Changes in gene expression during myogenic differentiation. II. Identification of the proteins encoded by myotube-specific complementary DNA sequences

Differentiation and proliferation of respiration-deficient human myoblasts

Replication and transcription of mitochondrial DNA were impaired in dividing human myoblasts exposed to ethidium bromide. MtDNA content decreased linearly per cell division and mitochondrial transcript levels declined rapidly, resulting in respiration-deficiency of the myoblasts. Despite the absence of functional mitochondria the cells remained able to proliferate when grown under specific culture conditions. However, the formation of myotubes was severely impaired in respiration-deficient myoblasts. We conclude that differentiation of myoblasts into myotubes is more dependent on mitochondrial function than proliferation of myoblasts.

Steady-state transcript levels of cytochrome c oxidase genes during human myogenesis indicate subunit switching of subunit VIa and co-expression of subunit VIIa isoforms

Steady-state levels of the mitochondrial rRNAs, of mRNAs for mitochondrially and nuclear-encoded subunits of cytochrome c oxidase and for the beta subunit of ATP synthase were assessed by Northern blot hybridizations during the in vitro differentiation of human myoblasts. Transcript levels of the so-called liver-type form of subunit VIa of cytochrome c oxidase diminished during the course of differentiation, while transcription of the so-called heart-type form was induced. Transcripts for the liver-type form and for the heart-type form of subunit VIIa of cytochrome c oxidase were detected in all myogenic cultures; the levels of the heart-type form progressively increased during the course of differentiation. The levels of the other transcripts studied did not change substantially. The results suggest subunit switching of subunit VIa and co-expression of subunit VIIa isoforms during myogenesis. The differential changes in mRNA levels of the heart-type subunits VIa and VIIa and the differential changes in mRNA levels of the liver-type subunits VIa and VIIa demonstrate that different transcriptional regulation mechanisms are present for both heart-type genes as well as for both liver-type genes.

Structural alterations in chromatin during myogenesis in the chicken

Differentiation of mononucleated myoblasts to multinucleated myotubes is accompanied by hypertrophy achieved by co-ordinated synthesis of muscle proteins. This process may be achieved by co-ordinated synthesis and translation of new mRNA or gradual accumulation of constitutively synthesized mRNA, followed by coordinated translational activation. If the former process occurs, many structural alterations should occur in chromatin, whereas in the latter scenario, no chromatin changes will be necessary. The results of our investigation into chromatin structure of myoblast and myotube nuclei show that according to techniques used, viz. chromatin solubilization by nucleases, thermal denaturation, in vitro transcription, nucleosome sizing, there are major structural changes in chromatin during muscle cell differentiation. Since these alterations were detectable at a fairly gross level, many genes must be affected which could account for the increase in RNA and proteins observed in myotubes. This evidence argus in favour of new mRNA synthesis for rapid translation, rather than a gradual accumulation of mRNA followed by co-ordinated translation.

Temperature-sensitive non-fusing myoblast variant and spontaneous revertant: isolation and characterization

A stable, temperature-sensitive, non-fusing variant of the L6 rat myoblast cell line has been isolated following mild EMS-induced mutagenesis. At the permissive temperature (37 degrees C), the growth characteristics and developmental pattern of the tsA1 variant are essentially identical to those of the parental L6D0 line at either 37 degrees C or 40 degrees C. At the nonpermissive temperature (40 degrees C), the tsA1 variant grows normally but does not align, fuse, or synthesize detectable amounts of beta-tropomyosin or myosin LC2. A peptide corresponding to myosin LClemb is barely detectable. The temperature-sensitive period spans the interval from 4 to 72 h post-plating with a midpoint at approximately 40 h. Under standard culture conditions, commitment to terminal differentiation occurs between days 3 and 4, and alignment and fusion begin on days 4 and 5, respectively. Thus, the temperature-sensitive event occurs very early in the L6 developmental program. A spontaneous revertant of the temperature-sensitive phenotype (tsA1 [R3]) exhibits recovery of the capacities to align, fuse, and synthesize the repertoire of muscle-specific proteins, suggesting that a single pleiotropic mutation in the tsA1 variant may regulate several stages in L6 myogenesis.

5-Azacytidine induction of stable mesodermal stem cell lineages from 10T1/2 cells: evidence for regulatory genes controlling determination

5-Azacytidine converts the mouse embryonic cell line C3H 10T1/2 into differentiated chondrocytes, adipocytes, and skeletal muscle. Clonal and 2D protein gel analyses demonstrate that 5-azacytidine converts 10T1/2 cells into three stably determined, but undifferentiated, stem cell lineages which can differentiate into myofibers, chondrocytes, and adipocytes. Conversion of 10T1/2 cells is accompanied by specific changes in protein synthetic patterns unique for each cell lineage. We propose that 5-azacytidine converts 10T1/2 cells by hypomethylation of "determination" regulatory loci which establish lineages of stem cells with a restricted potential to differentiate into muscle, cartilage, or fat cells. Our results suggest that these three lineages are specified by separate regulatory loci and that as few as 1-3 hypomethylation events per cell are sufficient to activate the hypothesized muscle regulatory locus. Conversion of 10T1/2 cells by 5-azacytidine provides a model for studying regulatory genes involved in cell lineage determination.

Evidence for transcriptional regulation of the myosin heavy chain gene during myogenesis

One of the changes accompanying skeletal muscle cell (myoblast) fusion is a dramatic increase in synthesis of muscle specific proteins, one of which is myosin. The underlying mechanism for this burst in synthesis is not yet understood but may occur by two mechanisms: (a) gradual storage of mRNA and translational control as found by others or (b) gene activation and rapid synthesis of mRNA for immediate translation. In this paper we show that the myosin gene changes its organization such that postfusion skeletal muscle cells show an increased susceptibility to DNase I, a recognized probe for gene activation. We also show that this change accompanies an increase in rate of transcription and an increased cell content of myosin heavy chain mRNA. This work shows that transcriptional control is an important mechanism during muscle cell development in addition to the translational control shown by other workers.

Molecular cloning of pea mRNAs encoding a shoot-specific polypeptide and light-induced polypeptides

The molecular cloning of cDNA corresponds to pea seedling mRNA sequences encoding a shoot-specific polypeptide, the small subunit of the ribulose 1,5 biphosphate carboxylase and a component of the light-harvesting chlorophyll a/b complex is described. cDNA prepared from polysomal poly(A)RNA of light-grown shoots was enriched for shoot-specific and light-induced sequences by heterologous liquid hybridization with mercurated polysomal poly(A)RNA of dark-grown roots, followed by sulfhydryl chromatography. Cloned shoot-specific sequences were identified by 2D electrophoretic analysis of hybrid release translation products. The cloned shoot-specific sequence corresponded to a mRNA of 850 nt present both in light-and dark-grown shoots, and produced anin vitro translation product of Mr27 500 and isoelectric point of 4.7.

Drosophila muscle myosin heavy chain encoded by a single gene in a cluster of muscle mutations

Drosophila muscle myosin heavy chain is encoded by a single-copy gene which is transcribed during both larval and adult development. This myosin gene maps to a chromosomal locus distant from any of the actin genes, but is within a cluster of flight muscle mutations.

cDNA clone analysis of six co-regulated mRNAs encoding skeletal muscle contractile proteins

A cDNA cloning approach was used to investigate muscle gene regulation during differentiation of cultured embryonic quail myoblasts. A cDNA clone library of cultured myofiber poly(A)+RNA was constructed and screened by colony hybridization with cDNA probes of myoblast and myofiber RNA. Twenty-eight myofiber-specific cDNA clones were identified and, by cross-hybridization analysis, these clones were found to represent, at most, 18 different myofiber-specific RNAs. Six of these RNAs were identified by sequence analysis of the cDNA clones. These six RNAs encode the contractile proteins alpha-actin, alpha-tropomyosin, myosin heavy chain, myosin light chain 2, troponin C, and troponin I. The embryonic muscle contractile protein sequences are identical with, or closely match, those of adult skeletal muscle proteins and include both fast fiber (myosin light chain 2 and troponin I) and slow fiber (troponin C) isotypes. RNA gel transfer hybridization analysis showed that the cellular abundances of these contractile protein mRNAs increase 20- to 30-fold or more during myoblast differentiation. These findings indicate that coordinate activation of contractile protein synthesis during myogenesis is controlled by mechanisms that direct the accumulation of contractile protein mRNAs rather than their translational utilization. Furthermore, with the possible exception of myosin heavy chain, the contractile protein genes expressed by cultured embryogenic muscle encode adult muscle proteins of both basal and slow fiber types, consistent with a co-activation-selective repression model of gene regulation during fiber type differentiation in developing skeletal muscle.

Verapamil preserves myocardial contractility in the hereditary cardiomyopathy of the Syrian hamster

We attempted to alter the inherited myocardial damage and loss of contractility of the cardiomyopathic Syrian hamster (strain U-MX7-1) by giving cardiac drugs that altered intracellular calcium and myocardial workload. Thirty-seven 21-day-old cardiomyopathic and thirty-seven 21-day-old normal hamsters were divided into five groups each: verapamil-, propranolol-, digoxin-, hydralazine-, and saline-injected. On their 90th day of life, the hamsters were killed. Of the five cardiomyopathic groups, only verapamil reduced myocardial damage. When both "control" and cardiomyopathic hamsters were treated with saline, digoxin, or propranolol, the cardiomyopathic hamsters had significantly less contractile force, maximal rate of force development, and maximum velocity of unloaded shortening. When both groups were treated with verapamil or hydralazine, there were no significant group differences in the indices of contractility. However, when saline-treated cardiomyopathic hamsters were compared with drug-treated cardiomyopathic hamsters, only verapamil preserved myocardial contractility. There was also a weak correlation between the Vmax and the actin-activated ATPase activity of the cardiomyopathic hamsters (r = 0.63, P less than 0.001). We conclude that verapamil helped protect the myocardium of genetically cardiomyopathic hamsters against structural damage, and helped preserve myocardial contractility.

Cell-free translation of messenger RNAs from adult and fetal human muscle. Characterization of neosynthesized glycogen phosphorylase, phosphofructokinase and glucose phosphate isomerase

Using a procedure of ethanol precipitation in concentrated guanidine . HCl solutions followed by chloroform/isoamylic alcohol extraction and washing in 3 M sodium acetate, we isolated high-molecular-weight cellular RNA from human fetal and adult skeletal muscle. About 500 micrograms RNA were obtained/g of fetal muscle and 50 micrograms RNA/g of adult muscle. Both RNA preparations were efficiently translated in a cell-free reticulocyte lysate system and directed synthesis of various polypeptides, one of them of Mr 200,000 probably corresponding to myosin heavy chains. Dodecylsulphate/polyacrylamide gel electrophoretic pattern of polypeptides neosynthesized using either fetal or adult RNA exhibited several differences. Three neosynthesized cytosolic muscle enzymes were purified from the translation mixtures using a micro-method of immunoaffinity chromatography; specificity of the neosynthesized polypeptides purified according to this procedure was checked by immunological competition with the corresponding unlabeled pure muscle enzymes. Successful cell-free translation of RNA from adult skeletal muscle and purification of neosynthesized human enzymes are reported for the first time. These methods, indispensable for further studies on human adult muscle gene expression, could also shed light on the mechanism of some inherited molecular diseases and tumoral or dystrophic processes.