Sequential development of hematopoietic and cardiac mesoderm during embryonic stem cell differentiation

High glucose impairs osteogenic differentiation of embryonic stem cells via early diversion of beta-catenin from Forkhead box O to T cell factor interaction

BACKGROUND

Diabetes, which is characterized by an increase in blood glucose concentration, is accompanied by low bone turnover, increased fracture risk, and the formation of embryonic skeletal malformations. Yet, there are few studies elucidating the underlying alterations in signaling pathways leading to these osteogenic defects. We hypothesized here that bone formation deficiencies in a high glucose environment result from altered activity of beta-catenin (CTNNB1), a key contributor to osteogenic differentiation, dysregulation of which has also been implicated in the development of diabetes.

METHODS

To test this hypothesis, we used a previously established embryonic stem cell (ESC) model of differentiation that mimics the diabetic environment of the developing embryo. We differentiated murine ESCs within osteogenic-inducing media containing either high (diabetic) or low (physiological) levels of D-glucose and performed time course analyses to study the influence of high glucose on early and late bone cell differentiation.

RESULTS

Endpoint measures for osteogenic differentiation were reduced in a glucose-dependent manner and expression of precursor-specific markers altered at multiple time points. Furthermore, transcriptional activity of the lymphoid enhancer factor (LEF)/T cell factor (TCF) transcription factors during precursor formation stages was significantly elevated while levels of CTNNB1 complexed with Forkhead box O 3a (FOXO3a) declined. Modulation of AKT, a known upstream regulator of both LEF/TCF and FOXO3a, as well as CTNNB1 rescued some of the reductions in osteogenic output seen in the high glucose condition.

CONCLUSIONS

Within our in vitro model, we found a clear involvement of LEF/TCF and FOXO3a signaling pathways in the regulation of osteogenic differentiation, which may account for the skeletal deficiencies found in newborns of diabetic mothers.

Don't Turn Off the Tap! The Importance of Discovery Science to the Australian Cardiovascular Sector and Improving Clinical Outcomes Into the Future

Despite significant advances in interventional and therapeutic approaches, cardiovascular disease (CVD) remains the leading cause of death and mortality. To lower this health burden, cardiovascular discovery scientists need to play an integral part in the solution. Successful clinical translation is achieved when built upon a strong foundational understanding of the disease mechanisms involved. Changes in the Australian funding landscape, to place greater emphasis on translation, however, have increased job insecurity for discovery science researchers and especially early-mid career researchers. To highlight the importance of discovery science in cardiovascular research, this review compiles six science stories in which fundamental discoveries, often involving Australian researchers, has led to or is advancing to clinical translation. These stories demonstrate the importance of the role of discovery scientists and the need for their work to be prioritised now and in the future. Australia needs to keep discovery scientists supported and fully engaged within the broader cardiovascular research ecosystem so they can help realise the next game-changing therapy or diagnostic approach that diminishes the burden of CVD on society.

Gastruloids: Pluripotent stem cell models of mammalian gastrulation and embryo engineering

Gastrulation is a fundamental and critical process of animal development whereby the mass of cells that results from the proliferation of the zygote transforms itself into a recognizable outline of an organism. The last few years have seen the emergence of a number of experimental models of early mammalian embryogenesis based on Embryonic Stem (ES) cells. One of this is the Gastruloid model. Gastruloids are aggregates of defined numbers of ES cells that, under defined culture conditions, undergo controlled proliferation, symmetry breaking, and the specification of all three germ layers characteristic of vertebrate embryos, and their derivatives. However, they lack brain structures and, surprisingly, reveal a disconnect between cell type specific gene expression and tissue morphogenesis, for example during somitogenesis. Gastruloids have been derived from mouse and human ES cells and several variations of the original model have emerged that reveal a hereto unknown modularity of mammalian embryos. We discuss the organization and development of gastruloids in the context of the embryonic stages that they represent, pointing out similarities and differences between the two. We also point out their potential as a reproducible, scalable and searchable experimental system and highlight some questions posed by the current menagerie of gastruloids.

Endocardial-Myocardial Interactions During Early Cardiac Differentiation and Trabeculation

Throughout the continuum of heart formation, myocardial growth and differentiation occurs in concert with the development of a specialized population of endothelial cells lining the cardiac lumen, the endocardium. Once the endocardial cells are specified, they are in close juxtaposition to the cardiomyocytes, which facilitates communication between the two cell types that has been proven to be critical for both early cardiac development and later myocardial function. Endocardial cues orchestrate cardiomyocyte proliferation, survival, and organization. Additionally, the endocardium enables oxygenated blood to reach the cardiomyocytes. Cardiomyocytes, in turn, secrete factors that promote endocardial growth and function. As misregulation of this delicate and complex endocardial-myocardial interplay can result in congenital heart defects, further delineation of underlying genetic and molecular factors involved in cardiac paracrine signaling will be vital in the development of therapies to promote cardiac homeostasis and regeneration. Herein, we highlight the latest research that has advanced the elucidation of endocardial-myocardial interactions in early cardiac morphogenesis, including endocardial and myocardial crosstalk necessary for cellular differentiation and tissue remodeling during trabeculation, as well as signaling critical for endocardial growth during trabeculation.

Dissecting the Complexity of Early Heart Progenitor Cells

Early heart development depends on the coordinated participation of heterogeneous cell sources. As pioneer work from Adriana C. Gittenberger-de Groot demonstrated, characterizing these distinct cell sources helps us to understand congenital heart defects. Despite decades of research on the segregation of lineages that form the primitive heart tube, we are far from understanding its full complexity. Currently, single-cell approaches are providing an unprecedented level of detail on cellular heterogeneity, offering new opportunities to decipher its functional role. In this review, we will focus on three key aspects of early heart morphogenesis: First, the segregation of myocardial and endocardial lineages, which yields an early lineage diversification in cardiac development; second, the signaling cues driving differentiation in these progenitor cells; and third, the transcriptional heterogeneity of cardiomyocyte progenitors of the primitive heart tube. Finally, we discuss how single-cell transcriptomics and epigenomics, together with live imaging and functional analyses, will likely transform the way we delve into the complexity of cardiac development and its links with congenital defects.

Improving Cardiac Reprogramming for Heart Regeneration in Translational Medicine

Direct reprogramming of fibroblasts into CM-like cells has emerged as an attractive strategy to generate induced CMs (iCMs) in heart regeneration. However, low conversion rate, poor purity, and the lack of precise conversion of iCMs are still present as significant challenges. In this review, we summarize the recent development in understanding the molecular mechanisms of cardiac reprogramming with various strategies to achieve more efficient iCMs. reprogramming. Specifically, we focus on the identified critical roles of transcriptional regulation, epigenetic modification, signaling pathways from the cellular microenvironment, and cell cycling regulation in cardiac reprogramming. We also discuss the progress in delivery system optimization and cardiac reprogramming in human cells related to preclinical applications. We anticipate that this will translate cardiac reprogramming-based heart therapy into clinical applications. In addition to optimizing the cardiogenesis related transcriptional regulation and signaling pathways, an important strategy is to modulate the pathological microenvironment associated with heart injury, including inflammation, pro-fibrotic signaling pathways, and the mechanical properties of the damaged myocardium. We are optimistic that cardiac reprogramming will provide a powerful therapy in heart regenerative medicine.

Drawing Inspiration from Developmental Biology for Cardiac Tissue Engineers

A sound understanding of developmental biology is part of the foundation of effective stem cell-derived tissue engineering. Here, the key concepts of cardiac development that are successfully applied in a bioinspired approach to growing engineered cardiac tissues, are reviewed. The native cardiac milieu is studied extensively from embryonic to adult phenotypes, as it provides a resource of factors, mechanisms, and protocols to consider when working toward establishing living tissues in vitro. It begins with the various cell types that constitute the cardiac tissue. It is discussed how myocytes interact with other cell types and their microenvironment and how they change over time from the embryonic to the adult states, with a view on how such changes affect the tissue function and may be used in engineered tissue models. Key embryonic signaling pathways that have been leveraged in the design of culture media and differentiation protocols are presented. The cellular microenvironment, from extracellular matrix chemical and physical properties, to the dynamic mechanical and electrical forces that are exerted on tissues is explored. It is shown that how such microenvironmental factors can inform the design of biomaterials, scaffolds, stimulation bioreactors, and maturation readouts, and suggest considerations for ongoing biomimetic advancement of engineered cardiac tissues and regeneration strategies for the future.

Long Non-coding RNAs and MicroRNAs Interplay in Osteogenic Differentiation of Mesenchymal Stem Cells

Long non-coding RNAs (lncRNAs) have gained great attention as epigenetic regulators of gene expression in many tissues. Increasing evidence indicates that lncRNAs, together with microRNAs (miRNAs), play a pivotal role in osteogenesis. While miRNA action mechanism relies mainly on miRNA-mRNA interaction, resulting in suppressed expression, lncRNAs affect mRNA functionality through different activities, including interaction with miRNAs. Recent advances in RNA sequencing technology have improved knowledge into the molecular pathways regulated by the interaction of lncRNAs and miRNAs. This review reports on the recent knowledge of lncRNAs and miRNAs roles as key regulators of osteogenic differentiation. Specifically, we described herein the recent discoveries on lncRNA-miRNA crosstalk during the osteogenic differentiation of mesenchymal stem cells (MSCs) derived from bone marrow (BM), as well as from different other anatomical regions. The deep understanding of the connection between miRNAs and lncRNAs during the osteogenic differentiation will strongly improve knowledge into the molecular mechanisms of bone growth and development, ultimately leading to discover innovative diagnostic and therapeutic tools for osteogenic disorders and bone diseases.

Long Noncoding RNAs in Human Stemness and Differentiation

There is increasing evidence that long noncoding RNAs (lncRNAs) are among the main regulatory factors of stem cell maintenance and differentiation. They act through various mechanisms and interactions with proteins, DNA, and RNA. This heterogeneity in function increases the capabilities of the lncRNome toolkit but also makes it difficult to predict the function of novel lncRNAs or even rely on biological information produced in animal models. As lncRNAs are species- and tissue-specific, the recent technical advances in self-renewal and differentiation of human embryonic stem cells (ESCs) make these cells the ideal system to identify key regulatory lncRNAs and study their molecular functions. Here we provide an overview of the functional versatility of lncRNA mechanistic heterogeneity in regulating pluripotency maintenance and human differentiation.

Fluorescent PSC-Derived Cardiomyocyte Reporter Lines: Generation Approaches and Their Applications in Cardiovascular Medicine

Recent advances have made pluripotent stem cell (PSC)-derived cardiomyocytes an attractive option to model both normal and diseased cardiac function at the single-cell level. However, in vitro differentiation yields heterogeneous populations of cardiomyocytes and other cell types, potentially confounding phenotypic analyses. Fluorescent PSC-derived cardiomyocyte reporter systems allow specific cell lineages to be labelled, facilitating cell isolation for downstream applications including drug testing, disease modelling and cardiac regeneration. In this review, the different genetic strategies used to generate such reporter lines are presented with an emphasis on their relative technical advantages and disadvantages. Next, we explore how the fluorescent reporter lines have provided insights into cardiac development and cardiomyocyte physiology. Finally, we discuss how exciting new approaches using PSC-derived cardiomyocyte reporter lines are contributing to progress in cardiac cell therapy with respect to both graft adaptation and clinical safety.

Cardiopharyngeal Progenitor Specification: Multiple Roads to the Heart and Head Muscles

During embryonic development, the heart arises from various sources of undifferentiated mesodermal progenitors, with an additional contribution from ectodermal neural crest cells. Mesodermal cardiac progenitors are plastic and multipotent, but are nevertheless specified to a precise heart region and cell type very early during development. Recent findings have defined both this lineage plasticity and early commitment of cardiac progenitors, using a combination of single-cell and population analyses. In this review, we discuss several aspects of cardiac progenitor specification. We discuss their markers, fate potential in vitro and in vivo, early segregation and commitment, and also intrinsic and extrinsic cues regulating lineage restriction from multipotency to a specific cell type of the heart. Finally, we also discuss the subdivisions of the cardiopharyngeal field, and the shared origins of the heart with other mesodermal derivatives, including head and neck muscles.

Induced pluripotent stem cell-derived vascular smooth muscle cells

The reproducible generation of human-induced pluripotent stem cell (hiPSC)-derived vascular smooth muscle cells (vSMCs) in vitro has been critical to overcoming many limitations of animal and primary cell models of vascular biology and disease. Since this initial advance, research in the field has turned toward recapitulating the naturally occurring subtype specificity found in vSMCs throughout the body, and honing functional models of vascular disease. In this review, we summarize vSMC derivation approaches, including current phenotype and developmental origin-specific methods, and applications of vSMCs in functional disease models and engineered tissues. Further, we discuss the challenges of heterogeneity in hiPSC-derived tissues and propose approaches to identify and isolate vSMC subtype populations.

Cardiac Progenitor Cells from Stem Cells: Learning from Genetics and Biomaterials

Cardiac Progenitor Cells (CPCs) show great potential as a cell resource for restoring cardiac function in patients affected by heart disease or heart failure. CPCs are proliferative and committed to cardiac fate, capable of generating cells of all the cardiac lineages. These cells offer a significant shift in paradigm over the use of human induced pluripotent stem cell (iPSC)-derived cardiomyocytes owing to the latter's inability to recapitulate mature features of a native myocardium, limiting their translational applications. The iPSCs and direct reprogramming of somatic cells have been attempted to produce CPCs and, in this process, a variety of chemical and/or genetic factors have been evaluated for their ability to generate, expand, and maintain CPCs in vitro. However, the precise stoichiometry and spatiotemporal activity of these factors and the genetic interplay during embryonic CPC development remain challenging to reproduce in culture, in terms of efficiency, numbers, and translational potential. Recent advances in biomaterials to mimic the native cardiac microenvironment have shown promise to influence CPC regenerative functions, while being capable of integrating with host tissue. This review highlights recent developments and limitations in the generation and use of CPCs from stem cells, and the trends that influence the direction of research to promote better application of CPCs.

GATA2 Promotes Hematopoietic Development and Represses Cardiac Differentiation of Human Mesoderm

In vertebrates, GATA2 is a master regulator of hematopoiesis and is expressed throughout embryo development and in adult life. Although the essential role of GATA2 in mouse hematopoiesis is well established, its involvement during early human hematopoietic development is not clear. By combining time-controlled overexpression of GATA2 with genetic knockout experiments, we found that GATA2, at the mesoderm specification stage, promotes the generation of hemogenic endothelial progenitors and their further differentiation to hematopoietic progenitor cells, and negatively regulates cardiac differentiation. Surprisingly, genome-wide transcriptional and chromatin immunoprecipitation analysis showed that GATA2 bound to regulatory regions, and repressed the expression of cardiac development-related genes. Moreover, genes important for hematopoietic differentiation were upregulated by GATA2 in a mostly indirect manner. Collectively, our data reveal a hitherto unrecognized role of GATA2 as a repressor of cardiac fates, and highlight the importance of coordinating the specification and repression of alternative cell fates.

Exogenous WNT5A and WNT11 proteins rescue CITED2 dysfunction in mouse embryonic stem cells and zebrafish morphants

Mutations and inadequate methylation profiles of CITED2 are associated with human congenital heart disease (CHD). In mouse, Cited2 is necessary for embryogenesis, particularly for heart development, and its depletion in embryonic stem cells (ESC) impairs cardiac differentiation. We have now determined that Cited2 depletion in ESC affects the expression of transcription factors and cardiopoietic genes involved in early mesoderm and cardiac specification. Interestingly, the supplementation of the secretome prepared from ESC overexpressing CITED2, during the onset of differentiation, rescued the cardiogenic defects of Cited2-depleted ESC. In addition, we demonstrate that the proteins WNT5A and WNT11 held the potential for rescue. We also validated the zebrafish as a model to investigate cited2 function during development. Indeed, the microinjection of morpholinos targeting cited2 transcripts caused developmental defects recapitulating those of mice knockout models, including the increased propensity for cardiac defects and severe death rate. Importantly, the co-injection of anti-cited2 morpholinos with either CITED2 or WNT5A and WNT11 recombinant proteins corrected the developmental defects of Cited2-morphants. This study argues that defects caused by the dysfunction of Cited2 at early stages of development, including heart anomalies, may be remediable by supplementation of exogenous molecules, offering the opportunity to develop novel therapeutic strategies aiming to prevent CHD.

Proteomic-based approaches to cardiac development and disease

Congenital malformations, or structural birth defects, are now the leading cause of infant mortality in the United States and Europe (Dolk et al., 2010; Heron et al., 2009). Of the congenital malformations, congenital heart disease (CHD) is the most common (Dolk et al., 2010; Heron et al., 2009). Thus, a molecular understanding of heart development is an essential goal for improving clinical approaches to CHD. However, CHDs are commonly a result of genetic defects that manifest themselves in a spatial and temporal manner during the early stages of embryogenesis, leaving them mostly intractable to mass spectrometry-based analysis. Here, we describe the technologies and advancements in the field of mass spectrometry over the past few years that have begun to provide insights into the molecular and cellular basis of CHD and prospects for these types of approaches in the future.

Etv2 as an essential regulator of mesodermal lineage development

The 'master regulatory factors' that position at the top of the genetic hierarchy of lineage determination have been a focus of intense interest, and have been investigated in various systems. Etv2/Etsrp71/ER71 is such a factor that is both necessary and sufficient for the development of haematopoietic and endothelial lineages. As such, genetic ablation of Etv2 leads to complete loss of blood and vessels, and overexpression can convert non-endothelial cells to the endothelial lineage. Understanding such master regulatory role of a lineage is not only a fundamental quest in developmental biology, but also holds immense possibilities in regenerative medicine. To harness its activity and utility for therapeutic interventions, it is essential to understand the regulatory mechanisms, molecular function, and networks that surround Etv2. In this review, we provide a comprehensive overview of Etv2 biology focused on mouse and human systems.

IP3R-mediated Ca2+ signals govern hematopoietic and cardiac divergence of Flk1+ cells via the calcineurin-NFATc3-Etv2 pathway

Ca2+ signals participate in various cellular processes with spatial and temporal dynamics, among which, inositol 1,4,5-trisphosphate receptors (IP3Rs)-mediated Ca2+ signals are essential for early development. However, the underlying mechanisms of IP3R-regulated cell fate decision remain largely unknown. Here we report that IP3Rs are required for the hematopoietic and cardiac fate divergence of mouse embryonic stem cells (mESCs). Deletion of IP3Rs (IP3R-tKO) reduced Flk1+/PDGFRα- hematopoietic mesoderm, c-Kit+/CD41+ hematopoietic progenitor cell population, and the colony-forming unit activity, but increased cardiac progenitor markers as well as cardiomyocytes. Concomitantly, the expression of a key regulator of hematopoiesis, Etv2, was reduced in IP3R-tKO cells, which could be rescued by the activation of Ca2+ signals and calcineurin or overexpression of constitutively active form of NFATc3. Furthermore, IP3R-tKO impaired specific targeting of Etv2 by NFATc3 via its evolutionarily conserved cis-element in differentiating ESCs. Importantly, the activation of Ca2+-calcineurin-NFAT pathway reversed the phenotype of IP3R-tKO cells. These findings reveal an unrecognized governing role of IP3Rs in hematopoietic and cardiac fate commitment via IP3Rs-Ca2+-calcineurin-NFATc3-Etv2 pathway.

Applications of genetically engineered human pluripotent stem cell reporters in cardiac stem cell biology

The advent of human pluripotent stem cells (hPSCs) has benefited many fields, from regenerative medicine to disease modeling, with an especially profound effect in cardiac research. Coupled with other novel technologies in genome engineering, hPSCs offer a great opportunity to delineate human cardiac lineages, investigate inherited cardiovascular diseases, and assess the safety and efficacy of cell-based therapies. In this review, we provide an overview of methods for generating genetically engineered hPSC reporters and a succinct synopsis of a variety of hPSC reporters, with a particular focus on their applications in cardiac stem cell biology.

Mesoderm Lineage 3D Tissue Constructs Are Produced at Large-Scale in a 3D Stem Cell Bioprocess

Various studies have presented different approaches to direct pluripotent stem cell differentiation such as applying defined sets of exogenous biochemical signals and genetic/epigenetic modifications. Although differentiation to target lineages can be successfully regulated, such conventional methods are often complicated, laborious, and not cost-effective to be employed to the large-scale production of 3D stem cell-based tissue constructs. A 3D-culture platform that could realize the large-scale production of mesoderm lineage tissue constructs from embryonic stem cells (ESCs) is developed. ESCs are cultured using our previously established 3D-bioprocess platform which is amenable to mass-production of 3D ESC-based tissue constructs. Hepatocarcinoma cell line conditioned medium is introduced to the large-scale 3D culture to provide a specific biomolecular microenvironment to mimic in vivo mesoderm formation process. After 5 days of spontaneous differentiation period, the resulting 3D tissue constructs are composed of multipotent mesodermal progenitor cells verified by gene and molecular expression profiles. Subsequently the optimal time points to trigger terminal differentiation towards cardiomyogenesis or osteogenesis from the mesodermal tissue constructs is found. A simple and affordable 3D ESC-bioprocess that can reach the scalable production of mesoderm origin tissues with significantly improved correspondent tissue properties is demonstrated.

Earlier and broader roles of Mesp1 in cardiovascular development

Mesoderm posterior 1 is one of earliest markers of the nascent mesoderm. Its best-known function is driving the onset of the cardiovascular system. In the past decade, new evidence supports that Mesp1 acts earlier with greater breadth in cell fate decisions, and through cell-autonomous and cell non-autonomous mechanisms. This review summarizes these new aspects, with an emphasis on the upstream and downstream regulation around Mesp1 and how they may guide cell fate reprogramming.

Identification of cardiac hemo-vascular precursors and their requirement of sphingosine-1-phosphate receptor 1 for heart development

The cardiac endothelium plays a crucial role in the development of a functional heart. However, the precise identification of the endocardial precursors and the mechanisms they require for their role in heart morphogenesis are not well understood. Using in vivo and in vitro cell fate tracing concomitant with specific cell ablation and embryonic heart transplantation studies, we identified a unique set of precursors which possess hemogenic functions and express the stem cell leukemia (SCL) gene driven by its 5' enhancer. These hemo-vascular precursors give rise to the endocardium, atrioventricular cushions and coronary vascular endothelium. Furthermore, deletion of the sphingosine-1-phosphate receptor 1 (S1P1) in these precursors leads to ventricular non-compaction cardiomyopathy, a poorly understood condition leading to heart failure and early mortality. Thus, we identified a distinctive population of hemo-vascular precursors which require S1P1 to exert their functions and are essential for cardiac morphogenesis.

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.

Real-time and non-invasive monitoring of embryonic stem cell survival during the development of embryoid bodies with smart nanosensor

Embryonic stem cells (ESCs)-derived embryoid body (EB) is a powerful model for the study of early embryonic development and the discovery of therapeutics for tissue regeneration. This article reports a smart nanosensor platform for labeling and tracking the survival and distribution of ESCs during the EB development in a real-time and non-invasive way. Compared with the cell tracker (i.e. DiO) and the green fluorescent protein (GFP), nanosensors provide the homogenous and highly-efficient ESC labeling. Following the internalization, intracellular nanosensors gradually release the non-fluorescent molecules that become fluorescent only in viable cells. This allows a continuous monitoring of ESC survival and distribution during the process of EB formation. Finally, we confirm that nanosensor labeling does not cause the significant influences to biological properties of the ESCs and EBs.

STATEMENT OF SIGNIFICANCE

The distribution pattern of viable embryonic stem cells (ESCs) within embryoid body (EB) is closely related with the maturation of EBs. Noninvasive and real-time monitoring of viable ESC distribution in EBs would allow researchers to optimize the culturing condition in time during the EB development and to select the suitable EBs for subsequent applications.

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

Hematopoietic (stem) cell development - how divergent are the roads taken?

The development of the hematopoietic system during early embryonic stages occurs in spatially and temporally distinct waves. Hematopoietic stem cells (HSC), the most potent and self‐renewing cells of this system, are produced in the final ‘definitive’ wave of hematopoietic cell generation. In contrast to HSCs in the adult, which differentiate via intermediate progenitor populations to produce functional blood cells, the generation of hematopoietic cells in the embryo prior to HSC generation occurs in the early waves by producing blood cells without intermediate progenitors (such as the ‘primitive’ hematopoietic cells). The lineage relationship between the early hematopoietic cells and the cells giving rise to HSCs, the genetic networks controlling their emergence, and the precise temporal determination of HSC fate remain topics of intense research and debate. This Review article discusses the current knowledge on the step‐wise embryonic establishment of the adult hematopoietic system, examines the roles of pivotal intrinsic regulators in this process, and raises questions concerning the temporal onset of HSC fate determination.

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.

A Simple Protocol for the Myocardial Differentiation of Human iPS Cells

We have developed a simple protocol for inducing the myocardial differentiation of human induced pluripotent stem (iPS) cells. Human iPS cell-derived embryonic bodies (EBs) were treated with a combination of activin-A, bone morphogenetic protein-4 and wnt-3a for one day in serum-free suspension culture, and were subsequently treated with noggin for three days. Thereafter, the EBs were subjected to adherent culture in media with 5% serum. All EBs were differentiated into spontaneously beating EBs, which were identified by the presence of striated muscles in transmission electron microscopy and the expression of the specific cardiomyocyte markers, NKX2-5 and TNNT2. The beating rate of the beating EBs was decreased by treatment with a rapidly activating delayed rectifier potassium current (Ikr) channel blocker, E-4031, an Ikr trafficking inhibitor, pentamidin, and a slowly activating delayed rectifier potassium current (Iks) channel blocker, chromanol 293B, and was increased by treatment with a beta-receptor agonist, isoproterenol. At a low concentration, verapamil, a calcium channel blocker, increased the beating rate of the beating EBs, while a high concentration decreased this rate. These findings suggest that the spontaneously beating EBs were myocardial cell clusters. This simple protocol for myocardial differentiation would be useful in providing a sufficient number of the beating myocardial cell clusters for studies requiring human myocardium.

Organoids and the genetically encoded self-assembly of embryonic stem cells

Understanding the mechanisms of early embryonic patterning and the timely allocation of specific cells to embryonic regions and fates as well as their development into tissues and organs, is a fundamental problem in Developmental Biology. The classical explanation for this process had been built around the notion of positional information. Accordingly the programmed appearance of sources of Morphogens at localized positions within a field of cells directs their differentiation. Recently, the development of organs and tissues from unpatterned and initially identical stem cells (adult and embryonic) has challenged the need for positional information and even the integrity of the embryo, for pattern formation. Here we review the emerging area of organoid biology from the perspective of Developmental Biology. We argue that the events underlying the development of these systems are not purely linked to self-organization, as often suggested, but rather to a process of genetically encoded self-assembly where genetic programs encode and control the emergence of biological structures.

The effects of gelatin–dopamine coating on polydimethylsiloxane substrates on pluripotency maintenance and myocardial differentiation of cultured mouse embryonic stem cells

Gelatin–dopamine coating to improve ESC culture and myocardial differentiation on PDMS substrates.

In Vitro Differentiation of Embryonic Stem Cells into Hematopoietic Lineage: Towards Erythroid Progenitor's Production

Embryonic stem cells (ESCs) differentiation via embryoid body (EB) formation is an established method that generates the three germ layers. However, EB differentiation poses several problems including formation of heterogeneous cell populations. Herein, we described a differentiation protocol on enhancing mesoderm derivation from murine ESCs (mESCs) using conditioned medium (CM) from HepG2 cells. We used this technique to direct hematopoiesis by generating "embryoid-like" colonies (ELCs) from murine (m) ESCs without following standard formation of EBs. Our CM-mESCs group yielded an almost fivefold increase in ELC formation (p ≤ 0.05) and higher expression of mesoderm genes;-Brachyury-T, Goosecoid, and Flk-1 compared with control mESCs group. Hematopoietic colony formation from CM-mESCs was also enhanced by twofold at days 7 and 14 with earlier colony commitment compared to control mESCs (p ≤ 0.05). This early clonogenic capacity was confirmed morphologically by the presence of nucleated erythrocytes and macrophages as early as day 7 in culture using standard 14-day colony-forming assay. Early expression of hematopoietic primitive (ζ-globin) and definitive (β-globin) erythroid genes and proteins was also observed by day 7 in the CM-treated culture. These data indicate that hematopoietic cells more quickly differentiate from CM-treated, compared with those using standard EB approaches, and provide an efficient bioprocess platform for erythroid-specific differentiation of ESCs.

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.

Derivation of cardiac progenitor cells from embryonic stem cells

Cardiac progenitor cells (CPCs) have the capacity to differentiate into cardiomyocytes, smooth muscle cells (SMC), and endothelial cells and hold great promise in cell therapy against heart disease. Among various methods to isolate CPCs, differentiation of embryonic stem cell (ESC) into CPCs attracts great attention in the field since ESCs can provide unlimited cell source. As a result, numerous strategies have been developed to derive CPCs from ESCs. In this protocol, differentiation and purification of embryonic CPCs from both mouse and human ESCs is described. Due to the difficulty of using cell surface markers to isolate embryonic CPCs, ESCs are engineered with fluorescent reporters activated by CPC-specific cre recombinase expression. Thus, CPCs can be enriched by fluorescence-activated cell sorting (FACS). This protocol illustrates procedures to form embryoid bodies (EBs) from ESCs for CPC specification and enrichment. The isolated CPCs can be subsequently cultured for cardiac lineage differentiation and other biological assays. This protocol is optimized for robust and efficient derivation of CPCs from both mouse and human ESCs.

Symmetry breaking, germ layer specification and axial organisation in aggregates of mouse embryonic stem cells

Mouse embryonic stem cells (mESCs) are clonal populations derived from preimplantation mouse embryos that can be propagated in vitro and, when placed into blastocysts, contribute to all tissues of the embryo and integrate into the normal morphogenetic processes, i.e. they are pluripotent. However, although they can be steered to differentiate in vitro into all cell types of the organism, they cannot organise themselves into structures that resemble embryos. When aggregated into embryoid bodies they develop disorganised masses of different cell types with little spatial coherence. An exception to this rule is the emergence of retinas and anterior cortex-like structures under minimal culture conditions. These structures emerge from the cultures without any axial organisation. Here, we report that small aggregates of mESCs, of about 300 cells, self-organise into polarised structures that exhibit collective behaviours reminiscent of those that cells exhibit in early mouse embryos, including symmetry breaking, axial organisation, germ layer specification and cell behaviour, as well as axis elongation. The responses are signal specific and uncouple processes that in the embryo are tightly associated, such as specification of the anteroposterior axis and anterior neural development, or endoderm specification and axial elongation. We discuss the meaning and implications of these observations and the potential uses of these structures which, because of their behaviour, we suggest to call ‘gastruloids’.

Metabolic suppression during mesodermal differentiation of embryonic stem cells identified by single-cell comprehensive gene expression analysis

Metabolic suppression has been revealed during mesodermal differentiation by using single-cell gene expression analysis.

How to make a cardiomyocyte

During development, cardiogenesis is orchestrated by a family of heart progenitors that build distinct regions of the heart. Each region contains diverse cell types that assemble to form the complex structures of the individual cardiac compartments. Cardiomyocytes are the main cell type found in the heart and ensure contraction of the chambers and efficient blood flow throughout the body. Injury to the cardiac muscle often leads to heart failure due to the loss of a large number of cardiomyocytes and its limited intrinsic capacity to regenerate the damaged tissue, making it one of the leading causes of morbidity and mortality worldwide. In this Primer we discuss how insights into the molecular and cellular framework underlying cardiac development can be used to guide the in vitro specification of cardiomyocytes, whether by directed differentiation of pluripotent stem cells or via direct lineage conversion. Additional strategies to generate cardiomyocytes in situ, such as reactivation of endogenous cardiac progenitors and induction of cardiomyocyte proliferation, will also be discussed.

Expression of podocalyxin separates the hematopoietic and vascular potentials of mouse embryonic stem cell-derived mesoderm

In the mouse embryo and differentiating embryonic stem cells, the hematopoietic, endothelial, and cardiomyocyte lineages are derived from Flk1+ mesodermal progenitors. Here, we report that surface expression of Podocalyxin (Podxl), a member of the CD34 family of sialomucins, can be used to subdivide the Flk1+ cells in differentiating embryoid bodies at day 4.75 into populations that develop into distinct mesodermal lineages. Definitive hematopoietic potential was restricted to the Flk1+Podxl+ population, while the Flk1-negative Podxl+ population displayed only primitive erythroid potential. The Flk1+Podxl-negative population contained endothelial cells and cardiomyocyte potential. Podxl expression distinguishes Flk1+ mesoderm populations in mouse embryos at days 7.5, 8.5, and 9.5 and is a marker of progenitor stage primitive erythroblasts. These findings identify Podxl as a useful tool for separating distinct mesodermal lineages.

Induced pluripotent stem cells for post-myocardial infarction repair: remarkable opportunities and challenges

Coronary artery disease with associated myocardial infarction continues to be a major cause of death and morbidity around the world, despite significant advances in therapy. Patients who have large myocardial infarctions are at highest risk for progressive heart failure and death, and cell-based therapies offer new hope for these patients. A recently discovered cell source for cardiac repair has emerged as a result of a breakthrough reprogramming somatic cells to induced pluripotent stem cells (iPSCs). The iPSCs can proliferate indefinitely in culture and can differentiate into cardiac lineages, including cardiomyocytes, smooth muscle cells, endothelial cells, and cardiac progenitors. Thus, large quantities of desired cell products can be generated without being limited by cellular senescence. The iPSCs can be obtained from patients to allow autologous therapy or, alternatively, banks of human leukocyte antigen diverse iPSCs are possible for allogeneic therapy. Preclinical animal studies using a variety of cell preparations generated from iPSCs have shown evidence of cardiac repair. Methodology for the production of clinical grade products from human iPSCs is in place. Ongoing studies for the safety of various iPSC preparations with regard to the risk of tumor formation, immune rejection, induction of arrhythmias, and formation of stable cardiac grafts are needed as the field advances toward the first-in-man trials of iPSCs after myocardial infarction.

Early exposure of murine embryonic stem cells to hematopoietic cytokines differentially directs definitive erythropoiesis and cardiomyogenesis in alginate hydrogel three-dimensional cultures

HepG2-conditioned medium (CM) facilitates early differentiation of murine embryonic stem cells (mESCs) into hematopoietic cells in two-dimensional cultures through formation of embryoid-like colonies (ELCs), bypassing embryoid body (EB) formation. We now demonstrate that three-dimensional (3D) cultures of alginate-encapsulated mESCs cultured in a rotating wall vessel bioreactor can be differentially driven toward definitive erythropoiesis and cardiomyogenesis in the absence of ELC formation. Three groups were evaluated: mESCs in maintenance medium with leukemia inhibitory factor (LIF, control) and mESCs cultured with HepG2 CM (CM1 and CM2). Control and CM1 groups were cultivated for 8 days in early differentiation medium with murine stem cell factor (mSCF) followed by 10 days in hematopoietic differentiation medium (HDM) containing human erythropoietin, m-interleukin (mIL)-3, and mSCF. CM2 cells were cultured for 18 days in HDM, bypassing early differentiation. In CM1, a fivefold expansion of hematopoietic colonies was observed at day 14, with enhancement of erythroid progenitors, hematopoietic genes (Gata-2 and SCL), erythroid genes (EKLF and β-major globin), and proteins (Gata-1 and β-globin), although ζ-globin was not expressed. In contrast, CM2 primarily produced beating colonies in standard hematopoietic colony assay and expressed early cardiomyogenic markers, anti-sarcomeric α-actinin and Gata-4. In conclusion, a scalable, automatable, integrated, 3D bioprocess for the differentiation of mESC toward definitive erythroblasts has been established. Interestingly, cardiomyogenesis was also directed in a specific protocol with HepG2 CM and hematopoietic cytokines making this platform a useful tool for the study of erythroid and cardiomyogenic development.

Microscale technologies for regulating human stem cell differentiation

During development and regeneration, tissues emerge from coordinated sequences of stem cell renewal, specialization, and assembly that are orchestrated by cascades of regulatory factors. This complex in vivo milieu, while necessary to fully recapitulate biology and to properly engineer progenitor cells, is difficult to replicate in vitro. We are just starting to fully realize the importance of the entire context of cell microenvironment-the other cells, three-dimensional matrix, molecular and physical signals. Bioengineered environments that combine tissue-specific transport and signaling are critical to study cellular responses at biologically relevant scales and in settings predictive of human condition. We therefore developed microbioreactors that couple the application of fast dynamic changes in environmental signals with versatile, high-throughput operation and imaging capability. Our base device is a microfluidic platform with an array of microwells containing cells or tissue constructs that are exposed to stable concentration gradients. Mathematical modeling of flow and mass transport can predict the shape of these gradients and the kinetic changes in local concentrations. A single platform, the size of a microscope slide, contains up to 120 biological samples. As an example of application, we describe studies of cell fate specification and mesodermal lineage commitment in human embryonic stem cells and induced pluripotent stem cells. The embryoid bodies formed from these cells were subjected to single and multiple concentration gradients of Wnt3a, Activin A, bone morphogenic protein 4 (BMP4), and their inhibitors, and the gene expression profiles were correlated to the concentration gradients of morphogens to identify the exact conditions for mesodermal differentiation.

Mesp1 patterns mesoderm into cardiac, hematopoietic, or skeletal myogenic progenitors in a context-dependent manner

Mesp1 is regarded as the master regulator of cardiovascular development, initiating the cardiac transcription factor cascade to direct the generation of cardiac mesoderm. To define the early embryonic cell population that responds to Mesp1, we performed pulse inductions of gene expression over tight temporal windows following embryonic stem cell differentiation. Remarkably, instead of promoting cardiac differentiation in the initial wave of mesoderm, Mesp1 binds to the Tal1 (Scl) +40 kb enhancer and generates Flk-1+ precursors expressing Etv2 (ER71) and Tal1 that undergo hematopoietic differentiation. The second wave of mesoderm responds to Mesp1 by differentiating into PDGFRα+ precursors that undergo cardiac differentiation. Furthermore, in the absence of serum-derived factors, Mesp1 promotes skeletal myogenic differentiation. Lineage tracing revealed that the majority of yolk sac and many adult hematopoietic cells derive from Mesp1+ precursors. Thus, Mesp1 is a context-dependent determination factor, integrating the stage of differentiation and the signaling environment to specify different lineage outcomes.

The modern primitives: applying new technological approaches to explore the biology of the earliest red blood cells

One of the most critical stages in mammalian embryogenesis is the independent production of the embryo's own circulating, functional red blood cells. Correspondingly, erythrocytes are the first cell type to become functionally mature during embryogenesis. Failure to achieve this invariably leads to in utero lethality. The recent application of technologies such as transcriptome analysis, flow cytometry, mutant embryo analysis, and transgenic fluorescent gene expression reporter systems has shed new light on the distinct erythroid lineages that arise early in development. Here, I will describe the similarities and differences between the distinct erythroid populations that must form for the embryo to survive. While much of the focus of this review will be the poorly understood primitive erythroid lineage, a discussion of other erythroid and hematopoietic lineages, as well as the cell types making up the different niches that give rise to these lineages, is essential for presenting an appropriate developmental context of these cells.

Embryonic heart progenitors and cardiogenesis

The mammalian heart is a highly specialized organ, comprised of many different cell types arising from distinct embryonic progenitor populations during cardiogenesis. Three precursor populations have been identified to contribute to different myocytic and nonmyocytic cell lineages of the heart: cardiogenic mesoderm cells (CMC), the proepicardium (PE), and cardiac neural crest cells (CNCCs). This review will focus on molecular cues necessary for proper induction, expansion, and lineage-specific differentiation of these progenitor populations during cardiac development in vivo. Moreover, we will briefly discuss how the knowledge gained on embryonic heart progenitor biology can be used to develop novel therapeutic strategies for the management of congenital heart disease as well as for improvement of cardiac function in ischemic heart disease.

Wnt/β-catenin signaling directs the regional expansion of first and second heart field-derived ventricular cardiomyocytes

In mammals, cardiac development proceeds from the formation of the linear heart tube, through complex looping and septation, all the while increasing in mass to provide the oxygen delivery demands of embryonic growth. The developing heart must orchestrate regional differences in cardiomyocyte proliferation to control cardiac morphogenesis. During ventricular wall formation, the compact myocardium proliferates more vigorously than the trabecular myocardium, but the mechanisms controlling such regional differences among cardiomyocyte populations are not understood. Control of definitive cardiomyocyte proliferation is of great importance for application to regenerative cell-based therapies. We have used murine and human pluripotent stem cell systems to demonstrate that, during in vitro cellular differentiation, early ventricular cardiac myocytes display a robust proliferative response to β-catenin-mediated signaling and conversely accelerate differentiation in response to inhibition of this pathway. Using gain- and loss-of-function murine genetic models, we show that β-catenin controls ventricular myocyte proliferation during development and the perinatal period. We further demonstrate that the differential activation of the Wnt/β-catenin signaling pathway accounts for the observed differences in the proliferation rates of the compact versus the trabecular myocardium during normal cardiac development. Collectively, these results provide a mechanistic explanation for the differences in localized proliferation rates of cardiac myocytes and point to a practical method for the generation of the large numbers of stem cell-derived cardiac myocytes necessary for clinical applications.

Generation of tissue constructs for cardiovascular regenerative medicine: from cell procurement to scaffold design

The ability of the human body to naturally recover from coronary heart disease is limited because cardiac cells are terminally differentiated, have low proliferation rates, and low turn-over rates. Cardiovascular tissue engineering offers the potential for production of cardiac tissue ex vivo, but is currently limited by several challenges: (i) Tissue engineering constructs require pure populations of seed cells, (ii) Fabrication of 3-D geometrical structures with features of the same length scales that exist in native tissue is non-trivial, and (iii) Cells require stimulation from the appropriate biological, electrical and mechanical factors. In this review, we summarize the current state of microfluidic techniques for enrichment of subpopulations of cells required for cardiovascular tissue engineering, which offer unique advantages over traditional plating and FACS/MACS-based enrichment. We then summarize modern techniques for producing tissue engineering scaffolds that mimic native cardiac tissue.

Materials science and tissue engineering: repairing the heart

Heart failure after a myocardial infarction continues to be a leading killer in the Western world. Currently, there are no therapies that effectively prevent or reverse the cardiac damage and negative left ventricular remodeling process that follows a myocardial infarction. Because the heart has limited regenerative capacity, there has been considerable effort to develop new therapies that could repair and regenerate the myocardium. Although cell transplantation alone was initially studied, more recently, tissue engineering strategies using biomaterial scaffolds have been explored. In this review, we cover the different approaches to engineering the myocardium, including cardiac patches, which are in vitro-engineered constructs of functional myocardium, and injectable scaffolds, which can either encourage endogenous repair and regeneration or act as vehicles to support the delivery of cells and other therapeutics.

Cardiac stem cell therapy and the promise of heart regeneration

Stem cell therapy for cardiac disease is an exciting but highly controversial research area. Strategies such as cell transplantation and reprogramming have demonstrated both intriguing and sobering results. Yet as clinical trials proceed, our incomplete understanding of stem cell behavior is made evident by numerous unresolved matters, such as the mechanisms of cardiomyocyte turnover or the optimal therapeutic strategies to achieve clinical efficacy. In this Perspective, we consider how cardiac stem cell biology has led us into clinical trials, and we suggest that achieving true cardiac regeneration in patients may ultimately require resolution of critical controversies in experimental cardiac regeneration.