DPP4 and type 2 diabetes mellitus: Abbreviations: GLP-1 R: glucagon-like peptide-1 receptor; PD: Parkinson disease; PI3K/Akt: phosphoinositide-3-kinase/protein kinase B; DPP4: dipeptidyl peptidase‐4; T2DM: type 2 diabetes mellitus; mTORC1: mechanistic target of rapamycin complex 1; GLP-1: glucagon-like peptide-1.