Increased expression and localization of the RNA-binding protein HuD and GAP-43 mRNA to cytoplasmic granules in DRG neurons during nerve regeneration.

The neuronal-specific RNA-binding protein, HuD, binds to a U-rich regulatory element of the 3' untranslated region (3' UTR) of the GAP-43 mRNA and delays the onset of its degradation. We have recently shown that overexpression of HuD in embryonic rat cortical cells accelerated the time course of normal neurite outgrowth and resulted in a twofold increase in GAP-43 mRNA levels. Given this evidence, we sought to investigate the involvement of HuD during nerve regeneration. It is known that HuD protein and GAP-43 mRNA are expressed in the dorsal root ganglia (DRG) of adult rat and that GAP-43 is upregulated in DRG neurons during regeneration. In this study, we examined the expression patterns and levels of HuD and GAP-43 mRNA in DRG neurons following sciatic nerve injury using a combination of in situ hybridization, immunocytochemistry, and quantitative RT-PCR. GAP-43 and HuD expression increased in the ipsilateral DRG during the first 3 weeks of regeneration, with peak values seen at 7 days postcrush. At this time point, the levels of HuD and GAP-43 mRNAs in the ipsilateral DRG increased by twofold and sixfold, respectively, relative to the contralateral DRG. Not only were the temporal patterns of expression of HuD protein and GAP-43 mRNA similar, but also they were found to colocalize in the cytoplasm of DRG neurons. Moreover, both molecules were distributed in cytoplasmic granules containing ribosomal RNA. In conclusion, our results suggest that HuD is involved in the upregulation of GAP-43 expression observed at early stages of peripheral nerve regeneration.