MFN1 and Alzheimer disease: Optic atrophy 1 (OPA1)—a mitochondrial GTPase—and mitofusin 1 and 2 (MFN1 and MFN2), the three main fusion proteins, modulate the fusion of the outer and inner membrane of adjacent mitochondria [84,87]: a decrease in their activity has been observed in the post mortem tissue of AD patients [88].