Tbx2 is essential for patterning the atrioventricular canal and for morphogenesis of the outflow tract during heart development

The Wilms' tumor suppressor Wt1 regulates Coronin 1B expression in the epicardium

Coronin 1B has been shown to be critical for cell motility and various actin-dependent processes. To understand its role more extensively, the expression and transcriptional regulation of Coro1b gene during mouse development were explored. Coronin 1B is ubiquitously expressed in the whole embryo but nevertheless shows distinct expression pattern in developing heart. In addition to the localization in endocardium, Coronin 1B is specifically expressed in the endocardial cushion and epicardium where cardiac EMT processes take place as the heart develops. Promoter deletion analysis identified the positions between -1038 and -681 is important for Coro1b basal promoter activity. In addition to a correlation of Coronin 1B localization with Wt1 expression in the epicardium, we also identified putative Wt1 binding sequences within Coro1b promoter. Direct binding of Wt1 to GC-rich sequences within the Coro1b promoter is required for the regulation of Coro1b gene expression. In accordance with the motility defect found in Coronin 1B-knockdown cells, a modest decrease in expression of Coronin 1B in the remaining epicardium of Wt1(EGFPCre/EGFPCre) mutant embryos was observed. These findings seem to shed light on the role of Wt1 during cell migration and suggest that, at least in part, this involves transcriptional control of Coro1b gene expression.

DiGeorge syndrome gene tbx1 functions through wnt11r to regulate heart looping and differentiation

DiGeorge syndrome (DGS) is the most common microdeletion syndrome, and is characterized by congenital cardiac, craniofacial and immune system abnormalities. The cardiac defects in DGS patients include conotruncal and ventricular septal defects. Although the etiology of DGS is critically regulated by TBX1 gene, the molecular pathways underpinning TBX1's role in heart development are not fully understood. In this study, we characterized heart defects and downstream signaling in the zebrafish tbx1^−/− mutant, which has craniofacial and immune defects similar to DGS patients. We show that tbx1^−/− mutants have defective heart looping, morphology and function. Defective heart looping is accompanied by failure of cardiomyocytes to differentiate normally and failure to change shape from isotropic to anisotropic morphology in the outer curvatures of the heart. This is the first demonstration of tbx1's role in regulating heart looping, cardiomyocyte shape and differentiation, and may explain how Tbx1 regulates conotruncal development in humans. Next we elucidated tbx1's molecular signaling pathway guided by the cardiac phenotype of tbx1^−/− mutants. We show for the first time that wnt11r (wnt11 related), a member of the non-canonical Wnt pathway, and its downstream effector gene alcama (activated leukocyte cell adhesion molecule a) regulate heart looping and differentiation similarly to tbx1. Expression of both wnt11r and alcama are downregulated in tbx1^−/− mutants. In addition, both wnt11r^−/− mutants and alcama morphants have heart looping and differentiation defects similar to tbx1^−/− mutants. Strikingly, heart looping and differentiation in tbx1^−/− mutants can be partially rescued by ectopic expression of wnt11r or alcama, supporting a model whereby heart looping and differentiation are regulated by tbx1 in a linear pathway through wnt11r and alcama. This is the first study linking tbx1 and non-canonical Wnt signaling and extends our understanding of DGS and heart development.

Signaling and transcriptional networks in heart development and regeneration

The mammalian heart is the first functional organ, the first indicator of life. Its normal formation and function are essential for fetal life. Defects in heart formation lead to congenital heart defects, underscoring the finesse with which the heart is assembled. Understanding the regulatory networks controlling heart development have led to significant insights into its lineage origins and morphogenesis and illuminated important aspects of mammalian embryology, while providing insights into human congenital heart disease. The mammalian heart has very little regenerative potential, and thus, any damage to the heart is life threatening and permanent. Knowledge of the developing heart is important for effective strategies of cardiac regeneration, providing new hope for future treatments for heart disease. Although we still have an incomplete picture of the mechanisms controlling development of the mammalian heart, our current knowledge has important implications for embryology and better understanding of human heart disease.

Tbx2 controls lung growth by direct repression of the cell cycle inhibitor genes Cdkn1a and Cdkn1b

Vertebrate organ development relies on the precise spatiotemporal orchestration of proliferation rates and differentiation patterns in adjacent tissue compartments. The underlying integration of patterning and cell cycle control during organogenesis is insufficiently understood. Here, we have investigated the function of the patterning T-box transcription factor gene Tbx2 in lung development. We show that lungs of Tbx2-deficient mice are markedly hypoplastic and exhibit reduced branching morphogenesis. Mesenchymal proliferation was severely decreased, while mesenchymal differentiation into fibrocytes was prematurely induced. In the epithelial compartment, proliferation was reduced and differentiation of alveolar epithelial cells type 1 was compromised. Prior to the observed cellular changes, canonical Wnt signaling was downregulated, and Cdkn1a (p21) and Cdkn1b (p27) (two members of the Cip/Kip family of cell cycle inhibitors) were strongly induced in the Tbx2-deficient lung mesenchyme. Deletion of both Cdkn1a and Cdkn1b rescued, to a large degree, the growth deficits of Tbx2-deficient lungs. Prolongation of Tbx2 expression into adulthood led to hyperproliferation and maintenance of mesenchymal progenitor cells, with branching morphogenesis remaining unaffected. Expression of Cdkn1a and Cdkn1b was ablated from the lung mesenchyme in this gain-of-function setting. We further show by ChIP experiments that Tbx2 directly binds to Cdkn1a and Cdkn1b loci in vivo, defining these two genes as direct targets of Tbx2 repressive activity in the lung mesenchyme. We conclude that Tbx2-mediated regulation of Cdkn1a and Cdkn1b represents a crucial node in the network integrating patterning information and cell cycle regulation that underlies growth, differentiation, and branching morphogenesis of this organ.

During organ formation, proliferation rates and differentiation patterns vary widely between different stages and tissue compartments. It is poorly understood how cell cycle progression is locally controlled and integrated with patterning processes in these developmental programs. Here, we used the mouse lung as a model to study how growth and differentiation are controlled on a transcriptional level. Combining genetic loss- and gain-of-function approaches, we show that the T-box transcription factor gene Tbx2 is required and sufficient to direct appropriate lung growth by maintaining proliferation and inhibiting differentiation in the mesenchymal compartment of the lung. We found that expression of the cell cycle inhibitor genes Cdkn1a (p21) and Cdkn1b (p27) inversely correlates with expression of Tbx2 and that deletion of both genes rescues, to a large degree, the growth deficits of Tbx2-mutant lungs. We further show by biochemical assays that Tbx2 directly binds to Cdkn1a and Cdkn1b loci in vivo, defining these two genes as direct targets of Tbx2 repressive activity in the lung mesenchyme. We conclude that Tbx2-mediated regulation of Cdkn1a and Cdkn1b represents a crucial module for the tissue-specific control of cell cycle progression that underlies growth, differentiation, and branching morphogenesis of this organ.

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.

Partitioning the heart: mechanisms of cardiac septation and valve development

Heart malformations are common congenital defects in humans. Many congenital heart defects involve anomalies in cardiac septation or valve development, and understanding the developmental mechanisms that underlie the formation of cardiac septal and valvular tissues thus has important implications for the diagnosis, prevention and treatment of congenital heart disease. The development of heart septa and valves involves multiple types of progenitor cells that arise either within or outside the heart. Here, we review the morphogenetic events and genetic networks that regulate spatiotemporal interactions between the cells that give rise to septal and valvular tissues and hence partition the heart.

Embryological origin of the endocardium and derived valve progenitor cells: from developmental biology to stem cell-based valve repair

The cardiac valves are targets of both congenital and acquired diseases. The formation of valves during embryogenesis (i.e., valvulogenesis) originates from endocardial cells lining the myocardium. These cells undergo an endothelial-mesenchymal transition, proliferate and migrate within an extracellular matrix. This leads to the formation of bilateral cardiac cushions in both the atrioventricular canal and the outflow tract. The embryonic origin of both the endocardium and prospective valve cells is still elusive. Endocardial and myocardial lineages are segregated early during embryogenesis and such a cell fate decision can be recapitulated in vitro by embryonic stem cells (ESC). Besides genetically modified mice and ex vivo heart explants, ESCs provide a cellular model to study the early steps of valve development and might constitute a human therapeutic cell source for decellularized tissue-engineered valves. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

Evolution and development of the building plan of the vertebrate heart

Early cardiac development involves the formation of a heart tube, looping of the tube and formation of chambers. These processes are highly similar among all vertebrates, which suggest the existence of evolutionary conservation of the building plan of the heart. From the jawless lampreys to man, T-box transcription factors like Tbx5 and Tbx20 are fundamental for heart formation, whereas Tbx2 and Tbx3 repress chamber formation on the sinu-atrial and atrioventricular borders. Also, electrocardiograms from different vertebrates are alike, even though the fish heart only has two chambers whereas the mammalian heart has four chambers divided by septa and in addition has much higher heart rates. We conclude that most features of the high-performance hearts of mammals and birds can be traced back to less developed traits in the hearts of ectothermic vertebrates. This article is part of a Special Issue entitled: Cardiomyocyte biology: Cardiac pathways of differentiation, metabolism and contraction.

The T box transcription factor TBX2 promotes epithelial-mesenchymal transition and invasion of normal and malignant breast epithelial cells

The T box transcription factor TBX2, a master regulator of organogenesis, is aberrantly amplified in aggressive human epithelial cancers. While it has been shown that overexpression of TBX2 can bypass senescence, a failsafe mechanism against cancer, its potential role in tumor invasion has remained obscure. Here we demonstrate that TBX2 is a strong cell-autonomous inducer of the epithelial-mesenchymal transition (EMT), a latent morphogenetic program that is key to tumor progression from noninvasive to invasive malignant states. Ectopic expression of TBX2 in normal HC11 and MCF10A mammary epithelial cells was sufficient to induce morphological, molecular, and behavioral changes characteristic of EMT. These changes included loss of epithelial adhesion and polarity gene (E-cadherin, ß-catenin, ZO1) expression, and abnormal gain of mesenchymal markers (N-cadherin, Vimentin), as well as increased cell motility and invasion. Conversely, abrogation of endogenous TBX2 overexpression in the malignant human breast carcinoma cell lines MDA-MB-435 and MDA-MB-157 led to a restitution of epithelial characteristics with reciprocal loss of mesenchymal markers. Importantly, TBX2 inhibition abolished tumor cell invasion and the capacity to form lung metastases in a Xenograft mouse model. Meta-analysis of gene expression in over one thousand primary human breast tumors further showed that high TBX2 expression was significantly associated with reduced metastasis-free survival in patients, and with tumor subtypes enriched in EMT gene signatures, consistent with a role of TBX2 in oncogenic EMT. ChIP analysis and cell-based reporter assays further revealed that TBX2 directly represses transcription of E-cadherin, a tumor suppressor gene, whose loss is crucial for malignant tumor progression. Collectively, our results uncover an unanticipated link between TBX2 deregulation in cancer and the acquisition of EMT and invasive features of epithelial tumor cells.

Periostin as a biomarker of the amniotic membrane

Tracing the precise developmental origin of amnion and amnion-derived stem cells is still challenging and depends chiefly on analyzing powerful genetic model amniotes like mouse. Profound understanding of the fundamental differences in amnion development in both the disc-shaped primate and human embryo and the cup-shaped mouse embryo is pivotal in particular when sampling amniotic membrane from nonprimate species for isolating candidate amniotic stem cells. The availability of molecular marker genes that are specifically expressed in the amniotic membrane and not in other extraembryonic membranes would be instrumental to validate unequivocally the starting material under investigation. So far such amniotic markers have not been reported. We postulated that bone morphogenetic protein (BMP) target genes are putative amniotic membrane markers mainly because deficiency in one of several components of the BMP signaling cascade in mice has been documented to result in defective development of the early amnion. Comparative gene expression analysis of acknowledged target genes for BMP in different extraembryonic tissues, combined with in situ hybridization, identified Periostin (Postn) mRNA enrichment in amnion throughout gestation. In addition, we identify and propose a combination of markers as transcriptional signature for the different extraembryonic tissues in mouse.

Coordinating tissue interactions: Notch signaling in cardiac development and disease

The Notch pathway is a crucial cell-fate regulator in the developing heart. Attention in the past centered on Notch function in cardiomyocytes. However, recent advances demonstrate that region-specific endocardial Notch activity orchestrates the patterning and morphogenesis of cardiac chambers and valves through regulatory interaction with multiple myocardial and neural crest signals. Notch also regulates cardiomyocyte proliferation and differentiation during ventricular chamber development and is required for coronary vessel specification. Here, we review these data and highlight disease connections, including evidence that Notch-Hey-Bmp2 interplay impacts adult heart valve disease and that Notch contributes to cardiac arrhythmia and pre-excitation syndromes.

Ectopic retinoic acid signaling affects outflow tract cushion development through suppression of the myocardial Tbx2-Tgfβ2 pathway

The progress of molecular genetics has enabled us to identify the genes responsible for congenital heart malformations. However, recent studies suggest that congenital heart diseases are induced not only by mutations in certain genes, but also by abnormal maternal factors. A high concentration of maternal retinoic acid (RA), the active derivative of vitamin A, is well known as a teratogenic agent that can cause developmental defects. Our previous studies have shown that the maternal administration of RA to mice within a narrow developmental window induces outflow tract (OFT) septum defects, a condition that closely resembles human transposition of the great arteries (TGA), although the responsible factors and pathogenic mechanisms of the TGA induced by RA remain unknown. We herein demonstrate that the expression of Tbx2 in the OFT myocardium is responsive to RA, and its downregulation is associated with abnormal OFT development. We found that RA could directly downregulate the Tbx2 expression through a functional retinoic acid response element (RARE) in the Tbx2 promoter region, which is also required for the initiation of Tbx2 transcription during OFT development. Tgfb2 expression was also downregulated in the RA-treated OFT region and was upregulated by Tbx2 in a culture system. Moreover, defective epithelial-mesenchymal transition caused by the excess RA was rescued by the addition of Tgfβ2 in an organ culture system. These data suggest that RA signaling participates in the Tbx2 transcriptional mechanism during OFT development and that the Tbx2-Tgfβ2 cascade is one of the key pathways involved in inducing the TGA phenotype.

Conditional ablation of Ezh2 in murine hearts reveals its essential roles in endocardial cushion formation, cardiomyocyte proliferation and survival

Ezh2 is a histone trimethyltransferase that silences genes mainly via catalyzing trimethylation of histone 3 lysine 27 (H3K27Me3). The role of Ezh2 as a regulator of gene silencing and cell proliferation in cancer development has been extensively investigated; however, its function in heart development during embryonic cardiogenesis has not been well studied. In the present study, we used a genetically modified mouse system in which Ezh2 was specifically ablated in the mouse heart. We identified a wide spectrum of cardiovascular malformations in the Ezh2 mutant mice, which collectively led to perinatal death. In the Ezh2 mutant heart, the endocardial cushions (ECs) were hypoplastic and the endothelial-to-mesenchymal transition (EMT) process was impaired. The hearts of Ezh2 mutant mice also exhibited decreased cardiomyocyte proliferation and increased apoptosis. We further identified that the Hey2 gene, which is important for cardiomyocyte proliferation and cardiac morphogenesis, is a downstream target of Ezh2. The regulation of Hey2 expression by Ezh2 may be independent of Notch signaling activity. Our work defines an indispensible role of the chromatin remodeling factor Ezh2 in normal cardiovascular development.

Bmp signaling exerts opposite effects on cardiac differentiation

RATIONALE

The importance for Bmp signaling during embryonic stem cell differentiation into myocardial cells has been recognized. The question when and where Bmp signaling in vivo regulates myocardial differentiation has remained largely unanswered.

OBJECTIVE

To identify when and where Bmp signaling regulates cardiogenic differentiation.

METHODS AND RESULTS

Here we have observed that in zebrafish embryos, Bmp signaling is active in cardiac progenitor cells prior to their differentiation into cardiomyocytes. Bmp signaling is continuously required during somitogenesis within the anterior lateral plate mesoderm to induce myocardial differentiation. Surprisingly, Bmp signaling is actively repressed in differentiating myocardial cells. We identified the inhibitory Smad6a, which is expressed in the cardiac tissue, to be required to inhibit Bmp signaling and thereby promote expansion of the ventricular myocardium.

CONCLUSION

Bmp signaling exerts opposing effects on myocardial differentiation in the embryo by promoting as well as inhibiting cardiac growth.

Localized and temporal gene regulation in heart development

The heart is a structurally complex and functionally heterogeneous organ. The repertoire of genes active in a given cardiac cell defines its shapes and function. This process of localized or heterogeneous gene expression is regulated to a large extent at the level of transcription, dictating the degree particular genes in a cell are active. Therefore, errors in the regulation of localized gene expression are at the basis of misregulation of the delicate process of heart development and function. In this review, we provide an overview of the origin of the different components of the vertebrate heart, and discuss our current understanding of the regulation of localized gene expression in the developing heart. We will also discuss where future research may lead to gain more insight into this process, which should provide much needed insight into the dysregulation of heart development and function, and the etiology of congenital defects.

Heart valve development, maintenance, and disease: the role of endothelial cells

Heart valves are dynamic structures that open and close during the cardiac cycle to maintain unidirectional blood flow throughout life. Insufficient valve function, commonly due to congenital malformations leads to disruptions in hemodynamics and eventual heart failure. Mature valve leaflets are composed of a heterogeneous population of interstitial cells and stratified extracellular matrix, surrounded by a layer of endothelial cells. This defined connective tissue "architecture" provides the valve with all the necessary biomechanical properties required to efficiently function while withstanding constant cyclic shear stress. Valvular endothelial cells (VECs) play essential roles in establishing the valve structures during embryonic development and are important for maintaining lifelong valve integrity and function. In contrast to a continuous endothelium over the surface of healthy valve leaflets, VEC disruption is commonly observed in malfunctioning valves and is associated with pathological processes that promote valve sclerosis and calcification. Increasing our understanding of the roles of VECs in development and disease has lead to promising advances in the development of endothelial cell-based therapies for treating valve disease.

Dynamic expression of Tbx2 subfamily genes in development of the mouse reproductive system

BACKGROUND

Tbx2, Tbx3, Tbx4, and Tbx5, members of the Tbx2 subfamily of T-box transcription factor genes, are important for many aspects of embryonic development and mutations in some human TBX2 subfamily genes cause developmental syndromes. In addition, TBX2 and TBX3 are overexpressed in a variety of cancers, including reproductive system cancers. This study characterizes the expression of Tbx2 subfamily genes during development of the reproductive system.

RESULTS

We show that these genes are expressed in both the internal and external reproductive systems. Tbx2 is expressed in gonads and genital ducts, the Wolffian and Müllerian ducts, while Tbx3 is only expressed in genital ducts. Tbx4 is expressed in embryonic and postnatal germ cells. All four genes are expressed in mesenchyme in external genitalia, with Tbx3 and Tbx5 expression in the epithelium as well.

CONCLUSION

This study lays the foundation for investigation of functional requirements for Tbx2 subfamily genes in development of the mammalian reproductive system.

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.

Tbx2 and Tbx3 induce atrioventricular myocardial development and endocardial cushion formation

A key step in heart development is the coordinated development of the atrioventricular canal (AVC), the constriction between the atria and ventricles that electrically and physically separates the chambers, and the development of the atrioventricular valves that ensure unidirectional blood flow. Using knock-out and inducible overexpression mouse models, we provide evidence that the developmentally important T-box factors Tbx2 and Tbx3, in a functionally redundant manner, maintain the AVC myocardium phenotype during the process of chamber differentiation. Expression profiling and ChIP-sequencing analysis of Tbx3 revealed that it directly interacts with and represses chamber myocardial genes, and induces the atrioventricular pacemaker-like phenotype by activating relevant genes. Moreover, mutant mice lacking 3 or 4 functional alleles of Tbx2 and Tbx3 failed to form atrioventricular cushions, precursors of the valves and septa. Tbx2 and Tbx3 trigger development of the cushions through a regulatory feed-forward loop with Bmp2, thus providing a mechanism for the co-localization and coordination of these important processes in heart development.

Identification of a Tbx1/Tbx2/Tbx3 genetic pathway governing pharyngeal and arterial pole morphogenesis

The 22q11.2 deletion syndrome (22q11.2DS) is the most common microdeletion disorder and is characterized by abnormal development of the pharyngeal apparatus and heart. Cardiovascular malformations (CVMs) affecting the outflow tract (OFT) are frequently observed in 22q11.2DS and are among the most commonly occurring heart defects. The gene encoding T-box transcription factor 1 (Tbx1) has been identified as a major candidate for 22q11.2DS. However, CVMs are generally considered to have a multigenic basis and single-gene mutations underlying these malformations are rare. The T-box family members Tbx2 and Tbx3 are individually required in regulating aspects of OFT and pharyngeal development. Here, using expression and three-dimensional reconstruction analysis, we show that Tbx1 and Tbx2/Tbx3 are largely uniquely expressed but overlap in the caudal pharyngeal mesoderm during OFT development, suggesting potential combinatorial requirements. Cross-regulation between Tbx1 and Tbx2/Tbx3 was analyzed using mouse genetics and revealed that Tbx1 deficiency affects Tbx2 and Tbx3 expression in neural crest-derived cells and pharyngeal mesoderm, whereas Tbx2 and Tbx3 function redundantly upstream of Tbx1 and Hh ligand expression in pharyngeal endoderm and bone morphogenetic protein- and fibroblast growth factor-signaling in cardiac progenitors. Moreover, in vivo, we show that loss of two of the three genes results in severe pharyngeal hypoplasia and heart tube extension defects. These findings reveal an indispensable T-box gene network governing pharyngeal and OFT development and identify TBX2 and TBX3 as potential modifier genes of the cardiopharyngeal phenotypes found in TBX1-haploinsufficient 22q11.2DS patients.

The Highly Homologous T-Box Transcription Factors, TBX2 and TBX3, Have Distinct Roles in the Oncogenic Process

The T-box transcription factors TBX2 and TBX3 are overexpressed in several cancers and are able to bypass senescence by repressing ARF and p21(WAF1/CIP1/SDII). Although these studies suggest that they may both contribute to the oncogenic process by repressing common targets, whether they have redundant or distinct roles in cancers where they are both overexpressed remains to be elucidated. Importantly, when Tbx2 function is inhibited in melanoma cells lacking Tbx3, the cells senesce, but whether this is possible in melanoma cells overexpressing both proteins is not known. An understanding of this issue may have important implications for the design of an effective pro-senescence therapy. In this study, the authors used a sh-RNA approach to knock down TBX2 and TBX3 individually in 2 human melanoma cell lines that overexpress both these factors and then examined their specific involvement in the oncogenic process. They demonstrate, using in vitro and in vivo cell proliferation, as well as colony- and tumor-forming ability and cell motility assays, that TBX2 and TBX3 have distinct roles in melanoma progression. In the tested lines, although TBX2 could promote proliferation and transformation and was required by primary melanoma cells for immortality, TBX3 was required for tumor formation and cell migration. These findings were reproducible in a human breast cancer cell line, which confirms that TBX2 and TBX3, although highly homologous, do not have redundant roles in the transformation process of cancers where they are both overexpressed. These results have important implications for the development of new cancer treatments and in particular for melanoma, which is a highly aggressive and intractable cancer.

Two new cases with microdeletion of 17q23.2 suggest presence of a candidate gene for sensorineural hearing loss within this region

Microdeletion of the 17q23.2 region has very recently been suggested as a new emerging syndrome based on the finding of 8 cases with common phenotypes including mild-to-moderate developmental delay, heart defects, microcephaly, postnatal growth retardation, and hand, foot, and limb abnormalities. In this report, we describe two new 17q23.2 deletion patients with mild intellectual disability and sensorineural hearing loss. They both had submicroscopic deletions smaller than the common deleted region for the 8 previously described 17q23.2 microdeletion cases. TBX4 was previously suggested as the responsible gene for the heart or limb defects observed in 17q23.2 deletion patients, but the present cases do not have these features despite deletion of this gene. The finding of sensorineural hearing loss in 5 of the 10 cases, including the present cases, with a microdeletion at17q23.2, strongly suggests the presence of a candidate gene for hearing loss within this region. We screened 41 patients with profound sensorineural hearing loss for mutations of TBX2 and detected no mutations.

Loss of Gata5 in mice leads to bicuspid aortic valve

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

Dynamic expression of Tbx2 and Tbx3 in developing mouse pancreas

Tbx2 and Tbx3 are closely related members of the T-box family of transcription factors that are important regulators during normal development as well as major contributors to human developmental syndromes when mutated. Although there is evidence for the involvement of Tbx2 and Tbx3 in pancreatic cancer, so far there are no reports characterizing the normal expression pattern of these genes in the pancreas. In this study, we examined spatial and temporal expression of Tbx2 and Tbx3 in mouse pancreas during development and in the adult using in situ hybridization and immunohistochemistry. Our results show that Tbx2 and Tbx3 are both expressed in the pancreatic mesenchyme throughout development beginning at embryonic day (E) 9.5. In addition, Tbx2 is expressed in pancreatic vasculature during development and in epithelial-derived endocrine and ductal cells during late fetal stages, postnatal development and in adult pancreas. In contrast, Tbx3 is expressed in exocrine tissue in the postnatal and adult pancreas. Further our results demonstrate that Tbx2 and Tbx3 are expressed in tumor-derived endocrine and exocrine cell lines, respectively. These dynamic changes in the expression pattern of these transcription factors lay the foundation for investigation of potential roles in pancreas development.

Developmental aspects of cardiac arrhythmogenesis

The transcriptional regulation orchestrating the development of the heart is increasingly recognized to play an essential role in the regulation of ion channel and gap junction gene expression and consequently the proper generation and conduction of the cardiac electrical impulse. This has led to the realization that in some instances, abnormal cardiac electrical function and arrhythmias in the postnatal heart may stem from a developmental abnormality causing maintained (epigenetic) changes in gene regulation. The role of developmental genes in the regulation of cardiac electrical function is further underscored by recent genome-wide association studies that provide strong evidence that common genetic variation, at loci harbouring these genes, modulates electrocardiographic indices of conduction and repolarization and susceptibility to arrhythmia. Here we discuss recent findings and provide background insight into these complex mechanisms.

Origin and development of the atrioventricular myocardial lineage: insight into the development of accessory pathways

Defects originating from the atrioventricular canal region are part of a wide spectrum of congenital cardiovascular malformations that frequently affect newborns. These defects include partial or complete atrioventricular septal defects, atrioventricular valve defects, and arrhythmias, such as atrioventricular re-entry tachycardia, atrioventricular nodal block, and ventricular preexcitation. Insight into the cellular origin of the atrioventricular canal myocardium and the molecular mechanisms that control its development will aid in the understanding of the etiology of the atrioventricular defects. This review discusses current knowledge concerning the origin and fate of the atrioventricular canal myocardium, the molecular mechanisms that determine its specification and differentiation, and its role in the development of certain malformations such as those that underlie ventricular preexcitation.

Notch signaling in cardiac valve development and disease

The Notch pathway is an intercellular signaling mechanism involved in multiple cell-to-cell communication processes that regulate cell fate specification, differentiation, and tissue patterning during embryogenesis and adulthood. Functional studies in the mouse have shown that a Hey-Bmp2 regulatory circuit restricts Bmp2 expression to presumptive valve myocardium (atrioventricular canal and outflow tract). Likewise, a Notch-Hey-Bmp2 axis represses Bmp2 in the endocardium. During cardiac valve formation, endocardial Notch signaling activates the epithelial-mesenchyme transition (EMT) that will give rise to the cardiac valve primordia. During this process, Notch integrates with myocardially derived signals (Bmp2 or Bmp4) to promote, via Snail1/2 activation a complete, invasive EMT in presumptive valve tissue. In humans, mutations in Notch signaling components are associated with several congenital disorders involving malformed valves, aortic arch, and defective chamber septation. Data suggest that the same embryonic Notch-Hey-Bmp2 regulatory axis is active in the adult valve. This review examines the experimental evidence supporting a role for Notch in heart valve development and homeostasis, and how altered Notch signaling may lead to valve disease in the newborn and adult.

Wnt signaling regulates atrioventricular canal formation upstream of BMP and Tbx2

In the developing heart, the atrioventricular canal (AVC) is essential for separation and alignment of the cardiac chambers, for valve formation, and serves to delay the electrical impulse from the atria to the ventricles. Defects in various aspects of its formation are the most common form of congenital heart defects. Using mutant and transgenic approaches in zebrafish, this study demonstrates that Wnt/β-catenin signaling is both sufficient and required for the induction of BMP4 and Tbx2b expression in the AVC and consequently the proper patterning of the myocardium. Furthermore, genetic analysis shows that Wnt/β-catenin signaling is upstream and in a linear pathway with BMP and Tbx2 during AVC specification.

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.

Heart chamber size in zebrafish is regulated redundantly by duplicated tbx2 genes

The Tbx2 transcription factor is implicated in growth control based on its association with human cancers. In the heart, Tbx2 represses cardiac differentiation to mediate development of the atrioventricular canal (AVC). The zebrafish genome retains two tbx2 genes, and both are required for formation of the AVC. Here, we show that both genes are also expressed earlier in the primitive heart tube, and we describe a previously unrecognized role for Tbx2 in promoting proliferation of presumptive myocardium at the heart tube stage. In contrast to single knockdowns, depletion of both gene products causes chamber defects, resulting in an expanded atrium and a smaller ventricle, associated with decreased proliferation of ventricular cardiomyocytes. The phenotype correlates with changes in the expression for known cardiac growth factors. Therefore, in zebrafish, two tbx2 genes are functionally redundant for regulating chamber development, while each gene is required independently for development of the AVC.

Role of Tbx2 in defining the territory of the pronephric nephron

Despite extensive study of the development of the nephron, which is the functional unit of the kidney, the molecular mechanisms underlying the determination of nephron size remain largely unknown. Using the Xenopus pronephros, we demonstrate here that Tbx2, a T-box transcriptional repressor, functions to demarcate the territory of the pronephric nephron. Tbx2 is specifically expressed around three distinct components of the pronephric nephron: the tubule, duct and glomus. Gain of function of Tbx2 inhibits nephric mesoderm formation. Conversely, Tbx2 loss of function expands the boundary of each component of the pronephric nephron, resulting in an enlarged pronephros. BMP signals induce Tbx2 in the non-nephric mesoderm, which inhibits the expression of the nephric markers Hey1 and Gremlin. Importantly, these pronephric molecules repress Tbx2 expression by antagonizing BMP signals in the nephric mesoderm. These results suggest that the negative regulatory loops between BMP/Tbx2 and Gremlin or Hey1 are responsible for defining the territory of the pronephric nephron.

Defective Tbx2-dependent patterning of the atrioventricular canal myocardium causes accessory pathway formation in mice

Ventricular preexcitation, a feature of Wolff-Parkinson-White syndrome, is caused by accessory myocardial pathways that bypass the annulus fibrosus. This condition increases the risk of atrioventricular tachycardia and, in the presence of atrial fibrillation, sudden death. The developmental mechanisms underlying accessory pathway formation are poorly understood but are thought to primarily involve malformation of the annulus fibrosus. Before birth, slowly conducting atrioventricular myocardium causes a functional atrioventricular activation delay in the absence of the annulus fibrosus. This myocardium remains present after birth, suggesting that the disturbed development of the atrioventricular canal myocardium may mediate the formation of rapidly conducting accessory pathways. Here we show that myocardium-specific inactivation of T-box 2 (Tbx2), a transcription factor essential for atrioventricular canal patterning, leads to the formation of fast-conducting accessory pathways, malformation of the annulus fibrosus, and ventricular preexcitation in mice. The accessory pathways ectopically express proteins required for fast conduction (connexin-40 [Cx40], Cx43, and sodium channel, voltage-gated, type V, α [Scn5a]). Additional inactivation of Cx30.2, a subunit for gap junctions with low conductance expressed in the atrioventricular canal and unaffected by the loss of Tbx2, did not affect the functionality of the accessory pathways. Our results suggest that malformation of the annulus fibrosus and preexcitation arise from the disturbed development of the atrioventricular myocardium.

Navigational error in the heart leads to premature ventricular excitation

In the normal heart, an insulating barrier separates the atria and ventricles. The only way in which electrical impulses can cross this barrier is via the atrioventricular (AV) node, which delays impulse conduction to ensure the forward flow of the blood. However, in some individuals, additional muscular bundles (accessory pathways) allow rapid conduction of electrical impulses from the atria to the ventricles, resulting in premature ventricular excitation and contraction. In this issue of the JCI, two independent research groups demonstrate that erroneous development of the embryonic AV canal, which performs a similar function to that of the adult AV node, is a novel mechanism by which accessory pathways can form.

Expression of Slit and Robo genes in the developing mouse heart

Development of the mammalian heart is mediated by complex interactions between myocardial, endocardial, and neural crest-derived cells. Studies in Drosophila have shown that the Slit-Robo signaling pathway controls cardiac cell shape changes and lumen formation of the heart tube. Here, we demonstrate by in situ hybridization that multiple Slit ligands and Robo receptors are expressed in the developing mouse heart. Slit3 is the predominant ligand transcribed in the early mouse heart and is expressed in the ventral wall of the linear heart tube and subsequently in chamber but not in atrioventricular canal myocardium. Furthermore, we identify that the homeobox gene Nkx2-5 is required for early ventral restriction of Slit3 and that the T-box transcription factor Tbx2 mediates repression of Slit3 in nonchamber myocardium. Our results suggest that patterned Slit-Robo signaling may contribute to the control of oriented cell growth during chamber morphogenesis of the mammalian heart.

TBX2 gene duplication associated with complex heart defect and skeletal malformations

We report on a patient with mild mental retardation, prenatal onset growth retardation, cerebellar hypoplasia, and complex heart defect including: interventricular septal defect, patent foramen ovale, aortic coarctation, tricuspid valve insufficiency, mitral valve stenosis, and minor skeletal anomalies with hypo-aplasia of the distal phalanges. A SNP-array analysis detected a de novo duplication of 17q23.2, encompassing the TBX2 gene. Animal models argue for a key role of Tbx2 during cardiac and limb development. Accordingly, we hypothesize that the heart malformation and mild digital anomalies found in this patient could be related to TBX2 gene overexpression, suggesting parallel consequences of TBX2 gene dosage imbalances in animals and in humans.

Generation of mice with a conditional null allele for Tbx2

The T-box transcription factor Tbx2 plays important roles in patterning and development, and has been implicated in cell-cycle regulation and cancer. Conventional disruption of Tbx2 results in abnormalities of the heart, limbs, eye and other structures, and early fetal lethality. To gain insight into the role of Tbx2 in different tissues and at different stages of development, we have generated a conditional null allele of Tbx2 by flanking Exon 2 with loxP sites (Tbx2(fl2)). Homozygous Tbx2(fl2) mice are viable and fertile, indicating that the Tbx2(fl2) allele is a fully functional Tbx2 allele. Cre-mediated recombination, using a ubiquitously active CMV-Cre line, results in deletion of Exon 2 and loss of protein expression. Embryos homozygous for the recombined allele (Tbx2(Delta2)) show the same heart and limb defects as conventional Tbx2-deficient embryos. This Tbx2 conditional null allele will be a valuable tool to uncover tissue-specific roles of Tbx2 in development and disease.

Sumoylation and regulation of cardiac gene expression

Sumoylation is a posttranslational modification process in which SUMO proteins are covalently and reversibly conjugated to their targets via enzymatic cascade reactions. Since the discovery of SUMO-1 in 1996, the SUMO pathway has garnered increased attention due to its role in a number of important biological activities such as cell cycle progression, epigenetic modulation, signal transduction, and DNA replication/repair, as well as its potential implication in human pathogenesis such as in cancer development and metastasis, neurodegenerative disorders and craniofacial defects. The role of the SUMO pathway in regulating cardiogenic gene activity, development and/or disorders is just emerging. Our review is based on recent advances that highlight the regulation of cardiac gene activity in cardiac development and disease by the SUMO conjugation pathway.

Molecular mechanisms of pituitary organogenesis: In search of novel regulatory genes

Defects in pituitary gland organogenesis are sometimes associated with congenital anomalies that affect head development. Lesions in transcription factors and signaling pathways explain some of these developmental syndromes. Basic research studies, including the characterization of genetically engineered mice, provide a mechanistic framework for understanding how mutations create the clinical characteristics observed in patients. Defects in BMP, WNT, Notch, and FGF signaling pathways affect induction and growth of the pituitary primordium and other organ systems partly by altering the balance between signaling pathways. The PITX and LHX transcription factor families influence pituitary and head development and are clinically relevant. A few later-acting transcription factors have pituitary-specific effects, including PROP1, POU1F1 (PIT1), and TPIT (TBX19), while others, such as NeuroD1 and NR5A1 (SF1), are syndromic, influencing development of other endocrine organs. We conducted a survey of genes transcribed in developing mouse pituitary to find candidates for cases of pituitary hormone deficiency of unknown etiology. We identified numerous transcription factors that are members of gene families with roles in syndromic or non-syndromic pituitary hormone deficiency. This collection is a rich source for future basic and clinical studies.

The retinoblastoma protein modulates Tbx2 functional specificity

This study demonstrates that Tbx2 binds Rb1. The interaction with Rb1 increases Tbx2 DNA-binding activity and enhances the ability of Tbx2 to repress transcription. The results show that Tbx2 regulates the expression of genes involved in cell division and DNA replication and that Rb1 modulates Tbx2 target gene recognition and specificity.

Tbx2 is a member of a large family of transcription factors defined by homology to the T-box DNA-binding domain. Tbx2 plays a key role in embryonic development, and in cancer through its capacity to suppress senescence and promote invasiveness. Despite its importance, little is known of how Tbx2 is regulated or how it achieves target gene specificity. Here we show that Tbx2 specifically associates with active hypophosphorylated retinoblastoma protein (Rb1), a known regulator of many transcription factors involved in cell cycle progression and cellular differentiation, but not with the Rb1-related proteins p107 or p130. The interaction with Rb1 maps to a domain immediately carboxy-terminal to the T-box and enhances Tbx2 DNA binding and transcriptional repression. Microarray analysis of melanoma cells expressing inducible dominant-negative Tbx2, comprising the T-box and either an intact or mutated Rb1 interaction domain, shows that Tbx2 regulates the expression of many genes involved in cell cycle control and that a mutation which disrupts the Rb1-Tbx2 interaction also affects Tbx2 target gene selectivity. Taken together, the data show that Rb1 is an important determinant of Tbx2 functional specificity.

Nucleocytoplasmic functions of the PDZ-LIM protein family: new insights into organ development

Recent work on the PDZ-LIM protein family has revealed that it has important activities at the cellular level, mediating signals between the nucleus and the cytoskeleton, with significant impact on organ development. We review and integrate current knowledge about the PDZ-LIM protein family and propose a new functional role, sequestering nuclear factors in the cytoplasm. Characterized by their PDZ and LIM domains, the PDZ-LIM family is comprised of evolutionarily conserved proteins found throughout the animal kingdom, from worms to humans. Combining two functional domains in one protein, PDZ-LIM proteins have wide-ranging and multi-compartmental cell functions during development and homeostasis. In contrast, misregulation can lead to cancer formation and progression. New emerging roles include interactions with integrins, T-box transcription factors, and receptor tyrosine kinases. Facilitating the assembly of protein complexes, PDZ-LIM proteins can act as signal modulators, influence actin dynamics, regulate cell architecture, and control gene transcription.

The function and regulation of TBX22 in avian frontonasal morphogenesis

The frontonasal mass gives rise to the facial midline and fuses with the maxillary prominence to form the upper lip. Here we focus on the regulation and function of TBX22, a repressor dynamically expressed in the frontonasal mass. Both FGF and Noggin (a BMP antagonist) strongly induce gTBX22, however, each has opposite effects on morphogenesis - Noggin inhibits whereas FGF stimulates growth. To determine whether TBX22 mediates these effects, we used retroviruses to locally increase expression levels. RCAS::hTBX22 decreased proliferation, reduced expression of MSX2 and DLX5 and caused cleft lip. Decreased levels of endogenous gTBX22 were also observed but were not the primary cause of the phenotype as determined in rescue experiments. Our data suggest that genetic or environmental insults such as those affecting the BMP pathway could lead to a gain-of-function of TBX22 and predispose an individual to cleft lip.

Pitx2-dependent occupancy by histone deacetylases is associated with T-box gene regulation in mammalian abdominal tissue

The homeodomain transcription factor Pitx2 and the T-box transcription factors are essential for organogenesis. Pitx2 and T-box genes are induced by growth factors and function as transcriptional activators or repressors. Gene expression analyses on abdominal tissue were used to identify seven of the T-box genes of the genome as Pitx2 target genes in the abdomen at embryonic day.10.5. Pitx2 activated Tbx4, Tbx15, and Mga and repressed Tbx1, Tbx2, Tbx5, and Tbx6 expression. As expected, activated genes showed reduced expression patterns, and repressed T-box genes showed increased expression patterns in the abdomen of Pitx2 mutants. Pitx2 occupied chromatin sites near all of these T-box genes. Co-occupancy by coactivators, corepressors, and histone acetylation at these sites was frequently Pitx2-dependent. Genes repressed by Pitx2 generally showed increased histone acetylation and decreased histone deacetylase (HDAC)/corepressor occupancy in Pitx2 mutants. The lower N-CoR, HDAC1, and HDAC3 occupancy observed at multiple sites along Tbx1 chromatin in mutants is consistent with the model that increased histone acetylation and gene expression of Tbx1 may result from a loss of recruitment of corepressors by Pitx2. Genes activated by Pitx2 showed less consistent patterns in chromatin analyses. Reduced H4 acetylation and increased HDAC1/nuclear receptor corepressor (N-CoR) occupancy at some Tbx4 sites were accompanied by increased H3 acetylation and reduced HDAC3 occupancy at the same or other more distal chromatin sites in mutants. Pitx2-dependent occupancy by corepressors resulted in alteration of the acetylation levels of several T-box genes, whereas Pitx2-dependent occupancy by coactivators was more site-localized. These studies will provide the basic scientific underpinning to understand abdominal wall syndromes.

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.

Expression of PSCA, PIWIL1 and TBX2 and its correlation with HPV16 infection in formalin-fixed, paraffin-embedded cervical squamous cell carcinoma specimens

The purpose of our study was to investigate the expression of prostate stem cell antigen (PSCA), piwi-like 1 (PIWIL1) and T-box 2 (TBX2) and its correlation with HPV16 infection in cervical squamous cell carcinoma (CSCC). HPV16 was detected by amplifying the HPV16 E7 gene by the polymerase chain reaction (PCR) method, and the expression of PSCA, PIWIL1, TBX2 and HPV16 E7 in 59 CSCCs and matched adjacent normal cervix (MANC) was examined by the streptavidin-peroxidase (SP) method. Fifty-two CSCCs and MANC specimens that were positive for the E7 gene and the E7 protein were identified as infected with HPV16 and included in present study. The rate of infection with HPV16 in CSCC was 52% (27/52), but that in matched adjacent normal cervix (MANC) samples was 4% (2/52). Infection with HPV16 was found to be statistically more frequent in CSCC (P = 0.000). The expression rates of PSCA, PIWIL1 and TBX2 in MANC were 6% (3/52), 8% (4/52) and 2% (1/52), respectively, but those in CSCC were 62% (32/52), 75% (39/52) and 52% (27/52), respectively. Higher expression rates of PSCA, PIWIL1 and TBX2 were observed in CSCC than in MANC (P = 0.000). HPV16 had a statistical positive correlation with PSCA, PIWIL1 and TBX2 in CSCC (P < 0.05). The increased expression of PSCA, PIWIL1 and TBX2 had no correlation with the patient's age or histological grade P > 0.05). The elevated expression of PSCA and PIWIL1 was associated with invasion of CSCC (P < 0.05). Up-regulated expression of TBX2 had a positive association with lymph node metastasis (P = 0.014). These findings demonstrate for the first time the expression of PSCA, PIWIL1 and TBX2 in CSCC. Their correlation with HPV16 might provide new basic information for investigating the molecular mechanism of HPV and help us to deepen our understanding of the interaction between HPV16 and host cells the carcinogenesis of CSCC.

Development of the Pacemaker Tissues of the Heart

Pacemaker and conduction system myocytes play crucial roles in initiating and regulating the contraction of the cardiac chambers. Genetic defects, acquired diseases, and aging cause dysfunction of the pacemaker and conduction tissues, emphasizing the clinical necessity to understand the molecular and cellular mechanisms of their development and homeostasis. Although all cardiac myocytes of the developing heart initially possess pacemaker properties, the majority differentiates into working myocardium. Only small populations of embryonic myocytes will form the sinus node and the atrioventricular node and bundle. Recent efforts have revealed that the development of these nodal regions is achieved by highly localized suppression of working muscle differentiation, and have identified transcriptional repressors that mediate this process. This review will summarize and reflect new experimental findings on the cellular origin and the molecular control of differentiation and morphogenesis of the pacemaker tissues of the heart. It will also shed light on the etiology of inborn and acquired errors of nodal tissues.