Elucidating the transcriptional mechanisms that control the expression of the SARS-CoV-2 receptor ACE2

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PROJECT SUMMARY There is a pressing urgency to understand the regulatory mechanisms that control the expression of ACE2, the cellular receptor for the new coronavirus SARS-CoV-2, in human cells under physiological and pathological conditions. The binding between ACE2 and the SARS-CoV-2 spike protein, along with proteolytic cleavage of ACE2, facilitates entry of the coronavirus into target cells, viral replication, and transmission. As such, ACE2 has been proposed as a key factor in virus infectivity and disease pathology. Although ACE2 is hijacked by SARS-CoV-2 in the pathogenesis of COVID-19, its primary physiological role is to counteract tissue injury and inflammation as part of the renin-angiotensin system (RAS). The expression of ACE2 is restricted to specific tissue-resident cell types, developmentally regulated, and highly responsive to physiological (e.g. sex hormones) and pathological (e.g. inflammation or virus infection) signals. By surveying the epigenetic landscapes at the human ACE2 locus in the major ACE2-expressing cell types, we identified a set of gene- distal cis-regulatory elements (CREs) displaying tissue-specific activity, consistent with putative cell type- specific enhancer elements for ACE2. The molecular basis for the tissue- and developmental stage-specific expression of ACE2 is unknown, and the regulatory mechanisms controlling ACE2 expression in response to physiological and pathological signals remain unexplored, highlighting a critical gap in knowledge. Without an in-depth understanding of the molecular and cellular pathways that control ACE2 expression, the genetic or pharmacological modulation of ACE2 as an approach to therapy for COVID-19 will likely remain unreachable. The objective of this project is to close a critical gap in our understanding of the transcriptional mechanisms controlling ACE2 expression under normal and pathological conditions. The central hypothesis is that ACE2 expression is controlled by combinations of tissue-specific CREs to recruit cell type-specific and/or signal- dependent transcription factors. This hypothesis has been formulated on the basis of the tissue-specific expression profiles of ACE2 in human cells, the presence of multiple gene-distal CREs demarcated by active chromatin signatures in ACE2-expressing cells, and the differential effects on ACE2 expression upon physiological or pathological stimulations. Guided by these findings, this hypothesis will be tested by two specific aims: 1) Determine the functional roles of tissue-specific cis-regulatory elements in controlling ACE2 expression and its responses to stimuli; 2) Identify and characterize chromatin regulatory complexes responsible for ACE2 expression by CRISPR/dCas9-mediated affinity purification. Together, by focusing on the validated entry receptor for SARS-CoV-2, our studies will not only advance our mechanistic understanding of critical aspects of viral susceptibility and post-infection pathology, but also establish a much-needed resource of candidate cis- and trans-regulatory factors required for ACE2 expression in human cells, thus facilitating ongoing development of innovative strategies to leverage ACE2 as targeted therapies against COVID-19.