Chick NKx-2.3 represents a novel family member of vertebrate homologues to the Drosophila homeobox gene tinman: differential expression of cNKx-2.3 and cNKx-2.5 during heart and gut development

Functional analysis of novel genetic variants of NKX2-5 associated with nonsyndromic congenital heart disease

NKX2-5, a master cardiac regulatory transcription factor was the first known genetic cause of congenital heart diseases (CHDs). To further investigate its role in CHD pathogenesis, we performed mutational screening of 285 CHD probands and 200 healthy controls. Five coding sequence variants were identified in six CHD cases (2.1%), including three in the N-terminal region (p.A61G, p.R95L, and p.E131K) and one each in homeodomain (HD) (p.A148E) and tyrosine-rich domain (p.P247A). Variant-p.A148E showed tertiary structure changes and differential DNA binding affinity of mutant compared to wild type. Two N-terminal variants-p.A61G and p.E131K along with HD variant p.A148E demonstrated significantly reduced transcriptional activity of Nppa and Actc1 promoters in dual luciferase promoter assay supported by their reduced expression in qRT-PCR. Nonetheless, variant p.R95L affected the synergy of NKX2-5 with serum response factor and TBX5 leading to significantly decreased Actc1 promoter activity depicting a distinctive role of this region. The aberrant expression of other target genes-Irx4, Mef2c, Bmp10, Myh6, Myh7, and Myocd is also observed in response to NKX2-5 variants, possibly due to the defective gene regulatory network. Severely impaired downstream promoter activities and abnormal expression of target genes due to N-terminal variants supports the emerging role of this region during cardiac-developmental pathways.

Cardiac remodeling in response to embryonic crude oil exposure involves unconventional NKX family members and innate immunity genes

Cardiac remodeling results from both physiological and pathological stimuli. Compared to mammals, fish hearts show a broader array of remodeling changes in response to environmental influences, providing exceptional models for dissecting the molecular and cellular bases of cardiac remodeling. We recently characterized a form of pathological remodeling in juvenile pink salmon (Oncorhynchus gorbuscha) in response to crude oil exposure during embryonic cardiogenesis. In the absence of overt pathology (cardiomyocyte death or inflammatory infiltrate), cardiac ventricles in exposed fish showed altered shape, reduced thickness of compact myocardium, and hypertrophic changes in spongy, trabeculated myocardium. Here we used RNA sequencing to characterize molecular pathways underlying these defects. In juvenile ventricular cardiomyocytes, antecedent embryonic oil exposure led to dose-dependent up-regulation of genes involved in innate immunity and two NKX homeobox transcription factors not previously associated with cardiomyocytes, nkx2.3 and nkx3.3. Absent from mammalian genomes, the latter is largely uncharacterized. In zebrafish embryos nkx3.3 demonstrated a potent effect on cardiac morphogenesis, equivalent to nkx2.5, the primary transcription factor associated with ventricular cardiomyocyte identity. The role of nkx3.3 in heart growth is potentially linked to the unique regenerative capacity of fish and amphibians. Moreover, these findings support a cardiomyocyte-intrinsic role for innate immune response genes in pathological hypertrophy. This study demonstrates how an expanding mechanistic understanding of environmental pollution impacts – i.e., the chemical perturbation of biological systems – can ultimately yield new insights into fundamental biological processes.

Developmental Mechanism of Limb Field Specification along the Anterior-Posterior Axis during Vertebrate Evolution

In gnathostomes, limb buds arise from the lateral plate mesoderm at discrete positions along the body axis. Specification of these limb-forming fields can be subdivided into several steps. The lateral plate mesoderm is regionalized into the anterior lateral plate mesoderm (ALPM; cardiac mesoderm) and the posterior lateral plate mesoderm (PLPM). Subsequently, Hox genes appear in a nested fashion in the PLPM and provide positional information along the body axis. The lateral plate mesoderm then splits into the somatic and splanchnic layers. In the somatic layer of the PLPM, the expression of limb initiation genes appears in the limb-forming region, leading to limb bud initiation. Furthermore, past and current work in limbless amphioxus and lampreys suggests that evolutionary changes in developmental programs occurred during the acquisition of paired fins during vertebrate evolution. This review presents these recent advances and discusses the mechanisms of limb field specification during development and evolution, with a focus on the role of Hox genes in this process.

Mesenchymal-epithelial interactions during digestive tract development and epithelial stem cell regeneration

The gastrointestinal tract develops from a simple and uniform tube into a complex organ with specific differentiation patterns along the anterior-posterior and dorso-ventral axes of asymmetry. It is derived from all three germ layers and their cross-talk is important for the regulated development of fetal and adult gastrointestinal structures and organs. Signals from the adjacent mesoderm are essential for the morphogenesis of the overlying epithelium. These mesenchymal-epithelial interactions govern the development and regionalization of the different gastrointestinal epithelia and involve most of the key morphogens and signaling pathways, such as the Hedgehog, BMPs, Notch, WNT, HOX, SOX and FOXF cascades. Moreover, the mechanisms underlying mesenchyme differentiation into smooth muscle cells influence the regionalization of the gastrointestinal epithelium through interactions with the enteric nervous system. In the neonatal and adult gastrointestinal tract, mesenchymal-epithelial interactions are essential for the maintenance of the epithelial regionalization and digestive epithelial homeostasis. Disruption of these interactions is also associated with bowel dysfunction potentially leading to epithelial tumor development. In this review, we will discuss various aspects of the mesenchymal-epithelial interactions observed during digestive epithelium development and differentiation and also during epithelial stem cell regeneration.

The lateral plate mesoderm: a novel source of skeletal muscle

It has been established in the last century that the skeletal muscle cells of vertebrates originate from the paraxial mesoderm. However, recently the lateral plate mesoderm has been identified as a novel source of the skeletal muscle. The branchiomeric muscles, such as masticatory and facial muscles, receive muscle progenitor cells from both the cranial paraxial mesoderm and lateral plate mesoderm. At the occipital level, the lateral plate mesoderm is the sole source of the muscle progenitors of the dorsolateral neck muscle, such as trapezius and sternocleidomastoideus in mammals and cucullaris in birds. The lateral plate mesoderm requires a longer time for generating skeletal muscle cells than the somites. The myogenesis of the lateral plate is determined early, but not cell autonomously and requires local signals. Lateral plate myogenesis is regulated by mechanisms controlling the cranial myogenesis. The connective tissue of the lateral plate-derived muscle is formed by the cranial neural crest. Although the cranial neural crest cells do not control the early myogenesis, they regulate the patterning of the branchiomeric muscles and the cucullaris muscle. Although satellite cells derived from the cranial lateral plate show distinct properties from those of the trunk, they can respond to local signals and generate myofibers for injured muscles in the limbs. In this review, we key feature in detail the muscle forming properties of the lateral plate mesoderm and propose models of how the myogenic fate may have arisen.

Enteric neural crest cells regulate vertebrate stomach patterning and differentiation

In vertebrates, the digestive tract develops from a uniform structure where reciprocal epithelial-mesenchymal interactions pattern this complex organ into regions with specific morphologies and functions. Concomitant with these early patterning events, the primitive GI tract is colonized by the vagal enteric neural crest cells (vENCCs), a population of cells that will give rise to the enteric nervous system (ENS), the intrinsic innervation of the GI tract. The influence of vENCCs on early patterning and differentiation of the GI tract has never been evaluated. In this study, we report that a crucial number of vENCCs is required for proper chick stomach development, patterning and differentiation. We show that reducing the number of vENCCs by performing vENCC ablations induces sustained activation of the BMP and Notch pathways in the stomach mesenchyme and impairs smooth muscle development. A reduction in vENCCs also leads to the transdifferentiation of the stomach into a stomach-intestinal mixed phenotype. In addition, sustained Notch signaling activity in the stomach mesenchyme phenocopies the defects observed in vENCC-ablated stomachs, indicating that inhibition of the Notch signaling pathway is essential for stomach patterning and differentiation. Finally, we report that a crucial number of vENCCs is also required for maintenance of stomach identity and differentiation through inhibition of the Notch signaling pathway. Altogether, our data reveal that, through the regulation of mesenchyme identity, vENCCs act as a new mediator in the mesenchymal-epithelial interactions that control stomach development.

Characteristics of Japanese inflammatory bowel disease susceptibility loci

BACKGROUND

There are substantial differences in inflammatory bowel disease (IBD) genetics depending on the populations examined. We aimed to identify Japanese population-specific or true culprit susceptibility genes through a meta-analysis of past genetic studies of Japanese IBD.

METHODS

For this study, we reviewed 2,703 articles. The review process consisted of three screening stages: we initially searched for relevant studies and then relevant single nucleotide polymorphisms (SNPs). Finally, we adjusted them for the meta-analysis. To maximize our chances of analysis, we introduced proxy SNPs during the first stage. To minimize publication bias, no significant SNPs and solitary SNPs without pairs were combined to be reconsidered during the third stage. Additionally, two SNPs were newly genotyped. Finally, we conducted a meta-analysis of 37 published studies in 50 SNPs located at 22 loci corresponding to the total number of 4,853 Crohn's disease (CD), 5,612 ulcerative colitis (UC) patients, and 14,239 healthy controls.

RESULTS

We confirmed that the NKX2-3 polymorphism is associated with common susceptibility to IBD and that HLA-DRB1*0450 alleles increase susceptibility to CD but reduce risk for UC while HLA-DRB1*1502 alleles increase susceptibility to UC but reduce CD risk. Moreover, we found individual disease risk loci: TNFSF15 and TNFα to CD and HLA-B*5201, and NFKBIL1 to UC. The genetic risk of HLA was substantially high (odds ratios ranged from 1.54 to 2.69) while that of common susceptibility loci to IBD was modest (odds ratio ranged from 1.13 to 1.24).

CONCLUSIONS

Results indicate that Japanese IBD susceptibility loci identified by the meta-analysis are closely associated with the HLA regions.

Molecular and evolutionary basis of limb field specification and limb initiation

Specification of limb field and initiation of limb development involve multiple steps, each of which is tightly regulated both spatially and temporally. Recent developmental analyses on various vertebrates have provided insights into the molecular mechanisms that specify limb field and have revealed several genetic interactions of signals involved in limb initiation processes. Furthermore, new approaches to the study of the developmental mechanisms of the lateral plate mesoderm of amphioxus and lamprey embryos have given us clues to understand the evolutionary scenarios that led to the acquisition of paired appendages during evolution. This review highlights such recent findings and discusses the mechanisms of limb field specification and limb bud initiation during development and evolution.

Genes differentially regulated by NKX2-3 in B cells between ulcerative colitis and Crohn's disease patients and possible involvement of EGR1

Ulcerative colitis (UC) and Crohn's disease (CD) are two related yet different forms of chronic intestinal inflammation. We investigated the genes regulated by NKX2-3 in B cells from a UC patient by cDNA microarray and compared the results to those genes regulated by NKX2-3 in B cells from a CD patient. Genes regulated by NKX2-3 in B cells from UC were mainly involved in cell growth, inflammation, and immune response. Among the genes regulated by NKX2-3 in both UC and CD, expression of 145 genes was similarly altered and 34 genes was differentially affected by NKX2-3 knockdown. EGR1 was up-regulated in NKX2-3 knockdown B cells from UC while down-regulated in NKX2-3 knockdown B cells from CD. mRNA expressions of NKX2-3 and EGR1 were increased in diseased intestinal tissues from 19 CD patients. NKX2-3 may play different roles in UC and CD pathogenesis by differential regulation of EGR1.

NKX2-3 variant rs11190140 is associated with IBD and alters binding of NFAT

NKX2-3 SNP rs11190140 is associated with inflammatory bowel disease (IBD). The T allele is over-transmitted in IBD and the C allele represents a potential CpG methylation site. We hypothesize that genetic variation and/or methylation of SNP rs11190140 may play a role in NKX2-3 gene expression by affecting transcription factor binding. We studied 233 IBD cases and 250 unrelated healthy individuals from an IBD population from central Pennsylvania and performed genotype analyses of the genetic variation and methylation status analysis using PCR-based RFLP. For transcription factor binding, nuclear extracts from human B cells were incubated with biotin-labeled oligonucleotide sequences of the NKX2-3 promoter region containing the genetic variation of T, non-methylated C or methylated C at rs11190140, followed by biotin pull-down and Western blot analysis for transcription factors SP1, NFAT1, NF-κB, and ETS-1. In case-control analysis, the genetic variation was significantly associated with IBD (OR=0.503, 95% CI=0.330-0.764, p<0.001). Methylation status analyses revealed that the C allele is subject to modification by DNA methylation. transcription factor binding assay indicated distinct differential binding of NFAT1 to the NKX2-3 promoter sequence, with higher binding to those with non-methylated and methylated C than to T. The binding of NFAT1 to the NKX2-3 promoter region with rs1190140 was confirmed by ChIP assay. We speculate that the rs11190140 may regulate NKX2-3 expression and have a role in IBD pathogenesis.

NKX2-3 transcriptional regulation of endothelin-1 and VEGF signaling in human intestinal microvascular endothelial cells

Background

NKX2-3 is associated with inflammatory bowel disease (IBD). NKX2-3 is expressed in microvascular endothelial cells and the muscularis mucosa of the gastrointestinal tract. Human intestinal microvascular endothelial cells (HIMECs) are actively involved in the pathogenesis of IBD and IBD-associated microvascular dysfunction. To understand the cellular function of NKX2-3 and its potential role underlying IBD pathogenesis, we investigated the genes regulated by NKX2-3 in HIMEC using cDNA microarray.

Methodology/Principal Findings

NKX2-3 expression was suppressed by shRNA in two HIMEC lines and gene expression was profiled by cDNA microarray. Pathway Analysis was used to identify gene networks according to biological functions and associated pathways. Validation of microarray and genes expression in intestinal tissues was assessed by RT-PCR. NKX2-3 regulated genes are involved in immune and inflammatory response, cell proliferation and growth, metabolic process, and angiogenesis. Several inflammation and angiogenesis related signaling pathways that play important roles in IBD were regulated by NKX2-3, including endothelin-1 and VEGF-PI3K/AKT-eNOS. Expression levels of NKX2-3, VEGFA, PI3K, AKT, and eNOS are increased in intestinal tissues from IBD patients and expression levels of EDN1 are decreased in intestinal tissues from IBD patients. These results demonstrated the important roles of NKX2-3, VEGF, PI3K, AKT, eNOS, and EDN1 in IBD pathogenesis. Correlation analysis showed a positive correlation between mRNA expression of NKX2-3 and VEGFA and a negative correlation between mRNA expression of NKX2-3 and EDN1 in intestinal tissues from IBD patients.

Conclusion/Relevance

NKX2-3 may play an important role in IBD pathogenesis by regulating endothelin-1 and VEGF signaling in HIMECs.

Genes regulated by Nkx2-3 in siRNA-mediated knockdown B cells: implication of endothelin-1 in inflammatory bowel disease

Nkx2-3 gene variants are strongly associated with inflammatory bowel disease (IBD) and its expression is up-regulated in Crohn's disease (CD). However, the nature of its role underlying IBD pathogenesis is unknown. We investigated the genes regulated by Nkx2-3 using cDNA microarray. A small interfering RNA (siRNA)-mediated knockdown of Nkx2-3 in a B cell line from a CD patient was generated. Gene expression was profiled on high-density cDNA microarrays representing over 25,000 genes. Ingenuity pathway analysis (IPA) was used to identify gene networks according to biological functions and associated pathways. Expression profiling analysis by cDNA microarray showed that 125 genes were regulated by Nkx2-3 knockdown (fold change >or=3.0, p<0.01), among which 51 genes were immune and inflammatory response genes. Microarray results were validated by RT-PCR and further confirmed in a B cell line expressing siRNA of Nkx2-3 from an additional CD patient. The results showed that Nkx2-3 was up-regulated (p<0.05) and EDN1 was down-regulated (p<0.05) in B cell lines from CD patients. mRNA expression levels of Nkx2-3 were negatively correlated with those of EDN1 (r=-0.6044, p<0.05). EDN1 was also down-regulated in intestinal tissues from UC patients (p<0.05). Our present results demonstrate that a decrease in Nkx2-3 gene expression level can profoundly alter the expression of genes and cellular functions relevant to the pathogenesis and progression of IBD, such as EDN1.

Dividing the tubular gut: generation of organ boundaries at the pylorus

The discrete organs that comprise the gastrointestinal tract (esophagus, stomach, small intestine, and large intestine) arise embryonically by regional differentiation of a single tube that is initially morphologically similar along its length. Regional organ differentiation programs, for example, for stomach or intestine, involve signaling cross-talk between epithelium and mesenchyme and result in the formation of precise boundaries between organs, across which dramatic differences in both morphology and gene expression are seen. The pylorus is a unique area of the gut tube because it not only marks an important organ boundary in the tubular gut (the stomach/intestinal boundary) but is also the hub for the development of multiple accessory organs (liver, pancreas, gall bladder, and spleen). This chapter examines: (a) our current understanding of the molecular and morphogenic processes that underlie the generation of the dramatic epithelial tissue boundary that compartmentalizes stomach and intestine; (b) the tissue interactions that promote development of the accessory organs in this area; and (c) the molecular interactions that specify patterning of the pyloric sphincter. Though the focus here is primarily on the mouse as a model organism, the molecular underpinnings of organ patterning near the pylorus are shared by chick and frog. Thus, further study of these conserved developmental programs could potentially shed light on the mechanisms underlying human pyloric malformations such as infantile hypertrophic pyloric stenosis.

Six2 activity is required for the formation of the mammalian pyloric sphincter

The functional activity of Six2, a member of the so/Six family of homeodomain-containing transcription factors, is required during mammalian kidney organogenesis. We have now determined that Six2 activity is also necessary for the formation of the pyloric sphincter, the functional gate at the stomach-duodenum junction that inhibits duodenogastric reflux. Our data reveal that several genes known to be important for pyloric sphincter formation in the chick (e.g., Bmp4, Bmpr1b, Nkx2.5, Sox9, and Gremlin) also appear to be required for the formation of this structure in mammals. Thus, we propose that Six2 activity regulates this gene network during the genesis of the pyloric sphincter in the mouse.

Association of a Nkx2-3 polymorphism with Crohn's disease and expression of Nkx2-3 is up-regulated in B cell lines and intestinal tissues with Crohn's disease

AIM

To replicate the association of Nkx2-3 rs10883365 SNP with Crohn's disease in patients from a familial IBD registry from the central Pennsylvania area and study mRNA and protein expression of Nkx2-3 in CD patients.

MATERIALS AND METHODS

We genotyped the Nkx2-3 rs10883365 SNP in 75 CD patients,137 non-CD family members and 118 unrelated healthy controls from EBV-transformed B cell lines of a familial IBD registry in central Pennsylvania. mRNA and protein expression levels of Nkx2-3 were measured by RT-PCR and Western blot, respectively.

RESULTS

rs10883365 was found to be associated with CD. A significant difference between the homozygous variant genotype (GG) compared to the wild type sequence (AA) was observed between CD and individuals without IBD, including both non-IBD family members from the familial IBD registry and unrelated healthy controls. However, there was not a significant difference between CD and non-IBD related family members. mRNA and protein expression levels of Nkx2-3 were increased in CD compared with non-CD sibling and healthy controls. A total of 16 sibling pairs were examined, and the mRNA and protein expression levels of Nkx2-3 from 12 of the sibling pairs (75%) were increased in the CD individual compared with the non-CD sibling. mRNA expression levels of Nkx2-3 were increased in diseased tissues compared with adjacent normal tissues in 7 of 9 patients (77.8%).

CONCLUSIONS

Nkx2-3 is genetically associated with CD and is up-regulated in CD, suggesting that Nkx2-3 is involved in the pathogenesis of CD.

The role of the visceral mesoderm in the development of the gastrointestinal tract

The gastrointestinal (GI) tract forms from the endoderm (which gives rise to the epithelium) and the mesoderm (which develops into the smooth muscle layer, the mesenchyme, and numerous other cell types). Much of what is known of GI development has been learned from studies of the endoderm and its derivatives, because of the importance of epithelial biology in understanding and treating human diseases. Although the necessity of epithelial-mesenchymal cross talk for GI development is uncontested, the role of the mesoderm remains comparatively less well understood. The transformation of the visceral mesoderm during development is remarkable; it differentiates from a very thin layer of cells into a complex tissue comprising smooth muscle cells, myofibroblasts, neurons, immune cells, endothelial cells, lymphatics, and extracellular matrix molecules, all contributing to the form and function of the digestive system. Understanding the molecular processes that govern the development of these cell types and elucidating their respective contribution to GI patterning could offer insight into the mechanisms that regulate cell fate decisions in the intestine, which has the unique property of rapid cell renewal for the maintenance of epithelial integrity. In reviewing evidence from both mammalian and nonmammalian models, we reveal the important role of the visceral mesoderm in the ontogeny of the GI tract.

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.

A functional nuclear localization signal in insulin-like growth factor binding protein-6 mediates its nuclear import

IGF binding protein (IGFBP)-6 is a member of the IGFBP family that regulates the actions of IGFs. Although IGFBPs exert their functions extracellularly in an autocrine/paracrine manner, several members of the family, such as IGFBP-3 and -5, possess nuclear localization signals (NLS). To date, no NLS has been described for IGFBP-6, an IGFBP that binds preferentially to IGF-II. We report here that both exogenous and endogenous IGFBP-6 could be imported into the nuclei of rhabdomyosarcoma and HEK-293 cells. Nuclear import of IGFBP-6 was mediated by a NLS sequence that bears limited homology to those found in IGFBP-3 and -5. IGFBP-6 nuclear translocation was an active process that required importins. A peptide corresponding to the IGFBP-6 NLS bound preferentially to importin-alpha. A comprehensive peptide array study revealed that, in addition to positively charged residues such as Arg and Lys, amino acids, notably Gly and Pro, within the NLS, played an important part in binding to importins. Overexpression of wild-type IGFBP-6 increased apoptosis, and the addition of IGF-II did not negate this effect. Only the deletion of the NLS segment abolished the apoptosis effect. Taken together, these results suggest that IGFBP-6 is translocated to the nucleus with functional consequences and that different members of the IGFBP family have specific nuclear import mechanisms.

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.

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.

The homeodomain of Tinman mediates homo- and heterodimerization of NK proteins

Cardiac development requires the action of transcription factors, which control the specification and differentiation of cardiac cell types. One of these factors, encoded by the homeobox gene tinman (tin), is essential for the specification of all cardiac cells in Drosophila. An increasing number of examples show that protein-protein interactions can be important for determining the specific transcriptional activities of homeodomain proteins, in addition to their binding to specific DNA target sites. Here, we show that Tin and Bagpipe (Bap), another homeodomain protein, form homo- and heterodimeric complexes. We demonstrate that homo- and heterodimerization of Tin is mediated through its homeodomain and that the region required for this interaction corresponds to the first two helices that are also necessary for DNA binding. We further show that, in the yeast system, the homeodomain can function as a transcriptional repressor domain. These findings suggest that protein-protein interactions of Tin play a role in its transcriptional and developmental functions.

Sox9 and Nkx2.5 determine the pyloric sphincter epithelium under the control of BMP signaling

The organs of the digestive tract are specified by coordinated signaling between the endoderm and mesoderm during development. These epithelial-mesenchymal interactions lead to the organ-specific morphogenesis and differentiation of regions along the gut tube. In this paper, we show that in the chick, the SRY-related transcription factor Sox9 is a marker for the posterior gizzard. Viral misexpression of Sox9 in the gizzard mesoderm is sufficient to specify epithelium characteristic of the pyloric sphincter. Sox9 expression is normally limited to the region of the posterior gizzard under the regulation of BMP signaling from the adjacent midgut. Misexpression of an activated form of BMPR1b in the gizzard upregulates Sox9 expression, while the BMP antagonist noggin down-regulates Sox9 expression in the gizzard mesoderm. Previously, Nkx2.5 was identified as a marker for the mesoderm of the pyloric sphincter. As with Sox9, BMP signaling appears to regulate Nkx2.5 and its ability to determine the pyloric epithelium. Despite these similarities, our evidence suggests that Sox9 and Nkx2.5 are regulated independently by BMP signaling, and act coordinately to specify the pyloric sphincter.

Spatial and temporal expression patterns of Xenopus Nkx-2.3 gene in skin epidermis during metamorphosis

Using PCR cloning, the mRNA of XNkx-2.3 gene, a Xenopus tinman homologue, was identified in a cDNA library prepared from thyroid hormone (T(3))-treated tadpole skin. Quantitative RT-PCR and RNase Protection Assay confirmed the expression of XNkx-2.3 in adult frog skin and its amount was similar to the amount found in heart. In situ hybridization indicated that XNkx-2.3 was expressed in the frog epidermis. Further analysis of XNkx-2.3 expression patterns demonstrates that it shares great similarities with a 63 kDa keratin, a well-characterized marker for skin maturation, in the following aspects. First, XNkx-2.3 was expressed in tadpole skin during metamorphosis (stages 55-59), but not in pre-metamorphic (stage 54) skin. Secondly, XNkx-2.3 expression in skin responded to T(3) stimulation because it could be precociously induced by T(3) at pre-metamorphic stage, both in tadpoles and in cultures of skin explants. Finally, the T(3)-induced appearance of XNkx-2.3 in head skin occurred earlier and at higher level than that in tail skin. These data suggest that XNkx-2.3 may be an important factor for skin maturation and may also serve as a good marker to indicate the maturation of Xenopus epidermis.

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.

Differential binding of an SRF/NK-2/MEF2 transcription factor complex in normal versus neoplastic smooth muscle tissues

The malignant potential of smooth muscle tumors correlates strongly with the disappearance of gamma-smooth muscle isoactin, a lineage-specific marker of smooth muscle development. In this paper, we identify a 36-base pair regulatory motif containing an AT-rich domain, CArG box, and a non-canonical NK-2 homeodomain-binding site that has the capacity to regulate smooth muscle-specific gene expression in cultured intestinal smooth muscle cells. Serum-response factor associates with an NK-2 transcription factor via protein-protein interactions and binds to the core CArG box element. Our studies suggest that the NK-2 transcription factor that associates with serum-response factor during smooth muscle differentiation is Nkx2-3. Myocyte-specific enhancer factor 2 binding to this regulatory complex was also observed but limited to uterine smooth muscle tissues. Smooth muscle neoplasms displayed altered transcription factor binding when compared with normal myometrium. Differential nuclear accessibility of serum-response factor protein during smooth muscle differentiation and neoplastic transformation was also observed. Thus, we have identified a unique regulatory complex whose differential binding properties and nuclear accessibility are associated with modulating gamma-smooth muscle isoactin-specific gene expression in both normal and neoplastic tissues.

Elevated expression of Nkx-2.5 in developing myocardial conduction cells

A number of different phenotypes emerge from the mesoderm-derived cardiomyogenic cells of the embryonic tubular heart, including those comprising the cardiac conduction system. The transcriptional regulation of this phenotypic divergence within the cardiomyogenic lineage remains poorly characterized. A relationship between expression of the transcription factor Nkx-2.5 and patterning to form cardiogenic mesoderm subsequent to gastrulation is well established. Nkx-2.5 mRNA continues to be expressed in myocardium beyond the looped, tubular heart stage. To investigate the role of Nkx-2.5 in later development, we have determined the expression pattern of Nkx-2.5 mRNA by in situ hybridization in embryonic chick, fetal mouse, and human hearts, and of Nkx-2.5 protein by immunolocalization in the embryonic chick heart. As development progresses, significant nonuniformities emerge in Nkx-2.5 expression levels. Relative to surrounding force-generating ("working") myocardium, elevated Nkx-2.5 mRNA signal becomes apparent in the specialized cells of the conduction system. Similar differences are found in developing chick, human, and mouse fetal hearts, and nuclear-localized Nkx-2.5 protein is prominently expressed in differentiating chick conduction cells relative to adjacent working myocytes. This tissue-restricted expression of Nkx-2.5 is transient and correlates with the timing of spatio-temporal recruitment of cells to the central and the peripheral conduction system. Our data represent the first report of a transcription factor showing a stage-dependent restriction to different parts of the developing conduction system, and suggest some commonality in this development between birds and mammals. This dynamic pattern of expression is consistent with the hypothesis that Nkx-2.5, and its level of expression, have a role in regulation and/or maintenance of specialized fate selection by embryonic myocardial cells.

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.

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.

Evolutionary relationships between the amphibian, avian, and mammalian stomachs

Although the gut is homologous among different vertebrates, morphological differences exist between different species. The most obvious variation in the guts of extant vertebrates appears in the stomach. To investigate the evolution of this structure, we compared the histology of the stomach and gastrointestinal tract in amphibian (Xenopus laevis), avian (Gallus gallus), and mammalian (Mus musculus) organisms, and defined the expression patterns of several genes within the developing guts of these lineages. In all three groups, we find that the anterior portion of the stomach has a similar glandular histology as well as a common embryonic expression of the secreted factors Wnt5a and BMP-4. Likewise, within the amniote lineages, the posterior nonglandular stomach and pyloric sphincter regions are also comparable in both histological and molecular phenotypes. The posterior stomach expresses Six2, BMPR1B, and Barx1, whereas the pyloric sphincter expresses Nkx2.5. Although the adult Xenopus stomach exhibits both glandular and aglandular regions and a distinct pyloric sphincter similar to that of the amniotic vertebrates, the histology of the Xenopus tadpole gut shows less distinct variation in differentiation in this region, which is most likely a derived condition. The molecular signature of the embryonic Xenopus gut correlates with the more derived morphology of the larval phase. We conclude that the global patterning of the gut is remarkably similar among the different vertebrate lineages. The distinct compartments of gene expression that we find in the gut be necessary for the unique morphological specializations that distinguish the stomachs from terrestrial vertebrates.

Roles of BMP signaling and Nkx2.5 in patterning at the chick midgut-foregut boundary

Patterning of the gut into morphologically distinct regions results from the appropriate factors being expressed in strict spatial and temporal patterns to assign cells their fates in development. Often, the boundaries of gene expression early in development correspond to delineations between different regions of the adult gut. For example, Bmp4 is expressed throughout the hindgut and midgut, but is not expressed in the early gizzard. Ectopic BMP4 in the gizzard caused a thinning of the muscularis. To understand this phenotype we examined the expression of the receptors transducing BMP signaling during gut development. We find that the BMP receptors are differentially expressed in distinct regions of the chicken embryonic gut. By using constitutively activated versions of the BMP type I receptors, we find that the BMP receptors act similarly to BMP4 in the gizzard when ectopically expressed. We show that the mesodermal thinning seen upon ectopic BMP signaling is due to an increase in apoptosis and a decrease in proliferation within the gizzard mesoderm. The mesodermal thinning is characterized by a disorganization and lack of differentiation of smooth muscle in the gizzard mesoderm. Further, ectopic BMP receptors cause an upregulation of Nkx2.5, the pyloric sphincter marker, similar to that seen with ectopic BMP4. This upregulation of Nkx2.5 is a cell-autonomous event within the mesoderm of the gizzard. We also find that Nkx2.5 is necessary and sufficient for establishing aspects of pyloric sphincter differentiation.

Homeodomain factor Nkx2-3 controls regional expression of leukocyte homing coreceptor MAdCAM-1 in specialized endothelial cells of the viscera

Regulated emigration of blood-borne leukocytes plays a defining role in lymphoid organ development, immune surveillance, and inflammatory responses. We report here that mice deficient in the homeobox gene Nkx2-3, expressed in developing visceral mesoderm, show a complex intestinal malabsorption phenotype and striking abnormalities of gut-associated lymphoid tissue and spleen suggestive of deranged leukocyte homing. Mutant Peyer's patches were reduced in number and size, intestinal villi contained few IgA(+) plasma cells, and mutant spleens were small and often atrophic, showing fused periarterial lymphoid sheaths, partially merged T and B cell zones, an absent marginal zone, and a dearth of macrophages in red pulp. Semiquantitative RT-PCR analysis and immunohistochemistry revealed down-regulation of mucosal addressin cell adhesion molecule-1 (MAdCAM-1) in endothelial cells in which Nkx2-3 is normally expressed. MAdCAM-1 is a member of the immunoglobulin superfamily, acting as an endothelial cell ligand for leukocyte homing receptors L-selectin and alpha4beta7 integrin. Our data suggest a role for a homeodomain factor in establishing the developmental and positional cues in endothelia that regulate leukocyte homing through local control of cellular adhesion and identify MAdCAM-1 as a candidate target gene of Nkx2-3.

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.

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.

The concentric structure of the developing gut is regulated by Sonic hedgehog derived from endodermal epithelium

The embryonic gut of vertebrates consists of endodermal epithelium, surrounding mesenchyme derived from splanchnic mesoderm and enteric neuronal components derived from neural crest cells. During gut organogenesis, the mesenchyme differentiates into distinct concentric layers around the endodermal epithelium forming the lamina propria, muscularis mucosae, submucosa and lamina muscularis (the smooth muscle layer). The smooth muscle layer and enteric plexus are formed at the outermost part of the gut, always some distance away from the epithelium. How this topographical organization of gut mesenchyme is established is largely unknown. Here we show the following: (1) Endodermal epithelium inhibits differentiation of smooth muscle and enteric neurons in adjacent mesenchyme. (2) Endodermal epithelium activates expression of patched and BMP4 in adjacent non-smooth muscle mesenchyme, which later differentiates into the lamina propria and submucosa. (3) Sonic hedgehog (Shh) is expressed in endodermal epithelium and disruption of Shh-signaling by cyclopamine induces differentiation of smooth muscle and a large number of neurons even in the area adjacent to epithelium. (4) Shh can mimic the effect of endodermal epithelium on the concentric stratification of the gut. Taken together, these data suggest that endoderm-derived Shh is responsible for the patterning across the radial axis of the gut through induction of inner components and inhibition of outer components, such as smooth muscle and enteric neurons.

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.

A conserved role for H15-related T-box transcription factors in zebrafish and Drosophila heart formation

T-box transcription factors are critical regulators of early embryonic development. We have characterized a novel zebrafish T-box transcription factor, hrT (H15-related T box) that is a close relative of Drosophila H15 and a recently identified human gene. We show that Drosophila H15 and zebrafish hrT are both expressed early during heart formation, in strong support of previous work postulating that vertebrate and arthropod hearts are homologous structures with conserved regulatory mechanisms. The timing and regulation of zebrafish hrT expression in anterior lateral plate mesoderm suggest a very early role for hrT in the differentiation of the cardiac precursors. hrT is coexpressed with gata4 and nkx2.5 not only in anterior lateral plate mesoderm but also in noncardiac mesoderm adjacent to the tail bud, suggesting that a conserved regulatory pathway links expression of these three genes in cardiac and noncardiac tissues. Finally, we analyzed hrT expression in pandora mutant embryos, since these have defects in many of the tissues that express hrT, including the heart. hrT expression is much reduced in the early heart fields of pandora mutants, whereas it is ectopically expressed subsequently. Using hrT expression as a marker, we describe a midline patterning defect in pandora affecting the anterior hindbrain and associated midline mesendodermal derivatives. We discuss the possibility that the cardiac ventricular defect previously described in pandora and the midline defects described here are related.

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.

Myoblast cell grafting into heart muscle: cellular biology and potential applications

This review surveys a wide range of cellular and molecular approaches to strengthening the injured or weakened heart, focusing on strategies to replace dysfunctional, necrotic, or apoptotic cardiomyocytes with new cells of mesodermal origin. A variety of cell types, including myogenic cell lines, adult skeletal myoblasts, immoratalized atrial cells, embryonic and adult cardiomyocytes, embryonic stem cells, tetratoma cells, genetically altered fibroblasts, smooth muscle cells, and bone marrow-derived cells have all been proposed as useful cells in cardiac repair and may have the capacity to perform cardiac work. We focus on the implantation of mesodermally derived cells, the best developed of the options. We review the developmental and cell biology that have stimulated these studies, examine the limitations of current knowledge, and identify challenges for the future, which we believe are considerable.

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.