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 a xonal growth-associated protein, GAP-43, is abundant during developmen t but absent in adults. Since optic fibers can be induced to regenerat e in culture, we examined whether this was associated with an increase d expression of GAP-43 in adult mouse optic fibers that were regenerat ing from organotypic retinal explants on to laminin substrates. We fou nd 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 n eurite outgrowth. In explants taken from animals in which the optic ne rve was crushed in the orbit eight days prior to culturing, GAP-43 was observed within one day, as was neurite outgrowth. This priming effec t 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 cultu re was comparable to that observed in optic neurites growing from embr yonic retinal explants and could be maintained for at least four weeks in culture. In the adult neurites, especially with longer times in cu lture, GAP-43 tended to be concentrated into varicosities that were of ten found in terminal-like arbors that formed in culture. Placing adul t retina in culture under serum-free conditions is sufficient to induc e re-expression of GAP-43 for an indefinite period of time. This sugge sts that GAP-43 expression and the propensity for growth in vivo may b e repressed by a factor that is absent in vitro.