Purification and pharmacological properties of eight sea anemone toxins from Anemonia sulcata, Anthopleura xanthogrammica, Stoichactis giganteus, and Actinodendron plumosum

The Sea Anemone Neurotoxins Modulating Sodium Channels: An Insight at Structure and Functional Activity after Four Decades of Investigation

Many human cardiovascular and neurological disorders (such as ischemia, epileptic seizures, traumatic brain injury, neuropathic pain, etc.) are associated with the abnormal functional activity of voltage-gated sodium channels (VGSCs/NaVs). Many natural toxins, including the sea anemone toxins (called neurotoxins), are an indispensable and promising tool in pharmacological researches. They have widely been carried out over the past three decades, in particular, in establishing different NaV subtypes functional properties and a specific role in various pathologies. Therefore, a large number of publications are currently dedicated to the search and study of the structure-functional relationships of new sea anemone natural neurotoxins-potential pharmacologically active compounds that specifically interact with various subtypes of voltage gated sodium channels as drug discovery targets. This review presents and summarizes some updated data on the structure-functional relationships of known sea anemone neurotoxins belonging to four structural types. The review also emphasizes the study of type 2 neurotoxins, produced by the tropical sea anemone Heteractis crispa, five structurally homologous and one unique double-stranded peptide that, due to the absence of a functionally significant Arg14 residue, loses toxicity but retains the ability to modulate several VGSCs subtypes.

Voltage-Gated Sodium Channels: A Prominent Target of Marine Toxins

Voltage-gated sodium channels (VGSCs) are considered to be one of the most important ion channels given their remarkable physiological role. VGSCs constitute a family of large transmembrane proteins that allow transmission, generation, and propagation of action potentials. This occurs by conducting Na⁺ ions through the membrane, supporting cell excitability and communication signals in various systems. As a result, a wide range of coordination and physiological functions, from locomotion to cognition, can be accomplished. Drugs that target and alter the molecular mechanism of VGSCs’ function have highly contributed to the discovery and perception of the function and the structure of this channel. Among those drugs are various marine toxins produced by harmful microorganisms or venomous animals. These toxins have played a key role in understanding the mode of action of VGSCs and in mapping their various allosteric binding sites. Furthermore, marine toxins appear to be an emerging source of therapeutic tools that can relieve pain or treat VGSC-related human channelopathies. Several studies documented the effect of marine toxins on VGSCs as well as their pharmaceutical applications, but none of them underlined the principal marine toxins and their effect on VGSCs. Therefore, this review aims to highlight the neurotoxins produced by marine animals such as pufferfish, shellfish, sea anemone, and cone snail that are active on VGSCs and discuss their pharmaceutical values.

Anemonia sulcata and Its Symbiont Symbiodinium as a Source of Anti-Tumor and Anti-Oxoxidant Compounds for Colon Cancer Therapy: A Preliminary in Vitro Study

Simple Summary

Colorectal cancer is one of the most frequent types of cancer in the population. Recently, invertebrate marine animals have been investigated for the presence of natural products which can damage tumor cells, prevent their spread to other tissues or avoid cancer develop. We analyzed the anemone Anemonia sulcata with and without the presence of its microalgal symbiont (Symbiodinium) as a source of bioactive molecules for the colorectal cancer therapy and prevention. Colon cancer tumor cells were exposed to Anemone extracts observing a remarkable cell death and a great antioxidant capacity. These preliminary results support that Anemonia sulcata could be a source of bioactive compounds against colorectal cancer and that the absence of its symbiont may enhance these properties. Further studies will be necessary to define the bioactive compounds of Anemonia sulcata and their mechanisms of action.

Abstract

Recently, invertebrate marine species have been investigated for the presence of natural products with antitumor activity. We analyzed the invertebrate Anemonia sulcata with (W) and without (W/O) the presence of its microalgal symbiont Symbiodinium as a source of bioactive compounds that may be applied in the therapy and/or prevention of colorectal cancer (CRC). Animals were mechanically homogenized and subjected to ethanolic extraction. The proximate composition and fatty acid profile were determined. In addition, an in vitro digestion was performed to study the potentially dialyzable fraction. The antioxidant and antitumor activity of the samples and the digestion products were analyzed in CRC cells in vitro. Our results show a high concentration of polyunsaturated fatty acid in the anemone and a great antioxidant capacity, which demonstrated the ability to prevent cell death and a high antitumor activity of the crude homogenates against CRC cells and multicellular tumor spheroids, especially W/O symbiont. These preliminary results support that Anemonia sulcata could be a source of bioactive compounds with antioxidant and antitumor potential against CRC and that the absence of its symbiont may enhance these properties. Further studies will be necessary to define the bioactive compounds of Anemonia sulcata and their mechanisms of action.

Sea Anemone Toxins: A Structural Overview

Sea anemones produce venoms of exceptional molecular diversity, with at least 17 different molecular scaffolds reported to date. These venom components have traditionally been classified according to pharmacological activity and amino acid sequence. However, this classification system suffers from vulnerabilities due to functional convergence and functional promiscuity. Furthermore, for most known sea anemone toxins, the exact receptors they target are either unknown, or at best incomplete. In this review, we first provide an overview of the sea anemone venom system and then focus on the venom components. We have organised the venom components by distinguishing firstly between proteins and non-proteinaceous compounds, secondly between enzymes and other proteins without enzymatic activity, then according to the structural scaffold, and finally according to molecular target.

The Anemonia viridis Venom: Coupling Biochemical Purification and RNA-Seq for Translational Research

Blue biotechnologies implement marine bio-resources for addressing practical concerns. The isolation of biologically active molecules from marine animals is one of the main ways this field develops. Strikingly, cnidaria are considered as sustainable resources for this purpose, as they possess unique cells for attack and protection, producing an articulated cocktail of bioactive substances. The Mediterranean sea anemone Anemonia viridis has been studied extensively for years. In this short review, we summarize advances in bioprospecting of the A. viridis toxin arsenal. A. viridis RNA datasets and toxin data mining approaches are briefly described. Analysis reveals the major pool of neurotoxins of A. viridis, which are particularly active on sodium and potassium channels. This review therefore integrates progress in both RNA-Seq based and biochemical-based bioprospecting of A. viridis toxins for biotechnological exploitation.

Sodium channels and pain: from toxins to therapies

Voltage-gated sodium channels (NaV channels) are essential for the initiation and propagation of action potentials that critically influence our ability to respond to a diverse range of stimuli. Physiological and pharmacological studies have linked abnormal function of NaV channels to many human disorders, including chronic neuropathic pain. These findings, along with the description of the functional properties and expression pattern of NaV channel subtypes, are helping to uncover subtype specific roles in acute and chronic pain and revealing potential opportunities to target these with selective inhibitors. High-throughput screens and automated electrophysiology platforms have identified natural toxins as a promising group of molecules for the development of target-specific analgesics. In this review, the role of toxins in defining the contribution of NaV channels in acute and chronic pain states and their potential to be used as analgesic therapies are discussed.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Sea Anemones: Quiet Achievers in the Field of Peptide Toxins

Sea anemones have been understudied as a source of peptide and protein toxins, with relatively few examined as a source of new pharmacological tools or therapeutic leads. This is surprising given the success of some anemone peptides that have been tested, such as the potassium channel blocker from Stichodactyla helianthus known as ShK. An analogue of this peptide, ShK-186, which is now known as dalazatide, has successfully completed Phase 1 clinical trials and is about to enter Phase 2 trials for the treatment of autoimmune diseases. One of the impediments to the exploitation of sea anemone toxins in the pharmaceutical industry has been the difficulty associated with their high-throughput discovery and isolation. Recent developments in multiple ‘omic’ technologies, including genomics, transcriptomics and proteomics, coupled with advanced bioinformatics, have opened the way for large-scale discovery of novel sea anemone toxins from a range of species. Many of these toxins will be useful pharmacological tools and some will hopefully prove to be valuable therapeutic leads.

Sodium Channels and Venom Peptide Pharmacology

Venomous animals including cone snails, spiders, scorpions, anemones, and snakes have evolved a myriad of components in their venoms that target the opening and/or closing of voltage-gated sodium channels to cause devastating effects on the neuromuscular systems of predators and prey. These venom peptides, through design and serendipity, have not only contributed significantly to our understanding of sodium channel pharmacology and structure, but they also represent some of the most phyla- and isoform-selective molecules that are useful as valuable tool compounds and drug leads. Here, we review our understanding of the basic function of mammalian voltage-gated sodium channel isoforms as well as the pharmacology of venom peptides that act at these key transmembrane proteins.

[Structural Features of Cysteine-Stabilized Polypeptides from Sea Anemones Venoms]

Nowadays, venom-based drug discovery becomes popular again: pharmaceutical companies evaluate animal venom potential as a combinatory library of biologically-active compounds. Collaborations with research groups from academia are intensified, new toxins are being investigated, among which polypeptides are of paramount importance. Sea anemones produce the most diversified, from structural point of view, polypep- tide venom components among other animals. This particular review considers known polypeptide toxins from sea anemones, basically taking into account its classification by primary structural features. The most important functional characteristics are analyzed in each structural class.

Evolution of an ancient venom: recognition of a novel family of cnidarian toxins and the common evolutionary origin of sodium and potassium neurotoxins in sea anemone

Despite Cnidaria (sea anemones, corals, jellyfish, and hydroids) being the oldest venomous animal lineage, structure-function relationships, phyletic distributions, and the molecular evolutionary regimes of toxins encoded by these intriguing animals are poorly understood. Hence, we have comprehensively elucidated the phylogenetic and molecular evolutionary histories of pharmacologically characterized cnidarian toxin families, including peptide neurotoxins (voltage-gated Na(+) and K(+) channel-targeting toxins: NaTxs and KTxs, respectively), pore-forming toxins (actinoporins, aerolysin-related toxins, and jellyfish toxins), and the newly discovered small cysteine-rich peptides (SCRiPs). We show that despite long evolutionary histories, most cnidarian toxins remain conserved under the strong influence of negative selection-a finding that is in striking contrast to the rapid evolution of toxin families in evolutionarily younger lineages, such as cone snails and advanced snakes. In contrast to the previous suggestions that implicated SCRiPs in the biomineralization process in corals, we demonstrate that they are potent neurotoxins that are likely involved in the envenoming function, and thus represent the first family of neurotoxins from corals. We also demonstrate the common evolutionary origin of type III KTxs and NaTxs in sea anemones. We show that type III KTxs have evolved from NaTxs under the regime of positive selection, and likely represent a unique evolutionary innovation of the Actinioidea lineage. We report a correlation between the accumulation of episodically adaptive sites and the emergence of novel pharmacological activities in this rapidly evolving neurotoxic clade.

The specificity of Av3 sea anemone toxin for arthropods is determined at linker DI/SS2-S6 in the pore module of target sodium channels

Av3 is a peptide neurotoxin from the sea anemone Anemonia viridis that shows specificity for arthropod voltage-gated sodium channels (Navs). Interestingly, Av3 competes with a scorpion α-toxin on binding to insect Navs and similarly inhibits the inactivation process, and thus has been classified as 'receptor site-3 toxin', although the two peptides are structurally unrelated. This raises questions as to commonalities and differences in the way both toxins interact with Navs. Recently, site-3 was partly resolved for scorpion α-toxins highlighting S1-S2 and S3-S4 external linkers at the DIV voltage-sensor module and the juxtaposed external linkers at the DI pore module. To uncover channel determinants involved in Av3 specificity for arthropods, the toxin was examined on channel chimaeras constructed with the external linkers of the mammalian brain Nav1.2a, which is insensitive to Av3, in the background of the Drosophila DmNav1. This approach highlighted the role of linker DI/SS2-S6, adjacent to the channel pore, in determining Av3 specificity. Point mutagenesis at DI/SS2-S6 accompanied by functional assays highlighted Trp404 and His405 as a putative point of Av3 interaction with DmNav1. His405 conservation in arthropod Navs compared with tyrosine in vertebrate Navs may represent an ancient substitution that explains the contemporary selectivity of Av3. Trp404 and His405 localization near the membrane surface and the hydrophobic bioactive surface of Av3 suggest that the toxin possibly binds at a cleft by DI/S6. A partial overlap in receptor site-3 of both toxins nearby DI/S6 may explain their binding competition capabilities.

Analysis of soluble protein contents from the nematocysts of a model sea anemone sheds light on venom evolution

The nematocyst is one of the most complex intracellular structures found in nature and is the defining feature of the phylum Cnidaria (sea anemones, corals, jellyfish, and hydroids). This miniature stinging organelle contains and delivers venom into prey and foe yet little is known about its toxic components. In the present study, we identified by tandem mass spectrometry 20 proteins released upon discharge from the nematocyst of the model sea anemone Nematostella vectensis. The availability of genomic and transcriptomic data for this species enabled accurate identification and phylogenetic study of these components. Fourteen of these proteins could not be identified in other animals suggesting that they might be the products of taxonomically restricted genes, a finding which fits well their origin from a taxon-specific organelle. Further, we studied by in situ hybridization the localization of two of the transcripts encoding the putative nematocyst venom proteins: a metallopeptidase related to the Tolloid family and a cysteine-rich protein. Both transcripts were detected in nematocytes, which are the cells containing nematocysts, and the metallopeptidase was found also in pharyngeal gland cells. Our findings reveal for the first time the possible venom components of a sea anemone nematocyst and suggest their evolutionary origins.

Peptide fingerprinting of the neurotoxic fractions isolated from the secretions of sea anemones Stichodactyla helianthus and Bunodosoma granulifera. New members of the APETx-like family identified by a 454 pyrosequencing approach

Sea anemones are known to contain a wide diversity of biologically active peptides, mostly unexplored according to recent peptidomic and transcriptomic studies. In the present work, the neurotoxic fractions from the exudates of Stichodactyla helianthus and Bunodosoma granulifera were analyzed by reversed-phase chromatography and mass spectrometry. The first peptide fingerprints of these sea anemones were assessed, revealing the largest number of peptide components (156) so far found in sea anemone species, as well as the richer peptide diversity of B. granulifera in relation to S. helianthus. The transcriptomic analysis of B. granulifera, performed by massive cDNA sequencing with 454 pyrosequencing approach allowed the discovery of five new APETx-like peptides (U-AITX-Bg1a-e - including the full sequences of their precursors for four of them), which together with type 1 sea anemone sodium channel toxins constitute a very distinguishable feature of studied sea anemone species belonging to genus Bunodosoma. The molecular modeling of these new APETx-like peptides showed a distribution of positively charged and aromatic residues in putative contact surfaces as observed in other animal toxins. On the other hand, they also showed variable electrostatic potentials, thus suggesting a docking onto their targeted channels in different spatial orientations. Moreover several crab paralyzing toxins (other than U-AITX-Bg1a-e), which induce a variety of symptoms in crabs, were isolated. Some of them presumably belong to new classes of crab-paralyzing peptide toxins, especially those with molecular masses below 2kDa, which represent the smallest peptide toxins found in sea anemones.

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.

Sea anemone toxins affecting voltage-gated sodium channels--molecular and evolutionary features

The venom of sea anemones is rich in low molecular weight proteinaceous neurotoxins that vary greatly in structure, site of action, and phyletic (insect, crustacean or vertebrate) preference. This toxic versatility likely contributes to the ability of these sessile animals to inhabit marine environments co-habited by a variety of mobile predators. Among these toxins, those that show prominent activity at voltage-gated sodium channels and are critical in predation and defense, have been extensively studied for more than three decades. These studies initially focused on the discovery of new toxins, determination of their covalent and folded structures, understanding of their mechanisms of action on different sodium channels, and identification of the primary sites of interaction of the toxins with their channel receptors. The channel binding site for Type I and the structurally unrelated Type III sea anemone toxins was identified as neurotoxin receptor site 3, a site previously shown to be targeted by scorpion alpha-toxins. The bioactive surfaces of toxin representatives from these two sea anemone types have been characterized by mutagenesis. These analyses pointed to heterogeneity of receptor site 3 at various sodium channels. A turning point in evolutionary studies of sea anemone toxins was the recent release of the genome sequence of Nematostella vectensis, which enabled analysis of the genomic organization of the corresponding genes. This analysis demonstrated that Type I toxins in Nematostella and other species are encoded by gene families and suggested that these genes developed by concerted evolution. The current review provides a brief historical description of the discovery and characterization of sea anemone toxins that affect voltage-gated sodium channels and delineates recent advances in the study of their structure-activity relationship and evolution.

Molecular analysis of the sea anemone toxin Av3 reveals selectivity to insects and demonstrates the heterogeneity of receptor site-3 on voltage-gated Na+ channels

Av3 is a short peptide toxin from the sea anemone Anemonia viridis shown to be active on crustaceans and inactive on mammals. It inhibits inactivation of Na(v)s (voltage-gated Na+ channels) like the structurally dissimilar scorpion alpha-toxins and type I sea anemone toxins that bind to receptor site-3. To examine the potency and mode of interaction of Av3 with insect Na(v)s, we established a system for its expression, mutagenized it throughout, and analysed it in toxicity, binding and electrophysiological assays. The recombinant Av3 was found to be highly toxic to blowfly larvae (ED50=2.65+/-0.46 pmol/100 mg), to compete well with the site-3 toxin LqhalphaIT (from the scorpion Leiurus quinquestriatus) on binding to cockroach neuronal membranes (K(i)=21.4+/-7.1 nM), and to inhibit the inactivation of Drosophila melanogaster channel, DmNa(v)1, but not that of mammalian Na(v)s expressed in Xenopus oocytes. Moreover, like other site-3 toxins, the activity of Av3 was synergically enhanced by ligands of receptor site-4 (e.g. scorpion beta-toxins). The bioactive surface of Av3 was found to consist mainly of aromatic residues and did not resemble any of the bioactive surfaces of other site-3 toxins. These analyses have portrayed a toxin that might interact with receptor site-3 in a different fashion compared with other ligands of this site. This assumption was corroborated by a D1701R mutation in DmNa(v)1, which has been shown to abolish the activity of all other site-3 ligands, except Av3. All in all, the present study provides further evidence for the heterogeneity of receptor site-3, and raises Av3 as a unique model for design of selective anti-insect compounds.

Sea anemone venom as a source of insecticidal peptides acting on voltage-gated Na+ channels

Sea anemones produce a myriad of toxic peptides and proteins of which a large group acts on voltage-gated Na+ channels. However, in comparison to other organisms, their venoms and toxins are poorly studied. Most of the known voltage-gated Na+ channel toxins isolated from sea anemone venoms act on neurotoxin receptor site 3 and inhibit the inactivation of these channels. Furthermore, it seems that most of these toxins have a distinct preference for crustaceans. Given the close evolutionary relationship between crustaceans and insects, it is not surprising that sea anemone toxins also profoundly affect insect voltage-gated Na+ channels, which constitutes the scope of this review. For this reason, these peptides can be considered as insecticidal lead compounds in the development of insecticides.

Voltage-gated ion channels and gating modifier toxins

Voltage-gated sodium, calcium, and potassium channels generate electrical signals required for action potential generation and conduction and are the molecular targets for a broad range of potent neurotoxins. These channels are built on a common structural motif containing six transmembrane segments and a pore loop. Their pores are formed by the S5/S6 segments and the pore loop between them, and they are gated by bending of the S6 segments at a hinge glycine or proline residue. The voltage sensor domain consists of the S1-S4 segments, with positively charged residues in the S4 segment serving as gating charges. The diversity of toxin action on these channels is illustrated by sodium channels, which are the molecular targets for toxins that act at six or more distinct receptor sites on the channel protein. Both hydrophilic low molecular weight toxins and larger polypeptide toxins physically block the pore and prevent sodium conductance. Hydrophobic alkaloid toxins and related lipid-soluble toxins act at intramembrane sites and alter voltage-dependent gating of sodium channels via an allosteric mechanism. In contrast, polypeptide toxins alter channel gating by voltage-sensor trapping through binding to extracellular receptor sites, and this toxin interaction has now been modeled at the atomic level for a beta-scorpion toxin. The voltage-sensor trapping mechanism may be a common mode of action for polypeptide gating modifier toxins acting on all of the voltage-gated ion channels.

Site-3 sea anemone toxins: Molecular probes of gating mechanisms in voltage-dependent sodium channels

Sea anemone toxins, whose biological function is the capture of marine prey, are invaluable tools for studying the structure and function of mammalian voltage-gated sodium channels. Their high degree of specificity and selectivity have allowed for detailed analysis of inactivation gating and assignment of molecular entities responsible for this process. Because of their ability to discriminate among channel isoforms, and their high degree of structural conservation, these toxins could serve as important lead compounds for future pharmaceutical design.

Peptide toxins in sea anemones: structural and functional aspects

Sea anemones are a rich source of two classes of peptide toxins, sodium channel toxins and potassium channel toxins, which have been or will be useful tools for studying the structure and function of specific ion channels. Most of the known sodium channel toxins delay channel inactivation by binding to the receptor site 3 and most of the known potassium channel toxins selectively inhibit Kv1 channels. The following peptide toxins are functionally unique among the known sodium or potassium channel toxins: APETx2, which inhibits acid-sensing ion channels in sensory neurons; BDS-I and II, which show selectivity for Kv3.4 channels and APETx1, which inhibits human ether-a-go-go-related gene potassium channels. In addition, structurally novel peptide toxins, such as an epidermal growth factor (EGF)-like toxin (gigantoxin I), have also been isolated from some sea anemones although their functions remain to be clarified.

Biochemical and physiological analyses of a hemolytic toxin isolated from a sea anemone Actineria villosa

A species of venomous sea anemone Actineria villosa was recently found inhabiting the coastal areas of Okinawa, Japan. This marine animal produces various proteinous toxins, so that a local health organization was called for medical treatment for those who had accidental contact with this animal. In this study we analyzed the biochemical and physiological properties of hemolytic protein from A. villosa. The toxin purified from the tentacles of the animals was found to be a protein with a molecular weight of approximately 19 kDa. We named this newly found hemolytic toxin of A. villosa, Avt-I. Incubation of the toxin with sphingomyelin inhibited hemolytic activity by up to 85%, showing that Avt-I may target sphingomyelin on the erythrocyte membrane. The hemolytic activity was stably maintained at temperatures below 45 degrees C, however, a sharp linear decrease in heat stability was observed within the range of 45-55 degrees C. Our results provide the first evidence that A. villosa produces a toxin with strong hemolytic activity similar in biochemical and physiological properties to other members of actinoporin family previously isolated from related species of sea anemones.

A new sea anemone peptide, APETx2, inhibits ASIC3, a major acid-sensitive channel in sensory neurons

From a systematic screening of animal venoms, we isolated a new toxin (APETx2) from the sea anemone Anthopleura elegantissima, which inhibits ASIC3 homomeric channels and ASIC3-containing heteromeric channels both in heterologous expression systems and in primary cultures of rat sensory neurons. APETx2 is a 42 amino-acid peptide crosslinked by three disulfide bridges, with a structural organization similar to that of other sea anemone toxins that inhibit voltage-sensitive Na+ and K+ channels. APETx2 reversibly inhibits rat ASIC3 (IC50=63 nM), without any effect on ASIC1a, ASIC1b, and ASIC2a. APETx2 directly inhibits the ASIC3 channel by acting at its external side, and it does not modify the channel unitary conductance. APETx2 also inhibits heteromeric ASIC2b+3 current (IC50=117 nM), while it has less affinity for ASIC1b+3 (IC50=0.9 microM), ASIC1a+3 (IC50=2 microM), and no effect on the ASIC2a+3 current. The ASIC3-like current in primary cultured sensory neurons is partly and reversibly inhibited by APETx2 with an IC50 of 216 nM, probably due to the mixed inhibitions of various co-expressed ASIC3-containing channels.

An epidermal growth factor-like toxin and two sodium channel toxins from the sea anemone Stichodactyla gigantea

Three peptide toxins (gigantoxins I-III) with crab toxicity were isolated from the sea anemone Stichodactyla gigantea by gel filtration on Sephadex G-50 and reverse-phase HPLC on TSKgel ODS-120T and their complete amino acid sequences were determined. Gigantoxins II (44 residues) and III (48 residues) have LD(50) (against crabs) of 70 and 120 microg/kg, respectively, and are analogous to the known type 1 and 2 sea anemone sodium channel toxins, respectively. On the other hand, gigantoxin I (48 residues) is potently paralytic to crabs (ED(50) 215 microg/kg), although its lethality is very weak (LD(50)>1000 microg/kg). Interestingly, gigantoxin I has 31-33% homologies with mammalian epidermal growth factors (EGFs), with the same location of six cysteine residues. In accordance with the sequence similarity, gigantoxin I exhibits EGF activity as evidenced by rounding of A431 cells and tyrosine phosphorylation of the EGF receptor in the cells, although much less potently than human EGF. Gigantoxin I is the first example of EGF-like toxins of natural origin.

The sea anemone Bunodosoma granulifera contains surprisingly efficacious and potent insect-selective toxins

Two sodium channel toxins, BgII and BgIII, isolated from the sea anemone Bunodosoma granulifera, have been subjected to an elaborate electrophysiological and pharmacological comparison between five different cloned sodium channels expressed in Xenopus laevis oocytes in order to determine their efficacy, potency and selectivity. Our results reveal large differences in toxin-induced effect between the different sodium channels. These toxins possess the highest efficacy for the insect sodium channel (para). Our data also show that BgII, generally known as a neurotoxin, is especially potent on the insect sodium channel with an EC(50) value of 5.5+/-0.5 nM. Therefore, this toxin can be used as a template for further development of new insecticides. Based on our findings, an evolutionary relationship between crustaceans and insects is also discussed.

Characterization of two Bunodosoma granulifera toxins active on cardiac sodium channels

1. Two sodium channel toxins, BgII and BgIII, have been isolated and purified from the sea anemone Bunodosoma granulifera. Combining different techniques, we have investigated the electrophysiological properties of these toxins. 2. We examined the effect of BgII and BgIII on rat ventricular strips. These toxins prolong action potentials with EC50 values of 60 and 660 nM and modify the resting potentials. 3. The effect on Na+ currents in rat cardiomyocytes was studied using the patch-clamp technique. BgII and BgIII slow the rapid inactivation process and increase the current density with EC50 values of 58 and 78 nM, respectively. 4. On the cloned hH1 cardiac Na+ channel expressed in Xenopus laevis oocytes, BgII and BgIII slow the inactivation process of Na+ currents (respective EC50 values of 0.38 and 7.8 microM), shift the steady-state activation and inactivation parameters to more positive potentials and the reversal potential to more negative potentials. 5. The amino acid sequences of these toxins are almost identical except for an asparagine at position 16 in BgII which is replaced by an aspartic acid in BgIII. In all experiments, BgII was more potent than BgIII suggesting that this conservative residue is important for the toxicity of sea anemone toxins. 6. We conclude that BgII and BgIII, generally known as neurotoxins, are also cardiotoxic and combine the classical effects of sea anemone Na+ channels toxins (slowing of inactivation kinetics, shift of steady-state activation and inactivation parameters) with a striking decrease on the ionic selectivity of Na+ channels.

AaIT: from neurotoxin to insecticide

AaIT is a single chain neurotoxic polypeptide derived from the venom of the Buthid scorpion Androctonus australis Hector, composed of 70 amino acids cross-linked by four disulfide bridges. Its strict selectivity for insects has been documented by toxicity, electrophysiological and ligand receptor binding assays. These last have shown that various insect neuronal membranes possess a single class of non-interacting AaIT binding sites of high affinity (K(D) = 1-3(n)M) and low capacity (0.5-2.0 pmol/mg prot.). The fast excitatory paralysis induced by AaIT is a result of a presynaptic effect, namely the induction of a repetitive firing in the terminal branches of the insect's motor nerves resulting in a massive and uncoordinated stimulation of the respective skeletal muscles. The neuronal repetitive activity is attributed to an exclusive and specific perturbation of sodium conductance as a consequence of toxin binding to external loops of the insect voltage-dependent sodium channel and modification of its gating mechanism. From a strictly agrotechnical point of view AaIT involvement in plant protection has taken the following two complementary forms: firstly, as a factor for the genetic engineering of insect infective baculoviruses resulting in potent and selective bio-insecticides. The efficacy of the AaIT-expressing, recombinant baculovirus is attributed mainly to its ability to continuously provide and translocate the gene of the expressed toxin to the insect central nervous system; secondly, based on the pharmacological flexibility of the voltage-gated sodium channel, as a device for insecticide resistance management. Channel mutations conferring resistance to a given class of insecticidal agents (such as the KDR phenomenon) may greatly increase susceptibility to the AaIT expressing bioinsecticides. Thus the AaIT is a pharmacological tool for the study of insect neuronal excitability and chemical ecology and the development of new approaches to insect control.

Regulation of Ca2+ homeostasis by atypical Na+ currents in cultured human coronary myocytes

Primary cultured human coronary myocytes (HCMs) derived from ischemic human hearts express an atypical voltage-gated tetrodotoxin (TTX)-sensitive sodium current (I(Na)). The whole-cell patch-clamp technique was used to study the properties of I(Na) in HCMs. The variations of intracellular calcium ([Ca2+]i) and sodium ([Na+]i) were monitored in non-voltage-clamped cells loaded with Fura-2 or benzofuran isophthalate, respectively, using microspectrofluorimetry. The activation and steady-state inactivation properties of I(Na) determined a "window" current between -50 and -10 mV suggestive of a steady-state Na+ influx at the cell resting membrane potential. Consistent with this hypothesis, the resting [Na+]i was decreased by TTX (1 micromol/L). In contrast, it was increased by Na+ channel agonists that also promoted a large rise in [Ca2+]i. Veratridine (10 micromol/L), toxin V from Anemonia sulcata (0.1 micromol/L), and N-bromoacetamide (300 micromol/L) increased [Ca2+]i by 7- to 15-fold. This increase was prevented by prior application of TTX or lidocaine (10 micromol/L) and by the use of Na(+)-free or Ca(2+)-free external solutions. The Ca(2+)-channel antagonist nicardipine (5 micromol/L) blocked the effect of veratridine on [Ca2+]i only partially. The residual component disappeared when external Na+ was replaced by Li+ known to block the Na+/Ca2+ exchanger. The resting [Ca2+]i was decreased by TTX in some cells. In conclusion, I(Na) regulates [Ca2+]i in primary cultured HCMs. This regulation, effective at baseline, involves a tonic control of Ca2+ influx via depolarization-gated Ca2+ channels and, to a lesser extent, via a Na+/Ca2+ exchanger working in the reverse mode.

Partial sequence and toxic effects of granulitoxin, a neurotoxic peptide from the sea anemone Bundosoma granulifera

A neurotoxic peptide, granulitoxin (GRX), was isolated from the sea anemone Bunodosoma granulifera. The N-terminal amino acid sequence of GRX is AKTGILDSDGPTVAGNSLSGT and its molecular mass is 4958 Da by electrospray mass spectrometry. This sequence presents a partial degree of homology with other toxins from sea anemones such as Bunodosoma caissarum, Anthopleura fuscoviridis and Anemonia sulcata. However, important differences were found: the first six amino acids of the sequence are different, Arg-14 was replaced by Ala and no cysteine residues were present in the partial sequence, while two cysteine residues were present in the first 21 amino acids of the other toxins described above. Purified GRX injected i.p. (800 micrograms/kg) into mice produced severe neurotoxic effects such as circular movements, aggressive behavior, dyspnea, tonic-clonic convulsion and death. The 2-h LD50 of GRX was 400 +/- 83 micrograms/kg.

125I-Labelled mapacalcine: a specific tool for a pharmacological approach to a receptor associated with a new calcium channel on mouse intestinal membranes

Mapacalcine is a small protein (Mr=19041) composed of two homologous chains purified from the marine sponge Cliona vastifica. Recently, we demonstrated that it was able to specifically block a Ca2+ channel which could not be related to already described channels on mouse intestinal myocytes. This Ca2+ current was insensitive to the known peptidic and organic calcium channel blockers. Mapacalcine was ineffective on T-type and L-type Ca2+ currents present on rat portal vein myocytes [Morel, Drobecq, Sautière, Tartar, Mironneau, Qar, Lavie, and Hugues (1997) Mol. Pharmacol. 51, 1042-1052]. We report here the preparation and purification of a monoiodo-derivative of mapa-calcine which retains its biological properties. Binding parameters of mapacalcine to its receptors have been characterized on mouse intestinal membranes. It binds to its receptors with a Kd=0. 8 nM, and a maximal binding capacity of 171 fmol/mg of protein on membrane preparations. Our data show that we have prepared a tool that is usable for pharmacological studies of a receptor associated with a new type of calcium channel for which no ligand was available until now.

Sea anemone peptides with a specific blocking activity against the fast inactivating potassium channel Kv3.4

Sea anemone venom is known to contain toxins that are active on voltage-sensitive Na+ channels, as well as on delayed rectifier K+ channels belonging to the Kv1 family. This report describes the properties of a new set of peptides from Anemonia sulcata that act as blockers of a specific member of the Kv3 potassium channel family. These toxins, blood depressing substance (BDS)-I and BDS-II, are 43 amino acids long and differ at only two positions. They share no sequence homologies with other K+ channel toxins from sea anemones, such as AsKS, AsKC, ShK, or BgK. In COS-transfected cells, the Kv3.4 current was inhibited in a reversible manner by BDS-I, with an IC50 value of 47 nM. This inhibition is specific because BDS-I failed to block other K+ channels in the Kv1, Kv2, Kv3, and Kv4 subfamilies. Inward rectifier K+ channels are also insensitive to BDS-I. BDS-I and BDS-II share the same binding site on brain synaptic membranes, with K0.5 values of 12 and 19 nM, respectively. We observed that BDS-I and BDS-II have some sequence homologies with other sea anemone Na+ channels toxins, such as AsI, AsII, and AxI. However, they had a weak effect on tetrodotoxin-sensitive Na+ channels in neuroblastoma cells and no effect on Na+ channels in cardiac and skeletal muscle cells. BDS-I and BDS-II are the first specific blockers identified so far for the rapidly inactivating Kv3.4 channel.

A specific interaction between the cardiac sodium channel and site-3 toxin anthopleurin B

The polypeptide neurotoxin anthopleurin B (ApB) isolated from the venom of the sea anemone Anthopleura xanthogrammica is one of a family of toxins that bind to the extracellular face of voltage-dependent sodium channels and retard channel inactivation. Because most regions of the sodium channel known to contribute to inactivation are located intracellularly or within the membrane bilayer, identification of the toxin/channel binding site is of obvious interest. Recently, mutation of a glutamic acid residue on the extracellular face of the fourth domain of the rat neuronal sodium channel (rBr2a) was shown to disrupt toxin/channel binding (Rogers, J. C., Qu, Y. S., Tanada, T. N., Scheuer, T., and Catterall, W. A. (1996) J. Biol. Chem. 271, 15950-15962). A negative charge at this position is highly conserved between mammalian sodium channel isoforms. We have constructed mutations of the corresponding residue (Asp-1612) in the rat cardiac channel isoform (rH1) and shown that the lowered affinity occurs primarily through an increase in the toxin/channel dissociation rate koff. Further, we have used thermodynamic mutant cycle analysis to demonstrate a specific interaction between this anionic amino acid and Lys-37 of ApB (DeltaDeltaG = 1.5 kcal/mol), a residue that is conserved among many sea anemone toxins. Reversal of the charge at Asp-1612, as in the mutant D1612R, also affects channel inactivation independent of toxin (-14 mV shift in channel availability). Binding of the toxin to Asp-1612 may therefore contribute both to toxin/channel affinity and to transduction of the effects of the toxin on channel kinetics.

Identification and characterization of novel sodium channel toxins from the sea anemone Anthopleura xanthogrammica

Six new toxins from the sea anemone Anthopleura xanthogrammica were identified using a molecular biological approach. Five of these novel isoforms resemble the 47 residue type I long polypeptides native to Anthopleura elegantissima, Anthopleura fuscoviridis and Anemonia sulcata, while one appears to be chimera of the two previously identified 49 residue toxins native to A. xanthogrammica. Four of these toxins were expressed in bacteria, purified and characterized by ion flux assays in RT4-B and N1E-115 cell lines expressing the cardiac and neuronal Na channel isoforms, respectively. The novel 47 residue toxin isoforms form a new subclass within the A. xanthogrammica neurotoxin family, although they are related to previously described anemone toxins. One of the three 47 residue toxins characterized, PCR2-10, enhances veratridine-dependent sodium uptake, displaying a K0.5 of 329 nM and 1354 nM in RT4-B and N1E-115 cell lines, respectively. The novel 49 residue toxin, PCR3-7, interacts with the sodium channel with even higher affinity, enhancing sodium uptake with a K0.5 of 47 nM and 108 nM in RT4-B and N1E-115 cells, respectively.

A simple biochemical method in the search for bioactive polypeptides in a sea anemone (Anemonia sulcata)

The sea anemone Anemonia sulcata is a well-known natural source of supply of biologically active polypeptides. So far, five toxins, ATX I, II, III, IV and AS V, several polyvalent protease inhibitors, an elastase inhibitor, two blood pressure-depressive polypeptides and very recently peptides that inhibit competitively the binding of 125I-dendrotoxin to rat brain membranes and block the voltage-sensitive K+ channels, have been isolated from it. The sea anemone toxins (especially toxin II of A. sulcata, ATX II) are very important tools in neurophysiological and pharmacological research, and their structure-function relationship has been investigated. Because of the great scientific value of the sea anemone toxins a simplification of their purification procedure was elaborated.

The role of exposed tryptophan residues in the activity of the cardiotonic polypeptide anthopleurin B

Scorpion and sea anemone venoms contain several polypeptides that delay inactivation of voltage-sensitive sodium channels via interaction with a common site. In this report, we target exposed hydrophobic residues at positions 33 and 45 of anthopleurin B (ApB) by polymerase chain reaction mutagenesis to ascertain their contribution to toxin activity. Nonconservative replacements are not permitted at position 33, indicating that Trp-33 may play an important structural role. Strikingly, the relatively conservative substitution of Trp-33 by phenylalanine results in major reductions in binding affinity for both the cardiac and neuronal channel isoforms as measured by ion flux, whereas substitution with tyrosine is tolerated and exhibits near wild-type affinities, suggesting that either the ability to form a hydrogen bond or the amphiphilic nature of the side chain are important at this position. Electrophysiological analysis of W33F indicates that its diminished affinity is primarily due to a decreased association rate. Analysis of a panel of mutants at Trp-45 shows only modest changes in apparent binding affinity for both channel isoforms but significant effects on Vmax. In neuronal channels, the maximal levels of uptake for W45A/S/F are about 50% those seen with ApB. This effect is also observed for W45A and W45S in the cardiac model, wherein W45F is normal. These results suggest that a hydrophobic contact is involved in toxin-induced stabilization of the open conformation of the cardiac sodium channel. We conclude that Trp-33 contributes significantly to apparent affinity, whereas Trp-45 does not appear to affect binding per se. Furthermore, W33F is the first ApB mutant that displays a significantly altered association rate and may prove to be a useful probe of the channel binding site.

Resistance and vulnerability of crustaceans to cytolytic sea anemone toxins

Crustaceans (Mithraculus, Neopetrolisthes, Periclimenes, Stenorhynchus sp.) living in association with sea anemones, shore crabs (Metopograpsus oceanicus) and brine shrimps (Artemia salina) were found to be resistent to the exposure of cytolytic sea anemone toxins (up to 100 micrograms/ml) and to other membrane-active compounds such as gramicidin A and saponin. The gill filaments of the crustaceans were not affected, indicating that the chitin layer protects the epithelium from the action of the cytolytic toxins. However, crustaceans are highly susceptible to sea anemone toxins when injected parenterally.

Preparation and characterization of a biologically active spin-labeled sea anemone toxin

A derivative of the polypeptide cardiostimulant anthopleurin-B(AP-B) labeled with the spin label 1-oxyl 2,2,6,6-tetramethyl-4-piperidinyloxycarbonyl azide has been prepared and characterized. The product was found by mass spectrometry to be labeled at a single site, which amino acid sequencing showed to be the N-terminus. It also retained positive inotropic activity when assayed on isolated guinea pig atria. The spin-labeled (SL) product was found to exist in two distinct conformations by reversed-phase HPLC and in at least two conformations by electron spin resonance spectroscopy (ESR) over the pH range 2-9. The ESR data also show evidence for multimetric states of SL-AP-B over the pH range 2-9, with maximum aggregation at pH 4.5-5, and a slow disaggregation when the pH is adjusted to 8-9. The presence of multiple conformers of SL-AP-B and its tendency to aggregate render it unsuitable for high-resolution NMR structural studies of the isolated ligand, but the retention of activity may make it useful for studies of the sodium-channel-bound form of the molecule.

Kalicludines and kaliseptine. Two different classes of sea anemone toxins for voltage sensitive K+ channels

New peptides have been isolated from the sea anemone Anemonia sulcata which inhibit competitively the binding of 125I-dendrotoxin I (a classical ligand for K+ channel) to rat brain membranes and behave as blockers of voltage-sensitive K+ channels. Sea anemone kalicludines are 58-59-amino acid peptides cross-linked with three disulfide bridges. They are structurally homologous both to dendrotoxins which are snake venom toxins and to the basic pancreatic trypsin inhibitor (Kunitz inhibitor) and have the unique property of expressing both the function of dendrotoxins in blocking voltage-sensitive K+ channels and the function of the Kunitz inhibitor in inhibiting trypsin. Kaliseptine is another structural class of peptide comprising 36 amino acids with no sequence homology with kalicludines or with dendrotoxins. In spite of this structural difference, it binds to the same receptor site as dendrotoxin and kalicludines and is as efficient as a K+ channel inhibitor as the most potent kalicludine.

Solution structure of the cardiostimulant polypeptide anthopleurin-B and comparison with anthopleurin-A

BACKGROUND

The polypeptide anthopleurin-B (AP-B) is one of a number of related toxins produced by sea anemones. AP-B delays inactivation of the voltage-gated sodium channel of excitable tissue. In the mammalian heart, this effect is manifest as an increase in the force of contraction. As a result, there is interest in exploiting the anthopleurins as lead compounds in the design of novel cardiac stimulants. Essential to this endeavour is a high-resolution solution structure of the molecule describing the positions of functionally important side chains.

RESULTS

AP-B exists in multiple conformations in solution as a result of cis-trans isomerization about the Gly40-Pro41 peptide bond. The solution structure of the major conformer of AP-B has been determined by two-dimensional 1H NMR at pH 4.5 and 25 degrees C. The core structure is a four-stranded, antiparallel beta-sheet (residues 2-4, 20-23, 34-37 and 45-48) and includes several beta-turns (6-9, 25-28, 30-33). Three loops connect the beta-strands, the longest and least well defined being the first loop, extending from residues 8-17. These features are shared by other members of this family of sea anemone toxins. The locations of a number of side chains which are important for the cardiac stimulatory activity of AP-B are well defined in the structures.

CONCLUSIONS

We have described the solution structure of AP-B and compared it with that of AP-A, from which it differs by substitutions at seven amino acid positions. It shares an essentially identical fold with AP-A yet is about 10-fold more active. Comparison of the structures, particularly in the region of residues essential for activity, gives a clearer indication of the location and extent of the cardioactive pharmacophore in these polypeptides.

Chemical modification of cationic groups in the polypeptide cardiac stimulant anthopleurin-A

Chemical modification studies have been carried out on the sea anemone polypeptide anthopleurin-A in order to clarify the role of Arg-14 in its cardiac stimulatory activity. Reaction with 1,2-cyclohexanedione at 37 degrees C produced a range of protein products, including some with amino group modifications. These side-reactions were eliminated by prior citraconylation of the amino groups, which, following reaction with cyclohexanedione, could be reversed under conditions which preserved the cyclohexanedione adduct. Citraconylation of the three amino groups, one from the N-terminus and two from Lys-37 and Lys-48, destroyed the cardiac stimulatory activity of the molecule, but this was fully recoverable upon reversal of this reaction. It appears that one or more of the amino groups is essential for activity. Anthopleurin-A contains only one arginine residue, and this was confirmed as the site of modification by cyclohexanedione by showing that the product was refractory to proteolysis by trypsin, which normally cleaves the molecule at this residue. The positive inotropic activity of the cyclohexanedione adduct on isolated guinea-pig atria was identical to that of unmodified anthopleurin-A, indicating that the side-chain of Arg-14 is not required for cardiotonic activity.

Actions of three structurally distinct sea anemone toxins on crustacean and insect sodium channels

The membrane actions of three recently isolated polypeptide neurotoxins from the sea anemones Stichodactyla helianthus (toxin ShI), Condylactis gigantea (toxin CgII) and Calliactis parasitica (toxin CpI) were investigated on action potentials and voltage-clamp membrane currents of the giant axon of the crayfish Procambarus clarkii. The first two toxins were also tested on the cockroach (Periplaneta americana) giant axon. All three toxins were particularly lethal to crustaceans, moderately toxic to an insect (cockroach), and essentially non-toxic to a mammal (mouse). ShI and CgII were 50- to 100-fold more potent on crayfish than on cockroach axons; this difference in activity was correlated with the relative reversibility of their effects on these arthropod axons. The crustacean selectivity of these toxins is therefore due largely to their greater affinity for crustacean sodium channels. All three toxins prolonged crayfish giant axon action potentials by selectively slowing Na channel inactivation without greatly affecting activation. Before toxin treatment, inactivation was nearly exponential, with a time constant less than 1 msec. After treatment, the inactivation time course could be described as the sum of two exponentially decaying components, plus a large steady-state component. The major component possessed the slower (10-20 msec) time constant. The steady-state component increased with depolarization, causing the sodium channel steady-state inactivation curve to reach a minimum between -60 and -20 mV and then increase at more positive potentials. All three toxins shifted the peak sodium current-voltage relation to the left. This voltage shift was greater at 20 degrees C than at 10 degrees C. Maintained membrane depolarization during toxin wash-in delayed the appearance of modified Na channels. Also, prolonged depolarization of toxin-treated axons converted modified sodium channels back to normal ones. The toxins did not affect potassium and leakage currents. Our results indicate that the three crustacean-active sea anemone toxins share a common electrophysiological action on arthropod sodium channels, at least at the macroscopic level.

Anti-muscarinic activity of a family of C11N5 compounds isolated from Agelas sponges

In a search for potential target sites for C11N5 compounds obtained from marine sponges of the genus Agelas we evaluated their interaction with muscarinic acetylcholine receptors from rat brain membranes. In competition experiments with 3H-QNB these compounds displayed the following rank order of potency: sceptrin greater than oroidin greater than or equal to dibromosceptrin greater than or equal to clathrodin. Sceptrin (50 microM) was shown to be a competitive inhibitor of 3H-QNB binding as revealed by Scatchard analysis. The results demonstrate the ability of these compounds to interact with multiple target molecules in the micromolar range.

Linear and cyclic peptide analogues of the polypeptide cardiac stimulant, anthopleurin-A. 1H-NMR and biological activity studies

A loop corresponding to residues 8-17 in the polypeptide cardiac stimulant anthopleurin-A is known to be important for the cardiostimulant activity of this molecule. To investigate the activity and possible conformations of this loop in isolation, two synthetic peptides have been studied. The first corresponds to residues 6-20 of anthopleurin-A with Cys6 replaced by Thr, and the second to residues 6-21 of anthopleurin-A, with Thr21 replaced by Cys. The introduction of an additional cysteine in the latter peptide enabled an intramolecular disulfide to be formed between the N- and C-terminal residues. Both linear peptides and the disulfide-containing analogue lack the cardiostimulant and Na(+-)-channel binding activity in the parent molecule, anthopleurin-A, indicating that although the loop is important for the function of anthopleurin-A, other regions of the molecule must also be involved in activity. Assignments of the 1H-NMR spectra of both peptides are presented, and their pH and temperature dependences investigated. The results show that the amide protons of Gly5 and Asn11 (corresponding to Gly10 and Asn16 in anthopleurin-A) sample hydrogen-bonded conformations in solution. Based on these NMR data, two regions of non-random structure, encompassing residues 2-5 and 8-11, respectively, are proposed, and the possible involvement of such structures in the activity of anthopleurin-A is discussed.