APP and neurodegenerative disease: This pathway is responsible for mediating the cleavage of APP and tau proteins, resulting in the formation of pathological fragments (e.g., APP C586 and tau N368) that promote Aβ and NFT formation, which are transmitted to the brain through the vagus nerve.244 In addition, activated C/EBPβ inhibits the expression of BDNF and netrin-1, leading to α-syn aggregation and dopaminergic neuronal loss.245 Eventually, microbial dysbiosis triggers chronic systemic inflammation, disrupting the BBB and exacerbating neuroinflammation and the progression of neurodegenerative diseases.246–248