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.

Adult mesenchymal stem cells (MSCs) and epithelial stem cells play essential roles in tissue repair and self-healing. Oral MSCs and epithelial stem cells can be isolated from adult human oral tissues, for example, teeth, periodontal ligament, and gingiva. Cocultivated adult oral epithelial stem cells and MSCs could represent some developmental events, such as epithelial invagination and tubular structure formation, signifying their potentials for tissue regeneration. Oral epithelial stem cells have been used in regenerative medicine over 1 decade. They are able to form a stratified cell sheet under three-dimensional culture conditions. Both experimental and clinical data indicate that the cell sheets can not only safely and effectively reconstruct the damaged cornea in humans, but also repair esophageal ulcer in animal models. Oral MSCs include dental pulp stem cells (DPSCs), stem cells from exfoliated deciduous teeth (SHED), stem cells from apical papilla (SCAP), periodontal ligament stem cells (PDLSCs), and mesenchymal stem cells from gingiva (GMSCs). They are widely applied in both regenerative dentistry and medicine. DPSCs, SHED, and SCAP are able to form dentin–pulp complex when being transplanted into immunodeficient animals. They have been experimentally used for the regeneration of dental pulp, neuron, bone muscle and blood vessels in animal models and have shown promising results. PDLSCs and GMSCs are demonstrated to be ideal cell sources for repairing the damaged tissues of periodontal, muscle, and tendon. Despite the abovementioned applications of oral stem cells, only a few human clinical trials are now underway to use them for the treatment of certain diseases. Since clinical use is the end goal, their true regenerative power and safety need to be further examined.