From metabolism to neurodegeneration: how microglial functional reprogramming drives neurodegenerative diseases

Microglia are brain-resident myeloid cells that maintain central nervous system homeostasis and respond dynamically to neuronal injury, protein aggregation, and alterations in the local metabolic environment. Single-cell and single-nucleus studies demonstrate that microglial responses in neurodegenerative diseases are highly heterogeneous and cannot be adequately explained by the classical M1/M2 polarization model. Increasing evidence further indicates that metabolic remodeling is not merely a consequence of activation but a determinant of microglial migration, phagocytosis, inflammatory signaling, redox balance, organelle function, and interactions with surrounding neural cells. In this review, we propose a microglial immunometabolic trajectory framework in which metabolic states are viewed as branching and potentially reversible determinants of cellular function rather than fixed stages of a universal disease pathway. We summarize how glucose metabolism, mitochondrial function, lipid metabolism, amino acid metabolism, lysosomal activity, and redox regulation shape microglial plasticity. We further examine relationships among transcriptionally defined states, including disease-associated microglia, microglia associated with neurodegeneration, lipid-droplet-accumulating microglia, and other disease-enriched populations, while emphasizing that transcriptional similarity does not necessarily imply metabolic function or lineage progression. Comparative evidence from Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis indicates that common metabolic regulators, including HIF-1α, mTOR, PKM2, TREM2, APOE, and NLRP3, exert disease-specific effects with unequal mechanistic support. We further distinguish associative metabolic signatures from intervention-based causal evidence and discuss limitations of animal models, immortalized cell lines, postmortem tissue, and induced pluripotent stem cell-derived microglia. Finally, we highlight the need for cell-specific, state-resolved, and temporally precise metabolic interventions that restore defined microglial functions without compromising physiological immune surveillance.