Microglia changes associated to Alzheimer's disease pathology in aged chimpanzees

Abstract

In Alzheimer's disease (AD), the brain's primary immune cells, microglia, become activated and are found in close apposition to amyloid beta (Aβ) protein plaques and neurofibrillary tangles (NFT). The present study evaluated microglia density and morphology in a large group of aged chimpanzees (n = 20, ages 37-62 years) with varying degrees of AD-like pathology. Using immunohistochemical and stereological techniques, we quantified the density of activated microglia and morphological variants (ramified, intermediate, and amoeboid) in postmortem chimpanzee brain samples from prefrontal cortex, middle temporal gyrus, and hippocampus, areas that show a high degree of AD pathology in humans. Microglia measurements were compared to pathological markers of AD in these cases. Activated microglia were consistently present across brain areas. In the hippocampus, CA3 displayed a higher density than CA1. Aβ42 plaque volume was positively correlated with higher microglial activation and with an intermediate morphology in the hippocampus. Aβ42-positive vessel volume was associated with increased hippocampal microglial activation. Activated microglia density and morphology were not associated with age, sex, pretangle density, NFT density, or tau neuritic cluster density. Aged chimpanzees displayed comparable patterns of activated microglia phenotypes as well as an association of increased microglial activation and morphological changes with Aβ deposition similar to AD patients. In contrast to human AD brains, activated microglia density was not significantly correlated with tau lesions. This evidence suggests that the chimpanzee brain may be relatively preserved during normal aging processes but not entirely protected from neurodegeneration as previously assumed.

Keywords: Alzheimer's disease; RRID: AB_223647; RRID: AB_2313890; RRID: AB_2313952; RRID: AB_2315150; RRID: AB_839504; amyloid beta protein; chimpanzee; microglia; neurofibrillary tangle; neuroinflammation.

Figure 1

Photomicrographs of activated microglia (Iba1-ir) morphologies in the MTG (a) and PFC (b–d) of a 39-year-old female (a; subject 2) and a 40-year-old female (b–d; subject 3) chimpanzee: (a) ramified morphology with small cell soma and fine processes, (b) intermediate morphology with enlarged cell soma and thickened, shorter processes, (c) amoeboid morphology with loss of nearly all processes, and (d) PHF-1/Iba1 expressing microglia with intermediate morphology (black arrows denote PHF-1 staining). Scale bar for each panel = 250 μm.

Figure 2

Photomicrographs of Iba1 (a,b, and f–h), PHF-1/Iba1 (c–e), and AT8/Iba1 (i–l) immunostaining in aged chimpanzees (subject 20: a–b; subject 13: c,i,k; subject 4: d; subject 8: e; subject 2: f–h; subject 19: j,l): (a) microglia in neocortex, (b) microglia in hippocampus, (c,d) PHF-1-ir microglia (black arrows), (e) dystrophic microglia in neocortex, (f–h) ramified microglia with spherical (f), triangular (g), and elongated (h) cell somas in neocortex, (i–j) AT8-ir pretangles surrounded by intermediate microglia (i, white arrows) in the neocortex and amoeboid microglia (j, red arrows) in the hippocampus, (k) AT8-ir NFT adjacent to amoeboid microglia in neocortex, and (l) tau neuritic cluster next to intermediate microglia. Scale bars = 250 μm (a–b) or 25 μm (c–l).

Figure 3

Photomicrographs showing activated Iba 1-ir microglia in the hippocampus of a 62-year old male chimpanzee (subject 20). Activated Iba1-ir microglia density (MGv, mm³) was significantly higher in CA3 compared to CA1 in the hippocampus of aged chimpanzees (a; * represents a significant difference, p = 0.01). Sex differences were not observed in activated Ibal-ir microglia density (b; p = 0.68). Whiskers represent 1 SD. Small circles represent outliers (1.5 × interquartile range). Hippocampal subfield CA1 (c–e) has significantly decreased microglial activation than subfield CA3 (f–h). Stratum oriens (so), stratum pyramidale (sp), stratum radiatum (sr). Scale bars = 250 μm (c–d, f–g) or 25 μm (e,h).

Figure 4

Proportion of Iba1-ir activated microglia (MGv, mm³) by morphology and region.

Figure 5

Iba1-ir ramified (a,d), intermediate (b,e), and amoeboid (c,f) microglia densities (MGv, mm³) did not differ by region (a–c; all p values ≥ 0.07) or sex (d–f; all p values ≥ 0.34). Whiskers represent 1 SD. Small circles represent outliers (1.5 × interquartile range).

Figure 6

Photomicrographs of tau (AT8, green) immunoreactivity in microglia (Iba1, red) in PFC of a 39-year-old male chimpanzee (subject 13): (a) microglial cell surrounded by a tau-ir (AT8, green) neuritic cluster with minimal tau localization intracellularly (white arrows, yellow), and (b,c) intracellular tau deposition in microglia (white arrows, yellow). Scale bars = 25 μm.

Figure 7

PHF-1/Iba1-ir microglia density (MGv, mm³) was significantly higher in PFC compared to CA1 and CA3 (a; all p values ≤ 0.04). Sex differences were not observed in aged chimpanzees (b; p = 0.10). Whiskers represent 1 SD. Small circles represent outliers (1.5 × interquartile range).

Figure 8

Scatter plots showing that colocalization of tau in activated microglia (PHF-1/Iba1-ir microglia density, MGv, mm³) was associated with increased intermediate morphology in the neocortex (a; R² = 0.24, p = 0.03). Aβ42 plaque volume (%) in the neocortex (b; R² = 0.25, p = 0.02) and hippocampus (c; R² = 0.18, p = 0.04) correlated with increased activation of microglia (Iba1-ir microglia density, mm³) and intermediate morphology (d; R² = 0.35, p = 0.03) in the hippocampus. Aβ42 vessel volume also was associated with greater activated microglial density in the hippocampus (e; R² = 0.19, p = 0.03).