RNAi-mediated HuR depletion leads to the inhibition of muscle cell differentiation

pADP-ribosylation regulates the cytoplasmic localization, cleavage, and pro-apoptotic function of HuR

HuR (ElavL1) is one of the main post-transcriptional regulators that determines cell fate. Although the role of HuR in apoptosis is well established, the post-translational modifications that govern this function remain elusive. In this study, we show that PARP1/2-mediated poly(ADP)-ribosylation (PARylation) is instrumental in the pro-apoptotic function of HuR. During apoptosis, a substantial reduction in HuR PARylation is observed. This results in the cytoplasmic accumulation and the cleavage of HuR, both of which are essential events for apoptosis. These effects are mediated by a pADP-ribose-binding motif within the HuR-HNS region (HuR PAR-binding site). Under normal conditions, the association of the HuR PAR-binding site with pADP-ribose is responsible for the nuclear retention of HuR. Mutations within this motif prevent the binding of HuR to its import factor TRN2, leading to its cytoplasmic accumulation and cleavage. Collectively, our findings underscore the role of PARylation in controlling the pro-apoptotic function of HuR, offering insight into the mechanism by which PARP1/2 enzymes regulate cell fate and adaptation to various assaults.

Tankyrase-1 regulates RBP-mediated mRNA turnover to promote muscle fiber formation

Poly(ADP-ribosylation) (PARylation) is a post-translational modification mediated by a subset of ADP-ribosyl transferases (ARTs). Although PARylation-inhibition based therapies are considered as an avenue to combat debilitating diseases such as cancer and myopathies, the role of this modification in physiological processes such as cell differentiation remains unclear. Here, we show that Tankyrase1 (TNKS1), a PARylating ART, plays a major role in myogenesis, a vital process known to drive muscle fiber formation and regeneration. Although all bona fide PARPs are expressed in muscle cells, experiments using siRNA-mediated knockdown or pharmacological inhibition show that TNKS1 is the enzyme responsible of catalyzing PARylation during myogenesis. Via this activity, TNKS1 controls the turnover of mRNAs encoding myogenic regulatory factors such as nucleophosmin (NPM) and myogenin. TNKS1 mediates these effects by targeting RNA-binding proteins such as Human Antigen R (HuR). HuR harbors a conserved TNKS-binding motif (TBM), the mutation of which not only prevents the association of HuR with TNKS1 and its PARylation, but also precludes HuR from regulating the turnover of NPM and myogenin mRNAs as well as from promoting myogenesis. Therefore, our data uncover a new role for TNKS1 as a key modulator of RBP-mediated post-transcriptional events required for vital processes such as myogenesis.

Post-transcriptional regulation of myogenic transcription factors during muscle development and pathogenesis

The transcriptional regulation of skeletal muscle (SKM) development (myogenesis) has been documented for over 3 decades and served as a paradigm for tissue-specific cell type determination and differentiation. Myogenic stem cells (MuSC) in embryos and adult SKM are regulated by the transcription factors Pax3 and Pax7 for their stem cell characteristics, while their lineage determination and terminal differentiation are both dictated by the myogenic regulatory factors (MRF) that comprise Mrf4, Myf5, Myogenin, and MyoD. The myocyte enhancer factor Mef2c is activated by MRF during terminal differentiation and collaborates with them to promote myoblast fusion and differentiation. Recent studies have found critical regulation of these myogenic transcription factors at mRNA level, including subcellular localization, stability, and translational regulation. Therefore, the regulation of Pax3/7, MRFs and Mef2c mRNAs by RNA-binding factors and non-coding RNAs (ncRNA), including microRNAs and long non-coding RNAs (lncRNA), will be the focus of this review and the impact of this regulation on myogenesis will be further addressed. Interestingly, the stem cell characteristics of MuSC has been found to be critically regulated by ncRNAs, implying the involvement of ncRNAs in SKM homeostasis and regeneration. Current studies have further identified that some ncRNAs are implicated in the etiology of some SKM diseases and can serve as valuable tools/indicators for prediction of prognosis. The roles of ncRNAs in the MuSC biology and SKM disease etiology will also be discussed in this review.

The novel RNA-RNA activation of H19 on MyoD transcripts promoting myogenic differentiation of goat muscle satellite cells

The elaborate interplay of coding and noncoding factors governs muscle growth and development. Here, we reported a mutual activation between long noncoding RNA (lncRNA) H19 and MyoD (myogenic determination gene number 1) in the muscle process. We successfully cloned the two isoforms of goat H19, which were significantly enriched and positively correlated with MyoD transcripts in skeletal muscles or differentiating muscle satellite cells (MuSCs). To systematically screen genes altered by H19, we performed RNA-seq using cDNA libraries of differentiating H19-deficiency MuSCs and consequently anchored MyoD as the critical genes in mediating H19 function. Intriguingly, some transcripts of MyoD and H19 overlapped in the cytoplasm, which was dramatically damaged when the core complementary nucleotides were mutated. Meanwhile, MyoD RNA successfully pulled down H19 in MS2-RIP experiments. Furthermore, HuR could bind both H19 and MyoD transcripts, while H19 or its truncated mutants successfully stabilized MyoD mRNA, with or without HuR deficiency. In turn, novel functional MyoD protein-binding sites were identified in the promoter and exons of the H19 gene. Our results suggest that MyoD activates H19 transcriptionally, and RNA-RNA hybridization is critical for H19-promoted MyoD expression, which extends our knowledge of the hierarchy of regulatory networks in muscle growth.

The Functional Role of Myogenin in Cardiomyoblast H9c2 Cells Treated with High Glucose and Palmitic Acid: Insights into No-Rejection Heart Transplantation

Various pathological alterations, including lipid-deposition-induced comparative cardiac lipotoxicity, contribute to cardiac aging in the failing heart. A decline in endogenous myogenin proteins can lead to the reversal of muscle cell differentiation and the creation of mononucleated muscle cells. Myogenin may be a specific regulator of adaptive responses to avoid pathological hypertrophy in the heart. Hence, it is important to understand the regulation of myogenin expression and functions in response to exposure to varied stresses. In this study, we first examined and verified the cytotoxic effect of palmitic acid on H9c2 cells. The reduction in myogenin mRNA and protein expression by palmitic acid was independent of the effect of glucose. Meanwhile, the induction of cyclooxygenase 2 and activating transcription factor 3 mRNAs and proteins by palmitic acid was dependent on the presence of glucose. In addition, palmitic acid failed to disrupt cell cycle progression when H9c2 cells were treated with no glucose. Next, we examined the functional role of myogenin in palmitic-acid-treated H9c2 cells and found that myogenin may be involved in palmitic-acid-induced mitochondrial and cytosolic ROS generation, cellular senescence, and mitochondrial membrane potential. Finally, the GSE150059 dataset was deposited in the Gene Expression Omnibus website and the dataset was further analyzed via the molecular microscope diagnostic system (MMDx), demonstrating that many heart transplant biopsies currently diagnosed as no rejection have mild molecular-antibody-mediated rejection-related changes. Our data show that the expression levels of myogenin were lower than the average level in the studied population. Combining these results, we uncover part of the functional role of myogenin in lipid- and glucose-induced cardiac cell stresses. This finding provides valuable insight into the differential role of fatty-acid-associated gene expression in cardiovascular tissues. Additionally, the question of whether this gene expression is regulated by myogenin also highlights the usefulness of a platform such as MMDx-Heart and can help elucidate the functional role of myogenin in heart transplantation.

HuR Promotes the Differentiation of Goat Skeletal Muscle Satellite Cells by Regulating Myomaker mRNA Stability

Human antigen R (HuR) is an RNA-binding protein that contributes to a wide variety of biological processes and diseases. HuR has been demonstrated to regulate muscle growth and development, but its regulatory mechanisms are not well understood, especially in goats. In this study, we found that HuR was highly expressed in the skeletal muscle of goats, and its expression levels changed during longissimus dorsi muscle development in goats. The effects of HuR on goat skeletal muscle development were explored using skeletal muscle satellite cells (MuSCs) as a model. The overexpression of HuR accelerated the expression of myogenic differentiation 1 (MyoD), Myogenin (MyoG), myosin heavy chain (MyHC), and the formation of myotubes, while the knockdown of HuR showed opposite effects in MuSCs. In addition, the inhibition of HuR expression significantly reduced the mRNA stability of MyoD and MyoG. To determine the downstream genes affected by HuR at the differentiation stage, we conducted RNA-Seq using MuSCs treated with small interfering RNA, targeting HuR. The RNA-Seq screened 31 upregulated and 113 downregulated differentially expressed genes (DEGs) in which 11 DEGs related to muscle differentiation were screened for quantitative real-time PCR (qRT-PCR) detection. Compared to the control group, the expression of three DEGs (Myomaker, CHRNA1, and CAPN6) was significantly reduced in the siRNA-HuR group (p < 0.01). In this mechanism, HuR bound to Myomaker and increased the mRNA stability of Myomaker. It then positively regulated the expression of Myomaker. Moreover, the rescue experiments indicated that the overexpression of HuR may reverse the inhibitory impact of Myomaker on myoblast differentiation. Together, our findings reveal a novel role for HuR in promoting muscle differentiation in goats by increasing the stability of Myomaker mRNA.

A splicing silencer in SMN2 intron 6 is critical in spinal muscular atrophy

Spinal muscular atrophy (SMA) is a fatal neuromuscular disease caused by homozygous deletions or mutations of the SMN1 gene. SMN2 is a paralogous gene of SMN1 and a modifying gene of SMA. A better understanding of how SMN2 exon 7 splicing is regulated helps discover new therapeutic targets for SMA therapy. Based on an antisense walk method to map exonic and intronic splicing silencers (ESSs and ISSs) in SMN2 exon 7 and the proximal regions of its flanking introns, we identified one ISS (ISS6-KH) at upstream of the branch point site in intron 6. By using mutagenesis-coupled RT-PCR with SMN1/2 minigenes, immunochromatography, overexpression and siRNA-knockdown, we found this ISS consists of a bipartite hnRNP A1 binding cis-element and a poly-U sequence located between the proximal hnRNP A1 binding site (UAGCUA) and the branch site. Both HuR and hnRNP C1 proteins promote exon 7 skipping through the poly-U stretch. Mutations or deletions of these motifs lead to efficient SMN2 exon 7 inclusion comparable to SMN1 gene. Furthermore, we identified an optimal antisense oligonucleotide that binds the intron six ISS and causes striking exon 7 inclusion in the SMN2 gene in patient fibroblasts and SMA mouse model. Our findings demonstrate that this novel ISS plays an important role in SMN2 exon 7 skipping and highlight a new therapeutic target for SMA therapy.

The formation of HuR/YB1 complex is required for the stabilization of target mRNA to promote myogenesis

mRNA stability is the mechanism by which cells protect transcripts allowing their expression to execute various functions that affect cell metabolism and fate. It is well-established that RNA binding proteins (RBPs) such as HuR use their ability to stabilize mRNA targets to modulate vital processes such as muscle fiber formation (myogenesis). However, the machinery and the mechanisms regulating mRNA stabilization are still elusive. Here, we identified Y-Box binding protein 1 (YB1) as an indispensable HuR binding partner for mRNA stabilization and promotion of myogenesis. Both HuR and YB1 bind to 409 common mRNA targets, 147 of which contain a U-rich consensus motif in their 3' untranslated region (3'UTR) that can also be found in mRNA targets in other cell systems. YB1 and HuR form a heterodimer that associates with the U-rich consensus motif to stabilize key promyogenic mRNAs. The formation of this complex involves a small domain in HuR (227-234) that if mutated prevents HuR from reestablishing myogenesis in siHuR-treated muscle cells. Together our data uncover that YB1 is a key player in HuR-mediated stabilization of pro-myogenic mRNAs and provide the first indication that the mRNA stability mechanism is as complex as other key cellular processes such as mRNA decay and translation.

Hu Antigen R (HuR) Protein Structure, Function and Regulation in Hepatobiliary Tumors

Hu antigen R (HuR) is a 36-kDa ubiquitous member of the ELAV/Hu family of RNA-binding proteins (RBPs), which plays an important role as a post-transcriptional regulator of specific RNAs under physiological and pathological conditions, including cancer. Herein, we review HuR protein structure, function, and its regulation, as well as its implications in the pathogenesis, progression, and treatment of hepatobiliary cancers. In particular, we focus on hepatocellular carcinoma (HCC) and cholangiocarcinoma (CCA), tumors where the increased cytoplasmic localization of HuR and activity are proposed, as valuable diagnostic and prognostic markers. An overview of the main regulatory axes involving HuR, which are associated with cell proliferation, invasion, metastasis, apoptosis, and autophagy in HCC, is provided. These include the transcriptional, post-transcriptional, and post-translational modulators of HuR function, in addition to HuR target transcripts. Finally, whereas studies addressing the relevance of targeting HuR in CCA are limited, in the past few years, HuR has emerged as a potential therapeutic target in HCC. In fact, the therapeutic efficacy of some pharmacological inhibitors of HuR has been evaluated, in early experimental models of HCC. We, further, discuss the major findings and future perspectives of therapeutic approaches that specifically block HuR interactions, either with post-translational modifiers or cognate transcripts in hepatobiliary cancers.

miR-18a-3p and Its Target Protein HuR May Regulate Myogenic Differentiation in Immune-Mediated Necrotizing Myopathy

Immune-mediated necrotizing myopathy (IMNM) is characterized by manifestation of myonecrosis and regeneration of muscle fibers; however, the underlying pathogenesis remains unclear. This study aimed to investigate the role and mechanism of miR-18a-3p and its target RNA-binding protein HuR in IMNM. HuR and miR-18a-3p levels were detected in the skeletal muscles of 18 patients with IMNM using quantitative reverse-transcription real-time polymerase chain reaction (qRT-PCR) and western blotting analysis. Human myoblasts were transfected with small interfering RNA targeting HuR and miR-18a-3p mimic or inhibitor. Myogenic differentiation markers, myogenin and myosin heavy chain, were analyzed by qRT-PCR, western blotting analysis, and immunofluorescence staining. The results showed that miR-18a-3p was upregulated (p=0.0002), whereas HuR was downregulated (p=0.002) in the skeletal muscles of patients with IMNM. The expression of miR-18a-3p in patients with IMNM was negatively correlated with those of HuR (r = -0.512, p = 0.029). We also found that disease activity was positively correlated with HuR expression (r = 0.576, p = 0.012) but muscle activity was negatively correlated with miR-18a-3p expression (r = -0.550, p = 0.017). Besides, bioinformatics analysis and dual-luciferase reporter assays suggested that miR-18a-3p could directly target HuR. Cellular experiments showed that overexpression of miR-18a-3p inhibited myogenic differentiation by targeting HuR, whereas inhibition of miR-18a-3p led to opposite results. Therefore, miR-18a-3p and its target protein HuR may be responsible for modulating the myogenic process in IMNM and can thus be therapeutic targets for the same.

HuR-targeted agents: An insight into medicinal chemistry, biophysical, computational studies and pharmacological effects on cancer models

The Human antigen R (HuR) protein is an RNA-binding protein, ubiquitously expressed in human tissues, that orchestrates target RNA maturation and processing both in the nucleus and in the cytoplasm. A survey of known modulators of the RNA-HuR interactions is followed by a description of its structure and molecular mechanism of action - RRM domains, interactions with RNA, dimerization, binding modes with naturally occurring and synthetic HuR inhibitors. Then, the review focuses on HuR as a validated molecular target in oncology and briefly describes its role in inflammation. Namely, we show ample evidence for the involvement of HuR in the hallmarks and enabling characteristics of cancer, reporting findings from in vitro and in vivo studies; and we provide abundant experimental proofs of a beneficial role for the inhibition of HuR-mRNA interactions through silencing (CRISPR, siRNA) or pharmacological inhibition (small molecule HuR inhibitors).

A novel CARM1-HuR axis involved in muscle differentiation and plasticity misregulated in spinal muscular atrophy

Spinal muscular atrophy (SMA) is characterized by the loss of alpha motor neurons in the spinal cord and a progressive muscle weakness and atrophy. SMA is caused by loss-of-function mutations and/or deletions in the survival of motor neuron (SMN) gene. The role of SMN in motor neurons has been extensively studied, but its function and the consequences of its loss in muscle has also emerged as a key aspect of SMA pathology. In this study, we explore the molecular mechanisms involved in muscle defects in SMA. First, we show in C2C12 myoblasts, that arginine methylation by CARM1 controls myogenic differentiation. More specifically, the methylation of HuR on K217 regulates HuR levels and subcellular localization during myogenic differentiation, and the formation of myotubes. Furthermore, we demonstrate that SMN and HuR interact in C2C12 myoblasts. Interestingly, the SMA-causing E134K point mutation within the SMN Tudor domain, and CARM1 depletion, modulate the SMN-HuR interaction. In addition, using the Smn2B/- mouse model, we report that CARM1 levels are markedly increased in SMA muscles and that HuR fails to properly respond to muscle denervation, thereby affecting the regulation of its mRNA targets. Altogether, our results show a novel CARM1-HuR axis in the regulation of muscle differentiation and plasticity as well as in the aberrant regulation of this axis caused by the absence of SMN in SMA muscle. With the recent developments of therapeutics targeting motor neurons, this study further indicates the need for more global therapeutic approaches for SMA.

Aberrant Post-Transcriptional Regulation of Protein Expression in the Development of Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is an incurable and prevalent respiratory disorder that is characterized by chronic inflammation and emphysema. COPD is primarily caused by cigarette smoke (CS). CS alters numerous cellular processes, including the post-transcriptional regulation of mRNAs. The identification of RNA-binding proteins (RBPs), microRNAs (miRNAs), and long non-coding RNAs (lncRNAs) as main factors engaged in the regulation of RNA biology opens the door to understanding their role in coordinating physiological cellular processes. Dysregulation of post-transcriptional regulation by foreign particles in CS may lead to the development of diseases such as COPD. Here we review current knowledge about post-transcriptional events that may be involved in the pathogenesis of COPD.

RNA-Binding Proteins in the Post-transcriptional Control of Skeletal Muscle Development, Regeneration and Disease

Embryonic myogenesis is a temporally and spatially regulated process that generates skeletal muscle of the trunk and limbs. During this process, mononucleated myoblasts derived from myogenic progenitor cells within the somites undergo proliferation, migration and differentiation to elongate and fuse into multinucleated functional myofibers. Skeletal muscle is the most abundant tissue of the body and has the remarkable ability to self-repair by re-activating the myogenic program in muscle stem cells, known as satellite cells. Post-transcriptional regulation of gene expression mediated by RNA-binding proteins is critically required for muscle development during embryogenesis and for muscle homeostasis in the adult. Differential subcellular localization and activity of RNA-binding proteins orchestrates target gene expression at multiple levels to regulate different steps of myogenesis. Dysfunctions of these post-transcriptional regulators impair muscle development and homeostasis, but also cause defects in motor neurons or the neuromuscular junction, resulting in muscle degeneration and neuromuscular disease. Many RNA-binding proteins, such as members of the muscle blind-like (MBNL) and CUG-BP and ETR-3-like factors (CELF) families, display both overlapping and distinct targets in muscle cells. Thus they function either cooperatively or antagonistically to coordinate myoblast proliferation and differentiation. Evidence is accumulating that the dynamic interplay of their regulatory activity may control the progression of myogenic program as well as stem cell quiescence and activation. Moreover, the role of RNA-binding proteins that regulate post-transcriptional modification in the myogenic program is far less understood as compared with transcription factors involved in myogenic specification and differentiation. Here we review past achievements and recent advances in understanding the functions of RNA-binding proteins during skeletal muscle development, regeneration and disease, with the aim to identify the fundamental questions that are still open for further investigations.

Targeting the RNA-Binding Protein HuR as Potential Thera-Peutic Approach for Neurological Disorders: Focus on Amyo-Trophic Lateral Sclerosis (ALS), Spinal Muscle Atrophy (SMA) and Multiple Sclerosis

The importance of precise co- and post-transcriptional processing of RNA in the regulation of gene expression has become increasingly clear. RNA-binding proteins (RBPs) are a class of proteins that bind single- or double-chain RNA, with different affinities and selectivity, thus regulating the various functions of RNA and the fate of the cells themselves. ELAV (embryonic lethal/abnormal visual system)/Hu proteins represent an important family of RBPs and play a key role in the fate of newly transcribed mRNA. ELAV proteins bind AU-rich element (ARE)-containing transcripts, which are usually present on the mRNA of proteins such as cytokines, growth factors, and other proteins involved in neuronal differentiation and maintenance. In this review, we focused on a member of ELAV/Hu proteins, HuR, and its role in the development of neurodegenerative disorders, with a particular focus on demyelinating diseases.

Interaction of OIP5-AS1 with MEF2C mRNA promotes myogenic gene expression

Long noncoding (lnc)RNAs potently regulate gene expression programs in physiology and disease. Here, we describe a key function for lncRNA OIP5-AS1 in myogenesis, the process whereby myoblasts differentiate into myotubes during muscle development and muscle regeneration after injury. In human myoblasts, OIP5-AS1 levels increased robustly early in myogenesis, and its loss attenuated myogenic differentiation and potently reduced the levels of the myogenic transcription factor MEF2C. This effect relied upon the partial complementarity of OIP5-AS1 with MEF2C mRNA and the presence of HuR, an RNA-binding protein (RBP) with affinity for both transcripts. Remarkably, HuR binding to MEF2C mRNA, which stabilized MEF2C mRNA and increased MEF2C abundance, was lost after OIP5-AS1 silencing, suggesting that OIP5-AS1 might serve as a scaffold to enhance HuR binding to MEF2C mRNA, in turn increasing MEF2C production. These results highlight a mechanism whereby a lncRNA promotes myogenesis by enhancing the interaction of an RBP and a myogenic mRNA.

AChR β-Subunit mRNAs Are Stabilized by HuR in a Mouse Model of Congenital Myasthenic Syndrome With Acetylcholinesterase Deficiency

Collagen Q (COLQ) is a specific collagen that anchors acetylcholinesterase (AChE) in the synaptic cleft of the neuromuscular junction. So far, no mutation has been identified in the ACHE human gene but over 50 different mutations in the COLQ gene are causative for a congenital myasthenic syndrome (CMS) with AChE deficiency. Mice deficient for COLQ mimic most of the functional deficit observed in CMS patients. At the molecular level, a striking consequence of the absence of COLQ is an increase in the levels of acetylcholine receptor (AChR) mRNAs and proteins in vivo and in vitro in murine skeletal muscle cells. Here, we decipher the mechanisms that drive AChR mRNA upregulation in cultured muscle cells deficient for COLQ. We show that the levels of AChR β-subunit mRNAs are post-transcriptionally regulated by an increase in their stability. We demonstrate that this process results from an activation of p38 MAPK and the cytoplasmic translocation of the nuclear RNA-binding protein human antigen R (HuR) that interacts with the AU-rich element located within AChR β-subunit transcripts. This HuR/AChR transcript interaction induces AChR β-subunit mRNA stabilization and occurs at a specific stage of myogenic differentiation. In addition, pharmacological drugs that modulate p38 activity cause parallel modifications of HuR protein and AChR β-subunit levels. Thus, our study provides new insights into the signaling pathways that are regulated by ColQ-deficiency and highlights for the first time a role for HuR and p38 in mRNA stability in a model of congenital myasthenic syndrome.

lncMGPF is a novel positive regulator of muscle growth and regeneration

BACKGROUND

Long non-coding RNAs (lncRNAs) play critical regulatory roles in diverse biological processes and diseases. While a large number of lncRNAs have been identified in skeletal muscles until now, their function and underlying mechanisms in skeletal myogenesis remain largely unclear.

METHODS

We characterized a novel functional lncRNA designated lncMGPF (lncRNA muscle growth promoting factor) using RACE, Northern blot, fluorescence in situ hybridization and quantitative real-time PCR. Its function was determined by gene overexpression, interference, and knockout experiments in C2C12 myoblasts, myogenic progenitor cells, and an animal model. The molecular mechanism by which lncMGPF regulates muscle differentiation was mainly examined by cotransfection experiments, luciferase reporter assay, RNA immunoprecipitation, RNA pull-down, and RNA stability analyses.

RESULTS

We report that lncMGPF, which is highly expressed in muscles and positively regulated by myoblast determination factor (MyoD), promotes myogenic differentiation of muscle cells in vivo and in vitro. lncMGPF knockout in mice substantially decreases growth rate, reduces muscle mass, and impairs muscle regeneration. Overexpression of lncMGPF in muscles can rescue the muscle phenotype of knockout mice and promote muscle growth of wild-type mice. Mechanistically, lncMGPF promotes muscle differentiation by acting as a molecular sponge of miR-135a-5p and thus increasing the expression of myocyte enhancer factor 2C (MEF2C), as well as by enhancing human antigen R-mediated messenger RNA stabilization of myogenic regulatory genes such as MyoD and myogenin (MyoG). We confirm that pig lncRNA AK394747 and human lncRNA MT510647 are homologous to mouse lncMGPF, with conserved function and mechanism during myogenesis.

CONCLUSIONS

Our data reveal that lncMGPF is a novel positive regulator of myogenic differentiation, muscle growth and regeneration in mice, pigs, and humans.

Cobalt chloride, a chemical hypoxia-mimicking agent, suppresses myoblast differentiation by downregulating myogenin expression

Cobalt chloride can create hypoxia-like state in vitro (referred to as chemical hypoxia). Several studies have suggested that chemical hypoxia may cause deleterious effects on myogenesis. The intrinsic underlying mechanisms of myoblast differentiation, however, are not fully understood. Here, we show that cobalt chloride strongly suppresses myoblast differentiation in a dose-dependent manner. The impaired myoblast differentiation is accompanied by downregulation of myogenic regulatory factor myogenin. Under chemical hypoxia, myogenin stability is decreased at mRNA and protein levels. A muscle-specific E3 ubiquitin ligase MAFbx, which can target myogenin protein for proteasomal degradation, is upregulated along with changes in Akt/Foxo and AMPK/Foxo signaling pathways. A proteasome inhibitor completely prevents cobalt chloride-mediated decrease in myogenin protein. These results suggest that cobalt chloride might modulate myogenin expression at post-transcriptional and post-translational levels, resulting in the failure of the myoblasts to differentiate into myotubes.

Understanding and targeting the disease-related RNA binding protein human antigen R (HuR)

Altered gene expression is a characteristic feature of many disease states such as tumorigenesis, and in most cancers, it facilitates cancer cell survival and adaptation. Alterations in global gene expression are strongly impacted by post-transcriptional gene regulation. The RNA binding protein (RBP) HuR (ELAVL1) is an established regulator of post-transcriptional gene regulation and is overexpressed in most human cancers. In many cancerous settings, HuR is not only overexpressed, but it is "overactive" as denoted by increased subcellular localization within the cytoplasm. This dysregulation of HuR expression and cytoplasmic localization allows HuR to stabilize and increase the translation of various prosurvival messenger RNA (mRNAs) involved in the pathogenesis of numerous cancers and various diseases. Based on almost 20 years of work, HuR is now recognized as a therapeutic target. Herein, we will review the role HuR plays in the pathophysiology of different diseases and ongoing therapeutic strategies to target HuR. We will focus on three ongoing-targeted strategies: (1) inhibiting HuR's translocation from the nucleus to the cytoplasm; (2) inhibiting the ability of HuR to bind target RNA; and (3) silencing HuR expression levels. In an oncologic setting, HuR has been demonstrated to be critical for a cancer cell's ability to survive a variety of cancer relevant stressors (including drugs and elements of the tumor microenvironment) and targeting this protein has been shown to sensitize cancer cells further to insult. We strongly believe that targeting HuR could be a powerful therapeutic target to treat different diseases, particularly cancer, in the near future. This article is categorized under: RNA in Disease and Development > RNA in Disease NRA Turnover and Surveillance > Regulation of RNA Stability Translation > Translation Regulation.

HuR counteracts miR-330 to promote STAT3 translation during inflammation-induced muscle wasting

Debilitating cancer-induced muscle wasting, a syndrome known as cachexia, is lethal. Here we report a posttranscriptional pathway involving the RNA-binding protein HuR as a key player in the onset of this syndrome. Under these conditions, HuR switches its function from a promoter of muscle fiber formation to become an inducer of muscle loss. HuR binds to the STAT3 (signal transducer and activator of transcription 3) mRNA, which encodes one of the main effectors of this condition, promoting its expression both in vitro and in vivo. While HuR does not affect the stability and the cellular movement of this transcript, HuR promotes the translation of the STAT3 mRNA by preventing miR-330 (microRNA 330)-mediated translation inhibition. To achieve this effect, HuR directly binds to a U-rich element in the STAT3 mRNA-3'untranslated region (UTR) located within the vicinity of the miR-330 seed element. Even though the binding sites of HuR and miR-330 do not overlap, the recruitment of either one of them to the STAT3-3'UTR negatively impacts the binding and the function of the other factor. Therefore, together, our data establish the competitive interplay between HuR and miR-330 as a mechanism via which muscle fibers modulate, in part, STAT3 expression to determine their fate in response to promoters of muscle wasting.

Post-transcriptional regulation of Pabpn1 by the RNA binding protein HuR

RNA processing is critical for proper spatial and temporal control of gene expression. The ubiquitous nuclear polyadenosine RNA binding protein, PABPN1, post-transcriptionally regulates multiple steps of gene expression. Mutations in the PABPN1 gene expanding an N-terminal alanine tract in the PABPN1 protein from 10 alanines to 11–18 alanines cause the muscle-specific disease oculopharyngeal muscular dystrophy (OPMD), which affects eyelid, pharynx, and proximal limb muscles. Previous work revealed that the Pabpn1 transcript is unstable, contributing to low steady-state Pabpn1 mRNA and protein levels in vivo, specifically in skeletal muscle, with even lower levels in muscles affected in OPMD. Thus, low levels of PABPN1 protein could predispose specific tissues to pathology in OPMD. However, no studies have defined the mechanisms that regulate Pabpn1 expression. Here, we define multiple cis-regulatory elements and a trans-acting factor, HuR, which regulate Pabpn1 expression specifically in mature muscle in vitro and in vivo. We exploit multiple models including C2C12 myotubes, primary muscle cells, and mice to determine that HuR decreases Pabpn1 expression. Overall, we have uncovered a mechanism in mature muscle that negatively regulates Pabpn1 expression in vitro and in vivo, which could provide insight to future studies investigating therapeutic strategies for OPMD treatment.

The RNA binding protein HuR influences skeletal muscle metabolic flexibility in rodents and humans

BACKGROUND

Metabolic flexibility can be assessed by changes in respiratory exchange ratio (RER) following feeding. Though metabolic flexibility (difference in RER between fasted and fed state) is often impaired in individuals with obesity or type 2 diabetes, the cellular processes contributing to this impairment are unclear.

MATERIALS AND METHODS

From several clinical studies we identified the 16 most and 14 least metabolically flexible male and female subjects out of >100 participants based on differences between 24-hour and sleep RER measured in a whole-room indirect calorimeter. Global skeletal muscle gene expression profiles revealed that, in metabolically flexible subjects, transcripts regulated by the RNA binding protein, HuR, are enriched. We generated and characterized mice with a skeletal muscle-specific knockout of the HuR encoding gene, Elavl1 (HuR^m-/-).

RESULTS

Male, but not female, HuR^m-/- mice exhibit metabolic inflexibility, with mild obesity, impaired glucose tolerance, impaired fat oxidation and decreased in vitro palmitate oxidation compared to HuR^fl/fl littermates. Expression levels of genes involved in mitochondrial fatty acid oxidation and oxidative phosphorylation are decreased in both mouse and human muscle when HuR is inhibited.

CONCLUSIONS

HuR inhibition results in impaired metabolic flexibility and decreased lipid oxidation, suggesting a role for HuR as an important regulator of skeletal muscle metabolism.

Muscle development and regeneration controlled by AUF1-mediated stage-specific degradation of fate-determining checkpoint mRNAs

AUF1 promotes rapid decay of mRNAs containing 3' untranslated region (3'UTR) AU-rich elements (AREs). AUF1 depletion in mice accelerates muscle loss and causes limb girdle muscular dystrophy. Here, we demonstrate that the selective, targeted degradation by AUF1 of key muscle stem cell fate-determining checkpoint mRNAs regulates each stage of muscle development and regeneration by reprogramming each myogenic stage. Skeletal muscle stem (satellite) cell explants show that Auf1 transcription is activated with satellite cell activation by stem cell regulatory factor CTCF. AUF1 then targets checkpoint ARE-mRNAs for degradation, progressively reprogramming the transcriptome through each stage of myogenesis. Transition steps in myogenesis, from stem cell proliferation to differentiation to muscle fiber development, are each controlled by fate-determining checkpoint mRNAs, which, surprisingly, were found to be controlled in their expression by AUF1-targeted mRNA decay. Checkpoint mRNAs targeted by AUF1 include Twist1, decay of which promotes myoblast development; CyclinD1, decay of which blocks myoblast proliferation and initiates differentiation; and RGS5, decay of which activates Sonic Hedgehog (SHH) pathway-mediated differentiation of mature myotubes. AUF1 therefore orchestrates muscle stem cell proliferation, self-renewal, myoblast differentiation, and ultimately formation of muscle fibers through targeted, staged mRNA decay.

RNA-binding protein, human antigen R regulates hypoxia-induced autophagy by targeting ATG7/ATG16L1 expressions and autophagosome formation

Autophagy, a prosurvival mechanism offers a protective role during acute kidney injury. We show novel findings on the functional role of RNA binding protein, HuR during hypoxia-induced autophagy in renal proximal tubular cells-2 (HK-2). HK-2 cells showed upregulated expressions of HuR and autophagy-related proteins such as autophagy related 7 (ATG7), autophagy related 16 like 1 (ATG16L1), and LC3II under hypoxia. Increased autophagosome formation was visualized as LC3 puncta in hypoxic cells. Further, short hairpin-RNA-mediated loss of HuR function in HK-2 cells significantly decreased ATG7 and ATG16L1 protein expressions. Bioinformatics prediction revealed HuR motif binding on the coding region of ATG7 and AU-rich element at 3'UTR ATG16L1 messnger RNA (mRNA). The RNA immunoprecipitation study showed that HuR was predominantly associated with ATG7 and ATG16L1 mRNAs under hypoxia. In addition, HuR enhanced autophagosome formation by regulating LC3II expressions. These results show that HuR regulates ATG7 and ATG16L1 expressions and thereby mediate autophagy in HK-2 cells. Importantly, HuR knockdown cells underwent apoptosis during hypoxia as observed through the terminal deoxynucleotidyl transferase dUTP nick end labeling assay. Collectively, these findings show the crucial role of HuR under hypoxia by regulating autophagy and suppressing apoptosis in renal tubular cells.

Conserved functions of RNA-binding proteins in muscle

Animals require different types of muscle for survival, for example for circulation, motility, reproduction and digestion. Much emphasis in the muscle field has been placed on understanding how transcriptional regulation generates diverse types of muscle during development. Recent work indicates that alternative splicing and RNA regulation are as critical to muscle development, and altered function of RNA-binding proteins causes muscle disease. Although hundreds of genes predicted to bind RNA are expressed in muscles, many fewer have been functionally characterized. We present a cross-species view summarizing what is known about RNA-binding protein function in muscle, from worms and flies to zebrafish, mice and humans. In particular, we focus on alternative splicing regulated by the CELF, MBNL and RBFOX families of proteins. We discuss the systemic nature of diseases associated with loss of RNA-binding proteins in muscle, focusing on mis-regulation of CELF and MBNL in myotonic dystrophy. These examples illustrate the conservation of RNA-binding protein function and the marked utility of genetic model systems in understanding mechanisms of RNA regulation.

eIF4A inhibition prevents the onset of cytokine-induced muscle wasting by blocking the STAT3 and iNOS pathways

Cachexia is a deadly muscle wasting syndrome that arises under conditions linked to chronic inflammation, such as cancer. Cytokines, including interferon γ (IFNγ), tumor necrosis factor α (TNFα) and interleukin-6 (IL-6), and their downstream effectors such as Signal Transducer and Activator of Transcription 3 (STAT3), have been shown to play a prominent role in muscle wasting. Previously, we demonstrated that Pateamine A (PatA), a compound that targets eukaryotic initiation factor 4A (eIF4A), could prevent muscle wasting by modulating the translation of the inducible Nitric Oxide Synthase (iNOS) mRNA. Here we show that hippuristanol, a compound that impedes eIF4A in a manner distinct from PatA, similarly inhibits the iNOS/NO pathway and cytokine-induced muscle wasting. Furthermore, we show that hippuristanol perturbs the activation of the STAT3 pathway and expression of STAT3-gene targets such as IL-6. The decreased activation of STAT3, which resulted from a decrease in STAT3 protein expression, was due to the inhibition of STAT3 translation as there were no changes in STAT3 mRNA levels. These effects are likely dependent on the inhibition of eIF4A activity since we observed similar results using PatA. Our results identify the inhibition of eIF4A-responsive transcripts, such as STAT3, as a viable approach to alleviate cachexia.

The requirement of Mettl3-promoted MyoD mRNA maintenance in proliferative myoblasts for skeletal muscle differentiation

Myogenic progenitor/stem cells retain their skeletal muscle differentiation potential by maintaining myogenic transcription factors such as MyoD. However, the mechanism of how MyoD expression is maintained in proliferative progenitor cells has not been elucidated. Here, we found that MyoD expression was reduced at the mRNA level by cell cycle arrest in S and G2 phases, which in turn led to the absence of skeletal muscle differentiation. The reduction of MyoD mRNA was correlated with the reduced expression of factors regulating RNA metabolism, including methyltransferase like 3 (Mettl3), which induces N⁶-methyladenosine (m⁶A) modifications of RNA. Knockdown of Mettl3 revealed that MyoD RNA was specifically downregulated and that this was caused by a decrease in processed, but not unprocessed, mRNA. Potential m⁶A modification sites were profiled by m⁶A sequencing and identified within the 5' untranslated region (UTR) of MyoD mRNA. Deletion of the 5' UTR revealed that it has a role in MyoD mRNA processing. These data showed that Mettl3 is required for MyoD mRNA expression in proliferative myoblasts.

Chemo-biologic combinatorial drug delivery using folate receptor-targeted dendrimer nanoparticles for lung cancer treatment

Co-administration of functionally distinct anti-cancer agents has emerged as an efficient strategy in lung cancer treatment. However, a specially designed drug delivery system is required to co-encapsulate functionally different agents, such as a combination of siRNA and chemotherapy, for targeted delivery. We developed a folic acid (FA)-conjugated polyamidoamine dendrimer (Den)-based nanoparticle (NP) system for co-delivery of siRNA against HuR mRNA (HuR siRNA) and cis-diamine platinum (CDDP) to folate receptor-α (FRA) -overexpressing H1299 lung cancer cells. The co-delivery of HuR siRNA and CDDP using the FRA-targeted NP had a significantly greater therapeutic effect than did individual therapeutics. Further, the FRA-targeted NP exhibited improved cytotoxicity compared to non-targeted NP against lung cancer cells. Finally, the NP showed negligible toxicity towards normal MRC9 lung fibroblast cells. Thus, the present study demonstrates FRA-targeted Den nanoparticle system as a suitable carrier for targeted co-delivery of siRNA and chemotherapy agents in lung cancer cells.

Translational Control of the Myogenic Program in Developing, Regenerating, and Diseased Skeletal Muscle

Translational control of genes that code for protein allows a cell to rapidly respond to changes in its environment, in part because translational control of gene expression does not depend on upstream events required to produce an mRNA molecule. The importance of translational control has been highlighted by studies concerning muscle development, regeneration, and disease. Translational control of specific mRNAs is achieved by microRNAs and RNA-binding proteins, which are particularly relevant to developmental myogenesis, where they ensure the stepwise differentiation of multipotent progenitors to committed myogenic progenitors that ultimately fuse into slow- or fast-type myofibers that make up skeletal muscle. The importance of translational control is also illustrated in muscle disease, where deregulated microRNA expression accelerates or delays progression of disease. Skeletal muscle is also unique for its remarkable capacity to regenerate after injury, which requires the activity of quiescent muscle stem cells, named satellite cells for their position underneath the basal lamina of the myofiber. Mitotically quiescent satellite cells are primed to activate the cell cycle and myogenic program, a unique feature that requires specific regulation of mRNA translation converging with pathways that regulate global protein synthesis. Emerging concepts in translational control of gene expression have shed light on multiple layers of control over the myogenic program. In parallel, the development and regeneration of skeletal muscle represents a unique, relevant, and highly defined context within which new concepts in translational control of gene expression should emerge.

The regulatory mechanisms of myogenin expression in doxorubicin-treated rat cardiomyocytes

Doxorubicin, an anthracycline antibiotic, has been used as an anti-neoplastic drug for almost 60 years. However, the mechanism(s) by which anthracyclines cause irreversible myocardial injury remains unclear. In order to delineate possible molecular signals involved in the myocardial toxicity, we assessed candidate genes using mRNA expression profiling in the doxorubicin-treated rat cardiomyocyte H9c2 cell line. In the study, it was confirmed that myogenin, an important transcriptional factor for muscle terminal differentiation, was significantly reduced by doxorubicin in a dose-dependent manner using both RT-PCR and western blot analyses. Also, it was identified that the doxorubicin-reduced myogenin gene level could not be rescued by most cardio-protectants. Furthermore, it was demonstrated how the signaling of the decreased myogenin expression by doxorubicin was altered at the transcriptional, post-transcriptional and translational levels. Based on these findings, a working model was proposed for relieving doxorubicin-associated myocardial toxicity by down-regulating miR-328 expression and increasing voltage-gated calcium channel β1 expression, which is a repressor of myogenin gene regulation. In summary, this study provides several lines of evidence indicating that myogenin is the target for doxorubicin-induced cardio-toxicity and a novel therapeutic strategy for doxorubicin clinical applications based on the regulatory mechanisms of myogenin expression.

Novel RNA-binding activity of MYF5 enhances Ccnd1/Cyclin D1 mRNA translation during myogenesis

Skeletal muscle contains long multinucleated and contractile structures known as muscle fibers, which arise from the fusion of myoblasts into multinucleated myotubes during myogenesis. The myogenic regulatory factor (MRF) MYF5 is the earliest to be expressed during myogenesis and functions as a transcription factor in muscle progenitor cells (satellite cells) and myocytes. In mouse C2C12 myocytes, MYF5 is implicated in the initial steps of myoblast differentiation into myotubes. Here, using ribonucleoprotein immunoprecipitation (RIP) analysis, we discovered a novel function for MYF5 as an RNA-binding protein which associated with a subset of myoblast mRNAs. One prominent MYF5 target was Ccnd1 mRNA, which encodes the key cell cycle regulator CCND1 (Cyclin D1). Biotin-RNA pulldown, UV-crosslinking and gel shift experiments indicated that MYF5 was capable of binding the 3' untranslated region (UTR) and the coding region (CR) of Ccnd1 mRNA. Silencing MYF5 expression in proliferating myoblasts revealed that MYF5 promoted CCND1 translation and modestly increased transcription of Ccnd1 mRNA. Accordingly, overexpressing MYF5 in C2C12 cells upregulated CCND1 expression while silencing MYF5 reduced myoblast proliferation as well as differentiation of myoblasts into myotubes. Moreover, MYF5 silencing reduced myogenesis, while ectopically restoring CCND1 abundance partially rescued the decrease in myogenesis seen after MYF5 silencing. We propose that MYF5 enhances early myogenesis in part by coordinately elevating Ccnd1 transcription and Ccnd1 mRNA translation.

HuR Mediates Changes in the Stability of AChR β-Subunit mRNAs after Skeletal Muscle Denervation

Acetylcholine receptors (AChRs) are heteromeric membrane proteins essential for neurotransmission at the neuromuscular junction. Previous work showed that muscle denervation increases expression of AChR mRNAs due to transcriptional activation of AChR subunit genes. However, it remains possible that post-transcriptional mechanisms are also involved in controlling the levels of AChR mRNAs following denervation. We examined whether post-transcriptional events indeed regulate AChR β-subunit mRNAs in response to denervation. First, in vitro stability assays revealed that the half-life of AChR β-subunit mRNAs was increased in the presence of denervated muscle protein extracts. A bioinformatics analysis revealed the existence of a conserved AU-rich element (ARE) in the 3'-untranslated region (UTR) of AChR β-subunit mRNA. Furthermore, denervation of mouse muscle injected with a luciferase reporter construct containing the AChR β-subunit 3'UTR, caused an increase in luciferase activity. By contrast, mutation of this ARE prevented this increase. We also observed that denervation increased expression of the RNA-binding protein human antigen R (HuR) and induced its translocation to the cytoplasm. Importantly, HuR binds to endogenous AChR β-subunit transcripts in cultured myotubes and in vivo, and this binding is increased in denervated versus innervated muscles. Finally, p38 MAPK, a pathway known to activate HuR, was induced following denervation as a result of MKK3/6 activation and a decrease in MKP-1 expression, thereby leading to an increase in the stability of AChR β-subunit transcripts. Together, these results demonstrate the important contribution of post-transcriptional events in regulating AChR β-subunit mRNAs and point toward a central role for HuR in mediating synaptic gene expression.

SIGNIFICANCE STATEMENT

Muscle denervation is a convenient model to examine expression of genes encoding proteins of the neuromuscular junction, especially acetylcholine receptors (AChRs). Despite the accepted model of AChR regulation, which implicates transcriptional mechanisms, it remains plausible that such events cannot fully account for changes in AChR expression following denervation. We show that denervation increases expression of the RNA-binding protein HuR, which in turn, causes an increase in the stability of AChR β-subunit mRNAs in denervated muscle. Our findings demonstrate for the first time the contribution of post-transcriptional events in controlling AChR expression in skeletal muscle, and points toward a central role for HuR in mediating synaptic development while also paving the way for developing RNA-based therapeutics for neuromuscular diseases.

Post-transcriptional regulation of satellite cell quiescence by TTP-mediated mRNA decay

Skeletal muscle satellite cells in their niche are quiescent and upon muscle injury, exit quiescence, proliferate to repair muscle tissue, and self-renew to replenish the satellite cell population. To understand the mechanisms involved in maintaining satellite cell quiescence, we identified gene transcripts that were differentially expressed during satellite cell activation following muscle injury. Transcripts encoding RNA binding proteins were among the most significantly changed and included the mRNA decay factor Tristetraprolin. Tristetraprolin promotes the decay of MyoD mRNA, which encodes a transcriptional regulator of myogenic commitment, via binding to the MyoD mRNA 3′ untranslated region. Upon satellite cell activation, p38α/β MAPK phosphorylates MAPKAP2 and inactivates Tristetraprolin, stabilizing MyoD mRNA. Satellite cell specific knockdown of Tristetraprolin precociously activates satellite cells in vivo, enabling MyoD accumulation, differentiation and cell fusion into myofibers. Regulation of mRNAs by Tristetraprolin appears to function as one of several critical post-transcriptional regulatory mechanisms controlling satellite cell homeostasis.

DOI:http://dx.doi.org/10.7554/eLife.03390.001

When muscles are damaged, they can repair themselves to some extent by making new muscle cells. These develop from groups of cells called satellite cells, which are found near the surface of muscle fibers. Once the muscle is injured, the satellite cells are activated and can divide to form two cells with different properties. One remains a satellite cell, while the other forms a ‘myoblast’ that eventually fuses into a mature muscle fiber. Under normal conditions the satellite cells remain in a dormant state and do not divide, but it is not clear how they maintain this dormant state.

To create a protein, the gene that encodes it is first ‘transcribed’ to produce a molecule called mRNA, which is then used as a template to build the protein. A protein called Tristetraprolin (TTP) can bind to mRNA molecules and cause them to break down or decay, and so TTP can prevent the mRNA from being used to make a protein.

Hausburg, Doles et al. analyzed satellite cells from uninjured muscle and compared them with those from injured tissue. This revealed that when injured, the satellite cells reduced the abundance of several mRNAs, including TTP. Further investigation found that in satellite cells from uninjured tissue, TTP causes the decay of mRNA molecules that are used to produce a protein called MyoD. As MyoD helps the satellite cells to specialize, this decay therefore prevents the formation of myoblasts and keeps the satellite cells in a dormant state. In contrast, damage to the muscle tissue activates a signaling pathway that ultimately inactivates TTP. This enables more of the MyoD protein to be made and the myoblast population to expand.

When Hausburg, Doles et al. experimentally reduced the levels of TTP inside satellite cells, the cells developed into myoblasts even when the tissue was uninjured. Thus, TTP is an important regulator that allows satellite cells to remain in a dormant state. In dormant adult stem cells, regulation of protein availability by RNA binding proteins, such as TTP, may co-ordinate rapid changes in metabolic state to promptly repair injured tissue. A major challenge will be to identify the group of proteins involved and determine the precise mechanisms involved in regulating their availability.

DOI:http://dx.doi.org/10.7554/eLife.03390.002

H19 long noncoding RNA controls the mRNA decay promoting function of KSRP

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.

The RNA-binding protein Staufen1 impairs myogenic differentiation via a c-myc-dependent mechanism

The expression pattern of Staufen1 during mouse skeletal muscle development is described. Sustained expression of Staufen1 in myoblasts prevents normal differentiation by causing decreases in the expression of key myogenic markers by an SMD-independent mechanism and by promoting the translational regulation of c-myc.

Recent work has shown that Staufen1 plays key roles in skeletal muscle, yet little is known about its pattern of expression during embryonic and postnatal development. Here we first show that Staufen1 levels are abundant in mouse embryonic muscles and that its expression decreases thereafter, reaching low levels in mature muscles. A similar pattern of expression is seen as cultured myoblasts differentiate into myotubes. Muscle degeneration/regeneration experiments revealed that Staufen1 increases after cardiotoxin injection before returning to the low levels seen in mature muscles. We next prevented the decrease in Staufen1 during differentiation by generating stable C2C12 muscle cell lines overexpressing Staufen1. Cells overexpressing Staufen1 differentiated poorly, as evidenced by reductions in the differentiation and fusion indices and decreases in MyoD, myogenin, MEF2A, and MEF2C, independently of Staufen-mediated mRNA decay. However, levels of c-myc, a factor known to inhibit differentiation, were increased in C2C12 cells overexpressing Staufen1 through enhanced translation. By contrast, the knockdown of Staufen1 decreased c-myc levels in myoblasts. Collectively our results show that Staufen1 is highly expressed during early stages of differentiation/development and that it can impair differentiation by regulating c-myc, thereby highlighting the multifunctional role of Staufen1 in skeletal muscle cells.

Destabilization of nucleophosmin mRNA by the HuR/KSRP complex is required for muscle fibre formation

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.

HuR and miR-1192 regulate myogenesis by modulating the translation of HMGB1 mRNA

Upon muscle injury, the high mobility group box 1 (HMGB1) protein is upregulated and secreted to initiate reparative responses. Here we show that HMGB1 controls myogenesis both in vitro and in vivo during development and after adult muscle injury. HMGB1 expression in muscle cells is regulated at the translational level: the miRNA miR-1192 inhibits HMGB1 translation and the RNA-binding protein HuR promotes it. HuR binds to a cis-element, HuR binding sites (HuRBS), located in the 3'UTR of the HMGB1 transcript, and at the same time miR-1192 is recruited to an adjacent seed element. The binding of HuR to the HuRBS prevents the recruitment of Argonaute 2 (Ago2), overriding miR-1192-mediated translation inhibition. Depleting HuR reduces myoblast fusion and silencing miR-1192 re-establishes the fusion potential of HuR-depleted cells. We propose that HuR promotes the commitment of myoblasts to myogenesis by enhancing the translation of HMGB1 and suppressing the translation inhibition mediated by miR-1192.

Mechanisms of exercise-induced mitochondrial biogenesis in skeletal muscle: implications for health and disease

Mitochondria have paradoxical functions within cells. Essential providers of energy for cellular survival, they are also harbingers of cell death (apoptosis). Mitochondria exhibit remarkable dynamics, undergoing fission, fusion, and reticular expansion. Both nuclear and mitochondrial DNA (mtDNA) encode vital sets of proteins which, when incorporated into the inner mitochondrial membrane, provide electron transport capacity for ATP production, and when mutated lead to a broad spectrum of diseases. Acute exercise can activate a set of signaling cascades in skeletal muscle, leading to the activation of the gene expression pathway, from transcription, to post-translational modifications. Research has begun to unravel the important signals and their protein targets that trigger the onset of mitochondrial adaptations to exercise. Exercise training leads to an accumulation of nuclear- and mtDNA-encoded proteins that assemble into functional complexes devoted to mitochondrial respiration, reactive oxygen species (ROS) production, the import of proteins and metabolites, or apoptosis. This process of biogenesis has important consequences for metabolic health, the oxidative capacity of muscle, and whole body fitness. In contrast, the chronic muscle disuse that accompanies aging or muscle wasting diseases provokes a decline in mitochondrial content and function, which elicits excessive ROS formation and apoptotic signaling. Research continues to seek the molecular underpinnings of how regular exercise can be used to attenuate these decrements in organelle function, maintain skeletal muscle health, and improve quality of life.

A novel role for the RNA-binding protein FXR1P in myoblasts cell-cycle progression by modulating p21/Cdkn1a/Cip1/Waf1 mRNA stability

The Fragile X-Related 1 gene (FXR1) is a paralog of the Fragile X Mental Retardation 1 gene (FMR1), whose absence causes the Fragile X syndrome, the most common form of inherited intellectual disability. FXR1P plays an important role in normal muscle development, and its absence causes muscular abnormalities in mice, frog, and zebrafish. Seven alternatively spliced FXR1 transcripts have been identified and two of them are skeletal muscle-specific. A reduction of these isoforms is found in myoblasts from Facio-Scapulo Humeral Dystrophy (FSHD) patients. FXR1P is an RNA–binding protein involved in translational control; however, so far, no mRNA target of FXR1P has been linked to the drastic muscular phenotypes caused by its absence. In this study, gene expression profiling of C2C12 myoblasts reveals that transcripts involved in cell cycle and muscular development pathways are modulated by Fxr1-depletion. We observed an increase of p21—a regulator of cell-cycle progression—in Fxr1-knocked-down mouse C2C12 and FSHD human myoblasts. Rescue of this molecular phenotype is possible by re-expressing human FXR1P in Fxr1-depleted C2C12 cells. FXR1P muscle-specific isoforms bind p21 mRNA via direct interaction with a conserved G-quadruplex located in its 3′ untranslated region. The FXR1P/G-quadruplex complex reduces the half-life of p21 mRNA. In the absence of FXR1P, the upregulation of p21 mRNA determines the elevated level of its protein product that affects cell-cycle progression inducing a premature cell-cycle exit and generating a pool of cells blocked at G0. Our study describes a novel role of FXR1P that has crucial implications for the understanding of its role during myogenesis and muscle development, since we show here that in its absence a reduced number of myoblasts will be available for muscle formation/regeneration, shedding new light into the pathophysiology of FSHD.

Muscle development is a complex process controlled by the timely expression of genes encoding crucial regulators of the muscle cell precursors called myoblasts. We know from previous studies that inactivation of the Fragile X related 1 (FXR1) gene in various animal models (mouse, frog, and zebrafish) causes muscular and cardiac abnormalities. Also, FXR1P is reduced in a human myopathy called Fascio-Scapulo Humeral Dystrophy (FSHD), suggesting its critical role in muscle that findings presented in this study contribute to elucidating. Cell-cycle arrest is a prerequisite to differentiation of myoblasts into mature myotubes, which will form the muscle. One key regulator is the p21/Cdkn1a/Cip1/Waf1 protein, which commands myoblasts to stop proliferating, and this action is particularly important during muscle regeneration. In this study, we have identified FXR1P as a novel regulator of p21 expression. We show that FXR1P absence in mouse myoblasts and FSHD-derived myopathic myoblasts increases abnormally the levels of p21, causing a premature cell cycle exit of myoblasts. Our study predicts that FXR1P absence leads to a reduced number of myoblasts available for muscle formation and regeneration. This explains the drastic effects of FXR1 inactivation on muscle and brings a better understanding of the molecular/cellular bases of FSHD.

CELFish ways to modulate mRNA decay

The CELF family of RNA-binding proteins regulates many steps of mRNA metabolism. Although their best characterized function is in pre-mRNA splice site choice, CELF family members are also powerful modulators of mRNA decay. In this review we focus on the different modes of regulation that CELF proteins employ to mediate mRNA decay by binding to GU-rich elements. After starting with an overview of the importance of CELF proteins during development and disease pathogenesis, we then review the mRNA networks and cellular pathways these proteins regulate and the mechanisms by which they influence mRNA decay. Finally, we discuss how CELF protein activity is modulated during development and in response to cellular signals. We conclude by highlighting the priorities for new experiments in this field. This article is part of a Special Issue entitled: RNA Decay mechanisms.

Regulation of ARE-mRNA Stability by Cellular Signaling: Implications for Human Cancer

During recent years, it has become clear that regulation of mRNA stability is an important event in the control of gene expression. The stability of a large class of mammalian mRNAs is regulated by AU-rich elements (AREs) located in the mRNA 3' UTRs. mRNAs with AREs are inherently labile but as a response to different cellular cues they can become either stabilized, allowing expression of a given gene, or further destabilized to silence their expression. These tightly regulated mRNAs include many that encode growth factors, proto-oncogenes, cytokines, and cell cycle regulators. Failure to properly regulate their stability can therefore lead to uncontrolled expression of factors associated with cell proliferation and has been implicated in several human cancers. A number of transfactors that recognize AREs and regulate the translation and degradation of ARE-mRNAs have been identified. These transfactors are regulated by signal transduction pathways, which are often misregulated in cancers. This chapter focuses on the function of ARE-binding proteins with an emphasis on their regulation by signaling pathways and the implications for human cancer.

A role for RNA post-transcriptional regulation in satellite cell activation

Background

Satellite cells are resident skeletal muscle stem cells responsible for muscle maintenance and repair. In resting muscle, satellite cells are maintained in a quiescent state. Satellite cell activation induces the myogenic commitment factor, MyoD, and cell cycle entry to facilitate transition to a population of proliferating myoblasts that eventually exit the cycle and regenerate muscle tissue. The molecular mechanism involved in the transition of a quiescent satellite cell to a transit-amplifying myoblast is poorly understood.

Methods

Satellite cells isolated by FACS from uninjured skeletal muscle and 12 h post-muscle injury from wild type and Syndecan-4 null mice were probed using Affymetrix 430v2 gene chips and analyzed by Spotfire^tm and Ingenuity Pathway analysis to identify gene expression changes and networks associated with satellite cell activation, respectively. Additional analyses of target genes identify miRNAs exhibiting dynamic changes in expression during satellite cell activation. The function of the miRNAs was assessed using miRIDIAN hairpin inhibitors.

Results

An unbiased gene expression screen identified over 4,000 genes differentially expressed in satellite cells in vivo within 12 h following muscle damage and more than 50% of these decrease dramatically. RNA binding proteins and genes involved in post-transcriptional regulation were significantly over-represented whereas splicing factors were preferentially downregulated and mRNA stability genes preferentially upregulated. Furthermore, six computationally identified miRNAs demonstrated novel expression through muscle regeneration and in satellite cells. Three of the six miRNAs were found to regulate satellite cell fate.

Conclusions

The quiescent satellite cell is actively maintained in a state poised to activate in response to external signals. Satellite cell activation appears to be regulated by post-transcriptional gene regulation.

HuR is necessary for mammary epithelial cell proliferation and polarity at least in part via ΔNp63

HuR, a RNA binding protein, is known to function as a tumor maintenance gene in breast cancer and associated with tumor growth and poor prognosis. However, the cellular function of this protein remains largely unknown in normal mammary epithelial cells. Here, we showed that in immortalized MCF10A mammary epithelial cells, HuR knockdown inhibits cell proliferation and enhances premature senescence. We also showed that in three-dimensional culture, MCF10A cells with HuR knockdown form abnormal acini with filled lumen and an aberrant expression pattern of the extracellular matrix protein laminin V. In addition, we showed that HuR knockdown increases ΔNp63, but decreases wild-type p53, expression in MCF10A cells. Moreover, we showed that ΔNp63 knockdown partially rescues the proliferative defect induced by HuR knockdown in MCF10A cells. Consistent with this, we identified two U-rich elements in the 3'-untranslated region of p63 mRNA, to which HuR specifically binds. Finally, we showed that HuR knockdown enhances ΔNp63 mRNA translation but has no effect on p63 mRNA turnover. Together, our data suggest that HuR maintains cell proliferation and polarity of mammary epithelial cells at least in part via ΔNp63.

Apoptotic-induced cleavage shifts HuR from being a promoter of survival to an activator of caspase-mediated apoptosis

Little is known about the cellular mechanisms modulating the shift in balance from a state of survival to cell death by caspase-mediated apoptosis in response to a lethal stress. Here we show that the RNA-binding protein HuR has an important function in mediating this switch. During caspase-mediated apoptosis, HuR is cleaved to generate two cleavage products (CPs). Our data demonstrate that the cleavage of HuR switches its function from being a prosurvival factor under normal conditions to becoming a promoter of apoptosis in response to a lethal stress. In the absence of an apoptotic stimuli, HuR associates with and promotes the expression of caspase-9 and prothymosin α (ProT) mRNAs, and pro- and antiapoptotic factors, respectively, both of which have been characterized as important players in determining cell fate. During the early steps of caspase-mediated apoptosis, however, the level of caspase-9 protein increases, while ProT remains unchanged. Under these conditions, the two HuR-CPs selectively bind to and stabilize caspase-9 mRNA, but do not bind to ProT. Hence, taken together, our data show that by maintaining a threshold of expression of proapoptotic factors such as caspase-9 in response to a lethal stress, the HuR-CPs help a cell to switch from resisting death to undergoing apoptosis.

The impact of mRNA turnover and translation on age-related muscle loss

The deterioration of skeletal muscle that develops slowly with age, termed sarcopenia, often leads to disability and mortality in the elderly population. As the proportion of elderly citizens continues to increase due to the dramatic rise in life expectancy, there are rising concerns about the healthcare cost and social burden of caring for geriatric patients. Thus, there is a growing need to understand the underlying mechanisms of sarcopenic muscle loss so that more efficacious therapies may be developed. Building evidence suggests that the onset of age-related muscle loss is linked to the age-related changes in gene expression that occur during sarcopenia. In recent work, the posttranscriptional regulation of gene expression by RNA-binding proteins (RBPs) and microRNA (miRNA) involved in the turnover and translation of mRNA were shown as key players believed to be involved in the induction of muscle wasting. Furthermore, posttranscriptional regulation may also be linked to the reduced ability of muscle satellite cells to contribute to muscle mass during ageing, a key contributing factor to sarcopenic progression. Here we highlight how the activation of pathways such as the p38 MAPK and the phosphoinositide 3-kinase (PI3K) pathways alter the ability of RBPs to regulate the expression of their target mRNAs encoding proteins involved in cell cycle (p21 and p16), as well as myogenesis (Pax7, myogenin and MyoD). Further investigation into the role of RBPs and miRNA during sarcopenia may provide new insights into the development and progression of this disorder, which may lead to the development of new treatment options for elderly patients suffering from sarcopenia.

The translation inhibitor pateamine A prevents cachexia-induced muscle wasting in mice

Cachexia, or muscle-wasting syndrome, is one of the major causes of death in patients affected by diseases such as cancer, AIDS and sepsis. However, no effective anti-cachectic treatment is currently available. Here we show that a low dose of pateamine A, an inhibitor of translation initiation, prevents muscle wasting caused by the cytokines interferon γ and tumour necrosis factor α or by C26-adenocarcinoma tumours. Surprisingly, although high doses of pateamine A abrogate general translation, low doses selectively inhibit the expression of pro-cachectic factors such as inducible nitric oxide synthase. This selectivity depends on the 5'UTR of inducible nitric oxide synthase messenger RNA (mRNA) that, unlike the 5'UTR of MyoD mRNA, promotes the recruitment of inducible nitric oxide synthase mRNA to stress granules, where its translation is repressed. Collectively, our data provide a proof of principle that nontoxic doses of compounds such as pateamine A could be used as novel drugs to combat cachexia-induced muscle wasting.

Integrating post-transcriptional regulation into the embryonic stem cell gene regulatory network

Stem cell behavior is orchestrated as a multilayered, concert of gene regulatory mechanisms collectively referred to as the gene regulatory network (GRN). Via cooperative mechanisms, transcriptional, epigenetic, and post-transcriptional regulators activate and repress gene expression to finely regulate stem cell self-renewal and commitment. Due to their tractability, embryonic stem cells (ESCs) serve as the model stem cell to dissect the complexities of the GRN, and discern its relation to stem cell fate. By way of high-throughput genomic analysis, targets of individual gene regulators have been established in ESCs. The compilation of these discrete networks has revealed convergent, multi-dimensional gene regulatory mechanisms involving transcription factors, epigenetic modifiers, non-coding RNA (ncRNA), and RNA-binding proteins. Here we highlight the seminal genomic studies that have shaped our understanding of the ESC GRN and describe alternate post-transcriptional gene regulatory mechanisms that require in depth analyses to draft networks that fully model ESC behavior.

HuR function in disease

The cytoplasmic events that control mammalian gene expression, primarily mRNA stability and translation, potently influence the cellular response to internal and external signals. The ubiquitous RNA-binding protein (RBP) HuR is one of the best-studied regulators of cytoplasmic mRNA fate. Through its post-transcriptional influence on specific target mRNAs, HuR can alter the cellular response to proliferative, stress, apoptotic, differentiation, senescence, inflammatory and immune stimuli. In light of its central role in important cellular functions, HuR's role in diseases in which these responses are aberrant is increasingly appreciated. Here, we review the mechanisms that control HuR function, its influence on target mRNAs, and how impairment in HuR-governed gene expression programs impact upon different disease processes. We focus on HuR's well-recognized implication in cancer and chronic inflammation, and discuss emerging studies linking HuR to cardiovascular, neurological, and muscular pathologies. We also discuss the progress, potential, and challenges of targeting HuR therapeutically.