HuR promotes myogenesis by stabilizing the MyoD, Myogenin and p21 mRNAs during the fusion of muscle cells to form myotubes. Here we show that HuR, via a novel mRNA destabilizing activity, promotes the early steps of myogenesis by reducing the expression of the cell cycle promoter nucleophosmin (NPM). Depletion of HuR stabilizes the NPM mRNA, increases NPM protein levels and inhibits myogenesis, while its overexpression elicits the opposite effects. NPM mRNA destabilization involves the association of HuR with the decay factor KSRP as well as the ribonuclease PARN and the exosome. The C-terminus of HuR mediates the formation of the HuR-KSRP complex and is sufficient for maintaining a low level of the NPM mRNA as well as promoting the commitment of muscle cells to myogenesis. We therefore propose a model whereby the downregulation of the NPM mRNA, mediated by HuR, KSRP and its associated ribonucleases, is required for proper myogenesis.

Long noncoding RNAs (lncRNAs) interact with protein factors to regulate different layers of gene expression transcriptionally or posttranscriptionally. Here we report on the functional consequences of the unanticipated interaction of the RNA binding protein K homology-type splicing regulatory protein (KSRP) with the H19 lncRNA (H19). KSRP directly binds to H19 in the cytoplasm of undifferentiated multipotent mesenchymal C2C12 cells, and this interaction favors KSRP-mediated destabilization of labile transcripts such as myogenin. AKT activation induces KSRP dismissal from H19 and, as a consequence, myogenin mRNA is stabilized while KSRP is repurposed to promote maturation of myogenic microRNAs, thus favoring myogenic differentiation. Our data indicate that H19 operates as a molecular scaffold that facilitates effective association of KSRP with myogenin and other labile transcripts, and we propose that H19 works with KSRP to optimize an AKT-regulated posttranscriptional switch that controls myogenic differentiation. Among small noncoding RNAs, microRNAs (miRNAs) repress gene expression by inhibiting translation and/or promoting decay of target mRNAs (1), with RBPs being able to control miRNA maturation from precursors or influence miRNA function (1, 2). A new class of transcripts referred to as long noncoding RNAs (lncRNAs, arbitrarily defined as longer than 200 nt) has recently moved to the forefront of regulatory RNA research (3). lncRNAs had been originally considered epigenetic regulators of gene expression, but the emphasis placed on the ways they regulate chromatin state likely obscures the full repertoire of their functions (4-6). Other roles of lncRNAs include posttranscriptional regulation, posttranslational control of protein activity, organization of protein complexes, as well as cell-cell signaling (4, 6). lncRNAs have been implicated in cellular events as different as cell cycle regulation, pluripotency, apoptosis, and DNA damage response, to name just a few (5, 6). Not surprisingly, lncRNA expression is altered in cancer, and it is becoming clear that some lncRNAs can control cell transformation by regulating vital cellular functions (7). Nevertheless, the composition and function of ribonucleoprotein complexes, including lncRNAs, is generally uncharacterized.Studies performed in primary and cultured cells as well as in mice proved that the K homology (KH)-type splicing regulatory protein (KSRP), a single-stranded RBP that interacts with nucleic acids through four hnRNP KH domains, is able to integrate different levels of gene expression and is required for proper immune response, lipid metabolism, cell fate decisions, and tissue regeneration (see ref. 8 for a recent review). We and others have found that KSRP negatively regulates gene expression via at least two distinct and integrated posttranscriptional mechanisms: (i) by promoting decay of unstable mRNAs [mainly targeting AU-rich elements (AREs) in their 3′UTRs] (8, 9) and (ii) by favoring maturation of select miRNAs from precursors (8, 10). Brie...

Skeletal myogenesis is orchestrated by distinct regulatory signaling pathways, including PI3K/AKT, that ultimately control muscle gene expression. Recently discovered myogenic micro-RNAs (miRNAs) are deeply implicated in muscle biology. Processing of miRNAs from their primary transcripts is emerging as a major step in the control of miRNA levels and might be well suited to be regulated by extracellular signals. Here we report that the RNA binding protein KSRP is required for the correct processing of primary myogenic miRNAs upon PI3K/AKT activation in myoblasts C2C12 and in the course of injury-induced muscle regeneration, as revealed by Ksrp knock-out mice analysis. PI3K/AKT activation regulates in opposite ways two distinct KSRP functions inhibiting its ability to promote decay of myogenin mRNA and activating its ability to favor maturation of myogenic miRNAs. This dynamic regulatory switch eventually contributes to the activation of the myogenic program.