GDNF and Parkinson disease: Since excessive microglial activation has a key role in the progressive loss of DA neurons in PD and GDNF does not cross the blood–brain barrier, our study reinforces the importance of developing new therapeutic approaches aiming at delivering GDNF more efficiently to the affected brain regions, using brain penetrant GDNF mimetic compounds or boosting the expression of endogenous GDNF in those regions, to protect or restore affected DA neurons in PD.