Lysosomal accumulation of lipids leads to autophagy inhibition in microglia and macrophages and contributes to inflammation after traumatic brain injury
Abstract Text: Protracted inflammation observed in both resident microglia and infiltrating macrophages after traumatic brain injury (TBI) correlates with poor prognosis. Our data suggest that autophagy is inhibited in those activated mononuclear phagocytes and that this inhibition contributes to the prolonged inflammatory phenotype after TBI. However, the mechanism of autophagy inhibition in microglia and macrophages after TBI has yet to be determined. Data from our laboratory including immunofluorescent staining, DESI-MSI lipid imaging, and LC-MS/MS lipidomic analyses of samples from controlled cortical impact (CCI) mouse model of TBI show that inhibition of autophagy correlates with accumulation of lipids and formation of lipid droplets in both infiltrating macrophages and resident microglia. Specifically, our lipidomic data demonstrate accumulation of neutral lipids typically found in lipid droplets in FACS-purified microglia and macrophages from the TBI brain. MSI data show accumulation of cholesteryl esters in perilesional tissue of the TBI brain. Finally, immunofluorescence data demonstrate lipid droplet accumulation in microglia and macrophages and correlation with inhibition of autophagy after TBI. Accumulation of lipid droplets in microglia and macrophages after TBI is reminiscent of myelin-laden foam macrophages reported in multiple sclerosis and lipid droplet associated microglia (LDAM) observed in the aged brain. We hypothesized that excessive phagocytosis of complex cholesterol species including myelin debris and oxidized LDL (oxLDL) that was also detected in the TBI brain, causes observed lysosomal dysfunction and autophagy impairment. We tested this in vitro in microglia and macrophage cell lines and bone marrow derived macrophages (BMDM). Treatment with either myelin or various cholesterol species including oxLDL, caused lipid accumulation, autophagy inhibition, and exacerbated proinflammatory responses induced by LPS. Our data suggest that myelin and cholesterol phagocytosis by myeloid cells inhibits autophagy and exacerbates inflammation after TBI.
