T-box genes and heart development: Putting the ?T? in heart

Using potential master regulator sites and paralogous expansion to construct tissue-specific transcriptional networks

Background

Transcriptional networks of higher eukaryotes are difficult to obtain. Available experimental data from conventional approaches are sporadic, while those generated with modern high-throughput technologies are biased. Computational predictions are generally perceived as being flooded with high rates of false positives. New concepts about the structure of regulatory regions and the function of master regulator sites may provide a way out of this dilemma.

Methods

We combined promoter scanning with positional weight matrices with a 4-genome conservativity analysis to predict high-affinity, highly conserved transcription factor (TF) binding sites and to infer TF-target gene relations. They were expanded to paralogous TFs and filtered for tissue-specific expression patterns to obtain a reference transcriptional network (RTN) as well as tissue-specific transcriptional networks (TTNs).

Results

When validated with experimental data sets, the predictions done showed the expected trends of true positive and true negative predictions, resulting in satisfying sensitivity and specificity characteristics. This also proved that confining the network reconstruction to the 1% top-ranking TF-target predictions gives rise to networks with expected degree distributions. Their expansion to paralogous TFs enriches them by tissue-specific regulators, providing a reasonable basis to reconstruct tissue-specific transcriptional networks.

Conclusions

The concept of master regulator or seed sites provides a reasonable starting point to select predicted TF-target relations, which, together with a paralogous expansion, allow for reconstruction of tissue-specific transcriptional networks.

T-box factors: insights into the evolutionary emergence of the complex heart

The heart as a functional organ first appeared in bilaterians as a single peristaltic pump and evolved through arthropods, fish, amphibians, and finally mammals into a four-chambered engine controlling blood-flow within the body. The acquisition of cardiac complexity in the evolving heart was a product of gene duplication events and the co-option of novel signaling pathways to an ancestral cardiac-specific gene network. T-box factors belong to an evolutionary conserved family of transcriptional regulators with diverse roles in development. Their regulatory functions are integral in the initiation and potentiation of heart development, and mutations in these genes are associated with congenital heart defects. In this review we will discuss the evolutionary conserved cardiac regulatory functions of this family as well as their implication in disease in an aim to facilitate future gene-targeted and regenerative therapeutic remedies.

Evolution of the tbx6/16 subfamily genes in vertebrates: insights from zebrafish

In any comparative studies striving to understand the similarities and differences of the living organisms at the molecular genetic level, the crucial first step is to establish the homology (orthology and paralogy) of genes between different organisms. Determination of the homology of genes becomes complicated when the genes have undergone a rapid divergence in sequence or when the involved genes are members of a gene family that has experienced a differential gain or loss of its constituents in different taxonomic groups. Organisms with duplicated genomes such as teleost fishes might have been especially prone to these problems because the functional redundancies provided by the duplicate copies of genes would have allowed a rapid divergence or loss of genes during evolution. In this study, we will demonstrate that much of the ambiguities in the determination of the homology between fish and tetrapod genes resulting from the problems like these can be eliminated by complementing the sequence-based phylogenies with nonsequence information, such as the exon-intron structure of a gene or the composition of a gene's genomic neighbors. We will use the Tbx6/16 subfamily genes of zebrafish (tbx6, tbx16, tbx24, and mga genes), which have been well known for the ambiguity of their evolutionary relationships to the Tbx6/16 subfamily genes of tetrapods, as an illustrative example. We will show that, despite the similarity of sequence and expression to the tetrapod Tbx6 genes, zebrafish tbx6 gene is actually a novel T-box gene more closely related to the tetrapod Tbx16 genes, whereas the zebrafish tbx24 gene, hitherto considered to be a novel gene due to the high level of sequence divergence, is actually an ortholog of tetrapod Tbx6 genes. We will also show that, after their initial appearance by the multiplication of a common ancestral gene at the beginning of vertebrate evolution, the Tbx6/16 subfamily of vertebrate T-box genes might have experienced differential losses of member genes in different vertebrate groups and gradual pooling of member gene's functions in surviving members, which might have prevented the revelation of the true identity of member genes by way of the comparison of sequence and function.

Genetic variation in T-box binding element functionally affects SCN5A/SCN10A enhancer

The contraction pattern of the heart relies on the activation and conduction of the electrical impulse. Perturbations of cardiac conduction have been associated with congenital and acquired arrhythmias as well as cardiac arrest. The pattern of conduction depends on the regulation of heterogeneous gene expression by key transcription factors and transcriptional enhancers. Here, we assessed the genome-wide occupation of conduction system-regulating transcription factors TBX3, NKX2-5, and GATA4 and of enhancer-associated coactivator p300 in the mouse heart, uncovering cardiac enhancers throughout the genome. Many of the enhancers colocalized with ion channel genes repressed by TBX3, including the clustered sodium channel genes Scn5a, essential for cardiac function, and Scn10a. We identified 2 enhancers in the Scn5a/Scn10a locus, which were regulated by TBX3 and its family member and activator, TBX5, and are functionally conserved in humans. We also provided evidence that a SNP in the SCN10A enhancer associated with alterations in cardiac conduction patterns in humans disrupts TBX3/TBX5 binding and reduces the cardiac activity of the enhancer in vivo. Thus, the identification of key regulatory elements for cardiac conduction helps to explain how genetic variants in noncoding regulatory DNA sequences influence the regulation of cardiac conduction and the predisposition for cardiac arrhythmias.

Tbx6 is a determinant of cardiac and neural cell fate decisions in multipotent P19CL6 cells

Multipotent P19CL6 cells differentiate into cardiac myocytes or neural lineages when stimulated with dimethyl sulfoxide (DMSO) or retinoic acid (RA), respectively. Expression of the transcription factor Tbx6 was found to increase during cardiac myocyte differentiation and to decrease during neural differentiation. Overexpression of Tbx6 was not sufficient to drive P19CL6 cells to a cardiac myocyte fate or to accelerate DMSO-induced differentiation. In contrast, knockdown of Tbx6 dramatically inhibited DMSO-induced differentiation of P19CL6 cells to cardiac myocytes, as evidenced by the loss of striated muscle-specific markers and spontaneous beating. Tbx6 knockdown was also accompanied by almost complete loss of Nkx2.5, a transcription factor involved in the specification of the cardiac myocyte lineage, indicating that Nkx2.5 is downstream of Tbx6. In distinction to its positive role in cardiac myocyte differentiation, Tbx6 knockdown augmented RA-induced differentiation of P19CL6 cells to both neurons and glia, and accelerated the rate of neurite formation. Conversely, Tbx6 overexpression attenuated differentiation to neural lineages. Thus, in the P19CL6 model, Tbx6 is required for cardiac myocyte differentiation and represses neural differentiation. We propose a model in which Tbx6 is a part of a molecular switch that modulates divergent differentiation programs within a single progenitor cell.

A new approach to transcription factor screening for reprogramming of fibroblasts to cardiomyocyte-like cells

The simultaneous overexpression of several transcription factors has emerged as a successful strategy to convert fibroblasts into other cell types including pluripotent cells, neurons, and cardiomyocytes. The selection and screening of factors are critical, and have often involved testing a large pool of transcription factors, followed by successive removal of single factors. Here, to identify a cardiac transcription factor combination facilitating mouse fibroblast reprogramming into cardiomyocytes, we directly screened all triplet combinations of 10 candidate factors combined with a Q-PCR assay reporting induction of multiple cardiac-specific genes. Through this screening method the combination of Tbx5, Mef2c, and Myocd was identified to upregulate a broader spectrum of cardiac genes compared to the combination of Tbx5, Mef2c, and Gata4 that was recently shown to induce reprogramming of fibroblasts into cardiomyocytes. Cells cotransduced with Tbx5, Mef2c, Myocd expressed cardiac contractile proteins, had cardiac-like potassium and sodium currents and action potentials could be elicited. In summary the alternative screening approach that is presented here avoided the elimination of transcription factors whose potency is masked in complex transcription factor mixes. Furthermore, our results point to the importance of verifying multiple lineage specific genes when assessing reprogramming.

Characterization of rainbow trout gonad, brain and gill deep cDNA repertoires using a Roche 454-Titanium sequencing approach

Rainbow trout, Oncorhynchus mykiss, is an important aquaculture species worldwide and, in addition to being of commercial interest, it is also a research model organism of considerable scientific importance. Because of the lack of a whole genome sequence in that species, transcriptomic analyses of this species have often been hindered. Using next-generation sequencing (NGS) technologies, we sought to fill these informational gaps. Here, using Roche 454-Titanium technology, we provide new tissue-specific cDNA repertoires from several rainbow trout tissues. Non-normalized cDNA libraries were constructed from testis, ovary, brain and gill rainbow trout tissue samples, and these different libraries were sequenced in 10 separate half-runs of 454-Titanium. Overall, we produced a total of 3million quality sequences with an average size of 328bp, representing more than 1Gb of expressed sequence information. These sequences have been combined with all publicly available rainbow trout sequences, resulting in a total of 242,187 clusters of putative transcript groups and 22,373 singletons. To identify the predominantly expressed genes in different tissues of interest, we developed a Digital Differential Display (DDD) approach. This approach allowed us to characterize the genes that are predominantly expressed within each tissue of interest. Of these genes, some were already known to be tissue-specific, thereby validating our approach. Many others, however, were novel candidates, demonstrating the usefulness of our strategy and of such tissue-specific resources. This new sequence information, acquired using NGS 454-Titanium technology, deeply enriched our current knowledge of the expressed genes in rainbow trout through the identification of an increased number of tissue-specific sequences. This identification allowed a precise cDNA tissue repertoire to be characterized in several important rainbow trout tissues. The rainbow trout contig browser can be accessed at the following publicly available web site (http://www.sigenae.org/).

Chromatin state signatures associated with tissue-specific gene expression and enhancer activity in the embryonic limb

The regulatory elements that direct tissue-specific gene expression in the developing mammalian embryo remain largely unknown. Although chromatin profiling has proven to be a powerful method for mapping regulatory sequences in cultured cells, chromatin states characteristic of active developmental enhancers have not been directly identified in embryonic tissues. Here we use whole-transcriptome analysis coupled with genome-wide profiling of H3K27ac and H3K27me3 to map chromatin states and enhancers in mouse embryonic forelimb and hindlimb. We show that gene-expression differences between forelimb and hindlimb, and between limb and other embryonic cell types, are correlated with tissue-specific H3K27ac signatures at promoters and distal sites. Using H3K27ac profiles, we identified 28,377 putative enhancers, many of which are likely to be limb specific based on strong enrichment near genes highly expressed in the limb and comparisons with tissue-specific EP300 sites and known enhancers. We describe a chromatin state signature associated with active developmental enhancers, defined by high levels of H3K27ac marking, nucleosome displacement, hypersensitivity to sonication, and strong depletion of H3K27me3. We also find that some developmental enhancers exhibit components of this signature, including hypersensitivity, H3K27ac enrichment, and H3K27me3 depletion, at lower levels in tissues in which they are not active. Our results establish histone modification profiling as a tool for developmental enhancer discovery, and suggest that enhancers maintain an open chromatin state in multiple embryonic tissues independent of their activity level.

Regulation of connexin expression by transcription factors and epigenetic mechanisms

Gap junctions are specialized cell-cell junctions that directly link the cytoplasm of neighboring cells. They mediate the direct transfer of metabolites and ions from one cell to another. Discoveries of human genetic disorders due to mutations in gap junction protein (connexin [Cx]) genes and experimental data on connexin knockout mice provide direct evidence that gap junctional intercellular communication is essential for tissue functions and organ development, and that its dysfunction causes diseases. Connexin-related signaling also involves extracellular signaling (hemichannels) and non-channel intracellular signaling. Thus far, 21 human genes and 20 mouse genes for connexins have been identified. Each connexin shows tissue- or cell-type-specific expression, and most organs and many cell types express more than one connexin. Connexin expression can be regulated at many of the steps in the pathway from DNA to RNA to protein. In recent years, it has become clear that epigenetic processes are also essentially involved in connexin gene expression. In this review, we summarize recent knowledge on regulation of connexin expression by transcription factors and epigenetic mechanisms including histone modifications, DNA methylation, and microRNA. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

MicroRNAs in heart development

MicroRNAs (miRNAs) are a class of small noncoding RNAs of ~22nt in length which are involved in the regulation of gene expression at the posttranscriptional level by degrading their target mRNAs and/or inhibiting their translation. Expressed ubiquitously or in a tissue-specific manner, miRNAs are involved in the regulation of many biological processes such as cell proliferation, differentiation, apoptosis, and the maintenance of normal cellular physiology. Many miRNAs are expressed in embryonic, postnatal, and adult hearts. Aberrant expression or genetic deletion of miRNAs is associated with abnormal cardiac cell differentiation, disruption of heart development, and cardiac dysfunction. This chapter will summarize the history, biogenesis, and processing of miRNAs as well as their function in heart development, remodeling, and disease.

Tbx20 regulation of cardiac cell proliferation and lineage specialization during embryonic and fetal development in vivo

TBX20 gain-of-function mutations in humans are associated with congenital heart malformations and myocardial defects. However the effects of increased Tbx20 function during cardiac chamber development and maturation have not been reported previously. CAG-CAT-Tbx20 transgenic mice were generated for Cre-dependent induction of Tbx20 in myocardial lineages in the developing heart. βMHCCre-mediated overexpression of Tbx20 in fetal ventricular cardiomyocytes results in increased thickness of compact myocardium, induction of cardiomyocyte proliferation, and increased expression of Bmp10 and pSmad1/5/8 at embryonic day (E) 14.5. βMHCCre-mediated Tbx20 overexpression also leads to increased expression of cardiac conduction system (CCS) genes Tbx5, Cx40, and Cx43 throughout the ventricular myocardium. In contrast, Nkx2.5Cre mediated overexpression of Tbx20 in the embryonic heart results in reduced cardiomyocyte proliferation, increased expression of a cell cycle inhibitor, p21(CIP1), and decreased expression of Tbx2, Tbx5, and N-myc1 at E9.5, concomitant with decreased phospho-ERK1/2 expression. Together, these analyses demonstrate that Tbx20 differentially regulates cell proliferation and cardiac lineage specification in embryonic versus fetal cardiomyocytes. Induction of pSmad1/5/8 at E14.5 and inhibition of dpERK expression at E9.5 are consistent with selective Tbx20 regulation of these pathways in association with stage-specific effects on cardiomyocyte proliferation. Together, these in vivo data support distinct functions for Tbx20 in regulation of cardiomyocyte lineage maturation and cell proliferation at embryonic and fetal stages of heart development.

Myocardial Tbx20 regulates early atrioventricular canal formation and endocardial epithelial-mesenchymal transition via Bmp2

During early embryogenesis, the formation of the cardiac atrioventricular canal (AVC) facilitates the transition of the heart from a linear tube into a chambered organ. However, the genetic pathways underlying this developmental process are poorly understood. The T-box transcription factor Tbx20 is expressed predominantly in the AVC of early heart tube. It was shown that Tbx20 activates Nmyc1 and suppresses Tbx2 expression to promote proliferation and specification of the atrial and ventricular chambers, yet it is not known if Tbx20 is involved in early AVC development. Here, we report that mice lacking Tbx20 in the AVC myocardium fail to form the AVC constriction, and the endocardial epithelial-mesenchymal transition (EMT) is severely perturbed. Tbx20 maintains expression of a variety of genes, including Bmp2, Tbx3 and Hand1 in the AVC myocardium. Intriguingly, we found Bmp2 downstream genes involved in the EMT initiation are also downregulated. In addition, re-expression of Bmp2 in the AVC myocardium substantially rescues the EMT defects resulting from the lack of Tbx20, suggesting Bmp2 is one of the key downstream targets of Tbx20 in AVC development. Our data support a complex signaling network with Tbx20 suppressing Tbx2 in the AVC myocardium but also indirectly promoting Tbx2 expression through Bmp2. The spatiotemporal expression of Tbx2 in the AVC appears to be balanced between these two opposing signals. Overall, our study provides genetic evidence that Tbx20 has essential roles in regulating AVC development that coordinate early cardiac chamber formation.

A decade of advances in the molecular embryology and genetics underlying congenital heart defects

Congenital heart defects (CHD) are the most common type of human birth defect and result in significant mortality worldwide. Despite numerous epidemiologic studies in the past decades, few genetic causes have been identified until recently. CHD result from abnormal morphogenesis of the systematic cardiovascular construction during development. Recent advances in molecular embryology, including the discovery of a new source of cardiac progenitor cells termed the second heart field (SHF), have revealed that the heart arises from multiple distinct embryonic origins. Cells derived from the SHF contribute to the development of the cardiac outflow tract, together with the other progenitor cell lineage called cardiac neural crest cells. Numerous cardiac transcription factors regulate these progenitor cells during heart development. Elucidation of the transcriptional network for these cardiac progenitor cells is essential for further understanding cardiac development and providing new insights into the morphogenesis of CHD. This review outlines the recent discoveries of the molecular embryology of the normal heart and the genetic basis of CHD.

Mechanisms of T-box gene function in the developing heart

The multi-chambered mammalian heart arises from a simple tube by polar elongation, myocardial differentiation and morphogenesis. Members of the large family of T-box (Tbx) transcription factors have been identified as crucial players that act in distinct subprogrammes during cardiac regionalization. Tbx1 and Tbx18 ensure elongation of the cardiac tube at the anterior and posterior pole, respectively. Tbx1 acts in the pharyngeal mesoderm to maintain proliferation of mesenchymal precursor cells for formation of a myocardialized and septated outflow tract. Tbx18 is expressed in the sinus venosus region and is required for myocardialization of the caval veins and the sinoatrial node. Tbx5 and Tbx20 function in the early heart tube and independently activate the chamber myocardial gene programme, whereas Tbx2 and Tbx3 locally repress this programme to favour valvuloseptal and conduction system development. Here, we summarize that these T-box factors act in different molecular circuits and control target gene expression using diverse molecular strategies including binding to distinct protein interaction partners.

Inactivation of Bmp4 from the Tbx1 expression domain causes abnormal pharyngeal arch artery and cardiac outflow tract remodeling

Maldevelopment of outflow tract and aortic arch arteries is among the most common forms of human congenital heart diseases. Both Bmp4 and Tbx1 are known to play critical roles during cardiovascular development. Expression of these two genes partially overlaps in pharyngeal arch areas in mouse embryos. In this study, we applied a conditional gene inactivation approach to test the hypothesis that Bmp4 expressed from the Tbx1 expression domain plays a critical role for normal development of outflow tract and pharyngeal arch arteries. We showed that inactivation of Bmp4 from Tbx1-expressing cells leads to the spectrum of deformities resembling the cardiovascular defects observed in human DiGeorge syndrome patients. Inactivation of Bmp4 from the Tbx1 expression domain did not cause patterning defects, but affected remodeling of outflow tract and pharyngeal arch arteries. Our further examination revealed that Bmp4 is required for normal recruitment/differentiation of smooth muscle cells surrounding the PAA4 and survival of outflow tract cushion mesenchymal cells.

The BMP pathway acts to directly regulate Tbx20 in the developing heart

TBX20 has been shown to be essential for vertebrate heart development. Mutations within the TBX20 coding region are associated with human congenital heart disease, and the loss of Tbx20 in a wide variety of model systems leads to cardiac defects and eventually heart failure. Despite the crucial role of TBX20 in a range of cardiac cellular processes, the signal transduction pathways that act upstream of Tbx20 remain unknown. Here, we have identified and characterized a conserved 334 bp Tbx20 cardiac regulatory element that is directly activated by the BMP/SMAD1 signaling pathway. We demonstrate that this element is both necessary and sufficient to drive cardiac-specific expression of Tbx20 in Xenopus, and that blocking SMAD1 signaling in vivo specifically abolishes transcription of Tbx20, but not that of other cardiac factors, such as Tbx5 and MHC, in the developing heart. We further demonstrate that activation of Tbx20 by SMAD1 is mediated by a set of novel, non-canonical, high-affinity SMAD-binding sites located within this regulatory element and that phospho-SMAD1 directly binds a non-canonical SMAD1 site in vivo. Finally, we show that these non-canonical sites are necessary and sufficient for Tbx20 expression in Xenopus, and that reporter constructs containing these sites are expressed in a cardiac-specific manner in zebrafish and mouse. Collectively, our findings define Tbx20 as a direct transcriptional target of the BMP/SMAD1 signaling pathway during cardiac maturation.

Transcriptional regulation of heart valve progenitor cells

The development and normal function of the heart valves requires complex interactions among signaling molecules, transcription factors and structural proteins that are tightly regulated in time and space. Here we review the roles of critical transcription factors that are required for specific aspects of normal valve development. The early progenitors of the heart valves are localized in endocardial cushions that express transcription factors characteristic of mesenchyme, including Twist1, Tbx20, Msx1 and Msx2. As the valve leaflets mature, they are composed of complex stratified extracellular matrix proteins that are regulated by the transcriptional functions of NFATc1, Sox9, and Scleraxis. Each of these factors has analogous functions in differentiation of related connective tissue lineages. Together, the precise timing and localized functions of specific transcription factors control cell proliferation, differentiation, elongation, and remodeling processes that are necessary for normal valve structure and function. In addition, there is increasing evidence that these same transcription factors contribute to congenital, as well as degenerative, valve disease.

The T-box transcription factor Tbx2: its role in development and possible implication in cancer

Tbx2 is a member of the T-box family of transcription factors that are crucial in embryonic development. Recent studies suggest that T-box factors may also play a role in controlling cell cycle progression and in the genesis of cancer. Tbx2 has been implicated in several developmental processes such as coordinating cell fate, patterning and morphogenesis of a wide range of tissues and organs including limbs, kidneys, lungs, mammary glands, heart, and craniofacial structures. Importantly, Tbx2 is overexpressed in several cancers including melanoma, small cell lung carcinoma, breast, pancreatic, liver, and bladder cancers and can suppress senescence, a cellular process, which serves as a barrier to cancer development. This review presents a state of the art overview of the role and regulation of Tbx2 in early embryonic development and in cancer.

Coronary vessel development and insight towards neovascular therapy

Formation of the coronary arteries consists of a precisely orchestrated series of morphogenetic and molecular events which can be divided into three distinct processes: vasculogenesis, angiogenesis and arteriogenesis (Risau 1997; Carmeliet 2000). Even subtle perturbations in this process may lead to congenital coronary artery anomalies, as occur in 0.2-1.2% of the general population (von Kodolitsch et al. 2004). Contrary to the previously held dogma, the process of vasculogenesis is not limited to prenatal development. Both vasculogenesis and angiogenesis are now known to actively occur within the adult heart. When the need for regeneration arises, for example in the setting of coronary artery disease, a reactivation of embryonic processes ensues, redeploying many of the same molecular regulators. Thus, an understanding of the mechanisms of embryonic coronary vasculogenesis and angiogenesis may prove invaluable in developing novel strategies for cardiovascular regeneration and therapeutic coronary angiogenesis.

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.

The outflow tract of the heart in fishes: anatomy, genes and evolution

A large number of congenital heart defects associated with mortality in humans are those that affect the cardiac outflow tract, and this provides a strong imperative to understand its development during embryogenesis. While there is wide phylogenetic variation in adult vertebrate heart morphology, recent work has demonstrated evolutionary conservation in the early processes of cardiogenesis, including that of the outflow tract. This, along with the utility and high reproductive potential of fish species such as Danio rerio, Oryzias latipes etc., suggests that fishes may provide ideal comparative biological models to facilitate a better understanding of this poorly understood region of the heart. In this review, the authors present the current understanding of both phylogeny and ontogeny of the cardiac outflow tract in fishes and examine how new molecular studies are informing the phylogenetic relationships and evolutionary trajectories that have been proposed. The authors also attempt to address some of the issues of nomenclature that confuse this area of research.

T-box 3 negatively regulates osteoblast differentiation by inhibiting expression of osterix and runx2

T-box (Tbx)3, a known transcriptional repressor, is a member of a family of transcription factors, which contain a highly homologous DNA binding domain known as the Tbx domain. Based on the knowledge that mutation of the Tbx3 gene results in limb malformation, Tbx3 regulates osteoblast proliferation and its expression increases during osteoblast differentiation, we predicted that Tbx3 is an important regulator of osteoblast cell functions. In this study, we evaluated the consequence of transgenic overexpression of Tbx3 on osteoblast differentiation. Retroviral overexpression increased Tbx3 expression >100-fold at the mRNA and protein level. Overexpression of Tbx3 blocked mineralized nodule formation (28 +/- 8 vs. 7 +/- 1%) in MC3T3-E1 cells. In support of these data, alkaline phosphatase (ALP) activity was reduced 33-70% (P < 0.05) in both MC3T3-E1 cells and primary calvaria osteoblasts overexpressing Tbx3. In contrast, Tbx3 overexpression did not alter ALP activity in bone marrow stromal cells. Tbx3 overexpression blocked the increase in expression of key osteoblast marker genes, ALP, bone sialoprotein, and osteocalcin that occurs during normal osteoblast differentiation, but had little or no effect on expression of proliferation genes p53 and Myc. In addition, Tbx3 overexpression abolished increased osterix and runx2 expression observed during normal osteoblast differentiation, but the change in Msx1 and Msx2 expression over time was similar between control and Tbx3 overexpressing cells. Interestingly, osterix and runx2, but not Msx1 and Msx2, contain Tbx binding site in the regulatory region. Based on these data and our previous findings, we conclude that Tbx3 promotes proliferation and suppresses differentiation of osteoblasts and may be involved in regulating expression of key transcription factors involved in osteoblast differentiation.

Hormonal regulation of cardiac KCNE2 gene expression

The KCNE2 gene encodes a single transmembrane domain protein that modulates a variety of K+ channel functions in various tissues. Here we show that cardiac KCNE2 transcript levels are approximately 10-fold upregulated at the end of pregnancy. This upregulation was mimicked by 17-beta estradiol but not by 5alpha-dihydrotestosterone treatments in ovariectomized mice. To investigate the mechanism of KCNE2 transcriptional regulation by estrogen, we experimentally identified KCNE2 transcription start sites, delineated its gene structure and characterized its promoter region. Estrogen treatment stimulated KCNE2 promoter activity in a dose-dependent manner and ICI 182,780 blocked estrogen stimulation. A direct genomic mechanism was demonstrated by (i) the loss of estrogen responsiveness in the presence of a DNA-binding domain mutant estrogen receptor alpha or mutant KCNE2 ERE and (ii) binding of ERalpha to the KCNE2 ERE. These findings show that a genomic mechanism of estrogen action alters KCNE2 expression, which may have important physiological implications.

Characterization of TBX20 in human hearts and its regulation by TFAP2

The T-box family of transcription factors has been shown to have major impact on human development and disease. In animal studies Tbx20 is essential for the development of the atrioventricular channel, the outflow tract and valves, suggesting its potential causative role for the development of Tetralogy of Fallot (TOF) in humans. In the presented study, we analyzed TBX20 in cardiac biopsies derived from patients with TOF, ventricular septal defects (VSDs) and normal hearts. Mutation analysis did not reveal any disease causing sequence variation, however, TBX20 is significantly upregulated in tissue samples of patients with TOF, but not VSD. In depth analysis of TBX20 transcripts lead to the identification of two new exons 3' to the known TBX20 message resembling the mouse variant Tbx20a, as well as an extended 5'UTR. Functional analysis of the human TBX20 promoter revealed a 100 bp region that contains strong activating elements. Within this core promoter region we recognized functional binding sites for TFAP2 transcription factors and identified TFAP2 as repressors of the TBX20 gene in vitro and in vivo. Moreover, decreased TFAP2C levels in cardiac biopsies of TOF patients underline the biological significance of the pathway described. In summary, we provide first insights into the regulation of TBX20 and show its potential for human congenital heart diseases.

T-box transcription factor TBX20 mutations in Chinese patients with congenital heart disease

Despite animal studies having demonstrated that Tbx20 is essential for heart development, few studies have been conducted about TBX20 and congenital heart disease (CHD) in humans. Recently two TBX20 mutations have been associated with human heart defects in two Caucasian families, but TBX20 mutations underlying the more common isolated forms of CHD are still unknown. To explore this question and to analyze the association between TBX20 and susceptibility to CHD 203 Chinese patients with a variety of predominantly sporadic CHD and 300 control subjects were investigated for TBX20 mutations. The exon 2-6 contributing to the T-box DNA-binding domain and their flanking intron sequences were amplified by polymerase chain reaction (PCR) and then were sequenced after purification. Three non-synonymous mutations (A63T, I121F, and T262M) were identified in 3 patients, which were not seen in 300 controls. I121F and T262M mutations occurred within the highly conserved T-box DNA-binding domain. Two synonymous sequence variants (N222N, T262T) and one intervening variant (IVS2-5insCT) were observed in 3 patients but not in the controls. In addition, eight SNPs were observed both in patients and controls and four (S167S, P177P, A181A, and I219I) of them are novel. These data indicate that the frequency of TBX20 missense mutations occurred in Chinese CHD children is low, but they probably contribute to the risk of atrial septal defect (ASD), total anomalous pulmonary venous connection (TAPVC) and tetralogy of Fallot (TOF) in a small subset of Chinese. The findings provide the first insight into TBX20 mutations for TOF and TAPVC. Functional study involved in the new sequence variants should be subject of further investigation.

T-cell factor 3 regulates embryonic stem cell pluripotency and self-renewal by the transcriptional control of multiple lineage pathways

The Wnt signaling pathway is necessary both for maintaining undifferentiated stem cells and for directing their differentiation. In mouse embryonic stem cells (ESCs), Wnt signaling preferentially maintains "stemness" under certain permissive conditions. T-cell factor 3 (Tcf3) is a component of the Wnt signaling and a dominant downstream effector in ESCs. Despite the wealth of knowledge regarding the importance of Wnt signaling underlying stem cells functions, the precise mechanistic explanation by which the effects are mediated is unknown. In this study, we identified new regulatory targets of Tcf3 using a whole-genome approach and found that Tcf3 transcriptionally represses many genes important for maintaining pluripotency and self-renewal, as well as those involved in lineage commitment and stem cell differentiation. This effect is in part mediated by the corepressors transducin-like enhancer of split 2 and C-terminal Binding Protein (CtBP). Notably, Tcf3 binds to and represses the Oct4 promoter, and this repressive effect requires both the Groucho and CtBP interacting domains of Tcf3. Interestingly, we find that in mouse preimplantation development embryos, Tcf3 expression is coregulated with Oct4 and Nanog and becomes localized to the inner cell mass of the blastocyst. These data demonstrate an important role for Tcf3 in modulating the appropriate level of gene transcription in ESCs and during embryonic development. Disclosure of potential conflicts of interest is found at the end of this article.

Ndrg4 is required for normal myocyte proliferation during early cardiac development in zebrafish

NDRG4 is a novel member of the NDRG family (N-myc downstream-regulated gene). The roles of NDRG4 in development have not previously been evaluated. We show that, during zebrafish embryonic development, ndrg4 is expressed exclusively in the embryonic heart, the central nervous system (CNS) and the sensory system. Ndrg4 knockdown in zebrafish embryos causes a marked reduction in proliferative myocytes and results in hypoplastic hearts. This growth defect is associated with cardiac phenotypes in morphogenesis and function, including abnormal heart looping, inefficient circulation and weak contractility. We reveal that ndrg4 is required for restricting the expression of versican and bmp4 to the developing atrioventricular canal. This constellation of ndrg4 cardiac defects phenocopies those seen in mutant hearts of heartstrings (hst), the tbx5 loss-of-function mutants in zebrafish. We further show that ndrg4 expression is significantly decreased in hearts with reduced tbx5 activities. Conversely, increased expression of tbx5 that is due to tbx20 knockdown leads to an increase in ndrg4 expression. Together, our studies reveal an essential role of ndrg4 in regulating proliferation and growth of cardiomyocytes, suggesting that ndrg4 may function downstream of tbx5 during heart development and growth.

An evolutionarily conserved nuclear export signal facilitates cytoplasmic localization of the Tbx5 transcription factor

During cardiac development, the T-box transcription factor Tbx5 displays dynamic changes in localization from strictly nuclear to both nuclear and cytoplasmic to exclusively cytoplasmic along the actin cytoskeleton in cells coexpressing its binding protein LMP4. Although nuclear localization signals (NLSs) have been described, the mechanism by which Tbx5 exits the nucleus remained elusive. Here, we describe for Tbx5 a nuclear export signal (NES) that is recognized by the CRM1 export protein. Site-directed mutagenesis of a critical amino acid(s) within this sequence determined the functionality of this NES. Confocal localization studies and luciferase transcriptional reporter assays with NES mutant Tbx5 forms demonstrated retention in the nucleus, regardless of the presence of LMP4. Coimmunoprecipitation and pharmacological interference studies demonstrated a direct interaction between Tbx5 and CRM1, revealing that Tbx5 is using the CRM1 pathway for nuclear export. In addition to Tbx5, we identified NESs in all T-box proteins and demonstrated interaction of the family members Tbx3 and Brachyury with the CRM1 exporter, suggesting general significance. This first demonstration of evolutionarily conserved NESs in all T-box proteins in conjunction with NLSs indicates a primordial function of T-box proteins to dynamically shuttle between nuclear and cytoplasmic compartments of the cell.

A role for Tbx2 in the regulation of the alpha2(1) collagen gene in human fibroblasts

The T-box gene family encodes highly conserved transcription factors that play important roles in embryonic development and have been implicated in carcinogenesis. One member of the family, Tbx2, is generally regarded as a transcriptional repressor but appears to be capable of functioning as an activator depending on the cellular context. This study shows that Tbx2 is expressed in normal human fibroblasts but is drastically reduced in several transformed fibroblast cell lines. This pattern of Tbx2 expression correlated with that observed for the human alpha2(1) collagen gene (COL1A2). Interestingly, stable expression of transfected Tbx2 in transformed fibroblast cell lines further reduces expression of the human endogenous COL1A2 gene. This ability of Tbx2 to repress the human COL1A2 gene was confirmed in luciferase reporter assays and shown to be independent of the consensus T-box binding element.

T-box factors determine cardiac design

The heart of higher vertebrates is a structurally complicated multi-chambered pump that contracts synchronously. For its proper function a number of distinct integrated components have to be generated, including force-generating compartments, unidirectional valves, septa and a system in charge of the initiation and coordinated propagation of the depolarizing impulse over the heart. Not surprisingly, a large number of regulating factors are involved in these processes that act in complex and intertwined pathways to regulate the activity of target genes responsible for morphogenesis and function. The finding that mutations in T-box transcription factor-encoding genes in humans lead to congenital heart defects has focused attention on the importance of this family of regulators in heart development. Functional and genetic analyses in a variety of divergent species has demonstrated the critical roles of multiple T-box factor gene family members, including Tbx11, −2, −3, −5, −18 and −20, in the patterning, recruitment, specification, differentiation and growth processes underlying formation and integration of the heart components. Insight into the roles of T-box factors in these processes will enhance our understanding of heart formation and the underlying molecular regulatory pathways.

Induction of proepicardial marker gene expression by the liver bud

Cells of the coronary vessels arise from a unique extracardiac mesothelial cell population, the proepicardium, which develops posterior to the sinoatrial region of the looping-stage heart. Although contribution of the proepicardial cells to cardiac development has been studied extensively, it remains unresolved how the proepicardium is induced and specified in the mesoderm during embryogenesis. It is known, however, that the proepicardium develops from the mesothelium that overlays the liver bud. Here, we show that the expression of proepicardial marker genes - Wt1, capsulin (epicardin, pod1, Tcf21) and Tbx18, can be induced in naïve mesothelial cells by the liver bud, both in vitro and in vivo. Lateral embryonic explants, when co-cultured with the liver bud, were induced to express these proepicardial marker genes. The same induction of the marker genes was detected in vivo when a quail liver bud was implanted in the posterior-lateral regions of a chick embryo. This ectopic induction of marker gene expression was not evident when other endodermal tissues, such as the lung bud or stomach, were implanted. This inductive response to the liver bud was not detectable in host embryos before stage 12 (16-somite stage). These results suggest that, after a specific developmental stage, a large area of the mesothelium becomes competent to express proepicardial marker genes in response to localized liver-derived signal(s). The developmentally regulated competency of mesothelium and a localized inductive signal might play a role in restricting the induction of the proepicardial marker gene expression to a specific region of the mesothelium. The data might also provide a foundation for future engineering of a coronary vascular progenitor population.

Bmp2 is essential for cardiac cushion epithelial-mesenchymal transition and myocardial patterning

Cardiac cushion development provides a valuable system to investigate epithelial to mesenchymal transition (EMT), a fundamental process in development and tumor progression. In the atrioventricular (AV) canal, endocardial cells lining the heart respond to a myocardial-derived signal, undergo EMT, and contribute to cushion mesenchyme. Here, we inactivated bone morphogenetic protein 2 (Bmp2) in the AV myocardium of mice. We show that Bmp2 has three functions in the AV canal: to enhance formation of the cardiac jelly, to induce endocardial EMT and to pattern the AV myocardium. Bmp2 is required for myocardial expression of Has2, a crucial component of the cardiac jelly matrix. During EMT, Bmp2 promotes expression of the basic helix-loop-helix factor Twist1, previously implicated in EMT in cancer metastases, and the homeobox genes Msx1 and Msx2. Deletion of the Bmp type 1A receptor, Bmpr1a, in endocardium also resulted in failed cushion formation, indicating that Bmp2 signals directly to cushion-forming endocardium to induce EMT. Lastly, we show that Bmp2 mutants failed to specify the AV myocardium with loss of Tbx2 expression uncovering a myocardial, planar signaling function for Bmp2. Our data indicate that Bmp2 has a crucial role in coordinating multiple aspects of AV canal morphogenesis.

Myocardium defects and ventricular hypoplasia in mice homozygous null for the Forkhead Box M1 transcription factor

The Forkhead Box m1 (Foxm1) transcription factor is expressed in cardiomyocytes and cardiac endothelial cells during heart development. In this study, we used a novel Foxm1 -/- mouse line to demonstrate that Foxm1-deletion causes ventricular hypoplasia and diminished DNA replication and mitosis in developing cardiomyocytes. Proliferation defects in Foxm1 -/- hearts were associated with a reduced expression of Cdk1-activator Cdc25B phosphatase and NFATc3 transcription factor, and with abnormal nuclear accumulation of the Cdk-inhibitor p21(Cip1) protein. Depletion of Foxm1 levels by siRNA caused altered expression of these genes in cultured HL-1 cardiomyocytes. Endothelial-specific deletion of the Foxm1 fl/fl allele in Tie2-Cre Foxm1 fl/fl embryos did not influence heart development and cardiomyocyte proliferation. Foxm1 protein binds to the -9,259/-9,288-bp region of the endogenous mouse NFATc3 promoter, indicating that Foxm1 is a transcriptional activator of the NFATc3 gene. Foxm1 regulates expression of genes essential for the proliferation of cardiomyocytes during heart development.

Erasure of Cellular Memory by Fusion with Pluripotent Cells

Pluripotent cells have been suggested as a prime source to reprogram somatic cells. We used F9 EC cells as a pluripotent partner to reprogram neurosphere cells (NSCs) because they exhibit a nonneural differentiation potential in the presence of retinoic acid. F9-NSC hybrid cells displayed various features of reprogramming, such as reactivation of pluripotency genes, inactivation of tissue-specific genes, and reactivation of the inactive X chromosome. As the hybrid cells undergo differentiation, the pluripotency markers Oct4 and Nanog were downregulated. Whereas neural marker genes were not upregulated, endodermal and mesodermal markers were, suggesting that NSCs lose memory of their neural origin and preferentially differentiate to the lineages corresponding to the F9 program. After fusion, the methylation status in the Xist region was similar to that of F9 EC cells. However, upon differentiation, the Xist region failed to resume the methylation patterns of differentiated cells, suggesting that the Xist in F9-NSC hybrids does not easily acquire a differentiated state.

Human TBX1 missense mutations cause gain of function resulting in the same phenotype as 22q11.2 deletions

Deletion 22q11.2 syndrome is the most frequent known microdeletion syndrome and is associated with a highly variable phenotype, including DiGeorge and Shprintzen (velocardiofacial) syndromes. Although haploinsufficiency of the T-box transcription factor gene TBX1 is thought to cause the phenotype, to date, only four different point mutations in TBX1 have been reported in association with six of the major features of 22q11.2 deletion syndrome. Although, for the two truncating mutations, loss of function was previously shown, the pathomechanism of the missense mutations remains unknown. We report a novel heterozygous missense mutation, H194Q, in a familial case of Shprintzen syndrome and show that this and the two previously reported missense mutations result in gain of function, possibly through stabilization of the protein dimer DNA complex. We therefore conclude that TBX1 gain-of-function mutations can result in the same phenotypic spectrum as haploinsufficiency caused by loss-of-function mutations or deletions.

Aortic root disease in tetralogy of Fallot

PURPOSE OF REVIEW

Progressive aortic root dilatation is a recognized feature of tetralogy of Fallot even in patients following initial reparative surgery. The underlying pathophysiology was initially attributed to altered hemodynamics resulting from longstanding volume overloading and stretching of the aortic root from increased right to left shunting. This review explores the pathophysiology and possible mechanisms for the aortic dilatation, and whether these changes are a reflection of the initial hemodynamic stress or a cellular expression of an unrecognized gene associated with conotruncal defects.

RECENT FINDINGS

The recent publication of two case reports of aortic aneurysm and dissection in tetralogy of Fallot patients re-emphasized the fact that aortic root dilatation can no longer be regarded as a benign problem in tetralogy of Fallot patients. Findings of intrinsic histological abnormalities in the aortic root and ascending aorta of tetralogy of Fallot patients suggest that intrinsic abnormalities may also play an important causative role.

SUMMARY

A better understanding of the pathophysiology will help to formulate future treatment and management strategies in the subgroup of tetralogy of Fallot patients with progressive aortic dilatation.

Microarray analysis of Tbx5-induced genes expressed in the developing heart

Tbx5 is a member of the T-box family of transcription factors and is associated with Holt-Oram syndrome (HOS), a congenital disorder characterized by heart and limb defects. Although implicated in several processes during development, only a few genes regulated by Tbx5 have been reported. To identify candidate genes regulated by Tbx5 during heart development, a microarray approach was used. A cardiac-derived mouse cell line (1H) was infected with adenoviruses expressing Tbx5 or beta-galactosidase and RNA was isolated for analysis using an Affymetrix gene chip representing over 39,000 transcripts. Real-time reverse transcriptase-polymerase chain reaction confirmed Tbx5 induction of a subset of the genes, including nppa, photoreceptor cadherin, brain creatine kinase, hairy/enhancer-of-split related 2, and gelsolin. In situ hybridization analysis indicated overlapping expression of these genes with tbx5 in the embryonic mouse heart. In addition, the effect of HOS-associated mutations on the ability of Tbx5 to induce target gene expression was evaluated. Together, these data identify several genes induced by Tbx5 that are potentially important during cardiac development. These genes represent new candidate gene targets of Tbx5 that may be related to congenital heart malformations associated with HOS.

When half is not enough: gene expression and dosage in the 22q11 deletion syndrome

The 22q11 Deletion Syndrome (22q11DS, also known as DiGeorge or Velo-Cardio-Facial Syndrome) has a variable constellation of phenotypes including life-threatening cardiac malformations, craniofacial, limb, and digit anomalies, a high incidence of learning, language, and behavioral disorders, and increased vulnerability for psychiatric diseases, including schizophrenia. There is still little clear understanding of how heterozygous microdeletion of approximately 30-50 genes on chromosome 22 leads to this diverse spectrum of phenotypes, especially in the brain. Three possibilities exist: 1) 22q11DS may reflect haploinsufficiency, homozygous loss of function, or heterozygous gain of function of a single gene within the deleted region; 2) 22q11DS may result from haploinsufficiency, homozygous loss of function, or heterozygous gain of function of a few genes in the deleted region acting at distinct phenotypically compromised sites; 3) 22q11DS may reflect combinatorial effects of reduced dosage of multiple genes acting in concert at all phenotypically compromised sites. Here, we consider evidence for each of these possibilities. Our review of the literature, as well as interpretation of work from our laboratory, favors the third possibility: 22q11DS reflects diminished expression of multiple 22q11 genes acting on common cellular processes during brain as well as heart, face, and limb development, and subsequently in the adolescent and adult brain.