GDNF and Parkinson disease: Moreover, Ren et al. (2013) found that implantation of mesenchymal stem cells (MSCs) with glial cell-derived neurotrophic factor (GDNF)-expressing MSCs into the striatum and substantia nigra improved motor function in PD monkeys, whereas transplantation of MSCs alone did not, suggesting that MSCs may be delivery vehicles for delivering GDNF to enhance nigrostriatal function.