Brain clearance of protein aggregates in degenerative diseases: the role of microglia and astrocytes

Abstract Protein aggregation and impaired clearance constitute central pathological hallmarks of numerous neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, and frontotemporal dementia. Misfolded proteins, both extracellular and intracellular, disrupt proteostasis and contribute to synaptic dysfunction, neuroinflammation, and neuronal loss. Within the central nervous system, glial cells serve as the primary mediators of pathological protein clearance. Microglia detect, phagocytose, and degrade aggregated proteins. However, under chronic pathological stress, microglial function turns from clearing pathological proteins to paradoxically facilitating their spread. Astrocytes, which provide metabolic and homeostatic support for neurons, also participate in the clearance of aggregates through phagocytosis and glymphatic drainage. Critically, crosstalk between astrocytes and microglia enhances the clearance: microglia can recruit and process aggregates internalized by astrocytes, while astrocyte-derived signals modulate the microglial phagocytic activity. Aging, genetic risk factors, or sustained neuroinflammation may reduce the efficiency of glial removal of aggregates and accelerate disease progression. Understanding the cellular and molecular underpinnings of microglial and astrocytic clearance pathways is essential for developing therapies aimed at restoring proteostasis and mitigating neurodegeneration.