Activation of different myogenic pathways: myf-5 is induced by the neural tube and MyoD by the dorsal ectoderm in mouse paraxial mesoderm

Actin-organizing protein palladin modulates C2C12 cell fate determination

Background

Cell confluency and serum deprivation promote the transition of C2C12 myoblasts into myocytes and subsequence fusion into myotubes. However, despite all myoblasts undergoing the same serum deprivation trigger, their responses vary: whether they become founder myocytes, remain proliferative, or evolve into fusion-competent myocytes remains unclear. We have previously shown that depletion of the scaffolding protein palladin in myoblasts inhibits cell migration and promotes premature muscle differentiation, pointing to its potential significance in muscle development and the necessity for a more in-depth examination of its function in cellular heterogeneity.

Methods and results

Here, we showed that the subcellular localization of palladin might contribute to founder-fate cell decision in the early differentiation process. Depleting palladin in C2C12 myoblasts depleted integrin-β3 plasma membrane localization of and focal adhesion formation at the early stage of myogenesis, decreased kindlin-2 and metavinculin expression during the myotube maturation process, leading to the inability of myocytes to fuse into preexisting mature myotubes. This aligns with previous findings where early differentiation into nascent myotubes occurred but compromised maturation. In contrast, wildtype C2C12 overexpressing the 140-kDa palladin isoform developed a polarized morphology with star-like structures toward other myoblasts. However, this behaviour was not observed in palladin-depleted cells, where the 140-kDa palladin overexpression could not recover cell migration capacity, suggesting other palladin isoforms are also needed to establish cell polarity.

Conclusion

Our study identifies a counter-intuitive role for palladin in regulating myoblast-to-myocyte cell fate decisions and impacting their ability to form mature multinucleated myotubes by influencing cell signalling pathways and cytoskeletal organization, necessary for skeletal muscle regeneration and repair studies.

L-arginine alters myogenic genes expression but does not affect breast muscle characteristics by in ovo feeding technique in slow-growing chickens

In ovo feeding (IOF) of nutrients is a viable method for increasing muscle mass through hyperplasia and hypertrophy. The objective of this study was to evaluate the effects of IOF of L-arginine (Arg) on breast muscle weight, muscle morphology, amino acid profile, and gene expression of muscle development in slow-growing chickens. Four hundred eighty fertilized eggs were randomly divided into two groups: the first group was the non-injected control group, and the second group was the Arg group, injected with 1% Arg (0.5 mL) into the amnion on day 18 of incubation. After hatching, 160 birds from each group were randomly divided into four replicates of 40 birds each. This experiment lasted for 63 days. The results showed that IOF of Arg did not affect (P > 0.05) breast muscle weight, muscle morphology, and mRNA expression of mammalian target of rapamycin (mTOR) signaling pathway in slow-growing chickens. However, the amino acid profile of breast muscle was altered (P < 0.05) on the day of hatching (DOH), day 21 (D21), and day 42 (D42) post-hatch, respectively. Myogenic factor 5 (Myf5) mRNA expression was upregulated (P < 0.05) on D21 post-hatch. Myogenic regulator 4 (MRF4) mRNA expression was increased (P < 0.05) on DOH. And myogenin (MyoG) was increased (P < 0.05) on DOH and D21 post-hatch, in the Arg group compared to the control group. Overall, IOF of 1% Arg improved the expression of myogenic genes but did not influence muscle morphology and BMW. These results indicate that in ovo Arg dosage (0.5 mL/egg) has no adverse effect on breast muscle development of slow-growing chickens.

Metabolic Contributions of Wnt Signaling: More Than Controlling Flight

The canonical Wnt signaling pathway is ubiquitous throughout the body and influences a diverse array of physiological processes. Following the initial discovery of the Wnt signaling pathway during wing development in Drosophila melanogaster, it is now widely appreciated that active Wnt signaling in mammals is necessary for the development and growth of various tissues involved in whole-body metabolism, such as brain, liver, pancreas, muscle, and adipose. Moreover, elegant gain- and loss-of-function studies have dissected the tissue-specific roles of various downstream effector molecules in the regulation of energy homeostasis. This review attempts to highlight and summarize the contributions of the Wnt signaling pathway and its downstream effectors on whole-body metabolism and their influence on the development of metabolic diseases, such as diabetes and obesity. A better understanding of the Wnt signaling pathway in these tissues may aid in guiding the development of future therapeutics to treat metabolic diseases.

Engineering Musculoskeletal Grafts for Multi-Tissue Unit Repair: Lessons From Developmental Biology and Wound Healing

In the musculoskeletal system, bone, tendon, and skeletal muscle integrate and act coordinately as a single multi-tissue unit to facilitate body movement. The development, integration, and maturation of these essential components and their response to injury are vital for conferring efficient locomotion. The highly integrated nature of these components is evident under disease conditions, where rotator cuff tears at the bone-tendon interface have been reported to be associated with distal pathological alterations such as skeletal muscle degeneration and bone loss. To successfully treat musculoskeletal injuries and diseases, it is important to gain deep understanding of the development, integration and maturation of these musculoskeletal tissues along with their interfaces as well as the impact of inflammation on musculoskeletal healing and graft integration. This review highlights the current knowledge of developmental biology and wound healing in the bone-tendon-muscle multi-tissue unit and perspectives of what can be learnt from these biological and pathological processes within the context of musculoskeletal tissue engineering and regenerative medicine. Integrating these knowledge and perspectives can serve as guiding principles to inform the development and engineering of musculoskeletal grafts and other tissue engineering strategies to address challenging musculoskeletal injuries and diseases.

Molecular Interaction of Bone Marrow Adipose Tissue with Energy Metabolism

PURPOSE OF REVIEW

The last decade has seen a resurgence in the study of bone marrow adipose tissue (BMAT) across diverse fields such as metabolism, haematopoiesis, skeletal biology and cancer. Herein, we review the most recent developments of BMAT research in both humans and rodents, including the distinct nature of BMAT; the autocrine, paracrine and endocrine interactions between BMAT and various tissues, both in physiological and pathological scenarios; how these interactions might impact energy metabolism; and the most recent technological advances to quantify BMAT.

RECENT FINDINGS

Though still dwarfed by research into white and brown adipose tissues, BMAT is now recognised as endocrine organ and is attracting increasing attention from biomedical researchers around the globe.

SUMMARY

We are beginning to learn the importance of BMAT both within and beyond the bone, allowing us to better appreciate the role of BMAT in normal physiology and disease.

Making muscle: Morphogenetic movements and molecular mechanisms of myogenesis in Xenopus laevis

Xenopus laevis offers unprecedented access to the intricacies of muscle development. The large, robust embryos make it ideal for manipulations at both the tissue and molecular level. In particular, this model system can be used to fate map early muscle progenitors, visualize cell behaviors associated with somitogenesis, and examine the role of signaling pathways that underlie induction, specification, and differentiation of muscle. Several characteristics that are unique to X. laevis include myogenic waves with distinct gene expression profiles and the late formation of dermomyotome and sclerotome. Furthermore, myogenesis in the metamorphosing frog is biphasic, facilitating regeneration studies. In this review, we describe the morphogenetic movements that shape the somites and discuss signaling and transcriptional regulation during muscle development and regeneration. With recent advances in gene editing tools, X. laevis remains a premier model organism for dissecting the complex mechanisms underlying the specification, cell behaviors, and formation of the musculature system.

Mapping the Chromatin State Dynamics in Myoblasts

Background

Genome-wide mapping reveals chromatin landscapes unique to cell states. Histone marks of regulatory genes involved in cell specification and organ development provide a powerful tool to map regulatory sequences. H3K4me3 marks promoter regions; H3K27me3 marks repressed regions, and Pol II presence indicates active transcription. The presence of both H3K4me3 and H3K27me3 characterize poised sequences, a common characteristic of genes involved in pattern formation during organogenesis.

Results

We used genome-wide profiling for H3K27me3, H3K4me3, and Pol II to map chromatin states in mouse embryonic day 12 forelimbs in wild type (control) and Pitx2-null mutant mice. We compared these data with previous gene expression studies from forelimb Lbx1⁺ migratory myoblasts and correlated Pitx2-dependent expression profiles and chromatin states. During forelimb development, several lineages including myoblast, osteoblast, neurons, angioblasts etc., require synchronized growth to form a functional limb. We identified 125 genes in the developing forelimb that are Pitx2-dependent. Genes involved in muscle specification and cytoskeleton architecture were positively regulated, while genes involved in axonal path finding were poised.

Conclusion

Our results have established histone modification profiles as a useful tool for identifying gene regulatory states in muscle development, and identified the role of Pitx2 in extending the time of myoblast progression, promoting formation of sarcomeric structures, and suppressing attachment of neuronal axons.

DNA Methylation in Skeletal Muscle Stem Cell Specification, Proliferation, and Differentiation

An unresolved and critically important question in skeletal muscle biology is how muscle stem cells initiate and regulate the genetic program during muscle development. Epigenetic dynamics are essential for cellular development and organogenesis in early life and it is becoming increasingly clear that epigenetic remodeling may also be responsible for the cellular adaptations that occur in later life. DNA methylation of cytosine bases within CpG dinucleotide pairs is an important epigenetic modification that reduces gene expression when located within a promoter or enhancer region. Recent advances in the field suggest that epigenetic regulation is essential for skeletal muscle stem cell identity and subsequent cell development. This review summarizes what is currently known about how skeletal muscle stem cells regulate the myogenic program through DNA methylation, discusses a novel role for metabolism in this process, and addresses DNA methylation dynamics in adult skeletal muscle in response to physical activity.

Somitogenesis: From somite to skeletal muscle

Myogenesis is controlled by an elaborate system of extrinsic and intrinsic regulatory mechanisms in all development stages. The aim of this review is to provide an overview of the different stages of myogenesis and muscle differentiation in mammals, starting from somitogenesis and analysis of the different portions that constitute the mature somite. Particular attention was paid to regulatory genes, in addition to mesodermal stem cells, which represent the earliest elements of myogenesis. Finally, the crucial role of growth factors, molecules of vital importance in contractile regulation, hormones and their function in skeletal muscle differentiation, growth and metabolism, and the role played by central nervous system, are discussed.

Cre recombinase-regulated Endothelin1 transgenic mouse lines: novel tools for analysis of embryonic and adult disorders

Endothelin-1 (EDN1) influences both craniofacial and cardiovascular development and a number of adult physiological conditions by binding to one or both of the known endothelin receptors, thus initiating multiple signaling cascades. Animal models containing both conventional and conditional loss of the Edn1 gene have been used to dissect EDN1 function in both embryos and adults. However, while transgenic Edn1 over-expression or targeted genomic insertion of Edn1 has been performed to understand how elevated levels of Edn1 result in or exacerbate disease states, an animal model in which Edn1 over-expression can be achieved in a spatiotemporal-specific manner has not been reported. Here we describe the creation of Edn1 conditional over-expression transgenic mouse lines in which the chicken β-actin promoter and an Edn1 cDNA are separated by a strong stop sequence flanked by loxP sites. In the presence of Cre, the stop cassette is removed, leading to Edn1 expression. Using the Wnt1-Cre strain, in which Cre expression is targeted to the Wnt1-expressing domain of the central nervous system (CNS) from which neural crest cells (NCCs) arise, we show that stable chicken β-actin-Edn1 (CBA-Edn1) transgenic lines with varying EDN1 protein levels develop defects in NCC-derived tissues of the face, though the severity differs between lines. We also show that Edn1 expression can be achieved in other embryonic tissues utilizing other Cre strains, with this expression also resulting in developmental defects. CBA-Edn1 transgenic mice will be useful in investigating diverse aspects of EDN1-mediated-development and disease, including understanding how NCCs achieve and maintain a positional and functional identity and how aberrant EDN1 levels can lead to multiple physiological changes and diseases.

Contrasting roles for MyoD in organizing myogenic promoter structures during embryonic skeletal muscle development

BACKGROUND

Among the complexities of skeletal muscle differentiation is a temporal distinction in the onset of expression of different lineage-specific genes. The lineage-determining factor MyoD is bound to myogenic genes at the onset of differentiation whether gene activation is immediate or delayed. How temporal regulation of differentiation-specific genes is established remains unclear.

RESULTS

Using embryonic tissue, we addressed the molecular differences in the organization of the myogenin and muscle creatine kinase (MCK) gene promoters by examining regulatory factor binding as a function of both time and spatial organization during somitogenesis. At the myogenin promoter, binding of the homeodomain factor Pbx1 coincided with H3 hyperacetylation and was followed by binding of co-activators that modulate chromatin structure. MyoD and myogenin binding occurred subsequently, demonstrating that Pbx1 facilitates chromatin remodeling and modification before myogenic regulatory factor binding. At the same time, the MCK promoter was bound by HDAC2 and MyoD, and activating histone marks were largely absent. The association of HDAC2 and MyoD was confirmed by co-immunoprecipitation, proximity ligation assay (PLA), and sequential ChIP.

CONCLUSIONS

MyoD differentially promotes activated and repressed chromatin structures at myogenic genes early after the onset of skeletal muscle differentiation in the developing mouse embryo.

Stage of perinatal development regulates skeletal muscle mitochondrial biogenesis and myogenic regulatory factor genes with little impact of growth restriction or cross-fostering

Foetal growth restriction impairs skeletal muscle development and adult muscle mitochondrial biogenesis. We hypothesized that key genes involved in muscle development and mitochondrial biogenesis would be altered following uteroplacental insufficiency in rat pups, and improving postnatal nutrition by cross-fostering would ameliorate these deficits. Bilateral uterine vessel ligation (Restricted) or sham (Control) surgery was performed on day 18 of gestation. Males and females were investigated at day 20 of gestation (E20), 1 (PN1), 7 (PN7) and 35 (PN35) days postnatally. A separate cohort of Control and Restricted pups were cross-fostered onto a different Control or Restricted mother and examined at PN7. In both sexes, peroxisome proliferator-activated receptor (PPAR)-γ coactivator-1α (PGC-1α), cytochrome c oxidase subunits 3 and 4 (COX III and IV) and myogenic regulatory factor 4 expression increased from late gestation to postnatal life, whereas mitochondrial transcription factor A, myogenic differentiation 1 (MyoD), myogenin and insulin-like growth factor I (IGF-I) decreased. Foetal growth restriction increased MyoD mRNA in females at PN7, whereas in males IGF-I mRNA was higher at E20 and PN1. Cross-fostering Restricted pups onto a Control mother significantly increased COX III mRNA in males and COX IV mRNA in both sexes above controls with little effect on other genes. Developmental age appears to be a major factor regulating skeletal muscle mitochondrial and developmental genes, with growth restriction and cross-fostering having only subtle effects. It therefore appears that reductions in adult mitochondrial biogenesis markers likely develop after weaning.

In vivo cell tracking of mouse embryonic myoblasts and fast fibers during development

Fast and slow TnI are co-expressed in E11.5 embryos, and fast TnI is present from the very beginning of myogenesis. A novel green fluorescent protein (GFP) reporter mouse lines (FastTnI/GFP lines) that carry the primary and secondary enhancer elements of the mouse fast troponin I (fast TnI), in which reporter expression correlates precisely with distribution of the endogenous fTnI protein was generated. Using the FastTnI/GFP mouse model, we characterized the early myogenic events in mice, analyzing the migration of GFP+ myoblasts, and the formation of primary and secondary myotubes in transgenic embryos. Interestingly, we found that the two contractile fast and slow isoforms of TnI are expressed during the migration of myoblasts from the somites to the limbs and body wall, suggesting that both participate in these events. Since no sarcomeres are present in myoblasts, we speculate that the function of fast TnI in early myogenesis is, like Myosin and Tropomyosin, to participate in cell movement during the initial myogenic stages. genesis

Making skeletal muscle from progenitor and stem cells: development versus regeneration

For locomotion, vertebrate animals use the force generated by contractile skeletal muscles. These muscles form an actin/myosin-based biomachinery that is attached to skeletal elements to affect body movement and maintain posture. The mechanics, physiology, and homeostasis of skeletal muscles in normal and disease states are of significant clinical interest. How muscles originate from progenitors during embryogenesis has attracted considerable attention from developmental biologists. How skeletal muscles regenerate and repair themselves after injury by the use of stem cells is an important process to maintain muscle homeostasis throughout lifetime. In recent years, much progress has been made toward uncovering the origins of myogenic progenitors and stem cells as well as the regulation of these cells during development and regeneration.

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

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

MyoD-expressing progenitors are essential for skeletal myogenesis and satellite cell development

Skeletal myogenesis in the embryo is regulated by the coordinated expression of the MyoD family of muscle regulatory factors (MRFs). MyoD and Myf-5, which are the primary muscle lineage-determining factors, function in a partially redundant manner to establish muscle progenitor cell identity. Previous diphtheria toxin (DTA)-mediated ablation studies showed that MyoD+ progenitors rescue myogenesis in embryos in which Myf-5-expressing cells were targeted for ablation, raising the possibility that the regulative behavior of distinct, MRF-expressing populations explains the functional compensatory activities of these MRFs. Using MyoD(iCre) mice, we show that DTA-mediated ablation of MyoD-expressing cells results in the cessation of myogenesis by embryonic day 12.5 (E12.5), as assayed by myosin heavy chain (MyHC) and Myogenin staining. Importantly, MyoD(iCre/+);R26(DTA/+) embryos exhibited a concomitant loss of Myf-5+ progenitors, indicating that the vast majority of Myf-5+ progenitors express MyoD, a conclusion consistent with immunofluorescence analysis of Myf-5 protein expression in MyoD(iCre) lineage-labeled embryos. Surprisingly, staining for the paired box transcription factor, Pax7, which functions genetically upstream of MyoD in the trunk and is a marker for fetal myoblasts and satellite cell progenitors, was also lost by E12.5. Specific ablation of differentiating skeletal muscle in ACTA1Cre;R26(DTA/+) embryos resulted in comparatively minor effects on MyoD+, Myf-5+ and Pax7+ progenitors, indicating that cell non-autonomous effects are unlikely to explain the rapid loss of myogenic progenitors in MyoD(iCre/+);R26(DTA/+) embryos. We conclude that the vast majority of myogenic cells transit through a MyoD+ state, and that MyoD+ progenitors are essential for myogenesis and stem cell development.

Pericytes in development and pathology of skeletal muscle

Increasing attention is currently devoted to the multiple roles that pericytes (also defined as mural, Rouget, or perivascular cells) may play during angiogenesis, vascular homeostasis, and pathology. Many recent excellent reviews thoroughly address these topics (see below); hence, we will not discuss them in detail here. However, not much is known about origin, heterogeneity, gene expression, and developmental potential of pericytes during fetal and postnatal development. This is likely because of the paucity of markers expressed by pericytes and the absence of truly unique ones. Thus, in vivo identification and ex perspective isolation are challenging and explain the relative little data available in comparison with neighbor but far more characterized cells such as the endothelium. Despite this preliminary knowledge, we will propose that contribution to growing mesoderm tissues may be an important role for pericytes. Thus, their ability to contribute to tissue regeneration may be a consequence of their role in tissue growth. However, in a severely damaged or diseased tissue, acute or chronic inflammation likely results in the production of signaling molecules that are different from those present in developing tissues, thus explaining why pericytes are easily diverted from a regenerative to a fibrotic fate.

Signal-dependent fra-2 regulation in skeletal muscle reserve and satellite cells

Activator protein-1 (AP-1) is a ubiquitous transcription factor that paradoxically also has some tissue-specific functions. In skeletal muscle cells, we document that the AP-1 subunit, Fra-2, is expressed in the resident stem cells (Pax7-positive satellite cells) and also in the analogous undifferentiated ‘reserve' cell population in myogenic cultures, but not in differentiated myofiber nuclei. Silencing of Fra-2 expression enhances the expression of differentiation markers such as muscle creatine kinase and myosin heavy chain, indicating a possible role of Fra-2 in undifferentiated myogenic progenitor cells. We observed that Fra-2 is a target of cytokine-mediated extracellular signal-regulated kinase-1/2 signaling in cultured muscle cells, and extensive mass spectrometry and mutational analysis identified S320 and T322 as regulators of Fra-2 protein stability. Interestingly, Fra-2 S320 phosphorylation occurs transiently in activated satellite cells and is extinguished in myogenin-positive differentiating cells. Thus, cytokine-mediated Fra-2 expression and stabilization is linked to regulation of myogenic progenitor cells having implications for the molecular regulation of adult muscle stem cells and skeletal muscle regeneration.

Itm2a is a Pax3 target gene, expressed at sites of skeletal muscle formation in vivo

The paired-box homeodomain transcription factor Pax3 is a key regulator of the nervous system, neural crest and skeletal muscle development. Despite the important role of this transcription factor, very few direct target genes have been characterized. We show that Itm2a, which encodes a type 2 transmembrane protein, is a direct Pax3 target in vivo, by combining genetic approaches and in vivo chromatin immunoprecipitation assays. We have generated a conditional mutant allele for Itm2a, which is an imprinted gene, by flanking exons 2–4 with loxP sites and inserting an IRESnLacZ reporter in the 3′ UTR of the gene. The LacZ reporter reproduces the expression profile of Itm2a, and allowed us to further characterize its expression at sites of myogenesis, in the dermomyotome and myotome of somites, and in limb buds, in the mouse embryo. We further show that Itm2a is not only expressed in adult muscle fibres but also in the satellite cells responsible for regeneration. Itm2a mutant mice are viable and fertile with no overt phenotype during skeletal muscle formation or regeneration. Potential compensatory mechanisms are discussed.

Dll4 and PDGF-BB convert committed skeletal myoblasts to pericytes without erasing their myogenic memory

Pericytes are endothelial-associated cells that contribute to vessel wall. Here, we report that pericytes may derive from direct conversion of committed skeletal myoblasts. When exposed to Dll4 and PDGF-BB, but not Dll1, skeletal myoblasts downregulate myogenic genes, except Myf5, and upregulate pericyte markers, whereas inhibition of Notch signaling restores myogenesis. Moreover, when cocultured with endothelial cells, skeletal myoblasts, previously treated with Dll4 and PDGF-BB, adopt a perithelial position stabilizing newly formed vessel-like networks in vitro and in vivo. In a transgenic mouse model in which cells expressing MyoD activate Notch, skeletal myogenesis is abolished and pericyte genes are activated. Even if overexpressed, Myf5 does not trigger myogenesis because Notch induces Id3, partially sequestering Myf5 and inhibiting MEF2 expression. Myf5-expressing cells adopt a perithelial position, as occasionally also observed in wild-type (WT) embryos. These data indicate that endothelium, via Dll4 and PDGF-BB, induces a fate switch in adjacent skeletal myoblasts.

Epigenetic regulation of skeletal muscle development and differentiation

Skeletal muscle cells have served as a paradigm for understanding mechanisms leading to cellular differentiation. Formation of skeletal muscle involves a series of steps in which cells are committed towards the myogenic lineage, undergo expansion to give rise to myoblasts that differentiate into multinucleated myotubes, and mature to form adult muscle fibers. The commitment, proliferation, and differentiation of progenitor cells involve both genetic and epigenetic changes that culminate in alterations in gene expression. Members of the Myogenic regulatory factor (MRF), as well as the Myocyte Enhancer Factor (MEF2) families control distinct steps of skeletal muscle proliferation and differentiation. In addition, -growing evidence indicates that chromatin modifying enzymes and remodeling complexes epigenetically reprogram muscle promoters at various stages that preclude or promote MRF and MEF2 activites. Among these, histone deacetylases (HDACs), histone acetyltransferases (HATs), histone methyltransferases (HMTs) and SWI/SNF complexes alter chromatin structure through post-translational modifications to impact MRF and MEF2 activities. With such new and emerging knowledge, we are beginning to develop a true molecular understanding of the mechanisms by which skeletal muscle development and differentiation is regulated. Elucidation of the mechanisms by which epigenetic regulators control myogenesis will likely provide a new foundation for the development of novel therapeutic drugs for muscle dystrophies, ageing-related regeneration defects that occur due to altered proliferation and differentiation, and other malignancies.

M-cadherin-inhibited phosphorylation of ß-catenin augments differentiation of mouse myoblasts

β-Catenin is essential for muscle development because it regulates both cadherin-mediated cell-cell adhesion and canonical Wingless and Int1 (Wnt) signaling. The phosphorylation of β-catenin by glycogen synthase kinase-3β (GSK-3β) at serine31/37/threonine41 regulates its stability and its role in canonical Wnt signaling. In this study, we have investigated whether the N-terminal phosphorylation of β-catenin is regulated by M-cadherin, and whether this regulation mediates the role of M-cadherin in myogenic differentiation. Our data show that the knockdown of M-cadherin expression by RNA interference (RNAi) in C2C12 myoblasts significantly increases the phosphorylation of β-catenin at Ser33/37/Thr41 and decreases the protein abundance of ser37/thr41-unphosphorylated active β-catenin. Furthermore, M-cadherin RNAi promotes TCF/LEF transcription activity but also blunts the initiation of the myogenic progress by Wnt pathway activator lithium chloride or Wnt-3a treatment. Knockdown of β-catenin expression by RNAi decreases myogenic induction in myoblasts. Forced expression of a phosphorylation-resistant β-catenin plasmid (S33Y-β-catenin) fails to enhance myogenic differentiation, but it partially rescues C2C12 cells from M-cadherin RNAi-induced apoptosis. These data show, for the first time, that M-cadherin-mediated signaling attenuates β-catenin phosphorylation at Ser31/37/Thr41 by GSK-3β, and that this regulation has a positive effect on myogenic differentiation induced by canonical Wnt signaling.

Production of functional classical brown adipocytes from human pluripotent stem cells using specific hemopoietin cocktail without gene transfer

Brown adipose tissue is attracting much attention due to its antiobestic effects; however, its development and involvement in metabolic improvement remain elusive. Here we established a method for a high-efficiency (>90%) differentiation of human pluripotent stem cells (hPSCs) into functional classical brown adipocytes (BAs) using specific hemopoietin cocktail (HC) without exogenous gene transfer. BAs were not generated without HC, and lack of a component of HC induced white adipocyte (WA) marker expressions. hPSC-derived BA (hPSCdBA) showed respiratory and thermogenic activation by β-adrenergic receptor (AdrRβ) stimuli and augmented lipid and glucose tolerance, whereas human multipotent stromal cell-derived WA (hMSCdWA) improved lipid but inhibited glucose metabolism. Cotransplantation of hPSCdBA normalized hMSCdWA-induced glucose intolerance. Surprisingly, hPSCdBAs expressed various hemopoietin genes, serving as stroma for myeloid progenitors. Moreover, AdrRβ stimuli enhanced recovery from chemotherapy-induced myelosuppression. Our study enhances our understanding of BA, identifying roles in metabolic and hemogenic regulation.

Epigenetic regulation of skeletal myogenesis

During embryogenesis a timely and coordinated expression of different subsets of genes drives the formation of skeletal muscles in response to developmental cues. In this review, we will summarize the most recent advances on the "epigenetic network" that promotes the transcription of selective groups of genes in muscle progenitors, through the concerted action of chromatin-associated complexes that modify histone tails and microRNAs (miRNAs). These epigenetic players cooperate to establish focal domains of euchromatin, which facilitates gene transcription, and large portions of heterochromatin, which precludes inappropriate gene expression. We also discuss the analogies and differences in the transcriptional and the epigenetic networks driving developmental and adult myogenesis. The elucidation of the epigenetic basis controlling skeletal myogenesis during development and adult life will facilitate experimental strategies toward generating muscle stem cells, either by reprogramming embryonic stem cells or by inducing pluripotency in adult skeletal muscles. During embryogenesis a timely and coordinated expression of different subsets of genes drives the formation of skeletal muscles in response to developmental cues. In this review, we will summarize the most recent advances on the "epigenetic network" that promotes the transcription of selective groups of genes in muscle progenitors, through the concerted action of chromatin-associated complexes that modify histone tails and microRNAs (miRNAs). These epigenetic players cooperate to establish focal domains of euchromatin, which facilitates gene transcription, and large portions of heterochromatin, which precludes inappropriate gene expression. We also discuss the analogies and differences in the transcriptional and the epigenetic networks driving developmental and adult myogenesis. The elucidation of the epigenetic basis controlling skeletal myogenesis during development and adult life will facilitate experimental strategies toward generating muscle stem cells, either by reprogramming embryonic stem cells or by inducing pluripotency in adult skeletal muscles.

Identification of differentially regulated secretome components during skeletal myogenesis

Myogenesis is a well-characterized program of cellular differentiation that is exquisitely sensitive to the extracellular milieu. Systematic characterization of the myogenic secretome (i.e. the ensemble of secreted proteins) is, therefore, warranted for the identification of novel secretome components that regulate both the pluripotency of these progenitor mesenchymal cells, and also their commitment and passage through the differentiation program. Previously, we have successfully identified 26 secreted proteins in the mouse skeletal muscle cell line C2C12 (1). In an effort to attain a more comprehensive picture of the regulation of myogenesis by its extracellular milieu, quantitative profiling employing stable isotope labeling by amino acids in cell culture was implemented in conjunction with two parallel high throughput online reverse phase liquid chromatography-tandem mass spectrometry systems. In summary, 34 secreted proteins were quantified, 30 of which were shown to be differentially expressed during muscle development. Intriguingly, our analysis has revealed several novel up- and down-regulated secretome components that may have critical biological relevance for both the maintenance of pluripotency and the passage of cells through the differentiation program. In particular, the altered regulation of secretome components, including follistatin-like protein-1, osteoglycin, spondin-2, and cytokine-induced apoptosis inhibitor-1, along with constitutively expressed factors, such as fibulin-2, illustrate dynamic changes in the secretome that take place when differentiation to a specific lineage occurs.

An evolutionarily acquired genotoxic response discriminates MyoD from Myf5, and differentially regulates hypaxial and epaxial myogenesis

Despite having distinct expression patterns and phenotypes in mutant mice, the myogenic regulatory factors Myf5 and MyoD have been considered to be functionally equivalent. Here, we report that these factors have a different response to DNA damage, due to the presence in MyoD and absence in Myf5 of a consensus site for Abl-mediated tyrosine phosphorylation that inhibits MyoD activity in response to DNA damage. Genotoxins failed to repress skeletal myogenesis in MyoD-null embryos; reintroduction of wild-type MyoD, but not mutant Abl phosphorylation-resistant MyoD, restored the DNA-damage-dependent inhibition of muscle differentiation. Conversely, introduction of the Abl-responsive phosphorylation motif converts Myf5 into a DNA-damage-sensitive transcription factor. Gene-dosage-dependent reduction of Abl kinase activity in MyoD-expressing cells attenuated the DNA-damage-dependent inhibition of myogenesis. The presence of a DNA-damage-responsive phosphorylation motif in vertebrate, but not in invertebrate MyoD suggests an evolved response to environmental stress, originated from basic helix-loop-helix gene duplication in vertebrate myogenesis.

Pitx2 defines alternate pathways acting through MyoD during limb and somitic myogenesis

The MyoD gene is part of the core regulatory network that governs skeletal myogenesis and acts as an essential determinant of the myogenic cell fate. Although generic regulatory networks converging on this gene have been described, the specific mechanisms leading to MyoD expression in muscles of different ontology remain misunderstood. We now show that the homeobox gene Pitx2 is required for initial activation of the MyoD gene in limb muscle precursors through direct binding of Pitx2 to the MyoD core enhancer. Whereas Myf5 and Mrf4 are dispensable for limb muscle progenitor fate, inactivation of Myf5 and Mrf4 in Pitx2 mutants results in a drastic decrease of limb MyoD expression. Thus, Pitx2 and Myf5 define parallel genetic pathways for limb myogenesis. We show a similar dependence on Pitx2 and Myf5(Mrf4) in myotome, where MyoD expression is initially activated by Myf5 and Mrf4. In their absence, MyoD expression is eventually rescued by a Pax3-dependent mechanism. We now provide evidence that Pitx2 contributes to the rescue of MyoD expression and that it acts downstream of Pax3. We thus propose that myogenic differentiation of somite-derived muscle cells relies on two parallel genetic pathways, with the Pitx2 pathway being of primary importance for limb myogenesis but the Myf5 and Mrf4 pathway predominating in myotome. Muscle-specific wiring of regulatory networks composed of similar transcription factors thus underlies development of distinct skeletal muscles.

Heterogeneity in the muscle satellite cell population

Satellite cells, the adult stem cells responsible for skeletal muscle regeneration, are defined by their location between the basal lamina and the fiber sarcolemma. Increasing evidence suggests that satellite cells represent a heterogeneous population of cells with distinct embryological origin and multiple levels of biochemical and functional diversity. This review focuses on the rich diversity of the satellite cell population based on studies across species. Ultimately, a more complete characterization of the heterogeneity of satellite cells will be essential to understand the functional significance in terms of muscle growth, homeostasis, tissue repair, and aging.

Impaired Wnt signaling in embryonal rhabdomyosarcoma cells from p53/c-fos double mutant mice

Rhabdomyosarcoma is a primitive neoplasm with a poorly understood etiology that exhibits features of fetal skeletal muscle. It represents the most frequent malignant soft tissue sarcoma affecting the pediatric population and is often treated very aggressively. Embryonal rhabdomyosarcoma (ERMS) and alveolar rhabdomyosarcoma constitute the two major subtypes and exhibit different molecular features. We investigated one potential molecular basis for ERMS by using cells derived from tumors produced in p53(-/-)/c-fos(-/-) mice. This model closely recapitulates the timing, location, molecular markers, and histology seen in human ERMS. A combined chromatin immunoprecipitation/promoter microarray approach was used to identify promoters bound by the c-Jun-containing AP-1 complex in the tumor-derived cells that lacked c-Fos. Identification of the Wnt2 gene and its overexpression in ERMS cells was confirmed in human rhabdomyosarcoma cell lines and prompted further analysis of the Wnt signaling pathway. Contrary to our expectations, the canonical Wnt/β-catenin signaling pathway was down-regulated in ERMS cells compared with normal myoblasts, and activating this pathway promoted myogenic differentiation. Furthermore, the identification of both survivin and sfrp2 through promoter and expression analyses suggested that increased resistance to apoptosis was associated with the inhibition of the Wnt signaling pathway. These results suggest that altered AP-1 activity that leads to the down-regulation of the Wnt pathway may contribute to the inhibition of myogenic differentiation and resistance to apoptosis in ERMS cases.

Myogenic regulatory factors regulate M-cadherin expression by targeting its proximal promoter elements

M- and N-cadherin are members of the Ca(2+)-dependent cell-cell adhesion molecule family. M-cadherin is expressed predominantly in developing skeletal muscles and has been implicated in terminal myogenic differentiation, particularly in myoblast fusion. N-cadherin-mediated cell-cell adhesion also plays an important role in skeletal myogenesis. In the present study, we found that both genes were differentially expressed in C2C12 and Sol8 myoblasts during myogenic differentiation and that the expression of M-cadherin was preferentially enhanced in slow-twitch muscle. Interestingly, most MRFs (myogenic regulatory factors) significantly activated the promoter of M-cadherin, but not that of N-cadherin. In line with this, overexpression of MyoD in C3H10T1/2 fibroblasts strongly induced endogenous M-cadherin expression. Promoter analysis in silico and in vitro identified an E-box (from -2 to +4) abutting the transcription initiation site within the M-cadherin promoter that is bound and differentially activated by different MRFs. The activation of the M-cadherin promoter by MRFs was also modulated by Bhlhe40 (basic helix-loop-helix family member e40). Finally, chromatin immunoprecipitation proved that MyoD as well as myogenin binds to the M-cadherin promoter in vivo. Taken together, these observations identify a molecular mechanism by which MRFs regulate M-cadherin expression directly to ensure the terminal differentiation of myoblasts.

P/CAF rescues the Bhlhe40-mediated repression of MyoD transactivation

Previously, we found that MRFs (myogenic regulatory factors) regulated the expression of PGC-1alpha (peroxisome-proliferator-activated receptor gamma co-activator 1alpha) by targeting a short region, from nt -49 to +2 adjacent to the transcription initiation site, that contained two E-boxes. However, only the E2-box had significant affinity for MRFs, and the E1-box was predicted to be the target of Bhlhe40 (basic helix-loop-helix family, member e40, also known as Stra13, Bhlhb2, DEC1 and Sharp2), a transcriptional repressor implicated in the regulation of several physiological processes. In the present study, by using EMSA (electrophoresis mobility-shift assay), we confirmed that Bhlhe40 targeted the E1-box and formed a complex with the basic helix-loop-helix transcription factor MyoD (myogenic differentiation factor D) on the PGC-1alpha core promoter. We demonstrate that Bhlhe40 binds to the promoters of PGC-1alpha and myogenic genes in vivo and that Bhlhe40 represses the MyoD-mediated transactivation of these promoters. Furthermore, we found that this repression could be relieved by P/CAF (p300/CBP-associated factor) in a dose-dependent manner, but not by CBP [CREB (cAMP-response-element-binding protein)-binding protein]. Bhlhe40 interacted with P/CAF and this interaction disrupted the interaction between P/CAF and MyoD. These results suggest that Bhlhe40 functions as a repressor of MyoD by binding to adjacent E-boxes and sequestering P/CAF from MyoD.

Muscle stem cell behavior is modified by microRNA-27 regulation of Pax3 expression

Skeletal muscle stem cells are regulated by Pax3/7. During development, Pax3 is required for the maintenance of these cells in the somite and their migration to sites of myogenesis; high levels of Pax3 interfere with muscle cell differentiation, both in the embryo and in the adult. Quantitative fine-tuning of Pax3 is critical, and microRNAs provide a potential mechanism. We identify microRNA-27b (miR-27b), which directly targets the 3'-UTR of Pax3 mRNA, as such a regulator. miR-27b is expressed in the differentiating skeletal muscle of the embryonic myotome and in activated satellite cells of adult muscle. In vivo overexpression of a miR-27b transgene in Pax3-positive cells in the embryo leads to down-regulation of Pax3, resulting in interference with progenitor cell migration and in premature differentiation. In a complementary experiment, miR-27b inhibitors were transfected into cultures of adult muscle satellite cells that normally express miR-27b at the onset of differentiation, when Pax3 protein levels undergo rapid down-regulation. Interference with miR-27b function results in continuing Pax3 expression leading to more proliferation and a delay in the onset of differentiation. Pax7 levels are not affected. Introduction of miR-27b antagomirs at a site of muscle injury in vivo also affects Pax3 expression and regeneration in vivo. We therefore conclude that miR-27b regulates Pax3 protein levels and this down-regulation ensures rapid and robust entry into the myogenic differentiation program.

Binding of sFRP-3 to EGF in the extra-cellular space affects proliferation, differentiation and morphogenetic events regulated by the two molecules

Background

sFRP-3 is a soluble antagonist of Wnts, widely expressed in developing embryos. The Wnt gene family comprises cysteine-rich secreted ligands that regulate cell proliferation, differentiation, organogenesis and oncogenesis of different organisms ranging from worms to mammals. In the canonical signal transduction pathway Wnt proteins bind to the extracellular domain of Frizzled receptors and consequently recruit Dishevelled (Dsh) to the cell membrane. In addition to Wnt membrane receptors belonging to the Frizzled family, several other molecules have been described which share homology in the CRD domain and lack the putative trans-membrane domain, such as sFRP molecules (soluble Frizzled Related Protein). Among them, sFRP-3 was originally isolated from bovine articular cartilage and also as a component of the Spemann organizer. sFRP-3 blocks Wnt-8 induced axis duplication in Xenopus embryos and binds to the surface of cells expressing a membrane-anchored form of Wnt-1. Injection of sFRP-3 mRNA blocks expression of XMyoD mRNA and leads to embryos with enlarged heads and shortened trunks.

Methodology/Principal Findings

Here we report that sFRP-3 specifically blocks EGF-induced fibroblast proliferation and foci formation. Over-expression of sFRP-3 reverts EGF-mediated inhibition of hair follicle development in the mouse ectoderm while its ablation in Xenopus maintains EGF-mediated inhibition of ectoderm differentiation. Conversely, over-expression of EGF reverts the inhibition of somitic myogenesis and axis truncation in Xenopus and mouse embryos caused by sFRP-3. In vitro experiments demonstrated a direct binding of EGF to sFRP-3 both on heparin and on the surface of CHO cells where the molecule had been membrane anchored.

Conclusions/Significance

sFRP-3 and EGF reciprocally inhibit their effects on cell proliferation, differentiation and morphogenesis and indeed are expressed in contiguous domains of the embryo, suggesting that in addition to their canonical ligands (Wnt and EGF receptor, respectively) these molecules bind to each other and regulate their activities during embryogenesis.

Beta-catenin interacts with MyoD and regulates its transcription activity

Wnt regulation of muscle development is thought to be mediated by the beta-catenin-TCF/LEF-dependent canonical pathway. Here we demonstrate that beta-catenin, not TCF/LEF, is required for muscle differentiation. We showed that beta-catenin interacts directly with MyoD, a basic helix-loop-helix transcription factor essential for muscle differentiation and enhances its binding to E box elements and transcriptional activity. MyoD-mediated transactivation is inhibited in muscle cells when beta-catenin is deficient or the interaction between MyoD and beta-catenin is disrupted. These results demonstrate that beta-catenin is necessary for MyoD function, identifying MyoD as an effector in the Wnt canonical pathway.

Beta catenin-independent activation of MyoD in presomitic mesoderm requires PKC and depends on Pax3 transcriptional activity

Early activation of myogenesis in the somite depends on signals from surrounding tissues. Canonical beta-catenin dependent Wnt signalling preferentially activates Myf5. We now show, in explant experiments with presomitic mesoderm, that the expression of another myogenic determination factor, MyoD, depends on non-canonical Wnt signalling, probably emanating from the dorsal ectoderm. Inhibitors of PKC block MyoD expression, indicating that the intracellular Wnt pathway depends on this kinase. In the absence of Myf5 and Mrf4, this activation is only minorily affected and we identify Pax3 as the transcriptional mediator responsible for MyoD expression. When embryos expressing a constitutively active form of Pax3, PAX3-FKHR, are used for these studies in the presence of PKC inhibitors, MyoD expression is not affected, suggesting that Wnt signalling acts on the transcriptional activity of Pax3.

The cytoskeleton-associated PDZ-LIM protein, ALP, acts on serum response factor activity to regulate muscle differentiation

In this report, an antisense RNA strategy has allowed us to show that disruption of ALP expression affects the expression of the muscle transcription factors myogenin and MyoD, resulting in the inhibition of muscle differentiation. Introduction of a MyoD expression construct into ALP-antisense cells is sufficient to restore the capacity of the cells to differentiate, illustrating that ALP function occurs upstream of MyoD. It is known that MyoD is under the control of serum response factor (SRF), a transcriptional regulator whose activity is modulated by actin dynamics. A dramatic reduction of actin filament bundles is observed in ALP-antisense cells and treatment of these cells with the actin-stabilizing drug jasplakinolide stimulates SRF activity and restores the capacity of the cells to differentiate. Furthermore, we show that modulation of ALP expression influences SRF activity, the level of its coactivator, MAL, and muscle differentiation. Collectively, these results suggest a critical role of ALP on muscle differentiation, likely via cytoskeletal regulation of SRF.

Amniote somite derivatives

Somites are segments of paraxial mesoderm that give rise to a multitude of tissues in the vertebrate embryo. Many decades of intensive research have provided a wealth of data on the complex molecular interactions leading to the formation of various somitic derivatives. In this review, we focus on the crucial role of the somites in building the body wall and limbs of amniote embryos. We give an overview on the current knowledge on the specification and differentiation of somitic cell lineages leading to the development of the vertebral column, skeletal muscle, connective tissue, meninges, and vessel endothelium, and highlight the importance of the somites in establishing the metameric pattern of the vertebrate body.

The eventful somite: patterning, fate determination and cell division in the somite

The segmental somites not only determine the vertebrate body plan, but also represent turntables of cell fates. The somite is initially naive in terms of its fate restriction as shown by grafting and rotation experiments whereby ectopically grafted or rotated tissue of newly formed somites yielded the same pattern of normal derivatives. Somitic derivatives are determined by local signalling between adjacent embryonic tissues, in particular the neural tube, notochord, surface ectoderm and the somitic compartments themselves. The correct spatio-temporal specification of the deriving tissues, skeletal muscle, cartilage, endothelia and connective tissue is achieved by a sequence of morphogenetic changes of the paraxial mesoderm, eventually leading to the three transitory somitic compartments: dermomyotome, myotome and sclerotome. These structures are specified along a double gradient from dorsal to ventral and from medial to lateral. The establishment and controlled disruption of the epithelial state of the somitic compartments are crucial for development. In this article, we give a synopsis of some of the most important signalling events involved in somite patterning and cell fate decisions. Particular emphasis has been laid on the issue of epithelio-mesenchymal transition and different types of cell division in the somite.

Skeletal muscle translocation in vertebrates

It is now over 30 years since Bodo Christ first demonstrated that the musculature of the limb originated from the somites and overturned the then prevailing view that limb muscle develops from a local source. Subsequently, using electron microscopy and histological procedures, Bodo Christ identified that cells of the somites undergo an epithelial to mesenchymal transition which enabled them to move from their paraxial point of origin to distal locations. These studies defined this translocation as one of the major mechanisms allowing myogenic cells to translocate around the body. The other means used to translocate muscle involves the movement of cells as a sheet. The deployment of one of these two mechanisms has been postulated to be involved in the formation of all the hypaxial musculature of the vertebrate body. In this paper we describe the formation of muscles both in the head and in the body, which use a translocatory mechanism during their development. We highlight recent data showing that muscle translocation is a far more complex process than first thought but which in itself can be used as a valuable tool to address questions regarding tissue patterning and development.

Myogenic basic helix-loop-helix proteins regulate the expression of peroxisomal proliferator activated receptor-gamma coactivator-1alpha

Peroxisomal proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha), a transcriptional coactivator, is selectively expressed in slow-twitch fibers in skeletal muscle. Ectopic expression of the PGC-1alpha gene in either a cell or an animal has been shown to promote fast to slow fiber-type switch. The expression of PGC-1alpha in muscle is regulated by myocyte enhancer factor 2 and Forkhead in rhabdomyosarcoma, two transcription factors implicated in terminal muscle differentiation. In this study we found that PGC-1alpha expression was activated during terminal muscle differentiation in both C2C12 and Sol8 myoblasts. Using retrovirus-mediated MyoD overexpression in C3H10T1/2 cells, we also demonstrated that MyoD, the master regulator of terminal differentiation, could activate PGC-1alpha expression in vivo. Our transient transfection results also show that myogenic basic helix-loop-helix (bHLH) proteins, especially MyoD, can activate PGC-1alpha expression by targeting its promoter. Myogenic bHLH protein target sites on PGC-1alpha promoter were localized to a short region (-49 to approximately +2) adjacent to the transcription start site, which contains two putative E boxes. Mutation of either site significantly reduced MyoD-mediated transactivation in the cells, suggesting that both sites are required for myogenic bHLH protein-mediated activation. However, only one site, the E2 box, was directly bound by glutathione-S-transferase-MyoD protein in EMSAs. Our results indicate that myogenic bHLH proteins not only are involved in lineage determination and terminal differentiation, but also are directly implicated in activation of the key fiber-type and metabolic switch gene, PGC-1alpha.

Follistatin induction by nitric oxide through cyclic GMP: a tightly regulated signaling pathway that controls myoblast fusion

The mechanism of skeletal myoblast fusion is not well understood. We show that endogenous nitric oxide (NO) generation is required for myoblast fusion both in embryonic myoblasts and in satellite cells. The effect of NO is concentration and time dependent, being evident only at the onset of differentiation, and direct on the fusion process itself. The action of NO is mediated through a tightly regulated activation of guanylate cyclase and generation of cyclic guanosine monophosphate (cGMP), so much so that deregulation of cGMP signaling leads to a fusion-induced hypertrophy of satellite-derived myotubes and embryonic muscles, and to the acquisition of fusion competence by myogenic precursors in the presomitic mesoderm. NO and cGMP induce expression of follistatin, and this secreted protein mediates their action in myogenesis. These results establish a hitherto unappreciated role of NO and cGMP in regulating myoblast fusion and elucidate their mechanism of action, providing a direct link with follistatin, which is a key player in myogenesis.

Ectodermal Wnt-6 promotes Myf5-dependent avian limb myogenesis

Limb muscles of vertebrates are derived from precursor cells that migrate from the lateral edge of the dermomyotome into the limb bud. Although several signaling molecules have been reported to be involved in the process of limb myogenesis, none of their activities has led to a consolidate idea about the limb myogenic pathway. Particularly, the role of ectodermal signals in limb myogenesis is still obscure. Here, we investigated the role of the ectoderm and ectodermal Wnt-6 during limb muscle development. We found that ectopic expression of Wnt-6 in the limb bud specifically extends the expression domains of Pax3, Paraxis, Myf5, Myogenin, Desmin and Myosin heavy chain (MyHC) but inhibits MyoD expression. Ectoderm removal results in a loss of expression of all of these myogenic markers. We show that Wnt-6 can compensate the absence of the ectoderm by rescuing the expression of Pax3, Paraxis, Myf5, Myogenin, Desmin and MyHC but not MyoD. These results show that, in chick, at least two signals from the limb ectoderm are necessary for muscle development. One of the signals is Wnt-6, which plays a unique role in promoting limb myogenesis via Pax3/Paraxis-Myf5, whereas the other putative signaling pathway involving MyoD expression is negatively regulated by Wnt-6 signaling.

Muscle function and dysfunction in health and disease

Skeletal muscles of the trunk and limbs developmentally originate from the cells of the dermomyotomal compartment of the somite. A wealth of knowledge has been accumulated with regard to understanding the molecular regulation of embryonic skeletal myogenesis. Myogenic induction is controlled through a complex series of spatiotemporal dependent signaling cascades. Secreted signaling molecules from surrounding structures not only initiate the myogenic program, but also influence proliferation and differentiation decisions. The proper coordination of these molecular events is thus critical for the formation of physiologically functional skeletal muscles. Hereditary congenital skeletal muscle defects arise due to genetics lesions in myogenic specific components. Understanding the mechanistic routes of congenital skeletal muscle disease therefore requires a comprehensive knowledge of the developmental system. Ultimately, the application of this knowledge will improve the diagnostic and therapeutic methodologies for such diseases. The aim of this review is to overview our current understanding of skeletal muscle development and associated human congenital diseases.

Cellular and molecular signatures of muscle regeneration: current concepts and controversies in adult myogenesis

Adult skeletal muscle generates force in a controlled and directed manner through the contraction of highly specialized, postmitotic, multinucleated myofibers. Life-long muscle function relies on maintenance and regeneration of myofibers through a highly regulated process beginning with activation of normally quiescent muscle precursor cells and proceeding with formation of proliferating progenitors that fuse to generate differentiated myofibers. In this review, we describe the historical basis and current evidence for the identification of satellite cells as adult muscle stem cells, critically evaluate contributions of other cells to adult myogenesis, and summarize existing data regarding the origins, genetic markers, and molecular regulation of satellite cells in normal, diseased, and aged muscle.

Regulation of vertebrate myotome development by the p38 MAP kinase-MEF2 signaling pathway

Biochemical and cell culture studies have characterized the myocyte enhancer factor 2 (MEF2) transcriptional regulatory proteins as obligatory partners for the myogenic regulatory factors (MRFs) in the differentiation of myogenic cells in culture. However, the role of MEF2 activation in somitic myogenesis has not been fully characterized. Here, we report a critical interaction between the p38 mitogen-activated protein kinase (p38 MAPK) and MEF2 in the developing somite myotome. We document expression of MEF2A and p38 MAPK proteins in the somite of 9.5 dpc mouse embryos concurrent with Myf 5 protein expression. We also observed that abrogation of p38 MAPK signaling blocks MEF2 activation using a MEF2 transgenic 'sensor' mouse. Inhibition of p38 MAPK signaling concurrently inhibited myogenic differentiation in somite cultures and in embryos in vivo using transplacental injection of a p38 inhibitor (SB203580). Finally, we document that commitment to the myogenic lineage is not appreciably affected by p38 MAPK inhibition since the activation of an early marker of myogenic commitment (Myf 5) occurs normally when p38 MAPK signaling is inhibited. Thus, we present novel evidence indicating a crucial role for p38 MAPK signaling to the MEF2 transcriptional regulators during early mammalian somite development and myotome formation.