GDNF and Parkinson disease: Our previous analysis of Gdnfwt/Hyper mice constitutively overexpressing endogenous GDNF in a proteasome inhibitor LC induced PD model and on aging revealed that a 2-fold increase in endogenous GDNF enhances motor coordination and brain dopamine levels without side-effects (Kumar et al., 2015; Mätlik et al., 2018; Turconi et al., 2020).