Effect of Clostridium perfringens phospholipase C(alpha-toxin) on the human diploid fibroblast membrane

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.

Effects of Clostridium perfringens phospholipase C in mammalian cells

Clostridium perfringens phospholipase C (Cp-PLC), the major virulence factor in the pathogenesis of gas gangrene, is a Zn(2+) metalloenzyme with lecithinase and sphingomyelinase activities. Its structure shows an N-terminal domain containing the active site, and a C-terminal Ca(2+) binding domain required for membrane interaction. Although the knowledge of the structure of Cp-PLC and its interaction with aggregated phospholipids has advanced significantly, an understanding of the effects of Cp-PLC in mammalian cells is still incomplete. Cp-PLC binds to artificial bilayers containing cholesterol and sphingomyelin or phosphatidylcholine (PC) and degrades them, but glycoconjugates present in biological membranes influence its binding or positioning toward its substrates. Studies with Cp-PLC variants harboring single amino-acid substitutions have revealed that the active site, the Ca(2+) binding region, and the membrane interacting surface are required for cytotoxic and haemolytic activity. Cp-PLC causes plasma membrane disruption at high concentrations, whereas at low concentrations it perturbs phospholipid metabolism, induces DAG generation, PKC activation, Ca(2+) mobilization, and activates arachidonic acid metabolism. The cellular susceptibility to Cp-PLC depends on the composition of the plasma membrane and the capacity to up-regulate PC synthesis. The composition of the plasma membrane determines whether Cp-PLC can bind and acquire its active conformation, and thus the extent of phospholipid degradation. The capacity of PC synthesis and the availability of precursors determine whether the cell can replace the degraded phospholipids. Whether the perturbations of signal transduction processes caused by Cp-PLC play a role in cytotoxicity is not clear. However, these perturbations in endothelial cells, platelets and neutrophils lead to the uncontrolled production of intercellular mediators and adhesion molecules, which inhibits bacterial clearance and induces thrombotic events, thus favouring bacterial growth and spread in the host tissues.

UDP-glucose deficiency causes hypersensitivity to the cytotoxic effect of Clostridium perfringens phospholipase C

A Chinese hamster cell line with a mutation in the UDP-glucose pyrophosphorylase (UDPG:PP) gene leading to UDP-glucose deficiency as well as a revertant cell were previously isolated. We now show that the mutant cell is 10(5) times more sensitive to the cytotoxic effect of Clostridium perfringens phospholipase C (PLC) than the revertant cell. To clarify whether there is a connection between the UDP-glucose deficiency and the hypersensitivity to C. perfringens PLC, stable transfectant cells were prepared using a wild type UDPG:PP cDNA. Clones of the mutant transfected with a construct having the insert in the sense orientation had increased their UDP-glucose level, whereas those of the revertant transfected with a UDPG:PP antisense had reduced their level of UDP-glucose compared with control clones transfected with the vector. Exposure of these two types of transfectant clones to C. perfringens PLC demonstrated that a cellular UDP-glucose deficiency causes hypersensitivity to the cytotoxic effect of this phospholipase. Further experiments with genetically engineered C. perfringens PLC variants showed that the sphingomyelinase activity and the C-domain are required for its cytotoxic effect in UDP-glucose-deficient cells.

Cellular UDP-glucose deficiency caused by a single point mutation in the UDP-glucose pyrophosphorylase gene

We previously isolated a mutant cell that is the only mammalian cell reported to have a persistently low level of UDP-glucose. In this work we obtained a spontaneous revertant whose UDP-glucose level lies between those found in the wild type and the mutant cell. The activity of UDP-glucose pyrophosphorylase (UDPG:PP), the enzyme that catalyzes the formation of UDP-glucose, was in the mutant 4% and in the revertant 56% of the activity found in the wild type cell. Sequence analysis of UDPG: PP cDNAs from the mutant cell showed one missense mutation, which changes amino acid residue 115 from glycine to aspartic acid. The substituted glycine is located within the largest stretch of strictly conserved residues among eukaryotic UDPG:PPs. The analysis of the cDNAs from the revertant cell indicated the presence of an equimolar mixture of the wild type and the mutated mRNAs, suggesting that the mutation has reverted in only one of the alleles. In summary, we demonstrate that the G115D substitution in the Chinese hamster UDPG:PP dramatically impairs its enzymatic activity, thereby causing cellular UDP-glucose deficiency.

Role of alpha-toxin in Clostridium perfringens infection determined by using recombinants of C. perfringens and Bacillus subtilis

Clostridium perfringens type A strains which differed in alpha-toxin (phospholipase C [PLC]) productivity were inoculated intraperitoneally or intravenously into mice, and then their 50% mouse lethal doses (LD50) were determined. Strain NCTC 8237 produced ninefold higher PLC activity than strain 13. The mean LD50 for the former was 1 log unit lower than that for the latter. Two isogenic strains were constructed from strain 13: strain 13(pJIR418 alpha) (pJIR418 alpha contains the plc gene), which produced ninefold higher PLC activity than strain 13; and strain 13 PLC-, which showed no PLC productivity at all because of transformation-mediated gene disruption. The mean LD50 for strain 13(pJIR418 alpha) was 1 log unit lower than those for strain 13 PLC- and strain 13. These results indicate that PLC functions as a virulence-determining factor when it is produced in a sufficient amount. Such a difference in LD50 was also observed between Bacillus subtilis with and without the cloned plc gene. Inoculation of B. subtilis PLC+ intravenously into mice caused marked thrombocytopenia and leukocytosis. Mice inoculated with B. subtilis at 2 LD50 died because of circulatory collapse. Histological examination revealed that intravascular coagulation and vascular congestion occurred most prominently in the lungs. These results suggest that PLC plays a key role in the systemic intoxication of clostridial myonecrosis, probably by affecting the functions of platelets and phagocytes.

Effects on cultured mammalian cells of myotoxin III, a phospholipase A2 isolated from Bothrops asper (terciopelo) venom

Myotoxin III (MT-III), a myotoxic phospholipase A2 from Bothrops asper, was studied with respect to interactions with cultured mammalian cells and red blood cells. Tests of the cytopathogenic effect of MT-III on different cell lines indicated that rat skeletal muscle L6 myoblasts were more sensitive to the toxin than chinese hamster ovary cells, human lung fibroblasts, mouse adrenal tumour cells and rat intestinal epithelial cells. Specific plasma-membrane permeabilization was assayed as release of a cytosolic [3H]uridine nucleotide marker from toxin-treated L6 cells. A dose- and time-related membrane permeabilization was induced at 37 degrees C, but not at 0 degree C. A half-maximal effect was obtained after 20 min. 30 micrograms/ml MT-III induced 50% marker release in 1 h, and the effect was not reversed by post-incubation for up to 48 h in toxin-free medium. The membrane permeabilization in L6 cells did not seem to require cellular internalisation of the toxin. The catalytic site of the toxin was inactivated by alkylation with p-bromophenacyl bromide (BPB). This treatment abolished the toxin's specific PLA2 activity, as assayed in vitro, and reduced the PLA2 activity on the myoblast membrane by more than 95%, as measured by release of [14C]arachidonic acid from prelabelled cells. However, the membrane-permeabilizing effect (release of cytosolic marker) was reduced only by 70% upon modification with BPB. We also report that MT-III is not directly haemolytic, and one reason for this is the inability of the toxin to associate with the membranes of human or mouse erythrocytes. Taken together, the data suggest that MT-III at 37 degrees C binds to and penetrates the plasma membrane of cultured myoblasts, thereby inducing a rapid, direct and irreversible membrane permeabilization. This effect apparently depends in part on the PLA2 activity of the toxin and in part on a molecular region which is separate from the catalytic site.

Bacterial phospholipases C

A variety of pathogenic bacteria produce phospholipases C, and since the discovery in 1944 that a bacterial toxin (Clostridium perfringens alpha-toxin) possessed an enzymatic activity, there has been considerable interest in this class of proteins. Initial speculation that all phospholipases C would have lethal properties has not been substantiated. Most of the characterized enzymes fall into one of four groups of structurally related proteins: the zinc-metallophospholipases C, the sphingomyelinases, the phosphatidylinositol-hydrolyzing enzymes, and the pseudomonad phospholipases C. The zinc-metallophospholipases C have been most intensively studied, and lethal toxins within this group possess an additional domain. The toxic phospholipases C can interact with eukaryotic cell membranes and hydrolyze phosphatidylcholine and sphingomyelin, leading to cell lysis. However, measurement of the cytolytic potential or lethality of phospholipases C may not accurately indicate their roles in the pathogenesis of disease. Subcytolytic concentrations of phospholipase C can perturb host cells by activating the arachidonic acid cascade or protein kinase C. Nonlethal phospholipases C, such as the Listeria monocytogenes PLC-A, appear to enhance the release of the organism from the host cell phagosome. Since some phospholipases C play important roles in the pathogenesis of disease, they could form components of vaccines. A greater understanding of the modes of action and structure-function relationships of phospholipases C will facilitate the interpretation of studies in which these enzymes are used as membrane probes and will enhance the use of these proteins as models for eukaryotic phospholipases C.

Stimulation of luteinizing hormone-Beta messenger ribonucleic Acid and post-translational modification of luteinizing hormone isoforms by second messengers mediating the action of gonadotrophin-releasing hormone

Abstract Several second messenger systems have been implicated in mediating the action of gonadotrophin-releasing hormone on the pituitary gonadotrophs and numerous studies have shown that activation of these systems induces luteinizing hormone (LH) secretion. However, it is not known how gonadotrophin-releasing hormone or the second messenger systems induce de novo LH biosynthesis and post-translational modification of the hormone. In these experiments hemipituitary glands have been perifused with drugs which activate second messengers or stimulate protein kinase C directly. The LH secretory responses have been correlated with measurements of common a and LHbeta mRNA and the molecular species of LH which were present in the pituitary perifusate after exposure to the drugs. Gonadotrophin-releasing hormone (50 ng/ml, 42 nM), with and without the presence of extracellular Ca(2+), the Ca(2+) ionophore, A23187 (10 muM), and phorbol 12-myristate (1 muM) all stimulated an increase in LHbeta mRNA compared with controls and the appearance of a different isoform of LH to that found stored in and released from the unstimulated pituitary gland. Phospholipase C was without effect on LHbeta mRNA levels and showed minimal efficacy in inducing the appearance of the different LH isoform.

Epitope mapping of the alpha-toxin of Clostridium perfringens

A panel of monoclonal antibodies specific for the Clostridium perfringens alpha-toxin was produced by the fusion of X63.Ag8-653 cells with splenocytes from mice immunized either intrasplenically or intraperitoneally with an alpha-toxoid. The toxin-binding activity of each monoclonal antibody was evaluated. The monoclonal antibodies were also screened for their toxin-neutralizing potential in vitro, as determined by the inhibition of phospholipase C and hemolytic activities. In vivo inhibition of toxicity was assessed by the survival of mice challenged with preincubated alpha-toxin-antibody mixtures. Only one monoclonal antibody (3A4D10) was protective in vivo and neutralizing in both in vitro assays. Since 3A4D10 could inhibit both activities, the evidence suggests that these are colocated in the same area of the toxin molecule. This paper identifies a significant continuous linear binding region for 3A4D10 at positions 193 to 198 in the primary amino acid sequence of alpha-toxin.

Molecular genetics and pathogenesis of Clostridium perfringens

Clostridium perfringens is the causative agent of a number of human diseases, such as gas gangrene and food poisoning, and many diseases of animals. Recently significant advances have been made in the development of C. perfringens genetics. Studies on bacteriocin plasmids and conjugative R plasmids have led to the cloning and analysis of many C. perfringens genes and the construction of shuttle plasmids. The relationship of antibiotic resistance genes to similar genes from other bacteria has been elucidated. A detailed physical map of the C. perfringens chromosome has been prepared, and numerous genes have been located on that map. Reproducible transformation methods for the introduction of plasmids into C. perfringens have been developed, and several genes coding for the production of extracellular toxins and enzymes have been cloned. Now that it is possible to freely move genetic information back and forth between C. perfringens and Escherichia coli, it will be possible to apply modern molecular methods to studies on the pathogenesis of C. perfringens infections.

Gene cloning shows the alpha-toxin of Clostridium perfringens to contain both sphingomyelinase and lecithinase activities

The plc gene encoding the alpha-toxin (phospholipase C), an important virulence factor of Clostridium perfringens, has been cloned, sequenced and expressed in Escherichia coli. Transcriptional analysis of mRNAs produced in vivo by C. perfringens and E. coli, and in vitro using purified RNA polymerase from C. perfringens revealed that plc is transcribed constitutively from a single promoter situated about 100 nucleotides from the coding sequence. A T7 expression system was used to overproduce alpha-toxin in E. coli; enzymological studies with the amplified plc gene product unambiguously demonstrated that both lecithinase (phospholipase C) and sphingomyelinase activities were associated with this 43,000 dalton cytotoxin. The 370-residue alpha-toxin is haemolytic and shares sequence and functional homology with the two components of Bacillus cereus haemolysin, cereolysin AB, in which phospholipase C and sphingomyelinase activities are associated with different polypeptides.

Phospholipase C and haemolytic activities of Clostridium perfringens alpha-toxin cloned in Escherichia coli: sequence and homology with a Bacillus cereus phospholipase C

The Clostridium perfringens alpha-toxin (phospholipase C) gene (cpa) has been cloned and expressed in Escherichia coli. The biological activities of the cloned gene product have been analysed and the complete nucleotide sequence of the cpa gene has been determined. The cloned cpa gene product, which is exported to the periplasm in E. coli, possesses both phospholipase C and haemolytic activities. Haemolysis is not apparent when cell extracts are incubated with isotonic suspensions of sheep erythrocytes, but can be detected and quantified readily when dilutions of the same extracts are placed in wells in sheep-blood agar plates. Like other sequenced clostridial genes, the cpa gene has a high AT content (66.4%), exhibits a strong bias for using codons with A or T in the wobble position, and the 350 base pairs upstream from the gene have a significantly higher AT content (79.5%) than the coding region. The cpa gene encodes a 398 amino acid polypeptide with a deduced molecular weight of 45,481 D. This is very similar to the estimated molecular weight (Mr) of the cpa primary gene product expressed in an in vitro transcription-translation system (Mr 46,000), but larger than the cpa gene product detected in E. coli minicells, E. coli whole cells or in C. perfringens cells (Mr 43,000), suggesting post-translational processing. The 28 N-terminal residues of the deduced alpha-toxin sequence possess the consensus features of a signal peptide and may be removed during secretion. The deduced alpha-toxin sequence shares significant structural homology with the phosphatidylcholine-preferring phospholipase C of Bacillus cereus.

Molecular cloning and nucleotide sequence of the alpha-toxin (phospholipase C) of Clostridium perfringens

A fragment of DNA containing the gene coding for the phospholipase C (alpha-toxin) of Clostridium perfringens was cloned into Escherichia coli. The cloned DNA appeared to code only for the alpha-toxin and contained both the coding region and its associated gene promoter. The nucleotide sequence of the cloned DNA was determined, and an open reading frame was identified which encoded a protein with a molecular weight of 42,528. By comparison of the gene sequence with the N-terminal amino acid sequence of the protein, a 28-amino-acid signal sequence was identified. The gene promoter showed considerable homology with the E. coli sigma 55 consensus promoter sequences, and this may explain why the gene was expressed by E. coli. The cloned gene product appeared to be virtually identical to the native protein. A 77-amino-acid stretch that was close to the N terminus of the alpha-toxin showed considerable homology with similarly located regions of the Bacillus cereus phosphatidylcholine, preferring phospholipase C and weaker homology with the phospholipase C from Pseudomonas aeruginosa.

Mitoses and microorganisms in the periodontal membrane after storage in milk or saliva

Milk and saliva were tested in vitro as potential storage media for avulsed teeth. Developing monkey teeth were extracted and stored in milk or saliva for periods ranging from 1 to 6 h. The osmolality, pH, conductivity and number of viable bacteria in the media were determined after predetermined intervals during the storage periods. After the storage periods the teeth were either prepared for scanning electron microscopy or cultured for 24 h in Eagle's medium supplemented with 3H-thymidine. In the scanning electron microscope numerous adherent bacteria were seen covering the periodontal membrane after storage in saliva but none were found after storage in milk. The cultured teeth were sectioned and evaluated with autoradiography. Superficial parts of the periodontal membrane were rapidly injured by storage in saliva while the epithelial root sheath and the apical pulpal cells were affected at a later stage. Cells neighboring the cementoblasts incorporated 3H-thymidine after 6 h storage in milk but not after storage in saliva for the same length of time. It was concluded that the low osmolality in combination with bacteria which adhered to the periodontal membrane made saliva less suited than milk for long time storage of avulsed teeth. Furthermore, a viable layer of cells close to the root surface seemed to be a prerequisite for a successful healing without root resorption after replantation.

Storage of experimentally avulsed teeth in milk prior to replantation

Extracted monkey teeth were endodontically treated, stored in milk or saliva for two or six h, and then replanted. Periodontal conditions were evaluated after eight wk. Teeth that had been stored for two or six h in milk or for two h in saliva showed periodontal healing almost as good as that of immediately replanted teeth. Teeth that had been kept in saliva for six h or bench-dried for one h showed extensive replacement resorption. Milk may thus be recommended as a storage medium for ex-articulated teeth prior to replantation in cases when immediate replantation is not possible.

Survival of cultured cells after functional and structural disorganization of plasma membrane by bacterial haemolysins and phospholipases

Lesions were induced in the plasma membranes of cultured human fibroblasts by membrane damaging toxins of bacterial origin (haemolysins). Structural disorganization of the membrane was measured as leakage of a radiolabelled small cytoplasmic marker and functional membrane damage was measured as decreased uptake of aminoisobutyrate. Cell survival was scored 24 and 48 hr later by measuring uptake of Trypan Blue and by light microscopical evaluation of cell morphology and proliferation. The membrane damage induced by most bacterial toxins was reversible upon removal of the toxin, since toxin-treated cells recovered and excluded Trypan Blue although they had been permeable to the dye immediately after the toxin treatment. Among ten bacterial toxins tested, the only exception of this general behavior was the Aeromonas hydrophila beta-haemolysin, which irreversibly damaged human fibroblasts. Thus, the action of bacterial haemolysins on cultured cells generally seems restricted to a plasma membrane permeabilization, which is reversible regardless of the mechanism of membrane damaging action of the toxin or of the relative size of the structural lesions induced. Furthermore, the use of Trypan Blue uptake as a measure of cell death caused by membrane damaging agents appears to be of limited value.

Combination chemotherapy with Clostridium perfringens phospholipase C and cytosine antimetabolites: complementary inhibition directed at membrane lipids

Tumor cell membranes were susceptible to the action of Clostridium perfringens phospholipase C, and this was reflected by inhibition of cellular replication in culture. The differential susceptibility of two neoplastic cell lines to this enzyme was studied in detail. The growth of sarcoma 180 cells cultured in Fischer's medium was markedly inhibited by phospholipase C; whereas, in contrast, cultured L1210 leukemia cells were relatively resistant to the cytotoxic effects of this enzyme. The differential sensitivity of these two neoplastic cell lines to phospholipase C was corroborated by dye-exclusion tests. Thus, leukemia L1210 cells exposed to a concentration of 0.2 mg of phospholipase C per ml of Fischer's medium for 30 min at 37 degrees C were able to exclude Trypan Blue; whereas, only about 21% of sarcoma 180 cells treated under identical conditions were able to exclude the dye. That the cytotoxicity of phospholipase C to sarcoma 180 was the result of hydrolysis of phospholipids of the plasma membrane was supported by measurements of the rate of hydrolysis of radioactivity from the phospholipid of neoplastic cells prelabeled with [3H]choline. Eighty-two percent of incorporated radioactive choline was released from sarcoma 180 cells treated with phospholipase C in Fischer's medium, whereas, only 20% of the label from [3H]choline was solubilized from L1210 leukemia cells treated with the enzyme under similar conditions. Scanning electron microscopy revealed significant damage to sarcoma 180 cells exposed to phospholipase C in Fischer's medium, which was characterized by alterations in size and shape of cells, disappearance of microvilli, and appearance of fistulas in cell membranes; relatively resistant L1210 leukemic cells did not appear to be markedly damaged by comparable enzyme treatment. Exposure of leukemia L1210 cells to phospholipase C in Puck's saline A increased the sensitivity of these cells to enzymatic action. Under these conditions, a comparable amount of phospholipid was hydrolyzed from surface membranes of sarcoma 180 and leukemia L1210 cells, and the degree of membrane damage appeared to be similar, as measured by the capacity of the tumor cell lines to exclude Trypan Blue and by scanning electron microscopy. The extensive damage to membranes by hydrolysis of phospholipids was not accompanied by a change in the degree of specific binding of [3H]concanavalin A(ConA).(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of osmolality and composition of some storage media on human periodontal ligament cells

The effect of media with different osmolalities and compositions on cell viability and integrity of the cell membrane has been studied. Physiologic media, such as milk, saline and physiologic sucrose solution, preserved cell viability and membrane morphology equally well. The cell membrane of cells stored in saliva was more extensively damaged than the cell membrane of cells stored in a sucrose solution with an hypotonic osmolality similar to saliva. Thus a hypotonic osmolality seemed to potentiate the damaging effect of an unfavourable composition or a bacterial contamination of the storage media.

Storage of human periodontal ligament cells in a combination of different media

Different types of storage media have been studied for use in the replantation of exarticulated teeth. It was found that human periodontal ligament cells survived well in milk. Fifty percent of the cells were viable after 12 hours' storage in milk, while no cells were viable after three hours' storage in saliva. Brief storage in saliva followed by subsequent storage in milk was better than storage in saliva only.

Inhibition of noradrenaline-stimulated lipolysis and cyclic AMP accumulation in isolated rat adipocytes by purified phospholipase C and theta-toxin from Clostridium perfringens

Purified phospholipae C (phosphatidylcholine cholinephosphohydrolase, EC 3.1.4.3) and theta-toxin from Clostridium perfringens both inhibited noradrenaline-stimulated lipolysis and cyclic AMP accumulation in isolated rat adipocytes in a dose-dependent manner. The action of phospholipase C was gradual in onset, while the effect of theta-toxin was almost immediate. Phospholipase C, but not theta-toxin, hydrolyzed membrane phospholipids and inhibited adenylate cyclase (EC 4.6.1.1) in a crude membrane fraction from fat cells. The inhibitory effects of phospholipase C were associated with morphological alterations detectable by electron microscopy, whereas effects of theta-toxin were observed at a time when no clearcut morphological alterations could be observed. It is concluded that the two purified principles from C. perfringens, which are both present in commercial preparations of phospholipase C, antagonize noradrenaline-stimulated cyclic AMP accumulation and lipolysis. Although their exact mechanisms of action have not been elucidated, phospholipase C and theta-toxin have different modes of attack.

Interaction of purified leukocidin from Pseudomonas aeruginosa with Bovine polymorphonuclear leukocytes

The interaction of purified leukocidin from Pseudomonas aeruginosa, strain 158, with polymorphonuclear leukocytes of cattle (PMLC) was studied by using 125I-labeled toxin. According to the Scatchard plot, PMLC offered two binding sites for leukocidin: one at the surface of the plasma membrane, and a second one that presumably became accessible to the toxin in the course of the cytotoxic action. Toxin once fixed to PMLC at 37 C could not be detached from the cells by either chemical or mechanical treatment. However, active leukocidin was liberated if it was bound to PMLC at 4 C and the temperature of the cell suspension was subsequently increased to 37 C. In the presence of Ca2+, the velocity of toxin fixation was accelerated and the rate of fixation was increased. Preliminary investigations on the identification of the leukocidin-binding material indicated the leukocidin receptor to be an integral protein of the plasma membrane.