The selective and inducible activation of endogenous PI 3-kinase in PC12 cells results in efficient NGF-mediated survival but defective neurite outgrowth

The Trk/Nerve Growth Factor receptor mediates the rapid activation of a num ber of intracellular signaling proteins, including phosphatidylinositol 3-k inase (PI 3-kinase). Here, we describe a novel, NGF-inducible system that w e used to specifically address the signaling potential of endogenous PI 3-k inase in NGF-mediated neuronal survival and differentiation processes. This system utilizes a Trk receptor mutant (Trk(def)) lacking sequences Y490, Y 785 and KFG important for the activation of the major Trk targets; SHC, PLC -gamma I, Ras, PI 3-kinase and SNT. TrKdef was kinase active but defective for NGF-induced responses when stably expressed in PC12nnr5 cells (which la ck detectable levels of TrkA and are non-responsive to NGF). The PI 3-kinas e consensus binding site, YxxM (YYPM), was introduced into the insert regio n within the kinase domain of Trk(def). NGF-stimulated tyrosine phosphoryla tion of the Trk(def)+PI 3-kinase addback receptor, resulted in the direct a ssociation and selective activation ;of PI 3-kinase in vitro and the produc tion of PI(3,4)P-2 and PI(3,4,5)P-3 in rivo (comparable to wild-type). PC12 nnr5 cells stably expressing Trk(def)+PI 3-kinase, initiated neurite outgro wth but failed to stably extend and maintain these neurites in response to NGF as compared to PC12 parental cells, or PC12nnr5 cells overexpressing wi ld-type Trk. However, Trk(def)+PI 3-kinase was fully competent in mediating NGF-induced survival processes. We propose that while endogenous PI 3-kina se can contribute in part to neurite initiation professes, its selective ac tivation and subsequent signaling to downstream effecters such as Akt, func tions mainly to promote cell survival in the PC12 system.