Down syndrome (DS) is a human genetic disease caused by trisomy of chromosome 21 and characterized by early developmental brain abnormalities. Dysfunctional endosomal pathway in neurons is an early event of DS and Alzheimer's disease. Recently, we have demonstrated that exosome secretion is upregulated in human DS postmortem brains, in the brain of the trisomic mouse model Ts[Rb(12.17¹⁶)]2Cje (Ts2) and by DS fibroblasts as compared with disomic controls. High levels of the tetraspanin CD63, a regulator of exosome biogenesis, were observed in DS brains. Partially blocking exosome secretion by DS fibroblasts exacerbated a pre‐existing early endosomal pathology. We thus hypothesized that enhanced CD63 expression induces generation of intraluminal vesicles (ILVs) in late endosomes/multivesicular bodies (MVBs), increasing exosome release as an endogenous mechanism to mitigate endosomal abnormalities in DS. Herein, we show a high‐resolution electron microscopy analysis of MVBs in neurons of the frontal cortex of 12‐month‐old Ts2 mice and littermate diploid controls. Our quantitative analysis revealed that Ts2 MVBs are larger, more abundant, and contain a higher number of ILVs per neuron compared to controls. These findings were further corroborated biochemically by Western blot analysis of purified endosomal fractions showing higher levels of ILVs proteins in the same fractions containing endosomal markers in the brain of Ts2 mice compared to controls. These data suggest that upregulation of ILVs production may be a key homeostatic mechanism to alleviate endosomal dysregulation via the endosomal–exosomal pathway.

中文翻译：

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唐氏综合症小鼠内体功能障碍的大脑神经元晚期内体中腔内囊泡生成的增强。

唐氏综合症（DS）是一种人类遗传疾病，由21号染色体的三体性引起，其特征是早期发育的大脑异常。神经元中的内体途径失调是DS和阿尔茨海默氏病的早期事件。最近，我们已经证明三体小鼠模型Ts [Rb（12.17 ^16）的人类DS死后大脑中，外泌体分泌被上调。）] 2Cje（Ts2）和DS成纤维细胞与二体组对照相比。在DS脑中观察到高水平的四跨膜蛋白CD63（一种外泌体生物发生的调节剂）。DS成纤维细胞部分阻止外泌体分泌加剧了先前存在的早期内体病理。因此，我们假设增强的CD63表达诱导了晚期内体/多囊体（MVB）中的腔内囊泡（ILV）生成，增加了外来体释放，作为减轻DS中内体异常的内源性机制。在本文中，我们显示了对12个月大的Ts2小鼠和同窝二倍体对照的额叶皮层神经元中MVB的高分辨率电子显微镜分析。我们的定量分析表明，与对照组相比，Ts2 MVB更大，更丰富，并且每个神经元包含更多的ILV。通过蛋白质印迹分析纯化的内体组分，进一步证实了这些发现，与对照相比，在Ts2小鼠的大脑中含有内体标记的相同组分中，ILVs蛋白水平更高。这些数据表明，ILVs产生的上调可能是通过内体-外体途径缓解内体失调的关键稳态机制。