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

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.

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.

Pore-forming toxins of foodborne pathogens

Pore-forming toxins (PFTs) are water-soluble molecules that have been identified as the most crucial virulence factors during bacterial pathogenesis. PFTs disrupt the host cell membrane to internalize or to deliver other bacterial or virulence factors for establishing infections. Disruption of the host cell membrane by PFTs can lead to uncontrollable exchanges between the extracellular and the intracellular matrix, thereby disturbing the cellular homeostasis. Recent studies have provided insights into the molecular mechanism of PFTs during pathogenesis. Evidence also suggests the activation of several signal transduction pathways in the host cell on recognition of PFTs. Additionally, numerous distinctive host defense mechanisms as well as membrane repair mechanisms have been reported; however, studies reveal that PFTs aid in host immune evasion of the bacteria through numerous pathways. PFTs have been primarily associated with foodborne pathogens. Infection and death from diseases by consuming contaminated food are a constant threat to public health worldwide, affecting socioeconomic development. Moreover, the emergence of new foodborne pathogens has led to the rise of bacterial antimicrobial resistance affecting the population. Hence, this review focuses on the role of PFTs secreted by foodborne pathogens. The review highlights the molecular mechanism of foodborne bacterial PFTs, assisting bacterial survival from the host immune responses and understanding the downstream mechanism in the activation of various signaling pathways in the host upon PFT recognition. PFT research is a remarkable and an important field for exploring novel and broad applications of antimicrobial compounds as therapeutics.

Membrane Activity and Channel Formation of the Adenylate Cyclase Toxin (CyaA) of Bordetella pertussis in Lipid Bilayer Membranes

The Gram-negative bacterium Bordetella pertussis is the cause of whooping cough. One of its pathogenicity factors is the adenylate cyclase toxin (CyaA) secreted by a Type I export system. The 1706 amino acid long CyaA (177 kDa) belongs to the continuously increasing family of repeat in toxin (RTX) toxins because it contains in its C-terminal half a high number of nine-residue tandem repeats. The protein exhibits cytotoxic and hemolytic activities that target primarily myeloid phagocytic cells expressing the αMβ2 integrin receptor (CD11b/CD18). CyaA represents an exception among RTX cytolysins because the first 400 amino acids from its N-terminal end possess a calmodulin-activated adenylate cyclase (AC) activity. The entry of the AC into target cells is not dependent on the receptor-mediated endocytosis pathway and penetrates directly across the cytoplasmic membrane of a variety of epithelial and immune effector cells. The hemolytic activity of CyaA is rather low, which may have to do with its rather low induced permeability change of target cells and its low conductance in lipid bilayer membranes. CyaA forms highly cation-selective channels in lipid bilayers that show a strong dependence on aqueous pH. The pore-forming activity of CyaA but not its single channel conductance is highly dependent on Ca²⁺ concentration with a half saturation constant of about 2 to 4 mM.

Functional Consequences of Calcium Influx Promoted by Bacterial Pore-Forming Toxins

Bacterial pore-forming toxins induce a rapid and massive increase in cytosolic Ca²⁺ concentration due to the formation of pores in the plasma membrane and/or activation of Ca²⁺-channels. As Ca²⁺ is an essential messenger in cellular signaling, a sustained increase in Ca²⁺ concentration has dramatic consequences on cellular behavior, eventually leading to cell death. However, host cells have adapted mechanisms to protect against Ca²⁺ intoxication, such as Ca²⁺ efflux and membrane repair. The final outcome depends upon the nature and concentration of the toxin and on the cell type. This review highlights the repercussions of Ca²⁺ overload on the induction of cell death, repair mechanisms, cellular adhesive properties, and the inflammatory response.

Colonization, resistance to bile, and virulence properties of Escherichia coli strains: Unusual characteristics associated with biliary tract diseases

Escherichia coli is the species that is most frequently isolated from bile of patients with biliary tract diseases. This study was aimed to investigate any association between resistance and virulence properties of these isolates with occurrence of the diseases. A total of 102 bile samples were obtained from patients subjected to endoscopic retrograde cholangiopancreatography for different biliary diseases. Clinical data were collected and culture of the bile samples was done on selective media. Resistance of characterized Escherichia coli isolates to deoxycholate sodium (0-7%) and nineteen antibiotics was determined and PCR using 16 pairs of primers targeting stx1, stx2, exhA, eae, bfp, agg, pcvd432, lt, st, ipaH, pic, pet, ast, set, sen, and cdtB genes was done. Our results showed a statistically significant association between E. coli colonization and existence of common bile duct and gallbladder stones (p value 0.028). Out of the 22 E. coli strains (22/102) multidrug resistance phenotype was present in 95.45%. None of the strains belonged to common E. coli pathotypes. However, bfp + EhxA-hly, bfp + astA, bfp + EhxA-hly + pic, and EhxA-hly + pic + astA, bfp, and astA genotypes were detected in these strains. bfp (7/22, 31.8%) and astA (5/22, 22.7%) were among most frequent virulence factors in these strains. Results of this study showed significant association between colonization of E. coli and choledocholithiasis. Unusual existence of virulence gene combinations in these strains and their resistance to DOC and multiple classes of antibiotics could be considered as possible causes of their persistence in this harsh microenvironment.

Channel formation by RTX-toxins of pathogenic bacteria: Basis of their biological activity

The pore-forming cytolysins of the RTX-toxin (Repeats in ToXin) family are a relatively small fraction of a steadily increasing family of proteins that contain several functionally important glycine-rich and aspartate containing nonapeptide repeats. These cytolysins produced by a variety of Gram-negative bacteria form ion-permeable channels in erythrocytes and other eukaryotic cells. Hemolytic and cytolytic RTX-toxins represent pathogenicity factors of the toxin-producing bacteria and are very often important key factors in pathogenesis of the bacteria. Channel formation by RTX-toxins lead to the dissipation of ionic gradients and membrane potential across the cytoplasmic membrane of target cells, which results in cell death. Here we discuss channel formation and channel properties of some of the best known RTX-toxins, such as α-hemolysin (HlyA) of Escherichia coli and the uropathogenic EHEC strains, the adenylate cyclase toxin (ACT, CyaA) of Bordetella pertussis and the RTX-toxins (ApxI, ApxII and ApxIII) produced by different strains of Actinobacillus pleuropneumoniae. The channels formed by these RTX-toxins in lipid bilayers share some common properties such as cation selectivity and voltage-dependence. Furthermore the channels are transient and show frequent switching between different ion-conducting states. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

[Ca2+]i Oscillations and IL-6 Release Induced by α-Hemolysin from Escherichia coli Require P2 Receptor Activation in Renal Epithelia

Urinary tract infections are commonly caused by α-hemolysin (HlyA)-producing Escherichia coli. In erythrocytes, the cytotoxic effect of HlyA is strongly amplified by P2X receptors, which are activated by extracellular ATP released from the cytosol of the erythrocytes. In renal epithelia, HlyA causes reversible [Ca(2+)]i oscillations, which trigger interleukin-6 (IL-6) and IL-8 release. We speculate that this effect is caused by HlyA-induced ATP release from the epithelial cells and successive P2 receptor activation. Here, we demonstrate that HlyA-induced [Ca(2+)]i oscillations in renal epithelia were completely prevented by scavenging extracellular ATP. In accordance, HlyA was unable to inflict any [Ca(2+)]i oscillations in 132-1N1 cells, which lack P2R completely. After transfecting these cells with the hP2Y2 receptor, HlyA readily triggered [Ca(2+)]i oscillations, which were abolished by P2 receptor antagonists. Moreover, HlyA-induced [Ca(2+)]i oscillations were markedly reduced in medullary thick ascending limbs isolated from P2Y2 receptor-deficient mice compared with wild type. Interestingly, the following HlyA-induced IL-6 release was absent in P2Y2 receptor-deficient mice. This suggests that HlyA induces ATP release from renal epithelia, which via P2Y2 receptors is the main mediator of HlyA-induced [Ca(2+)]i oscillations and IL-6 release. This supports the notion that ATP signaling occurs early during bacterial infection and is a key player in the further inflammatory response.

The deletion of several amino acid stretches of Escherichia coli alpha-hemolysin (HlyA) suggests that the channel-forming domain contains beta-strands

Escherichia coli α-hemolysin (HlyA) is a pore-forming protein of 110 kDa belonging to the family of RTX toxins. A hydrophobic region between the amino acid residues 238 and 410 in the N-terminal half of HlyA has previously been suggested to form hydrophobic and/or amphipathic α-helices and has been shown to be important for hemolytic activity and pore formation in biological and artificial membranes. The structure of the HlyA transmembrane channel is, however, largely unknown. For further investigation of the channel structure, we deleted in HlyA different stretches of amino acids that could form amphipathic β-strands according to secondary structure predictions (residues 71–110, 158–167, 180–203, and 264–286). These deletions resulted in HlyA mutants with strongly reduced hemolytic activity. Lipid bilayer measurements demonstrated that HlyA_Δ71–110 and HlyA_Δ264–286 formed channels with much smaller single-channel conductance than wildtype HlyA, whereas their channel-forming activity was virtually as high as that of the wildtype toxin. HlyA_Δ158–167 and HlyA_Δ180–203 were unable to form defined channels in lipid bilayers. Calculations based on the single-channel data indicated that the channels generated by HlyA_Δ71–110 and HlyA_Δ264–286 had a smaller size (diameter about 1.4 to 1.8 nm) than wildtype HlyA channels (diameter about 2.0 to 2.6 nm), suggesting that in these mutants part of the channel-forming domain was removed. Osmotic protection experiments with erythrocytes confirmed that HlyA, HlyA_Δ71–110, and HlyA_Δ264–286 form defined transmembrane pores and suggested channel diameters that largely agreed with those estimated from the single-channel data. Taken together, these results suggest that the channel-forming domain of HlyA might contain β-strands, possibly in addition to α-helical structures.

Mechanisms of cytolysin-induced cell damage -- a role for auto- and paracrine signalling

Cytolysins inflict cell damage by forming pores in the plasma membrane. The Na(+) conductivity of these pores results in an ion influx that exceeds the capacity of the Na(+) /K(+) -pump to extrude Na(+) . This net load of intracellular osmolytes results in swelling and eventual lysis of the attacked cell. Many nucleated cells have the capacity to reduce the potential damage of pore-forming proteins, whereas erythrocytes have been regarded as essentially defenceless against cytolysin-induced cell damage. This review addresses how autocrine/paracrine signalling and the cells intrinsic volume regulation markedly influence the fate of the cell after membrane insertion of cytolysins. Moreover, it regards the various steps that may explain the relative large degree of diversity between cell types and species as well as highlights some of the current gaps in the mechanistic understanding of cytolysin-induced cell injury.

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.

Down-regulation of annexin A1 in the urothelium decreases cell survival after bacterial toxin exposure

PURPOSE

We examined the role of annexins in bladder urothelium. We characterized expression and distribution in normal bladders, biopsies from patients with bladder pain syndrome, cultured human urothelium and urothelial TEU-2 cells.

MATERIALS AND METHODS

Annexin expression in bladder layers was analyzed by quantitative reverse transcriptase-polymerase chain reaction and immunofluorescence. We assessed cell survival after exposure to the pore forming bacterial toxin streptolysin O by microscopy and alamarBlue® assay. Bladder dome biopsies were obtained from 8 asymptomatic controls and 28 patients with symptoms of bladder pain syndrome.

RESULTS

Annexin A1, A2, A5 and A6 were differentially distributed in bladder layers. Annexin A6 was abundant in detrusor smooth muscle and low in urothelium, while annexin A1 was the highest in urothelium. Annexin A2 was localized to the lateral membrane of umbrella cells but excluded from tight junctions. TEU-2 cell differentiation caused up-regulation of annexin A1 and A2 and down-regulation of annexin A6 mRNA. Mature urothelium dedifferentiation during culture caused the opposite effect, decreasing annexin A1 and increasing annexin A6. Annexin A2 influenced TEU-2 cell epithelial permeability. siRNA mediated knockdown of annexin A1 in TEU-2 cells caused significantly decreased cell survival after streptolysin O exposure. Annexin A1 was significantly reduced in biopsies from patients with bladder pain syndrome.

CONCLUSIONS

Several annexins are expressed in human bladder and TEU-2 cells, in which levels are regulated during urothelial differentiation. Annexin A1 down-regulation in patients with bladder pain syndrome might decrease cell survival and contribute to compromised urothelial function.

Cytotoxic necrotizing factor 1 and hemolysin from uropathogenic Escherichia coli elicit different host responses in the murine bladder

Cytotoxic necrotizing factor 1 (CNF1) and hemolysin (HlyA1) are toxins produced by uropathogenic Escherichia coli (UPEC). We previously showed that these toxins contribute to the inflammation and tissue damage seen in a mouse model of ascending urinary tract infection. CNF1 constitutively activates small Rho GTPases by deamidation of a conserved glutamine residue, and HlyA1 forms pores in eukaryotic cell membranes. In this study, we used cDNA microarrays of bladder tissue isolated from mice infected intraurethrally with wild-type CP9, CP9cnf1, or CP9ΔhlyA to further evaluate the role that each toxin plays in the host response to UPEC. Regardless of the strain used, we found that UPEC itself elicited a significant change in host gene expression 24 h after inoculation. The largest numbers of upregulated genes were in the cytokine and chemokine signaling and Toll-like receptor signaling pathways. CNF1 exerted a strong positive influence on expression of genes involved in innate immunity and signal transduction and a negative impact on metabolism- and transport-associated genes. HlyA1 evoked an increase in expression of genes that encode innate immunity factors and a decrease in expression of genes involved in cytoskeletal and metabolic processes. Multiplex cytokine and myeloperoxidase assays corroborated our finding that a strong proinflammatory response was elicited by all strains tested. Bladders challenged intraurethrally with purified CNF1 displayed pathology similar to but significantly less intense than the pathology that we observed in CP9-challenged mice. Our data demonstrate substantial roles for CNF1 and HlyA1 in initiation of a strong proinflammatory response to UPEC in the bladder.

Staphylococcal leukotoxins trigger free intracellular Ca(2+) rise in neurones, signalling through acidic stores and activation of store-operated channels

Headache, muscle aches and chest pain of mild to medium intensity are among the most common clinical symptoms in moderate Staphylococcus aureus infections, with severe infections usually associated with worsening pain symptoms. These nociceptive responses of the body raise the question of how bacterial infection impinges on the nervous system. Does S. aureus, or its released virulence factors, act directly on neurones? To address this issue, we evaluated the potential effects on neurones of certain bi-component leukotoxins, which are virulent factors released by the bacterium. The activity of four different leukotoxins was verified by measuring the release of glutamate from rat cerebellar granular neurones. The bi-component γ-haemolysin HlgC/HlgB was the most potent leukotoxin, initiating transient rises in intracellular Ca²⁺ concentration in cerebellar neurones and in primary sensory neurones from dorsal root ganglia, as probed with the Fura-2 Ca²⁺ indicator dye. Using pharmacological antagonists of receptors and Ca²⁺ channels, the variations in intracellular Ca²⁺ concentration were found independent of the activation of voltage-operatedCa²⁺ channels or glutamate receptors. Drugs targeting Sarco-Endoplasmic Reticulum Ca²⁺-ATPase (SERCA) or H⁺-ATPase and antagonists of the store-operated Ca²⁺ entry complex blunted, or significantly reduced, the leukotoxin-induced elevation in intracellular Ca²⁺. Moreover, activation of the ADP-ribosyl cyclase CD38 was also required to initiate the release of Ca²⁺ from acidic stores. These findings suggest that, prior to forming a pore at the plasma membrane, leukotoxin HlgC/HlgB triggers a multistep process which initiates the release of Ca²⁺ from lysosomes, modifies the steady-state level of reticular Ca²⁺ stores and finally activates the Store-Operated Calcium Entry complex.

Bacteria differentially induce degradation of Bcl-xL, a survival protein, by human platelets

Bacteria can enter the bloodstream in response to infectious insults. Bacteremia elicits several immune and clinical complications, including thrombocytopenia. A primary cause of thrombocytopenia is shortened survival of platelets. We demonstrate that pathogenic bacteria induce apoptotic events in platelets that include calpain-mediated degradation of Bcl-x(L), an essential regulator of platelet survival. Specifically, bloodstream bacterial isolates from patients with sepsis induce lateral condensation of actin, impair mitochondrial membrane potential, and degrade Bcl-x(L) protein in platelets. Bcl-x(L) protein degradation is enhanced when platelets are exposed to pathogenic Escherichia coli that produce the pore-forming toxin α-hemolysin, a response that is markedly attenuated when the gene is deleted from E coli. We also found that nonpathogenic E coli gain degrading activity when they are forced to express α-hemolysin. Like α-hemolysin, purified α-toxin readily degrades Bcl-x(L) protein in platelets, as do clinical Staphylococcus aureus isolates that produce α-toxin. Inhibition of calpain activity, but not the proteasome, rescues Bcl-x(L) protein degradation in platelets coincubated with pathogenic E coli including α-hemolysin producing strains. This is the first evidence that pathogenic bacteria can trigger activation of the platelet intrinsic apoptosis program and our results suggest a new mechanism by which bacterial pathogens might cause thrombocytopenia in patients with bloodstream infections.

Pore-forming bacterial toxins and antimicrobial peptides as modulators of ADAM function

Membrane-perturbating proteins and peptides are widespread agents in biology. Pore-forming bacterial toxins represent major virulence factors of pathogenic microorganisms. Membrane-damaging peptides constitute important antimicrobial effectors of innate immunity. Membrane perturbation can incur multiple responses in mammalian cells. The present discussion will focus on the interplay between membrane-damaging agents and the function of cell-bound metalloproteinases of the ADAM family. These transmembrane enzymes have emerged as the major proteinase family that mediate the proteolytic release of membrane-associated proteins, a process designated as "shedding". They liberate a large spectrum of functionally active molecules including inflammatory cytokines, growth factor receptors and cell adhesion molecules, thereby regulating such vital cellular functions as cell-cell adhesion, cell proliferation and cell migration. ADAM activation may constitute part of the cellular recovery machinery on the one hand, but likely also promotes inflammatory processes on the other. The mechanisms underlying ADAM activation and the functional consequences thereof are currently the subject of intensive research. Attention here is drawn to the possible involvement of purinergic receptors and ceramide generation in the context of ADAM activation following membrane perturbation by membrane-active agents.

The UPEC pore-forming toxin α-hemolysin triggers proteolysis of host proteins to disrupt cell adhesion, inflammatory, and survival pathways

Uropathogenic Escherichia coli (UPEC), which are the leading cause of both acute and chronic urinary tract infections, often secrete a labile pore-forming toxin known as α-hemolysin (HlyA). We show that stable insertion of HlyA into epithelial cell and macrophage membranes triggers degradation of the cytoskeletal scaffolding protein paxillin and other host regulatory proteins, as well as components of the proinflammatory NFκB signaling cascade. Proteolysis of these factors requires host serine proteases, and paxillin degradation specifically involves the serine protease mesotrypsin. The induced activation of mesotrypsin by HlyA is preceded by redistribution of mesotrypsin precursors from the cytosol into foci along microtubules and within nuclei. HlyA intoxication also stimulated caspase activation, which occurred independently of effects on host serine proteases. HlyA-induced proteolysis of host proteins likely allows UPEC to not only modulate epithelial cell functions, but also disable macrophages and suppress inflammatory responses.

Molecular basis of early epithelial response to streptococcal exotoxin: role of STIM1 and Orai1 proteins

Streptolysin O (SLO) is a cholesterol-dependent cytolysin (CDC) from Streptococcus pyogenes. SLO induces diverse types of Ca(2+) signalling in host cells which play a key role in membrane repair and cell fate determination. The mechanisms behind SLO-induced Ca(2+) signalling remain poorly understood. Here, we show that in NCI-H441 cells, wild-type SLO as well as non-pore-forming mutant induces long-lasting intracellular Ca(2+) oscillations via IP(3) -mediated depletion of intracellular stores and activation of store-operated Ca(2+) (SOC) entry. SLO-induced activation of SOC entry was confirmed by Ca(2+) add-back experiments, pharmacologically and by overexpression as well as silencing of STIM1 and Orai1 expression. SLO also activated SOC entry in primary cultivated alveolar type II (ATII) cells but Ca(2+) oscillations were comparatively short-lived in nature. Comparison of STIM1 and Orai1 revealed a differential expression pattern in H441 and ATII cells. Overexpression of STIM1 and Orai1 proteins in ATII cells changed the short-lived oscillatory response into a long-lived one. Thus, we conclude that SLO-mediated Ca(2+) signalling involves Ca(2+) release from intracellular stores and STIM1/Orai1-dependent SOC entry. The phenotype of Ca(2+) signalling depends on STIM1 and Orai1 expression levels. Our findings suggest a new role for SOC entry-associated proteins in S. pyogenes-induced lung infection and pneumonia.

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.

Alpha hemolysin induces an increase of erythrocytes calcium: a FLIM 2-photon phasor analysis approach

α-Hemolysin (HlyA) from Escherichia coli is considered as the prototype of a family of toxins called RTX (repeat in toxin), a group of proteins that share genetic and structural features. HlyA is an important virulence factor in E. coli extraintestinal infections, such as meningitis, septicemia and urinary infections. High concentrations of the toxin cause the lysis of several cells such as erythrocytes, granulocytes, monocytes, endothelial and renal epithelial cells of different species. At low concentrations it induces the production of cytokines and apoptosis. Since many of the subcytolytic effects in other cells have been reported to be triggered by the increase of intracellular calcium, we followed the calcium concentration inside the erythrocytes while incubating with sublytic concentrations of HlyA. Calcium concentration was monitored using the calcium indicator Green 1, 2-photon excitation, and fluorescence lifetime imaging microscopy (FLIM). Data were analyzed using the phasor representation. In this report, we present evidence that, at sublytic concentrations, HlyA induces an increase of calcium concentration in rabbit erythrocytes in the first 10 s. Results are discussed in relation to the difficulties of measuring calcium concentrations in erythrocytes where hemoglobin is present, the contribution of the background and the heterogeneity of the response observed in individual cells.

Adenylate cyclase toxin promotes internalisation of integrins and raft components and decreases macrophage adhesion capacity

Bordetella pertussis, the bacterium that causes whooping cough, secretes an adenylate cyclase toxin (ACT) that must be post-translationally palmitoylated in the bacterium cytosol to be active. The toxin targets phagocytes expressing the CD11b/CD18 integrin receptor. It delivers a catalytic adenylate cyclase domain into the target cell cytosol producing a rapid increase of intracellular cAMP concentration that suppresses bactericidal functions of the phagocyte. ACT also induces calcium fluxes into target cells. Biochemical, biophysical and cell biology approaches have been applied here to show evidence that ACT and integrin molecules, along with other raft components, are rapidly internalized by the macrophages in a toxin-induced calcium rise-dependent process. The toxin-triggered internalisation events occur through two different routes of entry, chlorpromazine-sensitive receptor-mediated endocytosis and clathrin-independent internalisation, maybe acting in parallel. ACT locates into raft-like domains, and is internalised, also in cells devoid of receptor. Altogether our results suggest that adenylate cyclase toxin, and maybe other homologous pathogenic toxins from the RTX (Repeats in Toxin) family to which ACT belongs, may be endowed with an intrinsic capacity to, directly and efficiently, insert into raft-like domains, promoting there its multiple activities. One direct consequence of the integrin removal from the cell surface of the macrophages is the hampering of their adhesion ability, a fundamental property in the immune response of the leukocytes that could be instrumental in the pathogenesis of Bordetella pertussis.

RTX proteins: a highly diverse family secreted by a common mechanism

Repeats-in-toxin (RTX) exoproteins of Gram-negative bacteria form a steadily growing family of proteins with diverse biological functions. Their common feature is the unique mode of export across the bacterial envelope via the type I secretion system and the characteristic, typically nonapeptide, glycine- and aspartate-rich repeats binding Ca²⁺ ions. In this review, we summarize the current state of knowledge on the organization of rtx loci and on the biological and biochemical activities of therein encoded proteins. Applying several types of bioinformatic screens on the steadily growing set of sequenced bacterial genomes, over 1000 RTX family members were detected, with the biological functions of most of them remaining to be characterized. Activities of the so far characterized RTX family members are then discussed and classified according to functional categories, ranging from the historically first characterized pore-forming RTX leukotoxins, through the large multifunctional enzymatic toxins, bacteriocins, nodulation proteins, surface layer proteins, up to secreted hydrolytic enzymes exhibiting metalloprotease or lipase activities of industrial interest.

Bordetella adenylate cyclase toxin promotes calcium entry into both CD11b+ and CD11b- cells through cAMP-dependent L-type-like calcium channels

Adenylate cyclase toxin (ACT), a 200 kDa protein, is an essential virulence factor for Bordetella pertussis, the bacterium that causes whooping cough. ACT is a member of the pore-forming RTX (repeats-in-toxin) family of proteins that share a characteristic calcium-binding motif of Gly- and Asp-rich nonapeptide repeats and a marked cytolytic or cytotoxic activity. In addition, ACT exhibits a distinctive feature: it has an N-terminal calmodulin-dependent adenylate cyclase domain. Translocation of this domain into the host cytoplasm results in uncontrolled production of cAMP, and it has classically been assumed that this surge in cAMP is the basis for the toxin-mediated killing. Several members of the RTX family of toxins, including ACT, have been shown to induce intracellular calcium increases, through different mechanisms. We show here that ACT stimulates a raft-mediated calcium influx, through its cAMP production activity, that activates PKA, which in turn activates calcium channels with L-type properties. This process is shown to occur both in CD11b(+) and CD11b(-) cells, suggesting a common mechanism, independent of the toxin receptor. We also show that this ACT-induced calcium influx does not correlate with the toxin-induced cytotoxicity.

Pore-forming toxins activate MAPK p38 by causing loss of cellular potassium

Mitogen activated protein kinase (MAPK) p38 has emerged as a survival protein in cells that are attacked by bacterial toxins forming small membrane pores. Activation of p38 by pore forming toxins (PFT) has been attributed to osmotic stress, but here we show that loss of K+ is likely to be the critical parameter. Several lines of evidence support this conclusion: first, osmoprotection did not prevent p38-phosphorylation in alpha-toxin-loaded cells. Second, treatment of cells with a K+ ionophore, or simple incubation in K+-free medium sufficed to cause robust p38-phosphorylation. Third, media containing high [K+] prevented p38-activation by Staphylococcus aureus alpha-toxin, Vibrio cholerae cytolysin (VCC), Streptolysin O (SLO), or Escherichia coli hemolysin (HlyA), but did not impair activation by H2O2. Fourth, potential roles of LPS, TLR4, or calcium-influx were ruled out. Therefore, we propose that PFT trigger the p38 MAPK-pathway by causing loss of cellular K+.

Alpha-hemolysin from Escherichia coli uses endogenous amplification through P2X receptor activation to induce hemolysis

Escherichia coli is the dominant facultative bacterium in the normal intestinal flora. E. coli is, however, also responsible for the majority of serious extraintestinal infections. There are distinct serotypical differences between facultative and invasive E. coli strains. Invasive strains frequently produce virulence factors such as alpha-hemolysin (HlyA), which causes hemolysis by forming pores in the erythrocyte membrane. The present study reveals that this pore formation triggers purinergic receptor activation to mediate the full hemolytic action. Non-selective ATP-receptor (P2) antagonists (PPADS, suramin) and ATP scavengers (apyrase, hexokinase) concentration dependently inhibited HlyA-induced lysis of equine, murine, and human erythrocytes. The pattern of responsiveness to more selective P2-antagonists implies that both P2X(1) and P2X(7) receptors are involved in HlyA-induced hemolysis in all three species. In addition, our results also propose a role for the pore protein pannexin1 in HlyA-induced hemolysis, as non-selective inhibitors of this channel significantly reduced hemolysis in the three species. In conclusion, activation of P2X receptors and possibly also pannexins augment hemolysis induced by the bacterial toxin, HlyA. These findings potentially have clinical perspectives as P2 antagonists may ameliorate symptoms during sepsis with hemolytic bacteria.

Selective activation of human intestinal mast cells by Escherichia coli hemolysin

Mast cells (MCs) are recognized to play an important role in bacterial host defense in the murine system. In this study, we studied the interaction of human MCs, isolated from the intestine and purified to homogeneity, with different Escherichia coli and Shigella flexneri strains. We show that alpha-hemolysin (Hly)-producing E. coli strains induce the release of histamine, leukotrienes, and proinflammatory cytokines in intestinal MCs. In contrast, MCs were virtually unresponsive to S. flexneri and several Hly-negative E. coli strains, including the isogenic Hly-deficient mutants of Hly(+) strains. Hly(+) E. coli but not Hly(-) E. coli caused an increase in intracellular Ca(2+) levels. Blocking of extracellular Ca(2+) and of the calmodulin/calcineurin pathway by cyclosporin A inhibited the response to Hly(+) E. coli. Furthermore, inhibition of MAPKs p38 and ERK reduces activation of MCs by Hly(+) E. coli. In addition, using an ex vivo system, we directly record the histamine release by MCs located in the lamina propria after infection with Hly(+) E. coli. Our data indicate that human intestinal mast cells interact with selected Gram-negative bacteria, establish E. coli Hly as a factor regulating MC effector functions, and argue further for a role of human MCs in innate immunity.

Origins and virulence mechanisms of uropathogenic Escherichia coli

Strains of uropathogenic E. coli (UPEC) are the primary cause of urinary tract infections, including both cystitis and pyelonephritis. These bacteria have evolved a multitude of virulence factors and strategies that facilitate bacterial growth and persistence within the adverse settings of the host urinary tract. Expression of adhesive organelles like type 1 and P pili allow UPEC to bind and invade host cells and tissues within the urinary tract while expression of iron-chelating factors (siderophores) enable UPEC to pilfer host iron stores. Deployment of an array of toxins, including hemolysin and cytotoxic necrotizing factor 1, provide UPEC with the means to inflict extensive tissue damage, facilitating bacterial dissemination as well as releasing host nutrients and disabling immune effector cells. These toxins also have the capacity to modulate, in more subtle ways, host signaling pathways affecting myriad processes, including inflammatory responses, host cell survival, and cytoskeletal dynamics. Here, we discuss the mechanisms by which these and other virulence factors promote UPEC survival and growth within the urinary tract. Comparisons are also made between UPEC and other strains of extraintestinal pathogenic E. coli that, although closely related to UPEC, are distinct in their abilities to colonize the host and cause disease.

Inactivation of host Akt/protein kinase B signaling by bacterial pore-forming toxins

Uropathogenic Escherichia coli (UPEC) are the major cause of urinary tract infections (UTIs), and they have the capacity to induce the death and exfoliation of target uroepithelial cells. This process can be facilitated by the pore-forming toxin alpha-hemolysin (HlyA), which is expressed and secreted by many UPEC isolates. Here, we demonstrate that HlyA can potently inhibit activation of Akt (protein kinase B), a key regulator of host cell survival, inflammatory responses, proliferation, and metabolism. HlyA ablates Akt activation via an extracellular calcium-dependent, potassium-independent process requiring HlyA insertion into the host plasma membrane and subsequent pore formation. Inhibitor studies indicate that Akt inactivation by HlyA involves aberrant stimulation of host protein phosphatases. We found that two other bacterial pore-forming toxins (aerolysin from Aeromonas species and alpha-toxin from Staphylococcus aureus) can also markedly attenuate Akt activation in a dose-dependent manner. These data suggest a novel mechanism by which sublytic concentrations of HlyA and other pore-forming toxins can modulate host cell survival and inflammatory pathways during the course of a bacterial infection.

Ca(2+)/calmodulin-dependent invasion of microvascular endothelial cells of human brain by Escherichia coli K1

Escherichia coli K1 invasion of microvascular endothelial cells of human brain (HBMEC) is required for E. coli penetration into the central nervous system, but the microbial-host interactions that are involved in this invasion of HBMEC remain incompletely understood. We have previously shown that FimH, one of the E. coli determinants contributing to the binding to and invasion of HBMEC, induces Ca(2+) changes in HBMEC. In the present study, we have investigated in detail the role of cellular calcium signaling in the E. coli K1 invasion of HBMEC, the main constituents of the blood-brain barrier. Addition of the meningitis-causing E. coli K1 strain RS218 (O18:K1) to HBMEC results in transient increases of intracellular free Ca(2+). Inhibition of phospholipase C with U-73122 and the chelating of intracellular Ca(2+) by BAPTA/AM reduces bacterial invasion of HBMEC by approximately 50%. Blocking of transmembrane Ca(2+) fluxes by extracellular lanthanum ions also inhibits the E. coli invasion of HBMEC by approximately 50%. In addition, E. coli K1 invasion is significantly inhibited when HBMEC are pretreated by the calmodulin antagonists, trifluoperazine or calmidazolium, or by ML-7, a specific inhibitor of Ca(2+)/calmodulin-dependent myosin light-chain kinase. These findings indicate that host intracellular Ca(2+) signaling contributes in part to E. coli K1 invasion of HBMEC.

Role of the lipopolysaccharide-CD14 complex for the activity of hemolysin from uropathogenic Escherichia coli

Bacterial pathogens produce a variety of exotoxins, which often become associated with the bacterial outer membrane component lipopolysaccharide (LPS) during their secretion. LPS is a potent proinflammatory mediator; however, it is not known whether LPS contributes to cell signaling induced by those microbial components to which it is attached. This is partly due to the common view that LPS present in bacterial component preparations is an experimental artifact. The Escherichia coli exotoxin hemolysin (Hly) is a known inducer of proinflammatory signaling in epithelial cells, and the signal transduction pathway involves fluctuation of the intracellular-Ca(2+) concentration. Since LPS is known to interact with Hly, we investigated whether it is required as a cofactor for the activity of Hly. We found that the LPS/Hly complex exploits the CD14/LPS-binding protein recognition system to bring Hly to the cell membrane, where intracellular-Ca(2+) signaling is initiated via specific activation of the small GTPase RhoA. Hly-induced Ca(2+) signaling was found to occur independently of the LPS receptor TLR4, suggesting that the role of LPS/CD14 is to deliver Hly to the cell membrane. In contrast, the cytolytic effect triggered by exposure of cells to high Hly concentrations occurs independently of LPS/CD14. Collectively, our data reveal a novel molecular mechanism for toxin delivery in bacterial pathogenesis, where LPS-associated microbial compounds are targeted to the host cell membrane as a consequence of their association with LPS.

Third activity of Bordetella adenylate cyclase (AC) toxin-hemolysin. Membrane translocation of AC domain polypeptide promotes calcium influx into CD11b+ monocytes independently of the catalytic and hemolytic activities

The Bordetella adenylate cyclase toxin-hemolysin (CyaA) targets phagocytes expressing the alpha(M)beta2 integrin (CD11b/CD18), permeabilizes their membranes by forming small cation-selective pores, and delivers into cells a calmodulin-activated adenylate cyclase (AC) enzyme that dissipates cytosolic ATP into cAMP. We describe here a third activity of CyaA that yields elevation of cytosolic calcium concentration ([Ca2+]i) in target cells. The CyaA-mediated [Ca2+]i increase in CD11b+ J774A.1 monocytes was inhibited by extracellular La3+ ions but not by nifedipine, SK&F 96365, flunarizine, 2-aminoethyl diphenylborinate, or thapsigargin, suggesting that influx of Ca2+ into cells was not because of receptor signaling or opening of conventional calcium channels by cAMP. Compared with intact CyaA, a CyaA-AC- toxoid unable to generate cAMP promoted a faster, albeit transient, elevation of [Ca2+]i. This was not because of cell permeabilization by the CyaA hemolysin pores, because a mutant exhibiting a strongly enhanced pore-forming activity (CyaA-E509K/E516K), but unable to deliver the AC domain into cells, was also unable to elicit a [Ca2+]i increase. Further mutations interfering with AC translocation into cells, such as proline substitutions of glutamate residues 509 or 570 or deletion of the AC domain as such, reduced or ablated the [Ca2+]i-elevating capacity of CyaA. Moreover, structural alterations within the AC domain, because of insertion of various oligopeptides, differently modulated the kinetics and extent of Ca2+ influx elicited by the respective AC- toxoids. Hence, the translocating AC polypeptide itself appears to participate in formation of a novel type of membrane path for calcium ions, contributing to action of CyaA in an unexpected manner.