Analysis of the myogenin promoter reveals an indirect pathway for positive autoregulation mediated by the muscle-specific enhancer factor MEF-2

Dynamics of myogenic differentiation using a novel Myogenin knock-in reporter mouse

Background

Myogenin is a transcription factor that is expressed during terminal myoblast differentiation in embryonic development and adult muscle regeneration. Investigation of this cell state transition has been hampered by the lack of a sensitive reporter to dynamically track cells during differentiation.

Results

Here, we report a knock-in mouse line expressing the tdTOMATO fluorescent protein from the endogenous Myogenin locus. Expression of tdTOMATO in Myog^ntdTom mice recapitulated endogenous Myogenin expression during embryonic muscle formation and adult regeneration and enabled the isolation of the MYOGENIN⁺ cell population. We also show that tdTOMATO fluorescence allows tracking of differentiating myoblasts in vitro and by intravital imaging in vivo. Lastly, we monitored by live imaging the cell division dynamics of differentiating myoblasts in vitro and showed that a fraction of the MYOGENIN⁺ population can undergo one round of cell division, albeit at a much lower frequency than MYOGENIN⁻ myoblasts.

Conclusions

We expect that this reporter mouse will be a valuable resource for researchers investigating skeletal muscle biology in developmental and adult contexts.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-021-00260-x.

Mutant myogenin promoter-controlled oncolytic adenovirus selectively kills PAX3-FOXO1-positive rhabdomyosarcoma cells

The PAX3-FOXO1 fusion gene functions as a transactivator and increases expression of many cancer-related genes. These lead to metastases and other unfavorable outcomes for alveolar rhabdomyosarcoma (ARMS) patients. In order to target ARMS with the PAX3-FOXO1 transactivator, we developed an Oncolytic Adenovirus (OAd) regulated by the myogenin (pMYOG) promoter with a mutation in the Myocyte Enhancer Factor-2 binding site (mMEF2) in this study. The expression of MYOG in the two RMS cell lines (Rh30; PAX3-FOXO1-positive, RD; PAX3-FOXO1-negative) is about 1,000 times higher than normal skeletal muscle cell (SkMC). Ad5/3-pMYOG(S)-mMEF2 (short-length pMYOG-controlled OAd with mMEF2) showed strong replication and cytocidal effect in Rh30, but to a much lesser extent in RD. Ad5/3-pMYOG(S) (pMYOG-controlled OAd with native pMYOG) showed similar effects in RD and Rh30. Neither virus killed SkMC, indicating that Ad5/3-pMYOG(S)-mMEF2 selectively replicates and kills cells with PAX3-FOXO1. Additionally, Ad5/3-pMYOG(S)-mMEF2 showed replication and spread in vitro as well as tumor growth suppression and intratumoral viral spread in vivo, selectively in Rh30 not in RD. Our findings revealed that Ad5/3-pMYOG(S)-mMEF2 shows a promise as a safe and potent therapy to improve treatment in PAX3-FOXO1-positive ARMSs.

The calmodulin redox sensor controls myogenesis

Muscle aging is accompanied by blunted muscle regeneration in response to injury and disuse. Oxidative stress likely underlies this diminished response, but muscle redox sensors that act in regeneration have not yet been characterized. Calmodulin contains multiple redox sensitive methionines whose oxidation alters the regulation of numerous cellular targets. We have used the CRISPR-Cas9 system to introduce a single amino acid substitution M109Q that mimics oxidation of methionine to methionine sulfoxide in one or both alleles of the CALM1 gene, one of three genes encoding the muscle regulatory protein calmodulin, in C2C12 mouse myoblasts. When signaled to undergo myogenesis, mutated myoblasts failed to differentiate into myotubes. Although early myogenic regulatory factors were present, cells with the CALM1 M109Q mutation in one or both alleles were unable to withdraw from the cell cycle and failed to express late myogenic factors. We have shown that a single oxidative modification to a redox-sensitive muscle regulatory protein can halt myogenesis, suggesting a molecular target for mitigating the impact of oxidative stress in age-related muscle degeneration.

Calcineurin Broadly Regulates the Initiation of Skeletal Muscle-Specific Gene Expression by Binding Target Promoters and Facilitating the Interaction of the SWI/SNF Chromatin Remodeling Enzyme

Calcineurin (Cn) is a calcium-activated serine/threonine protein phosphatase that is broadly implicated in diverse cellular processes, including the regulation of gene expression. During skeletal muscle differentiation, Cn activates the nuclear factor of activated T-cell (NFAT) transcription factor but also promotes differentiation by counteracting the negative influences of protein kinase C beta (PKCβ) via dephosphorylation and activation of Brg1, an enzymatic subunit of the mammalian SWI/SNF ATP-dependent chromatin remodeling enzyme. Here we identified four major temporal patterns of Cn-dependent gene expression in differentiating myoblasts and determined that Cn is broadly required for the activation of the myogenic gene expression program. Mechanistically, Cn promotes gene expression through direct binding to myogenic promoter sequences and facilitating the binding of Brg1, other SWI/SNF subunit proteins, and MyoD, a critical lineage determinant for skeletal muscle differentiation. We conclude that the Cn phosphatase directly impacts the expression of myogenic genes by promoting ATP-dependent chromatin remodeling and formation of transcription-competent promoters.

Orai1 inhibitor STIM2β regulates myogenesis by controlling SOCE dependent transcriptional factors

Store-operated Ca²⁺ entry (SOCE), the fundamental Ca²⁺ signaling mechanism in myogenesis, is mediated by stromal interaction molecule (STIM), which senses the depletion of endoplasmic reticulum Ca²⁺ stores and induces Ca²⁺ influx by activating Orai channels in the plasma membrane. Recently, STIM2β, an eight-residue-inserted splice variant of STIM2, was found to act as an inhibitor of SOCE. Although a previous study demonstrated an increase in STIM2β splicing during in vitro differentiation of skeletal muscle, the underlying mechanism and detailed function of STIM2β in myogenesis remain unclear. In this study, we investigated the function of STIM2β in myogenesis using the C2C12 cell line with RNA interference-mediated knockdown and CRISPR-Cas-mediated knockout approaches. Deletion of STIM2β delayed myogenic differentiation through the MEF2C and NFAT4 pathway in C2C12 cells. Further, loss of STIM2β increased cell proliferation by altering Ca²⁺ homeostasis and inhibited cell cycle arrest mediated by the cyclin D1-CDK4 degradation pathway. Thus, this study identified a previously unknown function of STIM2β in myogenesis and improves the understanding of how cells effectively regulate the development process via alternative splicing.

Cytoplasmic sequestration of the RhoA effector mDiaphanous1 by Prohibitin2 promotes muscle differentiation

Muscle differentiation is controlled by adhesion and growth factor-dependent signalling through common effectors that regulate muscle-specific transcriptional programs. Here we report that mDiaphanous1, an effector of adhesion-dependent RhoA-signalling, negatively regulates myogenesis at the level of Myogenin expression. In myotubes, over-expression of mDia1ΔN3, a RhoA-independent mutant, suppresses Myogenin promoter activity and expression. We investigated mDia1-interacting proteins that may counteract mDia1 to permit Myogenin expression and timely differentiation. Using yeast two-hybrid and mass-spectrometric analysis, we report that mDia1 has a stage-specific interactome, including Prohibitin2, MyoD, Akt2, and β-Catenin, along with a number of proteosomal and mitochondrial components. Of these interacting partners, Prohibitin2 colocalises with mDia1 in cytoplasmic punctae in myotubes. We mapped the interacting domains of mDia1 and Phb2, and used interacting (mDia1ΔN3/Phb2 FL or mDia1ΔN3/Phb2-Carboxy) and non-interacting pairs (mDia1H + P/Phb2 FL or mDia1ΔN3/Phb2-Amino) to dissect the functional consequences of this partnership on Myogenin promoter activity. Co-expression of full-length as well as mDia1-interacting domains of Prohibitin2 reverse the anti-myogenic effects of mDia1ΔN3, while non-interacting regions do not. Our results suggest that Prohibitin2 sequesters mDia1, dampens its anti-myogenic activity and fine-tunes RhoA-mDia1 signalling to promote differentiation. Overall, we report that mDia1 is multi-functional signalling effector whose anti-myogenic activity is modulated by a differentiation-dependent interactome. The data have been deposited to the ProteomeXchange with identifier PXD012257.

Time-dependent Pax3-mediated chromatin remodeling and cooperation with Six4 and Tead2 specify the skeletal myogenic lineage in developing mesoderm

The transcriptional mechanisms driving lineage specification during development are still largely unknown, as the interplay of multiple transcription factors makes it difficult to dissect these molecular events. Using a cell-based differentiation platform to probe transcription function, we investigated the role of the key paraxial mesoderm and skeletal myogenic commitment factors-mesogenin 1 (Msgn1), T-box 6 (Tbx6), forkhead box C1 (Foxc1), paired box 3 (Pax3), Paraxis, mesenchyme homeobox 1 (Meox1), sine oculis-related homeobox 1 (Six1), and myogenic factor 5 (Myf5)-in paraxial mesoderm and skeletal myogenesis. From this study, we define a genetic hierarchy, with Pax3 emerging as the gatekeeper between the presomitic mesoderm and the myogenic lineage. By assaying chromatin accessibility, genomic binding and transcription profiling in mesodermal cells from mouse and human Pax3-induced embryonic stem cells and Pax3-null embryonic day (E)9.5 mouse embryos, we identified conserved Pax3 functions in the activation of the skeletal myogenic lineage through modulation of Hedgehog, Notch, and bone morphogenetic protein (BMP) signaling pathways. In addition, we demonstrate that Pax3 molecular function involves chromatin remodeling of its bound elements through an increase in chromatin accessibility and cooperation with sine oculis-related homeobox 4 (Six4) and TEA domain family member 2 (Tead2) factors. To our knowledge, these data provide the first integrated analysis of Pax3 function, demonstrating its ability to remodel chromatin in mesodermal cells from developing embryos and proving a mechanistic footing for the transcriptional hierarchy driving myogenesis.

Dual tandem promoter elements containing CCAC-like motifs from the tetrodotoxin-resistant voltage-sensitive Na+ channel (rSkM2) gene can independently drive muscle-specific transcription in L6 cells

cis-Elements in the -129/+124 promoter segment of the rat tetrodotoxin-resistant voltage-gated sodium channel (rSkM2) gene that are responsible for reporter gene expression in cultured muscle cells were identified by deletion and scanning mutations. Nested 5' deletion constructs, assayed in L6 myotubes and NIH3T3 cells, revealed that the minimum promoter allowing muscle-specific expression is contained within the -57 to +1 segment relative to the major transcription initiation site. In the context of the -129/+1 construct, however, scanning mutations in the -69/+1 segment failed to identify any critical promoter elements. In contrast, identical mutations in a minimal promoter (-57/+124) showed that all regions except -29/-20 are essential for expression, especially the -57/-40 segment, consistent with the 5' deletion analysis. Further experiments showed that the distal (-129/-58) and proximal promoter (-57/+1) elements can independently drive reporter expression in L6 myotubes, but not in NIH3T3 fibroblasts. This pair of elements is similar in sequence and contains Sp1 sites (CCGCCC), CCAC-like motifs, but no E-boxes or MEF-2 sites. The two segments form similarly migrating complexes with L6 myotube nuclear extracts in gel-shift assays. Critical elements within the distal promoter element were defined by 10 base pair scanning mutations in the -119 to -60 region in the context of the -129/+1 segment containing a mutated -59/-50 segment that inactivates the proximal promoter. Nucleotides in the -119/-90 region, especially -109/-100, were the most important regions for distal promoter function. We conclude that the -129/+1 segment contains two tandem promoter elements, each of which can independently drive muscle-specific transcription. Supershifts with antibodies to Sp1 and myocyte nuclear factor (MNF) implicate the involvement of Sp1, MNF, and other novel factors in the transcriptional regulation of rSkM2 gene expression.

Aligning Single-Cell Developmental and Reprogramming Trajectories Identifies Molecular Determinants of Myogenic Reprogramming Outcome

Cellular reprogramming through manipulation of defined factors holds great promise for large-scale production of cell types needed for use in therapy and for revealing principles of gene regulation. However, most reprogramming systems are inefficient, converting only a fraction of cells to the desired state. Here, we analyze MYOD-mediated reprogramming of human fibroblasts to myotubes, a well-characterized model system for direct conversion by defined factors, at pseudotemporal resolution using single-cell RNA-seq. To expose barriers to efficient conversion, we introduce a novel analytic technique, trajectory alignment, which enables quantitative comparison of gene expression kinetics across two biological processes. Reprogrammed cells navigate a trajectory with branch points that correspond to two alternative decision points, with cells that select incorrect branches terminating at aberrant or incomplete reprogramming outcomes. Analysis of these branch points revealed insulin and BMP signaling as crucial molecular determinants of reprogramming. Single-cell trajectory alignment enables rigorous quantitative comparisons between biological trajectories found in diverse processes in development, reprogramming, and other contexts.

Myogenic progenitor specification from pluripotent stem cells

Pluripotent stem cells represent important tools for both basic and translational science as they enable to study mechanisms of development, model diseases in vitro and provide a potential source of tissue-specific progenitors for cell therapy. Concomitantly with the increasing knowledge of the molecular mechanisms behind activation of the skeletal myogenic program during embryonic development, novel findings in the stem cell field provided the opportunity to begin recapitulating in vitro the events occurring during specification of the myogenic lineage. In this review, we will provide a perspective of the molecular mechanisms responsible for skeletal myogenic commitment in the embryo and how this knowledge was instrumental for specifying this lineage from pluripotent stem cells. In addition, we will discuss the current limitations for properly recapitulating skeletal myogenesis in the petri dish, and we will provide insights about future applications of pluripotent stem cell-derived myogenic cells.

Study on the transcriptional regulatory mechanism of the MyoD1 gene in Guanling bovine

The MyoD1 gene plays a key role in regulating the myoblast differentiation process in the early stage of skeletal muscle development. To understand the functional elements of the promoter region and transcriptional regulation of the bovine MyoD1 gene, we cloned eight fragments from the sequence region of the MyoD1 gene promoter and inserted them into eukaryotic expression vectors for cotransfection with the mouse myoblast cell line C2C12 and Madin-Darby bovine kidney (MDBK) line. A variety of transcription factor binding sites in the longest 5'-flanking fragment from Guanling cattle MyoD1-P1 were predicted by using the online software TFSEARCH and ALGGEN PROMO as well as validated by the promoter-binding TF profiling assay II and yeast one-hybrid (Y1H) assay, including MyoD, VDR, MEF1, MEF2, SF1, and Myf6. Myf6 strongly activated the MyoD1 promoter, while MyoD1 was also capable of efficiently activating the expression of its own promoter. The transcription factors MEF2A, SF1, and VDR were further confirmed to be capable of binding to MyoD1 by Y1H system experiments. The effects of the Guanling cattle MyoD1 gene on the mRNA expression of the MEF2A, SF1, and VDR genes were determined by using a lentivirus-mediated overexpression technique, confirming that overexpression of the MyoD1 gene upregulated the mRNA expression of MEF2A as well as downregulated the expression of SF1 and VDR in the process of muscle myogenesis. Our study revealed the effects of transcription factors including MEF2A, SF1 and VDR on regulatory aspects of MyoD1, providing abundant information for transcriptional regulation of MyoD1 in muscle differentiation.

Alternative Splicing of Transcription Factors Genes in Muscle Physiology and Pathology

Skeletal muscle formation is a multi-step process that is governed by complex networks of transcription factors. The regulation of their functions is in turn multifaceted, including several mechanisms, among them alternative splicing (AS) plays a primary role. On the other hand, altered AS has a role in the pathogenesis of numerous muscular pathologies. Despite these premises, the causal role played by the altered splicing pattern of transcripts encoding myogenic transcription factors in neuromuscular diseases has been neglected so far. In this review, we systematically investigate what has been described about the AS patterns of transcription factors both in the physiology of the skeletal muscle formation process and in neuromuscular diseases, in the hope that this may be useful in re-evaluating the potential role of altered splicing of transcription factors in such diseases.

Mef2 and the skeletal muscle differentiation program

Mef2 is a conserved and significant transcription factor in the control of muscle gene expression. In cell culture Mef2 synergises with MyoD-family members in the activation of gene expression and in the conversion of fibroblasts into myoblasts. Amongst its in vivo roles, Mef2 is required for both Drosophila muscle development and mammalian muscle regeneration. Mef2 has functions in other cell-types too, but this review focuses on skeletal muscle and surveys key findings on Mef2 from its discovery, shortly after that of MyoD, up to the present day. In particular, in vivo functions, underpinning mechanisms and areas of uncertainty are highlighted. We describe how Mef2 sits at a nexus in the gene expression network that controls the muscle differentiation program, and how Mef2 activity must be regulated in time and space to orchestrate specific outputs within the different aspects of muscle development. A theme that emerges is that there is much to be learnt about the different Mef2 proteins (from different paralogous genes, spliced transcripts and species) and how the activity of these proteins is controlled.

MyoD- and FoxO3-mediated hotspot interaction orchestrates super-enhancer activity during myogenic differentiation

Super-enhancers (SEs) are cis-regulatory elements enriching lineage specific key transcription factors (TFs) to form hotspots. A paucity of identification and functional dissection promoted us to investigate SEs during myoblast differentiation. ChIP-seq analysis of histone marks leads to the uncovering of SEs which remodel progressively during the course of differentiation. Further analyses of TF ChIP-seq enable the definition of SE hotspots co-bound by the master TF, MyoD and other TFs, among which we perform in-depth dissection for MyoD/FoxO3 interaction in driving the hotspots formation and SE activation. Furthermore, using Myogenin as a model locus, we elucidate the hierarchical and complex interactions among hotspots during the differentiation, demonstrating SE function is propelled by the physical and functional cooperation among hotspots. Finally, we show MyoD and FoxO3 are key in orchestrating the Myogenin hotspots interaction and activation. Altogether our results identify muscle-specific SEs and provide mechanistic insights into the functionality of SE.

Decreased myoblast differentiation in chronic binge alcohol-administered simian immunodeficiency virus-infected male macaques: role of decreased miR-206

Skeletal muscle stem cells play a critical role in regeneration of myofibers. We previously demonstrated that chronic binge alcohol (CBA) markedly attenuates myoblast differentiation potential and myogenic gene expression. Muscle-specific microRNAs (miRs) are implicated in regulation of myogenic genes. The aim of this study was to determine whether myoblasts isolated from asymptomatic CBA-administered simian immunodeficiency virus (SIV)-infected macaques treated with antiretroviral therapy (ART) showed similar impairments and, if so, to elucidate potential underlying mechanisms. Myoblasts were isolated from muscle at 11 mo after SIV infection from CBA/SIV macaques and from time-matched sucrose (SUC)-treated SIV-infected (SUC/SIV) animals and age-matched controls. Myoblast differentiation and myogenic gene expression were significantly decreased in myoblasts from SUC/SIV and CBA/SIV animals compared with controls. SIV and CBA decreased muscle-specific miR-206 in plasma and muscle and SIV decreased miR-206 expression in myoblasts, with no statistically significant changes in other muscle-specific miRs. These findings were associated with a significant increase in histone deacetylase 4 (HDAC4) and decrease in myogenic enhancer factor 2C (MEF2C) expression in CBA/SIV muscle. Transfection with miR-206 inhibitor decreased myotube differentiation, increased expression of HDAC4, and decreased MEF2C, suggesting a critical role of miR-206 in myogenesis. Moreover, HDAC4 was confirmed to be a direct miR-206 target. These results support a mechanistic role for decreased miR-206 in suppression of myoblast differentiation resulting from chronic alcohol and SIV infection. The parallel changes in skeletal muscle and circulating levels of miR-206 warrant studies to establish the possible use of plasma miR-206 as an indicator of impaired muscle function.

Developmental Stage, Muscle and Genetic Type Modify Muscle Transcriptome in Pigs: Effects on Gene Expression and Regulatory Factors Involved in Growth and Metabolism

Iberian pig production includes purebred (IB) and Duroc-crossbred (IBxDU) pigs, which show important differences in growth, fattening and tissue composition. This experiment was conducted to investigate the effects of genetic type and muscle (Longissimus dorsi (LD) vs Biceps femoris (BF)) on gene expression and transcriptional regulation at two developmental stages. Nine IB and 10 IBxDU piglets were slaughtered at birth, and seven IB and 10 IBxDU at four months of age (growing period). Carcass traits and LD intramuscular fat (IMF) content were measured. Muscle transcriptome was analyzed on LD samples with RNA-Seq technology. Carcasses were smaller in IB than in IBxDU neonates (p < 0.001), while growing IB pigs showed greater IMF content (p < 0.05). Gene expression was affected (p < 0.01 and Fold change > 1.5) by the developmental stage (5,812 genes), muscle type (135 genes), and genetic type (261 genes at birth and 113 at growth). Newborns transcriptome reflected a highly proliferative developmental stage, while older pigs showed upregulation of catabolic and muscle functioning processes. Regarding the genetic type effect, IBxDU newborns showed enrichment of gene pathways involved in muscle growth, in agreement with the higher prenatal growth observed in these pigs. However, IB growing pigs showed enrichment of pathways involved in protein deposition and cellular growth, supporting the compensatory gain experienced by IB pigs during this period. Moreover, newborn and growing IB pigs showed more active glucose and lipid metabolism than IBxDU pigs. Moreover, LD muscle seems to have more active muscular and cell growth, while BF points towards lipid metabolism and fat deposition. Several regulators controlling transcriptome changes in both genotypes were identified across muscles and ages (SIM1, PVALB, MEFs, TCF7L2 or FOXO1), being strong candidate genes to drive expression and thus, phenotypic differences between IB and IBxDU pigs. Many of the identified regulators were known to be involved in muscle and adipose tissues development, but others not previously associated with pig muscle growth were also identified, as PVALB, KLF1 or IRF2. The present study discloses potential molecular mechanisms underlying phenotypic differences observed between IB and IBxDU pigs and highlights candidate genes implicated in these molecular mechanisms.

Time course and side-by-side analysis of mesodermal, pre-myogenic, myogenic and differentiated cell markers in the chicken model for skeletal muscle formation

The chicken is a well-established model for amniote (including human) skeletal muscle formation because the developmental anatomy of chicken skeletal muscle matches that of mammals. The accessibility of the chicken in the egg as well as the sequencing of its genome and novel molecular techniques have raised the profile of this model. Over the years, a number of regulatory and marker genes have been identified that are suited to monitor the progress of skeletal myogenesis both in wildtype and in experimental embryos. However, in the various studies, differing markers at different stages of development have been used. Moreover, contradictory results on the hierarchy of regulatory factors are now emerging, and clearly, factors need to be able to cooperate. Thus, a reference paper describing in detail and side-by-side the time course of marker gene expression during avian myogenesis is needed. We comparatively analysed onset and expression patterns of the key markers for the chicken immature paraxial mesoderm, for muscle-competent cells, for cells committed to myogenesis and for cells entering terminal differentiation. We performed this analysis from stages when the first paraxial mesoderm is being laid down to the stage when mesoderm formation comes to a conclusion. Our data show that, although the sequence of marker gene expression is the same at the various stages of development, the timing of the expression onset is quite different. Moreover, marker gene expression in myogenic cells being deployed from the dorsomedial and ventrolateral lips of the dermomyotome is different from those being deployed from the rostrocaudal lips, suggesting different molecular programs. Furthermore, expression of Myosin Heavy Chain genes is overlapping but different along the length of a myotube. Finally, Mef2c is the most likely partner of Mrf proteins, and, in contrast to the mouse and more alike frog and zebrafish fish, chicken Mrf4 is co-expressed with MyoG as cells enter terminal differentiation.

Nfix Regulates Temporal Progression of Muscle Regeneration through Modulation of Myostatin Expression

Nfix belongs to a family of four highly conserved proteins that act as transcriptional activators and/or repressors of cellular and viral genes. We previously showed a pivotal role for Nfix in regulating the transcriptional switch from embryonic to fetal myogenesis. Here, we show that Nfix directly represses the Myostatin promoter, thus controlling the proper timing of satellite cell differentiation and muscle regeneration. Nfix-null mice display delayed regeneration after injury, and this deficit is reversed upon in vivo Myostatin silencing. Conditional deletion of Nfix in satellite cells results in a similar delay in regeneration, confirming the functional requirement for Nfix in satellite cells. Moreover, mice lacking Nfix show reduced myofiber cross sectional area and a predominant slow twitching phenotype. These data define a role for Nfix in postnatal skeletal muscle and unveil a mechanism for Myostatin regulation, thus providing insights into the modulation of its complex signaling pathway.

Comparative Analysis of Muscle Transcriptome between Pig Genotypes Identifies Genes and Regulatory Mechanisms Associated to Growth, Fatness and Metabolism

Iberian ham production includes both purebred (IB) and Duroc-crossbred (IBxDU) Iberian pigs, which show important differences in meat quality and production traits, such as muscle growth and fatness. This experiment was conducted to investigate gene expression differences, transcriptional regulation and genetic polymorphisms that could be associated with the observed phenotypic differences between IB and IBxDU pigs. Nine IB and 10 IBxDU pigs were slaughtered at birth. Morphometric measures and blood samples were obtained and samples from Biceps femoris muscle were employed for compositional and transcriptome analysis by RNA-Seq technology. Phenotypic differences were evident at this early age, including greater body size and weight in IBxDU and greater Biceps femoris intramuscular fat and plasma cholesterol content in IB newborns. We detected 149 differentially expressed genes between IB and IBxDU neonates (p < 0.01 and Fold-Change > 1. 5). Several were related to adipose and muscle tissues development (DLK1, FGF21 or UBC). The functional interpretation of the transcriptomic differences revealed enrichment of functions and pathways related to lipid metabolism in IB and to cellular and muscle growth in IBxDU pigs. Protein catabolism, cholesterol biosynthesis and immune system were functions enriched in both genotypes. We identified transcription factors potentially affecting the observed gene expression differences. Some of them have known functions on adipogenesis (CEBPA, EGRs), lipid metabolism (PPARGC1B) and myogenesis (FOXOs, MEF2D, MYOD1), which suggest a key role in the meat quality differences existing between IB and IBxDU hams. We also identified several polymorphisms showing differential segregation between IB and IBxDU pigs. Among them, non-synonymous variants were detected in several transcription factors as PPARGC1B and TRIM63 genes, which could be associated to altered gene function. Taken together, these results provide information about candidate genes, metabolic pathways and genetic polymorphisms potentially involved in phenotypic differences between IB and IBxDU pigs associated to meat quality and production traits.

AKAP6 inhibition impairs myoblast differentiation and muscle regeneration: Positive loop between AKAP6 and myogenin

Skeletal muscle regeneration occurs continuously to repair muscle damage incurred during normal activity and in chronic disease or injury. Herein, we report that A-kinase anchoring protein 6 (AKAP6) is important for skeletal myoblast differentiation and muscle regeneration. Compared with unstimulated skeletal myoblasts that underwent proliferation, differentiated cells show significant stimulation of AKAP6 expression. AKAP6 knockdown with siRNA effectively halts the formation of myotubes and decreases the expression of the differentiation markers myogenin and myosin heavy chain. When shAKAP6-lentivirus is delivered to mice with cardiotoxin (CTX)-induced muscle injury, muscle regeneration is impaired compared with that of mice injected with control shMock-lentivirus. The motor functions of mice infected with shAKAP6-lentivirus (CTX+shAK6) are significantly worse than those of mice infected with shMock-lentivirus (CTX+shMock). Mechanistic analysis showed that AKAP6 promotes myogenin expression through myocyte enhancer factor 2A (MEF2A). Notably, myogenin increases AKAP6 expression as well. The results of chromatin immunoprecipitation and luciferase assays showed that myogenin binds to an E-box site on the AKAP6 promoter. Taken together, our findings demonstrate a novel interplay between AKAP6 and myogenin, and we suggest that AKAP6 is an important regulator of myoblast differentiation, myotube formation, and muscle regeneration.

MEF2D drives photoreceptor development through a genome-wide competition for tissue-specific enhancers

Organismal development requires the precise coordination of genetic programs to regulate cell fate and function. MEF2 transcription factors (TFs) play essential roles in this process but how these broadly expressed factors contribute to the generation of specific cell types during development is poorly understood. Here we show that despite being expressed in virtually all mammalian tissues, in the retina MEF2D binds to retina-specific enhancers and controls photoreceptor cell development. MEF2D achieves specificity by cooperating with a retina-specific factor CRX, which recruits MEF2D away from canonical MEF2 binding sites and redirects it to retina-specific enhancers that lack the consensus MEF2-binding sequence. Once bound to retina-specific enhancers, MEF2D and CRX co-activate the expression of photoreceptor-specific genes that are critical for retinal function. These findings demonstrate that broadly expressed TFs acquire specific functions through competitive recruitment to enhancers by tissue-specific TFs and through selective activation of these enhancers to regulate tissue-specific genes.

Skeletal muscle tissue engineering: methods to form skeletal myotubes and their applications

Skeletal muscle tissue engineering (SMTE) aims to repair or regenerate defective skeletal muscle tissue lost by traumatic injury, tumor ablation, or muscular disease. However, two decades after the introduction of SMTE, the engineering of functional skeletal muscle in the laboratory still remains a great challenge, and numerous techniques for growing functional muscle tissues are constantly being developed. This article reviews the recent findings regarding the methodology and various technical aspects of SMTE, including cell alignment and differentiation. We describe the structure and organization of muscle and discuss the methods for myoblast alignment cultured in vitro. To better understand muscle formation and to enhance the engineering of skeletal muscle, we also address the molecular basics of myogenesis and discuss different methods to induce myoblast differentiation into myotubes. We then provide an overview of different coculture systems involving skeletal muscle cells, and highlight major applications of engineered skeletal muscle tissues. Finally, potential challenges and future research directions for SMTE are outlined.

NFATc1 controls skeletal muscle fiber type and is a negative regulator of MyoD activity

Skeletal muscle comprises a heterogeneous population of fibers with important physiological differences. Fast fibers are glycolytic and fatigue rapidly. Slow fibers utilize oxidative metabolism and are fatigue resistant. Muscle diseases such as sarcopenia and atrophy selectively affect fast fibers, but the molecular mechanisms regulating fiber type-specific gene expression remain incompletely understood. Here, we show that the transcription factor NFATc1 controls fiber type composition and is required for fast-to-slow fiber type switching in response to exercise in vivo. Moreover, MyoD is a crucial transcriptional effector of the fast fiber phenotype, and we show that NFATc1 inhibits MyoD-dependent fast fiber gene promoters by physically interacting with the N-terminal activation domain of MyoD and blocking recruitment of the essential transcriptional coactivator p300. These studies establish a molecular mechanism for fiber type switching through direct inhibition of MyoD to control the opposing roles of MyoD and NFATc1 in fast versus slow fiber phenotypes.

The Cavβ1a subunit regulates gene expression and suppresses myogenin in muscle progenitor cells

Ca_vβ_1a acts as a voltage-gated calcium channel-independent regulator of gene expression in muscle progenitor cells and is required for their normal expansion during myogenic development.

Voltage-gated calcium channel (Ca_v) β subunits are auxiliary subunits to Ca_vs. Recent reports show Ca_vβ subunits may enter the nucleus and suggest a role in transcriptional regulation, but the physiological relevance of this localization remains unclear. We sought to define the nuclear function of Ca_vβ in muscle progenitor cells (MPCs). We found that Ca_vβ_1a is expressed in proliferating MPCs, before expression of the calcium conducting subunit Ca_v1.1, and enters the nucleus. Loss of Ca_vβ_1a expression impaired MPC expansion in vitro and in vivo and caused widespread changes in global gene expression, including up-regulation of myogenin. Additionally, we found that Ca_vβ_1a localizes to the promoter region of a number of genes, preferentially at noncanonical (NC) E-box sites. Ca_vβ_1a binds to a region of the Myog promoter containing an NC E-box, suggesting a mechanism for inhibition of myogenin gene expression. This work indicates that Ca_vβ_1a acts as a Ca_v-independent regulator of gene expression in MPCs, and is required for their normal expansion during myogenic development.

Expression characterization and promoter activity analysis of the tilapia (Oreochromis niloticus) myosin light chain 3 promoter in skeletal muscle of fish

A tilapia (Oreochromis niloticus) myosin light chain 3 (Mlc3) promoter region (~4.3 kb) was isolated and characterized. Sequence analysis of the clone revealed high similarity with a tilapia gene encoding the Mlc3 promoter region, exon 1, and intron 1. The clone contained several putative binding sequences for transcription factors, including MEF-2, MYOG, MyoD, PKNOX1, and AREB6. Deletion of a region of the tilapia Mlc3 promoter (801 to -3,881 bp) enhanced promoter activity, as determined by direct injection of a luciferase reporter construct into skeletal muscle of Archocentrus nigrofasciatus. These findings suggest that the region between -801 and -3,881 bp may contain negative regulatory elements. Stable germline transgenic strains of the ornamental fish species A. nigrofasciatus var. carrying the Taiwan coral red fluorescent protein (TcRFP) driven by the Mlc3 promoter were established. F1 adult transgenic A. nigrofasciatus var. exhibited brilliant pink fluorescence in skeletal muscles in the daylight. Therefore, our current study demonstrates the feasibility of using the tilapia Mlc3 promoter to drive fluorescence in new fish species, such as Perciformes.

GATA factor switching from GATA2 to GATA1 contributes to erythroid differentiation

Transcription factor GATA2 is highly expressed in hematopoietic stem cells and progenitors, whereas its expression declines after erythroid commitment of progenitors. In contrast, the start of GATA1 expression coincides with the erythroid commitment and increases along with the erythroid differentiation. We refer this dynamic transition of GATA factor expression to as the 'GATA factor switching'. Here, we examined contribution of the GATA factor switching to the erythroid differentiation. In Gata1-knockdown embryos that concomitantly express Gata2-GFP reporter, high-level expression of GFP reporter was detected in accumulated immature hematopoietic cells with impaired differentiation, demonstrating that GATA1 represses Gata2 gene expression in hematopoietic progenitors in vivo. We have conducted chromatin immunoprecipitation (ChIP) on microarray analyses of GATA2 and GATA1, and results indicate that the GATA1-binding sites widely overlap with the sites pre-occupied by GATA2 before the GATA1 expression. Importantly, erythroid genes harboring GATA boxes bound by both GATA1 and GATA2 tend to be expressed in immature erythroid cells, whereas those harboring GATA boxes to which GATA1 binds highly but GATA2 binds only weakly are important for the mature erythroid cell function. Our results thus support the contention that preceding binding of GATA2 helps the following binding of GATA1 and thereby secures smooth expression of the transient-phase genes.

Distinct transcriptional networks in quiescent myoblasts: a role for Wnt signaling in reversible vs. irreversible arrest

Most cells in adult mammals are non-dividing: differentiated cells exit the cell cycle permanently, but stem cells exist in a state of reversible arrest called quiescence. In damaged skeletal muscle, quiescent satellite stem cells re-enter the cell cycle, proliferate and subsequently execute divergent programs to regenerate both post-mitotic myofibers and quiescent stem cells. The molecular basis for these alternative programs of arrest is poorly understood. In this study, we used an established myogenic culture model (C2C12 myoblasts) to generate cells in alternative states of arrest and investigate their global transcriptional profiles. Using cDNA microarrays, we compared G₀ myoblasts with post-mitotic myotubes. Our findings define the transcriptional program of quiescent myoblasts in culture and establish that distinct gene expression profiles, especially of tumour suppressor genes and inhibitors of differentiation characterize reversible arrest, distinguishing this state from irreversibly arrested myotubes. We also reveal the existence of a tissue-specific quiescence program by comparing G₀ C2C12 myoblasts to isogenic G₀ fibroblasts (10T1/2). Intriguingly, in myoblasts but not fibroblasts, quiescence is associated with a signature of Wnt pathway genes. We provide evidence that different levels of signaling via the canonical Wnt pathway characterize distinct cellular states (proliferation vs. quiescence vs. differentiation). Moderate induction of Wnt signaling in quiescence is associated with critical properties such as clonogenic self-renewal. Exogenous Wnt treatment subverts the quiescence program and negatively affects clonogenicity. Finally, we identify two new quiescence-induced regulators of canonical Wnt signaling, Rgs2 and Dkk3, whose induction in G₀ is required for clonogenic self-renewal. These results support the concept that active signal-mediated regulation of quiescence contributes to stem cell properties, and have implications for pathological states such as cancer and degenerative disease.

TFE3 inhibits myoblast differentiation in C2C12 cells via down-regulating gene expression of myogenin

Transcription factor E3 (TFE3) belongs to a basic helix-loop-helix family, and is involved in the biology of osteoclasts, melanocytes and their malignancies. We previously reported the metabolic effects of TFE3 on insulin in the liver and skeletal muscles in animal models. In the present study, we explored a novel role for TFE3 in a skeletal muscle cell line. When TFE3 was overexpressed in C2C12 myoblasts by adenovirus before induction of differentiation, myogenic differentiation of C2C12 cells was significantly inhibited. Adenovirus-mediated TFE3 overexpression also suppressed the gene expression of muscle regulatory factors (MRFs), such as MyoD and myogenin, during C2C12 differentiation. In contrast, knockdown of TFE3 using adenovirus encoding short-hairpin RNAi specific for TFE3 dramatically promoted myoblast differentiation associated with significantly increased expression of MRFs. Consistent with these findings, promoter analyses via luciferase reporter assay and electrophoretic mobility shift assay suggested that TFE3 negatively regulated myogenin promoter activity by direct binding to an E-box, E2, in the myogenin promoter. These findings indicated that TFE3 has a regulatory role in myoblast differentiation, and that transcriptional suppression of myogenin expression may be part of the mechanism of action.

αNAC interacts with histone deacetylase corepressors to control Myogenin and Osteocalcin gene expression

In the nucleus of differentiated osteoblasts, the DNA-binding αNAC protein acts as a transcriptional coactivator of the Osteocalcin gene. Chromatin immunoprecipitation-microarray assays (ChIP-chip) showed that αNAC binds the Osteocalcin promoter but also identified the Myogenin promoter as an αNAC target. Here, we confirm these array data using quantitative ChIP and further detected that αNAC binds to these promoters in myoblasts. Since these genes are differentially regulated during osteoblastogenesis or myogenesis, these results suggest cell- and promoter-context specific functions for αNAC. We hypothesized that αNAC dynamically recruits corepressors to inhibit Myogenin expression in cells committing to the osteoblastic lineage or to inhibit Osteocalcin transcription in differentiating myoblasts. Using co-immunoprecipitation assays, we detected complexes between αNAC and the corepressors HDAC1 and HDAC3, in myoblasts and osteoblasts. Sequential ChIP confirmed HDAC1 recruitment by αNAC at the Osteocalcin and Myogenin promoters. Interaction with the corepressors was detectable in pre-osteoblasts and in myoblasts but disappeared as the cells differentiate. Treatment with an HDAC inhibitor caused de-repression of Osteocalcin expression in myoblasts. Overexpression of αNAC in myoblasts inhibits expression of Myogenin and differentiation. However, overexpression of an N-terminus truncated αNAC mutant allowed myoblasts to express Myogenin and differentiate, and this mutant did not interact with HDAC1 or HDAC3. This study identified an additional DNA-binding target and novel protein-protein interactions for αNAC. We propose that αNAC plays a role in regulating gene transcription during mesenchymal cell differentiation by differentially recruiting corepressors at target promoters.

Effects of myogenin on expression of late muscle genes through MyoD-dependent chromatin remodeling ability of myogenin

MyoD and myogenin (Myog) recognize sets of distinct but overlapping target genes and play different roles in skeletal muscle differentiation. MyoD is sufficient for near-full expression of early targets, while Myog can only partially enhance expression of MyoD-initiated late muscle genes. However, the way in which Myog enhances the expression of MyoD-initiated late muscle genes remains unclear. Here, we examine the effects of Myog on chromatin remodeling at late muscle gene promoters and their activation within chromatin environment. Chromatin immunoprecipitation (ChIP) assay showed that Myog selectively bound to the regulatory sequences of late muscle genes. Overexpression of Myog was found to overcome sodium butyrateinhibited chromatin at late muscle genes in differentiating C2C12 myoblasts, shifting the transcriptional activation of these genes to an earlier time period. Furthermore, overexpression of Myog led to increased hyperacetylation of core histone H4 in differentiating C2C12 myoblasts but not NIH3T3 fibroblasts, and hyperacetylated H4 was associated directly with the late muscle genes in differentiating C2C12, indicating that Myog can induce chromatin remodeling in the presence of MyoD. In addition, co-immunoprecipitation (CoIP) revealed that Myog was associated with the nuclear protein Brd4 in differentiating C2C12 myoblasts. Together, these results suggest that Myog enhances the expression of MyoD-initiated late muscle genes through MyoD-dependent ability of Myog to induce chromatin remodeling, in which Myog-Brd4 interaction may be involved.

Turning on myogenin in muscle: a paradigm for understanding mechanisms of tissue-specific gene expression

Expression of the myogenin (Myog) gene is restricted to skeletal muscle cells where the transcriptional activator turns on a gene expression program that permits the transition from proliferating myoblasts to differentiating myotubes. The strict temporal and spatial regulation on Myog expression in the embryo makes it an ideal gene to study the developmental regulation of tissue-specific expression. Over the last 20 years, our knowledge of the regulation of Myog expression has evolved from the identification of the minimal promoter elements necessary for the gene to be transcribed in muscle, to a mechanistic understanding of how the proteins that bind these DNA elements work together to establish transcriptional competence. Here we present our current understanding of the developmental regulation of gene expression gained from studies of the Myog gene.

Myostatin inhibits proliferation but not differentiation of trout myoblasts

The muscle growth in mammals is regulated by several growth factors including myostatin (MSTN), a member of the transforming growth factor-beta (TGF-beta) superfamily. To date, it is unknown in fish whether MSTN could have any effect on proliferation or differentiation of myogenic cells. Using culture of trout satellite cells, we showed that mstn1a and mstn1b mRNA are expressed in myoblasts and that their expression decreased in differentiating myoblasts. We also demonstrated that a treatment with huMSTN decreased the proliferation of IGF1-stimulated myoblasts in a dose-dependent manner. By contrast, treatment of myoblasts with 100 nM of huMSTN for three days, did not affect the percentage of positive cells for myogenin neither the percentage of nuclei in myosin positive cells. Moreover, our results clearly indicated that huMSTN treatment had no effect on MyoD and myogenin protein levels, which suggests that huMSTN did not strongly affect MyoD activity. In conclusion, we showed that huMSTN inhibited proliferation but not differentiation of trout myoblasts, probably resulting from a lack of huMSTN effect on MyoD activity. Altogether, these results show high interspecies differences in the function of MSTN.

Role of myocyte enhancing factor 2B in epithelial myofibroblast transition of human gingival keratinocytes

It has recently emerged that the myogenic contribution of the epithelial mesenchymal transition plays a role in neoplastic invasion and metastasis. Myocyte enhancing factor 2B (MEF2B) is the only MEF2 isoform expressed during early embryonic development, and is herein proposed to transactivate the downstream target proteins of the epithelial myofibroblast transition (EMyT). We have previously generated eight preneoplastic cell lines with spindle and cobblestone morphology from human gingival mucosal keratinocytes immortalized by E6/E7 of human papillomavirus type 16. Spindle cells formed tubulogenic morphogenesis on Matrigel and exhibited contractility, anchorage-independent growth and invasiveness to a greater extent than cobblestone cells. Expression of MEF2B mRNA and myofibroblast proteins was higher in spindle cells compared with cobblestone cells. Epidermal growth factor (EGF) treatment of cobblestone cells also induced expression of these genes. Knockdown of MEF2B in a cobblestone cell line abolished EGF-induced upregulation of MEF2, vimentin and non-muscle caldesmon proteins, but enhanced basal expression of mesenchymal vimentin and fibronectin. Differential regulation of intermediate filaments revealed an unrecognized role of MEF2B in myogenic transformation of the epithelial to a myofibroblast phenotype, which occurs as epithelioid variants in some soft tissue sarcomas.

Role of JAK3 in myogenic differentiation

Skeletal muscle differentiation is regulated by transcription factors, including members of the myogenic regulatory factor (MRF) family and many signaling pathways. The JAK1 and JAK2 pathways are known to each have different effects on myoblast proliferation and differentiation; however, the role of JAK3 in myoblast differentiation remains unclear. In this study, we investigated the effect of JAK3 inhibition on myogenic differentiation in the C2C12 mouse myoblast cell line. During myogenic differentiation, treatment with the JAK3 inhibitor WHIp154 significantly increased the number of MHC-positive multinucleated myotubes and the expressions of myosin heavy chain (MHC), myogenin (MGN), MyoD, and myogenic enhancer factor 2 (MEF2). Knockdown of the JAK3 gene using siJAK3 also significantly increased MHC, MGN and MyoD mRNA expressions as well as insulin-like growth factor-II (IGF-II) gene expression. During differentiation, JAK3 was initially activated and later decreased. Differentiation decreased STAT1, which was further decreased by WHIp154. In contrast, STAT3 gradually was elevated during differentiation, and was increased by JAK3 inhibition. Moreover, we found that up-regulation of AKT activity and down-regulation of ERK activity cooperated to accelerate myogenic differentiation. Taken together, these data indicate that JAK3 inhibition potently facilitates myoblast differentiation through antagonistic STAT1/STAT3 activities. Additionally, JAK3 inhibition induced precocious differentiation and played important roles for terminal differentiation, including fusion, which is involved with regulation of AKT and ERK pathways.

Signal-dependent incorporation of MyoD-BAF60c into Brg1-based SWI/SNF chromatin-remodelling complex

Tissue-specific transcriptional activators initiate differentiation towards specialized cell types by inducing chromatin modifications permissive for transcription at target loci, through the recruitment of SWItch/Sucrose NonFermentable (SWI/SNF) chromatin-remodelling complex. However, the molecular mechanism that regulates SWI/SNF nuclear distribution in response to differentiation signals is unknown. We show that the muscle determination factor MyoD and the SWI/SNF subunit BAF60c interact on the regulatory elements of MyoD-target genes in myoblasts, prior to activation of transcription. BAF60c facilitates MyoD binding to target genes and marks the chromatin for signal-dependent recruitment of the SWI/SNF core to muscle genes. BAF60c phosphorylation on a conserved threonine by differentiation-activated p38α kinase is the signal that promotes incorporation of MyoD-BAF60c into a Brg1-based SWI/SNF complex, which remodels the chromatin and activates transcription of MyoD-target genes. Our data support an unprecedented two-step model by which pre-assembled BAF60c-MyoD complex directs recruitment of SWI/SNF to muscle loci in response to differentiation cues.

Transcriptional activity regulates alternative cleavage and polyadenylation

Transcriptomic and epigenomic data, as well as reporter and nuclear run-on assays collectively show that transcriptional activity regulates the relative abundance of alternative polyadenylation isoforms, indicating general coupling of 3′ end processing to transcription.

Using RNA-seq and exon array data for a large number of human and mouse tissues and cells, we identified a general correlation between relative expression of alternative polyadenylation (APA) isoforms and gene expression level: short 3′UTR isoforms are relatively more abundant when genes are highly expressed whereas long 3′UTR isoforms are relatively more abundant when genes are lowly expressed.

Using reporter assays with different promoters, we found that induction of transcription leads to more usage of promoter-proximal polyA sites, suggesting modulation of 3′ end processing efficiency by transcriptional activity. Global analysis and reporter-based assays further revealed that regulation of polyA site choice by transcription takes place when genes are regulated under different cell conditions.

Using global and reporter-based nuclear run-on assays, we found that highly expressed genes tend to have more RNA polymerase II pausing at promoter-proximal polyA sites, as compared with lowly expressed genes, supporting the notion that the efficiency of 3′ end processing is coupled to transcriptional activity.

Highly expressed genes have a lower nucleosome level but higher H3K4me3 and H3K36me3 levels at promoter-proximal polyA sites relative to distal ones, as compared with lowly expressed genes, indicating that transcriptional activity impacts 3′ end processing and regulation of APA leaves epigenetic signatures.

Genes containing multiple pre-mRNA cleavage and polyadenylation sites, or polyA sites, express mRNA isoforms with variable 3′ untranslated regions (UTRs). By systematic analysis of human and mouse transcriptomes, we found that short 3′UTR isoforms are relatively more abundant when genes are highly expressed whereas long 3′UTR isoforms are relatively more abundant when genes are lowly expressed. Reporter assays indicated that polyA site choice can be modulated by transcriptional activity through the gene promoter. Using global and reporter-based nuclear run-on assays, we found that RNA polymerase II is more likely to pause at the polyA site of highly expressed genes than that of lowly expressed ones. Moreover, highly expressed genes tend to have a lower level of nucleosome but higher H3K4me3 and H3K36me3 levels at promoter-proximal polyA sites relative to distal ones. Taken together, our results indicate that polyA site usage is generally coupled to transcriptional activity, leading to regulation of alternative polyadenylation by transcription.

The methyl-CpG-binding protein CIBZ suppresses myogenic differentiation by directly inhibiting myogenin expression

Postnatal growth and regeneration of skeletal muscle are carried out mainly by satellite cells, which, upon stimulation, begin to express myogenin (Myog), the critical determinant of myogenic differentiation. DNA methylation status has been associated with the expression of Myog, but the causative mechanism remains almost unknown. Here, we report that the level of CIBZ, a methyl-CpG-binding protein, decreases upon myogenic differentiation of satellite-derived C2C12 cells, and during skeletal muscle regeneration in mice. We present data showing that the loss of CIBZ promotes myogenic differentiation, whereas exogenous expression of CIBZ impairs it, in cultured cells. CIBZ binds to a Myog promoter-proximal region and inhibits Myog transcription in a methylation-dependent manner. These data suggest that the suppression of myogenic differentiation by CIBZ is dependent, at least in part, on the regulation of Myog. Our data show that the methylation status of this proximal Myog promoter inversely correlates with Myog transcription in cells and tissues, and during postnatal growth of skeletal muscle. Notably, induction of Myog transcription by CIBZ suppression is independent of the demethylation of CpG sites in the Myog promoter. These observations provide the first reported molecular mechanism illustrating how Myog transcription is coordinately regulated by a methyl-CpG-binding protein and the methylation status of the proximal Myog promoter.

Recombination activation gene-2-deficient blastocyst complementation analysis reveals an essential role for nuclear factor I-A transcription factor in T-cell activation

Nuclear factor I (NFI)-A is a member of the NFI family of transcription factors implicated in regulation of granulocyte differentiation. However, its role in the lymphoid lineage is not known. NFI-A deficiency results in perinatal lethality, thus precluding analysis of the role of NFI-A in lymphocyte development and function. Using recombination activation gene-2-deficient (RAG-2(-/-)) blastocysts and embryonic stem cells with homozygous NFI-A gene deletion, we show an essential role for NFI-A in T-cell activation. NFI-A(-/-)→RAG-2(-/-) chimeric mice had normal distributions of CD4(-)CD8(-) double negative, CD4(+)CD8(+) double positive, CD4(+)CD8(-) and CD4(-)CD8(+)-single positive cells in the thymus and CD4(+)CD8(-) and CD4(-)CD8(+) cells in spleen and lymph nodes. However, NFI-A(-/-)→RAG-2(-)(/)(-) mice had severely reduced thymus size and hypocellularity. The decrease in thymocytes and peripheral T cells in NFI-A(-/-)→RAG-2(-/-) chimeric mice is attributed to proliferative defects associated with decreased blast transformation, CD69 expression and DNA synthesis in response to T antigen receptor stimulation. Interestingly, NFI-A-null T cells showed increased levels of c-myc transcription that is inhibited in response to antigen receptor-mediated activation. These studies demonstrate for the first time a requirement for the NFI-A transcription factor in antigen receptor-induced T-cell activation events.

Lineage-based primary muscle fiber type diversification independent of MEF2 and NFAT in chick embryos

Differences in primary avian skeletal muscle fiber types are based on myoblast cell lineages and independent of innervation. To understand the basis for this mode of myogenesis, embryonic myoblasts specifically committed to the formation of either fast or fast/slow muscle fiber types were isolated, characterized, and examined for their capacities to transcriptionally regulate the slow myosin heavy chain 2 (MyHC2) gene. Myogenic basic helix-loop-helix protein binding sites within the slow MyHC2 promoter were mutated and did not direct fast versus fast/slow muscle fiber type development. Using promoter analyses coupled with overexpression studies and transcriptional sensors, the roles of Nuclear Factor of Activated T cells (NFATc1), and MEF2A in regulation of the slow MyHC2 gene were determined. MEF2A activated the slow MyHC2 promoter in both fast and fast/slow primary muscle fibers. In contrast, NFATc1 repressed promoter activity. These results do not support the roles of MEF2 and NFAT as direct regulators of primary muscle fiber type differences. Rather, the results reflect intrinsic differences in the modes of regulation of the slow MyHC2 gene in primary muscle fiber types.

Sodium arsenite delays the differentiation of C2C12 mouse myoblast cells and alters methylation patterns on the transcription factor myogenin

Epidemiological studies have correlated arsenic exposure with cancer, skin diseases, and adverse developmental outcomes such as spontaneous abortions, neonatal mortality, low birth weight, and delays in the use of musculature. The current study used C2C12 mouse myoblast cells to examine whether low concentrations of arsenic could alter their differentiation into myotubes, indicating that arsenic can act as a developmental toxicant. Myoblast cells were exposed to 20 nM sodium arsenite, allowed to differentiate into myotubes, and expression of the muscle-specific transcription factor myogenin, along with the expression of tropomyosin, suppressor of cytokine signaling 3 (Socs3), prostaglandin I2 synthesis (Ptgis), and myocyte enhancer 2 (Mef2), was investigated using QPCR and immunofluorescence. Exposing C2C12 cells to 20 nM sodium arsenite delayed the differentiation process, as evidenced by a significant reduction in the number of multinucleated myotubes, a decrease in myogenin mRNA expression, and a decrease in the total number of nuclei expressing myogenin protein. The expression of mRNA involved in myotube formation, such as Ptgis and Mef2 mRNA, was also significantly reduced by 1.6-fold and 4-fold during differentiation. This was confirmed by immunofluorescence for Mef2, which showed a 2.6-fold reduction in nuclear translocation. Changes in methylation patterns in the promoter region of myogenin (-473 to +90) were examined by methylation-specific PCR and bisulfite genomic sequencing. Hypermethylated CpGs were found at -236 and -126 bp, whereas hypomethylated CpGs were found at -207 bp in arsenic-exposed cells. This study indicates that 20 nM sodium arsenite can alter myoblast differentiation by reducing the expression of the transcription factors myogenin and Mef2c, which is likely due to changes in promoter methylation patterns. The delay in muscle differentiation may lead to developmental abnormalities.

Excitation-transcription coupling in skeletal muscle: the molecular pathways of exercise

Muscle fibres have different properties with respect to force, contraction speed, endurance, oxidative/glycolytic capacity etc. Although adult muscle fibres are normally post-mitotic with little turnover of cells, the physiological properties of the pre-existing fibres can be changed in the adult animal upon changes in usage such as after exercise. The signal to change is mainly conveyed by alterations in the patterns of nerve-evoked electrical activity, and is to a large extent due to switches in the expression of genes. Thus, an excitation-transcription coupling must exist. It is suggested that changes in nerve-evoked muscle activity lead to a variety of activity correlates such as increases in free intracellular Ca²⁺ levels caused by influx across the cell membrane and/or release from the sarcoplasmatic reticulum, concentrations of metabolites such as lipids and ADP, hypoxia and mechanical stress. Such correlates are detected by sensors such as protein kinase C (PKC), calmodulin, AMP-activated kinase (AMPK), peroxisome proliferator-activated receptor δ (PPARδ), and oxygen dependent prolyl hydroxylases that trigger intracellular signaling cascades. These complex cascades involve several transcription factors such as nuclear factor of activated T-cells (NFAT), myocyte enhancer factor 2 (MEF2), myogenic differentiation factor (myoD), myogenin, PPARδ, and sine oculis homeobox 1/eyes absent 1 (Six1/Eya1). These factors might act indirectly by inducing gene products that act back on the cascade, or as ultimate transcription factors binding to and transactivating/repressing genes for the fast and slow isoforms of various contractile proteins and of metabolic enzymes. The determination of size and force is even more complex as this involves not only intracellular signaling within the muscle fibres, but also muscle stem cells called satellite cells. Intercellular signaling substances such as myostatin and insulin-like growth factor 1 (IGF-1) seem to act in a paracrine fashion. Induction of hypertrophy is accompanied by the satellite cells fusing to myofibres and thereby increasing the capacity for protein synthesis. These extra nuclei seem to remain part of the fibre even during subsequent atrophy as a form of muscle memory facilitating retraining. In addition to changes in myonuclear number during hypertrophy, changes in muscle fibre size seem to be caused by alterations in transcription, translation (per nucleus) and protein degradation.

Differential gene expressions in atrial and ventricular myocytes: insights into the road of applying embryonic stem cell-derived cardiomyocytes for future therapies

Myocardial infarction has been the leading cause of morbidity and mortality in developed countries over the past few decades. The transplantation of cardiomyocytes offers a potential method of treatment. However, cardiomyocytes are in high demand and their supply is extremely limited. Embryonic stem cells (ESCs), which have been isolated from the inner cell mass of blastocysts, can self-renew and are pluripotent, meaning they have the ability to develop into any type of cell, including cardiomyocytes. This suggests that ESCs could be a good source of genuine cardiomyocytes for future therapeutic purposes. However, problems with the yield and purity of ESC-derived cardiomyocytes, among other hurdles for the therapeutic application of ESC-derived cardiomyocytes (e.g., potential immunorejection and tumor formation problems), need to be overcome before these cells can be used effectively for cell replacement therapy. ESC-derived cardiomyocytes consist of nodal, atrial, and ventricular cardiomyocytes. Specifically, for treatment of myocardial infarction, transplantation of a sufficient quantity of ventricular cardiomyocytes, rather than nodal or atrial cardiomyocytes, is preferred. Hence, it is important to find ways of increasing the yield and purity of specific types of cardiomyocytes. Atrial and ventricular cardiomyocytes have differential expression of genes (transcription factors, structural proteins, ion channels, etc.) and are functionally distinct. This paper presents a thorough review of differential gene expression in atrial and ventricular myocytes, their expression throughout development, and their regulation. An understanding of the molecular and functional differences between atrial and ventricular myocytes allows discussion of potential strategies for preferentially directing ESCs to differentiate into chamber-specific cells, or for fine tuning the ESC-derived cardiomyocytes into specific electrical and contractile phenotypes resembling chamber-specific cells.

Promoting developmental transcription

Animal growth and development depend on the precise control of gene expression at the level of transcription. A central role in the regulation of developmental transcription is attributed to transcription factors that bind DNA enhancer elements, which are often located far from gene transcription start sites. Here, we review recent studies that have uncovered significant regulatory functions in developmental transcription for the TFIID basal transcription factors and for the DNA core promoter elements that are located close to transcription start sites.

MyoD targets TAF3/TRF3 to activate myogenin transcription

Skeletal muscle differentiation requires a cascade of transcriptional events to control the spatial and temporal expression of muscle-specific genes. Until recently, muscle-specific transcription was primarily attributed to prototypic enhancer-binding factors, while the role of core promoter recognition complexes in directing myogenesis remained unknown. Here, we report the development of a purified reconstituted system to analyze the properties of a TAF3/TRF3 complex in directing transcription initiation at the Myogenin promoter. Importantly, this new complex is required to replace the canonical TFIID to recapitulate MyoD-dependent activation of Myogenin. In vitro and cell-based assays identify a domain of TAF3 that mediates coactivator functions targeted by MyoD. Our findings also suggest changes to CRSP/Mediator in terminally differentiated myotubes. This switching of the core promoter recognition complex during myogenesis allows a more balanced division of labor between activators and TAF coactivators, thus providing another strategy to accommodate cell-specific regulation during metazoan development.

p38 MAPK signaling regulates recruitment of Ash2L-containing methyltransferase complexes to specific genes during differentiation

Cell-specific patterns of gene expression are established through the antagonistic functions of trithorax group (TrxG) and Polycomb group (PcG) proteins. Several muscle-specific genes have previously been shown to be epigenetically marked for repression by PcG proteins in muscle progenitor cells. Here we demonstrate that these developmentally regulated genes become epigenetically marked for gene expression (trimethylated on histone H3 Lys4, H3K4me3) during muscle differentiation through specific recruitment of Ash2L-containing methyltransferase complexes. Targeting of Ash2L to specific genes is mediated by the transcriptional regulator Mef2d. Furthermore, this interaction is modulated during differentiation through activation of the p38 MAPK signaling pathway via phosphorylation of Mef2d. Thus, we provide evidence that signaling pathways regulate the targeting of TrxG-mediated epigenetic modifications at specific promoters during cellular differentiation.

MEF2: a central regulator of diverse developmental programs

The myocyte enhancer factor 2 (MEF2) transcription factor acts as a lynchpin in the transcriptional circuits that control cell differentiation and organogenesis. The spectrum of genes activated by MEF2 in different cell types depends on extracellular signaling and on co-factor interactions that modulate MEF2 activity. Recent studies have revealed MEF2 to form an intimate partnership with class IIa histone deacetylases, which together function as a point of convergence of multiple epigenetic regulatory mechanisms. We review the myriad roles of MEF2 in development and the mechanisms through which it couples developmental, physiological and pathological signals with programs of cell-specific transcription.

Necdin mediates skeletal muscle regeneration by promoting myoblast survival and differentiation

Regeneration of muscle fibers that are lost during pathological muscle degeneration or after injuries is sustained by the production of new myofibers. An important cell type involved in muscle regeneration is the satellite cell. Necdin is a protein expressed in satellite cell-derived myogenic precursors during perinatal growth. However, its function in myogenesis is not known. We compare transgenic mice that overexpress necdin in skeletal muscle with both wild-type and necdin null mice. After muscle injury the necdin null mice show a considerable defect in muscle healing, whereas mice that overexpress necdin show a substantial increase in myofiber regeneration. We also find that in muscle, necdin increases myogenin expression, accelerates differentiation, and counteracts myoblast apoptosis. Collectively, these data clarify the function and mechanism of necdin in skeletal muscle and show the importance of necdin in muscle regeneration.

The homeobox gene Arx is a novel positive regulator of embryonic myogenesis

Skeletal muscle fibers form in overlapping, but distinct phases that depend on the generation of temporally different lineages of myogenic cells. During primary myogenesis (E10.5-E12.5 in the mouse), embryonic myoblasts fuse homotypically to generate primary fibers, whereas during later development (E14.5-E17.5), fetal myoblasts differentiate into secondary fibers. How these myogenic waves are regulated remains largely unknown. Studies have been hampered by the lack of markers which would distinguish embryonic from fetal myoblast populations. We show here that the homeobox gene Arx is strongly expressed in differentiating embryonic muscle, downstream of myogenic basic helix-loop-helix (bHLH) genes. Its expression progressively decreases during development. When overexpressed in the C2C12 myogenic cell line, Arx enhances differentiation. Accordingly, it stimulates the transcriptional activity from the Myogenin promoter and from multimerized E-boxes when co-expressed with MyoD and Mef2C in CH310T1/2. Furthermore, Arx co-immunoprecipitates with Mef2C, suggesting that it participates in the transcriptional regulatory network acting in embryonic muscle. Finally, embryonic myoblasts isolated from Arx-deficient embryos show a delayed differentiation in vivo together with an enhanced clonogenic capacity in vitro. We propose here that Arx acts as a novel positive regulator of embryonic myogenesis by synergizing with Mef2C and MyoD and by establishing an activating loop with Myogenin.