Research Models

Plcg2*P522R x APP NL-G-F

Synonyms: Plcg2*P522R x APPNL-G-F, Plcg2R522 x APPNL-G-F, AppNL-G-FR522

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Species: Mouse
Genes: Plcg2, App
Mutations: APP K670_M671delinsNL (Swedish), APP I716F (Iberian), APP E693G (Arctic)
Modification: Plcg2: Knock-In; App: Knock-In
Disease Relevance: Alzheimer's Disease
Strain Name: B6.Cg-Plcg2em1Msasn/J x Apptm3.1Tcs/Apptm3.1Tcs
Genetic Background: C57BL/6
Availability: Plcg2*P522R mice are available from The Jackson Laboratory, JAX Stock# 029598; APPNL-G-F mice are available through Takaomi Saido.

The PLCG2 gene encodes the enzyme phospholipase C gamma 2 (PLCγ2), a mediator of transmembrane signaling in microglia that acts downstream of TREM2. A rare missense variant in this gene, P522R, has been associated with reduced risks of Alzheimer’s disease (Sims et al., 2017; Conway et al., 2018; van der Lee et al., 2019; Olive et al., 2020; Bellenguez et al., 2022), frontotemporal dementia (van der Lee et al., 2019), and dementia with Lewy bodies (van der Lee et al., 2019) (although it should be noted that other studies [Conway et al., 2018; Guerreiro et al., 2018; Orme et al., 2020; Strickland et al., 2020] did not find a statistically significant association with DLB). The P522R variant has also been reported to associate with a slower rate of cognitive decline and lower levels of total tau and p-tau181 in the cerebrospinal fluid of people with mild cognitive impairment (Kleineidam et al., 2020), less tau pathology in the brains of neuropathologically confirmed DLB and PSP cases (Strickland et al., 2020), and increased longevity (van der Lee et al., 2019).

The P522R variant is a mild hypermorph (i.e., the variant slightly increases the enzymatic activity of PLCγ2) (Magno et al., 2019).

CRISPR/Cas9 gene editing was used to introduce the P522R variant into the mouse Plcg2 gene. These Plcg2*P522R knock-in mice were then intercrossed with APPNL-G-F knock-in mice, in which the mouse App gene was modified to contain a humanized Aβ region and the Swedish (“NL”), Iberian (“F”), and Arctic (“G”) mutations linked to Alzheimer’s disease (Bevan et al., 2023).

APPNL-G-F mice homozygous for the P522R variant had increased amyloid plaque burdens and decreased microglia-plaque interactions, compared with mice carrying wild-type Plcg2. This finding contrasts with that seen in Plcg2*P522R x 5xFAD mice, where the P522R variant reduced plaque burdens and enhanced microglia-plaque interactions. Nonetheless, the P522R variant protected against synapse loss in both the APPNL-G-F and 5xFAD models of amyloidosis.

Hereafter, mice homozygous for both the Plcg2 variant and the modified App allele are referred to as APPNL-G-FP522R, while animals homozygous for the modified App allele without the Plcg2 mutation are referred to as APPNL-G-F. The descriptions below refer to 6-month-old mice of both sexes, unless stated otherwise.

Plaques. Plaque burden (percent area occupied by plaques) and plaque density were greater in the hippocampi of APPNL-G-FP522R mice, compared with APPNL-G-F animals, regardless of sex. Plaques in the P522R carriers were also larger and more intensely stained with Thioflavin S, perhaps indicating that they were more densely compacted. Amyloid pathology in the cortex was sex- and genotype-dependent: Female APPNL-G-F mice had higher plaque burdens and plaque densities than males, but APPNL-G-FP522R did not show sex differences in these measures. In males, plaque burdens and plaque densities were higher in APPNL-G-FP522R than APPNL-G-F mice, while females did not differ by genotype. Cortical plaques were more intensely stained in APPNL-G-FP522R than in APPNL-G-F mice, for both sexes.

Microglia. In the hippocampus, the density of Iba1-positive microglia was higher in APPNL-G-FP522R than APPNL-G-F mice, regardless of sex. Compared with APPNL-G-F mice, APPNL-G-FP522R mice displayed reduced microglia-plaque interactions, assessed as the amount of Iba1 immunoreactivity within 30 μm of plaque cores. Levels of the lysosomal marker CD68 in peri-plaque microglia were also lower in P522R carriers.

Similar to plaques, microglial abundance in the cortex depended on sex as well as genotype. Microglial density in female APPNL-G-F mice was higher than in males, but APPNL-G-FP522R mice did not show sex differences. In males, cortical microglial density was higher in APPNL-G-FP522R than APPNL-G-F mice, while females did not differ by genotype.

Synapse preservation. The PLCγ2 R522 variant appeared to protect against synapse loss in APPNL-G-F mice. Compared with wild-type mice, APPNL-G-F mice had fewer puncta immunoreactive for the presynaptic marker bassoon or the postsynaptic marker PSD95 and fewer DiOlistically labeled dendritic spines, assessed in the CA1 region of the hippocampus. Levels of these synaptic indicators were largely restored in APPNL-G-FP522R mice.

Furthermore, the PLCγ2 R522 variant had a dampening effect on microglial phagocytosis of synapses. More unengulfed synapses (represented by PSD95-immunoreactive puncta) and fewer engulfed synapses (co-localized PSD95 and CD68 immunoreactivity) were seen in peri-plaque regions in APPNL-G-FP522R mice, compared with APPNL-G-F mice.

Modification Details

CRISPR/Cas9 gene editing was used to introduce the P522R (c.1565 C>G ) mutation into the mouse Plcg2 gene in APOE4 Knock-In mice (JAX 027894). Correctly targeted mice were then backcrossed to C57BL/6J mice (JAX 000664) to remove the APOE4 sequence. The resulting Plcg2*R522 mice (JAX 029598) were then intercrossed with APPNL-G-F Knock-in mice to create animals homozygous for the Plcg2 P522R mutation and the App modifications (a humanized Aβ region, and the Swedish, Iberian, and Arctic mutations linked to Alzheimer’s disease).

 

Phenotype Characterization

When visualized, these models will distributed over a 18 month timeline demarcated at the following intervals: 1mo, 3mo, 6mo, 9mo, 12mo, 15mo, 18mo+.

Absent

  • Synaptic Loss

No Data

  • Tangles
  • Neuronal Loss
  • Changes in LTP/LTD
  • Cognitive Impairment

Plaques

ThioflavinS-positive amyloid plaques observed in mice studied at 6 months of age. Higher plaque burdens than APPNL-G-F.

Tangles

No data.

Neuronal Loss

No data.

Gliosis

Microgliosis observed in mice studied at 6 months of age. Attenuated microglia-plaque interactions in the hippocampus, compared with APPNL-G-F.

Synaptic Loss

The P522R variant attenuated the synapse loss observed in APPNL-G-F mice with wild-type PLCγ2.

Changes in LTP/LTD

No data.

Cognitive Impairment

No data.

Complementary Models

Microglial-like cells derived from human induced pluripotent stem cell lines (hIPSCs) have been used to study PLCγ2 biology in human cells in vitro and in vivo after transplantation into mouse brains.

CRISPR/Cas9 gene editing was used to introduce the PLCG2 P522R mutation into hIPSCs derived from skin cells of an apparently healthy, middle-aged Caucasian male. Isogenic clones homozygous for the wild-type P522 allele or mutant R522 allele were differentiated into microglia-like cells (Maguire et al., 2021). Stimulation of PLCγ2 by Fc receptor ligation led to a greater increase in intracellular Ca2+ in cells carrying the mutant allele, consistent with a hypermorphic effect of the mutation. Similar to microglia and macrophages isolated from Plcg2*P522R knock-in mice (Maguire et al., 2021), hIPSC-derived microglia carrying the mutant allele showed decreased phagocytosis (uptake of E. coli particles or zymosan) and increased endocytosis (uptake of Aβ42 oligomers or Dextrans), compared with isogenic hIPSC-derived microglia expressing wild-type PLCγ2.

A second study compared isogenic hIPSC-derived microglia that differed with regard to P522R gene dose—wild-type (PLCγ2WT), heterozygous for the P522R mutation (PLCγ2HET), and homozygous for the  mutation (PLCγ2HOM) (Solomon et al., 2022). In this case, the parental hIPSC line was derived from skin fibroblasts donated by a teenaged male (APOE3/4) of black or African-American ancestry with no diagnosed diseases. Here, too, CRISPR gene editing was used to introduce the PLCG2 P522R mutation. IPSC-derived microglia contained similar levels of PLCγ2 protein, regardless of PLCG2 genotype. However, the genotypes differed with regard to functional properties and gene expression—with PLCγ2HET showing more pronounced differences than PLCγ2HOM on several measures (compared with PLCγ2WT). PLCγ2HOM and PLCγ2HET showed increased uptake of fluorescently labeled Aβ42, but only PLCγ2HET cells showed increased uptake of Dextrans. Uptake of synaptosomes was reduced in P522R carriers, regardless of gene dose. LysoTracker staining—a marker for lysosomes—was elevated in P522R carriers, slightly more so in heterozygotes than homozygotes. When co-cultured with IPSC-derived neurons (heterozygous for the PLCG2 P522R mutation), PLCγ2HET microglia engaged in less synaptic pruning—as measured by PSD95 engulfment—than PLCγ2WT microglia, while PLCγ2HOM did not significantly differ from PLCγ2WT. When levels of expression of selected genes related to microglial function were compared between P522R carriers and wild-type cells, several genes were found to be upregulated in PLCγ2HET—in pathways related to lipid metabolism, lysosomal biogenesis, and immune function—while only APOE was upregulated in PLCγ2HOM. Microglial motility and intracellular Ca2+ levels were also greater in PLCγ2HET compared with the other two PLCG2 genotypes. Physiological studies showed a gene-dose-dependent increase in oxidative phosphorylation with PLCγ2HOM > PLCγ2HET > PLCγ2WT.

A third study focused on the effects of the P522R mutation on the transcriptomes of human microglia-like cells in vivo, in the context of amyloidosis (Claes et al., 2022). Once again, CRISPR gene editing was used to introduce the P522R mutation into the PLCG2 gene, this time in an (RFP)-α-tubulin expressing hIPSC line derived from fibroblasts donated by an apparently healthy 30-year-old Japanese man. IPSCs homozygous for the PLCG2 P522R mutation or isogenic hIPSCs with wild-type PLCG2 were differentiated into microglia-like cells in vitro, then grafted into the brains of neonatal immune-deficient 5xFAD or non-transgenic mice. Mice were aged to 7 months, a time when plaque deposition is well underway in 5xFAD brains, and the human cells were harvested for RNA sequencing. PLCG2 P522R microglia from 5xFAD brains showed increased levels of expression of multiple HLA and interferon genes and of genes encoding chemokines that mediate T-cell recruitment to the brain, compared with microglia expressing wild-type PLCG2. Gene Ontology analysis highlighted MHC class II antigen presentation, cytokine/chemokine signaling, interferon signaling, and regulation of T cell proliferation as pathways affected by the P522R mutation. PLCG2 P522R microglia isolated from non-transgenic hosts also showed increased expression of HLA genes, compared with microglia carrying wild-type PLCG2.

Chimeric 5xFAD brains were also examined histologically, and no differences were seen between those transplanted with P522R and wild-type PLCG2 hIPSC-derived microglia in the following measures: amyloid plaque burden, number, or size; microglial morphology, number of plaque-associated microglia, or microglial amyloid internalization; “amount” of plaque-associated dystrophic neurites; or numbers of total or plaque-associated astrocytes.

The lack of an effect of the P522R mutation on amyloid-related pathology in chimeric mice contrasted with findings in 5xFAD mice in which the P522R mutation was knocked into the endogenous Plcg2 gene. In the knock-in mice, the P522R mutation reduced amyloidosis, enhanced microglia-plaque interactions, and protected against plaque-associated pathology. The chimeric and knock-in models differ in several aspects that could potentially contribute to these discrepant findings, including intrinsic differences between human and mouse microglia, expression of P522R PLCγ2 in cells other than microglia in the knock-in mice, and lack of immune responses in chimeric hosts.

Last Updated: 27 Oct 2023

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References

Research Models Citations

  1. Plcg2*P522R
  2. APP NL-G-F Knock-in
  3. Plcg2*P522R x 5xFAD
  4. APOE4 Knock-In (JAX)
  5. Plcg2*P522R
  6. 5xFAD (C57BL6)
  7. Plcg2*P522R x 5xFAD

Paper Citations

  1. . Rare coding variants in PLCG2, ABI3, and TREM2 implicate microglial-mediated innate immunity in Alzheimer's disease. Nat Genet. 2017 Sep;49(9):1373-1384. Epub 2017 Jul 17 PubMed.
  2. . ABI3 and PLCG2 missense variants as risk factors for neurodegenerative diseases in Caucasians and African Americans. Mol Neurodegener. 2018 Oct 11;13(1):53. PubMed.
  3. . A nonsynonymous mutation in PLCG2 reduces the risk of Alzheimer's disease, dementia with Lewy bodies and frontotemporal dementia, and increases the likelihood of longevity. Acta Neuropathol. 2019 Aug;138(2):237-250. Epub 2019 May 27 PubMed.
  4. . Examination of the Effect of Rare Variants in TREM2, ABI3, and PLCG2 in LOAD Through Multiple Phenotypes. J Alzheimers Dis. 2020;77(4):1469-1482. PubMed.
  5. . New insights into the genetic etiology of Alzheimer's disease and related dementias. Nat Genet. 2022 Apr;54(4):412-436. Epub 2022 Apr 4 PubMed.
  6. . Investigating the genetic architecture of dementia with Lewy bodies: a two-stage genome-wide association study. Lancet Neurol. 2018 Jan;17(1):64-74. Epub 2017 Dec 16 PubMed.
  7. . Analysis of neurodegenerative disease-causing genes in dementia with Lewy bodies. Acta Neuropathol Commun. 2020 Jan 29;8(1):5. PubMed.
  8. . Association of ABI3 and PLCG2 missense variants with disease risk and neuropathology in Lewy body disease and progressive supranuclear palsy. Acta Neuropathol Commun. 2020 Oct 22;8(1):172. PubMed.
  9. . PLCG2 protective variant p.P522R modulates tau pathology and disease progression in patients with mild cognitive impairment. Acta Neuropathol. 2020 Jun;139(6):1025-1044. Epub 2020 Mar 12 PubMed.
  10. . Alzheimer's disease phospholipase C-gamma-2 (PLCG2) protective variant is a functional hypermorph. Alzheimers Res Ther. 2019 Feb 2;11(1):16. PubMed.
  11. . Uncoupling of synaptic loss from amyloid burden by an Alzheimer's disease protective variant of PLCγ2. 2023 Sep 22 10.1101/2023.09.22.558987 (version 1) bioRxiv.
  12. . PIP2 depletion and altered endocytosis caused by expression of Alzheimer's disease-protective variant PLCγ2 R522. EMBO J. 2021 Sep 1;40(17):e105603. Epub 2021 Jul 13 PubMed.
  13. . Heterozygous expression of the Alzheimer's disease-protective PLCγ2 P522R variant enhances Aβ clearance while preserving synapses. Cell Mol Life Sci. 2022 Jul 27;79(8):453. PubMed.
  14. . The P522R protective variant of PLCG2 promotes the expression of antigen presentation genes by human microglia in an Alzheimer's disease mouse model. Alzheimers Dement. 2022 Feb 9; PubMed.

External Citations

  1. JAX 027894
  2. JAX 000664
  3. JAX 029598
  4. The Jackson Laboratory, JAX Stock# 029598
  5. Takaomi Saido
  6. JAX 029598
  7. hIPSCs
  8. hIPSC
  9. hIPSC line

Further Reading

No Available Further Reading