ABSTRACT
In December 2019, SARS-CoV-2 emerged causing the COVID-19 pandemic. SARS-CoV, the agent responsible for the 2003 SARS outbreak, utilizes ACE2 and TMPRSS2 host molecules for viral entry. ACE2 and TMPRSS2 have recently been implicated in SARS-CoV-2 viral infection. Additional host molecules including ADAM17, cathepsin L, CD147, and GRP78 may also function as receptors for SARS-CoV-2.
To determine the expression and in situ localization of candidate SARS-CoV-2 receptors in the respiratory mucosa, we analyzed gene expression datasets from airway epithelial cells of 515 healthy subjects, gene promoter activity analysis using the FANTOM5 dataset containing 120 distinct sample types, single cell RNA sequencing (scRNAseq) of 10 healthy subjects, immunoblots on multiple airway epithelial cell types, and immunohistochemistry on 98 human lung samples.
We demonstrate absent to low ACE2 promoter activity in a variety of lung epithelial cell samples and low ACE2 gene expression in both microarray and scRNAseq datasets of epithelial cell populations. Consistent with gene expression, rare ACE2 protein expression was observed in the airway epithelium and alveoli of human lung. We present confirmatory evidence for the presence of TMPRSS2, CD147, and GRP78 protein in vitro in airway epithelial cells and confirm broad in situ protein expression of CD147 in the respiratory mucosa.
Collectively, our data suggest the presence of a mechanism dynamically regulating ACE2 expression in human lung, perhaps in periods of SARS-CoV-2 infection, and also suggest that alternate receptors for SARS-CoV-2 exist to facilitate initial host cell infection.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Funding: This work was supported by start-up funds from Hirota JA and a CIHR grant from Mossman K. Hirota JA is supported by the Canada Research Chairs program and an Ontario Early Researcher Award. Doxey AC is supported by NSERC and an Ontario Early Researcher Award.
This is the final version submitted for peer-review (European Respiratory Journal - ERJ). The version is double spaced as per ERJ submission guidelines. The final version now includes supplementary immunohistochemistry of human lung tissue microvasculature and human heart for positive ACE2 staining (previous version did not include this data). Paul Hanson and Bruce McManus were added as authors as they provided the human heart tissue for the ACE2 staining. A cartoon schematic was also added as a Figure 6 to summarize the characterization of the molecules.
