Role of myocyte-specific enhancer-binding factor (MEF-2) in transcriptional regulation of the alpha-cardiac myosin heavy chain gene

Cardiac maturation

The heart undergoes a dynamic maturation process following birth, in response to a wide range of stimuli, including both physiological and pathological cues. This process entails substantial re-programming of mitochondrial energy metabolism coincident with the emergence of specialized structural and contractile machinery to meet the demands of the adult heart. Many components of this program revert to a more "fetal" format during development of pathological cardiac hypertrophy and heart failure. In this review, emphasis is placed on recent progress in our understanding of the transcriptional control of cardiac maturation, encompassing the results of studies spanning from in vivo models to cardiomyocytes derived from human stem cells. The potential applications of this current state of knowledge to new translational avenues aimed at the treatment of heart failure is also addressed.

The MEF2A transcription factor interactome in cardiomyocytes

Transcriptional regulators encoded by the Myocyte Enhancer Factor 2 (MEF2) gene family play a fundamental role in cardiac development, homeostasis and pathology. Previous studies indicate that MEF2A protein-protein interactions serve as a network hub in several cardiomyocyte cellular processes. Based on the idea that interactions with regulatory protein partners underly the diverse roles of MEF2A in cardiomyocyte gene expression, we undertook a systematic unbiased screen of the MEF2A protein interactome in primary cardiomyocytes using an affinity purification-based quantitative mass spectrometry approach. Bioinformatic processing of the MEF2A interactome revealed protein networks involved in the regulation of programmed cell death, inflammatory responses, actin dynamics and stress signaling in primary cardiomyocytes. Further biochemical and functional confirmation of specific protein-protein interactions documented a dynamic interaction between MEF2A and STAT3 proteins. Integration of transcriptome level data from MEF2A and STAT3-depleted cardiomyocytes reveals that the balance between MEF2A and STAT3 activity exerts a level of executive control over the inflammatory response and cardiomyocyte cell survival and experimentally ameliorates Phenylephrine induced cardiomyocyte hypertrophy. Lastly, we identified several MEF2A/STAT3 co-regulated genes, including the MMP9 gene. Herein, we document the cardiomyocyte MEF2A interactome, which furthers our understanding of protein networks involved in the hierarchical control of normal and pathophysiological cardiomyocyte gene expression in the mammalian heart.

A family of microRNAs encoded by myosin genes governs myosin expression and muscle performance

Myosin is the primary regulator of muscle strength and contractility. Here we show that three myosin genes, Myh6, Myh7, and Myh7b, encode related intronic microRNAs (miRNAs), which, in turn, control muscle myosin content, myofiber identity, and muscle performance. Within the adult heart, the Myh6 gene, encoding a fast myosin, coexpresses miR-208a, which regulates the expression of two slow myosins and their intronic miRNAs, Myh7/miR-208b and Myh7b/miR-499, respectively. miR-208b and miR-499 play redundant roles in the specification of muscle fiber identity by activating slow and repressing fast myofiber gene programs. The actions of these miRNAs are mediated in part by a collection of transcriptional repressors of slow myofiber genes. These findings reveal that myosin genes not only encode the major contractile proteins of muscle, but act more broadly to influence muscle function by encoding a network of intronic miRNAs that control muscle gene expression and performance.

Cooperative activation of atrial naturetic peptide promoter by dHAND and MEF2C

An intricate array of cell-specific multiprotein complexes participate in programs of cell-specific gene expression through combinatorial interaction with different transcription factors and cofactors. The dHAND basic helix-loop-helix (bHLH) transcription factor, which is essential for heart development and extra embryonic structures, is thought to regulate cardiomyocyte-specific gene expression through combinatorial interactions with other cardiac-restricted transcription factors such as GATA4 and NKX2.5. Here, we determine that dHAND also interacts with the myocyte enhancer binding factor-2c (MEF2C) protein, which belongs to MADS-box transcription factors and is essential for heart development. dHAND and MEF2C synergistically activated expression of the atrial naturetic peptide gene (ANP) in transfected HeLa cells. GST-pulldown and immunoprecipitation assay demonstrate that full-length MEF2C protein is able to interact with dHAND in vitro and in vivo, just like MEF2A and bHLH transcription factors MyoD in skeletal muscle cells. In addition, electrophoretic mobility shift assays (EMSAs) demonstrate that MEF2C and dHAND do not influence each other's DNA binding activity. Using chromatin immunoprecipitation (ChIP) analysis in H9c2 cells we show that dHAND interact with MEF2C to form protein complex and bind A/T sequence in promoter of ANP. Taken together with previous observations, these results suggest the existence of large multiprotein transcriptional complex with core DNA binding proteins that physically interact with other transcriptional factors to form favorable conformation to potentiate transcription.

The p38 MAP kinase pathway and its biological function

p38 is a mitogen-activated protein (MAP) kinase with structural and functional characteristics that distinguish it from JNK and ERK MAP kinases. p38 activity is upregulated when cells are exposed to a variety of stimuli including bacterial pathogens, proinflammatory cytokines, certain growth factors, and other forms of environmental stress. By regulating downstream substrates that include protein kinases and transcription factors, p38 participates in transmission, amplification, and diversification of the extracellular signal, initiating several different cellular responses. Studies have revealed that activation of p38 pathway is related to many pathological changes that occur in the course of inflammatory/immunologic and cardiovascular diseases.

Identification and characterization of embryonic stem cell‐derived pacemaker and atrial cardiomyocytes

The aim of this study was to identify and functionally characterize cardiac subtypes during early stages of development. For this purpose, transgenic embryonic stem cells were generated using the alpha-myosin heavy chain promoter driving the expression of the enhanced green fluorescent protein (EGFP). EGFP-positive clusters of cells were first observed as early as 7 days of development, thus, even before the initiation of the contractile activity. Flow cytometry and single-cell fluorescence measurements evidenced large diversities of EGFP intensity. Patch-clamp experiments showed EGFP expression exclusively in pacemaker and atrial but not ventricular cells. The highest fluorescence intensities were detected in pacemaker-like cardiomyocytes. In accordance, multielectrode-array recordings of whole embryoid bodies confirmed that the pacemaker center coincided with strongly EGFP-positive areas. The cardiac subtypes displayed already at this early stage differential characteristics of electrical activity and ion channel expression. Thus, quantitation of the alpha-myosin heavy chain driven reporter gene expression allows identification and functional characterization of early cardiac subtypes.

Single-stranded DNA-binding proteins PURalpha and PURbeta bind to a purine-rich negative regulatory element of the alpha-myosin heavy chain gene and control transcriptional and translational regulation of the gene expression. Implications in the repression of alpha-myosin heavy chain during heart failure

The alpha-myosin heavy chain is a principal molecule of the thick filament of the sarcomere, expressed primarily in cardiac myocytes. The mechanism for its cardiac-restricted expression is not yet fully understood. We previously identified a purine-rich negative regulatory (PNR) element in the first intron of the gene, which is essential for its cardiac-specific expression (Gupta, M., Zak, R., Libermann, T. A., and Gupta, M. P. (1998) Mol. Cell. Biol. 18, 7243-7258). In this study we cloned and characterized muscle and non-muscle factors that bind to this element. We show that two single-stranded DNA-binding proteins of the PUR family, PURalpha and PURbeta, which are derived from cardiac myocytes, bind to the plus strand of the PNR element. In functional assays, PURalpha and PURbeta repressed alpha-myosin heavy chain (alpha-MHC) gene expression in the presence of upstream regulatory sequences of the gene. However, from HeLa cells an Ets family of protein, Ets-related protein (ERP), binds to double-stranded PNR element. The ERP.PNR complex inhibited the activity of the basal transcription complex from homologous as well as heterologous promoters in a PNR position-independent manner, suggesting that ERP acts as a silencer of alpha-MHC gene expression in non-muscle cells. We also show that PUR proteins are capable of binding to alpha-MHC mRNA and attenuate its translational efficiency. Furthermore, we show robust expression of PUR proteins in failing hearts where alpha-MHC mRNA levels are suppressed. Together, these results reveal that (i) PUR proteins participate in transcriptional as well as translational regulation of alpha-MHC expression in cardiac myocytes and (ii) ERP may be involved in cardiac-restricted expression of the alpha-MHC gene by preventing its expression in non-muscle cells.

Dose-dependent blockade to cardiomyocyte hypertrophy by histone deacetylase inhibitors

Postnatal cardiac myocytes respond to stress signals by hypertrophic growth and activation of a fetal gene program. Recently, we showed that class II histone deacetylases (HDACs) suppress cardiac hypertrophy, and mice lacking the class II HDAC, HDAC9, are sensitized to hypertrophic signals. To further define the roles of HDACs in cardiac hypertrophy, we analyzed the effects of HDAC inhibitors on the responsiveness of primary cardiomyocytes to hypertrophic agonists. Paradoxically, HDAC inhibitors imposed a dose-dependent blockade to hypertrophy and fetal gene activation. We conclude that distinct HDACs play positive or negative roles in the control of cardiomyocyte hypertrophy. HDAC inhibitors are currently being tested in clinical trials as anti-cancer agents. Our results suggest that these inhibitors may also hold promising clinical value as therapeutics for cardiac hypertrophy and heart failure.

Myocardial-directed overexpression of the human beta(1)-adrenergic receptor in transgenic mice

The beta(1)-adrenergic receptor (AR) is the dominant subtype in non-failing and failing myocardium. beta(1)-AR signaling, by the endogenous neurotransmitter norepinephrine, is central to the regulation of myocardial contractility. In heart failure, the beta(1)-AR undergoes subtype-selective downregulation which may protect against the increased cardiac adrenergic drive associated with this pathophysiological state. To examine the hypothesis that chronically increased beta(1)-AR mediated signaling has adverse myocardial effects, transgenic mice overexpressing the human beta(1)-AR in a cardiac-selective context were produced, utilizing an alpha-myosin heavy chain (MHC) promoter. In these mice, beta(1)-AR protein abundance was approximately 24-46-fold (1-2 pmol/mg protein) that of wild-type mice. Histopathological examination of young (4 months old) and old (approximately 9 months old) transgenic mouse hearts consistently demonstrated large areas of interstitial replacement fibrosis, marked myocyte hypertrophy and myofibrilar disarray. In addition, increased expression of the pre-apoptotic marker, Bax, was observed coincident with regions of fibrosis accompanied by an increased apoptotic index, as measured by TUNEL assay. Older non-transgenic mice exhibited a slight tendency towards a decreased fractional shortening, whereas older beta(1)-AR transgenic mice had a marked reduction in fractional shortening (%FS approximately 30) as determined by echocardiography. Additionally, older beta(1)-AR transgenic mice had an increased left ventricular chamber size. In summary, cardiac-directed overexpression of the human beta(1)-AR in transgenic mice leads to a significant histopathological phenotype with no apparent functional consequence in younger mice and a variable degree of cardiac dysfunction in older animals. This model system may ultimately prove useful for investigating the biological basis of adrenergically-mediated myocardial damage in humans.

Identification of cis elements in the cardiac troponin T gene conferring specific expression in cardiac muscle of transgenic mice

To investigate the underlying mechanism regulating cardiac gene expression, transgenic mice carrying the rat cardiac troponin T proximal promoter (-497 bp from the transcriptional start site) fused to a LacZ or chloramphenicol acetyltransferase (CAT) reporter gene were analyzed. The LacZ expression pattern throughout development was very similar to that of the endogenous cardiac troponin T gene. Within this promoter, a high degree of sequence homology was found at 2 sites, modules D (-335 to -289 bp) and F (-249 to -209 bp). Both regions contain at least a TCTG(G/C) direct repeat and an A/T-rich site, whereas only the F module has a muscle enhancer factor 2 (MEF2)-like motif. No significant decrease in CAT transgene expression was observed when only the MEF2 core sequence was mutated. However, when the MEF2 core sequence and its flanking TCTGG site were mutated (Mut5), CAT transgene expression was significantly decreased in the heart, and ectopic expression of the transgene was also observed. When mutations were introduced into this promoter to destroy all upstream TCTG(G/C) direct repeats in the D module (MutD), CAT expression remained cardiac specific, but the expression level was dramatically decreased. Relaxation of cardiac-specific transgene expression became even more severe in transgenic mice carrying double mutations (Mut[D+5]). In addition, CAT activity in the heart was nearly abolished. These results suggest that D and F modules have an additive function in determining the level of expression in the heart and only the F module confers cardiac-specific expression.

Transcriptional control of muscle development by myocyte enhancer factor-2 (MEF2) proteins

Metazoans contain multiple types of muscle cells that share several common properties, including contractility, excitability, and expression of overlapping sets of muscle structural genes that mediate these functions. Recent biochemical and genetic studies have demonstrated that members of the myocyte enhancer factor-2 (MEF2) family of MADS (MCM1, agamous, deficiens, serum response factor)-box transcription factors play multiple roles in muscle cells to control myogenesis and morphogenesis. Like other MADS-box proteins, MEF2 proteins act combinatorially through protein-protein interactions with other transcription factors to control specific sets of target genes. Genetic studies in Drosophila have also begun to reveal the upstream elements of myogenic regulatory hierarchies that control MEF2 expression during development of skeletal, cardiac, and visceral muscle lineages. Paradoxically, MEF2 factors also regulate cell proliferation by functioning as endpoints for a variety of growth factor-regulated intracellular signaling pathways that are antagonistic to muscle differentiation. We discuss the diverse functions of this family of transcription factors, the ways in which they are regulated, and their mechanisms of action.

Transcriptional coactivator p300 stimulates cell type-specific gene expression in cardiac myocytes

Terminal differentiation is characterized by cell cycle arrest and the expression of cell type-specific genes. Previous work has suggested that the p300 family of transcriptional coactivators plays an important role in preventing the re-initiation of DNA synthesis in terminally differentiated cardiac myocytes. In this study, we investigated whether p300 proteins are also involved in the transcriptional activation of cell type-specific genes in these cells. Since p300 function can be abrogated through direct binding by the adenovirus E1A protein, we overexpressed E1A in cardiac myocytes using recombinant adenoviral vectors. The expression of transfected reporter genes driven by alpha- or beta-myosin heavy chain promoters was markedly diminished by expression of the 12 S E1A protein. In contrast, the activity of a promoter derived from the ubiquitously expressed beta-actin gene was affected only modestly. While an E1A mutant unable to bind members of the retinoblastoma family of pocket proteins decreased the activity of alpha- and beta-myosin heavy chain promoters to nearly the same extent as wild type 12 S E1A, transcriptional repression by a mutant defective for p300 binding was severely impaired. Furthermore, overexpression of p300 and, to an even greater extent, p300del33, a mutant lacking residues required for binding by E1A, relieved E1A's repression of beta-myosin heavy chain promoter activity while having no effect on the activity of the beta-actin promoter. Thus, E1A's transcriptional repression of cell type-specific genes in cardiac myocytes is mediated through its binding of p300 proteins, and these proteins appear to be involved in maintaining both cell type-specific gene expression and cell cycle arrest in cardiac myocytes.

Transgenic A1 adenosine receptor overexpression increases myocardial resistance to ischemia

Activation of myocardial A1 adenosine receptors (A1AR) protects the heart from ischemic injury. In this study transgenic mice were created using the cardiac-specific alpha-myosin heavy chain promoter and rat A1AR cDNA. Heart membranes from two transgene positive lines displayed approximately 1,000-fold overexpression of A1AR (6,574 +/- 965 and 10,691 +/- 1,002 fmol per mg of protein vs. 8 +/- 5 fmol per mg of protein in control hearts). Compared with control hearts, transgenic Langendorff-perfused hearts had a significantly lower intrinsic heart rate (248 beats per min vs. 318 beats per min, P < 0. 05), lower developed tension (1.2 g vs. 1.6 g, P < 0.05), and similar coronary resistance. The difference in developed tension was eliminated by pacing. Injury of control hearts during global ischemia, indexed by time-to-ischemic contracture, was accelerated by blocking adenosine receptors with 50 microM 8-(p-sulfophenyl) theophylline but was unaffected by addition of 20 nM N6-cyclopentyladenosine, an A1AR agonist. Thus A1ARs in ischemic myocardium are presumably saturated by endogenous adenosine. Overexpressing myocardial A1ARs increased time-to-ischemic contracture and improved functional recovery during reperfusion. The data indicate that A1AR activation by endogenous adenosine affords protection during ischemia, but that the response is limited by A1AR number in murine myocardium. Overexpression of A1AR affords additional protection. These data support the concept that genetic manipulation of A1AR expression may improve myocardial tolerance to ischemia.

GATA-4 activates transcription via two novel domains that are conserved within the GATA-4/5/6 subfamily

GATA-4 is one of the earliest developmental markers of the precardiac mesoderm, heart, and gut and has been shown to activate regulatory elements controlling transcription of genes encoding cardiac-specific proteins. To elucidate the molecular mechanisms underlying the transcriptional activity of the GATA-4 protein, structure-function analyses were performed. These analyses revealed that the C-terminal zinc finger and adjacent basic domain of GATA-4 is bifunctional, modulating both DNA-binding and nuclear localization activities. The N terminus of the protein encodes two independent transcriptional Activation Domains (amino acids 1-74 and amino acids 130-177). Amino acid residues were identified within each domain that are required for transcriptional activation. Finally, we have shown that regions of Xenopus GATA-5 and -6 corresponding to Activation Domains I and II, respectively, function as potent transcriptional activators. The identification and functional characterization of two evolutionarily conserved transcriptional Activation Domains within the GATA-4/5/6 subfamily suggests that each of these domains modulates critical functions in the transcriptional regulatory program(s) encoded by GATA-4, -5, and -6 during vertebrate development. As such these data provide novel insights into the molecular mechanisms that control development of the heart.

Two domains of MyoD mediate transcriptional activation of genes in repressive chromatin: a mechanism for lineage determination in myogenesis

Genetic studies have demonstrated that MyoD and Myf5 establish the skeletal muscle lineage, whereas myogenin mediates terminal differentiation, yet the molecular basis for this distinction is not understood. We show that MyoD can remodel chromatin at binding sites in muscle gene enhancers and activate transcription at previously silent loci. TGF-beta, basic-FGF, and sodium butyrate blocked MyoD-mediated chromatin reorganization and the initiation of transcription. In contrast, TGF-beta and sodium butyrate did not block transcription when added after chromatin remodeling had occurred. MyoD and Myf-5 were 10-fold more efficient than myogenin at activating genes in regions of transcriptionally silent chromatin. Deletion mutagenesis of the MyoD protein demonstrated that the ability to activate endogenous genes depended on two regions: a region rich in cysteine and histidine residues between the acidic activation domain and the bHLH domain, and a second region in the carboxyl terminus of the protein. Neither region has been shown previously to regulate gene transcription and both have domains that are conserved in the Myf5 protein. Our results establish a mechanism for chromatin modeling in the skeletal muscle lineage and define domains of MyoD, independent of the activation domain, that participate in chromatin reorganization.

The mammalian myosin heavy chain gene family

Myosin is a highly conserved, ubiquitous protein found in all eukaryotic cells, where it provides the motor function for diverse movements such as cytokinesis, phagocytosis, and muscle contraction. All myosins contain an amino-terminal motor/head domain and a carboxy-terminal tail domain. Due to the extensive number of different molecules identified to date, myosins have been divided into seven distinct classes based on the properties of the head domain. One such class, class II myosins, consists of the conventional two-headed myosins that form filaments and are composed of two myosin heavy chain (MYH) subunits and four myosin light chain subunits. The MYH subunit contains the ATPase activity providing energy that is the driving force for contractile processes mentioned above, and numerous MYH isoforms exist in vertebrates to carry out this function. The MYHs involved in striated muscle contraction in mammals are the focus of the current review. The genetics, molecular biology, and biochemical properties of mammalian MYHs are discussed below. MYH gene expression patterns in developing and adult striated muscles are described in detail, as are studies of regulation of MYH genes in the heart. The discovery that mutant MYH isoforms have a causal role in the human disease familial hypertrophic cardiomyopathy (FHC) has implemented structure/function investigations of MYHs. The regulation of MYH genes expressed in skeletal muscle and the potential functional implications that distinct MYH isoforms may have on muscle physiology are addressed.

Isolation and characterization of an avian slow myosin heavy chain gene expressed during embryonic skeletal muscle fiber formation

We have isolated and begun characterization of the quail slow myosin heavy chain (MyHC) 3 gene, the first reported avian slow MyHC gene. Expression of slow MyHC 3 in skeletal muscle is restricted to the embryonic period of development, when the fiber pattern of future fast and slow muscle is established. In embryonic hindlimb development, slow MyHC 3 gene expression coincides with slow muscle fiber formation as distinguished by slow MyHC-specific antibody staining. In addition to expression in embryonic appendicular muscle, slow MyHC 3 is expressed continuously in the atria. Transfection of slow MyHC 3 promoter-reporter constructs into embryonic myoblasts that form slow MyHC-expressing fibers identified two regions regulating expression of this gene in skeletal muscle. The proximal promoter, containing potential muscle-specific regulatory motifs, permits expression of a reporter gene in embryonic slow muscle fibers, while a distal element, located greater than 2600 base pairs upstream, further enhances expression 3-fold. The slow muscle fiber-restricted expression of slow MyHC 3 during embryonic development, and expression of slow MyHC 3 promoter-reporter constructs in embryonic muscle fibers in vitro, makes this gene a useful marker to study the mechanism establishing the slow fiber lineage in the embryo.

Factors involved in cardiogenesis and the regulation of cardiac-specific gene expression

The delineation of the mechanisms that regulate cardiac gene expression is central to our understanding of cardiac growth and development. Much progress has been made toward the identification of factors involved in tissue-restricted gene expression, especially in skeletal muscle cells. However, the mechanisms regulating the expression of cardiac-specific genes remain less well understood. Certain homeodomain proteins have been implicated in commitment to the cardiac phenotype. Among the best characterized are the murine proteins Csx, Nkx-2.5, and Nkx-2.6, related to the protein tinman, which is essential for heart formation in Drosophila. The expression of these genes precedes that of cardiac-specific genes and is therefore believed to play a critical role in the development of the heart. The GATA proteins are a family of zinc finger proteins that are also expressed early in cardiac development and may act separately from, or in concert with, the homeodomain proteins as crucial regulators of heart development. The myosin heavy and light chain genes, the actin genes, the troponin genes, and the atrial natriuretic factor and muscle creatine kinase genes have served as excellent paradigms for the study of cardiac gene expression. Although differences in cis-acting elements and their behavior in binding assays have been observed between different genes, there exist similarities that are noteworthy. In this review, we will discuss the factors involved in the regulation of cardiac-specific gene expression in an attempt to provide a better understanding of the process of cardiogenesis.

Transgenic remodeling of the contractile apparatus in the mammalian heart

The structure-function relationships of the sarcomeric proteins in the mammalian cardiac compartment remain ill-defined because of the lack of a suitable model in which they can be readily manipulated or exchanged in vivo. To establish the validity of the transgenic paradigm for remodeling the mammalian heart, the murine alpha -cardiac myosin heavy chain gene promoter was used to express a ventricular myosin light chain-2 transgene (MLC2v) in both the atria and ventricles of the adult animal. Expression resulted in high levels of the transgene's transcript in both compartments. In the ventricle, the transgene was expressed against the background expression of the normal isoform. In the atrium, the transgene's expression would be ectopic, in that normally, MLC2v expression is restricted to the ventricle. Ectopic expression of the transgene in the atria resulted in a complete replacement of the atrial myosin light chain-2 protein isoform, although the endogenous isoform's steady state transcript levels were unchanged. In contrast, ventricular expression of the transgene had no effect at the protein level, despite an eightfold increase in MLC2v transcript levels. The data show that sarcomeric protein stoichiometry is maintained rigorously via posttransciptional regulation and that protein replacement can be achieved through a single transgenic manipulation.

Identification of a contractile-responsive element in the cardiac alpha-myosin heavy chain gene

The mechanisms by which the cardiac-specific alpha-myosin heavy chain (alpha-MHC) gene responds to contractile activity was studied in cultured cardiomyocytes and in vivo. Deletion analysis of the alpha-MHC promoter transiently transfected into neonatal rat cardiomyocytes localized the contractile-responsive element within -80 to -40 base pairs of the transcriptional start site. Mutational analysis of an E-box motif at position -47 showed that it was necessary for the contractile response both in cultured cardiomyocytes and in the intact heart. Competition gel mobility shift experiments indicated that the protein-DNA complex formed within the -39 to -59 base pair region could be competed by the E-box element at -309 of the alpha-MHC gene and that base substitutions within an E-box motif at -47 eliminated the protein-DNA complex. To identify the contractile-responsive nuclear protein, antibodies specific for E12/E47, an E-box binding basic-helix-loop-helix (bHLH) protein, and antibodies recognizing upstream stimulatory factor (USF), a widely expressed bHLH-leucine zipper transcription factor, were studied for their ability to inhibit cardiomyocyte nuclear protein binding to the E-box motif at -47. Anti-USF antibody abolished formation of the protein-DNA complex, thus identifying the protein as antigenically related to USF and demonstrating that bHLH-leucine zipper proteins are involved in the contractile-induced expression of the cardiac alpha-MHC gene.

An in vivo analysis of transcriptional elements in the mouse alpha-myosin heavy chain gene promoter

During development in the murine ventricle, there is a switch in myosin heavy chain gene (MyHC) transcription. The beta-MyHC is expressed in the ventricles during foetal development, but is shut down at or around birth, at which time alpha-MyHC transcription is activated. This antithetical switch is thought to be mediated by circulating levels of thyroid hormone (TH) and both low and high affinity thyroid response elements (TREs) have been identified in the proximal promoter region of the murine alpha-MyHC. Myosin gene expression in the atria is relatively unaffected by the TH status. Previously, we used site-directed mutagenesis of the promoter in a transgenic analysis to define those elements responsible for high levels of transcription in vivo. These analyses focused on the role(s) of two cis elements, TRE1 and TRE2 that are located at -129 to -149 and -102 to -120, respectively, on the alpha-MyHC promoter. Although the elements' ablation had differential effects on transgene expression, neither single mutation abolished transgene expression completely. Here, we show that mutating both elements results in a complete inactivation of the transgene in both ventricles and atria under euthyroid conditions. However, expression still can be detected in the hyperthyroid state, implying that, although the TRE1 and TRE2 elements are critical elements for high levels of alpha-MyHC transcription in vivo, other promoter sites can mediate at least some degree of transcriptional activation.

Localization of transcription factor GATA-4 to regions of the mouse embryo involved in cardiac development

To clarify the role of transcription factor GATA-4 in mammalian development, we have determined the pattern of expression of GATA-4 in early postimplantation mouse embryos. Using in situ hybridization and immunohistochemistry, we find that GATA-4 RNA and protein are expressed in cells associated with heart development. Intraembryonic expression of GATA-4 RNA is first apparent in coelomic epithelial cells of the primitive streak embryo (approximately 7.0-7.5 days postcoitum). During formation and bending of the heart tube (approximately 8 days postcoitum), GATA-4 RNA and protein are expressed in endocardium, myocardium, and embryonic structures containing precardiac mesoderm such as the septum transversum and intraembryonic coelomic epithelium. By the onset of cardiac septation (approximately 9 days postcoitum), abundant GATA-4 RNA expression is evident in endocardium, endocardial cushion tissue, and myocardium. Expression of GATA-4 by the myocardium continues through gestation and after birth. The temporal and spacial patterns of GATA-4 expression support a role for this factor in the regulation of cardiac differentiation, analogous to the established role of transcription factor GATA-1 in the regulation of hematopoiesis.