No mushroom for error – deadly fungi and the search for an antidote
When I am not in a lab growing nerve cells, I often wander out into forests and forage for edible plants and fungi. To my hiking companions’ dismay and annoyance, I stop by each mushroom to assess its edibility – or just simply to admire the only visible part of the vast underground network of mycelia under my feet, the crucial role of which in the ecosystems we are only just starting to appreciate. And, of course, I take many pictures of them. After all, the undisputed king of all toadstools, the beautiful but inedible fly agaric (Amanita muscaria), with its bright red cap adorned with white specks, looks better in pictures rather than on dinner plates. Aside from the need for pictures in atlases, do photography and imaging have anything else in common with deadly mushrooms? The unlikely link was discovered recently by a team of toxicologists in Australia and China, and comes in the form of a dye called indocyanine green.
Developed in the mid-1950s by Kodak researchers for use in near-infrared photography, indocyanine green (ICG for short) was approved for medical imaging purposes shortly after its discovery, although its routine use skyrocketed in the 1990s as the medical imaging and camera technology improved. Amongst ICG’s primary uses in its early days, and still the most popular applications today, are angiography and the study of liver blood flow and function.
The liver is also the site of havoc-wreckage where our anti-hero centres its powers. Enter alpha-amanitin, the toxic peptide which ended up giving Amanita, Galerina, and Lepiota mushrooms containing it such delightfully apt names like death cap (Amanita phalloides), destroying angel (Amanita virosa), deadly parasol (Lepiota subincarnata) or funeral bell (Galerina marginata). This potent hepatotoxin is responsible for the vast majority of mushroom poisoning cases worldwide now and throughout human fungivore history, ending, for example, the reign of the Roman Emperor Claudius in 54 CE, and the Holy Roman Emperor Charles VI in 1740. On the level of individual hepatocytes, alpha-amanitin inhibits RNA polymerase enzymes responsible for turning DNA into messenger RNA, thus preventing the production of new proteins and causing gradual cell death. The more hepatocytes die, the more liver enzymes and toxins associated with abrupt cell death are released into the bloodstream, causing a domino effect of organ damage. Alpha-amanitin poisoning survivors who recover from liver failure often suffer life-long chronic liver diseases – and that’s for those individuals who did not have to undergo liver or kidney transplantation.
There is no cure for alpha-amanitin poisoning and the success of supportive care depends on how fast medical assistance was sought, what treatment (if any) was administered, and, crucially, how much mushroom was eaten. Silibinin, a compound extracted from the milk thistle plant, showed promise in protecting the liver from poisoning and although it is effective in chronic and acute toxic liver disease, it is no match for deadly mushrooms. But here is where our hero, ICG, comes in. Using the famous, Nobel Prize-winning, gene-editing technology called CRISPR-Cas9, a team of scientists engineered human cells growing in a dish to have defects in different genes and then treated them with alpha-amanitin (1). One of those mutated genes, carrying a code for STT3B, made the cells resistant to amanitin. STT3B is an oligosaccharyltransferase, an enzyme responsible for adding sugar molecules to proteins, which ensures the cells function properly (but if you were picturing your Sunday roast getting coated in sugar, don’t worry – I did the same).
Identifying the key player in mitigating alpha-amanitin poisoning made the job of screening antidote candidates easier. From over 3000 tested compounds, one winner emerged – ICG, slashing the likelihood of death from alpha-amanitin poisoning down to 50% in mice whilst protecting the liver from necrosis. Although you might think that giving you a 50:50 chance of survival can hardly be called a miraculous cure (as opposed to a brand new potential snake antivenom which stopped all mice from dying [2]), the study was not yet done in human volunteers (or hapless foragers, for that matter). ICG is already used in people, is itself non-toxic, and was identified as a treatment candidate using a reliable method which the study authors used previously to find an antidote for jellyfish venom (3). With many other poisons with no remedy (like bacterial toxins which lead to sepsis), such results and potential applications of this technology are, indeed, very exciting.
One question remains – why did unrelated species of Amanita, Lepiota, and Galerina develop the ability to synthesise amanitin? Although recent studies showed that this likely occurred via horizontal gene transfer over the course of their evolution (4), the ecological benefit of such adaptation remains unknown. But then again, who wants to be eaten? Many species spanning both kingdoms of flora and fauna have evolved various ways of protecting themselves from being consumed. Whilst the early ancestors of our humble jalapeño, for example, could not predict that eventually no amount of capsaicinoids is going to stop the notorious Homo sapiens from eating them and challenging the limits of the Scoville Scale, mushrooms figured out that death or severe sickness might be the only feasible ways of keeping the hungry critters at bay. Unlike aposematic plants and animals which developed a colourful, often flamboyant, and above all, frankly, superbly polite system of warning their predators about their unpalatability, mushrooms look unassuming and have too many poisonous look-alikes to take the risk, even with potential treatments like ICG. With poisonings, like with most illnesses, time is the name of the game and symptoms might not develop or be recognised until it’s too late, when even the most potent antidote cannot reverse the damage.
Therefore, my message to all aspiring foragers is – never be less than absolutely certain that what you put in your basket is safe to consume. If not, a photo will make a nice memento – or a memento mori, if you like. That’s the morel (sorry, I had to) of the story.
References:
- Wang, B., Wan, A.H., Xu, Y. et al. Identification of indocyanine green as a STT3B inhibitor against mushroom α-amanitin cytotoxicity. Nat Commun 14, 2241 (2023).
- Irene S. Khalek et al. Synthetic development of a broadly neutralizing antibody against snake venom long-chain α-neurotoxins. Sci. Transl. Med.16, eadk1867 (2024).
- Lau M.T., Manion J., Littleboy J.B., Oyston L., Khuong T.M., Wang Q.P., Nguyen D.T., Hesselson D., Seymour J.E., Neely G.G. Nat Commun. 10(1):1655 (2019).
- Luo, H., Hallen-Adams, H.E., Lüli, Y., Sgambelluri, R.M., Li, X., Smith, M., Yang, Z.L., Martin, F.M. Genes and evolutionary fates of the amanitin biosynthesis pathway in poisonous mushrooms. Proc Natl Acad Sci U S A. 119(20), e2201113119 (2022).
Monika Myszczynska is a postdoctoral research associate in the group of Professor Guillaume Hautbergue at The University of Sheffield’s Neuroscience Institute. Her research focuses on the non-cell autonomous mechanisms of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), and in particular on the involvement of astrocytes. She completed her PhD in 2021 at The University of Sheffield under the supervision of Professor Laura Ferraiuolo, investigating the repurposing potential of various compounds in the context of ALS.