A homeo-paired domain-binding motif directs Myf5 expression in progenitor cells of limb muscle

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.

PAX3: A Molecule with Oncogenic or Tumor Suppressor Function Is Involved in Cancer

Metastasis is the most deadly aspect of cancer and results from acquired gene regulation abnormalities in tumor cells. Transcriptional regulation is an essential component of controlling of gene function and its failure could contribute to tumor progression and metastasis. During cancer progression, deregulation of oncogenic or tumor suppressive transcription factors, as well as master cell fate regulators, collectively influences multiple steps of the metastasis cascade, including local invasion and dissemination of the tumor to distant organs. Transcription factor PAX3/Pax3, which contributes to diverse cell lineages during embryonic development, plays a major role in tumorigenesis. Mutations in this gene can cause neurodevelopmental disease and the existing literature supports that there is a potential link between aberrant expression of PAX3 genes in adult tissues and a wide variety of cancers. PAX3 function is tissue-specific and could contribute to tumorigenesis either directly as oncogene or as a tumor suppressor by losing its function. In this review, we discuss comprehensively the differential role played by PAX3 in various tissues and how its aberrant expression is implicated in disease development. This review particularly highlights the oncogenic and tumor suppressor role played by PAX3 in different cancers and underlines the importance of precisely identifying tissue-specific role of PAX3 in order to determine its exact role in development of cancer.

SNAIL is a key regulator of alveolar rhabdomyosarcoma tumor growth and differentiation through repression of MYF5 and MYOD function

Rhabdomyosarcoma (RMS) is a mesenchymal tumor of soft tissue in children that originates from a myogenic differentiation defect. Expression of SNAIL transcription factor is elevated in the alveolar subtype of RMS (ARMS), characterized by a low myogenic differentiation status and high aggressiveness. In RMS patients SNAIL level increases with higher stage. Moreover, SNAIL level negatively correlates with MYF5 expression. The differentiation of human ARMS cells diminishes SNAIL level. SNAIL silencing in ARMS cells inhibits proliferation and induces differentiation in vitro, and thereby completely abolishes the growth of human ARMS xenotransplants in vivo. SNAIL silencing induces myogenic differentiation by upregulation of myogenic factors and muscle-specific microRNAs, such as miR-206. SNAIL binds to the MYF5 promoter suppressing its expression. SNAIL displaces MYOD from E-box sequences (CANNTG) that are associated with genes expressed during differentiation and G/C rich in their central dinucleotides. SNAIL silencing allows the re-expression of MYF5 and canonical MYOD binding, promoting ARMS cell myogenic differentiation. In differentiating ARMS cells SNAIL forms repressive complex with histone deacetylates 1 and 2 (HDAC1/2) and regulates their expression. Accordingly, in human myoblasts SNAIL silencing induces differentiation by upregulation of myogenic factors. Our data clearly point to SNAIL as a key regulator of myogenic differentiation and a new promising target for future ARMS therapies.

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.

β-Arrestin scaffolds and signaling elements essential for the obestatin/GPR39 system that determine the myogenic program in human myoblast cells

Obestatin/GPR39 signaling stimulates skeletal muscle repair by inducing the expansion of satellite stem cells as well as myofiber hypertrophy. Here, we describe that the obestatin/GPR39 system acts as autocrine/paracrine factor on human myogenesis. Obestatin regulated multiple steps of myogenesis: myoblast proliferation, cell cycle exit, differentiation and recruitment to fuse and form multinucleated hypertrophic myotubes. Obestatin-induced mitogenic action was mediated by ERK1/2 and JunD activity, being orchestrated by a G-dependent mechanism. At a later stage of myogenesis, scaffolding proteins β-arrestin 1 and 2 were essential for the activation of cell cycle exit and differentiation through the transactivation of the epidermal growth factor receptor (EGFR). Upon obestatin stimulus, β-arrestins are recruited to the membrane, where they functionally interact with GPR39 leading to Src activation and signalplex formation to EGFR transactivation by matrix metalloproteinases. This signalplex regulated the mitotic arrest by p21 and p57 expression and the mid- to late stages of differentiation through JNK/c-Jun, CAMKII, Akt and p38 pathways. This finding not only provides the first functional activity for β-arrestins in myogenesis but also identify potential targets for therapeutic approaches by triggering specific signaling arms of the GPR39 signaling involved in myogenesis.

Concise Review: Epigenetic Regulation of Myogenesis in Health and Disease

This review describes the recent findings on epigenetic regulation in satellite stem cells and committed myoblasts. It also addresses the potential of epigenetic drugs, such as histone deacetylase inhibitors, and their molecular mechanism of action in muscle cells.

Skeletal muscle regeneration is initiated by satellite cells, a population of adult stem cells that reside in the muscle tissue. The ability of satellite cells to self-renew and to differentiate into the muscle lineage is under transcriptional and epigenetic control. Satellite cells are characterized by an open and permissive chromatin state. The transcription factor Pax7 is necessary for satellite cell function. Pax7 is a nodal factor regulating the expression of genes associated with satellite cell growth and proliferation, while preventing differentiation. Pax7 recruits chromatin modifiers to DNA to induce expression of specific target genes involved in myogenic commitment following asymmetric division of muscle stem cells. Emerging evidence suggests that replacement of canonical histones with histone variants is an important regulatory mechanism controlling the ability of satellite cells and myoblasts to differentiate. Differentiation into the muscle lineage is associated with a global gene repression characterized by a decrease in histone acetylation with an increase in repressive histone marks. However, genes important for differentiation are upregulated by the specific action of histone acetyltransferases and other chromatin modifiers, in combination with several transcription factors, including MyoD and Mef2. Treatment with histone deacetylase (HDAC) inhibitors enhances muscle regeneration and is considered as a therapeutic approach in the treatment of muscular dystrophy. This review describes the recent findings on epigenetic regulation in satellite stem cells and committed myoblasts. The potential of epigenetic drugs, such as HDAC inhibitors, as well as their molecular mechanism of action in muscle cells, will be addressed.

Significance

This review summarizes recent findings concerning the epigenetic regulation of satellite cells in skeletal muscle.

Fine-tuning the onset of myogenesis by homeobox proteins that interact with the Myf5 limb enhancer

Skeletal myogenesis in vertebrates is initiated at different sites of skeletal muscle formation during development, by activation of specific control elements of the myogenic regulatory genes. In the mouse embryo, Myf5 is the first myogenic determination gene to be expressed and its spatiotemporal regulation requires multiple enhancer sequences, extending over 120 kb upstream of the Mrf4-Myf5 locus. An enhancer, located at −57/−58 kb from Myf5, is responsible for its activation in myogenic cells derived from the hypaxial domain of the somite, that will form limb muscles. Pax3 and Six1/4 transcription factors are essential activators of this enhancer, acting on a 145-bp core element. Myogenic progenitor cells that will form the future muscle masses of the limbs express the factors necessary for Myf5 activation when they delaminate from the hypaxial dermomyotome and migrate into the forelimb bud, however they do not activate Myf5 and the myogenic programme until they have populated the prospective muscle masses. We show that Msx1 and Meox2 homeodomain-containing transcription factors bind in vitro and in vivo to specific sites in the 145-bp element, and are implicated in fine-tuning activation of Myf5 in the forelimb. Msx1, when bound between Pax and Six sites, prevents the binding of these key activators, thus inhibiting transcription of Myf5 and consequent premature myogenic differentiation. Meox2 is required for Myf5 activation at the onset of myogenesis via direct binding to other homeodomain sites in this sequence. Thus, these homeodomain factors, acting in addition to Pax3 and Six1/4, fine-tune the entry of progenitor cells into myogenesis at early stages of forelimb development.

Summary: Homeodomain factors Msx1 and Meox2, acting in addition to Pax3 and Six1/4, fine-tune the entry of progenitor cells into myogenesis at early stages of forelimb development via modulation of limb enhancer gene Myf5.

Pax factors in transcription and epigenetic remodelling

The nine Pax transcription factors that constitute the mammalian family of paired domain (PD) factors play key roles in many developmental processes. As DNA binding transcription factors, they exhibit tremendous variability and complexity in their DNA recognition patterns. This is ascribed to the presence of multiple DNA binding structural domains, namely helix-turn-helix (HTH) domains. The PD contains two HTH subdomains and four of the nine Pax factors have an additional HTH domain, the homeodomain (HD). We now review these diverse DNA binding modalities together with their properties as transcriptional activators and repressors. The action of Pax factors on gene expression is also exerted through recruitment of chromatin remodelling complexes that introduce either activating or repressive chromatin marks. Interestingly, the recent demonstration that Pax7 has pioneer activity, the unique property to "open" chromatin, further underlines the mechanistic versatility and the developmental importance of these factors.

Expression of myogenic regulatory factors in chicken embryos during somite and limb development

The expression of the myogenic regulatory factors (MRFs), Myf5, MyoD, myogenin (Mgn) and MRF4 have been analysed during the development of chicken embryo somites and limbs. In somites, Myf5 is expressed first in somites and paraxial mesoderm at HH stage 9 followed by MyoD at HH stage 12, and Mgn and MRF4 at HH stage 14. In older somites, Myf5 and MyoD are also expressed in the ventrally extending myotome prior to Mgn and MRF4 expression. In limb muscles a similar temporal sequence is observed with Myf5 expression detected first in forelimbs at HH stage 22, MyoD at HH stage 23, Mgn at HH stage 24 and MRF4 at HH stage 30. This report describes the precise time of onset of expression of each MRF in somites and limbs during chicken embryo development, and provides a detailed comparative timeline of MRF expression in different embryonic muscle groups.

Human and mouse skeletal muscle stem cells: convergent and divergent mechanisms of myogenesis

Satellite cells are the chief contributor to skeletal muscle growth and regeneration. The study of mouse satellite cells has accelerated in recent years due to technical advancements in the isolation of these cells. The study of human satellite cells has lagged and thus little is known about how the biology of mouse and human satellite cells compare. We developed a flow cytometry-based method to prospectively isolate human skeletal muscle progenitors from the satellite cell pool using positive and negative selection markers. Results show that this pool is enriched in PAX7 expressing cells that possess robust myogenic potential including the ability to give rise to de novo muscle in vivo. We compared mouse and human satellite cells in culture and identify differences in the elaboration of the myogenic genetic program and in the sensitivity of the cells to cytokine stimulation. These results indicate that not all mechanisms regulating mouse satellite cell activation are conserved in human satellite cells and that such differences may impact the clinical translation of therapeutics validated in mouse models. Thus, the findings of this study are relevant to developing therapies to combat muscle disease.

Exceptional expansion and conservation of a CT-repeat complex in the core promoter of PAXBP1 in primates

Adaptive evolution may be linked with the genomic distribution and function of short tandem repeats (STRs). Proximity of the core promoter STRs to the +1 transcription start site (TSS), and their mutable nature are characteristics that highlight those STRs as a novel source of interspecies variation. The PAXBP1 gene (alternatively known as GCFC1) core promoter contains the longest STR identified in a Homo sapiens gene core promoter. Indeed, this core promoter is a stretch of four consecutive CT-STRs. In the current study, we used the Ensembl, NCBI, and UCSC databases to analyze the evolutionary trend and functional implication of this CT-STR complex in six major lineages across vertebrates, including primates, non-primate mammals, birds, reptiles, amphibians, and fish. We observed exceptional expansion (≥4-repeats) and conservation of this CT-STR complex across primates, except prosimians, Microcebus murinus and Otolemur garnettii (Fisher exact P<4.1×10(-7)). H. sapiens has the most complex STR formula, and longest repeats. Macaca mulatta and Callithrix jacchus monkeys have the simplest STR formulas, and shortest repeat numbers. CT≥4-repeats were not detected in non-primate lineages. Different length alleles across the PAXBP1 core promoter CT-STRs significantly altered gene expression in vitro (P<0.001, t-test). PAXBP1 has a crucial role in craniofacial development, myogenesis, and spine morphogenesis, properties that have been diverged between primates and non-primates. To our knowledge, this is the first instance of expansion and conservation of a STR complex co-occurring specifically with the primate lineage.

Pax3 synergizes with Gli2 and Zic1 in transactivating the Myf5 epaxial somite enhancer

Both Glis, the downstream effectors of hedgehog signaling, and Zic transcription factors are required for Myf5 expression in the epaxial somite. Here we demonstrate a novel synergistic interaction between members of both families and Pax3, a paired-domain transcription factor that is essential for both myogenesis and neural crest development. We show that Pax3 synergizes with both Gli2 and Zic1 in transactivating the Myf5 epaxial somite (ES) enhancer in concert with the Myf5 promoter. This synergy is dependent on conserved functional domains of the proteins, as well as on a novel homeodomain motif in the Myf5 promoter and the essential Gli motif in the ES enhancer. Importantly, overexpression of Zic1 and Pax3 in the 10T1/2 mesodermal cell model results in enrichment of these factors at the endogenous Myf5 locus and induction of Myf5 expression. In our previous work, we showed that by enhancing nuclear translocation of Gli factors, Zics provide spatiotemporal patterning for Gli family members in the epaxial induction of Myf5 expression. Our current study indicates a complementary mechanism in which association with DNA-bound Pax3 strengthens the ability of both Zic1 and Gli2 to transactivate Myf5 in the epaxial somite.

Functional dissection of Pax3 in paraxial mesoderm development and myogenesis

The paired box transcription factor Pax3 is well-known as a major regulator of embryonic myogenesis. Before Pax3 expression becomes restricted to the dermomyotome, this transcription factor is also expressed in the developing somites. The role of Pax3 at this early stage is unclear, in particular because of the scarce frequency of Pax3-positive cells in the early mouse embryo. Inducible gene expression in embryonic stem cells (ESCs) represents an excellent tool to overcome this limitation, since it can provide large quantities of otherwise rare embryonic populations expressing a factor of interest. Here we used engineered mouse ESCs to perform a functional analysis of Pax3 with the aim to identify the molecular determinants involved in the early functions of this transcription factor. We find that Pax3 induction during embryoid body differentiation results in the upregulation of genes expressed in the presomitic and somitic mesoderm. Moreover, we show that paraxial mesoderm induced by transient expression of Pax3 is not irreversibly committed to myogenesis rather requires sustained Pax3 expression. Using a series of deletion mutants of Pax3, which differentially affect its transcriptional activity, we map protein domains necessary for induction of paraxial mesoderm and induction of the myogenic program. The paired, homeo-, and transcriptional activation domains were each required for both processes, however, the paired-c-terminal RED domain showed a paraxial mesoderm-specific activity that was dispensable for myogenesis. These findings demonstrate and provide mechanistic insight into an early role for Pax3 in the generation of paraxial mesoderm.

Role of a polymorphism in a Hox/Pax-responsive enhancer in the evolution of the vertebrate spine

Patterning of the vertebrate skeleton requires the coordinated activity of Hox genes. In particular, Hox10 proteins are essential to set the transition from thoracic to lumbar vertebrae because of their rib-repressing activity. In snakes, however, the thoracic region extends well into Hox10-expressing areas of the embryo, suggesting that these proteins are unable to block rib formation. Here, we show that this is not a result of the loss of rib-repressing properties by the snake proteins, but rather to a single base pair change in a Hox/Paired box (Pax)-responsive enhancer, which prevents the binding of Hox proteins. This polymorphism is also found in Paenungulata, such as elephants and manatees, which have extended rib cages. In vivo, this modified enhancer failed to respond to Hox10 activity, supporting its role in the extension of rib cages. In contrast, the enhancer could still interact with Hoxb6 and Pax3 to promote rib formation. These results suggest that a polymorphism in the Hox/Pax-responsive enhancer may have played a role in the evolution of the vertebrate spine by differently modulating its response to rib-suppressing and rib-promoting Hox proteins.

Muscle development, regeneration and laminopathies: how lamins or lamina-associated proteins can contribute to muscle development, regeneration and disease

The aim of this review article is to evaluate the current knowledge on associations between muscle formation and regeneration and components of the nuclear lamina. Lamins and their partners have become particularly intriguing objects of scientific interest since it has been observed that mutations in genes coding for these proteins lead to a wide range of diseases called laminopathies. For over the last 10 years, various laboratories worldwide have tried to explain the pathogenesis of these rare disorders. Analyses of the distinct aspects of laminopathies resulted in formulation of different hypotheses regarding the mechanisms of the development of these diseases. In the light of recent discoveries, A-type lamins—the main building blocks of the nuclear lamina—together with other key elements, such as emerin, LAP2α and nesprins, seem to be of great importance in the modulation of various signaling pathways responsible for cellular differentiation and proliferation.

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity

How muscle diversity is generated in the vertebrate body is poorly understood. In the limb, dorsal and ventral muscle masses constitute the first myogenic diversification, as each gives rise to distinct muscles. Myogenesis initiates after muscle precursor cells (MPCs) have migrated from the somites to the limb bud and populated the prospective muscle masses. Here, we show that Sonic hedgehog (Shh) from the zone of polarizing activity (ZPA) drives myogenesis specifically within the ventral muscle mass. Shh directly induces ventral MPCs to initiate Myf5 transcription and myogenesis through essential Gli-binding sites located in the Myf5 limb enhancer. In the absence of Shh signaling, myogenesis is delayed, MPCs fail to migrate distally, and ventral paw muscles fail to form. Thus, Shh production in the limb ZPA is essential for the spatiotemporal control of myogenesis and coordinates muscle and skeletal development by acting directly to regulate the formation of specific ventral muscles.

Regulatory role for a conserved motif adjacent to the homeodomain of Hox10 proteins

Development of the vertebrate axial skeleton requires the concerted activity of several Hox genes. Among them, Hox genes belonging to the paralog group 10 are essential for the formation of the lumbar region of the vertebral column, owing to their capacity to block rib formation. In this work, we explored the basis for the rib-repressing activity of Hox10 proteins. Because genetic experiments in mice demonstrated that Hox10 proteins are strongly redundant in this function, we first searched for common motifs among the group members. We identified the presence of two small sequences flanking the homeodomain that are phylogenetically conserved among Hox10 proteins and that seem to be specific for this group. We show here that one of these motifs is required but not sufficient for the rib-repressing activity of Hox10 proteins. This motif includes two potential phosphorylation sites, which are essential for protein activity as their mutation to alanines resulted in a total loss of rib-repressing properties. Our data indicates that this motif has a significant regulatory function, modulating interactions with more N-terminal parts of the Hox protein, eventually triggering the rib-repressing program. In addition, this motif might also regulate protein activity by alteration of the protein's DNA-binding affinity through changes in the phosphorylation state of two conserved tyrosine residues within the homeodomain.

The MSX1 homeoprotein recruits G9a methyltransferase to repressed target genes in myoblast cells

Although the significance of lysine modifications of core histones for regulating gene expression is widely appreciated, the mechanisms by which these modifications are incorporated at specific regulatory elements during cellular differentiation remains largely unknown. In our previous studies, we have shown that in developing myoblasts the Msx1 homeoprotein represses gene expression by influencing the modification status of chromatin at its target genes. We now show that genomic binding by Msx1 promotes enrichment of the H3K9me2 mark on repressed target genes via recruitment of G9a histone methyltransferase, the enzyme responsible for catalyzing this histone mark. Interaction of Msx1 with G9a is mediated via the homeodomain and is required for transcriptional repression and regulation of cellular differentiation, as well as enrichment of the H3K9me2 mark in proximity to Msx1 binding sites on repressed target genes in myoblast cells as well as the developing limb. We propose that regulation of chromatin status by Msx1 recruitment of G9a and other histone modifying enzymes to regulatory regions of target genes represents an important means of regulating the gene expression during development.

Transcriptional repression by the Msx1 homeoprotein is associated with global redistribution of the H3K27me3 repressive mark to the nuclear periphery

The spatial and temporal regulation of gene expression during development requires the concerted actions of sequence-specific transcriptional regulators and epigenetic chromatin modifiers, which are thought to function within precise nuclear compartments. However, coordination of these activities within the dynamic context of the nuclear environment is still largely unresolved. Here we discuss the implications of our recent work showing that transcriptional repression by the Msx1 homeoprotein is associated with global redistribution of the H3K27me3 repressive mark to the nuclear periphery during development.

Barx homeobox family in muscle development and regeneration

Homeobox transcription factors are key intrinsic regulators of myogenesis. In studies spanning several years, we have characterized the homeobox factor Barx2 as a novel marker for muscle progenitor cells and an important regulator of muscle growth and repair. In this review, we place the expression and function of Barx2 and its paralogue Barx1 in context with other muscle-expressed homeobox factors in both embryonic and adult myogenesis. We also describe the structure and regulation of Barx genes and possible gene/disease associations. The functional domains of Barx proteins, their molecular interactions, and cellular functions are presented with particular emphasis on control of genes and processes involved in myogenic differentiation. Finally, we describe the patterns of Barx gene expression in vivo and the phenotypes of various Barx gene perturbation models including null mice. We focus on the Barx2 null mouse model, which has demonstrated the critical roles of Barx2 in postnatal myogenesis including muscle maintenance during aging, and regeneration of acute and chronic muscle injury.

The Msx1 Homeoprotein Recruits Polycomb to the Nuclear Periphery during Development

Control of gene expression during development requires the concerted action of sequence-specific transcriptional regulators and epigenetic modifiers, which are spatially coordinated within the nucleus through mechanisms that are poorly understood. Here we show that transcriptional repression by the Msx1 homeoprotein in myoblast cells requires the recruitment of Polycomb to target genes located at the nuclear periphery. Target genes repressed by Msx1 display an Msx1-dependent enrichment of Polycomb-directed trimethylation of lysine 27 on histone H3 (H3K27me3). Association of Msx1 with the Polycomb complex is required for repression and regulation of myoblast differentiation. Furthermore, Msx1 promotes a dynamic spatial redistribution of the H3K27me3 repressive mark to the nuclear periphery in myoblast cells and the developing limb in vivo. Our findings illustrate a hitherto unappreciated spatial coordination of transcription factors with the Polycomb complex for appropriate regulation of gene expression programs during development.

Transcriptional mechanisms regulating skeletal muscle differentiation, growth and homeostasis

Skeletal muscle is the dominant organ system in locomotion and energy metabolism. Postnatal muscle grows and adapts largely by remodelling pre-existing fibres, whereas embryonic muscle grows by the proliferation of myogenic cells. Recently, the genetic hierarchies of the myogenic transcription factors that control vertebrate muscle development - by myoblast proliferation, migration, fusion and functional adaptation into fast-twitch and slow-twitch fibres - have become clearer. The transcriptional mechanisms controlling postnatal hypertrophic growth, remodelling and functional differentiation redeploy myogenic factors in concert with serum response factor (SRF), JUNB and forkhead box protein O3A (FOXO3A). It has also emerged that there is extensive post-transcriptional regulation by microRNAs in development and postnatal remodelling.

Genome-wide identification of PAX3-FKHR binding sites in rhabdomyosarcoma reveals candidate target genes important for development and cancer

The PAX3-FKHR fusion protein is present in a majority of alveolar rhabdomyosarcomas associated with increased aggressiveness and poor prognosis. To better understand the molecular pathogenesis of PAX3-FKHR, we carried out the first, unbiased genome-wide identification of PAX3-FKHR binding sites and associated target genes in alveolar rhabdomyosarcoma. The data shows that PAX3-FKHR binds to the same sites as PAX3 at both MYF5 and MYOD enhancers. The genome-wide analysis reveals that the PAX3-FKHR sites are (a) mostly distal to transcription start sites, (b) conserved, (c) enriched for PAX3 motifs, and (d) strongly associated with genes overexpressed in PAX3-FKHR-positive rhabdomyosarcoma cells and tumors. There is little evidence in our data set for PAX3-FKHR binding at the promoter sequences. The genome-wide analysis further illustrates a strong association between PAX3 and E-box motifs in these binding sites, suggestive of a common coregulation for many target genes. We also provide the first direct evidence that FGFR4 and IGF1R are the targets for PAX3-FKHR. The map of PAX3-FKHR binding sites provides a framework for understanding the pathogenic roles of PAX3-FKHR, as well as its molecular targets to allow a systematic evaluation of agents against this aggressive rhabdomyosarcoma.

Hox genes and regional patterning of the vertebrate body plan

Several decades have passed since the discovery of Hox genes in the fruit fly Drosophila melanogaster. Their unique ability to regulate morphologies along the anteroposterior (AP) axis (Lewis, 1978) earned them well-deserved attention as important regulators of embryonic development. Phenotypes due to loss- and gain-of-function mutations in mouse Hox genes have revealed that the spatio-temporally controlled expression of these genes is critical for the correct morphogenesis of embryonic axial structures. Here, we review recent novel insight into the modalities of Hox protein function in imparting specific identity to anatomical regions of the vertebral column, and in controlling the emergence of these tissues concomitantly with providing them with axial identity. The control of these functions must have been intimately linked to the shaping of the body plan during evolution.

Evidence for a myotomal Hox/Myf cascade governing nonautonomous control of rib specification within global vertebral domains

Hox genes are essential for the patterning of the axial skeleton. Hox group 10 has been shown to specify the lumbar domain by setting a rib-inhibiting program in the presomitic mesoderm (PSM). We have now produced mice with ribs in every vertebra by ectopically expressing Hox group 6 in the PSM, indicating that Hox genes are also able to specify the thoracic domain. We show that the information provided by Hox genes to specify rib-containing and rib-less areas is first interpreted in the myotome through the regional-specific control of Myf5 and Myf6 expression. This information is then transmitted to the sclerotome by a system that includes FGF and PDGF signaling to produce vertebrae with or without ribs at different axial levels. Our findings offer a new perspective of how Hox genes produce global patterns in the axial skeleton and support a redundant nonmyogenic role of Myf5 and Myf6 in rib formation.

Distinct and dynamic myogenic populations in the vertebrate embryo

Myogenic cells in the body of vertebrates derive from the dorsal somite, the dermomyotome, where multipotent cells are present. Regulation of cell fate choice is discussed, as is that of progenitor cell self-renewal once cells have entered the myogenic programme. Ongoing research on the formation of the first skeletal muscle, the myotome, is presented with emphasis on mechanisms controlling the early segregation of slow and fast muscle lineages that characterizes this process in the zebrafish embryo. Further insights into myogenic populations that contribute to trunk and limb development at different stages are summarized and the distinct regulatory networks that underlie the formation of head muscles are discussed.

Credentialing a preclinical mouse model of alveolar rhabdomyosarcoma

The highly aggressive muscle cancer alveolar rhabdomyosarcoma (ARMS) is one of the most common soft tissue sarcoma of childhood, yet the outcome for the unresectable and metastatic disease is dismal and unchanged for nearly three decades. To better understand the pathogenesis of this disease and to facilitate novel preclinical approaches, we previously developed a conditional mouse model of ARMS by faithfully recapitulating the genetic mutations observed in the human disease, i.e., activation of Pax3:Fkhr fusion gene with either p53 or Cdkn2a inactivation. In this report, we show that this model recapitulates the immunohistochemical profile and the rapid progression of the human disease. We show that Pax3:Fkhr expression increases during late preneoplasia but tumor cells undergoing metastasis are under apparent selection for Pax3:Fkhr expression. At a whole-genome level, a cross-species gene set enrichment analysis and metagene projection study showed that our mouse model is most similar to human ARMS when compared with other pediatric cancers. We have defined an expression profile conserved between mouse and human ARMS, as well as a Pax3:Fkhr signature, including the target gene, SKP2. We further identified 7 "druggable" kinases overexpressed across species. The data affirm the accuracy of this genetically engineered mouse model.

The origin of embryonic and fetal myoblasts: a role of Pax3 and Pax7

Skeletal muscle is a heterogeneous tissue composed of individual muscle fibers, diversified in size, shape, and contractile protein content, to fulfill the different functional needs of the vertebrate body. This heterogeneity derives from and depends at least in part on distinct classes of myogenic progenitors; i.e., embryonic and fetal myoblasts and satellite cells whose origin and lineage relationship have been elusive so far. In this issue of Genes & Development, Hutcheson and colleagues (pp. 997-1013) provide a first answer to this question.

Embryonic and fetal limb myogenic cells are derived from developmentally distinct progenitors and have different requirements for beta-catenin

Vertebrate muscle arises sequentially from embryonic, fetal, and adult myoblasts. Although functionally distinct, it is unclear whether these myoblast classes develop from common or different progenitors. Pax3 and Pax7 are expressed by somitic myogenic progenitors and are critical myogenic determinants. To test the developmental origin of embryonic and fetal myogenic cells in the limb, we genetically labeled and ablated Pax3(+) and Pax7(+) cells. Pax3(+)Pax7(-) cells contribute to muscle and endothelium, establish and are required for embryonic myogenesis, and give rise to Pax7(+) cells. Subsequently, Pax7(+) cells give rise to and are required for fetal myogenesis. Thus, Pax3(+) and Pax7(+) cells contribute differentially to embryonic and fetal limb myogenesis. To investigate whether embryonic and fetal limb myogenic cells have different genetic requirements we conditionally inactivated or activated beta-catenin, an important regulator of myogenesis, in Pax3- or Pax7-derived cells. beta-Catenin is necessary within the somite for dermomyotome and myotome formation and delamination of limb myogenic progenitors. In the limb, beta-catenin is not required for embryonic myoblast specification or myofiber differentiation but is critical for determining fetal progenitor number and myofiber number and type. Together, these studies demonstrate that limb embryonic and fetal myogenic cells develop from distinct, but related progenitors and have different cell-autonomous requirements for beta-catenin.

The genetics of vertebrate myogenesis

The molecular, genetic and cellular bases for skeletal muscle growth and regeneration have been recently documented in a number of vertebrate species. These studies highlight the role of transient subcompartments of the early somite as a source of distinct waves of myogenic precursors. Individual myogenic progenitor populations undergo a complex series of cell rearrangements and specification events in different regions of the body, all of which are controlled by distinct gene regulatory networks. Collectively, these studies have opened a window into the morphogenetic and molecular bases of the different phases of vertebrate myogenesis, from embryo to adult.

Pax7 activates myogenic genes by recruitment of a histone methyltransferase complex

Satellite cells purified from adult skeletal muscle can participate extensively in muscle regeneration and can also re-populate the satellite cell pool, suggesting that they have direct therapeutic potential for treating degenerative muscle diseases. The paired-box transcription factor Pax7 is required for satellite cells to generate committed myogenic progenitors. In this study we undertook a multi-level approach to define the role of Pax7 in satellite cell function. Using comparative microarray analysis, we identified several novel and strongly regulated targets; in particular, we identified Myf5 as a gene whose expression was regulated by Pax7. Using siRNA, fluorescence-activated cell sorting (FACS) and chromatin immunoprecipitation (ChIP) studies we confirmed that Myf5 is directly regulated by Pax7 in myoblasts derived from satellite cells. Tandem affinity purification (TAP) and mass spectrometry were used to purify Pax7 together with its co-factors. This revealed that Pax7 associates with the Wdr5-Ash2L-MLL2 histone methyltransferase (HMT) complex that directs methylation of histone H3 lysine 4 (H3K4, refs 4-10). Binding of the Pax7-HMT complex to Myf5 resulted in H3K4 tri-methylation of surrounding chromatin. Thus, Pax7 induces chromatin modifications that stimulate transcriptional activation of target genes to regulate entry into the myogenic developmental programme.