The Alzheimer’s disease-associated protective Plcγ2-P522R variant promotes immune functions

Abstract

Background Microglia-specific genetic variants are enriched in several neurodegenerative diseases, including Alzheimer’s disease (AD), implicating a central role for alterations of the innate immune system in the disease etiology. A rare coding variant in the PLCG2 gene (rs72824905, p.P522R) expressed in myeloid lineage cells was recently identified and shown to reduce the risk for AD. Methods To assess the role of the protective variant in the context of immune cell functions, we generated a Plcγ2-P522R knock-in (KI) mouse model using CRISPR/Cas9 gene editing. Results Functional analyses of macrophages derived from homozygous KI mice and wild type (WT) littermates revealed that the P522R variant potentiates the primary function of Plcγ2 as a Pip2-metabolizing enzyme. This was associated with improved survival and increased acute inflammatory response of the KI macrophages. Enhanced phagocytosis was observed in mouse BV2 microglia-like cells overexpressing human PLCγ2-P522R, but not in PLCγ2-WT expressing cells. Immunohistochemical analyses did not reveal changes in the number or morphology of microglia in the cortex of Plcγ2-P522R KI mice. However, the brain mRNA signature together with microglia-related PET imaging suggested enhanced microglial functions in Plcγ2-P522R KI mice. Conclusion The AD-associated protective Plcγ2-P522R variant promotes protective functions associated with TREM2 signaling. Our findings provide further support for the idea that pharmacological modulation of microglia via TREM2-PLCγ2 pathway-dependent stimulation may be a novel therapeutic option for the treatment of AD.

TREM2/PLCγ2 signalling in immune cells: function, structural insight, and potential therapeutic modulation

“…In agreement with this finding, BMDMs from mice harbouring the p.P522R variant, were found to have increased basal IP 1 levels when compared to WT [35]. Furthermore, Positron Emission Tomography imaging, using the 18F-FPEA probe to detect Translocator Protein (TSPO) activity, revealed that P522R Knock-In (KI) mice showed an increase in microglial activity when compared to WT littermates [35]. This increased basal activity of the p.P522R variant has only been demonstrated in mouse models to date, and more work is therefore required to determine whether this is also true in human iPSC macrophages from engineered lines, and patient-derived cells.…”

“…TREM2 p.Q33X Loss of TREM2 expression [43], no studies into effect on microglia phenotype Heterozygous carriers show typical AD pathology with brain atrophy [44] Found in FTD patients [44] TREM2 p.H157Y Increased shedding from microglia [45], leading to phagocytosis deficits [46] Results in an increased risk of Alzheimer's disease, but the clinical phenotype is not characterised [47] TREM2 p.R62H Impaired phagocytosis of Abeta [17] Unclear due to rarity of this variant the P522R mutation might elicit an increase in basal PLCγ2 function [36]. In agreement with this finding, BMDMs from mice harbouring the p.P522R variant, were found to have increased basal IP 1 levels when compared to WT [35]. Furthermore, Positron Emission Tomography imaging, using the 18F-FPEA probe to detect Translocator Protein (TSPO) activity, revealed that P522R Knock-In (KI) mice showed an increase in microglial activity when compared to WT littermates [35].…”

“…Recent studies in mouse microglia have indicated that basal levels of PIP 2 may be decreased in cells harbouring the p.P522R variant, suggesting that in the mouse brain Protection against AD [1], DLB and FTD [9], promotion of healthy ageing [9], slower rate of cognitive decline [8], mitigation of tau pathology in presence of amyloid [8] Indirect measurements on BMDMs and BV2 cells [35].…”

“…This increased basal activity of the p.P522R variant has only been demonstrated in mouse models to date, and more work is therefore required to determine whether this is also true in human iPSC macrophages from engineered lines, and patient-derived cells. Whilst TREM2 LOAD variants typically induce a reduction in microglial phagocytosis and survival, the PLCγ2 p.P522R variant causes an increase in these beneficial microglial phenotypes [35,36]. Studies in mouse BMDMs revealed that cells derived from P522R KI mice had an increased capacity to phagocytose pHrodolabelled zymosan, in comparison to WT BMDMs [35].…”

“…Whilst TREM2 LOAD variants typically induce a reduction in microglial phagocytosis and survival, the PLCγ2 p.P522R variant causes an increase in these beneficial microglial phenotypes [35,36]. Studies in mouse BMDMs revealed that cells derived from P522R KI mice had an increased capacity to phagocytose pHrodolabelled zymosan, in comparison to WT BMDMs [35]. Furthermore, Maguire et al demonstrated that p.P522R increased Aβ-oligomer clearance when compared with WT expressing microglia, macrophages, and human derived iPSC macrophages [36].…”