Microglia in Brain Aging and Age-Related Diseases: Friends or Foes?

With the global rise in population aging, establishing effective strategies for the prevention and treatment of age-related neurodegenerative diseases, as well as their prodromal stage of cognitive frailty, has become an urgent challenge. Recent studies have revealed that the neural basis of both frailty and age-related disorders is closely associated with chronic neuroinflammation and impaired clearance of cellular debris, processes that are primarily regulated by microglia, the resident immune cells of the brain. As aging progresses, microglia exhibit reduced surveillance and motility, diminished phagocytic efficiency, and transition into a proinflammatory, hyperresponsive state. Such maladaptive microglia contribute to synaptic loss, white matter deterioration, and the spread of neurodegenerative pathology. Conversely, single-cell transcriptomic studies have identified distinct microglial subsets, including CD11c⁺ microglia, which show upregulation of lysosomal and lipid metabolism pathways, enhanced debris clearance, and elevated neurotrophic factor expression. These features suggest that certain microglial populations adopt protective or adaptive phenotypes that preserve neural integrity. However, under chronic inflammation or pathological conditions, even protective microglia may become inflammation-promoting. This review summarizes current evidence on microglial changes in aging, frailty, and neurodegeneration, emphasizing their dual roles and discussing strategies that modulate microglial function to maintain brain health and prevent or treat frailty and age-related diseases.