By Matthew DeGennaro, @mattdegennaro

Aedes aegypti
An Aedes aegypti female feeding on human blood

 Not all mosquitoes have a taste for human blood. But when they do bite us, they can potentially introduce a blood-borne infectious agent, like the virus that causes dengue fever or the microorganism that causes malaria. The goal of my research in the Vosshall Lab at The Rockefeller University is to identify the genes that cause mosquitoes to seek out humans so we can find more effective ways to repel them.

My most recent findings, published last week in Nature, focus on a gene called ORCO (Odorant Receptor Co-receptor) in the dengue virus carrying mosquito, Aedes aegypti. Specifically, my colleagues and I presented a mechanism for how mosquitoes use ORCO to locate and confirm the identity of a human host. Intriguingly, this same protein is also needed for mosquitoes to be repulsed by the insect repellent DEET.

During her post-doc in Richard Axel’s lab at Columbia University, Leslie Vosshall identified a family of proteins, called odorant receptors (ORs), that insects use to smell odors. Then, as head of the Laboratory for Neurogenetics and Behavior at The Rockefeller University, Leslie discovered how these OR proteins work together to sense odors. When a mosquito is exposed to a particular odor, specific ORs that form an ion channel with ORCO pick up the scent and activate the sensory neuron. When ORCO is absent, the OR protein can no longer reach the cell membrane, making the neuron unable to sense odors. In this way, we can disrupt the function of all 131 mosquito ORs by mutating a single gene, ORCO. But figuring out how to mutate ORCO was the first challenge.

Responding to Odors: ORCO is required for ORs to reach the cell membrane and trasport ions in order to transmit the order response.
Responding to Odors: ORCO is required for ORs to reach the cell membrane and transport ions in order to transmit the odor response.

The first task of my post-doc was to figure out a way to edit a mosquito’s genome, and create a mutation in ORCO. Because zinc-finger nucleases – proteins designed to cause a targeted break in DNA – are one of the molecular tools increasingly being used to modify the genomes of a wide variety of organisms such as fish, rats, pigs, and crickets, it seemed reasonable to hypothesize that this technique could also be applied to mosquitoes. The experiment was successful, and I was able to introduce a mutation into the ORCO gene.

We predicted that these ORCO mutant mosquitoes would have difficulty finding humans due to their loss of olfactory sensitivity. However, our results painted a more nuanced picture of ORCO’s role. The ORCO mutant mosquitoes we created were not attracted to human odor alone, but were attracted to human odor only when carbon dioxide was present. This suggested that mosquitoes possess other ways of sensing human odor, and stresses the importance of carbon dioxide in mosquito host-seeking. Since ORCO mutant mosquitoes could still be attracted to human odors mixed with carbon dioxide, it also gave us the opportunity to learn more about how mosquitoes pick their victims.

There are many different species of mosquitoes, and each species has its favorite host(s). The mosquito species that like us the most, Aedes aegypti and Anopheles gambiae in particular, also happen to spread disease. Understanding why these species prefer humans is a fascinating scientific puzzle that, when solved, can help us control mosquito populations and decrease our risk of getting mosquito-carried diseases. Carolyn (Lindy) McBride, another postdoc in the Vosshall lab, is studying this question, and has developed ways of testing whether mosquitoes prefer the smell of humans or the smell of other animals. Together, we decided to test the ORCO mutants and found that their preference for humans was much weaker than normal. This tells us that ORCO helps mosquitoes distinguish between humans and non-human animals.

Our next question was to learn more about how mosquitoes are repelled by DEET, and if ORCO played a role in this process. DEET was identified and tested by the United States military during the 1940s, but how it works is still a controversial subject. Our work has shown that DEET uses ORs to repel mosquitoes at a distance. In sharp contrast to our normal (wild-type) mosquitoes, ORCO mutants will target a human arm treated with DEET. However, if they have the chance to land on a DEET-treated arm, even ORCO mutant mosquitoes will not bite, and will just fly away instead. This shows mosquitoes sense DEET through something other than ORCO when they land on us.

 

ORCO mutant land on DEET-treated skin, but leaves without biting.
ORCO mutant land on DEET-treated skin, but leaves without biting.

Important details of how DEET works remain unexplored. From our work, we now know that DEET does not mask odors in the mosquito as it has been thought to do in the fly. Instead, DEET likely changes OR responses, causing the mosquitoes to avoid smells that are normally attractive. In my remaining time in Leslie’s lab, I hope to learn more about how volatile DEET hijacks ORs. I would also like to figure out which neurons in mosquitoes are involved in sensing DEET, and how they work together to make mosquitoes avoid people who are wearing this insect repellent. Our hope is that connecting the odors that attract and repel mosquitoes with their associated genes and neural circuits will open new possibilities in mosquito control, and help prevent the spread of disease.

***

Degennaro, M, McBride, CS, Seeholzer, L, Nakagawa, T, Dennis, EJ, Goldman, C, Jasinskiene, N, James, AA, and Voshall, LB. orco Mutant Mosquitoes Lose Stron Preference for Humans and are not Repelled by DEET. Nature, 2013 May 29. doi: 10.1038/nature12206 [PDF]