Synthesis-Driven Stereochemical Assignment of Marine Polycyclic Ether Natural Products

Marine polycyclic ether natural products have gained significant interest from the chemical community due to their impressively huge molecular architecture and diverse biological functions. The structure assignment of this class of extraordinarily complex natural products has mainly relied on NMR spectroscopic analysis. However, NMR spectroscopic analysis has its own limitations, including configurational assignment of stereogenic centers within conformationally flexible systems. Chemical shift deviation analysis of synthetic model compounds is a reliable means to assign the relative configuration of "difficult" stereogenic centers. The complete configurational assignment must be ultimately established through total synthesis. The aim of this review is to summarize the indispensable role of organic synthesis in stereochemical assignment of marine polycyclic ethers.

Rapid detection of ciguatoxins in Gambierdiscus and Fukuyoa with immunosensing tools

Consumption of seafood contaminated with ciguatoxins (CTXs) leads to a foodborne disease known as ciguatera. Primary producers of CTXs are epibenthic dinoflagellates of the genera Gambierdiscus and Fukuyoa. In this study, thirteen Gambierdiscus and Fukuyoa strains were cultured, harvested at exponential phase, and CTXs were extracted with an implemented rapid protocol. Microalgal extracts were obtained from pellets with a low cell abundance (20,000 cell/mL) and were then analyzed with magnetic bead (MB)-based immunosensing tools (colorimetric immunoassay and electrochemical immunosensor). It is the first time that these approaches are used to screen Gambierdiscus and Fukuyoa strains, providing not only a global indication of the presence of CTXs, but also the ability to discriminate between two series of congeners (CTX1B and CTX3C). Analysis of the microalgal extracts revealed the presence of CTXs in 11 out of 13 strains and provided new information about Gambierdiscus and Fukuyoa toxin profiles. The use of immunosensing tools in the analysis of microalgal extracts facilitates the elucidation of further knowledge regarding these dinoflagellate genera and can contribute to improved ciguatera risk assessment and management.

Toxicology of Gambierdiscus spp. (Dinophyceae) from Tropical and Temperate Australian Waters

Ciguatera Fish Poisoning (CFP) is a human illness caused by the consumption of marine fish contaminated with ciguatoxins (CTX) and possibly maitotoxins (MTX), produced by species from the benthic dinoflagellate genus Gambierdiscus. Here, we describe the identity and toxicology of Gambierdiscus spp. isolated from the tropical and temperate waters of eastern Australia. Based on newly cultured strains, we found that four Gambierdiscus species were present at the tropical location, including G. carpenteri, G. lapillus and two others which were not genetically identical to other currently described species within the genus, and may represent new species. Only G. carpenteri was identified from the temperate location. Using LC-MS/MS analysis we did not find any characterized microalgal CTXs (P-CTX-3B, P-CTX-3C, P-CTX-4A and P-CTX-4B) or MTX-1; however, putative maitotoxin-3 (MTX-3) was detected in all species except for the temperate population of G. carpenteri. Using the Ca²⁺ influx SH-SY5Y cell Fluorescent Imaging Plate Reader (FLIPR) bioassay we found CTX-like activity in extracts of the unidentified Gambierdiscus strains and trace level activity in strains of G. lapillus. While no detectable CTX-like activity was observed in tropical or temperate strains of G. carpenteri, all species showed strong maitotoxin-like activity. This study, which represents the most comprehensive analyses of the toxicology of Gambierdiscus strains isolated from Australia to date, suggests that CFP in this region may be caused by currently undescribed ciguatoxins and maitotoxins.

Application of solid phase adsorption toxin tracking (SPATT) devices for the field detection of Gambierdiscus toxins

Ciguatera fish poisoning is a food-borne illness caused by the consumption of seafood contaminated with ciguatoxins (CTXs) produced by dinoflagellates in the Gambierdiscus genus. Since most surveillance programs currently rely on the survey of Gambierdiscus cell densities and species composition, supplementary toxin-based methods allowing the time- and spatially integrated sampling of toxins in ciguateric environments are needed for a more reliable assessment and management of the risks associated with Gambierdiscus proliferation. Solid Phase Adsorption Toxin Tracking (SPATT) filters use porous synthetic resins capable of adsorbing toxins directly from the water column. To assess the ability of these passive monitoring devices to retain Gambierdiscus toxins, SPATT bags filled with 10g of HP20 resin were deployed for 48h in two French Polynesian locations at high (Nuku Hiva Island) vs. low to moderate (Kaukura Atoll) risk of ciguatera. CTXs could be detected in SPATT bags extracts from Nuku Hiva Island, as assessed by the mouse neuroblastoma cell-based assay (CBA-N2a) and liquid chromatography - tandem mass spectrometry (LC-MS/MS) analyses. Results of in vitro experiments suggest that the saturation limit of CTXs on HP20 resin, for a deployment time of 48h, is ≃ 55ng P-CTX-3C equiv. g^-1 resin. Despite the non detection of maitotoxin (MTX), LC-MS/MS analyses showed that two other compounds also produced by Gambierdiscus species were retained on SPATT bags, i.e. iso-P-CTX-3B/C and a putative MTX analogue, known as MTX-3. This study, the first to demonstrate the suitability of SPATT technology for the in situ monitoring of Gambierdiscus toxins, highlights the potential application of this tool for routine ciguatera risk assessment and management programs.

Ciguatoxins and Maitotoxins in Extracts of Sixteen Gambierdiscus Isolates and One Fukuyoa Isolate from the South Pacific and Their Toxicity to Mice by Intraperitoneal and Oral Administration

Ciguatoxins (CTXs), and possibly maitotoxins (MTXs), are responsible for Ciguatera Fish Poisoning, an important health problem for consumers of reef fish (such as inhabitants of islands in the South Pacific Ocean). The habitational range of the Gambierdiscus species is expanding, and new species are being discovered. In order to provide information on the potential health risk of the Gambierdiscus species, and one Fukuyoa species (found in the Cook Islands, the Kermadec Islands, mainland New Zealand, and New South Wales, Australia), 17 microalgae isolates were collected from these areas. Unialgal cultures were grown and extracts of the culture isolates were analysed for CTXs and MTXs by liquid chromatography tandem mass spectrometry (LC-MS/MS), and their toxicity to mice was determined by intraperitoneal and oral administration. An isolate of G. carpenteri contained neither CTXs nor MTXs, while 15 other isolates (including G. australes, G. cheloniae, G. pacificus, G.honu, and F. paulensis) contained only MTX-1 and/or MTX-3. An isolate of G. polynesiensis contained both CTXs and MTX-3. All the extracts were toxic to mice by intraperitoneal injection, but those containing only MTX-1 and/or -3 were much less toxic by oral administration. The extract of G. polynesiensis was highly toxic by both routes of administration.

Rapid Extraction and Identification of Maitotoxin and Ciguatoxin-Like Toxins from Caribbean and Pacific Gambierdiscus Using a New Functional Bioassay

Background

Ciguatera is a circumtropical disease produced by polyether sodium channel toxins (ciguatoxins) that enter the marine food chain and accumulate in otherwise edible fish. Ciguatoxins, as well as potent water-soluble polyethers known as maitotoxins, are produced by certain dinoflagellate species in the genus Gambierdiscus and Fukuyoa spp. in the Pacific but little is known of the potential of related Caribbean species to produce these toxins.

Methods

We established a simplified procedure for extracting polyether toxins from Gambierdiscus and Fukuyoa spp. based on the ciguatoxin rapid extraction method (CREM). Fractionated extracts from identified Pacific and Caribbean isolates were analysed using a functional bioassay that recorded intracellular calcium changes (Ca²⁺) in response to sample addition in SH-SY5Y cells. Maitotoxin directly elevated Ca²⁺_i, while low levels of ciguatoxin-like toxins were detected using veratridine to enhance responses.

Results

We identified significant maitotoxin production in 11 of 12 isolates analysed, with 6 of 12 producing at least two forms of maitotoxin. In contrast, only 2 Caribbean isolates produced detectable levels of ciguatoxin-like activity despite a detection limit of >30 pM. Significant strain-dependent differences in the levels and types of ciguatoxins and maitotoxins produced by the same Gambierdiscus spp. were also identified.

Conclusions

The ability to rapidly identify polyether toxins produced by Gambierdiscus spp. in culture has the potential to distinguish ciguatoxin-producing species prior to large-scale culture and in naturally occurring blooms of Gambierdiscus and Fukuyoa spp. Our results have implications for the evaluation of ciguatera risk associated with Gambierdiscus and related species.

Fluorescent Receptor Binding Assay for Detecting Ciguatoxins in Fish

Ciguatera fish poisoning is an illness suffered by > 50,000 people yearly after consumption of fish containing ciguatoxins (CTXs). One of the current methodologies to detect ciguatoxins in fish is a radiolabeled receptor binding assay (RBA_(R)). However, the license requirements and regulations pertaining to radioisotope utilization can limit the applicability of the RBA_(R) in certain labs. A fluorescence based receptor binding assay (RBA_(F)) was developed to provide an alternative method of screening fish samples for CTXs in facilities not certified to use radioisotopes. The new assay is based on competition binding between CTXs and fluorescently labeled brevetoxin-2 (BODIPY^®- PbTx-2) for voltage-gated sodium channel receptors at site 5 instead of a radiolabeled brevetoxin. Responses were linear in fish tissues spiked from 0.1 to 1.0 ppb with Pacific ciguatoxin-3C (P-CTX-3C) with a detection limit of 0.075 ppb. Carribean ciguatoxins were confirmed in Caribbean fish by LC-MS/MS analysis of the regional biomarker (C-CTX-1). Fish (N = 61) of six different species were screened using the RBA_(F). Results for corresponding samples analyzed using the neuroblastoma cell-based assay (CBA-N2a) correlated well (R² = 0.71) with those of the RBA_(F), given the low levels of CTX present in positive fish. Data analyses also showed the resulting toxicity levels of P-CTX-3C equivalents determined by CBA-N2a were consistently lower than the RBA_(F) affinities expressed as % binding equivalents, indicating that a given amount of toxin bound to the site 5 receptors translates into corresponding lower cytotoxicity. Consequently, the RBA_(F), which takes approximately two hours to perform, provides a generous estimate relative to the widely used CBA-N2a which requires 2.5 days to complete. Other RBA_(F) advantages include the long-term (> 5 years) stability of the BODIPY^®- PbTx-2 and having similar results as the commonly used RBA_(R). The RBA_(F) is cost-effective, allows high sample throughput, and is well-suited for routine CTX monitoring programs.

Clinical diagnosis and chemical confirmation of ciguatera fish poisoning in New South Wales, Australia

Ciguatera fish poisoning is common in tropical and sub-tropical areas and larger fish (> 10 kg) are more susceptible to toxin accumulation with age. Although the coastal climate of northern New South Wales is considered sub-tropical, prior to 2014 there has only been 1 documented outbreak of ciguatera fish poisoning from fish caught in the region. During February and March 2014, 2 outbreaks of ciguatera fish poisoning involved 4 and 9 individuals, respectively, both following consumption of Spanish mackerel from northern New South Wales coastal waters (Evans Head and Scotts Head). Affected individuals suffered a combination of gastrointestinal and neurological symptoms requiring hospital treatment. At least 1 individual was symptomatic up to 7 months later. Liquid chromatography-tandem mass spectrometry detected the compound Pacific ciguatoxin-1B at levels up to 1.0 µg kg(-1) in fish tissue from both outbreaks. During April 2015, another outbreak of ciguatera fish poisoning was reported in 4 individuals. The fish implicated in the outbreak was caught further south than the 2014 outbreaks (South West Rocks). Fish tissue was unavailable for analysis; however, symptoms were consistent with ciguatera fish poisoning. To our knowledge, these cases are the southernmost confirmed sources of ciguatera fish poisoning in Australia. Educational outreach to the fishing community, in particular recreational fishers was undertaken after the Evans Head outbreak. This highlighted the outbreak, species of fish involved and the range of symptoms associated with ciguatera fish poisoning. Further assessment of the potential for ciguatoxins to occur in previously unaffected locations need to be considered in terms of food safety.

Detection of ciguatoxin-like and paralysing toxins in Trichodesmium spp. from New Caledonia lagoon

Marine pelagic cyanobacteria Trichodesmium are widespread in the New Caledonia lagoon. Blooms of these Oscillatoriales are suspected to be a potential source of toxins in the ciguatera food chain and were previously reported to contain certain types of paralysing toxins. In the present study, toxicity experiments were conducted on lipid- and water-soluble extracts of freeze-dried samples of these cyanobacteria. Lipid-soluble fractions revealed a ciguatoxin-like activity in both in vivo (mouse bioassay) and in vitro (mouse neuroblastoma cells assay and receptor binding assay using tritiated brevetoxin-3) assays. The water-soluble fractions tested on mice exhibited neurotoxicity with paralytic symptoms. These toxicities have also been observed with benthic filamentous cyanobacteria within the Oscillatoriales order, also collected in New Caledonia. This study provides an unprecedented evidence of the toxicity of Trichodesmium species from the New Caledonia lagoon. This survey also demonstrates the possible role of these cyanobacteria in ciguatera fish poisoning.

The continuing saga of the marine polyether biotoxins

The unprecedented structure of the marine natural product brevetoxin B was elucidated by the research group of Nakanishi and Clardy in 1981. The ladderlike molecular architecture of this fused polyether molecule, its potent toxicity, and fascinating voltage-sensitive sodium channel based mechanism of action immediately captured the imagination of synthetic chemists. Synthetic endeavors resulted in numerous new methods and strategies for the construction of cyclic ethers, and culminated in several impressive total syntheses of this molecule and some of its equally challenging siblings. Of the marine polyethers, maitotoxin is not only the most complex and most toxic of the class, but is also the largest nonpolymeric natural product known to date. This Review begins with a brief history of the isolation of these biotoxins and highlights their biological properties and mechanism of action. Chemical syntheses are then described, with particular emphasis on new methods developed and applied to the total syntheses. The Review ends with a discussion of the, as yet unfinished, story of maitotoxin, and projects into the future of this area of research.

Recent advances in the synthesis of marine polycyclic ether natural products

For more than twenty years, chemists and biologists have been fascinated by the highly complex molecular architectures and the diverse and potent biological activities of marine polycyclic ether natural products. Given the scarce availability of these intriguing substances from natural sources, total chemical synthesis is the only way to obtain sufficient quantities for biological investigation. This review describes recent synthetic advances in the field of marine polycyclic ether natural products and their successful implementation in total synthesis endeavors.

Ciguatera risk assessment in two toxic sites of French Polynesia using the receptor-binding assay

Ciguatera Fish Poisoning (CFP) is a tropical syndrome well known in remote archipelagos where the population is still dependent on fish resources. In order to assess the ciguatera risk in two islands of French Polynesia, Tubuai (Australes) and Nuku Hiva (Marquesas), a study was carried out on both Gambierdiscus populations as well as on various fish species using the receptor-binding assay (RBA) to detect and quantify ciguatoxins. Relationship between RBA data and size or weight of fish was evaluated, and when only few individuals for a particular species were available the trophic level was used to help comparisons between studied areas. According to epidemiological data, toxic versus safe areas were explored and compared in both islands. In Tubuai Island, Gambierdiscus cells were surprisingly absent in the north area, considered as a toxic area, but almost 94% of fishes were classified as RBA+. In contrast, the south area, supposed to be safe, was evolving to be a risky area because of the presence of Gambierdiscus cells and 74% of fishes being RBA+. In Nuku Hiva Island, Gambierdiscus cells were present in the toxic areas, Anaho, Taiohae and Taipivei, with two toxic blooms in Anaho Bay, but none in Terre Déserte, the fishing area of this island. With RBA data, fishes were analyzed to be RBA+ at a high percentage in Anaho and Taiohae, higher than in Taipivei and Terre Déserte areas. In general, our findings were congruent with epidemiological data and the knowledge of local people only for risky fish species.

Optimization of ciguatoxin extraction method from blood for Pacific ciguatoxin (P-CTX-1)

Ciguatera diagnosis relies on clinical observations associated with a recent consumption of fish. Although needed, direct confirmation of exposure in subjects showing ciguatera disease symptoms is currently unavailable. We previously reported that ciguatoxins were measurable in the blood of mice exposed to extracts of Pacific ciguatoxins isolated from Gambierdiscus polynesiensis, and of Indian Ocean or Caribbean Sea ciguatoxins, isolated from fish. Although highly efficient for extracting spiked purified Caribbean-CTX-1, the methanolic extraction method previously described is found here to yield only 6% recovery of spiked Pacific-CTX-1 (P-CTX-1). We report in this short communication a substantially modified method for ciguatoxin extraction from both dried and fresh blood. With this method, toxin measurement is directly accomplished in acetonitrile deproteinated whole fresh blood or phosphate buffer solution (PBS) eluted dried blood using the N2A cell-based assay. Spike studies using increasing concentrations of purified ciguatoxins reveal linear (r2 above 0.87 for all toxins) and overall efficient toxin recoveries (62%, 96%, and 96% from fresh blood and 75%, 90%, and 74% from dried blood, for C-CTX-1, P-CTX-3C, and P-CTX-1, respectively). Comparative blood matrix analysis for P-CTX-1 recovery shows increased recovery of ciguatoxin activity from whole fresh blood than from dried blood, greater by 20% in P-CTX-1 spiked mice blood and by over 85% in P-CTX-1 exposed mouse blood. In conclusion, both Caribbean and Pacific ciguatoxins can be readily extracted from blood using this modified method; however, in the case of P-CTX-1 we find that fresh blood is optimal.

Tropical fish poisoning in temperate climates: food poisoning from ciguatera toxin presenting in Avonmouth

Ciguatera toxin causes a range of gastrointestinal, cardiovascular and neurological symptoms that occur within 1-6 h of ingesting fish with the toxin and can last for days, months or years. It is a well-recognized problem in the tropics. Avon Health Protection Team investigated food poisoning on a ship at Avonmouth, which was thought by the crew to be related to a white snapper fish from the Caribbean. The symptoms were initially thought to be scombroid fish poisoning but were consistent with ciguatera fish poisoning. Cases of fish poisoning from fish imported from the Caribbean and Pacific or travellers returning from tropical countries may be ciguatera fish poisoning, but mistakenly diagnosed as scombroid fish poisoning.

Identification of slow and fast-acting toxins in a highly ciguatoxic barracuda (Sphyraena barracuda) by HPLC/MS and radiolabelled ligand binding

A barracuda implicated in ciguatera fish poisoning in Guadeloupe was estimated to have an overall flesh toxicity of 15 MUg/g using mouse bioassay. A lipid soluble extract was separated into two toxic fractions, FrA and FrB, on a LH20 Sephadex column eluted with dichloromethane/methanol (1:1). When intraperitoneal injected into mice, FrA provoked symptoms characteristic of slow-acting ciguatoxins, whereas FrB produced symptoms indicative of fast-acting toxins (FAT). High performance liquid chromatography/mass spectrometry/radio-ligand binding (HPLC/MS/RLB) analysis confirmed the two fractions were distinct, because only a weak overlap of some compounds was observed. HPLC/MS/RLB analysis revealed C-CTX-1 as the potent toxin present in FrA, and two coeluting active compounds at m/z 809.43 and 857.42 in FrB, all displaying the characteristic pattern of ion formation for hydroxy-polyethers. Other C-CTX congeners and putative hydroxy-polyether-like compounds were detected in both fractions, however, the RLB found them inactive. C-CTX-1 accounted for > 90% of total toxicity in this barracuda and was confirmed to be a competitive inhibitor of brevetoxin binding to voltage-sensitive sodium channels (VSSCs) with a potency two-times lower than P-CTX-1. However, FAT active on VSSCs and < 900 Da were suspected to contribute to the overall toxicity.

Multiple ciguatoxins present in Indian Ocean reef fish

Optimised gradient reversed-phase high-performance liquid chromatography electrospray ionisation mass spectrometry (LC/MS) methods, in combination with a [3H]-brevetoxin binding assay (RLB), revealed multiple ciguatoxins in a partially purified extract of a highly toxic Lutjanus sebae (red emperor) from the Indian Ocean. Two major ciguatoxins of 1140.6 Da (I-CTX-1 and -2) and two minor ciguatoxins of 1156.6 Da (I-CTX-3 and -4) were identified. Accurate mass analysis revealed that I-CTX-1 and -2 and Caribbean C-CTX-1 had indistinguishable masses (1140.6316 Da, at 0.44 ppm resolution). Toxicity estimated from LC/MS/RLB responses indicated that I-CTX-1 and -2 were both approximately 60% the potency of Pacific ciguatoxin-1 (P-CTX-1). In contrast to ciguatoxins of the Pacific where the more oxidised ciguatoxins are more potent, I-CTX-3 and -4 were approximately 20% of P-CTX-1 potency. Interconversion in dilute acid or on storage, typical of spiroketal and hemiketal functionality found in P-CTXs and C-CTXs, respectively, was not observed to occur between I-CTX-1 and -2. The ratio of CTX-1 and -2 varied depending on the fish extract being analysed. These results suggest that I-CTX-1 and -2 may arise from separate dinoflagellate precursors that may be oxidatively biotransformed to I-CTX-3 and -4 in fish.

Characterisation of multiple Caribbean ciguatoxins and congeners in individual specimens of horse-eye jack (Caranx latus) by high-performance liquid chromatography/mass spectrometry

We studied the variation in toxin profiles of purified extracts of 10 individual specimens and two pools of ciguateric Caranx latus. High-performance liquid chromatography/mass spectrometry (HPLC/MS) identified in all individual samples at least seven Caribbean ciguatoxins (C-CTXs) comprising C-CTX-1 and its epimer C-CTX-2 ([M+H](+) m/z 1141.58), and five new C-CTX congeners with pseudo-molecular ions at m/z 1141.58, 1143.60, 1157.57, 1159.58, and 1127.57. In some samples, additional C-CTX isomers were detected with [M+H](+) ions at m/z 1141.58 (two), 1143.60 (one) and 1157.57 (two). The two low-toxic pools contained only four to six ciguatoxins. The comparison in relative proportions of four different mass classes ([M+H](+) at m/z 1141, 1143, 1157 and 1127) showed that the group at m/z 1157 increased (2-20%) with flesh toxicity. More than 80% of group m/z 1141 comprised C-CTX-1, C-CTX-2 and their isomer C-CTX-1a whose level in this group correlated with fish toxicity. Contrary to low-toxic fishes, high-risk specimens had C-CTX-1 levels <50% and were subjected to large losses of activity on purification indicating that unstable ciguatoxins were present. A possible conversion of C-CTX-1 into C-CTX-1a was identified when flesh was cooked, without changes in toxicity. In conclusion, HPLC/MS characterised 12 C-CTXs accumulated by C. latus at variable levels.

Isolation and characterisation of Indian Ocean ciguatoxin

We report the isolation and initial characterisation of Indian Ocean ciguatoxin (I-CTX) present in toxic lipid soluble extracts isolated from ciguateric fishes collected off the Republic of Mauritius in the Indian Ocean. Following i.p. injection of this extract, mice displayed symptoms that were similar, though not identical, to those produced by Pacific and Caribbean ciguatoxins (P-CTXs and C-CTXs). Using a radiolabelled brevetoxin (PbTx) binding assay and mouse bioassay guided fractionation, I-CTX was purified by Florisil, Sephadex LH-20 and TSK HW-40S chromatography with good recovery. Isolation to purity was not possible by preparative reversed phase high-performance liquid chromatography (HPLC) due to significant losses of toxicity. However, analytical reversed phase HPLC coupled to an electrospray mass spectrometry detector identified a [M + H](+) ion at m/z 1141.58 which co-eluted with activity that displaced [3H]-PbTx binding to rat brain. This mass corresponded to C-CTX-1, but the fragmentation pattern of I-CTX showed a different ratio of pseudo molecular and product ions. I-CTX was found to elute later than P-CTX-1 but was practically indistinguishable from C-CTX-1 on reversed phase HPLC, while the TSK HW-40S column chromatography differentiated I-CTX from the later eluting C-CTX-1. Taken together, these results indicate that I-CTX is a new ciguatoxin (CTX) responsible for ciguatera caused by reef fish in the Indian Ocean.

Isolation and characterisation of Caribbean ciguatoxins from the horse-eye jack (Caranx latus)

The toxins involved in ciguatera (fish poisoning) in the Caribbean Sea were isolated from Caranx latus, a pelagic fish often implicated in ciguatera in the Caribbean region, and purified by mouse bioassay directed fractionation. Five toxins were separated by reverse-phase high-performance liquid chromatography (HPLC). In order of increasing hydrophobicity, these toxins included a sleep-inducing fraction (< 1% of total toxicity), a major Caribbean ciguatoxin (C-CTX-1, 65% of toxicity), a minor Caribbean ciguatoxin (C-CTX-2, 13% of toxicity), a minor toxin (approximately 1% of toxicity) and a hydrophobic, fast-acting toxin (approximately 19% of toxicity). The i.p. injection into mice of each toxin induced signs typical of site-5 sodium channel activator toxins such as the Pacific ciguatoxins and brevetoxins. C-CTX-1 and C-CTX-2 were purified to homogeneity (LD50 = 3.6 and approximately 1 microgram/kg, respectively) and subjected to ion spray mass spectrometry. Both lost up to five H2O molecules and each had a [M+H]+ ion, m/z 1141.7, suggesting that C-CTX-1 and -2 are diastereomers that differ from the Pacific family of ciguatoxins. Turbo-assisted HPLC-mass spectrometry identified C-CTX-1, C-CTX-2 and three C-CTX-1-related compounds in an enriched fraction but no Pacific ciguatoxins were detected. The presence of different families of ciguatoxins in ciguateric fish from the Caribbean Sea and Pacific Ocean probably underlies the clinical differences in the ciguatera syndrome reported in these two regions. A Caribbean strain of the benthic dinoflagellate, Gambierdiscus toxicus, is suspected as source of these ciguatoxins. The extent to which these toxins are biotransformed as they pass through the marine food chain remains to be determined.

Characterization of ciguatoxins and ciguatoxin congeners present in ciguateric fish by gradient reverse-phase high-performance liquid chromatography/mass spectrometry

Reverse-phase high-performance liquid chromatography/mass spectrometry (HPLC/MS) was used to identify Pacific ciguatoxins (P-CTX) and P-CTX congeners present in a highly purified extract from the viscera of ciguateric moray eels (Lycodontis javanicus) collected in the central Pacific Ocean. Fourteen P-CTX or P-CTX congeners were identified with protonated molecular ions [M + H]+ m/z 1095.7 (two), 1111.6 (six) or 1127.7 (six), including dominant ions for P-CTX-1, -2 and -3. In addition to the protonated species, each of these ciguatoxins gave rise to prominent [M + NH4]+ and [M + Na]+ ions. The 11 new P-CTX congeners, not readily detected by mouse bioassay, were present in trace amounts (2-13% of P-CTX-2 levels) and identified as several oxidized P-CTX-1, -2 and -3, and a possible diasteriomer of P-CTX-1. Acetonitrile-water gradients buffered with 1 mM ammonium acetate improved the separation and detection of the minor ciguatoxins compared with an acetonitrile-water gradient modified with 0.1% TFA. Turbo-assisted HPLC/MS had sufficient sensitivity to detect P-CTX-1 in a crude extract from the flesh of an Australian ciguateric fish. Compounds with masses equivalent to other isolated ciguatoxins, including Caribbean-CTX-1, gambiertoxin-4A and P-CTX-3C, were not detected in these samples. HPLC/MS can readily identify multiple ciguatoxins accumulated by fish and has the potential to be used as a confirmatory analytical method for characterizing the low levels of ciguatoxins contaminating ciguateric fish.

Detection of sodium channel toxins: directed cytotoxicity assays of purified ciguatoxins, brevetoxins, saxitoxins, and seafood extracts

Neuroblastoma cells in culture were used to detect sodium channel-specific marine toxins based on an end-point determination of mitochondrial dehydrogenase activity. The assay responds in a dose-dependent manner to ciguatoxins, brevetoxins, and saxitoxins, and delineates the toxic activity as either sodium channel enhancing or sodium channel blocking. The assay responds rapidly to sodium channel activating toxins, allowing dose dependent detection in 4 to 6 h. Brevetoxins can be detected at 250 pg, and purified ciguatoxins are detected in the low picogram and subpicogram levels. The results obtained from cell bioassay of ciguatoxic finfish extracts correlates with those obtained from mouse bioassays. Sodium channel blocking toxins can also be detected with an approximate sensitivity of 20 pg in 24 to 48 h. This cell-based technique is simple, sensitive, demonstrates potential as an alternative to animal testing for sodium channel activating and blocking toxins, and can be automated.

Documented case of ciguatera on the Mexican Pacific coast

Serranidae and Labridae fish caught in 1993 at Alijos Rocks, 300 miles off East Magdalena Bay, Southern Baja California, caused severe illness of a fishing boat crew. The described symptoms resembled those of ciguatera. The presence of ciguatera-like toxins was confirmed on extracts from these fish using the mouse bioassay procedure, showing activities between 220 to 390 mouse units (M.U.). The founding of ciguatoxin at Alijos Rocks 24 degrees 57' N, 115 degrees 45' W) extends to the northeast its geographical distribution in the Pacific and locates the outbreak near the continental coastline.

Ionspray mass spectrometry of ciguatoxin-1, maitotoxin-2 and -3, and related marine polyether toxins

A range of marine polyether toxins from dinoflagellates were analysed by ionspray mass spectrometry. Ciguatoxin-1 ([M+H]+ m/z = 1,111.8) purified from several fish species yielded singly charged ions corresponding to the parent ion, sodium and H2O adducts and ions for the loss of up to five H2O molecules. Ciguatoxin-1 was detected to 1 ng; however, interference from fish lipids precluded direct detection of ciguatoxin-1 in crude extracts from fish flesh spiked with ciguatoxin-1 at a level equivalent to 1.5 ng ciguatoxin-1/g of extracted flesh. Maitotoxin-2 yielded doubly and triply charged ions for sodium and potassium salts and likely possessed only one sulphate ester (M(r) = 3,298 for the mono-sodium salt). Maitotoxin-3, a recently isolated small maitotoxin, yielded singly charged ions including ions for the loss of one sulphate and up to four H2O molecules. Maitotoxin-3 is proposed to be a polyether compound possessing two sulphate esters (M(r) = 1,060.5 for the disodium salt). Brevetoxin-A ([M+H]+ m/z = 867.5) and brevetoxin-B ([M+H]+ m/z = 895.5) yielded singly charged ions corresponding to the parent ion, Na+ adducts and the loss of up to four H2O molecules. Okadaic acid ([M+H]+ m/z = 805.5) yielded singly charged ions corresponding to the parent ion and ions for the loss of up to three H2O molecules. A signal for M + 18 Da species that may represent [M+NH4]+ was observed for ciguatoxin-1, brevetoxin-A and -B, and okadaic acid. For all polyethers examined, the orifice potential influenced the relative intensity of the ions detected in a predictable manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of rapid and sensitive high throughput pharmacologic assays for marine phycotoxins

The lack of rapid, high throughput assays is a major obstacle to many aspects of research on marine phycotoxins. Here we describe the application of microplate scintillation technology to develop high throughput assays for several classes of marine phycotoxin based on their differential pharmacologic actions. High throughput "drug discovery" format microplate receptor binding assays developed for brevetoxins/ciguatoxins and for domoic acid are described. Analysis for brevetoxins/ciguatoxins is carried out by binding competition with [3H] PbTx-3 for site 5 on the voltage dependent sodium channel in rat brain synaptosomes. Analysis of domoic acid is based on binding competition with [3H] kainic acid for the kainate/quisqualate glutamate receptor using frog brain synaptosomes. In addition, a high throughput microplate 45Ca flux assay for determination of maitotoxins is described. These microplate assays can be completed within 3 hours, have sensitivities of less than 1 ng, and can analyze dozens of samples simultaneously. The assays have been demonstrated to be useful for assessing algal toxicity and for assay-guided purification of toxins, and are applicable to the detection of biotoxins in seafood.

Evaluation of bioassay for toxicity of ciguateric fish and associated toxins

Evaluation of the mouse toxicity assay symptom of hind leg paralysis (HLP) with mouse death by statistical analysis is presented in this study. The fishes assessed were herbivores including Ctenochaetus strigosus (kole), Ctenochaetus hawaiiensis, Acanthurus sandvicensis (manini), and Mugil cephalus (mullet); and the carnivores, Cephalopholis argus (roi) and Cheilinus rhodochrous (po'ou). The latter can also be considered an omnivore. The extracts of both herbivore and carnivore species appeared to be most toxic when HLP occurred in the mice. Ninety-three percent of the mice with HLP died, whereas when no HLP (NHLP) occurred, only 51% of the mice died. Carnivore flesh extracts (po'ou and roi) were least toxic with one death out of a total 22 mice. The unidentified toxin associated with HLP appears to differ in biological properties from that of ciguatoxin(s) in that it was not found in the flesh tissues of carnivores. Further chemical studies of this toxin(s) is being addressed presently.

Recovery of ciguatoxin from fish flesh

A mouse bioassay, validated for the quantification of ciguatoxin in up to 20 mg of ether extract from fish flesh, revealed that 63 +/- 14% of spiked ciguatoxin was recovered using a standard extraction procedure. Except for extracts from the least toxic of ciguateric fish (0.1-0.5 nmol ciguatoxin-1/kg fish), signs in mice of intoxication by ciguatoxin (hypothermia to below 33 degrees C as well as at least severe diarrhoea or lachrymation or hypersalivation) could be distinguished from the toxic reaction that follows administration of ciguatoxin-free ether extracts. Ciguatoxin recovery was similar for four variants of the ether-water partition, with the 2 M NaC1/ether partition extracting half the contaminants. The method described is selective for ciguatoxin and could be used to quantify natural levels ciguatoxin in the flesh of fish in the absence of a validated in vitro test.

Multiple toxins in a specimen of the narrow-barred Spanish mackerel, Scomberomorus commersoni

Ciguatoxin-like, scaritoxin-like and other unidentified lipid-soluble toxins were detected in a specimen of Scomberomorus commersoni captured in sub-tropical Queensland. The fish was from a batch that had been involved in human poisonings. The ciguatoxin-like substance made a greater contribution to the total toxicity than did the scaritoxin-like substance. Water-soluble toxins were also present. The most important of these in terms of contribution to total toxicity was unidentified toxic material present in fractions eluted from a silicic acid column with 100% methanol or methanol:water (1:1). After TLC this material yielded a spot positive for alkaloids. Maitotoxin was also detected among the water-soluble toxins. The lethal potency of the fish flesh approximated 27.3 MU/100 g of flesh with water-soluble toxins contributing to a greater extent than the lipid-soluble toxins. (A MU is defined as the minimum amount of toxic material expressed in g required to kill a 20 g mouse within 24 hr after i.p. injection.) The 15 kg fish studied contained a total of approximately 4095 MU. The presence of several water-soluble and lipid-soluble toxins in the fish has implications for the detection of such ciguateric fishes and for the diagnosis and treatment of poisonings stemming from ingestion of these fishes.

Ciguatoxin-protein association in skeletal muscle of Spanish mackerel (Scomberomorus commersoni)

A study of soluble protein from skeletal muscle of Scomberomorus commersoni was undertaken to elucidate aspects of ciguatoxin (CTX) bioaccumulation in marine teleosts. Skeletal muscle tissue samples from toxic and non-toxic specimens were subjected to fractionation, centrifugation, (NH4)2SO4 precipitation and Sephacryl S-200 chromatography of soluble proteins. Toxicity associated with various fractions was assessed by mouse bioassay, and toxic and non-toxic soluble protein fractions were compared using SDS-PAGE. CTX eluted from Sephacryl S-200 with soluble proteins of apparent mol. wt between 35,500 and 59,500. The toxic eluate contained 1.4% of total sample protein and 15% of total sample toxicity, with an associated 7.2-fold increase in specific activity. SDS-PAGE comparisons show two protein bands in the 37,400 and 40,600 mol. wt range which appeared in toxic soluble protein fractions, but were not detectable in control (non-toxic) samples. These findings are interpreted as being consistent with the association of CTX with at least one monomeric soluble protein of 37,000 to 40,600 mol. wt from toxic S. commersoni skeletal muscle.

Multiple ciguatoxins in the flesh of fish

Most cases of ciguatera (fish poisoning) result from consumption of the flesh of fishes contaminated with ciguatoxin(s); however, the relatively low toxicity of ciguateric fish flesh has hindered attempts to identify these ciguatoxin(s). Utilising high performance liquid chromatography, mass spectroscopy and mouse bioassay signs we have determined that ciguatoxin-1 (MH+ m/z = 1112), ciguatoxin-2 and ciguatoxin-3 are the major ciguatoxins present in the flesh of ciguateric fish. Ciguatoxin-1, -2 and -3 were present in yields of 0.19, 0.09 and 0.02 microgram/kg flesh, respectively, in Scomberomorus commersoni; 0.08, 0.09 and 0.07 microgram/kg flesh, respectively, in Plectropomus spp. and; 0.67, 0.61 and 0.06 microgram/kg flesh, respectively, in Pomadasys maculatus. Two minor toxins, which may be further oxidised analogues of ciguatoxin-1 and ciguatoxin-2, were also identified. The presence of multiple ciguatoxins in fish flesh has important consequences for the detection of ciguateric fish and may be a contributing factor to the observed variability in the symptoms of ciguatera.

Effects of ciguatoxin on nerve excitability in rats (Part I)

Ciguatera poisoning is the most common fish food poisoning encountered in man. Electrophysiological studies were performed on the ventral nerve of the tail on adult rats following intraperitoneal injection of toxic fish extract. Significant slowing of both mixed and motor nerve conduction velocities and F wave responses were recorded. Motor and mixed nerve amplitudes were significantly reduced. Both absolute and supernormal periods were significantly prolonged together with an exaggeration of the supernormal response. These findings indirectly suggest that ciguatoxin acts on mammalian nerve by prolonging sodium channel activation.

Purification and characterization of ciguatoxins from moray eel (Lycodontis javanicus, Muraenidae)

Viscera (48.3 kg) from moray eels (Lycodontis javanicus) collected in a ciguatera endemic area were extracted and the ciguatoxins characterized. Three major ciguatoxins, CTX-1, CTX-2 and CTX-3, were isolated and purified to homogeneity on reverse phase high performance liquid chromatography. Several minor toxins were also detected. CTX-1 (490 micrograms) was comparable by both 1H nuclear magnetic resonance (1H NMR) and mass spectroscopy (MH+ m/z = 1111) to ciguatoxin isolated previously from moray eels. CTX-2 (280 micrograms) and CTX-3 (100 micrograms) were less polar ciguatoxins not previously characterized. CTX-2 and CTX-3 differed from CTX-1 by 16 mass units, suggesting that they were less oxygenated analogues. 1H NMR revealed that the hydroxyl at C54 in CTX-1 was absent in CTX-2 and CTX-3. An additional change in the chemistry of CTX-2 compared to CTX-1 and CTX-3 was also suggested on the basis of 1H NMR, indicating that CTX-2 may arise from a different precursor to CTX-1. CTX-3 is likely to be an intermediate in the oxidation of a gambiertoxin (sodium channel toxins from Gambierdiscus toxicus) to CTX-1. The i.p. LD50 values for CTX-1, CTX-2 and CTX-3 were 0.25, 2.3 and 0.9 micrograms/kg, respectively. The signs induced in mice by the ciguatoxins were similar, except that CTX-2 and CTX-3 induced hind-limb paralysis that was absent with CTX-1. Each ciguatoxin was potent orally. CTX-1, CTX-2 and CTX-3 competitively inhibited the binding of [3H]brevetoxin-3 to voltage-dependent sodium channels with relative potencies qualitatively (but not quantitatively) comparable to mouse lethality. This study reveals that the relatively small chemical differences between CTX-1, CTX-2 and CTX-3 give rise to significant structure-activity and pharmacokinetic differences.

Fractionation and purification of some muscular and visceral ciguatoxins extracted from Carribean fish

1. Cigautoxins (CTX) were extracted from flesh and viscera of seven large roving predatory fishes: Caranx bartholomaei, Caranx latus, Seriola dumerili, Alectis crinitus, Scomberomorus cavalla, Sphyraena barracuda and Gymnothorax funebris. 2. Generally each extract consisted of close-related CTX which were separated according to their polarity by Florisil column chromatography into a fast-acting CTX containing group and a slow-acting CTX containing group. 3. The shortest survival time of mice (ts) was low for the former group (less than 10 min) and high for the latter (greater than or equal to 29 min). 4. The level of purity had no influence on the range of ts values. The presence of these two CTX groups in different extracts did not from experimental conditions. 5. Attempts to convert fast-acting CTX to slow-acting CTX and vice-versa were negative. G. funebris and S. barracuda had an especially high content of unstable fast-acting CTX. 6. Purification of the slow-acting CTX was achieved by fast elution chromatography and Sephadex LH20 gel filtration. 7. The ts values of these CTX were identical for five species (40-44 min) but not for S. barracuda (29-32 min). 8. Thus ciguatoxic extracts from Caribbean fish were composed of several close-related CTX.

The mode of inotropic action of ciguatoxin on guinea-pig cardiac muscle

1. Ciguatoxin (CTX) caused a dose-dependent increase in the contractile force of the guinea-pig isolated left atria at concentrations ranging from 0.1 to 10 ng ml-1 with the ED50 value of 0.5 ng ml-1. 2. In the atria, tetrodotoxin (5 x 10(-7) M) inhibited markedly the inotropic action of CTX. The inotropic effect of CTX at low concentrations was abolished by practolol (10(-5) M) and reserpine (2 mg kg-1 daily, for 3 days), whereas that of CTX at high concentrations was partially inhibited by both drugs. 3. In single atrial cells, CTX (3 ng ml-1) produced a marked increase in the amplitude of longitudinal contractions. 4. CTX (3 ng ml-1) caused marked prolongation in the falling phase of action potentials of atrial strips without affecting the maximum rate of rise of action potentials and membrane resting potentials. The effect of CTX on action potentials was abolished by tetrodotoxin (10(-6) M). 5. The whole-cell patch-clamp experiments on myocytes revealed that CTX (20 ng ml-1) shifted the current-voltage curve of Na inward currents by 40 mV in the negative direction. CTX caused a small sustained Na inward current even at resting membrane potentials. 6. These results suggest that the inotropic action of lower concentrations of CTX is primarily due to an indirect action via noradrenaline release, whereas that of higher concentrations is caused not only by an indirect action but also by a direct action on voltage-dependent Na channels of cardiac muscle. It is also suggested that CTX activates cardiac muscle Na channels by modifying the voltage-dependence of channel activation to increase Na inward currents, thus producing cardiotonic actions.

[Heterogeneity of ciguatoxins extracted from fish caught in the French Antilles]

Ciguatoxin-like substances were extracted from the viscera or the flesh of eight Caribbean fish species, including small invertebrate feeders and large carnivores. The had similar properties, i.e. pharmacological action, solubility, chromatographic behaviour on silicic acid or Sephadex LH 20 column, stability in a weak acid solution and instability in alkaline medium. However, Florisil column and thin-layer chromatography showed different ciguatoxins whose number depended on tissue or species but not on fish trophic level. Less polar ciguatoxins appeared in salted and dried flesh. Thus, fish ciguatoxins are believed to be closely related substances, possibly changing in structure according to particular experimental conditions.

Ciguatoxin is a novel type of Na+ channel toxin

Purified ciguatoxin at 0.1 to 10 ng/ml inhibits the net accumulation of neurotransmitters (gamma-aminobutyric acid and dopamine) by brain synaptosomes. This action is due to a stimulation of neurotransmitter release. The half-maximum effect of the toxin is observed at 0.62 ng/ml. The effect of ciguatoxin is completely inhibited by tetrodotoxin (K0.5 = 4 nM). Electrophysiological studies on neuroblastoma cells indicate that ciguatoxin induces a membrane depolarization which is prevented by tetrodotoxin and which is due to an action that increases Na+ permeability. Under appropriate conditions ciguatoxin creates spontaneous oscillations in the membrane polarization level and repeated action potentials. Ciguatoxin stimulates 22Na+ entry through the voltage-dependent Na+ channels of neuroblastoma cells and rat skeletal myoblasts when it is used in synergy with veratridine, batrachotoxin, pyrethroids, sea anemone, or scorpion toxins. The half-maximum effect of ciguatoxin on 22Na+ flux in the presence of veratridine occurs at a concentration of 0.5 ng/ml. Stimulation of 22Na+ flux by ciguatoxin is abolished by tetrodotoxin. These results taken together indicate that ciguatoxin belongs to a new class of toxins acting on Na+ channels.

[Ciguatera: a rapid procedure for extraction of ciguatoxin]

A rapid procedure for extraction and partial purification of ciguatoxin has been achieved and compared to one of the routine methods. Fish from two species which provided extracts of differing purity by the routine method were used. From 8 g of raw flesh, 1.0 +/- 0.2 mg of a semi-purified extract of ciguatoxin of homogeneous quality was obtained within 1 hr, whatever the species or toxicity of the fish. The results were reproducible, making the procedure very promising.

Occurrence of a ciguatoxin-like substance in the Spanish mackerel (Scomberomorus commersoni)

A lipid-soluble toxin, similar to ciguatoxin as isolated by Scheuer et al. (1967), has been found in the flesh of the Spanish mackerel, Scomberomorus commersoni, caught in Queensland. The ciguatoxin-like substance was experimentally characterized by examination of specific biological and chromatographic properties of the lipid-soluble extract from a pooled sample of flesh from Spanish mackerel. Flesh from specimens known to have caused S. commersoni poisoning in humans was confirmed as toxic by cat bioassay. A toxin was extracted from S. commersoni which yielded, on partial purification, a clear, oily substance with an LD50 i.p. to mice of 0.72 mg/kg, and which had chromatographic properties similar to those of classical ciguatoxin. However, the Rf value on thin-layer chromatography plates was lower for S. commersoni toxin than for classical ciguatoxin. This is the first record of a ciguatoxin-like substance experimentally identified in S. commersoni, a pelagic fish that occurs throughout Queensland coastal waters. The majority of toxic S. commersoni are caught between latitudes 24 degrees and 26 degrees S.

[Assay for detection of ciguatoxin using an immunoenzyme technic]

The ELISA method has been used to detect possible anticiguatoxin antibodies in the tissues of toxic fishes. Thin lamellae of toxic fish muscles have been brought into contact on the one hand with antibodies obtained after immunization of the rabbit and the mouse by a human serumalbumine ciguatoxin conjugate and on the other hand with human sera of ciguatera convalescent. These various antibodies have been recognized by peroxydase labelled anti-rabbit, -mouse and -human antibodies. It has been not possible to show off specific antibodies of the ciguatoxin in the various immune sera tested by this immuno-enzymatic method.