In a new approach to combating malaria, a disease that affects half a billion people worlwide, US scientists successfully transplanted most of the “nose” of the disease-spreading Anopheles mosquito into frogs’ eggs and fruit flies so they could analyse the insect’s odorant receptors and find out how to lure it into traps and even prevent it being able to detect and thereby target humans.
You can read about the two studies by researchers from Yale University in New Haven, Connecticut, and Vanderbilt University in Nashville, Tennessee, in a report in the 3rd February online issue of the journal Nature and there is also a complementary article in the Proceedings of the National Academy of Sciences, PNAS.
The mosquito Anopheles gambiae is the major route through which humans in sub-Saharan Africa become infected with malaria. While we know that the insect uses its sense of smell to find human hosts, we know little about the underlying molecular process.
A mosquito’s “nose” is in its antennae which carry nerve cells covered with odorant receptors that react to different chemical compounds in the insect’s environment. These receptors are similar to those that give us our senses of smell and taste in our nose and on our taste buds.
Co-author Dr Laurence Zwiebel, professor of biological sciences at Vanderbilt, told the press that:
“We’ve successfully expressed about 80 percent of the Anopheles mosquito’s odorant receptors in frog’s eggs and in the fruit fly antennae.”
Zwiebel’s lab at Vanderbilt is where they successfully transplanted the receptors into frogs’ eggs. The transplant into fruit-fly (Drosophila melanogaster) eggs was done at the laboratory of John Carlson, Eugene Higgins Professor of Molecular, Cellular and Developmental Biology at Yale and is written up as a complementary study in PNAS.
Scientists have previously used frogs’ eggs to study olfactory receptors in moths, bees and fruit flies. For this study, the researchers injected DNA that codes for the mosquito’s olfactory receptors into a frog egg and waited for it to produce proteins. Eventually the surface of the egg became covered with mosquito odorant receptors.
They then tested the engineered egg’s reaction to being exposed to various odorant chemicals. They floated the egg in a buffer solution in a voltage clamp (so they could measure changes in the egg’s electrical properties) and dissolved the chemicals one by one in the solution. They detected a measurable electrical response in the egg.
Guirong Wang, lead author of the PNAS study, and a senior researcher in Zwiebel’s lab, said:
“The frog egg system is relatively rapid, highly sensitive and allows us to do very precise measurements of odorant response.”
Wang, who personally conducted several thousand measurements of egg responses to changes in odorant, described this method as a “medium throughput system”, because although they could set it up quite quickly, they had to make the odorant solutions by hand, which took much longer.
In the Drosophila fruit fly system at Yale instead of using eggs, the researchers focused on the antennae of the fruit fly. It took them about three months to engineer a fruit fly with a mosquito odorant receptor in its antennae, a lower throughput system compared to the frog egg one. The original work was done in Carlson’s lab, where graduate student Allison Carey inserted mosquito genes one at a time into fruit flies whose own odorant receptor had been knocked out so that the mosquito receptor would be expressed instead.
The researchers explained that although the fruit fly method takes a bit longer, it has advantages over the frog egg, the main one being that it works with vaporised odorants that don’t dissolve easily in water. The fruit fly method also appears to detect compounds that inhibit rather than excite the receptors.
Wang explained that in both methods they inserted the same set of 72 Anopheles odorant receptors and tested the same set of 110 odorants.
In the case of the frog egg method, the Vanderbilt team tested 6,300 odorant-receptor combinations and got 37 responses.
“The results of the two systems were quite similar. There were only a few small differences,” said Wang.
In both the frog egg and the fruit fly studies the researchers found that most of the mosquito receptors were generalists and reacted to a number of different odors, while a few were specialist and responded only to one or two. Conversely, they found that one chemical could trigger several receptors while other chemicals would only cause a response in one.
A particularly interesting discovery was that they found 27 Anopheles receptors responded strongly to chemicals present in human sweat.
This was a signicant step forward and they are now looking for other chemicals that these 27 receptors respond to: it could be that they find some that can lure the mosquito into traps or repel them away from humans. Another possible application could be a chemical that blocks the receptors and masks the presence of humans.
Zwiebel said they are calling these compounds that interact with the receptors BDOCs (behaviorally disruptive olfactory compounds):
“Ultimately we are looking for cocktails of multiple compounds that demonstrate activity in the field,” said Zwiebel.
The work has already produced a patented blend of BDOCs that attracts mosquitoes more effectively than humans, and the researchers have already found several BDOCs that repel mosquitoes. Discussions are currently under way with several private sector developers.
The studies are part of a five-year project supported by the Grand Challenges in Global Health Initiative, a fund sponsored by the Bill & Melinda Gates Foundation and whose goal is to find new ways to combat malaria.
“Odorant reception in the malaria mosquito Anopheles gambiae.”
Allison F. Carey, Guirong Wang, Chih-Ying Su, Laurence J. Zwiebel, John R. Carlson
DOI:10.1038/nature08834
Source: Vanderbilt University.
Written by: Catharine Paddock, PhD