A GATA-dependent right ventricular enhancer controls dHAND transcription in the developing heart

Oxidative damage and cardiotoxicity induced by 2-aminobenzothiazole in zebrafish (Danio rerio)

There is limited information available on cardiovascular toxicity of 2-Aminobenzothiazole (NTH), a derivative of benzothiazole (BTH) commonly used in tire production, in aquatic organisms. In the present study, the zebrafish embryos were exposed to varying concentrations of NTH (0, 0.05, 0.5, and 5 mg/L) until adulthood and the potential cardiovascular toxicity was assessed. NTH exposure resulted in striking aberrations in cardiac development, including heart looping failure and interference with atrioventricular canal differentiation. RNA-sequencing analysis indicated that NTH causes oxidative damage to the heart via ferroptosis, leading to oxygen supply disruption, cardiac malformation, and ultimately, zebrafish death. Quantitative real-time polymerase chain reaction (qPCR) analysis demonstrated the dysregulation of genes associated with early heart development, contraction, and oxidative stress. Additionally, reactive oxygen species accumulation and glutathione/malondialdehyde levels changes suggested a potential link between cardiac developmental toxicity and oxidative stress. In adult zebrafish, NTH exposure led to ventricular enlargement, decreased heart rate, reduced blood flow, and prolonged RR, QRS, and QTc intervals. To the best of our knowledge, this study is the first to provide evidence of cardiac toxicity and the adverse effects of ontogenetic NTH exposure in zebrafish, revealing the underlying toxic mechanisms connected with oxidative stress damage. These findings may provide crucial insights into the environmental risks associated with NTH and other BTHs.

Epigenetic Regulation of Mammalian Cardiomyocyte Development

The heart is the first organ formed during mammalian development and functions to distribute nutrients and oxygen to other parts of the developing embryo. Cardiomyocytes are the major cell types of the heart and provide both structural support and contractile function to the heart. The successful differentiation of cardiomyocytes during early development is under tight regulation by physical and molecular factors. We have reviewed current studies on epigenetic factors critical for cardiomyocyte differentiation, including DNA methylation, histone modifications, chromatin remodelers, and noncoding RNAs. This review also provides comprehensive details on structural and morphological changes associated with the differentiation of fetal and postnatal cardiomyocytes and highlights their differences. A holistic understanding of all aspects of cardiomyocyte development is critical for the successful in vitro differentiation of cardiomyocytes for therapeutic purposes.

Stromal Pbrm1 mediates chromatin remodeling necessary for embryo implantation in the mouse uterus

Early gestational loss occurs in approximately 20% of all clinically recognized human pregnancies and is an important cause of morbidity. Either embryonic or maternal defects can cause loss, but a functioning and receptive uterine endometrium is crucial for embryo implantation. We report that the switch/sucrose nonfermentable (SWI/SNF) remodeling complex containing polybromo-1 (PBRM1) and Brahma-related gene 1 (BRG1) is essential for implantation of the embryonic blastocyst on the wall of the uterus in mice. Although preimplantation development is unaffected, conditional ablation of Pbrm1 in uterine stromal cells disrupts progesterone pathways and uterine receptivity. Heart and neural crest derivatives expressed 2 (Hand2) encodes a basic helix-loop-helix (bHLH) transcription factor required for embryo implantation. We identify an enhancer of the Hand2 gene in stromal cells that requires PBRM1 for epigenetic histone modifications/coactivator recruitment and looping with the promoter. In Pbrm1cKO mice, perturbation of chromatin assembly at the promoter and enhancer sites compromises Hand2 transcription, adversely affects fibroblast growth factor signaling pathways, prevents normal stromal-epithelial crosstalk, and disrupts embryo implantation. The mutant female mice are infertile and provide insight into potential causes of early pregnancy loss in humans.

Identification and characterization of Hand2 upstream genomic enhancers active in developing stomach and limbs

BACKGROUND

The bHLH transcription factor HAND2 plays important roles in the development of the embryonic heart, face, limbs, and sympathetic and enteric nervous systems. To define how and when HAND2 regulates these developmental systems, requires understanding the transcriptional regulation of Hand2.

RESULTS

Remarkably, Hand2 is flanked by an extensive upstream gene desert containing a potentially diverse enhancer landscape. Here, we screened the regulatory interval 200 kb proximal to Hand2 for putative enhancers using evolutionary conservation and histone marks in Hand2-expressing tissues. H3K27ac signatures across embryonic tissues pointed to only two putative enhancer regions showing deep sequence conservation. Assessment of the transcriptional enhancer potential of these elements using transgenic reporter lines uncovered distinct in vivo enhancer activities in embryonic stomach and limb mesenchyme, respectively. Activity of the identified stomach enhancer was restricted to the developing antrum and showed expression within the smooth muscle and enteric neurons. Surprisingly, the activity pattern of the limb enhancer did not overlap Hand2 mRNA but consistently yielded a defined subectodermal anterior expression pattern within multiple transgenic lines.

CONCLUSIONS

Together, these results start to uncover the diverse regulatory potential inherent to the Hand2 upstream regulatory interval.

Modeling of large-scale hoxbb cluster deletions in zebrafish uncovers a role for segmentation pathways in atrioventricular boundary specification

Hox genes orchestrate the segmental specification of the muscular circulatory system in invertebrates but it has not proven straightforward to decipher segmental parallels in the vertebrate heart. Recently, patients with HOXB gene cluster deletion were found to exhibit abnormalities including atrioventricular canal defects. Using CRISPR, we established a mutant with the orthologous hoxbb cluster deletion in zebrafish. The mutant exhibited heart failure and atrioventricular regurgitation at 5 days. Analyzing the four genes in the hoxbb cluster, isolated deletion of hoxb1b-/- recapitulated the cardiac abnormalities, supporting hoxb1b as the causal gene. Both in situ and in vitro data indicated that hoxb1b regulates gata5 to inhibit hand2 expression and ultimately is required to pattern the vertebrate atrioventricular boundary. Together, these data reveal a role for segmental specification in vertebrate cardiac development and highlight the utility of CRISPR techniques for efficiently exploring the function of large structural genomic lesions.

Single-cell profiling of the developing embryonic heart in Drosophila

Drosophila is an important model for studying heart development and disease. Yet, single-cell transcriptomic data of its developing heart have not been performed. Here, we report single-cell profiling of the entire fly heart using ∼3000 Hand-GFP embryos collected at five consecutive developmental stages, ranging from bilateral migrating rows of cardiac progenitors to a fused heart tube. The data revealed six distinct cardiac cell types in the embryonic fly heart: cardioblasts, both Svp+ and Tin+ subtypes; and five types of pericardial cell (PC) that can be distinguished by four key transcription factors (Eve, Odd, Ct and Tin) and include the newly described end of the line PC. Notably, the embryonic fly heart combines transcriptional signatures of the mammalian first and second heart fields. Using unique markers for each heart cell type, we defined their number and location during heart development to build a comprehensive 3D cell map. These data provide a resource to track the expression of any gene in the developing fly heart, which can serve as a reference to study genetic perturbations and cardiac diseases.

Genomic approaches to identify and investigate genes associated with atrial fibrillation and heart failure susceptibility

Atrial fibrillation (AF) and heart failure (HF) contribute to about 45% of all cardiovascular disease (CVD) deaths in the USA and around the globe. Due to the complex nature, progression, inherent genetic makeup, and heterogeneity of CVDs, personalized treatments are believed to be critical. To improve the deciphering of CVD mechanisms, we need to deeply investigate well-known and identify novel genes that are responsible for CVD development. With the advancements in sequencing technologies, genomic data have been generated at an unprecedented pace to foster translational research. Correct application of bioinformatics using genomic data holds the potential to reveal the genetic underpinnings of various health conditions. It can help in the identification of causal variants for AF, HF, and other CVDs by moving beyond the one-gene one-disease model through the integration of common and rare variant association, the expressed genome, and characterization of comorbidities and phenotypic traits derived from the clinical information. In this study, we examined and discussed variable genomic approaches investigating genes associated with AF, HF, and other CVDs. We collected, reviewed, and compared high-quality scientific literature published between 2009 and 2022 and accessible through PubMed/NCBI. While selecting relevant literature, we mainly focused on identifying genomic approaches involving the integration of genomic data; analysis of common and rare genetic variants; metadata and phenotypic details; and multi-ethnic studies including individuals from ethnic minorities, and European, Asian, and American ancestries. We found 190 genes associated with AF and 26 genes linked to HF. Seven genes had implications in both AF and HF, which are SYNPO2L, TTN, MTSS1, SCN5A, PITX2, KLHL3, and AGAP5. We listed our conclusion, which include detailed information about genes and SNPs associated with AF and HF.

Neonatal Deletion of Hand1 and Hand2 within Murine Cardiac Conduction System Reveals a Novel Role for HAND2 in Rhythm Homeostasis

The cardiac conduction system, a network of specialized cells, is required for the functioning of the heart. The basic helix loop helix factors Hand1 and Hand2 are required for cardiac morphogenesis and have been implicated in cardiac conduction system development and maintenance. Here we use embryonic and post-natal specific Cre lines to interrogate the role of Hand1 and Hand2 in the function of the murine cardiac conduction system. Results demonstrate that loss of HAND1 in the post-natal conduction system does not result in any change in electrocardiogram parameters or within the ventricular conduction system as determined by optical voltage mapping. Deletion of Hand2 within the post-natal conduction system results in sex-dependent reduction in PR interval duration in these mice, suggesting a novel role for HAND2 in regulating the atrioventricular conduction. Surprisingly, results show that loss of both HAND factors within the post-natal conduction system does not cause any consistent changes in cardiac conduction system function. Deletion of Hand2 in the embryonic left ventricle results in inconsistent prolongation of PR interval and susceptibility to atrial arrhythmias. Thus, these results suggest a novel role for HAND2 in homeostasis of the murine cardiac conduction system and that HAND1 loss potentially rescues the shortened HAND2 PR phenotype.

The adult heart requires baseline expression of the transcription factor Hand2 to withstand RV pressure overload

AIMS

Research on the pathophysiology of right ventricular (RV) failure has, in spite of the associated high mortality and morbidity, lagged behind compared to the left ventricle (LV).Previous work from our lab revealed that the embryonic basic helix-loop-helix transcription factor heart and neural crest derivatives expressed-2 (Hand2) is re-expressed in the adult heart and activates a 'fetal gene program' contributing to pathological cardiac remodeling under conditions of LV pressure overload. As such, ablation of cardiac expression of Hand2 conferred protection to cardiac stress and abrogated the maladaptive effects that were observed upon increased expression levels. In this study, we aimed to understand the contribution of Hand2 to RV remodeling in response to pressure overload induced by pulmonary artery banding (PAB).

METHODS AND RESULTS

In the present study, Hand2F/F and MCM- Hand2F/F mice were treated with tamoxifen (control and knockout, respectively) and subjected to six weeks of RV pressure overload induced by PAB. Echocardiographic- and MRI-derived hemodynamic parameters as well as molecular remodeling were assessed for all experimental groups and compared to sham-operated controls. Six weeks after PAB, levels of Hand2 expression increased in the control banded animals but, as expected, remained absent in the knockout hearts. Despite the dramatic differences in Hand2 expression, pressure overload resulted in impaired cardiac function independently of the genotype. In fact, Hand2 depletion seems to sensitize the RV to pressure overload as these mice develop more hypertrophy and more severe cardiac dysfunction. Higher expression levels of HAND2 were also observed in RV samples of human hearts from patients with pulmonary hypertension. In turn, the LV of RV-pressure overloaded hearts was also dramatically affected as reflected by changes in shape, decreased LV mass and impaired cardiac function. RNA sequencing revealed a distinct set of genes that are dysregulated in the pressure-overloaded RV, compared to the previously described pressure-overloaded LV.

CONCLUSIONS

Cardiac-specific depletion of Hand2 is associated with severe cardiac dysfunction in conditions of RV pressure overload. While inhibiting Hand2 expression can prevent cardiac dysfunction in conditions of LV pressure overload, the same does not hold true for conditions of RV pressure overload. This study highlights the need to better understand the molecular mechanisms driving pathological remodeling of the RV in contrast to the LV, in order to better diagnose and treat patients with RV or LV failure.

TRANSLATIONAL PERSPECTIVE

RV failure associated with pulmonary hypertension reduces long-term survival rate to 55% within 3 years, suggesting that 3 years after diagnosis almost half of the patients will die. To revert these numbers an adequate RV-specific and, therefore, more efficient treatment is needed. Our work suggests that current therapies and potential mechanisms underlying LV failure may not be suitable for RV failure. While Hand2 deletion is favorable in LV response to stress, it is particularly detrimental in the RV under similar conditions, and thus, highlighting potential severe consequences of not differentiating therapeutic targets or treatment for RV or LV failure.

The evolutionary acquisition and mode of functions of promoter-associated non-coding RNAs (pancRNAs) for mammalian development

Increasing evidence has shown that many long non-coding RNAs (lncRNAs) are involved in gene regulation in a variety of ways such as transcriptional, post-transcriptional and epigenetic regulation. Promoter-associated non-coding RNAs (pancRNAs), which are categorized into the most abundant single-copy lncRNA biotype, play vital regulatory roles in finely tuning cellular specification at the epigenomic level. In short, pancRNAs can directly or indirectly regulate downstream genes to participate in the development of organisms in a cell-specific manner. In this review, we will introduce the evolutionarily acquired characteristics of pancRNAs as determined by comparative epigenomics and elaborate on the research progress on pancRNA-involving processes in mammalian embryonic development, including neural differentiation.

HAND transcription factors cooperatively specify the aorta and pulmonary trunk

Congenital heart defects (CHDs) affecting the cardiac outflow tract (OFT) constitute a significant cause of morbidity and mortality. The OFT develops from migratory cell populations which include the cardiac neural crest cells (cNCCs) and secondary heart field (SHF) derived myocardium and endocardium. The related transcription factors HAND1 and HAND2 have been implicated in human CHDs involving the OFT. Although Hand1 is expressed within the OFT, Hand1 NCC-specific conditional knockout mice (H1CKOs) are viable. Here we show that these H1CKOs present a low penetrance of OFT phenotypes, whereas SHF-specific Hand1 ablation does not reveal any cardiac phenotypes. Further, HAND1 and HAND2 appear functionally redundant within the cNCCs, as a reduction/ablation of Hand2 on an NCC-specific H1CKO background causes pronounced OFT defects. Double conditional Hand1 and Hand2 NCC knockouts exhibit persistent truncus arteriosus (PTA) with 100% penetrance. NCC lineage-tracing and Sema3c in situ mRNA expression reveal that Sema3c-expressing cells are mis-localized, resulting in a malformed septal bridge within the OFTs of H1CKO;H2CKO embryos. Interestingly, Hand1 and Hand2 also genetically interact within the SHF, as SHF H1CKOs on a heterozygous Hand2 background exhibit Ventricular Septal Defects (VSDs) with incomplete penetrance. Previously, we identified a BMP, HAND2, and GATA-dependent Hand1 OFT enhancer sufficient to drive reporter gene expression within the nascent OFT and aorta. Using these transcription inputs as a probe, we identify a novel Hand2 OFT enhancer, suggesting that a conserved BMP-GATA dependent mechanism transcriptionally regulates both HAND factors. These findings support the hypothesis that HAND factors interpret BMP signaling within the cNCCs to cooperatively coordinate OFT morphogenesis.

Heart Enhancers: Development and Disease Control at a Distance

Bound by lineage-determining transcription factors and signaling effectors, enhancers play essential roles in controlling spatiotemporal gene expression profiles during development, homeostasis and disease. Recent synergistic advances in functional genomic technologies, combined with the developmental biology toolbox, have resulted in unprecedented genome-wide annotation of heart enhancers and their target genes. Starting with early studies of vertebrate heart enhancers and ending with state-of-the-art genome-wide enhancer discovery and testing, we will review how studying heart enhancers in metazoan species has helped inform our understanding of cardiac development and disease.

GATA4-Twist1 Signalling in Disturbed Flow-Induced Atherosclerosis

BACKGROUND

Endothelial cell (EC) dysfunction (enhanced inflammation, proliferation and permeability) is the initial trigger for atherosclerosis. Atherosclerosis shows preferential development near branches and bends exposed to disturbed blood flow. By contrast, sites that are exposed to non-disturbed blood flow are atheroprotected. Disturbed flow promotes atherosclerosis by promoting EC dysfunction. Blood flow controls EC function through transcriptional and post-transcriptional mechanisms that are incompletely understood.

METHODS AND RESULTS

We identified the developmental transcription factors Twist1 and GATA4 as being enriched in EC at disturbed flow, atheroprone regions of the porcine aorta in a microarray study. Further work using the porcine and murine aortae demonstrated that Twist1 and GATA4 expression was enhanced at the atheroprone, disturbed flow sites in vivo. Using controlled in vitro flow systems, the expression of Twist1 and GATA4 was enhanced under disturbed compared to non-disturbed flow in cultured cells. Disturbed flow promoted Twist1 expression through a GATA4-mediated transcriptional mechanism as revealed by a series of in vivo and in vitro studies. GATA4-Twist1 signalling promoted EC proliferation, inflammation, permeability and endothelial-to-mesenchymal transition (EndoMT) under disturbed flow, leading to atherosclerosis development, as shown in a combination of in vitro and in vivo studies using GATA4 and Twist1-specific siRNA and EC-specific GATA4 and Twist1 Knock out (KO) mice.

CONCLUSIONS

We revealed that GATA4-Twist1-Snail signalling triggers EC dysfunction and atherosclerosis; this work could lead to the development of novel anti-atherosclerosis therapeutics.

The lncRNA Locus Handsdown Regulates Cardiac Gene Programs and Is Essential for Early Mouse Development

Precisely controlled gene regulatory networks are required during embryonic development to give rise to various structures, including those of the cardiovascular system. Long non-coding RNA (lncRNA) loci are known to be important regulators of these genetic programs. We have identified a novel and essential lncRNA locus Handsdown (Hdn), active in early heart cells, and show by genetic inactivation that it is essential for murine development. Hdn displays haploinsufficiency for cardiac development as Hdn-heterozygous adult mice exhibit hyperplasia in the right ventricular wall. Transcriptional activity of the Hdn locus, independent of its RNA, suppresses its neighboring gene Hand2. We reveal a switch in a topologically associated domain in differentiation of the cardiac lineage, allowing the Hdn locus to directly interact with regulatory elements of the Hand2 locus.

Functional analysis of the congenital heart disease‑associated GATA4 H436Y mutation in vitro

Congenital heart disease (CHD) is the most common type of developmental defect, with high rates of morbidity in infants. The transcription factor GATA‑binding factor 4 (GATA4) has been reported to serve a critical role in embryogenesis and cardiac development. Our previous study reported a heterozygous GATA4 c.1306C>T (p.H436Y) mutation in four Chinese infants with congenital heart defects. In the present study, functional analysis of the GATA4 H436Y mutation was performed in vitro. The functional effect of GATA4 mutation was compared with GATA4 wild‑type using a dual‑luciferase reporter assay system and immunofluorescence. Electrophoretic mobility‑shift assays were performed to explore the binding affinity of the mutated GATA4 to the heart and neural crest derivatives expressed 2 (HAND2) gene. The results revealed that the mutation had no effect on normal nuclear localization, but resulted in diminished GATA‑binding affinity to HAND2 and significantly decreased gene transcriptional activation. These results indicated that this GATA4 mutation may not influence cellular localization in transfected cells, but may affect the affinity of the GATA‑binding site on HAND2 and decrease transcriptional activity, thus suggesting that the GATA4 mutation may be associated with the pathogenesis of CHD.

The lncRNA Hand2os1/Uph locus orchestrates heart development through regulation of precise expression of Hand2

Exploration and dissection of potential actions and effects of long noncoding RNA (lncRNA) in animals remain challenging. Here, using multiple knockout mouse models and single cell RNA sequencing, we demonstrate that the divergent lncRNA Hand2os1/Uph has a key complex modulatory effect on the expression of its neighboring gene HAND2 and subsequently on heart development and function. Short deletion of the Hand2os1 promoter in mouse diminishes Hand2os1 transcription to ∼8-32%, but fails to affect HAND2 expression and yields no discernable heart phenotypes. Interestingly, full-length deletion of Hand2os1 in mouse causes moderate yet prevalent upregulation of HAND2 in hundreds of cardiac cells, leading to profound biological consequences, including dysregulated cardiac gene programs, congenital heart defects and perinatal lethality. We propose that the Hand2os1 locus dampens HAND2 expression to restrain cardiomyocyte proliferation, thereby orchestrating a balanced development of cardiac cell lineages. This study highlights the regulatory complexity of the lncRNA Hand2os1 on HAND2 expression, emphasizing the need for complementary genetic and single cell approaches to delineate the function and primary molecular effects of an lncRNA in animals.

Foxc2 is required for proper cardiac neural crest cell migration, outflow tract septation, and ventricle expansion

BACKGROUND

Proper development of the great vessels of the heart and septation of the cardiac outflow tract requires cardiac neural crest cells. These cells give rise to the parasympathetic cardiac ganglia, the smooth muscle layer of the great vessels, some cardiomyocytes, and the conotruncal cushions and aorticopulmonary septum of the outflow tract. Ablation of cardiac neural crest cells results in defective patterning of each of these structures. Previous studies have shown that targeted deletion of the forkhead transcription factor C2 (Foxc2), results in cardiac phenotypes similar to that derived from cardiac neural crest cell ablation.

RESULTS

We report that Foxc2^-/- embryos on the 129s6/SvEv inbred genetic background display persistent truncus arteriosus and hypoplastic ventricles before embryonic lethality. Foxc2 loss-of-function resulted in perturbed cardiac neural crest cell migration and their reduced contribution to the outflow tract as evidenced by lineage tracing analyses together with perturbed expression of the neural crest cell markers Sox10 and Crabp1. Foxc2 loss-of-function also resulted in alterations in PlexinD1, Twist1, PECAM1, and Hand1/2 expression in association with vascular and ventricular defects.

CONCLUSIONS

Our data indicate Foxc2 is required for proper migration of cardiac neural crest cells, septation of the outflow tract, and development of the ventricles. Developmental Dynamics 247:1286-1296, 2018. © 2018 Wiley Periodicals, Inc.

Venous identity requires BMP signalling through ALK3

Venous endothelial cells are molecularly and functionally distinct from their arterial counterparts. Although veins are often considered the default endothelial state, genetic manipulations can modulate both acquisition and loss of venous fate, suggesting that venous identity is the result of active transcriptional regulation. However, little is known about this process. Here we show that BMP signalling controls venous identity via the ALK3/BMPR1A receptor and SMAD1/SMAD5. Perturbations to TGF-β and BMP signalling in mice and zebrafish result in aberrant vein formation and loss of expression of the venous-specific gene Ephb4, with no effect on arterial identity. Analysis of a venous endothelium-specific enhancer for Ephb4 shows enriched binding of SMAD1/5 and a requirement for SMAD binding motifs. Further, our results demonstrate that BMP/SMAD-mediated Ephb4 expression requires the venous-enriched BMP type I receptor ALK3/BMPR1A. Together, our analysis demonstrates a requirement for BMP signalling in the establishment of Ephb4 expression and the venous vasculature.

The establishment of functional vasculatures requires the specification of newly formed vessels into veins and arteries. Here, Neal et al. use a combination of genetic approaches in mice and zebrafish to show that BMP signalling, via ALK3 and SMAD1/5, is required for venous specification during blood vessel development.

Loss of SMYD1 Results in Perinatal Lethality via Selective Defects within Myotonic Muscle Descendants

SET and MYND Domain 1 (SMYD1) is a cardiac and skeletal muscle-specific, histone methyl transferase that is critical for both embryonic and adult heart development and function in both mice and men. We report here that skeletal muscle-specific, myogenin (myoG)-Cre-mediated conditional knockout (CKO) of Smyd1 results in perinatal death. As early as embryonic day 12.5, Smyd1 CKOs exhibit multiple skeletal muscle defects in proliferation, morphology, and gene expression. However, all myotonic descendants are not afflicted equally. Trunk muscles are virtually ablated with excessive accumulation of brown adipose tissue (BAT), forelimb muscles are disorganized and improperly differentiated, but other muscles, such as the masseter, are normal. While expression of major myogenic regulators went unscathed, adaptive and innate immune transcription factors critical for BAT development/physiology were downregulated. Whereas classical mitochondrial BAT accumulation went unscathed following loss of SMYD1, key transcription factors, including PRDM16, UCP-1, and CIDE-a that control skeletal muscle vs. adipose fate, were downregulated. Finally, in rare adults that survive perinatal lethality, SMYD1 controls specification of some, but not all, skeletal muscle fiber-types.

Hand Factors in Cardiac Development

Congenital heart defects account for 1% of infant mortality and 10% of in utero deaths. As the vertebrate embryo develops, multiple tissue types develop in tandem to morphologically pattern the functional heart. Underlying cardiac development is a network of transcription factors known to tightly control these morphological events. Members of the Twist family of basic helix-loop-helix transcription factors, Hand1 and Hand2, are essential to this process. The expression patterns and functional role of Hand factors in neural crest cells, endocardium, myocardium, and epicardium is indicative of their importance during cardiogenesis; however, to date, an extensive understanding of the transcriptional targets of Hand proteins and their overall mechanism of action remain unclear. In this review, we summarize the recent findings that further outline the crucial functions of Hand factors during heart development and in post-natal heart function. Anat Rec, 302:101-107, 2019. © 2018 Wiley Periodicals, Inc.

The role of long non-coding RNAs in cardiac development and disease

Cells display a set of RNA molecules at one time point, reflecting thus the cellular transcriptional steady state, configuring therefore its transcriptome. It is basically composed of two different classes of RNA molecules; protein-coding RNAs (cRNAs) and protein non-coding RNAs (ncRNAs). Sequencing of the human genome and subsequently the ENCODE project identified that more than 80% of the genome is transcribed in some type of RNA. Importantly, only 3% of these transcripts correspond to protein-coding RNAs, pointing that ncRNAs are as important or even more as cRNAs. ncRNAs have pivotal roles in development, differentiation and disease. Non-coding RNAs can be classified into two distinct classes according to their length; i.e., small (<200 nt) and long (>200 nt) noncoding RNAs. The structure, biogenesis and functional roles of small non-coding RNA have been widely studied, particularly for microRNAs (miRNAs). In contrast to microRNAs, our current understanding of long non-coding RNAs (lncRNAs) is limited. In this manuscript, we provide state-of-the art review of the functional roles of long non-coding RNAs during cardiac development as well as an overview of the emerging role of these ncRNAs in distinct cardiac diseases.

Transcription of the non-coding RNA upperhand controls Hand2 expression and heart development

HAND2 is an ancestral regulator of heart development and one of four transcription factors that control the reprogramming of fibroblasts into cardiomyocytes. Deletion of Hand2 in mice results in right ventricle hypoplasia and embryonic lethality. Hand2 expression is tightly regulated by upstream enhancers that reside within a super-enhancer delineated by histone H3 acetyl Lys27 (H3K27ac) modifications. Here we show that transcription of a Hand2-associated long non-coding RNA, which we named upperhand (Uph), is required to maintain the super-enhancer signature and elongation of RNA polymerase II through the Hand2 enhancer locus. Blockade of Uph transcription, but not knockdown of the mature transcript, abolished Hand2 expression, causing right ventricular hypoplasia and embryonic lethality in mice. Given the substantial number of uncharacterized promoter-associated long non-coding RNAs encoded by the mammalian genome, the Uph-Hand2 regulatory partnership offers a mechanism by which divergent non-coding transcription can establish a permissive chromatin environment.

Specification of jaw identity by the Hand2 transcription factor

Acquisition of the lower jaw (mandible) was evolutionarily important for jawed vertebrates. In humans, syndromic craniofacial malformations often accompany jaw anomalies. The basic helix-loop-helix transcription factor Hand2, which is conserved among jawed vertebrates, is expressed in the neural crest in the mandibular process but not in the maxillary process of the first branchial arch. Here, we provide evidence that Hand2 is sufficient for upper jaw (maxilla)-to-mandible transformation by regulating the expression of homeobox transcription factors in mice. Altered Hand2 expression in the neural crest transformed the maxillae into mandibles with duplicated Meckel's cartilage, which resulted in an absence of the secondary palate. In Hand2-overexpressing mutants, non-Hox homeobox transcription factors were dysregulated. These results suggest that Hand2 regulates mandibular development through downstream genes of Hand2 and is therefore a major determinant of jaw identity. Hand2 may have influenced the evolutionary acquisition of the mandible and secondary palate.

The Isl1/Ldb1 Complex Orchestrates Genome-wide Chromatin Organization to Instruct Differentiation of Multipotent Cardiac Progenitors

Cardiac stem/progenitor cells hold great potential for regenerative therapies; however, the mechanisms regulating their expansion and differentiation remain insufficiently defined. Here we show that Ldb1 is a central regulator of genome organization in cardiac progenitor cells, which is crucial for cardiac lineage differentiation and heart development. We demonstrate that Ldb1 binds to the key regulator of cardiac progenitors, Isl1, and protects it from degradation. Furthermore, the Isl1/Ldb1 complex promotes long-range enhancer-promoter interactions at the loci of the core cardiac transcription factors Mef2c and Hand2. Chromosome conformation capture followed by sequencing identified specific Ldb1-mediated interactions of the Isl1/Ldb1 responsive Mef2c anterior heart field enhancer with genes that play key roles in cardiac progenitor cell function and cardiovascular development. Importantly, the expression of these genes was downregulated upon Ldb1 depletion and Isl1/Ldb1 haplodeficiency. In conclusion, the Isl1/Ldb1 complex orchestrates a network for heart-specific transcriptional regulation and coordination in three-dimensional space during cardiogenesis.

Epigenetic regulation of cardiac myocyte differentiation

Cardiac myocytes (CMs) proliferate robustly during fetal life but withdraw permanently from the cell cycle soon after birth and undergo terminal differentiation. This cell cycle exit is associated with the upregulation of a host of adult cardiac-specific genes. The vast majority of adult CMs (ACMs) do not reenter cell cycle even if subjected to mitogenic stimuli. The basis for this irreversible cell cycle exit is related to the stable silencing of cell cycle genes specifically involved in the progression of G2/M transition and cytokinesis. Studies have begun to clarify the molecular basis for this stable gene repression and have identified epigenetic and chromatin structural changes in this process. In this review, we summarize the current understanding of epigenetic regulation of CM cell cycle and cardiac-specific gene expression with a focus on histone modifications and the role of retinoblastoma family members.

Of mice and men: molecular genetics of congenital heart disease

Congenital heart disease (CHD) affects nearly 1 % of the population. It is a complex disease, which may be caused by multiple genetic and environmental factors. Studies in human genetics have led to the identification of more than 50 human genes, involved in isolated CHD or genetic syndromes, where CHD is part of the phenotype. Furthermore, mapping of genomic copy number variants and exome sequencing of CHD patients have led to the identification of a large number of candidate disease genes. Experiments in animal models, particularly in mice, have been used to verify human disease genes and to gain further insight into the molecular pathology behind CHD. The picture emerging from these studies suggest that genetic lesions associated with CHD affect a broad range of cellular signaling components, from ligands and receptors, across down-stream effector molecules to transcription factors and co-factors, including chromatin modifiers.

Identification of functional mutations in GATA4 in patients with congenital heart disease

Congenital heart disease (CHD) is one of the most prevalent developmental anomalies and the leading cause of noninfectious morbidity and mortality in newborns. Despite its prevalence and clinical significance, the etiology of CHD remains largely unknown. GATA4 is a highly conserved transcription factor that regulates a variety of physiological processes and has been extensively studied, particularly on its role in heart development. With the combination of TBX5 and MEF2C, GATA4 can reprogram postnatal fibroblasts into functional cardiomyocytes directly. In the past decade, a variety of GATA4 mutations were identified and these findings originally came from familial CHD pedigree studies. Given that familial and sporadic CHD cases allegedly share a basic genetic basis, we explore the GATA4 mutations in different types of CHD. In this study, via direct sequencing of the GATA4 coding region and exon-intron boundaries in 384 sporadic Chinese CHD patients, we identified 12 heterozygous non-synonymous mutations, among which 8 mutations were only found in CHD patients when compared with 957 controls. Six of these non-synonymous mutations have not been previously reported. Subsequent functional analyses revealed that the transcriptional activity, subcellular localization and DNA binding affinity of some mutant GATA4 proteins were significantly altered. Our results expand the spectrum of GATA4 mutations linked to cardiac defects. Together with the newly reported mutations, approximately 110 non-synonymous mutations have currently been identified in GATA4. Our future analysis will explore why the evolutionarily conserved GATA4 appears to be hypermutable.

GATA4 and GATA6 control mouse pancreas organogenesis

Recently, heterozygous mutations in GATA6 have been found in neonatal diabetic patients with failed pancreatic organogenesis. To investigate the roles of GATA4 and GATA6 in mouse pancreas organogenesis, we conditionally inactivated these genes within the pancreas. Single inactivation of either gene did not have a major impact on pancreas formation, indicating functional redundancy. However, double Gata4/Gata6 mutant mice failed to develop pancreata, died shortly after birth, and displayed hyperglycemia. Morphological defects in Gata4/Gata6 mutant pancreata were apparent during embryonic development, and the epithelium failed to expand as a result of defects in cell proliferation and differentiation. The number of multipotent pancreatic progenitors, including PDX1+ cells, was reduced in the Gata4/Gata6 mutant pancreatic epithelium. Remarkably, deletion of only 1 Gata6 allele on a Gata4 conditional knockout background severely reduced pancreatic mass. In contrast, a single WT allele of Gata4 in Gata6 conditional knockout mice was sufficient for normal pancreatic development, indicating differential contributions of GATA factors to pancreas formation. Our results place GATA factors at the top of the transcriptional network hierarchy controlling pancreas organogenesis.

Identification and characterization of the zebrafish pharyngeal arch-specific enhancer for the basic helix-loop-helix transcription factor Hand2

The development of the vertebrate jaw relies on a network of transcription factors that patterns the dorsal-ventral axis of the pharyngeal arches. Recent findings in both mouse and zebrafish illustrate that the basic-helix-loop-helix transcription factor, Hand2, is crucial in this patterning process. While Hand2 has functionally similar roles in these two species, little is known about the regulatory sequences controlling hand2 expression in zebrafish. Using bioinformatics and Tol2-mediated transgenesis, we have generated zebrafish transgenic reporter lines in which either the mouse or zebrafish arch-specific hand2 enhancer direct expression of a fluorescent reporter. We find that both the mouse and zebrafish enhancers drive early reporter expression in a hand2-specific pattern in the ventral pharyngeal arches of zebrafish embryos. These lines provide useful tools to follow ventral arch cells during vertebrate jaw development while also allowing dissection of hand2 transcriptional regulation during this process.

Twist factor regulation of non-cardiomyocyte cell lineages in the developing heart

The heart is a complex organ that is composed of numerous cell types, which must integrate their programs for proper specification, differentiation and cardiac morphogenesis. During cardiogenesis members of the Twist-family of basic helix-loop-helix (bHLH) transcription factors play distinct roles within cardiac lineages such as the endocardium and extra-cardiac lineages such as the cardiac neural crest (cNCC) and epicardium. While the study of these cell populations is often eclipsed by that of cardiomyocytes, the contributions of non-cardiomyocytes to development and disease are increasingly being appreciated as both dynamic and essential. This review summarizes what is known regarding Twist-family bHLH function in extra-cardiac cell populations and the endocardium, with a focus on regulatory mechanisms, downstream targets, and expression profiles. Improving our understanding of the molecular pathways that Twist-family bHLH factors mediate in these lineages will be necessary to ascertain how their dysfunction leads to congenital disease and adult pathologies such as myocardial infarctions and cardiac fibroblast induced fibrosis. Indeed, this knowledge will prove to be critical to clinicians seeking to improve current treatments.

Epigenetic repression of cardiac progenitor gene expression by Ezh2 is required for postnatal cardiac homeostasis

Adult-onset diseases can be associated with in utero events, but mechanisms for this remain unknown^1,2. The polycomb histone methyltransferase, Ezh2, stabilizes transcription by depositing repressive marks during development that persist into adulthood^3–9, but its function in postnatal organ homeostasis is unknown. We show that Ezh2 stabilizes cardiac gene expression and prevents cardiac pathology by repressing the homeodomain transcription factor Six1, which functions in cardiac progenitors but is stably silenced upon cardiac differentiation¹⁰. Ezh2 deletion in cardiac progenitors caused postnatal myocardial pathology and destabilized cardiac gene expression with activation of Six1-dependent skeletal muscle genes. Six1 induced cardiomyocyte hypertrophy and skeletal muscle gene expression. Furthermore, genetically reducing Six1 levels rescued the pathology of Ezh2-deficient hearts. Thus, Ezh2-mediated repression of Six1 in differentiating cardiac progenitors is essential for stable postnatal heart gene expression and homeostasis. Our results suggest that epigenetic dysregulation in embryonic progenitor cells predisposes to adult disease and dysregulated stress responses.

Regulation of GATA4 transcriptional activity in cardiovascular development and disease

Transcription factors regulate formation and function of the heart, and perturbation of transcription factor expression and regulation disrupts normal heart structure and function. Multiple mechanisms regulate the level and locus-specific activity of transcription factors, including transcription, translation, subcellular localization, posttranslational modifications, and context-dependent interactions with other transcription factors, chromatin remodeling enzymes, and epigenetic regulators. The zinc finger transcription factor GATA4 is among the best-studied cardiac transcriptional factors. This review focuses on molecular mechanisms that regulate GATA4 transcriptional activity in the cardiovascular system, providing a framework to investigate and understand the molecular regulation of cardiac gene transcription by other transcription factors.

GATA5 interacts with GATA4 and GATA6 in outflow tract development

Members of the GATA family of transcription factors are critical regulators of heart development and mutations in 2 of them, GATA4 and GATA6 are associated with outflow tract and septal defects in human. The heart expresses 3 GATA factors, GATA4, 5 and 6 in a partially overlapping pattern. Here, we report that compound Gata4/Gata5 and Gata5/Gata6 mutants die embryonically or perinatally due to severe congenital heart defects. Almost all Gata4(+/-)Gata5(+/-) mutant embryos have double outlet right ventricles (DORV), large ventricular septal defects (VSD) as well as hypertrophied mitral and tricuspid valves. Only 25% of double compound Gata4/Gata5 heterozygotes survive to adulthood and these mice have aortic stenosis. Compound loss of a Gata5 and a Gata6 allele also leads to DORVs associated with subaortic VSDs. Expression of several transcription factors important for endocardial and myocardial cell differentiation, such as Tbx20, Mef2c, Hey1 and Hand2, was reduced in compound heterozygote embryos. These findings suggest the existence of important genetic interactions between Gata5 and the 2 other cardiac GATA factors in endocardial cushion formation and outflow tract morphogenesis. The data identify GATA5 as a potential genetic modifier of congenital heart disease and provide insight for elucidating the genetic basis of an important class of human birth defects.

Hand factors as regulators of cardiac morphogenesis and implications for congenital heart defects

Almost 15 years of careful study have established the related basic Helix-Loop-Helix (bHLH) transcription factors Hand1 and Hand2 as critical for heart development across evolution. Hand factors make broad contributions, revealed through animal models, to the development of multiple cellular lineages that ultimately contribute to the heart. They perform critical roles in ventricular cardiomyocyte growth, differentiation, morphogenesis, and conduction. They are also important for the proper development of the cardiac outflow tract, epicardium, and endocardium. Molecularly, they function both through DNA binding and through protein-protein interactions, which are regulated transcriptionally, posttranscriptionally by microRNAs, and posttranslationally through phosphoregulation. Although direct Hand factor transcriptional targets are progressively being identified, confirmed direct targets of Hand factor transcriptional activity in the heart are limited. Identification of these targets will be critical to model the mechanisms by which Hand factor bHLH interactions affect developmental pathways. Improved understanding of Hand factor-mediated transcriptional cascades will be necessary to determine how Hand factor dysregulation translates to human disease phenotypes. This review summarizes the insight that animal models have provided into the regulation and function of these factors during heart development, in addition to the recent findings that suggest roles for HAND1 and HAND2 in human congenital heart disease.

Analysis of the Hand1 cell lineage reveals novel contributions to cardiovascular, neural crest, extra-embryonic, and lateral mesoderm derivatives

The basic Helix-Loop-Helix (bHLH) transcription factors Hand1 and Hand2 play critical roles in the development of multiple organ systems during embryogenesis. The dynamic expression patterns of these two factors within developing tissues obfuscate their respective unique and redundant organogenic functions. To define cell lineages potentially dependent upon Hand gene expression, we generated a mutant allele in which the coding region of Hand1 is replaced by Cre recombinase. Subsequent Cre-mediated activation of β-galactosidase or eYFP reporter alleles enabled lineage trace analyses that clearly define the fate of Hand1-expressing cells. Hand1-driven Cre marks specific lineages within the extra embryonic tissues, placenta, sympathetic nervous system, limbs, jaw, and several cell types within the cardiovascular system. Comparisons between Hand1 expression and Hand1-lineage greatly refine our understanding of its dynamic spatial-temporal expression domains and raise the possibility of novel Hand1 functions in structures not thought to be Hand1-dependent.

Mesp1: a key regulator of cardiovascular lineage commitment

In mammals, the heart arises from the differentiation of 2 sources of multipotent cardiovascular progenitors (MCPs). Different studies indicated that an evolutionary conserved transcriptional regulatory network controls cardiovascular development from flies to humans. Whereas in Drosophila, Tinman acts as a master regulator of cardiac development, the identification of such a master regulator in mammals remained elusive for a long time. In this review, we discuss the recent findings suggesting that Mesp1 acts as a key regulator of cardiovascular progenitors in vertebrates. Lineage tracing in mice demonstrated that Mesp1 represents the earliest marker of cardiovascular progenitors, tracing almost all the cells of the heart including derivatives of the primary and second heart fields. The inactivation of Mesp1/2 indicated that Mesp genes are essential for early cardiac mesoderm formation and MCP migration. Several recent studies have demonstrated that Mesp1 massively promotes cardiovascular differentiation during embryonic development and pluripotent stem cell differentiation and indicated that Mesp1 resides at the top of the cellular and transcriptional hierarchy that orchestrates MCP specification. In primitive chordates, Mesp also controls early cardiac progenitor specification and migration, suggesting that Mesp arises during chordate evolution to regulate the earliest step of cardiovascular development. Defining how Mesp1 regulates the earliest step of MCP specification and controls their migration is essential to understand the root of cardiovascular development and how the deregulation of these processes can lead to congenital heart diseases. In addition, these findings will be very useful to boost the production of cardiovascular cells for cellular therapy, drug and toxicity screening.

Myocardial lineage development

The myocardium of the heart is composed of multiple highly specialized myocardial lineages, including those of the ventricular and atrial myocardium, and the specialized conduction system. Specification and maturation of each of these lineages during heart development is a highly ordered, ongoing process involving multiple signaling pathways and their intersection with transcriptional regulatory networks. Here, we attempt to summarize and compare much of what we know about specification and maturation of myocardial lineages from studies in several different vertebrate model systems. To date, most research has focused on early specification, and although there is still more to learn about early specification, less is known about factors that promote subsequent maturation of myocardial lineages required to build the functioning adult heart.

Hand2 function in second heart field progenitors is essential for cardiogenesis

Cardiogenesis involves the contributions of multiple progenitor pools, including mesoderm-derived cardiac progenitors known as the first and second heart fields. Disruption of genetic pathways regulating individual subsets of cardiac progenitors likely underlies many forms of human cardiac malformations. Hand2 is a member of the basic helix loop helix (bHLH) family of transcription factors and is expressed in numerous cell lineages that contribute to the developing heart. However, the early embryonic lethality of Hand2-null mice has precluded lineage-specific study of its function in myocardial progenitors. Here, we generated and used a floxed allele of Hand2 to ablate its expression in specific cardiac cell populations at defined developmental points. We found that Hand2 expression within the mesoderm-derived second heart field progenitors was required for their survival and deletion in this domain recapitulated the complete Hand2-null phenotype. Loss of Hand2 at later stages of development and in restricted domains of the second heart field revealed a spectrum of cardiac anomalies resembling forms of human congenital heart disease. Molecular analyses of Hand2 mutant cells revealed several genes by which Hand2 may influence expansion of the cardiac progenitors. These findings demonstrate that Hand2 is essential for survival of second heart field progenitors and that the graded loss of Hand2 function in this cardiac progenitor pool can cause a spectrum of congenital heart malformation.

Hand2 regulates chondrogenesis in vitro and in vivo

Hand2 is a transcription factor of the basic helix-loop-helix family that plays essential roles during development. Hand2 determines the anterior-posterior axis during limb development and there is also substantial evidence that Hand2 regulates limb skeletogenesis. However, little is known about how Hand2 might regulate skeletogenesis. Here we show that, in a limb bud micromass culture system, over-expression of Hand2 represses chondrogenesis and the expression of chondrocytic genes, Sox9, type II collagen and aggrecan. Furthermore, we show that Hand2 is induced by the activation of canonical Wnt signaling, which strongly represses chondrogenesis. Surprisingly, Hand2 repressed chondrogenesis in a DNA binding- and dimer formation-independent manner. To examine the in vivo role of Hand2 in mice, we targeted the expression of Hand2 to the cartilage using regulatory elements from the collagen II gene. The resulting transgenic mice displayed a dwarf phenotype, with axial, appendicular and craniofacial skeletal deformities. Hand2 strongly inhibited chondrogenesis in the axial and cranial base skeleton. In the sternum, Hand2 inhibited endochondral ossification by slowing chondrocyte maturation. These data support a model of Hand2 regulating endochondral ossification via at least two steps: (1) determination of the site of chondrogenesis by outlining the region of the future cartilage template and (2) regulation of the rate of chondrocyte maturation.

Targeted deletion of Hand2 in cardiac neural crest-derived cells influences cardiac gene expression and outflow tract development

The basic helix-loop-helix DNA binding protein Hand2 has critical functions in cardiac development both in neural crest-derived and mesoderm-derived structures. Targeted deletion of Hand2 in the neural crest has allowed us to genetically dissect Hand2-dependent defects specifically in outflow tract and cardiac cushion independent of Hand2 functions in mesoderm-derived structures. Targeted deletion of Hand2 in the neural crest results in misalignment of the aortic arch arteries and outflow tract, contributing to development of double outlet right ventricle (DORV) and ventricular septal defects (VSD). These neural crest-derived developmental anomalies are associated with altered expression of Hand2-target genes we have identified by gene profiling. A number of Hand2 direct target genes have been identified using ChIP and ChIP-on-chip analyses. We have identified and validated a number of genes related to cell migration, proliferation/cell cycle and intracellular signaling whose expression is affected by Hand2 deletion in the neural crest and which are associated with development of VSD and DORV. Our data suggest that Hand2 is a multifunctional DNA binding protein affecting expression of target genes associated with a number of functional interactions in neural crest-derived cells required for proper patterning of the outflow tract, generation of the appropriate number of neural crest-derived cells for elongation of the conotruncus and cardiac cushion organization. Our genetic model has made it possible to investigate the molecular genetics of neural crest contributions to outflow tract morphogenesis and cell differentiation.

GATA4 mutations in Chinese patients with congenital cardiac septal defects

The object of the study was to elucidate the mutations of the GATA4 gene in Han ancestry patients with congenital cardiac septal defects. Fifty Han ancestry patients with sporadic and familial cardiac septal defects and 200 normal subjects of the same ethnical background were studied. A total of six exons and the intron-exon boundaries of GATA4 were amplified by polymerase chain reaction (PCR). The PCR products were purified and directly sequenced with an ABI PRISM 3730 Automatic DNA sequencer. Two novel heterozygous mutations were discovered in the GATA4 gene in five children with cardiac septal defects (10%, 5/50), His28Tyr in exon 2 and His436Tyr in exon 7, respectively, which were neither found in the control population nor reported in the SNP database at the website http://www.ncbi.nlm.nih.gov/SNP. In addition, we did not identify any mutations in GATA4 in three familial atrial septal defects and two familial ventricular septal defects. Our finding suggests that the mutations in the transcription factor GATA4 might be related to congenital cardiac septal defects in Han ancestry patients.