Overexpression of the RNA-binding protein HuD increases the stability of GAP-43mRNA in dentate granule cells of adult mice leading to aberrant synapse formation

With the sequencing of the human genome and the advent of proteomics a significant discrepancy has been brought to the attention of many scientists: there is a great disparity in the number of genes compared to the number of proteins we make. The difference is accounted for by post-transcriptional regulation, which is being recognized as an important mechanism for altering gene expression. Using GAP-43 as a model post-transcriptionally regulated mRNA, I examined the role of HuD-mediated stability of GAP-43 mRNA in the adult murine brain and a number of the downstream physiological consequences. Dentate granule cells (DGCs) of the adult mouse hippocampus transcribe the GAP-43 gene, yet do not express mRNA or protein. Interestingly, these cells also do not express the RNA-binding protein HuD. DGCs provide the perfect model system to test the function of HuD in post-transcriptional regulation of GAP-43. I found that HuD overexpressing (HUD-Tg) mice express HuD in DGCs and have high levels of GAP-43 mRNA, while levels of transcription were not changed. This suggests that the mRNA is more stable. Confirming this idea, in vitro transcribed GAP-43 mRNA was more stable in extracts from HuD-Tg mice. In addition, I found that GAP-43 is translated in these cells and the protein is targeted to their axons, the mossy fibers. There was also an increase in GAP-43 protein levels in the projections from the other brain areas to the hippocampus. While gross brain morphology was similar to that of wildtype mice, HuD-Tg mice exhibit an increase in mossy fibers of the infrapyramidal bundle that are Timm's stain positive, indicating that they make functional contact with nearby dendrites. The HuD-induced upregulation of GAP-43 and the structural changes highlight the importance of regulating mRNA turnover and stability.