Members of the Vector Borne Disease Research Group at Virginia Tech are using ancient pieces of genetic material as tools to explore the immune system of mosquitoes that play host to pathogens.
Assistant professors of entomology Zach Adelman, who researches the biology of the vector and Kevin Myles, who studies viruses, are collaborating to understand mosquito-virus interaction. They are testing the hypothesis that mosquitoes – just like plants, worms, humans, and other life forms – have an innate immunity mechanism based on RNA interference (RNAi).
RNA is created from a DNA template, then RNA in turn acts as a template for protein synthesis. Different forms of RNA create proteins with specific functions. Normally, RNA is single stranded, but many viruses produce double-stranded RNA while replicating in the infected cell.
“Double-stranded RNA is a sign that something is not right,” Adelman said. “The viruses we work with produce double-stranded RNA as part of their replication machinery,” said Myles. “So it is likely that the RNAi pathway is essential to the mosquito’s innate immune response against viruses. This may explain why some mosquitoes become infected with arthropod-borne viruses while others do not.”
“To make a mosquito immune to a particular virus, you can expose it to double-stranded RNA from a small part of the virus. Later, if that virus tries to invade, the mosquito immune system will recognize it and be prepared to destroy it,” said Adelman. “In the natural setting, we suspect that the immune response does not completely eradicate the virus. But the virus may be weakened. We want to enhance that ability so all mosquitoes have the ability to eliminate the virus.”
“We may someday be able to manipulate the RNAi pathway of the mosquito in order to reduce disease transmission. However, first we need to understand the immune pathway and how viruses overcome it,” Myles said.
Myles and Adelman have created a novel assay system for the study of virus-vector interactions, mosquito genetics, and virus genetics. In an experiment to test the hypothesis about innate immune response in mosquitoes, they have created a genetically-modified mosquito that serves as an RNAi sensor -- a sequence that will turn a mosquitoes’ eyes florescent green if the RNAi immune machinery stops working.
“If we knock down a gene important in the functioning of RNAi, it will turn on the green gene so the mosquito’s eyes turn green,” said Adelman.
“We used a transposable element to place three genes into the mosquito genome. One gene produces a red fluorescent protein that shows up in the mosquito’s eyes and lets you know the transposable element is present. The second gene produces a green fluorescent protein, also visible in the eyes. The third gene activates RNAi, which destroys the messenger RNA for the green fluorescent protein, and prevents the eyes from glowing green.
In other words, the eyes of this genetically altered mosquito serve as a sensor of the functional status of part of the mosquito’s immune system, glowing red to show that the introduced transposable element is present and glowing green if the RNAi immune machinery stops working — a situation that would favor the virus and could lead to increased transmission.
“The red and green eyes research is a way to discover genes involved in immunity,” said Myles.
“Today it is still too early to tell how well we might be able to manipulate the immune system,” he said. “However, as we learn more about this pathway, we validate the immunity hypothesis. In the process, we may also discover other ways to halt the transmission of disease that do not involve genetic engineering of mosquitoes.
“Some mosquitoes are better than others at transmitting disease, perhaps because they have weaker immune systems. If we can understand the reasons, we may be able to predict what populations are more likely to be involved in epidemic transmission,” said Myles. “One day we may have a rapid test to use in the field to determine if a mosquito population has some degree of immunity that makes it less competent. It is difficult to control mosquito populations. If you knew which ones you had to control, it would be easier to sustain control tactics for longer periods of time.”
Few Americans had heard of West Nile virus before it arrived in the United States, but now the spreading epidemic is more commonly known.
"[It] illustrates that arboviral diseases represent an emergent and resurgent threat to even the wealthiest countries,” said Myles.
Available options for addressing the problem are limited, he said.
The group's researchers approach the problem of infectious disease from a variety of perspectives.
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Members are also affiliated with Virginia Tech's Institute for Biomedical and Public Health Sciences.
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