Detection and quantification of maitotoxin-like compounds using a neuroblastoma (Neuro-2a) cell based assay. Application to the screening of maitotoxin-like compounds in Gambierdiscus spp

Characterization of New Gambierones Produced by Gambierdiscus balechii 1123M1M10

The benthic dinoflagellate genus Gambierdiscus is the primary producer of toxins responsible for ciguatera poisoning (CP), a food intoxication endemic in tropical and subtropical areas of the world. We used high-performance liquid chromatography tandem high-resolution mass spectrometry (HPLC-HRMS) to investigate the toxin profile of Gambierdiscus balechii 1123M1M10, which was obtained from Marakei Island (2°01'N, 173°15'E), Republic of Kiribati, located in the central Pacific Ocean. Four new gambierone analogues including 12,13-dihydro-44-methylgambierone, 38-dehydroxy-12,13-dihydro-44-methylgambierone, 38-dehydroxy-44-methylgambierone, and desulfo-hydroxyl gambierone, and two known compounds, gambierone and 44-methylgambierone, were proposed by analyzing their fragmentation behaviors and pathways. Our findings provide new insights into the toxin profile of Gambierdiscus balechii 1123M1M10, which can be used as a biomarker for species identification, and lay the foundation for further toxin isolation and bioactivity studies of gambierones.

Cytotoxic and Hemolytic Activities of Extracts of the Fish Parasite Dinoflagellate Amyloodinium ocellatum

The dinoflagellate Amyloodinium ocellatum is the etiological agent of a parasitic disease named amyloodiniosis. Mortalities of diseased fish are usually attributed to anoxia, osmoregulatory impairment, or opportunistic bacterial infections. Nevertheless, the phylogenetic proximity of A. ocellatum to a group of toxin-producing dinoflagellates from Pfiesteria, Parvodinium and Paulsenella genera suggests that it may produce toxin-like compounds, adding a new dimension to the possible cause of mortalities in A. ocellatum outbreaks. To address this question, extracts prepared from different life stages of the parasite were tested in vitro for cytotoxic effects using two cell lines derived from branchial arches (ABSa15) and the caudal fin (CFSa1) of the gilthead seabream (Sparus aurata), and for hemolytic effects using erythrocytes purified from the blood of gilthead seabream juveniles. Cytotoxicity and a strong hemolytic effect, similar to those observed for Karlodinium toxins, were observed for the less polar extracts of the parasitic stage (trophont). A similar trend was observed for the less polar extracts of the infective stage (dinospores), although cell viability was only affected in the ABSa15 line. These results suggest that A. ocellatum produces tissue-specific toxic compounds that may have a role in the attachment of the dinospores’ and trophonts’ feeding process.

Reductive Amination for LC-MS Signal Enhancement and Confirmation of the Presence of Caribbean Ciguatoxin-1 in Fish

Ciguatera poisoning is a global health concern caused by the consumption of seafood containing ciguatoxins (CTXs). Detection of CTXs poses significant analytical challenges due to their low abundance even in highly toxic fish, the diverse and in-part unclarified structures of many CTX congeners, and the lack of reference standards. Selective detection of CTXs requires methods such as liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) or high-resolution MS (LC-HRMS). While HRMS data can provide greatly improved resolution, it is typically less sensitive than targeted LC-MS/MS and does not reliably comply with the FDA guidance level of 0.1 µg/kg CTXs in fish tissue that was established for Caribbean CTX-1 (C-CTX-1). In this study, we provide a new chemical derivatization approach employing a fast and simple one-pot derivatization with Girard's reagent T (GRT) that tags the C-56-ketone intermediate of the two equilibrating C-56 epimers of C-CTX-1 with a quaternary ammonium moiety. This derivatization improved the LC-MS/MS and LC-HRMS responses to C-CTX-1 by approximately 40- and 17-fold on average, respectively. These improvements in sensitivity to the GRT-derivative of C-CTX-1 are attributable to: the improved ionization efficiency caused by insertion of a quaternary ammonium ion; the absence of adduct-ions and water-loss peaks for the GRT derivative in the mass spectrometer, and; the prevention of on-column epimerization (at C-56 of C-CTX-1) by GRT derivatization, leading to much better chromatographic peak shapes. This C-CTX-1-GRT derivatization strategy mitigates many of the shortcomings of current LC-MS analyses for C-CTX-1 by improving instrument sensitivity, while at the same time adding selectivity due to the reactivity of GRT with ketones and aldehydes.

Identification of New CTX Analogues in Fish from the Madeira and Selvagens Archipelagos by Neuro-2a CBA and LC-HRMS

Ciguatera Poisoning (CP) is caused by consumption of fish or invertebrates contaminated with ciguatoxins (CTXs). Presently CP is a public concern in some temperate regions, such as Macaronesia (North-Eastern Atlantic Ocean). Toxicity analysis was performed to characterize the fish species that can accumulate CTXs and improve understanding of the ciguatera risk in this area. For that, seventeen fish specimens comprising nine species were captured from coastal waters inMadeira and Selvagens Archipelagos. Toxicity was analysed by screening CTX-like toxicity with the neuroblastoma cell-based assay (neuro-2a CBA). Afterwards, the four most toxic samples were analysed with liquid chromatography-high resolution mass spectrometry (LC-HRMS). Thirteen fish specimens presented CTX-like toxicity in their liver, but only four of these in their muscle. The liver of one specimen of Muraena augusti presented the highest CTX-like toxicity (0.270 ± 0.121 µg of CTX1B equiv·kg−1). Moreover, CTX analogues were detected with LC-HRMS, for M. augusti and Gymnothorax unicolor. The presence of three CTX analogues was identified: C-CTX1, which had been previously described in the area; dihydro-CTX2, which is reported in the area for the first time; a putative new CTX m/z 1127.6023 ([M+NH4]+) named as putative C-CTX-1109, and gambieric acid A.

Gambierdiscus and Fukuyoa as potential indicators of ciguatera risk in the Balearic Islands

Gambierdiscus and Fukuyoa are genera of toxic dinoflagellates which were mainly considered as endemic to marine intertropical areas, and that are well known as producers of ciguatoxins (CTXs) and maitotoxins (MTXs). Ciguatera poisoning (CP) is a human poisoning occurring after the consumption of fish or more rarely, shellfish containing CTXs. The presence of these microalgae in a coastal area is an indication of potential risk of CP. This study assesses the risk of CP in the Balearic Islands (Western Mediterranean Sea) according to the distribution of both microalgae genera, and the presence of CTX-like and MTX-like toxicity in microalgal cultures as determined by neuro-2a cell based-assay (neuro-2a CBA). Genetic identification of forty-three cultured microalgal strains isolated from 2016 to 2019 revealed that all of them belong to the species G. australes and F. paulensis. Both species were widely distributed in Formentera, Majorca and Minorca. Additionally, all strains of G. australes and two of F. paulensis exhibited signals of CTX-like toxicity ranging respectively between 1 and 380 and 8-16 fg CTX1B equivalents (equiv.) • cell^-1. Four extracts of F. paulensis exhibited a novel toxicity response in neuro-2a cells consisting of the recovery of the cell viability in the presence of ouabain and veratridine. In addition, G. australes showed MTX-like toxicity while F. paulensis strains did not. Overall, the low CTX-like toxicities detected indicate that the potential risk of CP in the Balearic Islands is low, although, the presence of CTX-like and MTX-like toxicity in those strains reveal the necessity to monitor these genera in the Mediterranean Sea.

Further advance of Gambierdiscus Species in the Canary Islands, with the First Report of Gambierdiscus belizeanus

Ciguatera Poisoning (CP) is a human food-borne poisoning that has been known since ancient times to be found mainly in tropical and subtropical areas, which occurs when fish or very rarely invertebrates contaminated with ciguatoxins (CTXs) are consumed. The genus of marine benthic dinoflagellates Gambierdiscus produces CTX precursors. The presence of Gambierdiscus species in a region is one indicator of CP risk. The Canary Islands (North Eastern Atlantic Ocean) is an area where CP cases have been reported since 2004. In the present study, samplings for Gambierdiscus cells were conducted in this area during 2016 and 2017. Gambierdiscus cells were isolated and identified as G. australes, G. excentricus, G. caribaeus, and G. belizeanus by molecular analysis. In this study, G. belizeanus is reported for the first time in the Canary Islands. Gambierdiscus isolates were cultured, and the CTX-like toxicity of forty-one strains was evaluated with the neuroblastoma cell-based assay (neuro-2a CBA). G. excentricus exhibited the highest CTX-like toxicity (9.5-2566.7 fg CTX1B equiv. cell^-1) followed by G. australes (1.7-452.6.2 fg CTX1B equiv. cell^-1). By contrast, the toxicity of G. belizeanus was low (5.6 fg CTX1B equiv. cell^-1), and G. caribaeus did not exhibit CTX-like toxicity. In addition, for the G. belizeanus strain, the production of CTXs was evaluated with a colorimetric immunoassay and an electrochemical immunosensor resulting in G. belizeanus producing two types of CTX congeners (CTX1B and CTX3C series congeners) and can contribute to CP in the Canary Islands.

High sensitivity of rat cardiomyoblast H9c2(2-1) cells to Gambierdiscus toxic compounds

Ciguatera fish poisoning is a frequently reported non-bacterial food-borne illness related to the consumption of seafood contaminated with ciguatoxins, and possibly maitotoxins. These toxins are synthesized by marine dinoflagellate species of Gambierdiscus and Fukuyoa genera, and their abundance is a matter of great concern due to their adverse effects to aquatic life and human health. The present study aims to assess the sensitivity of rat cardiomyoblast H9c2(2-1) cells to Gambierdiscus toxic compounds using concentration- and time-dependent sulforhodamine B (SRB) colorimetric assays. Low concentrations of Gambierdiscus extracts (corresponding to 1.3-2.3 cells mL^-1) induced a concentration-dependent response. Specificity in time-dependent response of H9c2(2-1) cells was demonstrated for G. excentricus after a 180 min exposure compared to both G. cf. belizeanus and G. silvae species, with EC_50s obtained after 720 and 360 min, respectively. The sensitivity of H9c2(2-1) cells to dinoflagellate toxic compounds was also tested with other genera from benthic (Coolia malayensis, Ostreopsis cf. ovata, Prorocentrum hoffmannianum and P. lima) and planktonic (Amphidinium carterae and Lingulodinium polyedrum) habitats. Amphidinium, Coolia and Lingulodinium data did not present any concentration-response relationships, and EC₅₀ values could only be obtained after 720 and 1440 min of exposure to both Prorocentrum species and O. cf. ovata, respectively. This study demonstrated that the H9c2(2-1) SRB assay represents a promising and sensitive tool for the detection of Gambierdiscus toxic compounds present in water samples, particularly of G. excentricus at very low cell abundances.

Maitotoxin-4, a Novel MTX Analog Produced by Gambierdiscus excentricus

Maitotoxins (MTXs) are among the most potent toxins known. These toxins are produced by epi-benthic dinoflagellates of the genera Gambierdiscus and Fukuyoa and may play a role in causing the symptoms associated with Ciguatera Fish Poisoning. A recent survey revealed that, of the species tested, the newly described species from the Canary Islands, G. excentricus, is one of the most maitotoxic. The goal of the present study was to characterize MTX-related compounds produced by this species. Initially, lysates of cells from two Canary Island G. excentricus strains VGO791 and VGO792 were partially purified by (i) liquid-liquid partitioning between dichloromethane and aqueous methanol followed by (ii) size-exclusion chromatography. Fractions from chromatographic separation were screened for MTX toxicity using both the neuroblastoma neuro-2a (N2a) cytotoxicity and Ca2+ flux functional assays. Fractions containing MTX activity were analyzed using liquid chromatography coupled to high-resolution mass spectrometry (LC-HRMS) to pinpoint potential MTX analogs. Subsequent non-targeted HRMS analysis permitted the identification of a novel MTX analog, maitotoxin-4 (MTX4, accurate mono-isotopic mass of 3292.4860 Da, as free acid form) in the most toxic fractions. HRMS/MS spectra of MTX4 as well as of MTX are presented. In addition, crude methanolic extracts of five other strains of G. excentricus and 37 other strains representing one Fukuyoa species and ten species, one ribotype and one undetermined strain/species of Gambierdiscus were screened for the presence of MTXs using low resolution tandem mass spectrometry (LRMS/MS). This targeted analysis indicated the original maitotoxin (MTX) was only present in one strain (G. australes S080911_1). Putative maitotoxin-2 (p-MTX2) and maitotoxin-3 (p-MTX3) were identified in several other species, but confirmation was not possible because of the lack of reference material. Maitotoxin-4 was detected in all seven strains of G. excentricus examined, independently of their origin (Brazil, Canary Islands and Caribbean), and not detected in any other species. MTX4 may therefore serve as a biomarker for the highly toxic G. excentricus in the Atlantic area.

Genetic diversity and distribution of the ciguatera-causing dinoflagellate Gambierdiscus spp. (Dinophyceae) in coastal areas of Japan

Background

The marine epiphytic dinoflagellate genus Gambierdiscus produce toxins that cause ciguatera fish poisoning (CFP): one of the most significant seafood-borne illnesses associated with fish consumption worldwide. So far, occurrences of CFP incidents in Japan have been mainly reported in subtropical areas. A previous phylogeographic study of Japanese Gambierdiscus revealed the existence of two distinct phylotypes: Gambierdiscus sp. type 1 from subtropical and Gambierdiscus sp. type 2 from temperate areas. However, details of the genetic diversity and distribution for Japanese Gambierdiscus are still unclear, because a comprehensive investigation has not been conducted yet.

Methods/Principal Finding

A total of 248 strains were examined from samples mainly collected from western and southern coastal areas of Japan during 2006–2011. The SSU rDNA, the LSU rDNA D8–D10 and the ITS region were selected as genetic markers and phylogenetic analyses were conducted. The genetic diversity of Japanese Gambierdiscus was high since five species/phylotypes were detected: including two reported phylotypes (Gambierdiscus sp. type 1 and Gambierdiscus sp. type 2), two species of Gambierdiscus (G. australes and G. cf. yasumotoi) and a hitherto unreported phylotype Gambierdiscus sp. type 3. The distributions of type 3 and G. cf. yasumotoi were restricted to the temperate and the subtropical area, respectively. On the other hand, type 1, type 2 and G. australes occurred from the subtropical to the temperate area, with a tendency that type 1 and G. australes were dominant in the subtropical area, whereas type 2 was dominant in the temperate area. By using mouse bioassay, type 1, type 3 and G. australes exhibited mouse toxicities.

Conclusions/Significance

This study revealed a surprising diversity of Japanese Gambierdiscus and the distribution of five species/phylotypes displayed clear geographical patterns in Japanese coastal areas. The SSU rDNA and the LSU rDNA D8–D10 as genetic markers are recommended for further use.

Differences in the toxicity of six Gambierdiscus (Dinophyceae) species measured using an in vitro human erythrocyte lysis assay

This study examined the toxicity of six Gambierdiscus species (Gambierdiscus belizeanus, Gambierdiscus caribaeus, Gambierdiscus carolinianus, Gambierdiscus carpenteri, Gambierdiscus ribotype 2 and Gambierdiscus ruetzleri) using a human erythrocyte lysis assay. In all, 56 isolates were tested. The results showed certain species were significantly more toxic than others. Depending on the species, hemolytic activity consistently increased by ∼7-40% from log phase growth to late log - early stationary growth phase and then declined in mid-stationary growth phase. Increasing growth temperatures from 20 to 31 °C for clones of G. caribaeus showed only a slight increase in hemolytic activity between 20 and 27 °C. Hemolytic activity in the G. carolinianus isolates from different regions grown over the same 20-31 °C range remained constant. These data suggest that growth temperature is not a significant factor in modulating the inter-isolate and interspecific differences in hemolytic activity. The hemolytic activity of various isolates measured repeatedly over a 2 year period remained constant, consistent with the hemolytic compounds being constitutively produced and under strong genetic control. Depending on species, greater than 60-90% of the total hemolytic activity was initially associated with the cell membranes but diffused into solution over a 24 h assay incubation period at 4 °C. These findings suggest that hemolytic compounds produced by Gambierdiscus isolates were held in membrane bound vesicles as reported for brevetoxins produced by Karenia brevis. Gambierdiscus isolates obtained from other parts of the world exhibited hemolytic activities comparable to those found in the Caribbean and Gulf of Mexico confirming the range of toxicities is similar among Gambierdiscus species worldwide. Experiments using specific inhibitors of the MTX pathway and purified MTX, Gambierdiscus whole cell extracts, and hydrophilic cell extracts containing MTX, were consistent with MTX as the primary hemolytic compound produced by Gambierdiscus species. While the results from inhibition studies require validation by LC-MS analysis, the available data strongly suggest differences in hemolytic activity observed in this study reflect maitotoxicity.

Update on methodologies available for ciguatoxin determination: perspectives to confront the onset of ciguatera fish poisoning in Europe

Ciguatera fish poisoning (CFP) occurs mainly when humans ingest finfish contaminated with ciguatoxins (CTXs). The complexity and variability of such toxins have made it difficult to develop reliable methods to routinely monitor CFP with specificity and sensitivity. This review aims to describe the methodologies available for CTX detection, including those based on the toxicological, biochemical, chemical, and pharmaceutical properties of CTXs. Selecting any of these methodological approaches for routine monitoring of ciguatera may be dependent upon the applicability of the method. However, identifying a reference validation method for CTXs is a critical and urgent issue, and is dependent upon the availability of certified CTX standards and the coordinated action of laboratories. Reports of CFP cases in European hospitals have been described in several countries, and are mostly due to travel to CFP endemic areas. Additionally, the recent detection of the CTX-producing tropical genus Gambierdiscus in the eastern Atlantic Ocean of the northern hemisphere and in the Mediterranean Sea, as well as the confirmation of CFP in the Canary Islands and possibly in Madeira, constitute other reasons to study the onset of CFP in Europe [1]. The question of the possible contribution of climate change to the distribution of toxin-producing microalgae and ciguateric fish is raised. The impact of ciguatera onset on European Union (EU) policies will be discussed with respect to EU regulations on marine toxins in seafood. Critical analysis and availability of methodologies for CTX determination is required for a rapid response to suspected CFP cases and to conduct sound CFP risk analysis.