Life Sciences Division

One of the key elements in the fight against bioterrorism will be making research breakthroughs on high-priority biological agents as a platform for developing attack detection and decontamination tools and effective treatment protocols. Near the top of the danger list, the category B biological agent ricin has been implicated in cases of criminal intimidation, political assassinations, biocrime and suicides. Its effects are generally irreversible. Following exposure, symptoms appear within a few hours, and death follows within three to five days. There is currently no antidote, and treatment is purely symptomatic.

In order to find a specific treatment against ricin, and most importantly, molecules capable of blocking its cellular toxicity, the team ran cell-based high-throughput screening based on a databank containing 16,500 chemical compounds. They managed to pinpoint two molecules capable of blocking the toxic action of ricin. Looking to understand the mechanisms mobilized in this toxicity, the team studied the action of the two candidate compounds on the intracellular mechanisms governing toxin trafficking – including ricin – inside the cell. Their experiments showed that the two molecules, dubbed Retro-1 and Retro-2, selectively blocked toxin trafficking at the interface between early endosomes and the Golgi apparatus. These two cell compartments are involved in the transport process carrying molecules from outside into the cell interior – a process called retrograde transport (see diagram). Contrary to other known retrograde transport blockers, Retro-1 and Retro-2 leave the rest of the Golgi apparatus unaffected. Furthermore, they have the added advantage of being highly specific, and do not block the retrograde transport of proteins essential to cell functionality. They also act without affecting other transport systems or any of the organelle cargoes involved, thus demonstrating a complete lack of toxicity for cells or for the animals as whole. In vivo trials led by the team showed that at least one of the compounds, when injected before a lethal nasally-administered ricin challenge, clearly protected all mice exposed (with survival jumping from 15% to 100 % at the highest Retro-2 dose given).

These are the first molecules to show efficacy against the toxic effects of ricin in animal experiments, making them very strong candidates as therapeutic targets. The outreach from these discoveries extends way beyond countermeasures against bioterrorism. In the process of the study, the team showed that the effect of these molecules is not limited to ricin but also extends to other toxins, including Shiga toxins and cholera toxin. These findings open up perspectives for developing therapies against the Shiga-triggered haemolytic–uremic syndrome, a potentially fatal infection caused by E. coli strains and particularly virulent in infants under 36-months of age.

Caption: Toxins get captured by the cell and then transit through a series of cellular compartments (endosomes, Golgi apparatus, endoplasmic reticulum) until they penetrate the cytoplasm, where their toxicity develops. Retro-1 and Retro-2 block the trafficking at the endosome-to-Golgi apparatus interface, but without disturbing the pathways transporting cellular proteins across the cell compartments.

[1] Secretariat-General for National Defence and Security
[2] French Atomic Energy and Alternative Energies Commission, Institute of Biotechnology – Saclay (iBiTec-S), CEA Life Sciences Division.
[3] French Health Products Safety Agency (Afssaps), Testing Laboratories and Inspections Directorate.
[4] Shiga toxin: toxin produced by Shigella bacteria, and which causes an infectious bacillary dysentery disease called shigellosis.

These findings were covered in a press release dated 15 April 2010