INJURY-INDUCED EXPRESSION OF GROWTH-ASSOCIATED PROTEIN-43 IN ADULT-MOUSE RETINAL GANGLION-CELLS IN-VITRO

In optic fibers, as in most axons of the central nervous system, the axonal growth-associated protein, GAP-43, is abundant during development but absent in adults. Since optic fibers can be induced to regenerate in culture, we examined whether this was associated with an increased expression of GAP-43 in adult mouse optic fibers that were regenerating from organotypic retinal explants on to laminin substrates. We found that simply placing adult mouse retina in culture under serum-free conditions was sufficient to induce GAP-43, which was detectable after about four to five days in vitro, coincident with the initiation of neurite outgrowth. In explants taken from animals in which the optic nerve was crushed in the orbit eight days prior to culturing, GAP-43 was observed within one day, as was neurite outgrowth. This priming effect was also seen in vivo as an increased level of GAP-43 reactivity in retinal ganglion cells and optic fibers in histological sections taken eight days after nerve crush. Reactivity in the adult fibers in culture was comparable to that observed in optic neurites growing from embryonic retinal explants and could be maintained for at least four weeks in culture. In the adult neurites, especially with longer times in culture, GAP-43 tended to be concentrated into varicosities that were often found in terminal-like arbors that formed in culture. Placing adult retina in culture under serum-free conditions is sufficient to induce re-expression of GAP-43 for an indefinite period of time. This suggests that GAP-43 expression and the propensity for growth in vivo may be repressed by a factor that is absent in vitro.