Ultrastructural immunocytochemical localization of B-50/GAP43, a protein kinase C substrate, in isolated presynaptic nerve terminals and neuronal growth cones

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Abstract
Accumulating evidence indicates that the neuron-specific B-50/GAP43, a substrate for protein kinase C, plays a role in neuronal differentiation and neuritogenesis during nervous tissue development and axonal regeneration. An ultrastructural immunocytochemical study on the localization of B-50 in presynaptic terminals (synaptosomes) and neuronal growth cones was carried out by means of cryoultramicrotomy with affinity-purified B-50 antibodies. Detection was accomplished with colloidal gold, conjugated either to protein-A or goat anti-rabbit immunoglobulins. In synaptosomes, isolated from the frontal cortex of 6-week-old rats, and in neuronal growth cones, isolated from forebrains of 5-day-old rats, the majority of B-50 is detected at the surrounding neuronal plasma membrane. In both neuronal growth cones and synaptosomes, a relatively small fraction of B-50 in the cytoplasm was not evidently associated with internal membranes. Our results indicate that B-50 is mainly located at the cytoplasmic face of the synaptosomal and neuronal growth cone plasma membrane. The similar B-50 localization in neuronal growth cones and synaptosomes suggests that, both in extending axons and mature synaptic terminals, B-50 may exert identical functions as a protein kinase C substrate at the plasma membrane.

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