Chicken NKx2-8, a novel homeobox gene expressed during early heart and foregut development

Cardiac Development: A Glimpse on Its Translational Contributions

Cardiac development is a complex developmental process that is initiated soon after gastrulation, as two sets of precardiac mesodermal precursors are symmetrically located and subsequently fused at the embryonic midline forming the cardiac straight tube. Thereafter, the cardiac straight tube invariably bends to the right, configuring the first sign of morphological left–right asymmetry and soon thereafter the atrial and ventricular chambers are formed, expanded and progressively septated. As a consequence of all these morphogenetic processes, the fetal heart acquired a four-chambered structure having distinct inlet and outlet connections and a specialized conduction system capable of directing the electrical impulse within the fully formed heart. Over the last decades, our understanding of the morphogenetic, cellular, and molecular pathways involved in cardiac development has exponentially grown. Multiples aspects of the initial discoveries during heart formation has served as guiding tools to understand the etiology of cardiac congenital anomalies and adult cardiac pathology, as well as to enlighten novels approaches to heal the damaged heart. In this review we provide an overview of the complex cellular and molecular pathways driving heart morphogenesis and how those discoveries have provided new roads into the genetic, clinical and therapeutic management of the diseased hearts.

Homeobox Genes and Homeodomain Proteins: New Insights into Cardiac Development, Degeneration and Regeneration

Cardiovascular diseases are the most common cause of human death in the developing world. Extensive evidence indicates that various toxic environmental factors and unhealthy lifestyle choices contribute to the risk, incidence and severity of cardiovascular diseases. Alterations in the genetic level of myocardium affects normal heart development and initiates pathological processes leading to various types of cardiac diseases. Homeobox genes are a large and highly specialized family of closely related genes that direct the formation of body structure, including cardiac development. Homeobox genes encode homeodomain proteins that function as transcription factors with characteristic structures that allow them to bind to DNA, regulate gene expression and subsequently control the proper physiological function of cells, tissues and organs. Mutations in homeobox genes are rare and usually lethal with evident alterations in cardiac function at or soon after the birth. Our understanding of homeobox gene family expression and function has expanded significantly during the recent years. However, the involvement of homeobox genes in the development of human and animal cardiac tissue requires further investigation. The phenotype of human congenital heart defects unveils only some aspects of human heart development. Therefore, mouse models are often used to gain a better understanding of human heart function, pathology and regeneration. In this review, we have focused on the role of homeobox genes in the development and pathology of human heart as potential tools for the future development of targeted regenerative strategies for various heart malfunctions.

Mesodermal Nkx2.5 is necessary and sufficient for early second heart field development

The vertebrate heart develops from mesoderm and requires inductive signals secreted from early endoderm. During embryogenesis, Nkx2.5 acts as a key transcription factor and plays essential roles for heart formation from Drosophila to human. In mice, Nkx2.5 is expressed in the early first heart field, second heart field pharyngeal mesoderm, as well as pharyngeal endodermal cells underlying the second heart field. Currently, the specific requirements for Nkx2.5 in the endoderm versus mesoderm with regard to early heart formation are incompletely understood. Here, we performed tissue-specific deletion in mice to dissect the roles of Nkx2.5 in the pharyngeal endoderm and mesoderm. We found that heart development appeared normal after endodermal deletion of Nkx2.5 whereas mesodermal deletion engendered cardiac defects almost identical to those observed on Nkx2.5 null embryos (Nkx2.5(-/-)). Furthermore, re-expression of Nkx2.5 in the mesoderm rescued Nkx2.5(-/-) heart defects. Our findings reveal that Nkx2.5 in the mesoderm is essential while endodermal expression is dispensable for early heart formation in mammals.

Comparative Analysis of Nkx2-5/GATA4/TBX5 Expression in Chicken, Quail and Chicken-quail Hybrids during the Early Stage of Cardiac Development in Embryos

The present study makes an investigation into expression of genes related to cardiac development in chicken, quail and chicken-quail hybrids during the early stage of embryogenesis. Real-time PCR was used to detect mRNA expressions of Nkx2-5, GATA4 and TBX5 in the heart of chicken, quail and chicken-quail hybrids embryos during the 3rd to 7th days of incubation. Results showed that NKX2-5 mRNA displayed a similar expression trend in chicken, quail and chicken-quail hybrids. The initial and highest expression of Nkx2-5 was focused on the 3rd day of incubation, then it declined till 5th day of incubation, thereafter, it fluctuated. Expression of Nkx2-5 gene in quail was significantly higher than in chicken and chicken-quail hybrids, and no significant difference was observed between the two latter species. GATA4 mRNA showed a similar expression trend between chicken and quail, which displayed a steady increase from 3rd to 6th d, then, the expression level decreased. However, GATA4 mRNA expression in chicken-quail hybrids was significantly higher than that in chicken and quail from 3rd to 5th d (p<0.01), but significantly lower than that in chicken and quail during the later stage of the experiment (p<0.05), due to the dramatic drop from 5th d onwards (p<0.01). TBX5 mRNA expression in chicken and quail showed the same trend as GATA4 expressed in the two species. Furthermore, TBX5 expression in chicken-quail hybrids was significantly higher than that in chicken and quail during the whole course of experiment, although relatively lower TBX5 expression was detected in the early stage. In conclusion, Nkx2-5, GATA4 and TBX5 genes showed dynamic changes during the process of cardiac development in chicken, quail and their hybrids embryos. In addition, the expression trend in chicken was similar to that in quail, and there was no significant difference for gene expression level, except NKX2-5. However, expression of these genes in chicken-quail hybrids was significantly different from their parents, the difference mechanism needs to be further explored.

Therapeutic efficacy of human hepatocyte transplantation in a SCID/uPA mouse model with inducible liver disease

BACKGROUND

Severe Combined Immune Deficient (SCID)/Urokinase-type Plasminogen Activator (uPA) mice undergo liver failure and are useful hosts for the propagation of transplanted human hepatocytes (HH) which must compete with recipient-derived hepatocytes for replacement of the diseased liver parenchyma. While partial replacement by HH has proven useful for studies with Hepatitis C virus, complete replacement of SCID/uPA mouse liver by HH has never been achieved and limits the broader application of these mice for other areas of biomedical research. The herpes simplex virus type-1 thymidine kinase (HSVtk)/ganciclovir (GCV) system is a powerful tool for cell-specific ablation in transgenic animals. The aim of this study was to selectively eliminate murine-derived parenchymal liver cells from humanized SCID/uPA mouse liver in order to achieve mice with completely humanized liver parenchyma. Thus, we reproduced the HSVtk (vTK)/GCV system of hepatic failure in SCID/uPA mice.

METHODOLOGY/PRINCIPAL FINDINGS

In vitro experiments demonstrated efficient killing of vTK expressing hepatoma cells after GCV treatment. For in vivo experiments, expression of vTK was targeted to the livers of FVB/N and SCID/uPA mice. Hepatic sensitivity to GCV was first established in FVB/N mice since these mice do not undergo liver failure inherent to SCID/uPA mice. Hepatic vTK expression was found to be an integral component of GCV-induced pathologic and biochemical alterations and caused death due to liver dysfunction in vTK transgenic FVB/N and non-transplanted SCID/uPA mice. In SCID/uPA mice with humanized liver, vTK/GCV caused death despite extensive replacement of the mouse liver parenchyma with HH (ranging from 32-87%). Surprisingly, vTK/GCV-dependent apoptosis and mitochondrial aberrations were also localized to bystander vTK-negative HH.

CONCLUSIONS/SIGNIFICANCE

Extensive replacement of mouse liver parenchyma by HH does not provide a secure therapeutic advantage against vTK/GCV-induced cytotoxicity targeted to residual mouse hepatocytes. Functional support by engrafted HH may be secured by strategies aimed at limiting this bystander effect.

Nkx2.7 and Nkx2.5 function redundantly and are required for cardiac morphogenesis of zebrafish embryos

Background

Nkx2.7 is the tinman-related gene, as well as orthologs of Nkx2.5 and Nkx-2.3. Nkx2.7 and Nkx2.5 express in zebrafish heart fields of lateral plate mesoderm. The temporal and spatial expression patterns of Nkx2.7 are similar to those of Nkx2.5, but their functions during cardiogenesis remain unclear.

Methodology/Principal Findings

Here, Nkx2.7 is demonstrated to compensate for Nkx2.5 loss of function and play a predominant role in the lateral development of the heart, including normal cardiac looping and chamber formation. Knocking down Nkx2.5 showed that heart development was normal from 24 to 72 hpf. However, when knocking down either Nkx2.7 or Nkx2.5 together with Nkx2.7, it appeared that the heart failed to undergo looping and showed defective chambers, although embryos developed normally before the early heart tube stage. Decreased ventricular myocardium proliferation and defective myocardial differentiation appeared to result from late-stage up-regulation of bmp4, versican, tbx5 and tbx20, which were all expressed normally in hearts at an early stage. We also found that tbx5 and tbx20 were modulated by Nkx2.7 through the heart maturation stage because an inducible overexpression of Nkx2.7 in the heart caused down-regulation of tbx5 and tbx20. Although heart defects were induced by overexpression of an injection of 150-pg Nkx2.5 or 5-pg Nkx2.7 mRNA, either Nkx2.5 or Nkx2.7 mRNA rescued the defects induced by Nkx2.7-morpholino(MO) and Nkx2.5-MO with Nkx2.7-MO.

Conclusions and Significance

Therefore, we conclude that redundant activities of Nkx2.5 and Nkx2.7 are required for cardiac morphogenesis, but that Nkx2.7 plays a more critical function, specifically indicated by the gain-of-function and loss-of- function experiments where Nkx2.7 is observed to regulate the expressions of tbx5 and tbx20 through the maturation stage.

Complementary striped expression patterns of NK homeobox genes during segment formation in the annelid Platynereis

NK genes are related pan-metazoan homeobox genes. In the fruitfly, NK genes are clustered and involved in patterning various mesodermal derivatives during embryogenesis. It was therefore suggested that the NK cluster emerged in evolution as an ancestral mesodermal patterning cluster. To test this hypothesis, we cloned and analysed the expression patterns of the homologues of NK cluster genes Msx, NK4, NK3, Lbx, Tlx, NK1 and NK5 in the marine annelid Platynereis dumerilii, a representative of trochozoans, the third great branch of bilaterian animals alongside deuterostomes and ecdysozoans. We found that most of these genes are involved, as they are in the fly, in the specification of distinct mesodermal derivatives, notably subsets of muscle precursors. The expression of the homologue of NK4/tinman in the pulsatile dorsal vessel of Platynereis strongly supports the hypothesis that the vertebrate heart derived from a dorsal vessel relocated to a ventral position by D/V axis inversion in a chordate ancestor. Additionally and more surprisingly, NK4, Lbx, Msx, Tlx and NK1 orthologues are expressed in complementary sets of stripes in the ectoderm and/or mesoderm of forming segments, suggesting an involvement in the segment formation process. A potentially ancient role of the NK cluster genes in segment formation, unsuspected from vertebrate and fruitfly studies so far, now deserves to be investigated in other bilaterian species, especially non-insect arthropods and onychophorans.

New syndromic form of benign hereditary chorea is associated with a deletion of TITF-1 and PAX-9 contiguous genes

Benign hereditary chorea is a rare autosomal dominant disorder presenting with a childhood-onset and slowly progressive chorea. The objective of this study was to describe the clinical and genetic features of 3 patients who developed childhood-onset chorea. Three affected patients from three generations of a family with benign hereditary chorea associated with a multisystemic disorder of the basal ganglia, thyroid, lungs, salivary glands, bowels, and teeth. The TITF-1 gene was screened by microsatellite analysis, gene sequencing, and fluorescence in situ hybridization. Genetic analysis revealed a novel 0.9-Mb deletion on chromosome 14, which includes the TITF-1 and PAX9 genes. We have identified a novel deletion responsible for a new syndrome of benign hereditary chorea, including symptoms of brain-thyroid-lung syndrome associated with bowels, salivary glands, and teeth disorders. Associated signs, sometimes of slight expression, remain of high interest for the clinical and genetic diagnosis of benign hereditary chorea.

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

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

Origin, fate, and function of the components of the avian germ disc region and early blastoderm: Role of ooplasmic determinants

In the avian oocytal germ disc region, at the end of oogenesis, we discerned four ooplasms (alpha, beta, gamma, delta) presenting an onion-peel distribution (from peripheral and superficial to central and deep. Their fate was followed during early embryonic development. The most superficial and peripheral alpha ooplasm plays a fundamental role during cleavage. The beta ooplasm, originally localized in the peripheral region of the blastodisc, becomes mainly concentrated in the primitive streak. At the moment of bilateral symmetrization, a spatially oblique, sickle-shaped uptake of gamma and delta ooplasms occurs so that gamma and delta ooplasms become incorporated into the deeper part of the avian blastoderm. These ooplasms seem to contain ooplasmic determinants that initiate either early neurulation or gastrulation events. The early neural plate-inducing structure that forms a deep part of the blastoderm is the delta ooplasm-containing endophyll (primary hypoblast). Together with the primordial germ cells, it is derived from the superficial centrocaudal part of the nucleus of Pander, which also contains delta ooplasm. The other structure (gamma ooplasm) that is incorporated into the caudolateral deep part of the blastoderm forms Rauber's sickle. It induces gastrulation in the concavity of Rauber's sickle and blood island formation exterior to Rauber's sickle. Rauber's sickle develops by ingrowth of blastodermal cells into the gamma ooplasm, which surrounds the nucleus of Pander. Rauber's sickle constitutes the primary major organizer of the avian blastoderm and generates only extraembryonic tissues (junctional and sickle endoblast). By imparting positional information, it organizes and dominates the whole blastoderm (controlling gastrulation, neurulation, and coelom and cardiovascular system formation). Fragments of the horns of Rauber's sickle extend far cranially into the lateral quadrants of the unincubated blastoderm, so that often Rauber's sickle material forms three quarters of a circle. This finding explains the regulative capacities of isolated blastoderm parts, with the exception of the anti-sickle region and central blastoderm region, where no Rauber's sickle material is present. In avian blastoderms, there exists a competitive inhibition by Rauber's sickle on the primitive streak and neural plate-inducing effects of sickle endoblast. Avian primordial germ cells contain delta ooplasm derived from the superficial part of the nucleus of Pander. Their original deep and central ooplasmic localization has been confirmed by the use of a chicken vasa homologue. We conclude that the unincubated blastoderm consists of three elementary tissues: upper layer mainly containing beta ooplasm, endophyll containing delta ooplasm, and Rauber's sickle containing gamma ooplasm). These elementary tissues form before the three classic germ layers have developed.

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

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

Heart development: molecular insights into cardiac specification and early morphogenesis

The heart develops from two bilateral heart fields that are formed during early gastrulation. In recent years, signaling pathways that specify cardiac mesoderm have been extensively analyzed. In addition, a battery of transcription factors that regulate different aspects of cardiac morphogenesis and cytodifferentiation have been identified and characterized in model organisms. At the anterior pole, a secondary heart field is formed, which in its molecular make-up, appears to be similar to the primary heart field. The cardiac outflow tract and the right ventricle to a large extent are derivatives of this anterior heart field. Cardiac mesoderm receives positional information by which it is patterned along the three body axes. The molecular control of left-right axis development has received particular attention, and the underlying regulatory network begins to emerge. Cardiac chamber development involves the activation of a transcription program that is different from the one present in the primary heart field and regulates cardiac morphogenesis in a region-specific manner. This review also attempts to identify areas in which additional research is needed to fully understand early cardiac development.

Induction of the avian coelom with associated vitelline blood circulation by Rauber's sickle derived junctional endoblast and its fundamental role in heart formation

In histological sections through chicken blastoderms of different ages we describe the temporospatial relationship between junctional endoblast, the formation of blood islands (appearing first from a peripherally migrating mesoblastic blastema), and the formation of coelomic vesicles developing later in/and from a more superficially extending mesoblastic blastema (coelomic mesoblast). After unilateral removal of the Rauber's sickle-derived junctional endoblast in early streak blastoderms (stage 2-4; Vakaet [1970] Arch Biol 81:387-426) and culture to stage 11 (Hamburger and Hamilton [1951] J Morphol 88:49-92), we observed that the early formation of the coelomic cavity was locally or totally disturbed in the operated area. Besides the simultaneous absence of blood islands, the coelomic vesicles did not form normally. Instead of regularly aligned coelomic vesicles, progressively forming the coelomic cavity by fusion, some voluminous irregular cavities appeared. Thus, the extent of the coelomic cavity was greatly reduced and the operated side was considerably smaller than the unoperated side. Furthermore, in the youngest operated blastoderms the cranial portion of the involved coelomic cavity (hemipericardial cavity) exhibited rudimentary development and usually did not reach the region of the foregut endoderm. This resulted in the absence of the myoepicardium and associated endocardium at this side. In another experiment, after removal of the junctional endoblast at one side of the chicken blastoderm, a fragment of quail junctional endoblast was placed isotopically. This resulted, after further in vitro culture, in the restoration of the formation of coelomic vesicles and accompanying subjacent blood islands in the immediate neighborhood of the apposed quail junctional endoblast. Also, the pericardium and primary heart tube developed normally. Similarly, by using the quail-chicken chimera technique, we demonstrated that the splanchnic mesoderm cells of the pericardium develop in intimate association with the most cranial part of the junctional endoblast (derived from the Rauber's sickle horns). Our experiments indicate that the coelom and, in particular, the pericardium and primary heart tube form progressively (in time and space) under the inductory influence of Rauber's sickle and junctional endoblast.

Transcription factors in cardiogenesis: the combinations that unlock the mysteries of the heart

Heart formation is one of the first signs of organogenesis within the developing embryo and this process is conserved from flies to man. Completing the genetic roadmap of the molecular mechanisms that control the cell specification and differentiation of cells that form the developing heart has been an exciting and fast-moving area of research in the fields of molecular and developmental biology. At the core of these studies is an interest in the transcription factors that are responsible for initiation of a pluripotent cell to become programmed to the cardiac lineage and the subsequent transcription factors that implement the instructions set up by the cells commitment decision. To gain a better understanding of these pathways, cardiac-expressed transcription factors have been identified, cloned, overexpressed, and mutated to try to determine function. Although results vary depending on the gene in question, it is clear that there is a striking evolutionary conservation of the cardiogenic program among species. As we move up the evolutionary ladder toward man, we encounter cases of functional redundancy and combinatorial interactions that reflect the complex networks of gene expression that orchestrate heart development. This review focuses on what is known about the transcription factors implicated in heart formation and the role they play in this intricate genetic program.

Cell biology of cardiac development

Building a vertebrate heart is a complex task and involves several tissues, including the myocardium, endocardium, neural crest, and epicardium. Interactions between these tissues result in the changes in function and morphology (and also in the extracellular matrix, which serves as a substrate for morphological change) that are requisite for development of the heart. Some of the signaling pathways that mediate these changes have now been identified and several investigators are now filling in the missing pieces in these pathways in hopes of ultimately understanding the molecular mechanisms that govern healthy heart development. In addition, transcription factors that regulate various aspects of heart development have been identified. Transcription factors of the GATA and Nkx2 families are of particular importance for early specification of the heart field and for regulating expression of genes that encode proteins of the contractile apparatus. This chapter highlights some of the most significant discoveries made in the rapidly expanding field of heart development.

A unique combination of transcription factors controls differentiation of thyroid cells

The thyroid follicular cell type is devoted to the synthesis of thyroid hormones. Several genes, whose protein products are essential for efficient hormone biosynthesis, are uniquely expressed in this cell type. A set of transcriptional regulators, unique to the thyroid follicular cell type, has been identified as responsible for thyroid specific gene expression; it comprises three transcription factors, named TTF-1, TTF-2, and Pax8, each of which is expressed also in cell types different from the thyroid follicular cells. However, the combination of these factors is unique to the thyroid hormone producing cells, strongly suggesting that they play an important role in differentiation of these cells. An overview of the molecular and biological features of these transcription factors is presented here. Data demonstrating that all three play also an important role in early thyroid development, at stages preceding expression of the differentiated phenotype, are also reviewed. The wide temporal expression, from the beginning of thyroid organogenesis to the adult state, is suggestive of a recycling of the thyroid-specific transcription factors, that is, the control of different sets of target genes at diverse developmental stages. The identification of molecular mechanisms leading to specific gene expression in thyroid cells renders this cell type an interesting model in which to address several aspects of cell differentiation and organogenesis.

Characterization of homo- and heterodimerization of cardiac Csx/Nkx2.5 homeoprotein

Csx/Nkx2.5 is an evolutionarily conserved homeodomain (HD)-containing transcription factor that is essential for early cardiac development. We found that the HD of Csx/Nkx2.5 binds as a monomer as well as a dimer to its DNA binding sites in the promoter of the atrial natriuretic factor (ANF) gene, an in vivo target gene of Csx/Nkx2.5. Csx/Nkx2.5 physically interacts with each other in vitro as well as in cells, and the HD is critical for homodimerization. Lys(193) and Arg(194), located at the COOH-terminal end of HD, are essential for dimerization. Lys(193) is also required for a specific interaction with the zinc finger transcription factor GATA4. Csx/Nkx2.5 can heterodimerize with other NK2 homeodomain proteins, Nkx2.3 and Nkx2.6/Tix, with different affinities. A single missense mutation, Ile(183) to Pro in the HD of Csx/Nkx2.5, preserved homodimerization function, but totally abolished DNA binding. Ile(183) --> Pro mutant acts in an inhibitory manner on wild type Csx/Nkx2.5 transcriptional activity through the ANF promoter in 10T1/2 cells. However, Ile(183) --> Pro mutant does not inhibit wild type Csx/Nkx2.5 function on the ANF promoter in cultured neonatal cardiac myocytes, possibly due to failure of dimerization in the presence of the target DNA. These results suggest that complex protein-protein interactions of Csx/Nkx2.5 play a role in its transcriptional regulatory function.

Targeted disruption of the Nkx3.1 gene in mice results in morphogenetic defects of minor salivary glands: parallels to glandular duct morphogenesis in prostate

To investigate functions of the homeodomain-containing transcription factor Nkx3.1 a null mutation was generated by targeted gene disruption introducing the bacterial LacZ gene as reporter into the locus. In addition to defects in duct morphogenesis of the prostate and bulbourethral gland displaying progressive epithelial hyperplasia and reduced ductal branching (Bhatia-Gaur, R., Donjacour, A.A., Sciavolino, P.J., Kim, M., Desai, N., Young, P., Norton, C.R., Gridley, T., Cardiff, R.D., Cunha, G.R., Abate-Shen, C., Shen, M.M., 1999. Genes Dev. 13, 966-977), we observed a novel phenotype in minor salivary glands of Nkx3.1 null mutants. Minor salivary glands in the oral cavity of mutant mice appeared reduced in size and exhibited severely altered duct morphology. Other Nkx3.1 expressing regions were unaffected by the mutation. The activity of the Nkx3. 1/LacZ allele faithfully reflected the known expression domains of Nkx3.1 in sclerotome, a subset of blood vessels, Rathke's pouch, and ductal epithelium in prostate and minor salivary glands during pre- and postnatal mouse development. However, it was additionally expressed in the heart, duodenum and lung. These ectopic expression domains resemble the pattern of the Nkx2.6 gene which is closely linked to Nkx3.1 in the mouse genome and its regulation may therefore be affected by the mutation. In Nkx3.1/Shh compound mutant mice we found that Nkx3.1 expression in sclerotome and prostate was strictly dependent on sonic hedgehog (Shh) signaling, while other expression domains including heart and gut were independent of Shh. Expression in lung appeared augmented in the absence of Shh. Our results suggest that Nkx3.1 plays a unique role in regulating proliferation of glandular epithelium and in the formation of ducts in prostate and minor salivary glands.

tinman-related genes expressed during heart development in Xenopus

The tinman homeobox gene of Drosophila is absolutely required for development of the insect heart. This observation prompted the isolation of tinman-related genes from vertebrates, in the hope that the developmental function of the gene would be conserved between evolutionarily distinct species. The first vertebrate tinman gene, Nkx2-5, was isolated from mouse and subsequently, orthologues of Nkx2-5 have been isolated from a number of different species. In all cases, a conserved pattern of Nkx2-5 expression is observed in the developing heart, commencing prior to differentiation. Genetic ablation of Nkx2-5 in the mouse results in embryonic lethality due to heart defects, but most myocardial genes are expressed normally and a beating heart tube forms. This observation raises the possibility that additional genes related to Nkx2-5 are partially rescuing Nkx2-5 function in the null mouse. Recently, additional members of the tinman-related gene family have been discovered and characterized in a number of different species. Somewhat surprisingly, orthologous genes in different organisms can be rather divergent in sequence and may show completely different expression patterns. In at least some organisms, expression of the tinman-related genes is not observed in the heart. Due to the increasing number of family members and the somewhat divergent expression patterns, the precise role of the tinman-related genes in cardiac development remains an open question. In a search for additional tinman-related genes in the frog, Xenopus laevis, we have identified Nkx2-9, a novel member of the tinman-related gene family. Preliminary characterization reveals that Nkx2-9 is expressed in the cardiogenic region of the embryo prior to differentiation, but transcript levels decrease rapidly, in the heart, at about the time that differentiation commences.

Transcriptional regulation of human cardiac homeobox gene CSX1

Cardiac homeobox gene Csx/Nkx-2.5 is essential for normal heart development and morphogenesis and is the earliest marker for cardiogenesis. To elucidate the regulatory mechanisms of Csx/Nkx-2.5 expression, we have isolated and characterized the upstream regulatory region of human Csx/Nkx-2.5 (CSX1). Transfection of the reporter gene containing a 965-bp CSX1 5' flanking region indicated that this region confers cardiomyocyte-predominant expression of CSX1. Deletion and mutational analyses revealed two positive cis-regulatory elements in this region that are essential for CSX1 expression in cardiomyocytes. Electrophoretic mobility shift assay revealed that nuclear proteins prepared from cardiac myocytes bound to these elements in a sequence-specific manner. The identification of cis-regulatory sequences of the Csx/Nkx-2.5 gene will facilitate further analysis for the upstream regulatory factors that control the expression of Csx/Nkx-2.5 and the process of vertebrate heart development.

BMP2 is required for early heart development during a distinct time period

BMP2, like its Drosophila homologue dpp, is an important signaling molecule for specification of cardiogenic mesoderm in vertebrates. Here, we analyzed the time-course of BMP2-requirement for early heart formation in whole chick embryos and in explants of antero-lateral plate mesoderm. Addition of Noggin to explants isolated at stage 4 and cultured for 24 h resulted in loss of NKX2.5, GATA4, eHAND, Mef2A and vMHC expression. At stages 5-8 the individual genes showed differential sensitivity to Noggin addition. While expression of eHAND, NKX2.5 and Mef2A was clearly reduced by Noggin vMHC was only marginally affected. In contrast, GATA4 expression was enhanced after Noggin treatment. The developmental period during which cardiac mesoderm required the presence of BMP signaling in vivo was assessed by implantation of Noggin expressing cells into stage 4-8 embryos which were then cultured until stage 10-11. Complete loss of NKX2.5 and eHAND expression was observed in embryos implanted at stages 4-6, and expression was still suppressed in stages 7 and 8 implanted embryos. GATA4 expression was also blocked by Noggin at stage 4, however increased at stages 5, 6 and 7. Explants of central mesendoderm, that normally do not form heart tissue were employed to study the time-course of BMP2-induced cardiac gene expression. The induction of cardiac lineage markers in central mesendoderm of stage 5 embryos was distinct for different genes. While GATA4, -5, -6 and MEF2A were induced to maximal levels within 6 h after BMP2 addition, eHAND and dHAND required 12 h to reach maximum levels of expression. NKX2.5 was induced by 6 h and accumulated over 48 h. vMHC and titin were induced at significant levels only after 48 h of BMP2 addition. These results indicate that cardiac marker genes display distinct expression kinetics after BMP2 addition and differential response to Noggin treatment suggesting complex regulation of myocardial gene expression in the early tubular heart.

Transient cardiac expression of the tinman-family homeobox gene, XNkx2-10

In Drosophila, the tinman homeobox gene is absolutely required for heart development. In the vertebrates, a small family of tinman-related genes, the cardiac NK-2 genes, appear to play a similar role in the formation of the vertebrate heart. However, targeted gene ablation of one of these genes, Nkx2-5, results in defects in only the late stages of cardiac development suggesting the presence of a rescuing gene function early in development. Here, we report the characterization of a novel tinman-related gene, XNkx2-10, which is expressed during early heart development in Xenopus. Using in vitro assays, we show that XNkx2-10 is capable of transactivating expression from promoters previously shown to be activated by other tinman-related genes, including Nkx2-5. Furthermore, Xenopus Nkx2-10 can synergize with the GATA-4 and SRF transcription factors to activate reporter gene expression.

Induction and differentiation of the zebrafish heart requires fibroblast growth factor 8 (fgf8/acerebellar)

Vertebrate heart development is initiated from bilateral lateral plate mesoderm that expresses the Nkx2.5 and GATA4 transcription factors, but the extracellular signals specifying heart precursor gene expression are not known. We describe here that the secreted signaling factor Fgf8 is expressed in and required for development of the zebrafish heart precursors, particularly during initiation of cardiac gene expression. fgf8 is mutated in acerebellar (ace) mutants, and homozygous mutant embryos do not establish normal circulation, although vessel formation is only mildly affected. In contrast, heart development, in particular of the ventricle, is severely abnormal in acerebellar mutants. Several findings argue that Fgf8 has a direct function in development of cardiac precursor cells: fgf8 is expressed in cardiac precursors and later in the heart ventricle. Fgf8 is required for the earliest stages of nkx2.5 and gata4, but not gata6, expression in cardiac precursors. Cardiac gene expression is restored in acerebellar mutant embryos by injecting fgf8 RNA, or by implanting a Fgf8-coated bead into the heart primordium. Pharmacological inhibition of Fgf signalling during formation of the heart primordium phenocopies the acerebellar heart phenotype, confirming that Fgf signaling is required independently of earlier functions during gastrulation. These findings show that fgf8/acerebellar is required for induction and patterning of myocardial precursors.

Building the heart piece by piece: modularity of cis-elements regulating Nkx2-5 transcription

Heart formation in Drosophila is dependent on the homeobox gene tinman. The homeobox gene Nkx2-5 is closely related to tinman and is the earliest known marker for cardiogenesis in vertebrate embryos. Recent studies of cis-regulatory elements required for Nkx2-5 expression in the developing mouse heart have revealed an extraordinary array of independent cardiac enhancers, and associated negative regulatory elements, that direct transcription in distinct regions of the embryonic heart. These studies demonstrate the modularity in cardiac transcription, in which different regulatory elements respond to distinct sets of transcription factors to control gene expression in different compartments of the developing heart. We consider the potential mechanisms underlying such transcriptional complexity, its possible significance for cardiac function, and the implications for evolution of the multichambered heart.

Targeted disruption of the homeobox transcription factor Nkx2-3 in mice results in postnatal lethality and abnormal development of small intestine and spleen

The homeodomain transcription factor Nkx2-3 is expressed in gut mesenchyme and spleen of embryonic and adult mice. Targeted inactivation of the Nkx2-3 gene results in severe morphological alterations of both organs and early postnatal lethality in the majority of homozygous mutants. Villus formation in the small intestine appears considerably delayed in Nkx2-3(−)/- foetuses due to reduced proliferation of the epithelium, while massively increased growth of crypt cells ensues in surviving adult mutants. Interestingly, differentiated cell types of the intestinal epithelium are present in homozygous mutants, suggesting that Nkx2-3 is not required for their cell lineage allocation or migration-dependent differentiation. Hyperproliferation of the gut epithelium in adult mutants is associated with markedly reduced expression of BMP-2 and BMP-4, suggesting that these signalling molecules may be involved in mediating non-cell-autonomous control of intestinal cell growth. Spleens of Nkx2-3 mutants are generally smaller and contain drastically reduced numbers of lymphatic cells. The white pulp appears anatomically disorganized, possibly owing to a homing defect in the spleen parenchyme. Moreover, some of the Nkx2-3 mutants exhibit asplenia. Taken together these observations indicate that Nkx2-3 is essential for normal development and functions of the small intestine and spleen.

Control of early cardiac-specific transcription of Nkx2-5 by a GATA-dependent enhancer

The homeobox gene Nkx2-5 is the earliest known marker of the cardiac lineage in vertebrate embryos. Nkx2-5 expression is first detected in mesodermal cells specified to form heart at embryonic day 7.5 in the mouse and expression is maintained throughout the developing and adult heart. In addition to the heart, Nkx2-5 is transiently expressed in the developing pharynx, thyroid and stomach. To investigate the mechanisms that initiate cardiac transcription during embryogenesis, we analyzed the Nkx2-5 upstream region for regulatory elements sufficient to direct expression of a lacZ transgene in the developing heart of transgenic mice. We describe a cardiac enhancer, located about 9 kilobases upstream of the Nkx2-5 gene, that fully recapitulates the expression pattern of the endogenous gene in cardiogenic precursor cells from the onset of cardiac lineage specification and throughout the linear and looping heart tube. Thereafter, as the atrial and ventricular chambers become demarcated, enhancer activity becomes restricted to the developing right ventricle. Transcription of Nkx2-5 in pharynx, thyroid and stomach is controlled by regulatory elements separable from the cardiac enhancer. This distal cardiac enhancer contains a high-affinity binding site for the cardiac-restricted zinc finger transcription factor GATA4 that is essential for transcriptional activity. These results reveal a novel GATA-dependent mechanism for activation of Nkx2-5 transcription in the developing heart and indicate that regulation of Nkx2-5 is controlled in a modular manner, with multiple regulatory regions responding to distinct transcriptional networks in different compartments of the developing heart.

Tinman function is essential for vertebrate heart development: elimination of cardiac differentiation by dominant inhibitory mutants of the tinman-related genes, XNkx2-3 and XNkx2-5

In Drosophila, the tinman gene is absolutely required for development of the dorsal vessel, the insect equivalent of the heart. In vertebrates, the tinman gene is represented by a small family of tinman-related sequences, some of which are expressed during embryonic heart development. At present however, the precise importance of this gene family for vertebrate heart development is unclear. Using the Xenopus embryo, we have employed a dominant inhibitory strategy to interfere with the function of the endogenous tinman-related genes. In these experiments, suppression of tinman gene function can result in the complete elimination of myocardial gene expression and the absence of cell movements associated with embryonic heart development. This inhibition can be rescued by expression of wild-type tinman sequences. These experiments indicate that function of tinman family genes is essential for development of the vertebrate heart.

A GATA-dependent nkx-2.5 regulatory element activates early cardiac gene expression in transgenic mice

nkx-2.5 is one of the first genes expressed in the developing heart of early stage vertebrate embryos. Cardiac expression of nkx-2.5 is maintained throughout development and nkx-2.5 also is expressed in the developing pharyngeal arches, spleen, thyroid and tongue. Genomic sequences flanking the mouse nkx-2.5 gene were analyzed for early developmental regulatory activity in transgenic mice. Approximately 3 kb of 5′ flanking sequence is sufficient to activate gene expression in the cardiac crescent as early as E7.25 and in limited regions of the developing heart at later stages. Expression also was detected in the developing spleen anlage at least 24 hours before the earliest reported spleen marker and in the pharyngeal pouches and their derivatives including the thyroid. The observed expression pattern from the −3 kb construct represents a subset of the endogenous nkx-2.5 expression pattern which is evidence for compartment-specific nkx-2.5 regulatory modules. A 505 bp regulatory element was identified that contains multiple GATA, NKE, bHLH, HMG and HOX consensus binding sites. This element is sufficient for gene activation in the cardiac crescent and in the heart outflow tract, pharynx and spleen when linked directly to lacZ or when positioned adjacent to the hsp68 promoter. Mutation of paired GATA sites within this element eliminates gene activation in the heart, pharynx and spleen primordia of transgenic embryos. The dependence of this nkx-2. 5 regulatory element on GATA sites for gene activity is evidence for a GATA-dependent regulatory mechanism controlling nkx-2.5 gene expression. The presence of consensus binding sites for other developmentally important regulatory factors within the 505 bp distal element suggests that combinatorial interactions between multiple regulatory factors are responsible for the initial activation of nkx-2.5 in the cardiac, thyroid and spleen primordia.

Divergent roles for NK-2 class homeobox genes in cardiogenesis in flies and mice

Recent evidence suggests that cardiogenesis in organisms as diverse as insects and vertebrates is controlled by an ancient and evolutionarily conserved transcriptional pathway. In Drosophila, the NK-2 class homeobox gene tinman (tin) is expressed in cardiac and visceral mesodermal progenitors and is essential for their specification. In vertebrates, the tin homologue Nkx2-5/Csx and related genes are expressed in early cardiac and visceral mesodermal progenitors. To test for an early cardiogenic function for Nkx2-5 and to examine whether cardiogenic mechanisms are conserved, we introduced the mouse Nkx2-5 gene and various mutant and chimeric derivatives into the Drosophila germline, and tested for their ability to rescue the tin mutant phenotype. While tin itself strongly rescued both heart and visceral mesoderm, Nkx2-5 rescued only visceral mesoderm. Other vertebrate ‘non-cardiac’ NK-2 genes rescued neither. We mapped the cardiogenic domain of tin to a unique region at its N terminus and, when transferred to Nkx2-5, this region conferred a strong ability to rescue heart. Thus, the cardiac and visceral mesodermal functions of NK-2 homeogenes are separable in the Drosophila assay. The results suggest that, while tin and Nkx2-5 show close functional kinship, their mode of deployment in cardiogenesis has diverged possibly because of differences in their interactions with accessory factors. The distinct cardiogenic programs in vertebrates and flies may be built upon a common and perhaps more ancient program for specification of visceral muscle.

Homeobox genes in cardiovascular development

As summarized earlier, a surprisingly large number of different homeobox genes are expressed in the developing heart. Some are clearly important, as demonstrated by mouse gene ablation studies. For example, knockout of Nkx2-5 or Hoxa-3 function is embryonic lethal due to defects in cardiovascular development. However, gene ablation studies indicate that other homeobox genes that show cardiovascular expression are either not required for heart development or their function is effectively complemented by a redundant gene activity. Given the number of closely related homeobox genes that are expressed in the heart (and the rate at which new genes are being discovered), this is very likely to be the case for at least some homeobox gene activities. At present little is known of the precise mechanism of action of homeobox genes in embryonic development. This statement applies to homeobox genes in general, not just to genes involved in cardiovascular development. There is a popular view that homeobox genes are master regulators that control expression of a large number of downstream genes. In at least some cases, e.g., the eyeless gene of Drosophila (Holder et al., 1995), homeobox genes appear to be capable of activating and maintaining a very complex developmental program. Significantly, the eyeless gene is able to initiate eye development at numerous ectopic locations. Increasing evidence, however, suggests that genes of this type may be rather rare. Certainly there is no evidence to date that any of the homeobox genes expressed in the heart are able to initiate the complete heart development pathway. This is probably best understood in the case of the tinman gene in Drosophila, which, although absolutely required for heart development, is not capable of initiating the cardiac development pathway in ectopic locations (Bodmer, 1993). This conclusion is supported by studies of the vertebrate tinman-related gene Nkx2-5. Gene ablation studies show that Nkx2-5 is essential for correct cardiac development (Lyons et al., 1995) but is not able to initiate the regulatory pathway leading to cardiac development when expressed ectopically (Cleaver et al., 1996; Chen and Fishman, 1996). If most homeodomain proteins are not direct regulators of a differentiation pathway, what is their role during organogenesis? The cardiovascular homeobox gene about which most is known at the mechanistic level is gax (Smith et al., 1997). A number of experiments indicate that the Gax protein is involved in the regulation of cell proliferation and that it interacts with components of the cell cycle regulation machinery. Indeed, over recent years, the idea that at least some homeobox genes play their role in organogenesis through regulation of proliferation has been developed in some detail by Duboule (1995). Further evidence that this mechanism of homeobox activity is important, especially during organogenesis, comes from studies of the Hox11 homeobox gene, which is absolutely required for development of the spleen in mouse (Roberts et al., 1994). Studies indicate that Hox11 is able to interact with at least two different protein phosphatases, PP2A and PP1, which in turn, are involved in cell cycle regulation (Kawabe et al., 1997). It is quite clear that research in future years will need to focus on the precise mode of action of the different homeodomain proteins if we are to understand their role in the development of the cardiovascular system.

Molecular events involved in differentiation of thyroid follicular cells

Organogenesis is a complex event, often dependent on inductive tissue interactions, that ultimately promote expression and activation of a combinatorial sequence of transcription factors which are involved in controlling migration, growth and differentiation.

Expression of NK-2 class homeobox gene Nkx2-6 in foregut endoderm and heart

NK-2 class homeobox genes are candidate patterning and lineage regulators in diverse organisms. We report here the embryonic expression pattern of murine member, Nkx2-6. In keeping with its vertebrate relatives, Nkx2-6 was transcribed in ventrolateral embryonic structures. Expression was first detected at E8.0 in endodermal walls of the foregut pocket, tissue destined to become pharyngeal floor. From E8.0-10.5, transcripts were concentrated in pharyngeal pouches and juxtaposed arch ectoderm and mesoderm, as well as in more caudal gut segments. Expression was also seen at opposite poles of the developing heart from E8-8.5 in posterior myocardial progenitors, then sinus venosa and dorsal pericardium, and from E9.5 in outflow tract myocardium.

Nkx2-9 is a novel homeobox transcription factor which demarcates ventral domains in the developing mouse CNS

Nkx homeobox transcription factors are expressed in diverse embryonic cells and presumably control cell-type specification and morphogenetic events. Nkx2-9 is a novel family member of NK2 genes which lacks the conserved TN-domain found in all hitherto known murine Nkx2 genes. The prominent expression of Nkx2-9 in ventral brain and neural tube structures defines a subset of neuronal cells along the entire neuraxis. During embryonic development, Nkx2-9-expressing cells shift from the presumptive floor plate into a more dorsolateral position of the neuroectoderm and later become limited to the ventricular zone. Nkx2-9 expression overlaps with that of Nkx2-2 but is generally broader. While initially Nkx2-9 is expressed in close proximity to sonic hedgehog, its expression domain clearly segregates from sonic hedgehog at later developmental stages. The dynamic expression pattern of Nkx2-9 in ventral domains of the CNS is consistent with a possible role in the specification of a distinct subset of neurons.

BMP-2 induces ectopic expression of cardiac lineage markers and interferes with somite formation in chicken embryos

In Drosophila induction of the homeobox gene tinman and subsequent heart formation are dependent on dpp signaling from overlying ectoderm. In order to define vertebrate heart-inducing signals we screened for dpp-homologues expressed in HH stage 4 chicken embryos. The majority of transcripts were found to be BMP-2 among several other members of the BMP family. From embryonic HH stage 4 onwards cardiogenic mesoderm appeared to be in close contact to BMP-2 expressing cells which initially were present in lateral mesoderm and subsequently after headfold formation in the pharyngeal endoderm. In order to assess the role of BMP-2 for heart formation, gastrulating chick embryos in New culture were implanted with BMP-2 producing cells. BMP-2 implantation resulted in ectopic cardiac mesoderm specification. BMP-2 was able to induce Nkx2-5 expression ectopically within the anterior head domain, while GATA-4 was also induced more caudally. Cardiogenic induction by BMP-2, however remained incomplete, since neither Nkx2-8 nor the cardiac-restricted structural gene VMHC-1 became ectopically induced. BMP-2 expressing cells implanted adjacent to paraxial mesoderm resulted in impaired somite formation and blocked the expression of marker genes, such as paraxis, Pax-3, and the forkhead gene cFKH-1. These results suggest that BMP-2 is part of the complex of cardiogenic signals and is involved in the patterning of early mesoderm similar to the role of dpp in Drosophila.

Regulation of the twist target gene tinman by modular cis-regulatory elements during early mesoderm development

The Drosophila tinman homeobox gene has a major role in early mesoderm patterning and determines the formation of visceral mesoderm, heart progenitors, specific somatic muscle precursors and glia-like mesodermal cells. These functions of tinman are reflected in its dynamic pattern of expression, which is characterized by initial widespread expression in the trunk mesoderm, then refinement to a broad dorsal mesodermal domain, and finally restricted expression in heart progenitors. Here we show that each of these phases of expression is driven by a discrete enhancer element, the first being active in the early mesoderm, the second in the dorsal mesoderm and the third in cardioblasts. We provide evidence that the early-active enhancer element is a direct target of twist, a gene encoding a basic helix-loop-helix (bHLH) protein, which is necessary for tinman activation. This 180 bp enhancer includes three E-box sequences which bind Twist protein in vitro and are essential for enhancer activity in vivo. Ectodermal misexpression of twist causes ectopic activation of this enhancer in ectodermal cells, indicating that twist is the only mesoderm-specific activator of early tinman expression. We further show that the 180 bp enhancer also includes negatively acting sequences. Binding of Even-skipped to these sequences appears to reduce twist-dependent activation in a periodic fashion, thus producing a striped tinman pattern in the early mesoderm. In addition, these sequences prevent activation of tinman by twist in a defined portion of the head mesoderm that gives rise to hemocytes. We find that this repression requires the function of buttonhead, a head-patterning gene, and that buttonhead is necessary for normal activation of the hematopoietic differentiation gene serpent in the same area. Together, our results show that tinman is controlled by an array of discrete enhancer elements that are activated successively by differential genetic inputs, as well as by closely linked activator and repressor binding sites within an early-acting enhancer, which restrict twist activity to specific areas within the twist expression domain.

Nkx2.6 expression is transiently and specifically restricted to the branchial region of pharyngeal-stage mouse embryos

The Nkx2.6 gene belongs to the NK superfamily of homeobox genes (Harvey, 1996). We report here the expression pattern of the murine Nkx2.6 gene during early mouse development, which is unique among the NK family of homeobox genes in that its expression is restricted to the very narrow development period between stages E8.5 and E10.5 of embryogenesis. The distribution of Nkx2.6 transcripts is also quite restricted spatially, with expression detected uniquely within the caudal branchial arches. Nkx2.6 is expressed in all three layers comprising the caudal branchial arches (ectoderm, mesectoderm and endoderm) with the strongest expression being detected in the surface ectoderm.

The XHex homeobox gene is expressed during development of the vascular endothelium: overexpression leads to an increase in vascular endothelial cell number

The Hex/Prh homeobox gene is expressed in a subset of adult blood cell types and may play a role in the differentiation of the myeloid and B-cell lineages. In a search for homeobox genes involved in cardiovascular development, we have independently isolated a Xenopus laevis cDNA which appears to be the amphibian orthologue of Hex/Prh. Based on high sequence similarity in a number of regions, particularly the critical homeobox, we have named this gene XHex. This developmentally regulated gene is first expressed in the dorsal endomesoderm of the gastrula stage embryo. This tissue goes on to contribute to the structures of the embryonic liver and XHex continues to be expressed in the liver throughout development. From the tailbud stage, XHex is expressed in vascular endothelial cells throughout the developing vascular network. Vascular expression of XHex is transient and commences slightly after expression of the receptor tyrosine kinase gene, flk-1, which is known to be essential for vascular development. This observation raises the possibility that XHex is one of the transcription factors that responds to the VEGF/Flk-1 signal transduction pathway leading to differentiation of vascular endothelial cells. XHex is unique amongst homeobox genes in displaying expression in the endothelial layer throughout the developing vasculature. Overexpression of XHex sequences in the frog embryo causes disruption to developing vascular structures and an increase in the number of vascular endothelial cells, suggesting a possible role in regulation of cell proliferation.