Cx30.2 enhancer analysis identifies Gata4 as a novel regulator of atrioventricular delay

Development of the Cardiac Conduction System

The electrical impulses that coordinate the sequential, rhythmic contractions of the atria and ventricles are initiated and tightly regulated by the specialized tissues of the cardiac conduction system. In the mature heart, these impulses are generated by the pacemaker cardiomyocytes of the sinoatrial node, propagated through the atria to the atrioventricular node where they are delayed and then rapidly propagated to the atrioventricular bundle, right and left bundle branches, and finally, the peripheral ventricular conduction system. Each of these specialized components arise by complex patterning events during embryonic development. This chapter addresses the origins and transcriptional networks and signaling pathways that drive the development and maintain the function of the cardiac conduction system.

Protocol for inducing cellular ablation in the mouse atrioventricular conduction system

Damage to the atrioventricular conduction system (AVCS), the main electrical connection between the atrial and ventricular chambers, can result in a variety of cardiac conduction disorders. Here, we provide a protocol for selective damage of the mouse AVCS to study its response during injury. We describe tamoxifen-induced cellular ablation, detection of AV block through electrocardiography, and quantification of histological and immunofluorescence markers to analyze the AVCS. This protocol can be used to study mechanisms associated with AVCS injury repair and regeneration. For complete details on the use and execution of this protocol, please refer to Wang et al. (2021).1.

Global chromatin landscapes identify candidate noncoding modifiers of cardiac rhythm

Comprehensive cis-regulatory landscapes are essential for accurate enhancer prediction and disease variant mapping. Although cis-regulatory element (CRE) resources exist for most tissues and organs, many rare - yet functionally important - cell types remain overlooked. Despite representing only a small fraction of the heart's cellular biomass, the cardiac conduction system (CCS) unfailingly coordinates every life-sustaining heartbeat. To globally profile the mouse CCS cis-regulatory landscape, we genetically tagged CCS component-specific nuclei for comprehensive assay for transposase-accessible chromatin-sequencing (ATAC-Seq) analysis. Thus, we established a global CCS-enriched CRE database, referred to as CCS-ATAC, as a key resource for studying CCS-wide and component-specific regulatory functions. Using transcription factor (TF) motifs to construct CCS component-specific gene regulatory networks (GRNs), we identified and independently confirmed several specific TF sub-networks. Highlighting the functional importance of CCS-ATAC, we also validated numerous CCS-enriched enhancer elements and suggested gene targets based on CCS single-cell RNA-Seq data. Furthermore, we leveraged CCS-ATAC to improve annotation of existing human variants related to cardiac rhythm and nominated a potential enhancer-target pair that was dysregulated by a specific SNP. Collectively, our results established a CCS-regulatory compendium, identified novel CCS enhancer elements, and illuminated potential functional associations between human genomic variants and CCS component-specific CREs.

Changes in gap junction proteins Connexin30.2 and Connexin40 expression in the sinoatrial node of rats with dexmedetomidine-induced sinus bradycardia

BACKGROUND

Dexmedetomidine (Dex) is widely used, and its most common side effect is bradycardia. The complete mechanism through which Dex induces bradycardia has not been elucidated. This research investigates the expression of gap junction proteins Connexin30.2 (Cx30.2) and Connexin40 (Cx40) within the sinoatrial node of rats with Dex-induced sinus bradycardia.

METHODS

Eighty rats were randomly assigned to five groups. Saline was administered to rats in Group C. In the other four groups, the rats were administered Dex to induce bradycardia. In groups D₁ and D₂, the rats were administered Dex at a loading dose of 30 μg.kg^-1 and 100 μg.kg^-1 for 10 min, then at 15 μg.kg^-1.h^-1 and 50 μg.kg^-1.h^-1 for 120 min separately. The rats in group D₁A and D₂A were administered Dex in the same way as in group D₁ and D₂; however, immediately after the administration of the loading dose, 0.5 mg atropine was administered intravenously, and then at 0.5 mg.kg^-1.h^-1 for 120 min. The sinoatrial node was acquired after intravenous infusion was completed. Quantitative real-time polymerase chain reaction and western blot analyses were performed to measure mRNA and protein expression of Cx30.2 and Cx40, respectively.

RESULTS

The expression of Cx30.2 increased, whereas the expression of Cx40 decreased within the sinoatrial node of rats with Dex-induced sinus bradycardia. Atropine reversed the effects of Dex on the expression of gap junction proteins.

CONCLUSION

Dex possibly altered the expression of gap junction proteins to slow down cardiac conduction velocity in the sinoatrial node.

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.

Inducible cardiomyocyte injury within the atrioventricular conduction system uncovers latent regenerative capacity in mice

The cardiac conduction system (CCS) ensures regular contractile function, and injury to any of its components can cause cardiac dysrhythmia. Although all cardiomyocytes (CMs) originate from common progenitors, the CCS is composed of biologically distinct cell types with unique functional and developmental characteristics. In contrast to ventricular cardiomyocytes, which continue to proliferate after birth, most CCS cells terminally exit the cell cycle during fetal development. Although the CCS should thus provide a poor substrate for postnatal injury repair, its regenerative capacity remains untested. Here, we describe a genetic system for ablating CMs that reside within the atrioventricular conduction system (AVCS). Adult mouse AVCS ablation resulted in regenerative failure characterized by persistent atrioventricular conduction defects and contractile dysfunction. In contrast, AVCS injury in neonatal mice led to recovery in a subset of these mice, thus providing evidence for CCS plasticity. Furthermore, CM proliferation did not appear to completely account for the observed functional recovery, suggesting that mechanisms regulating recovery from dysrhythmia are likely to be distinct from cardiac regeneration associated with ventricular injury. Taken together, we anticipate that our results will motivate further mechanistic studies of CCS plasticity and enable the exploration of rhythm restoration as an alternative therapeutic strategy.

Cellular regeneration as a potential strategy to treat cardiac conduction disorders

Loss of atrioventricular conduction system (AVCS) cells due to either inherited or acquired deficits leads to conduction diseases, which can deteriorate into fatal cardiac arrhythmias and sudden death. In this issue of the JCI, Wang et al. constructed a mouse model of atrioventricular block (AVB) by inducing AVCS cell-specific injury using the Cx30.2 enhancer to drive expression of diphtheria toxin fragment A. AVCS cell ablation in adult mice led to irreversible AVB. jkjkIn contrast, AVCS cell injury in neonatal mice was followed by spontaneous recovery in a subset of mice, revealing a limited postnatal time window during which the regeneration of AVCS cells can occur as a result of cellular plasticity. This exciting study paves the way for future research into biological or cellular treatment approaches for cardiac conduction diseases by exploiting the regenerative potential of AVCS cells.

Zebrafish Models in Therapeutic Research of Cardiac Conduction Disease

Malfunction in the cardiac conduction system (CCS) due to congenital anomalies or diseases can cause cardiac conduction disease (CCD), which results in disturbances in cardiac rhythm, leading to syncope and even sudden cardiac death. Insights into development of the CCS components, including pacemaker cardiomyocytes (CMs), atrioventricular node (AVN) and the ventricular conduction system (VCS), can shed light on the pathological and molecular mechanisms underlying CCD, provide approaches for generating human pluripotent stem cell (hPSC)-derived CCS cells, and thus improve therapeutic treatment for such a potentially life-threatening disorder of the heart. However, the cellular and molecular mechanisms controlling CCS development remain elusive. The zebrafish has become a valuable vertebrate model to investigate early development of CCS components because of its unique features such as external fertilization, embryonic optical transparency and the ability to survive even with severe cardiovascular defects during development. In this review, we highlight how the zebrafish has been utilized to dissect the cellular and molecular mechanisms of CCS development, and how the evolutionarily conserved developmental mechanisms discovered in zebrafish could be applied to directing the creation of hPSC-derived CCS cells, therefore providing potential therapeutic strategies that may contribute to better treatment for CCD patients.

How Cardiac Embryology Translates into Clinical Arrhythmias

The electrophysiological signatures of the myocardium in cardiac structures, such as the atrioventricular node, pulmonary veins or the right ventricular outflow tract, are established during development by the spatial and temporal expression of transcription factors that guide expression of specific ion channels. Genome-wide association studies have shown that small variations in genetic regions are key to the expression of these transcription factors and thereby modulate the electrical function of the heart. Moreover, mutations in these factors are found in arrhythmogenic pathologies such as congenital atrioventricular block, as well as in specific forms of atrial fibrillation and ventricular tachycardia. In this review, we discuss the developmental origin of distinct electrophysiological structures in the heart and their involvement in cardiac arrhythmias.

Epigenetic Changes Governing Scn5a Expression in Denervated Skeletal Muscle

The SCN5A gene encodes the α-subunit of the voltage-gated cardiac sodium channel (Na_V1.5), a key player in cardiac action potential depolarization. Genetic variants in protein-coding regions of the human SCN5A have been largely associated with inherited cardiac arrhythmias. Increasing evidence also suggests that aberrant expression of the SCN5A gene could increase susceptibility to arrhythmogenic diseases, but the mechanisms governing SCN5A expression are not yet well understood. To gain insights into the molecular basis of SCN5A gene regulation, we used rat gastrocnemius muscle four days following denervation, a process well known to stimulate Scn5a expression. Our results show that denervation of rat skeletal muscle induces the expression of the adult cardiac Scn5a isoform. RNA-seq experiments reveal that denervation leads to significant changes in the transcriptome, with Scn5a amongst the fifty top upregulated genes. Consistent with this increase in expression, ChIP-qPCR assays show enrichment of H3K27ac and H3K4me3 and binding of the transcription factor Gata4 near the Scn5a promoter region. Also, Gata4 mRNA levels are significantly induced upon denervation. Genome-wide analysis of H3K27ac by ChIP-seq suggest that a super enhancer recently described to regulate Scn5a in cardiac tissue is activated in response to denervation. Altogether, our experiments reveal that similar mechanisms regulate the expression of Scn5a in denervated muscle and cardiac tissue, suggesting a conserved pathway for SCN5A expression among striated muscles.

GATA6 is a regulator of sinus node development and heart rhythm

The sinus node (SAN) is the primary pacemaker of the human heart, and abnormalities in its structure or function cause sick sinus syndrome, the most common reason for electronic pacemaker implantation. Here we report that transcription factor GATA6, whose mutations in humans are linked to arrhythmia, is highly expressed in the SAN and its haploinsufficiency in mice results in hypoplastic SANs and rhythm abnormalities. Cell-specific deletion reveals a requirement for GATA6 in various SAN lineages. Mechanistically, GATA6 directly activates key regulators of the SAN genetic program in conduction and nonconduction cells, such as TBX3 and EDN1, respectively. The data identify GATA6 as an important regulator of the SAN and provide a molecular basis for understanding the conduction abnormalities associated with GATA6 mutations in humans. They also suggest that GATA6 may be a potential modifier of the cardiac pacemaker.

Life Cycle of the Cardiac Voltage-Gated Sodium Channel NaV1.5

Cardiac voltage-gated sodium channel Na_V1.5, encoded by SCN5A, is crucial for the upstroke of action potential and excitation of cardiomyocytes. Na_V1.5 undergoes complex processes before it reaches the target membrane microdomains and performs normal functions. A variety of protein partners are needed to achieve the balance between SCN5A transcription and mRNA decay, endoplasmic reticulum retention and export, Golgi apparatus retention and export, selective anchoring and degradation, activation, and inactivation of sodium currents. Subtle alterations can impair Na_V1.5 in terms of expression or function, eventually leading to Na_V1.5-associated diseases such as lethal arrhythmias and cardiomyopathy.

Genome-Wide Analysis Identifies an Essential Human TBX3 Pacemaker Enhancer

RATIONALE

The development and function of the pacemaker cardiomyocytes of the sinoatrial node (SAN), the leading pacemaker of the heart, are tightly controlled by a conserved network of transcription factors, including TBX3 (T-box transcription factor 3), ISL1 (ISL LIM homeobox 1), and SHOX2 (short stature homeobox 2). Yet, the regulatory DNA elements (REs) controlling target gene expression in the SAN pacemaker cells have remained undefined.

OBJECTIVE

Identification of the regulatory landscape of human SAN-like pacemaker cells and functional assessment of SAN-specific REs potentially involved in pacemaker cell gene regulation.

METHODS AND RESULTS

We performed Assay for Transposase-Accessible Chromatin using sequencing on human pluripotent stem cell-derived SAN-like pacemaker cells and ventricle-like cells and identified thousands of putative REs specific for either human cell type. We validated pacemaker cell-specific elements in the SHOX2 and TBX3 loci. CRISPR-mediated homozygous deletion of the mouse ortholog of a noncoding region with candidate pacemaker-specific REs in the SHOX2 locus resulted in selective loss of Shox2 expression from the developing SAN and embryonic lethality. Putative pacemaker-specific REs were identified up to 1 Mbp upstream of TBX3 in a region close to MED13L harboring variants associated with heart rate recovery after exercise. The orthologous region was deleted in mice, which resulted in selective loss of expression of Tbx3 from the SAN and (cardiac) ganglia and in neonatal lethality. Expression of Tbx3 was maintained in other tissues including the atrioventricular conduction system, lungs, and liver. Heterozygous adult mice showed increased SAN recovery times after pacing. The human REs harboring the associated variants robustly drove expression in the SAN of transgenic mouse embryos.

CONCLUSIONS

We provided a genome-wide collection of candidate human pacemaker-specific REs, including the loci of SHOX2, TBX3, and ISL1, and identified a link between human genetic variants influencing heart rate recovery after exercise and a variant RE with highly conserved function, driving SAN expression of TBX3.

Development of the Cardiac Conduction System

The cardiac conduction system initiates and propagates each heartbeat. Specialized conducting cells are a well-conserved phenomenon across vertebrate evolution, although mammalian and avian species harbor specific components unique to organisms with four-chamber hearts. Early histological studies in mammals provided evidence for a dominant pacemaker within the right atrium and clarified the existence of the specialized muscular axis responsible for atrioventricular conduction. Building on these seminal observations, contemporary genetic techniques in a multitude of model organisms has characterized the developmental ontogeny, gene regulatory networks, and functional importance of individual anatomical compartments within the cardiac conduction system. This review describes in detail the transcriptional and regulatory networks that act during cardiac conduction system development and homeostasis with a particular emphasis on networks implicated in human electrical variation by large genome-wide association studies. We conclude with a discussion of the clinical implications of these studies and describe some future directions.

T-box transcription factor 3 governs a transcriptional program for the function of the mouse atrioventricular conduction system

Genome-wide association studies have identified noncoding variants near TBX3 that are associated with PR interval and QRS duration, suggesting that subtle changes in TBX3 expression affect atrioventricular conduction system function. To explore whether and to what extent the atrioventricular conduction system is affected by Tbx3 dose reduction, we first characterized electrophysiological properties and morphology of heterozygous Tbx3 mutant (Tbx3+/-) mouse hearts. We found PR interval shortening and prolonged QRS duration, as well as atrioventricular bundle hypoplasia after birth in heterozygous mice. The atrioventricular node size was unaffected. Transcriptomic analysis of atrioventricular nodes isolated by laser capture microdissection revealed hundreds of deregulated genes in Tbx3+/- mutants. Notably, Tbx3+/- atrioventricular nodes showed increased expression of working myocardial gene programs (mitochondrial and metabolic processes, muscle contractility) and reduced expression of pacemaker gene programs (neuronal, Wnt signaling, calcium/ion channel activity). By integrating chromatin accessibility profiles (ATAC sequencing) of atrioventricular tissue and other epigenetic data, we identified Tbx3-dependent atrioventricular regulatory DNA elements (REs) on a genome-wide scale. We used transgenic reporter assays to determine the functionality of candidate REs near Ryr2, an up-regulated chamber-enriched gene, and in Cacna1g, a down-regulated conduction system-specific gene. Using genome editing to delete candidate REs, we showed that a strong intronic bipartite RE selectively governs Cacna1g expression in the conduction system in vivo. Our data provide insights into the multifactorial Tbx3-dependent transcriptional network that regulates the structure and function of the cardiac conduction system, which may underlie the differences in PR duration and QRS interval between individuals carrying variants in the TBX3 locus.

Conversion of human cardiac progenitor cells into cardiac pacemaker-like cells

We used a screening strategy to test for reprogramming factors for the conversion of human cardiac progenitor cells (CPCs) into Pacemaker-like cells. Human transcription factors SHOX2, TBX3, TBX5, TBX18, and the channel protein HCN2, were transiently induced as single factors and in trio combinations into CPCs, first transduced with the connexin 30.2 (CX30.2) mCherry reporter. Following screens for reporter CX30.2 mCherry gene activation and FACS enrichment, we observed the definitive expression of many pacemaker specific genes; including, CX30.2, KCNN4, HCN4, HCN3, HCN1, and SCN3b. These findings suggest that the SHOX2, HCN2, and TBX5 (SHT5) combination of transcription factors is a much better candidate in driving the CPCs into Pacemaker-like cells than other combinations and single transcription factors. Additionally, single-cell RNA sequencing of SHT5 mCherry+ cells revealed cellular enrichment of pacemaker specific genes including TBX3, KCNN4, CX30.2, and BMP2, as well as pacemaker specific potassium and calcium channels (KCND2, KCNK2, and CACNB1). In addition, similar to human and mouse sinoatrial node (SAN) studies, we also observed the down-regulation of NKX2.5. Patch-clamp recordings of the converted Pacemaker-like cells exhibited HCN currents demonstrated the functional characteristic of pacemaker cells. These studies will facilitate the development of an optimal Pacemaker-like cell-based therapy within failing hearts through the recovery of SAN dysfunction.

Transcriptional regulation of the cardiac conduction system

The rate and rhythm of heart muscle contractions are coordinated by the cardiac conduction system (CCS), a generic term for a collection of different specialized muscular tissues within the heart. The CCS components initiate the electrical impulse at the sinoatrial node, propagate it from atria to ventricles via the atrioventricular node and bundle branches, and distribute it to the ventricular muscle mass via the Purkinje fibre network. The CCS thereby controls the rate and rhythm of alternating contractions of the atria and ventricles. CCS function is well conserved across vertebrates from fish to mammals, although particular specialized aspects of CCS function are found only in endotherms (mammals and birds). The development and homeostasis of the CCS involves transcriptional and regulatory networks that act in an embryonic-stage-dependent, tissue-dependent, and dose-dependent manner. This Review describes emerging data from animal studies, stem cell models, and genome-wide association studies that have provided novel insights into the transcriptional networks underlying CCS formation and function. How these insights can be applied to develop disease models and therapies is also discussed.

Shaping the nebulous enhancer in the era of high-throughput assays and genome editing

Since the 1st discovery of transcriptional enhancers in 1981, their textbook definition has remained largely unchanged in the past 37 years. With the emergence of high-throughput assays and genome editing, which are switching the paradigm from bottom-up discovery and testing of individual enhancers to top-down profiling of enhancer activities genome-wide, it has become increasingly evidenced that this classical definition has left substantial gray areas in different aspects. Here we survey a representative set of recent research articles and report the definitions of enhancers they have adopted. The results reveal that a wide spectrum of definitions is used usually without the definition stated explicitly, which could lead to difficulties in data interpretation and downstream analyses. Based on these findings, we discuss the practical implications and suggestions for future studies.

Using Gjd3-CreEGFP mice to examine atrioventricular node morphology and composition

The atrioventricular node (AVN) coordinates the timing of atrial and ventricular contraction to optimize cardiac performance. To study this critical function using mouse genetics, however, new reagents are needed that allow AVN-specific manipulation. Here we describe a novel Gjd3-CreEGFP mouse line that successfully recombines floxed alleles within the AVN beginning at E12.5. These mice have been engineered to express CreEGFP under the control of endogenous Gjd3 regulatory elements without perturbing native protein expression. Detailed histological analysis of Gjd3-CreEGFP mice reveals specific labeling of AVN cardiomyocytes and a subset of cardiac endothelial cells. Importantly, we show that Gjd3-CreEGFP mice have preserved cardiac mechanical and electrical function. In one application of our newly described mouse line, we provide a three-dimensional (3D) view of the AVN using tissue clearing combined with confocal microscopy. With this 3D model as a reference, we identify specific AVN sub-structures based on marker staining characteristics. In addition, we use our Gjd3-CreEGFP mice to guide microdissection of the AVN and construction of a single-cell atlas. Thus, our results establish a new transgenic tool for AVN-specific recombination, provide an updated model of AVN morphology, and describe a roadmap for exploring AVN cellular heterogeneity.

pouC Regulates Expression of bmp4 During Atrioventricular Canal Formation in Zebrafish

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Gene Expression Networks in the Murine Pulmonary Myocardium Provide Insight into the Pathobiology of Atrial Fibrillation

The pulmonary myocardium is a muscular coat surrounding the pulmonary and caval veins. Although its definitive physiological function is unknown, it may have a pathological role as the source of ectopic beats initiating atrial fibrillation. How the pulmonary myocardium gains pacemaker function is not clearly defined, although recent evidence indicates that changed transcriptional gene expression networks are at fault. The gene expression profile of this distinct cell type in situ was examined to investigate underlying molecular events that might contribute to atrial fibrillation. Via systems genetics, a whole-lung transcriptome data set from the BXD recombinant inbred mouse resource was analyzed, uncovering a pulmonary cardiomyocyte gene network of 24 transcripts, coordinately regulated by chromosome 1 and 2 loci. Promoter enrichment analysis and interrogation of publicly available ChIP-seq data suggested that transcription of this gene network may be regulated by the concerted activity of NKX2-5, serum response factor, myocyte enhancer factor 2, and also, at a post-transcriptional level, by RNA binding protein motif 20. Gene ontology terms indicate that this gene network overlaps with molecular markers of the stressed heart. Therefore, we propose that perturbed regulation of this gene network might lead to altered calcium handling, myocyte growth, and contractile force contributing to the aberrant electrophysiological properties observed in atrial fibrillation. We reveal novel molecular interactions and pathways representing possible therapeutic targets for atrial fibrillation. In addition, we highlight the utility of recombinant inbred mouse resources in detecting and characterizing gene expression networks of relatively small populations of cells that have a pathological significance.

Development and Function of the Cardiac Conduction System in Health and Disease

The generation and propagation of the cardiac impulse is the central function of the cardiac conduction system (CCS). Impulse initiation occurs in nodal tissues that have high levels of automaticity, but slow conduction properties. Rapid impulse propagation is a feature of the ventricular conduction system, which is essential for synchronized contraction of the ventricular chambers. When functioning properly, the CCS produces ~2.4 billion heartbeats during a human lifetime and orchestrates the flow of cardiac impulses, designed to maximize cardiac output. Abnormal impulse initiation or propagation can result in brady- and tachy-arrhythmias, producing an array of symptoms, including syncope, heart failure or sudden cardiac death. Underlying the functional diversity of the CCS are gene regulatory networks that direct cell fate towards a nodal or a fast conduction gene program. In this review, we will discuss our current understanding of the transcriptional networks that dictate the components of the CCS, the growth factor-dependent signaling pathways that orchestrate some of these transcriptional hierarchies and the effect of aberrant transcription factor expression on mammalian conduction disease.

Transcriptional regulation of the sodium channel gene (SCN5A) by GATA4 in human heart

Aberrant expression of the sodium channel gene (SCN5A) has been proposed to disrupt cardiac action potential and cause human cardiac arrhythmias, but the mechanisms of SCN5A gene regulation and dysregulation still remain largely unexplored. To gain insight into the transcriptional regulatory networks of SCN5A, we surveyed the promoter and first intronic regions of the SCN5A gene, predicting the presence of several binding sites for GATA transcription factors (TFs). Consistent with this prediction, chromatin immunoprecipitation (ChIP) and sequential ChIP (Re-ChIP) assays show co-occupancy of cardiac GATA TFs GATA4 and GATA5 on promoter and intron 1 SCN5A regions in fresh-frozen human left ventricle samples. Gene reporter experiments show GATA4 and GATA5 synergism in the activation of the SCN5A promoter, and its dependence on predicted GATA binding sites. GATA4 and GATA6 mRNAs are robustly expressed in fresh-frozen human left ventricle samples as measured by highly sensitive droplet digital PCR (ddPCR). GATA5 mRNA is marginally but still clearly detected in the same samples. Importantly, GATA4 mRNA levels are strongly and positively correlated with SCN5A transcript levels in the human heart. Together, our findings uncover a novel mechanism of GATA TFs in the regulation of the SCN5A gene in human heart tissue. Our studies suggest that GATA5 but especially GATA4 are main contributors to SCN5A gene expression, thus providing a new paradigm of SCN5A expression regulation that may shed new light into the understanding of cardiac disease.

GATA4 Regulates Blood-Testis Barrier Function and Lactate Metabolism in Mouse Sertoli Cells

Conditional deletion of Gata4 in Sertoli cells (SCs) of adult mice has been shown to increase permeability of the blood-testis barrier (BTB) and disrupt spermatogenesis. To gain insight into the molecular underpinnings of these phenotypic abnormalities, we assessed the impact of Gata4 gene silencing in cell culture models. Microarray hybridization identified genes dysregulated by siRNA-mediated inhibition of Gata4 in TM4 cells, an immortalized mouse SC line. Differentially expressed genes were validated by quantitative RT-PCR analysis of primary cultures of Gata4(flox/flox) mouse SCs that had been subjected to cre-mediated recombination in vitro. Depletion of GATA4 in TM4 cells and primary SCs was associated with altered expression of genes involved in key facets of BTB maintenance, including tight/adherens junction formation (Tjp1, Cldn12, Vcl, Tnc, Csk) and extracellular matrix reorganization (Lamc1, Col4a1, Col4a5, Mmp10, Mmp23, Timp2). Western blotting and immunocytochemistry demonstrated reduced levels of tight junction protein-1, a prototypical tight junction protein, in GATA4-depleted cells. These changes were accompanied by a loss of morphologically recognizable junctional complexes and a decline in epithelial membrane resistance. Furthermore, Gata4 gene silencing was associated with altered expression of Hk1, Gpi1, Pfkp, Pgam1, Gls2, Pdk3, Pkd4, and Ldhb, genes regulating the production of lactate, a key nutrient that SCs provide to developing germ cells. Comprehensive metabolomic profiling demonstrated impaired lactate production in GATA4-deficient SCs. We conclude that GATA4 plays a pivotal role in the regulation of BTB function and lactate metabolism in mouse SCs.

The formation and function of the cardiac conduction system

The cardiac conduction system (CCS) consists of distinctive components that initiate and conduct the electrical impulse required for the coordinated contraction of the cardiac chambers. CCS development involves complex regulatory networks that act in stage-, tissue- and dose-dependent manners, and recent findings indicate that the activity of these networks is sensitive to common genetic variants associated with cardiac arrhythmias. Here, we review how these findings have provided novel insights into the regulatory mechanisms and transcriptional networks underlying CCS formation and function.

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

Objectives:

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

Materials and Methods:

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

Results:

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

Conclusion:

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

GATA-dependent transcriptional and epigenetic control of cardiac lineage specification and differentiation

Heart progenitor cells differentiate into various cell types including pacemaker and working cardiomyocytes. Cell-type specific gene expression is achieved by combinatorial interactions between tissue-specific transcription factors (TFs), co-factors, and chromatin remodelers and DNA binding elements in regulatory regions. Dysfunction of these transcriptional networks may result in congenital heart defects. Functional analysis of the regulatory DNA sequences has contributed substantially to the identification of the transcriptional network components and combinatorial interactions regulating the tissue-specific gene programs. GATA TFs have been identified as central players in these networks. In particular, GATA binding elements have emerged as a platform to recruit broadly active histone modification enzymes and cell-type-specific co-factors to drive cell-type-specific gene programs. Here, we discuss the role of GATA factors in cell fate decisions and differentiation in the developing heart.

GATA-Binding Factor 6 Contributes to Atrioventricular Node Development and Function

BACKGROUND

Several transcription factors regulate cardiac conduction system (CCS) development and function but the role of each in specifying distinct CCS components remains unclear. GATA-binding factor 6 (GATA6) is a zinc-finger transcription factor that is critical for patterning the cardiovascular system. However, the role of GATA6 in the embryonic heart and CCS has never been shown.

METHODS AND RESULTS

We report that Gata6 is expressed abundantly in the proximal CCS during midgestation in mice. Myocardial-specific deletion of the carboxyl zinc-finger of Gata6 induces loss of hyperpolarizing cyclic nucleotide-gated channel, subtype 4 staining in the compact atrioventricular node with some retention of hyperpolarizing cyclic nucleotide-gated channel, subtype 4 staining in the atrioventricular bundle, but has no significant effect on the connexin-40-positive bundle branches. Furthermore, myocardial-specific deletion of the carboxyl zinc-finger of Gata6 alters atrioventricular conduction in postnatal life as assessed by surface and invasive electrophysiological evaluation, as well as decreasing the number of ventricular myocytes and inducing compensatory myocyte hypertrophy. Myocardial-specific deletion of the carboxyl zinc-finger of Gata6 is also associated with downregulation of the transcriptional repressor ID2 and the cardiac sodium-calcium exchanger NCX1 in the proximal CCS, where GATA6 transactivates both of these factors. Finally, carboxyl zinc-finger deletion of Gata6 reduces cell-cycle exit of TBX3+ myocytes in the developing atrioventricular bundle during the period of atrioventricular node specification, which results in fewer TBX3+ cells in the proximal CCS of mature mutant mice.

CONCLUSIONS

GATA6 contributes to the development and postnatal function of the murine atrioventricular node by promoting cell-cycle exit of specified cardiomyocytes toward a conduction system lineage.

Insights into cardiac conduction system formation provided by HCN4 expression

Specialized myocytes of the cardiac conduction system (CCS) are essential to coordinate sequential contraction of cardiac atria and ventricles. Anomalies of the CCS can result in lethal cardiac arrhythmias, including sick sinus syndrome and atrial or ventricular fibrillation. To develop future therapies and regenerative medicine aimed at cardiac arrhythmias, it is important to understand formation and function of distinct components of the CCS. Essential to this understanding is the development of CCS-specific markers. In this review, we briefly summarize available mouse models of CCS markers and focus on those involving the hyperpolarization cation-selective nucleotide-gated cation channel, HCN4, which selectively marks all components of the specialized CCS in adult heart. Recent studies have revealed, however, that HCN4 expression during development is highly dynamic in cardiac precursors. These studies have offered insights into the contributions of the first and second heart field to myocyte and conduction system lineages and suggested the timing of allocation of specific conduction system precursors during development. Altogether, they have highlighted the utility of HCN4 as a cell surface marker for distinct components of the CCS at distinct stages of development, which can be utilized to facilitate purification and characterization of CCS precursors in mouse and human model systems and pave the way for regenerative therapies.

MyoR modulates cardiac conduction by repressing Gata4

The cardiac conduction system coordinates electrical activation through a series of interconnected structures, including the atrioventricular node (AVN), the central connection point that delays impulse propagation to optimize cardiac performance. Although recent studies have uncovered important molecular details of AVN formation, relatively little is known about the transcriptional mechanisms that regulate AV delay, the primary function of the mature AVN. We identify here MyoR as a novel transcription factor expressed in Cx30.2(+) cells of the AVN. We show that MyoR specifically inhibits a Cx30.2 enhancer required for AVN-specific gene expression. Furthermore, we demonstrate that MyoR interacts directly with Gata4 to mediate transcriptional repression. Our studies reveal that MyoR contains two nonequivalent repression domains. While the MyoR C-terminal repression domain inhibits transcription in a context-dependent manner, the N-terminal repression domain can function in a heterologous context to convert the Hand2 activator into a repressor. In addition, we show that genetic deletion of MyoR in mice increases Cx30.2 expression by 50% and prolongs AV delay by 13%. Taken together, we conclude that MyoR modulates a Gata4-dependent regulatory circuit that establishes proper AV delay, and these findings may have wider implications for the variability of cardiac rhythm observed in the general population.

Reciprocal myocardial-endocardial interactions pattern the delay in atrioventricular junction conduction

Efficient blood flow depends on two developmental processes that occur within the atrioventricular junction (AVJ) of the heart: conduction delay, which entrains sequential chamber contraction; and valve formation, which prevents retrograde fluid movement. Defects in either result in severe congenital heart disease; however, little is known about the interplay between these two crucial developmental processes. Here, we show that AVJ conduction delay is locally assigned by the morphogenetic events that initiate valve formation. Our data demonstrate that physical separation from endocardial-derived factors prevents AVJ myocardium from becoming fast conducting. Mechanistically, this physical separation is induced by myocardial-derived factors that support cardiac jelly deposition at the onset of valve formation. These data offer a novel paradigm for conduction patterning, whereby reciprocal myocardial-endocardial interactions coordinate the processes of valve formation with establishment of conduction delay. This, in turn, synchronizes the electrophysiological and structural events necessary for the optimization of blood flow through the developing heart.

The functional diversity of essential genes required for mammalian cardiac development

Genes required for an organism to develop to maturity (for which no other gene can compensate) are considered essential. The continuing functional annotation of the mouse genome has enabled the identification of many essential genes required for specific developmental processes including cardiac development. Patterns are now emerging regarding the functional nature of genes required at specific points throughout gestation. Essential genes required for development beyond cardiac progenitor cell migration and induction include a small and functionally homogenous group encoding transcription factors, ligands and receptors. Actions of core cardiogenic transcription factors from the Gata, Nkx, Mef, Hand, and Tbx families trigger a marked expansion in the functional diversity of essential genes from midgestation onwards. As the embryo grows in size and complexity, genes required to maintain a functional heartbeat and to provide muscular strength and regulate blood flow are well represented. These essential genes regulate further specialization and polarization of cell types along with proliferative, migratory, adhesive, contractile, and structural processes. The identification of patterns regarding the functional nature of essential genes across numerous developmental systems may aid prediction of further essential genes and those important to development and/or progression of disease. genesis 52:713–737, 2014.

GATA-dependent regulatory switches establish atrioventricular canal specificity during heart development

The embryonic vertebrate heart tube develops an atrioventricular canal that divides the atrial and ventricular chambers, forms atrioventricular conduction tissue and organizes valve development. Here we assess the transcriptional mechanism underlying this localized differentiation process. We show that atrioventricular canal-specific enhancers are GATA-binding site-dependent and act as switches that repress gene activity in the chambers. We find that atrioventricular canal-specific gene loci are enriched in H3K27ac, a marker of active enhancers, in atrioventricular canal tissue and depleted in H3K27ac in chamber tissue. In the atrioventricular canal, Gata4 activates the enhancers in synergy with Bmp2/Smad signalling, leading to H3K27 acetylation. In contrast, in chambers, Gata4 cooperates with pan-cardiac Hdac1 and Hdac2 and chamber-specific Hey1 and Hey2, leading to H3K27 deacetylation and repression. We conclude that atrioventricular canal-specific enhancers are platforms integrating cardiac transcription factors, broadly active histone modification enzymes and localized co-factors to drive atrioventricular canal-specific gene activity.

The atrioventricular canal partitions the developing vertebrate heart. Here, the authors show that the cardiac transcription factor Gata4 together with histone modification enzymes and localized co-factors binds atrioventricular canal-specific enhancers, thereby repressing gene activity in the cardiac chambers.

Shox2 regulates the pacemaker gene program in embryoid bodies

The pacemaker tissues of the heart are a complex set of specialized cells that initiate the rhythmic heartbeat. The sinoatrial node (SAN) serves as the primary pacemaker, whereas the atrioventricular node can serve as a subsidiary pacemaker in cases of SAN failure or block. The elucidation of genetic networks regulating the development of these tissues is crucial for understanding the mechanisms underlying arrhythmias and for the design of targeted therapies. Here we report temporal and spatial self-organized formation of the pacemaker and contracting tissues in three-dimensional aggregate cultures of mouse embryonic stem cells termed embryoid bodies (EBs). Using genetic marker expression and electrophysiological analyses we demonstrate that in EBs the pacemaker potential originates from a localized population of cells and propagates into the adjacent contracting region forming a functional syncytium. When Shox2, a major determinant of the SAN genetic pathway, was ablated we observed substantial slowing of spontaneous contraction rates and an altered gene expression pattern including downregulation of HCN4, Cx45, Tbx2, Tbx3, and bone morphogenetic protein 4 (BMP4); and upregulation of Cx40, Cx43, Nkx2.5, and Tbx5. This phenotype could be rescued by adding BMP4 to Shox2 knockout EBs in culture from days 6 to 16 of differentiation. When wild-type EBs were treated with Noggin, a potent BMP4 inhibitor, we observed a phenotype consistent with the Shox2 knockout EB. Altogether, we have generated a reproducible in vitro model that will be an invaluable tool for studying the molecular pathways regulating the development of cardiac pacemaker tissues.

Genetic isolation of stem cell-derived pacemaker-nodal cardiac myocytes

Dysfunction of the cardiac pacemaker tissues due to genetic defects, acquired diseases, or aging results in arrhythmias. When arrhythmias occur, artificial pacemaker implants are used for treatment. However, the numerous limitations of electronic implants have prompted studies of biological pacemakers that can integrate into the myocardium providing a permanent cure. Embryonic stem (ES) cells cultured as three-dimensional (3D) spheroid aggregates termed embryoid bodies possess the ability to generate all cardiac myocyte subtypes. Here, we report the use of a SHOX2 promoter and a Cx30.2 enhancer to genetically identify and isolate ES cell-derived sinoatrial node (SAN) and atrioventricular node (AVN) cells, respectively. The ES cell-derived Shox2 and Cx30.2 cardiac myocytes exhibit a spider cell morphology and high intracellular calcium loading characteristic of pacemaker-nodal myocytes. These cells express abundant levels of pacemaker genes such as endogenous HCN4, Cx45, Cx30.2, Tbx2, and Tbx3. These cells were passaged, frozen, and thawed multiple times while maintaining their pacemaker-nodal phenotype. When cultured as 3D aggregates in an attempt to create a critical mass that simulates in vivo architecture, these cell lines exhibited an increase in the expression level of key regulators of cardiovascular development, such as GATA4 and GATA6 transcription factors. In addition, the aggregate culture system resulted in an increase in the expression level of several ion channels that play a major role in the spontaneous diastolic depolarization characteristic of pacemaker cells. We have isolated pure populations of SAN and AVN cells that will be useful tools for generating biological pacemakers.

Connexin diversity in the heart: insights from transgenic mouse models

Cardiac conduction is mediated by gap junction channels that are formed by connexin (Cx) protein subunits. The connexin family of proteins consists of more than 20 members varying in their biophysical properties and ability to combine with other connexins into heteromeric gap junction channels. The mammalian heart shows regional differences both in connexin expression profile and in degree of electrical coupling. The latter reflects functional requirements for conduction velocity which needs to be low in the sinoatrial and atrioventricular nodes and high in the ventricular conduction system. Over the past 20 years knowledge of the biology of gap junction channels and their role in the genesis of cardiac arrhythmias has increased enormously. This review focuses on the insights gained from transgenic mouse models. The mouse heart expresses Cx30, 30.2, 37, 40, 43, 45, and 46. For these connexins a variety of knock-outs, heart-specific knock-outs, conditional knock-outs, double knock-outs, knock-ins and overexpressors has been studied. We discuss the cardiac phenotype in these models and compare Cx expression between mice and men. Mouse models have enhanced our understanding of (patho)-physiological implications of Cx diversity in the heart. In principle connexin-specific modulation of electrical coupling in the heart represents an interesting treatment strategy for cardiac arrhythmias and conduction disorders.

Functional anatomy of distant-acting mammalian enhancers

Transcriptional enhancers are a major class of functional element embedded in the vast non-coding portion of the human genome. Acting over large genomic distances, enhancers play critical roles in the tissue and cell type-specific regulation of genes, and there is mounting evidence that they contribute to the aetiology of many human diseases. Methods for genome-wide mapping of enhancer regions are now available, but the functional architecture contained within human enhancer elements remains unclear. Here, we review recent approaches aimed at understanding the functional anatomy of individual enhancer elements, using systematic qualitative and quantitative assessments of mammalian enhancer variants in cultured cells and in vivo. These studies provide direct insight into common architectural characteristics of enhancers including the presence of multiple transcription factor-binding sites and the mixture of both transcriptionally activating and repressing domains within the same enhancer. Despite such progress in understanding the functional composition of enhancers, the inherent complexities of enhancer anatomy continue to limit our ability to predict the impact of sequence changes on in vivo enhancer function. While providing an initial glimpse into the mutability of mammalian enhancers, these observations highlight the continued need for experimental enhancer assessment as genome sequencing becomes routine in the clinic.

Spatiotemporal regulation of an Hcn4 enhancer defines a role for Mef2c and HDACs in cardiac electrical patterning

Regional differences in cardiomyocyte automaticity permit the sinoatrial node (SAN) to function as the leading cardiac pacemaker and the atrioventricular (AV) junction as a subsidiary pacemaker. The regulatory mechanisms controlling the distribution of automaticity within the heart are not understood. To understand regional variation in cardiac automaticity, we carried out an in vivo analysis of cis-regulatory elements that control expression of the hyperpolarization-activated cyclic-nucleotide gated ion channel 4 (Hcn4). Using transgenic mice, we found that spatial and temporal patterning of Hcn4 expression in the AV conduction system required cis-regulatory elements with multiple conserved fragments. One highly conserved region, which contained a myocyte enhancer factor 2C (Mef2C) binding site previously described in vitro, induced reporter expression specifically in the embryonic non-chamber myocardium and the postnatal AV bundle in a Mef2c-dependent manner in vivo. Inhibition of histone deacetylase (HDAC) activity in cultured transgenic embryos showed expansion of reporter activity to working myocardium. In adult animals, hypertrophy induced by transverse aortic constriction, which causes translocation of HDACs out of the nucleus, resulted in ectopic activation of the Hcn4 enhancer in working myocardium, recapitulating pathological electrical remodeling. These findings reveal mechanisms that control the distribution of automaticity among cardiomyocytes during development and in response to stress.

The effect of connexin40 deficiency on ventricular conduction system function during development

AIMS

The aim of this study was to characterize ventricular activation patterns in normal and connexin40-deficient mice in order to dissect the role of connexin40 in developing the conduction system.

METHODS AND RESULTS

We performed optical mapping of epicardial activation between ED9.5-18.5 and analysed ventricular activation patterns and times of left ventricular activation. Mouse embryos deficient for connexin40 were compared with normal and heterozygous littermates. Morphology of the primary interventricular ring (PIR) was delineated with the help of T3-LacZ transgene. Four major types of ventricular activation patterns characterized by primary breakthrough in different parts of the heart were detected during development: PIR, left ventricular apex, right ventricular apex, and dual right and left ventricular apices. Activation through PIR was frequently present at the early stages until ED12.5. From ED14.5, the majority of hearts showed dual left and right apical breakthrough, suggesting functionality of both bundle branches. Connexin40-deficient embryos showed initially a delay in left bundle branch function, but the right bundle branch block, previously described in the adults, was not detected in ED14.5 embryos and appeared only gradually with 80% penetrance at ED18.5.

CONCLUSION

The switch of function from the early PIR conduction pathway to the mature apex to base activation is dependent upon upregulation of connexin40 expression in the ventricular trabeculae. The early function of right bundle branch does not depend on connexin40. Quantitative analysis of normal mouse embryonic ventricular conduction patterns will be useful for interpretation of effects of mutations affecting the function of the cardiac conduction system.

Electrophysiological patterning of the heart

In the adult heart, electrophysiological heterogeneity is present to guide activation and contraction. A change in electrophysiological heterogeneity, for example, during disease, can contribute to arrhythmogenesis. During development, spatial and temporal patterns of transcriptional activity regulate the localized expression of ion channels that cause electrophysiological heterogeneity throughout the heart. If we gain insight into the regulating processes that generate the electrophysiological characteristics and factors involved during development, we can use this knowledge in the search for new therapeutic targets. In this review, we discuss which factors guide the electrical patterning of atrioventricular conduction system and ventricles and how this patterning relates to arrhythmogenic disease in patients.

Gene regulatory networks in cardiac conduction system development

The cardiac conduction system is a specialized tract of myocardial cells responsible for maintaining normal cardiac rhythm. Given its critical role in coordinating cardiac performance, a detailed analysis of the molecular mechanisms underlying conduction system formation should inform our understanding of arrhythmia pathophysiology and affect the development of novel therapeutic strategies. Historically, the ability to distinguish cells of the conduction system from neighboring working myocytes presented a major technical challenge for performing comprehensive mechanistic studies. Early lineage tracing experiments suggested that conduction cells derive from cardiomyocyte precursors, and these claims have been substantiated by using more contemporary approaches. However, regional specialization of conduction cells adds an additional layer of complexity to this system, and it appears that different components of the conduction system utilize unique modes of developmental formation. The identification of numerous transcription factors and their downstream target genes involved in regional differentiation of the conduction system has provided insight into how lineage commitment is achieved. Furthermore, by adopting cutting-edge genetic techniques in combination with sophisticated phenotyping capabilities, investigators have made substantial progress in delineating the regulatory networks that orchestrate conduction system formation and their role in cardiac rhythm and physiology. This review describes the connectivity of these gene regulatory networks in cardiac conduction system development and discusses how they provide a foundation for understanding normal and pathological human cardiac rhythms.

Myocardial Notch signaling reprograms cardiomyocytes to a conduction-like phenotype

BACKGROUND

Notch signaling has previously been shown to play an essential role in regulating cell fate decisions and differentiation during cardiogenesis in many systems including Drosophila, Xenopus, and mammals. We hypothesized that Notch may also be involved in directing the progressive lineage restriction of cardiomyocytes into specialized conduction cells.

METHODS AND RESULTS

In hearts where Notch signaling is activated within the myocardium from early development onward, Notch promotes a conduction-like phenotype based on ectopic expression of conduction system-specific genes and cell autonomous changes in electrophysiology. With the use of an in vitro assay to activate Notch in newborn cardiomyocytes, we observed global changes in the transcriptome, and in action potential characteristics, consistent with reprogramming to a conduction-like phenotype.

CONCLUSIONS

Notch can instruct the differentiation of chamber cardiac progenitors into specialized conduction-like cells. Plasticity remains in late-stage cardiomyocytes, which has potential implications for engineering of specialized cardiovascular tissues.

Regulation of connexin expression by transcription factors and epigenetic mechanisms

Gap junctions are specialized cell-cell junctions that directly link the cytoplasm of neighboring cells. They mediate the direct transfer of metabolites and ions from one cell to another. Discoveries of human genetic disorders due to mutations in gap junction protein (connexin [Cx]) genes and experimental data on connexin knockout mice provide direct evidence that gap junctional intercellular communication is essential for tissue functions and organ development, and that its dysfunction causes diseases. Connexin-related signaling also involves extracellular signaling (hemichannels) and non-channel intracellular signaling. Thus far, 21 human genes and 20 mouse genes for connexins have been identified. Each connexin shows tissue- or cell-type-specific expression, and most organs and many cell types express more than one connexin. Connexin expression can be regulated at many of the steps in the pathway from DNA to RNA to protein. In recent years, it has become clear that epigenetic processes are also essentially involved in connexin gene expression. In this review, we summarize recent knowledge on regulation of connexin expression by transcription factors and epigenetic mechanisms including histone modifications, DNA methylation, and microRNA. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

Localized and temporal gene regulation in heart development

The heart is a structurally complex and functionally heterogeneous organ. The repertoire of genes active in a given cardiac cell defines its shapes and function. This process of localized or heterogeneous gene expression is regulated to a large extent at the level of transcription, dictating the degree particular genes in a cell are active. Therefore, errors in the regulation of localized gene expression are at the basis of misregulation of the delicate process of heart development and function. In this review, we provide an overview of the origin of the different components of the vertebrate heart, and discuss our current understanding of the regulation of localized gene expression in the developing heart. We will also discuss where future research may lead to gain more insight into this process, which should provide much needed insight into the dysregulation of heart development and function, and the etiology of congenital defects.

Transcription factor pathways and congenital heart disease

Congenital heart disease is a major cause of morbidity and mortality throughout life. Mutations in numerous transcription factors have been identified in patients and families with some of the most common forms of cardiac malformations and arrhythmias. This review discusses transcription factor pathways known to be important for normal heart development and how abnormalities in these pathways have been linked to morphological and functional forms of congenital heart defects. A comprehensive, current list of known transcription factor mutations associated with congenital heart disease is provided, but the review focuses primarily on three key transcription factors, Nkx2-5, GATA4, and Tbx5, and their known biochemical and genetic partners. By understanding the interaction partners, transcriptional targets, and upstream activators of these core cardiac transcription factors, additional information about normal heart formation and further insight into genes and pathways affected in congenital heart disease should result.

Tbx2 and Tbx3 induce atrioventricular myocardial development and endocardial cushion formation

A key step in heart development is the coordinated development of the atrioventricular canal (AVC), the constriction between the atria and ventricles that electrically and physically separates the chambers, and the development of the atrioventricular valves that ensure unidirectional blood flow. Using knock-out and inducible overexpression mouse models, we provide evidence that the developmentally important T-box factors Tbx2 and Tbx3, in a functionally redundant manner, maintain the AVC myocardium phenotype during the process of chamber differentiation. Expression profiling and ChIP-sequencing analysis of Tbx3 revealed that it directly interacts with and represses chamber myocardial genes, and induces the atrioventricular pacemaker-like phenotype by activating relevant genes. Moreover, mutant mice lacking 3 or 4 functional alleles of Tbx2 and Tbx3 failed to form atrioventricular cushions, precursors of the valves and septa. Tbx2 and Tbx3 trigger development of the cushions through a regulatory feed-forward loop with Bmp2, thus providing a mechanism for the co-localization and coordination of these important processes in heart development.

Origin and development of the atrioventricular myocardial lineage: insight into the development of accessory pathways

Defects originating from the atrioventricular canal region are part of a wide spectrum of congenital cardiovascular malformations that frequently affect newborns. These defects include partial or complete atrioventricular septal defects, atrioventricular valve defects, and arrhythmias, such as atrioventricular re-entry tachycardia, atrioventricular nodal block, and ventricular preexcitation. Insight into the cellular origin of the atrioventricular canal myocardium and the molecular mechanisms that control its development will aid in the understanding of the etiology of the atrioventricular defects. This review discusses current knowledge concerning the origin and fate of the atrioventricular canal myocardium, the molecular mechanisms that determine its specification and differentiation, and its role in the development of certain malformations such as those that underlie ventricular preexcitation.

The emerging genetic landscape underlying cardiac conduction system function

Proper function of an organized Cardiac Conduction System (CCS) is vital to the survival of metazoans ranging from fly to man. The routine use of non-invasive electrocardiogram measures in the diagnosis and monitoring of cardiovascular health has established a trove of reliable CCS functional data in both normal and diseased cardiac states. Recent combination of echocardiogram (ECG) data with genome-wide association studies has identified genomic regions implicated in ECG variability which impact CCS function. In this study, we review the substantial recent progress in this area, highlighting the identification of novel loci, confirming the importance of previously implicated loci in CCS function, and exploring potential links between genes with important roles in developmental processes and variation in function of the CCS.