Deciphering the biosynthetic gene cluster of a potent freshwater toxin

Scientists from the Scripps Institution of Oceanography at the University of California San Diego, the University of São Paulo and UC Santa Cruz collaborated to discover and validate the enzymes responsible for the production of one of the neurotoxins the most toxic and fast-acting agents associated with harmful freshwater algal blooms in lakes and ponds.

The team combined genetic and biochemical studies to show how freshwater cyanobacteria produce the powerful neurotoxin called guanitoxin. This discovery revealed that guanitoxin-producing cyanobacteria are more prevalent than originally known in the United States, opening the possibility for new molecular diagnostic tests to better inform and protect the public from this naturally occurring toxin. ‘pure water. The results were described in an article published in the Journal of the American Chemical Society May 18, 2022.

The paper “also shows that guanitoxin is produced in freshwater bodies that have experienced highly toxic events in the past,” said the study’s lead author, Stella Lima, a former doctoral student at the University. from São Paulo and guest researcher at Scripps Oceanography.

Guanitoxin is one of the most potent neurotoxins made by cyanobacteria that actually have a similar mechanism of action to pesticides and chemical warfare agents, said Timothy Fallon, Scripps postdoctoral researcher in Scripps Marine Chemical Biologist Bradley’s lab. Moore., where Lima was a visiting scholar.

Harmful algal blooms (HABs) form in lakes and ponds when cyanobacteria, otherwise known as blue-green algae, become abundant. These freshwater HABs produce different cyanotoxins, which can harm nearby animals and people. Depending on the cyanotoxins involved, those exposed experience symptoms such as stomach pain, headaches, vomiting, liver damage or neurological disorders, according to federal health officials. Over the years, many areas have declared emergencies and issued “do not drink” advisories. Pet and animal deaths have also been reported after animals came into contact with contaminated water.

Freshwater HABs can cause a myriad of social and economic problems for communities and are a problematic public health issue, Lima said. Testing and monitoring for some cyanotoxins, such as microcystin, cylindrospermopsin, saxitoxin, and anatoxin-a, is done because methods are available to do so, but despite the fact that guanitoxin is the second most common cyanotoxin more toxic, “no one is researching it” because the right methods are not available for detection and monitoring, Lima added.

As a doctoral student in 2016, Lima discovered a set of genes she suspected were responsible for making guanitoxin by a cyanobacterium isolated from a large freshwater bloom in Brazil. The strain was isolated from the Tapacurá Reservoir in Pernambuco, Brazil, and was maintained and cultivated by Marli Fiore, former Lima doctoral advisor and co-author of the study.

After this discovery, Lima sought a partnership to confirm his suspicions. So in 2018, she traveled to UC San Diego to work with Moore, who had established the first biochemical studies of guanitoxin in the early 1990s. The team of scientists worked together to establish the precise functions of the nine enzymes that convert an ordinary amino acid into a neurotoxin, Lima said.

After discovering the genes involved in guanitoxin production and carefully validating their functions, the researchers searched thousands of publicly available environmental data samples for guanitoxin biosynthetic genes.

The researchers were able to detect toxin genes for guanitoxin in environmental hotspots in the United States in populated areas, said Moore, co-corresponding author of the study.. The two areas of concern, where toxin genes were consistently detected for guanitoxin, were in Lake Erie near Toledo, Ohio and in Lake Mendota, Wisconsin. Other detection areas include the Amazon River in Brazil, youe Columbia River in Oregon and the Delaware River in Delaware.

“We found these genes in many different freshwater sources, but no one has looked for or monitored this particular toxin in the environment,” said Shaun McKinnie, assistant professor of chemistry and biochemistry at UC Santa. Cruz and former postdoctoral researcher at Moore Lab, who contributed to the study.

“Here’s this neurotoxic potential in these lakes that people use recreationally, but this toxin went unnoticed until our work,” Fallon said.

Moore said follow-up work should include fieldwork to detect other areas where guanitoxin could be produced.

Cyanobacterial blooms are becoming more prevalent in the United States and around the world, primarily due to climate change and the introduction of fertilizers and other agriculture-related chemicals into water bodies.

While HABs may be visible on the surface of freshwater bodies, the Federal Environmental Protection Agency (EPA) states that “cyanotoxins may be present before and after blooms are visible. Therefore, it is recommended that cyanotoxin levels be confirmed by laboratory testing of the water.”

“Now that we understand the guanitoxin pathway at the genomic level, we can also give additional information to say, ‘This is a safe body of water, or this is a less safe body of water; Does it have the ability to become toxic and can we predict toxic events? “Said McKinnie.

The researchers filed a provisional patent application based on the concept of using the biosynthetic guanitoxin gene sequences they identified in the laboratory and applying molecular diagnostics using these sequences to find the genes in the environment.

In addition to Lima, Fallon, Moore, Fiore, and McKinnie, other study co-authors include Endrews Delbaje, Ernani Pinto, and Felipe Dörr of the University of São Paulo; Hanna Luhavaya, former Moore Lab scientist; Steffaney Wood, currently Scripps Oceanography PhD student; UC Santa Cruz researchers Jennifer Cordoza, Austin Hopiavuori, and Jackson Baumgartner; Jonathon Chekan of the University of North Carolina at Greensboro; Danillo Alvarenga from the University of Copenhagen; and Augusto Etchegaray from the Pontifical Catholic University of Campinas in Brazil.

The study was funded by the National Institute of Environmental Health Sciences, the Sao Paulo Research Foundation and the National Council for Scientific and Technological Development. Other funding came from the Life Sciences Research Foundation Simons Foundation Fellowship; the Brazilian Federal Agency for Support to Higher Education Evaluation; seed funding and a faculty research fellowship from UC Santa Cruz.

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