GDNF and Alzheimer disease: Interestingly, it has been also noticed that the levels of components of the amyloid β-peptide (Aβ)42-generating system in AD, such as β-site amyloid precursor protein-cleaving enzyme 1 (BACE-1), γ-secretase, soluble Aβ42, soluble amyloid precursor protein (sAPP)β, sAPPα, glial-derived neurotrophic factor (GDNF), and phosphorylated tau, are significantly higher in astrocyte-derived plasma exosomes than in neuron-derived exosomes.