Attaching and effacing pathogen-induced tight junction disruption in vivo

Host-Microbe Protein Interactions during Bacterial Infection

Interspecies protein-protein interactions are essential mediators of infection. While bacterial proteins required for host cell invasion and infection can be identified through bacterial mutant library screens, information about host target proteins and interspecies complex structures has been more difficult to acquire. Using an unbiased chemical crosslinking/mass spectrometry approach, we identified interspecies protein-protein interactions in human lung epithelial cells infected with Acinetobacter baumannii. These efforts resulted in identification of 3,076 crosslinked peptide pairs and 46 interspecies protein-protein interactions. Most notably, the key A. baumannii virulence factor, OmpA, was identified as crosslinked to host proteins involved in desmosomes, specialized structures that mediate host cell-to-cell adhesion. Co-immunoprecipitation and transposon mutant experiments were used to verify these interactions and demonstrate relevance for host cell invasion and acute murine lung infection. These results shed new light on A. baumannii-host protein interactions and their structural features, and the presented approach is generally applicable to other systems.

The cell surface receptor Slamf6 modulates innate immune responses during Citrobacter rodentium-induced colitis

The homophilic cell surface receptors CD150 (Slamf1) and CD352 (Slamf6) are known to modulate adaptive immune responses. Although the Th17 response was enhanced in Slamf6(-/-) C57BL/6 mice upon oral infection with Citrobacter rodentium, the pathologic consequences are indistinguishable from an infection of wild-type C57BL/6 mice. Using a reporter-based binding assay, we show that Slamf6 can engage structures on the outer cell membrane of several Gram(-) bacteria. Therefore, we examined whether Slamf6, like Slamf1, is also involved in innate responses to bacteria and regulates peripheral inflammation by assessing the outcome of C. rodentium infections in Rag(-/-) mice. Surprisingly, the pathology and immune responses in the lamina propria of C. rodentium-infected Slamf6(-/-) Rag(-/-) mice were markedly reduced as compared with those of Rag(-/-) mice. Infiltration of inflammatory phagocytes into the lamina propria was consistently lower in Slamf6(-/-) Rag(-/-) mice than in Rag(-/-) animals. Concomitant with the reduced systemic translocation of the bacteria was an enhanced production of IL-22, suggesting that Slamf6 suppresses a mucosal protective program. Furthermore, administering a mAb (330) that inhibits bacterial interactions with Slamf6 to Rag(-/-) mice ameliorated the infection compared with a control antibody. We conclude that Slamf6-mediated interactions of colonic innate immune cells with specific Gram(-) bacteria reduce mucosal protection and enhance inflammation, contributing to lethal colitis that is caused by C. rodentium infections in Rag(-/-) mice.

Enteropathogenic E. coli: breaking the intestinal tight junction barrier

Enteropathogenic E. coli (EPEC) causes acute intestinal infections in infants in the developing world. Infection typically spreads through contaminated food and water and leads to severe, watery diarrhea. EPEC attaches to the intestinal epithelial cells and directly injects virulence factors which modulate multiple signaling pathways leading to host cell dysfunction. However, the molecular mechanisms that regulate the onset of diarrhea are poorly defined. A major target of EPEC is the host cell tight junction complex which acts as a barrier and regulates the passage of water and solutes through the paracellular space. In this review, we focus on the EPEC effectors that target the epithelial barrier, alter its functions and contribute to leakage through the tight junctions.

Enteropathogenic E. coli: breaking the intestinal tight junction barrier

Enteropathogenic E. coli (EPEC) causes acute intestinal infections in infants in the developing world. Infection typically spreads through contaminated food and water and leads to severe, watery diarrhea. EPEC attaches to the intestinal epithelial cells and directly injects virulence factors which modulate multiple signaling pathways leading to host cell dysfunction. However, the molecular mechanisms that regulate the onset of diarrhea are poorly defined. A major target of EPEC is the host cell tight junction complex which acts as a barrier and regulates the passage of water and solutes through the paracellular space. In this review, we focus on the EPEC effectors that target the epithelial barrier, alter its functions and contribute to leakage through the tight junctions.

Mass Spectrometry-Based Proteomics Identification of Enteropathogenic Escherichia coli Pedestal Constituents

Enteropathogenic Escherichia coli (EPEC) co-opt host signaling pathways and recruit numerous host proteins to motile morphological structures, called pedestals, at sites of bacterial attachment. These pedestals are hallmarks of EPEC-based disease, and the identification and characterization of the functions of pedestal proteins continue to steadily increase. To identify additional constituents in an unbiased manner, we developed a strategy where EPEC pedestals were elongated artificially, severed, and then concentrated prior to their analysis by mass spectrometry (MS)-based proteomics. We identified >90 unique mammalian proteins over multiple experimental trials from our preparations. Seventeen predicted molecules were significantly higher in abundance (p < 0.05) when compared to both the negative controls and sample means. Validation of two identified proteins (cyclophilin A [nonactin-associated] and transgelin [actin-associated]) by immunolocalization was used to confirm our analysis, and both showed enrichment at EPEC pedestals. The EPEC pedestal concentration technique developed here together with the identification of novel pedestal proteins not only provides a resource for the further characterization of molecular components within these structures but also demonstrates that EPEC pedestals can be used as a model system for the identification of novel functions of proteins not normally thought to be at actin-based structures.

Giardia duodenalis-induced alterations of commensal bacteria kill Caenorhabditis elegans: a new model to study microbial-microbial interactions in the gut

Giardia duodenalis is the most common cause of parasitic diarrhea worldwide and a well-established risk factor for postinfectious irritable bowel syndrome. We hypothesized that Giardia-induced disruptions in host-microbiota interactions may play a role in the pathogenesis of giardiasis and in postgiardiasis disease. Functional changes induced by Giardia in commensal bacteria and the resulting effects on Caenorhabditis elegans were determined. Although Giardia or bacteria alone did not affect worm viability, combining commensal Escherichia coli bacteria with Giardia became lethal to C. elegans. Giardia also induced killing of C. elegans with attenuated Citrobacter rodentium espF and map mutant strains, human microbiota from a healthy donor, and microbiota from inflamed colonic sites of ulcerative colitis patient. In contrast, combinations of Giardia with microbiota from noninflamed sites of the same patient allowed for worm survival. The synergistic lethal effects of Giardia and E. coli required the presence of live bacteria and were associated with the facilitation of bacterial colonization in the C. elegans intestine. Exposure to C. elegans and/or Giardia altered the expression of 172 genes in E. coli. The genes affected by Giardia included hydrogen sulfide biosynthesis (HSB) genes, and deletion of a positive regulator of HSB genes, cysB, was sufficient to kill C. elegans even in the absence of Giardia. Our findings indicate that Giardia induces functional changes in commensal bacteria, possibly making them opportunistic pathogens, and alters host-microbe homeostatic interactions. This report describes the use of a novel in vivo model to assess the toxicity of human microbiota.

The study of n-3PUFAs protecting the intestinal barrier in rat HS/R model

BACKGROUND

N-3 PUFAs have been demonstrated in vitro it could prevent the intestinal tight junctions (TJs) from the ischemia/re-perfusion injury and the inflammatory reaction injury. The purpose of this study was to evaluate the protection of n-3 PUFAs on the intestinal TJs in the rat model of hemorrhagic shock followed by resuscitation.

METHODS

Male SD rats (n = 72; 250 ~ 300 g) were randomly divided into 6 groups: SHAM, hemorrhagic shock (HS), hemorrhagic shock/resuscitation (HS/R), ω-6 group, ω-3 group and ω-3 treatment group. Shock was induced, and a mean arterial pressure was maintained at 35 to 40 mmHg for 60 minutes. Resuscitation was carried out by returning half of the shed blood and Ringer's lactate solution. In ω-6 and ω-3 group, Intralipid or fish oil (0.2 g/Kg), respectively, was infused 30 minutes after shock. And fish oil was infused with resuscitation in ω-3 treatment group. Half of each group was killed at 30 minutes and 4 hours after resuscitation, respectively. The serum samples and the intestinal sample was collected for further examination.

RESULT

There is no difference between ω-3, ω-3 treatment and sham group in Chiu's score, but the other three groups have higher scores than they did. Compared with HS, HSR and ω-6 group, ω-3 and ω-3 treatment group showed most intact in intestinal mucoscal villi and TJs through HE, SEM and LSCM. The levels of IL-6 and TNF-α of bowel tissue in ω-3 and ω-3 treatment group were significantly lower than HS and HSR groups'. At the time point of 30 min, the levels of serum endotoxin were dramatically higher in HS、 HSR and ω-6 groups when compared with ω-3, ω-3 treatment and sham group. However, it in ω-3 group was greater than sham and HS group until 4 hours.

CONCLUSION

Fish oil pretreatment before resuscitation showed a beneficial effect to the intestinal TJs and atteunated inflammation after H/R in HS/R rat model and is better than ω-6 PUFAs did.

Rhubarb tannins extract inhibits the expression of aquaporins 2 and 3 in magnesium sulphate-induced diarrhoea model

Tannins, a group of major active components of Chinese rhubarb and widely distributed in nature, have a significant antidiarrhoeal activity. Aquaporins (AQPs) 2 and 3 play important roles in regulating water transfer during diarrhoea. The present study aims to determine the effect of the total tannins extract of rhubarb on aquaporins (AQPs) 2 and 3 in diarrhoea mice and HT-29 cells both induced by magnesium sulphate (MgSO₄). Our results showed that rhubarb tannins extract (RTE) significantly decreased the faecal water content in colon and evaluation index of defecation of diarrhoea mice. Interestingly, RTE could markedly reduce the mRNA and protein expression levels of AQPs 2 and 3 in apical and lateral mucosal epithelial cells in the colons of diarrhoea mice and HT-29 cells both induced by MgSO₄ in a dose-dependent manner. Furthermore, RTE suppressed the production of cyclic monophosphate- (cAMP-) dependent protein kinase A catalytic subunits α (PKA C-α) and phosphorylated cAMP response element-binding protein (p-CREB, Ser133) in MgSO₄-induced HT-29 cells. Our data showed for the first time that RTE inhibit AQPs 2 and 3 expression in vivo and in vitro via downregulating PKA/p-CREB signal pathway, which accounts for the antidiarrhoeal effect of RTE.

Enteropathogenic E. coli effectors EspG1/G2 disrupt microtubules, contribute to tight junction perturbation and inhibit restoration

Enteropathogenic Escherichia coli (EPEC) uses a type 3 secretion system to transfer effector proteins into the host intestinal epithelial cell. Several effector molecules contribute to tight junction disruption including EspG1 and its homologue EspG2 via a mechanism thought to involve microtubule destruction. The aim of this study was to investigate the contribution of EspG-mediated microtubule disruption to TJ perturbation. We demonstrate that wild type EPEC infection disassembles microtubules and induces the progressive movement of occludin away from the membrane and into the cytosol. Deletion of espG1/G2 attenuates both of these phenotypes. In addition, EPEC infection impedes barrier recovery from calcium switch, suggesting that inhibition of TJ restoration, not merely disruption, prolongs barrier loss. TJs recover more rapidly following infection with ΔespG1/G2 than with wild type EPEC, demonstrating that EspG1/G2 perpetuate barrier loss. Although EspG regulates ADP-ribosylation factor (ARF) and p21-activated kinase (PAK), these activities are not necessary for microtubule destruction or perturbation of TJ structure and function. These data strongly support a role for EspG1/G2 and its associated effects on microtubules in delaying the recovery of damaged tight junctions caused by EPEC infection.

Citrobacter rodentium: infection, inflammation and the microbiota

Citrobacter rodentium is a mucosal pathogen of mice that shares several pathogenic mechanisms with enteropathogenic Escherichia coli (EPEC) and enterohaemorrhagic E. coli (EHEC), which are two clinically important human gastrointestinal pathogens. Thus, C. rodentium has long been used as a model to understand the molecular basis of EPEC and EHEC infection in vivo. In this Review, we discuss recent studies in which C. rodentium has been used to study mucosal immunology, including the deregulation of intestinal inflammatory responses during bacteria-induced colitis and the role of the intestinal microbiota in mediating resistance to colonization by enteric pathogens. These insights should help to elucidate the roles of mucosal inflammatory responses and the microbiota in the virulence of enteric pathogens.

Epithelial microvilli establish an electrostatic barrier to microbial adhesion

Microvilli are membrane extensions on the apical surface of polarized epithelia, such as intestinal enterocytes and tubule and duct epithelia. One notable exception in mucosal epithelia is M cells, which are specialized for capturing luminal microbial particles; M cells display a unique apical membrane lacking microvilli. Based on studies of M cell uptake under different ionic conditions, we hypothesized that microvilli may augment the mucosal barrier by providing an increased surface charge density from the increased membrane surface and associated glycoproteins. Thus, electrostatic charges may repel microbes from epithelial cells bearing microvilli, while M cells are more susceptible to microbial adhesion. To test the role of microvilli in bacterial adhesion and uptake, we developed polarized intestinal epithelial cells with reduced microvilli ("microvillus-minus," or MVM) but retaining normal tight junctions. When tested for interactions with microbial particles in suspension, MVM cells showed greatly enhanced adhesion and uptake of particles compared to microvillus-positive cells. This preference showed a linear relationship to bacterial surface charge, suggesting that microvilli resist binding of microbes by using electrostatic repulsion. Moreover, this predicts that pathogen modification of electrostatic forces may contribute directly to virulence. Accordingly, the effacement effector protein Tir from enterohemorrhagic Escherichia coli O157:H7 expressed in epithelial cells induced a loss of microvilli with consequent enhanced microbial binding. These results provide a new context for microvillus function in the host-pathogen relationship, based on electrostatic interactions.

Vitamin D deficiency promotes epithelial barrier dysfunction and intestinal inflammation

BACKGROUND

Vitamin D, an important modulator of the immune system, has been shown to protect mucosal barrier homeostasis. This study investigates the effects of vitamin D deficiency on infection-induced changes in intestinal epithelial barrier function in vitro and on Citrobacter rodentium-induced colitis in mice.

METHODS

Polarized epithelial Caco2-bbe cells were grown in medium with or without vitamin D and challenged with enterohemorrhagic Escherichia coli O157:H7. Barrier function and tight junction protein expression were assessed. Weaned C57BL/6 mice were fed either a vitamin D-sufficient or vitamin D-deficient diet and then infected with C. rodentium. Disease severity was assessed by histological analysis, intestinal permeability assay, measurement of inflammatory cytokine levels, and microbiome analysis.

RESULTS

1,25(OH)2D3 altered E. coli O157:H7-induced reductions in transepithelial electrical resistance (P < .01), decreased permeability (P < .05), and preserved barrier integrity. Vitamin D-deficient mice challenged with C. rodentium demonstrated increased colonic hyperplasia and epithelial barrier dysfunction (P < .0001 and P < .05, respectively). Vitamin D deficiency resulted in an altered composition of the fecal microbiome both in the absence and presence of C. rodentium infection.

CONCLUSIONS

This study demonstrates that vitamin D is an important mediator of intestinal epithelial defenses against infectious agents. Vitamin D deficiency predisposes to more-severe intestinal injury in an infectious model of colitis.

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.

In vitro exposure to Escherichia coli decreases ion conductance in the jejunal epithelium of broiler chickens

Escherichia coli (E. coli) infections are very widespread in poultry. However, little is known about the interaction between the intestinal epithelium and E. coli in chickens. Therefore, the effects of avian non-pathogenic and avian pathogenic Escherichia coli (APEC) on the intestinal function of broiler chickens were investigated by measuring the electrogenic ion transport across the isolated jejunal mucosa. In addition, the intestinal epithelial responses to cholera toxin, histamine and carbamoylcholine (carbachol) were evaluated following an E. coli exposure. Jejunal tissues from 5-week-old broilers were exposed to 6×10(8) CFU/mL of either avian non-pathogenic E. coli IMT11322 (Ont:H16) or avian pathogenic E. coli IMT4529 (O24:H4) in Ussing chambers and electrophysiological variables were monitored for 1 h. After incubation with E. coli for 1 h, either cholera toxin (1 mg/L), histamine (100 μM) or carbachol (100 μM) were added to the incubation medium. Both strains of avian E. coli (non-pathogenic and pathogenic) reduced epithelial ion conductance (Gt) and short-circuit current (Isc). The decrease in ion conductance after exposure to avian pathogenic E. coli was, at least, partly reversed by the histamine or carbachol treatment. Serosal histamine application produced no significant changes in the Isc in any tissues. Only the uninfected control tissues responded significantly to carbachol with an increase of Isc, while the response to carbachol was blunted to non-significant values in infected tissues. Together, these data may explain why chickens rarely respond to intestinal infections with overt secretory diarrhea. Instead, the immediate response to intestinal E. coli infections appears to be a tightening of the epithelial barrier.

Vagus nerve stimulation attenuates intestinal epithelial tight junctions disruption in endotoxemic mice through α7 nicotinic acetylcholine receptors

We tested the effect of vagus nerve stimulation in endotoxin-induced intestinal tight junction injury in mice challenged with lipopolysaccharide (LPS) and examined the role of α7 nicotinic acetylcholine receptors (α7nAchR) in this process. Endotoxemia was induced by intraperitoneal injection of LPS (10 mg/kg) in male Balb/c mice. Samples were collected 12 h after LPS treatment. Endotoxemia was associated with intestinal barrier dysfunction, as evidenced by increased amount of fluorescein isothiocyanate-dextran in circulation. Western blot and immunofluorescence was performed, and the results demonstrated decreased expression of occludin and zonula occludens 1 along intestinal epithelium in endotoxemic mice. The ultrastructure of tight junction was disrupted as shown by transmission electron microscopy, which was associated with increased intestinal permeability. Stimulation of the right cervical vagus nerve ameliorated the damage of tight junction ultrastructure, which was consistent with decreased permeability to fluorescein isothiocyanate-dextran, and also reversed the decreased expression of tight junction proteins occludin and zonula occludens 1. Vagus nerve stimulation inhibited the upregulated activity of myosin light chain kinase and nuclear factor κB. In contrast, α-bungarotoxin (a specific α7nAchR antagonist, 0.1 μg/mouse) administered before vagus nerve stimulation significantly abolished these protective effects of vagus nerve stimulation. Our results for the first time confirmed that vagus nerve stimulation attenuated the disruption of tight junction in intestinal epithelium in endotoxemic mice, which was mediated through suppressing translocation of nuclear factor κB p65, downregulating myosin light chain kinase, and the α7nAchR may play an important role in this process.

Protein kinase C mediates enterohemorrhagic Escherichia coli O157:H7-induced attaching and effacing lesions

Enterohemorrhagic Escherichia coli serotype O157:H7 causes outbreaks of diarrhea, hemorrhagic colitis, and the hemolytic-uremic syndrome. E. coli O157:H7 intimately attaches to epithelial cells, effaces microvilli, and recruits F-actin into pedestals to form attaching and effacing lesions. Lipid rafts serve as signal transduction platforms that mediate microbe-host interactions. The aims of this study were to determine if protein kinase C (PKC) is recruited to lipid rafts in response to E. coli O157:H7 infection and what role it plays in attaching and effacing lesion formation. HEp-2 and intestine 407 tissue culture epithelial cells were challenged with E. coli O157:H7, and cell protein extracts were then separated by buoyant density ultracentrifugation to isolate lipid rafts. Immunoblotting for PKC was performed, and localization in lipid rafts was confirmed with an anti-caveolin-1 antibody. Isoform-specific PKC small interfering RNA (siRNA) was used to determine the role of PKC in E. coli O157:H7-induced attaching and effacing lesions. In contrast to uninfected cells, PKC was recruited to lipid rafts in response to E. coli O157:H7. Metabolically active bacteria and cells with intact lipid rafts were necessary for the recruitment of PKC. PKC recruitment was independent of the intimin gene, type III secretion system, and the production of Shiga toxins. Inhibition studies, using myristoylated PKCζ pseudosubstrate, revealed that atypical PKC isoforms were activated in response to the pathogen. Pretreating cells with isoform-specific PKC siRNA showed that PKCζ plays a role in E. coli O157:H7-induced attaching and effacing lesions. We concluded that lipid rafts mediate atypical PKC signal transduction responses to E. coli O157:H7. These findings contribute further to the understanding of the complex array of microbe-eukaryotic cell interactions that occur in response to infection.

Effect of Lactobacillus plantarum on diarrhea and intestinal barrier function of young piglets challenged with enterotoxigenic Escherichia coli K88

The present study was performed to investigate the preventative effect of Lactobacillus plantarum on diarrhea in relation to intestinal barrier function in young piglets challenged with enterotoxigenic Escherichia coli (ETEC) K88. Seventy-two male piglets (4 d old) were assigned to 2 diets (antibiotic-free basal diet with or without L. plantarum, 5 × 10(10) cfu/kg diet) and subsequently challenged or not with ETEC K88 (1 × 10(8) cfu per pig) on d 15 in a 2 × 2 factorial arrangement of treatments. Feed intake and BW were measured on d 15 and 18 (3 d after challenge) for determination of growth performance. On d 18, 1 piglet from each pen was slaughtered to evaluate small intestinal morphology and expression of tight junction proteins at the mRNA and protein levels while another piglet was used for the intestinal permeability test. Before and after ETEC K88 challenge, piglets fed L. plantarum had greater BW, ADG, and ADFI (P < 0.05) and marginally greater G:F (P < 0.10) compared to piglets fed the unsupplemented diet. After ETEC K88 challenge, the challenged piglets did not show an impaired growth performance but had greater incidence of diarrhea compared to the nonchallenged piglets. There was an interaction between dietary L. plantarum and ETEC K88 challenge (P < 0.05) as L. plantarum prevented the ETEC K88-induced diarrhea. Piglets challenged with ETEC K88 also had greater urinary lactulose:mannitol and plasma concentration of endotoxin, shorter villi, deeper crypt depth, and reduced villous height:crypt depth in the duodenum and jejunum and decreased zonula occludens-1 mRNA and occludin mRNA and protein expression in the jejunum (P < 0.05). These deleterious effects caused by ETEC K88 were inhibited by feeding L. plantarum (P < 0.05). There were no effects of either treatment on the morphology and expression of tight junction proteins in ileum. In conclusion, L. plantarum, given to piglets in early life, improved performance and effectively prevented the diarrhea in young piglets induced by ETEC K88 challenge by improving function of the intestinal barrier by protecting intestinal morphology and intestinal permeability and the expression of genes for tight junction proteins (zonula occludens-1 and occludin).

A balanced IL-1β activity is required for host response to Citrobacter rodentium infection

Microbial sensing plays essential roles in the innate immune response to pathogens. In particular, NLRP3 forms a multiprotein inflammasome complex responsible for the maturation of interleukin (IL)-1β. Our aim was to delineate the role of the NLRP3 inflammasome in macrophages, and the contribution of IL-1β to the host defense against Citrobacter rodentium acute infection in mice. Nlrp3(-/-) and background C57BL/6 (WT) mice were infected by orogastric gavage, received IL-1β (0.5 µg/mouse; ip) on 0, 2, and 4 days post-infection (DPI), and assessed on 6 and 10 DPI. Infected Nlrp3(-/-) mice developed severe colitis; IL-1β treatments reduced colonization, abrogated dissemination of bacteria to mesenteric lymph nodes, and protected epithelial integrity of infected Nlrp3(-/-) mice. In contrast, IL-1β treatments of WT mice had an opposite effect with increased penetration of bacteria and barrier disruption. Microscopy showed reduced damage in Nlrp3(-/-) mice, and increased severity of disease in WT mice with IL-1β treatments, in particular on 10 DPI. Secretion of some pro-inflammatory plasma cytokines was dissipated in Nlrp3(-/-) compared to WT mice. IL-1β treatments elevated macrophage infiltration into infected crypts in Nlrp3(-/-) mice, suggesting that IL-1β may improve macrophage function, as exogenous administration of IL-1β increased phagocytosis of C. rodentium by peritoneal Nlrp3(-/-) macrophages in vitro. As well, the exogenous administration of IL-1β to WT peritoneal macrophages damaged the epithelial barrier of C. rodentium-infected polarized CMT-93 cells. Treatment of Nlrp3(-/-) mice with IL-1β seems to confer protection against C. rodentium infection by reducing colonization, protecting epithelial integrity, and improving macrophage activity, while extraneous IL-1β appeared to be detrimental to WT mice. Together, these findings highlight the importance of balanced cytokine responses as IL-1β improved bacterial clearance in Nlrp3(-/-) mice but increased tissue damage when given to WT mice.

Berberine Reduces Rat Intestinal Tight Junction Injury Induced by Ischemia–Reperfusion Associated with the Suppression of Inducible Nitric Oxide Synthesis

Berberine (BBR) has been shown to attenuate the deleterious effects of ischemia/reperfusion (I/R) injury in the brain. We evaluated the effects of BBR on intestinal tight junction (TJ) changes during mesenteric I/R. I/R was induced in rats by the occlusion of the superior mesenteric artery and reperfusion. The rats were randomized into four groups: control, BBR, I/R, and I/R + BBR. Intestinal permeability was determined by the lactulose/mannitol test. The ileum and colon were harvested to assess mucosal injury and inducible nitric oxide synthase activity. The TJ ultrastructure was studied by transmission electron microscopy. The expressions and locations of the TJ proteins, occludin and ZO-1, in the epithelium were investigated by immunofluorescence microscopy. We also used Western blot analysis to detect the distribution of TJ proteins in lipid raft fractions. Our results suggest that I/R-induced intestinal TJ dysfunction can be improved by BBR, thereby demonstrating the therapeutic potential of BBR for intestinal I/R.

Human oral isolate Lactobacillus fermentum AGR1487 reduces intestinal barrier integrity by increasing the turnover of microtubules in Caco-2 cells

Lactobacillus fermentum is found in fermented foods and thought to be harmless. In vivo and clinical studies indicate that some L. fermentum strains have beneficial properties, particularly for gastrointestinal health. However, L. fermentum AGR1487 decreases trans-epithelial electrical resistance (TEER), a measure of intestinal barrier integrity. The hypothesis was that L. fermentum AGR1487 decreases the expression of intestinal cell tight junction genes and proteins, thereby reducing barrier integrity. Transcriptomic and proteomic analyses of Caco-2 cells (model of human intestinal epithelial cells) treated with L. fermentum AGR1487 were used to obtain a global view of the effect of the bacterium on intestinal epithelial cells. Specific functional characteristics by which L. fermentum AGR1487 reduces intestinal barrier integrity were examined using confocal microscopy, cell cycle progression and adherence bioassays. The effects of TEER-enhancing L. fermentum AGR1485 were investigated for comparison. L. fermentum AGR1487 did not alter the expression of Caco-2 cell tight junction genes (compared to L. fermentum AGR1485) and tight junction proteins were not able to be detected. However, L. fermentum AGR1487 increased the expression levels of seven tubulin genes and the abundance of three microtubule-associated proteins, which have been linked to tight junction disassembly. Additionally, Caco-2 cells treated with L. fermentum AGR1487 did not have defined and uniform borders of zona occludens 2 around each cell, unlike control or AGR1485 treated cells. L. fermentum AGR1487 cells were required for the negative effect on barrier integrity (bacterial supernatant did not cause a decrease in TEER), suggesting that a physical interaction may be necessary. Increased adherence of L. fermentum AGR1487 to Caco-2 cells (compared to L. fermentum AGR1485) was likely to facilitate this cell-to-cell interaction. These findings illustrate that bacterial strains of the same species can cause contrasting host responses and suggest that food-safe status should be given to individual strains not species.

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.

Recent advances in understanding enteric pathogenic Escherichia coli

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

Guanylate cyclase C limits systemic dissemination of a murine enteric pathogen

Background

Guanylate Cyclase C (GC-C) is an apically-oriented transmembrane receptor that is expressed on epithelial cells of the intestine. Activation of GC-C by the endogenous ligands guanylin or uroguanylin elevates intracellular cGMP and is implicated in intestinal ion secretion, cell proliferation, apoptosis, intestinal barrier function, as well as the susceptibility of the intestine to inflammation. Our aim was to determine if GC-C is required for host defense during infection by the murine enteric pathogen Citrobacter rodentium of the family Enterobacteriacea.

Methods

GC-C^+/+ control mice or those having GC-C genetically ablated (GC-C^−/−) were administered C. rodentium by orogastric gavage and analyzed at multiple time points up to post-infection day 20. Commensal bacteria were characterized in uninfected GC-C^+/+ and GC-C^−/− mice using 16S rRNA PCR analysis.

Results

GC-C^−/− mice had an increase in C. rodentium bacterial load in stool relative to GC-C^+/+. C. rodentium infection strongly decreased guanylin expression in GC-C^+/+ mice and, to an even greater degree, in GC-C^−/− animals. Fluorescent tracer studies indicated that mice lacking GC-C, unlike GC-C^+/+ animals, had a substantial loss of intestinal barrier function early in the course of infection. Epithelial cell apoptosis was significantly increased in GC-C^−/− mice following 10 days of infection and this was associated with increased frequency and numbers of C. rodentium translocation out of the intestine. Infection led to significant liver histopathology in GC-C^−/− mice as well as lymphocyte infiltration and elevated cytokine and chemokine expression. Relative to naïve GC-C^+/+ mice, the commensal microflora load in uninfected GC-C^−/− mice was decreased and bacterial composition was imbalanced and included outgrowth of the Enterobacteriacea family.

Conclusions

This work demonstrates the novel finding that GC-C signaling is an essential component of host defense during murine enteric infection by reducing bacterial load and preventing systemic dissemination of attaching/effacing-lesion forming bacterial pathogens such as C. rodentium.

The inhibition of COPII trafficking is important for intestinal epithelial tight junction disruption during enteropathogenic Escherichia coli and Citrobacter rodentium infection

Enterohemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC) are bacterial pathogens that cause severe illnesses in humans. Citrobacter rodentium is a related mouse pathogen that serves as a small animal model for EPEC and EHEC infections. EPEC, EHEC and C. rodentium translocate bacterial virulence proteins directly into host intestinal cells via a type III secretion system (T3SS). Non-LEE-encoded effector A (NleA) is a T3SS effector that is common to EPEC, EHEC and C. rodentium. NleA interacts with and inhibits the mammalian COPII complex, impairing cellular secretion; this interaction is required for bacterial virulence. Although diarrhea is a hallmark of EPEC, EHEC and C. rodentium infections, the underlying mechanisms are not well characterized. One of the essential functions of the intestine is to maintain a barrier between the lumen and submucosa. Tight junctions seal the space between adjacent epithelial cells creating this barrier. Consequently, it is thought that the disruption of intestinal epithelial tight junctions by EPEC, EHEC, and C. rodentium could result in a loss of barrier function. In this study, we demonstrate that NleA mediated COPII inhibition is required for EPEC- and C. rodentium-mediated disruption of tight junction proteins and increases in fecal water content.

Depolymerization of cytokeratin intermediate filaments facilitates intracellular infection of HeLa cells by Bartonella henselae

Bartonella henselae is capable of invading epithelial and endothelial cells by modulating the function of actin-dependent cytoskeleton proteins. Although understanding of the pathogenesis has been increased by the development of an in vitro infection model involving endothelial cells, little is known about the mechanism of interaction between B. henselae and epithelial cells. This study aims to identify the binding candidates of B. henselae in epithelial cells and explores their effect on B. henselae infection. Pull-down assays and mass spectrometry analysis confirmed that some of the binding proteins (keratin 14, keratin 6, and F-actin) are cytoskeleton associated. B. henselae infection significantly induces the expression of the cytokeratin genes. Chemical disruption of the keratin network by using ethylene glycol tetraacetic acid promotes the intracellular persistence of B. henselae in HeLa cells. However, cytochalasin B and phalloidin treatment inhibits B. henselae invasion. Immunofluorescent staining demonstrates that B. henselae infection induces an F-actin-dependent rearrangement of the cytoskeleton. However, we demonstrated via immunofluorescent staining and whole-mount cell electron microscopy that keratin intermediate filaments are depolymerized by B. henselae. The results indicate that B. henselae achieves an intracellular persistence in epithelial cells through the depolymerization of cytokeratin intermediate filaments that are protective against B. henselae invasion.

Intestinal antimicrobial peptides during homeostasis, infection, and disease

Antimicrobial peptides (AMPs), including defensins and cathelicidins, constitute an arsenal of innate regulators of paramount importance in the gut. The intestinal epithelium is exposed to myriad of enteric pathogens and these endogenous peptides are essential to fend off microbes and protect against infections. It is becoming increasingly evident that AMPs shape the composition of the commensal microbiota and help maintain intestinal homeostasis. They contribute to innate immunity, hence playing important functions in health and disease. AMP expression is tightly controlled by the engagement of pattern recognition receptors (PRRs) and their impairment is linked to abnormal host responses to infection and inflammatory bowel diseases (IBD). In this review, we provide an overview of the mucosal immune barriers and the intricate crosstalk between the host and the microbiota during homeostasis. We focus on the AMPs and pay particular attention to how PRRs promote their secretion in the intestine. Furthermore, we discuss their production and main functions in three different scenarios, at steady state, throughout infection with enteric pathogens and IBD.

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.

Diarrhea and colitis in mice require the Salmonella pathogenicity island 2-encoded secretion function but not SifA or Spv effectors

We investigated the roles of Salmonella pathogenicity island 2 (SPI-2) and two SPI-2 effectors in Salmonella colitis and diarrhea in genetically resistant BALB/c.D2(Slc11a1) congenic mice with the wild-type Nramp1 locus. Wild-type Salmonella enterica serovar Typhimurium 14028s caused a pan-colitis, and the infected mice developed frank diarrhea with a doubling of the fecal water content. An ssaV mutant caused only a 26% increase in fecal water content, without producing the pathological changes of colitis, and it did not cause weight loss over a 1-week period of observation. However, two SPI-2 effector mutants, the spvB and sifA mutants, and a double spvB sifA mutant caused diarrhea and colitis, even though the sifA mutant was sensitive to killing by bone marrow-derived macrophages from BALB/c.D2 mice and was severely impaired in extraintestinal growth but not in growth in the cecum. These results demonstrate that systemic S. enterica infection and diarrhea/colitis are distinct pathogenic processes and that only the former requires spvB and sifA.

Probiotics are effective for the prevention and treatment of Citrobacter rodentium-induced colitis in mice

BACKGROUND

Probiotics prevent disease induced by Citrobacter rodentium, a murine-specific enteric pathogen. Whether probiotics can be used to interrupt the infectious process following initiation of infection was determined.

METHODS

C57BL/6 adult and neonatal mice were challenged with C. rodentium, and a probiotic mixture containing Lactobacillus helveticus and Lactobacillus rhamnosus was provided 1 week before bacterial challenge, concurrently with infection, or 3 days and 6 days after infection. Mice were sacrificed 10 days after infection, and disease severity was assessed by histological analysis and in vivo intestinal permeability assay. Inflammatory pathways and the composition of the fecal microbiome were assessed in adult mice.

RESULTS

Preadministration and coadministration of probiotics ameliorated C. rodentium-induced barrier dysfunction, epithelial hyperplasia, and binding of the pathogen to host colonocytes in adults, with similar findings in neonatal mice. Upregulated tumor necrosis factor α and interferon γ transcripts were suppressed in the pretreated probiotic group, whereas interleukin 17 transcription was suppressed with probiotics given up to 3 days after infection. Probiotics promoted transcription of interleukin 10 and FOXP3, and increased follicular T-regulatory cells in pretreatment mice. C. rodentium infection resulted in an altered fecal microbiome, which was normalized with probiotic intervention.

CONCLUSIONS

This study provides evidence that probiotics can prevent illness and treat disease in an animal model of infectious colitis.

Inflammation and disintegration of intestinal villi in an experimental model for Vibrio parahaemolyticus-induced diarrhea

Vibrio parahaemolyticus is a leading cause of seafood-borne gastroenteritis in many parts of the world, but there is limited knowledge of the pathogenesis of V. parahaemolyticus-induced diarrhea. The absence of an oral infection-based small animal model to study V. parahaemolyticus intestinal colonization and disease has constrained analyses of the course of infection and the factors that mediate it. Here, we demonstrate that infant rabbits oro-gastrically inoculated with V. parahaemolyticus develop severe diarrhea and enteritis, the main clinical and pathologic manifestations of disease in infected individuals. The pathogen principally colonizes the distal small intestine, and this colonization is dependent upon type III secretion system 2. The distal small intestine is also the major site of V. parahaemolyticus-induced tissue damage, reduced epithelial barrier function, and inflammation, suggesting that disease in this region of the gastrointestinal tract accounts for most of the diarrhea that accompanies V. parahaemolyticus infection. Infection appears to proceed through a characteristic sequence of steps that includes remarkable elongation of microvilli and the formation of V. parahaemolyticus-filled cavities within the epithelial surface, and culminates in villus disruption. Both depletion of epithelial cell cytoplasm and epithelial cell extrusion contribute to formation of the cavities in the epithelial surface. V. parahaemolyticus also induces proliferation of epithelial cells and recruitment of inflammatory cells, both of which occur before wide-spread damage to the epithelium is evident. Collectively, our findings suggest that V. parahaemolyticus damages the host intestine and elicits disease via previously undescribed processes and mechanisms.

The marine bacterium Vibrio parahaemolyticus is a leading cause worldwide of gastroenteritis linked to the consumption of contaminated seafood. Despite the prevalence of V. parahaemolyticus-induced gastroenteritis, there is limited understanding of how this pathogen causes disease in the intestine. In part, the paucity of knowledge results from the absence of an oral infection-based animal model of the human disease. We developed a simple oral infection-based infant rabbit model of V. parahaemolyticus-induced intestinal pathology and diarrhea. This experimental model enabled us to define several previously unknown but key features of the pathology elicited by this organism. We found that V. parahaemolyticus chiefly colonizes the distal small intestine and that the organism's second type III secretion system is essential for colonization. The epithelial surface of the distal small intestine is also the major site of V. parahaemolyticus-induced damage, which arises via a characteristic sequence of events culminating in the formation of V. parahaemolyticus-filled cavities in the epithelial surface. This experimental model will transform future studies aimed at deciphering the bacterial and host factors/processes that contribute to disease, as well as enable testing of new therapeutics to prevent and/or combat infection.

Escherichia coli-induced epithelial hyporesponsiveness to secretagogues is associated with altered CFTR localization

Both pathogenic and commensal strains of Escherichia coli colonize the human intestinal tract. Pathogenic strains differ only in the expression of virulence factors, many of which comprise a type III secretion system (TTSS). Little is known regarding the effect of E. coli on the intestinal epithelial response to the secretagogues that drive ion secretion, despite its importance in causing clinically significant diarrhoea. Using Ussing chambers to measure electrogenic ion transport of T84 intestinal epithelial cell monolayers, we found that all strains of E. coli tested (pathogenic, commensal, probiotic and lab strain) significantly reduced cAMP-dependent ion secretion after 4-8 h exposure. Enteropathogenic E. coli mutants lacking a functional TTSS caused similar hyposecretion while not causing significant apoptosis (as shown by caspase-3 cleavage) or necrosis (lactate dehydrogenase release), as did the commensal strain F18, indicating that epithelial cell death was not the cause of hyposecretion. Enteropathogenic E. coli and the TTSS mutant significantly reduced cell surface expression of the apical anion channel, cystic fibrosis transmembrane conductance regulator, which is likely the mechanism behind the pathogen-induced hyposecretion. However, F18 did not cause cystic fibrosis transmembrane conductance regulator mislocalization and the commensal-induced mechanism remains unclear.

Impenetrable barriers or entry portals? The role of cell-cell adhesion during infection

Cell-cell adhesion plays a fundamental role in cell polarity and organogenesis. It also contributes to the formation and establishment of physical barriers against microbial infections. However, a large number of pathogens, from viruses to bacteria and parasites, have developed countless strategies to specifically target cell adhesion molecules in order to adhere to and invade epithelial cells, disrupt epithelial integrity, and access deeper tissues for dissemination. The study of all these processes has contributed to the characterization of molecular machineries at the junctions of eukaryotic cells that have been better understood by using pathogens as probes.

Calpain mediates epithelial cell microvillar effacement by enterohemorrhagic Escherichia coli

A member of the attaching and effacing (AE) family of pathogens, enterohemorrhagic Escherichia coli (EHEC) induces dramatic changes to the intestinal cell cytoskeleton, including effacement of microvilli. Effacement by the related pathogen enteropathogenic E. coli (EPEC) requires the activity of the Ca⁺²-dependent host protease, calpain, which participates in a variety of cellular processes, including cell adhesion and motility. We found that EHEC infection results in an increase in epithelial (CaCo-2a) cell calpain activity and that EHEC-induced microvillar effacement was blocked by ectopic expression of calpastatin, an endogenous calpain inhibitor, or by pretreatment of intestinal cells with a cell-penetrating version of calpastatin. In addition, ezrin, a known calpain substrate that links the plasma membrane to axial actin filaments in microvilli, was cleaved in a calpain-dependent manner during EHEC infection and lost from its normal locale within microvilli. Calpain may be a central conduit through which EHEC and other AE pathogens induce enterocyte cytoskeletal remodeling and exert their pathogenic effects.

The deubiquitinase activity of the Salmonella pathogenicity island 2 effector, SseL, prevents accumulation of cellular lipid droplets

To cause disease, Salmonella enterica serovar Typhimurium requires two type III secretion systems that are encoded by Salmonella pathogenicity islands 1 and 2 (SPI-1 and -2). These secretion systems serve to deliver specialized proteins (effectors) into the host cell cytosol. While the importance of these effectors to promote colonization and replication within the host has been established, the specific roles of individual secreted effectors in the disease process are not well understood. In this study, we used an in vivo gallbladder epithelial cell infection model to study the function of the SPI-2-encoded type III effector, SseL. The deletion of the sseL gene resulted in bacterial filamentation and elongation and the unusual localization of Salmonella within infected epithelial cells. Infection with the ΔsseL strain also caused dramatic changes in host cell lipid metabolism and led to the massive accumulation of lipid droplets in infected cells. This phenotype was directly attributable to the deubiquitinase activity of SseL, as a Salmonella strain carrying a single point mutation in the catalytic cysteine also resulted in extensive lipid droplet accumulation. The excessive buildup of lipids due to the absence of a functional sseL gene also was observed in murine livers during S. Typhimurium infection. These results suggest that SseL alters host lipid metabolism in infected epithelial cells by modifying the ubiquitination patterns of cellular targets.

Berberine ameliorates intestinal epithelial tight-junction damage and down-regulates myosin light chain kinase pathways in a mouse model of endotoxinemia

BACKGROUND

This study aimed to examine the protective effect of berberine in endotoxin-induced intestinal tight-junction injury in a mice model of endotoxinemia.

METHODS

Endotoxinemia was induced by intraperitoneal injection of lipopolysaccharide (10 mg/kg). Mice were randomized to 5 groups: control mice, berberine-treated mice, lipopolysaccharide (LPS)-injected mice, mice pretreated with berberine, and mice administered berberine following LPS injection. Samples were collected 12 h after LPS treatment.

RESULTS

Ileal mucosal permeability to fluorescein isothiocyanate dextran assay indicated that berberine reduced the permeability of the gut barrier in endotoxinemia. Transmission electron microscopy revealed that pretreatement with berberine partly prevented ultrastructural disruption of tight junctions by LPS. Immunofluorescence and Western blot analysis were performed, and the results demonstrated that pretreatement with berberine partially reversed the redistribution of tight-junction proteins in colon epithelium and in membrane microdomains. Our data also indicated that pretreatement with berberine could suppress translocation, from cytoplasm to the nucleus, of nuclear factor-κB and myosin light chain kinase activation in the intestinal epithelium.

CONCLUSIONS

Pretreatement with berberine attenuates disruption of tight junctions in intestinal epithelium in a mice model of endotoxinemia. This may possibly have been mediated through down-regulation of the nuclear factor-κB and myosin light chain kinase pathway.

Enteropathogenic and enterohaemorrhagic Escherichia coli: even more subversive elements

The human pathogens enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC) share a unique mechanism of colonization that results from the concerted action of effector proteins translocated into the host cell by a type III secretion system (T3SS). EPEC and EHEC not only induce characteristic attaching and effacing (A/E) lesions, but also subvert multiple host cell signalling pathways during infection. Our understanding of the mechanisms by which A/E pathogens hijack host cell signalling has advanced dramatically in recent months with the identification of novel activities for many effectors. In addition to further characterization of established effectors (Tir, EspH and Map), new effectors have emerged as important mediators of virulence through activities such as mimicry of Rho guanine nucleotide exchange factors (Map and EspM), inhibition of apoptosis (NleH and NleD), interference with inflammatory signalling pathways (NleB, NleC, NleE and NleH) and phagocytosis (EspF, EspH and EspJ). The findings have highlighted the multifunctional nature of the effectors and their ability to participate in redundant, synergistic or antagonistic relationships, acting in a co-ordinated spatial and temporal manner on different host organelles and cellular pathways during infection.

Bundle-forming pilus retraction enhances enteropathogenic Escherichia coli infectivity

Enteropathogenic Escherichia coli (EPEC) and other pathogenic bacteria use dynamic type IV pili to adhere to the host. Here we show that the capacity of the EPEC type IV pili to retract is required for the breakdown of the host epithelial tight-junction barrier, efficient actin-pedestal formation, and translocation of effectors via the type III secretion system.

Enteropathogenic Escherichia coli (EPEC) is an important human pathogen that causes acute infantile diarrhea. The type IV bundle-forming pili (BFP) of typical EPEC strains are dynamic fibrillar organelles that can extend out and retract into the bacterium. The bfpF gene encodes for BfpF, a protein that promotes pili retraction. The BFP are involved in bacterial autoaggregation and in mediating the initial adherence of the bacterium with its host cell. Importantly, BFP retraction is implicated in virulence in experimental human infection. How pili retraction contributes to EPEC pathogenesis at the cellular level remains largely obscure, however. In this study, an effort has been made to address this question using engineered EPEC strains with induced BFP retraction capacity. We show that the retraction is important for tight-junction disruption and, to a lesser extent, actin-rich pedestal formation by promoting efficient translocation of bacterial protein effectors into the host cells. A model is proposed whereby BFP retraction permits closer apposition between the bacterial and the host cell surfaces, thus enabling timely and effective introduction of bacterial effectors into the host cell via the type III secretion apparatus. Our studies hence suggest novel insights into the involvement of pili retraction in EPEC pathogenesis.

Using molecular tracers to assess the integrity of the intestinal epithelial barrier in vivo

The examination of the epithelial barrier has been a primary site of focused research for years. Despite the importance of this site to numerous intestinal diseases, the determination of the integrity of this barrier has been clouded by controversies as to the validity of certain techniques and the ease of use regarding others. To determine the barrier integrity in vivo, we have adapted a simple tracer-based microscopic assay that was initially used in other systems to the in vivo intestinal epithelium. This technique is widely adaptable to other tracer molecules that can be applied to the tissue and consequently generates images depicting barrier maintenance or functional breach.

Detection of tight junction barrier function in vivo by biotin

Tight junctions (TJs) are the most apical component of the junctional complexes in mammalian epithelial cells and form selective paracellular barriers restricting the passage of solutes and ions across the epithelial sheets. Claudins, a TJ integral membrane protein family, play a critical role in regulating paracellular barrier permeability. In the in vitro cell culture system, transepithelial electrical resistance (TER) measurement and the flux of radioisotope or fluorescent labeled molecules with different sizes have been widely used to determine the TJ barrier function. In the in vivo system, the tracer molecule Sulfo-NHS-Biotin was initially used in Xenopus embryo system and subsequently was successfully applied to a number of animal tissues in situ and in different organisms under the experimental conditions to examine the functional integrity of TJs by several laboratories. In this chapter, we will describe the detailed procedures of applying biotin as a paracellular tracer molecule to different in vivo systems to assay TJ barrier function.

Functional analysis of VopF activity required for colonization in Vibrio cholerae

Vibrio cholerae, a Gram-negative facultative pathogen, is the etiologic agent for the diarrheal disease cholera. We previously characterized a clinical isolate, AM-19226, that translocates a type III secretion system (T3SS) effector protein with actin-nucleating activity, VopF, into the host cells. From comparative genomic studies, we identified a divergent T3SS island in additional isolates which possess a VopF homolog, VopN. Unlike the VopF-mediated protrusion formation, VopN localizes to stress fiber in host cells similarly to VopL, which is present in the pandemic strain of Vibrio parahaemolyticus. Chimera and yeast two-hybrid studies indicated that the amino-terminal regions of VopF and VopN proteins interact with distinct host cell factors. We determined that AM-19226-infected cells are arrested at S phase of the cell cycle and that VopF/VopN are antiapoptotic factors. To understand how VopF may contribute to the pathogenesis of AM-19226, we examined the effect of VopF in an in vitro polarized-epithelial model and an in vivo adult rabbit diarrheal model. Within the T3SS pathogenicity island is VopE, a homolog of YopE from Yersinia, which has been shown to loosen tight junctions. In polarized intestinal epithelia, VopF and VopE compromised the integrity of tight junctions by inducing cortical actin depolymerization and aberrant localization of the tight-junction protein ZO-1. An assay for pathogenicity in the adult rabbit diarrhea model suggested that these effectors are involved in eliciting the diarrheal response in infected rabbits.

Vibrio cholerae is a bacterial pathogen that causes the diarrheal disease cholera, which remains a major public health problem in many developing countries. While the major virulence factors of the pandemic V. cholerae strains have been characterized, new clinical strains of V. cholerae have arisen, causing sporadic cholera-like diseases using unknown pathogenic mechanisms. Previously, we discovered the type III secretion system in a new clinical strain of V. cholerae and also identified an effector protein, VopF, which is injected into the host cells and induces changes in the actin cytoskeleton. In this work, we identified a homolog of VopF that causes a distinct cellular phenotype and interactions between the effectors and host proteins. We also discovered that both effectors prevent bacterium-induced cell death in infected cells. In our tissue culture and animal models, we showed that VopF contributes to the disruption of epithelial integrity and the diarrheal response.

The EspF effector, a bacterial pathogen's Swiss army knife

Central to the pathogenesis of many bacterial pathogens is the ability to deliver effector proteins directly into the cells of their eukaryotic host. EspF is one of many effector proteins exclusive to the attaching and effacing pathogen family that includes enteropathogenic (EPEC) and enterohemorrhagic (EHEC) Escherichia coli. Work in recent years has revealed EspF to be one of the most multifunctional effector proteins known, with defined roles in several host cellular processes, including disruption of the epithelial barrier, antiphagocytosis, microvillus effacement, host membrane remodelling, modulation of the cytoskeleton, targeting and disruption of the nucleolus, intermediate filament disruption, cell invasion, mitochondrial dysfunction, apoptosis, and inhibition of several important epithelial transporters. Surprisingly, despite this high number of functions, EspF is a relatively small effector protein, and recent work has begun to decipher the molecular events that underlie its multifunctionality. This review focuses on the activities of EspF within the host cell and discusses recent findings and molecular insights relating to the virulence functions of this fascinating bacterial effector.

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.

Evaluation of HIV protease and nucleoside reverse transcriptase inhibitors on proliferation, necrosis, apoptosis in intestinal epithelial cells and electrolyte and water transport and epithelial barrier function in mice

Background

Protease inhibitors (PI's) and reverse transcriptase drugs are important components of highly active antiretroviral therapy (HAART) for treating human acquired immunodeficiency syndrome (AIDS). Long-term clinical therapeutic efficacy and treatment compliance of these agents have been limited by undesirable side-effects, such as diarrhea. This study aims to investigate the effects of selected antiretroviral agents on intestinal histopathology and function in vivo and on cell proliferation and death in vitro.

Methods

Selected antiretroviral drugs were given orally over 7 days, to Swiss mice, as follows: 100 mg/kg of nelfinavir (NFV), indinavir (IDV), didanosine (DDI) or 50 mg/kg of zidovudine (AZT). Intestinal permeability measured by lactulose and mannitol assays; net water and electrolyte transport, in perfused intestinal segments; and small intestinal morphology and cell apoptosis were assessed in treated and control mice. In vitro cell proliferation was evaluated using the WST-1 reagent and apoptosis and necrosis by flow cytometry analysis.

Results

NFV, IDV, AZT and DDI caused significant reductions in duodenal and in jejunal villus length (p < 0.05). IDV and AZT increased crypt depth in the duodenum and AZT increased crypt depth in the jejunum. NFV, AZT and DDI significantly decreased ileal crypt depth. All selected antiretroviral drugs significantly increased net water secretion and electrolyte secretion, except for DDI, which did not alter water or chloride secretion. Additionally, only NFV significantly increased mannitol and lactulose absorption. NFV and IDV caused a significant reduction in cell proliferation in vitro at both 24 h and 48 h. DDI and AZT did not alter cell proliferation. There was a significant increase in apoptosis rates in IEC-6 cells after 24 h with 70 ug/mL of NFV (control: 4.7% vs NFV: 22%) while IDV, AZT and DDI did not show any significant changes in apoptosis compared to the control group. In jejunal sections, IDV and NFV significantly increased the number of TUNEL positive cells.

Conclusion

The PI's, NFV and IDV, increased cell apoptosis in vivo, water and electrolyte secretion and intestinal permeability and decreased villus length and cell proliferation. NFV was the only drug tested that increased cell apoptosis in vitro. The nucleoside reverse transcriptase inhibitors, AZT and DDI, did not affect cell apoptosis or proliferation. These findings may partly explain the intestinal side-effects associated with PI's.

Lactobacillus plantarum DSM 2648 is a potential probiotic that enhances intestinal barrier function

The aim of this research was to identify bacterial isolates having the potential to improve intestinal barrier function. Lactobacillus plantarum strains and human oral isolates were screened for their ability to enhance tight junction integrity as measured by the transepithelial electrical resistance (TEER) assay. Eight commercially used probiotics were compared to determine which had the greatest positive effect on TEER, and the best-performing probiotic strain, Lactobacillus rhamnosus HN001, was used as a benchmark to evaluate the isolates. One isolate, L. plantarum DSM 2648, was selected for further study because it increased TEER 135% more than L. rhamnosus HN001. The ability of L. plantarum DSM 2648 to tolerate gastrointestinal conditions and adhere to intestinal cells was determined, and L. plantarum DSM 2648 performed better than L. rhamnosus HN001 in all the assays. Lactobacillus plantarum DSM 2648 was able to reduce the negative effect of Escherichia coli [enteropathogenic E. coli (EPEC)] O127:H6 (E2348/69) on TEER and adherence by as much as 98.75% and 80.18%, respectively, during simultaneous or prior coculture compared with EPEC incubation alone. As yet, the precise mechanism associated with the positive effects exerted by L. plantarum DSM 2648 are unknown, and may influence its use to improve human health and wellness.

Role for CD2AP and other endocytosis-associated proteins in enteropathogenic Escherichia coli pedestal formation

Enteropathogenic Escherichia coli (EPEC) strains are extracellular pathogens that generate actin-rich structures (pedestals) beneath the adherent bacteria as part of their virulence strategy. Pedestals are hallmarks of EPEC infections, and their efficient formation in vitro routinely requires phosphorylation of the EPEC effector protein Tir at tyrosine 474 (Y474). This phosphorylation results in the recruitment and direct attachment of the host adaptor protein Nck to Tir at Y474, which is utilized for actin nucleation through a downstream N-WASP-Arp2/3-based mechanism. Recently, the endocytic protein clathrin was demonstrated to be involved in EPEC pedestal formation. Here we examine the organization of clathrin in pedestals and report that CD2AP, an endocytosis-associated and cortactin-binding protein, is a novel and important component of EPEC pedestal formation that also utilizes Y474 phosphorylation of EPEC Tir. We also demonstrate the successive recruitment of Nck and then clathrin prior to actin polymerization at pedestals during the Nck-dependent pathway of pedestal formation. This study further demonstrates that endocytic proteins are key components of EPEC pedestals and suggests a novel endocytosis subversion strategy employed by these extracellular bacteria.