Species: Mouse
Genes: Trem2, APP, PSEN1
Mutations: APP K670_M671delinsNL (Swedish), PSEN1 G384A
Modification: Trem2: Knock-In; APP: Transgenic; PSEN1: Transgenic
Disease Relevance: Alzheimer's Disease
Strain Name: N/A
Genetic Background: C57BL/6
Availability: TREM2-IPD and PS45 mice are available from Novartis Pharma AG under an MTA. Contact Ivan Galimberti (ivan.galimberti@novartis.com) or Derya Shimshek (derya.shimshek@novartis.com). APP23 mice are available through The Jackson Laboratory Stock# 030504, Live.

Summary

Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) is a transmembrane receptor found on microglia, where it modulates cell activity and survival. TREM2 is cleaved by ADAM proteases after amino acid 157 to release a soluble fragment, sTREM2, whose function is still being elucidated (Feuerbach et al., 2017; Schlepckow et al., 2017; Thornton et al., 2017; 31 Aug 2017 news). A rare variant in TREM2, H157Y, increases TREM2 shedding (Schlepckow et al., 2017; Thornton et al., 2017) and has been associated with an increased risk of Alzheimer’s disease (Jiang et al., 2016; Song et al., 2017).

In Trem2-IPD mice, the ADAM10/17 recognition site was disrupted through CRISPR/Cas9 gene editing—changing histidine-157 to isoleucine (I), serine-158 to proline (P), and threonine-159 to aspartate (D)—leading to both a reduction in sTREM2 and an increase in membrane-associated, signaling-competent TREM2 (Dhandapani et al. 2022). To study the effects of reducing TREM2 cleavage in the context of amyloid pathology, Trem2-IPD mice were intercrossed with APP23 and PS45 mice (Herzig et al., 2004), which carry transgenes for human APP with the AD-linked Swedish mutation and PSEN1 with the AD-linked G384A mutation, respectively. Compared with APP23xPS45 mice carrying wild-type Trem2, Trem2-IPDxAPP23xPS45 mice had higher plaque burdens and more severe plaque-associated pathology at an early—but not late—stage of plaque deposition.

The description below refers to animals homozygous for the Trem2 IPD mutation and hemizygous for the APP and PSEN1 transgenes. Mice were studied at 3 months of age, at the onset of plaque deposition, and 7 months, when plaque levels had plateaued.

qPCR on forebrain lysates from 3-month-old mice showed comparable levels of APP and PSEN1 mRNAs in mice expressing wild-type (APP23xPS45) and mutant (Trem2-IPDxAPP23xPS45 ) Trem2. Lower levels of TREM2 protein, presumably sTREM2, were found in forebrain extracts of Trem2-IPDxAPP23xPS45 prepared in Tris-buffered saline absent detergents, compared with APP23xPS45.

Amyloidosis | Microglia-plaque interaction | Plaque-associated neuritic dystrophies | Transcriptomics | Other | Modification details | Related Models

Amyloidosis

The Trem2 mutation increased cortical plaque load at an early—but not late—stage of plaque deposition. Methoxy-X04 revealed greater numbers of plaques, particularly small plaques, and larger areas occupied by plaques in 3-month-old Trem2-IPDxAPP23xPS45 mice, compared with APP23xPS45 animals. These genotype-dependent differences disappeared by 7 months. Biochemical findings were consistent with the histological results: Forebrain extracts from 3-month Trem2-IPDxAPP23xPS45 mice contained more insoluble Aβ than did APP23xPS45 extracts. Levels of insoluble Aβ40 and Aβ42 were similar in the two genotypes at 7 months.

Microglia-plaque interactions

The IPD substitution in TREM2 exacerbated microgliosis in the vicinity of plaques in 3-month-old mice. Compared with APP23xPS45 mice, Trem2-IPDxAPP23xPS45 mice had more TREM2-positive microglia surrounding plaques, more space occupied by the microglial marker Iba1, and increased levels of the autophagy marker CD68 around and within plaques.

Plaque-associated neuritic dystrophies

Compared with APP23xPS45 mice, Trem2-IPDxAPP23xPS45  mice showed more severe neuronal damage in the vicinity of plaques at 3 months of age. Mice carrying mutant Trem2 had more APP-positive puncta (an indicator of neuritic dystrophy) around plaques as well as fewer and smaller puncta stained for the presynaptic marker Sv2a (synaptic vesicle glycoprotein 2A). These genotype-dependent differences disappeared  by 7 months.

Transcriptomics

The IPD substitution appeared to accelerate microglial maturation in healthy, non-transgenic mice (discussed in more detail here), and this effect was also seen in the APP23xPS45 model of amyloidosis. Subsets of microglia in APP23xPS45 mice were classified by Dhandapini and colleagues as “homeostatic microglia,” “disease-associated microglia (DAM),” and “inflammatory DAM,” according to previously published transcriptomic signatures obtained in other disease models. Microglia from Trem2-IPDxAPP23xPS45 mice differed from those of transgenic mice with wild-type Trem2: The homeostatic, DAM, and inflammatory DAM profiles of 3-month-old Trem2-IPDxAPP23xPS45 mice more closely resembled those of 7-month APP23xPS45 mice than 3-month APP23xPS45 animals. Additionally, some microglia in Trem2-IPDxAPP23xPS45  were classified as interferon-response microglia (IRM), a category largely absent from APP23xPS45 brains.

Other

Consistent with the transcriptomic analysis, levels of cytokines and chemokines were elevated in the brains of 3-month-old Trem2-IPDxAPP23xPS45 mice, compared with APP23xPS45 mice.

Modification details

Trem2-IPD mice were created through CRISPR/Cas9 editing of the murine Trem2 gene—changing histidine-157 to isoleucine (I), serine-158 to proline (P), and threonine-159 to aspartate (D)—to disrupt the ADAM10/17 recognition site on TREM2 (Dhandapani et al. 2022).

APP23 mice express human APP (isoform 751) containing the Swedish (KM670/671NL) mutation under the control of the murine Thy1 promoter (Sturchler-Pierrat et al., 1997).

PS45 mice express human PSEN1 with the G384A mutation, also driven by the Thy-1 promoter (Herzig et al., 2004; Busche et al., 2008).

Related Models

The following models have been used to manipulate levels of soluble TREM2, through genetic alteration of the ADAM protease cleavage site or AAV-mediated expression of an extracellular fragment of TREM2.

Trem2-H157Y knock-in. CRISPR/Cas9 gene editing was used to introduce the H157Y mutation into the mouse Trem2 gene. This variant first gained attention when it was found to associate with an increased risk for Alzheimer’s disease in a Han Chinese cohort (Jiang et al., 2016). Elevated levels of sTREM2 were found in mice homozygous for the variant. The H157Y mutation did not affect microglial density or morphology, performance on a battery of behavioral tests, or levels of synaptic markers, but enhanced hippocampal synaptic plasticity. Perhaps surprisingly, when Trem2-H157Y mice were crossed with 5xFAD mice, the Trem2 variant decreased amyloid pathology, microgliosis, and plaque-associated neuritic damage.

Trem2-H157Y x 5xFAD. Trem2-H157Y mice were intercrossed with 5xFAD mice, a model of aggressive amyloidosis. Levels of Trem2 mRNA and TREM2 protein—both full-length, membrane-associated TREM2 and sTREM2—were lower in 5xFAD mice homozygous for Trem2-H157Y, compared with 5xFAD mice expressing wild-type Trem2. This difference may reflect the lower number of microglia in the brains of the Trem2 mutation carriers. However, the ratio of sTREM2 to full-length TREM2 was higher in the Trem2 mutation carriers, consistent with increased shedding of TREM2-H157Y.

Compared with 5xFAD mice homozygous for wild-type Trem2, mice homozygous for the H157Y variant showed age-dependent decreases in plaque burdens, microgliosis and plaque-associated neuritic damage. At 8.5 months, the only timepoint reported to date, expression of neuroinflammation-related genes was downregulated in the H157Y mutation carriers.

TREM2-sol. In an attempt to create a model that generates only sTREM2—but no full-length, signaling-competent receptor—CRISPR/Cas9 gene editing was used to introduce a stop codon after H157 of murine Trem2. These mice, called “TREM2-sol,” were found to express very low levels of sTREM2 and Trem2 mRNA. Nonetheless, differences between TREM2-sol and Trem2 knockout mice (Trem2-KO) were observed, including prolonged microglial responses to injury, increased vulnerability of bone marrow-derived macrophages to growth factor deprivation, and preservation of endo-lysosomal function in TREM2-sol compared with Trem2-KO mice.

TREM2-IPD. CRISPR/Cas9 gene editing was used to disrupt the ADAM10/17 recognition site on mouse TREM2, changing histidine-157 to isoleucine (I), serine-158 to proline (P), and threonine-159 to aspartate (D)—leading to both a reduction in sTREM2 and an increase in cell-surface TREM2. The IPD mutation accelerated microglial maturation and increased microglial phagocytic activity. In the cuprizone model of demyelination, the mutation resulted in persistent neuroinflammation during the recovery phase.

AAV-sTREM2 5xFAD. AAV-sTREM2 5xFAD mice were created to study the long-term effects of soluble TREM2 (sTREM2) in the context of amyloidosis. To generate this model, AAV carrying cDNA encoding EGFP- and FLAG-tagged sTREM2 (amino acids 1-171 of human TREM2) was injected into the brains of neonatal 5xFAD mice. Overexpression of sTREM2 led to decreased plaque burdens, increased the number of plaque-associated microglia, and rescued hippocampal long-term potentiation and performance in the Morris water maze in 5xFAD mice aged 6 to 7 months.

AAV-sTREM2 PS19. AAV-sTREM2 PS19 mice were created to study the long-term effects of soluble TREM2 (sTREM2) in the context of tauopathy. To generate this model, AAV carrying cDNA encoding EGFP- and FLAG-tagged sTREM2 (amino acids 1-171 of human TREM2) was injected into the brains of 3-month-old PS19 mice. AAV-mediated expression of sTREM2 protected against hippocampal synapse loss, improved performance in the Morris water maze and Y-maze, enhanced long-term potentiation, and reduced levels of p-tau202 and p-tau396 in PS19 mice studied at 7 months of age.

Phenotype Characterization

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References

News Citations

1. TREM2 Cleavage Site Pinpointed: A Gateway to New Therapies? 31 Aug 2017

Research Models Citations

1. APP23
2. TREM2-IPD
3. Trem2-H157Y knock-in
4. Trem2-H157Y x 5xFAD
5. 5xFAD (C57BL6)
6. TREM2-sol
7. Trem2 KO (KOMP)
8. AAV-sTREM2 5xFAD
9. AAV-sTREM2 PS19
10. Tau P301S (Line PS19)

Mutations Citations

1. APP K670_M671delinsNL (Swedish)
2. PSEN1 G384A

Paper Citations

1. Feuerbach D, Schindler P, Barske C, Joller S, Beng-Louka E, Worringer KA, Kommineni S, Kaykas A, Ho DJ, Ye C, Welzenbach K, Elain G, Klein L, Brzak I, Mir AK, Farady CJ, Aichholz R, Popp S, George N, Neumann U. ADAM17 is the main sheddase for the generation of human triggering receptor expressed in myeloid cells (hTREM2) ectodomain and cleaves TREM2 after Histidine 157. Neurosci Lett. 2017 Nov 1;660:109-114. Epub 2017 Sep 18 PubMed.
2. Schlepckow K, Kleinberger G, Fukumori A, Feederle R, Lichtenthaler SF, Steiner H, Haass C. An Alzheimer-associated TREM2 variant occurs at the ADAM cleavage site and affects shedding and phagocytic function. EMBO Mol Med. 2017 Oct;9(10):1356-1365. PubMed.
3. Thornton P, Sevalle J, Deery MJ, Fraser G, Zhou Y, Ståhl S, Franssen EH, Dodd RB, Qamar S, Gomez Perez-Nievas B, Nicol LS, Eketjäll S, Revell J, Jones C, Billinton A, St George-Hyslop PH, Chessell I, Crowther DC. TREM2 shedding by cleavage at the H157-S158 bond is accelerated for the Alzheimer's disease-associated H157Y variant. EMBO Mol Med. 2017 Oct;9(10):1366-1378. PubMed.
4. Jiang T, Hou JK, Gao Q, Yu JT, Zhou JS, Zhao HD, Zhang YD. TREM2 p.H157Y Variant and the Risk of Alzheimer's Disease: A Meta-Analysis Involving 14,510 Subjects. Curr Neurovasc Res. 2016;13(4):318-320. PubMed.
5. Song W, Hooli B, Mullin K, Jin SC, Cella M, Ulland TK, Wang Y, Tanzi RE, Colonna M. Alzheimer's disease-associated TREM2 variants exhibit either decreased or increased ligand-dependent activation. Alzheimers Dement. 2017 Apr;13(4):381-387. Epub 2016 Aug 9 PubMed.
6. Dhandapani R, Neri M, Bernhard M, Brzak I, Schweizer T, Rudin S, Joller S, Berth R, Kernen J, Neuhaus A, Waldt A, Cuttat R, Naumann U, Keller CG, Roma G, Feuerbach D, Shimshek DR, Neumann U, Gasparini F, Galimberti I. Sustained Trem2 stabilization accelerates microglia heterogeneity and Aβ pathology in a mouse model of Alzheimer's disease. Cell Rep. 2022 May 31;39(9):110883. PubMed.
7. Herzig MC, Winkler DT, Burgermeister P, Pfeifer M, Kohler E, Schmidt SD, Danner S, Abramowski D, Stürchler-Pierrat C, Bürki K, van Duinen SG, Maat-Schieman ML, Staufenbiel M, Mathews PM, Jucker M. Abeta is targeted to the vasculature in a mouse model of hereditary cerebral hemorrhage with amyloidosis. Nat Neurosci. 2004 Sep;7(9):954-60. PubMed.
8. Sturchler-Pierrat C, Abramowski D, Duke M, Wiederhold KH, Mistl C, Rothacher S, Ledermann B, Bürki K, Frey P, Paganetti PA, Waridel C, Calhoun ME, Jucker M, Probst A, Staufenbiel M, Sommer B. Two amyloid precursor protein transgenic mouse models with Alzheimer disease-like pathology. Proc Natl Acad Sci U S A. 1997 Nov 25;94(24):13287-92. PubMed.
9. Busche MA, Eichhoff G, Adelsberger H, Abramowski D, Wiederhold KH, Haass C, Staufenbiel M, Konnerth A, Garaschuk O. Clusters of hyperactive neurons near amyloid plaques in a mouse model of Alzheimer's disease. Science. 2008 Sep 19;321(5896):1686-9. PubMed.

External Citations

1. The Jackson Laboratory Stock# 030504

Further Reading