An HF-1a/HF-1b/MEF-2 combinatorial element confers cardiac ventricular specificity and established an anterior-posterior gradient of expression

Design and Production of Heart Chamber-Specific AAV9 Vectors

Adeno-associated virus serotype 9 (AAV9) is often used in heart research involving gene delivery due to its cardiotropism, high transduction efficiency, and little to no pathogenicity, making it highly applicable for gene manipulation, in vivo. However, current AAV9 technology is limited by the lack of strains that can selectively express and elucidate gene function in an atrial- and ventricular-specific manner. In fact, study of gene function in cardiac atria has been limited due to the lack of an appropriate tool to study atrial gene expression in vivo, hindering progress in the study of atrial-specific diseases such as atrial fibrillation, the most common cardiac arrhythmia in the USA.This chapter describes the method for the design and production of such chamber-specific AAV9 vectors, with the use of Nppa and Myl2 promoters to enhance atrial- and ventricular-specific expression. While several gene promoter candidates were considered and tested, Nppa and Myl2 were selected for use here because of their clearly defined regulatory elements that confer cardiac chamber-specific expression. Accordingly, Nppa (-425/+25) and Myl2 (-226/+36) promoter fragments are inserted into AAV9 vectors. The atrial- and ventricular-specific expression conferred by these new recombinant AAV9 was confirmed in a double-fluorescent Cre-dependent reporter mouse model. At only 450 and 262 base pairs of Nppa and Myl2 promoters, respectively, these AAV9 that drive chamber-specific AAV9 transgene expression address two major limitations of AAV9 technology, i.e., achieving chamber-specificity while maximizing space in the AAV genome for insertion of larger transgenes.

Generation of MLC-2v-tdTomato knock-in reporter mouse line

MLC-2v is a myosin light chain regulatory protein which is specifically expressed in ventricular cardiomyocytes and slow twitch skeletal muscle cells. MLC-2v plays critical roles in ventricular maturation during heart development. Mice lacking MLC-2v are embryonic lethal due to heart failure associated with abnormal myofibrillar organization of ventricular cardiomyocytes. To study the development of ventricular cardiac muscle and slow twitch skeletal muscle, we generated a new MLC-2v reporter mouse line by knocking-in a tdTomato reporter cassette into 3' UTR of the MLC-2v gene without disrupting the endogenous gene. Our results demonstrated specific MLC-2v-tdTomato knock-in reporter expression in ventricular cardiomyocytes and slow twitch muscle during myogenesis, precisely recapitulating the spatiotemporal expression pattern of endogenous MLC-2v. No tdTomato expression was observed in the atria, fast twitch muscle or other organs throughout development into adulthood. Isolated neonatal and adult ventricular cardiomyocytes uniformly express tdTomato. Taken together, MLC-2v-tdTomato knock-in reporter mouse model described in this article will serve as a valuable tool to study cardiac chamber and skeletal muscle specification during development and regeneration by overcoming the pitfalls of transgenic strategies.

The transcription factor MEF2A fine-tunes gene expression in the atrial and ventricular chambers of the adult heart

The distinct morphological and functional properties of the cardiac chambers arise from an elaborate developmental program involving cell lineage determination, morphogenesis, and dynamic spatiotemporal gene expression patterns. Although a number of transcription factors have been identified for proper gene regulation in the chambers, the complete transcriptional network that controls these patterns remains poorly defined. Previous studies have implicated the MEF2C transcription factor in the regulation of chamber-restricted enhancers. To better understand the mechanisms of MEF2-mediated regional gene regulation in the heart, we took advantage of MEF2A knock-out (KO) mice, a model that displays a predominantly ventricular chamber phenotype. Transcriptomic analysis of atrial and ventricular tissue from adult MEF2A KO hearts revealed a striking difference in chamber gene expression, with a larger proportion of dysregulated genes in the atrial chambers. Canonical pathway analysis of genes preferentially dysregulated in the atria and ventricles revealed distinct MEF2A-dependent cellular processes in each cardiac chamber. In the atria, MEF2A regulated genes involved in fibrosis and adhesion, whereas in the ventricles, it controlled inflammation and endocytosis. Finally, analysis of transcription factor-binding site motifs of differentially dysregulated genes uncovered distinct MEF2A co-regulators for the atrial and ventricular gene sets, and a subset of these was found to cooperate with MEF2A. In conclusion, our results suggest a mechanism in which MEF2 transcriptional activity is differentially recruited to fine-tune gene expression levels in each cardiac chamber. This regulatory mechanism ensures optimal output of these gene products for proper physiological function of the atrial and ventricular chambers.

Engineering human ventricular heart muscles based on a highly efficient system for purification of human pluripotent stem cell-derived ventricular cardiomyocytes

BACKGROUND

Most infarctions occur in the left anterior descending coronary artery and cause myocardium damage of the left ventricle. Although current pluripotent stem cells (PSCs) and directed cardiac differentiation techniques are able to generate fetal-like human cardiomyocytes, isolation of pure ventricular cardiomyocytes has been challenging. For repairing ventricular damage, we aimed to establish a highly efficient purification system to obtain homogeneous ventricular cardiomyocytes and prepare engineered human ventricular heart muscles in a dish.

METHODS

The purification system used TALEN-mediated genomic editing techniques to insert the neomycin or EGFP selection marker directly after the myosin light chain 2 (MYL2) locus in human pluripotent stem cells. Purified early ventricular cardiomyocytes were estimated by immunofluorescence, fluorescence-activated cell sorting, quantitative PCR, microelectrode array, and patch clamp. In subsequent experiments, the mixture of mature MYL2-positive ventricular cardiomyocytes and mesenchymal cells were cocultured with decellularized natural heart matrix. Histological and electrophysiology analyses of the formed tissues were performed 2 weeks later.

RESULTS

Human ventricular cardiomyocytes were efficiently isolated based on the purification system using G418 or flow cytometry selection. When combined with the decellularized natural heart matrix as the scaffold, functional human ventricular heart muscles were prepared in a dish.

CONCLUSIONS

These engineered human ventricular muscles can be great tools for regenerative therapy of human ventricular damage as well as drug screening and ventricular-specific disease modeling in the future.

Diastolic dysfunction and cardiac troponin I decrease in aging hearts

Cardiac tropnoin I (cTnI) plays a critical role in the regulation of diastolic function, and its low expression may result in cardiac diastolic dysfunction, which is the most common form of cardiovascular disorders in older adults. In this study, cTnI expression levels were determined in mice at various ages and cardiac function was measured and compared between young adult mice (3 and 10 months) and older mice (18 months). The data indicated that the cTnI levels reached a peak high in young adult hearts (3 months), but decreased in older hearts (18 months). Furthermore, the older hearts showed a significant diastolic dysfunction observed by P-V loop and echocardiography measurements. To further define the mechanism underlying the cTnI decrease in aging hearts, we tested DNA methylation and histone acetylation modifications of cTnI gene. We found that acetylation of histone near the promoter region of cTnI gene played an important role in regulation of cTnI expression in the heart at different ages. Our study indicates that epigenetic modification caused cTnI expression decrease is one of the possible causes that result in a reduced cTnI level and diastolic dysfunction in the older hearts.

Functions of myosin light chain-2 (MYL2) in cardiac muscle and disease

Myosin light chain-2 (MYL2, also called MLC-2) is an ~19kDa sarcomeric protein that belongs to the EF-hand calcium binding protein superfamily and exists as three major isoforms encoded by three distinct genes in mammalian striated muscle. Each of the three different MLC-2 genes (MLC-2f; fast twitch skeletal isoform, MLC-2v; cardiac ventricular and slow twitch skeletal isoform, MLC-2a; cardiac atrial isoform) has a distinct developmental expression pattern in mammals. Genetic loss-of-function studies in mice demonstrated an essential role for cardiac isoforms of MLC-2, MLC-2v and MLC-2a, in cardiac contractile function during early embryogenesis. In the adult heart, MLC-2v function is regulated by phosphorylation, which displays a specific 1`expression pattern (high in epicardium and low in endocardium) across the heart. These data along with new data from computational models, genetic mouse models, and human studies have revealed a direct role for MLC-2v phosphorylation in cross-bridge cycling kinetics, calcium-dependent cardiac muscle contraction, cardiac torsion, cardiac function and various cardiac diseases. This review focuses on the regulatory functions of MLC-2 in the embryonic and adult heart, with an emphasis on phosphorylation-driven actions of MLC-2v in adult cardiac muscle, which provide new insights into mechanisms regulating myosin cycling kinetics and human cardiac diseases.

Prenatal diagnosis of a 7p15-p21 deletion encompassing the TWIST1 gene involved in Saethre-Chotzen syndrome

Saethre-Chotzen syndrome is a craniosynostosis syndrome that is rarely diagnosed prenatally. It is caused by cytogenetic deletions or mutations of the TWIST1 gene. We report here a de novo prenatal case with clinically and molecularly well defined Saethre-Chotzen syndrome due to a TWIST1 deletion. This is the first reported case of a deletion encompassing the TWIST1 gene to be diagnosed prenatally. We recommend screening for a deletion of the TWIST1 gene if signs of coronal craniosynostosis with no clear etiology are observed on ultrasound examination.

Differential gene expressions in atrial and ventricular myocytes: insights into the road of applying embryonic stem cell-derived cardiomyocytes for future therapies

Myocardial infarction has been the leading cause of morbidity and mortality in developed countries over the past few decades. The transplantation of cardiomyocytes offers a potential method of treatment. However, cardiomyocytes are in high demand and their supply is extremely limited. Embryonic stem cells (ESCs), which have been isolated from the inner cell mass of blastocysts, can self-renew and are pluripotent, meaning they have the ability to develop into any type of cell, including cardiomyocytes. This suggests that ESCs could be a good source of genuine cardiomyocytes for future therapeutic purposes. However, problems with the yield and purity of ESC-derived cardiomyocytes, among other hurdles for the therapeutic application of ESC-derived cardiomyocytes (e.g., potential immunorejection and tumor formation problems), need to be overcome before these cells can be used effectively for cell replacement therapy. ESC-derived cardiomyocytes consist of nodal, atrial, and ventricular cardiomyocytes. Specifically, for treatment of myocardial infarction, transplantation of a sufficient quantity of ventricular cardiomyocytes, rather than nodal or atrial cardiomyocytes, is preferred. Hence, it is important to find ways of increasing the yield and purity of specific types of cardiomyocytes. Atrial and ventricular cardiomyocytes have differential expression of genes (transcription factors, structural proteins, ion channels, etc.) and are functionally distinct. This paper presents a thorough review of differential gene expression in atrial and ventricular myocytes, their expression throughout development, and their regulation. An understanding of the molecular and functional differences between atrial and ventricular myocytes allows discussion of potential strategies for preferentially directing ESCs to differentiate into chamber-specific cells, or for fine tuning the ESC-derived cardiomyocytes into specific electrical and contractile phenotypes resembling chamber-specific cells.

Characterization of promoter elements required for cardiac chamber-specific expression

Salmon cardiac natriuretic peptide (sCP, an A-type natriuretic peptide) is an excellent model for the study of cardiac chamber-specific gene expression because it is uniquely specific to the heart and its promoter drives gene expression effectively in mammalian cardiac atrial but not in ventricular cells. We have now prepared hybrid luciferase constructs containing specific sequences from both sCP and BNP 5' promoters. According to our results the simple addition of a short rat BNP proximal promoter fragment to the inert 846 nucleotide sCP proximal promoter increases 100 times the basal activity of the sCP promoter in rat ventricular cardiomyocytes, and also conveys inducibility by mechanical load and endothelin-1. Thus, a small rBNP promoter fragment can transform the prototypical A-type natriuretic peptide regulation of sCP to B-type regulation, a result which argues against a major role of repressors causing the low expression level of A-type peptides in ventricular cardiomyocytes.

Transient and transgenic analysis of the zebrafish ventricular myosin heavy chain (vmhc) promoter: an inhibitory mechanism of ventricle-specific gene expression

The zebrafish ventricular myosin heavy chain (vmhc) gene exhibits restricted expression in the ventricle. However, the molecular mechanism underlying this chamber-specific expression is unclear. Here, we exploited both transient and transgenic technologies to dissect the zebrafish vmhc promoter. We demonstrated that a combination of two transient assays in this animal model quickly identified chamber-specific cis-elements, isolating a 2.2 kb fragment upstream from the vmhc gene that can drive ventricle-specific expression. Furthermore, deletion analysis identified multiple cis-elements that exhibited cardiac-specific expression. To achieve chamber specificity, a distal element was required to coordinate with and suppress a proximal enhancer element. Finally, we discovered that Nkx2.5-binding sites (NKE) were essential for this repressive function. In summary, our study of the zebrafish vmhc promoter suggests that ventricle-specific expression is achieved through an inhibitory mechanism that suppresses expression in the atrium. Developmental Dynamics 238:1564-1573, 2009. (c) 2009 Wiley-Liss, Inc.

An Nkx2-5/Bmp2/Smad1 negative feedback loop controls heart progenitor specification and proliferation

During heart development the second heart field (SHF) provides progenitor cells for most cardiomyocytes and expresses the homeodomain factor Nkx2-5. We now show that feedback repression of Bmp2/Smad1 signaling by Nkx2-5 critically regulates SHF proliferation and outflow tract (OFT) morphology. In the cardiac fields of Nkx2-5 mutants, genes controlling cardiac specification (including Bmp2) and maintenance of the progenitor state were upregulated, leading initially to progenitor overspecification, but subsequently to failed SHF proliferation and OFT truncation. In Smad1 mutants, SHF proliferation and deployment to the OFT were increased, while Smad1 deletion in Nkx2-5 mutants rescued SHF proliferation and OFT development. In Nkx2-5 hypomorphic mice, which recapitulate human congenital heart disease (CHD), OFT anomalies were also rescued by Smad1 deletion. Our findings demonstrate that Nkx2-5 orchestrates the transition between periods of cardiac induction, progenitor proliferation, and OFT morphogenesis via a Smad1-dependent negative feedback loop, which may be a frequent molecular target in CHD.

Pan-myocardial expression of Cre recombinase throughout mouse development

Mouse-lines expressing Cre recombinase in a tissue-specific manner are a powerful tool in developmental biology. Here, we report that a 3 kb fragment of the Xenopus laevis myosin light-chain 2 (XMLC2) promoter drives Cre recombinase expression in a cardiac-restricted fashion in the mouse embryo. We have isolated two XMLC2-Cre lines that express recombinase exclusively within cardiomyocytes, from the onset of their differentiation in the cardiac crescent of the early embryo. Expression is maintained throughout the myocardium of the embryonic heart tube and subsequently the mature myocardium of the chambered heart. Recombinase activity is detected in all myocardial tissue, including the pulmonary veins. One XMLC2-Cre line shows uniform expression while the other only expresses recombinase in a mosaic fashion encompassing less than 50% of the myocardial cells. Both lines cause severe cardiac malformations when crossed to a conditional Tbx5 line, resulting in embryonic death at midgestation. Optical projection tomography reveals that the spectrum of developmental abnormalities includes a shortening of the outflow tract and its abnormal alignment, along with a dramatic reduction in trabeculation of the ventricular segment of the looping heart tube.

Patterning the heart, a template for human cardiomyocyte development

Although in mice, the dynamics of gene expression during heart development is well characterized, information on humans is scarce due to the limited availability of material. Here, we analyzed the transcriptional distribution of Mlc-2a, Mlc-1v, Mlc-2v, and atrial natriuretic factor (ANF) in human embryonic hearts between 7 and 18 weeks of gestation and in healthy and hypertrophic adult hearts by in situ hybridization and compared expression with that in mice. Strikingly, Mlc-2a, Mlc-1v, and ANF, which are essentially chamber-restricted in mice by mid-gestation, showed a broader distribution in humans. On the other hand, Mlc-2v may prove to be an adequate ventricular marker in humans in contrast to mouse. This study emphasizes the importance of careful comparative human-animal analyses during embryonic development and adulthood, as avoiding erroneous extrapolations may be critical to develop new and successful myocardial replacement therapies.

MEF2C is required for the normal allocation of cells between the ventricular and sinoatrial precursors of the primary heart field

Targeted deletion of the mef2c gene results in a small left ventricle and complete loss of the right ventricle (Lin et al. [1997] Science 276:1404-1407). Absence of the right ventricle is from defective differentiation of cells from the secondary heart field. Our studies of the dysmorphogenesis of the left ventricle uncovered morphological and transcriptional abnormalities at the transition from the cardiac crescent to the linear-tube stage heart. Use of the cgata6LacZ transgene demonstrated that lacZ-positive cells, which normally mark the precursors to the atrioventricular canal and adjacent regions of the left ventricle and atria, remain in the sinoatrial region of the mutant. This, along with the absence of a morphologically distinct atrioventricular canal, indicates a misapportioning of cells between the inflow and outflow segments. The underlying genetic program was also affected with altered expression of mlc2a, mlc2v, and irx4 in outflow segment precursors of the primary heart field. In addition, the sinoatrial-enriched transcription factor, tbx5, was ectopically expressed in the primitive ventricle and ventricle-specific splicing of mef2b was lost, suggesting that the mutant ventricle had acquired atrial-specific characteristics. Collectively, these results suggest a fundamental role of MEF2C in ventricular cardiomyocyte differentiation and apportioning of cells between inflow and outflow precursors in the primary heart field.

The Kruppel-like transcription factor KLF13 is a novel regulator of heart development

In humans, congenital heart defects occur in 1-2% of live birth, but the molecular mechanisms and causative genes remain unidentified in the majority of cases. We have uncovered a novel transcription pathway important for heart morphogenesis. We report that KLF13, a member of the Krüppel-like family of zinc-finger proteins, is expressed predominantly in the heart, binds evolutionarily conserved regulatory elements on cardiac promoters and activates cardiac transcription. KLF13 is conserved across species and knockdown of KLF13 in Xenopus embryos leads to atrial septal defects and hypotrabeculation similar to those observed in humans or mice with hypomorphic GATA-4 alleles. Physical and functional interaction with GATA-4, a dosage-sensitive cardiac regulator, provides a mechanistic explanation for KLF13 action in the heart. The data demonstrate that KLF13 is an important component of the transcription network required for heart development and suggest that KLF13 is a GATA-4 modifier; by analogy to other GATA-4 collaborators, mutations in KLF13 may be causative for congenital human heart disease.

Left and right ventricular contributions to the formation of the interventricular septum in the mouse heart

Mammalian heart development involves complex morphogenetic events which lead to the formation of fully separated left and right atrial and ventricular chambers from a tubular heart. Separation of left and right ventricular chambers is dependent on a single structure, the interventricular septum (IVS), which has both muscular and mesenchymal components. Little is known about the morphogenetic events that lead to the formation of the muscular component of the IVS. We have analyzed two transgenic mouse lines that display complementary nlacZ reporter gene expression patterns in the embryonic ventricles: the Mlc1v-nlacZ-24 transgene is expressed in right ventricular myocardium and the Mlc3f-nlacZ-2 transgene in left ventricular myocardium. Detailed analysis of these transgene expression patterns during IVS formation reveals a symmetric left and right myocardial identity within the developing IVS between embryonic days 9.5 and 11.5. From embryonic day 12.5 onwards, myocytes with a left ventricular identity dominate the IVS, particularly in its dorsal aspect. The T-box transcription factor encoding gene, Tbx18, is expressed in the left ventricle and left side of the developing IVS, providing additional support for the presence of left and right ventricular identities within the IVS. Analysis of clonally related cardiomyocyte clusters confirms that both left and right ventricular myocardial cell populations contribute to the forming IVS, in similar domains to those defined by the Mlc-nlacZ transgenes. Examination of the orientation as well as the distribution of labeled cells in clusters provides new insights into the morphogenesis of the septum.

Distinct roles of HF-1b/Sp4 in ventricular and neural crest cells lineages affect cardiac conduction system development

The heterogeneous cell types of the cardiac conduction system are responsible for coordinating and maintaining rhythmic contractions of the heart. While it has been shown that the cells of the conduction system are derived from myocytes, additional cell types, including neural crest cells, may play a role in the development and maturation of these specialized cell lineages. Previous work has shown that the expression of the hf-1b gene is required for specification of the cardiac conduction system. Using Cre-Lox technology, we conditionally mutated the hf-1b gene in the ventricular and the neural crest cell lineages. Cx40 immunohistochemistry on HF-1b tissue-restricted knockouts revealed a requirement for HF-1b in the cardiomyogenic lineage. Electrophysiological studies identified a second requirement for HF-1b in the neural crest-derived cells. Absence of HF-1b in the neural crest led to atrial and atrioventricular dysfunction resulting from deficiencies in the neurotrophin receptor trkC. Therefore, in this study, we document that a single transcription factor, HF-1b, acts through two separate cell types to direct distinct functions of the cardiac conduction system.

Adeno-associated viral vector delivers cardiac-specific and hypoxia-inducible VEGF expression in ischemic mouse hearts

It has been shown that the adeno-associated virus (AAV) vector can deliver the VEGF gene efficiently into the ischemic mouse myocardium. However, the AAV genomes can be found in extracardiac organs after intramyocardial injection. To limit unwanted VEGF expression in organs other than the heart, we tested the use of the cardiac myosin light chain 2v (MLC-2v) promoter and the hypoxia-response element to mediate cardiac-specific and hypoxia-inducible VEGF expression. An AAV vector, MLCVEGF, with 250 bp of the MLC-2v promoter and nine copies of the hypoxia-response element driving VEGF expression, was constructed. Gene expression was studied in vitro by infection of rat cardiomyocytes, rat skeletal myocytes, and mouse fibroblasts with the vector and in vivo by direct injection of the vector into normal and ischemic mouse hearts. With MLCVEGF infection, VEGF expression was higher in cardiomyocytes than the other two cell lines and was hypoxiainducible. VEGF expression was also higher in ischemic hearts than in normal hearts. No VEGF expression was detectable in organs with detectable MLCVEGF vectors other than the heart. MLCVEGF-injected ischemic hearts had more capillaries and small vessels around the injection site, smaller infarct size, and better cardiac function than the negative controls. Hence, MLCVEGF can mediate cardiac-specific and hypoxia-inducible VEGF expression, neoangiogenesis, infarct-size reduction, and cardiac functional improvement.

Right ventricular myocardium derives from the anterior heart field

The mammalian heart develops from a primary heart tube, which is formed by fusion of bilateral cardiac territories in which myocardial and endothelial cells have already begun to differentiate from splanchnic mesoderm. A population of myocardial precursors has been identified in pharyngeal mesoderm, anterior to the early heart tube. Cell labeling studies have indicated that this novel territory, called the anterior heart field (AHF), gives rise to the myocardial wall of the outflow tract. We now report that not only the myocardium of the outflow tract but also myocardial cells of the embryonic right ventricle are derived from this source. Explants of pharyngeal mesoderm or of the early heart tube were cultured from transgenic mice in which transgene expression marks different regions of the heart. Pharyngeal mesoderm from 5 to 7 somite embryos gives rise to cardiomyocytes with right ventricular and outflow tract identities, whereas the heart tube as this stage has an essentially left ventricular identity. DiI labeling confirms that the early heart tube is destined to contribute to the embryonic left ventricle and indicates that right ventricular myocardium is added from extracardiac mesoderm. Retrospective clonal analysis of the heart at embryonic day (E) 10.5 reveals the existence of a clonal boundary in the interventricular region, which appears during ventricular septation, underlining different origins of the two ventricular compartments. This study demonstrates the differences in the embryological origin of right and left ventricular myocardium, which has important implications for congenital heart disease.

Role of Hand1/eHAND in the dorso-ventral patterning and interventricular septum formation in the embryonic heart

Molecular mechanisms for the dorso-ventral patterning and interventricular septum formation in the embryonic heart are unknown. To investigate a role of Hand1/eHAND in cardiac chamber formation, we generated Hand1/eHAND knock-in mice where Hand1/eHAND cDNA was placed under the control of the MLC2V promoter. In Hand1/eHAND knock-in mice, the outer curvature of the right and left ventricles expanded more markedly. Moreover, there was no interventricular groove or septum formation, although molecularly, Hand1/eHAND knock-in hearts had two ventricles. However, the morphology of the inner curvature of the ventricles, the atrioventricular canal, and the outflow tract was not affected by Hand1/eHAND expression. Furthermore, expression of Hand1/eHAND in the whole ventricles altered the expression patterns of Chisel, ANF, and Hand2/dHAND but did not affect Tbx5 expression. In contrast, the interventricular septum formed normally in transgenic embryos overexpressing Hand1/eHAND in the right ventricle but not in the boundary region. These results suggested that Hand1/eHAND is involved in expansion of the ventricular walls and that absence of Hand1/eHAND expression in the boundary region between the right and left ventricles may be critical in the proper formation of the interventricular groove and septum. Furthermore, Hand1/eHAND is not a master regulatory gene that specifies the left ventricle myocyte lineage but may control the dorso-ventral patterning in concert with additional genes.

Molecular Regulation of Cardiac Chamber-Specific Gene Expression

This review focuses on recent studies investigating the genetic regulatory mechanisms leading to formation of morphologically, functionally, and molecularly distinct cardiac chambers. The regulation of four representative chamber-specific genes that have been studied in detail is reviewed. These genes include the atrial-specific genes, myosin light chain-2a (MLC2a), slow myosin heavy chain-3 (slow MyHC3), and atrial natriuretic factor (ANF) and the ventricular specific gene, myosin light chain-2v (MLC2v). Comparison of these promoters reveals some generalizations about the regulatory mechanisms involved in chamber-specific gene expression but, equally, indicates the large gaps in the knowledge concerning this intriguing genetic program.

Cardiac chamber formation: development, genes, and evolution

Concepts of cardiac development have greatly influenced the description of the formation of the four-chambered vertebrate heart. Traditionally, the embryonic tubular heart is considered to be a composite of serially arranged segments representing adult cardiac compartments. Conversion of such a serial arrangement into the parallel arrangement of the mammalian heart is difficult to understand. Logical integration of the development of the cardiac conduction system into the serial concept has remained puzzling as well. Therefore, the current description needed reconsideration, and we decided to evaluate the essentialities of cardiac design, its evolutionary and embryonic development, and the molecular pathways recruited to make the four-chambered mammalian heart. The three principal notions taken into consideration are as follows. 1) Both the ancestor chordate heart and the embryonic tubular heart of higher vertebrates consist of poorly developed and poorly coupled "pacemaker-like" cardiac muscle cells with the highest pacemaker activity at the venous pole, causing unidirectional peristaltic contraction waves. 2) From this heart tube, ventricular chambers differentiate ventrally and atrial chambers dorsally. The developing chambers display high proliferative activity and consist of structurally well-developed and well-coupled muscle cells with low pacemaker activity, which permits fast conduction of the impulse and efficacious contraction. The forming chambers remain flanked by slowly proliferating pacemaker-like myocardium that is temporally prevented from differentiating into chamber myocardium. 3) The trabecular myocardium proliferates slowly, consists of structurally poorly developed, but well-coupled, cells and contributes to the ventricular conduction system. The atrial and ventricular chambers of the formed heart are activated and interconnected by derivatives of embryonic myocardium. The topographical arrangement of the distinct cardiac muscle cells in the forming heart explains the embryonic electrocardiogram (ECG), does not require the invention of nodes, and allows a logical transition from a peristaltic tubular heart to a synchronously contracting four-chambered heart. This view on the development of cardiac design unfolds fascinating possibilities for future research.

Transcription factors in cardiogenesis: the combinations that unlock the mysteries of the heart

Heart formation is one of the first signs of organogenesis within the developing embryo and this process is conserved from flies to man. Completing the genetic roadmap of the molecular mechanisms that control the cell specification and differentiation of cells that form the developing heart has been an exciting and fast-moving area of research in the fields of molecular and developmental biology. At the core of these studies is an interest in the transcription factors that are responsible for initiation of a pluripotent cell to become programmed to the cardiac lineage and the subsequent transcription factors that implement the instructions set up by the cells commitment decision. To gain a better understanding of these pathways, cardiac-expressed transcription factors have been identified, cloned, overexpressed, and mutated to try to determine function. Although results vary depending on the gene in question, it is clear that there is a striking evolutionary conservation of the cardiogenic program among species. As we move up the evolutionary ladder toward man, we encounter cases of functional redundancy and combinatorial interactions that reflect the complex networks of gene expression that orchestrate heart development. This review focuses on what is known about the transcription factors implicated in heart formation and the role they play in this intricate genetic program.

Molecular embryogenesis of the heart

Development of the heart is a complex process involving primary and secondary heart fields that are set aside to generate myocardial and endocardial cell lineages. The molecular inductions that occur in the primary heart field appear to be recapitulated in induction and myocardial differentiation of the secondary heart field, which adds the conotruncal segments to the primary heart tube. While much is now known about the initial steps and factors involved in induction of myocardial differentiation, little is known about induction of endocardial development. Many of the genes expressed by nascent myocardial cells, which then become committed to a specific heart segment, have been identified and studied. In addition to the heart fields, several other "extracardiac" cell populations contribute to the fully functional mature heart. Less is known about the genetic programs of extracardiac cells as they enter the heart and take part in cardiogenesis. The molecular/genetic basis of many congenital cardiac defects has been elucidated in recent years as a result of new insights into the molecular control of developmental events.

Species-specific differences of myosin content in the developing cardiac chambers of fish, birds, and mammals

Key morphogenetic events during heart ontogenesis are similar in different vertebrate species. We report that in primitive vertebrates, i.e., cartilaginous fishes, both the embryonic and the adult heart show a segmental subdivision similar to that of the embryonic mammalian heart. Early morphogenetic events during cardiac development in the dogfish are long-lasting, providing a suitable model to study changes in pattern of gene expression during these stages. We performed a comparative study among dogfish, chicken, rat, and mouse to assess whether species-specific qualitative and/or quantitative differences in myosin heavy chain (MyHC) distribution arise during development, indicative of functional differences between species. MyHC RNA content was investigated by means of in situ hybridisation using an MyHC probe specific for a highly conserved domain, and MyHC protein content was assessed by immunohistochemistry. MyHC transcripts were found to be homogeneously distributed in the myocardium of the tubular and embryonic heart of dogfish and rodents. A difference between atrial and ventricular MyHC content (mRNA and protein) was observed in the adult stage. Interestingly, differences in the MyHC content were observed at the tubular heart stage in chicken. These differences in MyHC content illustrate the distinct developmental profiles of avian and mammalian species, which might be ascribed to distinct functional requirements of the myocardial segments during ontogenesis. The atrial myocardium showed the highest MyHC content in the adult heart of all species analysed (dogfish (S. canicula), mouse (M. musculus), rat (R. norvegicus), and chicken (G. gallus)). These observations indicate that in the adult heart of vertebrates the atrial myocardium contains more myosin than the ventricular myocardium.

Cooperative action of Tbx2 and Nkx2.5 inhibits ANF expression in the atrioventricular canal: implications for cardiac chamber formation

During heart development, chamber myocardium forms locally from the embryonic myocardium of the tubular heart. The atrial natriuretic factor (ANF) gene is specifically expressed in this developing chamber myocardium and is one of the first hallmarks of chamber formation. We investigated the regulatory mechanism underlying this selective expression. Transgenic analysis shows that a small fragment of the ANF gene is responsible for the developmental pattern of endogenous ANF gene expression. Furthermore, this fragment is able to repress cardiac troponin I (cTnI) promoter activity selectively in the embryonic myocardium of the atrioventricular canal (AVC). In vivo inactivation of a T-box factor (TBE)- or NK2-homeobox factor binding element (NKE) within the ANF fragment removed the repression in the AVC without affecting its chamber activity. The T-box family member Tbx2, encoding a transcriptional repressor, is expressed in the embryonic myocardium in a pattern mutually exclusive to ANF, thus suggesting a role in the suppression of ANF. Tbx2 formed a complex with Nkx2.5 on the ANF TBE-NKE, and was able to repress ANF promoter activity. Our data provide a potential mechanism for chamber-restricted gene activity in which the cooperative action of Tbx2 and Nkx2.5 inhibits expression in the AVC.

Wherefore heart thou? Embryonic origins of cardiogenic mesoderm

The developing heart in avian embryos has been examined extensively over the past several decades using classic embryologic and, more recently, molecular and genetic approaches. Still, conflicting reports arise as to the location and regulation of early heart progenitors in the embryo. In addition, a new source of cardiomyocytes has been identified recently that contributes to the outflow tract after the heart initially forms. The focus of this review is the examination of the tissue interactions, signaling molecules, and gene regulatory mechanisms that, together, control heart formation from primary and secondary heart forming fields of the embryo. Early studies of the induction and regulation of the secondary heart field indicate that at least some of the events of primary cardiomyogenesis are recapitulated when the conotruncal myocardium is recruited into the heart. The consideration of classic embryologic studies of the heart forming fields in terms of modern molecular and genetic tools provides reinforcing evidence for the location of cardiac progenitors in the embryo. The accurate definition of early cardiac regulatory events provides a necessary foundation for the generation of new therapeutic sources of cardiomyocytes.

[Regulation of myocardial gene expression during heart development]

The heart is an organ with special significance in medicine and developmental biology. The development of the heart and its vessels during embryogenesis is the result of numerous and complex processes. At present, our understanding is based on decades of meticulous anatomical studies. However, the spectacular progress of modern molecular biology and developmental biology has marked the beginning of a new era in embryology. The molecular bases for cardiogenesis are just emerging. Several families of genes with restricted expression to the heart have been identified in the last years, including genes encoding for contractile proteins, ion channels as well as transcription factors involved in tissue specific gene expression. Likewise, the analyses of regulatory elements have increased our understanding of the molecular mechanisms directing gene expression. In this review, we illustrate the different patterns of gene and transgene expression in the developing myocardium. These data demonstrate that the wide molecular heterogeneity observed in the developing myocardium is not restricted to embryogenesis but it also remains in the adulthood. Therefore, such molecular diversity should be taken into account on the design of future gene therapy approaches, having thus direct clinical implications.

The combinatorial activities of Nkx2.5 and dHAND are essential for cardiac ventricle formation

Nkx2.5/Csx and dHAND/Hand2 are conserved transcription factors that are coexpressed in the precardiac mesoderm and early heart tube and control distinct developmental events during cardiogenesis. To understand whether Nkx2.5 and dHAND may function in overlapping genetic pathways, we generated mouse embryos lacking both Nkx2.5 and dHAND. Mice heterozygous for mutant alleles of Nkx2.5 and dHAND were viable. Although single Nkx2.5 or dHAND mutants have a morphological atrial and single ventricular chamber, Nkx2.5(-/-)dHAND(-/-) mutants had only a single cardiac chamber which was molecularly defined as the atrium. Complete ventricular dysgenesis was observed in Nkx2.5(-/-)dHAND(-/-) mutants; however, a precursor pool of ventricular cardiomyocytes was identified on the ventral surface of the heart tube. Because Nkx2.5 mutants failed to activate eHAND expression even in the early precardiac mesoderm, the Nkx2.5(-/-)dHAND(-/-) phenotype appears to reflect an effectively null state of dHAND and eHAND. Cell fate analysis in dHAND mutants suggests a role of HAND genes in survival and expansion of the ventricular segment, but not in specification of ventricular cardiomyocytes. Our molecular analyses also revealed the cooperative regulation of the homeodomain protein, Irx4, by Nkx2.5 and dHAND. These studies provide the first demonstration of gene mutations that result in ablation of the entire ventricular segment of the mammalian heart, and reveal essential transcriptional pathways for ventricular formation.

Ablation of Cypher, a PDZ-LIM domain Z-line protein, causes a severe form of congenital myopathy

Cypher is a member of a recently emerging family of proteins containing a PDZ domain at their NH(2) terminus and one or three LIM domains at their COOH terminus. Cypher knockout mice display a severe form of congenital myopathy and die postnatally from functional failure in multiple striated muscles. Examination of striated muscle from the mutants revealed that Cypher is not required for sarcomerogenesis or Z-line assembly, but rather is required for maintenance of the Z-line during muscle function. In vitro studies demonstrated that individual domains within Cypher localize independently to the Z-line via interactions with alpha-actinin or other Z-line components. These results suggest that Cypher functions as a linker-strut to maintain cytoskeletal structure during contraction.

A GATA-6 gene heart-region-specific enhancer provides a novel means to mark and probe a discrete component of the mouse cardiac conduction system

The transcriptional programs that specify the distinct components of the cardiac conduction system are poorly understood, in part due to a paucity of definitive molecular markers. In the present study we show that a cGATA-6 gene enhancer can be used to selectively express transgenes in the atrioventricular (AV) conduction system as it becomes manifest in the developing multichambered mouse heart. Furthermore, our analysis of staged cGATA-6/lacZ embryos revealed that the activity of this heart-region-specific enhancer can be traced back essentially to the outset of the cardiogenic program. We provide evidence that this enhancer reads medial/lateral and anterior/posterior positional information before the heart tube forms and we show that the activity of this enhancer becomes restricted at the heart looping stage to AV myocardial cells that induce endocardial cushion formation. We infer that a deeply-rooted heart-region-specific transcriptional program serves to coordinate AV valve placement and AV conduction system formation. Lastly, we show that cGATA-6/Cre mice can be used to delete floxed genes in the respective subsets of specialized heart cells.

The outflow tract of the heart is recruited from a novel heart-forming field

As classically described, the precardiac mesoderm of the paired heart-forming fields migrate and fuse anteriomedially in the ventral midline to form the first segment of the straight heart tube. This segment ultimately forms the right trabeculated ventricle. Additional segments are added to the caudal end of the first in a sequential fashion from the posteriolateral heart-forming field mesoderm. In this study we report that the final major heart segment, which forms the cardiac outflow tract, does not follow this pattern of embryonic development. The cardiac outlet, consisting of the conus and truncus, does not derive from the paired heart-forming fields, but originates separately from a previously unrecognized source of mesoderm located anterior to the initial primitive heart tube segment. Fate-mapping results show that cells labeled in the mesoderm surrounding the aortic sac and anterior to the primitive right ventricle are incorporated into both the conus and the truncus. Conversely, if cells are labeled in the existing right ventricle no incorporation into the cardiac outlet is observed. Tissue explants microdissected from this anterior mesoderm region are capable of forming beating cardiac muscle in vitro when cocultured with explants of the primitive right ventricle. These findings establish the presence of another heart-forming field. This anterior heart-forming field (AHF) consists of mesoderm surrounding the aortic sac immediately anterior to the existing heart tube. This new concept of the heart outlet's embryonic origin provides a new basis for explaining a variety of gene-expression patterns and cardiac defects described in both transgenic animals and human congenital heart disease.

CHAMP, a novel cardiac-specific helicase regulated by MEF2C

MEF2C is a MADS-box transcription factor required for cardiac myogenesis and morphogenesis. In MEF2C mutant mouse embryos, heart development arrests at the looping stage (embryonic day 9.0), the future right ventricular chamber fails to form, and cardiomyocyte differentiation is disrupted. To identify genes regulated by MEF2C in the developing heart, we performed differential array analysis coupled with subtractive cloning using RNA from heart tubes of wild-type and MEF2C-null embryos. Here, we describe a novel MEF2C-dependent gene that encodes a cardiac-restricted protein, called CHAMP (cardiac helicase activated by MEF2 protein), that contains seven conserved motifs characteristic of helicases involved in RNA processing, DNA replication, and transcription. During mouse embryogenesis, CHAMP expression commences in the linear heart tube at embryonic day 8.0, shortly after initiation of MEF2C expression in the cardiogenic region. Thereafter, CHAMP is expressed specifically in embryonic and postnatal cardiomyocytes. At the trabeculation stage of heart development, CHAMP expression is highest in the trabecular region in which cardiomyocytes have exited the cell cycle and is lowest in the proliferative compact zone. These findings suggest that CHAMP acts downstream of MEF2C in a cardiac-specific regulatory pathway for RNA processing and/or transcriptional control.

Cell biology of cardiac development

Building a vertebrate heart is a complex task and involves several tissues, including the myocardium, endocardium, neural crest, and epicardium. Interactions between these tissues result in the changes in function and morphology (and also in the extracellular matrix, which serves as a substrate for morphological change) that are requisite for development of the heart. Some of the signaling pathways that mediate these changes have now been identified and several investigators are now filling in the missing pieces in these pathways in hopes of ultimately understanding the molecular mechanisms that govern healthy heart development. In addition, transcription factors that regulate various aspects of heart development have been identified. Transcription factors of the GATA and Nkx2 families are of particular importance for early specification of the heart field and for regulating expression of genes that encode proteins of the contractile apparatus. This chapter highlights some of the most significant discoveries made in the rapidly expanding field of heart development.

Inducible regulation of human brain natriuretic peptide promoter in transgenic mice

Studies have shown that brain natriuretic peptide (BNP) gene expression is rapidly induced in the infarcted heart and that plasma BNP levels reflect the degree of left ventricular dysfunction. Our previous in vitro work using transiently transfected neonatal rat cardiac myocytes has shown that the human BNP (hBNP) promoter, in particular a region extending from -127 to -40 relative to the start site of transcription, is more active in cardiac myocytes than in fibroblasts. To study tissue-specific and transcriptional regulation of the hBNP gene in vivo, we generated transgenic mice containing the proximal hBNP promoter (-408 to +100) coupled to a luciferase reporter gene. In four lines of transgenic mice, luciferase activity was approximately 33- to 100-fold higher in the heart than in other tissues, including the whole brain. To test whether the transgene responded to a pathophysiological stimulus, we induced infarction by coronary artery ligation. Luciferase activity was fivefold higher in the infarcted region of the left ventricle at 48 h than in sham-operated animals and remained elevated for 4 wk. Endogenous BNP mRNA was similarly increased in the infarcted hearts of a separate group of mice. We conclude that 1) the proximal 408-bp region of the hBNP promoter confers cardiac-specific expression and 2) myocardial infarction activates the proximal hBNP promoter in vivo. These data suggest that we have a valid model for the study of basal and inducible regulation of the hBNP gene in vivo.

MLC3F transgene expression iniv mutant mice reveals the importance of left-right signalling pathways for the acquisition of left and right atrial but not ventricular compartment identity

Abstract Transcriptional differences between left and right cardiac chambers are revealed by an nlacZ reporter transgene controlled by regulatory sequences of the MLC3F gene, which is expressed in the left ventricle (LV), atrioventricular canal (AVC), and right atrium (RA). To examine the role of left-right signalling in the acquisition of left and right chamber identity, we have investigated MLC3F transgene expression in iv mutant mice. iv/iv mice exhibit randomised direction of heart looping and an elevated frequency of associated laterality defects, including atrial isomerism. At fetal stages, 3F-nlacZ-2E transgene expression remains confined to the morphological LV, AVC, and RA in L-loop hearts, although these appear on the opposite side of the body. In cases of morphologically distinguishable right atrial appendage isomerism, both atrial appendages show strong transgene expression. Conversely, specimens with morphological left atrial appendage isomerism show only weak expression in both atrial appendages. The earliest left-right atrial differences in the expression of the 3F-nlacZ-2E transgene are observed at E8.5. DiI labelling experiments confirmed that transcriptional regionalisation of the 3F-nlacZ-2E transgene at this stage reflects future atrial chamber identity. In some iv/iv embryos at E8.5, the asymmetry of 3F-nlacZ-2E expression was lost, suggesting atrial isomerism at the transcriptional level prior to chamber formation. These data suggest that molecular specification of left and right atrial but not ventricular chambers is dependent on left-right axial cues.

Modular regulation of the MLC1F/3F gene and striated muscle diversity

Isoform diversity in striated muscle is largely controlled at the level of transcription. In this review we will concentrate on studies concerning transcriptional regulation of the alkali myosin light chain 1F/3F gene. Uncoupled activity of the MLC1F and 3F promoters, together with complex patterns of transcription in developing skeletal and cardiac muscle, combine to make analysis of this gene particularly intriguing. In vitro and transgenic studies of MLC1F/3F regulatory elements have revealed an array of cis-acting modules that each drive a subset of the expression pattern of the two promoters. These cis-acting regulatory modules, including the MLC1F and 3F promoter regions and two skeletal muscle enhancers, control tissue-specificity, cell or fibre-type specificity, and the spatiotemporal regulation of gene expression, including positional information. How each of these regulatory modules acts and how their individual activites are integrated to coordinate transcription at this locus are discussed.

A novel genetic pathway for sudden cardiac death via defects in the transition between ventricular and conduction system cell lineages

HF-1 b, an SP1 -related transcription factor, is preferentially expressed in the cardiac conduction system and ventricular myocytes in the heart. Mice deficient for HF-1 b survive to term and exhibit normal cardiac structure and function but display sudden cardiac death and a complete penetrance of conduction system defects, including spontaneous ventricular tachycardia and a high incidence of AV block. Continuous electrocardiographic recordings clearly documented cardiac arrhythmogenesis as the cause of death. Single-cell analysis revealed an anatomic substrate for arrhythmogenesis, including a decrease and mislocalization of connexins and a marked increase in action potential heterogeneity. Two independent markers reveal defects in the formation of ventricular Purkinje fibers. These studies identify a novel genetic pathway for sudden cardiac death via defects in the transition between ventricular and conduction system cell lineages.

Chamber formation and morphogenesis in the developing mammalian heart

In this study we challenge the generally accepted view that cardiac chambers form from an array of segmental primordia arranged along the anteroposterior axis of the linear and looping heart tube. We traced the spatial pattern of expression of genes encoding atrial natriuretic factor, sarcoplasmic reticulum calcium ATPase, Chisel, Irx5, Irx4, myosin light chain 2v, and beta-myosin heavy chain and related these to morphogenesis. Based on the patterns we propose a two-step model for chamber formation in the embryonic heart. First, a linear heart forms, which is composed of "primary" myocardium that nonetheless shows polarity in phenotype and gene expression along its anteroposterior and dorsoventral axes. Second, specialized ventricular chamber myocardium is specified at the ventral surface of the linear heart tube, while distinct left and right atrial myocardium forms more caudally on laterodorsal surfaces. The process of looping aligns these primordial chambers such that they face the outer curvature. Myocardium of the inner curvature, as well as that of inflow tract, atrioventricular canal, and outflow tract, retains the molecular signature originally found in linear heart tube myocardium. Evidence for distinct transcriptional programs which govern compartmentalization in the forming heart is seen in the patterns of expression of Hand1 for the dorsoventral axis, Irx4 and Tbx5 for the anteroposterior axis, and Irx5 for the distinction between primary and chamber myocardium.

Ventricular expression of tbx5 inhibits normal heart chamber development

The T-box gene tbx5 is expressed in the developing heart, forelimb, eye, and liver in vertebrate embryos during critical stages of morphogenesis and patterning. In humans, mutations in the TBX5 gene have been associated with Holt-Oram syndrome, which is characterized by developmental anomalies in the heart and forelimbs. In chicken and mouse embryos, tbx5 expression is initiated at the earliest stages of heart formation throughout the heart primordia and is colocalized with other cardiac transcription factors such as nkx-2.5 and GATA4. As the heart differentiates, tbx5 expression is restricted to the posterior sinoatrial segments of the heart, consistent with the timing of atrial chamber determination. The correlation between tbx5 expression and atrial lineage determination was examined in retinoic acid (RA)-treated chicken embryos. tbx5 expression is maintained throughout the hearts of RA-treated embryos under conditions that also expand atrial-specific gene expression. The downstream effects of persistent tbx5 expression in the ventricles were examined directly in transgenic mice. Embryos that express tbx5 driven by a beta-myosin heavy chain promoter throughout the primitive heart tube were generated. Loss of ventricular-specific gene expression and retardation of ventricular chamber morphogenesis were observed in these embryos. These studies provide direct evidence for an essential role for tbx5 in early heart morphogenesis and chamber-specific gene expression.

Expression patterns of FHL/SLIM family members suggest important functional roles in skeletal muscle and cardiovascular system

LIM domain containing proteins play critical roles in animal development and cellular differentiation. Here, we describe the cloning and expression patterns of three members of the four and a half LIM domain-only protein family, FHL1, 2, and 3, from mouse. A comparison of embryonic expression patterns of these three highly-related genes indicates that they are expressed in an overlapping pattern in the developing cardiovascular system, and skeletal muscle. In adult tissues, the three genes are expressed in a predominant and overlapping manner in cardiac and skeletal muscle. Of the three genes, FHL2 appears to have the most restricted expression pattern during development, in heart, blood vessels, and skeletal muscle. Expression in heart is highest in cardiac septa and in the region adjacent to the atrio-ventricular ring, suggesting a potential role in septation or conduction system development. In the heart, FHL1expression was observed strongly in developing outflow tract, and to a lesser extent in myocardium. FHL3 displays low and ubiquitous expression during mouse development. Cardiac ventricular expression of FHL1, but not FHL2 or FHL3, was upregulated in two mouse models of cardiac hypertrophic and dilated cardiomyopathy. Taken together, these data indicate the potential importance of this FHL family in the development and maintenance of the cardiovascular system and striated muscle, and suggest that FHL1 may play a role in the development of heart disease.

Sequential programs of retinoic acid synthesis in the myocardial and epicardial layers of the developing avian heart

Endogenous patterns of retinoic acid (RA) signaling in avian cardiac morphogenesis were characterized by localized expression of a key RA-synthetic enzyme, RALDH2, which displayed a biphasic pattern during heart development. RALDH2 immunoreactivity was initially apparent posterior to Hensen's node of stage 5-6 embryos and subsequently in somites and unsegmented paraxial and lateral plate mesoderm overlapping atrial precursors in the cardiogenic plate of stage 9- embryos. Initial RALDH2 synthesis in the posterior myocardium coincided with activation of the AMHC1 gene, a RA-responsive marker of inflow heart segments. A wave of RALDH2 synthesis then swept the myocardium in a posterior-to-anterior direction, reaching the outflow tract by stage 13, then fading from the myocardial layer. The second phase of RALDH2 expression, initiated at stage 18 in the proepicardial organ, persisted in migratory epicardial cells that completely enveloped the heart by stage 24. Early restriction of RALDH2 expression to the posterior cardiogenic plate, overlapping RA-inducible gene activation, provides evidence for commitment of posterior avian heart segments by localized production of RA, whereas subsequent RALDH2 expression exclusively in the migratory epicardium suggests a role for the morphogen in ventricular expansion and morphogenesis of underlying myocardial tissues.

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.

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.

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.