Identification of B cell recognized linear epitopes in a snake venom serine proteinase from the central American bushmaster Lachesis stenophrys.

Snake venom serine proteinases are toxins that perturb hemostasis acting on proteins from the blood coagulation cascade, the fibrinolytic or the kallikrein-kinin system. Despite the relevance of these enzymes in envenomations by viper bites, the characterization of the antibody response to these toxins at the molecular level has not been previously addressed. In this work surface-located B cell recognized linear epitopes from a Lachesis stenophrys venom serine proteinase (UniProt accession number Q072L7) were predicted using an artificial neuronal network at the ABCpred server, the corresponding peptides were synthesized and their immunoreactivity was analyzed against a panel of experimental and therapeutic antivenoms. A molecular model of the L. stenophrys enzyme was built using as a template the structure of the D. acutus Dav-PA serine proteinase (Q9I8X1), which displays the highest degree of sequence similarity to the L. stenophrys enzyme among proteins of known 3D structure, and the surface-located epitopes were identified in the protein model using iCn3D. A total of 13 peptides corresponding to the surface exposed predicted epitopes from L. stenophrys serine proteinase were synthesized and, their reactivity with a rabbit antiserum against the recombinant enzyme and a panel of antivenoms was evaluated by a capture ELISA. Some of the epitopes recognized by monospecific and polyspecific antivenoms comprise sequences overlapping motifs conserved in viper venom serine proteinases. The identification and characterization of relevant epitopes recognized by B cells in snake venom toxins may provide valuable information for the preparation of immunogens that help in the production of improved therapeutic antivenoms.

Lemnitoxin, the major component of Micrurus lemniscatus coral snake venom, is a myotoxic and pro-inflammatory phospholipase A(2)

The venom of Micrurus lemniscatus, a coral snake of wide geographical distribution in South America, was fractionated by reverse-phase HPLC and the fractions screened for phospholipase A(2) (PLA(2)) activity. The major component of the venom, a PLA(2), here referred to as 'Lemnitoxin', was isolated and characterized biochemically and toxicologically. It induces myotoxicity upon intramuscular or intravenous injection into mice. The amino acid residues Arg15, Ala100, Asn108, and a hydrophobic residue at position 109, which are characteristic of myotoxic class I phospholipases A(2), are present in Lemnitoxin. This PLA(2) is antigenically related to M. nigrocinctus nigroxin, Notechis scutatus notexin, Pseudechis australis mulgotoxin, and Pseudonaja textilis textilotoxin, as demonstrated with monoclonal and polyclonal antibodies. Lemnitoxin is highly selective in its targeting of cells, being cytotoxic for differentiated myotubes in vitro and muscle fibers in vivo, but not for undifferentiated myoblasts or endothelial cells. Lemnitoxin is not lethal after intravenous injection at doses up to 2 mu g/g in mice, evidencing its lack of significant neurotoxicity. Lemnitoxin displays anticoagulant effect on human plasma and proinflammatory activity also, as it induces paw edema and mast cell degranulation. Thus, the results of this work demonstrate that Lemnitoxin is a potent myotoxic and proinflammatory class I PLA(2). (C) 2016 Elsevier Ireland Ltd. All rights reserved.

Comparison of a nontoxic variant of Clostridium perfringens α-toxin with the toxic wild-type strain.

The α-toxin produced by Clostridium perfringens is one of the best-studied examples of a toxic phospholipase C. In this study, a nontoxic mutant protein from C. perfringens strain NCTC8237 in which Thr74 is substituted by isoleucine (T74I) has been characterized and is compared with the toxic wild-type protein. Thr74 is part of an exposed loop at the proposed membrane-interfacing surface of the toxin. The mutant protein had markedly reduced cytotoxic and myotoxic activities. However, this substitution did not significantly affect the catalytic activity towards water-soluble substrate or the overall three-dimensional structure of the protein. The data support the proposed role of the 70-90 loop in the recognition of membrane phospholipids. These findings also provide key evidence in support of the hypothesis that the hydrolysis of both phosphatidylcholine and sphingomyelin are required for the cytolytic and toxic activity of phospholipases.

Tyrosine 331 and phenylalanine 334 in Clostridium perfringens alpha-toxin are essential for cytotoxic activity.

Differences in the biological properties of the Clostridium perfringens phospholipase C (alpha-toxin) and the C. bifermentans phospholipase C (Cbp) have been attributed to differences in their carboxy-terminal domains. Three residues in the carboxy-terminal domain of alpha-toxin, which have been proposed to play a role in membrane recognition (D269, Y331 and F334), are not conserved in Cbp (Y, L and I respectively). We have characterised D269Y, Y331L and F334I variant forms of alpha-toxin. Variant D269Y had reduced phospholipase C activity towards aggregated egg yolk phospholipid but increased haemolytic and cytotoxic activity. Variants Y331L and F334I showed a reduction in phospholipase C, haemolytic and cytotoxic activities indicating that these substitutions contribute to the reduced haemolytic and cytotoxic activity of Cbp.

Identification of residues critical for toxicity in Clostridium perfringens phospholipase C, the key toxin in gas gangrene.

Clostridium perfringens phospholipase C (PLC), also called alpha-toxin, is the major virulence factor in the pathogenesis of gas gangrene. The toxic activities of genetically engineered alpha-toxin variants harboring single amino-acid substitutions in three loops of its C-terminal domain were studied. The substitutions were made in aspartic acid residues which bind calcium, and tyrosine residues of the putative membrane-interacting region. The variants D269N and D336N had less than 20% of the hemolytic activity and displayed a cytotoxic potency 103-fold lower than that of the wild-type toxin. The variants in which Tyr275, Tyr307, and Tyr331 were substituted by Asn, Phe, or Leu had 11-73% of the hemolytic activity and exhibited a cytotoxic potency 102- to 105-fold lower than that of the wild-type toxin. The results demonstrated that the sphingomyelinase activity and the C-terminal domain are required for myotoxicity in vivo and that the variants D269N, D336N, Y275N, Y307F, and Y331L had less than 12% of the myotoxic activity displayed by the wild-type toxin. This work therefore identifies residues critical for the toxic activities of C. perfringens PLC and provides new insights toward understanding the mechanism of action of this toxin at a molecular level.

Identification of residues in the carboxy-terminal domain of Clostridium perfringens alpha-toxin (phospholipase C) which are required for its biological activities.

A panel of random mutants within the DNA encoding the carboxy-terminal domain of Clostridium perfringens alpha-toxin was constructed. Three mutants were identified which encoded alpha-toxin variants (Lys330Glu, Asp305Gly, and Asp293Ser) with reduced hemolytic activity. These variants also had diminished phospholipase C activity toward aggregated egg yolk phospholipid and reduced cytotoxic and myotoxic activities. Asp305Gly showed a significantly increased enzymatic activity toward the monodisperse substrate rhoNPPC, whereas Asp293Ser displayed a reduced activity toward this phospholipid analogue. In addition, Asp293Ser showed an increased dependence on calcium for enzymatic activity toward aggregated phospholipid and appeared calcium-depleted in PAGE band-shift assays. In contrast, neither Lys330Glu nor Asp305Gly showed altered dependence on calcium for enzymatic activity toward aggregated phospholipid. Asp305 is located in the interface between the amino- and carboxy-terminal domains, whereas Asp293 and Lys330 are surface exposed residues which may play a role in the recognition of membrane phospholipids.

Elapid venom toxins: multiple recruitments of ancient scaffolds.

Nigroxins A and B, two myotoxic phospholipases A2 (PLA2s) from the venom of the American elapid Micrurus nigrocinctus, belong to a new PLA2 subclass. Their primary structures were established and compared with those of PLA2s that have already been studied with respect to myotoxic activity. The combination of amino acid residues Arg15, Ala100, Asn108 and a hydrophobic residue at position 109 is present exclusively in class I PLA2s that display myotoxic activity. These residues cluster within a surface region rich in positive charges and are suggested to play a role in the interaction with the target membrane of the muscle fibers. It is concluded that the myotoxic PLA2s resulted from recruitment of an ancient scaffold. Dendrotoxins and alpha-neurotoxins are similarly derived from other old structures, which are, however, now also present in nontoxic proteins that are widely distributed throughout the animal kingdom. The evolutionary pathways by which elapid PLA2s acquired myotoxicity and dendrotoxins acquired K+-channel blocker activity are traced. They demonstrate how existing scaffolds were adapted stepwise to serve toxic functions by exchange of a few surface-exposed residues.

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.

A comparison of in vitro methods for assessing the potency of therapeutic antisera against the venom of the coral snake Micrurus nigrocinctus.

Therapeutic antisera against Micrurus nigrocinctus venom were tested for protection against lethality, as well as for inhibition of the nicotinic acetylcholine receptor (AchR)-binding and neutralization of phospholipase A2 (PLA2) activities of the homologous venom. Protection against venom lethality did not correlate with inhibition of AchR-binding activity, whereas there was a significant correlation between antisera potency and inhibition of PLA2 activity (r = 0.82, n = 10, P < 0.02). Inhibition of PLA2 activity could be useful in assessing the protective efficacy of M. nigrocinctus antisera during antivenom production. Micrurus nigrocinctus nigrocinctus venom proteins were fractionated by cation-exchange chromatography on Mono S FPLC and fractions assayed for lethality, AchR-binding and PLA2 activities. Antisera were titrated by enzyme-linked immunoassay (ELISA) against a crude M. n. nigrocinctus venom, two FPLC lethal fractions containing AchR-binding activity, and two toxins purified from M. n. nigrocinctus venom. No correlation was found between protective efficacy and the ELISA titer against any of these antigens. Compared to other elapid venoms that contain few toxins as major components, M. n. nigrocinctus venom appears to be more complex and its lethal effect is likely to be due to the combined effect of several neurotoxins.

Development of immunoassays for determination of circulating venom antigens during envenomations by coral snakes (Micrurus species).

A reverse agglutination assay and two capture enzyme-linked immunoassays (ELISAs) for the quantitative determination of Micrurus nigrocinctus nigrocinctus venom antigens in fluids were developed using affinity-purified polyclonal antibodies and a cocktail of three monoclonal antibodies. The lower detection limit was 0.3 mg/ml for the reverse agglutination assay and 4 ng/ml for the capture ELISAs. The optical densities of both ELISAs correlated very well with venom concentrations in the range 4-333 ng/ml (r = 0.99). The ability of these assays to detect venoms of several medically important Micrurus species was studied. Besides detecting homologous venom, both ELISAs were also useful to quantitate venom from M. fulvius, M. dumerilii carinicauda and M. alleni. Using biotinylated polyclonal antibodies, M. n. nigrocinctus venom antigens were detected in sera or plasma from rabbits and mice during experimental envenomations with lethal and sublethal venom doses. The assays described in this work are promising tests to estimate the severity of poisoning in envenomations by the most important coral snakes of North and Central America.

Antibody-mediated neutralization and binding-reversal studies on alpha-neurotoxins from Micrurus nigrocinctus nigrocinctus (coral snake) venom.

An ELISA based, non-radioactive acetylcholine receptor (AchR) binding assay was used to detect the alpha-neurotoxins present in Micrurus nigrocinctus nigrocinctus venom. Sera from horses hyperimmunized against M. nigrocinctus venom contain antibodies which inhibit the binding of M. n. nigrocinctus alpha-neurotoxins to AchR and reverse the binding of toxins already complexed with the receptor. This result supports the importance of using antivenom therapeutically in M. n. nigrocinctus envenomations even after the onset of neurological symptoms. M. nigrocinctus antivenoms cross-reacted in an ELISA with several elapid alpha-neurotoxins and inhibited the binding of Bungarus multicinctus alpha-bungarotoxin and Naja naja oxiana neurotoxin II to AchR in vitro, suggesting the presence of short-chain and long-chain alpha-neurotoxins in M. nigrocinctus venom. In vivo neutralization experiments with M. nigrocinctus antivenom demonstrate that M. nigrocinctus venom contains short-chain alpha-neurotoxin(s) which share common neutralizing epitope(s) with Naja naja oxiana neurotoxin II.

Characterization of multiple nicotinic acetylcholine receptor-binding proteins and phospholipases A2 from the venom of the coral snake Micrurus nigrocinctus.

The presence of multiple nicotinic acetylcholine receptor (AchR)-binding proteins and phospholipases A2 was detected in the venom of a member of the Elapinae subfamily, Micrurus nigrocinctus nigrocinctus. Multi-step chromatographies were used to isolate four AchR-binding proteins (Mnn-9, Mnn-4, Mnn-3C and Mnn-1A) and five basic PLA2s (nigroxins A, B, C1, C2 and C3). The Micrurus AchR-binding proteins are antigenically and structurally related to short- and long-chain alpha-neurotoxins from Naja. The nigroxins are antigenically similar and constitute a new antigenic subclass of PLA(2)s. Nigroxins A and B are class I PLA(2)s, structurally more related to enzymes from Bungarinae than to those from Hydrophinae/Laticaudinae. These data contribute to clarify the relationships between Micrurus venom proteins and other elapid toxins and may be useful to improve the neutralizing efficiency of antivenoms.

Immunochemical characterization of Micrurus nigrocinctus nigrocinctus venom with monoclonal and polyclonal antibodies.

Eleven murine monoclonal antibodies (MAbs) against Micrurus nigrocinctus nigrocinctus venom were produced and partially characterized. When M. n. nigrocinctus venom proteins were separated by SDS-PAGE under non-reducing conditions four sharp and three diffuse bands were observed. The sharp bands had migration rates comparable to reduced standards of 10, 12, 50 and 72 kDa. The diffuse bands migrate in the range of reduced standards from 14.5 to 32 kDa. When venom proteins were separated under reducing conditions the same sharp bands and an additional prominent 14.5 kDa band were observed. Three antibodies (MAbs 4, 21 and 28) recognized the diffuse bands in western blots of non-reducing SDS-PAGE, whereas MAbs 7G, 22 and 26 reacted with only the 72 kDa protein. MAbs 21 and 28 reacted with the 14.5 kDa band whereas MAb 7G recognized the 72 kDa band in blots of reducing SDS-PAGE. Two M. nigrocinctus antivenoms cross-reacted by ELISA against nine neurotoxic snake venoms, as well as with gamma-toxin from Naja nigricollis and notexin. One antibody (MAb 9A) was used to affinity purify a fraction (called nigroxin) from M. n. nigrocinctus venom. Nigroxin showed phospholipase and myotoxic activities and appeared as a single 15 kDa band in SDS-PAGE under reducing conditions. However, three bands with slight differences in charge were resolved by urea-PAGE, representing isoforms named nigroxin a, b, and c. Nigroxin induced a dose-dependent release of peroxidase trapped in negatively charged liposomes. Nigroxin induced myonecrosis and increased the plasma creatine kinase levels in mice, when injected intramuscularly. The plasma membrane of cultured L6 myoblasts was permeabilized by nigroxin, as evidenced by the release of 3H-uridine nucleotides from prelabelled cells. This effect was completely abolished after preincubation with MAb 9A, although this antibody failed to neutralize the enzymatic activity of nigroxin. Nigroxin was also recognized by MAbs 4, 7H, 21, 27 and 28. Additionally, the epitope recognized by MAb 27 is also present in notexin and beta-bungarotoxin.

Electrophoretic and immunochemical studies of Micrurus snake venoms.

The electrophoretic mobilities of venom components from 15 Micrurus species were studied by polyacrylamide gel electrophoresis. The venoms showed species-specific protein patterns under native (PAGE) or denaturing (urea-PAGE) conditions. However, electrophoretic patterns obtained by SDS-PAGE under reducing conditions were similar. The proteins of all venoms had mol. wts either in the range of 45 to 75 kDa or lower than 14.5 kDa. PAGE and urea-PAGE of single extraction venom samples from 22 M. nigrocinctus nigrocinctus specimens revealed some proteins completely conserved, whereas others exhibited intraspecies variation. Based on ELISA cross-reactivity studies with 11 monoclonal antibodies against M. n. nigrocinctus venom, venoms from M. n. nigrocinctus, M. nigrocinctus mosquitensis, M. fulvius fulvius, M. dumerilii carnicauda and M. albicinctus were included in the same antigenic group, whereas M. frontalis frontalis and M. frontalis braziliensis venoms constituted a second group. Micrurus alleni and M. spixii spixii showed reactivity patterns similar to groups 1 and 2, respectively. Venoms from M. surinamensis surinamensis, M. corallinus, M. ibiboboca, M. hemiprichii ortoni, M. lemniscatus helleri and M. mipartitus had unique cross-reactivity patterns with monoclonal antibodies against M. n. nigrocinctus venom.