Impaired adult neurogenesis in the dentate gyrus of a triple transgenic mouse model of Alzheimer's disease

Abstract

It has become generally accepted that new neurones are added and integrated mainly in two areas of the mammalian CNS, the subventricular zone and the subgranular zone (SGZ) of the dentate gyrus (DG) of the hippocampus, which is of central importance in learning and memory. The newly generated cells display neuronal morphology, are able to generate action potentials and receive functional synaptic inputs, i.e. their properties are similar to those found in mature neurones. Alzheimer's disease (AD) is the primary and widespread cause of dementia and is an age-related, progressive and irreversible neurodegenerative disease that deteriorates cognitive functions. Here, we have used male and female triple transgenic mice (3xTg-AD) harbouring three mutant genes (beta-amyloid precursor protein, presenilin-1 and tau) and their respective non-transgenic (non-Tg) controls at 2, 3, 4, 6, 9 and 12 months of age to establish the link between AD and neurogenesis. Using immunohistochemistry we determined the area density of proliferating cells within the SGZ of the DG, measured by the presence of phosphorylated Histone H3 (HH3), and their possible co-localisation with GFAP to exclude a glial phenotype. Less than 1% of the HH3 labeled cells co-localised with GFAP. Both non-Tg and 3xTg-AD showed an age-dependent decrease in neurogenesis. However, male 3xTg-AD mice demonstrated a further reduction in the production of new neurones from 9 months of age (73% decrease) and a complete depletion at 12 months, when compared to controls. In addition, female 3xTg-AD mice showed an earlier but equivalent decrease in neurogenesis at 4 months (reduction of 63%) with an almost inexistent rate at 12 months (88% decrease) compared to controls. This reduction in neurogenesis was directly associated with the presence of beta-amyloid plaques and an increase in the number of beta-amyloid containing neurones in the hippocampus; which in the case of 3xgTg females was directly correlated. These results suggest that 3xTg-AD mice have an impaired ability to generate new neurones in the DG of the hippocampus, the severity of which increases with age and might be directly associated with the known cognitive impairment observed from 6 months of age onwards . The earlier reduction of neurogenesis in females, from 4 months, is in agreement with the higher prevalence of AD in women than in men. Thus it is conceivable to speculate that a recovery in neurogenesis rates in AD could help to rescue cognitive impairment.

Figure 1. Photomicrographs showing phosphorilated Histone H3 (HH3, a proliferating mitotic marker) within the dentate gyrus of Non Tg mice.

A–B: Single labelling of HH3 positive cells (arrows) in the dentate gyrus of 2 (A) and 12 months (B) Non Tg mice. C–D: Dual labeling of HH3 positive cells (arrows) and glial cells (GFAP, blue) in the dentate gyrus of 2 (C) and 12 months (D) Non Tg mice. E–F: Bar graphs showing the area density of HH3 positive cells within the dorsal dentate gyrus (all layers included) of Non-Tg males (E) and females (F) mice. GCL: Granular Cell Layer, ML: Molecular Layer. ** = p<0.01 compared to 2 months; *** = p<0.001 compared to 2 months;^•• = p<0.01 compared to 3 months; ## = p<0.01 compared to 2 and 3 months; ### = p<0.001 compared to 3 months; ◊ = p<0.05 compared to 4 months; ◊◊ = p<0.01 compared to 4 months; ∧ = p<0.05 compared to 4 and 6 months; + = p<0.05 compared to 2, 3 and 6 months.

Figure 2. Brightfield micrographs showing HH3 labelled cells within the dentate gyrus of 3xTg-AD e.

A–B: Dual labeling of HH3 positive cells (arrows) and glial cells (GFAP, red) in the dentate gyrus of 2 (A) and 12 months (B) 3xTg-AD mice. C–D: Bar graphs showing the area density of HH3 positive cells within the dorsal dentate gyrus (all layers included) of 3xTg-AD males (C) and females (D) mice. GCL: Granular Cell Layer, ML: Molecular Layer. ** = p<0.01 compared to 3 months; *** = p<0.001 compared to 3 months; ^• = p<0.05 compared to 2 months ^•• = p<0.001 compared to 2 months; ## = p<0.01 compared to 2 and 3 months; ◊ = p<0.05 compared to 4 months; ∧ = p<0.05 compared to 2 and 4 months.

Figure 3. Bar graphs showing the mean area density HH3 labelled cells within the GCL of the dentate gyrus of both 3xTg-AD and control nonTg-AD mice.

A–B: Males (A) and females (B) dorsal GCL. C–D: Males (C) and females (D) dorsal GCL. Asterisks indicate a significant difference in the means.

Figure 4. Photomicrographs showing the presence of β-amyloid within the pyramidal neurones of CA1 as well as the presence of a plaque in 12 months 3xTg-AD mice (B) compared to a nonTg control animal (A).

C–D: Bar graphs showing the number of cells containing β-amyloidin the hippocampal CA1 of males (C) and females (D) 3xTg-AD mice. E–F: Linear correlations between the mean number of cells containing β-amyloid in the hippocampal CA1 and the mean area density of HH3 positive cells in the GCL of the dentate gyrus of males (E) and females (F) 3xTg-AD mice. In G we can see the accumulation phosphorilated Tau within the CA1 of a 3xTg-AD mice. Or: CA1 Stratum Oriens, Py: CA1 stratum Pyramidale, Rad: CA1 Stratum Radiatum, LMol: CA1 Stratum Lacunosum Moleculare. * = p<0.05 compared to 2 months; ** = p<0.01 compared to 2 months; ∧ = p<0.05 compared to 3 months.