Endo- and Exocytosis of Zwitterionic Quantum Dot Nanoparticles by Live HeLa Cells

Uptake and intracellular transport of D-penicillamine coated quantum dots (DPA QDs) of 4 nm radius by live HeLa cells have been investigated systematically by spinning disk and 4Pi confocal microscopies Unlike larger nanoparticles, these small DPA Qds were observed to accumulate at the plasma membrane prior to internalization, and the uptake efficiency scaled nonlinearly with the nanoparticle concentration Both observations indicate that a critical threshold density has to be exceeded for triggering the internalization process By using specific inhibitors we showed that DPA QDs were predominantly internalized by clathnn mediated endocytosis and to smaller extent by macropinocytosis Clusters of DPA QDs were found in endosomes, which were actively transported along microtubules toward the pennuclear region Later on, a significant fraction of endocytosed DPA-QDs were found in lysosomes, while others were actively transported to the cell penphery and exocytosed with a half life of 21 min