that can tolerate substantial contractions. This was tested in vivo using the mouse pathogen Citrobacter rodentium (encoding 31 effectors). Sequential gene deletions showed that effector essentiality for infection was context dependent and that the network could tolerate 60% contraction while maintaining pathogenicity. Despite inducing very different colonic cytokine profiles (e.g., interleukin-22, interleukin-17, interferon-γ, or granulocyte-macrophage colony-stimulating factor), different networks induced protective immunity. Using data from >100 distinct mutant combinations, we built and trained a machine learning model able to predict colonization outcomes, which were confirmed experimentally. Furthermore, reproducing the human-restricted enteropathogenic E. coli effector repertoire in C. rodentium was not sufficient for efficient colonization, which implicates effector networks in host adaptation. These results unveil the extreme robustness of both T3SS effector networks and host responses.

Ribosomal protein uS3 in cell biology and human disease: Latest insights and prospects

The conserved ribosomal protein uS3 in eukaryotes has long been known as one of the essential components of the small (40S) ribosomal subunit, which is involved in the structure of the 40S mRNA entry pore, ensuring the functioning of the 40S subunit during translation initiation. Besides, uS3, being outside the ribosome, is engaged in various cellular processes related to DNA repair, NF-kB signaling pathway and regulation of apoptosis. This review is devoted to recent data opening new horizons in understanding the roles of uS3 in such processes as the assembly and maturation of 40S subunits, ensuring proper structure of 48S pre-initiation complexes, regulation of initiation and ribosome-based RNA quality control pathways. Besides, we summarize novel results on the participation of the protein in processes beyond translation and consider biomedical implications of previously known and recently found extra-ribosomal functions of uS3, primarily, in oncogenesis.

Attaching and effacing pathogens: the effector ABC of immune subversion

The innate immune response resembles an essential barrier to bacterial infection. Many bacterial pathogens have, therefore, evolved mechanisms to evade from or subvert the host immune response in order to colonize, survive and multiply. The attaching and effacing pathogens enteropathogenic Escherichia coli, enterohaemorrhagic E. coli, Escherichia albertii and Citrobacter rodentium are Gram-negative extracellular gastrointestinal pathogens. They use a type III secretion system to inject effector proteins into the host cell to manipulate a variety of cellular processes. Over the last decade, considerable progress was made in identifying and characterizing the effector proteins of attaching and effacing pathogens that are involved in the inhibition of innate immune signaling pathways, in determining their host cell targets and elucidating the mechanisms they employ. Their functions will be reviewed here.

Ribosomal protein S3 is a novel negative regulator of non-homologous end joining repair of DNA double-strand breaks

DNA double-strand breaks (DSBs) are one of the most serious types of DNA damage. However, multiple repair pathways are present in cells to ensure rapid and appropriate repair of DSBs. Pathway selection depends on several factors including cell type, cell cycle phase, and damage severity. Ribosomal protein S3 (rpS3), a component of the 40S small ribosomal subunit, is a multi-functional protein primarily involved in protein synthesis. rpS3 is also involved in the mediation of various extra-ribosomal pathways, including DNA damage processing and the stress response. Here, we report that rpS3 is a novel negative regulator of non-homologous end joining (NHEJ)-mediated repair of DSBs. We found that rpS3 interacts with the Ku heterodimers of the DNA-dependent protein kinase (DNA-PK) complex and slows down NHEJ ligation reactions, ultimately triggering p53-dependent cell death following treatment with high-dose ionizing radiation. After DSB formation, DNA-PK phosphorylates rpS3, which consequently reduces the binding of rpS3 to the Ku complex. We hypothesized that rpS3 may play a role in DSB repair by repressing NHEJ, while inducing other repair pathways, and by initiating DSB-induced cell death in response to severe DNA damage.

Programmed Cell Death in the Evolutionary Race against Bacterial Virulence Factors

Innate immune sensors can recognize when host cells are irrevocably compromised by pathogens, and in response can trigger programmed cell death (pyroptosis, apoptosis, and necroptosis). Innate sensors can directly bind microbial ligands; for example, NAIP/NLRC4 detects flagellin/rod/needle, whereas caspase-11 detects lipopolysaccharide. Other sensors are guards that monitor normal function of cellular proteins; for instance, pyrin monitors Rho GTPases, whereas caspase-8 and receptor-interacting protein kinase (RIPK)3 guards RIPK1 transcriptional signaling. Some proteins that need to be guarded can be duplicated as decoy domains, as seen in the integrated decoy domains within NLRP1 that watch for microbial attack. Here, we discuss the evolutionary battle between pathogens and host innate immune sensors/guards, illustrated by the Red Queen hypothesis. We discuss in depth four pathogens, and how they either fail in this evolutionary race (Chromobacterium violaceum, Burkholderia thailandensis), or how the evolutionary race generates increasingly complex virulence factors and host innate immune signaling pathways (Yersinia species, and enteropathogenic Escherichia coli [EPEC]).

NF-κB-dependent Luciferase Activation and Quantification of Gene Expression in Salmonella Infected Tissue Culture Cells

The dimeric transcription factor NF-κB regulates many cellular response pathways, including inflammatory pathways by inducing the expression of various cytokines and chemokines. NF-κB is constitutively expressed and is sequestered in the cytosol by the inhibitory protein nuclear factor of kappa light polypeptide gene enhancer in B cells inhibitor, alpha (IκBα). Activation of NF-κB requires the degradation of IκBα, which then exposes a nuclear localization signal on NF-κB and promotes its trafficking to the nucleus. Once in the nucleus, NF-κB binds to the promotor region of NF-κB target genes such as interleukin 6 (IL-6) and IL-23, to promote their expression. The activation of NF-κB occurs independently of transcription or translation. Therefore, the activation state of NF-κB must be measured either by quantifying NF-κB specifically in the nucleus, or by quantifying expression of NF-κB target genes. In this protocol, cells stably transfected with an NF-κB::luciferase reporter construct are assayed for NF-κB activation using in vitro tissue culture techniques. These cells are infected with Salmonella Typhimurium to activate NF-κB, which traffics to the nucleus and binds to κB sites in the promoter region of luciferase, inducing its expression. Cells are lysed and analyzed with the luciferase assay system. The amount of luciferase produced by the cells correlates with the intensity of the luminescence signal, which is detected by a plate reader. The luminescence signal generated by this procedure provides a quick and highly sensitive method by which to assess NF-κB activation under a range of conditions. This protocol also utilizes quantitative reverse transcription PCR (RT-qPCR) to detect relative mRNA levels that are indicative of gene expression.

A Bacterial Effector Mimics a Host HSP90 Client to Undermine Immunity

The molecular chaperone HSP90 facilitates the folding of several client proteins, including innate immune receptors and protein kinases. HSP90 is an essential component of plant and animal immunity, yet pathogenic strategies that directly target the chaperone have not been described. Here, we identify the HopBF1 family of bacterial effectors as eukaryotic-specific HSP90 protein kinases. HopBF1 adopts a minimal protein kinase fold that is recognized by HSP90 as a host client. As a result, HopBF1 phosphorylates HSP90 to completely inhibit the chaperone's ATPase activity. We demonstrate that phosphorylation of HSP90 prevents activation of immune receptors that trigger the hypersensitive response in plants. Consequently, HopBF1-dependent phosphorylation of HSP90 is sufficient to induce severe disease symptoms in plants infected with the bacterial pathogen, Pseudomonas syringae. Collectively, our results uncover a family of bacterial effector kinases with toxin-like properties and reveal a previously unrecognized betrayal mechanism by which bacterial pathogens modulate host immunity.

Pathogenic and non-pathogenic Escherichia coli colonization and host inflammatory response in a defined microbiota mouse model

Most Escherichia coli strains in the human intestine are harmless. However, enterohemorrhagic Ecoli (EHEC) is a foodborne pathogen that causes intestinal disease in humans. Conventionally reared (CONV) mice are inconsistent models for human infections with EHEC because they are often resistant to Ecoli colonization, in part due to their gastrointestinal (GI) microbiota. Although antibiotic manipulation of the mouse microbiota has been a common means to overcome colonization resistance, these models have limitations. Currently, there are no licensed treatments for clinical EHEC infections and, thus, new tools to study EHEC colonization need to be developed. Here, we used a defined microbiota mouse model, consisting of the altered Schaedler flora (ASF), to characterize intestinal colonization and compare host responses following colonization with EHEC strain 278F2 or non-pathogenic Ecoli strain MG1655. Significantly higher (P<0.05) levels of both strains were found in feces and cecal and colonic contents of C3H/HeN ASF compared to C3H/HeN CONV mice. GI inflammation was significantly elevated (P<0.05) in the cecum of EHEC 278F2-colonized compared to E. coli MG1655-colonized C3H/HeN ASF mice. In addition, EHEC 278F2 differentially modulated inflammatory-associated genes in colonic tissue of C3H/HeN ASF mice compared to E. coli MG1655-colonized mice. This approach allowed for prolonged colonization of the murine GI tract by pathogenic and non-pathogenic Ecoli strains, and for evaluation of host inflammatory processes. Overall, this system can be used as a powerful tool for future studies to assess therapeutics, microbe-microbe interactions, and strategies for preventing EHEC infections.

Hsp90 Interacts with the Bacterial Effector NleH1

Enterohemorrhagic Escherichia coli (EHEC) utilizes a type III secretion system (T3SS) to inject effector proteins into host cells. The EHEC NleH1 effector inhibits the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway by reducing the nuclear translocation of the ribosomal protein S3 (RPS3). NleH1 prevents RPS3 phosphorylation by the IκB kinase-β (IKKβ). IKKβ is a central kinase in the NF-κB pathway, yet NleH1 only restricts the phosphorylation of a subset of the IKKβ substrates. We hypothesized that a protein cofactor might dictate this inhibitory specificity. We determined that heat shock protein 90 (Hsp90) interacts with both IKKβ and NleH1 and that inhibiting Hsp90 activity reduces RPS3 nuclear translocation.

SseL Deubiquitinates RPS3 to Inhibit Its Nuclear Translocation

Many Gram-negative bacterial pathogens use type III secretion systems to deliver virulence proteins (effectors) into host cells to counteract innate immunity. The ribosomal protein S3 (RPS3) guides NF-κB subunits to specific κB sites and plays an important role in the innate response to bacterial infection. Two E. coli effectors inhibit RPS3 nuclear translocation. NleH1 inhibits RPS3 phosphorylation by IKK-β, an essential aspect of the RPS3 nuclear translocation process. NleC proteolysis of p65 generates an N-terminal p65 fragment that competes for full-length p65 binding to RPS3, thus also inhibiting RPS3 nuclear translocation. Thus, E. coli has multiple mechanisms by which to block RPS3-mediated transcriptional activation. With this in mind, we considered whether other enteric pathogens also encode T3SS effectors that impact this important host regulatory pathway. Here we report that the Salmonella Secreted Effector L (SseL), which was previously shown to function as a deubiquitinase and inhibit NF-κB signaling, also inhibits RPS3 nuclear translocation by deubiquitinating this important host transcriptional co-factor. RPS3 deubiquitination by SseL was restricted to K63-linkages and mutating the active-site cysteine of SseL abolished its ability to deubiquitinate and subsequently inhibit RPS3 nuclear translocation. Thus, Salmonella also encodes at least one T3SS effector that alters RPS3 activities in the host nucleus.

EPEC NleH1 is significantly more effective in reversing colitis and reducing mortality than NleH2 via differential effects on host signaling pathways

Enteropathogenic Escherichia coli (EPEC) is a foodborne pathogen that uses a type III secretion system to translocate effector molecules into host intestinal epithelial cells (IECs) subverting several host cell processes and signaling cascades. Interestingly, EPEC infection induces only modest intestinal inflammation in the host. The homologous EPEC effector proteins, NleH1 and NleH2, suppress the nuclear factor-κB (NF-κB) pathway and apoptosis in vitro. Increased apoptosis and activation of NF-κB and MAP kinases (MAPK) contribute to the pathogenesis of inflammatory bowel diseases (IBD). The aim of this study was to determine if NleH1 and NleH2 also block MAPK pathways in vitro and in vivo, and to compare the effects of these bacterial proteins on a murine model of colitis. Cultured IECs were infected with various strains of EPEC expressing NleH1 and NleH2, or not, and the activation of ERK1/2 and p38 was determined. In addition, the impact of infection with various strains of EPEC on murine DSS colitis was assessed by change in body weight, colon length, histology, and survival. Activation of apoptosis and MAPK signaling were also evaluated. Our data show that NleH1, but not NleH2, suppresses ERK1/2 and p38 activation in vitro. Interestingly, NleH1 affords significantly greater protection against and hastens recovery from dextran sodium sulfate (DSS)-induced colitis compared to NleH2. Strikingly, colitis-associated mortality was abolished by infection with EPEC strains expressing NleH1. Interestingly, in vivo NleH1 suppresses activation of ERK1/2 and p38 and blocks apoptosis independent of the kinase domain that inhibits NF-κB. In contrast, NleH2 suppresses only caspase-3 and p38, but not ERK1/2. We conclude that NleH1 affords greater protection against and improves recovery from DSS colitis compared to NleH2 due to its ability to suppress ERK1/2 in addition to NF-κB, p38, and apoptosis. These findings warrant further investigation of anti-inflammatory bacterial proteins as novel treatments for IBD.

Influence of Intestinal Microbiota Transplantation and NleH Expression on Citrobacter rodentium Colonization of Mice

The intestinal microbiota plays an important role in regulating host resistance to enteric pathogens. The relative abundance of the microbiota is dependent upon both genetic and environmental factors. The attaching and effacing pathogens enteropathogenic Escherichia coli, enterohemorrhagic E. coli, and Citrobacter rodentium cause diarrheal disease and translocate type III secretion system effector proteins into host cells to inhibit pro-inflammatory host responses. Here we determined the influence of both the intestinal microbiota and the expression of the C. rodentium NleH effector on C. rodentium colonization in different mouse models. We performed fecal transplantation experiments between C57BL/6J and C57BL/10ScNJ mice and found that such microbiota transfers altered both the host resistance to C. rodentium infection as well as the benefit or detriment of expressing NleH to C. rodentium intestinal colonization.

Cooperative Immune Suppression by Escherichia coli and Shigella Effector Proteins

The enteric attaching and effacing (A/E) pathogens enterohemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC) and the invasive pathogens enteroinvasive E. coli (EIEC) and Shigella encode type III secretion systems (T3SS) used to inject effector proteins into human host cells during infection. Among these are a group of effectors required for NF-κB-mediated host immune evasion. Recent studies have identified several effector proteins from A/E pathogens and EIEC/Shigella that are involved in suppression of NF-κB and have uncovered their cellular and molecular functions. A novel mechanism among these effectors from both groups of pathogens is to coordinate effector function during infection. This cooperativity among effector proteins explains how bacterial pathogens are able to effectively suppress innate immune defense mechanisms in response to diverse classes of immune receptor signaling complexes (RSCs) stimulated during infection.

Amino Acids As Mediators of Metabolic Cross Talk between Host and Pathogen

The interaction between host and pathogen decidedly shapes the outcome of an infection, thus understanding this interaction is critical to the treatment of a pathogen-induced infection. Although research in this area of cell biology has yielded surprising findings regarding interactions between host and pathogen, understanding of the metabolic cross talk between host and pathogen is limited. At the site of infection, host and pathogen share similar or identical nutritional substrates and generate common metabolic products, thus metabolic cross talk between host and pathogen could profoundly affect the pathogenesis of an infection. In this review, we present results of a recent discovery of a metabolic interaction between host and pathogen from an amino acid (AA) metabolism-centric point of view. The host depends on AA metabolism to support defensive responses against pathogens, while the pathogens modulate AA metabolism for its own advantage. Some AA, such as arginine, asparagine, and tryptophan, are central points of competition between the host and pathogen. Thus, a better understanding of AA-mediated metabolic cross talk between host and pathogen will provide insight into fruitful therapeutic approaches to manipulate and prevent progression of an infection.

Salmonella, E. coli, and Citrobacter Type III Secretion System Effector Proteins that Alter Host Innate Immunity

Bacteria deliver virulence proteins termed 'effectors' to counteract host innate immunity. Protein-protein interactions within the host cell ultimately subvert the generation of an inflammatory response to the infecting pathogen. Here we briefly describe a subset of T3SS effectors produced by enterohemorrhagic Escherichia coli (EHEC), enteropathogenic E. coli (EPEC), Citrobacter rodentium, and Salmonella enterica that inhibit innate immune pathways. These effectors are interesting for structural and mechanistic reasons, as well as for their potential utility in being engineered to treat human autoimmune disorders associated with perturbations in NF-κB signaling.

Modulation of Host Cell Processes by T3SS Effectors

Two of the enteric Escherichia coli pathotypes-enteropathogenic E. coli (EPEC) and enterohaemorrhagic E. coli (EHEC)-have a conserved type 3 secretion system which is essential for virulence. The T3SS is used to translocate between 25 and 50 bacterial proteins directly into the host cytosol where they manipulate a variety of host cell processes to establish a successful infection. In this chapter, we discuss effectors from EPEC/EHEC in the context of the host proteins and processes that they target-the actin cytoskeleton, small guanosine triphosphatases and innate immune signalling pathways that regulate inflammation and cell death. Many of these translocated proteins have been extensively characterised, which has helped obtain insights into the mechanisms of pathogenesis of these bacteria and also understand the host pathways they target in more detail. With increasing knowledge of the positive and negative regulation of host signalling pathways by different effectors, a future challenge is to investigate how the specific effector repertoire of each strain cooperates over the course of an infection.

Systematic Identification of Mycobacterium tuberculosis Effectors Reveals that BfrB Suppresses Innate Immunity

Mycobacterium tuberculosis (Mtb) has evolved multiple strategies to counter the human immune system. The effectors of Mtb play important roles in the interactions with the host. However, because of the lack of highly efficient strategies, there are only a handful of known Mtb effectors, thus hampering our understanding of Mtb pathogenesis. In this study, we probed Mtb proteome microarray with biotinylated whole-cell lysates of human macrophages, identifying 26 Mtb membrane proteins and secreted proteins that bind to macrophage proteins. Combining GST pull-down with mass spectroscopy then enabled the specific identification of all binders. We refer to this proteome microarray-based strategy as SOPHIE (Systematic unlOcking of Pathogen and Host Interacting Effectors). Detailed investigation of a novel effector identified here, the iron storage protein BfrB (Rv3841), revealed that BfrB inhibits NF-κB-dependent transcription through binding and reducing the nuclear abundance of the ribosomal protein S3 (RPS3), which is a functional subunit of NF- κB. The importance of this interaction was evidenced by the promotion of survival in macrophages of the mycobacteria, Mycobacterium smegmatis, by overexpression of BfrB. Thus, beyond demonstrating the power of SOPHIE in the discovery of novel effectors of human pathogens, we expect that the set of Mtb effectors identified in this work will greatly facilitate the understanding of the pathogenesis of Mtb, possibly leading to additional potential molecular targets in the battle against tuberculosis.

Epithelial cells detect functional type III secretion system of enteropathogenic Escherichia coli through a novel NF-κB signaling pathway

Enteropathogenic Escherichia coli (EPEC), a common cause of infant diarrhea, is associated with high risk of mortality in developing countries. The primary niche of infecting EPEC is the apical surface of intestinal epithelial cells. EPEC employs a type three secretion system (TTSS) to inject the host cells with dozens of effector proteins, which facilitate attachment to these cells and successful colonization. Here we show that EPEC elicit strong NF-κB activation in infected host cells. Furthermore, the data indicate that active, pore-forming TTSS per se is necessary and sufficient for this NF-κB activation, regardless of any specific effector or protein translocation. Importantly, upon infection with wild type EPEC this NF-κB activation is antagonized by anti-NF-κB effectors, including NleB, NleC and NleE. Accordingly, this NF-κB activation is evident only in cells infected with EPEC mutants deleted of nleB, nleC, and nleE. The TTSS-dependent NF-κB activation involves a unique pathway, which is independent of TLRs and Nod1/2 and converges with other pathways at the level of TAK1 activation. Taken together, our results imply that epithelial cells have the capacity to sense the EPEC TTSS and activate NF-κB in response. Notably, EPEC antagonizes this capacity by delivering anti-NF-κB effectors into the infected cells.

NleB/SseK effectors from Citrobacter rodentium, Escherichia coli, and Salmonella enterica display distinct differences in host substrate specificity

Many Gram-negative bacterial pathogens use a syringe-like apparatus called a type III secretion system to inject virulence factors into host cells. Some of these effectors are enzymes that modify host proteins to subvert their normal functions. NleB is a glycosyltransferase that modifies host proteins with N-acetyl-d-glucosamine to inhibit antibacterial and inflammatory host responses. NleB is conserved among the attaching/effacing pathogens enterohemorrhagic Escherichia coli (EHEC), enteropathogenic E. coli (EPEC), and Citrobacter rodentium Moreover, Salmonella enterica strains encode up to three NleB orthologs named SseK1, SseK2, and SseK3. However, there are conflicting reports regarding the activities and host protein targets among the NleB/SseK orthologs. Therefore, here we performed in vitro glycosylation assays and cell culture experiments to compare the activities and substrate specificities of these effectors. SseK1, SseK3, EHEC NleB1, EPEC NleB1, and Crodentium NleB blocked TNF-mediated NF-κB pathway activation, whereas SseK2 and NleB2 did not. C. rodentium NleB, EHEC NleB1, and SseK1 glycosylated host GAPDH. C. rodentium NleB, EHEC NleB1, EPEC NleB1, and SseK2 glycosylated the FADD (Fas-associated death domain protein). SseK3 and NleB2 were not active against either substrate. We also found that EHEC NleB1 glycosylated two GAPDH arginine residues, Arg¹⁹⁷ and Arg²⁰⁰, and that these two residues were essential for GAPDH-mediated activation of TNF receptor-associated factor 2 ubiquitination. These results provide evidence that members of this highly conserved family of bacterial virulence effectors target different host protein substrates and exhibit distinct cellular modes of action to suppress host responses.

Enterotoxigenic Escherichia coli Flagellin Inhibits TNF-Induced NF-κB Activation in Intestinal Epithelial Cells

Enterotoxigenic Escherichia coli (ETEC) causes childhood diarrhea in developing countries. ETEC strains produce the heat-labile enterotoxin (LT) and/or heat-stable enterotoxins (ST) and encode a diverse set of colonization factors used for adherence to intestinal epithelial cells. We previously found that ETEC secretes a heat-stable protein we designated as ETEC Secreted Factor (ESF) that inhibits the extent of NF-κB activation normally induced by tumor necrosis factor alpha (TNF). Here we fractionated ETEC supernatants using fast protein liquid chromatography (FPLC) and determined that ETEC flagellin was necessary and sufficient to protect IκBα from degradation in response to TNF stimulation. These data suggest a potentially novel mechanism by which ETEC may evade the host innate immune response by down-regulating NF-κB-dependent host responses.

T3SS-Independent Uptake of the Short-Trip Toxin-Related Recombinant NleC Effector of Enteropathogenic Escherichia coli Leads to NF-κB p65 Cleavage

Effector proteins secreted by the type 3 secretion system (T3SS) of pathogenic bacteria have been shown to precisely modulate important signaling cascades of the host for the benefit of the pathogens. Among others, the non-LEE encoded T3SS effector protein NleC of enteropathogenic Escherichia coli (EPEC) is a Zn-dependent metalloprotease and suppresses innate immune responses by directly targeting the NF-κB signaling pathway. Many pathogenic bacteria release potent bacterial toxins of the A-B type, which-in contrast to the direct cytoplasmic injection of T3SS effector proteins-are released first into the environment. In this study, we found that NleC displays characteristics of bacterial A-B toxins, when applied to eukaryotic cells as a recombinant protein. Although lacking a B subunit, that typically mediates the uptake of toxins, recombinant NleC (rNleC) induces endocytosis via lipid rafts and follows the endosomal-lysosomal pathway. The conformation of rNleC is altered by low pH to facilitate its escape from acidified endosomes. This is reminiscent of the homologous A-B toxin AIP56 of the fish pathogen Photobacterium damselae piscicida (Phdp). The recombinant protease NleC is functional inside eukaryotic cells and cleaves p65 of the NF-κB pathway. Here, we describe the endocytic uptake mechanism of rNleC, characterize its intracellular trafficking and demonstrate that its specific activity of cleaving p65 requires activation of host cells e.g., by IL1β. Further, we propose an evolutionary link between some T3SS effector proteins and bacterial toxins from apparently unrelated bacteria. In summary, these properties might suggest rNleC as an interesting candidate for future applications as a potential therapeutic against immune disorders.

L-Glutamine and L-arginine protect against enterotoxigenic Escherichia coli infection via intestinal innate immunity in mice

Dietary glutamine (Gln) or arginine (Arg) supplementation is beneficial for intestinal health; however, whether Gln or Arg may confer protection against Enterotoxigenic Escherichia coli (ETEC) infection is not known. To address this, we used an ETEC-infected murine model to investigate the protective effects of Gln and Arg. Experimentally, we pre-treated mice with designed diet of Gln or Arg supplementation prior to the oral ETEC infection and then assessed mouse mortality and intestinal bacterial burden. We also determined the markers of intestinal innate immunity in treated mice, including secretory IgA response (SIgA), mucins from goblet cells, as well as antimicrobial peptides from Paneth cells. ETEC colonized in mouse small intestine, including duodenum, jejunum, and ileum, and inhibited the mRNA expression of intestinal immune factors, such as polymeric immunoglobulin receptor (pIgR), cryptdin-related sequence 1C (CRS1C), and Reg3γ. We found that dietary Gln or Arg supplementation decreased bacterial colonization and promoted the activation of innate immunity (e.g., the mRNA expression of pIgR, CRS1C, and Reg3γ) in the intestine of ETEC-infected mice. Our results suggest that dietary arginine or glutamine supplementation may inhibit intestinal ETEC infection through intestinal innate immunity.

Modulation of host signaling in the inflammatory response by enteropathogenic Escherichia coli virulence proteins

To successfully infect host cells and evade the host immune response, a type III secretion system (T3SS) is commonly used by enteric bacterial pathogens such as enteropathogenic Escherichia coli (EPEC). Recent findings have revealed that various effectors are injected into host cells through the T3SS and exert an inhibitory effect on inflammatory signaling pathways, subverting the immune responses to these pathogens. Here we review recent studies aimed at addressing the modulation of several important inflammatory signaling pathways modulated by EPEC effector proteins, such as the nuclear factor-κB (NF-κB) and mitogen-activated protein kinase (MAPK) pathways, which provides insight into the unfinished work in this unexplored field and helps to identify novel positions in inflammatory signaling networks for EPEC effectors.Cellular & Molecular Immunology advance online publication, 31 October 2016; doi:10.1038/cmi.2016.52.

Modulation of the Inflammasome Signaling Pathway by Enteropathogenic and Enterohemorrhagic Escherichia coli

Innate immunity is an essential component in the protection of a host against pathogens. Enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC, respectively) are known to modulate the innate immune responses of infected cells. The interference is dependent on their type III secretion system (T3SS) and T3SS-dependent effector proteins. Furthermore, these cytosolically injected effectors have been demonstrated to engage multiple immune signaling pathways, including the IFN/STAT, MAPK, NF-κB, and inflammasome pathways. In this review, recent work describing the interaction between EPEC/EHEC and the inflammasome pathway will be discussed.

Citrobacter rodentium NleB Protein Inhibits Tumor Necrosis Factor (TNF) Receptor-associated Factor 3 (TRAF3) Ubiquitination to Reduce Host Type I Interferon Production

Interferon signaling plays important roles in both intestinal homeostasis and in the host response to pathogen infection. The extent to which bacterial pathogens inhibit this host pathway is an understudied area of investigation. We characterized Citrobacter rodentium strains bearing deletions in individual type III secretion system effector genes to determine whether this pathogen inhibits the host type I IFN response and which effector is responsible. The NleB effector limited host IFN-β production by inhibiting Lys(63)-linked ubiquitination of TNF receptor-associated factor 3 (TRAF3). Inhibition was dependent on the glycosyltransferase activity of NleB. GAPDH, a target of NleB during infection, bound to TRAF3 and was required for maximal TRAF3 ubiquitination. NleB glycosyltransferase activity inhibited GAPDH-TRAF3 binding, resulting in reduced TRAF3 ubiquitination. Collectively, our data reveal important interplay between GAPDH and TRAF3 and suggest a mechanism by which the NleB effector inhibits type I IFN signaling.

The Commonalities in Bacterial Effector Inhibition of Apoptosis

Antiapoptotic pathways of the host cell play integral roles in bacterial pathogenesis, with inhibition of those pathways resulting in halted disease pathology. Certain pathogens have developed elegant mechanisms to modulate the fate of the host cell, many of which target novel pathways that are poorly understood in the context of the cell biology. Bacterial pathogenesis research not only promotes the understanding of the role of antiapoptotic pathways in bacterial infection, but has a broader context in understanding the epitome of human disease, that is, developing the understanding of tumorigenic or inflammatory pathways. Here we review host antiapoptotic signalling pathways manipulated by translocated bacterial effectors that propagate the disease state, drawing common parallels and showing the novel differences.

The Locus of Enterocyte Effacement and Associated Virulence Factors of Enterohemorrhagic Escherichia coli

A subset of Shiga toxin-producing Escherichia coli strains, termed enterohemorrhagic E. coli (EHEC), is defined in part by the ability to produce attaching and effacing (A/E) lesions on intestinal epithelia. Such lesions are characterized by intimate bacterial attachment to the apical surface of enterocytes, cytoskeletal rearrangements beneath adherent bacteria, and destruction of proximal microvilli. A/E lesion formation requires the locus of enterocyte effacement (LEE), which encodes a Type III secretion system that injects bacterial proteins into host cells. The translocated proteins, termed effectors, subvert a plethora of cellular pathways to the benefit of the pathogen, for example, by recruiting cytoskeletal proteins, disrupting epithelial barrier integrity, and interfering with the induction of inflammation, phagocytosis, and apoptosis. The LEE and selected effectors play pivotal roles in intestinal persistence and virulence of EHEC, and it is becoming clear that effectors may act in redundant, synergistic, and antagonistic ways during infection. Vaccines that target the function of the Type III secretion system limit colonization of reservoir hosts by EHEC and may thus aid control of zoonotic infections. Here we review the features and functions of the LEE-encoded Type III secretion system and associated effectors of E. coli O157:H7 and other Shiga toxin-producing E. coli strains.

Interference with nuclear factor kappaB signaling pathway by pathogen-encoded proteases: global and selective inhibition

Pathogens have evolved a myriad of ways to abrogate and manipulate the host response to infections. Of the various mechanisms involved, pathogen-encoded and sometimes host-encoded proteases are an important category of virulence factors that cause robust changes on the host response by targeting key proteins along signaling cascades. The nuclear factor kappaB (NF-κB) signaling pathway is a crucial regulatory mechanism for the cell, controlling the expression of survival, immune and proliferation genes. Proteases from pathogens of almost all types have been demonstrated to target and cleave members of the NF-κB signaling pathway at nearly every level. This review provides discussion of proteases targeting the most abundant NF-κB subunit, p65, and the impact of protease-mediated p65 cleavage on the immune responses and survival of the infected host cell. After examining various examples of protease interference, it becomes evident that the cleavage fragments produced by pathogen-driven proteolytic processing should be further characterized to determine whether they have novel and unique functions within the cell. The selective targeting of p65 and its effect on gene transcription reveals unique mechanisms by which pathogens acutely alter their microenvironment, and further research may open new opportunities for novel therapeutics to combat pathogens.