Atypical microglial response to biodiesel exhaust in healthy and hypertensive rats

Abstract

Accumulating evidence suggests a deleterious role for urban air pollution in central nervous system (CNS) diseases and neurodevelopmental disorders. Microglia, the resident innate immune cells and sentinels in the brain, are a common source of neuroinflammation and are implicated in air pollution-induced CNS effects. While renewable energy, such as soy-based biofuel, is of increasing public interest, there is little information on how soy biofuel may affect the brain, especially in people with preexisting disease conditions. To address this, male spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) rats were exposed to 100% Soy-based Biodiesel Exhaust (100SBDE; 0, 50, 150 and 500μg/m³) by inhalation, 4h/day for 4 weeks (5 days/week). Ionized calcium-binding adapter molecule-1 (IBA-1) staining of microglia in the substantia nigra revealed significant changes in morphology with 100SBDE exposure in rats from both genotypes, where SHR were less sensitive. Aconitase activity was inhibited in the frontal cortex and cerebellum of WKY rats exposed to 100SBDE. No consistent changes occurred in pro-inflammatory cytokine expression, nitrated protein, or arginase1 expression in brain regions from either rat strain exposed to 100SBDE. However, while IBA-1 mRNA expression was not modified, CX3CR1 mRNA expression was lower in the striatum of 100SBDE exposed rats regardless of genotype, suggesting a downregulation of the fractalkine receptor on microglia in this brain region. Together, these data indicate that while microglia are detecting and responding to 100SBDE exposure with changes in morphology, there is reduced expression of CX3CR1 regardless of genetic background and the activation response is atypical without traditional inflammatory markers of M1 or M2 activation in the brain.

Keywords: Air pollution; Atypical activation; Biodiesel; Brain; Microglia; Neurotoxicity.

Figure 1.. Changes in Microglia Morphology in Response to Soy Biodiesel Exhaust in SHR and WKY Rats.

Young adult male Spontaneously Hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) rats were exposed for 4 h/day for 4 weeks (5 days/week) to 100% Soy Biodiesel Exhaust (100SBDE; 0, 50, 150 and 500 μg/m³). Three coronal sections (40 μm) from the substantia nigra per animal were stained with the IBA-1 antibody. Representative images of the substantia nigra were taken at 40X, the scale bar depicts 50 μM, and the red arrows depict activated microglia morphology (N=3).

Figure 2.. Striatum Changes in Fractalkine in Response to Soy Biodiesel Exhaust in SHR and WKY Rats.

Young adult male Spontaneously Hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) rats were exposed to 100% Soy Biodiesel Exhaust (100SBDE; 0, 50, 150 and 500 μg/m³) for 4 h/day for 4 weeks (5 days/week). Striatum fractalkine mRNA expression was evaluated by quantitative RT-PCR, values were normalized to GAPDH using the 2^-ΔΔCT method. An asterisk indicates significant decrease (P<0.05) from 0.0 μg/m³ control and an “Ϯ “indicates a significant difference between the rat strains (P<0.05, n=3).

Figure 3.. Striatum Changes in Fractalkine Receptor (CX3CR1) in Response to Soy Biodiesel Exhaust in SHR and WYK Rats.

Young adult male Spontaneously Hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) rats were exposed to 100% Soy Biodiesel Exhaust (100SBDE; 0, 50, 150 and 500 μg/m³) for 4 h/day for 4 weeks (5 days/week). Striatum CX3R1 mRNA expression was evaluated by quantitative RT-PCR, values were normalized to GAPDH using the 2^-ΔΔCT method. An asterisk indicates significant decrease (P<0.05) from 0.0 μg/m³ control in both the A.WKY rats and B. SHR rats.