Two-level regulation of cardiac actin gene transcription: muscle-specific modulating factors can accumulate before gene activation

Tissue-specific restriction of skeletal muscle troponin C gene expression

Expression of the skeletal muscle troponin C (TnC) gene is confined to fast-twitch skeletal muscle fibers (Gahlmann et al., 1988) and appears to be subject to an unexpected form of regulation. Unlike enhancers of other muscle genes, the TnC enhancer and basal promoter are muscle cell-specific only when linked to each other. We identified a strong classical enhancer element within the 5'-flanking sequence of this gene at -1.5 kb and a basal promoter near the transcription start site. Both elements are required for the transcriptional activity of TnC test constructs in myogenic cells. When the TnC enhancer was linked to the SV40 early basal promoter, or the TnC basal promoter was linked to the SV40 enhancer, each supported expression in non-muscle cells. Nuclear factors from both muscle and non-muscle cells bind to one CTF/NF1 binding site and to two functionally related MEF2-like A/T-rich binding sites in the enhancer element. It is currently unknown whether modifications of these nuclear factors, differences in their concentrations, or their interaction with additional factors restrict human fast-twitch TnC expression to skeletal muscle cells. However, it appears that the human fast-twitch skeletal troponin C gene is restricted in non-muscle cells in a distinctive way requiring communication between its enhancer and basal promoter.

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.

Fiber type conversion alters inactivation of voltage-dependent sodium currents in murine C2C12 skeletal muscle cells

Each skeletal muscle of the body contains a unique composition of "fast" and "slow" muscle fibers, each of which is specialized for certain challenges. This composition is not static, and the muscle fibers are capable of adapting their molecular composition by altered gene expression (i.e., fiber type conversion). Whereas changes in the expression of contractile proteins and metabolic enzymes in the course of fiber type conversion are well described, little is known about possible adaptations in the electrophysiological properties of skeletal muscle cells. Such adaptations may involve changes in the expression and/or function of ion channels. In this study, we investigated the effects of fast-to-slow fiber type conversion on currents via voltage-gated Na+ channels in the C(2)C(12) murine skeletal muscle cell line. Prolonged treatment of cells with 25 nM of the Ca2+ ionophore A-23187 caused a significant shift in myosin heavy chain isoform expression from the fast toward the slow isoform, indicating fast-to-slow fiber type conversion. Moreover, Na+ current inactivation was significantly altered. Slow inactivation less strongly inhibited the Na+ currents of fast-to-slow fiber type-converted cells. Compared with control cells, the Na+ currents of converted cells were more resistant to block by tetrodotoxin, suggesting enhanced relative expression of the cardiac Na+ channel isoform Na(v)1.5 compared with the skeletal muscle isoform Na(v)1.4. These results imply that fast-to-slow fiber type conversion of skeletal muscle cells involves functional adaptation of their electrophysiological properties.

The CACC box and myocyte enhancer factor-2 sites within the myosin light chain 2 slow promoter cooperate in regulating nerve-specific transcription in skeletal muscle

Previous experiments showed that activity of the -800-base pair MLC2slow promoter was 75-fold higher in the innervated soleus (SOL) compared with the noninnervated SOL muscles. Using in vivo DNA injection of MLC2slow promoter-luciferase constructs, the aim of this project was to identify regulatory sites and potential transcription factors important for slow nerve-dependent gene expression. Three sites within the proximal promoter (myocyte enhancer factor-2 (MEF2), E-box, and CACC box) were individually mutated, and the effect on luciferase expression was determined. There was no change in luciferase expression in the SOL and extensor digitorum longus (EDL) muscles when the E-box was mutated. In contrast, the MEF2 mutation resulted in a 30-fold decrease in expression in the innervated SOL muscles (10.3 versus 0.36 normalized relative light units (RLUs)). Transactivation of the MLC2slow promoter by overexpressing MEF2 was only seen in the innervated SOL (676,340 versus 2,225,957 RLUs; p < 0.01) with no effect in noninnervated SOL or EDL muscles. These findings suggest that the active MLC2slow promoter is sensitive to MEF2 levels, but MEF2 levels alone do not determine nerve-dependent expression. Mutation of the CACC box resulted in a significant up-regulation in the EDL muscles (0.23 versus 4.08 normalized RLUs). With the CACC box mutated, overexpression of MEF2 was sufficient to transactivate the MLC2slow promoter in noninnervated SOL muscles (27,536 versus 1, 605,797 RLUs). Results from electrophoretic mobility shift and supershift assays confirm MEF2 protein binding to the MEF2 site and demonstrate specific binding to the CACC sequence. These results suggest a model for nerve-dependent regulation of the MLC2slow promoter in which derepression occurs through the CACC box followed by quantitative expression through enhanced MEF2 activation.

ATF site of human RB gene promoter is a responsive element of myogenic differentiation

RB mRNA increases during terminal differentiation of C2 myoblasts. We demonstrate that RB promoter activity increases about 4-fold during differentiation. The increase of RB promoter activity was reduced when a point mutation was designed in the ATF site. In a gel shift assay of the ATF site, two specific bands were observed. One of them, with the lower mobility, disappeared during differentiation. This band reacted with an antibody against ATF-1. We cotransfected an RB promoter-luciferase plasmid with the TREB36/ATF-1 plasmid. ATF-l suppressed the activity of the wild-type RB promoter but not of that with a point mutation at the ATF site. These results suggest that the ATF site of the RB promoter is a responsive element during myogenic differentiation of C2 cells. We hypothesize that RB promoter activity is stimulated partially due to the dissociation of ATF-1, which suppresses the promoter activity through the ATF site in C2 myoblasts.

Altering creatine kinase isoenzymes in transgenic mouse muscle by overexpression of the B subunit

To change the levels of expression and isoenzyme distribution of creatine kinase (CK) in muscle, transgenic technology was used to express the B subunit of CK in mouse muscle. Normally, mammalian skeletal muscle contains the MM dimer of CK. The BB dimer and MB heterodimer of CK can be found in brain and heart, respectively. Heterologous genes consisting of skeletal and cardiac muscle-specific actin promoters fused to the genomic coding region of the B form of CK were used to create transgenic mice. Lines were established from the three highest expressing founders. Analysis of skeletal muscle extracts revealed that all three lines had an increase in total CK activity measured under maximal velocity conditions. The highest expressing line, 7001, had a CK activity 150% that of control muscle. Nuclear magnetic resonance saturation transfer was used to measure the in vivo rate of the CK reaction. In 7001 hindlimb muscles, the CK catalyzed reaction was 200% that of control muscle. The elevation in CK activity in transgenic muscle was accompanied by significant changes in the composition of the cytosolic isoenzyme ratio of CK. In control, 100% of CK was MM, whereas 7001 had 60 +/- 18% MM, 32 +/- 10% MB, and 8 +/- 2% BB. There were no changes in ATP, phosphocreatine, Pi, or creatine levels in transgenic muscle compared with control. Immunofluorescence of myofibrils isolated from control and transgenic muscle revealed specific association of CK to the M line. Small amounts of MB CK were detected on myofibrils from transgenic mice. Transgenic mice expressing the B subunit of CK in muscle represent a first step toward altering CK isoforms so as to elucidate the specific roles of these isoforms in energy metabolism.

Expression of wild-type and nuclear localization-deficient human lamin A in chick myogenic cells

Previous analysis of chick embryonic muscle (CEM) differentiation in vivo and in ovo demonstrated that lamin A accumulation to steady-state levels preceded the accumulation of muscle-specific proteins. These observations have suggested the appearance of A-type lamins may be important for differentiation. To test this hypothesis, we have temporally and quantitatively altered the expression of A-type lamins in CEM cells by transient transfection of wild-type (wt; pHLA) or nuclear localization-deficient (NLd; pHLA-del) human lamin A expression plasmids. Transfected CEM cells synthesized the wt and NLd human lamin As to high levels, both of which were resistant to high-salt extraction. The wt human lamin A localized to the nucleus, whereas the NLd protein showed cytoplasmic staining patterns, as well as time-dependent nuclear localization. The presence of endogenous chicken lamins A and B2 in NLd human lamin A cytoplasmic structures suggested the interspecies lamin copolymerization. Thus, this approach may provide a possible method for analysis of lamin-lamin or lamin-lamina component interactions in vivo. With regard to muscle differentiation, CEM cells transfected with either pHLA or pHLA-del demonstrated moderate and transient increased levels of the muscle-specific myosin heavy chain and creatine kinase activity. These increases appeared temporally and quantitatively to reflect the transient accumulation of the human lamin As. In contrast, beta-tubulin and skeletal tropomyosin protein accumulations appeared unaffected. On the basis of these results, we suggest that nuclear lamina content and structure may play a limited, permissive and indirect role in the temporally regulated expression of the myogenic phenotype.

Tissue-specific expression of the skeletal alpha-actin gene involves sequences that can function independently of MyoD and Id

The skeletal alpha-actin gene is a member of the sarcomeric contractile protein gene family and is specifically expressed in differentiated muscle. The skeletal alpha-actin gene is regulated efficiently by enhancer and regulatory sequences between nucleotide positions -1282 and -87. In the present study we have shown that the sequences 3' of nucleotide position -87 can functionally interact with the SV40 enhancer in a tissue-specific manner and can restrict the ubiquitous function of the SV40 enhancer to myogenic cells. Site-specific cassette mutagenesis was used to delimit the sequences upstream of the TATA motif (-32), between nucleotide positions -64 and -37, that mediate efficient expression in myogenic cells in the presence of the SV40 enhancer. The skeletal alpha-actin promoter was trans-activated by the helix-loop-helix (HLH) transcription factors MyoD, MRF-4, and Myogenin, in pluripotential 10T1/2 fibroblasts and trans-repressed by the HLH protein Id (inhibitor of differentiation) in myogenic C2C12 cells. This trans-regulation required sequences upstream of -87 and occurred independently of the two consensus E boxes (CANNTG) at positions +18 and +71. The -64/-37 region interacted with purified Sp1 and an unidentified protein(s), proximal regulatory factor(s) I (PRF-I). We conclude that the muscle-specific expression of the skeletal alpha-actin promoter is not simply determined by MyoD elements and enhancer and regulatory sequences, but that the minimal promoter contains important determinants of cell-specific transcription that can function independently of the helix-loop-helix transcription factors.

The human skeletal alpha-actin gene is regulated by a muscle-specific enhancer that binds three nuclear factors

The tissue-specific distal promoter of the human skeletal alpha-actin gene (-1282 to -708) induces transcription in myogenic cells approximately 10-fold and, with the most proximal promoter domain (-153 to -87), it synergistically increases transcription 100-fold (Muscat and Kedes 1987). We report here that it is a short fragment of the distal promoter, the distal regulatory element (DRE) from -1282 to -1177 that functions as a muscle-specific, composite enhancer. An internal deletion in the DRE (delta -1282/-1151) in the context of the full-length 2000 bp promoter, resulted in a 10-fold reduction in transcription. Three distinct nuclear proteins, DRF-1, DRF-2, and DRF-3, interact specifically with the DRE between positions -1260 and -1193. A site specific mutation that abolishes DRF-2 binding also results in a 10-fold reduction in transcriptional activity. The DRF-2 nuclear protein has characteristics similar to those of the muscle-specific regulatory factor, MEF-2 (Buskin and Hauschka 1989; Gossett et al., 1989). Like the MEF-2 binding site in the muscle creatine kinase enhancer, the critical DRF-2 binding site is also an A/T-rich sequence element. The DRF-2 nuclear protein binds equally well to the MCK MEF-2 binding site and to the A/T-rich regulatory element of the skeletal muscle fast-twitch troponin C gene (Gahlmann and Kedes 1990). Furthermore, this troponin C site competes in vivo for DRF-2 driven expression of the skeletal alpha-actin gene in C2 cells. The DRF-2 site alone, however, does not activate transcription in muscle cells when linked to the SV40 promoter. We conclude that the DRF-2 binding element is a MEF-2 binding site that is required but insufficient for regulation of muscle-specific skeletal alpha-actin gene expression by the DRE. Thus, muscle-specific regulation of the human skeletal alpha-actin gene appears to require interactions between the other elements of the composite DRE enhancer with the protein:DNA complex formed by DRF-2.

Isolation and characterization of an avian myogenic cell line

Myogenic cell lines have proven extremely valuable for studying myogenesis in vitro. Although a number of mammalian muscle cell lines have been isolated, attempts to produce cell lines from other classes of animals have met with only limited success. We report here the isolation and characterization of seven avian myogenic cell lines (QM1-4 and QM6-8), derived from the quail fibrosarcoma cell line QT6. A differentiation incompetent QM cell derivative was also isolated (QM5DI). The major features of QM cell differentiation in vitro closely resemble those of their mammalian counterparts. Mononucleated QM cells replicate in medium containing high concentrations of serum components. Upon switching to medium containing low serum components, cells withdraw from the cell cycle and fuse to form elongated multinucleated myotubes. Cultures typically obtain fusion indices of 43-49%. Northern blot and immunoblot analyses demonstrate that each differentiated QM cell line expresses a wide variety of genes encoding muscle specific proteins: desmin, cardiac troponin T, skeletal troponin T, cardiac troponin C, skeletal troponin I, alpha-tropomyosin, muscle creatine kinase, myosin light chain 2, and a ventricular isoform of myosin heavy chain. While all QM lines analyzed to date express at least some myosin light chain 2, only one line, QM7, expresses this gene at high levels. Surprisingly, none of the QM lines reported here express any known form of alpha-actin. The absence of sarcomeric actin expression may explain the absence of myofibrils in QM myotubes. These novel features of muscle gene expression in QM cells may prove useful for studying the role of specific muscle proteins during myogenesis. More importantly, however, the isolation of QM cell lines indicates that it may be feasible to isolate other avian myogenic cell lines with general utility for the study of muscle development.

Search of the 5' untranslated region of the human cardiac actin gene for segments controlling translation

We have examined the possibility that the 5'-UT region of the human cardiac actin (CH-actin) mRNA is responsible for regulating translation of this transcript during skeletal myogenesis. Genes were constructed which consisted of the murine leukaemia virus promoter driving the Escherichia coli LacZ coding region with and without the CH-actin 5'-UT region. These constructs were transfected into L6 myoblasts that were subsequently differentiated into myotubes. The presence of the CH-actin 5'-UT region appeared to have no effect on expression of the LacZ reporter gene. EGTA blocks myogenesis and inhibits translation of muscle-specific transcripts (Endo, T. and Nadal-Ginard, B. (1987) Cell 49, 515-526) but EGTA had no effect on expression of the chimaeric LacZ transcripts. Thus, if the CH-actin transcript is subject to translational regulation, it must be mediated by sequences other than those of the 5'-UT region.

Negative control of the helix-loop-helix family of myogenic regulators in the NFB mutant

We have characterized a nondifferentiating mouse muscle cell line, NFB, that represses the activity of the helix-loop-helix (HLH) family of myogenic regulators, yet expresses sarcomeric actins. The NFB MyoD gene is silent, but can be activated upon transfection of a long terminal region-controlled chicken MyoD cDNA, resulting in myogenesis. When NFB cells are fused with H9c2 rat muscle cells in heterokaryons, the level of rat MyoD transcripts declines. Thus, the stoichiometry of MyoD and the putative repressor controls myogenesis. Although NFB cells express myogenin and Myf-5 transcripts, the activity of these regulators is also repressed:myogenesis is not induced in 10T1/2 fibroblasts and is repressed in L6 muscle cells upon fusion with NFB cells. We conclude that the myogenic HLH regulators are not required for sarcomeric actin gene activation and that myogenesis is subject to dominant-negative control.

CC Ar GG boxes, cis-acting elements with a dual specificity. Muscle-specific transcriptional activation and serum responsiveness

The influence of different CC Ar GG boxes derived from either muscle-specific or serum-responsive genes, on the specificity of different promoters has been investigated. Inserted upstream from an 85 base-pair long minimal promoter of the human cardiac alpha-actin gene, a single copy of both the cognate CC Ar GG element (HCA1) and the c-fos gene serum response element (SRE) stimulate transcription four- to fivefold more efficiently in C2 myogenic cells than in L fibroblastic cells, SRE being two- to threefold more active than HCA1. Inserted upstream from the ubiquitous Herpes simplex thymidine kinase (HSV-tk) promoter, multimerized CC Ar GG boxes behave as strong muscle-specific activating elements, about 20-fold more active in myogenic C2 cells than in L fibroblasts and hepatoma HepG2 cells. They also confer serum responsiveness on the HSV-tk promoter. Efficiency of HCA1 and SRE tetramers in conferring both muscle specificity and serum responsiveness is roughly similar. It appears, therefore, that regardless of their origin (either muscle-specific or serum-responsive genes) CC Ar GG boxes behave by themselves as both muscle-specific activating and serum-responsive elements.

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.

Expression of actin and myosin heavy chain genes in skeletal, cardiac and uterine muscles of young and old rats

The steady-state levels of mRNA and transcription of alpha-skeletal actin (alpha-SKA) and adult myosin heavy chain (MHC) genes were measured in the skeletal, cardiac and uterine muscles of young (22-25 week) and old (123-135 week) female rats. The effects of 10(-8) M 17 beta-estradiol/dexamethasone/T3 alpha on their transcription were also studied. The data show that the alpha-SKA mRNA level is lower in the old skeletal muscle and uterus, but is higher in the old myocardium. The adult MHC mRNA level is not different in the three muscles of both the ages. The transcription of alpha-SKA gene is lower in the skeletal muscle and higher in the uterus of old rats. It is unaltered in the myocardium of old rats. The transcription of adult MHC gene is lower in the old uterus. The effects of hormones on transcription of both the genes are different in the three muscles. We show that the expression of alpha-SKA gene is tissue-specific and age-related. The over-expression of alpha-SKA gene in the old myocardium is possibly due to derepression of the gene caused by hypertrophy of cardiac myocytes, and continuous hemodynamic pressure overload on the old heart.

Smooth muscle cells of neural crest origin form the aorticopulmonary septum in the avian embryo

Previous studies have shown that the cells of the aorticopulmonary (AP) septum are similar to the smooth muscle cells of the mediae of the great vessels in their common origin from the cardiac neural crest and in their common expression of an elastic extracellular matrix. The purpose of this study was to test the cells of the AP septum for the presence of certain cytoplasmic proteins, especially smooth muscle alpha-actin (SMAA) whose presence is definitive of smooth muscle. A monoclonal antibody against SMAA was applied to normal chicken embryos at 3.5-8 days of incubation and to age-matched embryos from which the cardiac neural crest had been ablated surgically. Antibodies against the intermediate filaments desmin, cytokeratin, and vimentin also were applied. The results showed that the AP septal cells expressed SMAA during the process of septation, days 5-8; but when the cardiac neural crest was ablated and septation was defective, no cells in the conotruncal connective tissue expressed SMAA. None of the intermediate filament proteins were detected in the septum. These results indicate that the AP septal cells are smooth muscle and therefore may be hypothesized to have an active role in septation.

Mesodermal cell determination and differentiation

Many in vitro systems have been designed to study the processes governing cell determination and differentiation during development. Mammalian culture systems have been particularly helpful in elucidating the mechanisms regulating gene expression during differentiation in cells of mesodermal origin, namely, myoblasts, preadipocytes, and chondroblasts. Studies have shown that particular cis-acting sequences and trans-acting factors are important in determining tissue-specific and developmental gene expression in these systems. The role of growth factors, oncogenes, and other agents during differentiation has also been examined. Recently four putative muscle determination genes have been isolated and are being characterized. These studies have been useful in postulating models of how development proceeds in vivo and how differentiation and transformation to a neoplastic phenotype may be related.

RNA polymerase II-directed gene transcription by rat skeletal muscle nuclear extracts

A cell-free transcription system was developed using nuclear extracts of rat skeletal muscle to examine the transcription of specific genes involved in ribosome biogenesis and histone synthesis. Isolation and purification of muscle tissue nuclei were required prior to obtaining a transcriptionally active extract. The transcriptional abilities of myoblast, myotube, and muscle tissue nuclear extracts were then compared using the adenovirus major late promoter as a reporter gene. Transcription of r-protein L32 and histone H4 gene templates remained high in all extracts while histone H3 gene transcription was reduced in both myotube and muscle tissue extracts. These data indicate that transcription of these genes in myotubes and muscle tissue nuclear extracts is similar. Therefore, the L6 myoblast system accurately reflects the ability of intact muscle tissue to transcribe the genes concerned with histone production and ribosome biogenesis.

Expression of a transfected mouse muscle-creatine kinase gene is induced upon growth factor deprivation of myogenic but not of nonmyogenic cells

To determine whether mitogen-regulated expression of skeletal muscle genes is independent of cell type, muscle and nonmuscle cells were transfected with cloned 5'-flanking sequences of muscle creatine kinase (MCK) fused to a heterologous reporter gene and tested for expression in high and low mitogen culture conditions. Consistent with the behavior of endogenous MCK, a -3300MCK-CAT gene is expressed at high levels in differentiated muscle cells but at low to undetectable levels in proliferating myoblasts and in either mitogen-deprived or stimulated nonmuscle cells of mesodermal, ectodermal, or endodermal origin. A -776MCK-CAT gene behaves similarly with respect to its cell type specificity but it supports only an intermediate expression level in response to mitogen deprivation in skeletal muscle cells. These data suggest that the -3300 to +7 nucleotide region of mouse MCK contains one or more elements which are activable by mitogen deprivation only in myogenic cells.

Sequence and expression of human myosin alkali light chain isoforms

In order to initiate the study of the functional differences between myosin alkali light chain isoforms and to investigate the mechanisms of their differential expression, we have isolated cDNA clones for two human alkali light chain isoforms. Here we report DNA sequence and RNA blotting analyses that demonstrate that these cDNAs represent transcripts encoding human MLC3F and MLC1Sb. The sequence of the human MLC1Sb cDNA offers the first fully characterized example of a slow-fiber skeletal muscle alkali light chain isoform from any species. The sequence analysis of these two cDNAs allows an examination of evolutionarily conserved features of mammalian alkali light chain genes. Examination of the genomic organization of the human alkali light chain isoform genes revealed that, in contrast with some strains of mice, both are single copy genes. RNA blot analysis conclusively demonstrates that the human skeletal muscle MLC1Sb gene is also expressed in the heart ventricle but not the atria. In addition, we examined the expression of alkali light chain isoforms during the in vitro differentiation of a variety of human and rodent myogenic cells and found striking variation in the pattern of alkali light chain isoform gene expression in different myogenic cells.

Down-regulation of vimentin gene expression during myogenesis is controlled by a 5'-flanking sequence

During myogenesis, the intermediate filament proteins vimentin and desmin are differentially expressed. While desmin levels increase dramatically, vimentin mRNA levels decrease substantially. Here, we show that transfected whole- and mini-vimentin-coding genes (Vim) are expressed in fibroblasts (mouse L cells) and down-regulated during muscle cell differentiation in culture. Functional assays with 5'-end Vim::cat constructs demonstrate that this repression is controlled by a 5'-element (nt -321 to -160). This region is distinct from Vim promoter elements (nt -160 to +71) which do not contribute to vimentin's down-regulation during myogenesis.

Temporal resolution and sequential expression of muscle-specific genes revealed by in situ hybridization

The expression of muscle-specific mRNAs was analyzed directly within individual cells by in situ hybridization to chicken skeletal myoblasts undergoing differentiation in vitro. The probes detected mRNAs for sarcomeric myosin heavy chain (MHC) or the skeletal, cardiac, and beta isoforms of actin. Precise information as to the expression of these genes in individual cells was obtained and correlated directly with analyses of cell morphology and interactions, cell cycle stage, and immunofluorescence detection of the corresponding proteins. Results demonstrate that mRNAs for the two major muscle-specific proteins, myosin and actin, are not synchronously activated at the time of cell fusion. The mRNA for alpha-cardiac actin (CAct), known to be the predominant embryonic actin isoform in muscle, is expressed prior to cell fusion and prior to the expression of any isoform of muscle MHC mRNA. MHC mRNA accumulates rapidly immediately after fusion, whereas skeletal actin mRNA is expressed only in larger myofibers. Single cells expressing CAct mRNA have a characteristic short bipolar morphology, are in terminal G1, and do not contain detectable levels of the corresponding protein. In a pattern of expression reciprocal to that of CAct mRNA, beta-actin mRNA diminishes to low or undetectable levels in myofibers and in cells of the morphotype which expresses CAct mRNA. Finally, the intracellular distribution of mRNAs for different actin isoforms was compared using nonisotopic detection of isoform-specific oligonucleotide probes. This work illustrates a generally valuable approach to the analysis of cell differentiation and gene expression which directly integrates molecular, morphological, biochemical, and cell cycle information on individual cells.

Abundance of mRNAs encoding urea cycle enzymes in fetal and neonatal mouse liver

The relative abundances of mRNAs encoding the five urea cycle enzymes during development of mouse liver have been determined and compared with those of mRNAs encoding four other liver-specific proteins (phosphoenolpyruvate carboxykinase, tyrosine aminotransferase, alpha-fetoprotein, and albumin). Urea cycle enzyme mRNAs in fetal liver are expressed at 2-14% of the abundance in adult liver as early as 6 days before birth. Expression of the urea cycle enzyme mRNAs is not coordinate during the fetal and neonatal period. However, profiles of three urea cycle enzyme mRNAs are quite similar to that of alpha-fetoprotein mRNA, suggesting the possibility of a common response to regulatory signals during fetal development. With the exception of ornithine transcarbamylase mRNA, the urea cycle enzyme mRNAs have been shown previously to be inducible by cAMP and glucocorticoids. However, only argininosuccinate lyase mRNA exhibits any significant change in abundance at birth, resembling postnatal expression of tyrosine aminotransferase mRNA. The results indicate that the urea cycle enzyme mRNAs are potentially useful markers for elucidating various features of hepatocyte differentiation in mammals.

Conserved and unique sequences in the 3'-untranslated region of rat smooth-muscle alpha-actin mRNA

We have isolated a cDNA clone corresponding to rat smooth-muscle alpha-actin mRNA [Hsu and Frankel, J. Biol. Chem. 262 (1987) 9594-9600]. We present here the sequence of the 3'-untranslated region (3'-UTR) of the cDNA. By comparison with the reported sequence of the chicken gene, this 3'-UTR region contains a conserved 36-bp sequence and a unique 48-bp G + C-rich sequence. An RNA probe containing only the 3'-UTR of the cDNA was synthesized and shown to be specific for smooth-muscle alpha-actin message.

A third striated muscle actin gene is expressed during early development in the amphibian Xenopus laevis

During early embryonic development in the frog Xenopus laevis, several muscle-specific actin genes encoding distinct actin protein isoforms are activated in cells of the embryonic muscle. In addition to the cardiac (or alpha 1) and skeletal (or alpha 2) actin genes, a third muscle-specific actin gene is expressed in the same embryonic tissue. We have determined the complete nucleotide sequence of this third gene and examined its expression in embryonic and adult tissues. During embryogenesis, this femoral (alpha 3) actin gene is activated several hours later than its cardiac and skeletal counterparts and its transcripts are first detected after neurulation. The gene encodes a skeletal-type actin protein and is expressed exclusively in skeletal muscle in the adult frog. Two copies of this gene have been isolated from the tetraploid species Xenopus laevis, differing by only a few nucleotides in their protein-coding sequence. The related, diploid species, Xenopus tropicalis, possesses a single copy of the alpha 3 gene and its transcript is similarly conserved in nucleotide sequence. However, the X. tropicalis gene is expressed exclusively in embryonic stages of development. Comparison of the X. laevis and X. tropicalis alpha 3 gene promoters reveals extensive sequence homology, including several copies of a repeated motif that is common to other vertebrate striated-muscle actin gene promoters.

Myogenic lineage determination and differentiation: evidence for a regulatory gene pathway

Stable myogenic cell lines have been derived at a high frequency by transfection of a cloned multipotential mouse embryo cell line, C3H 10T1/2, with cloned human DNA linked to a selectable neomycin resistance gene. The myogenic phenotype remains linked to neomycin resistance during secondary transfections. Although proliferative in growth conditions, these cell lines maintain the ability to differentiate and express muscle-specific proteins. We conclude that there is a simple genetic basis for myogenic determination and that a single gene, myd, converts 10T1/2 cells to a myoblast lineage. Southern blot analysis demonstrates nonidentity of myd and the MyoD1 gene. Northern blot analysis shows that myd-transfected myogenic lineages express MyoD1 mRNA while parental 10T1/2 cells do not. These results suggest that a dependent regulatory gene pathway mediates myogenic determination and differentiation.

Hepatotoxicity and molecular aspects of hepatocyte function in primary culture

1. The application of primary cultures of hepatocytes in testing for hepatotoxicity of drugs is reviewed. 2. Hepatotoxicity results principally from the biotransformation of toxic agents. This process is very complex and specific and involves a powerful system of multigenic isozyme families for both phase I and phase II drug metabolizing reactions. Many of the isozymes are specifically expressed in the liver in relation to the maturation or differentiation state, and are specifically induced, possibly through a complex temporally programmed gene regulation. 3. This highly specific, coordinated, molecular regulation is difficult to maintain in vitro. Isolation of hepatocytes induces a prompt differential decline of liver-specific gene transcription, which leads to preferential loss of the most specific functions, including those of the drug metabolizing isozymes, whereas repair of cell damage remains active. 4. The use of serum-free, hormonally defined media stabilizes specific hepatic functions, but not transcriptional activity, for 4-5 days. Defined media retain active DNA replication but do not permit clonal growth of hepatocytes. Co-culturing hepatocytes with primitive biliary cells prolongs cell survival and their functional capacities for several weeks, including some of the transcriptional activity.

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.

Differential expression of slow and fast skeletal muscle troponin C. Slow skeletal muscle troponin C is expressed in human fibroblasts

We have isolated and sequenced the cDNAs for human slow and fast skeletal muscle troponin C (TnC). Each cDNA is encoded by one of the two TnC genes in the human genome. The fast skeletal muscle TnC gene appears to be expressed exclusively in skeletal muscle. Only the slow TnC gene is expressed in human cardiac ventricle. The slow skeletal TnC gene is also expressed in skeletal muscle and, surprisingly, in several human fibroblast cell lines. Thus, at least one of the three proteins of the troponin complex appears to be expressed in non-muscle cells of higher vertebrates. The relative steady-state amounts of the slow and fast skeletal TnC mRNAs in various adult and embryonic striated muscles are similar to the expected amounts of the corresponding protein, suggesting that the expression of TnC genes is controlled predominantly by the production or accumulation of mRNA rather than by translational or post-translational mechanisms.

The 'CC.Ar.GG' box. A protein-binding site common to transcription-regulatory regions of the cardiac actin, c-fos and interleukin-2 receptor genes

We have previously suggested that a repeated sequence motif in the upstream region of the human cardiac actin gene 'CC.Ar.GG', where Ar is an (A + T)-rich six-base-pair-sequence, may be important in the muscle-specific expression of this gene [Minty, A. & Kedes, L. (1986) Mol. Cell Biol. 6, 2125-2136]. Here we show that this sequence binds a nuclear protein, and that binding is abolished by mutating either the CC and GG dinucleotides or the (A + T)-rich centre. Mutation of the CC and GG nucleotides also abolishes the transcription-stimulating activity of this sequence on the cardiac actin promoter. A similar sequence has been implicated in the serum-response of the c-fos gene [Treisman, R. (1986) Cell 46, 567-574]. We show that this c-fos 'CC.Ar.GG' sequence competes with the cardiac actin sequence for factor binding. Our results suggest that the minimum sequence requirements for binding of the serum response factor may correspond to the 'CC.Ar.GG' box sequence. Using this criterion, we predict and confirm the existence of such a binding site in a regulatory region of the interleukin-2 receptor gene. It appears therefore that interactions between 'CC.Ar.GG' boxes and similar proteic factors could be involved in the control of different genes responding to different stimuli, e.g. muscle differentiation (cardiac actin gene) or growth stimulation (c-fos, cytoskeletal actin or interleukin-2 receptor genes).

Adenovirus E1A products suppress myogenic differentiation and inhibit transcription from muscle-specific promoters

The primary function of the adenovirus E1A-region genes is to activate other adenoviral genes during a permissive viral infection by modifying the host cell transcriptional apparatus. Host cell immortalization, or transformation by the whole adenoviral early region, presumably results as a consequence of these modifications. Both transcriptional activation and transcriptional repression of non-adenoviral genes by the E1A proteins have been reported. It is currently not clear which, if either, of these activities contributes to host cell transformation and immortalization. Although there may be a physiological impact of some E1A-stimulated host cell genes, in many cases the functional significance is unclear. No common target sequences have been recognized in stimulated cellular genes and it has recently been proposed that in many cases, particularly involving newly transfected genes, available 'TATA-box' sequences may be the opportunistic beneficiaries of E1A assistance as a secondary consequence of E1A primary functions within the host cell nucleus. E1A-mediated transcriptional repression appears to be a more specific process insofar as common core elements are shared by the E1A-suppressed SV40, polyoma B, IgG heavy-chain and insulin enhancers. In the present communication we report that the complete myogenic programme of L8 and C2 myoblasts can be blocked by the introduction of constitutively expressing E1A genes, and show that the transcriptional induction of muscle-specific genes is inhibited. In particular, the promoter-inducing activities of well-defined elements that are required for the muscle-specific expression of the two sarcomeric alpha-actins, and which normally bind cellular trans-acting factors, become targets for E1A suppression. The results support the hypothesis that the suppression of differentiation by E1A products is effected by an E1A-mediated block in the transcriptional activation of cellular genes by specific developmentally regulated cis-acting promoter elements.

Activation of human alpha 1-antitrypsin genes in rat hepatoma x human fibroblast hybrid cell lines is correlated with demethylation

Alpha-1-antitrypsin (AAT) is the major protease inhibitor in human serum and is primarily expressed in the liver. We have studied AAT expression in fusion hybrids between a rat hepatoma line and either human fetal liver fibroblasts (series XXII) or human skin fibroblasts (series XIX). While the human AAT gene was always activated in series XXII hybrids when it was present, it was only rarely activated in series XIX hybrids. RFLP analysis revealed that both parental AAT alleles in series XIX hybrids were capable of being activated. Molecular analysis of the AAT gene in expressing and nonexpressing hybrids revealed that active AAT genes were hypomethylated, while inactive AAT genes were highly methylated. However, differences in methylation patterns were confined to the 5' end of the gene, on both sides of the first exon. DNaseI sensitivity revealed no hypersensitive sites close to active or inactive AAT genes.

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.

Transfection of a DNA locus that mediates the conversion of 10T1/2 fibroblasts to myoblasts

Stable myoblast cell lines were isolated after a brief exposure of mouse fibroblasts (10T1/2 cells) to 5-azacytidine. We show that transfection of 10T1/2 cells with DNA from these azacytidine-induced myoblasts (or from mouse C2C12 myoblasts) results in myogenic conversion of approximately 1 in 15,000 transfected colonies. In contrast, transfection of 10T1/2 cells with DNA from nonmyogenic cells (parental 10T1/2 cell DNA) does not give rise to myoblast colonies. These results indicate that an azacytidine-induced structural modification (presumably demethylation) in the DNA of a single locus is sufficient to convert 10T1/2 cells into determined myoblasts.