Distribution of phosphorylated GAP-43 (neuromodulin) in growth cones directly reflects growth cone behavior.

Phosphorylation of GAP-43 (neuromodulin) by protein kinase C (PKC) occurs at a single site, serine41. In vivo, phosphorylation is induced after initiation of axonogenesis and is confined to distal axons and growth cones. Within individual growth cones, phosphorylation is nonuniformly distributed. Here, we have used high-resolution video-enhanced microscopy of cultured dorsal root ganglia neurons together with immunocytochemistry with a monoclonal antibody that recognizes PKC-phosphorylated GAP-43 to correlate the distribution of phosphorylated GAP-43 with growth cone behavior. In "quiescent," nontranslocating growth cones, phosphorylated GAP-43 was confined to the proximal neurite and the central organelle-rich region, and was low in organelle-poor lamellae. However, levels in lamellae were elevated when they became motile. Conversely, levels of phosphorylated GAP-43 were low in either lamellae that were actively retracting or in the central organelle-rich region and proximal neurite of growth cones that had totally collapsed. The results suggest a mechanism whereby phosphorylation of GAP-43 by PKC, potentially in response to extracellular signals, could direct the functional behavior of the growth cone.