DPP4 and type 2 diabetes mellitus: In this regard, human epidemiological studies have demonstrated a reduced incidence of PD in subjects with T2DM taking a DPP-4 inhibitor [8, 70] and results from our cell culture studies, herein, are in line with numerous prior studies demonstrating that elevated incretin levels associate with neurotrophic and neuroprotective actions in a human immortal neuronal cell line and, importantly, in rat primary cultures rich in dopaminergic neurons [6, 13, 14, 26–29, 32, 71].