Microglial Amyloid-β1-40 Phagocytosis Dysfunction Is Caused by High-Mobility Group Box Protein-1: Implications for the Pathological Progression of Alzheimer’s Disease

In Alzheimer disease (AD) patient brains, the accumulation of amyloid- β (A β ) peptides is associated with activated microglia. A β is derived from the amyloid precursor protein; two major forms of A β , that is, A β 1-40 (A β 40) and A β 1-42 (A β 42), exist. We previously reported that rat microglia phagocytose A β 42, and high mobility group box protein 1 (HMGB1), a chromosomal protein, inhibits phagocytosis. In the present study, we investigated the effects of exogenous HMGB1 on rat microglial A β 40 phagocytosis. In the presence of exogenous HMGB1, A β 40 markedly increased in microglial cytoplasm, and the reduction of extracellular A β 40 was inhibited. During this period, HMGB1 was colocalized with A β 40 in the cytoplasm. Furthermore, exogenous HMGB1 inhibited the degradation of A β 40 induced by the rat microglial cytosolic fraction. Thus, extracellular HMGB1 may internalize with A β 40 in the microglial cytoplasm and inhibit A β 40 degradation by microglia. This may subsequently delay A β 40 clearance. We further confirmed that in AD brains, the parts of senile plaques surrounded by activated microglia are composed of A β 40, and extracellular HMGB1 is deposited on these plaques. Taken together, microglial A β phagocytosis dysfunction may be caused by HMGB1 that accumulates extracellularly on A β plaques, and it may be critically involved in the pathological progression of AD.