Zebrafish tinman homolog demarcates the heart field and initiates myocardial differentiation

Csm, a cardiac-specific isoform of the RNA helicase Mov10l1, is regulated by Nkx2.5 in embryonic heart

Nkx2.5 (also called Csx) is an evolutionarily conserved cardiac transcription factor of the homeobox gene family. Nkx2.5 is required for early heart development, because Nkx2.5 null mice die before completion of cardiac looping. To identify genes regulated by Nkx2.5 in the developing heart, we performed differential screening in combination with suppression subtractive hybridization using RNA isolated from wild-type and Nkx2.5 null hearts at embryonic day 8.5. One gene that we found to be markedly down-regulated in the hearts from Nkx2.5 null embryos is an isoform of Mov10 like-1 (Mov10l1), a putative RNA helicase expressed in testis. We named this novel isoform as Csm (cardiac-specific isoform of Mov10l1). Csm is identical with the 3' region of the Mov10l1 gene, but its transcript starts from the exon 16 of Mov10l1. The conceptual protein encoded by Csm cDNA contains a helicase motif as well as ATPase and RNA interaction motifs. Csm is expressed specifically in the heart, and its expression in the heart is restricted to cardiac myocytes. Csm potentiated phenylephrine-induced hypertrophic response in cardiac myocytes. Furthermore, transient cotransfection analysis showed that Nkx2.5 transactivates the Csm promoter, suggesting that Nkx2.5 is essential for embryonic Csm expression.

The different cardiac expression of the type 2 iodothyronine deiodinase gene between human and rat is related to the differential response of the Dio2 genes to Nkx-2.5 and GATA-4 transcription factors

By producing T3 from T4, type 2 iodothyronine deiodinase (D2) catalyzes the first step in the cascade underlying the effect exerted by thyroid hormone. Type 2 iodothyronine deiodinase mRNA is expressed at high levels in human heart but is barely detectable in the corresponding rodent tissue. Although the heart is a major target of thyroid hormone, the role of cardiac D2 and the factors that regulate its expression are unknown. Here we report that the human Dio2 promoter is very sensitive to the cardiac transcription factors Nkx-2.5 and GATA-4. Nkx-2.5 transactivates a 6.5-kb human (h)Dio2-chloramphenicol acetyltransferase construct, with maximal induction reached with a 633-bp proximal promoter region. Interestingly, despite 73% identity with the corresponding human region, the rat Dio2 promoter is much less responsive to Nkx-2.5 induction. Using EMSA, we found that two sites in the human promoter (C and D) specifically bind Nkx-2.5. In coexpression studies, GATA-4 alone was a poor inducer of the hDio2 promoter; however in synergy with Nkx-2.5, it activated D2 reporter gene expression in the human, but not the rat promoter. Functional analysis showed that both C and D sites are required for the complete Nkx-2.5 response and for the Nkx-2.5/GATA-4 synergistic effect. In neonatal rat primary myocardiocytes, most of the hDio2-chloramphenicol acetyltransferase activity was suppressed by mutation of the Nkx-2.5 binding sites. Finally, a mutant Nkx-2.5 protein (N188K), which causes, in heterozygosity, congenital heart diseases, did not transactivate the Dio2 promoter and interfered with its activity in cardiomyocytes, possibly by titrating endogenous Nkx-2.5 protein away from the promoter. In conclusion, this study shows that Nkx-2.5 and GATA-4 play prime roles in Dio2 gene regulation in the human heart and suggests that it is their synergistic action in humans that causes the differential expression of the cardiac Dio2 gene between humans and rats.

Cardiac neural crest contributes to cardiomyogenesis in zebrafish

In birds and mammals, cardiac neural crest is essential for heart development and contributes to conotruncal cushion formation and outflow tract septation. The zebrafish prototypical heart lacks outflow tract septation, raising the question of whether cardiac neural crest exists in zebrafish. Here, results from three distinct lineage-labeling approaches identify zebrafish cardiac neural crest cells and indicate that these cells have the ability to generate MF20-positive muscle cells in the myocardium of the major chambers during development. Fate-mapping demonstrates that cardiac neural crest cells originate both from neural tube regions analogous to those found in birds, as well as from a novel region rostral to the otic vesicle. In contrast to other vertebrates, cardiac neural crest invades the myocardium in all segments of the heart, including outflow tract, atrium, atrioventricular junction, and ventricle in zebrafish. Three distinct groups of premigratory neural crest along the rostrocaudal axis have different propensities to contribute to different segments in the heart and are correspondingly marked by unique combinations of gene expression patterns. Zebrafish will serve as a model for understanding interactions between cardiac neural crest and cardiovascular development.

Myosin heavy chain expression in cranial, pectoral fin, and tail muscle regions of zebrafish embryos

To investigate whether different myosin heavy chain (MHC) isoforms may constitute myofibrils in the trunk and tail musculature and if their respective expression may be regulated by spadetail (spt) and no tail (brachyury), we identified and characterized mRNA expression patterns of an embryonic- and tail muscle-specific MHC gene (named myhz2) during zebrafish development in wild type, spt, and ntl mutant embryos. The identified myhz2 MHC gene encodes a polypeptide containing 1,935 amino acids. Deduced amino acid comparisons showed that myhz2 MHC shared 92.6% sequence identity with that of carp fast skeletal MHC. Temporal and spatial myhz2 MHC mRNA expression patterns were analyzed by quantitative RT-PCR and whole-mount in situ hybridization using primer pairs and probes designed from the 3'-untranslated region (UTR). Temporally myhz2 MHC mRNA appears in pharyngula embryos and peaks in protruding-mouth larvae. The expression level decreased in 7-day-old hatching larvae, and mRNA expression was not detectable in adult fish. Spatially in pharyngula embryos, mRNA was localized only in the tail somite region, while in long-pec embryos, transcripts were also expressed in the two cranial muscle elements of the adductor mandibulae and medial rectus, as well as in pectoral fin muscles and the tail muscle region. Myhz2 MHC mRNA was expressed in most cranial muscle elements, pectoral fin muscles, and the tail muscle region of 3-day-old hatching larvae. In contrast, no expression of myhz2 MHC mRNA could be observed in spt prim-15 mutant embryos. In spt long-pec mutant embryos, transcripts were expressed in two cranial muscle elements and the tail muscle region, but not in pectoral fin muscles, while only trace amounts of myhz2 MHC mRNA were expressed in the remaining tail muscle region of 38 hpf and long-pec ntl mutant embryos.

Early signals in cardiac development

The heart is the first organ to form during embryogenesis and its circulatory function is critical from early on for the viability of the mammalian embryo. Developmental abnormalities of the heart have also been widely recognized as the underlying cause of many congenital heart malformations. Hence, the developmental mechanisms that orchestrate the formation and morphogenesis of this organ have received much attention among classical and molecular embryologists. Due to the evolutionary conservation of many of these processes, major insights have been gained from the studies of a number of vertebrate and invertebrate models, including mouse, chick, amphibians, zebrafish, and Drosophila. In all of these systems, the heart precursors are generated within bilateral fields in the lateral mesoderm and then converge toward the midline to form a beating linear heart tube. The specification of heart precursors is a result of multiple tissue and cell-cell interactions that involve temporally and spatially integrated programs of inductive signaling events. In the present review, we focus on the molecular and developmental functions of signaling processes during early cardiogenesis that have been defined in both vertebrate and invertebrate models. We discuss the current knowledge on the mechanisms through which signals induce the expression of cardiogenic transcription factors and the relationships between signaling pathways and transcriptional regulators that cooperate to control cardiac induction and the formation of a linear heart tube.

Dual effects of the homeobox transcription factor Csx/Nkx2-5 on cardiomyocytes

A homeobox-containing transcription factor Csx/Nkx2-5 is an important regulator of cardiac development. Many different human CSX/NKX2-5 mutations have been reported to cause congenital heart disease. We here examined the effects of three representative CSX/NKX2-5 mutations on cardiomyocyte differentiation and death with the use of the P19CL6 cardiomyogenic cell lines. Stable overexpression of wild-type CSX/NKX2-5 enhanced expression of cardiac-specific genes such as MEF2C and MLC2v, the promoter activity of the atrial natriuretic peptide gene, and the terminal differentiation of P19CL6 into cardiomyocytes, while all CSX/NKX2-5 mutants attenuated them by different degrees. When exposed to H(2)O(2) or cultured without change of the medium, many differentiated P19CL6 cells overexpressing the mutants, especially the mutant which lacks the carboxyl terminal region just after the homeodomain, were dead, while most of the cells overexpressing wild-type CSX/NKX2-5 survived. Overexpression of the carboxyl terminus-deleted mutant down-regulated expression of an anti-apoptotic protein Bcl-x(L) and up-regulated that of a pro-apoptotic protein CAS, while in the cells overexpressing wild-type CSX/NKX2-5, expression of a pro-apoptotic protein RIP was reduced. Furthermore, overexpression of wild-type CSX/NKX2-5 decreased the number of H(2)O(2)-induced TUNEL-positive cultured cardiomyocytes of neonatal rats, whereas overexpression of the mutants enhanced it. These results suggest that Csx/Nkx2-5 not only regulates expression of cardiac-specific genes but protects cardiomyocytes from stresses and that cell death may be another cause for the cardiac defects induced by human CSX/NKX2-5 mutations.

Molecular embryogenesis of the heart

Development of the heart is a complex process involving primary and secondary heart fields that are set aside to generate myocardial and endocardial cell lineages. The molecular inductions that occur in the primary heart field appear to be recapitulated in induction and myocardial differentiation of the secondary heart field, which adds the conotruncal segments to the primary heart tube. While much is now known about the initial steps and factors involved in induction of myocardial differentiation, little is known about induction of endocardial development. Many of the genes expressed by nascent myocardial cells, which then become committed to a specific heart segment, have been identified and studied. In addition to the heart fields, several other "extracardiac" cell populations contribute to the fully functional mature heart. Less is known about the genetic programs of extracardiac cells as they enter the heart and take part in cardiogenesis. The molecular/genetic basis of many congenital cardiac defects has been elucidated in recent years as a result of new insights into the molecular control of developmental events.

Zebrafish M2 muscarinic acetylcholine receptor: cloning, pharmacological characterization, expression patterns and roles in embryonic bradycardia

1. A zebrafish M2 muscarinic acetylcholine receptor (mAChR) gene was cloned. It encodes 495 amino acids in a single exon. The derived amino acid sequence is 73.5% identical to its human homologue. 2. Competitive binding studies of the zebrafish M2 receptor and [(3)H]-NMS gave negative log dissociation constants (pK(i)) for each antagonist as follows: atropine (9.16)>himbacine (8.05)>/=4-DAMP (7.83)>AF-DX 116 (7.26)>/=pirenzepine (7.18)>/=tropicamide (6.97)>/=methoctramine (6.82)>/=p-F-HHSiD (6.67)>carbachol (5.20). The antagonist affinity profile correlated with the profile of the human M2 receptor, except for pirenzepine. 3. Reverse transcription polymerase chain reaction and Southern blotting analysis demonstrated that the M2 mAChR mRNA levels increased during the segmentation period (12 h post-fertilization; h.p.f.) in zebrafish. By whole-mount in situ hybridization, the M2 mAChR was first detectable in the heart, vagus motor ganglion, and vagus sensory ganglion at 30, 48 and 60 h.p.f., respectively. 4. The muscarinic receptor that mediates carbachol (CCh)-induced bradycardia was functionally mature at 72 h.p.f. The effect of CCh-induced bradycardia was antagonized by several muscarinic receptor antagonists with the order of potency (pIC(50) values): atropine (6.76)>methoctramine (6.47)>himbacine (6.10)>4-DAMP (5.72)>AF-DX 116 (4.77), however, not by pirenzepine, p-F-HHSiD, or tropicamide (<10 micro M). 5. The effect of CCh-induced bradycardia was abolished completely before 56 h.p.f. by M2 RNA interference, and the bradycardia effect gradually recovered after 72 h.p.f. The basal heart rate was increased in embryos injected with M2 mAChR morpholino antisense oligonucleotide (M2 MO) and the effect of CCh-induced bradycardia was abolished by M2 MO in a dose-dependent manner. In conclusion, the results suggest that the M2 mAChR inhibit basal heart rate in zebrafish embryo and the M2 mAChR mediates the CCh-induced bradycardia.

A protein disulfide isomerase expressed in the embryonic midline is required for left/right asymmetries

Although the vertebrate embryonic midline plays a critical role in determining the left/right asymmetric development of multiple organs, few genes expressed in the midline are known to function specifically in establishing laterality patterning. Here we show that a gene encoding protein disulfide isomerase P5 (PDI-P5) is expressed at high levels in the organizer and axial mesoderm and is required for establishing left/right asymmetries in the zebrafish embryo. pdi-p5 was discovered in a screen to detect genes down-regulated in the zebrafish midline mutant one-eyed pinhead and expressed predominantly in midline tissues of wild-type embryos. Depletion of the pdi-p5 product with morpholino antisense oligonucleotides results in loss of the asymmetric development of the heart, liver, pancreas, and gut. In addition, PDI-P5 depletion results in bilateral expression of all genes known to be expressed asymmetrically in the lateral plate mesoderm and the brain during embryogenesis. The laterality defects caused by pdi-p5 antisense treatment arise solely due to loss of the PDI-P5 protein, as they are reversed when treated embryos are supplied with an exogenous source of the PDI-P5 protein. Thus the spectrum of laterality defects resulting from depletion of the PDI-P5 protein fully recapitulates that resulting from loss of the midline. As loss of PDI-P5 does not appear to interfere with other aspects of midline development or function, we propose that PDI-P5 is specifically involved in the production of midline-derived signals required to establish left/right asymmetry.

Modulation of Cardiac Growth and Development by HOP, an Unusual Homeodomain Protein

We have discovered an unusual homeodomain protein, called HOP, which is comprised simply of a homeodomain. HOP is highly expressed in the developing heart where its expression is dependent on the cardiac-restricted homeodomain protein Nkx2.5. HOP does not bind DNA and acts as an antagonist of serum response factor (SRF), which regulates the opposing processes of proliferation and myogenesis. Mice homozygous for a HOP null allele segregate into two phenotypic classes characterized by an excess or deficiency of cardiac myocytes. We propose that HOP modulates SRF activity during heart development; its absence results in an imbalance between cardiomyocyte proliferation and differentiation with consequent abnormalities in cardiac morphogenesis.

Assessment of zebrafish cardiac performance using Doppler echocardiography and power angiography

The zebrafish (Danio rerio) has become a new animal model for cardiac researches. Although it is equipped with a prototypical vertebrate heart, the zebrafish studies for cardiac mutations and genetic control of development can reveal some hints for solving human problems. Despite the simplicity of the zebrafish heart, the objective parameters of cardiac performance are not easily available, except for the morphological description, due to its small size. Because the four components (sinus venosus, atrium, ventricle and bulbus arteriosus) of the zebrafish heart are connected in series, we studied it by applying ultrasonic imaging methods for the vascular system. A total of 20 fishes that were ages of 3 to 4 months were studied. Their mean body weight and height were 562 +/- 173 mg and 4.6 +/- 0.7 cm, respectively. Power angiography and routine Doppler echocardiography were used to evaluate the cardiac performance of zebrafish at 25 degrees C and 15 degrees C. The zebrafish hearts could be easily identified with color Doppler (8.5 MHz) or power angiography (7 MHz). The ventricular filling flow contained two components (E and A-flow). The E-flow velocities were lower than the A-flow velocities at both 25 and 15 degrees C. The cycle length was prolonged (p < 0.05) and the velocities of ventricular filling and bulbus arteriosus decreased significantly at 15 degrees C (p < 0.05). A significant decrease in early diastolic deceleration slope and significant prolongation in early diastolic and late-diastolic deceleration times were found at a lower temperature (15 degrees C). The acceleration:deceleration ratio for early and late diastole also showed a significant difference at 15 degrees C. In conclusion, the cardiac performance of the zebrafish could be approached using commercially available clinical instruments equipped with Doppler echocardiography and power angiography.

Csx/Nkx2-5 is required for homeostasis and survival of cardiac myocytes in the adult heart

Csx/Nkx2-5, which is essential for cardiac development of the embryo, is abundantly expressed in the adult heart. We here examined the role of Csx/Nkx2-5 in the adult heart using two kinds of transgenic mice. Transgenic mice that overexpress a dominant negative mutant of Csx/Nkx2-5 (DN-TG mice) showed degeneration of cardiac myocytes and impairment of cardiac function. Doxorubicin induced more marked cardiac dysfunction in DN-TG mice and less in transgenic mice that overexpress wild type Csx/Nkx2-5 (WT-TG mice) compared with non-transgenic mice. Doxorubicin induced cardiomyocyte apoptosis, and the number of apoptotic cardiomyocytes was high in the order of DN-TG mice, non-transgenic mice, and WT-TG mice. Overexpression of the dominant negative mutant of Csx/Nkx2-5 induced apoptosis in cultured cardiomyocytes, while expression of wild type Csx/Nkx2-5 protected cardiomyocytes from doxorubicin-induced apoptotic death. These results suggest that Csx/Nkx2-5 plays a critical role in maintaining highly differentiated cardiac phenotype and in protecting the heart from stresses including doxorubicin.

Organogenesis--heart and blood formation from the zebrafish point of view

Organs are specialized tissues used for enhanced physiology and environmental adaptation. The cells of the embryo are genetically programmed to establish organ form and function through conserved developmental modules. The zebrafish is a powerful model system that is poised to contribute to our basic understanding of vertebrate organogenesis. This review develops the theme of modules and illustrates how zebrafish have been particularly useful for understanding heart and blood formation.

Cardiac patterning and morphogenesis in zebrafish

Development of the embryonic vertebrate heart requires the precise coordination of pattern formation and cell movement. Taking advantage of the availability of zebrafish mutations that disrupt cardiogenesis, several groups have identified key regulators of specific aspects of cardiac patterning and morphogenesis. Several genes, including gata5, fgf8, bmp2b, one-eyed pinhead, and hand2, have been shown to be relevant to the patterning events that regulate myocardial differentiation. Studies of mutants with morphogenetic defects have indicated at least six genes that are essential for cardiac fusion and heart tube assembly, including casanova, bonnie and clyde, gata5, one-eyed pinhead, hand2, miles apart, and heart and soul. Furthermore, analysis of the jekyll gene has indicated its important role during the morphogenesis of the atrioventricular valve. Altogether, these data provide a substantial foundation for future investigations of cardiac patterning, cardiac morphogenesis, and the relationship between these processes.

BMP signaling regulates Nkx2-5 activity during cardiomyogenesis

Nkx2-5 regulates the transcription of muscle-specific genes during cardiomyogenesis. Nkx2-5 expression can induce cardiomyogenesis in aggregated P19 cells but not in monolayer cultures. In order to investigate the mechanism by which cellular aggregation regulates Nkx2-5 function, we examined the role of bone morphogenetic protein 4 (BMP4). We showed that the expression of the BMP inhibitor, noggin, was sufficient to inhibit the induction of cardiomyogenesis by Nkx2-5 during cellular aggregation. Furthermore, soluble BMP4 could activate Nkx2-5 function in monolayer cultures, resulting in the formation of cardiomyocytes. Therefore, BMP signaling is necessary and sufficient for the regulation of Nkx2-5 activity during cardiomyogenesis in P19 cells.

Ostia, the inflow tracts of the Drosophila heart, develop from a genetically distinct subset of cardial cells

The homeobox gene tinman and the nuclear receptor gene seven-up are expressed in mutually exclusive dorsal vessel cells in Drosophila, however, the physiological reason for this distinction is not known. We demonstrate that tin and svp-lacZ expression persists through the larval stage to the adult stage in the same pattern of cells expressing these genes in the embryo. In the larva, six pairs of Svp-expressing cells form muscular ostia, which permit hemolymph to enter the heart for circulation, however, more anterior Svp-expressing cells form the wall of the dorsal vessel. During pupation, the adult heart forms from a chimera of larval and imaginal muscle fibers. The portion of the dorsal vessel containing the larval ostia is histolyzed and the anterior Svp-expressing cells metamorphose into imaginal ostia. This is the first demonstration that the significant molecular diversity of cardial cells identified in the embryonic heart correlates with the formation of physiologically and functionally distinct muscle cells in the animal. Furthermore, our experiments define the cellular changes that occur as the larval heart is remodeled into an imaginal structure in an important model organism.

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.

Progressive atrioventricular conduction defects and heart failure in mice expressing a mutant Csx/Nkx2.5 homeoprotein

A DNA nonbinding mutant of the NK2 class homeoprotein Nkx2.5 dominantly inhibits cardiogenesis in Xenopus embryos, causing a small heart to develop or blocking heart formation entirely. Recently, ten heterozygous CSX/NKX2.5 homeoprotein mutations were identified in patients with congenital atrioventricular (AV) conduction defects. All four missense mutations identified in the human homeodomain led to markedly reduced DNA binding. To examine the effect of a DNA binding-impaired mutant of mouse Csx/Nkx2.5 in the embryonic heart, we generated transgenic mice expressing one such allele, I183P, under the beta-myosin heavy chain promoter. Unexpectedly, transgenic mice were born apparently normal, but the accumulation of Csx/Nkx2.5(I183P) mutant protein in the embryo, neonate, and adult myocardium resulted in progressive and profound cardiac conduction defects and heart failure. P-R prolongation observed at 2 weeks of age rapidly progressed into complete AV block as early as 4 weeks of age. Expression of connexins 40 and 43 was dramatically decreased in the transgenic heart, which may contribute to the conduction defects in the transgenic mice. This transgenic mouse model may be useful in the study of the pathogenesis of cardiac dysfunction associated with CSX/NKX2.5 mutations in humans.

Primary structure and developmental expression of zebrafish sodium channel Na(v)1.6 during neurogenesis

A zebrafish sodium channel cDNA encoding a 1949-amino acid polypeptide, Na(v)1.6, was isolated. Two transcripts were detected in zebrafish adult brain but not in cardiac or skeletal muscle. The RNase protection analysis confirmed the neural specificity of zebrafish Na(v)1.6 24 hours postfertilization (hpf) Na(v)1.6 was expressed in the trigeminal ganglion, anterior and posterior lateral line ganglia, rhombomeres, and Rohon-Beard neurons. This preferential localization suggests that Na(v)1.6 plays an important role in tactile sensitivity. The abundance of zebrafish Na(v) 1.6 mRNA in the central and peripheral nervous systems increased markedly between 48 and 72 hpf, during the maturation of the nervous system.

Identification of a novel stretch-responsive skeletal muscle gene (Smpx)

Skeletal muscle is able to respond to a range of stimuli, including stretch and increased load, by increasing in diameter and length in the absence of myofiber division. This type of cellular growth (hypertrophy) is a highly complex process involving division of muscle precursor cells (myoblasts) and their fusion to existing muscle fibers as well as increased protein synthesis and decreased protein degradation. Underlying the alterations in protein levels are increases in a range of specific mRNAs including those coding for structural proteins and proteins that regulate the hypertrophic process. Seven days of passive stretch in vivo of tibialis anterior (TA) muscle has been shown to elicit muscle hypertrophy. We have identified a cDNA corresponding to an mRNA that exhibits increased expression in response to 7 days of passive stretch imposed on TA muscles in vivo. This 944-bp novel murine transcript is expressed primarily in cardiac and skeletal muscle and to a lesser extent in brain. Translation of the transcript revealed an open reading frame of 85 amino acids encoding a nuclear localization signal and two overlapping casein kinase II phosphorylation sites. This gene has been called "small muscle protein (X chromosome)" (Smpx; HGMW-approved human gene symbol SMPX) and we hypothesize that it plays a role in skeletal muscle hypertrophy.

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.

Expression of zebrafish Hoxa1a in neuronal cells of the midbrain and anterior hindbrain

The expression pattern of zebrafish hoxa1a mRNA during embryonic development was studied. Herein, we show that hoxa1a mRNA is expressed in the ventral region of both the midbrain and anterior hindbrain during the developmental period from the pharyngula to the protruding-mouth stages via whole-mount in situ hybridization. Furthermore, double-labeling with anti-zHu antibody confirms that the zebrafish hoxa1a gene is expressed in neuronal cells. The observed temporal and spatial distributions of zebrafish hoxa1a mRNA differ greatly from the expression patterns of zebrafish hoxb1a and hoxb1b paralagous genes. In addition, in embryos injected with mouse ihh mRNA, hoxa1a-expressing cells increase in number with a dorsalized expression pattern in the midbrain.

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.

Molecular Mechanism of Cardiac Hypertrophy and Development

Congestive heart failure is a major issues for cardiologists and to fully understand heart failure, it is important to understand the mechanism of the development of cardiac hypertrophy. Hemodynamic overload, namely mechanical stress, is a major cause of cardiac hypertrophy and to dissect the signaling pathways from mechanical stress to cardiac hypertrophy, an in-vitro device by which mechanical stress can be imposed on cardiac myocytes of neonatal rats cultured in serum-free conditions has been developed. Passively stretching cardiac myocytes cultured on silicone membranes induced various hypertrophic responses, such as activation of the phosphorylation cascades of many protein kinases, expression of specific genes and an increase in protein synthesis. During this process, secretion and production of vasoactive peptides, such as angiotensin II and endothelin-1, were increased and they played critical roles in the induction of these hypertrophic responses. Candidates for the 'mechanoreceptor' that receives the mechanical stress and converts it into intracellular biochemical signals have been recently demonstrated. Gene therapy and cell transplantation are hopeful strategies for the treatment of heart failure and require an understanding of how normal cardiac myocytes are differentiated. A key gene that plays a critical role in cardiac development has been isolated. The cardiac homeobox-containing gene Csx is expressed in the heart and the heart progenitor cells from the very early developmental stage, and targeted disruption of the murine Csx results in embryonic lethality because of the abnormal looping morphogenesis of the primary heart tube. With a cardiac zinc finger protein GATA4, Csx induces cardiomyocyte differentiation of teratocarcinoma cells as well as upregulation of cardiac genes. Mutations of human CSX cause various congenital heart diseases including atrial septal defect, ventricular septal defect, tricuspid valve abnormalities and atrioventricular block.

Zebrafish genetics and vertebrate heart formation

Forward-genetic analyses in Drosophila and Caenorhabditis elegans have given us unprecedented insights into many developmental mechanisms. To study the formation of organs that contain cell types and structures not present in invertebrates, a vertebrate model system amenable to forward genetics would be very useful. Recent work shows that a newly initiated genetic approach in zebrafish is already making significant contributions to understanding the development of the vertebrate heart, an organ that contains several vertebrate-specific features. These and other studies point to the utility of the zebrafish system for studying a wide range of vertebrate-specific processes.

The endoderm plays an important role in patterning the segmented pharyngeal region in zebrafish (Danio rerio)

The development of the vertebrate head is a highly complex process involving tissues derived from all three germ layers. The endoderm forms pharyngeal pouches, the paraxial mesoderm gives rise to endothelia and muscles, and the neural crest cells, which originate from the embryonic midbrain and hindbrain, migrate ventrally to form cartilage, connective tissue, sensory neurons, and pigment cells. All three tissues form segmental structures: the hindbrain compartmentalizes into rhombomeres, the mesoderm into somitomeres, and the endoderm into serial gill slits. It is not known whether the different segmented tissues in the head develop by the same molecular mechanism or whether different pathways are employed. It is also possible that one tissue imposes segmentation on the others. Most recent studies have emphasized the importance of neural crest cells in patterning the head. Neural crest cells colonize the segmentally arranged arches according to their original position in the brain and convey positional information from the hindbrain into the periphery. During the screen for mutations that affect embryonic development of zebrafish, one mutant, called van gogh (vgo), in which segmentation of the pharyngeal region is absent, was isolated. In vgo, even though hindbrain segmentation is unaffected, the pharyngeal endoderm does not form reiterated pouches and surrounding mesoderm is not patterned correctly. Accordingly, migrating neural crest cells initially form distinct streams but fuse when they reach the arches. This failure to populate distinct pharyngeal arches is likely due to the lack of pharyngeal pouches. The results of our analysis suggest that the segmentation of the endoderm occurs without signaling from neural crest cells but that tissue interactions between the mesendoderm and the neural crest cells are required for the segmental appearance of the neural crest-derived cartilages in the pharyngeal arches. The lack of distinct patches of neural crest cells in the pharyngeal region is also seen in mutants of one-eyed pinhead and casanova, which are characterized by a lack of endoderm, as well as defects in mesodermal structures, providing evidence for the important role of the endoderm and mesoderm in governing head segmentation.

Multiple pathways in the midline regulate concordant brain, heart and gut left-right asymmetry

The embryonic midline in vertebrates has been implicated in left-right development, but the mechanisms by which it regulates left-right asymmetric gene expression and organ morphogenesis are unknown. Zebrafish embryos have three domains of left-right asymmetric gene expression that are useful predictors of organ situs. cyclops (nodal), lefty1 and pitx2 are expressed in the left diencephalon; cyclops, lefty2 and pitx2 are expressed in the left heart field; and cyclops and pitx2 are expressed in the left gut primordium. Distinct alterations of these expression patterns in zebrafish midline mutants identify four phenotypic classes that have different degrees of discordance among the brain, heart and gut. These classes help identify two midline domains and several genetic pathways that regulate left-right development. A cyclops-dependent midline domain, associated with the prechordal plate, regulates brain asymmetry but is dispensable for normal heart and gut left-right development. A second midline domain, associated with the anterior notochord, is dependent on no tail, floating head and momo function and is essential for restricting asymmetric gene expression to the left side. Mutants in spadetail or chordino give discordant gene expression among the brain, heart and gut. one-eyed pinhead and schmalspur are necessary for asymmetric gene expression and may mediate signaling from midline domains to lateral tissues. The different phenotypic classes help clarify the apparent disparity of mechanisms proposed to explain left-right development in different vertebrates.

tbx20, a new vertebrate T-box gene expressed in the cranial motor neurons and developing cardiovascular structures in zebrafish

The T-box genes constitute a family of transcriptional regulator genes that have been implicated in a variety of developmental processes ranging from the formation of germ layers to the regionalization of the central nervous system. In this report we describe the cloning and expression pattern of a new T-box gene from zebrafish, which we named tbx20. tbx20 is an ortholog of two other T-box genes isolated from animals of different phyla - H15 of Drosophila melanogaster and tbx-12 of Caenorhabditis elegans, suggesting that the evolutionary origin of this gene predates the divergence between the protostomes and deuterostomes. During development, tbx20 is expressed in embryonic structures of both mesodermal and ectodermal origins, including the heart, cranial motor neurons, and the roof of the dorsal aorta.

Loss of function and inhibitory effects of human CSX/NKX2.5 homeoprotein mutations associated with congenital heart disease

CSX/NKX2.5 is an evolutionarily conserved homeodomain-containing (HD-containing) transcription factor that is essential for early cardiac development. Recently, ten different heterozygous CSX/NKX2.5 mutations were found in patients with congenital heart defects that are transmitted in an autosomal dominant fashion. To determine the consequence of these mutations, we analyzed nuclear localization, DNA binding, transcriptional activation, and dimerization of mutant CSX/NKX2.5 proteins. All mutant proteins were translated and located to the nucleus, except one splice-donor site mutant whose protein did not accumulate in the cell. All mutants that had truncation or missense mutations in the HD had severely reduced DNA binding activity and little or no transcriptional activation function. In contrast, mutants with intact HDs exhibit normal DNA binding to the monomeric binding site but had three- to ninefold reduction in DNA binding to the dimeric binding sites. HD missense mutations that preserved homodimerization ability inhibited the activation of atrial natriuretic factor by wild-type CSX/NKX2.5. Although our studies do not characterize the genotype-phenotype relationship of the ten human mutations, they identify specific abnormalities of CSX/NKX2.5 function essential for transactivation of target genes.

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.

Combinatorial interactions regulating cardiac transcription

In vertebrates, heart development is a multistep process that starts with formation and patterning of the primitive heart tube and is followed by complex morphological events to give rise to the mature four-chambered heart. These various stages are characterized by distinct patterns of gene expression. Although chamber specificity and developmental regulation can be demonstrated in transgenic mice using short promoter fragments, the mechanism underlying spatial and temporal specificity within the heart remains largely unclear. Combinatorial interaction between a limited number of cardiac-specific and ubiquitous transcription factors may account for the diverse genetic inputs required to generate the complex transcriptional patterns that characterize the developing myocardium. We have used the cardiac atrial natriuretic peptide (ANP) promoter to test this hypothesis. The ANP gene is transcribed in a spatial- and temporal-specific manner in the heart, and a 500 bp promoter fragment is sufficient to recapitulate both chamber and developmental specificity. This promoter is composed of three modules, a "basal" cardiac promoter that is essential for transcription in embryonic and postnatal atrial and ventricular myocytes and two other independent modules that behave as chamber-specific enhancers. The basal cardiac promoter is the target of two cardiac-specific transcription factors, the zinc finger GATA-4 protein and the Nkx2-5 homeodomain, which bind to contiguous elements within this region. At low concentrations--a situation that likely occurs during the very first stages of cardiac cell fate determination--the two proteins synergistically activate transcription from the ANP promoter. This functional synergy requires physical interaction between the GATA-4 protein and an extended C-terminal homeodomain on Nkx2-5. This interaction, which unmasks an activation domain present just N-terminal of the homeodomain, is specific for GATA-4 and-5, but is not observed with the other cardiac GATA factor, GATA-6. Optimal synergy requires binding of both proteins to their cognate sites, although modest synergy also could be observed on heterologous promoters containing only multimerized Nkx binding sites, suggesting that Nkx2-5 is able to recruit GATA-4 into a transcriptionally active complex. The GATA/Nkx interaction, which appears to have been evolutionary conserved in nematode, fly, and mammals, provides a paradigm for analyzing transcription factor interaction during organogenesis. The data are also discussed in the context of our present knowledge of the roles of GATA and NK2 proteins in cardiac development.

Cloning and genetic mapping of zebrafish BMP-2

The BMP family of polypeptide growth factors has been shown to play diverse roles in establishing embryonic patterning and tissue fates. We report the cloning of the zebrafish homologue of BMP-2, examine its expression during embryogenesis, and find that it is localized to the distal end of the long arm of zebrafish chromosome 20. A missense mutation of the bmp2 gene has recently been shown to be responsible for the early dorsalized phenotype of the zebrafish swirl mutant [Kishimoto et al., 1997]. Given the dynamic expression of bmp2 in the developing embryo and the complex interactions of BMP signaling response in vertebrates, it is possible that other mutant phenotypes, due to altered bmp2 gene expression, will eventually map to or interact with this genetic locus.

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.

Screening mosaic F1 females for mutations affecting zebrafish heart induction and patterning

The genetic pathways underlying the induction and anterior-posterior patterning of the heart are poorly understood. The recent emergence of the zebrafish model system now allows a classical genetic approach to such challenging problems in vertebrate development. Two large-scale screens for mutations affecting zebrafish embryonic development have recently been completed; among the hundreds of mutations identified were several that affect specific aspects of cardiac morphogenesis, differentiation, and function. However, very few mutations affecting induction and/or anterior-posterior patterning of the heart were identified. We hypothesize that a directed approach utilizing molecular markers to examine these particular steps of heart development will uncover additional such mutations. To test this hypothesis, we are conducting two parallel screens for mutations that affect either the induction or the anterior-posterior patterning of the zebrafish heart. As an indicator of cardiac induction, we examine expression of nkx2.5, the earliest known marker of precardiac mesoderm; to assess anterior-posterior patterning, we distinguish ventricle from atrium with antibodies that recognize different myosin heavy chain isoforms. In order to expedite the examination of a large number of mutations, we are screening the haploid progeny of mosaic F1 females. In these ongoing screens, we have identified four mutations that affect nkx2.5 expression as well as 21 that disrupt either ventricular or atrial development and thus far have recovered several of these mutations, demonstrating the value of our approach. Future analysis of these and other cardiac mutations will provide further insight into the processes of induction and anterior-posterior patterning of the heart.

Regulation and function of tinman during dorsal mesoderm induction and heart specification in Drosophila

The homeobox gene tinman plays a key role in the specification of Drosophila heart progenitors and the visceral mesoderm of the midgut, both of which arise at defined positions within dorsal areas of the mesoderm. Here, we show that in addition to the heart and midgut visceral mesoderm, tinman is also required for the specification of all dorsal body wall muscles. Thus it appears that the precursors of the heart, visceral musculature, and dorsal somatic muscles are all specified within the same broad domain of dorsal mesodermal tinman expression. Locally restricted activities of tinman are also observed during its early, general mesodermal expression, where tinman is required for the activation of the homeobox gene buttonless in precursors of the "dorsal median" (DM) glial cells along the ventral midline. These observations, together with others showing only mild effects of ectopic tinman expression on heart development, indicate that tinman function is obligatory, but not sufficient to determine individual tissues within the mesoderm. Therefore, we propose that tinman has a role in integrating positional information that is provided by intersecting domains of additional regulators and signals, which may include Wingless, Sloppy Paired, and Hedgehog in the dorsal mesoderm and EGF-signaling at the ventral midline. Previous studies have shown that Dpp acts as an inductive signal from dorsal ectodermal cells to induce tinman expression in the dorsal mesoderm, which, in turn, is needed for heart and visceral mesoderm formation. In the present report, we show that Thickveins, a type I receptor of Dpp, is essential for the transmission of Dpp signals into the mesoderm. Constitutive activity of Tkv in the entire mesoderm induces ectopic tinman expression in the ventral mesoderm, and this results in the ectopic formation of heart precursors in a defined area of the ventrolateral mesoderm. We further show that Screw, a second BMP2/4-related gene product, Tolloid, a BMP1-related protein, and the zinc finger-containing protein Schnurri, are required to allow full levels of tinman induction during this process. It is likely that some of these functional and regulatory properties of tinman are shared by tinman-related genes from vertebrates that have similarly important roles in embryonic heart development.

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 bHLH transcription factor hand2 plays parallel roles in zebrafish heart and pectoral fin development

The precursors of several organs reside within the lateral plate mesoderm of vertebrate embryos. Here, we demonstrate that the zebrafish hands off locus is essential for the development of two structures derived from the lateral plate mesoderm - the heart and the pectoral fin. hands off mutant embryos have defects in myocardial development from an early stage: they produce a reduced number of myocardial precursors, and the myocardial tissue that does form is improperly patterned and fails to maintain tbx5 expression. A similar array of defects is observed in the differentiation of the pectoral fin mesenchyme: small fin buds form in a delayed fashion, anteroposterior patterning of the fin mesenchyme is absent and tbx5 expression is poorly maintained. Defects in these mesodermal structures are preceded by the aberrant morphogenesis of both the cardiogenic and forelimb-forming regions of the lateral plate mesoderm. Molecular analysis of two hands off alleles indicates that the hands off locus encodes the bHLH transcription factor Hand2, which is expressed in the lateral plate mesoderm starting at the completion of gastrulation. Thus, these studies reveal early functions for Hand2 in several cellular processes and highlight a genetic parallel between heart and forelimb development.

Up-regulation of natriuretic peptides in the ventricle of Csx/Nkx2-5 transgenic mice

A cardiac homeobox-containing gene Csx/Nkx2-5, which is essential for cardiac development, is abundantly expressed in the adult heart as well as in the heart primordia. Targeted disruption of this gene results in embryonic lethality due to abnormal heart morphogenesis. To elucidate the role of Csx/Nkx2-5 in the adult heart, we generated transgenic mice which overexpress human Csx/Nkx2-5. The transgene was expressed abundantly in the heart and the skeletal muscle. mRNA levels of several cardiac genes including natriuretic peptides, CARP, MLC2v, and endogenous Csx/Nkx2-5 were increased in the ventricle of the transgenic mice. Electron microscopic analysis revealed that the ventricular myocardium of the transgenic mice had many secretory granules, which disappeared after administration of vasopressin. These results suggest that Csx/Nkx2-5 regulates many cardiac genes and induces formation of secretory granules in the adult ventricle.

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.

Subdivision of the cardiac Nkx2.5 expression domain into myogenic and nonmyogenic compartments

Nkx2.5 is expressed in the cardiogenic mesoderm of avian, mouse, and amphibian embryos. To understand how various cardiac fates within this domain are apportioned, we fate mapped the mesodermal XNkx2.5 domain of neural tube stage Xenopus embryos. The lateral portions of the XNkx2.5 expression domain in the neural tube stage embryo (stage 22) form the dorsal mesocardium and roof of the pericardial cavity while the intervening ventral region closes to form the myocardial tube. XNkx2.5 expression is maintained throughout the period of heart tube morphogenesis and differentiation of myocardial, mesocardial, and pericardial tissues. A series of microsurgical experiments showed that myocardial differentiation in the lateral portion of the field is suppressed during normal development by signals from the prospective myocardium and by tissues located more dorsally in the embryo, in particular the neural tube. These signals combine to block myogenesis downstream of XNkx2.5 and at or above the level of contractile protein gene expression. We propose that the entire XNkx2.5/heart field is transiently specified as cardiomyogenic. Suppression of this program redirects lateral cells to adopt dorsal mesocardial and dorsal pericardial fates and subdivides the field into distinct myogenic and nonmyogenic compartments.

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

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

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

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

casanova plays an early and essential role in endoderm formation in zebrafish

The cellular and molecular mechanisms that regulate endoderm development in vertebrates have only recently begun to be explored. Here we show that the zebrafish locus casanova plays an early and essential role in this process. casanova mutants lack a gut tube and do not express any molecular markers of endoderm differentiation. The early endodermal expression of genes such as axial, gata5, and fkd2 does not initiate in casanova mutants, indicating that the endoderm is defective from the onset of gastrulation. Mosaic analysis demonstrates that casanova functions cell autonomously within the endodermal progenitors. We also report the isolation of a zebrafish homologue of Mixer, a gene important for early endoderm formation in Xenopus. casanova does not encode zebrafish Mixer, and mixer expression is normal in casanova mutants, indicating that casanova acts downstream of, or parallel to, mixer to promote endoderm formation. We further find that the forerunner cells, a specialized group of noninvoluting dorsal mesendodermal cells, do not form in casanova mutants. Studies of casanova mutants do not support an important role for the forerunner cells in either dorsal axis or tail development, as has been previously proposed. In addition, although different populations of mesodermal precursors are generated normally in casanova mutants, morphogenetic defects in the heart, vasculature, blood, and kidney are apparent, suggesting a possible role for the endoderm in morphogenesis of these organs.

Restricted expression of cardiac myosin genes reveals regulated aspects of heart tube assembly in zebrafish

The embryonic vertebrate heart is divided into two major chambers, an anterior ventricle and a posterior atrium. Although the fundamental differences between ventricular and atrial tissues are well documented, it is not known when and how cardiac anterior-posterior (A-P) patterning occurs. The expression patterns of two zebrafish cardiac myosin genes, cardiac myosin light chain 2 (cmlc2) and ventricular myosin heavy chain (vmhc), allow us to distinguish two populations of myocardial precursors at an early stage, well before the heart tube forms. These myocardial subpopulations, which may represent the ventricular and atrial precursors, are organized in a medial-lateral pattern within the precardiac mesoderm. Our examinations of cmlc2 and vmhc expression throughout the process of heart tube assembly indicate the important role of an intermediate structure, the cardiac cone, in the conversion of this early medial-lateral pattern into the A-P pattern of the heart tube. To gain insight into the genetic regulation of heart tube assembly and patterning, we examine cmlc2 and vmhc expression in several zebrafish mutants. Analyses of mutations that cause cardia bifida demonstrate that the achievement of a proper cardiac A-P pattern does not depend upon cardiac fusion. On the other hand, cardiac fusion does not ensure the proper A-P orientation of the ventricle and atrium, as demonstrated by the heart and soul mutation, which blocks cardiac cone morphogenesis. Finally, the pandora mutation interferes with the establishment of the early medial-lateral myocardial pattern. Altogether, these data suggest new models for the mechanisms that regulate the formation of a patterned heart tube and provide an important framework for future analyses of zebrafish mutants with defects in this process.

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.

Induction of the mesendoderm in the zebrafish germ ring by yolk cell-derived TGF-beta family signals and discrimination of mesoderm and endoderm by FGF

The endoderm forms the gut and associated organs, and develops from a layer of cells which emerges during gastrula stages in the vertebrate embryo. In comparison to mesoderm and ectoderm, little is known about the signals which induce the endoderm. The origin of the endoderm is intimately linked with that of mesoderm, both by their position in the embryo, and by the molecules that can induce them. We characterised a gene, zebrafish gata5, which is expressed in the endoderm from blastula stages and show that its transcription is induced by signals originating from the yolk cell. These signals also induce the mesoderm-expressed transcription factor no tail (ntl), whose initial expression coincides with gata5 in the cells closest to the blastoderm margin, then spreads to encompass the germ ring. We have characterised the induction of these genes and show that ectopic expression of activin induces gata5 and ntl in a pattern which mimics the endogenous expression, while expression of a dominant negative activin receptor abolishes ntl and gata5 expression. Injection of RNA encoding a constitutively active activin receptor leads to ectopic expression of gata5 and ntl. gata5 is activated cell-autonomously, whereas ntl is induced in cells distant from those which have received the RNA, showing that although expression of both genes is induced by a TGF-beta signal, expression of ntl then spreads by a relay mechanism. Expression of a fibroblast growth factor (eFGF) or a dominant negatively acting FGF receptor shows that ntl but not gata5 is regulated by FGF signalling, implying that this may be the relay signal leading to the spread of ntl expression. In embryos lacking both squint and cyclops, members of the nodal group of TGF-beta related molecules, gata5 expression in the blastoderm is abolished, making these factors primary candidates for the endogenous TGF-beta signal inducing gata5.

Regulation of left-right asymmetries in the zebrafish by Shh and BMP4

Left-right (LR) asymmetry of the heart in vertebrates is regulated by early asymmetric signals in the embryo, including the secreted signal Sonic hedgehog (Shh), but less is known about LR asymmetries of visceral organs. Here we show that Shh also specifies asymmetries in visceral precursors in the zebrafish and that cardiac and visceral sidedness are independent. The transcription factors fli-1 and Nkx-2.5 are expressed asymmetrically in the precardiac mesoderm and subsequently in the heart; an Eph receptor, rtk2, and an adhesion protein, DM-GRASP, mark early asymmetries in visceral endoderm. Misexpression of shh mRNA, or a dominant negative form of protein kinase A, on the right side reverses the expression of these asymmetries in precursors of both the heart and the viscera. Reversals in the heart and gut are uncoordinated, suggesting that each organ interprets the signal independently. Misexpression of Bone Morphogenetic Protein (BMP4) on the right side reverses the heart, but visceral organs are unaffected, consistent with a function for BMPs locally in the heart field. Zebrafish mutants with midline defects show independent reversals of cardiac and visceral laterality. Thus, hh signals influence the development of multiple organ asymmetries in zebrafish and different organs appear to respond to a central cascade of midline signaling independently, which in the heart involves BMP4.

Tbx5 is essential for heart development

Mutations in the Tbx5 transcription factor cause heart septal defects found in human Holt-Oram Syndrome. The complete extent to which Tbx5 functions in heart development, however, has not been established. Here we show that, in Xenopus embryos, Tbx5 is expressed in the early heart field, posterior to the cardiac homeobox transcription factor, Nkx2.5. During morphogenesis, Tbx5 is expressed throughout the heart tube except the anterior portion, the bulbus cordis. When Tbx5 activity is antagonized with a hormone-inducible, dominant negative version of the protein, the heart fails to develop. These results suggest that, in addition to its function in heart septation, Tbx5 has a more global role in cardiac specification and heart development in vertebrate embryos.