The microbiota-tryptophan-brain axis in neurodegenerative diseases: pathogenic mechanisms, disease-specific roles, and translational therapeutics

The pathogenesis of neurodegenerative diseases (NDDs) such as Alzheimer's disease (AD) and Parkinson's disease (PD) is very complex. Recent studies have shown that gut microbiota and their metabolites play a key role in the progression of these diseases. Tryptophan (Trp) is an essential amino acid, which mainly produces a variety of biologically active compounds in the intestine through the metabolism of indole pathway, Kynurenine pathway (KP) and serotonin pathway, including indole derivatives, Kynurenine (KYN) and serotonin (5-HT). These metabolites affect the central nervous system (CNS) through the Microbiota-gut-brain axis (MGBA) and affect CNS in a variety of mechanisms, including immune regulation, neuroprotection and maintenance of intestinal barrier function. They are involved in key pathological processes such as neuroinflammation, oxidative stress and pathological protein aggregation. This paper systematically reviews the mechanism of the role of Trp metabolites derived from gut microbiota in NDDs, and explores their specific roles in AD, PD, Amyotrophic Lateral Sclerosis (ALS) and Huntington's disease (HD), and summarizes the potential therapeutic value of the current pathway strategy. These strategies include nutritional intervention, targeted microbiome therapy [such as probiotic and fecal microbiota transplantation (FMT)], and metabolite-derived drugs. Future research must clarify its dynamic mechanism in the human body, develop relevant biomarkers, and promote personalized prevention and treatment strategies through clinical transformation, so as to provide a new direction for early intervention and treatment of NDDs.