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Apolipoprotein E2 protects against aging-induced neuronal endosomal dysfunction through enhanced exosome secretion

ABSTRACT: In 12-month-old APOE targeted-replacement mice, we report that overall differences in gene expression were the most prominent when comparing the protective APOE2 to the other two alleles, with fewer differences found when comparing the risk-neutral APOE3 and disease-promoting APOE4 alleles. When compared with either APOE3 or APOE4, differential expression of genes within the endosomal pathways is a prominent feature of APOE2 expression in the brain. We hypothesized that the protective effects of APOE2 are mediated through the endosomal pathway during aging. In contrast to Alzheimer’s disease and APOE4 models, we detected normal morphology and abundance of early endosomes within cortical neurons of APOE2 targeted-replacement mice during aging despite decreased rab5b recruitment to early endosomes. Similarly, the morphology and abundance of retromer-associated vesicles was normal in APOE2 mice, despite reduced recruitment of vesicle-associated VPS35. Significantly, we observed increased brain extracellular levels of endosome-derived exosomes in APOE2 compared with APOE3 mice during aging, indicative of an enhanced endosomal cargo clearance to the extracellular space that contributes to a homeostatic balance of endosomal functions. Our findings thus demonstrate that APOE2 effectively offsets endosomal pathway changes during aging to preserve its integrity by enhancing exosome biogenesis, mitigating age-driven endosomal dysfunction that contributes to Alzheimer’s disease risk.

ORGANISM(S): Mus musculus

PROVIDER: GSE188267 | GEO | 2023/07/31

REPOSITORIES: GEO

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Project description:The APOE gene is diversified by three alleles e2, e3, and e4 encoding corresponding apolipoprotein (apo) E isoforms. The e4 allele promotes age-related cognitive decline, risk of Alzheimer’s disease (AD), and the rate of AD dementia progression. ApoE is produced by astrocytes as high-density lipoprotein-like particles and these are internalized by neurons upon binding to neuron-expressed apoE receptors. ApoE isoforms differentially engage neuronal plasticity through poorly understood mechanisms. We examined here the effects of native apoE lipoproteins produced by immortalized astrocytes homozygous for e2, e3, and e4 alleles on the maturation and the transcriptomic profile of primary hippocampal neurons. Control neurons were grown in the presence of conditioned media from Apoe-/- astrocytes. ApoE2 and apoE3 significantly increase the dendritic arbor branching, the combined neurite length, and the total arbor surface of the hippocampal neurons, while apoE4 fails to produce similar effects and even significantly reduces the combined neurite length compared to the control. ApoE lipoproteins show no systemic effect on dendritic spine density, yet apoE2 and apoE3 increase the mature spines fraction, while apoE4 increases the immature spine fraction. This is associated with opposing effects of apoE2 or apoE3 and apoE4 on the expression of NR1 NMDA receptor subunit and PSD95. There are 1,062 genes differentially expressed across neurons cultured in the presence of apoE lipoproteins compared to the control. KEGG enrichment and gene ontology analyses show common for apoE2 and apoE3 activation of genes involved in neurite branching, and synaptic signaling. In contrast, apoE4 cultured neurons show upregulation of genes related to the glycolipid metabolism, which are involved in dendritic spine turnover and those which are usually silent in neurons and are related to cell cycle and DNA repair. In conclusion our work reveals that lipoprotein particles comprised of various apoE isoforms differentially regulate neuronal development through interaction with neuronal transcriptome. ApoE4 produces a functionally distinct transcriptomic profile, which is associated with attenuated neuronal development. Differential regulation of neuronal transcriptome by apoE isoforms is a newly identified biological mechanism, which has both implication in the development and aging of the CNS.

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Apolipoprotein E2 protects against aging-induced neuronal endosomal dysfunction through enhanced exosome secretion

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