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

GATA-4, GATA-5, and GATA-6 activate the rat liver fatty acid binding protein gene in concert with HNF-1alpha

Transcriptional regulation by GATA-4, GATA-5, and GATA-6 in intestine and liver was explored using a transgene constructed from the proximal promoter of the rat liver fatty acid binding protein gene (Fabpl). An immunohistochemical survey detected GATA-4 and GATA-6 in enterocytes, GATA-6 in hepatocytes, and GATA-5 in neither cell type in adult animals. In cell transfection assays, GATA-4 or GATA-5 but not GATA-6 activated the Fabpl transgene solely through the most proximal of three GATA binding sites in the Fabpl promoter. However, all three factors activated transgenes constructed from each Fabpl site upstream of a minimal viral promoter. GATA factors interact with hepatic nuclear factor (HNF)-1alpha, and the proximal Fabpl GATA site adjoins an HNF-1 site. GATA-4, GATA-5, or GATA-6 bounded to HNF-1alpha in solution, and all cooperated with HNF-1alpha to activate the Fabpl transgene. Mutagenizing all Fabpl GATA sites abrogated transgene activation by GATA factors, but GATA-4 activated the mutagenized transgene in the presence of HNF-1alpha. These in vitro results suggested GATA/HNF-1alpha interactions function in Fabpl regulation, and in vivo relevance was determined with subsequent experiments. In mice, the Fabpl transgene was active in enterocytes and hepatocytes, a transgene with mutagenized HNF-1 site was silent, and a transgene with mutagenized GATA sites had identical expression as the native transgene. Mice mosaic for biallelic Gata4 inactivation lost intestinal but not hepatic Fabpl expression in Gata4-deficient cells but not wild-type cells. These results demonstrate GATA-4 is critical for intestinal gene expression in vivo and suggest a specific GATA-4/HNF-1alpha physical and functional interaction in Fabpl activation.

Target gene-specific modulation of myocardin activity by GATA transcription factors

Myocardin is a transcriptional coactivator that regulates cardiac and smooth muscle gene expression by associating with serum response factor. We show that GATA transcription factors can either stimulate or suppress the transcriptional activity of myocardin, depending on the target gene. Modulation of myocardin activity by GATA4 is mediated by the physical interaction of myocardin with the DNA binding domain of GATA4 but does not require binding of GATA4 to DNA. Paradoxically, the transcription activation domain of GATA4 is dispensable for the stimulatory effect of GATA4 on myocardin activity but is required for repression of myocardin activity. The ability of GATA transcription factors to modulate myocardin activity provides a potential mechanism for fine tuning the expression of serum response factor target genes in a gene-specific manner.

Hairy-related transcription factors inhibit GATA-dependent cardiac gene expression through a signal-responsive mechanism

Combinatorial actions of transcription factors in multiprotein complexes dictate gene expression profiles in cardiac development and disease. The Hairy-related transcription factor (HRT) family of basic helix-loop-helix proteins is composed of transcriptional repressors highly expressed in the cardiovascular system. However, it has remained unclear whether HRT proteins modulate gene expression driven by cardiac transcriptional activators. Here, we have shown that HRT proteins inhibit cardiac gene transcription by interfering with GATA transcription factors that are implicated in cardiac development and hypertrophy. HRT proteins inhibited GATA-dependent transcriptional activation of cardiac gene promoters such as the atrial natriuretic factor (ANF) promoter. Adenovirus-mediated expression of Hrt2 suppressed mRNA expression of ANF and other cardiac-specific genes in cultured cardiomyocytes. Among various signaling molecules implicated in cardiomyocyte growth, constitutively active Akt1/protein kinase B alpha relieved Hrt2-mediated inhibition of GATA-dependent transcription. HRT proteins physically interacted with GATA proteins, and the basic domain of HRT was critical for physical association as well as transcriptional inhibition. These results suggest that HRT proteins may regulate specific sets of cardiac genes by modulating the function of GATA proteins and other cardiac transcriptional activators in a signal-dependent manner.

Transcription factor Nkx-2.5 induces sodium/iodide symporter gene expression and participates in retinoic acid- and lactation-induced transcription in mammary cells

The sodium/iodide symporter (NIS) is a plasma membrane protein that mediates active iodide transport in thyroid and mammary cells. It is a prerequisite for radioiodide treatment of thyroid cancer and a promising diagnostic and therapeutic tool for breast cancer. We investigated the molecular mechanisms governing NIS expression in mammary cells. Here we report that Nkx-2.5, a cardiac homeobox transcription factor that is also expressed in the thyroid primordium, is a potent inducer of the NIS promoter. By binding to two specific promoter sites (N2 and W), Nkx-2.5 induced the rNIS promoter (about 50-fold over the basal level). Interestingly, coincident with NIS expression, Nkx-2.5 mRNA and protein were present in lactating, but not virgin, mammary glands in two human breast cancer samples and in all-trans retinoic acid (tRA)-stimulated MCF-7 breast cancer cells. A cotransfected dominant-negative Nkx-2.5 mutant abolished tRA-induced endogenous NIS induction, which shows that Nkx-2.5 activity is critical for this process. Remarkably, in MCF-7 cells, Nkx-2.5 overexpression alone was sufficient to induce NIS and iodide uptake. In conclusion, Nkx-2.5 is a novel relevant transcriptional regulator of mammary NIS and could thus be exploited to manipulate NIS expression in breast cancer treatment strategies.

Complex regulation of the lactase-phlorizin hydrolase promoter by GATA-4

Lactase-phlorizin hydrolase (LPH), a marker of intestinal differentiation, is expressed in absorptive enterocytes on small intestinal villi in a tightly regulated pattern along the proximal-distal axis. The LPH promoter contains binding sites that mediate activation by members of the GATA-4, -5, and -6 subfamily, but little is known about their individual contribution to LPH regulation in vivo. Here, we show that GATA-4 is the principal GATA factor from adult mouse intestinal epithelial cells that binds to the mouse LPH promoter, and its expression is highly correlated with that of LPH mRNA in jejunum and ileum. GATA-4 cooperates with hepatocyte nuclear factor (HNF)-1alpha to synergistically activate the LPH promoter by a mechanism identical to that previously characterized for GATA-5/HNF-1alpha, requiring physical association between GATA-4 and HNF-1alpha and intact HNF-1 binding sites on the LPH promoter. GATA-4 also activates the LPH promoter independently of HNF-1alpha, in contrast to GATA-5, which is unable to activate the LPH promoter in the absence of HNF-1alpha. GATA-4-specific activation requires intact GATA binding sites on the LPH promoter and was mapped by domain-swapping experiments to the zinc finger and basic regions. However, the difference in the capacity between GATA-4 and GATA-5 to activate the LPH promoter was not due to a difference in affinity for binding to GATA binding sites on the LPH promoter. These data indicate that GATA-4 is a key regulator of LPH gene expression that may function through an evolutionarily conserved mechanism involving cooperativity with an HNF-1alpha and/or a GATA-specific pathway independent of HNF-1alpha.

A novel LIM protein Cal promotes cardiac differentiation by association with CSX/NKX2-5

The cardiac homeobox transcription factor CSX/NKX2-5 plays an important role in vertebrate heart development. Using a yeast two-hybrid screening, we identified a novel LIM domain-containing protein, named CSX-associated LIM protein (Cal), that interacts with CSX/NKX2-5. CSX/NKX2-5 and Cal associate with each other both in vivo and in vitro, and the LIM domains of Cal and the homeodomain of CSX/NKX2-5 were necessary for mutual binding. Cal itself possessed the transcription-promoting activity, and cotransfection of Cal enhanced CSX/NKX2-5-induced activation of atrial natriuretic peptide gene promoter. Cal contained a functional nuclear export signal and shuttled from the cytoplasm into the nucleus in response to calcium. Accumulation of Cal in the nucleus of P19CL6 cells promoted myocardial cell differentiation accompanied by increased expression levels of the target genes of CSX/NKX2-5. These results suggest that a novel LIM protein Cal induces cardiomyocyte differentiation through its dynamic intracellular shuttling and association with CSX/NKX2-5.

GATA-4 and MEF2C transcription factors control the tissue-specific expression of the alphaT-catenin gene CTNNA3

AlphaT-catenin is a recently identified member of the alpha-catenin family of cell-cell adhesion molecules. Its expression is restricted mainly to cardiomyocytes, although it is also expressed in skeletal muscle, testis and brain. Like other alpha-catenins, alphaT-catenin provides an indispensable link between a cadherin-based adhesion complex and the actin cytoskeleton, resulting in strong cell-cell adhesion. We show here that the tissue-specificity of alphaT-catenin expression is controlled by its promoter region. By in silico analysis, we found that the alphaT-catenin promoter contains several binding sites for cardiac and muscle-specific transcription factors. By co-transfection studies in P19 embryonal carcinoma cells, we demonstrated that MEF2C and GATA-4 each have an activating effect on the alphaT-catenin promoter. Transfections with wild-type and mutant promoter constructs in cardiac HL-1 cells indicated that one GATA box is absolutely required for high alphaT-catenin promoter activity in these cells. Furthermore, we showed that the GATA-4 transcription factor specifically binds and activates the alphaT-catenin promoter in vivo in cardiac HL-1 cells. In vivo promoter analysis in transgenic mice revealed that the isolated alphaT-catenin promoter region could direct the tissue-specific expression of a LacZ reporter gene in concordance with endogenous alphaT-catenin expression.

Characterization of GATA3 Mutations in the Hypoparathyroidism, Deafness, and Renal Dysplasia (HDR) Syndrome

The hypoparathyroidism, deafness, and renal dysplasia (HDR) syndrome is an autosomal dominant disorder caused by mutations of the dual zinc finger transcription factor, GATA3. The C-terminal zinc finger (ZnF2) binds DNA, whereas the N-terminal finger (ZnF1) stabilizes this DNA binding and interacts with other zinc finger proteins, such as the Friends of GATA (FOG). We have investigated seven HDR probands and their families for GATA3 abnormalities and have identified two nonsense mutations (Glu-228 --> Stop and Arg-367 --> Stop); two intragenic deletions that result in frameshifts from codons 201 and 355 with premature terminations at codons 205 and 370, respectively; one acceptor splice site mutation that leads to a frameshift from codon 351 and a premature termination at codon 367; and two missense mutations (Cys-318 --> Arg and Asn-320 --> Lys). The functional effects of these mutations, together with a previously reported GATA3 ZnF1 mutation and seven other engineered ZnF1 mutations, were assessed by electrophoretic mobility shift, dissociation, yeast two-hybrid and glutathione S-transferase pull-down assays. Mutations involving GATA3 ZnF2 or adjacent basic amino acids resulted in a loss of DNA binding, but those of ZnF1 either lead to a loss of interaction with specific FOG2 ZnFs or altered DNA-binding affinity. These findings are consistent with the proposed three-dimensional model of ZnF1, which has separate DNA and protein binding surfaces. Thus, our results, which expand the spectrum of HDR-associated GATA3 mutations and report the first acceptor splice site mutation, help to elucidate the molecular mechanisms that alter the function of this zinc finger transcription factor and its role in causing this developmental anomaly.

Interaction between hex and GATA transcription factors in vascular endothelial cells inhibits flk-1/KDR-mediated vascular endothelial growth factor signaling

Recent evidence supports a role for GATA transcription factors as important signal intermediates in differentiated endothelial cells. The goal of this study was to identify proteins that interact with endothelial-derived GATA transcription factors. Using yeast two-hybrid screening, we identified hematopoietically expressed homeobox (Hex) as a GATA-binding partner in endothelial cells. The physical association between Hex and GATA was confirmed with immunoprecipitation in cultured cells. Hex overexpression resulted in decreased flk-1/KDR expression, both at the level of the promoter and the endogenous gene, and attenuated vascular endothelial growth factor-mediated tube formation in primary endothelial cell cultures. In electrophoretic mobility shift assays, Hex inhibited the binding of GATA-2 to the flk-1/KDR 5'-untranslated region GATA motif. Finally, in RNase protection assays, transforming growth factor beta1, which has been previously shown to decrease flk-1 expression by interfering with GATA binding activity, was shown to increase Hex expression in endothelial cells. Taken together, the present study provides evidence for a novel association between Hex and GATA and suggests that transforming growth factor beta-mediated repression of flk-1/KDR and vascular endothelial growth factor signaling involves the inducible formation of inhibitory Hex-GATA complexes.

Differential expression and function of Tbx5 and Tbx20 in cardiac development

The T-box transcription factors play critical roles in embryonic development including cell type specification, tissue patterning, and morphogenesis. Several T-box genes are expressed in the heart and are regulators of cardiac development. At the earliest stages of heart development, two of these genes, Tbx5 and Tbx20, are co-expressed in the heart-forming region but then become differentially expressed as heart morphogenesis progresses. Although Tbx5 and Tbx20 belong to the same gene family and share a highly conserved DNA-binding domain, their transcriptional activities are distinct. The C-terminal region of the Tbx5 protein is a transcriptional activator, while the C terminus of Tbx20 can repress transcription. Tbx5, but not Tbx20, activates a cardiac-specific promoter (atrial natriuretic factor (ANF)) alone and synergistically with other transcription factors. In contrast, Tbx20 represses ANF promoter activity and also inhibits the activation mediated by Tbx5. Of the two T-box binding consensus sequences in the promoter of ANF, only T-box binding element 1 (TBE1) is required for the synergistic activation of ANF by Tbx5 and GATA4, but TBE2 is required for repression by Tbx20. To elucidate upstream signaling pathways that regulate Tbx5 and Tbx20 expression, recombinant bone morphogenetic protein-2 was added to cardiogenic explants from chick embryos. Using real time reverse transcription-PCR, it was demonstrated that Tbx20, but not Tbx5, is induced by bone morphogenetic protein-2. Collectively these data demonstrate clear differences in both the expression and function of two related transcription factors and suggest that the modulation of cardiac gene expression can occur as a result of combinatorial regulatory interactions of T-box proteins.

Roles of JUMONJI in mouse embryonic development

Cardiac development is a complex biological process requiring the integration of cell specification, differentiation, migration, proliferation, and morphogenesis. Although significant progress has been made recently in understanding the molecular basis of cardiac development, mechanisms of transcriptional control of cardiac development remain largely unknown. In search for the developmentally important genes, the jumonji gene (jmj) was identified by gene trap technology and characterized as a critical nuclear factor for mouse embryonic development. Jmj has been shown to play important roles in cardiovascular development, neural tube fusion process, hematopoiesis, and liver development in mouse embryos. The amino acid sequence of the JUMONJI protein (JMJ) reveals that JMJ belongs to the AT-rich interaction domain transcription factor family and more recently has been described as a member of the JMJ transcription factor family. Here, we review the roles of jmj in multiple organ development with a focus on cardiovascular development in mice.

Proliferation and Differentiation Processes in the Heart Muscle Elements in Different Phylogenetic Groups

This article reviews, discusses, and summarizes data about the generative behavior of muscle tissue cells, the mechanisms of its regulation, and the organization of the endocrine function of the heart in the main phylogenetic groups. With respect to the ratio of processes of proliferation and differentiation, cell organization, and growth mechanism, muscle tissues of propulsive organs can be divided into three types, each revealed in one of three main groups of animals, lophotrochozoans, ecdysozoans, and chordates. Ecdysterone is likely to play the key role in the regulation of proliferation and differentiation processes in the heart muscle of crustaceans, and, most probably, also of molluscs. In each of the three main phylogenetic groups the endocrine function of the heart consisting of secretion of natriuretic peptides has a peculiar organization. Vertebrate cardiomyocytes are known to combine contractile and endocrine differentiation. Such functional dualism is absent in heart muscle elements of Lophotrochozoa and Ecdysozoa; in the heart of lopfotrochozoans, secretion of natriuretic peptides is performed by endothelial cells and their derivatives. Homology of the heart muscle in the animal kingdom as well as possible mechanisms of genomic and epigenomic regulation of different types of cardiomyogenesis are discussed.

Stem cells and the formation of the myocardium in the vertebrate embryo

A major goal in cardiovascular biology is to repair diseased or damaged hearts with newly generated myocardial tissue. Stem cells offer a potential source of replacement myocytes for restoring cardiac function. Yet little is known about the nature of the cells that are able to generate myocardium and the conditions they require to form heart tissue. A source of information that may be pertinent to addressing these issues is the study of how the myocardium arises from progenitor cells in the early vertebrate embryo. Accordingly, this review will examine the initial events of cardiac developmental biology for insights into the identity and characteristics of the stem cells that can be used to generate myocardial tissue for therapeutic purposes.

Cardiac T-box factor Tbx20 directly interacts with Nkx2-5, GATA4, and GATA5 in regulation of gene expression in the developing heart

Tbx20 is a member of the T-box transcription factor family expressed in the forming hearts of vertebrate and invertebrate embryos. We report here analysis of Tbx20 expression during murine cardiac development and assessment of DNA-binding and transcriptional properties of Tbx20 isoforms. Tbx20 was expressed in myocardium and endocardium, including high levels in endocardial cushions. cDNAs generated by alternative splicing encode at least four Tbx20 isoforms, and Tbx20a uniquely carried strong transactivation and transrepression domains in its C terminus. Isoforms with an intact T-box bound specifically to DNA sites resembling the consensus brachyury half site, although with less avidity compared with the related factor, Tbx5. Tbx20 physically interacted with cardiac transcription factors Nkx2-5, GATA4, and GATA5, collaborating to synergistically activate cardiac gene expression. Among cardiac GATA factors, there was preferential synergy with GATA5, implicated in endocardial differentiation. In Xenopus embryos, enforced expression of Tbx20a, but not Tbx20b, led to induction of mesodermal and endodermal lineage markers as well as cell migration, indicating that the long Tbx20a isoform uniquely bears functional domains that can alter gene expression and developmental behaviour in an in vivo context. We propose that Tbx20 plays an integrated role in the ancient myogenic program of the heart, and has been additionally coopted during evolution of vertebrates for endocardial cushion development.

Cardiac chamber formation: development, genes, and evolution

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

Cardiac electrophysiological phenotypes in postnatal expression of Nkx2.5 transgenic mice

Nkx2.5 is a conserved homeodomain (HD) containing a transcription factor essential for early cardiac development. We generated several mutations modeling some patients with congenital heart disease. Transgenic mice (tg) expressing the wildtype Nkx2.5 under beta-myosin heavy chain (MHC) promoter died during the embryonic stage. However, tg mice expressing this mutation under beta-MHC promoter (beta-MHC-TG(I183P)), the wildtype Nkx2.5 (alpha-MHC-TG(wild)), and a putative transcriptionally active mutant (carboxyl-terminus deletion, alpha-MHC-TG(DeltaC)) under alpha-MHC promoter showed postnatal lethal heart failure. Given the profound atrioventricular conduction abnormalities we recently demonstrated in beta-MHC-TG(I183P) mice, the aim of this study was to determine whether alpha-MHC-TG(wild) and alpha-MHC-TG(DeltaC) mutant mice display similar cardiac electrophysiological phenotypes. Surface ECG recordings and in vivo electrophysiology studies were performed in alpha-MHC-TG(wild) mice and controls at 6 weeks of age, and in alpha-MHC-TG(DeltaC) mice and controls at 10 weeks of age. Ambulatory ECG recordings in alpha-MHC-TG(wild) and controls were obtained using an implantable radiofrequency telemetry system. PR prolongation and atrioventricular nodal dysfunction were detected in alpha-MHC-TG(wild) and alpha-MHC-TG(DeltaC) mice. Bradycardia and prolonged PR interval were seen in ambulatory ECG of alpha-MHC-TG(wild) mice compared to controls. Several alpha-MHC-TG(wild) mice died of bradycardia. Fetal and neonatal mutant Nkx2.5 expression causes severe cardiac conduction failure. Postnatal overexpression of nonmutant (wild) Nkx2.5 also causes conduction abnormalities, although the onset is after the neonatal stage. Bradycardia and AV conduction failure may contribute to the lethal heart failure and early mortality.

JUMONJI, a critical factor for cardiac development, functions as a transcriptional repressor

JUMONJI (JMJ) is a nuclear factor that is critical for normal cardiovascular development, evidenced by the analysis of jmj homozygous mutant mice. However, the molecular function of JMJ remains to be elucidated. In the present study, we investigated whether JMJ is a transcriptional modulator. Reporter gene assays using the GAL4-DNA binding domain fused to JMJ and a reporter gene consisting of the GAL4 binding sites upstream of a luciferase reporter gene indicated that JMJ functions as a powerful transcriptional repressor. The DNA binding motif of JMJ was determined using CASTing experiments by incubating a random oligonucleotide library with the GST-JMJ fusion protein coupled to agarose beads. Among the selected binding oligonucleotides, the high affinity DNA binding sequences were identified by gel retardation assays. JMJ repressed expression of the reporter genes containing the high affinity JMJ binding sequences, indicating that JMJ is a DNA-binding transcriptional repressor. The domains for transcriptional repression, DNA binding, and nuclear localization signal were mapped by mutational analyses using reporter gene assays, gel retardation assays, and immunostaining experiments, respectively. The present data demonstrate for the first time that JMJ functions as a DNA-binding transcriptional repressor. Therefore, JMJ may play a critical role in transcription factor cascade to regulate expression of heart-specific genes and normal cardiac development.

Transgenic analysis of the atrialnatriuretic factor (ANF) promoter: Nkx2-5 and GATA-4 binding sites are required for atrial specific expression of ANF

The atrial natriuretic factor (ANF) gene is initially expressed throughout the myocardial layer of the heart, but during subsequent development, expression becomes limited to the atrial chambers. Mouse knockout and mammalian cell culture studies have shown that the ANF gene is regulated by combinatorial interactions between Nkx2-5, GATA-4, Tbx5, and SRF; however, the molecular mechanisms leading to chamber-specific expression are currently unknown. We have isolated the Xenopus ANF promoter in order to examine the temporal and spatial regulation of the ANF gene in vivo using transgenic embryos. The mammalian and Xenopus ANF promoters show remarkable sequence similarity, including an Nkx2-5 binding site (NKE), two GATA sites, a T-box binding site (TBE), and two SRF binding sites (SREs). Our transgenic studies show that mutation of either SRE, the TBE or the distal GATA element, strongly reduces expression from the ANF promoter. However, mutations of the NKE, the proximal GATA, or both elements together, result in relatively minor reductions in transgene expression within the myocardium. Surprisingly, mutation of these elements results in ectopic ANF promoter activity in the kidneys, facial muscles, and aortic arch artery-associated muscles, and causes persistent expression in the ventricle and outflow tract of the heart. We propose that the NKE and proximal GATA elements serve as crucial binding sites for assembly of a repressor complex that is required for atrial-specific expression of the ANF gene.

Analysis of transcription regulatory regions of embryonic chicken pepsinogen (ECPg) gene

Genes encoding pepsinogens, zymogens of digestive enzyme pepsins, are expressed specifically in the gland epithelial cells of the vertebrate stomach, and their expression is also developmentally regulated, therefore providing a good model for the analysis of transcriptional regulation of genes. In the development of chicken embryonic stomach, the epithelium invaginates into the mesenchyme and forms glands and gland epithelial cells then begin to express embryonic chicken pepsinogen (ECPg) gene. It has been shown that cGATA5 binds directly GATA binding sites located within 1.1-kbp upstream of ECPg gene and activates its transcription. To find more precisely the sequences necessary for ECPg gene transcription, we carried out deletion and mutation analysis with 1.1-kbp upstream region. The results suggest that binding of GATA factor to three GATA binding sites within the upstream region -656 to -419 synergistically regulates ECPg expression in the gland epithelial cells.

Endothelin-converting enzyme-1 (ECE-1) is a downstream target of the homeobox transcription factor Nkx2-5

The homeobox transcription factor Nkx2-5 and the zinc metalloprotease endothelin-converting enzyme-1 (ECE-1) are essential for cardiac development. Here, we demonstrate for the first time a functional link between Nkx2-5 and ECE-1. In transiently transfected rat H9c2 cardiomyoblasts, the alternative promoters specific for ECE-1a, ECE-1b, and ECE-1c are activated by Nkx2-5 coexpression. Lack of a consensus sequence for Nkx2-5 binding within the ECE-1c promoter and mutational analyses of Nkx2-5 consensus sequences identified in the ECE-1a and ECE-1b promoters, respectively, reveal an indirect mechanism of activation that is supported by gel shift assays. Furthermore, we have evidence of an additional direct activation mechanism of the ECE-1b promoter by Nkx2-5. With the use of RNase protection assay, Northern blot, and real-time PCR, the activating effect of Nkx2-5 on mRNA expression of ECE-1 isoforms was confirmed in the chromatin context of H9c2 and endothelial EA.hy926 cells, respectively, by stable Nkx2-5 overexpression. The interaction presented in this work provides a possible explanation for distinct phenotypic aspects of patients carrying mutations in the Nkx2-5 gene and may also be of significance for the pathophysiology of heart failure.

The different cardiac expression of the type 2 iodothyronine deiodinase gene between human and rat is related to the differential response of the Dio2 genes to Nkx-2.5 and GATA-4 transcription factors

By producing T3 from T4, type 2 iodothyronine deiodinase (D2) catalyzes the first step in the cascade underlying the effect exerted by thyroid hormone. Type 2 iodothyronine deiodinase mRNA is expressed at high levels in human heart but is barely detectable in the corresponding rodent tissue. Although the heart is a major target of thyroid hormone, the role of cardiac D2 and the factors that regulate its expression are unknown. Here we report that the human Dio2 promoter is very sensitive to the cardiac transcription factors Nkx-2.5 and GATA-4. Nkx-2.5 transactivates a 6.5-kb human (h)Dio2-chloramphenicol acetyltransferase construct, with maximal induction reached with a 633-bp proximal promoter region. Interestingly, despite 73% identity with the corresponding human region, the rat Dio2 promoter is much less responsive to Nkx-2.5 induction. Using EMSA, we found that two sites in the human promoter (C and D) specifically bind Nkx-2.5. In coexpression studies, GATA-4 alone was a poor inducer of the hDio2 promoter; however in synergy with Nkx-2.5, it activated D2 reporter gene expression in the human, but not the rat promoter. Functional analysis showed that both C and D sites are required for the complete Nkx-2.5 response and for the Nkx-2.5/GATA-4 synergistic effect. In neonatal rat primary myocardiocytes, most of the hDio2-chloramphenicol acetyltransferase activity was suppressed by mutation of the Nkx-2.5 binding sites. Finally, a mutant Nkx-2.5 protein (N188K), which causes, in heterozygosity, congenital heart diseases, did not transactivate the Dio2 promoter and interfered with its activity in cardiomyocytes, possibly by titrating endogenous Nkx-2.5 protein away from the promoter. In conclusion, this study shows that Nkx-2.5 and GATA-4 play prime roles in Dio2 gene regulation in the human heart and suggests that it is their synergistic action in humans that causes the differential expression of the cardiac Dio2 gene between humans and rats.

GATA-4 is a nuclear mediator of mechanical stretch-activated hypertrophic program

In overloaded heart the cardiomyocytes adapt to increased mechanical and neurohumoral stress by activation of hypertrophic program, resulting in morphological changes of individual cells and specific changes in gene expression. Accumulating evidence suggests an important role for the zinc finger transcription factor GATA-4 in hypertrophic agonist-induced cardiac hypertrophy. However, its role in stretch-induced cardiomyocyte hypertrophy is not known. We employed an in vitro mechanical stretch model of cultured cardiomyocytes and used rat B-type natriuretic peptide promoter as stretch-sensitive reporter gene. Stretch transiently increased GATA-4 DNA binding activity and transcript levels, which was followed by increases in the expression of B-type natriuretic peptide as well as atrial natriuretic peptide and skeletal alpha-actin genes. The stretch inducibility mapped primarily to the proximal 520 bp of the B-type natriuretic peptide promoter. Mutational studies showed that the tandem GATA consensus sites of the proximal promoter in combination with an Nkx-2.5 binding element are critical for stretch-activated B-type natriuretic peptide transcription. Inhibition of GATA-4 protein production by adenovirus-mediated transfer of GATA-4 antisense cDNA blocked stretch-induced increases in B-type natriuretic peptide transcript levels and the sarcomere reorganization. The proportion of myocytes with assembled sarcomeres in control adenovirus-infected cultures increased from 14 to 59% in response to stretch, whereas the values for GATA-4 antisense-treated cells were 6 and 13%, respectively. These results show that activation of GATA-4, in cooperation with a factor binding on Nkx-2.5 binding element, is essential for mechanical stretch-induced cardiomyocyte hypertrophy.

PITX2 isoform-specific regulation of atrial natriuretic factor expression: synergism and repression with Nkx2.5

PITX2 and Nkx2.5 are two of the earliest known transcriptional markers of vertebrate heart development. Pitx2-/- mice present with severe cardiac malformations and embryonic lethality, demonstrating a role for PITX2 in heart development. However, little is known about the downstream targets of PITX2 in cardiogenesis. We report here that the atrial natriuretic factor (ANF) promoter is a target of PITX2. PITX2A, PITX2B, and PITX2C isoforms differentially activate the ANF promoter. However, only PITX2C can synergistically activate the ANF promoter in the presence of Nkx2.5. We further demonstrate that the procollagen lysyl hydroxylase (PLOD1) promoter is regulated by Nkx2.5. Mechanistically, PITX2C and Nkx2.5 synergistically regulate ANF and PLOD1 expression through binding to their respective DNA elements. Surprisingly, PITX2A activation of the ANF and PLOD1 promoters is repressed by co-transfection of Nkx2.5 in the C3H10T1/2 embryonic fibroblast cell line. Pitx2a and Pitx2c are endogenously expressed in C3H10T1/2 cells, and these cells express factors that differentially regulate PITX2 isoform activities. We provide a new mechanism for the regulation of heart development by PITX2 isoforms through the regulation of ANF and PLOD1 gene expression and Nkx2.5 transcriptional activity.

Novel roles for GATA transcription factors in the regulation of steroidogenesis

Steroidogenesis is a tightly regulated process that is dependent on pituitary hormones. In steroidogenic tissues, hormonal stimulation triggers activation of an intracellular signalling pathway that typically involves cAMP production, activation of PKA, and phosphorylation of target transcription factors. In the classic cAMP signalling pathway, phosphorylation of CREB (cAMP response element (CRE)-binding protein) and its subsequent binding to cAMP-response elements (CREs) in the regulatory regions of target genes play a key role in mediating cAMP responsiveness. However, the cAMP responsive regions of several genes expressed in steroidogenic tissues do not contain consensus CREs indicating that other transcription factors are also involved. We have been studying the role played by the GATA family of transcription factors. GATA factors are expressed in a variety of tissues including the adrenals and gonads. Since the regulatory regions of several steroidogenic genes contain GATA elements, we have proposed that GATA factors, particularly GATA-4 and GATA-6, might represent novel downstream effectors of hormonal signalling in steroidogenic tissues. In vitro experiments have revealed that GATA-4 is indeed phosphorylated in steroidogenic cells and that phosphorylation levels are rapidly induced by cAMP. GATA-4 phosphorylation is mediated by PKA. Phosphorylation increases GATA-4 DNA-binding activity and enhances its transcriptional properties on multiple steroidogenic promoters. We now define a new molecular mechanism whereby phospho-GATA factors contribute to increased transcription of steroidogenic genes in response to hormonal stimulation.

Roles of cardiac transcription factors in cardiac hypertrophy

Different cell types, equipped with unique structure and function, synthesize different sets of proteins on the basis of different patterns of gene expression, even though their genomes are identical. Cardiac transcription factors have been reported to control a cardiac gene program and thus to play a crucial role in transcriptional regulation during embryogenesis. Recently, postnatal roles of cardiac transcription factors have been extensively investigated. Consistent with the direct transactivation of numerous cardiac genes reactivated in response to hypertrophic stimulation, cardiac transcription factors are profoundly involved in the generation of cardiac hypertrophy or in cardioprotection from cytotoxic stress in the adult heart. In this review, the regulation of a cardiac gene program by cardiac transcription factors is summarized, with an emphasis on their potential role in the generation of cardiac hypertrophy.

Cell-specific expression of the glucose-dependent insulinotropic polypeptide gene functions through a GATA and an ISL-1 motif in a mouse neuroendocrine tumor cell line

BACKGROUND/AIMS

Glucose-dependent insulinotropic polypeptide (GIP) is a 42-amino acid gastrointestinal regulatory peptide that, in the presence of glucose, stimulates insulin secretion from beta-cells. GIP is expressed in gastrointestinal K-cells. Prior analysis of the GIP promoter demonstrated that 193 bases of the promoter are required to direct cell specific expression. Here we sought to identify and characterize the transcription factors involved.

RESULTS

By mutational analysis of the GIP promoter in a neuroendocrine cell line (STC-1), we identified two regions located between bases -193 and -182 and bases -156 and -151 that, when independently altered, were responsible for a 90% and 85% reduction in transcription, respectively. When we compared these two regions with known motifs from transcription factor databases, we identified the cis elements as potential GATA and ISL-1 binding sites. With subsequent electrophoretic mobility shift analysis (EMSA) using STC-1 nuclear extracts, we demonstrated the ability of these regions to form specific DNA protein complexes. Furthermore, we utilized antisera to confirm the specific binding of GATA-4 to the upstream site and ISL-1 to the downstream element.

CONCLUSION

These findings provide evidence for the involvement of the transcription factors GATA-4 and ISL-1 in the cell-specific expression of the GIP gene.

Retinoid X receptor alpha represses GATA-4-mediated transcription via a retinoid-dependent interaction with the cardiac-enriched repressor FOG-2

Dietary vitamin A and its derivatives, retinoids, regulate cardiac growth and development. To delineate mechanisms involved in retinoid-mediated control of cardiac gene expression, the regulatory effects of the retinoid X receptor alpha (RXR alpha) on atrial naturietic factor (ANF) gene transcription was investigated. The transcriptional activity of an ANF promoter-reporter in rat neonatal ventricular myocytes was repressed by RXR alpha in the presence of 9-cis-RA and by the constitutively active mutant RXR alpha F318A indicating that liganded RXR confers the regulatory effect. The RXR alpha-mediated repression mapped to the proximal 147 bp of the rat ANF promoter, a region lacking a consensus retinoid response element but containing several known cardiogenic cis elements including a well characterized GATA response element. Glutathione S-transferase "pull-down" assays revealed that RXR alpha interacts directly with GATA-4, in a ligand-independent manner, via the DNA binding domain of RXR alpha and the second zinc finger of GATA-4. Liganded RXR alpha repressed the activity of a heterologous promoter-reporter construct containing GATA-response element recognition sites in cardiac myocytes but not in several other cell types, suggesting that additional cardiac-enriched factors participate in the repression complex. Co-transfection of liganded RXR alpha and the known cardiac-enriched GATA-4 repressor, FOG-2, resulted in additive repression of GATA-4 activity in ventricular myocytes. In addition, RXR alpha was found to bind FOG-2, in a 9-cis-RA-dependent manner. These data reveal a novel mechanism by which retinoids regulate cardiogenic gene expression through direct interaction with GATA-4 and its co-repressor, FOG-2.

Cardiac ankyrin repeat protein (CARP) expression in human and murine atherosclerotic lesions: activin induces CARP in smooth muscle cells

OBJECTIVE

Cardiac ankyrin repeat protein (CARP) is a transcription factor-related protein that has been studied most extensively in the heart. In the present study, we investigated the expression and the potential function of CARP in human and murine atherosclerosis.

METHODS AND RESULTS

CARP expression was observed by in situ hybridization in endothelial cells lining human atherosclerotic plaques, whereas lesion macrophages were devoid of CARP. Furthermore, we established that CARP mRNA and smooth muscle (SM) alpha-actin antigen both colocalized in a subset of intimal smooth muscle cells (SMCs), whereas no CARP mRNA was encountered in quiescent SMCs in the media. The CARP mRNA-expressing intimal SMCs were distinct from intimal SMCs that synthesized the activation marker osteopontin or proliferating cell nuclear antigen. In addition, we showed that activin A, a member of the TGFbeta superfamily that prevents SMC-rich lesion formation, induced CARP mRNA expression in cultured SMCs.

CONCLUSIONS

Based on our data and the knowledge that CARP reduces the proliferation of cultured SMCs, we propose that CARP is involved in inhibition of vascular lesion formation.

Early signals in cardiac development

The heart is the first organ to form during embryogenesis and its circulatory function is critical from early on for the viability of the mammalian embryo. Developmental abnormalities of the heart have also been widely recognized as the underlying cause of many congenital heart malformations. Hence, the developmental mechanisms that orchestrate the formation and morphogenesis of this organ have received much attention among classical and molecular embryologists. Due to the evolutionary conservation of many of these processes, major insights have been gained from the studies of a number of vertebrate and invertebrate models, including mouse, chick, amphibians, zebrafish, and Drosophila. In all of these systems, the heart precursors are generated within bilateral fields in the lateral mesoderm and then converge toward the midline to form a beating linear heart tube. The specification of heart precursors is a result of multiple tissue and cell-cell interactions that involve temporally and spatially integrated programs of inductive signaling events. In the present review, we focus on the molecular and developmental functions of signaling processes during early cardiogenesis that have been defined in both vertebrate and invertebrate models. We discuss the current knowledge on the mechanisms through which signals induce the expression of cardiogenic transcription factors and the relationships between signaling pathways and transcriptional regulators that cooperate to control cardiac induction and the formation of a linear heart tube.

Dual effects of the homeobox transcription factor Csx/Nkx2-5 on cardiomyocytes

A homeobox-containing transcription factor Csx/Nkx2-5 is an important regulator of cardiac development. Many different human CSX/NKX2-5 mutations have been reported to cause congenital heart disease. We here examined the effects of three representative CSX/NKX2-5 mutations on cardiomyocyte differentiation and death with the use of the P19CL6 cardiomyogenic cell lines. Stable overexpression of wild-type CSX/NKX2-5 enhanced expression of cardiac-specific genes such as MEF2C and MLC2v, the promoter activity of the atrial natriuretic peptide gene, and the terminal differentiation of P19CL6 into cardiomyocytes, while all CSX/NKX2-5 mutants attenuated them by different degrees. When exposed to H(2)O(2) or cultured without change of the medium, many differentiated P19CL6 cells overexpressing the mutants, especially the mutant which lacks the carboxyl terminal region just after the homeodomain, were dead, while most of the cells overexpressing wild-type CSX/NKX2-5 survived. Overexpression of the carboxyl terminus-deleted mutant down-regulated expression of an anti-apoptotic protein Bcl-x(L) and up-regulated that of a pro-apoptotic protein CAS, while in the cells overexpressing wild-type CSX/NKX2-5, expression of a pro-apoptotic protein RIP was reduced. Furthermore, overexpression of wild-type CSX/NKX2-5 decreased the number of H(2)O(2)-induced TUNEL-positive cultured cardiomyocytes of neonatal rats, whereas overexpression of the mutants enhanced it. These results suggest that Csx/Nkx2-5 not only regulates expression of cardiac-specific genes but protects cardiomyocytes from stresses and that cell death may be another cause for the cardiac defects induced by human CSX/NKX2-5 mutations.

Molecular embryogenesis of the heart

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

Modulation of Cardiac Growth and Development by HOP, an Unusual Homeodomain Protein

We have discovered an unusual homeodomain protein, called HOP, which is comprised simply of a homeodomain. HOP is highly expressed in the developing heart where its expression is dependent on the cardiac-restricted homeodomain protein Nkx2.5. HOP does not bind DNA and acts as an antagonist of serum response factor (SRF), which regulates the opposing processes of proliferation and myogenesis. Mice homozygous for a HOP null allele segregate into two phenotypic classes characterized by an excess or deficiency of cardiac myocytes. We propose that HOP modulates SRF activity during heart development; its absence results in an imbalance between cardiomyocyte proliferation and differentiation with consequent abnormalities in cardiac morphogenesis.

Physical interaction between GATA-5 and hepatocyte nuclear factor-1alpha results in synergistic activation of the human lactase-phlorizin hydrolase promoter

GATA-4, -5, and -6 zinc finger and hepatocyte nuclear factor-1alpha (HNF-1alpha) homeodomain transcription factors are expressed in the intestinal epithelium and synergistically activate the promoter of intestinal genes. Here, we demonstrate that GATA-5 and HNF-1alpha physically associate both in vivo and in vitro and that this interaction is necessary for cooperative activation of the lactase-phlorizin hydrolase promoter. Furthermore, physical association is mediated by the C-terminal zinc finger of GATA factors and the homeodomain of HNF-1alpha. Deletion of HNF-1alpha activation domains or interruption of HNF-1-binding sites in the lactase-phlorizin hydrolase promoter resulted in a complete loss of cooperativity, whereas deletion of GATA-5 activation domains or interruption of GATA-binding sites resulted in a reduction, but not an elimination, of cooperativity. We hypothesize that GATA/HNF-1alpha cooperativity is mediated by HNF-1alpha through its activation domains, which are oriented for high levels of activation through binding to DNA and physical association with GATA factors. These data suggest a paradigm whereby intestine-specific gene expression is regulated by unique interactions among tissue-restricted transcription factors coexpressed in the intestine. Parallel mechanisms in other tissues as well as in Drosophila suggest that zinc finger/homeodomain interactions are an efficient pathway of cooperative activation of gene transcription that has been conserved throughout evolution.

Combinatorial expression of GATA4, Nkx2-5, and serum response factor directs early cardiac gene activity

Herein, the restricted expression of serum response factors (SRF) closely overlapped with Nkx2-5 and GATA4 transcripts in early chick embryos coinciding with the earliest appearance of cardiac alpha-actin (alphaCA) transcripts and nascent myocardial cells. The combinatorial expression of SRF, a MADS box factor Nkx2-5 (a NK4 homeodomain), and/or GATA4, a dual C4 zinc finger protein, in heterologous CV1 fibroblasts and Schneider 2 insect cells demonstrated synergistic induction of alphaCA promoter activity. These three factors induced endogenous alphaCA mRNA over a 100-fold in murine embryonic stem cells. In addition, the DNA-binding defective mutant Nkx2-5pm efficiently coactivated the alphaCA promoter in the presence of SRF and GATA4 in the presence of all four SREs and was substantially weakened when individual SREs were mutated and or serially deleted. In contrast, the introduction of SRFpm, a SRF DNA-binding mutant, blocked the activation with all of the alphaCA promoter constructions. These assays indicated a dependence upon cooperative SRF binding for facilitating the recruitment of Nkx2-5 and GATA4 to the alphaCA promoter. Furthermore, the recruitment of Nkx2-5 and GATA4 by SRF was observed to strongly enhance SRF DNA binding affinity. This mechanism allowed for the formation of higher ordered alphaCA promoter DNA binding complexes, led to a model of SRF physical association with Nkx2-5 and GATA4.

The transcription factors GATA4 and dHAND physically interact to synergistically activate cardiac gene expression through a p300-dependent mechanism

An intricate array of heterogeneous transcription factors participate in programming tissue-specific gene expression through combinatorial interactions that are unique to a given cell-type. The zinc finger-containing transcription factor GATA4, which is widely expressed in mesodermal and endodermal derived tissues, is thought to regulate cardiac myocyte-specific gene expression through combinatorial interactions with other semi-restricted transcription factors such as myocyte enhancer factor 2, nuclear factor of activated T-cells, serum response factor, and Nkx2.5. Here we determined that GATA4 also interacts with the cardiac-expressed basic helix-loop-helix transcription factor dHAND (also known as HAND2). GATA4 and dHAND synergistically activated expression of cardiac-specific promoters from the atrial natriuretic factor gene, the b-type natriuretic peptide gene, and the alpha-myosin heavy chain gene. Using artificial reporter constructs this functional synergy was shown to be GATA site-dependent, but E-box site-independent. A mechanism for the transcriptional synergy was suggested by the observation that the bHLH domain of dHAND physically interacted with the C-terminal zinc finger domain of GATA4 forming a higher order complex. This transcriptional synergy observed between GATA4 and dHAND was associated with p300 recruitment, but not with alterations in DNA binding activity of either factor. Moreover, the bHLH domain of dHAND directly interacted with the CH3 domain of p300 suggesting the existence of a higher order complex between GATA4, dHAND, and p300. Taken together with previous observations, these results suggest the existence of an enhanceosome complex comprised of p300 and multiple semi-restricted transcription factors that together specify tissue-specific gene expression in the heart.

Regulation of the human brain natriuretic peptide gene by GATA-4

Brain natriuretic peptide (BNP) is a cardiac hormone constitutively expressed in the adult heart. We previously showed that the human BNP (hBNP) proximal promoter region from -127 to -40 confers myocyte-specific expression. The proximal hBNP promoter contains several putative cis elements. Here we tested whether the proximal GATA element plays a role in basal and inducible regulation of the hBNP promoter. The hBNP promoter was coupled to a luciferase reporter gene (1818hBNPLuc) and transferred into neonatal ventricular myocytes (NVM), and luciferase activity was measured as an index of hBNP promoter activity. Mutation of the putative GATA element at -85 of the hBNP promoter [1818(mGATA)hBNPLuc] reduced activity by 97%. To study transactivation of the hBNP promoter, we co-transfected 1818hBNPLuc with the GATA-4 expression vector. GATA-4 activated 1818hBNPLuc, and this effect was eliminated by mutation of the proximal GATA element. Electrophoretic mobility shift assay showed that an oligonucleotide containing the hBNP GATA motif bound to cardiomyocyte nuclear protein, which was competed for by a consensus GATA oligonucleotide but not a mutated hBNP GATA element. The beta-adrenergic agonist isoproterenol and its second messenger cAMP stimulated hBNP promoter activity and binding of nuclear protein to the proximal GATA element. Thus the GATA element in the proximal hBNP promoter is involved in both basal and inducible transcriptional regulation in cardiac myocytes.

Differential regulation of the cardiac sodium calcium exchanger promoter in adult and neonatal cardiomyocytes by Nkx2.5 and serum response factor

Nkx2.5 and serum response factor (SRF) are critically important transcription factors in cardiac morphogenesis. They are also widely expressed in adult cardiomyocytes, but there is little data to indicate their possible role in adult cardiac cells. In this paper we demonstrate that the interaction of Nkx2.5 and SRF in cardiac-specific gene regulation is different between neonatal and adult cardiomyocytes. Our experimental model utilizes transient transfection and adenovirus mediated gene transfer of the proximal promoter fragment of the cardiac isoform of the sodium-calcium exchanger gene (NCX1). This promoter construct (NCX184) contains a single Nkx2.5-response element (NKE) and a single serum response element (CArG). In rat neonatal cardiomyocytes NCX184 activity is substantially induced with Nkx2.5 or SRF and additively with both. Mutagenesis of these NKE and CArG elements demonstrated the specificity of the interactions, which was confirmed with gel retardation analysis of cardiac ventricular tissue. In contrast, in adult cardiomyocytes, co-infection of Nkx2.5 and SRF adenovirus vectors showed Nkx2.5 induction but SRF did not have additive effects on NCX1 promoter regulation. As opposed to NCX1, the proximal atrial natriuretic factor (ANF) promoter was regulated identically in response to SRF and Nkx2.5 in both adult and neonatal cardiomyocytes. These results show that Nkx2.5-SRF interactions are capable of producing different transcriptional responses in adult versus neonatal cardiomyocytes, implying important differences in NCX1 promoter tertiary complex formation dependent on developmental stage.

Control of cardiac development by an evolutionarily conserved transcriptional network

Formation of the heart is dependent on an intricate cascade of developmental decisions. Analysis of the molecules and mechanisms involved in the specification of cardiac cell fates, differentiation and diversification of cardiac muscle cells, and morphogenesis and patterning of different cardiac cell types has revealed an evolutionarily conserved network of signaling pathways and transcription factors that underlies these processes. The regulatory network that controls the formation of the primitive heart in fruit flies has been elaborated upon to form the complex multichambered heart of mammals. We compare and contrast the mechanisms involved in heart formation in fruit flies and mammals in the context of a network of transcriptional interactions and point to unresolved questions for the future.

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

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

Multiple enhancers contribute to expression of the NK-2 homeobox gene ceh-22 in C. elegans pharyngeal muscle

Gene expression in the pharyngeal muscles of C. elegans is regulated in part by the NK-2 family homeodomain factor CEH-22, which is structurally and functionally related to Drosophila Tinman and the vertebrate Nkx2-5 factors. ceh-22 is expressed exclusively in the pharyngeal muscles and is the earliest gene known to be expressed in this tissue. Here we characterize the ceh-22 promoter region in transgenic C. elegans. A 1.9-kb fragment upstream of ceh-22 is sufficient to regulate reporter gene expression in a pattern identical to the endogenous gene. Within this promoter we identified two transcriptional enhancers and characterized their cell type and temporal specificity. The distal enhancer becomes active in the pharynx near the time that ceh-22 expression initiates; however, it becomes active more broadly later in development. The proximal enhancer becomes active after the onset of ceh-22 expression, but it is active specifically in the ceh-22-expressing pharyngeal muscles. We suggest these enhancers respond to distinct signals that initiate and maintain ceh-22 gene expression. Proximal enhancer activity requires a short segment containing a CEH-22 responsive element, suggesting that CEH-22 autoregulates its own expression.

Regulation of heart development and function through combinatorial interactions of transcription factors

Understanding the molecular mechanisms controlling cardiac-specific gene transcription requires the dissection of the cis-elements that govern the complex spatio-temporal expression of these genes. The four-chambered vertebrate heart is formed during the late phases of fetal development following a series of complex morphogenetic events that require the functional presence of different proteins. The gradient-like expression of some genes, as well as the chamber-specific expression of others, is tightly regulated by combinatorial interactions of several transcription factors and their cofactors. Chamber- and stage-specific cardiac myocyte cultures have been invaluable for identifying transcription factor binding sites involved in basal, chamber-specific, and inducible expression of many cardiac promoters; these studies, which were largely confirmed in vivo in transgenic mouse models, led to the isolation of key regulators of heart development. In addition, the use of pluripotent embryonic stem cells helped elucidate the early molecular events controlling cardiomyocyte differentiation. Together, these studies point to a major role for GATA transcription factors and their interacting partners in transcriptional control of heart development. In addition, members of the T-box family of transcription factors and homeodomain containing proteins, together with chamber-restricted transcriptional repressors and co-repressors play critical roles in heart septation and chamber specification. These fine-tuned cooperative interactions between different classes of proteins are at the basis of normal cardiac function, and alteration in their expression level or function leads to cardiac pathologies.

BMP signaling regulates Nkx2-5 activity during cardiomyogenesis

Nkx2-5 regulates the transcription of muscle-specific genes during cardiomyogenesis. Nkx2-5 expression can induce cardiomyogenesis in aggregated P19 cells but not in monolayer cultures. In order to investigate the mechanism by which cellular aggregation regulates Nkx2-5 function, we examined the role of bone morphogenetic protein 4 (BMP4). We showed that the expression of the BMP inhibitor, noggin, was sufficient to inhibit the induction of cardiomyogenesis by Nkx2-5 during cellular aggregation. Furthermore, soluble BMP4 could activate Nkx2-5 function in monolayer cultures, resulting in the formation of cardiomyocytes. Therefore, BMP signaling is necessary and sufficient for the regulation of Nkx2-5 activity during cardiomyogenesis in P19 cells.

Nkx2-5 activity is essential for cardiomyogenesis

The homeobox transcription factor tinman is essential for heart vessel formation in Drosophila. In contrast, mice lacking the murine homologue Nkx2-5 are defective in cardiac looping but not in cardiac myocyte development. The lack of an essential role for Nkx2-5 in cardiomyogenesis in mammalian systems is most likely the result of genetic redundancy with family members. In this study, we used a dominant negative mutant of Nkx2-5, created by fusing the repressor domain of engrailed 2 to the Nkx2-5 homeodomain, termed Nkx/EnR. Expression of Nkx/EnR inhibited Me(2)SO-induced cardiomyogenesis in P19 cells but not skeletal myogenesis. Nkx/EnR inhibited expression of cardiomyoblast markers, such as GATA-4 and MEF2C, but not of mesoderm markers, such as Brachyury T and Wnt5b, or of skeletal lineage markers, such as MyoD and Mox1. To identify the minimal region of Nkx2-5 that can trigger cardiomyogenesis, we analyzed the activity of various Nkx2-5 deletion mutants. The C-terminal domain was not necessary for the ability of Nkx2-5 to induce cardiomyogenesis and loss of this domain did not enhance myogenesis. Therefore, Nkx2-5 function is essential for commitment of mesoderm into the cardiac muscle lineage, and the N-terminal region, together with the homeodomain, is sufficient for cardiomyogenesis in P19 cells.

Endothelin-1 induces phosphorylation of GATA-4 transcription factor in the HL-1 atrial-muscle cell line

The transcription factor GATA-4 plays a central role in the regulation of cardiac-muscle gene transcription. The present study demonstrates that endothelin-1 (ET-1) induces GATA-4 activation and phosphorylation. The treatment of HL-1 adult mouse atrial-muscle cells with ET-1 (30 nM) caused a rapid increase in the DNA binding activity of GATA-4 within 3 min. The activation was associated with an upward mobility shift of the GATA-4 band on native PAGE in an electrophoretic- mobility-shift assay. The upward shift of the GATA-4 band also occurred on SDS/PAGE as monitored by immunoblotting. The in vitro treatment of nuclear extracts with lambda-protein phosphatase abolished the upward shift, indicating that GATA-4 was phosphorylated. ET-1 activated the p44/42 mitogen-activated protein kinase (MAPK) and the MAPK kinase (MEK) within 3 min, and PD98059 (a specific inhibitor of MEK) abolished the ET-1-induced GATA-4 phosphorylation. PMA also caused the rapid activation of MAPK and the phosphorylation of GATA-4. In contrast, the activation of MAPK by phenylephrine or H(2)O(2) was weak and did not lead to GATA-4 phosphorylation. Thus ET-1 induces a GATA-4 phosphorylation by activating a MEK-MAPK pathway.

p300 Functions as a coactivator of transcription factor GATA-4

Transcription factor GATA-4 plays critical roles in controlling heart development and cardiac hypertrophy. To understand how GATA-4 functions under diverse conditions, we sought to identify its coactivators. We tested p300 as a coactivator in GATA-4-dependent transient transcription assays in NIH3T3 cells and found that p300 synergistically activated GATA-4-dependent transcription on both synthetic and natural promoters. Direct physical interactions between the N- and C-zinc finger domains of GATA-4 and the cysteine/histidine-rich region 3 (C/H3) of p300 were identified in immunoprecipitation and glutathione S-transferase pull-down experiments. Deletion of the C/H3 region of p300 abolished its coactivator activity indicating that the physical interaction was required for functional synergy. Through the use of a series of GATA-4 zinc finger mutants, the amino acids WRR in the C finger were identified as critical to the interaction. The adenoviral E1A protein or a peptide encoding the C/H3 region of p300 could inhibit GATA-4-dependent transcription, presumably by competing for p300 binding. Furthermore, deletion of the region of p300 encoding the histone acetyltransferase activity abolished its effect on GATA-4-dependent transcriptional activity. These results establish that p300 acts as a GATA-4 coactivator and that the p300 histone acetyltransferase activity is necessary for the functional interaction.