Staphylococcal alpha-toxin kills human keratinocytes by permeabilizing the plasma membrane for monovalent ions

The Staphylococcus aureus regulatory program in a human skin-like environment

Staphylococcus aureus is a Gram-positive pathogen responsible for the majority of skin and soft tissue infections (SSTIs). S. aureus colonizes the anterior nares of approximately 20%-30% of the population and transiently colonizes the skin, thereby increasing the risk of developing SSTIs and more serious infections. Current laboratory models that mimic the skin surface environment are expensive, require substantial infrastructure, and limit the scope of bacterial physiology studies under human skin conditions. To overcome these limitations, we developed a cost-effective, open-source, chemically defined media recipe termed skin-like medium (SLM) that incorporates key aspects of the human skin surface environment and supports growth of several staphylococcal species. We utilized SLM to investigate the transcriptional response of methicillin-resistant Staphylococcus aureus (MRSA) following growth in SLM compared to a commonly used laboratory media. Through RNA-seq analysis, we observed the upregulation of several virulence factors, including genes encoding functions involved in adhesion, proteolysis, and cytotoxicity. To further explore these findings, we conducted quantitative reverse transcription-PCR (qRT-PCR) experiments to determine the influence of media composition, pH, and temperature on the transcriptional response of key factors involved in adhesion and virulence. We also demonstrated that MRSA primed in SLM adhered better to human corneocytes and demonstrated adhesin-specific phenotypes that previously required genetic manipulation. This improved adherence to corneocytes was dependent on both acidic pH and growth in SLM. These results support the potential utility of SLM as an in vitro model for assessing staphylococcal physiology and metabolism on human skin.

IMPORTANCE

Staphylococcus aureus is the major cause of skin diseases, and its increased prevalence in skin colonization and infections present a need to understand its physiology in this environment. The work presented here outlines S. aureus upregulation of colonization and virulence factors using a newly developed medium that strives to replicate the human skin surface environment and demonstrates roles for adhesins clumping factor A (ClfA), serine-rich repeat glycoprotein adhesin (SraP), and the fibronectin binding proteins (Fnbps) in human corneocyte adherence.

Long-term memory in Staphylococcus aureus α-hemolysin ion channel kinetics

The kinetics of an ion channel are classically understood as a random process. However, studies have shown that in complex ion channels, formed by multiple subunits, this process can be deterministic, presenting long-term memory. Staphylococcus aureus α-hemolysin (α-HL) is a toxin that acts as the major factor in Staphylococcus aureus virulence. α-HL is a water-soluble protein capable of forming ion channels into lipid bilayers, by insertion of an amphipathic  β-barrel. Here, the α-HL was used as an experimental model to study memory in ion channel kinetics. We applied the approximate entropy (ApEn) approach to analyze randomness and the Detrended Fluctuation Analysis (DFA) to investigate the existence of long memory in α-HL channel kinetics. Single-channel currents were measured through experiments with α-HL channels incorporated in planar lipid bilayers. All experiments were carried out under the following conditions: 1 M NaCl solution, pH 4.5; transmembrane potential of + 40 mV and temperature 25 ± 1 °C. Single-channel currents were recorded in real-time in the memory of a microcomputer coupled to an A/D converter and a patch-clamp amplifier. The conductance value of the α-HL channels was 0.82 ± 0.0025 nS (n = 128). The DFA analysis showed that the kinetics of α-HL channels presents long-term memory ([Formula: see text] = 0.63 ± 0.04). The ApEn outcomes showed low complexity to dwell times when open (ApEno = 0.5514 ± 0.28) and closed (ApEnc = 0.1145 ± 0.08), corroborating the results of the DFA method.

The Staphylococcus aureus regulatory program in a human skin-like environment

Staphylococcus aureus is a Gram-positive pathogen responsible for the majority of skin and soft tissue infections (SSTIs). S. aureus colonizes the anterior nares of approximately 20-30% of the population and transiently colonizes the skin, thereby increasing the risk of developing SSTIs and more serious infections. Current laboratory models that mimic the skin surface environment are expensive, require substantial infrastructure, and limit the scope of bacterial physiology studies under human skin conditions. To overcome these limitations, we developed a cost-effective, open-source, chemically defined media recipe termed skin-like media (SLM) that incorporates key aspects of the human skin surface environment and supports growth of several Staphylococcal species. We utilized SLM to investigate the transcriptional response of methicillin-resistant S. aureus (MRSA) following growth in SLM compared to a commonly used laboratory media. Through RNA-seq analysis, we observed the upregulation of several virulence factors, including genes encoding functions involved in adhesion, proteolysis, and cytotoxicity. To further explore these findings, we conducted qRT-PCR experiments to determine the influence of media composition, pH, and temperature on the transcriptional response of key factors involved in adhesion and virulence. We also demonstrated that MRSA primed in SLM adhered better to human corneocytes and demonstrated adhesin-specific phenotypes that previously required genetic manipulation. These results support the potential utility of SLM as an in vitro model for assessing Staphylococcal physiology and metabolism on human skin.

Importance

Staphylococcus aureus is the major cause of skin diseases, and its increased prevalence in skin colonization and infections present a need to understand its physiology in this environment. The work presented here outlines S. aureus upregulation of colonization and virulence factors using a newly developed media that strives to replicate the human skin surface environment, and demonstrates roles for adhesins ClfA, SraP, and Fnbps in human corneocyte adherence.

Inhibition of α-hemolysin activity of Staphylococcus aureus by theaflavin 3,3'-digallate

The ongoing rise in antibiotic resistance, and a waning of the introduction of new antibiotics, has resulted in limited treatment options for bacterial infections, including these caused by methicillin-resistant Staphylococcus aureus, leaving the world in a post-antibiotic era. Here, we set out to examine mechanisms by which theaflavin 3,3'-digallate (TF3) might act as an anti-hemolytic compound. In the presented study, we found that TF3 has weak bacteriostatic and bactericidal effects on Staphylococcus aureus, and strong inhibitory effect towards the hemolytic activity of its α-hemolysin (Hla) including its production and secretion. A supportive SPR assay reinforced these results and further revealed binding of TF3 to Hla with KD = 4.57×10-5 M. Interestingly, TF3 was also able to protect human primary keratinocytes from Hla-induced cell death, being at the same time non-toxic for them. Further analysis of TF3 properties revealed that TF3 blocked Hla-prompting immune reaction by inhibiting production and secretion of IL1β, IL6, and TNFα in vitro and in vivo, through affecting NFκB activity. Additionally, we observed that TF3 also markedly attenuated S. aureus-induced barrier disruption, by inhibiting Hla-triggered E-cadherin and ZO-1 impairment. Overall, by blocking activity of Hla, TF3 subsequently subdued the inflammation and protected the epithelial barrier, which is considered as beneficial to relieving skin injury.

Staphylococcus aureus Alpha-Toxin in Deep Tracheal Aspirates—Preliminary Evidence for Its Presence in the Lungs of Sepsis Patients

The pore forming alpha-toxin (hemolysin A, Hla) of Staphylococcus aureus (S. aureus) is a major virulence factor with relevance for the pathogenicity of this bacterium, which is involved in many cases of pneumonia and sepsis in humans. Until now, the presence of Hla in the body fluids of potentially infected humans could only be shown indirectly, e.g., by the presence of antibodies against Hla in serum samples or by hemolysis testing on blood agar plates of bacterial culture supernatants of the clinical isolates. In addition, nothing was known about the concentrations of Hla actually reached in the body fluids of the infected hosts. Western blot analyses on 36 samples of deep tracheal aspirates (DTA) isolated from 22 hospitalized sepsis patients using primary antibodies against different epitopes of the Hla molecule resulted in the identification of six samples from five patients containing monomeric Hla (approx. 33 kDa). Two of these samples showed also signals at the molecular mass of heptameric Hla (232 kDa). Semiquantitative analyses of the samples revealed that the concentrations of monomeric Hla ranged from 16 to 3200 ng/mL. This is, to our knowledge, the first study directly showing the presence of S. aureus Hla in samples of airway surface liquid in human patients.

Symbiotic microorganisms: prospects for treating atopic dermatitis

INTRODUCTION

Atopic dermatitis (AD) is a common chronic recurrent inflammatory skin disease. The pathogenesis is unclear but may be related to genetic, immune, and environmental factors and abnormal skin barrier function. Symbiotic microorganisms in the gut and on the skin are associated with AD occurrence.

AREAS COVERED

We discuss the metabolism and distribution of intestinal and skin flora and review their relationship with AD, summarizing the recent applications of intestinal and skin flora in AD treatment, and discussing the prospect of research on these two human microbiota systems and their influence on AD treatment. The PubMed database was searched to identify relevant publications from 1949 to 2020 for the bibliometric analysis of atopic dermatitis and symbiotic microorganisms.

EXPERT OPINION

Many studies have suggested a potential contribution of microbes in the intestine and on the skin to AD. Bacteria living on the skin can aggravate AD by secreting numerous virulence factors. Moreover, the metabolism of intestinal flora can influence AD occurrence and development via the circulatory system. Current evidence suggests that by regulating intestinal and skin flora, AD can be treated and prevented.

Oxysterols Protect Epithelial Cells Against Pore-Forming Toxins

Many species of bacteria produce toxins such as cholesterol-dependent cytolysins that form pores in cell membranes. Membrane pores facilitate infection by releasing nutrients, delivering virulence factors, and causing lytic cell damage - cytolysis. Oxysterols are oxidized forms of cholesterol that regulate cellular cholesterol and alter immune responses to bacteria. Whether oxysterols also influence the protection of cells against pore-forming toxins is unresolved. Here we tested the hypothesis that oxysterols stimulate the intrinsic protection of epithelial cells against damage caused by cholesterol-dependent cytolysins. We treated epithelial cells with oxysterols and then challenged them with the cholesterol-dependent cytolysin, pyolysin. Treating HeLa cells with 27-hydroxycholesterol, 25-hydroxycholesterol, 7α-hydroxycholesterol, or 7β-hydroxycholesterol reduced pyolysin-induced leakage of lactate dehydrogenase and reduced pyolysin-induced cytolysis. Specifically, treatment with 10 ng/ml 27-hydroxycholesterol for 24 h reduced pyolysin-induced lactate dehydrogenase leakage by 88%, and reduced cytolysis from 74% to 1%. Treating HeLa cells with 27-hydroxycholesterol also reduced pyolysin-induced leakage of potassium ions, prevented mitogen-activated protein kinase cell stress responses, and limited alterations in the cytoskeleton. Furthermore, 27-hydroxycholesterol reduced pyolysin-induced damage in lung and liver epithelial cells, and protected against the cytolysins streptolysin O and Staphylococcus aureus α-hemolysin. Although oxysterols regulate cellular cholesterol by activating liver X receptors, cytoprotection did not depend on liver X receptors or changes in total cellular cholesterol. However, oxysterol cytoprotection was partially dependent on acyl-CoA:cholesterol acyltransferase (ACAT) reducing accessible cholesterol in cell membranes. Collectively, these findings imply that oxysterols stimulate the intrinsic protection of epithelial cells against pore-forming toxins and may help protect tissues against pathogenic bacteria.

Scutellarin potentiates vancomycin against lethal pneumonia caused by methicillin-resistant Staphylococcus aureus through dual inhibition of sortase A and caseinolytic peptidase P

The strategy of targeting virulence factor has received great attention as it barely develops bacterial resistance. Sortase A (SrtA) and caseinolytic peptidase P (ClpP), as important virulence factors, are considered to be ideal pharmacological targets for methicillin-resistant Staphylococcus aureus (MRSA) infection. Through screening hundreds of compounds, we found scutellarin, a natural flavonoid, markedly inhibited SrtA and ClpP activities of MRSA strain USA300 with an IC₅₀ of 53.64 μg/mL and 107.00 μg/mL, respectively. Subsequently, we observed that scutellarin could inhibit the SrtA-related virulence of MRSA. To demonstrate whether scutellarin directly binding to SrtA, fluorescence quenching assay and molecular docking were performed and the results indicated that scutellarin directly bonded to SrtA molecule with a K_A value of 7.58 × 10⁴ L/mol. In addition to direct SrtA inhibition, scutellarin could also inhibit hemolytic activity of S. aureus by inhibiting the expression of Hla in a SrtA-independent manner. Further assays confirmed that scutellarin inhibited hemolysis by inhibiting ClpP. The combination of scutellarin and vancomycin showed enhancing inhibition of USA300 in vitro and in vivo, evidenced by decreased MIC from 3 μg/mL to 0.5 μg/mL and increased survival and improvement of lung pathology in pneumonia mice. Taken together, these results suggest that scutellarin exhibited di-inhibitory effects on SrtA and ClpP of USA300. The di-inhibition of virulence factors by scutellarin combined with vancomycin to prevent MRSA invasion of A549 cells and pneumonia in mice, indicating that scutellarin is expected to be a potential adjuvant against MRSA in the future.

Panorama of the Intracellular Molecular Concert Orchestrated by Actinoporins, Pore-Forming Toxins from Sea Anemones

Actinoporins (APs) are soluble pore-forming proteins secreted by sea anemones that experience conformational changes originating in pores in the membranes that can lead to cell death. The processes involved in the binding and pore-formation of members of this protein family have been deeply examined in recent years; however, the intracellular responses to APs are only beginning to be understood. Unlike pore formers of bacterial origin, whose intracellular impact has been studied in more detail, currently, we only have knowledge of a few poorly integrated elements of the APs’ intracellular action. In this review, we present and discuss an updated landscape of the studies aimed at understanding the intracellular pathways triggered in response to APs attack with particular reference to sticholysin II, the most active isoform produced by the Caribbean Sea anemone Stichodactyla helianthus. To achieve this, we first describe the major alterations these cytolysins elicit on simpler cells, such as non-nucleated mammalian erythrocytes, and then onto more complex eukaryotic cells, including tumor cells. This understanding has provided the basis for the development of novel applications of sticholysins such as the construction of immunotoxins directed against undesirable cells, such as tumor cells, and the design of a cancer vaccine platform. These are among the most interesting potential uses for the members of this toxin family that have been carried out in our laboratory.

Staphylococcus aureus α-Toxin Induces Acid Sphingomyelinase Release From a Human Endothelial Cell Line

Staphylococcus aureus (S. aureus) is well known to express a plethora of toxins of which the pore-forming hemolysin A (α-toxin) is the best-studied cytolysin. Pore-forming toxins (PFT) permeabilize host membranes during infection thereby causing concentration-dependent effects in host cell membranes ranging from disordered ion fluxes to cytolysis. Host cells possess defense mechanisms against PFT attack, resulting in endocytosis of the breached membrane area and delivery of repair vesicles to the insulted plasma membrane as well as a concurrent release of membrane repair enzymes. Since PFTs from several pathogens have been shown to recruit membrane repair components, we here investigated whether staphylococcal α-toxin is able to induce these mechanisms in endothelial cells. We show that S. aureus α-toxin induced increase in cytosolic Ca²⁺ in endothelial cells, which was accompanied by p38 MAPK phosphorylation. Toxin challenge led to increased endocytosis of an extracellular fluid phase marker as well as increased externalization of LAMP1-positive membranes suggesting that peripheral lysosomes are recruited to the insulted plasma membrane. We further observed that thereby the lysosomal protein acid sphingomyelinase (ASM) was released into the cell culture medium. Thus, our results show that staphylococcal α-toxin triggers mechanisms in endothelial cells, which have been implicated in membrane repair after damage of other cell types by different toxins.

Plasma membrane integrity: implications for health and disease

Plasma membrane integrity is essential for cellular homeostasis. In vivo, cells experience plasma membrane damage from a multitude of stressors in the extra- and intra-cellular environment. To avoid lethal consequences, cells are equipped with repair pathways to restore membrane integrity. Here, we assess plasma membrane damage and repair from a whole-body perspective. We highlight the role of tissue-specific stressors in health and disease and examine membrane repair pathways across diverse cell types. Furthermore, we outline the impact of genetic and environmental factors on plasma membrane integrity and how these contribute to disease pathogenesis in different tissues.

Major Determinants of Airway Epithelial Cell Sensitivity to S. aureus Alpha-Toxin: Disposal of Toxin Heptamers by Extracellular Vesicle Formation and Lysosomal Degradation

Alpha-toxin is a major virulence factor of Staphylococcus aureus. Monomer binding to host cell membranes results in the formation of heptameric transmembrane pores. Among human model airway epithelial cell lines, A549 cells were most sensitive toward the toxin followed by 16HBE14o^- and S9 cells. In this study we investigated the processes of internalization of pore-containing plasma membrane areas as well as potential pathways for heptamer degradation (lysosomal, proteasomal) or disposal (formation of exosomes/micro-vesicles). The abundance of toxin heptamers upon applying an alpha-toxin pulse to the cells declined both in extracts of whole cells and of cellular membranes of S9 cells, but not in those of 16HBE14o^- or A549 cells. Comparisons of heptamer degradation rates under inhibition of lysosomal or proteasomal degradation revealed that an important route of heptamer degradation, at least in S9 cells, seems to be the lysosomal pathway, while proteasomal degradation appears to be irrelevant. Exosomes prepared from culture supernatants of toxin-exposed S9 cells contained alpha-toxin as well as low amounts of exosome and micro-vesicle markers. These results indicate that lysosomal degradation of internalized toxin heptamers may be the most important determinant of toxin-resistance of some types of airway epithelial cells.

The Short Lipopeptides (C10)2-KKKK-NH2 and (C12)2-KKKK-NH2 Protect HaCaT Keratinocytes from Bacterial Damage Caused by Staphylococcus aureus Infection in a Co-Culture Model

The search for new antimicrobial strategies is of major importance since there is a growing resistance of both bacteria and fungi to existing antimicrobials. Lipopeptides are promising and potent antimicrobial compounds. For translation into clinically useful molecules, effectiveness of peptide treatment against human infections must be proved in complex in vitro wound models. The aim of this study was to examine if the synthesized short lipopeptides (C₁₀)₂-KKKK-NH₂ and (C₁₂)₂-KKKK-NH₂ can protect HaCaT keratinocytes from bacterial damage caused by Staphylococcus aureus infection in a coculture model. After 1 h, 24 h, and 48 h incubation, cellular ATP level and release of the cytotoxicity marker LDH as well as the proinflammatory cytokines interleukin-6 and interleukin-1α were measured. Infection of the keratinocytes resulted in strong bacterial damage of HaCaT cells along with low cellular ATP levels and high release of LDH, IL-6, and IL-1α after 24 h and 48 h. Incubation of the infected human keratinocytes with (C₁₀)₂-KKKK-NH₂ and (C₁₂)₂-KKKK-NH₂ resulted in protection of the keratinocytes from bacterial damage caused by Staphylococcus aureus infection with ATP, LDH, IL-6, and IL-1α levels comparable to the untreated control. Hence, both synthesized lipopeptides are promising candidates with high therapeutic potential in dermatology for the treatment of topical infections.

S. aureus alpha-toxin monomer binding and heptamer formation in host cell membranes - Do they determine sensitivity of airway epithelial cells toward the toxin?

Alpha-toxin (Hla) is a major virulence factor of Staphylococcus aureus (S. aureus) and plays an important role in S. aureus-induced pneumonia. It binds as a monomer to the cell surface of eukaryotic host cells and forms heptameric transmembrane pores. Sensitivities toward the toxin of various types of potential host cells have been shown to vary substantially, and the reasons for these differences are unclear. We used three human model airway epithelial cell lines (16HBE14o-, S9, A549) to correlate cell sensitivity (measured as rate of paracellular gap formation in the cell layers) with Hla monomer binding, presence of the potential Hla receptors ADAM10 or α5β1 integrin, presence of the toxin-stabilizing factor caveolin-1 as well as plasma membrane lipid composition (phosphatidylserine/choline, sphingomyelin). The abundance of ADAM10 correlated best with gap formation or cell sensitivities, respectively, when the three cell types were compared. Caveolin-1 or α5β1 integrin did not correlate with toxin sensitivity. The relative abundance of sphingomyelin in plasma membranes may also be used as a proxi for cellular sensitivity against alpha-toxin as sphingomyelin abundances correlated well with the intensities of alpha-toxin mediated gap formation in the cell layers.

Impact of Bacterial Toxins in the Lungs

Bacterial toxins play a key role in the pathogenesis of lung disease. Based on their structural and functional properties, they employ various strategies to modulate lung barrier function and to impair host defense in order to promote infection. Although in general, these toxins target common cellular signaling pathways and host compartments, toxin- and cell-specific effects have also been reported. Toxins can affect resident pulmonary cells involved in alveolar fluid clearance (AFC) and barrier function through impairing vectorial Na⁺ transport and through cytoskeletal collapse, as such, destroying cell-cell adhesions. The resulting loss of alveolar-capillary barrier integrity and fluid clearance capacity will induce capillary leak and foster edema formation, which will in turn impair gas exchange and endanger the survival of the host. Toxins modulate or neutralize protective host cell mechanisms of both the innate and adaptive immunity response during chronic infection. In particular, toxins can either recruit or kill central players of the lung's innate immune responses to pathogenic attacks, i.e., alveolar macrophages (AMs) and neutrophils. Pulmonary disorders resulting from these toxin actions include, e.g., acute lung injury (ALI), the acute respiratory syndrome (ARDS), and severe pneumonia. When acute infection converts to persistence, i.e., colonization and chronic infection, lung diseases, such as bronchitis, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF) can arise. The aim of this review is to discuss the impact of bacterial toxins in the lungs and the resulting outcomes for pathogenesis, their roles in promoting bacterial dissemination, and bacterial survival in disease progression.

Staphylococcus aureus α-toxin: small pore, large consequences

The small β-pore-forming α-toxin, also termed α-hemolysin or Hla is considered to be an important virulence factor of Staphylococcus aureus. Perforation of the plasma membrane (PM) by Hla leads to uncontrolled flux of ions and water. Already a small number of toxin pores seems to be sufficient to induce complex cellular responses, many of which depend on the efflux of potassium. In this article, we discuss the implications of secondary membrane lesions, for example, by endogenous channels, for Hla-mediated toxicity, for calcium-influx and membrane repair. Activation of purinergic receptors has been proposed to be a major contributor to the lytic effects of various pore forming proteins, but new findings raise doubts that this holds true for Hla. However, the recently discovered cellular pore forming proteins gasdermin D and Mixed lineage kinase domain-like pseudokinase (MLKL) which perforate the PM from the cytosolic side might contribute to both calcium-influx-dependent damage and membrane repair. Activation of endogenous pore forming proteins by Hla above a threshold concentration could explain the apparent dependence of pore characteristics on toxin concentrations. If secondary membrane damage in the aftermath of Hla-attack contributes significantly to overall PM permeability, it might be an interesting target for new therapeutic approaches.

Designing a light-activated recombinant alpha hemolysin for colorectal cancer targeting

Introduction: Colorectal cancer (CRC) is one of the main health burden worldwide, which can cause major economic and physiological problems along with relatively high rate of mortality. It is important to develop new methods for the localized delivery of recombinant protein therapeutics, in large part due to the failure of conventional therapies in most cases. Since E. coli Nissle 1917 (EcN) does not produce any virulence factors, here we used these bacteria with the light-activated promoter system to deliver therapeutic agents in the desired location and time. Methods: In this study, Staphylococcus aureus alpha hemolysin (SAH), after codon usage optimization, was cloned into blue light activating vector (pDawn) and transferred to EcN strain. Then, the functionality and cytotoxicity of secreted alpha hemolysin was evaluated in the SW480 colon cancer cell line by using different experiments, including blood agar test, flow cytometry analysis, and DAPI staining. Results: Our findings revealed that EcN can produce functional SAH under the blue light irradiation against SW480 cancer cells. Moreover, cytotoxicity assays confirmed the dose- and time-dependent toxicity of this payload (SAH) against SW480 cancer cells. Conclusion: Based on our results, EcN is proposed as an appropriate light-activated vehicle for delivery of anticancer agents to the target cancer cells/tissues.

Mechanistic Landscape of Membrane-Permeabilizing Peptides

Membrane permeabilizing peptides (MPPs) are as ubiquitous as the lipid bilayer membranes they act upon. Produced by all forms of life, most membrane permeabilizing peptides are used offensively or defensively against the membranes of other organisms. Just as nature has found many uses for them, translational scientists have worked for decades to design or optimize membrane permeabilizing peptides for applications in the laboratory and in the clinic ranging from antibacterial and antiviral therapy and prophylaxis to anticancer therapeutics and drug delivery. Here, we review the field of membrane permeabilizing peptides. We discuss the diversity of their sources and structures, the systems and methods used to measure their activities, and the behaviors that are observed. We discuss the fact that "mechanism" is not a discrete or a static entity for an MPP but rather the result of a heterogeneous and dynamic ensemble of structural states that vary in response to many different experimental conditions. This has led to an almost complete lack of discrete three-dimensional active structures among the thousands of known MPPs and a lack of useful or predictive sequence-structure-function relationship rules. Ultimately, we discuss how it may be more useful to think of membrane permeabilizing peptides mechanisms as broad regions of a mechanistic landscape rather than discrete molecular processes.

Mechanisms protecting host cells against bacterial pore-forming toxins

Pore-forming toxins (PFTs) are key virulence determinants produced and secreted by a variety of human bacterial pathogens. They disrupt the plasma membrane (PM) by generating stable protein pores, which allow uncontrolled exchanges between the extracellular and intracellular milieus, dramatically disturbing cellular homeostasis. In recent years, many advances were made regarding the characterization of conserved repair mechanisms that allow eukaryotic cells to recover from mechanical disruption of the PM membrane. However, the specificities of the cell recovery pathways that protect host cells against PFT-induced damage remain remarkably elusive. During bacterial infections, the coordinated action of such cell recovery processes defines the outcome of infected cells and is, thus, critical for our understanding of bacterial pathogenesis. Here, we review the cellular pathways reported to be involved in the response to bacterial PFTs and discuss their impact in single-cell recovery and infection.

Influences on allergic mechanisms through gut, lung, and skin microbiome exposures

In industrialized societies the incidence of allergic diseases like atopic dermatitis, food allergies, and asthma has risen alarmingly over the last few decades. This increase has been attributed, in part, to lifestyle changes that alter the composition and function of the microbes that colonize the skin and mucosal surfaces. Strategies that reverse these changes to establish and maintain a healthy microbiome show promise for the prevention and treatment of allergic disease. In this Review, we will discuss evidence from preclinical and clinical studies that gives insights into how the microbiota of skin, intestinal tract, and airways influence immune responses in the context of allergic sensitization.

Sphingomyelin Depletion from Plasma Membranes of Human Airway Epithelial Cells Completely Abrogates the Deleterious Actions of S. aureus Alpha-Toxin

Interaction of Staphylococcus aureus alpha-toxin (hemolysin A, Hla) with eukaryotic cell membranes is mediated by proteinaceous receptors and certain lipid domains in host cell plasma membranes. Hla is secreted as a 33 kDa monomer that forms heptameric transmembrane pores whose action compromises maintenance of cell shape and epithelial tightness. It is not exactly known whether certain membrane lipid domains of host cells facilitate adhesion of Ha monomers, oligomerization, or pore formation. We used sphingomyelinase (hemolysin B, Hlb) expressed by some strains of staphylococci to pre-treat airway epithelial model cells in order to specifically decrease the sphingomyelin (SM) abundance in their plasma membranes. Such a pre-incubation exclusively removed SM from the plasma membrane lipid fraction. It abrogated the formation of heptamers and prevented the formation of functional transmembrane pores. Hla exposure of rHlb pre-treated cells did not result in increases in [Ca²⁺]_i, did not induce any microscopically visible changes in cell shape or formation of paracellular gaps, and did not induce hypo-phosphorylation of the actin depolymerizing factor cofilin as usual. Removal of sphingomyelin from the plasma membranes of human airway epithelial cells completely abrogates the deleterious actions of Staphylococcus aureus alpha-toxin.

Multiple Parameters Beyond Lipid Binding Affinity Drive Cytotoxicity of Cholesterol-Dependent Cytolysins

The largest superfamily of bacterial virulence factors is pore-forming toxins (PFTs). PFTs are secreted by both pathogenic and non-pathogenic bacteria. PFTs sometimes kill or induce pro-pathogen signaling in mammalian cells, all primarily through plasma membrane perforation, though the parameters that determine these outcomes are unclear. Membrane binding, calcium influx, pore size, and membrane repair are factors that influence PFT cytotoxicity. To test the contribution of membrane binding to cytotoxicity and repair, we compared the closely related, similarly-sized PFTs Perfringolysin O (PFO) from Clostridium perfringens and Streptolysin O (SLO) from Streptococcus pyogenes. Cell death kinetics for PFO and SLO were different because PFO increased in cytotoxicity over time. We introduced known L3 loop mutations that swap binding affinity between toxins and measured hemolytic activity, nucleated cell death kinetics and membrane repair using viability assays, and live cell imaging. Altered hemolytic activity was directly proportional to toxin binding affinity. In contrast, L3 loop alterations reduced nucleated cell death, and they had limited effects on cytotoxicity kinetics and membrane repair. This suggests other toxin structural features, like oligomerization, drives these parameters. Overall, these findings suggest that repair mechanisms and toxin oligomerization add constraints beyond membrane binding on toxin evolution and activity against nucleated cells.

Microbe-derived extracellular vesicles as a smart drug delivery system

The human microbiome is known to play an essential role in influencing host health. Extracellular vesicles (EVs) have also been reported to act on a variety of signaling pathways, distally transport cellular components such as proteins, lipids, and nucleic acid, and have immunomodulatory effects. Here we shall review the current understanding of the intersectionality of the human microbiome and EVs in the emerging field of microbiota-derived EVs and their pharmacological potential. Microbes secrete several classes of EVs: outer membrane vesicles (OMVs), membrane vesicles (MVs), and apoptotic bodies. EV biogenesis is unique to each cell and regulated by sophisticated signaling pathways. EVs are primarily composed of lipids, proteins, nucleic acids, and recent evidence suggests they may also carry metabolites. These components interact with host cells and control various cellular processes by transferring their constituents. The pharmacological potential of microbiomederived EVs as vaccine candidates, biomarkers, and a smart drug delivery system is a promising area of future research. Therefore, it is necessary to elucidate in detail the mechanisms of microbiome-derived EV action in host health in a multi-disciplinary manner.

A potential key role for alpha-haemolysin of Staphylococcus aureus in mediating chondrocyte death in septic arthritis

Objectives

Staphylococcus aureus (S. aureus) is the most commonly implicated organism in septic arthritis, a condition that may be highly destructive to articular cartilage. Previous studies investigating laboratory and clinical strains of S. aureus have demonstrated that potent toxins induced significant chondrocyte death, although the precise toxin or toxins that were involved was unknown. In this study, we used isogenic S. aureus mutants to assess the influence of alpha (Hla)-, beta (Hlb)-, and gamma (Hlg)-haemolysins, toxins considered important for the destruction of host tissue, on in situ bovine chondrocyte viability.

Methods

Bovine cartilage explants were cultured with isogenic S. aureus mutants and/or their culture supernatants. Chondrocyte viability was then assessed within defined regions of interest in the axial and coronal plane following live- and dead-cell imaging using the fluorescent probes 5-chloromethylfluorescein diacetate and propidium iodide, respectively, and confocal laser-scanning microscopy.

Results

Hla-producing mutants caused substantial chondrocyte death compared with the toxin-deficient control (Hla-Hlb-Hlg-), whilst mutants producing Hlb and Hlg in the absence of Hla induced minimal chondrocyte death. Coronal studies established that Hla-induced chondrocyte death started in the superficial zone of cartilage and spread to deeper layers, whereas Hlb and Hlg toxins were without significant effect.

Conclusion

This study identified Hla as a highly potent S. aureus toxin that caused rapid chondrocyte death in bovine cartilage, with other toxins or metabolic products produced by the bacteria playing a minor role. The identification of Hla in mediating chondrocyte death may assist in the development of therapeutic strategies aimed at reducing the extent of cartilage damage during and after an episode of septic arthritis.

Cite this article: I. D. M. Smith, K. M. Milto, C. J. Doherty, S. G. B. Amyes, A. H. R. W. Simpson, A. C. Hall. A potential key role for alpha-haemolysin of Staphylococcus aureus in mediating chondrocyte death in septic arthritis. Bone Joint Res 2018;7:457–467. DOI: 10.1302/2046-3758.77.BJR-2017-0165.R1.

Prevalence of IgG and Neutralizing Antibodies against Staphylococcus aureus Alpha-Toxin in Healthy Human Subjects and Diverse Patient Populations

Staphylococcus aureus causes an array of serious infections resulting in high morbidity and mortality worldwide. This study evaluated naturally occurring serum anti-alpha-toxin (anti-AT) antibody levels in human subjects from various age groups, individuals with S. aureus dialysis and surgical-site infections, and S. aureus-colonized versus noncolonized subjects. Anti-AT immunoglobulin G (IgG) and neutralizing antibody (NAb) levels in infants (aged ≤1 year) were significantly lower than those in other populations. In comparison to adolescent, adult, and elderly populations, young children (aged 2 to 10 years) had equivalent anti-AT IgG levels but significantly lower anti-AT NAb levels. Therefore, the development of anti-AT NAbs appears to occur later than that of AT-specific IgG, suggesting a maturation of the immune response to AT. Anti-AT IgG levels were slightly higher in S. aureus-colonized subjects than in noncolonized subjects. The methicillin susceptibility status of colonizing isolates had no effect on anti-AT antibody levels in S. aureus-colonized subjects. The highest anti-AT IgG and NAb levels were observed in dialysis patients with acute S. aureus infection. Anti-AT IgG and NAb levels were well correlated in subjects aged >10 years, regardless of colonization or infection status. These data demonstrate that AT elicits a robust IgG humoral response in infants and young children that becomes stable prior to adolescence, matures into higher levels of NAbs in healthy adolescents, and becomes elevated during S. aureus infection. These findings may assist in identifying subjects and patient populations that could benefit from vaccination or immunoprophylaxis with anti-AT monoclonal antibodies.

Disruption of staphylococcal aggregation protects against lethal lung injury

Infection by Staphylococcus aureus strain USA300 causes tissue injury, multiorgan failure, and high mortality. However, the mechanisms by which the bacteria adhere to, then stabilize on, mucosal surfaces before causing injury remain unclear. We addressed these issues through the first real-time determinations of USA300-alveolar interactions in live lungs. We found that within minutes, inhaled USA300 established stable, self-associated microaggregates in niches at curved, but not at flat, regions of the alveolar wall. The microaggregates released α-hemolysin toxin, causing localized alveolar injury, as indicated by epithelial dye loss, mitochondrial depolarization, and cytosolic Ca2+ increase. Spread of cytosolic Ca2+ through intercellular gap junctions to adjoining, uninfected alveoli caused pulmonary edema. Systemic pretreatment with vancomycin, a USA300-cidal antibiotic, failed to protect mice infected with inhaled WT USA300. However, vancomycin pretreatment markedly abrogated mortality in mice infected with mutant USA300 that lacked the aggregation-promoting factor PhnD. We interpret USA300-induced mortality as having resulted from rapid bacterial aggregation in alveolar niches. These findings indicate, for the first time to our knowledge, that alveolar microanatomy is critical in promoting the aggregation and, hence, in causing USA300-induced alveolar injury. We propose that in addition to antibiotics, strategies for bacterial disaggregation may constitute novel therapy against USA300-induced lung injury.

Characterisation of anti-alpha toxin antibody levels and colonisation status after administration of an investigational human monoclonal antibody, MEDI4893, against Staphylococcus aureus alpha toxin

Objectives

MEDI4893 is a novel, long‐acting human monoclonal antibody targeting Staphylococcus aureus (SA) alpha toxin (AT). This report presents the results of the exploratory analyses from a randomised phase 1 dose‐escalation study in healthy human subjects receiving single intravenous MEDI4893 doses or placebo.

Methods

Anti‐AT antibodies and AT expression were measured as described previously. Nasal swabs were analysed by culture and PCR. Data were summarised by treatment groups and visits by using SAS System Version 9.3.

Results

Subjects receiving 2250 or 5000 mg of MEDI4893 had the highest serum anti‐AT neutralising antibody (NAb) levels: approximately 180‐ to 240‐, 70‐ to 100‐ and sevenfold to 10‐fold higher than respective baseline levels at peak, 30 and 360 days, respectively. In these subjects, levels of serum anti‐AT NAbs were >3.2 International Units (IU) mL⁻¹ for at least 211 days. In the upper respiratory tract, anti‐AT NAb levels increased with MEDI4893 dose. No apparent effect of MEDI4893 on SA nasal colonisation, hla gene sequence or AT expression was observed. Five AT variants were detected, their lytic activity was fully neutralised by MEDI4893.

Discussion

Our results indicate that (1) MEDI4893 administration at 2250 and 5000 mg would provide effective immunoprophylaxis against systemic SA disease; (2) MEDI4983 distributes to the upper respiratory tract and retains neutralising activity against AT; and (3) potential for emergence of MEDI4893 resistance is low.

Conclusion

Intravenous administration of MEDI4893 maintained levels of anti‐AT NAbs in serum and nasal mucosa that may provide effective immunoprophylaxis against SA disease and support continued clinical development of MEDI4893.

Infectious Complications in Atopic Dermatitis

Atopic dermatitis is characterized by the interplay of skin barrier defects with the immune system and skin microbiome that causes patients to be at risk for infectious complications. This article reviews the pathogenesis of atopic dermatitis and the mechanisms through which patients are at risk for infection from bacterial, viral, and fungal pathogens. Although these complications may be managed acutely, prevention of secondary infections depends on a multipronged approach in the maintenance of skin integrity, control of flares, and microbial pathogens.

Virulence analysis of Staphylococcus aureus in a rabbit model of infected full-thickness wound under negative pressure wound therapy

The aim of this study was to evaluate the virulence of Staphylococcus aureus in a controlled animal study using the standard sterile gauze and negative pressure wound therapy (NPWT), including activation of agr, gene expression and production of virulence foctors and depth of bacterial invasion. The tissue specimens were harvested on days 0 (6 h after bacterial inoculation), 2, 4, 6, and 8 at the center of wound beds. Laser scanning confocal microscopy was performed to obtain bioluminescent images which were used to measure the depth of bacterial invasion. The agrA expression of S.aureus and the transcription and production of virulence factors including Eap, Spa and α-toxin were significantly different. The bacterial invasion depth was significantly less with effect of NPWT. The markedly different activation of quorum sensing systems that enable cell-to-cell communication and regulation of numerous colonization and virulence factors result in distinct gene expression and pathogenicity over time in different microenvironment. Thus, the agr system represents a fundamental regulatory paradigm that can encompass different adaptive strategies and accommodate horizontally acquired virulence determinants.

Beyond Hygiene: Commensal Microbiota and Allergic Diseases

Complex communities of microorganisms, termed commensal microbiota, inhabit mucosal surfaces and profoundly influence host physiology as well as occurrence of allergic diseases. Perturbing factors such as the mode of delivery, dietary fibers and antibiotics can influence allergic diseases by altering commensal microbiota in affected tissues as well as in intestine. Here, we review current findings on the relationship between commensal microbiota and allergic diseases, and discuss the underlying mechanisms that contribute to the regulation of allergic responses by commensal microbiota.

Damage of eukaryotic cells by the pore-forming toxin sticholysin II: Consequences of the potassium efflux

Pore-forming toxins (PFTs) form holes in membranes causing one of the most catastrophic damages to a target cell. Target organisms have evolved a regulated response against PFTs damage including cell membrane repair. This ability of cells strongly depends on the toxin concentration and the properties of the pores. It has been hypothesized that there is an inverse correlation between the size of the pores and the time required to repair the membrane, which has been for long a non-intuitive concept and far to be completely understood. Moreover, there is a lack of information about how cells react to the injury triggered by eukaryotic PFTs. Here, we investigated some molecular events related with eukaryotic cells response against the membrane damage caused by sticholysin II (StII), a eukaryotic PFT produced by a sea anemone. We evaluated the change in the cytoplasmic potassium, identified the main MAPK pathways activated after pore-formation by StII, and compared its effect with those from two well-studied bacterial PFTs: aerolysin and listeriolysin O (LLO). Strikingly, we found that membrane recovery upon StII damage takes place in a time scale similar to LLO in spite of the fact that they form pores by far different in size. Furthermore, our data support a common role of the potassium ion, as well as MAPKs in the mechanism that cells use to cope with these toxins injury.

ATP Release from Human Airway Epithelial Cells Exposed to Staphylococcus aureus Alpha-Toxin

Airway epithelial cells reduce cytosolic ATP content in response to treatment with S. aureus alpha-toxin (hemolysin A, Hla). This study was undertaken to investigate whether this is due to attenuated ATP generation or to release of ATP from the cytosol and extracellular ATP degradation by ecto-enzymes. Exposure of cells to rHla did result in mitochondrial calcium uptake and a moderate decline in mitochondrial membrane potential, indicating that ATP regeneration may have been attenuated. In addition, ATP may have left the cells through transmembrane pores formed by the toxin or through endogenous release channels (e.g., pannexins) activated by cellular stress imposed on the cells by toxin exposure. Exposure of cells to an alpha-toxin mutant (H35L), which attaches to the host cell membrane but does not form transmembrane pores, did not induce ATP release from the cells. The Hla-mediated ATP-release was completely blocked by IB201, a cyclodextrin-inhibitor of the alpha-toxin pore, but was not at all affected by inhibitors of pannexin channels. These results indicate that, while exposure of cells to rHla may somewhat reduce ATP production and cellular ATP content, a portion of the remaining ATP is released to the extracellular space and degraded by ecto-enzymes. The release of ATP from the cells may occur directly through the transmembrane pores formed by alpha-toxin.

Staphylococcus aureus α-toxin-mediated cation entry depolarizes membrane potential and activates p38 MAP kinase in airway epithelial cells

Membrane potential (Vm)-, Na(+)-, or Ca(2+)-sensitive fluorescent dyes were used to analyze changes in Vm or intracellular ion concentrations in airway epithelial cells treated with Staphylococcus aureus α-toxin (Hla), a major virulence factor of pathogenic strains of these bacteria. Gramicidin, a channel-forming peptide causing membrane permeability to monovalent cations, a mutated form of Hla, rHla-H35L, which forms oligomers in the plasma membranes of eukaryotic cells but fails to form functional transmembrane pores, or the cyclodextrin-derivative IB201, a blocker of the Hla pore, were used to investigate the permeability of the pore. Na(+) as well as Ca(2+) ions were able to pass the Hla pore and accumulated in the cytosol. The pore-mediated influx of calcium ions was blocked by IB201. Treatment of cells with recombinant Hla resulted in plasma membrane depolarization as well as in increases in the phosphorylation levels of paxillin (signaling pathway mediating disruption of the actin cytoskeleton) and p38 MAP kinase (signaling pathway resulting in defensive actions). p38 MAP kinase phosphorylation, but not paxillin phosphorylation, was elicited by treatment of cells with gramicidin. Although treatment of cells with rHla-H35L resulted in the formation of membrane-associated heptamers, none of these cellular effects were observed in our experiments. This indicates that formation of functional Hla-transmembrane pores is required to induce the cell physiological changes mediated by α-toxin. Specifically, the changes in ion equilibria and plasma membrane potential are important activators of p38 MAP kinase, a signal transduction module involved in host cell defense.

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.

The Role of Staphylococcus aureus Virulence Factors in Skin Infection and Their Potential as Vaccine Antigens

Staphylococcus aureus (S. aureus) causes the vast majority of skin and soft tissue infections (SSTIs) in humans. S. aureus has become increasingly resistant to antibiotics and there is an urgent need for new strategies to tackle S. aureus infections. Vaccines offer a potential solution to this epidemic of antimicrobial resistance. However, the development of next generation efficacious anti-S. aureus vaccines necessitates a greater understanding of the protective immune response against S. aureus infection. In particular, it will be important to ascertain if distinct immune mechanisms are required to confer protection at distinct anatomical sites. Recent discoveries have highlighted that interleukin-17-producing T cells play a particularly important role in the immune response to S. aureus skin infection and suggest that vaccine strategies to specifically target these types of T cells may be beneficial in the treatment of S. aureus SSTIs. S. aureus expresses a large number of cell wall-anchored (CWA) proteins, which are covalently attached to the cell wall peptidoglycan. The virulence potential of many CWA proteins has been demonstrated in infection models; however, there is a paucity of information regarding their roles during SSTIs. In this review, we highlight potential candidate antigens for vaccines targeted at protection against SSTIs.

The role of the skin microbiome in atopic dermatitis

Atopic dermatitis (AD) is a common skin disease that affects a large proportion of the population worldwide. The incidence of AD has increased over the last several decades along with AD's burden on the physical and psychological health of the patient and family. However, current advances in understanding the mechanisms behind the pathophysiology of AD are leading to a hopeful outlook for the future. Staphylococcus aureus (S. aureus) colonization on AD skin has been directly correlated to disease severity but the functions of other members of the skin bacterial community may be equally important. Applying knowledge gained from understanding the role of the skin microbiome in maintaining normal skin immune function, and addressing the detrimental consequences of microbial dysbiosis in driving inflammation, is a promising direction for development of new treatments. This review discusses current preclinical and clinical research focused on determining how the skin microbiome may influence the development of AD.

Membrane Repair: Mechanisms and Pathophysiology

Eukaryotic cells have been confronted throughout their evolution with potentially lethal plasma membrane injuries, including those caused by osmotic stress, by infection from bacterial toxins and parasites, and by mechanical and ischemic stress. The wounded cell can survive if a rapid repair response is mounted that restores boundary integrity. Calcium has been identified as the key trigger to activate an effective membrane repair response that utilizes exocytosis and endocytosis to repair a membrane tear, or remove a membrane pore. We here review what is known about the cellular and molecular mechanisms of membrane repair, with particular emphasis on the relevance of repair as it relates to disease pathologies. Collective evidence reveals membrane repair employs primitive yet robust molecular machinery, such as vesicle fusion and contractile rings, processes evolutionarily honed for simplicity and success. Yet to be fully understood is whether core membrane repair machinery exists in all cells, or whether evolutionary adaptation has resulted in multiple compensatory repair pathways that specialize in different tissues and cells within our body.

The adherens junctions control susceptibility to Staphylococcus aureus α-toxin

Staphylococcus aureus is both a transient skin colonizer and a formidable human pathogen, ranking among the leading causes of skin and soft tissue infections as well as severe pneumonia. The secreted bacterial α-toxin is essential for S. aureus virulence in these epithelial diseases. To discover host cellular factors required for α-toxin cytotoxicity, we conducted a genetic screen using mutagenized haploid human cells. Our screen identified a cytoplasmic member of the adherens junctions, plekstrin-homology domain containing protein 7 (PLEKHA7), as the second most significantly enriched gene after the known α-toxin receptor, a disintegrin and metalloprotease 10 (ADAM10). Here we report a new, unexpected role for PLEKHA7 and several components of cellular adherens junctions in controlling susceptibility to S. aureus α-toxin. We find that despite being injured by α-toxin pore formation, PLEKHA7 knockout cells recover after intoxication. By infecting PLEKHA7(-/-) mice with methicillin-resistant S. aureus USA300 LAC strain, we demonstrate that this junctional protein controls disease severity in both skin infection and lethal S. aureus pneumonia. Our results suggest that adherens junctions actively control cellular responses to a potent pore-forming bacterial toxin and identify PLEKHA7 as a potential nonessential host target to reduce S. aureus virulence during epithelial infections.

Novel Staphylococcus aureus Secreted Protein Alters Keratinocyte Proliferation and Elicits a Proinflammatory Response In Vitro and In Vivo

Staphylococcus aureus is a leading cause of surgical site infections that results in increased hospital stays due to the development of chronic wounds. Little is known about factors involved in S. aureus' ability to prevent wounds from healing. We discovered a novel secreted protein produced by a surgical site isolate of S. aureus that prevents keratinocyte proliferation. The protein has a molecular weight of 15.7 kDa and an isoelectric point of 8.9. The cloned and purified protein has cytotoxic and proinflammatory properties, as shown in vitro and in vivo. Potent biological effects on keratinocytes and rabbit skin suggest that this protein may play an important role in preventing re-epithelialization. Its lack of homology to known exotoxins suggests that this protein is novel, and this observation is likely to open a new field of research in S. aureus exotoxins. Due to its cytotoxic activities, we call this new protein ε-cytotoxin.

Effect of enterohaemorrhagic Escherichia coli O157:H7-specific enterohaemolysin on interleukin-1β production differs between human and mouse macrophages due to the different sensitivity of NLRP3 activation

Enterohaemorrhagic Escherichia coli (EHEC) O157:H7 infection in humans can cause acute haemorrhagic colitis and severe haemolytic uraemic syndrome. The role of enterohaemolysin (Ehx) in the pathogenesis of O157:H7-mediated disease in humans remains undefined. Recent studies have revealed the importance of the inflammatory response in O157:H7 pathogenesis in humans. We previously reported that Ehx markedly induced interleukin-1β (IL-1β) production in human macrophages. Here, we investigated the disparity in Ehx-induced IL-1β production between human and mouse macrophages and explored the underlying mechanism regarding the activation of NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasomes. In contrast to the effects on human differentiated THP-1 cells and peripheral blood mononuclear cells, Ehx exerted no effect on IL-1β production in mouse macrophages and splenocytes because of a disparity in pro-IL-1β cleavage into mature IL-1β upon caspase-1 activation. Additionally, Ehx significantly contributed to O157:H7-induced ATP release from THP-1 cells, which was not detected in mouse macrophages. Confocal microscopy demonstrated that Ehx was a key inducer of cathepsin B release in THP-1 cells but not in mouse IC-21 cells upon O157:H7 challenge. Inhibitor experiments indicated that O157:H7-induced IL-1β production was largely dependent upon caspase-1 activation and partially dependent upon ATP signalling and cathepsin B release, which were both involved in NLRP3 activation. Moreover, inhibition of K(+) efflux drastically diminished O157:H7-induced IL-1β production and cytotoxicity. The findings in this study may shed light on whether and how the Ehx contributes to the development of haemolytic uraemic syndrome in human O157:H7 infection.

A critical role for hemolysin in Vibrio fluvialis-induced IL-1β secretion mediated by the NLRP3 inflammasome in macrophages

Vibrio fluvialis causes human diarrhea, but the pathogenesis is not well-studied. We hypothesized that V. fluvialis-secreted hemolysin (VFH) may induce IL-1β secretion through the activation of the NLRP3 inflammasome and contribute to the pathogenicity of V. fluvialis. To examine this possibility, we constructed VFH mutant and complement strains and demonstrated that V. fluvialis-induced IL-1β production and cytotoxicity in human monocytic THP-1 cells and mouse macrophages is attributed to VFH. To evaluate the role of VFH in vivo, we infected adult C57BL/6 mice intraperitoneally and suckling C57/B6 mice orally with various strains. The mice treated with 10⁸ CFU wild-type V. fluvialis or cell-free supernatant containing VFH induced significantly higher IL-1β production in peritoneal lavage fluid or in colon compared with those infected with the mutant strain, while no effect on TNF and IL-6 production was observed at day 5 or 24 h post-infection. VFH contributed to pathological changes and IL-1β release independent of colonization of V. fluvialis in the colon. VFH has no effect on the synthesis of pro-IL-1β, but rather it triggers the processing of pro-IL-1β into IL-1β. Furthermore, using deficient mouse strains, we verified that V. fluvialis-induced IL-1β is mediated through activation of Caspase-1 and the NLRP3 inflammasome ex vivo. Confocal microscopy suggests that VFH contributes to cathepsin B release. Furthermore, V. fluvialis-induced IL-1β secretion requires potassium (K⁺) efflux and reactive oxygen species production. Our results provide new evidence for the role of VFH in the activation of the NLRP3 inflammasome and pathogenesis in response to V. fluvialis infection.

Summary Sentence:Vibrio fluvialis-secreted hemolysin induces IL-1β secretion through the activation of the NLRP3 inflammasome and contributes to the pathogenicity of V. fluvialis.

Suppression of cell membrane permeability by suramin: involvement of its inhibitory actions on connexin 43 hemichannels

BACKGROUND AND PURPOSE

Suramin is a clinically prescribed drug for treatment of human African trypanosomiasis, cancer and infection. It is also a well-known pharmacological antagonist of P2 purinoceptors. Despite its clinical use and use in research, the biological actions of this molecule are still incompletely understood. Here, we investigated the effects of suramin on membrane channels, as exemplified by its actions on non-junctional connexin43 (Cx43) hemichannels, pore-forming α-haemolysin and channels involved in ATP release under hypotonic conditions.

EXPERIMENTAL APPROACH

Hemichannels were activated by removing extracellular Ca(2+) . The influences of suramin on hemichannel activities were evaluated by its effects on influx of fluorescent dyes and efflux of ATP. The membrane permeability and integrity were assessed through cellular retention of preloaded calcein and LDH release.

KEY RESULTS

Suramin blocked Cx43 hemichannel permeability induced by removal of extracellular Ca(2+) without much effect on Cx43 expression and gap junctional intercellular communication. This action of suramin was mimicked by its analogue NF023 and NF449 but not by another P2 purinoceptor antagonist PPADS. Besides hemichannels, suramin also significantly blocked intracellular and extracellular exchanges of small molecules caused by α-haemolysin from Staphylococcus aureus and by exposure of cells to hypotonic solution. Furthermore, it prevented α-haemolysin- and hypotonic stress-elicited cell injury.

CONCLUSION AND IMPLICATIONS

Suramin blocked membrane channels and protected cells against toxin- and hypotonic stress-elicited injury. Our finding provides novel mechanistic insights into the pharmacological actions of suramin. Suramin might be therapeutically exploited to protect membrane integrity under certain pathological situations.

A multi-omics approach identifies key hubs associated with cell type-specific responses of airway epithelial cells to staphylococcal alpha-toxin

Responsiveness of cells to alpha-toxin (Hla) from Staphylococcus aureus appears to occur in a cell-type dependent manner. Here, we compare two human bronchial epithelial cell lines, i.e. Hla-susceptible 16HBE14o- and Hla-resistant S9 cells, by a quantitative multi-omics strategy for a better understanding of Hla-induced cellular programs. Phosphoproteomics revealed a substantial impact on phosphorylation-dependent signaling in both cell models and highlights alterations in signaling pathways associated with cell-cell and cell-matrix contacts as well as the actin cytoskeleton as key features of early rHla-induced effects. Along comparable changes in down-stream activity of major protein kinases significant differences between both models were found upon rHla-treatment including activation of the epidermal growth factor receptor EGFR and mitogen-activated protein kinases MAPK1/3 signaling in S9 and repression in 16HBE14o- cells. System-wide transcript and protein expression profiling indicate induction of an immediate early response in either model. In addition, EGFR and MAPK1/3-mediated changes in gene expression suggest cellular recovery and survival in S9 cells but cell death in 16HBE14o- cells. Strikingly, inhibition of the EGFR sensitized S9 cells to Hla indicating that the cellular capacity of activation of the EGFR is a major protective determinant against Hla-mediated cytotoxic effects.

The Janus face of α-toxin: a potent mediator of cytoprotection in staphylococci-infected macrophages

After phagocytosis by macrophages, Staphylococcus aureus evades killing in an α-toxin-dependent manner, and then prevents apoptosis of infected cells by upregulating expression of antiapoptotic genes like MCL-1 (myeloid cell leukemia-1). Here, using purified α-toxin and a set of hla-deficient strains, we show that α-toxin is critical for the induction of MCL-1 expression and the cytoprotection of infected macrophages. Extracellular or intracellular treatment of macrophages with α-toxin alone did not induce cytoprotection conferred by increased Mcl-1, suggesting that the process is dependent on the production of α-toxin by intracellular bacteria. The increased expression of MCL-1 in infected cells was associated with enhanced NFκB activation, and subsequent IL-6 secretion. This effect was only partially inhibited by blocking TLR2, which suggests the participation of intracellular receptors in the specific recognition of S. aureus strains secreting α-toxin. Thus, S. aureus recognition by intracellular receptors and/or activation of downstream pathways leading to Mcl-1 expression is facilitated by α-toxin released by intracellular bacteria which permeabilize phagosomes, ensuring pathogen access to the cytoplasmatic compartment. Given that the intracellular survival of S. aureus depends on α-toxin, we propose a novel role for this agent in the protection of the intracellular niche, and further dissemination of staphylococci by infected macrophages.

An important role of α-hemolysin in extracellular vesicles on the development of atopic dermatitis induced by Staphylococcus aureus

Skin barrier disruption and dermal inflammation are key phenotypes of atopic dermatitis (AD). Staphylococcus aureus secretes extracellular vesicles (EVs), which are involved in AD pathogenesis. Here, we evaluated the role of EVs-associated α-hemolysin derived from S. aureus in AD pathogenesis. α-hemolysin production from S. aureus was detected using western blot analyses. The cytotoxic activity of α-hemolysin on HaCaT keratinocytes was evaluated by measuring cell viability after treating cells with soluble and EVs-associated α-hemolysin. To determine the type of cell death, HaCaT keratinocytes were stained with annexin V and 7-AAD. The in vivo effects of α-hemolysin were evaluated by application of soluble and EV-associated α-hemolysin on the mouse skin. The present study showed that increased α-hemolysin was produced by S. aureus colonized on AD patients compared to healthy subjects. α-hemolysin production was also related to AD severity. In addition, EV-associated α-hemolysin was more cytotoxic to HaCaT keratinocytes than soluble α-hemolysin, and α-hemolysin-negative EVs did not induce keratinocyte death. EV-associated α-hemolysin induced necrosis, but soluble α-hemolysin induced apoptosis of keratinocytes. In vivo, skin barrier disruption and epidermal hyperplasia were induced by soluble and EV-associated α-hemolysin. However, AD-like dermal inflammation was only caused by EV-associated α-hemolysin. Moreover, neither skin barrier disruption nor AD-like skin inflammation was induced by α-hemolysin-negative EVs. Taken together, α-Hemolysin secreted from S. aureus, particularly the EV-associated form, induces both skin barrier disruption and AD-like skin inflammation, suggesting that EV-associated α-hemolysin is a novel diagnostic and therapeutic target for the control of AD.