Cloning and expression of the Escherichia coli K1 outer membrane protein A receptor, a gp96 homologue

Involvement of Escherichia coli K1 ibeT in bacterial adhesion that is associated with the entry into human brain microvascular endothelial cells

IbeT is a downstream gene of the invasion determinant ibeA in the chromosome of a clinical isolate of Escherichia coli K1 strain RS218 (serotype 018:K1:H7). Both ibeT and ibeA are in the same operon. Our previous mutagenesis and complementation studies suggested that ibeT may coordinately contribute to E. coli K1 invasion with ibeA. An isogenic in-frame deletion mutant of ibeT has been made by chromosomal gene replacement with a recombinant suicide vector carrying a fragment with an ibeT internal deletion. The characteristics of the mutant in meningitic E. coli infection were examined in vitro [cell culture of human brain microvascular endothelial cells (HBMEC)] and in vivo (infant rat model of E. coli meningitis) in comparison with the parent strain. The ibeT deletion mutant was significantly less adhesive and invasive than its parent strain E. coli E44 in vitro, and the adhesion- and invasion-deficient phenotypes of the mutant can be complemented by the ibeT gene. Recombinant IbeT protein is able to block E. coli E44 invasion of HBMEC. Furthermore, the ibeT deletion mutant is less capable of colonizing intestine and less virulent in bacterial translocation across the blood-brain barrier (BBB) than its parent E. coli E44 in vivo. These data suggest that ibeT-mediated E. coli K1 adhesion is associated with the bacterial invasion process.

A molecular Swiss army knife: OmpA structure, function and expression

The OmpA outer membrane protein of Escherichia coli and other enterobacteria is a multifaceted protein. This protein is expressed to very high levels and ompA is tightly regulated at the posttranscriptional level. It can function as an adhesin and invasin, participate in biofilm formation, act as both an immune target and evasin, and serves as a receptor for several bacteriophages. Many of these properties are due to four short protein loops that emanate from the protein to the outside of the cell. Herein it is described how the structure of this protein relates to its many functions.

Identification of a new OmpA-like protein in Neisseria gonorrhoeae involved in the binding to human epithelial cells and in vivo colonization

Outer membrane protein As (OmpAs) are highly conserved proteins within the Enterobacteriaceae family. OmpA contributes to the maintenance of structural membrane integrity and invasion into mammalian cells. In Escherichia coli K1 OmpA also contributes to serum resistance and is involved in the virulence of the bacterium. Here we describe the identification of an OmpA-like protein in Neisseria gonorrhoeae (Ng-OmpA). We show that the gonococcal OmpA-like protein, similarly to E. coli OmpA, plays a significant role in the adhesion and invasion into human cervical carcinoma and endometrial cells and is required for entry into macrophages and intracellular survival. Furthermore, the isogenic knockout ompA mutant demonstrates reduced recovery in a mouse model of infection when compared with the wild-type strain, suggesting that Ng-OmpA plays an important role in the in vivo colonization. All together, these data suggest that the newly identified surface exposed protein Ng-OmpA represents a novel virulence factor of gonococcus.

PSF is an IbeA-binding protein contributing to meningitic Escherichia coli K1 invasion of human brain microvascular endothelial cells

During the development of Escherichia coli K1 meningitis, interaction between E. coli invasion protein IbeA and surface protein(s) on human brain microvascular endothelial cells (HBMEC) is required for invasion and IbeA-mediated signaling. Here, an IbeA-binding protein was identified as polypyrimidine tract-binding protein (PTB)-associated splicing factor (PSF). The specific binding was confirmed by ligand overlay assay. The cell surface-expressed PSF was verified by the confocal microscopy. Recombinant PSF blocked E. coli K1 invasion of HBMEC effectively. Overexpression of PSF in the lentivirus-transducted HBMEC significantly enhanced E. coli K1 invasion. These results suggest that IbeA interacts with PSF for the E. coli K1 invasion of HBMEC.

Fungal invasion of normally non-phagocytic host cells

Many fungi that cause invasive disease invade host epithelial cells during mucosal and respiratory infection, and subsequently invade endothelial cells during hematogenous infection. Most fungi invade these normally non-phagocytic host cells by inducing their own uptake. Candida albicans hyphae interact with endothelial cells in vitro by binding to N-cadherin on the endothelial cell surface. This binding induces rearrangement of endothelial cell microfilaments, which results in the endocytosis of the organism. The capsule of Cryptococcus neoformans is composed of glucuronoxylomannan, which binds specifically to brain endothelial cells, and appears to mediate both adherence and induction of endocytosis. The mechanisms by which other fungal pathogens induce their own uptake are largely unknown. Some angioinvasive fungi, such as Aspergillus species and the Zygomycetes, invade endothelial cells from the abluminal surface during the initiation of invasive disease, and subsequently invade the luminal surface of endothelial cells during hematogenous dissemination. Invasion of normally non-phagocytic host cells has different consequences, depending on the type of invading fungus. Aspergillus fumigatus blocks apoptosis of pulmonary epithelial cells, whereas Paracoccidioides brasiliensis induces apoptosis of epithelial cells. This review summarizes the mechanisms by which diverse fungal pathogens invade normally non-phagocytic host cells and discusses gaps in our knowledge that provide opportunities for future research.

Serum amyloid A is an innate immune opsonin for Gram-negative bacteria

Serum amyloid A (SAA) is the major acute-phase protein in man and most mammals. Recently we demonstrated that SAA binds to many Gram-negative bacteria including Escherichia coli and Pseudomonas aeruginosa through outer membrane protein A (OmpA) family members. Therefore we investigated whether SAA altered the response of innate phagocytic cells to bacteria. Both the percentage of neutrophils containing E coli and the number of bacteria per neutrophil were greatly increased by SAA opsonization, equivalent to the increase seen for serum opsonization. In contrast, no change was seen for Streptococcus pneumoniae, a bacteria that did not bind SAA. Neutrophil reactive oxygen intermediate production in response to bacteria was also increased by opsonization with SAA. SAA opsonization also increased phagocytosis of E coli by peripheral blood mononuclear cell-derived macrophages. These macrophages showed strong enhancement of TNF-alpha and IL-10 production in response to SAA-opsonized E coli and P aeruginosa. SAA did not enhance responses in the presence of bacteria to which it did not bind. These effects of SAA occur at normal concentrations consistent with SAA binding properties and a role in innate recognition. SAA therefore represents a novel innate recognition protein for Gram-negative bacteria.

Meningitis-Associated Escherichia coli

Escherichia coliis the most common Gram-negative organism causing neonatal meningitis. Neonatal E. colimeningitis continues to be an important cause of mortality and morbidity throughout the world. Our incomplete knowledge of its pathogenesis and pathophysiology contributes to such mortality and morbidity. Recent reports of neonatal meningitis caused by E. coli strains producing CTX-M-type or TEM-type extended-spectrum β-lactamases create a challenge. E. colipenetration into the brain, the essential step in the development of E. coli meningitis, requires a high-degree of bacteremia and penetration of the blood-brain barrier as live bacteria, but the underlying mechanisms remain incompletely understood. Recent functional genomic approaches of meningitis-causing E. coli in both in vitro and in vivo models of the blood-brain barrier (e.g., human brain microvascular endothelial cells and animal models of experimental hematogenousE. colimeningitis, respectively) have identified several E. coli factors contributing to a high-degree of bacteremia, as well as specific microbial factors contributing to E. coli invasion of the blood-brain barrier. In addition, E. coli penetration of the blood-brain barrier involves specific host factors as well as microbe- and host-specific signaling molecules. Blockade of such microbial and host factors and host cell signaling molecules is efficient in preventing E. coli penetration into the brain. Continued investigation of the microbial and host factors contributing to E. colibacteremia andinvasion of the blood-brain barrier is likely to identify new targets for prevention and therapy of E. coli meningitis, thereby limiting the exposure to emerging antimicrobial-resistant E. coli.

Invasion processes of pathogenic Escherichia coli

Pathogenic Escherichia coli causes extraintestinal infections such as urinary tract infection and meningitis, which are prevalent and associated with considerable morbidity. Previous investigations have identified common strategies evolved by pathogenic E. coli to exploit host cell function and cause extraintestinal infections, which include the invasion into non-phagocytic eukaryotic cells such as epithelial and endothelial cells and associated host cell actin cytoskeletal rearrangements. However, the mechanisms involved in pathogenic E. coli invasion of eukaryotic cells are shown to differ depending upon types of host tissues and microbial determinants. In this mini-review, invasion processes of pathogenic E. coli are discussed using E. coli K1 invasion of human brain microvascular endothelial cells (HBMEC) as a paradigm. E. coli K1 is the most common Gram-negative organism causing neonatal meningitis, and E. coli invasion of HBMEC is shown to be a prerequisite for E. coli traversal of the blood-brain barrier in vivo. Previous studies have demonstrated that E. coli translocation of the blood-brain barrier is the result of specific E. coli host interactions including specific signal transduction pathways and modulation of endocytic pathways. Recent studies using functional genomics have identified additional microbial determinants contributing to E. coli K1 invasion of HBMEC. Complete understanding of microbial-host interactions that are involved in E. coli K1 invasion of HBMEC should help in the development of new strategies to prevent E. coli meningitis.

Gp96 is a receptor for a novel Listeria monocytogenes virulence factor, Vip, a surface protein

By comparative genomics, we have identified a gene of the intracellular pathogen Listeria monocytogenes that encodes an LPXTG surface protein absent from nonpathogenic Listeria species. This gene, vip, is positively regulated by PrfA, the transcriptional activator of the major Listeria virulence factors. Vip is anchored to the Listeria cell wall by sortase A and is required for entry into some mammalian cells. Using a ligand overlay approach, we identified a cellular receptor for Vip, the endoplasmic reticulum (ER) resident chaperone Gp96 recently shown to interact with TLRs. The Vip-Gp96 interaction is critical for bacterial entry into some cells. Comparative infection studies using oral and intravenous inoculation of nontransgenic and transgenic mice expressing human E-cadherin demonstrated a role for Vip in Listeria virulence, not only at the intestine level but also in late stages of the infectious process. Vip thus appears as a new virulence factor exploiting Gp96 as a receptor for cell invasion and/or signalling events that may interfere with the host immune response in the course of the infection.

Adhesins and invasins of pathogenic bacteria: a structural view

Adhesion and invasion of pathogenic bacteria represent the important initial step of infection. Pathogens utilize surface-located adhesins/invasins for specific interaction with host cell receptors. The three-dimensional structures of a number of adhesins/invasins show that many are elongated molecules containing domains commonly found in eukaryotic proteins. Similar folds are employed repeatedly to target different receptors.

Stress cytokines: pivotal proteins in immune regulatory networks; Opinion

Stress proteins have three immunological regulatory functions: within the cell, on the cell membrane as signalling receptors, and in the extracellular environment as stress cytokines. They can activate the immune system by providing danger signals or they may downregulate immune and inflammatory responses. In addition, they can modulate immune responses by acting as chaperones for antigenic peptides while they themselves are processed and presented to T cells as self-peptides. We predict that the exploitation of the downregulatory properties of stress cytokines will have therapeutic applications in the treatment of human chronic inflammatory diseases, such as rheumatoid arthritis.

Serum amyloid A protein binds to outer membrane protein A of gram-negative bacteria

Serum amyloid A (SAA) is the major acute phase protein in man and most mammals. We observed SAA binding to a surprisingly large number of Gram-negative bacteria, including Escherichia coli, Salmonella typhimurium, Shigella flexneri, Klebsiella pneumoniae, Vibrio cholerae, and Pseudomonas aeruginosa. The binding was found to be high affinity and rapid. Importantly, this binding was not inhibited by high density lipoprotein with which SAA is normally complexed in serum. Binding was also observed when bacteria were offered serum containing SAA. Ligand blots following SDS-PAGE or two-dimensional gels revealed two major ligands of 29 and 35 kDa that bound SAA when probing with radiolabeled SAA or SAA and monoclonal anti-SAA. Following fractionation the ligand was found in the outer membrane fraction of E. coli and was identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry to be outer membrane protein A (OmpA). OmpA-deficient E. coli did not bind SAA, and following purification of OmpA the protein retained binding activity. The ligands on other bacteria were likely to be homologues of OmpA because wild type, but not OprF-deficient, P. aeruginosa bound SAA.

Escherichia coli K-1 interaction with human brain micro-vascular endothelial cells triggers phospholipase C-gamma1 activation downstream of phosphatidylinositol 3-kinase

Escherichia coli, the most common Gram-negative bacterium that causes meningitis in neonates, invades human brain microvascular endothelial cells (HBMEC) by rearranging host cell actin via the activation of phosphatidylinositol 3-kinase (PI3K) and PKC-alpha. Here, further, we show that phospholipase (PLC)-gamma1 is phosphorylated on tyrosine 783 and condenses at the HBMEC membrane beneath the E. coli entry site. Overexpression of a dominant negative (DN) form of PLC-gamma, the PLC-z fragment, in HBMEC inhibits PLC-gamma1 activation and significantly blocks E. coli invasion. PI3K activation is not affected in PLC-z/HBMEC upon infection, whereas PKC-alpha phosphorylation is completely abolished, indicating that PLC-gamma1 is downstream of PI3K. Concomitantly, the phosphorylation of PLC-gamma1 is blocked in HBMEC overexpressing a dominant negative form of the p85 subunit of PI3K but not in HBMEC overexpressing a dominant negative form of PKC-alpha. In addition, the recruitment of PLC-gamma1 to the cell membrane in both PLC-z/HBMEC and DN-p85/HBMEC is inhibited. Activation of PI3K is associated with the conversion of phosphatidylinositol 4,5-bisphosphate (PIP2) to phosphatidylinositol 1,4,5-trisphosphate (PIP3), which in turn recruits PLC-gamma1 to the cell membrane via its interaction with pleckstrin homology domain of PLC-gamma1. Utilizing the pleckstrin homology domains of PKC-delta and Btk proteins fused to green fluorescent protein (GFP), which specifically interact with PIP2 and PIP3, respectively, we show herein that E. coli invasion induces the breakdown of PIP2 at the plasma membrane near the site of E. coli interaction. PIP3, on the other hand, recruits the GFPBkt to the cell membrane beneath the sites of E. coli attachment. Our studies further show that E. coli invasion induces the release of Ca2+ from intracellular pools as well as the influx of Ca2+ from the extracellular medium. This elevation in Ca2+ levels is completely blocked both in PLC-z/HBMEC and DN-p85/HBMEC, but not in DN-PKC/HBMEC. Taken together, these results suggest that E. coli infection of HBMEC induces PLC-gamma1 activation in a PI3K-dependent manner to increase Ca2+ levels in HBMEC. This is the first report demonstrating the recruitment of activated PLC-gamma1 to the sites of bacterial entry.

Entry and intracellular replication of Escherichia coli K1 in macrophages require expression of outer membrane protein A

Interactions between Escherichia coli K1, which causes meningitis in neonates, and macrophages have not been explored well. In this study we found that E. coli K1 was able to enter, survive, and replicate intracellularly in both murine and human macrophage cell lines, as well as in monocytes and macrophages of newborn rats. In addition, we demonstrated that OmpA (+) E. coli also enters and replicates in human peripheral blood monocytes in vitro. Outer membrane protein A (OmpA) expression on E. coli contributes to binding to macrophages, phagocytosis, and survival within macrophages. Opsonization with either complement proteins or antibody is not required for uptake and survival of the bacteria within the macrophages. Transmission electron microscopy and immunocytochemistry studies with the infected macrophages indicated that OmpA(+) E. coli multiplies enormously in a single phagosome and bursts the cell. Internalization of OmpA(+) E. coli by RAW 264.7 cells occurred by both actin- and microtubule-dependent processes, which are independent of RGD-mediated integrin receptors. Internalization and intracellular survival within phagocytic cells thus may play an important role in the development of bacteremia, which is crucial for E. coli crossing of the blood-brain barrier.