Mosquitoes have a unique ability to sniff out humans, according to a new study

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A question we ask ourselves every summer, in vain: Why is it so difficult, even impossible, to escape ruthless detection by mosquitoes? Often accompanied by another question: Why do they bite me more than others? Scientists and insect repellant manufacturers have known for some time that carbon dioxide (CO₂), exhaled when you breathe, and octanol, a volatile compound present in sweat, form airborne highways that mosquitoes use to get to to their victims. What scientists didn’t know, but have now discovered, is that mosquitoes, unlike other creatures in the animal kingdom, have multiple odor and taste receptors in each of their thousands of olfactory neurons.

In 2004, researchers Richard Axel and Linda Buck won the Nobel Prize in Medicine for their discoveries related to “odor receptors and the organization of the olfactory system.” A decade earlier, Axel and Buck had discovered that there are approximately 1,000 genes involved in the process of smell, which are responsible for a similar number of olfactory receptors. Their work also showed that each olfactory sensory neuron expresses only one of these receptors, a phenomenon called the “one neuron, one receptor” rule, and that this information is then sent as an electrical signal to the olfactory bulb, the part of the brain. of mammals that processes and interprets aromas. But according to Leslie Vosshall, head of the Laboratory of Neurogenetics and Behavior at Rockefeller University in New York and a former postdoc in Axel’s lab, “all of Buck and Axel’s rules were thrown away by mosquitoes.”

Vosshall leads a research program aimed at understanding the olfactory system of mosquitoes. Specifically, his work focuses on mosquito species Aedes aegypti, commonly known as the “dengue mosquito” for its role in spreading the virus that causes dengue fever. But Aedes aegypti Not only is it responsible for transmitting dengue, but its bites can also introduce pathogens that cause yellow fever, chikungunya, Zika, and Mayaro virus disease. Find out how to block odor receptors from Aedes aegypti the females, the ones that bite, could have important implications for global health and disease prevention.

Detail of the antenna of the ‘Aedes aegypti’ mosquito, seen under an electron microscope. Fluorescent green corresponds to olfactory neurons stained using the CRISPR technique. margo here

The latest research findings from Vosshall and colleagues, published in the scientific journal Cell, show that mosquitoes, like all other animals, have some neurons with a single olfactory receptor. But they also found that oh Egypt mosquitoes have many neurons that co-express multiple receptor genes. “If you’re a human being and you lose a single odor receptor, all the neurons that express that receptor will lose the ability to smell that odor,” says Vosshall. “You have to do more to kill mosquitoes because getting rid of just one receptor has no effect. Any future attempts to control mosquitoes with repellants or anything else must take into account how unbreakable their attraction to us is.”

Once the mosquito genome was sequenced and the genes that express the olfactory receptors were identified, the researchers used various techniques to track the genes and locate them within individual neurons. Using the modern gene-editing technique known as CRISPR, they were able to introduce fluorescent proteins of different colors that corresponded to different receptors, allowing them to see that many neurons had more than one active receptor. Vosshall’s team found that neurons stimulated by the human odor octenol were also activated by other ammonia-derived chemicals, or amines, that also functioned to attract mosquitoes.

“Surprisingly, the neurons for human sensing through 1-octen-3-ol and amine receptors were not separate populations,” explains Meg Younger, a researcher at Boston University and co-author of the study. In an email to EL PAÍS, Rockefeller University researcher Margo Herre, lead author of the study, adds: “Mosquitoes also use decanal and undecanal aldehydes [volatile chemical compounds] and more research is needed to determine the exact composition of human odors and which of these odors mosquitoes are able to detect.”

The general picture painted by these findings is that oh Egypt they have a double or triple redundancy system, that is, if they fail to perceive an odour, they begin to detect a second or third odour. And if they detect all of them, the signal is amplified. As Vosshall explains: “Mosquitoes have Plan B after Plan B after Plan B. To me, the system is unbreakable.”

The findings could have far-reaching implications. For one thing, they could help explain why repeated attempts to control mosquitoes and limit their role in spreading pathogens have more or less ended in failure. As Younger explains, women oh Egypt they are hematophagous (they feed on blood) “because they need the proteins present in the blood to mature their ovules”. The eagerness and sophisticated biting ability of mosquitoes is the product of millions of years of evolution.

A researcher at the Butantan Institute in São Paulo, who is working on a dengue vaccine, shows his hand being bitten by mosquitoes.  Female 'Aedes aegypti' mosquitoes can transmit dengue, but also the viruses that cause yellow fever, chikungunya and zika.
A researcher at the Butantan Institute in São Paulo, who is working on a dengue vaccine, shows his hand being bitten by mosquitoes. Female ‘Aedes aegypti’ mosquitoes can transmit dengue, but also the viruses that cause yellow fever, chikungunya and zika.Diego Herculano (Getty)

Until now, attempts to block mosquitoes’ olfactory receptors through genetic modification have failed, perhaps because they all started from the commonly accepted idea, refuted by the new findings, that a given gene expressed only one receptor for each type. of neuron. This would also explain the relative, but not total, efficacy of DEET, the repellent discovered by the US military in 1946 that remains the main active ingredient in the vast majority of chemical insect repellents. Although its mechanism is not yet fully understood, N,N-Diethyl-meta-toluamide (DEET) is thought to inhibit CO₂ or lactic acid receptors, but we now know that there are other odor receptors on the same neuron. The good part of the bad news is that researchers now understand that they need to focus their efforts on multiple receptors at once, not just one.

It remains to be seen whether this new discovery applies to other species of biting mosquitoes as well: Aedes albopictus, for instance; other species of Anopheles, a genus of mosquitoes that transmit malaria; or Culexus mosquitoes, such as the common mosquito or the tiger mosquito, which usually inflict little more than discomfort or itching, except in exceptional cases. Christopher Potter, a neuroscientist at the Johns Hopkins University School of Medicine, fears so. In 2019, his lab discovered that fruit flies (Drosophila melanogaster) also had double or triple expression of receptors in a single neuron, and in the spring of this year they published findings indicating the same expression in a species of Anopheles mosquitoes

Potter, who was not involved in the study, says “this redundancy may be commonplace among insects.” Reflecting on the new findings, Potter explains how “the dogma before this was that an olfactory neuron would only express one type of olfactory receptor; that was the rule as far as we knew.” But, he says, “Dr. Vosshall’s work now suggests that a mosquito’s olfactory neurons may be much more adaptive, especially toward the key odors it needs to detect in order to locate its hosts.”

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