During more than 400 million years of evolution, scorpions have developed an efficient venom arsenal, composed of extremely diverse active components, to prey captures and deter competitors. The venom molecules are able to induce both toxicological and immunological responses, and also offer a tremendous resource for use in drug development. Usually transcriptome or proteome approach is employed to explore the complexity of venom components. Several recent studies performed on many venomous species demonstrate that venom proteome and transcriptome depart in their relative abundances of different toxin families[59, 60]. However, the ESTs-based transcriptome strategy has been shown to be effective in uncovering the real diversity of venom compositions[13, 61]. Not only sequences of known toxin types but also atypical venom molecules could be characterized by such a transcriptomic approach.
In this work, we have employed a transcriptomic approach to investigate possible venom components from the scorpion Scorpiops jendeki. Before RNA extraction, the scorpion specimens are milked by electrical stimulation. So the gene expression profiling obtained in this work represents the activated-state transcription of the venom glands. The transcripts for possible venom constitutes make up approximately 50% of the Scorpiops jendeki transcirptome. It is much higher than that observed for the scorpion Hadrurus gertschi (approximately 30%). Such difference may be attributed to genetic variations. This work could be used in comparative studies of gene expression profiling among different scorpion species.
Among different scorpion venoms, there are great variability in proportion of different types of venom peptides and proteins. A previous study conducted a comparative proteomic analysis of scorpion venom components with the method of mass finger print comparison among three different Tityus venoms. It shows that the proportion of molecular weight distribution is rather variable among Tityus cambridgei, Tityus costatus and Tityus discrepans. Until now, there is only one transcriptome study of scorpion venom glands. In the transcriptome of the Hadrurus gertschi venom gland, α-KTxs and scorpine-like peptides are most highly expressed, accounting 17.7% of the total ESTs. However, the most prevalent types of venom peptides and proteins are cytolytic peptides and SPSVs in Scorpiops jendeki. Approximately 19% of the total ESTs encode for the precursors of these two types of molecules. It is noteworthy that the four types (SPSVs, La1-like peptides, calcines, and jendins), with a high expression level in Scorpiops jendeki, were not detected in Hadrurus gertschi at all. Although different types of venom molecules couldn't arise in proteins at the same level of their mRNAs, we could definitely conclude that there is great difference in venom compositions between Scorpiops jendeki and Hadrurus gertschi. Furthermore, the venom compositions of Scorpiops jendeki must be different from that of Buthidae scorpions, whose major groups of venom constitutes are neurotoxins affecting Na+ channels (NaScTxs) and K+ channels (KTxs).
Great diversity has also been observed in primary sequences of most highly expressed venom peptides and proteins. We can exclude the possibility that such diversity is caused by the artifact in cDNA library construction or DNA sequencing. A negative control is that 31 ESTs from SJE009C encode one identical translated sequence. Such diversity may mainly be attributed to variations in scorpion population, as the cDNA library was constructed with the RNA extracted from about 50 specimens. However, a previous study demonstrates that such polymorphism could also arise at the level of individual scorpion. Whatever, such diversity extensively observed in different types of venom peptides and proteins reflects the dynamic process of diversification. It is beneficial for the survival of scorpions, as the more and more complex venom arsenal could meet their demands for interaction with their prey, predators, and competitors.
The most striking observation of this study is the absence of NaScTxs in Scorpiops jendeki. This phenomenon has also been observed in the non-Buthidae scorpion Hadrurus gertschi (Caraboctonidae), on which a transcriptomic analysis has been conducted. NaScTxs are peptides of 58–76 residues in length and characterized to possess a structure core, named Cysteine-Stabilized α/β motif (CS-αβ), tightly packed by three conserved disulfide bridges. They are a major group of venom components from Buthidae scorpions. NaScTxs and KTxs are suggested to evolve from a common progenitor, based their similarities in gene organizations, intron features and structure cores. But their evolutionary history is difficult to reconstruct, due to high diversity of each toxin types[63, 64]. Similar to NaScTxs, KTxs are also defined by the presence of the conserved CS-αβ motif. Distinct to NaScTxs, KTxs have been obtained from most scorpion species, both Buthidae and non-Buthidae, currently under investigated. The difference between the phylogeny distribution of NaScTxs and KTxs could provide some clues to their evolutionary relationship.
Until now, many types of venom peptides and proteins have been obtained from diverse scorpion species. Some types are found to be widely distributed among scorpion species from different families, in case of α-KTxs. However, some other types appear to be restricted to particular scorpion lineages. For instance, jendins haven't been detected in other scorpion species. Scorpine-like peptides have not been obtained from Buthidae scorpions, although some Buthidae scorpion species have been extensively studied. So far transcriptome studies are lacking even for the medically imprtant Buthidae scorpions. However, this work implies that the presence of additional, atypical toxin types in many scorpion lineages is most likely. The presence of these common and uncommon venom molecules among different lineages reflects the dynamic evolutionary process of the scorpion venom arsenal. In order to depict such a process, extensive studies should be conducted on diverse scorpion species, especially from the non-Buthidae families.