Intracelluar dynamics of a high affinity NGF receptor TrkA in PC12 cell

A high affinity nerve growth factor (NGF) receptor, tropomyosin-receptor kinase (TrkA), is visualized by expression of TrkA conjugated with cyan fluorescent protein (CFP) in PC12 cells. TrkA was distributed on the plasma membrane of PC12 cells almost uniformly in both differentiated cells and undifferentiated cells. NGF induced differentiation of PC12 cells transfected with TrkA-CFP normally as wild cells without transfection and the expression of TrkA was observed on the entire cell membrane which surrounds the cell body, axons and growth cones. Interestingly, TrkA-CFP was also present on the membrane of filopodia sticking out from the axon and growth cone. In the axonal region, transporting vesicles of TrkA with diameters ranging from 0.5 to 1.5 microm were observed. Some of these vesicles showed net directional movement along the axon in both directions, anterograde and retrograde. The mean velocity of anterograde and retrograde transport was 0.2+/-0.03 and 0.3+/-0.05 microm/s (mean+/-S.E.), respectively. Some vesicles moving anterogradely changed their direction occasionally although the net transport was anterograde. On the other hand, vesicles moving retrogradely seldom switched their direction in spite of occasional stops. These results demonstrated that the behavior of TrkA transporting vesicles in axons of PC12 cells was similar to that observed in the primary culture of sympathetic or sensory neuron. Therefore, it is suggested that the PC12 cell transfected with fluorescent protein-conjugated TrkA is a useful model for studying the signal transduction of NGF.