Comparison of venom composition and biological activities of the subspecies Crotalus lepidus lepidus, Crotalus lepidus klauberi and Crotalus lepidus morulus from Mexico

A Review of Rattlesnake Venoms

Venom components are invaluable in biomedical research owing to their specificity and potency. Many of these components exist in two genera of rattlesnakes, Crotalus and Sistrurus, with high toxicity and proteolytic activity variation. This review focuses on venom components within rattlesnakes, and offers a comparison and itemized list of factors dictating venom composition, as well as presenting their known characteristics, activities, and significant applications in biosciences. There are 64 families and subfamilies of proteins present in Crotalus and Sistrurus venom. Snake venom serine proteases (SVSP), snake venom metalloproteases (SVMP), and phospholipases A2 (PLA2) are the standard components in Crotalus and Sistrurus venom. Through this review, we highlight gaps in the knowledge of rattlesnake venom; there needs to be more information on the venom composition of three Crotalus species and one Sistrurus subspecies. We discuss the activity and importance of both major and minor components in biomedical research and drug development.

Crotalus morulus (Viperidae: Crotalinae) envenoming and treatment with F(ab')2 antivenom

We present the first published case of Tamaulipan Rock Rattlesnake (Crotalus morulus) envenoming. A 54-year-old male professional herpetologist was bitten on the left thumb by a captive C. morulus. Pain, swelling, bruising, and pressure in the thumb were experienced within minutes. On presentation, he reported 7/10 pain and had firm edema in his thumb and thenar eminence. Initial laboratory studies showed normal platelet count, PT, PTT, and creatine kinase. He was treated with pain medication and 10 vials of crotalidae immune F(ab')₂ (equine) antivenom approximately three hours post envenoming. Lymphangitic streaking and axillary lymphadenopathy developed, followed by progression of edema, emergence of a hemorrhagic bulla, and declining platelets, prompting treatment with two additional 10-vial antivenom doses. His platelet count declined to 125 × 10³/μL 24 hours post envenoming and he developed numbness in his thumb. Following antivenom therapy completion no further decline in platelets occurred and thrombocytopenia improved to 131 × 10³/μL prior to discharge 46 hours post envenoming; fibrinogen, PT, PTT, and CK remained normal. He had no residual signs or symptoms 5 months later. C. morulus venom includes proteolytic venom enzymes that induce local soft tissue destruction, pain, and edema with ecchymosis and blister formation. Although C. morulus venom contains a unique disintegrin, morulustatin, no fibrinogenolytic activity was observed.

A symphony of destruction: Dynamic differential fibrinogenolytic toxicity by rattlesnake (Crotalus and Sistrurus) venoms

What factors influence the evolution of a heavily selected functional trait in a diverse clade? This study adopts rattlesnakes as a model group to investigate the evolutionary history of venom coagulotoxicity in the wider context of phylogenetics, natural history, and biology. Venom-induced clotting of human plasma and fibrinogen was determined and mapped onto the rattlesnake phylogenetic tree to reconstruct the evolution of coagulotoxicity across the group. Our results indicate that venom phenotype is often independent of phylogenetic relationships in rattlesnakes, suggesting the importance of diet and/or other environmental variables in driving venom evolution. Moreover, the striking inter- and intraspecific variability in venom activity on human blood highlights the considerable variability faced by physicians treating envenomation. This study is the most comprehensive effort to date to describe and characterize the evolutionary and biological aspects of coagulotoxins in rattlesnake venom. Further research at finer taxonomic levels is recommended to elucidate patterns of variation within species and lineages.

A Meta-Analysis of the Protein Components in Rattlesnake Venom

The specificity and potency of venom components give them a unique advantage in developing various pharmaceutical drugs. Though venom is a cocktail of proteins, rarely are the synergy and association between various venom components studied. Understanding the relationship between various components of venom is critical in medical research. Using meta-analysis, we observed underlying patterns and associations in the appearance of the toxin families. For Crotalus, Dis has the most associations with the following toxins: PDE; BPP; CRL; CRiSP; LAAO; SVMP P-I and LAAO; SVMP P-III and LAAO. In Sistrurus venom, CTL and NGF have the most associations. These associations can predict the presence of proteins in novel venom and understand synergies between venom components for enhanced bioactivity. Using this approach, the need to revisit the classification of proteins as major components or minor components is highlighted. The revised classification of venom components is based on ubiquity, bioactivity, the number of associations, and synergies. The revised classification can be expected to trigger increased research on venom components, such as NGF, which have high biomedical significance. Using hierarchical clustering, we observed that the genera's venom compositions were similar, based on functional characteristics rather than phylogenetic relationships.

A Single Mutation Unlocks Cascading Exaptations in the Origin of a Potent Pitviper Neurotoxin

Evolutionary innovations and complex phenotypes seemingly require an improbable amount of genetic change to evolve. Rattlesnakes display two dramatically different venom phenotypes. Type I venoms are hemorrhagic with low systemic toxicity and high expression of tissue-destroying snake venom metalloproteinases. Type II venoms are highly neurotoxic and lack snake venom metalloproteinase expression and associated hemorrhagic activity. This dichotomy hinges on Mojave toxin (MTx), a phospholipase A2 (PLA2) based β-neurotoxin expressed in Type II venoms. MTx is comprised of a nontoxic acidic subunit that undergoes extensive proteolytic processing and allosterically regulates activity of a neurotoxic basic subunit. Evolution of the acidic subunit presents an evolutionary challenge because the need for high expression of a nontoxic venom component and the proteolytic machinery required for processing suggests genetic changes of seemingly little immediate benefit to fitness. We showed that MTx evolved through a cascading series of exaptations unlocked by a single nucleotide change. The evolution of one new cleavage site in the acidic subunit unmasked buried cleavage sites already present in ancestral PLA2s, enabling proteolytic processing. Snake venom serine proteases, already present in the venom to disrupt prey hemostasis, possess the requisite specificities for MTx acidic subunit proteolysis. The dimerization interface between MTx subunits evolved by exploiting a latent, but masked, hydrophobic interaction between ancestral PLA2s. The evolution of MTx through exaptation of existing functional and structural features suggests complex phenotypes that depend on evolutionary innovations can arise from minimal genetic change enabled by prior evolution.

Venom Ontogeny in the Mexican Lance-Headed Rattlesnake (Crotalus polystictus)

As trophic adaptations, rattlesnake venoms can vary in composition depending on several intrinsic and extrinsic factors. Ontogenetic changes in venom composition have been documented for numerous species, but little is known of the potential age-related changes in many rattlesnake species found in México. In the current study, venom samples collected from adult and neonate Crotalus polystictus from Estado de México were subjected to enzymatic and electrophoretic analyses, toxicity assays (LD₅₀), and MALDI-TOF mass spectrometry, and a pooled sample of adult venom was analyzed by shotgun proteomics. Electrophoretic profiles of adult males and females were quite similar, and only minor sex-based variation was noted. However, distinct differences were observed between venoms from adult females and their neonate offspring. Several prominent bands, including P-I and P-III snake venom metalloproteinases (SVMPs) and disintegrins (confirmed by MS/MS) were present in adult venoms and absent/greatly reduced in neonate venoms. Age-dependent differences in SVMP, kallikrein-like, phospholipase A₂ (PLA₂), and L-amino acid oxidase (LAAO) activity levels were confirmed by enzymatic activity assays, and like many other rattlesnake species, venoms from adult snakes have higher SVMP activity than neonate venoms. Conversely, PLA₂ activity was approximately 2.5 × greater in venoms from neonates, likely contributing to the increased toxicity (neonate venom LD₅₀ = 4.5 μg/g) towards non-Swiss albino mice when compared to adult venoms (LD₅₀ = 5.5 μg/g). Thrombin-like (TLE) and phosphodiesterase activities did not vary significantly with age. A significant effect of sex (between adult male and adult female venoms) was also observed for SVMP, TLE, and LAAO activities. Analysis of pooled adult venom by LC-MS/MS identified 14 toxin protein families, dominated by bradykinin-inhibitory peptides, SVMPs (P-I, P-II and P-III), disintegrins, PLA₂s, C-type-lectins, CRiSPs, serine proteinases, and LAAOs (96% of total venom proteins). Neonate and adult C. polystictus in this population consume almost exclusively mammals, suggesting that age-based differences in composition are related to physical differences in prey (e.g., surface-to-volume ratio differences) rather than taxonomic differences between prey. Venoms from adult C. polystictus fit a Type I pattern (high SVMP activity, lower toxicity), which is characteristic of many larger-bodied rattlesnakes of North America.

Canopy Venom: Proteomic Comparison among New World Arboreal Pit-Viper Venoms

Central and South American pitvipers, belonging to the genera Bothrops and Bothriechis, have independently evolved arboreal tendencies. Little is known regarding the composition and activity of their venoms. In order to close this knowledge gap, venom proteomics and toxin activity of species of Bothriechis, and Bothrops (including Bothriopsis) were investigated through established analytical methods. A combination of proteomics and bioactivity techniques was used to demonstrate a similar diversification of venom composition between large and small species within Bothriechis and Bothriopsis. Increasing our understanding of the evolution of complex venom cocktails may facilitate future biodiscoveries.

Molecular models of the Mojave rattlesnake (Crotalus scutulatus scutulatus) venom metalloproteinases reveal a structural basis for differences in hemorrhagic activities

Rattlesnake venom can differ in composition and in metalloproteinase-associated activities. The molecular basis for this intra-species variation in Crotalus scutulatus scutulatus (Mojave rattlesnake) remains an enigma. To understand the molecular basis for intra-species variation of metalloproteinase-associated activities, we modeled the three-dimensional structures of four metalloproteinases based on the amino acid sequence of four variations of the proteinase domain of the C. s. scutulatus metalloproteinase gene (GP1, GP2, GP3, and GP4). For comparative purposes, we modeled the atrolysin metalloproteinases of C. atrox as well. All molecular models shared the same topology. While the atrolysin metalloproteinase molecular models contained highly conserved substrate binding sites, the Mojave rattlesnake metalloproteinases showed higher structural divergence when superimposed onto each other. The highest structural divergence among the four C. s. scutulatus molecular models was located at the northern cleft wall and the S'1-pocket of the substrate binding site, molecular regions that modulate substrate selectivity. Molecular dynamics and field potential maps for each C. s. scutulatus metalloproteinase model demonstrated that the non-hemorrhagic metalloproteinases (GP2 and GP3) contain highly basic molecular and field potential surfaces while the hemorrhagic metalloproteinases GP1 and atrolysin C showed extensive acidic field potential maps and shallow but less dynamic active site pockets. Hence, differences in the spatial arrangement of the northern cleft wall, the S'1-pocket, and the physico-chemical environment surrounding the catalytic site contribute to differences in metalloproteinase activities in the Mojave rattlesnake. Our results provide a structural basis for variation of metalloproteinase-associated activities in the rattlesnake venom of the Mojave rattlesnake.