Rapid doubling of Alzheimer’s amyloid-β40 and 42 levels in brains of mice exposed to a nickel nanoparticle model of air pollution

Introduction

One common neurodegenerative disease, Parkinson’s disease, has been linked to exposure to MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and to inhaled manganese^1,2. Similarly, inhaled aluminum dust has been associated with neurotoxic effects and pre-clinical cognitive impairment³. Certain inhalation anesthetics have also been implicated in elevating AD risk, possibly by exacerbating the neurotoxic oligomerization of the amyloid-β (Aβ) peptide⁴. The early involvement of the olfactory cortex in AD has caused longtime speculation that some inhaled agent might play a role in AD risk⁵.

Recently, AD pathology was identified in young people living in areas with high levels of air pollution^6,7. Furthermore, impaired cognition has been recently attributed to air pollution exposure in some populations⁸. These converging lines of evidence led us to analyze brain levels of Aβ40 and Aβ42 in mice exposed to an inhaled particulate matter (nickel nanoparticle; Ni NP) model of air pollution.

Methods

All procedures involving animals were conducted in compliance with guidelines for ethical animal research and approved by the New York University School of Medicine Animal Care and Use Committee. Two-month-old male and female FVBN mice (Taconic Farm, Hudson, NY) were randomly assigned to Ni NP inhalation (count median diameter 54 nm, at 1 mg/m³, which is the current Occupational Safety and Health Administration’s Permissible Exposure Limit for nickel hydroxide [http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=standards&p_id=9992]) (n = 16 per group) or control filtered air (n = 5 per group) for 3 hours in a nose-only exposure chamber. This protocol has been established as a model for air pollution toxicity in pulmonary disease⁹, atherosclerosis¹⁰, and insulin resistance¹¹. Twenty-four hours post exposure, mice were given pentobarbital, bled out via the vena cava, and then their brains were harvested, snap frozen and stored at -80ºC until assay. For measurement of endogenous mouse brain Aβ40 and Aβ42, we employed the Schmidt method¹² and human/rat Aβ 1–40/1–42 ELISA kits (Wako, Richmond, VA). Statistical analysis was performed via Mann-Whitney test. #8 Ni NP is excluded from the analysis due to being more than 2 SD's away from mean or closest value.

Results

Both endogenous Aβ40 and Aβ42 were elevated in the brains of mice following Ni NP exposure (Figure 1). Aβ40 was increased by 1.72-fold (P = 0.0011, Mann-Whitney test), and Aβ42 was increased by 2.29-fold (P = 0.0005, Mann-Whitney test). Aβ42/40 ratio was also increased in the Ni NP-exposed group compared to the filtered air control group (0.27 ± 0.01 and 0.21 ± 0.007, respectively; P = 0.0093, Mann-Whitney test). Both male and female mice responded similarly to Ni NP exposure (male vs. female for Aβ40 and Aβ42 levels; P > 0.1, Mann-Whitney test).

Figure 1. Exposure to air pollution increases amyloid-β (Aβ) levels in the mouse brain.

Elevated endogenous mouse brain Aβ40 and Aβ42 in mice exposed to nickel nanoparticles (count median diameter 54 nm, at 1 mg/m³) (n = 16 per group) versus filtered air (n = 5 per group) for 3 hours in a nose-only exposure chamber. Data presented as mean + SEM. **P < 0.01, ***P < 0.001 (Mann-Whitney test).

Discussion

These data add credence to the proposal⁴ that one or more inhaled neurotoxin(s) might increase the risk for AD by elevating levels of brain Aβ. We have not identified whether this accumulation occurs at the level(s) of transcription, translation, or post-translational processing. It is tempting to speculate that the well-known links between inhaled toxins and brain inflammation, and other links between brain inflammation and AD established by Griffin and colleagues¹³ may underlie these phenomena.

The changes that we observed were dramatic, rapid, and unexpected. Human Aβ is more aggregatable than murine Aβ, making it conceivable that the effect on Aβ levels in human brain could be even greater. While elucidating the genesis and molecular underpinnings will be an important next step, an even more important step will be a rigorous application of environmental toxicology and epidemiology to determine whether the elevated brain Aβ caused in mice by this air pollution model corresponds to any situation of authentic human inhalation exposure that is linked to an increased risk for AD.