ENDOCYTOSIS OF ACTIVATED TRKA - EVIDENCE THAT NERVE GROWTH-FACTOR INDUCES FORMATION OF SIGNALING ENDOSOMES

The survival, differentiation, and maintenance of responsive neurons a re regulated by nerve growth factor (NGF), which is secreted by the ta rget and interacts with receptors on the axon tip. It is uncertain how the NGF signal is communicated retrogradely from distal axons to neur on cell bodies. Retrograde transport of activated receptors in endocyt ic vesicles could convey the signal. However, little is known about en docytosis of NGF receptors, and there is no evidence that NGF receptor s continue to signal after endocytosis. We have examined early events in the membrane traffic of NGF and its receptor, gp140(TrkA) (TrkA), i n PC12 cells. NGF induced rapid and extensive endocytosis of TrkA in t hese cells, and the receptor subsequently moved into small organelles located near the plasma membrane. Some of these organelles contained c lathrin and alpha-adaptin, which implies that TrkA is internalized by clathrin-mediated endocytosis. Using mechanical permeabilization and f ractionation, intracellular organelles derived from endocytosis were s eparated from the plasma membrane. After NGF treatment, NGF was bound to TrkA in endocytic organelles, and TrkA was tyrosine-phosphorylated and bound to PLC-gamma 1, suggesting that these receptors were compete nt to initiate signal transduction. These studies raise the possibilit y that NGF induces formation of signaling endosomes containing activat ed TrkA. They are an important first step in elucidating the molecular mechanism of NGF retrograde signaling.