A Protein in Microglia That Influences Alzheimer’s Disease Risk
Post by Trisha Vaidyanathan
The takeaway
Two variants of the gene encoding phospholipase C-gamma-2 (PLCG2) have opposing effects on Alzheimer’s disease pathology via their opposing effects on microglia. The first variant (M28L) results in lower PLCG2 levels which reduce the microglial response to plaques and elevate disease risk, while the second (P522R) protects against Alzheimer’s disease by increasing PLCG2 activity, enhancing the ability of microglia to remove plaques and protect synaptic function.
What's the science?
Genetic studies have linked a variant of the gene PLCG2, termed PLCG2-P522R, with reduced risk for Alzheimer’s disease. PLCG2 encodes an enzyme found only in microglia and acts as a critical component of immune signaling within the brain. However, the function of PLCG2 in Alzheimer’s disease is not well understood. This week in Immunity, Tsai and colleagues investigated the “protective” P522R variant and identified a new variant that increases Alzheimer’s disease risk, called PLCG2-M28L. The authors demonstrated that both variants differently alter microglia function, leading to opposing effects on Alzheimer’s disease pathology.
How did they do it?
To investigate the function of PLCG2, the authors first generated mice that had either the “protective” P522R variant or the “detrimental” M28L variant of PLCG2 and determined the effects of this variant on PLCG2 levels. These mice were then crossed to a well-established Alzheimer’s mouse model (called 5xFAD) that is known to develop amyloid plaques, a hallmark of Alzheimer’s pathology. Throughout the study, the authors compared the mice carrying the “protective” and “detrimental” variants of PLCG2 with the typical Alzheimer’s mouse model and healthy control mice.
First, the authors used magnetic resonance imaging (MRI) and immunohistochemistry to measure the buildup of amyloid plaques. Since microglia are known to clean up amyloid plaques, the authors then investigated the proximity of microglia to plaques and measured the ability of microglia to clean up plaque proteins.
Next, the authors tested the health of the neurons by measuring synaptic strength and plasticity with electrophysiology, and the mice’s cognitive ability and memory using a Y-Maze. Lastly, the authors used single nuclei RNA sequencing to identify distinct microglia subtypes and microglia functions that are altered by the PLCG2 variants.
What did they find?
The authors first determined that the “detrimental” M28L variant decreased PLCG2 levels, and is thus considered a loss-of-function mutation. In contrast, the “protective” P522R variant is known to increase PLCG2 activity and is considered a gain-of-function mutation.
Compared to the typical Alzheimer’s disease mouse model, the loss-of-function M28L variant had more plaque deposits, and the plaques were associated with fewer microglia. The gain-of-function P522R variant had fewer deposits and more microglia coverage. Next, the authors found that microglia with the M28L variant took up less fluorescent amyloid, while P522R took up more, demonstrating that PLCG2 is critical for microglia to eat plaques.
Next, the authors found that mice with the loss-of-function M28L variant had impaired synaptic plasticity (long-term potentiation) and worse cognitive performance than typical Alzheimer’s mice. In contrast, the P522R variant behaved more like healthy controls, confirming that the P522R variant of PLCG2 is protective in Alzheimer’s disease and preserves brain functionality.
Lastly, single nuclei RNA sequencing revealed several subtypes of microglia, including baseline homeostatic microglia, two types of disease-associated microglia, and microglia in states of transition from baseline to disease. The disease-associated microglia expressed several genes related to immune responsiveness and are likely critical to protect against Alzheimer’s disease. Interestingly, the loss-of-function M28L variant resulted in more baseline microglia and fewer disease-associated or transitioning microglia, suggesting that PLCG2 is necessary for microglia to transition into a responsive, disease-associated state.
What's the impact?
This study characterized two variants of PLCG2 with opposing effects on microglia and Alzheimer’s disease risk. Together, this demonstrated that PLCG2 is critical for mediating Alzheimer’s disease risk via its role in modulating the microglial response to disease. These findings may provide critical insight into PLCG2-directed therapies for Alzheimer’s disease that can enhance the protective ability of microglia to fight disease pathogenesis.