Gadd45α activity is the principal effector of Shigella mitochondria-dependent epithelial cell death in vitro and ex vivo

Septins promote caspase activity and coordinate mitochondrial apoptosis

Apoptosis is a form of regulated cell death essential for tissue homeostasis and embryonic development. Apoptosis also plays a key role during bacterial infection, yet some intracellular bacterial pathogens (such as Shigella flexneri, whose lipopolysaccharide can block apoptosis) can manipulate cell death programs as an important survival strategy. Septins are a component of the cytoskeleton essential for mitochondrial dynamics and host defense, however, the role of septins in regulated cell death is mostly unknown. Here, we discover that septins promote mitochondrial (i.e., intrinsic) apoptosis in response to treatment with staurosporine (a pan-kinase inhibitor) or etoposide (a DNA topoisomerase inhibitor). Consistent with a role for septins in mitochondrial dynamics, septins promote the release of mitochondrial protein cytochrome c in apoptotic cells and are required for the proteolytic activation of caspase-3, caspase-7, and caspase-9 (core components of the apoptotic machinery). Apoptosis of HeLa cells induced in response to infection by S. flexneri ΔgalU (a lipopolysaccharide mutant unable to block apoptosis) is also septin-dependent. In vivo, zebrafish larvae are significantly more susceptible to infection with S. flexneri ΔgalU (as compared to infection with wildtype S. flexneri), yet septin deficient larvae are equally susceptible to infection with S. flexneri ΔgalU and wildtype S. flexneri. These data provide a new molecular framework to understand the complexity of mitochondrial apoptosis and its ability to combat bacterial infection.

Type III secreted effectors that target mitochondria

A type III secretion system (T3SS) is used by Gram-negative bacterial pathogens to secrete and translocate a battery of proteins, termed effectors, from the bacteria directly into the host cells. These effectors, which are thought to play a key role in bacterial virulence, hijack and modify the activity of diverse host cell organelles, including mitochondria. Mitochondria-the energy powerhouse of the cell-are important cell organelles that play role in numerous critical cellular processes, including the initiation of apoptosis and the induction of innate immunity. Therefore, it is not surprising that pathogenic bacteria use mitochondrially targeted effectors to control host cell death and immunity pathways. Surprisingly, however, we found that despite their importance, only a limited number of type III secreted effectors have been characterised to target host mitochondria, and the mechanisms underlying their mitochondrial activity have not been sufficiently analysed. These include effectors secreted by the enteric attaching and effacing (A/E), Salmonella and Shigella bacterial pathogens. Here we give an overview of key findings, present gaps in knowledge and hypotheses concerning the mode by which these type III secreted effectors control the host and the bacterial cell life (and death) through targeting mitochondria.

Comparative analysis of the expression profiles of genes related to the Gadd45α signaling pathway in four kinds of liver diseases

Gadd45α (growth arrest and DNA damage inducible alpha) is a member of a group of genes whose transcript levels are increased following stressful conditions that lead to growth arrest and treatment with agents that lead to DNA damage. Gadd45α is upregulated in liver cirrhosis (LC), hepatic cancer (HC), acute liver failure (AHF) and non-alcoholic fatty liver disease(NAFLD). Here, we investigated the essential differences in the Gadd45α signaling pathway in these diseases at the transcriptional level. The results showed that 44, 46, 71 and 27 genes significant changes in these diseases, and the H-cluster showed that the expression of the Gadd45α signaling-related genes was significantly different in the four liver diseases. DAVID functional analysis showed that the Gadd45α signaling pathway-related genes were mainly involved in cell adhesion and migration, cell proliferation, apoptosis, stress and inflammatory responses, etc. Ingenuity pathway analysis (IPA) software was used to predict the functions of the Gadd45α signaling-related genes, and the results indicated that there were significant changes in cell differentiation, DNA damage repair, autophagy, apoptosis and necrosis. Gadd45α signaling pathway is involved in four kinds of liver disease and regulates a variety of activities via P38 MAPK, NF-κB, mTOR/STAT3, P21, PCNA, PI3K/Akt and other signaling pathways. Modulation of Gadd45α may be exploited to prevent the progression of liver disease, and to identify specific treatments for different stages of liver disease. In summary, the Gadd45α signaling pathway is involved in four kinds of liver disease and regulates a variety of physiological activities through various signaling pathways.

XIAP controls RIPK2 signaling by preventing its deposition in speck-like structures

This study provides evidence that the NOD1/2-associated kinase RIPK2 localizes to detergent insoluble cytosolic complexes upon activation and suggests novel regulatory mechanisms for RIPK2 signaling.

The receptor interacting serine/threonine kinase 2 (RIPK2) is essential for linking activation of the pattern recognition receptors NOD1 and NOD2 to cellular signaling events. Recently, it was shown that RIPK2 can form higher order molecular structures in vitro. Here, we demonstrate that RIPK2 forms detergent insoluble complexes in the cytosol of host cells upon infection with invasive enteropathogenic bacteria. Formation of these structures occurred after NF-κB activation and depended on the caspase activation and recruitment domain of NOD1 or NOD2. Complex formation upon activation required RIPK2 autophosphorylation at Y474 and was influenced by phosphorylation at S176. We found that the E3 ligase X-linked inhibitor of apoptosis (XIAP) counteracts complex formation of RIPK2, accordingly mutation of the XIAP ubiquitylation sites in RIPK2 enhanced complex formation. Taken together, our work reveals novel roles of XIAP in the regulation of RIPK2 and expands our knowledge on the function of RIPK2 posttranslational modifications in NOD1/2 signaling.

Telomeric epigenetic response mediated by Gadd45a regulates stem cell aging and lifespan

Progressive attrition of telomeres triggers DNA damage response (DDR) and limits the regenerative capacity of adult stem cells during mammalian aging. Intriguingly, telomere integrity is not only determined by telomere length but also by the epigenetic status of telomeric/sub-telomeric regions. However, the functional interplay between DDR induced by telomere shortening and epigenetic modifications in aging remains unclear. Here, we show that deletion of Gadd45a improves the maintenance and function of intestinal stem cells (ISCs) and prolongs lifespan of telomerase-deficient mice (G3Terc ^-/-). Mechanistically, Gadd45a facilitates the generation of a permissive chromatin state for DDR signaling by inducing base excision repair-dependent demethylation of CpG islands specifically at sub-telomeric regions of short telomeres. Deletion of Gadd45a promotes chromatin compaction in sub-telomeric regions and attenuates DDR initiation at short telomeres of G3Terc ^-/- ISCs. Treatment with a small molecule inhibitor of base excision repair reduces DDR and improves the maintenance and function of G3Terc ^-/- ISCs. Taken together, our study proposes a therapeutic approach to enhance stem cell function and prolong lifespan by targeting epigenetic modifiers.

Bradyrhizobium Lipid A: Immunological Properties and Molecular Basis of Its Binding to the Myeloid Differentiation Protein-2/Toll-Like Receptor 4 Complex

Lipopolysaccharides (LPS) are potent activator of the innate immune response through the binding to the myeloid differentiation protein-2 (MD-2)/toll-like receptor 4 (TLR4) receptor complexes. Although a variety of LPSs have been characterized so far, a detailed molecular description of the structure–activity relationship of the lipid A part has yet to be clarified. Photosynthetic Bradyrhizobium strains, symbiont of Aeschynomene legumes, express distinctive LPSs bearing very long-chain fatty acids with a hopanoid moiety covalently linked to the lipid A region. Here, we investigated the immunological properties of LPSs isolated from Bradyrhizobium strains on both murine and human immune systems. We found that they exhibit a weak agonistic activity and, more interestingly, a potent inhibitory effect on MD-2/TLR4 activation exerted by toxic enterobacterial LPSs. By applying computational modeling techniques, we also furnished a plausible explanation for the Bradyrhizobium LPS inhibitory activity at atomic level, revealing that its uncommon lipid A chemical features could impair the proper formation of the receptorial complex, and/or has a destabilizing effect on the pre-assembled complex itself.

To Eat and to Be Eaten: Mutual Metabolic Adaptations of Immune Cells and Intracellular Bacterial Pathogens upon Infection

Intracellular bacterial pathogens (IBPs) invade and replicate in different cell types including immune cells, in particular of the innate immune system (IIS) during infection in the acute phase. However, immune cells primarily function as essential players in the highly effective and integrated host defense systems comprising the IIS and the adaptive immune system (AIS), which cooperatively protect the host against invading microbes including IBPs. As countermeasures, the bacterial pathogens (and in particular the IBPs) have developed strategies to evade or reprogram the IIS at various steps. The intracellular replication capacity and the anti-immune defense responses of the IBP's as well as the specific antimicrobial responses of the immune cells of the innate and the AIS depend on specific metabolic programs of the IBPs and their host cells. The metabolic programs of the immune cells supporting or counteracting replication of the IBPs appear to be mutually exclusive. Indeed, recent studies show that upon interaction of naïve, metabolically quiescent immune cells with IBPs, different metabolic activation processes occur which may result in the provision of a survival and replication niche for the pathogen or its eradication. It is therefore likely that within a possible host cell population subsets exist that are metabolically programmed for pro- or anti-microbial conditions. These metabolic programs may be triggered by the interactions between different bacterial agonistic components and host cell receptors. In this review, we summarize the current status in the field and discuss metabolic adaptation processes within immune cells of the IIS and the IBPs that support or restrict the intracellular replication of the pathogens.

Infection of porcine colon explants with "Brachyspira hampsonii" leads to increased epithelial necrosis and catarrhal exudate

Mucohemorrhagic diarrhea in pigs caused by Brachyspira spp. has a global distribution, and an economic impact on affected farms due to poor performance of animals. Demonstrations that "Brachyspira hampsonii" is pathogenic have been achieved using in vivo animal models, but a critical knowledge gap exists regarding the pathogenic mechanisms employed by Brachyspira. Here, we used in vitro organ culture of porcine colon to investigate interactions between "B. hampsonii" and explants during the first 12 h of contact. Explants were either inoculated with "B. hampsonii" or sterile culture broth. Responses to infection were evaluated by optical microscopy and quantitative PCR. Significantly greater numbers of necrotic crypt cells and thicker catarrhal exudate were observed on infected explants compared to controls. Spirochaetes were observed in the mucus layer, in contact with necrotic exfoliated cells, in crypts and the lamina propria. Statistical differences were observed in mRNA levels between inoculated and control explants for IL-1α, TNF-α and ZO-1 using a Bayesian analysis, but not observed using the ΔΔCq method. These results provide a demonstration of a porcine colon explant model for investigating interactions of Brachyspira with its host and show that initial effects on the host are observed within the first 12 h of contact.

How Shigella Utilizes Ca(2+) Jagged Edge Signals during Invasion of Epithelial Cells

Shigella, the causative agent of bacillary dysentery invades intestinal epithelial cells using a type III secretion system (T3SS). Through the injection of type III effectors, Shigella manipulates the actin cytoskeleton to induce its internalization in epithelial cells. At early invasion stages, Shigella induces atypical Ca(2+) responses confined at entry sites allowing local cytoskeletal remodeling for bacteria engulfment. Global Ca(2+) increase in the cell triggers the opening of connexin hemichannels at the plasma membrane that releases ATP in the extracellular milieu, favoring Shigella invasion and spreading through purinergic receptor signaling. During intracellular replication, Shigella regulates inflammatory and death pathways to disseminate within the epithelium. At later stages of infection, Shigella downregulates hemichannel opening and the release of extracellular ATP to dampen inflammatory signals. To avoid premature cell death, Shigella activates cell survival by upregulating the PI3K/Akt pathway and downregulating the levels of p53. Furthermore, Shigella interferes with pro-apoptotic caspases, and orients infected cells toward a slow necrotic cell death linked to mitochondrial Ca(2+) overload. In this review, we will focus on the role of Ca(2+) responses and their regulation by Shigella during the different stages of bacterial infection.

The Orchestra and Its Maestro: Shigella's Fine-Tuning of the Inflammasome Platforms

Shigella spp. are the causative agents of bacillary dysentery, leading to extensive mortality and morbidity worldwide. These facultative intracellular bacteria invade the epithelium of the colon and the rectum, inducing a severe inflammatory response from which the symptoms of the disease originate. Shigella are human pathogens able to manipulate and subvert the innate immune system surveillance. Shigella dampens inflammasome activation in epithelial cells. In infected macrophages, inflammasome activation and IL-1β and IL-18 release lead to massive neutrophil recruitment and greatly contribute to inflammation. Here, we describe how Shigella hijacks and finely tunes inflammasome activation in the different cell populations involved in pathogenesis: epithelial cells, macrophages, neutrophils, DCs, and B and T lymphocytes. Shigella emerges as a "sly" pathogen that switches on/off the inflammasome mechanisms in order to optimize the interaction with the host and establish a successful infection.

Shigella Effector OspB Activates mTORC1 in a Manner That Depends on IQGAP1 and Promotes Cell Proliferation

The intracellular bacterial pathogen Shigella infects and spreads through the human intestinal epithelium. Effector proteins delivered by Shigella into cells promote infection by modulating diverse host functions. We demonstrate that the effector protein OspB interacts directly with the scaffolding protein IQGAP1, and that the absence of either OspB or IQGAP1 during infection leads to larger areas of S. flexneri spread through cell monolayers. We show that the effect on the area of bacterial spread is due to OspB triggering increased cell proliferation at the periphery of infected foci, thereby replacing some of the cells that die within infected foci and restricting the area of bacterial spread. We demonstrate that OspB enhancement of cell proliferation results from activation of mTORC1, a master regulator of cell growth, and is blocked by the mTORC1-specific inhibitor rapamycin. OspB activation of mTORC1, and its effects on cell proliferation and bacterial spread, depends on IQGAP1. Our results identify OspB as a regulator of mTORC1 and mTORC1-dependent cell proliferation early during S. flexneri infection and establish a role for IQGAP1 in mTORC1 signaling. They also raise the possibility that IQGAP1 serves as a scaffold for the assembly of an OspB-mTORC1 signaling complex.

During infection, Shigella spp. deliver into the cytoplasm of cells effector proteins that manipulate host cell processes in ways that promote infection and bacterial spread. We have discovered that the Shigella effector protein OspB interacts with the cellular scaffolding protein IQGAP1. OspB induces increased cell proliferation by activating mTORC1 kinase, a master regulator of cellular growth, in a manner that depends on IQGAP1. As IQGAP1 has been shown to interact with mTOR and with the mTORC1 activators ERK1/2, we propose that IQGAP1 serves as a scaffold for OspB activation of mTORC1. The presence of OspB and IQGAP1 lead to restricting the area of spread of S. flexneri in cell monolayers; our data support a model in which the effect of OspB and IQGAP1 on the area of S. flexneri spread is due to effects on cell proliferation locally within infected foci. As infection of cells and tissue by Shigella spp. leads to cell death, increased local cellular proliferation may serve to provide additional protective intracellular niches for the organism within infected tissue.

Manipulation of the host cell death pathway by Shigella

Host cells deploy multiple defences against microbial infection. One prominent host defence mechanism, the death of infected cells, plays a pivotal role in clearing damaged cells, eliminating pathogens, removing replicative niches, exposing intracellular bacterial pathogens to extracellular immune surveillance and presenting bacteria-derived antigens to the adaptive immune system. Although cell death can occur under either physiological or pathophysiological conditions, it acts as an innate defence mechanism against bacterial pathogens by limiting their persistent colonization. However, many bacterial pathogens, including Shigella, have evolved mechanisms that manipulate host cell death for their own benefit.

B lymphocytes undergo TLR2-dependent apoptosis upon Shigella infection

Shigella flexneri interacts with B cells and induces apoptosis via IpaD binding to TLR2.

Antibody-mediated immunity to Shigella, the causative agent of bacillary dysentery, requires several episodes of infection to get primed and is short-lasting, suggesting that the B cell response is functionally impaired. We show that upon ex vivo infection of human colonic tissue, invasive S. flexneri interacts with and occasionally invades B lymphocytes. The induction of a type three secretion apparatus (T3SA)–dependent B cell death is observed in the human CL-01 B cell line in vitro, as well as in mouse B lymphocytes in vivo. In addition to cell death occurring in Shigella-invaded CL-01 B lymphocytes, we provide evidence that the T3SA needle tip protein IpaD can induce cell death in noninvaded cells. IpaD binds to and induces B cell apoptosis via TLR2, a signaling receptor thus far considered to result in activation of B lymphocytes. The presence of bacterial co-signals is required to sensitize B cells to apoptosis and to up-regulate tlr2, thus enhancing IpaD binding. Apoptotic B lymphocytes in contact with Shigella-IpaD are detected in rectal biopsies of infected individuals. This study therefore adds direct B lymphocyte targeting to the diversity of mechanisms used by Shigella to dampen the host immune response.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

Dynamin-related protein Drp1 and mitochondria are important for Shigella flexneri infection

Shigella infection in epithelial cells induces cell death which is accompanied by mitochondrial dysfunction. In this study the role of the mitochondrial fission protein, Drp1 during Shigella infection in HeLa cells was examined. Significant lactate dehydrogenase (LDH) release was detected in the culture supernatant when HeLa cells were infected with Shigella at a high multiplicity of infection. Drp1 inhibition with Mdivi-1 and siRNA knockdown significantly reduced LDH release. HeLa cell death was also accompanied by mitochondrial fragmentation. Tubular mitochondrial networks were partially restored when Drp1 was depleted with either siRNA or inhibited with Mdivi-1. Surprisingly either Mdivi-1 treatment or Drp1 siRNA-depletion of HeLa cells also reduced Shigella plaque formation. The effect of Mdivi-1 on Shigella infection was assessed using the murine Sereny model, however it had no impact on ocular inflammation. Overall our results suggest that Drp1 and the mitochondria play important roles during Shigella infection.

Intracellular Shigella remodels its LPS to dampen the innate immune recognition and evade inflammasome activation

LPS is a potent bacterial effector triggering the activation of the innate immune system following binding with the complex CD14, myeloid differentiation protein 2, and Toll-like receptor 4. The LPS of the enteropathogen Shigella flexneri is a hexa-acylated isoform possessing an optimal inflammatory activity. Symptoms of shigellosis are produced by severe inflammation caused by the invasion process of Shigella in colonic and rectal mucosa. Here we addressed the question of the role played by the Shigella LPS in eliciting a dysregulated inflammatory response of the host. We unveil that (i) Shigella is able to modify the LPS composition, e.g., the lipid A and core domains, during proliferation within epithelial cells; (ii) the LPS of intracellular bacteria (iLPS) and that of bacteria grown in laboratory medium differ in the number of acyl chains in lipid A, with iLPS being the hypoacylated; (iii) the immunopotential of iLPS is dramatically lower than that of bacteria grown in laboratory medium; (iv) both LPS forms mainly signal through the Toll-like receptor 4/myeloid differentiation primary response gene 88 pathway; (v) iLPS down-regulates the inflammasome-mediated release of IL-1β in Shigella-infected macrophages; and (vi) iLPS exhibits a reduced capacity to prime polymorfonuclear cells for an oxidative burst. We propose a working model whereby the two forms of LPS might govern different steps of the invasive process of Shigella. In the first phases, the bacteria, decorated with hypoacylated LPS, are able to lower the immune system surveillance, whereas, in the late phases, shigellae harboring immunopotent LPS are fully recognized by the immune system, which can then successfully resolve the infection.

Should probiotics be tested on ex vivo organ culture models?

The use of probiotic strains as nutritional supplements has been gaining ground in the last decade. As the mechanisms with which they modulate innate and adaptive immunity start to unravel, probiotics have repeatedly been suggested as potential treatment for a wide variety of diseases, including inflammatory bowel disease (IBD). However, even though the benefits of probiotic treatment for conditions like atopic dermatitis are well established, very limited clinical benefit has been obtained on IBD treatment. This could be due to the lack of suitable models on which to obtain valid pre-clinical data to select the most appropriate strain for a given condition. We recently described a newly developed model for the culture and apical stimulation of whole human intestinal mucosal explants. We showed that the tissue was only viable if incubated in an O(2) chamber, but it was possible to stimulate the tissue with bacteria in a conventional incubator. We used the new set-up to test three different Lactobacilli strains, none of which appeared to be benign on inflamed IBD mucosa.

Preventing acute gut wall damage in infectious diarrhoeas with glycosylated dendrimers

Intestinal pathogens use the host's excessive inflammatory cytokine response, designed to eliminate dangerous bacteria, to disrupt epithelial gut wall integrity and promote their tissue invasion. We sought to develop a non-antibiotic-based approach to prevent this injury. Molecular docking studies suggested that glycosylated dendrimers block the TLR4-MD-2-LPS complex, and a 13.6 kDa polyamidoamine (PAMAM) dendrimer glucosamine (DG) reduced the induction of human monocyte interleukin (IL)-6 by Gram-negative bacteria. In a rabbit model of shigellosis, PAMAM-DG prevented epithelial gut wall damage and intestinal villous destruction, reduced local IL-6 and IL-8 expression, and minimized bacterial invasion. Computational modelling studies identified a 3.3 kDa polypropyletherimine (PETIM)-DG as the smallest likely bioactive molecule. In human monocytes, high purity PETIM-DG potently inhibited Shigella Lipid A-induced IL-6 expression. In rabbits, PETIM-DG prevented Shigella-induced epithelial gut wall damage, reduced local IL-6 and IL-8 expression, and minimized bacterial invasion. There was no change in β-defensin, IL-10, interferon-β, transforming growth factor-β, CD3 or FoxP3 expression. Small and orally delivered DG could be useful for preventing gut wall tissue damage in a wide spectrum of infectious diarrhoeal diseases.

–>See accompanying article http://dx.doi.org/10.1002/emmm.201201668

Calpain activation by the Shigella flexneri effector VirA regulates key steps in the formation and life of the bacterium's epithelial niche

The enteropathogen Shigella flexneri invades epithelial cells, leading to inflammation and tissue destruction. We report that Shigella infection of epithelial cells induces an early genotoxic stress, but the resulting p53 response and cell death are impaired due to the bacterium's ability to promote p53 degradation, mainly through calpain protease activation. Calpain activation is promoted by the Shigella virulence effector VirA and dependent on calcium flux and the depletion of the endogenous calpain inhibitor calpastatin. Further, although VirA-induced calpain activity is critical for regulating cytoskeletal events driving bacterial uptake, calpain activation ultimately leads to necrotic cell death, thereby restricting Shigella intracellular growth. Therefore, calpains work at multiple steps in regulating Shigella pathogenesis by disrupting the p53-dependent DNA repair response early during infection and regulating both formation and ultimate death of the Shigella epithelial replicative niche.