Enhanced generation of intraluminal vesicles in neuronal late endosomes in the brain of a Down syndrome mouse model with endosomal dysfunction

Abstract

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.

Exosome Production Is Key to Neuronal Endosomal Pathway Integrity in Neurodegenerative Diseases

“…Recent findings argue that it is brain exosome production, and more specifically neuronal exosome production, which is higher in DS compared to diploid controls. Using high-resolution electron microscopy and quantitative analyses of MVBs in neurons of the frontal cortex of Ts2 mice, we recently showed that Ts2 MVBs are larger, more abundant, and contain a higher number of ILVs per neuron compared to littermate diploid controls (D'Acunzo et al, 2019). While not neuron specific, higher levels in Ts2 mice compared to controls of ILV proteins within the brain subcellular fractions that also contained endosomal markers corroborated these electron microscopy findings (D'Acunzo et al, 2019).…”

Extracellular Vesicles as an Efficient and Versatile System for Drug Delivery

“…Exosomes are the small-size subtype of EVs associated with endosomal biogenesis pathways [ 35 ]. The formation of exosomes starts with the inward budding of early endosomal membrane which creates intraluminal vesicles [ 36 ]. The generation of these intraluminal vesicles in endosomes is known to be highly regulated by endosomal sorting complex required for transport (ESCRT) complexes.…”

Extracellular vesicles: where the amyloid precursor protein carboxyl‐terminal fragments accumulate and amyloid‐β oligomerizes

“…4 Accordingly, our previous research has supported a beneficial role for EVs in vulnerable neurons with endosomal pathology, one of the earliest insults observed in Alzheimer's disease (AD), 5 by relieving the cells from toxic waste that was not efficiently degraded intracellularly. 6,7 However, the identification of full-length amyloid-β precursor protein (flAPP) and amyloid-β precursor protein (APP) metabolites in EVs [8][9][10][11] has implicated EVs in AD amyloid pathology by carrying and spreading pathology throughout the brain. The accumulation of the amyloid-β (Aβ) peptide in extracellular senile plaques is one of the pathological hallmarks of AD.…”