NGF signaling in PC12 cells: the cooperation of p75 with TrkA is needed for the activation of both mTORC2 and the PI3K signalling cascade

PC12-27, a PC12 clone characterized by high levels of the transcription repressor REST and by very low mTORC2 activity, had been shown to be unresponsive to NGF, possibly because of its lack of the specific TrkA receptor. The neurotrophin receptor repressed by high REST in PC12-27 cells, however, is shown now to be not TrkA, which is normal, but p75, whose expression is inhibited at the transcriptional level. When treated with NGF, the PC12-27 cells lacking p75 exhibited a defective TrkA autophosphorylation restricted, however, to the TrkA(Y490) site, and an impairment of the PI3K signaling cascade. This defect was sustained in part by a mTORC1-dependent feed-back inhibition that in wtPC12 cells appeared marginal. Transfection of p75 to a level and surface distribution analogous to wtPC12 did not modify various high RESTdependent properties of PC12-27 cells such as high b-catenin, low TSC2 and high proliferation rate. In contrast, the defective PI3K signaling cascade and its associated mTORC2 activity were largely rescued together with the NGF-induced neurite outgrowth response. These changes were not due to p75 alone but required its cooperation with TrkA. Our results demonstrate that, in PC12, high REST induces alterations of NGF signaling which, however, are indirect, dependent on the repression of p75; and that the well-known potentiation by p75 of the TrkA signaling does not concern all the effects induced by NGF but primarily the PI3K cascade and its associated mTORC2, a complex known to play an important role in neural cell differentiation.