Alpha-hemolysin from Escherichia coli uses endogenous amplification through P2X receptor activation to induce hemolysis. Proc Natl Acad Sci U S A. 2009;106:4030-4035. [PMID: 19225107 DOI: 10.1073/pnas.0807044106]

Purinergic Receptor Antagonists Inhibit Hemolysis Induced by Clostridium perfringens Alpha Toxin

Clostridium perfringens alpha toxin (CPA), which causes yellow lamb disease in sheep and gas gangrene and food poisoning in humans, is produced by all types of C. perfringens and is the major virulence determinant of C. perfringens type A. CPA induces hemolysis in many species, including humans, murines, sheep and rabbits, through its enzymatic activity, which dissolves the cell membrane. Recent studies have shown that some pore-forming toxins cause hemolysis, which is achieved by the activation of purinergic receptors (P2). However, the relationship between P2 receptors and non-pore-forming toxin hemolysis has not been investigated. In the present study, we examined the function of P2 receptors in CPA toxin hemolysis and found that CPA-induced hemolysis was dependent on P2 receptor activation, and this was also true for Staphylococcus aureus β-Hemolysin, another non-pore-forming toxin. Furthermore, we use selective P2 receptor antagonists to demonstrate that P2X1 and P2X7 play important roles in the hemolysis of human and murine erythrocytes. In addition, we found that redox metabolism was mainly involved in CPA-induced hemolysis using metabolomic analysis. We further demonstrate that CPA activates P2 receptors and then activates NADPH oxidase through the PI3K/Akt and MEK1/ERK1 pathways, followed by the production of active oxygen to induce hemolysis. These findings contribute to our understanding of the pathological effects of CPA, clarify the relationship between P2 activation and non-pore-forming toxin-induced hemolysis, and provide new insights into CPA-induced hemolysis.

Induction of human-fetal-membrane remodeling in-vitro by the alpha hemolysin of Escherichia coli

INTRODUCTION

Almost 80% of urinary tract infections during pregnancy are caused by uropathogenic strains of Escherichia coli. Alpha-hemolysin, toxin secreted by them, has a fundamental role in this pathology development. Considering that urinary tract infections are related with premature rupture of fetal membranes, we proposed to evaluate the effects that alpha-hemolysin induces on human-fetal-membranes.

METHODS

Thirteen fetal membranes obtained from elective cesarean sections (>37 weeks) were mounted in a transwell-device generating two independent chambers. To mimic an ascendant-urinary-tract infection, membranes were incubated with different concentrations of pure alpha-hemolysin from the choriodecidual side during 24h. Extensive histological analyses were performed and transepithelial electrical-resistance were determined. Cell viability, metalloproteinase activity and cyclooxygenase-2- gene expression was estimated by lactate-dehydrogenase-release assay, zymography and RT-qPCR, respectively. Finally, four fetal membranes were treated with hemolysin preincubated with polyclonal anti-hemolysin antibodies. Cell viability and metalloproteinase activity were monitored.

RESULTS

After 24 h of treatment, fetal membranes evidenced a structural damage and a decrease in membrane resistance that progressed as the concentration of alpha hemolysin increased. While the amniotic-epithelial-layer remained practically unaffected, the chorion cells manifested an increase in vacuolization and necrosis. In addition, the extracellular matrix exhibited collagen-fiber disorganization, a marked decrease in fiber content, and became thicker in presence of the toxin. Cyclooxigenase-2 expression and metalloproteinase activity were also higher in the treated groups than in untreated ones. Finally, a preincubation of hemolysin with specific antibodies prevented the cytotoxicity on the chorion cells and the increase in metalloproteinase activity.

DISCUSSION

Hemolysin induces structural and molecular changes associated with the remodeling of human-fetal-membranes in-vitro.

Uropathogen and host responses in pyelonephritis

Urinary tract infections (UTIs) are among the most common bacterial infections seen in clinical practice. The ascent of UTI-causing pathogens to the kidneys results in pyelonephritis, which can trigger kidney injury, scarring and ultimately impair kidney function. Despite sizable efforts to understand how infections develop or are cleared in the bladder, our appreciation of the mechanisms by which infections develop, progress or are eradicated in the kidney is limited. The identification of virulence factors that are produced by uropathogenic Escherichia coli to promote pyelonephritis have begun to fill this knowledge gap, as have insights into the mechanisms by which kidney tubular epithelial cells oppose uropathogenic E. coli infection to prevent or eradicate UTIs. Emerging data also illustrate how specific cellular immune responses eradicate infection whereas other immune cell populations promote kidney injury. Insights into the mechanisms by which uropathogenic E. coli circumvent host immune defences or antibiotic therapy to cause pyelonephritis is paramount to the development of new prevention and treatment strategies to mitigate pyelonephritis and its associated complications.

ShlA toxin of Serratia induces P2Y2- and α5β1-dependent autophagy and bacterial clearance from host cells

Serratia marcescens is an opportunistic human pathogen involved in antibiotic-resistant hospital acquired infections. Upon contact with the host epithelial cell and prior to internalization, Serratia induces an early autophagic response that is entirely dependent on the ShlA toxin. Once Serratia invades the eukaryotic cell and multiples inside an intracellular vacuole, ShlA expression also promotes an exocytic event that allows bacterial egress from the host cell without compromising its integrity. Several toxins, including ShlA, were shown to induce ATP efflux from eukaryotic cells. Here, we demonstrate that ShlA triggered a nonlytic release of ATP from Chinese hamster ovary (CHO) cells. Enzymatic removal of accumulated extracellular ATP (eATP) or pharmacological blockage of the eATP-P2Y2 purinergic receptor inhibited the ShlA-promoted autophagic response in CHO cells. Despite the intrinsic ecto-ATPase activity of CHO cells, the effective concentration and kinetic profile of eATP was consistent with the established affinity of the P2Y2 receptor and the known kinetics of autophagy induction. Moreover, eATP removal or P2Y2 receptor inhibition also suppressed the ShlA-induced exocytic expulsion of the bacteria from the host cell. Blocking α5β1 integrin highly inhibited ShlA-dependent autophagy, a result consistent with α5β1 transactivation by the P2Y2 receptor. In sum, eATP operates as the key signaling molecule that allows the eukaryotic cell to detect the challenge imposed by the contact with the ShlA toxin. Stimulation of P2Y2-dependent pathways evokes the activation of a defensive response to counteract cell damage and promotes the nonlytic clearance of the pathogen from the infected cell.

Extracellular ATP: A powerful inflammatory mediator in the central nervous system

Nucleotides play a crucial role in extracellular signaling across species boundaries. All the three kingdoms of life (Bacteria, Archea and Eukariota) are responsive to extracellular ATP (eATP) and many release this and other nucleotides. Thus, eATP fulfills different functions, many related to danger-sensing or avoidance reactions. Basically all living organisms have evolved sensors for eATP and other nucleotides with very different affinity and selectivity, thus conferring a remarkable plasticity to this signaling system. Likewise, different intracellular transduction systems were associated during evolution to different receptors for eATP. In mammalian evolution, control of intracellular ATP (iATP) and eATP homeostasis has been closely intertwined with that of Ca²⁺, whether in the extracellular milieu or in the cytoplasm, establishing an inverse reciprocal relationship, i.e. high extracellular Ca²⁺ levels are associated to negligible eATP, while low intracellular Ca²⁺ levels are associated to high eATP concentrations. This inverse relationship is crucial for the messenger functions of both molecules. Extracellular ATP is sensed by specific plasma membrane receptors of widely different affinity named P2 receptors (P2Rs) of which 17 subtypes are known. This confers a remarkable plasticity to P2R signaling. The central nervous system (CNS) is a privileged site for purinergic signaling as all brain cell types express P2Rs. Accruing evidence suggests that eATP, in addition to participating in synaptic transmission, also plays a crucial homeostatic role by fine tuning microglia, astroglia and oligodendroglia responses. Drugs modulating the eATP concentration in the CNS are likely to be the new frontier in the therapy of neuroinflammation. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

Candidalysin Is the Hemolytic Factor of Candida albicans

Candida albicans produces an important virulence factor, the hypha-associated Ece1-derived secreted peptide toxin candidalysin, which is crucial for the establishment of mucosal and systemic infections. C. albicans has also long been known to be hemolytic, yet the hemolytic factor has not been clearly identified. Here, we show that candidalysin is the hemolytic factor of C. albicans. Its hemolytic activity is modulated by fragments of another Ece1 peptide, P7. Hemolysis by candidalysin can be neutralized by the purinergic receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). PPADS also affects candidalysin's ability to intercalate into synthetic membranes. We also describe the neutralization potential of two anti-candidalysin nanobodies, which are promising candidates for future anti-Candida therapy. This work provides evidence that the historically proposed hemolytic factor of C. albicans is in fact candidalysin and sheds more light on the complex roles of this toxin in C. albicans biology and pathogenicity.

P2R Inhibitors Prevent Antibody-Mediated Complement Activation in an Animal Model of Neuromyelitis Optica : P2R Inhibitors Prevent Autoantibody Injury

Purinergic 2 receptors (P2Rs) contribute to disease-related immune cell signaling and are upregulated in various pathological settings, including neuroinflammation. P2R inhibitors have been used to treat inflammatory diseases and can protect against complement-mediated cell injury. However, the mechanisms behind these anti-inflammatory properties of P2R inhibitors are not well understood, and their potential in CNS autoimmunity is underexplored. Here, we tested the effects of P2R inhibitors on glial toxicity in a mouse model of neuromyelitis optica spectrum disorder (NMOSD). NMOSD is a destructive CNS autoimmune disorder, in which autoantibodies against astrocytic surface antigen Aquaporin 4 (AQP4) mediate complement-dependent loss of astrocytes. Using two-photon microscopy in vivo, we found that various classes of P2R inhibitors prevented AQP4-IgG/complement-dependent astrocyte death. In vitro, these drugs inhibited the binding of AQP4-IgG or MOG-IgG to their antigen in a dose-dependent manner. Size-exclusion chromatography and circular dichroism spectroscopy revealed a partial unfolding of antibodies in the presence of various P2R inhibitors, suggesting a shared interference with IgG antibodies leading to their conformational change. Our study demonstrates that P2R inhibitors can disrupt complement activation by direct interaction with IgG. This mechanism is likely to influence the role of P2R inhibitors in autoimmune disease models and their therapeutic impact in human disease.

Kingella kingae RtxA Cytotoxin in the Context of Other RTX Toxins

The Gram-negative bacterium Kingella kingae is part of the commensal oropharyngeal flora of young children. As detection methods have improved, K. kingae has been increasingly recognized as an emerging invasive pathogen that frequently causes skeletal system infections, bacteremia, and severe forms of infective endocarditis. K. kingae secretes an RtxA cytotoxin, which is involved in the development of clinical infection and belongs to an ever-growing family of cytolytic RTX (Repeats in ToXin) toxins secreted by Gram-negative pathogens. All RTX cytolysins share several characteristic structural features: (i) a hydrophobic pore-forming domain in the N-terminal part of the molecule; (ii) an acylated segment where the activation of the inactive protoxin to the toxin occurs by a co-expressed toxin-activating acyltransferase; (iii) a typical calcium-binding RTX domain in the C-terminal portion of the molecule with the characteristic glycine- and aspartate-rich nonapeptide repeats; and (iv) a C-proximal secretion signal recognized by the type I secretion system. RTX toxins, including RtxA from K. kingae, have been shown to act as highly efficient ‘contact weapons’ that penetrate and permeabilize host cell membranes and thus contribute to the pathogenesis of bacterial infections. RtxA was discovered relatively recently and the knowledge of its biological role remains limited. This review describes the structure and function of RtxA in the context of the most studied RTX toxins, the knowledge of which may contribute to a better understanding of the action of RtxA in the pathogenesis of K. kingae infections.

Purinergic signaling is essential for full Psickle activation by hypoxia and by normoxic acid pH in mature human sickle red cells and in vitro-differentiated cultured human sickle reticulocytes

Paracrine ATP release by erythrocytes has been shown to regulate endothelial cell function via purinergic signaling, and this erythoid-endothelial signaling network is pathologically dysregulated in sickle cell disease. We tested the role of extracellular ATP-mediated purinergic signaling in the activation of Psickle, the mechanosensitive Ca²⁺-permeable cation channel of human sickle erythrocytes (SS RBC). Psickle activation increases intracellular [Ca²⁺] to stimulate activity of the RBC Gardos channel, KCNN4/KCa3.1, leading to cell shrinkage and accelerated deoxygenation-activated sickling.We found that hypoxic activation of Psickle recorded by cell-attached patch clamp in SS RBC is inhibited by extracellular apyrase, which hydrolyzes extracellular ATP. Hypoxic activation of Psickle was also inhibited by the pannexin-1 inhibitor, probenecid, and by the P2 antagonist, suramin. A Psickle-like activity was also activated in normoxic SS RBC (but not in control red cells) by bath pH 6.0. Acid-activated Psickle-like activity was similarly blocked by apyrase, probenecid, and suramin, as well as by the Psickle inhibitor, Grammastola spatulata mechanotoxin-4 (GsMTx-4).In vitro-differentiated cultured human sickle reticulocytes (SS cRBC), but not control cultured reticulocytes, also exhibited hypoxia-activated Psickle activity that was abrogated by GsMTx-4. Psickle-like activity in SS cRBC was similarly elicited by normoxic exposure to acid pH, and this acid-stimulated activity was nearly completely blocked by apyrase, probenecid, and suramin, as well as by GsMTx-4.Thus, hypoxia-activated and normoxic acid-activated cation channel activities are expressed in both SS RBC and SS cRBC, and both types of activation appear to be mediated or greatly amplified by autocrine or paracrine purinergic signaling.

Nucleotide receptors mediate protection against neonatal sepsis and meningitis caused by alpha-hemolysin expressing Escherichia coli K1

Neonatal meningitis-associated Escherichia coli (NMEC) is among the leading causes of bacterial meningitis and sepsis in newborn infants. Several virulence factors have been identified as common among NMEC, and have been shown to play an important role in the development of bacteremia and/or meningitis. However, there is significant variability in virulence factor expression between NMEC isolates, and relatively little research has been done to assess the impact of variable virulence factor expression on immune cell activation and the outcome of infection. Here, we investigated the role of NMEC strain-dependent P2X receptor (P2XR) signaling on the outcome of infection in neonatal mice. We found that alpha-hemolysin (HlyA)-expressing NMEC (HlyA⁺ ) induced robust P2XR-dependent macrophage cell death in vitro, while HlyA^- NMEC did not. P2XR-dependent cell death was inflammasome independent, suggesting an uncoupling of P2XR and inflammasome activation in the context of NMEC infection. In vivo inhibition of P2XRs was associated with increased mortality in neonatal mice infected with HlyA⁺ NMEC, but had no effect on the survival of neonatal mice infected with HlyA^- NMEC. Furthermore, we found that P2XR-dependent protection against HlyA⁺ NMEC in vivo required macrophages, but not neutrophils or NLRP3. Taken together, these data suggest that HlyA⁺ NMEC activates P2XRs which in turn confers macrophage-dependent protection against infection in neonates. In addition, our findings indicate that strain-dependent virulence factor expression should be taken into account when studying the immune response to NMEC.

Interactive Dynamics of Cell Volume and Cell Death in Human Erythrocytes Exposed to α-Hemolysin from Escherichia coli

α-hemolysin (HlyA) of E. coli binds irreversibly to human erythrocytes and induces cell swelling, ultimately leading to hemolysis. We characterized the mechanism involved in water transport induced by HlyA and analyzed how swelling and hemolysis might be coupled. Osmotic water permeability (P_f) was assessed by stopped-flow light scattering. Preincubation with HlyA strongly reduced P_f in control- and aquaporin 1-null red blood cells, although the relative P_f decrease was similar in both cell types. The dynamics of cell volume and hemolysis on RBCs was assessed by electrical impedance, light dispersion and hemoglobin release. Results show that HlyA induced erythrocyte swelling, which is enhanced by purinergic signaling, and is coupled to osmotic hemolysis. We propose a mathematical model of HlyA activity where the kinetics of cell volume and hemolysis in human erythrocytes depend on the flux of osmolytes across the membrane, and on the maximum volume that these cells can tolerate. Our results provide new insights for understanding signaling and cytotoxicity mediated by HlyA in erythrocytes.

The bacteria and the host: a story of purinergic signaling in urinary tract infections

The local environment forces a selection of bacteria that might invade the urinary tract, allowing only the most virulent to access the kidney. Quite similar to the diet in setting the stage for the gut microbiome, renal function determines the conditions for bacteria-host interaction in the urinary tract. In the kidney, the term local environment or microenvironment is completely justified because the environment literally changes within a few micrometers. The precise composition of the urine is a function of the epithelium lining the microdomain, and the microenvironment in the kidney shows more variation in the content of nutrients, ion composition, osmolality, and pH than any other site of bacteria-host interaction. This review will cover some of the aspects of bacterial-host interaction in this unique setting and how uropathogenic bacteria can alter the condition for bacteria-host interaction. There will be a particular focus on the recent findings regarding how bacteria specifically trigger host paracrine signaling, via release of extracellular ATP and activation of P2 purinergic receptors. These finding will be discussed from the perspective of severe urinary tract infections, including pyelonephritis and urosepsis.

P2RX7 at the Host-Pathogen Interface of Infectious Diseases

The P2X7 receptor (P2RX7) is an important molecule that functions as a danger sensor, detecting extracellular nucleotides from injured cells and thus signaling an inflammatory program to nearby cells. It is expressed in immune cells and plays important roles in pathogen surveillance and cell-mediated responses to infectious organisms. There is an abundance of literature on the role of P2RX7 in inflammatory diseases and the role of these receptors in host-pathogen interactions. Here, we describe the current knowledge of the role of P2RX7 in the host response to a variety of pathogens, including viruses, bacteria, fungi, protozoa, and helminths. We describe in vitro and in vivo evidence for the critical role these receptors play in mediating and modulating immune responses. Our observations indicate a role for P2X7 signaling in sensing damage-associated molecular patterns released by nearby infected cells to facilitate immunopathology or protection. In this review, we describe how P2RX7 signaling can play critical roles in numerous cells types in response to a diverse array of pathogens in mediating pathogenesis and immunity to infectious agents.

NusG, an Ancient Yet Rapidly Evolving Transcription Factor

Timely and accurate RNA synthesis depends on accessory proteins that instruct RNA polymerase (RNAP) where and when to start and stop transcription. Among thousands of transcription factors, NusG/Spt5 stand out as the only universally conserved family of regulators. These proteins interact with RNAP to promote uninterrupted RNA synthesis and with diverse cellular partners to couple transcription to RNA processing, modification or translation, or to trigger premature termination of aberrant transcription. NusG homologs are present in all cells that utilize bacterial-type RNAP, from endosymbionts to plants, underscoring their ancient and essential function. Yet, in stark contrast to other core RNAP components, NusG family is actively evolving: horizontal gene transfer and sub-functionalization drive emergence of NusG paralogs, such as bacterial LoaP, RfaH, and UpxY. These specialized regulators activate a few (or just one) operons required for expression of antibiotics, capsules, secretion systems, toxins, and other niche-specific macromolecules. Despite their common origin and binding site on the RNAP, NusG homologs differ in their target selection, interacting partners and effects on RNA synthesis. Even among housekeeping NusGs from diverse bacteria, some factors promote pause-free transcription while others slow the RNAP down. Here, we discuss structure, function, and evolution of NusG proteins, focusing on unique mechanisms that determine their effects on gene expression and enable bacterial adaptation to diverse ecological niches.

Role of astroglial Connexin 43 in pneumolysin cytotoxicity and during pneumococcal meningitis

Streptococcus pneumoniae or pneumococcus (PN) is a major causative agent of bacterial meningitis with high mortality in young infants and elderly people worldwide. The mechanism underlying PN crossing of the blood brain barrier (BBB) and specifically, the role of non-endothelial cells of the neurovascular unit that control the BBB function, remains poorly understood. Here, we show that the astroglial connexin 43 (aCx43), a major gap junctional component expressed in astrocytes, plays a predominant role during PN meningitis. Following intravenous PN challenge, mice deficient for aCx43 developed milder symptoms and showed severely reduced bacterial counts in the brain. Immunofluorescence analysis of brain slices indicated that PN induces the aCx43–dependent destruction of the network of glial fibrillary acid protein (GFAP), an intermediate filament protein specifically expressed in astrocytes and up-regulated in response to brain injury. PN also induced nuclear shrinkage in astrocytes associated with the loss of BBB integrity, bacterial translocation across endothelial vessels and replication in the brain cortex. We found that aCx4-dependent astrocyte damages could be recapitulated using in vitro cultured cells upon challenge with wild-type PN but not with a ply mutant deficient for the pore-forming toxin pneumolysin (Ply). Consistently, we showed that purified Ply requires Cx43 to promote host cell plasma membrane permeabilization in a process involving the Cx43-dependent release of extracellular ATP and prolonged increase of cytosolic Ca²⁺ in host cells. These results point to a critical role for astrocytes during PN meningitis and suggest that the cytolytic activity of the major virulence factor Ply at concentrations relevant to bacterial infection requires co-opting of connexin plasma membrane channels.

The role of non-endothelial cells constituting the neurovascular unit during infectious meningitis is poorly appreciated despite their key regulatory functions on the blood-brain barrier integrity. Here, we show that Streptococcus pneumoniae or pneumococcus, a major causative agent of bacterial meningitis, targets astroglial cells to translocate across brain endothelial vessels. We found that astroglial connexin 43, a gap junctional component, played a major role during PN meningitis in mice. PN translocation and replication in the brain cortex were associated with connexin-dependent fragmentation of astrocytic the GFAP network, a process associated with brain injury. These findings were recapitulated and extended in vitro using cultured primary astrocytes and the major PN virulence determinant Pneumolysin. Ply-mediated cytotoxicity was linked to Ca²⁺ increase and required aCx43, arguing against a direct toxin activity. The results reveal a key role for astroglial signaling during PN crossing of the BBB and shed light on the mechanism of Ply-mediated cytotoxicity during meningitis.

Prevention of P2 Receptor-Dependent Thrombocyte Activation by Pore-Forming Bacterial Toxins Improves Outcome in A Murine Model of Urosepsis

Urosepsis is a potentially life-threatening, systemic reaction to uropathogenic bacteria entering the bloodstream of the host. One of the hallmarks of sepsis is early thrombocyte activation with a following fall in circulating thrombocytes as a result of intravascular aggregation and sequestering of thrombocytes in the major organs. Development of a thrombocytopenic state is associated with a poorer outcome of sepsis. Uropathogenic Escherichia coli frequently produce the pore-forming, virulence factor α-haemolysin (HlyA), of which the biological effects are mediated by ATP release and subsequent activation of P2 receptors. Thus, we speculated that inhibition of thrombocyte P2Y₁ and P2Y₁₂ receptors might ameliorate the septic response to HlyA-producing E. coli. The study combined in vitro measurements of toxin-induced thrombocyte activation assessed as increased membrane abundance of P-selectin, fibronectin and CD63 and data from in vivo murine model of sepsis-induced by HlyA-producing E. coli under infusion of P2Y₁ and P2Y₁₂ antagonists. Our data show that the P2Y₁ receptor antagonist almost abolishes thrombocyte activation by pore-forming bacterial toxins. Inhibition of P2Y₁, by constant infusion of MRS2500, markedly increased the survival in mice with induced sepsis. Moreover, MRS2500 partially prevented the sepsis-induced depletion of circulating thrombocytes and dampened the sepsis-associated increase in proinflammatory cytokines. In contrast, P2Y₁₂ receptor inhibition had only a marginal effect in vivo and in vitro. Taken together, inhibition of the P2Y₁ receptor gives a subtle dampening of the thrombocyte activation and the cytokine response to bacteraemia, which may explain the improved survival observed by P2Y₁ receptor antagonists.

Extracellular ATP as an Inter-Kingdom Signaling Molecule: Release Mechanisms by Bacteria and Its Implication on the Host

The purine adenosine 5′-triphosphate (ATP) is not only a universal intracellular energy carrier but plays also an important role as extracellular signaling molecule. Purinergic signaling is involved in many physiological and pathological processes like coagulation, inflammation, or sepsis in mammals. ATP is well-known as a messenger for intercellular communications in multicellular organisms, but phylogenetically much older unicellular organisms like yeast or bacteria use ATP as an extracellular signaling molecule as well. However, the mechanisms of ATP secretion by bacteria and its extracellular implications still have to be elucidated. This review will provide an overview of the current knowledge about bacterial extracellular ATP (eATP) under homeostatic conditions and during growth. Possible secretion mechanisms of ATP by bacteria will be discussed and implications of bacterial ATP are shown, with a focus on bacteria–host interactions.

Interactions of Segmented Filamentous Bacteria (Candidatus Savagella) and bacterial drivers in colitis-associated colorectal cancer development

Colorectal cancer (CRC) risk is influenced by host genetics, sex, and the gut microbiota. Using a genetically susceptible mouse model of CRC induced via inoculation with pathobiont Helicobacter spp. and demonstrating variable tumor incidence, we tested the ability of the Th17-enhancing commensal Candidatus Savagella, more commonly denoted as Segmented Filamentous Bacteria (SFB), to influence the incidence and severity of colitis-associated CRC in male and female mice. To document the composition of the gut microbiota during CRC development and identify taxa associated with disease, fecal samples were collected before and throughout disease development and characterized via 16S rRNA sequencing. While there were no significant SFB-dependent effects on disease incidence or severity, SFB was found to exert a sex-dependent protective effect in male mice. Furthermore, SFB stabilized the GM against Helicobacter-induced changes post-inoculation, resulting in a shift in disease association from Helicobacter spp. to Escherichia coli. These data support sex-dependent SFB-mediated effects on CRC risk, and highlight the complex community dynamics within the GM during exposure to inflammatory pathobionts.

Induction of erythrocyte microvesicles by Escherichia Coli Alpha hemolysin

Alpha hemolysin (HlyA) is the major virulence factor of uropathogenic Escherichia coli (UPEC) strains. Once in circulation, a low concentration of the toxin induces an increase in intracellular calcium that activates calpains - which proteolyse cytoskeleton proteins - and also favours the exposure of phosphatidylserine (PS) in the outer leaflet of erythrocyte membranes. All these events are considered part of eryptosis, as well as the delivery of microvesicles (MVs). Within this context, we studied the delivery of MVs by erythrocytes treated with sublytic concentrations of HlyA and demonstrated that HlyA-treated erythrocytes secrete MVs of diameter ∼200 nm containing HlyA and PS by a mechanism involving an increment of intracellular calcium concentration and purinergic receptor activation. Despite the presence of toxin in their membrane, HlyA-MVs are not hemolytically active and do not induce ATP release in untreated erythrocytes, thus suggesting that the delivery of HlyA-MVs might act as a protective mechanism on the part of erythrocytes that removes the toxin from the membrane to prevent the spread of infection. Although erythrocytes have been found to eliminate denatured hemoglobin and several membrane proteins by shedding MVs, the present work has revealed for the first time that an exogenous protein, such as a toxin, is eliminated by this process. This finding sheds light on the mechanism of action of the toxin and serves to further elucidate the consequences of UPEC infection in patients exhibiting HlyA-related diseases.

Peptide-based targeting of connexins and pannexins for therapeutic purposes

INTRODUCTION

Connexin and pannexin (hemi)channels play an important role in paracrine and autocrine signaling pathways. The opening of these cellular pores is linked to a wide range of diseases. Therefore, pharmacological closing of connexin and pannexin (hemi)channels seems a promising therapeutic strategy. However, the currently available inhibitors cope with recurring problems concerning selectivity, specificity, stability and/or solubility.

AREAS COVERED

A number of peptides that mimic specific regions in the native sequence of connexins and pannexins have the potential to overcome some of these hurdles. In this paper, an overview is provided on these peptide-based inhibitors of connexin and pannexin (hemi)channels for therapeutic purposes. The authors also provide the reader with their expert perspectives on the future of these peptide-based inhibitors.

EXPERT OPINION

Peptide mimetics can become valuable tools in the treatment of connexin-related and pannexin-related diseases. This can be made possible provided that available peptides are optimized, and new peptide mimetics are designed based on knowledge of the mechanisms underlying the gating control of connexin and pannexin (hemi)channels.

Clostridium perfringens epsilon toxin binds to erythrocyte MAL receptors and triggers phosphatidylserine exposure

Epsilon toxin (ETX) is a 33-kDa pore-forming toxin produced by type B and D strains of Clostridium perfringens. We previously found that ETX caused haemolysis of human red blood cells, but not of erythrocytes from other species. The cellular and molecular mechanisms of ETX-mediated haemolysis are not well understood. Here, we investigated the effects of ETX on erythrocyte volume and the role of the putative myelin and lymphocyte (MAL) receptors in ETX-mediated haemolysis. We observed that ETX initially decreased erythrocyte size, followed by a gradual increase in volume until lysis. Moreover, ETX triggered phosphatidylserine (PS) exposure and enhanced ceramide abundance in erythrocytes. Cell shrinkage, PS exposure and enhanced ceramide abundance were preceded by increases in intracellular Ca²⁺ concentration. Interestingly, lentivirus-mediated RNA interference studies in the human erythroleukaemia cell line (HEL) cells confirmed that MAL contributes to ETX-induced cytotoxicity. Additionally, ETX was shown to bind to MAL in vitro. The results of this study recommend that ETX-mediated haemolysis is associated with MAL receptor activation in human erythrocytes. These data imply that interventions affecting local MAL-mediated autocrine and paracrine signalling may prevent ETX-mediated erythrocyte damage.

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.

RTX Toxins Ambush Immunity's First Cellular Responders

The repeats-in-toxin (RTX) family represents a unique class of bacterial exoproteins. The first family members described were toxins from Gram-negative bacterial pathogens; however, additional members included exoproteins with diverse functions. Our review focuses on well-characterized RTX family toxins from Aggregatibacteractinomycetemcomitans (LtxA), Mannheimiahaemolytica (LktA), Bordetella pertussis (CyaA), uropathogenic Escherichia coli (HlyA), and Actinobacillus pleuropneumoniae (ApxIIIA), as well as the studies that have honed in on a single host cell receptor for RTX toxin interactions, the β₂ integrins. The β₂ integrin family is composed of heterodimeric members with four unique alpha subunits and a single beta subunit. β₂ integrins are only found on leukocytes, including neutrophils and monocytes, the first responders to inflammation following bacterial infection. The LtxA, LktA, HlyA, and ApxIIIA toxins target the shared beta subunit, thereby targeting all types of leukocytes. Specific β₂ integrin family domains are required for the RTX toxin’s cytotoxic activity and are summarized here. Research examining the domains of the RTX toxins required for cytotoxic and hemolytic activity is also summarized. RTX toxins attack and kill phagocytic immune cells expressing a single integrin family, providing an obvious advantage to the pathogen. The critical question that remains, can the specificity of the RTX-β₂ integrin interaction be therapeutically targeted?

P2X1 receptor blockers reduce the number of circulating thrombocytes and the overall survival of urosepsis with haemolysin-producing Escherichia coli

Urosepsis is a severe condition often caused by Escherichia coli that spontaneously have ascended the urinary tract to the kidneys causing pyelonephritis and potentially bacteraemia. The number of sepsis cases has been steadily increasing over the last decades, and there are still no specific, molecular supportive therapies for sepsis to supplement antibiotic treatment. P2X₁ receptors are expressed by a number of immune cells including thrombocytes, which presently have been established as an important player in the acute immune response to bacterial infections. P2X₁ receptor-deficient mice have been shown to be relatively protected against urosepsis, with markedly reduced levels of circulating proinflammatory cytokines and intravascular coagulation. However, here we show that continuous intravenous infusion with P2X₁ receptor antagonist markedly accelerates development of a septic response to induced bacteraemia with uropathogenic E. coli. Mice exposed to the P2X₁ receptor antagonists die very early with haematuria, substantially elevated plasma levels of proinflammatory cytokines, massive intravascular coagulation and a concomitant reduction in circulating thrombocytes. Interestingly, infusion of P2X₁ receptor antagonists causes a marked acute reduction in circulating thrombocytes and a higher number of bacteria in the blood. These data support the notion that the number of functional thrombocytes is important for the acute defence against bacteria in the circulation and that the P2X₁ receptor potentially could be essential for this response.

Research articleHemolysis in human erythrocytes by Clostridium perfringens epsilon toxin requires activation of P2 receptors

Epsilon-toxin (ETX) is produced by types B and D strains of Clostridium perfringens, which cause fatal enterotoxaemia in sheep, goats and cattle. Previous studies showed that only a restricted number of cell lines are sensitive to ETX and ETX-induced hemolysis has not previously been reported. In this study, the hemolytic ability of ETX was examined using erythrocytes from 10 species including murine, rabbit, sheep, monkey and human. We found that ETX caused hemolysis in human erythrocytes (HC₅₀ = 0.2 μM) but not erythrocytes from the other test species. Moreover, the mechanism of ETX-induced hemolysis was further explored. Recent studies showed that some bacterial toxins induce hemolysis through purinergic receptor (P2) activation. Hence, the function of purinergic receptors in ETX-induced hemolysis was tested, and we found that the non-selective P2 receptor antagonists PPADS inhibited ETX-induced lysis of human erythrocytes in a concentration-dependent manner, indicating that ETX-induced hemolysis requires activation of purinergic receptors. P2 receptors comprise seven P2X (P2X1-7) and eight P2Y (P2Y1, P2Y2, P2Y4, P2Y6, and P2Y11-P2Y14) receptor subtypes. The pattern of responsiveness to more selective P2-antagonists implies that both P2Y13 and P2X7 receptors are involved in ETX-induced hemolysis in human species. Furthermore, we demonstrated that extracellular ATP is likely not involved in ETX-induced hemolysis and the activation of P2 receptors. These findings clarified the mechanism of ETX-induced hemolysis and provided new insight into the activities and ETX mode of action.

Lack of P2X7 Receptors Protects against Renal Fibrosis after Pyelonephritis with α-Hemolysin-Producing Escherichia coli

Severe urinary tract infections are commonly caused by sub-strains of Escherichia coli secreting the pore-forming virulence factor α-hemolysin (HlyA). Repeated or severe cases of pyelonephritis can cause renal scarring that subsequently can lead to progressive failure. We have previously demonstrated that HlyA releases cellular ATP directly through its membrane pore and that acute HlyA-induced cell damage is completely prevented by blocking ATP signaling. Local ATP signaling and P2X₇ receptor activation play a key role in the development of tissue fibrosis. This study investigated the effect of P2X₇ receptors on infection-induced renal scarring in a murine model of pyelonephritis. Pyelonephritis was induced by injecting 100 million HlyA-producing, uropathogenic E. coli into the urinary bladder of BALB/cJ mice. A similar degree of pyelonephritis and mortality was confirmed at day 5 after infection in P2X₇^+/+ and P2X₇^-/- mice. Fibrosis was first observed 2 weeks after infection, and the data clearly demonstrated that P2X₇^-/- mice and mice exposed to the P2X₇ antagonist, brillian blue G, show markedly less renal fibrosis 14 days after infection compared with controls (P < 0.001). Immunohistochemistry revealed comparable early neutrophil infiltration in the renal cortex from P2X₇^+/+ and P2X₇^-/- mice. Interestingly, lack of P2X₇ receptors resulted in diminished macrophage infiltration and reduced neutrophil clearance in the cortex of P2X₇^-/- mice. Hence, this study suggests the P2X₇ receptor to be an appealing antifibrotic target after renal infections.

α-Haemolysin production, as a single factor, causes fulminant sepsis in a model of Escherichia coli-induced bacteraemia

α-Haemolysin (HlyA) from uropathogenic Escherichia coli has been demonstrated to be a significant virulence factor for ascending urinary tract infections. Once the E. coli reach the well-vascularised kidneys, there is a high risk of bacteraemia and a subsequent septic host response. Despite this, HlyA has the potential to accelerate the host response both directly and via its ability to facilitate adenosine triphosphate release from cells. It has not been settled whether HlyA aggravates bacteraemia into a septic state. To address this, we used an E. coli strain in a model of acute urosepsis that was either transfected with a plasmid containing the full HlyA operon or one with deletion in the HlyA gene. Here, we show that HlyA accelerates the host response to E. coli in the circulation. Mice exposed to HlyA-producing E. coli showed massively increased proinflammatory cytokines, a substantial fall in circulating thrombocytes, extensive haematuria, and intravascular haemolysis. This was not seen in mice exposed to either E. coli that do not secrete HlyA or vehicle controls. Consistent with the massive host response to the bacteria, the mice exposed to HlyA-producing E. coli died exceedingly early, whereas mice exposed to E. coli without HlyA production and vehicle controls survived the entire observation period. These data allow us to conclude that HlyA is a virulence factor that accelerates a state of bacteraemia into fulminant sepsis in a mouse model.

Regulation of extracellular ATP of human erythrocytes treated with α-hemolysin. Effects of cell volume, morphology, rheology and hemolysis

Alpha-hemolysin (HlyA) of uropathogenic strains of Escherichia coli irreversibly binds to human erythrocytes (RBCs) and triggers activation of ATP release and metabolic changes ultimately leading to hemolysis. We studied the regulation of extracellular ATP (ATPe) of RBCs exposed to HlyA. Luminometry was used to assess ATP release and ATPe hydrolysis, whereas changes in cell volume and morphology were determined by electrical impedance, ektacytometry and aggregometry. Exposure of RBCs to HlyA induced a strong increase of [ATPe] (3-36-fold) and hemolysis (1-44-fold), partially compensated by [ATPe] hydrolysis by ectoATPases and intracellular ATPases released by dead cells. Carbenoxolone, a pannexin 1 inhibitor, partially inhibited ATP release (43-67%). The un-acylated toxin ProHlyA and the deletion analog HlyA∆914-936 were unable to induce ATP release or hemolysis. For HlyA treated RBCs, a data driven mathematical model showed that simultaneous lytic and non-lytic release mainly governed ATPe kinetics, while ATPe hydrolysis became important after prolonged toxin exposure. HlyA induced a 1.5-fold swelling, while blocking this swelling reduced ATP release by 77%. Blocking ATPe activation of purinergic P2X receptors reduced swelling by 60-80%. HlyA-RBCs showed an acute 1.3-2.2-fold increase of Ca²⁺i, increased crenation and externalization of phosphatidylserine. Perfusion of HlyA-RBCs through adhesion platforms showed strong adhesion to activated HMEC cells, followed by rapid detachment. HlyA exposed RBCs exhibited increased sphericity under osmotic stress, reduced elongation under shear stress, and very low aggregation in viscous media. Overall results showed that HlyA-RBCs displayed activated ATP release, high but weak adhesivity, low deformability and aggregability and high sphericity.

Erythrocyte P2X1 receptor expression is correlated with change in haematocrit in patients admitted to the ICU with blood pathogen-positive sepsis

Background

Pore-forming proteins released from bacteria or formed as result of complement activation are known to produce severe cell damage. Inhibition of purinergic P2X receptors markedly reduces damage inflicted by cytolytic bacterial toxin and after complement activation in both erythrocytes and monocytes. P2X expression generally shows variation throughout the population. Here, we investigate correlation between P2X receptor abundance in blood cell plasma membranes and haematocrit during sepsis, in patients admitted to the emergency department (ED) or intensive care unit (ICU).

Method

Patients admitted to the ED and successively transferred to ICU with the diagnosis sepsis (< 2 systemic inflammatory response syndrome (SIRS) criteria and suspected infection), were grouped as either blood pathogen-positive (14 patients) or blood pathogen-negative (20 patients). Blood samples drawn at ICU admission were analysed for P2X₁ and P2X₇ receptor abundance using indirect flow cytometry.

Results

Here, we find inverse correlation between P2X₁ receptor expression and change in haematocrit (r_s − 0.80) and haemoglobin (r_s − 0.78) levels from admission to ED to arrival at ICU in patients with pathogen-positive sepsis. This correlation was not found in patients without confirmed bacteraemia. Patients with high P2X₁ expression had a significantly greater change in both haematocrit (− 0.59 ± 0.36) and haemoglobin levels (− 0.182 ± 0.038 mg/dl) per hour, during the first hours after hospital admission compared to patients with low P2X₁ expression (0.007 ± 0.182 and − 0.020 ± 0.058 mg/dl, respectively).

Conclusion

High levels of P2X₁ are correlated with more pronounced reduction in haematocrit and haemoglobin in patients with confirmed bacteraemia. This supports previous in vitro findings of P2X activation as a significant component in cell damage caused by pore-forming bacterial toxins and complement-dependent major attack complex. These data suggest a new potential target for future therapeutics in initial stages of sepsis.

Electronic supplementary material

The online version of this article (10.1186/s13054-018-2100-3) contains supplementary material, which is available to authorized users.

RtxA like protein contributes to infection of Francisella novicida in silkworm and human macrophage THP-1

Tularemia is a zoonosis caused by CDC-declared Tier 1 threat agent Francisella tularensis. F. tularensis subsp. novicida (F. novicida) is virulent in mice but non-pathogenic in immunocompetent humans and serves as a potential surrogate organism. In a recent study, we established a silkworm (Bombyx mori) model of infection for F. novicida. Francisella secretes its virulence factors through various mechanisms that modify the intracellular environment to ensure its replication and survival. To identify new pathogenic factors, we focused on the type I secretory system (T1SS) of Francisella. In silico analysis revealed a RtxA (Repeats-in-toxin) like protein in the Francisella genome. The characteristics of RtxA like protein were investigated using mutant analysis. Firstly, the role of rtxA in silkworms was investigated by infecting them with F. novicida strains into the hemocoel. The rtxA mutant failed to kill the silkworms, whereas F. novicida wild-type (WT) strain killed silkworms within 3-7 days post infection. The arrested growth of the mutant strain in silkworms was observed using a whole-body CFU count assay. We also investigated the growth characteristics of the rtxA mutant in hemocytes, one of the primary multiplication sites of Francisella within silkworms. Interrupted growth of the rtxA mutant with significantly reduced cytotoxicity was observed in hemocytes via confocal microscopy. Next, we analyzed the effect of rtxA in human monocyte cell line THP-1. The mutant strain showed significantly decreased growth and reduced cytotoxicity compared with its parental strain in THP-1 cells. This study newly identified RtxA like protein of F. novicida as an important lethal pathogenic factor in silkworm and mammalian cells.

A new role for Zinc limitation in bacterial pathogenicity: modulation of α-hemolysin from uropathogenic Escherichia coli

Metal limitation is a common situation during infection and can have profound effects on the pathogen’s success. In this report, we examine the role of zinc limitation in the expression of a virulence factor in uropathogenic Escherichia coli. The pyelonephritis isolate J96 carries two hlyCABD operons that encode the RTX toxin α-hemolysin. While the coding regions of both operons are largely conserved, the upstream sequences, including the promoters, are unrelated. We show here that the two hlyCABD operons are differently regulated. The hly_II operon is efficiently silenced in the presence of zinc and highly expressed when zinc is limited. In contrast, the hly_I operon does not respond to zinc limitation. Genetic studies reveal that zinc-responsive regulation of the hly_II operon is controlled by the Zur metalloregulatory protein. A Zur binding site was identified in the promoter sequence of the hly_II operon, and we observe direct binding of Zur to this promoter region. Moreover, we find that Zur regulation of the hly_II operon modulates the ability of E. coli J96 to induce a cytotoxic response in host cell lines in culture. Our report constitutes the first description of the involvement of the zinc-sensing protein Zur in directly modulating the expression of a virulence factor in bacteria.

P2X-Receptor Antagonists Inhibit the Interaction of S. aureus Hemolysin A with Membranes

The pore forming hemolysin A, Hla, is a major virulence factor of Staphylococcus aureus. Apparently, 1–2 pore(s) per cell suffice(s) to cause cell death. Accumulated experimental evidence points towards a major role of ATP-gated purinergic receptors (P2XR) for hemolysis caused by Hla, complement and other pore forming proteins, presumably by increasing membrane permeability. Indeed, in experiments employing rabbit erythrocytes, inhibitory concentrations of frequently employed P2XR-antagonists were in a similar range as previously reported for erythrocytes of other species and other toxins. However, Hla-dependent hemolysis was not enhanced by extracellular ATP, and oxidized adenosinetriphosphate (oxATP) had only a minor inhibitory effect. Unexpectedly, P2XR-inhibitors also prevented Hla-induced lysis of pure lipid membranes, demonstrating that the inhibition did not even depend on the presence of P2XR. Fluorescence microscopy and gel-electrophoresis clearly revealed that P2XR-inhibitors interfere with binding and subsequent oligomerisation of Hla with membranes. Similar results were obtained employing HaCaT-cells. Furthermore, calorimetric data and hemolysis experiments with Hla pre-treated with pyridoxal phosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS) showed that this compound directly binds to Hla. Our results call for a critical re-assessment of the appealing concept, which suggests that P2XR are general amplifiers of damage by pore-forming proteins.

Uropathogenic Escherichia coli-Associated Exotoxins

Escherichia coli are a common cause of infectious disease outside of the gastrointestinal tract. Several independently evolved E. coli clades are common causes of urinary tract and bloodstream infections. There is ample epidemiological and in vitro evidence that several different protein toxins common to many, but not all, of these strains are likely to aid the colonization and immune-evasion ability of these bacteria. This review discusses our current knowledge and areas of ignorance concerning the contribution of the hemolysin; cytotoxic-necrotizing factor-1; and the autotransporters, Sat, Pic, and Vat, to extraintestinal human disease.

Interaction of acylated and unacylated forms of E. coli alpha-hemolysin with lipid monolayers: a PM-IRRAS study

Uropathogenic strains of Escherichia coli produce virulence factors, such as the protein toxin alpha-hemolysin (HlyA), that enable the bacteria to colonize the host and establish an infection. HlyA is synthetized as a protoxin (ProHlyA) that is transformed into the active form in the bacterial cytosol by the covalent linkage of two fatty-acyl moieties to the polypeptide chain before the secretion of HlyA into the extracellular medium. The aim of this work was to investigate the effect of the fatty acylation of HlyA on protein conformation and protein-membrane interactions. Polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS) experiments were performed at the air-water interface, and lipid monolayers mimicking the outer leaflet of red-blood-cell membranes were used as model systems for the study of protein-membrane interaction. According to surface-pressure measurements, incorporation of the acylated protein into the lipid films was faster than that of the nonacylated form. PM-IRRAS measurements revealed that the adsorption of the proteins to the lipid monolayers induced disorder in the lipid acyl chains and also changed the elastic properties of the films independently of protein acylation. No significant difference was observed between HlyA and ProHlyA in the interaction with the model lipid monolayers; but when these proteins became adsorbed on a bare air-water interface, they adopted different secondary structures. The assumption of the correct protein conformation at a hydrophobic-hydrophilic interface could constitute a critical condition for biologic activity.

P2X1, P2X4, and P2X7 Receptor Knock Out Mice Expose Differential Outcome of Sepsis Induced by α-Haemolysin Producing Escherichia coli

α-haemolysin (HlyA)-producing Escherichia coli commonly inflict severe urinary tract infections, including pyelonephritis, which comprises substantial risk for sepsis. In vitro, the cytolytic effect of HlyA is mainly mediated by ATP release through the HlyA pore and subsequent P2X₁/P2X₇ receptor activation. This amplification of the lytic process is not unique to HlyA but is observed by many other pore-forming proteins including complement-induced haemolysis. Since free hemoglobin in the blood is known to be associated with a worse outcome in sepsis one could speculate that inhibition of P2X receptors would ameliorate the course of sepsis. Surprisingly, this study demonstrates that [Formula: see text] and [Formula: see text] mice are exceedingly sensitive to sepsis with uropathogenic E. coli. These mice have markedly lower survival, higher cytokine levels and activated intravascular coagulation. Quite the reverse is seen in [Formula: see text] mice, which had markedly lower cytokine levels and less coagulation activation compared to controls after exposure to uropathogenic E. coli. The high cytokine levels in the [Formula: see text] mouse are unexpected, since P2X₇ is implicated in caspase-1-dependent IL-1β production. Here, we demonstrate that IL-1β production during sepsis with uropathogenic E. coli is mediated by caspase-8, since caspase-8 and RIPK3 double knock out mice show substantially lower cytokine during sepsis and increased survival after injection of TNFα. These data support that P2X₇ and P2X₄ receptor activation has a protective effect during severe E. coli infection.

Zinc treatment is efficient against Escherichia coli α-haemolysin-induced intestinal leakage in mice

Zinc homoeostasis exerts protective effects in inflammatory intestinal diseases and zinc supplementation has been successfully used for treating infectious diarrhoea. This study aimed at a characterisation of zinc effects on focal leak induction by α-haemolysin (HlyA)-producing Escherichia coli (E. coli) as protective mechanism for colitis. We conducted in vivo experiments by oral challenge of gnotobiotic mice colonised with HlyA-expressing E. coli-536. Mice were either fed a defined normal or high zinc diet to analyse effects of zinc as a therapeutic regimen. HlyA-deficient E. coli-536 mutants were used as controls. Mice infected with HlyA-producing E. coli showed impaired barrier integrity when receiving normal zinc. High zinc supplementation in HlyA-producing E. coli-infected mice reduced epithelial dysfunction as indicated by ameliorated macromolecule permeability. Reduced size of focal leaks with diminished bacterial translocation was observed as inherent mechanisms of this zinc action. In human colon cell monolayers application of zinc rescued the HlyA-dependent decline in transepithelial electrical resistance via reduction of the calcium entry into HlyA-exposed cells. Calcium-dependent cell exfoliation was identified as mechanism for focal leak induction. In conclusion, zinc supplementation protects from HlyA-induced barrier dysfunction in vivo and in vitro, providing an explanation for the protective efficacy of zinc in intestinal disorders.

F199E substitution reduced toxicity of Clostridium perfringens epsilon toxin by depriving the receptor binding capability

Epsilon toxin (ETX), a potent toxin, is produced by types B and D strains of Clostridium perfringens, which could cause severe diseases in humans and domestic animals. Mutant rETX^F199E was previously demonstrated to be a good vaccine candidate. However, the mechanism concerned remains unknown. To clarify how F199E substitution reduced ETX toxicity, we performed a series of experiments. The results showed that the cell-binding and pore-forming ability of rETX^F199E was almost abolished. We speculated that F199E substitution reduced toxicity by depriving the receptor binding capability of ETX, which contributed to the hypothesis that domain I of ETX is responsible for cell binding. In addition, our data suggested that ETX could cause Ca²⁺ release from intracellular Ca²⁺ stores, which may underlie an alternate pathway leading to cell death. Furthermore, ETX induced crenation of the MDCK cells was observed, with sags and crests first appearing on the surface of condensed MDCK cells, according to scanning electron microscopy. The data also demonstrated the safety and potentiality of rETX^F199E as a vaccine candidate for humans. In summary, findings of this work potentially contribute to a better understanding of the pathogenic mechanism of ETX and the development of vaccine against diseases caused by ETX, using mutant proteins.

Lys49 myotoxin from the Brazilian lancehead pit viper elicits pain through regulated ATP release

Pain-producing animal venoms contain evolutionarily honed toxins that can be exploited to study and manipulate somatosensory and nociceptive signaling pathways. From a functional screen, we have identified a secreted phospholipase A2 (sPLA2)-like protein, BomoTx, from the Brazilian lancehead pit viper (Bothrops moojeni). BomoTx is closely related to a group of Lys49 myotoxins that have been shown to promote ATP release from myotubes through an unknown mechanism. Here we show that BomoTx excites a cohort of sensory neurons via ATP release and consequent activation of P2X₂ and/or P2X₃ purinergic receptors. We provide pharmacological and electrophysiological evidence to support pannexin hemichannels as downstream mediators of toxin-evoked ATP release. At the behavioral level, BomoTx elicits nonneurogenic inflammatory pain, thermal hyperalgesia, and mechanical allodynia, of which the latter is completely dependent on purinergic signaling. Thus, we reveal a role of regulated endogenous nucleotide release in nociception and provide a detailed mechanism of a pain-inducing Lys49 myotoxin from Bothrops species, which are responsible for the majority of snake-related deaths and injuries in Latin America.

P2X7 receptor activation causes phosphatidylserine exposure in canine erythrocytes

AIM

To determine if activation of the ATP-gated P2X7 receptor channel induces phosphatidylserine (PS) exposure in erythrocytes from multiple dog breeds.

METHODS

Peripheral blood was collected from 25 dogs representing 13 pedigrees and seven crossbreeds. ATP-induced PS exposure on canine erythrocytes in vitro was assessed using a flow cytometric Annexin V binding assay.

RESULTS

ATP induced PS exposure in erythrocytes from all dogs studied. ATP caused PS exposure in a concentration-dependent manner with an EC₅₀ value of 395 μmol/L. The non-P2X7 agonists, ADP or AMP, did not cause PS exposure. The P2X7 antagonist, AZ10606120, but not the P2X1 antagonist, NF449, blocked ATP-induced PS exposure.

CONCLUSION

The results indicate that ATP induces PS exposure in erythrocytes from various dog breeds and that this process is mediated by P2X7 activation.

Inhibition of P2X Receptors Protects Human Monocytes against Damage by Leukotoxin from Aggregatibacter actinomycetemcomitans and α-Hemolysin from Escherichia coli

α-Hemolysin (HlyA) from Escherichia coli and leukotoxin A (LtxA) from Aggregatibacter actinomycetemcomitans are important virulence factors in ascending urinary tract infections and aggressive periodontitis, respectively. The extracellular signaling molecule ATP is released immediately after insertion of the toxins into plasma membranes and, via P2X receptors, is essential for the erythrocyte damage inflicted by these toxins. Moreover, ATP signaling is required for the ensuing recognition and phagocytosis of damaged erythrocytes by the monocytic cell line THP-1. Here, we investigate how these toxins affect THP-1 monocyte function. We demonstrate that both toxins trigger early ATP release and a following increase in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in THP-1 monocytes. The HlyA- and LtxA-induced [Ca²⁺]_i response is diminished by the P2 receptor antagonist in a pattern that fits the functional P2 receptor expression in these cells. Both toxins are capable of lysing THP-1 cells, with LtxA being more aggressive. Either desensitization or blockage of P2X₁, P2X₄, or P2X₇ receptors markedly reduces toxin-induced cytolysis. This pattern is paralleled in freshly isolated human monocytes from healthy volunteers. Interestingly, only a minor fraction of the toxin-damaged THP-1 monocytes eventually lyse. P2X₇ receptor inhibition generally prevents cell damage, except from a distinct cell shrinkage that prevails in response to the toxins. Moreover, we find that preexposure to HlyA preserves the capacity of THP-1 monocytes to phagocytose damaged erythrocytes and may induce readiness to discriminate between damaged and healthy erythrocytes. These findings suggest a new pharmacological target for protecting monocytes during exposure to pore-forming cytolysins during infection or injury.

Dissecting the self-assembly kinetics of multimeric pore-forming toxins

Pore-forming toxins are ubiquitous cytotoxins that are exploited by both bacteria and the immune response of eukaryotes. These toxins kill cells by assembling large multimeric pores on the cell membrane. However, a quantitative understanding of the mechanism and kinetics of this self-assembly process is lacking. We propose an analytically solvable kinetic model for stepwise, reversible oligomerization. In biologically relevant limits, we obtain simple algebraic expressions for the rate of pore formation, as well as for the concentration of pores as a function of time. Quantitative agreement is obtained between our model and time-resolved kinetic experiments of Bacillus thuringiensis Cry1Ac (tetrameric pore), aerolysin, Staphylococcus aureus α-haemolysin (heptameric pores) and Escherichia coli cytolysin A (dodecameric pore). Furthermore, our model explains how rapid self-assembly can take place with low concentrations of oligomeric intermediates, as observed in recent single-molecule fluorescence experiments of α-haemolysin self-assembly. We propose that suppressing the concentration of oligomeric intermediates may be the key to reliable, error-free, self-assembly of pores.

Role of pannexin 1 in Clostridium perfringens beta-toxin-caused cell death

BACKGROUND

Beta-toxin produced by Clostridium perfringens is a key virulence factor of fatal hemorrhagic enterocolitis and enterotoxemia. This toxin belongs to a family of β-pore-forming toxins (PFTs). We reported recently that the ATP-gated P2X₇ receptor interacts with beta-toxin. The ATP-release channel pannexin 1 (Panx1) is an important contributor to P2X₇ receptor signaling. Hence, we investigated the involvement of Panx1 in beta-toxin-caused cell death.

METHODS

We examined the effect of Panx1 in beta-toxin-induced cell death utilizing selective antagonists, knockdown of Panx1, and binding using dot-blot analysis. Localization of Panx1 and the P2X₇ receptor after toxin treatment was determined by immunofluorescence staining.

RESULTS

Selective Panx1 antagonists (carbenoxolone [CBX], probenecid, and Panx1 inhibitory peptide) prevented beta-toxin-caused cell death in THP-1 cells. CBX did not block the binding of the toxin to cells. Small interfering knockdown of Panx1 blocked beta-toxin-mediated cell death through inhibiting the oligomer formation of the toxin. Beta-toxin triggered a transient ATP release from THP-1 cells, but this early ATP release was blocked by CBX. ATP scavengers (apyrase and hexokinase) inhibited beta-toxin-induced cytotoxicity. Furthermore, co-administration of ATP with beta-toxin enhanced the binding and cytotoxicity of the toxin.

CONCLUSIONS

Based on our results, Panx1 activation is achieved through the interaction of beta-toxin with the P2X₇ receptor. Then, ATP released by the Panx1 channel opening promotes oligomer formation of the toxin, leading to cell death.

GENERAL SIGNIFICANCE

Pannexin 1 is a novel candidate therapeutic target for beta-toxin-mediated disease.

Purinergic control of lysenin's transport and voltage-gating properties

Lysenin, a pore-forming protein extracted from the coelomic fluid of the earthworm Eisenia foetida, manifests cytolytic activity by inserting large conductance pores in host membranes containing sphingomyelin. In the present study, we found that adenosine phosphates control the biological activity of lysenin channels inserted into planar lipid membranes with respect to their macroscopic conductance and voltage-induced gating. Addition of ATP, ADP, or AMP decreased the macroscopic conductance of lysenin channels in a concentration-dependent manner, with ATP being the most potent inhibitor and AMP the least. ATP removal from the bulk solutions by buffer exchange quickly reinstated the macroscopic conductance and demonstrated reversibility. Single-channel experiments pointed to an inhibition mechanism that most probably relies on electrostatic binding and partial occlusion of the channel-conducting pathway, rather than ligand gating induced by the highly charged phosphates. The Hill analysis of the changes in macroscopic conduction as a function of the inhibitor concentration suggested cooperative binding as descriptive of the inhibition process. Ionic screening significantly reduced the ATP inhibitory efficacy, in support of the electrostatic binding hypothesis. In addition to conductance modulation, purinergic control over the biological activity of lysenin channels has also been observed to manifest as changes of the voltage-induced gating profile. Our analysis strongly suggests that not only the inhibitor's charge but also its ability to adopt a folded conformation may explain the differences in the observed influence of ATP, ADP, and AMP on lysenin's biological activity.

A Fatal Case of Necrotizing Fasciitis Caused by a Highly Virulent Escherichia coli Strain

Necrotizing fasciitis is a serious disease characterized by the necrosis of the subcutaneous tissues and fascia. E. coli as the etiologic agent of necrotizing fasciitis is a rare occurrence. A 66-year-old woman underwent total abdominal hysterectomy with bilateral salpingo-oophorectomy. She rapidly developed necrotizing fasciitis which led to her death 68 hours following surgery. An E. coli strain was isolated from blood and fascia cultures. DNA microarray revealed the presence of 20 virulence genes.

Blood cells: an historical account of the roles of purinergic signalling

The involvement of purinergic signalling in the physiology of erythrocytes, platelets and leukocytes was recognised early. The release of ATP and the expression of purinoceptors and ectonucleotidases on erythrocytes in health and disease are reviewed. The release of ATP and ADP from platelets and the expression and roles of P1, P2Y(1), P2Y(12) and P2X1 receptors on platelets are described. P2Y(1) and P2X(1) receptors mediate changes in platelet shape, while P2Y(12) receptors mediate platelet aggregation. The changes in the role of purinergic signalling in a variety of disease conditions are considered. The successful use of P2Y(12) receptor antagonists, such as clopidogrel and ticagrelor, for the treatment of thrombosis, myocardial infarction and stroke is discussed.