From alpha to beta: identification of amino acids required for the N-acetyllactosamine-specific lectin-like activity of bundlin

Indispensable functions of ABL and PDGF receptor kinases in epithelial adherence of attaching/effacing pathogens under physiological conditions

Enteropathogenic Escherichia coli (EPEC) and Citrobacter rodentium are attaching-and-effacing (A/E) pathogens that cause intestinal inflammation and diarrhea. The bacteria adhere to the intestinal epithelium, destroy microvilli, and induce actin-filled membranous pedestals but do not invade the mucosa. Adherence leads to activation of several host cell kinases, including FYN, n-SRC, YES, ABL, and ARG, phosphorylation of the bacterial translocated intimin receptor, and actin polymerization and pedestal formation in cultured cells. However, marked functional redundancy appears to exist between kinases, and their physiological importance in A/E pathogen infections has remained unclear. To address this question, we employed a novel dynamic in vitro infection model that mimics transient and short-term interactions in the intestinal tract. Screening of a kinase inhibitor library and RNA interference experiments in vitro revealed that ABL and platelet-derived growth factor (PDGF) receptor (PDGFR) kinases, as well as p38 MAP kinase, have unique, indispensable roles in early attachment of EPEC to epithelial cells under dynamic infection conditions. Studies with mutant EPEC showed that the attachment functions of ABL and PDGFR were independent of the intimin receptor but required bacterial bundle-forming pili. Furthermore, inhibition of ABL and PDGFR with imatinib protected against infection of mice with modest loads of C. rodentium, whereas the kinases were dispensable for high inocula or late after infection. These results indicate that ABL and PDGFR have indispensable roles in early A/E pathogen attachment to intestinal epithelial cells and for in vivo infection with limiting inocula but are not required for late intimate bacterial attachment or high inoculum infections.

The hypervariable region of meningococcal major pilin PilE controls the host cell response via antigenic variation

Type IV pili (Tfp) are expressed by many Gram-negative bacteria to promote aggregation, adhesion, internalization, twitching motility, or natural transformation. Tfp of Neisseria meningitidis, the causative agent of cerebrospinal meningitis, are involved in the colonization of human nasopharynx. After invasion of the bloodstream, Tfp allow adhesion of N. meningitidis to human endothelial cells, which leads to the opening of the blood-brain barrier and meningitis. To achieve firm adhesion, N. meningitidis induces a host cell response that results in elongation of microvilli surrounding the meningococcal colony. Here we study the role of the major pilin subunit PilE during host cell response using human dermal microvascular endothelial cells and the pharynx carcinoma-derived FaDu epithelial cell line. We first show that some PilE variants are unable to induce a host cell response. By engineering PilE mutants, we observed that the PilE C-terminus domain, which contains a disulfide bonded region (D-region), is critical for the host cell response and that hypervariable regions confer different host cell specificities. Moreover, the study of point mutants of the pilin D-region combined with structural modeling of PilE revealed that the D-region contains two independent regions involved in signaling to human dermal microvascular endothelial cells (HDMECs) or FaDu cells. Our results indicate that the diversity of the PilE D-region sequence allows the induction of the host cell response via several receptors. This suggests that Neisseria meningitidis has evolved a powerful tool to adapt easily to many niches by modifying its ability to interact with host cells.

Type IV pili (Tfp) are long appendages expressed by many Gram-negative bacteria, including Neisseria meningitidis, the causative agent of cerebrospinal meningitis. These pili are involved in many aspects of pathogenesis: natural competence, aggregation, adhesion, and twitching motility. More specifically, Neisseria meningitidis, which is devoid of a secretion system to manipulate its host, has evolved its Tfp to signal to brain endothelial cells and open the blood-brain barrier. In this report, we investigate, at the molecular level, the involvement of the major pilin subunit PilE in host cell response. Our results indicate that the PilE C-terminal domain, which contains a disulfide bonded region (D-region), is critical for the host cell response and contains two independent regions involved in host cell signaling.

Type IV pilin proteins: versatile molecular modules

Type IV pili (T4P) are multifunctional protein fibers produced on the surfaces of a wide variety of bacteria and archaea. The major subunit of T4P is the type IV pilin, and structurally related proteins are found as components of the type II secretion (T2S) system, where they are called pseudopilins; of DNA uptake/competence systems in both Gram-negative and Gram-positive species; and of flagella, pili, and sugar-binding systems in the archaea. This broad distribution of a single protein family implies both a common evolutionary origin and a highly adaptable functional plan. The type IV pilin is a remarkably versatile architectural module that has been adopted widely for a variety of functions, including motility, attachment to chemically diverse surfaces, electrical conductance, acquisition of DNA, and secretion of a broad range of structurally distinct protein substrates. In this review, we consider recent advances in this research area, from structural revelations to insights into diversity, posttranslational modifications, regulation, and function.

Bundle-forming pilus retraction enhances enteropathogenic Escherichia coli infectivity

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

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

Sticky situation: localized adherence of enteropathogenic Escherichia coli to the small intestine epithelium

Enteropathogenic Escherichia coli (EPEC) primarily cause gastrointestinal illness in neonates. They accomplish this by a complex coordinated multistage strategy, whereby the organisms colonize the epithelial lining of the small intestine. This process can be divided into four stages: first, localized, nonintimate adherence; second, type III secretion-mediated injection of effector proteins, third effacement of microvilli and, finally, intimate adherence. In this article, we review the history and current state of knowledge, as well as present potential future directions for further investigating the fascinating processes by which EPEC and related organisms colonize the human intestine and cause disease.

N-acetyllactosamine-induced retraction of bundle-forming pili regulates virulence-associated gene expression in enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) are a major cause of infant morbidity and mortality due to diarrhoea in developing countries. The pathogenesis of EPEC is dependent on a coordinated multi-step process culminating in the intimate adherence of the organisms to the host's intestinal mucosa. During the initial stages of the EPEC colonization process, the fimbrial adhesin, bundle-forming pili (BFP), plays an integral role. We previously reported that the major BFP structural subunit, bundlin, displays lectin-like properties, which enables BFP to initially tether EPEC to N-acetyllactosamine (LacNAc) glycan receptors on host cell surfaces. We also reported that incubating EPEC with synthetic LacNAc-bearing neoglycoconjugates not only inhibits their adherence to host cells, but also induces BFP retraction and subsequent degradation of the bundlin subunits. Herein, we demonstrate that the periplasmic serine protease, DegP, is required for degrading bundlin during this process. We also show that DegP appears to act as a bundlin chaperone during BFP assembly and that LacNAc-BSA-induced BFP retraction is followed by transcriptional upregulation of the BFP operon and downregulation of the locus of enterocyte effacement operons in EPEC.

Interactions of enteropathogenic Escherichia coli with pediatric and adult intestinal biopsy specimens during early adherence

Enteropathogenic Escherichia coli (EPEC) strains cause watery diarrhea almost exclusively in young children. The basis for this age discrimination has never been determined, but it may be related to host cell receptors. During infection, EPEC strains express type IV bundle-forming pili composed of repeating subunits of the protein called bundlin. The very first interaction between EPEC and in vitro-cultured epithelial cells is mediated by the binding of alpha-bundlin to a carbohydrate receptor that contains, at a minimum, the N-acetyllactosamine (LacNAc) glycan sequence. However, bundlins expressed from the beta-bundlin allele do not bind LacNAc glycan sequences. Herein, we investigated whether EPEC strains use alpha-bundlin to mediate early adherence to human intestinal biopsy specimens cultured in vitro by assessing the ability of isogenic EPEC mutants expressing either the alpha(1)- or beta(1)-bundlin allele or a bundlin-deficient EPEC strain to bind to these specimens. Furthermore, we directly compared the abilities of a wild-type EPEC strain to bind to the epithelial surfaces of both human adult and pediatric biopsy specimens. Our results demonstrate that beta-bundlin does not act as an adhesin during early EPEC adherence to adult duodenal biopsy specimens. The results also indicate that EPEC binds equally well to adult and pediatric biopsy specimens in an early adherence assay. This result is supported by the finding that the early adherence of EPEC to both adult and pediatric biopsy specimens was inhibited by LacNAc neoglycoconjugates, suggesting that organisms expressing alpha-bundlin-type bundle-forming pili initially bind to related glycan receptors in both age groups.