Neurogenesis and alterations of neural stem cells in mouse models of cerebral amyloidosis

Abstract

The hippocampus in Alzheimer's disease is burdened with amyloid plaques and is one of the few locations where neurogenesis continues throughout adult life. To evaluate the impact of amyloid-beta deposition on neural stem cells, hippocampal neurogenesis was assessed using bromodeoxyuridine incorporation and doublecortin staining in two amyloid precursor protein (APP) transgenic mouse models. In 5-month-old APP23 mice prior to amyloid deposition, neurogenesis showed no robust difference relative to wild-type control mice, but 25-month-old amyloid-depositing APP23 mice showed significant increases in neurogenesis compared to controls. In contrast, 8-month-old amyloid-depositing APPPS1 mice revealed decreases in neurogenesis compared to controls. To study whether alterations in neurogenesis are the result of amyloid-induced changes at the level of neural stem cells, APPPS1 mice were crossed with mice expressing green fluorescence protein (GFP) under a central nervous system-specific nestin promoter. Eight-month-old nestin-GFP x APPPS1 mice exhibited decreases in quiescent nestin-positive astrocyte-like stem cells, while transient amplifying progenitor cells did not change in number. Strikingly, both astrocyte-like and transient-amplifying progenitor cells revealed an aberrant morphologic reaction toward congophilic amyloid-deposits. A similar reaction toward the amyloid was no longer observed in doublecortin-positive immature neurons. Results provide evidence for a disruption of neural stem cell biology in an amyloidogenic environment and support findings that neurogenesis is differently affected among various transgenic mouse models of Alzheimer's disease.

Figure 1

Neurogenesis in the GCL of adult and aged APP23 mice. A: Three weeks after BrdU injections no difference was found in the number of BrdU-positive cells in the GCL in 5-month-old (adult) APP23 mice (6 mice/group). In contrast, in 25-month-old (aged) APP23 mice, number of BrdU-positive cells were significantly increased compared to aged-matched WT mice (6 mice/group; *P < 0.001). B: Most BrdU-positive cells (closed arrows) were found close to the SGZ of the dentate gyrus. In the aged APP23 mice additional BrdU-positive cells were found in the periphery of amyloid plaques (asterisk) and identified as microglia cells (open arrows). C: When BrdU-positive cells were phenotyped using the neuronal marker NeuN no significant difference was again found between adult APP23 and WT mice (6 mice/group). In aged APP23 mice a significantly higher number of newly generated neurons was found compared to WT (6 mice/group; *P < 0.05). D: This phenotyping was done by capturing multiple confocal images in the Z-dimension, allowing the evaluation of the co-localization of different markers in all three dimensions. (Green: NeuN; Red: BrdU; Blue: GFAP). E: To estimate the numbers of immature neurons adjacent sections were stained for DCX. In adult APP23 mice the pool of DCX-positive cells was smaller compared to WT mice (6 mice/group; *P < 0.05). In aged APP23 mice the population of immature neurons was significantly increased compared to WT mice (6 mice/group; *P < 0.001). This increase in the aged APP23 mice was most pronounced in the caudal pole of the dentate gyrus. F: Representative micrographs of DCX-positive cells in the GCL. Asterisks indicate amyloid plaques. Scale bar: 50 μm in B and F; 10 μm in C.

Figure 2

Amyloid deposition in APP23 and APPPS1 mice. Immunohistochemical staining for Aβ in the hippocampus of a 25-month-old APP23 mouse (A) and of an 8-month-old APPPS1 mouse (B). Note the high amyloid load in the dentate gyrus in both mouse models at that age. Scale bars = 200 μm.

Figure 3

Neurogenesis in the GCL of adult APPPS1 mice. A: Three weeks after BrdU injections a decrease was found in the number of BrdU-positive cells in the GCL in 8-month-old APPPS1 mice compared to age-matched WT control mice (6 mice/group). However this decrease was statistically not significant. B: When BrdU-positive cells were phenotyped using the neuronal marker NeuN a similar not significant decrease was found. C: In contrast the decrease in the number of DCX-positive immature neurons reached statistical significance (7 to 8 mice/group; *P < 0.05). Distribution and morphology of DCX-positive cells in the control mice (D) and APPPS1 mice (E). Amyloid is stained with Congo red. Note that DCX-positive dendrites are neither attracted nor sprout to the amyloid plaques (E–H). Scale bar = 50 μm.

Figure 4

Neural stem cells in the GCL interact with amyloid plaques. A: Adult nestin-positive stem cells in the GCL of aged 8-month-old GFP-WT mice. Quiescent astrocyte-like stem cells (open arrows) were located along the GCL, with their cell body in the SGZ and dendritic-like processes in the molecular layer perpendicular to the GCL. In contrast, transient amplifying precursor cells (closed arrows) in the SGZ displayed a horizontal orientation. B: In GFP-APPPS1 mice, amyloid deposits (Congo red staining in red) strongly attracted these dendritic-like processes of quiescent astrocyte-like (open arrows) and transient amplifying cells (closed arrow). C: High magnification of the typical morphological alterations of a quiescent astrocyte-like cell attracted to Aβ (red). Note the thickened main branch of the tree of processes (closed arrow) and the thickened fibers in close vicinity to the plaque (open arrows). D: Transient amplifying cells were found to react to the presence of Aβ similar to quiescent astrocyte-like cells with thickened processes (closed arrows) and aberrant dendritic-like processes sheathing the amyloid plaque with dystrophic bouton-like structures (open arrow). E: Stereological estimates of quiescent astrocyte-like stem cells revealed a significant reduction in 8-month-old GFP-APPPS1 as compared with age-matched GFP-WT mice control mice (n = 7 to 8 mice/group; *P < 0.05). F: No difference occurred in the number of transient amplifying progenitor cells in GFP-APPPS1 compared to GFP-WT mice. Scale bars: 50 μm in B; 10 μm in D. A and B and C and D have the same magnification.