Theheartstringsmutation in zebrafish causes heart/fin Tbx5 deficiency syndrome

Assessing the potential risks to zebrafish posed by environmentally relevant copper and silver nanoparticles

The manufacture of large quantities of engineered nanomaterials (NMs) may lead to unintended contamination of aquatic ecosystems. Biologically based monitoring techniques need to be developed to detect these emerging NMs. The purpose of this study was to develop a risk-based probability model to predict the potential hazards of nanoecotoxicity toward aquatic organisms posed by waterborne copper and silver nanoparticles (Cu/Ag NPs). Published experimental evidence based on Cu/Ag NP-zebrafish (Danio rerio) systems was adopted as the study data. A Hill model was used to reconstruct a concentration-mortality response profile. A cumulative Weibull predictive model was employed to estimate exposure thresholds. The derived probabilistic model can predict the potential risk of environmentally relevant Cu/Ag NPs for major Taiwanese rivers with predicted environmental concentrations of 0.06 (95% confidence interval (CI): 0.01-0.92) mgL(-1) for Cu NPs and 0.04 (0.01-0.11) mgL(-1) for Ag NPs. The results indicated that estimated thresholds were 0.10-0.48mgL(-1) (95% CI) for Cu NPs and 2.69-2.73mgL(-1) for Ag NPs. The probabilities of a risk quotient (RQ) of >1 ranged 17%-81% for zebrafish exposed to Cu NPs. This study found that Ag NP exposure scenarios posed no significant risks to zebrafish (RQ≪0.1).

The Osr1 and Osr2 genes act in the pronephric anlage downstream of retinoic acid signaling and upstream of Wnt2b to maintain pectoral fin development

Vertebrate odd-skipped related genes (Osr) have an essential function during the formation of the intermediate mesoderm (IM) and the kidney structures derived from it. Here, we show that these genes are also crucial for limb bud formation in the adjacent lateral plate mesoderm (LPM). Reduction of zebrafish Osr function impairs fin development by the failure of tbx5a maintenance in the developing pectoral fin bud. Osr morphant embryos show reduced wnt2b expression, and increasing Wnt signaling in Osr morphant embryos partially rescues tbx5a expression. Thus, Osr genes control limb bud development in a non-cell-autonomous manner, probably through the activation of Wnt2b. Finally, we demonstrate that Osr genes are downstream targets of retinoic acid (RA) signaling. Therefore, Osr genes act as a relay within the genetic cascade of fin bud formation: by controlling the expression of the signaling molecule Wnt2ba in the IM they play an essential function transmitting the RA signaling originated in the somites to the LPM.

Transmembrane protein 2 (Tmem2) is required to regionally restrict atrioventricular canal boundary and endocardial cushion development

The atrioventricular canal (AVC) physically separates the atrial and ventricular chambers of the heart and plays a crucial role in the development of the valves and septa. Defects in AVC development result in aberrant heart morphogenesis and are a significant cause of congenital heart malformations. We have used a forward genetic screen in zebrafish to identify novel regulators of cardiac morphogenesis. We isolated a mutant, named wickham (wkm), that was indistinguishable from siblings at the linear heart tube stage but exhibited a specific loss of cardiac looping at later developmental stages. Positional cloning revealed that the wkm locus encodes transmembrane protein 2 (Tmem2), a single-pass transmembrane protein of previously unknown function. Expression analysis demonstrated myocardial and endocardial expression of tmem2 in zebrafish and conserved expression in the endocardium of mouse embryos. Detailed phenotypic analysis of the wkm mutant identified an expansion of expression of known myocardial and endocardial AVC markers, including bmp4 and has2. By contrast, a reduction in the expression of spp1, a marker of the maturing valvular primordia, was observed, suggesting that an expansion of immature AVC is detrimental to later valve maturation. Finally, we show that immature AVC expansion in wkm mutants is rescued by depleting Bmp4, indicating that Tmem2 restricts bmp4 expression to delimit the AVC primordium during cardiac development.

Pitx1 is necessary for normal initiation of hindlimb outgrowth through regulation of Tbx4 expression and shapes hindlimb morphologies via targeted growth control

The forelimbs and hindlimbs of vertebrates are morphologically distinct. Pitx1, expressed in the hindlimb bud mesenchyme, is required for the formation of hindlimb characteristics and produces hindlimb-like morphologies when misexpressed in forelimbs. Pitx1 is also necessary for normal expression of Tbx4, a transcription factor required for normal hindlimb development. Despite the importance of this protein in these processes, little is known about its mechanism of action. Using a transgenic gene replacement strategy in a Pitx1 mutant mouse, we have uncoupled two discrete functions of Pitx1. We show that, firstly, this protein influences hindlimb outgrowth by regulating Tbx4 expression levels and that, subsequently, it shapes hindlimb bone and soft tissue morphology independently of Tbx4. We provide the first description of how Pitx1 sculpts the forming hindlimb skeleton by localised modulation of the growth rate of discrete elements.

Restraint of Fgf8 signaling by retinoic acid signaling is required for proper heart and forelimb formation

Cardiomelic or heart-hand syndromes include congenital defects affecting both the forelimb and heart, suggesting a hypothesis where similar signals may coordinate their development. In support of this hypothesis, we have recently defined a mechanism by which retinoic acid (RA) signaling acts on the forelimb progenitors to indirectly restrict cardiac cell number. However, we still do not have a complete understanding of the mechanisms downstream of RA signaling that allow for the coordinated development of these structures. Here, we test the hypothesis that appropriate Fgf signaling in the cardiac progenitor field downstream of RA signaling is required for the coordinated development of the heart and forelimb. Consistent with this hypothesis, we find that increasing Fgf signaling can autonomously increase cardiac cell number and non-autonomously inhibit forelimb formation over the same time period that embryos are sensitive to loss of RA signaling. Furthermore, we find that Fgf8a, which is expressed in the cardiac progenitors, is expanded into the posterior in RA signaling-deficient zebrafish embryos. Reducing Fgf8a function in RA signaling-deficient embryos is able to rescue both heart and forelimb development. Together, these results are the first to directly support the hypothesis that RA signaling is required shortly after gastrulation in the forelimb field to temper Fgf8a signaling in the cardiac field, thus coordinating the development of the heart and forelimb.

miR-196 regulates axial patterning and pectoral appendage initiation

Vertebrate Hox clusters contain protein-coding genes that regulate body axis development and microRNA (miRNA) genes whose functions are not yet well understood. We overexpressed the Hox cluster microRNA miR-196 in zebrafish embryos and found four specific, viable phenotypes: failure of pectoral fin bud initiation, deletion of the 6th pharyngeal arch, homeotic aberration and loss of rostral vertebrae, and reduced number of ribs and somites. Reciprocally, miR-196 knockdown evoked an extra pharyngeal arch, extra ribs, and extra somites, confirming endogenous roles of miR-196. miR-196 injection altered expression of hox genes and the signaling of retinoic acid through the retinoic acid receptor gene rarab. Knocking down rarab mimicked the pectoral fin phenotype of miR-196 overexpression, and reporter constructs tested in tissue culture and in embryos showed that the rarab 3'UTR is a miR-196 target for pectoral fin bud initiation. These results show that a Hox cluster microRNA modulates development of axial patterning similar to nearby protein-coding Hox genes, and acts on appendicular patterning at least in part by modulating retinoic acid signaling.

Xenopus: An emerging model for studying congenital heart disease

Congenital heart defects affect nearly 1% of all newborns and are a significant cause of infant death. Clinical studies have identified a number of congenital heart syndromes associated with mutations in genes that are involved in the complex process of cardiogenesis. The African clawed frog, Xenopus, has been instrumental in studies of vertebrate heart development and provides a valuable tool to investigate the molecular mechanisms underlying human congenital heart diseases. In this review, we discuss the methodologies that make Xenopus an ideal model system to investigate heart development and disease. We also outline congenital heart conditions linked to cardiac genes that have been well studied in Xenopus and describe some emerging technologies that will further aid in the study of these complex syndromes.

Chromatin remodelling complex dosage modulates transcription factor function in heart development

Dominant mutations in cardiac transcription factor genes cause human inherited congenital heart defects (CHDs); however, their molecular basis is not understood. Interactions between transcription factors and the Brg1/Brm-associated factor (BAF) chromatin remodelling complex suggest potential mechanisms; however, the role of BAF complexes in cardiogenesis is not known. In this study, we show that dosage of Brg1 is critical for mouse and zebrafish cardiogenesis. Disrupting the balance between Brg1 and disease-causing cardiac transcription factors, including Tbx5, Tbx20 and Nkx2–5, causes severe cardiac anomalies, revealing an essential allelic balance between Brg1 and these cardiac transcription factor genes. This suggests that the relative levels of transcription factors and BAF complexes are important for heart development, which is supported by reduced occupancy of Brg1 at cardiac gene promoters in Tbx5 haploinsufficient hearts. Our results reveal complex dosage-sensitive interdependence between transcription factors and BAF complexes, providing a potential mechanism underlying transcription factor haploinsufficiency, with implications for multigenic inheritance of CHDs.

Inherited congenital heart defects are prevalent in the human population, but the molecular mechanisms are poorly understood. In this article, deficiency in the chromatin remodelling factor, Brg1, is shown to alter cardiac development in both mouse and zebrafish laboratory models.

Dermoskeleton morphogenesis in zebrafish fins

Zebrafish fins have a proximal skeleton of endochondral bones and a distal skeleton of dermal bones. Recent experimental and genetic studies are discovering mechanisms to control fin skeleton morphogenesis. Whereas the endochondral skeleton has been extensively studied, the formation of the dermal skeleton requires further revision. The shape of the dermal skeleton of the fin is generated in its distal growing margin and along a proximal growing domain. In these positions, dermoskeletal fin morphogenesis can be explained by intertissue interactions and the function of several genetic pathways. These pathways regulate patterning, size, and cell differentiation along three axes. Finally, a common genetic control of late development, regeneration, and tissue homeostasis of the fin dermoskeleton is currently being analyzed. These pathways may be responsible for the similar shape obtained after each morphogenetic process. This provides an interesting conceptual framework for future studies on this topic. Developmental Dynamics 239:2779–2794, 2010. © 2010 Wiley-Liss, Inc.

The secreted integrin ligand nephronectin is necessary for forelimb formation in Xenopus tropicalis

While limb regeneration has been extensively studied in amphibians, little is known about the initial events in limb formation in metamorphosing anurans. The small secreted integrin ligand nephronectin (npnt) is necessary for development of the metanephros in mouse. Although expressed in many tissues, its role in other developmental processes is not well-studied. Here we show that a transgene insertion that disrupts this gene ablates forelimb formation in Xenopus tropicalis. Our results suggest a novel role for integrin signalling in limb development, and represent the first insertional phenotype to be cloned in amphibians.

Pdlim7 is required for maintenance of the mesenchymal/epidermal Fgf signaling feedback loop during zebrafish pectoral fin development

Background

Vertebrate limb development involves a reciprocal feedback loop between limb mesenchyme and the overlying apical ectodermal ridge (AER). Several gene pathways participate in this feedback loop, including Fgf signaling. In the forelimb lateral plate mesenchyme, Tbx5 activates Fgf10 expression, which in turn initiates and maintains the mesenchyme/AER Fgf signaling loop. Recent findings have revealed that Tbx5 transcriptional activity is regulated by dynamic nucleocytoplasmic shuttling and interaction with Pdlim7, a PDZ-LIM protein family member, along actin filaments. This Tbx5 regulation is critical in heart formation, but the coexpression of both proteins in other developing tissues suggests a broader functional role.

Results

Knock-down of Pdlim7 function leads to decreased pectoral fin cell proliferation resulting in a severely stunted fin phenotype. While early gene induction and patterning in the presumptive fin field appear normal, the pectoral fin precursor cells display compaction and migration defects between 18 and 24 hours post-fertilization (hpf). During fin growth fgf24 is sequentially expressed in the mesenchyme and then in the apical ectodermal ridge (AER). However, in pdlim7 antisense morpholino-treated embryos this switch of expression is prevented and fgf24 remains ectopically active in the mesenchymal cells. Along with the lack of fgf24 in the AER, other critical factors including fgf8 are reduced, suggesting signaling problems to the underlying mesenchyme. As a consequence of perturbed AER function in the absence of Pdlim7, pathway components in the fin mesenchyme are misregulated or absent, indicating a breakdown of the Fgf signaling feedback loop, which is ultimately responsible for the loss of fin outgrowth.

Conclusion

This work provides the first evidence for the involvement of Pdlim7 in pectoral fin development. Proper fin outgrowth requires fgf24 downregulation in the fin mesenchyme with subsequent activation in the AER, and Pdlim7 appears to regulate this transition, potentially through Tbx5 regulation. By controlling Tbx5 subcellular localization and transcriptional activity and possibly additional yet unknown means, Pdlim7 is required for proper development of the heart and the fins. These new regulatory mechanisms may have important implications how we interpret Tbx5 function in congenital hand/heart syndromes in humans.

Shox2 mediates Tbx5 activity by regulating Bmp4 in the pacemaker region of the developing heart

Heart formation requires a highly balanced network of transcriptional activation of genes. The homeodomain transcription factor, Shox2, is essential for the formation of the sinoatrial valves and for the development of the pacemaking system. The elucidation of molecular mechanisms underlying the development of pacemaker tissue has gained clinical interest as defects in its patterning can be related to atrial arrhythmias. We have analyzed putative targets of Shox2 and identified the Bmp4 gene as a direct target. Shox2 interacts directly with the Bmp4 promoter in chromatin immunoprecipitation assays and activates transcription in luciferase-reporter assays. In addition, ectopic expression of Shox2 in Xenopus embryos stimulates transcription of the Bmp4 gene, and silencing of Shox2 in cardiomyocytes leads to a reduction in the expression of Bmp4. In Tbx5^del/+ mice, a model for Holt-Oram syndrome, and Shox2^−/− mice, we show that the T-box transcription factor Tbx5 is a regulator of Shox2 expression in the inflow tract and that Bmp4 is regulated by Shox2 in this compartment of the embryonic heart. In addition, we could show that Tbx5 acts cooperatively with Nkx2.5 to regulate the expression of Shox2 and Bmp4. This work establishes a link between Tbx5, Shox2 and Bmp4 in the pacemaker region of the developing heart and thus contributes to the unraveling of the intricate interplay between the heart-specific transcriptional machinery and developmental signaling pathways.

Nucleocytoplasmic functions of the PDZ-LIM protein family: new insights into organ development

Recent work on the PDZ-LIM protein family has revealed that it has important activities at the cellular level, mediating signals between the nucleus and the cytoskeleton, with significant impact on organ development. We review and integrate current knowledge about the PDZ-LIM protein family and propose a new functional role, sequestering nuclear factors in the cytoplasm. Characterized by their PDZ and LIM domains, the PDZ-LIM family is comprised of evolutionarily conserved proteins found throughout the animal kingdom, from worms to humans. Combining two functional domains in one protein, PDZ-LIM proteins have wide-ranging and multi-compartmental cell functions during development and homeostasis. In contrast, misregulation can lead to cancer formation and progression. New emerging roles include interactions with integrins, T-box transcription factors, and receptor tyrosine kinases. Facilitating the assembly of protein complexes, PDZ-LIM proteins can act as signal modulators, influence actin dynamics, regulate cell architecture, and control gene transcription.

The tbx/bHLH transcription factor mga regulates gata4 and organogenesis

The mga gene encodes a unique transcription factor containing both TBOX and bHLHzip DNA-binding domains. Here we describe the structure, expression pattern, and loss-of-function phenotype for zebrafish mga. The mga gene is conserved with mammalian homologs for both DNA-binding domains. It is expressed maternally, and subsequently in the developing brain, heart, and gut, and its depletion causes morphogenetic defects in each of these organ systems. The heart and gut phenotypes are similar to those described previously for loss of gata4, and the mga morphant shows increased levels of gata4 transcripts in lateral mesoderm. Knockdown of gata4 rescues the early heart-looping defect (but not the gut defect), indicating that mga restricts the normal levels of Gata4 required for heart tube looping, while both genes are important for gut development. Transcript profiling experiments show that mga functions early to influence key regulators of mesendoderm, including tbx6, cas, and sox17.

Tbx5 and Bmp signaling are essential for proepicardium specification in zebrafish

RATIONALE

The proepicardial organ (PE) contributes to the cellular diversity of the developing heart by giving rise to the epicardium as well as vascular smooth muscle cells and fibroblasts. Despite the importance of these cells in cardiac development, function and regeneration, the signals required for the specification of the PE remain largely unexplored.

OBJECTIVE

We aim to identify the signaling molecules and transcription factors that regulate PE specification.

METHODS AND RESULTS

Here, we present the first genetic evidence that bone morphogenetic protein (Bmp) signaling in conjunction with the T-box transcription factor Tbx5a is essential for PE specification in zebrafish. Specifically, Bmp4 from the cardiac region, but not the liver bud, acting through the type I BMP receptor Acvr1l, is required for PE specification. By overexpressing a dominant-negative form of a Bmp receptor at various embryonic stages, we determined when Bmp signaling was required for PE specification. We also found that overexpression of bmp2b right before PE specification led to the ectopic expression of PE specific markers including tbx18. Furthermore, using loss-of-function approaches, we discovered a previously unappreciated PE specification role for Tbx5a at early somite stages; this role occurs earlier than, and appears to be independent from, the requirement for Bmp signaling in this process.

CONCLUSION

Altogether, these data lead us to propose that Tbx5a confers anterior lateral plate mesodermal cells the competence to respond to Bmp signals and initiate PE development.

Structural and functional effects of developmental exposure to ethanol on the zebrafish heart

BACKGROUND

Fetal alcohol exposure during development results in a host of cardiac abnormalities including atrial and ventricular septal defects, teratology of Fallot, d-transposition of the great arteries, truncus arteriosus communis, and aortico-pulmonary window. The mechanisms behind these ethanol-induced deficits are unknown. The purpose of this study was to determine whether the zebrafish, a simple model in which heart development and the sequence of gene expression is well elucidated and comparable to that in higher vertebrates, is sensitive to developmental exposure of pharmacologically relevant concentrations of ethanol.

METHODS

Zebrafish eggs of the AB strain were raised in egg water or in 0.5% (v/v) ethanol solution for either 54 hpf (hours postfertilization) or 72 hpf. Heart pathology and volumes were evaluated on the latter group at 5 dpf (days postfertilization) on tissue sections from fixed larvae embedded in glycolmethacrylate. Heart rates were determined in embryos of 54 hpf and larvae of 5 dpf. The functional maturity of the heart's conducting system was measured by determining the response of ethanol-treated and control embryos and larvae to the adrenergic agonist, isoproterenol, and the cholinergic agonist, carbachol.

RESULTS

Ethanol-induced alterations occurred in heart morphology and heart volume. A developmental lag in the isoproterenol response and the absence of carbachol-mediated bradycardia were also observed following ethanol treatment.

CONCLUSIONS

These results show that exposure of the zebrafish to ethanol during development results in structural and functional changes in the heart that mimic malformations that occur in patients with fetal alcohol syndrome (FAS). These findings promote the zebrafish heart as a future model for investigating the mechanisms responsible for ethanol's adverse effects on vertebrate heart development.

Tbx4/5 gene duplication and the origin of vertebrate paired appendages

Paired fins/limbs are one of the most successful vertebrate innovations, since they are used for numerous fundamental activities, including locomotion, feeding, and breeding. Gene duplication events generate new genes with the potential to acquire novel functions, and two rounds of genome duplication took place during vertebrate evolution. The cephalochordate amphioxus diverged from other chordates before these events and is widely used to deduce the functions of ancestral genes, present in single copy in amphioxus, compared to the functions of their duplicated vertebrate orthologues. The T-box genes Tbx5 and Tbx4 encode two closely related transcription factors that are the earliest factors required to initiate forelimb and hind limb outgrowth, respectively. Since the genetic components proposed to be responsible for acquiring a trait during evolution are likely to be involved in the formation of that same trait in living organisms, we investigated whether the duplication of an ancestral, single Tbx4/5 gene to give rise to distinct Tbx4 and Tbx5 genes has been instrumental in the acquisition of limbs during vertebrate evolution. We analyzed whether the amphioxus Tbx4/5 gene is able to initiate limb outgrowth, and assayed the amphioxus locus for the presence of limb-forming regulatory regions. We show that AmphiTbx4/5 is able to initiate limb outgrowth and, in contrast, that the genomic locus lacks the regulatory modules required for expression that would result in limb formation. We propose that changes at the level of Tbx5 and Tbx4 expression, rather than the generation of novel protein function, have been necessary for the acquisition of paired appendages during vertebrate evolution.

Identification and characterisation of the developmental expression pattern of tbx5b, a novel tbx5 gene in zebrafish

Tbx5 is a T-box transcription factor that has been characterised in most vertebrate lineages and is widely expressed during the development of various embryonic structures, including the heart, the eyes and the anterior set of paired appendages (tetrapod forelimbs and fish pectoral fins). Mutations in TBX5 cause Holt-Oram syndrome, an autosomal dominant human "heart-hand" condition characterised by upper limb and heart malformations. In zebrafish, embryos with compromised tbx5 function show a complete absence of pectoral fins, whereas heart and eye development are not so highly disturbed. Here, we identify a new tbx5 gene in zebrafish that we have called tbx5b. This duplicate gene is present in all teleost genomes whose sequence is available, suggesting it resulted from the teleost-specific genome duplication event that took place during fish evolution. We show that tbx5b has lost the characteristic forelimb/pectoral fin expression of Tbx5 genes but has retained the eye and heart expression, partially overlapping with that of its paralogue, now referred to as tbx5a. Functional redundancy of tbx5a and tbx5b in the eye and heart would therefore explain the mild phenotypes observed during development of these organs in fish embryos with compromised tbx5a function.

Pdlim7 (LMP4) regulation of Tbx5 specifies zebrafish heart atrio-ventricular boundary and valve formation

Tbx5 is involved in congenital heart disease, however, the mechanisms leading to organ malformation are greatly unknown. We hypothesized a model by which the Tbx5 binding protein Pdlim7 controls nuclear/cytoplasmic shuttling and function of the transcription factor. Using the zebrafish, we present in vivo significance for an essential role of Tbx5/Pdlim7 protein interaction in the regulation of cardiac formation. Knock-down of Pdlim7 results in a non-looped heart, strikingly reminiscent of the tbx5 heartstrings mutant phenotype. However, while misregulation of Pdlim7 and Tbx5 produce similar aberrant cardiac morphology, molecular and histological analysis uncovered that the Pdlim7 and Tbx5 cardiac phenotypes are due to opposite effects on valve development. Loss of Pdlim7 function causes no valve tissue to develop while lack of Tbx5 results in increased valve tissue. These opposing defects are evident before valve formation and are the result of distinct gene misregulation during specification of the atrio-ventricular (AV) boundary. We show that Pdlim7/Tbx5 interactions affect the expression of Tbx5 target genes nppa and tbx2b at the AV boundary, and their domains of misexpression directly correlate with the identified valve defects. These studies demonstrate that controlling the correct balance of Tbx5 activity is crucial for the specification of the AV boundary and valve formation.

The erythropoietin and regenerative medicine: a lesson from fish

BACKGROUND

Erythropoietin (EPO), the main haematopoietic growth factor for the proliferation and differentiation of erythroid progenitor cells, is also known for its angiogenic and regenerative properties.

MATERIALS AND METHODS

In this study, we aimed to test the regenerative effects of EPO administration in an experimental model of Sea bass (Dicentrarchus labrax) subjected to amputation of the caudal fin.

RESULTS

Erythropoietin-treated fishes (3000 UI of human recombinant EPO-alpha immediately after cutting and after 15 days) showed an increased growth rate of their fins compared with those untreated (anova variance: P: 0.01 vs. P: 0.04). By analysing fin length at established times (15 and 30 days after cut), EPO-treated fishes always showed an increased length compared with untreated ones (T-15: 1.1 +/- 0.2 vs. 0.7 +/- 0.2 cm, P: 0.03; T-30: 1.9 +/- 0.3 vs. 1.2 +/- 0.2 cm, P: 0.01). Moreover, exogenous EPO administration induced an enormous increase in EPO-blood levels at each observation time (T-15: 2240 +/- 210 vs. 16.7 +/- 1.8 mU mL(-1), P < 0.001; T-30: 2340 +/- 190 vs. 17.1 +/- 1.9 mU mL(-1), P < 0.001), whereas these levels remained quite unmodified in untreated fishes. Immunochemical analyses performed by confocal laser scanning microscopic observations showed an increased expression of EPO-receptors and PECAM-1 (an endothelial surface marker of vessels sprout) in the regenerating tissue, whereas no signs of inflammation or fibrosis were recognisable.

CONCLUSIONS

All these findings confirm EPO as a new factor involved in regenerative processes, also suggesting a potential, future utility for new therapeutical applications in the field of human regenerative medicine.

Fishing for the genetic basis of cardiovascular disease

Cardiovascular disease (CVD) has recently overtaken infectious disease to become the biggest global killer. Genetic factors have emerged as being of major importance in the pathogenesis of CVD. Owing to disease heterogeneity, variable penetrance and high mortality, human genetic studies alone are not sufficient to elucidate the genetic basis of CVD. Animal models are needed to identify novel genes that are involved in cardiovascular pathology and to verify the effect of suspected disease genes on cardiovascular function. An intriguing model organism is the zebrafish danio rerio. Several features of the zebrafish, such as a closed cardiovascular system, transparency at embryonal stages, rapid and external development, and easily tractable genetics make it ideal for cardiovascular research. Moreover, zebrafish are suitable for forward genetics approaches, which allow the unbiased identification of novel and unanticipated cardiovascular genes. Zebrafish mutants with various cardiovascular phenotypes that closely correlate with human disease, such as congenital heart disease, cardiomyopathies and arrhythmias, have been isolated. The pool of zebrafish mutants, for which the causal gene mutation has been identified, is constantly growing. The human orthologues of several of these zebrafish genes have been shown to be involved in the pathogenesis of human CVD. Cardiovascular zebrafish models also provide the opportunity to develop and test novel therapeutic strategies, using innovative technologies such as high throughput in vivo small molecule screens.

Gdf6a is required for the initiation of dorsal-ventral retinal patterning and lens development

Dorsal-ventral patterning of the vertebrate retina is essential for accurate topographic mapping of retinal ganglion cell (RGC) axons to visual processing centers. Bone morphogenetic protein (Bmp) growth factors regulate dorsal retinal identity in vertebrate models, but the developmental timing of this signaling and the relative roles of individual Bmps remain unclear. In this study, we investigate the functions of two zebrafish Bmps, Gdf6a and Bmp4, during initiation of dorsal retinal identity, and subsequently during lens differentiation. Knockdown of zebrafish Gdf6a blocks initiation of retinal Smad phosphorylation and dorsal marker expression, while knockdown of Bmp4 produces no discernable retinal phenotype. These data, combined with analyses of embryos ectopically expressing Bmps, demonstrate that Gdf6a is necessary and sufficient for initiation of dorsal retinal identity. We note a profound expansion of ventral retinal identity in gdf6a morphants, demonstrating that dorsal BMP signaling antagonizes ventral marker expression. Finally, we demonstrate a role for Gdf6a in non-neural ocular tissues. Knockdown of Gdf6a leads to defects in lens-specific gene expression, and when combined with Bmp signaling inhibitors, disrupts lens fiber cell differentiation. Taken together, these data indicate that Gdf6a initiates dorsal retinal patterning independent of Bmp4, and regulates lens differentiation.

A new method for detection and quantification of heartbeat parameters in Drosophila, zebrafish, and embryonic mouse hearts

The genetic basis of heart development is remarkably conserved from Drosophila to mammals, and insights from flies have greatly informed our understanding of vertebrate heart development. Recent evidence suggests that many aspects of heart function are also conserved and the genes involved in heart development also play roles in adult heart function. We have developed a Drosophila heart preparation and movement analysis algorithm that allows quantification of functional parameters. Our methodology combines high-speed optical recording of beating hearts with a robust, semi-automated analysis to accurately detect and quantify, on a beat-to-beat basis, not only heart rate but also diastolic and systolic intervals, systolic and diastolic diameters, percent fractional shortening, contraction wave velocity, and cardiac arrhythmicity. Here, we present a detailed analysis of hearts from adult Drosophila, 2-3-day-old zebrafish larva, and 8-day-old mouse embryos, indicating that our methodology is potentially applicable to an array of biological models. We detect progressive age-related changes in fly hearts as well as subtle but distinct cardiac deficits in Tbx5 heterozygote mutant zebrafish. Our methodology for quantifying cardiac function in these genetically tractable model systems should provide valuable insights into the genetics of heart function.

The outflow tract of the heart in fishes: anatomy, genes and evolution

A large number of congenital heart defects associated with mortality in humans are those that affect the cardiac outflow tract, and this provides a strong imperative to understand its development during embryogenesis. While there is wide phylogenetic variation in adult vertebrate heart morphology, recent work has demonstrated evolutionary conservation in the early processes of cardiogenesis, including that of the outflow tract. This, along with the utility and high reproductive potential of fish species such as Danio rerio, Oryzias latipes etc., suggests that fishes may provide ideal comparative biological models to facilitate a better understanding of this poorly understood region of the heart. In this review, the authors present the current understanding of both phylogeny and ontogeny of the cardiac outflow tract in fishes and examine how new molecular studies are informing the phylogenetic relationships and evolutionary trajectories that have been proposed. The authors also attempt to address some of the issues of nomenclature that confuse this area of research.

Physical interaction between TBX5 and MEF2C is required for early heart development

TBX5 is a transcription factor which plays important roles in the development of the heart and upper limbs. Mutations in this gene produce the inherited disorder Holt-Oram syndrome. Here, we report a physical interaction between TBX5 and MEF2C leading to a synergistic activation of the alpha-cardiac myosin heavy chain (MYH6). Mutants of TBX5, TBX5G80R, and TBX5R279X that produce severe cardiac phenotypes impair the synergy. Using fluorescence resonance energy transfer, we demonstrate the interaction of TBX5 and MEF2C in living cells. We also show that they physically associate through their DNA-binding domains to form a complex on the MYH6 promoter. Morpholino-mediated knockdowns of Tbx5 and Mef2c in zebrafish suggest that the genetic interaction of these proteins is not only required for MYH6 expression but also essential for the early stages of heart development and survival. This is the first report of a functional interaction between a T-box protein and a MADS box factor that may be crucial in cardiomyocyte differentiation.

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

Background

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

Methodology/Principal Findings

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

Conclusions and Significance

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

Hoxb5b acts downstream of retinoic acid signaling in the forelimb field to restrict heart field potential in zebrafish

How adjacent organ fields communicate during development is not understood. Here, we identify a mechanism in which signaling within the forelimb field restricts the potential of the neighboring heart field. In zebrafish embryos deficient in retinoic acid (RA) signaling, the pectoral fins (forelimbs) are lost while both chambers of the heart are enlarged. We provide evidence that both of these phenotypes are due to RA signaling acting directly within the forelimb field. hoxb5b, an RA-responsive gene expressed within the forelimb field, is required to restrict the number of atrial cells arising from the adjacent heart field, although its function is dispensable for forelimb formation. Together, these data indicate nonautonomous influences downstream of RA signaling that act to limit individual chamber size. Therefore, our results offer new perspectives on the mechanisms regulating organ size and the possible causes of congenital syndromes affecting both the heart and forelimb.

Acute and long-term effects after single loading of functionalized multi-walled carbon nanotubes into zebrafish (Danio rerio)

Carbon nanotubes (CNTs) are widely explored for biomedical applications, but there is very limited information regarding their in vivo biodistribution and biocompatibility. Here, we report the in vivo biodistribution and long-term effects of functionalized multi-walled carbon nanotubes (MWCNTs) in developing zebrafish. The fluorescent-labeled MWCNTs were introduced into zebrafish embryos at 1-cell stage and at 72 h post fertilization through microinjection. After single injection, both acute and long-term interactions between zebrafish and functionalized MWCNTs were studied. The injected FITC-BSA-MWCNTs (at 1-cell stage) were allocated to all blastoderm cells of the embryos through proliferation, and were distinctively excluded from the yolk cell. When introduced into the circulation system, FITC-BSA-MWCNTs moved easily in the compartments and finally were cleaned out by the body at 96 h after the loading. At early stages, the treated zebrafish embryos generated immune response by accumulating circulating white blood cells at the trunk region. Under transmission electron microscope, many lysosome-like vesicles were observed in the blastoderm cells of the treated embryos. The zebrafish loaded with MWCNTs had normal primordial germ cells at early stage and produced second generation later on. However, the larvae of the second generation had obviously lower survival rates as compared to the untreated groups, suggesting a negative effect on the reproduction potential. These results suggest that extensive purification and functionalization processes can help improve the biocompatibility of CNTs. This study also indicates that purified CNTs may have long-term toxicity effects when they were delivered into the body.

The role of insulin receptor signaling in zebrafish embryogenesis

Insulin receptor (IR) signaling is considered to be important in growth and development in addition to its major role in metabolic homeostasis. The metabolic role of insulin in carbohydrate and lipid metabolism is extensively studied. In contrast, the role of IR activation during embryogenesis is less understood. To address this, we examined the function of the IR during zebrafish development. Zebrafish express two isoforms of IR (insra and insrb). Both isoforms were cloned and show high homology to the human insulin receptor and can functionally substitute for the human IR in fibroblasts derived from insr gene-deleted mice. Gene expression studies reveal that these receptors are expressed at moderate levels in the central nervous system during development. Morpholino-mediated selective knockdown of each of the IR isoforms causes growth retardation and profound morphogenetic defects in the brain and eye. These results clearly demonstrate that IR signaling plays essential roles in vertebrate embryogenesis and growth.

Combinatorial roles for zebrafish retinoic acid receptors in the hindbrain, limbs and pharyngeal arches

Retinoic acid (RA) signaling regulates multiple aspects of vertebrate embryonic development and tissue patterning, in part through the local availability of nuclear hormone receptors called retinoic acid receptors (RARs) and retinoid receptors (RXRs). RAR/RXR heterodimers transduce the RA signal, and loss-of-function studies in mice have demonstrated requirements for distinct receptor combinations at different stages of embryogenesis. However, the tissue-specific functions of each receptor and their individual contributions to RA signaling in vivo are only partially understood. Here we use morpholino oligonucleotides to deplete the four known zebrafish RARs (raraa, rarab, rarga, and rargb). We show that while all four are required for anterior-posterior patterning of rhombomeres in the hindbrain, there are unique requirements for rarga in the cranial mesoderm for hindbrain patterning, and rarab in lateral plate mesoderm for specification of the pectoral fins. In addition, the alpha subclass (raraa, rarab) is RA inducible, and of these only raraa expression is RA-dependent, suggesting that these receptors establish a region of particularly high RA signaling through positive-feedback. These studies reveal novel tissue-specific roles for RARs in controlling the competence and sensitivity of cells to respond to RA.

Cascade effect of cardiac myogenesis gene expression during cardiac looping in tbx5 knockdown zebrafish embryos

Zebrafish tbx5 expresses in the heart, pectoral fins and eyes of zebrafish during embryonic development. In zebrafish, injection of tbx5 morpholino antisense RNA caused changes of heart conformation, defect of heart looping, pericardium effusion, dropsy of ventral position and decreased heart rate. We suggested that cardiac myogenesis genes might be responsible for this phenomenon. Morpholino antisense RNA which against the initiation site of tbx5 gene was designed in order to knockdown the expression of tbx5, and the results were analyzed by whole-mount in situ hybridization and quantitative real-time PCR. Expression of cardiac myogenesis genes amhc, vmhc and cmlc2 were expressed constantly at the early embryonic development and reached its highest rate right before cardiac looping initiated. These cardiac myogenesis genes showed insufficient expressions within different heart defect embryos. Moreover, vmhc showed ectopic expression in addition to heart looping defect in heart defective embryos at 36 hpf. Our data suggests that the heart failure caused by the knockdown of tbx5 gene might result from the down-regulation of cardiac myogenesis genes.

Tri-phasic expression of posterior Hox genes during development of pectoral fins in zebrafish: implications for the evolution of vertebrate paired appendages

During development of the limbs, Hox genes belonging to the paralogous groups 9-13 are expressed in three distinct phases, which play key roles in the segmental patterning of limb skeletons. In teleost fishes, which have a very different organization in their fin skeletons, it is not clear whether a similar patterning mechanism is at work. To determine whether Hox genes are also expressed in several distinct phases during teleost paired fin development, we re-analyzed the expression patterns of hox9-13 genes during development of pectoral fins in zebrafish. We found that, similar to tetrapod Hox genes, expression of hoxa/d genes in zebrafish pectoral fins occurs in three distinct phases, in which the most distal/third phase is correlated with the development of the most distal structure of the fin, the fin blade. Like in tetrapods, hox gene expression in zebrafish pectoral fins during the distal/third phase is dependent upon sonic hedgehog signaling (hoxa and hoxd genes) and the presence of a long-range enhancer (hoxa genes), which indicates that the regulatory mechanisms underlying tri-phasic expression of Hox genes have remained relatively unchanged during evolution. Our results suggest that, although simpler in organization, teleost fins do have a distal structure that might be considered comparable to the autopod region of limbs.

Distinct expression and function of alternatively spliced Tbx5 isoforms in cell growth and differentiation

Mutations in the T-box transcription factor Tbx5 cause Holt-Oram syndrome, an autosomal dominant disease characterized by a wide spectrum of cardiac and upper limb defects with variable expressivity. Tbx5 haploinsufficiency has been suggested to be the underlying mechanism, and experimental models are consistent with a dosage-sensitive requirement for Tbx5 in heart development. Here, we report that Tbx5 levels are regulated through alternative splicing that generates, in addition to the known 518-amino-acid protein, a C-terminal truncated isoform. This shorter isoform retains the capacity to bind DNA, but its interaction with Tbx5 collaborators such as GATA-4 is altered. In vivo, the two spliced isoforms are oppositely regulated in a temporal and growth factor-dependent manner and are present in distinct DNA-binding complexes. The expression of the long isoform correlates with growth stimulation, and its reexpression in postnatal transgenic mouse hearts promotes hypertrophy. Conversely, the upregulation of the short but not the long isoform in C2C12 myoblasts leads to growth arrest and cell death. The results provide novel insight into posttranscriptional Tbx5 regulation and point to an important role not only in cell differentiation but also in cell proliferation and organ growth. The data may help analyze genotype-phenotype relations in patients with Holt-Oram syndrome.

How to create the vascular tree? (Latest) help from the zebrafish

The cardiovascular system provides oxygen, nutrients and hormones to organs, it directs traffic of metabolites and it maintains tissue homeostasis. It is one of the first organs assembled during vertebrate development and it is essential to life from early stages to adult. For these reasons, the process of vessel formation has being studied for more than a century, but it is only in the late eighties that there has been an explosion of research in the field with the employment of various in vitro and in vivo model systems. The zebrafish (Danio rerio) offers several advantages for in vivo studies; it played a fundamental role in new discoveries and helped to refine our knowledge of the vascular system. This review recapitulates the zebrafish data on vasculogenesis and angiogenesis, including the specification of the haemangioblasts from the mesoderm, their migration to form the vascular cord followed by axial vessels specification, the primary and secondary sprouting of intersomitic vessels, the formation of the lumen, the arterial versus venous specification and patterning. To emphasize the strengths of the zebrafish system in the vascular field, we summarize main tools, such as gene expression and mutagenesis screens, knock down technologies, transgenic lines and imaging, which played a major role in the development of the field and allowed significant discoveries, for instance the recent visualization of the lymphatic system in zebrafish. This information contributes to the prospective of drug discovery to cure human diseases linked to angiogenesis, not last tumours.

Ndrg4 is required for normal myocyte proliferation during early cardiac development in zebrafish

NDRG4 is a novel member of the NDRG family (N-myc downstream-regulated gene). The roles of NDRG4 in development have not previously been evaluated. We show that, during zebrafish embryonic development, ndrg4 is expressed exclusively in the embryonic heart, the central nervous system (CNS) and the sensory system. Ndrg4 knockdown in zebrafish embryos causes a marked reduction in proliferative myocytes and results in hypoplastic hearts. This growth defect is associated with cardiac phenotypes in morphogenesis and function, including abnormal heart looping, inefficient circulation and weak contractility. We reveal that ndrg4 is required for restricting the expression of versican and bmp4 to the developing atrioventricular canal. This constellation of ndrg4 cardiac defects phenocopies those seen in mutant hearts of heartstrings (hst), the tbx5 loss-of-function mutants in zebrafish. We further show that ndrg4 expression is significantly decreased in hearts with reduced tbx5 activities. Conversely, increased expression of tbx5 that is due to tbx20 knockdown leads to an increase in ndrg4 expression. Together, our studies reveal an essential role of ndrg4 in regulating proliferation and growth of cardiomyocytes, suggesting that ndrg4 may function downstream of tbx5 during heart development and growth.

An evolutionarily conserved nuclear export signal facilitates cytoplasmic localization of the Tbx5 transcription factor

During cardiac development, the T-box transcription factor Tbx5 displays dynamic changes in localization from strictly nuclear to both nuclear and cytoplasmic to exclusively cytoplasmic along the actin cytoskeleton in cells coexpressing its binding protein LMP4. Although nuclear localization signals (NLSs) have been described, the mechanism by which Tbx5 exits the nucleus remained elusive. Here, we describe for Tbx5 a nuclear export signal (NES) that is recognized by the CRM1 export protein. Site-directed mutagenesis of a critical amino acid(s) within this sequence determined the functionality of this NES. Confocal localization studies and luciferase transcriptional reporter assays with NES mutant Tbx5 forms demonstrated retention in the nucleus, regardless of the presence of LMP4. Coimmunoprecipitation and pharmacological interference studies demonstrated a direct interaction between Tbx5 and CRM1, revealing that Tbx5 is using the CRM1 pathway for nuclear export. In addition to Tbx5, we identified NESs in all T-box proteins and demonstrated interaction of the family members Tbx3 and Brachyury with the CRM1 exporter, suggesting general significance. This first demonstration of evolutionarily conserved NESs in all T-box proteins in conjunction with NLSs indicates a primordial function of T-box proteins to dynamically shuttle between nuclear and cytoplasmic compartments of the cell.

Genetic analysis of the two zebrafish patched homologues identifies novel roles for the hedgehog signaling pathway

Background

Aberrant activation of the Hedgehog (Hh) signaling pathway in different organisms has shown the importance of this family of morphogens during development. Genetic screens in zebrafish have assigned specific roles for Hh in proliferation, differentiation and patterning, but mainly as a result of a loss of its activity. We attempted to fully activate the Hh pathway by removing both receptors for the Hh proteins, called Patched1 and 2, which are functioning as negative regulators in this pathway.

Results

Here we describe a splice-donor mutation in Ptc1, called ptc1^hu1602, which in a homozygous state results in a subtle eye and somite phenotype. Since we recently positionally cloned a ptc2 mutant, a ptc1;ptc2 double mutant was generated, showing severely increased levels of ptc1, gli1 and nkx2.2a, confirming an aberrant activation of Hh signaling. As a consequence, a number of phenotypes were observed that have not been reported previously using Shh mRNA overexpression. Somites of ptc1;ptc2 double mutants do not express anteroposterior polarity markers, however initial segmentation of the somites itself is not affected. This is the first evidence that segmentation and anterior/posterior (A/P) patterning of the somites are genetically uncoupled processes. Furthermore, a novel negative function of Hh signaling is observed in the induction of the fin field, acting well before any of the previously reported function of Shh in fin formation and in a way that is different from the proposed early role of Gli3 in limb/fin bud patterning.

Conclusion

The generation and characterization of the ptc1;ptc2 double mutant assigned novel and unexpected functions to the Hh signaling pathway. Additionally, these mutants will provide a useful system to further investigate the consequences of constitutively activated Hh signaling during vertebrate development.

What causes partial F1 hybrid viability? Incomplete penetrance versus genetic variation

BACKGROUND

Interspecific hybrid crosses often produce offspring with reduced but non-zero survivorship. In this paper we ask why such partial inviability occurs. This partial inviability could arise from incomplete penetrance of lethal Dobzhansky-Muller incompatibilities (DMIs) shared by all members of a hybrid cross. Alternatively, siblings may differ with respect to the presence or number of DMIs, leading to genotype-dependent variation in viability and hence non-Mendelian segregation of parental alleles in surviving F1 hybrids.

METHODOLOGY/PRINCIPAL FINDINGS

We used amplified fragment length polymorphisms (AFLPs) to test for segregation distortion in one hybrid cross between green and longear sunfish (Lepomis cyanellus and L. megalotis). Hybrids showed partial viability, and twice as much segregation distortion (36.8%) of AFLPs as an intraspecific control cross (18.8%). Incomplete penetrance of DMIs, which should cause genotype-independent mortality, is insufficient to explain the observed segregation distortion.

CONCLUSIONS/SIGNIFICANCE

We conclude that F1 hybrid sunfish are polymorphic for DMIs, either due to sex-linked DMI loci (causing Haldane's Rule), or polymorphic autosomal DMI loci. Because few AFLP markers were sex-linked (2%), the most parsimonious conclusion is that parents may have been heterozygous for loci causing hybrid inviability.

Development of the proepicardial organ in the zebrafish

The epicardium is the last layer of the vertebrate heart to form, surrounding the heart muscle during embryogenesis and providing signaling cues essential to the continued growth and differentiation of the heart. This outer layer of the heart develops from a transient structure, the proepicardial organ (PEO). Despite its essential roles, the early signals required for the formation of the PEO and the epicardium remain poorly understood. The molecular markers wt1 and tcf21 are used to identify the epicardial layer in the zebrafish heart, to trace its development and to determine genes required for its normal development. Disruption of lateral plate mesoderm (LPM) migration through knockdown of miles apart or casanova leads to cardia bifida with each bilateral heart associated with its own PEO, suggesting that the earliest progenitors of the epicardium lie in the LPM. Using a gene knockdown approach, a genetic framework for PEO development is outlined. The pandora/spt6 gene is required for multiple cardiac lineages, the zinc-finger transcription factor wt1 is required for the epicardial lineage only and finally, the cell polarity genes heart and soul and nagie oko are required for proper PEO morphogenesis.

Glycogen synthase kinase 3 alpha and 3 beta have distinct functions during cardiogenesis of zebrafish embryo

Background

Glycogen synthase kinase 3 (GSK3) encodes a serine/threonine protein kinase, is known to play roles in many biological processes. Two closely related GSK3 isoforms encoded by distinct genes: GSK3α (51 kDa) and GSK3β (47 kDa). In previously studies, most GSK3 inhibitors are not only inhibiting GSK3, but are also affecting many other kinases. In addition, because of highly similarity in amino acid sequence between GSK3α and GSK3β, making it difficult to identify an inhibitor that can be selective against GSK3α or GSK3β. Thus, it is relatively difficult to address the functions of GSK3 isoforms during embryogenesis. At this study, we attempt to specifically inhibit either GSK3α or GSK3β and uncover the isoform-specific roles that GSK3 plays during cardiogenesis.

Results

We blocked gsk3α and gsk3β translations by injection of morpholino antisense oligonucleotides (MO). Both gsk3α- and gsk3β-MO-injected embryos displayed similar morphological defects, with a thin, string-like shaped heart and pericardial edema at 72 hours post-fertilization. However, when detailed analysis of the gsk3α- and gsk3β-MO-induced heart defects, we found that the reduced number of cardiomyocytes in gsk3α morphants during the heart-ring stage was due to apoptosis. On the contrary, gsk3β morphants did not exhibit significant apoptosis in the cardiomyocytes, and the heart developed normally during the heart-ring stage. Later, however, the heart positioning was severely disrupted in gsk3β morphants. bmp4 expression in gsk3β morphants was up-regulated and disrupted the asymmetry pattern in the heart. The cardiac valve defects in gsk3β morphants were similar to those observed in axin1 and apc^mcr mutants, suggesting that GSK3β might play a role in cardiac valve development through the Wnt/β-catenin pathway. Finally, the phenotypes of gsk3α mutant embryos cannot be rescued by gsk3β mRNA, and vice versa, demonstrating that GSK3α and GSK3β are not functionally redundant.

Conclusion

We conclude that (1) GSK3α, but not GSK3β, is necessary in cardiomyocyte survival; (2) the GSK3β plays important roles in modulating the left-right asymmetry and affecting heart positioning; and (3) GSK3α and GSK3β play distinct roles during zebrafish cardiogenesis.

Early steps of paired fin development in zebrafish compared with tetrapod limb development

The development of zebrafish paired fins and tetrapod forelimbs and hindlimbs show striking similarities at the molecular level. In recent years, the zebrafish, Danio rerio has become a valuable model for the study of the development of vertebrate paired appendages and several large-scale mutagenesis screens have identified novel fin mutants. This review summarizes recent advances in research into zebrafish paired fin development and highlights features that are shared with and distinct from limb development in other main animal models.

Tbx2 and Tbx3 regulate the dynamics of cell proliferation during heart remodeling

Background

The heart forms from a linear tube that is subject to complex remodeling during embryonic development. Hallmarks of this remodeling are the looping of the heart tube and the regionalization into chamber and non-chamber myocardium. Cardiomyocytes in the future chamber myocardium acquire different cellular and physiological characteristics through activation of a chamber-specific genetic program, which is in part mediated by T-box genes.

Methodology/Principal Finding

We characterize two new zebrafish T-box transcription factors, tbx3b and tbx2a, and analyze their role during the development of the atrioventricular canal. Loss- and gain-of-function analyses demonstrate that tbx3b and tbx2a are necessary to repress the chamber-genetic program in the non-chamber myocardium. We also show that tbx3b and tbx2a are required to control cell proliferation in the atrioventricular canal and that misregulation of cell proliferation in the heart tube influences looping. Furthermore, we characterize the heart phenotype of a novel Tbx3 mutation in mice and show that both the control of cell proliferation and the repression of chamber-specific genetic program in the non-chamber myocardium are conserved roles of Tbx3 in this species.

Conclusions/Significance

Taken together, our results uncover an evolutionarily conserved role of Tbx2/3 transcription factors during remodeling of the heart myocardium and highlight the importance of controlling cell proliferation as a driving force of morphogenesis.