Microglial complement receptor 3 regulates brain Aβ levels through secreted proteolytic activity

Recent genetic evidence supports a link between microglia and the complement system in Alzheimer's disease (AD). In this study, we uncovered a novel role for the microglial complement receptor 3 (CR3) in the regulation of soluble beta-amyloid (A beta) clearance independent of phagocytosis. Unexpectedly, ablation of CR3 in human amyloid precursor protein-transgenic mice results in decreased, rather than increased, A beta accumulation. In line with these findings, cultured microglia lacking CR3 are more efficient than wild-type cells at degrading extracellular A beta by secreting enzymatic factors, including tissue plasminogen activator. Furthermore, a small molecule modulator of CR3 reduces soluble A beta levels and A beta half-life in brain interstitial fluid (ISF), as measured by in vivo microdialysis. These results suggest that CR3 limits A beta clearance from the ISF, illustrating a novel role for CR3 and microglia in brain A beta metabolism and defining a potential new therapeutic target in AD.