Smell loss emerges as the early behavioural sign of Alzheimer’s

By Dr. Priyom Bose, Ph.D.Reviewed by Lauren HardakerSep 4 2025

New research reveals that immune cells attack brainstem nerve fibers linked to smell, making loss of smell the earliest detectable sign of Alzheimer’s disease, and a potential window for earlier diagnosis.

Study: Early Locus Coeruleus noradrenergic axon loss drives olfactory dysfunction in Alzheimer’s disease. Image credit: Ground Picture/Shutterstock.com

A recent study in Nature Communications examines how the early loss of the brainstem locus coeruleus (LC) noradrenergic axon influences olfactory dysfunction in Alzheimer’s disease (AD).

Reduced olfactory sensitivity in Alzheimer’s disease

AD is the most severe form of dementia, characterized by extracellular deposition of β-amyloid (Aβ), microtubule-associated protein tau aggregation, and Aβ plaque formation. Importantly, therapeutic success relies heavily on the earliest possible diagnosis. Therefore, developing a detailed understanding of the mechanisms preceding the first onset of cognitive symptoms is crucial.

The LC-NA system is affected particularly early in AD. Aberrant tau hyperphosphorylation (pTau) is first detected at this site, which has led researchers to focus mainly on the effects of pTau on LC physiology. Conversely, the literature on the impact of Aβ on LC dysfunction is scarce. Forebrain noradrenalin (NA) regulates various physiological processes and is almost completely derived from the LC.

Symptomatically, the early onset of AD is often marked by olfactory dysfunction, with patients remaining otherwise well and cognitively normal. Despite the common prevalence of decreased olfactory sensitivity in AD cases, the exact mechanisms of action remain unclear.

About the study

The App^NL-G-F mouse line was selected, in which pathogenic Aβ has been enhanced by incorporating three different mutations linked with AD. Both male and female mice, aged one, two, three, and six months, were used. App^NL-G-F mice were crossed with Dbh-Cre mice to manipulate the locus coeruleus-noradrenergic system. App^NL-G-F mice were also crossed with a global TSPO knock-out, which preserved LC axons and normalised olfactory behaviour.

Mouse brain tissues were fixed and subjected to immunostaining analysis. Three-dimensional (3D) images of these samples were acquired via confocal microscopy. Z-stack images of eight microglia per mouse were acquired from three animals per group in the external plexiform layer.

NET fibre density, Iba1-microglia, and NAB228-Aβ-plaque area were quantified.  Colocalization of phosphatidylserine (PS) on NET⁺ LC axon, C1q on NET⁺ LC axon, milk fat globule-EGF factor 8 protein (MFG-E8) on NET⁺ LC axon, and Translocator protein 18 kDa (TSPO) in Iba1⁺ microglia was analyzed. Colocalization was determined in volume and normalized to the NET axon density.

Brain tissue from nine healthy unaffected humans, eight prodromal AD subjects, and six AD patients was obtained from the Munich Brain Bank. Demographic details of all the subjects were collected.

Study findings

The current study observed early LC axon degeneration, particularly to the olfactory bulb (OB), starting between 1 and 2 months in AppNL-G-F mice. Compared to wild-type (WT) animals, one-month-old AppNL-G-F mice exhibited unaltered LC axon density, which gradually altered. For instance, when the mice were two months old, they underwent a 14% fibre loss, which progressed to 27% at 3 months and 33% at 6 months.

Furthermore, LC axons began to degenerate in the piriform cortex, hippocampus, and medial prefrontal cortex between 6 and 12 months at the earliest. At the age of three months, no decrease in the density of choline-acetyl-transferase (ChAT+) nor of serotonergic transporter (SERT+) neurites was recorded. These findings suggest that the loss of axons in the OB was specific to the LC-NA system at this age.

The internal plexiform layer of the OB was determined to be the region with the most prominent axon loss, followed by the external plexiform layer. Even without significant Aβ plaque deposition, OB microglia increased between 2 and 3 months of age.

The current study highlighted LC fibre loss as independent of the extracellular Aβ amount. A consistent olfactory phenotype was detected in App^NL-G-F mice at 3 months of age, which could be deemed the earliest behavioural manifestation linked to AD. A decrease in NA release was estimated in AppNL-G-F mice compared to WT animals for all odours tested. These findings were validated in immunohistochemical assays.

A differential effect on mitral cell membrane potential was observed in WT and App^NL-G-F mice. The patch-clamp findings confirmed that Clozapine-N-Oxide (CNO) application readily activates LC neurons. However, chemogenetic LC activation did not rescue olfactory behaviour, underscoring a structure-to-function dependency on intact axons. Experimental findings robustly indicated a structure-to-function relationship of LC axons in the OB in the context of olfaction.

RNA sequencing (RNA-seq) of microglia isolated from OBs of WT and App^NL-G-F mice at 2 months was performed. This sequencing analysis revealed increased microglia cells isolated from bulbi of App^NL-G-F animals.

Gene ontology analysis indicated that several genes play roles in synapse and plasticity, while only two differentially expressed genes were linked to phagocytosis. Functional assays, including in vitro uptake and in vivo CD68 colocalization, demonstrated increased phagocytic activity. A higher volume of NET⁺ immunosignal was recorded in single microglia cells from App^NL-G-F mice compared to WT animals, further indicating an increase in phagocytic activity. The current study observed increased phagocytic activity in App^NL-G-F mice compared to WT animals of the same age.

OB LC axons showed increased PS externalisation and MFG-E8 decoration, marking them for microglial phagocytosis. LC hyperactivity was linked to Ca²⁺-dependent PS externalisation, providing a mechanistic trigger for clearance. No significant change in complement component 1q (C1q) to NET⁺ axons in the OBs of App^NL-G-F mice compared to WT mice.

Experimental findings indicated that microglial phagocytosis of NA axons in the OB could be the underlying cause of the progressive early axon loss in App^NL-G-F mice. LC-restricted APP overexpression was sufficient to induce OB axon loss and hyposmia, confirming a causal link.

The current study also observed increased TSPO signals in the OBs of patients with prodromal AD, likely reflecting increased microglial density rather than single-cell activation.

Conclusions

The current study highlighted that the underlying mechanism for hyposmia could be an underestimated sensory deficit in AD. In the future, combined assessments, including olfactory testing and CSF and blood biomarkers, could be used for earlier AD diagnosis. This combined approach could also be used to predict disease progression and outcome.

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