Pax3 induces differentiation of juvenile skeletal muscle stem cells without transcriptional upregulation of canonical myogenic regulatory factors

An Enzyme-Free Method for Isolation and Expansion of Muscle Stem Cells for Cultivated Meat Applications

Cultivated meat, an alternative to conventional meat, holds great promise in alleviating environmental and ethical concerns. Skeletal muscle stem cell isolation is a critical phase in cultivated meat production, and efficiency is a major determinant in the final differentiated muscle cell yield. The conventional enzymatic dissociation method for cell isolation presents drawbacks, including added costs and the destruction of vital extracellular matrix components. We developed an alternative cell isolation technique, explant cell isolation, to isolate muscle stem cells from muscle tissue. The present protocol yields myogenic cell populations, mainly composed of skeletal muscle stem cells without the use of enzymes, and through a simplified process. Overall, the explant method allows for propagation of cells in their natural environment, preserving intricate cell-cell and cell-matrix interactions, resulting in both economic efficiency and consistent generation of high-quality cells.

Impacts of the 1918 flu on survivors' nutritional status: A double quasi-natural experiment

Robust empirical evidence supports the idea that embryonic and, more generally, intrauterine disruptions induced by the 1918-flu pandemic had long-term consequences on adult health status and other conditions. In this paper we assess the 1918-flu long-term effects not just of in utero exposure but also during infancy and early childhood. A unique set of events that took place in Puerto Rico during 1918-1919 generated conditions of a "double quasi-natural experiment". We exploit these conditions to empirically identify effects of exposure to the 1918 flu pandemic and those of the devastation left by an earthquake-tsunami that struck the island in 1918. Because the earthquake-tsunami affected mostly the Western coast of the island whereas early (in utero and postnatal) exposure to the flu was restricted to those born in the interval 1917-1920, we use geographic variation to identify the effects of the quake and timing of birth variation to identify those of the flu. We benefit from availability of information on markers of nutritional status in a nationally representative sample of individuals aged 75 and older in 2002. We make two contributions. First, unlike most fetal-origins research that singles out early nutritional status as a determinant of adult health, we hypothesize that the 1918 flu damaged the nutritional status of adult survivors who, at the time of the flu, were in utero or infants. Second, we target markers of nutritional status largely set when the adult survivors were infants and young children. Estimates of effects of the pandemic are quite large mostly among females and those who were exposed to the earthquake-tsunami. Impacts of the flu in areas less affected by the earthquake are smaller but do vary by area flu severity. These findings constitute empirical evidence supporting the conjecture that effects of the 1918 flu and/or the earthquake are associated not just with disruption experienced during the fetal period but also postnatally.

Reductive stress impairs myogenic differentiation

Myo-satellite cells regenerate and differentiate into skeletal muscle (SM) after acute or chronic injury. Changes in the redox milieu towards the oxidative arm at the wound site are known to compromise SM regeneration. Recently, we reported that abrogation of Nrf2/antioxidant signaling promotes oxidative stress and impairs SM regeneration in C57/Bl6 mice. Here, we investigated whether the activation of intracellular Nrf2 signaling favors antioxidant transcription and promotes myoblast differentiation. Satellite cell-like C2C12 myoblasts were treated with sulforaphane (SF; 1.0 & 5.0 μM) to activate Nrf2/antioxidant signaling during proliferation and differentiation (i.e. formation of myotubes/myofibers). SF-mediated Nrf2 activation resulted in increased expression of Nrf2-antioxidants (e.g. GCLC and G6PD) and augmented the production of reduced glutathione (GSH) leading to a reductive redox state. Surprisingly, this resulted in significant inhibition of myoblast differentiation, as observed from morphological changes and reduced expression of MyoD, Pax7, and Myh2, due to reductive stress (RS). Furthermore, supplementation of N-acetyl-cysteine (NAC) or GSH-ester or genetic knock-down of Keap1 (using siRNA) also resulted in RS-driven inhibition of differentiation. Interestingly, withdrawing Nrf2 activation rescued differentiation potential and formation of myotubes/myofibers from C2C12 myoblasts. Thus, abrogation of physiological ROS signaling through over-activation of Nrf2 (i.e. RS) and developing RS hampers differentiation of muscle satellite cells.

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Sulforaphane activates Nrf2 and establishes reductive stress (RS) in C2C12 myoblasts.

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RS abolishes basal ROS and significantly impede the differentiation of myoblasts.

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Augmentation of glutathione using pharmacological agents (NAC and GSH-ester) promotes RS and impairs differentiation.

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Precluding RS restores the myoblast differentiation process.

Core Transcription Factors Promote Induction of PAX3-Positive Skeletal Muscle Stem Cells

The use of adult skeletal muscle stem cells (MuSCs) for cell therapy has been attempted for decades, but still encounters considerable difficulties. MuSCs derived from human induced pluripotent stem cells (hiPSCs) are promising candidates for stem cell therapy to treat Duchenne muscular dystrophy (DMD). Here we report that four transcription factors, HEYL, KLF4, MYOD, and PAX3, selected by comprehensive screening of different MuSC populations, enhance the derivation of PAX3-positive myogenic progenitors from fibroblasts and hiPSCs, using medium that promotes the formation of presomitic mesoderm. These induced PAX3-positive cells contribute efficiently to the repair of DMD-damaged myofibers and also reconstitute the MuSC population. These studies demonstrate how a combination of core transcription factors can fine-tune the derivation of MuSCs capable of contributing to the repair of adult skeletal muscle.

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Persistent single MyoD can induce myogenic cells, not muscle stem cells

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The combination of Heyl, Klf4, Pax3, and transient MyoD can induce muscle stem cells

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Induced PAX3⁺ cells revealed incorporation into regenerating myofibers of DMD mice

Bmi1 Suppresses Adipogenesis in the Hematopoietic Stem Cell Niche

Bone marrow stromal cells (BMSCs) that express high levels of stem cell factor (SCF) and CXC chemokine ligand 12 (CXCL12) are one crucial component of the hematopoietic stem cell (HSC) niche. While the secreted factors produced by BMSCs to support HSCs have been well described, little is known regarding the transcriptional regulators controlling the cell fate of BMSCs and thus indirectly maintaining HSCs. BMI1 is a polycomb group protein that regulates HSCs both cell intrinsically and extrinsically, but it is unknown in which cell type and how BMI1 functions to maintain HSCs extrinsically. Here we show that Bmi1 maintains HSCs by preventing adipogenic differentiation of BMSCs. Bmi1 is highly expressed in BMSCs but becomes downregulated upon adipogenic differentiation and during aging. Deleting Bmi1 from BMSCs increased marrow adipocytes, induced HSC quiescence and depletion, and impaired hematopoiesis. We found that BMI1 repressed multiple developmental programs in BMSCs by safeguarding the repressive epigenetic marks histone H2A ubiquitylation and H3 lysine 27 trimethylation. We identified a novel adipogenic program governed by Pax3, which BMI1 repressed in BMSCs. Our results establish Bmi1 as a critical regulator of BMSC cell fate that suppresses marrow adipogenesis to create a supportive niche for HSCs.

Multiple transcription factors mediating the expressional regulation of myosin heavy chain gene involved in the indeterminate muscle growth of fish

Torafugu myosin heavy chain gene, MYH_M2528-1, is specifically expressed in neonatal slow and fast muscle fibers, suggesting its functional role in indeterminate muscle growth in fish. However, the transcriptional regulatory mechanisms of MYH_M2528-1 involved in indeterminate muscle growth in fish remained unknown. We previously isolated a 2100 bp 5'- flanking sequence of torafugu MYH_M2528-1 that showed sufficient promoter activity to allow specific gene expression in neonatal muscle fibers of zebrafish. Here, we examined the cis-regulatory mechanism of 2100 bp 5'-flanking region of torafugu MYH_M2528-1 using deletion-mutation analysis in zebrafish embryo. We discovered that myoblast determining factor (MyoD) binding elements play a key role and participate in the transcriptional regulation of MYH_M2528-1 expression in zebrafish embryos. We further discovered that paired box protein (Pax3) are required for promoting MYH_M2528-1 expression and myocyte enhancer factor-2 (MEF2) binding sites participate in the transcriptional regulation of MYH_M2528-1 expression in slow/fast skeletal muscles. Our study also confirmed that the nuclear factor of activated T-cell (NFAT) binding sites take part in the transcriptional regulation of MYH_M2528-1 expression in slow and fast muscles fiber in relation to indeterminate muscle growth. These results obviously confirmed that multiple cis-elements in the 5'-flanking region of MYH_M2528-1 function in the transcriptional regulation of its expression.

Muscle injury, impaired muscle function and insulin resistance in Chromogranin A-knockout mice

Chromogranin A (CgA) is widely expressed in endocrine and neuroendocrine tissues as well as in the central nervous system. We observed CgA expression (mRNA and protein) in the gastrocnemius (GAS) muscle and found that performance of CgA-deficient Chga-KO mice in treadmill exercise was impaired. Supplementation with CgA in Chga-KO mice restored exercise ability suggesting a novel role for endogenous CgA in skeletal muscle function. Chga-KO mice display (i) lack of exercise-induced stimulation of pAKT, pTBC1D1 and phospho-p38 kinase signaling, (ii) loss of GAS muscle mass, (iii) extensive formation of tubular aggregates (TA), (iv) disorganized cristae architecture in mitochondria, (v) increased expression of the inflammatory cytokines Tnfα, Il6 and Ifnγ, and fibrosis. The impaired maximum running speed and endurance in the treadmill exercise in Chga-KO mice correlated with decreased glucose uptake and glycolysis, defects in glucose oxidation and decreased mitochondrial cytochrome C oxidase activity. The lack of adaptation to endurance training correlated with the lack of stimulation of p38MAPK that is known to mediate the response to tissue damage. As CgA sorts proteins to the regulated secretory pathway, we speculate that lack of CgA could cause misfolding of membrane proteins inducing aggregation of sarcoplasmic reticulum (SR) membranes and formation of tubular aggregates that is observed in Chga-KO mice. In conclusion, CgA deficiency renders the muscle energy deficient, impairs performance in treadmill exercise and prevents regeneration after exercise-induced tissue damage.

Noggin inactivation affects the number and differentiation potential of muscle progenitor cells in vivo

Inactivation of Noggin, a secreted antagonist of Bone Morphogenetic Proteins (BMPs), in mice leads, among others, to severe malformations of the appendicular skeleton and defective skeletal muscle fibers. To determine the molecular basis of the phenotype, we carried out a histomorphological and molecular analysis of developing muscles Noggin(-/-) mice. We show that in 18.5 dpc embryos there is a marked reduction in muscle fiber size and a failure of nuclei migration towards the cell membrane. Molecularly, the absence of Noggin results in an increased BMP signaling in muscle tissue as shown by the increase in SMAD1/5/8 phosphorylation, concomitant with the induction of BMP target genes such as Id1, 2, 3 as well as Msx1. Finally, upon removal of Noggin, the number of mesenchymal Pax7(+) muscle precursor cells is reduced and they are more prone to differentiate into adipocytes in vitro. Thus, our results highlight the importance of Noggin/BMP balance for myogenic commitment of early fetal progenitor cells.

PAX3+ skeletal muscle satellite cells retain long-term self-renewal and proliferation

INTRODUCTION

Different populations of satellite cells (SCs) have been identified, but their functional difference remains unclear.

METHODS

We used cell-surface markers and paired box transcription factor 3 (Pax3)/paired box transcription factor 7 (Pax7) expression to separate SC populations. In addition, self-renewal, proliferation, and differentiation abilities of each population were analyzed.

RESULTS

Pax3⁺ /Pax7^- SCs exhibited higher proliferation ability characterized by forming clusters of myogenic colonies with more self-renewing cells after several passages, while Pax3^- /Pax7⁺ SCs had faster differentiation. The myotubes derived from Pax3⁺ /Pax7^- SCs tended to express slow-myosin heavy chain and exhibited rhythmic contraction, while myotubes originating from Pax3^- /Pax7⁺ SCs primarily formed fast-myosin heavy chains characterized by transitory contraction.

CONCLUSIONS

Pax3⁺ /Pax7^- SCs exhibited the ability of long-term self-renewal and proliferation, whereas Pax3^- /Pax7⁺ SCs demonstrated faster differentiation. Muscle Nerve 54: 943-951, 2016.

PAX3 and PAX7 as upstream regulators of myogenesis

Like other subclasses within the PAX transcription factor family, PAX3 and PAX7 play important roles in the emergence of a number of different tissues during development. PAX3 regulates neural crest and, together with its orthologue PAX7, is also expressed in parts of the central nervous system. In this chapter we will focus on their role in skeletal muscle. Both factors are key regulators of myogenesis where Pax3 plays a major role during early skeletal muscle formation in the embryo while Pax7 predominates during post-natal growth and muscle regeneration in the adult. We review the expression and functions of these factors in the myogenic context. We also discuss mechanistic aspects of PAX3/7 function and modulation of their activity by interaction with other proteins, as well as the post-transcriptional and transcriptional regulation of their expression.

Genome-wide association study identifies new susceptibility loci for adolescent idiopathic scoliosis in Chinese girls

Adolescent idiopathic scoliosis (AIS) is a structural deformity of the spine affecting millions of children. As a complex disease, the genetic aetiology of AIS remains obscure. Here we report the results of a four-stage genome-wide association study (GWAS) conducted in a sample of 4,317 AIS patients and 6,016 controls. Overall, we identify three new susceptibility loci at 1p36.32 near AJAP1 (rs241215, Pcombined=2.95 × 10(-9)), 2q36.1 between PAX3 and EPHA4 (rs13398147, Pcombined=7.59 × 10(-13)) and 18q21.33 near BCL-2 (rs4940576, Pcombined=2.22 × 10(-12)). In addition, we refine a previously reported region associated with AIS at 10q24.32 (rs678741, Pcombined=9.68 × 10(-37)), which suggests LBX1AS1, encoding an antisense transcript of LBX1, might be a functional variant of AIS. This is the first GWAS investigating genetic variants associated with AIS in Chinese population, and the findings provide new insight into the multiple aetiological mechanisms of AIS.

Extended 2D myotube culture recapitulates postnatal fibre type plasticity

BACKGROUND

The traditional problems of performing skeletal muscle cell cultures derived from mammalian or avian species are limited myotube differentiation, and transient myotube persistence which greatly restricts the ability of myotubes to undergo phenotypic maturation. We report here on a major technical breakthrough in the establishment of a simple and effective method of extended porcine myotube cultures (beyond 50 days) in two-dimension (2D) that recapitulates key features of postnatal fibre types.

RESULTS

Primary porcine muscle satellite cells (myoblasts) were isolated from the longissimus dorsi of 4 to 6 weeks old pigs for 2D cultures to optimise myotube formation, improve surface adherence and characterise myotube maturation. Over 95 % of isolated cells were myoblasts as evidenced by the expression of Pax3 and Pax7. Our relatively simple approach, based on modifications of existing surface coating reagents (Maxgel), and of proliferation and differentiation (Ultroser G) media, typically achieved by 5 days of differentiation fusion index of around 80 % manifested in an abundance of discrete myosin heavy chain (MyHC) slow and fast myotubes. There was little deterioration in myotube viability over 50 days, and the efficiency of myotube formation was maintained over seven myoblast passages. Regular spontaneous contractions of myotubes were frequently observed throughout culture. Myotubes in extended cultures were able to undergo phenotypic adaptation in response to different culture media, including the adoption of a dominant postnatal phenotype of fast-glycolytic MyHC 2x and 2b expression by about day 20 of differentiation. Furthermore, fast-glycolytic myotubes coincided with enhanced expression of the putative porcine long intergenic non-coding RNA (linc-MYH), which has recently been shown to be a key coordinator of MyHC 2b expression in vivo.

CONCLUSIONS

Our revised culture protocol allows the efficient differentiation and fusion of porcine myoblasts into myotubes and their prolonged adherence to the culture surface. Furthermore, we are able to recapitulate in 2D the maturation process of myotubes to resemble postnatal fibre types which represent a major technical advance in opening access to the in vitro study of coordinated postnatal muscle gene expression.

Satellite cells: regenerative mechanisms and applicability in muscular dystrophy

The satellite cells are long regarded as heterogeneous cell population, which is intimately linked to the processes of muscular recovery. The heterogeneous cell population may be classified by specific markers. In spite of the significant amount of variation amongst the satellite cell populations, it seems that their activity is tightly bound to the paired box 7 transcription factor expression, which is, therefore, used as a canonical marker for these cells. Muscular dystrophic diseases, such as Duchenne muscular dystrophy, elicit severe tissue injuries leading those patients to display a very specific pattern of muscular recovery abnormalities. There have been works on the application of precursors cells as a therapeutic alternative for Duchenne muscular dystrophy and initial attempts have proven the cells inefficient; however later endeavours have proposed solutions for the experiments improving significantly the results. The presence of a range of satellite cells populations indicates the existence of specific cells with enhanced capability of muscular recovery in afflicted muscles.

Nrf2 deficiency promotes apoptosis and impairs PAX7/MyoD expression in aging skeletal muscle cells

Skeletal muscle redox homeostasis is transcriptionally regulated by nuclear erythroid-2-p45-related factor-2 (Nrf2). We recently demonstrated that age-associated stress impairs Nrf2-ARE (antioxidant-response element) transcriptional signaling. Here, we hypothesize that age-dependent decline or genetic ablation of Nrf2 leads to accelerated apoptosis and skeletal muscle degeneration. Under basal-physiological conditions, disruption of Nrf2 significantly downregulates antioxidants and causes oxidative stress. Surprisingly, Nrf2-null mice had enhanced antioxidant capacity identical to wild-type (WT) upon acute endurance exercise stress (AEES), suggesting activation of Nrf2-independent mechanisms (i.e., PGC1α) against oxidative stress. Analysis of prosurvival pathways in the basal state reveals decreased AKT levels, whereas p-p53, a repressor of AKT, was increased in Nrf2-null vs WT mice. Upon AEES, AKT and p-AKT levels were significantly (p < 0.001) increased (>10-fold) along with profound downregulation of p-p53 (p < 0.01) in Nrf2-null vs WT skeletal muscle, indicating the onset of prosurvival mechanisms to compensate for the loss of Nrf2 signaling. However, we found a decreased stem cell population (PAX7) and MyoD expression (differentiation) along with profound activation of ubiquitin and apoptotic pathways in Nrf2-null vs WT mice upon AEES, suggesting that compensatory prosurvival mechanisms failed to overcome the programmed cell death and degeneration in skeletal muscle. Further, the impaired regeneration was sustained in Nrf2-null vs WT mice after 1 week of post-AEES recovery. In an age-associated oxidative stress condition, ablation of Nrf2 results in induction of apoptosis and impaired muscle regeneration.

Transcriptional targets of Foxd3 in murine ES cells

Understanding gene regulatory networks controlling properties of pluripotent stem cells will facilitate development of stem cell-based therapies. The transcription factor Foxd3 is critical for maintenance of self-renewal, survival, and pluripotency in murine embryonic stem cells (ESCs). Using a conditional deletion of Foxd3 followed by gene expression analyses, we demonstrate that genes required for several developmental processes including embryonic organ development, epithelium development, and epithelial differentiation were misregulated in the absence of Foxd3. Additionally, we identified 6 novel targets of Foxd3 (Sox4, Safb, Sox15, Fosb, Pmaip1 and Smarcd3). Finally, we present data suggesting that Foxd3 functions upstream of genes required for skeletal muscle development. Together, this work provides further evidence that Foxd3 is a critical regulator of murine development through the regulation of lineage specific differentiation.

Silencing Pax3 by shRNA inhibits the proliferation and differentiation of duck (Anas platyrhynchos) myoblasts

The Pax3 gene has been proven to play a crucial role in determining myogenic progenitor cell fate during embryonic myogenesis; however, the molecular role of Pax3 in myoblast development during later stages of myogenesis is unknown. We hypothesized that Pax3 would function in myoblast proliferation and differentiation; therefore, we employed three short hairpin RNAs (shRNAs) (shRNA1, shRNA2, and shRNA3) that target Pax3 to characterize the function of Pax3 in duck myoblast development. The mRNA and protein expression levels of Pax3 in duck myoblasts were detected using real-time PCR and Western blotting. Cell proliferation was assessed using the MTT and BrdU assays, while cell differentiation was assayed using immunofluorescence labeling with a MyoG antibody. Additionally, folic acid (FA), which is a rescue tool, was added into the medium of duck myoblasts to indirectly examine the function of Pax3 on duck myoblast proliferation and differentiation. The results revealed that one of the shRNA vectors, shRNA1, could significantly and stably reduce the expression of Pax3 (P < 0.05). Silencing Pax3 by shRNA1 significantly reduced the proliferation and differentiation of duck myoblasts (P < 0.05) due to downregulated expression of myogenic regulator factors. These trends could be rescued by adding FA; and Pax7, a paralog gene of Pax3, was involved in those processes. Overall, Pax3 had a positive function in duck myoblast proliferation and differentiation by modulating the expression of myogenic regulation factors, and shRNA targeting of Pax3 might be a new approach for understanding the function of Pax3 in the development of diverse tissues.

In vitro indeterminate teleost myogenesis appears to be dependent on Pax3

The zebrafish (Danio rerio) has been used extensively as a model system for developmental studies but, unlike most teleost fish, it grows in a determinate-like manner. A close relative, the giant danio (Devario cf. aequipinnatus), grows indeterminately, displaying both hyperplasia and hypertrophy of skeletal myofibers as an adult. To better understand adult muscle hyperplasia, a postlarval/postnatal process that closely resembles secondary myogenesis during development, we characterized the expression of Pax3/7, c-Met, syndecan-4, Myf5, MyoD1, myogenin, and myostatin during in vitro myogenesis, a technique that allows for the complete progression of myogenic precursor cells to myotubes. Pax7 appears to be expressed only in newly activated MPCs while Pax3 is expressed through most of the myogenic program, as are c-Met and syndecan-4. MyoD1 appears important in all stages of myogenesis, while Myf5 is likely expressed at low to background levels, and myogenin expression is enriched in myotubes. Myostatin, like MyoD1, appears to be ubiquitous at all stages. This is the first comprehensive report of key myogenic factor expression patterns in an indeterminate teleost, one that strongly suggests that Pax3 and/or Myf5 may be involved in the regulation of this paradigm. Further, it validates this species as a model organism for studying adult myogenesis in vitro, especially mechanisms underlying nascent myofiber recruitment.

The origin and fate of muscle satellite cells

Satellite cells represent the primary population of stem cells resident in skeletal muscle. These adult muscle stem cells facilitate the postnatal growth, remodeling, and regeneration of skeletal muscle. Given the remarkable regenerative potential of satellite cells, there is great promise for treatment of muscle pathologies such as the muscular dystrophies with this cell population. Various protocols have been developed which allow for isolation, enrichment, and expansion of satellite cell derived muscle stem cells. However, isolated satellite cells have yet to translate into effective modalities for therapeutic intervention. Broadening our understanding of satellite cells and their niche requirements should improve our in vivo and ex vivo manipulation of these cells to expedite their use for regeneration of diseased muscle. This review explores the fates of satellite cells as determined by their molecular signatures, ontogeny, and niche dependent programming.

Barx homeobox family in muscle development and regeneration

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

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.

Efficient generation of iPS cells from skeletal muscle stem cells

Reprogramming of somatic cells into inducible pluripotent stem cells generally occurs at low efficiency, although what limits reprogramming of particular cell types is poorly understood. Recent data suggest that the differentiation status of the cell targeted for reprogramming may influence its susceptibility to reprogramming as well as the differentiation potential of the induced pluripotent stem (iPS) cells that are derived from it. To assess directly the influence of lineage commitment on iPS cell derivation and differentiation, we evaluated reprogramming in adult stem cell and mature cell populations residing in skeletal muscle. Our data using clonal assays and a second-generation inducible reprogramming system indicate that stem cells found in mouse muscle, including resident satellite cells and mesenchymal progenitors, reprogram with significantly greater efficiency than their more differentiated daughters (myoblasts and fibroblasts). However, in contrast to previous reports, we find no evidence of biased differentiation potential among iPS cells derived from myogenically committed cells. These data support the notion that adult stem cells reprogram more efficiently than terminally differentiated cells, and argue against the suggestion that "epigenetic memory" significantly influences the differentiation potential of iPS cells derived from distinct somatic cell lineages in skeletal muscle.

Identification of telocytes in skeletal muscle interstitium: implication for muscle regeneration

Skeletal muscle interstitium is crucial for regulation of blood flow, passage of substances from capillaries to myocytes and muscle regeneration. We show here, probably, for the first time, the presence of telocytes (TCs), a peculiar type of interstitial (stromal) cells, in rat, mouse and human skeletal muscle. TC features include (as already described in other tissues) a small cell body and very long and thin cell prolongations-telopodes (Tps) with moniliform appearance, dichotomous branching and 3D-network distribution. Transmission electron microscopy (TEM) revealed close vicinity of Tps with nerve endings, capillaries, satellite cells and myocytes, suggesting a TC role in intercellular signalling (via shed vesicles or exosomes). In situ immunolabelling showed that skeletal muscle TCs express c-kit, caveolin-1 and secrete VEGF. The same phenotypic profile was demonstrated in cell cultures. These markers and TEM data differentiate TCs from both satellite cells (e.g. TCs are Pax7 negative) and fibroblasts (which are c-kit negative). We also described non-satellite (resident) progenitor cell niche. In culture, TCs (but not satellite cells) emerge from muscle explants and form networks suggesting a key role in muscle regeneration and repair, at least after trauma.

CD133 positive embryonal rhabdomyosarcoma stem-like cell population is enriched in rhabdospheres

Cancer stem cells (CSCs) have been identified in a number of solid tumors, but not yet in rhabdomyosarcoma (RMS), the most frequently occurring soft tissue tumor in childhood. Hence, the aim of this study was to identify and characterize a CSC population in RMS using a functional approach. We found that embryonal rhabdomyosarcoma (eRMS) cell lines can form rhabdomyosarcoma spheres (short rhabdospheres) in stem cell medium containing defined growth factors over several passages. Using an orthotopic xenograft model, we demonstrate that a 100 fold less sphere cells result in faster tumor growth compared to the adherent population suggesting that CSCs were enriched in the sphere population. Furthermore, stem cell genes such as oct4, nanog, c-myc, pax3 and sox2 are significantly upregulated in rhabdospheres which can be differentiated into multiple lineages such as adipocytes, myocytes and neuronal cells. Surprisingly, gene expression profiles indicate that rhabdospheres show more similarities with neuronal than with hematopoietic or mesenchymal stem cells. Analysis of these profiles identified the known CSC marker CD133 as one of the genes upregulated in rhabdospheres, both on RNA and protein levels. CD133(+) sorted cells were subsequently shown to be more tumorigenic and more resistant to commonly used chemotherapeutics. Using a tissue microarray (TMA) of eRMS patients, we found that high expression of CD133 correlates with poor overall survival. Hence, CD133 could be a prognostic marker for eRMS. These experiments indicate that a CD133(+) CSC population can be enriched from eRMS which might help to develop novel targeted therapies against this pediatric tumor.

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.

Protein kinase D2 is an essential regulator of murine myoblast differentiation

Muscle differentiation is a highly conserved process that occurs through the activation of quiescent satellite cells whose progeny proliferate, differentiate, and fuse to generate new myofibers. A defined pattern of myogenic transcription factors is orchestrated during this process and is regulated via distinct signaling cascades involving various intracellular signaling pathways, including members of the protein kinase C (PKC) family. The protein kinase D (PKD) isoenzymes PKD1, -2, and -3, are prominent downstream targets of PKCs and phospholipase D in various biological systems including mouse and could hence play a role in muscle differentiation. In the present study, we used a mouse myoblast cell line (C2C12) as an in vitro model to investigate the role of PKDs, in particular PKD2, in muscle stem cell differentiation. We show that C2C12 cells express all PKD isoforms with PKD2 being highly expressed. Furthermore, we demonstrate that PKD2 is specifically phosphorylated/activated during the initiation of mouse myoblast differentiation. Selective inhibition of PKCs or PKDs by pharmacological inhibitors blocked myotube formation. Depletion of PKD2 by shRNAs resulted in a marked inhibition of myoblast cell fusion. PKD2-depleted cells exhibit impaired regulation of muscle development-associated genes while the proliferative capacity remains unaltered. Vice versa forced expression of PKD2 increases myoblast differentiation. These findings were confirmed in primary mouse satellite cells where myotube fusion was also decreased upon inhibition of PKDs. Active PKD2 induced transcriptional activation of myocyte enhancer factor 2D and repression of Pax3 transcriptional activity. In conclusion, we identify PKDs, in particular PKD2, as a major mediator of muscle cell differentiation in vitro and thereby as a potential novel target for the modulation of muscle regeneration.