DPP4 and type 2 diabetes mellitus: Dipeptidyl peptidase-4 inhibitors (DPP-4i) are a growing class of clinically used T2D drugs [19, 83] that have also shown beneficial effects in the CNS of animal models of AD [15, 22, 45–47, 77] and in T2D patients with AD [37], even independently from glycemic regulation [27, 70].