Microglia are the main immune cells of the brain, and under some circumstances they can play an important role in removal of fibrillar Alzheimer amyloid ␤ peptide (fA␤). Primary mouse microglia can internalize fA␤, but they do not degrade it efficiently. We compared the level of lysosomal proteases in microglia and J774 macrophages, which can degrade fA␤ efficiently, and we found that microglia actually contain higher levels of many lysosomal proteases than macrophages. However, the microglial lysosomes are less acidic (average pH of ϳ6), reducing the activity of lysosomal enzymes in the cells. Proinflammatory treatments with macrophage colony-stimulating factor (MCSF) or interleukin-6 acidify the lysosomes of microglia and enable them to degrade fA␤. After treatment with MCSF, the pH of microglial lysosomes is similar to J774 macrophages (pH of ϳ5), and the MCSF-induced acidification can be partially reversed upon treatment with an inhibitor of protein kinase A or with an anion transport inhibitor. Microglia also degrade fA␤ if lysosomes are acidified by an ammonia pulse-wash or by treatment with forskolin, which activates protein kinase A. Our results indicate that regulated lysosomal acidification can potentiate fA␤ degradation by microglia.

Cultured microglia internalize fibrillar amyloid Aβ (fAβ) and deliver it to lysosomes. Degradation of fAβ by microglia is incomplete, but macrophages degrade fAβ efficiently. When mannose-6 phosphorylated lysosomal enzymes were added to the culture medium of microglia, degradation of fAβ was increased, and the increased degradation was inhibited by excess mannose-6-phosphate, which competes for binding and endocytic uptake. This suggests that low activity of one or more lysosomal enzymes in the microglia was responsible for the poor degradation of fAβ. To further characterize the degradation of fAβ in late endosomes and lysosomes, we analyzed fAβ-derived intracellular degradation products in macrophages and microglia by mass spectrometry. Fragments with truncations in the first 12 N-terminal residues were observed in extracts from both cell types. We also analyzed material released by the cells. Microglia released mainly intact Aβ 1-42 , whereas macrophages released a variety of N-terminal truncated fragments. These results indicate that initial proteolysis near the N-terminus is similar in both cell types, but microglia are limited in their ability to make further cuts in the fAβ.