Gene expression during the differentiation of myogenic cells of the L6 line

A role for alphaV integrin subunit in TGF-beta-stimulated osteoclastogenesis

TGF-beta increases bone resorption in vivo and greatly increases osteoclast formation stimulated by receptor activator of NF-kappaB ligand (RANKL) in vitro. TGF-beta does not independently affect the differentiation state of RAW264.7 preosteoclasts, but increases cell attachment to vitronectin. This effect is mediated by increased expression of alphaV integrin subunit mRNA and protein. Concomitant with induction of osteoclast differentiation, RANKL causes relocation of alphaV to focal sites in the cell. This effect is potentiated by TGF-beta. Integrin blockade disrupts both attachment to vitronectin and RANKL-induced osteoclast formation, but culture on vitronectin has little effect. Ectopic expression of alphaV stimulates multinucleation of RAW264.7 cells and increases the number of osteoclasts formed in the presence of RANKL. These data suggest that TGF-beta potentiates RANKL-induced osteoclast formation, in part by increased expression of the alphaV integrin subunit, which may contribute to cell fusion.

Characterization of sarcoplasmic reticulum in skinned muscle cultures

The plasma membranes of chick or rat skeletal muscles, grown in cell culture, were made permeable with saponin in a solution lacking calcium. The cells were then supplied with a medium resembling the cytosol and the ATP-dependent Ca2+ sequestration was performed. Based on the low concentration of free Ca2+ in the medium (below 5 microM), the presence of mitochondrial inhibitors and the effect of drugs that interfere with sarcoplasmic reticulum (SR) function, we assume that the measured Ca2+ accumulation expresses SR function on the saponin-treated myotubes. The development of the SR in muscle cultures is augmented as myogenesis proceeds and depends on its occurrence. Whereas creatine kinase activity is elevated immediately following cell fusion, there is a delay of at least 1 day between myoblast fusion and the increase in Ca2+ accumulation in the SR. Thyroxine or triiodothyronine caused an inhibition of Ca2+ accumulation in rat or chick muscle cultures. This inhibition could explain some of the muscle abnormalities caused by excess of thyroid hormones. A comparison was made between a white-type (fast) and heterogeneous muscle, differentiated in cell culture. There was no significant difference in SR function, indicating the important role of innervation in specifying the properties of muscle fiber types.

Metalloendoprotease inhibitors that block fusion also prevent biochemical differentiation in L6 myoblasts

The effect of metalloendoprotease inhibitors on the biochemical differentiation of the rat skeletal muscle line, L6, was investigated. Confluent unfused L6 cells exposed briefly to 1,10-phenanthroline, a chelator of divalent metal cations, or continuously to dipeptide amide metalloendoprotease substrates that are blocked at the NH2-terminals, N-carbobenzyloxyserylleucyl amide and N-carbobenzyloxyglycylleucyl amide, did not fuse or express creatine kinase, myosin heavy chain, or alpha-actin. These effects were reversible and dose-dependent. Exposure to N-carbobenzyloxylglycylglycyl amide, which is not a metalloendoprotease inhibitor, had no effect. As the differentiation in a culture progressed, 1,10-phenanthroline became less effective in blocking the accumulation of creatine kinase and myosin heavy chain. Exposure of partially fused cultures to N-carbobenzyloxyserylleucyl amide prevented any further accumulation of muscle-specific proteins. In confluent cultures where cell division was blocked before the onset of differentiation, N-carbobenzyloxyserylleucyl amide still prevented fusion and the induction of creatine kinase. This indicates that these inhibitors do not act by interfering with the cell cycle. Experiments that measured DNA synthesis rates, plating efficiencies, and the effects of sequential dipeptide and dimethyl sulfoxide treatments indicate that L6 myoblasts do not irreversibly withdraw from the cell cycle when exposed to N-carbobenzyloxyserylleucyl amide. These results are consistent with the role of a metalloendoprotease in initiating the terminal differentiation of cultured muscle cells.

Metabolic changes in the extracellular matrix during differentiation of myoblasts of the L6 line and of a Myo- non-fusing mutant

In the present study we have characterized by biochemical and immunochemical methods the changes which take place in collagen, laminin and fibronectin biosynthesis during the differentiation of clonal skeletal myoblasts of the L6 line. Time-course experiments showed that the relative rate of synthesis of collagen increased significantly during the cell-cell contact step of myogenesis and decreased later on. The major collagens synthesized were types I and III, found mainly as soluble precursors in the culture medium. Types IV and V collagens were detected exclusively in the cell layer. The relative amounts of types I and III collagens remained unchanged during myogenesis, while types IV and V collagens increased as the cells of the L6 line fused. In a non-fusing alpha-amanitin-resistant mutant of the L6 line (Ama 102), the rate of collagen synthesis was largely depressed and its rate of degradation was increased as compared with the fusing wild type. The synthesis of laminin was very low in cells of the fusing wild type, but abundant and associated with the cell layer of the Myo- mutant. The appearance of a muscle-specific extracellular matrix is a complex process involving changes in the organization, the biosynthesis and remodelling of its macromolecules of the extracellular matrix.

Expression of a developmentally regulated antigen on the surface of skeletal and cardiac muscle cells

H36 is a species-specific, cell-surface antigen on differentiating newborn rat skeletal myoblasts and myogenic lines. This membrane antigen has been defined by a monoclonal antibody raised by the fusion of SP 2/0-Ag14 myeloma cells with spleen cells from mice immunized with myotubes derived from the myogenic E63 line. H36 antigen, isolated by immunoaffinity chromatography, is comprised of two polypeptides with apparent molecular weights of 98,000 and 117,000. Fluorescence photometry and radioimmunoassays have been used to follow quantitative and topographic changes in the H36 determinant during myogenesis. H36 is present at a basal level on replicating myoblasts; it increases on prefusion myoblasts and persists on myotubes. At or near the time of prefusion, it becomes concentrated between adjacent aligned myoblasts and localized on membrane "blebs". H36 is present on both skeletal and cardiac cells but absent from a variety of cells that include fibroblasts, neuronal cells, and smooth muscle. There are approximately 4 x 10(5) determinants per myoblast, and the Ka of the antibody is 3.8 x 10(8) liters/mol. The distributions of H36 on the top and attached surfaces of myoblasts and myotubes are distinct, which suggests localized specialization of these surfaces. H36 is an integral membrane component and upon cross-linking, it associates with the detergent-insoluble cytoskeletal framework. Inhibition of myogenesis by 5-bromodeoxyuridine or by calcium deprivation prevents the developmentally associated changes in the expression of H36. H36 is also absent or markedly reduced on the fu- and Ama102 developmentally defective mutant myoblast lines. We conclude that H36 is a muscle-specific, developmentally regulated cell-surface antigen that may have a role in myoblast differentiation and that can be used to determine the embryonic lineages of skeletal and cardiac muscle.

Stimulation and inhibition of myoblast differentiation by hormones

The growth and differentiation of L6 myoblasts are subject to control by two proteins secreted by cells of the Buffalo rat liver line. The first of these, rat insulinlike growth factor-II (formerly designated multiplication stimulating activity) is a potent stimulator of myoblast proliferation and differentiation, as well as associated processes such as amino acid uptake and incorporation into protein, RNA synthesis, and thymidine incorporation into DNA. In addition, this hormone causes a significant decrease in the rate of protein degradation. All of these actions seem to be attributable to a single molecular species, although their time courses and sensitivity to the hormone differ substantially. The second protein, the differentiation inhibitor (DI), is a nonmitogenic inhibitor of all tested aspects of myoblast differentiation, including fusion and the elevation of creatine kinase. Indirect immunofluorescence experiments demonstrated that DI also blocks accumulation of myosin heavy chain and myomesin. Upon removal of DI after 72 h incubation, all of these effects were reversed and normal myotubes containing the usual complement of muscle-specific proteins were formed. Thus, this system makes it possible to achieve specific stimulation or inhibition of muscle cell differentiation by addition of purified proteins to cloned cells in serum-free medium.

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

Cell cycle parameters and expression of myoblast and myocyte antigens were investigated during exponential growth and during the differentiation phase of rat L8( E63 ) myoblasts by an integrated approach involving microspectrophotometry with DNA fluorochromes, [3H]thymidine autoradiography, and immunofluorescent staining with monoclonal antibodies. In addition to the majority of cells which are recruited into myotubes, two distinct populations of mononucleate cells were resolved in cultures of rat myoblasts undergoing differentiation. These mononucleate cells consist of (1) a population of proliferating cells with a prolonged G1 transit time; (2) a population of non-proliferating cells which remain arrested in G1 for more than 72 h. The latter group was examined with respect to the expression of two marker antigens recognized by two monoclonal antibodies: antibody B58 reacts with a macromolecular component present in undifferentiated myoblasts but not in mature myotubes, and antibody XMlb reacts with a muscle-specific isoform of myosin. All four possible combinations of expression of these antigens by single cells were found: B58 +XM1b -, B58 +XM1b +, B58 - XM1b -, and B58 - XMlb +. The implication of these findings with respect to the transition from the proliferative to the differentiative phase of myogenesis is discussed.

Analysis of muscle protein expression in polyethylene glycol-induced chicken: rat myoblast heterokaryons

Heterokaryons derived from polyethylene glycol-mediated fusion of myoblasts at different stages of development were used to investigate the transition of cells in the skeletal muscle lineage from the determined to the differentiated state. Heterokaryons were analyzed by immunofluorescence, using rabbit antibodies against the skeletal muscle isoforms of chicken creatine kinase and myosin, and a mouse monoclonal antibody that cross-reacts with chicken and rat skeletal muscle myosin. When cytochalasin B-treated rat L8(E63) myocytes (Konieczny S.F., J. McKay, and J. R. Coleman, 1982, Dev. Biol., 91:11-26) served as the differentiated parental component and chicken limb myoblasts from stage 23-26 or 10-12-d embryos were used as the determined, undifferentiated parental cell, heterokaryons exhibited a progressive extinction of rat skeletal muscle myosin during a 4-6-d culture period, and no precocious expression of chicken differentiated gene products was detected. In the reciprocal experiment, 85-97% of rat myoblast X chicken myocyte heterokaryons ceased expression of chicken skeletal muscle myosin and the M subunit of chicken creatine kinase within 7 d of culture. Extinction was not observed in heterokaryons produced by fusion of differentiated chicken and differentiated rat myocytes and thus is not due to species incompatibility or to the polyethylene glycol treatment itself. The results suggest that, when confronted in a common cytoplasm, the regulatory factors that maintain myoblasts in a proliferating, undifferentiated state are dominant over those that govern expression of differentiated gene products.

Modulation of differentiation in vitro. I. Influence of the attachment surface on myogenesis

Myogenic cells of the L6 line proliferate and fuse in culture to form myotubes that actively synthesize muscle-specific proteins such as myosin. We show that the expression of the differentiated phenotype can be influenced by the electrical charges of the substratum on which the cells were grown. Negatively charged surfaces did not influence the developmental program of the cells although positively charged ones interfered with myogenesis. The interaction operates primarily by interfering with the mitotic cycle, which is slowed down, with fusion which is blocked, and with myosin synthesis, which is reduced. Our results show that growth of the cells on positively charged surfaces prevents the switching of a large fraction of the population from a proliferative state to a differentiating program. We postulate that this interference might operate through the slowdown in DNA replication. The cell culture method described represents a good model for studying the different steps involved in the differentiation of L6 cells.

Manipulation of myogenesis in vitro: reversible inhibition by DMSO

A system has been developed for the detailed analysis of the transition from proliferative myoblast to differentiated muscle cell. Dimethylsulfoxide (DMSO) prevents the terminal differentiation of L8 myoblasts in vitro, and its effect is reversible. DMSO (2%) inhibits the fusion of myoblasts to form multinucleate myotubes, the normal increases in activity of creatine phosphokinase (CPK) and acetylcholinesterase, and the synthesis of alpha-actin and acetylcholine receptor protein. Upon removal of DMSO from the medium, a lag precedes the onset of differentiation. The potential to inhibit muscle differentiation reversibly is not specific to DMSO, but is shared by a number of compounds, including dimethylformamide, hexamethylbisacetamide and butyric acid, all potent inducers of gene expression in Friend erythroleukemia cells. L8 cells routinely cease DNA synthesis and initiate fusion and muscle protein synthesis once they are confluent. In the presence of DMSO, however, nearly all cells continue DNA synthesis, even several days after reaching confluence. Protein synthetic patterns of DMSO-inhibited cells are almost indistinguishable from those of untreated myoblasts and distinct from differentiated myotubes. It appears that cells exposed to DMSO are locked indefinitely in a proliferative myoblast stage of development and are unable to enter the Go phase of the cell cycle necessary for initiation of differentiation. DMSO coordinately inhibits all the differentiative parameters measured. In contrast, cytochalasin B uncouples normally linked differentiative events so that fusion is inhibited while muscle-specific protein synthesis proceeds. DMSO has similar effects on both cytochalasin B-treated and fusing control cultures, suggesting that its primary effect is exerted not at the level of fusion but earlier in the differentiative time-table. Once fusion and the synthesis of muscle-specific proteins are well under way, the addition of DMSO is ineffective and differentiation continues in its presence. The potential to manipulate muscle gene expression in vitro makes this system particularly useful for the detailed analysis of the processes involved in the transition to the differentiated state and for determining the linkage of developmental events.

Hormone responsiveness of adenylate cyclase activity of cultured myogenic cells

We studied the effect of catecholamines on the adenylate cyclase activity of myogenic cells of the L6 line during the differentiation process. The enzyme of mononucleated myoblasts was found to be activated by adrenaline L, noradrenaline L, and to a small extent, by adrenaline D and dihydrocymandelic acid. The adenylate cyclase of the differentiated cells responded to adrenaline L and noradrenaline L but not to adrenaline D or dihydroxymandelic acid. This activation could be inhibited by the addition of DOPA and propranolol, a specific beta adrenergic blocker. alpha adrenergic compounds such as phentolamine did not have any effect. It is concluded that beta adrenergic receptors are present on myogenic cells before and after differentiation.