GDNF and Parkinson disease: The founding member of GFLs, GDNF, was discovered as a potent survival factor for dopamine neurons [9], and, therefore, it was tested in several clinical trials in patients with Parkinson’s disease (PD), as PD is characterized by the profound degeneration of dopamine neurons in the brain, resulting in motor symptoms in the disease [10,11,12,13,14,15].