APP and neurodegenerative disease: AD is a primary progressive neurodegenerative disease, characterized by neuronal cell death caused by an aberrant metabolism of the amyloid precursor protein (APP), resulting in the production and deposition of amyloid beta (Aβ) peptides (Aβ 1-40 and Aβ 1-42 isoforms), that are the main constituents of the extracellular amyloid plaques, a hallmark of this disease.