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. P C12 cells derived from the pheochromocytoma of the rat adrenal medulla are widely used for studying cell signaling in response to numerous growth factors, neurotrophins, and hormones. These cells serve as a useful model to study the proliferation, differentiation, and survival of sympathetic neurons (1). PC12 cells stop dividing and differentiate to cells with neuronal extensions when cultured in the presence of nerve growth factor (NGF) and low levels of serum (2). Differentiated PC12 cells show changes in their electrical excitability, sensitivity to acetylcholine, and choline acetyltransferase activity (3). NGF-, brain-derived neurotrophic factor (BDNF)-, or cyclic AMP-treated, differentiated PC12 cells are connected to each other, although it is not clear whether newly formed neuronal extensions or neurites form functional synapses or not (4). The paradoxical finding that the Src and Ras oncogene products enhanced rather than blocked NGF-induced differentiation led to the identification of signaling pathways involving both Ras and Src as part of the total differentiation response to NGF (5). In PC12 cells, Ras accounts for the immediate effects of NGF, mediated through extracellular signal-regulated kinase (ERK) (1).MicroRNAs (miRNAs), a class of 21-nucleotide-long noncoding RNAs, can reversibly repress the translation of their target messages by binding the mRNA with imperfect complementarities in metazoan cells. miRNA-targeted messages and the components of the miRNA machinery, including the Argonaute (Ago) proteins and miRNAs, accumulate in mammalian P bodies (6-8). miRNAs can control neuronal differentiat...

No abstract

RNA processing bodies (P-bodies) are cytoplasmic RNA granules in eukaryotic cells that regulate gene expression by executing the translation suppression and degradation of mRNAs that are targeted to these bodies. P-bodies can also serve as storage sites for translationally repressed mRNAs both in mammalian cells and yeast cells. In this report, a unique role of mammalian P-bodies is documented. Depletion of P-body components dedifferentiate nerve growth factor-treated PC12 cells, whereas ectopic expression of P-body components induces the neuronal differentiation of precursor cells. Trophic factor withdrawal from differentiated cells induces a decrease in cellular P-body size and numbers that are coupled with dedifferentiation and cell death. Here, we report how the expression of P-body proteins-by ensuring the phosphorylation of argonaute protein 2 and the subsequent inactivation let-7a miRNPs-prevents the apoptotic death of growth factor-depleted neuronal cells.-Patranabis, S., Bhattacharyya, S. N. P-body-induced inactivation of let-7a miRNP prevents the death of growth factor-deprived neuronal cells.