Transcriptional and cell cycle-mediated regulation of myosin heavy chain gene expression during muscle cell differentiation

RNA-biology ruling cancer progression? Focus on 3'UTRs and splicing

The protein-coding regions of mRNAs have the information to make proteins and hence have been at the center of attention for understanding altered protein functions in disease states, including cancer. Indeed, the discovery of genomic alterations and driver mutations that change protein levels and/or activity has been pivotal in our understanding of cancer biology. However, to better understand complex molecular mechanisms that are deregulated in cancers, we also need to look at non-coding parts of mRNAs, including 3'UTRs (untranslated regions), which control mRNA stability, localization, and translation efficiency. Recently, these rather overlooked regions of mRNAs are gaining attention as mounting evidence provides functional links between 3'UTRs, protein functions, and cancer-related molecular mechanisms. Here, roles of 3'UTRs in cancer biology and mechanisms that result in cancer-specific 3'-end isoform variants will be reviewed. An increased appreciation of 3'UTRs may help the discovery of new ways to explain as of yet unknown oncogene activation and tumor suppressor inactivation cases in cancers, and provide new avenues for diagnostic and therapeutic applications.

Uncoupling of expression of an intronic microRNA and its myosin host gene by exon skipping

The ancient MYH7b gene, expressed in striated muscle and brain, encodes a sarcomeric myosin and the intronic microRNA miR-499. We find that skipping of an exon introduces a premature termination codon in the transcript that downregulates MYH7b protein production without affecting microRNA expression. Among other genes, endogenous miR-499 targets the 3' untranslated region of the transcription factor Sox6, which in turn acts as a repressor of MYH7b transcriptional activity. Thus, concerted transcription and alternative splicing uncouple the level of expression of MYH7b and miR-499 when their coexpression is not required.

Studies of acetylcholine receptor subunit gene expression: chromatin structural changes during myogenesis

Myogenesis proceeds stepwise from pluripotential stem cell to differentiated myotube. The precise number of transitions that occur along the developmental pathway remains to be determined. We examined the myogenic pathway as modelled by mouse mesodermal stem cell and muscle cell lines for stage-specific alterations in the chromatin structure of the acetylcholine receptor delta and gamma subunit genes. We reasoned that such an analysis would allow us to observe either the primary events in the activation of these muscle-specific genes or processes secondary to the binding of muscle-specific regulatory proteins. We probed chromatin structure with DNase I (deoxyribonuclease I) and precisely mapped to the 5' ends of the delta and gamma genes DNase I hypersensitive (DH) sites whose induction is unique to each myogenic stage. Putative mesodermal stem cells have the simplest pattern of DH sites with no sites near the 5' ends of the delta and gamma genes, whereas differentiated myotubes express the most complex pattern; the myoblast pattern is intermediate and of two types. In muscle cell lines where differentiation must be induced the myoblasts have a simple pattern (one more site than stem cells); in muscle lines where differentiation is spontaneous the myoblasts express a complex pattern of DH sites (one less site than myotubes). Inducible myoblasts seem to be arrested in an earlier step in the myogenic pathway than spontaneously differentiating myoblasts. Thus, myogenic activation of acetylcholine receptor subunit genes appears to be a stepwise process that can be detected by chromatin structural changes specific to four distinct stages of muscle development: stem cell, early myoblast, late myoblast, and differentiated myotube.

Human fast skeletal myosin light chain 2 cDNA: isolation, tissue specific expression of the single copy gene, comparative sequence analysis of isoforms and evolutionary relationships

A cDNA clone encoding human fast skeletal myosin regulatory light chain (HSRLC) has been isolated and characterized from a fetal muscle cDNA library. The cDNA contains the coding sequence of 170 amino acids (aa) and 58 and 91 nucleotides in the 5' and 3' untranslated regions (UTRs), respectively. HSRLC is encoded by a single copy gene in the human genome and shows a tissue-specific pattern of expression in skeletal muscle. Comparison of derived amino acid sequence of HSRLC with database sequences reveals highly conserved 12 amino acid residues in a putative calcium-binding region. HSRLC is unique among all RLC sequences in having three consecutive potential phosphorylatable serine residues. The Cys-129 of HSRLC corresponds to the critical Gly-117 of scallop RLC that is essential for its regulatory function. The clusters of hydrophobic residues that are believed to stabilize the binding of NH2-terminal of RLC with myosin heavy chain show high sequence conservation in RLCs. Besides identifying specific targets for functional studies of HSRLC by mutagenesis, the results support the concept of an ancestral gene from which the RLC genes have evolved.

Expression of the utrophin gene during myogenic differentiation

The process of myogenic differentiation is known to be accompanied by large increases ( approximately 10-fold) in the expression of genes encoding cytoskeletal and membrane proteins including dystrophin and the acetylcholine receptor (AChR) subunits, via the effects of transcription factors belonging to the MyoD family. Since in skeletal muscle (i) utrophin is a synaptic homolog to dystrophin, and (ii) the utrophin promoter contains an E-box, we examined, in the present study, expression of the utrophin gene during myogenic differentiation using the mouse C2 muscle cell line. We observed that in comparison to myoblasts, the levels of utrophin and its transcript were approximately 2-fold higher in differentiated myotubes. In order to address whether a greater rate of transcription contributed to the elevated levels of utrophin transcripts, we performed nuclear run-on assays. In these studies we determined that the rate of transcription of the utrophin gene was approximately 2-fold greater in myotubes as compared to myoblasts. Finally, we examined the stability of utrophin mRNAs in muscle cultures by two separate methods: following transcription blockade with actinomycin D and by pulse-chase experiments. Under these conditions, we determined that the half-life of utrophin mRNAs in myoblasts was approximately 20 h and that it remained largely unaffected during myogenic differentiation. Altogether, these results show that in comparison to other synaptic proteins and to dystrophin, expression of the utrophin gene is only moderately increased during myogenic differentiation.

Interleukin-1beta is a negative transcriptional regulator of alpha1-adrenergic induced gene expression in cultured cardiac myocytes

We recently reported that interleukin-1beta (IL-1beta) induces a novel form of cardiac myocyte hypertrophy characterized by an increase in protein content but an absence of the fetal program of skeletal alpha-actin or beta-myosin heavy chain (beta-MHC) gene expression (Palmer, J. N., Hartogensis, W. E., Patten, M., Fortuin, F. D., and Long, C. S. (1995) J. Clin. Invest. 95, 2555-2564). Because of the apparent disparity between this myocardial phenotype and that seen with other hypertrophic agents in culture, such as catecholamines, we investigated the effect of IL-1beta on alpha1-induced cardiomyocyte hypertrophy. Although there was no augmentation in total protein when IL-1beta and phenylephrine were given simultaneously, IL-1beta attenuated the increase in contractile protein mRNAs (skeletal alpha-actin and beta-MHC) in response to phenylephrine. Transient transfection studies with skeletal alpha-actin and beta-MHC promoter constructs linked to the chloramphenicol acetyltransferase (CAT)-reporter gene indicate that repression occurred at the level of gene transcription. In view of the previously reported activity of the zinc finger protein YY1 in the negative regulation of the skeletal alpha-actin promoter in cardiomyocytes (MacLellan, W. R., Lee, T. C., Schwartz, R. J., and Schneider, M. D. (1994) J. Biol. Chem. 269, 16754-16760), we investigated the potential role of this factor in the IL-1beta-mediated effects. Using transient transfection, we found that a mutation in the YY1 binding site of the skeletal alpha-actin promoter abolished the inhibitory effect of IL-1beta. We further found that the 127-base pair fragment of the skeletal alpha-actin promoter required for the IL-1beta effect is also required for inhibition by the overexpression of YY1 in the myocytes. Furthermore, increased levels of YY1 protein are found in IL-1beta treated myocytes. Taken together these results suggest that the repression of contractile protein gene transcription by IL-1beta may be due, at least in part, to activation of the negative transcription factor YY1.

Alteration of translation and stability of mRNA for the poly(A)-binding protein during myogenesis

The regulation of synthesis of various factors involved in mRNA translation during differentiation of muscle cells was examined. The steady-state levels of mRNAs coding for eukaryotic initiation factor (eIF) 2 alpha, 2 beta and elongation factor (eEF)-1 alpha were measured in both proliferating rat L6 myoblast and differentiated myotubes. The steady-state levels of these mRNAs were not altered during myogenesis. Furthermore, the distribution of these mRNAs between repressed and translated populations remained unchanged. Recent studies suggest a role for poly(A)-binding protein (PABP) in translation initiation. Therefore, we also examined the expression of PABP mRNA during myogenesis. The PABP mRNA was less abundant in myotubes compared to myoblasts. However, the synthesis of PABP remained unchanged. In myoblasts, approximately 50-60% of the total mRNA was associated with polyribosomes, whereas in myotubes more than 80% of the mRNA was associated with polyribosomes. These results, therefore, suggest that the PABP mRNA was more efficiently translated in differentiated myotubes than in the proliferating myoblasts. Measurement of the stability and transcription of PABP mRNA showed that, while transcription was not affected during myogenesis, the stability of the mRNA was reduced in differentiated cells. The t1/2 of PABP mRNA in myoblasts was 13 h compared to 7.5 h in myotubes. This observation suggests that the reduced steady-state level of PABP mRNA in myotube were largely due to the change in stability of this mRNA during myogenesis.

Mechanisms for intracellular distribution of mRNA: in situ hybridization studies in muscle

The intracellular distribution of mRNA in striated muscle fibers is highly ordered, as is the structural organization of the fibers' contractile apparatus. Results from in situ hybridization of muscle mRNA are reviewed in an attempt to discern the mechanisms involved in mRNA distribution and to determine its relationship to developmental, growth, and repair processes in muscle. Nonradioactively labeled complementary RNA probes allow anatomic localization of mRNA at the light and electron microscopic level. Myosin mRNA in striated muscle is concentrated around transcriptionally active nuclei, myosin mRNA is excluded by the myofibrillar mass, myosin mRNA distribution correlates with that of cytoskeletal elements, and myosin mRNA is concentrated in regions of rapid growth and repair. The even distribution of myosin mRNA along the length of myofibrils gives no indication of specific association with either the thick or thin filaments. Of the possible mechanisms directing mRNA distribution, results from in situ hybridization and other analyses support a restricted diffusion model. Diffusion of mRNA (and polysomes) is severely limited by the myofibrillar lattice. It is possible that myosin mRNA is also associated with a cytoskeletal element, which may direct the mRNA to specific intracellular locations and affect translational activity.

Gene-specific DNA repair in terminally differentiating rat myoblasts

Preferential DNA repair in expressed genes has been well documented in proliferating mammalian cells following ultraviolet irradiation. It was of interest to learn whether excision repair is similarly selective in terminally differentiating cells. We have measured the removal of ultraviolet-induced cyclobutane pyrimidine dimers (detected as T4 endonuclease V-sensitive sites) from various genes in cultured L8 rat skeletal myoblasts. In these cells, the transcription of muscle-specific genes such as the embryonic myosin heavy chain (MHCemb) gene can be regulated by inducing cells to differentiate. L8 myoblasts are somewhat more sensitive than Chinese hamster ovary cells to ultraviolet radiation, and they exhibit relatively poor overall DNA-repair rates throughout differentiation. Irradiation severely reduces the rates of transcription and steady-state RNA levels for the genes studied. Although differences in kinetics are seen between the repair of active and inactive genes, repair rates are low relative to those previously measured in proliferating rodent cell lines. Repair efficiency in the MHCemb gene increases as it is activated during differentiation and, in fact, approaches 100% within 5 days, while that in the silent GAP43 gene is much lower. While repair efficiencies generally correlate with expression in the genes studied, the overall time course of repair appears to be prolonged in these cells compared to that in proliferating cells. These terminally differentiating cells seem to maintain a DNA damage surveillance and repair capacity for selected genes and/or genomic domains.

Reprogramming of myosin light chain 1/3 expression in muscle heterokaryons

Fast myosin light chain (MLC) 1/3 is one of the few genes which regulates transcript production at both transcriptional and post-transcriptional levels, utilizing two functionally distinct promoters coupled with alternatively spliced exons. The transcriptional process controlling expression from this single gene locus is developmentally regulated, such that MLC 1 precedes MLC 3 during myogenesis. Results from our RNA analyses demonstrate that in differentiated rat L6E9 muscle, MLC 3 is the sole isoform expressed from the MLC 1/3 locus. However, we also show that by generating rat L6E9:mouse C2 muscle heterokaryons, MLC 1 expression from the L6E9 MLC locus can be induced. In addition to novel rat MLC 1 expression in the C2:L6E9 heterokaryons, we show that the synthesis profile of rat MLC 3 mRNA is also altered relative to L6E9 muscle cultured alone. Additional experiments demonstrate that the reprogramming of rat MLC 1 and 3 expression in the muscle heterokaryons requires that C2 and L6E9 nuclei be contained within a common cytoplasm. These results demonstrate that expression from the MLC 1/3 gene is "plastic," and is not under the control of a strict developmental program but, rather, can be modified by the environmental milieu.

Activation of alpha-myosin heavy chain gene expression by cAMP in cultured fetal rat heart myocytes

The effect of cAMP on cardiac myosin heavy chain (MHC) gene expression in primary cultures of 18-day-old fetal rat heart myocytes was investigated. When myocytes were treated with either 10 microM forskolin or 1 mM 8-bromo-cAMP for 48 h, the relative amount of the V1 to -V3 myosin isoform ratio increased 3-fold. The abundance of alpha-MHC mRNA was also increased 3-to-4-fold in forskolin treated vs control cells. However, no appreciable change was observed in the level of beta-MHC mRNA. In addition, a 70% increase in the transcription rate of the cardiac MHC gene was observed by nuclear run-on assay following treatment of cells with 10 microM forskolin for 12 h. These results demonstrate the preferential induction of alpha-MHC mRNA by cAMP which is, in part, mediated by transcriptional activation of the gene.

Redistribution of myosin heavy chain mRNA in the midregion of stretched muscle fibers

Myosin mRNA distribution was compared to the distribution of striations, nuclei, and cytoskeletal components in normal fibers and in fibers undergoing growth and repair processes in response to stretch. Plantarflexion of rabbit lower hindlimb for 4 or 6 days resulted in a 35% increase in weight of the tibialis anterior muscle. Slow myosin expression in stretched fibers increased such that the proportion of fibers shifted from the fast type towards an intermediate type. Semi-quantitative in situ hybridization revealed a large increase in concentration of slow myosin mRNA in stretched fibers. Polysomes translating myosin heavy chain were excluded from the intact myofibrillar lattice. Significant increases of myosin mRNA concentration occurred only in the outer 8 microns subsarcolemmal annulus of these stretched fibers (P less than 0.001) where myofibril formation also was evident. In some fibers, stretch caused myofibrillar disorder where nuclei became centrally located, and focal concentrations of myosin mRNA also occurred. We discuss mechanisms for mRNA accumulation and favor free diffusion to loosely packed cytoplasmic regions where myosin is needed for myofibrillar growth and repair.

Three linked myosin heavy chain genes clustered within 370 kb of each other show independent transcriptional and post-transcriptional regulation during differentiation of a mouse muscle cell line

We have examined myosin heavy chain gene transcription in the mouse muscle cell line C2/7 under different culture conditions. Gene-specific probes for embryonic (MHCemb), perinatal (MHCpn), and adult (MHCIIB) MHC sequences were used in nuclear run-on experiments, and transcriptional levels compared with cytoplasmic RNA accumulation of the transcripts during muscle cell differentiation. Transcripts are not detectable in myoblasts. These three MHC genes are physically linked within 370 kb of each other. However, they are not activated coordinately, but show independent transcriptional regulation as muscle cells differentiate into myotubes and as myotubes mature in culture. Post-transcriptional mechanisms also regulate cytoplasmic RNA accumulation of these MHC genes.

Regulation of contractile protein gene family mRNA pool sizes during myogenesis

During myogenesis, muscle contractile protein gene expression is induced and the products are used to assemble the contractile apparatus characteristic of striated muscle. The different muscle proteins are accumulated in a fixed stoichiometric ratio related to their organization in the contractile apparatus. We have examined the relationship between contractile protein gene expression and the maintenance of stoichiometry at different stages of human myogenesis. Essentially all of the known components of adult human skeletal muscle thick and thin filaments have been cloned in the form of cDNAs and used to generate isoform-specific DNA probes. The expression of fast, slow, and cardiac isoforms was measured in human myogenic primary culture and in fetal and adult human skeletal muscle. We observed that neither fast nor slow nor cardiac isoforms are coordinately regulated at the level of comparative transcript accumulation throughout myogenesis. Thus, the stoichiometry of contractile protein levels cannot be explained by coordination of expression in each of these isoform classes. However, we find that the stoichiometry of mRNA accumulation of each gene family is very similar among three developmental stages: myotubes, fetal skeletal muscle, and adult skeletal muscle. This is consistent with the possibility that the maintenance of stoichiometry between the contractile proteins could be largely regulated by the total accumulation of mRNA from each of these gene families.

Differentiation-dependent expression of apolipoprotein A-I in chicken myogenic cells in culture

Northern blot hybridization experiments showed that Apolipoprotein A-I (Apo A-I) mRNA is present at high concentration in chicken myotubes cultured in vitro, while it is virtually absent in fibroblasts and myoblasts. Myotubes are also capable of translating and secreting in the culture medium a protein which is specifically immunoprecipitated by anti-Apo A-I antibodies and has the same electrophoretic mobility as Apo A-I purified from circulating high-density lipoproteins. The appearance of Apo A-I mRNA in myotubes depends on the transcriptional activation of the corresponding gene, as it was shown by hybridizing 32P-labeled RNA synthesized in isolated nuclei to Apo A-I cDNA. The activation of the Apo A-I gene is regulated by the muscle cell coordinately with muscle-specific genes. In fact, treatment with TPA, a powerful inhibitor of differentiation, efficiently prevents myoblasts from producing Apo A-I mRNA, as well as muscle actin mRNA, and causes myotubes to quickly cease Apo A-I mRNA synthesis. The existence of a strict relationship between Apo A-I mRNA concentration and myogenic cell differentiation was also confirmed by experiments with quail myoblasts transformed with a temperature-sensitive mutant of the Rous Sarcoma Virus. Cells raised at the permissive temperature (undifferentiated phenotype) do not contain Apo A-I as well as alpha-actin mRNAs, while shifting to the nonpermissive temperature (differentiated phenotype) causes a rapid increase in Apo A-I and alpha-actin mRNA concentration.

Transcriptional regulation of actin and myosin genes during differentiation of a mouse muscle cell line

During terminal differentiation of skeletal muscle cells in vitro there is a transition from a predominantly nonmuscle contractile protein phenotype to a sarcomeric contractile protein phenotype. In order to investigate whether this transition and subsequent changes in expression are primarily transcriptionally regulated, we have analysed the rate of transcription and level of corresponding RNA accumulation of actin and myosin light chain genes during differentiation of a mouse muscle cell line under different culture conditions (low-serum and serum-free). We have found by 'nuclear run-on' analysis, that the alpha-cardiac actin, alpha-skeletal actin, myosin light chain 1F/3F and embryonic myosin light chain genes are transcriptionally activated as myoblasts begin to fuse to form myotubes. In contrast the nonsarcomeric beta-actin gene is transcribed at high levels in myoblasts and is transcriptionally down-regulated during differentiation. There is a sequential transition in transcription and RNA accumulation from predominantly alpha-cardiac to predominantly alpha-skeletal actin during subsequent myotube maturation, which reflects the pattern of expression found during development in vivo. A similar transition from embryonic to adult patterns of myosin light chain expression does not occur. RNA accumulation of actin and myosin light chains is regulated at both transcriptional and post-transcriptional levels. In our culture system the expression of myosin light chains 1F and 3F, which are encoded by a single gene, is uncoupled, 3F predominating. These data are discussed in the context of gene regulation mechanisms.

Identification and developmental expression of a novel embryonic myosin heavy-chain gene in chicken

The developmental expression of an embryonic chicken myosin heavy-chain (MHC) gene homologous to the genomic clone pCM4.1 was examined by S1 analysis. Transcripts homologous to pCM4.1 are first detected at day 12 in ovo, and are maximally expressed between days 15-17 in ovo. No pCM4.1 transcripts are detected at earlier stages of embryogenesis or at high levels in posthatch stages. This unique pattern of expression has led to the proposal that pCM4.1 represents a previously uncharacterized MHC gene, which is confined in its expression to late embryogenesis. Genomic hybridization data, in addition to a comparison between the DNA and amino acid sequences of pCM4.1 and other characterized chicken MHC 3' end clones, provide further evidence for this proposal. We also present observations made during the sequence analysis of pCM4.1 that may be relevant to our understanding of the 3'-end processing of homologous primary transcripts, and of the mechanism controlling developmental MHC isoform transitions.

Molecular genetics in basic myology: a rapidly evolving perspective

Myology has greatly benefited from the recent unification of concepts in molecular, cellular, and developmental biology. The interplay between intrinsic and extrinsic factors in determining the physiologic characteristics of individual myofibers has emerged as an important theme. Of special note is the manner in which the study of contractile protein gene structure and expression has contributed to our understanding of the development and ultimate plasticity of the contractile apparatus. As mechanistic models of normal myogenesis achieve increasing sophistication, the opportunities for understanding the pathogenesis of progressive muscle disfunction improve. In this article we review recent progress in basic myology which will be of interest to clinicians studying the heritable neuromuscular disorders.

Control of myogenesis in the mouse myogenic C2 cell line by medium composition and by insulin: characterization of permissive and inducible C2 myoblasts

Using subcloning and manipulations of culture conditions we have isolated from the mouse myogenic cell line C2 a variant cell line that we named inducible. Unlike the progenitor cells that are referred to as permissive, inducible myoblasts differentiate poorly in Dulbecco modified Eagle medium plus fetal calf serum (FCS) and require the presence of insulin at a high concentration (1.6 10(-6) M) or insulin-like growth factor I (IGFI) at a lower concentration (2.5 10(-8) M) to differentiate. Permissive and inducible myoblasts fail to differentiate when grown in MCDB202 medium plus 20% FCS, even after a prolonged arrest in G1 phase. This shows that an arrest in G1 is in itself insufficient to trigger terminal differentiation. Both cell types also exhibit distinct patterns of accumulation of muscle mRNAs corresponding to sarcomeric actins and myosin light chain MLC1A. The possibility that these two cell lines might represent two different stages of the progression of myoblasts toward terminal differentiation is discussed.

Transcriptional repression of an embryo-specific muscle gene

Skeletal muscle involves both the induction and repression of gene expression. Although activation and up-regulation of several contractile protein genes has been shown to occur via transcriptional mechanisms, the mechanisms by which contractile protein genes are repressed during muscle development remain unknown. However, a post-transcriptional mechanism has been implicated in the repression of thymidine kinase expression during muscle development. The chicken cardiac troponin T (cTNT) gene is expressed in early embryonic skeletal muscle but is abruptly repressed in late embryonic/fetal development. Using run-on transcription assays we demonstrate here that cTNT gene repression occurs at the level of transcription. Thus, transcriptional as well as post-transcriptional mechanisms operate both to activate and repress gene expression during skeletal muscle development.

Regulation of translation and stability of an mRNA coding for a 40-kDa polypeptide in rat L6 muscle cells

The mRNA coding for a 40-kDa polypeptide (P-40) was previously cloned and sub-cellular distribution of this mRNA was examined in rat L6 myoblast cells under different conditions [Pramanik, S. & Bag, J. (1987) Eur. J. Biochem. 170, 59-67]. The translation of this mRNA was found to be regulated during differentiation of myoblasts. This mRNA was translated in proliferating myoblasts but not in the non-dividing differentiated myotubes. We have further examined whether the mRNA present in the polysomal fraction of myoblasts and that in the free non-polysomal fraction in myotubes was identical by nuclease S1 mapping. The coding strand of the 600-base-pair PstI fragment of the recombinant clone was 3'-end-labeled with cordycepin 5'-[alpha-32P]triphosphate and hybridized with RNA from either myoblasts or myotubes. The results of these studies have shown that RNA from both preparations was fully able to hybridize with the probe DNA and, therefore, protected the 600-nucleotide-long fragment from nuclease S1 digestion, thus suggesting that the sequence of 600 nucleotides at 3' ends of both translationally active polysomal mRNA of myoblasts and repressed free mRNA of myotubes are identical. These results also confirmed the results of our earlier studies on the subcellular distribution of this mRNA by Northern blot analysis. Further studies were also performed to determine whether withdrawal of muscle cells from the cell cycle during differentiation to form myotubes alone was responsible for regulating translation of P-40 mRNA. The results of the subcellular distribution of this mRNA in proliferating myoblasts following inhibition of DNA synthesis by cytosine arabinoside have shown that translation of P-40 mRNA continued in absence of DNA synthesis. This observation suggests that an additional signal is necessary to block the translation of P-40 mRNA in myotubes. The relationship between the translation of P-40 mRNA and its stability was examined. Two different methods were used to determine the stability of mRNAs. The first approach was by determining the steady-state levels of this mRNA following inhibition of RNA synthesis by actinomycin D. In the second method, we have determined the amount of 3H-labeled P-40 mRNA during pulse and chase experiments. Both methods produced similar results. It was found that the stability of P-40 mRNA was not altered during differentiation of rat L6 cells. The results of pulse and chase studies have also shown that P-40 mRNA was synthesized in both myoblasts and myotubes.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of myogenic differentiation by cellular oncogenes

The establishment of a differentiated phenotype in skeletal muscle cells requires withdrawal from the cell cycle and termination of DNA synthesis. Myogenesis can be inhibited by serum components, purified mitogens, and transforming growth factors, but the intracellular signaling pathways utilized by these molecules are unknown. Recent studies have confirmed a role for proteins encoded by cellular proto-oncogenes in transduction of growth factor effects that lead to cell proliferation. To test the contrasting hypothesis that cellular oncogenes might also regulate tissue-specific gene expression in developing muscle cells, myoblasts have been modified by incorporation of the cognate viral oncogenes, the corresponding normal or oncogenic cellular homologs, and chimeric oncogenes, whose expression can be induced reversibly. Regulation of the endogenous cellular oncogenes also has been examined in detail. Down-regulation of c-myc is not obligatory for myogenesis; rather, inhibitory effects of myc on muscle differentiation are contingent on sustained proliferation. In contrast, activated src and ras genes block myocyte differentiation directly, through a mechanism that is independent of DNA synthesis and is rapidly reversible, resembling the effects of inhibitory growth factors. The coordinate regulation of diverse tissue-specific gene products including muscle creatine kinase, nicotinic acetylcholine receptors, sarcomeric proteins, and voltage-gated ion channels, raises the hypothesis that inhibitors such as transforming growth factor-beta and ras proteins might exert their effects through a transacting transcriptional signal shared by multiple muscle-specific genes.

Regulation of cardiac myosin synthesis: studies of RNA content in cultured heart cells

Contraction regulates the myosin content and the rate of myosin synthesis in cultured neonatal rat heart cells. To further explore the mechanism for this regulation we examined various parameters of RNA content and RNA synthesis in contacting versus noncontracting myocytes. While contraction stimulated myosin heavy chain (MHC) synthesis by 72% compared to that of KCl-arrested cells, simultaneous analyses of polysome profiles were no different under the two culture conditions. Incorporation of [3H]uridine monophosphate into cellular RNA revealed no change in the rate of total RNA or ribosomal subunits synthesis. In vitro translation of cellular RNA yielded similar incorporation of [35S]methionine into trichloroacetic acid precipitable protein. Specific transcription of the MHC gene was examined by dot-blot analysis and was unaltered by contraction. Northern blot analysis of the MHC sequences detected by a cDNA probe revealed an mRNA sequence corresponding to a molecular weight of approximately 30 S. These data suggest that RNA synthesis and RNA content are unaltered by contraction in cultured heart cells and therefore the changes in myosin synthesis may be mediated at a post-transcriptional control level.

Three types of muscle-specific gene expression in fusion-blocked rat skeletal muscle cells: translational control in EGTA-treated cells

When rat skeletal muscle cells were treated with EGTA, an inhibitor of cell fusion, a battery of muscle-specific mRNAs was synthesized but not translated despite the synthesis of many other proteins. Most of the muscle-specific mRNAs were associated with polysomes in fused myotubes, whereas they were found in postpolysomal fractions in EGTA-treated cells. Therefore, in addition to the well-documented transcriptional and posttranscriptional control of muscle-specific genes, translational control of this specific group of genes, presumably involving a Ca2+-dependent process, is also observed in these fusion-blocked cells. These findings and results obtained with other fusion inhibitors demonstrate that three types of muscle-specific gene expression take place in the fusion-blocked cells depending on the inhibitors used: one, neither muscle-specific mRNAs nor proteins are synthesized; two, the mRNAs are synthesized but not translated; and three, both the mRNAs and the proteins are synthesized.

Myosin isozyme synthesis and mRNA levels in pressure-overloaded rabbit hearts

The in vivo synthesis rates of myosin isozyme heavy chains beta and alpha were measured in right ventricular (RV) muscle at 2 and 4 days following pulmonary artery constriction in rabbits, together with measurements of their relative mRNA levels. The synthesis rate of beta-myosin heavy chains was elevated in 2-day (0.27 +/- 0.06 day-1 or 2.5 +/- 0.7 mg/g RV/day, mean +/- SD) and in 4-day (0.25 +/- 0.08 day-1 or 2.8 +/- 1.0 mg/g RV/day) pressure overload, when compared to untreated rabbits (0.15 +/- 0.04 day-1 or 1.5 +/- 0.4 mg/g RV/day). However, the synthesis rates of alpha-myosin heavy chains in the same hearts were not altered significantly. There was a differential increase in the fractional synthesis rate of beta vs. alpha heavy chains in 2-day and 4-day pressure overload and in 2-day shams, suggesting switching toward beta heavy chain synthesis had occurred at these time points. beta heavy chain synthesis, as a proportion of total (alpha + beta) heavy chain synthesis, was significantly higher in 4-day pressure overload (78 +/- 9%) than in 4-day sham rabbits (63 +/- 6%). This increase in relative beta-synthesis was associated with a significant increase in the relative proportion of beta heavy chain mRNA level (76 +/- 13% vs. 56 +/- 7%). Furthermore, relative beta-synthesis and the beta-mRNA levels correlated linearly with each other in all experimental groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Complete nucleotide and encoded amino acid sequence of a mammalian myosin heavy chain gene. Evidence against intron-dependent evolution of the rod

The complete nucleotide sequence and exon/intron structure of the rat embryonic skeletal muscle myosin heavy chain (MHC) gene has been determined. This gene comprises 24 X 10(3) bases of DNA and is split into 41 exons. The exons encode a 6035 nucleotide (nt) long mRNA consisting of 90 nt of 5' untranslated, 5820 nt of protein coding and 125 nt of 3' untranslated sequence. The rat embryonic MHC polypeptide is encoded by exons 3 to 41 and contains 1939 amino acid residues with a calculated Mr of 223,900. Its amino acid sequence displays the structural features typical for all sarcomeric MHCs, i.e. an amino-terminal "globular" head region and a carboxy-terminal alpha-helical rod portion that shows the characteristics of a coiled coil with a superimposed 28-residue repeat pattern interrupted at only four positions by "skip" residues. The complex structure of the rat embryonic MHC gene and the conservation of intron locations in this and other MHC genes are indicative of a highly split ancestral sarcomeric MHC gene. Introns in the rat embryonic gene interrupt the coding sequence at the boundaries separating the proteolytic subfragments of the head, but not at the head/rod junction or between the 28-residue repeats present within the rod. Therefore, there is little evidence for exon shuffling and intron-dependent evolution by gene duplication as a mechanism for the generation of the ancestral MHC gene. Rather, intron insertion into a previously non-split ancestral MHC rod gene consisting of multiple tandemly arranged 28-residue-encoding repeats, or convergent evolution of an originally non-repetitive ancestral MHC rod gene must account for the observed structure of the rod-encoding portion of present-day MHC genes.

All members of the MHC multigene family respond to thyroid hormone in a highly tissue-specific manner

In mammals different isoforms of myosin heavy chain are encoded by the members of a multigene family. The expression of each gene of this family is regulated in a tissue- and developmental stage-specific manner as well as by hormonal and various pathological stimuli. In this study the molecular basis of isoform switches induced in myosin heavy chain by thyroid hormone was investigated. The expression of the myosin heavy chain gene family was analyzed in seven different muscles of adult rats subjected to hypo- or hyperthyroidism with complementary DNA probes specific for six different myosin heavy chain genes. The results demonstrate that all six genes are responsive to thyroid hormone. More interestingly, the same myosin heavy chain gene can be regulated by thyroid hormone in highly different modes, even in opposite directions, depending on the tissue in which it is expressed. Furthermore, the skeletal embryonic and neonatal myosin heavy chain genes, so far considered specific to these two developmental stages, can be reinduced by hypothyroidism in specific adult muscles.

Differentiation of rat myoblasts. Regulation of turnover of ribosomal proteins and their mRNAs

The regulation of ribosomal proteins (r-proteins) and their mRNAs (rp-mRNAs) was studied in the L6 myoblast, a mammalian cell line which can undergo myogenesis. Upon terminal differentiation, the rate of accumulation of mature ribosomes dropped to approximately 25% of the rate found in undifferentiated myoblasts. Despite the drop in the rate of ribosome accumulation and the rate of rRNA synthesis following terminal differentiation, the rate of r-protein synthesis remained constant. The excess r-protein synthesized in myotubes was quickly degraded. The levels of rp-mRNAs were assessed before and after differentiation. Over 90% of the rp-mRNAs were found on polysomes in both myoblasts and myotubes and represented similar fractions of total poly(A)-rich mRNA. The half-lives of the rp-mRNAs averaged approximately 11 h in both myoblasts and myotubes. In vitro nuclear transcription measurements of a representative rp-mRNA (L32 mRNA) revealed that following differentiation, its rate of synthesis relative to the overall transcription rate dropped by approximately 26% in myotubes while the rate of transcription of rRNA dropped by approximately 77%. These results indicate that the coordination of r-protein and rRNA synthesis observed in myoblasts was uncoupled in myotubes at the level of transcription.

Genetic modifications during cellular aging

We review evidence that biological aging is a genetic process related to development and cytodifferentiation and thus may involve alterations of DNA structure and gene expression. We conclude that although determined to a high degree aging also involves stochastic features which lead to progressive somatic cell diversification during the life span. These considerations may help to explain the unevenness of physiological decline and the clonal emergence of certain age-dependent diseases such as cancer.

Changes in actin synthesis and alpha-actin-mRNA content in rat muscle during immobilization

The fractional rates of actin synthesis in adult rat gastrocnemius muscle from control and 6-h hindlimb-immobilized animals were determined by the constant-infusion technique. The rate of actin synthesis in gastrocnemius muscle was significantly decreased from control values during the 6th h of hindlimb immobilization. The content of alpha-actin-specific mRNA was then estimated in adult rat gastrocnemius muscle from control, 6-h, 72-h, and 7-day immobilized animals by "dot blot" hybridization. RNA extracted from control and immobilized animals was secured on nitrocellulose filters and hybridized to 32P-labeled plasmid p749 (containing a cDNA sequence produced from rat alpha-actin mRNA). The relative content of alpha-actin-specific mRNA in gastrocnemius muscle was significantly decreased at 7 days of immobilization but not at 6 or 72 h of immobilization. It is concluded that a change in the content of alpha-actin mRNA does not contribute significantly to the rapid onset of the decrease in actin synthesis rate observed after 6 h of immobilization. An alteration in the translation of alpha-actin-specific mRNA must occur to account for the early decline in actin synthesis during immobilization.

A transient species of poly(A)+ RNA detected by a myosin heavy chain cDNA probe in muscle cell culture during terminal differentiation

Primary cell cultures were prepared from breast muscles of 11 day 4 hour-embryonic chicks. Cytoplasmic RNAs were isolated from the cultured cells at various time intervals from day 3 to day 8. A [P32] DNA probe complementary to messenger RNA of myosin heavy chain was used to hybridize with the RNAs after gel electrophoresis. A transient species of polyadenylated RNA with a decreased mobility in electrophoresis was detected during a period of time when contractions of syncytial fibers were first observed.

Reversibility of muscle differentiation in the absence of commitment: analysis of a myogenic cell line temperature-sensitive for commitment

The interrelationship between commitment (irreversible withdrawal from the cell cycle) and muscle-specific gene expression was analyzed with the myogenic cell line ts 3b-2, which is temperature sensitive for commitment and cell fusion. The rates of synthesis and levels of accumulation of muscle-specific mRNAs and proteins in the ts 3b-2 cells at permissive and nonpermissive temperatures are comparable, indicating that neither commitment nor cell fusion is required for induction of muscle-specific gene expression. In the absence of commitment, the cells are reversibly withdrawn from the cell cycle during gene induction, and expression of the muscle-specific genes is deinduced upon the switch to growth-stimulating conditions. The deinduction reflects coordinate and preferential cessation of muscle-specific mRNA synthesis, coupled with destabilization of the muscle-specific mRNAs in the cytoplasm, without effect on constitutively expressed housekeeping protein genes. The phenotype of the ts 3b-2 cells demonstrates that commitment and muscle-specific gene expression are both required, but alone are insufficient, to produce the terminally differentiated muscle phenotype.