Ligand-bound quantum dot probes for studying the molecular scale dynamics of receptor endocytic trafficking in live cells

Endocytic receptor trafficking is a complex, dynamic process underlying fundamental cell function. An integrated understanding of endocytosis at the level of single or small numbers of ligand bound-receptor complexes inside live cells is currently hampered by technical limitations. Here, we develop and test ligand nerve growth factor-bound quantum dot (NGF-QD) bioconjugates for imaging discrete receptor endocytic events inside live NGF-responsive PC12 cells. Using single particle tracking, QD hybrid gel coimmunoprecipitation, and immunocolocalization, we illustrate and validate the use of QD-receptor complexes for imaging receptor trafficking at synchronized time points after QD-ligand - receptor binding and internalization (t = 15-150 min). The unique value of these probes is illustrated by new dynamic observations: (1) that endocytosis proceeds at strikingly regulated fashion, and (2) that diffusive and active forms of transport inside cells are rapid and efficient. QDs are powerful intracellular probes that can provide biologists with new capabilities and fresh insight for studying endocytic receptor signaling events, in real time, and at the resolution of single or small numbers of receptors in live cells.