APOE4 increases energy metabolism in APOE-isogenic iPSC-derived neurons
The apolipoprotein E4 (APOE4) allele represents the major genetic risk factor for Alzheimers disease (AD). In contrast, APOE2 is known to lower the AD risk while APOE3 is defined as risk neutral. APOE plays a prominent role in the bioenergetic homeostasis of the brain, and early-stage metabolic changes have been detected in brains of AD patients. Although APOE is primarily expressed by astrocytes in the brain, neurons also have been shown as source for APOE. However, little is known about the differential role of the three APOE isoforms for neuronal energy homeostasis. In this study, we generated pure human neurons (iN cells) from APOE-isogenic induced pluripotent stem cells (iPSCs), expressing either APOE2, APOE3, APOE4 or carrying an APOE-knockout (KO) to investigate APOE isoform-specific effects on neuronal energy metabolism. We showed that endogenously produced APOE4 enhanced mitochondrial ATP production in APOE-isogenic iN cells but not in the corresponding iPS cell line. This effect neither correlated with the expression levels of mitochondrial fission or fusion proteins, nor with the intracellular or secreted levels of APOE, which were similar for APOE2, APOE3 and APOE4 iN cells. ATP production and basal respiration in APOE-KO iN cells strongly differed from APOE4 and more closely resembled APOE2 and APOE3 iN cells indicating a gain-of-function mechanism of APOE4 rather than a loss-of-function. Taken together, our findings in APOE isogenic iN cells reveal an APOE genotype-dependent and neuron-specific regulation of oxidative energy metabolism.