Gene expression of nutrient-sensing molecules in I cells of CCK reporter male mice

Cholecystokinin (CCK) is secreted from enteroendocrine I cells in response to fat, carbohydrate, and protein ingestion. Gene expression of nutrient-sensing molecules in I cells remains unclear, primarily due to the difficulty in distinguishing I cells from intestinal epithelial cells in vivo. In this study, we generated CCK reporter male mice in which the red fluorescence protein tdTomato (Tomato) is produced by activation of the native murine Cck promoter. Fluorescence microscopy revealed the presence of Tomato-positive cells in upper small intestine (SI), lower SI, and colon. Flow cytometer analysis revealed that Tomato-positive cells among epithelial cells of upper SI, lower SI, and colon occurred at the rate of 0.95, 0.54, and 0.06%, respectively. In upper SI and lower SI, expression levels of Cck mRNA were higher in Tomato-positive cells than those in Tomato-negative cells. The fatty acid receptors Gpr120, Gpr40, and Gpr43 and the oleoylethanolamide receptor Gpr119 were highly expressed in Tomato-positive cells isolated from SI, but were not found in Tomato-positive cells from colon. The glucose and fructose transporters Sglt1, Glut2, and Glut5 were expressed in both Tomato-positive cells and -negative cells, but these expression levels tended to be decreased in Tomato-positive cells from upper SI to colon. The peptide transporter Pept1 and receptor Gpr93 were expressed in both Tomato-positive cells and -negative cells, whereas Casr was expressed only in Tomato-positive cells isolated from SI. Thus, this transgenic mouse reveals that I cell number and gene expression in I cells vary according to region in the gastrointestinal tract.