Understanding the molecular mechanism underlying the presynaptic toxicity of secreted phospholipases A2: An update

Neurotoxic snakebites in Africa: Clinical implications, therapeutic strategies, and antivenom efficacy

Snakebite is a significant health concern in Africa, particularly due to neurotoxic envenomation which can lead to neuromuscular paralysis and respiratory failure. In Nigeria, snakes from the Elapidae family are a notable cause of envenomation cases, though these incidents are underreported. This review examined case reports of neurotoxic envenomation in Africa, highlighting the clinical impacts and the efficacy of available antivenoms. Preclinical studies showed that the polyvalent antivenom from the South African Institute for Medical Research (SAIMR) was highly effective against neurotoxicity with a protective efficacy (R) of 1346.80 mg/mL, while clinical assessment emphasized the need for high-dose antivenom therapy along with supportive measures like mechanical ventilation. Unlike hemorrhagic envenomation, where antivenom promptly resolves bleeding, neurotoxic cases often require additional interventions. The review underscores the necessity for tailored approaches in antivenom therapy to address the complexities of neurotoxic snakebites and reduce their public health burden in Africa.

Comparative electrophysiological characterization of ammodytoxin A, a β-neurotoxin from the nose-horned viper venom, and its enzymatically inactive mutant

Presynaptic- or β-neurotoxicity of secreted phospholipases A2 (sPLA2) is a complex process. For full expression of β-neurotoxicity, the enzymatic activity of the toxin is essential. However, it has been shown that not all toxic effects of a β-neurotoxin depend on its enzymatic activity, for example, the inhibition of mitochondrial cytochrome c oxidase. The main objective of this study was to verify whether it is possible to observe and study the phospholipase-independent actions of β-neurotoxins by a standard ex vivo twitch-tension experimental approach. To this end, we compared the effects of a potent snake venom β-neurotoxin, ammodytoxin A (AtxA), and its enzymatically inactive mutant AtxA(D49S) on muscle contraction of the mouse phrenic nerve-hemidiaphragm preparation. While AtxA significantly affected the amplitude of the indirectly evoked isometric muscle contraction, the resting tension of the neuromuscular (NM) preparation, the amplitude of the end-plate potential (EPP), the EPP half decay time and the resting membrane potential, AtxA(D49S) without enzymatic activity did not. From this, we can conclude that the effects of AtxA independent of enzymatic activity cannot be studied with classical electrophysiological measurements on the isolated NM preparation. Our results also suggest that the inhibition of cytochrome c oxidase activity by AtxA is not involved in the rapid NM blockade by this β-neurotoxin, but that its pathological consequences are rather long-term. Interestingly, in our experimental setup, AtxA upon direct stimulation reduced the amplitude of muscle contraction and induced contracture of the hemidiaphragm, effects that could be interpreted as myotoxic.

An agonist of CXCR4 induces a rapid recovery from the neurotoxic effects of Vipera ammodytes and Vipera aspis venoms

People bitten by Alpine vipers are usually treated with antivenom antisera to prevent the noxious consequences caused by the injected venom. However, this treatment suffers from a number of drawbacks and additional therapies are necessary. The venoms of Vipera ammodytes and of Vipera aspis are neurotoxic and cause muscle paralysis by inducing neurodegeneration of motor axon terminals because they contain a presynaptic acting sPLA2 neurotoxin. We have recently found that any type of damage to motor axons is followed by the expression and activation of the intercellular signaling axis consisting of the CXCR4 receptor present on the membrane of the axon stump and of its ligand, the chemokine CXCL12 released by activated terminal Schwann cells. We show here that also V. ammodytes and V. aspis venoms cause the expression of the CXCL12-CXCR4 axis. We also show that a small molecule agonist of CXCR4, dubbed NUCC-390, induces a rapid regeneration of the motor axon terminal with functional recovery of the neuromuscular junction. These findings qualify NUCC-390 as a promising novel therapeutics capable of improving the recovery from the paralysis caused by the snakebite of the two neurotoxic Alpine vipers.

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.

What Are the Neurotoxins in Hemotoxic Snake Venoms?

Snake venoms as tools for hunting are primarily aimed at the most vital systems of the prey, especially the nervous and circulatory systems. In general, snakes of the Elapidae family produce neurotoxic venoms comprising of toxins targeting the nervous system, while snakes of the Viperidae family and most rear-fanged snakes produce hemotoxic venoms directed mainly on blood coagulation. However, it is not all so clear. Some bites by viperids results in neurotoxic signs and it is now known that hemotoxic venoms do contain neurotoxic components. For example, viperid phospholipases A₂ may manifest pre- or/and postsynaptic activity and be involved in pain and analgesia. There are other neurotoxins belonging to diverse families ranging from large multi-subunit proteins (e.g., C-type lectin-like proteins) to short peptide neurotoxins (e.g., waglerins and azemiopsin), which are found in hemotoxic venoms. Other neurotoxins from hemotoxic venoms include baptides, crotamine, cysteine-rich secretory proteins, Kunitz-type protease inhibitors, sarafotoxins and three-finger toxins. Some of these toxins exhibit postsynaptic activity, while others affect the functioning of voltage-dependent ion channels. This review represents the first attempt to systematize data on the neurotoxins from "non-neurotoxic" snake venom. The structural and functional characteristic of these neurotoxins affecting diverse targets in the nervous system are considered.

Secretory Phospholipases A2, from Snakebite Envenoming to a Myriad of Inflammation Associated Human Diseases-What Is the Secret of Their Activity?

Secreted phospholipases of type A2 (sPLA2s) are proteins of 14-16 kDa present in mammals in different forms and at different body sites. They are involved in lipid transformation processes, and consequently in various immune, inflammatory, and metabolic processes. sPLA2s are also major components of snake venoms, endowed with various toxic and pharmacological properties. The activity of sPLA2s is not limited to the enzymatic one but, through interaction with different types of molecules, they exert other activities that are still little known and explored, both outside and inside the cells, as they can be endocytosed. The aim of this review is to analyze three features of sPLA2s, yet under-explored, knowledge of which could be crucial to understanding the activity of these proteins. The first feature is their disulphide bridge pattern, which has always been considered immutable and necessary for their stability, but which might instead be modulable. The second characteristic is their ability to undergo various post-translational modifications that would control their interaction with other molecules. The third feature is their ability to participate in active molecular condensates both on the surface and within the cell. Finally, the implications of these features in the design of anti-inflammatory drugs are discussed.

In Vitro Efficacy of Antivenom and Varespladib in Neutralising Chinese Russell's Viper (Daboia siamensis) Venom Toxicity

The venom of the Russell's viper (Daboia siamensis) contains neurotoxic and myotoxic phospholipase A₂ toxins which can cause irreversible damage to motor nerve terminals. Due to the time delay between envenoming and antivenom administration, antivenoms may have limited efficacy against some of these venom components. Hence, there is a need for adjunct treatments to circumvent these limitations. In this study, we examined the efficacy of Chinese D. siamensis antivenom alone, and in combination with a PLA₂ inhibitor, Varespladib, in reversing the in vitro neuromuscular blockade in the chick biventer cervicis nerve-muscle preparation. Pre-synaptic neurotoxicity and myotoxicity were not reversed by the addition of Chinese D. siamensis antivenom 30 or 60 min after venom (10 µg/mL). The prior addition of Varespladib prevented the neurotoxic and myotoxic activity of venom (10 µg/mL) and was also able to prevent further reductions in neuromuscular block and muscle twitches when added 60 min after venom. The addition of the combination of Varespladib and antivenom 60 min after venom failed to produce further improvements than Varespladib alone. This demonstrates that the window of time in which antivenom remains effective is relatively short compared to Varespladib and small-molecule inhibitors may be effective in abrogating some activities of Chinese D. siamensis venom.

Neuromuscular Weakness and Paralysis Produced by Snakebite Envenoming: Mechanisms and Proposed Standards for Clinical Assessment

Respiratory and airway-protective muscle weakness caused by the blockade of neuromuscular transmission is a major cause of early mortality from snakebite envenoming (SBE). Once weakness is manifest, antivenom appears to be of limited effectiveness in improving neuromuscular function. Herein, we review the topic of venom-induced neuromuscular blockade and consider the utility of adopting clinical management methods originally developed for the safe use of neuromuscular blocking agents by anesthesiologists in operating rooms and critical care units. Failure to quantify neuromuscular weakness in SBE is predicted to cause the same significant morbidity that is associated with failure to do so in the context of using a clinical neuromuscular block in surgery and critical care. The quantitative monitoring of a neuromuscular block, and an understanding of its neurophysiological characteristics, enables an objective measurement of weakness that may otherwise be overlooked by traditional clinical examination at the bedside. This is important for the initial assessment and the monitoring of recovery from neurotoxic envenoming. Adopting these methods will also be critical to the conduct of future clinical trials of toxin-inhibiting drugs and antivenoms being tested for the reversal of venom-induced neuromuscular block.

Rat Group IIA Secreted Phospholipase A2 Binds to Cytochrome c Oxidase and Inhibits Its Activity: A Possible Episode in the Development of Alzheimer's Disease

Alzheimer's disease (AD), a progressive form of dementia, is characterized by the increased expression of secreted phospholipase A₂ group IIA (GIIA) in the affected tissue and the dysfunction of neuronal mitochondria, similar to that induced by an orthologous GIIA from snake venom, β-neurotoxic ammodytoxin (Atx), in the motor neurons. To advance our knowledge about the role of GIIA in AD, we studied the effect of rat GIIA on the neuronal mitochondria and compared it with that of the Atx. We produced recombinant rat GIIA (rGIIA) and its enzymatically inactive mutant, rGIIA(D49S), and demonstrated that they interact with the subunit II of cytochrome c oxidase (CCOX-II) as Atx. rGIIA and rGIIA(D49S) bound to this essential constituent of the respiratory chain complex with an approximately 100-fold lower affinity than Atx; nevertheless, both rGIIA molecules potently inhibited the CCOX activity in the isolated rat mitochondria. Like Atx, rGIIA was able to reach the mitochondria in the PC12 cells from the extracellular space, independent of its enzymatic activity. Consistently, the inhibition of the CCOX activity in the intact PC12 cells and in the rat's brain tissue sections was clearly demonstrated using rGIIA(D49S). Our results show that the effects of mammalian and snake venom β-neurotoxic GIIA on the neuronal mitochondria have similar molecular backgrounds. They suggest that the elevated extracellular concentration of GIIA in the AD tissue drives the translocation of this enzyme into local neurons and their mitochondria to inhibit the activity of the CCOX in the respiratory chain. Consequently, the process of oxidative phosphorylation in the neurons is attenuated, eventually leading to their degeneration. Atx was thus revealed as a valuable molecular tool for further investigations of the role of GIIA in AD.

Diversity of Phospholipases A2 from Bothrops atrox Snake Venom: Adaptive Advantages for Snakes Compromising Treatments for Snakebite Patients

The evolution of snake venoms resulted in multigene toxin families that code for structurally similar isoforms eventually harboring distinct functions. PLA₂s are dominant toxins in viper venoms, and little is known about the impact of their diversity on human envenomings and neutralization by antivenoms. Here, we show the isolation of three distinct PLA₂s from B. atrox venom. FA1 is a Lys-49 homologue, and FA3 and FA4 are catalytic Asp-49 PLA₂s. FA1 and FA3 are basic myotoxic proteins, while FA4 is an acid non-myotoxic PLA₂. FA3 was the most potent toxin, inducing higher levels of edema, inflammatory nociception, indirect hemolysis, and anticoagulant activity on human, rat, and chicken plasmas. FA4 presented lower anticoagulant activity, and FA1 had only a slight effect on human and rat plasmas. PLA₂s presented differential reactivities with antivenoms, with an emphasis on FA3, which was not recognized or neutralized by the antivenoms used in this study. Our findings reveal the functional and antigenic diversity among PLA₂s from B. atrox venom, highlighting the importance of assessing venom variability for understanding human envenomations and treatment with antivenoms, particularly evident here as the antivenom fails to recognize FA3, the most active multifunctional toxin described.

Therapeutic potential of venom peptides: insights in the nanoparticle-mediated venom formulations

Background

Venoms are the secretions produced by animals, generally for the purpose of self-defense or catching a prey. Biochemically venoms are mainly composed of proteins, lipids, carbohydrates, ions, etc., and classified into three major classes, viz. neurotoxic, hemotoxic and cytotoxic based upon their mode of action. Venoms are composed of different specific peptides/toxins which are responsible for their unique biological actions. Though venoms are generally seen as a source of death, scientifically venom is a complex biochemical substance having a specific pharmacologic action which can be used as agents to diagnose and cure a variety of diseases in humans.

Main body

Many of these venoms have been used since centuries, and their specified therapies can also be found in ancient texts such as Charka Samhita. The modern-day example of such venom therapeutic is captopril, an antihypertensive drug developed from venom of Bothrops jararaca. Nanotechnology is a modern-day science of building materials on a nanoscale with advantages like target specificity, increased therapeutic response and diminished side effects. In the present review we have introduced the venom, sources and related constituents in brief, by highlighting the therapeutic potential of venom peptides and focusing more on the nanoformulations-based approaches. This review is an effort to compile all such report to have an idea about the future direction about the nanoplatforms which should be focused to have more clinically relevant formulations for difficult to treat diseases.

Conclusion

Venom peptides which are fatal in nature if used cautiously and effectively can save life. Several research findings suggested that many of the fatal diseases can be effectively treated with venom peptides. Nanotechnology has emerged as novel strategy in diagnosis, treatment and mitigation of diseases in more effective ways. A variety of nanoformulation approaches have been explored to enhance the therapeutic efficacy and reduce the toxicity and targeted delivery of the venom peptide conjugated with it. We concluded that venom peptides along with nanoparticles can evolve as the new era for potential treatments of ongoing and untreatable diseases.

Graphical Abstract

The Phospholipase Activity of Ammodytoxin, a Prototype Snake Venom β-Neurotoxin, Is Not Obligatory for Cell Internalisation and Translocation to Mitochondria

β-Neurotoxins are secreted phospholipase A2 molecules that inhibit transmission in neuromuscular synapses by poisoning the motor neurons. These toxins specifically and rapidly internalise into the nerve endings of motor neurons. Ammodytoxin (Atx) is a prototype β-neurotoxin from the venom of the nose-horned viper (Vipera ammodytes ammodytes). Here, we studied the relevance of the enzymatic activity of Atx in cell internalisation and subsequent intracellular movement using transmission electron microscopy (TEM). We prepared a recombinant, enzymatically inactive mutant of Atx, Atx(D49S), labelled with gold nanoparticles (GNP), and incubated this with PC12 cells, to analyse its localisation by TEM. Atx(D49S)-GNP internalised into the cells. Inside the cells, Atx(D49S)-GNP was detected in different vesicle-like structures, cytosol, endoplasmic reticulum and mitochondria, where it was spotted in the intermembrane space and matrix. Co-localization of fluorescently labelled Atx(D49S) with mitochondria in PC12 cells by confocal fluorescence microscopy confirmed the reliability of results generated using Atx(D49S)-GNP and TEM and allowed us to conclude that the phospholipase activity of Atx is not obligatory for its cell internalisation and translocation into the mitochondrial intermembrane space and matrix.

Secreted Phospholipases A2 - not just Enzymes: Revisited

Secreted phospholipases A₂ (sPLA₂s) participate in a very broad spectrum of biological processes through their enzymatic activity and as ligands for membrane and soluble receptors. The physiological roles of sPLA₂s as enzymes have been very well described, while their functions as ligands are still poorly known. Since the last overview of sPLA₂-binding proteins (sPLA₂-BPs) 10 years ago, several important discoveries have occurred in this area. New and more sensitive analytical tools have enabled the discovery of additional sPLA₂-BPs, which are presented and critically discussed here. The structural diversity of sPLA₂-BPs reveals sPLA₂s as very promiscuous proteins, and we offer some structural explanations for this nature that makes these proteins evolutionarily highly advantageous. Three areas of physiological engagement of sPLA₂-BPs have appeared most clearly: cellular transport and signalling, and regulation of the enzymatic activity of sPLA₂s. Due to the multifunctionality of sPLA₂s, they appear to be exceptional pharmacological targets. We reveal the potential to exploit interactions of sPLA₂s with other proteins in medical terms, for the development of original diagnostic and therapeutic procedures. We conclude this survey by suggesting the priority questions that need to be answered.

β-micrustoxin (Mlx-9), a PLA2 from Micrurus lemniscatus snake venom: biochemical characterization and anti-proliferative effect mediated by p53

Background

Endogenous phospholipases A₂ (PLA₂) play a fundamental role in inflammation, neurodegenerative diseases, apoptosis and cellular senescence. Neurotoxins with PLA₂ activity are found in snake venoms from the Elapidae and Viperidae families. The mechanism of action of these neurotoxins have been studied using hippocampal and cerebellar neuronal cultures showing [Ca²⁺]i increase, mitochondrial depolarization and cell death. Astrocytes are rarely used as a model, despite being modulators at the synapses and responsible for homeostasis and defense in the central nervous system. Preserving the cell division ability, they can be utilized to study the cell proliferation process. In the present work cultured astrocytes and glioblastoma cells were employed to characterize the action of β-micrustoxin (previously named Mlx-9), a PLA₂ isolated from Micrurus lemniscatus snake venom. The β-micrustoxin structure was determined and the cell proliferation, cell cycle phases and the regulatory proteins p53, p21 and p27 were investigated.

Methods

β-micrustoxin was characterized biochemically by a proteomic approach. Astrocytes were obtained by dissociation of pineal glands from Wistar rats; glioblastoma tumor cells were purchased from ATCC and Sigma and cultured in DMEM medium. Cell viability was evaluated by MTT assay; cell proliferation and cell cycle phases were analyzed by flow cytometry; p53, p21 and p27 proteins were studied by western blotting and immunocytochemistry.

Results

Proteomic analysis revealed fragments on β-micrustoxin that aligned with a PLA₂ from Micrurus lemniscatus lemniscatus previously identified as transcript ID DN112835_C3_g9_i1/m.9019. β-micrustoxin impaired the viability of astrocytes and glioblastoma tumor cells. There was a reduction in cell proliferation, an increase in G2/M phase and activation of p53, p21 and p27 proteins in astrocytes.

Conclusion

These findings indicate that β-micrustoxin from Micrurus lemniscatus venom could inhibit cell proliferation through p53, p21 and p27 activation thus imposing cell cycle arrest at the checkpoint G2/M.

What do secreted phospholipases A2 have to offer in combat against different viruses up to SARS-CoV-2?

Secreted phospholipases A2 (sPLA2s) form a widespread group of structurally-related enzymes that catalyse the hydrolysis of the sn-2 ester bond of glycerophospholipids to produce free fatty acids and lysophospholipids. In humans, nine catalytically active and two inactive sPLA2 proteins have been identified. These enzymes play diverse biological roles, including host defence against bacteria, parasites and viruses. Several of these endogenous sPLA2s may play a defensive role in viral infections, as they display in vitro antiviral activity by both direct and indirect mechanisms. However, endogenous sPLA2s may also exert an offensive and negative role, dampening the antiviral response or promoting inflammation in animal models of viral infection. Similarly, several exogenous sPLA2s, most of them from snake venoms and other animal venoms, possess in vitro antiviral activities. Thus, both endogenous and exogenous sPLA2s may be exploited for the development of new antiviral substances or as therapeutic targets for antagonistic drugs that may promote a more robust antiviral response. In this review, the antiviral versus proviral role of both endogenous and exogenous sPLA2s against various viruses including coronaviruses is presented. Based on the highlighted developments in this area of research, possible directions of future investigation are envisaged. One of them is also a possibility of exploiting sPLA2s as biological markers of the severity of the Covid-19 pandemic caused by SARS-CoV-2 infection.

A Meta-Analysis of the Protein Components in Rattlesnake Venom

The specificity and potency of venom components give them a unique advantage in developing various pharmaceutical drugs. Though venom is a cocktail of proteins, rarely are the synergy and association between various venom components studied. Understanding the relationship between various components of venom is critical in medical research. Using meta-analysis, we observed underlying patterns and associations in the appearance of the toxin families. For Crotalus, Dis has the most associations with the following toxins: PDE; BPP; CRL; CRiSP; LAAO; SVMP P-I and LAAO; SVMP P-III and LAAO. In Sistrurus venom, CTL and NGF have the most associations. These associations can predict the presence of proteins in novel venom and understand synergies between venom components for enhanced bioactivity. Using this approach, the need to revisit the classification of proteins as major components or minor components is highlighted. The revised classification of venom components is based on ubiquity, bioactivity, the number of associations, and synergies. The revised classification can be expected to trigger increased research on venom components, such as NGF, which have high biomedical significance. Using hierarchical clustering, we observed that the genera's venom compositions were similar, based on functional characteristics rather than phylogenetic relationships.

Proteomic analysis of three medically important Nigerian Naja (Naja haje, Naja katiensis and Naja nigricollis) snake venoms

Proteomics technologies enable a comprehensive study of complex proteins and their functions. The venom proteomes of three medically important Nigerian Elapidae snakes Naja haje, Naja katiensis and Naja nigricollis was studied using HILIC coupled with LC-MS/MS analysis. Results revealed a total of 57, 55, and 46 proteins in the venoms of N. haje, N. katiensis, and N. nigricollis, respectively, with molecular mass ranging between 5 and 185  kDa. These snakes have 38 common proteins in addition to 3 uncommon proteins: actiflagelin, cathelicidin, and cystatin identified in their venoms. The identified proteins belonged to 14 protein families in N. haje and N. katiensis, and 12 protein families in N. nigricollis. Of the total venom proteins, 3FTx was the most abundant protein family, constituting 52% in N. haje and N. katiensis, and 41% in N. nigricollis, followed by PLA₂, constituting 37% in N. nigricollis, 26% in N. haje, and 24% in N. katiensis. Other protein families, including LAAO, CRISPs, VEGF, PLB, CVF, SVMP, SVH, AMP, PI, Globin, Actin, and C-type lectins, were also detected, although, at very low abundances. Quantification of the relative abundance of each protein revealed that alpha and beta fibrinogenase and PLA_2, which constituted 18-26% of the total proteome, were the most abundant. The 3 uncommon proteins have no known function in snake venom. However, actiflagelin activates sperm motility; cystatin inhibits angiogenesis, while cathelicidin exerts antimicrobial effects. The three Nigerian Naja genus proteomes displayed 70% similarity in composition, which suggests the possibility of formulating antivenom that may cross-neutralise the venoms of cobra species found in Nigeria. These data provide insights into clinically relevant peptides/proteins present in the venoms of these snakes. Data are available via ProteomeXchange with identifier PXD024627.

Cytotoxicity of snake venom enzymatic toxins: phospholipase A2 and l-amino acid oxidase

The phospholipase A2 (PLA2) and l-amino acid oxidase (LAAO) are two major enzymes found in the venoms from most snake species. These enzymes have been structurally and functionally characterised for their pharmacological activities. Both PLA2 and LAAO from different venoms demonstrate considerable cytotoxic effects on cancer cells via induction of apoptosis, cell cycle arrest and suppression of proliferation. These enzymes produce more pronounced cytotoxic effects in cancer cells than normal cells, thus they can be potential sources as chemotherapeutic agents. It is proposed that PLA2 and LAAO contribute to an elevated oxidative stress due to their catalytic actions, for instance, the ability of PLA2 to produce reactive oxygen species during lipolysis and formation of H2O2 from LAAO catalytic activity which consequently lead to cell death. Nonetheless, the cell-death signalling pathways associated with exposure to these enzymatic toxins are not fully elucidated yet. Here in this review, we will discuss the cytotoxic effects of PLA2 and LAAO in relationship to their catalytic mechanisms and the underlying mechanisms of cytotoxic actions.

In-Vitro Neutralization of the Neurotoxicity of Coastal Taipan Venom by Australian Polyvalent Antivenom: The Window of Opportunity

Coastal taipan (Oxyuranus scutellatus) envenoming causes life-threatening neuromuscular paralysis in humans. We studied the time period during which antivenom remains effective in preventing and arresting in vitro neuromuscular block caused by taipan venom and taipoxin. Venom showed predominant pre-synaptic neurotoxicity at 3 µg/mL and post-synaptic neurotoxicity at 10 µg/mL. Pre-synaptic neurotoxicity was prevented by addition of Australian polyvalent antivenom before the venom and taipoxin and, reversed when antivenom was added 5 min after venom and taipoxin. Antivenom only partially reversed the neurotoxicity when added 15 min after venom and had no significant effect when added 30 min after venom. In contrast, post-synaptic activity was fully reversed when antivenom was added 30 min after venom. The effect of antivenom on pre-synaptic neuromuscular block was reproduced by washing the bath at similar time intervals for 3 µg/mL, but not for 10 µg/mL. We found an approximate 10–15 min time window in which antivenom can prevent pre-synaptic neuromuscular block. This time window is likely to be longer in envenomed patients due to the delay in venom absorption. Similar effectiveness of antivenom and washing with 3 µg/mL venom suggests that antivenom most likely acts by neutralizing pre-synaptic toxins before they interfere with neurotransmission inside the motor nerve terminals.

Varespladib (LY315920) inhibits neuromuscular blockade induced by Oxyuranus scutellatus venom in a nerve-muscle preparation

The phospholipase A₂ (PLA₂) inhibitors varespladib (LY315920) and its orally available derivative methyl-varespladib (LY333013) have been proposed as potential therapies for the treatment of snakebite envenomings in which toxicity depends on the action of PLA₂s. In this study, the ability of LY315920 to abrogate the effect of the potent neurotoxic venom of Oxyuranus scutellatus (taipan) was assessed using the mouse phrenic nerve-diaphragm preparation. LY315920 inhibited the venom when (a) incubated with venom before addition to the medium; (b) added to the medium before addition of venom, and; (c) added to the medium within 30 min after addition of venom, and even after the onset of decline in twitch response. This contrasts with previous results with antivenom using the same experimental model, in which the window of time when antibodies are effective is shorter than 10 min. It is proposed that such differences may depend either on the higher affinity of the inhibitor for PLA₂s or on the possibility that LY315920 reaches the cytosol of the nerve terminals, inhibiting neurotoxins that have been internalized. Our findings bear implications on the therapeutic potential of varespladib in neurotoxic snakebite envenomings mediated by presynaptically-acting PLA₂s.

An agonist of the CXCR4 receptor accelerates the recovery from the peripheral neuroparalysis induced by Taipan snake envenomation

Envenomation by snakes is a major neglected human disease. Hospitalization and use of animal-derived antivenom are the primary therapeutic supports currently available. There is consensus that additional, not expensive, treatments that can be delivered even long after the snake bite are needed. We recently showed that the drug dubbed NUCC-390 shortens the time of recovery from the neuroparalysis caused by traumatic or toxic degeneration of peripheral motor neurons. These syndromes are characterized by the activation of a pro-regenerative molecular axis, consisting of the CXCR4 receptor expressed at the damaged site in neuronal axons and by the release of its ligand CXCL12α, produced by surrounding Schwann cells. This intercellular signaling axis promotes axonal growth and functional recovery from paralysis. NUCC-390 is an agonist of CXCR4 acting similarly to CXCL12α. Here, we have tested its efficacy in a murine model of neuroparalytic envenoming by a Papuan Taipan (Oxyuranus scutellatus) where a degeneration of the motor axon terminals caused by the presynaptic PLA2 toxin Taipoxin, contained in the venom, occurs. Using imaging of the neuromuscular junction and electrophysiological analysis, we found that NUCC-390 administration after injection of either the purified neuroparalytic Taipoxin or the whole Taipan venom, significantly accelerates the recovery from paralysis. These results indicate that NUCC-390, which is non-toxic in mice, should be considered for trials in humans to test its efficacy in accelerating the recovery from the peripheral neuroparalysis induced by Taipans. NUCC-390 should be tested as well in the envenomation by other snakes that cause neuroparalytic syndromes in humans. NUCC-390 could become an additional treatment, common to many snake envenomings, that can be delivered after the bite to reduce death by respiratory deficits and to shorten and improve functional recovery.

Snake three-finger α-neurotoxins and nicotinic acetylcholine receptors: molecules, mechanisms and medicine

Snake venom three-finger α-neurotoxins (α-3FNTx) act on postsynaptic nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction (NMJ) to produce skeletal muscle paralysis. The discovery of the archetypal α-bungarotoxin (α-BgTx), almost six decades ago, exponentially expanded our knowledge of membrane receptors and ion channels. This included the localisation, isolation and characterization of the first receptor (nAChR); and by extension, the pathophysiology and pharmacology of neuromuscular transmission and associated pathologies such as myasthenia gravis, as well as our understanding of the role of α-3FNTxs in snakebite envenomation leading to novel concepts of targeted treatment. Subsequent studies on a variety of animal venoms have yielded a plethora of novel toxins that have revolutionized molecular biomedicine and advanced drug discovery from bench to bedside. This review provides an overview of nAChRs and their subtypes, classification of α-3FNTxs and the challenges of typifying an increasing arsenal of structurally and functionally unique toxins, and the three-finger protein (3FP) fold in the context of the uPAR/Ly6/CD59/snake toxin superfamily. The pharmacology of snake α-3FNTxs including their mechanisms of neuromuscular blockade, variations in reversibility of nAChR interactions, specificity for nAChR subtypes or for distinct ligand-binding interfaces within a subtype and the role of α-3FNTxs in neurotoxic envenomation are also detailed. Lastly, a reconciliation of structure-function relationships between α-3FNTx and nAChRs, derived from historical mutational and biochemical studies and emerging atomic level structures of nAChR models in complex with α-3FNTxs is discussed.

Varespladib (LY315920) and Methyl Varespladib (LY333013) Abrogate or Delay Lethality Induced by Presynaptically Acting Neurotoxic Snake Venoms

The phospholipase A₂ (PLA₂) inhibitor Varespladib (LY315920) and its orally bioavailable prodrug, methyl-Varespladib (LY333013) inhibit PLA₂ activity of a wide variety of snake venoms. In this study, the ability of these two forms of Varespladib to halt or delay lethality of potent neurotoxic snake venoms was tested in a mouse model. The venoms of Notechis scutatus, Crotalus durissus terrificus, Bungarus multicinctus, and Oxyuranus scutellatus, all of which have potent presynaptically acting neurotoxic PLA₂s of variable quaternary structure, were used to evaluate simple dosing regimens. A supralethal dose of each venom was injected subcutaneously in mice, followed by the bolus intravenous (LY315920) or oral (LY333013) administration of the inhibitors, immediately and at various time intervals after envenoming. Control mice receiving venom alone died within 3 h of envenoming. Mice injected with O. scutellatus venom and treated with LY315920 or LY333013 survived the 24 h observation period, whereas those receiving C. d. terrificus and B. multicinctus venoms survived at 3 h or 6 h with a single dose of either form of Varespladib, but not at 24 h. In contrast, mice receiving N. scutatus venom and then the inhibitors died within 3 h, similarly to the control animals injected with venom alone. LY315920 was able to reverse the severe paralytic manifestations in mice injected with venoms of O. scutellatus, B. multicinctus, and C. d. terrificus. Overall, results suggest that the two forms of Varespladib are effective in abrogating, or delaying, neurotoxic manifestations induced by some venoms whose neurotoxicity is mainly dependent on presynaptically acting PLA₂s. LY315920 is able to reverse paralytic manifestations in severely envenomed mice, but further work is needed to understand the significance of species-specific differences in animal models as they compare to clinical syndromes in human and for potential use in veterinary medicine.

Amplification of Snake Venom Toxicity by Endogenous Signaling Pathways

The active components of snake venoms encompass a complex and variable mixture of proteins that produce a diverse, but largely stereotypical, range of pharmacologic effects and toxicities. Venom protein diversity and host susceptibilities determine the relative contributions of five main pathologies: neuromuscular dysfunction, inflammation, coagulopathy, cell/organ injury, and disruption of homeostatic mechanisms of normal physiology. In this review, we describe how snakebite is not only a condition mediated directly by venom, but by the amplification of signals dysregulating inflammation, coagulation, neurotransmission, and cell survival. Although venom proteins are diverse, the majority of important pathologic events following envenoming follow from a small group of enzyme-like activities and the actions of small toxic peptides. This review focuses on two of the most important enzymatic activities: snake venom phospholipases (svPLA2) and snake venom metalloproteases (svMP). These two enzyme classes are adept at enabling venom to recruit homologous endogenous signaling systems with sufficient magnitude and duration to produce and amplify cell injury beyond what would be expected from the direct impact of a whole venom dose. This magnification produces many of the most acutely important consequences of envenoming as well as chronic sequelae. Snake venom PLA2s and MPs enzymes recruit prey analogs of similar activity. The transduction mechanisms that recruit endogenous responses include arachidonic acid, intracellular calcium, cytokines, bioactive peptides, and possibly dimerization of venom and prey protein homologs. Despite years of investigation, the precise mechanism of svPLA2-induced neuromuscular paralysis remains incomplete. Based on recent studies, paralysis results from a self-amplifying cycle of endogenous PLA2 activation, arachidonic acid, increases in intracellular Ca2+ and nicotinic receptor deactivation. When prolonged, synaptic suppression supports the degeneration of the synapse. Interaction between endothelium-damaging MPs, sPLA2s and hyaluronidases enhance venom spread, accentuating venom-induced neurotoxicity, inflammation, coagulopathy and tissue injury. Improving snakebite treatment requires new tools to understand direct and indirect effects of envenoming. Homologous PLA2 and MP activities in both venoms and prey/snakebite victim provide molecular targets for non-antibody, small molecule agents for dissecting mechanisms of venom toxicity. Importantly, these tools enable the separation of venom-specific and prey-specific pathological responses to venom.

Neurotoxicity of Micrurus lemniscatus lemniscatus (South American coralsnake) venom in vertebrate neuromuscular preparations in vitro and neutralization by antivenom

We investigated the effect of South American coralsnake (Micrurus lemniscatus lemniscatus) venom on neurotransmission in vertebrate nerve-muscle preparations in vitro. The venom (0.1-30 µg/ml) showed calcium-dependent PLA₂ activity and caused irreversible neuromuscular blockade in chick biventer cervicis (BC) and mouse phrenic nerve-diaphragm (PND) preparations. In BC preparations, contractures to exogenous acetylcholine and carbachol (CCh), but not KCl, were abolished by venom concentrations ≥ 0.3 µg/ml; in PND preparations, the amplitude of the tetanic response was progressively attenuated, but with little tetanic fade. In low Ca²⁺ physiological solution, venom (10 µg/ml) caused neuromuscular blockade in PND preparations within ~ 10 min that was reversible by washing; the addition of Ca²⁺ immediately after the blockade temporarily restored the twitch responses, but did not prevent the progression to irreversible blockade. Venom (10 µg/ml) did not depolarize diaphragm muscle, prevent depolarization by CCh, or cause muscle contracture or histological damage. Venom (3 µg/ml) had a biphasic effect on the frequency of miniature end-plate potentials, but did not affect their amplitude; there was a progressive decrease in the amplitude of evoked end-plate potentials. The amplitude of compound action potentials in mouse sciatic nerve was unaffected by venom (10 µg/ml). Pre-incubation of venom with coralsnake antivenom (Instituto Butantan) at the recommended antivenom:venom ratio did not neutralize the neuromuscular blockade in PND preparations, but total neutralization was achieved with a tenfold greater volume of antivenom. The addition of antivenom after 50% and 80% blockade restored the twitch responses. These results show that M. lemniscatus lemniscatus venom causes potent, irreversible neuromuscular blockade, without myonecrosis. This blockade is apparently mediated by pre- and postsynaptic neurotoxins and can be reversed by coralsnake antivenom.

A Novel Phospholipase A2 Isolated from Palythoa caribaeorum Possesses Neurotoxic Activity

Zoanthids of the genus Palythoa are distributed worldwide in shallow waters around coral reefs. Like all cnidarians, they possess nematocysts that contain a large diversity of toxins that paralyze their prey. This work was aimed at isolating and functionally characterizing a cnidarian neurotoxic phospholipase named A2-PLTX-Pcb1a for the first time. This phospholipase was isolated from the venomous extract of the zoanthid Palythoa caribaeorum. This enzyme, which is Ca²⁺-dependent, is a 149 amino acid residue protein. The analysis of the A2-PLTX-Pcb1a sequence showed neurotoxic domain similitude with other neurotoxic sPLA₂´s, but a different catalytic histidine domain. This is remarkable, since A2-PLTX-Pcb1a displays properties like those of other known PLA₂ enzymes.

The neurotoxic secreted phospholipase A2 from the Vipera a. ammodytes venom targets cytochrome c oxidase in neuronal mitochondria

The β-neurotoxic secreted phospholipases A2 (sPLA2s) block neuro-muscular transmission by poisoning nerve terminals. Damage inflicted by such sPLA2s (β-ntx) on neuronal mitochondria is characteristic, very similar to that induced by structurally homologous endogenous group IIA sPLA2 when its activity is elevated, as, for example, in the early phase of Alzheimer's disease. Using ammodytoxin (Atx), the β-ntx from the venom of the nose-horned viper (Vipera a. ammodytes), the sPLA2 receptor R25 has been detected in neuronal mitochondria. This receptor has been purified from porcine cerebral cortex mitochondria by a new Atx-affinity-based chromatographic procedure. Mass spectrometry analysis revealed R25 to be the subunit II of cytochrome c oxidase (CCOX), an essential constituent of the respiratory chain complex. CCOX was confirmed as being the first intracellular membrane receptor for sPLA2 by alternative Atx-affinity-labellings of purified CCOX, supported also by the encounter of Atx and CCOX in PC12 cells. This discovery suggests the explanation of the mechanism by which β-ntx hinders production of ATP in poisoned nerve endings. It also provides a new insight into the potential function and dysfunction of endogenous GIIA sPLA2 in mitochondria.

Unresolved issues in the understanding of the pathogenesis of local tissue damage induced by snake venoms

Snakebite envenoming by viperid species, and by some elapids, is characterized by a complex pattern of tissue damage at the anatomical site of venom injection. In severe cases, tissue destruction may be so extensive as to lead to permanent sequelae, with serious pathophysiological, social and psychological consequences. Significant advances have been performed in the study of venom-induced tissue damage, including identification and characterization of the toxins involved, insights into the mechanisms of action of venoms and toxins, and study of tissue responses to venom-induced injury. Nevertheless, much remains to be known and understood on the pathogenesis of these alterations. This review focuses on some of the pending issues in the topic of snake venom-induced local tissue damage. The traditional 'reductionist' approach, which has predominated in the study of snake venoms and their actions, needs to be complemented by more integrative and holistic perspectives aimed at capturing the complexity of these pathological alterations. Future advances in the study of these topics will certainly pave the way for innovative therapeutic interventions, with the goal of reducing the impact of this aspect of snakebite envenoming.

Synergistic strategies of predominant toxins in snake venoms

Synergism is a significant phenomenon present in snake venoms that may be an evolving strategy to potentiate toxicities. Synergism exists between different toxins or toxin complexes in various snake venoms, with phospholipaseA₂s (PLA₂s) (toxins or subunits) the main enablers. The predominant toxins, snake venom PLA₂s, metalloproteases (SVMPs), serine proteases (SVSPs) and three-finger toxins (3FTxs), play essential roles in synergistic processes. The hypothetical mechanisms of synergistic effect can be generalized under the effects of amplification and chaperoning. The Toxicity Score is among the few quantitative methods to assess synergism. Selection of toxins involved in synergistically enhanced toxicity as the targets are important for development of novel antivenoms or inhibitors.

Broad-spectrum antiviral agents: secreted phospholipase A2 targets viral envelope lipid bilayers derived from the endoplasmic reticulum membrane

Hepatitis C virus (HCV), dengue virus (DENV) and Japanese encephalitis virus (JEV) belong to the family Flaviviridae. Their viral particles have the envelope composed of viral proteins and a lipid bilayer acquired from budding through the endoplasmic reticulum (ER). The phospholipid content of the ER membrane differs from that of the plasma membrane (PM). The phospholipase A2 (PLA2) superfamily consists of a large number of members that specifically catalyse the hydrolysis of phospholipids at a particular position. Here we show that the CM-II isoform of secreted PLA2 obtained from Naja mossambica mossambica snake venom (CM-II-sPLA2) possesses potent virucidal (neutralising) activity against HCV, DENV and JEV, with 50% inhibitory concentrations (IC50) of 0.036, 0.31 and 1.34 ng/ml, respectively. In contrast, the IC50 values of CM-II-sPLA2 against viruses that bud through the PM (Sindbis virus, influenza virus and Sendai virus) or trans-Golgi network (TGN) (herpes simplex virus) were >10,000 ng/ml. Moreover, the 50% cytotoxic (CC50) and haemolytic (HC50) concentrations of CM-II-sPLA2 were >10,000 ng/ml, implying that CM-II-sPLA2 did not significantly damage the PM. These results suggest that CM-II-sPLA2 and its derivatives are good candidates for the development of broad-spectrum antiviral drugs that target viral envelope lipid bilayers derived from the ER membrane.

Ammodytin L is the main cardiotoxic component of the Vipera ammodytes ammodytes venom

Venom of the nose-horned viper (V. a. ammodytes) as also venoms of some related European viperids can induce also cardiotoxic effects in mammals. In this work we demonstrated that the protein in the V. a. ammodytes venom acting on heart is a myotoxic secreted phospholipase A₂ analogue ammodytin L (AtnL). In the isolated perfused rat heart AtnL induced significant and irreversible cardiotoxicity characterized by atrioventricular (AV) blockade. This venom protein induced appearance of high levels of creatine kinase, lactate dehydrogenase, aspartate aminotransferase and troponin I in the sinus effluent of the isolated heart, indicative for myocardial damage, which is obviously the primary cause of its cardiotoxic action. Gel filtration chromatography subfractions C1 and C2 of the V. a. ammodytes venom harboured most of the venom cardiotoxicity. As we showed, just these two subfractions contained also AtnL. Subfraction C1 in the final CF concentration 11.3 μg/mL (containing 3.1 μg/mL AtnL) induced a complete cardiac arrest while subfraction C2 in the final CF concentration 6.0 μg/mL (containing 0.8 μg/mL AtnL) and the pure AtnL (1.0 μg/mL) did not. Contrary to AtnL, subfraction C1 at 11.3 μg/mL was not able to induce the AV blockade. This exposed the only other cardiotoxic subfractions-specific venom protein, a cysteine-rich secretory protein (CRISP), as an additional venom component potentially involved in modulation of the heart activity. Cardiotoxicity reported in some cases of the adder (V. berus) venom and the asp viper (V. aspis) venom poisonings may be assigned to AtnL in these venoms.

Synthetic peptide antigens derived from long-chain alpha-neurotoxins: Immunogenicity effect against elapid venoms

Three-finger toxins (3FTXs), especially α-neurotoxins, are the most poorly neutralized elapid snake toxins by current antivenoms. In this work, the conserved structural similarity and motif arrangements of long-chain α-neurotoxins led us to design peptides with consensus sequences. Eight long-chain α-neurotoxins (also known as Type II) were used to generate a consensus sequence from which two peptides were chemically synthesized, LCP1 and LCP2. Rabbit sera raised against them were able to generate partially-neutralizing antibodies, which delayed mice mortality in neutralization assays against Naja haje, Dendrospis polylepis and Ophiophagus hannah venoms.

Cardiotoxic effects of the Vipera ammodytes ammodytes venom fractions in the isolated perfused rat heart

The nose-horned viper (Vipera ammodytes ammodytes) is the most venomous European snake. Its venom is known as haematotoxic, myotoxic and neurotoxic but it exerts also cardiotoxic effects. To further explore the cardiotoxicity of the venom we separated it into four fractions by gel filtration chromatography. Three fractions that contain polypeptides (A, B, and C) were tested for their effects on isolated rat heart. Heart rate (HR), incidence of arrhythmias (atrioventricular (AV) blocks, ventricular tachycardia, ventricular fibrillation, and asystolia), coronary flow (CF), systolic, developed and diastolic left ventricular pressure (LVP) were measured before, during, and after the application of venom fractions in three different concentrations. Fraction A, containing proteins of 60-100 kDa, displayed no effect on the rat heart. Fractions B and C disturbed heart functioning in similar way, but with different potency that was higher by the latter. This was manifested by significant decrease of HR and CF, the increase of diastolic, and the decrease of systolic and developed LVPs. All hearts treated with fraction C in the final CF concentrations 22.5 and 37.5 μg/mL suffered rapid and irreversible asystolia without AV blockade. They underwent also ventricular fibrillation and ventricular tachycardia. Fraction B affected hearts only at the highest dose inducing asystolia in all hearts, ventricular fibrillation in 80% and ventricular tachycardia in 70% of the hearts. Venom fraction C induced 71% of all recorded heart rhythm disturbances, significantly more than fraction B, which induced 29%. Most abundant proteins in fraction C were secreted phospholipases A₂ among which the venom component acting on the heart is most probably to be looked for.

RNA-Seq Analysis Reveals Candidate Targets for Curcumin against Tetranychus cinnabarinus

Tetranychus cinnabarinus is an important agricultural pest with a broad host range. We previously identified curcumin as a promising acaricidal compound against T. cinnabarinus. However, the acaricidal mechanism of curcumin remains unknown. In this study, RNA-seq was employed to analyze the transcriptome changes in T. cinnabarinus treated with curcumin or the solvent. A total of 105,706,297 clean sequence reads were generated by sequencing, with more than 90% of the reads successfully mapped to the reference sequence. The RNA-seq identified 111 and 96 differentially expressed genes between curcumin- and solvent-treated mites at 24 and 48 h after treatment, respectively. GO enrichment analysis of differentially expressed genes showed that the cellular process was the dominant group at both time points. Finally, we screened 23 differentially expressed genes that were functionally identical or similar to the targets of common insecticide/acaricides or genes that were associated with mite detoxification and metabolism. Calmodulin, phospholipase A₂, and phospholipase C were activated upon curcumin treatment suggesting that the calcium channel related genes might play important roles in mite's response to curcumin. Overall our results revealed the global transcriptional changes in T. cinnabarinus after curcumin treatment to enable further identification of the targets of curcumin in mites.

Neurotoxic phospholipase A2 from rattlesnake as a new ligand and new regulator of prokaryotic receptor GLIC (proton-gated ion channel from G. violaceus)

Neurotoxic phospholipases A2 (sPLA2) from snake venoms interact with various protein targets with high specificity and potency. They regulate function of multiple receptors or channels essential to life processes including neuronal or neuromuscular chemoelectric signal transduction. These toxic sPLA2 exhibit high pharmacological potential and determination of PLA2-receptor binding sites represents challenging part in the receptor-channel biochemistry and pharmacology. To investigate the mechanism of interaction of neurotoxic PLA2 with its neuronal receptor at the molecular level, we used as a model crotoxin, a heterodimeric sPLA2 from rattlesnake venom and proton-gated ion channel GLIC, a bacterial homolog of pentameric ligand-gated ion channels. The three-dimensional structures of both partners, crotoxin and GLIC have been solved by X-ray crystallography and production of full-length pentameric GLIC (with ECD and TM domains) is well established. In the present study, for the first time, we demonstrated physical and functional interaction of full-length purified and solubilized GLIC with CB, (PLA2 subunit of crotoxin). We identified GLIC as a new protein target of CB and CB as a new ligand of GLIC, and showed that this non covalent interaction (PLA2-GLIC) involves the extracellular domain of GLIC. We also determined a novel function of CB as an inhibitor of proton-gated ion channel activity. In agreement with conformational changes observed upon formation of the complex, CB appears to be negative allosteric modulator (NAM) of GLIC. Finally, we proposed a possible stoichiometric model for CB - GLIC interaction based on analytical ultracentrifugation.

Neutralization of the neuromuscular inhibition of venom and taipoxin from the taipan (Oxyuranus scutellatus) by F(ab')2 and whole IgG antivenoms

The neuromuscular junction activity of Oxyuranus scutellatus venom and its presynaptic neurotoxin, taipoxin, and their neutralization by two antivenoms were examined in mouse phrenic nerve-diaphragm preparations. The action of taipoxin was also studied at 21°C. The efficacy of the antivenoms was also assessed in an in vivo mouse model. Both antivenoms were effective in neutralizing the neuromuscular blocking activity in preincubation-type experiments. In experiments involving independent addition of venom and antivenoms, neutralization depended on the time interval between venom addition and antivenom application. When taipoxin was incubated for 5, 10 or 20min at 21°C, and antivenom added and temperature increased to 37°C, neutralization was achieved only when the toxin was incubated for 5 or 10min. The neutralization by the two antivenoms in an in vivo model showed that both whole IgG and F(ab')2 antivenoms were effective in neutralizing lethality. Our findings highlight the very rapid action of taipan venom at the nerve terminal, and the poor capacity of antivenoms to revert neurotoxicity as the time interval between venom or taipoxin application and antivenom addition increased. Additionally the disparity between molecular masses of the active substances of the two antivenoms did not result in differences in neutralization.