Protein kinase C stimulates HuD-mediated mRNA stability and protein expression of neurotrophic factors and enhances dendritic maturation of hippocampal neurons in culture.

HuD protein is an RNA-binding protein involved in post-transcriptional regulation of gene expression for synaptogenesis, neuronal differentiation, and learning and memory, and is up-regulated and redistributed by a protein kinase C (PKC)-dependent pathway in neurons. Here, we show a PKC-regulated mechanism on HuD-mediated mRNA stability and expression of several neurotrophic factors (NTFs) in cultured hippocampal neurons. HuD pull-down assays showed that HuD is associated with brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin (NT)-3 mRNAs. Reduction of HuD expression with short hairpin RNAs decreased BDNF, NGF, and NT-3 mRNAs and NTFs expression. Bryostatin, a PKC activator, treatment enhanced their association with HuD and increased these transcripts' stability. Bryostatin induced HuD phosphorylation, which was inhibited by Ro 32-0432, a specific PKC inhibitor. Activated PKC specifically phosphorylated coactivator-associated arginine methyltransferase 1 (CARM1), which methylates HuD and negatively modulates HuD-mRNA interactions during neuronal differentiation, and inhibited its methyltransferase activity, resulting in decrease in CARM1-mediated HuD methylation. Furthermore cotreatment of bryostatin and AMI-1, a specific CARM1 inhibitor, potentiated PKC-dependent HuD-mRNA interactions and enhanced dendritic arborization. These results demonstrate that PKC may play an important role in neuronal differentiation and synaptogenesis via stimulating HuD-mediated mRNA stability and inhibiting CARM1 in hippocampal neurons.