GDNF and Parkinson disease: NTFs can exert protective or even restorative effects on PD animal models and dopaminergic cell cultures; key examples include brain-derived neurotrophic factor (BDNF) [63], glial-derived neurotrophic factor (GDNF) [64], mesencephalic-astrocyte-derived neurotrophic factor (MANF) [65], and cerebral dopamine neurotropic factor (CDNF) [66].