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

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. Overall design: Hemibrain genetic profiles of targeted-replacement APOE2, APOE3, and APOE4 mice at 12 months of age

在12月龄的载脂蛋白E（APOE）靶向替换小鼠中，我们发现将具有神经保护作用的APOE2与另外两种等位基因进行比较时，基因表达的整体差异最为显著；而比较风险中性的APOE3与促疾病APOE4等位基因时，差异则相对较少。与APOE3或APOE4相比，内体通路（endosomal pathways）内基因的差异表达是APOE2在大脑中表达的显著特征。我们提出假说：APOE2的神经保护作用在衰老过程中通过内体通路介导。
与阿尔茨海默病（Alzheimer’s disease）及APOE4模型不同，尽管rab5b向早期内体（early endosomes）的招募水平降低，但在衰老过程中，APOE2靶向替换小鼠的皮层神经元内早期内体的形态与丰度均保持正常。同样地，尽管囊泡相关VPS35（VPS35，囊泡分选蛋白35）的招募水平降低，但APOE2小鼠体内与逆传体（retromer）相关的囊泡形态与丰度同样维持正常。
值得注意的是，在衰老过程中，与APOE3小鼠相比，APOE2小鼠的脑内源自内体的外泌体（exosomes）水平升高，这提示APOE2可增强内体所载物质向细胞外间隙的清除，从而维持内体功能的稳态平衡。综上，我们的研究结果表明，APOE2可通过增强外泌体的生物发生，有效抵消衰老过程中内体通路的异常变化，维持内体通路的完整性，进而缓解衰老诱导的、可增加阿尔茨海默病发病风险的内体功能障碍。
整体实验设计：对12月龄的APOE2、APOE3及APOE4靶向替换小鼠的半脑进行基因表达谱分析。