Background/Aims: Quantitative and qualitative alterations in the sense of smell are well established symptoms of COVID-19. Some reports have shown that non-neuronal supporting (also named sustentacular) cells of the human olfactory epithelium co-express ACE2 and TMPRSS2 necessary for SARS-CoV-2 infection. In COVID-19, syncytia were found in many tissues but were not investigated in the olfactory epithelium. Some studies have shown that syncytia in some tissues are formed when SARS-CoV-2 Spike expressed at the surface of an infected cell binds to ACE2 on another cell, followed by activation of the scramblase TMEM16F (also named ANO6) which exposes phosphatidylserine to the external side of the membrane. Furthermore, niclosamide, an approved antihelminthic drug, inhibits Spike-induced syncytia by blocking TMEM16F activity. The aim of this study was to investigate if proteins involved in Spike-induced syncytia formation, i.e., ACE2 and TMEM16F, are expressed in the human olfactory epithelium. Methods: We analysed a publicly available single-cell RNA-seq dataset from human nasal epithelium and performed immunohistochemistry in human nasal tissues from biopsies. Results: We found that ACE2 and TMEM16F are co-expressed both at RNA and protein levels in non-neuronal supporting cells of the human olfactory epithelium. Conclusion: Our results provide the first evidence that TMEM16F is expressed in human olfactory supporting cells and indicate that syncytia formation, that could be blocked by niclosamide, is one of the pathogenic mechanisms worth investigating in COVID-19 smell loss.

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The COVID-19 pandemic brought attention to our limited understanding of human olfactory physiology. While the cellular composition of the human olfactory epithelium is similar to that of other vertebrates, its functional properties are largely unknown. We prepared acute slices of human olfactory epithelium from nasal biopsies and used the whole-cell patch-clamp technique to record electrical properties of cells. We measured voltage-gated currents in human olfactory sensory neurons and supporting cells, and action potentials in neurons. Additionally, inward currents and action potentials responses of neurons to a phosphodiesterase inhibitor indicated that the transduction cascade involves cAMP as a second messenger. Furthermore, responses to odorant mixtures demonstrated that the transduction cascade was intact in this preparation. This study provides the first electrophysiological characterization of olfactory sensory neurons in acute slices of the human olfactory epithelium, paving the way for future research to expand our knowledge of human olfactory physiology.