Discoveries Featured

On the Scent of the Malaria Mosquito

By: Carol A. Rouzer, VICB Communications
Published: February 19, 2010

New detailed investigations of the function of the mosquito olfactory system provide the foundation for battling malaria in sub-Saharan Africa.

For most people living in the developed world, mosquitoes are just a summer nuisance, causing itchy irritating bites that usually heal without serious consequences. But for those living in sub-Saharan Africa, a mosquito bite may bring debilitating illness or even death. In that region of the world, the Anopheles gambiae species of mosquito (Figure 1) carries malaria, a devastating disease that kills 3,000 children per day and costs $12 billion in lost wages for adult victims. Treatments for malaria are expensive and not always effective so it is becoming evident that the key to controlling malaria may be in controlling the mosquito.

Figure 1. Anopheles gambiae taking a blood meal.
(Image courtesy of Wikimedia Commons under the GNU Free Documentation License.)

An. gambiae is characteristically anthropophilic, meaning that the mosquitoes take their blood meal primarily from humans. Like most insects, they rely heavily on the sense of smell to locate their targets. Thus, understanding the mosquito olfactory system may provide the necessary foundation for the development of new compounds that will repel the insects from their human hosts or lure them into a trap for extermination. To this end, VICB member Larry Zwiebel and his laboratory have been studying the signal transduction pathways in An. gambiae olfactory receptor neurons (ORNs), which are the cells primarily responsible for detecting odors in insects. ORNs express specialized membrane proteins, the olfactory receptors, which bind to odorants in the environment and trigger a signaling response. The Zwiebel lab was the first to clone the gene for an An. gambiae olfactory receptor (AgOR), and they have used bioinformatics to identify a total of 79 distinct AgOR genes in the An. gambiae genome. Now, in collaboration with John Carlson’s lab at Yale, the Zwiebel lab has reported the functional characterization of over two-thirds of the An. gambiae AgOR proteins [Wang et al. (2010) Proc. Natl. Acad. Sci. U.S.A., published online Feb. 16, DOI: 10.1073/pnas.1000738107, and Carey et al. (2010) Nature, published online Feb. 3, DOI: 10.1038/nature08834].

To obtain active AgOR proteins for study, the Zwiebel lab expressed the genes for 72 of the 79 receptors in oocytes from the African clawed frog, Xenopus laevus. Use of these exceptionally large cells allowed the investigators to easily measure whole cell electric currents in response to a collection of 88 odorants representing a wide range of chemical structures. In this system, 37 out of the 72 expressed receptors gave rise to specific odorant response profiles. At the same time, the Carlson lab expressed the receptors in a mutant ORN located in the antenna of the fruit fly, D. melanogaster. The mutant ORN lacks a functional odorant receptor gene, so transfection of this “empty neuron” with the gene for an AgOR protein allowed the investigators to assess the function of only the expressed protein. By measuring the responses as electrical impulses in the fly’s antenna, the Yale group obtained specific odorantresponse profiles to a library of 110 odorants in 50 of the 72 transfected genes.

The results from both cell systems revealed that each AgOR possesses a distinctive response pattern to the odorant library. Some receptors, the specialists, respond strongly to one or just a few compounds, while others, the generalists, respond to a larger number of structurally diverse odorants. Conversely, some odorants excite only one or a few AgORs, while others elicit a response in many receptors. Of particular interest is the finding that the specialist AgORs, those responding to a limited number of odorants, detect compounds of great importance to the mosquito. For example, four of the most specialized AgORs respond to indole, a major volatile component of human sweat, 1-octen-3-ol, a human volatile that has a strong attraction for mosquitoes, 2,3-butane-dione, which is produced by human skin microflora, and 2-ethyl phenol, a component of human urine (Figure 2). Not only are these receptors highly specific, they are also exquisitely sensitive, being able to detect their target odorant at concentrations of 1 part per million or less. Similarly, the odorants to which the fewest receptors respond are of particular importance to mosquito physiology. Some of these include 3-methylindole, an oviposition site volatile important in sexual attraction, indole, which is both an oviposition site and a human sweat volatile, geranyl acetate and citronellal, which are plant-derived mosquito repellants, and dimethylsulfide, a mosquito attractant found in human breath. These results indicate that An. gambiae has evolved highly sensitive and specific receptors that enable it to identify its human target and carry out other basic physiologic functions, such as reproduction.

Classification of the odorants by chemical structure revealed that structurally similar odorants tend to elicit similar responses among the AgORs. An exception is seen in the case of aromatic compounds, such as indole and 3-ethylphenol, which are often found in human sweat, breath, and urine. This class of odorants elicit a diverse range of responses, thereby providing the mosquito with an enhanced ability to discriminate among these compounds. In fact, AgORs appear to be particularly well-equipped to detect aromatic and heterocyclic odorants, and less able to discriminate other classes of compounds, such as amines, esters, and aldehydes.

The use of the empty neuron system allowed the investigators to directly compare the response pattern of AgORs with those of the odorant receptors (DmORs) of the fruit fly. DmORs were the first insect receptors to be characterized in the empty neuron system. A striking finding was that many of the specialist DmORs detect esters and aldehydes that enable the fly to locate fruit, while being less sensitive to aromatic and heterocyclic compounds. Thus, each insect’s olfactory system has evolved to be highly responsive to a diverse range of the compounds of greatest importance to that insect’s lifestyle. Figure 2. High specificity odorants for AgORs.

Figure 2. High specificity odorants for AgORs.

Together these results provide the foundation for understanding the function of the An. gambiae olfactory system. We now know the specific AgORs that the mosquito uses to identify its human prey and to attract a mate. With ongoing funding from the Foundation for the National Institutes of Health, the Bill and Melinda Gates Foundation, and the NIH, the Zwiebel and Carlson labs are looking forward to exploiting their newfound knowledge in the never-ending battle against the malaria mosquito.