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Genes Found for Mosquitoes' Sense of Scents

Discovery could lead to better, safer repellents

MONDAY, Nov. 26, 2001 (HealthDayNews) -- Once a bloodthirsty mosquito has zeroed in on your scent, there's little outside a skilled swat that can stop her. But a new discovery raises hopes that man can thwart the pest's ability to smell you.

A team of American biologists has isolated four genes that are crucial to a mosquito's sense of smell. The discovery of these genes, which contain the instructions for proteins called odorant receptors, could lead to new methods of repelling disease-carrying mosquitoes and other insects.

In the Dec. 4 issue of the Proceedings of the National Academy of Sciences, researchers describe four genes called Anopheles gambiae odorant receptors (AgORs), which they found in the family of anopheline mosquitoes known to spread malaria.

The genes -- AgOR1, AgOR2, AgOR3, and AgOR4 -- are activated by odor chemicals that trigger the sensation of smell. "Insects, and in this case, mosquitoes, use their sense of smell to find their way in the world," says lead author Laurence Zwiebel, an associate professor of biological sciences at Vanderbilt University in Nashville, Tenn.

"Because insects don't have any consciousness that we know of, they have to respond very simply to chemical signals in the environment," says Marc Klowden, an expert on insect olfaction and behavior at the University of Idaho in Moscow, Idaho.

"In order to find a host for a blood meal, which is what makes Anopheles gambiae such a serious vector of malaria and human suffering, it's able to identify the components in the human host and initiate behaviors that bring the mosquito closer to the host," says Klowden, who was not involved in the study.

Zwiebel views mosquitoes as having particular significance in terms of public health, so his team set out to study the molecular aspects of their sense of smell.

Using data from the Anopheles gambiae genome project, the researchers scanned for genes with characteristics matching known olfactory genes from the common fruit fly and mammals.

They found that when an odor crosses the antenna of a mosquito, a porous structure allows the odor chemicals to enter. Then they meet a membrane that is part of a nerve that extends all the way into the mosquito's head. On that membrane, the AgOR proteins wait to encounter these chemical signals and translate them into electrical signals -- the language spoken in the brain.

"These [Anopheles gambiae odorant] receptors are essentially the first elements of this signal transduction cascade, as it is called," says Zwiebel.

After searching for receptors that had the right structure and were located in the right place, the researchers tried to find those that were exclusive to female mosquitoes. "Female mosquitoes are the only ones that bite," says Zwiebel, explaining that they use the blood meal for making their eggs. They found that the AgOR1 gene was expressed in the antennae of only the female mosquitoes.

Based on previous studies that showed that females were not as interested in pre-feeding odors after a blood meal, Zwiebel found that AgOR1 expression dropped sharply after females had a blood meal. "It's a very intriguing parallel between the behavior and a molecular switch," he says.

Learning how to block or trigger these receptors could have many uses, says Zwiebel. "Designing a molecule that would act as a repellant would have obvious benefits in the sense that it would keep mosquitoes from biting people," he says. Conversely, attractants could be used to draw mosquitoes away from people and into insecticide-laden traps.

That could have important public health implications in both the developing and developed world, says Zwiebel. "Anopheles gambiae is the primary vector for malaria in sub-Saharan Africa," he says. Malaria is caused by a single-celled parasite spread through the bites of mosquitoes carrying infected blood. The World Health Organization estimates that there are between 300 million and 500 million cases of malaria every year, causing more than 1 million deaths.

However, he says, these findings could also be relevant to public health efforts against mosquitoes implicated in the spread of West Nile virus in the United States. And since other insects use the same genetic system, similar repellants or attractants could also be applied to nuisance insects and agricultural pests.

Klowden notes that existing repellents confuse the mosquitoes' sensory systems but that the most effective repellents, such as DEET, have potential neurotoxic side effects.

What To Do

For more information on mosquitoes, visit Enchanted Learning, or Ohio State University.

You can also check out the Centers for Disease Control and Prevention Web site on mosquito-borne diseases.

SOURCES: Interviews with Laurence J. Zwiebel, Ph.D., assistant professor, Department of Biological Sciences, Vanderbilt University, Nashville, Tenn., and Marc J. Klowden, Ph.D., professor, Division of Entomology, Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow, Idaho; Dec. 4, 2001 Proceedings of the National Academy of Sciences
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