Human GAP-43 Gene Expression: Multiple Start Sites for Initiation of Transcription in Differentiating Human Neuroblastoma Cells

Abstract

Phorbol ester treatment of human SH-SY5Y neuroblastoma cells, which leads to mature neuron-like cells with a sympathetic phenotype, induces outgrowth of neurites which are terminated by growth cones. The neurite extension is parallelled by an increased expression of the growth-associated protein, GAP-43. At the mRNA level, two GAP-43 mRNA species of 1.4 and 1.6 kb, respectively, were detected in SH-SY5Y cells. Both the low- and high-molecular-weight GAP-43 transcripts cosedimented with a polysomal fraction, indicating translation of both types of transcripts. To structurally characterize these GAP-43 mRNAs, several cDNA clones were isolated. The only difference identified corresponded to various size extensions in the 5′-untranslated region. A human genomic DNA fragment extending 1145 bp 5′ of the GAP-43 translation start site, including a putative promoter region of the GAP-43 gene, was also characterized. Comparison of human and rat GAP-43 genomic sequences revealed an 85% identity between the first 900 bp 5′ of translation start site. By RNase protection analysis, two clusters of putative transcription start sites, located approximately 200 bp apart, were identified. DNaseI footprinting analyses, using nuclear extracts prepared from untreated and TPA-treated SH-SY5Y cells, revealed specific footprints primarily detected in extracts prepared from differentiating cells. These clustered at positions immediately 5′ of the two putative transcription start site regions. GAP-43 mRNA expression was finally studied using a probe which specifically recognizes the high-molecular-weight GAP-43 transcripts. Five tested human neuroblastoma cell lines and human fetal brain tissue expressed these transcripts. Furthermore, during differentiation of SH-SY5Y and LA-N-5 cells, both sizes of GAP-43 transcripts were transiently induced with the larger slightly preceeding the smaller mRNA species.