Intracellular transport of nanoparticles is an issue of considerable current interest in developing advanced nanoparticle agents for molecular imaging and therapy. It has been technically challenging to directly follow nanoparticle transport in live cells, however. We have taken advantage of the bright and stable fluorescence of quantum dots (QDs), which are used as the prototype of nanoparticles of many kinds, and the dynamic confocal imaging capability of spinning disk confocal microscope to track the cellular uptake of nanoparticles and the downstream events of nanoparticle transport. Specifically, we have examined the intracellular transport of Tat peptide conjugated quantum dots (Tat-QDs). Tat peptide is a peptide sequence playing critical role in the infection of HIV virus, and has been used as a novel delivery agent for a variety of cargos, although its delivery mechanism remains controversial.

We have found that Tat-QDs are internalized by an 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 actively transported by motor proteins along cytoskeleton tracks and end up being accumulated at a perinuclear region. 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.