Cloning and characterisation of three novel disintegrin precursors from the venoms of three Atheris species: Atheris chlorechis, Atheris nitschei and Atheris squamigera

Rapid Identification of Phospholipase A₂ Transcripts from Snake Venoms

Phospholipase A₂ (PLA₂) is a major component in snake venoms and it is found in many different isoforms. To identify transcripts encoding different PLA₂ isoforms, we developed a simple, rapid procedure. Total RNA was extracted from the venoms of three cottonmouth snakes and two diamondback rattlesnakes, and further reverse-transcribed into complementary DNA (cDNA). Using one pair of cottonmouth PLA₂-specific primers and a Reverse Transcription Polymerase Chain Reaction (RT-PCR) technique, we identified 27 unique full-length PLA₂ transcripts, including nine sequences identical to the previously documented ones in the database and one novel GIII-like PLA₂. Two common transcripts respectively encoding Asp49 and Lys49 PLA₂ isoforms were identified in all three cottonmouth venoms, that contain more PLA₂ transcripts than the diamondback rattlesnake venoms. The placement of cloned PLA₂ transcripts in snake venom PLA₂s was further discussed by phylogenetic analysis. The procedure developed in this study paves the way for accelerated acquisition of transcriptome data on any other venom toxin families. The results obtained are crucial for insight into the structure and function of PLA₂ isoforms for scientific and potential therapeutic purposes.

Comparative Profiling of Three Atheris Snake Venoms: A. squamigera, A. nitschei and A. chlorechis

A proteomic and transcriptomic comparative analysis of the venoms of three Atheris species (A. squamigera, A. nitschei and A. chlorechis) was carried out by size exclusion liquid chromatography, gel electrophoresis, mass spectrometry, and mRNA sequencing. The improved proteomic profiling utilised in this work was combined with transcript studies, advancing our insights into venom composition, protein distribution and inter-species variation among the three bush vipers. Crude venoms of all three samples contained at least 10-20 protein components, ranging in size from ≤ 3 to > 98 kDa. Both approaches yielded converging overall information, pointing to phospholipases, disintegrins, serine proteases and metalloproteases as the major toxin classes, which are likely to explain the local and systemic symptoms observed in envenomation by Atheris genus. Being considered as the main factors involved in the distinct venom-induced pathologies, these identified snake venom proteins are of particular interest in terms of understanding their physiological and biological function as well as for their contribution in potential medical treatments.

Full-Length Venom Protein cDNA Sequences from Venom-Derived mRNA: Exploring Compositional Variation and Adaptive Multigene Evolution

Envenomation of humans by snakes is a complex and continuously evolving medical emergency, and treatment is made that much more difficult by the diverse biochemical composition of many venoms. Venomous snakes and their venoms also provide models for the study of molecular evolutionary processes leading to adaptation and genotype-phenotype relationships. To compare venom complexity and protein sequences, venom gland transcriptomes are assembled, which usually requires the sacrifice of snakes for tissue. However, toxin transcripts are also present in venoms, offering the possibility of obtaining cDNA sequences directly from venom. This study provides evidence that unknown full-length venom protein transcripts can be obtained from the venoms of multiple species from all major venomous snake families. These unknown venom protein cDNAs are obtained by the use of primers designed from conserved signal peptide sequences within each venom protein superfamily. This technique was used to assemble a partial venom gland transcriptome for the Middle American Rattlesnake (Crotalus simus tzabcan) by amplifying sequences for phospholipases A₂, serine proteases, C-lectins, and metalloproteinases from within venom. Phospholipase A₂ sequences were also recovered from the venoms of several rattlesnakes and an elapid snake (Pseudechis porphyriacus), and three-finger toxin sequences were recovered from multiple rear-fanged snake species, demonstrating that the three major clades of advanced snakes (Elapidae, Viperidae, Colubridae) have stable mRNA present in their venoms. These cDNA sequences from venom were then used to explore potential activities derived from protein sequence similarities and evolutionary histories within these large multigene superfamilies. Venom-derived sequences can also be used to aid in characterizing venoms that lack proteomic profiles and identify sequence characteristics indicating specific envenomation profiles. This approach, requiring only venom, provides access to cDNA sequences in the absence of living specimens, even from commercial venom sources, to evaluate important regional differences in venom composition and to study snake venom protein evolution.

This work demonstrates that full-length venom protein messenger RNAs are present in secreted venoms and can be used to acquire full-length protein sequences of toxins from both front-fanged (Elapidae, Viperidae) and rear-fanged (Colubridae) snake venoms, eliminating the need to use venom glands. Full-length transcripts were obtained from venom samples that were fresh, newly lyophilized, old, field desiccated or commercially prepared, representing a significant advance over previous attempts which produced only partial sequence transcripts. Transcripts for all major venom protein families (metalloproteinases, serine proteases, C-type lectins, phospholipases A₂ and three-finger toxins) responsible for clinically significant snakebite symptoms were obtained from venoms. These sequences aid in the identification and characterization of venom proteome profiles, allowing for the identification of peptide sequences, specific isoforms, and novel venom proteins. The application of this technique will help to provide venom protein sequences for many snake species, including understudied rear-fanged snakes. Venom protein transcripts offer important insights into potential snakebite envenomation profiles and the molecular evolution of venom protein multigene families. By requiring only venom to obtain venom protein cDNAs, the approach detailed here will provide access to cDNA-based protein sequences from commercial and other venom sources, facilitating study of snake venom protein composition and evolution.

Molecular Characterization of Three Novel Phospholipase A₂ Proteins from the Venom of Atheris chlorechis, Atheris nitschei and Atheris squamigera

Secretory phospholipase A₂ (sPLA₂) is known as a major component of snake venoms and displays higher-order catalytic hydrolysis functions as well as a wide range of pathological effects. Atheris is not a notoriously dangerous genus of snakes although there are some reports of fatal cases after envenomation due to the effects of coagulation disturbances and hemorrhaging. Molecular characterization of Atheris venom enzymes is incomplete and there are only a few reports in the literature. Here, we report, for the first time, the cloning and characterization of three novel cDNAs encoding phospholipase A₂ precursors (one each) from the venoms of the Western bush viper (Atheris chlorechis), the Great Lakes bush viper (Atheris nitschei) and the Variable bush viper (Atheris squamigera), using a “shotgun cloning” strategy. Open-reading frames of respective cloned cDNAs contained putative 16 residue signal peptides and mature proteins composed of 121 to 123 amino acid residues. Alignment of mature protein sequences revealed high degrees of structural conservation and identity with Group II venom PLA₂ proteins from other taxa within the Viperidae. Reverse-phase High Performance Liquid Chromatography (HPLC) profiles of these three snake venoms were obtained separately and chromatographic fractions were assessed for phospholipase activity using an egg yolk suspension assay. The molecular masses of mature proteins were all identified as approximately 14 kDa. Mass spectrometric analyses of the fractionated oligopeptides arising from tryptic digestion of intact venom proteins, was performed for further structural characterization.