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Mouse Footpad Inoculation Model to Study Viral-Induced Neuroinflammatory Responses
Published on: June 14, 2020
Basic Science and Pathogenesis
Min Sung Gee1, Shimako Kawauchi1, Giedre Milinkeviciute1
1University of California, Irvine, Irvine, CA, USA.
Background:
The TREM2 R47H (rs75932628) coding variant has been identified as one of the most strongly associated genetic risk factors for late-onset Alzheimer's disease (LOAD). Previous studies revealed that this missense mutation causes microglia to be less reactive to the amyloid beta (Aβ) plaques, which might exacerbate the progression of Alzheimer's disease (AD) pathologies. In an effort to develop mouse models for LOAD, we have generated human TREM2 (hTREM2) knock-in and hTREM2-R47H knock-in mouse lines, fully replacing the murine Trem2 (mTrem2) sequence with its human TREM2 counterpart between start and stop codons, and crossed them with 5xFAD mouse line, a commonly used AD mouse model with robust amyloid-beta pathology. We anticipate that these mouse models will shed light on the impact of microglial TREM2-R47H variant in AD pathology development.
Method:
6 genotype groups were developed (WT, WT;hTREM2, WT;hTREM2-R47H, 5xFAD, 5xFAD;hTREM2, 5xFAD;hTREM2-R47H) with two age time points (4mo, 12mo). Mouse brains were isolated and cut in half; one hemisphere for immunohistochemistry (IHC) and the other for spatial transcriptomic analysis. Amyloid pathology and glial responses were measured via IHC. In spatial transcriptomic analysis using CosMx, transcriptomic characteristics and changes in each glial cell type were analyzed.
Result:
At 4 months of age, 5xFAD mice with the hTREM2-R47H variant showed increased amyloid plaque load in subiculum, and a significant spread of amyloid plaque burden in cortex area relative to both 5xFAD with murine Trem2, as well as with hTREM2. Microglia also failed to respond to plaques. CosMx results indicated that hTREM2-R47H knock-in microglia failed to downregulate homeostatic gene expression (i.e., P2ry12, Csf1r, Tgfbr1, and Tmem119) compared to hTREM2 knock-in line.
Conclusion:
Microglial reactivity to amyloid plaques was less in hTREM2 knock-in line compared to mTrem2, suggesting that hTREM2 itself is not fully functional in the murine biologic system. Nevertheless, compared to mTrem2 and hTREM2, hTREM2-R47H microglia do not down-regulate their homeostatic genes in response to amyloid-beta. Further, the magnitude of the effect of R47H in hTREM2 appears stronger than R47H in mTREM2, suggesting that humanization of disease relevant genes offers additional insights into disease mechanisms.
Insights
The TREM2 R47H variant exacerbates Alzheimer's disease pathology by impairing microglial response to amyloid plaques. Human TREM2 R47H knock-in mice show increased plaque load and failed homeostatic gene regulation.
Area of Science:
- Neuroscience
- Genetics
- Immunology
Background:
- The TREM2 R47H variant is a significant genetic risk factor for late-onset Alzheimer's disease (LOAD).
- This variant impairs microglial reactivity to amyloid-beta (Aβ) plaques, potentially accelerating Alzheimer's disease (AD) progression.
Purpose of the Study:
- To develop and characterize novel mouse models for LOAD using human TREM2 (hTREM2) and hTREM2-R47H knock-in strategies.
- To investigate the impact of the microglial TREM2-R47H variant on AD pathology development in vivo.
Main Methods:
- Generated hTREM2 and hTREM2-R47H knock-in mouse lines, crossed with 5xFAD mice.
- Analyzed amyloid pathology and glial responses using immunohistochemistry (IHC) at 4 and 12 months.
- Performed spatial transcriptomic analysis using CosMx to assess glial cell type-specific gene expression.
Main Results:
- 5xFAD mice with hTREM2-R47H exhibited increased amyloid plaque load and spread in the cortex and subiculum at 4 months.
- Microglia in hTREM2-R47H mice showed impaired response to plaques and failed to downregulate homeostatic genes (e.g., P2ry12, Tmem119).
- Human TREM2 (hTREM2) alone showed reduced functionality compared to murine Trem2 (mTrem2) in the mouse system.
Conclusions:
- Human TREM2 R47H microglia exhibit a distinct lack of homeostatic gene downregulation in response to amyloid-beta.
- The R47H variant's detrimental effect on microglial function appears stronger in the humanized TREM2 context.
- Humanization of disease-relevant genes like TREM2 provides valuable insights into Alzheimer's disease mechanisms.
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