Infectivity of Human Olfactory Neurons to SARS-CoV-2: A Link to Anosmia
Year: 2021
Type: article
Abstract: Background: Most reports regarding SARS-CoV-2 infection have been dominated by the presence of severe respiratory symptoms: cough, breathlessness, and fever. Initial anecdotal reports suggested anosmia may also be a manifestation. Indeed, recent data suggests that up to 85% of infected individuals may experience an interference in gustation and olfaction, resulting in loss of smell and taste. In many individuals, inability to smell may be the only symptom of infection. We sought to investigate if the loss of smell is primarily due to direct infection of olfactory neurons. Methods: We utilized two olfactory neuronal cell lines, one male and the other female in origin. In addition, we used four undifferentiated and two partially differentiated, human developing neuronal cell lines derived from neuroblastoma of male and female origin to determine the permissiveness of these neurons to SARS-CoV-2. All infections were confirmed by quantitative-reverse transcriptase PCR (RT-qPCR), immunofluorescence assay (IFA) probing with anti-SARS-CoV-2 antibody, and evaluation of cytopathic effects and neurite formation. Since, both olfactory cell lines are terminally differentiated we induced partial differentiation of all cell lines with retinoic acid (RA) to determine if differentiation is a factor in viral permissiveness. To determine the mechanism of viral entry we evaluated the expression of serine protease and transmembrane serine protease 2 (TMPRSS2) by RT-qPCR and IFA. Results: Infection of two olfactory cell lines, JFEN and TC-268, with SARS-CoV-2 was determined by IFA and confirmed by RT-qPCR and morphological observations where dysregulation in neurite formation was documented. Four-to-five days after exposure to the virus both olfactory cell lines exhibited morphological evidence of infection while IFA analyses indicated that ~30% of the neurons were positive. Two weeks following infection between 70-80% of the neurons exhibited positive expression of SARS-CoV-2 antigens. Two partially differentiated: CRL-2266 and CRL-2267 and four undifferentiated cell lines: CRL 2142 CRL 2149, CRL-127 and CDL-2271 were essentially non-permissive when compared against the two olfactory cell lines and only rare neurons (<1:1,000) exhibited positive cells by IFA but no apparent morphological changes could be discerned. The RT-qPCR results showed low levels of infection. After RA treatment only one cell line, CRL-127, exhibited slight permissiveness which was confirmed by RT-qPCR. The TMPRSS2 receptor showed high expression in olfactory neurons but low expression in CRL-127 after RA treatment. Conclusion: Our findings strengthen the reported association of SARS-CoV-2 infection with loss of smell in individuals infected with the virus. Our results suggest that olfactory neurons, originating from both genders, were highly permissive to SARS-CoV-2 infection. Moreover, there were no differences in permissiveness to infection between male and female neurons. The infrequent susceptibility of other undifferentiated or partially differentiated neurons suggest it would be unlikely that SARS-CoV-2 infects other mature human brain neurons since, after birth, they have mostly completed neurogenesis. Our studies confirm the neurotropism of the SARS-CoV-2 to olfactory neurons. Preliminary observations indicate that viral entry is mediated by the TMPRSS2 receptor. Funding Statement: None to declare Declaration of Interests: All authors declare no conflict of interest.
Source: Oman Medical Journal
DOI: 10.5001/omj.2021.128
Institutions: Claflin University, Prisma Health, University of South Carolina
Language: English
Subfield: Sensory Systems
Field: Neuroscience
Domain: Life Sciences
Open Access status: diamond
OpenAlex ID: W3210893544