Their findings should facilitate the design and use of Bt toxins to prevent insects, which the researchers believe also possess the molecule, from developing resistance to Bt, extending the life of this natural pesticide.
The study, published February 11 in the journal Science, details the structure of a molecule to which Bt attaches, or "binds," in the lining of the intestines of insects and roundworms. The molecule is a glycolipid--a lipid attached to a tree-like arrangement of sugars. Because changes in the sugars impact Bt's ability to bind, the researchers believe that their discovery will make it possible to develop better pesticides and lead to new treatments for parasitic infections that affect close to two billion people worldwide.
"Our previous findings with the roundworm C. elegans strongly suggested that specific sugar structures are likely critical for Bt toxin susceptibility," said Joel Griffitts, the first author on the paper and a former graduate student with UCSD biology professor Raffi Aroian. "This latest paper demonstrates what these sugars actually do. They provide a receptor for the toxin that allows the toxin to recognize its "victim"--a roundworm or an insect. This paper also brings us from the conceptual realm to the chemical nature of these sugar structures--how their atoms are arranged, and how the toxin binds to them."
"Bt toxin, which is produced by a soil bacterium, is toxic to insects and roundworms, but not to vertebrates, which accounts for its popularity as a pesticide," explained Aroian, who led the team. "But the development of insect resistance to Bt is a major threat to its long term use. Our findings make it possible to understand resistance at the molecular level and should improve resistance management."
In collaboration with Paul Cremer and Tinglu Yang, coauthors on the paper and chemists at Texas A&M University, Griffitts and Aroian found that Bt toxin directly binds glycolipids. However, in each of the four Bt resistant mutants tested--bre-2, bre-3, bre-4 and bre-5--the researchers found that there was either zero or dramatically reduced binding of glycolipids to Bt toxin. They concluded that the defective sugar structure of the glycolipid receptor in each of the mutants prevents Bt from binding.
Other members of the research team, coauthors Stuart Haslam and Anne Dell, biologists at Imperial College London; Barbara Mulloy, a biochemist at the Laboratory for Molecular Structure, National Institute for Biological Standards and Control in Hertfordshire, England; and Howard Morris, a biochemist at the M-SCAN Mass Spectrometry Research and Training Centre in Berkshire England, determined the chemical structure of the normal glycolipid receptor that binds Bt toxin.
Elements of this structure are found in both insects and nematodes, but are not found in vertebrates at all, which may be one reason these proteins are safe to vertebrates. This work furthermore opens up the possibility of using Bt toxins against roundworms that parasitize humans.
"These parasites infect nearly one-third of the human population and pose a significant health problem in developing countries," said Aroian. "Perhaps one-day vertebrate-safe Bt toxins could be used as human therapies against these parasites."
Griffitts and Aroian credit the flexibility of the roundworm C. elegans as an experimental system, particularly the ease of manipulating it genetically, in making it possible to find and characterize the structure of the long sought-after Bt receptor. However, their results apply to insects as well. Michael Adang and Stephan Garczyn ski, coauthors and entomologists at the University of Georgia, showed that the glycolipid receptor is present in the tobacco hornworm, an insect pest that is susceptible to Bt toxins used commercially in plants.
"It will now be possible to monitor insect populations near fields where Bt is used and catch insect resistance in its early stages by looking for changes in glycolipids," said Aroian. "If changes are detected, switching to another pesticide, perhaps even another variety of Bt that works through a different mechanism, could prevent the resistance genes from becoming widespread."
According to the researchers, prior work indicates that there are other receptors that also contribute to Bt resistance. Combining pesticides that work through different receptors or designing pesticides that can work through more than one receptor type could thwart the development of resistance.
"This paper presents an intriguing question," said Griffitts. "In light of findings by insect biologists that certain proteins function as important Bt toxin receptors in some cases, how might glycolipid and protein receptors cooperate to engage this intoxication program? If the field can figure this out, it might allow for the engineering of toxins that can utilize either type of receptor alternatively, such that host resistance would require the mutation of both receptor types. This means that resistance would be exponentially less probable."