The Binding of Staphylococcal 125I-α-Toxin (B) to Erythrocytes

Subinhibitory concentrations of resveratrol reduce alpha-hemolysin production in Staphylococcus aureus isolates by downregulating saeRS

Resveratrol is a natural phytoalexin. In recent studies, it has been shown to have beneficial effects on cardiovascular disease and cancer and has been deemed to have effective antiviral and immunomodulatory activities. Methicillin-resistant Staphylococcus aureus is a multidrug-resistant pathogen associated with skin and soft tissue infections. Alpha-hemolysin is known to play a key role in the symptoms caused by S. aureus, and the saeRS two-component system has been shown to be a major regulatory system of S. aureus virulence. The present study was designed to determine the effect of subinhibitory concentrations of resveratrol on the production of alpha-hemolysin in S. aureus. The effect of resveratrol on the transcription of S. aureus was studied by transcriptome sequencing. A total of 760 genes with >2-fold changes in expression were selected, including 479 upregulated genes and 281 downregulated genes. On the basis of transcriptome sequencing, the expression of alpha-hemolysin in the S. aureus strains of the resveratrol-treated group was downregulated. Our results showed that resveratrol weakly inhibited the growth of S. aureus strains, and subinhibitory concentration of resveratrol decreased the expression of hla and inhibited the regulation of saeRS. Hemolysis testing confirmed that resveratrol had an inhibitory effect on the hemolysis of rabbit erythrocytes infected with S. aureus strains in a dose-dependent manner. Resveratrol also decreased the hemolytic capacity by reducing the production of alpha-hemolysin. We found that resveratrol could decrease the expression of hla and reduce the secretion of alpha-hemolysin by downregulating saeRS. These findings have provided more evidence of the potential of resveratrol as a drug for resisting S. aureus infections.

Atypical sensors for direct and rapid neuronal detection of bacterial pathogens

Bacterial infection can threaten the normal biological functions of a host, often leading to a disease. Hosts have developed complex immune systems to cope with the danger. Preceding the elimination of pathogens, selective recognition of the non-self invaders is necessary. At the forefront of the body’s defenses are the innate immune cells, which are equipped with particular sensor molecules that can detect common exterior patterns of invading pathogens and their secreting toxins as well as with phagocytic machinery. Inflammatory mediators and cytokines released from these innate immune cells and infected tissues can boost the inflammatory cascade and further recruit adaptive immune cells to maximize the elimination and resolution. The nervous system also seems to interact with this process, mostly known to be affected by the inflammatory mediators through the binding of neuronal receptors, consequently activating neural circuits that tune the local and systemic inflammatory states. Recent research has suggested new contact points: direct interactions of sensory neurons with pathogens. Latest findings demonstrated that the sensory neurons not only share pattern recognition mechanisms with innate immune cells, but also utilize endogenous and exogenous electrogenic components for bacterial pathogen detection, by which the electrical firing prompts faster information flow than what could be achieved when the immune system is solely involved. As a result, rapid pain generation and active accommodation of the immune status occur. Here we introduced the sensory neuron-specific detector molecules for directly responding to bacterial pathogens and their signaling mechanisms. We also discussed extended issues that need to be explored in the future.

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.

Cell targeting by the Staphylococcus aureus pore-forming toxins: it's not just about lipids

Staphylococcus aureus employs numerous pore-forming cytotoxins to injure host immune cells and promote infection. Until recently, it was unclear how these cytotoxins targeted specific cell types for lysis. Membrane lipids were initially postulated to be cytotoxin receptor candidates. However, the cell-type specificity and species-dependent targeting of these toxins did not support lipids as sole receptors. The recent identification of proteinaceous receptors for several S. aureus cytotoxins now provides an explanation for the observed tropism. These findings also have important implications for the implementation of animal models to study S. aureus pathogenesis, and for the development of novel therapeutics.

Identification of anti-alpha toxin monoclonal antibodies that reduce the severity of Staphylococcus aureus dermonecrosis and exhibit a correlation between affinity and potency

Staphylococcus aureus alpha toxin (AT) is an important virulence determinant and may be a valid target for immunoprophylaxis against staphylococcal disease. Here we report the identification of potent inhibitory anti-AT monoclonal antibodies (MAbs) derived using B-cell hybridoma technology from VelocImmune mice engineered to produce IgG with a human variable domain. A small panel of inhibitory MAbs blocked AT-mediated lysis of rabbit red blood cells, A549 human lung epithelial cells, and THP-1 human monocytic cells, in a dose-dependent manner. Binding studies indicated that these MAbs recognize a similar epitope on AT and exhibit dissociation constants (K(D)) ranging from 0.50 to 15 nM. In an S. aureus dermonecrosis model, mice passively immunized with anti-AT inhibitory MAbs exhibited significant reductions of lesion size relative to mice treated with an irrelevant IgG control. Interestingly, there was a correlation between MAb affinity for a single epitope, the 50% inhibitory concentration (IC(50)) in the AT hemolytic assay, and lesion size reduction in the dermonecrosis model. A representative high-affinity MAb, 2A3.1, was demonstrated to significantly reduce lesion size following infection with three different clinical isolates (USA300, CC30, and CC5). Taken together, these results indicate that in vitro potency of anti-AT MAbs predicts in vivo potency in this model, supporting their continued preclinical evaluation as molecules for immunoprophylaxis against staphylococcal skin and soft tissue infections caused by diverse clinical isolates.

Python erythrocytes are resistant to α-hemolysin from Escherichia coli

α-Hemolysin (HlyA) from Escherichia coli lyses mammalian erythrocytes by creating nonselective cation pores in the membrane. Pore insertion triggers ATP release and subsequent P2X receptor and pannexin channel activation. Blockage of either P2X receptors or pannexin channels reduces HlyA-induced hemolysis. We found that erythrocytes from Python regius and Python molurus are remarkably resistant to HlyA-induced hemolysis compared to human and Trachemys scripta erythrocytes. HlyA concentrations that induced maximal hemolysis of human erythrocytes did not affect python erythrocytes, but increasing the HlyA concentration 40-fold did induce hemolysis. Python erythrocytes were more resistant to osmotic stress than human erythrocytes, but osmotic stress tolerance per se did not confer HlyA resistance. Erythrocytes from T. scripta, which showed higher osmotic resistance than python erythrocytes, were as susceptible to HlyA as human erythrocytes. Therefore, we tested whether python erythrocytes lack the purinergic signalling known to amplify HlyA-induced hemolysis in human erythrocytes. P. regius erythrocytes increased intracellular Ca²⁺ concentration and reduced cell volume when exposed to 3 mM ATP, indicating the presence of a P2X₇-like receptor. In addition, scavenging extracellular ATP or blocking P2 receptors or pannexin channels reduced the HlyA-induced hemolysis. We tested whether the low HlyA sensitivity resulted from low affinity of HlyA to the python erythrocyte membrane. We found comparable incorporation of HlyA into human and python erythrocyte membranes. Taken together, the remarkable HlyA resistance of python erythrocytes was not explained by increased osmotic resistance, lack of purinergic hemolysis amplification, or differences in HlyA affinity.

Haemolysis induced by α-toxin from Staphylococcus aureus requires P2X receptor activation

Recently, it was documented that α-haemolysin (HlyA) from Escherichia coli uses erythrocyte P2 receptors cause lysis. This finding was surprising as it appeared firmly established that HlyA-dependent pore formation per se is sufficient for full cell lysis. We discovered that HlyA induced a sequential process of shrinkage and swelling and that the final haemolysis is completely prevented by blockers of P2X receptors and pannexin channels. This finding has potential clinical relevance as it may offer specific pharmacological interference to ameliorate haemolysis inflicted by pore-forming bacterial toxins. In this context, it is essential to know whether this is specific to HlyA-induced cell damage or if other bacterial pore-forming toxins involve purinergic signals to orchestrate haemolysis. Here, we investigate if the haemolysis produced by α-toxin from Staphylococcus aureus involves P2 receptor activation. We observed that α-toxin-induced haemolysis is completely blocked by the unselective P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid. Moreover, several selective blockers of P2X(1) and P2X(7) ionotropic receptors abolished haemolysis in murine and equine erythrocytes. Inhibitors of pannexin channels partially reduced the α-toxin induced lysis. Thus, we conclude that α-toxin, similar to HlyA from E. coli produces cell damage by specific activation of a purinergic signalling cascade. These data indicate that pore-forming toxins in general require purinergic signalling to elicit their toxicity.

Role of a disintegrin and metalloprotease 10 in Staphylococcus aureus alpha-hemolysin-mediated cellular injury

Staphylococcus aureus alpha-hemolysin (Hla), a potent cytotoxin, plays an important role in the pathogenesis of staphylococcal diseases, including those caused by methicillin-resistant epidemic strains. Hla is secreted as a water-soluble monomer that undergoes a series of conformational changes to generate a heptameric, beta-barrel structure in host membranes. Structural maturation of Hla depends on its interaction with a previously unknown proteinaceous receptor in the context of the cell membrane. It is reported here that a disintegrin and metalloprotease 10 (ADAM10) interacts with Hla and is required to initiate the sequence of events whereby the toxin is transformed into a cytolytic pore. Hla binding to the eukaryotic cell requires ADAM10 expression. Further, ADAM10 is required for Hla-mediated cytotoxicity, most notably when the toxin is present at low concentrations. These data thus implicate ADAM10 as the probable high-affinity toxin receptor. Upon Hla binding, ADAM10 relocalizes to caveolin 1-enriched lipid rafts that serve as a platform for the clustering of signaling molecules. It is demonstrated that the Hla-ADAM10 complex initiates intracellular signaling events that culminate in the disruption of focal adhesions.

Adsorption of sphingomyelinase of Bacillus cereus onto erythrocyte membranes

Sphingomyelinase of Bacillus cereus proved to be specifically adsorbed onto mammalian erythrocyte membranes in the presence of either Ca2+ or Ca2+ plus Mg2+ in the order of sphingomyelin content; i.e., sheep, bovine greater than porcine greater than rat erythrocytes. No appreciable adsorption was observed in the presence of Mg2+ alone nor in the absence of divalent metal ions. The enzyme adsorption onto bovine erythrocytes was dependent upon the incubation temperature. By shifting the temperature from 37 to 0 degrees C, sphingomyelinase once adsorbed onto the surface of bovine erythrocytes was released into the supernatant. Ca2+ proved to be an essential factor for the enzyme adsorption: The addition of 1 mM Ca2+ enhanced the adsorptive process, but inhibited sphingomyelin hydrolysis and hot or hot-cold hemolysis of erythrocytes, while the addition of 1 mM Ca2+ plus 1 mM Mg2+ enhanced sphingomyelin breakdown and hemolysis as well as the enzyme adsorption. However, when the amount of sphingomyelin fell off to 0.2-0.7 nmol/ml or less by the action of sphingomyelinase, the enzyme once adsorbed was completely released from the surface of erythrocytes. The result indicates that the major binding site for sphingomyelinase is sphingomyelin. In the presence of 1 mM Mg2+ alone, the enzymatic hydrolysis of sphingomyelin and hemolysis proceeded whereas the enzyme adsorption was not encountered during 60 min incubation at 37 degrees C. The change in the molar ratio of Ca2+ to Mg2+ affected the enzyme adsorption and sphingomyelin breakdown; the higher Ca2+ enhanced the adsorption whereas the higher Mg2+ stimulated sphingomyelin hydrolysis.

Specific binding of staphylococcal alpha-toxin to isolated rabbit vagus nerves in vitro

The binding of staphylococcal [125I]alpha-toxin to rabbit vagus nerves in vitro was a saturable process. The radiolabeled alpha-toxin binding was reduced by the coaddition of added navive alpha-toxin, indicating that the binding is specific. Sucrose gradient analysis of detergent-extracted complexes of [125I]alpha-toxin-rabbit vagus nerves showed both high and low S-value peaks analogous to those observed with similarly treated alpha-toxin-rabbit erythrocyte preparations (P. Cassidy and S. Harshman, Biochemistry, in press).

Studies on the binding of staphylococcal 125I-labeled alpha-toxin to rabbit erythrocytes

Staphylococcal alpha-toxin, a hemolytic exotoxin, can be iodinated using the lactoperoxidase method. 125 I-Labeled alpha-toxin binds to rabbit erythrocytes in an apparently irreversible and highly specific manner. The binding of 125 I-labeled alpha-toxin to erythrocytes of rabbit and human reflects the species specificity of native alpha-toxin. Binding of 125I-labeled alpha-toxin is blocked by the presence of native alpha-toxin, 127I-labeled alpha-toxin, or anti-alpha-toxin antibody. Simultaneous assays of 125I-labeled alpha-toxin binding and leakage of intracellular 86Rb+ suggest that toxin binding and membrane damage are separate, sequential functions. Both the rate and extent of binding are temperature dependent. Rabbit erythrocytes possess 5 X 10(3) binding sites/cell, while human erythrocytes possess no detectable binding sites. Treatment of rabbit erythrocytes with 125I-labeled alpha-toxin appears to decrease the number of unoccupied binding sites. Chaotropic ions can inhibit 125I-labeled alpha-toxin binding and cause bound 125I-labeled alpha-toxin to dissociate from rabbit erythrocyte membranes. Treatment of intact rabbit erythrocytes with pronase reduces both the binding capacity of the cells for 125I-labeled alpha-toxin, and the cells' sensitivity to hemolysis by native alpha-toxin. It is proposed that the primary binding site for alpha-toxin in biomembranes is a surface membrane protein.