Molecular Evolution of Vertebrate Neurotrophins: Co-Option of the Highly Conserved Nerve Growth Factor Gene into the Advanced Snake Venom Arsenalf

Abstract

Neurotrophins are a diverse class of structurally related proteins, essential for neuronal development, survival, plasticity and regeneration. They are characterized by major family members, such as the nerve growth factors (NGF), brain-derived neurotrophic factors (BDNF) and neurotrophin-3 (NT-3), which have been demonstrated here to lack coding sequence variations and follow the regime of negative selection, highlighting their extremely important conserved role in vertebrate homeostasis. However, in stark contrast, venom NGF secreted as part of the chemical arsenal of the venomous advanced snake family Elapidae (and to a lesser extent Viperidae) have characteristics consistent with the typical accelerated molecular evolution of venom components. This includes a rapid rate of diversification under the significant influence of positive-selection, with the majority of positively-selected sites found in the secreted β-polypeptide chain (74%) and on the molecular surface of the protein (92%), while the core structural and functional residues remain highly constrained. Such focal mutagenesis generates active residues on the toxin molecular surface, which are capable of interacting with novel biological targets in prey to induce a myriad of pharmacological effects. We propose that caenophidian NGFs could participate in prey-envenoming by causing a massive release of chemical mediators from mast cells to mount inflammatory reactions and increase vascular permeability, thereby aiding the spread of other toxins and/or by acting as proapoptotic factors. Despite their presence in reptilian venom having been known for over 60 years, this is the first evidence that venom-secreted NGF follows the molecular evolutionary pattern of other venom components, and thus likely participates in prey-envenomation.

Snake Genome Sequencing: Results and Future Prospects

“…At first sight, it may seem to be nothing more than an innocuous neurotrophin apparently occurring in the venom for no good reason. However, NGF is an extremely potent inducer of mast cell degranulation; thus it is possible that it may produce increased local vascular permeability and toxin absorption; it may also produce or enhance anaphylaxis [52,53]. The possibility that venom nerve growth factor may contribute to the toxicity of venom is further suggested by the fact that, like other true venom toxins, it is under positive selection in at least some snakes [52,53].…”

“…However, NGF is an extremely potent inducer of mast cell degranulation; thus it is possible that it may produce increased local vascular permeability and toxin absorption; it may also produce or enhance anaphylaxis [52,53]. The possibility that venom nerve growth factor may contribute to the toxicity of venom is further suggested by the fact that, like other true venom toxins, it is under positive selection in at least some snakes [52,53]. Nerve growth factor may also play an ancillary (non-toxic) role while the venom is stored in the venom gland by inhibiting metalloprotease-mediated degradation [54].…”

A Critique of the Toxicoferan Hypothesis

“…Additionally, it now seems certain that many of the proposed shared venom toxins within the Toxicofera actually result from the confusion of orthologs and paralogs, where non-toxic relatives of toxin genes have been identified (Hargreaves et al 2014a). For example, genes encoding complement c3 and nerve growth factor have been shown to have undergone an Elapid-specific gene duplication (Sunagar et al 2013;Hargreaves et al 2014a, b) to give rise to the putatively toxic "cobra venom factor" and nerve growth factor b (Hargreaves et al 2014b). This mis-identification of physiological orthologs as toxin-encoding paralogs has led to the conclusion that all Toxicoferan reptiles produce toxins in their oral secretions, and are therefore descended from a common venomous ancestor.…”

“…The occurrence of lineage-specific gene duplications (for example complement c3 and nerve growth factor in Elapids (Sunagar et al 2013;Hargreaves et al 2014a;Hargreaves et al 2014b)) would indicate that these genes may confer some preyspecific effects (as seen in the Mangrove catsnake, Boiga dendrophilia (Pawlak et al 2006)), or may have allowed adaptation to a new ecological niche.…”

Evolution and diversification of the cnidarian venom system

“…Thus, a majority of positively selected or episodically adaptive sites in neurotoxins are surface exposed. RAVER has been reported in a myriad of animal lineages and in a plethora of venom proteins, including scorpion neurotoxins [91,181,182,184,[188][189][190][191].…”