Olfactory Function as an Early Biomarker: Testing, Physiology & Nasal Brushing

Measuring smell (olfaction) is a window into brain health and an early warning sign for neuro-cognitive disorders.
Key research goal: map the relationship between olfactory deficits and future cognitive decline.
Core principles stressed by the lecturer:
- Diverse outcome battery: Do not rely on a single task (e.g.
  identification) just because it is easy; different tasks stress different neural circuits.
- Longitudinal baselines: Because smell varies widely between individuals, each person should be compared to their own baseline, ideally collected in mid-life.
- Compare with gold-standard biomarkers: When developing any new test/biomarker it is vital to run it in parallel with established clinical measures (neurology work-ups, imaging, CSF, etc.) to benchmark effect size and timing.
- Disentangle peripheral vs. central causes: Changes may reflect the nose itself (peripheral) and/or brain-level interpretation (central). Study designs must isolate the focus of interest.

Example cohort: Harvard ADRC follows participants starting at 55\,\text{years} until death, testing annually.
Objective: Catch olfactory changes before neurologists can detect clinical disease.
Longitudinal smell testing + biomarker comparisons provide a guard-rail for validation (effect size, time-course).

Peripheral influences
- Nasal congestion, viral infection (e.g. SARS-CoV-2 damaging support cells), toxic exposure, etc.
- These block odorant access or impair receptor renewal.
Central influences
- Learning, culture, language, expectations, and broader cognitive frameworks used to interpret odor signals.
Exclusion criteria in “central” studies: rule out respiratory illness or other peripheral problems to enrich for brain-related smell deficits.

Blue = central; black = mixed or peripheral

Subjective self-report (blue)
- “How good is your smell overall?” “Has it declined?”
- Controversial: people often under-estimate impairment because olfaction lacks a ‘blurry-vision’ equivalent.
Objective, forced-choice tasks
- Identification (name the odor)
- Detection threshold (lowest concentration detectable)
- Discrimination (same/different pairs)
Evaluative measures (mixed)
- Intensity ratings ("How strong is it?" from “nothing” to “strongest imaginable”)
- Hedonic valence (pleasant, disgusting, neutral, etc.)
Each measure probes distinct neural computations; combining them yields a richer functional signature.

Students received a pack of 6 laminated cards, each containing microencapsulated odors beneath peel-back circles.
Classroom exercise used Card A (first 3 odors):
1. Peel half-way, sniff, reseal.
2. Record intensity on a personal scale (subjective evaluator component).
3. Provide free-association: concrete label, abstract memory, emotional tone, etc.
Demonstrates blend of perceptual detection and cognitive/affective interpretation.

Olfactory epithelium (OE): Yellowish strip high in nasal vault; first point of contact for airborne chemicals.
Cellular composition (from lumen inward):
- Mature olfactory receptor neurons (ORNs/OSNs)
- Immature ORNs
- Sustentacular (support) cells
- Basal stem cells (globose + horizontal)
Unique feature – adult neurogenesis: Entire OSN population self-renews roughly every 60\,\text{days}.
- Protective adaptation because OSNs are directly exposed to environment/toxins.
- Provides natural model for studying cell turnover, aging, and neurodegeneration.

Purpose: harvest living OE cells + surrounding fluid for molecular analysis while patient is awake.
Procedure overview:
1. Topical \text{lidocaine} (anesthetic) + \text{afrin} (vasoconstrictor) to open nasal passages.
2. ENT physician inserts endoscope for real-time video guidance.
3. Flexible cytology brush advanced to high posterior OE region.
4. Brush rotated/withdrawn, capturing cells and extracellular fluid.
Video shows brush navigating up nasal cavity under endoscopic view.
Yields heterogeneous sample: mature/immature OSNs, sustentacular cells, basal stem cells, mucus.

Single-cell RNA sequencing (scRNA-seq): profile transcriptomes, identify cell types, quantify inflammatory markers, measure odorant-receptor gene (OR) expression.
Cytology & immunohistochemistry: protein localization (e.g. tau phosphorylation, \alpha-synuclein aggregation).
Fluid biomarker assays: same neurodegeneration panels used in CSF/blood—\text{A}\beta, \text{p}-\tau, \text{TDP-43}, etc.

Healthy control study: OSNs naturally express neurodegenerative-associated proteins (TDP-43, tau, \beta-amyloid, etc.)—baseline signature.
Open questions:
- Do expression levels or mis-localization patterns shift in pre-clinical or symptomatic patients?
- Does rapid 60\,\text{day} turnover hinder toxic aggregation, or can OSNs seed brain pathology through axonal spread?
- Could OE sampling replace or complement \$
  >$12500 PET scans or lumbar punctures for early diagnosis?

Long COVID cohort: n=1 persistent chemesthetic loss, n=2 recovered, n=1$$ healthy control.
- scRNA-seq aims: detect inflammatory transcripts vs. OR down-regulation.
- Clinical goal: tailor interventions—olfactory training vs. anti-inflammatory therapy.
Neurodegenerative recruitment (Summer): brush Parkinson’s, Alzheimer’s, Lewy-body, FTD cases; perform multiplex immunoassays to correlate molecular profile with symptom clusters.

Non-invasive smell tests are cheap, mail-friendly, accessible to underserved populations; can be repeated at home for remote monitoring.
Nasal brushing is more invasive but still outpatient and cheaper than imaging.
Potential future workflow:
1. Annual home smell kit → detect deviation from baseline.
2. Clinic visit → nasal brush if score crosses risk threshold.
3. Molecular read-out → personalized prognosis & treatment plan.
Could narrow clinical heterogeneity, accelerate trials (enrich for molecularly defined sub-types), and democratize early detection.

Sensory tests = functional probe of distributed neural network.
Nasal brushing = molecular/ cellular snapshot of peripheral-central interface.
Combining both offers a scalable, tiered biomarker pipeline: screen widely, zoom in precisely, intervene earlier.
Vision: shift from late-stage symptom management to proactive, biology-informed neuro-protection using the nose as an accessible extension of the brain.