Accumulation and Elimination of Tetrodotoxin in the Pufferfish Takifugu obscurus by Dietary Administration of the Wild Toxic Gastropod Nassarius semiplicata

Tetrodotoxin: The State-of-the-Art Progress in Characterization, Detection, Biosynthesis, and Transport Enrichment

Tetrodotoxin (TTX) is a neurotoxin that binds to sodium channels and blocks sodium conduction. Importantly, TTX has been increasingly detected in edible aquatic organisms. Because of this and the lack of specific antidotes, TTX poisoning is now a major threat to public health. However, it is of note that ultra-low dose TTX is an excellent analgesic with great medicinal value. These contradictory effects highlight the need for further research to elucidate the impacts and functional mechanisms of TTX. This review summarizes the latest research progress in relation to TTX sources, analogs, mechanisms of action, detection methods, poisoning symptoms, therapeutic options, biosynthesis pathways, and mechanisms of transport and accumulation in pufferfish. This review also provides a theoretical basis for reducing the poisoning risks associated with TTX and for establishing an effective system for its use and management to ensure the safety of fisheries and human health.

Where Does All the Poison Go? Investigating Toxicokinetics of Newt (Taricha) Tetrodotoxin (TTX) in Garter Snakes (Thamnophis)

Animals that consume toxic diets provide models for understanding the molecular and physiological adaptations to ecological challenges. Garter snakes (Thamnophis) in western North America prey on Pacific newts (Taricha), which employ tetrodotoxin (TTX) as an antipredator defense. These snakes possess mutations in voltage-gated sodium channels (Nav), the molecular targets of TTX, that decrease the binding ability of TTX to sodium channels (target-site resistance). However, genetic variation at these loci that cannot explain all the phenotypic variation in TTX resistance in Thamnophis. We explored a separate means of resistance, toxin metabolism, to determine if TTX-resistant snakes either rapidly remove TTX or sequester TTX. We examined the metabolism and distribution of TTX in the body (toxicokinetics), to determine differences between TTX-resistant and TTX-sensitive snakes in the rates at which TTX is eliminated from organs and the whole body (using TTX half-life as our metric). We assayed TTX half-life in snakes from TTX-resistant and TTX-sensitive populations of three garter snake species with a coevolutionary history with newts (T. atratus, T. couchii, T. sirtalis), as well as two non-resistant "outgroup" species (T. elegans, Pituophis catenifer) that seldom (if ever) engage newts. We found TTX half-life varied across species, populations, and tissues. Interestingly, TTX half-life was shortest in T. elegans and P. catenifer compared to all other snakes. Furthermore, TTX-resistant populations of T. couchii and T. sirtalis eliminated TTX faster (shorter TTX half-life) than their TTX-sensitive counterparts, while populations of TTX-resistant and TTX-sensitive T. atratus showed no difference rates of TTX removal (same TTX half-life). The ability to rapidly eliminate TTX may have permitted increased prey consumption, which may have promoted the evolution of additional resistance mechanisms. Finally, snakes still retain substantial amounts of TTX, and we projected that snakes could be dangerous to their own predators days to weeks following the ingestion of a single newt. Thus, aspects of toxin metabolism may have been key in driving predator-prey relationships, and important in determining other ecological interactions.

Evolution of ion channels in cetaceans: a natural experiment in the tree of life

Cetaceans represent a natural experiment within the tree of life in which a lineage changed from terrestrial to aquatic habitats. This shift involved phenotypic modifications, representing an opportunity to explore the genetic bases of phenotypic diversity. Among the different molecular systems that maintain cellular homeostasis, ion channels are crucial for the proper physiological functioning of all living species. This study aims to explore the evolution of ion channels during the evolutionary history of cetaceans. To do so, we created a bioinformatic pipeline to annotate the repertoire of ion channels in the genome of the species included in our sampling. Our main results show that cetaceans have, on average, fewer protein-coding genes and a higher percentage of annotated ion channels than non-cetacean mammals. Signals of positive selection were detected in ion channels related to the heart, locomotion, visual and neurological phenotypes. Interestingly, we predict that the NaV1.5 ion channel of most toothed whales (odontocetes) is sensitive to tetrodotoxin, similar to NaV1.7, given the presence of tyrosine instead of cysteine, in a specific position of the ion channel. Finally, the gene turnover rate of the cetacean crown group is more than three times faster than that of non-cetacean mammals.

Sudden peak in tetrodotoxin in French oysters during the summer of 2021: Source investigation using microscopy, metabarcoding and droplet digital PCR

Tetrodotoxin (TTX) is a potent neurotoxin causing human intoxications from contaminated seafood worldwide and is of emerging concern in Europe. Shellfish have been shown to contain varying TTX concentrations globally, with concentrations typically higher in Pacific oysters Crassostrea gigas in Europe. Despite many decades of research, the source of TTX remains unknown, with bacterial or algal origins having been suggested. The aim of this study was to identify potential source organisms causing TTX contamination in Pacific oysters in French coastal waters, using three different techniques. Oysters were deployed in cages from April to September 2021 in an estuary where TTX was previously detected. Microscopic analyses of water samples were used to investigate potential microalgal blooms present prior or during the peak in TTX. Differences in the bacterial communities from oyster digestive glands (DG) and remaining flesh were explored using metabarcoding, and lastly, droplet digital PCR assays were developed to investigate the presence of Cephalothrix sp., one European TTX-bearing species in the DG of toxic C. gigas. Oysters analysed by liquid chromatography-tandem mass spectrometry contained quantifiable levels of TTX over a three-week period (24 June-15 July 2021), with concentrations decreasing in the DG from 424 μg/kg for the first detection to 101 μg/kg (equivalent to 74 to 17 μg/kg of total flesh), and trace levels being detected until August 13, 2021. These concentrations are the first report of the European TTX guidance levels being exceeded in French shellfish. Microscopy revealed that some microalgae bloomed during the TTX peak, (e.g., Chaetoceros spp., reaching 40,000 cells/L). Prokaryotic metabarcoding showed increases in abundance of Rubritaleaceae (genus Persicirhabdus) and Neolyngbya, before and during the TTX peak. Both phyla have previously been described as possible TTX-producers and should be investigated further. Droplet digital PCR analyses were negative for the targeted TTX-bearing genus Cephalothrix.

Construction of a high-density genetic linkage map and QTL mapping of growth and cold tolerance traits in Takifugu fasciatus

Takifugu fasciatus is an aquaculture species with high economic value. In recent years, problems such as environmental pollution and inbreeding have caused a serious decline in T. fasciatus germplasm resources. In this study, a high-density genetic linkage map was constructed by whole-genome resequencing. The map consists of 4891 bin markers distributed across 22 linkage groups (LGs), with a total genetic coverage of 2381.353 cM and a mean density of 0.535 cM. Quantitative trait locus (QTL) localization analysis showed that a total of 19 QTLs associated with growth traits of T. fasciatus in the genome-wide significance threshold range, distributed on 11 LGs. In addition, 11 QTLs associated with cold tolerance traits were identified, each scattered on a different LG. Furthermore, we used QTL localization analysis to screen out three candidate genes (IGF1, IGF2, ADGRB) related to growth in T. fasciatus. Meanwhile, we screened three candidate genes (HSP90, HSP70, and HMGB1) related to T. fasciatus cold tolerance. Our study can provide a theoretical basis for the selection and breeding of cold-tolerant or fast-growing T. fasciatus.

Assessing the Toxicity of Lagocephalus sceleratus Pufferfish from the Southeastern Aegean Sea and the Relationship of Tetrodotoxin with Gonadal Hormones

Given the dramatic increase in the L. sceleratus population in the southeastern Aegean Sea, there is growing interest in assessing the toxicity of this pufferfish and the factors controlling its tetrodotoxin (TTX) content. In the present study, liver, gonads, muscle and skin of 37 L. sceleratus specimens collected during May and June 2021 from the island of Rhodes, Greece, were subjected to multi-analyte profiling using liquid chromatography-tandem mass spectrometry (LC-MS/MS) in order to quantitate TTX and evaluate whether this biotoxin interrelates with hormones. TTX and its analogues 4-epiTTX, 11-deoxyTTX, 11-norTTX-6-ol, 4,9-anhydroTTX and 5,11/6,11-dideoxyTTX were detected in all tissue types. Liver and gonads were the most toxic tissues, with the highest TTX concentrations being observed in the ovaries of female specimens. Only 22% of the analyzed muscle samples were non-toxic according to the Japanese toxicity threshold (2.2 μg TTX eq g-1), confirming the high poisoning risk from the inadvertent consumption of this species. Four steroid hormones (i.e., cortisol, testosterone, androstenedione and β-estradiol) and the gonadotropin-releasing hormone (GnRH) were detected in the gonads. Androstenedione dominated in female specimens, while GnRH was more abundant in males. A positive correlation of TTX and its analogues with β-estradiol was observed. However, a model incorporating sex rather than β-estradiol as the independent variable proven to be more efficient in predicting TTX concentration, implying that other sex-related characteristics are more important than specific hormone-regulated processes.

Hepatorenal Toxicity after 7-Day Oral Administration of Low-Dose Tetrodotoxin and Its Distribution in the Main Tissues in Mice

Tetrodotoxin (TTX) is a highly toxic compound detected in various edible marine animals even in European waters. To characterize the hazard by oral exposure to TTX, its tissue distribution was evaluated after single (75 μg/kg) or 7-day (25-125 μg/kg) oral administration in mice. Moreover, TTX liver and renal toxicity was evaluated after 7-day oral administration. The elimination cycle of a single oral dose of TTX (75 µg/kg) was found to be approximately 168 h (7 days). Daily oral administration of TTX at doses of 25, 75, and 125 µg/kg for 7 consecutive days revealed dose-dependent toxic effects on the liver and kidney. Histopathological examination showed increased inflammatory cell infiltration in the liver and kidney with higher TTX doses, along with disorganization of the hepatic cord and renal tubular cell arrangement. The study results indicated that TTX had more hepatotoxicity than nephrotoxicity in mice. These findings provide insights into the unintentional ingestion of a low dose of TTX in mammals, including humans, and emphasize the importance of food safety.

Transcriptomic analysis reveals the genes involved in tetrodotoxin (TTX) accumulation, translocation, and detoxification in the pufferfish Takifugu rubripes

Tetrodotoxin (TTX) is a potent marine neurotoxin that exists in a variety of aquatic and terrestrial organisms. Pufferfish in different habitats show great variation in their TTX contents. Exploring the genes involved in TTX metabolism could contribute to our understanding of the molecular mechanisms underlying TTX accumulation, translocation, and detoxification in pufferfish. In this study, transcriptomic analysis was used to identify the functional genes related to TTX metabolism in the blood, liver, and muscle of the toxic and non-toxic tiger puffer (Takifugu rubripes). A total of 6101 differentially expressed genes (DEGs) were obtained after transcriptomic analysis; of these, 2401 were identified in the blood, 2262 in the liver, and 1438 in the muscle. After enrichment analysis, fourteen genes encoding glutathione S-transferases (GSTs), glutathione peroxidase (GPx), thioredoxins (TXNs), superoxide dismutase (SOD), ATP-binding cassettes (ABCs), apolipoproteins (APOs), inhibitors of apoptosis protein (IAP), and solute carrier (SLC), which are mainly antioxidant enzymes, membrane transporters, or anti-apoptotic factors, were revealed in the blood. Thirty-six genes encoding SLCs, ABCs, long-chain-fatty-acid-CoA ligases (ACSLs), interleukin 6 cytokine family signal transducer (IL6ST), endoplasmic reticulum (ER), and heat shock protein family A (Hsp70) were involved in transmembrane transporter activity and innate immune response. Notably, a large number of slc genes were found to play critical and diverse roles in TTX accumulation and translocation in the liver of T. rubripes. Nine genes from the slc, hsp70, complement C5 (c5), acsl, er, and serpin peptidase inhibitor (serpin) gene families were found to participate in the regulation of protein processing and anti-apoptosis. These results reflect the diverse functions of genes closely related to TTX accumulation, translocation, and detoxification in T. rubripes.

An Overview of the Anatomical Distribution of Tetrodotoxin in Animals

Tetrodotoxin (TTX), a potent paralytic sodium channel blocker, is an intriguing marine toxin. Widely distributed in nature, TTX has attracted attention in various scientific fields, from biomedical studies to environmental safety concerns. Despite a long history of studies, many issues concerning the biosynthesis, origin, and spread of TTX in animals and ecosystems remain. This review aims to summarize the current knowledge on TTX circulation inside TTX-bearing animal bodies. We focus on the advances in TTX detection at the cellular and subcellular levels, providing an expanded picture of intra-organismal TTX migration mechanisms. We believe that this review will help address the gaps in the understanding of the biological function of TTX and facilitate the development of further studies involving TTX-bearing animals.

A review of chemical defense in harlequin toads (Bufonidae: Atelopus)

Toads of the genus Atelopus are chemically defended by a unique combination of endogenously synthesized cardiotoxins (bufadienolides) and neurotoxins which may be sequestered (guanidinium alkaloids). Investigation into Atelopus small-molecule chemical defenses has been primarily concerned with identifying and characterizing various forms of these toxins while largely overlooking their ecological roles and evolutionary implications. In addition to describing the extent of knowledge about Atelopus toxin structures, pharmacology, and biological sources, we review the detection, identification, and quantification methods used in studies of Atelopus toxins to date and conclude that many known toxin profiles are unlikely to be comprehensive because of methodological and sampling limitations. Patterns in existing data suggest that both environmental (toxin availability) and genetic (capacity to synthesize or sequester toxins) factors influence toxin profiles. From an ecological and evolutionary perspective, we summarize the possible selective pressures acting on Atelopus toxicity and toxin profiles, including predation, intraspecies communication, disease, and reproductive status. Ultimately, we intend to provide a basis for future ecological, evolutionary, and biochemical research on Atelopus.

Response of gut microbiota to feed-borne bacteria depends on fish growth rate: a snapshot survey of farmed juvenile Takifugu obscurus

Environmental bacteria have a great impact on fish gut microbiota, yet little is known as to where fish acquire their gut symbionts, and how gut microbiota response to the disturbance from environmental bacteria. Through the integrative analysis by community profiling and source tracking, we show that feed-associated bacteria can impose a strong disturbance upon the hindgut microbiota of cultured fugu. Consequently, marked alterations in the composition and function of gut microbiota in slow growth fugu were observed, implying a reduced stability upon bacterial disturbance from feed. Moreover, quantitative ecological analyses indicated that homogeneous selection and dispersal limitation largely contribute to the community stability and partial variations among hosts in the context of lower degree of disturbance. While the disturbance peaked, variable selection leads to an augmented interaction within gut microbiota, entailing community unstability and shift. Our findings emphasized the intricate linkage between feed and gut microbiota and highlighted the importance of resolving the feed source signal before the conclusion of comparative analysis of microbiota can be drawn. Our results provide a deeper insight into aquaculture of fugu and other economically important fishes and have further implications for an improved understanding of host-microbe interactions in the vertebrate gastrointestinal tract.

The biology and ecology of the invasive silver-cheeked toadfish (Lagocephalus sceleratus), with emphasis on the Eastern Mediterranean

Invasive species pose threats to either human health or inflict ecological and/or economic damage. The silver-cheeked toadfish (Lagocephalus sceleratus), a Lessepsian species, is one of the most harmful species in the Mediterranean Sea, because of its potent neurotoxin, impacts on marine biodiversity, and the increased costs and labor they inflict on fishers. Since the catch and consumption of this pufferfish is prohibited by almost all countries bordering the Mediterranean, they have now expanded into the entire Mediterranean and Black Sea. We performed a comprehensive study of L. sceleratus covering ecological aspects, growth, reproduction, diet and trophic level based on samples from southwestern coasts of Turkey. The estimated growth parameters were L∞ = 88.7 cm, K = 0.27 year-1, C = 0.6 and WP = 0.1. Their sex-ratio was M/F = 1:0.69. Lagocephalus sceleratus appears to be a batch spawner with discontinuous oocyte recruitment and has different spawning seasons in the Eastern Mediterranean which seem to be based on temperature cues which get shorter in duration as one moves north from the Suez. We also report their first positive ecological trait, that they are controlling some other invasive species through their diets, such as lionfish, Red Sea goatfish, rabbitfish and longspine sea urchins, in addition to controlling themselves through cannibalism, which appears to be density-dependent. They are indeed a top predator in the region with a trophic level of 4.1. We suggest that targeted fishing using improved gear-types to reduce fishing gear damages are initiated, and that finding commercial markets for pufferfish could help to naturally fund ongoing control efforts.

Tetrodotoxin and Its Analogues in Cephalothrix cf. simula (Nemertea: Palaeonemertea) from the Sea of Japan (Peter the Great Gulf): Intrabody Distribution and Secretions

Some nemertean species from the genus Cephalothrix accumulate tetrodotoxin (TTX) in extremely high concentrations. The current study is the first to provide high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) data on tetrodotoxin and its analogues (TTXs) profile and concentration in different regions and organs of Cephalothrix cf. simula, and its secretions produced in response to stimulation. Different specimens of C. cf. simula possessed 7-11 analogues, including nine previously found in this species and two new for nemerteans-4,9-anhydro-8-epi-5,6,11-trideoxyTTX and 1-hydroxy-8-epi-5,6,11-trideoxyTTX. The study of the toxins' distribution in different regions and organs of nemerteans revealed the same qualitative composition of TTXs throughout the body but differences in the total concentration of the toxins. The total concentration of TTXs was highest in the anterior region of the body and decreased towards the posterior; the ratio of the analogues also differed between regions. The data obtained suggest a pathway of TTXs uptake in C. cf. simula and the role of toxins in the life activity of nemerteans.

Phylogeny and Toxin Profile of Freshwater Pufferfish (Genus Pao) Collected from 2 Different Regions in Cambodia

The species classification of Cambodian freshwater pufferfish is incomplete and confusing, and scientific information on their toxicity and toxin profile is limited. In the present study, to accumulate information on the phylogeny and toxin profile of freshwater pufferfish, and to contribute to food safety in Cambodia, we conducted simultaneous genetic-based phylogenetic and toxin analyses using freshwater pufferfish individuals collected from Phnom Penh and Kratie (designated PNH and KTI, respectively). Phylogenetic analysis of partial sequences of three mitochondrial genes (cytochrome b, 16S rRNA, and cytochrome c oxidase subunit I) determined for each fish revealed that PNH and KTI are different species in the genus Pao (designated Pao sp. A and Pao sp. B, respectively). A partial sequence of the nuclear tributyltin-binding protein type 2 (TBT-bp2) gene differentiated the species at the amino acid level. Instrumental analysis of the toxin profile revealed that both Pao sp. A and Pao sp. B possess saxitoxins (STXs), comprising STX as the main component. In Pao sp. A, the toxin concentration in each tissue was extremely high, far exceeding the regulatory limit for STXs set by the Codex Committee, whereas in Pao sp. B, only the skin contained high toxin concentrations. The difference in the STX accumulation ability between the two species with different TBT-bp2 sequences suggests that TBT-bp2 is involved in STX accumulation in freshwater pufferfish.