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

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

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

Progesterone Receptor Modulates Extraembryonic Mesoderm and Cardiac Progenitor Specification during Mouse Gastrulation

Progesterone treatment is commonly employed to promote and support pregnancy. While maternal tissues are the main progesterone targets in humans and mice, its receptor (PGR) is expressed in the murine embryo, questioning its function during embryonic development. Progesterone has been previously associated with murine blastocyst development. Whether it contributes to lineage specification is largely unknown. Gastrulation initiates lineage specification and generation of the progenitors contributing to all organs. Cells passing through the primitive streak (PS) will give rise to the mesoderm and endoderm. Cells emerging posteriorly will form the extraembryonic mesodermal tissues supporting embryonic growth. Cells arising anteriorly will contribute to the embryonic heart in two sets of distinct progenitors, first (FHF) and second heart field (SHF). We found that PGR is expressed in a posterior–anterior gradient in the PS of gastrulating embryos. We established in vitro differentiation systems inducing posterior (extraembryonic) and anterior (cardiac) mesoderm to unravel PGR function. We discovered that PGR specifically modulates extraembryonic and cardiac mesoderm. Overexpression experiments revealed that PGR safeguards cardiac differentiation, blocking premature SHF progenitor specification and sustaining the FHF progenitor pool. This role of PGR in heart development indicates that progesterone administration should be closely monitored in potential early-pregnancy patients undergoing infertility treatment.

Systematic reconstruction of cellular trajectories across mouse embryogenesis

Mammalian embryogenesis is characterized by rapid cellular proliferation and diversification. Within a few weeks, a single-cell zygote gives rise to millions of cells expressing a panoply of molecular programs. Although intensively studied, a comprehensive delineation of the major cellular trajectories that comprise mammalian development in vivo remains elusive. Here, we set out to integrate several single-cell RNA-sequencing (scRNA-seq) datasets that collectively span mouse gastrulation and organogenesis, supplemented with new profiling of ~150,000 nuclei from approximately embryonic day 8.5 (E8.5) embryos staged in one-somite increments. Overall, we define cell states at each of 19 successive stages spanning E3.5 to E13.5 and heuristically connect them to their pseudoancestors and pseudodescendants. Although constructed through automated procedures, the resulting directed acyclic graph (TOME (trajectories of mammalian embryogenesis)) is largely consistent with our contemporary understanding of mammalian development. We leverage TOME to systematically nominate transcription factors (TFs) as candidate regulators of each cell type's specification, as well as 'cell-type homologs' across vertebrate evolution.

Spatiotemporal expression of long noncoding RNA Moshe modulates heart cell lineage commitment

The roles of long non-coding RNA (LncRNA) have been highlighted in various development processes including congenital heart defects (CHD). Here, we characterized the molecular function of LncRNA, Moshe (1010001N08ik-203), one of the Gata6 antisense transcripts located upstream of Gata6, which is involved in both heart development and the most common type of congenital heart defect, atrial septal defect (ASD). During mouse embryonic development, Moshe was first detected during the cardiac mesoderm stage (E8.5 to E9.5) where Gata6 is expressed and continues to increase at the atrioventricular septum (E12.5), which is involved in ASD. Functionally, the knock-down of Moshe during cardiogenesis caused significant repression of Nkx2.5 in cardiac progenitor stages and resulted in the increase in major SHF lineage genes, such as cardiac transcriptional factors (Isl1, Hand2, Tbx2), endothelial-specific genes (Cd31, Flk1, Tie1, vWF), a smooth muscle actin (a-Sma) and sinoatrial node-specific genes (Shox2, Tbx18). Chromatin Isolation by RNA Purification showed Moshe activates Nkx2.5 gene expression via direct binding to its promoter region. Of note, Moshe was conserved across species, including human, pig and mouse. Altogether, this study suggests that Moshe is a heart-enriched lncRNA that controls a sophisticated network of cardiogenesis by repressing genes in SHF via Nkx2.5 during cardiac development and may play an important role in ASD.

Xenopus: Experimental Access to Cardiovascular Development, Regeneration Discovery, and Cardiovascular Heart-Defect Modeling

Xenopus has been used to study a wide array of developmental processes, benefiting from vast quantities of relatively large, externally developing eggs. Xenopus is particularly amenable to examining the cardiac system because many of the developmental processes and genes involved in cardiac specification, differentiation, and growth are conserved between Xenopus and human and have been characterized in detail. Furthermore, compared with other higher vertebrate models, Xenopus embryos can survive longer without a properly functioning heart or circulatory system, enabling investigation of later consequences of early embryological manipulations. This biology is complemented by experimental technology, such as embryonic explants to study the heart, microinjection of overexpression constructs, and, most recently, the generation of genetic mutations through gene-editing technologies. Recent investigations highlight Xenopus as a powerful experimental system for studying injury/repair and regeneration and for congenital heart disease (CHD) modeling, which reinforces why this model system remains ideal for studying heart development.

Cardiac toxicity of acrolein exposure in embryonic zebrafish (Danio rerio)

Acrolein is a widely distributed pollutant produced from various sources such as industrial waste, organic combustion, and power plant emissions. It is also intentionally released into irrigation canals to control invasive aquatic plants. Zebrafish (Danio rerio) has a good reputation for being an attractive model organism for developmental and toxicological research. In this study, zebrafish embryos were exposed to acrolein to investigate the cardiotoxic effects. The 96-h LC₅₀ (median lethal concentration) value of acrolein was determined as 654.385 μg/L. Then, the embryos were treated with the sublethal experimental concentrations of acrolein (1, 4, 16, 64, and 256 μg/L) for 96 h. Embryos were examined at 48, 72, and 96 h post-fertilization (hpf). Acrolein affected the cardiac morphology and function of the embryos. Sinus venosus-bulbus arteriosus (SV-BA) distance of 64 μg/L and 256 μg/L acrolein groups was elongated compared with the control samples. Immunostaining with MF20 antibody clearly exhibited that the atrium positioned posterior to the ventricle which indicated cardiac looping inhibition. Histological preparations also showed the mispositioning and the lumens of the chambers narrowed. Acrolein-induced increased heart rate was noted in the 4, 16, 64, and 256 μg/L treatment groups. Taken together, these results indicated that acrolein disrupted the heart development and cardiac function in zebrafish, suggesting that its water-borne risks should be considered seriously.

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.

A Human iPSC Double-Reporter System Enables Purification of Cardiac Lineage Subpopulations with Distinct Function and Drug Response Profiles

The diversity of cardiac lineages contributes to the heterogeneity of human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes (CMs). Here, we report the generation of a hiPSC TBX5^Clover2 and NKX2-5^TagRFP double reporter to delineate cardiac lineages and isolate lineage-specific subpopulations. Molecular analyses reveal that four different subpopulations can be isolated based on the differential expression of TBX5 and NKX2-5, TBX5+NKX2-5+, TBX5+NKX2-5-, TBX5-NKX2-5+, and TBX5-NKX2-5-, mimicking the first heart field, epicardial, second heart field, and endothelial lineages, respectively. Genetic and functional characterization indicates that each subpopulation differentiates into specific cardiac cells. We further identify CORIN as a cell-surface marker for isolating the TBX5+NKX2-5+ subpopulation and demonstrate the use of lineage-specific CMs for precise drug testing. We anticipate that this tool will facilitate the investigation of cardiac lineage specification and isolation of specific cardiac subpopulations for drug screening, tissue engineering, and disease modeling.

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

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

Irx4 Marks a Multipotent, Ventricular-Specific Progenitor Cell

While much progress has been made in the resolution of the cellular hierarchy underlying cardiogenesis, our understanding of chamber-specific myocardium differentiation remains incomplete. To better understand ventricular myocardium differentiation, we targeted the ventricle-specific gene, Irx4, in mouse embryonic stem cells to generate a reporter cell line. Using an antibiotic-selection approach, we purified Irx4⁺ cells in vitro from differentiating embryoid bodies. The isolated Irx4⁺ cells proved to be highly proliferative and presented Cxcr4, Pdgfr-alpha, Flk1, and Flt1 on the cell surface. Single Irx4⁺ ventricular progenitor cells (VPCs) exhibited cardiovascular potency, generating endothelial cells, smooth muscle cells, and ventricular myocytes in vitro. The ventricular specificity of the Irx4⁺ population was further demonstrated in vivo as VPCs injected into the cardiac crescent subsequently produced Mlc2v⁺ myocytes that exclusively contributed to the nascent ventricle at E9.5. These findings support the existence of a newly identified ventricular myocardial progenitor. This is the first report of a multipotent cardiac progenitor that contributes progeny specific to the ventricular myocardium. Stem Cells 2016;34:2875-2888.

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

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

Micromanaging cardiac regeneration: Targeted delivery of microRNAs for cardiac repair and regeneration

The loss of cardiomyocytes during injury and disease can result in heart failure and sudden death, while the adult heart has a limited capacity for endogenous regeneration and repair. Current stem cell-based regenerative medicine approaches modestly improve cardiomyocyte survival, but offer neglectable cardiomyogenesis. This has prompted the need for methodological developments that crease de novo cardiomyocytes. Current insights in cardiac development on the processes and regulatory mechanisms in embryonic cardiomyocyte differentiation provide a basis to therapeutically induce these pathways to generate new cardiomyocytes. Here, we discuss the current knowledge on embryonic cardiomyocyte differentiation and the implementation of this knowledge in state-of-the-art protocols to the direct reprogramming of cardiac fibroblasts into de novo cardiomyocytes in vitro and in vivo with an emphasis on microRNA-mediated reprogramming. Additionally, we discuss current advances on state-of-the-art targeted drug delivery systems that can be employed to deliver these microRNAs to the damaged cardiac tissue. Together, the advances in our understanding of cardiac development, recent advances in microRNA-based therapeutics, and innovative drug delivery systems, highlight exciting opportunities for effective therapies for myocardial infarction and heart failure.

Direct nkx2-5 transcriptional repression of isl1 controls cardiomyocyte subtype identity

During cardiogenesis, most myocytes arise from cardiac progenitors expressing the transcription factors Isl1 and Nkx2-5. Here, we show that a direct repression of Isl1 by Nkx2-5 is necessary for proper development of the ventricular myocardial lineage. Overexpression of Nkx2-5 in mouse embryonic stem cells (ESCs) delayed specification of cardiac progenitors and inhibited expression of Isl1 and its downstream targets in Isl1(+) precursors. Embryos deficient for Nkx2-5 in the Isl1(+) lineage failed to downregulate Isl1 protein in cardiomyocytes of the heart tube. We demonstrated that Nkx2-5 directly binds to an Isl1 enhancer and represses Isl1 transcriptional activity. Furthermore, we showed that overexpression of Isl1 does not prevent cardiac differentiation of ESCs and in Xenopus laevis embryos. Instead, it leads to enhanced specification of cardiac progenitors, earlier cardiac differentiation, and increased cardiomyocyte number. Functional and molecular characterization of Isl1-overexpressing cardiomyocytes revealed higher beating frequencies in both ESC-derived contracting areas and Xenopus Isl1-gain-of-function hearts, which associated with upregulation of nodal-specific genes and downregulation of transcripts of working myocardium. Immunocytochemistry of cardiomyocyte lineage-specific markers demonstrated a reduction of ventricular cells and an increase of cells expressing the pacemaker channel Hcn4. Finally, optical action potential imaging of single cardiomyocytes combined with pharmacological approaches proved that Isl1 overexpression in ESCs resulted in normally electrophysiologically functional cells, highly enriched in the nodal subtype at the expense of the ventricular lineage. Our findings provide an Isl1/Nkx2-5-mediated mechanism that coordinately regulates the specification of cardiac progenitors toward the different myocardial lineages and ensures proper acquisition of myocyte subtype identity.

Pax3 and Tbx5 specify whether PDGFRα+ cells assume skeletal or cardiac muscle fate in differentiating embryonic stem cells

Embryonic stem cells (ESCs) represent an ideal model to study how lineage decisions are established during embryonic development. Using a doxycycline-inducible mouse ESC line, we have previously shown that expression of the transcriptional activator Pax3 in early mesodermal cells leads to the robust generation of paraxial mesoderm progenitors that ultimately differentiate into skeletal muscle precursors. Here, we show that the ability of this transcription factor to induce the skeletal myogenic cell fate occurs at the expenses of the cardiac lineage. Our results show that the PDGFRα+FLK1--subfraction represents the main population affected by Pax3, through downregulation of several transcripts encoding for proteins involved in cardiac development. We demonstrate that although Nkx2-5, Tbx5, and Gata4 negatively affect Pax3 skeletal myogenic activity, the cardiac potential of embryoid body-derived cultures is restored solely by forced expression of Tbx5. Taking advantage of this model, we used an unbiased genome-wide approach to identify genes whose expression is rescued by Tbx5, and which could represent important regulators of cardiac development. These findings elucidate mechanisms regulating the commitment of mesodermal cells in the early embryo and identify the Tbx5 cardiac transcriptome.

SPREDs (Sprouty related proteins with EVH1 domain) promote self-renewal and inhibit mesodermal differentiation in murine embryonic stem cells

BACKGROUND

Pluripotency, self-renewal, and differentiation are special features of embryonic stem (ES) cells, thereby providing valuable perspectives in regenerative medicine. Developmental processes require a fine-tuned organization, mainly regulated by the well-known JAK/STAT, PI3K/AKT, and ERK/MAPK pathways. SPREDs (Sprouty related proteins with EVH1 domain) were discovered as inhibitors of the ERK/MAPK signaling pathway, whereas nothing was known about their functions in ES cells and during early differentiation, so far.

RESULTS

We generated SPRED1 and SPRED2 overexpressing and SPRED2 knockout murine ES cells to analyze the functions of SPRED proteins in ES cells and during early differentiation. Overexpression of SPREDs increases significantly the self-renewal and clonogenicity of murine ES cells, whereas lack of SPRED2 reduces proliferation and increases apoptosis. During early differentiation in embryoid bodies, SPREDs promote the pluripotent state and inhibit differentiation whereby mesodermal differentiation into cardiomyocytes is considerably delayed and inhibited. LIF- and growth factor-stimulation revealed that SPREDs inhibit ERK/MAPK activation in murine ES cells. However, no effects were detectable on LIF-induced activation of the JAK/STAT3, or PI3K/AKT signaling pathway by SPRED proteins.

CONCLUSIONS

We show that SPREDs promote self-renewal and inhibit mesodermal differentiation of murine ES cells by selective suppression of the ERK/MAPK signaling pathway in pluripotent cells.

The T-box gene family: emerging roles in development, stem cells and cancer

The T-box family of transcription factors exhibits widespread involvement throughout development in all metazoans. T-box proteins are characterized by a DNA-binding motif known as the T-domain that binds DNA in a sequence-specific manner. In humans, mutations in many of the genes within the T-box family result in developmental syndromes, and there is increasing evidence to support a role for these factors in certain cancers. In addition, although early studies focused on the role of T-box factors in early embryogenesis, recent studies in mice have uncovered additional roles in unsuspected places, for example in adult stem cell populations. Here, I provide an overview of the key features of T-box transcription factors and highlight their roles and mechanisms of action during various stages of development and in stem/progenitor cell populations.

Irx4 identifies a chamber-specific cell population that contributes to ventricular myocardium development

BACKGROUND

The ventricular myocardium is the most prominent layer of the heart, and the most important for mediating cardiac physiology. Although the ventricular myocardium is critical for heart function, the cellular hierarchy responsible for ventricle-specific myocardium development remains unresolved.

RESULTS

To determine the pattern and time course of ventricular myocardium development, we investigated IRX4 protein expression, which has not been previously reported. We identified IRX4+ cells in the cardiac crescent, and these cells were positive for markers of the first or second heart fields. From the onset of chamber formation, IRX4+ cells were restricted to the ventricular myocardium. This expression pattern persisted into adulthood. Of interest, we observed that IRX4 exhibits developmentally regulated dynamic intracellular localization. Throughout prenatal cardiogenesis, and up to postnatal day 4, IRX4 was detected in the cytoplasm of ventricular myocytes. However, between postnatal days 5–6, IRX4 translocated to the nucleus of ventricular myocytes.

CONCLUSIONS

Given the ventricle-specific expression of Irx4 in later stages of heart development, we hypothesize that IRX4+ cells in the cardiac crescent represent the earliest cell population in the cellular hierarchy underlying ventricular myocardium development.

Histone deacetylase 3 modulates Tbx5 activity to regulate early cardiogenesis

Congenital heart defects often result from improper differentiation of cardiac progenitor cells. Although transcription factors involved in cardiac progenitor cell differentiation have been described, the associated chromatin modifiers in this process remain largely unknown. Here we show that mouse embryos lacking the chromatin-modifying enzyme histone deacetylase 3 (Hdac3) in cardiac progenitor cells exhibit precocious cardiomyocyte differentiation, severe cardiac developmental defects, upregulation of Tbx5 target genes and embryonic lethality. Hdac3 physically interacts with Tbx5 and modulates its acetylation to repress Tbx5-dependent activation of cardiomyocyte lineage-specific genes. These findings reveal that Hdac3 plays a critical role in cardiac progenitor cells to regulate early cardiogenesis.

Comparative analysis reveals distinct and overlapping functions of Mef2c and Mef2d during cardiogenesis in Xenopus laevis

The family of vertebrate Mef2 transcription factors is comprised of four members named Mef2a, Mef2b, Mef2c, and Mef2d. These transcription factors are regulators of the myogenic programs with crucial roles in development of skeletal, cardiac and smooth muscle cells. Mef2a and Mef2c are essential for cardiac development in mice. In Xenopus, mef2c and mef2d but not mef2a were recently shown to be expressed during cardiogenesis. We here investigated the function of Mef2c and Mef2d during Xenopus laevis cardiogenesis. Knocking down either gene by corresponding antisense morpholino oligonucleotides led to profound heart defects including morphological abnormalities, pericardial edema, and brachycardia. Marker gene expression analyses and rescue experiments revealed that (i) both genes are required for proper cardiac gene expression, (ii) Mef2d can compensate for the loss of Mef2c but not vice versa, and (iii) the γ domain of Mef2c is required for early cardiac development. Taken together, our data provide novel insights into the function of Mef2 during cardiogenesis, highlight evolutionary differences between species and might have an impact on attempts of direct reprogramming.

Islet-1 may function as an assistant factor for histone acetylation and regulation of cardiac development-related transcription factor Mef2c expression

Objective

Islet-1 is an important transcription factor for cardiac development through mediating extensive interactions between DNA and proteins. The present study was to investigate the role of Islet-1 in regulating the expression of cardiac development-related transcription factors and mechanism.

Methods and Results

The expression of Islet-1 and histone acetylases (HATs) subtype p300 was determined in newborn mouse hearts and mouse embryonic hearts at different development stages using Western blot. The expression of Islet-1 and cardiac development-related transcription factors Mef2c, GATA4 and Tbx5 as well as histone H3 acetylation level were determined in cardiac progenitor cells with and without transfection of Islet-1 interference RNA (RNAi) in lentivirus using PCR and Western blot. Islet-1 peak expression occurred on day E14.5 in mouse embryonic heart, and was present in the promoter regions of Mef2c, GATA4 and Tbx5 that were precipitated with p300 antibody. When Islet-1 was inhibited with specific RNAi in cardiac progenitor cells, the expression of Mef2c and Tbx5, but not GATA4, was significantly suppressed along with selective reduction in histone H3 acetylation in the promoter region of Mef2c, but not GATA4 and Tbx5. The level of Mef2c DNA, not GATA4 and Tbx5, in the complex associated with p300 was significantly decreased in the cells with Islet-1 knockdown.

Conclusions

These data suggested that Islet-1 might function as an assistant factor that was involved in the regulation of histone acetylation and Mef2c expression via assisting p300 on specifically targeting the promoter of Mef2c.

Effects of low-level hexabromocyclododecane (HBCD) exposure on cardiac development in zebrafish embryos

Hexabromocyclododecane (HBCD) is one of the most widely used brominated flame retardants. In the present study, zebrafish embryos were exposed to HBCD at the low concentrations of 0, 2, 20 and 200 nM. The results showed HBCD exposure resulted in an increase in heart rate and cardiac arrhythmia after exposure for 72 h, though the survival rate and the whole malformation rate were not significantly affected. These results demonstrated that the heart might be a target of HBCD. Low-level HBCD exposure may not share the same mechanisms as exposure to high concentrations, since no obvious increase of apoptotic cells around the heart was observed in the HBCD-treated groups. It was observed that the expression of Tbx5 and Nkx2.5 was significantly elevated by HBCD treatment in a dose-dependent manner using real-time quantitative PCR, which may be mainly responsible for the alteration of heart rate, given that Tbx5 and Nkx2.5 are two factors regulating ventricle conduction. The mRNA expression of RyR2 and Atp2a2b (SERCA2a) was up-regulated in the exposure group, which may be one of reasons to affect the normal heart rate, since SERCA2a and RyR2 play an important role in calcium ion transport of cadiomyocytes. However, HBCD exposure did not significantly change the expression of Actc1l, Tnnt2, and Myh6, which are mainly muscle contractile genes that play key roles in the formation of cardiac structure. These results were consistent with the lack of effect seen on the other measurements of cardiac function, end diastolic volume, end-systolic volume, stroke volume, and cardiac output.