Species: Mouse
Genes: TREM2, MAPT
Mutations: MAPT P301S
Modification: TREM2: Virus; MAPT: Transgenic
Disease Relevance: Frontotemporal Dementia, Alzheimer's Disease
Strain Name: N/A
Genetic Background: Tau P301S (Line PS19)
Availability: PS19 mice are available from The Jackson Lab: Stock# 008169; Live. Research with PS19 mice is available from Scantox Neuro.

Summary

Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) is a transmembrane receptor found on microglia, where it modulates cell activity and survival. In addition to membrane-associated TREM2, there are soluble forms of the protein—generated by protease cleavage of the extracellular domain or expression of alternative transcripts that lack a transmembrane domain.

AAV-sTREM2 PS19 mice were created to study the long-term effects of soluble TREM2 (sTREM2) in the context of tauopathy (Zhang et al., 2023). To generate this model, AAV carrying cDNA encoding EGFP- and FLAG-tagged sTREM2 (amino acids 1-171 of human TREM2) was injected into the hippocampi of 3-month-old PS19 mice. The PS19 line carries human a transgene encoding human tau (4R/1N) with the P301S mutation linked to frontotemporal dementia. PS19 mice accumulate neurofibrillary tangles, and display age-dependent synapse and neuron loss, gliosis, deficits in synaptic plasticity and behavioral impairments. AAV-mediated expression of sTREM2 protected against hippocampal synapse loss, improved performance in the Morris water maze and Y-maze, reduced levels p-tau202 and p-tau396, and decreased activity of glycogen synthase kinase-3 beta (GSK3β) in PS19 mice examined at 7 months of age.

Expression of sTREM2 | Markers of tauopathy | Synapse loss | Long-term potentiation | Behavior | Modification Details | Related Models

Expression of sTREM2

EGFP was expressed in neurons in the hippocampi of 7-month-old mice injected with AAV-EGFP-sTREM2 or AAV-EGFP four months previously. It was reported that levels of expression of TREM2 in the hippocampus were approximately 100 times higher in AAV-sTREM2 PS19 mice than mice who received the control vector. However, the ELISA kit used to measure TREM2 is marketed as human-specific, so it is unclear whether the assay accurately measured endogenous mouse TREM2.

Markers of tauopathy

Levels of p-tau202 and p-tau396 were reduced in AAV-sTREM2 PS19 mice, compared with PS19 mice who received control vector, as was the activity of GSK3β, a kinase known to phosphorylate tau.

Synapse loss

AAV-mediated expression of sTREM2 protected against hippocampal synapse loss in PS19 mice. The number of synapses observed by electron microscopy, the number of dendritic spines counted on Golgi-stained neurons, and levels of the presynaptic marker Vlgut1 and postsynaptic marker PSD95 were all greater in AAV-sTREM2 PS19 mice, compared with PS19 mice who received control vector.

Long-term potentiation

LTP at Shaeffer collateral-CA1 synapses was slightly enhanced in hippocampal slices from 7-month-old AAV-sTREM2 PS19 mice, compared with PS19 mice who had received control vector. Notably, AAV-sTREM2 also rescued LTP in 5xFAD mice, a transgenic model of amyloidosis (Zhong et al., 2019). These findings are congruent with the observation of enhanced LTP in Trem2-H157Y knock-in mice, in which levels of sTREM2 are elevated (Qiao et al., 2023).

Behavior

By 7 months of age, PS19 mice show deficits in spatial learning and memory in the Morris water maze. AAV-mediated expression of sTREM2 eliminated these deficits, with AAV-sTREM2 PS19 mice performing similarly to non-transgenic mice during the acquisition (learning) and probe (memory) trials. AAV-sTREM2 PS19 mice also performed better in the Y-maze test of working memory than did PS19 mice who received control vector, although their performance in this task did not rise to the level of non-transgenic mice.

Modification details

Adeno-associated viruses (AAV2/8) expressing EGFP- and FLAG-tagged human TREM2 (amino acids 1-171) under the control of the CAG promoter were injected bilaterally into the hippocampi of 3-month-old male PS19 mice. Control mice received injections of AAV expressing only EGFP.

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.

Trem2-IPDxAPP23xPS45. 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 early, but not late, stages of plaque deposition.

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.

Phenotype Characterization

Complementary Models

The AAV-sTREM2 PS19 model employed AAV-mediated over expression of an extracellular fragment of TREM2 in the brains of PS19 mice to study the effects of chronically elevated soluble TREM2 (sTREM2) in the context of tauopathy (Zhang et al., 2023).

Another approach to investigating the role of sTREM2 is the direct application of the protein to animals or cells, but these studies generally look at short-term effects. sTREM2-Fc is a recombinant, chimeric protein intended to mimic sTREM2, consisting of the human TREM2 extracellular domain (amino acids 1-171) fused to human IgG-Fc to aid in purification of the recombinant protein (Zhong et al., 2017). When applied to primary neurons from PS19 mice or HEK293 cells stably overexpressing GFP-Tau, sTREM2-Fc led to a dose-dependent decrease in levels of tau phosphorylated at sites targeted by GSK3β (Zhang et al., 2023). Conditioned medium from BV2 cells infected with the same AAV-sTREM2 vector used in the PS19 mice also reduced tau phosphorylation in the GFP-Tau-expressing HEK293 cells (Zhang et al., 2023).

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References

Research Models Citations

1. Tau P301S (Line PS19)
2. Trem2-H157Y knock-in
3. Trem2-H157Y x 5xFAD
4. TREM2-sol
5. Trem2 KO (KOMP)
6. TREM2-IPD
7. Trem2-IPDxAPP23xPS45
8. APP23
9. AAV-sTREM2 5xFAD

Mutations Citations

1. MAPT P301S
2. APP K670_M671delinsNL (Swedish)
3. PSEN1 G384A

Paper Citations

1. Zhang X, Tang L, Yang J, Meng L, Chen J, Zhou L, Wang J, Xiong M, Zhang Z. Soluble TREM2 ameliorates tau phosphorylation and cognitive deficits through activating transgelin-2 in Alzheimer's disease. Nat Commun. 2023 Oct 21;14(1):6670. PubMed.
2. Zhong L, Xu Y, Zhuo R, Wang T, Wang K, Huang R, Wang D, Gao Y, Zhu Y, Sheng X, Chen K, Wang N, Zhu L, Can D, Marten Y, Shinohara M, Liu CC, Du D, Sun H, Wen L, Xu H, Bu G, Chen XF. Soluble TREM2 ameliorates pathological phenotypes by modulating microglial functions in an Alzheimer's disease model. Nat Commun. 2019 Mar 25;10(1):1365. PubMed.
3. Qiao W, Chen Y, Zhong J, Madden BJ, Charlesworth CM, Martens YA, Liu CC, Knight J, Ikezu TC, Aishe K, Zhu Y, Meneses A, Rosenberg CL, Kuchenbecker LA, Vanmaele LK, Li F, Chen K, Shue F, Dacquel MV, Fryer J, Pandey A, Zhao N, Bu G. Trem2 H157Y increases soluble TREM2 production and reduces amyloid pathology. Mol Neurodegener. 2023 Jan 31;18(1):8. PubMed. Correction.
4. 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.
5. Zhang X, Tang L, Yang J, Meng L, Chen J, Zhou L, Wang J, Xiong M, Zhang Z. Soluble TREM2 ameliorates tau phosphorylation and cognitive deficits through activating transgelin-2 in Alzheimer's disease. Nat Commun. 2023 Oct 21;14(1):6670. PubMed.
6. Zhong L, Chen XF, Wang T, Wang Z, Liao C, Wang Z, Huang R, Wang D, Li X, Wu L, Jia L, Zheng H, Painter M, Atagi Y, Liu CC, Zhang YW, Fryer JD, Xu H, Bu G. Soluble TREM2 induces inflammatory responses and enhances microglial survival. J Exp Med. 2017 Mar 6;214(3):597-607. Epub 2017 Feb 16 PubMed.

External Citations

1. The Jackson Lab: Stock# 008169
2. Scantox Neuro

Further Reading