FIS1 and Alzheimer disease: Coordinated cycles of fission and fusion are essential for maintaining mitochondrial morphology and functions and requires the adequate expression of genes coding proteins such as dynamic-related protein-1 (Drp-1), mitochondrial fission-1 (Fis-1), fusion proteins (Mfn1, Mfn2 and Opa1), that are disturbed by both Aβ and CTFs already in early-stage AD, resulting in mitochondrial fragmentation [105,106].