A unique pattern of expression of the four muscle regulatory factor proteins distinguishes somitic from embryonic, fetal and newborn mouse myogenic cells

Inhibition of myotube formation by platelet-derived growth factor subunit B in QM7 cells

OBJECTIVE

The primary objective of this study was to investigate the role and regulatory mechanisms of platelet-derived growth factor subunit B (PDGFB) in muscle differentiation.

METHODS

In this study, a vector for PDGFB was designed and transfected into quail muscle cells to investigate its role and regulatory mechanism during muscle formation. To investigate the inhibitory mechanisms of PDGFB on myogenic differentiation, the mRNA expression levels of various genes and the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK 1/2), both known to regulate muscle development and differentiation were compared.

RESULTS

PDGFB-overexpressed (OE) cells formed morphologically shorter and thinner myotubes and demonstrated a smaller total myotube area than did the control cells. This result was also confirmed at the molecular level by a reduced amount of myosin heavy chain protein in the PDGFB-OE cells. Therefore, PDGFB inhibits the differentiation of muscle cells. Additionally, the expression of myogenin (MYOG) significantly decreased in the PDGFBOE cells on days 2 and 4 compared with that in the control cells. The phosphorylation of ERK 1/2, an upstream protein that inhibits MYOG expression, increased in the PDGFB-OE cells on day 4 compared with that in the control cells. The decreased expression of MYOG in the PDGFB-OE cells increased by inhibition ERK 1/2 phosphorylation.

CONCLUSION

PDGFB may suppress myogenesis by reducing MYOG expression through ERK 1/2 phosphorylation. These findings can help understand muscle differentiation and potentially improve poultry meat production.

Current Research, Industrialization Status, and Future Perspective of Cultured Meat

Expectations for the industrialization of cultured meat are growing due to the increasing support from various sectors, such as the food industry, animal welfare organizations, and consumers, particularly vegetarians, but the progress of industrialization is slower than initially reported. This review analyzes the main issues concerning the industrialization of cultured meat, examines research and media reports on the development of cultured meat to date, and presents the current technology, industrialization level, and prospects for cultured meat. Currently, over 30 countries have companies industrializing cultured meat, and around 200 companies that are developing or industrializing cultured meat have been surveyed globally. By country, the United States has over 50 companies, accounting for more than 20% of the total. Acquiring animal cells, developing cell lines, improving cell proliferation, improving the efficiency of cell differentiation and muscle production, or developing cell culture media, including serum-free media, are the major research themes related to the development of cultured meat. In contrast, the development of devices, such as bioreactors, which are crucial in enabling large-scale production, is relatively understudied, and few of the many companies invested in the development of cultured meat have presented products for sale other than prototypes. In addition, because most information on key technologies is not publicly available, it is not possible to determine the level of technology in the companies, and it is surmised that the technology of cultured meat-related startups is not high. Therefore, further research and development are needed to promote the full-scale industrialization of cultured meat.

The Satellite Cell at 60: The Foundation Years

The resident stem cell for skeletal muscle is the satellite cell. On the 50th anniversary of its discovery in 1961, we described the history of skeletal muscle research and the seminal findings made during the first 20 years in the life of the satellite cell (Scharner and Zammit 2011, doi: 10.1186/2044-5040-1-28). These studies established the satellite cell as the source of myoblasts for growth and regeneration of skeletal muscle. Now on the 60th anniversary, we highlight breakthroughs in the second phase of satellite cell research from 1980 to 2000. These include technical innovations such as isolation of primary satellite cells and viable muscle fibres complete with satellite cells in their niche, together with generation of many useful reagents including genetically modified organisms and antibodies still in use today. New methodologies were combined with description of endogenous satellite cells markers, notably Pax7. Discovery of the muscle regulatory factors Myf5, MyoD, myogenin, and MRF4 in the late 1980s revolutionized understanding of the control of both developmental and regerenative myogenesis. Emergence of genetic lineage markers facilitated identification of satellite cells in situ, and also empowered transplantation studies to examine satellite cell function. Finally, satellite cell heterogeneity and the supportive role of non-satellite cell types in muscle regeneration were described. These major advances in methodology and in understanding satellite cell biology provided further foundations for the dramatic escalation of work on muscle stem cells in the 21st century.

Linkages between changes in the 3D organization of the genome and transcription during myotube differentiation in vitro

Background

The spatial organization of eukaryotic genomes facilitates and reflects the underlying nuclear processes that are occurring in the cell. As such, the spatial organization of a genome represents a window on the genome biology that enables analysis of the nuclear regulatory processes that contribute to mammalian development.

Methods

In this study, Hi-C and RNA-seq were used to capture the genome organization and transcriptome in mouse muscle progenitor cells (C2C12 myoblasts) before and after differentiation to myotubes, in the presence or absence of the cytidine analogue AraC.

Results

We observed significant local and global developmental changes despite high levels of correlation between the myotubes and myoblast genomes. Notably, the genes that exhibited the greatest variation in transcript levels between the different developmental stages were predominately within the euchromatic compartment. There was significant re-structuring and changes in the expression of replication-dependent histone variants within the HIST1 locus. Finally, treating terminally differentiated myotubes with AraC resulted in additional changes to the transcriptome and 3D genome organization of sets of genes that were all involved in pyroptosis.

Conclusions

Collectively, our results provide evidence for muscle cell-specific responses to developmental and environmental stimuli mediated through a chromatin structure mechanism.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-017-0122-1) contains supplementary material, which is available to authorized users.

Carbon Dioxide Water Bathing Enhances Myogenin but Not MyoD Protein Expression after Skeletal Muscle Injury

[Purpose] We reported that carbon dioxide (CO₂) water bathing accelerates skeletal muscle regeneration; however, the underlying mechanism was unclear. MyoD and myogenin play roles in muscle regeneration, and the purpose of this study was to determine changes in MyoD and myogenin caused by CO₂ water bathing after injury. [Subjects] Sixteen female Wistar rats (n = 4 per group) were used. [Methods] The rats were divided into four groups: no-injury (NI), injury (IC), injury + tap water bathing (ITW), and injury + CO₂ water bathing (ICO₂). Muscle injury was induced by injection of bupivacaine hydrochloride into the left tibial anterior (TA) muscles. Tap water and CO₂ (1,000 ppm) water bathing were performed at 37 °C for 30 minutes once a day. The left TA muscles were removed 4 days after injury, and the expressions of MyoD and myogenin were measured. [Results] MyoD and myogenin were increased in the IC, ITW, and ICO₂ groups compared with the NI group. Although the MyoD level was similar in the IC, ITW, and ICO₂ groups, myogenin increased more in the ICO₂ group than in the IC and ITW groups. [Conclusion] CO₂ water bathing after muscle injury appears to induce an increase in the expression of myogenin.

Growth and sex effects on the expression of syndecan-4 and glypican-1 in turkey myogenic satellite cell populations

The adult skeletal muscle stem cells, satellite cells, are responsible for skeletal muscle growth and regeneration. Satellite cells represent a heterogeneous cell population that differentially express cell surface markers. The membrane-associated heparan sulfate proteoglycans, syndecan-4, and glypican-1, are differentially expressed by satellite cells during the proliferation and differentiation stages of satellite cells. However, how the population of syndecan-4- or glypican-1-positive satellite cells changes during proliferation and differentiation, and how sex and muscle growth potential affect the expression of these genes is unknown. Differences in the amount of satellite cells positive for syndecan-4 or glypican-1 would affect the process of proliferation and differentiation which would impact both muscle mass accretion and the regeneration of muscle. In the current study, the percentage of satellite cells positive for syndecan-4 or glypican-1 from male and female turkeys from a Randombred Control Line 2 and a line (F) selected for increased 16-week body weight were measured during proliferation and differentiation. Growth selection altered the population of syndecan-4- and glypican-1-positive satellite cells and there were sex differences in the percentage of syndecan-4- and glypican-1-positive satellite cells. This study provides new information on dynamic changes in syndecan-4- and glypican-1-positive satellite cells showing that they are differentially expressed during myogenesis and growth selection and sex affects their expression.

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.

Generation of skeletal muscle stem/progenitor cells from murine induced pluripotent stem cells

Induced pluripotent stem (iPS) cells, which are a type of pluripotent stem cell generated from reprogrammed somatic cells, are expected to have potential for patient-oriented disease investigation, drug screening, toxicity tests, and transplantation therapies. Here, we demonstrated that murine iPS cells have the potential to develop in vitro into skeletal muscle stem/progenitor cells, which are almost equivalent to murine embryonic stem cells. Cells with strong in vitro myogenic potential effectively were enriched by fluorescence-activated cell sorting using the anti-satellite cell antibody SM/C-2.6. Furthermore, on transplantation into mdx mice, SM/C-2.6(+) cells exerted sustained myogenic lineage differentiation in injured muscles, while providing long-lived muscle stem cell support. Our data suggest that iPS cells have the potential to be used in clinical treatment of muscular dystrophies.

The origin of intermuscular adipose tissue and its pathophysiological implications

The intermuscular adipose tissue (IMAT) is a depot of adipocytes located between muscle bundles. Several investigations have recently been carried out to define the phenotype, the functional characteristics, and the origin of the adipocytes present in this depot. Among the different mechanisms that could be responsible for the accumulation of fat in this site, the dysdifferentiation of muscle-derived stem cells or other mesenchymal progenitors has been postulated, turning them into cells with an adipocyte phenotype. In particular, muscle satellite cells (SCs), a heterogeneous stem cell population characterized by plasticity and self-renewal that allow muscular growth and regeneration, can acquire features of adipocytes, including the abilities to express adipocyte-specific genes and accumulate lipids. Failure to express the transcription factors that direct mesenchymal precursors into fully differentiated functionally specialized cells may be responsible for their phenotypic switch into the adipogenic lineage. We proved that human SCs also possess a clear adipogenic potential that could explain the presence of mature adipocytes within skeletal muscle. This occurs under some pathological conditions (i.e., primary myodystrophies, obesity, hyperglycemia, high plasma free fatty acids, hypoxia, etc.) or as a consequence of thiazolidinedione treatment or simply because of a sedentary lifestyle or during aging. Several pathways and factors (PPARs, WNT growth factors, myokines, GEF-GAP-Rho, p66(shc), mitochondrial ROS production, PKCβ) could be implicated in the adipogenic conversion of SCs. The understanding of the molecular pathways that regulate muscle-to-fat conversion and SC behavior could explain the increase in IMAT depots that characterize many metabolic diseases and age-related sarcopenia.

Ku70 regulates Bax-mediated pathogenesis in laminin-alpha2-deficient human muscle cells and mouse models of congenital muscular dystrophy

The severely debilitating disease Congenital Muscular Dystrophy Type 1A (MDC1A) is caused by mutations in the gene encoding laminin-alpha2. Bax-mediated muscle cell death is a significant contributor to the severe neuromuscular pathology seen in the Lama2-null mouse model of MDC1A. To extend our understanding of pathogenesis due to laminin-alpha2-deficiency, we have now analyzed molecular mechanisms of Bax regulation in normal and laminin-alpha2-deficient muscles and cells, including myogenic cells obtained from patients with a clinical diagnosis of MDC1A. In mouse myogenic cells, we found that, as in non-muscle cells, Bax co-immunoprecipitated with the multifunctional protein Ku70. In addition, cell permeable pentapeptides designed from Ku70, termed Bax-inhibiting peptides (BIPs), inhibited staurosporine-induced Bax translocation and cell death in mouse myogenic cells. We also found that acetylation of Ku70, which can inhibit binding to Bax and can be an indicator of increased susceptibility to cell death, was more abundant in Lama2-null than in normal mouse muscles. Furthermore, myotubes formed in culture from human laminin-alpha2-deficient patient myoblasts produced high levels of activated caspase-3 when grown on poly-L-lysine, but not when grown on a laminin-alpha2-containing substrate or when treated with BIPs. Finally, cytoplasmic Ku70 in human laminin-alpha2-deficient myotubes was both reduced in amount and more highly acetylated than in normal myotubes. Increased susceptibility to cell death thus appears to be an intrinsic property of human laminin-alpha2-deficient myotubes. These results identify Ku70 as a regulator of Bax-mediated pathogenesis and a therapeutic target in laminin-alpha2-deficiency.

A distinct profile of myogenic regulatory factor detection within Pax7+ cells at S phase supports a unique role of Myf5 during posthatch chicken myogenesis

Satellite cells are skeletal muscle stem cells that provide myogenic progeny for myofiber growth and repair. Temporal expression of muscle regulatory factors (MRFs) and the paired box transcription factor Pax7 defines characteristic phases of proliferation (Pax7(+)/MyoD(+)/myogenin(-)) and differentiation (Pax7(-)/MyoD(+)/myogenin(+)) during myogenesis of satellite cells. Here, using bromodeoxyuridine (BrdU) labeling and triple immunodetection, we analyzed expression patterns of Pax7 and the MRFs MyoD, Myf5, or myogenin within S phase myoblasts prepared from posthatch chicken muscle. Essentially, all BrdU incorporation was restricted to Pax7(+) cells, of which the majority also expressed MyoD. The presence of a minor BrdU(+)/Pax7(+)/myogenin(+) population in proliferation stage cultures suggests that myogenin up-regulation is alone insufficient for terminal differentiation. Myf5 was detected strictly within Pax7(+) cells and decreased during S phase while MyoD presence persisted in cycling cells. This study provides novel data in support of a unique role for Myf5 during posthatch myogenesis.

Mechanisms of lipase maturation

Lipases are acyl hydrolases that represent a diverse group of enzymes present in organisms ranging from prokaryotes to humans. This article focuses on an evolutionarily related family of extracellular lipases that include lipoprotein lipase, hepatic lipase and endothelial lipase. As newly synthesized proteins, these lipases undergo a series of co- and post-translational maturation steps occurring in the endoplasmic reticulum, including glycosylation and glycan processing, and protein folding and subunit assembly. This article identifies and discusses mechanisms that direct early and late events in lipase folding and assembly. Lipase maturation employs the two general chaperone systems operating in the endoplasmic reticulum, as well as a recently identified lipase-specific chaperone termed lipase maturation factor 1. We propose that the two general chaperone systems act in a coordinated manner early in lipase maturation in order to help create partially folded monomers; lipase maturation factor 1 then facilitates final monomer folding and subunit assembly into fully functional homodimers. Once maturation is complete, the lipases exit the endoplasmic reticulum and are secreted to extracellular sites, where they carry out a number of functions related to lipoprotein and lipid metabolism.

Generation of transplantable, functional satellite-like cells from mouse embryonic stem cells

Satellite cells are myogenic stem cells responsible for the postnatal regeneration of skeletal muscle. Here we report the successful in vitro induction of Pax7-positive satellite-like cells from mouse embryonic stem (mES) cells. Embryoid bodies were generated from mES cells and cultured on Matrigel-coated dishes with Dulbecco's modified Eagle medium containing fetal bovine serum and horse serum. Pax7-positive satellite-like cells were enriched by fluorescence-activated cell sorting using a novel anti-satellite cell antibody, SM/C-2.6. SM/C-2.6-positive cells efficiently differentiate into skeletal muscle fibers both in vitro and in vivo. Furthermore, the cells demonstrate satellite cell characteristics such as extensive self-renewal capacity in subsequent muscle injury model, long-term engraftment up to 24 wk, and the ability to be secondarily transplanted with remarkably high engraftment efficiency compared to myoblast transplantation. This is the first report of transplantable, functional satellite-like cells derived from mES cells and will provide a foundation for new therapies for degenerative muscle disorders.

Identification and expression profile of a novel alternative splicing of Pax7 in chick skeletal muscle

Pax7 is essential for skeletal muscle myogenesis. The alternative splicing forms of Pax7 were found in human and mouse. In this study, we identified a new alternative splicing of chick Pax7. We named it Pax7-2 and localized it in the nucleus of chick myoblast. In Pax7-2 mRNA, exon 8 of chick Pax7 gene was spliced out. That led to a 22-amino acid deletion in the COOH terminal of Pax7-2 protein compared with Pax7 protein. Luciferase assays demonstrated that chick Pax7 could act as a transactivator and the deleted 22 amino acids in Pax7-2 may belong to the transactivation domain of Pax7. Pax7-2 lost transactivation ability. We detected the expression levels of Pax7-2 in chick pectoralis muscles at different developmental stages and found that the expression of Pax7-2 was at its peak in d-12 chick embryos. The expression levels of Pax7 and Pax-2 in chick pectoralis muscles at different developmental stages had a similar trend across the period under study, although the changes of their expression levels were different. As chicks grew up, Pax7 and Pax7-2 were expressed at much lower levels.

The skeletal muscle satellite cell: the stem cell that came in from the cold

The muscle satellite cell was first described and actually named on the basis of its anatomic location under the basement membrane surrounding each myofiber. For many years following its discovery, electron microscopy provided the only definitive method of identification. More recently, several molecular markers have been described that can be used to detect satellite cells, making them more accessible for study at the light microscope level. Satellite cells supply myonuclei to growing myofibers before becoming mitotically quiescent in muscle as it matures. They are then activated from this quiescent state to fulfill their roles in routine maintenance, hypertrophy, and repair of adult muscle. Because muscle is able to efficiently regenerate after repeated bouts of damage, systems must be in place to maintain a viable satellite cell pool, and it was proposed over 30 years ago that self-renewal was the primary mechanism. Self-renewal entails either a stochastic event or an asymmetrical cell division, where one daughter cell is committed to differentiation whereas the second continues to proliferate or becomes quiescent. This classic model of satellite cell self-renewal and the importance of satellite cells in muscle maintenance and repair have been challenged during the past few years as bone marrow-derived cells and various intramuscular populations were shown to be able to contribute myonuclei and occupy the satellite cell niche. This is a fast-moving and dynamic field, however, and in this review we discuss the evidence that we think puts this enigmatic cell firmly back at the center of adult myogenesis.

Inhibition of mechanosensitive cation channels inhibits myogenic differentiation by suppressing the expression of myogenic regulatory factors and caspase-3 activity

Mechanosensitive cation channels (MSC) are ubiquitous in eukaryotic cell types. However, the physiological functions of MSC in several tissues remain in question. In this study we have investigated the role of MSC in skeletal myogenesis. Treatment of C2C12 myoblasts with gadolinium ions (MSC blocker) inhibited myotube formation and the myogenic index in differentiation medium (DM). The enzymatic activity of creatine kinase (CK) and the expression of myosin heavy chain-fast twitch (MyHCf) in C2C12 cultures were also blocked in response to gadolinium. Treatment of C2C12 myoblasts with gadolinium ions did not affect the expression of either cyclin A or cyclin D1 in DM. Other inhibitors of MSC such as streptomycin and GsTMx-4 also suppressed the expression of CK and MyHCf in C2C12 cultures. The inhibitory effect of gadolinium ions on myogenic differentiation was reversible and independent of myogenic cell type. Real-time-polymerase chain reaction analysis revealed that inhibition of MSC decreases the expression of myogenic transcription factors MyoD, myogenin, and Myf-5. Furthermore, the activity of skeletal alpha-actin promoter was suppressed on MSC blockade. Treatment of C2C12 myoblasts with gadolinium ions prevented differentiation-associated cell death and inhibited the cleavage of poly (ADP-ribose) polymerase and activation of caspase-3. On the other hand, delivery of active caspase-3 protein to C2C12 myoblasts reversed the inhibitory effect of gadolinium ions on myogenesis. Our data suggest that inhibition of MSC suppresses myogenic differentiation by inhibiting the caspase-3 activity and the expression of myogenic regulatory factors.

Sarcopenia is not due to lack of regenerative drive in senescent skeletal muscle

Sarcopenia, loss of skeletal muscle mass, is a hallmark of aging commonly attributed to a decreased capacity to maintain muscle tissue in senescence, yet the mechanism behind the muscle wasting remains unresolved. To address these issues we have explored a rodent model of sarcopenia and age-related sensorimotor impairment, allowing us to discriminate between successfully and unsuccessfully aged cohort members. Immunohistochemistry and staining of cell nuclei revealed that senescent muscle has an increased density of cell nuclei, occurrence of aberrant fibers and fibers expressing embryonic myosin. Using real-time PCR we extend the findings of increased myogenic regulatory factor mRNA to show that very high levels are found in unsuccessfully aged cohort members. This pattern is also reflected in the number of embryonic myosin-positive fibers, which increase with the degree of sarcopenia. In addition, we confirm that there is no local down-regulation of IGF-I and IGF-IR mRNA in aged muscle tissue; on the contrary, the most sarcopenic individuals showed significantly higher local expression of IGF-I mRNA. Combined, our results show that the initial drive to regenerate myofibers is most marked in cases with the most advanced loss of muscle mass, a pattern that may have its origin in differences in the rate of tissue deterioration and/or that regenerating myofibers in these cases fail to mature into functional fibers. Importantly, the genetic background is a determinant of the pace of progression of sarcopenia.

Expression of late myogenic differentiation markers in sarcoplasmic masses of patients with myotonic dystrophy

Sarcoplasmic masses contain disorganized myofibrillar material and are a striking feature of myotonic dystrophy. However their significance is still unclear. Using immunocytochemistry we studied the expression of cytoskeletal proteins (desmin and vimentin), dystrophin, markers of myogenic differentiation (foetal myosin, neural cell adhesion molecule, bcl-2, insulin-like growth factor-I, fibroblast growth factor, retinoblastoma protein and myoD1), cell cycle regulators (Cdk2, p16, p27 and p57) and muscle proteases (ubiquitin, micro and m calpain and cathepsin D) in muscle biopsies from four patients with myotonic dystrophy. Sarcoplasmic masses were strongly positive for desmin, neural cell adhesion molecule, bcl-2, insulin-like growth factor I, retinoblastoma protein and p57, weakly positive for dystrophin and p16 and negative for vimentin, fibroblast growth factor, myoD1, Cdk2 and p27. Immunoreactivity for foetal myosin was detected only in a few fibres (< 1%). Our data suggest that the late myogenic differentiation programme is activated in sarcoplasmic masses although these areas do not reach complete maturation.

The modulation of caveolin-1 expression controls satellite cell activation during muscle repair

We have previously shown that caveolin-1, the principal structural protein component of caveolar membrane domains, inhibits cellular proliferation and induces cell cycle arrest. We demonstrate here for the first time that caveolin-1 is expressed in satellite cells but not in mature muscle fibers. Satellite cells are quiescent myogenic precursors that, after muscle injury, become mitotically active, proliferate, and fuse together or, to existing myofibers, to form new muscle fibers. We show that down-regulation of caveolin-1 expression occurs in satellite cells/myogenic precursor cells (MPCs) during muscle regeneration and that hepatocyte growth factor, which is produced after muscle injury, down-regulates caveolin-1. We also demonstrate that down-regulation of endogenous caveolin-1 expression activates ERK and that activation of the p42/44 MAP kinase pathway is necessary to promote muscle regeneration. Finally, we show that overexpression of caveolin-1 inhibits muscle repair mechanisms both in vitro and in vivo. Taken together, these results propose caveolin-1 as a novel regulator of satellite cell functions and suggest that the following signaling pathway modulates satellite cell activation during muscle repair: injured fibers release HGF --> HGF down-regulates caveolin-1 protein expression --> down-regulation of caveolin-1 activates ERK --> activation of ERK promotes muscle repair by stimulating the proliferation and migration of MPCs toward the wounded area.

In ovo temperature manipulation influences post-hatch muscle growth in the turkey

1. The effect of manipulating egg incubation temperature for short periods on turkey muscle development was determined using the M. semitendinosus, a thigh muscle, as the model. 2. Experiment 1. Eggs were incubated at a control temperature of 37.5 degrees C. For a 4-d period of 0 to 4, 5 to 8, 9 to 12, 13 to 16, 17 to 20 or 21 to 24 embryonic days (ED) eggs were transferred to either 38.5 or 35.5 degrees C. A regime of 38.5 degrees C at 5 to 8 and 9 to 12 ED caused an increased myonuclei number and muscle fibre number, respectively. 3. Experiment 2. Eggs were incubated at a control temperature of 37.5 degrees C. At 5 to 8 ED eggs were transferred to 38.5 or 35.5 degrees C. Temperature-manipulated embryos showed a delay in differentiation (myogenin expression) of the semitendinosus muscle compared to controls. 4. Manipulating the incubation temperature for 4 d in early incubation alters muscle development in the turkey with no observation of deformities or reduction in hatchability. We speculate that this increase in temperature may result in an improved muscle growth in the post-hatch bird.

Alternative usages of multiple promoters of the acetyl-CoA carboxylase beta gene are related to differential transcriptional regulation in human and rodent tissues

Acetyl-CoA carboxylase beta (ACCbeta) is a critical enzyme in the regulation of fatty acid oxidation and is dominantly expressed in the skeletal muscle, heart, and liver. It has been established that two promoters, P-I and P-II, control the transcription of the ACCbeta gene. However, the precise mechanism involved in controlling tissue-specific gene expression of ACCbeta is largely unknown yet. In this study we revealed that promoter P-I, active in the skeletal muscle and heart but not in the liver, could be activated by myogenic regulatory factors and retinoid X receptors in a synergistic manner. Moreover, P-I was also activated markedly by the cardiac-specific transcription factors, Csx/Nkx2.5 and GATA4. These results suggest that the proper stimulation of P-I by these tissue-specific transcription factors is important for the expression of ACCbeta according to the tissue types. In addition, CpG sites around human exon 1a transcribed by P-I are half-methylated in muscle but completely methylated in the liver, where P-I is absolutely inactive. In humans, the skeletal muscle uses P-II as well as P-I, whereas only P-I is active in rat skeletal muscle. The proximal myogenic regulatory factor-binding sites in human P-II, which are not conserved in rat P-II, might contribute to this difference in P-II usage between human and rat skeletal muscle. Hepatoma-derived cell lines primarily use another novel promoter located about 3 kilobases upstream of P-I, designated as P-O. This study is the first to explain the mechanisms underlying the differential regulation of ACCbeta gene expression between tissues in living organisms.

Integrins in the mouse myotome: Developmental changes and differences between the epaxial and hypaxial lineage

Integrins are cellular adhesion receptors that mediate signaling and play key roles in the development of multicellular organisms. However, their role in the cellular events leading to myotome formation is completely unknown. Here, we describe the expression patterns of the alpha1, alpha4, alpha5, alpha6, and alpha7 integrin subunits in the mouse myotome and correlate them with the expression of several differentiation markers. Our results indicate that these integrin subunits may be differentially involved in the various phases of myogenic determination and differentiation. A detailed characterization of the myogenic cell types expressing the alpha4 and alpha6 subunits showed a regionalization of the myotome and dermomyotome based on cell-adhesion properties. We conclude that alpha6beta1 may be an early marker of epaxial myogenic progenitor cells. In contrast, alpha4beta1 is up-regulated in the intercalated myotome after myocyte differentiation. Furthermore, alpha4beta1 is expressed in the hypaxial dermomyotome and is maintained by early hypaxial myogenic progenitor cells colonizing the myotome.

RNAi-mediated HuR depletion leads to the inhibition of muscle cell differentiation

The formation of muscle fibers involves the sequential expression of many proteins that regulate key steps during myoblast-to-myotube transition. MyoD, myogenin, and the cyclin-dependent kinase inhibitor p21cip1 are major players in the initiation and maintenance of the differentiated state of mouse embryonic muscle cells (C2C12). The messenger RNAs encoding these three proteins contain typical AU-rich elements (AREs) in their 3'-untranslated regions (3'-UTRs), which are known to affect the half-life of many short-lived mRNAs. HuR, an RNA-binding protein that regulates both the stability and cellular movement of ARE-containing mRNAs, interacts and stabilizes the p21cip1 message under UV stress in human RKO colorectal carcinoma cells. Here, by the use of gel shift experiments and immunoprecipitation followed by reverse transcription-PCR analysis, we show that HuR interacts with MyoD, myogenin, and p21cip1 mRNAs through specific sequences in their 3'-UTRs. To demonstrate the implication of endogenous HuR in myogenesis, we knocked down its expression in myoblasts using RNA interference and observed a significant reduction of HuR expression, associated with complete inhibition of myogenesis. Moreover, the expression of MyoD and myogenin mRNAs, as well as proteins, is significantly reduced in the HuR knockdown C2C12 cells. We were able to completely re-establish the myogenic process of these defective cells by introducing back HuR protein conjugated to a cell-permeable peptide. Finally, HuR accumulates in the cytoplasm during myogenesis. Thus, our results clearly demonstrated that endogenous HuR plays a crucial role in muscle differentiation by regulating the expression and/or the nuclear export of ARE-containing mRNAs that are essential for this process.

Myostatin-deficient mice lose more skeletal muscle mass than wild-type controls during hindlimb suspension

Myostatin inhibits myogenesis. Therefore, we sought to determine if mice lacking the myostatin gene [Mstn(-/-)] would lose less muscle mass than wild-type mice during 7 days of hindlimb suspension (HS). Male Mstn(-/-) and wild-type (C57) mice were subjected to HS or served as ground-based controls (n = 6/group). Wild-type mice lost 8% of body mass and approximately 13% of wet mass from biceps femoris, quadriceps femoris, and soleus, whereas the mass of extensor digitorum longus (EDL) was unchanged after HS. Unexpectedly, Mstn(-/-) mice lost more body (13%, P < 0.05) and quadriceps femoris (17%, P < 0.05) mass than wild-type mice and lost 33% of EDL mass (P < 0.01) after HS. Protein expression of myostatin in biceps femoris and quadriceps femoris was not altered, whereas expression of MyoD, Myf-5, and myogenin increased in wild-type mice and tended to decrease in muscles of Mstn(-/-) mice. These data suggest that HS induced myogenesis in wild-type mice to counter atrophy, whereas myogenesis was not induced in Mstn(-/-) mice, thereby resulting in a greater loss of muscle mass.

Regeneration of transgenic skeletal muscles with altered timing of expression of the basic helix-loop-helix muscle regulatory factor MRF4

In regenerating muscle cells, muscle regulatory factor (MRF) 4 is normally the last of the four MRFs to be expressed. To analyze how the timing of MRF4 expression affects muscle regeneration, we compared regeneration after local freeze injury of muscles from wild-type mice with muscles from transgenic mice in which MRF4 expression was under control of an approximately 1.6-kb fragment of the myogenin promoter. Three days after injury, masseter and tibialis anterior (TA) muscles in wild-type mice expressed little or no MRF4 mRNA; whereas these muscles in transgenic mice expressed abundant MRF4 mRNA from both the transgene and the endogenous gene. Thus, MRF4 up-regulation was accelerated in transgenic compared to wild-type regenerating muscles, and expression of the transgene appeared to activate, perhaps indirectly, expression of the endogenous MRF4 gene. At 11 days after injury, regeneration, as measured by cross-sectional area and density of regenerated fibers, was significantly impaired in transgenic TA compared to wild-type TA, whereas at 19 days after injury both transgenic and TA muscle fibers had fully recovered to preinjury values. Regeneration of masseter muscles, which normally regenerate much less completely than TA muscles, was unaffected by the transgene. Thus, the timing of MRF4 up-regulation, as well as additional muscle-specific factors, can determine the progress of muscle regeneration.

The regeneration process of the striated urethral sphincter involves activation of intrinsic satellite cells

The regeneration of adult skeletal muscle is mediated by satellite cells. Classically, these are considered to be somitically derived cells that colonize the limbs during early embryogenesis. The striated urethral sphincter presents specific developmental characteristics that distinguish it from skeletal muscles, such as the non-somitic origin of its precursor cells and the late formation of its myofibers. This prompted us to determine whether the striated urethral sphincter can regenerate after injury by the same mechanism as skeletal muscles. By means of the single myofiber explant culture technique we investigated the presence of satellite cells in the striated urethral sphincter of male mice and evaluated their ability to recapitulate a myogenic program. In addition, a myotoxic substance (notexin) was injected into the sphincter in order to provoke rapid destruction of the myofibers; the regeneration process was studied by means of electrophysiological and histological techniques. Satellite cells expressing pax7 were found attached to the sphincteric myofibers. They proliferated and expressed MyoD, Myf5 and desmin after 2 days in culture. After 10 days, they formed multinucleated myotubes expressing alpha-actinin-2. In vivo, complete recovery of the striated urethral sphincter, as assessed by normalization of muscle strength and of myofiber number and diameter, was observed after 3 weeks, and resulted from the fusion of myogenic cells. These results demonstrate that the striated urethral sphincter can regenerate by means of a myogenic program involving intrinsic satellite cells. The therapeutic implications of this knowledge and the possible origin of the sphincteric satellite cells are discussed.

Genome-wide examination of myoblast cell cycle withdrawal during differentiation

Skeletal and cardiac myocytes cease division within weeks of birth. Although skeletal muscle retains limited capacity for regeneration through recruitment of satellite cells, resident populations of adult myocardial stem cells have not been identified. Because cell cycle withdrawal accompanies myocyte differentiation, we hypothesized that C2C12 cells, a mouse myoblast cell line previously used to characterize myocyte differentiation, also would provide a model for studying cell cycle withdrawal during differentiation. C2C12 cells were differentiated in culture medium containing horse serum and harvested at various time points to characterize the expression profiles of known cell cycle and myogenic regulatory factors by immunoblot analysis. BrdU incorporation decreased dramatically in confluent cultures 48 hr after addition of horse serum, as cells started to form myotubes. This finding was preceded by up-regulation of MyoD, followed by myogenin, and activation of Bcl-2. Cyclin D1 was expressed in proliferating cultures and became undetectable in cultures containing 40% fused myotubes, as levels of p21(WAF1/Cip1) increased and alpha-actin became detectable. Because C2C12 myoblasts withdraw from the cell cycle during myocyte differentiation following a course that recapitulates this process in vivo, we performed a genome-wide screen to identify other gene products involved in this process. Using microarrays containing approximately 10,000 minimally redundant mouse sequences that map to the UniGene database of the National Center for Biotechnology Information, we compared gene expression profiles between proliferating, differentiating, and differentiated C2C12 cells and verified candidate genes demonstrating differential expression by RT-PCR. Cluster analysis of differentially expressed genes revealed groups of gene products involved in cell cycle withdrawal, muscle differentiation, and apoptosis. In addition, we identified several genes, including DDAH2 and Ly-6A, whose expression specifically was up-regulated during cell cycle withdrawal coincident with early myoblast differentiation.

A fast fiber enhancer exists in the muscle regulatory factor 4 gene promoter

The development of skeletal muscle is a highly regulated process governed by the four myogenic regulatory factors (MRFs) MyoD, myf-5, myogenin, and MRF4. While these factors exhibit some unique functions, part of their individual activity can be attributed to different temporal and spatial expression patterns. To delineate the factors that control expression of the MRFs, we have begun a molecular dissection of the MRF4 gene promoter. Through the generation of promoter/reporter gene constructs, we show that an 853bp fragment, residing 4kb upstream of the MRF4 transcriptional start site (853AV), is able to enhance expression of the basal MRF4 promoter 3-4-fold in myogenic cell cultures. Analysis of the 853AV enhancer in transgenic mice indicates that this region drives MRF4 gene expression primarily in fast muscle fibers, suggesting that the normal adult MRF4 expression pattern is regulated by a variety of control elements that may dictate fiber-type specificity.

Effects of microgravity on myogenic factor expressions during postnatal development of rat skeletal muscle

To clarify the role of gravity in the postnatal development of skeletal muscle, we exposed neonatal rats at 7 days of age to microgravity. After 16 days of spaceflight, tibialis anterior, plantaris, medial gastrocnemius, and soleus muscles were removed from the hindlimb musculature and examined for the expression of MyoD-family transcription factors such as MyoD, myogenin, and MRF4. For this purpose, we established a unique semiquantitative method, based on RT-PCR, using specific primers tagged with infrared fluorescence. The relative expression of MyoD in the tibialis anterior and plantaris muscles and that of myogenin in the plantaris and soleus muscles were significantly reduced (P < 0.001) in the flight animals. In contrast, MRF4 expression was not changed in any muscle. These results suggest that MyoD and myogenin, but not MRF4, are sensitive to gravity-related stimuli in some skeletal muscles during postnatal development.

Apoptosis and syncytial fusion in human placental trophoblast and skeletal muscle

Skeletal muscle fibers and placental villous trophoblast are the main representatives of syncytia in the human. Both syncytia are derived from fusion of mononucleated stem cells, show a high degree of differentiation, and have lost their generative potency. Consequently, for their growth both depend on fusion of additional stem cells. There is evidence that syncytial fusion is directly or indirectly related to apoptotic events: As early as in the differentiated stages of the mononucleated stem cells, initiation stages of the apoptosis cascade have been observed. After syncytial fusion progression of the cascade is retarded or blocked by a variety of mechanisms. In this review we emphasize the links between apoptosis cascade, differentiation pathways and syncytial fusion. It needs to be elucidated whether these processes simply take place in parallel, both temporally and spatially, or whether there are causal connections between apoptosis cascade and syncytial fusion. Based on recent data obtained for placental villous trophoblast, it is tempting to speculate that early molecular mechanisms of the apoptosis cascade are involved in differentiation and syncytial fusion. Data obtained in skeletal muscles support this assumption and reveal a considerable degree of homology in genesis, maintenance and turnover of both tissues.

Myogenic satellite cells: physiology to molecular biology

Adult skeletal muscle has a remarkable ability to regenerate following myotrauma. Because adult myofibers are terminally differentiated, the regeneration of skeletal muscle is largely dependent on a small population of resident cells termed satellite cells. Although this population of cells was identified 40 years ago, little is known regarding the molecular phenotype or regulation of the satellite cell. The use of cell culture techniques and transgenic animal models has improved our understanding of this unique cell population; however, the capacity and potential of these cells remain ill-defined. This review will highlight the origin and unique markers of the satellite cell population, the regulation by growth factors, and the response to physiological and pathological stimuli. We conclude by highlighting the potential therapeutic uses of satellite cells and identifying future research goals for the study of satellite cell biology.

A new immunodeficient mouse model for human myoblast transplantation

Design of efficient transplantation strategies for myoblast-based gene therapies in humans requires animal models in which xenografts are tolerated for long periods of time. In addition, such recipients should be able to withstand pretransplantation manipulations for enhancement of graft growth. Here we report that a newly developed immunodeficient mouse carrying two known mutations (the recombinase activating gene 2, RAG2, and the common cytokine receptor gamma, gammac) is a candidate fulfilling these requirements. Skeletal muscles from RAG2(-/-)/gammac(-/-) double mutant mice recover normally after myotoxin application or cryolesion, procedures commonly used to induce regeneration and improve transplantation efficiency. Well-differentiated donor-derived muscle tissue could be detected up to 9 weeks after transplantation of human myoblasts into RAG2(-/-)/gammac(-/-) muscles. These results suggest that the RAG2(-/-)/gammac(-/-) mouse model will provide new opportunities for human muscle research.

MyoD and myogenin expression patterns in cultures of fetal and adult chicken myoblasts

Isolated chicken myoblasts had previously been utilized in many studies aiming at understanding the emergence and regulation of the adult myogenic precursors (satellite cells). However, in recent years only a small number of chicken satellite cell studies have been published compared to the increasing number of studies with rodent satellite cells. In large part this is due to the lack of markers for tracing avian myogenic cells before they become terminally differentiated and express muscle-specific structural proteins. We previously demonstrated that myoblasts isolated from fetal and adult chicken muscle display distinct schedules of myosin heavy-chain isoform expression in culture. We further showed that myoblasts isolated from newly hatched and young chickens already possess the adult myoblast phenotype. In this article, we report on the use of polyclonal antibodies against the chicken myogenic regulatory factor proteins MyoD and myogenin for monitoring fetal and adult chicken myoblasts as they progress from proliferation to differentiation in culture. Fetal-type myoblasts were isolated from 11-day-old embryos and adult-type myoblasts were isolated from 3-week-old chickens. We conclude that fetal myoblasts express both MyoD and myogenin within the first day in culture and rapidly transit into the differentiated myosin-expressing state. In contrast, adult myoblasts are essentially negative for MyoD and myogenin by culture Day 1 and subsequently express first MyoD and then myogenin before expressing sarcomeric myosin. The delayed MyoD-to-myogenin transition in adult myoblasts is accompanied by a lag in the fusion into myotubes, compared to fetal myoblasts. We also report on the use of a commercial antibody against the myocyte enhancer factor 2A (MEF2A) to detect terminally differentiated chicken myoblasts by their MEF2+ nuclei. Collectively, the results support the hypothesis that fetal and adult myoblasts represent different phenotypic populations. The fetal myoblasts may already be destined for terminal differentiation at the time of their isolation, and the adult myoblasts may represent progenitors that reside in an earlier compartment of the myogenic lineage.

Pro- and anti-apoptotic members of the Bcl-2 family in skeletal muscle: a distinct role for Bcl-2 in later stages of myogenesis

Apoptotic myonuclei appear during myogenesis and in diseased muscles. To investigate cell death regulation in skeletal muscle, we examined how members of the Bcl-2 family of apoptosis regulators are expressed and function in the C2C12 muscle cell line and in primary muscle cells at different stages of development. Both anti-apoptotic (Bcl-W, Bcl-X(L)) and pro-apoptotic (Bad, Bak, Bax) members of the Bcl-2 family were expressed in developing skeletal muscle in vivo. Each was also expressed in embryonic (E11-12), fetal (E15-16), and neonatal muscle stem cells, myoblasts, and myotubes in vitro. In contrast, Bcl-2 expression was limited to a small group of mononucleate, desmin-positive, myogenin-negative muscle cells that were seen in fetal and neonatal, but not embryonic, muscle cell cultures. The cell surface protein Sca-1, which is associated with muscle and blood stem cells, was found on approximately 1/2 of these Bcl-2-positive cells. Loss of Bcl-2 did not affect expression of other family members, because neonatal muscles of wild-type and Bcl-2-null mice had similar amounts of Bcl-X(L), Bcl-W, Bad, Bak, and Bax mRNAs. Loss of Bcl-2 did have functional consequences; however, because neonatal muscles of Bcl-2-null mice had only approximately 2/3 as many fast muscle fibers as muscles in wild-type mice. Thus, Bcl-2 function is required for particular stages of fetal and postnatal myogenesis.

Differentiation of avian craniofacial muscles: I. Patterns of early regulatory gene expression and myosin heavy chain synthesis

Myogenic populations of the avian head arise within both epithelial (somitic) and mesenchymal (unsegmented) mesodermal populations. The former, which gives rise to neck, tongue, laryngeal, and diaphragmatic muscles, show many similarities to trunk axial, body wall, and appendicular muscles. However, muscle progenitors originating within unsegmented head mesoderm exhibit several distinct features, including multiple ancestries, the absence of several somite lineage-determining regulatory gene products, diverse locations relative to neuraxial and pharyngeal tissues, and a prolonged and necessary interaction with neural crest cells. The object of this study has been to characterize the spatial and temporal patterns of early muscle regulatory gene expression and subsequent myosin heavy chain isoform appearance in avian mesenchyme-derived extraocular and branchial muscles, and compare these with expression patterns in myotome-derived neck and tongue muscles. Myf5 and myoD transcripts are detected in the dorsomedial (epaxial) region of the occipital somites before stage 12, but are not evident in the ventrolateral domain until stage 14. Within unsegmented head mesoderm, myf5 expression begins at stage 13.5 in the second branchial arch, followed within a few hours in the lateral rectus and first branchial arch myoblasts, then other eye and branchial arch muscles. Expression of myoD is detected initially in the first branchial arch beginning at stage 14.5, followed quickly by its appearance in other arches and eye muscles. Multiple foci of myoblasts expressing these transcripts are evident during the early stages of myogenesis in the first and third branchial arches and the lateral rectus-pyramidalis/quadratus complex, suggesting an early patterned segregation of muscle precursors within head mesoderm. Myf5-positive myoblasts forming the hypoglossal cord emerge from the lateral borders of somites 4 and 5 by stage 15 and move ventrally as a cohort. Myosin heavy chain (MyHC) is first immunologically detectable in several eye and branchial arch myofibers between stages 21 and 22, although many tongue and laryngeal muscles do not initiate myosin production until stage 24 or later. Detectable synthesis of the MyHC-S3 isoform, which characterizes myofibers as having "slow" contraction properties, occurs within 1-2 stages of the onset of MyHC synthesis in most head muscles, with tongue and laryngeal muscles being substantially delayed. Such a prolonged, 2- to 3-day period of regulatory gene expression preceding the onset of myosin production contrasts with the interval seen in muscles developing in axial (approximately 18 hr) and wing (approximately 1-1.5 days) locations, and is unique to head muscles. This finding suggests that ongoing interactions between head myoblasts and their surroundings, most likely neural crest cells, delay myoblast withdrawal from the mitotic pool. These descriptions define a spatiotemporal pattern of muscle regulatory gene and myosin heavy chain expression unique to head muscles. This pattern is independent of origin (somitic vs. unsegmented paraxial vs. prechordal mesoderm), position (extraocular vs. branchial vs. subpharyngeal), and fiber type (fast vs. slow) and is shared among all muscles whose precursors interact with cephalic neural crest populations. Dev Dyn 1999;216:96-112.

In vivo satellite cell activation via Myf5 and MyoD in regenerating mouse skeletal muscle

Regeneration of adult skeletal muscle is an asynchronous process requiring the activation, proliferation and fusion of satellite cells, to form new muscle fibres. This study was designed to determine the pattern of expression in vivo of the two myogenic regulatory factors, Myf5 and MyoD during this process. Cardiotoxin was used to induce regeneration in the gastrocnemius and soleus muscles of heterozygous Myf5-nlacZ mice, and the muscles were assayed for the presence of (beta)-galactosidase (Myf5) and MyoD. Adult satellite cells identified by M-cadherin labelling, when activated, initially express either MyoD or Myf5 or both myogenic factors. Subsequently all proliferating myoblasts express MyoD and part of the population is (beta)-galactosidase (Myf5) positive. Furthermore, we demonstrate that activated satellite cells, which express either Myf5 or MyoD, do not accumulate selectively on fast or slow muscle fibres.

The transition from proliferation to differentiation is delayed in satellite cells from mice lacking MyoD

Satellite cells from adult rat muscle coexpress proliferating cell nuclear antigen and MyoD upon entry into the cell cycle, suggesting that MyoD plays a role during the recruitment of satellite cells. Moreover, the finding that muscle regeneration is compromised in MyoD-/- mice, has provided evidence for the role of MyoD during myogenesis in adult muscle. In order to gain further insight into the role of MyoD during myogenesis in the adult, we compared satellite cells from MyoD-/- and wildtype mice as they progress through myogenesis in single-myofiber cultures and in tissue-dissociated cell cultures (primary cultures). Satellite cells undergoing proliferation and differentiation were traced immunohistochemically using antibodies against various regulatory proteins. In addition, an antibody against the mitogen-activated protein kinases ERK1 and ERK2 was used to localize the cytoplasm of the fiber-associated satellite cells regardless of their ability to express specific myogenic regulatory factor proteins. We show that during the initial days in culture the myofibers isolated from both the MyoD-/- and the wildtype mice contain the same number of proliferating, ERK+ satellite cells. However, the MyoD-/- satellite cells continue to proliferate and only a very small number of cells transit into the myogenin+ state, whereas the wildtype cells exit the proliferative compartment and enter the myogenin+ stage. Analyzing tissue-dissociated cultures of MyoD-/- satellite cells, we identified numerous cells whose nuclei were positive for the Myf5 protein. In contrast, quantification of Myf5+ cells in the wildtype cultures was difficult due to the low level of Myf5 protein present. The Myf5+ cells in the MyoD-/- cultures were often positive for desmin, similar to the MyoD+ cells in the wildtype cultures. Myogenin+ cells were identified in the MyoD-/- primary cultures, but their appearance was delayed compared to the wildtype cells. These "delayed" myogenin+ cells can express other differentiation markers such as MEF2A and cyclin D3 and fuse into myotubes. Taken together, our studies suggest that the presence of MyoD is critical for the normal progression of satellite cells into the myogenin+, differentiative state. It is further proposed that the Myf5+/MyoD- phenotype may represent the myogenic stem cell compartment which is capable of maintaining the myogenic precursor pool in the adult muscle.

Dynamics of myoblast transplantation reveal a discrete minority of precursors with stem cell-like properties as the myogenic source

Myoblasts, the precursors of skeletal muscle fibers, can be induced to withdraw from the cell cycle and differentiate in vitro. Recent studies have also identified undifferentiated subpopulations that can self-renew and generate myogenic cells (Baroffio, A., M. Hamann, L. Bernheim, M.-L. Bochaton-Pillat, G. Gabbiani, and C.R. Bader. 1996. Differentiation. 60:47-57; Yoshida, N., S. Yoshida, K. Koishi, K. Masuda, and Y. Nabeshima. 1998. J. Cell Sci. 111:769-779). Cultured myoblasts can also differentiate and contribute to repair and new muscle formation in vivo, a capacity exploited in attempts to develop myoblast transplantation (MT) for genetic modification of adult muscle. Our studies of the dynamics of MT demonstrate that cultures of myoblasts contain distinct subpopulations defined by their behavior in vitro and divergent responses to grafting. By comparing a genomic and a semiconserved marker, we have followed the fate of myoblasts transplanted into muscles of dystrophic mice, finding that the majority of the grafted cells quickly die and only a minority are responsible for new muscle formation. This minority is behaviorally distinct, slowly dividing in tissue culture, but rapidly proliferative after grafting, suggesting a subpopulation with stem cell-like characteristics.

Seeking muscle stem cells

Skeletal muscle development requires the formation of myoblasts that can fuse with each other to form multinucleate myofibers. Distinct primary and secondary, slow and fast, populations of myofibers form by the time of birth. At embryonic, fetal, and perinatal stages of development, temporally distinct lineages of myogenic cells arise and contribute to the formation of these multiple types of myofibers. In addition, spatially distinct lineages of myogenic cells arise and form the anterior head muscles, limb (hypaxial) muscles, and dorsal (epaxial) muscles. There is strong evidence that myoblasts are produced from muscle stem cells, which are self-renewing cells that do not themselves terminally differentiate but produce progeny that are capable of becoming myoblasts and myofibers. Muscle stem cells, which may be multipotent, appear to be distinguishable from myoblasts by a number of indirect and direct criteria. Muscle stem cells arise either in unsegmented paraxial mesoderm (anterior head muscle progenitors) or in segmented mesoderm of the somites (epaxial and hypaxial muscle progenitors). These initial stages of myogenesis are regulated by positive and negative signals, including Wnt, BMP, and Shh family members, from nearby notochord, neural tube, ectoderm, and lateral mesoderm tissues. The formation of skeletal muscles, therefore, depends on the generation of spatially and temporally distinct lineages of myogenic cells. Myogenic cell lineages begin with muscle stem cells which produce the myoblasts that fuse to form myofibers.

Fibroblast growth factor promotes recruitment of skeletal muscle satellite cells in young and old rats

Although the role of satellite cells in muscle growth and repair is well recognized, understanding of the molecular events that accompany their activation and proliferation is limited. In this study, we used the single myofiber culture model for comparing the proliferative dynamics of satellite cells from growing (3-week-old), young adult (8- to 10-week-old), and old (9- to 11-month-old) rats. In these fiber cultures, the satellite cells are maintained in their in situ position underneath the fiber basement membrane. We first demonstrate that the cytoplasm of fiber-associated satellite cells can be monitored with an antibody against the extracellular signal regulated kinases 1 and 2 (ERK1 and ERK2), which belong to the mitogen-activated protein kinase (MAPK) superfamily. With this immunocytological marker, we show that the satellite cells from all three age groups first proliferate and express PCNA and MyoD, and subsequently, about 24 hr later, exit the PCNA+/MyoD+ state and become positive for myogenin. For all three age groups, fibroblast growth factor 2 (FGF2) enhances by about twofold the number of satellite cells that are capable of proliferation, as determined by monitoring the number of cells that transit from the MAPK+ phenotype to the PCNA+/MAPK+ or MyoD+/MAPK+ phenotype. Furthermore, contrary to the commonly accepted convention, we show that in the fiber cultures FGF2 does not suppress the subsequent transition of the proliferating cells into the myogenin+ compartment. Although myogenesis of satellite cells from growing, young adult, and old rats follows a similar program, two distinctive features were identified for satellite cells in fiber cultures from the old rats. First, a large number of MAPK+ cells do not appear to enter the MyoD-myogenin expression program. Second, the maximal number of proliferating satellite cells is attained a day later than in cultures from the young adults. This apparent "lag" in proliferation was not affected by hepatocyte growth factor (HGF), which has been implicated in accelerating the first round of satellite cell proliferation. HGF and FGF2 were equally efficient in promoting proliferation of satellite cells in fibers from old rats. Collectively, the investigation suggests that FGF plays a critical role in the recruitment of satellite cells into proliferation.

mRNA levels of myogenic regulatory factors in rat slow and fast muscles regenerating from notexin-induced necrosis

The transcript levels of the myogenic regulatory factors (myoD, myf5, myogenin and MRF4) were measured by RT PCR in rat soleus (slow) and EDL (fast) muscles which were regenerating from notexin-induced necrosis. Some muscle fibers in the EDL were more resistant to the toxin, therefore the necrosis and the dominance of myoblasts were delayed for two days in EDL compared to soleus. In spite of this shift in time-course of necrosis, both types of muscle presented roughly similar, although variable, changes in the expression pattern of MRF mRNA levels. For both muscles, the myoD mRNA was upregulated on the first day after administration of the toxin, whereas concomitantly myf-5 mRNA disappeared but showed a substantial increase in later stages of regeneration. In contrast, the mRNA levels of the late MRFs myogenin and MRF4 decreased on day one only in the soleus, then increased on day three in both types of muscle. Meanwhile in EDL the level of MRF4 mRNA remained relatively normal. Four weeks after administration of the toxin the mRNA levels for each of the MRFs returned to nearly control levels. This shows that in spite of the different time course of the necrosis and regeneration, also documented by the microscopical morphology and the skeletal actin mRNA levels of the muscles, the level of MRF transcripts changed according to a quite predictable pattern; the upregulation corresponded to myoblast activation and the downregulation to the reinnervation.

Temporal restriction of MyoD induction and autocatalysis during Xenopus mesoderm formation

In Xenopus, the activation of the myogenic determination factors MyoD and Myf-5 in the muscle-forming region of the embryo occurs in response to mesoderm-inducing factors (MIFs). Different members of the FGF, TGF-beta, and Wnt protein families have been implicated in this process, but how MIFs induce the myogenic regulators is not known. For MyoD, the induction process may serve to locally stabilize a transient burst of ubiquitous transcription at the midblastula transition, possibly by triggering MyoD's autocatalytic loop. Here we have sought to distinguish separate activating functions during MyoD induction by analyzing when MyoD responds to different MIF signaling or to MyoD autoactivation. We show that MyoD induction depends on the developmental age of the induced cells, rather than on the type or time point of inducer application. At the permissive time, de novo MyoD induction by Activin requires less than 90 min, arguing for an immediate response, rather than a series of inductive events. MyoD autoactivation is direct, but subject to the same temporal restriction as MyoD induction by MIF signaling. Further evidence implicating MyoD autocatalysis as an essential component of the induction process comes from the observation that both autocatalysis and induction of MyoD are selectively repressed by a dominant-negative MyoD mutant. In summary, our observations let us conclude that MyoD's expression domain in the embryo results from an interplay of timed changes in cellular competence, pleiotropic signaling pathways, and autocatalysis.

Bcl-2 expression identifies an early stage of myogenesis and promotes clonal expansion of muscle cells

We show that Bcl-2 expression in skeletal muscle cells identifies an early stage of the myogenic pathway, inhibits apoptosis, and promotes clonal expansion. Bcl-2 expression was limited to a small proportion of the mononucleate cells in muscle cell cultures, ranging from approximately 1-4% of neonatal and adult mouse muscle cells to approximately 5-15% of the cells from the C2C12 muscle cell line. In rapidly growing cultures, some of the Bcl-2-positive cells coexpressed markers of early stages of myogenesis, including desmin, MyoD, and Myf-5. In contrast, Bcl-2 was not expressed in multinucleate myotubes or in those mononucleate myoblasts that expressed markers of middle or late stages of myogenesis, such as myogenin, muscle regulatory factor 4 (MRF4), and myosin. The small subset of Bcl-2-positive C2C12 cells appeared to resist staurosporine-induced apoptosis. Furthermore, though myogenic cells from genetically Bcl-2-null mice formed myotubes normally, the muscle colonies produced by cloned Bcl-2-null cells contained only about half as many cells as the colonies produced by cells from wild-type mice. This result suggests that, during clonal expansion from a muscle progenitor cell, the number of progeny obtained is greater when Bcl-2 is expressed.

Cell heterogeneity upon myogenic differentiation: down-regulation of MyoD and Myf-5 generates ‘reserve cells’

When a proliferating myoblast culture is induced to differentiate by deprivation of serum in the medium, a significant proportion of cells escape from terminal differentiation, while the rest of the cells differentiate. Using C2C12 mouse myoblast cells, this heterogeneity observed upon differentiation was investigated with an emphasis on the myogenic regulatory factors. The differentiating part of the cell population followed a series of well-described events, including expression of myogenin, p21(WAF1), and contractile proteins, permanent withdrawal from the cell cycle and cell fusion, whereas the rest of the cells did not initiate any of these events. Interestingly, the latter cells showed an undetectable or greatly reduced level of MyoD and Myf-5 expression, which had been originally expressed in the undifferentiated proliferating myoblasts. When these undifferentiated cells were isolated and returned to the growth conditions, they progressed through the cell cycle and regained MyoD expression. These cells demonstrated identical features with the original culture on the deprivation of serum. They produced both MyoD-positive differentiating and MyoD-negative undifferentiated populations once again. Thus the undifferentiated cells in the serum-deprived culture were designated ‘reserve cells’. Upon serum deprivation, MyoD expression rapidly decreased as a result of down-regulation in approximately 50% of the cells. After this heterogenization, MyoD positive cells expressed myogenin, which is the earliest known event of terminal differentiation and marks irreversible commitment to this, while MyoD-negative cells did not differentiate and became the reserve cells. We also demonstrated that ectopic expression of MyoD converted the reserve cells to differentiating cells, indicating that down-regulation of MyoD is a causal event in the formation of reserve cells.