A significant reduction of the diaphragm in mdx:MyoD-/-(9th) embryos suggests a role for MyoD in the diaphragm development

To further investigate the role of MyoD during skeletal myogenesis, we backcrossed mdx mutant mice (lacking dystrophin) with MyoD knock-out mice to obtain viable mice with MyoD allele on a pure mdx background. However, after nine generations of backcrossing, it was not possible to obtain a viable mdx:MyoD-/- phenotype (designated as: mdx:MyoD-/-(9th)). The compound-mutant embryos were examined just before birth. Essentially normal Myf5-dependent and most of the MyoD-dependent musculature was observed. By contrast, the skeletal muscle compartment of the diaphragm was significantly reduced. The mesenchymal compartment of the diaphragm was intact and no herniations were observed. Other examined organs (e.g., liver, kidney, brain, etc.) showed no histological abnormalities. Pulmonary hypoplasia was determined as the cause of neonatal death. Therefore, using a different approach, our new data supplement our previous findings and suggest an essential role for MyoD in development of skeletal muscle of the diaphragm. The failure of mdx:MyoD-/-(9th) diaphragm to develop normally is not caused by a reduced number of satellite cells, but from the inability of stem cells to progress through the myogenic program. Our data also suggest that functions of MyoD and Myf5 (and the respective muscle precursor cell sub-populations) are not entirely redundant by term, as previously suggested, since Myf5 is not capable of fully substituting for MyoD in the diaphragm development.     

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.     

Purification and cell-surface marker characterization of quiescent satellite cells from murine skeletal muscle by a novel monoclonal antibody

A novel monoclonal antibody, SM/C-2.6, specific for mouse muscle satellite cells was established. SM/C-2.6 detects mononucleated cells beneath the basal lamina of skeletal muscle, and the cells co-express M-cadherin. Single fiber analyses revealed that M-cadherin+ mononucleated cells attaching to muscle fibers are stained with SM/C-2.6. SM/C-2.6+ cells, which were freshly purified by FACS from mouse skeletal muscle, became MyoD+ in vitro in proliferating medium, and the cells differentiated into desmin+ and nuclear-MyoD+ myofibers in vitro when placed under differentiation conditions. When the sorted cells were injected into mdx mouse muscles, donor cells differentiated into muscle fibers. Flow cytometric analyses of SM/C-2.6+ cells showed that the quiescent satellite cells were c-kit-, Sca-1-, CD34+, and CD45-. More, SM/C-2.6+ cells were barely included in the side population but in the main population of cells in Hoechst dye efflux assay. These results suggest that SM/C-2.6 identifies and enriches quiescent satellite cells from adult mouse muscle, and that the antibody will be useful as a powerful tool for the characterization of cellular and molecular mechanisms of satellite cell activation and proliferation.     

Satellite cells from dystrophic (mdx) mouse muscle are stimulated by fibroblast growth factor in vitro

Satellite cells cultured from dystrophic (mdx) and from control mouse hindlimb muscles grow and fuse to form muscle fibers within 4-5 days. Total cell number and muscle-fiber formation are stimulated by bovine fibroblast growth factor (FGF). At low FGF levels (0.02-0.20 ng/ml) control satellite cells as well as fibroblasts are unresponsive, while mdx satellite cells show three- to four-fold increases in growth. Control cells do not begin to respond until FGF levels reach 1-5 ng/ml. Heparin, a major constituent of muscle fiber basal lamina, inhibits myogenesis in these mouse muscle cultures. The heightened sensitivity of mdx satellite cells to FGF may permit high rates of new fiber formation in vivo without a parallel hyperplasia in the muscle fibroblast population. This finding may be important in explaining successful regeneration in mdx muscle in vivo and the fact that mdx animals escape the catastrophic symptoms seen in the related human Duchenne muscular dystrophy.     

BM stem cell transplantation rescues pathophysiologic features of aged dystrophic mdx muscle

BACKGROUND: The value of transplantation of BM stem cells in aged (12-month-old) mdx was evaluated because it is thought to be a more ideal model for studying the praxiology of Duchenne muscular dystrophy (DMD). The possible mechanisms of stem cell differentiation were then discussed. METHODS: BM was isolated from 8-10-week-old male C57 BL/10 mice. After injecting BM cells into 12-month-old female mdx mice through the tail vein, the expression of dystrophin and MyoD was detected at different time points by immunofluorescence staining, RT-PCR and Western blot. RESULTS: The C57 male mice donor-specific and Y-chromosome-specific sequence could be detected in all female aged mdx mice, implying the success of the transplantation. Expression of dystrophin and MyoD was detected and increased over time. DISCUSSION: BM cells were recruited to the muscle and partially restored specific pathophysiologic features of the dystrophic muscle in aged mdx mice. Muscle differentiation of BM cells recapitulated embryonic myogenesis.     

IFN-γ promotes muscle damage in the mdx mouse model of Duchenne muscular dystrophy by suppressing M2 macrophage activation and inhibiting muscle cell proliferation

Duchenne muscular dystrophy is a degenerative disorder that leads to death by the third decade of life. Previous investigations have shown that macrophages that invade dystrophic muscle are a heterogeneous population consisting of M1 and M2 macrophages that promote injury and repair, respectively. In the present investigation, we tested whether IFN-γ worsens the severity of mdx dystrophy by activating macrophages to a cytolytic M1 phenotype and by suppressing the activation of proregenerative macrophages to an M2 phenotype. IFN-γ is a strong inducer of the M1 phenotype and is elevated in mdx dystrophy. Contrary to our expectations, null mutation of IFN-γ caused no reduction of cytotoxicity of macrophages isolated from mdx muscle and did not reduce muscle fiber damage in vivo or improve gross motor function of mdx mice at the early, acute peak of pathology. In contrast, ablation of IFN-γ reduced muscle damage in vivo during the regenerative stage of the disease and increased activation of the M2 phenotype and improved motor function of mdx mice at that later stage of the disease. IFN-γ also inhibited muscle cell proliferation and differentiation in vitro, and IFN-γ mutation increased MyoD expression in mdx muscle in vivo, showing that IFN-γ can have direct effects on muscle cells that could impair repair. Taken together, the findings show that suppression of IFN-γ signaling in muscular dystrophy reduces muscle damage and improves motor performance by promoting the M2 macrophage phenotype and by direct actions on muscle cells.     

Highly efficient, functional engraftment of skeletal muscle stem cells in dystrophic muscles

Satellite cells reside beneath the basal lamina of skeletal muscle fibers and include cells that act as precursors for muscle growth and repair. Although they share a common anatomical localization and typically are considered a homogeneous population, satellite cells actually exhibit substantial heterogeneity. We used cell-surface marker expression to purify from the satellite cell pool a distinct population of skeletal muscle precursors (SMPs) that function as muscle stem cells. When engrafted into muscle of dystrophin-deficient mdx mice, purified SMPs contributed to up to 94% of myofibers, restoring dystrophin expression and significantly improving muscle histology and contractile function. Transplanted SMPs also entered the satellite cell compartment, renewing the endogenous stem cell pool and participating in subsequent rounds of injury repair. Together, these studies indicate the presence in adult skeletal muscle of prospectively isolatable muscle-forming stem cells and directly demonstrate the efficacy of myogenic stem cell transplant for treating muscle degenerative disease.     

成肌调节因子MyoD和myogenin在不同月龄DMD模型鼠mdx小鼠的表达

目的探讨成肌调节因子MyoD和myogenin在不同月龄DMD模型鼠mdx鼠的表达情况。方法取不同月龄DMD模型鼠mdx鼠以及相应的同龄正常C57鼠的腓肠肌,冰冻切片后用HE染色显示肌肉病理,SABC-DAB染色检测成肌调节因子MyoD和myogenin的表达。结果不同月龄mdx鼠肌肉坏死和再生程度不同,MyoD和myogenin在1月龄mdx鼠表达最强,在13月龄mdx鼠仍有表达,在正常同龄C57鼠不表达。结论MyoD与Myogenin在肌肉损伤后的再生修复过程中起作用,可作为鉴定肌肉前体细胞和反映肌肉再生的指标。     

Oxygen-mediated regulation of skeletal muscle satellite cell proliferation and adipogenesis in culture

Major problems in stem cell biology revolve around defining the developmental potential of cell populations and understanding how their potential is maintained or progressively restricted. Oxygen (O(2)) is an obvious environmental factor which has received little attention in culturing skeletal muscle progenitor cells. In this work, we examine the effects of O(2) levels on the developmental potential, proliferative capacity, and phenotype of the adult skeletal muscle fiber progenitor population (satellite cells), and cell lines that model multipotential embryonic paraxial mesoderm from which skeletal muscle develops. Both satellite cell proliferation and survival of mature fibers increased in physiologic (6%) O(2) vs. non-physiologic 20% O(2) used in virtually all traditional cell culture. Six percent O(2) conditions also accelerated the up-regulation of multiple MyoD family myogenic regulatory factors (MRFs). An unexpected finding was that fiber-adherent satellite cells could assume a non-myogenic phenotype. By the criteria of molecular markers and gross lipid accumulation, satellite cells were found to assume an adipocyte phenotype, and did so more prominently in 20% O(2) than in physiologic O(2). Selection of the adipogenic fate and execution of adipogenesis by multipotential mesenchymal cell lines was also dramatically higher in traditional 20 vs. 6% O(2), and decreased adipogenesis in physiologic O(2) was associated with significantly less expression of the adipogenic regulator, PPAR gamma. These results suggest that regulatory pathways affected by O(2) are important for satellite cell proliferation, execution of cell fate, and parent muscle survival in culture, and so may play a role in vivo under normal or pathologic conditions.     

Syndecan-3 and syndecan-4 specifically mark skeletal muscle satellite cells and are implicated in satellite cell maintenance and muscle regeneration.

Myogenesis in the embryo and the adult mammal consists of a highly organized and regulated sequence of cellular processes to form or repair muscle tissue that include cell proliferation, migration, and differentiation. Data from cell culture and in vivo experiments implicate both FGFs and HGF as critical regulators of these processes. Both factors require heparan sulfate glycosaminoglycans for signaling from their respective receptors. Since syndecans, a family of cell-surface transmembrane heparan sulfate proteoglycans (HSPGs) are implicated in FGF signaling and skeletal muscle differentiation, we examined the expression of syndecans 1-4 in embryonic, fetal, postnatal, and adult muscle tissue, as well as on primary adult muscle fiber cultures. We show that syndecan-1, -3, and -4 are expressed in developing skeletal muscle tissue and that syndecan-3 and -4 expression is highly restricted in adult skeletal muscle to cells retaining myogenic capacity. These two HSPGs appear to be expressed exclusively and universally on quiescent adult satellite cells in adult skeletal muscle tissue, suggesting a role for HSPGs in satellite cell maintenance or activation. Once activated, all satellite cells maintain expression of syndecan-3 and syndecan-4 for at least 96 h, also implicating these HSPGs in muscle regeneration. Inhibition of HSPG sulfation by treatment of intact myofibers with chlorate results in delayed proliferation and altered MyoD expression, demonstrating that heparan sulfate is required for proper progression of the early satellite cell myogenic program. These data suggest that, in addition to providing potentially useful new markers for satellite cells, syndecan-3 and syndecan-4 may play important regulatory roles in satellite cell maintenance, activation, proliferation, and differentiation during skeletal muscle regeneration.     

Hesr1 and Hesr3 are essential to generate undifferentiated quiescent satellite cells and to maintain satellite cell numbers

Satellite cells, which are skeletal muscle stem cells, divide to provide new myonuclei to growing muscle fibers during postnatal development, and then are maintained in an undifferentiated quiescent state in adult skeletal muscle. This state is considered to be essential for the maintenance of satellite cells, but their molecular regulation is unknown. We show that Hesr1 (Hey1) and Hesr3 (Heyl) (which are known Notch target genes) are expressed simultaneously in skeletal muscle only in satellite cells. In Hesr1 and Hesr3 single-knockout mice, no obvious abnormalities of satellite cells or muscle regenerative potentials are observed. However, the generation of undifferentiated quiescent satellite cells is impaired during postnatal development in Hesr1/3 double-knockout mice. As a result, myogenic (MyoD and myogenin) and proliferative (Ki67) proteins are expressed in adult satellite cells. Consistent with the in vivo results, Hesr1/3-null myoblasts generate very few Pax7(+) MyoD(-) undifferentiated cells in vitro. Furthermore, the satellite cell number gradually decreases in Hesr1/3 double-knockout mice even after it has stabilized in control mice, and an age-dependent regeneration defect is observed. In vivo results suggest that premature differentiation, but not cell death, is the reason for the reduced number of satellite cells in Hesr1/3 double-knockout mice. These results indicate that Hesr1 and Hesr3 are essential for the generation of adult satellite cells and for the maintenance of skeletal muscle homeostasis.     

Satellite cells and utrophin are not directly correlated with the degree of skeletal muscle damage in mdx mice

To determine whether muscle satellite cells and utrophin are correlated with the degree of damage in mdx skeletal muscles, we measured the area of the degenerative region as an indicator of myofiber degeneration in the masseter, gastrocnemius, soleus, and diaphragm muscles of mdx mice. Furthermore, we analyzed the expression levels of the paired box homeotic gene 7 (pax7), m-cadherin (the makers of muscle satellite cells), and utrophin mRNA. We also investigated the immunolocalization of m-cadherin and utrophin proteins in the muscles of normal C57BL/10J (B10) and mdx mice. The expression level of pax7 mRNA and the percentage of m-cadherin-positive cells among the total number of cell nuclei in the muscle tissues in all four muscles studied were greater in the mdx mice than in the B10 mice. However, there was no significant correlation between muscle damage and expression level for pax7 mRNA (R = –0.140), nor was there a correlation between muscle damage and the percentage of satellite cells among the total number of cell nuclei (R = –0.411) in the mdx mice. The expression level of utrophin mRNA and the intensity of immunostaining for utrophin in all four muscles studied were greater in the mdx mice than in the B10 mice. However, there also was not a significant correlation between muscle damage and expression level of utrophin mRNA (R = 0.231) in the mdx mice, although upregulated utrophin was incorporated into the sarcolemma. These results suggest that satellite cells and utrophin are not directly correlated with the degree of skeletal muscle damage in mdx mice. dystrophy; pax7; m-cadherin; dystrophin-related proteins     

New fiber formation in the interstitial spaces of rat skeletal muscle during postnatal growth.

The purpose of this study was to determine whether fiber hyperplasia occurs in the rat plantaris muscle during postnatal weeks 3-20. Total muscle fiber number, obtained via the nitric acid digestion method, increased by 28% during the early postnatal rapid growth phase (3-10 weeks), whereas the number of branched fibers was consistently low. Whole-muscle mitotic activity and amino acid uptake levels showed an inverse relationship to the increase in total fiber number. The expression of MyoD mRNA (RT-PCR) levels decreased from 3 to 20 weeks of age, as did the detection of anti-BrdU- and MyoD-positive cells in histological sections. Immunohistochemical staining patterns for MyoD, myogenin, or developmental myosin heavy chain on sections stained for laminin (identification of the basal lamina) and electron micrographs clearly indicate that de novo fiber formation occurred in the interstitial spaces. Myogenic cells in the interstitial spaces were negative for the reliable specific satellite cell marker M-cadherin. In contrast, CD34 (an established marker for hematopoietic stem cells)-positive cells were located only in the interstitial spaces, and their frequency and location were similar to those of MyoD- and/or myogenin-positive cells. These findings are consistent with fiber hyperplasia occurring in the interstitial spaces of the rat plantaris muscle during the rapid postnatal growth phase. Furthermore, these data suggest that the new fibers may be formed from myogenic cells in the interstitial spaces of skeletal muscle and may express CD34 that is distinct from satellite cells.