Differential expression of the myocyte enhancer factor 2 family of transcription factors in development: the cardiac factor BBF-1 is an early marker for cardiogenesis

Heavy and light roles: myosin in the morphogenesis of the heart

Myosin is an essential component of cardiac muscle, from the onset of cardiogenesis through to the adult heart. Although traditionally known for its role in energy transduction and force development, recent studies suggest that both myosin heavy-chain and myosin light-chain proteins are required for a correctly formed heart. Myosins are structural proteins that are not only expressed from early stages of heart development, but when mutated in humans they may give rise to congenital heart defects. This review will discuss the roles of myosin, specifically with regards to the developing heart. The expression of each myosin protein will be described, and the effects that altering expression has on the heart in embryogenesis in different animal models will be discussed. The human molecular genetics of the myosins will also be reviewed.

Repression of the cardiac myosin light chain-2 gene in skeletal muscle requires site-specific association of antithetic regulator, Nished, and HDACs

The transcriptional activation mechanisms that regulate tissue-specific expression of cardiac muscle genes have been extensively investigated, but little is known of the regulatory events involved in repression of cardiac-specific genes in non-cardiac cells. We have previously reported that Nished, a ubiquitous transcription factor, interacts with a positive sequence element, the Intron Regulatory Element (IRE) as well as a negatively acting element, the Cardiac-Specific Sequence (CSS), in myosin light chain-2 (MLC2v) gene to promote activation and repression of the gene in cardiac and skeletal muscle cells respectively. Here, we show that the negative regulation of cardiac MLC2v gene in skeletal muscle cells is mediated via the interaction of Nished with histone deacetylase (HDAC) co-repressor. Treatment of cells with the HDAC inhibitor, Trichostatin A (TSA), alleviates the repressor activity of Nished in a dose-dependent manner. Co-transfection studies in primary muscle cells in culture and in Nished expressing stable skeletal muscle cell line demonstrate that Nished down-regulates the cardiac MLC2 gene expression when its association is restricted to CSS alone. Chromatin immunoprecipitation data suggest that the CSS-mediated repression of cardiac MLC2v gene in skeletal muscle cells excludes the participation of the positive element IRE despite the presence of an identical Nished binding site. Taken together, it appears that the negative control of MLC2v transcription is based on a dual mode of regulations, one that affords inaccessibility of IRE to Nished and second that promotes the formation of the transcription repression complex at the inhibitory CSS site to silence the cardiac gene in skeletal muscle cell.

Morphological and molecular analysis of the early developing chick requires an expanded series of primitive streak stages

A detailed analysis of the gastrulating chick embryo was performed using three methods : time-lapse videotaping of embryos in culture, histological semithin sections, and in situ hybridization with 10 mRNA signals expressed during gastrulation. The results suggest that the gene expression pattern of Goosecoid, Hex, Crescent, and Bmp7 may be involved in the axial establishment of the temporal and spatial arrangement of cells forming the prechordal plate endoderm, and that Chordin, cNot1, Noggin, and Brachyury are precocious markers of cells coming from Hensen's node, which contribute to the rostralmost tip of the notochord, its arrowhead, the head process, and, later, the elongating notochord. These results explain several earlier descriptions based only on morphological analyses of the axial mesodermal structures characteristic of the gastrulation stages. The data, carefully observed and compared with the whole-mount observation in time-lapse video, show that the changes in cell populations, movements, and cell differentiation occur step-by-step over a precise temporal range, which requires the establishment of a subdivision of the stages usually employed. Knowledge of new aspects of avian gastrulation, including gene expression patterns, immunocytochemical analyses, and the great number of recent experiments based on microinjections or transplants of groups of cells to analyze processes of induction or regulation, need the support of a precisely defined scheme of primitive streak stages (PS-stages), and a correlation of these stages with other approaches to provide a finer resolution of the staging steps, and thus to facilitate a better understanding of the initial gastrulation period.

A ternary complex of transcription factors, Nishéd and NFATc4, and co-activator p300 bound to an intronic sequence, intronic regulatory element, is pivotal for the up-regulation of myosin light chain-2v gene in cardiac hypertrophy

Transcriptional up-regulation of the myosin light chain-2 (MLC-2v) gene is an established marker for hypertrophic response in cardiomyocytes. Despite the documentation on the role of several cis-elements in the MLC-2v gene and their cognate proteins in transcription, the mechanism that dictates the preferential increase in MLC-2v gene expression during myocardial hypertrophy has not been delineated. Here we describe the properties of a cardiac specific intronic activator element (IRE) that shares sequence homology with the repressor element, the cardiac specific sequence, in the chicken MLC-2v gene. The transcription factor, Nishéd, that recognizes both IRE and the cardiac specific sequence potentiates the transcription of the MLC-2v gene via interaction with another transcription factor, nuclear factor of activated T cells, and the co-activator p300 at the IRE site. Angiotensin II (Ang II), a potent agonist of hypertrophy, causes induction of the MLC-2v gene transcription, which correlates well with the enhanced binding of Nishéd-nuclear factor of the activated T cells-p300 complex to IRE in the gel mobility shift assay. Losartan, an antagonist of Ang II receptor (AT1), abolishes the agonist-dependent stimulation of IRE/protein interaction and the consequent increase in MLC-2v gene transcription. These results together have thus established a transcriptional role of IRE as a direct target sequence of Ang II-mediated signaling that appears to be pivotal in the mechanism underlying the up-regulation of the MLC-2v gene during cardiac hypertrophy.

Structure, expression, and functional characterization of the mouse CLP-1 gene

Mouse CLP-1, a potential cardiac transcriptional regulatory factor, is encoded by a single copy gene lacking introns that is expressed into two mRNAs via alternative polyadenylation. Both mRNAs encode the same 41 kDa protein, a novel protein that is 85.3% homologous with a human homologue called HIS1. Mouse CLP-1 is widely expressed in a number of tissues as well as in early development and is localized to the nucleus. The CLP-1 gene promoter is active in different cell types and sequence analysis shows a number of potential binding sites for cardiogenic transcription factors such as Nkx2.5 and GATA-4, indicating a potential role in development. CLP-1 appears to "squelch" the cardiac MLC-2v promoter in a concentration-dependent manner in cardiac but not other cell types, suggesting that CLP-1 may be interacting with a cardiac-specific factor to regulate cardiac MLC-2v expression. The overall expression pattern of CLP-1 is similar to that of LCR-F1 and Oct-1, two widely expressed transcription factors that also play specific roles in the transcription of cell-specific genes. CLP-1 may be a transcriptional mediator capable of interacting with and potentiating cell-specific transcription factors.

Purkinje fibers of the avian heart express a myogenic transcription factor program distinct from cardiac and skeletal muscle

A rhythmic heart beat is coordinated by conduction of pacemaking impulses through the cardiac conduction system. Cells of the conduction system, including Purkinje fibers, terminally differentiate from a subset of cardiac muscle cells that respond to signals from endocardial and coronary arterial cells. A vessel-associated paracrine factor, endothelin, can induce embryonic heart muscle cells to differentiate into Purkinje fibers both in vivo and in vitro. During this phenotypic conversion, the conduction cells down-regulate genes characteristic of cardiac muscle and up-regulate subsets of genes typical of both skeletal muscle and neuronal cells. In the present study, we examined the expression of myogenic transcription factors associated with the switch of the gene expression program during terminal differentiation of heart muscle cells into Purkinje fibers. In situ hybridization analyses and immunohistochemistry of embryonic and adult hearts revealed that Purkinje fibers up-regulate skeletal and atrial muscle myosin heavy chains, connexin-42, and neurofilament protein. Concurrently, a cardiac muscle-specific myofibrillar protein, myosin-binding protein-C (cMyBP-C), is down-regulated. During this change in transcription, however, Purkinje fibers continue to express cardiac muscle transcription factors, such as Nkx2.5, GATA4, and MEF2C. Importantly, significantly higher levels of Nkx2.5 and GATA4 mRNAs were detected in Purkinje fibers as compared to ordinary heart muscle cells. No detectable difference was observed in MEF2C expression. In culture, endothelin-induced Purkinje fibers from embryonic cardiac muscle cells dramatically down-regulated cMyBP-C transcription, whereas expression of Nkx2.5 and GATA4 persisted. In addition, myoD, a skeletal muscle transcription factor, was up-regulated in endothelin-induced Purkinje cells, while Myf5 and MRF4 transcripts were undetectable in these cells. These results show that during and after conversion from heart muscle cells, Purkinje fibers express a unique myogenic transcription factor program. The mechanism underlying down-regulation of cardiac muscle genes and up-regulation of skeletal muscle genes during conduction cell differentiation may be independent from the transcriptional control seen in ordinary cardiac and skeletal muscle cells.

Muscle-specific transcription factors in fibroblasts expressing the alpha-striated tropomyosin 3' untranslated region

The alpha-striated tropomyosin 3' untranslated region (TM UTR) promotes differentiation of fibroblasts into cells resembling skeletal muscle. To investigate the mechanism of this observation, RNA harvested from transfected primary fibroblasts was used for semiquantitative RT-PCR with primers specific for muscle transcription factors, showing that myoD and myogenin transcripts are detected in these cells, but that differentiation after TM UTR expression is independent of a detectable increase in these transcripts. Double immunofluorescent staining with antibodies to myoD family members and to titin confirms that muscle differentiation in TM UTR-transfected fibroblasts is independent of production of any transcription factor in this family. In contrast, the muscle transcription factor myocyte enhancer factor 2 (mef-2) is strongly expressed after transfection of fibroblasts with the TM UTR. The increase in mef-2 protein is due to an increase in the steady-state level of its mRNA, as shown by Northern analysis. The expression of p21 ordinarily observed in skeletal myogenesis before the expression of muscle-specific proteins is not seen in fibroblasts induced to differentiate by the TM UTR. These results demonstrate that post-transcriptional regulation of myoD family members is seen in fibroblasts, and that the TM UTR induces muscle differentiation independent of the myoD transcription factors and without expressing proteins characteristic of terminal withdrawal from the cell cycle. Finally, an increase in the steady-state level of mef-2 transcripts appears in the proximal pathway of myogenic activation in response to expression of the TM UTR. These results imply that fibroblasts can utilize an additional differentiation route upon TM UTR expression resulting in mature muscle other than that requiring myoD family members.

Cardiac hypertrophy: signal transduction, transcriptional adaptation, and altered growth control

Cardiac hypertrophy results from the enlargement of cardiac muscle and fibroblast cells. This abnormal pattern of growth can be elicited by a number of hypertrophic agents, such as cytokines and hormones that participate in normal cell-cell signaling events during development. Under conditions yet to be defined, these same signaling molecules can cause hypertrophy of the heart. Intracellular signal transduction pathways appear to be the prime means by which the hypertrophic signal is transduced in cardiomyocytes. There is no evidence that the signal transduction pathways in hypertrophic cardiomyocytes differ from those of normal cardiomyocytes. Perhaps the signal itself is aberrant, mistimed, misplaced, or occurring at non-physiological concentrations. Alternatively, as a quiescent cell, the cardiomyocyte may not be able to respond completely to a growth signal by turning on its proliferative machinery. Three avenues of research are described: (1) the study of the upregulation of the cardiac MLC-2 gene, (2) STAT proteins and activation of angiotensin II, and (3) hypertrophy as a perturbation of cell cycle controls.

Signal transduction and transcriptional adaptation in embryonic heart development and during myocardial hypertrophy

In comparing the pathological state of cardiac hypertrophy with early embryonic growth and development of the primitive heart, important and informative aspects of mechanisms that underlie activation of the gene expression pattern become apparent. Interestingly, in both cases the muscle phenotypes share the expression of a 'fetal' gene expression program, raising the question whether the same genetic mechanism is being called upon by signals associated with the onsets of cardiogenesis and myocardial hypertrophy. A cell specific transcription factor, CLP-1, was recently identified in our laboratory that is likely to play a crucial role, in conjunction with other known regulatory factors, in early cardiac events leading to cardiogenic cell specification and differentiation. We have also identified a novel mechanism that involves activation of the Jak/Stat signaling pathway that is linked to the autocrine angiotensin-II loop associated with the hypertrophic response in cardiomyocytes. Since early cardiac cell development and the hypertrophic state involve the expression of the same battery of genes, one may speculate that common transcription factors may account for assembling a competent apparatus responsible for transcribing the genes. Our present studies are designed to investigate the potential role of these factors in control of both processes.

Osteogenic protein-1 up-regulation of the collagen X promoter activity is mediated by a MEF-2-like sequence and requires an adjacent AP-1 sequence

Bone morphogenetic proteins induce chondrogenesis and osteogenesis in vivo. To investigate molecular mechanisms involved in chondrocyte induction, we examined the effect of osteogenic protein (OP)-1/bone morphogenetic protein-7 on the collagen X promoter. In rat calvaria-derived chondrogenic C5.18 cells, OP-1 up-regulates collagen X mRNA levels and its promoter activity in a cell type- specific manner. Deletion analysis localizes the OP-1 response region to 33 bp (-310/-278), which confers OP-1 responsiveness to both the minimal homologous and heterologous Rous sarcoma virus promoter. Transforming growth factor-beta2 or activin, which up-regulates the expression of a transforming growth factor-beta-inducible p3TP-Lux construct, has little effect on collagen X mRNA and on this 33-bp region. Mutational analysis shows that both an AP-1 like sequence (-294/-285, TGAATCATCA) and an A/T-rich myocyte enhancer factor (MEF)-2 like sequence (-310/-298, TTAAAAATAAAAA) in the 33-bp region are necessary for the OP-1 effect. Gel shift assays show interaction of distinct nuclear proteins from C5.18 cells with the AP-1-like and the MEF-2-like sequences. OP-1 rapidly induces nuclear protein interaction with the MEF-2-like sequence but not with the AP-1 like sequence. MEF-2-like binding activity induced by OP-1 is distinct from the MEF-2 family proteins present in C2C12 myoblasts, in which OP-1 does not induce collagen X mRNA or up-regulate its promoter activity. In conclusion, we identified a specific response region for OP-1 in the mouse collagen X promoter. Mutational and gel shift analyses suggest that OP-1 induces nuclear protein interaction with an A/T-rich MEF-2 like sequence, distinct from the MEF-2 present in myoblasts, and up-regulates collagen X promoter activity, which also requires an AP-1 like sequence.

Adenovirus E1A inhibits cardiac myocyte-specific gene expression through its amino terminus

Adenovirus E1A oncoproteins inhibit muscle-specific gene expression and myogenic differentiation by suppressing the transcriptional activating functions of basic helix-loop-helix proteins. As one approach to identifying cardiac-specific gene regulatory proteins, we analyzed the functional regions of E1A proteins that are required for muscle gene repression in cardiac cells. Myocyte-specific promoters, including the alpha-actins and alpha-myosin heavy chain, were selectively and potently inhibited (>90%) by E1A, while the ubiquitously expressed beta-actin promoter was only partially ( approximately 30%) repressed; endogenous gene expression was also affected. Distinct E1A protein binding sites mediated repression of muscle-specific and ubiquitous actin promoters. E1A-mediated inhibition of beta-actin required both an intact binding site for the tumor repressor proteins pRb and p107 and a second E1A domain (residues 15-35). In contrast, cardiac-specific promoter repression required the E1A amino-terminal residues 2-36. The proximal skeletal actin promoter (3' to base pair -153) was a target for repression by E1A. Although E1A binding to p300 was not required for inhibition of either promoter, co-expression of p300 partially reversed E1A-mediated transcriptional repression. We conclude that cardiac-specific and general promoter inhibition by E1A occurs by distinct mechanisms and that cardiac-specific gene expression is modulated by cellular factors interacting with the E1A p300/CBP-binding domain.

Two distinct factor-binding DNA elements in cardiac myosin light chain 2 gene are essential for repression of its expression in skeletal muscle. Isolation of a cDNA clone for repressor protein Nished

The expression of the cardiac myosin light chain 2 (MLC2) gene is repressed in skeletal muscle as a result of the negative regulation of its transcription. Two regulatory elements, the cardiac specific sequence (CSS) located upstream (-360 base pairs) and a downstream negative modulatory sequence (NMS), which function in concert with each other, are required for repression of the MLC2 promoter activity in skeletal muscle. Individually, CSS and NMS have no effect. Transient transfection analysis with recombinant plasmids indicated that CSS- and NMS-mediated repression of transcription is position- and orientation-dependent and is transferable to heterologous promoters. A minimal conserved motif, GAAG/CTTC, present in both CSS and NMS, is responsible for repression as the mutation in the core CTTC sequence alone was sufficient to abrogate its repressor activity. The DNA binding assay by gel mobility shift analysis revealed that one of the two complexes, CSSBP2, is significantly enriched in embryonic skeletal muscle relative to cardiac muscle. In extracts from adult skeletal muscle, where the cardiac MLC2 expression is suppressed, both complexes, CSSBP1 and CSSBP2, were present, whereas the cardiac muscle extracts contained CSSBP1 alone, suggesting that the protein(s) in the CSSBP2 complex accounts for the negative regulation of cardiac MLC2 in skeletal muscle. A partial cDNA clone (Nished) specific for the candidate repressor factor was isolated by expression screening of the skeletal muscle cDNA library by multimerized CSS-DNA as probe. The recombinant Nished protein binds to the CSS-DNA, but not to DeltaCSS-DNA where the core CTTC sequence was mutated. The amino acid sequence of Nished showed a significant structural similarity to the sequence of transcription factor "runt," a known repressor of gap and pair-rule gene expression in Drosophila.

Evidence for serum response factor-mediated regulatory networks governing SM22alpha transcription in smooth, skeletal, and cardiac muscle cells

SM22alpha is an adult smooth muscle-specific protein that is expressed in the smooth, cardiac, and skeletal muscle lineages during early embryogenesis before becoming restricted specifically to all vascular and visceral smooth muscle cells (SMC) in late fetal development and adulthood. We have used the SM22alpha gene as a marker to define the regulatory mechanisms that control muscle-specific gene expression in SMCs. Previously, we reported that the 445-base-pair promoter of SM22alpha was sufficient to direct transcription of a lacZ reporter gene in early cardiac and skeletal muscle cell lineages and in a subset of arterial SMCs, but not in venous nor visceral SMCs in transgenic mice. Here we describe two evolutionarily conserved CArG (CC(A/T)6GG) boxes in the SM22alpha promoter, both of which are essential for full promoter activity in cultured SMCs. In contrast, only the promoter-proximal CArG box is essential for specific expression in developing smooth, skeletal, and cardiac muscle lineages in transgenic mice. Both CArG boxes bind serum response factor (SRF), but SRF binding is not sufficient for SM22alpha promoter activity, since overexpression of SRF in the embryonal teratocarcinoma cell line F9, which normally expresses low levels of SRF, fails to activate the promoter. However, a chimeric protein in which SRF was fused to the transcription activation domain of the viral coactivator VP16 is able to activate the SM22alpha promoter in F9 cells. These results demonstrate the SM22alpha promoter-proximal CArG box is a target for the regulatory programs that confer smooth, skeletal, and cardiac muscle specificity to the SM22alpha promoter and they suggest that SRF activates SM22alpha transcription in conjunction with additional regulatory factors that are cell type-restricted.

Catecholamines and cardiac growth

The present knowledge concerning the alpha- and beta-adrenergic systems in the regulation of cardiac growth and gene expression is reviewed. To investigate the mechanism by which cAMP regulates the expression of cardiac genes we have used cultured myocytes derived from fetal rat hearts. We have shown previously that the addition of Br cAMP to the culture medium produced an increase in alpha-myosin heavy chain (alpha-MHC) mRNA level, in its rate of transcription as well as in the amount of V1 isomyosin. To characterize the promoter element(s) involved in cAMP responsive regulation of alpha-MHC expression we performed transient transfection analysis with a series of alpha-MHC gene promoter-CAT constructs. We have identified a 13 bp E-box/M-CAT hybrid motif (EM element) which conferred a basal muscle specific and cAMP inducible expression of the alpha-MHC gene. Using mobility shift assay we have documented that one of the EM element binding protein is TEF-1. Moreover, by incubating cardiac nuclear extracts with the catalytic subunit of PK-A we have found that factor(s) binding to the EM element is a substrate for cAMP dependent phosphorylation.

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.

Adaptational response in transcription factors during development of myocardial hypertrophy

Cardiac hypertrophy is characterized, among others, by the molecular events which selectively activate the expression of genes for contractile proteins within individual myocardial cells. As such, myosin light chain 2 (MLC-2), which is upregulated in the hypertrophic state in both rat and human, serves as a marker for hypertrophy. In an attempt to investigate the gene regulatory mechanisms of this phenomenon, we tested the hypothesis that certain transcription factors are directly involved in the development of cardiac hypertrophy by demonstrating the presence of cardiac tissue-specific regulatory elements in the 5'-flanking region of the MLC-2 promoter and testing them in the gel mobility shift assay for their binding activity to nuclear proteins from hypertrophied and normal cardiac tissue. In nuclear extracts from the ventricular tissues of the spontaneously hypertensive rat (SHR), distinctive changes in two families of activator proteins, the A/T-rich DNA-binding transcription factors, myocyte enhancer factor (MEF-2) and CArG-binding factor, manifested in a developmentally dictated manner paralleling the evolution of cardiac hypertrophy in these animals. Extracts isolated from brains and skeletal muscle tissues from the same animals did not exhibit the changes in binding activity. Also, the changes were not apparent when a distal negative regulatory element (CSS), which confers cardiac-specific expression, was tested in gel mobility shift assays. The ubiquitous TATA-binding proteins remained unchanged in comparing SHR with the control strain WKY in the same assay. These data support the notion that the expression of specific transcription factors is modulated in response to hypertrophy related signals which execute changes at the gene level effecting the enrichment of certain contractile proteins in an effort discrete and estranged from the basal transcription machinery.