GDNF and Parkinson disease: Brain-derived neurotrophic factor (BDNF) and glial-derived neurotrophic factor (GDNF) play an important role in the pathophysiology of neurodegenerative diseases, demonstrating a potential for therapeutic applications [28,29,30]: in a preclinical model of Parkinson’s disease, systemic administration of GDNF-expressing macrophages significantly improved the lifespan of mice [31].