Air Pollution Particulate Matter Amplifies White Matter Vascular Pathology and Demyelination Caused by Hypoperfusion

Abstract

Cerebrovascular pathologies are commonly associated with dementia. Because air pollution increases arterial disease in humans and rodent models, we hypothesized that air pollution would also contribute to brain vascular dysfunction. We examined the effects of exposing mice to nanoparticulate matter (nPM; aerodynamic diameter ≤200 nm) from urban traffic and interactions with cerebral hypoperfusion. C57BL/6 mice were exposed to filtered air or nPM with and without bilateral carotid artery stenosis (BCAS) and analyzed by multiparametric MRI and histochemistry. Exposure to nPM alone did not alter regional cerebral blood flow (CBF) or blood brain barrier (BBB) integrity. However, nPM worsened the white matter hypoperfusion (decreased CBF on DSC-MRI) and exacerbated the BBB permeability (extravascular IgG deposits) resulting from BCAS. White matter MRI diffusion metrics were abnormal in mice subjected to cerebral hypoperfusion and worsened by combined nPM+BCAS. Axonal density was reduced equally in the BCAS cohorts regardless of nPM status, whereas nPM exposure caused demyelination in the white matter with or without cerebral hypoperfusion. In summary, air pollution nPM exacerbates cerebrovascular pathology and demyelination in the setting of cerebral hypoperfusion, suggesting that air pollution exposure can augment underlying cerebrovascular contributions to cognitive loss and dementia in susceptible elderly populations.

Keywords: MRI; air pollution; blood brain barrier; carotid artery stenosis; hypoperfusion.

Figure 1

Design of the study and physicochemical properties of nPM used in exposure studies. (A) Experimental timeline of air pollution exposure and surgical procedures. (B) Particle size distribution of the collected exposure aerosol. (C) Particulate matter mass concentration (µg/m³), (D) Total particulate number concentration (x10⁵ #/m³), (E) Particle mode diameter (nm), (F) Total mass of organic matter (% of particulate matter mass) and (G) Mass fractions of trace elements and metals (ng/µg of particulate matter mass) during exposures. Data is presented as mean ± standard deviation in panel (B) and mean ± standard error in panels (C–H). (n)PM, (nano)particulate matter; BCAS, bilateral carotid artery stenosis; MRI, magnetic resonance imaging; PNC, particulate number concentration; TOM, total mass of organic material.

Figure 2

nPM exposure worsens white matter hypoperfusion after bilateral carotid artery stenosis. (A, B) Corpus callosum cerebral blood flow (CBF) was measured in vivo by using dynamic susceptibility weighted MRI and gadolinium-based contrast agent. Regional cerebral blood flow values (CBF in mL/100g/min) were mapped (A) and quantified (B) in the corpus callosum area. In panel (B) n=6 in filter and nPM groups and n=5 in filter+BCAS and nPM+BCAS groups. (C) Time-of-flight angiography was used to visualize arterial angioarchitecture in mice and is shown as maximum intensity projections. Yellow arrows indicate placement of microcoils around carotid arteries of hypoperfused mice and yellow square indicates the middle cerebral artery area, which was quantified in (D). In panel (D) n=6 in filter, nPM and nPM+BCAS groups and n=5 in filter+BCAS group. P-value indicates BCAS effect in panel (D). Data is presented as violin plot with median and quartiles. 1-way ANOVA (B) and 2-way ANOVA (D) and Bonferroni Post Hoc tests were used for statistical testing. CC, corpus callosum; MCA, middle cerebral artery.

Figure 3

nPM exposure exacerbates white matter blood brain barrier leakage after bilateral carotid artery stenosis. (A, B) Corpus callosum blood brain barrier (BBB) permeability was measured in vivo by using dynamic contrast enhanced MRI. Distribution of gadolinium-based contrast agent was followed during the scan and regional BBB transfer constant (K_trans, x10^-3 min^-1) was mapped (A) and quantified (B) in the corpus callosum area. In panel (B) n=6 in filter, nPM and nPM+BCAS groups and n=5 in filter+BCAS group. (C) BBB leakage was visualized postmortem by staining for extravascular IgG deposits. IgG positive deposits (red) outside lectin (white) positive blood vessels were quantified and presented as integrated density (D). In panel (D) n=12 in all groups. Data is presented as violin plot with median and quartiles. 1-way ANOVA and Bonferroni Post Hoc tests were used for statistical testing. Scale bar = 50 µm in panel (C). CC, corpus callosum; nPM, nanoparticulate matter; IgG, Immunoglobulin G; IntDen, integrated density.

Figure 4

Altered diffusion metrics in white matter of nPM exposed BCAS mice. (A, B) Diffusion weighted MRI was used to measure integrity of white matter in mice and FA maps were generated (A) and quantified within the corpus callosum area (B). (C, D) ADC maps were generated using diffusion weighted MRI (C) and values (x10^-3 mm²/s) were quantified in the corpus callosum area (D). Notice lack of white and gray matter contrast especially in nPM+BCAS group in the corpus callosum area [arrows in (A, C)]. In panels (B, D) n = 6 in filter, nPM and nPM+BCAS groups and n = 5 in filter+BCAS group. Data is presented as violin plot with median and quartiles. 1-way ANOVA and Bonferroni Post Hoc tests were used for statistical testing. CC, corpus callosum; FA, fractional anisotropy; ADC, apparent diffusion coefficient.

Figure 5

Increased demyelination in white matter of nPM exposed mice. (A) White matter axons were visualized by staining with SMI312 antibody (red) and quantified in the corpus callosum area (B). In panel (B) n=12 in filter, nPM and nPM+BCAS groups and n=11 in filter+BCAS group. (C) White matter myelin was visualized by staining with myelin basic protein (red) antibody and quantified in the corpus callosum area (D). In panel (D) n=12 in filter, nPM and nPM+BCAS groups and n=9 in filter+BCAS group. Data is presented as violin plot with median and quartiles. 1-way ANOVA and Bonferroni Post Hoc tests were used for statistical testing. Scale bars = 50 µm in panels (A, C). CC, corpus callosum; IntDen, integrated density; MBP, myelin basic protein.