Neural selective activation and temporal regulation of a mammalian GAP-43 promoter in zebrafish

Neurons throughout the vertebrate nervous system selectively activate the gene for a growth cone component, GAP-43, during embryonic development, and then decrease its expression abruptly as they form synapses. Distal interruption of mature axons in the central nervous system (CNS) of fish and amphibians, but not in the mammalian CNS reverses the developmental down-regulation of GAP-43 expression. To explore functional conservation and divergence of cis-acting elements that regulate expression of the GAP-43 gene, we studied activation, in transgenic zebrafish embryos, of mammalian GAP-43 genomic sequences fused to a marker gene. The DNA fragments containing the GAP-43 promoter, including a short fragment of 386 base pairs, were preferentially activated in the embryonic fish nervous system at times when extensive neuronal differentiation and neurite outgrowth take place. After 2 days of development, expression of the mammalian transgenes was specifically downregulated in the fish spinal cord but increased in more rostral regions of the CNS. This expression pattern was well correlated with the regulation of the endogenous fish GAP-43 gene revealed by in situ hybridization. Elements of the mammalian gene located a substantial distance upstream of the minimal promoter directed additional expression of the marker gene in a specific set of non-neural cells in zebrafish embryos. Our results indicate that cis-acting elements of the GAP-43 gene, and signaling pathways controlling these elements during embryonic development, have been functionally conserved in vertebrate evolution.