Causes and Consequences of Snake Venom Variation

Abstract

Snake venoms are mixtures of toxins that vary extensively between and within snake species. This variability has serious consequences for the management of the world's 1.8 million annual snakebite victims. Advances in 'omic' technologies have empowered toxinologists to comprehensively characterize snake venom compositions, unravel the molecular mechanisms that underpin venom variation, and elucidate the ensuing functional consequences. In this review, we describe how such mechanistic processes have resulted in suites of toxin isoforms that cause diverse pathologies in human snakebite victims and we detail how variation in venom composition can result in treatment failure. Finally, we outline current therapeutic approaches designed to circumvent venom variation and deliver next-generation treatments for the world's most lethal neglected tropical disease.

Keywords: antivenom; gene duplication; protein neofunctionalization; snakebite; toxins; venom evolution.

Figure 1. The Molecular and Evolutionary Mechanisms That Underpin the Origin and Diversification ofSnake Venom Toxins.

This figure depicts various evolutionary mechanisms that underpin the origin and diversification of snake venom coding genes. Here, introns are shown in grey, while exons are depicted in various colors. Following their origin from (endo)physiological homologues (P) via (1) duplication, snake venom coding genes (V) rapidly accumulate variation under the influence of (2) positive Darwinian selection. On rare occasions, this process results in (3) the origin of novel functions, while it more commonly leads to (4) pseudogenization/degeneration. Snake venom diversity can also be generated via (5) alternative- and (6) trans-splicing, while increased expression can be achieved through (7) repeated gene duplications.

Figure 2. The Structural and Functional Diversity of Three-Finger Toxins (3FTxs).

(A) Structural model for a ‘typical’ 3FTx, the short-chain -neurotoxin cobrotoxin (Protein Data Bank: 1COE, from Naja atra), highlighting the multiple β-hairpin loops extending from the disulfide bond-stabilized hydrophobic core. Disulfide bond numbers are colored red. (B) Via the processes of gene duplication and positive selection, 3FTxs have diversified from a plesiotypic form found in basal henophidian snakes (boas and pythons) into a paralogous suite of functionally diverse toxins in ‘advanced snakes’, many of which act on sites at the neuromuscular junction to cause neuromuscular paralysis. Homology models for various subclasses of 3FTx are displayed, with their variable disulfide bond numbers colored red, and their differential sites of action at the neuromuscular junction shown. (1) Calliotoxin activates the voltage-gated sodium channel, Nav1.4; (2) calciseptine selectively blocks L-type calcium channels; (3) fasciculins exert inhibitory activities against acetylcholinesterase; (4) muscarinic 3FTxs antagonize muscarinic acetylcholine receptors (mAChR); (5) both short-chain (top) and long-chain (bottom) -neurotoxins antagonize a variety of different nicotinic acetylcholine receptor (nAChR) subtypes. Note that there are a number of other, functionally distinct, 3FTxs that are not shown here (e.g., cytotoxins, anticoagulant 3FTxs, etc.).

Figure 3. Conceptual Representation of Next-Generation Antivenoms as Hybrid Products Comprising Mixtures of Antibodies, Antibody Fragments, and Small Molecule Inhibitors.

These different modalities have different pharmacodynamics and pharmacokinetics, which may be suitable for neutralizing different families of venom toxins with distinct functions and toxicokinetics. Note that this schematic is all encompassing and that future hybrid products seem likely to contain a small number of the different modalities presented, rather than all of them simultaneously. Abbreviations: 3FTx, three-finger toxins; PLA2, phospholipases A2; SVMP, snake venom metalloproteinases; SVSP, snake venom serine proteases. Figure courtesy of: Tulika (Technical University of Denmark).