Regulation of the human cardiac/slow-twitch troponin C gene by multiple, cooperative, cell-type-specific, and MyoD-responsive elements

Identification of teleost tnnc1a enhancers for specific pan-cardiac transcription

Troponin C regulates muscle contraction by forming the troponin complex with troponin I and troponin T. Different muscle types express different troponin C genes. The mechanisms of such differential transcription are not fully understood. The Zebrafish tnnc1a gene is restrictively expressed in cardiac muscles. We here identify the enhancers and promoters of the zebrafish and medaka tnnc1a genes, including intronic enhancers in zebrafish and medaka and an upstream enhancer in the medaka. The intronic and upstream enhancers are likely functionally redundant. The GFP transgenic reporter driven by these enhancers is expressed more strongly in the ventricle than in the atrium, recapitulating the expression pattern of the endogenous zebrafish tnnc1a gene. Our study identifies a new set of enhancers for cardiac-specific transgenic expression in zebrafish. These enhancers can serve as tools for future identification of transcription factor networks that drive cardiac-specific gene transcription.

The myogenic regulatory circuit that controls cardiac/slow twitch troponin C gene transcription in skeletal muscle involves E-box, MEF-2, and MEF-3 motifs

We have characterized the specific DNA regulatory elements responsible for the function of the human cardiac troponin C gene (cTnC) muscle-specific enhancer in myogenic cells. We used functional transient transfection assays with deletional and site-specific mutagenesis to evaluate the role of the conserved sequence elements. Gel electrophoresis mobility shift assays (EMSA) demonstrated the ability of the functional sites to interact with nuclear proteins. We demonstrate that three distinct transcription activator binding sites commonly found in muscle-specific enhancers (a MEF-2 site, a MEF-3 site, and at least four redundant E-box sites) all contribute to full enhancer activity but a CArG box does not. Mutation of either the MEF-2 or MEF-3 sites or deletion of the E-boxes reduces expression by 70% or more. Furthermore, the MEF-2 site and the E-boxes specifically bind, respectively, to MEF-2 and myogenic determination factors derived from nuclear extracts. EMSA assays using a MEF-3 containing oligonucleotide revealed indistinguishable separation patterns with extracts from myogenic cells and nonmyogenic cells. These data suggest that expression of the cTnC gene in slow-twitch skeletal muscle is sustained through complex interactions at the 3'Ile enhancer between muscle-specific and nontissue-specific transcription factors: either a myogenic bHLH complex or MEF-2 can activate transcription but only in the presence of a third transcriptional activator that appears not to be muscle specific. We conclude from these observations that the cTnC 3'Ile element is a composite enhancer that functions through the combined interactions of at least five regulatory elements and their cognate binding factors: three or four E-boxes, a MEF-2 site, and a MEF-3 site. The data support the notion that all of these sites contribute to enhancer function in cell systems in an additive way but that none are absolutely required for enhancer activity. The data imply that the levels of transcription of cTnC in myogenic tissues in which the activities of one of the transcriptional factors is lacking would be partially but not wholly suppressed. Our data support the critical role of E-box sites in conjunction with the adjacent elements. Hence, we assign CTnC gene regulation to the "ordinary" rather than to the "novel" category of transcriptional regulation during skeletal myogenesis.

The association between cortisol response to mental stress and high-sensitivity cardiac troponin T plasma concentration in healthy adults

Objectives

The objective of this study was to examine the association between cortisol response to mental stress and high-sensitivity cardiac troponin T (hs-cTnT) in healthy older individuals without history of cardiovascular disease (CVD).

Background

Mental stress is a recognized risk factor for CVD, although the mechanisms remain unclear. Cortisol, a key stress hormone, is associated with coronary atherosclerosis and may accentuate structural and functional cardiac disease.

Methods

This cross-sectional study involved 508 disease-free men and women aged 53 to 76 years drawn from the Whitehall II epidemiological cohort. We evaluated salivary cortisol response to standardized mental stress tests (exposure) and hs-cTnT plasma concentration using a high-sensitivity assay (outcome). We measured coronary calcification using electron-beam dual-source computed tomography and Agatston scores.

Results

After adjustment for demographic and clinical variables associated with CVD as well as for inflammatory factors, we found a robust association between cortisol response and detectable hs-cTnT (odds ratio [OR]: 3.98; 95% confidence interval [CI]: 1.60 to 9.92; p = 0.003). The association remained when we restricted the analysis to participants without coronary calcification (n = 222; OR: 4.77; 95% CI: 1.22 to 18.72; p = 0.025) or when we further adjusted for coronary calcification in participants with positive Agatston scores (n = 286; OR: 7.39; 95% CI: 2.22 to 26.24; p = 0.001).

Conclusions

We found that heightened cortisol response to mental stress was associated with detectable plasma levels of cTnT using high-sensitivity assays in healthy participants, independently of coronary atherosclerosis. Further research is needed to understand the role of psychosocial stress in the pathophysiology of cardiac cell damage.

Identification of gene co-regulatory modules and associated cis-elements involved in degenerative heart disease

BACKGROUND

Cardiomyopathies, degenerative diseases of cardiac muscle, are among the leading causes of death in the developed world. Microarray studies of cardiomyopathies have identified up to several hundred genes that significantly alter their expression patterns as the disease progresses. However, the regulatory mechanisms driving these changes, in particular the networks of transcription factors involved, remain poorly understood. Our goals are (A) to identify modules of co-regulated genes that undergo similar changes in expression in various types of cardiomyopathies, and (B) to reveal the specific pattern of transcription factor binding sites, cis-elements, in the proximal promoter region of genes comprising such modules.

METHODS

We analyzed 149 microarray samples from human hypertrophic and dilated cardiomyopathies of various etiologies. Hierarchical clustering and Gene Ontology annotations were applied to identify modules enriched in genes with highly correlated expression and a similar physiological function. To discover motifs that may underly changes in expression, we used the promoter regions for genes in three of the most interesting modules as input to motif discovery algorithms. The resulting motifs were used to construct a probabilistic model predictive of changes in expression across different cardiomyopathies.

RESULTS

We found that three modules with the highest degree of functional enrichment contain genes involved in myocardial contraction (n = 9), energy generation (n = 20), or protein translation (n = 20). Using motif discovery tools revealed that genes in the contractile module were found to contain a TATA-box followed by a CACC-box, and are depleted in other GC-rich motifs; whereas genes in the translation module contain a pyrimidine-rich initiator, Elk-1, SP-1, and a novel motif with a GCGC core. Using a naïve Bayes classifier revealed that patterns of motifs are statistically predictive of expression patterns, with odds ratios of 2.7 (contractile), 1.9 (energy generation), and 5.5 (protein translation).

CONCLUSION

We identified patterns comprised of putative cis-regulatory motifs enriched in the upstream promoter sequence of genes that undergo similar changes in expression secondary to cardiomyopathies of various etiologies. Our analysis is a first step towards understanding transcription factor networks that are active in regulating gene expression during degenerative heart disease.

Myocyte-specific M-CAT and MEF-1 elements regulate G-protein gamma 3 gene (gamma3) expression in cardiac myocytes

Little is known regarding the mechanisms that control the expression of G-protein alpha, beta, and gamma subtypes. We have previously shown that the G-protein gamma(3) gene is expressed in the heart, brain, lung, spleen, kidney, muscle, and testis in mice. We have also reported that the G-protein gamma(3) subunit is expressed in rat cardiac myocytes, but not in cardiac fibroblasts. Other studies have shown that the gamma(3) subunit couples to the angiotensin A1A receptor in portal vein myocytes, and has been shown to mediate beta-adrenergic desensitization in cardiac myocytes treated with atorvastatin. In the present study, we evaluated G-protein gamma(3) promoter-luciferase reporter constructs in primary myocytes to identify key regulatory promoter regions. We identified two important regions of the promoter (upstream promoter region [UPR] and downstream promoter region [DPR]), which are required for expression in cardiac myocytes. We observed that removal of 48 bp in the UPR diminished gene transcription by 75%, and that the UPR contains consensus elements for myocyte-specific M-CAT and myocyte enhancer factor 1 (MEF-1) elements. The UPR and DPR share transcription factor elements for myocyte-specific M-CAT element. We observed that cardiac myocyte proteins bind to gamma(3) oligonucleotides containing transcription factor elements for myocyte-specific M-CAT and MEF-1. Myocyte-specific M-CAT proteins were supershifted with transcriptional enhancer factor-1 (TEF-1) antibodies binding to the gamma(3) M-CAT element, which is in agreement with reports showing that the M-CAT element binds the TEF-1 family of transcription factors. The 150 bp DPR contains three M-CAT elements, an INR element, an upstream stimulatory factor 1 element, and the transcription start site. We have shown that myocyte gamma(3) gene expression is regulated by myocyte-specific M-CAT and MEF-1 elements.

The Ankrd2, Cdkn1c and Calcyclin Genes are Under the Control of MyoD During Myogenic Differentiation

Skeletal muscle development requires the coordinated expression of numerous transcription factors to control the specification of the muscle fate in mesodermal cells and the differentiation of the committed myoblasts into functional contractile fibers. The bHLH transcription factor MyoD plays a key role in these processes, since its forced expression is sufficient to induce the myogenesis in a variety of non-muscle cells in culture. Consistent with this observation, the majority of skeletal muscle genes require MyoD to activate their own transcription. In order to identify novel MyoD-target genes we generated C2C12 MyoD-silenced clones, and used a muscle-specific cDNA microarray to study the induced modifications of the transcriptional profile. Gene expression was analyzed at three different stages in differentiating MyoD(-)C2C12 myoblasts. These microarray data sets identified many additional uncharacterized downstream MyoD transcripts that may play important functions in muscle cell differentiation. Among these genes, we concentrated our study on the cell cycle regulators Cdkn1c and calcyclin and on the muscle-specific putative myogenic regulator Ankrd2. Bioinformatic and functional studies on the promoters of these genes clarified their dependence on MyoD activity. Clues of other regulatory mechanisms that might interact with the principal bHLH transcription factor have been revealed by the unexpected up-regulation in MyoD(-) cells of these novel (and other) target transcripts, at the differentiation stage in which MyoD became normally down-regulated.

Normalization of muscle plasmid uptake by Southern blot: application to SERCA1 promoter analysis

Direct injection of plasmid DNA into muscle allows the study of promoters in a physiological environment. Because of the variability of reporter gene activity, attempts have been made to normalize activity to muscle plasmid uptake by coinjection of a second "control" plasmid whose reporter gene is constitutively expressed by a viral promoter. The purpose of this study was to evaluate the use of a control plasmid vs. Southern blot to normalize for differences in uptake of plasmids containing promoter fragments of the skeletal muscle-specific sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA1) gene. Results showed that the correlation of luciferase activity from control vs. SERCA1 plasmids is poor and that normalization by a virally driven control plasmid increased variability of SERCA1 luciferase activity. In several cases, the presence of a control plasmid inhibited SERCA1 reporter expression. When Southern blot analysis was used to normalize for differences in plasmid uptake there was less variability than with coinjection, and correlations between plasmid uptake and SERCA1 luciferase activity were better. Moreover, there were no inhibitory effects of a control plasmid allowing for optimization of injection conditions of the SERCA1 deletion constructs. The use of Southern analysis is suggested to determine whether plasmid uptake is differentially affected by physiological stimuli, muscle types, or plasmid sizes under study.

Characterization of the human glycogenin-1 gene: identification of a muscle-specific regulatory domain

The de-novo synthesis of glycogen is now known to involve a novel class of self-glucosylating protein primers. In mammalian skeletal muscle, glycogenin-1 is the protein responsible for this initiation step. Northern blot analysis revealed that glycogenin-1 gene transcription is differentially regulated in the C2C12 mouse muscle cell line. To define the regulatory elements that control expression of the glycogenin-1 gene, we have cloned and characterized the genomic structure of the human glycogenin-1 gene and its promoter region. This gene consists of seven exons and six introns, and spans over 13kb. Transcription of human glycogenin-1 is initiated at two major sites, 80 and 86bp upstream from the initiation of translation codon. Nucleotide sequence analysis of 2.1kb of the 5'-flanking region revealed the proximal promoter contains both a TATA box and two putative Sp1 binding sites located in a CpG island. There are numerous binding sites for developmental and cell-type-specific transcription factors, including AP-1, AP-2, GATA, and several potential Oct 1 binding domains. There are also nine consensus E-boxes that bind the basic helix-loop-helix family of muscle-specific transcription factors. The transcriptional activity of the glycogenin-1 gene was investigated by transient transfection of the 5'-flanking region in HepG2 cells and C2C12 myoblasts and myotubes. These results permitted the definition of a minimal 232bp promoter fragment that is responsible for basal level transcription in a cell-type-independent manner. Furthermore, we have identified a regulatory region located between -2076 and -1736 of the 5'-flanking region of the human glycogenin-1 gene that allows myotube-specific expression in C2C12 cells.

Definition of regulatory sequence elements in the promoter region and the first intron of the myotonic dystrophy protein kinase gene

Myotonic dystrophy is the most common inherited adult neuromuscular disorder with a global frequency of 1/8000. The genetic defect is an expanding CTG trinucleotide repeat in the 3'-untranslated region of the myotonic dystrophy protein kinase gene. We present the in vitro characterization of cis regulatory elements controlling transcription of the myotonic dystrophy protein kinase gene in myoblasts and fibroblasts. The region 5' to the initiating ATG contains no consensus TATA or CCAAT box. We have mapped two transcriptional start sites by primer extension. Deletion constructs from this region fused to the bacterial chloramphenicol acetyltransferase reporter gene revealed only subtle muscle specific cis elements. The strongest promoter activity mapped to a 189-base pair fragment. This sequence contains a conserved GC box to which the transcription factor Sp1 binds. Reporter gene constructs containing a 2-kilobase pair first intron fragment of the myotonic dystrophy protein kinase gene enhances reporter activity up to 6-fold in the human rhabdomyosarcoma myoblast cell line TE32 but not in NIH 3T3 fibroblasts. Co-transfection of a MyoD expression vector with reporter constructs containing the first intron into 10 T1/2 fibroblasts resulted in a 10-20-fold enhancement of expression. Deletion analysis of four E-box elements within the first intron reveal that these elements contribute to enhancer activity similarly in TE32 myoblasts and 10 T1/2 fibroblasts. These data suggest that E-boxes within the myotonic dystrophy protein kinase first intron mediate interactions with upstream promoter elements to up-regulate transcription of this gene in myoblasts.

A novel site, Mt, in the human desmin enhancer is necessary for maximal expression in skeletal muscle

Previous investigations have shown that expression of the muscle-specific intermediate filament desmin gene in skeletal muscle is controlled in part by a 5' muscle-specific enhancer. This enhancer activity can be divided into myoblast-specific and myotube-specific activation domains. The myotube-specific region contains a MyoD and MEF2 sites, whereas the myoblast-specific region contains Sp1, Krox, and Mb sites. In the present study, we designed mutations in the conserved portion of the myotube-specific region; transfection analysis of these mutations showed that a novel site located between the MyoD and MEF2 sites, named Mt (GGTATTT), is required for full transcriptional activity of the desmin enhancer in skeletal muscle. Although gel mobility shift assays demonstrate that myotube, myoblast, fibroblast, and HeLa nuclear extracts contain a nuclear factor that binds specifically to Mt, four copies of the Mt site function as the native enhancer only in myotubes. Functional synergism among the MyoD, MEF2, and Mt sites in myotubes has been demonstrated. These results show that the novel Mt site cooperates with MyoD and MEF2 to give maximal expression of the desmin gene.

A novel cardiac-restricted target for doxorubicin. CARP, a nuclear modulator of gene expression in cardiac progenitor cells and cardiomyocytes

Doxorubicin (Dox), a cardiotoxic antineoplastic drug, disrupts the cardiac-specific program of gene expression (Kurabayashi, M., Dutta, S., Jeyaseelan, R., and Kedes, L. (1995) Mol. Cell. Biol. 15, 6386-6397; Jeyaseelan, R., Poizat, C., Wu, H. Y., and Kedes, L. (1997) J. Biol. Chem. 272, 5828-5832). To determine whether this drug might interfere with the function of cardiac-specific regulatory pathways, we used a differential display strategy to clone from neonatal rat cardiomyocyte candidate mRNAs that were rapidly sensitive to Dox. We report here the identification of a constitutively expressed, cardiac-restricted, nuclear protein whose mRNA level is exquisitely sensitive to Dox. Hence we have named this protein cardiac adriamycin-responsive protein (CARP). CARP mRNA is present at the earliest stages of cardiac morphogenesis. It was detected by in situ hybridization within the cardiogenic plate of 7. 5-day post coitum (p.c.) embryos, and in 8.5-day p.c. embryos CARP transcripts are present in uniformly high levels in the myocardium. Throughout cardiac development, CARP expression is specific for the myocardium; endocardial cushions and valves exhibit only background levels of signal. Transcript levels persist but gradually decrease in neonatal, 2-week-old, and adult hearts. There were no stages when CARP mRNA could not be detected. The pattern and timing of CARP mRNA expression, including transient expression in the tongue at 14.5 days p.c., coincides with that of Nkx2.5/Csx (a putative homolog of tinman, the Drosophila melanogaster gene responsible for cardiac development). The cloned full-length 1749 nucleotide CARP cDNA encodes a 319-amino acid 40-kDa polypeptide containing five tandem ankyrin repeats. CARP appears to be the rat homolog of a previously reported human single-copy gene (C-193; Chu, W., Burns, D. K., Swerlick, R. A., and Presky, D. H. (1995) J. Biol. Chem. 270, 10236-10245), whose mRNA is inducible by cytokines only in human endothelial cells. CARP appears to function as a negative regulator of cardiac-specific gene expression. Overexpression of CARP in cardiomyocytes suppresses cardiac troponin C and atrial natriuretic factor transcription. Cotransfection experiments in HeLa cells indicate that CARP inhibits Nkx2.5 transactivation of atrial natriuretic factor promoter. When fused to a GAL4 DNA-binding domain, CARP has transcriptional inhibitory properties in noncardiac cells. CARP thus represents the first example of a cardiac-restricted transcriptional regulatory protein that is sensitive to Dox.

Differential expression of troponin C genes during tongue myogenesis

Determination of muscle fiber type is related to the developmental stage of the tissue. Ordinarily the final distribution of fast and slow fibers in a muscle is determined postnatally. Tongue muscle, however, is composed solely of fast-twitch fibers that express only troponin C fast mRNA and fast (type II) myosin heavy chain (MHC) proteins in both the adult and the one-day-old mouse. The fiber-type determination of this muscle was examined during fetal development. Both troponin C fast and slow mRNAs were expressed at initial stages of tongue development at embryonic day 18. However, by embryonic day 16 the troponin C fast transcripts predominated. AT 17 days of embryonic development, TnC fast mRNA was 10 times more abundant than TnC slow, and at 18 days of development the TnC slow mRNA was barely detectable. The tongue muscle myotubes expressed fast, slow, and embryonic MHC isoforms during early embryonic development. At 18 days of gestation, the MHC isoform expressed by the majority of the myotubes was the fast isoform, whereas the slow isoform was present in very few fibers. RT-PCR analysis of the MHC transcripts present throughout tongue development demonstrated expression of the mdms or type IIx MHC in both late fetal and postnatal stages of development. In contrast, the type I/beta slow MHC mRNA was undetectable in the postnatal and adult tongue. The absence of TnC and MHC slow-isoform mRNAs in the newborn mouse tongue suggests that slow isoform genes become dominantly repressed with the TnC-F and MHC type IIx genes remaining transcriptionally active, giving rise to an unusually homogeneous fast-twitch phenotype. The tongue muscle fibers acquire their specific adult-type fiber characteristics during fetal development rather than postnatally.

Molecular mechanisms of doxorubicin-induced cardiomyopathy. Selective suppression of Reiske iron-sulfur protein, ADP/ATP translocase, and phosphofructokinase genes is associated with ATP depletion in rat cardiomyocytes

Doxorubicin, a cardiotoxic antineoplastic, disrupts the cardiac-specific program of gene expression (Kurabayashi, M., Dutta, S., Jeyaseelan, R., and Kedes, L. (1995) Mol. Cell. Biol. 15, 6386-6397). We have now identified neonatal rat cardiomyocyte mRNAs rapidly sensitive to doxorubicin, or its congener daunomycin, including transcripts of nuclear genes encoding enzymes critical in production of energy in cardiomyocytes: ADP/ATP translocase, a heart- and muscle-specific isoform; Reiske iron-sulfur protein (RISP), a ubiquitously expressed electron transport chain component; and a muscle isozyme of phosphofructokinase. Loss of these mRNAs following doxorubicin or daunomycin is evident as early as 2 h and precedes significant reduction of intracellular ATP. ATP levels in control cardiomyocytes (17.9 +/- 2.9 nM/mg of protein) fall only after 14 h and reach residual levels of 10.4 +/- 0.9 nM (doxorubicin; p = <0.006) and 6.7 +/- 1.9 nM (daunomycin; p = <0. 001) by 24 h. Loss of mRNAs generating ATP was highly selective since mRNAs for other energy production enzymes, (cytochrome c, cytochrome b, and malate dehydrogenase), and genes important in glycolysis (pyruvate kinase and glyceraldehyde-3-phosphate dehydrogenase) were unaffected even at 24 and 48 h. The drugs had no effect on levels of ubiquitously expressed RISP mRNA in fibroblasts. These findings could link doxorubicin-induced damage to membranes and signaling pathways with 1) suppression of transcripts encoding myofibrillar proteins and proteins of energy production pathways and 2) depletion of intracellular ATP stores, myofibrillar degeneration, and related cardiotoxic effects.

Identification and characterization of the cell type-specific and developmentally regulated alpha7 integrin gene promoter

Expression of alpha7 is mainly confined to skeletal and cardiac muscle in which it appears to be the major laminin-binding integrin. When myoblasts differentiate to myotubes, alpha7 mRNA and protein expression is up-regulated. To explore the mechanisms involved in the tissue-specific and developmentally regulated expression of alpha7, we isolated and characterized a genomic clone containing approximately 2.8 kilobase pairs (kb) of the 5'-flanking region of the murine alpha7 gene. The 5'-flanking region lacks both TATA and CCAAT boxes but contains five putative Sp1 binding sites located in a CpG island. Two transcription start sites, located near an initiator-like sequence, are 176 and 170 base pairs upstream of the translation start site. There are numerous binding sites for developmental and cell type-specific transcription factors, including AP-1, AP-2, GATA, and several AT-rich sites. There are also eight consensus E-boxes that bind the basic helix-loop-helix family of muscle-specific transcription factors. The approximately 2.8-kb 5'-flanking region was an active promoter in C2C12 skeletal myoblasts and exhibited increased expression upon conversion to myotubes but was inactive in HtLM2 cells, a mouse breast carcinoma epithelial cell line that does not express alpha7. Deletion analysis identified both positive and negative regulatory elements within the approximately 2.8-kb fragment. In 10T1/2 fibroblasts the approximately 2.8-kb alpha7 promoter was trans-activated by the myogenic basic helix-loop-helix proteins myogenin and MyoD but not by MRF4 and myf5. These results suggest that muscle-specific transcription factors play a role in regulating the cell-type expression of the alpha7 gene during development.

Atrial chamber-specific expression of the slow myosin heavy chain 3 gene in the embryonic heart

The quail slow myosin heavy chain 3 (slow MyHC 3) gene is expressed in the developing heart and in slow muscles of the developing limb. It is first expressed in the pulsatile cardiac tube in the embryo, and as the heart chamberizes its expression becomes restricted to the atria. To identify regulatory elements responsible for atrial-specific expression, the 5' upstream region of slow MyHC 3 gene was investigated. An atrial regulatory domain (ARD1) between -840 and -680 acts as an atrial cell-specific enhancer in primary cardiocyte cultures. ARD1 also specifies atrial-specific expression in vivo when the ARD1/heterologous promoter was introduced into developing chick embryos by a replication-competent retroviral vector. ARD1 is the first atrial cell-specific enhancer to be identified. Fine deletion and mutation analysis within ARD1 defined a 40-base pair vitamin D3 receptor-like element that controls atrial cell-specific expression of the slow MyHC 3 gene by inhibiting its expression in ventricular cardiocytes.

Slow troponin C gene expression in chicken heart and liver is regulated by similar enhancers

Two isoforms of troponin C (TnC) are encoded by distinct single copy genes. Expression of fast TnC is restricted to the skeletal muscle, whereas the slow isoform is expressed in both skeletal and cardiac muscle. Chicken slow TnC (cTnC) gene is also expressed in some non-muscle tissues like the liver and the brain. Expression of cTnC gene is regulated by two distinct enhancers in cardiac and skeletal muscles. The cardiac specific enhancer is located in the immediate 5' flanking region (bp-124 to -79) of the murine cTnC gene whereas the skeletal enhancer is located within the first intron (bp 997 to 1141). In the present study we have examined how cTnC gene expression is regulated in the chicken liver. Transient transfection of liver cells with CTnC-CAT reporter constructs containing various regions of the murine cTnC gene showed that its expression in chicken liver is regulated by the cardiac specific enhancer. Furthermore, electrophoretic mobility shift assays using synthetic oligonucleotides corresponding to both CEF-1 and CEF-2 regions of the murine cardiac enhancer revealed formation of specific DNA-protein complexes. Ultraviolet light induced covalent linking of nuclear proteins to CEF-1 and CEF-2 oligomers were used to examine the nature of the cardiac enhancer binding polypeptides; one polypeptide of 48 kDa appeared to bind to both CEF-1 and CEF-2 sequences.

Regulation of tissue-specific expression of the skeletal muscle ryanodine receptor gene

The ryanodine receptors (RYR) are a family of calcium release channels that are expressed in a variety of tissues. Three genes, i. e. ryr1, ryr2, and ryr3, have been identified coding for a skeletal muscle, cardiac muscle, and brain isoform, respectively. Although, the skeletal muscle isoform (RYR1) was shown to be expressed predominantly in skeletal muscle, expression was also detected in the esophagus and brain. To analyze the transcriptional regulation of the RYR1 gene, we have constructed chimeric genes composed of the upstream region of the RYR1 gene and the bacterial chloramphenicol acetyltransferase (CAT) gene and transiently transfected them into primary cultured porcine myoblasts, myotubes, and fibroblasts. A 443-base pair region upstream from the transcription start site was sufficient to direct CAT activity without tissue specificity. Deletion of a 61-base pair fragment from the 5'-end of the promoter resulted in a marked reduction of CAT activity in all three tissue types. A similar reduction of expression was observed when using a construct with the first intron in antisense orientation upstream from the promoter. In contrast, the first intron in sense orientation enhanced expression only in myotubes, while expression was repressed in fibroblasts and myoblasts. Gel retardation analyses showed DNA binding activity in nuclear extracts for two upstream DNA sequence elements. Our data suggest that (i) RYR1 gene expression is regulated by at least two novel transcription factors (designated RYREF-1 and RYREF-2), and (ii) tissue specificity results from a transcriptional repression in nonmuscle cells mediated by the first intron.

Transcriptional control of muscle plasticity: differential regulation of troponin I genes by electrical activity

Plasticity of the skeletal muscle phenotype can result from the selective repression and activation of gene expression in response to innervation patterns. Motoneurons, eliciting different patterns of depolarization, regulate the contractile properties of the myofibers they innervate by selectively activating expression of genes encoding fiber-type-specific (fast vs. slow) contractile proteins. We have analyzed the regulation of the troponin I slow (TnIs) and fast (TnIf) genes as a model to study the molecular mechanisms regulating fiber-type plasticity. We found that expression of the two TnI isoforms is downregulated by denervation. Moreover, TnI expression is upregulated by specific patterns of electrical activity [10 Hz vs. 100 Hz] used to depolarize muscle. We previously isolated the rat TnIs gene and demonstrated that regulatory sequences reside in its upstream region and second intron [Banerjee-Basu S, Buonanno A (1993), Mol Cell Biol 12:5024-5032]. Using transgenic mice, we show that the upstream region of the TnIs gene extending from -949 to +50 is sufficient to confer transcription specifically in slowtwitch muscles. Serial deletions of the TnIs upstream and intronic regions were generated in a CAT reporter vector to delineate transcriptional regulatory elements in transiently transfected Sol8 myotubes. Sequences necessary to confer the highest levels of TnIs transcription mapped to the upstream region between -0.95 and -0.72 kb, and to a 56 bp sequence located in the second intron. Comparison of the at sequence between -0.95 and -0.72 to the human TnIs gene identified a highly homologous region of 128 bp that we named the TnI SURE (slow upstream regulatory element). Alignment of these two SURE sequences with the quail TnI fast intronic regulatory element identified common motifs, namely, two A/T-rich sequences (A/T1 and A/T2) with homology to homeotic protein and MEF2 binding sites, a CACC box, an E box, and a novel motif (GCAGGCA) that we denoted the CAGG box. Mutation of either the A/T2 site, E box, or CAGG box practically abolish the SURE function in transfected myotubes; mutation of the A/T1 and CACC sites has a lesser effect. Using competitive electrophoretic mobility shift assays with nuclear extracts derived from Sol8 myotubes, we demonstrate specific binding to these motifs. The A/T1 and A/T2 sites are shown to form different complexes. The A/T2 site, which bears extensive homology to a MEF2 site, forms complexes that are super shifted by MEF2A antisera and that are competed by a consensus MEF2 site present in the MCK enhancer. Our results demonstrate that the linear arrangement of DNA sequence motifs is conserved in the regulatory elements of the TnI slow and fast genes and suggest that the interaction of multiple protein-DNA complexes are necessary for enhancer function.