Primary structure of two cytolysin isoforms from Stichodactyla helianthus differing in their hemolytic activity

Unveiling Sticholysin II and plasmid DNA interaction: Implications for developing non-viral vectors

Non-viral gene delivery systems offer significant potential for gene therapy due to their versatility, safety, and cost advantages over viral vectors. However, their effectiveness can be hindered by the challenge of efficiently releasing the genetic cargo from endosomes to prevent degradation in lysosomes. To overcome this obstacle, functional components can be incorporated into these systems. Sticholysin II (StII) is one of the pore-forming proteins derived from the sea anemone Stichodactyla helianthus, known for its high ability to permeabilize cellular and model membranes. In this study, we aimed to investigate the interaction between StII, and a model plasmid (pDNA) as an initial step towards designing an improved vector with enhanced endosomal escape capability. The electrophoretic mobility shift assay (EMSA) confirmed the formation of complexes between StII and pDNA. Computational predictions identified specific residues involved in the StII-DNA interaction interface, highlighting the importance of electrostatic interactions and hydrogen bonds in mediating the binding. Atomic force microscopy (AFM) of StII-pDNA complexes revealed the presence of nodular fiber and toroid shapes. These complexes were found to have a predominantly micrometer size, as confirmed by dynamic light scattering (DLS) measurements. Despite increase in the overall charge, the complexes formed at the evaluated nitrogen-to-phosphorus (N/P) ratios still maintained a negative charge. Moreover, StII retained its pore-forming capacity regardless of its binding to the complexes. These findings suggest that the potential ability of StII to permeabilize endosomal membranes could be largely maintained when combined with nucleic acid delivery systems. Additionally, the still remaining negative charge of the complexes would enable the association of another positively charged component to compact pDNA. However, to minimize non-specific cytotoxic effects, it is advisable to explore methods to regulate the protein's activity in response to the microenvironment.

Panorama of the Intracellular Molecular Concert Orchestrated by Actinoporins, Pore-Forming Toxins from Sea Anemones

Actinoporins (APs) are soluble pore-forming proteins secreted by sea anemones that experience conformational changes originating in pores in the membranes that can lead to cell death. The processes involved in the binding and pore-formation of members of this protein family have been deeply examined in recent years; however, the intracellular responses to APs are only beginning to be understood. Unlike pore formers of bacterial origin, whose intracellular impact has been studied in more detail, currently, we only have knowledge of a few poorly integrated elements of the APs’ intracellular action. In this review, we present and discuss an updated landscape of the studies aimed at understanding the intracellular pathways triggered in response to APs attack with particular reference to sticholysin II, the most active isoform produced by the Caribbean Sea anemone Stichodactyla helianthus. To achieve this, we first describe the major alterations these cytolysins elicit on simpler cells, such as non-nucleated mammalian erythrocytes, and then onto more complex eukaryotic cells, including tumor cells. This understanding has provided the basis for the development of novel applications of sticholysins such as the construction of immunotoxins directed against undesirable cells, such as tumor cells, and the design of a cancer vaccine platform. These are among the most interesting potential uses for the members of this toxin family that have been carried out in our laboratory.

Sticholysin II shows similar immunostimulatory properties to LLO stimulating dendritic cells and MHC-I restricted T cell responses of heterologous antigen

Induction of CD8⁺ T cell responses against tumor cells and intracellular pathogens is an important goal of modern vaccinology. One approach of translational interest is the use of liposomes encapsulating pore-forming proteins (PFPs), such as Listeriolysin O (LLO), which has shown efficacy at priming strong and sustained CD8⁺ T cell responses. Recently, we have demonstrated that Sticholysin II (StII), a PFP from the sea anemone Stichodactyla helianthus, co-encapsulated into liposomes with ovalbumin (OVA) was able to stimulate, antigen presenting cells, antigen-specific CD8⁺ T cells and anti-tumor activity in mice. In the present study, we aimed to compare StII and LLO in terms of their abilities to stimulate dendritic cells and to induce major histocompatibility complex (MHC) class I restricted T cell responses against OVA. Interestingly, StII exhibited similar abilities to LLO in vitro of inducing dendritic cells maturation, as measured by increased expression of CD40, CD80, CD86 and MHC-class II molecules, and of stimulating OVA cross-presentation to a CD8⁺ T cell line. Remarkably, using an ex vivo Enzyme-Linked ImmunoSpot Assay (ELISPOT) to monitor gamma interferon (INF-γ) producing effector memory CD8⁺ T cells, liposomal formulations containing either StII or LLO induced comparable frequencies of OVA-specific INF-γ producing CD8⁺ T cells in mice that were sustained in time. However, StII-containing liposomes stimulated antigen-specific memory CD8⁺ T cells with a higher potential to secrete IFN-γ than liposomes encapsulating LLO. This StII immunostimulatory property further supports its use for the rational design of T cell vaccines against cancers and intracellular pathogens. In summary, this study indicates that StII has immunostimulatory properties similar to LLO, despite being evolutionarily distant PFPs.

Sticholysins, pore-forming proteins from a marine anemone can induce maturation of dendritic cells through a TLR4 dependent-pathway

Sticholysins (Sts) I and II (StI and StII) are pore-forming proteins (PFPs), purified from the Caribbean Sea anemone Stichodactyla helianthus. StII encapsulated into liposomes induces a robust antigen-specific cytotoxic CD8⁺ T lymphocytes (CTL) response and in its free form the maturation of bone marrow-derived dendritic cells (BM-DCs). It is probable that the latter is partially supporting in part the immunomodulatory effect on the CTL response induced by StII-containing liposomes. In the present work, we demonstrate that the StII's ability of inducing maturation of BM-DCs is also shared by StI, an isoform of StII. Using heat-denatured Sts we observed a significant reduction in the up-regulation of maturation markers indicating that both PFP's ability to promote maturation of BM-DCs is dependent on their conformational characteristics. StII-mediated DC maturation was abrogated in BM-DCs from toll-like receptor (TLR) 4 and myeloid differentiation primary response gene 88 (MyD88)-knockout mice but not in cells from TLR2-knockout mice. Furthermore, the antigen-specific CTL response induced by StII-containing liposomes was reduced in TLR4-knockout mice. These results indicate that StII, and probably by extension StI, has the ability to induce maturation of DCs through a TLR4/MyD88-dependent pathway, and that this activation contributes to the CTL response generated by StII-containing liposomes.

A Review of Toxins from Cnidaria

Cnidarians have been known since ancient times for the painful stings they induce to humans. The effects of the stings range from skin irritation to cardiotoxicity and can result in death of human beings. The noxious effects of cnidarian venoms have stimulated the definition of their composition and their activity. Despite this interest, only a limited number of compounds extracted from cnidarian venoms have been identified and defined in detail. Venoms extracted from Anthozoa are likely the most studied, while venoms from Cubozoa attract research interests due to their lethal effects on humans. The investigation of cnidarian venoms has benefited in very recent times by the application of omics approaches. In this review, we propose an updated synopsis of the toxins identified in the venoms of the main classes of Cnidaria (Hydrozoa, Scyphozoa, Cubozoa, Staurozoa and Anthozoa). We have attempted to consider most of the available information, including a summary of the most recent results from omics and biotechnological studies, with the aim to define the state of the art in the field and provide a background for future research.

Dissecting the mechanism of action of actinoporins. Role of the N-terminal amphipathic α-helix in membrane binding and pore activity of sticholysins I and II

Actinoporins sticholysin I and sticholysin II (St I, St II) are proposed to lyse model and biomembranes via toroidal pore formation by their N-terminal domain. Based on the hypothesis that peptide fragments can reproduce the structure and function of this domain, the behavior of peptides containing St I residues 12–31 (StI_12-31), St II residues 11–30 (StII_11-30), and its TOAC-labeled analogue (N-TOAC-StII_11-30) was examined. Molecular modeling showed a good match with experimental structures, indicating amphipathic α-helices in the same regions as in the toxins. CD spectra revealed that the peptides were essentially unstructured in aqueous solution, acquiring α-helical conformation upon interaction with micelles and large unilamellar vesicles (LUV) of variable lipid composition. Fluorescence quenching studies with NBD-containing lipids indicated that N-TOAC-StII_11-30’s nitroxide moiety is located in the membranes polar head group region. Pyrene-labeled phospholipid inter-leaflet redistribution suggested that the peptides form toroidal pores, according to the mechanism of action proposed for the toxins. Binding occurred only to negatively charged LUV, indicating the importance of electrostatic interactions; in contrast the peptides bound to both negatively charged and zwitterionic micelles, pointing to a lesser influence of these interactions. In addition, differences between bilayers and micelles in head group packing and in curvature led to differences in peptide-membrane interaction. We propose that the peptides topography in micelles resembles that of the toxins in the toroidal pore. The peptides mimicked the toxins permeabilizing activity, St II peptides being more effective than StI_12-31. To our knowledge, this is the first demonstration that differences in the toxins N-terminal amphipathic α-helix play a role in the difference between St I and St II activities.

Pharmacological characterization of cnidarian extracts from the Caribbean Sea: evaluation of anti-snake venom and antitumor properties

BACKGROUND

Cnidarians produce toxins, which are composed of different polypeptides that induce pharmacological effects of biotechnological interest, such as antitumor, antiophidic and anti-clotting activities. This study aimed to evaluate toxicological activities and potential as antitumor and antiophidic agents contained in total extracts from five cnidarians: Millepora alcicornis, Stichodactyla helianthus, Plexaura homomalla, Bartholomea annulata and Condylactis gigantea (total and body wall).

METHODS

The cnidarian extracts were evaluated by electrophoresis and for their phospholipase, proteolytic, hemorrhagic, coagulant, fibrinogenolytic, neuromuscular blocking, muscle-damaging, edema-inducing and cytotoxic activities.

RESULTS

All cnidarian extracts showed indirect hemolytic activity, but only S. helianthus induced direct hemolysis and neurotoxic effect. However, the hydrolysis of NBD-PC, a PLA2 substrate, was presented only by the C. gigantea (body wall) and S. helianthus. The extracts from P. homomalla and S. helianthus induced edema, while only C. gigantea and S. helianthus showed intensified myotoxic activity. The proteolytic activity upon casein and fibrinogen was presented mainly by B. annulata extract and all were unable to induce hemorrhage or fibrinogen coagulation. Cnidarian extracts were able to neutralize clotting induced by Bothrops jararacussu snake venom, except M. alcicornis. All cnidarian extracts were able to inhibit hemorrhagic activity induced by Bothrops moojeni venom. Only the C. gigantea (body wall) inhibited thrombin-induced coagulation. All cnidarian extracts showed antitumor effect against Jurkat cells, of which C. gigantea (body wall) and S. helianthus were the most active; however, only C. gigantea (body wall) and M. alcicornis were active against B16F10 cells.

CONCLUSION

The cnidarian extracts analyzed showed relevant in vitro inhibitory potential over the activities induced by Bothrops venoms; these results may contribute to elucidate the possible mechanisms of interaction between cnidarian extracts and snake venoms.

Cloning, purification and characterization of nigrelysin, a novel actinoporin from the sea anemone Anthopleura nigrescens

Actinoporins constitute a unique class of pore-forming toxins found in sea anemones that being secreted as soluble monomers are able to bind and permeabilize membranes leading to cell death. The interest in these proteins has risen due to their high cytotoxicity that can be properly used to design immunotoxins against tumor cells and antigen-releasing systems to cell cytosol. In this work we describe a novel actinoporin produced by Anthopleura nigrescens, an anemone found in the Central American Pacific Ocean. Here we report the amino acid sequence of an actinoporin as deduced from cDNA obtained from total body RNA. The synthetic DNA sequence encoding for one cytolysin variant was expressed in BL21 Star (DE3) Escherichia coli and the protein purified by chromatography on CM Sephadex C-25 with more than 97% homogeneity as verified by MS-MS and HPLC analyses. This actinoporin comprises 179 amino acid residues, consistent with its observed isotope-averaged molecular mass of 19 661 Da. The toxin lacks Cys and readily permeabilizes erythrocytes, as well as L1210 cells. CD spectroscopy revealed that its secondary structure is dominated by beta structure (58.5%) with 5.5% of α-helix, and 35% of random structure. Moreover, binding experiments to lipidic monolayers and to liposomes, as well as permeabilization studies in vesicles, revealed that the affinity of this toxin for sphingomyelin-containing membranes is quite similar to sticholysin II (StII). Comparison by spectroscopic techniques and modeling the three-dimensional structure of nigrelysin (Ng) showed a high homology with StII but several differences were also detectable. Taken together, these results reinforce the notion that Ng is a novel member of the actinoporin pore-forming toxin (PFT) family with a HA as high as that of StII, the most potent actinoporin so far described, but with peculiar structural characteristics contributing to expand the understanding of the structure-function relationship in this protein family.

Stichodactyla helianthus' de novo transcriptome assembly: Discovery of a new actinoporin isoform

Transcriptomic profiling of venom producing tissues from different animals is an effective approach for discovering new toxins useful in biotechnological and pharmaceutical applications, as well in evolutionary comparative studies of venomous animals. Stichodactyla helianthus is a Caribbean sea anemone which produces actinoporins as part of its toxic venom. This family of pore forming toxins is multigenic and at least two different isoforms, encoded by separate genes, are produced by S. helianthus. These isoforms, sticholysins I and II, share 93% amino acid identity but differ in their pore forming activity and act synergistically. This observation suggests that other actinoporin isoforms, if present in the venomous mixture, could offer an advantageous strategy to modulate whole venom activity. Using high-throughput sequencing we generated a de novo transcriptome of S. helianthus and determined the relative expression of assembled transcripts using RNA-Seq to better characterize components of this species' venom, focusing on actinoporin diversity. Applying this approach, we have discovered at least one new actinoporin variant from S. helianthus in addition to several other putative venom components.

Biophysical and biochemical strategies to understand membrane binding and pore formation by sticholysins, pore-forming proteins from a sea anemone

Actinoporins constitute a unique class of pore-forming toxins found in sea anemones that are able to bind and oligomerize in membranes, leading to cell swelling, impairment of ionic gradients and, eventually, to cell death. In this review we summarize the knowledge generated from the combination of biochemical and biophysical approaches to the study of sticholysins I and II (Sts, StI/II), two actinoporins largely characterized by the Center of Protein Studies at the University of Havana during the last 20 years. These approaches include strategies for understanding the toxin structure-function relationship, the protein-membrane association process leading to pore formation and the interaction of toxin with cells. The rational combination of experimental and theoretical tools have allowed unraveling, at least partially, of the complex mechanisms involved in toxin-membrane interaction and of the molecular pathways triggered upon this interaction. The study of actinoporins is important not only to gain an understanding of their biological roles in anemone venom but also to investigate basic molecular mechanisms of protein insertion into membranes, protein-lipid interactions and the modulation of protein conformation by lipid binding. A deeper knowledge of the basic molecular mechanisms involved in Sts-cell interaction, as described in this review, will support the current investigations conducted by our group which focus on the design of immunotoxins against tumor cells and antigen-releasing systems to cell cytosol as Sts-based vaccine platforms.

Novel Adjuvant Based on the Pore-Forming Protein Sticholysin II Encapsulated into Liposomes Effectively Enhances the Antigen-Specific CTL-Mediated Immune Response

Vaccine strategies to enhance CD8⁺ CTL responses remain a current challenge because they should overcome the plasmatic and endosomal membranes for favoring exogenous Ag access to the cytosol of APCs. As a way to avoid this hurdle, sticholysin (St) II, a pore-forming protein from the Caribbean Sea anemone Stichodactyla helianthus, was encapsulated with OVA into liposomes (Lp/OVA/StII) to assess their efficacy to induce a CTL response. OVA-specific CD8⁺ T cells transferred to mice immunized with Lp/OVA/StII experienced a greater expansion than when the recipients were injected with the vesicles without St, mostly exhibiting a memory phenotype. Consequently, Lp/OVA/StII induced a more potent effector function, as shown by CTLs, in vivo assays. Furthermore, treatment of E.G7-OVA tumor-bearing mice with Lp/OVA/StII significantly reduced tumor growth being more noticeable in the preventive assay. The contribution of CD4⁺ and CD8⁺ T cells to CTL and antitumor activity, respectively, was elucidated. Interestingly, the irreversibly inactive variant of the StI mutant StI W111C, encapsulated with OVA into Lp, elicited a similar OVA-specific CTL response to that observed with Lp/OVA/StII or vesicles encapsulating recombinant StI or the reversibly inactive StI W111C dimer. These findings suggest the relative independence between StII pore-forming activity and its immunomodulatory properties. In addition, StII-induced in vitro maturation of dendritic cells might be supporting these properties. These results are the first evidence, to our knowledge, that StII, a pore-forming protein from a marine eukaryotic organism, encapsulated into Lp functions as an adjuvant to induce a robust specific CTL response.

Pore-forming toxins: Properties, diversity, and uses as tools to image sphingomyelin and ceramide phosphoethanolamine

Pore-forming toxins (PFTs) represent a unique class of highly specific lipid-binding proteins. The cytotoxicity of these compounds has been overcome through crystallographic structure and mutation studies, facilitating the development of non-toxic lipid probes. As a consequence, non-toxic PFTs have been utilized as highly specific probes to visualize the diversity and dynamics of lipid nanostructures in living and fixed cells. This review is focused on the application of PFTs and their non-toxic analogs as tools to visualize sphingomyelin and ceramide phosphoethanolamine, two major phosphosphingolipids in mammalian and insect cells, respectively. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

A RNA-seq approach to identify putative toxins from acrorhagi in aggressive and non-aggressive Anthopleura elegantissima polyps

BACKGROUND

The use of venom in intraspecific aggression is uncommon and venom-transmitting structures specifically used for intraspecific competition are found in few lineages of venomous taxa. Next-generation transcriptome sequencing allows robust characterization of venom diversity and exploration of functionally unique tissues. Using a tissue-specific RNA-seq approach, we investigate the venom composition and gene ontology diversity of acrorhagi, specialized structures used in intraspecific competition, in aggressive and non-aggressive polyps of the aggregating sea anemone Anthopleura elegantissima (Cnidaria: Anthozoa: Hexacorallia: Actiniaria: Actiniidae).

RESULTS

Collectively, we generated approximately 450,000 transcripts from acrorhagi of aggressive and non-aggressive polyps. For both transcriptomes we identified 65 candidate sea anemone toxin genes, representing phospholipase A2s, cytolysins, neurotoxins, and acrorhagins. When compared to previously characterized sea anemone toxin assemblages, each transcriptome revealed greater within-species sequence divergence across all toxin types. The transcriptome of the aggressive polyp had a higher abundance of type II voltage gated potassium channel toxins/Kunitz-type protease inhibitors and type II acrorhagins. Using toxin-like proteins from other venomous taxa, we also identified 612 candidate toxin-like transcripts with signaling regions, potentially unidentified secretory toxin-like proteins. Among these, metallopeptidases and cysteine rich (CRISP) candidate transcripts were in high abundance. Furthermore, our gene ontology analyses identified a high prevalence of genes associated with "blood coagulation" and "positive regulation of apoptosis", as well as "nucleoside: sodium symporter activity" and "ion channel binding". The resulting assemblage of expressed genes may represent synergistic proteins associated with toxins or proteins related to the morphology and behavior exhibited by the aggressive polyp.

CONCLUSION

We implement a multifaceted approach to investigate the assemblage of expressed genes specifically within acrorhagi, specialized structures used only for intraspecific competition. By combining differential expression, phylogenetic, and gene ontology analyses, we identify several candidate toxins and other potentially important proteins in acrorhagi of A. elegantissima. Although not all of the toxins identified are used in intraspecific competition, our analysis highlights some candidates that may play a vital role in intraspecific competition. Our findings provide a framework for further investigation into components of venom used exclusively for intraspecific competition in acrorhagi-bearing sea anemones and potentially other venomous animals.

A pore-forming toxin requires a specific residue for its activity in membranes with particular physicochemical properties

The physicochemical landscape of the bilayer modulates membrane protein function. Actinoporins are a family of potent hemolytic proteins from sea anemones acting at the membrane level. This family of cytolysins preferentially binds to target membranes containing sphingomyelin, where they form lytic pores giving rise to cell death. Although the cytolytic activity of the actinoporin fragaceatoxin C (FraC) is sensitive to vesicles made of various lipid compositions, it is far from clear how this toxin adjusts its mechanism of action to a broad range of physiochemical landscapes. Herein, we show that the conserved residue Phe-16 of FraC is critical for pore formation in cholesterol-rich membranes such as those of red blood cells. The interaction of a panel of muteins of Phe-16 with model membranes composed of raft-like lipid domains is inactivated in cholesterol-rich membranes but not in cholesterol-depleted membranes. These results indicate that actinoporins recognize different membrane environments, resulting in a wider repertoire of susceptible target membranes (and preys) for sea anemones. In addition, this study has unveiled promising candidates for the development of protein-based biosensors highly sensitive to the concentration of cholesterol within the membrane.

Differential effect of solution conditions on the conformation of the actinoporins Sticholysin II and Equinatoxin II

Actinoporins are a family of pore-forming proteins with hemolytic activity. The structural basis for such activity appears to depend on their correct folding. Such folding encompasses a phosphocholine binding site, a tryptophan-rich region and the activity-related N-terminus segment. Additionally, different solution conditions are known to be able to influence the pore formation by actinoporins, as for Sticholysin II (StnII) and Equinatoxin II (EqtxII). In this context, the current work intends to characterize the influence of distinct solution conditions in the conformational behavior of these proteins through molecular dynamics (MD) simulations. The obtained data offer structural insights into actinoporins dynamics in solution, characterizing its conformational behavior at the atomic level, in accordance with previous experimental data on StnII and EqtxII hemolytic activities.

Searching for a toxic key to unlock the mystery of anemonefish and anemone symbiosis

Twenty-six species of anemonefish of the genera Amphiprion and monospecific Premnas, use only 10 species of anemones as hosts in the wild (Families: Actiniidae, Stichodactylidae and Thalassianthidae). Of these 10 anemone species some are used by multiple species of anemonefish while others have only a single anemonefish symbiont. Past studies have explored the different patterns of usage between anemonefish species and anemone species; however the evolution of this relationship remains unknown and has been little studied over the past decade. Here we reopen the case, comparing the toxicity of crude venoms obtained from anemones that host anemonefish as a way to investigate why some anemone species are used as a host more than others. Specifically, for each anemone species we investigated acute toxicity using Artemia francisca (LC₅₀), haemolytic toxicity using ovine erythrocytes (EC₅₀) and neurotoxicity using shore crabs (Ozius truncatus). We found that haemolytic and neurotoxic activity varied among host anemone species. Generally anemone species that displayed greater haemolytic activity also displayed high neurotoxic activity and tend to be more toxic on average as indicated by acute lethality analysis. An overall venom toxicity ranking for each anemone species was compared with the number of anemonefish species that are known to associate with each anemone species in the wild. Interestingly, anemones with intermediate toxicity had the highest number of anemonefish associates, whereas anemones with either very low or very high toxicity had the fewest anemonefish associates. These data demonstrate that variation in toxicity among host anemone species may be important in the establishment and maintenance of anemonefish anemone symbiosis.

Hydra actinoporin-like toxin-1, an unusual hemolysin from the nematocyst venom of Hydra magnipapillata which belongs to an extended gene family

Cnidarians rely on their nematocysts and the venom injected through these unique weaponry systems to catch prey and protect themselves from predators. The development and physiology of the nematocysts of Hydra magnipapillata, a classic model organism, have been intensively studied, yet the composition and biochemical activity of their venom components are mostly unknown. Here, we show that hydra actinoporin-like toxins (HALTs), which have previously been associated with Hydra nematocysts, belong to a multigene family comprising six genes, which have diverged from a single common ancestor. All six genes are expressed in a population of Hydra magnipapillata. When expressed recombinantly, HALT-1 (Δ-HYTX-Hma1a), an actinoporin-like protein found in the stenoteles (the main penetrating nematocysts used in prey capture), reveals hemolytic activity, albeit about two-thirds lower than that of the anemone actinoporin equinatoxin II (EqTII, Δ-AITX-Aeq1a). HALT-1 also differs from EqTII in the size of its pores, and likely does not utilize sphingomyelin as a membrane receptor. We describe features of the HALT-1 sequence which may contribute to this difference in activity, and speculate on the role of this unusual family of pore-forming toxins in the ecology of Hydra.

Sea anemone (Cnidaria, Anthozoa, Actiniaria) toxins: an overview

The Cnidaria phylum includes organisms that are among the most venomous animals. The Anthozoa class includes sea anemones, hard corals, soft corals and sea pens. The composition of cnidarian venoms is not known in detail, but they appear to contain a variety of compounds. Currently around 250 of those compounds have been identified (peptides, proteins, enzymes and proteinase inhibitors) and non-proteinaceous substances (purines, quaternary ammonium compounds, biogenic amines and betaines), but very few genes encoding toxins were described and only a few related protein three-dimensional structures are available. Toxins are used for prey acquisition, but also to deter potential predators (with neurotoxicity and cardiotoxicity effects) and even to fight territorial disputes. Cnidaria toxins have been identified on the nematocysts located on the tentacles, acrorhagi and acontia, and in the mucous coat that covers the animal body. Sea anemone toxins comprise mainly proteins and peptides that are cytolytic or neurotoxic with its potency varying with the structure and site of action and are efficient in targeting different animals, such as insects, crustaceans and vertebrates. Sea anemones toxins include voltage-gated Na⁺ and K⁺ channels toxins, acid-sensing ion channel toxins, Cytolysins, toxins with Kunitz-type protease inhibitors activity and toxins with Phospholipase A2 activity. In this review we assessed the phylogentic relationships of sea anemone toxins, characterized such toxins, the genes encoding them and the toxins three-dimensional structures, further providing a state-of-the-art description of the procedures involved in the isolation and purification of bioactive toxins.

Exploiting the nephrotoxic effects of venom from the sea anemone, Phyllodiscus semoni, to create a hemolytic uremic syndrome model in the rat

In the natural world, there are many creatures with venoms that have interesting and varied activities. Although the sea anemone, a member of the phylum Coelenterata, has venom that it uses to capture and immobilise small fishes and shrimp and for protection from predators, most sea anemones are harmless to man. However, a few species are highly toxic; some have venoms containing neurotoxins, recently suggested as potential immune-modulators for therapeutic application in immune diseases. Phyllodiscus semoni is a highly toxic sea anemone; the venom has multiple effects, including lethality, hemolysis and renal injuries. We previously reported that venom extracted from Phyllodiscus semoni induced acute glomerular endothelial injuries in rats resembling hemolytic uremic syndrome (HUS), accompanied with complement dysregulation in glomeruli and suggested that the model might be useful for analyses of pathology and development of therapeutic approaches in HUS. In this mini-review, we describe in detail the venom-induced acute renal injuries in rat and summarize how the venom of Phyllodiscus semoni could have potential as a tool for analyses of complement activation and therapeutic interventions in HUS.

The behavior of sea anemone actinoporins at the water-membrane interface

Actinoporins constitute a group of small and basic α-pore forming toxins produced by sea anemones. They display high sequence identity and appear as multigene families. They show a singular behaviour at the water-membrane interface: In aqueous solution, actinoporins remain stably folded but, upon interaction with lipid bilayers, become integral membrane structures. These membranes contain sphingomyelin, display phase coexistence, or both. The water soluble structures of the actinoporins equinatoxin II (EqtII) and sticholysin II (StnII) are known in detail. The crystalline structure of a fragaceatoxin C (FraC) nonamer has been also determined. The three proteins fold as a β-sandwich motif flanked by two α-helices, one of them at the N-terminal end. Four regions seem to be especially important: A cluster of aromatic residues, a phosphocholine binding site, an array of basic amino acids, and the N-terminal α-helix. Initial binding of the soluble monomers to the membrane is accomplished by the cluster of aromatic amino acids, the array of basic residues, and the phosphocholine binding site. Then, the N-terminal α-helix detaches from the β-sandwich, extends, and lies parallel to the membrane. Simultaneously, oligomerization occurs. Finally, the extended N-terminal α-helix penetrates the membrane to build a toroidal pore. This model has been however recently challenged by the cryo-EM reconstruction of FraC bound to phospholipid vesicles. Actinoporins structural fold appears across all eukaryotic kingdoms in other functionally unrelated proteins. Many of these proteins neither bind to lipid membranes nor induce cell lysis. Finally, studies focusing on the therapeutic potential of actinoporins also abound.

Validation of a mutant of the pore-forming toxin sticholysin-I for the construction of proteinase-activated immunotoxins

The use of pore-forming toxins from sea anemones (actinoporins) in the construction of immunotoxins (ITs) against tumour cells is an alternative for cancer therapy. However, the main disadvantage of actinoporin-based ITs obtained so far has been the poor cellular specificity associated with the toxin's ability to bind and exert its activity in almost any cell membrane. Our final goal is the construction of tumour proteinase-activated ITs using a cysteine mutant at the membrane binding region of sticholysin-I (StI), a cytolysin isolated from the sea anemone Stichodactyla helianthus. The mutant and the ligand moiety would be linked by proteinase-sensitive peptides through the StI cysteine residue blocking the toxin binding region and hence the IT non-specific killing activity. To accomplish this objective the first step was to obtain the mutant StI W111C, and to evaluate the impact of mutating tryptophan 111 by cysteine on the toxin pore-forming capacity. After proteolysis of the cleavage sequence, a short peptide would remain attached to the toxin. The next step was to evaluate whether this mutant is able to form pores even with a residual peptide linked to cysteine 111. In this work we demonstrated that (i) StI W111C shows pore-forming capacity in a nanomolar range, although it is 8-fold less active than the wild-type recombinant StI, corroborating the previously reported importance of residue 111 for the binding of StI to membranes, and (ii) the mutant is able to form pores even with a residual seven-residue peptide linked to cysteine 111. In addition, it was demonstrated that binding of a large molecule to cysteine 111 renders an inactive toxin that is no longer able to bind to the membrane. These results validate the mutant StI W111C for its use in the construction of tumour proteinase-activated ITs.

Purification, cloning and characterization of fragaceatoxin C, a novel actinoporin from the sea anemone Actinia fragacea

Actinia fragacea is commonly called the "strawberry" anemone because of the distinctive yellow or green spots displayed on its red column. Its venom contains several haemolytic proteins with a molecular mass of approximately 20 kDa that can be separated by ion-exchange column chromatography. One of them was purified to homogeneity and was named fragaceatoxin C (FraC). Its 15 N-terminal residues were identified by Edman degradation and served to obtain its complete DNA coding sequence by RT-PCR. The coding region of FraC was amplified and cloned in the expression vector pBAT-4. Purified recombinant FraC consists of 179 amino acids and multiple sequence alignment with other actinoporins clearly indicates that FraC belongs to this protein family. The secondary structure, thermal stability and lytic activity of native and recombinant FraC were practically identical and exhibit the same basic features already described for equinatoxin-II and sticholysin-II.

Marine toxins: an overview

Oceans provide enormous and diverse space for marine life. Invertebrates are conspicuous inhabitants in certain zones such as the intertidal; many are soft-bodied, relatively immobile and lack obvious physical defenses. These animals frequently have evolved chemical defenses against predators and overgrowth by fouling organisms. Marine animals may accumulate and use a variety of toxins from prey organisms and from symbiotic microorganisms for their own purposes. Thus, toxic animals are particularly abundant in the oceans. The toxins vary from small molecules to high molecular weight proteins and display unique chemical and biological features of scientific interest. Many of these substances can serve as useful research tools or molecular models for the design of new drugs and pesticides. This chapter provides an initial survey of these toxins and their salient properties.

Correlations between differences in amino-terminal sequences and different hemolytic activity of sticholysins

Sticholysins I and II (St I and St II) are cytolysins produced by the sea anemone Stichodactyla helianthus. In spite of their 93% sequence homology, St II is more hemolytic against human erythrocytes than St I. In order to establish the possible causes of this difference, we studied the hemolytic activity of synthetic peptides containing sequences from the N-termini of both proteins. The results demonstrated that the differences in hemolytic activity of the toxins could be ascribed at least partly to differences in their N-termini.

Sticholysins I and II interaction with cationic micelles promotes toxins’ conformational changes and enhanced hemolytic activity

The effect of three cationic surfactants bearing the same polar head group and different chain length (cetyltrimethyl ammonium bromide (CTAB); tetradecyltrimethylammonium bromide (TTAB); dodecyltrimethylammonium bromide (DTAB)) on the conformation and function of the sea anemone pore-forming toxins sticholysins I and II (St I and St II) was studied by fluorescence and circular dichroism spectroscopy and evaluation of hemolytic activity (HA). Preincubation of the toxins with the longer chain surfactants CTAB and TTAB at concentrations slightly above their critical micelle concentration (CMC) leads to an enhancement of their HA. Significant increases in the fluorescence intensity with a slightly red shift in lambda(max) were observed at concentrations close to the surfactants' CMC, suggesting changes in the environment of the tryptophan residues. The changes in the fluorescence intensity are more noticeable and take place at lower surfactant concentrations for St I, irrespective of the surfactant alkyl chain length, although the differences between St I and St II increase as the surfactant alkyl chain length increases. This is evinced not only by the higher fluorescence intensity values and the lower surfactant concentrations required to reach them, but also by the higher acrylamide-quenching constant values (Ksv) for St I. However, the surfactant's effects on the toxins' HA were not found to be directly related to the observed changes in fluorescence intensity, as well as near- and far-UV-CD spectra. In particular, the latter spectra indicate that changes in HA and in fluorescence behavior take place without noticeable modifications in St I and St II secondary and tertiary structures. The results suggest that the interaction with the surfactants induces only subtle conformational changes in the toxins that favor the formation of lytic competent structures.

A protein toxin from the sea anemone Phyllodiscus semoni targets the kidney and causes a severe renal injury with predominant glomerular endothelial damage

Envenomation by the sea anemone Phyllodiscus semoni causes fulminant dermatitis and, rarely, acute renal failure in humans. Here, we investigated whether the venom extracted from the nematocysts (PsTX-T) was nephrotoxic when administered intravenously in rats and whether PsTX-T induced activation of the complement system. Although small dose of PsTX-T induced acute tubular necrosis in rats resembling pathology seen in patients, kidneys displayed glomerular injury with glomerular endothelial damage, thrombus formation, mesangiolysis, and partial rupture of glomerular basement membrane, accompanied by severe tubular necrosis at 24 hours after administration of 0.03 mg of PsTX-T per animal, similar to the glomerular findings typical of severe hemolytic uremic syndrome. The early stage injury was accompanied by specific PsTX-T binding, massive complement C3b, and membrane attack complex deposition in glomeruli in the regions of injury and decreased glomerular expression of complement regulators. A pathogenic role for complement was confirmed by demonstrating that systemic complement inhibition reduced renal injury. The isolated nephrotoxic component, a 115-kd protein toxin (PsTX-115), was shown to cause identical renal pathology. The demonstration that PsTX-T and PsTX-115 were highly nephrotoxic acting via induction of complement activation suggests that inhibition of complement might be used to prevent acute renal damage following envenomation by P. semoni.

The crude venom from the sea anemone Stichodactyla helianthus induces haemolysis and slight peroxidative damage in rat and human erythrocytes

The haemolytic and peroxidative effects of crude venom of the sea anemone Stichodactyla helianthus were evaluated in rat and human erythrocytes. Venom extract caused a significant concentration-dependent effect on haemolysis (release of haemoglobin). Human erythrocytes were more sensitive (0.094 mg protein/ml) than those of the rats (0.3787 mg protein/ml). In contrast, a light effect on lipid peroxidation (LP, an index of oxidative damage to membrane lipids) was recorded. The concentrations needed to produce a significant effect on LP in rat and human erythrocytes were, respectively, 2-fold and 7-fold higher than those required to produce significant haemolysis. The differential effect of S. helianthus venom on haemolysis and oxidation of membrane lipids is not common for venoms of other sea anemones, which usually show a tightly related effect on LP and haemolytic damage.

Effect of sphingomyelin and cholesterol on the interaction of St II with lipidic interfaces

Sticholysin II (St II) is a cytolysin produced by the sea anemone Stichodactyla helianthus, characterized by forming oligomeric pores in natural and artificial membranes. In the present work the influence of the membrane lipidic components sphingomyelin (SM) and cholesterol (Cho) on binding and functional activity of St II, was evaluated using ELISA, lipid monolayers and liposomes. The aim of this work was to establish the promoting role of Cho and SM, both in St II binding and pore formation efficiency. In general the association (evaluated by ELISA and incorporation to phospholipid monolayers) of St II to lipids mixtures was better than to any one of the single components. Regarding the unique role of SM, it was found that, albeit inefficiently, St II binds to phosphatidylcholine (PC):Cho monolayers and liposomes, and is able to form active pores in these bilayers. The results in monolayers and liposomes show that the presence of SM and large amounts of Cho leads to the highest values of critical pressure and rate of association to monolayers, the most favorable interaction with liposomes, and the fastest rate of pore formation, in spite of the rigidity of the layers as suggested by the high generalized polarization (GP) of Laurdan incorporated to liposomes and FTIR data. Taken together, the present results show that the joint presence of SM and Cho, both in binary and ternary (PC containing) mixtures provide conditions particularly suitable for St II binding and function. We suggest that microdomains present in the bilayers could be important for toxin-membrane association.

Structural and functional characterization of a recombinant sticholysin I (rSt I) from the sea anemone Stichodactyla helianthus

Sticholysins I and II (Sts I and II) are two potent cytolysins from the sea anemone Stichodactyla helianthus. These isoforms present 13 substitutions, with three non-conservative located at the N-terminus. St II is considerably more hemolytic than St I in human red blood cells, a result explained by the smaller number of negatively charged groups present at St II's N-terminus. In the present work, we have obtained a recombinant St I (rSt I), differing from the wild type in a single amino acid residue (E16Q). This pseudo-wild type is structurally similar to St I and shows a similar capacity to interact with and form pores in model membranes. This was assessed by the intrinsic fluorescence increase in the presence of liposomes, their adsorption to bilayers (measured by SPR), their concentration at the air-water interface, their interaction with lipid monolayers and their capacity to promote the release of carboxyfluorescein entrapped in liposomes. In spite of these similarities, rSt I presents a larger hemolytic activity in human red blood cells than St I, being intermediate in activity between Sts I and II. The results obtained in the present work emphasize that even the change of one single E by Q at the N-terminal segment may modify the toxin HA and show that this functional property is the most sensitive to subtle changes in the protein primary structure.

Amino acid sequence of RTX-A's isoform actinoporin from the sea anemone, Radianthus macrodactylus

The amino acid sequence of actinoporin RTX-A (175 aa) from the sea anemone Radianthus macrodactylus was determined by sequencing of clones obtained via amplification of cDNA. It was established that RTX-A possessed high homology with HmgIII from Heteractis magnifica (87%) and StI, StII from Stichodactyla helianthus (84 and 87%, respectively). The analysis of structural and functional relationships within RTX-A was carried out. The some disagreement concerning to significant role of several amino acid residues for actinoporins exhibition of hemolytic activity was found.

Model peptides mimic the structure and function of the N-terminus of the pore-forming toxin sticholysin II

To investigate the role of the N-terminal region in the lytic mechanism of the pore-forming toxin sticholysin II (St II), we studied the conformational and functional properties of peptides encompassing the first 30 residues of the protein. Peptides containing residues 1-30 (P1-30) and 11-30 (P11-30) were synthesized and their conformational properties were examined in aqueous solution as a function of peptide concentration, pH, ionic strength, and addition of the secondary structure-inducing solvent trifluoroethanol (TFE). CD spectra showed that increasing concentration, pH, and ionic strength led to aggregation of P1-30; as a consequence, the peptide acquired beta-sheet conformation. In contrast, P11-30 exhibited practically no conformational changes under the same conditions, remaining essentially structureless. Moreover, this peptide did not undergo aggregation. These differences clearly point to the modulating effect of the first 10 hydrophobic residues on the peptides aggregation and conformational properties. In TFE both the first ten hydrophobic peptides acquired alpha-helical conformation, albeit to a different extent, P11-30 displayed lower alpha-helical content. P1-30 presented a larger fraction of residues in alpha-helical conformation in TFE than that found in St II's crystal structure for that portion of the protein. Since TFE mimics the membrane environment, such increase in helical content could also occur upon toxin binding to membranes and represent a step in the mechanism of pore formation. The peptides conformational properties correlated well with their functional behavior. Thus, P1-30 exhibited much higher hemolytic activity than P11-30. In addition, P11-30 was able to block the toxin's hemolytic activity. The size of pores formed in red blood cells by P1-30 was estimated by measuring the permeability to PEGs of different molecular mass. The pore radius (0.95 +/- 0.01 nm) was very similar to that of the pore formed by the toxin. The results demonstrate that the synthetic peptide P1-30 is a good model of St II conformation and function and emphasize the contribution of the toxin's N-terminal region, and, in particular, the hydrophobic residues 1-10 to pore formation.

Conformational Stability and Hemolytic Activity of Actinoporin RTX-SII from the Sea Anemone Radianthus macrodactylus

The spatial organization of actinoporin RTX-SII from the sea anemone Radianthus macrodactylus on the level of tertiary and secondary structures was studied by UV and CD spectroscopy and intrinsic protein fluorescence. The specific and molar extinction coefficients of RTX-SII were determined. The percentages of canonical secondary structures of actinoporin were calculated. The tertiary structure of the polypeptide is well developed and its secondary structure is highly ordered and contains about 50% antiparallel folded beta-sheets. The irreversible thermal denaturation of RTX-SII was studied by CD spectroscopy; a conformational transition occurs at 53 degrees C. Above this temperature irreversible conformational changes are observed in the secondary and tertiary structures. This is accompanied by redistribution of the content of regular and distorted forms of beta-sheet and also by increase in the content of an unordered form. It is suggested that an intermediate is formed in the process of thermal denaturation. Acid-base titration of RTX-SII results in irreversible conformational changes at pH below 2.0 and above 12.0. As shown by intrinsic protein fluorescence, tyrosine residues of RTX-SII make a fundamental contribution to emission, and the total fluorescence depends more on temperature and ionic strength of the solution than tryptophan fluorescence. The data on conformational stability of actinoporin are correlated with data on its hemolytic activity. Activity of RTX-SII significantly decreases at increased temperature and slightly decreases at low pH. Hemolytic activity drastically increases at high pH. Increase in the actinoporin activity at pH above 10 seems to be caused by ionization of the molecule.

Construction of an immunotoxin with the pore forming protein StI and ior C5, a monoclonal antibody against a colon cancer cell line

Sticholysin I (StI), a potent cytolysin isolated from the sea anemone Stichodactyla helianthus, was linked to the monoclonal antibody (mAb) ior C5. StI acts by forming hydrophilic pores in the membrane of the attacked cells leading to osmotic lysis. ior C5 is a murine IgG1, which recognizes the tumor associated antigen (TAA) ior C2. The cytolysin and the mAb were coupled by using the heterobifunctional cross-linking reagent sulfosuccinimidyl 4-(N-maleimidomethyl)-cyclohexane-1-carboxylate (SMCC). Two hybrid molecules composed by one ior C5 and one or two StI molecules were obtained (named conjugated I and II, respectively). The purified conjugates were evaluated by a binding affinity assay against an ior C2-positive colon cancer cell line (SW948). Both molecules were able to recognize the antigen (Ag) in the same way that unconjugated ior C5 does. The activity of both conjugates against human erythrocytes and SW948 cells was assessed. They lost most of their hemolytic activity but their residual activity was very similar. Nevertheless, when their cytotoxicity was studied on the SW948 cell line, only conjugate II killed efficiently the cells, indicating a specific mAb-Ag interaction. In this chimeric molecule the ratio between the cytotoxic and the hemolytic activity was larger than that of the free cytolysin. This fact indicates an increase of the specificity of the toxic effect toward the SW948 cell line and consequently an increase of the difference between its hemolytic and cytotoxic doses. The results herein support the feasibility of directing StI to the surface of cancer cells expressing ior C2 Ag via the mAb ior C5.

Cross-reactivity and inhibition of haemolysis by polyclonal antibodies raised against St II, a cytolysin from the sea anemone Stichodactyla helianthus

The immunogenicity of sticholysin II (St II), a pore-forming polypeptide from the sea anemone Stichodactyla helianthus, was studied in rabbits using two adjuvants, Freund's and aluminium hydroxide. High titres of antibodies were raised against St II with Freund's adjuvant (FA). The structural homology between sticholysins I and II was also revealed by cross-reactivity assays. Since the oil constituent of FA neutralized the St II haemolytic activity, immunizations with St II-Freund's emulsions were carried out with the inactivated cytolysin. Purified anti-St II IgG also neutralized the St II haemolytic activity.

Primary structure of echotoxin 2, an actinoporin-like hemolytic toxin from the salivary gland of the marine gastropod Monoplex echo

Echotoxins are 25 kDa proteins with both hemolytic and lethal activities, previously purified from the salivary gland of the marine gastropod Monoplex echo. In this study, a cDNA encoding echotoxin 2 was cloned by RT-PCR, 3'-RACE and 5'-RACE, based on its partial amino acid sequence. The full-length echotoxin 2 cDNA (1000 bp) obtained contains an open reading frame (825 bp) coding for a precursor protein of 274 amino acid residues. Mature echotoxin 2 composed of 226 amino acid residues is assumed to be produced by post-translational removal of N-terminal 23 residues (predicted as a signal peptide) and C-terminal 25 residues from the precursor protein. Very interestingly, a homology search revealed that echotoxin 2 is analogous to actinoporins, 20 kDa pore-forming hemolysins reported from various sea anemones. In addition to the similarities in biological activity, molecular size and basicity between echotoxin 2 and actinoporins, two prominent structural features, an N-terminal amphiphilic alpha-helix and an aromatic patch comprising Trp and Tyr residues, both of which are important for the pore-forming activity of actinoporins, are also recognized in echotoxin 2. However, echotoxin 2 is distinguishable from actinoporins in having Cys residues and lacking an RGD motif.