Two new studies offer hope in the fight against malaria, a disease that kills a million people a year. Malaria and other diseases are transmitted by mosquitoes, and the insects can use smell to track humans, even when they're hundreds of meters away. Researchers are working to figure out how mosquitoes pick up smells, and then use this information to stop the pesky insects.
"It's been known for some time that mosquitoes find us through their sense of smell," says biology professor John Carlson, at Yale University. "But how they do this has been a mystery." The mystery has been complicated by the challenges of working with mosquitoes in labs. They need periodic blood meals, for one. And they're not as well understood as some other organisms.
A Different Kind Of Nose Job
To study smell in mosquitoes, Carlson is using the workhorse of many labs: fruit flies. Specifically, he's using a fruit fly that has no odor receptors (molecules that recognize specific chemical smells and signal their presence to the brain). In the lab, he isolates the genes that are the blueprints for the mosquitoes' noses — the odor receptors on the mosquitoes' antennae. He transplants those genes into fruit flies.
"We found a method of turning on a mosquito gene inside a fruit fly nerve cell that doesn't have its own odor receptor," Carlson says. This way, he could test what odors the mosquito "noses" pick up. He and his colleagues, primarily Allison Carey, Carlson says, have been attaching electrodes to the fruit fly near the smell receptor cells. When the receptor cells recognize a smell, the attached loudspeakers emit a series of beeps, and the researchers know they have a "hit."
"Some of these receptors responded quite nicely to human odors — they gave screaming responses — so we got quite excited," Carlson says. He says these receptors may help the mosquito figure out where people are and navigate toward them.
Frog Studies Prop Flies' Findings
At Vanderbilt University, biology professor Larry Zwiebel is going at the question a bit differently, with the help of senior research associate Guirong Wang. Where Carlson's lab focuses primarily on fruit flies, Zwiebel's lab works primarily with frog eggs. They're easier, Zwiebel says — they express their genes within days. With fruit flies, it can be three months.
"We've basically tried to take apart the mosquito's nose and find the genes and the proteins they encode that are responsible for that [smelling] process," he says. "It's hard to ask a mosquito what she smells."
Both research labs are funded by the Bill and Melinda Gates Foundation, as part of an initiative called Grand Challenges in Global Health. The idea behind Grand Challenges is that basic research should be done with the idea that it could lead to practical approaches.
Zwiebel and Carlson are both hoping that understanding how a mosquito uses its sense of smell will lead to a way to block or override that sense of smell, so that malaria-carrying mosquitoes couldn't find human prey. Another option is to find an attractant so strong that it will make mosquitoes forget about humans and go to the attractant instead.
"The key thing here," says Carlson, "is we want to find new ways of controlling mosquitoes, ways that are effective and environmentally friendly."
Carlson's latest research appears in the online version of Nature published earlier this month. Zwiebel's research is published online in the Proceedings of the National Academy of Sciences.
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