Cloning and characterization of a novel neurotoxin from the sea anemone Anthopleura sp

Sequencing and de novo transcriptome assembly of Anthopleura dowii Verrill (1869), from Mexico

Next-generation technologies for determination of genomics and transcriptomics composition have a wide range of applications. Moreover, the development of tools for big data set analysis has allowed the identification of molecules and networks involved in metabolism, evolution or behavior. By natural habitats aquatic organisms have implemented molecular strategies for survival, including the production and secretion of toxic compounds for their predators; therefore these organisms are possible sources of proteins or peptides with potential biotechnological application. In the last decade anthozoans, mainly octocorals but also sea anemones, have been proben to be a source of natural products. Members of the genus Anthopleura are one of the best known and most studied sea anemones because they are common constituents of rocky intertidal communities and show interesting ecological and biological phenomena (e.g. intraespecific competition, symbiosis, etc.); however, many aspects of these taxa remain in need to be analyzed. This work describes the transcriptome sequencing of Anthopleura dowii Verrill, 1869 (Cnidaria: Anthozoa: Actiniaria); this is the first report of this kind for these species. The data set used to construct the transcriptome has been deposited on NCBI's database. Illumina sequence reads are available under BioProject accession number PRJNA329297 and Sequence Read Archive under accession number SRP078992.

Evidence of accelerated evolution and ectodermal-specific expression of presumptive BDS toxin cDNAs from Anemonia viridis

Anemonia viridis is a widespread and extensively studied Mediterranean species of sea anemone from which a large number of polypeptide toxins, such as blood depressing substances (BDS) peptides, have been isolated. The first members of this class, BDS-1 and BDS-2, are polypeptides belonging to the β-defensin fold family and were initially described for their antihypertensive and antiviral activities. BDS-1 and BDS-2 are 43 amino acid peptides characterised by three disulfide bonds that act as neurotoxins affecting Kv3.1, Kv3.2 and Kv3.4 channel gating kinetics. In addition, BDS-1 inactivates the Nav1.7 and Nav1.3 channels. The development of a large dataset of A. viridis expressed sequence tags (ESTs) and the identification of 13 putative BDS-like cDNA sequences has attracted interest, especially as scientific and diagnostic tools. A comparison of BDS cDNA sequences showed that the untranslated regions are more conserved than the protein-coding regions. Moreover, the K_A/K_S ratios calculated for all pairwise comparisons showed values greater than 1, suggesting mechanisms of accelerated evolution. The structures of the BDS homologs were predicted by molecular modelling. All toxins possess similar 3D structures that consist of a triple-stranded antiparallel β-sheet and an additional small antiparallel β-sheet located downstream of the cleavage/maturation site; however, the orientation of the triple-stranded β-sheet appears to differ among the toxins. To characterise the spatial expression profile of the putative BDS cDNA sequences, tissue-specific cDNA libraries, enriched for BDS transcripts, were constructed. In addition, the proper amplification of ectodermal or endodermal markers ensured the tissue specificity of each library. Sequencing randomly selected clones from each library revealed ectodermal-specific expression of ten BDS transcripts, while transcripts of BDS-8, BDS-13, BDS-14 and BDS-15 failed to be retrieved, likely due to under-representation in our cDNA libraries. The calculation of the relative abundance of BDS transcripts in the cDNA libraries revealed that BDS-1, BDS-3, BDS-4, BDS-5 and BDS-6 are the most represented transcripts.

Effects of recombinant neurotoxins on single Na(+) channels in isolated rat hippocampal neurons

Four recombinant neurotoxins Hk2a, Hk7a, Hk8a, Hk16a, originally from a sea anemone species Anthopleura sp., were obtained by fusion expression of their genes in Escherichia coli. These neurotoxins were composed of 47 amino acid residues, among which the differences were found at positions 14, 22, 25, and 37, respectively. The effects of the four neurotoxins on single-channel current of sodium in rat hippocampal neurons were studied by cell-attached patch clamp. Each neurotoxin 2 microM could modulate the sodium channel by prolonging the opening dwell time and increasing the open probability, but did not change the amplitude of sodium channel currents. Based on the studies of the structure-function relationship, we found that Hk7a displayed the biggest increase of the open probability because His14 (from Arg14) makes its structure seem more compact in comparison with the other three toxins and Ap-A. Phe25 (Hk8a, Hk16a), which varied from Ala25 (Hk2a, Hk7a), showed that phenyl group might interfere with other key amino acid residue to decrease the activity of toxins. Arg37 (from His37) in Hk8a contributed to decrease of open probability. In our work, it was shown that these important amino acid sites might provide a reliable proof for the future pharmaceutical design.

Fusion and retrotransposition events in the evolution of the sea anemone Anemonia viridis neurotoxin genes

Sea anemones are sessile predators that use a variety of toxins to paralyze prey and foe. Among these toxins, Types I, II and III are short peptides that affect voltage-gated sodium channels. Anemonia viridis is the only sea anemone species that produces both Types I and III neurotoxin. Although the two toxin types are unrelated in sequence and three-dimensional structure, cloning and comparative analysis of their loci revealed a highly similar sequence at the 5' region, which encodes a signal peptide. This similarity was likely generated by gene fusion and could be advantageous in transcript stability and intracellular trafficking and secretion. In addition, these analyses identified the processed pseudogenes of the two gene families in the genome of A. viridis, probably resulting from retrotransposition events. As presence of processed pseudogenes in the genome requires transcription in germ-line cells, we analyzed oocyte-rich ovaries and found that indeed they contain Types I and III transcripts. This result raises questions regarding the role of toxin transcripts in these tissues. Overall, the retrotransposition and gene fusion events suggest that the genes of both Types I and III neurotoxins evolved in a similar fashion and share a partial common ancestry.

Novel peptide toxins from the sea anemone Stichodactyla haddoni

Four peptide toxins, SHTX I-III with crab-paralyzing activity and SHTX IV with crab lethality, were isolated from the sea anemone Stichodactyla haddoni and their primary structures elucidated by protein sequencing and cDNA cloning. SHTX I (new toxin, 28 residues), II (analogue of SHTX I, 28 residues) and III (Kunitz-type protease inhibitor, 62 residues) are potassium channel toxins and SHTX IV (48 residues) is a member of the type 2 sea anemone sodium channel toxins. The precursor protein of SHTX IV is composed of a signal peptide, propart and mature peptide, while the propart is missing in that of SHTX III. In addition to these four toxins, an epidermal growth factor-like peptide was detected in S. haddoni by RT-PCR.

Expression and purification of a recombinant antibacterial peptide, cecropin, from Escherichia coli

Insect cecropins are small basic polypeptides synthesized in fat body and hemocytes in response to bacterial infections or hypodermic injuries. To explore a new approach for high expression of soluble cecropin in Escherichia coli cells, we fused the sequence encoding Musca domestica mature cecropin (named Mdmcec) in-frame to thioredoxin (TRX) gene to construct an expression vector pTRX-6His-Mdmcec. An enterokinase cleavage site was introduced between the 6xHis-tag and Mdmcec to facilitate final release of the recombinant Mdmcec. The fusion protein TRX-6His-Mdmcec was purified successfully by HisTrap HP affinity column and a high yield of 48.0mg purified fusion protein was obtained from 1L culture. Recombinant Mdmcec was readily obtained by enterokinase cleavage of the fusion protein followed by HPLC chromatography, and 11.2mg pure active recombinant Mdmcec was obtained from 1L E. coli culture. The molecular mass of recombinant Mdmcec determined by electrospray ionization-mass spectrometry (ESI-MS) is identical to that of native cecropin. Analysis of recombinant Mdmcec by circular dichroism (CD) indicated that recombinant Mdmcec contained predominantly alpha-helix with some random coil. Antimicrobial activity assays demonstrated that recombinant Mdmcec had a broad spectrum of activity against fungi, Gram-positive and negative bacteria. The procedure described in this study will provide a reliable and simple method for production of different cationic peptides for biological studies.

Isolation and cDNA cloning of a potassium channel peptide toxin from the sea anemone Anemonia erythraea

A potassium channel peptide toxin (AETX K) was isolated from the sea anemone Anemonia erythraea by gel filtration on Sephadex G-50, reverse-phase HPLC on TSKgel ODS-120T and anion-exchange HPLC on Mono Q. AETX K inhibited the binding of (125)I-alpha-dendrotoxin to rat synaptosomal membranes, although much less potently than alpha-dendrotoxin. Based on the determined N-terminal amino acid sequence, the nucleotide sequence of the full-length cDNA (609bp) encoding AETX K was elucidated by a combination of degenerate RT-PCR, 3'RACE and 5'RACE. The precursor protein of AETX K is composed of a signal peptide (22 residues), a propart (27 residues) ended with a pair of basic residues (Lys-Arg) and a mature peptide (34 residues). AETX K is the sixth member of the type 1 potassium channel toxins from sea anemones, showing especially high sequence identities with HmK from Heteractis magnifica and ShK from Stichodactyla helianthus. It has six Cys residues at the same position as the known type 1 toxins. In addition, the dyad comprising Lys and Tyr, which is considered to be essential for the binding of the known type 1 toxins to potassium channels, is also conserved in AETX K.

Novel peptide toxins from acrorhagi, aggressive organs of the sea anemone Actinia equina

Two peptide toxins, acrorhagin I (50 residues) and II (44 residues), were isolated from special aggressive organs (acrorhagi) of the sea anemone Actinia equina by gel filtration on Sephadex G-50 and reverse-phase HPLC on TSKgel ODS-120T. The LD50 against crabs of acrorhagin I and II were estimated to be 520 and 80 microg/kg, respectively. 3'- and 5'-RACE established the amino acid sequences of the acrorhagin precursors. The precursor of acrorhagin I is composed of both signal and mature peptides and that of acrorhagin II has an additional sequence (propart) between signal and mature peptides. Acrorhagin I has no sequence homologies with any toxins, while acrorhagin II is somewhat similar to spider neurotoxins (hainantoxin-I from Selenocosmia hainana and Tx 3-2 from Phoneutria nigriventer) and cone snail neurotoxin (omega-conotoxin MVIIB from Conus magus). In addition, analogous peptides (acrorhagin Ia and IIa) were also cloned during RT-PCR experiments performed to confirm the nucleotide sequences of acrorhagins. This is the first to demonstrate the existence of novel peptide toxins in the sea anemone acrorhagi.

Molecular cloning of an epidermal growth factor-like toxin and two sodium channel toxins from the sea anemone Stichodactyla gigantea

An epidermal growth factor (EGF)-like toxin (gigantoxin I) and two sodium channel toxins (gigantoxins II and III), previously isolated from the sea anemone Stichodactyla gigantea, were cloned for their cDNAs. The precursor protein of gigantoxin I is composed of a signal peptide, propart and mature peptide, similar to those of gigantoxins II and III, and is much simpler in structure than those of mammalian EGFs. In addition, gigantoxin I as well as gigantoxins II and III was demonstrated to be contained in nematocysts, suggesting that gigantoxin I functions as a toxin in S. gigantea.

Functional expression and characterization of four novel neurotoxins from sea anemone Anthopleura sp

The genes of four novel neurotoxins, named Hk2a, Hk7a, Hk8a, and Hk16a, were obtained from sea anemone Anthopleura sp. All four neurotoxins were composed of 47 amino acid residues and the variable residues among them were found in positions 14, 22, 25, and 37. To study their activities, the four toxins fused to the Escherichia coli thioredoxin were overexpressed by BL21 (DE3), cleaved off from the fusion partner, purified, and characterized with MALDI-TOF and CD assays. Contractile force studies of isolated SD atria indicated that rHk2a had the strongest and rHk7a the longest heart stimulation effect. Consequently, the Arg14, a highly conserved residue in various sea anemone neurotoxins, can be inferred to contribute to the duration but not the intensity of contraction-stimulating activity. Our work renders useful information to studies of sea anemone neurotoxins, especially to the clarification of the function of the disputative Arg14.

Expression and purification of Src I from sea anemone Sagartia rosea as a recombinant non-fusion protein

The cDNA coding for an acidic actinoporin, Sagartia rosea cytolysin I (Src I), has been isolated, cloned into pGEM-T Easy Vector, and sequenced. The region encoding matured Src I was also cloned into prokaryotic expression vector pBV220 and expressed in Escherichia coli as a non-fusion protein in the form of inclusion body. Through washing and denaturation-renaturation step, the recombinant Src I was purified by Q Sepharose Fast Flow ion exchange chromatography and Phenyl Sepharose hydrophobic interaction chromatography. The two-step purification of Src I was effective, simple, and suitable for isolating large amount of high purity (above 95%) recombinant Src I.