Microbiota–gut–brain axis in neurodegenerative diseases: molecular mechanisms and therapeutic targets

The microbiota-gut-brain axis (MGBA) is an intricate bidirectional communication network that links intestinal microbiota with the central nervous system (CNS) through immune, neural, endocrine, and metabolic pathways. Emerging evidence suggests that dysregulation of the MGBA plays pivotal roles in the onset and progression of neurodegenerative diseases. This review outlines the key molecular mechanisms by which gut microbes modulate neuroinflammation, blood-brain barrier integrity, protein misfolding, and neuronal homeostasis. We discuss how microbial metabolites, such as short-chain fatty acids, tryptophan derivatives, and bile acids, interact with host to influence CNS functions. Disease-specific features are described across Alzheimer's disease, Parkinson's disease, Multiple sclerosis, and Amyotrophic lateral sclerosis, emphasizing the distinct and overlapping pathways through which gut dysbiosis may contribute to pathogenesis. We further explore the translational potential of microbiota-targeted therapies, including probiotics, fecal microbiota transplantation, dietary interventions, and small-molecule modulators. While preclinical results are promising, clinical trials reveal considerable variability, highlighting the need for personalized approaches and robust biomarkers. Challenges remain in deciphering causal relationships, accounting for inter-individual variability, and ensuring reproducibility in therapeutic outcomes. Future research should integrate multi-omics strategies, longitudinal human cohorts, and mechanistic models to clarify the role of the MGBA in neurodegeneration. Collectively, understanding the MGBA provides a transformative perspective on neurodegenerative disease mechanisms and offers innovative therapeutic avenues that bridge neurology, microbiology, and precision medicine.