GDNF and Parkinson disease: Microbubbles loaded with glial cell-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) have been shown to specifically accelerate the cell survival of dopaminergic neurons and protect neurons in treatment of many diseases, such as stroke, Alzheimer’s, Parkinson’s disease, seizure disorders, brain or spinal injuries, and other neurological disorders [8, 15, 44, 52–54].