Sex-dependent synergy between O3 exposure, APOE4 e4 genotype, and aging in the onset of Alzheimer's disease

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The etiology for late-onset Alzheimer’s disease (LOAD), which accounts for >95% of AD cases, is unknown. Aging is the greatest risk factor for LOAD, whereas APOE4 is believed to be a major genetic risk factor in acquiring LOAD, with female APOE4 carriers at the greatest risk. Yet, not all of APOE4 carriers, even older female carriers, acquire AD, suggesting that other factors including environmental exposure must play a role. Ozone (O3) is a highly reactive oxidant and one of the most abundant urban pollutants. Recent epidemiology studies show that exposure to high levels of O3 is associated with an increased incidence of AD in the elderly and that APOE4 carriers are more sensitive to O3-induced memory decline than non-APOE4 carriers are, although no study has addressed sex-differences in response to O3. In an R21 project, we tested the hypothesis that O3 exposure synergies with aging and APOE4 leading to AD, using male apoE4 target replacement (TR) mice (only male mice were proposed due to time and budget limitation). We found, surprisingly, that O3 exposure impaired memory in old apoE3 (represents the majority of human population who carry the APOE3 allele) male mice, while old apoE4 or young apoE3 and apoE4 male mice were spared 6. Associated with memory loss, old apoE3 male mice exhibit increased protein oxidative modifications (glutathionylation) and neuroinflammation, compared to other groups6. Old apoE4 male mice, on the other hand, have significantly increased expression/activities of several antioxidant enzymes and diminished protein oxidation as well as neuroinflammation upon O3 exposure6. Our data suggest that an elevated antioxidant capacity may underlie the increased resistance of old male apoE4 mice to O3-induced memory loss. Our data also suggest that APOE4 may affect the sensitivity to O3-induced memory loss in a sex-dependent manner, just as it does to the toxicities of other toxicants. This R01 will continue our R21 project to further test sex-dependent effects of ApoE4 and O3. As oxidative stress plays a critical role in AD pathophysiology and plasma level of estrogen, an inducer of many antioxidant enzymes, decreases with age in females, our new data-supported hypothesis is that O3 exposure synergizes with aging and APOE4 leading to LOAD in sex-dependent manner, which is to promote AD in females but not in males. We will test this hypothesis in two specific Aims, using apoE3 and apoE4 TR mice and a cyclic O3 exposure protocol, which mimics human exposure scenarios. In Aim 1, we will test the hypothesis that O3 exposure induces AD-like pathophysiology in female apoE4 TR mice compared to their male counterparts and this is exacerbated by aging. In Aim 2, we will test the hypothesis that restoration of brain antioxidant capacity with tert-butyl hydroquinone (TBHQ), a canonical activator of nuclear factor erythroid 2-related factor 2 (Nrf2), will eliminate sex-APOE genotype-aging-dependent sensitivity to O3-induced neuropathophysiology. The results from these studies will not only shed new light on the etiology of LOAD but may also lead to the development of new strategies for the prevention and treatment of this devastating disease.