Isolation and Pharmacological Characterization of α-Elapitoxin-Ot1a, a Short-Chain Postsynaptic Neurotoxin from the Venom of the Western Desert Taipan, Oxyuranus temporalis

In Vitro neurotoxicity and myotoxicity of Malaysian Naja sumatrana and Naja kaouthia venoms: Neutralization by monovalent and Neuro Polyvalent Antivenoms from Thailand

Naja sumatrana and Naja kaouthia are medically important elapids species found in Southeast Asia. Snake bite envenoming caused by these species may lead to morbidity or mortality if not treated with the appropriate antivenom. In this study, the in vitro neurotoxic and myotoxic effects N. sumatrana and N. kaouthia venoms from Malaysian specimens were assessed and compared. In addition, the neutralizing capability of Cobra Antivenom (CAV), King Cobra Antivenom (KCAV) and Neuro Polyvalent Antivenom (NPAV) from Thailand were compared. Both venoms produced concentration-dependent neurotoxic and myotoxic effects in the chick biventer cervicis nerve-muscle preparation. Based on the time to cause 90% inhibition of twitches (i.e. t₉₀) N. kaouthia venom displayed more potent neurotoxic and myotoxic effects than N. sumatrana venom. All three of the antivenoms significantly attenuated venom-induced twitch reduction of indirectly stimulated tissues when added prior to venom. When added after N. sumatrana venom, at the t₉₀ time point, CAV and NPAV partially restored the twitch height but has no significant effect on the reduction in twitch height caused by N. kaouthia venom. The addition of KCAV, at the t₉₀ time point, did not reverse the attenuation of indirectly stimulated twitches caused by either venom. In addition, none of the antivenoms, when added prior to venom, prevented attenuation of directly stimulated twitches. Differences in the capability of antivenoms, especially NPAV and CAV, to reverse neurotoxicity and myotoxicity indicate that there is a need to isolate and characterize neurotoxins and myotoxins from Malaysian N. kaouthia and N. sumatrana venoms to improve neutralization capability of the antivenoms.

Isolation and Characterization of Two Postsynaptic Neurotoxins From Indian Cobra (Naja Naja) Venom

The Indian Cobra (Naja naja) is among the “Big Four” responsible for most of the snakebite envenoming cases in India. Although recent proteomic studies suggest the presence of postsynaptic neurotoxins in N. naja venom, little is known about the pharmacology of these toxins. We isolated and characterized α-Elapitoxin-Nn2a (α-EPTX-Nn2a; 7020 Da) and α-Elapitoxin-Nn3a (α-EPTX-Nn3a; 7807 Da), a short-chain and long-chain postsynaptic neurotoxin, respectively, which constitute 1 and 3% of N. naja venom. α-EPTX-Nn2a (100–300 nM) and α-EPTX-Nn3a (100–300 nM) both induced concentration-dependent inhibition of indirect twitches and abolished contractile responses of tissues to exogenous acetylcholine and carbachol, in the chick biventer cervicis nerve-muscle preparation. The prior incubation of tissues with Indian polyvalent antivenom (1 ml/0.6 mg) prevented the in vitro neurotoxic effects of α-EPTX-Nn2a (100 nM) and α-EPTX-Nn3a (100 nM). The addition of Indian polyvalent antivenom (1 ml/0.6 mg), at the t₉₀ time point, could not reverse the in vitro neurotoxicity of α-EPTX-Nn2a (100 nM). The in vitro neurotoxicity of α-EPTX-Nn3a (100 nM) was partially reversed by the addition of Indian polyvalent antivenom (1 ml/0.6 mg), as well as repeated washing of the tissue. α-EPTX-Nn2a displayed non-competitive antagonism of concentration-response curves to carbachol, with a pA₂ of 8.01. In contrast, α-EPTX-Nn3a showed reversible antagonism of concentration-response curves to carbachol, with a pA₂ of 8.17. De novo sequencing of α-EPTX-Nn2a and α-EPTX-Nn3a showed a short-chain and long-chain postsynaptic neurotoxin, respectively, with 62 and 71 amino acids. The important observation made in this study is that antivenom can reverse the neurotoxicity of the clinically important long-chain neurotoxin, but not the short-chain neurotoxin, from N. naja venom.

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.

Nicotinic acetylcholine receptor inhibitors derived from snake and snail venoms

The nicotinic acetylcholine receptor (nAChR) represents the prototype of ligand-gated ion channels. It is vital for neuromuscular transmission and an important regulator of neurotransmission. A variety of toxic compounds derived from diverse species target this receptor and have been of elemental importance in basic and applied research. They enabled milestone discoveries in pharmacology and biochemistry ranging from the original formulation of the receptor concept, the first isolation and structural analysis of a receptor protein (the nAChR) to the identification, localization, and differentiation of its diverse subtypes and their validation as a target for therapeutic intervention. Among the venom-derived compounds, α-neurotoxins and α-conotoxins provide the largest families and still represent indispensable pharmacological tools. Application of modified α-neurotoxins provided substantial structural and functional details of the nAChR long before high resolution structures were available. α-bungarotoxin represents not only a standard pharmacological tool and label in nAChR research but also for unrelated proteins tagged with a minimal α-bungarotoxin binding motif. A major advantage of α-conotoxins is their smaller size, as well as superior selectivity for diverse nAChR subtypes that allows their development into ligands with optimized pharmacological and chemical properties and potentially novel drugs. In the following, these two groups of nAChR antagonists will be described focusing on their respective roles in the structural and functional characterization of nAChRs and their development into research tools. In addition, we provide a comparative overview of the diverse α-conotoxin selectivities that can serve as a practical guide for both structure activity studies and subtype classification. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'