The olfactory epithelium: a critical gateway for pathological tau propagation and a target for mitigating tauopathy in the central nervous system.

Olfactory impairment is a recognized early indicator of neurodegenerative diseases (NDs), such as Alzheimer's disease (AD). Intracellular aggregates of hyperphosphorylated tau protein, referred to as neurofibrillary tangles (NFTs), are a hallmark of AD. NFTs are found in the olfactory bulb (OB) and entorhinal cortex (EC), both crucial for processing olfactory information. We explored the hypothesis that typical tau lesions could appear early and progress along olfactory regions to reach connected areas critically affected in AD (e.g., EC and hippocampal formation). To that end, we used transgenic PS19 mice expressing mutated human tau protein (1N4R isoform, P301S mutation). They recapitulate major phenotypes of AD, such as accumulation of NFTs, synaptic dysfunction, cognitive impairment, and neuronal loss. The presence of pathological hyperphosphorylated human tau protein (pTau) was monitored in olfactory regions: olfactory epithelium (OE), OB, piriform cortex (PC), and in connected regions of the hippocampal formation (hippocampus and EC). pTau was detected in the OE's middle stratum and in the OB's olfactory nerve layer (ONL) at 1.5 months. At 6 months of age, tau accumulations were found in the PC and EC, along with the CA3 region and dentate gyrus of the hippocampus. We found that olfactory function remained unaffected in PS19 mice, despite the presence of tau pathology in key regions of the olfactory system. Targeted treatments (ZnSO and AAVs) were applied at the OE level to assess the impact on tau pathology in the CNS. Complete stripping of the OE by intranasal administration of ZnSO led to a significant reduction in pretangle-like tau pathology within the PC, amygdala, and EC of 6-month-old PS19 mice. Finally, we observed in human postmortem samples that pTau signal was present in the olfactory regions (OE and OB) of patients at early Braak stages (I/II). Based on these observations, we propose that pTau could appear, due to aging or environmental agents, in the OE and subsequently spread in a prion-like manner to the hippocampal formation along neuroanatomical connections. These findings also indicate the interest of the OE as a target for intervention aimed at mitigating the progression of tauopathy in the CNS.