DPP4 and type 2 diabetes mellitus: In addition to GLP-1R agonists, orally administrated DPP4-inhibitors, e.g., sitagliptin and saxagliptin, which are known to potentiate the effect of endogenous GLP-1 by inhibiting its degradation, have also been clinically studied as protective agents against retinopathy (Chung et al., 2016), Alzheimer’s disease (Isik et al., 2017), and Parkinson’s disease (Svenningsson et al., 2016) in T2D patients, although DPP4-inhibitors cannot cross the blood-brain barrier (Mousa and Ayoub, 2019).