TJP1 and neurodegenerative disease: Microbial metabolites, especially short-chain fatty acids (SCFAs), modulate the immune system by promoting regulatory T cell differentiation, reducing inflammatory cytokines such as IL-6 and TNF-α, and strengthening the epithelial barrier via proteins like claudins and ZO-1, with alterations linked to inflammatory, autoimmune, metabolic, and neurodegenerative diseases [162,163,164].