[10, 11] Released PGE2 then increases epithelial intracellular pH (pHi), HCO3− secretion, and mucus output, all important mucosal defense factors to luminal STI571 in vivo acid.[7, 12, 13] How luminal acid increases epithelial PGE2 synthesis is, however, still uncertain. Furthermore, whether other luminal stimuli increase PGE2 synthesis and

release is also unknown. Here, we introduce our novel hypothesis that epithelial H2O2 production is related to duodenal acid-induced PGE2 synthesis, a mechanism that can also be extrapolated to luminal bacterial sensing. We will show how the PG pathway is essential for duodenal acid and bacterial sensing, augmenting mucosal and host defense mechanisms. Duodenal defense factors include HCO3− and mucus secretion (pre-epithelial), pHi regulation with ion transporters and ecto- and cytosolic enzyme activities (epithelial), and blood flow regulated via afferent nerves and mediator releases (subepithelial). Rapid changes in these defense factors in response to topical application of luminal chemicals imply the presence of mucosal recognition of luminal chemicals via the pathways depicted in Fig. 1. We have assessed duodenal mucosal defense

factors using microscopic mucosal imaging in vivo, enabling the anti-PD-1 antibody measurement of mucosal defense factors such as mucosal blood flow, mucus secretion, and enterocyte pHi in response to luminal chemicals, in addition to measuring the rate of HCO3− secretion using a duodenal loop perfusion system. These approaches enable us to observe a rapid response

to luminal compounds and identify the mechanisms using pharmacological or genetic tools. The second pattern of luminal chemosensing is brush border ecto-enzyme-related signals, including duodenal ATP-P2Y receptors and pH-dependent intestinal alkaline phosphatase (IAP) activity[14, 15] (Fig. 1b). Since the optimal pH of IAP is 8–9 and IAP activity is closely 上海皓元 correlated to the HCO3− secretory rate,[14] IAP may act as a surface pH (pHs) sensor in the duodenum. At neutral luminal pH, extracellular ATP, non-lytically released from the epithelial cells, is rapidly degraded to adenosine (ADO), which is further degraded to inosine by adenosine deaminase. Once pHs is lowered by gastric acid, surface ATP concentrations increase due to the decreased degradation by IAP or the increased release of ATP, since IAP activity is reduced at acidic pH. Ecto-ATPases, also known as ectonucleoside triphosphate diphosphohydrolases (CD39 family), and 5′-nucleotidase (CD73) are also involved in the degradation of ATP to ADO. Increased surface ATP concentration stimulates P2Y receptors expressed on the apical membrane of epithelial cells, increasing HCO3− secretion. Increased surface HCO3− concentration increases the pHs, increasing IAP activity, which degrades surface ATP, terminating ATP-P2Y signaling. Luminal ADO additionally increases HCO3− secretion via A2B receptors.