The Cellular basis of loss of smell in 2019-nCoV-infected individuals.

A prominent clinical symptom of 2019-novel coronavirus (nCoV) infection is hyposmia/anosmia (decrease or loss of sense of smell), along with general symptoms such as fatigue, shortness of breath, fever and cough. The identity of the cell lineages that underpin the infection-associated loss of olfaction could be critical for the clinical management of 2019-nCoV-infected individuals. Recent research has confirmed the role of angiotensin-converting enzyme 2 (ACE2) and transmembrane protease serine 2 (TMPRSS2) as key host-specific cellular moieties responsible for the cellular entry of the virus. Accordingly, the ongoing medical examinations and the autopsy reports of the deceased individuals indicate that organs/tissues with high expression levels of ACE2, TMPRSS2 and other putative viral entry-associated genes are most vulnerable to the infection. We studied if anosmia in 2019-nCoV-infected individuals can be explained by the expression patterns associated with these host-specific moieties across the known olfactory epithelial cell types, identified from a recently published single-cell expression study. Our findings underscore selective expression of these viral entry-associated genes in a subset of sustentacular cells (SUSs), Bowman's gland cells (BGCs) and stem cells of the olfactory epithelium. Co-expression analysis of ACE2 and TMPRSS2 and protein-protein interaction among the host and viral proteins elected regulatory cytoskeleton protein-enriched SUSs as the most vulnerable cell type of the olfactory epithelium. Furthermore, expression, structural and docking analyses of ACE2 revealed the potential risk of olfactory dysfunction in four additional mammalian species, revealing an evolutionarily conserved infection susceptibility. In summary, our findings provide a plausible cellular basis for the loss of smell in 2019-nCoV-infected patients.

Anosmia: a missing link in the neuroimmunology of coronavirus disease 2019 (COVID-19).

Just before 2020 began, a novel coronavirus, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), brought for humans a potentially fatal disease known as coronavirus disease 2019 (COVID-19). The world has thoroughly been affected by COVID-19, while there has been little progress towards understanding the pathogenesis of COVID-19. Patients with a severe phenotype of disease and those who died from the disease have shown hyperinflammation and were more likely to develop neurological manifestations, linking the clinical disease with neuroimmunological features. Anosmia frequently occurs early in the course of COVID-19. The prevalence of anosmia would be influenced by self-diagnosis as well as self-misdiagnosis in patients with COVID-19. Despite this, the association between anosmia and COVID-19 has been a hope for research, aiming to understand the pathogenesis of COVID-19. Studies have suggested differently probable mechanisms for the development of anosmia in COVID-19, including olfactory cleft syndrome, postviral anosmia syndrome, cytokine storm, direct damage of olfactory sensory neurons, and impairment of the olfactory perception center in the brain. Thus, the observation of anosmia would direct us to find the pathogenesis of COVID-19 in the central nervous system, and this is consistent with numerous neurological manifestations related to COVID-19. Like other neurotropic viruses, SARS-CoV-2 might be able to enter the central nervous system via the olfactory epithelium and induce innate immune responses at the site of entry. Viral replication in the nonneural olfactory cells indirectly causes damage to the olfactory receptor nerves, and as a consequence, anosmia occurs. Further studies are required to investigate the neuroimmunology of COVID-19 in relation to anosmia.