Trem2 Deletion Reduces Late-Stage Amyloid Plaque Accumulation, Elevates the Aβ42:Aβ40 Ratio, and Exacerbates Axonal Dystrophy and Dendritic Spine Loss in the PS2APP Alzheimer's Mouse Model
- William J. Meilandt1,
- Hai Ngu2,
- Alvin Gogineni3,
- Guita Lalehzadeh1,
- Seung-Hye Lee1,
- Karpagam Srinivasan1,
- Jose Imperio1,
- Tiffany Wu1,
- Martin Weber1,
- Agatha J. Kruse3,
- Kimberly L. Stark1,
- Pamela Chan5,
- Mandy Kwong5,
- Zora Modrusan6,
- Brad A. Friedman4,
- Justin Elstrott3,
- Oded Foreman2,
- Amy Easton1,
- Morgan Sheng1, and
- David V. Hansen1
- 1Departments of Neuroscience,
- 2Pathology,
- 3Biomedical Imaging,
- 4Bioinformatics,
- 5Biochemical and Cellular Pharmacology, and
- 6Molecular Biology, Genentech, Inc., South San Francisco, California 94080
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Author contributions: W.J.M., K.L.S., J.E., A.E., M.S., and D.V.H. designed research; W.J.M., A.G., G.L., S.-H.L., K.S., J.I., T.W., M.W., A.J.K., P.C., M.K., and Z.M. performed research; W.J.M., H.N., A.G., S.-H.L., P.C., B.A.F., O.F., and D.V.H. analyzed data; W.J.M., H.N., M.W., A.E., M.S., and D.V.H. edited the paper; W.J.M. and D.V.H. wrote the paper; H.N. contributed unpublished reagents/analytic tools; D.V.H. wrote the first draft of the paper.
Abstract
TREM2 is an Alzheimer's disease (AD) risk gene expressed in microglia. To study the role of Trem2 in a mouse model of β-amyloidosis, we compared PS2APP transgenic mice versus PS2APP mice lacking Trem2 (PS2APP;Trem2ko) at ages ranging from 4 to 22 months. Microgliosis was impaired in PS2APP;Trem2ko mice, with Trem2-deficient microglia showing compromised expression of proliferation/Wnt-related genes and marked accumulation of ApoE. Plaque abundance was elevated in PS2APP;Trem2ko females at 6–7 months; but by 12 or 19–22 months of age, it was notably diminished in female and male PS2APP;Trem2ko mice, respectively. Across all ages, plaque morphology was more diffuse in PS2APP;Trem2ko brains, and the Aβ42:Aβ40 ratio was elevated. The amount of soluble, fibrillar Aβ oligomers also increased in PS2APP;Trem2ko hippocampi. Associated with these changes, axonal dystrophy was exacerbated from 6 to 7 months onward in PS2APP;Trem2ko mice, notwithstanding the reduced plaque load at later ages. PS2APP;Trem2ko mice also exhibited more dendritic spine loss around plaque and more neurofilament light chain in CSF. Thus, aggravated neuritic dystrophy is a more consistent outcome of Trem2 deficiency than amyloid plaque load, suggesting that the microglial packing of Aβ into dense plaque is an important neuroprotective activity.
SIGNIFICANCE STATEMENT Genetic studies indicate that TREM2 gene mutations confer increased Alzheimer's disease (AD) risk. We studied the effects of Trem2 deletion in the PS2APP mouse AD model, in which overproduction of Aβ peptide leads to amyloid plaque formation and associated neuritic dystrophy. Interestingly, neuritic dystrophies were intensified in the brains of Trem2-deficient mice, despite these mice displaying reduced plaque accumulation at later ages (12–22 months). Microglial clustering around plaques was impaired, plaques were more diffuse, and the Aβ42:Aβ40 ratio and amount of soluble, fibrillar Aβ oligomers were elevated in Trem2-deficient brains. These results suggest that the Trem2-dependent compaction of Aβ into dense plaques is a protective microglial activity, limiting the exposure of neurons to toxic Aβ species.
- Received August 1, 2019.
- Revision received December 8, 2019.
- Accepted December 23, 2019.
- Copyright © 2020 Meilandt et al.
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