Original articleTraffic-related air pollutants (TRAP-PM) promote neuronal amyloidogenesis through oxidative damage to lipid rafts
Graphical abstract
Traffic-related air pollutants (TRAP) of nano-particle size (nPM) increase the risk of Alzheimer Disease (AD). We propose a new molecular mechanism in the pro-amyloidogenic effect of TRAP. Exposure of mice or cells to nPM (1) increases the oxidative damage to cell membranes (2). In particular, the oxidation of lipid raft domains (3) alters APP processing (4) to increase production of Aβ fragments (5). Treatment with anti-oxidant N-acetyl-cysteine (NAC) attenuated nPM-induced oxidative damage to lipid rafts and pro-amyloidogenic alterations of APP processing. These findings identify lipid raft alterations of APP processing as a novel target of air pollution relevant to cognitive impairments.
Introduction
Alzheimer disease (AD) and accelerated cognitive aging are associated with traffic-related air pollution particulate matter (TRAP-PM) in several studies [[1], [2], [3], [4], [5]] and other reports critically reviewed in Refs. [6,7]. However, the subcellular mechanisms of TRAP-PM induced neurodegeneration are obscure. We show lipid raft alterations that increase production of the neurotoxic amyloid peptide Aβ as a novel mechanism in air pollution toxicology.
Premature brain deposits of Aβ in children and young adults from a highly polluted Mexican city suggested increased production of brain Aβ as a possible mechanism in TRAP-PM associated AD risk [8]. Correspondingly, young wildtype rodents had increased levels of soluble Aβ peptides from exposure to various TRAP components: diesel exhaust [9], nickel nano-particles [10], and TRAP-PM [11]. While these wildtype rodents do not acquire fibrillar deposits of the endogenous species-specific Aβ, mice carrying the APPswe mutation for familial AD together with human ApoE genes (EFAD genotype) responded to TRAP-nPM with increased Aβ oligomers and fibrillar Aβ plaque load [1]. In vitro, TRAP-nPM also increased Aβ production in neuronal N2a cells transgenic for APPswe [1]. TRAP-nPM is a subfraction of ultrafine PM, <0.2μ in size, collected from a local freeway, which is neurotoxic at low levels in vitro [[12], [13], [14]].
The present study analyzed subcellular mechanisms of nPM-induced Aβ increase with J20 mice and neuronal N2a cells transgenic for APPswe. Processing of the amyloid precursor protein (APP) occurs on lipid rafts, where neuronal APP is sequentially cleaved by the endoproteases α- or β-secretase, and by γ-secretase [15]. The resulting soluble APP fragments (sAPPα and sAPPβ) are then processed to Aβ38-43 peptides [16]. Pro-amyloidogenic processing is also characterized by an increased sAPPβ:sAPPα ratio [16]. Cerebral cortex was compared with the cerebellum, a brain region with minimal amyloid deposits and neurodegeneration in AD [8].
Because TRAP-nPM induces oxidative stress [[12], [13], [14]] and because the lipid oxidation product 4-HNE can increase neuronal Aβ levels [17], we considered oxidative mechanisms in altered APP processing. We also assayed nitric oxide (NO) which is rapidly induced by nPM in vitro together with 4-HNE [13,14]. In vivo and in vitro models tested the hypothesis that nPM exposure would cause lipid raft oxidation and stimulate pro-amyloidogenic APP processing.
Section snippets
nPM collection and extraction
Nano-sized particulate matter (nPM, <0.2 μm in diameter) was collected on Teflon filters by a High-Volume Ultrafine Particle (HVUP) Sampler [18] at 400 L/min flow, 150 m downwind of the I-110 Freeway in central Los Angeles. These TRAP-PM0.2 particles are a mix of fresh ambient PM from vehicular traffic and industrial sources [19,20]. Filter-trapped nPM were eluted by sonication into deionized water and stored at −20 °C. Their chemical composition matched prior studies, with enrichment of ions
Results
Chronic nPM exposure of J20-hAPPswe mice increased Aβ, APP, oxidation in lipid rafts.
Cerebral cortex of male mice exposed to nPM for 150 total hours over 10 weeks had increased soluble Aβ and plaque Aβ load: total SDS-soluble Aβ40, +5-fold, p < 0.05; Aβ42, +10-fold, p < 0.05 (Fig. 1a); plaque Aβ, + 45%, p < 0.05 (Fig. 1b). Lipid raft enriched subcellular fractions had increased APP (65%, p < 0.05) (Fig. 1c and d). In contrast, no changes were detected in the endoproteases responsible for the
Discussion
This is the first evidence that air pollutants alter lipid rafts in neuronal cells in vivo and in vitro, and the first evidence associating lipid raft oxidation from air pollutants with pro-amyloidogenic processes relevant to AD. The relation of pro-amyloidogenic processes to membrane oxidation to lipid rafts was shown in vivo and in vitro with J20 mice and N2a cells carrying the familial AD gene, APPswe, that increases Aβ42 production. The mouse and cell findings are summarized in Table 1.
Funding
This study was supported by Cure Alzheimer's Fund and National Institute on Aging R21AG050201 and R01AG051521 and P50AG05142-31 to CEF, and National Institute of Environmental Health Sciences (NIEHS) ES023864 to HJF.
Acknowledgments
We thank Prof. Myron Goodman (USC) for use of the ultracentrifuge. This study was supported by Cure Alzheimer's Fund and National Institute on Aging R21AG050201 and R01AG051521and P50AG05142-31 to CEF, and National Institute of Environmental Health Sciences (NIEHS) ES023864 to HJF.
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