Hilo Yen

NleC, a type III secretion protease, compromises NF-κB activation by targeting p65/RelA.

The NF-κB signaling pathway is central to the innate and adaptive immune responses. Upon their detection of pathogen-associated molecular patterns, Toll-like receptors on the cell surface initiate signal transduction and activate the NF-κB pathway, leading to the production of a wide array of inflammatory cytokines, in attempt to eradicate the invaders. As a countermeasure, pathogens have evolved ways to subvert and manipulate this system to their advantage. Enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC) are closely related bacteria responsible for major food-borne diseases worldwide. Via a needle-like protein complex called the type three secretion system (T3SS), these pathogens deliver virulence factors directly to host cells and modify cellular functions, including by suppressing the inflammatory response. Using gain- and loss-of-function screenings, we identified two bacterial effectors, NleC and NleE, that down-regulate the NF-κB signal upon being injected into a host cell via the T3SS. A recent report showed that NleE inhibits NF-κB activation, although an NleE-deficient pathogen was still immune-suppressive, indicating that other anti-inflammatory effectors are involved. In agreement, our present results showed that NleC was also required to inhibit inflammation. We found that NleC is a zinc protease that disrupts NF-κB activation by the direct cleavage of NF-κBs p65 subunit in the cytoplasm, thereby decreasing the available p65 and reducing the total nuclear entry of active p65. More importantly, we showed that a mutant EPEC/EHEC lacking both NleC and NleE (ΔnleC ΔnleE) caused greater inflammatory response than bacteria carrying ΔnleC or ΔnleE alone. This effect was similar to that of a T3SS-defective mutant. In conclusion, we found that NleC is an anti-inflammatory bacterial zinc protease, and that the cooperative function of NleE and NleC disrupts the NF-κB pathway and accounts for most of the immune suppression caused by EHEC/EPEC.

Enteropathogenic Escherichia coli Uses NleA to Inhibit NLRP3 Inflammasome Activation.

Enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC) are related strains capable of inducing severe gastrointestinal disease. For optimal infection, these pathogens actively modulate cellular functions through the deployment of effector proteins in a type three secretion system (T3SS)-dependent manner. In response to enteric pathogen invasion, the Nod-like receptor pyrin domain containing (NLRP) inflammasome has been increasingly recognized as an important cytoplasmic sensor against microbial infection by activating caspase-1 and releasing IL-1β. EPEC and EHEC are known to elicit inflammasome activation in macrophages and epithelial cells; however, whether the pathogens actively counteract such innate immune responses is unknown. Using a series of compound effector-gene deletion strains of EPEC, we screened and identified NleA, which could subdue host IL-1β secretion. It was found that the reduction is not because of blocked NF-κB activity; instead, the reduction results from inhibited caspase-1 activation by NleA. Immunostaining of human macrophage-like cells following infection revealed limited formation of inflammasome foci with constituents of total caspase-1, ASC and NLRP3 in the presence of NleA. Pulldown of PMA-induced differentiated THP-1 lysate with purified MBP-NleA reveals that NLRP3 is a target of NleA. The interaction was verified by an immunoprecipitation assay and direct interaction assay in which purified MBP-NleA and GST-NLRP3 were used. We further showed that the effector interacts with regions of NLRP3 containing the PYD and LRR domains. Additionally, NleA was found to associate with non-ubiquitinated and ubiquitinated NLRP3 and to interrupt de-ubiquitination of NLRP3, which is a required process for inflammasome activation. Cumulatively, our findings provide the first example of EPEC-mediated suppression of inflammasome activity in which NieA plays a novel role in controlling the host immune response through targeting of NLRP3.

LeuO enhances butyrate‐induced virulence expression through a positive regulatory loop in enterohaemorrhagic Escherichia coli

Enterohaemorrhagic Escherichia coli (EHEC) causes bloody diarrhoea and other severe symptoms such as haemorrhagic uraemic syndrome. The expression of virulence genes on the locus for enterocyte effacement (LEE) and associated genes is regulated by a variety of factors, including transcriptional regulators and environmental signals. Butyrate, one of the major short‐chain fatty acids present in the intestine, enhances expression of LEE genes and flagella biosynthesis genes in EHEC O157:H7, resulting in increased bacterial adherence and motility. Here, we show that expression of the leuO gene, which encodes a LysR‐type transcriptional regulator, is enhanced by butyrate via Lrp, which is also necessary for butyrate‐induced responses of LEE genes. LeuO expression induces prolonged activation of the promoter of LEE1 operon, including the ler gene, as well as virulence mechanisms such as microcolony formation. Activation of the LEE1 promoter by LeuO depends on another regulator, called Pch. The response of the leuO promoter to butyrate requires two virulence regulators, Pch and Ler, in addition to Lrp. Pch, Ler and Lrp bind the upstream region of the leuO promoter. Thus, leuO is involved in butyrate‐enhanced expression of LEE genes through a positive feedback mechanism, but its expression and action on the LEE1 promoter are dependent on the virulence regulators Pch and Ler.

Antisense transcription regulates the expression of the enterohemorrhagic Escherichia coli virulence regulatory gene ler in response to the intracellular iron concentration.

Enteric pathogens, such as enterohemorrhagic E. coli (EHEC) O157:H7, encounter varying concentrations of iron during their life cycle. In the gastrointestinal tract, the amount of available free iron is limited because of absorption by host factors. EHEC and other enteric pathogens have developed sophisticated iron-responsive systems to utilize limited iron resources, and these systems are primarily regulated by the Fur repressor protein. The iron concentration could be a signal that controls gene expression in the intestines. In this study, we explored the role of iron in LEE (locus for enterocyte effacement) virulence gene expression in EHEC. In contrast to the expression of Fur-regulated genes, the expression of LEE genes was greatly reduced in fur mutants irrespective of the iron concentration. The expression of the ler gene, the LEE-encoded master regulator, was affected at a post-transcription step by fur mutation. Further analysis showed that the loss of Fur affected the translation of the ler gene by increasing the intracellular concentration of free iron, and the transcription of the antisense strand was necessary for regulation. The results indicate that LEE gene expression is closely linked to the control of intracellular free iron homeostasis.

DNA looping-dependent autorepression of LEE1 P1 promoters by Ler in enteropathogenic Escherichia coli (EPEC).

Significance Ler [locus of enterocyte effacement (LEE)-encoded regulator], encoded by the first gene of the LEE1 operon in enteropathogenic Escherichia coli (EPEC), represses its own transcription driven by two promoters separated by 10 bp. We found that Ler does this repression through a DNA loop of 16 helical turns, in which RNA polymerase is trapped as open promoter complex, although this complex should be most readily transformed into productive initiation complex. Ler, a homolog of H-NS in enteropathogenic Escherichia coli (EPEC), plays a critical role in the expression of virulence genes encoded by the pathogenic island, locus of enterocyte effacement (LEE). Although Ler acts as an antisilencer of multiple LEE operons by alleviating H-NS–mediated silencing, it represses its own expression from two LEE1 P1 promoters, P1A and P1B, that are separated by 10 bp. Various in vitro biochemical methods were used in this study to elucidate the mechanism underlying transcription repression by Ler. Ler acts through two AATT motifs, centered at position −111.5 on the coding strand and at +65.5 on the noncoding strand, by simultaneously repressing P1A and P1B through DNA-looping. DNA-looping was visualized using atomic force microscopy. It is intriguing that an antisilencing protein represses transcription, not by steric exclusion of RNA polymerase, but by DNA-looping. We propose that the DNA-looping prevents further processing of open promoter complex (RPO) at these promoters during transcription initiation.

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.

Activation of lpxR gene through enterohemorrhagic Escherichia coli virulence regulators mediates lipid A modification to attenuate innate immune response

During the course of infection, pathogens must overcome a variety of host defence systems. Modulation of lipid A, which is a strong stimulant for host immune systems, is one of the strategies used by microorganisms to evade the host response. The lpxR gene, which encodes a lipid A 3′‐O‐deacylase, is commonly found in several pathogens and has been shown to reduce the inflammatory response. Here, we demonstrated that the lpxR gene of enterohaemorrhagic Escherichia coli (EHEC) was positively regulated by two virulence regulators, Pch and Ler, and that this regulation was coordinated with the locus of enterocyte effacement genes, which encode major virulence factors for colonisation. The lpxR promoter was repressed by the binding of H‐NS, but the competitive binding of both regulators resulted in transcription activation. Next, we showed that lipid A from the lpxR mutant was more stimulatory of the inflammatory response in macrophage‐like cells than lipid A from wild‐type EHEC. Furthermore, phagocytic activity and phagosome maturation in host cells infected with the lpxR mutant were increased in a p38 mitogen‐activated protein kinase‐dependent manner in comparison with wild‐type EHEC infection. Finally, we demonstrated that the pch mutant, which is deficient in activation of the locus of enterocyte effacement genes, was phagocytised more efficiently than the wild type. Thus, EHEC modulates lipid A to dampen the host immune response when activating virulence genes for colonisation.

Enterohaemorrhagic Escherichia coli produces outer membrane vesicles as an active defence system against antimicrobial peptide LL‐37

Antimicrobial peptides (AMPs) are important components of the innate immune system. Enterohaemorrhagic Escherichia coli (EHEC), a food‐borne pathogen causing serious diarrheal diseases, must overcome attack by AMPs. Here, we show that resistance of EHEC against human cathelicidin LL‐37, a primary AMP, was enhanced by butyrate, which has been shown to act as a stimulant for the expression of virulence genes. The increase of resistance depended on the activation of the ompT gene, which encodes the outer membrane protease OmpT for LL‐37. The expression of the ompT gene was enhanced through the activation system for virulence genes. The increase in ompT expression did not result in an increase in OmpT protease in bacteria but in enhancement of the production of OmpT‐loaded outer membrane vesicles (OMVs), which primarily contributed to the increase in LL‐37‐resistance. Furthermore, a sublethal dosage of LL‐37 stimulated the production of OMVs. Finally, we showed that OMVs produced by OmpT‐positive strains protect the OmpT‐negative strain, which is susceptible to LL‐37 by itself more efficiently than OMVs from the ompT mutant. These results indicate that EHEC enhances the secretion of OmpT‐loaded OMVs in coordination with the activation of virulence genes during infection and blocks bacterial cell attack by LL‐37.