ApoE2-Mediated Neuroprotective Mechanism Through Up-regulation of Glycolysis

Abstract

Studies presented here aim at gaining an insight into mechanisms of how human apolipoprotein E (ApoE) isoforms impact glucose metabolism, particularly through regulation of glycolysis, which may ultimately result in pathophysiological alterations in the brain. Consistent with our previous findings, hexokinase, the enzyme that catalyzes the initial and irreversible conversion of glucose to 6-phophoglucose in the first step of glycolysis, is significantly affected by ApoE isoforms in stably human Apo E2, 3, or 4 expressing Neuro-2a cell lines. Results from a time course study indicate that the regulation of hexokinase by ApoE is typically through a chronic pattern. Additionally, glycolytic function was also differentially regulated by three ApoE isoforms with ApoE2 group shows the most robust profile. The data indicate that hApoE2-expressing cells exhibited significantly enhanced glycolytic activity compared to ApoE4-expressing cells, possibly through the upregulation of hexokinase. With the evidence that ApoE isoforms differentially regulate glycolytic function via hexokinase, cell health status was further assessed by both metabolic activity and morphological phenotype. In line with our prediction, the regulation of hexokinase and the differential glycolytic profiles directly correlated to the overall health status of the three ApoE isoforms-expressing cells. Meanwhile, we observed no significant alteration in apoptotic markers and the insulin-regulated glucose transporter, which further supports a neuroprotective role of ApoE2 through up-regulation of glucose metabolism. Furthermore, this thesis employed a differentiated neuronal model to determine the influence of ApoE on the regulation of neuronal glycolysis. In transfected neurons, differential regulation of hexokinase and glycolytic function by hApoE2/3/4 was also observed. hApoE2-transfected neurons exhibited a significantly higher expression and activity of hexokinase as well as lactate production than cells transfected with ApoE3 or ApoE4. Taken together, results from these studies indicate that human ApoE isoforms differentially modulate neuronal glycolysis via regulation of hexokinase, which directly correlates to neuronal metabolic activity and health status. The ApoE2-mediated glycolytic robustness may suggest a mechanistic rationale for its neuroprotective role and consequently provides a novel therapeutic approach against the onset of AD.