Microglia, the brain’s resident immune cells, normally clear harmful molecules, such as amyloid, and remove damaged cells from the brain. In Alzheimer’s disease (AD), however, microglia become dysfunctional and can’t perform these cleanup duties effectively. Chronic exposure to amyloid can drive the accumulation of fat stores within the cell (known as lipid droplets), thereby amplifying microglial metabolic stress. As microglia encounter excess fat from multiple sources, including membrane-rich cellular debris and extracellular lipid, they struggle to properly process, store, and export this lipid burden. Together, this accumulation of lipid droplets impairs the ability of the microglia to function properly. This problem is even worse in people carrying the APOE4 gene variant, a major genetic risk factor for Alzheimer’s.
Lipid droplets inside microglia are coated by a protein called perilipin-2 (PLIN2), which stabilizes lipid droplets and prevents microglia from breaking them down. It also sequesters the material mitochondria need for fuel, which creates an energy deficit. This creates a negative feedback loop: energy-starved cells can’t break down lipids, so lipid droplets accumulate and trigger damaging inflammatory signaling that causes further damage. Preliminary work by Dr. Johnson’s team, led by graduate student Isaiah Stephens, showed that reducing PLIN2 levels destabilizes lipid droplets and almost completely restores microglial function. Other research groups found similar results in other immune cells.
Dr. Johnson hypothesizes that removing PLIN2 will relieve the stress caused by lipid droplet accumulation in microglia, allowing microglia to perform their normal critical functions, including the removal of amyloid beta from the brain. To test this, the group has deleted PLIN2 from microglia in an amyloid mouse model (5XFAD). In the first aim of their project, they will assess how removing PLIN2 from microglia affects markers of AD in the mice, including amyloid plaques and inflammation. They will also perform cognitive and behavioral tests to look for improvements. In their second aim, the team will examine individual microglia from the mice to study the impact of PLIN2 deletion. They will analyze the RNA content of individual microglia to determine which genes they are using to make proteins. This provides clues into how the microglia change their behavior in response to PLIN2 loss. They will also study how PLIN2 loss in microglia changes the lipid content and metabolic behavior of the brain. For the third aim, they will use induced pluripotent stem cells (iPSCs) derived from human donors and modified to carry the APOE4 risk gene in place of the original APOE3 gene. This ensures that the only genetic difference between the microglia they generate from these cells will be in the APOE genes. They will then test if deleting PLIN2 from these cells can correct the increased lipid droplet accumulation and impaired amyloid beta clearance associated with APOE4.
This project has great potential to identify mechanisms underlying microglial dysfunction in AD and APOE4 carriers and to provide new drug targets.
