Interaction of Ipa proteins of Shigella flexneri with alpha5beta1 integrin promotes entry of the bacteria into mammalian cells

Pathogenicity and virulence of Shigella sonnei: A highly drug-resistant pathogen of increasing prevalence

Shigella spp. are the causative agent of shigellosis (or bacillary dysentery), a diarrhoeal disease characterized for the bacterial invasion of gut epithelial cells. Among the 4 species included in the genus, Shigella flexneri is principally responsible for the disease in the developing world while Shigella sonnei is the main causative agent in high-income countries. Remarkably, as more countries improve their socioeconomic conditions, we observe an increase in the relative prevalence of S. sonnei. To date, the reasons behind this change in aetiology depending on economic growth are not understood. S. flexneri has been widely used as a model to study the pathogenesis of the genus, but as more research data are collected, important discrepancies with S. sonnei have come to light. In comparison to S. flexneri, S. sonnei can be differentiated in numerous aspects; it presents a characteristic O-antigen identical to that of one serogroup of the environmental bacterium Plesiomonas shigelloides, a group 4 capsule, antibacterial mechanisms to outcompete and displace gut commensal bacteria, and a poorer adaptation to an intracellular lifestyle. In addition, the World Health Organization (WHO) have recognized the significant threat posed by antibiotic-resistant strains of S. sonnei, demanding new approaches. This review gathers knowledge on what is known about S. sonnei within the context of other Shigella spp. and aims to open the door for future research on understanding the increasing spread of this pathogen.

Genome-wide characterization of integrin (ITG) gene family and their expression profiling in half-smooth tongue sole (Cynoglossus semilaevis) upon Vibrio anguillarum infection

Integrins (ITGs) are transmembrane heterodimer receptors with ITGα subunit and ITGβ subunit, participating in various physiological processes, including immunity. At present, systematic research on ITGs in teleost is scarce, especially in half-smooth tongue sole (Cynoglossus semilaevis). In this study, a set of 28 ITG genes in half-smooth tongue sole have been identified and characterized. The phylogenetic analysis showed that ITGα and ITGβ subunits were respectively classified into five and two clusters, consistent with previous studies. The selection pressure analysis indicated that most of ITG genes were under purifying selection, except for ITGα11b and ITGαL with positive selection. The expression profiles of eight selected ITG genes, including ITGα1, ITGα5, ITGα8, ITGα11, ITGβ1, ITGβ2, ITGβ3, and ITGβ8, were analyzed in healthy tissues and after infection with Vibrio anguillarum, revealed their implications in immune response. The study provided a comprehensive characterization and expression analysis of ITG genes in half-smooth tongue sole, setting a solid foundation for further functional studies and promising potential in disease control.

Full-length transcriptome sequencing and comparative transcriptome analysis of Eriocheir sinensis in response to infection by the microsporidian Hepatospora eriocheir

As a new generation of high-throughput sequencing technology, PacBio Iso-Seq technology (Iso-Seq) provides a better alternative sequencing method for the acquisition of full-length unigenes. In this study, a total of 22.27 gigabyte (Gb) subread bases and 128,614 non-redundant unigenes (mean length: 2,324 bp) were obtained from six main tissues of Eriocheir sinensis including the heart, nerve, intestine, muscle, gills and hepatopancreas. In addition, 74,732 unigenes were mapped to at least one of the following databases: Non-Redundant Protein Sequence Database (NR), Gene Ontology (GO), Kyoto Encyclopaedia of Genes and Genomes (KEGG), KEGG Orthology (KO) and Protein family (Pfam). In addition, 6696 transcription factors (TFs), 28,458 long non-coding RNAs (lncRNAs) and 94,230 mRNA-miRNA pairs were identified. Hepatospora eriocheir is the primary pathogen of E. sinensis and can cause hepatopancreatic necrosis disease (HPND); the intestine is the main target tissue. Here, we attempted to identify the key genes related to H. eriocheir infection in the intestines of E. sinensis. By combining Iso-Seq and Illumina RNA-seq analysis, we identified a total of 12,708 differentially expressed unigenes (DEUs; 6,696 upregulated and 6,012 downregulated) in the crab intestine following infection with H. eriocheir. Based on the biological analysis of these DEUs, several key processes were identified, including energy metabolism-related pathways, cell apoptosis and innate immune-related pathways. Twelve selected genes from these DEUs were subsequently verified by quantitative real-time PCR (qRT-PCR) analysis. Our findings enhance our understanding of the E. sinensis transcriptome and the specific association between E. sinensis and H. eriocheir infection.

The Shigella Type III Secretion Effector IpaH4.5 Targets NLRP3 to Activate Inflammasome Signaling

Activation of the NLRP3 inflammasome requires the expression of NLRP3, which is strictly regulated by its capacity to directly recognize microbial-derived substances. Even though the involvement of caspase-1 activation in macrophages via NLRP3 and NLRC4 has been discovered, the accurate mechanisms by which Shigella infection triggers NLRP3 activation remain inadequately understood. Here, we demonstrate that IpaH4.5, a Shigella T3SS effector, triggers inflammasome activation by regulating NLRP3 expression through the E3 ubiquitin ligase activity of IpaH4.5. First, we found that IpaH4.5 interacted with NLRP3. As a result, IpaH4.5 modulated NLRP3 protein stability and inflammasome activation. Bacteria lacking IpaH4.5 had dramatically reduced ability to induce pyroptosis. Our results identify a previously unrecognized target of IpaH4.5 in the regulation of inflammasome signaling and clarify the molecular basis for the cytosolic response to the T3SS effector.

The anti-adhesive and anti-invasive effects of recombinant azurin on the interaction between enteric pathogens (invasive/non-invasive) and Caco-2 cells

Anti-adhesion therapy and anti-adhesin immunity are meant to diminish the interaction between pathogens and host tissues, either by prevention or by exclusion of bacterial adhesion and entrance to cells. Azurin is a scaffold protein possessing antiviral, antiparasitic, and anticancer activities. The purpose of the present study was to determine the effect of recombinant Azurin (rAzurin) on the adhesion and invasion capacity of invasive (Shigella sonnei, Shigella flexneri, Campylobacter jejuni) and non-invasive (Vibrio cholerae) enteric bacteria to cells. The non-toxic dose of rAzurin and the best MOI (Multiplicity of Infection) of bacterial species was assessed by MTT assay. Bacterial species were used at MOIs of 20:1 and Azurin was applied at the concentrations of 5 and 25 μg/mL and added to Caco-2 cells in competition and replacement assay to assess the anti-adhesion and anti-invasion properties of rAzurin. The protein caused significant decrease in the adhesion rate of S. sonnei, S. flexneri, C. jejuni, and V. cholerae strains to Caco-2 cells by 43, 39, 72, and 38% in competition and 45, 46, 75, and 48% in replacement assays, respectively. Also, S. sonnei, S. flexneri, and C. jejuni strains invasion rate was reduced to 50, 50, and 70% in anti-invasion assay, respectively. The inhibitory effect of Azurin against C. jejuni and V. cholerae strains adhesion was more significant (p < .001) compared to Shigella spp. (p < .05) which may be due to smaller size of the former bacteria. On the contrary, in invasion assay, rAzurin showed a greater inhibitory effect against Shigella spp. (p < .001) compared to C. jejuni (p < .05), which may probably be due to the interaction of rAzurin with several effectors or ligands, involved in Shigella invasion and internalization. The findings of the present study opens new insights of rAzurin as a new and potent candidate for reducing or probably preventing enteric bacterial attachment, invasion, and pathogenesis.

Cell Adhesion by Integrins

Integrins are heterodimeric cell surface receptors ensuring the mechanical connection between cells and the extracellular matrix. In addition to the anchorage of cells to the extracellular matrix, these receptors have critical functions in intracellular signaling, but are also taking center stage in many physiological and pathological conditions. In this review, we provide some historical, structural, and physiological notes so that the diverse functions of these receptors can be appreciated and put into the context of the emerging field of mechanobiology. We propose that the exciting journey of the exploration of these receptors will continue for at least another new generation of researchers.

Peptostreptococcus anaerobius promotes colorectal carcinogenesis and modulates tumour immunity

Emerging evidence implicates a role of the gut microbiota in colorectal cancer (CRC). Peptostreptococcus anaerobius (P. anaerobius) is an anaerobic bacterium selectively enriched in the faecal and mucosal microbiota from patients with CRC, but its causative role and molecular mechanism in promoting tumorigenesis remain unestablished. We demonstrate that P. anaerobius adheres to the CRC mucosa and accelerates CRC development in Apc^Min/+ mice. In vitro assays and transmission electron microscopy revealed that P. anaerobius selectively adheres to CRC cell lines (HT-29 and Caco-2) compared to normal colonic epithelial cells (NCM460). We identified a P. anaerobius surface protein, putative cell wall binding repeat 2 (PCWBR2), which directly interacts with colonic cell lines via α₂/β₁ integrin, a receptor frequently overexpressed in human CRC tumours and cell lines. Interaction between PCWBR2 and integrin α₂/β₁ induces the activation of the PI3K-Akt pathway in CRC cells via phospho-focal adhesion kinase, leading to increased cell proliferation and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation. NF-κB in turn triggers a pro-inflammatory response as indicated by increased levels of cytokines, such as interleukin-10 and interferon-γ in the tumours of P. anaerobius-treated Apc^Min/+ mice. Analyses of tumour-infiltrating immune cell populations in P. anaerobius-treated Apc^Min/+ mice revealed significant expansion of myeloid-derived suppressor cells, tumour-associated macrophages and granulocytic tumour-associated neutrophils, which are associated with chronic inflammation and tumour progression. Blockade of integrin α₂/β₁ by RGDS peptide, small interfering RNA or antibodies all impair P. anaerobius attachment and abolish P. anaerobius-mediated oncogenic response in vitro and in vivo. Collectively, we show that P. anaerobius drives CRC via a PCWBR2-integrin α₂/β₁-PI3K-Akt-NF-κB signalling axis and identify the PCWBR2-integrin α₂/β₁ axis as a potential therapeutic target for CRC.

Shigella Pathogenesis: New Insights through Advanced Methodologies

Shigella is a genus of Gram-negative enteropathogens that have long been, and continue to be, an important public health concern worldwide. Over the past several decades, Shigella spp. have also served as model pathogens in the study of bacterial pathogenesis, and Shigella flexneri has become one of the best-studied pathogens on a molecular, cellular, and tissue level. In the arms race between Shigella and the host immune system, Shigella has developed highly sophisticated mechanisms to subvert host cell processes in order to promote infection, escape immune detection, and prevent bacterial clearance. Here, we give an overview of Shigella pathogenesis while highlighting innovative techniques and methods whose application has significantly advanced our understanding of Shigella pathogenesis in recent years.

Novel insights into the immune regulatory effects of ferritins from blunt snout bream, Megalobrama amblycephala

Ferritins play vital roles in maintenance of iron homeostasis as iron storage proteins. Recently, the immune function of ferritins have attracted increasing attention, especially their roles in defense against pathogenic infections. However, the immune regulatory mechanism of fish ferritins are not well known. In the present study, comparative digital gene expression (DGE) profiling was performed to explore the regulatory effects of the Megalobrama amblycephala ferritins (MamFers) using MamFers overexpressed and control L8824 cells (Ctenopharyngodon idella hepatic cell line). Clean reads were aligned to the C. idella genome and differential expression analysis was conducted with representative differentially expressed genes pointed out. On that basis, further studies were performed to verify two pivotal regulated pathways in L8824 and EPC (Epithelioma Papulosum Cyprini cell line) cells, respectively. The results showed that NLRC5 (NOD-like Receptor Family CARD Domain Containing 5) mediated the regulation of MamFers on expression of MHC I (Major Histocompatibility Complex Class I) and its chaperone β2M (Beta-2-Microglobulin) in L8824 cells. Then, β2M further mediated the regulation of MamFers on hepcidin expression, indicating that MamFers regulated the expression of hepcidin via NLRC5/MHC I/β2M axis. In addition, MamFers regulated the adhesion of Aeromonas hydrophila to EPC cells by regulating the expression of two extracellular matrix proteins Intgβ1 (integrin β1) and FN (fibronectin). In a word, the present study provided novel insights into the immune regulatory functions of fish ferritins.

Thinking Outside the Bug: Molecular Targets and Strategies to Overcome Antibiotic Resistance

Since their discovery in the early 20th century, antibiotics have been used as the primary weapon against bacterial infections. Due to their prophylactic effect, they are also used as part of the cocktail of drugs given to treat complex diseases such as cancer or during surgery, in order to prevent infection. This has resulted in a decrease of mortality from infectious diseases and an increase in life expectancy in the last 100 years. However, as a consequence of administering antibiotics broadly to the population and sometimes misusing them, antibiotic-resistant bacteria have appeared. The emergence of resistant strains is a global health threat to humanity. Highly-resistant bacteria like Staphylococcus aureus (methicillin-resistant) or Enterococcus faecium (vancomycin-resistant) have led to complications in intensive care units, increasing medical costs and putting patient lives at risk. The appearance of these resistant strains together with the difficulty in finding new antimicrobials has alarmed the scientific community. Most of the strategies currently employed to develop new antibiotics point towards novel approaches for drug design based on prodrugs or rational design of new molecules. However, targeting crucial bacterial processes by these means will keep creating evolutionary pressure towards drug resistance. In this review, we discuss antibiotic resistance and new options for antibiotic discovery, focusing in particular on new alternatives aiming to disarm the bacteria or empower the host to avoid disease onset.

Quercetin Prevents Escherichia coli O157:H7 Adhesion to Epithelial Cells via Suppressing Focal Adhesions

The attachment of Escherichia coli O157:H7 to intestinal epithelial cells is indispensable for its pathogenesis. Besides translocated-intimin receptor (Tir), E. coli O157:H7 interacts with host cell surface receptors to promote intimate adhesion. This study showed that integrin β1 was increased in Caco-2 cells upon E. coli O157:H7 infection, while Caco-2 cells subjected to integrin β1 antibody blocking or CRISPR/Cas9 knockout had reduced bacterial attachment. Infection of E. coli O157:H7 inactivated focal adhesion kinase (FAK) and paxillin, increased focal adhesion (FA) and actin polymerization, and decreased cell migration in Caco-2 cells, which were rescued by integrin β1 antibody blocking or knockout. Pre-treatment with quercetin, known for its anti-oxidant and anti-inflammatory activity, reduced bacterial infection to Caco-2 cells, which might be partially via interfering integrin β1 and FAK association augmented by E. coli O157:H7. In addition, quercetin decreased FA formation induced by bacterial infection and recovered host cell motility. Taken together, data showed that E. coli O157:H7 interacts with integrin β1 to facilitate its adhesion to host cells. Quercetin inhibits bacterial infection possibly by blocking the interaction between E. coli O157:H7 and integrin β1. Collectively, these data indicate that quercetin provides an alternative antimicrobial to mitigate and control E. coli O157:H7 intestinal infection, and suggest potential broad benefits of quercetin and related polyphenols in fighting other enteric pathogen infections.

Glycan-Glycan Interaction Determines Shigella Tropism toward Human T Lymphocytes

Direct interactions between bacterial and host glycans have been recently reported to be involved in the binding of pathogenic bacteria to host cells. In the case of Shigella, the Gram-negative enteroinvasive bacterium responsible for acute rectocolitis, such interactions contribute to bacterial adherence to epithelial cells. However, the role of glycans in the tropism of Shigella for immune cells whose glycosylation pattern varies depending on their activation state is unknown. We previously reported that Shigella targets activated, but not nonactivated, human CD4⁺ T lymphocytes. Here, we show that nonactivated CD4⁺ T lymphocytes can be turned into Shigella-targetable cells upon loading of their plasma membrane with sialylated glycosphingolipids (also termed gangliosides). The Shigella targeting profile of ganglioside-loaded nonactivated T cells is similar to that of activated T cells, with a predominance of injection of effectors from the type III secretion system (T3SS) not resulting in cell invasion. We demonstrate that gangliosides interact with the O-antigen polysaccharide moiety of lipopolysaccharide (LPS), the major bacterial surface antigen, thus promoting Shigella binding to CD4⁺ T cells. This binding step is critical for the subsequent injection of T3SS effectors, a step which we univocally demonstrate to be dependent on actin polymerization. Altogether, these findings highlight the critical role of glycan-glycan interactions in Shigella pathogenesis.

Glycosylation of host cell surface varies with species and location in the body, thus contributing to species specificity and tropism of microorganisms. Cross talk by Shigella, the Gram-negative enteroinvasive bacterium responsible for bacillary dysentery, with its exclusively human host has been extensively studied. However, the molecular determinants of the step of binding to host cells are poorly defined. Taking advantage of the observation that human-activated CD4⁺ T lymphocytes, but not nonactivated cells, are targets of Shigella, we succeeded in rendering the refractory cells susceptible to targeting upon loading of their plasma membrane with sialylated glycosphingolipids (gangliosides) that are abundantly present on activated cells. We show that interactions between the sugar polar part of gangliosides and the polysaccharide moiety of Shigella lipopolysaccharide (LPS) promote bacterial binding, which results in the injection of effectors via the type III secretion system. Whereas LPS interaction with gangliosides was proposed long ago and recently extended to a large variety of glycans, our findings reveal that such glycan-glycan interactions are critical for Shigella pathogenesis by driving selective interactions with host cells, including immune cells.

Beyond the Matrix: The Many Non-ECM Ligands for Integrins

The traditional view of integrins portrays these highly conserved cell surface receptors as mediators of cellular attachment to the extracellular matrix (ECM), and to a lesser degree, as coordinators of leukocyte adhesion to the endothelium. These canonical activities are indispensable; however, there is also a wide variety of integrin functions mediated by non-ECM ligands that transcend the traditional roles of integrins. Some of these unorthodox roles involve cell-cell interactions and are engaged to support immune functions such as leukocyte transmigration, recognition of opsonization factors, and stimulation of neutrophil extracellular traps. Other cell-cell interactions mediated by integrins include hematopoietic stem cell and tumor cell homing to target tissues. Integrins also serve as cell-surface receptors for various growth factors, hormones, and small molecules. Interestingly, integrins have also been exploited by a wide variety of organisms including viruses and bacteria to support infectious activities such as cellular adhesion and/or cellular internalization. Additionally, the disruption of integrin function through the use of soluble integrin ligands is a common strategy adopted by several parasites in order to inhibit blood clotting during hematophagy, or by venomous snakes to kill prey. In this review, we strive to go beyond the matrix and summarize non-ECM ligands that interact with integrins in order to highlight these non-traditional functions of integrins.

Shigella and Enteroinvasive Escherichia Coli

Shigella and enteroinvasive Escherichia coli (EIEC) are gram-negative bacteria responsible for bacillary dysentery (shigellosis) in humans, which is characterized by invasion and inflammatory destruction of the human colonic epithelium. Different EIEC and Shigella subgroups rose independently from commensal E. coli through patho-adaptive evolution that included loss of functional genes interfering with the virulence and/or with the intracellular lifestyle of the bacteria, as well as acquisition of genetic elements harboring virulence genes. Among the latter is the large virulence plasmid encoding for a type three secretion system (T3SS), which enables translocation of virulence proteins (effectors) from the bacterium directly into the host cell cytoplasm. These effectors enable the pathogen to subvert epithelial cell functions, promoting its own uptake, replication in the host cytosol, and dissemination to adjacent cells while concomitantly inhibiting pro-inflammatory cell death. Furthermore, T3SS effectors are directly involved in Shigella manipulation of immune cells causing their dysfunction and promoting cell death. In the current chapter, we first describe the evolution of the enteroinvasive pathovars and then summarize the overall knowledge concerning the pathogenesis of these bacteria, with a particular focus on Shigella flexneri. Subversion of host cell functions in the human gut, both epithelial and immune cells, by different virulence factors is especially highlighted.

Manipulation of host membranes by the bacterial pathogens Listeria, Francisella, Shigella and Yersinia

Bacterial pathogens display an impressive arsenal of molecular mechanisms that allow survival in diverse host niches. Subversion of plasma membrane and cytoskeletal functions are common themes associated to infection by both extracellular and intracellular pathogens. Moreover, intracellular pathogens modify the structure/stability of their membrane-bound compartments and escape degradation from phagocytic or autophagic pathways. Here, we review the manipulation of host membranes by Listeria monocytogenes, Francisella tularensis, Shigella flexneri and Yersinia spp. These four bacterial model pathogens exemplify generalized strategies as well as specific features observed during bacterial infection processes.

Macropinosomes are Key Players in Early Shigella Invasion and Vacuolar Escape in Epithelial Cells

Intracellular pathogens include all viruses, many bacteria and parasites capable of invading and surviving within host cells. Key to survival is the subversion of host cell pathways by the pathogen for the purpose of propagation and evading the immune system. The intracellular bacterium Shigella flexneri, the causative agent of bacillary dysentery, invades host cells in a vacuole that is subsequently ruptured to allow growth of the pathogen within the host cytoplasm. S. flexneri invasion has been classically described as a macropinocytosis-like process, however the underlying details and the role of macropinosomes in the intracellular bacterial lifestyle have remained elusive. We applied dynamic imaging and advanced large volume correlative light electron microscopy (CLEM) to study the highly transient events of S. flexneri's early invasion into host epithelial cells and elucidate some of its fundamental features. First, we demonstrate a clear distinction between two compartments formed during the first step of invasion: the bacterial containing vacuole and surrounding macropinosomes, often considered identical. Next, we report a functional link between macropinosomes and the process of vacuolar rupture, demonstrating that rupture timing is dependent on the availability of macropinosomes as well as the activity of the small GTPase Rab11 recruited directly to macropinosomes. We go on to reveal that the bacterial containing vacuole and macropinosomes come into direct contact at the onset of vacuolar rupture. Finally, we demonstrate that S. flexneri does not subvert pre-existing host endocytic vesicles during the invasion steps leading to vacuolar rupture, and propose that macropinosomes are the major compartment involved in these events. These results provide the basis for a new model of the early steps of S. flexneri epithelial cell invasion, establishing a different view of the enigmatic process of cytoplasmic access by invasive bacterial pathogens.

The genomic signatures of Shigella evolution, adaptation and geographical spread

Shigella spp. are some of the key pathogens responsible for the global burden of diarrhoeal disease. These facultative intracellular bacteria belong to the family Enterobacteriaceae, together with other intestinal pathogens, such as Escherichia coli and Salmonella spp. The genus Shigella comprises four different species, each consisting of several serogroups, all of which show phenotypic similarity, including invasive pathogenicity. DNA sequencing suggests that this similarity results from the convergent evolution of different Shigella spp. founders. Here, we review the evolutionary relationships between Shigella spp. and E . coli, and we highlight how the genomic plasticity of these bacteria and their acquisition of a distinctive virulence plasmid have enabled the development of such highly specialized pathogens. Furthermore, we discuss the insights that genotyping and whole-genome sequencing have provided into the phylogenetics and intercontinental spread of Shigella spp.

The Many Faces of IpaB

The type III secretion system (T3SS) is Shigella's most important virulence factor. The T3SS apparatus (T3SA) is comprised of an envelope-spanning basal body and an external needle topped by a tip complex protein called IpaD. This nanomachine is used to deliver effector proteins into host cells to promote pathogen entry. A key component of the matured T3SS needle tip complex is the translocator protein IpaB. IpaB can exist in multiple states when prepared as a recombinant protein, however, it has also been described as having additional roles in Shigella pathogenesis. This mini-review will briefly describe some of the features of IpaB as a T3SS needle tip protein, as a pore-forming translocator protein and as an effector protein. Reflection on the potential importance of the different in vitro states of IpaB on its function and importance in serotype-independent vaccines is also provided.

Adenylate Cyclase Toxin promotes bacterial internalisation into non phagocytic cells

Bordetella pertussis causes whooping cough, a respiratory infectious disease that is the fifth largest cause of vaccine-preventable death in infants. Though historically considered an extracellular pathogen, this bacterium has been detected both in vitro and in vivo inside phagocytic and non-phagocytic cells. However the precise mechanism used by B. pertussis for cell entry, or the putative bacterial factors involved, are not fully elucidated. Here we find that adenylate cyclase toxin (ACT), one of the important toxins of B. pertussis, is sufficient to promote bacterial internalisation into non-phagocytic cells. After characterization of the entry route we show that uptake of "toxin-coated bacteria" proceeds via a clathrin-independent, caveolae-dependent entry pathway, allowing the internalised bacteria to survive within the cells. Intracellular bacteria were found inside non-acidic endosomes with high sphingomyelin and cholesterol content, or "free" in the cytosol of the invaded cells, suggesting that the ACT-induced bacterial uptake may not proceed through formation of late endolysosomes. Activation of Tyr kinases and toxin-induced Ca(2+)-influx are essential for the entry process. We hypothesize that B. pertussis might use ACT to activate the endocytic machinery of non-phagocytic cells and gain entry into these cells, in this way evading the host immune system.

The roles of the virulence factor IpaB in Shigella spp. in the escape from immune cells and invasion of epithelial cells

Shigellosis is an acute invasive enteric infection by the Gram negative pathogen Shigella, which causes human diarrhea. Shigella, which are highly epidemic and pathogenic, have become a serious public health problem. The virulence plasmid is a large plasmid essential to the infected host cells. Many virulence factors are encoded in the ipa-mxi-spa region by the virulence plasmid. IpaB is a multifunctional and essential virulence factor in the infection process. In this review article, we introduce the recent studies of the effect of IpaB in Shigella-infected host cells. IpaB is involved in a type III secretion system (T3SS) structure. It also controls the secretion of virulence factors and Shigella adhesion to host cells. In addition, it forms the ion pore, destroys phagosomes, and induces the immune cell's apoptosis or necrosis. Moreover, IpaB can become a potential antigen for Shigella vaccine development.

Molecular mechanisms of host cytoskeletal rearrangements by Shigella invasins

Pathogen-induced reorganization of the host cell cytoskeleton is a common strategy utilized in host cell invasion by many facultative intracellular bacteria, such as Shigella, Listeria, enteroinvasive E. coli and Salmonella. Shigella is an enteroinvasive intracellular pathogen that preferentially infects human epithelial cells and causes bacillary dysentery. Invasion of Shigella into intestinal epithelial cells requires extensive remodeling of the actin cytoskeleton with the aid of pathogenic effector proteins injected into the host cell by the activity of the type III secretion system. These so-called Shigella invasins, including IpaA, IpaC, IpgB1, IpgB2 and IpgD, modulate the actin-regulatory system in a concerted manner to guarantee efficient entry of the bacteria into host cells.

TNF-α augmented Porphyromonas gingivalis invasion in human gingival epithelial cells through Rab5 and ICAM-1

Background

Tumor necrosis factor alpha (TNF-α) plays a central role in the initiation and maintenance of immune responses to periodontopathic bacteria. However, excess TNF-α leads to dysregulated immune responses and progression of periodontitis. Porphyromonas gingivalis (P. gingivalis) invades gingival epithelial cells and then multiplies and survives for a long period. Additionally, increment of TNF-α in periodontal sites is associated with a high prevalence of gram-negative anaerobes such as P. gingivalis. However, it has not been determined whether TNF-α affects invasion of P. gingivalis in periodontal tissues.

Results

We examined the effect of TNF-α on invasion of P. gingivalis in gingival epithelial cells and clarified the mechanism by which TNF-α augments invasion of P. gingivalis. Invasion of P. gingivalis into Ca9-22 cells was augmented by stimulation with TNF-α and it was inhibited by treatment with an antibody to TNF receptor-1. TNF-α increased production of ICAM-1, and P. gingivalis invasion was inhibited by an antibody to ICAM-1 in Ca9-22 cells. Silencing of Rab5 mRNA inhibited P. gingivalis invasion. Furthermore, the JNK inhibitor SP600125 inhibited invasion of P. gingivalis and also decreased the active form of Rab5 in Ca9-22 cells.

Conclusion

TNF-α augments invasion of P. gingivalis in human gingival epithelial cells through increment of ICAM-1 and activation of Rab5. These phenomena may contribute to persistent infection of P. ginigvalis and prolongation of immune responses in periodontal tissues.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-014-0229-z) contains supplementary material, which is available to authorized users.

Functional characterizations of effector protein BipC, a type III secretion system protein, in Burkholderia pseudomallei pathogenesis

OBJECTIVES

The bsa locus of Burkholderia pseudomallei encodes several proteins that are components of the type III secretion system (TTSS). BipC was postulated as one of the TTSS-3 effector proteins, but its role in the pathogenesis of B. pseudomallei infection is not well understood. Thus, the aim of this study was to determine its role(s) in the virulence of B. pseudomallei pathogenesis.

METHODS

A bipC TTSS-3-deficient strain of B. pseudomallei and complemented strains were generated to assess the role of BipC as a type III translocation apparatus. Human cell lines and a mouse model of melioidosis were used for in vitro and in vivo assays, respectively.

RESULTS

A significant 2-fold reduction was demonstrated in the percentage of adherence, invasion, intracellular survival, and phagosomal escape of the bipC mutant. Interestingly, microscopic studies have shown that BipC was capable of delayed B. pseudomallei actin-based motility. The virulence of the mutant strain in a murine model of melioidosis demonstrated that the bipC mutant was less virulent, compared with the wild type.

CONCLUSION

The results suggested that BipC possesses virulence determinants that play significant roles in host cell invasion and immune evasion.

Reconstituted human polyclonal plasma-derived secretory-like IgM and IgA maintain the barrier function of epithelial cells infected with an enteropathogen

Intravenous administration of polyclonal and monoclonal antibodies has proven to be a clinically valid approach in the treatment, or at least relief, of many acute and chronic pathologies, such as infection, immunodeficiency, and a broad range of autoimmune conditions. Plasma-derived IgG or recombinant IgG are most frequently used for intravenous or subcutaneous administration, whereas a few IgM-based products are available as well. We have established recently that secretory-like IgA and IgM can be produced upon association of plasma-derived polymeric IgA and IgM with a recombinant secretory component. As a next step toward potential future mucosal administration, we sought to unravel the mechanisms by which these secretory Igs protect epithelial cells located at the interface between the environment and the inside of the body. By using polarized epithelial Caco-2 cell monolayers and Shigella flexneri as a model enteropathogen, we found that polyspecific plasma-derived SIgA and SIgM fulfill many protective functions, including dose-dependent recognition of the antigen via formation of aggregated immune complexes, reduction of bacterial infectivity, maintenance of epithelial cell integrity, and inhibition of proinflammatory cytokine/chemokine production by epithelial cells. In this in vitro model devoid of other cellular or molecular interfering partners, IgM and secretory IgM showed stronger bacterial neutralization than secretory IgA. Together, these data suggest that mucosally delivered antibody preparations may be most effective when combining both secretory-like IgA and IgM, which, together, play a crucial role in preserving several levels of epithelial cell integrity.

Pathogenicity of Shigella in chickens

Shigellosis in chickens was first reported in 2004. This study aimed to determine the pathogenicity of Shigella in chickens and the possibility of cross-infection between humans and chickens. The pathogenicity of Shigella in chickens was examined via infection of three-day-old SPF chickens with Shigella strain ZD02 isolated from a human patient. The virulence and invasiveness were examined by infection of the chicken intestines and primary chicken intestinal epithelial cells. The results showed Shigella can cause death via intraperitoneal injection in SPF chickens, but only induce depression via crop injection. Immunohistochemistry and transmission electron microscopy revealed the Shigella can invade the intestinal epithelia. Immunohistochemistry of the primary chicken intestinal epithelial cells infected with Shigella showed the bacteria were internalized into the epithelial cells. Electron microscopy also confirmed that Shigella invaded primary chicken intestinal epithelia and was encapsulated by phagosome-like membranes. Our data demonstrate that Shigella can invade primary chicken intestinal epithelial cells in vitro and chicken intestinal mucosa in vivo, resulting in pathogenicity and even death. The findings suggest Shigella isolated from human or chicken share similar pathogenicity as well as the possibility of human-poultry cross-infection, which is of public health significance.

The periplasmic enzyme, AnsB, of Shigella flexneri modulates bacterial adherence to host epithelial cells

S. flexneri strains, most frequently linked with endemic outbreaks of shigellosis, invade the colonic and rectal epithelium of their host and cause severe tissue damage. Here we have attempted to elucidate the contribution of the periplasmic enzyme, L-asparaginase (AnsB) to the pathogenesis of S. flexneri. Using a reverse genetic approach we found that ansB mutants showed reduced adherence to epithelial cells in vitro and attenuation in two in vivo models of shigellosis, the Caenorhabditis elegans and the murine pulmonary model. To investigate how AnsB affects bacterial adherence, we compared the proteomes of the ansB mutant with its wild type parental strain using two dimensional differential in-gel electrophoresis and identified the outer membrane protein, OmpA as up-regulated in ansB mutant cells. Bacterial OmpA, is a prominent outer membrane protein whose activity has been found to be required for bacterial pathogenesis. Overexpression of OmpA in wild type S. flexneri serotype 3b resulted in decreasing the adherence of this virulent strain, suggesting that the up-regulation of OmpA in ansB mutants contributes to the reduced adherence of this mutant strain. The data presented here is the first report that links the metabolic enzyme AnsB to S. flexneri pathogenesis.

The involvement of FAK and Src in the invasion of cardiomyocytes by Trypanosoma cruzi

The activation of signaling pathways involving protein tyrosine kinases (PTKs) has been demonstrated during Trypanosoma cruzi invasion. Herein, we describe the participation of FAK/Src in the invasion of cardiomyocytes by T. cruzi. The treatment of cardiomyocytes with genistein, a PTK inhibitor, significantly reduced T. cruzi invasion. Also, PP1, a potent Src-family protein inhibitor, and PF573228, a specific FAK inhibitor, also inhibited T. cruzi entry; maximal inhibition was achieved at concentrations of 25μM PP1 (53% inhibition) and 40μM PF573228 (50% inhibition). The suppression of FAK expression in siRNA-treated cells and tetracycline-uninduced Tet-FAK(WT)-46 cells significantly reduced T. cruzi invasion. The entry of T. cruzi is accompanied by changes in FAK and c-Src expression and phosphorylation. An enhancement of FAK activation occurs during the initial stages of T. cruzi-cardiomyocyte interaction (30 and 60min), with a concomitant increase in the level of c-Src expression and phosphorylation, suggesting that FAK/Src act as an integrated signaling pathway that coordinates parasite entry. These data provide novel insights into the signaling pathways that are involved in cardiomyocyte invasion by T. cruzi. A better understanding of the signal transduction networks involved in T. cruzi invasion may contribute to the development of more effective therapies for the treatment of Chagas' disease.

Systems-level overview of host protein phosphorylation during Shigella flexneri infection revealed by phosphoproteomics

The enteroinvasive bacterium Shigella flexneri invades the intestinal epithelium of humans. During infection, several injected effector proteins promote bacterial internalization, and interfere with multiple host cell responses. To obtain a systems-level overview of host signaling during infection, we analyzed the global dynamics of protein phosphorylation by liquid chromatography-tandem MS and identified several hundred of proteins undergoing a phosphorylation change during the first hours of infection. Functional bioinformatic analysis revealed that they were mostly related to the cytoskeleton, transcription, signal transduction, and cell cycle. Fuzzy c-means clustering identified six temporal profiles of phosphorylation and a functional module composed of ATM-phosphorylated proteins related to genotoxic stress. Pathway enrichment analysis defined mTOR as the most overrepresented pathway. We showed that mTOR complex 1 and 2 were required for S6 kinase and AKT activation, respectively. Comparison with a published phosphoproteome of Salmonella typhimurium-infected cells revealed a large subset of coregulated phosphoproteins. Finally, we showed that S. flexneri effector OspF affected the phosphorylation of several hundred proteins, thereby demonstrating the wide-reaching impact of a single bacterial effector on the host signaling network.

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.

Interplay of host-pathogen microvesicles and their role in infectious disease

The release of extracellular vesicles, whether MVs (microvesicles) or exosomes, from host cells or intracellular pathogens is likely to play a significant role in the infection process. Host MVs may fuse with pathogen surfaces to deliver host complement regulatory proteins. They may also deliver cytokines that enhance invasion. Decoy functions are also possible. Whereas host MVs may direct pathogens away from their target cells, pathogen MVs may in turn redirect complement membrane-attack complexes away from their target pathogen. An understanding of the mechanisms of this interplay, bringing about both immune evasion and enhanced invasion, will help to direct future research with a view to rendering pathogens more susceptible to immune attack or in improving drug efficacy. It should also be possible to use MVs or exosomes isolated directly from the pathogens, or from the cells infected with pathogens, to provide alternative vaccination strategies.

Integrins and small GTPases as modulators of phagocytosis

Phagocytosis is the mechanism whereby cells engulf large particles. This process has long been recognized as a critical component of the innate immune response, which constitutes the organism's defense against microorganisms. In addition, phagocytic internalization of apoptotic cells or cell fragments plays important roles in tissue homeostasis and remodeling. Phagocytosis requires target interactions with receptors on the plasma membrane of the phagocytic cell. Integrins have been identified as important mediators of particle clearance, in addition to their well-established roles in cell adhesion, migration and mechanotransduction. Indeed, these ubiquitously expressed proteins impart phagocytic capacity to epithelial, endothelial and mesenchymal cell types. The importance of integrins in particle internalization is emphasized by the ability of microbial and viral pathogens to exploit their signaling pathways to invade host cells, and by the wide variety of disorders that arise from abnormalities in integrin-dependent phagocytic uptake.

Chlamydial biology and its associated virulence blockers

Chlamydiales are obligate intracellular parasites of eukaryotic cells. They can be distinguished from other Gram-negative bacteria through their characteristic developmental cycle, in addition to special biochemical and physical adaptations to subvert the eukaryotic host cell. The host spectrum includes humans and other mammals, fish, birds, reptiles, insects and even amoeba, causing a plethora of diseases. The first part of this review focuses on the specific chlamydial infection biology and metabolism. As resistance to classical antibiotics is emerging among Chlamydiae as well, the second part elaborates on specific compounds and tools to block chlamydial virulence traits, such as adhesion and internalization, Type III secretion and modulation of gene expression.

Filopodium retraction is controlled by adhesion to its tip

Filopodia are thin cell extensions sensing the environment. They play an essential role during cell migration, cell-cell or cell-matrix adhesion, by initiating contacts and conveying signals to the cell cortex. Pathogenic microorganisms can hijack filopodia to invade cells by inducing their retraction towards the cell body. Because their dynamics depend on a discrete number of actin filaments, filopodia provide a model of choice to study elementary events linked to adhesion and downstream signalling. However, the determinants controlling filopodial sensing are not well characterized. In this study, we used beads functionalized with different ligands that triggered filopodial retraction when in contact with filopodia of epithelial cells. With optical tweezers, we were able to measure forces stalling the retraction of a single filopodium. We found that the filopodial stall force depends on the coating of the bead. Stall forces reached 8 pN for beads coated with the β1 integrin ligand Yersinia Invasin, whereas retraction was stopped with a higher force of 15 pN when beads were functionalized with carboxyl groups. In all cases, stall forces increased in relation to the density of ligands contacting filopodial tips and were independent of the optical trap stiffness. Unexpectedly, a discrete and small number of Shigella type three secretion systems induced stall forces of 10 pN. These results suggest that the number of receptor-ligand interactions at the filopodial tip determines the maximal retraction force exerted by filopodia but a discrete number of clustered receptors is sufficient to induce high retraction stall forces.

Temperature Depended Role of Shigella flexneri Invasion Plasmid on the Interaction with Acanthamoeba castellanii

Shigella flexneri is a Gram-negative bacterium causing the diarrhoeal disease shigellosis in humans. The virulence genes required for invasion are clustered on a large 220 kb plasmid encoding type three secretion system (TTSS) apparatus and virulence factors such as adhesions and invasion plasmid antigens (Ipa). The bacterium is transmitted by contaminated food, water, or from person to person. Acanthamoebae are free-living amoebae (FLA) which are found in diverse environments and isolated from various water sources. Different bacteria interact differently with FLA since Francisella tularensis, Vibrio cholerae, Shigella sonnei, and S. dysenteriae are able to grow inside A. castellanii. In contrast, Pseudomonas aeruginosa induces both necrosis and apoptosis to kill A. castellanii. The aim of this study is to examine the role of invasion plasmid of S. flexneri on the interaction with A. castellanii at two different temperatures. A. castellanii in the absence or presence of wild type, IpaB mutant, or plasmid-cured strain S. flexneri was cultured at 30°C and 37°C and the interaction was analysed by viable count of both bacteria and amoebae, electron microscopy, flow cytometry, and statistical analysis. The outcome of the interaction was depended on the temperature since the growth of A. castellanii was inhibited at 30°C, and A. castellanii was killed by invasion plasmid mediated necrosis at 37°C.

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.

Integrin-mediated uptake of fibronectin-binding bacteria

Invasion of mammalian cells via cell adhesion molecules of the integrin family is a common theme in bacterial pathogenesis. Whereas some microorganisms directly bind to integrins, other pathogens such as Staphylococcus aureus indirectly engage these receptors via fibronectin-binding proteins (FnBPs). In this review, we summarize the structure-function relationship of FnBPs and the current view of the role of these proteins during pathogenesis in vivo. A major focus will be on recent findings on the role of cholesterol- and sphingolipid-rich membrane microdomains for integrin-initiated uptake of fibronectin-binding bacteria and the surprising inhibitory function of caveolin-1 in this process. The detailed mechanistic understanding of host cell invasion by fibronectin-binding S. aureus can not only serve as a paradigm for other fibronectin-binding pathogenic bacteria, but might also reveal the physiological regulation of endocytosis of ligand-occupied integrins.

Polymeric immunoglobulin receptor-mediated invasion of Streptococcus pneumoniae into host cells requires a coordinate signaling of SRC family of protein-tyrosine kinases, ERK, and c-Jun N-terminal kinase

Streptococcus pneumoniae are commensals of the human nasopharynx with the capacity to invade mucosal respiratory cells. PspC, a pneumococcal surface protein, interacts with the human polymeric immunoglobulin receptor (pIgR) to promote bacterial adherence to and invasion into epithelial cells. Internalization of pneumococci requires the coordinated action of actin cytoskeleton rearrangements and the retrograde machinery of pIgR. Here, we demonstrate the involvement of Src protein-tyrosine kinases (PTKs), focal adhesion kinase (FAK), extracellular signal-regulated kinase (ERK), and c-Jun N-terminal kinase (JNK) but not p38 mitogen-activated protein kinases (MAPK) in pneumococcal invasion via pIgR. Pharmacological inhibitors of PTKs and MAPKs and genetic interference with Src PTK and FAK functions caused a significant reduction of pIgR-mediated pneumococcal invasion but did not influence bacterial adhesion to host cells. Furthermore, pneumococcal ingestion by host cells induces activation of ERK1/2 and JNK. In agreement with activated JNK, its target molecule and DNA-binding protein c-Jun was phosphorylated. We also show that functionally active Src PTK is essential for activation of ERK1/2 upon pneumococcal infections. In conclusion, these data illustrate the importance of a coordinated signaling between Src PTKs, ERK1/2, and JNK during PspC-pIgR-mediated uptake of pneumococci by host epithelial cells.

Enteropathogenic Escherichia coli, Samonella, Shigella and Yersinia: cellular aspects of host-bacteria interactions in enteric diseases

A successful infection of the human intestine by enteropathogenic bacteria depends on the ability of bacteria to attach and colonize the intestinal epithelium and, in some cases, to invade the host cell, survive intracellularly and disseminate from cell to cell. To accomplish these processes bacteria have evolved an arsenal of molecules that are mostly secreted by dedicated type III secretion systems, and that interact with the host, subverting normal cellular functions. Here we overview the most important molecular strategies developed by enteropathogenic Escherichia coli, Salmonella enterica, Shigella flexneri, and Yersinia enterocolitica to cause enteric infections. Despite having evolved different effectors, these four microorganisms share common host cellular targets.

Listeria monocytogenes internalin B activates junctional endocytosis to accelerate intestinal invasion

Listeria monocytogenes (Lm) uses InlA to invade the tips of the intestinal villi, a location at which cell extrusion generates a transient defect in epithelial polarity that exposes the receptor for InlA, E-cadherin, on the cell surface. As the dying cell is removed from the epithelium, the surrounding cells reorganize to form a multicellular junction (MCJ) that Lm exploits to find its basolateral receptor and invade. By examining individual infected villi using 3D-confocal imaging, we uncovered a novel role for the second major invasin, InlB, during invasion of the intestine. We infected mice intragastrically with isogenic strains of Lm that express or lack InlB and that have a modified InlA capable of binding murine E-cadherin and found that Lm lacking InlB invade the same number of villi but have decreased numbers of bacteria within each infected villus tip. We studied the mechanism of InlB action at the MCJs of polarized MDCK monolayers and find that InlB does not act as an adhesin, but instead accelerates bacterial internalization after attachment. InlB locally activates its receptor, c-Met, and increases endocytosis of junctional components, including E-cadherin. We show that MCJs are naturally more endocytic than other sites of the apical membrane, that endocytosis and Lm invasion of MCJs depends on functional dynamin, and that c-Met activation by soluble InlB or hepatocyte growth factor (HGF) increases MCJ endocytosis. Also, in vivo, InlB applied through the intestinal lumen increases endocytosis at the villus tips. Our findings demonstrate a two-step mechanism of synergy between Lm's invasins: InlA provides the specificity of Lm adhesion to MCJs at the villus tips and InlB locally activates c-Met to accelerate junctional endocytosis and bacterial invasion of the intestine.

Inactivated and subunit vaccines to prevent shigellosis

Shigellosis remains a formidable disease globally, with children of the developing world bearing the greatest number of infections. The need for an affordable, safe and efficacious vaccine has persisted for decades. Vaccines to prevent shigellosis can be divided into living and nonliving approaches. Several nonliving Shigella vaccines are currently at different stages of development and show substantial promise. Outlined here is an overview of multiple nonliving vaccine technologies, highlighting their current status and recent advances in testing. In addition, gaps in the knowledge base regarding immune mechanisms of protection are explored.

Campylobacter jejuni FlpA binds fibronectin and is required for maximal host cell adherence

Campylobacter jejuni is one of the most frequent bacterial causes of food-borne gastrointestinal disease in developed countries. Previous work indicates that the binding of C. jejuni to human intestinal cells is crucial for host colonization and disease. Fibronectin (Fn), a major constituent of the extracellular matrix, is a approximately 250-kDa glycoprotein present at regions of cell-to-cell contact in the intestinal epithelium. Fn is composed of three types of repeating units: type I (approximately 45 amino acids), type II (approximately 60 amino acids), and type III (approximately 90 amino acids). The deduced amino acid sequence of C. jejuni flpA (Cj1279c) contains at least three Fn type III domains. Based on the presence of the Fn type III domains, we hypothesized that FlpA contributes to the binding of C. jejuni to human INT 407 epithelial cells and Fn. We assessed the contribution of FlpA in C. jejuni binding to host cells by in vitro adherence assays with a C. jejuni wild-type strain and a C. jejuni flpA mutant and binding of purified FlpA protein to Fn by enzyme-linked immunosorbent assay (ELISA). Adherence assays revealed the binding of the C. jejuni flpA mutant to INT 407 epithelial cells was significantly reduced compared with that for a wild-type strain. In addition, rabbit polyclonal serum generated against FlpA blocked C. jejuni adherence to INT 407 cells in a concentration-dependent manner. Binding of FlpA to Fn was found to be dose dependent and saturable by ELISA, demonstrating the specificity of the interaction. Based on these data, we conclude that FlpA mediates C. jejuni attachment to host epithelial cells via Fn binding.

UNC119 inhibits dynamin and dynamin-dependent endocytic processes

Unc119 is an adapter signaling molecule, which regulates activation of tyrosine kinases in T cells, eosinophils and fibroblasts. It plays an important role in the photoreceptor synapses of the retina. Recently, we have shown that it inhibits bacterial uptake through macropinocytosis. In this paper we demonstrate a role for Unc119 in clathrin- and caveolae-based endocytosis as well as macropinocytosis. Depletion of Unc119 in fibroblasts increases, whereas overexpression inhibits uptake of transferrin, FM4-64, albumin, viruses, and ligand-coated beads. Physiological stimuli that upregulate the expression of Unc119 also inhibits endocytosis. Unc119 has the opposite effect on cholera toxin B uptake, which represents a clathrin- and dynamin-independent endocytic process. Unc119 interacts with dynamin, a key effector molecule of many endocytic processes. More importantly, Unc119 inhibits the GTPase activity of dynamin. Binding of Unc119 to dynamin decreases the association with its binding partner amphiphysin, a known regulator of dynamin activation. Thus, Unc119 regulates various endocytic pathways through dynamin and sets a threshold point for vesicular trafficking.

Unc119 protects from Shigella infection by inhibiting the Abl family kinases

Background

Bacteria engage cell surface receptors and intracellular signaling molecules to enter the cell. Unc119 is an adaptor protein, which interacts with receptors and tyrosine kinases. Its role in bacterial invasion of cells is unknown.

Methodology/Principal Findings

We used biochemical, molecular and cell biology approaches to identify the binding partners of Unc119, and to study the effect of Unc119 on Abl family kinases and Shigella infection. We employed loss-of-function and gain-in-function approaches to study the effect of Unc119 in a mouse model of pulmonary shigellosis. Unc119 interacts with Abl family kinases and inhibits their kinase activity. As a consequence, it inhibits Crk phosphorylation, which is essential for Shigella infection. Unc119 co-localizes with Crk and Shigella in infected cells. Shigella infectivity increases in Unc119-deficient epithelial and macrophage cells. In a mouse model of shigellosis cell-permeable TAT-Unc119 inhibits Shigella infection. Conversely, Unc119 knockdown in vivo results in enhanced bacterial invasion and increased lethality. Unc119 is an inducible protein. Its expression is upregulated by probacteria and bacterial products such as lipopolysacharide and sodium butyrate. The latter inhibits Shigella infection in mouse lungs but is ineffective in Unc119 deficiency.

Conclusions

Unc119 inhibits signaling pathways that are used by Shigella to enter the cell. As a consequence it provides partial but significant protection from Shigella infections. Unc119 induction in vivo boosts host defense against infections.