Plasma protein binding of tetrodotoxin in the marine puffer fish Takifugu rubripes

Epidermal distribution of tetrodotoxin-rich cells in newly hatched larvae of Takifugu spp

Pufferfish of the genus Takifugu possess tetrodotoxin (TTX), known as "pufferfish toxin" and it is believed that pufferfish eggs and newly hatched larvae utilize TTX as a defensive substance against predators. However, the mechanism for the placement of TTX to specific cells on the larval body surface during the developmental process remains unknown. In this study, we clarify the distribution and characteristics of TTX-rich cells. We performed whole-mount immunohistochemistry (IHC) using anti-TTX monoclonal antibody on larvae of two pufferfish species, Takifugu rubripes and Takifugu alboplumbeus, just after hatching. This allowed observation of the TTX location and compared it with those of wheat germ agglutinin (WGA)-positive (periodic acid-Schiff (PAS)-positive) cells for mucous cells and IHC using anti-Na+/K+-ATPase (NKA) monoclonal antibody for ionocytes. As a result, uniformly scattered localization of TTX-rich cells was commonly observed in the epidermis of the larvae of the two Takifugu species. TTX-rich cells were WGA-negative (PAS-negative) and structurally distinct from NKA-positive cells, suggesting that TTX-rich cells are unreported small cells unique to pufferfish skin, but not mucous cells nor ionocytes.

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.

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.

Molecular Characterization of a New Tetrodotoxin-Binding Protein, Peroxiredoxin-1, from Takifugu bimaculatus

Pufferfish are considered a culinary delicacy but require careful preparation to avoid ingestion of the highly toxic tetrodotoxin (TTX), which accumulates in certain tissues. In this study, the tissue distribution of peroxiredoxin-1 from Takifugu bimaculatus was investigated. The peroxiredoxin-1 protein was obtained by in vitro recombinant expression and purification. The recombinant protein had a strong ability to scavenge hydroxyl radicals, protect superhelical DNA plasmids from oxidative damage, and protect L929 cells from H₂O₂ toxicity through in vitro antioxidant activity. In addition, we verified its ability to bind to tetrodotoxin using surface plasmon resonance techniques. Further, recombinant proteins were found to facilitate the entry of tetrodotoxin into cells. Through these analyses, we identified, for the first time, peroxiredoxin-1 protein from Takifugu bimaculatus as a potential novel tetrodotoxin-binding protein. Our findings provide a basis for further exploration of the application of peroxiredoxin-1 protein and the molecular mechanisms of tetrodotoxin enrichment in pufferfish.

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.

Molecular Characterization of the Von Willebrand Factor Type D Domain of Vitellogenin from Takifugu flavidus

The von Willebrand factor type D (VWD) domain in vitellogenin has recently been found to bind tetrodotoxin. The way in which this protein domain associates with tetrodotoxin and participates in transporting tetrodotoxin in vivo remains unclear. A cDNA fragment of the vitellogenin gene containing the VWD domain from pufferfish (Takifugu flavidus) (TfVWD) was cloned. Using in silico structural and docking analyses of the predicted protein, we determined that key amino acids (namely, Val¹¹⁵, ASP¹¹⁶, Val¹¹⁷, and Lys¹²²) in TfVWD mediate its binding to tetrodotoxin, which was supported by in vitro surface plasmon resonance analysis. Moreover, incubating recombinant rTfVWD together with tetrodotoxin attenuated its toxicity in vivo, further supporting protein–toxin binding and indicating associated toxicity-neutralizing effects. Finally, the expression profiling of TfVWD across different tissues and developmental stages indicated that its distribution patterns mirrored those of tetrodotoxin, suggesting that TfVWD may be involved in tetrodotoxin transport in pufferfish. For the first time, this study reveals the amino acids that mediate the binding of TfVWD to tetrodotoxin and provides a basis for further exploration of the molecular mechanisms underlying the enrichment and transfer of tetrodotoxin in pufferfish.

Detoxification roles of tributyltin-binding protein type 2 in Japanese medaka (Oryzias latipes) exposed to tributyltin

Tributyltin-binding protein type 2 (TBT-bp2), a homolog of α1-acid glycoprotein, may contribute to both accumulation and detoxification of TBT in fish. In this study, we conducted acute TBT exposure tests using both wide-type (WT) and TBT-bp2^-/- (KO) strains of medaka and compared their responses in survival time and accumulation of TBT. Deficiency of TBT-bp2 significantly accelerated the time to death of medaka and decreased the LC₅₀ of TBT, indicating that the KO-strain is more sensitive to TBT. No significant difference in the intrinsic TBT concentration in surviving fish was observed between the two strains. However, the intrinsic TBT concentration in dead KO-strain was significantly lower than that in WT-strain. These findings provide direct evidence, supporting the hypothesis that TBT-bp2 plays a critical role in the detoxification of TBT in fish.

The evolution and origin of tetrodotoxin acquisition in the blue-ringed octopus (genus Hapalochlaena)

Tetrodotoxin is a potent non-proteinaceous neurotoxin, which is commonly found in the marine environment. Synthesised by bacteria, tetrodotoxin has been isolated from the tissues of several genera including pufferfish, salamanders and octopus. Believed to provide a defensive function, the independent evolution of tetrodotoxin sequestration is poorly understood in most species. Two mechanisms of tetrodotoxin resistance have been identified to date, tetrodotoxin binding proteins in the circulatory system and mutations to voltage gated sodium channels, the binding target of tetrodotoxin with the former potentially succeeding the latter in evolutionary time. This review focuses on the evolution of tetrodotoxin acquisition, in particular how it may have occurred within the blue-ringed octopus genus (Hapalochlaena) and the subsequent impact on venom evolution.

Urinary Excretion of Tetrodotoxin Modeled in a Porcine Renal Proximal Tubule Epithelial Cell Line, LLC-PK₁

This study examined the urinary excretion of tetrodotoxin (TTX) modeled in a porcine renal proximal tubule epithelial cell line, LLC-PK₁. Time course profiles of TTX excretion and reabsorption across the cell monolayers at 37 °C showed that the amount of TTX transported increased linearly for 60 min. However, at 4 °C, the amount of TTX transported was approximately 20% of the value at 37 °C. These results indicate that TTX transport is both a transcellular and carrier-mediated process. Using a transport inhibition assay in which cell monolayers were incubated with 50 µM TTX and 5 mM of a transport inhibitor at 37 °C for 30 min, urinary excretion was significantly reduced by probenecid, tetraethylammonium (TEA), l-carnitine, and cimetidine, slightly reduced by p-aminohippuric acid (PAH), and unaffected by 1-methyl-4-phenylpyridinium (MPP+), oxaliplatin, and cefalexin. Renal reabsorption was significantly reduced by PAH, but was unaffected by probenecid, TEA and l-carnitine. These findings indicate that TTX is primarily excreted by organic cation transporters (OCTs) and organic cation/carnitine transporters (OCTNs), partially transported by organic anion transporters (OATs) and multidrug resistance-associated proteins (MRPs), and negligibly transported by multidrug and toxic compound extrusion transporters (MATEs).

The Bacterial (Vibrio alginolyticus) Production of Tetrodotoxin in the Ribbon Worm Lineus longissimus-Just a False Positive?

We test previous claims that the bacteria Vibrio alginolyticus produces tetrodotoxin (TTX) when living in symbiosis with the nemertean Lineus longissimus by a setup with bacteria cultivation for TTX production. Toxicity experiments on the shore crab, Carcinus maenas, demonstrated the presence of a paralytic toxin, but evidence from LC-MS and electrophysiological measurements of voltage-gated sodium channel-dependent nerve conductance in male Wistar rat tissue showed conclusively that this effect did not originate from TTX. However, a compound of similar molecular weight was found, albeit apparently non-toxic, and with different LC retention time and MS/MS fragmentation pattern than those of TTX. We conclude that C. maenas paralysis and death likely emanate from a compound <5 kDa, and via a different mechanism of action than that of TTX. The similarity in mass between TTX and the Vibrio-produced low-molecular-weight, non-toxic compound invokes that thorough analysis is required when assessing TTX production. Based on our findings, we suggest that re-examination of some published claims of TTX production may be warranted.

Distribution of tetrodotoxin in the ribbon worm Lineus alborostratus (Takakura, 1898) (nemertea): Immunoelectron and immunofluorescence studies

Transmission electron and confocal laser scanning (CLSM) microscopies with monoclonal anti-tetrodotoxin antibodies were used to locate tetrodotoxin (TTX) in tissues and gland cells of the ribbon worm Lineus alborostratus. CLSM studies have shown that the toxin is primarily localized in the cutis (special subepidermal layer) of the body wall and in the glandular epithelium of the proboscis. Immunoelectron micrographs have shown that only subepidermal bacillary gland cells type I in cutis and pseudocnidae-containing and mucoid gland cells manifested TTX-gold labeling. TTX was associated with the nuclear envelope, endoplasmic reticulum membrane, and secretory granules of TTX-positive gland cells. These studies indicate that ТТХ is brought into the cytoplasm of the glandular cells of the cutis and proboscis epithelium, where it is associated with membrane-enclosed organelles involved in protein secretion and then concentrated in glandular granules.

Investigating diet as the source of tetrodotoxin in Pleurobranchaea maculata

The origin of tetrodotoxin (TTX) is highly debated; researchers have postulated either an endogenous or exogenous source with the host accumulating TTX symbiotically or via food chain transmission. The aim of this study was to determine whether the grey side-gilled sea slug (Pleurobranchaea maculata) could obtain TTX from a dietary source, and to attempt to identify this source through environmental surveys. Eighteen non-toxic P. maculata were maintained in aquariums and twelve were fed a TTX-containing diet. Three P. maculata were harvested after 1 h, 24 h, 17 days and 39 days and TTX concentrations in their stomach, gonad, mantle and remaining tissue/fluids determined using liquid chromatography-mass spectrometry. Tetrodotoxin was detected in all organs/tissue after 1 h with an average uptake of 32%. This decreased throughout the experiment (21%, 15% and 9%, respectively). Benthic surveys at sites with dense populations of toxic P. maculata detected very low or no TTX in other organisms. This study demonstrates that P. maculata can accumulate TTX through their diet. However, based on the absence of an identifiable TTX source in the environment, in concert with the extremely high TTX concentrations and short life spans of P. maculata, it is unlikely to be the sole TTX source for this species.

Localization of pufferfish saxitoxin and tetrodotoxin binding protein (PSTBP) in the tissues of the pufferfish, Takifugu pardalis, analyzed by immunohistochemical staining

Pufferfish saxitoxin and tetrodotoxin binding protein (PSTBP) was previously isolated from the plasma of the marine pufferfish, Takifugu pardalis. In this study, we investigated distribution pattern of PSTBP in intestine, liver, ovary, skin, and skeletal muscle of T. pardalis by immunohistochemical staining for the study of functions of this protein. In the skin, dermis around the tetrodotoxin secreting gland was positive, while this secreting gland itself was negative. In the ovary containing vitellogenic oocytes, ovarian wall and vitelline envelope were positive, while yolk and nucleus were negative. In the liver, hepatocytes with large fat droplets and capillaries were positive. In the intestine, the lamina propria mucosae were positive, while the mucosal epithelium was negative. In the skeletal muscle, only capillaries were positive. Furthermore, liver specific expression of PSTBP was confirmed by Northern blot analysis. Based on these results together with reported tetrodotoxin localization pattern in pufferfish, PSTBP was assumed to be a carrier protein to transfer tetrodotoxin among the tissues, especially liver, ovary, and skin.

Distribution of homologous proteins to puffer fish saxitoxin and tetrodotoxin binding protein in the plasma of puffer fish and among the tissues of Fugu pardalis examined by Western blot analysis

Puffer fish saxitoxin and tetrodotoxin binding protein (PSTBP) is a glycoprotein (200 kDa as a dimer) that we previously isolated from the plasma of Fugu pardalis (Yotsu-Yamashita et al., 2001). For the study on functions of PSTBP, here we examined distribution of homologous proteins to PSTBP in the plasma of seven species of puffer fish, and among the tissues of F. pardalis by Western blot analysis probed with a polyclonal IgG against unglycosylated PSTBP1 expressed in Echelichia coli. One or two major positive broad bands were detected at 105-140 kDa molecular weight range in the plasma (0.5 microg protein) of all species of puffer fish tested, while no band was detected in the plasma (5 microg protein) of fish other than puffer fish. Glycopeptidase F treated plasma of all species of puffer fish tested commonly showed the bands at approximately 42 kDa that was consistent to the molecular weight of unglycosylated PSTBP. These data suggest that puffer fish commonly possess glycoproteins homologous to PSTBP, but the sizes of N-glycan are specific to the species. Among soluble protein extracts (5 microg protein) from the tissues of F. pardalis, PSTBP was detected in all tissues examined, most prominently in heart, skin, and gall.