Crystallization of the Lys49 PLA2 homologue, myotoxin II, from the venom of Atropoides nummifer

Venom composition, toxicity and cross-neutralization by PoliVal-ICP antivenom, of Mesoamerican jumping pitvipers genus Metlapilcoatlus (Viperidae: Crotalinae)

BACKGROUND

The genus Metlapilcoatlus was recently erected to include six species of stout venomous snakes, known as the jumping pitvipers, which inhabit mountainous areas of Mesoamerica. This group maintains affinity with Atropoides picadoi, another jumping pitviper with restricted distribution in Costa Rica and Panama. Although the venom of A. picadoi and a couple of Metlapilcoatlus species has previously been characterized, little is known about the interspecific and intraspecific variation of the other species that comprise the genus. In this work, we characterize the venoms of five out of the six species that make up the genus Metlapilcoatlus: Metlapilcoatlus indomitus, Metlapilcoatlus mexicanus, Metlapilcoatlus nummifer, Metlapilcoatlus occiduus and Metlapilcoatlus olmec, and for three of them, we analyze whether ontogenetic change occurs in the composition of their venoms. Additionally, we evaluated the cross-neutralizing capacity of the antivenom PoliVal-ICP used in Central American countries to treat viper envenomation.

METHODS

We utilized sodium dodecyl sulfate-polyacrylamide gel electrophoresis and reverse-phase HPLC for venom characterization. Toxin identification was conducted using a bottom-up shotgun proteomic approach. We also estimated venom toxicity based on average lethality estimates in a murine model. The PoliVal-ICP neutralizing capacity on lethal activity was evaluated for all venoms. Using the venom of M. mexicanus as a model, we also tested the neutralizing capacity of this antivenom on hemorrhagic, myotoxic, proteolytic, phospholipase and coagulant activities.

RESULTS

Our analysis revealed that the venoms of jumping vipers are composed of proteins belonging to approximately 8-17 families, typically shared with other crotalines. Despite these general similarities, we observed variations at both intraspecific, including ontogenetic, and interspecific levels in venom composition and toxicity. The chromatographic pattern of Metlapilcoatlus venom exhibited peaks in the PLA2/PLA2-like eluting region, likely responsible for the myotoxic activity of these venoms. By contrast, these peaks were almost negligible in the chromatogram of A. picadoi, whose venom is significantly more hemorrhagic. Among the Metlapilcoatlus species, M. indomitus venom stood out as notably different from the others, and it was also the most lethal. The antivenom demonstrated its effectiveness in neutralizing the lethal activity of all the venoms tested, as well as the various biological activities studied in the venom of M. mexicanus.

CONCLUSIONS

Beyond the scope of the variation revealed here, our preclinical results demonstrate that PoliVal-ICP antivenom effectively neutralizes toxins from the venom of all Mesoamerican jumping vipers, despite not including venom from any of them in its immunization mixture. This cross-neutralization capacity predicts ICP antivenom's effectiveness in treating snake envenoming in the Neotropical region.

Lys49 myotoxins, secreted phospholipase A2-like proteins of viperid venoms: A comprehensive review

Muscle necrosis is a potential clinical complication of snakebite envenomings, which in severe cases can lead to functional or physical sequelae such as disability or amputation. Snake venom proteins with the ability to directly damage skeletal muscle fibers are collectively referred to as myotoxins, and include three main types: cytolysins of the "three-finger toxin" protein family expressed in many elapid venoms, the so-called "small" myotoxins found in a number of rattlesnake venoms, and the widespread secreted phospholipase A₂ (sPLA₂) molecules. Among the latter, protein variants that conserve the sPLA₂ structure, but lack such enzymatic activity, have been increasingly found in the venoms of many viperid species. Intriguingly, these sPLA₂-like proteins are able to induce muscle necrosis by a mechanism independent of phospholipid hydrolysis. They are commonly referred to as "Lys49 myotoxins" since they most often present, among other substitutions, the replacement of the otherwise invariant residue Asp49 of sPLA₂s by Lys. This work comprehensively reviews the historical developments and current knowledge towards deciphering the mechanism of action of Lys49 sPLA₂-like myotoxins, and points out main gaps to be filled for a better understanding of these multifaceted snake venom proteins, to hopefully lead to improved treatments for snakebites.

Short Linear Motifs Characterizing Snake Venom and Mammalian Phospholipases A2

Snake venom phospholipases A2 (PLA2s) have sequences and structures very similar to those of mammalian group I and II secretory PLA2s, but they possess many toxic properties, ranging from the inhibition of coagulation to the blockage of nerve transmission, and the induction of muscle necrosis. The biological properties of these proteins are not only due to their enzymatic activity, but also to protein–protein interactions which are still unidentified. Here, we compare sequence alignments of snake venom and mammalian PLA2s, grouped according to their structure and biological activity, looking for differences that can justify their different behavior. This bioinformatics analysis has evidenced three distinct regions, two central and one C-terminal, having amino acid compositions that distinguish the different categories of PLA2s. In these regions, we identified short linear motifs (SLiMs), peptide modules involved in protein–protein interactions, conserved in mammalian and not in snake venom PLA2s, or vice versa. The different content in the SLiMs of snake venom with respect to mammalian PLA2s may result in the formation of protein membrane complexes having a toxic activity, or in the formation of complexes whose activity cannot be blocked due to the lack of switches in the toxic PLA2s, as the motif recognized by the prolyl isomerase Pin1.

Structure of myotoxin II, a catalytically inactive Lys49 phospholipase A2 homologue from Atropoides nummifer venom

Lys49 snake-venom phospholipase A2 (PLA2) homologues are highly myotoxic proteins which, although lacking catalytic activity, possess the ability to disrupt biological membranes, inducing significant muscle-tissue loss and permanent disability in severely envenomed patients. Since the structural basis for their toxic activity is still only partially understood, the structure of myotoxin II, a monomeric Lys49 PLA2 homologue from Atropoides nummifer, has been determined at 2.08 angstroms resolution and the anion-binding site has been characterized.