The cardiac tissue-restricted homeobox protein Csx/Nkx2.5 physically associates with the zinc finger protein GATA4 and cooperatively activates atrial natriuretic factor gene expression

Xenopus: An emerging model for studying congenital heart disease

Congenital heart defects affect nearly 1% of all newborns and are a significant cause of infant death. Clinical studies have identified a number of congenital heart syndromes associated with mutations in genes that are involved in the complex process of cardiogenesis. The African clawed frog, Xenopus, has been instrumental in studies of vertebrate heart development and provides a valuable tool to investigate the molecular mechanisms underlying human congenital heart diseases. In this review, we discuss the methodologies that make Xenopus an ideal model system to investigate heart development and disease. We also outline congenital heart conditions linked to cardiac genes that have been well studied in Xenopus and describe some emerging technologies that will further aid in the study of these complex syndromes.

Mutations in the NKX2.5 gene and the PAX8 promoter in a girl with thyroid dysgenesis

CONTEXT

Screening of the known candidate genes involved in thyroid organogenesis has revealed mutations in a small subset of patients with congenital hypothyroidism due to thyroid dysgenesis (TD).

OBJECTIVE

We studied a girl with TD who had mutations in two transcription factors involved in thyroid development.

RESULTS

Sequencing analysis of candidate genes involved in thyroid gland development revealed a new paternally inherited heterozygous mutation in the NKX2.5 gene (S265R) and a new maternally inherited heterozygous mutation in the PAX8 promoter region (-456C>T). Both parents and a brother, who was also heterozygous for both mutations, were phenotypically normal. Immunofluorescence microscopy showed a correct nuclear localization of both wild-type (WT) and mutant NKX2.5 proteins. EMSA demonstrated that the mutant NKX2.5 binds to the NKE_2, DIO2, TG, and TPO promoter elements equally well as the WT protein. However, the mutant NKX2.5 protein showed a 30-40% reduced transactivation of the thyroglobulin and the thyroid peroxidase promoters and a dominant-negative effect of the mutant NKX2.5. EMSA studies of the WT and mutant PAX8 promoter sequences incubated with nuclear extracts from PCCL3 cells exhibited a loss of protein binding capacity of the mutant promoter. In addition, the mutant PAX8 promoter showed a significantly reduced transcriptional activation of a luciferase reporter gene in vitro. Thus, this promoter mutation is expected to lead to reduced PAX8 expression.

CONCLUSIONS

We identified new heterozygous mutations in both NKX2.5 and PAX8 genes of a girl with TD. Both defects might contribute to the phenotype.

Co-occupancy by multiple cardiac transcription factors identifies transcriptional enhancers active in heart

Identification of genomic regions that control tissue-specific gene expression is currently problematic. ChIP and high-throughput sequencing (ChIP-seq) of enhancer-associated proteins such as p300 identifies some but not all enhancers active in a tissue. Here we show that co-occupancy of a chromatin region by multiple transcription factors (TFs) identifies a distinct set of enhancers. GATA-binding protein 4 (GATA4), NK2 transcription factor-related, locus 5 (NKX2-5), T-box 5 (TBX5), serum response factor (SRF), and myocyte-enhancer factor 2A (MEF2A), here referred to as "cardiac TFs," have been hypothesized to collaborate to direct cardiac gene expression. Using a modified ChIP-seq procedure, we defined chromatin occupancy by these TFs and p300 genome wide and provided unbiased support for this hypothesis. We used this principle to show that co-occupancy of a chromatin region by multiple TFs can be used to identify cardiac enhancers. Of 13 such regions tested in transient transgenic embryos, seven (54%) drove cardiac gene expression. Among these regions were three cardiac-specific enhancers of Gata4, Srf, and swItch/sucrose nonfermentable-related, matrix-associated, actin-dependent regulator of chromatin, subfamily d, member 3 (Smarcd3), an epigenetic regulator of cardiac gene expression. Multiple cardiac TFs and p300-bound regions were associated with cardiac-enriched genes and with functional annotations related to heart development. Importantly, the large majority (1,375/1,715) of loci bound by multiple cardiac TFs did not overlap loci bound by p300. Our data identify thousands of prospective cardiac regulatory sequences and indicate that multiple TF co-occupancy of a genomic region identifies developmentally relevant enhancers that are largely distinct from p300-associated enhancers.

A de novo mutation in NKX2.5 associated with atrial septal defects, ventricular noncompaction, syncope and sudden death

BACKGROUND

Mutations in transcription factor NKX2.5 cause congenital heart disease (CHD). We identified a CHD family with atrial septal defects (ASDs), atrioventricular block, ventricular noncompaction, syncope and sudden death. Our objective is to identify the disease-causing mutation in the CHD family.

METHODS

Direct DNA sequence analysis was used to identify the CHD mutation. The functional effects of the mutation were characterized by a luciferase reporter assay and immunostaining.

RESULTS

A novel, de novo 2-bp insertion (c.512insGC) was identified in exon 2 of NKX2.5. Mutation c.512insGC co-segregates with CHD in the family, and is not present in 200 controls. Functional studies indicate that the c.512insGC mutation impedes nuclear localization of NKX2.5 and causes a total loss of transactivation activity of NKX2.5. Furthermore, no NKX2.5 mutation was identified in 125 sporadic Chinese CHD patients.

CONCLUSIONS

(1) NKX2.5 mutation c.512insGC is associated with ASDs, syncope and sudden death. It is the second de novo mutation identified in NKX2.5. (2) NKX2.5 mutations are rare in sporadic CHD patients. (3) This study for the first time identifies association between a NKX2.5 mutation and ventricular noncompaction. Our results significantly expand the phenotypic spectrum of NKX2.5 mutations.

Design and testing of regulatory cassettes for optimal activity in skeletal and cardiac muscles

Gene therapy for muscular dystrophies requires efficient gene delivery to the striated musculature and specific, high-level expression of the therapeutic gene in a physiologically diverse array of muscles. This can be achieved by the use of recombinant adeno-associated virus vectors in conjunction with muscle-specific regulatory cassettes. We have constructed several generations of regulatory cassettes based on the enhancer and promoter of the muscle creatine kinase gene, some of which include heterologous enhancers and individual elements from other muscle genes. Since the relative importance of many control elements varies among different anatomical muscles, we are aiming to tailor these cassettes for high-level expression in cardiac muscle, and in fast and slow skeletal muscles. With the achievement of efficient intravascular gene delivery to isolated limbs, selected muscle groups, and heart in large animal models, the design of cassettes optimized for activity in different muscle types is now a practical goal. In this protocol, we outline the key steps involved in the design of regulatory cassettes for optimal activity in skeletal and cardiac muscle, and testing in mature muscle fiber cultures. The basic principles described here can also be applied to engineering tissue-specific regulatory cassettes for other cell types.

Mutations of the GATA4 and NKX2.5 genes in Chinese pediatric patients with non-familial congenital heart disease

A number of mutations in GATA4 and NKX2.5 have been identified to be causative for a subset of familial congenital heart defects (CHDs) and a small number of sporadic CHDs. In this study, we evaluated common GATA4 and NKX2.5 mutations in 135 Chinese pediatric patients with non-familial congenital heart defects. Two novel mutations in the coding region of GATA4 were identified, namely, 487C >T (Pro163Ser) in exon 1 in a child with tetralogy of Fallot and 1220C >A (Pro407Gln) in exon 6 in a pediatric patient with outlet membranous ventricular septal defect. We also found 848C >A (Pro283Gln) in exon 2 of the NKX2.5 gene in a pediatric patient with ventricular septal defect, patent ductus arteriosus and aortic isthmus stenosis. None of the mutations was detected in healthy control subjects (n = 114). This study suggests that GATA4 and NKX2.5 missense mutations may be associated with congenital heart defects in pediatric Chinese patients. Further clinical studies with large samples are warranted.

A novel functional DEC1 promoter polymorphism -249T>C reduces risk of squamous cell carcinoma of the head and neck

Human DEC1 (deleted in esophageal cancer 1) gene is located on chromosome 9q, a region frequently deleted in various types of human cancers, including squamous cell carcinoma of the head and neck (SCCHN). However, only one epidemiological study has evaluated the association between DEC1 polymorphisms and cancer risk. In this hospital-based case-control study, four potentially functional single-nucleotide polymorphisms -1628 G>A (rs1591420), -606 T>C [rs4978620, in complete linkage disequilibrium with -249T>C (rs2012775) and -122 G>A(rs2012566)], c.179 C>T p.Ala60Val (rs2269700) and 3' untranslated region-rs3750505 as well as the TP53 tumor suppressor gene codon 72 (Arg72Pro, rs1042522) polymorphism were genotyped in 1111 non-Hispanic Whites SCCHN patients and 1130 age-and sex-matched cancer-free controls. After adjustment for age, sex and smoking and drinking status, the variant -606CC (i.e. -249CC) homozygotes had a significantly reduced SCCHN risk (adjusted odds ratio = 0.71, 95% confidence interval = 0.52-0.99) compared with the -606TT homozygotes. Stratification analyses showed that a reduced risk associated with the -606CC genotype was more pronounced in subgroups of non-smokers, non-drinkers, younger subjects (defined as ≤57 years), carriers of the TP53 Arg/Arg (rs1042522) genotype, patients with oropharyngeal cancer or late-stage SCCHN. Further in silico analysis revealed that the -249 T-to-C change led to a gain of a transcription factor-binding site. Additional functional analysis showed that the -249T-to-C change significantly enhanced transcriptional activity of the DEC1 promoter and the DNA-protein-binding activity. We conclude that the DEC1 promoter -249 T>C (rs2012775) polymorphism is functional, modulating susceptibility to SCCHN among non-Hispanic Whites.

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.

From molecular mechanisms of cardiac development to genetic substrate of congenital heart diseases

Congenital heart disease is one of the most important chapters in medicine because its incidence is increasing and nowadays it is close to 1.2%. Most congenital heart disorders are the result of defects during embryogenesis, which implies that they are due to alterations in genes involved in cardiac development. This review summarizes current knowledge regarding the molecular mechanisms involved in cardiac development in order to clarify the genetic basis of congenital heart disease.

Myogenic differentiation in atrium-derived adult cardiac pluripotent cells and the transcriptional regulation of GATA4 and myogenin on ANP promoter

We established cardiac pluripotent stem-like cells from the left atrium (LA-PCs) of adult rat hearts. These cells could differentiate not only into beating myocytes but also into cells of other lineages, including adipocytes and endothelial cells in the methylcellulose-based medium containing interleukin-3 (IL-3), interleukin-6 (IL-6), and stem cell factor (SCF). In particular, IL-3 and SCF contributed to the differentiation into cardiac troponin I-positive cells. Notably, small population of LA-PCs coexpressed GATA4 and myogenin, which are markers specific to cardiomyocytes and skeletal myocytes, respectively, and could differentiate into both cardiac and skeletal myocytes. Therefore, we investigated the involvement of these two tissue-specific transcription factors in the cardiac transcriptional activity. Coexpression of GATA4 and myogenin synergistically activated GATA4-specific promoter of the atrial natriuretic peptide gene. This combinatorial function was shown to be dependant on the GATA site, but independent of the E-box. The results of chromatin immunoprecipitation and electrophoretic mobility shift assays suggested that myogenin bound to GATA4 on the GATA elements and the C-terminal Zn-finger domain of GATA4 and the N-terminal region of myogenin were required for this synergistic activation of transcription. Taken together, these two transcription factors could be involved in the myogenesis of LA-PCs.

Micropatterned matrix directs differentiation of human mesenchymal stem cells towards myocardial lineage

Stem cell response can be influenced by a multitude of chemical, topological and mechanical physiochemical cues. While extensive studies have been focused on the use of soluble factors to direct stem cell differentiation, there are growing evidences illustrating the potential to modulate stem cell differentiation via precise engineering of cell shape. Fibronectin were printed on poly(lactic-co-glycolic acid) (PLGA) thin film forming spatially defined geometries as a means to control the morphology of bone marrow derived human mesenchymal stem cells (hMSCs). hMSCs that were cultured on unpatterned substrata adhered and flattened extensively (approximately 10,000 microm(2)) while cells grown on 20 microm micropatterend wide adhesive strips were highly elongated with much smaller area coverage of approximately 2000 microm(2). Gene expression analysis revealed up-regulation of several hallmark markers associated to neurogenesis and myogenesis for cells that were highly elongated while osteogenic markers were specifically down-regulated or remained at its nominal level. Even though there is clearly upregulated levels of both neuronal and myogenic lineages but at the functionally relevant level of protein expression, the myogenic lineage is dominant within the time scale studied as determined by the exclusive expression of cardiac myosin heavy chain for the micropatterned cells. Enforced cell shape distortion resulting in large scale rearrangement of cytoskeletal network and altered nucleus shape has been proposed as a physical impetus by which mechanical deformation is translated into biochemical response. These results demonstrated for the first time that cellular shape modulation in the absence of any induction factors may be a viable strategy to coax lineage-specific differentiation of stem cells.

Regulation and significance of atrial and brain natriuretic peptides as cardiac hormones

Atrial and brain natriuretic peptides (ANP and BNP, respectively) are cardiac hormones. During cardiac development, their expression is a maker of cardiomyocyte differentiation and is under tight spatiotemporal regulation. After birth, however, their ventricular expression is only up-regulated in response to various cardiovascular diseases. As a result, analysis of ANP and BNP gene expression has led to discoveries of transcriptional regulators and signaling pathways involved in both cardiac differentiation and cardiac disease. Studies using genetically engineered mice have shed light on the molecular mechanisms regulating ANP and BNP gene expression, as well as the physiological and pathophysiological relevance of the cardiac natriuretic peptide system. In this review we will summarize what is currently known about their regulation and the significance of ANP and BNP as hormones derived from the heart.

GATA4 mutations in Chinese patients with congenital cardiac septal defects

The object of the study was to elucidate the mutations of the GATA4 gene in Han ancestry patients with congenital cardiac septal defects. Fifty Han ancestry patients with sporadic and familial cardiac septal defects and 200 normal subjects of the same ethnical background were studied. A total of six exons and the intron-exon boundaries of GATA4 were amplified by polymerase chain reaction (PCR). The PCR products were purified and directly sequenced with an ABI PRISM 3730 Automatic DNA sequencer. Two novel heterozygous mutations were discovered in the GATA4 gene in five children with cardiac septal defects (10%, 5/50), His28Tyr in exon 2 and His436Tyr in exon 7, respectively, which were neither found in the control population nor reported in the SNP database at the website http://www.ncbi.nlm.nih.gov/SNP. In addition, we did not identify any mutations in GATA4 in three familial atrial septal defects and two familial ventricular septal defects. Our finding suggests that the mutations in the transcription factor GATA4 might be related to congenital cardiac septal defects in Han ancestry patients.

Cardiac transcription factors driven lineage-specification of adult stem cells

Differentiation of human bone marrow mesenchymal stem cells (hBMSC) into the cardiac lineage has been assayed using different approaches such as coculture with cardiac or embryonic cells, treatment with factors, or by seeding cells in organotypic cultures. In most cases, differentiation was evaluated in terms of expression of cardiac-specific markers at protein or molecular level, electrophysiological properties, and formation of sarcomers in differentiated cells. As observed in embryonic stem cells and cardiac progenitors, differentiation of MSC towards the cardiac lineage was preceded by translocation of NKX2.5 and GATA4 transcription factors to the nucleus. Here, we induce differentiation of hBMSC towards the cardiac lineage using coculture with neonatal rat cardiomyocytes. Although important ultrastructural changes occurred during the course of differentiation, sarcomerogenesis was not fully achieved even after long periods of time. Nevertheless, we show that the main cardiac markers, NKX2.5 and GATA4, translocate to the nucleus in a process characteristic of cardiac specification.

Poor vessel formation in embryos from knock-in mice expressing ALK5 with L45 loop mutation defective in Smad activation

Transforming growth factor (TGF)-beta regulates vascular development through two type I receptors: activin receptor-like kinase (ALK) 1 and ALK5, each of which activates a different downstream Smad pathway. The endothelial cell (EC)-specific ALK1 increases EC proliferation and migration, whereas the ubiquitously expressed ALK5 inhibits both of these processes. As ALK1 requires the kinase activity of ALK5 for optimal activation, the lack of ALK5 in ECs results in defective phosphorylation of both Smad pathways on TGF-beta stimulation. To understand why TGF-beta signaling through ALK1 and ALK5 has opposing effects on ECs and whether this takes place in vivo, we carefully compared the phenotype of ALK5 knock-in (ALK5(KI/KI)) mice, in which the aspartic acid residue 266 in the L45 loop of ALK5 was replaced by an alanine residue, with the phenotypes of ALK5 knock-out (ALK5(-/-)) and wild-type mice. The ALK5(KI/KI) mice showed angiogenic defects with embryonic lethality at E10.5-11.5. Although the phenotype of the ALK5(KI/KI) mice was quite similar to that of the ALK5(-/-) mice, the hierarchical structure of blood vessels formed in the ALK5(KI/KI) embryos was more developed than that in the ALK5(-/-) mutants. Thus, the L45 loop mutation in ALK5 partially rescued the earliest vascular defects in the ALK5(-/-) embryos. This study supports our earlier observation that vascular maturation in vivo requires both TGF-beta/ALK1/BMP-Smad and TGF-beta/ALK5/activin-Smad pathways for normal vascular development.

Conformational stability and DNA binding specificity of the cardiac T-box transcription factor Tbx20

The transcription factor Tbx20 acts within a hierarchy of T-box factors in lineage specification and morphogenesis in the mammalian heart and is mutated in congenital heart disease. T-box family members share a approximately 20-kDa DNA-binding domain termed the T-box. The question of how highly homologous T-box proteins achieve differential transcriptional control in heart development, while apparently binding to the same DNA sequence, remains unresolved. Here we show that the optimal DNA recognition sequence for the T-box of Tbx20 corresponds to a T-half-site. Furthermore, we demonstrate using purified recombinant domains that distinct T-boxes show significant differences in the affinity and kinetics of binding and in conformational stability, with the T-box of Tbx20 displaying molten globule character. Our data highlight unique features of Tbx20 and suggest mechanistic ways in which cardiac T-box factors might interact synergistically and/or competitively within the cardiac regulatory network.

Requirement of TCTG(G/C) Direct Repeats and Overlapping GATA Site for Maintaining the Cardiac-Specific Expression of Cardiac troponin T in Developing and Adult Mice

The cardiac-specific -497 bp promoter of rat cardiac troponin T (cTnT) contains two similar modules, D and F, each of which possesses TCTG(G/C) direct repeats and A/T-rich sites. To identify cis-elements critical for cardiac specificity, a -249 bp promoter containing only module F and its site-directed mutations were used to generate transgenic mice. Transgene expression of the -249 bp promoter remained cardiac-specific, despite low and nonuniform expression. The nonuniform expression pattern of the transgene coincided with differential expression of HMGB1, which appeared to be the predominant form of HMGB family proteins in the heart. The HMGB1 binds to the A/T-rich/MEF2-like sites of the cTnT promoter, as determined by chromatin immunoprecipitation assays. Mice carrying the -249 bp promoter with point mutations disrupting the direct repeats expressed transgene at lower levels in the heart and ectopically in the brain. Ectopic expression of transgene was also observed in developing limbs and head. These results suggest an important role for the direct repeat in determining the cardiac specificity. Furthermore, mice carrying a mutant promoter simultaneously disrupting the direct repeats and overlapping GATA site failed to express the transgene in any tissues tested. Therefore, the direct repeat and overlapping GATA site are critical for the expression level and cardiac specificity. The F module controls one level of cardiac specificity. For a uniform and high level of cardiac-specific expression, the upstream element (-497 to -250 bp) is further required, possibly through the D enhancer module and the combination of Nkx2.5 and GATA sites.

GATA4 mutations in 486 Chinese patients with congenital heart disease

Recent studies have reported germline mutations in GATA4 gene in some types of congenital heart disease (CHD). However, the prevalence of GATA4 mutations in CHD and the correlation between the GATA4 genotype and CHD phenotype have not been extensively studied. We screened germline mutations in the coding exons and the flanking intron sequences of the GATA4 gene in 486 CHD patients by denaturing high-performance liquid chromatography (DHPLC), and confirmed the mutations by sequencing. Nine distinct mutations including one small deletion mutation (46delS), two small insertion mutations (118-119insA and 125-126insAA), and six non-synonymous mutations (A6V, P163S, E359K, P407Q, S429T and A442V) were identified in 12 of the 486 patients (nine with ventricular septal defect, two with Tetralogy of Fallot, and one with endocardial cushion defect). Of them, two patients carrying E359K mutation were from two generations in one family with ventricular septal defect (VSD). Interestingly, a nucleotide insertion of c.1146+25insA in exon 6 was detected in five VSD patients, but not in 486 normal healthy controls. Our findings are useful in understanding the prevalence of GATA4 mutations and the correlation between the GATA4 genotype and the CHD phenotype in Chinese patients.

Role of the GATA family of transcription factors in endocrine development, function, and disease

The WGATAR motif is a common nucleotide sequence found in the transcriptional regulatory regions of numerous genes. In vertebrates, these motifs are bound by one of six factors (GATA1 to GATA6) that constitute the GATA family of transcriptional regulatory proteins. Although originally considered for their roles in hematopoietic cells and the heart, GATA factors are now known to be expressed in a wide variety of tissues where they act as critical regulators of cell-specific gene expression. This includes multiple endocrine organs such as the pituitary, pancreas, adrenals, and especially the gonads. Insights into the functional roles played by GATA factors in adult organ systems have been hampered by the early embryonic lethality associated with the different Gata-null mice. This is now being overcome with the generation of tissue-specific knockout models and other knockdown strategies. These approaches, together with the increasing number of human GATA-related pathologies have greatly broadened the scope of GATA-dependent genes and, importantly, have shown that GATA action is not necessarily limited to early development. This has been particularly evident in endocrine organs where GATA factors appear to contribute to the transcription of multiple hormone-encoding genes. This review provides an overview of the GATA family of transcription factors as they relate to endocrine function and disease.

Human heart LIM protein activates atrial-natriuretic-factor gene expression by interacting with the cardiac-restricted transcription factor Nkx2.5

hhLIM [human heart LIM (Lin-11/IsI-1/Mec-3) protein] is a muscle-specific LIM-only protein that consists of two LIM motifs. hhLIM functions as a positive regulator for cardiac hypertrophy. Here we report that hhLIM serves as a cofactor regulating the expression of the ANF (atrial natriuretic factor) gene in H9c2 rat cardiomyoblast cells. We found that hhLIM promoted the expression of the ANF gene in H9c2 cells, but not in A293 human embryonic kidney cells. Furthermore, we showed that hhLIM interacted with Nkx2.5 (a cardiac-restricted transcription factor) in vivo and in vitro using its N-terminal LIM domain and enhanced the binding ability of Nkx2.5 to the NKE (Nkx2.5-binding element) boxes in the ANF promoter. These results suggest that hhLIM promotes the specific expression of the ANF gene by co-operating with Nkx2.5.

Mutations in the 3'-untranslated region of GATA4 as molecular hotspots for congenital heart disease (CHD)

Background

The 3'-untranslated region (3'-UTR) of mRNA contains regulatory elements that are essential for the appropriate expression of many genes. These regulatory elements are involved in the control of nuclear transport, polyadenylation status, subcellular targetting as well as rates of translation and degradation of mRNA. Indeed, 3'-UTR mutations have been associated with disease, but frequently this region is not analyzed. To gain insights into congenital heart disease (CHD), we have been analyzing cardiac-specific transcription factor genes, including GATA4, which encodes a zinc finger transcription factor. Germline mutations in the coding region of GATA4 have been associated with septation defects of the human heart, but mutations are rather rare. Previously, we identified 19 somatically-derived zinc finger mutations in diseased tissues of malformed hearts. We now continued our search in the 609 bp 3'-UTR region of GATA4 to explore further molecular avenues leading to CHD.

Methods

By direct sequencing, we analyzed the 3'-UTR of GATA4 in DNA isolated from 68 formalin-fixed explanted hearts with complex cardiac malformations encompassing ventricular, atrial, and atrioventricular septal defects. We also analyzed blood samples of 12 patients with CHD and 100 unrelated healthy individuals.

Results

We identified germline and somatic mutations in the 3'-UTR of GATA4. In the malformed hearts, we found nine frequently occurring sequence alterations and six dbSNPs in the 3'-UTR region of GATA4. Seven of these mutations are predicted to affect RNA folding. We also found further five nonsynonymous mutations in exons 6 and 7 of GATA4. Except for the dbSNPs, analysis of tissue distal to the septation defect failed to detect sequence variations in the same donor, thus suggesting somatic origin and mosaicism of mutations. In a family, we observed c.+119A > T in the 3'-UTR associated with ASD type II.

Conclusion

Our results suggest that somatic GATA4 mutations in the 3'-UTR may provide an additional molecular rationale for CHD.

Characterization of zinc finger protein 496 that interacts with Jumonji/Jarid2

Jumonij (JMJ)/Jarid2 plays important roles in embryonic development and functions as a transcriptional repressor. Using yeast two-hybrid screening, we have identified a cofactor of JMJ, the zinc finger protein 496 (Zfp496) that contains a SCAN, KRAB and zinc finger domain. Our molecular analyses indicate that Zfp496 functions as a transcriptional activator. Further, Zfp496 inhibits the transcriptional repression of JMJ and JMJ represses the transcriptional activation of Zfp496. This study demonstrates that JMJ physically and functionally interacts with Zfp496, which will provide important insights into endogenous target gene regulation by both factors.

Biochemical Analyses of Csx/Nkx2.5 Mutants and Their Structure–Function Relationship

A homeodomain-containing transcription factor Csx/Nkx2.5 is an important regulator of cardiogensis in mammals. There has been considerable interest in understanding determinants of the diverse cardiac phenotypes associated with Csx/Nkx2.5 mutations situated within or around the homeodomain in patients. To make clear of the functional properties of the regions out of homeodomain, we found that mutants locate outside of the homeodomain retained intact nuclear localization and have nearly normal or increased transcriptional activity but impaired DNA binding capability, the C-terminus region exhibits an inhibitory function on transcriptional activity of wild type Csx/Nkx2.5, and the NK2-Specific Domain is likely to facilitate both DNA binding and protein-protein interaction. In the current study, deletion mutant in homeodomain displayed extremely different biological appearance from the mutants with deletion outside of the homeodomain, these may explain the clinical observation that patients with missense situated outside the homeodomain were not associated with atrioventricular conduction disturbance.

Genetically selected stem cells from human adipose tissue express cardiac markers

In the present study, the potential of human adipose-derived stem cells to differentiate into cells with characteristics of cardiomyocytes was investigated. Adipose tissue-derived stem cells (ADSCs) were transduced with two different lentiviral vectors simultaneously: (1) a lentiviral vector expressing eGFP controlled by the Nkx2.5 promoter and (2) a lentiviral vector expressing DsRed2 controlled by the myosin light chain-2v promoter (MLC-2v). Nkx2.5-eGFP and MLC-2v-DsRed2 dual positive cells were isolated by FACS. Immunostaining and RT-PCR analysis of the dual positive cells revealed that these cells are positive for Nkx2.5, cardiac troponin I, and L-type calcium channel alpha-1c subunit. Electrophysiology studies demonstrated the presence of functional voltage-dependent calcium and potassium channels. These observations confirm that cardiac progenitor cells can be isolated and enriched from human adipose-derived stem cells using lentiviral selection, and they might represent a new source for cell therapy for myocardial infarction and heart failure.

Re-employment of developmental transcription factors in adult heart disease

A finite number of transcription factors constitute a combinatorial code that orchestrates cardiac development and the specification and differentiation of myocytes. Many, if not all of these same transcription factors are re-employed in the adult heart in response to disease stimuli that promote hypertrophic enlargement and/or dilated cardiomyopathy, as part of the so-called "fetal gene program". This review will discuss the transcription factors that regulate the hypertrophic growth response of the adult heart, with a special emphasis on those regulators that participate in cardiac development.

The human Cx40 promoter polymorphism -44G-->A differentially affects transcriptional regulation by Sp1 and GATA4

Expression of the tissue-specific gap junction protein connexin(Cx)40 is regulated by the interaction of ubiquitous and tissue-specific factors such as Sp1 and GATA4. Cardiac Cx40 expression is altered under pathological conditions such as atrial fibrillation. A human promoter polymorphism, a G-->A change at position -44 that has been associated with atrial-specific arrhythmias, is located between the TBE-NKE-Sp and GATA consensus transcription factor binding sites important for the regulation of the mouse Cx40 gene. The presence of the A-allele at position -44 in promoter-reporter constructs significantly reduces promoter activity. Using electrophoretic mobility shift assays and luciferase reporter assays in various cell types, we show that Sp1 and GATA4 are important regulators of human Cx40 gene transcription and that the -44 G-->A polymorphism negatively affects the promoter regulation by the transcription factors Sp1 and GATA4.

Development of heart valves requires Gata4 expression in endothelial-derived cells

Cardiac malformations due to aberrant development of the atrioventricular (AV) valves are among the most common forms of congenital heart disease. At localized swellings of extracellular matrix known as the endocardial cushions, the endothelial lining of the heart undergoes an epithelial to mesenchymal transition (EMT) to form the mesenchymal progenitors of the AV valves. Further growth and differentiation of these mesenchymal precursors results in the formation of portions of the atrial and ventricular septae, and the generation of thin, pliable valves. Gata4, which encodes a zinc finger transcription factor, is expressed in the endothelium and mesenchyme of the AV valves. Using a Tie2-Cre transgene, we selectively inactivated Gata4 within endothelial-derived cells. Mutant endothelium failed to undergo EMT, resulting in hypocellular cushions. Mutant cushions had decreased levels of Erbb3, an EGF-family receptor essential for EMT in the atrioventricular cushions. In Gata4 mutant embryos, Erbb3 downregulation was associated with impaired activation of Erk, which is also required for EMT. Expression of a Gata4 mutant protein defective in interaction with Friend of Gata (FOG) cofactors rescued the EMT defect, but resulted in a decreased proliferation of mesenchyme and hypoplastic cushions that failed to septate the ventricular inlet. We demonstrate two novel functions of Gata4 in development of the AV valves. First, Gata4 functions as an upstream regulator of an Erbb3-Erk pathway necessary for EMT, and second, Gata4 acts to promote cushion mesenchyme growth and remodeling.

Bridging the gap between anatomy and molecular genetics for an improved understanding of congenital heart disease

Birth defects are the leading cause of infant mortality and malformations in congenital heart disease (CHD) are among the most prevalent and fatal of all birth defects. Yet the molecular mechanisms leading to CHD are complex and the causes of the cardiac malformations observed in humans are still unclear. In recent years, the pivotal role of certain transcription factors in heart development has been demonstrated, and gene targeting of cardiac-specific transcription factor genes in animal models has provided valuable insights into heart anomalies. Nonetheless results in these models can be species specific, and in humans, germline mutations in transcription factor genes can only account for some cases of CHD. Furthermore, most patients do not have family history of CHD. There is, therefore, a need for a better understanding of the mechanisms in both normal cardiac development and the formation of malformations. The combining of expertise in cardiac anatomy, pathology, and molecular genetics is essential to adequately comprehend developmental abnormalities associated with CHD. To help elucidate genetic alterations in affected tissues of malformed hearts, we carried out genetic analysis of cardiac-specific transcription factor genes from the Leipzig collection of formalin-fixed malformed hearts. Working with this morphologically well-characterized archival material not only provided valuable genetic information associated with disease, but enabled us to put forward a hypothesis of somatic mutations as a novel molecular cause of CHD. Knowledge of cause and disease mechanism may allow for intervention that could modify the degree of cardiac malformations or development of new approaches for prevention of CHD.

The ATP-sensitive potassium (KATP) channel-encoded dSUR gene is required for Drosophila heart function and is regulated by tinman

The homeobox transcription factor Tinman plays an important role in the initiation of heart development. Later functions of Tinman, including the target genes involved in cardiac physiology, are less well studied. We focused on the dSUR gene, which encodes an ATP-binding cassette transmembrane protein that is expressed in the heart. Mammalian SUR genes are associated with K(ATP) (ATP-sensitive potassium) channels, which are involved in metabolic homeostasis. We provide experimental evidence that Tinman directly regulates dSUR expression in the developing heart. We identified a cis-regulatory element in the first intron of dSUR, which contains Tinman consensus binding sites and is sufficient for faithful dSUR expression in the fly's myocardium. Site-directed mutagenesis of this element shows that these Tinman sites are critical to dSUR expression, and further genetic manipulations suggest that the GATA transcription factor Pannier is synergistically involved in cardiac-restricted dSUR expression in vivo. Physiological analysis of dSUR knock-down flies supports the idea that dSUR plays a protective role against hypoxic stress and pacing-induced heart failure. Because dSUR expression dramatically decreases with age, it is likely to be a factor involved in the cardiac aging phenotype of Drosophila. dSUR provides a model for addressing how embryonic regulators of myocardial cell commitment can contribute to the establishment and maintenance of cardiac performance.

A threshold of GATA4 and GATA6 expression is required for cardiovascular development

The zinc-finger transcription factors GATA4 and GATA6 play critical roles in embryonic development. Mouse embryos lacking GATA4 die at embryonic day (E) 8.5 because of failure of ventral foregut closure and cardiac bifida, whereas GATA6 is essential for development of the visceral endoderm. Although mice that are heterozygous for either a GATA4 or GATA6 null allele are normal, we show that compound heterozygosity of GATA4 and GATA6 results in embryonic lethality by E13.5 accompanied by a spectrum of cardiovascular defects, including thin-walled myocardium, ventricular and aortopulmonary septal defects, and abnormal smooth muscle development. Myocardial hypoplasia in GATA4/GATA6 double heterozygous mutant embryos is associated with reduced proliferation of cardiomyocytes, diminished expression of the myogenic transcription factor MEF2C (myocyte enhancer factor 2C), and down-regulation of beta-myosin heavy chain expression, a key determinant of cardiac contractility. These findings reveal a threshold of GATA4 and GATA6 activity that is required for gene expression in the developing cardiovascular system and underscore the potential of recessive mutations to perturb the delicate regulation of cardiovascular development.

Transcription factors and congenital heart defects

Although there have been important advances in diagnostic modalities and therapeutic strategies for congenital heart defects (CHD), these malformations still lead to significant morbidity and mortality in the human population. Over the past 10 years, characterization of the genetic causes of CHD has begun to elucidate some of the molecular causes of these defects. Linkage analysis and candidate-gene approaches have been used to identify gene mutations that are associated with both familial and sporadic cases of CHD. Complementation of the human studies with developmental studies in mouse models provides information for the roles of these genes in normal development as well as indications for disease pathogenesis. Biochemical analysis of these gene mutations has provided further insight into the molecular effects of these genetic mutations. Here we review genetic, developmental, and biochemical studies of six cardiac transcription factors that have been identified as genetic causes for CHD in humans.

p204 Is Required for the Differentiation of P19 Murine Embryonal Carcinoma Cells to Beating Cardiac Myocytes

Among 10 adult mouse tissues tested, the p204 protein levels were highest in heart and skeletal muscle. We described previously that the MyoD-inducible p204 protein is required for the differentiation of cultured murine C2C12 skeletal muscle myoblasts to myotubes. Here we report that p204 was also required for the differentiation of cultured P19 murine embryonal carcinoma stem cells to beating cardiac myocytes. As shown by others, this process can be triggered by dimethyl sulfoxide (DMSO). We established that DMSO induced the formation of 204RNA and p204. Ectopic p204 could partially substitute for DMSO in inducing differentiation, whereas ectopic 204 antisense RNA inhibited the differentiation. Experiments with reporter constructs, including regulatory regions from the Ifi204 gene (encoding p204) in P19 cells and in cultured newborn rat cardiac myocytes, as well as chromatin coimmunoprecipitations with transcription factors, revealed that p204 expression was synergistically transactivated by the cardiac Gata4, Nkx2.5, and Tbx5 transcription factors. Furthermore, ectopic p204 triggered the expression of Gata4 and Nkx2.5 in P19 cells. p204 contains a nuclear export signal and was partially translocated to the cytoplasm during the differentiation. p204 from which the nuclear export signal was deleted was not translocated, and it did not induce differentiation. The various mechanisms by which p204 promoted the differentiation are reported in the accompanying article (Ding, B., Liu, C., Huang, Y., Yu, J., Kong, W., and Lengyel, P. (2006) J. Biol. Chem. 281, 14893-14906).

Transcription factor CHF1/Hey2 suppresses cardiac hypertrophy through an inhibitory interaction with GATA4

Pathological cardiac hypertrophy is considered a precursor to clinical heart failure. Understanding the transcriptional regulators that suppress the hypertrophic response may have profound implications for the treatment of heart disease. We report the generation of transgenic mice that overexpress the transcription factor CHF1/Hey2 in the myocardium. In response to the alpha-adrenergic agonist phenylephrine, they show marked attenuation in the hypertrophic response compared with wild-type controls, even though blood pressure is similar in both groups. Isolated myocytes from transgenic mice demonstrate a similar resistance to phenylephrine-induced hypertrophy in vitro, providing further evidence that the protective effect of CHF1/Hey2 is mediated at the myocyte level. Induction of the hypertrophy marker genes ANF, BNP, and beta-MHC in the transgenic cells is concurrently suppressed in vivo and in vitro, demonstrating that the induction of hypertrophy-associated genes is repressed by CHF1/Hey2. Transfection of CHF1/Hey2 into neonatal cardiomyocytes suppresses activation of an ANF reporter plasmid by the transcription factor GATA4, which has previously been shown to activate a hypertrophic transcriptional program. Furthermore, CHF1/Hey2 binds GATA4 directly in coimmunoprecipitation assays and inhibits the binding of GATA4 to its recognition sequence within the ANF promoter. Our findings demonstrate that CHF1/Hey2 functions as an antihypertrophic gene, possibly through inhibition of a GATA4-dependent hypertrophic program.

Tbx1 is regulated by forkhead proteins in the secondary heart field

Transcriptional regulation in a tissue-specific and quantitative manner is essential for developmental events, including those involved in cardiovascular morphogenesis. Tbx1 is a T-box-containing transcription factor that is responsible for many of the defects observed in 22q11 deletion syndrome in humans. Tbx1 is expressed in the secondary heart field (SHF) and is essential for cardiac outflow tract (OFT) development. We previously reported that Tbx1 is regulated by sonic hedgehog by means of forkhead (Fox) transcription factors in the head mesenchyme and pharyngeal endoderm, but how it is regulated in the SHF is unknown. Here, we show that Tbx1 expression in the SHF is regulated by Fox proteins through a combination of two evolutionarily conserved Fox binding sites in a dose-dependent manner. Cell fate analysis using the Tbx1 enhancer suggests that SHF-derived Tbx1-expressing cells contribute extensively to the right ventricular myocardium as well as the OFT during early development and ultimately give rise to the right ventricular infundibulum, pulmonary trunk, and pulmonary valves. These results suggest that Fox proteins are involved in most, if not all, Tbx1 expression domains and that Tbx1 marks a subset of SHF-derived cells, particularly those that uniquely contribute to the right-sided outflow tract and proximal pulmonary artery.

Differentiation of cardiac Purkinje fibers requires precise spatiotemporal regulation of Nkx2-5 expression

Nkx2-5 gene mutations cause cardiac abnormalities, including deficits of function in the atrioventricular conduction system (AVCS). In the chick, Nkx2-5 is elevated in Purkinje fiber AVCS cells relative to working cardiomyocytes. Here, we show that Nkx2-5 expression rises to a peak as Purkinje fibers progressively differentiate. To disrupt this pattern, we overexpressed Nkx2-5 from embryonic day 10, as Purkinje fibers are recruited within developing chick hearts. Overexpression of Nkx2-5 caused inhibition of slow tonic myosin heavy chain protein (sMHC), a late Purkinje fiber marker but did not affect Cx40 levels. Working cardiomyocytes overexpressing Nkx2-5 in these hearts ectopically up-regulated Cx40 but not sMHC. Isolated embryonic cardiomyocytes overexpressing Nkx2-5 also displayed increased Cx40 and suppressed sMHC. By contrast, overexpression of a human NKX2-5 mutant did not effect these markers in vivo or in vitro, suggesting one possible mechanism for clinical phenotypes. We conclude that a prerequisite for normal Purkinje fiber maturation is precise regulation of Nkx2-5 levels.

Cardiac transcription factor Csx/Nkx2-5: Its role in cardiac development and diseases

During the past decade, an emerging body of evidence has accumulated that cardiac transcription factors control a cardiac gene program and play a critical role in transcriptional regulation during cardiogenesis and during the adaptive process in adult hearts. Especially, an evolutionally conserved homeobox transcription factor Csx/Nkx2-5 has been in the forefront in the field of cardiac biology, providing molecular insights into the mechanisms of cardiac development and diseases. Csx/Nkx2-5 is indispensable for normal cardiac development, and mutations of the gene are associated with human congenital heart diseases (CHD). In the present review, the regulation of a cardiac gene program by Csx/Nkx2-5 is summarized, with an emphasis on its role in the cardiac development and diseases.

GATA factors and transcriptional regulation of cardiac natriuretic peptide genes

The A- and B-natriuretic peptides (ANP and BNP) are the heart major secretory products. ANF and BNP expression is a marker of cardiomyocyte differentiation, and is regulated spatially, developmentally and hormonally. Analysis of the ANP and BNP promoters has contributed in a major way to our present understanding of the key regulators of cardiac development. It has also started to unravel the complex combinatorial interactions required for proper regulation of the cardiac genetic program. The GATA family of transcription factors initially identified as essential regulators of the two natriuretic peptide genes appears to be at the heart of the molecular circuits governing cardiac growth and differentiation. In particular, GATA-4 has emerged as the nuclear effector of several signaling pathways which modulate its function through post-translational modifications and protein-protein interactions. This review will cover our current knowledge of cardiac transcription and the role of GATA factors in embryonic and postnatal heart development.

GATA factors in Drosophila heart and blood cell development

GATA transcription factors comprise an evolutionarily conserved family of proteins that function in the specification and differentiation of various cell types during animal development. In this review, we examine current knowledge of the structure, expression, and function of the Pannier and Serpent GATA factors as they relate to cardiogenesis and hematopoiesis in the Drosophila system. We also assess the molecular and genetic characteristics of the Friend of GATA protein U-shaped, which serves as a regulator of Pannier and Serpent function in these two developmental processes.

Phenotypes with GATA4 or NKX2.5 mutations in familial atrial septal defect

Recently, GATA4 and NKX2.5 were reported as the disease genes of atrial septal defect (ASD) but the relationship between the locations of their mutations and phenotypes is not clear. We analyzed GATA4 and NKX2.5 mutations in 16 familial ASD cases, including four probands with atrioventricular conduction disturbance (AV block) and two with pulmonary stenosis (PS), by PCR and direct sequencing, and examined their phenotypes clinically. Five mutations, including two GATA4 and three NKX2.5 mutations, were identified in 31.3% of the probands with ASD, and three of them were novel. The two GATA4 mutations in the probands without AV block were S52F and E359Xfs (c.1075delG) that was reported previously, and three NKX2.5 mutations in the probands with AV block were A88Xfs (c.262delG), R190C, and T178M. Additionally, we observed some remarkable phenotypes, i.e., dextrocardia with E359Xfs (c.1075delG) and cribriform type ASD with R190C, both of which are expected to be clues for further investigations. Furthermore, progressive, most severe AV block was closely related with a missense mutation in a homeodomain or with a nonsense/frame-shift mutation of NKX2.5 for which classification has not been clearly proposed. This pinpoints essential sites of NKX2.5 in the development of the conduction system.

Functional dissection of sequence-specific NKX2-5 DNA binding domain mutations associated with human heart septation defects using a yeast-based system

Human heart development requires an orderly coordination of transcriptional programs, with the homeodomain protein NKX2-5 being one of the key transcription factors required for the differentiation of mesodermal progenitor cells. Indeed, lack of Nkx2-5 in mice arrests heart development prior to looping, resulting in embryonic lethality. There are 28 germline NKX2-5 mutations identified in humans that are associated with congenital heart disease, and we recently reported multiple somatic mutations in patients with complex cardiac malformations. To address the functional consequences of single and multiple mutations of NKX2-5, we developed a functional assay in the budding yeast Saccharomyces cerevisiae, which could determine transactivation capacity and specificity of expressed NKX2-5 alleles towards targeted response element (RE) sequences. We focused on mutants of the third helix, which provides DNA binding specificity, and characterized mutations that were highly associated with either ventricular (VSD) or atrioventricular (AVSD) septal defects. Individual mutants exhibited partial to complete loss of function and differences in transactivation capacity between the various REs. The mutants also exhibited gene dosage rather than dominant effects on transcription. Surprisingly, all AVSD patients (22/23) had a single K183E mutation in the DNA binding domain, which resulted in transcriptional inactivation. None of the VSD patients had this mutation; yet 14/29 had at least one mutation in the third helix leading to either inactivation or reduction of NKX2-5 transactivation. Therefore, mutations of somatic origin in the binding domains of NKX2-5 were associated specifically with AVSD or VSD and resulted in loss of protein function.

Jumonji represses α-cardiac myosin heavy chain expression via inhibiting MEF2 activity

Expression of alpha-cardiac myosin heavy chain gene (alphaMHC) is developmentally regulated in normal embryonic hearts and down-regulated in cardiac myopathy and failing hearts. Jumonji (JMJ) has been shown to be critical for normal cardiovascular development and functions as a transcriptional repressor. Here, we demonstrate that JMJ represses alphaMHC expression through inhibition of myocyte enhancer factor 2 (MEF2) activity. In primary cardiomyocytes, overexpression of JMJ leads to marked reduction of endogenous alphaMHC expression. JMJ represses the synergistic activation of alphaMHC by MEF2 and thyroid hormone receptor (TR). Interestingly, JMJ inhibits transcriptional activities of all MEF2 isoforms, but not the TR-dependent activation. The transcriptional repression domain of JMJ interacts with the N-terminal part of MEF2A, resulting in the repression of MEF2A activities. These results suggest that JMJ represses alphaMHC expression via protein-protein interaction with MEF2A.

Mutation in myosin heavy chain 6 causes atrial septal defect

Atrial septal defect is one of the most common forms of congenital heart malformation. We identified a new locus linked with atrial septal defect on chromosome 14q12 in a large family with dominantly inherited atrial septal defect. The underlying mutation is a missense substitution, I820N, in alpha-myosin heavy chain (MYH6), a structural protein expressed at high levels in the developing atria, which affects the binding of the heavy chain to its regulatory light chain. The cardiac transcription factor TBX5 strongly regulates expression of MYH6, but mutant forms of TBX5, which cause Holt-Oram syndrome, do not. Morpholino knock-down of expression of the chick MYH6 homolog eliminates the formation of the atrial septum without overtly affecting atrial chamber formation. These data provide evidence for a link between a transcription factor, a structural protein and congenital heart disease.

Jumonji represses atrial natriuretic factor gene expression by inhibiting transcriptional activities of cardiac transcription factors

Mice with a homozygous knockout of the jumonji (jmj) gene showed abnormal heart development and defective regulation of cardiac-specific genes, including the atrial natriuretic factor (ANF). ANF is one of the earliest markers of cardiac differentiation and a hallmark for cardiac hypertrophy. Here, we show that JMJ represses ANF gene expression by inhibiting transcriptional activities of Nkx2.5 and GATA4. JMJ represses the Nkx2.5- or GATA4-dependent activation of the reporter genes containing the ANF promoter-enhancer or containing the Nkx2.5 or GATA4-binding consensus sequence. JMJ physically associates with Nkx2.5 and GATA4 in vitro and in vivo as determined by glutathione S-transferase pull-down and immunoprecipitation assays. Using mutational analyses, we mapped the protein-protein interaction domains in JMJ, Nkx2.5, and GATA4. We identified two DNA-binding sites of JMJ in the ANF enhancer by gel mobility shift assays. However, these JMJ-binding sites do not seem to mediate ANF repression by JMJ. Mutational analysis of JMJ indicates that the protein-protein interaction domain of JMJ mediates the repression of ANF gene expression. Therefore, JMJ may play important roles in the down-regulation of ANF gene expression and in heart development.

Cardiac Developmental Biology: From Flies to Humans

The heart is the first organ to form during embryogenesis, and heart formation is essential for subsequent embryonic development. Since the identification of a cardiac-restricted transcription factor Csx/Nkx-2.5 in the early 1990s, extensive studies on cardiac development have been done in various species ranging from flies to humans. Molecular dissection of regulatory pathways that control multiple steps of cardiogenesis will not only advance our understanding of cardiac development and congenital heart diseases, but will also provide an important clue to novel therapeutic strategies for heart diseases.

Cell-specific activation of the atrial natriuretic factor promoter by PITX2 and MEF2A

The PITX2 homeodomain protein is mutated in patients with Axenfeld-Rieger syndrome and is involved in the development of multiple organ systems, including the heart. We have examined the interaction of PITX2 isoforms with myocyte-enhancing factor 2A (MEF2A), which is a known regulator of cardiac development. A direct interaction between PITX2a and MEF2A was demonstrated using yeast two-hybrid and GST pull-down assays. To study the functional significance of this interaction, we used the atrial natriuretic factor (ANF) promoter. Coexpression of MEF2A and PITX2a or Pitx2c resulted in a strong synergistic activation of the ANF promoter in LS8 oral epithelial cells but not in other cell lines (NIH/3T3, Chinese hamster ovary, or C2C12). The synergism was dependent on promoter context, because it required MEF2 binding sites and was not seen with two other PITX2 target promoters. DNA binding by MEF2A was required but not sufficient for synergism. Upstream activators of p38 MAP kinases, MKK3 and MKK6, increased PITX2a and Pitx2c activity to yield up to 90-fold activation of the ANF promoter in LS8 cells. Because Axenfeld-Rieger syndrome is autosomal dominant and affects development of the oral epithelium, we tested one of the known PITX2 mutants. The PITX2a-K88E mutant protein suppressed wild type PITX2a synergism with MEF2A. These results demonstrate a promoter- and cell-specific functional interaction between PITX2 and MEF2A and suggest the possibility of coordinate control by these factors in the oral epithelium.

Alternative usages of multiple promoters of the acetyl-CoA carboxylase beta gene are related to differential transcriptional regulation in human and rodent tissues

Acetyl-CoA carboxylase beta (ACCbeta) is a critical enzyme in the regulation of fatty acid oxidation and is dominantly expressed in the skeletal muscle, heart, and liver. It has been established that two promoters, P-I and P-II, control the transcription of the ACCbeta gene. However, the precise mechanism involved in controlling tissue-specific gene expression of ACCbeta is largely unknown yet. In this study we revealed that promoter P-I, active in the skeletal muscle and heart but not in the liver, could be activated by myogenic regulatory factors and retinoid X receptors in a synergistic manner. Moreover, P-I was also activated markedly by the cardiac-specific transcription factors, Csx/Nkx2.5 and GATA4. These results suggest that the proper stimulation of P-I by these tissue-specific transcription factors is important for the expression of ACCbeta according to the tissue types. In addition, CpG sites around human exon 1a transcribed by P-I are half-methylated in muscle but completely methylated in the liver, where P-I is absolutely inactive. In humans, the skeletal muscle uses P-II as well as P-I, whereas only P-I is active in rat skeletal muscle. The proximal myogenic regulatory factor-binding sites in human P-II, which are not conserved in rat P-II, might contribute to this difference in P-II usage between human and rat skeletal muscle. Hepatoma-derived cell lines primarily use another novel promoter located about 3 kilobases upstream of P-I, designated as P-O. This study is the first to explain the mechanisms underlying the differential regulation of ACCbeta gene expression between tissues in living organisms.