The unbreakable attraction of mosquitoes to humans
More than a decade ago, Leslie Vosshall, then a relatively new researcher at the Howard Hughes Medical Institute (HHMI), decided to switch from studying harmless fruit flies to a far more deadly creature: the mosquito. Perhaps her in-depth knowledge of how the fruit fly sniffs out its food could be applied to mosquitoes, she wondered, discovering new ways to blunt the blood-sucking insect’s uncanny ability to find human prey. “I wanted to do something that could excite the public,” she says.
Her new job would indeed turn out to have a major impact, but not what she had anticipated. It was “a huge and amazing surprise,” she says. As she and the team she leads at Rockefeller University’s Laboratory of Neurogenetics and Behavior now report in an article published August 18, 2022, in Cell , his research overturned the conventional model of the neural circuits that animals use to detect and distinguish thousands of distinct odors in their olfactory systems. ‘It’s a big deal,’ says neuroscientist Christopher Potier from the Johns Hopkins University School of Medicine. “It really changes the way we think the insect olfactory system works.”
What’s more, the unexpected new result shows that it’s even harder than previously thought to confuse mosquitoes as they relentlessly search for human blood. In the fight to reduce the huge number of illnesses and deaths from mosquito-borne diseases, “this is not good news,” says Vosshall, now also HHMI’s vice president and chief scientific officer.
When Vosshall’s HHMI lab at Rockefeller turned to the mosquito, one of the initial tasks they successfully tackled was to assemble the insect’s first complete genome. “No one had done genome editing before, partly because the genome was so fragmented,” Vosshall says. Then, with the genome in hand, Meg Young, a former postdoc at the lab, attempted to answer a puzzling question. Mosquitoes are attracted to both CO2 that people exhale and to human body odors. “But there’s something magical about adding those two ingredients together, where one plus one equals twenty,” Vosshall says. The bugs get super excited, becoming very focused and fierce human hunters. So how do the two signals add up and amplify so much in the olfactory system?
“Mosquitoes have plan B after plan B after plan B. To me, the system is unbreakable.”
Leslie Vosshall, Vice President and Scientific Director of HHMI
To try to find out, Younger figured they could identify the olfactory neurons that responded to CO2 and who to body odor, then trace the signal pathways to the brain. So they used the gene-editing tool CRISPR to slip a fluorescent marker protein into neurons that had receptors for CO2 and another marker in those that could detect body odor chemicals.
That’s when the search took an unexpected turn. “It was like ‘Alice in Wonderland’ – where nothing makes sense,” Vosshall says.
Scientific dogma, based on the Nobel Prize-winning research of Linda Buck (now at Fred Hutchinson Cancer Center) and Richard Axel of Columbia University in mice, was that odor detection systems in animals are extremely specialized and organized. Each olfactory neuron has a single type of receptor, which detects a specific set of chemicals and then connects to a single structure (called a glomerulus) in the olfactory bulb. According to this logic, there would be distinct types of neurons that respond to the smell of strawberries, for example, others for peanut butter, still others for gasoline, and so on. “As a field, we were so influenced by Buck and Axel,” says Vosshall (who was a postdoc in Axel’s lab). “Those were the rules.”
By probing receptor genes with different fluorescent colors, Marguerite Herre, a former medical/doctoral student in the lab, found that individual neurons are teeming with multiple types of receptors, not just one. We found that “all of Buck and Axel’s rules were thrown in the trash by mosquitoes,” says Vosshall.
The results were so startling that Vosshall’s lab spent years painstakingly proving they were actually real, using several additional sources of evidence. For instance, Olivia Goldman, a PhD student in the lab, exploited a relatively new and revolutionary technique called single-nucleus RNA sequencing (snRNA-seq) to probe which genes are turned on in individual neurons. The approach confirmed that each neuronal cell indeed manufactures many types of receptors.
They also teamed up with scientists from the Swedish University of Agricultural Sciences, who had done groundbreaking work to figure out how to stick electrodes into individual mosquito olfactory neurons and measure the cells’ responses to various odors. This method also confirmed that a single mosquito neuron can detect different odors, even two different flavors of body odor, fragrant odor and stinky foot odor, which require two entirely different classes of receptors. These results “were a huge relief,” says Vosshall. She anticipated widespread skepticism about her findings, “so the number of levels of evidence that we used to prove it was intense,” she says.
As news and preprints of Vosshall’s team’s results spread through the community, in fact “there was a lot of skepticism at first,” says Potter. But not only was the evidence overwhelming, but in fact similar findings were also emerging from Potter’s lab at Johns Hopkins. Working with both the fruit fly and a species of mosquito, Potter’s team published a paper in eLifein April suggesting that “co-expression of chemosensory receptors is common in insect olfactory neurons”. In the past, the conventional wisdom of one receptor per smell and one receptor per neuron was so strong that there was no reason to probe multiple receptors, Potter says. “Now we know to look for it.”
In retrospect, the added complexity of the insect olfactory system makes sense in evolution, especially for mosquitoes that must find humans to survive. Having multiple types of receptors in each neuron increases insects’ ability to detect exhaled CO2 and the whole assortment of body odors. And when people try to repel biting insects by blocking certain receptors, mosquitoes can still easily head for the bloodstream using their other receptors. “It’s a really good round,” says Vosshall. “Mosquitoes have plan B after plan B after plan B. To me, the system is unbreakable.” This is obviously not good news for efforts to reduce the number of mosquito-borne diseases, such as malaria, yellow fever and dengue fever, by trying to block the receptors. But perhaps an alternate strategy could be to flood the entire system with alternate smells, Potter adds. At least now “we have a more realistic view of what we’re up against,” he says.
In the meantime, Vosshall aims to compare the olfactory neurons of blood-eating mosquitoes with those of purely vegetarian mosquito relatives to see if the more extreme complexity of the receptors is a unique adaptation for species that only hunt humans. And as far as the puzzle goes, Vosshall first started probing – how the combined detection of the two CO2 and body odor greatly amplifies the message to the brain? One of his former postdocs, Meg Younger, is tackling the question in her new lab at Boston University.
Margaret Herre, Olivia V. Goldman et al. “Non-canonical olfactory coding in the mosquito.” Cell. Published August 18, 2022. doi: 10.1016/j.cell.2022.07.024