Facilitated intracellular transport of TrkA by an interaction with nerve growth factor.

Intracellular transport of neurotrophin receptors together with neurotrophins is one of the key events of neurotrophin signaling for the growth and the survival of neurons. However, the involvement of neurotrophin signaling in the regulation of intracellular transport of neurotrophin receptors has been remained unclear. We visualized the behavior of TrkA, a receptor of nerve growth factor (NGF), by labeling with GFP in PC12 cells. We found remarkable changes of the behavior of TrkA-GFP upon the application of NGF. Before the application, only ~37% of the fluorescent dots of TrkA showed translocations along neurites of PC12 cells. After the application, number of the dots showing the directional movement increased to ~65%. The averaged velocities of the directional movement of TrkA-GFP dots became higher after the application of NGF. We tested the idea whether NGF binding accelerated the translocations of TrkA by simultaneously observing TrkA-GFP and fluorescently labeled NGF, Cy3.5-NGF. The velocity of TrkA-GFP dots associated with Cy3.5-NGF was remarkably higher than that of TrkA-GFP dots without Cy3.5-NGF. On the basis of these observations, we hypothesize that there is a signaling mechanism within a single vesicle that facilitates the intracellular transport of each vesicle containing the activated TrkA.