Imaging and Tracking of Tat Peptide-Conjugated Quantum Dots in Living Cells:  New Insights into Nanoparticle Uptake, Intracellular Transport, and Vesicle Shedding

We report the use of Tat peptide-conjugated quantum dots (Tat-QDs) to examine the complex behavior of nanoparticle probes in live cells, a topic that is of considerable current interest in developing advanced nanoparticle agents for molecular and cellular imaging. Dynamic confocal imaging studies indicate that the peptide-conjugated QDs are internalized by macropinocytosis, a fluid-phase endocytosis process triggered by Tat-QD binding to negatively charged cell membranes. The internalized Tat-QDs are tethered to the inner vesicle surfaces and are trapped in cytoplasmic organelles. The QD loaded vesicles are found to be actively transported by molecular machines (such as dyneins) along microtubule tracks. The destination of this active transport is an asymmetric perinuclear region (outside the cell nucleus) known as the microtubule organizing center (MTOC). We also find that Tat-QDs strongly bind to cellular membrane structures such as filopodia and that large QD-containing vesicles are released from the tips of filopodia by vesicle shedding. These results provide new insights into the mechanisms of Tat peptide-mediated delivery as well as toward the design of functionalized nanoparticles for molecular imaging and targeted therapy.