Clostridium perfringens α-toxin up-regulates plasma membrane CD11b expression on murine neutrophils by changing intracellular localization

Gas gangrene caused by Clostridium perfringens type A infection is a highly lethal infection of soft tissue characterized by rapid spread of tissue necrosis. This tissue destruction is related to profound attenuation of blood flow accompanied by formation of platelet-leukocyte aggregates in the blood vessels. Several studies have identified α-toxin, which has both sphingomyelinase and phospholipase C activities, as a major virulence factor in the aggregate formation via activation of the platelet gpIIbIIIa. Here, we show that α-toxin greatly and rapidly increases plasma membrane localization of CD11b, which binds to the platelet gpIIbIIIa via fibrinogen, in mouse neutrophils. Interestingly, short-term treatment of α-toxin has little effect on gene expression profiles in neutrophils, and the toxin does not change the total protein expression levels of CD11b in whole cell lysates. The following analysis demonstrated that CD11b localizes to intracellular vesicles in intact cells, but the localization changed to the cytoplasmic membrane in α-toxin-treated cells. These results suggest that CD11b is recruited to the cytoplasmic membrane by α-toxin. Previously, we reported that α-toxin promotes the formation of ceramide by its sphingomyelinase activity in mouse neutrophils. Interestingly, a synthetic cell-permeable ceramide analog, C₂-ceramide, increases plasma membrane localization of CD11b, suggesting that ceramide production by α-toxin recruits CD11b to the cytoplasmic membrane to promote platelet-leukocyte aggregation. Together, our results illustrate that the increase of cell membrane CD11b expression by α-toxin might be crucial for the pathogenesis of C. perfringens to promote formation of platelet-leukocyte aggregates, leading to rapid tissue necrosis due to ischemia.

Simultaneous action of microbial phospholipase C and lipase on model bacterial membranes - Modeling the processes crucial for bioaugmentation

Bioaugmentation is a promising method of the remediation of soils polluted by persistent organic pollutants (POP). Unfortunately, it happens frequently that the microorganisms inoculated into the soil die out due to the presence of enzymes secreted by autochthonous microorganisms. Especially destructive are here phospholipases C (PLC) and lipases which destruct the microorganism's cellular membrane. The composition of bacterial membranes differs between species, so it is highly possible that depending on the membrane constitution some bacteria are more resistant to PLCs and lipases than other. To shed light on these problems we applied phospholipid Langmuir monolayers as model microbial membranes and studied their interactions with α-toxin (model bacterial PLC) and the lipase isolated from soil fungus Candida rugosa. Membrane phospholipids differing in their headgroup (phosphatidylcholines, phosphatidylethanolamines, phosphatidylglycerols and cardiolipins) and in their tail structure were applied. The monolayers were characterized by the Langmuir technique, visualized by Brewster angle microscopy, and the packing mode of the phospholipid molecules was verified by the application of the diffraction of synchrotron radiation. We also studied the mutual miscibility of diacylglycerols and the native phospholipids as their interaction is crucial for the understanding of the PLC and lipase activity. It turned out that all the investigated phospholipid classes can be hydrolyzed by PLC; however, they differ profoundly in the hydrolysis degree. Depending on the effects of the initial PLC action and the mutual organization of the diacylglycerol and phospholipid molecules the lipase can ruin the model membranes or can be completely neutral to them.

Toll-Like Receptor 4 Protects Against Clostridium perfringens Infection in Mice

Toll-like receptor 4 (TLR4) has been reported to protect against Gram-negative bacteria by acting as a pathogen recognition receptor that senses mainly lipopolysaccharide (LPS) from Gram-negative bacteria. However, the role of TLR4 in Gram-positive bacterial infection is less well understood. Clostridium perfringens type A is a Gram-positive bacterium that causes gas gangrene characterized by severe myonecrosis. It was previously demonstrated that C. perfringens θ-toxin is a TLR4 agonist, but the role of TLR4 in C. perfringens infection is unclear. Here, TLR4-defective C3H/HeJ mice infected with C. perfringens showed a remarkable decrease in survival rate, an increase in viable bacterial counts, and accelerated destruction of myofibrils at the infection site compared with wild-type C3H/HeN mice. These results demonstrate that TLR4 plays an important role in the elimination of C. perfringens. Remarkable increases in levels of inflammatory cytokines, such as interleukin-1β (IL-1β), interleukin-6 (IL-6), and granulocyte colony-stimulating factor (G-CSF), were observed in C. perfringens-infected C3H/HeN mice, whereas the increases were limited in C3H/HeJ mice. Generally, increased G-CSF accelerates granulopoiesis in the bone marrow and the spleen to exacerbate neutrophil production, resulting in elimination of bacteria. The number of neutrophils in the spleen was increased in C. perfringens-infected C3H/HeN mice compared with non-infected mice, while the increase was lower in C. perfringens-infected C3H/HeJ mice. Furthermore, DNA microarray analysis revealed that the mutation in TLR4 partially affects host gene expression during C. perfringens infection. Together, our results illustrate that TLR4 is crucial for the innate ability to eliminate C. perfringens.

Clostridium perfringens phospholipase C impairs innate immune response by inducing integrated stress response and mitochondrial-induced epigenetic modifications

Clostridium perfringens, a rod-shaped, gram-positive, anaerobic, spore-forming bacterium is one of the most widely occurring bacterial pathogens, associated with a spectrum of diseases in humans. A major virulence factor during its infection is the enzyme phospholipase C encoded by the plc gene, known as Clostridium perfringens phospholipase C (CpPLC). The present study was designed to understand the role of CpPLC in inducing survival mechanisms and mitochondrial-induced epigenetic changes in a human lymphocyte cell culture model. Following exposure to CpPLC, a significant generation of mitochondrial reactive oxygen species was observed, which coincided with the changes in the expression of vital components of MAP/ERK/RTK signaling cascade that regulates the downstream cellular functions. These disturbances further led to alterations in the mitochondrial genome and functioning. This was supported by the observed upregulation in the expression of mitochondrial fission genes Drp1, Fis1, and Mff, and mitochondrial fusion genes MFN1, MFN2, and OPA1 following CpPLC exposure. CpPLC exposed cells showed upregulation of OMA1, DELE1, and HRI genes involved in the integrated stress response (ISR), which suggests that it may induce the ISR that provides a pro-survival mechanism to the host cell. CpPLC also initiated immune patho-physiologic mechanisms including mitochondrial-induced epigenetic modifications through a mitochondrial-ROS driven signaling pathway. Interestingly, epigenetic machinery not only play a pivotal role in lymphocyte homeostasis by contributing to cell-fate decisions but thought to be one of the mechanisms by which intracellular pathogens survive within the host cells. Importantly, the impairment of mtDNA repair among the CpPLC exposed cells, induced alterations within mtDNA methylation, and led to the deregulation of MT-CO1, MT-ND6, MT-ATPase 6, and MT-ATPase8 gene expression profiles that are important for mitochondrial bioenergetics and subsequent metabolic pathways. This was further confirmed by the changes in the activity of mitochondrial electron chain complexes (complex I, II, III, IV and V). The altered mtDNA methylation profile was also found to be closely associated with the varied expression of mitomiRs and their targets. CpPLC exposed cells showed up-regulation of miR24 expression and down-regulation of miR34a, miR150, and miR155, while the increased expression of mitomiR target genes i.e. of K-Ras, MYC, EGFR, and NF-kβ was also observed in these cells. Altogether, our findings provide novel insights into the derailment of redox signaling machinery in CpPLC treated lymphocytes and its role in the induction of survival mechanisms and mitochondrial-induced epigenetic modifications.

Clostridium perfringens α-toxin inhibits myogenic differentiation of C2C12 myoblasts

Clostridium perfringens type A is the causative agent of clostridial myonecrosis, and α-toxin has been reported to be responsible for the pathogenesis. Recently, it was reported that regeneration of skeletal muscle after C. perfringens-induced muscle disorders is delayed, but the detailed mechanisms have not been elucidated. Here, we tested whether α-toxin impairs the differentiation of C2C12 myoblasts, a useful cell line to study muscle growth, maturation, and regeneration in vitro. α-Toxin dose-dependently inhibited myotube formation in C2C12 cultures after induction of their differentiation by horse serum. Also, immunoblot analysis revealed that α-toxin dose-dependently decreases the expressions of two skeletal muscle differentiation markers, myogenic differentiation 1 (MyoD) and myogenin. These results demonstrate that α-toxin impairs the myogenic differentiation of C2C12 myoblasts. To reveal the mechanism behind α-toxin-mediated impairment of myogenic differentiation, we focused on ceramide production since α-toxin is known to promote the formation of ceramide by its sphingomyelinase activity. Immunofluorescent analysis revealed that ceramide production is accelerated by treatment with α-toxin. Furthermore, a synthetic cell-permeable ceramide analog, C₂-ceramide, inhibited myotube formation in C2C12 cells and decreased the expressions of MyoD and myogenin, suggesting that accelerated ceramide production is involved in the α-toxin-mediated blockage of myogenic differentiation. Together, our results illustrate that the impairment of myogenic differentiation by α-toxin might be crucial for the pathogenesis of C. perfringens to delay regeneration of severely damaged skeletal muscles.

Clostridium perfringens α-toxin specifically induces endothelial cell death by promoting ceramide-mediated apoptosis

Clostridium perfringens type A-induced gas gangrene is characterized by severe myonecrosis, and α-toxin has been revealed to be a major virulence factor involved in the pathogenesis. However, the detailed mechanism is unclear. Here, we show that CD31⁺ endothelial cell counts decrease in muscles infected with C. perfringens in an α-toxin-dependent manner. In vitro experiments revealed that α-toxin preferentially and rapidly induces the death of human umbilical vein endothelial cells (HUVECs) compared with C2C12 murine muscle cells. The toxin induces apoptosis of HUVECs by increasing ceramide. Furthermore, the specificity might be dependent on differences in the sensitivity to ceramide between these cell lines. Together, our results suggest that α-toxin-induced endothelial cell death promotes severe myonecrosis and is involved in the pathogenesis of C. perfringens.

Impact of Plasma Membrane Domains on IgG Fc Receptor Function

Lipid cell membranes not only represent the physical boundaries of cells. They also actively participate in many cellular processes. This contribution is facilitated by highly complex mixtures of different lipids and incorporation of various membrane proteins. One group of membrane-associated receptors are Fc receptors (FcRs). These cell-surface receptors are crucial for the activity of most immune cells as they bind immunoglobulins such as immunoglobulin G (IgG). Based on distinct mechanisms of IgG binding, two classes of Fc receptors are now recognized: the canonical type I FcγRs and select C-type lectin receptors newly referred to as type II FcRs. Upon IgG immune complex induced cross-linking, these receptors are known to induce a multitude of cellular effector responses in a cell-type dependent manner, including internalization, antigen processing, and presentation as well as production of cytokines. The response is also determined by specific intracellular signaling domains, allowing FcRs to either positively or negatively modulate immune cell activity. Expression of cell-type specific combinations and numbers of receptors therefore ultimately sets a threshold for induction of effector responses. Mechanistically, receptor cross-linking and localization to lipid rafts, i.e., organized membrane microdomains enriched in intracellular signaling proteins, were proposed as major determinants of initial FcR activation. Given that immune cell membranes might also vary in their lipid compositions, it is reasonable to speculate, that the cell membrane and especially lipid rafts serve as an additional regulator of FcR activity. In this article, we aim to summarize the current knowledge on the interplay of lipid rafts and IgG binding FcRs with a focus on the plasma membrane composition and receptor localization in immune cells, the proposed mechanisms underlying this localization and consequences for FcR function with respect to their immunoregulatory capacity.

Molecular structure and function of the carboxy-terminus of the alpha-toxin from Clostridium perfringens type A

In order to interpret the molecular structure and biological characteristics of Clostridium perfringens alpha-toxin (CPA), the CPA251-370 gene was cloned and the 120 amino acid carboxy terminal of CPA (CPA251-370) was obtained. The secondary and three-dimensional (3D) structures of CPA251-370 were predicted. The secondary structure of CPA251-370 consisted primarily of 35.48% β-sheets and 44.35% random coils. Compared with the CPA toxin consisting of 10 α-helices and eight β-sheets, the 3D structure of CPA251-370 only contained eight β-sheets. The circular dichroism (CD) spectrum detection showed that the CD spectrum of CPA251-370 changed slightly compared with the CD spectrum of CPA. Biological activity assays showed that CPA251-370 had lost the phospholipase C (PLC) activity and haemolytic activity of CPA. More importantly, the mice immunized with CPA251-370 were protected against a challenge with 1 MLD C. perfringens type A strain C57-1. This study laid a solid foundation for explaining the relationship between molecular structure and biological characteristics of CPA in the future. Our research also provides CPA251-370 as a candidate strains for genetic engineering subunit vaccines of C. perfringens type A.

Granulocyte Colony-Stimulating Factor Does Not Influence Clostridium Perfringens α-Toxin-Induced Myonecrosis in Mice

Clostridium perfringens type A causes gas gangrene characterized by myonecrosis and development of an effective therapy for treating affected patients is of clinical importance. It was recently reported that the expression of granulocyte colony-stimulating factor (G-CSF) is greatly up-regulated by C. perfringens infection. However, the role of G-CSF in C. perfringens-mediated myonecrosis is still unclear. Here, we assessed the destructive changes in C. perfringens-infected skeletal muscles and tested whether inhibition of G-CSF receptor (G-CSFR) signaling or administration of recombinant G-CSF affects the tissue injury. Severe edema, contraction of muscle fiber diameter, and increased plasma creatine kinase activity were observed in mice intramuscularly injected with C. perfringens type A, and the destructive changes were α-toxin-dependent, indicating that infection induces the destruction of skeletal muscle in an α-toxin-dependent manner. G-CSF plays important roles in the protection of tissue against damage and in the regeneration of injured tissue. However, administration of a neutralizing antibody against G-CSFR had no profound impact on the destructive changes to skeletal muscle. Moreover, administration of recombinant human G-CSF, filgrastim, imparted no inhibitory effect against the destructive changes caused by C. perfringens. Together, these results indicate that G-CSF is not beneficial for treating C. perfringens α-toxin-mediated myonecrosis, but highlight the importance of revealing the mechanism by which C. perfringens negates the protective effects of G-CSF in skeletal muscle.

Study of the Structure and Biological Activity of the Amino-Terminus of the α-Toxin from Clostridium welchii Type A

To explore the biological activity of Clostridium welchii α-toxin (CPA), the Asp56 residue of CPA was mutated to glycine (CPA D56G) by site-directed mutagenesis, and the 250 amino acid amino-terminal phospholipase C (PLC)-containing domain of CPA (PLC1-250) was isolated. The secondary and three-dimensional (3D) structures of CPA D56G and PLC1-250 were predicted, and the results showed that the secondary structures of CPA D56G and PLC1-250 were composed of α-helices and random coils. The 3D structures of CPA D56G and PLC1-250 were similar to the 3D structures of CPA. The circular dichroism (CD) spectrum of CPA D56G differed from the CD spectrum of CPA, but the CD spectrum of PLC1-250 was similar to the CD spectrum of CPA. Biological activity assays showed that CPA D56G lost the PLC activity of CPA and that mice immunized with CPA D56G were protected against a challenge with 1 MLD C. welchii type A strain C57-1. In addition, PLC1-250 contained the PLC activity of CPA. This study laid a solid foundation for future studies on the relationship between the molecular structure and biological function of CPA and its molecular mechanism. Our study also provided CPA D56G as a candidate strain for engineering a CPA subunit vaccine for C. welchii type A.

Role of Lipid Rafts in Hematopoietic Stem Cells Homing, Mobilization, Hibernation, and Differentiation

Hematopoietic stem cells (HSCs) are multipotent, self-renewing cells that can differentiate into myeloid or lymphoid cells. The mobilization and differentiation processes are affected by the external environment, such as extracellular matrix and soluble molecules in the niche, where the lipid rafts (LRs) of the HSCs act as the receptors and control platforms for these effectors. LRs are membrane microdomains that are enriched in cholesterol, sphingolipid, and proteins. They are involved in diverse cellular processes including morphogenesis, cytokinesis, signaling, endocytic events, and response to the environment. They are also involved in different types of diseases, such as cancer, Alzheimer's, and prion disease. LR clustering and disruption contribute directly to the differentiation, homing, hibernation, or mobilization of HSCs. Thus, characterization of LR integrity may provide a promising approach to controlling the fate of stem cells for clinical applications. In this review, we show the critical role of LR modification (clustering, disruption, protein incorporation, and signal responding) in deciding the fate of HSCs, under the effect of soluble cytokines such as stem cell factor (SCF), transforming growth factor- β (TGF-β), hematopoietic-specific phospholipase Cβ2 (PLC-β2), and granulocyte colony-stimulating factor (G-CSF).

Clostridium perfringens α-toxin impairs granulocyte colony-stimulating factor receptor-mediated granulocyte production while triggering septic shock

During bacterial infection, granulocyte colony-stimulating factor (G-CSF) is produced and accelerates neutrophil production from their progenitors. This process, termed granulopoiesis, strengthens host defense, but Clostridium perfringens α-toxin impairs granulopoiesis via an unknown mechanism. Here, we tested whether G-CSF accounts for the α-toxin-mediated impairment of granulopoiesis. We find that α-toxin dramatically accelerates G-CSF production from endothelial cells in response to Toll-like receptor 2 (TLR2) agonists through activation of the c-Jun N-terminal kinase (JNK) signaling pathway. Meanwhile, α-toxin inhibits G-CSF-mediated cell proliferation of Ly-6G+ neutrophils by inducing degradation of G-CSF receptor (G-CSFR). During sepsis, administration of α-toxin promotes lethality and tissue injury accompanied by accelerated production of inflammatory cytokines in a TLR4-dependent manner. Together, our results illustrate that α-toxin disturbs G-CSF-mediated granulopoiesis by reducing the expression of G-CSFR on neutrophils while augmenting septic shock due to excess inflammatory cytokine release, which provides a new mechanism to explain how pathogenic bacteria modulate the host immune system.

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.

Peptidoglycan accelerates granulopoiesis through a TLR2- and MyD88-dependent pathway

Granulopoiesis is accelerated during Gram-negative bacterial infection through activation of toll-like receptor 4 (TLR4). In this study, we tested whether activation of TLR2 promotes granulopoiesis by using the well-known TLR2 agonist, peptidoglycan (PGN). Neutrophils in bone marrow and spleen, and plasma granulocyte colony-stimulating factor (G-CSF) were increased in mice that had received intraperitoneal PGN administration. Incorporation of BrdU into bone marrow neutrophils increased, demonstrating that PGN accelerated granulopoiesis. Treatment of bone marrow cells (BMCs) with PGN increased neutrophils in vitro and promoted the secretion of G-CSF from Ly-6G^-Ly-6C⁺ monocytes. The accelerated granulopoiesis caused by PGN was not seen in TLR2-deficient and MyD88-deficient BMCs. Additionally, PGN induced G-CSF production in human umbilical vein endothelial cells. These findings demonstrate that PGN promotes the secretion of G-CSF from monocytes and endothelial cells, leading to the acceleration of granulopoiesis. Our results illustrate that bacterial recognition by TLR2 facilitates granulopoiesis during Gram-positive bacterial infection.