Formation and maintenance of Alzheimer's disease beta-amyloid plaques in the absence of microglia

Abstract

In Alzheimer's disease, microglia cluster around beta-amyloid deposits, suggesting that these cells are important for amyloid plaque formation, maintenance and/or clearance. We crossed two distinct APP transgenic mouse strains with CD11b-HSVTK mice, in which nearly complete ablation of microglia was achieved for up to 4 weeks after ganciclovir application. Neither amyloid plaque formation and maintenance nor amyloid-associated neuritic dystrophy depended on the presence of microglia.

Figure 1

(a,b) Male 3-month-old amyloid-bearing APPPS1; TK and APPPS1 mice were treated with icv GCV for 2 or 4 weeks. Double staining for Iba1 and congophilic amyloid (top), congophilic amyloid alone (middle) and Aβ immunohistochemistry (bottom) are shown. (c) Quantitative stereological analysis of the number of Iba-1 positive microglia (n = 5 per group, *** P < 0.001). (d,e) No change in congophilic (d) or total Aβ load (e) was noted in GCV-treated APPPS1; TK mice compared with GCV-treated APPPS1 mice. (f) Plaque size distribution did not change after 4 weeks of GCV. (g) For western blotting, male APPPS1; TK mice were icv GCV-treated for 3 weeks. Two mice of each genotype are shown with GAPDH as a loading control. Densitometric analysis of all mice (n = 4–5 per group) revealed no difference in human APP and Aβ (combined Aβ40 and Aβ42) between microglia-depleted APPPS1; TK and control APPPS1 mice. Aβ ELISA of the soluble fraction revealed a 3–4-fold increase in Aβ40 (P < 0.05) and Aβ42 (P = 0.05) in microglia-deficient APPPS1; TK mice (data not shown); however, this may represent an artifact resulting from an increased release of ‘insoluble’ Aβ into the soluble fraction in the absence of microglia cells. (h) APP-immunoreactive dystrophic boutons (black) surrounding congophilic amyloid plaques (red) appeared to be unchanged in number and morphology in APPPS1; TK mice lacking microglia compared with APPPS1 mice after 4 weeks of GCV treatment (n = 5 per group, P > 0.05). Error bars represent s.e.m. Scale bars represent 100 μm (a,b), 25 μm (inserts) and 20 μm (h).

Figure 2

(a) Numbers of Iba1-positive microglia in 6-week-old male APPPS1; TK and APPPS1 mice did not differ. The mice did not exhibit amyloid plaque deposition. Double staining for microglia (Iba1) and compact congophilic amyloid (top panels), congophilic amyloid alone (middle panels) and Aβ immunohistochemistry (bottom panels) are shown. (b) Both APPPS1; TK and APPPS1 mice develop amyloid plaques irrespective of microglia 3 weeks after icv GCV treatment. (c) Quantitative stereological analysis of total microglia revealed identical numbers of cells at baseline (0 weeks) in both genotypes and a 95% reduction in microglia 3 weeks after GCV treatment in APPPS1; TK mice compared with APPPS1 mice (n = 5 per group, *** P < 0.001). (d,e) No group difference in congophilic amyloid and Aβ load was seen after GCV treatment. (f) APP-immunoreactive dystrophic boutons (black) surrounding congophilic amyloid plaques (red) were indistinguishable in number and morphology in APPPS1; TK mice lacking microglia and APPPS1 control mice 3 weeks after GCV application (n = 5 per group, P > 0.05). Error bars represent s.e.m. Scale bars represent 100 μm (a), 25 μm (inserts) and 20 μm (f).