Digestive system infection by SARS­CoV­2: Entry mechanism, clinical symptoms and expression of major receptors (Review).

Besides causing severe acute respiratory syndrome (SARS), SARS­coronavirus 2 (SARS­CoV­2) also harms the digestive system. Given the appearance of numerous cases of SARS­CoV­2, it has been demonstrated that SARS­CoV­2 is able to harm target organs such as the gastrointestinal tract, liver and pancreas, and either worsen the condition of patients with basic digestive illnesses or make their prognosis poor. According to several previously published studies, angiotensin­converting enzyme II (ACE2) and transmembrane serine protease II (TMPRSS2) are expressed either singly or in combination in the digestive system and in other regions of the human body. In order to change the viral conformation, create a fusion hole and release viral RNA into the host cell for replication and transcription, SARS­CoV­2 is capable of binding to these two proteins through the spike protein on its surface. As a result, the body experiences an immune reaction and an inflammatory reaction, which may lead to nausea, diarrhea, abdominal pain and even gastrointestinal bleeding, elevated levels of liver enzymes, acute liver injury, pancreatitis and other serious lesions. In order to provide possible strategies for the clinical diagnosis and treatment of digestive system diseases during the COVID­19 pandemic, the molecular structure of SARS­CoV­2 and the mechanism via which SARS­CoV­2 enters the human body through ACE2 and TMPRSS2 were discussed in the present review, and the clinical manifestations of SARS­CoV­2 infection in the digestive system were also summarized. Finally, the expression characteristics of ACE2 and TMPRSS2 in the main target organs of the digestive system were described.