Ultrastructural and dynamic studies of the endosomal compartment in Down syndrome | Acta Neuropathologica Communications | Full Text

Ultrastructural and dynamic studies of the endosomal compartment in Down syndrome | Acta Neuropathologica Communications | Full Text

Ultrastructural and dynamic studies of the endosomal compartment in Down syndrome

• Alexandra Botté, 
• Jeanne Lainé, 
• Laura Xicota, 
• Xavier Heiligenstein, 
• Gaëlle Fontaine, 
• Amal Kasri, 
• Isabelle Rivals, 
• Pollyanna Goh, 
• Orestis Faklaris, 
• Jack-Christophe Cossec, 
• Etienne Morel, 
• Anne-Sophie Rebillat, 
• Dean Nizetic, 
• Graça Raposo & 
• Marie-Claude Potier 

Acta Neuropathologica Communications volume 8, Article number: 89 (2020) Cite this article

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Abstract

Enlarged early endosomes have been visualized in Alzheimer’s disease (AD) and Down syndrome (DS) using conventional confocal microscopy at a resolution corresponding to endosomal size (hundreds of nm). In order to overtake the diffraction limit, we used super-resolution structured illumination microscopy (SR-SIM) and transmission electron microscopies (TEM) to analyze the early endosomal compartment in DS.

By immunofluorescence and confocal microscopy, we confirmed that the volume of Early Endosome Antigen 1 (EEA1)-positive puncta was 13–19% larger in fibroblasts and iPSC-derived neurons from individuals with DS, and in basal forebrain cholinergic neurons (BFCN) of the Ts65Dn mice modelling DS. However, EEA1-positive structures imaged by TEM or SR-SIM after chemical fixation had a normal size but appeared clustered. In order to disentangle these discrepancies, we imaged optimally preserved High Pressure Freezing (HPF)-vitrified DS fibroblasts by TEM and found that early endosomes were 75% denser but remained normal-sized.

RNA sequencing of DS and euploid fibroblasts revealed a subgroup of differentially-expressed genes related to cargo sorting at multivesicular bodies (MVBs). We thus studied the dynamics of endocytosis, recycling and MVB-dependent degradation in DS fibroblasts. We found no change in endocytosis, increased recycling and delayed degradation, suggesting a “traffic jam” in the endosomal compartment.

Finally, we show that the phosphoinositide PI (3) P, involved in early endosome fusion, is decreased in DS fibroblasts, unveiling a new mechanism for endosomal dysfunctions in DS and a target for pharmacotherapy.