FNDC5 and Alzheimer disease: ICV injection of Aβ caused the following events: (1) decreased FNDC5 gene expression in the hippocampus of AD rats, (2) decreased irisin and BDNF levels in the hippocampus of AD rats, and (3) increase Aβ plaques in the hippocampal CA1, CA3, and DG areas in AD rats.