Post-COVID-19 Hyposmia Does Not Exhibit Main Neurodegeneration Markers in the Olfactory Pathway

Abstract

The biological substrate of persistent post-COVID-19 hyposmia is still unclear. However, as many neurodegenerative diseases present with smell impairment at onset, it may theoretically reflect degeneration within the central olfactory circuits. However, no data still exist regarding the post-COVID-19 patients. As the olfactory neurons (ONs) mirror pathological changes in the brain, allowing for tracking the underlying molecular events, here, we performed a broad analysis of ONs from patients with persistent post-COVID-19 OD to identify traces of potential neurodegeneration. ONs were collected through the non-invasive brushing of the olfactory mucosa from ten patients with persistent post-COVID-19 hyposmia (lasting > 6 months after infection) and ten age/sex-matched controls. Immunofluorescence staining for protein quantification and RT-PCR for gene expression levels were combined to measure ONs markers of α-synuclein, amyloid-β, and tau pathology, axonal injury, and mitochondrial network. Patients and controls had similar ONs levels of oligomeric α-synuclein, amyloid-β peptide, tau protein, neurofilament light chain (NfL), cytochrome C oxidase subunit 3 (COX3), and the heat shock protein 60 (HSP60). Our findings thus did not provide evidence for synucleinopathy and amyloid-β mismetabolism or gross traces of neuronal injury and mitochondrial dysfunction within the olfactory system in the early phase of persistent post-COVID-19 hyposmia.

Keywords: Hyposmia; Long-COVID; Neurodegeneration; Olfactory dysfunction; Olfactory neurons; Post-COVID.

Fig. 1

Neurodegeneration-associated marker expression levels. Representative immunofluorescent images showing the expression pattern of oligomeric α-syn (green) (A), tau protein (green) (C) and Aβ (green) (E) in ONs from healthy controls (CTRL) and post-COVID-19 OD patients. Nuclei were stained with DAPI (blue). Scale bar, 10 µm. Graphs showing immunofluorescence signal quantification of oligomeric α-syn (B), tau protein (D), and Aβ (F) in each group evaluated by ImageJ software. Data points represent the mean value ± SEM

Fig. 2

Neurofilament light chain (NfL) expression levels. A The NfL mRNA expression level in ONs from post-COVID-19 OD patients and controls. B Representative immunofluorescence analysis of NfL protein (green) in ONs. Nuclei were stained with DAPI (blue). Scale bar, 10 µm. C Graphs showing immunofluorescence intensity of NfL staining in each group evaluated by ImageJ software. Data points represent the mean value ± SEM

Fig. 3

Mitochondrial network markers. A COX3 mRNA expression levels in ONs from post-COVID-19 OD patients and controls. Data points represent the mean value ± SEM. B Representative immunofluorescence analysis of the mitochondrial protein HSP60 (green) with the β3-tubulin (red) and in ONs. Nuclei were stained with DAPI (blue). Scale bar, 10 µm