Temporal gene profiling of the 5XFAD transgenic mouse model highlights the importance of microglial activation in Alzheimer's disease

Abstract

Background: The 5XFAD early onset mouse model of Alzheimer's disease (AD) is gaining momentum. Behavioral, electrophysiological and anatomical studies have identified age-dependent alterations that can be reminiscent of human AD. However, transcriptional changes during disease progression have not yet been investigated. To this end, we carried out a transcriptomic analysis on RNAs from the neocortex and the hippocampus of 5XFAD female mice at the ages of one, four, six and nine months (M1, M4, M6, M9).

Results: Our results show a clear shift in gene expression patterns between M1 and M4. At M1, 5XFAD animals exhibit region-specific variations in gene expression patterns whereas M4 to M9 mice share a larger proportion of differentially expressed genes (DEGs) that are common to both regions. Analysis of DEGs from M4 to M9 underlines the predominance of inflammatory and immune processes in this AD mouse model. The rise in inflammation, sustained by the overexpression of genes from the complement and integrin families, is accompanied by an increased expression of transcripts involved in the NADPH oxidase complex, phagocytic processes and IFN-γ related pathways.

Conclusions: Overall, our data suggest that, from M4 to M9, sustained microglial activation becomes the predominant feature and point out that both detrimental and neuroprotective mechanisms appear to be at play in this model. Furthermore, our study identifies a number of genes already known to be altered in human AD, thus confirming the use of the 5XFAD strain as a valid model for understanding AD pathogenesis and for screening potential therapeutic molecules.

Figure 1

Overview of gene expression profiles in cortex and hippocampus of 5XFAD mice, at 4 different ages, reveals a shift in expression patterns between M1 and M4. (A) Number of up- (red) and down- (green) regulated genes in cortex and hippocampus of 5XFAD mice compared with wild type animals at M1, M4, M6 and M9. Total number of DEGs for each brain region at each age is reported in brackets. (B) Number of overlapping and non-overlapping DEGs in cortex (blue) and hippocampus (yellow) at M1, M4, M6 and M9. The center of the Venn diagram (green) illustrates the proportion of shared DEGs between both brain regions. (C and D) Number of shared and specific DEGs across all ages in cortex (C) and hippocampus (D). Analysis was based on the total number of DEGs, both up- and down-regulated at each age. Fold change (FC) cut-off used for above analyses was −1.5 > FC > 1.5 when comparing signals from 5XFAD mice with wild type controls.

Figure 2

The top ten most up-regulated genes at each age, in cortex and hippocampus, illustrate the alteration of inflammatory and immune processes from M4 onwards. Venn diagrams representing the top ten up-regulated genes in cortex (A) and hippocampus (B) of 5XFAD mice at M1 (blue), M4 (yellow), M6 (green) and M9 (red). Genes are listed by rank of fold change, the cut-off for analysis being FC > 1.5 when comparing gene probe signals from 5XFAD with wild type mice. Note that only Ttr dysregulation is present at both M1 and M4 in the cortex while the other 9 genes are specific to M1. However, at M4, M6 and M9, 60% and 40% of these genes are consistently dysregulated in the cortex and the hippocampus, respectively.

Figure 3

Metabolic pathways associated to gene dysregulation in 5XFAD cortex and hippocampus at M4, M6 and M9: common versus age-specific processes. Biological functions and metabolic pathways associated to gene expression dysregulation were identified using Ingenuity Pathway Analysis (IPA). (A) Data from both the cortex and hippocampus were analyzed as one dataset and the main metabolic pathways affected were clustered according to age (M4, blue; M6, yellow; M9, red). The metabolic pathways affected across all three ages are represented in white and are related to inflammatory and immune responses. (B) The main metabolic pathway affected at M4, M6 and M9 relates to infectious disease, cellular function and antigen presentation.

Figure 4

The complement-induced inflammation pathway: an important mediator of neuroinflammatory processes from M4 onwards. This functional network was designed using the text-mining software, PredictSearch, based on more stringent criteria than for the above figure: only upregulated DEGs (with a FC > 1.5) found in both the cortex and hippocampus and at consecutive time points (M4/M6; M6/M9 or M4/M6/M9) were considered for analysis. Top right corner: legend for Figures 4, 5, 6, 7.

Figure 5

A large proportion of upregulated genes belong to the family of IFNγ-induced genes: engagement of inflammatory, immune, autophagic and phagocytic pathways. Legend for this figure is located in the top right corner of Figure 4.

Figure 6

Increased expression of genes involved in the NADPH oxidase complex. Legend for this figure is located in the top right corner of Figure 4.

Figure 7

Increased microglial activation and induction of associated neuroprotective signaling pathways. Legend for this figure is located in the top right corner of Figure 4.