NGF signals through TrkA to increase clathrin at the plasma membrane and enhance clathrin-mediated membrane trafficking

Neurotrophin (NT) signals may be moved from axon terminals to neuron cell b odies via signaling endosomes-organelles in which NTs continue to be bound to their activated receptors. Suggesting that clathrin-coated membranes ser ve as one source of signaling endosomes, in earlier studies we showed that nerve growth factor (NGF) treatment increased clathrin at the plasma membra ne and resulted in colocalization of clathrin with TrkA, the receptor tyros ine kinase for NGF. Strikingly, however, we also noted that most clathrin p uncta at the surface of NGF-treated cells did not colocalize with TrkA, rai sing the possibility that NGF induces a general increase in clathrin-coated membrane formation. To explore this possibility further, we examined the d istribution of clathrin in NGF- and BDNF-treated cells. NGF signaling in PC 12 cells robustly redistributed the adaptor protein AP2 and the clathrin he avy chain (CHC) to surface membranes. Using confocal and epifluorescence mi croscopy, as well as biochemical assays, we showed the redistribution of cl athrin to be attributable to the activation of TrkA. Significantly, NGF sig naled through TrkA to induce an increase in clathrin-mediated membrane traf ficking, as revealed in the increased endocytosis of transferrin. In that B DNF treatment increased AP2 and clathrin at the surface membranes of hippoc ampal neurons, these findings may represent a physiologically significant r esponse to NTs. We conclude that NT signaling increases clathrin-coated mem brane formation and clathrin-mediated membrane trafficking and speculate th at this effect contributes to their trophic actions via the increased inter nalization of receptors and other proteins that are present in clathrin-coa ted membranes.