APOE4 Increases Energy Metabolism in APOE-Isogenic iPSC-Derived Neurons.

The apolipoprotein E4 (APOE4) allele represents the major genetic risk factor for Alzheimer's 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 the brains of AD patients. Although APOE is primarily expressed by astrocytes in the brain, neurons have also been shown as source for APOE. However, the distinct roles of the three APOE isoforms in neuronal energy homeostasis remain poorly understood. 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.

Effects of postmortem delays on protein composition and oxidation.

Human autopsy brain tissue is widely used to study neurodegenerative diseases such as Alzheimer's, Parkinson's and other diseases. However, when it comes to an evaluation of data obtained from such tissue, it is essential to consider potential postmortem effects on protein composition, posttranslational modification and proteolysis with increasing postmortem delays. In this study, we analyzed mouse brain tissues with different postmortem delays (pmd) of 0 h, 6h and 24h, for changes in protein composition, proteolysis and modifications such as S-nitrosylation, carbonylation and ubiquitination. Proteins involved in Alzheimer's disease (AD) were of special interest, including cytoskeletal and synaptic proteins or proteins involved in inflammation. Several proteins were fairly resistant to degradation during the first 6h but started to degrade thereafter. S-nitrosylation and carbonylation showed not much variation, except for those proteins that were susceptible to degradation. Brain spectrin was S-nitrosylated at death, and S-nitrosylated degradation fragments were measured at a pmd of 24h, indicating a susceptibility of brain spectrin to degradation. Furthermore, the physiological role of S-nitrosylation remains to be investigated. When studying human brain tissue, some proteins are more susceptible to degradation than others, while ubiquitination and carbonylation were little affected during the first 24h after death.