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Gymnodimine-A (also known as GYM A and (-)-Gymnodimine) is a marine lipophilic biotoxin that blocks nicotinic acetylcholine receptors at a neuromuscular junction. It is a secondary metabolic product of microalgae species; when consumed by shellfish, it accumulates in their flesh and therefore becomes a potential trigger of shellfish poisoning. The reason behind gymnodimine-A synthesis by microalgae is unknown; however, it is suggested that this secondary toxic metabolite is used as defense or in resource competition.

Etymology and history
The name for the gymnodimine family of toxins comes from the genus of dinoflagellates Gymnodiminum.

Gymnodimine-A, alongside other toxins of the gymnodimine family, was discovered in 1994, during a routine toxin monitoring in bivalve mollusks in the South Island of New Zealand. The toxin was later also detected in molluscs harvested in Tunisia, as well as near European and North American coasts.

Gymnodimine-A is known to be a fast-acting toxin, which leads to rapid death when injected in mice. The reason behind its neurotoxic activity is its high affinity for nicotinic acetylcholine receptors (nAChRs), including human receptors. Still, no human poisoning has been attributed to gymnodimine-A consumption and its toxicity to humans is considered to be of low danger. Despite that and the fact that the maximum level of gymnodimine-A found in mass consumption shellfish was lower than the regulatory standards, a potential risk of consumer poisoning might still be present.

Source
Gymnodimine-A, as well as other gymnodimines, has first been extracted from shellfish contaminated with the toxin by the intake of the dinoflagellate Karenia selliformis. Besides Karenia selliformis, the other algal bloom species that is known to produce gymnodimine-A is a dinoflagellate Alexandrium ostenfeldii. As a result of consumption of algae, the presence of gymnodimine-A has been observed in various molluscs, such as Donax deltoides, Modiolus proclivis and Saccostrea glomerata, as well as in seawater across the world.

Chemistry
The first molecule of the gymnodimine family, was gymnodimine-A, the structure of which got revealed with the use of nuclear magnetic resonance method.

Gymnodimines belong to a cyclic imine toxins family sharing the spirocyclic imine ring system, but in contrast to spirolides (other shellfish toxins, presenting the largest group cyclic imine toxins family), gymnodimines contain a trisubstituted tetrahydrofuran embedded within a 16-membered macrocycle. The spirocyclic imine ring is common for spirolides, pinnatoxins , pteriatoxins and prorocentrolides.

Gymnodimines, including gymnodimine-A, change the radical for a CH3 (R1 on the picture) (image reference: Bourne, 2010).

Gymnodimines form homopentameric assemblies of subunits homologous to the N-terminal of ligand-binding domain of nAChR. The aromatic side chains of these subunits form a positively-charged substrate structure, which has the functional residues required to match a negatively-charged nAChR ligand-binding domain, which is the reason behind the high affinity of gymnodimine-A to nAChR.

Target and mode of action
The nanomolar concentration (10-100 nM) of gymnodimine blocks neuromuscular transmission in a concentration and time-dependent manner. Gymnodimine disrupts interaction of acetylcholine (ACh) quanta and nAChRs. Gymnodimine decreases the amplitude or, at higher concentrations, completely blocks spontaneous [https://en.wikipedia.org/wiki/End-plate_potential#:~:text=In%20the%20absence%20of%20an,by%20one%20acetylcholine%2Dcontaining%20vesicle. miniature endplate potentials]. Gymnodimine has no impact on muscle resting potential, that is why directly stimulated muscles fully contract nonetheless. Gymnodimine prevents |voltage-gated sodium channels in muscle fibers from reaching the threshold to open, therefore a muscle action potential would not be triggered by nerve stimulation.

Gymnodimine-A targets both neuronal and muscle nAChRs receptors with low selectivity. It has been shown that gymnodimine-A is capable of acting as an antagonist with high affinity towards homomeric human α7 nAChRs, as well as towards muscular α3β2 and neuronal α4β2 nAChRs. In neuronal nAChRs gymnodimine inhibits Ca2+ influx, which happens under normal conditions after nicotinic stimulation of the α7 nAChRs.

The dissociation equilibrium constant Kd for gymnodimine-A to the AChR binding pocket was shown to be as low as 0.0047 nM in Aplysia californica and 0.0013 nM in Lymnaea stagnalis. Compared to the dissociation equilibrium constant for ACh in mice, which is 0.16 nM, it suggests that gymnodimine-A dissociates from nAChRs with less propensity than ACh. In addition to that, the competition binding constant for gymnodimine-A in a competition assay for binding muscle- and neuronal-type nAChRs expressed in HEK293 cells was found to be 0.62 nM for human neuronal α4β2 receptor and 0.24 nM for human muscular α3β2 receptor, with the competition being against an alkaloid agonist epibatidine and a peptidic antagonist α-bungarotoxin. These toxicokinetic properties of gymnodimine-A provide further evidence for the toxin’s high affinity to nAChRs.

Toxicity
Gymnodimine leads to fast neurotoxic mortality in mice, disrupting the respiratory neuromuscular transmission. The way it exhibits its toxic activity is by preventing nerve-evoked twitch responses, without impacting muscle contraction. Mice injected with a lethal dose (80-96 μg/kg intraperitoneally or 3 μg/kg [https://en.wikipedia.org/wiki/Intracerebroventricular_injection#:~:text=Intracerebroventricular%20injection%20(also%20called%20ICV,bypass%20the%20blood%E2%80%93brain%20barrier. intracerebroventricularly]) of gymnodimine-A immediately express hyperactivity, followed by a decrease in exploratory behaviors; subsequently, hind legs become paralyzed, and later animals completely stop responding to stimuli; dyspnea follows and death occurs within 3 minutes post-injection . Gymnodimine-A is less toxic in the case of subcutaneous injection, and even less toxic when taken orally. The administration of the toxin decreases the number of neurons and makes them more susceptible to apoptosis triggered by the okadaic acid, often found in the same shellfish as gymnodimine-A.