Chronic Hypoxia Exaggerates Alzheimer’s Disease And The Underlying Mechanistic Investigation

Alzheimer’s disease is the most common form of dementia. The pathological hallmarks of AD are neuritic plaques, neurofibrillary tangles (NFTs) and neuron degeneration in the brain. Aberrant hyperphosphorylation of tau protein is the main constituent of NFTs. Neuritic plaques are composed of β-amyloid (Aβ) peptide. Most cases of AD are sporadic late-onset form of AD (LOAD), with no specific genetic inheritance. The etiology of LOAD has not yet been elucidated. More and more evidences support that environmental risk factors and epigenetic alterations contribute greatly to the onset and progression of AD.Our previous studies documented that chronic hypoxia is a risk factor that may trigger AD development and aggravate the disease progression. We previously demonstrated that chronic hypoxia increased the neuritic plaques formation and Aβ generation by altering β-and γ-cleavage of APP in AD mice. Recently, we documented that prenatal hypoxia may aggravate the cognitive impairment and AD neuropathology in offspring adult AD mice by significantly inducing Aβ accumulation through increasing APP and decreasing NEP We proposed that chronic hypoxia may induce certain epigenetic modifications to histones or DNA, which may result in sustained changes in Aβ metabolism. While, autophagy, which is a highly conserved cellular catabolic process via lysosomal pathway for the turnover of organelles and abnormal aggregated proteins, is important for the clearance of Aβ and P-tau. More and more evidences implicate that autophagic dysfunction contributes to the progression of AD pathogenesis. In the present study, we also investigate the molecular mechanisms under which chronic hypoxia induced autophagy and contribute to AD pathogenesis in in APPSwe/PS1dE9 transgenic mice.The present study consists of two parts:Part 1:Chronic hypoxia facilitates Alzheimer’s disease through demethylation of y-secretase by down-regulating DNA methyltransferase 3bEnvironmental factors and epigenetic mechanisms are believed to contribute to the etiology AD. We have previously demonstrated that hypoxia increased Aβ generation by altering β-and γ-cleavage of APP in aged AD mice. Our latest study demonstrated prenatal hypoxia may aggravate the cognitive impairment and AD neuropathology in adult offspring in AD mice. Here, we further investigate the mechanisms under which chronic hypoxia facilitates AD, mainly focusing on epigenetic modifications. The APPSwe/PS1dE9 mice were exposed to hypoxic environment for 6 hours per day for 30 days since 3-month of age, and were fed in normobaric environment till the age of 4-,6-and 9-month. Morris water maze, Neuritic plaque and NTF immunostaining were used to detect the effects of chronic hypoxia on behavior and AD neuropathology. Western blot was performed to detect the proteins of interest. Dot blot and Bisulfite sequencing PCR were performed to detect the methylation levels of genomic DNA and CpG site of interest. We found that chronic hypoxia preceded the AD neuropathology and aggravate the cognitive impairment of the mice. We also found that chronic hypoxia induced demethylation on genomic DNA and decreased the expression of DNA methyltransferases (DNMTs). Inhibition of DNMTs by 5-Aza-CdR elevated the protein level of APP, β-and y-secretase, while overexpression of DNMT3b suppressed the levels of them in vitro. Our study suggests chronic hypoxia can aggravate AD progression through demethylation of y-secretase by down-regulation of DNMT3b. Epigenetic modifications may offer new treatment strategy for the disease.Part 2:Chronic hypoxia-induced autophagy activation aggravates the neuropathology of Alzheimer’s disease by AMPK-mTOR signaling in APPSwe/PS1dE9 mouse modelAutophagy is the most important mechanism for mammalian cells to degrade damaged organelles and abnormal proteins. Enormous evidence suggests that the autophagy pathway is impaired in AD and autophagy impairment can contribute to the pathogenesis of AD. In this study, we investigated the effect of chronic hypoxia on AD and explored whether autophagic dysfunction was involved in the hypoxia mediated-pathogenesis of AD in APPSwe/PS1E9 transgenic mice. We found that chronic hypoxia induced significant activation of autophagy in brain of the experimental mice. We also found that autophagy was defect in Tg mice and chronic hypoxia aggrevated the impairment of autophagy. Furthermore, we discovered that chronic hypoxia activated AMPK and mTOR signaling pathway, while inhibition of AMPK reduced the autophagy by increasing the phosphorylation of mTOR. In short, our study provides a new insight into the mechanism underlying chronic hypoxia-mediated AD pathogenesis. Our findings suggest that chronic hypoxia can aggravate the pathology of AD through induction of autophagy by AMPK-mTOR signaling.