The modulation of MFG-E8 mediated microglial phagocytosisPublic Deposited
Phagocytosis is an essential mechanism for clearance of pathogens, dying cells, and other unwanted debris in order to maintain tissue health in the body. Macrophages execute this process in the peripheral immune system, but in the brain microglia act as resident macrophages to accomplish this function. In the peripheral immune system, macrophages secrete Milk Fat Globule Factor-E8 (MFG-E8) that recognizes phosphatidylserine expressed on the surface of apoptotic cells. MFG-E8 acts as a tether to join the apoptotic cell and the macrophage and trigger a signaling cascade that stimulates phagocyte development, allowing the macrophage to engulf the dying cell. When this process becomes disrupted, inflammation can result. MFG-E8 resides in the brain as well as in the periphery, and microglia express MFG-E8. However, the function of MFG-E8 in the brain has not been elucidated. We measured MFG-E8 production in the BV-2 microglial cell line and the role of this protein in the recognition and engulfment of apoptotic SY5Y neuroblastoma cells. BV-2 cells produced and released MFG-E8, which apoptotic SY5Y cells and the chemokine fractalkine further stimulated. Conversely, lipopolysaccharide (LPS) and proinflammatory cytokines reduced the production of MFG-E8 by BV-2 cells. Furthermore, MFG-E8 increased phagocytosis of apoptotic SY5Y cells, and a dominant negative form of MFG-E8 inhibited phagocytosis by BV-2 cells, while LPS reduced the phagocytic activity of BV-2 cells. Finally, brain MFG-E8 levels were altered in a mouse model of the amyloid pathology found in Alzheimer's disease. We hypothesize that MFG-E8 acts in the brain via microglia to aid in clearance of apoptotic neurons. Our data provide a foundation for further exploration of the involvement of MFG-E8 in the proper clearance of apoptotic cells, debris, and pathogenic material by microglia and suggest that a dysregulation of this protein may be involved in neurodegenerative disease.