Diesel engine exhaust accelerates plaque formation in a mouse model of Alzheimer's disease

Abstract

Background: Increasing evidence from toxicological and epidemiological studies indicates that the central nervous system is an important target for ambient air pollutants. We have investigated whether long-term inhalation exposure to diesel engine exhaust (DEE), a dominant contributor to particulate air pollution in urban environments, can aggravate Alzheimer's Disease (AD)-like effects in female 5X Familial AD (5XFAD) mice and their wild-type female littermates. Following 3 and 13 weeks exposures to diluted DEE (0.95 mg/m³, 6 h/day, 5 days/week) or clean air (controls) behaviour tests were performed and amyloid-β (Aβ) plaque formation, pulmonary histopathology and systemic inflammation were evaluated.

Results: In a string suspension task, assessing for grip strength and motor coordination, 13 weeks exposed 5XFAD mice performed significantly less than the 5XFAD controls. Spatial working memory deficits, assessed by Y-maze and X-maze tasks, were not observed in association with the DEE exposures. Brains of the 3 weeks DEE-exposed 5XFAD mice showed significantly higher cortical Aβ plaque load and higher whole brain homogenate Aβ42 levels than the clean air-exposed 5XFAD littermate controls. After the 13 weeks exposures, with increasing age and progression of the AD-phenotype of the 5XFAD mice, DEE-related differences in amyloid pathology were no longer present. Immunohistochemical evaluation of lungs of the mice revealed no obvious genetic background-related differences in tissue structure, and the DEE exposure did not cause histopathological changes in the mice of both backgrounds. Luminex analysis of plasma cytokines demonstrated absence of sustained systemic inflammation upon DEE exposure.

Conclusions: Inhalation exposure to DEE causes accelerated plaque formation and motor function impairment in 5XFAD transgenic mice. Our study provides further support that the brain is a relevant target for the effects of inhaled DEE and suggests that long-term exposure to this ubiquitous air pollution mixture may promote the development of Alzheimer's disease.

Keywords: 5XFAD mice; Alzheimer’s disease; Amyloid-β; Behaviour; Diesel engine exhaust; Inhalation; Particulate matter.

Fig. 1

Study design. Female 5X Familial AD (5XFAD) mice and wild-type (WT) female littermates were exposed for 3 or 13 weeks to clean air or diluted diesel engine exhaust (DEE). Animals were born within a time range of 4 days, and age was 10 weeks at exposure start

Fig. 2

Behaviour tasks performances of WT and 5XFAD mice following inhalation exposure to DEE or clean air. Data represent mean ± standard error of the % alternation in the Y-maze task (panels a and b), % alternation in the X-maze task (c and d) and the string suspension task score (e and f) following 3 weeks exposure (a, c and e) or 13 weeks exposure (b, d and f) to clean air or DEE as indicated in the figures

Fig. 3

Representative images showing amyloid-β plaque staining in cortex (a, b, e and f) and hippocampus (c, d, g and h) of brains sections from 5XFAD mice. The accumulation of Aβ42 (reddish-brown colour) was localised by immunohistochemistry in sections of paraffin-embedded brain hemispheres. Hippocampus (50 x magnification) and cortex (100 x magnification) from the same animal are shown for each time point and exposure. i.e. 3 weeks to clean air (a and c), 3 weeks to DEE (b and d), 13 weeks to clean air (e and g) and 13 weeks to DEE (f and h), respectively

Fig. 4

Plaque load in cortex (a, b) and hippocampus (c, d) of 5XFAD mice following 3 weeks (a, c) or 13 weeks (b, d) exposure to DEE or clean air. Quantitative Aβ42 plaque analyses were performed via calculation of the % of total plaque load in the analysed area of the section (n = 11–16 mice per group)

Fig. 5

Amyloid-β protein levels in mouse brain homogenates. Human Aβ42 protein levels were determined by ELISA following 3 (a) or 13 weeks (b) exposure to DEE or clean air (n = 5–8 mice per group)

Fig. 6

Representative images of haematoxylin-eosin stained mouse lungs. Tissue sections shown are from: a 3 weeks clean air-exposed 5XFAD mouse; b 3 weeks clean air-exposed WT mouse; c 3 weeks DEE-exposed FAD mouse; d 3 weeks DEE-exposed WT mouse; e 13 weeks clean air-exposed 5XFAD mouse; f 13 weeks clean air-exposed WT mouse; g 13 weeks DEE-exposed 5XFAD mouse; h 13 weeks DEE-exposed WT mouse. Original magnification 640×

Fig. 7

Plasma levels of IL-1β (panel a), G-CSF (b), RANTES (c) and MCP-1 (d) in blood of mice following 13 weeks exposure to DEE or clean air. Data were obtained by Bio-Plex murine cytokine platform from blood collected at sacrifice (n = 5 mice per group)