GAP-43 promoter elements in transgenic zebrafish reveal a difference in signals for axon growth during CNS development and regeneration.

A pivotal event in neural development is the point at which differentiating neurons become competent to extend long axons. Initiation of axon growth is equally critical for regeneration. Yet we have a limited understanding of the signaling pathways that regulate the capacity for axon growth during either development or regeneration. Expression of a number of genes encoding growth associated proteins (GAPs) accompanies both developmental and regenerative axon growth and has led to the suggestion that the same signaling pathways regulate both modes of axon growth. We have tested this possibility by asking whether a promoter fragment from a well characterized GAP gene, GAP-43, is sufficient to activate expression in both developing and regenerating neurons. We generated stable lines of transgenic zebrafish that express green fluorescent protein (GFP) under regulation of a 1 kb fragment of the rat GAP-43 gene, a fragment that contains a number of evolutionarily conserved elements. Analysis of GFP expression in these lines confirms that the rat 1 kb region can direct growth-associated expression of the transgene in differentiating neurons that extend long axons. Furthermore, this region supports developmental down-regulation of transgene expression which, like the endogenous gene, coincides with neuronal maturation. Strikingly, these same sequences are insufficient for directing expression in regenerating neurons. This finding suggests that signaling pathways regulating axon growth during development and regeneration are not the same. While these results do not exclude the possibility that pathways involved in developmental axon growth are also active in regenerative growth, they do indicate that signaling pathway(s) controlling activation of the GAP-43 gene after CNS injury differ in at least one key component from the signals controlling essential features of developmental axon growth.