Mesp1 acts as a master regulator of multipotent cardiovascular progenitor specification

Generation of multipotent induced cardiac progenitor cells from mouse fibroblasts and potency testing in ex vivo mouse embryos

Here we describe a protocol to generate expandable and multipotent induced cardiac progenitor cells (iCPCs) from mouse adult fibroblasts using forced expression of Mesp1, Tbx5, Gata4, Nkx2.5 and Baf60c (MTGNB) along with activation of Wnt and JAK/STAT signaling. This method does not use iPS cell factors and thus differs from cell activation and signaling-directed (CASD) reprogramming to cardiac progenitors. Our method is specific to direct CPC reprogramming, whereas CASD reprogramming can generate various cell types depending on culture conditions and raises the possibility of transitioning through a pluripotent cell state. The protocol describes how to isolate and infect primary fibroblasts; induce reprogramming and observe iCPC colonies; expand and characterize reprogrammed iCPCs by immunostaining, flow cytometry and gene expression; differentiate iCPCs in vitro into cardiac-lineage cells; and test the embryonic potency of iCPCs via injection into the cardiac crescent of mouse embryos. A scientist experienced in molecular cell biology and embryology can reproduce this protocol in 12-16 weeks. iCPCs can be used for studying cardiac biology, drug discovery and regenerative medicine.

Turning Potential Into Action: Using Pluripotent Stem Cells to Understand Heart Development and Function in Health and Disease

Pluripotent stem cells hold enormous potential for regenerative therapies, however their ability to provide insight into early human development and the origins of disease could arguably provide an even greater outcome. This is primarily due to their contribution to the establishment of a powerful knowledge base of human development, something which all researchers and clinicians can potentially benefit from. Modeling human heart development and disease using pluripotent stem cells has already provided many important insights into cardiogenesis and cardiovascular disease mechanisms however, it is important to be aware of the complexities of this model system. Thorough contemplation of experimental models and specialized techniques is required to provide high‐quality evidence of the intricacies of both normal early development, and when this process goes awry in disease states. Stem Cells Translational Medicine2017;6:1452–1457

SOXF transcription factors in cardiovascular development

Cardiovascular development during embryogenesis involves complex changes in gene regulatory networks regulated by a variety of transcription factors. In this review we discuss the various reported roles of the SOXF factors: SOX7, SOX17 and SOX18 in cardiac, vascular and lymphatic development. SOXF factors have pleiotropic roles during these processes, and there is significant redundancy and functional compensation between SOXF family members. Despite this, evidence suggests that there is some specificity in the transcriptional programs they regulate which is necessary to control the differentiation and behaviour of endothelial subpopulations. Furthermore, SOXF factors appear to have an indirect role in regulating cardiac mesoderm specification and differentiation. Understanding how SOXF factors are regulated, as well as their downstream transcriptional target genes, will be important for unravelling their roles in cardiovascular development and related diseases.

Thyroid Progenitors Are Robustly Derived from Embryonic Stem Cells through Transient, Developmental Stage-Specific Overexpression of Nkx2-1

The clinical importance of anterior foregut endoderm (AFE) derivatives, such as thyrocytes, has led to intense research efforts for their derivation through directed differentiation of pluripotent stem cells (PSCs). Here, we identify transient overexpression of the transcription factor (TF) NKX2-1 as a powerful inductive signal for the robust derivation of thyrocyte-like cells from mouse PSC-derived AFE. This effect is highly developmental stage specific and dependent on FOXA2 expression levels and precise modulation of BMP and FGF signaling. The majority of the resulting cells express thyroid TFs (Nkx2-1, Pax8, Foxe1, Hhex) and thyroid hormone synthesis-related genes (Tg, Tpo, Nis, Iyd) at levels similar to adult mouse thyroid and give rise to functional follicle-like epithelial structures in Matrigel culture. Our findings demonstrate that NKX2-1 overexpression converts AFE to thyroid epithelium in a developmental time-sensitive manner and suggest a general methodology for manipulation of cell-fate decisions of developmental intermediates.

The Calcineurin Variant CnAβ1 Controls Mouse Embryonic Stem Cell Differentiation by Directing mTORC2 Membrane Localization and Activation

Embryonic stem cells (ESC) have the potential to generate all the cell lineages that form the body. However, the molecular mechanisms underlying ESC differentiation and especially the role of alternative splicing in this process remain poorly understood. Here, we show that the alternative splicing regulator MBNL1 promotes generation of the atypical calcineurin Aβ variant CnAβ1 in mouse ESCs (mESC). CnAβ1 has a unique C-terminal domain that drives its localization mainly to the Golgi apparatus by interacting with Cog8. CnAβ1 regulates the intracellular localization and activation of the mTORC2 complex. CnAβ1 knockdown results in delocalization of mTORC2 from the membrane to the cytoplasm, inactivation of the AKT/GSK3β/β-catenin signaling pathway, and defective mesoderm specification. In summary, here we unveil the structural basis for the mechanism of action of CnAβ1 and its role in the differentiation of mESCs to the mesodermal lineage.

CITED2 Cooperates with ISL1 and Promotes Cardiac Differentiation of Mouse Embryonic Stem Cells

The transcriptional regulator CITED2 is essential for heart development. Here, we investigated the role of CITED2 in the specification of cardiac cell fate from mouse embryonic stem cells (ESC). The overexpression of CITED2 in undifferentiated ESC was sufficient to promote cardiac cell emergence upon differentiation. Conversely, the depletion of Cited2 at the onset of differentiation resulted in a decline of ESC ability to generate cardiac cells. Moreover, loss of Cited2 expression impairs the expression of early mesoderm markers and cardiogenic transcription factors (Isl1, Gata4, Tbx5). The cardiogenic defects in Cited2-depleted cells were rescued by treatment with recombinant CITED2 protein. We showed that Cited2 expression is enriched in cardiac progenitors either derived from ESC or mouse embryonic hearts. Finally, we demonstrated that CITED2 and ISL1 proteins interact physically and cooperate to promote ESC differentiation toward cardiomyocytes. Collectively, our results show that Cited2 plays a pivotal role in cardiac commitment of ESC.

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Overexpression of CITED2 in ESC promotes cardiogenesis upon differentiation

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Cited2 depletion reduces ESC ability to generate cardiac cells

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Cited2 expression is enriched in cardiac progenitors

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CITED2 and ISL1 cooperate to promote ESC differentiation toward cardiomyocytes

Development of Bipotent Cardiac/Skeletal Myogenic Progenitors from MESP1+ Mesoderm

The branchiomeric skeletal muscles co-evolved with new chambers of the heart to enable predatory feeding in chordates. These co-evolved tissues develop from a common population in anterior splanchnic mesoderm, referred to as cardiopharyngeal mesoderm (CPM). The regulation and development of CPM are poorly understood. We describe an embryonic stem cell-based system in which MESP1 drives a PDGFRA+ population with dual cardiac and skeletal muscle differentiation potential, and gene expression resembling CPM. Using this system, we investigate the regulation of these bipotent progenitors, and find that cardiac specification is governed by an antagonistic TGFβ-BMP axis, while skeletal muscle specification is enhanced by Rho kinase inhibition. We define transcriptional signatures of the first committed CPM-derived cardiac and skeletal myogenic progenitors, and discover surface markers to distinguish cardiac (PODXL+) from the skeletal muscle (CDH4+) CPM derivatives. These tools open an accessible window on this developmentally and evolutionarily important population.

Zrf1 controls mesoderm lineage genes and cardiomyocyte differentiation

In the present study we addressed the function of the transcriptional activator Zrf1 in the generation of the 3 germ layers during in vitro development. Currently, Zrf1 is rather regarded as a factor that drives the expression of neuronal genes. Here, we have employed mouse embryonic stem cells and P19 cells to understand the role of Zrf1 in the generation of mesoderm-derived tissues like adipocytes, cartilage and heart. Our data shows that Zrf1 is essential for the transcriptional activation of genes that give rise to mesoderm and in particular heart development. In both, the mESC and P19 systems, we provide evidence that Zrf1 contributes to the generation of functional cardiomyocytes. We further demonstrate that Zrf1 binds to the transcription start sites (TSSs) of heart tissue-specific genes from the first and second heart field where it drives their temporal expression during differentiation. Taken together, we have identified Zrf1 as a novel regulator of the mesodermal lineage that might facilitate spatiotemporal expression of genes.

Probing early heart development to instruct stem cell differentiation strategies

Scientists have studied organs and their development for centuries and, along that path, described models and mechanisms explaining the developmental principles of organogenesis. In particular, with respect to the heart, new fundamental discoveries are reported continuously that keep changing the way we think about early cardiac development. These discoveries are driven by the need to answer long-standing questions regarding the origin of the earliest cells specified to the cardiac lineage, the differentiation potential of distinct cardiac progenitor cells, and, very importantly, the molecular mechanisms underlying these specification events. As evidenced by numerous examples, the wealth of developmental knowledge collected over the years has had an invaluable impact on establishing efficient strategies to generate cardiovascular cell types ex vivo, from either pluripotent stem cells or via direct reprogramming approaches. The ability to generate functional cardiovascular cells in an efficient and reliable manner will contribute to therapeutic strategies aimed at alleviating the increasing burden of cardiovascular disease and morbidity. Here we will discuss the recent discoveries in the field of cardiac progenitor biology and their translation to the pluripotent stem cell model to illustrate how developmental concepts have instructed regenerative model systems in the past and promise to do so in the future. Developmental Dynamics 245:1130-1144, 2016. © 2016 Wiley Periodicals, Inc.

CIBZ Regulates Mesodermal and Cardiac Differentiation of by Suppressing T and Mesp1 Expression in Mouse Embryonic Stem Cells

The molecular mechanisms underlying mesodermal and cardiac specification from embryonic stem cells (ESCs) are not fully understood. Here, we showed that the BTB domain-containing zinc finger protein CIBZ is expressed in mouse ESCs but is dramatically downregulated during ESC differentiation. CIBZ deletion in ESCs induced specification toward mesoderm phenotypes and their differentiation into cardiomyocytes, whereas overexpression of CIBZ delayed these processes. During ESC differentiation, CIBZ loss-and-gain-of-function data indicate that CIBZ negatively regulates the expressions of Brachyury (T) and Mesp1, the key transcriptional factors responsible for the specification of mammalian mesoderm and cardiac progenitors, respectively. Chromatin immunoprecipitation assays showed that CIBZ binds to T and Mesp1 promoters in undifferentiated ESCs, and luciferase assays indicate that CIBZ suppresses T and Mesp1 promoters. These findings demonstrate that CIBZ is a novel regulator of mesodermal and cardiac differentiation of ESCs, and suggest that CIBZ-mediated cardiac differentiation depends on the regulation of these two genes.

Genetic and Epigenetic Regulation of Human Cardiac Reprogramming and Differentiation in Regenerative Medicine

Regeneration or replacement of lost cardiomyocytes within the heart has the potential to revolutionize cardiovascular medicine. Numerous methodologies have been used to achieve this aim, including the engraftment of bone marrow- and heart-derived cells as well as the identification of modulators of adult cardiomyocyte proliferation. Recently, the conversion of human somatic cells into induced pluripotent stem cells and induced cardiomyocyte-like cells has transformed potential approaches toward this goal, and the engraftment of cardiac progenitors derived from human embryonic stem cells into patients is now feasible. Here we review recent advances in our understanding of the genetic and epigenetic control of human cardiogenesis, cardiac differentiation, and the induced reprogramming of somatic cells to cardiomyocytes. We also cover genetic programs for inducing the proliferation of endogenous cardiomyocytes and discuss the genetic state of cells used in cardiac regenerative medicine.

Conditional Creation and Rescue of Nipbl-Deficiency in Mice Reveals Multiple Determinants of Risk for Congenital Heart Defects

Elucidating the causes of congenital heart defects is made difficult by the complex morphogenesis of the mammalian heart, which takes place early in development, involves contributions from multiple germ layers, and is controlled by many genes. Here, we use a conditional/invertible genetic strategy to identify the cell lineage(s) responsible for the development of heart defects in a Nipbl-deficient mouse model of Cornelia de Lange Syndrome, in which global yet subtle transcriptional dysregulation leads to development of atrial septal defects (ASDs) at high frequency. Using an approach that allows for recombinase-mediated creation or rescue of Nipbl deficiency in different lineages, we uncover complex interactions between the cardiac mesoderm, endoderm, and the rest of the embryo, whereby the risk conferred by genetic abnormality in any one lineage is modified, in a surprisingly non-additive way, by the status of others. We argue that these results are best understood in the context of a model in which the risk of heart defects is associated with the adequacy of early progenitor cell populations relative to the sizes of the structures they must eventually form.

A G-Rich Motif in the lncRNA Braveheart Interacts with a Zinc-Finger Transcription Factor to Specify the Cardiovascular Lineage

Long non-coding RNAs (lncRNAs) are an emerging class of transcripts that can modulate gene expression; however, their mechanisms of action remain poorly understood. Here, we experimentally determine the secondary structure of Braveheart (Bvht) using chemical probing methods and show that this ∼590 nt transcript has a modular fold. Using CRISPR/Cas9-mediated editing of mouse embryonic stem cells, we find that deletion of 11 nt in a 5' asymmetric G-rich internal loop (AGIL) of Bvht (bvht^dAGIL) dramatically impairs cardiomyocyte differentiation. We demonstrate a specific interaction between AGIL and cellular nucleic acid binding protein (CNBP/ZNF9), a zinc-finger protein known to bind single-stranded G-rich sequences. We further show that CNBP deletion partially rescues the bvht^dAGIL mutant phenotype by restoring differentiation capacity. Together, our work shows that Bvht functions with CNBP through a well-defined RNA motif to regulate cardiovascular lineage commitment, opening the door for exploring broader roles of RNA structure in development and disease.

Developmental origin and lineage plasticity of endogenous cardiac stem cells

Over the past two decades, several populations of cardiac stem cells have been described in the adult mammalian heart. For the most part, however, their lineage origins and in vivo functions remain largely unexplored. This Review summarizes what is known about different populations of embryonic and adult cardiac stem cells, including KIT(+), PDGFRα(+), ISL1(+)and SCA1(+)cells, side population cells, cardiospheres and epicardial cells. We discuss their developmental origins and defining characteristics, and consider their possible contribution to heart organogenesis and regeneration. We also summarize the origin and plasticity of cardiac fibroblasts and circulating endothelial progenitor cells, and consider what role these cells have in contributing to cardiac repair.

Mespaa can potently induce cardiac fates in zebrafish

The Mesp family of transcription factors have been implicated in the early formation and migration of the cardiac lineage, although the precise molecular mechanisms underlying this process remain unknown. In this study we examine the function of Mesp family members in zebrafish cardiac development and find that Mespaa is remarkably efficient at promoting cardiac fates in normally non-cardiogenic cells. However, Mespaa is dispensable for normal cardiac formation. Despite no overt defects in cardiovascular specification, we find a consistent defect in cardiac laterality in mespaa null embryos. This is further exacerbated by the depletion of other mesp paralogues, highlighting a conserved role for the mesp family in left-right asymmetry, distinct from a function in cardiac specification. Despite an early requirement for mespaa to promote cardiogenesis, cells over-expressing mespaa are found to both exhibit unique cellular behaviors and activate the transcription of gata5 only after the completion of gastrulation. We propose that while mespaa remains capable of driving cardiac progenitor formation in zebrafish, it may not play an essential role in the cardiac regulatory network. Furthermore, the late activation of migration and cardiac gene transcription in mespaa over-expressing cells challenges previous studies on the timing of these events and provides intriguing questions for future study.

Mesp1 Marked Cardiac Progenitor Cells Repair Infarcted Mouse Hearts

Mesp1 directs multipotential cardiovascular cell fates, even though it's transiently induced prior to the appearance of the cardiac progenitor program. Tracing Mesp1-expressing cells and their progeny allows isolation and characterization of the earliest cardiovascular progenitor cells. Studying the biology of Mesp1-CPCs in cell culture and ischemic disease models is an important initial step toward using them for heart disease treatment. Because of Mesp1's transitory nature, Mesp1-CPC lineages were traced by following EYFP expression in murine Mesp1(Cre/+); Rosa26(EYFP/+) ES cells. We captured EYFP+ cells that strongly expressed cardiac mesoderm markers and cardiac transcription factors, but not pluripotent or nascent mesoderm markers. BMP2/4 treatment led to the expansion of EYFP+ cells, while Wnt3a and Activin were marginally effective. BMP2/4 exposure readily led EYFP+ cells to endothelial and smooth muscle cells, but inhibition of the canonical Wnt signaling was required to enter the cardiomyocyte fate. Injected mouse pre-contractile Mesp1-EYFP+ CPCs improved the survivability of injured mice and restored the functional performance of infarcted hearts for at least 3 months. Mesp1-EYFP+ cells are bona fide CPCs and they integrated well in infarcted hearts and emerged de novo into terminally differentiated cardiac myocytes, smooth muscle and vascular endothelial cells.

Paracrine nitric oxide induces expression of cardiac sarcomeric proteins in adult progenitor cells through soluble guanylyl cyclase/cyclic-guanosine monophosphate and Wnt/β-catenin inhibition

AIM

Cardiac progenitor cells (CPC) from adult hearts can differentiate to several cell types composing the myocardium but the underlying molecular pathways are poorly characterized. We examined the role of paracrine nitric oxide (NO) in the specification of CPC to the cardiac lineage, particularly through its inhibition of the canonical Wnt/β-catenin pathway, a critical step preceding cardiac differentiation.

METHODS AND RESULTS

Sca1 + CPC from adult mouse hearts were isolated by magnetic-activated cell sorting and clonally expanded. Pharmacologic NO donors increased their expression of cardiac myocyte-specific sarcomeric proteins in a concentration and time-dependent manner. The optimal time window for NO efficacy coincided with up-regulation of CPC expression of Gucy1a3 (coding the alpha1 subunit of guanylyl cyclase). The effect of paracrine NO was reproduced in vitro upon co-culture of CPC with cardiac myocytes expressing a transgenic NOS3 (endothelial nitric oxide synthase) and in vivo upon injection of CPC in infarcted hearts from cardiac-specific NOS3 transgenic mice. In mono- and co-cultures, this effect was abrogated upon inhibition of soluble guanylyl cyclase or nitric oxide synthase, and was lost in CPC genetically deficient in Gucy1a3. Mechanistically, NO inhibits the constitutive activity of the canonical Wnt/β-catenin in CPC and in cell reporter assays in a guanylyl cyclase-dependent fashion. This was paralleled with decreased expression of β-catenin and down-regulation of Wnt target genes in CPC and abrogated in CPC with a stabilized, non-inhibitable β-catenin.

CONCLUSIONS

Exogenous or paracrine sources of NO promote the specification towards the myocyte lineage and expression of cardiac sarcomeric proteins of adult CPC. This is contingent upon the expression and activity of the alpha1 subunit of guanylyl cyclase in CPC that is necessary for NO-mediated inhibition of the canonical Wnt/β-catenin pathway.

miR-322/-503 cluster is expressed in the earliest cardiac progenitor cells and drives cardiomyocyte specification

Understanding the mechanisms of early cardiac fate determination may lead to better approaches in promoting heart regeneration. We used a mesoderm posterior 1 (Mesp1)-Cre/Rosa26-EYFP reporter system to identify microRNAs (miRNAs) enriched in early cardiac progenitor cells. Most of these miRNA genes bear MESP1-binding sites and active histone signatures. In a calcium transient-based screening assay, we identified miRNAs that may promote the cardiomyocyte program. An X-chromosome miRNA cluster, miR-322/-503, is the most enriched in the Mesp1 lineage and is the most potent in the screening assay. It is specifically expressed in the looping heart. Ectopic miR-322/-503 mimicking the endogenous temporal patterns specifically drives a cardiomyocyte program while inhibiting neural lineages, likely by targeting the RNA-binding protein CUG-binding protein Elav-like family member 1 (Celf1). Thus, early miRNAs in lineage-committed cells may play powerful roles in cell-fate determination by cross-suppressing other lineages. miRNAs identified in this study, especially miR-322/-503, are potent regulators of early cardiac fate.

BRG1 interacts with GLI2 and binds Mef2c gene in a hedgehog signalling dependent manner during in vitro cardiomyogenesis

Background

The Hedgehog (HH) signalling pathway regulates cardiomyogenesis in vivo and in differentiating P19 embryonal carcinoma (EC) cells, a mouse embryonic stem (mES) cell model. To further assess the transcriptional role of HH signalling during cardiomyogenesis in stem cells, we studied the effects of overexpressing GLI2, a primary transducer of the HH signalling pathway, in mES cells.

Results

Stable GLI2 overexpression resulted in an enhancement of cardiac progenitor-enriched genes, Mef2c, Nkx2-5, and Tbx5 during mES cell differentiation. In contrast, pharmacological blockade of the HH pathway in mES cells resulted in lower expression of these genes. Mass spectrometric analysis identified the chromatin remodelling factor BRG1 as a protein which co-immunoprecipitates with GLI2 in differentiating mES cells. We then determined that BRG1 is recruited to a GLI2-specific Mef2c gene element in a HH signalling-dependent manner during cardiomyogenesis in P19 EC cells, a mES cell model.

Conclusions

Thus, we propose a mechanism where HH/GLI2 regulates the expression of Mef2c by recruiting BRG1 to the Mef2c gene, most probably via chromatin remodelling, to ultimately regulate in vitro cardiomyogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-016-0127-8) contains supplementary material, which is available to authorized users.

Mechanisms of Cardiac Regeneration

Adult humans fail to regenerate their hearts following injury, and this failure to regenerate myocardium is a leading cause of heart failure and death worldwide. Although all adult mammals appear to lack significant cardiac regeneration potential, some vertebrates can regenerate myocardium throughout life. In addition, new studies indicate that mammals have cardiac regeneration potential during development and very soon after birth. The mechanisms of heart regeneration among model organisms, including neonatal mice, appear remarkably similar. Orchestrated waves of inflammation, matrix deposition and remodeling, and cardiomyocyte proliferation are commonly seen in heart regeneration models. Understanding why adult mammals develop extensive scarring instead of regeneration is a crucial goal for regenerative biology.

Generation of Induced Cardiospheres via Reprogramming of Skin Fibroblasts for Myocardial Regeneration

Recent pre-clinical and clinical studies have suggested that endogenous cardiospheres (eCS) are potentially safe and effective for cardiac regeneration following myocardial infarction (MI). Nevertheless the preparation of autologous eCS requires invasive myocardial biopsy with limited yield. We describe a novel approach to generate induced cardiospheres (iCS) from adult skin fibroblasts via somatic reprogramming. After infection with Sox2, Klf4, and Oct4, iCS were generated from mouse adult skin fibroblasts treated with Gsk3β inhibitor-(2'Z,3'E)- 6-Bromoindirubin-3'-oxime and Oncostatin M. They resembled eCS, but contained a higher percentage of cells expressing Mesp1, Isl1, and Nkx2.5. They were differentiated into functional cardiomyocytes in vitro with similar electrophysiological properties, calcium transient and contractile function to eCS and mouse embryonic stem cell-derived cardiomyocytes. Transplantation of iCS (1 × 10⁶ cells) into mouse myocardium following MI had similar effects to transplantation of eCS but significantly better than saline or fibroblast in improving left ventricular ejection fraction, increasing anterior/septal ventricular wall thickness and capillary density in the infarcted region 4 weeks after transplantation. No tumor formation was observed. iCS generated from adult skin fibroblasts by somatic reprogramming and a cocktail of Gsk3β inhibitor-6-Bromoindirubin-3'-oxime and Oncostatin M may represent a novel source for cell therapy in MI. Stem Cells 2016;34:2693-2706.

Human Induced Pluripotent Stem Cells as a Platform for Personalized and Precision Cardiovascular Medicine

Human induced pluripotent stem cells (hiPSCs) have revolutionized the field of human disease modeling, with an enormous potential to serve as paradigm shifting platforms for preclinical trials, personalized clinical diagnosis, and drug treatment. In this review, we describe how hiPSCs could transition cardiac healthcare away from simple disease diagnosis to prediction and prevention, bridging the gap between basic and clinical research to bring the best science to every patient.

How Mesp1 makes a move

The transcription factors Mesp1 and Mesp2 have essential roles in the migration and specification of multipotent progenitor cells at the onset of cardiogenesis. Chiapparo et al. (2016. J. Cell Biol http://dx.doi.org/10.1083/jcb.201505082) identify common Mesp functions in fate specification and Mesp1-specific targets controlling the speed and direction of progenitor cell migration.

Mesp1 controls the speed, polarity, and directionality of cardiovascular progenitor migration

During embryonic development, Mesp1 marks the earliest cardiovascular progenitors (CPs) and promotes their specification, epithelial-mesenchymal transition (EMT), and cardiovascular differentiation. However, Mesp1 deletion in mice does not impair initial CP specification and early cardiac differentiation but induces cardiac malformations thought to arise from a defect of CP migration. Using inducible gain-of-function experiments during embryonic stem cell differentiation, we found that Mesp2, its closest homolog, was as efficient as Mesp1 at promoting CP specification, EMT, and cardiovascular differentiation. However, only Mesp1 stimulated polarity and directional cell migration through a cell-autonomous mechanism. Transcriptional analysis and chromatin immunoprecipitation experiments revealed that Mesp1 and Mesp2 activate common target genes that promote CP specification and differentiation. We identified two direct Mesp1 target genes, Prickle1 and RasGRP3, that are strongly induced by Mesp1 and not by Mesp2 and that control the polarity and the speed of cell migration. Altogether, our results identify the molecular interface controlled by Mesp1 that links CP specification and cell migration.

Heterogeneity of Mesp1+ mesoderm revealed by single-cell RNA-seq

Mesp1 is a transcription factor that promotes differentiation of pluripotent cells into different mesoderm lineages including hematopoietic, cardiac and skeletal myogenic. This occurs via at least two transient cell populations: a common hematopoietic/cardiac progenitor population and a common cardiac/skeletal myogenic progenitor population. It is not established whether Mesp1-induced mesoderm cells are intrinsically heterogeneous, or are simply capable of multiple lineage decisions. In the current study, we applied single-cell RNA-seq to analyze Mesp1+ mesoderm. Initial whole transcriptome analysis showed a surprising homogeneity among Mesp1-induced mesoderm cells. However, this apparent global homogeneity masked an intrinsic heterogeneity revealed by interrogating a panel of early mesoderm patterning factors. This approach enabled discovery of subpopulations primed for hematopoietic or cardiac development. These studies demonstrate the heterogeneic nature of Mesp1+ mesoderm.

The Apelin receptor enhances Nodal/TGFβ signaling to ensure proper cardiac development

The Apelin receptor (Aplnr) is essential for heart development, controlling the early migration of cardiac progenitors. Here we demonstrate that in zebrafish Aplnr modulates Nodal/TGFβ signaling, a key pathway essential for mesendoderm induction and migration. Loss of Aplnr function leads to a reduction in Nodal target gene expression whereas activation of Aplnr by a non-peptide agonist increases the expression of these same targets. Furthermore, loss of Aplnr results in a delay in the expression of the cardiogenic transcription factors mespaa/ab. Elevating Nodal levels in aplnra/b morphant and double mutant embryos is sufficient to rescue cardiac differentiation defects. We demonstrate that loss of Aplnr attenuates the activity of a point source of Nodal ligands Squint and Cyclops in a non-cell autonomous manner. Our results favour a model in which Aplnr is required to fine-tune Nodal output, acting as a specific rheostat for the Nodal/TGFβ pathway during the earliest stages of cardiogenesis.

DOI:http://dx.doi.org/10.7554/eLife.13758.001

In one of the first events that happens as an embryo develops, cells become the different stem cell populations that form the body’s organs. So what makes a cell become one stem cell type rather than another? In the case of the heart, the first important event is the activity of a signaling pathway called the Nodal/TGFβ pathway. Nodal signaling can drive cells to become many different stem cell types depending on its level of activity. Many different levels of regulation fine-tune Nodal signaling to produce these activity thresholds.

Zebrafish that have a mutation in the gene that encodes a protein called the Apelin receptor have no heart. The loss of this receptor interferes with how heart stem cells (called cardiac progenitors) are made and how they move to where heart development occurs.

Deshwar et al. have now studied mutant zebrafish in order to investigate how the Apelin receptor influences early heart development. This revealed that Nodal signaling levels are slightly lower in the mutant zebrafish embryos than in normal fish at the time when Nodal activity induces cardiac progenitors to form. When Nodal activity is experimentally boosted in zebrafish that lack the Apelin receptor, they become able to develop hearts.

Deshwar et al. also found that the Apelin receptor does not work in cells that produce or receive Nodal signals. This suggests that the Apelin receptor modulates Nodal signaling levels by acting in cells that lie between the cells that release Nodal signals and the cardiac progenitors. An important question for future work to address is how this modulation works. As Nodal is a key determinant of many cell types in developing embryos, learning how Apelin receptors regulate its activity could help researchers to derive specific cell types from cultured stem cells for use in regenerative medicine.

DOI:http://dx.doi.org/10.7554/eLife.13758.002

Maturation of pluripotent stem cell derived cardiomyocytes: The new challenge

Stem cell therapy appears to be a promising area of research for cardiac regeneration following ischemic heart failure. However, in vitro differentiation of cardiomyocytes from pluripotent stem cells, or directly from somatic cells, leads to generation of "immature" cardiomyocytes that differ from their adult counterparts in various ways. This immaturity triggers some challenges for their potential clinical use, and multiple techniques reviewed here have been developed for in vitro maturation of those cells. Nevertheless, full maturity of cardiomyocytes remains elusive and will remain the main challenge for stem cell therapy in the near future.

Rbm24 Regulates Alternative Splicing Switch in Embryonic Stem Cell Cardiac Lineage Differentiation

The transition of embryonic stem cell (ESC) pluripotency to differentiation is accompanied by an expansion of mRNA and proteomic diversity. Post-transcriptional regulation of ESCs is critically governed by cell type-specific splicing. However, little is known about the splicing factors and the molecular mechanisms directing ESC early lineage differentiation. Our study identifies RNA binding motif protein 24 (Rbm24) as a key splicing regulator that plays an essential role in controlling post-transcriptional networks during ESC transition into cardiac differentiation. Using an inducible mouse ESC line in which gene expression could be temporally regulated, we demonstrated that forced expression of Rbm24 in ESCs dramatically induced a switch to cardiac specification. Genome-wide RNA sequencing analysis identified more than 200 Rbm24-regulated alternative splicing events (AS) which occurred in genes essential for the ESC pluripotency or differentiation. Remarkably, AS genes regulated by Rbm24 composed of transcriptional factors, cytoskeleton proteins, and ATPase gene family members which are critical components required for cardiac development and functionality. Furthermore, we show that Rbm24 regulates ESC differentiation by promoting alternative splicing of pluripotency genes. Among the Rbm24-regulated events, Tpm1, an actin filament family gene, was identified to possess ESC/tissue specific isoforms. We demonstrated that these isoforms were functionally distinct and that their exon AS switch was essential for ESC differentiation. Our results suggest that ESC's switching into the differentiation state can be initiated by a tissue-specific splicing regulator, Rbm24. This finding offers a global view on how an RNA binding protein influences ESC lineage differentiation by a splicing-mediated regulatory mechanism. Stem Cells 2016;34:1776-1789.

Non-neural and cardiac differentiating properties of Tbx6-expressing mouse embryonic stem cells

T-box transcription factors play important roles in vertebrate mesoderm formation. Eomesodermin is involved in the initial step of the prospective mesodermal cells recruited near the primitive streak. Then T or Brachyury gene is responsible for general and axial mesodermal development. Tbx6, on the other hand, promotes paraxial mesodermal development while suppressing neural differentiation. Here, we studied differentiative properties of mouse ES cells (mESCs) with its Tbx6 expression regulated under the Tet-off system. mESCs were treated with noggin to promote neural differentiation. When Tbx6 was simultaneously turned on, later neural differentiation of these cells hardly occurred. Next, mESCs were subjected to formation of the embryoid bodies (EBs). When Tbx6 was turned on during EB formation, the rate of later cardiac troponin T (cTnT)-positive cells increased. If the cells were further treated with a wnt inhibitor KY02111 after EB formation, a synergistic increase of cTnT-positive cells occurred. Tbx6 expression in mESCs influenced the constituent ratio of the cardiac myosin light chain types, such that atrial species markedly increased over ventricular ones. These results are coincident with the function of Tbx6 in normal development, in that Tbx6 strongly suppressed neural differentiation while promoting cardiac development in a cooperative manner with wnt inhibition.

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Tbx6 expression in mouse ES cells (mESCs) inhibited neural differentiation.

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Tbx6 expression in mESCs increased cardiac muscle synergistically with wnt inhibitor.

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Tbx6 expression increased atrial myosin light chains over ventricular chains.

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.

Apelin: an endogenous peptide essential for cardiomyogenic differentiation of mesenchymal stem cells via activating extracellular signal-regulated kinase 1/2 and 5

Growing evidence has shown that apelin/APJ system functions as a critical mediator of cardiac development as well as cardiovascular function. Here, we investigated the role of apelin in the cardiomyogenic differentiation of mesenchymal stem cells derived from Wharton's jelly of human umbilical cord in vitro. In this research, we used RNA interference methodology and gene transfection technique to regulate the expression of apelin in Wharton's jelly-derived mesenchymal stem cells and induced cells with a effective cardiac differentiation protocol including 5-azacytidine and bFGF. Four weeks after induction, induced cells assumed a stick-like morphology and myotube-like structures except apelin-silenced cells and the control group. The silencing expression of apelin in Wharton's jelly-derived mesenchymal stem cells decreased the expression of several critical cardiac progenitor transcription factors (Mesp1, Mef2c, NKX2.5) and cardiac phenotypes (cardiac α-actin, β-MHC, cTnT, and connexin-43). Meanwhile, endogenous compensation of apelin contributed to differentiating into cells with characteristics of cardiomyocytes in vitro. Further experiment showed that exogenous apelin peptide rescued the cardiomyogenic differentiation of apelin-silenced mesenchymal stem cells in the early stage (1-4 days) of induction. Remarkably, our experiment indicated that apelin up-regulated cardiac specific genes in Wharton's jelly-derived mesenchymal stem cells via activating extracellular signal-regulated kinase (ERK) 1/2 and 5.

Desmin enters the nucleus of cardiac stem cells and modulates Nkx2.5 expression by participating in transcription factor complexes that interact with the nkx2.5 gene

The transcription factor Nkx2.5 and the intermediate filament protein desmin are simultaneously expressed in cardiac progenitor cells during commitment of primitive mesoderm to the cardiomyogenic lineage. Up-regulation of Nkx2.5 expression by desmin suggests that desmin may contribute to cardiogenic commitment and myocardial differentiation by directly influencing the transcription of the nkx2.5 gene in cardiac progenitor cells. Here, we demonstrate that desmin activates transcription of nkx2.5 reporter genes, rescues nkx2.5 haploinsufficiency in cardiac progenitor cells, and is responsible for the proper expression of Nkx2.5 in adult cardiac side population stem cells. These effects are consistent with the temporary presence of desmin in the nuclei of differentiating cardiac progenitor cells and its physical interaction with transcription factor complexes bound to the enhancer and promoter elements of the nkx2.5 gene. These findings introduce desmin as a newly discovered and unexpected player in the regulatory network guiding cardiomyogenesis in cardiac stem cells.

MESP1 Mutations in Patients with Congenital Heart Defects

Identifying the genetic etiology of congenital heart disease (CHD) has been challenging despite being one of the most common congenital malformations in humans. We previously identified a microdeletion in a patient with a ventricular septal defect containing over 40 genes including MESP1 (mesoderm posterior basic helix-loop-helix transcription factor 1). Because of the importance of MESP1 as an early regulator of cardiac development in both in vivo and in vitro studies, we tested for MESP1 mutations in 647 patients with congenital conotruncal and related heart defects. We identified six rare, nonsynonymous variants not seen in ethnically matched controls and one likely race-specific nonsynonymous variant. Functional analyses revealed that three of these variants altered activation of transcription by MESP1. Two of the deleterious variants are located within the conserved HLH domain and thus impair the protein-protein interaction of MESP1 and E47. The third deleterious variant was a loss-of-function frameshift mutation. Our results suggest that pathologic variants in MESP1 may contribute to the development of CHD and that additional protein partners and downstream targets could likewise contribute to the wide range of causes for CHD.

Ionizing Radiation Impacts on Cardiac Differentiation of Mouse Embryonic Stem Cells

Little is known about the effects of ionizing radiation on the earliest stages of embryonic development although it is well recognized that ionizing radiation is a natural part of our environment and further exposure may occur due to medical applications. The current study addresses this issue using D3 mouse embryonic stem cells as a model system. Cells were irradiated with either X-rays or carbon ions representing sparsely and densely ionizing radiation and their effect on the differentiation of D3 cells into spontaneously contracting cardiomyocytes through embryoid body (EB) formation was measured. This study is the first to demonstrate that ionizing radiation impairs the formation of beating cardiomyocytes with carbon ions being more detrimental than X-rays. However, after prolonged culture time, the number of beating EBs derived from carbon ion irradiated cells almost reached control levels indicating that the surviving cells are still capable of developing along the cardiac lineage although with considerable delay. Reduced EB size, failure to downregulate pluripotency markers, and impaired expression of cardiac markers were identified as the cause of compromised cardiomyocyte formation. Dysregulation of cardiac differentiation was accompanied by alterations in the expression of endodermal and ectodermal markers that were more severe after carbon ion irradiation than after exposure to X-rays. In conclusion, our data show that carbon ion irradiation profoundly affects differentiation and thus may pose a higher risk to the early embryo than X-rays.

The ETS Factor, ETV2: a Master Regulator for Vascular Endothelial Cell Development

Appropriate vessel development and its coordinated function is essential for proper embryogenesis and homeostasis in the adult. Defects in vessels cause birth defects and are an important etiology of diseases such as cardiovascular disease, tumor and diabetes retinopathy. The accumulative data indicate that ETV2, an ETS transcription factor, performs a potent and indispensable function in mediating vessel development. This review discusses the recent progress of the study of ETV2 with special focus on its regulatory mechanisms and cell fate determining role in developing mouse embryos as well as somatic cells.

Long Non-Coding RNAs as Master Regulators in Cardiovascular Diseases

Cardiovascular disease is the leading cause of death in the United States, accounting for nearly one in every seven deaths. Over the last decade, various targeted therapeutics have been introduced, but there has been no corresponding improvement in patient survival. Since the mortality rate of cardiovascular disease has not been significantly decreased, efforts have been made to understand the link between heart disease and novel therapeutic targets such as non-coding RNAs. Among multiple non-coding RNAs, long non-coding RNA (lncRNA) has emerged as a novel therapeutic in cardiovascular medicine. LncRNAs are endogenous RNAs that contain over 200 nucleotides and regulate gene expression. Recent studies suggest critical roles of lncRNAs in modulating the initiation and progression of cardiovascular diseases. For example, aberrant lncRNA expression has been associated with the pathogenesis of ischemic heart failure. In this article, we present a synopsis of recent discoveries that link the roles and molecular interactions of lncRNAs to cardiovascular diseases. Moreover, we describe the prevalence of circulating lncRNAs and assess their potential utilities as biomarkers for diagnosis and prognosis of heart disease.

FOXF1 inhibits hematopoietic lineage commitment during early mesoderm specification

The molecular mechanisms orchestrating early mesoderm specification are still poorly understood. In particular, how alternate cell fate decisions are regulated in nascent mesoderm remains mostly unknown. In the present study, we investigated both in vitro in differentiating embryonic stem cells, and in vivo in gastrulating embryos, the lineage specification of early mesodermal precursors expressing or not the Forkhead transcription factor FOXF1. Our data revealed that FOXF1-expressing mesoderm is derived from FLK1(+) progenitors and that in vitro this transcription factor is expressed in smooth muscle and transiently in endothelial lineages, but not in hematopoietic cells. In gastrulating embryos, FOXF1 marks most extra-embryonic mesoderm derivatives including the chorion, the allantois, the amnion and a subset of endothelial cells. Similarly to the in vitro situation, FOXF1 expression is excluded from the blood islands and blood cells. Further analysis revealed an inverse correlation between hematopoietic potential and FOXF1 expression in vivo with increased commitment toward primitive erythropoiesis in Foxf1-deficient embryos, whereas FOXF1-enforced expression in vitro was shown to repress hematopoiesis. Altogether, our data establish that during gastrulation, FOXF1 marks all posterior primitive streak extra-embryonic mesoderm derivatives with the remarkable exception of the blood lineage. Our study further suggests that this transcription factor is implicated in actively restraining the specification of mesodermal progenitors to hematopoiesis.

Porous nanofibrous poly(L-lactic acid) scaffolds supporting cardiovascular progenitor cells for cardiac tissue engineering

Myocardial infarction (MI) is the irreversible necrosis of heart with approximately 1.5 million cases every year in the United States. Tissue engineering offers a promising strategy for cardiac repair after MI. However, the optimal cell source for heart tissue regeneration and the ideal scaffolds to support cell survival, differentiation, and integration, remain to be developed. To address these issues, we developed the technology to induce cardiovascular progenitor cells (CPCs) derived from mouse embryonic stem cells (ESCs) towards desired cardiomyocytes as well as smooth muscle cells and endothelial cells. We fabricated extracellular matrix (ECM)-mimicking nanofibrous poly(l-lactic acid) (PLLA) scaffolds with porous structure of high interconnection for cardiac tissue formation. The CPCs were seeded into the scaffolds to engineer cardiac constructs in vitro. Fluorescence staining and RT-PCR assay showed that the scaffolds facilitated cell attachment, extension, and differentiation. Subcutaneous implantation of the cell/scaffold constructs in a nude mouse model showed that the scaffolds favorably supported survival of the grafted cells and their commitment to the three desired lineages in vivo. Thus, our study suggested that the porous nanofibrous PLLA scaffolds support cardiac tissue formation from CPCs. The integration of CPCs with the nanofibrous PLLA scaffolds represents a promising tissue engineering strategy for cardiac repair.

STATEMENT OF SIGNIFICANCE

Myocardial infarction is the irreversible necrosis of heart with approximately 1.5 million cases every year in the United States. Tissue engineering offers a promising strategy for cardiac repair after MI. However, the optimal cell source for heart tissue regeneration and the ideal scaffolds to support cell survival, differentiation, and integration, remain to be developed. To address these issues, we developed porous nanofibrous PLLA scaffolds that mimic natural extracellular matrix to support cardiac tissue formation from CPCs. The integration of CPCs with the nanofibrous PLLA scaffolds represents a promising tissue engineering strategy for cardiac repair.

Lncing epigenetic control of transcription to cardiovascular development and disease

Transcriptional and epigenetic regulation is critical for proper heart development, cardiac homeostasis, and pathogenesis. Long noncoding RNAs have emerged as key components of the transcriptional regulatory pathways that govern cardiac development as well as stress response, signaling, and remodeling in cardiac pathologies. Within the past few years, studies have identified many long noncoding RNAs in the context of cardiovascular biology and have begun to reveal the key functions of these transcripts. In this review, we discuss the growing roles of long noncoding RNAs in different aspects of cardiovascular development as well as pathological responses during injury or disease. In addition, we discuss diverse mechanisms by which long noncoding RNAs orchestrate cardiac transcriptional programs. Finally, we explore the exciting potential of this novel class of transcripts as biomarkers and novel therapeutic targets for cardiovascular diseases.

Human fetal cardiac progenitors: The role of stem cells and progenitors in the fetal and adult heart

The human fetal heart is formed early during embryogenesis as a result of cell migrations, differentiation, and formative blood flow. It begins to beat around gestation day 22. Progenitor cells are derived from mesoderm (endocardium and myocardium), proepicardium (epicardium and coronary vessels), and neural crest (heart valves, outflow tract septation, and parasympathetic innervation). A variety of molecular disturbances in the factors regulating the specification and differentiation of these cells can cause congenital heart disease. This review explores the contribution of different cardiac progenitors to the embryonic heart development; the pathways and transcription factors guiding their expansion, migration, and functional differentiation; and the endogenous regenerative capacity of the adult heart including the plasticity of cardiomyocytes. Unfolding these mechanisms will become the basis for understanding the dynamics of specific congenital heart disease as well as a means to develop therapy for fetal as well as postnatal cardiac defects and heart failure.

Genome-Wide Identification of MESP1 Targets Demonstrates Primary Regulation Over Mesendoderm Gene Activity

MESP1 is considered the first sign of the nascent cardiac mesoderm and plays a critical role in the appearance of cardiac progenitors, while exhibiting a transient expression in the developing embryo. We profiled the transcriptome of a pure population of differentiating MESP1-marked cells and found that they chiefly contribute to the mesendoderm lineage. High-throughput sequencing of endogenous MESP1-bound DNA revealed that MESP1 preferentially binds to two variants of E-box sequences and activates critical mesendoderm modulators, including Eomes, Gata4, Wnt5a, Wnt5b, Mixl1, T, Gsc, and Wnt3. These mesendoderm markers were enriched in the MESP1 marked population before the appearance of cardiac progenitors and myocytes. Further, MESP1-binding is globally associated with H(3)K(27) acetylation, supporting a novel pivotal role of it in regulating target gene epigenetics. Therefore, MESP1, the pioneer cardiac factor, primarily directs the appearance of mesendoderm, the intermediary of the earliest progenitors of mesoderm and endoderm organogenesis.

Dysregulation of Mitochondrial Functions and Osteogenic Differentiation in Cisd2-Deficient Murine Induced Pluripotent Stem Cells

Wolfram syndrome 2 (WFS2) is a premature aging syndrome caused by an irreversible mitochondria-mediated disorder. Cisd2, which regulates mitochondrial electron transport, has been recently identified as the causative gene of WFS2. The mouse Cisd2 knockout (KO) (Cisd2(-/-)) recapitulates most of the clinical manifestations of WFS2, including growth retardation, osteopenia, and lordokyphosis. However, the precise mechanisms underlying osteopenia in WFS2 and Cisd2 KO mice remain unknown. In this study, we collected embryonic fibroblasts from Cisd2-deficient embryos and reprogrammed them into induced pluripotent stem cells (iPSCs) via retroviral transduction with Oct4/Sox2/Klf4/c-Myc. Cisd2-deficient mouse iPSCs (miPSCs) exhibited structural abnormalities in their mitochondria and an impaired proliferative capability. The global gene expression profiles of Cisd2(+/+), Cisd2(+/-), and Cisd2(-/-) miPSCs revealed that Cisd2 functions as a regulator of both mitochondrial electron transport and Wnt/β-catenin signaling, which is critical for cell proliferation and osteogenic differentiation. Notably, Cisd2(-/-) miPSCs exhibited impaired Wnt/β-catenin signaling, with the downregulation of downstream genes, such as Tcf1, Fosl1, and Jun and the osteogenic regulator Runx2. Several differentiation markers for tridermal lineages were globally impaired in Cisd2(-/-) miPSCs. Alizarin red S staining and flow cytometry analysis further revealed that Cisd2(-/-) miPSCs failed to undergo osteogenic differentiation. Taken together, our results, as determined using an miPSC-based platform, have demonstrated that Cisd2 regulates mitochondrial function, proliferation, intracellular Ca(2+) homeostasis, and Wnt pathway signaling. Cisd2 deficiency impairs the activation of Wnt/β-catenin signaling and thereby contributes to the pathogeneses of osteopenia and lordokyphosis in WFS2 patients.

Mixl1 and Flk1 Are Key Players of Wnt/TGF-β Signaling During DMSO-Induced Mesodermal Specification in P19 cells

Dimethyl sulfoxide (DMSO) is widely used to induce multilineage differentiation of embryonic and adult progenitor cells. To date, little is known about the mechanisms underlying DMSO-induced mesodermal specification. In this study, we investigated the signaling pathways and lineage-determining genes involved in DMSO-induced mesodermal specification in P19 cells. Wnt/β-catenin and TGF-β superfamily signaling pathways such as BMP, TGF-β and GDF1 signaling were significantly activated during DMSO-induced mesodermal specification. In contrast, Nodal/Cripto signaling pathway molecules, required for endoderm specification, were severely downregulated. DMSO significantly upregulated the expression of cardiac mesoderm markers but inhibited the expression of endodermal and hematopoietic lineage markers. Among the DMSO-activated cell lineage markers, the expression of Mixl1 and Flk1 was dramatically upregulated at both the transcript and protein levels, and the populations of Mixl1+, Flk1+ and Mixl1+/Flk1+ cells also increased significantly. DMSO modulated cell cycle molecules and induced cell apoptosis, resulting in significant cell death during EB formation of P19 cells. An inhibitor of Flk1, SU5416 significantly blocked expressions of TGF-β superfamily members, mesodermal cell lineage markers and cell cycle molecules but it did not affect Wnt molecules. These results demonstrate that Mixl1 and Flk1 play roles as key downstream or interacting effectors of Wnt/TGF-β signaling pathway during DMSO-induced mesodermal specification in P19 cells.

NKX2-5 mutations causative for congenital heart disease retain functionality and are directed to hundreds of targets

We take a functional genomics approach to congenital heart disease mechanism. We used DamID to establish a robust set of target genes for NKX2-5 wild type and disease associated NKX2-5 mutations to model loss-of-function in gene regulatory networks. NKX2-5 mutants, including those with a crippled homeodomain, bound hundreds of targets including NKX2-5 wild type targets and a unique set of "off-targets", and retained partial functionality. NKXΔHD, which lacks the homeodomain completely, could heterodimerize with NKX2-5 wild type and its cofactors, including E26 transformation-specific (ETS) family members, through a tyrosine-rich homophilic interaction domain (YRD). Off-targets of NKX2-5 mutants, but not those of an NKX2-5 YRD mutant, showed overrepresentation of ETS binding sites and were occupied by ETS proteins, as determined by DamID. Analysis of kernel transcription factor and ETS targets show that ETS proteins are highly embedded within the cardiac gene regulatory network. Our study reveals binding and activities of NKX2-5 mutations on WT target and off-targets, guided by interactions with their normal cardiac and general cofactors, and suggest a novel type of gain-of-function in congenital heart disease.

The roles of Mesp family proteins: functional diversity and redundancy in differentiation of pluripotent stem cells and mammalian mesodermal development

Mesp family proteins comprise two members named mesodermal posterior 1 (Mesp1) and mesodermal posterior 2 (Mesp2). Both Mesp1 and Mesp2 are transcription factors and they share an almost identical basic helix-loop-helix motif. They have been shown to play critical regulating roles in mammalian heart and somite development. Mesp1 sits in the core of the complicated regulatory network for generation of cardiovascular progenitors while Mesp2 is central for somitogenesis. Here we summarize the similarities and differences in their molecular functions during mammalian early mesodermal development and discuss possible future research directions for further study of the functions of Mesp1 and Mesp2. A comprehensive knowledge of molecular functions of Mesp family proteins will eventually help us better understand mammalian heart development and somitogenesis as well as improve the production of specific cell types from pluripotent stem cells for future regenerative therapies.

"The state of the heart": Recent advances in engineering human cardiac tissue from pluripotent stem cells

The pressing need for effective cell therapy for the heart has led to the investigation of suitable cell sources for tissue replacement. In recent years, human pluripotent stem cell research expanded tremendously, in particular since the derivation of human-induced pluripotent stem cells. In parallel, bioengineering technologies have led to novel approaches for in vitro cell culture. The combination of these two fields holds potential for in vitro generation of high-fidelity heart tissue, both for basic research and for therapeutic applications. However, this new multidisciplinary science is still at an early stage. Many questions need to be answered and improvements need to be made before clinical applications become a reality. Here we discuss the current status of human stem cell differentiation into cardiomyocytes and the combined use of bioengineering approaches for cardiac tissue formation and maturation in developmental studies, disease modeling, drug testing, and regenerative medicine.

Transdifferentiation of Fibroblasts by Defined Factors

Cellular differentiation is usually considered to be an irreversible process during development due to robust lineage commitment. Feedback and feed-forward loops play a significant role in maintaining lineage-specific gene expression processes in various cell types, and, in turn, factors secreted by cells may regulate the homeostatic balance of these cycles during development and differentiation. The output of biological responses is controlled by such mechanisms in many regulatory pathways through gene networks involved in transcription, RNA metabolism, signal transduction, micromolecular synthesis, and degradation. The pluripotent stage during cellular conversion may be avoided through ectopic expression of lineage-specific factors. Lineage-specific transcription factors produced during development may strengthen cell type-specific gene expression patterns. Cellular phenotypes are further stabilized by epigenetic modifications. This reprogramming approach could have important implications for disease modeling and regenerative and personalized medicine.

PDGFRα demarcates the cardiogenic clonogenic Sca1+ stem/progenitor cell in adult murine myocardium

Cardiac progenitor/stem cells in adult hearts represent an attractive therapeutic target for heart regeneration, though (inter)-relationships among reported cells remain obscure. Using single-cell qRT-PCR and clonal analyses, here we define four subpopulations of cardiac progenitor/stem cells in adult mouse myocardium all sharing stem cell antigen-1 (Sca1), based on side population (SP) phenotype, PECAM-1 (CD31) and platelet-derived growth factor receptor-α (PDGFRα) expression. SP status predicts clonogenicity and cardiogenic gene expression (Gata4/6, Hand2 and Tbx5/20), properties segregating more specifically to PDGFRα(+) cells. Clonal progeny of single Sca1(+) SP cells show cardiomyocyte, endothelial and smooth muscle lineage potential after cardiac grafting, augmenting cardiac function although durable engraftment is rare. PDGFRα(-) cells are characterized by Kdr/Flk1, Cdh5, CD31 and lack of clonogenicity. PDGFRα(+)/CD31(-) cells derive from cells formerly expressing Mesp1, Nkx2-5, Isl1, Gata5 and Wt1, distinct from PDGFRα(-)/CD31(+) cells (Gata5 low; Flk1 and Tie2 high). Thus, PDGFRα demarcates the clonogenic cardiogenic Sca1(+) stem/progenitor cell.