A role for the myogenic determination gene Myf5 in adult regenerative myogenesis

Chromatin modifications as determinants of muscle stem cell quiescence and chronological aging

The ability to maintain quiescence is critical for the long-term maintenance of a functional stem cell pool. To date, the epigenetic and transcriptional characteristics of quiescent stem cells and how they change with age remain largely unknown. In this study, we explore the chromatin features of adult skeletal muscle stem cells, or satellite cells (SCs), which reside predominantly in a quiescent state in fully developed limb muscles of both young and aged mice. Using a ChIP-seq approach to obtain global epigenetic profiles of quiescent SCs (QSCs), we show that QSCs possess a permissive chromatin state in which few genes are epigenetically repressed by Polycomb group (PcG)-mediated histone 3 lysine 27 trimethylation (H3K27me3), and a large number of genes encoding regulators that specify nonmyogenic lineages are demarcated by bivalent domains at their transcription start sites (TSSs). By comparing epigenetic profiles of QSCs from young and old mice, we also provide direct evidence that, with age, epigenetic changes accumulate and may lead to a functional decline in quiescent stem cells. These findings highlight the importance of chromatin mapping in understanding unique features of stem cell identity and stem cell aging.

Transthyretin is a key regulator of myoblast differentiation

Transthyretin (TTR) is a known carrier protein for thyroxine (T₄) and retinol-binding protein in the blood that is primarily synthesized in the liver and choroid plexus of the brain. Herein, we report that the TTR gene is expressed in skeletal muscle tissue and up-regulated during myotube formation in C2C12 cells. TTR silencing (TTR_kd) significantly reduced myogenin expression and myotube formation, whereas myogenin silencing (MYOG_kd) did not have any effect on TTR gene expression. Both TTR_kd and MYOG_kd led to a decrease in calcium channel related genes including Cav1.1, STIM1 and Orai1. A significant decrease in intracellular T₄ uptake during myogenesis was observed in TTR_kd cells. Taken together, the results of this study suggest that TTR initiates myoblast differentiation via affecting expression of the genes involved during early stage of myogenesis and the genes related to calcium channel.

Fetal skeletal muscle progenitors have regenerative capacity after intramuscular engraftment in dystrophin deficient mice

Muscle satellite cells (SCs) are stem cells that reside in skeletal muscles and contribute to regeneration upon muscle injury. SCs arise from skeletal muscle progenitors expressing transcription factors Pax3 and/or Pax7 during embryogenesis in mice. However, it is unclear whether these fetal progenitors possess regenerative ability when transplanted in adult muscle. Here we address this question by investigating whether fetal skeletal muscle progenitors (FMPs) isolated from Pax3^GFP/+ embryos have the capacity to regenerate muscle after engraftment into Dystrophin-deficient mice, a model of Duchenne muscular dystrophy. The capacity of FMPs to engraft and enter the myogenic program in regenerating muscle was compared with that of SCs derived from adult Pax3^GFP/+ mice. Transplanted FMPs contributed to the reconstitution of damaged myofibers in Dystrophin-deficient mice. However, despite FMPs and SCs having similar myogenic ability in culture, the regenerative ability of FMPs was less than that of SCs in vivo. FMPs that had activated MyoD engrafted more efficiently to regenerate myofibers than MyoD-negative FMPs. Transcriptome and surface marker analyses of these cells suggest the importance of myogenic priming for the efficient myogenic engraftment. Our findings suggest the regenerative capability of FMPs in the context of muscle repair and cell therapy for degenerative muscle disease.

Embryonic founders of adult muscle stem cells are primed by the determination gene Mrf4

Skeletal muscle satellite cells play a critical role during muscle growth, homoeostasis and regeneration. Selective induction of the muscle determination genes Myf5, Myod and Mrf4 during prenatal development can potentially impact on the reported functional heterogeneity of adult satellite cells. Accordingly, expression of Myf5 was reported to diminish the self-renewal potential of the majority of satellite cells. In contrast, virtually all adult satellite cells showed antecedence of Myod activity. Here we examine the priming of myogenic cells by Mrf4 throughout development. Using a Cre-lox based genetic strategy and novel highly sensitive Pax7 reporter alleles compared to the ubiquitous Rosa26-based reporters, we show that all adult satellite cells, independently of their anatomical location or embryonic origin, have been primed for Mrf4 expression. Given that Mrf4Cre and Mrf4nlacZ are active exclusively in progenitors during embryogenesis, whereas later expression is restricted to differentiated myogenic cells, our findings suggest that adult satellite cells emerge from embryonic founder cells in which the Mrf4 locus was activated. Therefore, this level of myogenic priming by induction of Mrf4, does not compromise the potential of the founder cells to assume an upstream muscle stem cell state. We propose that embryonic myogenic cells and the majority of adult muscle stem cells form a lineage continuum.

Satellite cells and the muscle stem cell niche

Adult skeletal muscle in mammals is a stable tissue under normal circumstances but has remarkable ability to repair after injury. Skeletal muscle regeneration is a highly orchestrated process involving the activation of various cellular and molecular responses. As skeletal muscle stem cells, satellite cells play an indispensible role in this process. The self-renewing proliferation of satellite cells not only maintains the stem cell population but also provides numerous myogenic cells, which proliferate, differentiate, fuse, and lead to new myofiber formation and reconstitution of a functional contractile apparatus. The complex behavior of satellite cells during skeletal muscle regeneration is tightly regulated through the dynamic interplay between intrinsic factors within satellite cells and extrinsic factors constituting the muscle stem cell niche/microenvironment. For the last half century, the advance of molecular biology, cell biology, and genetics has greatly improved our understanding of skeletal muscle biology. Here, we review some recent advances, with focuses on functions of satellite cells and their niche during the process of skeletal muscle regeneration.

Six1 regulates stem cell repair potential and self-renewal during skeletal muscle regeneration

Six1 in satellite cells is important for muscle regeneration and homeostasis of the stem cell niche by regulating MyoD, Myogenin, and Dusp6-ERK signaling.

Satellite cells (SCs) are stem cells that mediate skeletal muscle growth and regeneration. Here, we observe that adult quiescent SCs and their activated descendants expressed the homeodomain transcription factor Six1. Genetic disruption of Six1 specifically in adult SCs impaired myogenic cell differentiation, impaired myofiber repair during regeneration, and perturbed homeostasis of the stem cell niche, as indicated by an increase in SC self-renewal. Six1 regulated the expression of the myogenic regulatory factors MyoD and Myogenin, but not Myf5, which suggests that Six1 acts on divergent genetic networks in the embryo and in the adult. Moreover, we demonstrate that Six1 regulates the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway during regeneration via direct control of Dusp6 transcription. Muscles lacking Dusp6 were able to regenerate properly but showed a marked increase in SC number after regeneration. We conclude that Six1 homeoproteins act as a rheostat system to ensure proper regeneration of the tissue and replenishment of the stem cell pool during the events that follow skeletal muscle trauma.

Identification of Map4k4 as a novel suppressor of skeletal muscle differentiation

Myoblast differentiation into mature myotubes is a critical step in the development and repair of human skeletal muscle. Here we show that small interfering RNA (siRNA)-based silencing of the Ste20-like mitogen-activated protein 4 kinase 4 (Map4k4) in C2C12 myoblasts markedly enhances expression of myogenic differentiation genes, myoblast fusion, and myotube diameter. In contrast, adenovirus-mediated expression of native Map4k4 in C2C12 cells attenuates each of these processes, indicating that Map4k4 is a negative regulator of myogenic differentiation and hypertrophy. Expression of a Map4k4 kinase-inactive mutant enhances myotube formation, suggesting that the kinase activity of Map4k4 is essential for its inhibition of muscle differentiation. Map4k4 regulation of myogenesis is unlikely to be mediated by classic mitogen-activated protein kinase (MAPK) signaling pathways, because no significant difference in phosphorylation of extracellular signal-regulated kinase (ERK), p38, or c-Jun N-terminal kinase (JNK) is observed in Map4k4-silenced cells. Furthermore, silencing of these other MAPKs does not result in a hypertrophic myotube phenotype like that seen with Map4k4 depletion. Uniquely, Map4k4 silencing upregulates the expression of the myogenic regulatory factor Myf5, whose depletion inhibits myogenesis. Furthermore, Myf5 is required for enhancement of myotube formation in Map4k4-silenced cells, while Myf5 overexpression rescues Map4k4-mediated inhibition of myogenic differentiation. These results demonstrate that Map4k4 is a novel suppressor of skeletal muscle differentiation, acting through a Myf5-dependent mechanism.

Donor satellite cell engraftment is significantly augmented when the host niche is preserved and endogenous satellite cells are incapacitated

Stem cell transplantation is already in clinical practice for certain genetic diseases and is a promising therapy for dystrophic muscle. We used the mdx mouse model of Duchenne muscular dystrophy to investigate the effect of the host satellite cell niche on the contribution of donor muscle stem cells (satellite cells) to muscle regeneration. We found that incapacitation of the host satellite cells and preservation of the muscle niche promote donor satellite cell contribution to muscle regeneration and functional reconstitution of the satellite cell compartment. But, if the host niche is not promptly refilled, or is filled by competent host satellite cells, it becomes nonfunctional and donor engraftment is negligible. Application of this regimen to aged host muscles also promotes efficient regeneration from aged donor satellite cells. In contrast, if the niche is destroyed, yet host satellite cells remain proliferation-competent, donor-derived engraftment is trivial. Thus preservation of the satellite cell niche, concomitant with functional impairment of the majority of satellite cells within dystrophic human muscles, may improve the efficiency of stem cell therapy.

Possible correlation between selenoprotein W and myogenic regulatory factors in chicken embryonic myoblasts

The biological function of selenium (Se) is mainly elicited through Se-containing proteins. Selenoprotein W (SelW), one member of the selenoprotein family, is essential for the normal function of the skeletal muscle system. To investigate the possible relationship of Se in the process of differentiation in chicken myoblasts and the expression of SelW, the cultured chicken embryonic myoblasts were incubated with sodium selenite at different concentrations for 72 h, and then the mRNA levels of SelW and myogenic regulatory factors (MRFs) in myoblasts were determined at 12, 24, 48, and 72 h, respectively. Furthermore, the correlation between SelW mRNA expression and MRF mRNA expression was assessed. The results showed that the sodium selenite medium enhanced the mRNA expression of SelW, Myf-5, MRF4, and myogenin in chicken myoblasts. The mRNA expression levels of MRFs were significantly correlated with those of SelW at 24, 48, and 72 h. These data demonstrate that Se is involved in the differentiation of chicken embryonic myoblasts, and SelW showed correlation with MRFs.

Muscle satellite cells are primed for myogenesis but maintain quiescence with sequestration of Myf5 mRNA targeted by microRNA-31 in mRNP granules

Regeneration of adult tissues depends on stem cells that are primed to enter a differentiation program, while remaining quiescent. How these two characteristics can be reconciled is exemplified by skeletal muscle in which the majority of quiescent satellite cells transcribe the myogenic determination gene Myf5, without activating the myogenic program. We show that Myf5 mRNA, together with microRNA-31, which regulates its translation, is sequestered in mRNP granules present in the quiescent satellite cell. In activated satellite cells, mRNP granules are dissociated, relative levels of miR-31 are reduced, and Myf5 protein accumulates, which initially requires translation, but not transcription. Conditions that promote the continued presence of mRNP granules delay the onset of myogenesis. Manipulation of miR-31 levels affects satellite cell differentiation ex vivo and muscle regeneration in vivo. We therefore propose a model in which posttranscriptional mechanisms hold quiescent stem cells poised to enter a tissue-specific differentiation program.

Compromised genomic integrity impedes muscle growth after Atrx inactivation

ATR-X syndrome is a severe intellectual disability disorder caused by mutations in the ATRX gene. Many ancillary clinical features are attributed to CNS deficiencies, yet most patients have muscle hypotonia, delayed ambulation, or kyphosis, pointing to an underlying skeletal muscle defect. Here, we identified a cell-intrinsic requirement for Atrx in postnatal muscle growth and regeneration in mice. Mice with skeletal muscle-specific Atrx conditional knockout (Atrx cKO mice) were viable, but by 3 weeks of age presented hallmarks of underdeveloped musculature, including kyphosis, 20% reduction in body mass, and 34% reduction in muscle fiber caliber. Atrx cKO mice also demonstrated a marked regeneration deficit that was not due to fewer resident satellite cells or their inability to terminally differentiate. However, activation of Atrx-null satellite cells from isolated muscle fibers resulted in a 9-fold reduction in myoblast expansion, caused by delayed progression through mid to late S phase. While in S phase, Atrx colocalized specifically to late-replicating chromatin, and its loss resulted in rampant signs of genomic instability. These observations support a model in which Atrx maintains chromatin integrity during the rapid developmental growth of a tissue.

Endocrine and metabolic regulation of muscle growth and body composition in cattle

Muscle metabolism (in interaction with other organs and tissues, including adipose tissue) plays an important role in the control of growth and body composition. Muscle ontogenesis has been described in different genotypes of cattle for myofibres, connective tissue and intramuscular depots. The ontogenesis or the action of putatively important factors controlling muscle development (IGF-II expression, IGF receptors, growth hormone (GH) receptor, myostatin, basic fibroblast growth factor, transforming growth factor-β1, insulin and thyroid hormones) has also been studied on bovine foetal muscle samples and satellite cells. The glucose/insulin axis has been specifically studied in both the bovine adipose tissue and heart. Clearly, cattle, like sheep, are mature species at birth based on their muscle characteristics compared to other mammalian or farm animal species. The different myoblast generations have been well characterised in cattle, including the second generation which is liable to be affected by foetal undernutrition at least in sheep. Interesting genotypes, for example, double-muscled genotype, have been characterised by an altered metabolic and endocrine status associated with a reduced fat mass, specific muscle traits and different foetal characteristics. Finally, the recent development of genomics in cattle has allowed the identification of novel genes controlling muscle development during foetal and postnatal life. Generally, a high muscle growth potential is associated with a reduced fat mass and a switch of muscle fibres towards the glycolytic type. The possibility and the practical consequences of manipulating muscle growth and, hence, body composition by nutritional and hormonal factors are discussed for bovines based on our current biological knowledge.

Notch signaling is necessary to maintain quiescence in adult muscle stem cells

Satellite cells (SCs) are myogenic stem cells found in skeletal muscle that function to repair tissue damaged by injury or disease. SCs are quiescent at rest, although the signaling pathways required to maintain quiescence are unknown. Using a transgenic Notch reporter mouse and quantitative reverse-transcription polymerase chain reaction analysis of Notch target genes, we determined that Notch signaling is active in quiescent SCs. SC-specific deletion of recombining binding protein-Jκ (RBP-Jκ), a nuclear factor required for Notch signaling, resulted in the depletion of the SC pool and muscles that lacked any ability to regenerate in response to injury. SC depletion was not due to apoptosis. Rather, RBP-Jκ-deficient SCs spontaneously activate, fail to self-renew, and undergo terminal differentiation. Intriguingly, most of the cells differentiate without first dividing. They then fuse with adjacent myofibers, leading to the gradual disappearance of SCs from the muscle. These results demonstrate the requirement of Notch signaling for the maintenance of the quiescent state and for muscle stem cell homeostasis by the regulation of self-renewal and differentiation, processes that are all critical for normal postnatal myogenesis.

Overexpression of Akt1 enhances adipogenesis and leads to lipoma formation in zebrafish

Background

Obesity is a complex, multifactorial disorder influenced by the interaction of genetic, epigenetic, and environmental factors. Obesity increases the risk of contracting many chronic diseases or metabolic syndrome. Researchers have established several mammalian models of obesity to study its underlying mechanism. However, a lower vertebrate model for conveniently performing drug screening against obesity remains elusive. The specific aim of this study was to create a zebrafish obesity model by over expressing the insulin signaling hub of the Akt1 gene.

Methodology/Principal Findings

Skin oncogenic transformation screening shows that a stable zebrafish transgenic of Tg(krt4Hsa.myrAkt1)^cy18 displays severely obese phenotypes at the adult stage. In Tg(krt4:Hsa.myrAkt1)^cy18, the expression of exogenous human constitutively active Akt1 (myrAkt1) can activate endogenous downstream targets of mTOR, GSK-3α/β, and 70S6K. During the embryonic to larval transitory phase, the specific over expression of myrAkt1 in skin can promote hypertrophic and hyperplastic growth. From 21 hour post-fertilization (hpf) onwards, myrAkt1 transgene was ectopically expressed in several mesenchymal derived tissues. This may be the result of the integration position effect. Tg(krt4:Hsa.myrAkt1)^cy18 caused a rapid increase of body weight, hyperplastic growth of adipocytes, abnormal accumulation of fat tissues, and blood glucose intolerance at the adult stage. Real-time RT-PCR analysis showed the majority of key genes on regulating adipogenesis, adipocytokine, and inflammation are highly upregulated in Tg(krt4:Hsa.myrAkt1)^cy18. In contrast, the myogenesis- and skeletogenesis-related gene transcripts are significantly downregulated in Tg(krt4:Hsa.myrAkt1)^cy18, suggesting that excess adipocyte differentiation occurs at the expense of other mesenchymal derived tissues.

Conclusion/Significance

Collectively, the findings of this study provide direct evidence that Akt1 signaling plays an important role in balancing normal levels of fat tissue in vivo. The obese zebrafish examined in this study could be a new powerful model to screen novel drugs for the treatment of human obesity.

A subpopulation of adult skeletal muscle stem cells retains all template DNA strands after cell division

Satellite cells are adult skeletal muscle stem cells that are quiescent and constitute a poorly defined heterogeneous population. Using transgenic Tg:Pax7-nGFP mice, we show that Pax7-nGFP(Hi) cells are less primed for commitment and have a lower metabolic status and delayed first mitosis compared to Pax7-nGFP(Lo) cells. Pax7-nGFP(Hi) can give rise to Pax7-nGFP(Lo) cells after serial transplantations. Proliferating Pax7-nGFP(Hi) cells exhibit lower metabolic activity, and the majority performs asymmetric DNA segregation during cell division, wherein daughter cells retaining template DNA strands express stem cell markers. Using chromosome orientation-fluorescence in situ hybridization, we demonstrate that all chromatids segregate asymmetrically, whereas Pax7-nGFP(Lo) cells perform random DNA segregation. Therefore, quiescent Pax7-nGFP(Hi) cells represent a reversible dormant stem cell state, and during muscle regeneration, Pax7-nGFP(Hi) cells generate distinct daughter cell fates by asymmetrically segregating template DNA strands to the stem cell. These findings provide major insights into the biology of stem cells that segregate DNA asymmetrically.

Genetic biomarkers for ALS disease in transgenic SOD1(G93A) mice

The pathophysiological mechanisms of both familial and sporadic Amyotrophic Lateral Sclerosis (ALS) are unknown, although growing evidence suggests that skeletal muscle tissue is a primary target of ALS toxicity. Skeletal muscle biopsies were performed on transgenic SOD1^G93A mice, a mouse model of ALS, to determine genetic biomarkers of disease longevity. Mice were anesthetized with isoflurane, and three biopsy samples were obtained per animal at the three main stages of the disease. Transcriptional expression levels of seventeen genes, Ankrd1, Calm1, Col19a1, Fbxo32, Gsr, Impa1, Mef2c, Mt2, Myf5, Myod1, Myog, Nnt, Nogo A, Pax7, Rrad, Sln and Snx10, were tested in each muscle biopsy sample. Total RNA was extracted using TRIzol Reagent according to the manufacturer's protocol, and variations in gene expression were assayed by real-time PCR for all of the samples. The Pearson correlation coefficient was used to determine the linear correlation between transcriptional expression levels throughout disease progression and longevity. Consistent with the results obtained from total skeletal muscle of transgenic SOD1^G93A mice and 74-day-old denervated mice, five genes (Mef2c, Gsr, Col19a1, Calm1 and Snx10) could be considered potential genetic biomarkers of longevity in transgenic SOD1^G93A mice. These results are important because they may lead to the exploration of previously unexamined tissues in the search for new disease biomarkers and even to the application of these findings in human studies.

Heparan sulfate proteoglycans, syndecan-4 and glypican-1, differentially regulate myogenic regulatory transcription factors and paired box 7 expression during turkey satellite cell myogenesis: implications for muscle growth

The heparan sulfate proteoglycans have been shown to play essential roles in the proliferation and differentiation of myogenic satellite cells. Myogenic regulatory factors (MRF) and paired box 7 (Pax7) are essential transcription factors for satellite cell myogenesis. The objective of the current study was to investigate whether the expression of the MRF and Pax7 is, in part, regulated by the heparan sulfate proteoglycans, syndecan-4, and glypican-1, whose expression has been shown to differentially affect satellite cell proliferation and differentiation. To test this objective, small interfering RNA was used to knockdown the gene expression of glypican-1 and syndecan-4. The effect on the expression of MRF and Pax7 was measured at the mRNA level by real-time quantitative PCR. The knockdown of the glypican-1 gene decreased mRNA expression of MyoD, myogenin, MRF4, and Pax7 expression during proliferation and differentiation of turkey satellite cells; whereas knockdown of the syndecan-4 gene increased mRNA expression of MyoD and MRF4 expression during cell proliferation but not during differentiation and had no effect on myogenin and Pax7 expression. These data suggested that the precise expression of the MRF are dependent upon the appropriate expression of glypican-1 and syndecan-4 during satellite cell proliferation and differentiation, and Pax7 expression is influenced by glypican-1.

Myf5 haploinsufficiency reveals distinct cell fate potentials for adult skeletal muscle stem cells

Skeletal muscle stem cell fate in adult mice is regulated by crucial transcription factors, including the determination genes Myf5 and Myod. The precise role of Myf5 in regulating quiescent muscle stem cells has remained elusive. Here we show that most, but not all, quiescent satellite cells express Myf5 protein, but at varying levels, and that resident Myf5 heterozygous muscle stem cells are more primed for myogenic commitment compared with wild-type satellite cells. Paradoxically however, heterotypic transplantation of Myf5 heterozygous cells into regenerating muscles results in higher self-renewal capacity compared with wild-type stem cells, whereas myofibre regenerative capacity is not altered. By contrast, Pax7 haploinsufficiency does not show major modifications by transcriptome analysis. These observations provide a mechanism linking Myf5 levels to muscle stem cell heterogeneity and fate by exposing two distinct and opposing phenotypes associated with Myf5 haploinsufficiency. These findings have important implications for how stem cell fates can be modulated by crucial transcription factors while generating a pool of responsive heterogeneous cells.

The skeletal muscle satellite cell: still young and fascinating at 50

The skeletal muscle satellite cell was first described and named based on its anatomic location between the myofiber plasma and basement membranes. In 1961, two independent studies by Alexander Mauro and Bernard Katz provided the first electron microscopic descriptions of satellite cells in frog and rat muscles. These cells were soon detected in other vertebrates and acquired candidacy as the source of myogenic cells needed for myofiber growth and repair throughout life. Cultures of isolated myofibers and, subsequently, transplantation of single myofibers demonstrated that satellite cells were myogenic progenitors. More recently, satellite cells were redefined as myogenic stem cells given their ability to self-renew in addition to producing differentiated progeny. Identification of distinctively expressed molecular markers, in particular Pax7, has facilitated detection of satellite cells using light microscopy. Notwithstanding the remarkable progress made since the discovery of satellite cells, researchers have looked for alternative cells with myogenic capacity that can potentially be used for whole body cell-based therapy of skeletal muscle. Yet, new studies show that inducible ablation of satellite cells in adult muscle impairs myofiber regeneration. Thus, on the 50th anniversary since its discovery, the satellite cell's indispensable role in muscle repair has been reaffirmed.

The APC/C Ubiquitin Ligase: From Cell Biology to Tumorigenesis

The ubiquitin proteasome system (UPS) is required for normal cell proliferation, vertebrate development, and cancer cell transformation. The UPS consists of multiple proteins that work in concert to target a protein for degradation via the 26S proteasome. Chains of an 8.5-kDa protein called ubiquitin are attached to substrates, thus allowing recognition by the 26S proteasome. Enzymes called ubiquitin ligases or E3s mediate specific attachment to substrates. Although there are over 600 different ubiquitin ligases, the Skp1-Cullin-F-box (SCF) complexes and the anaphase promoting complex/cyclosome (APC/C) are the most studied. SCF involvement in cancer has been known for some time while APC/C's cancer role has recently emerged. In this review we will discuss the importance of APC/C to normal cell proliferation and development, underscoring its possible contribution to transformation. We will also examine the hypothesis that modulating a specific interaction of the APC/C may be therapeutically attractive in specific cancer subtypes. Finally, given that the APC/C pathway is relatively new as a cancer target, therapeutic interventions affecting APC/C activity may be beneficial in cancers that are resistant to classical chemotherapy.

WAT is a functional adipocyte?

In vertebrates, adipose tissue is the main storage site for lipids within specialized lipid-laden mature adipocytes. While many species have evolved cells capable of lipid storage, the adipocyte represents a unique specialized cell involved in fuel storage, endocrine, nervous and immune function. However, the adipocytes are not the only cell type in mammals that can accumulate lipid droplets. The ectopic accumulation of lipid in non-adipose tissues including the liver, skeletal muscle, bone, pancreas, and heart in combination with its excessive accumulation in adipose tissue contributes to metabolic disease. Determining the lipid processing components that are necessary and sufficiently for lipid accumulation in adipose and non-adipose tissues, in addition to endocrine function, will lead to a clearer definition of an adipocyte.

Skeletal muscle satellite cells: background and methods for isolation and analysis in a primary culture system

Repair of adult skeletal muscle depends on satellite cells, myogenic stem cells located between the basal lamina and the plasmalemma of the myofiber. Standardized protocols for the isolation and culture of satellite cells are key tools for understanding cell autonomous and extrinsic factors that regulate their performance. Knowledge gained from such studies can contribute important insights to developing strategies for the improvement of muscle repair following trauma and in muscle wasting disorders. This chapter provides an introduction to satellite cell biology and further describes the basic protocol used in our laboratory to isolate and culture satellite cells from adult skeletal muscle. The cell culture conditions detailed herein support proliferation and differentiation of satellite cell progeny and the development of reserve cells, which are thought to reflect the in vivo self-renewal ability of satellite cells. Additionally, this chapter describes our standard immunostaining protocol that allows the characterization of satellite cell progeny by the temporal expression of characteristic transcription factors and structural proteins associated with different stages of myogenic progression. Although emphasis is given here to the isolation and characterization of satellite cells from mouse hindlimb muscles, the protocols are suitable for other muscle types (such as diaphragm and extraocular muscles) and for muscles from other species, including chicken and rat. Altogether, the basic protocols described are straightforward and facilitate the study of diverse aspects of skeletal muscle stem cells.

Intramuscular adipose is derived from a non-Pax3 lineage and required for efficient regeneration of skeletal muscles

Ectopic accumulation of adipose in the skeletal muscle is associated with muscle wasting, insulin resistance and diabetes. However, the developmental origin of postnatal intramuscular adipose and its interaction with muscle tissue are unclear. We report here that compared to the fast EDL muscles, slow SOL muscles are more enriched with adipogenic progenitors and have higher propensity to form adipose. Using Cre/LoxP mediated lineage tracing in mice, we show that intramuscular adipose in both EDL and SOL muscles is exclusively derived from a Pax3(-) non-myogenic lineage. In contrast, inter-scapular brown adipose is derived from the Pax3(+) lineage. To dissect the interaction between adipose and skeletal muscle tissues, we used Myf5-Cre and aP2-Cre mice in combination with ROSA26-iDTR mice to genetically ablate myogenic and adipogenic cell lineages, respectively. Whereas ablation of the myogenic cell lineage facilitated adipogenic differentiation, ablation of the adipogenic cell lineage surprisingly impaired the regeneration of acutely injured skeletal muscles. These results reveal striking heterogeneity of tissue-specific adipose and a previously unappreciated role of intramuscular adipose in skeletal muscle regeneration.

Leucine limitation regulates myf5 and myoD expression and inhibits myoblast differentiation

Satellite cells are the major pool of muscle stem cells after birth; they represent an important component required to maintain muscle mass and functionality during life. The molecular mechanisms involved in myogenic differentiation are relatively well-known. However, the role of extracellular stimulus in the control of differentiation remains largely unresolved. Notably little is known about the impact of nutrients on this process. Here we have studied the role of leucine, an essential amino acid, in the control of myogenic differentiation. Leucine is a well-known regulator of muscle protein synthesis. It acts not only as a substrate for translation but also as a regulator of gene expression and signaling pathways such as those involving mTOR and GCN2. In this study we demonstrated that the lack of leucine abolishes the differentiation of both C2C12 myoblasts and primary satellite cells. This effect is associated with a modification of the pattern of expression of the myogenic regulatory factors (MRF) myf5 and myoD. We report an up-regulation of myf5 mRNA and a decrease of myoD protein level during leucine starvation. This study demonstrates the importance of a nutrient, leucine, in the control of the myogenic differentiation program.

Fibrosis and adipogenesis originate from a common mesenchymal progenitor in skeletal muscle

Accumulation of adipocytes and collagen type-I-producing cells (fibrosis) is observed in muscular dystrophies. The origin of these cells had been largely unknown, but recently we identified mesenchymal progenitors positive for platelet-derived growth factor receptor alpha (PDGFRα) as the origin of adipocytes in skeletal muscle. However, the origin of muscle fibrosis remains largely unknown. In this study, clonal analyses show that PDGFRα(+) cells also differentiate into collagen type-I-producing cells. In fact, PDGFRα(+) cells accumulated in fibrotic areas of the diaphragm in the mdx mouse, a model of Duchenne muscular dystrophy. Furthermore, mRNA of fibrosis markers was expressed exclusively in the PDGFRα(+) cell fraction in the mdx diaphragm. Importantly, TGF-β isoforms, known as potent profibrotic cytokines, induced expression of markers of fibrosis in PDGFRα(+) cells but not in myogenic cells. Transplantation studies revealed that fibrogenic PDGFRα(+) cells mainly derived from pre-existing PDGFRα(+) cells and that the contribution of PDGFRα(-) cells and circulating cells was limited. These results indicate that mesenchymal progenitors are the main origin of not only fat accumulation but also fibrosis in skeletal muscle.

PPARδ regulates satellite cell proliferation and skeletal muscle regeneration

Peroxisome proliferator-activated receptors (PPARs) are a class of nuclear receptors that play important roles in development and energy metabolism. Whereas PPARδ has been shown to regulate mitochondrial biosynthesis and slow-muscle fiber types, its function in skeletal muscle progenitors (satellite cells) is unknown. Since constitutive mutation of Pparδ leads to embryonic lethality, we sought to address this question by conditional knockout (cKO) of Pparδ using Myf5-Cre/Pparδflox/flox alleles to ablate PPARδ in myogenic progenitor cells. Although Pparδ-cKO mice were born normally and initially displayed no difference in body weight, muscle size or muscle composition, they later developed metabolic syndrome, which manifested as increased body weight and reduced response to glucose challenge at age nine months. Pparδ-cKO mice had 40% fewer satellite cells than their wild-type littermates, and these satellite cells exhibited reduced growth kinetics and proliferation in vitro. Furthermore, regeneration of Pparδ-cKO muscles was impaired after cardiotoxin-induced injury. Gene expression analysis showed reduced expression of the Forkhead box class O transcription factor 1 (FoxO1) gene in Pparδ-cKO muscles under both quiescent and regenerating conditions, suggesting that PPARδ acts through FoxO1 in regulating muscle progenitor cells. These results support a function of PPARδ in regulating skeletal muscle metabolism and insulin sensitivity, and they establish a novel role of PPARδ in muscle progenitor cells and postnatal muscle regeneration.

Myosin heavy chain mRNA isoforms are expressed in two distinct cohorts during C2C12 myogenesis

The regulation of muscle fibre transitions has mainly been studied in vivo using conventional histological or immunohistochemical techniques. In order to investigate the molecular regulation of myosin heavy chain (MyHC) isoform expression in cell culture studies, we first characterised the normal transitions in endogenous expression of the MyHC isoforms and the myogenic regulatory factors during differentiation of C2C12 muscle cells. Interestingly, across the time course of differentiation, MyHC mRNA isoforms were expressed in a distinct temporal pattern as two distinct cohorts, one including MyHC I, embryonic and neonatal, the other including MyHC IIa, IIx and IIb. The pattern of expression suggests a transition in MyHC isoforms, from one cohort to another, occurs during muscle cell differentiation and that these transitions occur independent of nerve innervation. To our knowledge, this is the most comprehensive analysis of in vitro MyHC mRNA isoform transitions and provides important information for studying the regulation of transitions in MyHC isoforms in cell culture systems.

Satellite cells, connective tissue fibroblasts and their interactions are crucial for muscle regeneration

Muscle regeneration requires the coordinated interaction of multiple cell types. Satellite cells have been implicated as the primary stem cell responsible for regenerating muscle, yet the necessity of these cells for regeneration has not been tested. Connective tissue fibroblasts also are likely to play a role in regeneration, as connective tissue fibrosis is a hallmark of regenerating muscle. However, the lack of molecular markers for these fibroblasts has precluded an investigation of their role. Using Tcf4, a newly identified fibroblast marker, and Pax7, a satellite cell marker, we found that after injury satellite cells and fibroblasts rapidly proliferate in close proximity to one another. To test the role of satellite cells and fibroblasts in muscle regeneration in vivo, we created Pax7(CreERT2) and Tcf4(CreERT2) mice and crossed these to R26R(DTA) mice to genetically ablate satellite cells and fibroblasts. Ablation of satellite cells resulted in a complete loss of regenerated muscle, as well as misregulation of fibroblasts and a dramatic increase in connective tissue. Ablation of fibroblasts altered the dynamics of satellite cells, leading to premature satellite cell differentiation, depletion of the early pool of satellite cells, and smaller regenerated myofibers. Thus, we provide direct, genetic evidence that satellite cells are required for muscle regeneration and also identify resident fibroblasts as a novel and vital component of the niche regulating satellite cell expansion during regeneration. Furthermore, we demonstrate that reciprocal interactions between fibroblasts and satellite cells contribute significantly to efficient, effective muscle regeneration.

Pax7-expressing satellite cells are indispensable for adult skeletal muscle regeneration

Distinct cell populations with regenerative capacity have been reported to contribute to myofibres after skeletal muscle injury, including non-satellite cells as well as myogenic satellite cells. However, the relative contribution of these distinct cell types to skeletal muscle repair and homeostasis and the identity of adult muscle stem cells remain unknown. We generated a model for the conditional depletion of satellite cells by expressing a human diphtheria toxin receptor under control of the murine Pax7 locus. Intramuscular injection of diphtheria toxin during muscle homeostasis, or combined with muscle injury caused by myotoxins or exercise, led to a marked loss of muscle tissue and failure to regenerate skeletal muscle. Moreover, the muscle tissue became infiltrated by inflammatory cells and adipocytes. This localised loss of satellite cells was not compensated for endogenously by other cell types, but muscle regeneration was rescued after transplantation of adult Pax7(+) satellite cells alone. These findings indicate that other cell types with regenerative potential depend on the presence of the satellite cell population, and these observations have important implications for myopathic conditions and stem cell-based therapeutic approaches.

Establishment of bipotent progenitor cell clone from rat skeletal muscle

The present study describes the isolation, cloning and characterization of adipogenic progenitor cells from rat skeletal muscle. Among the obtained 10 clones, the most highly adipogenic progenitor, 2G11 cells, were further characterized. In addition to their adipogenicity, 2G11 cells retain myogenic potential as revealed by formation of multinucleated myotubes when co-cultured with myoblasts. 2G11 cells were resistant to an inhibitory effect of basic fibroblast growth factor on adipogenesis, while adipogenesis of widely used preadipogenic cell line, 3T3-L1 cells, was suppressed almost completely by the same treatment. In vivo transplantation experiments revealed that 2G11 cells are able to possess both adipogenicity and myogenicity in vivo. These results indicate the presence of bipotent progenitor cells in rat skeletal muscle, and suggest that such cells may contribute to ectopic fat formation in skeletal muscle.

The expression of myogenic regulatory factors and muscle growth factors in the masticatory muscles of dystrophin-deficient (mdx) mice

The activities of myogenic regulatory factors (MRF) and muscle growth factors increase in muscle that is undergoing regeneration, and may correspond to some specific changes. Little is known about the role of MRFs in masticatory muscles in mdx mice (the model of Duchenne muscular dystrophy) and particularly about their mRNA expression during the process of muscle regeneration. Using Taqman RT-PCR, we examined the mRNA expression of the MRFs myogenin and MyoD1 (myogenic differentiation 1), and of the muscle growth factors myostatin, IGF1 (insulin-like growth factor) and MGF (mechano-growth factor) in the masseter, temporal and tongue masticatory muscles of mdx mice (n = 6 to 10 per group). The myogenin mRNA expression in the mdx masseter and temporal muscle was found to have increased (P < 0.05), whereas the myostatin mRNA expressions in the mdx masseter (P < 0.005) and tongue (P < 0.05) were found to have diminished compared to those for the controls. The IGF and MGF mRNA amounts in the mdx mice remained unchanged. Inside the mdx animal group, gender-related differences in the mRNA expressions were also found. A higher mRNA expression of myogenin and MyoD1 in the mdx massterer and temporal muscles was found in females in comparison to males, and the level of myostatin was higher in the masseter and tongue muscle (P < 0.001 for all comparisons). Similar gender-related differences were also found within the control groups. This study reveals the intermuscular differences in the mRNA expression pattern of myogenin and myostatin in mdx mice. The existence of these differences implies that dystrophinopathy affects the skeletal muscles differentially. The finding of gender-related differences in the mRNA expression of the examined factors may indicate the importance of hormonal influences on muscle regeneration.

Skeletal muscle satellite cells are committed to myogenesis and do not spontaneously adopt nonmyogenic fates

The developmental potential of skeletal muscle stem cells (satellite cells) remains controversial. The authors investigated satellite cell developmental potential in single fiber and clonal cultures derived from MyoD(iCre/+);R26R(EYFP/+) muscle, in which essentially all satellite cells are permanently labeled. Approximately 60% of the clones derived from cells that co-purified with muscle fibers spontaneously underwent adipogenic differentiation. These adipocytes stained with Oil-Red-O and expressed the terminal differentiation markers, adipsin and fatty acid binding protein 4, but did not express EYFP and were therefore not of satellite cell origin. Satellite cells mutant for either MyoD or Myf-5 also maintained myogenic programming in culture and did not adopt an adipogenic fate. Incorporation of additional wash steps prior to muscle fiber plating virtually eliminated the non-myogenic cells but did not reduce the number of adherent Pax7+ satellite cells. More than half of the adipocytes observed in cultures from Tie2-Cre mice were recombined, further demonstrating a non-satellite cell origin. Under adipogenesis-inducing conditions, satellite cells accumulated cytoplasmic lipid but maintained myogenic protein expression and did not fully execute the adipogenic differentiation program, distinguishing them from adipocytes observed in muscle fiber cultures. The authors conclude that skeletal muscle satellite cells are committed to myogenesis and do not spontaneously adopt an adipogenic fate.

Sequential association of myogenic regulatory factors and E proteins at muscle-specific genes

Background

Gene expression in skeletal muscle is controlled by a family of basic helix-loop-helix transcription factors known as the myogenic regulatory factors (MRFs). The MRFs work in conjunction with E proteins to regulate gene expression during myogenesis. However, the precise mechanism by which the MRFs activate gene expression is unclear. In this work, we sought to define the binding profiles of MRFs and E proteins on muscle-specific genes throughout a time course of differentiation.

Results

We performed chromatin immunoprecipitation (ChIP) assays for myogenin, MyoD, Myf5 and E proteins over a time course of C₂C₁₂ differentiation, resulting in several surprising findings. The pattern of recruitment is specific to each promoter tested. The recruitment of E proteins often coincides with the arrival of the MRFs, but the binding profile does not entirely overlap with the MRF binding profiles. We found that E12/E47 is bound to certain promoters during proliferation, but every gene tested is preferentially bound by HEB during differentiation. We also show that MyoD, myogenin and Myf5 have transient roles on each of these promoters during muscle differentiation. We also found that RNA polymerase II occupancy correlates with the transcription profile of these promoters. ChIP sequencing assays confirmed that MyoD, myogenin and Myf5 co-occupy promoters.

Conclusions

Our data reveal the sequential association of MyoD, myogenin, Myf5 and HEB on muscle-specific promoters. These data suggest that each of the MRFs, including Myf5, contribute to gene expression at each of the geness analyzed here.. The dynamic binding profiles observed suggest that MRFs and E proteins are recruited independently to promoters.

Many routes to the same destination: lessons from skeletal muscle development

The development and differentiation of vertebrate skeletal muscle provide an important paradigm to understand the inductive signals and molecular events controlling differentiation of specific cell types. Recent findings show that a core transcriptional network, initiated by the myogenic regulatory factors (MRFs; MYF5, MYOD, myogenin and MRF4), is activated by separate populations of cells in embryos in response to various signalling pathways. This review will highlight how cells from multiple distinct starting points can converge on a common set of regulators to generate skeletal muscle.

A WNT/beta-catenin signaling activator, R-spondin, plays positive regulatory roles during skeletal myogenesis

R-spondins (RSPOs) are a recently characterized family of secreted proteins that activate WNT/β-catenin signaling. In this study, we investigated the potential roles of the RSPO proteins during myogenic differentiation. Overexpression of the Rspo1 gene or administration of recombinant RSPO2 protein enhanced mRNA and protein expression of a basic helix-loop-helix (bHLH) class myogenic determination factor, MYF5, in both C2C12 myoblasts and primary satellite cells, whereas MYOD or PAX7 expression was not affected. RSPOs also promoted myogenic differentiation and induced hypertrophic myotube formation in C2C12 cells. In addition, Rspo2 and Rspo3 gene knockdown by RNA interference significantly compromised MYF5 expression, myogenic differentiation, and myotube formation. Furthermore, Myf5 expression was reduced in the developing limbs of mouse embryos lacking the Rspo2 gene. Finally, we demonstrated that blocking of WNT/β-catenin signaling by DKK1 or a dominant-negative form of TCF4 reversed MYF5 expression, myogenic differentiation, and hypertrophic myotube formation induced by RSPO2, indicating that RSPO2 exerts its activity through the WNT/β-catenin signaling pathway. Our results provide strong evidence that RSPOs are key positive regulators of skeletal myogenesis acting through the WNT/β-catenin signaling pathway.

Origin of vertebrate limb muscle: the role of progenitor and myoblast populations

Muscle development, growth, and regeneration take place throughout vertebrate life. In amniotes, myogenesis takes place in four successive, temporally distinct, although overlapping phases. Understanding how embryonic, fetal, neonatal, and adult muscle are formed from muscle progenitors and committed myoblasts is an area of active research. In this review we examine recent expression, genetic loss-of-function, and genetic lineage studies that have been conducted in the mouse, with a particular focus on limb myogenesis. We synthesize these studies to present a current model of how embryonic, fetal, neonatal, and adult muscle are formed in the limb.

Genetic regulation of skeletal muscle development

During development, skeletal muscles are established in a highly organized manner, which persists throughout life. Molecular and genetic experiments over the last decades have identified many developmental control genes critical for skeletal muscle formation. Developmental studies have shown that skeletal muscles of the body, limb and head have distinct embryonic and cellular origin, and the genetic regulation at work in these domains and during adult myogenesis are starting to be identified. In this review we will summarize the current knowledge on the regulatory circuits that lead to the establishment of skeletal muscle in these different anatomical regions.

Role of Annexin A1 in mouse myoblast cell differentiation

Annexin A1 (ANXA1) is a calcium- and phospholipid-binding protein involved in a broad range of cellular events. This study used molecular and microscopy approaches to explore the role of ANXA1 in mouse myoblast C2C12 cell differentiation. We report that ANXA1 expression increases during differentiation and that the down-regulation of ANXA1 significantly inhibits the differentiation process. ANXA1 is expressed in vivo in both quiescent and activated satellite cells and is highly localized in the cells that migrate in the lumen of regenerating fibers after an acute injury. Endogenous ANXA1 co-localizes with actin fibers at the protruding ends of undifferentiated but not differentiated cells suggesting a role of the protein in cell migration. Furthermore, ANXA1 neutralizing antibody reduces MyHC expression, decreases myotube formation and significantly inhibits cell migration. The data reported here suggest for the first time that ANXA1 plays a role in myogenic differentiation.

Loss of LAP2 alpha delays satellite cell differentiation and affects postnatal fiber-type determination

Lamina-associated polypeptide 2 alpha (LAP2 alpha) is a nucleoplasmic protein implicated in cell cycle regulation through its interaction with A-type lamins and the retinoblastoma protein. Mutations in lamin A/C and LAP2 alpha cause late onset striated muscle diseases, but the molecular mechanisms are poorly understood. To study the role of LAP2 alpha in skeletal muscle function and postnatal tissue homeostasis, we generated complete and muscle-specific LAP2 alpha knockout mice. Whereas overall muscle morphology, function, and regeneration were not detectably affected, the myofiber-associated muscle stem cell pool was increased in complete LAP2 alpha knockout animals. At molecular level, the absence of LAP2 alpha preserved the stem cell-like phenotype of Lap2 alpha(-/-) primary myoblasts and delayed their in vitro differentiation. In addition, loss of LAP2 alpha shifted the myofiber-type ratios of adult slow muscles toward fast fiber types. Conditional Cre-mediated late muscle-specific ablation of LAP2 alpha affected early stages of in vitro myoblast differentiation, and also fiber-type determination, but did not change myofiber-associated stem cell numbers in vivo. Our data demonstrate multiple and distinct functions of LAP2 alpha in muscle stem cell maintenance, early phases of myogenic differentiation, and muscle remodeling.

Dietary lipid levels have a remarkable impact on the expression of growth-related genes in Senegalese sole (Solea senegalensis Kaup)

In Senegalese sole (Solea senegalensis Kaup), growth is negatively correlated to dietary lipid levels. To understand the molecular basis of this effect a molecular toolbox of 12 genes, including fgf6, fst, mstn1, myf5, mrf4, myod1, myod2, myog, myHC, mylc2, igf1r and insr, was developed. The expression profiles of these genes were investigated in white muscle and liver of fish fed with three dietary lipid levels (4%, 12% and 20%). The expression of igf-I and igf-II was also examined. MRFs and myosins were only expressed in the muscle and, except for myf5, the general trend was a decrease in expression with an increase in dietary lipids. Fgf6 was identified for the first time in liver and its expression augmented in hepatic tissues with increasing dietary lipid levels. A similar tendency was observed for mstn1 and igf-I. The opposite was observed for igf1r expression in muscle and liver. Myog, mrf4, mylc2 and igf1r were highly correlated with growth and nutrient utilisation indices. In addition to its practical implications, this work provides a valuable contribution towards our understanding of the genetic networks controlling growth in teleosts.

Impact of treadmill locomotor training on skeletal muscle IGF1 and myogenic regulatory factors in spinal cord injured rats

The objective of this study was to determine the impact of treadmill locomotor training on the expression of insulin-like growth factor I (IGF1) and changes in myogenic regulatory factors (MRFs) in rat soleus muscle following spinal cord injury (SCI). Moderate, midthoracic (T(8)) contusion SCIs were produced using a NYU (New York University) impactor. Animals were randomly assigned to treadmill training or untrained groups. Rats in the training group were trained starting at 1 week after SCI, for either 3 bouts of 20 min over 1.5 days or 10 bouts over 5 days. Five days of treadmill training completely prevented the decrease in soleus fiber size resulting from SCI. In addition, treadmill training triggered increases in IGF1, MGF and IGFBP4 mRNA expression, and a concurrent reduction of IGFBP5 mRNA in skeletal muscle. Locomotor training also caused an increase in markers of muscle regeneration, including small muscle fibers expressing embryonic myosin and Pax7 positive nuclei and increased expression of the MRFs, myogenin and MyoD. We concluded that treadmill locomotor training ameliorated muscle atrophy in moderate contusion SCI rats. Training-induced muscle regeneration and fiber hypertrophy following SCI was associated with an increase in IGF1, an increase in Pax7 positive nuclei, and upregulation of MRFs.

Inefficient skeletal muscle repair in inhibitor of differentiation knockout mice suggests a crucial role for BMP signaling during adult muscle regeneration

The bone morphogenetic protein (BMP) pathway is known to be involved in limb myogenesis during development, but whether it is involved in postnatal muscle regeneration is unclear. We have found that adult inhibitor of differentiation (Id)-mutant (Id1(+/-)Id3(-/-)) mice display delayed and reduced skeletal muscle regeneration after injury compared with either wild-type littermates or Id3-null mice. Immunoblotting of wild-type muscle lysates revealed that, not only were Id1 and Id3 highly upregulated within 24 h after injury, but other upstream components of the BMP pathway were as well, including the BMP receptor type II and phosphorylated Smad1/5/8 (pSmad1/5/8). Inhibition of BMP signaling in injured skeletal muscle by Noggin injection reduced pSmad1/5/8, Id1, and Id3 protein levels. The mouse myoblast-derived cell line C2C12 also expressed Id1, Id3, BMP receptor type II, and pSmad1/5/8 during proliferation, but all were reduced upon differentiation into myotubes. In addition, these cells secreted mature BMP-4, and BMP signaling could be inhibited with exogenous Noggin, causing a reduction in pSmad1/5/8, Id1, and Id3 levels. Confocal immunofluorescence microscopy revealed that activated Pax7(+) myoblasts coexpressed nuclear pSmad1/5/8, Id1, and Id3 in injured mouse skeletal muscle sections. Although we did not observe differences in the numbers of quiescent Pax7(+) satellite cells in adult uninjured hindlimb muscles, we did observe a significant reduction in the number of proliferating Pax7(+) cells in the Id-mutant mice after muscle injury compared with either wild-type or Id3-null mice. These data suggest a model in which BMP signaling regulates Id1 and Id3 in muscle satellite cells, which directs their proper proliferation before terminal myogenic differentiation after skeletal muscle injury in postnatal animals.

Dynamics of muscle fibre growth during postnatal mouse development

Background

Postnatal growth in mouse is rapid, with total skeletal muscle mass increasing several-fold in the first few weeks. Muscle growth can be achieved by either an increase in muscle fibre number or an increase in the size of individual myofibres, or a combination of both. Where myofibre hypertrophy during growth requires the addition of new myonuclei, these are supplied by muscle satellite cells, the resident stem cells of skeletal muscle.

Results

Here, we report on the dynamics of postnatal myofibre growth in the mouse extensor digitorum longus (EDL) muscle, which is essentially composed of fast type II fibres in adult. We found that there was no net gain in myofibre number in the EDL between P7 and P56 (adulthood). However, myofibre cross-sectional area increased by 7.6-fold, and length by 1.9-fold between these ages, resulting in an increase in total myofibre volume of 14.1-fold: showing the extent of myofibre hypertrophy during the postnatal period. To determine how the number of myonuclei changes during this period of intense muscle fibre hypertrophy, we used two complementary mouse models: 3F-nlacZ-E mice express nlacZ only in myonuclei, while Myf5^nlacZ/+ mice have β-galactosidase activity in satellite cells. There was a ~5-fold increase in myonuclear number per myofibre between P3 and P21. Thus myofibre hypertrophy is initially accompanied by a significant addition of myonuclei. Despite this, the estimated myonuclear domain still doubled between P7 and P21 to 9.2 × 10³ μm³. There was no further addition of myonuclei from P21, but myofibre volume continued to increase, resulting in an estimated ~3-fold expansion of the myonuclear domain to 26.5 × 10³ μm³ by P56. We also used our two mouse models to determine the number of satellite cells per myofibre during postnatal growth. Satellite cell number in EDL was initially ~14 satellite cells per myofibre at P7, but then fell to reach the adult level of ~5 by P21.

Conclusions

Postnatal fast muscle fibre type growth is divided into distinct phases in mouse EDL: myofibre hypertrophy is initially supported by a rapid increase in the number of myonuclei, but nuclear addition stops around P21. Since the significant myofibre hypertrophy from P21 to adulthood occurs without the net addition of new myonuclei, a considerable expansion of the myonuclear domain results. Satellite cell numbers are initially stable, but then decrease to reach the adult level by P21. Thus the adult number of both myonuclei and satellite cells is already established by three weeks of postnatal growth in mouse.

DUX4c is up-regulated in FSHD. It induces the MYF5 protein and human myoblast proliferation

Facioscapulohumeral muscular dystrophy (FSHD) is a dominant disease linked to contractions of the D4Z4 repeat array in 4q35. We have previously identified a double homeobox gene (DUX4) within each D4Z4 unit that encodes a transcription factor expressed in FSHD but not control myoblasts. DUX4 and its target genes contribute to the global dysregulation of gene expression observed in FSHD. We have now characterized the homologous DUX4c gene mapped 42 kb centromeric of the D4Z4 repeat array. It encodes a 47-kDa protein with a double homeodomain identical to DUX4 but divergent in the carboxyl-terminal region. DUX4c was detected in primary myoblast extracts by Western blot with a specific antiserum, and was induced upon differentiation. The protein was increased about 2-fold in FSHD versus control myotubes but reached 2-10-fold induction in FSHD muscle biopsies. We have shown by Western blot and by a DNA-binding assay that DUX4c over-expression induced the MYF5 myogenic regulator and its DNA-binding activity. DUX4c might stabilize the MYF5 protein as we detected their interaction by co-immunoprecipitation. In keeping with the known role of Myf5 in myoblast accumulation during mouse muscle regeneration DUX4c over-expression activated proliferation of human primary myoblasts and inhibited their differentiation. Altogether, these results suggested that DUX4c could be involved in muscle regeneration and that changes in its expression could contribute to the FSHD pathology.

A home away from home: challenges and opportunities in engineering in vitro muscle satellite cell niches

Satellite cells are skeletal muscle stem cells with a principal role in postnatal skeletal muscle regeneration. Satellite cells, like many tissue-specific adult stem cells, reside in a quiescent state in an instructive, anatomically defined niche. The satellite cell niche constitutes a distinct membrane-enclosed compartment within the muscle fiber, containing a diversity of biochemical and biophysical signals that influence satellite cell function. A major limitation to the study and clinical utility of satellite cells is that upon removal from the muscle fiber and plating in traditional plastic tissue culture platforms, their muscle stem cell properties are rapidly lost. Clearly, the maintenance of stem cell function is critically dependent on in vivo niche signals, highlighting the need to create novel in vitro microenvironments that allow for the maintenance and propagation of satellite cells while retaining their potential to function as muscle stem cells. Here, we discuss how emerging biomaterials technologies offer great promise for engineering in vitro microenvironments to meet these challenges. In engineered biomaterials, signaling molecules can be presented in a manner that more closely mimics cell-cell and cell-matrix interactions, and matrices can be fabricated with diverse rigidities that approximate in vivo tissues. The development of in vitro microenvironments in which niche features can be systematically modulated will be instrumental not only to future insights into muscle stem cell biology and therapeutic approaches to muscle diseases and muscle wasting with aging, but also will provide a paradigm for the analysis of numerous adult tissue-specific stem cells.

Distinct regulatory cascades govern extraocular and pharyngeal arch muscle progenitor cell fates

Genetic regulatory networks governing skeletal myogenesis in the body are well understood, yet their hierarchical relationships in the head remain unresolved. We show that either Myf5 or Mrf4 is necessary for initiating extraocular myogenesis. Whereas Mrf4 is dispensable for pharyngeal muscle progenitor fate, Tbx1 and Myf5 act synergistically for governing myogenesis in this location. As in the body, Myod acts epistatically to the initiating cascades in the head. Thus, complementary pathways, governed by Pax3 for body, and Tbx1 for pharyngeal muscles, but absent for extraocular muscles, activate the core myogenic network. These diverse muscle progenitors maintain their respective embryonic regulatory signatures in the adult. However, these signatures are not sufficient to ensure the specific muscle phenotypes, since the expected differentiated phenotype is not manifested when satellite cells are engrafted heterotopically. These findings identify novel genetic networks that may provide insights into myopathies which often affect only subsets of muscles.