APP and Alzheimer disease: The presence of extracellular senile plaques and intracellular neurofibrillary tangles (NFTs), mainly constituted by amyloid-β (Aβ) peptides and tau proteins, respectively, is the main histological evidence of Alzheimer’s disease [53,54,55]; the most abundant Aβ isoforms terminating either at amino acid 40 or at amino acid 42 (Aβ(1–40) and Aβ(1–42), respectively) are produced from the amyloid precursor protein through proteolytic cleavage and then released into the cortex and cerebrospinal fluid (CSF).