APP and Parkinson disease: In line with the prion-like spreading hypothesis (Prusiner, 1982), their implications in the transmission of infectious particles, prions (in Creutzfeldt-Jakob disease, CJD), amyloid precursor protein (APP; in Alzheimer’s disease (AD)), α-synuclein (α-syn; in Parkinson’s disease (PD)) and superoxide dismutase 1 (SOD1; in amyotrophic lateral sclerosis (ALS)) between cells in the nervous system are currently being explored.