Phosphorylation of Ago2 and Subsequent Inactivation of let-7a RNP-Specific MicroRNAs Control Differentiation of Mammalian Sympathetic Neurons

Abstract

microRNAs are small regulatory RNAs that regulate gene expression post-transcriptionally by base pairing to the target mRNAs in animal cells. Kras, an oncogene known to be repressed by let-7a miRNAs, is expressed and needed for the differentiation of mammalian sympathetic neurons and PC12 cells. We documented a loss of let-7a activity during this differentiation process without any significant change in cellular level of let-7a miRNA. However, the level of Ago2, an essential component that is associated with miRNAs to form microRNP complexes, shows an increase with neuronal differentiation. In this report, differentiation-induced phosphorylation and subsequent loss of miRNA from Ago2 was noted, which accounted for the loss of miRNA activity in differentiating neurons. Neuronal differentiation induces phosphorylation of MAPK/p38 and downstream kinase MSK1. This in turn, upregulates phosphorylation of Ago2 and ensures miRNA dissociation from Ago2 in neuronal cells. MSK1 mediated miRNP inactivation is a prerequisite for differentiation of neuronal cells, where let-7a miRNA gets unloaded from Ago2 to ensure upregulation of Kras, a target of let-7a. We noted that inactivation of let-7a is both necessary and sufficient for differentiation of sympathetic neurons.