Could Nasal Mucus Help Explain Age-Related Changes in Smell?

When we think about how smell works, we tend to focus on receptors in the nose or on the brain. But the first thing inhaled molecules encounter is a thin layer of mucus covering the upper nasal cavity.

A 2018 study broadly examined the proteins in this mucus and identified approximately 3,000 types across 24 people. By exploring differences between nasal locations and age groups, as well as associations with sensitivity to odors, it identified candidate mechanisms operating just before odors reach their sensory receptors.

Mucus: the entrance that odor molecules pass through

Near the top of the nose is a narrow space called the olfactory cleft. Sensory neurons that detect odors lie in the nearby olfactory epithelium. Its surface is covered in mucus, which comes into contact with molecules and microorganisms entering from outside.

This mucus has been thought to do more than protect tissue: it may transport odor molecules or chemically alter them through enzymes. However, the proteins present in human mucus were not yet well characterized.

The researchers therefore carried out proteomic analysis, a comprehensive examination of proteins. Their goals were to build a catalogue of components, determine whether the olfactory cleft differed from the front of the nose, compare younger and older people, and look for candidates associated with olfactory sensitivity.

Sampling two locations in 12 younger and 12 older participants

The study included 12 younger and 12 older participants. The main text gives age ranges of 21–40 and 65–80, but Supplementary Table S1 also lists participants aged 19, 84, and 85, so the reported age ranges are inconsistent. Medical interviews and nasal endoscopy confirmed the absence of obvious nasal or sinus inflammation. Smokers and pregnant people were excluded.

Using an endoscope to check placement, the researchers positioned small sponges in the olfactory cleft and the front of the nose to collect mucus. The design compared two locations within the same person.

The collected proteins were broken into small fragments with an enzyme and examined using liquid chromatography and mass spectrometry. Matching information from those fragments to a database enabled many proteins to be identified together.

The participants also underwent smell testing. For the main association analyses, sensitivity was represented by the average of their detection-threshold scores for two odors, phenylethyl alcohol and n-butanol: how dilute each odor could become before it was no longer detected.

A catalogue of about 3,000 proteins and differences between nasal locations

Across all 24 participants, 2,987 types of proteins were identified in olfactory cleft mucus. The average per person was 1,236 in the younger group and 1,227 in the older group. The figure does not mean that every person individually had about 3,000 identified proteins.

The catalogue included 75 of the 83 proteins reported in an earlier analysis, substantially extending the scope of identification. However, the method can struggle to detect small proteins present in low amounts, among other limitations, so it does not represent an exhaustive measurement of every mucus component.

Overall, protein composition was very similar in the olfactory cleft and the front of the nose. Rather than entirely separate fluids occupying each location, the authors suggest that a continuous mucus layer may allow some mixing.

Within that overall similarity, criteria combining correction for multiple statistical comparisons and a greater than 1.3-fold difference in mean concentration identified 87 proteins enriched in the olfactory cleft in the younger group and 10 in the older group. These included metabolic enzymes, providing candidates for investigating functions particular to the site of smell.

Age differences appeared in certain proteins rather than overall quantities

Total mucus protein concentration and the number of identified protein types did not differ significantly between the younger and older groups. The differences involved the amounts of particular proteins, rather than a uniform decrease across the entire composition.

Twenty olfactory cleft proteins showed age differences meeting the multiple-comparison correction and 1.3-fold criteria. The older group had higher levels of proteins including S100A8 and S100A9, which are involved in inflammation.

Conversely, levels of some metabolic enzymes, including glutathione S-transferase P (GSTP), and some proteins associated with cellular protection were lower.

Metabolic enzymes are candidates for altering odor molecules before they reach receptors or processing molecules after detection. The authors discuss whether changes in inflammation and molecular processing capacity may be important for understanding age-related changes in smell. The experiment compared protein amounts; it did not directly measure the reactions of individual enzymes.

Looking for candidates that vary with olfactory sensitivity

In the smell tests, the older group had lower sensitivity on average than the younger group. There were also individual differences within each group.

When all 24 participants were analyzed together to explore relationships between protein abundance and sensitivity, associations were reported for 83 proteins. For 52, higher abundance was associated with lower sensitivity; for 31, it was associated with higher sensitivity.

Examples associated with higher sensitivity included LCN15 and BPIFB4. These proteins have been proposed as candidates for binding and transporting odor molecules. Alongside earlier information about gene expression in nasal olfactory tissue, the findings make them targets for future functional studies.

Candidates associated with lower sensitivity included inflammation-related components and keratins that support epithelial cell structure. They provide clues linking the condition of mucus with differences in smell.

What is needed to move from associations to demonstrated functions?

The sensitivity association analysis needs particularly careful interpretation. Many tests were performed across approximately 3,000 protein types, but the 83 candidates were selected using a 5% significance threshold without correction for multiple comparisons. Testing many items at once makes it more likely that some associations will arise by chance.

Because younger and older participants were analyzed together, age may also be a common factor behind the relationships between proteins and sensitivity. The 83 proteins should be treated as exploratory candidates for confirmation with additional participants or functional experiments.

LCN15 and BPIFB4 also showed enrichment in the olfactory cleft when compared individually, but they did not meet the initial, stricter selection criteria after correction. Distinguishing which analysis produced each result makes the next steps for verification clearer.

The study’s contribution is a large catalogue of components that can serve as a starting point for such verification, along with candidates organized by location, age, and sensitivity. It does not establish a single cause of aging-related changes, a diagnostic method for neurological disease, or a way to improve the sense of smell.

Summary

The mucus covering the nasal olfactory cleft contained a diverse range of proteins, with about 3,000 types identified across 24 people. Its overall composition resembled that of mucus at the front of the nose, but some components were more abundant in the olfactory cleft, and 20 showed differences in amount between age groups.

The study also identified candidates associated with olfactory sensitivity, opening the way to investigate functions such as transporting and altering odor molecules and protecting tissue. Understanding how we experience smell is worth approaching not only through receptors and the brain but also through the mucus that lies before them.

The research was funded by Kao and conducted under a collaborative research agreement with the Monell Chemical Senses Center. The paper identifies four authors as Kao employees; the other authors declared no competing interests.

References

Yoshikawa K, et al. (2018). The human olfactory cleft mucus proteome and its age-related changes. Scientific Reports, 8, 17170. Original article and open-access full text