Tight junctions and enteropathogenic E. coli

Enterohemorrhagic Escherichia coli Effector Protein EspF Interacts With Host Protein ANXA6 and Triggers Myosin Light Chain Kinase (MLCK)-Dependent Tight Junction Dysregulation

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is an important foodborne pathogen that can cause bloody diarrhea and hemolytic uremic syndrome (HUS) in humans. EspF is one of the best-characterized effector proteins secreted from the type three secretion system to hijack host cell functions. However, the crucial pathogen-host interactions and the basis for the intestinal barrier disruption during infections remain elusive. Our previous study screened and verified the interaction between host protein ANXA6 and EspF protein. Here, by fluorescence resonance energy transfer (FRET) and co-immunoprecipitation (CO-IP), we verified that EspF interacts with ANXA6 through its C-terminal domain. Furthermore, we found that both the constitutive expression of EspF or ANXA6 and the co-expression of EspF-ANXA6 could decrease the levels of tight junction (TJ) proteins ZO-1 and occludin, and disrupt the distribution of ZO-1. Moreover, we showed that EspF-ANXA6 activated myosin light chain kinase (MLCK), induced the phosphorylation of myosin light chain (MLC) and PKCα, and down-regulated the expression level of Calmodulin protein. Collectively, this study revealed a novel interaction between the host protein (ANXA6) and EspF. The binding of EspF to ANXA6 may perturb TJs in an MLCK-MLC-dependent manner, and thus may be involved in EHEC pathogenic function.

Dietary genistein supplementation improves intestinal mucosal barrier function in Escherichia coli O78-challenged broilers

Genistein has multiple biological activities in both humans and animals. However, a protective effect of genistein on Escherichia coli (E. coli)-induced intestinal mucosal barrier dysfunction remains unknown. In the present study, a total of 288 1-day-old male Arbor Acre broilers fed a corn-soybean basal diet unsupplemented or supplemented with 20 mg genistein/kg diet were subjected to E. coli serotype O78 (10⁸ cfu per bird) infection or equal volume of sodium chloride at 19 days of age. Sera and tissue samples were collected 2 days after E. coli infection. Growth performance, index of immune-related organs, intestinal barrier permeability, protein level of inflammatory cytokines, sIgA, tight junction protein, and mRNA level of apoptotic genes in jejunum were determined. Mortality rate at 7 days post infection was recorded. The results showed that E. coli challenge led to a reduced average daily gain, a decreased thymus index, and bursal index in broilers, an increase of fluorescein isothiocyanate (FITC)-dextran in serum, and a decreased sIgA in jejunum. These effects were abrogated by genistein administration. Western blot results showed that E. coli infection led to increased protein level of claudin-1 and zonula occludens (ZO)-1, which was largely abolished by genistein. Moreover, E. coli infection resulted increased protein level of TNF-α and IL-6, enhanced mRNA level of Bax and caspase-3, as well as decreased mRNA level of Bcl-2 were abrogated by genistein in jujunum of broilers. In conclusion, the results indicate that genistein supplementation improves intestinal mucosal barrier function which is associated with a regulatory effect on tight junction proteins, sIgA, apoptosis, and secretion of inflammatory cytokines in jejunum of E. coli-challenged broilers.

Extracellular vesicles and soluble factors secreted by Escherichia coli Nissle 1917 and ECOR63 protect against enteropathogenic E. coli-induced intestinal epithelial barrier dysfunction

Background

Enteric pathogens have developed mechanisms to disrupt tight junctions and increase gut permeability. Many studies have analysed the ability of live probiotics to protect intestinal epithelial cells against tight junction damage caused by bacterial pathogens. Escherichia coli Nissle 1917 (EcN) is among the probiotics that positively modulates the intestinal epithelial barrier by regulating expression and distribution of tight junction proteins. We previously reported that regulation of ZO-1, claudin-14 and claudin-2 is mediated by EcN secreted factors, either free-released or associated with outer membrane vesicles (OMVs). Factors secreted by commensal ECOR63 elicited comparable effects in intact epithelial T-84 and Caco-2 cell monolayers.

Results

Here we analyse the ability of OMVs and soluble secreted factors to protect epithelial barrier function in polarized T-84 and Caco-2 cells infected with enteropathogenic Escherichia coli (EPEC). Transepithelial electrical resistance, paracellular permeability, mRNA levels and subcellular distribution of tight junction proteins were monitored in the absence or presence of EcN and ECOR63 extracellular fractions. EPEC downregulated expression of ZO-1 ZO-2, occludin and claudin-14 and altered the subcellular localization of ZO-1, occludin and F-actin cytoskeleton. OMVs and soluble factors secreted by EcN and ECOR63 counteracted EPEC-altered transepithelial resistance and paracellular permeability, preserved occludin and claudin-14 mRNA levels, retained ZO-1 and occludin at tight junctions in the cell boundaries and ameliorated F-actin disorganization. Redistribution of ZO-1 was not accompanied by changes at mRNA level.

Conclusion

This study provides new insights on the role of microbiota secreted factors on the modulation of intestinal tight junctions, expanding their barrier-protective effects against pathogen-induced disruption.

Clever Cooperation: Interactions Between EspF and Host Proteins

EspF is a central effector protein of enterohemorrhagic Escherichia coli (EHEC), enteropathogenic E. coli (EPEC), and Citrobacter rodentium (CR) that is secreted through the type III secretion system to host cells. The interaction between EspF and host proteins plays an important role in bacterial pathogenesis. EspF protein binds to host SNX9 and N-WASP proteins to promote the colonization of pathogenic bacteria in intestinal epithelial cells; combines with cytokeratin 18, actin, 14-3-3ζ, Arp2/3, profilin, and ZO-1 proteins to intervene in the redistribution of intermediate filaments, the rearrangement of actin, and the disruption of tight junctions; acts together with Abcf2 to boost host cell intrinsic apoptosis; and collaborates with Anxa6 protein to inhibit phagocytosis. The interaction between EspF and host proteins is key to the pathogenic mechanism of EHEC and EPEC. Here, we review how EspF protein functions through interactions with these 10 host proteins and contributes to the pathogenicity of EHEC/EPEC.

Modulation of Intestinal Paracellular Transport by Bacterial Pathogens

The passive and regulated movement of ions, solutes, and water via spaces between cells of the epithelial monolayer plays a critical role in the normal intestinal functioning. This paracellular pathway displays a high level of structural and functional specialization, with the membrane-spanning complexes of the tight junctions, adherens junctions, and desmosomes ensuring its integrity. Tight junction proteins, like occludin, tricellulin, and the claudin family isoforms, play prominent roles as barriers to unrestricted paracellular transport. The past decade has witnessed major advances in our understanding of the architecture and function of epithelial tight junctions. While it has been long appreciated that microbes, notably bacterial and viral pathogens, target and disrupt junctional complexes and alter paracellular permeability, the precise mechanisms remain to be defined. Notably, renewed efforts will be required to interpret the available data on pathogen-mediated barrier disruption in the context of the most recent findings on tight junction structure and function. While much of the focus has been on pathogen-induced dysregulation of junctional complexes, commensal microbiota and their products may influence paracellular permeability and contribute to the normal physiology of the gut. Finally, microbes and their products have become important tools in exploring host systems, including the junctional properties of epithelial cells. © 2018 American Physiological Society. Compr Physiol 8:823-842, 2018.

Tricellular Tight Junction Protein Tricellulin Is Targeted by the Enteropathogenic Escherichia coli Effector EspG1, Leading to Epithelial Barrier Disruption

Enteropathogenic Escherichia coli (EPEC)-induced diarrhea is often associated with disruption of intestinal epithelial tight junctions. Although studies have shown alterations in the expression and localization of bicellular tight junction proteins during EPEC infections, little is known about whether tricellular tight junction proteins (tTJs) are affected. Using Caco-2 cell monolayers, we investigated if EPEC is capable of targeting the tTJ protein tricellulin. Our results demonstrated that at 4 h postinfection, EPEC induced a significant reduction in tricellulin levels, accompanied by a significant loss of transepithelial resistance (TEER) and a corresponding increase in paracellular permeability. Conversely, cells overexpressing tricellulin were highly resistant to EPEC-induced barrier disruption. Confocal microscopy revealed the distribution of tricellulin into the plasma membrane of infected epithelial cells and confirmed the localization of EPEC aggregates in close proximity to tTJs. Moreover, infections with EPEC strains lacking genes encoding specific type III secreted effector proteins demonstrated a crucial role for the effector EspG1 in modulating tricellulin expression. Complementation studies suggest that the EspG-induced depletion of tricellulin is microtubule dependent. Overall, our results show that EPEC-induced epithelial barrier dysfunction is mediated in part by EspG1-induced microtubule-dependent depletion of tricellulin.

A pathogen's journey in the host cell: Bridges between actin and traffic

Manipulation of the actin cytoskeleton is a commonly used process by which bacterial pathogens and viruses are able to neutralize host defense mechanisms and subvert them in order to replicate in a hostile environment. Diverse bacteria display a wide array of mechanisms of regulation of microfilaments to enter, move within or exit the host cell. A less studied subject is how pathogens may co-opt the actin cytoskeleton to disturb vesicle trafficking pathways, namely phagolysosomal fusion, and avoid degradation. In fact, although actin plays a role in endosomal trafficking and phagosome maturation, the knowledge on the exact mechanisms and additional players is still scarce. Recently, we found that the Legionella pneumophila virulence factor VipA is an actin nucleator, associates with actin filaments and early endosomes during infection, and interferes in yeast organelle trafficking pathways, suggesting it may be linking actin dynamics to endosome biogenesis. Further studies on this protein, together with work on other bacterial effectors, may help shed light in the role of actin in endosomal maturation.

The role of epithelial tight junctions involved in pathogen infections

Tight junctions (TJs) are sealing complexes between adjacent epithelial cells, functioning by controlling paracellular passage and maintaining cell polarity. These functions of TJs are primarily based on structural integrity as well as dynamic regulatory balance, indicating plasticity of TJ in response to external stimuli. An indispensable role of TJs involved in pathogen infection has been widely demonstrated since disruption of TJs leads to a distinct increase in paracellular permeability and polarity defects which facilitate viral or bacterial entry and spread. In addition to pathological changes in TJ integrity, TJ proteins such as occludin and claudins can either function as receptors for pathogen entry or interact with viral/bacterial effector molecules as an essential step for characterizing an infective stage. This suggests a more complicated role for TJ itself and especially specific TJ components. Thus, this review surveys the role of the epithelial TJs involved in various pathogen infections, and extends TJ targeted therapeutic and pharmacological application prospects.

Enteropathogenic Escherichia coli inhibits type I interferon- and RNase L-mediated host defense to disrupt intestinal epithelial cell barrier function

Enteropathogenic Escherichia coli (EPEC) primarily infects children in developing countries and causes diarrhea that can be deadly. EPEC pathogenesis occurs through type III secretion system (T3SS)-mediated injection of effectors into intestinal epithelial cells (IECs); these effectors alter actin dynamics, modulate the immune response, and disrupt tight junction (TJ) integrity. The resulting compromised barrier function and increased gastrointestinal (GI) permeability may be responsible for the clinical symptoms of infection. Type I interferon (IFN) mediates anti-inflammatory activities and serves essential functions in intestinal immunity and homeostasis; however, its role in the immune response to enteric pathogens, such as EPEC, and its impact on IEC barrier function have not been examined. Here, we report that IFN-β is induced following EPEC infection and regulates IEC TJ proteins to maintain barrier function. The EPEC T3SS effector NleD counteracts this protective activity by inhibiting IFN-β induction and enhancing tumor necrosis factor alpha to promote barrier disruption. The endoribonuclease RNase L is a key mediator of IFN induction and action that promotes TJ protein expression and IEC barrier integrity. EPEC infection inhibits RNase L in a T3SS-dependent manner, providing a mechanism by which EPEC evades IFN-induced antibacterial activities. This work identifies novel roles for IFN-β and RNase L in IEC barrier functions that are targeted by EPEC effectors to escape host defense mechanisms and promote virulence. The IFN-RNase L axis thus represents a potential therapeutic target for enteric infections and GI diseases involving compromised barrier function.

Real-time sensing of enteropathogenic E. coli-induced effects on epithelial host cell height, cell-substrate interactions, and endocytic processes by infrared surface plasmon spectroscopy

Enteropathogenic Escherichia coli (EPEC) is an important, generally non-invasive, bacterial pathogen that causes diarrhea in humans. The microbe infects mainly the enterocytes of the small intestine. Here we have applied our newly developed infrared surface plasmon resonance (IR-SPR) spectroscopy approach to study how EPEC infection affects epithelial host cells. The IR-SPR experiments showed that EPEC infection results in a robust reduction in the refractive index of the infected cells. Assisted by confocal and total internal reflection microscopy, we discovered that the microbe dilates the intercellular gaps and induces the appearance of fluid-phase-filled pinocytic vesicles in the lower basolateral regions of the host epithelial cells. Partial cell detachment from the underlying substratum was also observed. Finally, the waveguide mode observed by our IR-SPR analyses showed that EPEC infection decreases the host cell's height to some extent. Together, these observations reveal novel impacts of the pathogen on the host cell architecture and endocytic functions. We suggest that these changes may induce the infiltration of a watery environment into the host cell, and potentially lead to failure of the epithelium barrier functions. Our findings also indicate the great potential of the label-free IR-SPR approach to study the dynamics of host-pathogen interactions with high spatiotemporal sensitivity.

Diarrhea among children in developing countries

Diarrhea continues to stand among the most important causes of global morbidity and mortality in children under 5 years of age. Although the introduction of oral rehydration and other case-management strategies have reduced acute diarrhea fatalities, many of the survivors develop persistent diarrhea and/or deficiencies of growth and cognition. Thus understanding the true global burden of diarrhea requires attention to acute diarrhea as well is its sequelae. To understand the etiology of moderate to severe diarrhea among children in high mortality areas of sub-Saharan Africa and south Asia we performed a comprehensive case-control study of children under 5 years of age at seven sites. Each site employed an identical case-control study design and each utilized a uniform comprehensive set of microbiological assays to identify the likely bacterial, viral and protozoal etiologies. Results of the studies will inform diarrhea prevention and management efforts worldwide.

Redistribution of tight junction proteins during EPEC infection in vivo

Enteropathogenic Escherichia coli (EPEC) is a leading cause of diarrhea among infants. Tight junction plays a vital role in intestinal paracellular permeability by forming physical intercellular barriers in epithelial cells. However, the impact of this enteric pathogen on tight junctions in vivo has not been fully investigated. In the present study, the alterations in tight junctions following EPEC infection in vivo were investigated. Western blot analysis revealed that the tight junction proteins, occludin and claudin-1, were displaced from tight junction membrane microdomains to Triton X-100 soluble fractions after EPEC infection. Changes in intestinal paracellular permeability were determined using the molecular tracer biotin, which was observed to penetrate the epithelia and extended into the lamina propria, indicating disruption in tight junction barrier function. Our results suggested that redistribution of tight junction proteins plays an important role in the disruption of epithelial barrier function induced by EPEC infection, which may provide new insight into the pathogenesis of diarrhea caused by EPEC.

A tannic acid-based medical food, Cesinex(®), exhibits broad-spectrum antidiarrheal properties: a mechanistic and clinical study

BACKGROUND

The purpose of this investigation was to evaluate the efficacy and tolerability of a tannic acid-based medical food, Cesinex(®), in the treatment of diarrhea and to investigate the mechanisms underlying its antidiarrheal effect.

METHODS

Cesinex(®) was prescribed to six children and four adults with diarrhea. Patient records were retrospectively reviewed for the primary outcome. Cesinex(®) and its major component, tannic acid, were tested for their effects on cholera toxin-induced intestinal fluid secretion in mice. Polarized human gut epithelial cells (HT29-CL19A cells) were used to investigate the effects of tannic acid on epithelial barrier properties, transepithelial chloride secretion, and cell viability.

RESULTS

Successful resolution of diarrheal symptoms was reported in nine of ten patients receiving Cesinex(®). The treatment of HT29-CL19A cells with clinically relevant concentrations of tannic acid (0.01-1 mg/ml) significantly increased transepithelial resistance (TER) and inhibited the cystic fibrosis transmembrane conductance regulator (CFTR)-dependent or the calcium-activated Cl(-) secretion. Tannic acid could also improve the impaired epithelial barrier function induced by tumor necrosis factor alpha (TNFα) and inhibited the disrupting effect of TNFα on the epithelial barrier function in these cells. Cholera toxin (CTX)-induced mouse intestinal fluid secretion was significantly reduced by the administration of Cesinex(®) or tannic acid. Cesinex(®) has high antioxidant capacity.

CONCLUSIONS

Cesinex(®) demonstrates efficacy and a good safety profile in the treatment of diarrhea. The broad-spectrum antidiarrheal effect of Cesinex(®) can be attributed to a combination of factors: its ability to improve the epithelial barrier properties, to inhibit intestinal fluid secretion, and the high antioxidant capacity.

The unexpected link between infection-induced apoptosis and a TH17 immune response

Microbial pathogens can initiate MOMP in host cells and as such, initiate the mitochondrial pathway of apoptosis. Innate immune recognition of cells dying in this way by infection-induced apoptosis would involve recognition of ligands derived from the apoptotic host cell simultaneously with those derived from the infecting pathogen. The resultant signal transduction pathways engaged direct DCs to concomitantly synthesize TGF-β and IL-6, two cytokines that subsequently favor the differentiation of naïve CD4 T cells into T(h)17 cells. Citrobacter rodentium is one rodent pathogen that targets mitochondria and induces apoptosis, and blockade of apoptosis during enteric Citrobacter infection impairs the characteristic T(h)17 response in the intestinal LP. Here, we review these original findings. We discuss microbial infections other than Citrobacter that have been shown to induce T(h)17 responses, and we examine what is known about the ability of those pathogens to induce apoptosis. We also consider types of cell death other than apoptosis that can be triggered by microbial infection, and we highlight how little we know about the impact of various forms of cell death on the ensuing adaptive immune response.

Responding to infection and apoptosis--a task for TH17 cells

Two of the critical cytokines required for the differentiation of T helper 17 (T(H)17) cells from naive CD4 T cells are transforming growth factor-beta (TGF-β) and interleukin-6 (IL-6). Innate recognition of apoptotic cells in the presence of Toll-like receptor engagement directs the simultaneous synthesis of these cytokines by antigen-presenting cells (APCs), and as such provides a cytokine milieu that favors T(H)17 cell induction. In this situation, APCs are activated in response to ligands derived from apoptotic cells, but also to those from the infecting pathogen. Induction of a T(H)17 response against Citrobacter rodentium infection was dependent on the ability of Citrobacter to induce apoptosis of intestinal epithelial cells. In this review, we will discuss how simultaneous activation of inflammatory and noninflammatory pattern recognition receptors on APCs impacts T helper cell differentiation, and what relevance this effect has on the immune response generated against bacterial infections that cause host cell apoptosis.

Bacterial interactions with the host epithelium

The gastrointestinal epithelium deploys multiple innate defense mechanisms to fight microbial intruders, including epithelial integrity, rapid epithelial cell turnover, quick expulsion of infected cells, autophagy, and innate immune responses. Nevertheless, many bacterial pathogens are equipped with highly evolved infectious stratagems that circumvent these defense systems and use the epithelium as a replicative foothold. During replication on and within the gastrointestinal epithelium, gastrointestinal bacterial pathogens secrete various components, toxins, and effectors that can subvert, usurp, and exploit host cellular functions to benefit bacterial survival. In addition, bacterial pathogens use a variety of mechanisms that balance breaching the epithelial barrier with maintaining the epithelium in order to promote bacterial colonization. These complex strategies represent a new paradigm of bacterial pathogenesis.

PKA-mediated phosphorylation of EPEC-Tir at serine residues 434 and 463: A novel pathway in regulating Rac1 GTPase function

Type-III or type-IV secretion systems of many Gram-negative bacterial pathogens inject effector proteins into host cells that modulate cellular functions in their favour. A preferred target of these effectors is the actin-cytoskeleton as shown by studies using the gastric pathogens Helicobacter pylori (H. pylori) and enteropathogenic Escherichia coli (EPEC). We recently developed a co-infection approach to study effector protein function and molecular mechanisms by which they highjack cellular signalling cascades. This is exemplified by our observation that EPEC profoundly blocks H. pylori-induced epithelial cell scattering and elongation, a disease-related event requiring the activity of small Rho GTPase Rac1. While this suppressive effect is dependent on the effector protein Tir and the outer-membrane protein Intimin, it unexpectedly revealed evidence for Tir-signalling independent of phosphorylation of Tir at tyrosine residues 454 and 474. Instead, our studies revealed a previously unidentified function for protein kinase A (PKA)-mediated phosphorylation of Tir at serine residues 434 and 463. We demonstrated that EPEC infection activates PKA for Tir phosphorylation. Activated PKA then phosphorylates Rac1 at its serine residue 71 associated with reduced GTP-load and inhibited cell elongation. Phosphorylation of Rho GTPases such as Rac1 might be an interesting novel strategy in microbial pathogenesis.

PKA-mediated phosphorylation of EPEC-Tir at serine residues 434 and 463: A novel pathway in regulating Rac1 GTPase function

Type-III or type-IV secretion systems of many Gram-negative bacterial pathogens inject effector proteins into host cells that modulate cellular functions in their favour. A preferred target of these effectors is the actin-cytoskeleton as shown by studies using the gastric pathogens Helicobacter pylori (H. pylori) and enteropathogenic Escherichia coli (EPEC). We recently developed a co-infection approach to study effector protein function and molecular mechanisms by which they highjack cellular signalling cascades. This is exemplified by our observation that EPEC profoundly blocks H. pylori-induced epithelial cell scattering and elongation, a disease-related event requiring the activity of small Rho GTPase Rac1. While this suppressive effect is dependent on the effector protein Tir and the outer-membrane protein Intimin, it unexpectedly revealed evidence for Tir-signalling independent of phosphorylation of Tir at tyrosine residues 454 and 474. Instead, our studies revealed a previously unidentified function for protein kinase A (PKA)-mediated phosphorylation of Tir at serine residues 434 and 463. We demonstrated that EPEC infection activates PKA for Tir phosphorylation. Activated PKA then phosphorylates Rac1 at its serine residue 71 associated with reduced GTP-load and inhibited cell elongation. Phosphorylation of Rho GTPases such as Rac1 might be an interesting novel strategy in microbial pathogenesis.

Tight junctions: molecular structure meets function

Tight junctions of epithelial and endothelial cells form selective barriers that regulate paracellular transport of solutes, immune cells, and drugs. Tight junctions consist of proteins that physically "seal" the tight junction but also form channels that allow for permeation between the cells, resulting in epithelial surfaces of different tightness. The tight junction proteins occludin, tricellulin, and at least 24 members of the claudin family are characterized by four transmembranal domains and two extracellular loops that, like teeth of a zipper, contact the appropriate loops from opposing cell membranes. Tight junctions are regulated in their molecular composition, ultrastructure, and function by intracellular scaffolding proteins and the cytoskeleton; such regulation serves normal, physiologic adaptation but also occurs in numerous diseases.