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

An inherited TBX3 alteration in a prenatal case of ulnar-mammary syndrome: Clinical assessment and functional characterization in Drosophila melanogaster

Ulnar mammary syndrome (UMS) results from heterozygous variants in the TBX3 gene and impacts limb, tooth, hair, apocrine gland, and genitalia development. The expressivity of UMS is highly variable with no established genotype-phenotype correlations. TBX3 belongs to the Tbx gene family, which encodes transcription factors characterized by the presence of a T-box DNA-binding domain. We describe a fetus exhibiting severe upper limb defects and harboring the novel TBX3:c.400 C > T (p.P134S) variant inherited from the mother who remained clinically misdiagnosed until prenatal diagnosis. Literature revision was conducted to uncover the TBX3 clinical and mutational spectrum. Moreover, we generated a Drosophila humanized model for TBX3 to study the developmental consequences of the p.P134S as well as of other variants targeting different regions of the protein. Phenotypic analysis in flies, coupled with in silico modeling on the TBX3 variants, suggested that the c.400 C > T is UMS-causing and impacts TBX3 localization. Comparative analyses of the fly phenotypes caused by the expression of all variants, demonstrated that missense changes in the T-box domain affect more significantly TBX3 activity than variants outside this domain. To improve the clinicians' recognition of UMS, we estimated the frequency of the main clinical features of the disease. Core features often present pre-pubertally include defects of the ulna and/or of ulnar ray, hypoplastic nipples and/or areolas and, less frequently, genitalia anomalies in young males. These results enhance our understanding of the molecular basis and the clinical spectrum of UMS, shedding light on the functional consequences of TBX3 variants in a developmental context.

pIGLET: Safe harbor landing sites for reproducible and efficient transgenesis in zebrafish

Standard zebrafish transgenesis involves random transgene integration with resource-intensive screening. While phiC31 integrase-based attP/attB recombination has streamlined transgenesis in mice and Drosophila, validated attP-based landing sites for universal applications are lacking in zebrafish. Here, we developed phiC31 Integrase Genomic Loci Engineered for Transgenesis (pIGLET) as transgenesis approach, with two attP landing sites pIGLET14a and pIGLET24b from well-validated Tol2 transgenes. Both sites facilitate diverse transgenesis applications including reporters and Cre/loxP transgenes. The pIGLET14a and pIGLET24b landing sites consistently yield 25 to 50% germline transmission, substantially reducing the resources needed for transgenic line generation. Transgenesis into these sites enables reproducible expression patterns in F0 zebrafish embryos for enhancer discovery and testing of gene regulatory variants. Together, our new landing sites streamline targeted, reproducible zebrafish transgenesis as a robust platform for various applications while minimizing the workload for generating transgenic lines.

Emerging disinfection byproducts 3-bromine carbazole induces cardiac developmental toxicity via aryl hydrocarbon receptor activation in zebrafish larvae

3-bromine carbazole (3-BCZ) represents a group of emerging aromatic disinfection byproducts (DBP) detected in drinking water; however, limited information is available regarding its potential cardiotoxicity. To assess its impacts, zebrafish embryos were exposed to 0, 0.06, 0.14, 0.29, 0.58, 1.44 or 2.88 mg/L of 3-BCZ for 120 h post fertilization (hpf). Our results revealed that ≥1.44 mg/L 3-BCZ exposure induced a higher incidence of heart malformation and an elevated pericardial area in zebrafish larvae; it also decreased the number of cardiac muscle cells and thins the walls of the ventricle and atrium while increasing cardiac output and impeding cardiac looping. Furthermore, 3-BCZ exposure also exhibited significant effects on the transcriptional levels of genes related to both cardiac development (nkx2.5, vmhc, gata4, tbx5, tbx2b, bmp4, bmp10, and bmp2b) and cardiac function (cacna1ab, cacna1da, atp2a1l, atp1b2b, atp1a3b, and tnnc1a). Notably, N-acetyl-L-cysteine, a reactive oxygen species scavenger, may alleviate the failure of cardiac looping induced by 3-BCZ but not the associated cardiac dysfunction or malformation; conversely, the aryl hydrocarbon receptor agonist CH131229 can completely eliminate the cardiotoxicity caused by 3-BCZ. This study provides new evidence for potential risks associated with ingesting 3-BCZ as well as revealing underlying mechanisms responsible for its cardiotoxic effects on zebrafish embryos.

pIGLET: Safe harbor landing sites for reproducible and efficient transgenesis in zebrafish

Standard methods for transgenesis in zebrafish depend on random transgene integration into the genome followed by resource-intensive screening and validation. Targeted vector integration into validated genomic loci using phiC31 integrase-based attP/attB recombination has transformed mouse and Drosophila transgenesis. However, while the phiC31 system functions in zebrafish, validated loci carrying attP-based landing or safe harbor sites suitable for universal transgenesis applications in zebrafish have not been established. Here, using CRISPR-Cas9, we converted two well-validated single insertion Tol2-based zebrafish transgenes with long-standing genetic stability into two attP landing sites, called phiC31 Integrase Genomic Loci Engineered for Transgenesis (pIGLET). Generating fluorescent reporters, loxP-based Switch lines, CreERT2 drivers, and gene-regulatory variant reporters in the pIGLET14a and pIGLET24b landing site alleles, we document their suitability for transgenesis applications across cell types and developmental stages. For both landing sites, we routinely achieve 25-50% germline transmission of targeted transgene integrations, drastically reducing the number of required animals and necessary resources to generate individual transgenic lines. We document that phiC31 integrase-based transgenesis into pIGLET14a and pIGLET24b reproducibly results in representative reporter expression patterns in injected F0 zebrafish embryos suitable for enhancer discovery and qualitative and quantitative comparison of gene-regulatory element variants. Taken together, our new phiC31 integrase-based transgene landing sites establish reproducible, targeted zebrafish transgenesis for numerous applications while greatly reducing the workload of generating new transgenic zebrafish lines.

Space microgravity increases expression of genes associated with proliferation and differentiation in human cardiac spheres

Efficient generation of cardiomyocytes from human-induced pluripotent stem cells (hiPSCs) is important for their application in basic and translational studies. Space microgravity can significantly change cell activities and function. Previously, we reported upregulation of genes associated with cardiac proliferation in cardiac progenitors derived from hiPSCs that were exposed to space microgravity for 3 days. Here we investigated the effect of long-term exposure of hiPSC-cardiac progenitors to space microgravity on global gene expression. Cryopreserved 3D hiPSC-cardiac progenitors were sent to the International Space Station (ISS) and cultured for 3 weeks under ISS microgravity and ISS 1 G conditions. RNA-sequencing analyses revealed upregulation of genes associated with cardiac differentiation, proliferation, and cardiac structure/function and downregulation of genes associated with extracellular matrix regulation in the ISS microgravity cultures compared with the ISS 1 G cultures. Gene ontology analysis and Kyoto Encyclopedia of Genes and Genomes mapping identified the upregulation of biological processes, molecular function, cellular components, and pathways associated with cell cycle, cardiac differentiation, and cardiac function. Taking together, these results suggest that space microgravity has a beneficial effect on the differentiation and growth of cardiac progenitors.

Nr2f1a maintains atrial nkx2.5 expression to repress pacemaker identity within venous atrial cardiomyocytes of zebrafish

Maintenance of cardiomyocyte identity is vital for normal heart development and function. However, our understanding of cardiomyocyte plasticity remains incomplete. Here, we show that sustained expression of the zebrafish transcription factor Nr2f1a prevents the progressive acquisition of ventricular cardiomyocyte (VC) and pacemaker cardiomyocyte (PC) identities within distinct regions of the atrium. Transcriptomic analysis of flow-sorted atrial cardiomyocytes (ACs) from nr2f1a mutant zebrafish embryos showed increased VC marker gene expression and altered expression of core PC regulatory genes, including decreased expression of nkx2.5, a critical repressor of PC differentiation. At the arterial (outflow) pole of the atrium in nr2f1a mutants, cardiomyocytes resolve to VC identity within the expanded atrioventricular canal. However, at the venous (inflow) pole of the atrium, there is a progressive wave of AC transdifferentiation into PCs across the atrium toward the arterial pole. Restoring Nkx2.5 is sufficient to repress PC marker identity in nr2f1a mutant atria and analysis of chromatin accessibility identified an Nr2f1a-dependent nkx2.5 enhancer expressed in the atrial myocardium directly adjacent to PCs. CRISPR/Cas9-mediated deletion of the putative nkx2.5 enhancer leads to a loss of Nkx2.5-expressing ACs and expansion of a PC reporter, supporting that Nr2f1a limits PC differentiation within venous ACs via maintaining nkx2.5 expression. The Nr2f-dependent maintenance of AC identity within discrete atrial compartments may provide insights into the molecular etiology of concurrent structural congenital heart defects and associated arrhythmias.

Space microgravity improves proliferation of human iPSC-derived cardiomyocytes

In microgravity, cells undergo profound changes in their properties. However, how human cardiac progenitors respond to space microgravity is unknown. In this study, we evaluated the effect of space microgravity on differentiation of human induced pluripotent stem cell (hiPSC)-derived cardiac progenitors compared with 1G cultures on the International Space Station (ISS). Cryopreserved 3D cardiac progenitors were cultured for 3 weeks on the ISS. Compared with 1G cultures, the microgravity cultures had 3-fold larger sphere sizes, 20-fold higher counts of nuclei, and increased expression of proliferation markers. Highly enriched cardiomyocytes generated in space microgravity showed improved Ca²⁺ handling and increased expression of contraction-associated genes. Short-term exposure (3 days) of cardiac progenitors to space microgravity upregulated genes involved in cell proliferation, survival, cardiac differentiation, and contraction, consistent with improved microgravity cultures at the late stage. These results indicate that space microgravity increased proliferation of hiPSC-cardiomyocytes, which had appropriate structure and function.

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Cryopreserved 3D hiPSC-cardiac progenitors differentiated efficiently in space

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Microgravity cultures had increased sphere sizes and cellular proliferation

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Beating cardiomyocytes in microgravity cultures had improved Ca²⁺ handling

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Microgravity cultures had upregulated genes in cardiac contraction

Association of T-box gene polymorphisms with the risk of Wolff-Parkinson-White syndrome in a Han Chinese population

Abnormal development of the atrioventricular ring can lead to the formation of a bypass pathway and the occurrence of Wolff-Parkinson-White (WPW) syndrome. The genetic mechanism underlying the sporadic form of WPW syndrome remains unclear. Existing evidence suggests that both T-box transcription factor 3 (TBX3) and T-box transcription factor 2 (TBX2) genes participate in regulating annulus fibrosus formation and atrioventricular canal development. Thus, we aimed to examine whether single-nucleotide polymorphisms (SNPs) in the TBX3 and TBX2 genes confer susceptibility to WPW syndrome in a Han Chinese Population. We applied a SNaPshot SNP assay to analyze 5 selected tagSNPs of TBX3 and TBX2 in 230 patients with sporadic WPW syndrome and 231 sex- and age-matched controls. Haplotype analysis was performed using Haploview software. Allele C of TBX3 rs1061657 was associated with a higher risk of WPW syndrome (odds ratio [OR] = 1.41, 95% confidence interval [CI]: 1.08-1.83, P = .011) and left-sided accessory pathways (OR = 1.40, 95% CI: 1.07-1.84, P = .016). However, allele C of TBX3 rs8853 was likely to reduce these risks (OR = 0.71, 95% CI: 0.54-0.92, P = .011; OR = 0.70, 95% CI: 0.53-0.92, P = .011, respectively). The data revealed no association between TBX3 rs77412687, TBX3 rs2242442, or TBX2 rs75743672 and WPW syndrome. TBX3 rs1061657 and rs8853 are significantly associated with sporadic WPW syndrome among a Han Chinese population. To verify our results, larger sample sizes are required in future studies.

An integrative multiscale view of early cardiac looping

The heart is the first organ to form and function during the development of an embryo. Heart development consists of a series of events believed to be highly conserved in vertebrates. Development of heart begins with the formation of the cardiac fields followed by a linear heart tube formation. The straight heart tube then undergoes a ventral bending prior to further bending and helical torsion to form a looped heart. The looping phase is then followed by ballooning, septation, and valve formation giving rise to a four-chambered heart in avians and mammals. The looping phase plays a central role in heart development. Successful looping is essential for proper alignment of the future cardiac chambers and tracts. As aberrant looping results in various congenital heart diseases, the mechanisms of cardiac looping have been studied for several decades by various disciplines. Many groups have studied anatomy, biology, genetics, and mechanical processes during heart looping, and have proposed multiple mechanisms. Computational modeling approaches have been utilized to examine the proposed mechanisms of the looping process. Still, the exact underlying mechanism(s) controlling the looping phase remain poorly understood. Although further experimental measurements are obviously still required, the need for more integrative computational modeling approaches is also apparent in order to make sense of the vast amount of experimental data and the complexity of multiscale developmental systems. Indeed, there needs to be an iterative interaction between experimentation and modeling in order to properly find the gap in the existing data and to validate proposed hypotheses. This article is categorized under: Cardiovascular Diseases > Genetics/Genomics/Epigenetics Cardiovascular Diseases > Computational Models Cardiovascular Diseases > Molecular and Cellular Physiology.

Genomic frontiers in congenital heart disease

The application of next-generation sequencing to study congenital heart disease (CHD) is increasingly providing new insights into the causes and mechanisms of this prevalent birth anomaly. Whole-exome sequencing analysis identifies damaging gene variants altering single or contiguous nucleotides that are assigned pathogenicity based on statistical analyses of families and cohorts with CHD, high expression in the developing heart and depletion of damaging protein-coding variants in the general population. Gene classes fulfilling these criteria are enriched in patients with CHD and extracardiac abnormalities, evidencing shared pathways in organogenesis. Developmental single-cell transcriptomic data demonstrate the expression of CHD-associated genes in particular cell lineages, and emerging insights indicate that genetic variants perturb multicellular interactions that are crucial for cardiogenesis. Whole-genome sequencing analyses extend these observations, identifying non-coding variants that influence the expression of genes associated with CHD and contribute to the estimated ~55% of unexplained cases of CHD. These approaches combined with the assessment of common and mosaic genetic variants have provided a more complete knowledge of the causes and mechanisms of CHD. Such advances provide knowledge to inform the clinical care of patients with CHD or other birth defects and deepen our understanding of the complexity of human development. In this Review, we highlight known and candidate CHD-associated human genes and discuss how the integration of advances in developmental biology research can provide new insights into the genetic contributions to CHD.

Modeling Transposition of the Great Arteries with Patient-Specific Induced Pluripotent Stem Cells

The dextro-transposition of the great arteries (d-TGA) is one of the most common congenital heart diseases. To identify biological processes that could be related to the development of d-TGA, we established induced pluripotent stem cell (iPSC) lines from two patients with d-TGA and from two healthy subjects (as controls) and differentiated them into endothelial cells (iPSC-ECs). iPSC-EC transcriptome profiling and bioinformatics analysis revealed differences in the expression level of genes involved in circulatory system and animal organ development. iPSC-ECs from patients with d-TGA showed impaired ability to develop tubular structures in an in vitro capillary-like tube formation assay, and interactome studies revealed downregulation of biological processes related to Notch signaling, circulatory system development and angiogenesis, pointing to alterations in vascular structure development. Our study provides an iPSC-based cellular model to investigate the etiology of d-TGA.

Twisting of the zebrafish heart tube during cardiac looping is a tbx5-dependent and tissue-intrinsic process

Organ laterality refers to the left-right asymmetry in disposition and conformation of internal organs and is established during embryogenesis. The heart is the first organ to display visible left-right asymmetries through its left-sided positioning and rightward looping. Here, we present a new zebrafish loss-of-function allele for tbx5a, which displays defective rightward cardiac looping morphogenesis. By mapping individual cardiomyocyte behavior during cardiac looping, we establish that ventricular and atrial cardiomyocytes rearrange in distinct directions. As a consequence, the cardiac chambers twist around the atrioventricular canal resulting in torsion of the heart tube, which is compromised in tbx5a mutants. Pharmacological treatment and ex vivo culture establishes that the cardiac twisting depends on intrinsic mechanisms and is independent from cardiac growth. Furthermore, genetic experiments indicate that looping requires proper tissue patterning. We conclude that cardiac looping involves twisting of the chambers around the atrioventricular canal, which requires correct tissue patterning by Tbx5a.

BMP9 and BMP10: two close vascular quiescence partners that stand out

Bone morphogenetic proteins (BMPs) are dimeric transforming growth factor ß (TGFß) family cytokines that were first described in bone and cartilage formation but have since been shown to be involved in many pleiotropic functions. In human, there are 15 BMP ligands, which initiate their cellular signaling by forming a complex with two copies of type I receptors and two copies of type II receptors, both of which are transmembrane receptors with an intracellular serine/threonine kinase domain. Within this receptor family, ALK1 (Activin receptor-Like Kinase 1), which is a type I receptor mainly expressed on endothelial cells, and BMPRII (BMP Receptor type II), a type II receptor also highly expressed on endothelial cells, have been directly linked to two rare vascular diseases: hereditary haemorrhagic telangiectasia (HHT), and pulmonary arterial hypertension (PAH), respectively. BMP9 (gene name GDF2) and BMP10, two close members of the BMP family, are the only known ligands for the ALK1 receptor. This specificity gives them a unique role in physiological and pathological angiogenesis and tissue homeostasis. The aim of this current review is to present an overview of what is known about BMP9 and BMP10 on vascular regulation with a particular emphasis on recent results and the many questions that remain unanswered regarding the roles and specificities between BMP9 and BMP10. This article is protected by copyright. All rights reserved.

3D Bioprinting the Cardiac Purkinje System Using Human Adipogenic Mesenchymal Stem Cell Derived Purkinje Cells

PURPOSE

The objective of this study was to reprogram human adipogenic mesenchymal stem cells (hADMSCs) to form Purkinje cells and to use the reprogrammed Purkinje cells to bioprint Purkinje networks.

METHODS

hADMSCs were reprogrammed to form Purkinje cells using a multi-step process using transcription factors ETS2 and MESP1 to first form cardiac progenitor stem cells followed by SHOX2 and TBX3 to form Purkinje cells. A novel bioprinting method was developed based on Pluronic acid as the sacrificial material and type I collagen as the structural material. The reprogrammed Purkinje cells were used in conjunction with the novel bioprinting method to bioprint Purkinje networks. Printed constructs were evaluated for retention of functional protein connexin 40 (Cx40) and ability to undergo membrane potential changes in response to physiologic stimulus.

RESULTS

hADMSCs were successfully reprogrammed to form Purkinje cells based on the expression pattern of IRX3, IRX5, SEMA and SCN10. Reprogrammed purkinje cells were incorporated into a collagen type-1 bioink and the left ventricular Purkinje network was printed using anatomical images of the bovine Purkinje system as reference. Optimization studies demonstrated that 1.8 mg/mL type-I collagen at a seeding density of 300,000 cells per 200 µL resulted in the most functional bioprinted Purkinje networks. Furthermore, bioprinted Purkinje networks formed continuous syncytium, retained expression of vital functional gap junction protein Cx40 post-print, and exhibited membrane potential changes in response to electric stimulation and acetylcholine evaluated by DiBAC₄(5), an electrically responsive dye.

CONCLUSION

Based on the results of this study, hADMSCs were successfully reprogrammed to form Purkinje cells and bioprinted to form Purkinje networks.

Loss of Asb2 Impairs Cardiomyocyte Differentiation and Leads to Congenital Double Outlet Right Ventricle

Defining the pathways that control cardiac development facilitates understanding the pathogenesis of congenital heart disease. Herein, we identify enrichment of a Cullin5 Ub ligase key subunit, Asb2, in myocardial progenitors and differentiated cardiomyocytes. Using two conditional murine knockouts, Nkx^+/Cre.Asb2^fl/fl and AHF-Cre.Asb2^fl/fl, and tissue clarifying technique, we reveal Asb2 requirement for embryonic survival and complete heart looping. Deletion of Asb2 results in upregulation of its target Filamin A (Flna), and concurrent Flna deletion partially rescues embryonic lethality. Conditional AHF-Cre.Asb2 knockouts harboring one Flna allele have double outlet right ventricle (DORV), which is rescued by biallelic Flna excision. Transcriptomic and immunofluorescence analyses identify Tgfβ/Smad as downstream targets of Asb2/Flna. Finally, using CRISPR/Cas9 genome editing, we demonstrate Asb2 requirement for human cardiomyocyte differentiation suggesting a conserved mechanism between mice and humans. Collectively, our study provides deeper mechanistic understanding of the role of the ubiquitin proteasome system in cardiac development and suggests a previously unidentified murine model for DORV.

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Flna removal partially rescues embryonic lethality of Asb2-heart-specific knockout

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AHF-Asb2 knockouts harboring one Flna allele have double outlet right ventricle

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Asb2-Flna regulate TGFβ-Smad2 signaling in the heart

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Conserved role of Asb2 in heart morphogenesis between mice and humans

The roles and regulation of TBX3 in development and disease

TBX3, a member of the ancient and evolutionary conserved T-box transcription factor family, is a critical developmental regulator of several structures including the heart, mammary glands, limbs and lungs. Indeed, mutations in the human TBX3 lead to ulnar mammary syndrome which is characterized by several clinical malformations including hypoplasia of the mammary and apocrine glands, defects of the upper limb, areola, dental structures, heart and genitalia. In contrast, TBX3 has no known function in adult tissues but is frequently overexpressed in a wide range of epithelial and mesenchymal derived cancers. This overexpression greatly impacts several hallmarks of cancer including bypass of senescence, apoptosis and anoikis, promotion of proliferation, tumour formation, angiogenesis, invasion and metastatic capabilities as well as cancer stem cell expansion. The debilitating consequences of having too little or too much TBX3 suggest that its expression levels need to be tightly regulated. While we have a reasonable understanding of the mutations that result in low levels of functional TBX3 during development, very little is known about the factors responsible for the overexpression of TBX3 in cancer. Furthermore, given the plethora of oncogenic processes that TBX3 impacts, it must be regulating several target genes but to date only a few have been identified and characterised. Interestingly, while there is compelling evidence to support oncogenic roles for TBX3, a few studies have indicated that it may also have tumour suppressor functions in certain contexts. Together, the diverse functional elasticity of TBX3 in development and cancer is thought to involve, in part, the protein partners that it interacts with and this area of research has recently received some attention. This review provides an insight into the significance of TBX3 in development and cancer and identifies research gaps that need to be explored to shed more light on this transcription factor.

Differential chamber-specific expression and regulation of long non-coding RNAs during cardiac development

Cardiovascular development is governed by a complex interplay between inducting signals such as Bmps and Fgfs leading to activation of cardiac specific transcription factors such as Nkx2.5, Mef2c and Srf that orchestrate the initial steps of cardiogenesis. Over the last decade we have witnessed the discovery of novel layers of gene regulation, i.e. post-transcriptional regulation exerted by non-coding RNAs. The function role of small non coding RNAs has been widely demonstrated, e.g. miR-1 knockout display several cardiovascular abnormalities during embryogenesis. More recently long non-coding RNAs have been also reported to modulate gene expression and function in the developing heart, as exemplified by the embryonic lethal phenotypes of Fendrr and Braveheart knock out mice, respectively. In this study, we investigated the differential expression profile during cardiogenesis of previously reported lncRNAs in heart development. Our data revealed that Braveheart, Fendrr, Carmen display a preferential adult expression while Miat, Alien, H19 preferentially display chamber-specific expression at embryonic stages. We also demonstrated that these lncRNAs are differentially regulated by Nkx2.5, Srf and Mef2c, Pitx2 > Wnt > miRNA signaling pathway and angiotensin II and thyroid hormone administration. Importantly isoform-specific expression and distinct nuclear vs cytoplasmic localization of Braveheart, Carmen and Fendrr during chamber morphogenesis is observed, suggesting distinct functional roles of these lncRNAs in atrial and ventricular chambers. Furthermore, we demonstrate by in situ hybridization a dynamic epicardial, myocardial and endocardial expression of H19 during cardiac development. Overall our data support novel roles of these lncRNAs in different temporal and tissue-restricted fashion during cardiogenesis.

Maintenance of spatial gene expression by Polycomb-mediated repression after formation of a vertebrate body plan

Polycomb group (PcG) proteins are transcriptional repressors that are important regulators of cell fate during embryonic development. Among them, Ezh2 is responsible for catalyzing the epigenetic repressive mark H3K27me3 and is essential for animal development. The ability of zebrafish embryos lacking both maternal and zygotic ezh2 to form a normal body plan provides a unique model for comprehensively studying Ezh2 function during early development in vertebrates. By using a multi-omics approach, we found that Ezh2 is required for the deposition of H3K27me3 and is essential for proper recruitment of Polycomb group protein Rnf2. However, despite the complete absence of PcG-associated epigenetic mark and proteins, only minor changes in H3K4me3 deposition and gene and protein expression occur. These changes were mainly due to local dysregulation of transcription factors outside their normal expression boundaries. Altogether, our results in zebrafish show that Polycomb-mediated gene repression is important immediately after the body plan is formed to maintain spatially restricted expression profiles of transcription factors, and we highlight the differences that exist in the timing of PcG protein action between vertebrate species.

Summary: Our unique zebrafish model of a maternal and zygotic mutant for the Polycomb group gene ezh2 reveals major conserved and divergent mechanisms in epigenetic gene repression during vertebrate development.

pouC Regulates Expression of bmp4 During Atrioventricular Canal Formation in Zebrafish

BACKGROUND

Many human gene mutations have been linked to congenital heart disease (CHD), yet CHD remains a major health issue worldwide due in part to an incomplete understanding of the molecular basis for cardiac malformation.

RESULTS

Here we identify the orthologous mouse Pou6f1 and zebrafish pouC as POU homeodomain transcription factors enriched in the developing heart. We find that pouC is a multi-functional transcriptional regulator containing separable activation, repression, protein-protein interaction, and DNA binding domains. Using zebrafish heart development as a model system, we demonstrate that pouC knockdown impairs cardiac morphogenesis and affects cardiovascular function. We also find that levels of pouC expression must be fine-tuned to enable proper heart formation. At the cellular level, we demonstrate that pouC knockdown disrupts atrioventricular canal (AVC) cardiomyocyte maintenance, although chamber myocyte specification remains intact. Mechanistically, we show that pouC binds a bmp4 intronic regulatory element to mediate transcriptional activation.

CONCLUSIONS

Taken together, our study establishes pouC as a novel transcriptional input into the regulatory hierarchy that drives AVC morphogenesis in zebrafish. We anticipate that these findings will inform future efforts to explore functional conservation in mammals and potential association with atrioventricular septal defects in humans. Developmental Dynamics 248:173-188, 2019. © 2018 Wiley Periodicals, Inc.

Left-right asymmetry in heart development and disease: forming the right loop

Extensive studies have shown how bilateral symmetry of the vertebrate embryo is broken during early development, resulting in a molecular left-right bias in the mesoderm. However, how this early asymmetry drives the asymmetric morphogenesis of visceral organs remains poorly understood. The heart provides a striking model of left-right asymmetric morphogenesis, undergoing rightward looping to shape an initially linear heart tube and align cardiac chambers. Importantly, abnormal left-right patterning is associated with severe congenital heart defects, as exemplified in heterotaxy syndrome. Here, we compare the mechanisms underlying the rightward looping of the heart tube in fish, chick and mouse embryos. We propose that heart looping is not only a question of direction, but also one of fine-tuning shape. This is discussed in the context of evolutionary and clinical perspectives.

Identification of TBX2 and TBX3 variants in patients with conotruncal heart defects by target sequencing

Background

Conotruncal heart defects (CTDs) are heterogeneous congenital heart malformations that result from outflow tract dysplasia; however, the genetic determinants underlying CTDs remain unclear. Increasing evidence demonstrates that dysfunctional TBX2 and TBX3 result in outflow tract malformations, implying that both of them are involved in CTD pathogenesis. We screened for TBX2 and TBX3 variants in a large cohort of CTD patients (n = 588) and population-matched healthy controls (n = 300) by target sequencing and genetically analyzed the expression and function of these variants.

Results

The probably damaging variants p.R608W, p.T249I, and p.R616Q of TBX2 and p.A192T, p.M65L, and p.A562V of TBX3 were identified in CTD patients, but none in controls. All altered amino acids were highly conserved evolutionarily. Moreover, our data suggested that mRNA and protein expressions of TBX2 and TBX3 variants were altered compared with those of the wild-type. We screened PEA3 and MEF2C as novel downstream genes of TBX2 and TBX3, respectively. Functional analysis revealed that TBX2R608W and TBX2R616Q variant proteins further activated HAS2 promoter but failed to activate PEA3 promoter and that TBX3A192T and TBX3A562V variant proteins showed a reduced transcriptional activity over MEF2C promoter.

Conclusions

Our results indicate that the R608W and R616Q variants of TBX2 as well as the A192T and A562V variants of TBX3 contribute to CTD etiology; this was the first association of variants of TBX2 and TBX3 to CTDs based on a large population.

Electronic supplementary material

The online version of this article (10.1186/s40246-018-0176-0) contains supplementary material, which is available to authorized users.

A predictive model of asymmetric morphogenesis from 3D reconstructions of mouse heart looping dynamics

How left-right patterning drives asymmetric morphogenesis is unclear. Here, we have quantified shape changes during mouse heart looping, from 3D reconstructions by HREM. In combination with cell labelling and computer simulations, we propose a novel model of heart looping. Buckling, when the cardiac tube grows between fixed poles, is modulated by the progressive breakdown of the dorsal mesocardium. We have identified sequential left-right asymmetries at the poles, which bias the buckling in opposite directions, thus leading to a helical shape. Our predictive model is useful to explore the parameter space generating shape variations. The role of the dorsal mesocardium was validated in Shh^-/- mutants, which recapitulate heart shape changes expected from a persistent dorsal mesocardium. Our computer and quantitative tools provide novel insight into the mechanism of heart looping and the contribution of different factors, beyond the simple description of looping direction. This is relevant to congenital heart defects.

Heart morphogenesis gene regulatory networks revealed by temporal expression analysis

During embryogenesis the heart forms as a linear tube that then undergoes multiple simultaneous morphogenetic events to obtain its mature shape. To understand the gene regulatory networks (GRNs) driving this phase of heart development, during which many congenital heart disease malformations likely arise, we conducted an RNA-seq timecourse in zebrafish from 30 hpf to 72 hpf and identified 5861 genes with altered expression. We clustered the genes by temporal expression pattern, identified transcription factor binding motifs enriched in each cluster, and generated a model GRN for the major gene batteries in heart morphogenesis. This approach predicted hundreds of regulatory interactions and found batteries enriched in specific cell and tissue types, indicating that the approach can be used to narrow the search for novel genetic markers and regulatory interactions. Subsequent analyses confirmed the GRN using two mutants, Tbx5 and nkx2-5, and identified sets of duplicated zebrafish genes that do not show temporal subfunctionalization. This dataset provides an essential resource for future studies on the genetic/epigenetic pathways implicated in congenital heart defects and the mechanisms of cardiac transcriptional regulation.

TBX2 over-expression promotes nasopharyngeal cancer cell proliferation and invasion

TBX2 is a member of the T box transcription factor family. Its expression and potential biological functions in nasopharyngeal cancer (NPC) cells are studied here. We showed that TBX2 mRNA and protein expression was significantly elevated in multiple human NPC tissues, as compared with that in adjacent normal tissues. Knockdown of TBX2 by targeted-siRNA significantly inhibited proliferation and invasion of NPC cells (CNE-1 and HONE-1 lines). Meanwhile, TBX2 knockdown also induced G1-phase cell cycle arrest. At the molecular level, we discovered that expressions of several tumor suppressor genes, including p21, p27, phosphatase with tensin homology (PTEN) and E-Cadherin, were increased dramatically after TBX2 knockdown in above NPC cells. Collectively, our results imply that TBX2 over-expression promotes NPC cell proliferation and invasion, possibly via silencing several key tumor suppressor genes.

Novel adverse outcome pathways revealed by chemical genetics in a developing marine fish

Crude oil spills are a worldwide ocean conservation threat. Fish are particularly vulnerable to the oiling of spawning habitats, and crude oil causes severe abnormalities in embryos and larvae. However, the underlying mechanisms for these developmental defects are not well understood. Here, we explore the transcriptional basis for four discrete crude oil injury phenotypes in the early life stages of the commercially important Atlantic haddock (Melanogrammus aeglefinus). These include defects in (1) cardiac form and function, (2) craniofacial development, (3) ionoregulation and fluid balance, and (4) cholesterol synthesis and homeostasis. Our findings suggest a key role for intracellular calcium cycling and excitation-transcription coupling in the dysregulation of heart and jaw morphogenesis. Moreover, the disruption of ionoregulatory pathways sheds new light on buoyancy control in marine fish embryos. Overall, our chemical-genetic approach identifies initiating events for distinct adverse outcome pathways and novel roles for individual genes in fundamental developmental processes.

The tbx2a/b transcription factors direct pronephros segmentation and corpuscle of Stannius formation in zebrafish

The simplified and genetically conserved zebrafish pronephros is an excellent model to examine the cryptic processes of cell fate decisions during the development of nephron segments as well as the origins of associated endocrine cells that comprise the corpuscles of Stannius (CS). Using whole mount in situ hybridization, we found that transcripts of the zebrafish genes t-box 2a (tbx2a) and t-box 2b (tbx2b), which belong to the T-box family of transcription factors, were expressed in the caudal intermediate mesoderm progenitors that give rise to the distal pronephros and CS. Deficiency of tbx2a, tbx2b or both tbx2a/b reduced the size of the distal late (DL) segment, which was accompanied by a proximal convoluted segment (PCT) expansion. Further, tbx2a/b deficiency led to significantly larger CS clusters. These phenotypes were also observed in embryos with the from beyond (fby)c144 mutation, which encodes a premature stop codon in the tbx2b T-box sequence. Conversely, overexpression of tbx2a and tbx2b in wild-type embryos expanded the DL segment where cells were comingled with the adjacent DE, and also decreased CS cell number, but notably did not alter PCT development-providing independent evidence that tbx2a and tbx2b are each necessary and sufficient to promote DL fate and suppress CS genesis. Epistasis studies indicated that tbx2a acts upstream of tbx2b to regulate the DL and CS fates, and likely has other targets as well. Retinoic acid (RA) addition and inhibition studies revealed that tbx2a and tbx2b are negatively regulated by RA signaling. Interestingly, the CS cell expansion that typifies tbx2a/b deficiency also occurred when blocking Notch signaling with the chemical DAPT (N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester). Ectopic activation of Notch in Tg(hsp70::Gal4; UAS::NICD)(NICD) embryos led to a reduced CS post heat-shock induction. To further examine the link between the tbx2a/b genes and Notch during CS formation, DAPT treatment was used to block Notch activity in tbx2a/b deficient embryos, and tbx2a/b knockdown was performed in NICD transgenic embryos. Both manipulations caused similar CS expansions, indicating that Notch functions upstream of the tbx2a/b genes to suppress CS ontogeny. Taken together, these data reveal for the first time that tbx2a/b mitigate pronephros segmentation downstream of RA, and that interplay between Notch signaling and tbx2a/b regulate CS formation, thus providing several novel insights into the genetic regulatory networks that influence these lineages.

Transcriptomics of two evolutionary novelties: how to make a sperm-transfer organ out of an anal fin and a sexually selected "sword" out of a caudal fin

Swords are exaggerated male ornaments of swordtail fishes that have been of great interest to evolutionary biologists ever since Darwin described them in the Descent of Man (1871). They are a novel sexually selected trait derived from modified ventral caudal fin rays and are only found in the genus Xiphophorus. Another phylogenetically more widespread and older male trait is the gonopodium, an intromittent organ found in all poeciliid fishes, that is derived from a modified anal fin. Despite many evolutionary and behavioral studies on both traits, little is known so far about the molecular mechanisms underlying their development. By investigating transcriptomic changes (utilizing a RNA-Seq approach) in response to testosterone treatment in the swordtail fish, Xiphophorus hellerii, we aimed to better understand the architecture of the gene regulatory networks underpinning the development of these two evolutionary novelties. Large numbers of genes with tissue-specific expression patterns were identified. Among the "sword genes" those involved in embryonic organ development, sexual character development and coloration were highly expressed, while in the gonopodium rather more morphogenesis-related genes were found. Interestingly, many genes and genetic pathways are shared between both developing novel traits derived from median fins: the sword and the gonopodium. Our analyses show that a larger set of gene networks was co-opted during the development and evolution of the "older" gonopodium than in the "younger," and morphologically less complex trait, the sword. We provide a catalog of candidate genes for future efforts to dissect the development of those sexually selected exaggerated male traits in swordtails.

Zebrafish cul4a, but not cul4b, modulates cardiac and forelimb development by upregulating tbx5a expression

CUL4A and CUL4B are closely related cullin family members and can each assemble a Cullin-RING E3 ligase complex (CRL) and participate in a variety of biological processes. While the CRLs formed by the two cullin members may have common targets, the two appeared to have very different consequences when mutated or disrupted in mammals. We here investigated the roles of cul4a and cul4b during zebrafish embryogenesis by using the morpholino knockdown approach. We found that cul4a is essential for cardiac development as well as for pectoral fin development. Whereas cul4a morphants appeared to be unperturbed in chamber specification, they failed to undergo heart looping. The failures in heart looping and pectoral fin formation in cul4a morphants were accompanied by greatly reduced proliferation of cardiac cells and pectoral fin-forming cells. We demonstrated that tbx5a, a transcription factor essential for heart and limb development, is transcriptionally upregulated by cul4a and mediates the function of cul4a in cardiac and pectoral fin development. In contrast to the critical importance of cul4a, cul4b appeared to be dispensable for zebrafish development and was incapable of compensating for the loss of cul4a. This work provides the first demonstration of an essential role of cul4a, but not cul4b, in cardiac development and in the regulation of tbx5a in zebrafish. These findings justify exploring the functional role of CUL4A in human cardiac development.

T-box transcription factors in cancer biology

The evolutionarily conserved T-box family of transcription factors have critical and well-established roles in embryonic development. More recently, T-box factors have also gained increasing prominence in the field of cancer biology where a wide range of cancers exhibit deregulated expression of T-box factors that possess tumour suppressor and/or tumour promoter functions. Of these the best characterised is TBX2, whose expression is upregulated in cancers including breast, pancreatic, ovarian, liver, endometrial adenocarcinoma, glioblastomas, gastric, uterine cervical and melanoma. Understanding the role and regulation of TBX2, as well as other T-box factors, in contributing directly to tumour progression, and especially in suppression of senescence and control of invasiveness suggests that targeting TBX2 expression or function alone or in combination with currently available chemotherapeutic agents may represent a therapeutic strategy for cancer.

Cardiac looping may be driven by compressive loads resulting from unequal growth of the heart and pericardial cavity. Observations on a physical simulation model

The transformation of the straight embryonic heart tube into a helically wound loop is named cardiac looping. Such looping is regarded as an essential process in cardiac morphogenesis since it brings the building blocks of the developing heart into an approximation of their definitive topographical relationships. During the past two decades, a large number of genes have been identified which play important roles in cardiac looping. However, how genetic information is physically translated into the dynamic form changes of the looping heart is still poorly understood. The oldest hypothesis of cardiac looping mechanics attributes the form changes of the heart loop (ventral bending → simple helical coiling → complex helical coiling) to compressive loads resulting from growth differences between the heart and the pericardial cavity. In the present study, we have tested the physical plausibility of this hypothesis, which we call the growth-induced buckling hypothesis, for the first time. Using a physical simulation model, we show that growth-induced buckling of a straight elastic rod within the confined space of a hemispherical cavity can generate the same sequence of form changes as observed in the looping embryonic heart. Our simulation experiments have furthermore shown that, under bilaterally symmetric conditions, growth-induced buckling generates left- and right-handed helices (D-/L-loops) in a 1:1 ratio, while even subtle left- or rightward displacements of the caudal end of the elastic rod at the pre-buckling state are sufficient to direct the buckling process toward the generation of only D- or L-loops, respectively. Our data are discussed with respect to observations made in biological “models.” We conclude that compressive loads resulting from unequal growth of the heart and pericardial cavity play important roles in cardiac looping. Asymmetric positioning of the venous heart pole may direct these forces toward a biased generation of D- or L-loops.

AcvR1-mediated BMP signaling in second heart field is required for arterial pole development: implications for myocardial differentiation and regional identity

BMP signaling plays an essential role in second heart field-derived heart and arterial trunk development, including myocardial differentiation, right ventricular growth, and interventricular, outflow tract and aortico-pulmonary septation. It is mediated by a number of different BMP ligands, and receptors, many of which are present simultaneously. The mechanisms by which they regulate morphogenetic events and degree of redundancy amongst them have still to be elucidated. We therefore assessed the role of BMP Type I receptor AcvR1 in anterior second heart field-derived cell development, and compared it with that of BmpR1a. By removing Acvr1 using the driver Mef2c[AHF]-Cre, we show that AcvR1 plays an essential role in arterial pole morphogenesis, identifying defects in outflow tract wall and cushion morphology that preceded a spectrum of septation defects from double outlet right ventricle to common arterial trunk in mutants. Its absence caused dysregulation in gene expression important for myocardial differentiation (Isl1, Fgf8) and regional identity (Tbx2, Tbx3, Tbx20, Tgfb2). Although these defects resemble to some degree those in the equivalent Bmpr1a mutant, a novel gene knock-in model in which Bmpr1a was expressed in the Acvr1 locus only partially restored septation in Acvr1 mutants. These data show that both BmpR1a and AcvR1 are needed for normal heart development, in which they play some non-redundant roles, and refine our understanding of the genetic and morphogenetic processes underlying Bmp-mediated heart development important in human congenital heart disease.

The atypical Rho GTPase, RhoU, regulates cell-adhesion molecules during cardiac morphogenesis

The vertebrate heart undergoes early complex morphologic events in order to develop key cardiac structures that regulate its overall function (Fahed et al., 2013). Although many genetic factors that participate in patterning the heart have been elucidated (Tu and Chi, 2012), the cellular events that drive cardiac morphogenesis have been less clear. From a chemical genetic screen to identify cellular pathways that control cardiac morphogenesis in zebrafish, we observed that inhibition of the Rho signaling pathways resulted in failure to form the atrioventricular canal and loop the linear heart tube. To identify specific Rho proteins that may regulate this process, we analyzed cardiac expression profiling data and discovered that RhoU was expressed at the atrioventricular canal during the time when it forms. Loss of RhoU function recapitulated the atrioventricular canal and cardiac looping defects observed in the ROCK inhibitor treated zebrafish. Similar to its family member RhoV/Chp (Tay et al., 2010), we discovered that RhoU regulates the cell junctions between cardiomyocytes through the Arhgef7b/Pak kinase pathway in order to guide atrioventricular canal development and cardiac looping. Inhibition of this pathway resulted in similar underlying cardiac defects and conversely, overexpression of a PAK kinase was able to rescue the loss of RhoU cardiac defect. Finally, we found that Wnt signaling, which has been implicated in atrioventricular canal development (Verhoeven et al., 2011), may regulate the expression of RhoU at the atrioventricular canal. Overall, these findings reveal a cardiac developmental pathway involving RhoU/Arhgef7b/Pak signaling, which helps coordinate cell junction formation between atrioventricular cardiomyocytes to promote cell adhesiveness and cell shapes during cardiac morphogenesis. Failure to properly form these cell adhesions during cardiac development may lead to structural heart defects and mechanistically account for the cellular events that occur in certain human congenital heart diseases.

Tbx2a is required for specification of endodermal pouches during development of the pharyngeal arches

Tbx2 is a member of the T-box family of transcription factors essential for embryo- and organogenesis. A deficiency in the zebrafish paralogue tbx2a causes abnormalities of the pharyngeal arches in a p53-independent manner. The pharyngeal arches are formed by derivatives of all three embryonic germ layers: endodermal pouches, mesenchymal condensations and neural crest cells. While tbx2a expression is restricted to the endodermal pouches, its function is required for the normal morphogenesis of the entire pharyngeal arches. Given the similar function of Tbx1 in craniofacial development, we explored the possibility of an interaction between Tbx1 and Tbx2a. The use of bimolecular fluorescence complementation revealed the interaction between Tbx2a and Tbx1, thus providing support for the idea that functional interaction between different, co-expressed Tbx proteins could be a common theme across developmental processes in cell lineages and tissues. Together, this work provides mechanistic insight into the role of TBX2 in human disorders affecting the face and neck.

MicroRNA-17-92, a direct Ap-2α transcriptional target, modulates T-box factor activity in orofacial clefting

Among the most common human congenital anomalies, cleft lip and palate (CL/P) affects up to 1 in 700 live births. MicroRNA (miR)s are small, non-coding RNAs that repress gene expression post-transcriptionally. The miR-17-92 cluster encodes six miRs that have been implicated in human cancers and heart development. We discovered that miR-17-92 mutant embryos had severe craniofacial phenotypes, including incompletely penetrant CL/P and mandibular hypoplasia. Embryos that were compound mutant for miR-17-92 and the related miR-106b-25 cluster had completely penetrant CL/P. Expression of Tbx1 and Tbx3, the DiGeorge/velo-cardio-facial (DGS) and Ulnar-mammary syndrome (UMS) disease genes, was expanded in miR-17-92 mutant craniofacial structures. Both Tbx1 and Tbx3 had functional miR seed sequences that mediated gene repression. Analysis of miR-17-92 regulatory regions uncovered conserved and functional AP-2α recognition elements that directed miR-17-92 expression. Together, our data indicate that miR-17-92 modulates expression of critical T-box transcriptional regulators during midface development and is itself a target of Bmp-signaling and the craniofacial pioneer factor AP-2α. Our data are the first genetic evidence that an individual miR or miR cluster is functionally important in mammalian CL/P.

Cardiotoxicity of mycotoxin citrinin and involvement of microRNA-138 in zebrafish embryos

Citrinin (CTN) is a fungal secondary metabolite that contaminates various foodstuffs and animal feeds; it also exhibits organotoxicity in several animal models. In this study, the zebrafish was used to elucidate the mechanism of CTN cardiotoxicity in developing embryos. Following CTN administration, the gross morphology of the embryonic heart was apparently altered, including heart malformation, pericardial edema, and red blood accumulation. Whole-mount immunostaining and histological analysis of ventricle and atrium indicated incorrect heart looping and reduced size of heart chambers. From the perspective of cardiac function, the heartbeat and blood flow rate of embryos were significantly decreased in the presence of CTN. CTN also modulated the expression of tbx2a and jun B genes, but not that of bmp4 and nkx2.5. Furthermore, the heart areas of CTN-exposed embryos demonstrated an elevated levels of aldh1a2 and cspg2 messenger RNA; these 2 cardiac-related genes are known to be involved in retinoic acid (RA) pathway as well as downstream targets of microRNA-138 (miR-138) in zebrafish. CTN treatment also downregulated the expression of miR-138. Moreover, overexpression of miR-138 was able to rescue the heart defects generated by CTN. These results support the notion that CTN exposure has a severe impact on heart development, affecting heart morphogenesis through the dysregulation of miR-138, RA signaling, and tbx2a.

Dpp signaling inhibits proliferation in the Drosophila wing by Omb-dependent regional control of bantam

The control of organ growth is a fundamental aspect of animal development but remains poorly understood. The morphogen Dpp has long been considered as a general promoter of cell proliferation during Drosophila wing development. It is an ongoing debate whether the Dpp gradient is required for the uniform cell proliferation observed in the wing imaginal disc. Here, we investigated how the Dpp signaling pathway regulates proliferation during wing development. By systematic manipulation of Dpp signaling we observed that it controls proliferation in a region-specific manner: Dpp, via omb, promoted proliferation in the lateral and repressed proliferation in the medial wing disc. Omb controlled the regional proliferation rate by oppositely regulating transcription of the microRNA gene bantam in medial versus lateral wing disc. However, neither the Dpp nor Omb gradient was essential for uniform proliferation along the anteroposterior axis.

Lack of genetic interaction between Tbx20 and Tbx3 in early mouse heart development

Members of the T-box family of transcription factors are important regulators orchestrating the complex regionalization of the developing mammalian heart. Individual mutations in Tbx20 and Tbx3 cause distinct congenital heart abnormalities in the mouse: Tbx20 mutations result in failure of heart looping, developmental arrest and lack of chamber differentiation, while hearts of Tbx3 mutants progress further, loop normally but show atrioventricular convergence and outflow tract defects. The two genes have overlapping areas of expression in the atrioventricular canal and outflow tract of the heart but their potential genetic interaction has not been previously investigated. In this study we produced compound mutants to investigate potential genetic interactions at the earliest stages of heart development. We find that Tbx20; Tbx3 double heterozygous mice are viable and fertile with no apparent abnormalities, while double homozygous mutants are embryonic lethal by midgestation. Double homozygous mutant embryos display abnormal cardiac morphogenesis, lack of heart looping, expression patterns of cardiac genes and time of death that are indistinguishable from Tbx20 homozygous mutants. Prior to death, the double homozygotes show an overall developmental delay similar to Tbx3 homozygous mutants. Thus the effects of Tbx20 are epistatic to Tbx3 in the heart but Tbx3 is epistatic to Tbx20 with respect to developmental delay.

Mouse TBX3 mutants suggest novel molecular mechanisms for Ulnar-mammary syndrome

The transcription factor TBX3 plays critical roles in development and TBX3 mutations in humans cause Ulnar-mammary syndrome. Efforts to understand how altered TBX3 dosage and function disrupt the development of numerous structures have been hampered by embryonic lethality of mice bearing presumed null alleles. We generated a novel conditional null allele of Tbx3: after Cre-mediated recombination, no mRNA or protein is detectable. In contrast, a putative null allele in which exons 1-3 are deleted produces a truncated protein that is abnormally located in the cytoplasm. Heterozygotes and homozygotes for this allele have different phenotypes than their counterparts bearing a true null allele. Our observations with these alleles in mice, and the different types of TBX3 mutations observed in human ulnar-mammary syndrome, suggest that not all mutations observed in humans generate functionally null alleles. The possibility that mechanisms in addition to TBX3 haploinsufficiency may cause UMS or other malformations merits investigation in the human UMS population.

Proliferation of embryonic cardiomyocytes in zebrafish requires the sodium channel scn5Lab

In mice, homozygous deletion of the cardiac sodium channel Scn5a results in defects in cardiac morphology and embryonic death before robust sodium current can be detected. In zebrafish, morpholino knockdown of cardiac sodium channel orthologs scn5Laa and scn5Lab perturbs specification of precardiac mesoderm and inhibits growth of the embryonic heart. It is not known which developmental processes are perturbed by sodium channel knockdown and whether reduced cell number is from impaired migration of cardiac progenitors into the heart, impaired myocyte proliferation, or both. We found that embryos deficient in scn5Lab displayed defects in primary cardiogenesis specific to loss of nkx2.5, but not nkx2.7. We generated kaede reporter fish and demonstrated that embryos treated with anti-scn5Lab morpholino showed normal secondary differentiation of cardiomyocytes at the arterial pole between 30 and 48 h post-fertilization. However, while proliferating myocytes were readily detected at 48 hpf in wild type embryos, there were no BrdU-positive cardiomyocytes in embryos subjected to anti-scn5Lab treatment. Proliferating myocytes were present in embryos injected with anti-tnnt2 morpholino to phenocopy the silent heart mutation, and absent in embryos injected with anti-tnnt2 and anti-scn5Lab morpholinos, indicating cardiac contraction is not required for the loss of proliferation. These data demonstrate that the role of scn5Lab in later heart growth does not involve contribution of the secondary heart field, but rather proliferation of cardiomyocytes, and appears unrelated to the role of the channel in cardiac electrogenesis.

DiGeorge syndrome gene tbx1 functions through wnt11r to regulate heart looping and differentiation

DiGeorge syndrome (DGS) is the most common microdeletion syndrome, and is characterized by congenital cardiac, craniofacial and immune system abnormalities. The cardiac defects in DGS patients include conotruncal and ventricular septal defects. Although the etiology of DGS is critically regulated by TBX1 gene, the molecular pathways underpinning TBX1's role in heart development are not fully understood. In this study, we characterized heart defects and downstream signaling in the zebrafish tbx1^−/− mutant, which has craniofacial and immune defects similar to DGS patients. We show that tbx1^−/− mutants have defective heart looping, morphology and function. Defective heart looping is accompanied by failure of cardiomyocytes to differentiate normally and failure to change shape from isotropic to anisotropic morphology in the outer curvatures of the heart. This is the first demonstration of tbx1's role in regulating heart looping, cardiomyocyte shape and differentiation, and may explain how Tbx1 regulates conotruncal development in humans. Next we elucidated tbx1's molecular signaling pathway guided by the cardiac phenotype of tbx1^−/− mutants. We show for the first time that wnt11r (wnt11 related), a member of the non-canonical Wnt pathway, and its downstream effector gene alcama (activated leukocyte cell adhesion molecule a) regulate heart looping and differentiation similarly to tbx1. Expression of both wnt11r and alcama are downregulated in tbx1^−/− mutants. In addition, both wnt11r^−/− mutants and alcama morphants have heart looping and differentiation defects similar to tbx1^−/− mutants. Strikingly, heart looping and differentiation in tbx1^−/− mutants can be partially rescued by ectopic expression of wnt11r or alcama, supporting a model whereby heart looping and differentiation are regulated by tbx1 in a linear pathway through wnt11r and alcama. This is the first study linking tbx1 and non-canonical Wnt signaling and extends our understanding of DGS and heart development.

On the form problem of embryonic heart loops, its geometrical solutions, and a new biophysical concept of cardiac looping

BACKGROUND

Cardiac looping is an essential process in the morphogenesis of embryonic hearts. Unfortunately, relatively little is known about the form and biophysics of embryonic heart loops. Thompson regarded the form of an object as "a 'diagram of forces' … from it we can … deduce the forces that are acting or have acted upon it." Therefore, the present study was conducted to uncover the best geometrical solution of the form problem of embryonic heart loops. This approach may help to identify the biophysics of cardiac looping.

RESULTS

Analysis of the tendrils of climbing plants disclosed striking resemblance between the configurations of embryonic heart loops and a form motif named helical perversion. Helical perversion occurs in helically wound objects where they connect two helical segments of opposite handedness (two-handed helix). Helical perversion evolves in living and non-living filamentary objects such as the tendrils of climbing plants and helical telephone cords.

CONCLUSIONS

Helical perversion may be the best geometrical solution of the form problem of embryonic heart loops. The dynamics and mechanics of the emergence of helical perversions are relatively well known. The behavior of looping embryonic hearts may be interpreted in light of this knowledge.

PRSS23 is essential for the Snail-dependent endothelial-to-mesenchymal transition during valvulogenesis in zebrafish

AIMS

Cardiac valve disease is a common cause of congenital heart failure. Cardiac valve development requires a complex regulation of assorted protease activities. Nevertheless, the regulation of these proteases during atrioventricular (AV) valve formation is poorly understood. Previously, PRSS23, a novel vascular protease, is shown to be highly expressed at the AV canal during murine heart development; however, its function remains unknown. In this study, we sought to characterize the functional role of PRSS23 during cardiac valve formation.

METHODS AND RESULTS

We used a transgenic zebrafish line with fluorescently labelled vasculature as a tool to study the function of PRSS23. We first cloned the zebrafish prss23 and confirmed its sequence conservation with other vertebrate orthologues. Expression of prss23 was detected in the ventricle, atrium, and AV canal during zebrafish embryonic development. We found that morpholino knockdown of Prss23 inhibited the endothelial-to-mesenchymal transition (EndoMT) at the AV canal. Moreover, in human aortic endothelial cell-based assays, PRSS23 knockdown by short-hairpin RNA not only repressed the transforming growth factor-β-induced EndoMT, but also reduced Snail transcription, suggesting that Snail signalling is downstream of PRSS23 during EndoMT. We further demonstrated that human PRSS23 and SNAIL could rescue the prss23 morpholino-induced AV canal defect in zebrafish embryos, indicating that the function of PRSS23 in valvulogenesis is evolutionarily conserved.

CONCLUSION

We demonstrated for the first time that the initiation of EndoMT in valvulogenesis depends on PRSS23-Snail signalling and that the functional role of PRSS23 during AV valve formation is evolutionarily conserved.

T-box factors: insights into the evolutionary emergence of the complex heart

The heart as a functional organ first appeared in bilaterians as a single peristaltic pump and evolved through arthropods, fish, amphibians, and finally mammals into a four-chambered engine controlling blood-flow within the body. The acquisition of cardiac complexity in the evolving heart was a product of gene duplication events and the co-option of novel signaling pathways to an ancestral cardiac-specific gene network. T-box factors belong to an evolutionary conserved family of transcriptional regulators with diverse roles in development. Their regulatory functions are integral in the initiation and potentiation of heart development, and mutations in these genes are associated with congenital heart defects. In this review we will discuss the evolutionary conserved cardiac regulatory functions of this family as well as their implication in disease in an aim to facilitate future gene-targeted and regenerative therapeutic remedies.

The T box transcription factor TBX2 promotes epithelial-mesenchymal transition and invasion of normal and malignant breast epithelial cells

The T box transcription factor TBX2, a master regulator of organogenesis, is aberrantly amplified in aggressive human epithelial cancers. While it has been shown that overexpression of TBX2 can bypass senescence, a failsafe mechanism against cancer, its potential role in tumor invasion has remained obscure. Here we demonstrate that TBX2 is a strong cell-autonomous inducer of the epithelial-mesenchymal transition (EMT), a latent morphogenetic program that is key to tumor progression from noninvasive to invasive malignant states. Ectopic expression of TBX2 in normal HC11 and MCF10A mammary epithelial cells was sufficient to induce morphological, molecular, and behavioral changes characteristic of EMT. These changes included loss of epithelial adhesion and polarity gene (E-cadherin, ß-catenin, ZO1) expression, and abnormal gain of mesenchymal markers (N-cadherin, Vimentin), as well as increased cell motility and invasion. Conversely, abrogation of endogenous TBX2 overexpression in the malignant human breast carcinoma cell lines MDA-MB-435 and MDA-MB-157 led to a restitution of epithelial characteristics with reciprocal loss of mesenchymal markers. Importantly, TBX2 inhibition abolished tumor cell invasion and the capacity to form lung metastases in a Xenograft mouse model. Meta-analysis of gene expression in over one thousand primary human breast tumors further showed that high TBX2 expression was significantly associated with reduced metastasis-free survival in patients, and with tumor subtypes enriched in EMT gene signatures, consistent with a role of TBX2 in oncogenic EMT. ChIP analysis and cell-based reporter assays further revealed that TBX2 directly represses transcription of E-cadherin, a tumor suppressor gene, whose loss is crucial for malignant tumor progression. Collectively, our results uncover an unanticipated link between TBX2 deregulation in cancer and the acquisition of EMT and invasive features of epithelial tumor cells.

Zebrafish models in cardiac development and congenital heart birth defects

The zebrafish has become an ideal vertebrate animal system for investigating cardiac development due to its genetic tractability, external fertilization, early optical clarity and ability to survive without a functional cardiovascular system during development. In particular, recent advances in imaging techniques and the creation of zebrafish transgenics now permit the in vivo analysis of the dynamic cellular events that transpire during cardiac morphogenesis. As a result, the combination of these salient features provides detailed insight as to how specific genes may influence cardiac development at the cellular level. In this review, we will highlight how the zebrafish has been utilized to elucidate not only the underlying mechanisms of cardiac development and human congenital heart diseases (CHDs), but also potential pathways that may modulate cardiac regeneration. Thus, we have organized this review based on the major categories of CHDs-structural heart, functional heart, and vascular/great vessel defects, and will conclude with how the zebrafish may be further used to contribute to our understanding of specific human CHDs in the future.

Epigenetic regulation of cardiac development and function by polycomb group and trithorax group proteins

Heart disease is a leading cause of death and disability in developed countries. Heart disease includes a broad range of diseases that affect the development and/or function of the cardiovascular system. Some of these diseases, such as congenital heart defects, are present at birth. Others develop over time and may be influenced by both genetic and environmental factors. Many of the known heart diseases are associated with abnormal expression of genes. Understanding the factors and mechanisms that regulate gene expression in the heart is essential for the detection, treatment, and prevention of heart diseases. Polycomb Group (PcG) and Trithorax Group (TrxG) proteins are special families of chromatin factors that regulate developmental gene expression in many tissues and organs. Accumulating evidence suggests that these proteins are important regulators of development and function of the heart as well. A better understanding of their roles and functional mechanisms will translate into new opportunities for combating heart disease.

The ulnar-mammary syndrome gene, Tbx3, is a direct target of the retinoic acid signaling pathway, which regulates its expression during mouse limb development

TBX3, a member of the T-box transcription factor gene family, is a transcriptional repressor that is required for the development of the heart, limbs, and mammary glands. Mutations in TBX3 that result in reduced functional protein lead to ulnar-mammary syndrome, a developmental disorder characterized by limb, mammary gland, tooth, and genital abnormalities. Increased levels of TBX3 have been shown to contribute to the oncogenic process, and TBX3 is overexpressed in several cancers, including breast cancer, liver cancer, and melanoma. Despite its important role in development and postnatal life, little is known about the signaling pathways that modulate TBX3 expression. Here we show, using in vitro and in vivo assays, that retinoic acid (RA) activates endogenous TBX3 expression, which is mediated by an RA-receptor complex directly binding and activating the TBX3 promoter, and we provide evidence that this regulation may be functionally relevant in mouse embryonic limb development. Our data identify TBX3 as a direct target of the RA signaling pathway and extend our understanding of the role and regulation of TBX3 in limb development.

Multiple influences of blood flow on cardiomyocyte hypertrophy in the embryonic zebrafish heart

Cardiomyocyte hypertrophy is a complex cellular behavior involving coordination of cell size expansion and myofibril content increase. Here, we investigate the contribution of cardiomyocyte hypertrophy to cardiac chamber emergence, the process during which the primitive heart tube transforms into morphologically distinct chambers and increases its contractile strength. Focusing on the emergence of the zebrafish ventricle, we observed trends toward increased cell surface area and myofibril content. To examine the extent to which these trends reflect coordinated hypertrophy of individual ventricular cardiomyocytes, we developed a method for tracking cell surface area changes and myofibril dynamics in live embryos. Our data reveal a previously unappreciated heterogeneity of ventricular cardiomyocyte behavior during chamber emergence: although cardiomyocyte hypertrophy was prevalent, many cells did not increase their surface area or myofibril content during the observed timeframe. Despite the heterogeneity of cell behavior, we often found hypertrophic cells neighboring each other. Next, we examined the impact of blood flow on the regulation of cardiomyocyte behavior during this phase of development. When blood flow through the ventricle was reduced, cell surface area expansion and myofibril content increase were both dampened, and the behavior of neighboring cells did not seem coordinated. Together, our studies suggest a model in which hemodynamic forces have multiple influences on cardiac chamber emergence: promoting both cardiomyocyte enlargement and myofibril maturation, enhancing the extent of cardiomyocyte hypertrophy, and facilitating the coordination of neighboring cell behaviors.