Expression of myoblast and myocyte antigens in relation to differentiation and the cell cycle

Comparison of lipid deposition of intramuscular preadipocytes in Tan sheep co-cultured with satellite cells or alone

The purpose of this study was to investigate the effect of the skeletal muscle satellite cells (SMSCs) on the lipid deposition of the intramuscular preadipocytes (IMPs) in a co-culture system of the Tan sheep cells. The SMSCs and IMPs from Tan sheep were separated and cultured. After the two kinds of cells were separated and cultured, they were inoculated onto a transwell cell chamber co-culture plate for co-cultivation. When the cell density reached more than 90%, the cells were induced to differentiate. After the induction of the SMSCs differentiation for 8 days, the level of the IMPs differentiation and the expression levels of the differentiation marker genes and the key enzymes of the lipid metabolism were assessed. The results showed that the number and area of the lipid droplets in the IMPs in the co-culture system were significantly reduced compared to those in the IMPs culture alone (p < 0.05). Meanwhile, the expression levels of the PPARγ, c/EBPα, ACC, FAS mRNA in the IMPs were significantly decreased (p < 0.05); the expression level of aP2 mRNA was decreased, but the difference was not significant (p > 0.05).These findings indicate that the SMSCs of the Tan sheep in the co-culture system inhibited the lipid deposition by the IMPs.

Dormancy and quiescence of skeletal muscle stem cells

The skeletal muscle of vertebrates has a huge regenerative capacity. When destroyed after different types of injury, this organ can regenerate very quickly (less than 20 days following myotoxin injection in the mouse) ad integrum and repeatedly. The cell responsible for this regeneration is the so-called satellite cell, the muscle stem cell that lies on top of the muscle fibre, a giant, multinucleated cell that contains the contractile material. When injected in the muscle, satellite cells can efficiently differentiate into contractile muscle fibres. The satellite cell shows great therapeutic potential; and its regenerative capacity has triggered particular interest in the field of muscular degeneration. In this review we will focus on one particular property of the satellite cell: its quiescence and dormancy. Indeed adult satellite cells are quiescent; they lie between the basal lamina and the basement membrane of the muscle fibre, ready to proliferate, and fuse in order to regenerate myofibers upon injury. It has recently been shown that a subpopulation of satellite cells is able to enter dormancy in human and mice cadavers. Dormancy is defined by a low metabolic state, low mobility, and a long lag before division when plated in vitro, compared to quiescent cells. This definition is also based on current knowledge about long-term hematopoietic stem cells, a subpopulation of stem cells that are described as dormant based on the same criteria (rare division and low metabolism when compared to progeny which are dividing more often). In the first part of this review, we will provide a description of satellite cells which addresses their quiescent state. We will then focus on the uneven distribution of satellite cells in the muscle and describe evidence that suggests that their dormancy differs from one muscle to the next and that one should be cautious when making generalisations regarding this cellular state. In a second part, we will discuss the transition between active dividing cells in developing animals to quiescence. This mechanism could be used or amplified in the switch from quiescence to dormancy. In a third part, we will review the signals and dynamics that actively maintain the satellite cell quiescent. The in-depth understanding of these mechanisms is key to describing how dormancy relies on quiescent state of the cells. In a fourth part, we will deal with dormancy per se: how dormant satellite cells can be obtained, their characteristics, their metabolic profile, and their molecular signature as compared to quiescent cells. Here, we will highlight one of the most important recent findings: that quiescence is a prerequisite for the entry of the satellite cell into dormancy. Since dormancy is a newly discovered phenomenon, we will review the mechanisms responsible for quiescence and activation, as these two cellular states are better known and key to understanding satellite cell dormancy. This will allow us to describe dormancy and its prerequisites.

Coordinate regulation of cell cycle and apoptosis during myogenesis

During myogenesis, precursor cells irreversibly withdraw from the cell cycle as they differentiate into mature myotubes. The state of myocyte differentiation also influences the propensity of these cells to undergo apoptosis. Proliferative precursor cells are far more susceptible to apoptotic cell death than are terminally differentiated myotubes. The upregulation of the cdk inhibitor p21 and the dephosphorylation of pRb are critical regulatory events that establish both the post-mitotic and apoptosis-resistant states. The coordinate regulation of cell proliferation and death provides the organism with a mechanism to control the deposition of muscle mass during embryonic development.

Myogenin expression, cell cycle withdrawal, and phenotypic differentiation are temporally separable events that precede cell fusion upon myogenesis

During terminal differentiation of skeletal myoblasts, cells fuse to form postmitotic multinucleated myotubes that cannot reinitiate DNA synthesis. Here we investigated the temporal relationships among these events during in vitro differentiation of C2C12 myoblasts. Cells expressing myogenin, a marker for the entry of myoblasts into the differentiation pathway, were detected first during myogenesis, followed by the appearance of mononucleated cells expressing both myogenin and the cell cycle inhibitor p21. Although expression of both proteins was sustained in mitogen-restimulated myocytes, 5-bromodeoxyuridine incorporation experiments in serum-starved cultures revealed that myogenin-positive cells remained capable of replicating DNA. In contrast, subsequent expression of p21 in differentiating myoblasts correlated with the establishment of the postmitotic state. Later during myogenesis, postmitotic (p21-positive) mononucleated myoblasts activated the expression of the muscle structural protein myosin heavy chain, and then fused to form multinucleated myotubes. Thus, despite the asynchrony in the commitment to differentiation, skeletal myogenesis is a highly ordered process of temporally separable events that begins with myogenin expression, followed by p21 induction and cell cycle arrest, then phenotypic differentiation, and finally, cell fusion.

A unique subset of developmentally regulated surface proteins turns over rapidly during fusion of the L6 rat myoblast cell line

We have previously identified several developmentally regulated surface polypeptides in the L6 rat myoblast cell line, on the basis of their susceptibility to lactoperoxidase catalyzed iodination. An analysis of the turnover rates of these polypeptides now indicates that while the bulk of the iodinated polypeptides have a half-life of 20-30 h, four low-molecular-weight polypeptides have half lives of 2-7 h. The half-lives of all of the rapid turnover class surface polypeptides were greatly increased in cultures where fusion was inhibited by chloroquine and in nonfusing variants of the L6 cell line. In contrast, inhibition of fusion by the metalloendoprotease inhibitor 1, 10-phenanthroline did not alter the turnover of any iodinatable surface proteins. We propose that some or all of the rapid turnover class of polypeptides may be surface receptors which control cell surface alterations involved in the acquisition of fusion competence or in fusion itself.

A model of myogenesis in vivo, derived from detailed autoradiographic studies of regenerating skeletal muscle, challenges the concept of quantal mitosis

We have recently shown that myogenesis following severe injury is prolonged compared with minor injury (McGeachie and Grounds 1987). In this previous autoradiographic study 44 mice were injected with tritiated thymidine at various times after muscle injury (0 to 120 h), and samples were taken 9 d after injury to determine the percentage of labelled myotube nuclei. In the present study the same experimental data are analysed in detail to reveal how many times labelled muscle precursors divided before fusing to form myotubes. Additional mice were prepared and samples removed 1 h after injection of tritiated thymidine to determine the maximum grain counts of premitotic nuclei. When a labelled premitotic nucleus divides, each of the two daughter nuclei will contain half of the original label. The grain counts of nuclei resulting from sequential divisions of a maximally labelled premitotic nucleus, forms the basis for our detailed analysis which can reveal how many times a muscle precursor has divided after labelling. Nine days after injury the autoradiographic grain counts of labelled myotube nuclei were analysed in detail. The results describe an in vivo model of myogenesis which we use to evaluate quantitatively observations derived from tissue culture studies. The analysis shows that, at the onset of myogenesis in regenerating muscle (30 h after injury), muscle precursors divide only twice before fusing to form myotubes. This observation challenges the concept of quantal mitosis as defined by the tissue culture studies of Quinn et al. (1984, 1985).

DNA replication and differentiation in rat myoblasts studied with monoclonal antibodies against 5-bromodeoxyuridine, actin, and alpha 2-macroglobulin

During the differentiation of skeletal muscle, mononucleate myoblasts proliferate, then stop replicating, spontaneously fuse, and express a large number of genes which encode the muscle phenotype. We have used monoclonal antibodies specific for 5-bromodeoxyuridine, myoactin, and equine alpha 2-macroglobulin to follow and establish the sequence of events that surround the transition from a replicating to a differentiating population. Triple-label immunofluorescence microscopy was used to visualize the changes in DNA synthesis, formation of myoactin fibers, and the cessation of endocytosis of alpha 2-macroglobulin that accompany myogenesis. Our results indicate that myoblasts cease actively endocytosing alpha 2-macroglobulin after stopping DNA synthesis but prior to fusion. Formation of myoactin fibers rarely occurs in mononucleate myoblasts and only in post-mitotic cells, but they are common in multinucleate myotubes. We suggest that the regulation of DNA synthesis is critical to normal myogenesis and that detection of incorporated BrdUrd by immunofluorescence, in conjunction with other antibodies and nucleic acid probes, is a convenient method with which to study and sequence the molecular events in single cells as they relate to the transition in DNA synthesis that accompanies differentiation.