A protein induced during nerve growth (GAP-43) is a major component of growth-cone membranes.

Growth cones are specialized structures that form the distal tips of growing axons. During both normal development of the nervous system and regeneration of injured nerves, growth cones are essential for elongation and guidance of growing axons. Developmental and regenerative axon growth is frequently accompanied by elevated synthesis of a protein designated GAP-43. GAP-43 has now been found to be a major component of growth-cone membranes in developing rat brains. Relative to total protein, GAP-43 is approximately 12 times as abundant in growth-cone membranes as in synaptic membranes from adult brains. Immunohistochemical localization of GAP-43 in frozen sections of developing brain indicates that the protein is specifically associated with neuropil areas containing growth cones and immature synaptic terminals. The results support the proposal that GAP-43 plays a role in axon growth.

Light-microscopic immunolocalization of the growth- and plasticity-associated protein GAP-43 in the developing rat brain.

Growth-associated protein-43 (GAP-43) is a developmentally regulated, fast-axonally transported phosphoprotein whose synthesis and transport are enhanced during periods of growth and synaptic terminal formation. GAP-43 is a substrate of protein kinase C and is identical to protein F1, a phosphoprotein which is regulated during long-term potentiation in the hippocampus. In order to characterize the cellular localization of GAP-43, we have raised a specific antiserum against it, and used this as a probe to show that GAP-43 is neuron-specific, and is localized to growing neuronal processes in developing rat brain, and to presynaptic terminals in both the peripheral and central nervous system. In the mature CNS, GAP-43 immunoreactivity is present in most neuropil areas, but is especially dense in the molecular layers of the cerebellum, neocortex, and the hippocampus, structures known to exhibit synaptic plasticity. Its localization, together with biochemical data concerning the dynamics of its synthesis and its identity as protein F1, suggest that GAP-43 may be involved in axon growth in the developing nervous system, and in some aspect of synaptic plasticity in the mature CNS. These data also suggest that axon growth and synaptic plasticity in the brain may be regulated by a common mechanism, both involving the protein kinase C-mediated phosphorylation of GAP-43.

Nerve injury stimulates the secretion of apolipoprotein E by nonneuronal cells.

Nerve trauma initiates significant changes in the composition of proteins secreted by nonneuronal cells. The most prominent of these proteins is a 37-kDa protein, whose expression correlates with the time course of nerve development, degeneration, and regeneration. We now report that the 37-kDa protein is apolipoprotein E (apoE). We produced a specific antiserum against the 37-kDa protein isolated from previously crushed nerves. This antiserum recognizes a 36-kDa protein in rat serum that we have purified and identified as apoE. The anti-37-kDa antiserum also recognizes apoE on electrophoretic transfer blots of authentic samples of high and very low density lipoproteins. The nerve 37-kDa protein comigrates with apoE by two-dimensional electrophoresis, shares a similar amino acid composition, and reacts with an antiserum against authentic apoE. The purified apoE specifically blocks the immunoprecipitation of [35S]methionine-labeled 37-kDa protein synthesized by nonneuronal cells. Thus, on the basis of its molecular mass, isoelectric point, amino acid composition, and immunological properties, we conclude that the 37-kDa protein is apoE. We also used light microscopic immunohistochemistry to localize apoE following nerve injury. In rats with optic nerve lesions, the 37-kDa antiserum bound specifically to the degenerating optic tracts and to the retino-recipient layers of the lateral geniculate nucleus and the superior colliculus. We propose that apoE is synthesized by phagocytic cells in response to nerve injury for the purpose of mobilizing lipids produced as a consequence of axon degeneration.

Evidence for the coidentification of GAP-43, a growth-associated protein, and F1, a plasticity-associated protein.

GAP-43 is a fast-axonally transported protein whose expression correlates with periods of axon growth both during development and during regeneration. Similarities in molecular weight (43-47 kDa), pI (4.3-4.5), and aberrant behavior in acrylamide gels suggested that GAP-43 might be related or identical to protein F1, a protein kinase C substrate that has been shown to undergo a change in phosphorylation state during long-term potentiation in the hippocampus. Here we show that GAP-43 and protein F1 comigrate by two-dimensional PAGE and that antiserum raised against GAP-43 specifically immunoprecipitates protein F1. More direct evidence that GAP-43 and protein F1 are identical proteins was obtained by performing S. aureus V8 protease digests of a mixture of purified 32P-labeled protein F1 and purified GAP-43. Under these conditions, 2 phosphorylated peptide fragments of protein F1 corresponded exactly to 2 Coomassie-stainable bands from purified GAP-43. We conclude on the basis of these data that GAP-43 and protein F1 are identical proteins. Using light-microscopic immunocytochemistry, we also show that GAP-43/protein F1 immunoreactivity is localized to neuropil areas of the hippocampus consistent with its roles as a protein kinase C substrate in vivo and in long-term potentiation. These findings suggest that nerve growth during development and regeneration, and synaptic plasticity in the adult mammalian brain, may be mediated by a common mechanism involving the phosphorylation of GAP-43/protein F1.