(Image caption: Primitive olfactory receptors of the jumping bristletail are thought to be amongst the most evolutionarily ancient versions of olfactory receptors in insects. Credit: Katja Schulz, CC BY 2.0)

Study reveals how smell receptors work

All senses must reckon with the richness of the world, but nothing matches the challenge faced by the olfactory system that underlies our sense of smell. We need only three receptors in our eyes to sense all the colors of the rainbow—that’s because different hues emerge as light-waves that vary across just one dimension, their frequency. The vibrant colorful world, however, pales in comparison to the complexity of the chemical world, with its many millions of odors, each composed of hundreds of molecules, all varying greatly in shape, size and properties. The smell of coffee, for instance, emerges from a combination of more than 200 chemical components, each of which are structurally diverse, and none of which actually smells like coffee on its own.

“The olfactory system has to recognize a vast number of molecules with only a few hundred odor receptors or even less,” says Rockefeller neuroscientist Vanessa Ruta. “It’s clear that it had to evolve a different kind of logic than other sensory systems.”

In a new study, Ruta and her colleagues offer answers to the decades-old question of odor recognition by providing the first-ever molecular views of an olfactory receptor at work.

The findings, published in Nature, reveal that olfactory receptors indeed follow a logic rarely seen in other receptors of the nervous system. While most receptors are precisely shaped to pair with only a few select molecules in a lock-and-key fashion, most olfactory receptors each bind to a large number of different molecules. Their promiscuity in pairing with a variety of odors allows each receptor to respond to many chemical components. From there, the brain can figure out the odor by considering the activation pattern of combinations of receptors.

Holistic recognition

Olfactory receptors were discovered 30 years ago. But scientists have not been able to see them up close and decipher their structural and mechanistic workings, in part because these receptors didn’t lend themselves to commonly available molecular imaging methods. Complicating the matter, there seems to be no rhyme or reason to the receptors’ preferences—an individual odor receptor can respond to compounds that are both structurally and chemically different.

“To form a basic understanding of odorant recognition we need to know how a single receptor can recognize multiple different chemicals, which is a key feature of how the olfactory system works and has been a mystery,” says Josefina del Mármol, a postdoc in Ruta’s lab.

So Ruta and del Mármol, along with Mackenzie Yedlin, a research assistant in the lab, set out to solve an odor receptor’s structure taking advantage of recent advances in cryo-electron microscopy. This technique, which involves beaming electrons at a frozen specimen, can reveal extremely small molecular constructs in 3D, down to their individual atoms.

The team turned to the jumping bristletail, a ground-dwelling insect whose genome has been recently sequenced and has only five kinds of olfactory receptors. Although the jumping bristletail’s olfactory system is simple, its receptors belong to a large family of receptors with tens of millions of variants thought to exist in hundreds of thousands of different insect species. Despite their diversity, these receptors function the same way: They form an ion channel—a pore through which charged particles flow—that opens only when the receptor encounters its target odorant, ultimately activating the sensory cells that initiate the sense of smell.

The researchers chose OR5, a receptor from the jumping bristletail with broad recognition ability, responding to 60 percent of the small molecules they tested.

They then examined OR5’s structure alone and also bound to a chemical, either eugenol, a common odor molecule, or DEET, the insect repellent. “We learned a lot from comparing these three structures,” Ruta says. “One of the beautiful things you can see is that in the unbound structure the pore is closed, but in the structure where it’s bound with either eugenol or DEET, the pore has dilated and provides a pathway for ions to flow.”

With the structures in hand, the team looked closer to see exactly where and how the two chemically different molecules bind to the receptor. There have been two ideas about odor receptors’ interactions with molecules. One is that the receptors have evolved to distinguish large swaths of molecules by responding to a partial but defining feature of a molecule, such as a part of its shape. Other researchers have proposed that each receptor packs multiple pockets on its surface at once, allowing it to accommodate a number of different molecules.

But what Ruta found fit neither of those scenarios. It turned out that both DEET and eugenol bind at the same location and fit entirely inside a simple pocket within the receptor. And surprisingly, the amino acids lining the pocket didn’t form strong, selective chemical bonds with the odorants, but only weak bonds. Whereas in most other systems, receptors and their target molecules are good chemical matches, here they seemed more like friendly acquaintances. “These kinds of nonspecific chemical interactions allow different odorants to be recognized,” Ruta says. “In this way, the receptor is not selective to a specific chemical feature. Rather, it’s recognizing the more general chemical nature of the odorant,” Ruta says.

And as computational modeling revealed, the same pocket could accommodate many other odor molecules in just the same way.

But the receptor’s promiscuity doesn’t mean it has no specificity, Ruta says. Although each receptor responds to a large number of molecules, it is insensitive to others. Moreover, a simple mutation in the amino acids of the binding site would broadly reconfigure the receptor, changing the molecules with which it prefers to bind. This latter finding also helps to explain how insects have been able to evolve many millions of odor receptor varieties suited for the wide range of lifestyles and habitats they encounter.

The findings are likely representative of many olfactory receptors, Ruta says. “They point to key principles in odorant recognition, not only in insects’ receptors but also in receptors within our own noses that must also detect and discriminate the rich chemical world.”

Sense of smell is our most rapid warning system

The ability to detect and react to the smell of a potential threat is a precondition of our and other mammals’ survival. Using a novel technique, researchers at Karolinska Institutet have been able to study what happens in the brain when the central nervous system judges a smell to represent danger. The study, which is published in PNAS, indicates that negative smells associated with unpleasantness or unease are processed earlier than positive smells and trigger a physical avoidance response.

“The human avoidance response to unpleasant smells associated with danger has long been seen as a conscious cognitive process, but our study shows for the first time that it’s unconscious and extremely rapid,” says the study’s first author Behzad Iravani, researcher at the Department of Clinical Neuroscience, Karolinska Institutet.

The olfactory organ takes up about five per cent of the human brain and enables us to distinguish between many million different smells. A large proportion of these smells are associated with a threat to our health and survival, such as that of chemicals and rotten food. Odour signals reach the brain within 100 to 150 milliseconds after being inhaled through the nose.

Measuring the olfactory response

The survival of all living organisms depends on their ability to avoid danger and seek rewards. In humans, the olfactory sense seems particularly important for detecting and reacting to potentially harmful stimuli.

It has long been a mystery just which neural mechanisms are involved in the conversion of an unpleasant smell into avoidance behaviour in humans.

One reason for this is the lack of non-invasive methods of measuring signals from the olfactory bulb, the first part of the rhinencephalon (literally “nose brain”) with direct (monosynaptic) connections to the important central parts of the nervous system that helps us detect and remember threatening and dangerous situations and substances.

Researchers at Karolinska Institutet have now developed a method that for the first time has made it possible to measure signals from the human olfactory bulb, which processes smells and in turn can transmits signals to parts of the brain that control movement and avoidance behaviour. 

The fastest warning system

Their results are based on three experiments in which participants were asked to rate their experience of six different smells, some positive, some negative, while the electrophysiological activity of the olfactory bulb when responding to each of the smells was measured.

“It was clear that the bulb reacts specifically and rapidly to negative smells and sends a direct signal to the motor cortex within about 300 ms,” says the study’s last author Johan Lundström, associate professor at the Department of Clinical Neuroscience, Karolinska Institutet. “The signal causes the person to unconsciously lean back and away from the source of the smell.”

He continues:

“The results suggest that our sense of smell is important to our ability to detect dangers in our vicinity, and much of this ability is more unconscious than our response to danger mediated by our senses of vision and hearing.” 

“Caramel receptor” identified—New insights from the world of chemical senses

Who doesn’t like the smell of caramel? However, the olfactory receptor that contributes decisively to this sensory impression was unknown until now. Researchers at the Leibniz Institute for Food Systems Biology at the Technical University of Munich (LSB) have now solved the mystery of its existence and identified the “caramel receptor”. The new knowledge contributes to a better understanding of the molecular coding of food flavors.

Furaneol is a natural odorant that gives numerous fruits such as strawberries, but also coffee or bread, a caramel-like scent. Likewise, the substance has long played an important role as a flavoring agent in food production. Nevertheless, until now it was unknown which of the approximately 400 different types of olfactory receptors humans use to perceive this odorant.

Odorant receptors put to the test

This is not an isolated case. Despite intensive research, it is still only known for about 20 percent of human olfactory receptors which odorant spectrum they recognize. To help elucidate the recognition spectra, the team led by Dietmar Krautwurst at LSB is using a collection of all human olfactory receptor genes and their most common genetic variants to decipher their function using a test cell system.

“The test system we developed is unique in the world. We have genetically modified the test cells so that they act like small biosensors for odorants. In doing so, we specify exactly which type of odorant receptor they present on their cell surface. In this way, we can specifically investigate which receptor reacts how strongly to which odorant,” explains Dietmar Krautwurst. In the present study, the researchers examined a total of 391 human odorant receptor types and 225 of their most common variants. 

Only two odorants for one receptor

“As our results show, furaneol activated only the OR5M3 odorant receptor. Even one thousandth of a gram of the odorant per liter is sufficient to generate a signal,” says first author of the study Franziska Haag. In addition, the team investigated whether the receptor also reacts to other odorants. To this end, the team examined 186 other substances that are key odorants and therefore play a major role in shaping the aroma of food. Of these, however, only homofuraneol was able to significantly activate the receptor.

This odorant is structurally closely related to furaneol. As shown by previous LSB studies, it imparts a caramel-like aroma to fruits such as durian. “We hypothesize that the receptor we identified, OR5M3, has a very specific recognition spectrum for food ingredients that smell caramel-like. In the future, this knowledge could be used to develop new biotechnologies that can be used to quickly and easily check the sensory quality of foods along the entire value chain,” says Dietmar Krautwurst. Although there is still a long way to go to understand the complex interplay between the approximately 230 key food-related odorants and human olfactory receptors, a start has been made, the molecular biologist adds.

Veronika Somoza, Director of the Leibniz Institute adds: “In the future, we will continue to use our extensive odorant and receptor collections at the Institute to help elucidate the molecular basis of human olfactory perception. After all, this significantly influences our food choices and thus our health.”

(Image credit: G. Olias / LSB)

How scents take on meaning

A Bochum-based research team triggered artificial odour sensations in rats – and looked at what happens in the brain as a result.

Once a scent is detected, different areas of the brain are activated. A team from the Department of Neuroscience at Ruhr-Universität Bochum (RUB) has recently discovered that structures of the olfactory sense work closely together with the brain’s reward and aversion systems. This means that scents are processed not only by the olfactory centre but also by regions responsible for emotions and valence determination. The findings were published in the journal “Cerebral Cortex”.

Dr. Christina Strauch, PhD student Thu-Huong Hoang, and Professor Denise Manahan-Vaughan from the Department of Neurophysiology collaborated on the study with Professor Frank Angenstein from the German Center for Neurodegenerative Diseases (DZNE) in Magdeburg.

Olfactory perception outside the olfactory bulb and the olfactory cortex

The researchers studied how the processing of scents affects structures in the brain. They used electrical impulses to stimulate the olfactory bulbs of test animals. Then, they analysed the activity in the olfactory cortex, where olfactory stimuli are processed. “We already knew that there is a connection between the olfactory bulb and the piriform cortex, a part of the olfactory cortex, in the perception of scents,” explains Dr. Christina Strauch, lead author of the study. “But our goal was to go deeper into the brain structures and find out which regions we had underestimated or overlooked until now.” “So far, only a few studies on olfactory perception have analysed regions outside the olfactory bulb and olfactory cortex regions in rodents,” says Professor Denise Manahan-Vaughan, spokesperson of Collaborative Research Centre 874 Integration and Representation of Sensory Processes. “It is still not completely understood how olfactory memories are formed. Our goal was to clarify to what extent brain structures that aren’t part of the olfactory system are involved in olfactory memory formation.”

Evidence of olfactory processing in the rodent brain

In their study, the researchers combined electrophysiological stimulation with functional magnetic resonance imaging (fMRI). Following this approach, the team obtained a detailed picture of the neuronal structures that responded to the stimulation of the olfactory bulb. Highly responsive structures were then analysed in more depth using fluorescence in situ hybridisation analysis of neuronal gene expression. This technique helps researchers determine whether neurons do indeed store the olfactory stimulus: This event serves as evidence of memory formation.

Sure enough, stimulation of the olfactory bulb had led to altered gene activity. This happened even in the nerve cells of the limbic cortex – that is, in a functional unit attributed with the processing of emotions. “The involvement of these non-olfactory structures probably plays a key role in the storage of olfactory experiences,” as Christina Strauch interprets the findings. “We deduce from this that rodents quickly categorise perceived scents as pleasant or unpleasant while smelling them.”

Overall, the results prove that the olfactory system works closely with the brain’s reward and aversion systems in both learning and memory formation.

“The study provides us an additional theoretical basis for understanding why the sense of smell plays such a unique role in the formation and retrieval of memories,” says Denise Manahan-Vaughan, who together with Christina Strauch has been exploring how memories are formed from scents since 2010.

Editor’s note: Hi all! It’s been a while - but I’m back with a post about how we smell things! This is reposted from a fragrance review + perfumery blog I created, Aromatic Aethers - do consider checking it out and subscribing to it if you enjoyed this post and want to learn more about olfaction/perfumery, or are simply passionate about fragrances :)

Envision yourself in an endless hallway with countless locked doors. Behind each door is a room filled with a scent - vanilla, jasmine, sea spray, you name it - even some that you’ve never smelled before in your life!

Each lock is unique, but it doesn’t require the key to be a perfect fit to be opened; as long as the key is of roughly the right shape and size, it’ll be able to open the door, sending the hallway awash with its contained aroma - and sometimes, even an unfurled paperclip would suffice.

In some cases, the locks are similar, too - a key that opens one door can open some others, and as the scents mix in the hallway, you pick up a mixture of smells.

That is the lock-and-key mechanism of olfaction; the keys are the molecules that make their way into your nose, which are known as ligands, and the locks are the olfactory receptors in your nasal cavity, which have a complex 3D structure that the ligand will need to be complementary to in order to bind.

Just like regular keys, the teeth are what controls whether the key will be able to turn the lock; that is analogous to the osmophore and profile, key parts (see what I did there?) of the molecule which enables it to be inserted into and bind to the receptor, triggering nervous signals that your brain interprets as a scent. Without it, the ligand would be completely unable to engage the receptor.

On the other hand, while necessary for the functioning of the key, you could get away with a locksmith making you a duplicate key with a slightly misshapen stem - the key would probably still be able to fit inside the lock, and as long as the tip can be inserted and the teeth line up with the mechanism, you’ll be able to open the door - given that the stem isn’t completely out of shape or of the wrong size. This is akin to the rest of the ligand besides the osmophore and profile.

(Note: While the lock-and-key analogy suggests an all-or-none response, the “stem” in odourants or the positioning of the osmophore can help the molecule achieve better binding, which in turn triggers a stronger response - so imagine a key with a poor fit allowing you to open the door by a gap and pick up wisps of the aroma, while a perfect fit would enable you to throw the door wide open, completely flooding the hallway with its scent.)

As you can see, the molecular structures of limonene, which smells like citruses, and cedrol, which is found in cedarwood, which look nothing alike, also smell nothing alike - cedrol is too large to fit into the limonene receptor, while limonene can’t achieve good binding to the cedrol receptors as the teeth of the key don’t match.

On the other hand, considering beta-santalol, which is one of the principal odourants of sandalwood, and sandalore, which is a synthetic molecule but also smells like sandalwood, you can see that they share a similar general shape; a bulky cyclic structure on one end of the molecule and an OH group on the other, separated by a chain of carbon atoms (they look just like keys, don’t they?). This allows them to both fit into the same receptor and elicit the same response from our brain (“oh, this smells like sandalwood!”).

With the knowledge of the receptor’s shape, new odourant molecules can also be designed, and may even achieve better binding than the natural ligand - javanol, for example, is an extremely powerful sandalwood molecule that was discovered through intentional molecular design.

Back to the titular question: What makes a rose smell like a rose?

If you analysed the components of rose oil, you’d find that it has myriad compounds that contribute to its scent - citronellol, geraniol, phenylethyl alcohol, beta-damascone… You get the idea. None of them are singularly responsible for the complex, rich scent of roses, but instead contribute to the various olfactory facets that rose possesses - lemony, floral, balsamic, et cetera. It is important to note that smells are usually highly complex mixtures of hundreds or thousands of molecules, and oftentimes, no single molecule can realistically replicate a natural scent.

If you attempted to recreate the scent of a rose by taking the most abundant molecules like geraniol and phenethyl alcohol, you’d end up with a rather flat caricature; somewhat counterintuitively, relative abundance isn’t correlated to the impact that they have on the overall scent! That is why even though citronellol makes up 38% of rose oil, it’s only responsible for 4.3% of the olfactory impact, whereas beta-damascenone is a trace component (0.14%) but is responsible for 70% of the olfactory impact (Source: Scent and Chemistry, by Ohloff, Pickenhagen, and Kraft)!

This caused a lot of headaches for chemists a century ago, as they only had the techniques to identify the more abundant compounds back then, and were thus unable to convincingly recreate the scent of roses as a cheaper alternative to rose oil!

While analytical techniques have advanced immensely since then, many questions remain - even a commonly used perfumery material like vetiver oil only recently had its odour principle, which is present in trace amounts, discovered by Kraft et al. Nevertheless, new discoveries are being made every day, and I’m excited to see what the future brings!

Photo sources: [1][2][3] [4, 5 - my own] [6]

I hope you enjoyed this first part of a series on olfaction. This post was reposted from my perfumery & olfaction blog, Aromatic Aethers - do consider paying it a visit and subscribing if you liked this post! :)