Post-transcriptional regulation of GAP-43 rnRNA levels during neuronal differentiation and nerve regeneration

The expression of GAP-43 is a well-characterized correlate of the growth and reorganization of neural connections. Although the expression of this protein is known to depend on the levels of the mRNA, the mechanisms that control the latter are poorly understood. Using Northern blots and nuclear run-on assays, we measured the levels of synthesis and accumulation of GAP-43 mRNA in (a) developing rat cortical neurons, (b) NGF-induced PC12 cells, and (c) regenerating goldfish optic nerve. In all of these instances, significant levels of the nascent transcript were detected even when cells were not growing and this changed little during differentiation, though the steady-state levels of GAP-43 mRNA increased 6- to 20-fold under these conditions. The lack of correlation between the levels of transcription and accumulation of GAP-43 mRNA suggested that the levels of this mRNA are regulated via a post-transcriptional mechanism. To investigate the nature of the mechanism involved, we measured the turnover of the mRNA in control and NGF-treated PC12 cells. Experiments using pulse-chase labeling or actinomycin D treatment showed that NGF induced a 2-fold increase in the half-life of GAP-43 mRNA, which is consistent with a 7-fold increase in the accumulation of the mRNA within 24 h. Thus, changes in the rate of degradation of the mRNA appear to be the major mechanism that controls GAP-43 levels during neurite outgrowth. The presence of some highly conserved putative instability-conferring motifs in the 3' untranslated region (3' UTR) of GAP-43 mRNA lends further support to this possibility.