Human ACE2 expression, a major tropism determinant for SARS-CoV-2, is regulated by upstream and intragenic elements

Author summarySARS-CoV-2, the virus responsible for COVID-19, infects the human respiratory tract by binding to angiotensin-converting enzyme 2 (ACE2), a protein on the outer surface of cells that is exposed to the air we inhale. Differences in the structure of ACE2 between mouse and man prevent the virus from infecting cells lining the airways of the mouse, limiting the usefulness of wildtype mice as a model system for studying COVID-19. To circumvent this problem, we have created two mouse lines in which the chromosomal segment that encodes the mouse ACE2 protein has been replaced with the equivalent segment of human DNA. In the first of these lines, expression of human ACE2 is regulated by the mouse upstream promoter, while in the second line it is regulated by the human upstream promoter. Using these mice, we show both qualitative and quantitative differences between ACE2 expression driven by the mouse and human promoters that are likely to impact disease progression in the two species. Mice expressing the full-length human ACE2 gene under the control of its own promoter should provide a useful model system for understanding the complex pathological processes and immune responses associated with COVID-19 in humans. Angiotensin-converting enzyme 2 (ACE2), part of the renin-angiotensin system (RAS), serves as an entry point for SARS-CoV-2, leading to viral proliferation in permissive cell types. Using mouse lines in which the Ace2 locus has been humanized by syntenic replacement, we show that regulation of basal and interferon induced ACE2 expression, relative expression levels of different ACE2 transcripts, and sexual dimorphism in ACE2 expression are unique to each species, differ between tissues, and are determined by both intragenic and upstream promoter elements. Our results indicate that the higher levels of expression of ACE2 observed in the lungs of mice relative to humans may reflect the fact that the mouse promoter drives expression of ACE2 in populous airway club cells while the human promoter drives expression in alveolar type 2 (AT2) cells. In contrast to transgenic mice in which human ACE2 is expressed in ciliated cells under the control of the human FOXJ1 promoter, mice expressing ACE2 in club cells under the control of the endogenous Ace2 promoter show a robust immune response after infection with SARS-CoV-2, leading to rapid clearance of the virus. This supports a model in which differential expression of ACE2 determines which cell types in the lung are infected, and this in turn modulates the host response and outcome of COVID-19.

Automated summary

To study COVID-19, researchers have created two mouse lines with the mouse ACE2 protein replaced by the human equivalent. The expression of human ACE2 is regulated by either the mouse or human promoter, leading to qualitative and quantitative differences in ACE2 expression between mice and humans. These mouse models expressing the full-length human ACE2 gene provide a valuable tool for understanding the pathology and immune response associated with COVID-19.