STIMULATION OF L-TYPE CA2+ CHANNEL IN GROWTH CONES ACTIVATES 2 INDEPENDENT SIGNALING PATHWAYS

Although growth cones respond to various modulators of neurite outgrow th, such as neurotrophins, neurotransmitters, and cell adhesion molecu les, the signal-transducing mechanisms for these modulators in growth cones are unclear, Since recent studies have suggested that the signal s of these modulators are mediated by Ca2+ influx through L-type volta ge-sensitive Ca2+ channels (VSCCs) in the growth cone, we examined L-t ype VSCC-dependent signaling pathways, using isolated growth cones (IG Cs) from developing rat forebrains, Binding assays revealed that L-typ e VSCC is enriched in growth cone membrane and gradually decreased in amount developmentally, while N-type VSCC has the opposite tendency, I n intact IGCs, Bay K 8644 (BK, an L-type agonist) induced much more ra pid elevation of [Ca2+](i) than that in adult synaptosomes. Ca2+-depen dent phosphorylation of GAP-43 and MARCKS protein by protein kinase C (PKC) was enhanced in the IGC by BK, resulting in the release of these proteins from the membrane, which is consistent with our recent repor t, In addition, the Ca2+-dependent degradation of brain spectrin (fodr in) by calpain was also enhanced by BK or GABA, consequently inducing the release of alpha-actinin from the membrane skeleton of the growth cones, The activities of PKC and calpain were not inhibited by inhibit ors of the other, indicating that these reactions occur independently, Our results suggest that Ca2+ influx through L-type VSCCs activates t wo distinct signaling branches, probably in the different domains of t he growth cone, i.e., Ca2+-dependent phosphorylation of GAP-43 and MAR CKS protein, and Ca2+-dependent degradation of brain spectrin and the release of alpha-actinin by calpain. (C) 1998 Wiley-Liss, Inc.