The Dragon’s Paralysing Spell: Evidence of Sodium and Calcium Ion Channel Binding Neurotoxins in Helodermatid and Varanid Lizard Venoms

Abstract

Bites from helodermatid lizards can cause pain, paresthesia, paralysis, and tachycardia, as well as other symptoms consistent with neurotoxicity. Furthermore, in vitro studies have shown that Heloderma horridum venom inhibits ion flux and blocks the electrical stimulation of skeletal muscles. Helodermatids have long been considered the only venomous lizards, but a large body of robust evidence has demonstrated venom to be a basal trait of Anguimorpha. This clade includes varanid lizards, whose bites have been reported to cause anticoagulation, pain, and occasionally paralysis and tachycardia. Despite the evolutionary novelty of these lizard venoms, their neuromuscular targets have yet to be identified, even for the iconic helodermatid lizards. Therefore, to fill this knowledge gap, the venoms of three Heloderma species (H. exasperatum, H. horridum and H. suspectum) and two Varanus species (V. salvadorii and V. varius) were investigated using Gallus gallus chick biventer cervicis nerve–muscle preparations and biolayer interferometry assays for binding to mammalian ion channels.. Incubation with Heloderma venoms caused the reduction in nerve-mediated muscle twitches post initial response of avian skeletal muscle tissue preparation assays suggesting voltage-gated sodium (NaV) channel binding. Congruent with the flaccid paralysis inducing blockage of electrical stimulation in the skeletal muscle preparations, the biolayer interferometry tests with Heloderma suspectum venom revealed binding to the S3–S4 loop within voltage-sensing domain IV of the skeletal muscle channel subtype, NaV1.4. Consistent with tachycardia reported in clinical cases, the venom also bound to voltage-sensing domain IV of the cardiac smooth muscle calcium channel, CaV1.2. While Varanus varius venom did not have discernable effects in the avian tissue preparation assay at the concentration tested, in the biointerferometry assay both V. varius and V. salvadorii bound to voltage-sensing domain IV of both NaV1.4 and CaV1.2, similar to H. suspectum venom. The ability of varanid venoms to bind to mammalian ion channels but not to the avian tissue preparation suggests prey-selective actions, as did the differential potency within the Heloderma venoms for avian versus mammalian pathophysiological targets. This study thus presents the detailed characterization of Heloderma venom ion channel neurotoxicity and offers the first evidence of varanid lizard venom neurotoxicity. In addition, the data not only provide information useful to understanding the clinical effects produced by envenomations, but also reveal their utility as physiological probes, and underscore the potential utility of neglected venomous lineages in the drug design and development pipeline.

A Russian Doll of Resistance: Nested Gains and Losses of Venom Immunity in Varanid Lizards

“…Addressing this knowledge gap through focused future research remains imperative for a comprehensive appreciation of these intricate evolutionary processes. In addition to further work investigating varanid lizard resistance to other venomous animals (whether as predators or prey), future work should also investigate the resistance of Anguimorph lizards to their own venoms, including presynaptic ion-channel toxins [ 49 ], and toxins that act upon the blood clotting cascade [ 50 , 51 , 52 ].…”

Venom biotechnology: casting light on nature’s deadliest weapons using synthetic biology

“…BLI based on nAChR mimotopes has been highly successful ( Harris et al, 2020a ; Harris et al, 2020b ; Harris et al, 2020c ; Harris et al, 2020d ; Youngman et al, 2021 ), but other ion channel receptor domains have been tested in preliminary experiments, including domain IV Ca V 1.2 ( Harris et al, 2021b ) and domain IV Na V 1.4 ( Dobson et al, 2021 ). Further investigations are needed to confirm the initial results.…”

“…BLI has allowed the binding of crude venoms rich in α-neurotoxins to be investigated by the assessment of taxon-specific representative sequences across a range of prey (amphibian, mammalian, and reptilian). Prey-specific targeting of taxon-specific mimotopes has been assessed in snakes ( Harris et al, 2020a ; Harris et al, 2020b ; Harris et al, 2020c ; Youngman et al, 2021 ), lizards ( Dobson et al, 2021 ), and even fish ( Harris et al, 2021b ). All these studies provided insight into the biochemistry and evolution of venoms.…”

Neglected Venomous Animals and Toxins: Underrated Biotechnological Tools in Drug Development

“…The venom gland transcriptome analysis from H. horridum horridum revealed the presence of metalloproteases, lipases, vespryns, waspryns, lectins, cystatins and serine peptidase inhibitors, but none of these proteins were actually isolated from the venom [71]. Furthermore, Heloderma contains neurotoxins in its venom, and these toxins are able to bind sodium and calcium channels [72]. An important work by Fry et al [73] evaluated phylogeny between snakes and lizards and demonstrated that the venom delivery system of these animals could have evolved from the same common ancestor.…”

“…Venoms from different species of the Varanus genus were evaluated for the ability to prevent blood clotting by thromboelastography, and the venoms differ regarding the activity; the most potent effects were found in arboricole species, probably due to the selective pressure, according to the authors [75]. Similar to Heloderma, Varanid lizards possess neurotoxins that are able to bind sodium and calcium channels [72].…”