Fig. 2.

Role of ACE2 in the intestinal epithelium. ACE2 is necessary for the surface expression of the amino acid transporter B⁰AT1 in the epithelium of the small intestine. Tryptophan has to be obtained from the diet and uptake of tryptophan mainly depends on B⁰AT1. Tryptophan levels regulate via mTOR pathway activation the secretion of antimicrobial peptides which in turn influence the composition of the intestinal microbiota. In the absence of ACE2, tryptophan cannot get absorbed efficiently, which leads to aberrant secretion of antimicrobial peptides and consequently to an altered microbiota which confers susceptibility to inflammation of the large intestine.