Chicken Nkx-2.8: a novel homeobox gene expressed in early heart progenitor cells and pharyngeal pouch-2 and -3 endoderm

NK2 homeobox gene cluster: Functions and roles in human diseases

NK2 genes (NKX2 gene cluster in humans) encode for homeodomain-containing transcription factors that are conserved along the phylogeny. According to the most detailed classifications, vertebrate NKX2 genes are classified into two distinct families, NK2.1 and NK2.2. The former is constituted by NKX2-1 and NKX2-4 genes, which are homologous to the Drosophila scro gene; the latter includes NKX2-2 and NKX2-8 genes, which are homologous to the Drosophila vnd gene. Conservation of these genes is not only related to molecular structure and expression, but also to biological functions. In Drosophila and vertebrates, NK2 genes share roles in the development of ventral regions of the central nervous system. In vertebrates, NKX2 genes have a relevant role in the development of several other organs such as the thyroid, lung, and pancreas. Loss-of-function mutations in NKX2-1 and NKX2-2 are the monogenic cause of the brain-lung-thyroid syndrome and neonatal diabetes, respectively. Alterations in NKX2-4 and NKX2-8 genes may play a role in multifactorial diseases, autism spectrum disorder, and neural tube defects, respectively. NKX2-1, NKX2-2, and NKX2-8 are expressed in various cancer types as either oncogenes or tumor suppressor genes. Several data indicate that evaluation of their expression in tumors has diagnostic and/or prognostic value.

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.

Channelopathy as a SUDEP Biomarker in Dravet Syndrome Patient-Derived Cardiac Myocytes

Dravet syndrome (DS) is a severe developmental and epileptic encephalopathy with a high incidence of sudden unexpected death in epilepsy (SUDEP). Most DS patients carry de novo variants in SCN1A, resulting in Nav1.1 haploinsufficiency. Because SCN1A is expressed in heart and in brain, we proposed that cardiac arrhythmia contributes to SUDEP in DS. We generated DS patient and control induced pluripotent stem cell-derived cardiac myocytes (iPSC-CMs). We observed increased sodium current (INa) and spontaneous contraction rates in DS patient iPSC-CMs versus controls. For the subject with the largest increase in INa, cardiac abnormalities were revealed upon clinical evaluation. Generation of a CRISPR gene-edited heterozygous SCN1A deletion in control iPSCs increased INa density in iPSC-CMs similar to that seen in patient cells. Thus, the high risk of SUDEP in DS may result from a predisposition to cardiac arrhythmias in addition to seizures, reflecting expression of SCN1A in heart and brain.

NKL homeobox gene activities in hematopoietic stem cells, T-cell development and T-cell leukemia

T-cell acute lymphoblastic leukemia (T-ALL) cells represent developmentally arrested T-cell progenitors, subsets of which aberrantly express homeobox genes of the NKL subclass, including TLX1, TLX3, NKX2-1, NKX2-5, NKX3-1 and MSX1. Here, we analyzed the transcriptional landscape of all 48 members of the NKL homeobox gene subclass in CD34+ hematopoietic stem and progenitor cells (HSPCs) and during lymphopoiesis, identifying activities of nine particular genes. Four of these were expressed in HSPCs (HHEX, HLX1, NKX2-3 and NKX3-1) and three in common lymphoid progenitors (HHEX, HLX1 and MSX1). Interestingly, our data indicated downregulation of NKL homeobox gene transcripts in late progenitors and mature T-cells, a phenomenon which might explain the oncogenic impact of this group of genes in T-ALL. Using MSX1-expressing T-ALL cell lines as models, we showed that HHEX activates while HLX1, NKX2-3 and NKX3-1 repress MSX1 transcription, demonstrating the mutual regulation and differential activities of these homeobox genes. Analysis of a public T-ALL expression profiling data set comprising 117 patient samples identified 20 aberrantly activated members of the NKL subclass, extending the number of known NKL homeobox oncogene candidates. While 7/20 genes were also active during hematopoiesis, the remaining 13 showed ectopic expression. Finally, comparative analyses of T-ALL patient and cell line profiling data of NKL-positive and NKL-negative samples indicated absence of shared target genes but instead highlighted deregulation of apoptosis as common oncogenic effect. Taken together, we present a comprehensive survey of NKL homeobox genes in early hematopoiesis, T-cell development and T-ALL, showing that these genes generate an NKL-code for the diverse stages of lymphoid development which might be fundamental for regular differentiation.

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.

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.

Missense mutation in the transcription factor NKX2-5: a novel molecular event in the pathogenesis of thyroid dysgenesis

CONTEXT

Congenital hypothyroidism (CH) is a common endocrine disorder with an incidence of 1:3000-4000 at birth. In 80-85% of cases, CH is caused by defects in thyroid organogenesis, resulting in absent, ectopically located, and/or severely reduced gland [thyroid dysgenesis (TD)]. Mutations in genes controlling thyroid development have demonstrated that in a few cases, TD is a Mendelian trait. However, accumulating evidence supports the view that the genetics of TD are complex, possibly with a polygenic/multifactorial basis. A higher prevalence of congenital heart disease has been documented in children with CH than in the general population. Such an association suggests a possible pathogenic role of genes involved in both heart and thyroid development. NKX2-5 encodes a homeodomain-containing transcription factor with a major role in heart development, and mutations affecting this gene have been reported in individuals with congenital heart disease.

OBJECTIVE

In the present work we investigated the possible involvement of NKX2-5 mutations in TD.

RESULTS

Our results indicate that Nkx2-5(-/-) embryos exhibit thyroid bud hypoplasia, providing evidence that NKX2-5 plays a role in thyroid organogenesis and that NKX2-5 mutations contribute to TD. NKX2-5 mutational screening in 241 patients with TD allowed the identification of three heterozygous missense changes (R25C, A119S, and R161P) in four patients with TD. Functional characterization of the three mutations demonstrated reduced DNA binding and/or transactivation properties, with a dominant-negative effect on wild-type NKX2-5.

CONCLUSION

Our results suggest a previously unknown role of NKX2-5 in the pathogenesis of TD.

The genesis and evolution of homeobox gene clusters

Once called the 'Rosetta stone' of developmental biology, the homeobox continues to fascinate both evolutionary and developmental biologists. The birth of the homeotic, or Hox, gene cluster, and its subsequent evolution, has been crucial in mediating the major transitions in metazoan body plan. Comparative genomics studies indicate that the more recently discovered ParaHox and NK clusters were linked to the Hox cluster early in evolution, and that together they constituted a 'megacluster' of homeobox genes that conspicuously contributed to body-plan evolution.

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.

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.

Xenopus bagpipe-related gene, koza, may play a role in regulation of cell proliferation

The homeobox gene koza is a new member of the vertebrate bagpipe-related gene family. Embryonic expression of koza is observed at highest levels in the muscle layer of the somites and, during later development, is restricted to the lateral somitic cells, which correspond to slow twitch muscle tissue. Expression of koza is also observed in the myocardial layer of the heart and in the cement gland. In each of these tissues, koza transcription commences only after the expression of terminal differentiation markers. By injection of synthetic mRNA, we show that overexpression of koza leads to an apparent decrease in the number of cells in the somites. No reduction in cell number is observed when koza is present in neural tissues, suggesting that koza exhibits some tissue specificity in regulation of cell proliferation. Embryonic manipulations show that restriction of koza expression to the slow twitch muscle layer is independent of axial structures but is, at least partly, regulated by signals arising in ectodermal tissue. Finally, in Drosophila, bagpipe expression is regulated by the hedgehog signaling pathway. By using ectopic expression, we show that koza transcription is positively regulated by banded hedgehog. This result indicates that regulation of bagpipe expression by hedgehog signaling is evolutionarily conserved.

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.

Murine Tbx2 contains domains that activate and repress gene transcription

T-box (Tbx) genes represent a phylogenetically conserved family of transcription factors that play important roles during embryonic development. Tbx family members have been shown to either activate or inhibit gene expression. However, little is known about the domains within Tbx proteins responsible for mediating gene transcription. While Tbx2 is known to repress gene expression, the domain(s) within Tbx2 remains poorly defined. Deletion of the carboxy-terminus of Tbx2, which contains a domain that is highly conserved with Tbx3 and ET, which has been demonstrated to contain a repression domain, only minimally diminishes the ability of Tbx2 to repress gene expression. However, in combination with the carboxy-terminal truncation, deletion of the amino acids located amino-terminal to the T-box abolished the ability of Tbx2 to repress gene expression. Both of these domains were capable of repressing gene expression when linked to the GAL4 DNA binding domain. In contrast to these two repression domains, the T-box was capable of weakly activating gene expression depending on the promoter context. Deletion analysis of the T-box suggests that this activation domain is located in the amino-terminal end of the T-box. These results reveal a novel transcription repression domain, confirm the presence of a previously implicated domain, and suggest a novel role for the T-box. Taken together, these results provide the basis for understanding the molecular mechanism whereby Tbx2 regulates gene expression and subsequently controls embryonic development.

Characterization of the mouse peroxisome proliferator-activated receptor delta gene

Peroxisome proliferator-activated receptors (alpha, delta and gamma) are ligand-activated transcription factors that are involved in multiple cellular responses. The PPARdelta subtype is the least understood of all PPAR subtypes. PPARdelta is activated by unsaturated fatty acids, PGI2, and by synthetic ligands. PPARdelta-regulated genes have not been identified and the factors that control PPARdelta expression are not known. The gene that encodes the mouse PPARdelta gene is contained in >30 kb DNA sequence and organized in eight exons, six of which encode the PPARdelta receptor. A PPARdelta-luciferase reporter containing 694 bp 5' upstream regulatory and 127 bp untranslated was introduced to primary brain cultures to begin a characterization of the DNA sequences that mediate transcriptional regulation of PPARdelta. PPARdelta-luciferase expression was 10 times higher in oligodendrocyte-containing mature cultures than in immature cultures, indicating that PPARdelta may play a role during oligodendrocyte migration, proliferation, and/or maturation.

The DNA glycosylase T:G mismatch-specific thymine DNA glycosylase represses thyroid transcription factor-1-activated transcription

The transcription factor thyroid transcription factor-1 (TTF-1) is a homeodomain-containing protein that belongs to the NK2 family of genes involved in organogenesis. TTF-1 is required for normal development of the forebrain, lung, and thyroid. In a search for factors that regulate TTF-1 transcriptional activity, we isolated three genes (T:G mismatch-specific thymine DNA glycosylase (TDG), homeodomain-interacting protein kinase 2 (HIPK2), and Ajuba), whose products can interact with TTF-1 in yeast and in mammalian cells. TDG is an enzyme involved in base excision repair. In the present paper, we show that TDG acts as a strong repressor of TTF-1 transcriptional activity in a dose-dependent manner, while HIPK2 and Ajuba display no effect on TTF-1 activity, at least under the tested conditions. TDG-mediated inhibition occurs specifically on TTF-1-responsive promoters in thyroid and non thyroid cells. TDG associates with TTF-1 in mammalian cells through the TTF-1 carboxyl-terminal activation domain and is independent of the homeodomain. These findings reveal a previously unsuspected role for the repair enzyme TDG as a transcriptional repressor and open new routes toward the understanding of the regulation of TTF-1 transcriptional activity.

Initiation and early patterning of the endoderm

We review the early stages of endoderm formation in the major animal models. In Amphibia maternal molecules are important in initiating endoderm formation. This is followed by successive signaling events that establish and then pattern the endoderm. In other organisms there are differences in endodermal development, particularly in the initial, prephylotypic stages. Later many of the same key families of transcription factors and signaling cassettes are used in all animals, but more work will be needed to establish exact evolutionary homologies.

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.

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.

Vertebrate homologs of tinman and bagpipe: roles of the homeobox genes in cardiovascular development

In Drosophila, dorsal mesodermal specification is regulated by the homeobox genes tinman and bagpipe. Vertebrate homologs of tinman and bagpipe have been isolated in various species. Moreover, there are at least four different genes related to tinman in the vertebrate, which indicates that this gene has been duplicated during evolution. One of the murine homologs of tinman is the cardiac homeobox gene Csx or Nkx2.5. Gene targeting of Csx/Nkx2.5 showed that this gene is required for completion of the looping morphogenesis of the heart. However, it is not essential for the specification of the heart cell lineage. Early cardiac development might therefore be regulated by other genes, which may act either independently or in concert with Csx/Nkx2.5. Possible candidates might be other members of the NK2 class of homeobox proteins like Tix/Nkx2.6, Nkx2.3, nkx2.7, or cNkx2.8. Murine Tix/Nkx2.6 mRNA has been detected in the heart and pharyngeal endoderm (this study). Xenopus XNkx2.3 and chicken cNkx2.3 are expressed in the heart as well as in pharyngeal and gut endoderm. In contrast, murine Nkx2.3 is expressed in the gut and pharyngeal arches but not the heart. In zebrafish and chicken, two new NK-2 class homeoproteins, nkx2.7 and cNkx2.8, have been identified. Zebrafish nkx2.7 is expressed in both, the heart and pharyngeal endoderm. In the chicken, cNkx2.8 is expressed in the heart primordia and the primitive heart tube and becomes undetectable after looping. No murine homologs of nkx2.7 or cNkx2.8 have been found so far. The overlapping expression pattern of NK2 class homeobox genes in the heart and the pharynx may suggest a common origin of these two organs. In the Drosophila genome, the tinman gene is linked to another NK family gene named bagpipe. A murine homolog of bagpipe, Bax/Nkx3.1, is expressed in somites, blood vessels, and the male reproductive system during embryogenesis (this study), suggesting that this gene's function may be relevant for the development of these organs. A bagpipe homolog in Xenopus, Xbap, is expressed in the gut masculature and a region of the facial cartilage during development. In this paper, we discuss molecular mechanisms of cardiovascular development with particular emphasis on roles of transcription factors.

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.

Heart development in Drosophila and vertebrates: conservation of molecular mechanisms

Vertebrate and insect (Drosophila) hearts look and function quite differently from each other. Nevertheless, during embryogenesis their mesodermal origin and initial assembly into a linear heart tube are comparable in many respects. In the past few years, numerous gene functions have been identified that are utilized by both vertebrates and Drosophila for the specification and differentiation of the heart progenitor cells. These studies have begun with the discovery of the homeobox gene tinman in Drosophila and its vertebrate counterparts. By now, there is also evidence that MEF2 transcription factors and TGF-beta signaling have cardiogenic functions in both these systems. Perhaps in a few years, the GATA and HAND transcription factors and Wnt signaling, which currently only have a demonstrated cardiogenic function in one of the systems, may also be part of this group. One of the pressing but still wide open questions is if the spectrum of targets for these transcription factors and signaling pathways is also conserved.

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.

Phenotypic characterization of the murine Nkx2.6 homeobox gene by gene targeting

The NK-2 homeobox genes have been shown to play critical roles in the development of specific organs and tissues. Nkx2.6 is a member of the NK-2 homeobox gene family and is most closely related to the Drosophila tinman gene. Nkx2.6 is expressed in the caudal pharyngeal pouches, the caudal heart progenitors, the sinus venosus, and the outflow tract of the heart and in a short segment of the gut at early stages of embryogenesis. To investigate the function of Nkx2.6 in vivo, we generated mice with null mutations of Nkx2.6 by the gene targeting technique. Homozygous Nkx2.6 mutant mice were viable and fertile. There were no obvious abnormalities in the caudal pharyngeal pouch derivatives (the thymus, parathyroid glands, and thyroid gland), heart, and gut. Expression of Nkx2.6 overlaps that of Nkx2.5 in the pharynx and heart and that of Nkx2.3 in the pharynx. Interestingly, in mutant embryos homozygous for Nkx2.6, Nkx2.5 expression extended to the lateral side of the pharynx, suggesting a compensatory function of Nkx2.5 in the mutant pharyngeal pouches.

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.

An Nkx-dependent enhancer regulates cGATA-6 gene expression during early stages of heart development

The evolutionarily conserved GATA-6 transcription factor is an early and persistent marker of heart development in diverse vertebrate species. We previously found evidence for a functionally conserved heart-specific enhancer upstream of the chicken GATA-6 (cGATA-6) gene and in the present study we used transgenic mouse assays to further characterize this regulatory module. We show that this enhancer is activated in committed precursor cells within the cardiac crescent and that it remains active in essentially all cardiogenic cells through the linear heart stage. Although this enhancer can account for cGATA-6 gene expression early in the cardiogenic program, it is not able to maintain expression throughout the heart later in development. In particular, the enhancer is sequentially downregulated along the posterior to anterior axis, with activity becoming confined to outflow tract myocardium. Enhancers with similar properties have been shown to regulate the early heart-restricted expression of the mouse Nkx2.5 transcription factor gene. Whereas these Nkx2.5 enhancers are GATA-dependent, we show that the cGATA-6 enhancer is Nkx-dependent. We speculate that these enhancers are silenced to allow GATA-6 and Nkx2.5 gene expression to be governed by region-specific enhancers in the multichambered heart.

Expression and functional analysis of Cititf1, an ascidian NK-2 class gene, suggest its role in endoderm development

In solitary ascidians the fate of endoderm is determined at a very early stage of development and depends on cytoplasmic factors whose nature has not been determined. We have isolated a member of the NK-2 gene family, Cititf1, from the ascidian Ciona intestinalis, showing high sequence homology to mammalian TITF1. The Cititf1 gene was expressed in all endodermal precursors at the pregastrula and gastrula stages, and is thus the first specific regulatory endodermal marker to be isolated from an ascidian. Cititf1 expression was downregulated at the end of gastrulation to reappear at middle tailbud and larval stages in the most anterior and ventral parts of head endoderm, regions which give rise, after metamorphosis, to the adult endostyle, where Cititf1 mRNA was still present. Microinjection of Cititf1 mRNA into fertilized eggs resulted in tadpole larvae with abnormalities in head-trunk development consequent to the formation of excess endoderm, perhaps due to recruitment of notochord precursors to an endodermal fate. These data suggest that Cititf1 plays an important role in normal endoderm differentiation during ascidian embryogenesis.

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.

A novel role for cardiac neural crest in heart development

It is well known that cardiac neural crest participates in development of the cardiac outflow septation and patterning of the great arteries. Less well known is that ablation of the cardiac neural crest leads to a primary myocardial dysfunction. Recent data suggests that the myocardial dysfunction occurs because of the absence of an interaction of neural crest and pharyngeal endoderm to alter signaling from the endoderm. Continuation of an FGF-like signal from the endoderm past a precise time in development appears to be detrimental to myocardial maturation.

WNT11 promotes cardiac tissue formation of early mesoderm

Cardiac tissue in the bird is derived from paired regions of lateral mesoderm within the anterior half of the embryo (Rawles [1943] Physiol. Zool. 16:22-42; Stalsberg and DeHaan [1969] Dev. Biol. 19:128-159). Previously, we reported that WNT11 is expressed in early avian mesoderm in a pattern that overlaps with the precardiac regions. To examine whether this molecule may play a role in promoting cardiogenesis, we cultured tissue explants from microdissected HH stage 4, 5, and 6 quail embryos. The isolated tissue consisted of both the mesoderm and endoderm layers from either anterior precardiac or posterior noncardiogenic regions of the embryo. As a necessary control for examining the ability of WNT11 to convert noncardiogenic mesoderm to cardiac tissue, we compared the cardiogenic potential of anterior and posterior regions. For stages 5 and 6, our results were consistent with what has been previously reported (Rawles [1943] Physiol. Zool. 16:22-42; Sugi and Lough [1994] Dev. Dyn. 200:155-162); as anterior mesoderm becomes contractile, while posterior mesoderm does not produce cardiac tissue. Surprisingly, when we examined stage 4 embryos both anterior and posterior regions gave rise to cardiac tissue in culture. To determine whether WNT11 could promote cardiac differentiation in tissue that was noncardiogenic, this molecule was ectopically expressed or added to mesoderm/endoderm explants obtained from stage 5 or 6 posterior tissue. Transfection of stage 5 posterior tissue with a WNT11 expression plasmid provoked the appearance of cardiomyocytes in 33% of the explants; half of which were contractile. Similarly transfected stage 6 posterior explants did not demonstrate cardiac differentiation. More dramatic results were obtained when noncardiogenic tissue was exposed to conditioned media containing soluble WNT11; as 63% and 33% of posterior stage 5- or stage 6-derived explants underwent cardiac differentiation. Together, these results indicate that WNT11 can promote cardiac development within noncardiac tissue. The expression of WNT11 in anterior mesoderm of early gastrula stage embryos suggests it may play a role in the formation of the vertebrate heart. Dev Dyn 1999;216:45-58.

Identification of upstream regulatory regions in the heart-expressed homeobox gene Nkx2-5

Nkx2-5 marks the earliest recognizable cardiac progenitor cells, and is activated in response to inductive signals involved in lineage specification. Nkx2-5 is also expressed in the developing foregut, thyroid, spleen, stomach and tongue. One approach to elucidate the signals involved in cardiogenesis was to examine the transcriptional regulation of early lineage markers such as Nkx2-5. We generated F0 transgenic mice, which carry Nkx2-5 flanking sequences linked to a lacZ reporter gene. We identified multiple regulatory regions located within the proximal 10.7 kb of the Nkx2-5 gene. In addition to a proximal promoter, we identified a second promoter and a novel upstream exon that could participate in the regulation of Nkx2-5 transcription. Although used rarely in normal development, this novel exon could be spliced into the Nkx2-5 coding region in several ways, thereby potentially creating novel Nkx2-5 protein isoforms, whose transcriptional activity is greatly diminished as compared to wild-type Nkx2-5. An enhancer that directs expression in pharynx, spleen, thyroid and stomach was identified within 3.5 kb of exon 1 between the coding exon 1 and the novel upstream exon 1a. Two or more enhancers upstream of exon 1a were capable of driving expression in the cardiac crescent, throughout the myocardium of the early heart tube, then in the outflow tract and right ventricle of the looped heart tube. A negative element was also located upstream of exon1a, which interacted in complex ways with enhancers to direct correct spatial expression. In addition, potential autoregulatory elements can be cooperatively stimulated by Nkx2-5 and GATA-4. Our results demonstrate that a complex suite of interacting regulatory domains regulate Nkx2-5 transcription. Dissection of these elements should reveal essential features of cardiac induction and positive and negative signaling within the cardiac field.

Identification of the in vivo casein kinase II phosphorylation site within the homeodomain of the cardiac tisue-specifying homeobox gene product Csx/Nkx2.5

Csx/Nkx2.5, a member of the homeodomain-containing transcription factors, serves critical developmental functions in heart formation in vertebrates and nonvertebrates. In this study the putative nuclear localization signal (NLS) of Csx/Nkx2.5 was identified by site-directed mutagenesis to the amino terminus of the homeodomain, which is conserved in almost all homeodomain proteins. When the putative NLS of Csx/Nkx2.5 was mutated a significant amount of the cytoplasmically localized Csx/Nkx2.5 was unphosphorylated, in contrast to the nuclearly localized Csx/Nkx2.5, which is serine- and threonine-phosphorylated, suggesting that Csx/Nkx2.5 phosphorylation is regulated, at least in part, by intracellular localization. Tryptic phosphopeptide mapping indicated that Csx/Nkx2.5 has at least five phosphorylation sites. Using in-gel kinase assays, we detected a Csx/Nkx2.5 kinase whose molecular mass is approximately 40 kDa in both cytoplasmic and nuclear extracts. Mutational analysis and in vitro kinase assays suggested that this 40-kDa Csx/Nkx2.5 kinase is a catalytic subunit of casein kinase II (CKII) that phosphorylates the serine residue between the first and second helix of the homeodomain. This CKII site is phosphorylated in vivo. CKII-dependent phosphorylation of the homeodomain increased Csx/Nkx2. 5 DNA binding. Serine-to-alanine mutation at the CKII phosphorylation site reduced transcriptional activity when the carboxyl-terminal repressor domain was deleted. Although the precise biological function of Csx/Nkx2.5 phosphorylation by CKII remains to be determined, it may play an important role, as this CKII phosphorylation site within the homeodomain is fully conserved in all known members of the NK2 family of the homeobox genes.

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.

GATA-4 and Nkx-2.5 coactivate Nkx-2 DNA binding targets: role for regulating early cardiac gene expression

The cardiogenic homeodomain factor Nkx-2.5 and serum response factor (SRF) provide strong transcriptional coactivation of the cardiac alpha-actin (alphaCA) promoter in fibroblasts (C. Y. Chen and R. J. Schwartz, Mol. Cell. Biol. 16:6372-6384, 1996). We demonstrate here that Nkx-2.5 also cooperates with GATA-4, a dual C-4 zinc finger transcription factor expressed in early cardiac progenitor cells, to activate the alphaCA promoter and a minimal promoter, containing only multimerized Nkx-2.5 DNA binding sites (NKEs), in heterologous CV-1 fibroblasts. Transcriptional activity requires the N-terminal activation domain of Nkx-2.5 and Nkx-2.5 binding activity through its homeodomain but does not require GATA-4's activation domain. The minimal interactive regions were mapped to the homeodomain of Nkx-2.5 and the second zinc finger of GATA-4. Removal of Nkx-2.5's C-terminal inhibitory domain stimulated robust transcriptional activity, comparable to the effects of GATA-4 on wild-type Nkx-2.5, which in part facilitated Nkx-2.5 DNA binding activity. We postulate the following simple model: GATA-4 induces a conformational change in Nkx-2.5 that displaces the C-terminal inhibitory domain, thus eliciting transcriptional activation of promoters containing Nkx-2.5 DNA binding targets. Therefore, alphaCa promoter activity appears to be regulated through the combinatorial interactions of at least three cardiac tissue-enriched transcription factors, Nkx-2.5, GATA-4, and SRF.

Cardiac and extracardiac expression of Csx/Nkx2.5 homeodomain protein

Csx/Nkx2.5 is an evolutionary conserved homeobox gene related to the Drosophila tinman gene, which is essential for the dorsal mesoderm formation. Expression of Csx/Nkx2.5 mRNA is the earliest marker for heart precursor cells in all vertebrates so far examined. Previous studies have demonstrated that Csx/Nkx2.5 mRNA is highly expressed in the heart and at lower levels in the spleen, tongue, stomach, and thyroid in the murine embryo. Since some developmental genes are regulated by posttranscriptional mechanisms, we analyzed the developmental pattern of Csx protein expression at the single-cell level using Csx-specific antibodies. Immunohistochemical analysis of murine embryos at 7.8 days post coitum revealed that Csx protein is strongly expressed in the nucleus of endodermal and mesodermal cells in the cardiogenic plate. Subsequently, in the heart, Csx protein was detected only in the nucleus of myocytes of the atrium and the ventricle through the adult stage. During the fetal period, Csx protein expression in the nucleus was also noted in the spleen, stomach, liver, tongue, and anterior larynx. Unexpectedly, confocal microscopy revealed that Csx immunoreactivity was detected only in the cytoplasm of a subset of cranial skeletal muscles. Csx protein was not detected in the thyroid glands. The expression of Csx protein in all organs was markedly downregulated after birth except in the heart. These results raise the possibility that Csx/Nkx2.5 may play a role in the early developmental process of multiple tissues in addition to its role in early heart development.

Site-directed mutations in the vnd/NK-2 homeodomain. Basis of variations in structure and sequence-specific DNA binding

Secondary structures, DNA binding properties, and thermal denaturation behavior of six site-directed mutant homeodomains encoded by the vnd/NK-2 gene from Drosophila melanogaster are described. Three single site H52R, Y54M, and T56W mutations, two double site H52R/T56W and Y54M/T56W mutations, and one triple site H52R/Y54M/T56W mutation were investigated. These positions were chosen based on their variability across homeodomains displaying differences in secondary structure and DNA binding specificity. Multidimensional NMR, electrophoretic mobility shift assays, and circular dichroism spectropolarimetry studies were carried out on recombinant 80-amino acid residue proteins containing the homeodomain. Position 56, but more importantly position 56 in combination with position 52, plays an important role in determining the length of the recognition helix. The H52R mutation alone does not affect the length of this helix but does increase the thermal stability. Introduction of site mutations at positions 52 and 56 in vnd/NK-2 does not modify their high affinity binding to the 18-base pair DNA fragment containing the vnd/NK-2 consensus binding sequence, CAAGTG. Site mutations involving position 54 (Y54M, Y54M/T56W, and H52R/Y54M/T56W) all show a decrease of 1 order of magnitude in their binding affinity. The roles in structure and sequence specificity of individual atom-atom interactions are described.

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.