Ultrastructural and dynamic studies of the endosomal compartment in Down syndrome

• Published in: Acta neuropathologica communications Vol. 8; no. 1; pp. 89 - 22
• Main Authors: Botté, Alexandra, Lainé, Jeanne, Xicota, Laura, Heiligenstein, Xavier, Fontaine, Gaëlle, Kasri, Amal, Rivals, Isabelle, Goh, Pollyanna, Faklaris, Orestis, Cossec, Jack-Christophe, Morel, Etienne, Rebillat, Anne-Sophie, Nizetic, Dean, Raposo, Graça, Potier, Marie-Claude
• Format: Journal Article
• Language: English
• Published: England BioMed Central Ltd 24.06.2020; BioMed Central; BioMed Central part of Springer Science; BMC
• Subjects: Advertising executives; Alzheimer's disease; Analysis; Animals; Antibodies; Brain; Cloning; Deoxyribonucleic acid; DNA; Down syndrome; Down Syndrome - metabolism; Down Syndrome - pathology; Early endosomes; Electron microscopy; Endocytosis; Endosomes - metabolism; Endosomes - ultrastructure; Fibroblasts; Fibroblasts - metabolism; Fibroblasts - ultrastructure; Florbetapir; Genes; Humans; Induced Pluripotent Stem Cells; Laboratories; Life Sciences; Males; Mice; Microscopy; Microscopy, Confocal; Microscopy, Electron, Transmission; Morphology; Mutation; Neurobiology; Neurons; Neurons and Cognition; Peptides; Stem cells; Super-resolution microscopy; Tissue Fixation; Traffic congestion; Vitrification; Super-resolution microscopy; Endocytosis; Down syndrome; Alzheimer’s disease; Electron microscopy; Early endosomes; Alzheimer's disease
• ISSN: 2051-5960; 2051-5960
• DOI: 10.1186/s40478-020-00956-z

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.