Christine Stadelmann

The majority of patients with Parkinson disease (PD) experience a loss in their sense of smell and accumulate insoluble alpha-synuclein aggregates in their olfactory bulbs (OB). Subjects affected by a SARS-CoV-2-linked illness (COVID-19) frequently experience hyposmia. We previously hypothesized that alpha-synuclein and tau misprocessing could occur following host responses to microbial triggers. Using semiquantitative measurements of immunohistochemical signals, we examined OB and olfactory tract specimens collected serially at autopsies between 2020 and 2023. Deceased subjects comprised 50 adults, which included COVID19+ patients (n=22), individuals with Lewy body disease (e.g., PD and dementia with Lewy bodies (DLB; n=6)), Alzheimer disease (AD; n=3), other non-synucleinopathy-linked degenerative diseases (e.g., progressive supranuclear palsy (PSP; n=2) and multisystem atrophy (MSA; n=1)). Further, we included neurologically healthy controls (HCO; n=9) and those with an inflammation-rich brain disorder as neurological controls (NCO; n=7). When probing for inflammatory changes focusing on anterior olfactory nuclei (AON) using anti-CD68 immunostaining, scores were consistently elevated in NCO and AD cases. In contrast, inflammation on average was not significantly altered in COVID19+ patients relative to controls, although anti-CD68 reactivity in their OB and tracts declined with progression in age. Mild-to-moderate increases in phospho-alpha-Syn and phospho-tau signals were detected in the AON of tauopathy- and synucleinopathy-afflicted brains, respectively, consistent with mixed pathology, as described by others. Lastly, when both sides were available for comparison in our case series, we saw no asymmetry in the degree of pathology of the left versus right OB and tracts. We concluded from our autopsy series that after a fatal course of COVID-19, microscopic changes, when present, in the rostral, intracranial portion of the olfactory circuitry generally reflected neurodegenerative processes seen elsewhere in the brain. In general, inflammation correlated best with the degree of Alzheimer's-linked tauopathy and declined with progression of age in COVID19+ patients.

This study leverages X-ray phase-contrast tomography for detailed analysis of neurodegenerative diseases focusing on the 3D visualization and quantification of neuropathological features within fixed human postmortem tissue. X-ray phase-contrast tomography with synchrotron radiation offers micrometer and even sub-micron resolution, enabling us to examine intra- and extraneuronal aggregates and inclusions such as Lewy bodies, granulovacuolar degeneration, Hirano bodies, neurofibrillary tangles, β-amyloid plaques and vascular amyloid deposits in three dimensions (3D). In the reconstructions, we identified the highest electron densities in Hirano bodies and Lewy bodies while neurofibrillary tangles exhibit only slight alterations in X-ray phase-contrast tomography contrast. Using cutting edge high-resolution X-ray synchrotron beamlines we are now able to even detect subcellular differences of electron densities found in granulovacuolar degeneration. Small scale inhomogeneities of the electron density are also detected in Lewy bodies potentially relating to inclusions of organelles. Additionally, we reveal a peculiar 3D geometry of Hirano bodies and demonstrate the co-occurence with granulovacuolar degeneration in the same neuron. Utilizing X-ray phase-contrast tomography in a complementary fashion to traditional technologies, a quantitative, systematic and disease-overarching understanding of inclusions and aggregates in neurodegeneration can be achieved.