Research Models

Trem2 KO (Colonna) x 5XFAD

Synonyms: Trem2-/-5XFAD, mTrem2-/-5XFAD 

Species: Mouse
Genes: Trem2, APP, PSEN1
Mutations: APP KM670/671NL (Swedish), APP I716V (Florida), APP V717I (London), PSEN1 M146L (A>C), PSEN1 L286V
Modification: Trem2: Knock-Out; APP: Transgenic; PSEN1: Transgenic
Disease Relevance: Alzheimer's Disease
Strain Name: C57BL/6 -TREM2tm1cln; B6SJL-Tg(APPSwFlLon,PSEN1*M146L*L286V)6799Vas/Mmja
Genetic Background: C57BL/6
Availability: Trem2 KO: available through Marco Colonna. 5XFAD: The Jackson Lab; available through the JAX MMRRC Stock# 034848; Live

Summary

Loss-of-function mutations in TREM2 cause Nasu-Hakola disease (also known as polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy) (Paloneva et al., 2002), a rare, autosomal-recessive disorder characterized by bone fractures and early onset frontotemporal dementia (Paloneva et al., 2002). TREM2 variants also have been associated with frontotemporal dementia in the absence of bone abnormalities (Chouery et al., 2008; Guerreiro et al., 2013; Guerreiro et al., 2013; LaBer et al., 2014). Some variants may confer increased risk for Alzheimer’s disease and other neurodegenerative disorders (Jay et al., 2017; Yeh et al., 2017).

To investigate the influence of loss of TREM2 function on amyloid pathology and plaque-associated neuroinflammation, 5XFAD mice were crossed with Trem2-/- mice (Turnbull et al., 2006). 5XFAD mice express human APP and human presenilin-1, with a total of five AD-linked mutations, and aggressively deposit amyloid plaques, starting at six weeks of age. Trem2-/- mice were generated by targeted deletion of Trem2 exons 3 and 4. All mice were on a C57BL/6 background (Ulland et al., 2017).

Microglial Phenotype

The numbers of microglia are similar in Trem2+/+5XFAD and Trem2-/-5XFAD at four months of age (Wang et al., 2016), but at eight months of age, Trem2-/-5XFAD have fewer total microglia (Wang et al., 2016) and an increased number of apoptotic microglia (Wang et al., 2015).

TREM2 insufficiency in 5XFAD mice leads to an altered microglial response to amyloid plaques, already apparent by four months. Compared with Trem2+/+5XFAD, significantly fewer microglia surround plaques in 5XFAD mice lacking or haploinsufficient for TREM2 (Wang et al., 2016). Microglial processes in Trem2+/+5XFAD orient toward and envelop plaques, and processes contacting plaques are enriched in phospho-tyrosine, a marker of kinase activation; microglia in Trem2-/-5XFAD and Trem2+/-5XFAD are less polarized, with dysmorphic processes that do not follow plaque borders and do not show evidence of kinase activation (Yuan et al., 2016). The failure of microglia to concentrate around plaques persists in eight-month Trem2-/-5XFAD and Trem2+/-5XFAD mice (Wang et al., 2015).

Microglia in eight-month Trem2-/-5XFAD mice contain abundant autophagosomes, rarely seen in Trem2+/+5XFAD (Ulland et al., 2017).

Microglia from Trem2-/-5XFAD have a transcriptional profile intermediate between Trem2+/+5XFAD and wild-type mice (Wang et al., 2015).

In vitro, microglia derived from Trem2-/-5XFAD mice are less viable than cells from Trem2+/+5XFAD (Wang et al., 2015).Trem2-/-5XFAD microglia less efficiently phagocytize Aβ than do microglia from Trem2+/+5XFAD or Trem2+/-5XFAD (Wang et al., 2016; Yuan et al., 2016).

Aβ Accumulation

At four months of age, Trem2+/+5XFAD, Trem2+/-5XFAD and Trem2-/-5XFAD mice do not differ in terms of total plaque burden or levels of insoluble Aβ40 and Aβ42 (Wang et al., 2016; Yuan et al., 2016). However, the decreased microglial coverage in mice lacking TREM2 is accompanied by changes in plaque morphology. While Thioflavin S-labeled plaques have distinct borders in Trem2+/+5XFAD, plaques inTREM2-deficient mice lack sharp borders and sport “spike-like” extensions. Trem2+/-5XFAD and Trem2-/-5XFAD also have a significantly larger proportion of filamentous plaques—Thioflavin S-labeled plaques without a discernible core—than do Trem2+/+5XFAD mice.

At eight months of age, Trem2-/-5XFAD have greater plaque burdens and elevated levels of insoluble Aβ40 and Aβ42 in the hippocampus, but not in cortex, compared with Trem2+/+5XFAD (Wang et al., 2015). The relative abundance of modified Aβ42 peptides (e.g., Aβp3-42) within plaques also differs between Trem2+/+5XFAD and 5XFAD deficient in TREM2 (Wang et al., 2016).

Neuron Health

The loss of layer V neurons seen in 5XFAD mice (Oakley et al., 2006; Jawhar et al., 2012) is exaggerated in Trem2-/-5XFAD and Trem2+/-5XFAD animals, in a gene-dose-dependent manner (Wang et al., 2015). Plaque-associated neuritic dystrophy is more severe in Trem2-/-5XFAD than in Trem2+/+5XFAD (Wang et al., 2016Yuan et al., 2016).

Modification Details

5XFAD mice were crossed with Trem2 KO (Colonna) mice. All mice were on a C57BL/6 background.

Related Strains

Trem2 KO (Colonna) x APPPS1 - The effects of TREM2 deficiency also have been studied in APPPS1 mice, which express human APP and PSEN1, with the AD-linked Swedish and L166P mutations, respectively. Trem2  haploinsufficiency had no effect on cortical plaque deposition in three- or seven-month mice (Ulrich et al., 2014), but Trem2-/-APPPS1mice had lower plaque burdens than APPPS1 mice, studied at four months of age (Krasemann et al., 2017). A reduction in microglial clustering around plaques was seen in APPPS1 mice deficient in TREM2, studied at three months of age (Ulrich et al., 2014; Wang et al., 2015). The microglia that did associate with neuritic plaques in Trem2-/-APPPS1 mice did not undergo the switch from a homeostatic to a “microglial neurodegenerative phenotype,” seen in microglia in APPPS1 mice that express Trem2 (Krasemann et al., 2017).

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

No Data

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

Plaques

Plaques present by 4 months, the earliest age studied.

Tangles

No data.

Neuronal Loss

Loss of cortical layer V neurons by 8 months, the earliest age studied.

Gliosis

MIcrogliosis by 4 months, the earliest age studied.

Synaptic Loss

No data.

Changes in LTP/LTD

No data.

Cognitive Impairment

No data.

COMMENTS / QUESTIONS

  1. Neuroinflammatory mechanisms have emerged as an important feature and pathogenic event in Alzheimer’s disease. Recently, mutations in TREM2 have been identified as risk factors for the development of the sporadic type of this neurodegenerative disease. In this study, the Colonna group publishes the first profound analysis of microglial TREM2 function in murine AD models. One of the most intriguing findings of this study is that TREM2-deficient microglia seem to show an impaired reaction to Aβ deposition. Since TREM2-deficient 5xFAD mice showed an increase in the overall Aβ load in the hippocampus, this suggest that, at the investigated time point, TREM2 mediated mechanisms that restrict the deposition of Aβ. In keeping with this, TREM2 deficiency impaired microglial recruitment to the site of Aβ deposition. Importantly, the authors excluded, at least by in vitro experiments, that TREM2 deficiency affects microglia Aβ phagocytosis or degradation directly.

    Instead TREM2 seems to be involved in microglial survival mechanisms and TREM2 deficiency increased microglial apoptosis, possibly linked to restricted colony-stimulating factor 1 levels. Alternatively, TREM2-deficient cells may harm themselves by an increased release of TNFα, although several types of microglial cell death need to be considered (Kim and Li , 2013; Jung et al.  2005). Thus, TREM2-deficient microglia seem to not  survive the Aβ challenge and therefore fail to mount an appropriate clearance response, in line with previous findings showing that improving microglial phagocytosis in vivo can restrict Aβ deposition (Heneka et al., 2013).

    Another important finding of this study is that TREM2 is not activated by Aβ itself, as previously suggested, but by certain anionic membrane phospholipids, a a response that  was severely limited by the human R47H mutation, which has been linked to sporadic AD. Therefore, TREM2 expression at Aβ plaque sites (Frank et al., 2008Lue et al., 2014) can be interpreted as an attempt to survive the local inflammatory and toxic milieu, a prerequisite to restrict Aβ accumulation by phagocytosis or release of degrading proteases.

    Overall this study further highlights the role of microglia in neurodegeneration and in particular in Alzheimer’s disease. Similar to previous studies, (e.g., Bradshaw et al., 2013) it points to microglial uptake and degradation of Aβ as an important method for restricting the peptide's accumulation. Given the plethora of GWAS-identified mutations that are potentially linked to immune function (Lambert et al.,  2013), it can be expected that further disease-relevant microglial functions will be discovered.

    Naturally, these findings fuel the hope of developing therapeutics that modify microglia functions. For this to be successful, we need to consider not only the different innate immune mechanisms, but the precise disease stage when they manifest.

    References:

    . Caspase blockade induces RIP3-mediated programmed necrosis in Toll-like receptor-activated microglia. Cell Death Dis. 2013 Jul 11;4:e716. PubMed.

    . Pro-apoptotic activity of N-myc in activation-induced cell death of microglia. J Neurochem. 2005 Jul;94(1):249-56. PubMed.

    . NLRP3 is activated in Alzheimer's disease and contributes to pathology in APP/PS1 mice. Nature. 2013 Jan 31;493(7434):674-8. Epub 2012 Dec 19 PubMed.

    . TREM2 is upregulated in amyloid plaque-associated microglia in aged APP23 transgenic mice. Glia. 2008 Oct;56(13):1438-47. PubMed.

    . TREM2 Protein Expression Changes Correlate with Alzheimer's Disease Neurodegenerative Pathologies in Post-Mortem Temporal Cortices. Brain Pathol. 2014 Sep 3; PubMed.

    . CD33 Alzheimer's disease locus: altered monocyte function and amyloid biology. Nat Neurosci. 2013 Jul;16(7):848-50. PubMed.

    . Meta-analysis of 74,046 individuals identifies 11 new susceptibility loci for Alzheimer's disease. Nat Genet. 2013 Dec;45(12):1452-8. Epub 2013 Oct 27 PubMed.

    View all comments by Michael Heneka
  2. The intriguing finding of the Wang et al. study is that lipids “activate” wild-type TREM2 and turn on NFAT signaling pathways whereas the R47H variant of TREM2, which is a risk allele for AD, PD, FTD, and ALS, is almost completely inactive in the NFAT reporter assay.

    NFAT singling regulates expression of pro-inflammatory cytokines TNF-α, IL-2, IFNg, etc. Thus, the charged lipids—presumably released from apoptotic neurons and coating the amyloid plaquesshould activate WT microglia and stimulate the release of inflammatory cytokines, whereas those expressing R47H-TREM2 should not. By implication, WT-TREM2 should be proinflammatory and R47H-TREM2 should not promote inflammation in response to apoptotic cells. This seems to run counterintuitive to the common finding that increased inflammation is observed in the brains of patients with all four neurodegenerative diseases indicated above, and there is increasing evidence that chronic neuroinflammation is toxic to the brain function and initiates neurodegeneration.

    One thing to keep in mind is that the NFAT reporter assay was performed by overexpressing WT-TREM2 or R47H-TREM2 in 2B4 reporter T-cells. TREM2 has an extremely short cytoplasmic tail and is known to signal by binding another membrane protein ,DAP12/TYRO-BP, which possesses a longer cytoplasmic tail with an immunoreceptor tyrosine-based activation (ITAM) motif. From the information in the manuscript, it seems that Wang et al. transfected TREM2 alone and not TREM2+DAP12. Also unclear is whether 2B4 T-cells express endogenous DAP12 and if so, what the stoichiometry of overexpressed TREM2 to endogenous DAP12 is. Thus, at present it remains an open question whether the effects of R47H mutation on NFAT signaling reported here shed any light on the role of TREM2 in AD pathogenesis or are due to overexpression of the protein in a non-microglial cell line. Future studies will need to resolve the conundrum of how R47H-TREM2, which does not seem to promote inflammation, increases the risk for neurodegeneration.

    View all comments by Sanjay Pimplikar

Make a comment or submit a question

To make a comment you must login or register.

References

Research Models Citations

  1. 5xFAD (C57BL6)
  2. Trem2 KO (Colonna)
  3. APPPS1

Paper Citations

  1. . Mutations in two genes encoding different subunits of a receptor signaling complex result in an identical disease phenotype. Am J Hum Genet. 2002 Sep;71(3):656-62. Epub 2002 Jun 21 PubMed.
  2. . Polycystic Lipomembranous Osteodysplasia with Sclerosing Leukoencephalopathy (PLOSL). In: Adam MP, Ardinger HH, Pagon RA, Wallace SE, Bean LJH, Mefford HC, Stephens K, Amemiya A, Ledbetter N, editors. SourceGeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2017. 2002 Jan 24 [updated 2015 Mar 12].
  3. . Mutations in TREM2 lead to pure early-onset dementia without bone cysts. Hum Mutat. 2008 Sep;29(9):E194-204. PubMed.
  4. . Using exome sequencing to reveal mutations in TREM2 presenting as a frontotemporal dementia-like syndrome without bone involvement. JAMA Neurol. 2013 Jan;70(1):78-84. PubMed.
  5. . Novel compound heterozygous mutation in TREM2 found in a Turkish frontotemporal dementia-like family. Neurobiol Aging. 2013 Dec;34(12):2890.e1-5. Epub 2013 Jul 17 PubMed.
  6. . Homozygous TREM2 mutation in a family with atypical frontotemporal dementia. Neurobiol Aging. 2014 Oct;35(10):2419.e23-2419.e25. Epub 2014 Apr 18 PubMed.
  7. . TREM2 in Neurodegenerative Diseases. Mol Neurodegener. 2017 Aug 2;12(1):56. PubMed.
  8. . TREM2, Microglia, and Neurodegenerative Diseases. Trends Mol Med. 2017 Jun;23(6):512-533. Epub 2017 Apr 22 PubMed.
  9. . Cutting edge: TREM-2 attenuates macrophage activation. J Immunol. 2006 Sep 15;177(6):3520-4. PubMed.
  10. . TREM2 Maintains Microglial Metabolic Fitness in Alzheimer's Disease. Cell. 2017 Aug 10;170(4):649-663.e13. PubMed.
  11. . TREM2-mediated early microglial response limits diffusion and toxicity of amyloid plaques. J Exp Med. 2016 May 2;213(5):667-75. Epub 2016 Apr 18 PubMed.
  12. . TREM2 lipid sensing sustains the microglial response in an Alzheimer's disease model. Cell. 2015 Mar 12;160(6):1061-71. Epub 2015 Feb 26 PubMed.
  13. . TREM2 Haplodeficiency in Mice and Humans Impairs the Microglia Barrier Function Leading to Decreased Amyloid Compaction and Severe Axonal Dystrophy. Neuron. 2016 May 18;90(4):724-39. PubMed.
  14. . Intraneuronal beta-amyloid aggregates, neurodegeneration, and neuron loss in transgenic mice with five familial Alzheimer's disease mutations: potential factors in amyloid plaque formation. J Neurosci. 2006 Oct 4;26(40):10129-40. PubMed.
  15. . Motor deficits, neuron loss, and reduced anxiety coinciding with axonal degeneration and intraneuronal Aβ aggregation in the 5XFAD mouse model of Alzheimer's disease. Neurobiol Aging. 2012 Jan;33(1):196.e29-40. PubMed.
  16. . Altered microglial response to Aβ plaques in APPPS1-21 mice heterozygous for TREM2. Mol Neurodegener. 2014 Jun 3;9:20. PubMed.
  17. . The TREM2-APOE Pathway Drives the Transcriptional Phenotype of Dysfunctional Microglia in Neurodegenerative Diseases. Immunity. 2017 Sep 19;47(3):566-581.e9. PubMed.

Other Citations

  1. Swedish

External Citations

  1. JAX MMRRC Stock# 034848

Further Reading

Papers

  1. . 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.