Novel conopeptides of the I-superfamily occur in several clades of cone snails

Principal Component and Structural Element Analysis Provide Insights into the Evolutionary Divergence of Conotoxins

Predatory cone snails (Conus) developed a sophisticated neuropharmacological mechanism to capture prey, escape against other predators, and deter competitors. Their venom's remarkable specificity for various ion channels and receptors is an evolutionary feat attributable to the venom's variety of peptide components (conotoxins). However, what caused conotoxin divergence remains unclear and may be related to the role of prey shift. Principal component analysis revealed clustering events within diet subgroups indicating peptide sequence similarity patterns based on the prey they subdue. Molecular analyses using multiple sequence alignment and structural element analysis were conducted to observe the events at the molecular level that caused the subgrouping. Three distinct subgroups were identified. Results showed homologous regions and conserved residues within diet subgroups but divergent between other groups. We specified that these structural elements caused subgrouping in alpha conotoxins that may play a role in function specificity. In each diet subgroup, amino acid character, length of intervening amino acids between cysteine residues, and polypeptide length influenced subgrouping. This study provides molecular insights into the role of prey shift, specifically diet preference, in conotoxin divergence.

Pathophysiological Responses to Conotoxin Modulation of Voltage-Gated Ion Currents

Voltage-gated ion channels are plasma membrane proteins that generate electrical signals following a change in the membrane voltage. Since they are involved in several physiological processes, their dysfunction may be responsible for a series of diseases and pain states particularly related to neuronal and muscular systems. It is well established for decades that bioactive peptides isolated from venoms of marine mollusks belonging to the Conus genus, collectively known as conotoxins, can target different types and isoforms of these channels exerting therapeutic effects and pain relief. For this reason, conotoxins are widely used for either therapeutic purposes or studies on ion channel mechanisms of action disclosure. In addition their positive property, however, conotoxins may generate pathological states through similar ion channel modulation. In this narrative review, we provide pieces of evidence on the pathophysiological impacts that different members of conotoxin families exert by targeting the three most important voltage-gated channels, such as sodium, calcium, and potassium, involved in cellular processes.

Molecular Diversity and Gene Evolution of the Venom Arsenal of Terebridae Predatory Marine Snails

Venom peptides from predatory organisms are a resource for investigating evolutionary processes such as adaptive radiation or diversification, and exemplify promising targets for biomedical drug development. Terebridae are an understudied lineage of conoidean snails, which also includes cone snails and turrids. Characterization of cone snail venom peptides, conotoxins, has revealed a cocktail of bioactive compounds used to investigate physiological cellular function, predator-prey interactions, and to develop novel therapeutics. However, venom diversity of other conoidean snails remains poorly understood. The present research applies a venomics approach to characterize novel terebrid venom peptides, teretoxins, from the venom gland transcriptomes of Triplostephanus anilis and Terebra subulata. Next-generation sequencing and de novo assembly identified 139 putative teretoxins that were analyzed for the presence of canonical peptide features as identified in conotoxins. To meet the challenges of de novo assembly, multiple approaches for cross validation of findings were performed to achieve reliable assemblies of venom duct transcriptomes and to obtain a robust portrait of Terebridae venom. Phylogenetic methodology was used to identify 14 teretoxin gene superfamilies for the first time, 13 of which are unique to the Terebridae. Additionally, basic local algorithm search tool homology-based searches to venom-related genes and posttranslational modification enzymes identified a convergence of certain venom proteins, such as actinoporin, commonly found in venoms. This research provides novel insights into venom evolution and recruitment in Conoidean predatory marine snails and identifies a plethora of terebrid venom peptides that can be used to investigate fundamental questions pertaining to gene evolution.

Molecular phylogeny and evolution of the cone snails (Gastropoda, Conoidea)

We present a large-scale molecular phylogeny that includes 320 of the 761 recognized valid species of the cone snails (Conus), one of the most diverse groups of marine molluscs, based on three mitochondrial genes (COI, 16S rDNA and 12S rDNA). This is the first phylogeny of the taxon to employ concatenated sequences of several genes, and it includes more than twice as many species as the last published molecular phylogeny of the entire group nearly a decade ago. Most of the numerous molecular phylogenies published during the last 15years are limited to rather small fractions of its species diversity. Bayesian and maximum likelihood analyses are mostly congruent and confirm the presence of three previously reported highly divergent lineages among cone snails, and one identified here using molecular data. About 85% of the species cluster in the single Large Major Clade; the others are divided between the Small Major Clade (∼12%), the Conus californicus lineage (one species), and a newly defined clade (∼3%). We also define several subclades within the Large and Small major clades, but most of their relationships remain poorly supported. To illustrate the usefulness of molecular phylogenies in addressing specific evolutionary questions, we analyse the evolution of the diet, the biogeography and the toxins of cone snails. All cone snails whose feeding biology is known inject venom into large prey animals and swallow them whole. Predation on polychaete worms is inferred as the ancestral state, and diet shifts to molluscs and fishes occurred rarely. The ancestor of cone snails probably originated from the Indo-Pacific; rather few colonisations of other biogeographic provinces have probably occurred. A new classification of the Conidae, based on the molecular phylogeny, is published in an accompanying paper.

[Conotoxins: from the biodiversity of gastropods to new drugs]

A review describes general trends in research of conotoxins that are peptide toxins isolated from sea gastropods of the Conus genus, since the toxins were discovered in 1970th. There are disclosed a conotoxin classification, their structure diversity and different ways of action to their molecular targets, mainly, ion channels. In the applied aspect of conotoxin research, drug discovery and development is discussed, the drugs being based on conotoxin structure. A first exemplary drug is a ziconotide, which is an analgesic of new generation.

A Conus regularis conotoxin with a novel eight-cysteine framework inhibits CaV2.2 channels and displays an anti-nociceptive activity

A novel peptide, RsXXIVA, was isolated from the venom duct of Conus regularis, a worm-hunting species collected in the Sea of Cortez, México. Its primary structure was determined by mass spectrometry and confirmed by automated Edman degradation. This conotoxin contains 40 amino acids and exhibits a novel arrangement of eight cysteine residues (C-C-C-C-CC-CC). Surprisingly, two loops of the novel peptide are highly identical to the amino acids sequence of ω-MVIIA. The total length and disulfide pairing of both peptides are quite different, although the two most important residues for the described function of ω-MVIIA (Lys2 and Tyr13) are also present in the peptide reported here. Electrophysiological analysis using superior cervical ganglion (SCG) neurons indicates that RsXXIVA inhibits Ca_V2.2 channel current in a dose-dependent manner with an EC₅₀ of 2.8 μM, whose effect is partially reversed after washing. Furthermore, RsXXIVA was tested in hot-plate assays to measure the potential anti-nociceptive effect to an acute thermal stimulus, showing an analgesic effect in acute thermal pain at 30 and 45 min post-injection. Also, the toxin shows an anti-nociceptive effect in a formalin chronic pain test. However, the low affinity for Ca_V2.2 suggests that the primary target of the peptide could be different from that of ω-MVIIA.

Conopeptide characterization and classifications: an analysis using ConoServer

Cone snails are carnivorous marine gastropods that have evolved potent venoms to capture their prey. These venoms comprise a rich and diverse cocktail of peptide toxins, or conopeptides, whose high diversity has arisen from an efficient hypermutation mechanism, combined with a high frequency of post-translational modifications. Conopeptides bind with high specificity to distinct membrane receptors, ion channels, and transporters of the central and muscular nervous system. As well as serving their natural function in prey capture, conopeptides have been utilized as versatile tools in neuroscience and have proven valuable as drug leads that target the nervous system in humans. This paper examines current knowledge on conopeptide sequences based on an analysis of gene and peptide sequences in ConoServer (http://www.conoserver.org), a specialized database of conopeptide sequences and three-dimensional structures. We describe updates to the content and organization of ConoServer and discuss correlations between gene superfamilies, cysteine frameworks, pharmacological families targeted by conopeptides, and the phylogeny, habitat, and diet of cone snails. The study identifies gaps in current knowledge of conopeptides and points to potential directions for future research.

Identification, by molecular cloning, of a novel type of I2-superfamily conotoxin precursor and two novel I2-conotoxins from the worm-hunter snail Conus spurius from the Gulf of México

cDNA was prepared from the venom duct of a single Conus spurius specimen collected near the coast of Campeche, México. From it, PCR products were generated aiming to clone I-conotoxin precursors. Thirty clones were sequenced and predicted to encode ten distinct precursors: seven of I(2)-conotoxins and three of I(2)-like-conotoxins. These precursors contain three different, mature toxins, sr11a, sr11b and sr11c, of which two are novel and one (sr11a) has been previously purified and characterized from the venom of this species. The precursors include a 26- (I(2)) or 23- residue signal peptide (I(2)-like), a 31-residue "pro" region (I(2)-like), and a 32-residue mature toxin region (I(2) and I(2)-like). In addition, all the precursors have a 13-residue "post" region which contains a gamma-carboxylation recognition sequence that directs the gamma-carboxylation of Glu-9 and Glu-10 of toxin sr11a and, possibly, Glu-13 of toxin sr11b and Glu-9 of toxin sr11c. This is the first time that a "post" region has been found in precursors of I-conotoxins that also contain a "pro" region. The "post" peptide is enzymatically processed to yield the amidated mature toxin sr11a, which implies that gamma-carboxylation occurs before amidation. Phylogenetic analysis at the whole precursor level indicates that the I(2)-like-conotoxins of C. spurius are more related to I(2)-conotoxins than to I(1)- and I(3)-conotoxins from other species, and that they might represent a new subgroup of the I(2)-superfamily. The three I-conotoxins from C. spurius have charge differences at seven to nine positions, suggesting that they might have different molecular target types or subtypes.

Identification of novel I-superfamily conopeptides from several clades of Conus species found in the South China Sea

The I-superfamily of Conus peptides represents a new class of peptides with four disulfide bridges (-C-C-CC-CC-C-C-) that falls into three (I1, I2 and I3) categories according to the different signal peptide sequences. The I-superfamily has received increasing attention because it targets K+ ion channels, a function that is relatively rare in conotoxins. Herein we report 11 novel I-superfamily conotoxins from the venom ducts of five Cone snails (Conus eburneus, Conus imperialis, Conus vitulinus, Conus emaciatus and Conus litteratus) native to the South China Sea using a primer designed according to the N-terminus of the signal sequence of I2-superfamily conotoxins. The alignment of sequences revealed that signal regions exhibited moderate conservation with the exception of Eb11.3 from C. eburneus with homologies of 21.1%, 38.5% and 30.0% to the signal peptides of I1, I2 and I3 superfamily conotoxins, respectively. The mature peptides ranged from almost identical to highly divergent between species. Analyses of the evolutionary trees of these peptides with those of reported I-superfamily conotoxins showed that nine of them fall in I2 superfamily clades, but two of them were neither I1- and I2- nor I3-superfamily clades. Notably, some peptides exhibited significantly different amino acid residues in the intercysteine loops compared with group A, B and C of I-superfamily conopeptides, suggesting that they may have different bioactivities and functions.

New conotoxins define the novel I3-superfamily

We purified two novel conotoxins, designated as ca11a and ca11b, from the venom of Conus caracteristicus. Based on the amino acid sequence of mature ca11a, we cloned its full-length cDNA. Based on the signal peptide of ca11a, several ca11a-like conotoxins were cloned from C. caracteristicus and C. pulicarius. These novel conotoxins have an I-superfamily cysteine pattern but with a novel signal peptide sequence, suggesting they belong to a new branch of I-superfamily, designated as I(3)-superfamily. Additionally, two O-superfamily conotoxins were also cloned based on the signal peptide of ca11a, suggesting a possible evolutionary relationship between O- and I-superfamilies.

Isolation and cloning of a conotoxin with a novel cysteine pattern from Conus caracteristicus

A new conotoxin, ca16a, containing 8 cysteine residues was purified, sequenced, and cloned from a worm-hunting snail, Conus caracteristicus. This conotoxin is an extremely hydrophilic peptide comprising 34 residues, with 4 acidic and 4 basic residues. It is rich in polar Gly, Ser, and Thr residues and includes a hydroxylated Pro residue. The cysteine arrangement pattern of ca16a (-C-C-CC-C-CC-C-, designated as framework #16) is distinct from that of other known conotoxins. Furthermore, the signal peptide sequence of this conotoxin does not share any homology with those of other conotoxins. Leu residues account for almost 50% of its 20-residue signal peptide. The unique cysteine framework and signal peptide sequence of ca16a suggest that it belongs to a new conotoxin superfamily.

Identification of a novel class of conotoxins defined as V-conotoxins with a unique cysteine pattern and signal peptide sequence

Cone snails are predatory gastropod mollusks distributed in all tropical marine habitats with a highly sophisticated defense strategy using small peptides in their venoms. Here, we report the discovery and initial characterization of the V-superfamily conotoxins. A novel conotoxin vi15a was purified from the venom of a worm-hunting species Conus virgo. The sequence of vi15a was determined to have a unique arrangement of cysteine residues (C-C-CC-C-C-C-C), which defines the new V-superfamily conotoxins. The cDNA of vi15a was cloned with RACE method. Its unique signal peptide sequence led to the cloning of another V-superfamily conotoxin, Vt15.1, from Conus vitulinus. These results, as well as the existence of Lt15.1 from Conus litteratus and ca15a from Conus caracteristicus with the same cysteine pattern, suggest that V-superfamily might be a large and diverse group of peptides widely distributed in different Conus species. Like other eight Cys-containing toxins, V-superfamily conotoxins might also adopt an "ICK+1" disulfide bond connectivity. The identification of this novel class of conotoxins will certainly improve our understanding of the structure diversity of disulfide rich toxins.

Conus ventricosus venom peptides profiling by HPLC-MS: a new insight in the intraspecific variation

Conus is a genus of predatory marine gastropods that poison the prey with a complex mixture of compounds active on muscle and nerve cells. An individual cone snail's venom contains a mixture of pharmacological agents, mostly short, structurally constrained peptides. This study is focused on the composition of the venom employed by Conus ventricosus Gmelin, 1791, a worm-hunting cone snail living in the Mediterranean Sea. For this purpose, LC coupled to MS techniques has been successfully used to establish qualitative and quantitative differences in conopeptides from minute amounts of venom ducts. We were able to prove variability in the venom conopeptide complement, possibly related to different trophic habits of the species in the Mediterranean Sea. Moreover, the information-rich MS techniques enabled us to identify two novel C. ventricosus peptides, here named Conotoxin-Vn and -Conotoxin-Vn. On the basis of the structural data collected so far, we suggest that Conotoxin-Vn is a conopeptide belonging to the -family that recognizes calcium channels through a specific pharmacophore. Similarly, molecular modeling data suggest that -Conotoxin-Vn should represent a competitive antagonist of neuronal nicotinic acetylcholine receptors (nAChRs).

Structure and sodium channel activity of an excitatory I1-superfamily conotoxin

Conotoxin iota-RXIA, from the fish-hunting species Conus radiatus, is a member of the recently characterized I1-superfamily, which contains eight cysteine residues arranged in a -C-C-CC-CC-C-C- pattern. iota-RXIA (formerly designated r11a) is one of three characterized I1 peptides in which the third last residue is posttranslationally isomerized to the d configuration. Naturally occurring iota-RXIA with d-Phe44 is significantly more active as an excitotoxin than the l-Phe analogue both in vitro and in vivo. We have determined the solution structures of both forms by NMR spectroscopy, the first for an I1-superfamily member. The disulfide connectivities were determined from structure calculations and confirmed chemically as 5-19, 12-22, 18-27, and 21-38, suggesting that iota-RXIA has an ICK structural motif with one additional disulfide (21-38). Indeed, apart from the first few residues, the structure is well defined up to around residue 35 and does adopt an ICK structure. The C-terminal region, including Phe44, is disordered. Comparison of the d-Phe44 and l-Phe44 forms indicates that the switch from one enantiomer to the other has very little effect on the structure, even though it is clearly important for receptor interaction based on activity data. Finally, we identify the target of iota-RXIA as a voltage-gated sodium channel; iota-RXIA is an agonist, shifting the voltage dependence of activation of mouse NaV1.6 expressed in Xenopus oocytes to more hyperpolarized potentials. Thus, there is a convergence of structure and function in iota-RXIA, as its disulfide pairing and structure resemble those of funnel web spider toxins that also target sodium channels.

I-conotoxins in vermivorous species of the West Atlantic: peptide sr11a from Conus spurius

Peptide sr11a was purified from the venom of Conus spurius, a vermivorous cone snail collected in the Yucatan Channel, in the Western Atlantic. Its primary structure was determined by automatic Edman degradation after reduction and alkylation. Its molecular mass, as determined by MALDI-TOF mass spectrometry (average mass 3650.77 Da), confirmed the chemical data (calculated average mass, 3651.13 Da). The sequence of peptide sr11a (CRTEGMSCgamma gamma NQQCCWRSCCRGECEAPCRFGP&; gamma, gamma-carboxy-Glu; &, amidated C-terminus) shows eight Cys residues arranged in the pattern that defines the I-superfamily of conotoxins. Peptide sr11a contains two gamma-carboxy-Glu residues, a post-translational modification that has been found in other I-conotoxins from species that live in the West Pacific: r11e from the piscivorous Conus radiatus, and kappa-BtX from the vermivorous Conus betulinus. Peptide sr11a is the eighth I-conotoxin isolated from a Conus venom and the first I-conotoxin from a species from the Western Atlantic. Peptide sr11a produced stiffening of body, limbs and tail when injected intracranially into mice.

Diversity and evolution of conotoxins based on gene expression profiling of Conus litteratus

Cone snails are attracting increasing scientific attention due to their unprecedented diversity of invaluable channel-targeted peptides. As arguably the largest and most successful evolutionary genus of invertebrates, Conus also may become the model system to study the evolution of multigene families and biodiversity. Here, a set of 897 expressed sequence tags (ESTs) derived from a Conus litteratus venom duct was analyzed to illuminate the diversity and evolution mechanism of conotoxins. Nearly half of these ESTs represent the coding sequences of conotoxins, which were grouped into 42 novel conotoxin cDNA sequences (seven superfamilies), with T-superfamily conotoxins being the dominant component. The gene expression profile of conotoxin revealed that transcripts are expressed with order-of-magnitude differences, sequence divergence within a superfamily increases from the N to the C terminus of the open reading frame, and even multiple scaffold-different mature peptides exist in a conotoxin gene superfamily. Most excitingly, we identified a novel conotoxin superfamily and three novel cysteine scaffolds. These results give an initial insight into the C. litteratus transcriptome that will contribute to a better understanding of conotoxin evolution and the study of the cone snail genome in the near future.

Identification of a Novel Class of Nicotinic Receptor Antagonists

The venoms of predatory marine snails (Conus spp.) contain diverse mixtures of peptide toxins with high potency and selectivity for a variety of voltage-gated and ligand-gated ion channels. Here we describe the chemical and functional characterization of three novel conotoxins, alphaD-VxXIIA, alphaD-VxXIIB, and alphaD-VxXIIC, purified from the venom of Conus vexillum. Each toxin was observed as an approximately 11-kDa protein by LC/MS, size exclusion chromatography, and SDS-PAGE. After reduction, the peptide sequences were determined by Edman degradation chemistry and tandem MS. Combining the sequence data together with LC/MS and NMR data revealed that in solution these toxins are pseudo-homodimers of paired 47-50-residue peptides. The toxin subunits exhibited a novel arrangement of 10 conserved cystine residues, and additional post-translational modifications contributed heterogeneity to the proteins. Binding assays and two-electrode voltage clamp analyses showed that alphaD-VxXIIA, alphaD-VxXIIB, and alphaD-VxXIIC are potent inhibitors of nicotinic acetylcholine receptors (nAChRs) with selectivity for alpha7 and beta2 containing neuronal nAChR subtypes. These dimeric conotoxins represent a fifth and highly divergent structural class of conotoxins targeting nAChRs.

I-conotoxin superfamily revisited

The I-conotoxin superfamily (I-Ctx) is known to have four disulfide bonds with the cysteine arrangement C-C-CC-CC-C-C, and the members inhibit or modify ion channels of nerve cells. Recently, Olivera and co-workers (FEBS J. 2005; 272: 4178-4188) have suggested that the previously described I-Ctx should now be divided into two different gene superfamilies, namely, I1 and I2, in view of their having two different types of signal peptides and exhibiting distinct functions. We have revisited the 28 entries presently grouped as I-Ctx in UniProt Swiss-Prot knowledgebase, and on the basis of in silico analysis have divided them into I1 and I2 superfamilies. The sequence analysis has provided a framework for in silico annotation enabling us to carry out computer-based functional characterization of the UniProtKB/TrEMBL entry Q59AA4 from Conus miles and to predict it as a member of the I2 superfamily. Furthermore, we have predicted the mature toxin of this entry and have proposed that it may be an inhibitor of voltage-gated potassium channels.

Characterization of D-amino-acid-containing excitatory conotoxins and redefinition of the I-conotoxin superfamily

Post-translational isomerization of l-amino acids to d-amino acids is a subtle modification, not detectable by standard techniques such as Edman sequencing or MS. Accurate predictions require more sequences of modified polypeptides. A 46-amino-acid-long conotoxin, r11a, belonging to the I-superfamily was previously shown to have a d-Phe residue at position 44. In this report, we characterize two related peptides, r11b and r11c, with d-Phe and d-Leu, respectively, at the homologous position. Electrophysiological tests show that all three peptides induce repetitive activity in frog motor nerve, and epimerization of the single amino acid at the third position from the C-terminus attenuates the potency of r11a and r11b, but not that of r11c. Furthermore, r11c (but neither r11a nor r11b) also acts on skeletal muscle. We identified more cDNA clones encoding conopeptide precursors with Cys patterns similar to r11a/b/c. Although the predicted mature toxins have the same cysteine patterns, they belong to two different gene superfamilies. A potential correlation between the identity of the gene superfamily to which the I-conotoxin belongs and the presence or absence of a d-amino acid in the primary sequence is discussed. The great diversity of I-conopeptide sequences provides a rare opportunity for defining parameters that may be important for this most stealthy of all post-translational modifications. Our results indicate that neither the chemical nature of the side chain nor the precise vicinal sequence around the modified residue seem to be critical, but there may be favored loci for isomerization to a d-amino acid.

Direct cDNA cloning of novel conopeptide precursors of the O-superfamily

Conotoxins from the venom of marine cone snails (genus Conus) represent large families of proteins exhibiting a similar precursor organization, but highly diverse pharmacological activities. A directed PCR-based approach using primers according to the conserved signal sequence was applied to investigate the diversity of conotoxins from the O-superfamily. Using 3' RACE, cDNA sequences encoding precursor peptides were identified in five Conus species (Conus capitaneus, Conus imperialis, Conusstriatus, Conus vexillum and Conus virgo). In all cases, the sequence of the signal region exhibited high conservancy, whereas the sequence of the mature peptides was either almost identical or highly divergent among the five species. These findings demonstrate that beside a common genetic pattern divergent evolution of toxins occurred in a highly mutating peptide family.