Temporal and brain region-specific elevations of soluble Amyloid-β40-42 in the Ts65Dn mouse model of Down syndrome and Alzheimer's disease

Abstract

Down syndrome (DS) is a leading cause of intellectual disability that also results in hallmark Alzheimer's disease (AD) pathologies such as amyloid beta (Aβ) plaques and hyperphosphorylated tau. The Ts65Dn mouse model is commonly used to study DS, as trisomic Ts65Dn mice carry 2/3 of the triplicated gene homologues as occur in human DS. The Ts65Dn strain also allows investigation of mechanisms common to DS and AD pathology, with many of these triplicated genes implicated in AD; for example, trisomic Ts65Dn mice overproduce amyloid precursor protein (APP), which is then processed into soluble Aβ_40-42 fragments. Notably, Ts65Dn mice show alterations to the basal forebrain, which parallels the loss of function in this region observed in DS and AD patients early on in disease progression. However, a complete picture of soluble Aβ_40-42 accumulation in a region-, age-, and sex-specific manner has not yet been characterized in the Ts65Dn model. Here, we show that trisomic mice accumulate soluble Aβ_40-42 in the basal forebrain, frontal cortex, hippocampus, and cerebellum in an age-specific manner, with elevation in the frontal cortex and hippocampus as early as 4 months of age. Furthermore, we detected sex differences in accumulation of Aβ_40-42 within the basal forebrain, with females having significantly higher Aβ_40-42 at 7-8 months of age. Lastly, we show that APP expression in the basal forebrain and hippocampus inversely correlates with Aβ_40-42 levels. This spatial and temporal characterization of soluble Aβ_40-42 in the Ts65Dn model allows for further exploration of the role soluble Aβ plays in the progression of other AD-like pathologies in these key brain regions.

Keywords: Amyloid-β40-42; Ts65Dn; basal forebrain; down syndrome; hippocampus.

FIGURE 1

Soluble Aβ_40‐42 levels at 4 months of age across 4 separate brain regions. (a, b) At 4 months of age, soluble Aβ_40‐42 is not elevated in the BF of trisomic (3N) vs. diploid (2N) Ts65Dn mice. (c, d) 3N mice showed elevated Aβ_40‐42 in both the FCtx and (e, f) the Hp compared to their age‐matched 2N controls. (g, h) Cere tissue showed no significant differences in Aβ_40‐42 between 2N and 3N mice. Data are presented as means ± SEM. 2N vs 3N bars to the right illustrate the observed main effects of genotype. *p < 0.05, **p < 0.01, ***p < 0.001

FIGURE 2

Soluble Aβ_40‐42 levels are elevated in all brain regions of 3N mice compared to the 2N counterparts by 7–8 months of age, with observed sex‐specific differences. At 7–8 months of age, soluble Aβ_40‐42 is elevated in 3N mice in all four brain regions. (a) In the BF, soluble Aβ₄₀ is significantly elevated in 3N vs. 2N mice and in females vs. males; a significant interaction was also shown, with 3N females showing the highest levels. (b) Aβ₄₂ is significantly elevated in 3N vs. 2N mice and in females vs. males. (c–h) 3N mice showed elevated Aβ_40‐42 levels in the FCtx, the Hp, and the Cere compared to 2N mice. Data are presented as means ± SEM. 2N vs 3N bars to the right illustrate the observed main effects of genotype. **p < 0.01, ***p < 0.001, ****p < 0.0001

FIGURE 3

Soluble Aβ_40‐42 levels are elevated in 3N mice compared to 2N counterparts at 12 months of age. At 12 months of age, soluble Aβ_40‐42 is significantly elevated in 3N vs. 2N mice in all four brain regions: (a–h) BF, FCtx, Hp, and Cere. No significant sex effects or interactions were observed at this age. Data are presented as means ± SEM. 2N vs 3N bars to the right illustrate the observed main effects of genotype. ****p < 0.0001

FIGURE 4

Soluble Aβ_40‐42 in 3N mice changes as a function of age. (a) Aβ₄₀ in the BF rises across age, with females showing a dramatic increase from 4 to 7–8 months. (b) Aβ₄₂ in the BF rises significantly but less dramatically over time, particularly from 4 to 7–8 months. (c) Aβ₄₀ rises significantly in the FCtx from 4 to 7–8 months before plateauing. (d) Aβ₄₂ in the FCtx shows a significant increase between 4 and 7–8 months before plateauing. (e) Aβ₄₀ in the Hp rises from 4 to 7–8 months in both sexes but also rises significantly in females from 7–8 to 12 months. (f) Aβ₄₂ rises significantly in the Hp from 7–8 to 12 months. (g‐h) In the Cere, Aβ₄₀ but not Aβ₄₂ rises significantly across age in males, while significant increases in Aβ_40‐42 are seen only in females from 4 to 7–8 months. Data are presented as means ± SEM. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001

FIGURE 5

Full‐length APP is inversely correlated with soluble Aβ_40‐42 in the BF and Hp. (a, b) Soluble Aβ₄₀ and Aβ₄₂ are each significantly inversely correlated with APP levels in the BF of 3N mice. (c, d) Soluble Aβ₄₀ and Aβ₄₂ are each significantly inversely correlated with APP levels in the Hp of 3N mice. (e–h) Neither soluble Aβ₄₀ nor Aβ₄₂ in the FCtx or Cere are correlated to APP levels in 3N mice

FIGURE 6

Brain region dissection protocol. Schematic of tissue extraction protocol showing isolation of the basal forebrain (BF), frontal cortex (FCtx), hippocampus (Hp), and cerebellum (Cere) from whole mouse brain