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. 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（alleles）进行比较时，基因表达的整体差异最为显著；而将风险中性的APOE3与致病型APOE4等位基因进行比较时，检测到的表达差异则相对更少。 相较于APOE3或APOE4，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。我们据此提出假说：APOE2的神经保护作用在衰老过程中是通过内体通路介导的。 与阿尔茨海默病（Alzheimer's disease）模型及APOE4模型不同，尽管早期内体（early endosomes）上的rab5b招募水平有所下降，但在衰老的APOE靶向替换小鼠的皮层神经元（cortical neurons）中，我们仍观察到早期内体的形态与丰度均维持在正常水平。 同样，尽管囊泡相关的液泡蛋白分选相关蛋白35（VPS35）的招募水平有所降低，但APOE2小鼠体内与分选返送复合体（retromer）相关的囊泡的形态与丰度仍保持正常。 值得注意的是，在衰老过程中，相较于APOE3小鼠，APOE2小鼠脑内的内体衍生外泌体（exosomes）的细胞外水平有所升高，这提示内体所载货物向细胞外空间的清除过程得到增强，进而有助于维持内体功能的稳态平衡。 综上，本研究结果表明，APOE2可通过增强外泌体生物发生（exosome biogenesis），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，并减轻衰老诱导的内体功能障碍——此类功能障碍会增加阿尔茨海默病的发病风险。 12月龄APOE靶向替换小鼠的半脑遗传表达谱