FNDC5 and Alzheimer disease: FNDC5/irisin is a biological target that regulates AD and its expression level is negatively correlated with AD-related symptoms, such as improving neuroinflammation and promoting increased synaptogenesis and plasticity (Lourenco et al., 2019), Another ability of irisin is its permeability to the BBB, and irisin crosses the BBB to upregulate BDNF expression (Guo P. et al., 2022).