TBX5 overexpression stimulates differentiation of chamber myocardium in P19C16 embryonic carcinoma cells

The benign nature and rare occurrence of cardiac myxoma as a possible consequence of the limited cardiac proliferative/ regenerative potential: a systematic review

BACKGROUND

Cardiac Myxoma is a primary tumor of heart. Its origins, rarity of the occurrence of primary cardiac tumors and how it may be related to limited cardiac regenerative potential, are not yet entirely known. This study investigates the key cardiac genes/ transcription factors (TFs) and signaling pathways to understand these important questions.

METHODS

Databases including PubMed, MEDLINE, and Google Scholar were searched for published articles without any date restrictions, involving cardiac myxoma, cardiac genes/TFs/signaling pathways and their roles in cardiogenesis, proliferation, differentiation, key interactions and tumorigenesis, with focus on cardiomyocytes.

RESULTS

The cardiac genetic landscape is governed by a very tight control between proliferation and differentiation-related genes/TFs/pathways. Cardiac myxoma originates possibly as a consequence of dysregulations in the gene expression of differentiation regulators including Tbx5, GATA4, HAND1/2, MYOCD, HOPX, BMPs. Such dysregulations switch the expression of cardiomyocytes into progenitor-like state in cardiac myxoma development by dysregulating Isl1, Baf60 complex, Wnt, FGF, Notch, Mef2c and others. The Nkx2-5 and MSX2 contribute predominantly to both proliferation and differentiation of Cardiac Progenitor Cells (CPCs), may possibly serve roles based on the microenvironment and the direction of cell circuitry in cardiac tumorigenesis. The Nkx2-5 in cardiac myxoma may serve to limit progression of tumorigenesis as it has massive control over the proliferation of CPCs. The cardiac cell type-specific genetic programming plays governing role in controlling the tumorigenesis and regenerative potential.

CONCLUSION

The cardiomyocytes have very limited proliferative and regenerative potential. They survive for long periods of time and tightly maintain the gene expression of differentiation genes such as Tbx5, GATA4 that interact with tumor suppressors (TS) and exert TS like effect. The total effect such gene expression exerts is responsible for the rare occurrence and benign nature of primary cardiac tumors. This prevents the progression of tumorigenesis. But this also limits the regenerative and proliferative potential of cardiomyocytes. Cardiac Myxoma develops as a consequence of dysregulations in these key genes which revert the cells towards progenitor-like state, hallmark of CM. The CM development in carney complex also signifies the role of TS in cardiac cells.

PinX1t, a Novel PinX1 Transcript Variant, Positively Regulates Cardiogenesis of Embryonic Stem Cells

Background

Pin2/TRF1‐interacting protein, PinX1, was previously identified as a tumor suppressor. Here, we discovered a novel transcript variant of mPinX1 (mouse PinX1), mPinX1t (mouse PinX1t), in embryonic stem cells (ESCs). The aims of this investigation were (1) to detect the presence of mPinX1 and mPinX1t in ESCs and their differentiation derivatives; (2) to investigate the role of mPinX1 and mPinX1t on regulating the characteristics of undifferentiated ESCs and the cardiac differentiation of ESCs; (3) to elucidate the molecular mechanisms of how mPinX1 and mPinX1t regulate the cardiac differentiation of ESCs.

Methods and Results

By 5′ rapid amplification of cDNA ends, 3′ rapid amplification of cDNA ends, and polysome fractionation followed by reverse transcription–polymerase chain reaction, mPinX1t transcript was confirmed to be an intact mRNA that is actively translated. Western blot confirmed the existence of mPinX1t protein. Overexpression or knockdown of mPinX1 (both decreased mPinX1t expression) both decreased while overexpression of mPinX1t increased the cardiac differentiation of ESCs. Although both mPinX1 and mPinX1t proteins were found to bind to cardiac transcription factor mRNAs, only mPinX1t protein but not mPinX1 protein was found to bind to nucleoporin 133 protein, a nuclear pore complex component. In addition, mPinX1t‐containing cells were found to have a higher cytosol‐to‐nucleus ratio of cardiac transcription factor mRNAs when compared with that in the control cells. Our data suggested that mPinX1t may positively regulate cardiac differentiation by enhancing export of cardiac transcription factor mRNAs through interacting with nucleoporin 133.

Conclusions

We discovered a novel transcript variant of mPinX1, the mPinX1t, which positively regulates the cardiac differentiation of ESCs.

Single-Cell Expression Profiling Reveals a Dynamic State of Cardiac Precursor Cells in the Early Mouse Embryo

In the early vertebrate embryo, cardiac progenitor/precursor cells (CPs) give rise to cardiac structures. Better understanding their biological character is critical to understand the heart development and to apply CPs for the clinical arena. However, our knowledge remains incomplete. With the use of single-cell expression profiling, we have now revealed rapid and dynamic changes in gene expression profiles of the embryonic CPs during the early phase after their segregation from the cardiac mesoderm. Progressively, the nascent mesodermal gene Mesp1 terminated, and Nkx2-5+/Tbx5+ population rapidly replaced the Tbx5low+ population as the expression of the cardiac genes Tbx5 and Nkx2-5 increased. At the Early Headfold stage, Tbx5-expressing CPs gradually showed a unique molecular signature with signs of cardiomyocyte differentiation. Lineage-tracing revealed a developmentally distinct characteristic of this population. They underwent progressive differentiation only towards the cardiomyocyte lineage corresponding to the first heart field rather than being maintained as a progenitor pool. More importantly, Tbx5 likely plays an important role in a transcriptional network to regulate the distinct character of the FHF via a positive feedback loop to activate the robust expression of Tbx5 in CPs. These data expands our knowledge on the behavior of CPs during the early phase of cardiac development, subsequently providing a platform for further study.

The role of transcription factors in atrial fibrillation

Atrial fibrillation (AF) is a complex disease that results from genetic and environmental factors and their interactions. In recent years, genome-wide association studies (GWAS) and family-based linkage analysis have found amounts of genetic variants associated with AF. Some of them lie in coding sequences and thus mediate the encoded proteins, some in non-coding regions and influence the expression of adjacent genes. These variants exert influence on the development of cardiovascular system and normal cardiac electrical activity in different levels, and eventually contribute to the occurrence of AF. Among these affected genes, as a crucial means of transcriptional regulation, several transcription factors play important roles in the pathogenesis of AF. In this review, we will focus on the potential role of PITX2, PRRX1, ZHFX3, TBX5, and NKX2.5 in AF.

ISL1 protein transduction promotes cardiomyocyte differentiation from human embryonic stem cells

BACKGROUND

Human embryonic stem cells (hESCs) have the potential to provide an unlimited source of cardiomyocytes, which are invaluable resources for drug or toxicology screening, medical research, and cell therapy. Currently a number of obstacles exist such as the insufficient efficiency of differentiation protocols, which should be overcome before hESC-derived cardiomyocytes can be used for clinical applications. Although the differentiation efficiency can be improved by the genetic manipulation of hESCs to over-express cardiac-specific transcription factors, these differentiated cells are not safe enough to be applied in cell therapy. Protein transduction has been demonstrated as an alternative approach for increasing the efficiency of hESCs differentiation toward cardiomyocytes.

METHODS

We present an efficient protocol for the differentiation of hESCs in suspension by direct introduction of a LIM homeodomain transcription factor, Islet1 (ISL1) recombinant protein into the cells.

RESULTS

We found that the highest beating clusters were derived by continuous treatment of hESCs with 40 µg/ml recombinant ISL1 protein during days 1-8 after the initiation of differentiation. The treatment resulted in up to a 3-fold increase in the number of beating areas. In addition, the number of cells that expressed cardiac specific markers (cTnT, CONNEXIN 43, ACTININ, and GATA4) doubled. This protocol was also reproducible for another hESC line.

CONCLUSIONS

This study has presented a new, efficient, and reproducible procedure for cardiomyocytes differentiation. Our results will pave the way for scaled up and controlled differentiation of hESCs to be used for biomedical applications in a bioreactor culture system.

Tbx6 is a determinant of cardiac and neural cell fate decisions in multipotent P19CL6 cells

Multipotent P19CL6 cells differentiate into cardiac myocytes or neural lineages when stimulated with dimethyl sulfoxide (DMSO) or retinoic acid (RA), respectively. Expression of the transcription factor Tbx6 was found to increase during cardiac myocyte differentiation and to decrease during neural differentiation. Overexpression of Tbx6 was not sufficient to drive P19CL6 cells to a cardiac myocyte fate or to accelerate DMSO-induced differentiation. In contrast, knockdown of Tbx6 dramatically inhibited DMSO-induced differentiation of P19CL6 cells to cardiac myocytes, as evidenced by the loss of striated muscle-specific markers and spontaneous beating. Tbx6 knockdown was also accompanied by almost complete loss of Nkx2.5, a transcription factor involved in the specification of the cardiac myocyte lineage, indicating that Nkx2.5 is downstream of Tbx6. In distinction to its positive role in cardiac myocyte differentiation, Tbx6 knockdown augmented RA-induced differentiation of P19CL6 cells to both neurons and glia, and accelerated the rate of neurite formation. Conversely, Tbx6 overexpression attenuated differentiation to neural lineages. Thus, in the P19CL6 model, Tbx6 is required for cardiac myocyte differentiation and represses neural differentiation. We propose a model in which Tbx6 is a part of a molecular switch that modulates divergent differentiation programs within a single progenitor cell.

Regulation of connexin expression by transcription factors and epigenetic mechanisms

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

Developmental aspects of cardiac arrhythmogenesis

The transcriptional regulation orchestrating the development of the heart is increasingly recognized to play an essential role in the regulation of ion channel and gap junction gene expression and consequently the proper generation and conduction of the cardiac electrical impulse. This has led to the realization that in some instances, abnormal cardiac electrical function and arrhythmias in the postnatal heart may stem from a developmental abnormality causing maintained (epigenetic) changes in gene regulation. The role of developmental genes in the regulation of cardiac electrical function is further underscored by recent genome-wide association studies that provide strong evidence that common genetic variation, at loci harbouring these genes, modulates electrocardiographic indices of conduction and repolarization and susceptibility to arrhythmia. Here we discuss recent findings and provide background insight into these complex mechanisms.

Embryonic stem cells overexpressing Pitx2c engraft in infarcted myocardium and improve cardiac function

This study investigated the effects on cardiomyocyte differentiation of embryonic stem cells by the overexpression of the transcription factor, Pitx2c, and examined the effects of transplantation of these differentiated cells on cardiac function in a mouse model of myocardial infarction. Pitx2c overexpressing embryonic stem cells were characterized for cardiac differentiation by immunocytochemistry, RNA analysis, and electrophysiology. Differentiated cells were transplanted by directed injection into the infarcted murine myocardium and functional measurements of blood pressure, contractility, and relaxation were performed. Histochemistry and FISH analysis performed on these mice confirmed the engraftment and cardiac nature of the transplanted cells. Pitx2c overexpressing embryonic stem cells robustly differentiated into spontaneously contracting cells which acquired cardiac protein markers and exhibited action potentials resembling that of cardiomyocytes. These cells could also be synchronized to an external pacemaker. Significant improvements (P < 0.01) in blood pressure (56%), contractility (57%), and relaxation (59%) were observed in infarcted mice with transplants of these differentiated cells but not in mice which were transplanted with control cells. The Pitx2c overexpressing cells secrete paracrine factors which when adsorbed onto a heparinated gel and injected into the infarcted myocardium produce a comparable and significant (P < 0.01) functional recovery. Pitx2c overexpression is a valuable method for producing cardiomyocytes from embryonic stem cells, and transplantation of these cardiomyocytes into infracted myocardium restores cardiac function through multiple mechanisms.

Msx1 and Msx2 are functional interacting partners of T-box factors in the regulation of Connexin43

AIMS

T-box factors Tbx2 and Tbx3 play key roles in the development of the cardiac conduction system, atrioventricular canal, and outflow tract of the heart. They regulate the gap-junction-encoding gene Connexin43 (Cx43) and other genes critical for heart development and function. Discovering protein partners of Tbx2 and Tbx3 will shed light on the mechanisms by which these factors regulate these gene programs.

METHODS AND RESULTS

Employing an yeast 2-hybrid screen and subsequent in vitro pull-down experiments we demonstrate that muscle segment homeobox genes Msx1 and Msx2 are able to bind the cardiac T-box proteins Tbx2, Tbx3, and Tbx5. This interaction, as that of the related Nkx2.5 protein, is supported by the T-box and homeodomain alone. Overlapping spatiotemporal expression patterns of Msx1 and Msx2 together with the T-box genes during cardiac development in mouse and chicken underscore the biological significance of this interaction. We demonstrate that Msx proteins together with Tbx2 and Tbx3 suppress Cx43 promoter activity and down regulate Cx43 gene activity in a rat heart-derived cell line. Using chromatin immunoprecipitation analysis we demonstrate that Msx1 can bind the Cx43 promoter at a conserved binding site located in close proximity to a previously defined T-box binding site, and that the activity of Msx proteins on this promoter appears dependent in the presence of Tbx3.

CONCLUSION

Msx1 and Msx2 can function in concert with the T-box proteins to suppress Cx43 and other working myocardial genes.

T-box factors determine cardiac design

The heart of higher vertebrates is a structurally complicated multi-chambered pump that contracts synchronously. For its proper function a number of distinct integrated components have to be generated, including force-generating compartments, unidirectional valves, septa and a system in charge of the initiation and coordinated propagation of the depolarizing impulse over the heart. Not surprisingly, a large number of regulating factors are involved in these processes that act in complex and intertwined pathways to regulate the activity of target genes responsible for morphogenesis and function. The finding that mutations in T-box transcription factor-encoding genes in humans lead to congenital heart defects has focused attention on the importance of this family of regulators in heart development. Functional and genetic analyses in a variety of divergent species has demonstrated the critical roles of multiple T-box factor gene family members, including Tbx11, −2, −3, −5, −18 and −20, in the patterning, recruitment, specification, differentiation and growth processes underlying formation and integration of the heart components. Insight into the roles of T-box factors in these processes will enhance our understanding of heart formation and the underlying molecular regulatory pathways.

Microarray analysis of Tbx5-induced genes expressed in the developing heart

Tbx5 is a member of the T-box family of transcription factors and is associated with Holt-Oram syndrome (HOS), a congenital disorder characterized by heart and limb defects. Although implicated in several processes during development, only a few genes regulated by Tbx5 have been reported. To identify candidate genes regulated by Tbx5 during heart development, a microarray approach was used. A cardiac-derived mouse cell line (1H) was infected with adenoviruses expressing Tbx5 or beta-galactosidase and RNA was isolated for analysis using an Affymetrix gene chip representing over 39,000 transcripts. Real-time reverse transcriptase-polymerase chain reaction confirmed Tbx5 induction of a subset of the genes, including nppa, photoreceptor cadherin, brain creatine kinase, hairy/enhancer-of-split related 2, and gelsolin. In situ hybridization analysis indicated overlapping expression of these genes with tbx5 in the embryonic mouse heart. In addition, the effect of HOS-associated mutations on the ability of Tbx5 to induce target gene expression was evaluated. Together, these data identify several genes induced by Tbx5 that are potentially important during cardiac development. These genes represent new candidate gene targets of Tbx5 that may be related to congenital heart malformations associated with HOS.

Embryonic and adult stem cell-derived cardiomyocytes: lessons from in vitro models

For years, research has focused on how to treat heart failure by sustaining the overloaded remaining cardiomyocytes. Recently, the concept of cell replacement therapy as a treatment of heart diseases has opened a new area of investigation. In vitro-generated cardiomyocytes could be injected into the heart to rescue the function of a damaged myocardium. Embryonic and/or adult stem cells could provide cardiac cells for this purpose. Knowledge of fundamental cardiac differentiation mechanisms unraveled by studies on animal models has been improved using in vitro models of cardiogenesis such as mouse embryonal carcinoma cells, mouse embryonic stem cells and, recently, human embryonic stem cells. On the other hand, studies suggesting the existence of cardiac stem cells and the potential of adult stem cells from bone marrow or skeletal muscle to differentiate toward unexpected phenotypes raise hope and questions about their potential use for cardiac cell therapy. In this review, we compare the specificities of embryonic vs adult stem cell populations regarding their cardiac differentiation potential, and we give an overview of what in vitro models have taught us about cardiogenesis.

Regulation of connexin expression

Gap junctions contain cell-cell communicating channels that consist of multimeric proteins called connexins and mediate the exchange of low-molecular-weight metabolites and ions between contacting cells. Gap junctional communication has long been hypothesized to play a crucial role in the maintenance of homeostasis, morphogenesis, cell differentiation, and growth control in multicellular organisms. The recent discovery that human genetic disorders are associated with mutations in connexin genes and experimental data on connexin knockout mice have provided direct evidence that gap junctional communication is essential for tissue functions and organ development. Thus far, 21 human genes and 20 mouse genes for connexins have been identified. Each connexin shows tissue- or cell-type-specific expression, and most organs and many cell types express more than one connexin. Cell coupling via gap junctions is dependent on the specific pattern of connexin gene expression. This pattern of gene expression is altered during development and in several pathological conditions resulting in changes of cell coupling. Connexin expression can be regulated at many of the steps in the pathway from DNA to RNA to protein. However, transcriptional control is one of the most important points. In this review, we summarize recent knowledge on transcriptional regulation of connexin genes by describing the structure of connexin genes and transcriptional factors that regulate connexin expression.

T-box genes and heart development: Putting the ?T? in heart

Members of the T-box gene family (Tbx) are essential for normal heart development, and mutations in human TBX genes cause congenital cardiovascular malformations. T-box genes have been implicated in early cardiac lineage determination, chamber specification, valvuloseptal development, and diversification of the specialized conduction system in vertebrate embryos. These genes include Tbx1, Tbx2, Tbx3, Tbx5, Tbx18, and Tbx20, all of which exhibit complex temporal spatial regulation in developing cardiac structures. Less is known about T-box genes in invertebrate heart development, but multiple T-box genes are expressed in Drosophila cardiac lineages. The molecular hierarchies and developmental processes controlled by T-box genes in the heart are the focus of this review.