Staphylococcal alpha-toxin: repair of a calcium-impermeable pore in the target cell membrane

Membrane Dynamics and Remodelling in Response to the Action of the Membrane-Damaging Pore-Forming Toxins

Pore-forming protein toxins (PFTs) represent a diverse class of membrane-damaging proteins that are produced by a wide variety of organisms. PFT-mediated membrane perforation is largely governed by the chemical composition and the physical properties of the plasma membranes. The interaction between the PFTs with the target membranes is critical for the initiation of the pore-formation process, and can lead to discrete membrane reorganization events that further aids in the process of pore-formation. Punching holes on the plasma membranes by the PFTs interferes with the cellular homeostasis by disrupting the ion-balance inside the cells that in turn can turn on multiple signalling cascades required to restore membrane integrity and cellular homeostasis. In this review, we discuss the physicochemical attributes of the plasma membranes associated with the pore-formation processes by the PFTs, and the subsequent membrane remodelling events that may start off the membrane-repair mechanisms.

Ca2+ dependence and kinetics of cell membrane repair after electropermeabilization

Electroporation, in particular with nanosecond pulses, is an efficient technique to generate nanometer-size membrane lesions without the use of toxins or other chemicals. The restoration of the membrane integrity takes minutes and is only partially dependent on [Ca²⁺]. We explored the impact of Ca²⁺ on the kinetics of membrane resealing by monitoring the entry of a YO-PRO-1 dye (YP) in BPAE and HEK cells. Ca²⁺ was promptly removed or added after the electric pulse (EP) by a fast-step perfusion. YP entry increased sharply after the EP and gradually slowed down following either a single- or a double-exponential function. In BPAE cells permeabilized by a single 300- or 600-ns EP at 14 kV/cm in a Ca²⁺-free medium, perfusion with 2 mM of external Ca²⁺ advanced the 90% resealing and reduced the dye uptake about twofold. Membrane restoration was accomplished by a combination of fast, Ca²⁺-independent resealing (τ = 13-15 s) and slow, Ca²⁺-dependent processes (τ ~70 s with Ca²⁺ and ~ 110 s or more without it). These time constants did not change when the membrane damage was doubled by increasing EP duration from 300 to 600 ns. However, injury by microsecond-range EP (300 and 600 μs) took longer to recover even when the membrane initially was less damaged, presumably because of the larger size of pores made in the membrane. Full membrane recovery was not prevented by blocking both extra- and intracellular Ca²⁺ (by loading cells with BAPTA or after Ca²⁺ depletion from the reticulum), suggesting the recruitment of unknown Ca²⁺-independent repair mechanisms.

Beyond pore formation: reorganization of the plasma membrane induced by pore-forming proteins

Pore-forming proteins (PFPs) are a heterogeneous group of proteins that are expressed and secreted by a wide range of organisms. PFPs are produced as soluble monomers that bind to a receptor molecule in the host cell membrane. They then assemble into oligomers that are incorporated into the lipid membrane to form transmembrane pores. Such pore formation alters the permeability of the plasma membrane and is one of the most common mechanisms used by PFPs to destroy target cells. Interestingly, PFPs can also indirectly manipulate diverse cellular functions. In recent years, increasing evidence indicates that the interaction of PFPs with lipid membranes is not only limited to pore-induced membrane permeabilization but is also strongly associated with extensive plasma membrane reorganization. This includes lateral rearrangement and deformation of the lipid membrane, which can lead to the disruption of target cell function and finally death. Conversely, these modifications also constitute an essential component of the membrane repair system that protects cells from the lethal consequences of pore formation. Here, we provide an overview of the current knowledge on the changes in lipid membrane organization caused by PFPs from different organisms.

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.

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.

Phobalysin: Fisheye View of Membrane Perforation, Repair, Chemotaxis and Adhesion

Phobalysin P (PhlyP, for photobacterial lysin encoded on a plasmid) is a recently described small β-pore forming toxin of Photobacterium damselae subsp. damselae (Pdd). This organism, belonging to the family of Vibrionaceae, is an emerging pathogen of fish and various marine animals, which occasionally causes life-threatening soft tissue infections and septicemia in humans. By using genetically modified Pdd strains, PhlyP was found to be an important virulence factor. More recently, in vitro studies with purified PhlyP elucidated some basic consequences of pore formation. Being the first bacterial small β-pore forming toxin shown to trigger calcium-influx dependent membrane repair, PhlyP has advanced to a revealing model toxin to study this important cellular function. Further, results from co-culture experiments employing various Pdd strains and epithelial cells together with data on other bacterial toxins indicate that limited membrane damage may generally enhance the association of bacteria with target cells. Thereby, remodeling of plasma membrane and cytoskeleton during membrane repair could be involved. In addition, a chemotaxis-dependent attack-and track mechanism influenced by environmental factors like salinity may contribute to PhlyP-dependent association of Pdd with cells. Obviously, a synoptic approach is required to capture the regulatory links governing the interaction of Pdd with target cells. The characterization of Pdd’s secretome may hold additional clues because it may lead to the identification of proteases activating PhlyP’s pro-form. Current findings on PhlyP support the notion that pore forming toxins are not just killer proteins but serve bacteria to fulfill more subtle functions, like accessing their host.

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.

Membrane Repair Mechanisms against Permeabilization by Pore-Forming Toxins

Permeabilization of the plasma membrane represents an important threat for any cell, since it compromises its viability by disrupting cell homeostasis. Numerous pathogenic bacteria produce pore-forming toxins that break plasma membrane integrity and cause cell death by colloid-osmotic lysis. Eukaryotic cells, in turn, have developed different ways to cope with the effects of such membrane piercing. Here, we provide a short overview of the general mechanisms currently proposed for plasma membrane repair, focusing more specifically on the cellular responses to membrane permeabilization by pore-forming toxins and presenting new data on the effects and cellular responses to the permeabilization by an RTX (repeats in toxin) toxin, the adenylate cyclase toxin-hemolysin secreted by the whooping cough bacterium Bordetella pertussis, which we have studied in the laboratory.

Repair of a Bacterial Small β-Barrel Toxin Pore Depends on Channel Width

Membrane repair emerges as an innate defense protecting target cells against bacterial pore-forming toxins. Here, we report the first paradigm of Ca²⁺-dependent repair following attack by a small β-pore-forming toxin, namely, plasmid-encoded phobalysin of Photobacterium damselae subsp. damselae. In striking contrast, Vibrio cholerae cytolysin, the closest ortholog of phobalysin, subverted repair. Mutational analysis uncovered a role of channel width in toxicity and repair. Thus, the replacement of serine at phobalysin´s presumed channel narrow point with the bulkier tryptophan, the corresponding residue in Vibrio cholerae cytolysin (W318), modulated Ca²⁺ influx, lysosomal exocytosis, and membrane repair. And yet, replacing tryptophan (W318) with serine in Vibrio cholerae cytolysin enhanced toxicity. The data reveal divergent strategies evolved by two related small β-pore-forming toxins to manipulate target cells: phobalysin leads to fulminant perturbation of ion concentrations, closely followed by Ca²⁺ influx-dependent membrane repair. In contrast, V. cholerae cytolysin causes insidious perturbations and escapes control by the cellular wounded membrane repair-like response.

Previous studies demonstrated that large transmembrane pores, such as those formed by perforin or bacterial toxins of the cholesterol-dependent cytolysin family, trigger rapid, Ca²⁺ influx-dependent repair mechanisms. In contrast, recovery from attack by the small β-pore-forming Staphylococcus aureus alpha-toxin or aerolysin is slow in comparison and does not depend on extracellular Ca²⁺. To further elucidate the scope of Ca²⁺ influx-dependent repair and understand its limitations, we compared the cellular responses to phobalysin and V. cholerae cytolysin, two related small β-pore-forming toxins which create membrane pores of slightly different sizes. The data indicate that the channel width of a small β-pore-forming toxin is a critical determinant of both primary toxicity and susceptibility to Ca²⁺-dependent repair.

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.

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.

The RTX pore-forming toxin α-hemolysin of uropathogenic Escherichia coli: progress and perspectives

Members of the RTX family of protein toxins are functionally conserved among an assortment of bacterial pathogens. By disrupting host cell integrity through their pore-forming and cytolytic activities, this class of toxins allows pathogens to effectively tamper with normal host cell processes, promoting pathogenesis. Here, we focus on the biology of RTX toxins by describing salient properties of a prototype member, α-hemolysin, which is often encoded by strains of uropathogenic Escherichia coli. It has long been appreciated that RTX toxins can have distinct effects on host cells aside from outright lysis. Recently, advances in modeling and analysis of host-pathogen interactions have led to novel findings concerning the consequences of pore formation during host-pathogen interactions. We discuss current progress on longstanding questions concerning cell specificity and pore formation, new areas of investigation that involve toxin-mediated perturbations of host cell signaling cascades and perspectives on the future of RTX toxin investigation.

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.

Membrane damage during Listeria monocytogenes infection triggers a caspase-7 dependent cytoprotective response

The cysteine protease caspase-7 has an established role in the execution of apoptotic cell death, but recent findings also suggest involvement of caspase-7 during the host response to microbial infection. Caspase-7 can be cleaved by the inflammatory caspase, caspase-1, and has been implicated in processing and activation of microbial virulence factors. Thus, caspase-7 function during microbial infection may be complex, and its role in infection and immunity has yet to be fully elucidated. Here we demonstrate that caspase-7 is cleaved during cytosolic infection with the intracellular bacterial pathogen, Listeria monocytogenes. Cleavage of caspase-7 during L. monocytogenes infection did not require caspase-1 or key adaptors of the primary pathways of innate immune signaling in this infection, ASC, RIP2 and MyD88. Caspase-7 protected infected macrophages against plasma membrane damage attributable to the bacterial pore-forming toxin Listeriolysin O (LLO). LLO-mediated membrane damage could itself trigger caspase-7 cleavage, independently of infection or overt cell death. We also detected caspase-7 cleavage upon treatment with other bacterial pore-forming toxins, but not in response to detergents. Taken together, our results support a model where cleavage of caspase-7 is a consequence of toxin-mediated membrane damage, a common occurrence during infection. We propose that host activation of caspase-7 in response to pore formation represents an adaptive mechanism by which host cells can protect membrane integrity during infection.

Phospholipid scramblase 1 mediates type i interferon-induced protection against staphylococcal α-toxin

The opportunistic gram-positive pathogen Staphylococcus aureus is a leading cause of pneumonia and sepsis. Staphylococcal α-toxin, a prototypical pore-forming toxin, is a major virulence factor of S. aureus clinical isolates, and lung epithelial cells are highly sensitive to α-toxin's cytolytic activity. Type I interferon (IFN) signaling activated in response to S. aureus increases pulmonary cell resistance to α-toxin, but the underlying mechanisms are uncharacterized. We show that IFNα protects human lung epithelial cells from α-toxin-induced intracellular ATP depletion and cell death by reducing extracellular ATP leakage. This effect depends on protein palmitoylation and induction of phospholipid scramblase 1 (PLSCR1). IFNα-induced PLSCR1 associates with the cytoskeleton after exposure to α-toxin, and cellular depletion of PLSCR1 negates IFN-induced protection from α-toxin. PLSCR1-deficient mice display enhanced sensitivity to inhaled α-toxin and an α-toxin-producing S. aureus strain. These results uncover PLSCR1 activity as part of an innate protective mechanism to a bacterial pore-forming toxin.

Elimination of a bacterial pore-forming toxin by sequential endocytosis and exocytosis

Staphylococcus aureus alpha-toxin is the archetype of bacterial pore forming toxins and a key virulence factor secreted by the majority of clinical isolates of S. aureus. Toxin monomers bind to target cells and oligomerize to form small beta-barrel pores in the plasma membrane. Many nucleated cells are able to repair a limited number of lesions by unknown, calcium-independent mechanisms. Here we show that cells can internalize alpha-toxin, that uptake is essential for cellular survival, and that pore-complexes are not proteolytically degraded, but returned to the extracellular milieu in the context of exosome-like structures, which we term toxosomes.

Membrane insertion of the Bacillus thuringiensis Cry1Ab toxin: single mutation in domain II block partitioning of the toxin into the brush border membrane

The umbrella and penknife models hypothesize that insecticidal Bacillus thuringiensis Cry toxins partition into the apical membrane of the insect midgut by insertion of only two alpha-helices from domain I of the protein, alpha-helices 4 and 5 in the case of the umbrella model and alpha-helices 5 and 6 in the case of the penknife model. Neither model envisages membrane partitioning by domains II and III. In this study, we present data suggesting that mutations in the domain II residue, F371, affect insertion of the whole toxin into Manduca sexta brush border membrane vesicles (BBMVs). Using steady state fluorescence measurements combined with a proteinase K protection assay, we show that mutants of F371 have lost their ability to insert into the BBMV, even though binding to cadherin is almost unaffected. The study also identifies a difference in partitioning of toxins into artificial lipid vesicles (SUVs) as opposed to native BBMVs. While the F371 mutations block insertion of domains I and II into BBMVs, they only block domain II insertion into SUVs. Bioassay and voltage clamping of midguts also confirm the fluorescence data that the noninserting mutants are nontoxic. Our study leads us to propose that, in contrast to previous models of individual free helices inserting into the membrane, the toxin enters into the membrane as a whole molecule or oligomers of the molecule, wherein the domain II residue F371 has a vital role to play in membrane insertion.

Interferons increase cell resistance to Staphylococcal alpha-toxin

Many bacterial pathogens, including Staphylococcus aureus, use a variety of pore-forming toxins as important virulence factors. Staphylococcal alpha-toxin, a prototype beta-barrel pore-forming toxin, triggers the release of proinflammatory mediators and induces primarily necrotic death in susceptible cells. However, whether host factors released in response to staphylococcal infections may increase cell resistance to alpha-toxin is not known. Here we show that prior exposure to interferons (IFNs) prevents alpha-toxin-induced membrane permeabilization, the depletion of ATP, and cell death. Moreover, pretreatment with IFN-alpha decreases alpha-toxin-induced secretion of interleukin 1beta (IL-1beta). IFN-alpha, IFN-beta, and IFN-gamma specifically protect cells from alpha-toxin, whereas tumor necrosis factor alpha (TNF-alpha), IL-6, and IL-4 have no effects. Furthermore, we show that IFN-alpha-induced protection from alpha-toxin is not dependent on caspase-1 or mitogen-activated protein kinases, but requires protein synthesis and fatty acid synthase activity. Our results demonstrate that IFNs may increase cell resistance to staphylococcal alpha-toxin via the regulation of lipid metabolism and suggest that interferons play a protective role during staphylococcal infections.

Why Escherichia coli alpha-hemolysin induces calcium oscillations in mammalian cells--the pore is on its own

Escherichia coli alpha-hemolysin (HlyA), archetype of a bacterial pore-forming toxin, has been reported to deregulate physiological Ca2+ channels, thus inducing periodic low-frequency Ca2+ oscillations that trigger transcriptional processes in mammalian cells. The present study was undertaken to delineate the mechanisms underlying the Ca2+ oscillations. Patch-clamp experiments were combined with single cell measurements of intracellular Ca2+ and with flowcytometric analyses. Application of HlyA at subcytocidal concentrations provoked Ca2+ oscillations in human renal and endothelial cells. However, contrary to the previous report, the phenomenon could not be inhibited by the Ca2+ channel blocker nifedipine and Ca2+ oscillations showed no constant periodicity at all. Ca2+ oscillations were dependent on the pore-forming activity of HlyA: application of a nonhemolytic but bindable toxin had no effect. Washout experiments revealed that Ca2+ oscillations could not be maintained in the absence of toxin in the medium. Analogously, propidium iodide flux into cells occurred in the presence of HlyA, but cells rapidly became impermeable toward the dye after toxin washout, indicating resealing or removal of the membrane lesions. Finally, patch-clamp experiments revealed temporal congruence between pore formation and Ca2+ influx. We conclude that the nonperiodic Ca2+ oscillations induced by HlyA are not due to deregulation of physiological Ca2+ channels but derive from pulsed influxes of Ca2+ as a consequence of formation and rapid closure of HlyA pores in mammalian cell membranes.

Alpha-toxin is produced by skin colonizing Staphylococcus aureus and induces a T helper type 1 response in atopic dermatitis

BACKGROUND

Staphylococcus aureus is a well known trigger factor of atopic dermatitis (AD). Besides the superantigens, further exotoxins are produced by S. aureus and may have an influence on the eczema.

OBJECTIVE

To explore the impact of staphylococcal alpha-toxin on human T cells, as those represent the majority of skin infiltrating cells in AD.

METHODS

Adult patients with AD were screened for cutaneous colonization with alpha-toxin producing S. aureus. As alpha-toxin may induce necrosis, CD4(+) T cells were incubated with sublytic alpha-toxin concentrations. Proliferation and up-regulation of IFN-gamma on the mRNA and the protein level were assessed. The induction of t-bet translocation in CD4(+) T cells was detected with the Electrophoretic Mobility Shift Assay.

RESULTS

Thirty-four percent of the patients were colonized with alpha-toxin producing S. aureus and alpha-toxin was detected in lesional skin of these patients by immunohistochemistry. Sublytic alpha-toxin concentrations induced a marked proliferation of isolated CD4(+) T cells. Microarray analysis indicated that alpha-toxin induced particularly high amounts of IFN-gamma transcripts. Up-regulation of IFN-gamma was confirmed both on the mRNA and the protein level. Stimulation of CD4(+) T cells with alpha-toxin resulted in DNA binding of t-bet, known as a key transcription factor involved into primary T helper type 1 (Th1) commitment.

CONCLUSION

alpha-toxin is produced by S. aureus isolated from patients with AD. We show here for the first time that sublytic alpha-toxin concentrations activate T cells in the absence of antigen-presenting cells. Our results indicate that alpha-toxin is relevant for the induction of a Th1 like cytokine response. In AD, this facilitates the development of Th1 cell dominated chronic eczema.

A rivet model for channel formation by aerolysin-like pore-forming toxins

The bacterial toxin aerolysin kills cells by forming heptameric channels, of unknown structure, in the plasma membrane. Using disulfide trapping and cysteine scanning mutagenesis coupled to thiol-specific labeling on lipid bilayers, we identify a loop that lines the channel. This loop has an alternating pattern of charged and uncharged residues, suggesting that the transmembrane region has a beta-barrel configuration, as observed for Staphylococcal alpha-toxin. Surprisingly, we found that the turn of the beta-hairpin is composed of a stretch of five hydrophobic residues. We show that this hydrophobic turn drives membrane insertion of the developing channel and propose that, once the lipid bilayer has been crossed, it folds back parallel to the plane of the membrane in a rivet-like fashion. This rivet-like conformation was modeled and sequence alignments suggest that such channel riveting may operate for many other pore-forming toxins.

Cytotoxicity of Bacillus thuringiensis Cry1Ab toxin depends on specific binding of the toxin to the cadherin receptor BT-R1 expressed in insect cells

The specific role of cadherin receptors in cytotoxicity involving Cry toxins of Bacillus thuringiensis and their interactions with cell membrane has not been defined. To elucidate the involvement of toxin-membrane and toxin-receptor interactions in cytotoxicity, we established a cell-based system utilizing High Five insect cells stably expressing BT-R1, the cadherin receptor for Cry1Ab toxin. Cry1Ab toxin is incorporated into cell membrane in both oligomeric and monomeric form. Monomeric toxin binds specifically to BT-R1 whereas incorporation of oligomeric toxin is nonspecific and lipid dependent. Toxin oligomers in the cell membrane do not produce lytic pores and do not kill insect cells. Rather, cell death correlates with binding of the Cry1Ab toxin monomer to BT-R1, which apparently activates a Mg2+-dependent cellular signaling pathway.

Responses of fish chromatophore-based cytosensor to a broad range of biological agents

A cytosensor based on living chromatophores from Betta splendens Siamese fighting fish was used to test several classes of biologically active agents. Tested agents include neurotransmitters, adenyl cyclase activators, cytoskeleton effectors, cell membrane effectors and protein synthesis inhibitors. Characteristic cell responses were analyzed, and potential cytosensor applications were considered. Streptococcus pyogenes toxins streptolysin S and streptolysin O, Clostridium tetani tetanolysin, Staphylococcus aureus alpha-toxin and Vibrio parahemolyticus hemolysin, all bacterial toxins that act on cell membranes, elicited a strong response from chromatophores. A comparison of purified toxin to actual bacterial culture from Vibrio parahemolyticus demonstrated a nearly identical chromatophore cell response pattern. This suggests that the cytosensor response is reflective of bacterial toxin production.

Membrane lipid composition protects Entamoeba histolytica from self-destruction by its pore-forming toxins

The protozoan parasite and human pathogen Entamoeba histolytica is protected against killing by its own lytic effector proteins. Amoebae withstand doses of amoebapores, their pore-forming polypeptides, that readily kill human Jurkat T cells. Moreover, the polypeptides do not bind to the amoebic surface membrane as evidenced by using fluorescently labelled amoebapores and confocal laser microscopy. Experiments employing liposomes as a minimalistic membrane system and the major isoform amoebapore A revealed that the lipid composition of amoebic membranes prevents binding of the cytolytic molecule and that both the phospholipid ingredients and the high content of cholesterol contributes to the protection of the toxin-producing cell.

The streptococcal exotoxin streptolysin O activates mast cells to produce tumor necrosis factor alpha by p38 mitogen-activated protein kinase- and protein kinase C-dependent pathways

Streptolysin O (SLO), a major virulence factor of pyogenic streptococci, binds to cholesterol in the membranes of eukaryotic cells and oligomerizes to form large transmembrane pores. While high toxin doses are rapidly cytocidal, low doses are tolerated because a limited number of lesions can be resealed. Here, we report that at sublethal doses, SLO activates primary murine bone marrow-derived mast cells to degranulate and to rapidly induce or enhance the production of several cytokine mRNAs, including tumor necrosis factor alpha (TNF-alpha). Mast cell-derived TNF-alpha plays an important protective role in murine models of acute inflammation, and the production of this cytokine was analyzed in more detail. Release of biologically active TNF-alpha peaked approximately 4 h after stimulation with SLO. Production of TNF-alpha was blunted upon depletion of protein kinase C by pretreatment of the cells with phorbol-12 myristate-13 acetate. Transient permeabilization of mast cells with SLO also led to the activation of the stress-activated protein kinases p38 mitogen-activated protein (MAP) kinase and c-jun N-terminal kinase (JNK), and inhibition of p38 MAP kinase markedly reduced production of TNF-alpha. In contrast, secretion of preformed granule constituents triggered by membrane permeabilization was not dependent on p38 MAP kinase or on protein kinase C. Thus, transcriptional activation of mast cells following transient permeabilization might contribute to host defense against infections via the beneficial effects of TNF-alpha. However, hyperstimulation of mast cells might also lead to overproduction of TNF-alpha, which would then promote the development of toxic streptococcal syndromes.

Plasma Membrane Disruption: Repair, Prevention, Adaptation

Many metazoan cells inhabit mechanically stressful environments and, consequently, their plasma membranes are frequently disrupted. Survival requires that the cell rapidly repair or reseal the disruption. Rapid resealing is an active and complex structural modification that employs endomembrane as its primary building block, and cytoskeletal and membrane fusion proteins as its catalysts. Endomembrane is delivered to the damaged plasma membrane through exocytosis, a ubiquitous Ca2+-triggered response to disruption. Tissue and cell level architecture prevent disruptions from occurring, either by shielding cells from damaging levels of force, or, when this is not possible, by promoting safe force transmission through the plasma membrane via protein-based cables and linkages. Prevention of disruption also can be a dynamic cell or tissue level adaptation triggered when a damaging level of mechanical stress is imposed. Disease results from failure of either the preventive or resealing mechanisms.

Resealing of large transmembrane pores produced by streptolysin O in nucleated cells is accompanied by NF-kappaB activation and downstream events

Streptolysin O (SLO), archetype of a cholesterol-binding bacterial cytolysin, forms large pores in the plasma membrane of mammalian cells. We have recently reported that when a limited number of pores are generated in a cell, they can be sealed in a Ca++-dependent process. Here, we show that resealing is followed by the release of IL-6 and IL-8 from keratinocytes and from endothelial cells, both relevant targets for SLO attack. Production of cytokines by these cells was preceded by activation of transcription factor nuclear factor kappaB, which thus emerges as a common denominator of stress responses to various pore-forming agents, including alpha-toxin of Staphylococcus aureus and complement. Furthermore, we show that activation and cytokine release in response to an agent that forms a pore in the plasma membrane do not depend on paracrine effects, because supernatants of cells perforated by SLO did not activate bystander cells. The study provides definitive evidence that a transient transmembrane pore suffices to trigger productive transcriptional activation in a target cell.

Not as simple as just punching a hole

Like a variety of other pathogenic bacteria, Aeromonas hydrophila secretes a pore-forming toxin that contribute to its virulence. The last decade has not only increased our knowledge about the structure of this toxin, called aerolysin, but has also shed light on how it interacts with its target cell and how the cell reacts to this stress. Whereas pore-forming toxins are generally thought to lead to brutal death by osmotic lysis of the cell, based on what is observed for erythrocytes, recent studies have started to reveal far more complicated pathways leading to death of nucleated mammalian cells.

Subcytocidal attack by staphylococcal alpha-toxin activates NF-kappaB and induces interleukin-8 production

Formation of transmembrane pores by staphylococcal alpha-toxin can provoke a spectrum of events depending on target cell species and toxin dose, and in certain cases, repair of the lesions has been observed. Here, we report that transcriptional processes are activated as a response of cells to low toxin doses. Exposure of monocytic (THP-1) or epithelial (ECV304) cells to 40 to 160 ng/ml alpha-toxin provoked a drop in cellular ATP level that was followed by secretion of substantial amounts of interleukin-8 (IL-8). Cells transfected with constructs comprising the proximal IL-8 promoter fused to luciferase or to green fluorescent protein cDNA exhibited enhanced reporter gene expression following toxin treatment. Electrophoretic mobility shift and immunofluorescence assays demonstrated that IL-8 secretion was preceded by activation of NF-kappaB. Transfection experiments conducted with p65/p50 double-deficient cells showed that activation of the IL-8 promoter/reporter by toxin was absolutely dependent on NF-kappaB. In contrast, this transcription factor was not required for lesion repair. Attack of cells by low doses of a pore-forming toxin can lead to transcriptional gene activation, which is followed by production of mediators that may contribute to the initiation and propagation of inflammatory lesions.