GDNF and Parkinson disease: For instance, levels of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and glial cell line-derived neurotrophic factor (GDNF) are reduced in AD, FTD, PD, LBD, and VD (Mitra et al., 2021; Biane et al., 2014; Ventriglia et al., 2013).