Regulation of muscle transcription by the MyoD family. The heart of the matter

Vertebrate homologs of tinman and bagpipe: roles of the homeobox genes in cardiovascular development

In Drosophila, dorsal mesodermal specification is regulated by the homeobox genes tinman and bagpipe. Vertebrate homologs of tinman and bagpipe have been isolated in various species. Moreover, there are at least four different genes related to tinman in the vertebrate, which indicates that this gene has been duplicated during evolution. One of the murine homologs of tinman is the cardiac homeobox gene Csx or Nkx2.5. Gene targeting of Csx/Nkx2.5 showed that this gene is required for completion of the looping morphogenesis of the heart. However, it is not essential for the specification of the heart cell lineage. Early cardiac development might therefore be regulated by other genes, which may act either independently or in concert with Csx/Nkx2.5. Possible candidates might be other members of the NK2 class of homeobox proteins like Tix/Nkx2.6, Nkx2.3, nkx2.7, or cNkx2.8. Murine Tix/Nkx2.6 mRNA has been detected in the heart and pharyngeal endoderm (this study). Xenopus XNkx2.3 and chicken cNkx2.3 are expressed in the heart as well as in pharyngeal and gut endoderm. In contrast, murine Nkx2.3 is expressed in the gut and pharyngeal arches but not the heart. In zebrafish and chicken, two new NK-2 class homeoproteins, nkx2.7 and cNkx2.8, have been identified. Zebrafish nkx2.7 is expressed in both, the heart and pharyngeal endoderm. In the chicken, cNkx2.8 is expressed in the heart primordia and the primitive heart tube and becomes undetectable after looping. No murine homologs of nkx2.7 or cNkx2.8 have been found so far. The overlapping expression pattern of NK2 class homeobox genes in the heart and the pharynx may suggest a common origin of these two organs. In the Drosophila genome, the tinman gene is linked to another NK family gene named bagpipe. A murine homolog of bagpipe, Bax/Nkx3.1, is expressed in somites, blood vessels, and the male reproductive system during embryogenesis (this study), suggesting that this gene's function may be relevant for the development of these organs. A bagpipe homolog in Xenopus, Xbap, is expressed in the gut masculature and a region of the facial cartilage during development. In this paper, we discuss molecular mechanisms of cardiovascular development with particular emphasis on roles of transcription factors.

Genetics of muscle determination and development

Skeletal muscles in vertebrates develop from somites as the result of patterning and cell type specification events. Here, we review the current knowledge of genes and signals implicated in these processes. We discuss in particular the role of the myogenic determination genes as deduced from targeted gene disruptions in mice and how their expression may be controlled. We also refer to other transcription factors which collaborate with the myogenic regulators in positive or negative ways to control myogenesis. Moreover, we review experiments that demonstrate the influence of tissues surrounding the somites on the process of muscle formation and provide model views on the underlying mechanisms. Finally, we present recent evidence on genes that play a role in regeneration of muscle in adult organisms.

Evidence for the expression of neonatal skeletal myosin heavy chain in primary myocardium and cardiac conduction tissue in the developing chick heart

We isolated a neonatal skeletal myosin heavy chain (MHC) cDNA clone, CV11E1, from a cDNA library of embryonic chick ventricle. At early cardiogenesis, diffuse expression of neonatal skeletal MHC mRNA was first detected in the heart tube at stage 10. During subsequent embryonic stages, the expression of the mRNA in the atrium was upregulated until shortly after birth. It then diminished, dramatically, and disappeared in the adult. On the other hand, in the ventricle, only a trace of the expression was detected throughout embryonic life and in the adult. However, transient expression of mRNA in the ventricle was observed, post-hatching. At the protein level, during the embryonic stage, the atrial myocardium was stained diffusely with monoclonal antibody 2E9, specific for chick neonatal skeletal MHC, whereas the ventricles showed weak reactivity with 2E9. At the late embryonic and newly hatched stages, 2E9-positive cells were located clearly in the subendocardial layer, and around the blood vessels of the atrial and ventricular myocardium. These results provide the first evidence that the neonatal skeletal MHC gene is expressed in developing chick hearts. This MHC appears during early cardiogenesis and is then localized in cardiac conduction cells. Dev Dyn 2000;217:37-49.

Gene transfer and models of gene therapy for the myocardium

1. Gene transfer into the myocardium can be achieved through direct injection of plasmid DNA or through the delivery of viral vectors, either directly or through the coronary vasculature. Direct DNA injection has proven extremely valuable in studies aimed at characterizing the activities of promoter elements in cardiac tissue and for examining the influence of the pathophysiological state of the myocardium on expression of transferred foreign genes. 2. Viral vectors, in particular adenoviruses and adeno-associated virus, are capable of transfecting genetic material with high transduction efficiencies and have been applied to a range of model systems for in vivo gene transfer. Efficient gene transfer has been achieved into the coronary vessels and surrounding myocardium by intracoronary infusion of adenovirus. 3. Because the immunogenicity of viral vectors can limit transgene expression, much attention has been paid to strategies for circumventing this, including the development of new modified adenovirus and adeno-associated virus vectors that do not elicit significant inflammatory responses. While cellular transplantation may prove valuable for the repair of myocardial tissue, confirmation of its value awaits establishment of a functional improvement in the myocardium following cell grafting. 4. Because gene transfer into the myocardium can now be achieved with high efficiency in the absence of significant inflammatory responses, the ability to regulate foreign gene expression in response to an endogenous disease phenotype will enable the development of new effective viral vectors with direct clinical applicability for specified therapeutic targets.

Phosphatidylinositol 3-kinase regulates differentiation of H9c2 cardiomyoblasts mainly through the protein kinase B/Akt-independent pathway

Phosphatidylinositol 3-kinase (PI3-kinase) is known to be a crucial regulator of muscle differentiation. However, its downstream pathway for this function is quite obscure. In this experiment we demonstrated the regulatory mechanism of the differentiation of H9c2 cardiomyoblasts, focusing on PI3-kinase, protein kinase B/Akt (PKB/Akt) and p42/44 mitogen-activated protein kinase (p42/44 MAPK). When H9c2 cells stably transfected with a constitutively active p110 (H9c2-p110*), a constitutively active PKB/Akt (H9c2-Akt), and an empty vector (H9c2-con) were induced to differentiate, H9c2-p110* cells differentiated fastest, followed by H9c2-Akt cells. H9c2-con cells differentiated at the slowest rate. Consistent with this result, LY294002 completely blocked differentiation of all these transfected cell lines, whereas PD098059 had no effect on their differentiation. When H9c2-p110* cells were transiently transfected with a dominant negative form of PKB/Akt, differentiation was not affected. Taken together, we concluded that PI3-kinase, but not p42/44 MAPK, regulates differentiation of H9c2 cardiomyoblasts mainly through the PKB/Akt-independent pathway.

Differential activation of the SMalphaA promoter in smooth vs. skeletal muscle cells by bHLH factors

E-box/basic helix-loop-helix (bHLH)-dependent regulation of promoters for skeletal muscle-specific genes is well established, but similar regulation of smooth muscle-selective promoters has not been reported. Using transient transfection assays of smooth muscle alpha-actin (SMalphaA) promoter-chloramphenicol acetyltransferase (CAT) reporter constructs in rat vascular smooth muscle cells (SMCs) and L6 skeletal myotubes, we identified two activator elements, smE1 and smE2, with sequences corresponding to E-box (5'-CAnnTG-3') motifs. In L6 myotubes, 4-bp mutations of smE1 or smE2 E-box motif alone completely abolished promoter activity. In contrast, mutation of smE1 and smE2 was required to reduce promoter activity in SMCs. Supershift analyses identified a myogenin-containing complex as the predominant smE1 and smE2 binding activity in skeletal muscle, and myogenin overexpression transactivated the promoter. Supershift analyses with SMC extracts demonstrated that the bHLH protein upstream stimulatory factor (USF) bound smE1, and USF overexpression transactivated the promoter in an smE1-dependent manner. In summary, our results provide novel evidence implicating E-box elements in directing expression of the SMalphaA promoter through distinct bHLH factor complexes in skeletal vs. smooth muscle.

Properties and expression of Ca2+-activated K+ channels in H9c2 cells derived from rat ventricle

H9c2 is a clonal myogenic cell line derived from embryonic rat ventricle that can serve as a surrogate for cardiac or skeletal muscle in vitro. Using whole cell clamp with H9c2 myotubes, we observed that depolarizing pulses activated slow outward K+ currents and then slow tail currents. The K+ currents were abolished in a Ca2+-free external solution, indicating that they were Ca2+-activated K+ currents. They were blocked by apamin, a small-conductance Ca2+-activated K+ (SK) channel antagonist (IC50 = 6.2 nM), and by d-tubocurarine (IC50 = 49.4 microM). Activation of SK channels exhibited a bell-shaped voltage dependence that paralleled the current-voltage relation for L-type Ca2+ currents (ICa,L). ICa,L exhibited a slow time course similar to skeletal ICa, L, were unaffected by apamin, and were only slightly depressed by d-tubocurarine. RT-PCR analysis of the mRNAs revealed that rSK3, but not rSK1 or rSK2, was expressed in H9c2 myotubes but not in myoblasts. These results suggest that rSK3 channels are expressed in H9c2 myotubes and are primarily activated by ICa,L directly or indirectly via Ca2+-induced Ca2+ release from sarcoplasmic reticulum.

Organization of human and mouse skeletal myosin heavy chain gene clusters is highly conserved

Myosin heavy chains (MyHCs) are highly conserved ubiquitous actin-based motor proteins that drive a wide range of motile processes in eukaryotic cells. MyHC isoforms expressed in skeletal muscles are encoded by a multigene family that is clustered on syntenic regions of human and mouse chromosomes 17 and 11, respectively. In an effort to gain a better understanding of the genomic organization of the skeletal MyHC genes and its effects on the regulation, function, and molecular genetics of this multigene family, we have constructed high-resolution physical maps of both human and mouse loci using PCR-based marker content mapping of P1-artificial chromosome clones. Genes encoding six MyHC isoforms have been mapped with respect to their linear order and transcriptional orientations within a 350-kb region in both human and mouse. These maps reveal that the order, transcriptional orientation, and relative intergenic distances of these genes are remarkably conserved between these species. Unlike many clustered gene families, this order does not reflect the known temporal expression patterns of these genes. However, the conservation of gene organization since the estimated divergence of these species (approximately 75-110 million years ago) suggests that the physical organization of these genes may be significant for their regulation and function.

Molecular control of vascular smooth muscle cell differentiation

Changes in the differentiated state of the vascular smooth muscle cell (SMC) including enhanced growth responsiveness, altered lipid metabolism, and increased matrix production are known to play a key role in development of atherosclerotic disease. As such, there has been extensive interest in understanding the molecular mechanisms and factors that regulate differentiation of vascular SMC, and how this regulation might be disrupted in vascular disease. Key questions include determination of mechanisms that control the coordinate expression of genes required for the differentiated function of the smooth muscle cell, and determination as to how these regulatory processes are influenced by local environmental cues known to be important to control of smooth muscle differentiation. Of particular interest, a number of common cis regulatory elements including highly conserved CArG [CC(A/T)6GG] motifs or CArG-like motifs and a TGF beta control element have been identified in the promoters of virtually all smooth muscle differentiation marker genes characterized to date including smooth muscle alpha-actin, smooth muscle myosin heavy chain, telokin, and SM22 alpha and shown to be required for expression of these genes both in vivo and in vitro. In addition, studies have identified a number of trans factors that interact with these cis elements, and shown how the expression or activity of these factors is modified by local environmental cues such as contractile agonists that are known to influence differentiation of smooth muscle.

Two MCAT elements of the SM alpha-actin promoter function differentially in SM vs. non-SM cells

Transcriptional activity of the smooth muscle (SM) alpha-actin gene is differentially regulated in SM vs. non-SM cells. Contained within the rat SM alpha-actin promoter are two MCAT motifs, binding sites for transcription enhancer factor 1 (TEF-1) transcriptional factors implicated in the regulation of many muscle-specific genes. Transfections of SM alpha-actin promoter-CAT constructs containing wild-type or mutagenized MCAT elements were performed to evaluate their functional significance. Mutation of the MCAT elements resulted in increased transcriptional activity in SM cells, whereas these mutations either had no effect or decreased activity in L6 myotubes or endothelial cells. High-resolution gel shift assays resolved several complexes of different mobilities that were formed between MCAT oligonucleotides and nuclear extracts from the different cell types, although no single band was unique to SM. Western blot analysis of nuclear extracts with polyclonal antibodies to conserved domains of the TEF-1 gene family revealed multiple reactive bands, some that were similar and others that differed between SM and non-SM. Supershift assays with a polyclonal antibody to the TEF-related protein family demonstrated that TEF-1 or TEF-1-related proteins were contained in the shifted complexes. Results suggest that the MCAT elements may contribute to cell type-specific regulation of the SM alpha-actin gene. However, it remains to be determined whether the differential transcriptional activity of MCAT elements in SM vs. non-SM is due to differences in expression of TEF-1 or TEF-1-related proteins or to unique (cell type specific) combinatorial interactions of the MCAT elements with other cis-elements and trans-factors.

Lung smooth muscle differentiation

The vascular and visceral smooth muscle tissues of the lung perform a number of tasks that are critical to pulmonary function. Smooth muscle function often is compromised as a result of lung disease. Though a great deal is known about regulation of smooth muscle cell replication and cell and tissue contractility, much less is understood regarding the phenotype of the contractile protein machinery of lung smooth muscle cells. This review focuses on the expression of cytoskeletal and contractile proteins of lung vascular and airway smooth muscle cells during development, in the adult and during vascular and airway remodeling. Emphasis is placed on the expression of the heavy chain of smooth muscle myosin, as well as the regulation of its gene. Important areas for future research are discussed.

Hypertrophy-stimulated myogenic regulatory factor mRNA increases are attenuated in fast muscle of aged quails

Myogenic regulatory factors (MRFs) are a family of skeletal muscle-specific transcription factors that regulate the expression of several muscle genes. This study was designed to determine whether MRF transcripts were increased in hypertrophy-stimulated muscle of adult quails and whether equivalent increases occurred in muscles of older quails. Slow-tonic anterior latissimus dorsi and fast-twitch patagialis muscles of adult, middle-aged, aged, and senescent quails were stretch overloaded for 6, 24, or 72 h, with contralateral muscles serving as controls. RNase protection assays showed that MRF4 and MyoD transcript levels were increased and myogenin and Myf5 transcripts were induced in stretch-overloaded muscles. However, MRF4 and MyoD increases were significantly attenuated in patagialis muscles of older quails. RT-PCR analyses of three MRF-regulated genes showed that increases in the transcription of these genes occurred with stretch overload, but the increases were less in muscles of older quails. In summary, attenuated MRF responses in muscles from aged animals may partially explain why muscles from older animals do not hypertrophy to the same extent as muscles from younger animals.

Structure of the human sarco/endoplasmic reticulum Ca2+-ATPase 3 gene. Promoter analysis and alternative splicing of the SERCA3 pre-mRNA

Human chromosome 17-specific genomic clones extending over 90 kilobases (kb) of DNA and coding for sarco/endoplasmic reticulum Ca2+-ATPase 3 (SERCA3) were isolated. The presence of the D17S1828 genetic marker in the cosmid contig enabled us to map the SERCA3 gene (ATP2A3) 11 centimorgans from the top of the short arm p of chromosome 17, in the vicinity of the cystinosis gene locus. The SERCA3 gene contains 22 exons spread over 50 kb of genomic DNA. The exon/intron boundaries are well conserved between human SERCA3 and SERCA1 genes, except for the junction between exons 8 and 9 which is found in the SERCA1 gene but not in SERCA3 and SERCA2 genes. The transcription start site (+1) is located 152 nucleotides (nt) upstream of the AUG codon. The 5'-flanking region, including exon 1, is embedded in a 1.5-kb CpG island and is characterized by the absence of a TATA box and by the presence of 14 putative Sp1 sites, 11 CACCC boxes, 5 AP-2-binding motifs, 3 GGCTGGGG motifs, 3 CANNTG boxes, a GATA motif, as well as single sites for Ets-1, c-Myc, and TFIIIc. Functional promoter analysis indicated that the GC-rich region (87% G + C) from -135 to -31 is of critical importance in initiating SERCA3 gene transcription in Jurkat cells. Exon 21 (human, 101 base pairs; mouse, 86 base pairs) can be alternatively excluded, partially included, or totally included, thus generating, respectively, SERCA3a (human and mouse, 999 amino acids (aa)), SERCA3b (human, 1043 aa; mouse, 1038 aa), or SERCA3c (human, 1024 aa; mouse, 1021 aa) isoforms with different C termini. Expression of the mouse SERCA3 isoforms in COS-1 cells demonstrated their ability to function as active pumps, although with different apparent affinities for Ca2+.

Molecular genetics of succinate:quinone oxidoreductase in eukaryotes

Succinate:quinone oxidoreductase is a membrane-associated complex in mitochondria, often referred to as complex II, based on the fractionation scheme developed by Y. Hatefi and colleagues. It consists of four peptides, two of which are integral membrane proteins (15 and 12-13 kDa, respectively) and two others that are peripheral membrane proteins, i.e., a flavoprotein (Fp, 70 kDa) and an iron-protein (Ip, 27 kDa). The mature, functional complex contains a cytochrome in association with the membrane proteins, a flavin linked covalently to the largest peptide, and three iron-sulfur clusters in the 27-kDa subunit. The present review touches only briefly on the biochemical and biophysical properties of this complex. Instead, the focus is on the molecular-genetic studies that have become possible since the first genes from eukaryotes were cloned in 1989. The evolutionary conservation of the amino acid sequence of both the Fp and the Ip peptides has facilitated the cloning of these genes from a large variety of eukaryotic organisms by PCR-based methods. The review addresses questions related to the regulation of the expression of these genes, with an emphasis on mammals and yeast, for which most of the information is available. Four different genes have to be co-ordinately regulated. Transcriptional as well as posttranscriptional regulatory mechanisms have been observed in diverse organisms. Intriguing observations have been made in studies of this enzyme during the life cycle of organisms existing alternately under aerobic and anaerobic conditions. Naturally occurring or induced mutations in these genes have shed light on several questions related to the assembly of this complex, and on the relationship between structure and function. Four different peptides are imported into the mitochondria. They have to be modified, folded, and assembled. The stage is set for the exploration of highly specific changes introduced by site-directed mutagenesis. Until recently the genes were believed to be exclusively nuclear in all eukaryotes, but exceptions have since been found. This finding has relevance in the discussion of the evolution of mitochondria from prokaryotes. A highly conserved set of genes is found in prokaryotes, and some informative comparisons on gene organization and expression in prokaryotes and eukaryotes have been included.

Transcriptional regulation of the human nonmuscle myosin II heavy chain-A gene. Identification of three clustered cis-elements in intron-1 which modulate transcription in a cell type- and differentiation state-dependent manner

In an attempt to identify cis-acting elements for transcriptional regulation of the human nonmuscle myosin II heavy chain (MHC)-A gene, the region extending 20 kilobases (kb) upstream and 40 kb downstream from the transcription start sites, which includes the entire 37-kb intron 1, was examined. Using transient transfection analysis of luciferase reporter constructs, a 100-base pair (bp) region (N2d) in intron 1, located 23 kb downstream from the transcriptional start sites, has been found to activate transcription in a cell type- and differentiation state-dependent manner. Maximum activity (approximately 20-fold) is seen in NIH 3T3 fibroblasts and intermediate activity (7-fold) in proliferating and undifferentiated C2C12 myoblasts. In contrast, this region is almost inactive in terminally differentiated C2C12 myotubes, in which endogenous nonmuscle MHC-A expression is down-regulated. Gel mobility shift assays and methylation interference analyses were performed using NIH 3T3 nuclear extracts to determine the protein-binding elements for transcription factors. Three binding elements have been identified within the N2d region. Antibody-supershift experiments, as well as competition experiments using consensus binding sequences for specific transcription factors, revealed that the most 5'-element, C (GGGAGGGGCC) is recognized specifically and exclusively by Sp1 and Sp3 transcriptional factors. Element C is immediately followed by a novel element, A (GTGACCC). A third element, F (GTGTCAGGTG), which contains an E-box, is located 50 bp 3' to element A. Element F can be recognized partially by upstream stimulatory factors, USF1 and/or USF2. Transfection studies with luciferase reporter constructs which include mutations in all three elements in various combinations demonstrate that the A and C binding factors cooperatively activate transcriptional activity in NIH 3T3 cells. The F binding factor shows an additive effect on transcription.

The pufferfish SLP-1 gene, a new member of the SCL/TAL-1 family of transcription factors

The SCL/TAL-1 gene encodes a basic helix-loop-helix (bHLH) transcription factor essential for the development of all hemopoietic lineages and also acts as a T-cell oncogene. Four related genes have been described in mammals (LYL-1, TAL-2, NSCL1, and NSCL2), all of which exhibit a high degree of sequence similarity to SCL/TAL-1 in the bHLH domain and two of which (LYL-1 and TAL-2) have also been implicated in the pathogenesis of T-cell acute lymphoblastic leukemia. In this study we describe the identification and characterization of a pufferfish gene termed SLP-1, which represents a new member of this gene family. The genomic structure and sequence of SLP-1 suggests that it forms a subfamily with SCL/TAL-1 and LYL-1 and is most closely related to SCL/TAL-1. However, unlike SCL/TAL-1, SLP-1 is widely expressed. Sequence analysis of a whole cosmid containing SLP-1 shows that SLP-1 is flanked upstream by a zinc finger gene and a fork-head-domain gene and downstream by a heme-oxygenase and a RING finger gene.

The developmentally regulated expression of serum response factor plays a key role in the control of smooth muscle-specific genes

Serum response factor (SRF) is a MADS box transcription factor that has been shown to be important in the regulation of a variety of muscle-specific genes. We have previously shown SRF to be a major component of multiple cis/trans interactions found along the smooth muscle gamma-actin (SMGA) promoter. In the studies reported here, we have further characterized the role of SRF in the regulation of the SMGA gene in the developing gizzard. EMSA analyses, using nuclear extracts derived from gizzards at various stages in development, showed that the SRF-containing complexes were not present early in gizzard smooth muscle development, but appeared as development progressed. We observed an increase in SRF protein and mRNA levels during gizzard development by Western and Northern blot analyses, with a large increase just preceding an increase in SMGA expression. Thus, changes in SRF DNA-binding activity were paralleled with increased SRF gene expression. Immunohistochemical analyses demonstrated a correspondence of SRF and SMGA expression in differentiating visceral smooth muscle cells (SMCs) during gizzard tissue development. This correspondence of SRF and SMGA expression was also observed in cultured smooth muscle mesenchyme induced to express differentiated gene products in vitro. In gene transfer experiments with SMGA promoter-luciferase reporter gene constructs we observed four- to fivefold stronger SMGA promoter activity in differentiated SMCs relative to replicating visceral smooth muscle cells. Further, we demonstrate the ability of a dominant negative SRF mutant protein to specifically inhibit transcription of the SMGA promoter in visceral smooth muscle, directly linking SRF with the control of SMGA gene expression. Taken together, these data suggest that SRF plays a prominent role in the developmental regulation of the SMGA gene.

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.

Transcription enhancer factor 1 interacts with a basic helix-loop-helix zipper protein, Max, for positive regulation of cardiac alpha-myosin heavy-chain gene expression

The M-CAT binding factor transcription enhancer factor 1 (TEF-1) has been implicated in the regulation of several cardiac and skeletal muscle genes. Previously, we identified an E-box-M-CAT hybrid (EM) motif that is responsible for the basal and cyclic AMP-inducible expression of the rat cardiac alpha-myosin heavy chain (alpha-MHC) gene in cardiac myocytes. In this study, we report that two factors, TEF-1 and a basic helix-loop-helix leucine zipper protein, Max, bind to the alpha-MHC EM motif. We also found that Max was a part of the cardiac troponin T M-CAT-TEF-1 complex even when the DNA template did not contain an apparent E-box binding site. In the protein-protein interaction assay, a stable association of Max with TEF-1 was observed when glutathione S-transferase (GST)-TEF-1 or GST-Max was used to pull down in vitro-translated Max or TEF-1, respectively. In addition, Max was coimmunoprecipitated with TEF-1, thus documenting an in vivo TEF-1-Max interaction. In the transient transcription assay, overexpression of either Max or TEF-1 resulted a mild activation of the alpha-MHC-chloramphenicol acetyltransferase (CAT) reporter gene at lower concentrations and repression of this gene at higher concentrations. However, when Max and TEF-1 expression plasmids were transfected together, the repression mediated by a single expression plasmid was alleviated and a three- to fourfold transactivation of the alpha-MHC-CAT reporter gene was observed. This effect was abolished once the EM motif in the promoter-reporter construct was mutated, thus suggesting that the synergistic transactivation function of the TEF-1-Max heterotypic complex is mediated through binding of the complex to the EM motif. These results demonstrate a novel association between Max and TEF-1 and indicate a positive cooperation between these two factors in alpha-MHC gene regulation.

Identification of a novel marker for primordial smooth muscle and its differential expression pattern in contractile vs noncontractile cells

The assembly of the vessel wall from its cellular and extracellular matrix components is an essential event in embryogenesis. Recently, we used the descending aorta of the embryonic quail to define the morphological events that initiate the formation of a multilayered vessel wall from a nascent endothelial cell tube (Hungerford, J.E., G.K. Owens, W.S. Argraves, and C.D. Little. 1996. Dev. Biol. 178:375-392). We generated an mAb, 1E12, that specifically labels smooth muscle cells from the early stages of development to adulthood. The goal of our present study was to characterize further the 1E12 antigen using both cytological and biochemical methods. The 1E12 antigen colocalizes with the actin cytoskeleton in smooth muscle cells grown on planar substrates in vitro; in contrast, embryonic vascular smooth muscle cells in situ contain 1E12 antigen that is distributed in threadlike filaments and in cytoplasmic rosette-like patterns. Initial biochemical analysis shows that the 1E12 mAb recognizes a protein, Mr = 100,000, in lysates of adult avian gizzard. An additional polypeptide band, Mr = 40,000, is also recognized in preparations of lysate, when stronger extraction conditions are used. We have identified the 100-kD polypeptide as smooth muscle alpha-actinin by tandem mass spectroscopy analysis. The 1E12 antibody is an IgM isotype. To prepare a more convenient 1E12 immunoreagent, we constructed a single chain antibody (sFv) using recombinant protein technology. The sFv recognizes a single 100-kD protein in gizzard lysates. Additionally, the recombinant antibody recognizes purified smooth muscle alpha-actinin. Our results suggest that the 1E12 antigen is a member of the alpha-actinin family of cytoskeletal proteins; furthermore, the onset of its expression defines a primordial cell restricted to the smooth muscle lineage.

Repression of muscle-specific gene activation by the murine Twist protein

Inhibition of myogenic differentiation can be achieved by various mechanisms. The murine bHLH protein Twist has been shown to inhibit muscle differentiation in mammalian cells. Here we demonstrate that this inhibition is cell autonomous and does not alter cell proliferation. By overexpression of E12, we can distinguish the inhibitory mechanisms of Twist and the dominant negative HLH factor Id. A difference is seen both for the native muscle-specific enhancers of myogenin and myosin light chain 1/3 and for an enhancer consisting only of four E-boxes. Mutagenesis experiments revealed that both the basic region and an evolutionarily conserved carboxy-terminal domain are required for the Twist-specific type of inhibition. Loss of either of these regions renders Twist less efficient and more similar to Id. Gel mobility shift assays demonstrate that Twist can bind to the muscle creatine kinase E-box and inhibit DNA binding of heterodimers of E12 with myogenic bHLH transcription factors like MyoD. However, a fourfold excess of Twist compared to MyoD is required for both effects. Our results suggest that Twist inhibits muscle-specific gene activation by formation of actively inhibitory complexes rather than by sequestering E-proteins.

Regulation of differentiation/maturation in vascular smooth muscle cells by hormones and growth factors

Smooth muscle cells (SMC) within atherosclerotic lesions show marked alterations in their differentiated properties as compared to normal medial SMC. This process of de-differentiation of SMC has been referred to as "phenotypic modulation", and is characterized by increased growth responsiveness, altered lipid metabolism, increased matrix production, and loss of contractile proteins, all of which can contribute to the development and/or progression of atherosclerotic disease. As such there has been much interest in understanding mechanisms and factors that control the differentiation of the vascular SMC. This paper reviews the effects of growth factors, growth inhibitors, and other extrinsic factors on differentiation/maturation of SMC, with a particular emphasis on consideration of factors that may contribute to abnormal control of SMC differentiation in vascular disease. In addition, we will briefly summarize what is currently known regarding molecular mechanisms that control the coordinate expression of genes encoding for SMC-selective/specific proteins that are required for the differentiated function of the vascular SMC.

Tropomyosin isoforms in nonmuscle cells

Vertebrate nonmuscle cells, such as human and rat fibroblasts, express multiple isoforms of tropomyosin, which are generated from four different genes and a combination of alternative promoter activities and alternative splicing. The amino acid variability among these isoforms is primarily restricted to three alternatively spliced exon regions; an amino-terminal region, an internal exon, and a carboxyl-terminal exon. Recent evidence reveals that these variable exon regions encode amino acid sequences that may dictate isoform-specific functions. The differential expression of tropomyosin isoforms found in cell transformation and cell differentiation, as well as the differential localization of tropomyosin isoforms in some types of culture cells and developing neurons suggest a differential isoform function in vivo. Tropomyosin in striated muscle works together with the troponin complex to regulate muscle contraction in a Ca(2+)-dependent fashion. Both in vitro and in vivo evidence suggest that multiple isoforms of tropomyosin in nonmuscle cells may be required for regulating actin filament stability, intracellular granule movement, cell shape determination, and cytokinesis. Tropomyosin-binding proteins such as caldesmon, tropomodulin, and other unidentified proteins may be required for some of these functions. Strong evidence for the distinct functions carried out by different tropomyosin isoforms has been generated from genetic analysis of yeast and Drosophila tropomyosin mutants.

Zebrafish tinman homolog demarcates the heart field and initiates myocardial differentiation

The fashioning of a vertebrate organ requires integration of decisions of cell fate by individual cells with those that regulate organotypic form. Logical candidates for this role, in an organ such as the heart, are genes that initiate the differentiation process leading to heart muscle and those that define the earliest embryonic heart field, but for neither class are genes defined. We cloned zebrafish Nkx2.5, a homolog of the tinman homeodomain gene needed for visceral and cardiac mesoderm formation in Drosophila. In the zebrafish, its expression is associated with cardiac precursor cells throughout development, even in the early gastrula, where the level of zebrafish Nkx2.5 is in a gradient which spatially matches the regional propensity of ventral-marginal cells to become heart. Overexpression of Nkx2.5 causes formation of disproportionally larger hearts in otherwise apparently normal embryos. Transplanted cell expressing high levels of Nkx2.5 express cardiac genes even in ectopic locales. Fibroblasts transfected with myc-tagged Nkx2.5 express cardiac genes. These effects require the homeodomain. Thus, Nkx2.5 appears to mark the earliest embryonic heart field and to be capable of initiating the cardiogenic differentiation program. Because ectopic cells or transfected fibroblasts do not beat, Nkx2.5 is likely to be but one step in the determination of cardiac myocyte cell fate. Its overexpression increases heart size, perhaps by bringing cells on the edge of the field to a threshold level for initiation of cardiac differentiation.

Overexpression of the tinman-related genes XNkx-2.5 and XNkx-2.3 in Xenopus embryos results in myocardial hyperplasia

Drosophila tinman is an NK-class homeobox gene required for formation of the dorsal vessel, the insect equivalent of the vertebrate heart. Vertebrate sequences related to tinman, such as mouse Nkx-2.5, chicken cNkx-2.5, Xenopus XNkx-2.5 and XNkx-2.3 are expressed in cardiac precursors and in tissues involved in induction of cardiac mesoderm. Mice which lack a functional Nkx-2.5 gene die due to cardiac defects. To determine the role of tinman-related sequences in heart development, we have overexpressed both XNkx-2.3 and XNkx-2.5 in Xenopus laevis embryos. The resulting embryos are morphologically normal except that they have enlarged hearts. The enlarged heart phenotype is due to a thickening of the myocardium caused by an increase in the overall number of myocardial cells (hyperplasia). Neither ectopic nor precocious expression of cardiac differentiation markers is detectable in overexpressing embryos. These results suggest that both XNkx-2.3 and XNkx-2.5 are functional homologues of tinman, responsible for maintenance of the heart field.

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.

Myocardial infarction as a problem of growth control: cell cycle therapy for cardiac myocytes?

Pump failure after myocardial infarction ultimately can be ascribed, in large part, to the inability of ventricular muscle to regenerate functional mass through cell proliferation. Recent studies using adenoviral gene transfer have provided direct evidence for the operation of two growth-suppressing pathways in cardiac muscle, via "pocket proteins," including the retinoblastoma gene product, and via a less well understood protein, p300. An understanding of molecular mechanisms that confer a virtually irreversible lock to the proliferative cell cycle in "postmitotic" cardiac muscle, together with improved means for delivery of exogenous genes to the heart, suggests the long-term potential for manipulating cardiac growth to achieve a therapeutic benefit.

Identification of a novel cardiac-specific transcript critical for cardiac myocyte differentiation

A novel cDNA, pCMF1, which is expressed exclusively and transiently in the myogenic cells of the differentiating chicken heart was isolated and characterized. The full-length cDNA of pCMF1 has one open reading frame encoding 1538 predicted amino acids. While computer analysis predicts the presence of specific structural motifs, the overall sequence of pCMF1 is unique. The pattern of pCMF1 gene expression during heart formation was determined by whole-mount in situ hybridization. pCMF1 is transiently expressed within the myogenic cells of the primitive heart tube from stages 9 to 18 and is not detected in the heart or any other tissue thereafter. A replication-deficient retrovirus was used to mediate pCMF1 antisense expression in cardiogenic mesoderm. These analyses determined that the presence of pCMF1 antisense sequences disrupted myosin heavy chain expression during cardiac mesoderm differentiation. pCMF1 antisense had no effect on myosin heavy chain expression in differentiated cardiac myocytes. These data suggest a potential function for pCMF1 during cardiac myogenesis.

Molecular pathways controlling heart development

Heart formation requires complex interactions among cells from multiple embryonic origins. Recent studies have begun to reveal the genetic pathways that control cardiac morphogenesis. Many of the genes within these pathways are conserved across vast phylogenetic distances, which has allowed cardiac development to be dissected in organisms ranging from flies to mammals. Studies of cardiac development have also revealed the molecular defects underlying several congenital cardiac malformations in humans and may ultimately provide opportunities for genetic testing and intervention.

DTEF-1, a novel member of the transcription enhancer factor-1 (TEF-1) multigene family

M-CAT motifs mediate muscle-specific transcriptional activity via interaction with binding factors that are antigenically and biochemically related to vertebrate transcription enhancer factor-1 (TEF-1), a member of the TEA/ATTS domain family of transcription factors. M-CAT binding activities present in cardiac and skeletal muscle tissues cannot be fully accounted for by existing cloned isoforms of TEF-1. TEF-1-related cDNAs isolated from heart libraries indicate that at least three classes of TEF-1-related cDNAs are expressed in these and other tissues. One class are homologues of the human TEF-1 originally cloned from HeLa cells (Xiao, J. H., Davidson, I., Matthes, H., Garnier, J. M., and Chambon, P. (1991) Cell 65, 551-568). A second class represents homologues of the avian TEF-1-related gene previously isolated (Stewart, A. F., Larkin, S. B., Farrance, I. K., Mar, J. H., Hall, D. E., and Ordahl, C. P. (1994) J. Biol. Chem. 269, 3147-3150). The third class consists of a novel, divergent TEF-1 cDNA, named DTEF-1, and its preliminary characterization is described here. Two isoforms of DTEF-1 (DTEF-1A and DTEF-1B) were isolated as 1.9-kilobase pair clones with putative open reading frames of 433 and 432 amino acids whose differences are attributable to alternative splicing at the C terminus of the TEA DNA binding domain. Cardiac muscle contains high levels of DTEF-1 transcripts, but unexpectedly low levels are detected in skeletal muscle. DTEF-1 transcripts are present at intermediate levels in gizzard and lung, and at low levels in kidney. DTEF-1A is a sequence-specific M-CAT-binding factor. The distinct spatial pattern of expression, and unusual amino acid sequence in its DNA binding domain, may indicate a particular role for DTEF-1 in cell-specific gene regulation. Recent work also suggests that at least one more TEF-1-related gene exists in vertebrates. We propose a naming system for the four TEF-1 gene family members identified to date that preserves existing nomenclature and provides a means for extending that nomenclature as additional family members may be identified.

The role of transcription enhancer factor-1 (TEF-1) related proteins in the formation of M-CAT binding complexes in muscle and non-muscle tissues

M-CAT sites are required for the activity of many promoters in cardiac and skeletal muscle. M-CAT binding activity is muscle-enriched, but is found in many tissues and is immunologically related to the HeLa transcription enhancer factor-1 (TEF-1). TEF-1-related cDNAs (RTEF-1) have been cloned from chick heart. RTEF-1 mRNA is muscle-enriched, consistent with a role for RTEF-1 in the regulation of muscle-specific gene expression. Here, we have examined the tissue distribution of TEF-1-related proteins and of M-CAT binding activity by Western analysis and mobility shift polyacrylamide gel electrophoresis. TEF-1-related proteins of 57, 54 and 52 kDa were found in most tissues with the highest levels in muscle tissues. All of these TEF-1-related proteins bound M-CAT DNA and the 57- and 54-kDa TEF-1-related polypeptides were phosphorylated. Proteolytic digestion mapping showed that the 54-kDa TEF-1-related polypeptide is encoded by a different gene than the 52- and 57-kDa TEF-1-related polypeptides. A comparison of the migration and proteolytic digestion of the 54-kDa TEF-1-related polypeptide with proteins encoded by the cloned RTEF-1 cDNAs showed that the 54-kDa TEF-1-related polypeptide is encoded by RTEF-1A. High resolution mobility shift polyacrylamide gel electrophoresis showed multiple M-CAT binding activities in tissues. All of these activities contained TEF-1-related proteins. One protein-M-CAT DNA complex was muscle-enriched and was up-regulated upon differentiation of a skeletal muscle cell line. This complex contained the 54-kDa TEF-1-related polypeptide. Therefore, RTEF1-A protein is a component of a muscle-enriched transcription complex that forms on M-CAT sites and may play a key role in the regulation of transcription in muscle.

Induction of DNA synthesis and apoptosis in cardiac myocytes by E1A oncoprotein

Beginning during the second half of gestation, increasing numbers of cardiac myocytes withdraw from the cell cycle such that DNA synthesis is no longer detectable in these cells by neonatal day 17 in vivo. The mechanisms that exclude these and other terminally differentiated cells from the cell division cycle are poorly understood. To begin to explore the molecular basis of the barrier to G1/S progression in cardiac myocytes, we used adenoviruses to express wild-type and mutant E1A proteins in primary cultures from embryonic day 20 rats. While most of these cardiac myocytes are ordinarily refractory to DNA synthesis, even in the presence of serum growth factors, expression of wild-type E1A stimulates DNA synthesis in up to 94% or almost all successfully transduced cells. Rather than complete the cell cycle, however, these cells undergo apoptosis. Apoptosis is limited to those cells that engage in DNA synthesis, and the kinetics of the two processes suggest that DNA synthesis precedes apoptosis. Mutations in E1A that disable it from binding Rb and related pocket proteins have little effect on its ability to stimulate DNA synthesis in cardiac myocytes. In contrast, mutants that are defective in binding the cellular protein p300 stimulate DNA synthesis 2.4-4.1-fold less efficiently, even in the context of retained E1A pocket protein binding. In the absence of ElA pocket protein binding, the usual situation in the cell, loss of p300 binding severely decreases the ability of ElA to stimulate DNA synthesis. These results suggest that the barrier to G1/S progression in cardiac myocytes is mediated. at least in part, by the same molecules that gate the G1/S transition in actively cycling cells, and that p300 or related family members play an important role in this process.

Analysis of muscle creatine kinase gene regulatory elements in skeletal and cardiac muscles of transgenic mice

Regulatory regions of the mouse muscle creatine kinase (MCK) gene, previously discovered by analysis in cultured muscle cells, were analyzed in transgenic mice. The 206-bp MCK enhancer at nt-1256 was required for high-level expression of MCK-chloramphenicol acetyltransferase fusion genes in skeletal and cardiac muscle; however, unlike its behavior in cell culture, inclusion of the 1-kb region of DNA between the enhancer and the basal promoter produced a 100-fold increase in skeletal muscle activity. Analysis of enhancer control elements also indicated major differences between their properties in transgenic muscles and in cultured muscle cells. Transgenes in which the enhancer right E box or CArG element were mutated exhibited expression levels that were indistinguishable from the wild-type transgene. Mutation of three conserved E boxes in the MCK 1,256-bp 5' region also had no effect on transgene expression in thigh skeletal muscle expression. All these mutations significantly reduced activity in cultured skeletal myocytes. However, the enhancer AT-rich element at nt - 1195 was critical for expression in transgenic skeletal muscle. Mutation of this site reduced skeletal muscle expression to the same level as transgenes lacking the 206-bp enhancer, although mutation of the AT-rich site did not affect cardiac muscle expression. These results demonstrate clear differences between the activity of MCK regulatory regions in cultured muscles cells and in whole adult transgenic muscle. This suggests that there are alternative mechanism of regulating the MCK gene in skeletal and cardiac muscle under different physiological states.

Transcription factors and the cardiac gene programme

During the past decade, major advances have been made in uncovering the mechanisms that switch genes on and off. Gene methylation and histones play an important role in gene (in)activation. Following gene activation, the initiation of transcription by RNA polymerase requires the assembly of multiple protein complexes on the promoter region of a gene. How a cell type-specific gene expression pattern can be induced is a key question in cardiovascular biology today. Members of the helix-loop-helix-family of the transcription factors play a dominant role in skeletal muscle formation. In cardiac muscle the situation is less obvious. Recent studies identified muscle transcription factors like MEF-2, TEF-1 and MNF, which are common to both the skeletal and cardiac muscle lineages. A few transcription factors, among which Nkx 2.5 and GATA-4, are expressed predominantly in the heart. The absence of master regulators in the heart points to the importance of interaction between ubiquitous factors and tissue restricted factors to initiate the cardiac gene programme and to lock these cells in their differentiated state. The recent development of murine transgenic and gene-targeting technology provides tools to study the role of mammalian transcription factors in vivo. Interesting cardiac phenotypes are found in gene targeted mice, indicating a crucial role for retinoic acid and homeobox genes in murine cardiogenesis.

cDNA cloning and characterization of murine transcriptional enhancer factor-1-related protein 1, a transcription factor that binds to the M-CAT motif

The M-CAT motif is a cis-regulatory DNA sequence that is essential for muscle-specific transcription of several genes. Previously, we had shown that both muscle-specific (A1) and ubiquitous (A2) factors bind to an essential M-CAT motif in the myosin heavy chain beta gene and that the ubiquitous factor is transcriptional enhancer factor (TEF)-1. Here we report the isolation of mouse cDNAs encoding two forms (a and b) of a TEF-1-related protein, TEFR1. The TEFR1a cDNA encodes a 427-amino acid protein. The coding region of TEFR1b is identical to 1a in both nucleotide and predicted amino acid sequence except for the absence of 43 amino acids downstream of the TEA DNA-binding domain. Three TEFR1 transcripts (approximately 7, approximately 3.5, and approximately 2 kilobase pairs) are enriched in differentiated skeletal muscle (myotubes) relative to undifferentiated skeletal muscle (myoblasts) and non-muscle cells in culture. In situ hybridization analysis indicated that TEFR1 transcripts are enriched in the skeletal muscle lineage during mouse embryogenesis. Transient expression of fusion proteins of TEFR1 and the yeast GAL4 DNA-binding domain in cell lines activated the expression of chloramphenicol acetyltransferase (CAT) reporter constructs containing GAL4 binding sites, indicating that TEFR1 contains an activation domain. An anti-TEFR1 polyclonal antibody supershifted the muscle-specific M-CAT.A1 factor complex in gel mobility shift assays, suggesting that TEFR1 is a major component of this complex. Our results suggest that TEFR1 might play a role in the embryonic development of skeletal muscle in the mouse.

Activation of the cardiac alpha-actin promoter depends upon serum response factor, Tinman homologue, Nkx-2.5, and intact serum response elements

A murine cardiac specific homeoboxgene, Nkx-2.5/CSX, a potential Drosophila tinman homologue, may have a fundamental role in cardiac myocyte differentiation. DNA binding targets for Nkx-2.5 were recently shown to represent novel homeodomain binding sequences, some of which resembled serum response elements (SREs); [Chen CY, Schwartz RJ (1995): J Biol Chem 270: 15628-15633]. In this study, Nkx-2.5 facilitated serum response factor (SRF) DNA-binding activity to the multiple SREs found on the cardiac alpha-actin promoter and together stimulated cardiac alpha-actin promoter dependent transcription in 10T1/2 fibroblasts. Analysis of cardiac alpha-actin promoter mutants demonstrated the importance of the multiple upstream SREs and an obligatory requirement for an intact proximal SRE1, for providing high levels of activity in the presence of Nkx-2.5 and SRF coexpression. Transfection assays with mutant SRF species indicated that the C-terminal activation domain and DNA-binding MADS box were necessary for transcriptional activity in the presence of Nkx-2.5. Expression of Nkx-2.5 mutants also demonstrated that the homeodomain alone was insufficient for directing promoter activity in the presence of SRF. The central role of SRF in regulating striated alpha-actin gene activity also was revealed by its embryonic expression restricted primarily to myocardium of the developing heart and the myotomal portion of somites. Thus the function of the cardiac actin promoter SREs appeared to provide binding sites for SRF and Nkx-2.5 to interact and elicit striated muscle specific transcription that was independent of the MyoD family.

A subclass of bHLH proteins required for cardiac morphogenesis

Skeletal muscle development is controlled by a family of muscle-specific basic helix-loop-helix (bHLH) transcription factors. Two bHLH genes, dHAND and eHAND, have now been isolated that are expressed in the bilateral heart primordia and subsequently throughout the primitive tubular heart and its derivatives during chick and mouse embryogenesis. Incubation of stage 8 chick embryos with dHAND and eHAND antisense oligonucleotides revealed that either oligonucleotide alone had no effect on embryonic development, whereas together they arrested development at the looping heart tube stage. Thus, dHAND and eHAND may play redundant roles in the regulation of the morphogenetic events of vertebrate heart development.

Nitric oxide synthase in human placenta and umbilical cord from normal, intrauterine growth-retarded and pre-eclamptic pregnancies

1. It has been suggested that a deficiency of nitric oxide (NO) may explain many of the pathophysiological features of pre-eclampsia (PE) and intra-uterine (foetal) growth retardation (IUGR). To elucidate further the role of NO in the pathophysiology of pregnancy we have determined the relative amount and activity of NO synthase (NOS) in first trimester and normal-term placental tissues, as well as in the placenta and umbilical cord in pregnancies complicated by PE and IUGR, using NG-nitro-L-[2,3,4,5(-3)H]-arginine ([3H]-L-NOARG) binding, quantitative in vitro autoradiography, [3H]-arginine to [3H]-citrulline conversion and Western blotting. 2. Specific, high affinity (KD = 38 nM) [3H]-L-NOARG binding was demonstrated in the villous trophoblast of normal-term placentae. Binding was calcium-independent, stereoselective and exhibited a rank order of inhibition by NOS inhibitors and substrate (L-NOARG > or = L-NMMA > or = 7-NI > L-NAME > L-Arg > or = L-NIO > ADMA). 3. [3H]-L-NOARG binding density and NOS activity were both significantly greater in placental tissues from first trimester and PE or IUGR complicated pregnancies compared to normal-term placentae. 4. Western blotting, using an endothelial NOS peptide antiserum, demonstrated a approximately 140 KDa protein band in placental extracts and indicated that the amount of immunoreactive material was significantly greater in first trimester compared to normal-term placentae. 5. Specific [3H]-L-NOARG binding was also localized to the endothelial lining of umbilical arteries and veins, binding density being greater in the artery than the vein. [3H]-L-NOARG binding to the umbilical artery endothelium was significantly lower in PE and IUGR complicated pregnancies compared to normal-term controls. 6. The role of trophoblast-derived NO in human placental pathophysiology remains to be established, but differences in the amount of placental [3H]-L-NOARG binding, NOS activity and immunoreactive material indicate that expression of NOS in the villous trophoblast falls during pregnancy. Conversely, the apparent reduction in NOS in the umbilical artery endothelium in PE and IUGR complicated pregnancies may be indicative of endothelial dysfunction.

Structural characterization and regulatory element analysis of the heart isoform of cytochrome c oxidase VIa

In order to investigate the mechanism(s) governing the striated muscle-specific expression of cytochrome c oxidase VIaH we have characterized the murine gene and analyzed its transcriptional regulatory elements in skeletal myogenic cell lines. The gene is single copy, spans 689 base pairs (bp), and is comprised of three exons. The 5'-ends of transcripts from the gene are heterogeneous, but the most abundant transcript includes a 5'-untranslated region of 30 nucleotides. When fused to the luciferase reporter gene, the 3.5-kilobase 5'-flanking region of the gene directed the expression of the heterologous protein selectively in differentiated Sol8 cells and transgenic mice, recapitulating the pattern of expression of the endogenous gene. Deletion analysis identified a 300-bp fragment sufficient to direct the myotube-specific expression of luciferase in Sol8 cells. The region lacks an apparent TATA element, and sequence motifs predicted to bind NRF-1, NRF-2, ox-box, or PPAR factors known to regulate other nuclear genes encoding mitochondrial proteins are not evident. Mutational analysis, however, identified two cis-elements necessary for the high level expression of the reporter protein: a MEF2 consensus element at -90 to -81 bp and an E-box element at -147 to -142 bp. Additional E-box motifs at closely located positions were mutated without loss of transcriptional activity. The dependence of transcriptional activation of cytochrome c oxidase VIaH on cis-elements similar to those found in contractile protein genes suggests that the striated muscle-specific expression is coregulated by mechanisms that control the lineage-specific expression of several contractile and cytosolic proteins.

Stable fetal cardiomyocyte grafts in the hearts of dystrophic mice and dogs

This report documents the formation of stable fetal cardiomyocyte grafts in the myocardium of dystrophic dogs. Preliminary experiments established that the dystrophin gene product could be used to follow the fate of engrafted cardiomyocytes in dystrophic mdx mice. Importantly, ultrastructural analyses revealed the presence of intercalated discs consisting of fascia adherens, desmosomes, and gap junctions at the donor-host cardiomyocyte border. To determine if isolated cardiomyocytes could form stable intracardiac grafts in a larger species, preparations of dissociated fetal canine cardiomyocytes were delivered into the hearts of adult CXMD (canine X-linked muscular dystrophy) dogs. CXMD dogs, like Duchenne muscular dystrophy patients and mdx mice, fail to express dystrophin in both cardiac and skeletal muscle. Engrafted fetal cardiomyocytes, identified by virtue of dystrophin immunoreactivity, were observed to be tightly juxtaposed with host cardiomyocytes as long as 10 wk after engraftment, the latest date analyzed. Confocal laser scanning microscopy revealed the presence of connexin43, a major constituent of the gap junction, at the donor-host cardiomyocyte border. The presence of intracardiac grafts was not associated with arrhythmogenesis in the CXMD model. These results indicate that fetal cardiomyocyte grafting can successfully augment cardiomyocyte number in larger animals.

Cell death in the failing heart: role of an unnatural growth response to overload

Hypertrophy of the overloaded heart, characterized by an increased number of sarcomeres, provides an adaptive, short-term response. However, when cardiac overload is long-standing, the hypertrophic response appears to cause shortened myocyte survival. The mechanisms responsible for the deleterious effects of chronic myocardial hypertrophy may include a maladaptive growth response of the mature heart. Because terminally differentiated adult cardiac myocytes have little or no capacity to divide, stimuli that promote growth in the overloaded adult heart cannot lead to normal cell division. Instead, overload initiates an unnatural growth response that appears to shorten cardiac myocyte survival, possibly because the same growth factors that mediate the hypertrophic response of the adult heart can also induce programmed cell death (apoptosis). The converting enzyme inhibitors and nitrates, which have growth-inhibitory as well as vasodilator effects, may improve prognosis in heart failure by inhibiting the production of transcription factors. These transcription factors stimulate both the unnatural growth response to overload and stimuli that lead to apoptosis. Since both beta-adrenergic agonists and cytokines, such as tumor necrosis factor-alpha, can stimulate production of similar transcription factors, evidence suggests that beta blockers and vesnarinone improve the prognosis in patients with heart failure possibly because of their ability to inhibit maladaptive growth.

Strategies for myocardial repair

The therapeutic recourse for end-stage heart disease is currently limited to cardiac transplantation. The ability to augment cardiomyocyte number in an end-stage heart might facilitate myocardial function. Augmentation of cardiomyocyte number may be achievable by the targeted expression of cell cycle regulatory genes to the myocardium. Alternatively, intracardiac grafting of exogenous cardiomyocytes might also provide a viable approach to augment cardiomyocyte number. Potential strategies for heart muscle regeneration via gene therapy and cellular transplantation are discussed.

Myogenic and morphogenetic defects in the heart tubes of murine embryos lacking the homeo box gene Nkx2-5

The murine homeo box gene Nkx2-5 is expressed in precardiac mesoderm and in the myocardium of embryonic and fetal hearts. Targeted interruption of Nkx2-5 resulted in abnormal heart morphogenesis, growth retardation and embryonic lethality at approximately 9-10 days postcoitum (p.c.). Heart tube formation occurred normally in mutant embryos, but looping morphogenesis, a critical determinant of heart form, was not initiated at the linear heart tube stage (8.25-8.5 days p.c.). Commitment to the cardiac muscle lineage, expression of most myofilament genes and myofibrillogenesis were not compromised. However, the myosin light-chain 2V gene (MLC2V) was not expressed in mutant hearts nor in mutant ES cell-derived cardiocytes. MLC2V expression normally occurs only in ventricular cells and is the earliest known molecular marker of ventricular differentiation. The regional expression in mutant hearts of two other ventricular markers, myosin heavy-chain beta and cyclin D2, indicated that not all ventricle-specific gene expression is dependent on Nkx2-5. The data demonstrate that Nkx2-5 is essential for normal heart morphogenesis, myogenesis, and function. Furthermore, this gene is a component of a genetic pathway required for myogenic specialization of the ventricles.

Structure and expression of a smooth muscle cell-specific gene, SM22 alpha

SM22 alpha is expressed exclusively in smooth muscle-containing tissues of adult animals and is one of the earliest markers of differentiated smooth muscle cells (SMCs). To examine the molecular mechanisms that regulate SMC-specific gene expression, we have isolated and structurally characterized the murine SM22 alpha gene. SM22 alpha is a 6.2-kilobase single copy gene composed of five exons. SM22 alpha mRNA is expressed at high levels in the aorta, uterus, lung, and intestine, and in primary cultures of rat aortic SMCs, and the SMC line, A7r5. In contrast to genes encoding SMC contractile proteins, SM22 alpha gene expression is not decreased in proliferating SMCs. Transient transfection experiments demonstrated that 441 base pairs of SM22 alpha 5'-flanking sequence was necessary and sufficient to program high level transcription of a luciferase reporter gene in both primary rat aortic SMCs and A7r5 cells. DNA sequence analyses revealed that the 441-base pair promoter contains two CArG/SRF boxes, a CACC box, and one potential MEF-2 binding site, cis-acting elements which are each important regulators of striated muscle transcription. Taken together, these studies have identified the murine SM22 alpha promoter as an excellent model system for studies of developmentally regulated, lineage-specific gene expression in SMCs.

Myb and Ets proteins cooperate to transactivate an early myeloid gene

The earliest progenitor cell committed to the granulocyte/monocyte developmental pathway can be identified by the appearance of a 150-kDa glycoprotein on the cell surface (CD13/aminopeptidase N (CD13/APN), EC 3.4.11.2). A 455-base pair genomic fragment from the CD13/APN gene containing a Myb consensus-binding site as well as three potential Ets-binding sites was found to regulate tissue-appropriate expression of reporter genes in hematopoietic cell lines. Transactivation experiments with plasmids expressing either a full-length or truncated Myb protein and the full-length Ets-1 or Ets-2 protein demonstrated that these proteins cooperate to positively regulate CD13/APN gene expression. This cooperation is synergistic, as levels of transcriptional activity produced by Myb and Ets in combination were higher than those expected from a purely additive effect. Mutation of the Myb consensus-binding site completely abolished CD13/APN promoter activity in myeloid cells. Introduction of a dominant interfering Myb allele disrupted the ability of endogenous c-Myb in myeloid cells to transactivate the CD13/APN construct. Other myeloid cell-expressed Ets family members (PU.1, Fli-1, and Elf-1) failed to produce a cooperative transactivating effect when combined with the Myb expression construct. These data contrast with previous studies indicating that full-length c-Myb is unable to positively cooperate with Ets proteins in the regulation of myeloid genes. Because intact c-Myb and Ets-2 proteins, both endogenously expressed in myeloid cells, act synergistically to transactivate the CD13/APN promoter, this gene may represent a physiological target for dissection of the roles of these transcription factors in normal and malignant myelopoiesis.

The smooth muscle alpha-actin gene promoter is differentially regulated in smooth muscle versus non-smooth muscle cells

To identify potential regulators of smooth muscle cell (SMC) differentiation, we studied the molecular mechanisms that control the tissue-specific transcriptional expression of SM alpha-actin, the most abundant protein in fully differentiated SMCs. A construct containing the region from -1 to -125 of the promoter (p125CAT) had high transcriptional activity in SMCs (57-fold > promoterless) and endothelial cells (ECs) (18-fold) but not in skeletal myoblasts or myotubes. Mutation of either of two highly conserved CC(AT-rich)6GG (CArG) motifs at -62 and -112 abolished the activity of p125CAT in SMCs but had no effect in ECs. In contrast, high transcriptional activity in skeletal myotubes, which also express SM alpha-actin, required at least 271 base pairs of the promoter (-1 to > or = -271). Constructs containing 547 base pairs or more of the promoter were transcriptionally active in SMCs and skeletal myotubes but had no activity in skeletal myoblasts or ECs, cell types that do not express SM alpha-actin. Electrophoretic mobility shift assays provided evidence for binding of a unique serum response factor-containing complex of factors to the CArG box elements in SMCs. Results indicate that: 1) transcriptional expression of SM alpha-actin in SMCs requires the interaction of the CArG boxes with SMC nucleoprotein(s); 2) expression of SM alpha-actin in skeletal myotubes requires different cis-elements and trans-factors than in SMCs; and 3) negative-acting cis-elements are important in restricting transcription in cells that do not express SM alpha-actin.