Early astrocytic atrophy in the entorhinal cortex of a triple transgenic animal model of Alzheimer's disease

Abstract

The EC (entorhinal cortex) is fundamental for cognitive and mnesic functions. Thus damage to this area appears as a key element in the progression of AD (Alzheimer's disease), resulting in memory deficits arising from neuronal and synaptic alterations as well as glial malfunction. In this paper, we have performed an in-depth analysis of astroglial morphology in the EC by measuring the surface and volume of the GFAP (glial fibrillary acidic protein) profiles in a triple transgenic mouse model of AD [3xTg-AD (triple transgenic mice of AD)]. We found significant reduction in both the surface and volume of GFAP-labelled profiles in 3xTg-AD animals from very early ages (1 month) when compared with non-Tg (non-transgenic) controls (48 and 54%, reduction respectively), which was sustained for up to 12 months (33 and 45% reduction respectively). The appearance of Aβ (amyloid β-peptide) depositions at 12 months of age did not trigger astroglial hypertrophy; nor did it result in the close association of astrocytes with senile plaques. Our results suggest that the AD progressive cognitive deterioration can be associated with an early reduction of astrocytic arborization and shrinkage of the astroglial domain, which may affect synaptic connectivity within the EC and between the EC and other brain regions. In addition, the EC seems to be particularly vulnerable to AD pathology because of the absence of evident astrogliosis in response to Aβ accumulation. Thus we can consider that targeting astroglial atrophy may represent a therapeutic strategy which might slow down the progression of AD.

Figure 1. Schematic illustration of entorhinal inputs to the HC

In brief, the superficial layers (I–III) project to the hippocampal formation and subiculum, while the deep layers (V/VI) receive reciprocal inputs from these areas. Meanwhile, deep layers also innervate to parahippocampal areas and the HC.

Figure 2. Comparison of GFAP-IR cell distribution and density in the EC of non-Tg and 3xTg-AD animals

Bright-field micrographs show the distribution of GFAP-IR astrocytes in the EC of non-Tg and 3xTg-AD animals (A and B respectively). Confocal imaging shows the organization of GFAP-IR astrocytes in the EC of non-Tg and 3xTg-AD animals at 1 month (C and D respectively) and 12 months (E and F respectively) at higher magnification. The histogram shows the Nv (number of cells per mm³) of GFAP-IR cells in the EC of 3xTg-AD and non-Tg controls (G). Results are means±S.E.M.

Figure 3. Comparison of astrocytic GFAP surface and volume in the whole EC of non-Tg and 3xTg-AD animals across ages

The histograms show a comparison of GFAP (A) surface, (B) volume and (C) body volume in the global EC at the age of 1, 3, 6, 9 and 12 months between 3xTg-AD and non-Tg animals. Results are means±S.E.M. (*P≤0.05 compared with the age-matched non-Tg control). Confocal micrographs show astrocytic atrophy in 3xTg-AD at 1 month (E) and 12 months (G) compared with the control animals (D and F).

Figure 4. Comparison of astrocytic GFAP EC surface and volume at different ages

The histograms show decreased GFAP surface (A, C, E) and volume (B, D, F) within specific layers between 3xTg-AD animals at the ages of 1, 6 and 12 months respectively. Results are means±S.E.M. (*P≤0.05; **P≤0.01 compared with the age-matched non-Tg control).

Figure 5. Distribution and relationship of GFAP immunoreactive astrocytes and β-amyloid presence

Confocal images of dual labelling of GFAP (red) and Aβ (green) show that GFAP-IR astrocytes are distant from intracellular Aβ deposits (with a distance of 86 μm) at 12 months and distant from the Aβ plaques (with a distance of 125 μm) at 18 months. Arrowheads indicate astrocytes.