Recently, the SARS-CoV-2 induced disease COVID-19 has spread all over the world. Nearly 20% of the patients have severe or critical conditions. SARS-CoV-2 exploits ACE2 for host cell entry. ACE2 plays an essential role in the renin-angiotensin-aldosterone system (RAAS), which regulates blood pressure and fluid balance. ACE2 also protects organs from inflammatory injuries and regulates intestinal functions. ACE2 can be shed by two proteases, ADAM17 and TMPRSS2. TMPRSS2-cleaved ACE2 allows SARS-CoV-2 cell entry, whereas ADAM17-cleaved ACE2 offers protection to organs. SARS-CoV-2 infection-caused ACE2 dysfunction worsens COVID-19 and could initiate multi-organ failure. Here, we will explain the role of ACE2 in the pathogenesis of severe and critical conditions of COVID-19 and discuss auspicious strategies for controlling the disease.Viruses 2020, 12, 491 2 of 10
Profile of ACE2Human angiotensin-converting enzyme-related carboxypeptidase ACE2 is encoded by the ACE2 gene which maps to chromosome Xp22 [6]. ACE2 is a type I transmembrane protein, comprised of an extracellular heavily N-glycosylated N-terminal domain containing the carboxypeptidase site and a short intracellular C-terminal cytoplasmic tail [7]. The N-terminal peptidase domain is also the SARS-CoV binding site [8]. There are two forms of ACE2 protein: cellular (membrane-bound) form and circulating (soluble) form. Cellular ACE2 protein is the full-length protein which is expressed abundantly in pneumocytes and enterocytes of the small intestine [9]. ACE2 is also expressed in vascular endothelial cells of the heart, the kidneys, and other organs, such as the brain. However, ACE2 is absent in the spleen, thymus, lymph nodes, bone marrow, and cells of the immune system (including B and T lymphocytes, and macrophages) [10,11].Circulating ACE2 (with the N-terminal peptidase domain) is cleaved from the full-length ACE2 on the cell membrane by the metalloprotease ADAM17 and then released into the extracellular environment [7]. The type II transmembrane serine protease, TMPRSS2 was found to compete with ADAM17 for ACE2 shedding but cleaves ACE2 differently. Both ADAM17 and TMPRSS2 remove a short C-terminal fragment from ACE2. Arginine and lysine residues within amino acids 652 to 659 are critical for ADAM17 shedding, whereas arginine and lysine residues within amino acids 697 to 716 are essential for TMPRSS2 shedding. Only cleavage by TMPRSS2 results in augmented SARS-CoV cell entry [12][13][14][15]. There are two ways for SARS-CoV to enter the target cell: endocytosis, and fusion of the viral membrane with a membrane of the target cell, which is 100 times more efficient than endocytosis for viral replication [16]. With the help of TMPRSS2, ADAM17-regulated ectodomain shedding of ACE2 could induce SARS-CoV cell entry through endocytosis [7,12]; however, ADAM17 activity is not required for SARS-CoV cell entry through fusion [12]. As the N-terminal domain is the coronavirus binding site, circulating ACE2 also binds to the virus. Iwata-Yoshikawa et al. infected both ...