Age Worsens the Cognitive Phenotype in Mice Carrying the Thr92Ala-DIO2 Polymorphism

Abstract

The Thr92Ala-Dio2 polymorphism has been associated with reduced cognition in 2-month-old male mice and increased risk for cognitive impairment and Alzheimer's disease in African Americans. This has been attributed to reduced thyroid hormone (TH) signaling and endoplasmic reticulum (ER) stress in the brain. Here we studied the Thr92Ala-Dio2 mouse model and saw that older male mice (7-8-month-old) exhibited a more severe cognition impairment, which extended to different aspects of declarative and working memories. A similar phenotype was observed in 4-5-month-old female mice. There were no structural alterations in the prefrontal cortex (PFC) and hippocampus of the Thr92Ala-Dio2 mouse. Nonetheless, in both male and female PFC, there was an enrichment in genes associated with TH-dependent processes, ER stress, and Golgi apparatus, while in the hippocampus there was additional enrichment in genes associated with inflammation and apoptosis. Reduced TH signaling remains a key mechanism of disease given that short-term treatment with L-T3 rescued the cognitive phenotype observed in males and females. We conclude that in mice, age is an additional risk factor for cognitive impairment associated with the Thr92Ala-Dio2 polymorphism. In addition to reduced TH signaling, ER-stress, and involvement of the Golgi apparatus, hippocampal inflammation and apoptosis were identified as potentially important mechanisms of a disease.

Keywords: cognition; thyroid hormone; type 2 deiodinase.

Figure 1

Cognitive tests in 2–3 (2 m) and 7–8-month-old (7 m) male mice. The social recognition test are displayed as time (% of total time) to assess sociability (A,C) and social preference (B,D) for Thr92-Dio2 and Ala92-Dio2 mice and mice treated with T3 for ten days; in each plot, “+” represents a co-specific animal and “−“ represents an empty cage and k represents a familiar mouse and uk represents an unknown mouse; n = 6–7/group. * p < 0.05 and *** p < 0.001 as calculated through the two-way ANOVA followed by the Bonferroni post-test. These experiments were repeated once with similar results; only one set of results is shown in the figure.

Figure 2

Cognitive tests in 3–4-month-old female mice. The social recognition tests are displayed as time (% of total time) to assess sociability (A) and social preference (B) for Thr92-Dio2 and Ala92-Dio2 mice and mice treated with T3 for ten days; in each plot, “+” represents a co-specific animal and “−” represents an empty cage, and k represents a familiar mouse and uk represents an unknown mouse; n = 6–7/group. **** p < 0.001 as calculated through the two-way ANOVA followed by the Bonferroni post-test. These experiments were repeated once with similar results; only one set of results is shown in the figure.

Figure 3

Pre-frontal cortex and hippocampus gene expression profile in Ala92-Dio2 male and female mice. (A) RNA-seq coverage breakdown; distribution reflects the average of all samples; (B) Pre-frontal cortex differential gene expression (Volcano plot) in Ala92-Dio2 male vs. Thr92-Dio2 male; each point represents the average of 4 mice for each transcript; shown in red are upregulated genes with p < 0.05; shown in blue are downregulated genes with p < 0.05; n = 4–6/group (C) Same as A, except that shown is male hippocampus; (D) Same as A, except that shown, is the female prefrontal cortex; (E) Same as A, except that shown is the female hippocampus. The statistical analysis was done using one-way ANOVA.

Figure 4

Heat map of genes from Ala92-Dio2 mice shown in Figure 3. (A) heat map of 63 genes (47-up and 16-down regulated genes) differentially expressed in Ala92-Dio2 pre-frontal cortex that overlapped in male and female mice; (B) same as A, except that shown is 34 genes (23-up and 11-down regulated genes) that overlapped in the hippocampus of male and female mice; and (C) Same as A, except that shown is 29 genes (23-up and 6-down regulated) overlapped in the pre-frontal cortex and hippocampus of male and female mice; relative gene expression is indicated by the degree of color saturation (red: higher level; blue: lower level).

Figure 5

Gene expression coverage in the Ala92-Dio2 mice. (A–D) browser Integrative Genomics Viewer (IGV)-display of transcript areas of selected genes (4 representative genes out of a total of 29 genes that overlapped in the pre-frontal cortex and hippocampus of male and female mice) that are upregulated in Ala92-Dio2; the gene names are indicated at the top of each panel along with the blue arrow indicating the direction of transcription; the red tracks reflect the coverage for the Ala92-Dio2 RNA-seq data, and the blue tracks reflect the Thr92-Dio2 Rna-seq data.