PIP2 depletion and altered endocytosis caused by expression of Alzheimer's disease‐protective variant PLCγ2 R522

Abstract

Variants identified in genome‐wide association studies have implicated immune pathways in the development of Alzheimer’s disease (AD). Here, we investigated the mechanistic basis for protection from AD associated with PLCγ2 R522, a rare coding variant of the PLCG2 gene. We studied the variant's role in macrophages and microglia of newly generated PLCG2‐R522‐expressing human induced pluripotent cell lines (hiPSC) and knockin mice, which exhibit normal endogenous PLCG2 expression. In all models, cells expressing the R522 mutation show a consistent non‐redundant hyperfunctionality in the context of normal expression of other PLC isoforms. This manifests as enhanced release of cellular calcium ion stores in response to physiologically relevant stimuli like Fc‐receptor ligation or exposure to Aβ oligomers. Expression of the PLCγ2‐R522 variant resulted in increased stimulus‐dependent PIP2 depletion and reduced basal PIP2 levels in vivo. Furthermore, it was associated with impaired phagocytosis and enhanced endocytosis. PLCγ2 acts downstream of other AD‐related factors, such as TREM2 and CSF1R, and alterations in its activity directly impact cell function. The inherent druggability of enzymes such as PLCγ2 raises the prospect of PLCγ2 manipulation as a future therapeutic approach in AD.

Heterozygous expression of the Alzheimer’s disease protective PLCγ2 P522R variant enhances Aβ clearance while preserving synapses.

“…Several other polymorphisms implicated as risk factors in AD, such as the R47H TREM2 mutation, result in alterations to microglial phagocytic capacity that lead to reduced clearance of Aβ aggregates and neuronal decline [18,19]. In our hiPSC derived microglia assay, PLCγ2 P522R enhanced the uptake of Aβ compared to PLCγ2 WT microglia, suggesting that the variant may positively impact directly on disease relevant pathology clearance, as previously suggested [12]. Consistent with this increased Aβ clearance, lysotracker levels, which indicate acidic vesicles such as lysosomes, were higher in both PLCγ2 HET and PLCγ2 HOM microglia variants compared to PLCγ2 WT cells.…”

“…PLCγ2 P522R differential selectivity is driven by cargo size. PLCγ2 P522R has previously been reported to in uence selective cargo uptake, suggesting a shift towards endocytic pathways [12]. Given the differential impact of heterozygous and homozygous PLCγ2 P522R on Aβ and synaptosome uptake, we sought to further investigate whether similar cargo selectivity differences were observed in our microglial cell model.…”

“…The protein product of PLCG2 (PLCγ2) is recruited to the cell membrane upon activation by tyrosine kinase and modulates immune signals impacting cell fate and function, both in the brain and periphery [9,10]. Recent studies have identi ed PLCγ2 downstream of TREM2 signalling, modulating different microglial functions such as phagocytosis, lipid metabolism, survival and cytokine release [11,12]. It also appears to play a role in TLR4 signalling idenpdent of its role in the TREM2 pathway (10), highlighting it's role in facilitating in ammatory responses.…”

PLCG2 is associated with the inflammatory response and is induced by amyloid plaques in Alzheimer’s disease

“…Importantly, approximately 40% of the susceptibility genes identified in genetic studies of LOAD are immune- and microglia-related, suggesting that microglia are involved in modulating AD pathology [ 8 ]. To date, it is known that phospholipase C γ 2 ( PLCG2) might be important in AD due to the pervious findings that a hypermorphic variant in PLCG2 , rs72824905, is protective against AD risk [ 9 – 11 ]. However, the role of PLCG2 has not yet been comprehensively explored.…”

Plcg2M28Linteracts with high fat-high sugar diet to accelerate Alzheimer’s disease-relevant phenotypes in mice

“…PLCG2 is expected to be important in AD due to the previous findings that a hypermorphic variant in PLCG2, rs72824905, is protective against AD risk. However, the role of PLCG2 has not yet been comprehensively explored [11, 39]. A previous study has reported that reduced PLCG2 gene expression alter microglial phenotypes in 5XFAD mice, affect plaque pathology, and drive distinct transcriptional phenotypes of microglia in the presence of amyloid pathology[11].…”