Exosomes derived from induced cardiopulmonary progenitor cells alleviate acute lung injury in mice

Cardiopulmonary progenitor cells (CPPs) constitute a minor subpopulation of cells that are commonly associated with heart and lung morphogenesis during embryonic development but completely subside after birth. This fact offers the possibility for the treatment of pulmonary heart disease (PHD), in which the lung and heart are both damaged. A reliable source of CPPs is urgently needed. In this study, we reprogrammed human cardiac fibroblasts (HCFs) into CPP-like cells (or induced CPPs, iCPPs) and evaluated the therapeutic potential of iCPP-derived exosomes for acute lung injury (ALI). iCPPs were created in passage 3 primary HCFs by overexpressing GLI1, WNT2, ISL1 and TBX5 (GWIT). Exosomes were isolated from the culture medium of passage 6-8 GWIT-iCPPs. A mouse ALI model was established by intratracheal instillation of LPS. Four hours after LPS instillation, ALI mice were treated with GWIT-iCPP-derived exosomes (5 × 109, 5 × 1010 particles/mL) via intratracheal instillation. We showed that GWIT-iCPPs could differentiate into cell lineages, such as cardiomyocyte-like cells, endothelial cells, smooth muscle cells and alveolar epithelial cells, in vitro. Transcription analysis revealed that GWIT-iCPPs have potential for heart and lung development. Intratracheal instillation of iCPP-derived exosomes dose-dependently alleviated LPS-induced ALI in mice by attenuating lung inflammation, promoting endothelial function and restoring capillary endothelial cells and the epithelial cells barrier. This study provides a potential new method for the prevention and treatment of cardiopulmonary injury, especially lung injury, and provides a new cell model for drug screening.

Zebrafish smarcc1a mutants reveal requirements for BAF chromatin remodeling complexes in distinguishing the atrioventricular canal from the cardiac chambers

BACKGROUND

Essential patterning processes transform the heart tube into a compartmentalized organ with distinct chambers separated by an atrioventricular canal (AVC). This transition involves the refinement of expression of genes that are first found broadly throughout the heart tube and then become restricted to the AVC. Despite the importance of cardiac patterning, we do not fully understand the mechanisms that limit gene expression to the AVC.

RESULTS

We show that the zebrafish gene smarcc1a, encoding a BAF chromatin remodeling complex subunit homologous to mammalian BAF155, is critical for cardiac patterning. In smarcc1a mutants, myocardial differentiation and heart tube assembly appear to proceed normally. Subsequently, the smarcc1a mutant heart fails to exhibit refinement of gene expression patterns to the AVC, and the persistence of broad gene expression is accompanied by failure of chamber expansion. In addition to their cardiac defects, smarcc1a mutants lack pectoral fins, indicating similarity to tbx5a mutants. However, comparison of smarcc1a and tbx5a mutants suggests that perturbation of tbx5a function is not sufficient to cause the smarcc1a mutant phenotype.

CONCLUSIONS

Our data indicate an important role for Smarcc1a-containing chromatin remodeling complexes in regulating the changes in gene expression and morphology that distinguish the AVC from the cardiac chambers.

Super-enhancer-driven lncRNA Snhg7 aggravates cardiac hypertrophy via Tbx5/GLS2/ferroptosis axis

Long non-coding RNAs (lncRNAs) are expressed aberrantly in cardiac disease, but their roles in cardiac hypertrophy are still unknown. Here we sought to identify a specific lncRNA and explore the mechanisms underlying lncRNA functions. Our results revealed that lncRNA Snhg7 was a super-enhancer-driven gene in cardiac hypertrophy by using chromatin immunoprecipitation sequencing (ChIP-Seq). We next found that lncRNA Snhg7 induced ferroptosis by interacting with T-box transcription factor 5 (Tbx5), a cardiac transcription factor. Moreover, Tbx5 bound to the promoter of glutaminase 2 (GLS2) and regulated cardiomyocyte ferroptosis activity in cardiac hypertrophy. Importantly, extra-terminal domain inhibitor JQ1 could suppress super-enhancers in cardiac hypertrophy. Inhibition of lncRNA Snhg7 could block the expressions of Tbx5, GLS2 and levels of ferroptosis in cardiomyocytes. Furthermore, we verified that Nkx2-5 as a core transcription factor, directly bound the super-enhancer of itself and lncRNA Snhg7, increasing both of their activation. Collectively, we are the first to identify lncRNA Snhg7 as a novel functional lncRNA in cardiac hypertrophy, might regulate cardiac hypertrophy via ferroptosis. Mechanistically, lncRNA Snhg7 could transcriptionally regulate Tbx5/GLS2/ferroptosis in cardiomyocytes.

Tbx5 overexpression in embryoid bodies increases TAK1 expression but does not enhance the differentiation of sinoatrial node cardiomyocytes

Genetic studies place Tbx5 at the apex of the sinoatrial node (SAN) transcriptional program. To understand its role in SAN differentiation, clonal embryonic stem (ES) cell lines were made that conditionally overexpress Tbx5, Tbx3, Tbx18, Shox2, Islet-1, and MAP3k7/TAK1. Cardiac cells differentiated using embryoid bodies (EBs). EBs overexpressing Tbx5, Islet1, and TAK1 beat faster than cardiac cells differentiated from control ES cell lines suggesting possible roles in SAN differentiation. Tbx5 overexpressing EBs showed increased expression of TAK1, but cardiomyocytes did not differentiate as SAN cells. EBs showed no change in the expression of the SAN transcription factors Shox2 and Islet1 and decreased expression of the SAN channel protein HCN4. EBs constitutively overexpressing TAK1 direct cardiac differentiation to the SAN fate but have reduced phosphorylation of its targets, p38 and Jnk. This opens the possibility that blocking the phosphorylation of TAK1 targets may have the same impact as forced overexpression. To test this, we treated EBs with 5z-7-Oxozeanol (OXO), an inhibitor of TAK1 phosphorylation. Like TAK1 overexpressing cardiac cells, cardiomyocytes differentiated in the presence of OXO beat faster and showed increased expression of SAN genes (Shox2, HCN4, and Islet1). This suggests that activation of the SAN transcriptional network can be accomplished by blocking the phosphorylation of TAK1.

Case report: Novel TBX5-related pathogenic mechanism of Holt-Oram syndrome

Introduction: Holt-Oram syndrome (HOS) is a rare genetic disorder characterized by upper limb abnormalities, congenital heart defects, and/or conduction abnormalities. Sequence alteration of T-box transcription factor 5 (TBX5) is correlated with the incidence of HOS. Case description: We present the case of a 24-year-old female with upper limb alterations (congenital dysplasia in the wrist and elbow joints) and an anomalous left main trunk arising from the right coronary sinus. The patient inherited a base T (reference C) at rs883079 from her mother and base C (reference T) at rs10850326 from her father, both of which belong to the 3'-untranslated region (UTR) of the TBX5 gene; no alterations in TBX5 expression or single-nucleotide polymorphisms (SNPs) in other exon areas were found. We explored the effects of TBX5 on cardiomyocytes using the HL-1 cell line and TBX5-knockdown cells. Discussion: Quantitative polymerase chain reaction analysis demonstrated that TEKT2, TEKT4, and SPTB expression decreased after TBX5 knockdown, while chromatin immunoprecipitation analysis further revealed that TBX5 binds to the TEKT2, TEKT4, and SPTB promoter regions to promote gene transcription. Our findings support a novel TBX5-related pathogenic mechanism in HOS.

Zfhx3 Transcription Factor Represses the Expression of SCN5A Gene and Decreases Sodium Current Density (INa)

The ZFHX3 and SCN5A genes encode the zinc finger homeobox 3 (Zfhx3) transcription factor (TF) and the human cardiac Na⁺ channel (Nav1.5), respectively. The effects of Zfhx3 on the expression of the Nav1.5 channel, and in cardiac excitability, are currently unknown. Additionally, we identified three Zfhx3 variants in probands diagnosed with familial atrial fibrillation (p.M1260T) and Brugada Syndrome (p.V949I and p.Q2564R). Here, we analyzed the effects of native (WT) and mutated Zfhx3 on Na⁺ current (I_Na) recorded in HL-1 cardiomyocytes. ZFHX3 mRNA can be detected in human atrial and ventricular samples. In HL-1 cardiomyocytes, transfection of Zfhx3 strongly reduced peak I_Na density, while the silencing of endogenous expression augmented it (from −65.9 ± 8.9 to −104.6 ± 10.8 pA/pF; n ≥ 8, p < 0.05). Zfhx3 significantly reduced the transcriptional activity of human SCN5A, PITX2, TBX5, and NKX25 minimal promoters. Consequently, the mRNA and/or protein expression levels of Nav1.5 and Tbx5 were diminished (n ≥ 6, p < 0.05). Zfhx3 also increased the expression of Nedd4-2 ubiquitin-protein ligase, enhancing Nav1.5 proteasomal degradation. p.V949I, p.M1260T, and p.Q2564R Zfhx3 produced similar effects on I_Na density and time- and voltage-dependent properties in WT. WT Zfhx3 inhibits I_Na as a result of a direct repressor effect on the SCN5A promoter, the modulation of Tbx5 increasing on the I_Na, and the increased expression of Nedd4-2. We propose that this TF participates in the control of cardiac excitability in human adult cardiac tissue.

Hedgehog signalling controls sinoatrial node development and atrioventricular cushion formation

Smoothened is a key receptor of the hedgehog pathway, but the roles of Smoothened in cardiac development remain incompletely understood. In this study, we found that the conditional knockout of Smoothened from the mesoderm impaired the development of the venous pole of the heart and resulted in hypoplasia of the atrium/inflow tract (IFT) and a low heart rate. The blockage of Smoothened led to reduced expression of genes critical for sinoatrial node (SAN) development in the IFT. In a cardiac cell culture model, we identified a Gli2–Tbx5–Hcn4 pathway that controls SAN development. In the mutant embryos, the endocardial-to-mesenchymal transition (EndMT) in the atrioventricular cushion failed, and Bmp signalling was downregulated. The addition of Bmp2 rescued the EndMT in mutant explant cultures. Furthermore, we analysed Gli2⁺ scRNAseq and Tbx5^−/− RNAseq data and explored the potential genes downstream of hedgehog signalling in posterior second heart field derivatives. In conclusion, our study reveals that Smoothened-mediated hedgehog signalling controls posterior cardiac progenitor commitment, which suggests that the mutation of Smoothened might be involved in the aetiology of congenital heart diseases related to the cardiac conduction system and heart valves.

Individual Limb Muscle Bundles Are Formed through Progressive Steps Orchestrated by Adjacent Connective Tissue Cells during Primary Myogenesis

Although the factors regulating muscle cell differentiation are well described, we know very little about how differentiating muscle fibers are organized into individual muscle tissue bundles. Disruption of these processes leads to muscle hypoplasia or dysplasia, and replicating these events is vital in tissue engineering approaches. We describe the progressive cellular events that orchestrate the formation of individual limb muscle bundles and directly demonstrate the role of the connective tissue cells that surround muscle precursors in controlling these events. We show how disruption of gene activity within or genetic ablation of connective tissue cells impacts muscle precursors causing disruption of muscle bundle formation and subsequent muscle dysplasia and hypoplasia. We identify several markers of the populations of connective tissue cells that surround muscle precursors and provide a model for how matrix-modifying proteoglycans secreted by these cells may influence muscle bundle formation by effects on the local extracellular matrix (ECM) environment.

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Characterization of the events that prefigure the formation of individual muscle bundles

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Direct demonstration of the role of connective tissue cells in muscle morphogenesis

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Identification of markers of limb irregular connective tissue (ICT)

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Demonstration of molecularly distinct ICT subdomains in the limb

Tbx5 variants disrupt Nav1.5 function differently in patients diagnosed with Brugada or Long QT Syndrome

AIMS

The transcription factor Tbx5 controls cardiogenesis and drives Scn5a expression in mice. We have identified two variants in TBX5 encoding p.D111Y and p.F206L Tbx5, respectively, in two unrelated patients with structurally normal hearts diagnosed with Long QT (LQTS) and Brugada (BrS) Syndrome. Here we characterized the consequences of each variant to unravel the underlying disease mechanisms.

METHODS AND RESULTS

We combined clinical analysis with in vivo and in vitro electrophysiological and molecular techniques in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), HL-1 cells, and cardiomyocytes from mice trans-expressing human wildtype (WT) or mutant proteins. Tbx5 increased transcription of SCN5A encoding cardiac Nav1.5 channels, while repressing CAMK2D and SPTBN4 genes encoding Ca-calmodulin kinase IIδ (CaMKIIδ) and βIV-spectrin, respectively. These effects significantly increased Na current (INa) in hiPSC-CMs and in cardiomyocytes from mice trans-expressing Tbx5. Consequently, action potential (AP) amplitudes increased and QRS interval narrowed in the mouse electrocardiogram. p.F206L Tbx5 bound to the SCN5A promoter failed to transactivate it, thus precluding the pro-transcriptional effect of WT Tbx5. Therefore, p.F206L markedly decreased INa in hiPSC-CM, HL-1 cells, and mouse cardiomyocytes. The INa decrease in p.F206L trans-expressing mice translated into QRS widening and increased flecainide sensitivity. p.D111Y Tbx5 increased SCN5A expression but failed to repress CAMK2D and SPTBN4. The increased CaMKIIδ and βIV-spectrin significantly augmented the late component of INa (INaL) which, in turn, significantly prolonged AP duration in both hiPSC-CMs and mouse cardiomyocytes. Ranolazine, a selective INaL inhibitor, eliminated the QT and QTc intervals prolongation seen in p.D111Y trans-expressing mice.

CONCLUSIONS

In addition to peak INa, Tbx5 critically regulates INaL and the duration of repolarization in human cardiomyocytes. Our original results suggest that TBX5 variants associate with and modulate the intensity of the electrical phenotype in LQTS and BrS patients.

Dorsoventral inversion of the air-filled organ (lungs, gas bladder) in vertebrates: RNAsequencing of laser capture microdissected embryonic tissue

How modification of gene expression generates novel traits is key to understanding the evolutionary process. We investigated the genetic basis for the origin of the piscine gas bladder from lungs of ancestral bony vertebrates. Distinguishing these homologous organs is the direction of budding from the foregut during development; lungs bud ventrally and the gas bladder buds dorsally.

Transcriptional Patterning of the Ventricular Cardiac Conduction System

RATIONALE

The heartbeat is organized by the cardiac conduction system (CCS), a specialized network of cardiomyocytes. Patterning of the CCS into atrial node versus ventricular conduction system (VCS) components with distinct physiology is essential for the normal heartbeat. Distinct node versus VCS physiology has been recognized for more than a century, but the molecular basis of this regional patterning is not well understood.

OBJECTIVE

To study the genetic and genomic mechanisms underlying node versus VCS distinction and investigate rhythm consequences of failed VCS patterning.

METHODS AND RESULTS

Using mouse genetics, we found that the balance between T-box transcriptional activator, Tbx5, and T-box transcriptional repressor, Tbx3, determined the molecular and functional output of VCS myocytes. Adult VCS-specific removal of Tbx5 or overexpression of Tbx3 re-patterned the fast VCS into slow, nodal-like cells based on molecular and functional criteria. In these cases, gene expression profiling showed diminished expression of genes required for VCS-specific fast conduction but maintenance of expression of genes required for nodal slow conduction physiology. Action potentials of Tbx5-deficient VCS myocytes adopted nodal-specific characteristics, including increased action potential duration and cellular automaticity. Removal of Tbx5 in vivo precipitated inappropriate depolarizations in the atrioventricular (His)-bundle associated with lethal ventricular arrhythmias. TBX5 bound and directly activated cis-regulatory elements at fast conduction channel genes required for fast physiological characteristics of the VCS action potential, defining the identity of the adult VCS.

CONCLUSIONS

The CCS is patterned entirely as a slow, nodal ground state, with a T-box dependent, physiologically dominant, fast conduction network driven specifically in the VCS. Disruption of the fast VCS gene regulatory network allowed nodal physiology to emerge, providing a plausible molecular mechanism for some lethal ventricular arrhythmias.

The Role of Retinoic Acid in Establishing the Early Limb Bud

Retinoic acid (RA) was one of the first molecules in the modern era of experimental embryology to be shown capable of generating profound effects on limb development. In this review, we focus on the earliest events of limb development and specifically on the role of RA in establishing the domain of cells that will go on to form the limb itself. Although there is some consensus on the role of RA during the earliest stages of limb formation, some controversy remains on the mechanism of RA action and the requirement for RA signaling in forming the hindlimb buds.

Genomic Knockout of Two Presumed Forelimb Tbx5 Enhancers Reveals They Are Nonessential for Limb Development

A standard approach in the identification of transcriptional enhancers is the use of transgenic animals carrying DNA elements joined to reporter genes inserted randomly in the genome. We examined elements near Tbx5, a gene required for forelimb development in humans and other vertebrates. Previous transgenic studies reported a mammalian Tbx5 fore-limb enhancer located in intron 2 containing a putative retinoic acid response element and a zebrafish tbx5a forelimb (pectoral fin) enhancer located downstream that is conserved from fish to mammals. We used CRISPR/Cas9 gene editing to knockout the endogenous elements and unexpectedly found that deletion of the intron 2 and downstream elements, either singly or together in double knockouts, resulted in no effect on forelimb development. Our findings show that reporter transgenes may not identify endogenous enhancers and that in vivo genetic loss-of-function studies are required, such as CRISPR/Cas9, which is similar in effort to production of animals carrying reporter transgenes.

Cardiac Reprogramming Factors Synergistically Activate Genome-wide Cardiogenic Stage-Specific Enhancers

The cardiogenic transcription factors (TFs) Mef2c, Gata4, and Tbx5 can directly reprogram fibroblasts to induced cardiac-like myocytes (iCLMs), presenting a potential source of cells for cardiac repair. While activity of these TFs is enhanced by Hand2 and Akt1, their genomic targets and interactions during reprogramming are not well studied. We performed genome-wide analyses of cardiogenic TF binding and enhancer profiling during cardiac reprogramming. We found that these TFs synergistically activate enhancers highlighted by Mef2c binding sites and that Hand2 and Akt1 coordinately recruit other TFs to enhancer elements. Intriguingly, these enhancer landscapes collectively resemble patterns of enhancer activation during embryonic cardiogenesis. We further constructed a cardiac reprogramming gene regulatory network and found repression of EGFR signaling pathway genes. Consistently, chemical inhibition of EGFR signaling augmented reprogramming. Thus, by defining epigenetic landscapes these findings reveal synergistic transcriptional activation across a broad landscape of cardiac enhancers and key signaling pathways that govern iCLM reprogramming.

A calcium transport mechanism for atrial fibrillation in Tbx5-mutant mice

Risk for Atrial Fibrillation (AF), the most common human arrhythmia, has a major genetic component. The T-box transcription factor TBX5 influences human AF risk, and adult-specific Tbx5-mutant mice demonstrate spontaneous AF. We report that TBX5 is critical for cellular Ca²⁺ homeostasis, providing a molecular mechanism underlying the genetic implication of TBX5 in AF. We show that cardiomyocyte action potential (AP) abnormalities in Tbx5-deficient atrial cardiomyocytes are caused by a decreased sarcoplasmic reticulum (SR) Ca²⁺ ATPase (SERCA2)-mediated SR calcium uptake which was balanced by enhanced trans-sarcolemmal calcium fluxes (calcium current and sodium/calcium exchanger), providing mechanisms for triggered activity. The AP defects, cardiomyocyte ectopy, and AF caused by TBX5 deficiency were rescued by phospholamban removal, which normalized SERCA function. These results directly link transcriptional control of SERCA2 activity, depressed SR Ca²⁺ sequestration, enhanced trans-sarcolemmal calcium fluxes, and AF, establishing a mechanism underlying the genetic basis for a Ca²⁺-dependent pathway for AF risk.

The human heart contains four distinct chambers that work together to pump blood around the body. In individuals with a condition called atrial fibrillation, two of the chambers (known as the atria) beat irregularly and are unable to push all the blood they hold into the other two chambers of the heart. This can cause heart failure and increases the likelihood of blood clots, which may lead to stroke and heart attacks.

Small molecules called calcium ions play a crucial role in regulating how and when the atria contract by driving electrical activity in heart cells. To contract the atria, a storage compartment within heart cells known as the sarcoplasmic reticulum releases calcium ions into the main compartment of the cells. Calcium ions also enter the cell from the surrounding tissue. As the atria relax, calcium ions are pumped back into the sarcoplasmic reticulum or out of the cell by specific transport proteins.

Individuals with mutations in a gene called Tbx5 are more likely to develop atrial fibrillation than other people, but it was not clear how such gene mutations contribute to the disease. Here, Dai, Laforest et al. used mice with a mutation in the Tbx5 gene to study how defects in Tbx5 affect electrical activity in heart cells.

The experiments found that the Tbx5 gene was critical for calcium ions to drive normal electrical activity in mouse heart cells. Compared with heart cells from normal mice, the heart cells from the mutant mice had decreased flow of calcium ions into the sarcoplasmic reticulum and increased flow of calcium ions out of the cell.

These findings provide a direct link between atrial fibrillation and the flow of calcium ions in heart cells. Together with previous work, these findings indicate that multiple different mechanisms could lead to atrial fibrillation, but that many of these involve changes in the flow of calcium ions. Therefore, personalized medicine, where clinicians uncover the specific mechanisms responsible for atrial fibrillation in individual patients, may play an important role in treating this condition in the future.

Timed Collinear Activation of Hox Genes during Gastrulation Controls the Avian Forelimb Position

Limb position along the body is highly consistent within one species but very variable among vertebrates. Despite major advances in our understanding of limb patterning in three dimensions, how limbs reproducibly form along the antero-posterior axis remains largely unknown. Hox genes have long been suspected to control limb position; however, supporting evidences are mostly correlative and their role in this process is unclear. Here, we show that limb position is determined early in development through the action of Hox genes. Dynamic lineage analysis revealed that, during gastrulation, the forelimb, interlimb, and hindlimb fields are progressively generated and concomitantly patterned by the collinear activation of Hox genes in a two-step process. First, the sequential activation of Hoxb genes controls the relative position of their own collinear domains of expression in the forming lateral plate mesoderm, as demonstrated by functional perturbations during gastrulation. Then, within these collinear domains, we show that Hoxb4 anteriorly and Hox9 genes posteriorly, respectively, activate and repress the expression of the forelimb initiation gene Tbx5 and instruct the definitive position of the forelimb. Furthermore, by comparing the dynamics of Hoxb genes activation during zebra finch, chicken, and ostrich gastrulation, we provide evidences that changes in the timing of collinear Hox gene activation might underlie natural variation in forelimb position between different birds. Altogether, our results that characterize the cellular and molecular mechanisms underlying the regulation and natural variation of forelimb positioning in avians show a direct and early role for Hox genes in this process.

Regulatory integration of Hox factor activity with T-box factors in limb development

In tetrapods, Tbx4, Tbx5 and Hox cluster genes are crucial for forelimb and hindlimb development and mutations in these genes are responsible for congenital limb defects. The molecular basis of their integrated mechanisms of action in the context of limb development remains poorly understood. We studied Tbx4 and Hoxc10 owing to their overlapping loss-of-function phenotypes and colocalized expression in mouse hindlimb buds. We report an extensive overlap between Tbx4 and Hoxc10 genome occupancy and their putative target genes. Tbx4 and Hoxc10 interact directly with each other, have the ability to bind to a previously unrecognized T-box-Hox composite DNA motif and show synergistic activity when acting on reporter genes. Pitx1, the master regulator for hindlimb specification, also shows extensive genomic colocalization with Tbx4 and Hoxc10. Genome occupancy by Tbx4 in hindlimb buds is similar to Tbx5 occupancy in forelimbs. By contrast, another Hox factor, Hoxd13, also interacts with Tbx4/Tbx5 but antagonizes Tbx4/Tbx5-dependent transcriptional activity. Collectively, the modulation of Tbx-dependent activity by Hox factors acting on common DNA targets may integrate different developmental processes for the balanced formation of proportionate limbs.

Transcription-factor-dependent enhancer transcription defines a gene regulatory network for cardiac rhythm

The noncoding genome is pervasively transcribed. Noncoding RNAs (ncRNAs) generated from enhancers have been proposed as a general facet of enhancer function and some have been shown to be required for enhancer activity. Here we examine the transcription-factor-(TF)-dependence of ncRNA expression to define enhancers and enhancer-associated ncRNAs that are involved in a TF-dependent regulatory network. TBX5, a cardiac TF, regulates a network of cardiac channel genes to maintain cardiac rhythm. We deep sequenced wildtype and Tbx5-mutant mouse atria, identifying ~2600 novel Tbx5-dependent ncRNAs. Tbx5-dependent ncRNAs were enriched for tissue-specific marks of active enhancers genome-wide. Tbx5-dependent ncRNAs emanated from regions that are enriched for TBX5-binding and that demonstrated Tbx5-dependent enhancer activity. Tbx5-dependent ncRNA transcription provided a quantitative metric of Tbx5-dependent enhancer activity, correlating with target gene expression. We identified RACER, a novel Tbx5-dependent long noncoding RNA (lncRNA) required for the expression of the calcium-handling gene Ryr2. We illustrate that TF-dependent enhancer transcription can illuminate components of TF-dependent gene regulatory networks.

Developmental toxicity and cardiac effects of butyl benzyl phthalate in zebrafish embryos

Phthalic acid esters (PAEs), commonly called phthalates, have become ubiquitous environment pollutants. Studies have focused on reproductive toxicity, neurotoxicity, teratogenicity, tumourigenesis, and mutagenesis of phthalates. However, relatively little is known about the phthalates effects on the heart. Butyl benzyl phthalate (BBP), a member of PAEs, is classified by the US Environmental Protection Agency as a priority environmental pollutant. We studied the developmental toxicity of BBP, especially its effects on the heart development, in zebrafish (Danio rerio) embryos. Embryos at 4hr post-fertilization (hpf) were exposed to 0, 0.1, 0.6 and 1.2mg/L BBP until 72hpf. BBP caused abnormalities in embryo morphology, including yolk-sac edema, spinal curvature, tail deformity, uninflated swim bladder and cardiac defects. Exposure to 0.6mg/L BBP significantly increased the malformation rate, caused growth inhibition, increased the cardiac malformation rate as well as the distance between the sinus venosus (SV) and bulbus arteriosus (BA), and reduced the heart rate of embryos. Exposure to 1.2mg/L BBP significantly affected all endpoints, except survival rate at 24hpf. To preliminarily elucidate the potential mechanism of heart developmental toxicity caused by BBP, we examined the expression of two genes related to heart development, Nkx2.5 and T-box transcription factor 5, by real-time quantitative PCR. The expression of the two genes was dose-dependently downregulated with BBP. BBP could induce developmental toxicity, with adverse effects on the heart development in zebrafish embryos, and alter the expression of genes related to heart development.

Pitx1 directly modulates the core limb development program to implement hindlimb identity

Forelimbs (FLs) and hindlimbs (HLs) develop complex musculoskeletal structures that rely on the deployment of a conserved developmental program. Pitx1, a transcription factor gene with expression restricted to HL and absent from FL, plays an important role in generating HL features. The genomic mechanisms by which Pitx1 effects HL identity remain poorly understood. Here, we use expression profiling and analysis of direct Pitx1 targets to characterize the HL- and FL-restricted genetic programs in mouse and situate the Pitx1-dependent gene network within the context of limb-specific gene regulation. We show that Pitx1 is a crucial component of a narrow network of HL-restricted regulators, acting on a developmental program that is shared between FL and HL. Pitx1 targets sites that are in a similar chromatin state in FL and HL and controls expression of patterning genes as well as the chondrogenic program, consistent with impaired chondrogenesis in Pitx1^-/- HL. These findings support a model in which multifactorial actions of a limited number of HL regulators redirect the generic limb development program in order to generate the unique structural features of the limb.

Developmental Mechanism of Limb Field Specification along the Anterior-Posterior Axis during Vertebrate Evolution

In gnathostomes, limb buds arise from the lateral plate mesoderm at discrete positions along the body axis. Specification of these limb-forming fields can be subdivided into several steps. The lateral plate mesoderm is regionalized into the anterior lateral plate mesoderm (ALPM; cardiac mesoderm) and the posterior lateral plate mesoderm (PLPM). Subsequently, Hox genes appear in a nested fashion in the PLPM and provide positional information along the body axis. The lateral plate mesoderm then splits into the somatic and splanchnic layers. In the somatic layer of the PLPM, the expression of limb initiation genes appears in the limb-forming region, leading to limb bud initiation. Furthermore, past and current work in limbless amphioxus and lampreys suggests that evolutionary changes in developmental programs occurred during the acquisition of paired fins during vertebrate evolution. This review presents these recent advances and discusses the mechanisms of limb field specification during development and evolution, with a focus on the role of Hox genes in this process.

Bmi1 Is a Key Epigenetic Barrier to Direct Cardiac Reprogramming

Direct reprogramming of induced cardiomyocytes (iCMs) suffers from low efficiency and requires extensive epigenetic repatterning, although the underlying mechanisms are largely unknown. To address these issues, we screened for epigenetic regulators of iCM reprogramming and found that reducing levels of the polycomb complex gene Bmi1 significantly enhanced induction of beating iCMs from neonatal and adult mouse fibroblasts. The inhibitory role of Bmi1 in iCM reprogramming is mediated through direct interactions with regulatory regions of cardiogenic genes, rather than regulation of cell proliferation. Reduced Bmi1 expression corresponded with increased levels of the active histone mark H3K4me3 and reduced levels of repressive H2AK119ub at cardiogenic loci, and de-repression of cardiogenic gene expression during iCM conversion. Furthermore, Bmi1 deletion could substitute for Gata4 during iCM reprogramming. Thus, Bmi1 acts as a critical epigenetic barrier to iCM production. Bypassing this barrier simplifies iCM generation and increases yield, potentially streamlining iCM production for therapeutic purposes.

Tbx5 and Osr1 interact to regulate posterior second heart field cell cycle progression for cardiac septation

RATIONALE

Mutations of TBX5 cause Holt-Oram syndrome (HOS) in humans, a disease characterized by atrial or occasionally ventricular septal defects in the heart and skeletal abnormalities of the upper extremity. Previous studies have demonstrated that Tbx5 regulates Osr1 expression in the second heart field (SHF) of E9.5 mouse embryos. However, it is unknown whether and how Tbx5 and Osr1 interact in atrial septation.

OBJECTIVE

To determine if and how Tbx5 and Osr1 interact in the posterior SHF for cardiac septation.

METHODS AND RESULTS

In the present study, genetic inducible fate mapping showed that Osr1-expressing cells contribute to atrial septum progenitors between E8.0 and E11.0. Osr1 expression in the pSHF was dependent on the level of Tbx5 at E8.5 and E9.5 but not E10.5, suggesting that the embryo stage before E10.5 is critical for Tbx5 interacting with Osr1 in atrial septation. Significantly more atrioventricular septal defects (AVSDs) were observed in embryos with compound haploinsufficiency for Tbx5 and Osr1. Conditional compound haploinsufficiency for Tbx5 and Osr1 resulted in a significant cell proliferation defect in the SHF, which was associated with fewer cells in the G2 and M phases and a decreased level of Cdk6 expression. Remarkably, genetically targeted disruption of Pten expression in atrial septum progenitors rescued AVSDs caused by Tbx5 and Osr1 compound haploinsufficiency. There was a significant decrease in Smo expression, which is a Hedgehog (Hh) signaling pathway modulator, in the pSHF of Osr1 knockout embryos at E9.5, implying a role for Osr1 in regulating Hh signaling.

CONCLUSIONS

Tbx5 and Osr1 interact to regulate posterior SHF cell cycle progression for cardiac septation.

Fbxo25 controls Tbx5 and Nkx2-5 transcriptional activity to regulate cardiomyocyte development

The ubiquitin-proteasome system (UPS) plays an important role in protein quality control, cellular signalings, and cell differentiation through the regulated turnover of key transcription factors in cardiac tissue. However, the molecular mechanism underlying Fbxo25-mediated ubiquitination of cardiac transcription factors remains elusive. We report that an Fbxo25-mediated SCF ubiquitination pathway regulates the protein levels and activities of Tbx5 and Nkx2-5 based on our studies using MG132, proteasome inhibitor, and the temperature sensitive ubiquitin system in ts20 cells. Our data indicate that Fbxo25 directly interacts with Tbx5 and Nkx2-5 in vitro and in vivo. In support of our findings, a dominant-negative mutant of Fbxo25, Fbxo251-236, prevents Tbx5 degradation and increases Tbx5 transcriptional activity in a Tbx5 responsive luciferase assay. Therefore, Fbxo25 facilitates Tbx5 degradation in an SCF-dependent manner. In addition, the silencing of endogenous Fbxo25 increases Tbx5 and Nkx2-5 mRNA levels and suppresses mESC-derived cardiomyocyte differentiation. Likewise, the exogenous expression of FBXO25 downregulates NKX2-5 level in human ESC-derived cardiomyocytes. In myocardial infarction model, Fbxo25 mRNA decreases, whereas the mRNA and protein levels of Tbx5 and Nkx2-5 increase. The protein levels of Tbx5 and Nkx2-5 are regulated negatively by Fbxo25-mediated SCF ubiquitination pathway. Thus, our findings reveal a novel mechanism for regulation of SCFFbox25-dependent Nkx2-5 and Tbx5 ubiquitination in cardiac development and provide a new insight into the regulatory mechanism of Nkx2-5 and Tbx5 transcriptional activity.

Regulatory modulation of the T-box gene Tbx5 links development, evolution, and adaptation of the sternum

The sternum bone lies at the ventral midline of the thorax where it provides a critical attachment for the pectoral muscles that allow the forelimbs to raise the body from the ground. Among tetrapods, sternum morphology is correlated with the mode of locomotion: Avians that fly have a ventral extension, or keel, on their sterna, which provides an increased area for flight muscle attachment. The sternum is fused with the ribs attaching on either side; however, unlike the ribs, the sternal precursors do not originate from the somites. Despite the crucial role of the sternum in tetrapod locomotion, little attention has been given to its acquisition, evolution, and embryological development. We demonstrate an essential role for the T-box transcription factor gene Tbx5 in sternum and forelimb formation and show that both structures share an embryological origin within the lateral plate mesoderm. Consistent with this shared origin and role of Tbx5, sternum defects are a characteristic feature of Holt-Oram Syndrome (OMIM 142900) caused by mutations in TBX5. We demonstrate a link between sternum size and forelimb use across avians and provide evidence that modulation of Tbx5 expression underlies the reduction in sternum and wing size in a flightless bird, the emu. We demonstrate that Tbx5 is a common node in the genetic pathways regulating forelimb and sternum development, enabling specific adaptations of these features without affecting other skeletal elements and can also explain the linked adaptation of sternum and forelimb morphology correlated with mode of locomotion.

Tbx5 is required for avian and Mammalian epicardial formation and coronary vasculogenesis

RATIONALE

Holt-Oram syndrome is an autosomal dominant heart-hand syndrome caused by mutations in the TBX5 gene. Overexpression of Tbx5 in the chick proepicardial organ impaired coronary blood vessel formation. However, the potential activity of Tbx5 in the epicardium itself, and the role of Tbx5 in mammalian coronary vasculogenesis, remains largely unknown.

OBJECTIVE

To evaluate the consequences of altered Tbx5 gene dosage during proepicardial organ and epicardial development in the embryonic chick and mouse.

METHODS AND RESULTS

Retroviral-mediated knockdown or upregulation of Tbx5 expression in the embryonic chick proepicardial organ and proepicardial-specific deletion of Tbx5 in the embryonic mouse (Tbx5(epi-/)) impaired normal proepicardial organ cell development, inhibited epicardial and coronary blood vessel formation, and altered developmental gene expression. The generation of epicardial-derived cells and their migration into the myocardium were impaired between embryonic day (E) 13.5 to 15.5 in mutant hearts because of delayed epicardial attachment to the myocardium and subepicardial accumulation of epicardial-derived cells. This caused defective coronary vasculogenesis associated with impaired vascular smooth muscle cell recruitment and reduced invasion of cardiac fibroblasts and endothelial cells into myocardium. In contrast to wild-type hearts that exhibited an elaborate ventricular vascular network, Tbx5(epi-/-) hearts displayed a marked decrease in vascular density that was associated with myocardial hypoxia as exemplified by hypoxia inducible factor-1α upregulation and increased binding of hypoxyprobe-1. Tbx5(epi-/-) mice with such myocardial hypoxia exhibited reduced exercise capacity when compared with wild-type mice.

CONCLUSIONS

Our findings support a conserved Tbx5 dose-dependent requirement for both proepicardial and epicardial progenitor cell development in chick and in mouse coronary vascular formation.