Molecular pathways controlling heart development

Transcriptional regulation of human cardiac homeobox gene CSX1

Cardiac homeobox gene Csx/Nkx-2.5 is essential for normal heart development and morphogenesis and is the earliest marker for cardiogenesis. To elucidate the regulatory mechanisms of Csx/Nkx-2.5 expression, we have isolated and characterized the upstream regulatory region of human Csx/Nkx-2.5 (CSX1). Transfection of the reporter gene containing a 965-bp CSX1 5' flanking region indicated that this region confers cardiomyocyte-predominant expression of CSX1. Deletion and mutational analyses revealed two positive cis-regulatory elements in this region that are essential for CSX1 expression in cardiomyocytes. Electrophoretic mobility shift assay revealed that nuclear proteins prepared from cardiac myocytes bound to these elements in a sequence-specific manner. The identification of cis-regulatory sequences of the Csx/Nkx-2.5 gene will facilitate further analysis for the upstream regulatory factors that control the expression of Csx/Nkx-2.5 and the process of vertebrate heart development.

Expression of replication factor C 40-kDa subunit is down-regulated during neonatal development in rat ventricular myocardium

During neonatal development, cardiac myocytes undergo a transition from hyperplastic to hypertrophic growth. Whether these cells are terminally differentiated and permanently withdrawn from the cell cycle shortly after birth is controversial. Nevertheless, the clinical observation that functionally significant myocardial regeneration has not been documented in cardiovascular disease or injury during adulthood seems to support the notion that the vast majority of cardiac myocytes do not proliferate once they differentiate. Regardless of the controversy, the elucidation on how mitosis is blocked in cardiac myocytes may facilitate development of new cardiovascular therapies, based on the regeneration of the adult myocardium. To better understand postnatal myocardial development, we performed suppression subtractive hybridization to isolate genes that are differentially expressed in day one or day seven postnatal rat ventricular myocardium. Here we report the down-regulated mRNA expression of the 40-kDa subunit of replication factor C (RFC p40 or RFC2), which is an essential processive factor for proliferating cellular nuclear antigen-dependent DNA replication during neonatal myocardial development.

The rostro-caudal position of cardiac myocytes affect their fate

During chick embryogenesis, cells destined to form cardiac myocytes are located within the primitive streak at stage 3 in the same relative anterior-posterior distribution as in the prelooped heart. The most rostral cells contribute to the extreme anterior pole of the heart, the bulbus cordis, and the most caudal to the extreme posterior end, the sinoatrial region. After gastrulation, these cells commit to the myocyte lineage and, retaining their relative positions, migrate to the anterior lateral plate. From stages 5 to 10 they diversify into atrial and ventricular myocytes, with the former located posteriorly and the latter, anteriorly. To determine the effect of a change in the rostro-caudal position of these cells on their diversification, anterior lateral plate mesoderm and the underlying endoderm were cut and rotated 180 degrees along the longitudinal axis, at stages 4-8. The subsequent diversification of these precursor cells into atrial and ventricular myocytes was examined using lineage-specific markers. Our results showed that altering location along the longitudinal axis through stage 6 changed the normal fate of a precursor cell. The orientation of the overlying ectoderm did not alter normal morphogenesis or determination of fate.

Up-regulation of natriuretic peptides in the ventricle of Csx/Nkx2-5 transgenic mice

A cardiac homeobox-containing gene Csx/Nkx2-5, which is essential for cardiac development, is abundantly expressed in the adult heart as well as in the heart primordia. Targeted disruption of this gene results in embryonic lethality due to abnormal heart morphogenesis. To elucidate the role of Csx/Nkx2-5 in the adult heart, we generated transgenic mice which overexpress human Csx/Nkx2-5. The transgene was expressed abundantly in the heart and the skeletal muscle. mRNA levels of several cardiac genes including natriuretic peptides, CARP, MLC2v, and endogenous Csx/Nkx2-5 were increased in the ventricle of the transgenic mice. Electron microscopic analysis revealed that the ventricular myocardium of the transgenic mice had many secretory granules, which disappeared after administration of vasopressin. These results suggest that Csx/Nkx2-5 regulates many cardiac genes and induces formation of secretory granules in the adult ventricle.

Anterior endoderm is sufficient to rescue foregut apoptosis and heart tube morphogenesis in an embryo lacking retinoic acid

The vitamin A deficient (VAD) quail embryo lacks active retinoids, fails to express normally GATA-4, and develops a nonlooping heart tube morphogenetic defect that is a model for congenital cardiomyopathy. VAD quail embryos, or chick embryos depleted specifically for GATA factors, show in addition abnormal foregut development, characterized by apoptosis of the endoderm cells associated with presumptive myocardium during the process of heart tube formation. Exogenous retinoic acid or transplantation of normal chick embryo anterior endoderm is sufficient to rescue apoptosis as well as GATA-4 expression and results in normal development and heart tube morphogenesis. Normal posterior endoderm also contains retinoids but is unable to rescue the VAD defect. Our results indicate that a retinoid-dependent transcriptional program, mediated at least in part by GATA factors, is critical in presumptive foregut endoderm for normal heart tube morphogenesis.

Complete sequence and characterization of chick ventricular myosin heavy chain in the developing atria

We isolated five complementary DNA (cDNA) clones, encoding the chick ventricular myosin heavy chain (MyHC) by reverse transcription polymerase chain reaction (RT-PCR). The entire cDNA consists of 5995 nucleotides with the 52 bp 5'-untranslated region and the 129 bp 3'-untranslated region. The complete cDNA encodes 1937 amino acids. Expression of the chick ventricular MyHC gene was also studied by Northern blot analysis. This gene continued to be strongly expressed in the ventricle during cardiac development. On the other hand, its expression was moderate in the early embryonic atria, and was down-regulated during development. In the adult atria, this gene was expressed at very low levels. To determine the localization of the ventricular MyHC protein, an immunohistochemical study was performed. The ventricular MyHC was present in early embryonic atrial myocytes. During development, the expression of this protein in the atrial myocytes was down-regulated, but continued to be present in the atrial conduction system. Our results indicate that the ventricular MyHC appears in the primary atrial myocardium and is then localized in the conduction cells of the atria.

Neural crest-derived defects in experimental esophageal atresia

Esophageal atresia (EA) is often associated with cardiovascular and other malformations that are likely neural crest derived. The present study tests the hypothesis that the heart and great vessels and the thymus and parathyroids may be abnormal in the rat model of EA as a result of disturbed neural crest development. Time-mated pregnant rats received intraperitoneally on d 8 and 9 of gestation either 2 mg/kg adriamycin or vehicle. Esophageal, heart, and thymic malformations were sought under the microscope in term fetuses. The parathyroids were histologically investigated. Control fetuses had no malformations, whereas 69 of 109 fetuses exposed to adriamycin had EA and 45 of 69 had 15 right aortic arches, nine aberrant right subclavia, eight ventricular septal defects, six narrow pulmonary outflow tracts, five tetralogies of Fallot, three double outflow right ventricles, three double aortic arches, three atrial septal defects, three right ductus arteriosus, and two truncus. The thymus was absent in 19, hypoplastic in 12, and ectopic in five out of 36 fetuses with EA in which it was studied, whereas the parathyroid glands were absent in 16, single in four, and ectopic in one of the 23 fetuses with EA in which they were studied. In conclusion, the nature of the cardiovascular, thymic, and parathyroid malformations associated with EA in rats is consistent with the hypothesis of neural crest participation in their pathogenesis. Mechanisms simultaneously disturbing foregut septation, somitic segmentation, and neural crest development should be sought to explain the combined occurrence of malformations in EA.

Direct activation of a GATA6 cardiac enhancer by Nkx2.5: evidence for a reinforcing regulatory network of Nkx2.5 and GATA transcription factors in the developing heart

The zinc finger transcription factors GATA4, -5, and -6 and the homeodomain protein Nkx2.5 are expressed in the developing heart and have been shown to activate a variety of cardiac-specific genes. To begin to define the regulatory relationships between these cardiac transcription factors and to understand the mechanisms that control their expression during cardiogenesis, we analyzed the mouse GATA6 gene for regulatory elements sufficient to direct cardiac expression during embryogenesis. Using beta-galactosidase fusion constructs in transgenic mice, a 4.3-kb 5' regulatory region that directed transcription specifically in the cardiac lineage, beginning at the cardiac crescent stage, was identified. Thereafter, transgene expression became compartmentalized to the outflow tract, a portion of the right ventricle, and a limited region of the common atrial chamber of the embryonic heart. Further dissection of this regulatory region identified a 1.8-kb cardiac-specific enhancer that recapitulated the expression pattern of the larger region when fused to a heterologous promoter and a smaller 500-bp subregion that retained cardiac expression, but was quantitatively weaker. The GATA6 cardiac enhancer contained a binding site for Nkx2.5 that was essential for cardiac-specific expression in transgenic mice. These studies demonstrate that GATA6 is a direct target gene for Nkx2.5 in the developing heart and reveal a mutually reinforcing regulatory network of Nkx2.5 and GATA transcription factors during cardiogenesis.

Cardiac expression of the ventricle-specific homeobox gene Irx4 is modulated by Nkx2-5 and dHand

We report the isolation and characterization of the cDNAs encoded by the murine and human homeobox genes, Irx4 (Iroquois homeobox gene 4). Mouse and human Irx4 proteins are highly conserved (83%) and their 63-aa homeodomain is more than 93% identical to that of the Drosophila Iroquois patterning genes. Human IRX4 maps to chromosome 5p15.3, which is syntenic to murine chromosome 13. Irx4 transcripts are present in the developing central nervous system, skin, and vibrissae, but are predominantly expressed in the cardiac ventricles. In mice at embryonic day (E) 7.5, Irx4 transcripts are found in the chorion and at low levels in a discrete anterior domain of the cardiac primordia. During the formation of the linear heart tube and its subsequent looping (E8.0-8.5), Irx4 expression is restricted to the ventricular segment and is absent from both the posterior (eventual atrial) and the anterior (eventual outflow tract) segments of the heart. Throughout all subsequent stages in which the chambers of the heart become morphologically distinct (E8.5-11) and into adulthood, cardiac Irx4 expression is found exclusively in the ventricular myocardium. Irx4 gene expression was also assessed in embryos with aberrant cardiac development: mice lacking RXRalpha or MEF2c have normal Irx4 expression, but mice lacking the homeobox transcription factor Nkx2-5 (Csx) have markedly reduced levels of Irx4 transcripts. dHand-null embryos initiate Irx4 expression, but cannot maintain normal levels. These data indicate that the homeobox gene Irx4 is likely to be an important mediator of ventricular differentiation during cardiac development, which is downstream of Nkx2-5 and dHand.

An Nkx-dependent enhancer regulates cGATA-6 gene expression during early stages of heart development

The evolutionarily conserved GATA-6 transcription factor is an early and persistent marker of heart development in diverse vertebrate species. We previously found evidence for a functionally conserved heart-specific enhancer upstream of the chicken GATA-6 (cGATA-6) gene and in the present study we used transgenic mouse assays to further characterize this regulatory module. We show that this enhancer is activated in committed precursor cells within the cardiac crescent and that it remains active in essentially all cardiogenic cells through the linear heart stage. Although this enhancer can account for cGATA-6 gene expression early in the cardiogenic program, it is not able to maintain expression throughout the heart later in development. In particular, the enhancer is sequentially downregulated along the posterior to anterior axis, with activity becoming confined to outflow tract myocardium. Enhancers with similar properties have been shown to regulate the early heart-restricted expression of the mouse Nkx2.5 transcription factor gene. Whereas these Nkx2.5 enhancers are GATA-dependent, we show that the cGATA-6 enhancer is Nkx-dependent. We speculate that these enhancers are silenced to allow GATA-6 and Nkx2.5 gene expression to be governed by region-specific enhancers in the multichambered heart.

Induction and differentiation of the zebrafish heart requires fibroblast growth factor 8 (fgf8/acerebellar)

Vertebrate heart development is initiated from bilateral lateral plate mesoderm that expresses the Nkx2.5 and GATA4 transcription factors, but the extracellular signals specifying heart precursor gene expression are not known. We describe here that the secreted signaling factor Fgf8 is expressed in and required for development of the zebrafish heart precursors, particularly during initiation of cardiac gene expression. fgf8 is mutated in acerebellar (ace) mutants, and homozygous mutant embryos do not establish normal circulation, although vessel formation is only mildly affected. In contrast, heart development, in particular of the ventricle, is severely abnormal in acerebellar mutants. Several findings argue that Fgf8 has a direct function in development of cardiac precursor cells: fgf8 is expressed in cardiac precursors and later in the heart ventricle. Fgf8 is required for the earliest stages of nkx2.5 and gata4, but not gata6, expression in cardiac precursors. Cardiac gene expression is restored in acerebellar mutant embryos by injecting fgf8 RNA, or by implanting a Fgf8-coated bead into the heart primordium. Pharmacological inhibition of Fgf signalling during formation of the heart primordium phenocopies the acerebellar heart phenotype, confirming that Fgf signaling is required independently of earlier functions during gastrulation. These findings show that fgf8/acerebellar is required for induction and patterning of myocardial precursors.

Cardiac defects and renal failure in mice with targeted mutations in Pkd2

PKD2, mutations in which cause autosomal dominant polycystic kidney disease (ADPKD), encodes an integral membrane glycoprotein with similarity to calcium channel subunits. We induced two mutations in the mouse homologue Pkd2 (ref.4): an unstable allele (WS25; hereafter denoted Pkd2WS25) that can undergo homologous-recombination-based somatic rearrangement to form a null allele; and a true null mutation (WS183; hereafter denoted Pkd2-). We examined these mutations to understand the function of polycystin-2, the protein product of Pkd2, and to provide evidence that kidney and liver cyst formation associated with Pkd2 deficiency occurs by a two-hit mechanism. Pkd2-/- mice die in utero between embryonic day (E) 13.5 and parturition. They have structural defects in cardiac septation and cyst formation in maturing nephrons and pancreatic ducts. Pancreatic ductal cysts also occur in adult Pkd2WS25/- mice, suggesting that this clinical manifestation of ADPKD also occurs by a two-hit mechanism. As in human ADPKD, formation of kidney cysts in adult Pkd2WS25/- mice is associated with renal failure and early death (median survival, 65 weeks versus 94 weeks for controls). Adult Pkd2+/- mice have intermediate survival in the absence of cystic disease or renal failure, providing the first indication of a deleterious effect of haploinsufficiency at Pkd2on long-term survival. Our studies advance our understanding of the function of polycystin-2 in development and our mouse models recapitulate the complex human ADPKD phenotype.

Suppression of atrial myosin gene expression occurs independently in the left and right ventricles of the developing mouse heart

Many cardiac genes are broadly expressed in the early heart and become restricted to the atria or ventricles as development proceeds. Additional transcriptional differences between left and right compartments of the embryonic heart have been described recently, in particular for a number of transgenes containing cardiac regulatory elements. We now demonstrate that three myosin genes which become transcriptionally restricted to the atria between embryonic day (E) 12.5 and birth, alpha-myosin heavy chain (MHC), myosin light chain (MLC) 1A and MLC2A, are coordinately downregulated in the compact myocardium of the left ventricle before that of the right ventricle. alpha-MHC protein also accumulates in the right, but not left, compact ventricular myocardium during this period, suggesting that this transient regionalization contributes to fktal heart function. dHAND and eHAND, basic helix-loop-helix transcription factors known to be expressed in the right and left ventricles respectively at E10. 5, remain regionalized between E12.5 and E14.5. Downregulation of alpha-MHC, MLC1A, and MLC2A in iv/iv embryos, which have defective left/right patterning, initiates in the morphological left (systemic) ventricle regardless of its anatomical position on the right or left hand side of the heart. This points to the importance of left/right ventricular differences in sarcomeric gene expression patterns during fktal cardiogenesis and indicates that these differences originate in the embryo in response to anterior-posterior patterning of the heart tube rather than as a result of cardiac looping. Dev Dyn 2000;217:75-85.

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.

Modular regulation of cGATA-5 gene expression in the developing heart and gut

The evolutionarily conserved GATA-5 transcription factor is an early and persistent marker of heart and gut development in diverse vertebrate species. To search for control regions that might regulate the chicken GATA-5 (cGATA-5) gene, we assayed a set of cGATA-5/lacZ constructs in transgenic mice and found evidence for two functionally conserved control regions that regulate different facets of cGATA-5 gene expression. The more distal control region is activated in embryonic endoderm at the head-fold stage, whereas the other control region contains a regulatory module that is activated in a restricted region of endoderm following closure of the gut tube. Remarkably, the latter control region also contains a complex regulatory module that is activated in the cardiac crescent at the head-fold stage and subsequently functions in several mesodermal components of the developing heart, including the outer (epicardial) layer. We discuss these results in terms of possible contributions of epicardial-derived cells to the formation of heart valves, conduction tissue, and compact myocardium. These transgenes thus reveal, and provide a means to further analyze, transcriptional programs for several facets of heart morphogenesis and gut development.

Synergistic roles of neuregulin-1 and insulin-like growth factor-I in activation of the phosphatidylinositol 3-kinase pathway and cardiac chamber morphogenesis

Cardiac chamber morphogenesis requires the coordinated growth of both cardiac muscle and endocardial cell lineages. Paracrine growth factors may modulate the coordinated cellular specification and differentiation during cardiac chamber morphogenesis, as suggested by the essential role of endothelial-derived growth factors, neuregulin-1, and insulin-like growth factor-I. Using the whole mouse embryo culture system for delivery of diffusible factors into the cardiac chamber, neuregulin-1 was shown to promote trabeculation of the ventricular wall. Another factor, insulin-like growth factor-I, had no apparent effect by itself. Combined treatment with neuregulin-1 and insulin-like growth factor-I strongly induced DNA synthesis of cardiomyocytes and expansion of both the ventricular compact zone and the atrioventricular cushions leading to chamber growth and maturation. In cultured cardiomyocytes, combined neuregulin-1 and insulin-like growth factor-I also had a synergistic effect to promote DNA synthesis and cellular growth, which were prevented by wortmannin, an inhibitor of phosphatidylinositol 3-kinase. Adenoviral delivery of dominant negative Rac1, which acts downstream of phosphatidylinositol 3-kinase, blocked the effect of combined neuregulin-1/insulin-like growth factor-I treatment. These studies support the concept that the interaction of neuregulin-1 and insulin-like growth factor-I pathways plays an important role in coordinating cardiac chamber morphogenesis and may occur through convergent activation of phosphatidylinositol 3-kinase.

An intrinsic timer that controls cell-cycle withdrawal in cultured cardiac myocytes

Developing cardiac myocytes divide a limited number of times before they stop and terminally differentiate, but the mechanism that stops their division is unknown. To help study the stopping mechanism, we defined conditions under which embryonic rat cardiac myocytes cultured in serum-free medium proliferate and exit the cell cycle on a schedule that closely resembles that seen in vivo. The culture medium contains FGF-1 and FGF-2, which stimulate cell proliferation, and thyroid hormone, which seems to be necessary for stable cell-cycle exit. Time-lapse video recording shows that the cells within a clone tend to divide a similar number of times before they stop, whereas cells in different clones divide a variable number of times before they stop. Cells cultured at 33 degrees C divide more slowly but stop dividing at around the same time as cells cultured at 37 degrees C, having undergone fewer divisions. Together, these findings suggest that an intrinsic timer helps control when cardiac myocytes withdraw from the cell cycle and that the timer does not operate by simply counting cell divisions. We provide evidence that the cyclin-dependent kinase inhibitors p18 and p27 may be part of the timer and that thyroid hormone may help developing cardiac myocytes stably withdraw from the cell cycle.

The zinc finger proteins Pannier and GATA4 function as cardiogenic factors in Drosophila

The regulation of cardiac gene expression by GATA zinc finger transcription factors is well documented in vertebrates. However, genetic studies in mice have failed to demonstrate a function for these proteins in cardiomyocyte specification. In Drosophila, the existence of a cardiogenic GATA factor has been implicated through the analysis of a cardial cell enhancer of the muscle differentiation gene D-mef2. We show that the GATA gene pannier is expressed in the dorsal mesoderm and required for cardial cell formation while repressing a pericardial cell fate. Ectopic expression of Pannier results in cardial cell overproduction, while co-expression of Pannier and the homeodomain protein Tinman synergistically activate cardiac gene expression and induce cardial cells. The related GATA4 protein of mice likewise functions as a cardiogenic factor in Drosophila, demonstrating an evolutionarily conserved function between Pannier and GATA4 in heart development.

Mutations in the cardiac transcription factor NKX2.5 affect diverse cardiac developmental pathways

Heterozygous mutations in NKX2.5, a homeobox transcription factor, were reported to cause secundum atrial septal defects and result in atrioventricular (AV) conduction block during postnatal life. To further characterize the role of NKX2.5 in cardiac morphogenesis, we sought additional mutations in groups of probands with cardiac anomalies and first-degree AV block, idiopathic AV block, or tetralogy of Fallot. We identified 7 novel mutations by sequence analysis of the NKX2.5-coding region in 26 individuals. Associated phenotypes included AV block, which was the primary manifestation of cardiac disease in nearly a quarter of affected individuals, as well as atrial septal defect and ventricular septal defect. Ventricular septal defect was associated with tetralogy of Fallot or double-outlet right ventricle in 3 individuals. Ebstein's anomaly and other tricuspid valve abnormalities were also present. Mutations in human NKX2.5 cause a variety of cardiac anomalies and may account for a clinically significant portion of tetralogy of Fallot and idiopathic AV block. The coinheritance of NKX2.5 mutations with various congenital heart defects suggests that this transcription factor contributes to diverse cardiac developmental pathways.

HRT1, HRT2, and HRT3: a new subclass of bHLH transcription factors marking specific cardiac, somitic, and pharyngeal arch segments

Members of the Hairy/Enhancer of Split family of basic helix-loop-helix (bHLH) transcription factors are regulated by the Notch signaling pathway in vertebrate and Drosophila embryos and control cell fates and establishment of sharp boundaries of gene expression. Here, we describe a new subclass of bHLH proteins, HRT1 (Hairy-related transcription factor 1), HRT2, and HRT3, that share high homology with the Hairy family of proteins yet have characteristics that are distinct from those of Hairy and other bHLH proteins. Each HRT gene was expressed in distinct cell types within numerous organs, particularly in those patterned along the anterior-posterior axis. HRT1 and HRT2 were expressed in atrial and ventricular precursors, respectively, and were also expressed in the cardiac outflow tract and aortic arch arteries. HRT1 and HRT2 transcripts were also detected in precursors of the pharyngeal arches and subsequently in the pharyngeal clefts. Within somitic precursors, HRT1 and HRT3 exhibited dynamic expression in the presomitic mesoderm, mirroring the expression of other components of Notch-Delta signaling pathways. The HRT genes were expressed in other sites of epithelial-mesenchymal interactions, including the developing kidneys, brain, limb buds, and vasculature. The unique and complementary expression patterns of this novel subfamily of bHLH proteins suggest a previously unrecognized role for Hairy-related pathways in segmental patterning of the heart and pharyngeal arches, among other organs.

Transcription of the myogenic regulatory gene Mef2 in cardiac, somatic, and visceral muscle cell lineages is regulated by a Tinman-dependent core enhancer

The MADS-box transcription factor MEF2 is expressed specifically in developing cardiac, somatic, and visceral muscle cell lineages during Drosophila embryogenesis and is required for myoblast differentiation and muscle morphogenesis. To define the mechanisms that regulate Mef2 transcription, we have analyzed the Mef2 upstream region for sequences sufficient to recapitulate the expression pattern of the gene in Drosophila embryos. Here we describe a complex enhancer located 5.8 kb upstream of the Drosophila Mef2 gene that controls transcription in cardial cells of the dorsal vessel, a subset of somatic muscle founder cells, and the visceral muscle cells. The core of this enhancer contains two evolutionarily conserved binding sites for the homeodomain protein Tinman (Tin), expressed in developing cardiac, somatic, and visceral muscle lineages. Both Tin binding sites are required for enhancer activity in all three muscle cell lineages. Whereas the 285-bp enhancer core alone is sufficient for expression in cardiac cells, expression in somatic founder cells and visceral muscle is dependent on the core enhancer plus unique flanking sequences that include an evolutionarily conserved E box. These results reveal an essential role for Tin in activation of Mef2 transcription in multiple myogenic lineages and demonstrate that transcriptional activity of Tin is dependent on combinatorial interactions with other factors unique to different muscle cell types.

Radiographic evaluation of the neonate with congenital heart disease

Remarkable advances in pediatric cardiology have been spurred by the explosion of technologies both in interventional and surgical techniques and the ability to manipulate the genome of experimental animals. After a brief discussion concerning the striking advances in the molecular understanding of congenital heart disease, this article focuses on clues to the diagnosis of congenital heart disease and on chest radiography and common, specific lesions of the neonate such as hypoplastic left heart, transposition of the great vessels, and severe tetralogy of Fallot. The impact of treatment protocols involving interventional cardiology in the neonate also are discussed.

MEF2 is upregulated during cardiac hypertrophy and is required for normal post-natal growth of the myocardium

In mammals, growth of the fetal heart is regulated by proliferation of cardiac muscle cells. At later stages of pre-natal life, this proliferation diminishes profoundly [1] [2] and the dramatic expansion in heart size during the transition to adulthood is due exclusively to hypertrophy of individual cardiomyocytes [3] [4] [5]. Cardiomyocyte hypertrophy also contributes to the pathology of most post-natal heart disease [6] [7] [8] [9] [10]. Within this context, numerous signal transduction pathways have been implicated as the link between the effector(s) and altered cardiac gene expression [11] [12] [13] [14] [15] [16]. A common pathway has yet to be discovered, however. Here, we found that the activity of the stress-activated kinase p38 was enhanced in both types of cardiomyocyte hypertrophy. We also found that a target of the activated p38 kinase is the cardiac transcription factor MEF2. Transgenic mice expressing a dominant-negative form of MEF2C displayed attenuated post-natal growth of the myocardium. These results provide the first evidence for a single pathway regulating both normal and pathologic cardiomyocyte hypertrophy.

Bone morphogenetic proteins induce cardiomyocyte differentiation through the mitogen-activated protein kinase kinase kinase TAK1 and cardiac transcription factors Csx/Nkx-2.5 and GATA-4

Bone morphogenetic proteins (BMPs) have been shown to induce ectopic expression of cardiac transcription factors and beating cardiomyocytes in nonprecardiac mesodermal cells in chicks, suggesting that BMPs are inductive signaling molecules that participate in the development of the heart. However, the precise molecular mechanisms by which BMPs regulate cardiac development are largely unknown. In the present study, we examined the molecular mechanisms by which BMPs induce cardiac differentiation by using the P19CL6 in vitro cardiomyocyte differentiation system, a clonal derivative of P19 embryonic teratocarcinoma cells. We established a permanent P19CL6 cell line, P19CL6noggin, which constitutively overexpresses the BMP antagonist noggin. Although almost all parental P19CL6 cells differentiate into beating cardiomyocytes when treated with 1% dimethyl sulfoxide, P19CL6noggin cells did not differentiate into beating cardiomyocytes nor did they express cardiac transcription factors or contractile protein genes. The failure of differentiation was rescued by overexpression of BMP-2 or addition of BMP protein to the culture media, indicating that BMPs were indispensable for cardiomyocyte differentiation in this system. Overexpression of TAK1, a member of the mitogen-activated protein kinase kinase kinase superfamily which transduces BMP signaling, restored the ability of P19CL6noggin cells to differentiate into cardiomyocytes and concomitantly express cardiac genes, whereas overexpression of the dominant negative form of TAK1 in parental P19CL6 cells inhibited cardiomyocyte differentiation. Overexpression of both cardiac transcription factors Csx/Nkx-2.5 and GATA-4 but not of Csx/Nkx-2.5 or GATA-4 alone also induced differentiation of P19CL6noggin cells into cardiomyocytes. These results suggest that TAK1, Csx/Nkx-2.5, and GATA-4 play a pivotal role in the cardiogenic BMP signaling pathway.

PPAR gamma is required for placental, cardiac, and adipose tissue development

The nuclear hormone receptor PPAR gamma promotes adipogenesis and macrophage differentiation and is a primary pharmacological target in the treatment of type II diabetes. Here, we show that PPAR gamma gene knockout results in two independent lethal phases. Initially, PPAR gamma deficiency interferes with terminal differentiation of the trophoblast and placental vascularization, leading to severe myocardial thinning and death by E10.0. Supplementing PPAR gamma null embryos with wild-type placentas via aggregation with tetraploid embryos corrects the cardiac defect, implicating a previously unrecognized dependence of the developing heart on a functional placenta. A tetraploid-rescued mutant surviving to term exhibited another lethal combination of pathologies, including lipodystrophy and multiple hemorrhages. These findings both confirm and expand the current known spectrum of physiological functions regulated by PPAR gamma.

Building the heart piece by piece: modularity of cis-elements regulating Nkx2-5 transcription

Heart formation in Drosophila is dependent on the homeobox gene tinman. The homeobox gene Nkx2-5 is closely related to tinman and is the earliest known marker for cardiogenesis in vertebrate embryos. Recent studies of cis-regulatory elements required for Nkx2-5 expression in the developing mouse heart have revealed an extraordinary array of independent cardiac enhancers, and associated negative regulatory elements, that direct transcription in distinct regions of the embryonic heart. These studies demonstrate the modularity in cardiac transcription, in which different regulatory elements respond to distinct sets of transcription factors to control gene expression in different compartments of the developing heart. We consider the potential mechanisms underlying such transcriptional complexity, its possible significance for cardiac function, and the implications for evolution of the multichambered heart.

Control of segmental expression of the cardiac-restricted ankyrin repeat protein gene by distinct regulatory pathways in murine cardiogenesis

Although accumulating evidence suggests that the heart develops in a segmental fashion, the molecular mechanisms that control regional specification of cardiomyocytes in the developing heart remain largely unknown. In this study, we have used the mouse cardiac-restricted ankyrin repeat protein (CARP) gene as a model system to study these mechanisms. The CARP gene encodes a nuclear co-regulator for cardiac gene expression, which lies downstream of the cardiac homeobox gene, Nkx 2.5, and is an early marker of the cardiac muscle cell lineage. We have demonstrated that the expression of the gene is developmentally down regulated and dramatically induced as part of the embryonic gene program during cardiac hypertrophy. Using a lacZ/knock-in mouse and three lines of transgenic mouse harboring various CARP promoter/lacZ reporters, we have identified distinct 5′ cis regulatory elements of the gene that can direct heart segment-specific transgene expression, such as atrial versus ventricular and left versus right. Most interestingly, a 213 base pair sequence element of the gene was found to confer conotruncal segment-specific transgene expression. Using the transgene as a conotruncal segment-specific marker, we were able to document the developmental fate of a subset of cardiomyocytes in the conotruncus during cardiogenesis. In addition, we have identified an essential GATA-4 binding site in the proximal upstream regulatory region of the gene and cooperative transcriptional regulation mediated by Nkx2.5 and GATA-4. We have shown that this cooperative regulation is dependent on binding of GATA-4 to its cognate DNA sequence in the promoter, which suggests that Nkx2.5 controls CARP expression, at least in part, through GATA-4.

The novel Cer-like protein Caronte mediates the establishment of embryonic left-right asymmetry

In the chick embryo, left-right asymmetric patterns of gene expression in the lateral plate mesoderm are initiated by signals located in and around Hensen's node. Here we show that Caronte (Car), a secreted protein encoded by a member of the Cerberus/Dan gene family, mediates the Sonic hedgehog (Shh)-dependent induction of left-specific genes in the lateral plate mesoderm. Car is induced by Shh and repressed by fibroblast growth factor-8 (FGF-8). Car activates the expression of Nodal by antagonizing a repressive activity of bone morphogenic proteins (BMPs). Our results define a complex network of antagonistic molecular interactions between Activin, FGF-8, Lefty-1, Nodal, BMPs and Car that cooperate to control left-right asymmetry in the chick embryo.

Roles for the winged helix transcription factors MF1 and MFH1 in cardiovascular development revealed by nonallelic noncomplementation of null alleles

The murine Mf1 and Mfh1 genes have overlapping patterns of expression in the embryo and encode forkhead/winged helix transcription factors with virtually identical DNA binding domains. Previous studies have shown that Mfh1 null mutants have severe cardiovascular defects, including interruptions and coarctations of the aortic arch and ventricular septal defects (Iida et al., Development 124, 4627-4638, 1997). Here, we show that Mf1(lacZ) homozygous null mutants also have a similar spectrum of cardiovascular abnormalities. Moreover, most embryos doubly heterozygous for Mfh1(tm1) and Mf1(lacZ) die before birth with interruptions and coarctations of the aortic arch, dysgenesis of the aortic and pulmonary valves, ventricular septal defects, and other cardiac anomalies. This nonallelic noncomplementation and the similar patterns of expression of the two genes in the mesenchyme and endothelial cells of the branchial arches, outflow tract, and heart suggest that Mf1 and Mfh1 play interactive roles in the morphogenesis of the cardiovascular system. Implications for the development of human congenital heart defects are discussed.

Cardiovascular malformations in congenital diaphragmatic hernia: human and experimental studies

BACKGROUND/PURPOSE

Cardiovascular malformations (CVM) associated with congenital diaphragmatic hernia (CDH) account in part for the high mortality caused by this defect. The aim of this study is to examine the nature of these malformations in a large series of autopsies and to assess if similar defects are also present in rat fetuses with experimental CDH.

METHODS

The incidence of CVM and their nature were examined in the autopsy records of 136 stillborns and neonates with CDH admitted to our institution in the last 30 years. Experimental CDH was induced in rat fetuses by giving 100 mg of nitrofen to their mothers on gestational day 9.5, and the fetuses were harvested on day 21 (near full term). The presence of CDH and the anatomy of the heart and great vessels were studied under dissecting microscope after formalin fixation. Unexposed fetuses were used as controls.

RESULTS

Thirty-three newborns with CDH (24%) had CVM, either isolated or associated with other defects, and 7 had heart hypoplasia. Most CVM (ventricular septal defect, tetralogy of Fallot, transposition of the great vessels, double-outlet right ventricle) involved the outflow tract. In our animal experiments, no malformations were found in 21 control pups. Conversely, 80 of 130 nitrofen-exposed fetuses (61%) had CDH, and 59 of them (74%) had CVM. A significant association (Fisher's Exact test, P<.01) was found between CDH and CVM because only 25 of the 50 exposed animals without CDH (50%) had CVM. Again, most defects involved the outflow tract and were similar to those seen in human CDH (tetralogy of Fallot, persistent truncus, ventricular septal defect, double-outlet right ventricle, aberrant right subclavian artery, agenetic ductus, and interrupted aortic arch). Animals with CDH had significantly decreased heart weight to fetal weight ratio in comparison with controls and with those without CDH.

CONCLUSIONS

The similar nature of the cardiovascular defects found in babies succumbing to CDH and in nitrofen-exposed rats suggests that a similar disturbance of the regional organogenesis related to the neural crest might be involved in both settings, and further validates the use of this animal model for clarifying the cellular and molecular pathogenetic mechanisms.

The Drosophila homeobox genes zfh-1 and even-skipped are required for cardiac-specific differentiation of a numb-dependent lineage decision

A series of inductive signals are necessary to subdivide the mesoderm in order to allow the formation of the progenitor cells of the heart. Mesoderm-endogenous transcription factors, such as those encoded by twist and tinman, seem to cooperate with these signals to confer correct context and competence for a cardiac cell fate. Additional factors are likely to be required for the appropriate specification of individual cell types within the forming heart. Similar to tinman, the zinc finger- and homeobox-containing gene, zfh-1, is expressed in the early mesoderm and later in the forming heart, suggesting a possible role in heart development. Here, we show that zfh-1 is specifically required for formation of the even-skipped (eve)-expressing subset of pericardial cells (EPCs), without affecting the formation of their siblings, the founders of a dorsal body wall muscle (DA1). In addition to zfh-1, mesodermal eve itself appears to be needed for correct EPC differentiation, possibly as a direct target of zfh-1. Epistasis experiments show that zfh-1 specifies EPC development independently of numb, the lineage gene that controls DA1 founder versus EPC cell fate. We discuss the combinatorial control mechanisms that specify the EPC cell fate in a spatially precise pattern within the embryo.

The mammalian Tolloid-like 1 gene, Tll1, is necessary for normal septation and positioning of the heart

Mammalian Tolloid-like 1 (mTLL-1) is an astacin-like metalloprotease, highly similar in domain structure to the morphogenetically important proteases bone morphogenetic protein-1 (BMP-1) and Drosophila Tolloid. To investigate possible roles for mTLL-1 in mammalian development, we have used gene targeting in ES cells to produce mice with a disrupted allele for the corresponding gene, Tll1. Homozygous mutants were embryonic lethal, with death at mid-gestation from cardiac failure and a unique constellation of developmental defects that were apparently confined solely to the heart. Constant features were incomplete formation of the muscular interventricular septum and an abnormal and novel positioning of the heart and aorta. Consistent with roles in cardiac development, Tll1 expression was specific to precardiac tissue and endocardium in 7.5 and 8.5 days p.c. embryos, respectively. Tll1 expression was also high in the developing interventricular septum, where expression of the BMP-1 gene, Bmp1, was not observed. Cardiac structures that were not affected in Tll1−/− embryos either showed no Tll1 expression (atrio-ventricular cushions) or showed overlapping expression of Tll1 and Bmp1 (aortico-pulmonary septum), suggesting that products of the Bmp1 gene may be capable of functionally substituting for mTLL-1 at sites in which they are co-expressed. Together, the various data show that mTLL-1 plays multiple roles in formation of the mammalian heart and is essential for formation of the interventricular septum.

Functions of c-Jun in liver and heart development

Mice lacking the AP-1 transcription factor c-Jun die around embryonic day E13.0 but little is known about the cell types affected as well as the cause of embryonic lethality. Here we show that a fraction of mutant E13.0 fetal livers exhibits extensive apoptosis of both hematopoietic cells and hepatoblasts, whereas the expression of 15 mRNAs, including those of albumin, keratin 18, hepatocyte nuclear factor 1, beta-globin, and erythropoietin, some of which are putative AP-1 target genes, is not affected. Apoptosis of hematopoietic cells in mutant livers is most likely not due to a cell-autonomous defect, since c-jun-/- fetal liver cells are able to reconstitute all hematopoietic compartments of lethally irradiated recipient mice. A developmental analysis of chimeras showed contribution of c-jun-/- ES cell derivatives to fetal, but not to adult livers, suggesting a role of c-Jun in hepatocyte turnover. This is in agreement with the reduced mitotic and increased apoptotic rates found in primary liver cell cultures derived from c-jun-/- fetuses. Furthermore, a novel function for c-Jun was found in heart development. The heart outflow tract of c-jun-/- fetuses show malformations that resemble the human disease of a truncus arteriosus persistens. Therefore, the lethality of c-jun mutant fetuses is most likely due to pleiotropic defects reflecting the diversity of functions of c-Jun in development, such as a role in neural crest cell function, in the maintenance of hepatic hematopoiesis and in the regulation of apoptosis.

Combinatorial interactions regulate cardiac expression of the murine adenylosuccinate synthetase 1 gene

The mammalian heart begins contracting at the linear tube stage during embryogenesis and continuously pumps, nonstop, throughout the entire lifetime of the animal. Therefore, the cardiac energy metabolizing pathways must be properly established and efficiently functioning. While the biochemistry of these pathways is well defined, limited information regarding the regulation of cardiac metabolic genes is available. Previously, we reported that 1.9 kilobase pairs of murine adenylosuccinate synthetase 1 gene (Adss1) 5'-flanking DNA directs high levels of reporter expression to the adult transgenic heart. In this report, we define the 1.9-kilobase pair fragment as a cardiac-specific enhancer that controls correct spatiotemporal expression of a reporter similar to the endogenous Adss1 gene. A 700-base pair fragment within this region activates a heterologous promoter specifically in adult transgenic hearts. Proteins present in a cardiac nuclear extract interact with potential transcription factor binding sites of this region and these cis-acting sites play important regulatory roles in the cardiac expression of this reporter. Finally, we report that several different cardiac transcription factors trans-activate the 1.9HSCAT construct through these sites and that combinations result in enhanced reporter expression. Adss1 appears to be one of the first target genes identified for the bHLH factors Hand1 and Hand2.

Developmental and genetic aspects of congenital heart disease

Congenital heart defects (CHDs) are the result of abnormal cardiac mesoderm or cardiac neural crest development. The molecular cause of most congenital heart disease remains unknown, although numerous cardiac regulatory factors have recently been described. dHAND and eHAND are basic helix-loop-helix transcription factors expressed differentially in the right and left ventricles, respectively, and in the cardiac neural crest. Mice lacking dHAND have a hypoplastic right ventricle and abnormal development of vessels arising from the heart and cell death of craniofacial precursors. By searching for dHAND-dependent genes, a gene likely responsible for the cardiac and craniofacial defects associated with chromosome 22q11 deletion has been identified. A systematic dissection of molecular pathways involved in cardiogenesis should allow for further identification of genes responsible for CHD.

Familial atrial septal defect and atrioventricular conduction disturbance associated with a point mutation in the cardiac homeobox gene CSX/NKX2-5 in a Japanese patient

Atrial septal defect (ASD) is the most common form of congenital cardiac defect in humans. Recently, point mutations in the cardiac homeobox gene CSX/NKX2-5 have been reported to cause the autosomal dominant form of familial ASD. Notably, all the affected patients exhibit atrioventricular conduction disturbance and some of them died suddenly. The first case of familial ASD with a mutation of the CSX/NKX2-5 gene in a Japanese patient is reported here. Identification of CSX/NKX2-5 mutations in ASD patients would be very important because the existence of such mutations may predict sudden cardiac death.

The orphan nuclear receptor COUP-TFII is required for angiogenesis and heart development

The embryonic expression of COUP-TFII, an orphan nuclear receptor, suggests that it may participate in mesenchymal-epithelial interactions required for organogenesis. Targeted deletion of the COUP-TFII gene results in embryonic lethality with defects in angiogenesis and heart development. COUP-TFII mutants are defective in remodeling the primitive capillary plexus into large and small microcapillaries. In the COUP-TFII mutant heart, the atria and sinus venosus fail to develop past the primitive tube stage. Reciprocal interactions between the endothelium and the mesenchyme in the vascular system and heart are essential for normal development of these systems. In fact, the expression of Angiopoietin-1, a proangiogenic soluble factor thought to mediate the mesenchymal-endothelial interactions during heart development and vascular remodeling, is down-regulated in COUP-TFII mutants. This down-regulation suggests that COUP-TFII may be required for bidirectional signaling between the endothelial and mesenchymal compartments essential for proper angiogenesis and heart development.

Embryonic retinoic acid synthesis is essential for early mouse post-implantation development

A number of studies have suggested that the active derivative of vitamin A, retinoic acid (RA), may be important for early development of mammalian embryos. Severe vitamin A deprivation in rodents results in maternal infertility, precluding a thorough investigation of the role of RA during embryogenesis. Here we show that production of RA by the retinaldehyde dehydrogenase-2 (Raldh2) enzyme is required for mouse embryo survival and early morphogenesis. Raldh2 is an NAD-dependent aldehyde dehydrogenase with high substrate specificity for retinaldehyde. Its pattern of expression during mouse development has suggested that it may be responsible for embryonic RA synthesis. We generated a targeted disruption of the mouse Raldh2 gene and found that Raldh2-/- embryos, which die at midgestation without undergoing axial rotation (body turning), exhibit shortening along the anterioposterior axis and do not form limb buds. Their heart consists of a single, medial, dilated cavity. Their frontonasal region is truncated and their otocysts are severely reduced. These defects result from a block in embryonic RA synthesis, as shown by the lack of activity of RA-responsive transgenes, the altered expression of an RA-target homeobox gene and the near full rescue of the mutant phenotype by maternal RA administration. Our data establish that RA synthesized by the post-implantation mammalian embryo is an essential developmental hormone whose lack leads to early embryo death.

Myoblast cell grafting into heart muscle: cellular biology and potential applications

This review surveys a wide range of cellular and molecular approaches to strengthening the injured or weakened heart, focusing on strategies to replace dysfunctional, necrotic, or apoptotic cardiomyocytes with new cells of mesodermal origin. A variety of cell types, including myogenic cell lines, adult skeletal myoblasts, immoratalized atrial cells, embryonic and adult cardiomyocytes, embryonic stem cells, tetratoma cells, genetically altered fibroblasts, smooth muscle cells, and bone marrow-derived cells have all been proposed as useful cells in cardiac repair and may have the capacity to perform cardiac work. We focus on the implantation of mesodermally derived cells, the best developed of the options. We review the developmental and cell biology that have stimulated these studies, examine the limitations of current knowledge, and identify challenges for the future, which we believe are considerable.

Context-dependent transcriptional cooperation mediated by cardiac transcription factors Csx/Nkx-2.5 and GATA-4

Although the cardiac homeobox gene Csx/Nkx-2.5 is essential for normal heart development, little is known about its regulatory mechanisms. In a search for the downstream target genes of Csx/Nkx-2. 5, we found that the atrial natriuretic peptide (ANP) gene promoter was strongly transactivated by Csx/Nkx-2.5. Deletion and mutational analyses of the ANP promoter revealed that the Csx/Nkx-2.5-binding element (NKE2) located at -240 was required for high level transactivation by Csx/Nkx-2.5. We also found that Csx/Nkx-2.5 and GATA-4 displayed synergistic transcriptional activation of the ANP promoter, and in contrast to previous reports (Durocher, D., Charron, F., Warren, R., Schwartz, R. J., and Nemer, M. (1997) EMBO J. 16, 5687-5696; Lee, Y., Shioi, T., Kasahara, H., Jobe, S. M., Wiese, R. J., Markham, B., and Izumo, S (1998) Mol. Cell. Biol. 18, 3120-3129), this synergism was dependent on binding of Csx/Nkx-2.5 to NKE2, but not on GATA-4-DNA interactions. Although GATA-4 also potentiated the Csx/Nkx-2.5-induced transactivation of the artificial promoter that contains multimerized Csx/Nkx-2.5-binding sites, Csx/Nkx-2.5 reduced the GATA-4-induced transactivation of the GATA-4-dependent promoters. These findings indicate that the cooperative transcriptional regulation mediated by Csx/Nkx-2.5 and GATA-4 is promoter context-dependent and suggest that the complex cis-trans interactions may fine-tune gene expression in cardiac myocytes.

The cardiac homeobox gene Csx/Nkx2.5 lies genetically upstream of multiple genes essential for heart development

Csx/Nkx2.5 is a vertebrate homeobox gene with a sequence homology to the Drosophila tinman, which is required for the dorsal mesoderm specification. Recently, heterozygous mutations of this gene were found to cause human congenital heart disease (Schott, J.-J., Benson, D. W., Basson, C. T., Pease, W., Silberbach, G. M., Moak, J. P., Maron, B. J., Seidman, C. E. and Seidman, J. G. (1998) Science 281, 108–111). To investigate the functions of Csx/Nkx2.5 in cardiac and extracardiac development in the vertebrate, we have generated and analyzed mutant mice completely null for Csx/Nkx2.5. Homozygous null embryos showed arrest of cardiac development after looping and poor development of blood vessels. Moreover, there were severe defects in vascular formation and hematopoiesis in the mutant yolk sac. Interestingly, TUNEL staining and PCNA staining showed neither enhanced apoptosis nor reduced cell proliferation in the mutant myocardium. In situ hybridization studies demonstrated that, among 20 candidate genes examined, expression of ANF, BNP, MLC2V, N-myc, MEF2C, HAND1 and Msx2 was disturbed in the mutant heart. Moreover, in the heart of adult chimeric mice generated from Csx/Nkx2.5 null ES cells, there were almost no ES cell-derived cardiac myocytes, while there were substantial contributions of Csx /Nkx2.5-deficient cells in other organs. Whole-mount β-gal staining of chimeric embryos showed that more than 20% contribution of Csx/Nkx2. 5-deficient cells in the heart arrested cardiac development. These results indicate that (1) the complete null mutation of Csx/Nkx2.5 did not abolish initial heart looping, (2) there was no enhanced apoptosis or defective cell cycle entry in Csx/Nkx2.5 null cardiac myocytes, (3) Csx/Nkx2.5 regulates expression of several essential transcription factors in the developing heart, (4) Csx/Nkx2.5 is required for later differentiation of cardiac myocytes, (5) Csx/Nkx2. 5 null cells exert dominant interfering effects on cardiac development, and (6) there were severe defects in yolk sac angiogenesis and hematopoiesis in the Csx/Nkx2.5 null embryos.

Calreticulin is essential for cardiac development

Calreticulin is a ubiquitous Ca2+ binding protein, located in the endoplasmic reticulum lumen, which has been implicated in many diverse functions including: regulation of intracellular Ca2+ homeostasis, chaperone activity, steroid-mediated gene regulation, and cell adhesion. To understand the physiological function of calreticulin we used gene targeting to create a knockout mouse for calreticulin. Mice homozygous for the calreticulin gene disruption developed omphalocele (failure of absorption of the umbilical hernia) and showed a marked decrease in ventricular wall thickness and deep intertrabecular recesses in the ventricular walls. Transgenic mice expressing a green fluorescent protein reporter gene under the control of the calreticulin promoter were used to show that the calreticulin gene is highly activated in the cardiovascular system during the early stages of cardiac development. Calreticulin protein is also highly expressed in the developing heart, but it is only a minor component of the mature heart. Bradykinin-induced Ca2+ release by the InsP3-dependent pathway was inhibited in crt-/- cells, suggesting that calreticulin plays a role in Ca2+ homeostasis. Calreticulin-deficient cells also exhibited impaired nuclear import of nuclear factor of activated T cell (NF-AT3) transcription factor indicating that calreticulin plays a role in cardiac development as a component of the Ca2+/calcineurin/NF-AT/GATA-4 transcription pathway.

Congenital heart disease: six decades of progress

Congenital heart malformations occur in approximately 1 in 100 live births. Since Robert Gross successfully ligated a persistent ductus arteriosus 60 years ago, there has been enormous progress in the surgical management of even the most complex lesions. More recently, the interventional cardiologist armed with balloons, stents, coils, umbrellas, and laser beams is providing an alternative to surgery for many lesions. Echocardiography combined with Doppler studies is now the most informative diagnostic modality, and its usefulness is being expanded by transesophageal studies. Magnetic resonance imaging is helpful for some lesions not well visualized on echo. Some complex lesions continue to challenge caregivers, especially the hypoplastic left heart syndrome and several variants of pulmonary atresia. Neurological complications in patients with congenital heart disease are not uncommon and are the main theme of this issue.

Molecular advances in cardiac and cardiovascular disease

Recent advances in the field of molecular biology have led to a better understanding of the pathological mechanisms of cardiovascular disease. The impact of these findings will shape the future of treatment modalities for cardiovascular disorders. Postulated targets and biological rationale of new techniques are being developed in a race towards molecular therapies for vascular diseases. Whether it is modulation of transmembrane cell receptors or phenotypic changes via vectors that mediate gene transfer, there is no doubt that molecular strategies will be an integral part of the future. Here we examine past and recent perspectives, describe directions and challenges in cardiac and cardiovascular areas of research, and discuss relevance to the field of cardiovascular perfusion.

Cardiomyocytes can be generated from marrow stromal cells in vitro

We have isolated a cardiomyogenic cell line (CMG) from murine bone marrow stromal cells. Stromal cells were immortalized, treated with 5-azacytidine, and spontaneously beating cells were repeatedly screened. The cells showed a fibroblast-like morphology, but the morphology changed after 5-azacytidine treatment in approximately 30% of the cells; they connected with adjoining cells after one week, formed myotube-like structures, began spontaneously beating after two weeks, and beat synchronously after three weeks. They expressed atrial natriuretic peptide and brain natriuretic peptide and were stained with anti-myosin, anti-desmin, and anti-actinin antibodies. Electron microscopy revealed a cardiomyocyte-like ultrastructure, including typical sarcomeres, a centrally positioned nucleus, and atrial granules. These cells had several types of action potentials, such as sinus node-like and ventricular cell-like action potentials. All cells had a long action potential duration or plateau, a relatively shallow resting membrane potential, and a pacemaker-like late diastolic slow depolarization. Analysis of the isoform of contractile protein genes, such as myosin heavy chain, myosin light chain, and alpha-actin, indicated that their muscle phenotype was similar to that of fetal ventricular cardiomyocytes. These cells expressed Nkx2.5/Csx, GATA4, TEF-1, and MEF-2C mRNA before 5-azacytidine treatment and expressed MEF-2A and MEF-2D after treatment. This new cell line provides a powerful model for the study of cardiomyocyte differentiation.

Molecular cloning of FOG-2: a modulator of transcription factor GATA-4 in cardiomyocytes

GATA transcription factors are important regulators of both hematopoiesis (GATA-1/2/3) and cardiogenesis (GATA-4) in mammals. The transcriptional activities of the GATA proteins are modulated by their interactions with other transcription factors and with transcriptional coactivators and repressors. Recently, two related zinc finger proteins, U-shaped (USH) and Friend of GATA-1 (FOG) have been reported to interact with the GATA proteins Pannier and GATA-1, respectively, and to modulate their transcriptional activities in vitro and in vivo. In this report, we describe the molecular cloning and characterization of a third FOG-related protein, FOG-2. FOG-2 is an 1,151 amino acid nuclear protein that contains eight zinc finger motifs that are structurally related to those of both FOG and USH. FOG-2 is first expressed in the mouse embryonic heart and septum transversum at embryonic day 8.5 and is subsequently expressed in the developing neuroepithelium and urogenital ridge. In the adult, FOG-2 is expressed predominately in the heart, brain, and testis. FOG-2 associates physically with the N-terminal zinc finger of GATA-4 both in vitro and in vivo. This interaction appears to modulate specifically the transcriptional activity of GATA-4 because overexpression of FOG-2 in both NIH 3T3 cells and primary rat cardiomyocytes represses GATA-4-dependent transcription from multiple cardiac-restricted promoters. Taken together, these results implicate FOG-2 as a novel modulator of GATA-4 function during cardiac development and suggest a paradigm in which tissue-specific interactions between different FOG and GATA proteins regulate the differentiation of distinct mesodermal cell lineages.