Changing the energy substrate balance: Does APOE2 promote glucose usage to protect from Alzheimer's Disease?

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Metabolic dysfunction, as in the case of obesity, contributes to the development of several age-related diseases, including Alzheimer’s disease (AD). Apolipoprotein E (apoE) is a critical component of circulating lipoproteins found both in the periphery and brain, and the APOE gene 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. Like obesity, the E4 isoform is also associated with an increased risk of AD. Paradoxically, mice and humans with E4 have an increased risk of AD despite having reduced adiposity, while those with E2 are protected from AD despite increased adiposity. Our preliminary findings in mice suggest that these two factors – obesity and apoE – may be linked by a unifying biological mechanism related to energy substrate preference, and have broad implications for the prevention and personalized (genetic) treatment of AD. Our central hypothesis is that the apoE isoforms differentially shift the Randle cycle balance: E2 in favor of glucose, E4 in favor of FA. Specifically, we hypothesize that E2 exerts its neuroprotective effects through a metabolic preference for glucose utilization at the expense of fatty acid oxidation. To test the hypothesis that E2 affords neuroprotection by decreasing fatty acid oxidation and increasing glucose utilization, we will quantitatively track substrate entry and metabolism through the unique precursor-product “tracing” afforded by Stable Isotope Resolved Metabolomics (SIRM). We will then use a multi-omics approach to integrate SIRM results with transcriptomic profiling of human apoE expressing mice and cells in order to identify the specific metabolic pathways altered by E2 expression in both the brain and periphery. Finally, we will translate our findings by measuring basal metabolic rates in E2-, E3- and E4-expressing individuals at rest, and during a cognitive challenge. Using O2 and CO2 recordings, we will calculate the respiratory exchange ratio (RER), which reflects the ratio of carbohydrate/lipid oxidation, across each trial. Analyzing these data based on APOE status will provide a new understanding of substrate utilization in E2+ individuals, and offer insight into potential apoE effects on peripheral and neural energy responses. If successful, this proposal will provide novel targets by characterizing the neuroprotective metabolic profile of E2 individuals and designing future therapies to mimic these effects. Enhancing cerebral metabolism by making “E2-like” changes to energy balance could have great impact in preventing or delaying the onset of AD.