Life Sciences Division

Marine biotoxins are produced naturally by several species of single-cell algae. They can accumulate in the flesh of fish and shellfish, and are then referred to as phycotoxins. In humans, the consumption of shellfish contaminated by these substances can cause diarrheal, paralytic, neurological and other symptoms. Phycotoxins can spread rapidly throughout the world, especially via merchant ships emptying ballast tanks. As early as 1991, shellfish contamination outbreaks were observed in Canada, then along the coasts of Norway, Spain and Tunisia. In 2005, contaminated oysters were detected in the Arcachon Bay on the west coast of France, prompting the health authorities to impose a temporary ban on their sale.

A Franco-American collaborative project involving two joint CNRS/University laboratories in Marseille, a CNRS-owned laboratory, a CEA laboratory at Gif-sur-Yvette and an American laboratory at the University of California, has studied the mode of action of two types of phycotoxins, a spirolide and a gymnodimine. Both a “fast-acting” neurotoxins: injecting them into lab mice triggers severe neurological symptoms that prove fatal within minutes. The researchers have managed to characterize the target these toxins attack: a receptor that is essential in all living organism, the nicotinic acetylcholine receptor [3] (nRACh), a channel receptor situated on the membrane of muscle or nerve cells that allows the passage of small ionized molecules into and out of the cell. nRACh plays a crucial role in neuromuscular and neuronal transmission. More precisely, these toxins act by rapidly and almost irreversibly blocking the channel receptor function of nRACh. This inhibition then causes muscle and/or brain dysfunctions reminiscent of those observed during certain muscle diseases or cognitive disorders.

Moving on, the scientists then characterized how the two phycotoxins bind to the receptor. Resolved using X-ray crystallography, the 3D structures of the complexes that form between the phycotoxins and the receptor revealed that each toxin inserts itself at the heart of the binding site for acetylcholine, the natural neurotransmitter [4] of this receptor. This is a key strategic position in that it can block the channel receptor function of nRAChs. Of particular interest [5] was the discovery that the binding mode of these toxins could provide new opportunities for developing novel therapeutic agents able to act on nRAChs.

These in vitro results explain the neurotoxicity of these phycotoxins in numerous animal species. A clearer understanding of their mode of action constitutes the first step towards the development of antidotes that might become a public health and/or economic necessity. These findings therefore raise hopes for the design of new, reliable, sensitive, practical and inexpensive tests that could detect the presence of phycotoxins in shellfish offered to consumers.

AChBP, which is the protein the researchers have used as their nRACh model, is formed of five identical subunits (shown here in different colours). These subunits assemble into a ring, as seen from above (image left) and seen from the side, opposite the blue and yellow subunits (middle). The neurotransmitter inserts itself into the inter-subunit interfaces, i.e. precisely where each phycotoxin, one of which is shown at image-right, binds to block the nRACh channel-receptor function.

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Notes:

[1] Three CNRS units are involved: the Neurobiology–Neurophysiology Research Centre in Marseille (CNRS/Aix-Marseille Universities 2 and 3), the “Architecture and Function of Biological Macromolecules” Laboratory (CNRS/Aix-Marseille Universities 1 and 2) and the CNRS Cellular and Molecular Neurobiology Laboratory
[2] iBiTecS, Department of Molecular Protein Engineering, Laboratory of Molecular Toxicology, CEA/Life Sciences Division
[3] Acetylcholine was the first neurotransmitter to be discovered. It plays an important role in both the central nervous system, where it is involved in memory and learning, and the peripheral nervous system, where it controls muscle functioning.
[4] The chemical compounds released by neurons and which act on other neurons or on muscles, one of which is acetylcholine.
[5] The binding mode of these toxins is interesting, as it differs from that of other nRACh effectors.

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[These findings were covered in a press release dated 3 March 2010.]