Notably, chronic treatment with intraperitoneal injection of a 5-HT4R agonist in WT mice with diabetes repairs the TJ barrier and promotes TJ protein expressions, including occludin, claudin-1 and ZO-1, in the colon, whereas a 5-HT4R agonist does not improve TJ barrier function or TJ protein expressions in 5-HT4R KO mice with diabetes. Here, CLDN1 is linked to diabetes mellitus.