Neurotoxins From Australo-Papuan Elapids: A Biochemical and Pharmacological Perspective

Involvement of phospholipase A2 in the neuromuscular blockade caused by coralsnake (Micrurus spp.) venoms in mouse phrenic nerve-diaphragm preparations in vitro

In this work, we examined the neuromuscular blockade caused by venoms from four South-American coralsnakes (Micrurus altirostris - MA, M. corallinus - MC, M. spixii - MS, and M. dumerilii carinicauda - MDC) and the ability of varespladib (VPL), a phospholipase A2 (PLA2) inhibitor, to attenuate this blockade. PLA2 activity was determined using a colorimetric assay and a fixed amount of venom (10 μg). Neurotoxicity was assayed using a single concentration of venom (10 μg/ml) in mouse phrenic nerve-diaphragm (PND) preparations mounted for myographic recordings and then subjected to histological analysis. All venoms showed PLA2 activity, with MS and MA venoms having the highest (15.53 ± 1.9 A425 nm/min) and lowest (0.23 ± 0.14 A425 nm/min) activities, respectively. VPL (292 and 438 μM) inhibited the PLA2 activity of all venoms, although that of MA venom was least affected. All venoms caused neuromuscular blockade, with MS and MDC venoms causing the fastest and slowest 100% blockade [in 40 ± 3 min and 120 ± 6 min (n = 4), respectively]; MA and MC produced complete blockade within 90-100 min. Preincubation of venoms with 292 μM VPL attenuated the blockade to varying degrees: the greatest inhibition was seen with MDC venom and blockade by MS venom was unaffected by this inhibitor. These results indicate that PLA2 has a variable contribution to coralsnake venom-induced neuromuscular blockade in vitro, with the insensitivity of MS venom to VPL suggesting that blockade by this venom is mediated predominantly by post-synaptically-active α-neurotoxins.

In Vitro Neurotoxicity of Chinese Krait (Bungarus multicinctus) Venom and Neutralization by Antivenoms

Bungarus multicinctus, the Chinese krait, is a highly venomous elapid snake which causes considerable morbidity and mortality in southern China. B. multicinctus venom contains pre-synaptic PLA₂ neurotoxins (i.e., β-bungarotoxins) and post-synaptic neurotoxins (i.e., α-bungarotoxins). We examined the in vitro neurotoxicity of B. multicinctus venom, and the efficacy of specific monovalent Chinese B. multicinctus antivenom, and Australian polyvalent elapid snake antivenom, against venom-induced neurotoxicity. B. multicinctus venom (1-10 μg/mL) abolished indirect twitches in the chick biventer cervicis nerve-muscle preparation as well as attenuating contractile responses to exogenous ACh and CCh, but not KCl. This indicates a post-synaptic neurotoxic action but myotoxicity was not evident. Given that post-synaptic α-neurotoxins have a more rapid onset than pre-synaptic neurotoxins, the activity of the latter in the whole venom will be masked. The prior addition of Chinese B. multicinctus antivenom (12 U/mL) or Australian polyvalent snake antivenom (15 U/mL), markedly attenuated the neurotoxic actions of B. multicinctus venom (3 μg/mL) and prevented the inhibition of contractile responses to ACh and CCh. The addition of B. multicinctus antivenom (60 U/mL), or Australian polyvalent snake antivenom (50 U/mL), at the t₉₀ time point after the addition of B. multicinctus venom (3 μg/mL), did not restore the twitch height over 180 min. The earlier addition of B. multicinctus antivenom (60 U/mL), at the t₂₀ or t₅₀ time points, also failed to prevent the neurotoxic effects of the venom but did delay the time to abolish twitches based on a comparison of t₉₀ values. Repeated washing of the preparation with physiological salt solution, commencing at the t₂₀ time point, failed to reverse the neurotoxic effects of venom or delay the time to abolish twitches. This study showed that B. multicinctus venom displays marked in vitro neurotoxicity in a skeletal muscle preparation which is not reversed by antivenom. This does not appear to be related to antivenom efficacy, but due to the irreversible/pseudo-irreversible nature of the neurotoxins.

Isolation and pharmacological characterization of α-Elapitoxin-Na1a, a novel short-chain postsynaptic neurotoxin from the venom of the Chinese Cobra (Naja atra)

The Chinese Cobra (Naja atra) is an elapid snake of major medical importance in southern China. Although previous studies have shown that postsynaptic neurotoxins account for 11-23% of N. atra venom, envenomed patients do not display marked signs of neurotoxicity. We have previously shown that the lack of clinical neurotoxicity following snake envenoming by some species with 'neurotoxic' venoms may be related to the high prevalence of short-chain postsynaptic neurotoxins in these venoms. In this study, we describe the isolation and characterization of α-Elapitoxin-Na1a (α-EPTX-Na1a; 6949 Da), a short-chain postsynaptic neurotoxin, which accounts for approximately 9% of N. atra crude venom. α-EPTX-Na1a (30-300 nM) produced concentration-dependent inhibition of indirect-twitches, with a t₉₀ value of 17 ± 2 min at 300 nM, and abolished contractile responses to exogenous acetylcholine and carbachol, in the chick biventer cervicis nerve-muscle preparation. The prior addition of either Chinese N. atra monovalent antivenom (0.3 U/ml) or Australian polyvalent snake antivenom (2.4 U/ml), prevented the in vitro neurotoxic effects of α-EPTX-Na1a (30 nM). Addition of each of these antivenoms at the t₉₀ time point partially reversed the in vitro neurotoxicity caused by α-EPTX-Na1a (30 nM). The inhibition of indirect twitches by α-EPTX-Na1a (30 nM) was not reversed by repeatedly washing the tissue. α-EPTX-Na1a displayed pseudo-irreversible antagonism of concentration-response curves to carbachol with a pA₂ value of 8.21. De novo protein sequencing of α-EPTX-Na1a revealed a typical short-chain postsynaptic neurotoxin profile of 62 amino acids which shared >98% amino acid sequence similarity with short-chain postsynaptic neurotoxins from other Naja species. When compared to short-chain neurotoxins isolated from cobras in China, α-EPTX-Na1a contained novel residues K47Q (i.e. lysine to glutamine), N48T (i.e. asparagine to threonine) and G49A (i.e. glycine to alanine). In conclusion, α-EPTX-Na1a is a potent, pseudo-irreversible, short-chain neurotoxin. The high prevalence of α-EPTX-Na1a in Chinese N. atra venom is likely to explain the mild neurotoxicity experienced by envenomed patients.

An in vivo examination of the differences between rapid cardiovascular collapse and prolonged hypotension induced by snake venom

We investigated the cardiovascular effects of venoms from seven medically important species of snakes: Australian Eastern Brown snake (Pseudonaja textilis), Sri Lankan Russell’s viper (Daboia russelii), Javanese Russell’s viper (D. siamensis), Gaboon viper (Bitis gabonica), Uracoan rattlesnake (Crotalus vegrandis), Carpet viper (Echis ocellatus) and Puff adder (Bitis arietans), and identified two distinct patterns of effects: i.e. rapid cardiovascular collapse and prolonged hypotension. P. textilis (5 µg/kg, i.v.) and E. ocellatus (50 µg/kg, i.v.) venoms induced rapid (i.e. within 2 min) cardiovascular collapse in anaesthetised rats. P. textilis (20 mg/kg, i.m.) caused collapse within 10 min. D. russelii (100 µg/kg, i.v.) and D. siamensis (100 µg/kg, i.v.) venoms caused ‘prolonged hypotension’, characterised by a persistent decrease in blood pressure with recovery. D. russelii venom (50 mg/kg and 100 mg/kg, i.m.) also caused prolonged hypotension. A priming dose of P. textilis venom (2 µg/kg, i.v.) prevented collapse by E. ocellatus venom (50 µg/kg, i.v.), but had no significant effect on subsequent addition of D. russelii venom (1 mg/kg, i.v). Two priming doses (1 µg/kg, i.v.) of E. ocellatus venom prevented collapse by E. ocellatus venom (50 µg/kg, i.v.). B. gabonica, C. vegrandis and B. arietans (all at 200 µg/kg, i.v.) induced mild transient hypotension. Artificial respiration prevented D. russelii venom induced prolonged hypotension but not rapid cardiovascular collapse from E. ocellatus venom. D. russelii venom (0.001–1 μg/ml) caused concentration-dependent relaxation (EC₅₀ = 82.2 ± 15.3 ng/ml, R_max = 91 ± 1%) in pre-contracted mesenteric arteries. In contrast, E. ocellatus venom (1 µg/ml) only produced a maximum relaxant effect of 27 ± 14%, suggesting that rapid cardiovascular collapse is unlikely to be due to peripheral vasodilation. The prevention of rapid cardiovascular collapse, by ‘priming’ doses of venom, supports a role for depletable endogenous mediators in this phenomenon.

Biotoxins in muscle regeneration research

Skeletal muscles are characterized by their unique regenerative capacity following injury due to the presence of muscle precursor cells, satellite cells. This characteristic allows researchers to study muscle regeneration using experimental injury models. These injury models should be stable and reproducible. Variety of injury models have been used, among which the intramuscular injection of myotoxic biotoxins is considered the most common and widespread method in muscle regeneration research. By using isolated biotoxins, researchers could induce acute muscle damage and regeneration in a controlled and reproducible manner. Therefore, it is considered an easy method for inducing muscle injury in order to understand the different mechanisms involved in muscle injuries and tissue response following injury. However, different toxins and venoms have different compositions and subsequently the possible effects of these toxins on skeletal muscle vary according to their composition. Moreover, regeneration of injured muscle by venoms and toxins varies according to the target of toxin or venom. Therefore, it is essential for researcher to be aware of the mechanism and possible target of toxin-induced injury. The current paper provides an overview of the biotoxins used in skeletal muscle research.

D. russelii Venom Mediates Vasodilatation of Resistance Like Arteries via Activation of Kv and KCa Channels

Russell's viper (Daboia russelii) venom causes a range of clinical effects in humans. Hypotension is an uncommon but severe complication of Russell's viper envenoming. The mechanism(s) responsible for this effect are unclear. In this study, we examined the cardiovascular effects of Sri Lankan D. russelii venom in anaesthetised rats and in isolated mesenteric arteries. D. russelii venom (100 μg/kg, i.v.) caused a 45 ± 8% decrease in blood pressure within 10 min of administration in anaesthetised (100 μg/kg ketamine/xylazine 10:1 ratio, i.p.) rats. Venom (1 ng/mL⁻1 μg/mL) caused concentration-dependent relaxation (EC50 = 145.4 ± 63.6 ng/mL, Rmax = 92 ± 2%) in U46619 pre-contracted rat small mesenteric arteries mounted in a myograph. Vasorelaxant potency of venom was unchanged in the presence of the nitric oxide synthase inhibitor, L-NAME (100 µM), or removal of the endothelium. In the presence of high K⁺ (30 mM), the vasorelaxant response to venom was abolished. Similarly, blocking voltage-dependent (Kv: 4-aminopryidine; 1000 µM) and Ca2+-activated (KCa: tetraethylammonium (TEA; 1000 µM); SKCa: apamin (0.1 µM); IKCa: TRAM-34 (1 µM); BKCa; iberiotoxin (0.1 µM)) K⁺ channels markedly attenuated venom-induced relaxation. Responses were unchanged in the presence of the ATP-sensitive K⁺ channel blocker glibenclamide (10 µM), or H1 receptor antagonist, mepyramine (0.1 µM). Venom-induced vasorelaxtion was also markedly decreased in the presence of the transient receptor potential cation channel subfamily V member 4 (TRPV4) antagonist, RN-1734 (10 µM). In conclusion, D. russelii-venom-induced hypotension in rodents may be due to activation of Kv and KCa channels, leading to vasorelaxation predominantly via an endothelium-independent mechanism. Further investigation is required to identify the toxin(s) responsible for this effect.

Addressing the Issue of Tetrodotoxin Targeting

This review is devoted to the medical application of tetrodotoxin (TTX), a potent non-protein specific blocker of voltage-gated sodium (NaV) channels. The selectivity of action, lack of affinity with the heart muscle NaV channels, and the inability to penetrate the blood–brain barrier make this toxin an attractive candidate for anesthetic and analgesic drug design. The efficacy of TTX was shown in neuropathic, acute and inflammatory pain models. The main emphasis of the review is on studies focused on the improvement of TTX efficacy and safety in conjunction with additional substances and drug delivery systems. A significant improvement in the effectiveness of the toxin was demonstrated when used in tandem with vasoconstrictors, local anesthetics and chemical permeation enhancers, with the best results obtained with the encapsulation of TTX in microparticles and liposomes conjugated to gold nanorods.

Effects of Animal Venoms and Toxins on Hallmarks of Cancer

Animal venoms are a cocktail of proteins and peptides, targeting vital physiological processes. Venoms have evolved to assist in the capture and digestion of prey. Key venom components often include neurotoxins, myotoxins, cardiotoxins, hematoxins and catalytic enzymes. The pharmacological activities of venom components have been investigated as a source of potential therapeutic agents. Interestingly, a number of animal toxins display profound anticancer effects. These include toxins purified from snake, bee and scorpion venoms effecting cancer cell proliferation, migration, invasion, apoptotic activity and neovascularization. Indeed, the mechanism behind the anticancer effect of certain toxins is similar to that of agents currently used in chemotherapy. For example, Lebein is a snake venom disintegrin which generates anti-angiogenic effects by inhibiting vascular endothelial growth factors (VEGF). In this review article, we highlight the biological activities of animal toxins on the multiple steps of tumour formation or hallmarks of cancer. We also discuss recent progress in the discovery of lead compounds for anticancer drug development from venom components.

Venomous and poisonous Australian animals of veterinary importance: a rich source of novel therapeutics

Envenomation and poisoning by terrestrial animals (both vertebrate and invertebrate) are a significant economic problem and health risk for domestic animals in Australia. Australian snakes are some of the most venomous animals in the world and bees, wasps, ants, paralysis ticks, and cane toads are also present as part of the venomous and poisonous fauna. The diagnosis and treatment of envenomation or poisoning in animals is a challenge and can be a traumatic and expensive process for owners. Despite the potency of Australian venoms, there is potential for novel veterinary therapeutics to be modeled on venom toxins, as has been the case with human pharmaceuticals. A comprehensive overview of envenomation and poisoning signs in livestock and companion animals is provided and related to the potential for venom toxins to act as therapeutics.

Comparative studies of the venom of a new Taipan species, Oxyuranus temporalis, with other members of its genus

Taipans are highly venomous Australo-Papuan elapids. A new species of taipan, the Western Desert Taipan (Oxyuranus temporalis), has been discovered with two specimens housed in captivity at the Adelaide Zoo. This study is the first investigation of O. temporalis venom and seeks to characterise and compare the neurotoxicity, lethality and biochemical properties of O. temporalis venom with other taipan venoms. Analysis of O. temporalis venom using size-exclusion and reverse-phase HPLC indicated a markedly simplified "profile" compared to other taipan venoms. SDS-PAGE and agarose gel electrophoresis analysis also indicated a relatively simple composition. Murine LD50 studies showed that O. temporalis venom is less lethal than O. microlepidotus venom. Venoms were tested in vitro, using the chick biventer cervicis nerve-muscle preparation. Based on t90 values, O. temporalis venom is highly neurotoxic abolishing indirect twitches far more rapidly than other taipan venoms. O. temporalis venom also abolished responses to exogenous acetylcholine and carbachol, indicating the presence of postsynaptic neurotoxins. Prior administration of CSL Taipan antivenom (CSL Limited) neutralised the inhibitory effects of all taipan venoms. The results of this study suggest that the venom of the O. temporalis is highly neurotoxic in vitro and may contain procoagulant toxins, making this snake potentially dangerous to humans.

In vitro toxic effects of puff adder (Bitis arietans) venom, and their neutralization by antivenom

This study investigated the in vitro toxic effects of Bitis arietans venom and the ability of antivenom produced by the South African Institute of Medical Research (SAIMR) to neutralize these effects. The venom (50 µg/mL) reduced nerve-mediated twitches of the chick biventer muscle to 19% ± 2% of initial magnitude (n = 4) within 2 h. This inhibitory effect of the venom was significantly attenuated by prior incubation of tissues with SAIMR antivenom (0.864 µg/µL; 67% ± 4%; P < 0.05; n = 3–5, unpaired t-test). Addition of antivenom at t₅₀ failed to prevent further inhibition or reverse the inhibition of twitches and responses to agonists. The myotoxic action of the venom (50 µg/mL) was evidenced by a decrease in direct twitches (30% ± 6% of the initial twitch magnitude) and increase in baseline tension (by 0.7 ± 0.3 g within 3 h) of the chick biventer. Antivenom failed to block these effects. Antivenom however prevented the venom induced cytotoxic effects on L6 skeletal muscle cells. Venom induced a marginal but significant reduction in plasma clotting times at concentrations above 7.8 µg/100 µL of plasma, indicating poor procoagulant effects. In addition, the results of western immunoblotting indicate strong immunoreactivity with venom proteins, thus warranting further detailed studies on the neutralization of the effects of individual venom toxins by antivenom.

Inhibition of presynaptic neurotoxins in taipan venom by suramin

Taipans are amongst the most venomous snakes in the world, and neurotoxicity is a major life-threatening symptom of envenoming by these snakes. Three species of taipans exist, and the venom from each species contains a presynaptic neurotoxin which accounts for much of the neurotoxicity observed following human envenoming. The high cost of antivenom used to treat neurotoxicity has resulted in the need to develop alternative but effective therapies. Therefore, in this study, we examined the ability of the P2Y receptor antagonist suramin to prevent the in vitro neurotoxic effects of the three presynaptic neurotoxins in taipan venoms: taipoxin, paradoxin and cannitoxin. Toxins were purchased from commercial sources or purified in house, using multiple steps of gel filtration chromatography. All three toxins (11 nM) inhibited nerve-mediated twitches in the chick biventer cervicis nerve-muscle preparation within 300 min. The presence of suramin (0.3 mM) completely blocked the taipoxin and cannitoxin-mediated inhibition of nerve-mediated twitches within the course of the experiment (P < 0.0001). However, paradoxin induced a 32 % decrease in twitch height even in the presence of suramin within 360 min. This was significantly different compared to toxin alone (P < 0.0001). We also examined the effect of suramin on the neurotoxic effects of textilotoxin and the products of phospholipase A2 action. Each toxin alone or in the presence of suramin failed to inhibit the responses to exogenous agonists ACh, CCh or KCl. Our results warrant clinical studies aimed determining the efficacy of suramin in preventing the onset of neurotoxicity following taipan envenoming.

Characterization of monomeric and multimeric snake neurotoxins and other bioactive proteins from the venom of the lethal Australian common copperhead (Austrelaps superbus)

Envenomation by Australian copperheads results mainly in muscle paralysis largely attributed to the presence of postsynaptic α-neurotoxins. However, poorly reversible neurotoxic effects suggest that these venoms may contain snake presynaptic phospholipase A2 neurotoxins (SPANs) that irreversibly inhibit neurotransmitter release. Using size-exclusion liquid chromatography, the present study isolated the first multimeric SPAN complex from the venom of the Australian common copperhead, Austrelaps superbus. The multimeric SPAN P-elapitoxin-As1a (P-EPTX-As1a) along with two novel monomeric SPANs and a new postsynaptic α-neurotoxin were then pharmacologically characterized using the chick biventer cervicis nerve-muscle preparation. All SPANs inhibited nerve-evoked twitch contractions at the neuromuscular junction without inhibiting contractile responses to cholinergic agonists or KCl. These actions are consistent with a prejunctional action to inhibit neurotransmitter release, without direct myotoxicity. Furthermore, the multimeric P-EPTX-As1a caused tetanic 'fade' in muscle tension under high frequency nerve stimulation, and produced a triphasic alteration to neurotransmitter release. These actions have been previously noted with other multimeric SPAN complexes such as taipoxin. Moreover, the neurotoxic α-subunit of P-EPTX-As1a shows high homology to taipoxin α-chain. Several other coagulopathic and myotoxic high mass proteins including a class PIII snake venom metalloproteinase, C-type lectin, l-amino acid oxidase, acetylcholinesterase and phospholipase B were also identified that may contribute to the overall toxicity of A. superbus venom. In conclusion, clinicians should be aware that early antivenom intervention might be necessary to prevent the onset of irreversible presynaptic neurotoxicity caused by multimeric and monomeric SPANs and that A. superbus venom is potentially capable of producing coagulopathic and myotoxic effects.

Clinical effects and antivenom dosing in brown snake (Pseudonaja spp.) envenoming--Australian snakebite project (ASP-14)

Background

Snakebite is a global health issue and treatment with antivenom continues to be problematic. Brown snakes (genus Pseudonaja) are the most medically important group of Australian snakes and there is controversy over the dose of brown snake antivenom. We aimed to investigate the clinical and laboratory features of definite brown snake (Pseudonaja spp.) envenoming, and determine the dose of antivenom required.

Methods and Finding

This was a prospective observational study of definite brown snake envenoming from the Australian Snakebite Project (ASP) based on snake identification or specific enzyme immunoassay for Pseudonaja venom. From January 2004 to January 2012 there were 149 definite brown snake bites [median age 42y (2–81y); 100 males]. Systemic envenoming occurred in 136 (88%) cases. All envenomed patients developed venom induced consumption coagulopathy (VICC), with complete VICC in 109 (80%) and partial VICC in 27 (20%). Systemic symptoms occurred in 61 (45%) and mild neurotoxicity in 2 (1%). Myotoxicity did not occur. Severe envenoming occurred in 51 patients (38%) and was characterised by collapse or hypotension (37), thrombotic microangiopathy (15), major haemorrhage (5), cardiac arrest (7) and death (6). The median peak venom concentration in 118 envenomed patients was 1.6 ng/mL (Range: 0.15–210 ng/mL). The median initial antivenom dose was 2 vials (Range: 1–40) in 128 patients receiving antivenom. There was no difference in INR recovery or clinical outcome between patients receiving one or more than one vial of antivenom. Free venom was not detected in 112/115 patients post-antivenom with only low concentrations (0.4 to 0.9 ng/ml) in three patients.

Conclusions

Envenoming by brown snakes causes VICC and over a third of patients had serious complications including major haemorrhage, collapse and microangiopathy. The results of this study support accumulating evidence that giving more than one vial of antivenom is unnecessary in brown snake envenoming.

In vitro neurotoxic effects of Pseudechis spp. venoms: A comparison of avian and murine skeletal muscle preparations

Two common in vitro skeletal muscle preparations used for the study of venom neurotoxicity are the indirectly stimulated chick isolated biventer cervicis nerve-muscle preparation and the rat isolated phrenic nerve-diaphragm preparation. The aim of the current study was to compare the in vitro neurotoxicity of six Pseudechis spp. (Black snakes) venoms in both avian (chicken) and mammalian (rat) skeletal muscle preparations to determine differences in sensitivity. All Pseudechis spp. venoms significantly inhibited indirect twitches, in both preparations, indicating the presence of post synaptic neurotoxins. The inhibitory effects of all venoms were more rapid in the avian preparation, except for Pseudechis colletti venom where no significant difference was seen between the murine and avian muscles. Time taken to produce 50% reduction in stimulated twitches (i.e. t(50)) was markedly shorter in the avian preparation. We have shown that the avian in vitro preparation is more sensitive to the neurotoxic activity of Pseudechis spp. than the murine preparation. This difference is likely to be due to species differences in the interaction between the neurotoxins and the nicotinic receptor binding sites as well as differences in the 'safety factor' between the preparations.

Differential evolution and neofunctionalization of snake venom metalloprotease domains

Snake venom metalloproteases (SVMP) are composed of five domains: signal peptide, propeptide, metalloprotease, disintegrin, and cysteine-rich. Secreted toxins are typically combinatorial variations of the latter three domains. The SVMP-encoding genes of Psammophis mossambicus venom are unique in containing only the signal and propeptide domains. We show that the Psammophis SVMP propeptide evolves rapidly and is subject to a high degree of positive selection. Unlike Psammophis, some species of Echis express both the typical multidomain and the unusual monodomain (propeptide only) SVMP, with the result that a lower level of variation is exerted upon the latter. We showed that most mutations in the multidomain Echis SVMP occurred in the protease domain responsible for proteolytic and hemorrhagic activities. The cysteine-rich and disintegrin-like domains, which are putatively responsible for making the P-III SVMPs more potent than the P-I and P-II forms, accumulate the remaining variation. Thus, the binding sites on the molecule's surface are evolving rapidly whereas the core remains relatively conserved. Bioassays conducted on two post-translationally cleaved novel proline-rich peptides from the P. mossambicus propeptide domain showed them to have been neofunctionalized for specific inhibition of mammalian a7 neuronal nicotinic acetylcholine receptors. We show that the proline rich postsynaptic specific neurotoxic peptides from Azemiops feae are the result of convergent evolution within the precursor region of the C-type natriuretic peptide instead of the SVMP. The results of this study reinforce the value of studying obscure venoms for biodiscovery of novel investigational ligands.

Cross-neutralisation of the neurotoxic effects of Egyptian cobra venom with commercial tiger snake antivenom

Cross-neutralisation has been demonstrated for haemorrhagic venoms including Echis spp. and Cerastes spp. and for Australia elapid procoagulant toxins. A previous study showed that commercial tiger snake antivenom (TSAV) was able to neutralise the systemic effects of the Egyptian cobra, Naja haje, in vivo but it is unclear if this was true cross-neutralisation. The aim of the current study was to determine whether TSAV can neutralise the in vitro neurotoxic effects of N. haje venom. Both Notechis scutatus (10 μg/ml) and N. haje (10 μg/ml) venoms caused inhibition of indirect (supramaximal V, 0.1 Hz, 0.2 msec.) twitches of the chick biventer cervicis nerve-muscle preparation with t(90) values (i.e. the time to produce 90% inhibition of the original twitch height) of 26 ± 1 min. (n = 4) and 36 ± 4 min.; (n = 4). This effect at 10 μg/ml was significantly attenuated by the prior addition of TSAV (5 U/ml). A comparison of the reverse-phase HPLC profiles of both venoms showed some similarities with peak elution times, and SDS-PAGE analysis elucidated comparable bands across both venoms. Further analysis using Western immunoblotting indicated TSAV was able to detect N. haje venom, and enzyme immunoassay showed that in-house biotinylated polyclonal monovalent N. scutatus antibodies were able to detect N. haje venom. These findings demonstrate cross-neutralisation between different and geographically separated snakes supporting potential immunological similarities in snake toxin groups for a large range of snakes. This provides more evidence that antivenoms could be developed against specific toxin groups to cover a large range of snakes.

Isolation and characterisation of P-EPTX-Ap1a and P-EPTX-Ar1a: pre-synaptic neurotoxins from the venom of the northern (Acanthophis praelongus) and Irian Jayan (Acanthophis rugosus) death adders

The neurotoxicity observed following death adder envenoming has been thought to be solely due to the presence of potent post-synaptic neurotoxins. Clinically, these effects are often poorly reversed by death adder antivenom or anticholinesterase, particularly when patients present with established paralysis. This suggests that either the post-synaptic neurotoxins are irreversible/'pseudo' irreversible, or the venom contains pre-synaptic neurotoxins that do not respond to antivenom. To support the later hypothesis, a pre-synaptic neurotoxin (P-EPTX-Aa1a) has recently been isolated from the venom of Acanthophis antarcticus. We examined Acanthophis praelongus and Acanthophis rugosus venoms for the presence of pre-synaptic neurotoxins. P-EPTX-Ap1a (40,719Da) and P-EPTX-Ar1a (40,879Da) were isolated from A. praelongus and A. rugosus venoms, respectively. P-EPTX-Ap1a and P-EPTX-Ar1a are comprised of three different subunits, alpha, beta1 and beta2. The two toxins displayed similar levels of PLA(2) activity which was almost solely attributed to the alpha subunit in both toxins. P-EPTX-Ap1a (20-100nM) and P-EPTX-Ar1a (20-100nM) caused inhibition of indirect twitches of the skeletal muscle preparation without affecting contractile responses to nicotinic receptor agonists. Interestingly, only the alpha subunit of both toxins (300nM) displayed neurotoxic activity. Inhibition of PLA(2) activity markedly reduced the effect of the toxins on muscle twitch height. These results confirm that P-EPTX-Ap1a and P-EPTX-Ar1a are pre-synaptic neurotoxins and represent the second and third such toxins to be isolated from death adder venom. The presence of pre-synaptic neurotoxins in Acanthophis sp. venoms indicates that treatment strategies for envenoming by these snakes needs to be reassessed given the likelihood of irreversible neurotoxicity.

Characterisation of the heterotrimeric presynaptic phospholipase A(2) neurotoxin complex from the venom of the common death adder (Acanthophis antarcticus)

While Australo-Papuan death adder neurotoxicity is generally considered to be due to the actions of reversible competitive postsynaptic alpha-neurotoxins, the neurotoxic effects are often poorly reversed by antivenom or anticholinesterases. This suggests that the venom may contain a snake presynaptic phospholipase A(2) (PLA(2)) neurotoxin (SPAN) that binds irreversibly to motor nerve terminals to inhibit neurotransmitter release. Using size-exclusion liquid chromatography under non-reducing conditions, we report the isolation and characterisation of a high molecular mass SPAN complex, P-elapitoxin-Aa1a (P-EPTX-Aa1a), from the venom of the common death adder Acanthophis antarcticus. Using the chick biventer-cervicis nerve-muscle preparation, P-EPTX-Aa1a (44,698Da) caused inhibition of nerve-evoked twitch contractions while responses to cholinergic agonists and KCl remained unaffected. P-EPTX-Aa1a also produced significant fade in tetanic contractions and a triphasic timecourse of neuromuscular blockade. These actions are consistent with other SPANs that inhibit acetylcholine release. P-EPTX-Aa1a was found to be a heterotrimeric complex composed of alpha, beta and gamma-subunits in a 1:1:1 stoichiometry with each subunit showing significant N-terminal sequence homology to the subunits of taipoxin, a SPAN from Oxyuranus s. scutellatus. Like taipoxin, only the alpha-chain produced any signs of neurotoxicity or displayed significant PLA(2) enzymatic activity. Preincubation with monovalent death adder antivenom or suramin, or inhibition of PLA(2) activity by incubation with 4-bromophenacyl bromide, either prevented or significantly delayed the onset of toxicity by P-EPTX-Aa1a. However, antivenom failed to reverse neurotoxicity. Early intervention with antivenom may therefore be important in severe cases of envenomation by A. antarcticus, given the presence of potent irreversible presynaptic neurotoxins.

Cross-neutralisation of Australian brown snake, taipan and death adder venoms by monovalent antibodies

An understanding of the cross-neutralisation of snake venoms by antibodies is important for snake antivenom development. We investigated the cross-neutralisation of brown snake (Pseudonaja textilis) venom, taipan (Oxyuranus scutellatus) venom and death adder (Acanthophis antarcticus) with commercial antivenoms and monovalent anti-snake IgG, using enzyme immunoassays, in vitro clotting and neurotoxicity assays. Each commercial antivenom bound all three venoms, and neutralised clotting activity of brown snake and taipan venoms and neurotoxicity of death adder venom. The 'in-house' monovalent anti-snake venom IgG raised against procoagulant brown snake and taipan venoms, did not neutralise the neurotoxic effects of death adder venom. However, they did cross-neutralise the procoagulant effects of both procoagulant venoms. This supports the idea of developing antivenoms against groups of snake toxins rather than individual snake venoms.

Antivenom efficacy or effectiveness: the Australian experience

Despite widespread use of antivenoms, many questions remain about their effectiveness in the clinical setting. The almost universal acceptance of their value is based mainly on in vitro studies, animal studies and human observational studies. Numerous examples exist where they demonstrate clear benefit, such as consumption coagulopathy in viper envenoming, prevention of neurotoxicity in Australasian elapid bites, systemic effects in scorpion and funnel-web spider envenoming. There are also concerns about the quality and efficacy of some antivenoms. However, it is important not to confuse the efficacy of antivenom, defined as its ability to bind and neutralise venom-mediated effects under ideal conditions, and the effectiveness of antivenom, defined as its ability to reverse or prevent envenoming in human cases. There are numerous potential reasons for antivenom failure in human envenoming, of which antivenom inefficacy is only one. Other important reasons include venom-mediated effects being irreversible, antivenom being unable to reach the site of toxin-mediated injury, or the rapidity of onset of venom-mediated effects. A number of recent studies in Australia bring into question the effectiveness of some antivenoms, including snake antivenom for coagulopathy, redback spider and box jellyfish antivenoms. Despite brown snake antivenom being able to neutralise venom induced clotting in vitro, use of the antivenom in human envenoming does not appear to change the time course of coagulopathy. However, it is important that apparent antivenom ineffectiveness in specific cases is correctly interpreted and does not lead to a universal belief that antivenom is ineffective. It should rather encourage further studies to investigate the underlying pathophysiology of envenoming, the pharmacokinetics of venoms and antivenoms, and ultimately the effectiveness of antivenom based on snake type, clinical effects and timing of administration.

An examination of the activity of expired and mistreated commercial Australian antivenoms

Expired antivenoms may be useful in countries where snake envenoming is common and supplies are limited. This study examined the activity of expired Australasian antivenoms. Expired CSL snake antivenoms, including taipan, brown snake and polyvalent antivenoms, were used. The most current antivenom was used as the reference to compare expired antivenoms. Binding activity was assessed by enzyme immunoassay. Neutralisation of venom clotting effects was assessed by a modified clotting test using changes in optical density. Neutralisation of the in vitro neurotoxic effects of taipan venom was determined using a chick biventer cervicis nerve-muscle preparation. All antivenom batches remained active, with gradual deterioration in activity and binding over time. All batches of taipan antivenom at concentrations equivalent to the administration of one vial (including one 15 years expired) prevented clotting by taipan venom. Brown snake antivenoms also prevented clotting, except two that were 10 years old. All expired taipan/polyvalent antivenom prevented in vitro neurotoxicity at concentrations consistent with antivenom treatment. Freeze-thawing the antivenom or leaving it at room temperature for 3 days caused only small decreases in activity. CSL antivenoms are more robust than indicated on their label and maintain useful activity long past their nominated expiry dates.