Clostridium perfringens alpha-toxin activates the sphingomyelin metabolism system in sheep erythrocytes

Preparation and Application of Clostridium perfringens Alpha Toxin Nanobodies

All subtypes of Clostridium perfringens (C. perfringens) produce the alpha toxin (CPA), which can cause enteritis or enterotoxemia in lambs, cattle, pigs, and horses, as well as traumatic clostridial myonecrosis in humans and animals. CPA acts on cell membranes, ultimately leading to endocytosis and cell death. Therefore, the neutralization of CPA is crucial for the prevention and treatment of diseases caused by C. perfringens. In this study, utilizing CPA as an antigen, a nanobody (CPA-VHH) with a half-life of 2.9 h, an affinity constant (KD) of 0.9 nmol/L, and good stability below 60 °C was prepared from a natural nanobody library from alpacas. The biological activity analysis of CPA-VHH revealed its ability to effectively neutralize the phospholipase and hemolytic activity of CPA at a 15-fold ratio. In Vero cells, 9.8 μg/mL CPA-VHH neutralized the cytotoxicity of CPA at two times the half-maximal inhibitory concentration (IC50). In a mouse model, 35.7 ng/g body weight (BW) of CPA-VHH neutralized 90% of the lethality caused by a 2× median lethal dose (LD50) of CPA. It was found that CPA-VHH protected 80% of mice within 30 min at 2 × LD50 CPA, but this dropped below 50% after 2 h and to 0% after 4 h. Rescue trials indicated that using CPA-VHH within 30 min post-infection with 2 × LD50 CPA achieved an 80% rescue rate, which decreased to 10% after 2 h. Furthermore, CPA-VHH effectively mitigated the reduction in the expression levels of zonula occludens-1 (ZO-1), Occludin, and Claudin-1, while also attenuating the upregulation of the pro-inflammatory cytokines interleukin-1β (IL-1β), interleukin-6 (IL-6), interleukin-7 (IL-7), interleukin-8 (IL-8), tumor necrosis factor α (TNF-α), and interferon-γ (IFN-γ) induced by CPA infection. Overall, this study has identified a specific nanobody, CPA-VHH, that effectively neutralizes CPA toxins in vitro and in animal models, providing a new tool for inhibiting the pathogenicity resulting from these toxins and laying an important foundation for the development of new anti-C. perfringens toxin-related therapeutic products.

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.

Yellow Lamb Disease (Clostridium perfringens Type A Enterotoxemia of Sheep): A Review

Simple Summary

Yellow lamb disease is a poorly researched and understood condition that seems to affect young sheep. The disease is characterized by hemolysis and is thought to be caused by alpha toxin-producing Clostridium perfringens type A, although a definitive association with this microorganism has not been confirmed. This is due, in part, to the ubiquitous nature of C. perfringens type A, which is naturally present in the intestine of healthy sheep, a fact that complicates the diagnosis. In this review, we summarize the available information on the etiology, clinical signs, lesions, diagnosis, prevention and prophylaxis of yellow lamb disease.

Abstract

Yellow lamb disease is an infrequent disease in sheep for which there is scant literature, and that has been reported in the US, Australia, New Zealand, South Africa and Europe, although anecdotal evidence indicates that it may have also been diagnosed in South America. The disease is produced by some strains of Clostridium perfringens type A that produce unusually high levels of alpha- toxin. Because C. perfringens type A is ubiquitous and is found in the intestine of most clinically healthy sheep, diagnosis of yellow lamb disease is challenging and requires quantitating the amount of this microorganism present in feces and/or intestinal content. Clinically, yellow lamb disease is characterized by depression, anemia, icterus and hemoglobinuria. Occasionally, sudden death may occur. Gross findings include generalized icterus, red urine in the bladder, enlarged, pale, and friable spleen, enlarged liver with an acinar pattern, and dark, swollen kidneys. Microscopically, yellow lamb disease is characterized by centrilobular necrosis of the liver, hemoglobinuria-associated acute tubular injury, splenic congestion, pulmonary congestion and edema. Although there are no vaccines specifically designed to prevent yellow lamb disease, several vaccines against the different types of C. perfringens may afford at least some level of protection against yellow lamb disease.

Navarro M, Li J, Shrestha A, Uzal F, A. McClane B. Pathogenicity and virulence of Clostridium perfringens

Clostridium perfringens is an extremely versatile pathogen of humans and livestock, causing wound infections like gas gangrene (clostridial myonecrosis), enteritis/enterocolitis (including one of the most common human food-borne illnesses), and enterotoxemia (where toxins produced in the intestine are absorbed and damage distant organs such as the brain). The virulence of this Gram-positive, spore-forming, anaerobe is largely attributable to its copious toxin production; the diverse actions and roles in infection of these toxins are now becoming established. Most C. perfringens toxin genes are encoded on conjugative plasmids, including the pCW3-like and the recently discovered pCP13-like plasmid families. Production of C. perfringens toxins is highly regulated via processes involving two-component regulatory systems, quorum sensing and/or sporulation-related alternative sigma factors. Non-toxin factors, such as degradative enzymes like sialidases, are also now being implicated in the pathogenicity of this bacterium. These factors can promote toxin action in vitro and, perhaps in vivo, and also enhance C. perfringens intestinal colonization, e.g. NanI sialidase increases C. perfringens adherence to intestinal tissue and generates nutrients for its growth, at least in vitro. The possible virulence contributions of many other factors, such as adhesins, the capsule and biofilms, largely await future study.

Bacterial phospholipases C with dual activity: phosphatidylcholinesterase and sphingomyelinase

Bacterial phospholipases and sphingomyelinases are lipolytic esterases that are structurally and evolutionarily heterogeneous. These enzymes play crucial roles as virulence factors in several human and animal infectious diseases. Some bacterial phospholipases C (PLCs) have both phosphatidylcholinesterase and sphingomyelinase C activities. Among them, Listeria  monocytogenes PlcB, Clostridium perfringens PLC, and Pseudomonas aeruginosa PlcH are the most deeply understood. In silico predictions of substrates docking with these three bacterial enzymes provide evidence that they interact with different substrates at the same active site. This review discusses structural aspects, substrate specificity, and the mechanism of action of those bacterial enzymes on target cells and animal infection models to shed light on their roles in pathogenesis.

PET Study of Sphingosine-1-phosphate Receptor 1 Expression in Response to S. aureus Infection

Sphingosine-1-phosphate receptor 1 (S1PR1) plays a crucial role in infectious diseases. Targeting S1PR1 provides protection against pathogens, such as influenza viruses. This study is aimed at investigating S1PR1 in response to bacterial infection by assessing S1PR1 expression in S. aureus-infected mice. A rodent local muscle bacterial infection model was developed by injecting S. aureus to the lower hind limb of Balb/c mice. The changes of S1PR1 expression in response to bacterial infection and blocking treatment were assessed using ex vivo biodistribution and in vivo positron emission tomography (PET) after intravenous injection of an S1PR1-specific radiotracer [18F]TZ4877. The specificity of [18F]TZ4877 was assessed using S1PR1-specific antagonist, NIBR-0213, and S1PR1-specific DsiRNA pretreated the animals. Immunohistochemical studies were performed to confirm the increase of S1PR1 expression in response to infection. Ex vivo biodistribution data showed that the uptake of [18F]TZ4877 was increased 30.6%, 54.3%, 74.3%, and 115.3% in the liver, kidney, pancreas, and thymus of the infected mice, respectively, compared to that in normal control mice, indicating that S1PR1 is involved in the early immune response to bacterial infection. NIBR-0213 or S1PR1-specific DsiRNA pretreatment reduced the tissue uptake of [18F]TZ4877, suggesting that uptake of [18F]TZ4877 is specific. Our PET/CT study data also confirmed that infected mice have increased [18F]TZ4877 uptake in several organs comparing to that in normal control mice. Particularly, compared to control mice, a 39% increase of [18F]TZ4877 uptake was observed in the infected muscle of S. aureus mice, indicating that S1PR1 expression was directly involved in the inflammatory response to infection. Overall, our study suggested that S1PR1 plays an important role in the early immune response to bacterial infection. The uptake of [18F]TZ4877 is tightly correlated with the S1R1 expression in response to S. aureus infection. PET with S1PR1-specific radiotracer [18F]TZ4877 could provide a noninvasive tool for detecting the early S1PR1 immune response to infectious diseases.

An EGFP-marked recombinant lactobacillus oral tetravalent vaccine constitutively expressing α, ε, β1, and β2 toxoids for Clostridium perfringens elicits effective anti-toxins protective immunity

Clostridium perfringens is a common opportunistic pathogen endangering livestock and poultry breeds. Here, using enhanced green fluorescent protein as screening marker, a recombinant lactobacillus tetravalent vaccine constitutively expressing α, ϵ, β1, and β2 toxoids of C. perfringens was developed, and its immunogenicity in mice was investigated via oral administration. This probiotic vaccine could effectively induce antigen-specific secretory IgA (sIgA)-based mucosal and IgG-based humoral immune responses, and significantly high levels (p< 0.05) of cytokines IL-2, IL-4, IL-10, IL-12, IL-17, and IFN-γ were produced in immunized mice. Moreover, lymphoproliferation and percentage of CD4⁺ and CD8⁺ T cells significantly increased in mice of the probiotic vaccine group. Challenge experiments were performed in mice with C. perfringens toxinotypes A, C, and D crude toxins to evaluate protection efficiency of the probiotic vaccine, using a commercial inactivated C. perfringens vaccine made by C. perfringens toxinotypes A, C, and D as vaccine control. We observed 80% protection rate in the probiotic vaccine group, which was higher than commercial vaccine group, whereas all mice in control groups died and obvious histopathological changes were observed in liver, spleen, kidney, and intestines of mice. Significantly, we compared the immunogenicity and protection efficiency of lactobacillus constitutive expression system and lactobacillus inducible expression system, and results showed that lactobacillus constitutive expression system has obvious advantages. Our study clearly demonstrated that the probiotics vaccine could effectively induce mucosal, humoral, and cellular immunity, and provide effective protection against C. perfringens toxins, suggesting a promising strategy for the development of oral vaccine against C. perfringens.

Clostridium perfringens Epsilon-Toxin Impairs the Barrier Function in MDCK Cell Monolayers in a Ca2+-Dependent Manner

Epsilon-toxin produced by Clostridium perfringens significantly contributes to the pathogeneses of enterotoxemia in ruminants and multiple sclerosis in humans. Epsilon-toxin forms a heptameric oligomer in the host cell membrane, promoting cell disruption. Here, we investigate the effect of epsilon-toxin on epithelial barrier functions. Epsilon-toxin impairs the barrier integrity of Madin-Darby Canine Kidney (MDCK) cells, as demonstrated by decreased transepithelial electrical resistance (TEER), increased paracellular flux marker permeability, and the decreased cellular localization of junctional proteins, such as occludin, ZO-1, and claudin-1. U73122, an endogenous phospholipase C (PLC) inhibitor, inhibited the decrease in TEER and the increase in the permeability of flux marker induced by epsilon-toxin. The application of epsilon-toxin to MDCK cells resulted in the biphasic formation of 1,2-diacylglycerol (DAG) and inositol-1,4,5-triphosphate (IP₃). U73122 blocked the formation of DAG and IP₃ induced by the toxin. Epsilon-toxin also specifically activated endogenous PLC-γ1. Epsilon-toxin dose-dependently increased the cytosolic calcium ion concentration ([Ca²⁺]i). The toxin-induced elevation of [Ca²⁺]i was inhibited by U73122. Cofilin is a key regulator of actin cytoskeleton turnover and tight-junction (TJ) permeability regulation. Epsilon-toxin caused cofilin dephosphorylation. These results demonstrate that epsilon-toxin induces Ca²⁺ influx through activating the phosphorylation of PLC-γ1 and then causes TJ opening accompanied by cofilin dephosphorylation.

Histotoxic Clostridial Infections

The pathogenesis of clostridial myonecrosis or gas gangrene involves an interruption to the blood supply to the infected tissues, often via a traumatic wound, anaerobic growth of the infecting clostridial cells, the production of extracellular toxins, and toxin-mediated cell and tissue damage. This review focuses on host-pathogen interactions in Clostridium perfringens-mediated and Clostridium septicum-mediated myonecrosis. The major toxins involved are C. perfringens α-toxin, which has phospholipase C and sphingomyelinase activity, and C. septicum α-toxin, a β-pore-forming toxin that belongs to the aerolysin family. Although these toxins are cytotoxic, their effects on host cells are quite complex, with a range of intracellular cell signaling pathways induced by their action on host cell membranes.

T-Cell Accumulation in the Hypertensive Brain: A Role for Sphingosine-1-Phosphate-Mediated Chemotaxis

Hypertension is considered the major modifiable risk factor for the development of cognitive impairment. Because increased blood pressure is often accompanied by an activation of the immune system, the concept of neuro-inflammation gained increasing attention in the field of hypertension-associated neurodegeneration. Particularly, hypertension-associated elevated circulating T-lymphocyte populations and target organ damage spurred the interest to understanding mechanisms leading to inflammation-associated brain damage during hypertension. The present study describes sphingosine-1-phosphate (S1P) as major contributor to T-cell chemotaxis to the brain during hypertension-associated neuro-inflammation and cognitive impairment. Using Western blotting, flow cytometry and mass spectrometry approaches, we show that hypertension stimulates a sphingosine kinase 1 (SphK1)-dependent increase of cerebral S1P concentrations in a mouse model of angiotensin II (AngII)-induced hypertension. The development of a distinct S1P gradient between circulating blood and brain tissue associates to elevated CD3+ T-cell numbers in the brain. Inhibition of S1P₁-guided T-cell chemotaxis with the S1P receptor modulator FTY720 protects from augmentation of brain CD3 expression and the development of memory deficits in hypertensive WT mice. In conclusion, our data highlight a new approach to the understanding of hypertension-associated inflammation in degenerative processes of the brain during disease progression.

The emerging alliance of sphingosine-1-phosphate signalling and immune cells: from basic mechanisms to implications in hypertension

The immune system plays a considerable role in hypertension. In particular, T-lymphocytes are recognized as important players in its pathogenesis. Despite substantial experimental efforts, the molecular mechanisms underlying the nature of T-cell activation contributing to an onset of hypertension or disease perpetuation are still elusive. Amongst other cell types, lymphocytes express distinct profiles of GPCRs for sphingosine-1-phosphate (S1P) - a bioactive phospholipid that is involved in many critical cell processes and most importantly majorly regulates T-cell development, lymphocyte recirculation, tissue-homing patterns and chemotactic responses. Recent findings have revealed a key role for S1P chemotaxis and T-cell mobilization for the onset of experimental hypertension, and elevated circulating S1P levels have been linked to several inflammation-associated diseases including hypertension in patients. In this article, we review the recent progress towards understanding how S1P and its receptors regulate immune cell trafficking and function and its potential relevance for the pathophysiology of hypertension. LINKED ARTICLES: This article is part of a themed section on Immune Targets in Hypertension. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.12/issuetoc.

Mechanisms of Action and Cell Death Associated with Clostridium perfringens Toxins

Clostridium perfringens uses its large arsenal of protein toxins to produce histotoxic, neurologic and intestinal infections in humans and animals. The major toxins involved in diseases are alpha (CPA), beta (CPB), epsilon (ETX), iota (ITX), enterotoxin (CPE), and necrotic B-like (NetB) toxins. CPA is the main virulence factor involved in gas gangrene in humans, whereas its role in animal diseases is limited and controversial. CPB is responsible for necrotizing enteritis and enterotoxemia, mostly in neonatal individuals of many animal species, including humans. ETX is the main toxin involved in enterotoxemia of sheep and goats. ITX has been implicated in cases of enteritis in rabbits and other animal species; however, its specific role in causing disease has not been proved. CPE is responsible for human food-poisoning and non-foodborne C. perfringens-mediated diarrhea. NetB is the cause of necrotic enteritis in chickens. In most cases, host–toxin interaction starts on the plasma membrane of target cells via specific receptors, resulting in the activation of intracellular pathways with a variety of effects, commonly including cell death. In general, the molecular mechanisms of cell death associated with C. perfringens toxins involve features of apoptosis, necrosis and/or necroptosis.

Clostridium perfringens α-toxin impairs erythropoiesis by inhibition of erythroid differentiation

Clostridium perfringens α-toxin induces hemolysis of erythrocytes from various species, but it has not been elucidated whether the toxin affects erythropoiesis. In this study, we treated bone marrow cells (BMCs) from mice with purified α-toxin and found that TER119⁺ erythroblasts were greatly decreased by the treatment. A variant α-toxin defective in enzymatic activities, phospholipase C and sphingomyelinase, had no effect on the population of erythroblasts, demonstrating that the decrease in erythroblasts was dependent of its enzymatic activities. α-Toxin reduced the CD71⁺TER119⁺ and CD71^–TER119⁺ cell populations but not the CD71⁺TER119⁻ cell population. In addition, α-toxin decreased the number of colony-forming unit erythroid colonies but not burst-forming unit erythroid colonies, indicating that α-toxin preferentially reduced mature erythroid cells compared with immature cells. α-Toxin slightly increased annexinV⁺ cells in TER119⁺ cells. Additionally, simultaneous treatment of BMCs with α-toxin and erythropoietin greatly attenuated the reduction of TER119⁺ erythroblasts by α-toxin. Furthermore, hemin-induced differentiation of human K562 erythroleukemia cells was impaired by α-toxin, whereas the treatment exhibited no apparent cytotoxicity. These results suggested that α-toxin mainly inhibited erythroid differentiation. Together, our results provide new insights into the biological activities of α-toxin, which might be important to understand the pathogenesis of C. perfringens infection.

Lipidomic profile of GM95 cell death induced by Clostridium perfringens alpha-toxin

Clostridium perfringens alpha-toxin (ATX) is considered as a prototype of cytotoxic bacterial phospholipases C, and is the major virulence factor in C. perfringens-induced gas gangrene. It is known that, depending on the dose, ATX causes membrane disruption and cytolysis or only limited hydrolysis of its substrates. In the latter case, toxin activity leads to the unregulated generation of bioactive lipids that can ultimately induce cell death. We have characterized apoptosis and necrosis in highly ATX-sensitive, ganglioside-deficient cells exposed to different concentrations of ATX and we have studied the lipidomic profile of cells treated with ATX as compared to native cells to detect the main changes in the lipidomic profile and the possible involvement of lipid signals in cell death. ATX causes both apoptosis and necrosis, depending on dose and time. ATX activates cell death, stimulating the release of cytochrome C from mitochondria and the consequent activation of caspases-3. Moreover GM95 cells treated with ATX showed important lipidomic alterations, among them we detected a general decrease in several phospholipid species and important changes in lipids involved in programmed cell death e.g. ceramide. The data suggest two different mechanisms of cell death caused by ATX, one leading to (mainly saturated) glycerophospholipid hydrolysis related to an increase in diacylglycerols and associated to membrane damage and necrosis, and a second mechanism involving chiefly sphingomyelin hydrolysis and generation of proapoptotic lipidic mediators such as ceramide, N-acylethanolamine and saturated non-esterified fatty acids.

Membrane-Binding Mechanism of Clostridium perfringens Alpha-Toxin

Clostridium perfringens alpha-toxin is a key mediator of gas gangrene, which is a life-threatening infection that manifests as fever, pain, edema, myonecrosis, and gas production. Alpha-toxin possesses phospholipase C and sphingomyelinase activities. The toxin is composed of an N-terminal domain (1-250 aa, N-domain), which is the catalytic site, and a C-terminal domain (251-370 aa, C-domain), which is the membrane-binding site. Immunization of mice with the C-domain of alpha-toxin prevents the gas gangrene caused by C. perfringens, whereas immunization with the N-domain has no effect. The central loop domain (55-93 aa), especially H….SW(84)Y(85)….G, plays an important role in the interaction with ganglioside GM1a. The toxin binds to lipid rafts in the presence of a GM1a/TrkA complex, and metabolites from phosphatidylcholine to diacylglycerol through the enzymatic activity of alpha-toxin itself. These membrane dynamics leads to the activation of endogenous PLCγ-1 via TrkA. In addition, treatment with alpha-toxin leads to the formation of diacylglycerol at membrane rafts in ganglioside-deficient DonQ cells; this in turn triggers endocytosis and cell death. This article summarizes the current the membrane-binding mechanism of alpha-toxin in detail.

Sphingosine-1-phosphate signaling: unraveling its role as a drug target against infectious diseases

Sphingosine-1-phosphate (S1P) signaling is reported in variety of cell types, including immune, endothelial and cancerous cells. It is emerging as a crucial regulator of cellular processes, such as apoptosis, cell proliferation, migration, differentiation and so on. This signaling pathway is initiated by the intracellular production and secretion of S1P through a cascade of enzymatic reactions. Binding of S1P to different S1P receptors (S1PRs) activates different downstream signaling pathways that regulate the cellular functions differentially depending upon the cell type. An accumulating body of evidence suggests that S1P metabolism and signaling is often impaired during infectious diseases; thus, its manipulation might be helpful in the treatment of such diseases. In this review, we summarize recent advances in our understanding of the S1P signaling pathway and its candidature as a novel drug target against infectious diseases.

Clostridium perfringens Alpha-Toxin Induces Gm1a Clustering and Trka Phosphorylation in the Host Cell Membrane

Clostridium perfringens alpha-toxin elicits various immune responses such as the release of cytokines, chemokines, and superoxide via the GM1a/TrkA complex. Alpha-toxin possesses phospholipase C (PLC) hydrolytic activity that contributes to signal transduction in the pathogenesis of gas gangrene. Little is known about the relationship between lipid metabolism and TrkA activation by alpha-toxin. Using live-cell fluorescence microscopy, we monitored transbilayer movement of diacylglycerol (DAG) with the yellow fluorescent protein-tagged C1AB domain of protein kinase C-γ (EYFP-C1AB). DAG accumulated at the marginal region of the plasma membrane in alpha toxin-treated A549 cells, which also exhibited GM1a clustering and TrkA phosphorylation. Annexin V binding assays showed that alpha-toxin induced the exposure of phosphatidylserine on the outer leaflet of the plasma membrane. However, H148G, a variant toxin which binds cell membrane and has no enzymatic activity, did not induce DAG translocation, GM1a clustering, or TrkA phosphorylation. Alpha-toxin also specifically activated endogenous phospholipase Cγ-1 (PLCγ-1), a TrkA adaptor protein, via phosphorylation. U73122, an endogenous PLC inhibitor, and siRNA for PLCγ-1 inhibited the formation of DAG and release of IL-8. GM1a accumulation and TrkA phosphorylation in A549 cells treated with alpha-toxin were also inhibited by U73122. These results suggest that the flip-flop motion of hydrophobic lipids such as DAG leads to the accumulation of GM1a and TrkA. We conclude that the formation of DAG by alpha-toxin itself (first step) and activation of endogenous PLCγ-1 (second step) leads to alterations in membrane dynamics, followed by strong phosphorylation of TrkA.

Clostridium perfringens phospholipase C induced ROS production and cytotoxicity require PKC, MEK1 and NFκB activation

Clostridium perfringens phospholipase C (CpPLC), also called α-toxin, is the most toxic extracellular enzyme produced by this bacteria and is essential for virulence in gas gangrene. At lytic concentrations, CpPLC causes membrane disruption, whereas at sublytic concentrations this toxin causes oxidative stress and activates the MEK/ERK pathway, which contributes to its cytotoxic and myotoxic effects. In the present work, the role of PKC, ERK 1/2 and NFκB signalling pathways in ROS generation induced by CpPLC and their contribution to CpPLC-induced cytotoxicity was evaluated. The results demonstrate that CpPLC induces ROS production through PKC, MEK/ERK and NFκB pathways, the latter being activated by the MEK/ERK signalling cascade. Inhibition of either of these signalling pathways prevents CpPLC's cytotoxic effect. In addition, it was demonstrated that NFκB inhibition leads to a significant reduction in the myotoxicity induced by intramuscular injection of CpPLC in mice. Understanding the role of these signalling pathways could lead towards developing rational therapeutic strategies aimed to reduce cell death during a clostridialmyonecrosis.

In silico, in vitro and in vivo analysis of binding affinity between N and C-domains of Clostridium perfringens alpha toxin

Clostridium perfringens alpha toxin/phospholipase C (CP-PLC) is one of the most potent bacterial toxins known to cause soft tissue infections like gas gangrene in humans and animals. It is the first bacterial toxin demonstrated to be an enzyme with phospholipase, sphingomyelinase and lecithinase activities. The toxin is comprised of an enzymatic N-domain and a binding C-domain interconnected by a flexible linker. The N-domain alone is non-toxic to mammalian cells, but incubation with C-domain restores the toxicity, the mechanism of which is still not elucidated. The objectives of the current study were to investigate the formation of a stable N and C-domain complex, to determine possible interactions between the two domains in silico and to characterize the in vitro and in vivo correlates of the interaction. To establish the existence of a stable N and C-domain hybrid, in vitro pull down assay and dot-Far Western blotting assays were employed, where it was clearly revealed that the two domains bound to each other to form an intermediate. Using bioinformatics tools like MetaPPISP, PatchDock and FireDock, we predicted that the two domains may interact with each other through electrostatic interactions between at least six pairs of amino acids. This N and C-domains interacted with each other in 1:1 ratio and the hybrid lysed mouse erythrocytes in a slower kinetics when compared with wild type native Cp-PLC. BALB/c mice when challenged with N and C-domain hybrid demonstrated severe myonecrosis at the site of injection while no death was observed. Our results provide further insight into better understanding the mechanism for the toxicity of Cp-PLC N and C-domain mixture.

Reactive oxygen species and the MEK/ERK pathway are involved in the toxicity of clostridium perfringens α-toxin, a prototype bacterial phospholipase C

Clostridium perfringens, the most broadly distributed pathogen in nature, produces a prototype phospholipase C, also called α-toxin, which plays a key role in the pathogenesis of gas gangrene. α-Toxin causes plasma membrane disruption at high concentrations, but the role of intracellular mediators in its toxicity at low concentrations is unknown. This work demonstrates that α-toxin causes oxidative stress and activates the MEK/ERK pathway in cultured cells and furthermore provides compelling evidence that O(2)(-.), hydrogen peroxide, and the OH(.) radical are involved in its cytotoxic and myotoxic effects. The data show that antioxidants and MEK1 inhibitors reduce the cytotoxic and myotoxic effects of α-toxin and demonstrate that edaravone, a clinically used hydroxyl radical trap, reduces the myonecrosis and the mortality caused by an experimental infection with C. perfringens in a murine model of gas gangrene. This knowledge provides new insights for the development of novel therapies to reduce tissue damage during clostridial myonecrosis.

Clostridium perfringens alpha-toxin recognizes the GM1a-TrkA complex

Clostridium perfringens alpha-toxin is the major virulence factor in the pathogenesis of gas gangrene. Alpha-toxin is a 43-kDa protein with two structural domains; the N-domain contains the catalytic site and coordinates the divalent metal ions, and the C-domain is a membrane-binding site. The role of the exposed loop region (72-93 residues) in the N-domain, however, has been unclear. Here we show that this loop contains a ganglioside binding motif (H … SXWY … G) that is the same motif seen in botulinum neurotoxin and directly binds to a specific conformation of the ganglioside Neu5Acα2-3(Galβ1-3GalNAcβ1-4)Galβ1-4Glcβ1Cer (GM1a) through a carbohydrate moiety. Confocal microscopy analysis using fluorescently labeled BODIPY-GM1a revealed that the toxin colocalized with GM1a and induced clustering of GM1a on the cell membranes. Alpha-toxin was only slightly toxic in β1,4-N-acetylgalactosaminyltransferase knock-out mice, which lack the a-series gangliosides that contain GM1a, but was highly toxic in α2,8-sialyltransferase knock-out mice, which lack both b-series and c-series gangliosides, similar to the control mice. Moreover, experiments with site-directed mutants indicated that Trp-84 and Tyr-85 in the exposed alpha-toxin loop play an important role in the interaction with GM1a and subsequent activation of TrkA. These results suggest that binding of alpha-toxin to GM1a facilitates the activation of the TrkA receptor and induces a signal transduction cascade that promotes the release of chemokines. Therefore, we conclude that GM1a is the primary cellular receptor for alpha-toxin, which can be a potential target for drug developed against this pathogen.

Role of sphingomyelinase in infectious diseases caused by Bacillus cereus

Bacillus cereus (B. cereus) is a pathogen in opportunistic infections. Here we show that Bacillus cereus sphingomyelinase (Bc-SMase) is a virulence factor for septicemia. Clinical isolates produced large amounts of Bc-SMase, grew in vivo, and caused death among mice, but ATCC strains isolated from soil did not. A transformant of the ATCC strain carrying a recombinant plasmid containing the Bc-SMase gene grew in vivo, but that with the gene for E53A, which has little enzymatic activity, did not. Administration of an anti-Bc-SMase antibody and immunization against Bc-SMase prevented death caused by the clinical isolates, showing that Bc-SMase plays an important role in the diseases caused by B. cereus. Treatment of mouse macrophages with Bc-SMase resulted in a reduction in the generation of H₂O₂ and phagocytosis of macrophages induced by peptidoglycan (PGN), but no effect on the release of TNF-α and little release of LDH under our experimental conditions. Confocal laser microscopy showed that the treatment of mouse macrophages with Bc-SMase resulted in the formation of ceramide-rich domains. A photobleaching analysis suggested that the cells treated with Bc-SMase exhibited a reduction in membrane fluidity. The results suggest that Bc-SMase is essential for the hydrolysis of SM in membranes, leading to a reduction in phagocytosis.

The relationship between calcium and the metabolism of plasma membrane phospholipids in hemolysis induced by brown spider venom phospholipase-D toxin

Brown spider venom phospholipase-D belongs to a family of toxins characterized as potent bioactive agents. These toxins have been involved in numerous aspects of cell pathophysiology including inflammatory response, platelet aggregation, endothelial cell hyperactivation, renal disorders, and hemolysis. The molecular mechanism by which these toxins cause hemolysis is under investigation; literature data have suggested that enzyme catalysis is necessary for the biological activities triggered by the toxin. However, the way by which phospholipase-D activity is directly related with human hemolysis has not been determined. To evaluate how brown spider venom phospholipase-D activity causes hemolysis, we examined the impact of recombinant phospholipase-D on human red blood cells. Using six different purified recombinant phospholipase-D molecules obtained from a cDNA venom gland library, we demonstrated that there is a correlation of hemolytic effect and phospholipase-D activity. Studying recombinant phospholipase-D, a potent hemolytic and phospholipase-D recombinant toxin (LiRecDT1), we determined that the toxin degrades synthetic sphingomyelin (SM), lysophosphatidylcholine (LPC), and lyso-platelet-activating factor. Additionally, we determined that the toxin degrades phospholipids in a detergent extract of human erythrocytes, as well as phospholipids from ghosts of human red blood cells. The products of the degradation of synthetic SM and LPC following recombinant phospholipase-D treatments caused hemolysis of human erythrocytes. This hemolysis, dependent on products of metabolism of phospholipids, is also dependent on calcium ion concentration because the percentage of hemolysis increased with an increase in the dose of calcium in the medium. Recombinant phospholipase-D treatment of human erythrocytes stimulated an influx of calcium into the cells that was detected by a calcium-sensitive fluorescent probe (Fluo-4). This calcium influx was shown to be channel-mediated rather than leak-promoted because the influx was inhibited by L-type calcium channel inhibitors but not by a T-type calcium channel blocker, sodium channel inhibitor or a specific inhibitor of calcium activated potassium channels. Finally, this inhibition of hemolysis following recombinant phospholipase-D treatment occurred in a concentration-dependent manner in the presence of L-type calcium channel blockers such as nifedipine and verapamil. The data provided herein, suggest that the brown spider venom phospholipase-D-induced hemolysis of human erythrocytes is dependent on the metabolism of membrane phospholipids, such as SM and LPC, generating bioactive products that stimulate a calcium influx into red blood cells mediated by the L-type channel.

Induction of neurite-outgrowth in PC12 cells by alpha-toxin from Clostridium perfringens

Alpha-toxin-induced phosphorylation of PDK1 via the tyrosine kinase A (TrkA) receptor signaling pathway plays an important role in the activation of rabbit neutrophils. The relation between the toxin and TrkA, however, remains poorly understood. Here, we show that the toxin-induced phosphorylation of TrkA is closely related to the induction of neurite-outgrowth in PC12 cells. The toxin induced neurite-outgrowth and phosphorylation of TrkA in the cells in a dose-dependent manner. K252a, a TrkA inhibitor, and shRNA for TrkA inhibited the toxin-induced neurite-outgrowth, and phosphorylation of TrkA and ERK1/2. PD98059, an inhibitor of the ERK1/2 cascade, inhibited phosphorylation of ERK1/2 and the neurite-outgrowth induced by alpha-toxin. The wild-type toxin induced the formation of diacylglycerol, and neurite-outgrowth, but H148G, a variant toxin which binds to cell membranes and has lost the enzymatic activity did not. We demonstrated that the phosphorylation of TrkA through the phospholipid metabolism induced by the toxin synergistically play a key role in neurite-outgrowth.

Effect of erythromycin on biological activities induced by clostridium perfringens alpha-toxin

Clostridium perfringens alpha-toxin, an important agent of gas gangrene with inflammatory myopathies, possesses lethal, hemolytic, and necrotic activities. Here, we show that alpha-toxin-induced lethality in mice was inhibited by i.v. preadministration of erythromycin (ERM). Administration of ERM resulted in a drastic reduction in the release of tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, and IL-6 and systemic hemolysis induced by alpha-toxin, whereas the administration of kitasamycin did not. Furthermore, the lethality and systemic hemolysis caused by alpha-toxin were blocked by the preinjection of anti-TNF-alpha, but not the anti-IL-1beta- or anti-IL-6-antibody. In addition, TNF-alpha-deficient mice were resistant to alpha-toxin, indicating that TNF-alpha plays an important role in the lethality. ERM inhibited the toxin-induced release of TNF-alpha from neutrophils and phosphorylation of toropomyosin-related kinase receptor A (TrkA) and extracellular-regulated kinase (ERK) 1/2. Furthermore, K252a, a TrkA inhibitor, and PD98059 (2'-amino-3'-methoxyflavone), an ERK1/2 inhibitor, inhibited the toxin-induced release of TNF-alpha from neutrophils. The observation shows that the toxin-induced release of TNF-alpha is dependent on the activation of ERK/mitogen-activated protein kinase signal transduction via TrkA in neutrophils and that ERM specifically blocks the toxin-induced events through the activation of neutrophils.

Ceramide-enriched membrane domains in red blood cells and the mechanism of sphingomyelinase-induced hot-cold hemolysis

Hot-cold hemolysis is the phenomenon whereby red blood cells, preincubated at 37 degrees C in the presence of certain agents, undergo rapid hemolysis when transferred to 4 degrees C. The mechanism of this phenomenon is not understood. PlcHR 2, a phospholipase C/sphingomyelinase from Pseudomonas aeruginosa, that is the prototype of a new phosphatase superfamily, induces hot-cold hemolysis. We found that the sphingomyelinase, but not the phospholipase C activity, is essential for hot-cold hemolysis because the phenomenon occurs not only in human erythrocytes that contain both phosphatidylcholine (PC) and sphingomyelin (SM) but also in goat erythrocytes, which lack PC. However, in horse erythrocytes, with a large proportion of PC and almost no SM, hot-cold hemolysis induced by PlcHR 2 is not observed. Fluorescence microscopy observations confirm the formation of ceramide-enriched domains as a result of PlcHR 2 activity. After cooling down to 4 degrees C, the erythrocyte ghost membranes arising from hemolysis contain large, ceramide-rich domains. We suggest that formation of these rigid domains in the originally flexible cell makes it fragile, thus highly susceptible to hemolysis. We also interpret the slow hemolysis observed at 37 degrees C as a phenomenon of gradual release of aqueous contents, induced by the sphingomyelinase activity, as described by Ruiz-Arguello et al. [(1996) J. Biol. Chem. 271, 26616]. These hypotheses are supported by the fact that ceramidase, which is known to facilitate slow hemolysis at 37 degrees C, actually hinders hot-cold hemolysis. Differential scanning calorimetry of erytrocyte membranes treated with PlcHR 2 demonstrates the presence of ceramide-rich domains that are rigid at 4 degrees C but fluid at 37 degrees C. Ceramidase treatment causes the disapperance of the calorimetric signal assigned to ceramide-rich domains. Finally, in liposomes composed of SM, PC, and cholesterol, which exhibit slow release of aqueous contents at 37 degrees C, addition of 10 mol % ceramide and transfer to 4 degrees C cause a large increase in the rate of solute efflux.

"Inside-out" signaling of sphingosine-1-phosphate: therapeutic targets

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid metabolite involved in many critical cellular processes including proliferation, survival, and migration, as well as angiogenesis and allergic responses. S1P levels inside cells are tightly regulated by the balance between its synthesis by sphingosine kinases and degradation. S1P is interconvertible with ceramide, which is a critical mediator of apoptosis. It has been postulated that the ratio between S1P and ceramide determines cell fate. Activation of sphingosine kinase by a variety of agonists increases intracellular S1P, which in turn can function intracellularly as a second messenger or be secreted out of the cell and act extracellularly by binding to and signaling through S1P receptors in autocrine and/or paracrine manners. Recent studies suggest that this "inside-out" signaling by S1P may play a role in many human diseases, including cancer, atherosclerosis, inflammation, and autoimmune disorders such as multiple sclerosis. In this review we summarize metabolism of S1P, mechanisms of sphingosine kinase activation, and S1P receptors and their downstream signaling pathways and examine relationships to multiple disease processes. In particular, we describe recent preclinical and clinical trials of therapies targeting S1P signaling, including 2-amino-2-propane-1,3-diol hydrochloride (FTY720, fingolimod), S1P receptor agonists, sphingosine kinase inhibitors, and anti-S1P monoclonal antibody.

Effect of unsaturated bonds in the sn-2 acyl chain of phosphatidylcholine on the membrane-damaging action of Clostridium perfringens alpha-toxin toward liposomes

Clostridium perfringens alpha-toxin degrades phosphatidylcholine (PC) in the bilayer of liposomes and destroys the membrane. The effect of the type and position of unsaturation in the fatty acyl chain of PC (18:0/18:1 PC) synthesized on the toxin-induced leakage of carboxyfluorescein (CF) from PC liposomes was examined. Differential scanning calorimetry showed that the phase transition temperature (T(m)) was minimal when the triple bond was positioned at C (9) in the sn-2 acyl chain. The toxin-induced CF leakage decreased with the migration of the bond from C (9) to either end of the acyl chain in PC. The PC containing the cis-double bond had a similar T(m) to that with the triple bond, but a lower value than the PC containing the trans-double bond. Furthermore, the toxin-induced leakage from liposomes composed of PC containing the cis-double bond resembled that with PC having the triple bond and was greater than that from liposomes with PC having the trans-double bond. The binding of a H148G mutant to PC liposomes showed a reciprocal relationship in terms of the T(m) value of PC containing the triple bond. These results indicate that the toxin-induced membrane damage is closely related to membrane fluidity in liposomes.

Signal transduction mechanism involved in Clostridium perfringens alpha-toxin-induced superoxide anion generation in rabbit neutrophils

Clostridium perfringens alpha-toxin induces the generation of superoxide anion (O2(-)) via production of 1,2-diacylglycerol (DG) in rabbit neutrophils. The mechanism of the generation, however, remains poorly understood. Here we report a novel mechanism for the toxin-induced production of O2(-) in rabbit neutrophils. Treatment of the cells with the toxin resulted in tyrosine phosphorylation of a protein of about 140 kDa. The protein reacted with anti-TrkA (nerve growth factor high-affinity receptor) antibody and bound nerve growth factor. Anti-TrkA antibody inhibited the production of O2(-) and binding of the toxin to the protein. The toxin induced phosphorylation of 3-phosphoinositide-dependent protein kinase 1 (PDK1). K252a, an inhibitor of TrkA receptor, and LY294002, an inhibitor of phosphatidylinositol 3-kinase (PI3K), reduced the toxin-induced production of O2(-) and phosphorylation of PDK1, but not the formation of DG. These inhibitors inhibited the toxin-induced phosphorylation of protein kinase C theta (PKCtheta). U73122, a phospholipase C (PLC) inhibitor, and pertussis toxin inhibited the toxin-induced generation of O2(-) and formation of DG, but not the phosphorylation of PDK1. These observations show that the toxin independently induces production of DG through activation of endogenous PLC and phosphorylation of PDK1 via the TrkA receptor signaling pathway and that these events synergistically activate PKCtheta in stimulating an increase in O2(-). In addition, we show the participation of mitogen-activated protein kinase-associated signaling events via activation of PKCtheta in the toxin-induced generation of O2(-).

Structural basis of the sphingomyelin phosphodiesterase activity in neutral sphingomyelinase from Bacillus cereus

Sphingomyelinase (SMase) from Bacillus cereus (Bc-SMase) hydrolyzes sphingomyelin to phosphocholine and ceramide in a divalent metal ion-dependent manner. Bc-SMase is a homologue of mammalian neutral SMase (nSMase) and mimics the actions of the endogenous mammalian nSMase in causing differentiation, development, aging, and apoptosis. Thus Bc-SMase may be a good model for the poorly characterized mammalian nSMase. The metal ion activation of sphingomyelinase activity of Bc-SMase was in the order Co2+ > or = Mn2+ > or = Mg2+ >> Ca2+ > or = Sr2+. The first crystal structures of Bc-SMase bound to Co2+, Mg2+, or Ca2+ were determined. The water-bridged double divalent metal ions at the center of the cleft in both the Co2+- and Mg2+-bound forms were concluded to be the catalytic architecture required for sphingomyelinase activity. In contrast, the architecture of Ca2+ binding at the site showed only one binding site. A further single metal-binding site exists at one side edge of the cleft. Based on the highly conserved nature of the residues of the binding sites, the crystal structure of Bc-SMase with bound Mg2+ or Co2+ may provide a common structural framework applicable to phosphohydrolases belonging to the DNase I-like folding superfamily. In addition, the structural features and site-directed mutagenesis suggest that the specific beta-hairpin with the aromatic amino acid residues participates in binding to the membrane-bound sphingomyelin substrate.

Cloning of α-β fusion gene from Clostridium perfringens and its expression

AIM: To study the cloning of α-β fusion gene from Clostridium perfringens and the immunogenicity of α-β fusion expression.

METHODS: Cloning was accomplished after PCR amplification from strains NCTC64609 and C58-1 of the protective antigen genes of α-toxin and β-toxin. The fragment of the gene was cloned using plasmid pZCPAB. This fragment coded for the gene with the stable expression of α-β fusion gene binding. In order to verify the exact location of the α-β fusion gene, domain plasmids were constructed. The two genes were fused into expression vector pBV221. The expressed α-β fusion protein was identified by ELISA, SDS-PAGE, Western blotting and neutralization assay.

RESULTS: The protective α-toxin gene (cpa906) and the β-toxin gene (cpb930) were obtained. The recombinant plasmid pZCPAB carrying α-β fusion gene was constructed and transformed into BL21(DE3). The recombinant strain BL21(DE3)(pZCPAB) was obtained. After the recombinant strain BL21(DE3)(pZCPAB) was induced by 42°C，its expressed product was about 22.14% of total cellular protein at SDS-PAGE and thin-layer gel scanning analysis. Neutralization assay indicated that the antibody induced by immunization with α-β fusion protein could neutralize the toxicity of α-toxin and β-toxin.

CONCLUSION: The obtained α-toxin and β-toxin genes are correct. The recombinant strain BL21(DE3)(pZCPAB) could produce α-β fusion protein. This protein can be used for immunization and is immunogenic. The antibody induced by immunization with α-β fusion protein could neutralize the toxicity of α-toxin and β-toxin.

Immunization with an alphatoxin variant 121A/91-R212H protects mice against Clostridium perfringens alphatoxin

As shown previously, a recombinant alphatoxin variant (rAT121A/91) constructed from the naturally occurring Clostridium perfringens mutant strain 121A/91, was devoid of enzymatic (PLC), hemolytic and lethal activity (18). In the present study, the recombinant variant was altered by an oligonucleotide-directed reversion of an arginine in position 212 for a histidine residue, corresponding to the sequence of the wild-type alphatoxin. The new variant rAT121A/91R212H proved to be negative in enzymatic, hemolytic and lethal activity as well. RAT121A/91 as well as rAT121A/91R212H was used for i.p. immunization of balb/c mice. The immune response was studied in ELISA as well as in the mouse neutralization test. Furthermore, immunized mice were challenged by i.p. application of active C. perfringens alphatoxin. In all immunized groups, mice developed high anti-alphatoxin titers (up to 1:128000). Antisera of both groups were able to reduce the hemolytic effect of native alphatoxin with predominance of anti-rAT121A/91R212H sera. During neutralization experiments, mice receiving a mixture of anti-rAT121A/91R212H and wild-type toxin were protected completely, whereas an anti-rAT121A/91/toxin mixture prolonged time until death but failed in protection. I.p immunization with rAT121A/91R212H yielded a significant protection rate (76%) when mice were challenged intraperitoneal with wild-type toxin. Our cumulative data indicates that the reversion of arginine in position 212 to histidine for rAT121A/91R212H was necessary to induce production of protective antibodies against wild-type alphatoxin of C. perfringens.

Role of tyrosine-57 and -65 in membrane-damaging and sphingomyelinase activities of Clostridium perfringens alpha-toxin

Clostridium perfringens alpha-toxin (370 residues) is a major virulence factor in the pathogenesis of gas gangrene. The toxin is composed of an N-terminal domain (1-250 residues) where lies the catalytic site and a C-terminal domain (251-370 residues), the Ca(2+)-binding domain, responsible for binding to membranes. The role of Tyr-57 and Tyr-65 close to the catalytic pocket (site) in the N-domain was investigated. Replacement of Tyr-57 and -65 with alanine, leucine, or phenylalanine did not affect the sphingomyelinase activity of the toxin for sodium deoxycholate-solubilized shingomyelin. However, the substitution of Tyr-57 and -65 with alanine or leucine resulted in a radical reduction in the hemolysis of sheep erythrocytes, the release of carboxyfluorescein from shingomyelin-cholesterol (1:1) liposomes, and a significant decrease in binding to the liposomes. The binding of variant toxins, Y57C/C169L and Y65C/C169L, labeled with the environmentally sensitive fluorophore, acrylodan, to the liposomes suggested insertion of the variants in a hydrophobic environment in the bilayer. These observations suggested that Tyr-57 and -65 play a role in the penetration of the toxin into the bilayer of membranes and access of the catalytic site to sphingomyelin in membranes, but do not participate in the enzymatic activity.

Involvement of neutral ceramidase in ceramide metabolism at the plasma membrane and in extracellular milieu

Neutral ceramidase is a type II integral membrane protein, which is occasionally secreted into the extracellular milieu after the processing of its N-terminal anchor. We found that when overexpressed in CHOP cells, neutral ceramidase hydrolyzed cell surface ceramide, which increased in amount after the treatment of cells with bacterial sphingomyelinase, leading to an increase in the cellular level of sphingosine and sphingosine 1-phosphate. On the other hand, knockdown of the endogenous enzyme by siRNA decreased the cellular level of both sphingolipid metabolites. The treatment of cells with bovine serum albumin significantly reduced the cellular level of sphingosine, but not sphingosine 1-phosphate, generated by overexpression of the enzyme. The cellular level of sphingosine 1-phosphate increased with overexpression of the cytosolic sphingosine kinase. These results suggest that sphingosine 1-phosphate is mainly produced inside of the cell after the incorporation of sphingosine generated on the plasma membranes. The enzyme also seems to participate in the hydrolysis of serum-derived ceramide in the vascular system. Significant amounts of sphingosine as well as sphingosine 1-phosphate were generated in the cell-free conditioned medium of ceramidase transfectants, compared with mock transfectants. No increase in these metabolites was observed if serum or bacterial sphingomyelinase was omitted from the conditioned medium, suggesting that the major source of ceramide is the serum-derived sphingomyelin. A sphingosine 1-phosphate receptor, S1P(1), was internalized much faster by the treatment of S1P(1)-overexpressing cells with conditioned medium of ceramidase transfectants than that of mock transfectants. Collectively, these results clearly indicate that the enzyme is involved in the metabolism of ceramide at the plasma membrane and in the extracellular milieu, which could regulate sphingosine 1-phosphate-mediated signaling through the generation of sphingosine.