MAPT and Alzheimer disease: What is more critical, CCAAT/enhancer binding protein β/asparagine endopeptidase (C/EBPβ/AEP) signaling pathway, cleaving Tau and β-amyloid precursor protein in the brain and resulting in the pathogenesis of AD, was proved to have existed in the gut of AD mouse model and possess a mutual interaction with AD progression in the brain.