TxXIIIA, an atypical homodimeric conotoxin found in the Conus textile venom

Venom Diversity and Evolution in the Most Divergent Cone Snail Genus Profundiconus

Profundiconus is the most divergent cone snail genus and its unique phylogenetic position, sister to the rest of the family Conidae, makes it a key taxon for examining venom evolution and diversity. Venom gland and foot transcriptomes of Profundiconus cf. vaubani and Profundiconus neocaledonicus were de novo assembled, annotated, and analyzed for differential expression. One hundred and thirty-seven venom components were identified from P. cf. vaubani and 82 from P. neocaledonicus, with only four shared by both species. The majority of the transcript diversity was composed of putative peptides, including conotoxins, profunditoxins, turripeptides, insulin, and prohormone-4. However, there were also a significant percentage of other putative venom components such as chymotrypsin and L-rhamnose-binding lectin. The large majority of conotoxins appeared to be from new gene superfamilies, three of which are highly different from previously reported venom peptide toxins. Their low conotoxin diversity and the type of insulin found suggested that these species, for which no ecological information are available, have a worm or molluscan diet associated with a narrow dietary breadth. Our results indicate that Profundiconus venom is highly distinct from that of other cone snails, and therefore important for examining venom evolution in the Conidae family.

Identification of a Novel O-Conotoxin Reveals an Unusual and Potent Inhibitor of the Human α9α10 Nicotinic Acetylcholine Receptor

Conotoxins are a pool of disulfide-rich peptide neurotoxins produced by cone snails for predation and defense. They are a rich reservoir of novel ligands for ion channels, neurotransmitter receptors and transporters in the nervous system. In this study, we identified a novel conotoxin component, O-conotoxin GeXXVIIA, from the venom of Conus generalis. The native form of this component is a disulfide-linked homodimer of a 5-Cys-containing peptide. Surprisingly, our electrophysiological studies showed that, in comparison to the folded monomers, the linear peptide of this toxin had the highest inhibitory activity at the human α9α10 nicotinic acetylcholine receptor (nAChR), with an IC₅₀ of 16.2 ± 1.4 nM. The activities of the N-terminal and C-terminal halves of the linear toxin are markedly reduced compared with the full-length toxin, suggesting that the intact sequence is required to potently inhibit the hα9α10 nAChR. α9α10 nAChRs are expressed not only in the nervous system, but also in a variety of non-neuronal cells, such as cochlear hair cells, keratinocytes, epithelial and immune cells. A potent inhibitor of human α9α10 nAChRs, such as GeXXVIIA, would facilitate unraveling the functions of this nAChR subtype. Furthermore, this unusual nAChR inhibitor may lead to the development of novel α9α10 nAChR-targeting drugs.

High-throughput expression of animal venom toxins in Escherichia coli to generate a large library of oxidized disulphide-reticulated peptides for drug discovery

BACKGROUND

Animal venoms are complex molecular cocktails containing a wide range of biologically active disulphide-reticulated peptides that target, with high selectivity and efficacy, a variety of membrane receptors. Disulphide-reticulated peptides have evolved to display improved specificity, low immunogenicity and to show much higher resistance to degradation than linear peptides. These properties make venom peptides attractive candidates for drug development. However, recombinant expression of reticulated peptides containing disulphide bonds is challenging, especially when associated with the production of large libraries of bioactive molecules for drug screening. To date, as an alternative to artificial synthetic chemical libraries, no comprehensive recombinant libraries of natural venom peptides are accessible for high-throughput screening to identify novel therapeutics.

RESULTS

In the accompanying paper an efficient system for the expression and purification of oxidized disulphide-reticulated venom peptides in Escherichia coli is described. Here we report the development of a high-throughput automated platform, that could be adapted to the production of other families, to generate the largest ever library of recombinant venom peptides. The peptides were produced in the periplasm of E. coli using redox-active DsbC as a fusion tag, thus allowing the efficient formation of correctly folded disulphide bridges. TEV protease was used to remove fusion tags and recover the animal venom peptides in the native state. Globally, within nine months, out of a total of 4992 synthetic genes encoding a representative diversity of venom peptides, a library containing 2736 recombinant disulphide-reticulated peptides was generated. The data revealed that the animal venom peptides produced in the bacterial host were natively folded and, thus, are putatively biologically active.

CONCLUSIONS

Overall this study reveals that high-throughput expression of animal venom peptides in E. coli can generate large libraries of recombinant disulphide-reticulated peptides of remarkable interest for drug discovery programs.

Conotoxin gene superfamilies

Conotoxins are the peptidic components of the venoms of marine cone snails (genus Conus). They are remarkably diverse in terms of structure and function. Unique potency and selectivity profiles for a range of neuronal targets have made several conotoxins valuable as research tools, drug leads and even therapeutics, and has resulted in a concerted and increasing drive to identify and characterise new conotoxins. Conotoxins are translated from mRNA as peptide precursors, and cDNA sequencing is now the primary method for identification of new conotoxin sequences. As a result, gene superfamily, a classification based on precursor signal peptide identity, has become the most convenient method of conotoxin classification. Here we review each of the described conotoxin gene superfamilies, with a focus on the structural and functional diversity present in each. This review is intended to serve as a practical guide to conotoxin superfamilies and to facilitate interpretation of the increasing number of conotoxin precursor sequences being identified by targeted-cDNA sequencing and more recently high-throughput transcriptome sequencing.

Peptidomics combined with cDNA library unravel the diversity of centipede venom

Centipedes are one of the oldest venomous arthropods using toxin as their weapon to capture prey. But little attention was focused on them and only few centipede toxins were demonstrated with activity on ion channels. Therefore, more deep works are needed to understand the diversity of centipede venom. In the present study, we use peptidomics combined with cDNA library to uncover the diversity of centipede Scolopendra subspinipes mutilans L. Koch. 192 peptides were identified by LC-MS/MS and 79 precursors were deduced by cDNA library. Surprisingly, the signal peptides of centipede toxins were more complicated than any other animal toxins and even exhibited large differences in homologues. Meanwhile, a large number of variants generated by alternative cleavage sites were detected by mass spectra. Odd number of cystein (3, 5, 7) found in the mature peptides were seldom seen in peptide toxins. In additional, two novel cysteine frameworks (C-C-C-CCC, C-C-C-C-CC-CC) were identified from 16 different cysteine frameworks from centipede peptides. Only 29 precursors have clear targets, while others may provide a potential diversity function for centipede. These findings highlight the extensive diversity of centipede toxins and provide powerful tools to understand the capture and defense weapon of centipede.

BIOLOGICAL SIGNIFICANCE

Peptide toxins from venomous animal have attracted increasing attentions due to their extraordinary chemical and pharmacological diversity. Centipedes are one of the most used Chinese traditional medicines, but little was known about the active components. The venom of Scolopendra subspinipes mutilans L. Koch is first deeply analyzed in this work and most of peptides were never discovered before. Interestingly, the number and arrangement of cysteine showed a larger different to known peptide toxins such spider or scorpion toxins. Moreover, only 29 peptides from this centipede venom were identified with known function. It suggested that our work not only important to understand the composition of centipede venom, but also provide many valuable peptides for potential biological functions.

Diversity of conotoxin gene superfamilies in the venomous snail, Conus victoriae

Animal venoms represent a vast library of bioactive peptides and proteins with proven potential, not only as research tools but also as drug leads and therapeutics. This is illustrated clearly by marine cone snails (genus Conus), whose venoms consist of mixtures of hundreds of peptides (conotoxins) with a diverse array of molecular targets, including voltage- and ligand-gated ion channels, G-protein coupled receptors and neurotransmitter transporters. Several conotoxins have found applications as research tools, with some being used or developed as therapeutics. The primary objective of this study was the large-scale discovery of conotoxin sequences from the venom gland of an Australian cone snail species, Conus victoriae. Using cDNA library normalization, high-throughput 454 sequencing, de novo transcriptome assembly and annotation with BLASTX and profile hidden Markov models, we discovered over 100 unique conotoxin sequences from 20 gene superfamilies, the highest diversity of conotoxins so far reported in a single study. Many of the sequences identified are new members of known conotoxin superfamilies, some help to redefine these superfamilies and others represent altogether new classes of conotoxins. In addition, we have demonstrated an efficient combination of methods to mine an animal venom gland and generate a library of sequences encoding bioactive peptides.

On the importance of oxidative folding in the evolution of conotoxins: cysteine codon preservation through gene duplication and adaptation

Conotoxin genes are among the most rapidly evolving genes currently known; however, despite the well-established hypervariability of the intercysteine loops, the cysteines demonstrate significant conservation, with a site-specific codon bias for each cysteine in a family of conotoxins. Herein we present a novel rationale behind the codon-level conservation of the cysteines that comprise the disulfide scaffold. We analyze cysteine codon conservation using an internal reference and phylogenetic tools; our results suggest that the established codon conservation can be explained as the result of selective pressures linked to the production efficiency and folding of conotoxins, driving the conservation of cysteine at the amino-acid level. The preservation of cysteine has resulted in maintenance of the ancestral codon in most of the daughter lineages, despite the hypervariability of adjacent residues. We propose that the selective pressures acting on the venom components of cone snails involve an interplay of biosynthetic efficiency, activity at the target receptor and the importance of that activity to effective prey immobilization. Functional redundancy in the venom can thus serve as a buffer for the energy expenditure of venom production.

Peptide backbone fragmentation initiated by side-chain loss at cysteine residue in matrix-assisted laser desorption/ionization in-source decay mass spectrometry

Matrix-assisted laser desorption/ionization in-source decay (MALDI-ISD) is initiated by hydrogen transfer from matrix molecules to the carbonyl oxygen of peptide backbone with subsequent radical-induced cleavage leading to c'/z• fragments pair. MALDI-ISD is a very powerful method to obtain long sequence tags from proteins or to do de novo sequencing of peptides. Besides classical fragmentation, MALDI-ISD also shows specific fragments for which the mechanism of formation enlightened the MALDI-ISD process. In this study, the MALDI-ISD mechanism is reviewed, and a specific mechanism is studied in details: the N-terminal side of Cys residue (Xxx-Cys) is described to promote the generation of c' and w fragments in MALDI-ISD. Our data suggest that for sequences containing Xxx-Cys motifs, the N-Cα bond cleavage occurs following the hydrogen attachment to the thiol group of Cys side-chain. The c•/w fragments pair is formed by side-chain loss of the Cys residue with subsequent radical-induced cleavage at the N-Cα bond located at the left side (N-terminal direction) of the Cys residue. This fragmentation pathway preferentially occurs at free Cys residue and is suppressed when the cysteines are involved in disulfide bonds. Hydrogen attachment to alkylated Cys residues using iodoacetamide gives free Cys residue by the loss of •CH2CONH2 radical. The presence of alkylated Cys residue also suppress the formation of c•/w fragments pair via the (Cβ)-centered radical, whereas w fragment is still observed as intense signal. In this case, the z• fragment formed by hydrogen attachment of carbonyl oxygen followed side-chain loss at alkylated Cys leads to a w fragment. Hydrogen attachment on peptide backbone and side-chain of Cys residue occurs therefore competitively during MALDI-ISD process.

Deep venomics reveals the mechanism for expanded peptide diversity in cone snail venom

Cone snails produce highly complex venom comprising mostly small biologically active peptides known as conotoxins or conopeptides. Early estimates that suggested 50-200 venom peptides are produced per species have been recently increased at least 10-fold using advanced mass spectrometry. To uncover the mechanism(s) responsible for generating this impressive diversity, we used an integrated approach combining second-generation transcriptome sequencing with high sensitivity proteomics. From the venom gland transcriptome of Conus marmoreus, a total of 105 conopeptide precursor sequences from 13 gene superfamilies were identified. Over 60% of these precursors belonged to the three gene superfamilies O1, T, and M, consistent with their high levels of expression, which suggests these conotoxins play an important role in prey capture and/or defense. Seven gene superfamilies not previously identified in C. marmoreus, including five novel superfamilies, were also discovered. To confirm the expression of toxins identified at the transcript level, the injected venom of C. marmoreus was comprehensively analyzed by mass spectrometry, revealing 2710 and 3172 peptides using MALDI and ESI-MS, respectively, and 6254 peptides using an ESI-MS TripleTOF 5600 instrument. All conopeptides derived from transcriptomic sequences could be matched to masses obtained on the TripleTOF within 100 ppm accuracy, with 66 (63%) providing MS/MS coverage that unambiguously confirmed these matches. Comprehensive integration of transcriptomic and proteomic data revealed for the first time that the vast majority of the conopeptide diversity arises from a more limited set of genes through a process of variable peptide processing, which generates conopeptides with alternative cleavage sites, heterogeneous post-translational modifications, and highly variable N- and C-terminal truncations. Variable peptide processing is expected to contribute to the evolution of venoms, and explains how a limited set of ∼ 100 gene transcripts can generate thousands of conopeptides in a single species of cone snail.

Secretion and maturation of conotoxins in the venom ducts of Conus textile

The 700 or more species of cone snail attack prey by employing complex venom which can vary considerably both within species and from one species to another. Cone snail venom is remarkable for the high proportion of conotoxins with varied post-translational modifications (PTMs) and for the production of more diverse toxin scaffolds than any other known venomous animal. The venom gland, which is several times longer than its shell, is also unique in being tubular. These unusual characteristics both raise questions, and provide the opportunity for research, concerning the secretion and maturation of conotoxins along the venom duct, a process which is currently not fully understood. This research uses the two mass spectrometric techniques of isotope Coded Affinity Tagging (ICAT) and label-free quantification to study each of five portions of the venom duct of Conus textile snails from New Caledonia. Fifteen conotoxins, several with different post-translational modifications (PTMs) were identified and quantified. One hundred and forty three non-identified conotoxins were also quantified. Distinctive patterns emerged, with the largest group of conotoxins increasing, then peaking in the central-proximal part, before decreasing; whilst the second largest group peaked in the distal region, generally displaying nothing in the first parts. Conotoxins from different superfamilies were commonly found to have similar distributions. A new conotoxin, PCCSKLHDNSCCGL*, was sequenced. A comparison is made with other studies to see how the process varies in cone snails from different regions.

Molecular phylogeny, classification and evolution of conopeptides

Conopeptides are toxins expressed in the venom duct of cone snails (Conoidea, Conus). These are mostly well-structured peptides and mini-proteins with high potency and selectivity for a broad range of cellular targets. In view of these properties, they are widely used as pharmacological tools and many are candidates for innovative drugs. The conopeptides are primarily classified into superfamilies according to their peptide signal sequence, a classification that is thought to reflect the evolution of the multigenic system. However, this hypothesis has never been thoroughly tested. Here we present a phylogenetic analysis of 1,364 conopeptide signal sequences extracted from GenBank. The results validate the current conopeptide superfamily classification, but also reveal several important new features. The so-called "cysteine-poor" conopeptides are revealed to be closely related to "cysteine-rich" conopeptides; with some of them sharing very similar signal sequences, suggesting that a distinction based on cysteine content and configuration is not phylogenetically relevant and does not reflect the evolutionary history of conopeptides. A given cysteine pattern or pharmacological activity can be found across different superfamilies. Furthermore, a few conopeptides from GenBank do not cluster in any of the known superfamilies, and could represent yet-undefined superfamilies. A clear phylogenetically based classification should help to disentangle the diversity of conopeptides, and could also serve as a rationale to understand the evolution of the toxins in the numerous other species of conoideans and venomous animals at large.

9.3 KDa components of the injected venom of Conus purpurascens define a new five-disulfide conotoxin framework

The 83-residue conopeptide (p21a) and its corresponding nonhydroxylated analog were isolated from the injected venom of Conus purpurascens. The complete conopeptide sequences were derived from Edman degradation sequencing of fragments from tryptic, chymotryptic and cyanogen bromide digestions, p21a has a unique, 10-cystine/5-disulfide 7-loop framework with extended 10-residue N-terminus and a 5-residue C-terminus tails, respectively. p21a has a 48% sequence homology with a recently described 13-cystine conopeptide, con-ikot-ikot, isolated from the dissected venom of the fish-hunting snail Conus striatus. However, unlike con-ikot-ikot, p21a does not form a dimer of dimers. MALDI-TOF mass spectrometry suggests that p21a may form a noncovalent dimer. p21a is an unusually large conotoxin and in so far is the largest isolated from injected venom. p21a provides evidence that the Conus venom arsenal includes larger molecules that are directly injected into the prey. Therefore, cone snails can utilize toxins that are comparable in size to the ones commonly found in other venomous animals.

Accessing novel conoidean venoms: Biodiverse lumun-lumun marine communities, an untapped biological and toxinological resource

Cone snail venoms have yielded pharmacologically active natural products of exceptional scientific interest. However, cone snails are a small minority of venomous molluscan biodiversity, the vast majority being tiny venomous morphospecies in the family Turridae. A novel method called lumun-lumun opens access to these micromolluscs and their venoms. Old fishing nets are anchored to the sea bottom for a period of 1-6months and marine biotas rich in small molluscs are established. In a single lumun-lumun community, we found a remarkable gastropod biodiversity (155 morphospecies). Venomous predators belonging to the superfamily Conoidea (36 morphospecies) were the largest group, the majority being micromolluscs in the family Turridae. We carried out an initial analysis of the most abundant of the turrid morphospecies recovered, Clathurella (Lienardia) cincta (Dunker, 1871). In contrast to all cDNA clones characterized from cone snail venom ducts, one of the C. cincta clones identified encoded two different peptide precursors presumably translated from a single mRNA. The prospect of easily accessing so many different morphospecies of venomous marine snails raises intriguing toxinological possibilities: the 36 conoidean morphospecies in this one net alone have the potential to yield thousands of novel pharmacologically active compounds.