Relevance of enriched expression of SARS-CoV-2 binding receptor ACE2 in gastrointestinal tissue with pathogenesis of digestive symptoms, diabetes-associated mortality, and disease recurrence in COVID-19 patients

Ashutosh Kumar; Muneeb A. Faiq; Vikas Pareek; Khursheed Raza; Ravi K. Narayan; Pranav Prasoon; Pavan Kumar; Maheswari Kulandhasamy; Chiman Kumari; Kamla Kant; Himanshu N. Singh; Rizwana Qadri; Sada N. Pandey; Santosh Kumar

We found enriched genomic and proteomic expression of SARS-CoV-2 binding receptor ACE2 in lower GIT (small intestine, colon, and rectum) and GB (Fig. 1-3 . Table 1 ). The digestive system specific functional enrichment map of the ACE2 gene suggests its role in regulating secretory/absorptive functions at the brush border membrane of the enterocytes in the intestinal lining epithelium (Fig.S1, Table S1 ). ACE2 is a homologue of angiotensin-I converting enzyme (ACE), the key enzyme of the renin-angiotensin system (RAS). It is an integral membrane protein and localizes predominantly at the apical surface of polarized epithelial cells where it is proteolytically cleaved within its ectodomain to release a soluble form. 17, 18 Currently, SARS-CoV-2 mediated binding of ACE2 and the following downstream events leading to tissue damage are little known. Presumptive understanding of SARS-CoV-2 driven pathology is being borrowed from SARS-CoV-1 which was the etiological basis of SARS pandemic in 2003. Uniquely, it acted on the same receptor as SARS-CoV-2 and led to many clinical manifestations similar to COVID-19. 19 Studies utilizing cell lines to decipher SARS pathology in lung tissue showed that the spike protein of SARS-CoV-1 (SARS-S) induced TNFα production which facilitated virus entry. 20 TNFα also led to inflammation of the cell membrane and consequently tissue damage. 18,20 SARS-CoV-1 was also showed to cause downregulation of ACE2 expression at the cell membrane level. 18, 21 Existing literature regarding expression of ACE2 in human tissues are rare. Hamming et al, studied ACE2 protein expression in human tissues in reference to SARS-CoV-1. 22 Our findings for ACE2 protein expression in digestive system components are in line with the findings of their study. 22 Enriched expression of SARS-CoV-2 binding receptor ACE2 in the mucosal glands and enterocytes (including brush border cells) in the lining epithelium ( Fig. 2e-h, Table 1 ) of the lower GIT indicates that GI cells are potential sites for virus replication. Evidence of the viral shedding in the feces shown in some studies indicates possible replication of the virus inside the GI cells which, in turn may explain GI manifestations of COVID-19 in addition to disease recurrence. 23, 24 How the virus reaches the GI is arguable. Some authors speculated a fecal-oral route of entry. 6 We examined possibility of this route of entry based on the expression pattern of ACE2 along the length of the GIT (Fig. 1, 2 , Table 1 ). Negligible or very low mRNA expression and undetectable proteomic expression of ACE2 in the mouth cavity (including tongue, oral mucosa, and salivary glands), esophagus, and stomach (Fig. 1 , 2a-d, Table 1 ) indicate these parts of GIT can be resistant for the virus entry. But this observation does not negate a possible site of virus entry through the ACE2 receptors present in the lower GIT in case of fecal-oral transmission. It is then intriguing that how SARS-CoV-2 survives extremes of pH within the digestive system milieu (gastric-1.5 to 3.5, pancreatic-7.5, bile acid-7-8) while passing along the length of GIT. Recently, Chin et al., 2020 showed in vitro that SARS-CoV-2 can survive at wide range of pH values at room temperature (pH3-10). 25 This can be further explained by an earlier study by Hirose et al, who, in an experimental, model demonstrated that RNA viruses like influenza A and B (when swallowed) can survive extremes of pH and maintain infectivity with help of