Can quantum dots (QDs) act as relevant intracellular probes to investigate routing of ligands in live cells? To answer this
question we studied intracellular trafficking of QDs that were coupled to the plant toxin ricin, Shiga toxin or the ligand
transferrin (Tf) by confocal fluorescence microscopy in three different cell lines. The Tf:QDs were internalized but
instead of being recycled they accumulated within endosomes in all cell lines. However, for the HEp-2 and SW480 cells
a higher fraction colocalized with a lysosomal marker as compared with HeLa cells. The Shiga:QD bioconjugate was
internalized slowly and with poor efficiency in the HEp-2 and SW480 cells as compared with HeLa cells, and was not
routed to the Golgi apparatus in any of the cell lines. The internalized ricin:QD bioconjugates localized to the same
endosomes as ricin itself, but could in contrast to ricin not be visualized in the Golgi apparatus. Importantly, we find that
the endosomal accumulation of either ricin:QDs or transferrin:QDs affects endosome-to-Golgi transport of both ricin and
Shiga toxin: Transport of ricin was reduced whereas transport of Shiga toxin was increased. In conclusion, the data from
different cells reveal that in general these ligand-coupled QD nanoparticles are arrested within endosomes, and somehow
perturb the normal endosomal sorting in cells.
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