Phagocytic clearance of presynaptic dystrophies by reactive astrocytes in Alzheimer's disease

Reactive astrogliosis, a complex process characterized by cell hypertrophy and upregulation ofcomponents of intermediate filaments, is a common feature in brains of Alzheimer’s patients. Reac-tive astrocytes are found in close association with neuritic plaques; however, the precise role ofthese glial cells in disease pathogenesis is unknown. In this study, using immunohistochemical tech-niques and light and electron microscopy, we report that plaque-associated reactive astrocytesenwrap, engulf and may digest presynaptic dystrophies in the hippocampus of amyloid precursorprotein/presenilin-1 (APP/PS1) mice. Microglia, the brain phagocytic population, was apparentlynot engaged in this clearance. Phagocytic reactive astrocytes were present in 35% and 67% ofamyloid plaques at 6 and 12 months of age, respectively. The proportion of engulfed dystrophicneurites was low, around 7% of total dystrophies around plaques at both ages. This fact, alongwith the accumulation of dystrophic neurites during disease course, suggests that the efficiency ofthe astrocyte phagocytic process might be limited or impaired. Reactive astrocytes surroundingand engulfing dystrophic neurites were also detected in the hippocampus of Alzheimer’spatientsby confocal and ultrastructural analysis. We posit that the phagocytic activity of reactive astrocytesmight contribute to clear dysfunctional synapses or synaptic debris, thereby restoring impairedneural circuits and reducing the inflammatory impact of damaged neuronal parts and/or limitingthe amyloid pathology. Therefore, potentiation of the phagocytic properties of reactive astrocytesmay represent a potential therapy in Alzheimer s disease.