Microglial TLR4 Mediates White Matter Injury in a Combined Model of Diesel Exhaust Exposure and Cerebral Hypoperfusion

Background: Air pollution particulate matter (PM) exposure and chronic cerebral hypoperfusion (CCH) contribute to white matter toxicity through shared mechanisms of neuroinflammation, oxidative stress, and myelin breakdown. Prior studies showed that exposure of mice to joint PM and CCH caused supra-additive injury to corpus callosum white matter. This study examines the role of toll-like receptor 4 (TLR4) signaling in mediating neurotoxicity and myelin damage observed in joint PM and CCH exposures. Methods: Experiments utilized a novel murine model of inducible monocyte/microglia-specific TLR4 knockout (i-mTLR4-ko). Bilateral carotid artery stenosis (BCAS) was induced surgically to model CCH. TLR4-intact (control) and i-mTLR4-ko mice were exposed to 8 weeks of either aerosolized diesel exhaust particulate (DEP) or filtered air (FA) in eight experimental groups: 1) control/FA (n=10), 2) control/DEP (n=10), 3) control/FA+BCAS (n=9), 4) control/DEP+BCAS (n=10), 5) i-mTLR4-ko/FA (n=9), 6) i-mTLR4-ko/DEP (n=8), 7) i-mTLR4-ko/FA+BCAS (n=8), and 8) i-mTLR4-ko/DEP+BCAS (n=10). Corpus callosum levels of 4-hydroxynonenal (4-HNE), 8-Oxo-2'-deoxyguanosine (8-OHdG), ionized calcium-binding adapter molecule 1 (Iba-1), and degraded myelin basic protein (dMBP) were assayed via immunofluorescence to measure oxidative stress, neuroinflammation, and myelin breakdown, respectively. Results: Compared with control/FA mice, control/DEP+BCAS mice exhibited increased dMBP (52% (p<0.01), Iba-1 (68%; p<0.0001), 4-HNE (200%; p<0.0001), and 8-OhdG (96%; p<0.05). I-mTLR4 knockout attenuated responses to DEP/BCAS for all markers. Conclusion: i-mTLR4-ko markedly reduced neuroinflammation and oxidative stress and attenuated white matter degradation following DEP and CCH exposure. This suggests a potential role for targeting TLR4 signaling in individuals with vascular cognitive impairment, particularly those exposed to substantial ambient air pollution.