Cooperative interaction between GATA5 and NF-ATc regulates endothelial-endocardial differentiation of cardiogenic cells

Molecular Pathways and Animal Models of Tricuspid Atresia and Univentricular Heart

The process of valve formation is a complex process that involves intricate interplay between various pathways at precise times. Although we have not completely elucidated the molecular pathways that lead to normal valve formation, we have identified a few major players in this process. We are now able to implicate TGF-ß, BMP, and NOTCH as suspects in tricuspid atresia (TA), as well as their downstream targets: NKX2-5, TBX5, NFATC1, GATA4, and SOX9. We know that the TGF-ß and the BMP pathways converge on the SMAD4 molecule, and we believe that this molecule plays a very important role to tie both pathways to TA. Similarly, we look at the NOTCH pathway and identify the HEY2 as a potential link between this pathway and TA. Another transcription factor that has been implicated in TA is NFATC1. While several mouse models exist that include part of the TA abnormality as their phenotype, no true mouse model can be said to represent TA. Bridging this gap will surely shed light on this complex molecular pathway and allow for better understanding of the disease process.

Towards Understanding the Gene-Specific Roles of GATA Factors in Heart Development: Does GATA4 Lead the Way?

Transcription factors play crucial roles in the regulation of heart induction, formation, growth and morphogenesis. Zinc finger GATA transcription factors are among the critical regulators of these processes. GATA4, 5 and 6 genes are expressed in a partially overlapping manner in developing hearts, and GATA4 and 6 continue their expression in adult cardiac myocytes. Using different experimental models, GATA4, 5 and 6 were shown to work together not only to ensure specification of cardiac cells but also during subsequent heart development. The complex involvement of these related gene family members in those processes is demonstrated through the redundancy among them and crossregulation of each other. Our recent identification at the genome-wide level of genes specifically regulated by each of the three family members and our earlier discovery that gata4 and gata6 function upstream, while gata5 functions downstream of noncanonical Wnt signalling during cardiac differentiation, clearly demonstrate the functional differences among the cardiogenic GATA factors. Such suspected functional differences are worth exploring more widely. It appears that in the past few years, significant advances have indeed been made in providing a deeper understanding of the mechanisms by which each of these molecules function during heart development. In this review, I will therefore discuss current evidence of the role of individual cardiogenic GATA factors in the process of heart development and emphasize the emerging central role of GATA4.

Functional coordination of non-myocytes plays a key role in adult zebrafish heart regeneration

Cardiac regeneration occurs primarily through proliferation of existing cardiomyocytes, but also involves complex interactions between distinct cardiac cell types including non-cardiomyocytes (non-CMs). However, the subpopulations, distinguishing molecular features, cellular functions, and intercellular interactions of non-CMs in heart regeneration remain largely unexplored. Using the LIGER algorithm, we assemble an atlas of cell states from 61,977 individual non-CM scRNA-seq profiles isolated at multiple time points during regeneration. This analysis reveals extensive non-CM cell diversity, including multiple macrophage (MC), fibroblast (FB), and endothelial cell (EC) subpopulations with unique spatiotemporal distributions, and suggests an important role for MC in inducing the activated FB and EC subpopulations. Indeed, pharmacological perturbation of MC function compromises the induction of the unique FB and EC subpopulations. Furthermore, we developed computational algorithm Topologizer to map the topological relationships and dynamic transitions between functional states. We uncover dynamic transitions between MC functional states and identify factors involved in mRNA processing and transcriptional regulation associated with the transition. Together, our single-cell transcriptomic analysis of non-CMs during cardiac regeneration provides a blueprint for interrogating the molecular and cellular basis of this process.

Decoding an organ regeneration switch by dissecting cardiac regeneration enhancers

Heart regeneration in regeneration-competent organisms can be accomplished through the remodeling of gene expression in response to cardiac injury. This dynamic transcriptional response relies on the activities of tissue regeneration enhancer elements (TREEs); however, the mechanisms underlying TREEs are poorly understood. We dissected a cardiac regeneration enhancer in zebrafish to elucidate the mechanisms governing spatiotemporal gene expression during heart regeneration. Cardiac lepb regeneration enhancer (cLEN) exhibits dynamic, regeneration-dependent activity in the heart. We found that multiple injury-activated regulatory elements are distributed throughout the enhancer region. This analysis also revealed that cardiac regeneration enhancers are not only activated by injury, but surprisingly, they are also actively repressed in the absence of injury. Our data identified a short (22 bp) DNA element containing a key repressive element. Comparative analysis across Danio species indicated that the repressive element is conserved in closely related species. The repression mechanism is not operational during embryogenesis and emerges when the heart begins to mature. Incorporating both activation and repression components into the mechanism of tissue regeneration constitutes a new paradigm that might be extrapolated to other regeneration scenarios.

BMP10 Signaling Promotes the Development of Endocardial Cells from Human Pluripotent Stem Cell-Derived Cardiovascular Progenitors

The embryonic endocardium is essential for early heart development as it functions to induce trabecular myocardium, the first heart tissue to form, and is the source of the cells that make up the valves and a portion of the coronary vasculature. With this potential, human endocardial cells could provide unique therapeutic opportunities that include engineering biological valves and cell-based therapy strategies to replace coronary vasculature in damaged hearts. To access human endocardial cells, we generated a human pluripotent stem cell (hPSC)-derived endothelial population that displays many characteristics of endocardium, including expression of the cohort of genes that identifies this lineage in vivo, the capacity to induce a trabecular fate in immature cardiomyocytes in vitro, and the ability to undergo an endothelial-to-mesenchymal transition. Analyses of the signaling pathways required for development of the hPSC-derived endocardial cells identified a novel role for BMP10 in the specification of this lineage from cardiovascular mesoderm.

From embryogenesis to adulthood: Critical role for GATA factors in heart development and function

Cardiac development is governed by a complex network of transcription factors (TFs) that regulate cell fates in a spatiotemporal manner. Among these, the GATA family of zinc finger TFs plays prominent roles in regulating the development of the myocardium, endocardium, and outflow tract. This family comprises six members three of which, GATA4, 5, and 6, are predominantly expressed in cardiac cells where they activate specific downstream gene targets via interactions with one another and with other TFs and signaling molecules. Their critical function in heart formation is evidenced by the phenotypes of animal models lacking these factors and by the broad spectrum of human congenital heart diseases associated with mutations in their genes. Similarly, in the postnatal heart, these proteins play significant and nonredundant roles in cardiac function, regulating adaptive stress responses including cardiomyocyte hypertrophy and survival, as well as endothelial homeostasis and angiogenesis. As such, decreased expression of either GATA4, 5, or 6 results in impaired cardiovascular homeostasis and increased risk of premature and serious cardiovascular events such as hypertension, arrhythmia, aortopathy, and heart failure. Although a great deal of progress has been made in understanding GATA-dependent regulatory processes in the heart, the molecular mechanisms underlying the specificity of GATA factors and their upstream regulation remain incompletely understood. The knowledge and tools developed since their discovery 25 years ago should accelerate progress toward further elucidation of their mechanisms of action in health and disease. This in turn will greatly improve diagnosis and care for the millions of individuals affected by congenital and acquired cardiac disease worldwide.

Human pluripotent stem cell-derived epicardial progenitors can differentiate to endocardial-like endothelial cells

During heart development, epicardial progenitors contribute various cardiac lineages including smooth muscle cells, cardiac fibroblasts, and endothelial cells. However, their specific contribution to the human endothelium has not yet been resolved, at least in part due to the inability to expand and maintain human primary or pluripotent stem cell (hPSC)‐derived epicardial cells. Here we first generated CDH5‐2A‐eGFP knock‐in hPSC lines and differentiated them into self‐renewing WT1+ epicardial cells, which gave rise to endothelial cells upon VEGF treatment in vitro. In addition, we found that the percentage of endothelial cells correlated with WT1 expression in a WT1‐2A‐eGFP reporter line. The resulting endothelial cells displayed many endocardium‐like endothelial cell properties, including high expression levels of endocardial‐specific markers, nutrient transporters and well‐organized tight junctions. These findings suggest that human epicardial progenitors may have the capacity to form endocardial endothelium during development and have implications for heart regeneration and cardiac tissue engineering.

KLF13 is a genetic modifier of the Holt-Oram syndrome gene TBX5

TBX5, a member of the T-box family of transcription factors, is a dosage sensitive regulator of heart development. Mutations in TBX5 are responsible for Holt-Oram Syndrome, an autosomal dominant disease with variable and partially penetrant cardiac defects suggestive of the existence of genetic and environmental modifiers. KLF13, a member of the Krüppel-like family of zinc finger proteins is co-expressed with TBX5 in several cardiac cells including atrial cardiomyocytes and cells of the interatrial septum. We report that KLF13 interacts physically and functionally with TBX5 to synergistically activate transcription of cardiac genes. We show that TBX5 contacts KLF13 via its T-domain and find that several disease-causing mutations therein have decreased KLF13 interaction. Whereas Klf13 heterozygote mice have no detectable cardiac defects, loss of a Klf13 allele in Tbx5 heterozygote mice significantly increases the penetrance of TBX5-dependent cardiac abnormalities including atrial, atrial-ventricular and ventricular septal defects. The results reveal for the first time combinatorial interaction between a T-box protein and a KLF family member and its importance for heart and possibly other organ development. The data also suggest that, in human, KLF13 may be a genetic modifier of the Holt-Oram Syndrome gene TBX5.

GATA5 mutation homozygosity linked to a double outlet right ventricle phenotype in a Lebanese patient

Background

GATA transcription factors are evolutionary conserved zinc finger proteins with multiple roles in cell differentiation/proliferation and organogenesis. GATA5 is only transiently expressed in the embryonic heart, and the inactivation of both Gata5 alleles results in a partially penetrant bicuspid aortic valve (BAV) phenotype in mice. We hypothesized that only biallelic mutations in GATA5 could be disease causing.

Methods

A total of 185 patients with different forms of congenital heart disease (CHD) were screened along 150 healthy individuals for GATA4, 5, and 6. All patients' phenotypes were diagnosed with echocardiography.

Results

Sequencing results revealed eight missense variants (three of which are novel) in cases with various conotruncal and septal defects. Out of these, two were inherited in recessive forms: the p.T67P variant, which was found both in patients and in healthy individuals, and the previously described p.Y142H variant which was only found in a patient with a double outlet right ventricle (DORV). We characterized the p.Y142H variant and showed that it significantly reduced the transcriptional activity of the protein over cardiac promoters by 30–40%.

Conclusion

Our results do prove that p.Y142H is associated with DORV and suggests including GATA5 as a potential gene to be screened in patients with this phenotype.

Endothelial Gata5 transcription factor regulates blood pressure

Despite its high prevalence and economic burden, the aetiology of human hypertension remains incompletely understood. Here we identify the transcription factor GATA5, as a new regulator of blood pressure (BP). GATA5 is expressed in microvascular endothelial cells and its genetic inactivation in mice (Gata5-null) leads to vascular endothelial dysfunction and hypertension. Endothelial-specific inactivation of Gata5 mimics the hypertensive phenotype of the Gata5-null mice, suggestive of an important role for GATA5 in endothelial homeostasis. Transcriptomic analysis of human microvascular endothelial cells with GATA5 knockdown reveals that GATA5 affects several genes and pathways critical for proper endothelial function, such as PKA and nitric oxide pathways. Consistent with a role in human hypertension, we report genetic association of variants at the GATA5 locus with hypertension traits in two large independent cohorts. Our results unveil an unsuspected link between GATA5 and a prominent human condition, and provide a new animal model for hypertension.

Unravelling the molecular basis of hypertension remains a major challenge. Here, the authors identify the transcription factor GATA5 as a novel regulator of blood pressure and potential genetic determinant of human hypertension and describe a unique mouse model for research of salt-sensitive hypertension.

Ageing is a risk factor in imatinib mesylate cardiotoxicity

AIMS

Chemotherapy-induced heart failure is increasingly recognized as a major clinical challenge. Cardiotoxicity of imatinib mesylate, a highly selective and effective anticancer drug belonging to the new class of tyrosine kinase inhibitors, is being reported in patients, some progressing to congestive heart failure. This represents an unanticipated challenge that could limit effective drug use. Understanding the mechanisms and risk factors of imatinib mesylate cardiotoxicity is crucial for prevention of cardiovascular complications in cancer patients.

METHODS AND RESULTS

We used genetically engineered mice and primary rat neonatal cardiomyocytes to analyse the action of imatinib on the heart. We found that treatment with imatinib (200 mg/kg/day for 5 weeks) leads to mitochondrial-dependent myocyte loss and cardiac dysfunction, as confirmed by electron microscopy, RNA analysis, and echocardiography. Imatinib cardiotoxicity was more severe in older mice, in part due to an age-dependent increase in oxidative stress. Mechanistically, depletion of the transcription factor GATA4 resulting in decreased levels of its prosurvival targets Bcl-2 and Bcl-XL was an underlying cause of imatinib toxicity. Consistent with this, GATA4 haploinsufficient mice were more susceptible to imatinib, and myocyte-specific up-regulation of GATA4 or Bcl-2 protected against drug-induced cardiotoxicity.

CONCLUSION

The results indicate that imatinib action on the heart targets cardiomyocytes and involves mitochondrial impairment and cell death that can be further aggravated by oxidative stress. This in turn offers a possible explanation for the current conflicting data regarding imatinib cardiotoxicity in cancer patients and suggests that cardiac monitoring of older patients receiving imatinib therapy may be especially warranted.

MicroRNA-208a Silencing Attenuates Doxorubicin Induced Myocyte Apoptosis and Cardiac Dysfunction

AIMS

GATA4 depletion is a distinct mechanism by which doxorubicin leads to cardiomyocyte apoptosis, and preservation of GATA4 mitigates doxorubicin induced myocyte apoptosis and cardiac dysfunction. We investigated a novel approach of attenuating doxorubicin induced cardiac toxicity by silencing miR-208a, a heart specific microRNA known to target GATA4.

METHODS AND RESULTS

Eight-week-old female Balb/C mice were randomly assigned to sham, antagomir, and control groups. Antagomir group were pretreated with miR-208a antagomir 4 days before doxorubicin administration. At day 0, control and antagomir groups received 20 mg/kg of doxorubicin, while sham mice received phosphate buffered solution. Echocardiography was done at day 7, after which animals were sacrificed and hearts harvested and assessed for apoptosis and expression of miR-208a, GATA4, and BCL-2. Doxorubicin significantly upregulated miR-208a, downregulated GATA4, and increased myocyte apoptosis, with resulting decrease in cardiac function. In contrast, therapeutic silencing of miR-208a salvaged GATA4 and BCL-2 and decreased apoptosis, with improvement in cardiac function.

CONCLUSION

Doxorubicin upregulates miR-208a and promotes cardiomyocyte apoptosis, while therapeutic silencing of miR-208a attenuates doxorubicin induced myocyte apoptosis with subsequent improvement in cardiac function. These novel results highlight the therapeutic potential of targeting miR-208a to prevent doxorubicin cardiotoxicity.

Caspase-1 cleavage of transcription factor GATA4 and regulation of cardiac cell fate

Caspase-1 or interleukin-1β (IL-1β) converting enzyme is a pro-inflammatory member of the caspase family. An IL-1β-independent role for caspase-1 in cardiomyocyte cell death and heart failure has emerged but the mechanisms underlying these effects are incompletely understood. Here, we report that transcription factor GATA4, a key regulator of cardiomyocyte survival and adaptive stress response is an in vivo and in vitro substrate for caspase-1. Caspase-1 mediated cleavage of GATA4 generates a truncated protein that retains the ability to bind DNA but lacks transcriptional activation domains and acts as a dominant negative regulator of GATA4. We show that caspase-1 is rapidly activated in cardiomyocyte nuclei treated with the cell death inducing drug Doxorubicin. We also find that inhibition of caspase-1 alone is as effective as complete caspase inhibition at rescuing GATA4 degradation and myocyte cell death. Caspase-1 inhibition of GATA4 transcriptional activity is rescued by HSP70, which binds directly to GATA4 and masks the caspase recognition motif. The data identify a caspase-1 nuclear substrate and suggest a direct role for caspase-1 in transcriptional regulation. This mechanism may underlie the inflammation-independent action of caspase-1 in other organs.

Novel and functional DNA sequence variants within the GATA5 gene promoter in ventricular septal defects

BACKGROUND

Congenital heart disease (CHD) is the most common human birth defect. Genetic causes for CHD remain largely unknown. GATA transcription factor 5 (GATA 5) is an essential regulator for the heart development. Mutations in the GATA5 gene have been reported in patients with a variety of CHD. Since misregulation of gene expression have been associated with human diseases, we speculated that changed levels of cardiac transcription factors, GATA5, may mediate the development of CHD.

METHODS

In this study, GATA5 gene promoter was genetically and functionally analyzed in large cohorts of patients with ventricular septal defect (VSD) (n=343) and ethnic-matched healthy controls (n=348).

RESULTS

Two novel and heterozygous DNA sequence variants (DSVs), g.61051165A>G and g.61051463delC, were identified in three VSD patients, but not in the controls. In cultured cardiomyocytes, GATA5 gene promoter activities were significantly decreased by DSV g.61051165A>G and increased by DSV g.61051463delC. Moreover, fathers of the VSD patients carrying the same DSVs had reduced diastolic function of left ventricles. Three SNPs, g.61051279C>T (rs77067995), g.61051327A>C (rs145936691) and g.61051373G>A (rs80197101), and one novel heterozygous DSV, g.61051227C>T, were found in both VSD patients and controls with similar frequencies.

CONCLUSION

Our data suggested that the DSVs in the GATA5 gene promoter may increase the susceptibility to the development of VSD as a risk factor.

Transcriptional inhibition of etv2 expression is essential for embryonic cardiac development

E-twenty six variant 2 (Etv2) transcription factor participates in cardiac, vascular-endothelial and blood cell lineage specification decisions during embryonic development. Previous studies have identified genomic elements in the etv2 locus responsible for vascular endothelial cell specification. Using transgenic analysis in zebrafish, we report here an etv2 proximal promoter fragment that prevents transgene misexpression in myocardial progenitor cells. This inhibition of etv2 expression in the cardiac progenitor population is partly mediated by Scl and Nkx2.5, likely through direct binding to the etv2 promoter, and cis-regulatory elements located in the first and second introns. The results identify an etv2 cis-regulatory mechanism controlling cardiovascular fate choice implying that etv2 participates in a transcriptional network mediating developmental plasticity of endothelial progenitor cells during embryonic development.

Rare GATA5 sequence variants identified in individuals with bicuspid aortic valve

BACKGROUND

Bicuspid aortic valve (BAV) is the most common type of congenital heart disease (CHD) and has a proposed genetic etiology. BAV is categorized by cusp fusion, with right-left (R-L) cusp fusion being associated with additional CHD, and right-noncoronary cusp (R-NC) fusion being associated with aortic valve dysfunction. Loss of murine Gata5, which encodes a cardiac transcription factor, results in a partially penetrant R-NC BAV, and we hypothesize that mutations in GATA5 are associated with R-NC BAV in humans.

METHODS

A cohort of 78 BAV patients (50 with isolated BAV and 28 with associated aortic coarctation) was analyzed using Sanger sequencing to identify GATA5 sequence variants. Biochemical assays were performed to identify functional deficits of identified sequence variants.

RESULTS

We identified two rare heterozygous nonsynonymous variants, p.Gln3Arg and p.Leu233Pro, for a frequency of 2.6% (2/78). Both individuals with nonsynonymous variants had BAV and aortic coarctation, one R-L and one R-NC subtype. Of the nonsynonymous variants, only p.Gln3Arg demonstrated decreased transcriptional activity in vitro.

CONCLUSION

Rare sequence variants in GATA5 are associated with human BAV. Our findings suggest a genotype-phenotype correlation in regards to associated CHD but not cusp fusion.

GATA5 loss-of-function mutations associated with congenital bicuspid aortic valve

Bicuspid aortic valve (BAV) is the most common form of congenital cardiovascular defect in humans worldwide and is responsible for substantial morbidity and mortality. Accumulating evidence has demonstated that genetic risk factors are involved in the pathogenesis of BAV. However, BAV is genetically heterogeneous and the genetic basis underlying BAV in a large number of patients remains unknown. In the present study, the coding regions and splice junction sites of the GATA5 gene, which codes for a zinc-finger transcription factor crucial for the normal development of the aortic valve, was sequenced initially in 110 unrelated patients with BAV. The available relatives of the mutation carriers and 200 unrelated healthy individuals used as controls were subsequently genotyped for GATA5. The functional effect of the mutations was characterized by using a luciferase reporter assay system. As a result, two novel heterozygous GATA5 mutations, p.Y16D and p.T252P, were identified in two families with autosomal dominant inheritance of BAV, respectively. The variations were absent in 400 control chromosomes and the altered amino acids were completely conserved evolutionarily. Functional assays revealed that the two GATA5 mutants were associated with significantly reduced transcriptional activity compared with their wild-type counterpart. To the best of our knowledge, this is the first study on the association of GATA5 loss-of-function mutations with enhanced susceptibility to BAV, providing novel insight into the molecular mechanism involved in human BAV and suggesting a potential role for the early prophylaxis and personalized treatment of this common congenital heart disease.

Isolation, characterization, and transplantation of cardiac endothelial cells

Isolation and ex vivo expansion of cardiac endothelial cells have been a recurrent challenge due to difficulties in isolation, cell heterogeneity, lack of specific markers to identify myocardial endothelial cells, and inadequate conditions to maintain long-term cultures. Herein, we developed a method for isolation, characterization, and expansion of cardiac endothelial cells applicable to study endothelial cell biology and clinical applications such as neoangiogenesis. First, we dissociated the cells from murine heart by mechanical disaggregation and enzymatic digestion. Then, we used flow cytometry coupled with specific markers to isolate endothelial cells from murine hearts. CD45+ cells were gated out to eliminate the hematopoietic cells. CD31+/Sca-1+ cells were isolated as endothelial cells. Cells isolated from atrium grew faster than those from ventricle. Cardiac endothelial cells maintain endothelial cell function such as vascular tube formation and acetylated-LDL uptake in vitro. Finally, cardiac endothelial cells formed microvessels in dorsal matrigel plug and engrafted in cardiac microvessels following intravenous and intra-arterial injections. In conclusion, our multicolor flow cytometry method is an effective method to analyze and purify endothelial cells from murine heart, which in turn can be ex vivo expanded to study the biology of endothelial cells or for clinical applications such as therapeutic angiogenesis.

Expression and potential role of GATA factors in trophoblast development

Despite exhaustive studies, molecular mechanisms governing blastocyst formation, implantation to the uterine endometrium and placentation have not been definitively characterized. GATA family proteins are a group of zinc finger transcription factors, for which gene ablations eventually result in embryonic death later in pregnancy. These findings suggested that GATA factors are not essential for early embryonic development. However, recent studies from our laboratory and others have revealed that GATA proteins are involved in the regulation of key genes expressed by the trophectoderm that underpin the transition from the morula to trophoblast, and trophectoderm maintenance. Consequently, it is important to consider the current understanding how GATA factors govern early trophectoderm development.

Atrial identity is determined by a COUP-TFII regulatory network

Atria and ventricles exhibit distinct molecular profiles that produce structural and functional differences between the two cardiac compartments. However, the factors that determine these differences remain largely undefined. Cardiomyocyte-specific COUP-TFII ablation produces ventricularized atria that exhibit ventricle-like action potentials, increased cardiomyocyte size, and development of extensive T tubules. Changes in atrial characteristics are accompanied by alterations of 2,584 genes, of which 81% were differentially expressed between atria and ventricles, suggesting that a major function of myocardial COUP-TFII is to determine atrial identity. Chromatin immunoprecipitation assays using E13.5 atria identified classic atrial-ventricular identity genes Tbx5, Hey2, Irx4, MLC2v, MLC2a, and MLC1a, among many other cardiac genes, as potential COUP-TFII direct targets. Collectively, our results reveal that COUP-TFII confers atrial identity through direct binding and by modulating expression of a broad spectrum of genes that have an impact on atrial development and function.

Regulation of de novo ceramide synthesis: the role of dihydroceramide desaturase and transcriptional factors NFATC and Hand2 in the hypoxic mouse heart

We have previously shown that ceramide, a proapoptotic molecule decreases in the mouse heart as it adapts to hypoxia. We have also shown that its precursor, dihydroceramide, accumulates with hypoxia. This implicates the enzyme dihydroceramide desaturase (DHC-DS), which converts dihydroceramide to ceramide, in a potential regulatory checkpoint in cardiomyocytes. We hypothesised that the regulation of de novo ceramide synthesis plays an important role in the cardiomyocyte adaptation to hypoxia. We used an established mouse model to induce acute and chronic hypoxia. Cardiac tissues were extracted and quantitative real-time polymerase chain reaction (qRT-PCR) was used to evaluate the expression levels of DHC-DS. Electrophoretic Mobility Shift Assays (EMSAs) and qRT-PCR were used to evaluate the activity and expression levels of an array of transcription factors that might regulate DEGS1 gene expression. We demonstrated that DEGS1 mRNA levels decrease with time in hypoxic mice concurrent with the decrease in HAND2 transcripts. Interestingly, the DEGS1 promoter harbors overlapping sites for Hand2 and Nuclear Factor of Activated T-cells (NFATC) transcription factors. We have demonstrated a physical interaction between NFATC1 and the E-Box proteins with EMSA and coimmunoprecipitation assays. The regulation of de novo ceramide synthesis in response to hypoxia and this newly described interaction between E-box and NFATC transcription factors will pave the way to identify new pathways in the adaptation of the cardiomyocyte to stress. The elucidation of these pathways will in the long-term provide insights into potential targets for novel therapeutic regimens.

Nfatc1 directs the endocardial progenitor cells to make heart valve primordium

Heart valves arise from the cardiac endocardial cushions located at the atrioventricular canal (AVC) and cardiac outflow tract (OFT) during development. A subpopulation of cushion endocardial cells undergoes endocardial to mesenchymal transformation (EMT) and generates the cushion mesenchyme, which is then remodeled into the interstitial tissue of the mature valves. The cushion endocardial cells that do not undertake EMT proliferate to elongate valve leaflets. During EMT and the post-EMT valve remodeling, endocardial cells at the cushions highly express nuclear factor in activated T cell, cytoplasmic 1 (Nfatc1), a transcription factor required for valve formation in mice. In this review, we present the current knowledge of Nfatc1 roles in the ontogeny of heart valves with a focus on the fate decision of the endocardial cells in the processes of EMT and valve remodeling.

Two heterozygous mutations in NFATC1 in a patient with Tricuspid Atresia

Tricuspid Atresia (TA) is a rare form of congenital heart disease (CHD) with usually poor prognosis in humans. It presents as a complete absence of the right atrio-ventricular connection secured normally by the tricuspid valve. Defects in the tricuspid valve are so far not associated with any genetic locus, although mutations in numerous genes were linked to multiple forms of congenital heart disease. In the last decade, Knock-out mice have offered models for cardiologists and geneticists to study the causes of congenital disease. One such model was the Nfatc1(-/-) mice embryos which die at mid-gestation stage due to a complete absence of the valves. NFATC1 belongs to the Rel family of transcription factors members of which were shown to be implicated in gene activation, cell differentiation, and organogenesis. We have previously shown that a tandem repeat in the intronic region of NFATC1 is associated with ventricular septal defects. In this report, we unravel for the first time a potential link between a mutation in NFATC1 and TA. Two heterozygous missense mutations were found in the NFATC1 gene in one indexed-case out of 19 patients with TA. The two amino-acids changes were not found neither in other patients with CHDs, nor in the control healthy population. Moreover, we showed that these mutations alter dramatically the normal function of the protein at the cellular localization, DNA binding and transcriptional levels suggesting they are disease-causing.

Genetic insights into bicuspid aortic valve formation

Bicuspid aortic valve (BAV) is the most common congenital heart defect, affecting 1-2% of the population. It is generally diagnosed late in adulthood when deterioration of the abnormal leaflet becomes clinically evident. BAV patients have an increased risk of developing serious complications, including stenosis, regurgitation, endocarditis, dilation of the aorta, aortic dissection, and aneurysm. BAV is a heritable trait, but the genetic basis underlying this cardiac malformation remains poorly understood. In the last decade, thanks to studies in animal models as well as genetic and biochemical approaches, a large number of genes that play important roles in heart development have been identified. These discoveries provided valuable insight into cardiac morphogenesis and uncovered congenital-heart-disease-causing genes. This paper will summarize the current knowledge of valve morphogenesis as well as our current understanding of the genetic pathways involved in BAV formation. The impact of these advances on human health including diagnosis of BAV and prevention of cardiovascular complications in individuals with BAV or with a family history of BAV is also discussed.

Distinct compartments of the proepicardial organ give rise to coronary vascular endothelial cells

The proepicardial organ is an important transient structure that contributes cells to various cardiac lineages. However, its contribution to the coronary endothelium has been disputed, with conflicting data arising in chick and mouse. Here we resolve this conflict by identifying two proepicardial markers, Scleraxis (Scx) and Semaphorin3D (Sema3D), that genetically delineate heretofore uncharacterized proepicardial subcompartments. In contrast to previously fate-mapped Tbx18/WT-1-expressing cells that give rise to vascular smooth muscle, Scx- and Sema3D-expressing proepicardial cells give rise to coronary vascular endothelium both in vivo and in vitro. Furthermore, Sema3D(+) and Scx(+) proepicardial cells contribute to the early sinus venosus and cardiac endocardium, respectively, two tissues linked to vascular endothelial formation at later stages. Taken together, our studies demonstrate that the proepicardial organ is a molecularly compartmentalized structure, reconciling prior chick and mouse data and providing a more complete understanding of the progenitor populations that establish the coronary vascular endothelium.

Mechanisms of cardiogenesis in cardiovascular progenitor cells

Self-renewing cells of the vertebrate heart have become a major subject of interest in the past decade. However, many researchers had a hard time to argue against the orthodox textbook view that defines the heart as a postmitotic organ. Once the scientific community agreed on the existence of self-renewing cells in the vertebrate heart, their origin was again put on trial when transdifferentiation, dedifferentiation, and reprogramming could no longer be excluded as potential sources of self-renewal in the adult organ. Additionally, the presence of self-renewing pluripotent cells in the peripheral blood challenges the concept of tissue-specific stem and progenitor cells. Leaving these unsolved problems aside, it seems very desirable to learn about the basic biology of this unique cell type. Thus, we shall here paint a picture of cardiovascular progenitor cells including the current knowledge about their origin, basic nature, and the molecular mechanisms guiding proliferation and differentiation into somatic cells of the heart.

Loss of Gata5 in mice leads to bicuspid aortic valve

Bicuspid aortic valve (BAV), the leading congenital heart disease, occurs in 1%-2% of the population. Genetic studies suggest that BAV is an autosomal-dominant disease with reduced penetrance. However, only 1 gene, NOTCH1, has been linked to cases of BAV. Here, we show that targeted deletion of Gata5 in mice leads to hypoplastic hearts and partially penetrant BAV formation. Endocardial cell-specific inactivation of Gata5 led to BAV, similar to that observed in Gata5-/- mice. In all cases, the observed BAVs resulted from fusion of the right-coronary and noncoronary leaflets, the subtype associated with the more severe valve dysfunction in humans. Neither endocardial cell proliferation nor cushion formation was altered in the absence of Gata5. Rather, defective endocardial cell differentiation, resulting from the deregulation of several components of the Notch pathway and other important endocardial cell regulators, may be the underlying mechanism of disease. The results unravel a critical cell-autonomous role for endocardial Gata5 in aortic valve formation and identify GATA5 as a potential gene responsible for congenital heart disease in humans. Mice with mutated Gata5 alleles represent unique models to dissect the mechanisms underlying degenerative aortic valve disease and to develop much-needed preventive and therapeutic interventions.

GATA5 interacts with GATA4 and GATA6 in outflow tract development

Members of the GATA family of transcription factors are critical regulators of heart development and mutations in 2 of them, GATA4 and GATA6 are associated with outflow tract and septal defects in human. The heart expresses 3 GATA factors, GATA4, 5 and 6 in a partially overlapping pattern. Here, we report that compound Gata4/Gata5 and Gata5/Gata6 mutants die embryonically or perinatally due to severe congenital heart defects. Almost all Gata4(+/-)Gata5(+/-) mutant embryos have double outlet right ventricles (DORV), large ventricular septal defects (VSD) as well as hypertrophied mitral and tricuspid valves. Only 25% of double compound Gata4/Gata5 heterozygotes survive to adulthood and these mice have aortic stenosis. Compound loss of a Gata5 and a Gata6 allele also leads to DORVs associated with subaortic VSDs. Expression of several transcription factors important for endocardial and myocardial cell differentiation, such as Tbx20, Mef2c, Hey1 and Hand2, was reduced in compound heterozygote embryos. These findings suggest the existence of important genetic interactions between Gata5 and the 2 other cardiac GATA factors in endocardial cushion formation and outflow tract morphogenesis. The data identify GATA5 as a potential genetic modifier of congenital heart disease and provide insight for elucidating the genetic basis of an important class of human birth defects.

An endocardial pathway involving Tbx5, Gata4, and Nos3 required for atrial septum formation

In humans, septal defects are among the most prevalent congenital heart diseases, but their cellular and molecular origins are not fully understood. We report that transcription factor Tbx5 is present in a subpopulation of endocardial cells and that its deletion therein results in fully penetrant, dose-dependent atrial septal defects in mice. Increased apoptosis of endocardial cells lacking Tbx5, as well as neighboring TBX5-positive myocardial cells of the atrial septum through activation of endocardial NOS (Nos3), is the underlying mechanism of disease. Compound Tbx5 and Nos3 haploinsufficiency in mice worsens the cardiac phenotype. The data identify a pathway for endocardial cell survival and unravel a cell-autonomous role for Tbx5 therein. The finding that Nos3, a gene regulated by many congenital heart disease risk factors including stress and diabetes, interacts genetically with Tbx5 provides a molecular framework to understand gene-environment interaction in the setting of human birth defects.

Endothelin-1 gene regulation

Over two decades of research have demonstrated that the peptide hormone endothelin-1 (ET-1) plays multiple, complex roles in cardiovascular, neural, pulmonary, reproductive, and renal physiology. Differential and tissue-specific production of ET-1 must be tightly regulated in order to preserve these biologically diverse actions. The primary mechanism thought to control ET-1 bioavailability is the rate of transcription from the ET-1 gene (edn1). Studies conducted on a variety of cell types have identified key transcription factors that govern edn1 expression. With few exceptions, the cis-acting elements bound by these factors have been mapped in the edn1 regulatory region. Recent evidence has revealed new roles for some factors originally believed to regulate edn1 in a tissue or hormone-specific manner. In addition, other mechanisms involved in epigenetic regulation and mRNA stability have emerged as important processes for regulated edn1 expression. The goal of this review is to provide a comprehensive overview of the specific factors and signaling systems that govern edn1 activity at the molecular level.

Gene expression programs of mouse endothelial cells in kidney development and disease

Endothelial cells are remarkably heterogeneous in both morphology and function, and they play critical roles in the formation of multiple organ systems. In addition endothelial cell dysfunction can contribute to disease processes, including diabetic nephropathy, which is a leading cause of end stage renal disease. In this report we define the comprehensive gene expression programs of multiple types of kidney endothelial cells, and analyze the differences that distinguish them. Endothelial cells were purified from Tie2-GFP mice by cell dissociation and fluorescent activated cell sorting. Microarrays were then used to provide a global, quantitative and sensitive measure of gene expression levels. We examined renal endothelial cells from the embryo and from the adult glomerulus, cortex and medulla compartments, as well as the glomerular endothelial cells of the db/db mutant mouse, which represents a model for human diabetic nephropathy. The results identified the growth factors, receptors and transcription factors expressed by these multiple endothelial cell types. Biological processes and molecular pathways were characterized in exquisite detail. Cell type specific gene expression patterns were defined, finding novel molecular markers and providing a better understanding of compartmental distinctions. Further, analysis of enriched, evolutionarily conserved transcription factor binding sites in the promoters of co-activated genes begins to define the genetic regulatory network of renal endothelial cell formation. Finally, the gene expression differences associated with diabetic nephropathy were defined, providing a global view of both the pathogenic and protective pathways activated. These studies provide a rich resource to facilitate further investigations of endothelial cell functions in kidney development, adult compartments, and disease.

Development of the endocardium

The endocardium, the endothelial lining of the heart, plays complex and critical roles in heart development, particularly in the formation of the cardiac valves and septa, the division of the truncus arteriosus into the aortic and pulmonary trunks, the development of Purkinje fibers that form the cardiac conduction system, and the formation of trabecular myocardium. Current data suggest that the endocardium is a regionally specialized endothelium that arises through a process of de novo vasculogenesis from a distinct population of mesodermal cardiogenic precursors in the cardiac crescent. In this article, we review recent developments in the understanding of the embryonic origins of the endocardium. Specifically, we summarize vasculogenesis and specification of endothelial cells from mesodermal precursors, and we review the transcriptional pathways involved in these processes. We discuss the lineage relationships between the endocardium and other endothelial populations and between the endocardium and the myocardium. Finally, we explore unresolved questions about the lineage relationships between the endocardium and the myocardium. One of the central questions involves the timing with which mesodermal cells, which arise in the primitive streak and migrate to the cardiac crescent, become committed to an endocardial fate. Two competing conceptual models of endocardial specification have been proposed. In the first, mesodermal precursor cells in the cardiac crescent are prespecified to become either endocardial or myocardial cells, while in the second, fate plasticity is retained by bipotential cardiogenic cells in the cardiac crescent. We propose a third model that reconciles these two views and suggest future experiments that might resolve this question.

Cardiogenesis: an embryological perspective

Cardiogenesis, considered as the formation of new heart tissue from embryonic, postnatal, or adult cardiac progenitors, is a pivotal concept to understand the rationale of advanced therapies to repair the damaged heart. In this review, we focus on the cellular and molecular regulation of cardiogenesis in the developing embryo, and we dissect the complex interactions that control the diversification and maturation of a variety of cardiac cell lineages. Our aim is to show how the sophisticated anatomical structure of the adult four-chambered heart strongly depends on the fine regulation of the differentiation of cardiac progenitor cells. These events are shown to be progressive and dynamic as well as plastic, so that the patterned differentiation of distinct heart domains is highly dependent on signals provided by nonmyocardial heart components and extracardiac tissues. Finally, we present the core of our knowledge on cardiac embryogenesis in a biomedical context to provide a critical analysis on the logic of cell therapies designed to treat the failing heart.

GATA4 and GATA5 are essential for heart and liver development in Xenopus embryos

Background

GATA factors 4/5/6 have been implicated in the development of the heart and endodermal derivatives in vertebrates. Work in zebrafish has indicated that GATA5 is required for normal development earlier than GATA4/6. However, the GATA5 knockout mouse has no apparent embryonic phenotype, thereby questioning the importance of the gene for vertebrate development.

Results

In this study we show that in Xenopus embryos GATA5 is essential for early development of heart and liver precursors. In addition, we have found that in Xenopus embryos GATA4 is important for development of heart and liver primordia following their specification, and that in this role it might interact with GATA6.

Conclusion

Our results suggest that GATA5 acts earlier than GATA4 to regulate development of heart and liver precursors, and indicate that one early direct target of GATA5 is homeobox gene Hex.

Distinct expression and function of alternatively spliced Tbx5 isoforms in cell growth and differentiation

Mutations in the T-box transcription factor Tbx5 cause Holt-Oram syndrome, an autosomal dominant disease characterized by a wide spectrum of cardiac and upper limb defects with variable expressivity. Tbx5 haploinsufficiency has been suggested to be the underlying mechanism, and experimental models are consistent with a dosage-sensitive requirement for Tbx5 in heart development. Here, we report that Tbx5 levels are regulated through alternative splicing that generates, in addition to the known 518-amino-acid protein, a C-terminal truncated isoform. This shorter isoform retains the capacity to bind DNA, but its interaction with Tbx5 collaborators such as GATA-4 is altered. In vivo, the two spliced isoforms are oppositely regulated in a temporal and growth factor-dependent manner and are present in distinct DNA-binding complexes. The expression of the long isoform correlates with growth stimulation, and its reexpression in postnatal transgenic mouse hearts promotes hypertrophy. Conversely, the upregulation of the short but not the long isoform in C2C12 myoblasts leads to growth arrest and cell death. The results provide novel insight into posttranscriptional Tbx5 regulation and point to an important role not only in cell differentiation but also in cell proliferation and organ growth. The data may help analyze genotype-phenotype relations in patients with Holt-Oram syndrome.

Ca2+-Operated Transcriptional Networks: Molecular Mechanisms and In Vivo Models

Calcium is the most universal signal used by living organisms to convey information to many different cellular processes. In this review we present well-known and recently identified proteins that sense and decode the calcium signal and are key elements in the nucleus to regulate the activity of various transcriptional networks. When possible, the review also presents in vivo models in which the genes encoding these calcium sensors-transducers have been modified, to emphasize the critical role of these Ca2+-operated mechanisms in many physiological functions.

Redundancy and evolution of GATA factor requirements in development of the myocardium

The transcription factors, GATA4, 5 and 6, recognize the same DNA sequence and are all expressed in the developing myocardium. However, knockout studies in the mouse have indicated that none of them are absolutely required for the specification of the myocardium. Here we present evidence for redundancy in this family for the first time. Using morpholinos in both Xenopus and zebrafish embryos, we show that GATA4 knockdown, for example, only affects cardiac marker expression in the absence of either GATA5 or GATA6. A similar situation pertains for GATA5 in Xenopus whereas, in zebrafish, GATA5 (faust) plays a major role in driving the myocardial programme. This requirement for GATA5 in zebrafish is for induction of the myocardium, in contrast to the GATA6 requirement in both species, which is for differentiation. This early role for GATA5 in zebrafish correlates with its earlier expression and with an earlier requirement for BMP signalling, suggesting that a mutual maintenance loop for GATA, BMP and Nkx expression is the evolutionarily conserved entity.

Early endocardial morphogenesis requires Scl/Tal1

The primitive heart tube is composed of an outer myocardial and an inner endocardial layer that will give rise to the cardiac valves and septa. Specification and differentiation of these two cell layers are among the earliest events in heart development, but the embryonic origins and genetic regulation of early endocardial development remain largely undefined. We have analyzed early endocardial development in the zebrafish using time-lapse confocal microscopy and show that the endocardium seems to originate from a region in the lateral plate mesoderm that will give rise to hematopoietic cells of the primitive myeloid lineage. Endocardial precursors appear to rapidly migrate to the site of heart tube formation, where they arrive prior to the bilateral myocardial primordia. Analysis of a newly discovered zebrafish Scl/Tal1 mutant showed an additional and previously undescribed role of this transcription factor during the development of the endocardium. In Scl/Tal1 mutant embryos, endocardial precursors are specified, but migration is severely defective and endocardial cells aggregate at the ventricular pole of the heart. We further show that the initial fusion of the bilateral myocardial precursor populations occurs independently of the endocardium and tal1 function. Our results suggest early separation of the two components of the primitive heart tube and imply Scl/Tal1 as an indispensable component of the molecular hierarchy that controls endocardium morphogenesis.

In its earliest functional form, the embryonic heart of all vertebrates is a simple linear tube consisting of two cell types. An outer muscular cell layer called the myocardium surrounds an inner vascular cell layer called the endocardium that connects the heart to the vascular system. The integration of both cell types is an important step during heart development, but the formation of the endocardial component of the heart tube is poorly understood. Here, we analyze the formation of the endocardium in the zebrafish embryo and show using time-lapse imaging that it is a highly dynamic structure. In addition, we have identified a zebrafish mutant with a specific defect during endocardial development. This defect is caused by a mutation in T cell acute leukemia 1, a gene that—when misexpressed—causes many cases of childhood leukemias. Here, we show an additional role for this gene during heart development. In mutant embryos, both endocardial and myocardial precursors are specified, but integration of both cell types does not occur properly due to a defective migration of the endocardial precursors. Given the many interactions that occur between the endocardium and the myocardium, our results will provide a more comprehensive understanding of heart development.

Molecular and cellular basis of congenital heart disease

The cellular and molecular basis of congenital heart disease (CHD) is an evolving area of rapid discovery. This article introduced the basic mechanisms underlying cardiac development and CHD in order to permit a clear understanding of current diagnostics and therapeutics and their future development. It is clear that although significant advances have been made in understanding mechanisms controlling heart formation, the direct causes of CHD remain poorly defined. Future studies tha delineate the complexity of these mechanisms are required to provide a comprehensive understanding of the etiologies of CHD. Such understanding will lead to the development of novel approaches to prevention and therapy.

Rosiglitazone antagonizes vascular endothelial growth factor signaling and nuclear factor of activated T cells activation in cardiac valve endothelium

Nuclear factor of activated T cells, Cytoplasmic 1 (NFATc1) is required for heart valve formation. Vascular endothelial growth factor (VEGF) signaling, mediated by NFATc1 activation, positively regulates growth of valvular endothelial cells. However, regulators of VEGF/NFATc1 signaling in valve endothelium are poorly understood. Peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits NFATc1 activity in T cells and cardiomyocytes, but it is not known if PPARgamma controls NFATc1 function in endothelial cells. The authors hypothesize PPARgamma antagonizes VEGF signaling in valve endothelium by inhibiting NFATc1. Endothelial cells isolated from human valve leaflet tissue were shown by immunocytochemistry to express the endothelial-specific markers von Willebrand factor (vWF) and platelet endothelial cell adhesion molecule (PECAM)-1. VEGF-induced proliferation and migration of human pulmonary valve endothelial cells (HPVECs) were inhibited by rosiglitazone (ROSI), a specific ligand of PPARgamma activation, suggesting that PPARgamma disrupts VEGF signaling in the valve endothelium. ROSI also antagonized VEGF-mediated NFATc1 nuclear translocation in HPVECs, suggesting that PPARgamma inhibits VEGF signaling of NFATc1 activation in the valve. The effect of ROSI on nonvalve human umbilical vein endothelial cells (HUVECs) was tested in parallel and a similar inhibition of NFATc1 activation was observed. These data provide the first demonstration that ROSI negatively regulates VEGF signaling in the valve endothelium by a mechanism involving NFATc1 activation and nuclear translocation.

NFATc1 expression in the developing heart valves is responsive to the RANKL pathway and is required for endocardial expression of cathepsin K

NFATc1 is necessary for remodeling endocardial cushions into mature heart valve leaflets and is also an essential effector of receptor activator of NFkappaB ligand (RANKL) signaling required for transcriptional activation of bone matrix remodeling enzymes during osteoclast differentiation. Therefore, developing heart valves were examined to determine if NFATc1 functions in the RANKL pathway during leaflet remodeling. Key components of RANKL signal transduction including RANKL, its receptor RANK, and the downstream remodeling enzyme cathepsin K (Ctsk) are expressed in the heart during valve remodeling and colocalize with NFATc1 in developing valve endocardium. However, the absence of tartrate-resistant acid phosphatase (TRAP) activity and the lack of F4/80-positive macrophage lineage contribution to the remodeling valves demonstrate that certain aspects of osteoclast RANKL function are not shared during valve formation. Analysis of NFATc1-/- mouse embryos shows that NFATc1 is specifically required for endocardial expression of RANKL and Ctsk during valve formation. In addition, RANKL treatment augments expression of NFATc1 and Ctsk in embryonic heart cultures, and the RANKL-mediated increase in Ctsk expression is dependent on NFATc1. Together, these results support a role for RANKL signaling during heart valve development and suggest that valve leaflet morphogenesis involves NFATc1-dependent expression of remodeling enzymes including Ctsk.

MEF2-dependent Recruitment of the HAND1 Transcription Factor Results in Synergistic Activation of Target Promoters

HAND proteins are tissue-restricted members of the basic helix-loop-helix transcription factor family that play critical roles in cell differentiation and organogenesis including placental, cardiovascular, and craniofacial development. Nevertheless, the molecular basis underlying the developmental action of HAND proteins remains undefined. Within the embryo, HAND1 is first detected in the developing heart where it becomes restricted to the atrial and left ventricular compartments, a pattern identical to that of the Nppa gene, which encodes atrial natriuretic factor, the major secretory product of the heart. We hereby report that the cardiac atrial natriuretic factor promoter is directly activated by HAND1, making it the first known HAND1 transcriptional target. The action of HAND1 does not require heterodimerization with class I basic helix-loop-helix factors or DNA binding through E-box elements. Instead, HAND1 is recruited to the promoter via physical interaction with MEF2 proteins. MEF2/HAND1 interaction results in synergistic activation of MEF2-dependent promoters, and MEF2 binding sites are sufficient to mediate this synergy. MEF2 binding to DNA is not enhanced in the presence of HAND1. Instead, cooperativity likely results from corecruitment of co-activators such as CREB-binding protein. The related HAND2 protein can also synergize with MEF2. Thus, HAND proteins act as cell-specific developmental co-activators of the MEF2 family of transcription factors. These findings identify a novel mechanism for HAND action in the heart and provide a general paradigm to understand the mechanism of HAND action in organogenesis.

Regulation of calcineurin through transcriptional induction of the calcineurin A beta promoter in vitro and in vivo

The calcineurin-nuclear factor of activated T cells (NFAT) signaling pathway has been shown to be of critical importance in regulating the growth response of cardiac myocytes. We have previously demonstrated that calcineurin A(beta) (CnA(beta)) mRNA and protein are increased in response to growth stimulation, although the precise regulatory mechanism underlying CnA(beta) upregulation is not clear. Here, we isolated the mouse CnA(beta) promoter and characterized its responsiveness to growth stimuli in vitro and in vivo. A 2.3-kb promoter fragment was strongly activated by phenylephrine and endothelin-1 stimulation and by cotransfection with constitutively active CnA, NFATc4, and GATA4. Using chromatin immunoprecipitation, sequence regions were identified within the 2.3-kb promoter that associated with NFAT and GATA4, as well as with acetylated histone H3, following agonist stimulation. Consistent with the chromatin immunoprecipitation experiments, deletion of the distal half of the CnA(beta) promoter severely reduced NFAT, GATA4, and hypertrophic agonist-mediated activation. To investigate in vivo activity, we generated beta-galactosidase (LacZ) containing transgenic mice under the control of the CnA(beta) 2.3-kb promoter. CnA(beta)-LacZ mice showed expression in the heart that was cyclosporine sensitive, as well as expression in the central nervous system and skeletal muscle from early embryonic stages through adulthood. CnA(beta)-LacZ mice were subjected to cardiac pressure overload stimulation and crossbreeding with mice containing cardiac-specific transgenes for activated calcineurin and NFATc4, which revealed inducible expression in the heart. These results indicate that the CnA(beta) 2.3-kb promoter is specifically activated by hypertrophic stimuli through a positive feedback mechanism involving NFAT and GATA4 transcription factors, suggesting transcriptional induction of CnA(beta) expression as an additional means of regulating calcineurin activity in the heart.

Development of heart valve leaflets and supporting apparatus in chicken and mouse embryos

Abnormalities in valvuloseptal development significantly contribute to congenital heart defects, yet the underlying causes are complex and poorly understood. Early cardiac regulatory genes are differentially expressed during valvuloseptal development, consistent with novel functions during heart chamber formation in chicken and mouse embryos. Distinct valve cell lineages were identified in the leaflets, chordae tendineae, and myotendinous junctions with the papillary muscles based on restricted expression of extracellular matrix molecules. Specific cell types within these structures demonstrate characteristics of chondrogenesis and tendon development, identified by scleraxis, type II collagen, and tenascin expression. In chicken embryos, valve remodeling and maturation accompanies a decrease in mitotic index indicated by reduced bromodeoxyuridine incorporation. Analysis of Tie2-cre x ROSA26R mice demonstrates that mature valve structures, including the atrioventricular and outflow tract semilunar valve leaflets, chordae tendineae, and the fibrous continuity that connects the septal leaflets of mitral and tricuspid valves, arise from endothelial cells of the endocardial cushions. Together, these studies provide novel insights into the origins and cell lineage diversity of mature valve structures in the developing vertebrate heart.

The roles of GATA-4, -5 and -6 in vertebrate heart development

The transcription factors GATA-4, -5 and -6 are expressed very early in heart tissue. Essential GATA sites have been detected in several cardiac genes and the cardiac GATA factors interact with a wide variety of cofactors which synergistically increase gene expression. These multi-protein transcriptional complexes confer promoter-specificity on the GATA factors and also on the more broadly expressed cofactors. Here we summarise the data on these interactions and represent the conclusions as a GATA factor-based genetic regulatory network for the heart. Of the three cardiac GATAs, GATA-4 is by far the most extensively studied, however, loss-of-function data question its presumed dominance during heart development as opposed to hypertrophy.

Using the zebrafish model to study GATA transcription factors

The zebrafish is an established animal model system that profits from the availability of strong experimental approaches in both genetics and embryology. As a vertebrate, zebrafish can be used to model many aspects of human development and disease. GATA transcription factors play important roles in the development of many organ systems, including those for hematopoietic, cardiovascular, reproductive, and gut-endoderm derived tissues. The six vertebrate GATA factors are highly conserved in zebrafish at the level of sequence, expression pattern, and function. The identification of mutants, establishment of transgenic GFP reporter fish, and the ease of performing loss- and gain-of-function experiments have all contributed new insight into our understanding of the regulation and function of GATA factors. We review recent advances toward this goal using the zebrafish system with a focus on hematopoiesis and cardiogenesis, and suggest how comparative genetics using the zebrafish genes might reveal core conserved properties, as well as changes in gene function that reflect different morphogenetic programs utilized by various vertebrate embryos.