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

Abstract

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.

Keywords: TTX biosynthesis; TTX transporter accumulation; puffer fish; tetrodotoxin.

Figure 4

Tetrodotoxin (TTX) biosynthesis processes [93,94,95]. (A): arginine synthesis; (1, the synthetic process of polyketone derivatives; 2, the sugar-derived TTX carbon backbone process); “?” represents an as yet unspecified step in the biosynthesis of TTX; (B): monoterpene compound synthesis process (labeled red functional group changes position for TTX biosynthesis); The parts marked in red refer to changes in functional groups;C5-O-C10: An ether bond is formed between the fifth and tenth carbons; and (C): intermediate compounds synthesis (Tb-210B, Tb-226, Tb-242A, Tb-242C, Tb-258), which are isolated intermediate in the biosynthesis of pufferfish TTX. These figures are re-published with permission from copyright (2018) Chemistry-A European Journal, copyright (2014) Angewandte Chemie International Edition, and copyright (2019) American Chemical Society.

Figure 4

Tetrodotoxin (TTX) biosynthesis processes [93,94,95]. (A): arginine synthesis; (1, the synthetic process of polyketone derivatives; 2, the sugar-derived TTX carbon backbone process); “?” represents an as yet unspecified step in the biosynthesis of TTX; (B): monoterpene compound synthesis process (labeled red functional group changes position for TTX biosynthesis); The parts marked in red refer to changes in functional groups;C5-O-C10: An ether bond is formed between the fifth and tenth carbons; and (C): intermediate compounds synthesis (Tb-210B, Tb-226, Tb-242A, Tb-242C, Tb-258), which are isolated intermediate in the biosynthesis of pufferfish TTX. These figures are re-published with permission from copyright (2018) Chemistry-A European Journal, copyright (2014) Angewandte Chemie International Edition, and copyright (2019) American Chemical Society.

Figure 5

Hypothetical transporter accumulation pathway of TTX in pufferfish liver [117,119,125]. Solid line: physiological processes occurring in the liver of the puffer fish; dashed line: indicates that this process is currently hypothesized and has not yet been confirmed by sufficient data. ➀ Hepatocytes take up only TTX and consume energy through membrane proteins, and the binding protein PSTBP does not enter the cell; ➁ the TTX-PSTBP conjugate enters the hepatocyte through the alkaline compounds transferring the membrane proteins and is then dissociated by acidic catabolic factors into free PSTBP and TTX, and PSTBP then diffuses to the extracellular space through the correlated protein channel; ➂ TTX-PSTBP enters the hepatocyte through cytosolization into hepatocytes, then dissociates into free PSTBP and TTX via acidic catabolic factors, and PSTBP is secreted extracellularly via the cytosol or exosomes. The figure was drawn using figdraw 2.0.

Figure 1

The structure of TTX and its analogs.

Figure 2

Amino acid sequences of the binding sites (α-subunit) for human and pufferfish sodium channels to TTX [15,70,71]. DI, DII, DIII, DIV: four homologous protein structural domains of the sodium channel α-subunit; S1–S6: six hydrophobic transmembrane segments of each homologous protein structural domains. Between S5 and S6 is the TTX binding site (green dot); DEKA: Amino acid sequences, Green color represents different amino acids.

Figure 3

Enhanced TTX nerve block effect and targeted anesthesia. (A): Schematic illustration of aptamer/TTX complexes and the block time on the rat sciatic nerve [86]; a. Peripheral nerve blockade with 42 µM TTX, free or complexed with aptamers (PO, PS, Scr-PS); b. Peripheral nerve blockade with varying molar ratios of PS/TTX complexes, using 42 µM TTX; c. Sciatic nerve blockade with free TTX and PS/TTX (2:1) in the injected hindpaws. The dagger indicates 100% mortality; d. Frequency of nerve block in the contralateral (uninjected) hindpaws; (B): schematic illustration of liposome-encapsulated TTX and the block time on the rat sciatic nerve [87]; a. Duration of sensory nerve block from different formulations. Daggers indicate 100% mortality; b. Frequency of block in the contralateral (uninjected) leg; c. Mortality from different formulations; d. Sensory nerve block with different molar percentages of branched lipids in the liposomal formulation, using 25 μg TTX; (C): schematic illustration of polymer and TTX system, and the block time on the rat sciatic nerve [88]; and (D): schematic illustration of hollow silica nanoparticles capped with TTX, and the block time on the rat sciatic nerve [89]; Sciatic nerve blockade with free TTX and TTX-HSN30. Effect of TTX dose on a. the median duration of sensory nerve blocks, b. the frequency of successful blocks, c. the frequency of nerve block in the uninjected (contralateral) extremity, and d. of death.

Figure 3

Enhanced TTX nerve block effect and targeted anesthesia. (A): Schematic illustration of aptamer/TTX complexes and the block time on the rat sciatic nerve [86]; a. Peripheral nerve blockade with 42 µM TTX, free or complexed with aptamers (PO, PS, Scr-PS); b. Peripheral nerve blockade with varying molar ratios of PS/TTX complexes, using 42 µM TTX; c. Sciatic nerve blockade with free TTX and PS/TTX (2:1) in the injected hindpaws. The dagger indicates 100% mortality; d. Frequency of nerve block in the contralateral (uninjected) hindpaws; (B): schematic illustration of liposome-encapsulated TTX and the block time on the rat sciatic nerve [87]; a. Duration of sensory nerve block from different formulations. Daggers indicate 100% mortality; b. Frequency of block in the contralateral (uninjected) leg; c. Mortality from different formulations; d. Sensory nerve block with different molar percentages of branched lipids in the liposomal formulation, using 25 μg TTX; (C): schematic illustration of polymer and TTX system, and the block time on the rat sciatic nerve [88]; and (D): schematic illustration of hollow silica nanoparticles capped with TTX, and the block time on the rat sciatic nerve [89]; Sciatic nerve blockade with free TTX and TTX-HSN30. Effect of TTX dose on a. the median duration of sensory nerve blocks, b. the frequency of successful blocks, c. the frequency of nerve block in the uninjected (contralateral) extremity, and d. of death.