Tbx5 is essential for heart development

In Vitro Models of Cardiovascular Disease: Embryoid Bodies, Organoids and Everything in Between

Cardiovascular disease comprises a group of disorders affecting or originating within tissues and organs of the cardiovascular system; most, if not all, will eventually result in cardiomyocyte dysfunction or death, negatively impacting cardiac function. Effective models of cardiac disease are thus important for understanding crucial aspects of disease progression, while recent advancements in stem cell biology have allowed for the use of stem cell populations to derive such models. These include three-dimensional (3D) models such as stem cell-based models of embryos (SCME) as well as organoids, many of which are frequently derived from embryoid bodies (EB). Not only can they recapitulate 3D form and function, but the developmental programs governing the self-organization of cell populations into more complex tissues as well. Many different organoids and SCME constructs have been generated in recent years to recreate cardiac tissue and the complex developmental programs that give rise to its cellular composition and unique tissue morphology. It is thus the purpose of this narrative literature review to describe and summarize many of the recently derived cardiac organoid models as well as their use for the recapitulation of genetic and acquired disease. Owing to the cellular composition of the models examined, this review will focus on disease and tissue injury associated with embryonic/fetal tissues.

Generation of heart and vascular system in rodents by blastocyst complementation

Generating organs from stem cells through blastocyst complementation is a promising approach to meet the clinical need for transplants. In order to generate rejection-free organs, complementation of both parenchymal and vascular cells must be achieved, as endothelial cells play a key role in graft rejection. Here, we used a lineage-specific cell ablation system to produce mouse embryos unable to form both the cardiac and vascular systems. By mouse intraspecies blastocyst complementation, we rescued heart and vascular system development separately and in combination, obtaining complemented hearts with cardiomyocytes and endothelial cells of exogenous origin. Complemented chimeras were viable and reached adult stage, showing normal cardiac function and no signs of histopathological defects in the heart. Furthermore, we implemented the cell ablation system for rat-to-mouse blastocyst complementation, obtaining xenogeneic hearts whose cardiomyocytes were completely of rat origin. These results represent an advance in the experimentation towards the in vivo generation of transplantable organs.

Prioritizing cardiovascular disease-associated variants altering NKX2-5 and TBX5 binding through an integrative computational approach

Cardiovascular diseases (CVDs) are the leading cause of death worldwide and are heavily influenced by genetic factors. Genome-wide association studies have mapped >90% of CVD-associated variants within the noncoding genome, which can alter the function of regulatory proteins, such as transcription factors (TFs). However, due to the overwhelming number of single-nucleotide polymorphisms (SNPs) (>500,000) in genome-wide association studies, prioritizing variants for in vitro analysis remains challenging. In this work, we implemented a computational approach that considers support vector machine (SVM)-based TF binding site classification and cardiac expression quantitative trait loci (eQTL) analysis to identify and prioritize potential CVD-causing SNPs. We identified 1535 CVD-associated SNPs within TF footprints and putative cardiac enhancers plus 14,218 variants in linkage disequilibrium with genotype-dependent gene expression in cardiac tissues. Using ChIP-seq data from two cardiac TFs (NKX2-5 and TBX5) in human-induced pluripotent stem cell-derived cardiomyocytes, we trained a large-scale gapped k-mer SVM model to identify CVD-associated SNPs that altered NKX2-5 and TBX5 binding. The model was tested by scoring human heart TF genomic footprints within putative enhancers and measuring in vitro binding through electrophoretic mobility shift assay. Five variants predicted to alter NKX2-5 (rs59310144, rs6715570, and rs61872084) and TBX5 (rs7612445 and rs7790964) binding were prioritized for in vitro validation based on the magnitude of the predicted change in binding and are in cardiac tissue eQTLs. All five variants altered NKX2-5 and TBX5 DNA binding. We present a bioinformatic approach that considers tissue-specific eQTL analysis and SVM-based TF binding site classification to prioritize CVD-associated variants for in vitro analysis.

Statistical learning quantifies transposable element-mediated cis-regulation

BACKGROUND

Transposable elements (TEs) have colonized the genomes of most metazoans, and many TE-embedded sequences function as cis-regulatory elements (CREs) for genes involved in a wide range of biological processes from early embryogenesis to innate immune responses. Because of their repetitive nature, TEs have the potential to form CRE platforms enabling the coordinated and genome-wide regulation of protein-coding genes by only a handful of trans-acting transcription factors (TFs).

RESULTS

Here, we directly test this hypothesis through mathematical modeling and demonstrate that differences in expression at protein-coding genes alone are sufficient to estimate the magnitude and significance of TE-contributed cis-regulatory activities, even in contexts where TE-derived transcription fails to do so. We leverage hundreds of overexpression experiments and estimate that, overall, gene expression is influenced by TE-embedded CREs situated within approximately 500 kb of promoters. Focusing on the cis-regulatory potential of TEs within the gene regulatory network of human embryonic stem cells, we find that pluripotency-specific and evolutionarily young TE subfamilies can be reactivated by TFs involved in post-implantation embryogenesis. Finally, we show that TE subfamilies can be split into truly regulatorily active versus inactive fractions based on additional information such as matched epigenomic data, observing that TF binding may better predict TE cis-regulatory activity than differences in histone marks.

CONCLUSION

Our results suggest that TE-embedded CREs contribute to gene regulation during and beyond gastrulation. On a methodological level, we provide a statistical tool that infers TE-dependent cis-regulation from RNA-seq data alone, thus facilitating the study of TEs in the next-generation sequencing era.

Prioritizing Cardiovascular Disease-Associated Variants Altering NKX2-5 Binding through an Integrative Computational Approach

Cardiovascular diseases (CVDs) are the leading cause of death worldwide and are heavily influenced by genetic factors. Genome-wide association studies (GWAS) have mapped > 90% of CVD-associated variants within the non-coding genome, which can alter the function of regulatory proteins, like transcription factors (TFs). However, due to the overwhelming number of GWAS single nucleotide polymorphisms (SNPs) (>500,000), prioritizing variants for in vitro analysis remains challenging. In this work, we implemented a computational approach that considers support vector machine (SVM)-based TF binding site classification and cardiac expression quantitative trait loci (eQTL) analysis to identify and prioritize potential CVD-causing SNPs. We identified 1,535 CVD-associated SNPs that occur within human heart footprints/enhancers and 9,309 variants in linkage disequilibrium (LD) with differential gene expression profiles in cardiac tissue. Using hiPSC-CM ChIP-seq data from NKX2-5 and TBX5, two cardiac TFs essential for proper heart development, we trained a large-scale gapped k-mer SVM (LS-GKM-SVM) predictive model that can identify binding sites altered by CVD-associated SNPs. The computational predictive model was tested by scoring human heart footprints and enhancers in vitro through electrophoretic mobility shift assay (EMSA). Three variants (rs59310144, rs6715570, and rs61872084) were prioritized for in vitro validation based on their eQTL in cardiac tissue and LS-GKM-SVM prediction to alter NKX2-5 DNA binding. All three variants altered NKX2-5 DNA binding. In summary, we present a bioinformatic approach that considers tissue-specific eQTL analysis and SVM-based TF binding site classification to prioritize CVD-associated variants for in vitro experimental analysis.

Use of Frogs as a Model to Study the Etiology of HLHS

A frog is a classical model organism used to uncover processes and regulations of early vertebrate development, including heart development. Recently, we showed that a frog also represents a useful model to study a rare human congenital heart disease, hypoplastic left heart syndrome. In this review, we first summarized the cellular events and molecular regulations of vertebrate heart development, and the benefit of using a frog model to study congenital heart diseases. Next, we described the challenges in elucidating the etiology of hypoplastic left heart syndrome and discussed how a frog model may contribute to our understanding of the molecular and cellular bases of the disease. We concluded that a frog model offers its unique advantage in uncovering the cellular mechanisms of hypoplastic left heart syndrome; however, combining multiple model organisms, including frogs, is needed to gain a comprehensive understanding of the disease.

Effect of hyperglycemia on tbx5a and nppa gene expression and its correlation to structural and functional changes in developing zebrafish heart

The objective of the current study is to analyze the effects of gestational diabetes on structural and functional changes in correlation with these two essential regulators of developing hearts in vivo using zebrafish embryos. We employed fertilized zebrafish embryos exposed to a hyperglycemic condition of 25 mM glucose for 96 h postfertilization. The embryos were subjected to various structural and functional analyses in a time-course manner. The data showed that exposure to high glucose significantly affected the embryo's size, heart length, heart rate, and looping of the heart compared to the control. Further, we observed an increased incidence of ventricular standstill and valvular regurgitation with a marked reduction of peripheral blood flow in the high glucose-exposed group compared to the control. In addition, the histological data showed that the high-glucose exposure markedly reduced the thickness of the wall and the number of cardiomyocytes in both atrium and ventricles. We also observed striking alterations in the pericardium like edema, increase in diameter with thinning of the wall compared to the control group. Interestingly, the expression of tbx5a and nppa was increased in the early development and found to be repressed in the later stage of development in the hyperglycemic group compared to the control. In conclusion, the developing heart is more susceptible to hyperglycemia in the womb, thereby showing various developmental defects possibly by altering the expression of crucial gene regulators such as tbx5a and nppa.

The negative regulation of gene expression by microRNAs as key driver of inducers and repressors of cardiomyocyte differentiation

Cardiac muscle damage-induced loss of cardiomyocytes (CMs) and dysfunction of the remaining ones leads to heart failure, which nowadays is the number one killer worldwide. Therapies fostering effective cardiac regeneration are the holy grail of cardiovascular research to stop the heart failure epidemic. The main goal of most myocardial regeneration protocols is the generation of new functional CMs through the differentiation of endogenous or exogenous cardiomyogenic cells. Understanding the cellular and molecular basis of cardiomyocyte commitment, specification, differentiation and maturation is needed to devise innovative approaches to replace the CMs lost after injury in the adult heart. The transcriptional regulation of CM differentiation is a highly conserved process that require sequential activation and/or repression of different genetic programs. Therefore, CM differentiation and specification have been depicted as a step-wise specific chemical and mechanical stimuli inducing complete myogenic commitment and cell-cycle exit. Yet, the demonstration that some microRNAs are sufficient to direct ESC differentiation into CMs and that four specific miRNAs reprogram fibroblasts into CMs show that CM differentiation must also involve negative regulatory instructions. Here, we review the mechanisms of CM differentiation during development and from regenerative stem cells with a focus on the involvement of microRNAs in the process, putting in perspective their negative gene regulation as a main modifier of effective CM regeneration in the adult heart.

Tbx5 drives Aldh1a2 expression to regulate a RA-Hedgehog-Wnt gene regulatory network coordinating cardiopulmonary development

The gene regulatory networks that coordinate the development of the cardiac and pulmonary systems are essential for terrestrial life but poorly understood. The T-box transcription factor Tbx5 is critical for both pulmonary specification and heart development, but how these activities are mechanistically integrated remains unclear. Here using Xenopus and mouse embryos, we establish molecular links between Tbx5 and retinoic acid (RA) signaling in the mesoderm and between RA signaling and sonic hedgehog expression in the endoderm to unveil a conserved RA-Hedgehog-Wnt signaling cascade coordinating cardiopulmonary (CP) development. We demonstrate that Tbx5 directly maintains expression of aldh1a2, the RA-synthesizing enzyme, in the foregut lateral plate mesoderm via an evolutionarily conserved intronic enhancer. Tbx5 promotes posterior second heart field identity in a positive feedback loop with RA, antagonizing a Fgf8-Cyp regulatory module to restrict FGF activity to the anterior. We find that Tbx5/Aldh1a2-dependent RA signaling directly activates shh transcription in the adjacent foregut endoderm through a conserved MACS1 enhancer. Hedgehog signaling coordinates with Tbx5 in the mesoderm to activate expression of wnt2/2b, which induces pulmonary fate in the foregut endoderm. These results provide mechanistic insight into the interrelationship between heart and lung development informing CP evolution and birth defects.

The transcription factor CREB1 is a mechanistic driver of immunogenicity and reduced HIV-1 acquisition following ALVAC vaccination

Development of effective human immunodeficiency virus 1 (HIV-1) vaccines requires synergy between innate and adaptive immune cells. Here we show that induction of the transcription factor CREB1 and its target genes by the recombinant canarypox vector ALVAC + Alum augments immunogenicity in non-human primates (NHPs) and predicts reduced HIV-1 acquisition in the RV144 trial. These target genes include those encoding cytokines/chemokines associated with heightened protection from simian immunodeficiency virus challenge in NHPs. Expression of CREB1 target genes probably results from direct cGAMP (STING agonist)-modulated p-CREB1 activity that drives the recruitment of CD4+ T cells and B cells to the site of antigen presentation. Importantly, unlike NHPs immunized with ALVAC + Alum, those immunized with ALVAC + MF59, the regimen in the HVTN702 trial that showed no protection from HIV infection, exhibited significantly reduced CREB1 target gene expression. Our integrated systems biology approach has validated CREB1 as a critical driver of vaccine efficacy and highlights that adjuvants that trigger CREB1 signaling may be critical for efficacious HIV-1 vaccines.

Preclinical evidence for the therapeutic value of TBX5 normalization in arrhythmia control

AIMS

Arrhythmias and sudden cardiac death (SCD) occur commonly in patients with heart failure. We found T-box 5 (TBX5) dysregulated in ventricular myocardium from heart failure patients and thus we hypothesized that TBX5 reduction contributes to arrhythmia development in these patients. To understand the underlying mechanisms, we aimed to reveal the ventricular TBX5-dependent transcriptional network and further test the therapeutic potential of TBX5 level normalization in mice with documented arrhythmias.

METHODS AND RESULTS

We used a mouse model of TBX5 conditional deletion in ventricular cardiomyocytes. Ventricular (v) TBX5 loss in mice resulted in mild cardiac dysfunction and arrhythmias and was associated with a high mortality rate (60%) due to SCD. Upon angiotensin stimulation, vTbx5KO mice showed exacerbated cardiac remodelling and dysfunction suggesting a cardioprotective role of TBX5. RNA-sequencing of a ventricular-specific TBX5KO mouse and TBX5 chromatin immunoprecipitation was used to dissect TBX5 transcriptional network in cardiac ventricular tissue. Overall, we identified 47 transcripts expressed under the control of TBX5, which may have contributed to the fatal arrhythmias in vTbx5KO mice. These included transcripts encoding for proteins implicated in cardiac conduction and contraction (Gja1, Kcnj5, Kcng2, Cacna1g, Chrm2), in cytoskeleton organization (Fstl4, Pdlim4, Emilin2, Cmya5), and cardiac protection upon stress (Fhl2, Gpr22, Fgf16). Interestingly, after TBX5 loss and arrhythmia development in vTbx5KO mice, TBX5 protein-level normalization by systemic adeno-associated-virus (AAV) 9 application, re-established TBX5-dependent transcriptome. Consequently, cardiac dysfunction was ameliorated and the propensity of arrhythmia occurrence was reduced.

CONCLUSIONS

This study uncovers a novel cardioprotective role of TBX5 in the adult heart and provides preclinical evidence for the therapeutic value of TBX5 protein normalization in the control of arrhythmia.

Mending a broken heart: In vitro, in vivo and in silico models of congenital heart disease

Birth defects contribute to ∼0.3% of global infant mortality in the first month of life, and congenital heart disease (CHD) is the most common birth defect among newborns worldwide. Despite the significant impact on human health, most treatments available for this heterogenous group of disorders are palliative at best. For this reason, the complex process of cardiogenesis, governed by multiple interlinked and dose-dependent pathways, is well investigated. Tissue, animal and, more recently, computerized models of the developing heart have facilitated important discoveries that are helping us to understand the genetic, epigenetic and mechanobiological contributors to CHD aetiology. In this Review, we discuss the strengths and limitations of different models of normal and abnormal cardiogenesis, ranging from single-cell systems and 3D cardiac organoids, to small and large animals and organ-level computational models. These investigative tools have revealed a diversity of pathogenic mechanisms that contribute to CHD, including genetic pathways, epigenetic regulators and shear wall stresses, paving the way for new strategies for screening and non-surgical treatment of CHD. As we discuss in this Review, one of the most-valuable advances in recent years has been the creation of highly personalized platforms with which to study individual diseases in clinically relevant settings.

Heart malformation is an early response to valproic acid in developing zebrafish

Valproic acid (VPA) is a branched short-chain fatty acid primarily used in epilepsy, but is also used in bipolar disorder, migraine, and psychotic disorders. Despite its wide range of use, it is a teratogen resulting in various congenital abnormalities. Although a large number of scientific studies evidenced the teratogenic effects, there are limited data on embryonic exposure to VPA at specific or different stages of early embryogenesis. Based on this, the present study was planned to investigate the embryonic exposure to VPA at specific and different hours post fertilization (hpf) in zebrafish embryonic model. In first set of experiments, embryos from spawning groups of adult zebrafish were exposed to different molar concentrations of VPA at 2.5 hpf, and in the second set of experiments, embryos were exposed to VPA 100 μM at 24 hpf, 36 hpf, 48 hpf, 72 hpf, and 96 hpf. The parameters examined were hatching rate, mortality, morphology, body length, pericardial sac size, heartrate, anatomical changes in heart, skeletal and notochord till 120 hpf. It was observed that the embryos exposed to VPA at 2.5 hpf suffered from cardiac abnormalities including heart malformation, bradycardia, circulatory failure, and pericardial sac enlargement which was more apparent in embryos exposed to 100 μM VPA. In the second set of experiments, embryos exposed to VPA 100 μM at 24 hpf and 36 hpf suffered from heart malformations, but there was no incidence of cardiac malformation in embryos exposed to VPA at 48 hpf, 72 hpf, and 96 hpf. From the results, it was evident that exposure to VPA at early developmental stage of embryogenesis produced congenital cardiac abnormalities. Since VPA is a selective HDAC inhibitor, histone acetylation with aberrant gene expression during cardiogenesis might be the underlying cause of cardiac malformation.

Conservation and divergence of protein pathways in the vertebrate heart

Heart disease is the leading cause of death in the western world. Attaining a mechanistic understanding of human heart development and homeostasis and the molecular basis of associated disease states relies on the use of animal models. Here, we present the cardiac proteomes of 4 model vertebrates with dual circulatory systems: the pig (Sus scrofa), the mouse (Mus musculus), and 2 frogs (Xenopus laevis and Xenopus tropicalis). Determination of which proteins and protein pathways are conserved and which have diverged within these species will aid in our ability to choose the appropriate models for determining protein function and to model human disease. We uncover mammalian- and amphibian-specific, as well as species-specific, enriched proteins and protein pathways. Among these, we find and validate an enrichment in cell-cycle-associated proteins within Xenopus laevis. To further investigate functional units within cardiac proteomes, we develop a computational approach to profile the abundance of protein complexes across species. Finally, we demonstrate the utility of these data sets for predicting appropriate model systems for studying given cardiac conditions by testing the role of Kielin/chordin-like protein (Kcp), a protein found as enriched in frog hearts compared to mammals. We establish that germ-line mutations in Kcp in Xenopus lead to valve defects and, ultimately, cardiac failure and death. Thus, integrating these findings with data on proteins responsible for cardiac disease should lead to the development of refined, species-specific models for protein function and disease states.

Interspecies Organogenesis for Human Transplantation

Blastocyst complementation combined with gene editing is an emerging approach in the field of regenerative medicine that could potentially solve the worldwide problem of organ shortages for transplantation. In theory, blastocyst complementation can generate fully functional human organs or tissues, grown within genetically engineered livestock animals. Targeted deletion of a specific gene(s) using gene editing to cause deficiencies in organ development can open a niche for human stem cells to occupy, thus generating human tissues. Within this review, we will focus on the pancreas, liver, heart, kidney, lung, and skeletal muscle, as well as cells of the immune and nervous systems. Within each of these organ systems, we identify and discuss (i) the common causes of organ failure; (ii) the current state of regenerative therapies; and (iii) the candidate genes to knockout and enable specific exogenous organ development via the use of blastocyst complementation. We also highlight some of the current barriers limiting the success of blastocyst complementation.

Retinoic acid signaling in heart development

Retinoic acid (RA) is a vitamin A metabolite that acts as a morphogen and teratogen. Excess or defective RA signaling causes developmental defects including in the heart. The heart develops from the anterior lateral plate mesoderm. Cardiogenesis involves successive steps, including formation of the primitive heart tube, cardiac looping, septation, chamber development, coronary vascularization, and completion of the four-chambered heart. RA is dispensable for primitive heart tube formation. Before looping, RA is required to define the anterior/posterior boundaries of the heart-forming mesoderm as well as to form the atrium and sinus venosus. In outflow tract elongation and septation, RA signaling is required to maintain/differentiate cardiogenic progenitors in the second heart field at the posterior pharyngeal arches level. Epicardium-secreted insulin-like growth factor, the expression of which is regulated by hepatic mesoderm-derived erythropoietin under the control of RA, promotes myocardial proliferation of the ventricular wall. Epicardium-derived RA induces the expression of angiogenic factors in the myocardium to form the coronary vasculature. In cardiogenic events at different stages, properly controlled RA signaling is required to establish the functional heart.

Exposure to DBP induces the toxicity in early development and adverse effects on cardiac development in zebrafish (Danio rerio)

Dibutyl phthalate (DBP) is one of the most ubiquitous plasticizers used worldwide and has been frequently detected in soil, water, atmosphere, and other environmental media. DBP has become a ubiquitous environment contaminant and causes serious pollution. However, much attention has been paid to the toxicity of DBP, with only limited attention paid to its detrimental effects on the heart. In the present study, we investigated the toxicity of DBP in zebrafish embryo development, especially adverse effects on cardiac development. Embryos at 4-h post-fertilization (hpf) were exposed to different concentrations of DBP (0, 0.36, 1.8 and 3.6 μM) until 72 hpf. Exposure to DBP resulted in morphological abnormalities in zebrafish embryos. Exposure to 1.8 μM DBP significantly affected the growth, malformation rate, cardiac malformation rate and cardiac looping. Exposure to 3.6 μM DBP significantly affected all endpoints. To preliminarily understand the underlying mechanisms of toxic effects of DBP on the embryo heart, we examined the expression of master cardiac transcription factors such as NKX2.5 and TBX5. The expression of this two transcription factors was significantly reduced with DBP treatment in a dose-dependent manner. Our results demonstrate that exposure to DBP resulted in zebrafish developmental toxicity, pericardial edema, cardiac structure deformities and function alteration, and changed the expression of master cardiac transcription factors such as NKX2.5 and TBX5.

Cardiac septation in heart development and evolution

The heart is one of the vital organs and is functionalized for blood circulation from its early development. Some vertebrates have altered their living environment from aquatic to terrestrial life over the course of evolution and obtained circulatory systems well adapted to their lifestyles. The morphology of the heart has been changed together with the acquisition of a sophisticated respiratory organ, the lung. Adaptation to a terrestrial environment requires the coordination of heart and lung development due to the intake of oxygen from the air and the production of the large amount of energy needed for terrestrial life. Therefore, vertebrates developed pulmonary circulation and a septated heart (four-chambered heart) with venous and arterial blood completely separated. In this review, we summarize how vertebrates change the structures and functions of their circulatory systems according to environmental changes.

Evolutionarily conserved Tbx5-Wnt2/2b pathway orchestrates cardiopulmonary development

Codevelopment of the lungs and heart underlies key evolutionary innovations in the transition to terrestrial life. Cardiac specializations that support pulmonary circulation, including the atrial septum, are generated by second heart field (SHF) cardiopulmonary progenitors (CPPs). It has been presumed that transcription factors required in the SHF for cardiac septation, e.g., Tbx5, directly drive a cardiac morphogenesis gene-regulatory network. Here, we report instead that TBX5 directly drives Wnt ligands to initiate a bidirectional signaling loop between cardiopulmonary mesoderm and the foregut endoderm for endodermal pulmonary specification and, subsequently, atrial septation. We show that Tbx5 is required for pulmonary specification in mice and amphibians but not for swim bladder development in zebrafish. TBX5 is non-cell-autonomously required for pulmonary endoderm specification by directly driving Wnt2 and Wnt2b expression in cardiopulmonary mesoderm. TBX5 ChIP-sequencing identified cis-regulatory elements at Wnt2 sufficient for endogenous Wnt2 expression domains in vivo and required for Wnt2 expression in precardiac mesoderm in vitro. Tbx5 cooperated with Shh signaling to drive Wnt2b expression for lung morphogenesis. Tbx5 haploinsufficiency in mice, a model of Holt-Oram syndrome, caused a quantitative decrement of mesodermal-to-endodermal Wnt signaling and subsequent endodermal-to-mesodermal Shh signaling required for cardiac morphogenesis. Thus, Tbx5 initiates a mesoderm-endoderm-mesoderm signaling loop in lunged vertebrates that provides a molecular basis for the coevolution of pulmonary and cardiac structures required for terrestrial life.

Thalidomide remodels developing heart in chick embryo: discovery of a thalidomide mediated hematoma in heart muscle

Despite of medical disaster caused by thalidomide in 1960s, the drug came to clinical use again for the treatment of erythema nodosum leprosum (ENL) and multiple myeloma. Recently, a new generation of children affected by thalidomide intake by their mothers during pregnancy has been identified in Brazil. In the past few years, there is the great enhancement in our understanding of the molecular mechanisms and targets of thalidomide with the help of modern OMICS technologies. However, understanding of cardiac-specific anomalies in fetus due to thalidomide intake by the respective mother has not been explored fully. At organ level, thalidomide causes congenital heart diseases, limb deformities in addition to ocular, and neural and ear abnormalities. The period of morning sickness and cardiogenesis is synchronized in pregnant women. Therefore, thalidomide intake during the first trimester could affect cardiogenesis severely. Thalidomide intake in pregnant women either causes miscarriage or heart abnormalities such as patent ductus arteriosus, ventricular septal defect (VSD), atrial septal defect (ASD), and pulmonary stenosis in survivors. In the present study, we identified a novel morphological defect (lump) in the heart of thalidomide-treated chick embryos. We characterized the lump at morphological, histo-pathological, oxidative stress, electro-physiological, and gene expression level. To our knowledge, here, we report the very first electrophysiological characterization of embryonic heart affected by thalidomide treatment.

An inactivating mutation in the histone deacetylase SIRT6 causes human perinatal lethality

Ferrer et al. demonstrate that a homozygous inactivating mutation in the histone deacetylase SIRT6 results in severe congenital anomalies and perinatal lethality in four affected fetuses. Human induced pluripotent stem cells derived from D63H homozygous fetuses fail to differentiate into embryoid bodies, functional cardiomyocytes, and neural progenitor cells due to a failure to repress pluripotent genes.

It has been well established that histone and DNA modifications are critical to maintaining the equilibrium between pluripotency and differentiation during early embryogenesis. Mutations in key regulators of DNA methylation have shown that the balance between gene regulation and function is critical during neural development in early years of life. However, there have been no identified cases linking epigenetic regulators to aberrant human development and fetal demise. Here, we demonstrate that a homozygous inactivating mutation in the histone deacetylase SIRT6 results in severe congenital anomalies and perinatal lethality in four affected fetuses. In vitro, the amino acid change at Asp63 to a histidine results in virtually complete loss of H3K9 deacetylase and demyristoylase functions. Functionally, SIRT6 D63H mouse embryonic stem cells (mESCs) fail to repress pluripotent gene expression, direct targets of SIRT6, and exhibit an even more severe phenotype than Sirt6-deficient ESCs when differentiated into embryoid bodies (EBs). When terminally differentiated toward cardiomyocyte lineage, D63H mutant mESCs maintain expression of pluripotent genes and fail to form functional cardiomyocyte foci. Last, human induced pluripotent stem cells (iPSCs) derived from D63H homozygous fetuses fail to differentiate into EBs, functional cardiomyocytes, and neural progenitor cells due to a failure to repress pluripotent genes. Altogether, our study described a germline mutation in SIRT6 as a cause for fetal demise, defining SIRT6 as a key factor in human development and identifying the first mutation in a chromatin factor behind a human syndrome of perinatal lethality.

Discovery of Small Molecules Targeting the Synergy of Cardiac Transcription Factors GATA4 and NKX2-5

Transcription factors are pivotal regulators of gene transcription, and many diseases are associated with the deregulation of transcriptional networks. In the heart, the transcription factors GATA4 and NKX2-5 are required for cardiogenesis. GATA4 and NKX2-5 interact physically, and the activation of GATA4, in cooperation with NKX2-5, is essential for stretch-induced cardiomyocyte hypertrophy. Here, we report the identification of four small molecule families that either inhibit or enhance the GATA4-NKX2-5 transcriptional synergy. A fragment-based screening, reporter gene assay, and pharmacophore search were utilized for the small molecule screening, identification, and optimization. The compounds modulated the hypertrophic agonist-induced cardiac gene expression. The most potent hit compound, N-[4-(diethylamino)phenyl]-5-methyl-3-phenylisoxazole-4-carboxamide (3, IC₅₀ = 3 μM), exhibited no activity on the protein kinases involved in the regulation of GATA4 phosphorylation. The identified and chemically and biologically characterized active compound, and its derivatives may provide a novel class of small molecules for modulating heart regeneration.

A HAND to TBX5 Explains the Link Between Thalidomide and Cardiac Diseases

Congenital heart disease is the leading cause of death in the first year of life. Mutations only in few genes have been linked to some cases of CHD. Thalidomide was used by pregnant women for morning sickness but was removed from the market because it caused severe malformations including CHDs. We used both in silico docking software, and in vitro molecular and biochemical methods to document a novel interaction involving Thalidomide, TBX5, and HAND2. Thalidomide binds readily to TBX5 through amino acids R81, R82, and K226 all implicated in DNA binding. It reduces TBX5 binding to DNA by 40%, and suppresses TBX5 mediated activation of the NPPA and VEGF promoters by 70%. We documented a novel interaction between TBX5 and HAND2, and showed that a p.G202V HAND2 variant associated with CHD and coronary artery diseases found in a large Lebanese family with high consanguinity, drastically inhibited this interaction by 90%. Similarly, thalidomide inhibited the TBX5/HAND2 physical interaction, and the in silico docking revealed that the same amino acids involved in the interaction of TBX5 with DNA are also involved in its binding to HAND2. Our results establish a HAND2/TBX5 pathway implicated in heart development and diseases.

Overexpression of T-box Transcription Factor 5 (TBX5) Inhibits Proliferation and Invasion in Non-Small Cell Lung Carcinoma Cells

T-box transcription factor 5 (TBX5), a member of the conserved T-box transcription factor family that functions in organogenesis and embryogenesis, has recently been identified as a critical player in cancer development. The aim of this study was to determine the role of TBX5 in non-small cell lung carcinoma (NSCLC). Immunohistochemistry was used to detect the correlation between levels of TBX5 and clinicopathological features of NSCLC patients in tissue microarray. Expression of TBX5 in NSCLC tissues and cell lines was evaluated by quantitative PCR and Western blot. The role of TBX5 in regulating proliferation, colony formation, invasion, and apoptosis of NSCLC cells was evaluated in vitro. Finally, a tumorigenicity assay was performed to determine the effect of TBX5 on tumor growth in vivo. The levels of TBX5 in NSCLC tissues were significantly correlated with the TNM stage (p = 0.016), histopathologic type (p = 0.029), and lymph node status (p = 0.035) of NSCLC. TBX5 overexpression markedly suppressed in vitro NSCLC cell proliferation, colony formation, and invasion and induced apoptosis. In vivo tumor growth was significantly suppressed by TBX5. TBX5 has a tumor-suppressing effect in NSCLC and may serve as a therapeutic target for diagnoses and treatment of NSCLC.

microRNA-10a Targets T-box 5 to Inhibit the Development of Cardiac Hypertrophy

The mechanism of cardiac hypertrophy involving microRNAs (miRNAs) is attracting increasing attention. Our study aimed to investigate the role of miR-10a in cardiac hypertrophy development and the underlying regulatory mechanism.Transverse abdominal aortic constriction (TAAC) surgery was performed to establish a cardiac hypertrophy rat model, and angiotensin II (AngII) was used to induce cardiac hypertrophy in cultured neonatal rat cardiomyocytes. Expression of T-box 5 (TBX5) and miR-10a was altered by cell transfection of siRNA or miRNA mimic/inhibitor. Leucine incorporation assay, histological and cytological examination, quantitative real-time PCR (qRT-PCR), and Western blot were performed to detect the effects of miR-10a and TBX5 on cardiac hypertrophy. Dual-luciferase reporter assay was conducted to verify the regulation of TBX5 by miR-10a.miR-10a was down-regulated, and TBX5 was up-regulated in the rat model and AngII-stimulated cardiomyocytes. miR-10a inhibited TBX5 expression by directly targeting the binding site in Tbx5 3'UTR. Overexpression of miR-10a in AngII-treated cardiomyocytes decreased relative cell area, and significantly reduced the mRNA levels of natriuretic peptide A (Nppa), myosin heavy chain 7 cardiac muscle beta (Myh7), and leucine incorporation (P < 0.01 or P < 0.001). Knockdown of Tbx5 had similar effects on AngII-induced cardiomyocytes.Our findings indicate that miR-10a may inhibit cardiac hypertrophy via targeting Tbx5. Thus, miR-10a provides promising therapeutic strategies for the treatment of cardiac hypertrophy.

TBX5: A Key Regulator of Heart Development

TBX5 is a member of the T-box transcription factor family and is primarily known for its role in cardiac and forelimb development. Human patients with dominant mutations in TBX5 are characterized by Holt-Oram syndrome, and show defects of the cardiac septa, cardiac conduction system, and the anterior forelimb. The range of cardiac defects associated with TBX5 mutations in humans suggests multiple roles for the transcription factor in cardiac development and function. Animal models demonstrate similar defects and have provided a useful platform for investigating the roles of TBX5 during embryonic development. During early cardiac development, TBX5 appears to act primarily as a transcriptional activator of genes associated with cardiomyocyte maturation and upstream of morphological signals for septation. During later cardiac development, TBX5 is required for patterning of the cardiac conduction system and maintenance of mature cardiomyocyte function. A comprehensive understanding of the integral roles of TBX5 throughout cardiac development and adult life will be critical for understanding human cardiac morphology and function.

Tbx3 represses bmp4 expression and, with Pax6, is required and sufficient for retina formation

Vertebrate eye formation begins in the anterior neural plate in the eye field. Seven eye field transcription factors (EFTFs) are expressed in eye field cells and when expressed together are sufficient to generate retina from pluripotent cells. The EFTF Tbx3 can regulate the expression of some EFTFs; however, its role in retina formation is unknown. Here, we show that Tbx3 represses bmp4 transcription and is required in the eye field for both neural induction and normal eye formation in Xenopus laevis Although sufficient for neural induction, Tbx3-expressing pluripotent cells only form retina in the context of the eye field. Unlike Tbx3, the neural inducer Noggin can generate retina both within and outside the eye field. We found that the neural and retina-inducing activity of Noggin requires Tbx3. Noggin, but not Tbx3, induces Pax6 and coexpression of Tbx3 and Pax6 is sufficient to determine pluripotent cells to a retinal lineage. Our results suggest that Tbx3 represses bmp4 expression and maintains eye field neural progenitors in a multipotent state; then, in combination with Pax6, Tbx3 causes eye field cells to form retina.

Discovering miRNA Regulatory Networks in Holt-Oram Syndrome Using a Zebrafish Model

MicroRNAs (miRNAs) are small non-coding RNAs that play an important role in the post-transcriptional regulation of gene expression. miRNAs are involved in the regulation of many biological processes such as differentiation, apoptosis, and cell proliferation. miRNAs are expressed in embryonic, postnatal, and adult hearts, and they have a key role in the regulation of gene expression during cardiovascular development and disease. Aberrant expression of miRNAs is associated with abnormal cardiac cell differentiation and dysfunction. Tbx5 is a member of the T-box gene family, which acts as transcription factor involved in the vertebrate heart development. Alteration of Tbx5 level affects the expression of hundreds of genes. Haploinsufficiency and gene duplication of Tbx5 are at the basis of the cardiac abnormalities associated with Holt–Oram syndrome (HOS). Recent data indicate that miRNAs might be an important part of the regulatory circuit through which Tbx5 controls heart development. Using high-throughput technologies, we characterized genome-widely the miRNA and mRNA expression profiles in WT- and Tbx5-depleted zebrafish embryos at two crucial developmental time points, 24 and 48 h post fertilization (hpf). We found that several miRNAs, which are potential effectors of Tbx5, are differentially expressed; some of them are already known to be involved in cardiac development and functions, such as miR-30, miR-34, miR-190, and miR-21. We performed an integrated analysis of miRNA expression data with gene expression profiles to refine computational target prediction approaches by means of the inversely correlation of miRNA–mRNA expressions, and we highlighted targets, which have roles in cardiac contractility, cardiomyocyte proliferation/apoptosis, and morphogenesis, crucial functions regulated by Tbx5. This approach allowed to discover complex regulatory circuits involving novel miRNAs and protein coding genes not considered before in the HOS such as miR-34a and miR-30 and their targets.

A Matter of the Heart: The African Clawed Frog Xenopus as a Model for Studying Vertebrate Cardiogenesis and Congenital Heart Defects

The African clawed frog, Xenopus, is a valuable non-mammalian model organism to investigate vertebrate heart development and to explore the underlying molecular mechanisms of human congenital heart defects (CHDs). In this review, we outline the similarities between Xenopus and mammalian cardiogenesis, and provide an overview of well-studied cardiac genes in Xenopus, which have been associated with congenital heart conditions. Additionally, we highlight advantages of modeling candidate genes derived from genome wide association studies (GWAS) in Xenopus and discuss commonly used techniques.

Effects of obesity on the association between common variations in the TBX5 gene and matrix metalloproteinase 9 levels in Taiwanese

OBJECTIVES

The TBX5 gene, a member of the T-box family, is associated with congenital heart disease, electrocardiographic parameters, and development of atrial fibrillation in the general population. This study aimed to elucidate the role of TBX5 gene polymorphisms in metabolic and inflammatory profiles possibly linked to TBX5-related pathologies.

MATERIALS AND METHODS

A sample population of 597 individuals having routine health examinations was enrolled. Five tagging TBX5 single nucleotide polymorphisms (SNPs) were analyzed using polymerase chain reaction and restriction enzyme digestion or TaqMan SNP genotyping assays. Associations between genotypes/haplotypes and matrix metalloproteinase 9 (MMP9) levels were investigated using generalized linear model analysis. Interactions between each genotype/haplotype, MMP9 level, and obesity status were tested using two-way analysis of variance with Golden Helix SVS Win32 7.3.1 software.

RESULTS

After adjusting for clinical covariates, TBX5 genotypes were found to be associated with MMP9 levels (p = 0.002 and p = 0.001 for rs4113925 and rs3825214, respectively) in a dominant inheritance model. Haplotype analysis using three tag SNPs (rs11067101, rs1247973, and rs3825214) revealed a significant association between TBX5 haplotype GCG and MMP9 levels (uncorrected p = 0.0093 and the corrected false discovery rate p = 0.0435). Multivariate analysis identified that SNP rs3825214, in addition to the MMP9 and E-selectin genotypes, was independently associated with MMP9 levels (p < 0.001). Using a dominant inheritance model, subgroup and interaction analysis showed associations between the rs4113925, rs3825214, and MMP9 levels only in nonobese individuals (p = 1.04 × 10-4 and p = 7.11 × 10-5, respectively; interaction p = 0.009 and 0.018, respectively). Subgroup analysis showed a borderline significant association between haplotype GCG and MMP9 levels (uncorrected p = 0.020 and corrected false discovery rate p = 0.073), but with no evidence of interaction.

CONCLUSION

TBX5 genotypes/haplotypes are independently associated with MMP9 in Taiwanese individuals and occur predominantly in nonobese people. These associations may broaden our understanding of the mechanism underlying T-box family gene activity and related cardiovascular pathologies.

Mesenchymal Stem Cells for Cardiac Regenerative Therapy: Optimization of Cell Differentiation Strategy

With the high mortality rate, coronary heart disease (CHD) has currently become a major life-threatening disease. The main pathological change of myocardial infarction (MI) is the induction of myocardial necrosis in infarction area which finally causes heart failure. Conventional treatments cannot regenerate the functional cell efficiently. Recent researches suggest that mesenchymal stem cells (MSCs) are able to differentiate into multiple lineages, including cardiomyocyte-like cells in vitro and in vivo, and they have been used for the treatment of MI to repair the injured myocardium and improve cardiac function. In this review, we will focus on the recent progress on MSCs derived cardiomyocytes for cardiac regeneration after MI.

in vitro development of bioimplants made up of elastomeric scaffolds with peptide gel filling seeded with human subcutaneous adipose tissue-derived progenitor cells

Myocardial tissue lacks the ability to regenerate itself significantly following a myocardial infarction. Thus, new strategies that could compensate this lack are of high interest. Cardiac tissue engineering (CTE) strategies are a relatively new approach that aims to compensate the tissue loss using combination of biomaterials, cells and bioactive molecules. The goal of the present study was to evaluate cell survival and growth, seeding capacity and cellular phenotype maintenance of subcutaneous adipose tissue-derived progenitor cells in a new synthetic biomaterial scaffold platform. Specifically, here we tested the effect of the RAD16-I peptide gel in microporous poly(ethyl acrylate) polymers using two-dimensional PEA films as controls. Results showed optimal cell adhesion efficiency and growth in the polymers coated with the self-assembling peptide RAD16-I. Importantly, subATDPCs seeded into microporous PEA scaffolds coated with RAD16-I maintained its phenotype and were able to migrate outwards the bioactive patch, hopefully toward the infarcted area once implanted. These data suggest that this bioimplant (scaffold/RAD16-I/cells) can be suitable for further in vivo implantation with the aim to improve the function of affected tissue after myocardial infarction.

Systems Chemo-Biology and Transcriptomic Meta-Analysis Reveal the Molecular Roles of Bioactive Lipids in Cardiomyocyte Differentiation

Lipids, which are essential constituents of biological membranes, play structural and functional roles in the cell. In recent years, certain lipids have been identified as regulatory signaling molecules and have been termed "bioactive lipids". Subsequently, the importance of bioactive lipids in stem cell differentiation and cardiogenesis has gained increasing recognition. Therefore, the aim of this study was to identify the biological processes underlying murine cardiac differentiation and the mechanisms by which bioactive lipids affect these processes. For this purpose, a transcriptomic meta-analysis of microarray and RNA-seq data from murine stem cells undergoing cardiogenic differentiation was performed. The differentially expressed genes identified via this meta-analysis, as well as bioactive lipids, were evaluated using systems chemo-biology tools. These data indicated that bioactive lipids are associated with the regulation of cell motility, cell adhesion, cytoskeletal rearrangement, and gene expression. Moreover, bioactive lipids integrate the signaling pathways involved in cell migration, the secretion and remodeling of extracellular matrix components, and the establishment of the cardiac phenotype. In conclusion, this study provides new insights into the contribution of bioactive lipids to the induction of cellular responses to various stimuli, which may originate from the extracellular environment and morphogens, and the manner in which this contribution directly affects murine heart morphogenesis.

Dual developmental role of transcriptional regulator Ets1 in Xenopus cardiac neural crest vs. heart mesoderm

AIMS

Ets1 is an important transcription factor that is expressed in both the cardiac neural crest (NC) and heart mesoderm of vertebrate embryos. Moreover, Ets1 deletion in humans results in congenital heart abnormalities. To clarify the functional contributions of Ets1 in cardiac NC vs. heart mesoderm, we performed tissue-targeted loss-of-function analysis to compare the relative roles of Ets1 in these two tissues during heart formation using Xenopus embryos as a model system.

METHODS AND RESULTS

We confirmed by in situ hybridization analysis that Ets1 is expressed in NC and heart mesoderm during embryogenesis. Using a translation-blocking antisense morpholino to knockdown Ets1 protein selectively in the NC, we observed defects in NC delamination from the neural tube, collective cell migration, as well as segregation of NC streams in the cranial and cardiac regions. Many cardiac NC cells failed to reach their destination in the heart, resulting in defective aortic arch artery formation. A different set of defects was noted when Ets1 knockdown was targeted to heart mesoderm. The formation of the primitive heart tube was dramatically delayed and the endocardial tissue appeared depleted. As a result, the conformation of the heart was severely disrupted. In addition, the outflow tract septum was missing, and trabeculae formation in the ventricle was abolished.

CONCLUSION

Our study shows that Ets1 is required in both the cardiac NC and heart mesoderm, albeit for different aspects of heart formation. Our results reinforce the suggestion that proper interaction between these tissues is critical for normal heart development.

Diversity of congenital cardiac defects and skeletal deformities associated with the Holt-Oram syndrome

INTRODUCTION

The Holt–Oram syndrome is a rare congenital disorder involving the skeletal and cardiovascular systems. It is characterized by upper limb deformities and cardiac malformations, atrial septal defects in particular.

PRESENTATION OF CASE

Four consecutive patients 1–15 years old with the Holt–Oram syndrome presented over a 10 year span for surgical treatment of their cardiac maladies. The spectrum of the heart defects and skeletal deformities encountered in these patients are described and discussed.

DISCUSSION

The Holt–Oram syndrome is an autosomal dominant condition; however absence of the morphological features of the trait in close family members is not rare. Although patients are known to predominately present with atrial septal defects, other cardiovascular anomalies, including rhythm abnormalities, are not uncommon. Skeletal disorders vary as well.

CONCLUSION

Cardiovascular disorders, skeletal malformations and familial expression of the Holt–Oram syndrome, vary widely.

Proteomic profiling of cardiac tissue by isolation of nuclei tagged in specific cell types (INTACT)

The proper dissection of the molecular mechanisms governing the specification and differentiation of specific cell types requires isolation of pure cell populations from heterogeneous tissues and whole organisms. Here, we describe a method for purification of nuclei from defined cell or tissue types in vertebrate embryos using INTACT (isolation of nuclei tagged in specific cell types). This method, previously developed in plants, flies and worms, utilizes in vivo tagging of the nuclear envelope with biotin and the subsequent affinity purification of the labeled nuclei. In this study we successfully purified nuclei of cardiac and skeletal muscle from Xenopus using this strategy. We went on to demonstrate the utility of this approach by coupling the INTACT approach with liquid chromatography-tandem mass spectrometry (LC-MS/MS) proteomic methodologies to profile proteins expressed in the nuclei of developing hearts. From these studies we have identified the Xenopus orthologs of 12 human proteins encoded by genes, which when mutated in human lead to congenital heart disease. Thus, by combining these technologies we are able to identify tissue-specific proteins that are expressed and required for normal vertebrate organ development.

Regulation of organogenesis and stem cell properties by T-box transcription factors

T-box transcription factors containing the common DNA-binding domain T-box contribute to the organization of multiple tissues in vertebrates and invertebrates. In mammals, 17 T-box genes are divided into five subfamilies depending on their amino acid homology. The proper distribution and expression of individual T-box transcription factors in different tissues enable regulation of the proliferation and differentiation of tissue-specific stem cells and progenitor cells in a suitable time schedule for tissue organization. Consequently, uncontrollable expressions of T-box genes induce abnormal tissue organization, and eventually cause various diseases with malformation and malfunction of tissues and organs. Furthermore, some T-box transcription factors are essential for maintaining embryonic stem cell pluripotency, improving the quality of induced pluripotent stem cells, and inducing cell-lineage conversion of differentiated cells. These lines of evidence indicate fundamental roles of T-box transcription factors in tissue organization and stem cell properties, and suggest that these transcription factors will be useful for developing therapeutic approaches in regenerative medicine.

The small molecule Wnt signaling modulator ICG-001 improves contractile function in chronically infarcted rat myocardium

The adult mammalian heart has limited capability for self-repair after myocardial infarction. Therefore, therapeutic strategies that improve post-infarct cardiac function are critically needed. The small molecule ICG-001 modulates Wnt signaling and increased the expression of genes beneficial for cardiac regeneration in epicardial cells. Lineage tracing experiments, demonstrated the importance of β-catenin/p300 mediated transcription for epicardial progenitor contribution to the myocardium. Female rats given ICG-001 for 10 days post-occlusion significantly improved ejection fraction by 8.4%, compared to controls (P<0.05). Taken together, Wnt modulation via β-catenin/CBP inhibition offers a promising therapeutic strategy towards restoration of myocardial tissues and an enhancement of cardiac functions following infarction.

Experimental approaches for gene regulatory network construction: the chick as a model system

Setting up the body plan during embryonic development requires the coordinated action of many signals and transcriptional regulators in a precise temporal sequence and spatial pattern. The last decades have seen an explosion of information describing the molecular control of many developmental processes. The next challenge is to integrate this information into logic "wiring diagrams" that visualize gene actions and outputs, have predictive power and point to key control nodes. Here, we provide an experimental workflow on how to construct gene regulatory networks using the chick as model system.

MicroRNA 218 mediates the effects of Tbx5a over-expression on zebrafish heart development

tbx5, a member of the T-box gene family, encodes one of the key transcription factors mediating vertebrate heart development. Tbx5 function in heart development appears to be exquisitely sensitive to gene dosage, since both haploinsufficiency and gene duplication generate the cardiac abnormalities associated with Holt−Oram syndrome (HOS), a highly penetrant autosomal dominant disease characterized by congenital heart defects of varying severity and upper limb malformation. It is suggested that tight integration of microRNAs and transcription factors into the cardiac genetic circuitry provides a rich and robust array of regulatory interactions to control cardiac gene expression. Based on these considerations, we performed an in silico screening to identify microRNAs embedded in genes highly sensitive to Tbx5 dosage. Among the identified microRNAs, we focused our attention on miR-218-1 that, together with its host gene, slit2, is involved in heart development. We found correlated expression of tbx5 and miR-218 during cardiomyocyte differentiation of mouse P19CL6 cells. In zebrafish embryos, we show that both Tbx5 and miR-218 dysregulation have a severe impact on heart development, affecting early heart morphogenesis. Interestingly, down-regulation of miR-218 is able to rescue the heart defects generated by tbx5 over-expression supporting the notion that miR-218 is a crucial mediator of Tbx5 in heart development and suggesting its possible involvement in the onset of heart malformations.

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.

Regulatory variation in a TBX5 enhancer leads to isolated congenital heart disease

Recent studies have identified the genetic underpinnings of a growing number of diseases through targeted exome sequencing. However, this strategy ignores the large component of the genome that does not code for proteins, but is nonetheless biologically functional. To address the possible involvement of regulatory variation in congenital heart diseases (CHDs), we searched for regulatory mutations impacting the activity of TBX5, a dosage-dependent transcription factor with well-defined roles in the heart and limb development that has been associated with the Holt-Oram syndrome (heart-hand syndrome), a condition that affects 1/100 000 newborns. Using a combination of genomics, bioinformatics and mouse genetic engineering, we scanned ∼700 kb of the TBX5 locus in search of cis-regulatory elements. We uncovered three enhancers that collectively recapitulate the endogenous expression pattern of TBX5 in the developing heart. We re-sequenced these enhancer elements in a cohort of non-syndromic patients with isolated atrial and/or ventricular septal defects, the predominant cardiac defects of the Holt-Oram syndrome, and identified a patient with a homozygous mutation in an enhancer ∼90 kb downstream of TBX5. Notably, we demonstrate that this single-base-pair mutation abrogates the ability of the enhancer to drive expression within the heart in vivo using both mouse and zebrafish transgenic models. Given the population-wide frequency of this variant, we estimate that 1/100 000 individuals would be homozygous for this variant, highlighting that a significant number of CHD associated with TBX5 dysfunction might arise from non-coding mutations in TBX5 heart enhancers, effectively decoupling the heart and hand phenotypes of the Holt-Oram syndrome.

Tbx5 overexpression favors a first heart field lineage in murine embryonic stem cells and in Xenopus laevis embryos

The T-box transcription factor Tbx5 is involved in several developmental processes including cardiogenesis. Early steps of cardiac development are characterised by the formation of two cardiogenic lineages, the first (FHF) and the second heart field (SHF) lineage, which arise from a common cardiac progenitor cell population. To further investigate the function of Tbx5 during cardiogenesis, we generated a murine embryonic stem cell line constitutively overexpressing Tbx5. Differentiation of these cells is characterised by an earlier and increased appearance of contracting cardiomyocytes that beat with a higher frequency than control cells. In semi-quantitative and quantitative RT-PCR analyses, we observed an up-regulation of cardiac marker genes such as Troponin T, endogenous Tbx5, and Nkx2.5 and a down-regulation of others like BMP4 and Hand2. Similar data were gained in Xenopus laevis arguing for a conserved function of Tbx5. Furthermore, markers of the conduction system and atrial cardiomyocytes were increased.

Inhibition of heart formation by lithium is an indirect result of the disruption of tissue organization within the embryo

Lithium is a commonly used drug for the treatment of bipolar disorder. At high doses, lithium becomes teratogenic, which is a property that has allowed this agent to serve as a useful tool for dissecting molecular pathways that regulate embryogenesis. This study was designed to examine the impact of lithium on heart formation in the developing frog for insights into the molecular regulation of cardiac specification. Embryos were exposed to lithium at the beginning of gastrulation, which produced severe malformations of the anterior end of the embryo. Although previous reports characterized this deformity as a posteriorized phenotype, histological analysis revealed that the defects were more comprehensive, with disfigurement and disorganization of all interior tissues along the anterior-posterior axis. Emerging tissues were poorly segregated and cavity formation was decreased within the embryo. Lithium exposure also completely ablated formation of the heart and prevented myocardial cell differentiation. Despite the complete absence of cardiac tissue in lithium treated embryos, exposure to lithium did not prevent myocardial differentiation of precardiac dorsal marginal zone explants. Moreover, precardiac tissue freed from the embryo subsequent to lithium treatment at gastrulation gave rise to cardiac tissue, as demonstrated by upregulation of cardiac gene expression, display of sarcomeric proteins, and formation of a contractile phenotype. Together these data indicate that lithium's effect on the developing heart was not due to direct regulation of cardiac differentiation, but an indirect consequence of disrupted tissue organization within the embryo.

Stem-cell-like embryonic explants to study cardiac development

Cells from the animal pole of Xenopus blastula embryo possess pluripotency that can be used to generate various tissues and even functional organs ex vivo. This finding has sparkled development of a variety of experimental protocols to study mechanisms that underlie formation of various organs and explore strategies for organ engineering for clinical applications. In this chapter, key methods are described for using Xenopus stem-cell-like embryonic explants as an assay system for studying organ development, with a focus on cardiogenesis. This assay allows investigation of cardiogenesis in isolation from neighboring tissues, minimizes interference with other developmental processes, and presents the further advantage of a heterologous system to study cardiogenesis in isolation of endogenous development of the heart. The cardiogenic assays can be exploited to investigate molecular mechanisms and cellular processes that underlie function of different molecules involved in cardiogenesis.