A type III effector antagonizes death receptor signalling during bacterial gut infection.

Successful infection by enteric bacterial pathogens depends on the ability of the bacteria to colonize the gut, replicate in host tissues and disseminate to other hosts. Pathogens such as Salmonella, Shigella and enteropathogenic and enterohaemorrhagic (EPEC and EHEC, respectively) Escherichia coli use a type III secretion system (T3SS) to deliver virulence effector proteins into host cells during infection that promote colonization and interfere with antimicrobial host responses. Here we report that the T3SS effector NleB1 from EPEC binds to host cell death-domain-containing proteins and thereby inhibits death receptor signalling. Protein interaction studies identified FADD, TRADD and RIPK1 as binding partners of NleB1. NleB1 expressed ectopically or injected by the bacterial T3SS prevented Fas ligand or TNF-induced formation of the canonical death-inducing signalling complex (DISC) and proteolytic activation of caspase-8, an essential step in death-receptor-induced apoptosis. This inhibition depended on the N-acetylglucosamine transferase activity of NleB1, which specifically modified Arg 117 in the death domain of FADD. The importance of the death receptor apoptotic pathway to host defence was demonstrated using mice deficient in the FAS signalling pathway, which showed delayed clearance of the EPEC-like mouse pathogen Citrobacter rodentium and reversion to virulence of an nleB mutant. The activity of NleB suggests that EPEC and other attaching and effacing pathogens antagonize death-receptor-induced apoptosis of infected cells, thereby blocking a major antimicrobial host response.

The bacterial arginine glycosyltransferase effector NleB preferentially modifies Fas-associated death domain protein (FADD).

The inhibition of host innate immunity pathways is essential for the persistence of attaching and effacing pathogens such as enteropathogenic Escherichia coli (EPEC) and Citrobacter rodentium during mammalian infections. To subvert these pathways and suppress the antimicrobial response, attaching and effacing pathogens use type III secretion systems to introduce effectors targeting key signaling pathways in host cells. One such effector is the arginine glycosyltransferase NleB1 (NleBCR in C. rodentium) that modifies conserved arginine residues in death domain-containing host proteins with N-acetylglucosamine (GlcNAc), thereby blocking extrinsic apoptosis signaling. Ectopically expressed NleB1 modifies the host proteins Fas-associated via death domain (FADD), TNFRSF1A-associated via death domain (TRADD), and receptor-interacting serine/threonine protein kinase 1 (RIPK1). However, the full repertoire of arginine GlcNAcylation induced by pathogen-delivered NleB1 is unknown. Using an affinity proteomic approach for measuring arginine-GlcNAcylated glycopeptides, we assessed the global profile of arginine GlcNAcylation during ectopic expression of NleB1, EPEC infection in vitro, or C. rodentium infection in vivo NleB overexpression resulted in arginine GlcNAcylation of multiple host proteins. However, NleB delivery during EPEC and C. rodentium infection caused rapid and preferential modification of Arg117 in FADD. This FADD modification was extremely stable and insensitive to physiological temperatures, glycosidases, or host cell degradation. Despite its stability and effect on the inhibition of apoptosis, arginine GlcNAcylation did not elicit any proteomic changes, even in response to prolonged NleB1 expression. We conclude that, at normal levels of expression during bacterial infection, NleB1/NleBCR antagonizes death receptor-induced apoptosis of infected cells by modifying FADD in an irreversible manner.

Metformin Protects Against Noise-Induced Hearing Loss in Male Mice.

HYPOTHESIS: Metformin treatment will protect mice from noise-induced hearing loss (NIHL). BACKGROUND: We recently identified metformin as the top-ranking, Food and Drug Administration-approved drug to counter inner ear molecular changes induced by permanent threshold shift-inducing noise. This study is designed to functionally test metformin as a potential otoprotective drug against NIHL. METHODS: Male and female B6CBAF1/J mice were obtained at 7 to 8 weeks of age. A cohort of the females underwent ovariectomy to simulate menopause and eliminate the effect of ovarian-derived estrogens. At 10 weeks of age, mice underwent a permanent threshold shift-inducing noise exposure (102.5 or 105 dB SPL, 8-16 kHz, 2 h). Auditory brainstem response (ABR) thresholds were obtained at baseline, 24 h after noise exposure, and 1 week after noise exposure. Mice were administered metformin (200 mg/kg/d) or a saline control in their drinking water after the baseline ABR and for the remainder of the study. After the 1-week ABR, mice were euthanized and cochlear tissue was analyzed. RESULTS: Metformin treatment reduced the 1-week ABR threshold shift at 16 kHz ( p < 0.01; d = 1.20) and 24 kHz ( p < 0.01; d = 1.15) as well as outer hair cell loss in the 32-45.5 kHz range ( p < 0.0001; d = 2.37) in male mice. In contrast, metformin treatment did not prevent hearing loss or outer hair cell loss in the intact or ovariectomized female mice. CONCLUSIONS: Metformin exhibits sex-dependent efficacy as a therapeutic for NIHL. These data compel continued investigation into metformin's protective effects and demonstrate the importance of evaluating the therapeutic efficacy of drugs in subjects of both sexes.

The molecular pathogenesis of STAT3-driven gastric tumourigenesis in mice is independent of IL-17.

Chronic activation of the gastric mucosal adaptive immune response is a characteristic trait of gastric cancer. It has recently emerged that a new class of T helper (Th) cells, defined by their ability to produce interleukin (IL)-17A (Th17), is associated with a host of inflammatory responses, including gastritis. However, the role of these Th17 cells in the pathogenesis of gastric cancer is less clear. To formally address this, we employed gp130(F/F) mice, which spontaneously develop gastric inflammation-associated tumours akin to human intestinal-type gastric cancer. At the molecular level, these tumours demonstrate hyper-activation of the latent transcription factor signal transducer and activator of transcription (STAT)3 via the IL-6 cytokine family member, IL-11. In gp130(F/F) mice, the generation of Th17 cells, as well as the gastric expression of IL-17a and other Th17-related factors (Rorγt, IL-23), were augmented compared to wild-type gp130(+/+) mice. Consistent with a role for IL-6 and STAT3 in regulating IL-17A, increased Th17 generation and gastric expression of Th17-related factors in gp130(F/F) mice were reduced to wild-type levels in gp130(F/F) :Stat3(-/+) mice displaying normalized STAT3 activity, and also in gp130(F/F) :IL-6(-/-) mice. Importantly, genetic ablation of IL-17A in gp130(F/F) :IL-17a(-/-) mice did not suppress the initiation and growth of gastric tumours. Furthermore, IL-17A and RORC gene expression was strongly increased in human gastric biopsies from patients with gastritis, but not gastric cancer. Collectively, our data suggest that increased expression of Th17-related factors does not correlate with the molecular pathogenesis of gastric tumourigenesis.

Prospective validation of a brief questionnaire for predicting the severity of pediatric obstructive sleep apnea.

INTRODUCTION: Pediatric obstructive sleep apnea (OSA) is diagnosed and stratified by polysomnography. However, due to cost and inaccessibility, up to 90% of children undergo tonsillectomy and adenoidectomy (T&A) solely based on clinical criteria. We previously developed a data-driven brief screening questionnaire ('Selected Features,' SF) that predicted OSA severity than alternatives. The SF asks the parent whether a child: (i) has had breath-holding spells at night over the past 4 weeks, (ii) is a mouth-breather during the day, (iii) has stopped growing at a normal rate any time since birth, and (iv) is overweight. This study sought prospectively validate the SF questionnaire. METHODS: We conducted a prospective assessment of the predictive accuracy of SF compared to the Pediatric Sleep Questionnaire-Sleep Related Breathing Disorder (PSQ-SRBD) scale in otherwise healthy children with sleep disordered breathing referred for T&A. We compared the model fits of PSQ-SRDB and SF for (i) a linear regression model for the prediction of OSA, and (ii) a logistic regression model for severe OSA, defined as apnea hypopnea index (AHI) > 10. P < 0.05 was significant. RESULTS: A total of 124 patients were included. The average age was 7.3 years (95% confidence interval, 6.6-8.0) and 66 (54%) were male. The racial composition was 54 (44%) black, 41 (33%) white, and 28 (23%) other. The median AHI was 4.8 (interquartile range 12) and 43 (35%) of patients had severe OSA. In linear and logistic regression models, SF outperformed the PSQ-SRBD and null models as measured by Akaike Information Criteria. The overall accuracy in predicting AHI >10 for PSQ-SRBD was 0.65 (0.56-0.73, P = 0.54) compared to 0.73 (0.64-0.80, P = 0.04) for SF. CONCLUSION: By eliminating redundancy, we have developed a questionnaire with improved prediction of OSA and its severity, in children with high pre-test probability of the condition. While multi-site validation is necessary, SF demonstrates value in screening children prior to T&A in resource-limited environments.

Programmed cellular necrosis mediated by the pore-forming alpha-toxin from Clostridium septicum.

Programmed necrosis is a mechanism of cell death that has been described for neuronal excitotoxicity and ischemia/reperfusion injury, but has not been extensively studied in the context of exposure to bacterial exotoxins. The alpha-toxin of Clostridium septicum is a beta-barrel pore-forming toxin and a potent cytotoxin; however, the mechanism by which it induces cell death has not been elucidated in detail. We report that alpha-toxin formed Ca(2+)-permeable pores in murine myoblast cells, leading to an increase in intracellular Ca(2+) levels. This Ca(2+) influx did not induce apoptosis, as has been described for other small pore-forming toxins, but a cascade of events consistent with programmed necrosis. Ca(2+) influx was associated with calpain activation and release of cathepsins from lysosomes. We also observed deregulation of mitochondrial activity, leading to increased ROS levels, and dramatically reduced levels of ATP. Finally, the immunostimulatory histone binding protein HMGB1 was found to be released from the nuclei of alpha-toxin-treated cells. Collectively, these data show that alpha-toxin initiates a multifaceted necrotic cell death response that is consistent with its essential role in C. septicum-mediated myonecrosis and sepsis. We postulate that cellular intoxication with pore-forming toxins may be a major mechanism by which programmed necrosis is induced.

Molecular and cellular basis of microvascular perfusion deficits induced by Clostridium perfringens and Clostridium septicum.

Reduced tissue perfusion leading to tissue ischemia is a central component of the pathogenesis of myonecrosis caused by Clostridium perfringens. The C. perfringens alpha-toxin has been shown capable of inducing these changes, but its potential synergy with perfringolysin O (theta-toxin) is less well understood. Similarly, Clostridium septicum is a highly virulent causative agent of spontaneous gas gangrene, but its effect on the microcirculation has not been examined. Therefore, the aim of this study was to use intravital microscopy to examine the effects of C. perfringens and C. septicum on the functional microcirculation, coupled with the use of isogenic toxin mutants to elucidate the role of particular toxins in the resultant microvascular perfusion deficits. This study represents the first time this integrated approach has been used in the analysis of the pathological response to clostridial toxins. Culture supernatants from wild-type C. perfringens induced extensive cell death within 30 min, as assessed by in vivo uptake of propidium iodide. Furthermore, significant reductions in capillary perfusion were observed within 60 min. Depletion of either platelets or neutrophils reduced the alteration in perfusion, consistent with a role for these blood-borne cells in obstructing perfusion. In addition, mutation of either the alpha-toxin or perfringolysin O structural genes attenuated the reduction in perfusion, a process that was reversed by genetic complementation. C. septicum also induced a marked reduction in perfusion, with the degree of microvascular compromise correlating with the level of the C. septicum alpha-toxin. Together, these data indicate that as a result of its ability to produce alpha-toxin and perfringolysin O, C. perfringens rapidly induces irreversible cellular injury and a marked reduction in microvascular perfusion. Since C. septicum induces a similar reduction in microvascular perfusion, it is postulated that this function is central to the pathogenesis of clostridial myonecrosis, irrespective of the causative bacterium.

Pore-forming activity of alpha-toxin is essential for clostridium septicum-mediated myonecrosis.

Clostridium septicum alpha-toxin is a beta-barrel pore-forming cytolysin that is functionally similar to aerolysin. Residues important in receptor binding, oligomerization, and pore formation have been identified; however, little is known about the activity of the toxin in an infection, although it is essential for disease. We have now shown that deletion of a small portion of the transmembrane domain, so that the toxin is no longer able to form pores, completely abrogates its ability to contribute to disease, as does replacement of the sole cysteine residue with leucine. However, although previous biochemical and cytotoxicity assays clearly indicated that mutations in residues important in oligomerization, binding, and prepore conversion greatly reduced activity or rendered the toxin inactive, once the mutated toxins were overexpressed by the natural host in the context of an infection it was found they were able to cause disease in a mouse model of myonecrosis. These results highlight the importance of testing the activity of virulence determinants in the normal host background and in an infectious disease context and provide unequivocal evidence that it is the ability of alpha-toxin to form a pore that confers its toxicity in vivo.

The NanI and NanJ sialidases of Clostridium perfringens are not essential for virulence.

The essential toxin in Clostridium perfringens-mediated gas gangrene or clostridial myonecrosis is alpha-toxin, although other toxins and extracellular enzymes may also be involved. In many bacterial pathogens extracellular sialidases are important virulence factors, and it has been suggested that sialidases may play a role in gas gangrene. C. perfringens strains have combinations of three different sialidase genes, two of which, nanI and nanJ, encode secreted sialidases. The nanI and nanJ genes were insertionally inactivated by homologous recombination in derivatives of sequenced strain 13 and were shown to encode two functional secreted sialidases, NanI and NanJ. Analysis of these derivatives showed that NanI was the major sialidase in this organism. Mutation of nanI resulted in loss of most of the secreted sialidase activity, and the residual activity was eliminated by subsequent mutation of the nanJ gene. Only a slight reduction in the total sialidase activity was observed in a nanJ mutant. Cytotoxicity assays using the B16 melanoma cell line showed that supernatants containing NanI or overexpressing NanJ enhanced alpha-toxin-mediated cytotoxicity. Finally, the ability of nanI, nanJ, and nanIJ mutants to cause disease was assessed in a mouse myonecrosis model. No attenuation of virulence was observed for any of these strains, providing evidence that neither the NanI sialidase nor the NanJ sialidase is essential for virulence.

mTORC1 underlies age-related muscle fiber damage and loss by inducing oxidative stress and catabolism.

Aging leads to skeletal muscle atrophy (i.e., sarcopenia), and muscle fiber loss is a critical component of this process. The mechanisms underlying these age-related changes, however, remain unclear. We show here that mTORC1 signaling is activated in a subset of skeletal muscle fibers in aging mouse and human, colocalized with fiber damage. Activation of mTORC1 in TSC1 knockout mouse muscle fibers increases the content of morphologically abnormal mitochondria and causes progressive oxidative stress, fiber damage, and fiber loss over the lifespan. Transcriptomic profiling reveals that mTORC1's activation increases the expression of growth differentiation factors (GDF3, 5, and 15), and of genes involved in mitochondrial oxidative stress and catabolism. We show that increased GDF15 is sufficient to induce oxidative stress and catabolic changes, and that mTORC1 increases the expression of GDF15 via phosphorylation of STAT3. Inhibition of mTORC1 in aging mouse decreases the expression of GDFs and STAT3's phosphorylation in skeletal muscle, reducing oxidative stress and muscle fiber damage and loss. Thus, chronically increased mTORC1 activity contributes to age-related muscle atrophy, and GDF signaling is a proposed mechanism.

Citrobacter rodentium Infection Model for the Analysis of Bacterial Pathogenesis and Mucosal Immunology.

Citrobacter rodentium is a mouse restricted pathogen that was originally isolated from laboratory mouse colonies and causes transmissible colonic hyperplasia, characterized by thickening of the colon and inflammation. As a natural pathogen of mice, the infection model has proven critical to the development of our understanding of the pathogenesis of enteric disease and the mucosal immune response. In addition to this, some features of disease such as dysbiosis, inflammation, and wound healing replicate features of human inflammatory bowel diseases. As such, the C. rodentium infection model has become a key tool in investigations of many aspects of mucosal immunology.

Clinical Utility of a STAT3-Regulated miRNA-200 Family Signature with Prognostic Potential in Early Gastric Cancer.

Purpose: The majority of gastric cancer patients are diagnosed with late-stage disease, for which distinct molecular subtypes have been identified that are potentially amenable to targeted therapies. However, there exists no molecular classification system with prognostic power for early-stage gastric cancer (EGC) because the molecular events promoting gastric cancer initiation remain ill-defined.Experimental Design: miRNA microarrays were performed on gastric tissue from the gp130F/F preclinical EGC mouse model, prior to tumor initiation. Computation prediction algorithms were performed on multiple data sets and independent gastric cancer patient cohorts. Quantitative real-time PCR expression profiling was undertaken in gp130F/F-based mouse strains and human gastric cancer cells genetically engineered for suppressed activation of the oncogenic latent transcription factor STAT3. Human gastric cancer cells with modulated expression of the miR-200 family member miR-429 were also assessed for their proliferative response.Results: Increased expression of miR-200 family members is associated with both tumor initiation in a STAT3-dependent manner in gp130F/F mice and EGC (i.e., stage IA) in patient cohorts. Overexpression of miR-429 also elicited contrasting pro- and antiproliferative responses in human gastric cancer cells depending on their cellular histologic subtype. We also identified a miR-200 family-regulated 15-gene signature that integrates multiple key current indicators of EGC, namely tumor invasion depth, differentiation, histology, and stage, and provides superior predictive power for overall survival compared with each EGC indicator alone.Conclusions: Collectively, our discovery of a STAT3-regulated, miR-200 family-associated gene signature specific for EGC, with predictive power, provides a molecular rationale to classify and stratify EGC patients for endoscopic treatment. Clin Cancer Res; 24(6); 1459-72. ©2018 AACR.

Imbalanced gp130 signalling in ApoE-deficient mice protects against atherosclerosis.

OBJECTIVE: Interleukin (IL)-6 is a key modulator of the acute phase response (APR), and while both are implicated in atherosclerosis, the pathological role of specific IL-6 signalling cascades is ill-defined. Since IL-6 employs the cytokine receptor gp130 to primarily activate the STAT3 pathway, here we evaluate whether gp130-dependent STAT3 activation modulates atherosclerosis. METHODS: High-fat diet-induced atherosclerosis was established in ApoE(-/-) mice crossed with gp130(F/F) knock-in mice displaying elevated gp130-dependent STAT3 activation and production of the APR protein, serum amyloid A (SAA). Also generated were gp130(F/F):Stat3(-/+):ApoE(-/-) mice displaying genetically-normalised STAT3 activation and SAA levels, and bone marrow chimeras involving ApoE(-/-) and gp130(F/F):ApoE(-/-) mice. At 10 weeks post high-fat diet, aortic atherosclerotic lesions, including the presence of CD68(+) macrophages, and plasma lipid and SAA profiles, were assessed. RESULTS: Aortic plaque development and plasma triglyceride levels in gp130(F/F):ApoE(-/-) mice were significantly reduced (3-fold, P < 0.001) compared to ApoE(-/-) littermates. By contrast, in gp130(F/F):ApoE(-/-) mice, atherosclerotic plaques contained augmented CD68(+) macrophage infiltrates, and plasma SAA levels were elevated, compared to ApoE(-/-) mice. Atherosclerotic lesion development and plasma triglyceride levels in gp130(F/F):ApoE(-/-) and gp130(F/F):Stat3(-/+):ApoE(-/-) mice were comparable, despite a significant (P < 0.05) reduction in macrophage numbers in lesions, and also plasma SAA levels, in gp130(F/F):Stat3(-/+):ApoE(-/-) mice. Aortic plaque development and plasma triglyceride levels were comparable in ApoE(-/-) mice reconstituted with gp130(F/F):ApoE(-/-) (ApoE(F/F:ApoE)) or ApoE(-/-) (ApoE(ApoE)) bone marrow cells. CONCLUSIONS: Deregulation of gp130/STAT3 signalling augments the APR and macrophage infiltration during atherosclerosis without impacting on the development of aortic plaques.

Fas regulates neutrophil lifespan during viral and bacterial infection.

The regulation of neutrophil lifespan is critical for a circumscribed immune response. Neutrophils are sensitive to Fas/CD95 death receptor signaling in vitro, but it is unknown if Fas regulates neutrophil lifespan in vivo. We hypothesized that FasL-expressing CD8(+) T cells, which kill antigen-stimulated T cells during chronic viral infection, can also induce neutrophil death in tissues during infection. With the use of LysM-Cre Fas(fl/fl) mice, which lack Fas expression in macrophages and neutrophils, we show that Fas regulates neutrophil lifespan during lymphocytic choriomeningitis virus (LCMV) infection in the lung, peripheral blood, and spleen. Fas also contributed to the regulation of neutrophil numbers in the colon of Citrobacter rodentium-infected mice. To examine the effects of infection on Fas activation in neutrophils, we primed neutrophils with TLR ligands or IL-18, resulting in ablation of Fas death receptor signaling. These data provide the first in vivo genetic evidence that neutrophil lifespan is controlled by death receptor signaling and provide a mechanism to account for neutrophil resistance to Fas stimulation during infection.

A RIPK2 inhibitor delays NOD signalling events yet prevents inflammatory cytokine production.

Intracellular nucleotide binding and oligomerization domain (NOD) receptors recognize antigens including bacterial peptidoglycans and initiate immune responses by triggering the production of pro-inflammatory cytokines through activating NF-κB and MAP kinases. Receptor interacting protein kinase 2 (RIPK2) is critical for NOD-mediated NF-κB activation and cytokine production. Here we develop and characterize a selective RIPK2 kinase inhibitor, WEHI-345, which delays RIPK2 ubiquitylation and NF-κB activation downstream of NOD engagement. Despite only delaying NF-κB activation on NOD stimulation, WEHI-345 prevents cytokine production in vitro and in vivo and ameliorates experimental autoimmune encephalomyelitis in mice. Our study highlights the importance of the kinase activity of RIPK2 for proper immune responses and demonstrates the therapeutic potential of inhibiting RIPK2 in NOD-driven inflammatory diseases.

STAT3-driven upregulation of TLR2 promotes gastric tumorigenesis independent of tumor inflammation.

Gastric cancer (GC) is associated with chronic inflammation; however, the molecular mechanisms promoting tumorigenesis remain ill defined. Using a GC mouse model driven by hyperactivation of the signal transducer and activator of transcription (STAT)3 oncogene, we show that STAT3 directly upregulates the epithelial expression of the inflammatory mediator Toll-like receptor (TLR)2 in gastric tumors. Genetic and therapeutic targeting of TLR2 inhibited gastric tumorigenesis, but not inflammation, characterized by reduced proliferation and increased apoptosis of the gastric epithelium. Increased STAT3 pathway activation and TLR2 expression were also associated with poor GC patient survival. Collectively, our data reveal an unexpected role for TLR2 in the oncogenic function of STAT3 that may represent a therapeutic target in GC.

Cross-complementation of Clostridium perfringens PLC and Clostridium septicum alpha-toxin mutants reveals PLC is sufficient to mediate gas gangrene.

Clostridium perfringens and Clostridium septicum are the most common causes of clostridial myonecrosis or gas gangrene. Although they mediate a similar disease pathology, they elaborate functionally very different alpha-toxins. We used a reciprocal complementation approach to assess the contribution of the primary toxin of each species to disease and found that C. perfringens alpha-toxin (PLC) was able to mediate the gross pathology of myonecrosis even in a C. septicum background, although it could not induce vascular leukostasis. Conversely, while C. septicum alpha-toxin restored some virulence to a C. perfringens plc mutant, it was less active than in its native background.

The alpha-toxin of Clostridium septicum is essential for virulence.

Clostridium septicum is the causative agent of spontaneous gas gangrene or atraumatic myonecrosis, a sudden and frequently fatal infection that is increasingly associated with malignancy of the colon. Little is known about the disease process although the focus of virulence studies has been the alpha-toxin, a pore-forming cytolysin that is encoded by the csa gene and secreted as an inactive protoxin. Until now a lack of techniques for the genetic manipulation of C. septicum has hindered the use of molecular approaches to understand pathogenesis. By introducing plasmids by conjugation from Escherichia coli, we have developed methods for the genetic manipulation of C. septicum and constructed a chromosomal csa mutant by allelic exchange. Virulence testing of an isogenic series of strains consisting of the wild type, the csa mutant, and a csa mutant complemented with the wild-type csa gene revealed that the development of fulminant myonecrosis in mice was dependent on the ability to produce a functional haemolytic alpha-toxin. Furthermore, the inhibition of leukocyte influx into the lesion, which is very typical of clostridial myonecrosis, was also dependent on the ability to produce alpha-toxin. This study represents the first definitive identification of a virulence factor in this organism and opens the way for further studies that will delineate the role of other putative virulence factors in this significant pathogen.