Complex environment experience rescues impaired neurogenesis, enhances synaptic plasticity, and attenuates neuropathology in familial Alzheimer's disease-linked APPswe/PS1DeltaE9 mice

Abstract

Experience in complex environments induces numerous forms of brain plasticity, improving structure and function. It has been long debated whether brain plasticity can be induced under neuropathological conditions, such as Alzheimer's disease (AD), to an extent that would reduce neuropathology, rescue brain structure, and restore its function. Here we show that experience in a complex environment rescues a significant impairment of hippocampal neurogenesis in transgenic mice harboring familial AD-linked mutant APPswe/PS1DeltaE9. Proliferation of hippocampal cells is enhanced significantly after enrichment, and these proliferating cells mature to become new neurons and glia. Enhanced neurogenesis was accompanied by a significant reduction in levels of hyperphosphorylated tau and oligomeric Abeta, the precursors of AD hallmarks, in the hippocampus and cortex of enriched mice. Interestingly, enhanced expression of the neuronal anterograde motor kinesin-1 was observed, suggesting enhanced axonal transport in hippocampal and cortical neurons after enrichment. Examination of synaptic physiology revealed that environmental experience significantly enhanced hippocampal long-term potentiation, without notable alterations in basal synaptic transmission. This study suggests that environmental modulation can rescue the impaired phenotype of the Alzheimer's brain and that induction of brain plasticity may represent therapeutic and preventive avenues in AD.

Figure 1

Environmental enrichment. A) Enrichment cage, composed of running wheels, colorful tunnels, and toys. Components in the cage were reconfigured every day to ensure novelty. Control mice were singly housed in standard laboratory cages. B) Experimental design. Mice were weaned at P21 and housed either 3 or 4 animals/cage (experimental group). Mice in the experimental group experienced enriched environmental conditions for 3 h every day. To examine cell proliferation and early differentiation in the SGL, mice were injected with BrdU on the last 3 d of enrichment (P58–P60) and sacrificed 3 h after the last injection (experimental group 1). To examine newly differentiated neurons in the SGL, animals were allowed to experience an enriched environment for another month and sacrificed at P90 (experimental group 2). Mice in the control groups were singly housed in standard laboratory cages and were subjected to same BrdU regimen.

Figure 2

Impaired number of proliferating cells in the SGL of the DG of FAD-linked APPswe/PS1ΔE9 can be rescued by experience in an enriched environment. A) APPswe/PS1ΔE9 mice exhibit significant impairments in neurogenesis. Total number of proliferating (BrdU⁺) cells and newly formed neurons (BrdU⁺/DCX⁺) is significantly decreased in transgenic mice harboring FAD-linked APPswe/PS1ΔE9 (APP/PS1) maintained in standard housing conditions compared with their NonTg littermates. *P = 0.0339, **P = 0.0026; Student’s t test. B) Total number of BrdU⁺ cells significantly increased in APP/PS1 and NonTg mice after experience in an enriched environment. Two-way ANOVA analysis shows a significant effect of housing conditions (F_1,16=54.88, P<0.0001) and genotype (F_1,16=10.84, P<0.01), with no significant interaction among the two parameters (F_1,16=0.08642, P=0.7726), suggesting that the extent of increase in the enriched groups is comparable. Data are means ± se. *P = 0.0014, **P = 0.0004; Student’s t test. C) Representative confocal images of BrdU⁺ cells in the DG of mice maintained in standard housing (left) and in an enriched environment (right). Scale bar = 100 μm.

Figure 3

Increased number of new neurons and enhanced survival of new mature neurons in the DG of FAD-linked APPswe/PS1ΔE9 after experience in an enriched environment. A) Number of newly formed neurons (BrdU⁺/DCX⁺) is increased significantly after environmental enrichment. *P = 0.0452, **P = 0.0042; Student’s t test. B) More mature neurons (BrdU⁺/NeuN⁺) survive in the DG of APP/PS1 and NonTg mice that experienced environmental enrichment. Data are presented as means ± se. Two-way ANOVA shows no significant interaction among genotypes and housing conditions (F_1,16=0.6603, P=0.4284 for BrdU⁺/DCX⁺; F_1,12=1.353, P=0.2673 for BrdU⁺/NeuN⁺), suggesting that the extent of increase in the enriched groups is comparable. *P = 0.0022, **P = 0.0382; Student’s t test. C) High-power confocal images of cells immunolabeled for BrdU (red) and DCX (blue) in the DG of APP/PS1 and NonTg mice. D) High-power confocal images of cells immunolabeled for BrdU (red), NeuN (blue), and GFAP (green) in the DG of APP/PS1 and NonTg mice. Scale bars = 20 μm.

Figure 4

A selective increase in the number of glia in the brains of APPswe/PS1ΔE9 mice after experience in an enriched environment. A) Number of BrdU⁺/GFAP⁺ newly formed astrocytes in the DG is greatly increased in both NonTg and APPswe/PS1ΔE9 enriched groups compared with standard-housing controls. *P < 0.0001,**P = 0.0058; Student’s t test. B) No significant increase in the number of BrdU⁺/S100β⁺ cells was observed after experience in an enriched environment in the NonTg or the APPswe/PS1ΔE9 group (P=0.5945 and P=0.2575, respectively), suggesting that enrichment induces signaling that up-regulate specific subtypes of the glial population. Data are means ± se. C) Comparison of the extent of increase in the number of lineage-specific BrdU⁺ cells examined in the DG of mice that experienced enriched environmental conditions or standard housing. D, E) Representative confocal image of cells coexpressing BrdU⁺ (red) and GFAP⁺ (green) (D) and BrdU⁺ (red) and S100β⁺ (green) (E) in the DG of APPswe/PS1ΔE9 mice. F, G) Proliferating astrocytes in the SGL of APPswe/PS1ΔE9 mice colocalize with neural stem cell markers. BrdU⁺/GFAP⁺ cells coimmunostain with sox2 (F) and nestin (G). Scale bars = 20 μm (D, E); 10 μm (F, G).

Figure 5

Environmental enrichment attenuates levels of soluble oligomeric Aβ levels in the cortex and hippocampus of FAD-linked APPswe/ PS1ΔE9 mice. A) Representative dot-blot analysis of levels of oligomeric Aβ in protein extract samples of cortex and hippocampus of APPswe/PS1ΔE9 mice maintained in standard housing or an enriched environment. Immunoreactivity to A11 of protein extracts of representative animals from each group is shown. B) Quantification of dot-blot analyses reveals a significant reduction in levels of oligomeric Aβ in mice that experienced enriched environmental conditions. *P = 0.009, **P = 0.0359; Student’s t test.

Figure 6

Synaptic transmission and short-term plasticity is normal in APPswe/PS1ΔE9 mice and unaffected by environmental enrichment. A) fEPSP input-output curves in the CA1 field of hippocampal slices from mice in standard housing conditions and after environmental enrichment. SC synapses were stimulated with stimulus intensities of 2.5, 4.0, 6.3, 10.0, 16, 25, 40, 63, 100, and 160 mA. Graphs plot fEPSP amplitude (4 consecutive responses at each intensity in each slice). Values are means ± se obtained from 1 slice from each NonTg (left; n=9 for standard housing, n=9 for enrichment) and APPswe/PS1ΔE9 (right; n=10 for standard housing, n=15 for enrichment) mouse. ANOVA did not show any significant differences attributable to genotype (F_1,39=0.44, P>0.40) or housing condition (F_1,39=1.47, P>0.20); no significant interactions were observed between these factors and stimulus intensity. B) Representative waveforms evoked by paired-pulse stimulation in mice with indicated genotype and housing conditions. C) Facilitation curves generated by paired-pulse stimulation at indicated interpulse intervals (IPIs) in the same slices shown in A. No significant differences in facilitation were observed to be attributable to genotype (F_1,39=0.35, P>0.55) or housing conditions (F_1,39=0.70, P>0.40), and no interaction was observed between these factors and IPI.

Figure 7

LTP is enhanced after environmental enrichment. TBS consisted of 4 (left panels) or 8 (right panels) high-frequency bursts. A) Comparison of LTP in NonTg mice in standard housing or after environmental enrichment. Graphs show fEPSP slope (mean±se) before and after TBS. B) LTP in APPswe/PS1ΔE9 mice with standard housing or environmental enrichment. Graphs show fEPSP slope (means±se) before and after TBS. C) Histograms show mean + se potentiation at 60 min after TBS in each group.

Figure 8

Environmental enrichment attenuates tau pathology by reducing hyperphosphorylated tau levels in the cortex. A) Representative Western blot analysis of protein extracts from the cortex of standard-housing (SH) and enriched (EE) APPswe/PS1ΔE9 mice. FL-APP, FL-APP using anti-APP-C^′-terminal 369 antibody; Tau5, total tau protein using Tau5 antibody; PHF-1, phosphorylated-tau protein using PHF-1 monoclonal antibody; KHC, kinesin-1 heavy chain using KHC (H2) antibody; KLC, kinesin-1 light chain using KLC (63–90) antibody. B) Densitometric quantification of protein expression levels in the cortex of standard-housing and enriched mice, as detected by Western blot analysis. Data are means ± se (arbitrary units).

Figure 9

Environmental enrichment attenuates tau pathology by reducing hyperphosphorylated tau levels in the hippocampus. A) Representative Western blot analysis of protein extracts from the hippocampus of standard-housing (SH) and enriched (EE) APPswe/PS1ΔE9 mice. FL-APP, FL-APP using anti-APP-C^′-terminal 369 antibody; Tau5^, total tau protein using Tau5 antibody; PHF-1^, phosphorylated-tau protein using PHF-1 monoclonal antibody; KHC^, kinesin-1 heavy chain using KHC (H2) antibody; KLC^, kinesin-1 light chain using KLC (63–90) antibody. B) Densitometric quantification of protein expression levels in the hippocampus of standard-housing and enriched mice, as detected by Western blot analyses. Data are means ± se (arbitrary units).