APOE and the PPP: Glucose Metabolism and Oxidative Stress in Alzheimer's Disease

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ABSTRACT Metabolic dysfunction may contribute to the development of several age-related diseases, including Alzheimer's disease (AD). The gene Apolipoprotein E (APOE) encodes three major isoforms in the human population: E2, E3, and E4. E4 is the most significant genetic risk factor for sporadic AD, while E2 is protective. An understudied hallmark of AD patients – and of cognitively normal E4 individuals – is cerebral glucose hypometabolism. E4-associated reductions in glucose uptake begin decades prior to cognitive impairment, however the mechanism by which it occurs and its relevance to AD risk remain unknown. The brain predominantly metabolizes glucose, a substantial amount of which is shunted to the pentose phosphate pathway (PPP) in both neurons and astrocytes. The PPP generates antioxidant reducing factors such as NADPH and glutathione, and decreased PPP activity increases oxidative stress and cell death. Interestingly, our novel preliminary data describe a murine model with human apoE that recapitulates an E4-associated decrease in glucose metabolism and also documents decreases in multiple PPP metabolites. Thus, the central hypothesis of this proposal is that APOE influences neuronal function and survival through isoform-specific changes in glucose metabolism. Specifically, we hypothesize that E4 contributes to cognitive impairment through metabolic reprogramming in which glucose uptake is decreased and redox management via the PPP is reduced. Our preliminary data in mice show a stepwise decrease in brain glucose uptake (E2>E3>E4), and in vitro results suggest these differences are due to changes in astrocytic uptake via GLUT-1. Therefore, in the first Aim, we will test the hypothesis that E4 decreases cerebral glucose uptake through downregulation of the astrocytic glucose transporter GLUT-1 using a scintillation proximity assay with targeted manipulation of apoE isoforms, total protein concentrations and glucose transporters. To test the hypothesis that E4 decreases glucose entry into the PPP, we will quantitatively track glucose entry and metabolism in the cell through the unique precursor-product “tracing” afforded by Stable Isotope Resolved Metabolomics (SIRM), and translate our results through analysis of human brain tissue. Finally, we will test the hypothesis that E4 exacerbates oxidative damage and cell death due to a reduction in PPP-mediated management of oxidative stress. This will be accomplished in vitro through pharmacological manipulation of PPP enzymes and in vivo by assessing cognitive function, AD pathology, and oxidative damage using redox proteomics analysis of brain tissue from human apoE mice treated with a PPP stimulant. If successful, this proposal will provide novel therapeutic targets to normalize glucose metabolism in high-risk individuals. Enhancing cerebral metabolism by increasing glucose uptake and entry into the PPP could have great impact in preventing or delaying the onset of AD.