Key residues for membrane binding, oligomerization, and pore forming activity of staphylococcal alpha-hemolysin identified by cysteine scanning mutagenesis and targeted chemical modification

High hemolytic activity in Staphylococcus aureus t1081/ST45 due to increased hla protein production and potential RNAIII-independent regulation

BACKGROUND

α-Hemolysin, encoded by hla, is a major virulence factor of Staphylococcus aureus. Sequence type (ST) 45 is a globally spread clone with increasing clinical prevalence in Taiwan. Our previous study showed that among the CC45 isolates, the spa type t1081 isolates presented greater hemolytic activity.

MATERIALS AND METHODS

The hemolytic activity of 67 CC45 isolates (44 t1081 and 23 non-t1081) from clinical blood cultures was assessed using rabbit red blood cells. The sequences of hla and its upstream regulatory regions and RNAIII were compared between the two groups. The expression of hla and its regulators RNAIII, mgrA, and saeR was analyzed via qRT‒PCR, while Hla protein levels were measured via Western blotting.

RESULTS

Compared with non-t1081 isolates, t1081 isolates presented increased hemolytic activity. No significant differences in hla sequences, upstream regulatory regions, or gene expression levels were detected between the two groups. The expression of the transcriptional regulators mgrA and saeR was also similar between the two groups. Western blotting revealed increased Hla protein in the t1081 isolates. However, neither the sequence or expression of RNAIII, a regulator of hla at both the transcriptional and posttranscriptional levels, differed between the groups.

CONCLUSION

Our study revealed that, compared with other CC45 isolates, the t1081/ST45 isolates presented greater hemolytic activity. This heightened activity was due mainly to increased Hla protein levels. Moreover, the higher translation levels may be independent of the known regulator RNAIII, indicating a potential RNAIII-independent mechanism for Hla regulation.

α-Hemolysin from Staphylococcus aureus Changes the Epigenetic Landscape of Th17 Cells

The human body harbors a substantial population of bacteria, which may outnumber host cells. Thus, there are multiple interactions between both cell types. Given the common presence of Staphylococcus aureus in the human body and the role of Th17 cells in controlling this pathogen on mucous membranes, we sought to investigate the effect of α-hemolysin, which is produced by this bacterium, on differentiating Th17 cells. RNA sequencing analysis revealed that α-hemolysin influences the expression of signature genes for Th17 cells as well as genes involved in epigenetic regulation. We observed alterations in various histone marks and genome methylation levels via whole-genome bisulfite sequencing. Our findings underscore how bacterial proteins can significantly influence the transcriptome, epigenome, and phenotype of human Th17 cells, highlighting the intricate and complex nature of the interaction between immune cells and the microbiota.

Bacillus velezensis iturins inhibit the hemolytic activity of Staphylococcus aureus

Bovine mastitis caused by S. aureus has a major economic impact on the dairy sector. With the crucial need for new therapies, anti-virulence strategies have gained attention as alternatives to antibiotics. Here we aimed to identify novel compounds that inhibit the production/activity of hemolysins, a virulence factor of S. aureus associated with mastitis severity. We screened Bacillus strains obtained from diverse sources for compounds showing anti-hemolytic activity. Our results demonstrate that lipopeptides produced by Bacillus spp. completely prevented the hemolytic activity of S. aureus at certain concentrations. Following purification, both iturins, fengycins, and surfactins were able to reduce hemolysis caused by S. aureus, with iturins showing the highest anti-hemolytic activity (up to 76% reduction). The lipopeptides showed an effect at the post-translational level. Molecular docking simulations demonstrated that these compounds can bind to hemolysin, possibly interfering with enzyme action. Lastly, molecular dynamics analysis indicated general stability of important residues for hemolysin activity as well as the presence of hydrogen bonds between iturins and these residues, with longevous interactions. Our data reveals, for the first time, an anti-hemolytic activity of lipopeptides and highlights the potential application of iturins as an anti-virulence therapy to control bovine mastitis caused by S. aureus.

Investigation of Staphylococcus aureus Biofilm-Associated Toxin as a Potential Squamous Cell Carcinoma Therapeutic

Cancer therapies developed using bacteria and their components have been around since the 19th century. Compared to traditional cancer treatments, the use of bacteria-derived compounds as cancer therapeutics could offer a higher degree of specificity, with minimal off-target effects. Here, we explored the use of soluble bacteria-derived toxins as a potential squamous cell carcinoma (SCC) therapeutic. We optimized a protocol to generate Staphylococcus aureus biofilm-conditioned media (BCM), where soluble bacterial products enriched in the development of biofilms were isolated from a bacterial culture and applied to SCC cell lines. Bioactive components of S. aureus ATCC 29213 (SA29213) BCM display selective toxicity towards cancerous human skin SCC-12 at low doses, while non-cancerous human keratinocyte HaCaT and fibroblast BJ-5ta are minimally affected. SA29213 BCM treatment causes DNA damage to SCC-12 and initiates Caspase 3-dependent-regulated cell death. The use of the novel SA29213 bursa aurealis transposon mutant library led to the identification of S. aureus alpha hemolysin as the main bioactive compound responsible for the observed SCC-12-specific toxicity. The antibody neutralisation of Hla eradicates the cytotoxicity of SA29213 BCM towards SCC-12. Hla displays high SCC-12-specific toxicity, which is exerted primarily through Hla-ADAM10 interaction, Hla oligomerisation, and pore formation. The high target specificity and potential to cause cell death in a controlled manner highlight SA29213 Hla as a good candidate as an alternative SCC therapeutic.

Enhanced Hemolytic Activity of Mesophilic Aeromonas salmonicida SRW-OG1 Is Brought about by Elevated Temperatures

Aeromonas salmonicida is a well-known cold-water pathogenic bacterium. Previously, we reported the first isolation of pathogenic A. salmonicida from diseased Epinephelus coioides, a kind of warm-water fish, and it was proved to be a putative mesophilic strain with potent pathogenicity to humans. In order to investigate the mechanisms underlying mesophilic growth ability and virulence, the transcriptome of A. salmonicida SRW-OG1 at 18, 28, and 37 °C was analyzed. The transcriptome of A. salmonicida SRW-OG1 at different temperatures showed a clear separation boundary, which might provide valuable information for the temperature adaptation and virulence regulation of A. salmonicida SRW-OG1. Interestingly, aerA and hlyA, the hemolytic genes encoding aerolysin and hemolysin, were found to be significantly up-regulated at 28 and 37 °C. Since aerolysin and hemolysin are the most well-known and -characterized virulence factors of pathogenic Aeromonas strains, the induction of aerA and hlyA was associated with the mesophilic virulence. Further study proved that the extracellular products (ECPs) purchased from A. salmonicida SRW-OG1 cultured at 28 and 37 °C showed elevated hemolytic activity and virulence than those at 18 °C. Moreover, the silence of aerA and hlyA led to significantly decreased hemolysis and virulence. Taken together, our results revealed that the mesophilic virulence of A. salmonicida SRW-OG1 might be due to the enhanced expression of aerA and hlyA induced by elevated temperatures.

Molecular Targets for Antibody-Based Anti-Biofilm Therapy in Infective Endocarditis

Infective endocarditis (IE) is a heart disease caused by the infection of heart valves, majorly caused by Staphilococcus aureus. IE is initiated by bacteria entering the blood circulation in favouring conditions (e.g., during invasive procedures). So far, the conventional antimicrobial strategies based on the usage of antibiotics remain the major intervention for treating IE. Nevertheless, the therapeutic efficacy of antibiotics in IE is limited not only by the bacterial drug resistance, but also by the formation of biofilms, which resist the penetration of antibiotics into bacterial cells. To overcome these drawbacks, the development of anti-biofilm treatments that can expose bacteria and make them more susceptible to the action of antibiotics, therefore resulting in reduced antimicrobial resistance, is urgently required. A series of anti-biofilm strategies have been developed, and this review will focus in particular on the development of anti-biofilm antibodies. Based on the results previously reported in the literature, several potential anti-biofilm targets are discussed, such as bacterial adhesins, biofilm matrix and bacterial toxins, covering their antigenic properties (with the identification of potential promising epitopes), functional mechanisms, as well as the antibodies already developed against these targets and, where feasible, their clinical translation.

Inhibitory Effect of Andrographis paniculata Lactone on Staphylococcus aureus α-Hemolysin

We investigated the effect of andrographolide (AP) on the hemolytic capacity of Staphylococcus aureus (S. aureus) isolated from our region. AP is a labdane diterpenoid isolated from the stem and leaves of Andrographis paniculata. The hla gene from 234 S. aureus strains and the quality control standard strain ATCC29213 in dairy cows in some areas of Ningxia was analyzed. Evolutionary analysis, homology modeling, and functional enrichment annotation of α-hemolysin Hla detected from our region were performed through bioinformatics. The hemolytic ability of S. aureus isolates from the region was examined using the hemolysis test, and the effect of AP on S. aureus was quantified. Moreover, the effect of AP on the transcript levels of hla and genes highly related to hla (i.e., clfA and fnbA) was examined through fluorescence quantitative PCR. The mode of action of AP on the detected Hla was analyzed through molecular docking and dynamic simulation. The results showed that S. aureus in our region has a high rate of hla carriage. The hemolytic activity of strains NM98 and XF10 was significant, and ATCC29213 also exhibited some hemolytic activity. AP could inhibit the expression of Hla and its related proteins by downregulating hla, clfA, and fnbA transcript levels, which in turn attenuated the S. aureus hemolytic activity. Meanwhile, the AP molecule can form three hydrogen bonds with residues ASN105, SER106, and THR155 of Hla protein; bind with PRO103 through alkyl intermolecular forces; and form carbon hydrogen bonds with LYS154, reflecting that the AP molecule has a comparatively ideal theoretical binding activity with Hla protein. Among them, PRO103 and LYS154 are highly conserved in Hla protein molecules and play pivotal roles in the biological functions of Hla, and their binding may affect these functions. Their binding may also prevent the conformational transition of Hla from a monomer to an oligomer, thus inhibiting Hla hemolytic activity. This study offers a molecular basis for use of AP as an antivirulence drug and new ideas for developing novel drugs against S. aureus infection.

Antigenic landscapes on Staphylococcus aureus pore-forming toxins reveal insights into specificity and cross-neutralization

Staphylococcus aureus carries an exceptional repertoire of virulence factors that aid in immune evasion. Previous single-target approaches for S. aureus-specific vaccines and monoclonal antibodies (mAbs) have failed in clinical trials due to the multitude of virulence factors released during infection. Emergence of antibiotic-resistant strains demands a multi-target approach involving neutralization of different, non-overlapping pathogenic factors. Of the several pore-forming toxins that contribute to S. aureus pathogenesis, efforts have largely focused on mAbs that neutralize α-hemolysin (Hla) and target the receptor-binding site. Here, we isolated two anti-Hla and three anti-Panton-Valentine Leukocidin (LukSF-PV) mAbs, and used a combination of hydrogen deuterium exchange mass spectrometry (HDX-MS) and alanine scanning mutagenesis to delineate and validate the toxins’ epitope landscape. Our studies identified two novel, neutralizing epitopes targeted by 2B6 and CAN6 on Hla that provided protection from hemolytic activity in vitro and showed synergy in rodent pneumonia model against lethal challenge. Of the anti-LukF mAbs, SA02 and SA131 showed specific neutralization activity to LukSF-PV while SA185 showed cross-neutralization activity to LukSF-PV, γ-hemolysin HlgAB, and leukotoxin ED. We further compared these antigen-specific mAbs to two broadly neutralizing mAbs, H5 (targets Hla, LukSF-PV, HlgAB, HlgCB, and LukED) and SA185 (targeting LukSF-PV, HlgAB, and LukED), and identified molecular level markers for broad-spectrum reactivity among the pore-forming toxins by HDX-MS. To further underscore the need to target the cross-reactive epitopes on leukocidins for the development of broad-spectrum therapies, we annotated Hla sequences isolated from patients in multiple countries for genomic variations within the perspective of our defined epitopes.

Applications and potentials of nanopore sequencing in the (epi)genome and (epi)transcriptome era

The Human Genome Project opened an era of (epi)genomic research, and also provided a platform for the development of new sequencing technologies. During and after the project, several sequencing technologies continue to dominate nucleic acid sequencing markets. Currently, Illumina (short-read), PacBio (long-read), and Oxford Nanopore (long-read) are the most popular sequencing technologies. Unlike PacBio or the popular short-read sequencers before it, which, as examples of the second or so-called Next-Generation Sequencing platforms, need to synthesize when sequencing, nanopore technology directly sequences native DNA and RNA molecules. Nanopore sequencing, therefore, avoids converting mRNA into cDNA molecules, which not only allows for the sequencing of extremely long native DNA and full-length RNA molecules but also document modifications that have been made to those native DNA or RNA bases. In this review on direct DNA sequencing and direct RNA sequencing using Oxford Nanopore technology, we focus on their development and application achievements, discussing their challenges and future perspective. We also address the problems researchers may encounter applying these approaches in their research topics, and how to resolve them.

Using multiscale molecular dynamics simulations to obtain insights into pore forming toxin mechanisms

Pore forming toxins (PFTs) are virulent proteins released by several species, including many strains of bacteria, to attack and kill host cells. In this article, we focus on the utility of molecular dynamics (MD) simulations and the molecular insights gleaned from these techniques on the pore forming pathways of PFTs. In addition to all-atom simulations which are widely used, coarse-grained MARTINI models and structure-based models have also been used to study PFTs. Here, the emphasis is on methods and techniques involved while setting up, monitoring, and evaluating properties from MD simulations of PFTs in a membrane environment. We draw from several case studies to illustrate how MD simulations have provided molecular insights into protein-protein and protein-lipid interactions, lipid dynamics, conformational transitions and structures of both the oligomeric intermediates and assembled pore structures.

How protein engineering has revealed the molecular mechanisms of pore-forming toxins

Pore-forming proteins are found in prokaryotes, vertebrates, and invertebrates, and when involved in pathogenic processes they are classified as pore-forming toxins (PFTs). The use of gene engineering methods in combination with the information provided by the high-resolution crystal structures of the PFTs have allowed investigators to gain a deep understanding of their pore-forming mechanisms. In this chapter, we discuss how protein engineering has helped us and others to reveal the molecular mechanisms of pore formation by prokaryotic PFTs with an emphasis on our experiences with the cholesterol-dependent cytolysins (CDCs).

Pyrene Excimer-Based Fluorescent Labeling of Cysteines Brought into Close Proximity by Protein Dynamics: ASEM-Induced Thiol-Ene Click Reaction for High Spatial Resolution CLEM

Fluorescence microscopy (FM) has revealed vital molecular mechanisms of life. Mainly, molecules labeled by fluorescent probes are imaged. However, the diversity of labeling probes and their functions remain limited. We synthesized a pyrene-based fluorescent probe targeting SH groups, which are important for protein folding and oxidative stress sensing in cells. The labeling achieved employs thiol-ene click reactions between the probes and SH groups and is triggered by irradiation by UV light or an electron beam. When two tagged pyrene groups were close enough to be excited as a dimer (excimer), they showed red-shifted fluorescence; theoretically, the proximity of two SH residues within ~30 Å can thus be monitored. Moreover, correlative light/electron microscopy (CLEM) was achieved using our atmospheric scanning electron microscope (ASEM); radicals formed in liquid by the electron beam caused the thiol-ene click reactions, and excimer fluorescence of the labeled proteins in cells and tissues was visualized by FM. Since the fluorescent labeling is induced by a narrow electron beam, high spatial resolution labeling is expected. The method can be widely applied to biological fields, for example, to study protein dynamics with or without cysteine mutagenesis, and to beam-induced micro-fabrication and the precise post-modification of materials.

Structure-based discovery of a small-molecule inhibitor of methicillin-resistant Staphylococcus aureus virulence

The rapid emergence and dissemination of methicillin-resistant Staphylococcus aureus (MRSA) strains poses a major threat to public health. MRSA possesses an arsenal of secreted host-damaging virulence factors that mediate pathogenicity and blunt immune defenses. Panton-Valentine leukocidin (PVL) and α-toxin are exotoxins that create lytic pores in the host cell membrane. They are recognized as being important for the development of invasive MRSA infections and are thus potential targets for antivirulence therapies. Here, we report the high-resolution X-ray crystal structures of both PVL and α-toxin in their soluble, monomeric, and oligomeric membrane-inserted pore states in complex with n-tetradecylphosphocholine (C₁₄PC). The structures revealed two evolutionarily conserved phosphatidylcholine-binding mechanisms and their roles in modulating host cell attachment, oligomer assembly, and membrane perforation. Moreover, we demonstrate that the soluble C₁₄PC compound protects primary human immune cells in vitro against cytolysis by PVL and α-toxin and hence may serve as the basis for the development of an antivirulence agent for managing MRSA infections.

Staphylococcus argenteus and Staphylococcus schweitzeri are cytotoxic to human cells in vitro due to high expression of alpha-hemolysin Hla

Staphylococcus argenteus and Staphylococcus schweitzeri are newly identified species of the S. aureus-related complex. S. argenteus, as occurring globally and showing significant prevalence and comparable infection and morbidity rates compared to S. aureus, is becoming clinically important. Whole genome sequencing has revealed the presence of several virulence genes but the molecular mechanisms of S. argenteus infection and virulence are largely unknown. Here, we studied the effect of a previously characterized clinical S. argenteus isolate on human cells in vitro. The clinical isolate, together with the S. argenteus type strain MSHR1132T and the S. schweitzeri type strain FSA084T, had a cytotoxic effect on the cells, which showed necrotic cell death after a few hours of treatment. The protein causing the cytotoxic effect was purified and identified by mass spectrometry as alpha-hemolysin, Hla, which is awell-known pore-forming toxin in S.aureus. The cytotoxic effect could be blocked with an antibody against Hla. S.argenteus showed 12–15 fold higher expression levels of hla at the RNA level and 4–6 fold higher expression levels at the protein level compared to S.aureus. The higher expression levels of hla were supported by higher RNA levels of the regulatory factors sarA and saeR. Also, the RNAIII component of the accessory gene regulator (agr) quorum sensing system was 8,000–10,000 fold higher in the S.argenteus isolates compared to S.aureus. This is the first study on the effect of S.argenteus on ahuman cell line and strengthens the idea of significant virulence of S.argenteus.

Surface coatings for solid-state nanopores

Since their introduction in 2001, solid-state nanopores have been increasingly exploited for the detection and characterization of biomolecules ranging from single DNA strands to protein complexes. A major factor that enables the application of nanopores to the analysis and characterization of a broad range of macromolecules is the preparation of coatings on the pore wall to either prevent non-specific adhesion of molecules or to facilitate specific interactions of molecules of interest within the pore. Surface coatings can therefore be useful to minimize clogging of nanopores or to increase the residence time of target analytes in the pore. This review article describes various coatings and their utility for changing pore diameters, increasing the stability of nanopores, reducing non-specific interactions, manipulating surface charges, enabling interactions with specific target molecules, and reducing the noise of current recordings through nanopores. We compare the coating methods with respect to the ease of preparing the coating, the stability of the coating and the requirement for specialized equipment to prepare the coating.

Cellular demolition: Proteins as molecular players of programmed cell death

Apoptosis, a well-characterized and regulated cell death programme in eukaryotes plays a fundamental role in developing or later-life periods to dispose of unwanted cells to maintain typical tissue architecture, homeostasis in a spatiotemporal manner. This silent cellular death occurs without affecting any neighboring cells/tissue and avoids triggering of immunological response. Furthermore, diminished forms of apoptosis result in cancer and autoimmune diseases, whereas unregulated apoptosis may also lead to the development of a myriad of neurodegenerative diseases. Unraveling the mechanistic events in depth will provide new insights into understanding physiological control of apoptosis, pathological consequences of abnormal apoptosis and development of novel therapeutics for diseases. Here we provide a brief overview of molecular players of programmed cell death with discussion on the role of caspases, modifications, ubiquitylation in apoptosis, removal of the apoptotic body and its relevance to diseases.

Redirecting Pore Assembly of Staphylococcal α-Hemolysin by Protein Engineering

α-Hemolysin (αHL), a β-barrel pore-forming toxin (βPFT), is secreted as a water-soluble monomer by Staphylococcus aureus. Upon binding to receptors on target cell membranes, αHL assembles to form heptameric membrane-spanning pores. We have previously engineered αHL to create a protease-activatable toxin that is activated by site-specific proteolysis including by tumor proteases. In this study, we redesigned αHL so that it requires 2-fold activation on target cells through (i) binding to specific receptors, and (ii) extracellular proteolytic cleavage. To assess our strategy, we constructed a fusion protein of αHL with galectin-1 (αHLG1, αHL-Galectin-1 chimera). αHLG1 was cytolytic toward cells that lack a receptor for wild-type αHL. We then constructed protease-activatable mutants of αHLG1 (PAMαHLG1s). PAMαHLG1s were activated by matrix metalloproteinase 2 (MMP-2) and had approximately 50-fold higher cytolytic activity toward MMP-2 overexpressing cells (HT-1080 cells) than toward non-overexpressing cells (HL-60 cells). Our approach provides a novel strategy for tailoring pore-forming toxins for therapeutic applications.

GPER activation protects against epithelial barrier disruption by Staphylococcus aureus α-toxin

Sex bias in innate defense against Staphylococcus aureus skin and soft tissue infection (SSTI) is dependent on both estrogen production by the host and S. aureus secretion of the virulence factor, α-hemolysin (Hla). The impact of estrogen signaling on the immune system is most often studied in terms of the nuclear estrogen receptors ERα and ERβ. However, the potential contribution of the G protein-coupled estrogen receptor (GPER) to innate defense against infectious disease, particularly with respect to skin infection, has not been addressed. Using a murine model of SSTI, we found that GPER activation with the highly selective agonist G-1 limits S. aureus SSTI and Hla-mediated pathogenesis, effects that were absent in GPER knockout mice. Specifically, G-1 reduced Hla-mediated skin lesion formation and pro-inflammatory cytokine production, while increasing bacterial clearance. In vitro, G-1 reduced surface expression of the Hla receptor, ADAM10, in a human keratinocyte cell line and increased resistance to Hla-mediated permeability barrier disruption. This novel role for GPER activation in skin innate defense against infectious disease suggests that G-1 may have clinical utility in patients with epithelial permeability barrier dysfunction or who are otherwise at increased risk of S. aureus infection, including those with atopic dermatitis or cancer.

Pharmacological Targeting of Pore-Forming Toxins as Adjunctive Therapy for Invasive Bacterial Infection

For many of the most important human bacterial infections, invasive disease severity is fueled by the cell damaging and pro-inflammatory effects of secreted pore-forming toxins (PFTs). Isogenic PFT-knockout mutants, e.g., Staphylococcus aureus lacking α-toxin or Streptococcus pneumoniae deficient in pneumolysin, show attenuation in animal infection models. This knowledge has inspired multi-model investigations of strategies to neutralize PFTs or counteract their toxicity as a novel pharmacological approach to ameliorate disease pathogenesis in clinical disease. Promising examples of small molecule, antibody or nanotherapeutic drug candidates that directly bind and neutralize PFTs, block their oligomerization or membrane receptor interactions, plug establishment membrane pores, or boost host cell resiliency to withstand PFT action have emerged. The present review highlights these new concepts, with a special focus on β-PFTs produced by leading invasive human Gram-positive bacterial pathogens. Such anti-virulence therapies could be applied as an adjunctive therapy to antibiotic-sensitive and -resistant strains alike, and further could be free of deleterious effects that deplete the normal microflora.

Alanine Scanning Mutagenesis of the MEDI4893 (Suvratoxumab) Epitope Reduces Alpha Toxin Lytic Activity In Vitro and Staphylococcus aureus Fitness in Infection Models

Alpha toxin (AT) is a cytolytic pore-forming toxin that plays a key role in Staphylococcus aureus pathogenesis; consequently, extensive research was undertaken to understand the AT mechanism of action and its utility as a target for novel prophylaxis and treatment strategies against S. aureus infections. MEDI4893 (suvratoxumab) is a human anti-AT IgG1 monoclonal antibody (MAb) that targets AT and is currently in phase 2 clinical development.

Alpha toxin (AT) is a cytolytic pore-forming toxin that plays a key role in Staphylococcus aureus pathogenesis; consequently, extensive research was undertaken to understand the AT mechanism of action and its utility as a target for novel prophylaxis and treatment strategies against S. aureus infections. MEDI4893 (suvratoxumab) is a human anti-AT IgG1 monoclonal antibody (MAb) that targets AT and is currently in phase 2 clinical development. As shown previously, the MEDI4893-binding epitope on AT is comprised of the highly conserved amino acid regions 177 to 200 and 261 to 271, suggesting these amino acids are important for AT function. To test this hypothesis and gain insight into the effect of mutations in the epitope on AT neutralization by MEDI4893, nine MEDI4893 contact residues in AT were individually mutated to alanine. Consistent with our hypothesis, 8 out of 9 mutants exhibited >2-fold loss in lytic activity resulting from a defect in cell binding and pore formation. MEDI4893 binding affinity was reduced >2-fold (2- to 27-fold) for 7 out of 9 mutants, and no binding was detected for the W187A mutant. MEDI4893 effectively neutralized all of the lytic mutants in vitro and in vivo. When the defective mutants were introduced into an S. aureus clinical isolate, the mutant-expressing strains exhibited less severe disease in mouse models and were effectively neutralized by MEDI4893. These results indicate the MEDI4893 epitope is highly conserved due in part to its role in AT pore formation and bacterial fitness, thereby decreasing the likelihood for the emergence of MAb-resistant variants.

Defined Bilayer Interactions of DNA Nanopores Revealed with a Nuclease-Based Nanoprobe Strategy

DNA nanopores are a recent class of bilayer-puncturing nanodevices that can help advance biosensing, synthetic biology, and nanofluidics. Here, we create archetypal lipid-anchored DNA nanopores and characterize them with a nanoprobe-based approach to gain essential information about their interactions with bilayers. The strategy determines the molecular accessibility of DNA pores with a nuclease and can thus distinguish between the nanopores' membrane-adhering and membrane-spanning states. The analysis reveals, for example, that pores interact with bilayers in two steps whereby fast initial membrane tethering is followed by slower reorientation to the puncturing state. Tethering occurs for pores with one anchor, while puncturing requires multiple anchors. Both low and high-curvature membranes are good substrates for tethering, but efficient insertion proceeds only for high-curvature bilayers of the examined lipid composition. This is likely due to the localized lipid misalignments and the associated lower energetic barrier for pore permeation. Our study advances the fields of DNA nanotechnology and nanopores by overcoming the considerable experimental hurdle of efficient membrane insertion. It also provides mechanistic insights to aid the design of advanced nanopores, and offers a useful route to probe bilayer orientation of DNA nanostructures.

Molecular Mechanism of the Flavonoid Natural Product Dryocrassin ABBA against Staphylococcus aureus Sortase A

The intractability of bacterial resistance presents a dilemma for therapies against Staphylococcus aureus (S. aureus) infection. Effective anti-virulence strategies are urgently needed, reflecting the proliferation of resistant strains. Inhibitors of sortase A (SrtA), enzymes that anchor virulence-related surface proteins, are regarded as promising candidates for countermeasures against bacterial infections. In the present study, the inhibitory effect of dryocrassin ABBA (ABBA) against SrtA and its molecular basis has been examined. Fluorescence resonance energy transfer (FRET) assays were used to determine the inhibitory activity of ABBA against SrtA. To identify the mechanism underlying this activity, molecular dynamics simulations and mutagenesis assays were applied, and the results revealed that the direct engagement of SrtA via ABBA through binding to V166 and V168 significantly attenuated the catalytic activity of SrtA. Taken together, these findings indicated that ABBA is a potential novel antimicrobial agent for S. aureus infection via targeting SrtA.

Dissecting the role of ADAM10 as a mediator of Staphylococcus aureus α-toxin action

This work elucidates the role of the transmembrane protease ADAM10 (a disintegrin and metalloprotease 10) for the action of Staphylococcus aureus α-toxin, by showing that the cytotoxicity of α-toxin does not depend on ADAM10’s catalytic activity but on the chaperone function of its prodomain.

Curcumin protects mice from Staphylococcus aureus pneumonia by interfering with the self-assembly process of α-hemolysin

α-hemolysin (Hla) is a self-assembling extracellular protein secreted as a soluble monomer by most Staphylococcus aureus strains and is an essential virulence factor for the pathogenesis of various S. aureus infections. Here, we show that curcumin (CUR), a natural compound with weak anti-S. aureus activity, can inhibit the hemolysis induced by Hla. Molecular dynamics simulations, free energy calculations, and mutagenesis assays were further employed for the Hla-CUR complex to determine the mechanism of such inhibition. The analysis of this combined approach indicated that the direct binding CUR to Hla blocks the conformational transition of Hla from the monomer to the oligomer, leading to an inhibition of Hla hemolytic activity. We also found that the addition of CUR significantly attenuated Hla-mediated injury of human alveolar cell (A549) co-cultured with S. aureus. The in vivo data further demonstrated that treatment with CUR protects mice from pneumonia caused by S. aureus, including methicillin-resistant strains (MRSA). These findings suggest that CUR inhibits the pore-forming activity of Hla through a novel mechanism, which would pave the way for the development of new and more effective antibacterial agents to combat S. aureus pneumonia.

Adaptive processes of Staphylococcus aureus isolates during the progression from acute to chronic bone and joint infections in patients

Staphylococcus aureus bone and joint infection (BJI) is associated with significant rates of chronicity and relapse. In this study, we investigated how S. aureus is able to adapt to the human environment by comparing isolates from single patients with persisting or relapsing BJIs that were recovered during the initial and recurrent BJI episodes. In vitro and in vivo assays and whole-genome sequencing analyses revealed that the recurrent isolates induced a reduced inflammatory response, formed more biofilms, persisted longer in the intracellular compartments of host bone cells, were less cytotoxic and induced less mortality in a mouse infection model compared with the initial isolates despite the lack of significant changes at the genomic level. These findings suggest that S. aureus BJI chronicization is associated with an in vivo bacterial phenotypical adaptation that leads to decreased virulence and host immune escape, which is linked to increased intraosteoblastic persistence and biofilm formation.

Genotypic Diversity of Staphylococcus aureus α-Hemolysin Gene (hla) and Its Association with Clonal Background: Implications for Vaccine Development

The α-hemolysin, encoded by the hla gene, is a major virulence factor in S. aureus infections. Changes in key amino acid residues of α-hemolysin can result in reduction, or even loss, of toxicity. The aim of this study was to investigate the diversity of the hla gene sequence and the relationship of hla variants to the clonal background of S. aureus isolates. A total of 47 clinical isolates from China were used in this study, supplemented with in silico analysis of 318 well-characterized whole genome sequences from globally distributed isolates. A total of 28 hla genotypes were found, including three unique to isolates from China, 20 found only in the global genomes and five found in both. The hla genotype generally correlated with the clonal background, particularly the multilocus sequence type, but was not related to geographic origin, host source or methicillin-resistance phenotype. In addition, the hla gene showed greater diversity than the seven loci utilized in the MLST scheme for S. aureus. Our investigation has provided genetic data which may be useful for future studies of toxicity, immunogenicity and vaccine development.

More Than a Pore: The Interplay of Pore-Forming Proteins and Lipid Membranes

Pore-forming proteins (PFPs) punch holes in their target cell membrane to alter their permeability. Permeabilization of lipid membranes by PFPs has received special attention to study the basic molecular mechanisms of protein insertion into membranes and the development of biotechnological tools. PFPs act through a general multi-step mechanism that involves (i) membrane partitioning, (ii) insertion into the hydrophobic core of the bilayer, (iii) oligomerization, and (iv) pore formation. Interestingly, PFPs and membranes show a dynamic interplay. As PFPs are usually produced as soluble proteins, they require a large conformational change for membrane insertion. Moreover, membrane structure is modified upon PFPs insertion. In this context, the toroidal pore model has been proposed to describe a pore architecture in which not only protein molecules but also lipids are directly involved in the structure. Here, we discuss how PFPs and lipids cooperate and remodel each other to achieve pore formation, and explore new evidences of protein-lipid pore structures.

An intermolecular electrostatic interaction controls the prepore-to-pore transition in a cholesterol-dependent cytolysin

β-Barrel pore-forming toxins (βPFTs) form an obligatory oligomeric prepore intermediate before the formation of the β-barrel pore. The molecular components that control the critical prepore-to-pore transition remain unknown for βPFTs. Using the archetype βPFT perfringolysin O, we show that E183 of each monomer within the prepore complex forms an intermolecular electrostatic interaction with K336 of the adjacent monomer on completion of the prepore complex. The signal generated throughout the prepore complex by this interaction irrevocably commits it to the formation of the membrane-inserted giant β-barrel pore. This interaction supplies the free energy to overcome the energy barrier (determined here to be ∼ 19 kcal/mol) to the prepore-to-pore transition by the coordinated disruption of a critical interface within each monomer. These studies provide the first insight to our knowledge into the molecular mechanism that controls the prepore-to-pore transition for a βPFT.

Identification and validation of a linear protective neutralizing epitope in the β-pore domain of alpha toxin

The plethora of virulence factors associated with Staphylococcus aureus make this bacterium an attractive candidate for a molecularly-designed epitope-focused vaccine. This approach, which necessitates the identification of neutralizing epitopes for incorporation into a vaccine construct, is being evaluated for pathogens where conventional approaches have failed to elicit protective humoral responses, like HIV-1 and malaria, but may also hold promise for pathogens like S. aureus, where the elicitation of humoral immunity against multiple virulence factors may be required for development of an effective vaccine. Among the virulence factors employed by S. aureus, animal model and epidemiological data suggest that alpha toxin, a multimeric β-pore forming toxin like protective antigen from Bacillus anthracis, is particularly critical, yet no candidate neutralizing epitopes have been delineated in alpha toxin to date. We have previously shown that a linear determinant in the 2β2-2β3 loop of the pore forming domain of B. anthracis protective antigen is a linear neutralizing epitope. Antibody against this site is highly potent for neutralizing anthrax lethal toxin in vitro and for protection of rabbits in vivo from virulent B. anthracis. We hypothesized that sequences in the β-pore of S. aureus alpha toxin that share structural and functional homology to β-pore sequences in protective antigen would contain a similarly critical neutralizing epitope. Using an in vivo mapping strategy employing peptide immunogens, an optimized in vitro toxin neutralization assay, and an in vivo dermonecrosis model, we have now confirmed the presence of this epitope in alpha toxin, termed the pore neutralizing determinant. Antibody specific for this determinant neutralizes alpha toxin in vitro, and is highly effective for mitigating dermonecrosis and bacterial growth in a mouse model of S. aureus USA300 skin infection. The delineation of this linear neutralizing determinant in alpha toxin could facilitate the development of an epitope-focused vaccine against S. aureus.

In vitro directed evolution of alpha-hemolysin by liposome display

We have developed a method to enable in vitro directed evolution that can be applied to membrane proteins. This method, termed liposome display, uses liposomes as compartments in which membrane proteins are synthesized and as scaffolds for membrane protein integration. Thus, the synthesized membrane proteins are displayed on the surface of the liposome and exhibit their functions. A randomly mutated DNA library of the membrane protein was generated, encapsulated in the liposomes at the single-molecule level, and used to generate a liposome library. Liposomes displaying the desired membrane protein function were selected, thus accumulating the DNA molecule encoding the desired membrane protein. We have applied this method to alpha-hemolysin, a membrane protein derived from Staphylococcus aureus. Alpha-hemolysin forms a nanopore in the membrane, which allows the penetration of small molecules. We aimed to improve this nanopore activity by using the liposome display method. Consequently, alpha-hemolysin evolved and attained a higher specific affinity for the liposome membrane. In this review, we describe the essential characteristics of liposome display and the properties of the evolved alpha-hemolysin obtained by this method.

Mechanisms of neutralization of a human anti-α-toxin antibody

MEDI4893 is a neutralizing human monoclonal antibody that targets α-toxin (AT) and is currently undergoing evaluation in the field of Staphylococcus aureus-mediated diseases. We have solved the crystal structure of MEDI4893 Fab bound to monomeric AT at a resolution of 2.56 Å and further characterized its epitope using various engineered AT variants. We have found that MEDI4893 recognizes a novel epitope in the so-called “rim” domain of AT and exerts its neutralizing effect through a dual mechanism. In particular, MEDI4893 not only sterically blocks binding of AT to its cell receptor but also prevents it from adopting a lytic heptameric trans-membrane conformation.

Insights into alpha-hemolysin (Hla) evolution and expression among Staphylococcus aureus clones with hospital and community origin

Background

Alpha-hemolysin (Hla) is a major virulence factor in the pathogenesis of Staphylococcus aureus infection, being active against a wide range of host cells. Although hla is ubiquitous in S. aureus, its genetic diversity and variation in expression in different genetic backgrounds is not known. We evaluated nucleotide sequence variation and gene expression profiles of hla among representatives of hospital (HA) and community-associated (CA) S. aureus clones.

Methods

51 methicillin-resistant S. aureus and 22 methicillin-susceptible S. aureus were characterized by PFGE, spa typing, MLST and SCCmec typing. The internal regions of hla and the hla promoter were sequenced and gene expression was assessed by RT-PCR.

Results

Alpha-hemolysin encoding- and promoter sequences were diverse, with 12 and 23 different alleles, respectively. Based on phylogenetic analysis, we suggest that hla may have evolved together with the S. aureus genetic background, except for ST22, ST121, ST59 and ST93. Conversely, the promoter region showed lack of co-evolution with the genetic backgrounds. Four non-synonymous amino acid changes were identified close to important regions of hla activity. Amino acid changes in the RNAIII binding site were not associated to hla expression. Although expression rates of hla were in general strain-specific, we observed CA clones showed significantly higher hla expression (p = 0.003) when compared with HA clones.

Conclusion

We propose that the hla gene has evolved together with the genetic background. Overall, CA genetic backgrounds showed higher levels of hla expression than HA, and a high strain-to-strain variation of gene expression was detected in closely related strains.

Discovery of protective B-cell epitopes for development of antimicrobial vaccines and antibody therapeutics

Protective antibodies play an essential role in immunity to infection by neutralizing microbes or their toxins and recruiting microbicidal effector functions. Identification of the protective B-cell epitopes, those parts of microbial antigens that contact the variable regions of the protective antibodies, can lead to development of antibody therapeutics, guide vaccine design, enable assessment of protective antibody responses in infected or vaccinated individuals, and uncover or localize pathogenic microbial functions that could be targeted by novel antimicrobials. Monoclonal antibodies are required to link in vivo or in vitro protective effects to specific epitopes and may be obtained from experimental animals or from humans, and their binding can be localized to specific regions of antigens by immunochemical assays. The epitopes are then identified with mapping methods such as X-ray crystallography of antigen-antibody complexes, antibody inhibition of hydrogen-deuterium exchange in the antigen, antibody-induced alteration of the nuclear magnetic resonance spectrum of the antigen, and experimentally validated computational docking of antigen-antibody complexes. The diversity in shape, size and structure of protective B-cell epitopes, and the increasing importance of protective B-cell epitope discovery to development of vaccines and antibody therapeutics are illustrated through examples from different microbe categories, with emphasis on epitopes targeted by broadly neutralizing antibodies to pathogens of high antigenic variation. Examples include the V-shaped Ab52 glycan epitope in the O-antigen of Francisella tularensis, the concave CR6261 peptidic epitope in the haemagglutinin stem of influenza virus H1N1, and the convex/concave PG16 glycopeptidic epitope in the gp120 V1/V2 loop of HIV type 1.

Identification of a key residue for oligomerisation and pore-formation of Clostridium perfringens NetB

Necrotic enteritis toxin B (NetB) is a β-pore-forming toxin produced by Clostridium perfringens and has been identified as a key virulence factor in the pathogenesis of avian necrotic enteritis, a disease causing significant economic damage to the poultry industry worldwide. In this study, site-directed mutagenesis was used to identify amino acids that play a role in NetB oligomerisation and pore-formation. NetB K41H showed significantly reduced toxicity towards LMH cells and human red blood cells relative to wild type toxin. NetB K41H was unable to oligomerise and form pores in liposomes. These findings suggest that NetB K41H could be developed as a genetic toxoid vaccine to protect against necrotic enteritis.

Staphylococcus aureus α-toxin: nearly a century of intrigue

Staphylococcus aureus secretes a number of host-injurious toxins, among the most prominent of which is the small β-barrel pore-forming toxin α-hemolysin. Initially named based on its properties as a red blood cell lytic toxin, early studies suggested a far greater complexity of α-hemolysin action as nucleated cells also exhibited distinct responses to intoxication. The hemolysin, most aptly referred to as α-toxin based on its broad range of cellular specificity, has long been recognized as an important cause of injury in the context of both skin necrosis and lethal infection. The recent identification of ADAM10 as a cellular receptor for α-toxin has provided keen insight on the biology of toxin action during disease pathogenesis, demonstrating the molecular mechanisms by which the toxin causes tissue barrier disruption at host interfaces lined by epithelial or endothelial cells. This review highlights both the historical studies that laid the groundwork for nearly a century of research on α-toxin and key findings on the structural and functional biology of the toxin, in addition to discussing emerging observations that have significantly expanded our understanding of this toxin in S. aureus disease. The identification of ADAM10 as a proteinaceous receptor for the toxin not only provides a greater appreciation of truths uncovered by many historic studies, but now affords the opportunity to more extensively probe and understand the role of α-toxin in modulation of the complex interaction of S. aureus with its human host.

An engineered dimeric protein pore that spans adjacent lipid bilayers

The bottom-up construction of artificial tissues is an underexplored area of synthetic biology. An important challenge is communication between constituent compartments of the engineered tissue, and between the engineered tissue and additional compartments, including extracellular fluids, further engineered tissue and living cells. Here we present a dimeric transmembrane pore that can span two adjacent lipid bilayers, and thereby allow aqueous compartments to communicate. Two heptameric staphylococcal α-hemolysin pores were covalently linked in an aligned cap-to-cap orientation. The structure of the dimer, (α7)2, was confirmed by biochemical analysis, transmission electron microscopy and single-channel electrical recording. We show that one of two β-barrels of (α7)2 can insert into the lipid bilayer of a small unilamellar vesicle, while the other spans a planar lipid bilayer. The (α7)2 pores spanning two bilayers were also observed by transmission electron microscopy.

Distinct localization of the complement C5b-9 complex on Gram-positive bacteria

The plasma proteins of the complement system fulfil important immune defence functions, including opsonization of bacteria for phagocytosis, generation of chemo-attractants and direct bacterial killing via the Membrane Attack Complex (MAC or C5b-9). The MAC is comprised of C5b, C6, C7, C8, and multiple copies of C9 that generate lytic pores in cellular membranes. Gram-positive bacteria are protected from MAC-dependent lysis by their thick peptidoglycan layer. Paradoxically, several Gram-positive pathogens secrete small proteins that inhibit C5b-9 formation. In this study, we found that complement activation on Gram-positive bacteria in serum results in specific surface deposition of C5b-9 complexes. Immunoblotting revealed that C9 occurs in both monomeric and polymeric (SDS-stable) forms, indicating the presence of ring-structured C5b-9. Surprisingly, confocal microscopy demonstrated that C5b-9 deposition occurs at specialized regions on the bacterial cell. On Streptococcus pyogenes, C5b-9 deposits near the division septum whereas on Bacillus subtilis the complex is located at the poles. This is in contrast to C3b deposition, which occurs randomly on the bacterial surface. Altogether, these results show a previously unrecognized interaction between the C5b-9 complex and Gram-positive bacteria, which might ultimately lead to a new model of MAC assembly and functioning.

Structural Insights into Clostridium perfringens Delta Toxin Pore Formation

Clostridium perfringens Delta toxin is one of the three hemolysin-like proteins produced by C. perfringens type C and possibly type B strains. One of the others, NetB, has been shown to be the major cause of Avian Nectrotic Enteritis, which following the reduction in use of antibiotics as growth promoters, has become an emerging disease of industrial poultry. Delta toxin itself is cytotoxic to the wide range of human and animal macrophages and platelets that present GM2 ganglioside on their membranes. It has sequence similarity with Staphylococcus aureus β-pore forming toxins and is expected to heptamerize and form pores in the lipid bilayer of host cell membranes. Nevertheless, its exact mode of action remains undetermined. Here we report the 2.4 Å crystal structure of monomeric Delta toxin. The superposition of this structure with the structure of the phospholipid-bound F component of S. aureus leucocidin (LukF) revealed that the glycerol molecules bound to Delta toxin and the phospholipids in LukF are accommodated in the same hydrophobic clefts, corresponding to where the toxin is expected to latch onto the membrane, though the binding sites show significant differences. From structure-based sequence alignment with the known structure of staphylococcal α-hemolysin, a model of the Delta toxin pore form has been built. Using electron microscopy, we have validated our model and characterized the Delta toxin pore on liposomes. These results highlight both similarities and differences in the mechanism of Delta toxin (and by extension NetB) cytotoxicity from that of the staphylococcal pore-forming toxins.

Structural and functional analysis of the pore-forming toxin NetB from Clostridium perfringens

Clostridium perfringens is an anaerobic bacterium that causes numerous important human and animal diseases, primarily as a result of its ability to produce many different protein toxins. In chickens, C. perfringens causes necrotic enteritis, a disease of economic importance to the worldwide poultry industry. The secreted pore-forming toxin NetB is a key virulence factor in the pathogenesis of avian necrotic enteritis and is similar to alpha-hemolysin, a β-barrel pore-forming toxin from Staphylococcus aureus. To address the molecular mechanisms underlying NetB-mediated tissue damage, we determined the crystal structure of the monomeric form of NetB to 1.8 Å. Structural comparisons with other members of the alpha-hemolysin family revealed significant differences in the conformation of the membrane binding domain. These data suggested that NetB may recognize different membrane receptors or use a different mechanism for membrane-protein interactions. Consistent with this idea, electrophysiological experiments with planar lipid bilayers revealed that NetB formed pores with much larger single-channel conductance than alpha-hemolysin. Channel conductance varied with phospholipid net charge. Furthermore, NetB differed in its ion selectivity, preferring cations over anions. Using hemolysis as a screen, we carried out a random-mutagenesis study that identified several residues that are critical for NetB-induced cell lysis. Mapping of these residues onto the crystal structure revealed that they were clustered in regions predicted to be required for oligomerization or membrane binding. Together these data provide an insight into the mechanism of NetB-mediated pore formation and will contribute to our understanding of the mode of action of this important toxin.

Necrotic enteritis is an economically important disease of the worldwide poultry industry and is mediated by Clostridium perfringens strains that produce NetB, a β-pore-forming toxin. We carried out structural and functional studies of NetB to provide a mechanistic insight into its mode of action and to assist in the development of a necrotic enteritis vaccine. We determined the structure of the monomeric form of NetB to 1.8 Å, used both site-directed and random mutagenesis to identify key residues that are required for its biological activity, and analyzed pore formation by NetB and its substitution-containing derivatives in planar lipid bilayers.

Oroxylin A inhibits hemolysis via hindering the self-assembly of α-hemolysin heptameric transmembrane pore

Alpha-hemolysin (α-HL) is a self-assembling, channel-forming toxin produced by most Staphylococcus aureus strains as a 33.2-kDa soluble monomer. Upon binding to a susceptible cell membrane, the monomer self-assembles to form a 232.4-kDa heptamer that ultimately causes host cell lysis and death. Consequently, α-HL plays a significant role in the pathogenesis of S. aureus infections, such as pneumonia, mastitis, keratitis and arthritis. In this paper, experimental studies show that oroxylin A (ORO), a natural compound without anti-S. aureus activity, can inhibit the hemolytic activity of α-HL. Molecular dynamics simulations, free energy calculations, and mutagenesis assays were performed to understand the formation of the α-HL-ORO complex. This combined approach revealed that the catalytic mechanism of inhibition involves the direct binding of ORO to α-HL, which blocks the conformational transition of the critical “Loop” region of the α-HL protein thereby inhibiting its hemolytic activity. This mechanism was confirmed by experimental data obtained from a deoxycholate-induced oligomerization assay. It was also found that, in a co-culture system with S. aureus and human alveolar epithelial (A549) cells, ORO could protect against α-HL-mediated injury. These findings indicate that ORO hinders the lytic activity of α-HL through a novel mechanism, which should facilitate the design of new and more effective antibacterial agents against S. aureus.

The mechanism controlling protein-ligand interactions is one of the most important processes in rational drug design. X-ray crystallography is a traditional tool used to investigate the interaction of ligands and proteins in a complex. However, protein crystallography is inefficient, and the development of crystal technology and research remains unequally distributed. Thus, it seems impractical to explore the structure of the α-hemolysin-ORO monomer complex by crystallography. Therefore, we used molecular dynamics simulations to investigate the receptor-ligand interaction in the α-HL-ORO monomer complex. In this study, we found that oroxylin A (ORO), a natural compound with little anti-S. aureus activity, can inhibit the hemolytic activity of α-HL at low concentrations. Through molecular docking and molecular dynamics simulations, we determined the potential binding mode of the protein-ligand interaction. The data revealed that ORO directly binds to α-HL, an interaction that blacks the conformational transition of the critical “Loop” region in α-HL and thus prevents the formation of the α-HL heptameric transmembrane pore, which ultimately inhibits the hemolytic activity of α-HL. This mechanism was confirmed by experimental data. Furthermore, we demonstrated that ORO could protect against α-HL-mediated injury in human alveolar epithelial (A549) cells.

Molecular insight into the inhibition mechanism of cyrtominetin to α-hemolysin by molecular dynamics simulation

The protein α-hemolysin (α-HL) is a self-assembling exotoxin that binds to the membrane of a susceptible host cell. In this paper, experimental studies show that cyrtominetin (CTM) can inhibit the hemolytic activity of α-HL. To understand how CTM can affect hemolytic activity, molecular dynamics simulations were carried out for α-HL-CTM complex and these results were compared with the crystal structure of monomeric α-HL. With this approach, the analysis revealed that the inhibition of CTM involves CTM directly binding to α-HL. Due to the binding of CTM, the conformation of the critical "Loop" region was restrained. This mechanism was confirmed by the experimental data. These findings indicate that CTM hinders the lysis activity of α-HL through a novel mechanism.

Molecular architecture and functional analysis of NetB, a pore-forming toxin from Clostridium perfringens

NetB is a pore-forming toxin produced by Clostridium perfringens and has been reported to play a major role in the pathogenesis of avian necrotic enteritis, a disease that has emerged due to the removal of antibiotics in animal feedstuffs. Here we present the crystal structure of the pore form of NetB solved to 3.9 Å. The heptameric assembly shares structural homology to the staphylococcal α-hemolysin. However, the rim domain, a region that is thought to interact with the target cell membrane, shows sequence and structural divergence leading to the alteration of a phosphocholine binding pocket found in the staphylococcal toxins. Consistent with the structure we show that NetB does not bind phosphocholine efficiently but instead interacts directly with cholesterol leading to enhanced oligomerization and pore formation. Finally we have identified conserved and non-conserved amino acid positions within the rim loops that significantly affect binding and toxicity of NetB. These findings present new insights into the mode of action of these pore-forming toxins, enabling the design of more effective control measures against necrotic enteritis and providing potential new tools to the field of bionanotechnology.

A subunit of eukaryotic translation initiation factor 2α-phosphatase (CreP/PPP1R15B) regulates membrane traffic

The constitutive reverter of eIF2α phosphorylation (CReP)/PPP1r15B targets the catalytic subunit of protein phosphatase 1 (PP1c) to phosphorylated eIF2α (p-eIF2α) to promote its dephosphorylation and translation initiation. Here, we report a novel role and mode of action of CReP. We found that CReP regulates uptake of the pore-forming Staphylococcus aureus α-toxin by epithelial cells. This function was independent of PP1c and translation, although p-eIF2α was involved. The latter accumulated at sites of toxin attack and appeared conjointly with α-toxin in early endosomes. CReP localized to membranes, interacted with phosphomimetic eIF2α, and, upon overexpression, induced and decorated a population of intracellular vesicles, characterized by accumulation of N-(lissamine rhodamine B sulfonyl)phosphatidylethanolamine (N-Rh-PE), a lipid marker of exosomes and intralumenal vesicles of multivesicular bodies. By truncation analysis, we delineated the CReP vesicle induction/association region, which comprises an amphipathic α-helix and is distinct from the PP1c interaction domain. CReP was also required for exocytosis from erythroleukemia cells and thus appears to play a broader role in membrane traffic. In summary, the mammalian traffic machinery co-opts p-eIF2α and CReP, regulators of translation initiation.

Cytotoxin CctA, a major virulence factor of Clostridium chauvoei conferring protective immunity against myonecrosis

OBJECTIVE

The purpose of this study was to determine the identity of the major toxin of Clostridium chauvoei, an important pathogen of cattle causing black leg and to determine its value as a protective antigen in vaccines against myonecrosis.

METHODS

Genomic sequence analysis was used to determine potential virulence genes of C. chauvoei. Subsequently, the putative toxin candidate gene was cloned and expressed to obtain recombinant toxin. This toxin was investigated for its cytotoxic activity, hemolysis and its potential as a protective antigen in the guinea pig potency assay.

RESULTS

A novel protein toxin, named Clostridium chauvoei toxin A (CctA) that belongs to the family of β-barrel pore forming toxins of the leucocidin superfamily of bacterial toxins was discovered by whole genome sequence analysis. The corresponding gene cctA was found in all strains of C. chauvoei analyzed, isolated from various geographical areas over the globe during the last 50 years, but not in other pathogenic Clostridium species. Native CctA and recombinant rCctA produced in Escherichia coli in the form of a rCctA::NusA fusion protein or thrombin processed rCctA were highly cytotoxic for Embryonic Calf Nasal Epithelial (ECaNEp) cells and had high haemolytic activity against sheep erythrocytes in standard haemolysis assays. Polyclonal anti-rCctA rabbit antibodies fully neutralized the cytotoxic and haemolytic activity, not only of rCctA but also of supernatants from cultures of the various C. chauvoei strains, indicating that CctA is the main cytotoxic and haemolytic substance secreted by C. chauvoei. Using a standard vaccine release procedure, we demonstrated that vaccination of guinea pigs with CctA in the form of a fusion protein with the E. coli heat labile toxin B subunit (rCctA::LTB) as a peptide adjuvant protected the animals against challenge with spores of virulent C. chauvoei.

CONCLUSIONS

CctA is the major virulence factor of C. chauvoei and the main protective antigen in vaccines against blackleg.

Crystal structure of the octameric pore of staphylococcal γ-hemolysin reveals the β-barrel pore formation mechanism by two components

Staphylococcal γ-hemolysin is a bicomponent pore-forming toxin composed of LukF and Hlg2. These proteins are expressed as water-soluble monomers and then assemble into the oligomeric pore form on the target cell. Here, we report the crystal structure of the octameric pore form of γ-hemolysin at 2.5 Å resolution, which is the first high-resolution structure of a β-barrel transmembrane protein composed of two proteins reported to date. The octameric assembly consists of four molecules of LukF and Hlg2 located alternately in a circular pattern, which explains the biochemical data accumulated over the past two decades. The structure, in combination with the monomeric forms, demonstrates the elaborate molecular machinery involved in pore formation by two different molecules, in which interprotomer electrostatic interactions using loops connecting β2 and β3 (loop A: Asp43-Lys48 of LukF and Lys37-Lys43 of Hlg2) play pivotal roles as the structural determinants for assembly through unwinding of the N-terminal β-strands (amino-latch) of the adjacent protomer, releasing the transmembrane stem domain folded into a β-sheet in the monomer (prestem), and interaction with the adjacent protomer.