Neuronal Apolipoprotein E4 Expression Results in Proteome-Wide Alterations and Compromises Bioenergetic Capacity by Disrupting Mitochondrial Function

Abstract

Apolipoprotein (apo) E4, the major genetic risk factor for Alzheimer’s disease (AD), alters mitochondrial function and metabolism early in AD pathogenesis. When injured or stressed, neurons increase apoE synthesis. Because of its structural difference from apoE3, apoE4 undergoes neuron-specific proteolysis, generating fragments that enter the cytosol, interact with mitochondria, and cause neurotoxicity. However, apoE4’s effect on mitochondrial respiration and metabolism is not understood in detail. Here we used biochemical assays and proteomic profiling to more completely characterize the effects of apoE4 on mitochondrial function and cellular metabolism in Neuro-2a neuronal cells stably expressing apoE4 or apoE3. Under basal conditions, apoE4 impaired respiration and increased glycolysis, but when challenged or stressed, apoE4-expressing neurons had 50% less reserve capacity to generate ATP to meet energy requirements than apoE3-expressing neurons. ApoE4 expression also decreased the NAD + /NADH ratio and increased the levels of reactive oxygen species and mitochondrial calcium. Global proteomic profiling revealed widespread changes in mitochondrial processes in apoE4 cells, including reduced levels of numerous respiratory complex subunits and major disruptions to all detected subunits in complex V (ATP synthase). Also altered in apoE4 cells were levels of proteins related to mitochondrial endoplasmic reticulum–associated membranes, mitochondrial fusion/fission, mitochondrial protein translocation, proteases, and mitochondrial ribosomal proteins. ApoE4-induced bioenergetic deficits led to extensive metabolic rewiring, but despite numerous cellular adaptations, apoE4-expressing neurons remained vulnerable to metabolic stress. Our results provide insights into potential molecular targets of therapies to correct apoE4-associated mitochondrial dysfunction and altered cellular metabolism.

E2F4 as a single multifactorial target against Alzheimer’s disease

“…In our case, functional recovery would be obtained by expressing an E2F4 form unable to become Thr phosphorylated, thus counteracting the pathological environment that favors E2F4 phosphorylation. In this way, neuronal E2F4DN expression could efficiently target the complex etiology of AD (15)(16)(17)(18), thus becoming a promising molecule for a successful therapy against this devastating disease.…”

Apolipoprotein E4, inhibitory network dysfunction, and Alzheimer’s disease

“…An intriguing explanation for GABAergic interneuron selective vulnerability to apoE4, then, is that they have unique demands for high energy production which, in turn, makes them vulnerable to any perturbation of mitochondrial function [189, 190]. A recent study reports that apoE4-expressing neuronal cells have 50% less reserve capacity to generate ATP than apoE3-expressing neuronal cells as well as widespread changes in mitochondrial protein production and translocation, which makes apoE4-expressing neuronal cells more vulnerable to metabolic stress [191]. Building off these data, a testable hypothesis is that apoE4-induced mitochondrial dysfunction is especially damaging to GABAergic interneurons because of their especially high demands for metabolic energy.…”

APOE4 Lowers Energy Expenditure and Impairs Glucose Oxidation by Increasing Flux through Aerobic Glycolysis

“…These results are congruent with other studies of APOE4 and AD. For example, a recent study by our group demonstrated that E4 astrocytes have increased lactate production 53 , and neurons expressing E4 exhibit increased reliance on glycolysis for ATP production with apparent deficits in mitochondrial respiration 54 . Interestingly, another study showed that fibroblasts from AD patients show a ‘Warburg-type’ (aerobic glycolysis) shift from oxidative phosphorylation to glycolysis with increased lactate production 55 .…”