GDNF and Parkinson disease: Extrinsic cues such as glial derived neurotrophic factor (GDNF) impact dopaminergic axon terminal structures and modify signaling through the PI3K pathway [37], [38]; it will be of interest to pursue the role of macroautophagy in such changes, which have been implicated clinically in pathological movements associated with experimental therapeutics for Parkinson's disease [39]–[42].