Alzheimer's Protection by PLCγ2 Compacts Plaques, Redistributes Microglia, and Protects Synapses in App^{NL -G-F} Mice

The Alzheimer's disease protective P522R PLCG2 coding variant (rs72824905) is downstream of TREM2, but how it confers disease protection is poorly understood. Using a Plcg2-R522 knock-in mouse and Plcg2-P522 control on both wildtype and Alzheimer's disease-like App^NL-G-F amyloidosis mouse backgrounds, aged mice were assayed for amyloid load, microglial activity, and synaptic integrity. In the absence of Alzheimer's disease-like pathology, the R522 variant increased microglial coverage and was associated with reduced ramification complexity, fewer terminal points, and elevated lysosomal CD68 expression. On the App^NL-G-F background, total amyloid burden was unaffected, but expression of the R522 variant led to increased plaque compaction compared to the P522 common variant. The protective R522 variant was also associated with: enhanced microglial engagement with less compact amyloid plaques; reduced microglial localisation around highly compacted plaques; protection from amyloid-induced synapse loss; and decreased engulfment of synaptic material by microglia. Our data indicate a significant direct PLCγ2 role in controlling microglial-plaque interactions and synaptic protection downstream of amyloid deposition, prioritizing it as a therapeutic target, potentially as an adjunct to other approaches, such as those targeting amyloid.