Activated Cyclin-Dependent Kinase 5 Promotes Microglial Phagocytosis of Fibrillar β-Amyloid by Up-regulating Lipoprotein Lipase Expression

Amyloid plaques are crucial for the pathogenesis of Alzheimer disease (AD). Phagocytosis of fibrillar beta-amyloid (A beta) by activated microglia is essential for A beta clearance in Alzheimer disease. However, the mechanism underlying A beta clearance in the microglia remains unclear. In this study, we performed stable isotope labeling of amino acids in cultured cells for quantitative proteomics analysis to determine the changes in protein expression in BV2 microglia treated with or without A beta. Among 2742 proteins identified, six were significantly up-regulated and seven were down-regulated by A beta treatment. Bioinformatic analysis revealed strong over-representation of membrane proteins, including lipoprotein lipase (LPL), among proteins regulated by the A beta stimulus. We verified that LPL expression increased at both mRNA and protein levels in response to A beta treatment in BV2 microglia and primary microglial cells. Silencing of LPL reduced microglial phagocytosis of A beta, but did not affect degradation of internalized A beta. Importantly, we found that enhanced cyclin- dependent kinase 5 (CDK5) activity by increasing p35-to-p25 conversion contributed to LPL up-regulation and promoted A beta phagocytosis in microglia, whereas inhibition of CDK5 reduced LPL expression and A beta internalization. Furthermore, A beta plaques was increased with reducing p25 and LPL level in APP/PS1 mouse brains, suggesting that CDK5/p25 signaling plays a crucial role in microglial phagocytosis of A beta. In summary, our findings reveal a potential role of the CDK5/p25-LPL signaling pathway in A beta phagocytosis by microglia and provide a new insight into the molecular pathogenesis of Alzheimer disease.