Endosome abnormalities are another common cellular phenotype in AD. Enlarged RAB5-positive early endosomes have been observed in mouse models and in the brains of sAD and fAD patients [70,71]. Such enlarged endosomes may contain aberrantly phosphorylated APP and aggregated Aβ [72]. iPSC-derived neurons created from sAD, fAD, and frontotemporal dementia (FTD) patients also show increases in the number of medium to large RAB5-positive endosomes [44] and defects in endosome trafficking [43,48]. To examine endosome phenotypes in iPSC-derived organoids, we labeled control and fAD organoid sections with antibodies against the early endosome marker 1 (EEA1; Fig 1G). The fAD organoids trended towards an increase in the number of small endosomes (<1 μm in diameter) and exhibited a significantly higher number of large (1–2 μm) endosomes compared to controls. Additionally, we observed a trend towards an increased overall size for EEA1-positive endosome particles in the fAD organoids (Fig 1H). To functionally assess endosome trafficking in the control and fAD organoids, we employed the transferrin endocytosis assay, in which labeled transferrin is taken up by live cells via clathrin-mediated endocytosis [73]. Following incubation
