Functional Impairment of Microglia Coincides with Beta-Amyloid Deposition in Mice with Alzheimer-Like Pathology

<div><p>Microglial cells closely interact with senile plaques in Alzheimer’s disease and acquire the morphological appearance of an activated phenotype. The significance of this microglial phenotype and the impact of microglia for disease progression have remained controversial. To uncover and characterize putative changes in the functionality of microglia during Alzheimer’s disease, we directly assessed microglial behavior in two mouse models of Alzheimer’s disease. Using <i>in vivo</i> two-photon microscopy and acute brain slice preparations, we found that important microglial functions - directed process motility and phagocytic activity - were strongly impaired in mice with Alzheimer’s disease-like pathology compared to age-matched non-transgenic animals. Notably, impairment of microglial function temporally and spatially correlated with Aβ plaque deposition, and phagocytic capacity of microglia could be restored by interventionally decreasing amyloid burden by Aβ vaccination. These data suggest that major microglial functions progressively decline in Alzheimer’s disease with the appearance of Aβ plaques, and that this functional impairment is reversible by lowering Aβ burden, e.g. by means of Aβ vaccination.</p></div

Phagocytic capacity of cortical microglia is impaired in two mouse models of cerebral amyloidosis.

<p>(<b>A</b>) Representative images (left) and microglial phagocytic index (in arbitrary units, a.u., right) of 9 month old <i>APPPS1</i> mice and wildtype littermate controls (3 mice per genotype; p<0.001). Images show microglia (Iba-1, red), Aβ (4G8, blue) and fluorescent microspheres (green). Orthogonal views of z-stack images are shown in the bottom panel. (<b>B</b>) Representative images (left) and microglial phagocytic index of 20 month old <i>APP23</i> and age-matched control mice (3 mice per genotype, p<0.001, right) are shown. Data are mean ± s.e.m, ***p<0.001. Scale bars: 10 µm.</p

Passive anti-Aβ vaccination reduces plaque burden and restores hippocampal microglial phagocytic activity.

<p>5 month old <i>APPPS1</i> mice (n = 3 mice per group) and wildtype littermates (n = 2 mice per group) were biweekly injected intraperitoneally with IgG (black bar) or anti-Aβ antibody (Ab9, grey bar) for 6 weeks. The area covered by Thiazine Red-positive Aβ plaques in cortex (<b>A</b>) and hippocampus (<b>B</b>) of 6.5 month old <i>APPPS1</i> mice or age-matched controls treated with IgG or Ab9 is shown in the left panel. Absolute values of microglial phagocytic indices in the cortex (<b>A</b>) and hippocampus (<b>B</b>) of the same mice are depicted on the right panel. All data are mean ± s.e.m, *p<0.05, **p<0.01. a.u. = arbitrary units.</p

Impairment of microglial phagocytosis in <i>APPPS1</i> mice correlates with Aβ plaque deposition.

<p>(<b>A</b>) Aβ plaque load (brain area covered by Thiazine red-positive plaques) and relative microglial phagocytic activity normalized to corresponding wildtype littermate in the cortex of 7–9 week, 4 and 9 month old <i>APPPS1</i> mice. 7–9 week old mice were sub-classified according to apparent 4G8 positive plaque deposition as with (+) or without (−) detectable plaque load. (<b>B</b>) Correlation between extent of plaque load and relative microglial phagocytic activity in the cortex of <i>APPPS1</i> mice. (<b>C, D</b>) Thiazine red-covered area and relative phagocytic activity of microglia in the hippocampus of 7–9 week and 4 month old mice (<b>C</b>) and in the cerebellum of 4 month old <i>APPPS1</i> mice (<b>D</b>). Absolute values of microglial phagocytic indices from <i>APPPS1</i> mice were normalized to wildtype littermate controls (3–4 mice per age group and genotype, ***p<0.001). (<b>E</b>) Phagocytic index (3 independent experiments, p = 0.181) and representative images of primary microglial cultures from wildtype and <i>APPPS1</i> mice. Microglia (Iba-1, red), nuclei (DRAQ5, blue) and microspheres (green). All data are mean ± s.e.m, *p<0.05, **p<0.01. a.u. = arbitrary units. Scale bars: 10 µm.</p

Lesion-directed microglial process movement is impaired in a mouse model of cerebral amyloidosis.

<p>(<b>A</b>) Representative intravital two-photon microscopy images and (<b>B</b>) time course of microglial process movement towards a laser-induced micro-lesion (dashed circle) in 8 month old live anaesthetized <i>APPPS1</i>-<i>Cx3cr1</i>^+/gfp (n = 6) and <i>Cx3cr1</i>^+/gfp mice (n = 8). Aβ plaques are stained with Methoxy-XO4 (blue, *). (<b>C</b>) Representative images and (<b>D</b>) relative microglial response to laser lesions in acute cortical cerebral slices of 10 month old <i>APPPS1</i>-<i>Cx3cr1</i>^+/gfp (n = 8) and <i>Cx3cr1</i>^+/gfp (n = 7) mice. Aβ plaques are stained with Thiazine Red (red, *). Data are mean ± s.e.m, *p<0.05. Scale bars: 10 µm. a.u. = arbitrary units.</p

Essential role of interleukin-6 in post-stroke angiogenesis

Ambivalent effects of interleukin-6 on the pathogenesis of ischaemic stroke have been reported. However, to date, the long-term actions of interleukin-6 after stroke have not been investigated. Here, we subjected interleukin-6 knockout (IL-6(−/−)) and wild-type control mice to mild brain ischaemia by 30-min filamentous middle cerebral artery occlusion/reperfusion. While ischaemic tissue damage was comparable at early time points, IL-6(−/−) mice showed significantly increased chronic lesion volumes as well as worse long-term functional outcome. In particular, IL-6(−/−) mice displayed an impaired angiogenic response to brain ischaemia with reduced numbers of newly generated endothelial cells and decreased density of perfused microvessels along with lower absolute regional cerebral blood flow and reduced vessel responsivity in ischaemic striatum at 4 weeks. Similarly, the early genomic activation of angiogenesis-related gene networks was strongly reduced and the ischaemia-induced signal transducer and activator of transcription 3 activation observed in wild-type mice was almost absent in IL-6(−/−) mice. In addition, systemic neoangiogenesis was impaired in IL-6(−/−) mice. Transplantation of interleukin-6 competent bone marrow into IL-6(−/−) mice (IL-6(chi)) did not rescue interleukin-6 messenger RNA expression or the early transcriptional activation of angiogenesis after stroke. Accordingly, chronic stroke outcome in IL-6(chi) mice recapitulated the major effects of interleukin-6 deficiency on post-stroke regeneration with significantly enhanced lesion volumes and reduced vessel densities. Additional in vitro experiments yielded complementary evidence, which showed that after stroke resident brain cells serve as the major source of interleukin-6 in a self-amplifying network. Treatment of primary cortical neurons, mixed glial cultures or immortalized brain endothelia with interleukin 6-induced robust interleukin-6 messenger RNA transcription in each case, whereas oxygen–glucose deprivation did not. However, oxygen–glucose deprivation of organotypic brain slices resulted in strong upregulation of interleukin-6 messenger RNA along with increased transcription of key angiogenesis-associated genes. In conclusion, interleukin-6 produced locally by resident brain cells promotes post-stroke angiogenesis and thereby affords long-term histological and functional protection