Abstract
Phosphorylation of the nervous system-specific growth cone protein GAP- 43 by kinase C in vivo occurs exclusively in growth cones and distal axons, and the onset of this phosphorylation is delayed relative to the onset of axonogenesis, with the delay predicted on the time needed for axons to reach the vicinity of their targets (Meiri et al., 1991). We have used a subcellular fraction of intact growth cones (IGCs) to investigate whether this induction of GAP-43 phosphorylation can be influenced by target-derived substances, and show here that increased phosphorylation of GAP-43 can be both stimulated and maintained by NGF at concentrations of 2 x 10(-10) M. This low concentration of NGF and the subsequent phosphorylation of GAP-43 are both consistent with the interpretation that phosphorylation is due to the binding of NGF to a biologically active high-affinity receptor. Second, we used the monoclonal antibody 2G12 to show that the NGF-stimulated phosphorylation of GAP-43 occurs on serine, the kinase C phosphorylation site, consistent with the results seen in vivo. Levels of phosphorylated GAP-43 in the intact IGCs are also modulated by calcium-stimulated dephosphorylation that could be inhibited by EGTA but not okadaic acid and that therefore resembled the calcineurin- stimulated dephosphorylation reported in vitro. The results suggest that the spatial and temporal regulation of GAP-43 phosphorylation that occurs during axonogenesis in vivo can be regulated by target-derived neurotropic molecules, specifically NGF.