Particulate air pollutants, APOE alleles and their contributions to cognitive impairment in older women and to amyloidogenesis in experimental models

Abstract

Exposure to particulate matter (PM) in the ambient air and its interactions with APOE alleles may contribute to the acceleration of brain aging and the pathogenesis of Alzheimer's disease (AD). Neurodegenerative effects of particulate air pollutants were examined in a US-wide cohort of older women from the Women's Health Initiative Memory Study (WHIMS) and in experimental mouse models. Residing in places with fine PM exceeding EPA standards increased the risks for global cognitive decline and all-cause dementia respectively by 81 and 92%, with stronger adverse effects in APOE ɛ4/4 carriers. Female EFAD transgenic mice (5xFAD^+/-/human APOE ɛ3 or ɛ4^+/+) with 225 h exposure to urban nanosized PM (nPM) over 15 weeks showed increased cerebral β-amyloid by thioflavin S for fibrillary amyloid and by immunocytochemistry for Aβ deposits, both exacerbated by APOE ɛ4. Moreover, nPM exposure increased Aβ oligomers, caused selective atrophy of hippocampal CA1 neurites, and decreased the glutamate GluR1 subunit. Wildtype C57BL/6 female mice also showed nPM-induced CA1 atrophy and GluR1 decrease. In vitro nPM exposure of neuroblastoma cells (N2a-APP/swe) increased the pro-amyloidogenic processing of the amyloid precursor protein (APP). We suggest that airborne PM exposure promotes pathological brain aging in older women, with potentially a greater impact in ɛ4 carriers. The underlying mechanisms may involve increased cerebral Aβ production and selective changes in hippocampal CA1 neurons and glutamate receptor subunits.

Figure 1

Adverse effects of PM_2.5 exposure on cognitive impairment in older women, stratified by APOE alleles. Horizontal bars represent the effect measures (hazard ratios (HRs) and 95% confidence intervals) estimated from the Cox proportional hazard models, comparing high (exceeding the US National Ambient Air Quality Standard with 3-year averages PM_2.5>12 μg m⁻³) versus low exposure for their associated incidence rates of global cognitive decline (a) and all-cause dementia (b), stratified by APOE alleles (ɛ3/3 vs ɛ3/4 vs ɛ4/4). The dotted vertical lines denote no statistically significant adverse effects (with HR=1). The presented crude estimates were adjusted for APOE alleles. The adjusted estimates further accounted for age, geographic region, spatial random effect, years of education, household income, employment status, lifestyle factors (smoking; alcohol use; physical activities) and clinical characteristics (use of hormone treatment; depression; body mass index; hypercholesterolemia; hypertension, diabetes; and histories of cardiovascular disease). At any time during 1999–2010, if older women were residing at locations with high PM_2.5, their hazards for accelerated global cognitive decline and all-cause dementia respectively would be 81% (HR=1.81; 1.42–2.32) and 92% (HR=1.92; 1.32–2.80) greater than if they had low exposure. This increase in hazard for all-cause dementia associated with high PM_2.5 exposure was 68% (HR=1.68; 0.97–2.92), 91% (HR=1.91; 1.17–3.14), and 295% (HR=3.95; 1.18–13.19), respectively, in ɛ3/3, ɛ3/4, and ɛ4/4 carriers. High PM_2.5 exposure also increase the hazard for global cognitive decline by 65% (HR=1.65; 1.23–2.23), 93% (HR=1.93; 1.29–2.90), and 264% (HR=3.64; 1.36–9.69) in women of ɛ3/3, ɛ3/4, and ɛ4/4 alleles.

Figure 2

In vivo and in vitro nPM exposure on Aβ levels. (a,b) In vivo nPM exposure of female EFAD mice (N=5 mice per experimental group). (a–c). Cerebral cortex sagittal sections were analyzed for Aβ plaque load using two independent staining: (a) Thioflavin S, (b) 4G8 antibody. Both reagents showed responses to nPM in E4FAD mice but not in E3FAD. For Thioflavin S, E3FAD coeff, −0.09, P=0.79; E4FAD, coeff 1.06, P=0.048. E4FAD mice had 2.8-fold greater increased total plaque load after nPM than E3FAD mice (E3FAD coeff, 0.49, P=0.27; E4FAD, coeff 1.39, P=0.04). (c) Aβ oligomers in soluble extracts of cerebral cortex were increased by nPM exposure in E4FAD mice (coeff 0.03, P=0.03), with trend of increase in E3FAD (coeff 0.07, P=0.07). (d,e) In vitro nPM exposure (N2a-APP/swe cells). Cells exposed to 10 μg ml⁻¹ nPM for 24 h showed 35% increased sAPPβ/α ratio (P=0.02). Culture media Aβ42 levels increased twofold (P<0.001). White bar, control; black bar, nPM exposed. Mean±s.e. *P<0.05, ***P<0.0001.

Figure 3

In vivo nanosized particulate matter (nPM) exposure decreased hippocampal CA1 neurite density. (a–d) Silver histochemistry for neurodegeneration in hippocampal subregions CA1 pyramidal neuron layer and dentate gyrus (DG). EFAD mice, N=5 mice per group; B6 mice, N=9 mice/group. (a) Whole hippocampus; scale bar, 500 μm. (b) Hippocampal subregions: scale bar, 100 μm; left panel: CA1, Nissl-stained CA1 neuron layer; center, detail of silver staining from Figure 3a to show neurites; right, density filtered to resolve neurites. (c, d) nPM caused decreased neurite density in CA1 neurons of EFAD and wild-type mice (C57BL/6J) without affecting dentate gyrus neurons. Overall nPM effects on CA1, combining E3FAD and E4FAD, was significant (P=0.02, adjusting for genetic effect): E3FAD (coeff=−0.09, P=0.03); C57BL/6J, (coeff=−0.25 P=0.003). White bar, control; black bar, nPM exposed. Mean±s.e. *P<0.05, ***P<0.005.

Figure 4

Chronic nPM exposure of female EFAD and C57BL/6J mice alters the GluR1 receptor subunit, but not other synaptic proteins. (a) Hippocampus glutamatergic receptor protein subunit GluR1 was decreased by nPM in both E3FAD (coeff=−0.26, P=0.01), E4FAD (coeff=−0.32, P=0.01) and C57BL/6J (coeff=−0.42, P=0.002) mice. (b–d) GluR2, NR2A and NR2B were unchanged. White bar, control; black bar, nPM exposed. EFAD, N = 5 mice per experimental group; B57BL/6, N = 9 mice per experimental group. Mean±s.e. **P<0.01, ***P<0.005.