Making sense of scents: Deciphering our sense of smell

Breaking a longstanding impasse in our understanding of olfaction, scientists at UC San Francisco (UCSF) have created the first molecular-level, 3D picture of how an odor molecule activates a human odorant receptor, a crucial step in deciphering the sense of smell.

The findings, appearing online March 15, 2023, in Nature, are poised to reignite interest in the science of smell with implications for fragrances, food science, and beyond. Odorant receptors -- proteins that bind odor molecules on the surface of olfactory cells -- make up half of the largest, most diverse family of receptors in our bodies; A deeper understanding of them paves the way for new insights about a range of biological processes.

"This has been a huge goal in the field for some time," said Aashish Manglik, MD, PhD, an associate professor of pharmaceutical chemistry and a senior author of the study. The dream, he said, is to map the interactions of thousands of scent molecules with hundreds of odorant receptors, so that a chemist could design a molecule and predict what it would smell like.

"But we haven't been able to make this map because, without a picture, we don't know how odor molecules react with their corresponding odor receptors," Manglik said.

A Picture Paints the Scent of Cheese

Smell involves about 400 unique receptors. Each of the hundreds of thousands of scents we can detect is made of a mixture of different odor molecules. Each type of molecule may be detected by an array of receptors, creating a puzzle for the brain to solve each time the nose catches a whiff of something new.

"It's like hitting keys on a piano to produce a chord," said Hiroaki Matsunami, PhD, professor of molecular genetics and microbiology at Duke University and a close collaborator of Manglik. Matsunami's work over the past two decades has focused on decoding the sense of smell. "Seeing how an odorant receptor binds an odorant explains how this works at a fundamental level."

To create that picture, Manglik's lab used a type of imaging called cryo-electron microscopy (cryo-EM), that allows researchers to see atomic structure and study the molecular shapes of proteins. But before Manglik's team could visualize the odorant receptor binding a scent molecule, they first needed to purify a sufficient quantity of the receptor protein.

Odorant receptors are notoriously challenging, some say impossible, to make in the lab for such purposes.

The Manglik and Matsunami teams looked for an odorant receptor that was abundant in both the body and the nose, thinking it might be easier to make artificially, and one that also could detect water-soluble odorants. They settled on a receptor called OR51E2, which is known to respond to propionate -- a molecule that contributes to the pungent smell of Swiss cheese.

But even OR51E2 proved hard to make in the lab. Typical cryo-EM experiments require a milligram of protein to produce atomic-level images, but co-first author Christian Billesbøelle, PhD, a senior scientist in the Manglik Lab, developed approaches to use only 1/100th of a milligram of OR51E2, putting the snapshot of receptor and odorant within reach.

"We made this happen by overcoming several technical impasses that have stifled the field for a long time," said Billesbøelle. "Doing that allowed us to catch the first glimpse of an odorant connecting with a human odorant receptor at the very moment a scent is detected."

This molecular snapshot showed that propionate sticks tightly to OR51E2 thanks to a very specific fit between odorant and receptor. The finding jibes with one of the duties of the olfactory system as a sentinel for danger.

While propionate contributes to the rich, nutty aroma of Swiss cheese, on its own, its scent is much less appetizing.

"This receptor is laser focused on trying to sense propionate and may have evolved to help detect when food has gone bad," said Manglik. Receptors for pleasing smells like menthol or caraway might instead interact more loosely with odorants, he speculated.

Just a Whiff

Along with employing a large number of receptors at a time, another interesting quality of the sense of smell is our ability to detect tiny amounts of odors that can come and go. To investigate how propionate activates this receptor, the collaboration enlisted quantitative biologist Nagarajan Vaidehi, PhD, at City of Hope, who used physics-based methods to simulate and make movies of how OR51E2 is turned on by propionate.

"We performed computer simulations to understand how propionate causes a shape change in the receptor at an atomic level," said Vaidehi. "These shape changes play a critical role in how the odorant receptor initiates the cell signaling process leading to our sense of smell."

The team is now developing more efficient techniques to study other odorant-receptor pairs, and to understand the non-olfactory biology associated with the receptors, which have been implicated in prostate cancer and serotonin release in the gut.

Manglik envisions a future where novel smells can be designed based on an understanding of how a chemical's shape leads to a perceptual experience, not unlike how pharmaceutical chemists today design drugs based on the atomic shapes of disease-causing proteins.

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How mosquito brains encode human odor so they can seek us out

Mosquitoes. Bane of backyard picnics -- and deadly in Zika- and dengue-prone regions.

Most of the world's mosquitos are opportunistic, willing to drink blood from any nearby source. But in some regions, the mosquitoes that carry Zika, dengue and yellow fever -- Aedes aegypti -- have evolved to bite humans almost exclusively. But to succeed as a specialized feeder, depending on just one species -- ours -- to survive, they must have evolved incredibly precise targeting strategies. How do they do it?

"We set out to try to understand how these mosquitoes distinguish human and animal odor," said Carolyn "Lindy" McBride, an assistant professor of ecology and evolutionary biology and neuroscience, "both in terms of what it is about human odor that they cue in on and what part of their brain allows them to cue in on those signals."

After years of dedicated work, including countless scientific and technological challenges, her team has discovered answers to both parts of this equation. What is it that the mosquitos are detecting, and how do they detect it? Their results appear in the current issue of Nature.

McBride described their mosquito-centric approach: "We sort of dove into the brain of the mosquito and asked, 'What can you smell? What lights up your brain? What's activating your neurons? And how is your brain activated differently when you smell human odor versus animal odor?'"

Then-graduate student Zhilei Zhao, a 2021 Ph.D. alumnus who is now at Cornell, pioneered their novel approach: imaging mosquito brains at very high resolution to watch how the mosquito identifies its next victim. To do that, he had to first genetically engineer mosquitos whose brains lit up when active, and then the team had to deliver human- and animal-flavored air in ways that the mosquitos could detect while inside the team's custom-built imaging equipment.

Human odor is composed of dozens of different compounds, and those same compounds, in slightly different ratios, are present in most mammal odors. None of those compounds is attractive to mosquitoes by itself, so the challenge was to determine the exact blend of components that mosquitos use to recognize human odor.

The team concluded that two chemicals, decanal and undecanal, are enriched in human odor. They patented a blend featuring decanal that they hope could lead to baits attracting mosquitoes to lethal traps, or repellents that interrupt the signal.

To provide comparison mammals to test, graduate student Jessica Zung worked with former research specialists Alexis Kriete and Azwad Iqbal to collect hair, fur and wool samples. For this paper, the team used odor from sixteen humans, two rats, two guinea pigs, two quail, one sheep and four dogs. Howell Living History Farm in Hopewell, N.J., donated several fleeces from their spring sheep shearing; for another domesticated mammal, Zung went to a grooming salon and gathered trimmed hairs from recently groomed pet dogs.

"For the human samples, we had a bunch of great volunteers," Zung said. "We had them not shower for a few days, then strip down naked and lie down in a Teflon bag." Why naked? Because cotton, polyester and other clothing fibers have their own smells that would distort the data.

Once they conquered the technical challenges -- retrieving the human and animal odors nondestructively, designing a system that allowed them to puff human odor at the mosquitos in the imaging setup, creating a wind tunnel to test simple blends or single compounds, and breeding viable strains of mosquitos whose brains respond to the equipment -- they began gathering data.

Very surprising data.

Before this study, researchers speculated that mosquito brains must have a complicated, sophisticated technique for distinguishing humans from other animals. Quite the opposite, it turned out.

"The simplicity surprised us," said McBride. "Despite the complexity of human odor, and the fact that it doesn't really have any kind of human-specific compounds in it, the mosquitoes have evolved a surprisingly simple mechanism for recognizing us. To me, it's an evolutionary story: if we created a statistical test to differentiate human odor, it would be very complex, but the mosquito does something remarkably simple, and simple usually works pretty well, when it comes to evolution."

In other words, simple solutions tend to breed true, over evolutionary time.

Mosquito brains have 60 nerve centers called glomeruli (singular: glomerulus). The team had hypothesized that many -- maybe even most -- would be involved in helping these human-dependent mosquitos find their favorite food.

"When I first saw the brain activity, I couldn't believe it -- just two glomeruli were involved," Zhao said. "That contradicted everything we expected, so I repeated the experiment several times, with more humans, more animals. I just couldn't believe it. It's so simple."

Of the two nerve centers, one responds to many smells including human odor, essentially saying, "Hey, look, there's something interesting nearby you should check out," while the other responds only to humans. Having two may help the mosquitos home in on their targets, the researchers suggest.

That was one of the biggest "Eureka!" moments in the project, said McBride. "Zhilei had worked for a couple years to get the transgenic mosquitoes that he needed, and then we found that we didn't have a good way to deliver human odor. So we worked for another year or two, coming up with ideas to try to figure out how to deliver enough human odor in a controlled enough way to see a response. Then, the first time we tried this new technology that we described in the paper -- this new way of delivering odors -- he actually saw a brain respond. It was incredible."

By determining the glomeruli that mosquitos use to detect humans, and identifying what it is they are detecting -- decanal and undecanal -- the team has an elegantly straightforward answer to their questions, noted Zung.

"If this were purely a neuro imaging paper, there would be some questions remaining," she said. "If this were purely an odor analysis paper, there would still be unanswered questions. A purely behavior paper, same thing. But one real strength of this project is that we were able to bring in so many different methods and the expertise of so many people. And Lindy was just amazing and willing to learn about and invest in all these different methods."

Read more at Science Daily