Abstract
A novel coronavirus (COVID-2019) was first identified in Wuhan, Hubei Province, and then spreads to the other Provinces of China. COVID-2019 was reported to share the same receptor, Angiotensin-converting enzyme 2 (ACE2), with SARS-CoV. But the infection rate of COVID-2019 is much higher than SARS-CoV. The biophysical and structural evidence showed that the COVID-2019 binds ACE2 with 10~20 times affinity than SARS-CoV. TMPRSS2 cleaves ACE2 and facilitates the entry of the virus into host cells. The presence of SLC6A19 may block the access of TMPRSS2 to the cutting site on ACE2 and weaken the entry of COVID-2019 into host cells. Here based on the public single-cell RNA-Seq datasets, we analyzed the ACE2 expression in the nasal, mouth, lung, and colon tissues. We find that the number of ACE2-expressing cells in the nasal and mouth tissues is comparable to the number of ACE2-expressing cells in the lung and colon tissues. We also find that ACE2 tends to be co-expressed with TMPRSS2 and not co-expressed with SLC6A19 in the nasal and mouth tissues. With the results, we infer that nasal and mouth tissues may be the first host cells of COVID-2019 infection. In our previous report in medRxiv and a recurrent report in New England Journal of Medicine, the COVID-2019 load tends to be higher in the nasal-swabs than in throat-swabs. We believe the roles of nasal and mouth tissues in COVID-2019 infection should be investigated, and we need to pay more attention to protect nose and mouth from COVID-2019 infection.