Mutations: APP KM670/671NL (Swedish), APP E693G (Arctic), APP T714I (Austrian)
Modification: App: Knock-In
Disease Relevance: Alzheimer's Disease
Strain Name: B6(Cg)-Apptm1.1Dnli/J
Genetic Background: C57BL/6J
Availability: Available from The Jackson Laboratory Stock# 034711.
The AppSAA mouse joins a list of knock-in models that carry a humanized Aβ sequence within the murine App gene (App knock-in (humanized Aβ), APP NL-F Knock-in, APP NL-G-F Knock-in). As in the latter two models, the App gene was further modified to carry Alzheimer’s disease-linked mutations to accelerate plaque formation—in this case the Swedish (KM670/671NL), Arctic (E693G), and Austrian (T714I) mutations. AppSAA mice exhibit an age-dependent accumulation of amyloid plaques, cerebral amyloid angiopathy, and plaque-associated dystrophic neurites and microgliosis. The transcriptome of plaque-associated, phagocytic microglia from AppSAA mice was found to overlap with that of microglia from human AD brains.
In these knock-in mice, expression of App is driven by its natural promoter. App is expressed at normal levels (i.e., levels of App mRNA and full-length protein in homozygous and heterozygous knock-in brains are similar to wild-type mice) and is expected to show normal cell-type and temporal specificity. Levels of humanized APP and APP C-terminal fragments are elevated in a gene dose-dependent manner, with levels in heterozygotes about half those in homozygotes.
From 2 months of age, the Aβ42:Aβ40 ratio is higher in the brains of AppSAA homozygotes, compared with wild-type mice. Prior to plaque deposition, this difference is due to a reduction in the levels of Aβ40 in the knock-in mice relative to wild-type; at 4 months, when plaques begin to appear, the increased ratio reflects higher levels of Aβ42 in the knock-in brains.
Levels of TREM2 and various cytokines are also elevated in brain lysates from AppSAA homozygous mice.
Amyloid plaques appear in AppSAA homozygous mice at 4 months of age and in heterozygous mice by 16 months. Plaques are seen in multiple brain regions, with the most pronounced pathology in the cortex and hippocampus. Plaque deposition is accompanied by microgliosis and plaque-associated dystrophic neurites.
Cerebral amyloid angiopathy is observed in AppSAA homozygous mice from 8 months of age.
Transcriptomic analysis performed on microglia isolated from 8-month-old mice showed that more than 600 genes are differentially expressed in AppSAA homozygous and wild-type mice. Among the genes upregulated in the knock-in mice are disease-associated microglia (DAM) genes (Keren-Shaul et al., 2017; June 2017 news) and genes related to cholesterol metabolism, glycolysis, and phagocytic and lysosomal function. Few genes are differentially expressed in heterozygous AppSAA mice and wild-type mice at this age.
Microglia from AppSAA homozygous mice were further sorted into plaque-associated cells and non-plaque-associated cells. (Plaque-associated microglia were identified after phagocytosis in situ of amyloid fibrils labeled with the dye methoxy-X04.) Compared with unlabeled cells, methoxy-X04-labeled microglia showed increased expression of DAM genes, as well as activation of plaque-induced genes (Chen et al., 2020; July 2020 news). Genes involved in innate immunity and lipid clearance and metabolism were upregulated, while those involved in neuronal development, axon guidance, and spine morphogenesis were downregulated in the methoxy-X04-positive cells. Notably, there was overlap between the transcriptomic signature of the phagocytic microglia from AppSAA homozygous mice and microglia from human AD brains (Mathys et al., 2019; May 2019 news).
Lipidomics and Metabolomics
Significant alterations of the lipidome were identified in microglia from AppSAA homozygous mice, including upregulation of the ganglioside GM3 (d36:1), various species of triglyceride (TG) (including species of arachidonate-containing TG), as well as a variety of phospholipid species (i.e., phosphatidylglycerol, phosphatidylserine, and phosphatidylinositol).
Metabolomic analysis showed a pronounced increase in the levels of the polyamine spermine in methoxy-X04-labeled microglia.
At 2 and 4 months of age, the Aβ42:Aβ40 ratio in CSF and plasma are higher in AppSAA homozygous mice than in wild-type mice. It is not known whether levels of Aβ42 drop in the CSF with increasing plaque deposition in the knock-in animals.
CSF levels of total tau and neurofilament light chain are increased in the AppSAA homozygotes at 8 months of age, compared with wild-type mice.
The AppSAA knock-in mouse model was engineered by insertion of six mutations into the genomic App locus via homologous recombination. Three amino acids were substituted to humanize the mouse Aβ region: G676R (G5R), F681Y (F10Y), and R684H (R13H), numbered according to the 770-amino-acid isoform of human APP (position within the Aβ sequence). Additionally, the following three AD-linked mutations were inserted: KM670/671NL (Swedish), E693G (Arctic), and T714I (Austrian). This mouse model was created on a C57BL/6J background.
When visualized, these models will distributed over a 18 month timeline demarcated at the following intervals: 1mo, 3mo, 6mo, 9mo, 12mo, 15mo, 18mo+.
- Neuronal Loss
- Synaptic Loss
- Changes in LTP/LTD
- Cognitive Impairment
Amyloid plaques seen in AppSAA homozygous mice from 4 months of age and heterozygous mice at 16 months of age.
AT8-positive dystrophic neurites, but no neurofibrillary tangles, detected in AppSAA homozygous mice at 8 months of age.
Plaque-associated microgliosis observed by 4 months of age.
Changes in LTP/LTD
Last Updated: 16 Dec 2020
Research Models Citations
- Hot DAM: Specific Microglia Engulf Plaques
- Paper Alert: Those PIGs! Spatial Transcriptomics Add Human Data
- When It Comes to Alzheimer’s Disease, Do Human Microglia Even Give a DAM?
- 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.
- Chen WT, Lu A, Craessaerts K, Pavie B, Sala Frigerio C, Corthout N, Qian X, Laláková J, Kühnemund M, Voytyuk I, Wolfs L, Mancuso R, Salta E, Balusu S, Snellinx A, Munck S, Jurek A, Fernandez Navarro J, Saido TC, Huitinga I, Lundeberg J, Fiers M, De Strooper B. Spatial Transcriptomics and In Situ Sequencing to Study Alzheimer's Disease. Cell. 2020 Aug 20;182(4):976-991.e19. Epub 2020 Jul 22 PubMed.
- Mathys H, Davila-Velderrain J, Peng Z, Gao F, Mohammadi S, Young JZ, Menon M, He L, Abdurrob F, Jiang X, Martorell AJ, Ransohoff RM, Hafler BP, Bennett DA, Kellis M, Tsai LH. Single-cell transcriptomic analysis of Alzheimer's disease. Nature. 2019 Jun;570(7761):332-337. Epub 2019 May 1 PubMed.
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