Transcriptome analysis of the venom gland of the scorpion Scorpiops jendeki: implication for the evolution of the scorpion venom arsenal
- Yibao Ma†1,
- Ruiming Zhao†1,
- Yawen He1,
- Songryong Li1,
- Jun Liu1,
- Yingliang Wu1,
- Zhijian Cao1Email author and
- Wenxin Li1Email author
© Ma et al; licensee BioMed Central Ltd. 2009
Received: 26 February 2009
Accepted: 01 July 2009
Published: 01 July 2009
The family Euscorpiidae, which covers Europe, Asia, Africa, and America, is one of the most widely distributed scorpion groups. However, no studies have been conducted on the venom of a Euscorpiidae species yet. In this work, we performed a transcriptomic approach for characterizing the venom components from a Euscorpiidae scorpion, Scorpiops jendeki.
There are ten known types of venom peptides and proteins obtained from Scorpiops jendeki. Great diversity is observed in primary sequences of most highly expressed types. The most highly expressed types are cytolytic peptides and serine proteases. Neurotoxins specific for sodium channels, which are major groups of venom components from Buthidae scorpions, are not detected in this study. In addition to those known types of venom peptides and proteins, we also obtain nine atypical types of venom molecules which haven't been observed in any other scorpion species studied to date.
This work provides the first set of cDNAs from Scorpiops jendeki, and one of the few transcriptomic analyses from a scorpion. This allows the characterization of a large number of venom molecules, belonging to either known or atypical types of scorpion venom peptides and proteins. Besides, our work could provide some clues to the evolution of the scorpion venom arsenal by comparison with venom data from other scorpion lineages.
Based on cladistic morphological analysis, the extant scorpions can be phylogenetically divided into 14 families. All scorpions possess a homologous venom apparatus which consists of the vesicle holding a pair of venom glands and the hypodermic aculeus used to inject the venom. Scorpion venom is a combinatorial library of peptides and proteins which could cause toxicological responses and can be candidates for drug design and development. The general compositions of scorpion venoms vary among different families. For instance, in a comparative LC/MS analysis of two scorpion species from the families Buthidae and Ischnuridae, vast abundance difference was observed in venom components with molecular size from 5000 to 10,000 Da. Furthermore, such differences in venom compositions could also be observed from genus to genus, and even between different species within a genus[5, 6].
Hundreds of venom peptides and proteins have been characterized from various scorpion species. It is noteworthy that most of these venom molecules are obtained by either bioassay-guided fractionation or PCR-based methods conducted with cDNA libraries. Due to their medical importance, most research performed to date has focused on Buthidae scorpions. Buthid venoms mainly consist of four different families of neurotoxins which specifically target ion channels, including sodium channels, potassium channels, chloride channels, and calcium channels [8–10]. However, in contrary to buthids, little attention has been paid to the other thirteen non-Buthidae families. As several classes of venom peptides and proteins from non-Buthidae scorpions were shown to possess unique primary sequences and biological activity, it is worth exploring the venom compositions of non-Buthidae scorpions.
The scorpion Scorpiops jendeki is distributed in Yunnan province, Southwest China. It was once considered to be a member of the family Scorpiopidae, but now it is classified into the family Euscorpiidae after a very thorough phylogenetic analysis. The Euscorpiidae family is among the most widely distributed groups of extant scorpions, and it covers Europe, Asia, Africa, and America. Euscorpiids are considered to be harmless scorpions which possess no threat to human health. So far, euscorpiid venoms haven't been studied yet.
Different from bioassay-guided isolation, an "-ome" approach such as transcriptomic or proteomic analysis could help uncover the real diversity of scorpion venom components. Not only known types of venom peptides and proteins but also atypical venom molecules could be obtained by such an approach. Until now, proteomic studies have been employed in assessing the diversity of venom compositions from several scorpion species. Only one transcriptomic analysis has been conducted on the venom gland of a scorpion. An extensive knowledge of venom compositions from different scorpion species is helpful in understanding the envenomation and providing candidate molecules for drug development. Furthermore, comparative analysis of venom constituents from different scorpion lineages could also provide a clue to the evolutionary track of scorpion venom arsenal, as illustrated in the snake venom systems [14–16].
In this work, we carried an EST approach to overview the transcriptome of the Scorpiops jendeki venom gland. A great number of venom peptides and proteins, belonging to known and atypical toxin types, were identified through the first transcriptome study on the venom gland of a Euscorpiidae scorpion. Besides, venom data comparison among different scorpion lineages provides some clues to the evolutionary track of the scorpion venom arsenal.
EST sequencing and clustering
Distribution of 293 clusers assembled from the scorpion Scorpiops jendeki
Similar to venom peptide transcripts
Not similar to venom peptide transcripts
Known toxin types
known toxin types have been characterized from the scorpion Scorpiops jendeki. They are encoded by 359 ESTs (33 clusters), accounting for approximately 40% of the total venom gland transcripts (Table 1).
It is noteworthy that SJE042C consists of two almost identical ESTs differing by only a few nucleotides. The translated sequences are named SJE042C.1 and SJE042C.2, respectively. Similar phenomenon has also been extensively observed in other types of venom peptides and proteins discussed followingly. The possibility that these minor differences are derived in the course of cDNA library construction and sequencing could be excluded, as the phenomenon can hardly be observed in the clusters encoding common cellular proteins. Such subtle differences in EST sequences reflect the polymorphism of scorpion venom peptide genes.
Interestingly, although SJE009C have four disulfide bridges, it shows closer relationship with SJE093C and SJE094C, the α-KTxs with three disulfide bridges. This highlights the evolutionary relationship between α-KTxs with 3 disulfide bridges and those with 4 disulfide bridges.
Scorpine-like peptides show obvious sequence similarity to β-family of KTxs. But distinct to β-KTxs, they don't possess a putative short pro-sequence following the signal peptide. Until now, all scorpine-like peptides are exclusively obtained from non-Buthidae scorpions, whereas all β-KTxs are from Buthidae scorpions (Figure 3B). The Scorpine-like peptide Tco 41.46-2, which is isolated from Tityus costatus (Buthidae), should be classified into β-KTxs, based on sequence similarity and the presence of a pro-peptide. As scorpion neurotoxins are paralogous genes of defensins, scorpion defensins were used to root the phylogeny tree[26, 27]. The reconstructed phylogeny relationship strongly suggests that β-KTxs and scorpine-like peptides share a common ancestor before the lineage split between Buthidae and the non-Buthidae families. After the lineage split, β-KTxs and scorpine-like peptides evolve independently in different scorpion families.
The first cytolytic linear peptide, named IsCT, was got from the scorpion Opisthacanthus madagascariensis, a member of the family Scorpionidae[34, 35]. Then this type of venom peptides were later found in the scorpion Mesobuthus martensii (Buthidae). Their precursors consist of a signal peptide, a mature peptide and a C-terminal propeptide rich in acidic amino acids. Cytolytic peptides possess broad activity spectra against microbes and hemolytic activity. They are suggested to lyse cell membranes via pore formation or destabilization of membrane phospholipid packing, based on their amphiphilic α-helical structures.
Trypsin inhibitor like (TIL) peptide
Of note, SJE017C is almost identical to SJE037C, except for a 72 bp insertion into the former. Which molecular mechanism causes this phenomenon would depend on uncovering their genomic organizations and structures. Interestingly, a nonsense mutation in the 72 bp insertion of SJE017C results in a premature stop codon. Three ESTs in SJE017C represent different transcripts of the same gene, as they are not completely identical. So the possibility of an error in the sequencing is excluded. Resequencing these three clones further supports the nonsense mutation. So the cluster SJE017C may represent a pseudogene.
Secretory peptides with trypsin inhibitor like domain can also be found in the venom glands of mosquito[41–43]. They function as serine protease inhibitors or antimicrobial peptides[44, 45]. So convergent evolution has repeatedly selected genes coding for proteins containing the trypsin inhibitor like cysteine rich domain as templates for venom molecules.
Opistoporin like peptide
SPSVs (serine proteases from scorpion venoms)
The atypical possible toxin types
In addition to those known types of venom peptides and proteins as described above, there are also several clusters supposed to encode novel venom peptide types, base on their high expression level and the presence of the signal peptide.
Besides, there are several medium-abundant clusters which are deduced to encode eight novel types of scorpion venom peptides [see Additional file 1]. They are either cysteine-free or cysteine-rich. Similar to jendins, they have not homologs found from public database. The presence of atypical venom peptides and proteins indicates that scorpion venoms are a rather complex pool, and multiple currently unkown types of venom peptides and proteins remain to be characterized from different scorpion lineages.
Common cellular protein ESTs
The scorpion venom gland is a specialized organ for synthesizing and secreting venom components. As demonstrated in Scorpiops jendeki, transcripts for different types of venom peptides and proteins account for more than 50% of the full transcriptome. So it is interesting to overview the physiological state of the venom gland when it highly expresses venom peptides and proteins.
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.
In conclusion, we conducted a transcriptomic analysis of Scorpiops jendeki venom gland. Scorpiops jendeki belong to the family Euscorpiidae whose venoms have never been investigated. So our work greatly expanded the current knowledge of scorpion venoms. We obtained ten known types and nine atypical types of venom peptides and proteins. These molecules provide a rich hitherto unexplored resource for drugdevelopment. Besides, some clues can be provided into the evolution of scorpion venom arsenal by comparing the presence of common and umcomon types of venom peptides and proteins among different scorpion lineages.
cDNA library construction
50 specimens of Scorpiops jendeki were collected in Yunnan province, Southwest China. They were milked 2 days before RNA isolation as described previously. Total RNA was extracted with TRIZOL Reagent (Invitrogen, Carlsbad, CA, USA), and then mRNA was purified with FastTrack 2.0 mRNA Isolation Kit(Invitrogen). The cDNA library was constructed from 5 μg of mRNA using the Creator SMART cDNA Library Construction Kit (Clontech Laboratories, Palo Alto, CA). cDNA inserts were directionally cloned into the plasmids pDNR-LIB digested by restriction enzymes Sfi IA and Sfi IB. The recombinant plasmids were transformed into electrocompenent Escherichia coli DH10B (Invitrogen).
To obtain an unbiased overview of the venom gland transcriptome, random colonies were selected and cultured in appropriate Luria Broth culture medium containing 30 μg/ml of chloramphenicol. After overnight culture, plasmid DNA was isolated using alkaline lysis method. Purified plasmids were single-pass sequenced on an ABI 3730xl sequencer using the standard M13 forward primer and BigDye terminator v3.1 cycle sequencing kit (Applied Biosystems, Foster City, CA, USA).
The trace files of sequenced clones were subjected to Phred program, the cutoff Phred score was set to 40. After these sequences were strictly trimmed, the got high-quality sequences were processed on the website EGassembler http://egassembler.hgc.jp/ with the default parameter. Vector and adaptor sequences were removed using the program Cross_Match. After removing the PolyA tail, we discarded those sequences shorter than 100 bp. The resulted sequences were deposited into the dbEST, and then assembled into clusters with the program CAP3.
Each cluster was annotated by being searched against SWISS-PROT http://www.expasy.org/tools/blast/ and GenBank NCBI database http://www.ncbi.nlm.nih.gov/blast with BLAST algorithms. After BLAST search, the unmatched clusters were further identified for open reading frames using the ORFfinder http://www.ncbi.nlm.nih.gov/projects/gorf/. Considering the extreme diversity of scorpion toxins, those clusters putative to encode venom peptides was reexamined manually to pick out individual different isoforms.
All clusters were checked for the existence of signal peptides using the SignalP 3.0 program http://www.cbs.dtu.dk/services/SignalP/. All types of venom peptides and proteins are annotated by searching against Pfam protein families database http://pfam.sanger.ac.uk/.
Alignment and phylogeny analysis
The sequences used for alignment and phylogeny analysis were retrieved from SWISS-PROT databsae http://www.expasy.org/tools/blast/. The alignment was performed by Clustal_X 1.83 software followed by manual adjustment, and viewed by the software Jalview. Phylogeny analysis was carried out with Neighbor joining method implemented in MEGA3.1.
This work was supported by grants from the National Natural Sciences Foundation of China to Li WX, Cao ZJ and Wu YL (Nos. 30530140, 30570045 and 30770519), the Basic Project of Ministry of Science and Technology of China to Li WX (No. 2007FY210800) and the Youth Chenguang Project of Science and Technology of Wuhan City to Cao ZJ (No. 20065004116-06).
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