Species: Mouse
Genes: Trem2
Mutations: TREM2 R47H
Modification: Trem2: Knock-In
Disease Relevance: Alzheimer's Disease
Strain Name: B6(SJL)-Trem2^em1Aduci/J
Genetic Background: C57BL/6J
Availability: Available from The Jackson Laboratory, Stock No. 034036.

Summary

R47H is a rare variant in TREM2 that triples the risk of Alzheimer’s disease in heterozygous carriers. In order to study the effects of this variant, several mouse models were created in which the R47H mutation was introduced into the mouse Trem2 gene (Trem2 R47H KI (JAX), Trem2 R47H KI (Haass), Trem2 R47H KI (Lamb/Landreth)). Unfortunately, these early R47H knock-in mice had decreased expression of Trem2, unlike human carriers of the R47H mutation. This difference in expression was traced to aberrant splicing of the mutant allele (Xiang et al., 2018; see Sep 2018 news): Introduction of the R47H point mutation into the mouse Trem2 sequence created a cryptic splice acceptor site in exon 2, generating a transcript with a premature stop codon subject to nonsense-mediated decay.

The Trem2*R47H^NSS line—for “normal splice site”—was created without the problem of aberrant splicing of the mouse Trem2 gene. Trem2*R47H^NSS mice generate the same Trem2 isoforms as wild-type mice, in normal amounts. Homozygous R47H carriers exhibit age-dependent differences in microglial morphology, synaptic features, and gene expression, compared with wild-type mice. Trem2*R47H^NSS mice generated an inflammatory response to experimental demyelination largely similar to that seen in wild-type animals. When crossed with 5xFAD, a model of aggressive amyloidosis, inflammatory responses were suppressed in younger animals, but exacerbated in older animals homozygous for the R47H variant. This model is available to academic and commercial users from The Jackson Laboratory, without restrictions.

The following description refers to mice homozygous for the Trem2 R47H allele. (It should be noted that the vast majority of human R47H carriers reported in the literature are heterozygotes.)

Trem2 mRNA expression and splicing were normal in the brains of Trem2*R47H^NSS mice. Mice carrying the R47H variant generated the same three Trem2 isoforms, at comparable levels, as animals homozygous for the wild-type Trem2 allele. Trem2*R47H^NSS mice did not express the aberrant transcript seen in prior Trem2 R47H knock-in lines.

In the brain, TREM2 is expressed primarily by microglia, where it regulates cell activity and survival. Microglial morphology differed between homozygous Trem2*R47H^NSS and wild-type mice at 4 months of age: R47H homozygotes had smaller microglia, with longer, thinner processes. At 12 months, microglial volume no longer differed between genotypes.

Trem2*R47H^NSS mice exhibited age-dependent synaptic deficits. At 4 months of age, paired-pulse facilitation, long-term potentiation at Schaffer collateral-CA1 synapses, and co-localization of the presynaptic marker bassoon and postsynaptic marker PSD95 were similar in in the hippocampi of homozygous Trem2*R47H^NSS and wild-type mice. However, by 12 months, homozygous Trem2*R47H^NSS showed impaired basal synaptic transmission and LTP, as well as synapse loss.

Degenerating myelin and aggregated Aβ can act as TREM2 ligands. To test whether R47H acts as a loss-of-function allele that impairs microglial responses to stimuli, homozygous Trem2*R47H^NSS mice were subjected to experimental demyelination or crossed with amyloid-plaque-depositing mice (discussed below).

Administration of the copper chelator cuprizone caused similar degrees of demyelination in Trem2*R47H^NSS and wild-type mice and led to pronounced microgliosis in white-matter tracts. Expression of inflammatory genes—including Trem2 itself and the DAM (disease-associated microglia; Keren-Shaul et al., 2017) signature genes Apoe, Clec7a, and Itgax—were similarly upregulated in homozygous Trem2*R47H^NSS and wild-type mice. Genes associated with myelin and oligodendrocytes were downregulated in both genotypes, but more so in the R47H carriers, whose response resembled that of Trem2 knockout mice in the same study. In contrast to wild-type mice, Trem2*R47H^NSS mice downregulated expression of a set of genes related to striatal dopaminergic signaling.

Trem2*R47H^NSS/5xFAD

Trem2*R47H^NSS mice were intercrossed with 5xFAD mice to study the effects of the R47H variant in the context of amyloidosis. Progeny homozygous for the Trem2 R47H allele and hemizygous for the 5xFAD transgenes (hereafter referred to as “Trem2*R47H^NSS/5xFAD”) were compared with mice carrying the 5xFAD transgenes on a wild-type Trem2 background (“5xFAD”).

The effects of the R47H variant on plaque density and size were sex-, age-, and region-dependent. At 4 months, male Trem2*R47H^NSS/5xFAD mice had lower plaque densities in the cortex and subiculum than their 5xFAD counterparts. Plaques in the subiculum of R47H carriers were smaller for both sexes combined, but female R47H carriers had larger plaques in the cortex, compared with mice expressing wild-type Trem2. These genotype-dependent differences in plaque size disappeared by 12 months, but plaque density was greater in the subiculum of R47H carriers.

The effects of the R47H variant on microglial responses to plaques also varied by sex, age, and region. At 4 months, microglial density in the cortices of female Trem2*R47H^NSS/5xFAD was higher than 5xFAD, while the opposite relationship was seen in the subiculum (when both sexes were combined). Microglial-plaque interactions, quantified as co-localization of Thioflavin S staining (plaques) and Iba1 immunoreactivity (microglia), were impaired in R47H carriers, with females being more strongly affected. These genotype-dependent differences disappeared by 12 months.

Plaque-associated neuronal injury, assessed as the volume of dystrophic neurites normalized to plaque volume, was exacerbated in R47H carriers at 4 months of age. At one year, the genotypes no longer differed with regard to this measure. However, levels of plasma neurofilament-light chain, a putative biomarker for neuronal injury, were higher in Trem2*R47H^NSS/5xFAD than 5xFAD at both 4 and 12 months of age.

5xFAD mice show impaired long-term potentiation and hippocampal synapse loss, compared with wild-type mice not carrying the APP and PSEN1 transgenes. Surprisingly homozygosity for the Trem2 R47H variant appeared to protect against these abnormalities in 5xFAD brains, ameliorating the deficits in LTP and restoring synapse numbers—assessed as the co-localization of bassoon and PSD95—to wild-type levels.

Transcriptomic analysis revealed that the R47H allele suppressed the upregulation of inflammation-related genes seen in 5xFAD at 4 months of age but exacerbated this inflammatory response in 12-month-old mice. In addition, Trem2*R47H^NSS/5xFAD mice upregulated expression of genes involved in interferon signaling, a response not seen in 5xFAD. Sex differences in differentially expressed genes were also apparent: Compared with 5xFAD females, Trem2*R47H^NSS/5xFAD females showed downregulation of genes associated with dendritic development, synaptic transmission, circadian rhythms, and axonal guidance.

Modification Details

CRISPR/Cas9 was used to edit the mouse Trem2 gene, introducing the R47H point mutation and 10 silent mutations in exon 2. No off-target mutations were found on chromosome 17 (i.e. where Trem2 is located) in mice backcrossed to wild-type C57BL/6J mice for three generations.

Phenotype Characterization

Q&A with Model Creator

Q&A with Angela Gomez‑Arboledas, Grant MacGregor, and Kim Green.

What would you say are the unique advantages of this model?

This model contains the LOAD risk coding variant for TREM2 R47H. Previous R47H variants in the murine Trem2 locus have induced artefactual splice variants and as a result show markedly diminished Trem2 levels. This model allows for the exploration of this variant without this confound.

What do you think this model is best used for?

Exploration of the impacts of the TREM2 R47H variant on aging, development of neuropathology in appropriate models, inflammation etc.

What caveats are associated with this model?

This model contains the R47H variant in the murine Trem2/TREM2 sequence. It is possible that the same variant in the human sequence has additional or different effects. We have also produced mice in which the murine Trem2 locus is replaced with the human TREM2 locus, with either a common variant (Jax # 038103) or the R47H variant (Jax # 037497) to explore this.

COMMENTS / QUESTIONS

No Available Comments

Make a comment or submit a question

To make a comment you must login or register.

References

Research Models Citations

1. Trem2 R47H KI (JAX)
2. Trem2 R47H KI (Haass)
3. Trem2 R47H KI (Lamb/Landreth)
4. 5xFAD (C57BL6)

News Citations

1. Model Morass? R47H Mutation Scuttles TREM2 Expression in Mice, Not People 13 Sep 2018

Paper Citations

1. Xiang X, Piers TM, Wefers B, Zhu K, Mallach A, Brunner B, Kleinberger G, Song W, Colonna M, Herms J, Wurst W, Pocock JM, Haass C. The Trem2 R47H Alzheimer's risk variant impairs splicing and reduces Trem2 mRNA and protein in mice but not in humans. Mol Neurodegener. 2018 Sep 6;13(1):49. PubMed.
2. Keren-Shaul H, Spinrad A, Weiner A, Matcovitch-Natan O, Dvir-Szternfeld R, Ulland TK, David E, Baruch K, Lara-Astaiso D, Toth B, Itzkovitz S, Colonna M, Schwartz M, Amit I. A Unique Microglia Type Associated with Restricting Development of Alzheimer's Disease. Cell. 2017 Jun 15;169(7):1276-1290.e17. Epub 2017 Jun 8 PubMed.

External Citations

1. The Jackson Laboratory, Stock No. 034036
2. Jax # 038103
3. Jax # 037497

Further Reading

No Available Further Reading