Transcription factor ISL1 is essential for pacemaker development and function

Molecular Regulation of Cardiac Conduction System Development

PURPOSE OF REVIEW

The cardiac conduction system, composed of pacemaker cells and conducting cardiomyocytes, orchestrates the propagation of electrical activity to synchronize heartbeats. The conduction system plays a crucial role in the development of cardiac arrhythmias. In the embryo, the cells of the conduction system derive from the same cardiac progenitors as the contractile cardiomyocytes and and the key question is how this choice is made during development.

RECENT FINDINGS

This review focuses on recent advances in developmental biology using the mouse as animal model to better understand the cellular origin and molecular regulations that control morphogenesis of the cardiac conduction system, including the latest findings in single-cell transcriptomics. The conducting cell fate is acquired during development starting with pacemaking activity and last with the formation of a complex fast-conducting network. Cardiac conduction system morphogenesis is controlled by complex transcriptional and gene regulatory networks that differ in the components of the cardiac conduction system.

TBX3 transfection and nodal signal pathway inhibition promote differentiation of adipose mesenchymal stem cell to cardiac pacemaker-like cells

BACKGROUND

Mesenchymal stem cells (MSCs) are known as one of the best candidate cells to produce cardiac pacemaker-like cells (CPLCs). Upregulation of TBX3 transcription factor and inhibition of the nodal signal pathway have a significant role in the formation of cardiac pacemaker cells such as sinoatrial and atrioventricular nodes, which initiate the heartbeat and control the rhythm of heart contractions. This study aimed to confirm the effects of transfection of TBX3 transcription factor and inhibition of the nodal signal pathway on differentiating adipose-derived MSCs (AD-MSCs) to CPLCs. AD-MSCs were characterized using flow cytometry and three-lineage differentiation staining.

METHODS

The transfection of TBX3 plasmid was carried out using lipofectamine, and inhibition of the nodal signal pathway was done using the small-molecule SB431542. The morphology of the cells was observed using a light microscope. Pacemaker-specific markers, including TBX3, Cx30, HCN4, HCN1, HCN3, and KCNN4, were evaluated using the qRT-PCR method. For protein level, TBX3 and Cx30 were evaluated using ELISA and immunofluorescence staining. The electrophysiology of cells was evaluated using a patch clamp.

RESULTS

The TBX3 expression in the TBX3, SM, and TBX + SM groups significantly higher (p < 0.05) compared to the control group and cardiomyocytes. The expression of Cx40 and Cx43 genes were lower in TBX3, SM, TBX + SM groups. In contrast, Cx30 gene showed higher expression in TBX3 group. The expression HCN1, HCN3, and HCN4 genes are higher in TBX3 group.

CONCLUSION

The transfection of TBX3 and inhibition of the nodal signal pathway by small-molecule SB431542 enhanced differentiation of AD-MSCs to CPLCs.

TAD boundary deletion causes PITX2-related cardiac electrical and structural defects

While 3D chromatin organization in topologically associating domains (TADs) and loops mediating regulatory element-promoter interactions is crucial for tissue-specific gene regulation, the extent of their involvement in human Mendelian disease is largely unknown. Here, we identify 7 families presenting a new cardiac entity associated with a heterozygous deletion of 2 CTCF binding sites on 4q25, inducing TAD fusion and chromatin conformation remodeling. The CTCF binding sites are located in a gene desert at 1 Mb from the Paired-like homeodomain transcription factor 2 gene (PITX2). By introducing the ortholog of the human deletion in the mouse genome, we recapitulate the patient phenotype and characterize an opposite dysregulation of PITX2 expression in the sinoatrial node (ectopic activation) and ventricle (reduction), respectively. Chromatin conformation assay performed in human induced pluripotent stem cell-derived cardiomyocytes harboring the minimal deletion identified in family#1 reveals a conformation remodeling and fusion of TADs. We conclude that TAD remodeling mediated by deletion of CTCF binding sites causes a new autosomal dominant Mendelian cardiac disorder.

Resequencing Analyses Revealed Genetic Diversity and Selection Signatures during Rabbit Breeding and Improvement

Domestication has shaped the diverse characteristics of rabbits, including coat color, fur structure, body size, and various physiological traits. Utilizing whole-genome resequencing (DNBSEQ-T7), we analyzed the genetic diversity, population structure, and genomic selection across 180 rabbits from 17 distinct breeds to uncover the genetic basis of these traits. We conducted whole-genome sequencing on 17 rabbit breeds, identifying 17,430,184 high-quality SNPs and analyzing genomic diversity, patterns of genomic variation, population structure, and selection signatures related to coat color, coat structure, long hair, body size, reproductive capacity, and disease resistance. Through PCA and NJ tree analyses, distinct clusters emerged among Chinese indigenous rabbits, suggesting varied origins and domestication histories. Selective sweep testing pinpointed regions and genes linked to domestication and key morphological and economic traits, including those affecting coat color (TYR, ASIP), structure (LIPH), body size (INSIG2, GLI3), fertility (EDNRA, SRD5A2), heat stress adaptation (PLCB1), and immune response (SEC31A, CD86, LAP3). Our study identified key genomic signatures of selection related to traits such as coat color, fur structure, body size, and fertility; these findings highlight the genetic basis underlying phenotypic diversification in rabbits and have implications for breeding programs aiming to improve productive, reproductive, and adaptive traits. The detected genomic signatures of selection also provide insights into rabbit domestication and can aid conservation efforts for indigenous breeds.

Canonical Wnt signaling directs the generation of functional human PSC-derived atrioventricular canal cardiomyocytes in bioprinted cardiac tissues

The creation of a functional 3D bioprinted human heart remains challenging, largely due to the lack of some crucial cardiac cell types, including the atrioventricular canal (AVC) cardiomyocytes, which are essential to slow down the electrical impulse between the atrium and ventricle. By utilizing single-cell RNA sequencing analysis and a 3D bioprinting technology, we discover that stage-specific activation of canonical Wnt signaling creates functional AVC cardiomyocytes derived from human pluripotent stem cells. These cardiomyocytes display morphological characteristics and express molecular markers of AVC cardiomyocytes, including transcription factors TBX2 and MSX2. When bioprinted in prefabricated cardiac tissues, these cardiomyocytes successfully delay the electrical impulse, demonstrating their capability of functioning as the AVC cardiomyocytes in vitro. Thus, these findings not only identify canonical Wnt signaling as a key regulator of the AVC cardiomyocyte differentiation in vitro, but, more importantly, provide a critical cellular source for the biofabrication of a functional human heart.

The cardiac conduction system: History, development, and disease

The heart is the first organ to form during embryonic development, establishing the circulatory infrastructure necessary to sustain life and enable downstream organogenesis. Critical to the heart's function is its ability to initiate and propagate electrical impulses that allow for the coordinated contraction and relaxation of its chambers, and thus, the movement of blood and nutrients. Several specialized structures within the heart, collectively known as the cardiac conduction system (CCS), are responsible for this phenomenon. In this review, we discuss the discovery and scientific history of the mammalian cardiac conduction system as well as the key genes and transcription factors implicated in the formation of its major structures. We also describe known human diseases related to CCS development and explore existing challenges in the clinical context.

Cardiac Development at a Single-Cell Resolution

Mammalian cardiac development is a complex, multistage process. Though traditional lineage tracing studies have characterized the broad trajectories of cardiac progenitors, the advent and rapid optimization of single-cell RNA sequencing methods have yielded an ever-expanding toolkit for characterizing heterogeneous cell populations in the developing heart. Importantly, they have allowed for a robust profiling of the spatiotemporal transcriptomic landscape of the human and mouse heart, revealing the diversity of cardiac cells-myocyte and non-myocyte-over the course of development. These studies have yielded insights into novel cardiac progenitor populations, chamber-specific developmental signatures, the gene regulatory networks governing cardiac development, and, thus, the etiologies of congenital heart diseases. Furthermore, single-cell RNA sequencing has allowed for the exquisite characterization of distinct cardiac populations such as the hard-to-capture cardiac conduction system and the intracardiac immune population. Therefore, single-cell profiling has also resulted in new insights into the regulation of cardiac regeneration and injury repair. Single-cell multiomics approaches combining transcriptomics, genomics, and epigenomics may uncover an even more comprehensive atlas of human cardiac biology. Single-cell analyses of the developing and adult mammalian heart offer an unprecedented look into the fundamental mechanisms of cardiac development and the complex diseases that may arise from it.

Cadherin-5 facilitated the differentiation of human induced pluripotent stem cells into sinoatrial node-like pacemaker cells by regulating β-catenin

Our study was conducted to investigate whether cadherin-5 (CDH5), a vascular endothelial cell adhesion glycoprotein, could facilitate the differentiation of human induced pluripotent stem cells (hiPSCs) into sinoatrial node-like pacemaker cells (SANLPCs), following previous findings of silk-fibroin hydrogel-induced direct conversion of quiescent cardiomyocytes into pacemaker cells in rats through the activation of CDH5. In this study, the differentiating hiPSCs were treated with CDH5 (40 ng/mL) between Day 5 and 7 during cardiomyocytes differentiation. The findings in the present study demonstrated that CDH5 stimulated the expression of pacemaker-specific markers while suppressing markers associated with working cardiomyocytes, resulting in an increased proportion of SANLPCs among hiPSCs-derived cardiomyocytes (hiPSC-CMs) population. Moreover, CDH5 induced typical electrophysiological characteristics resembling cardiac pacemaker cells in hiPSC-CMs. Further mechanistic investigations revealed that the enriched differentiation of hiPSCs into SANLPCs induced by CDH5 was partially reversed by iCRT14, an inhibitor of β-catenin. Therefore, based on the aforementioned findings, it could be inferred that the regulation of β-catenin by CDH5 played a crucial role in promoting the enriched differentiation of hiPSCs into SANLPCs, which presents a novel avenue for the construction of biological pacemakers in forthcoming research.

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.

The method of sinus node-like pacemaker cells from human induced pluripotent stem cells by BMP and Wnt signaling

The embryonic development of sinus nodes (SAN) is co-regulated by multiple signaling pathways. Among these, the bone morphogenetic protein (BMP) and Wnt signaling pathways are involved in the development of SAN. In this study, the effects of BMP and Wnt signaling on the differentiation of SAN-like pacemaker cells (SANLPCs) were investigated. Human induced pluripotent stem cells (hiPSCs) were divided into four groups: control, BMP4, CHIR-3, and BMP4 + CHIR (CHIR: a Wnt signaling activator). The samples were tested at day (D) 15 of differentiation. The final protocol for the activation of BMP signaling at D0-D3 and reactivation of Wnt signaling at D5-D7 in the differentiation of hiPSCs were determined. The results showed that the mRNA levels of pacemaker markers (TBX18, SHOX2, TBX3, HCN4, and HCN1) were higher in the BMP4 + CHIR group than in the control group, and working myocardial genes were downregulated. The immunofluorescence assay revealed that the expression of SHOX2 and HCN4 increased in the BMP4 + CHIR group compared to that in the other groups. In addition, the results of patch clamps revealed that a funny current of higher density and typical SAN action potentials were recorded, except in the control group, in which the L-type calcium current was higher in the BMP4 + CHIR group than in the other groups. Finally, the proportion of SANLPCs (cTnT+ NKX2.5-) was further enhanced by the combination of BMP4 and CHIR treatment. In summary, the combination of BMP and Wnt signaling promotes the differentiation of SANLPCs from hiPSCs.

The multi-lineage transcription factor ISL1 controls cardiomyocyte cell fate through interaction with NKX2.5

Congenital heart disease often arises from perturbations of transcription factors (TFs) that guide cardiac development. ISLET1 (ISL1) is a TF that influences early cardiac cell fate, as well as differentiation of other cell types including motor neuron progenitors (MNPs) and pancreatic islet cells. While lineage specificity of ISL1 function is likely achieved through combinatorial interactions, its essential cardiac interacting partners are unknown. By assaying ISL1 genomic occupancy in human induced pluripotent stem cell-derived cardiac progenitors (CPs) or MNPs and leveraging the deep learning approach BPNet, we identified motifs of other TFs that predicted ISL1 occupancy in each lineage, with NKX2.5 and GATA motifs being most closely associated to ISL1 in CPs. Experimentally, nearly two-thirds of ISL1-bound loci were co-occupied by NKX2.5 and/or GATA4. Removal of NKX2.5 from CPs led to widespread ISL1 redistribution, and overexpression of NKX2.5 in MNPs led to ISL1 occupancy of CP-specific loci. These results reveal how ISL1 guides lineage choices through a combinatorial code that dictates genomic occupancy and transcription.

Harnessing cell reprogramming for cardiac biological pacing

Electrical impulses from cardiac pacemaker cardiomyocytes initiate cardiac contraction and blood pumping and maintain life. Abnormal electrical impulses bring patients with low heart rates to cardiac arrest. The current therapy is to implant electronic devices to generate backup electricity. However, complications inherent to electronic devices remain unbearable suffering. Therefore, cardiac biological pacing has been developed as a hardware-free alternative. The approaches to generating biological pacing have evolved recently using cell reprogramming technology to generate pacemaker cardiomyocytes in-vivo or in-vitro. Different from conventional methods by electrical re-engineering, reprogramming-based biological pacing recapitulates various phenotypes of de novo pacemaker cardiomyocytes and is more physiological, efficient, and easy for clinical implementation. This article reviews the present state of the art in reprogramming-based biological pacing. We begin with the rationale for this new approach and review its advances in creating a biological pacemaker to treat bradyarrhythmia.

Local tissue mechanics control cardiac pacemaker cell embryonic patterning

Cardiac pacemaker cells (CPCs) initiate the electric impulses that drive the rhythmic beating of the heart. CPCs reside in a heterogeneous, ECM-rich microenvironment termed the sinoatrial node (SAN). Surprisingly, little is known regarding the biochemical composition or mechanical properties of the SAN, and how the unique structural characteristics present in this region of the heart influence CPC function remains poorly understood. Here, we have identified that SAN development involves the construction of a "soft" macromolecular ECM that specifically encapsulates CPCs. In addition, we demonstrate that subjecting embryonic CPCs to substrate stiffnesses higher than those measured in vivo results in loss of coherent electrical oscillation and dysregulation of the HCN4 and NCX1 ion channels required for CPC automaticity. Collectively, these data indicate that local mechanics play a critical role in maintaining the embryonic CPC function while also quantitatively defining the range of material properties that are optimal for embryonic CPC maturation.

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

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

Sinus node dysfunction: current understanding and future directions

The sinoatrial node (SAN) is the primary pacemaker of the heart. Normal SAN function is crucial in maintaining proper cardiac rhythm and contraction. Sinus node dysfunction (SND) is due to abnormalities within the SAN, which can affect the heartbeat frequency, regularity, and the propagation of electrical pulses through the cardiac conduction system. As a result, SND often increases the risk of cardiac arrhythmias. SND is most commonly seen as a disease of the elderly given the role of degenerative fibrosis as well as other age-dependent changes in its pathogenesis. Despite the prevalence of SND, current treatment is limited to pacemaker implantation, which is associated with substantial medical costs and complications. Emerging evidence has identified various genetic abnormalities that can cause SND, shedding light on the molecular underpinnings of SND. Identification of these molecular mechanisms and pathways implicated in the pathogenesis of SND is hoped to identify novel therapeutic targets for the development of more effective therapies for this disease. In this review article, we examine the anatomy of the SAN and the pathophysiology and epidemiology of SND. We then discuss in detail the most common genetic mutations correlated with SND and provide our perspectives on future research and therapeutic opportunities in this field.

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.

A single cell transcriptional roadmap of human pacemaker cell differentiation

Each heartbeat is triggered by the sinoatrial node (SAN), the primary pacemaker of the heart. Studies in animal models have revealed that pacemaker cells share a common progenitor with the (pro)epicardium, and that the pacemaker cardiomyocytes further diversify into ‘transitional’, ‘tail’, and ‘head’ subtypes. However, the underlying molecular mechanisms, especially of human pacemaker cell development, are poorly understood. Here, we performed single cell RNA sequencing (scRNA-seq) and trajectory inference on human induced pluripotent stem cells (hiPSCs) differentiating to SAN-like cardiomyocytes (SANCMs) to construct a roadmap of transcriptional changes and lineage decisions. In differentiated SANCM, we identified distinct clusters that closely resemble different subpopulations of the in vivo SAN. Moreover, the presence of a side population of proepicardial cells suggested their shared ontogeny with SANCM, as also reported in vivo. Our results demonstrate that the divergence of SANCM and proepicardial lineages is determined by WNT signaling. Furthermore, we uncovered roles for TGFβ and WNT signaling in the branching of transitional and head SANCM subtypes, respectively. These findings provide new insights into the molecular processes involved in human pacemaker cell differentiation, opening new avenues for complex disease modeling in vitro and inform approaches for cell therapy-based regeneration of the SAN.

Comparative Study of Transcriptome in the Hearts Isolated from Mice, Rats, and Humans

The heart is a significant organ in mammalian life, and the heartbeat mechanism has been an essential focus of science. However, few studies have focused on species differences. Accordingly, challenges remain in studying genes that have universal functions across species and genes that determine species differences. Here, we analyzed transcriptome data in mouse, rat, and human atria, ventricles, and sinoatrial nodes (SA) obtained from different platforms and compared them by calculating specificity measure (SPM) values in consideration of species differences. Among the three heart regions, the species differences in SA were the greatest, and we searched for genes that determined the essential characteristics of SA, which was SHOX2 in our criteria. The SPM value of SHOX2 was prominently high across species. Similarly, by calculating SPM values, we identified 3 atrial-specific, 11 ventricular-specific, and 17 SA-specific markers. Ontology analysis identified 70 cardiac region- and species-specific ontologies. These results suggest that reanalyzing existing data by calculating SPM values may identify novel tissue-specific genes and species-dependent gene expression. This study identified the importance of SHOX2 as an SA-specific transcription factor, a novel cardiac regional marker, and species-dependent ontologies.

A dual SHOX2:GFP; MYH6:mCherry knockin hESC reporter line for derivation of human SAN-like cells

The sinoatrial node (SAN) is the primary pacemaker of the heart. The human SAN is poorly understood due to limited primary tissue access and limitations in robust in vitro derivation methods. We developed a dual SHOX2:GFP; MYH6:mCherry knockin human embryonic stem cell (hESC) reporter line, which allows the identification and purification of SAN-like cells. Using this line, we performed several rounds of chemical screens and developed an efficient strategy to generate and purify hESC-derived SAN-like cells (hESC-SAN). The derived hESC-SAN cells display molecular and electrophysiological characteristics of bona fide nodal cells, which allowed exploration of their transcriptional profile at single-cell level. In sum, our dual reporter system facilitated an effective strategy for deriving human SAN-like cells, which can potentially be used for future disease modeling and drug discovery.

GOing Forward With the Cardiac Conduction System Using Gene Ontology

The cardiac conduction system (CCS) comprises critical components responsible for the initiation, propagation, and coordination of the action potential. Aberrant CCS development can cause conduction abnormalities, including sick sinus syndrome, accessory pathways, and atrioventricular and bundle branch blocks. Gene Ontology (GO; http://geneontology.org/) is an invaluable global bioinformatics resource which provides structured, computable knowledge describing the functions of gene products. Many gene products are known to be involved in CCS development; however, this information is not comprehensively captured by GO. To address the needs of the heart development research community, this study aimed to describe the specific roles of proteins reported in the literature to be involved with CCS development and/or function. 14 proteins were prioritized for GO annotation which led to the curation of 15 peer-reviewed primary experimental articles using carefully selected GO terms. 152 descriptive GO annotations, including those describing sinoatrial node and atrioventricular node development were created and submitted to the GO Consortium database. A functional enrichment analysis of 35 key CCS development proteins confirmed that this work has improved the in-silico interpretation of this CCS dataset. This work may improve future investigations of the CCS with application of high-throughput methods such as genome-wide association studies analysis, proteomics, and transcriptomics.

Modeling Human Cardiac Arrhythmias: Insights from Zebrafish

Cardiac arrhythmia, or irregular heart rhythm, is associated with morbidity and mortality and is described as one of the most important future public health challenges. Therefore, developing new models of cardiac arrhythmia is critical for understanding disease mechanisms, determining genetic underpinnings, and developing new therapeutic strategies. In the last few decades, the zebrafish has emerged as an attractive model to reproduce in vivo human cardiac pathologies, including arrhythmias. Here, we highlight the contribution of zebrafish to the field and discuss the available cardiac arrhythmia models. Further, we outline techniques to assess potential heart rhythm defects in larval and adult zebrafish. As genetic tools in zebrafish continue to bloom, this model will be crucial for functional genomics studies and to develop personalized anti-arrhythmic therapies.

A novel PRRX1 loss-of-function variation contributing to familial atrial fibrillation and congenital patent ductus arteriosus

Atrial fibrillation (AF) represents the most common type of sustained cardiac arrhythmia in humans and confers a significantly increased risk for thromboembolic stroke, congestive heart failure and premature death. Aggregating evidence emphasizes the predominant genetic defects underpinning AF and an increasing number of deleterious variations in more than 50 genes have been involved in the pathogenesis of AF. Nevertheless, the genetic basis underlying AF remains incompletely understood. In the current research, by whole-exome sequencing and Sanger sequencing analysis in a family with autosomal-dominant AF and congenital patent ductus arteriosus (PDA), a novel heterozygous variation in the PRRX1 gene encoding a homeobox transcription factor critical for cardiovascular development, NM_022716.4:c.373G>T;p.(Glu125^*), was identified to be in co-segregation with AF and PDA in the whole family. The truncating variation was not detected in 306 unrelated healthy individuals employed as controls. Quantitative biological measurements with a reporter gene analysis system revealed that the Glu125^*-mutant PRRX1 protein failed to transactivate its downstream target genes SHOX2 and ISL1, two genes that have been causally linked to AF. Conclusively, the present study firstly links PRRX1 loss-of-function variation to AF and PDA, suggesting that AF and PDA share a common abnormal developmental basis in a proportion of cases.

Spotlight on Isl1: A Key Player in Cardiovascular Development and Diseases

Cardiac transcription factors orchestrate a regulatory network controlling cardiovascular development. Isl1, a LIM-homeodomain transcription factor, acts as a key player in multiple organs during embryonic development. Its crucial roles in cardiovascular development have been elucidated by extensive studies, especially as a marker gene for the second heart field progenitors. Here, we summarize the roles of Isl1 in cardiovascular development and function, and outline its cellular and molecular modes of action, thus providing insights for the molecular basis of cardiovascular diseases.

PRRX1 Loss-of-Function Mutations Underlying Familial Atrial Fibrillation

Background

Atrial fibrillation (AF) is the most common form of clinical cardiac dysrhythmia responsible for thromboembolic cerebral stroke, congestive heart failure, and death. Aggregating evidence highlights the strong genetic basis of AF. Nevertheless, AF is of pronounced genetic heterogeneity, and in an overwhelming majority of patients, the genetic determinants underpinning AF remain elusive.

Methods and Results

By genome‐wide screening with polymorphic microsatellite markers and linkage analysis in a 4‐generation Chinese family affected with autosomal‐dominant AF, a novel locus for AF was mapped to chromosome 1q24.2–q25.1, a 3.20‐cM (≈4.19 Mbp) interval between markers D1S2851 and D1S218, with the greatest 2‐point logarithm of odds score of 4.8165 for the marker D1S452 at recombination fraction=0.00. Whole‐exome sequencing and bioinformatics analyses showed that within the mapping region, only the mutation in the paired related homeobox 1 (PRRX1) gene, NM_022716.4:c.319C>T;(p.Gln107*), cosegregated with AF in the family. In addition, sequencing analyses of PRRX1 in another cohort of 225 unrelated patients with AF revealed a new mutation, NM_022716.4:c.437G>T; (p.Arg146Ile), in a patient. The 2 mutations were absent in 908 control subjects. Biological analyses in HeLa cells demonstrated that the 2 mutants had significantly diminished transactivation on the target genes ISL1 and SHOX2 and markedly decreased ability to bind the promoters of ISL1 and SHOX2 (2 genes causally linked to AF), although with normal intracellular distribution.

Conclusions

This study first indicates that PRRX1 loss‐of‐function mutations predispose to AF, which provides novel insight into the molecular pathogenesis underpinning AF, implying potential implications for precisive prophylaxis and management of AF.

TUB and ZNF532 Promote the Atoh1-Mediated Hair Cell Regeneration in Mouse Cochleae

Hair cell (HC) regeneration is a promising therapy for permanent sensorineural hearing loss caused by HC loss in mammals. Atoh1 has been shown to convert supporting cells (SCs) to HCs in neonatal cochleae; its combinations with other factors can improve the efficiency of HC regeneration. To identify additional transcription factors for efficient Atoh1-mediated HC regeneration, here we optimized the electroporation procedure for explant culture of neonatal mouse organs of Corti and tested multiple transcription factors, Six2, Ikzf2, Lbh, Arid3b, Hmg20 a, Tub, Sall1, and Znf532, for their potential to promote Atoh1-mediated conversion of SCs to HCs. These transcription factors are expressed highly in HCs but differentially compared to the converted HCs based on previous studies, and are also potential co-reprograming factors for Atoh1-mediated SC-to-HC conversion by literature review. P0.5 cochlear explants were electroporated with these transcription factors alone or jointly with Atoh1. We found that Sox2+ progenitors concentrated within the lateral greater epithelial ridge (GER) can be electroporated efficiently with minimal HC damage. Atoh1 ectopic expression promoted HC regeneration in Sox2+ lateral GER cells. Transcription factors Tub and Znf532, but not the other six tested, promoted the HC regeneration mediated by Atoh1, consistent with previous studies that Isl1 promotes Atoh1-mediated HC conversionex vivo and in vivo and that both Tub and Znf532 are downstream targets of Isl1. Thus, our studies revealed an optimized electroporation method that can transfect the Sox2+ lateral GER cells efficiently with minimal damage to the endogenous HCs. Our results also demonstrate the importance of the Isl1/Tub/Znf532 pathway in promoting Atoh1-mediated HC regeneration.

Further insights into the molecular complexity of the human sinus node - The role of 'novel' transcription factors and microRNAs

RESEARCH PURPOSE

The sinus node (SN) is the heart's primary pacemaker. Key ion channels (mainly the funny channel, HCN4) and Ca2+-handling proteins in the SN are responsible for its function. Transcription factors (TFs) regulate gene expression through inhibition or activation and microRNAs (miRs) do this through inhibition. There is high expression of macrophages and mast cells within the SN connective tissue. 'Novel'/unexplored TFs and miRs in the regulation of ion channels and immune cells in the SN are not well understood. Using RNAseq and bioinformatics, the expression profile and predicted interaction of key TFs and cell markers with key miRs in the adult human SN vs. right atrial tissue (RA) were determined.

PRINCIPAL RESULTS

68 and 60 TFs significantly more or less expressed in the SN vs. RA respectively. Among those more expressed were ISL1 and TBX3 (involved in embryonic development of the SN) and 'novel' RUNX1-2, CEBPA, GLI1-2 and SOX2. These TFs were predicted to regulate HCN4 expression in the SN. Markers for different cells: fibroblasts (COL1A1), fat (FABP4), macrophages (CSF1R and CD209), natural killer (GZMA) and mast (TPSAB1) were significantly more expressed in the SN vs. RA. Interestingly, RUNX1-3, CEBPA and GLI1 also regulate expression of these cells. MiR-486-3p inhibits HCN4 and markers involved in immune response.

MAJOR CONCLUSIONS

In conclusion, RUNX1-2, CSF1R, TPSAB1, COL1A1 and HCN4 are highly expressed in the SN but not miR-486-3p. Their complex interactions can be used to treat SN dysfunction such as bradycardia. Interestingly, another research group recently reported miR-486-3p is upregulated in blood samples from severe COVID-19 patients who suffer from bradycardia.

Transcriptome profile of the sinoatrial ring reveals conserved and novel genetic programs of the zebrafish pacemaker

BACKGROUND

Sinoatrial Node (SAN) is part of the cardiac conduction system, which controls the rhythmic contraction of the vertebrate heart. The SAN consists of a specialized pacemaker cell population that has the potential to generate electrical impulses. Although the SAN pacemaker has been extensively studied in mammalian and teleost models, including the zebrafish, their molecular nature remains inadequately comprehended.

RESULTS

To characterize the molecular profile of the zebrafish sinoatrial ring (SAR) and elucidate the mechanism of pacemaker function, we utilized the transgenic line sqet33mi59BEt to isolate cells of the SAR of developing zebrafish embryos and profiled their transcriptome. Our analyses identified novel candidate genes and well-known conserved signaling pathways involved in pacemaker development. We show that, compared to the rest of the heart, the zebrafish SAR overexpresses several mammalian SAN pacemaker signature genes, which include hcn4 as well as those encoding calcium- and potassium-gated channels. Moreover, genes encoding components of the BMP and Wnt signaling pathways, as well as members of the Tbx family, which have previously been implicated in pacemaker development, were also overexpressed in the SAR. Among SAR-overexpressed genes, 24 had human homologues implicated in 104 different ClinVar phenotype entries related to various forms of congenital heart diseases, which suggest the relevance of our transcriptomics resource to studying human heart conditions. Finally, functional analyses of three SAR-overexpressed genes, pard6a, prom2, and atp1a1a.2, uncovered their novel role in heart development and physiology.

CONCLUSION

Our results established conserved aspects between zebrafish and mammalian pacemaker function and revealed novel factors implicated in maintaining cardiac rhythm. The transcriptome data generated in this study represents a unique and valuable resource for the study of pacemaker function and associated heart diseases.

Implementing Biological Pacemakers: Design Criteria for Successful

Each heartbeat that pumps blood throughout the body is initiated by an electrical impulse generated in the sinoatrial node (SAN). However, a number of disease conditions can hamper the ability of the SAN's pacemaker cells to generate consistent action potentials and maintain an orderly conduction path, leading to arrhythmias. For symptomatic patients, current treatments rely on implantation of an electronic pacing device. However, complications inherent to the indwelling hardware give pause to categorical use of device therapy for a subset of populations, including pediatric patients or those with temporary pacing needs. Cellular-based biological pacemakers, derived in vitro or in situ, could function as a therapeutic alternative to current electronic pacemakers. Understanding how biological pacemakers measure up to the SAN would facilitate defining and demonstrating its advantages over current treatments. In this review, we discuss recent approaches to creating biological pacemakers and delineate design criteria to guide future progress based on insights from basic biology of the SAN. We emphasize the need for long-term efficacy in vivo via maintenance of relevant proteins, source-sink balance, a niche reflective of the native SAN microenvironment, and chronotropic competence. With a focus on such criteria, combined with delivery methods tailored for disease indications, clinical implementation will be attainable.

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.

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.

New Insights into the Development and Morphogenesis of the Cardiac Purkinje Fiber Network: Linking Architecture and Function

The rapid propagation of electrical activity through the ventricular conduction system (VCS) controls spatiotemporal contraction of the ventricles. Cardiac conduction defects or arrhythmias in humans are often associated with mutations in key cardiac transcription factors that have been shown to play important roles in VCS morphogenesis in mice. Understanding of the mechanisms of VCS development is thus crucial to decipher the etiology of conduction disturbances in adults. During embryogenesis, the VCS, consisting of the His bundle, bundle branches, and the distal Purkinje network, originates from two independent progenitor populations in the primary ring and the ventricular trabeculae. Differentiation into fast-conducting cardiomyocytes occurs progressively as ventricles develop to form a unique electrical pathway at late fetal stages. The objectives of this review are to highlight the structure–function relationship between VCS morphogenesis and conduction defects and to discuss recent data on the origin and development of the VCS with a focus on the distal Purkinje fiber network.

A BMP4-p38 MAPK signaling axis controls ISL1 protein stability and activity during cardiogenesis

During development, cells respond rapidly to intra- and intercellular signals, which induce signaling cascades regulating the activity of transcription factors at the transcriptional and/or post-translational level. The transcription factor ISL1 plays a key role in second heart field development and cardiac differentiation, and its mRNA levels are tightly regulated during cardiogenesis. Here, we show that a BMP-p38 MAPK signaling axis controls ISL1 protein function at the post-translational level. BMP-mediated activation of p38 MAPK leads to ISL1 phosphorylation at S269 by p38, which prevents ISL1 degradation and ensures its transcriptional activity during cardiogenesis. Interfering with p38 MAPK signaling leads to the degradation of ISL1 by the proteasome, resulting in defects in cardiomyocyte differentiation and impaired zebrafish and mouse heart morphogenesis and function. Given the critical role of the tight control of ISL1 activity during cardiac lineage diversification, modulation of BMP4-p38 MAPK signaling could direct differentiation into specific cardiac cell subpopulations.

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ISL1 is phosphorylated by p38 MAPK at serine 269

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A BMP4-p38 MAPK signaling axis controls ISL1 protein stability

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Phosphorylation of ISL1 by p38 regulates its activity during cardiogenesis

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p38 Inhibition in vivo results in ISL1 degradation and second heart field defects

Transcriptional and Epigenetic Landscape of Cardiac Pacemaker Cells: Insights Into Cellular Specialization in the Sinoatrial Node

Cardiac pacemaker cells differentiate and functionally specialize early in embryonic development through activation of critical gene regulatory networks. In general, cellular specification and differentiation require that combinations of cell type-specific transcriptional regulators activate expression of key effector genes by binding to DNA regulatory elements including enhancers and promoters. However, because genomic DNA is tightly packaged by histones that must be covalently modified in order to render DNA regulatory elements and promoters accessible for transcription, the process of development and differentiation is intimately connected to the epigenetic regulation of chromatin accessibility. Although the difficulty of obtaining sufficient quantities of pure populations of pacemaker cells has limited progress in this field, the advent of low-input genomic technologies has the potential to catalyze a rapid growth of knowledge in this important area. The goal of this review is to outline the key transcriptional networks that control pacemaker cell development, with particular attention to our emerging understanding of how chromatin accessibility is modified and regulated during pacemaker cell differentiation. In addition, we will discuss the relevance of these findings to adult sinus node function, sinus node diseases, and origins of genetic variation in heart rhythm. Lastly, we will outline the current challenges facing this field and promising directions for future investigation.

Molecular Profiling of the Cardiac Conduction System: the Dawn of a New Era

PURPOSE OF REVIEW

Recent technological advances have led to an increased ability to define the gene expression profile of the cardiac conduction system (CCS). Here, we review the most salient studies to emerge in recent years and discuss existing gaps in our knowledge as well as future areas of investigation.

RECENT FINDINGS

Molecular profiling of the CCS spans several decades. However, the advent of high-throughput sequencing strategies has allowed for the discovery of unique transcriptional programs of the many diverse CCS cell types. The CCS, a diverse structure with significant inter- and intra-component cellular heterogeneity, is essential to the normal function of the heart. Progress in transcriptomic profiling has improved the resolution and depth of characterization of these unique and clinically relevant CCS cell types. Future studies leveraging this big data will play a crucial role in improving our understanding of CCS development and function as well as translating these findings into tangible translational tools for the improved detection, prevention, and treatment of cardiac arrhythmias.

Cellular and Molecular Mechanisms of Functional Hierarchy of Pacemaker Clusters in the Sinoatrial Node: New Insights into Sick Sinus Syndrome

The sinoatrial node (SAN), the primary pacemaker of the heart, consists of a heterogeneous population of specialized cardiac myocytes that can spontaneously produce action potentials, generating the rhythm of the heart and coordinating heart contractions. Spontaneous beating can be observed from very early embryonic stage and under a series of genetic programing, the complex heterogeneous SAN cells are formed with specific biomarker proteins and generate robust automaticity. The SAN is capable to adjust its pacemaking rate in response to environmental and autonomic changes to regulate the heart's performance and maintain physiological needs of the body. Importantly, the origin of the action potential in the SAN is not static, but rather dynamically changes according to the prevailing conditions. Changes in the heart rate are associated with a shift of the leading pacemaker location within the SAN and accompanied by alterations in P wave morphology and PQ interval on ECG. Pacemaker shift occurs in response to different interventions: neurohormonal modulation, cardiac glycosides, pharmacological agents, mechanical stretch, a change in temperature, and a change in extracellular electrolyte concentrations. It was linked with the presence of distinct anatomically and functionally defined intranodal pacemaker clusters that are responsible for the generation of the heart rhythm at different rates. Recent studies indicate that on the cellular level, different pacemaker clusters rely on a complex interplay between the calcium (referred to local subsarcolemmal Ca²⁺ releases generated by the sarcoplasmic reticulum via ryanodine receptors) and voltage (referred to sarcolemmal electrogenic proteins) components of so-called "coupled clock pacemaker system" that is used to describe a complex mechanism of SAN pacemaking. In this review, we examine the structural, functional, and molecular evidence for hierarchical pacemaker clustering within the SAN. We also demonstrate the unique molecular signatures of intranodal pacemaker clusters, highlighting their importance for physiological rhythm regulation as well as their role in the development of SAN dysfunction, also known as sick sinus syndrome.

Assembly of the Cardiac Pacemaking Complex: Electrogenic Principles of Sinoatrial Node Morphogenesis

Cardiac pacemaker cells located in the sinoatrial node initiate the electrical impulses that drive rhythmic contraction of the heart. The sinoatrial node accounts for only a small proportion of the total mass of the heart yet must produce a stimulus of sufficient strength to stimulate the entire volume of downstream cardiac tissue. This requires balancing a delicate set of electrical interactions both within the sinoatrial node and with the downstream working myocardium. Understanding the fundamental features of these interactions is critical for defining vulnerabilities that arise in human arrhythmic disease and may provide insight towards the design and implementation of the next generation of potential cellular-based cardiac therapeutics. Here, we discuss physiological conditions that influence electrical impulse generation and propagation in the sinoatrial node and describe developmental events that construct the tissue-level architecture that appears necessary for sinoatrial node function.

Embryonic Expression of NrasG 12 D Leads to Embryonic Lethality and Cardiac Defects

Ras proteins control a complex intracellular signaling network. Gain-of-function mutations in RAS genes lead to RASopathy disorders in humans, including Noonan syndrome (NS). NS is the second most common syndromic cause of congenital heart disease. Although conditional expression of the Nras^G12D/+ mutation in adult hematopoietic system is leukemogenic, its effects on embryonic development remain unclear. Here, we report that pan-embryonic expression of endogenous Nras^G12D/+ by Mox2-Cre in mice caused embryonic lethality from embryonic day (E) 15.5 and developmental defects predominantly in the heart. At E13.5, Nras^G12D/+; Mox2^Cre/+ embryos displayed a moderate expansion of hematopoietic stem and progenitor cells without a significant impact on erythroid differentiation in the fetal liver. Importantly, the mutant embryos exhibited cardiac malformations resembling human congenital cardiac defects seen in NS patients, including ventricular septal defects, double outlet right ventricle, the hypertrabeculation/thin myocardium, and pulmonary valve stenosis. The mutant heart showed dysregulation of ERK, BMP, and Wnt pathways, crucial signaling pathways for cardiac development. Endothelial/endocardial-specific expression of Nras^G12D/+ caused the cardiac morphological defects and embryonic lethality as observed in Nras^G12D/+; Mox2^Cre/+ mutants, but myocardial-specific expression of Nras^G12D/+ did not. Thus, oncogenic Nras^G12D mutation may not be compatible with embryonic survival.

Atrial and Sinoatrial Node Development in the Zebrafish Heart

Proper development and function of the vertebrate heart is vital for embryonic and postnatal life. Many congenital heart defects in humans are associated with disruption of genes that direct the formation or maintenance of atrial and pacemaker cardiomyocytes at the venous pole of the heart. Zebrafish are an outstanding model for studying vertebrate cardiogenesis, due to the conservation of molecular mechanisms underlying early heart development, external development, and ease of genetic manipulation. Here, we discuss early developmental mechanisms that instruct appropriate formation of the venous pole in zebrafish embryos. We primarily focus on signals that determine atrial chamber size and the specialized pacemaker cells of the sinoatrial node through directing proper specification and differentiation, as well as contemporary insights into the plasticity and maintenance of cardiomyocyte identity in embryonic zebrafish hearts. Finally, we integrate how these insights into zebrafish cardiogenesis can serve as models for human atrial defects and arrhythmias.

Rathke's cleft-like cysts arise from Isl1 deletion in murine pituitary progenitors

The transcription factor ISL1 is expressed in pituitary gland stem cells and the thyrotrope and gonadotrope lineages. Pituitary-specific Isl1 deletion causes hypopituitarism with increased stem cell apoptosis, reduced differentiation of thyrotropes and gonadotropes, and reduced body size. Conditional Isl1 deletion causes development of multiple Rathke's cleft-like cysts, with 100% penetrance. Foxa1 and Foxj1 are abnormally expressed in the pituitary gland and associated with a ciliogenic gene-expression program in the cysts. We confirmed expression of FOXA1, FOXJ1, and stem cell markers in human Rathke's cleft cyst tissue, but not craniopharyngiomas, which suggests these transcription factors are useful, pathological markers for diagnosis of Rathke's cleft cysts. These studies support a model whereby expression of ISL1 in pituitary progenitors drives differentiation into thyrotropes and gonadotropes and without it, activation of FOXA1 and FOXJ1 permits development of an oral epithelial cell fate with mucinous cysts. This pituitary-specific Isl1 mouse knockout sheds light on the etiology of Rathke's cleft cysts and the role of ISL1 in normal pituitary development.

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.

ATAC-Seq Reveals an Isl1 Enhancer That Regulates Sinoatrial Node Development and Function

RATIONALE

Cardiac pacemaker cells (PCs) in the sinoatrial node (SAN) have a distinct gene expression program that allows them to fire automatically and initiate the heartbeat. Although critical SAN transcription factors, including Isl1 (Islet-1), Tbx3 (T-box transcription factor 3), and Shox2 (short-stature homeobox protein 2), have been identified, the cis-regulatory architecture that governs PC-specific gene expression is not understood, and discrete enhancers required for gene regulation in the SAN have not been identified.

OBJECTIVE

To define the epigenetic profile of PCs using comparative ATAC-seq (assay for transposase-accessible chromatin with sequencing) and to identify novel enhancers involved in SAN gene regulation, development, and function.

METHODS AND RESULTS

We used ATAC-seq on sorted neonatal mouse SAN to compare regions of accessible chromatin in PCs and right atrial cardiomyocytes. PC-enriched assay for transposase-accessible chromatin peaks, representing candidate SAN regulatory elements, were located near established SAN genes and were enriched for distinct sets of TF (transcription factor) binding sites. Among several novel SAN enhancers that were experimentally validated using transgenic mice, we identified a 2.9-kb regulatory element at the Isl1 locus that was active specifically in the cardiac inflow at embryonic day 8.5 and throughout later SAN development and maturation. Deletion of this enhancer from the genome of mice resulted in SAN hypoplasia and sinus arrhythmias. The mouse SAN enhancer also directed reporter activity to the inflow tract in developing zebrafish hearts, demonstrating deep conservation of its upstream regulatory network. Finally, single nucleotide polymorphisms in the human genome that occur near the region syntenic to the mouse enhancer exhibit significant associations with resting heart rate in human populations.

CONCLUSIONS

(1) PCs have distinct regions of accessible chromatin that correlate with their gene expression profile and contain novel SAN enhancers, (2) cis-regulation of Isl1 specifically in the SAN depends upon a conserved SAN enhancer that regulates PC development and SAN function, and (3) a corresponding human ISL1 enhancer may regulate human SAN function.

ISL1 loss-of-function variation causes familial atrial fibrillation

Atrial fibrillation (AF) represents the most frequent form of sustained cardiac rhythm disturbance, affecting approximately 1% of the general population worldwide, and confers a substantially enhanced risk of cerebral stroke, heart failure, and death. Increasing epidemiological studies have clearly demonstrated a strong genetic basis for AF, and variants in a wide range of genes, including those coding for ion channels, gap junction channels, cardiac structural proteins and transcription factors, have been identified to underlie AF. Nevertheless, the genetic pathogenesis of AF is complex and still far from completely understood. Here, whole-exome sequencing and bioinformatics analyses of a three-generation family with AF were performed, and after filtering variants by multiple metrics, we identified a heterozygous variant in the ISL1 gene (encoding a transcription factor critical for embryonic cardiogenesis and postnatal cardiac remodeling), NM_002202.2: c.481G > T; p.(Glu161*), which was validated by Sanger sequencing and segregated with autosome-dominant AF in the family with complete penetrance. The nonsense variant was absent from 284 unrelated healthy individuals used as controls. Functional assays with a dual-luciferase reporter assay system revealed that the truncating ISL1 protein lost transcriptional activation on the verified target genes MEF2C and NKX2-5. Additionally, the variant nullified the synergistic transactivation between ISL1 and TBX5 as well as GATA4, two other transcription factors that have been implicated in AF. The findings suggest ISL1 as a novel gene contributing to AF, which adds new insight to the genetic mechanisms underpinning AF, implying potential implications for genetic testing and risk stratification of the AF family members.

Hand2 Selectively Reorganizes Chromatin Accessibility to Induce Pacemaker-like Transcriptional Reprogramming

Gata4, Hand2, Mef2c, and Tbx5 (GHMT) can reprogram transduced fibroblasts into induced pacemaker-like myocytes (iPMs), but the underlying mechanisms remain obscure. Here, we explore the role of Hand2 in iPM formation by using a combination of transcriptome, genome, and biochemical as-says. We found many shared transcriptional signatures between iPMs and the endogenous sinoatrial node (SAN), yet key regulatory networks remain missing. We demonstrate that Hand2 augments chromatin accessibility at loci involved in sarcomere organization, electrical coupling, and membrane depolarization. Focusing on an established cardiac Hand2 cistrome, we observe selective reorganization of chromatin accessibility to promote pacemaker-specific gene expression. Moreover, we identify a Hand2 cardiac subtype diversity (CSD) domain through biochemical analysis of the N terminus. By integrating our RNA-seq and ATAC-seq datasets, we highlight desmosome organization as a hallmark feature of iPM formation. Collectively, our results illuminate Hand2-dependent mechanisms that may guide future efforts to rationally improve iPM formation.

Gata4, Hand2, Mef2c, and Tbx5 can reprogram fibroblasts into cardiomyocyte-like cells, including induced pacemakers (iPMs). Fernandez-Perez et al. show that Hand2 coordinates this process by influencing chromatin accessibility and gene expression in fibroblasts undergoing iPM lineage conversion. These insights could eventually inform the production of superior replacement cells.

Silencing miR-370-3p rescues funny current and sinus node function in heart failure

Bradyarrhythmias are an important cause of mortality in heart failure and previous studies indicate a mechanistic role for electrical remodelling of the key pacemaking ion channel HCN4 in this process. Here we show that, in a mouse model of heart failure in which there is sinus bradycardia, there is upregulation of a microRNA (miR-370-3p), downregulation of the pacemaker ion channel, HCN4, and downregulation of the corresponding ionic current, If, in the sinus node. In vitro, exogenous miR-370-3p inhibits HCN4 mRNA and causes downregulation of HCN4 protein, downregulation of If, and bradycardia in the isolated sinus node. In vivo, intraperitoneal injection of an antimiR to miR-370-3p into heart failure mice silences miR-370-3p and restores HCN4 mRNA and protein and If in the sinus node and blunts the sinus bradycardia. In addition, it partially restores ventricular function and reduces mortality. This represents a novel approach to heart failure treatment.

Correlations between ISL1 rs1017 polymorphism and congenital heart disease risk: A PRISMA-compliant meta-analysis

BACKGROUND

ISL1 promotes cardiomyocyte differentiation and plays important roles in heart development. However, whether ISL1 rs1017 polymorphism is associated with the congenital heart disease (CHD) risk remains controversial.

METHODS

Five database including PubMed, Cochrane Library, ISI Web of Science, CNKI, and Wan Fang were searched by using key words "Insulin Gene Enhancer Protein ISL1" and "Single Nucleotide Polymorphism," and "Congenital Heart Disease." Five relative articles including 6 independent studies containing 2132 cases and 3812 controls were finally recruited to our study. Meta-analyses were performed by pooling odds ratios (ORs) and 95% confidence interval (CI) from included studies using STATA 12.0 software.

RESULTS

The associations between ISL1 rs1017 polymorphism and the risk of CHD were statistically significant under the allele model (T vs A; OR: 1.421; 95% CI: 1.072-1.882), heterozygous model (AT vs AA; OR: 1.342; 95% CI: 1.019-1.767), and dominant model (AT+ TT vs AA; OR: 1.466; 95% CI: 1.059-2.028). Sensitivity analysis indicated that the results were not stable. Subgroup analysis demonstrated that associations were found in Caucasians under the allele model and the heterozygous model (P < .05), but not the dominant model (P > .05).

CONCLUSION

In summary, our meta-analysis results suggest that the T allele of ISL1 rs1017 is a risk factor for CHD. However, further studies based on large sample size and multi-ethnic population should be conducted to further prove this correlation.

Transcription Factor TBX18 Reprograms Vascular Smooth Muscle Cells of Ascending Aorta to Pacemaker-Like Cells

Vascular smooth muscle cells (VSMCs) of ascending aorta and TBX18⁺ sinus node both originated from the second heart field. The study explored whether ascending aortic smooth muscle cells in vitro could be reprogrammed into pacemaker-like cells with human TBX18. In the study, VSMCs were infected with TBX18, and then cocultured with neonatal rat ventricular cardiomyocytes (NRVMs) in vitro. By overexpressing TBX18, the transfected VSMCs expressed high levels of hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4), insulin gene enhancer binding protein 1, and human dwarf homeobox gene SHOX2, cardiac troponin I, and low level of connexin 43. In addition, funny current (I_f) was recorded by patch clamp appeared the time and voltage dependence in TBX18 group, which the amplitude of I_f density was from -5.164 ± 0.662 pA/pF to -0.765 ± 0.358 pA/pF (n = 14). Furthermore, TBX18-transfected VSMCs coupled with NRVMs showed typical action potential of pacemaker-like cells and the beating rate was faster (178.00 ± 7.55 bpm, p < 0.05) compared with other groups. In conclusion, our study indicated that transcription factor TBX18 could reprogram VSMCs into pacemaker-like cells in vitro.