Deoxyribonucleic acid synthesis, mitosis, and skeletal muscle differentiation

Involvement of ribonucleotide reductase in cellular differentiation

L6 and L8 rat myoblast cell lines have been selected for resistance to hydroxyurea, an antineoplastic agent whose intracellular target is the rate-limiting enzyme activity of DNA synthesis, ribonucleotide reductase. In contrast to the differentiation-competent parental lines from which they were selected, the drug-resistant lines exhibit a grossly altered or absent myogenic capacity. Independent selections have revealed a strong correlation between changes in ribonucleotide reductase, as determined by velocity levels and product pool analyses, and altered myogenic potential. These results provide the first indication that alterations in this key enzyme activity and its accompanying deoxyribonucleoside triphosphate pools can affect cellular differentiation.

Hormonal stimulation of myoblast differentiation in the absence of DNA synthesis

The role of DNA synthesis in the final stages of muscle cell differentiation has been a subject of controversy for more than a decade. In an attempt to resolve disagreements over the necessity for a unique (or "quantal") mitosis just prior to the conversion of proliferating myoblasts to form postmitotic myotubes, we have studied the effects of insulin and somatomedin on the stimulation of myoblast differentiation with or without DNA synthesis. Under conditions in which at least 95% of [3H]thymidine incorporation was blocked by cytosine arabinoside, there was a 5- to 10-fold increase in the extent of differentiation (determined as fusion or creatine kinase elevation) on addition of insulin or multiplication-stimulating activity. The effect of the hormones was on myoblast differentiation, not enzyme induction; insulin did not cause any increase in creatine kinase when it was added to performed myotubes. These studies were done using two different cell types, Yaffe's L6 cell line and Japanese quail myoblasts in serum-free media; we obtained similar results in both. Our results are not compatible with the view that a quantal mitosis is required at a late stage of muscle cell differentiation.

Quantitation of changes in cell surface determinants during skeletal muscle cell differentiation using monospecific antibody

The differentiation of skeletal muscle is characterized by recognition, alignment, and subsequent fusion of myoblast cells at their surfaces to form large, multinucleated myotubes. Monoclonal antibodies were used to investigate antigenic changes in the cell surface membrane specific for various stages of myogenesis. Chick embryonic skeletal muscle cells were cultured in vitro to the desired stage of differentiation and then injected into BALB/c mice. Spleen cells from the immunized mice were hybridized with NS-1 or P3 8653 mouse myeloma cells. Hybrid cell clones were selected in HAT medium and screened using an indirect radioimmunoassay for the production of monoclonal antibodies specific to myogenic cell surfaces. Target cells for the radioimmunoassay included three stages of myogenesis (myoblasts, midfusion, myoblasts, and myotubes) and chick lung cells as a control for polymorphic antigens. Sixty-one clones were obtained which produced antibodies specific for myogenic cells. Thirty-five of these clones were generated from mice immunized with midfusion myoblast stages of myogenesis and 26 were obtained from mice immunized with the later myotube stage of myogenesis. Quantitative measurements by RIA of myogenic determinants per cell surface area on each target cell type revealed that most of the determinants decrease during myogenesis when midfusion myoblasts are used as the immunogen. When myotube stages are used as the immunogen, more determinants increase with cell differentiation. Therefore, the most common pattern of determinant change is for them to be present at all stages of myogenesis but to vary quantitatively through development. There are determinants unique to each stage of myogenesis and marked quantitative differences within a cell stage for each determinant.

Altered protein synthesis and creatine kinase in breast muscle cell cultures from dystrophic chick embryos

The total protein synthesis (TPS), myosin synthesis (MS) and creatine kinase (CK) levels in muscle cell cultures obtained from 400 normal (strain 454) and 400 dystrophic chick embryos (strain 455) were investigated. The cultures were obtained from breast muscles of 12 day chick embryos by dissociation in 0.25% trypsin, preplating and plating of 5 x 10(5) floating cells on gelatin coated dishes in Minimal Essential Medium, 10% horse serum and 2% chick embryo extract. After 6 days, when electron-microscopic studies demonstrated good muscle differentiation, cell cultures were labeled with [3H]leucine. TPS and MS, respectively, showed 85% and 65% increases in breast muscle cell cultures from dystrophic chick embryos. The half-life times for total protein and myosin from dystrophics were 19 and 32 hr, respectively as compared with 36 and 48 hr from controls. Noncollagen protein content (NCP) showed 27% decrease in postfusion stage (12 days) of cell cultures from dystrophics. The CK level showed 30% lower values in the cells from dystrophics but 50% higher values in their culture medium. The addition of leupeptin plus pepstatin (50 microgram/ml) to these cultures resotred NCP content, total protein and myosin turnover to normal values and significantly increased TPS and MS. The addition of diphenylhydantoin (DPH) (20 microgram/ml) to cell cultures from dystrophics did not change the NCP content nor the turnover for total protein and myosin but significantly increased TPS, MS and CK while medium CK significantly decreased. The addition of leupeptin plus pepstatin or DPH to muscle cell cultures from normal chick embryos also significantly stimulated TPS and MS.

DNA polymerase activity in muscle cultures

Nuclei within myotubes do not synthesize DNA for replication. Accordingly, cultures of myotubes display low levels of DNA polymerase activity. The coincidental decline in DNA polymerase activity and increased formation of multinucleated myotubes during culture does not prove that the loss of capacity to synthesize DNA is a consequence of fusion. Tne experiments described demonstrate that myogenic cells prevented from fusing have low levels of DNA polymerase activity. This is consistent with the notion that, in myogenic cultures, there is a population of mononucleated cells, the myoblasts, which have withdrawn from the mitotic cycle before fusion.

Lineages, quantal cell cycles, and the generation of cell diversity

Most theories of determination or differentiation assume that embryonic cells differ from mature cells. Embryonic cells are thought to have metastable control mechanisms. These labile controls are believed to become progressively more stabilized as the cells differentiate. Zygote, blastula, neural plate, limb bud, somite, or ‘stem’ cells are conceived of as undifferentiated, totipotent, or multipotential cells. As such, these cells supposedly have available for activation a larger repertoire of phenotypic programmes than their progeny. A necessary corollary to this view is that the activation of one particular phenotypic programme out of the many available is a function of instructive exogenous inducing molecules.

Incorporation of fucose and glucosamine into cell bound and medium released macromolecules

(1) Determinations were carried out on the incorporation of fucose-6-(3H) and glucosamine-6-(3H) into trichloracetic acid insoluble macromolecules which remained bound to the cells or were released into the medium of chick embryo muscle cell cultures. The radioactivity determined in the medium was corrected for unspecific binding of label to components of the medium. (2) During an incorporation period of six hours the incorporation per microgram DNA with fucose as label into cell bound macromolecules is about twice as high as the incorporation into macromolecules released into medium. With glucosamine about twice as much is incorporated into medium released into the cell bound macromolecules. (3) The incorporation per microgram DNA increased during a culture period of three days but the increase ceases at different times during this culture period when determined with fucose or glucosamine or for cell bound and medium released material. (4) An increase in cell density increases the incorporation per DNA of fucose and to a much slighter extent that of glucosamine. Reduction of cell density by addition of cytosine arabinoside to the medium does not increase the incorporation per microgram DNA. (5) The effect of changes of fibroblast/myoblast ratios on the incorporation of fucose and glucosamine were examined. No significant effect was observed for a ratio of 10-30% fibroblasts when control cultures or cultures after cell sedimentation were maintained in complete medium. Marked changes were observed after culture in medium without protein components. Under these conditions an increase in the fibroblast/myoblast ratios were observed as well as an increase in the incorporation of label into medium released and a decrease into cell bound macromolecules.

Effects of cytochaslasin B and colcemide on myogenic cultures

Muscle cultures treated with cytochalasin B yield mono- and oligonucleated cells of two kinds: (i) arborized, replicating precursor myogenic cells and fibroblasts; and (ii) round, post-mitotic, terminally differentiating myoblasts and myotubes. The arborized cells do not bind fluorescein-labeled antibody against myosin, do not contract rhythmically, and do not display hexagonally stacked thick and thin filaments. The round, mono-nucleated myoblasts and round, oligonucleated myotubes bind the fluorescein-labeled antibody against myosin, contract rhythmically, and display clusters of hexagonally-stacked thick and thin filaments. When cytochalasin B is removed and replaced by colcemide, the arborized cells, but not the post-mitotic muscle cells, acquire a radial symmetry and are induced to assemble massive, meandering cables that may occupy over 25% of the cell volume. These tortuous calbes are positively birefringent and consist exclusively of enormous numbers of 100-A, intermediate-sized filaments.