CLDN1 and diabetes mellitus: Western blot analysis showed that occludin and claudin-1 protein levels in colonic mucosa were lower in diabetic mice than in nondiabetic mice with or without 5-HT4R. As expected, stimulation of 5-HT4R by administration of its agonist enhanced occludin and claudin-1 protein levels in colonic mucosa in WT mice with diabetes, but the agonist did not block diabetes-induced decreases in occludin and claudin-1 protein levels in colonic mucosa in 5-HT4R KO mice (Figure 5 ).