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

When the end modifies its means: the origins of novelty and the evolution of innovation

The origin of novel complex traits constitutes a central yet largely unresolved challenge in evolutionary biology. Intriguingly, many of the most promising breakthroughs in understanding the genesis of evolutionary novelty in recent years have occurred not in evolutionary biology itself, but through the comparative study of development and, more recently, the interface of developmental biology and ecology. Here, I discuss how these insights are changing our understanding of what matters in the origin of novel, complex traits in ontogeny and evolution. Specifically, my essay has two major objectives. First, I discuss how the nature of developmental systems biases the production of phenotypic variation in the face of novel or stressful environments toward functional, integrated and, possibly, adaptive variants. This, in turn, allows the production of novel phenotypes to precede (rather than follow) changes in genotype and allows developmental processes that are the product of past evolution to shape evolutionary change that has yet to occur. Second, I explore how this nature of developmental systems has itself evolved over time, increasing the repertoire of ontogenies to pursue a wider range of objectives across an expanding range of conditions, thereby creating an increasingly extensive affordance landscape in development and developmental evolution. Developmental systems and their evolution can thus be viewed as dynamic processes that modify their own means across ontogeny and phylogeny. The study of these dynamics necessitates more than the strict reductionist approach that currently dominates the fields of developmental and evolutionary developmental biology.

foxF-1 Controls Specification of Non-body Wall Muscle and Phagocytic Cells in Planarians

Planarians are flatworms capable of regenerating any missing body part in a process requiring stem cells and positional information. Muscle is a major source of planarian positional information and consists of several types of fibers with distinct regulatory roles in regeneration. The transcriptional regulatory programs used to specify different muscle fibers are poorly characterized. Using single-cell RNA sequencing, we define the transcriptomes of planarian dorsal-ventral muscle (DVM), intestinal muscle (IM), and pharynx muscle. This analysis identifies foxF-1, which encodes a broadly conserved Fox-family transcription factor, as a master transcriptional regulator of all non-body wall muscle. The transcription factors encoded by nk4 and gata4/5/6-2 specify two different subsets of DVM, lateral and medial, respectively, whereas gata4/5/6-3 specifies IM. These muscle types all express planarian patterning genes. Both lateral and medial DVM are required for medial-lateral patterning in regeneration, whereas medial DVM and IM have a role in maintaining and regenerating intestine morphology. In addition to the role in muscle, foxF-1 is required for the specification of multiple cell types with transcriptome similarities, including high expression levels of cathepsin genes. These cells include pigment cells, glia, and several other cells with unknown function. cathepsin⁺ cells phagocytose E. coli, suggesting these are phagocytic cells. In conclusion, we describe a regulatory program for planarian muscle cell subsets and phagocytic cells, both driven by foxF-1. FoxF proteins specify different mesoderm-derived tissues in other organisms, suggesting that FoxF regulates formation of an ancient and broadly conserved subset of mesoderm derivatives in the Bilateria.

Disorders of thyroid morphogenesis

Developmental anomalies of the thyroid gland, defined as thyroid dysgenesis, underlie the majority of cases of congenital hypothyroidism. Thyroid dysgenesis is predominantly a sporadic disorder although a reported familial enrichment, variation of incidence by ethnicity and the monogenic defects associated mainly with athyreosis or orthotopic thyroid hypoplasia, suggest a genetic contribution. Of note, the most common developmental anomaly, thyroid ectopy, remains unexplained. Ectopy may result from multiple genetic or epigenetic variants in the germline and/or at the somatic level. This review provides a brief overview of the monogenic defects in candidate genes that have been identified so far and of the syndromes which are known to be associated with thyroid dysgenesis.

miR-30c regulates proliferation, apoptosis and differentiation via the Shh signaling pathway in P19 cells

MicroRNAs (miRNAs) are small, non-coding single-stranded RNAs that suppress protein expression by binding to the 3′ untranslated regions of their target genes. Many studies have shown that miRNAs have important roles in congenital heart diseases (CHDs) by regulating gene expression and signaling pathways. We previously found that miR-30c was highly expressed in the heart tissues of aborted embryos with ventricular septal defects. Therefore, this study aimed to explore the effects of miR-30c in CHDs. miR-30c was overexpressed or knocked down in P19 cells, a myocardial cell model that is widely used to study cardiogenesis. We found that miR-30c overexpression not only increased cell proliferation by promoting cell entry into S phase but also suppressed cell apoptosis. In addition, we found that miR-30c inhibited dimethyl sulfoxide-induced differentiation of P19 cells. miR-30c knockdown, in contrast, inhibited cell proliferation and increased apoptosis and differentiation. The Sonic hedgehog (Shh) signaling pathway is essential for normal embryonic development. Western blotting and luciferase assays revealed that Gli2, a transcriptional factor that has essential roles in the Shh signaling pathway, was a potential target gene of miR-30c. Ptch1, another important player in the Shh signaling pathway and a transcriptional target of Gli2, was downregulated by miR-30c overexpression and upregulated by miR-30c knockdown. Collectively, our study revealed that miR-30c suppressed P19 cell differentiation by inhibiting the Shh signaling pathway and altered the balance between cell proliferation and apoptosis, which may result in embryonic cardiac malfunctions.

Copy number variation of GATA4 and NKX2-5 in Chinese fetuses with congenital heart disease

BACKGROUND

Congenital heart disease (CHD) is one of the most common birth defects in newborns. The etiology of CHD has remained largely unknown, but it is assumed to result from the combined effects of genetic and environmental factors. Recent investigations have detected potentially pathogenic copy number variations (CNV) in a proportion of patients with CHD. The present case-control study evaluated whether CNV in the GATA4 and NKX2-5 genes contribute to the pathogenesis of CHD in Chinese fetuses (n = 117), by comparing them with non-CHD control subjects (n = 100).

METHODS

Multiplex ligation-dependent probe amplification with the P311A probe mixture was used to detect CNV.

RESULTS

The normalized signals were within the normal range for all exons in all CHD patients and non-CHD control subjects. Of the 117 CHD patients, three had a deletion of 22q11, and two had a duplication of 22q11.

CONCLUSIONS

There was no evidence of a role for NKX2-5 and GATA4 CNV in fetal CHD; therefore, these CNV may not be common in fetal CHD in China.

Investigation of Somatic NKX2-5 Mutations in Chinese Children with Congenital Heart Disease

The purposes of this study are to investigate somatic NKX2-5 mutations in Chinese children with congenital heart disease (CHD) and assess the reliability of somatic mutation detection in formalin-fixed, paraffin-embedded (FFPE) tissues. The study cohort included frozen and FFPE cardiac tissues as well as blood samples from 85 Chinese children with CHD who had the cardiac operations. The right atrial appendage far from the diseased heart was used as normal control. Genomic DNA was isolated from cardiac tissues and blood samples using TIANamp Blood DNA kit. Two exons and exon-intron boundaries of NKX2-5 were amplified by polymerase chain reaction (PCR) and sequenced by dideoxynucleotide chain termination approach. The acquired sequences were aligned with GenBank sequences to identify the sequence variations. No somatic mutation in the NKX2-5 gene was observed in both frozen and FFPE cardiac tissues in 85 Chinese children with CHD. Nonetheless, a common single nucleotide polymorphism (SNP), c.63 A > G (E21E), was identified in all the three kinds of DNA samples with the same allele frequency 82.3%. Moreover, another common SNP c.606 G > C (L202L) was found in 2.3% of our patients. There were no significant differences in the allele frequencies of two SNPs between the cardiac diseased tissues and right atrial appendage (P > 0.05). PCR artefact as mutations was not found in the FFPE tissues stored for one year. Our findings demonstrate that somatic NKX2-5 mutations do not represent an important aetiologic pathway in Chinese children with congenital heart disease.

Expression of sarcomeric tropomyosin in striated muscles in axolotl treated with shz-1, a small cardiogenic molecule

We evaluated the effect of shz-1, a cardiogenic molecule, on the expression of various tropomyosin (TM) isoforms in the Mexican axolotl (Ambystoma mexicanum) hearts. qRT-PCR data show a ~1.5-fold increase in cardiac transcripts of the Nkx2.5 gene, which plays a crucial role in cardiogenesis in vertebrates. Shz-1 augments the expression of transcripts of the total sarcomeric TPM1 (both TPM1α & TPM1κ) and sarcomeric TPM4α. In order to understand the mechanism by which shz-1 augments the expression of sarcomeric TPM transcription in axolotl hearts, we transfected C2C12 cells with pGL3.axolotl. We transfected C2C12 cells with pGL3-axolotl TPM4 promoter constructs containing the firefly luciferase reporter gene. The transfected C2C12 cells were grown in the absence or presence of shz-1 (5 μM). Subsequently, we determined the firefly luciferase activity in the extracts of transfected cells. The results suggest that shz-1 activates the axolotl TPM4 promoter-driven ectopic expression in C2C12 cells. Also, we transfected C2C12 cells with a pGL3.1 vector containing the promoter of the mouse skeletal muscle troponin-I and observed a similar increase in the luciferase activity in shz-1-treated cells. We conclude that shz-1 activates the promoters of a variety of genes including axolotl TPM4. We have quantified the expression of the total sarcomeric TPM1 and observed a 1.5-fold increase in treated cells. Western blot analyses with CH1 monoclonal antibody specific for sarcomeric isoforms show that shz-1 does not increase the expression of TM protein in axolotl hearts, whereas it does in C2C12 cells. These findings support our hypothesis that cardiac TM expression in axolotl undergoes translational control.

Ectopic activation of the canonical wnt signaling pathway affects ectodermal patterning along the primary axis during larval development in the anthozoan Nematostella vectensis

The primary axis of cnidarians runs from the oral pole to the apical tuft and defines the major body axis of both the planula larva and adult polyp. In the anthozoan cnidarian Nematostella vectensis, the primary oral-aboral (O-Ab) axis first develops during the early embryonic stage. Here, we present evidence that pharmaceutical activators of canonical wnt signaling affect molecular patterning along the primary axis of Nematostella. Although not overtly morphologically complex, molecular investigations in Nematostella reveal that the O-Ab axis is demarcated by the expression of differentially localized signaling molecules and transcription factors that may serve roles in establishing distinct ectodermal domains. We have further characterized the larval epithelium by determining the position of a nested set of molecular boundaries, utilizing several newly characterized as well as previously reported epithelial markers along the primary axis. We have assayed shifts in their position in control embryos and in embryos treated with the pharmacological agents alsterpaullone and azakenpaullone, Gsk3β inhibitors that act as canonical wnt agonists, and the Wnt antagonist iCRT14, following gastrulation. Agonist drug treatments result in an absence of aboral markers, a shift in the expression boundaries of oral markers toward the aboral pole, and changes in the position of differentially localized populations of neurons in a dose-dependent manner, while antagonist treatment had the opposite effect. These experiments are consistent with canonical wnt signaling playing a role in an orally localized wnt signaling center. These findings suggest that in Nematostella, wnt signaling mediates O-Ab ectodermal patterning across a surprisingly complex epithelium in planula stages following gastrulation in addition to previously described roles for the wnt signaling pathway in endomesoderm specification during gastrulation and overall animal-vegetal patterning at earlier stages of anthozoan development.

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.

Mesp1: a key regulator of cardiovascular lineage commitment

In mammals, the heart arises from the differentiation of 2 sources of multipotent cardiovascular progenitors (MCPs). Different studies indicated that an evolutionary conserved transcriptional regulatory network controls cardiovascular development from flies to humans. Whereas in Drosophila, Tinman acts as a master regulator of cardiac development, the identification of such a master regulator in mammals remained elusive for a long time. In this review, we discuss the recent findings suggesting that Mesp1 acts as a key regulator of cardiovascular progenitors in vertebrates. Lineage tracing in mice demonstrated that Mesp1 represents the earliest marker of cardiovascular progenitors, tracing almost all the cells of the heart including derivatives of the primary and second heart fields. The inactivation of Mesp1/2 indicated that Mesp genes are essential for early cardiac mesoderm formation and MCP migration. Several recent studies have demonstrated that Mesp1 massively promotes cardiovascular differentiation during embryonic development and pluripotent stem cell differentiation and indicated that Mesp1 resides at the top of the cellular and transcriptional hierarchy that orchestrates MCP specification. In primitive chordates, Mesp also controls early cardiac progenitor specification and migration, suggesting that Mesp arises during chordate evolution to regulate the earliest step of cardiovascular development. Defining how Mesp1 regulates the earliest step of MCP specification and controls their migration is essential to understand the root of cardiovascular development and how the deregulation of these processes can lead to congenital heart diseases. In addition, these findings will be very useful to boost the production of cardiovascular cells for cellular therapy, drug and toxicity screening.

Mutations of the GATA4 and NKX2.5 genes in Chinese pediatric patients with non-familial congenital heart disease

A number of mutations in GATA4 and NKX2.5 have been identified to be causative for a subset of familial congenital heart defects (CHDs) and a small number of sporadic CHDs. In this study, we evaluated common GATA4 and NKX2.5 mutations in 135 Chinese pediatric patients with non-familial congenital heart defects. Two novel mutations in the coding region of GATA4 were identified, namely, 487C >T (Pro163Ser) in exon 1 in a child with tetralogy of Fallot and 1220C >A (Pro407Gln) in exon 6 in a pediatric patient with outlet membranous ventricular septal defect. We also found 848C >A (Pro283Gln) in exon 2 of the NKX2.5 gene in a pediatric patient with ventricular septal defect, patent ductus arteriosus and aortic isthmus stenosis. None of the mutations was detected in healthy control subjects (n = 114). This study suggests that GATA4 and NKX2.5 missense mutations may be associated with congenital heart defects in pediatric Chinese patients. Further clinical studies with large samples are warranted.

Conservation of gene linkage in dispersed vertebrate NK homeobox clusters

Nk homeobox genes are important regulators of many different developmental processes including muscle, heart, central nervous system and sensory organ development. They are thought to have arisen as part of the ANTP megacluster, which also gave rise to Hox and ParaHox genes, and at least some NK genes remain tightly linked in all animals examined so far. The protostome-deuterostome ancestor probably contained a cluster of nine Nk genes: (Msx)-(Nk4/tinman)-(Nk3/bagpipe)-(Lbx/ladybird)-(Tlx/c15)-(Nk7)-(Nk6/hgtx)-(Nk1/slouch)-(Nk5/Hmx). Of these genes, only NKX2.6-NKX3.1, LBX1-TLX1 and LBX2-TLX2 remain tightly linked in humans. However, it is currently unclear whether this is unique to the human genome as we do not know which of these Nk genes are clustered in other vertebrates. This makes it difficult to assess whether the remaining linkages are due to selective pressures or because chance rearrangements have "missed" certain genes. In this paper, we identify all of the paralogs of these ancestrally clustered NK genes in several distinct vertebrates. We demonstrate that tight linkages of Lbx1-Tlx1, Lbx2-Tlx2 and Nkx3.1-Nkx2.6 have been widely maintained in both the ray-finned and lobe-finned fish lineages. Moreover, the recently duplicated Hmx2-Hmx3 genes are also tightly linked. Finally, we show that Lbx1-Tlx1 and Hmx2-Hmx3 are flanked by highly conserved noncoding elements, suggesting that shared regulatory regions may have resulted in evolutionary pressure to maintain these linkages. Consistent with this, these pairs of genes have overlapping expression domains. In contrast, Lbx2-Tlx2 and Nkx3.1-Nkx2.6, which do not seem to be coexpressed, are also not associated with conserved noncoding sequences, suggesting that an alternative mechanism may be responsible for the continued clustering of these genes.

Evolving enhancer-promoter interactions within the tinman complex of the flour beetle, Tribolium castaneum

Modifications of cis-regulatory DNAs, particularly enhancers, underlie changes in gene expression during animal evolution. Here, we present evidence for a distinct mechanism of regulatory evolution, whereby a novel pattern of gene expression arises from altered gene targeting of a conserved enhancer. The tinman gene complex (Tin-C) controls the patterning of dorsal mesodermal tissues, including the dorsal vessel or heart in Drosophila. Despite broad conservation of Tin-C gene expression patterns in the flour beetle (Tribolium castaneum), the honeybee (Apis mellifera) and the fruit fly (Drosophila melanogaster), the expression of a key pericardial determinant, ladybird, is absent from the dorsal mesoderm of Tribolium embryos. Evidence is presented that this loss in expression is replaced by expression of C15, the neighboring gene in the complex. This switch in expression from ladybird to C15 appears to arise from an inversion within the tinman complex, which redirects a conserved ladybird 3' enhancer to regulate C15. In Drosophila, this enhancer fails to activate C15 expression owing to the activity of an insulator at the intervening ladybird early promoter. By contrast, a chromosomal inversion allows the cardiac enhancer to bypass the ladybird insulator in Tribolium. Given the high frequency of genome rearrangements in insects, it is possible that such enhancer switching might be widely used in the diversification of the arthropods.

Myocardin-dependent activation of the CArG box-rich smooth muscle gamma-actin gene: preferential utilization of a single CArG element through functional association with the NKX3.1 homeodomain protein

Serum response factor (SRF) is a ubiquitously expressed transcription factor that binds a 10-bp element known as the CArG box, located in the proximal regulatory region of hundreds of target genes. SRF activates target genes in a cell- and context-dependent manner by assembling unique combinations of cofactors over CArG elements. One particularly strong SRF cofactor, myocardin (MYOCD), acts as a component of a molecular switch for smooth muscle cell (SMC) differentiation by activating cytoskeletal and contractile genes harboring SRF-binding CArG elements. Here we report that the human ACTG2 promoter, containing four conserved CArG elements, displays SMC-specific basal activity and is highly induced in the presence of MYOCD. Stable transfection of a non-SMC cell type with Myocd elicits elevations in endogenous Actg2 mRNA. Gel shift and luciferase assays reveal a strong bias for MYOCD-dependent transactivation through CArG2 of the human ACTG2 promoter. Substitution of CArG2 with other CArGs, including a consensus CArG element, fails to reconstitute full MYOCD-dependent ACTG2 promoter stimulation. Mutation of an adjacent binding site for NKX3.1 reduces MYOCD-dependent transactivation of the ACTG2 promoter. Co-immunoprecipitation, glutathione S-transferase pulldown, and luciferase assays show a physical and functional association between MYOCD and NKX3.1; no such functional relationship is evident with the related NKX2.5 transcription factor despite its interaction with MYOCD. These results demonstrate the ability of MYOCD to discriminate among several juxtaposed CArG elements, presumably through its novel partnership with NKX3.1, to optimally transactivate the human ACTG2 promoter.

Tissue-specific and transcription factor-mediated nuclear entry of DNA

Low levels of gene transfer and a lack of tissue-specific targeting of vectors have limited the therapeutic potential of non-viral gene therapy. This is due to the numerous cellular barriers that hinder nuclear delivery of vectors and the paucity of methods that restrict expression to specific cells types. In non-dividing cells, the nuclear envelope is an especially problematic hurdle to gene transfer. Given that the majority of target tissues are non-dividing in vivo, the nuclear membrane is a major obstacle to therapeutic gene transfer. In this review, the various barriers to gene transfer are discussed. In particular, the role of the nuclear pore complex (NPC) in regulating passage of plasmid vectors during interphase is reviewed. Several methods of modifying plasmid (pDNA) vectors to enhance nuclear import through the NPC are also discussed, including the use of tissue-specific transcription factors to mediate nuclear entry of pDNA in a cell-specific manner.

NKX2.5 mutations and congenital heart disease: is it a marker of cardiac anomalies?

Congenital heart defects (CHD) are the leading cause of morbidity and mortality in infants, the etiology of most CHD remains unknown, with the influence of genetics still topic of debate. Heterozygous mutations in the transcription factor, NKX2.5, were among the first evidence of genetic cause for congenital heart disease. For the prevention of CHD identification of specific genetic causes for congenital cardiac malformations will provide insight into the developmental mechanisms that result in normal and abnormal cardiac development and will allow for improved family counseling.

Cell-specific nuclear import of plasmid DNA in smooth muscle requires tissue-specific transcription factors and DNA sequences

Two shortcomings of nonviral gene therapy are a lack of tissue-specific targeting of vectors and low levels of gene transfer. Our laboratory has begun to address these limitations by designing plasmids that enter the nucleus of specific cell types in the absence of cell division, thereby enhancing expression in a controlled manner. We have shown that a 176 bp portion of the smooth muscle gamma-actin (SMGA) promoter can mediate plasmid nuclear import specifically in smooth muscle cells (SMCs). Here, we demonstrate that the binding sites for serum response factor (SRF) and NKX3-1/3-2 within this DNA nuclear targeting sequence (DTS) are required for plasmid nuclear import. Knockdown of these factors with siRNA abrogates plasmid nuclear import, indicating that they are necessary cofactors. In addition, coinjection of recombinant SRF and Nkx3.2 with the vector in TC7 epithelial cells rescues import. Finally, we show that the SRF nuclear localization sequence (NLS) is required for vector nuclear import. We propose that SRF and NKX3-1/3-2 bind the SMGA DTS in the cytoplasm, thus coating the plasmid with NLSs that mediate translocation across the nuclear pore complex. This discovery could aid in the development of more efficient nonviral vectors for gene transfer to SMCs.

Ultrastructural and immunocharacterization of undifferentiated myocardial cells in the developing mouse heart

The recent discovery of several myogenic cardiac progenitor cells in the post-natal heart suggests that some myocardial cells may remain undifferentiated during embryonic development. In this study, we examined the subcellular characteristics of the embryonic (E) mouse ventricular myocardial cells using transmission electron microscopy (TEM). At the ultrastructural level, we identified three different cell populations within the myocardial layer of the E11.5 heart. These cells were designated as undifferentiated cells (43 ± 6%), moderately differentiated cells (43 ± 2%) and mature cardiomyocytes (14 ± 4%). Undifferentiated cells contained a large nucleus and sparse cytoplasm with no myofibrillar bundles. Moderately differentiated cells contained randomly arranged myofilaments in the cytoplasm. In contrast, mature cardiomyocytes contained well-developed sarcomere structures. We also confirmed the presence of similar undifferentiated cells albeit at low levels in the E16.5 (∼20%) and E18.5 (∼7%) myocardium. Further we used immunogold labeling technique to test whether these distinct cell populations were also positive for markers such as Nkx2.5, ISL1 and ANF. A preponderance of anti-Nkx2.5 label was found in the undifferentiated and moderately differentiated cell types. Anti-ANF label was found only in the cytoplasmic compartment of moderately differentiated and mature myocardial cells. All of the undifferentiated cells were negative for anti-ANF labeling. We did not find immuno-gold labeling with ISL1 in any of the three myocardial cell types. Based on these results, we suggest that embryonic myocardial cell differentiation is a gradual process and undifferentiated cells expressing Nkx2.5 in post-chamber myocardium may represent a progenitor cell population while cells expressing Nkx2.5 and ANF represent differentiating myocytes.

Ciona intestinalis as a model for cardiac development

The primitive chordate Ciona intestinalis has emerged as a significant model system for the study of heart development. The Ciona embryo employs a conserved heart gene network in the context of extremely low cell numbers and reduced genetic redundancy. Here, I review recent studies on the molecular genetics of Ciona cardiogenesis as well as classic work on heart anatomy and physiology. I also discuss the potential of employing Ciona to decipher a comprehensive chordate gene network and to determine how this network controls heart morphogenesis.

Cardioblast-intrinsic Tinman activity controls proper diversification and differentiation of myocardial cells in Drosophila

The NK homeobox gene tinman (tin) is required for the specification of the cardiac, visceral muscle and somatic muscle progenitors in the early dorsal mesoderm of Drosophila. Like its vertebrate counterpart Nkx2.5, the expression of tin is maintained in cardiac cells during cardiac maturation and differentiation; however, owing to the complete lack of a dorsal vessel in tin mutant embryos, the function of tin in these cells has not been defined. Here we show that myocardial cells and dorsal vessels can form even though they lack Tin, and that viable adults can develop, as long as Tin is provided in the embryonic precardiac mesoderm. However, embryos in which tin expression is specifically missing from cardial cells show severe disruptions in the normal diversification of the myocardial cells, and adults exhibit severe defects in cardiac remodeling and function. Our study reveals that the normal expression and activity of Tin in four of the six bilateral cardioblasts within each hemisegment of the heart allows these cells to adopt a cell fate as ;working' myocardium, as opposed to a fate as inflow tract (ostial) cells. This function of tin involves the repression of Dorsocross (Doc) T-box genes and, hence, the restriction of Doc to the Tin-negative cells that will form ostia. We conclude that tin has a crucial role within myocardial cells that is required for the proper diversification, differentiation, and post-embryonic maturation of cardiomyocytes, and we present a pathway involving regulatory interactions among seven-up, midline, tinman and Dorsocross that establishes these developmental events upon myocardial cell specification.

Retinoic acid regulates morphogenesis and patterning of posterior foregut derivatives

Retinoic acid (RA) is an embryonic signaling molecule regulating a wide array of target genes, thereby being a master regulator of patterning and differentiation in a variety of organs. Here we show that mouse embryos deficient for the RA-synthesizing enzyme retinaldehyde dehydrogenase 2 (RALDH2), if rescued from early lethality by maternal RA supplementation between E7.5 and E8.5, lack active RA signaling in the foregut region. The resulting mutants completely fail to develop lungs. Development of more posterior foregut derivatives (stomach and duodenum), as well as liver growth, is also severely affected. A primary lung bud is specified in the RA-deficient embryos, which fails to outgrow due to defective FGF10 signaling and lack of activation of FGF-target genes, such as Pea3 and Bmp4 in the epithelium. Specific Hox and Tbx genes may mediate these RA regulatory effects. Development of foregut derivatives can be partly restored in mutants by extending the RA supplementation until at least E10.5, but lung growth and branching remain defective and a hypoplastic lung develops on the right side only. Such conditions poorly restore FGF10 signaling in the lung buds. Explant culture of RALDH2-deficient foreguts show a capacity to undergo lung budding and early branching in the presence of RA or FGF10. Our data implicate RA as a regulator of gene expression in the early embryonic lung and stomach region upstream of Hox, Tbx and FGF10 signaling.

Patterning of the third pharyngeal pouch into thymus/parathyroid by Six and Eya1

Previous studies have suggested a role of the homeodomain Six family proteins in patterning the developing vertebrate head that involves appropriate segmentation of three tissue layers, the endoderm, the paraxial mesoderm and the neural crest cells; however, the developmental programs and mechanisms by which the Six genes act in the pharyngeal endoderm remain largely unknown. Here, we examined their roles in pharyngeal pouch development. Six1-/- mice lack thymus and parathyroid and analysis of Six1-/- third pouch endoderm demonstrated that the patterning of the third pouch into thymus/parathyroid primordia is initiated. However, the endodermal cells of the thymus/parathyroid rudiments fail to maintain the expression of the parathyroid-specific gene Gcm2 and the thymus-specific gene Foxn1 and subsequently undergo abnormal apoptosis, leading to a complete disappearance of organ primordia by E12.5. This thus defines the thymus/parathyroid defects present in the Six1 mutant. Analyses of the thymus/parathyroid development in Six1-/-;Six4-/- double mutant show that both Six1 and Six4 act synergistically to control morphogenetic movements of early thymus/parathyroid tissues, and the threshold of Six1/Six4 appears to be crucial for the regulation of the organ primordia-specific gene expression. Previous studies in flies and mice suggested that Eya and Six genes may function downstream of Pax genes. Our data clearly show that Eya1 and Six1 expression in the pouches does not require Pax1/Pax9 function, suggesting that they may function independently from Pax1/Pax9. In contrast, Pax1 expression in all pharyngeal pouches requires both Eya1 and Six1 function. Moreover, we show that the expression of Tbx1, Fgf8 and Wnt5b in the pouch endoderm was normal in Six1-/- embryos and slightly reduced in Six1-/-;Six4-/- double mutant, but was largely reduced in Eya1-/- embryos. These results indicate that Eya1 appears to be upstream of very early events in the initiation of thymus/parathyroid organogenesis, while Six genes appear to act in an early differentiation step during thymus/parathyroid morphogenesis. Together, these analyses establish an essential role for Eya1 and Six genes in patterning the third pouch into organ-specific primordia.

Pendrin is a novel in vivo downstream target gene of the TTF-1/Nkx-2.1 homeodomain transcription factor in differentiated thyroid cells

Thyroid transcription factor gene 1 (TTF-1) is a homeobox-containing gene involved in thyroid organogenesis. During early thyroid development, the homeobox gene Nkx-2.5 is expressed in thyroid precursor cells coincident with the appearance of TTF-1. The aim of this study was to investigate the molecular mechanisms underlying thyroid-specific gene expression. We show that the Nkx-2.5 C terminus interacts with the TTF-1 homeodomain and, moreover, that the expression of a dominant-negative Nkx-2.5 isoform (N188K) in thyroid cells reduces TTF-1-driven transcription by titrating TTF-1 away from its target DNA. This process reduced the expression of several thyroid-specific genes, including pendrin and thyroglobulin. Similarly, down-regulation of TTF-1 by RNA interference reduced the expression of both genes, whose promoters are sensitive to and directly associate with TTF-1 in the chromatin context. In conclusion, we demonstrate that pendrin and thyroglobulin are downstream targets in vivo of TTF-1, whose action is a prime factor in controlling thyroid differentiation in vivo.

The Drosophila melanogaster T-box genes midline and H15 are conserved regulators of heart development

The Drosophila melanogaster genes midline and H15 encode predicted T-box transcription factors homologous to vertebrate Tbx20 genes. All identified vertebrate Tbx20 genes are expressed in the embryonic heart and we find that both midline and H15 are expressed in the cardioblasts of the dorsal vessel, the insect organ equivalent to the vertebrate heart. The midline mRNA is first detected in dorsal mesoderm at embryonic stage 12 in the two progenitors per hemisegment that will divide to give rise to all six cardioblasts. Expression of H15 mRNA in the dorsal mesoderm is detected first in four to six cells per hemisegment at stage 13. The expression of midline and H15 in the dorsal vessel is dependent on Wingless signaling and the transcription factors tinman and pannier. We find that the selection of two midline-expressing cells from a pool of competent progenitors is dependent on Notch signaling. Embryos deleted for both midline and H15 have defects in the alignment of the cardioblasts and associated pericardial cells. Embryos null for midline have weaker and less penetrant phenotypes while embryos deficient for H15 have morphologically normal hearts, suggesting that the two genes are partially redundant in heart development. Despite the dorsal vessel defects, embryos mutant for both midline and H15 have normal numbers of cardioblasts, suggesting that cardiac cell fate specification is not disrupted. However, ectopic expression of midline in the dorsal mesoderm can lead to dramatic increases in the expression of cardiac markers, suggesting that midline and H15 participate in cardiac fate specification and may normally act redundantly with other cardiogenic factors. Conservation of Tbx20 expression and function in cardiac development lends further support for a common ancestral origin of the insect dorsal vessel and the vertebrate heart.

GATA-6 can act as a positive or negative regulator of smooth muscle-specific gene expression

The GATA-4/5/6 family of transcription factors is important for the development of the cardiovascular system and the visceral endoderm. GATA-6 is the only family member expressed in vascular smooth muscle cells and has been shown to be important for controlling the phenotype of these cells following vascular injury. To clarify further the role of GATA-6 in regulating vascular smooth muscle differentiation, we directly examined its ability to regulate the promoters of smooth muscle-specific genes. This analysis revealed that GATA-6 strongly repressed telokin promoter activity. In contrast, GATA-6 activated the smooth muscle myosin heavy chain and smooth muscle alpha-actin promoters and had no significant effect on the SM22alpha promoter. Gel mobility shift assays demonstrate that GATA-6 binds to a consensus site adjacent to the CArG box in the telokin promoter. GATA-6 did not interfere with the serum-response factor-stimulated promoter activity but blocked myocardin-induced activation of the telokin promoter. In contrast, GATA-6 and myocardin resulted in synergistic activation of the smooth muscle myosin heavy chain promoter. Consistent with these findings, overexpression of GATA-6 in smooth muscle cells selectively inhibited expression of endogenous telokin, while simultaneously increasing expression of other smooth muscle proteins. These data suggest that GATA-6 selectively inhibits telokin expression by triggering the displacement of myocardin from the serum-response factor. As GATA-6 is expressed at high levels in vascular smooth muscle, this finding may explain the relatively low levels of telokin expression in the vascular system. These data also reveal a novel transcription regulatory mechanism by which GATA-6 can modulate the activity of the myocardin-serum-response factor complexes.

Proliferation and Differentiation Processes in the Heart Muscle Elements in Different Phylogenetic Groups

This article reviews, discusses, and summarizes data about the generative behavior of muscle tissue cells, the mechanisms of its regulation, and the organization of the endocrine function of the heart in the main phylogenetic groups. With respect to the ratio of processes of proliferation and differentiation, cell organization, and growth mechanism, muscle tissues of propulsive organs can be divided into three types, each revealed in one of three main groups of animals, lophotrochozoans, ecdysozoans, and chordates. Ecdysterone is likely to play the key role in the regulation of proliferation and differentiation processes in the heart muscle of crustaceans, and, most probably, also of molluscs. In each of the three main phylogenetic groups the endocrine function of the heart consisting of secretion of natriuretic peptides has a peculiar organization. Vertebrate cardiomyocytes are known to combine contractile and endocrine differentiation. Such functional dualism is absent in heart muscle elements of Lophotrochozoa and Ecdysozoa; in the heart of lopfotrochozoans, secretion of natriuretic peptides is performed by endothelial cells and their derivatives. Homology of the heart muscle in the animal kingdom as well as possible mechanisms of genomic and epigenomic regulation of different types of cardiomyogenesis are discussed.

NKX2.5 mutations in patients with congenital heart disease

OBJECTIVES

The purpose of this study was to estimate the frequency of NKX2.5 mutations in specific cardiovascular anomalies and investigate genotype-phenotype correlations in individuals with NKX2.5 mutations.

BACKGROUND

Recent reports have implicated mutations in the transcription factor NKX2.5 as a cause of various congenital heart defects (CHD).

METHODS

We tested genomic deoxyribonucleic acid from 608 prospectively recruited patients with conotruncal anomalies (n = 370), left-sided lesions (n = 160), secundum atrial septal defect (ASD) (n = 71), and Ebstein's malformation (n = 7) for NKX2.5 mutations.

RESULTS

Twelve distinct mutations in the NKX2.5 coding region were identified in 18 of 608 patients (3%), including 9 of 201 (4%) with tetralogy of Fallot, 3 of 71 (4%) with a secundum ASD, one each with truncus arteriosus, double-outlet right ventricle, L-transposition of the great arteries, interrupted aortic arch, hypoplastic left heart syndrome, and aortic coarctation, but in no patients with D-transposition of the great arteries (n = 86) or valvar aortic stenosis (n = 21). Eleven of the mutations were amino acid-altering missense nucleotide substitutions or deletions, and one was predicted to cause premature termination of translation. None of the mutations were in the homeodomain. Sixteen of the 18 individuals with NKX2.5 mutations in this study had no family history of congenital cardiovascular anomalies, and one had first-degree atrioventricular (AV) block.

CONCLUSIONS

NKX2.5 mutations occur in a small percentage of patients with various CHD. Most of the mutations identified in this study were missense, outside the homeodomain, and not associated with AV block. These findings suggest that NKX2.5 mutations in non-homeodomain regions may be important in the development of human structural cardiac defects.

Lineage choice and differentiation in mouse embryos and embryonic stem cells

The use of embryonic stem (ES) cells for generating healthy tissues has the potential to revolutionize therapies for human disease or injury, for which there are currently no effective treatments. Strategies for manipulating stem cell differentiation should be based on knowledge of the mechanisms by which lineage decisions are made during early embryogenesis. Here, we review current research into the factors influencing lineage differentiation in the mouse embryo and the application of this knowledge to in vitro differentiation of ES cells. In the mouse embryo, specification of tissue lineages requires cell-cell interactions that are influenced by coordinated cell migration and cellular neighborhood mediated by the key WNT, FGF, and TGFbeta signaling pathways. Mimicking the cellular interactions of the embryo by providing appropriate signaling molecules in culture has enabled the differentiation of ES cells to be directed predominately toward particular lineages. Multistep strategies incorporating the provision of soluble factors known to influence lineage choices in the embryo, coculture with other cells or tissues, genetic modification, and selection for desirable cell types have allowed the production of ES cell derivatives that produce beneficial effects in animal models. Increasing the efficiency of this process can only result from a better understanding of the molecular control of cell lineage determination in the embryo.

AmphiNk2-tin, an amphioxus homeobox gene expressed in myocardial progenitors: insights into evolution of the vertebrate heart

We isolated a full-length cDNA clone of amphioxus AmphiNk2-tin, an NK2 gene similar in sequence to vertebrate NK2 cardiac genes, suggesting a potentially similar function to Drosophila tinman and to vertebrate NK2 cardiac genes during heart development. During the neurula stage of amphioxus, AmphiNk2-tin is expressed first within the foregut endoderm, then transiently in muscle precursor cells in the somites, and finally in some mesoderm cells of the visceral peritoneum arranged in an approximately midventral row running beneath the midgut and hindgut. The peritoneal cells that express AmphiNk2-tin are evidently precursors of the myocardium of the heart, which subsequently becomes morphologically detectable ventral to the gut. The amphioxus heart is a rostrocaudally extended tube consisting entirely of myocardial cells (at both the larval and adult stages); there are no chambers, valves, endocardium, epicardium, or other differentiated features of vertebrate hearts. Phylogenetic analysis of the AmphiNk2-tin sequence documents its close relationship to vertebrate NK2 class cardiac genes, and ancillary evidence suggests a relationship with the Drosophila NK2 gene tinman. Apparently, an amphioxus-like heart, and the developmental program directing its development, was the foundation upon which the vertebrate heart evolved by progressive modular innovations at the genetic and morphological levels of organization.

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.

Molecular dissection of craniofacial development using zebrafish

The zebrafish, Danio rerio, is a small, freshwater teleost that only began to be used as a vertebrate genetic model by the late George Streisinger in the early 1980s. The strengths of the zebrafish complement genetic studies in mice and embryological studies in avians. Its advantages include high fecundity, externally fertilized eggs and transparent embryos that can be easily manipulated, inexpensive maintenance, and the fact that large-scale mutagenesis screens can be performed. Here we review studies that have used the zebrafish as a model for craniofacial development. Lineage studies in zebrafish have defined the origins of the cranial skeleton at the single-cell level and followed the morphogenetic behaviors of these cells in skeletal condensations. Furthermore, genes identified by random mutational screening have now revealed genetic pathways controlling patterning of the jaw and other pharyngeal arches, as well as the midline of the skull, that are conserved between fish and humans. We discuss the potential impact of specialized mutagenesis screens and the future applications of this versatile, vertebrate developmental model system in the molecular dissection of craniofacial development.

The last common bilaterian ancestor

Many regulatory genes appear to be utilized in at least superficially similar ways in the development of particular body parts in Drosophila and in chordates. These similarities have been widely interpreted as functional homologies, producing the conventional view of the last common protostome-deuterostome ancestor (PDA) as a complex organism that possessed some of the same body parts as modern bilaterians. Here we discuss an alternative view, in which the last common PDA had a less complex body plan than is frequently conceived. This reconstruction alters expectations for Neoproterozoic fossil remains that could illustrate the pathways of bilaterian evolution.

Novel point mutation in the cardiac transcription factor CSX/NKX2.5 associated with congenital heart disease

The homeobox transcription factor CSX/NKX2.5, which is a vertebrate homologue of the Drosophila gene tinman, is essential for cardiac development. It is expressed in the early cardiac mesoderm and in heart muscle lineage throughout life. Homozygous deletion of CSX/NKX2.5 causes early embryonic lethality in mice because cardiac development is arrested at the linear heart tube stage. Heterozygous mutation of human CSX/NKX2.5 has been associated with various congenital heart diseases such as atrial septal defect (ASD), ventricular septal defect, tetralogy of Fallot, and tricuspid valve abnormalities, including Ebstein's anomaly. Additionally, CSX/NKX2.5 mutation causes atrioventricular (AV) conduction block with or without associated congenital heart diseases. Ten different heterozygous mutations have been already reported and a new point mutation, which is a C-to-A transition (Cys264ter) at nucleotide 901 of CSX/NKX2.5, results in the production of a truncated protein occurring COOH-terminal to the homeodomain of CSX/NKX2.5. The mutation was found in a patient with familial ASD and first-degree AV block; 4 members from 3 generations had secundum-type ASD and first-degree AV block.

Cooperative E-box regulation of human GLI1 by TWIST and USF

Sonic hedgehog signaling plays a critical role in vertebrate patterning, and signaling defects are associated with severe birth defects and cancer in man. GLI1 encodes a critical transcription activator in this pathway. GLI1 is expressed in human basal cell carcinomas and sarcomas. Despite the significance of the GLI1 gene in human disease, few immediate upstream regulators of GLI1 expression are known. We previously demonstrated that a 5' region, including 5' flanking sequence, an untranslated exon, and 425 bp of the first intron, regulates the human GLI1 gene. Here we show that inactivating mutations in E-box, GC box, AP-2, GATA, GSG, PuF, and Zeste sites identified three critical regulatory elements, including a GC box that binds Sp1 and two intronic E-boxes that bind USF proteins or Twist. Expression of Twist but not a frame shift mutation of Twist activates the wild-type human GLI1 regulatory sequences but not with inactivating mutations of the E-boxes. Twist activates GLI1 reporter expression through E-box +482 but requires binding of USF proteins to E-box +157. Twist mutations cause human birth defects and Twist is overexpressed in many rhabdomyosarcomas, suggesting that one of Twist's primary roles is the regulation of GLI1.

Transcriptional regulation of vertebrate cardiac morphogenesis

Transcription factors can regulate the expression of other genes in a tissue-specific and quantitative manner and are thus major regulators of embryonic developmental processes. Several transcription factors that regulate cardiac genes specifically have been described, and the recent discovery that dominant inherited transcription factor mutations cause congenital heart defects in humans has brought direct medical relevance to the study of cardiac transcription factors in heart development. Although this field of study is extensive, several major gaps in our knowledge of the transcriptional control of heart development still exist. This review will concentrate on recent developments in the field of cardiac transcription factors and their roles in heart formation.

The smooth muscle gamma-actin gene is androgen responsive in prostate epithelia

Nkx 3.1 is an evolutionarily conserved vertebrate homolog of the Drosophila Nk-3 homeodomain gene bagpipe that is expressed by a variety of cells during early mammalian development and has been shown to be a critical factor for prostate development and function. Previous studies utilizing a heterologous cell transfection strategy from our laboratory identified the smooth muscle gamma-actin (SMGA) gene as a novel molecular target of Nkx 3.1 regulatory activity. In the studies presented here, SMGA gene activity and regulation were evaluated in normal and cancerous prostate epithelial cells. SMGA transcripts were demonstrated in prostate epithelia and SMGA mRNA levels were increased in androgen-responsive LNCaP cancer and normal prostate epithelial cells. SMGA gene transcriptional activity was androgen responsive in these cells and required a segment of the human SMGA promoter containing NKE and SRF (serum response factor) binding elements. This region of the human SMGA proximal promoter is well conserved across species and is synergistically activated by coexpression of Nkx 3.1 and SRF in heterologous CV-1 cells. SMGA transcription was not responsive to steroid in PC-3 prostate epithelial cancer cells, which do not express Nkx 3.1. However, SMGA transcription was influenced by expression of androgen receptor in these cells, a situation that allows the androgen-dependent expression of Nkx 3.1. Furthermore, SMGA gene activity was influenced by direct Nkx 3.1 expression in the PC-3 cells. Thus, SMGA gene activity in prostate epithelia is due, in part, to the androgen-dependent expression of Nkx 3.1. As such, our studies provide the initial description of Nkx 3.1 target gene regulatory activity in the prostate.

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.

Molecular evolution of the homeodomain family of transcription factors

The homeodomain family of transcription factors plays a fundamental role in a diverse set of functions that include body plan specification, pattern formation and cell fate determination during metazoan development. Members of this family are characterized by a helix-turn-helix DNA-binding motif known as the homeodomain. Homeodomain proteins regulate various cellular processes by specifically binding to the transcriptional control region of a target gene. These proteins have been conserved across a diverse range of species, from yeast to human. A number of inherited human disorders are caused by mutations in homeodomain-containing proteins. In this study, we present an evolutionary classification of 129 human homeodomain proteins. Phylogenetic analysis of these proteins, whose sequences were aligned based on the three-dimensional structure of the homeodomain, was performed using a distance matrix approach. The homeodomain proteins segregate into six distinct classes, and this classification is consistent with the known functional and structural characteristics of these proteins. An ancestral sequence signature that accurately describes the unique sequence characteristics of each of these classes has been derived. The phylogenetic analysis, coupled with the chromosomal localization of these genes, provides powerful clues as to how each of these classes arose from the ancestral homeodomain.

Nkx2.5 and Nkx2.6, homologs of Drosophila tinman, are required for development of the pharynx

Nkx2.5 and Nkx2.6 are murine homologs of Drosophila tinman. Their genes are expressed in the ventral region of the pharynx at early stages of embryogenesis. However, no abnormalities in the pharynges of embryos with mutations in either Nkx2.5 or Nkx2.6 have been reported. To examine the function of Nkx2.5 and Nkx2.6 in the formation of the pharynx, we generated and analyzed Nkx2.5 and Nkx2.6 double-mutant mice. Interestingly, in the double-mutant embryos, the pharynx did not form properly. Pharyngeal endodermal cells were largely missing, and the mutant pharynx was markedly dilated. Moreover, we observed enhanced apoptosis and reduced proliferation in pharyngeal endodermal cells of the double-mutant embryos. These results demonstrated a critical role of the NK-2 homeobox genes in the differentiation, proliferation, and survival of pharyngeal endodermal cells. Furthermore, the development of the atrium was less advanced in the double-mutant embryos, indicating that these two genes are essential for both pharyngeal and cardiac development.

Tbx5 associates with Nkx2-5 and synergistically promotes cardiomyocyte differentiation

The cardiac homeobox protein Nkx2-5 is essential in cardiac development, and mutations in Csx (which encodes Nkx2-5) cause various congenital heart diseases. Using the yeast two-hybrid system with Nkx2-5 as the 'bait', we isolated the T-box-containing transcription factor Tbx5; mutations in TBX5 cause heart and limb malformations in Holt-Oram syndrome (HOS). Co-transfection of Nkx2-5 and Tbx5 into COS-7 cells showed that they also associate with each other in mammalian cells. Glutathione S-transferase (GST) 'pull-down' assays indicated that the N-terminal domain and N-terminal part of the T-box of Tbx5 and the homeodomain of Nkx2-5 were necessary for their interaction. Tbx5 and Nkx2-5 directly bound to the promoter of the gene for cardiac-specific natriuretic peptide precursor type A (Nppa) in tandem, and both transcription factors showed synergistic activation. Deletion analysis showed that both the N-terminal domain and T-box of Tbx5 were important for this transactivation. A G80R mutation of Tbx5, which causes substantial cardiac defects with minor skeletal abnormalities in HOS, did not activate Nppa or show synergistic activation, whereas R237Q, which causes upper-limb malformations without cardiac abnormalities, activated the Nppa promoter to a similar extent to that of wildtype Tbx5. P19CL6 cell lines overexpressing wildtype Tbx5 started to beat earlier and expressed cardiac-specific genes more abundantly than did parental P19CL6 cells, whereas cell lines expressing the G80R mutant did not differentiate into beating cardiomyocytes. These results indicate that two different types of cardiac transcription factors synergistically induce cardiac development.

Gizzard formation and the role of Bapx1

The anterior-posterior gut pattern is formed from three broad domains: fore-, mid-, and hindgut that have distinct functional, morphological, and molecular boundaries. The stomach demarcates the posterior boundary of the foregut. Avian stomachs are composed of two chambers: the anterior chamber (proventriculus) and the thick muscular posterior chamber (gizzard). Expression of candidate pattern formation control factors are restricted in the chick stomach regions such that Bmp4 and Wnt5a are not expressed in the gizzard. We previously implicated Bmp4 as controlling growth and differentiation of the gut musculature. Bmp4 is not expressed in the developing gizzard but is expressed in the rest of the gut including the adjacent proventriculus and midgut. Bapx1 (Nkx3.2) is expressed in the gizzard musculature but not in the proventriculus or midgut. We show ectopic expression of Bapx1 in the proventriculus results in a gizzard-like morphology and inhibits the normal proventricular expression of Bmp4 and Wnt5a. Overexpression of a reverse-function Bapx1 construct can result in a small stomach and ectopic extension of Bmp4 and Wnt5a expression into the gizzard. We suggest the role of Bapx1 is to regulate the expression of Bmp4 and Wnt5a to pattern the avian stomach.

A cluster of Drosophila homeobox genes involved in mesoderm differentiation programs

Although genes involved in common developmental programs are usually scattered throughout the metazoan genome, there are some important examples of functionally interconnected regulatory genes that display close physical linkage. In particular the homeotic genes, which determine the identities of body parts, are clustered in the Hox complexes and clustering is thought to be crucial for the proper execution of their developmental programs. Here we describe the organization and functional properties of a more recently identified cluster of six homeobox genes at 93DE on the third chromosome of Drosophila. These genes, which include tinman, bagpipe, ladybird early, ladybird late, C15, and slouch, all participate in mesodermal patterning and differentiation programs and show multiple regulatory interactions among each other. We propose that their clustering, through unknown mechanisms, is functionally significant and discuss the similarities and differences between the 93DE homeobox gene cluster and the Hox complexes.

The smooth muscle gamma-actin gene promoter is a molecular target for the mouse bagpipe homologue, mNkx3-1, and serum response factor

An evolutionarily conserved vertebrate homologue of the Drosophila NK-3 homeodomain gene bagpipe, Nkx3-1, is expressed in vascular and visceral mesoderm-derived muscle tissues and may influence smooth muscle cell differentiation. Nkx3-1 was evaluated for mediating smooth muscle gamma-actin (SMGA) gene activity, a specific marker of smooth muscle differentiation. Expression of mNkx3-1 in heterologous CV-1 fibroblasts was unable to elicit SMGA promoter activity but required the coexpression of serum response factor (SRF) to activate robust SMGA transcription. A novel complex element containing a juxtaposed Nkx-binding site (NKE) and an SRF-binding element (SRE) in the proximal promoter region was found to be necessary for the Nkx3-1/SRF coactivation of SMGA transcription. Furthermore, Nkx3-1 and SRF associate through protein-protein interactions and the homeodomain region of Nkx3-1 facilitated SRF binding to the complex NKE.SRE. Mutagenesis of Nkx3-1 revealed an inhibitory domain within its C-terminal segment. In addition, mNkx3-1/SRF cooperative activity required an intact Nkx3-1 homeodomain along with the MADS box of SRF, which contains DNA binding and dimerization structural domains, and the contiguous C-terminal SRF activation domain. Thus, SMGA is a novel target for Nkx3-1, and the activity of Nkx3-1 on the SMGA promoter is dependent upon SRF.

MetaHox gene clusters

Homeobox genes encode important developmental control proteins. The Drosophila fruit fly HOM complex genes are clustered in region 84-89 of chromosome 3. Probably due to large-scale genome duplication events, their human HOX orthologs belong to four paralogous regions. A series of 13 other homeobox genes are also clustered in region 88-94, on the same chromosome of Drosophila. We suggest that they also duplicated during vertebrate evolution and belong to paralogous regions in humans. These regions are on chromosome arms 4p, 5q, 10q, and 2p or 8p. We coined the term "paralogon" to designate paralogous regions in general. We propose to call these genes "meta Hox" genes. Like Hox genes, metaHox genes are present in one cluster in Drosophila and four clusters (metaHox A-D) in humans on the 4p/5q/10q paralogon.

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.

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.