Thermodynamically reengineering the listerial invasion complex InlA/E-cadherin

Two Permeases Associated with the Multifunctional CtaP Cysteine Transport System in Listeria monocytogenes Play Distinct Roles in Pathogenesis

The soil-dwelling bacterium Listeria monocytogenes survives a multitude of conditions when residing in the outside environment and as a pathogen within host cells. Key to survival within the infected mammalian host is the expression of bacterial gene products necessary for nutrient acquisition. Similar to many bacteria, L. monocytogenes uses peptide import to acquire amino acids. Peptide transport systems play an important role in nutrient uptake as well as in additional functions that include bacterial quorum sensing and signal transduction, recycling of peptidoglycan fragments, adherence to eukaryotic cells, and alterations in antibiotic susceptibility. It has been previously described that CtaP, encoded by lmo0135, is a multifunctional protein associated with activities that include cysteine transport, resistance to acid, membrane integrity, and bacterial adherence to host cells. ctaP is located next to two genes predicted to encode membrane-bound permeases lmo0136 and lmo0137, termed CtpP1 and CtpP2, respectively. Here, we show that CtpP1 and CtpP2 are required for bacterial growth in the presence of low concentrations of cysteine and for virulence in mouse infection models. Taken together, the data identify distinct nonoverlapping roles for two related permeases that are important for the growth and survival of L. monocytogenes within host cells. IMPORTANCE Bacterial peptide transport systems are important for nutrient uptake and may additionally function in a variety of other roles, including bacterial communication, signal transduction, and bacterial adherence to eukaryotic cells. Peptide transport systems often consist of a substrate-binding protein associated with a membrane-spanning permease. The environmental bacterial pathogen Listeria monocytogenes uses the substrate-binding protein CtaP not only for cysteine transport but also for resistance to acid, maintenance of membrane integrity, and bacterial adherence to host cells. In this study, we demonstrate complementary yet distinct functional roles for two membrane permeases, CtpP1 and CtpP2, that are encoded by genes linked to ctaP and that contribute to bacterial growth, invasion, and pathogenicity.

Dual-Use Quickscan: A Web-Based Tool to Assess the Dual-Use Potential of Life Science Research

Research on pathogenic organisms is crucial for medical, biological and agricultural developments. However, biological agents as well as associated knowledge and techniques, can also be misused, for example for the development of biological weapons. Potential malicious use of well-intended research, referred to as “dual-use research”, poses a threat to public health and the environment. There are various international resources providing frameworks to assess dual-use potential of the research concerned. However, concrete instructions for researchers on how to perform a dual-use risk assessment is largely lacking. The international need for practical dual-use monitoring and risk assessment instructions, in addition to the need to raise awareness among scientists about potential dual-use aspects of their research has been identified over the last years by the Netherlands Biosecurity Office, through consulting national and international biorisk stakeholders. We identified that Biorisk Management Advisors and researchers need a practical tool to facilitate a dual-use assessment on their specific research. Therefore, the Netherlands Biosecurity Office developed a web-based Dual-Use Quickscan (www.dualusequickscan.com), that can be used periodically by researchers working with microorganisms to assess potential dual-use risks of their research by answering a set of fifteen yes/no questions. The questions for the tool were extracted from existing international open resources, and categorized into three themes: characteristics of the biological agent, knowledge and technology about the biological agent, and consequences of misuse. The results of the Quickscan provide the researcher with an indication of the dual-use potential of the research and can be used as a basis for further discussions with a Biorisk Management Advisor. The Dual-Use Quickscan can be embedded in a broader system of biosafety and biosecurity that includes dual-use monitoring and awareness within organizations. Increased international attention to examine pathogens with pandemic potential has been enhanced by the current COVID-19 pandemic, hence monitoring of dual-use potential urgently needs to be encouraged.

A Structural Study on the Listeria Monocytogenes Internalin A-Human E-cadherin Interaction: A Molecular Tool to Investigate the Effects of Missense Mutations

Listeria monocytogenes is a widespread foodborne pathogen of high concern and internalin A is an important virulence factor that mediates cell invasion upon the interaction with the host protein E-cadherin. Nonsense mutations of internalin A are known to reduce virulence. Although missense mutations are largely overlooked, they need to be investigated in respect to their effects in cell invasion processes. This work presented a computational workflow to early characterize internalin A missense mutations. The method reliably estimated the effects of a set of engineered missense mutations in terms of their effects on internalin A–E-cadherin interaction. Then, the effects of mutations of an internalin A variant from a L. monocytogenes isolate were calculated. Mutations showed impairing effects on complex stability providing a mechanistic explanation of the low cells invasion capacity previously observed. Overall, our results provided a rational approach to explain the effects of internalin A missense mutations. Moreover, our findings highlighted that the strength of interaction may not directly relate to the cell invasion capacity reflecting the non-exclusive role of internalin A in determining the virulence of L. monocytogenes. The workflow could be extended to other virulence factors providing a promising platform to support a better molecular understanding of L. monocytogenes epidemiology.

An extracellular Leptospira interrogans leucine-rich repeat protein binds human E- and VE-cadherins

Pathogenic Leptospira bacteria are the causative agents of leptospirosis, a zoonotic disease affecting animals and humans worldwide. These pathogenic species have the ability to rapidly cross host tissue barriers by a yet unknown mechanism. A comparative analysis of pathogens and saprophytes revealed a higher abundance of genes encoding proteins with leucine-rich repeat (LRR) domains in the genomes of pathogens. In other bacterial pathogens, proteins with LRR domains have been shown to be involved in mediating host cell attachment and invasion. One protein from the pathogenic species Leptospira interrogans, LIC10831, has been previously analysed via X-ray crystallography, with findings suggesting it may be an important bacterial adhesin. Herein we show that LIC10831 elicits an antibody response in infected animals, is actively secreted by the bacterium, and binds human E- and VE-cadherins. These results provide biochemical and cellular evidences of LRR protein-mediated host-pathogen interactions and identify a new multireceptor binding protein from this infectious Leptospira species.

Evaluation of Caco-2 cells response to Listeria monocytogenes virulence factors by RT-PCR

Listeria monocytogenes expresses various virulence factors enabling the invasion and multiplying in host cells, and together induces cytokines transcription. In order to explore the relationship between virulence factors of L. monocytogenes wild-type EGD-e and cellular response in human colonic epithelial cell line(Caco-2), we constructed mutant strains with in-frame deletions of critical virulence genes of inlA, inlB, hly, actA and virulence regulatory factor prfA from EGD-e, respectively. Compared with EGD-e, mutant strains showed significantly decreased invasion and apoptosis in Caco-2 cells. However, mutant strains were capable to evoke cytokines transcription of interleukin-8 (IL-8), mononuclear chemoattractant protein-1 (MCP-1), tumor necrosis factor-a (TNF-a), interleukin-1β (IL-1β), interleukin-6 (IL-6) and CXCL-2 production in Caco-2 cells. Interestingly, EGD-e Δhly-infected Caco-2 cells showed a significant decrease of IL-6, IL-8 and MCP-1 transcription compared with EGD-e at 1 h post-infection. Simultaneously, EGD-e ΔinlB-infected cells showed a decrease in IL-6 transcription, while EGD-e ΔactA-infected cells reflected a decrease in MCP-1 transcription. Virulence genes play a role in inflammatory transcription, but the interaction between pathogenic bacteria and the host cells predominates in inflammatory transcription. Overall, the data showed cellular response of Caco-2 cells infected with EGD-e mutant strains.

Listeria monocytogenes: towards a complete picture of its physiology and pathogenesis

Listeria monocytogenes is a food-borne pathogen responsible for a disease called listeriosis, which is potentially lethal in immunocompromised individuals. This bacterium, first used as a model to study cell-mediated immunity, has emerged over the past 20 years as a paradigm in infection biology, cell biology and fundamental microbiology. In this Review, we highlight recent advances in the understanding of human listeriosis and L. monocytogenes biology. We describe unsuspected modes of hijacking host cell biology, ranging from changes in organelle morphology to direct effects on host transcription via a new class of bacterial effectors called nucleomodulins. We then discuss advances in understanding infection in vivo, including the discovery of tissue-specific virulence factors and the 'arms race' among bacteria competing for a niche in the microbiota. Finally, we describe the complexity of bacterial regulation and physiology, incorporating new insights into the mechanisms of action of a series of riboregulators that are critical for efficient metabolic regulation, antibiotic resistance and interspecies competition.

Structural characterization of a novel subfamily of leucine-rich repeat proteins from the human pathogen Leptospira interrogans

Pathogenic Leptospira spp. are the agents of leptospirosis, an emerging zoonotic disease. Analyses of Leptospira genomes have shown that the pathogenic leptospires (but not the saprophytes) possess a large number of genes encoding proteins containing leucine-rich repeat (LRR) domains. In other pathogenic bacteria, proteins with LRR domains have been shown to be involved in mediating host-cell attachment and invasion, but their functions remain unknown in Leptospira. To gain insight into the potential function of leptospiral LRR proteins, the crystal structures of four LRR proteins that represent a novel subfamily with consecutive stretches of a 23-amino-acid LRR repeat motif have been solved. The four proteins analyzed adopt the characteristic α/β-solenoid horseshoe fold. The exposed residues of the inner concave surfaces of the solenoid, which constitute a putative functional binding site, are not conserved. The various leptospiral LRR proteins could therefore recognize distinct structural motifs of different host proteins and thus serve separate and complementary functions in the physiology of these bacteria.

Of mice and men: Dissecting the interaction between Listeria monocytogenes Internalin A and E-cadherin

We report a study of the interaction between internalin A (inlA) and human or murine E-cadherin (Ecad). inlA is used by Listeria monocytogenes to internalize itself into host cell, but the bacterium is unable to invade murine cells, which has been attributed to the difference in sequence between hEcad and mEcad. Using molecular dynamics simulations, MM/GBSA free energy calculations, hydrogen bond analysis, water characterization and umbrella sampling, we provide a complete atomistic picture of the binding between inlA and Ecad. We dissect key residues in the protein–protein interface and analyze the energetics using MM/GBSA. From this analysis it is clear that the binding of inlA–mEcad is weaker than inlA–hEcad, on par with the experimentally observed inability of inlA to bind to mEcad. However, extended MD simulations of 200 ns in length show no destabilization of the inlA–mEcad complex and the estimation of the potential of mean force (PMF) using umbrella sampling corroborates this conclusion. The binding strength computed from the PMFs show no significant difference between the two protein complexes. Hence, our study suggests that the inability of L. monocytogenes to invade murine cells cannot be explained by processes at the nanosecond to sub-microsecond time scale probed by the simulations performed here.

Fusobacterium nucleatum adhesin FadA binds vascular endothelial cadherin and alters endothelial integrity

Fusobacterium nucleatum is a Gram-negative oral anaerobe, capable of systemic dissemination causing infections and abscesses, often in mixed-species, at different body sites. We have shown previously that F. nucleatum adheres to and invades host epithelial and endothelial cells via a novel FadA adhesin. In this study, vascular endothelial (VE)-cadherin, a member of the cadherin family and a cell-cell junction molecule, was identified as the endothelial receptor for FadA, required for F. nucleatum binding to the cells. FadA colocalized with VE-cadherin on endothelial cells, causing relocation of VE-cadherin away from the cell-cell junctions. As a result, the endothelial permeability was increased, allowing the bacteria to cross the endothelium through loosened junctions. This crossing mechanism may explain why the organism is able to disseminate systemically to colonize in different body sites and even overcome the placental and blood-brain barriers. Co-incubation of F. nucleatum and Escherichia coli enhanced penetration of the endothelial cells by the latter in the transwell assays, suggesting F. nucleatum may serve as an 'enabler' for other microorganisms to spread systemically. This may explain why F. nucleatum is often found in mixed infections. This study reveals a possible novel dissemination mechanism utilized by pathogens.

Aromatic rings in chemical and biological recognition: energetics and structures

This review describes a multidimensional treatment of molecular recognition phenomena involving aromatic rings in chemical and biological systems. It summarizes new results reported since the appearance of an earlier review in 2003 in host-guest chemistry, biological affinity assays and biostructural analysis, data base mining in the Cambridge Structural Database (CSD) and the Protein Data Bank (PDB), and advanced computational studies. Topics addressed are arene-arene, perfluoroarene-arene, S⋅⋅⋅aromatic, cation-π, and anion-π interactions, as well as hydrogen bonding to π systems. The generated knowledge benefits, in particular, structure-based hit-to-lead development and lead optimization both in the pharmaceutical and in the crop protection industry. It equally facilitates the development of new advanced materials and supramolecular systems, and should inspire further utilization of interactions with aromatic rings to control the stereochemical outcome of synthetic transformations.

Directed evolution and targeted mutagenesis to murinize Listeria monocytogenes internalin A for enhanced infectivity in the murine oral infection model

BACKGROUND

Internalin A (InlA) is a critical virulence factor which mediates the initiation of Listeria monocytogenes infection by the oral route in permissive hosts. The interaction of InlA with the host cell ligand E-cadherin efficiently stimulates L. monocytogenes entry into human enterocytes, but has only a limited interaction with murine cells.

RESULTS

We have created a surface display library of randomly mutated InlA in a non-invasive heterologous host Lactococcus lactis in order to create and screen novel variants of this invasion factor. After sequential passage through a murine cell line (CT-26), multiple clones with enhanced invasion characteristics were identified. Competitive index experiments were conducted in mice using selected mutations introduced into L. monocytogenes EGD-e background. A novel single amino acid change was identified which enhanced virulence by the oral route in the murine model and will form the basis of further engineering approaches. As a control a previously described EGD-InlA(m) murinized strain was also re-created as part of this study with minor modifications and designated EGD-e InlA(m)*. The strain was created using a procedure that minimizes the likelihood of secondary mutations and incorporates Listeria-optimized codons encoding the altered amino acids. L. monocytogenes EGD-e InlA(m)* yielded consistently higher level murine infections by the oral route when compared to EGD-e, but did not display the two-fold increased invasion into a human cell line that was previously described for the EGD-InlA(m) strain.

CONCLUSIONS

We have used both site-directed mutagenesis and directed evolution to create variants of InlA which may inform future structure-function analyses of this protein. During the course of the study we engineered a murinized strain of L. monocytogenes EGD-e which shows reproducibly higher infectivity in the intragastric murine infection model than the wild type, but does not display enhanced entry into human cells as previously observed. This murinized L. monocytogenes strain will provide a useful tool for the analysis of the gastrointestinal phase of listeriosis.

Listeria monocytogenes: at the coalface of host-pathogen research

Listeria monocytogenes is a highly adaptable food-borne pathogen that causes the life threatening illness listeriosis in infected individuals. Within the host this bacterium invades cells, escapes into the host cell cytosol and replicates intracellularly. To achieve this L. monocytogenes has evolved a sophisticated set of molecular weaponry that allows it to interact with and manipulate the cell biology of the host to its own advantage. Many of these interactions are well understood, putting this pathogen at the forefront of host-pathogen research, but fascinating new interactions are still emerging. The seventeenth International Symposium on Problems of Listeriosis (ISOPOL) was held in Portugal (Porto) in May of this year and this report describes some of the exciting developments that were presented at the meeting. The report focuses on developments in understanding the molecular interactions between L. monocytogenes and the host; it describes novel uses for L. monocytogenes as an anti-cancer treatment; and it describes some innovative uses of transcriptional profiling and reporter gene fusions that are helping illuminate our understanding of the basic biology of this important pathogen.

Listeria monocytogenes internalin and E-cadherin: from bench to bedside

Listeria monocytogenes is a Gram-positive bacterium responsible for a severe infection associated with different clinical features (gastroenteritis, meningoencephalitis, and abortion in pregnant women). These pathologies are caused by the unusual capacity of the bacterium to cross three host barriers during infection and to invade nonphagocytic cells. To invade host cells, Listeria uses two proteins, InlA and InlB, which have specific receptors on the host-cell surface, E-cadherin and Met, respectively. Here, we discuss the specificity of the InlA-E-cadherin interaction, the signaling cascade activated on E-cadherin engagement by InlA, and the role of InlA and E-cadherin in the breaching of host barriers and the dissemination of the infection.

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.

Protective capacities of cell surface-associated proteins of Streptococcus suis mutants deficient in divalent cation-uptake regulators

Many cell surface-associated, divalent cation-regulated proteins are immunogenic, and some of them confer protection against the bacterial species from which they are derived. In this work, two Streptococcus suis divalent cation uptake regulator genes controlling zinc/manganese and iron uptake (adcR and fur, respectively) were inactivated in order to study the protective capacities of their cell surface-associated proteins. The results obtained showed overexpression of a set of immunogenic proteins (including members of the pneumococcal histidine triad family previously reported to confer protection against streptococcal pathogens) in S. suis adcR mutant cell surface extracts. Likewise, genes encoding zinc transporters, putative virulence factors and a ribosomal protein paralogue related to zinc starvation appeared to be derepressed in this mutant strain. Moreover, protection assays in mice showed that although neither adcR- nor fur-regulated cell surface-associated proteins were sufficient to confer protection in mice, the combination of both adcR- and fur-regulated cell surface-associated proteins is able to confer significant protection (50 %, P=0.038) against a challenge to mice vaccinated with them.

A survey of the year 2007 literature on applications of isothermal titration calorimetry

Elucidation of the energetic principles of binding affinity and specificity is a central task in many branches of current sciences: biology, medicine, pharmacology, chemistry, material sciences, etc. In biomedical research, integral approaches combining structural information with in-solution biophysical data have proved to be a powerful way toward understanding the physical basis of vital cellular phenomena. Isothermal titration calorimetry (ITC) is a valuable experimental tool facilitating quantification of the thermodynamic parameters that characterize recognition processes involving biomacromolecules. The method provides access to all relevant thermodynamic information by performing a few experiments. In particular, ITC experiments allow to by-pass tedious and (rarely precise) procedures aimed at determining the changes in enthalpy and entropy upon binding by van't Hoff analysis. Notwithstanding limitations, ITC has now the reputation of being the "gold standard" and ITC data are widely used to validate theoretical predictions of thermodynamic parameters, as well as to benchmark the results of novel binding assays. In this paper, we discuss several publications from 2007 reporting ITC results. The focus is on applications in biologically oriented fields. We do not intend a comprehensive coverage of all newly accumulated information. Rather, we emphasize work which has captured our attention with originality and far-reaching analysis, or else has provided ideas for expanding the potential of the method.

Ligand binding by repeat proteins: natural and designed

Repeat proteins contain tandem arrays of small structural motifs. As a consequence of this architecture, they adopt non-globular, extended structures that present large, highly specific surfaces for ligand binding. Here we discuss recent advances toward understanding the functional role of this unique modular architecture. We showcase specific examples of natural repeat proteins interacting with diverse ligands and also present examples of designed repeat protein-ligand interactions.

Crystal structure and standardized geometric analysis of InlJ, a listerial virulence factor and leucine-rich repeat protein with a novel cysteine ladder

We report on the crystal structure of the internalin domain of InlJ, a virulence-associated surface protein of Listeria monocytogenes, at 2.7-A resolution. InlJ is a member of the internalin family of listerial cell surface proteins characterized by a common N-terminal domain. InlJ bears 15 leucine-rich repeats (LRRs), the same number as in InlA, the prototypical internalin family member. The LRRs of InlJ differ from those of other internalins by having 21, rather than 22, residues and by replacing 1 LRR-defining hydrophobic residue with a conserved cysteine. These cysteines stack to form an intramolecular ladder and regular hydrophobic interactions in consecutive repeats. Analyzing the curvature, twist, and lateral bending angles of InlJ and comparing these with several other LRR proteins, we provide a systematic geometric comparison of LRR protein structures (http://bragi2.helmholtz-hzi.de/Angulator/). These indicate that both cysteine and asparagine ladders stabilize the LRR fold, whereas substitutions in some repeat positions are more likely than others to induce changes in LRR geometry.