Deafferentation of Olfactory Bulb in Subjects Dying with COVID-19

There have been clinical descriptions of diverse neurological effects in COVID-19 disease, involving up to 36% of patients. It appears likely that most of these are not caused by viral brain invasion but by systemic accompaniments of critical illness such as coagulopathy, deleteriously upregulated immune response, autoimmune mechanisms, hypoxia or multiorgan failure. Anosmia or hyposmia is present in a majority of COVID-19 patients, and there is early and severe involvement of the nasopharyngeal mucosa and olfactory epithelium. Preliminary studies by our group have found massive gene expression changes in olfactory bulb, but the magnitude of these changes are not different between subjects with detectable versus non-detectable olfactory bulb SARS-CoV-2 RNA. As spontaneous discharge of olfactory epithelial afferents dictates intra-olfactory bulb neurophysiological activity and connectivity, we hypothesized that olfactory bulb deafferentation during COVID-19 is responsible for a large fraction of our observed olfactory bulb transcriptional changes. As the olfactory marker protein (OMP-1) is a specific marker of olfactory epithelial afferents to the olfactory bulb and is severely depleted in animal model lesions of olfactory epithelium, we quantified OMP-1-immunoreactivity in the olfactory bulb of subjects dying with or without COVID-19. Additionally, we quantified olfactory bulb tyrosine hydroxylase (TH), which is often also reduced after olfactory epithelium lesions, and SNAP-25, a pan-synaptic marker. COVID-19 cases (n = 18) were generally elderly and were not significantly different in age or gender distribution from the non-COVID-19 cases (n = 28). Both COVID-19 and non-COVID-19 cases had a wide range of neuropathological diagnoses. The area occupied by OMP-1 immunoreactivity in COVID-19 cases was significantly less, about 60% of that in control cases but amongst subjects with COVID-19, there was no significant difference between OBT-SARS-CoV-2-PCR-positive and negative cases. There were no significant group differences for TH or SNAP-25, supporting a selective effect for OMP-1. We suggest that olfactory dysfunction, and some of the COVID-19-associated transcriptional changes that we have reported for the olfactory bulb and amygdala, may be due to olfactory bulb deafferentation and subsequent transsynaptic effects. Additionally, animal models of olfactory bulb deafferentation or bulbectomy indicate a possibility for widespread changes in interconnected brain regions, providing a possible substrate for diverse post-acute COVID-19 neurological sequelae. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study was funded by This project was supported by a Covid-19 Supplement to a National Institute on Aging grant, (3P30AG019610-20S1), submitted in response to a Notice of Special Interest (NOSI) issued by the National Institute on Aging (NOT-AG-20-022), to highlight the urgent need for research on Coronavirus Disease 2019. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The ethics committee IRB of Banner Sun Health Research Institute, Western IRB of Puyallup, WA, USA, gave ethical approval for this work (Protocols 1132516 and 20201852). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors.