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

MicroRNAs (miRNAs) are small regulatory RNAs that regulate gene expression posttranscriptionally 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 the cellular level of let-7a miRNA. However, the level of Ago2, an essential component that is associated with miRNAs to form RNP-specific miRNA (miRNP) complexes, shows an increase with neuronal differentiation. In this study, differentiation-induced phosphorylation and the subsequent loss of miRNA from Ago2 were noted, and these accounted for the loss of miRNA activity in differentiating neurons. Neuronal differentiation induces the phosphorylation of mitogen-activated protein kinase p38 and the downstream kinase mitogen-and stress-activated protein kinase 1 (MSK1). This in turn upregulates the phosphorylation of Ago2 and ensures the dissociation of miRNA from Ago2 in neuronal cells. MSK1-mediated miRNP inactivation is a prerequisite for the differentiation of neuronal cells, where let-7a miRNA gets unloaded from Ago2 to ensure the upregulation of KRas, a target of let-7a. We noted that the inactivation of let-7a is both necessary and sufficient for the differentiation of sympathetic neurons.