Expression of A chain and B chain of beta-bungarotoxin from taiwan banded krait: the functional implication of the interchain disulfide bond between A chain and B chain

Development and optimization of a DNA aptamer to delay β-bungarotoxin-induced lethality in a rodent model

Current treatment of snakebite relies on immunoglobulin-rich antivenoms. However, production of these antivenoms is complicated and costly. Aptamers - single-stranded DNAs or RNAs with specific folding structures that bind to specific target molecules - represent excellent alternatives or complements to antibody-based therapeutics. However, no studies have systematically assessed the feasibility of using aptamers to mitigate venom-induced toxicity in vivo. β-bungarotoxin is the predominant protein responsible for the toxicity of the venom of Bungarus multicinctus, a prominent venomous snake inhabiting Taiwan. In this study, we reported the screening and optimization of a DNA aptamer against β-bungarotoxin and tested its utility in a mouse model. After 14 rounds of directed evolution of ligands by exponential enrichment, an aptamer, called BB3, displaying remarkable binding affinity and specificity for β-bungarotoxin was obtained. Following structural prediction and point-modification experiments, BB3 underwent truncation and was modified with 2'-O-methylation and a 3'-inverted dT. This optimized aptamer showed sustained, high-affinity binding for β-bungarotoxin and exhibited remarkable nuclease resistance in plasma. Importantly, administration of this optimized aptamer extended the survival time of mice treated with a lethal dose of β-bungarotoxin. Collectively, our data provide a compelling illustration of the potential of aptamers as promising candidates for development of recombinant antivenom therapies.

Development of sandwich ELISA and lateral flow strip assays for diagnosing clinically significant snakebite in Taiwan

Taiwan is an island located in the south Pacific, a subtropical region that is home to 61 species of snakes. Of these snakes, four species—Trimeresurus stejnegeri, Protobothrops mucrosquamatus, Bungarus multicinctus and Naja atra—account for more than 90% of clinical envenomation cases. Currently, there are two types of bivalent antivenom: hemorrhagic antivenom against the venom of T. stejnegeri and P. mucrosquamatus, and neurotoxic antivenom for treatment of envenomation by B. multicinctus and N. atra. However, no suitable detection kits are available to precisely guide physicians in the use of antivenoms. Here, we sought to develop diagnostic assays for improving the clinical management of snakebite in Taiwan. A two-step affinity purification procedure was used to generate neurotoxic species-specific antibodies (NSS-Abs) and hemorrhagic species-specific antibodies (HSS-Abs) from antivenoms. These two SSAbs were then used to develop a sandwich ELISA (enzyme-linked immunosorbent assay) and a lateral flow assay comprising two test lines. The resulting ELISAs and lateral flow strip assays could successfully discriminate between neurotoxic and hemorrhagic venoms. The limits of quantification (LOQ) of the ELISA for neurotoxic venoms and hemorrhagic venoms were determined to be 0.39 and 0.78 ng/ml, respectively, and the lateral flow strips were capable of detecting neurotoxic and hemorrhagic venoms at concentrations lower than 5 and 50 ng/ml, respectively, in 10–15 min. Tests of lateral flow strips in 21 clinical snakebite cases showed 100% specificity and 100% sensitivity for neurotoxic envenomation, whereas the sensitivity for detecting hemorrhagic envenomation samples was 36.4%. We herein presented a feasible strategy for developing a sensitive sandwich ELISA and lateral flow strip assay for detecting and differentiating venom proteins from hemorrhagic and neurotoxic snakes. A useful snakebite diagnostic guideline according to the lateral flow strip results and clinical symptoms was proposed to help physicians to use antivenoms appropriately. The two-test-line lateral flow strip assay could potentially be applied in an emergency room setting to help physicians diagnose and manage snakebite victims.

Snakebite is a public health issue that causes life-threatening medical emergencies. Rapid diagnosis of snakebite in the clinic is a critical necessity in many tropical and subtropical countries, where various venomous snakes are common. Venoms from different snake species contain distinct protein components that require treatment with different antivenoms. However, given the similarity in clinical symptoms among some snake envenomations, it is often challenging for physicians to precisely define the snake species responsible for envenomation. Thus, a reliable method or assay for rapidly diagnosing envenoming species is urgently needed. Here, we present a two-step affinity purification procedure for generating species-specific antibodies (SSAbs) from antivenom, followed by the development of a sandwich ELISA (enzyme-linked immunosorbent assay) and lateral flow strip assay using these SSAbs. This feasible and cost-effective strategy allowed us to develop workable assays for distinguishing between venom proteins from hemorrhagic and neurotoxic snakes in Taiwan. The usefulness of this strategy was demonstrated in the clinic, where both diagnostic assays were shown capable of detecting venoms in blood samples from snakebite patients. Together with the observation of clinical symptoms, the two-test-line lateral flow strip assay is potentially applicable in an emergency room setting to improve snakebite diagnosis and management.

Isolation and characterization of a presynaptic neurotoxin, P-elapitoxin-Bf1a from Malaysian Bungarus fasciatus venom

Presynaptic neurotoxins are one of the major components in Bungarus venom. Unlike other Bungarus species that have been studied, β-bungarotoxin has never been isolated from Bungarus fasciatus venom. It was hypothesized that the absence of β-bungarotoxin in this species was due to divergence during evolution prior to evolution of β-bungarotoxin. In this study, we have isolated a β-bungarotoxin isoform we named P-elapitoxin-Bf1a by using gel filtration, cation-exchange and reverse-phase chromatography from Malaysian B. fasciatus venom. The toxin consists of two heterogeneous subunits, subunit A and subunit B. LCMS/MS data showed that subunit A was homologous to acidic phospholipase A2 subunit A3 from Bungarus candidus and B. multicinctus venoms, whereas subunit B was homologous with subunit B1 from B. fasciatus venom that was previously detected by cDNA cloning. The toxin showed concentration- and time-dependent reduction of indirect-twitches without affecting contractile responses to ACh, CCh or KCl at the end of experiment in the chick biventer preparation. Toxin modification with 4-BPB inhibited the neurotoxic effect suggesting the importance of His-48. Tissue pre-incubation with monovalent B. fasciatus (BFAV) or neuro-polyvalent antivenom (NPV), at the recommended titer, was unable to inhibit the twitch reduction induced by the toxin. This study indicates that Malaysian B. fasciatus venom has a unique β-bungarotoxin isoform which was not neutralized by antivenoms. This suggests that there might be other presynaptic neurotoxins present in the venom and there is a variation in the enzymatic neurotoxin composition in venoms from different localities.

Divergence of genes encoding B chains of beta-bungarotoxins

The structural organization of the genes encoding B2, B4, B5 and B6 chains of beta-bungarotoxins are reported in this study. These genes shared virtually identical overall organization with three exons interrupted by two introns in similar positions. On the contrary, intron 1 of these genes had a similar size, a notable variation with the size of intron 2 was observed. It was found that two regions at the second intron of B1 and B2 chains were absent in that of B4, B5 and B6 chains. RT-PCR analyses indicated that Bungarus multicinctus venom gland, heart, liver and muscle expressed the RNA transcripts showing sequence similarity with the intronic segment being deleted in B4, B5 and B6 chain genes. This reflects that the ancestral gene of the intronic segment might insert in multiple loci of B. multicinctus genome. Comparative analyses of B chain genes showed that the protein-coding regions of the exons are more diverse than introns, except for in the signal peptide domain. These results suggest that intron insertions or deletions occur with the evolution of B chains, and that accelerated evolution may diversify the protein-coding sequence of B chain genes same as snake phospholipase A2, neurotoxin and cardiotoxin genes.

KChIP3: a binding protein for Taiwan banded krait beta-bungarotoxin

Using B1 chain of beta-bungarotoxin (beta-Bgt) as bait in yeast two-hybrid screen, we found that KChIP3 was a binding protein of B1 chain. Thus, protein-protein interaction between beta-Bgt and KChIP3 is investigated in the present study. Pull-down assay showed that recombinant KChIP3 proteins were associated with beta-Bgt as well as B1 chain, whereas the inability of KChIPs 1, 2 and 4 to bind with beta-Bgt was observed. Although Ca2+ was not a crucial factor essential for the binding of KChIP3 with beta-Bgt and B1 chain, their interaction could be enhanced by the addition of Ca2+. Alternatively, the association of A1 chain of beta-Bgt with KChIP3 was marginally detected. The dissociation constant of beta-Bgt with KChIP3 were 12.2 and 6.08 microM in the absence and presence of 2mM Ca2+, respectively. Moreover, native KChIP3 from rat brain was to be isolated by beta-Bgt-Sepharose. These observations indicate that KChIP3 is a binding protein of beta-Bgt. In view of the multiple functions of KChIP3 in neuronal cells, the interaction of KChIP3 with beta-Bgt may represent an event for the manifestation of the biological activities of beta-Bgt.

The structural events associated with the binding of divalent cations to β-bungarotoxin

In order to address the mechanism why the Ca2+ was crucial for the manifestation of the phospholipase A2 (PLA2) activity of beta-bungarotoxin (beta-BuTx), four divalent cations were used to assess their influences on the catalytic activity and the fine structures of beta-BuTx. Substitution Mg2+ or Sr2+ for Ca2+ in the substrate solution was found to cause a decrease in the PLA2 activity to approximately 15 or 6% of that in the presence of Ca2+. However, only marginally detectable PLA2 activity was observed with the addition of Ba2+. The nonpolarity of 8-anilinonaphthalene-1-sulfonate (ANS)-binding site of beta-BuTx markedly increased with the binding of cations to beta-BuTx. The negative ellipticity noted with the CD spectra of beta-BuTx increased upon the binding of cations too. With the exception of Ba2+, the order of the ability of cations to enhance the intensity of ANS fluorescence or increase the increment of negative ellipticity was Sr2+ > Ca2+ > Mg2+, which was the same order as the increase in their atomic radii. However, the energy transfer from Trp fluorescence emission to ANS was most effective upon the addition of Ca2+. Moreover, the extent of glutaraldehyde crosslinking between A chain and B chain decreased in the presence of cations. Nevertheless, the binding affinities of beta-BuTx for the four cations were similar. These results, together with the findings that the ANS molecule binds at the active site of the A chain in beta-BuTx, suggest that the binding of Ca2+ to beta-BuTx induces subtly conformational changes occurred at the active site for exerting the activity of beta-BuTx. Moreover, the change in the gross conformation induced by the binding of Ca2+ may affect the interaction between A chain and B chain, and consequently the activity of beta-BuTx as well.

Both A chain and B chain of β-bungarotoxin are functionally involved in the facilitation of spontaneous transmitter release in Xenopus nerve–muscle cultures

In the present study, Xenopus nerve-muscle cultures were used to explore the functional roles of A chain (a phospholipase A(2) subunit) and B chain (a non-phospholipase A(2) subunit) of Bungarus multicinctus beta-bungarotoxin. It was found that beta-bungarotoxin induced an increment of the frequency of spontaneous synaptic currents (SSCs) in the nerve-muscle cultures. Modification of beta-bungarotoxin with pyridoxal-5'-phosphate or substitution of Ca(2+) with Ba(2+) in buffer abolished the phospholipase A(2) activity of beta-bungarotoxin and the facilitatory phase of SSC as well. Antibodies that were directed specifically against A chain or B chain effectively inhibited phospholipase A(2) activity, and as a consequence the SSC frequency was not greatly different from the control rate. These results suggest that both A and B chains are indispensable parts of beta-bungarotoxin for inducing the facilitation of SSC frequency with Xenopus nerve-muscle cultures.

The organization of the genes encoding the A chains of beta-bungarotoxins: evidence for the skipping of exon

Bungarus multicinctus (Taiwan banded krait) beta-bungarotoxins consist of two dissimilar polypeptide chains, A and B. The A chain is structurally homologous to phospholipase A(2) (PLA(2)) enzymes. The structural organization of the genes encoding A1, A2 and A8 chains are reported in this study. Their nucleotide sequences shared up to 97.5% identity. Alignment of the determined A chain genes with their cDNAs revealed that A1 chain gene organized with four exons and three introns, while A2 chain gene comprised three exons and two introns. When A2 chain is expressed, the region corresponding to the first exon of A1 chain gene is skipped instead of the inclusion of intronic sequence adjacent to the second exon. The resulting A2 chain mRNA encoded a 25 residue signal peptide, which is different from A1 chain mRNA with a 27 residue signal peptide. Nevertheless, expression of the A chain genes was partly regulated by a common mechanism as evidenced by sequence conservation of their promoter region and consensus transcriptional factor binding-sites inside this region. 5'-RACE analyses revealed that A chain mRNAs with 27 residue signal peptide represented the predominant species in the preparation of B. multicinctus venom gland mRNAs. Comparative analyses on PLA(2) genes and cDNAs suggest that this is the first report on the skipping of exon which changes the signal peptide sequence of snake venom proteins.