The Pathological Roles And Mechanistic Investigation Of Chronic Sleep Deprivation And Acute Hypoxia In Alzheimer’s Disease

Alzheimer’s disease(AD)is a progressive neurodegenerative disease featured by cognitive impairment and behavior disorders.It is the most common cause of dementia in the elderly.The pathological hallmarks of AD are the presence of amyloid-β(Aβ)deposition as senile plaques(SPs),neurofibrillary tangles(NFTs)caused by hyperphosphorylated tau protein,and neurodegeneration.Although the etiology and pathological m echanisms of AD are still unclear,it is generally considered that AD arises through the interaction among various genetic and environmental factors.Genetic factors include dominantly inherited mutations in Aβ precursor protein(APP),presenilin 1(PS1),and presenilin 2(PS2),which cause mostly early onset AD,and risk genes associated with familial and sporadic AD including apolipoprotein E(APOE).Despite these AD-related genes,only a small proportion(<1%)of patients have autosomal dominant inherited AD2.Most patients(>95%)are sporadic AD,which is considered as the consequence of interactions between genetic risk and non-genetic factors.There are several major non-genetic factors such as cerebrovascular diseases,type 2 diabetes,brain trauma,intellectual activity,hypoxia,sleep disorders,and others,which may contribute to the disease onset and progression.Besides Aβ and tau pathologies,dysregulated autophagy,mitochondrial dysfunction and neuroinflammation also play important roles in the pathogenesis of AD.Autophagy is a cellular process that uses double-membrane vesicles to deliver cytoplasmic contents to lysosomes for degradation.It is essential to clear toxic or aggregated proteins and dysfunctional or damaged organelles such as mitochondria.It is reported that autophagy in microglia degrades extracellular Aβ fibrils and the autophagy-lysosome system degrades tau protein in various forms.Autophagic degradation system is reported to be compromised in AD and could be disrupted by AD-related PS1 mutations.Mitochondrial dysfunction is another feature of AD.It has been shown that mitochondrial dysfunction in AD brain with reduced membrane potential,increased permeability,and excessive ROS production,which is believed to contribute to neurodegeneration.In addition,neuroinflammation also plays an important role in AD pathogenesis.SPs and NFTs can activate immune cells,leading to the release of inflammatory factors and neurotoxins which further cause neuronal loss.Sleep disorders,as one of the important environmental risk factors,are believed to contribute to the pathogenesis of AD,and hypoxia,as another important environmental risk factor,is reported to facilitate Aβ production,increase tau phosphorylation and induce neuroinflammation in AD.In the current study,we investigated the pathological roles of chronic sleep deprivation(CSD)and acute hypoxia in the pathogenesis of AD and their underlying mechanisms.In clinical patients,comorbidities such as respiratory dysfunction,cardiovascular and cerebral vascular diseases can cause sleep disorders and hypoxia conditions.A better understanding of the mechanisms of chronic hypoxia in AD pathogenesis will provide useful information for AD prevention and therapeutics.The current study consists of two parts:PART Ⅰ The pathological roles and underlying mechanisms of chronic sleep d epri vation in Alzheimer’s diseaseIn this part,we investigated the effect of CSD on AD-related pathological changes and its roles in the epigenetic modulation of tau pathology.We will also investigate the neuroinflammation response induced by CSD.APPswe/PS1dE9 AD mice and their wild type(WT)littermates at 4～4.5 months of age were included in this study.CSD was carried out according to small platform method for 2 months with minor modifications(deprived from 12:00 to next 8:00).After CSD,sleep electroencephalography(EEG)was monitored and the mice were tested with cognitive behavioral tests.Pathological and biochemical tests for AD-related pathological changes were performed in hippocampus and cortex of AD and WT mice.We found that CSD changed sleep patterns and decreased sleep quality,sleep maintenance,and sleep stability,increased arousal in sleep and fragmentation of sleep in AD and WT mice.CSD facilitated the decline of learning ability and memory cognitive function in AD and WT mice.CSD increased Aβ and tau pathologies in hippocampus and cortex of AD and WT mice.CSD caused decreases in genomic DNA methylation and the methylation of CpGs in promotors of tau kinases genes in hippocampus and cortex of AD and WT mice,which led to hyperphosphorylation of tau.This might be associated with down-regulation of DNA methyltransferases(DNMT3A)and 3B.CSD increased microglia in hippocampus and cortex of AD and WT mice along with an imbalanced activation of M1/M2,which might be associated with the activation of nuclear factor-κB(NF-κB)signaling.PART Ⅱ The pathological roles and underlying mechanisms of acute hypoxia in Alzheimer’s diseaseIn this part,we investigated the pathological roles of acute hypoxia in AD.APPswe/PS1dE9 AD mice and their WT littermates at 6 months of age were included in this study.The hypoxia groups were exposed to a continued hypoxic condition(oxygen 7%)in a hypoxic chamber for 24 hours.After acute hypoxia treatment,Pathological and biochemical tests for AD-related pathological changes were carried out in hippocampus and cortex of AD and WT mice.We found that acute hypoxia increased APP γ-cleavage and Aβ production through up-regulation of APP and anterior pharynx-defective 1α(APH1α),a component of γ-secretase.Acute hypoxia up-related tau kinase cyclin-dependent-like kinase-5(CDK5)and led to phosphorylation of tau at T181 and T231 sites.Acute hypoxia activated autophagy with an impaired autophagy flux,which was associated with inhibition of mTOR pathway.Acute hypoxia elicited mitochondrial dysfunction,as evidenced by increased cytosolic levels of cytochrome C(Cyt C)and cytochrome C oxidase subunit Ⅳ(COX Ⅳ).Acute hypoxia increased microglia M1 activation and attenuated M2 activation,along with an increase of pro-inflammatory cytokines and chemokines and a decrease of anti-inflammatory cytokines.