Role of phosphatidylinositol 3-kinase in neuronal survival and axonal outgrowth of adult mouse dorsal root ganglia explants

Adult ganglionic peripheral neurons have lost dependence on target-derived neurotrophin signaling for survival and regeneration after injury. To understand the mechanisms required to sustain such processes at maturity, we are studying neuronal survival and axonal outgrowth of adult mouse dorsal root ganglia (DRG) explants. We have here examined the role of phosphatidylinositol 3-kinase (P13-K) activity. Both neuronal survival and axonal outgrowth of spontaneously growing preparations were decreased significantly by the P13-K inhibitor LY294002 as was the increased outgrowth caused by nerve growth factor or glial cell line-derived factor. Inhibition of P13-K activity promoted neuronal cell death to the same extent in the presence as in the absence of a growth factor, whereas inhibition of mitogen-activated protein kinase, MAPK, lacked effect. Using a compartmentalized system, it could be shown that only axonal outgrowth was decreased when the outgrowth region only was exposed to LY294002. Already-formed growth cones showed morphological changes within 5-10 min after exposure to LY294002. Akt (PKB) is one downstream effector of P13-K. Immunofluorescence revealed the presence of activated Akt in DRG cell bodies and in axonal growth cones. Immunoreactivity was decreased by P13-K inhibition. The results suggest that Akt is constitutively active in adult DRG neurons, and that P13-K mediated processes are involved in neuronal survival of one or more DRG neuronal subpopulations and also in axonal elongation. The possible significance of Akt signaling for these effects is discussed. (C) 2003 Wiley-Liss, Inc.