Their findings, which appear this week in an early online edition of the journal Proceedings of the National Academy of Sciences, show that fruit flies can be used to study the link between the biochemical activities and physiological effects of anthrax toxins.
Learning how these toxins attack developing and adult tissues is important because it can help scientists understand how they function at the molecular level and may lead to new therapeutic strategies for neutralizing their effects in humans.
Annabel Guichard, a biologist at UCSD and lead author of the study, tracked the ways that two active anthrax toxins, known as lethal factor, or LF, and edema factor, EF, cause cellular damage and death in the fruit fly Drosophila melanogaster. These toxins are required for the anthrax bacterium Bacillus anthracis to evade the host immune system and cause disease.
Using a combination of biochemical, genetic and cell biological approaches, the biologists tested whether or not the anthrax toxins were active in living Drosophila and, if so, whether they acted in the same way as they do in humans. The biologists found that anthrax toxins do alter the same signaling pathways used for cell communication in fruit flies and humans.
"Drosophila is an excellent tool to understand the effect of a toxin on its host and to determine the molecular mechanism underlying its toxicity, because the fly system is already so well characterized," Guichard said. "We knew how anthrax toxins acted on human cells, but this study is the first to show that these toxins are active in fruit flies, suggesting that this fast breeding laboratory animal could also be used to determine the function of a variety of bacterial and viral pathogenic factors. "
Anthrax bacterium secretes three toxins, including LF and EF, and is only known to infect mammals. Because fruit flies lack components required for toxin entry into cells, they cannot actually contract the anthrax disease. However, the study finds that fruit flies can be used to test the effects of a single virulence factor, such as the LF or EF toxins, on signaling pathways shared by flies and humans.
Guichard and her co-authors applied lethal factor toxin to fruit fly embryos and larvae and observed that a component of the expected signaling pathway was inactivated, disrupting the whole molecular system and leading to death. When applied in a more limited fashion, LF interfered with the formation of sutures in the epidermis, resulting in a hole or cleft in thoracic regions of embryos and adults. This developmental process disrupted by LF treatment is similar mechanistically to wound healing, which is mediated by the same signaling pathway in humans.
The EF toxin is also lethal when applied to fly larvae and can cause severe malformation in the wings of adult fruit flies--an effect that can also be understood as an interruption of another unrelated signaling process common to flies and humans.
"We asked the simple question of whether anthrax toxins affecting mammals could act on the fly counterparts of proteins affected in humans, and the answer is yes," said Ethan Bier, a professor of biology at UCSD who was the senior author of the study. "What this means is that similar types of analyses might identify yet unknown proteins shared by flies and humans that can be acted on by anthrax toxins. More generally, this study suggests that flies can be used as a rapid whole organism system to determine the function of a variety of bacterial and viral pathogens of unknown function. One could then test hypotheses obtained from these studies with flies in mammalian organisms such as mice."
The biologists also anticipate that toxins such as the anthrax lethal factor toxin that have multiple host target proteins may be used to simultaneously reduce or eliminate the activities of several related proteins that perform overlapping functions in other diseases or biological processes. Such experimental tools could accelerate progress in various areas of biomedical research.
Guichard first conceived this study in 2002, to apply her experience in molecular genetic studies to research that had medical application. At that time, after the spate of anthrax-laced letters in 2001, anthrax became a charged topic in the research community and the general public.
"Anthrax is still a hot subject because of its possible use as a weapon of bioterrorism and remains a health threat in third world countries," Guichard said. "Anthrax infections can be cured by antibiotics when detected early. But after a certain point, the toxins released in the bloodstream can kill the patient even after antibiotic treatment. The more we understand about how the toxins function, the better we'll be able to design effective co-operative or "adjunctive" therapies."