The renewal and differentiation of Isl1+ cardiovascular progenitors are controlled by a Wnt/beta-catenin pathway

Advancements in Induced Pluripotent Stem Cell Technology for Cardiac Regenerative Medicine

Cardiovascular diseases remain the leading causes of morbidity and mortality in the developed world. Cellular-based cardiac regenerative therapy serves as a potential approach to treating cardiovascular diseases. Although various cellular types have been tested, induced pluripotent stem cells (iPSCs) are regarded as a promising cell source for therapy. In this review, we will highlight some of the advances in generating iPSCs and differentiation to cardiac cells. We will also discuss the progress in modeling cardiovascular diseases using iPSCs-derived cardiac cells. As we continue to make progress in iPSC and cardiac differentiation technology, we will come closer to the application of cardiac regenerative medicine.

Small molecules targeting in vivo tissue regeneration

The field of regenerative medicine has boomed in recent years thanks to milestone discoveries in stem cell biology and tissue engineering, which has been driving paradigm shifts in the pharmacotherapy of degenerative and ischemic diseases. Small molecule-mediated replenishment of lost and/or dysfunctional tissue in vivo, however, is still in its infancy due to a limited understanding of mechanisms that control such endogenous processes of tissue homeostasis or regeneration. Here, we discuss current progress using small molecules targeting in vivo aspects of regeneration, including adult stem cells, stem cell niches, and mechanisms of homing, mobilization, and engraftment as well as somatic cell proliferation. Many of these compounds derived from both knowledge-based design and screening campaigns, illustrating the feasibility of translating in vitro discovery to in vivo regeneration. These early examples of drug-mediated in vivo regeneration provide a glimpse of the future directions of in vivo regenerative medicine approaches.

Distinct populations within Isl1 lineages contribute to appendicular and facial skeletogenesis through the β-catenin pathway

Isl1 expression marks progenitor populations in developing embryos. In this study, we investigated the contribution of Isl1-expressing cells that utilize the β-catenin pathway to skeletal development. Inactivation of β-catenin in Isl1-expressing cells caused agenesis of the hindlimb skeleton and absence of the lower jaw (agnathia). In the hindlimb, Isl1-lineages broadly contributed to the mesenchyme; however, deletion of β-catenin in the Isl1-lineage caused cell death only in a discrete posterior domain of nascent hindlimb bud mesenchyme. We found that the loss of posterior mesenchyme, which gives rise to Shh-expressing posterior organizer tissue, caused loss of posterior gene expression and failure to expand chondrogenic precursor cells, leading to severe truncation of the hindlimb. In facial tissues, Isl1-expressing cells broadly contributed to facial epithelium. We found reduced nuclear β-catenin accumulation and loss of Fgf8 expression in mandibular epithelium of Isl1(-/-) embryos. Inactivating β-catenin in Isl1-expressing epithelium caused both loss of epithelial Fgf8 expression and death of mesenchymal cells in the mandibular arch without affecting epithelial proliferation and survival. These results suggest a Isl1→β-catenin→Fgf8 pathway that regulates mesenchymal survival and development of the lower jaw in the mandibular epithelium. By contrast, activating β-catenin signaling in Isl1-lineages caused activation of Fgf8 broadly in facial epithelium. Our results provide evidence that, despite its broad contribution to hindlimb mesenchyme and facial epithelium, the Isl1-β-catenin pathway regulates skeletal development of the hindlimb and lower jaw through discrete populations of cells that give rise to Shh-expressing posterior hindlimb mesenchyme and Fgf8-expressing mandibular epithelium.

Insulin inhibits cardiac mesoderm, not mesendoderm, formation during cardiac differentiation of human pluripotent stem cells and modulation of canonical Wnt signaling can rescue this inhibition

The study of the regulatory signaling hierarchies of human heart development is limited by a lack of model systems that can reproduce the precise developmental events that occur during human embryogenesis. The advent of human pluripotent stem cell (hPSC) technology and robust cardiac differentiation methods affords a unique opportunity to monitor the full course of cardiac induction in vitro. Here, we show that stage-specific activation of insulin signaling strongly inhibited cardiac differentiation during a monolayer-based differentiation protocol that used transforming growth factor β superfamily ligands to generate cardiomyocytes. However, insulin did not repress cardiomyocyte differentiation in a defined protocol that used small molecule regulators of canonical Wnt signaling. By examining the context of insulin inhibition of cardiomyocyte differentiation, we determined that the inhibitory effects by insulin required Wnt/β-catenin signaling and that the cardiomyocyte differentiation defect resulting from insulin exposure was rescued by inhibition of Wnt/β-catenin during the cardiac mesoderm (Nkx2.5+) stage. Thus, insulin and Wnt/β-catenin signaling pathways, as a network, coordinate to influence hPSC differentiation to cardiomyocytes, with the Wnt/β-catenin pathway dominant to the insulin pathway. Our study contributes to the understanding of the regulatory hierarchies of human cardiomyocyte differentiation and has implications for modeling human heart development.

In vitro multilineage differentiation and self-renewal of single pancreatic colony-forming cells from adult C57BL/6 mice

In a previous study we established colony assays suitable for studying murine adult (2-4 months) pancreatic progenitor cells plated in semisolid media containing methylcellulose and extracellular matrix proteins. Using these assays, we found robust in vitro progenitor cell activities (multilineage differentiation and self-renewal) from pancreatic cells of adult mice in the CD-1 outbred background. However, it was not clear whether progenitor cell activities can be detected from inbred mice, a preferred mouse model for various genetic studies. It was also not clear whether a single cell is sufficient to self-renew. Here, we show that fluorescent activated cell sorting pancreatic CD133(+) but not CD133(-) cells from adult C57Bl/6 inbred mice are enriched for progenitor cells that self-renew and give rise to multilineage colonies in vitro. The number of cells in a colony is in proportion to its diameter. Around 60% of single handpicked 3-week-old colonies express trilineage markers, indicating most progenitors are tripotent for ductal, acinar, and endocrine lineage differentiation. Approximately 80% of primary (freshly sorted) colony-forming progenitor cells are capable of giving rise to secondary progenitors in vitro, indicating that a majority of the primary progenitors self-renew. A single cell is sufficient for self-renewal and a Wnt agonist, R-Spondin1, enhances the number of secondary progenitors from the primary progenitors. Together, our pancreatic colony assays allow quantitative analyses of progenitors at a single-cell level from inbred mice. These assays will be useful for elucidating in vitro mechanisms of pancreatic progenitor cell biology.

Tissue engineering approaches to heart repair

The heart is a large organ containing many cell types, each of which is necessary for normal function. Because of this, cardiac regenerative medicine presents many unique challenges. Because each of the many types of cells within the heart has unique physiological and electrophysiological characteristics, donor cells must be well matched to the area of the heart into which they are grafted to avoid mechanical dysfunction or arrhythmia. In addition, grafted cells must be functionally integrated into host tissue to effectively repair cardiac function. Because of its size and physiological function, the metabolic needs of the heart are considerable. Therefore grafts must contain not only cardiomyocytes but also a functional vascular network to meet their needs for oxygen and nutrition. In this article we review progress in the use of pluripotent stem cells as a source of donor cardiomyocytes and highlight current unmet needs in the field. We also examine recent tissue engineering approaches integrating cells with various engineered materials that should address some of these unmet needs.

Expanding mouse ventricular cardiomyocytes through GSK-3 inhibition

Controlled proliferation of cardiomyocytes remains a major limitation in cell biology and one of the main underlying hurdles for true modern regenerative medicine. Here, a technique is described for robust expansion of early fetal-derived mouse ventricular cardiomyocytes on a platform usable for high-throughput molecular screening, tissue engineering and, potentially, in vivo translational experiments. This method provides a small-molecule approach to control proliferation or differentiation of early beating cardiomyocytes through modulation of the Wnt/β-catenin signaling pathway. Moreover, isolation and expansion of fetal cardiomyocytes takes less than 3 weeks, yields a relatively pure (∼70%) functional myogenic population, and is highly reproducible.

The small molecule Wnt signaling modulator ICG-001 improves contractile function in chronically infarcted rat myocardium

The adult mammalian heart has limited capability for self-repair after myocardial infarction. Therefore, therapeutic strategies that improve post-infarct cardiac function are critically needed. The small molecule ICG-001 modulates Wnt signaling and increased the expression of genes beneficial for cardiac regeneration in epicardial cells. Lineage tracing experiments, demonstrated the importance of β-catenin/p300 mediated transcription for epicardial progenitor contribution to the myocardium. Female rats given ICG-001 for 10 days post-occlusion significantly improved ejection fraction by 8.4%, compared to controls (P<0.05). Taken together, Wnt modulation via β-catenin/CBP inhibition offers a promising therapeutic strategy towards restoration of myocardial tissues and an enhancement of cardiac functions following infarction.

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.

Modified mRNA directs the fate of heart progenitor cells and induces vascular regeneration after myocardial infarction

In a cell-free approach to regenerative therapeutics, transient application of paracrine factors in vivo could be used to alter the behavior and fate of progenitor cells to achieve sustained clinical benefits. Here we show that intramyocardial injection of synthetic modified RNA (modRNA) encoding human vascular endothelial growth factor-A (VEGF-A) results in the expansion and directed differentiation of endogenous heart progenitors in a mouse myocardial infarction model. VEGF-A modRNA markedly improved heart function and enhanced long-term survival of recipients. This improvement was in part due to mobilization of epicardial progenitor cells and redirection of their differentiation toward cardiovascular cell types. Direct in vivo comparison with DNA vectors and temporal control with VEGF inhibitors revealed the greatly increased efficacy of pulse-like delivery of VEGF-A. Our results suggest that modRNA is a versatile approach for expressing paracrine factors as cell fate switches to control progenitor cell fate and thereby enhance long-term organ repair.

Transmembrane protein 88: a Wnt regulatory protein that specifies cardiomyocyte development

Genetic regulation of the cell fate transition from lateral plate mesoderm to the specification of cardiomyocytes requires suppression of Wnt/β-catenin signaling, but the mechanism for this is not well understood. By analyzing gene expression and chromatin dynamics during directed differentiation of human embryonic stem cells (hESCs), we identified a suppressor of Wnt/β-catenin signaling, transmembrane protein 88 (TMEM88), as a potential regulator of cardiovascular progenitor cell (CVP) specification. During the transition from mesoderm to the CVP, TMEM88 has a chromatin signature of genes that mediate cell fate decisions, and its expression is highly upregulated in advance of key cardiac transcription factors in vitro and in vivo. In early zebrafish embryos, tmem88a is expressed broadly in the lateral plate mesoderm, including the bilateral heart fields. Short hairpin RNA targeting of TMEM88 during hESC cardiac differentiation increases Wnt/β-catenin signaling, confirming its role as a suppressor of this pathway. TMEM88 knockdown has no effect on NKX2.5 or GATA4 expression, but 80% of genes most highly induced during CVP development have reduced expression, suggesting adoption of a new cell fate. In support of this, analysis of later stage cell differentiation showed that TMEM88 knockdown inhibits cardiomyocyte differentiation and promotes endothelial differentiation. Taken together, TMEM88 is crucial for heart development and acts downstream of GATA factors in the pre-cardiac mesoderm to specify lineage commitment of cardiomyocyte development through inhibition of Wnt/β-catenin signaling.

Fibronectin mediates mesendodermal cell fate decisions

Non-cell-autonomous signals often play crucial roles in cell fate decisions during animal development. Reciprocal signaling between endoderm and mesoderm is vital for embryonic development, yet the key signals and mechanisms remain unclear. Here, we show that endodermal cells efficiently promote the emergence of mesodermal cells in the neighboring population through signals containing an essential short-range component. The endoderm-mesoderm interaction promoted precardiac mesoderm formation in mouse embryonic stem cells and involved endodermal production of fibronectin. In vivo, fibronectin deficiency resulted in a dramatic reduction of mesoderm accompanied by endodermal expansion in zebrafish embryos. This event was mediated by regulation of Wnt signaling in mesodermal cells through activation of integrin-β1. Our findings highlight the importance of the extracellular matrix in mediating short-range signals and reveal a novel function of endoderm, involving fibronectin and its downstream signaling cascades, in promoting the emergence of mesoderm.

Highly efficient induction and long-term maintenance of multipotent cardiovascular progenitors from human pluripotent stem cells under defined conditions

Cardiovascular progenitor cells (CVPCs) derived from human pluripotent stem cells (hPSCs), including human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs), hold great promise for the study of cardiovascular development and cell-based therapy of heart diseases, but their applications are challenged by the difficulties in their efficient generation and stable maintenance. This study aims to develop chemically defined systems for robust generation and stable propagation of hPSC-derived CVPCs by modulating the key early developmental pathways involved in human cardiovascular specification and CVPC self-renewal. Herein we report that a combination of bone morphogenetic protein 4 (BMP4), glycogen synthase kinase 3 (GSK3) inhibitor CHIR99021 and ascorbic acid is sufficient to rapidly convert monolayer-cultured hPSCs, including hESCs and hiPSCs, into homogeneous CVPCs in a chemically defined medium under feeder- and serum-free culture conditions. These CVPCs stably self-renewed under feeder- and serum-free conditions and expanded over 10(7)-fold when the differentiation-inducing signals from BMP, GSK3 and Activin/Nodal pathways were simultaneously eliminated. Furthermore, these CVPCs exhibited expected genome-wide molecular features of CVPCs, retained potentials to generate major cardiovascular lineages including cardiomyocytes, smooth muscle cells and endothelial cells in vitro, and were non-tumorigenic in vivo. Altogether, the established systems reported here permit efficient generation and stable maintenance of hPSC-derived CVPCs, which represent a powerful tool to study early embryonic cardiovascular development and provide a potentially safe source of cells for myocardial regenerative medicine.

Temporal modulation of β-catenin signaling by multicellular aggregation kinetics impacts embryonic stem cell cardiomyogenesis

Pluripotent stem cell differentiation recapitulates aspects of embryonic development, including the regulation of morphogenesis and cell specification via precise spatiotemporal signaling. The assembly and reorganization of cadherins within multicellular aggregates may similarly influence β-catenin signaling dynamics and the associated cardiomyogenic differentiation of pluripotent embryonic stem cells (ESCs). In this study, dynamic changes in β-catenin expression and transcriptional activity were analyzed in response to altered cell adhesion kinetics during embryoid body (EB) formation and differentiation. Modulation of intercellular adhesion kinetics by rotary orbital mixing conditions led to temporal modulation of T-cell factor/lymphoid enhancer-binding factor activity, as well as changes in the spatial localization and phosphorylation state of β-catenin expression. Slower rotary speeds, which promoted accelerated ESC aggregation, resulted in the early accumulation of nuclear dephosphorylated β-catenin, which was followed by a decrease in β-catenin transcriptional activity and an increase in the gene expression of Wnt inhibitors such as Dkk-1. In addition, EBs that exhibited increased β-catenin transcriptional activity at early stages of differentiation subsequently demonstrated increased expression of genes related to cardiomyogenic phenotypes, and inhibition of the Wnt pathway during the initial 4 days of differentiation significantly decreased cardiomyogenic gene expression. Together, the results of this study indicate that the expression and transcriptional activity of β-catenin are temporally regulated by multicellular aggregation kinetics of pluripotent ESCs and influence mesoderm and cardiomyocyte differentiation.

Tcf7l1 proteins cell autonomously restrict cardiomyocyte and promote endothelial specification in zebrafish

Tcf7l1 (formerly Tcf3) proteins are conserved transcription factors whose function as transcriptional repressors is relieved through interactions with β-catenin. Although the functions of Tcf7l1 proteins have been studied in many developmental contexts, whether this conserved mediator of Wnt signaling is required for appropriate cardiomyocyte (CM) development has not been investigated. We find that Tcf7l1 proteins are necessary during two developmental periods to limit CM number in zebrafish embryos: prior to gastrulation and after the initial wave of CM differentiation. In contrast to partially redundant roles in anterior neural patterning, we find that Tcf7l1a and Tcf7l1b have non-redundant functions with respect to restricting CM specification during anterior mesodermal patterning, suggesting that between the two zebrafish Tcf7l1 paralogs there is a limit to the transcriptional repression provided during early CM specification. Using cell transplantation experiments, we determine that the Tcf7l1 paralogs are required cell autonomously to restrict CM specification and promote endothelial cell (EC) specification, which is overtly similar to the ability of Wnt signaling to direct a transformation between these progenitors in embryonic stem cells. Therefore, these results argue that during anterior-posterior patterning of the mesoderm Tcf7l1 proteins are cell autonomously required to limit Wnt signaling, which balances CM and EC progenitor specification within the anterior lateral plate mesoderm. This study expands our understanding of the in vivo developmental requirements of Tcf7l1 proteins and the mechanisms directing CM development in vertebrates.

Autophagy in stem cells

Autophagy is a highly conserved cellular process by which cytoplasmic components are sequestered in autophagosomes and delivered to lysosomes for degradation. As a major intracellular degradation and recycling pathway, autophagy is crucial for maintaining cellular homeostasis as well as remodeling during normal development, and dysfunctions in autophagy have been associated with a variety of pathologies including cancer, inflammatory bowel disease and neurodegenerative disease. Stem cells are unique in their ability to self-renew and differentiate into various cells in the body, which are important in development, tissue renewal and a range of disease processes. Therefore, it is predicted that autophagy would be crucial for the quality control mechanisms and maintenance of cellular homeostasis in various stem cells given their relatively long life in the organisms. In contrast to the extensive body of knowledge available for somatic cells, the role of autophagy in the maintenance and function of stem cells is only beginning to be revealed as a result of recent studies. Here we provide a comprehensive review of the current understanding of the mechanisms and regulation of autophagy in embryonic stem cells, several tissue stem cells (particularly hematopoietic stem cells), as well as a number of cancer stem cells. We discuss how recent studies of different knockout mice models have defined the roles of various autophagy genes and related pathways in the regulation of the maintenance, expansion and differentiation of various stem cells. We also highlight the many unanswered questions that will help to drive further research at the intersection of autophagy and stem cell biology in the near future.

Studying arrhythmogenic right ventricular dysplasia with patient-specific iPSCs

Cellular reprogramming of somatic cells to patient-specific induced pluripotent stem cells (iPSCs) enables in vitro modelling of human genetic disorders for pathogenic investigations and therapeutic screens. However, using iPSC-derived cardiomyocytes (iPSC-CMs) to model an adult-onset heart disease remains challenging owing to the uncertainty regarding the ability of relatively immature iPSC-CMs to fully recapitulate adult disease phenotypes. Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) is an inherited heart disease characterized by pathological fatty infiltration and cardiomyocyte loss predominantly in the right ventricle, which is associated with life-threatening ventricular arrhythmias. Over 50% of affected individuals have desmosome gene mutations, most commonly in PKP2, encoding plakophilin-2 (ref. 9). The median age at presentation of ARVD/C is 26 years. We used previously published methods to generate iPSC lines from fibroblasts of two patients with ARVD/C and PKP2 mutations. Mutant PKP2 iPSC-CMs demonstrate abnormal plakoglobin nuclear translocation and decreased β-catenin activity in cardiogenic conditions; yet, these abnormal features are insufficient to reproduce the pathological phenotypes of ARVD/C in standard cardiogenic conditions. Here we show that induction of adult-like metabolic energetics from an embryonic/glycolytic state and abnormal peroxisome proliferator-activated receptor gamma (PPAR-γ) activation underlie the pathogenesis of ARVD/C. By co-activating normal PPAR-alpha-dependent metabolism and abnormal PPAR-γ pathway in beating embryoid bodies (EBs) with defined media, we established an efficient ARVD/C in vitro model within 2 months. This model manifests exaggerated lipogenesis and apoptosis in mutant PKP2 iPSC-CMs. iPSC-CMs with a homozygous PKP2 mutation also had calcium-handling deficits. Our study is the first to demonstrate that induction of adult-like metabolism has a critical role in establishing an adult-onset disease model using patient-specific iPSCs. Using this model, we revealed crucial pathogenic insights that metabolic derangement in adult-like metabolic milieu underlies ARVD/C pathologies, enabling us to propose novel disease-modifying therapeutic strategies.

High density sphere culture of adult cardiac cells increases the levels of cardiac and progenitor markers and shows signs of vasculogenesis

3D environment and high cell density play an important role in restoring and supporting the phenotypes of cells represented in cardiac tissues. The aim of this study was therefore to investigate the suitability of high density sphere (HDS) cultures for studies of cardiomyocyte-, endothelial-, and stem-cell biology. Primary adult cardiac cells from nine human biopsies were cultured using different media for up to 9 weeks. The possibilities to favor a certain cell phenotype and induce production of extra cellular matrix (ECM) were studied by histology, immunohistochemistry, and quantitative real-time PCR. Defined media gave significant increase in both cardiac- and progenitor-specific markers and also an intraluminal position of endothelial cells over time. Cardiac media showed indication of differentiation and maturity of HDS considering the ECM production and activities within NOTCH regulation but no additional cardiac differentiation. Endothelial media gave no positive effects on endothelial phenotype but increased proliferation without fibroblast overgrowth. In addition, indications for early vasculogenesis were found. It was also possible to affect the Wnt signaling in HDS by addition of a glycogen synthase kinase 3 (GSK3) inhibitor. In conclusion, these findings show the suitability of HDS as in vitro model for studies of cardiomyocyte-, endothelial-, and stem-cell biology.

Islet-1 is a dual regulator of fibrogenic epithelial-to-mesenchymal transition in epicardial mesothelial cells

Recent reports suggest that the adult epicardium is a source of cardiac progenitor cells having the ability to undergo epithelial-to-mesenchymal transition (EMT) and predominantly differentiate into myofibroblasts, thereby contributing to fibrosis of the stressed myocardium. Islet-1 (Isl1) is a widely applied marker of progenitor cells, including the epicardial mesothelial cells (EMCs). However, little is known of the general biological function of Islet-1, let alone its role in EMT of EMCs. Using rat-derived adult EMC cultures we therefore investigated the role of Isl1 expression in both non-stimulated EMCs and during TGF-β-induced EMT. We found that Isl1 had a dual role by promoting mesenchymal features in non-stimulated EMCs, while a loss of Isl1 associated with EMT acted as a negative modulator of EMT progression as assessed on phenotype. We furthermore found that the loss of Isl1 expression during EMT was, in addition to transcriptional regulation by β-catenin, mediated through direct targeting by microRNA-31 (miR-31). Through manipulations of miR-31 bioactivity in EMCs, we thus report that miR-31 is a negative modulator of cardiac fibrogenic EMT, primarily via targeting Isl1. Our data show that Isl1 is a key regulatory molecule in adult cardiac EMT.

Directed cardiomyocyte differentiation from human pluripotent stem cells by modulating Wnt/β-catenin signaling under fully defined conditions

The protocol described here efficiently directs human pluripotent stem cells (hPSCs) to functional cardiomyocytes in a completely defined, growth factor- and serum-free system by temporal modulation of regulators of canonical Wnt signaling. Appropriate temporal application of a glycogen synthase kinase 3 (GSK3) inhibitor combined with the expression of β-catenin shRNA or a chemical Wnt inhibitor is sufficient to produce a high yield (0.8-1.3 million cardiomyocytes per cm(2)) of virtually pure (80-98%) functional cardiomyocytes in 14 d from multiple hPSC lines without cell sorting or selection. Qualitative (immunostaining) and quantitative (flow cytometry) characterization of differentiated cells is described to assess the expression of cardiac transcription factors and myofilament proteins. Flow cytometry of BrdU incorporation or Ki67 expression in conjunction with cardiac sarcomere myosin protein expression can be used to determine the proliferative capacity of hPSC-derived cardiomyocytes. Functional human cardiomyocytes differentiated via these protocols may constitute a potential cell source for heart disease modeling, drug screening and cell-based therapeutic applications.

Small molecules, big roles -- the chemical manipulation of stem cell fate and somatic cell reprogramming

Despite the great potential of stem cells for basic research and clinical applications, obstacles - such as their scarce availability and difficulty in controlling their fate - need to be addressed to fully realize their potential. Recent achievements of cellular reprogramming have enabled the generation of induced pluripotent stem cells (iPSCs) or other lineage-committed cells from more accessible and abundant somatic cell types by defined genetic factors. However, serious concerns remain about the efficiency and safety of current genetic approaches to cell reprogramming and traditional culture systems that are used for stem cell maintenance. As a complementary approach, small molecules that target specific signaling pathways, epigenetic processes and other cellular processes offer powerful tools for manipulating cell fate to a desired outcome. A growing number of small molecules have been identified to maintain the self-renewal potential of stem cells, to induce lineage differentiation and to facilitate reprogramming by increasing the efficiency of reprogramming or by replacing genetic reprogramming factors. Furthermore, mechanistic investigations of the effects of these chemicals also provide new biological insights. Here, we examine recent achievements in the maintenance of stem cells, including pluripotent and lineage-specific stem cells, and in the control of cell fate conversions, including iPSC reprogramming, conversion of primed to naïve pluripotency, and transdifferentiation, with an emphasis on manipulation with small molecules.

Reawakening atlas: chemical approaches to repair or replace dysfunctional musculature

Muscle diseases are major health concerns. For example, ischemic heart disease is the third most common cause of death. Cell therapy is an attractive approach for treating muscle diseases, although this is hampered by the need to generate large numbers of functional muscle cells. Small molecules have become established as attractive tools for modulating cell behavior and, in this review, we discuss the recent, rapid research advances made in the development of small molecule methods to facilitate the production of functional cardiac, skeletal, and smooth muscle cells. We also describe how new developments in small molecule strategies for muscle disease aim to induce repair and remodelling of the damaged tissues in situ. Recent progress has been made in developing small molecule cocktails that induce skeletal muscle regeneration, and these are discussed in a broader context, because a similar phenomenon occurs in the early stages of salamander appendage regeneration. Although formidable technical hurdles still remain, these new advances in small molecule-based methodologies should provide hope that cell therapies for patients suffering from muscle disease can be developed in the near future.

A novel genetic variant in the transcription factor Islet-1 exerts gain of function on myocyte enhancer factor 2C promoter activity

AIMS

The transcription factor Islet-1 (ISL1) is a marker of cardiovascular progenitors and is essential for mammalian cardiogenesis. An ISL1 haplotype has recently been associated with congenital heart disease. In this study we evaluated whether ISL1 variants are associated with hypertrophic (HCM), dilated (DCM), arrhythmogenic right ventricular cardiomyopathy (ARVC), or with Emery-Dreifuss muscular dystrophy (EDMD).

METHODS AND RESULTS

The six exon and intron boundaries of ISL1 were screened for genetic variants in a cohort of 454 index cases. Eleven exonic variants were identified in HCM, DCM, ARVC, and/or EDMD. Out of the five novel variants, two are located in the 5'-untranslated region, two are silent (p.Arg171Arg and p.Asn189Asn), and one is a missense (p.Asn252Ser). The latter was identified in the homozygous state in one DCM patient, and in the heterozygous state in 11 relatives, who did not present with DCM but often with cardiovascular features. This variant was found in one HCM patient also carrying a MYH7 mutation and in 3/96 North-African Caucasian control individuals, but was absent in 138 European Caucasian control individuals. We investigated the effect of the ISL1 wild type and p.Asn252Ser mutant on myocyte enhancer factor 2C (Mef2c) promoter activity, an established ISL1 target. Mef2c promoter activity was ∼4-fold higher in the presence of wild-type and ∼6-fold higher in the presence of mutant ISL1 in both HEK and CHO cells.

CONCLUSION

This study describes a new gain-of-function p.Asn252Ser variant in the human ISL1 gene, which could potentially lead to greater activation of downstream targets involved in cardiac development, dilation, and hypertrophy.

A small molecule that promotes cardiac differentiation of human pluripotent stem cells under defined, cytokine- and xeno-free conditions

Human pluripotent stem cells (hPSCs), including embryonic stem cells and induced pluripotent stem cells, are potentially useful in regenerative therapies for heart disease. For medical applications, clinical-grade cardiac cells must be produced from hPSCs in a defined, cost-effective manner. Cell-based screening led to the discovery of KY02111, a small molecule that promotes differentiation of hPSCs to cardiomyocytes. Although the direct target of KY02111 remains unknown, results of the present study suggest that KY02111 promotes differentiation by inhibiting WNT signaling in hPSCs but in a manner that is distinct from that of previously studied WNT inhibitors. Combined use of KY02111 and WNT signaling modulators produced robust cardiac differentiation of hPSCs in a xeno-free, defined medium, devoid of serum and any kind of recombinant cytokines and hormones, such as BMP4, Activin A, or insulin. The methodology has potential as a means for the practical production of human cardiomyocytes for regeneration therapies.

From pluripotency to distinct cardiomyocyte subtypes

Differentiated adult cardiomyocytes (CMs) lack significant regenerative potential, which is one reason why degenerative heart diseases are the leading cause of death in the western world. For future cardiac repair, stem cell-based therapeutic strategies may become alternatives to donor heart transplantation. The principle of reprogramming adult terminally differentiated cells (iPSC) had a major impact on stem cell biology. One can now generate autologous pluripotent cells that highly resemble embryonic stem cells (ESC) and that are ethically inoffensive as opposed to human ESC. Yet, due to genetic and epigenetic aberrations arising during the full reprogramming process, it is questionable whether iPSC will enter the clinic in the near future. Therefore, the recent achievement of directly reprogramming fibroblasts into cardiomyocytes via a milder approach, thereby avoiding an initial pluripotent state, may become of great importance. In addition, various clinical scenarios will depend on the availability of specific cardiac cellular subtypes, for which a first step was achieved via our own programming approach to achieve cardiovascular cell subtypes. In this review, we discuss recent progress in the cardiovascular stem cell field addressing the above mentioned aspects.

Scl represses cardiomyogenesis in prospective hemogenic endothelium and endocardium

Endothelium in embryonic hematopoietic tissues generates hematopoietic stem/progenitor cells; however, it is unknown how its unique potential is specified. We show that transcription factor Scl/Tal1 is essential for both establishing the hematopoietic transcriptional program in hemogenic endothelium and preventing its misspecification to a cardiomyogenic fate. Scl(-/-) embryos activated a cardiac transcriptional program in yolk sac endothelium, leading to the emergence of CD31+Pdgfrα+ cardiogenic precursors that generated spontaneously beating cardiomyocytes. Ectopic cardiogenesis was also observed in Scl(-/-) hearts, where the disorganized endocardium precociously differentiated into cardiomyocytes. Induction of mosaic deletion of Scl in Scl(fl/fl)Rosa26Cre-ER(T2) embryos revealed a cell-intrinsic, temporal requirement for Scl to prevent cardiomyogenesis from endothelium. Scl(-/-) endothelium also upregulated the expression of Wnt antagonists, which promoted rapid cardiomyocyte differentiation of ectopic cardiogenic cells. These results reveal unexpected plasticity in embryonic endothelium such that loss of a single master regulator can induce ectopic cardiomyogenesis from endothelial cells.

Stem cell factor gene transfer promotes cardiac repair after myocardial infarction via in situ recruitment and expansion of c-kit+ cells

RATIONALE

There is growing evidence that the myocardium responds to injury by recruiting c-kit(+) cardiac progenitor cells to the damage tissue. Even though the ability of exogenously introducing c-kit(+) cells to injured myocardium has been established, the capability of recruiting these cells through modulation of local signaling pathways by gene transfer has not been tested.

OBJECTIVE

To determine whether stem cell factor gene transfer mediates cardiac regeneration in a rat myocardial infarction model, through survival and recruitment of c-kit(+) progenitors and cell-cycle activation in cardiomyocytes, and explore the mechanisms involved.

METHODS AND RESULTS

Infarct size, cardiac function, cardiac progenitor cells recruitment, fibrosis, and cardiomyocyte cell-cycle activation were measured at different time points in controls (n=10) and upon stem cell factor gene transfer (n=13) after myocardial infarction. We found a regenerative response because of stem cell factor overexpression characterized by an enhancement in cardiac hemodynamic function: an improvement in survival; a reduction in fibrosis, infarct size and apoptosis; an increase in cardiac c-kit(+) progenitor cells recruitment to the injured area; an increase in cardiomyocyte cell-cycle activation; and Wnt/β-catenin pathway induction.

CONCLUSIONS

Stem cell factor gene transfer induces c-kit(+) stem/progenitor cell expansion in situ and cardiomyocyte proliferation, which may represent a new therapeutic strategy to reverse adverse remodeling after myocardial infarction.

Electrophysiological and contractile function of cardiomyocytes derived from human embryonic stem cells

Human embryonic stem cells have emerged as the prototypical source from which cardiomyocytes can be derived for use in drug discovery and cell therapy. However, such applications require that these cardiomyocytes (hESC-CMs) faithfully recapitulate the physiology of adult cells, especially in relation to their electrophysiological and contractile function. We review what is known about the electrophysiology of hESC-CMs in terms of beating rate, action potential characteristics, ionic currents, and cellular coupling as well as their contractility in terms of calcium cycling and contraction. We also discuss the heterogeneity in cellular phenotypes that arises from variability in cardiac differentiation, maturation, and culture conditions, and summarize present strategies that have been implemented to reduce this heterogeneity. Finally, we present original electrophysiological data from optical maps of hESC-CM clusters.

Wnt signaling and injury repair

Wnt signaling is activated by wounding and participates in every subsequent stage of the healing process from the control of inflammation and programmed cell death, to the mobilization of stem cell reservoirs within the wound site. In this review we summarize recent data elucidating the roles that the Wnt pathway plays in the injury repair process. These data provide a foundation for potential Wnt-based therapeutic strategies aimed at stimulating tissue regeneration.

DMH1, a novel BMP small molecule inhibitor, increases cardiomyocyte progenitors and promotes cardiac differentiation in mouse embryonic stem cells

The possibility of using cell-based therapeutics to treat cardiac failure has generated significant interest since the initial introduction of stem cell-based technologies. However, the methods to quickly and robustly direct stem cell differentiation towards cardiac cell types have been limited by a reliance on recombinant growth factors to provide necessary biological cues. We report here the use of dorsomorphin homologue 1 (DMH1), a second-generation small molecule BMP inhibitor based on dorsomorphin, to efficiently induce beating cardiomyocyte formation in mouse embryonic stem cells (ESCs) and to specifically upregulate canonical transcriptional markers associated with cardiac development. DMH1 differs significantly from its predecessor by its ability to enrich for pro-cardiac progenitor cells that respond to late-stage Wnt inhibition using XAV939 and produce secondary beating cardiomyocytes. Our study demonstrates the utility of small molecules to complement existing in vitro cardiac differentiation protocols and highlights the role of transient BMP inhibition in cardiomyogenesis.

ILK induces cardiomyogenesis in the human heart

Background

Integrin-linked kinase (ILK) is a widely conserved serine/threonine kinase that regulates diverse signal transduction pathways implicated in cardiac hypertrophy and contractility. In this study we explored whether experimental overexpression of ILK would up-regulate morphogenesis in the human fetal heart.

Methodology/Principal Findings

Primary cultures of human fetal myocardial cells (19–22 weeks gestation) yielded scattered aggregates of cardioblasts positive for the early cardiac lineage marker nk×2.5 and containing nascent sarcomeres. Cardiac cells in colonies uniformly expressed the gap junction protein connexin 43 (C×43) and displayed a spectrum of differentiation with only a subset of cells exhibiting the late cardiomyogenic marker troponin T (cTnT) and evidence of electrical excitability. Adenovirus-mediated overexpression of ILK potently increased the number of new aggregates of primitive cardioblasts (p<0.001). The number of cardioblast colonies was significantly decreased (p<0.05) when ILK expression was knocked down with ILK targeted siRNA. Interestingly, overexpression of the activation resistant ILK mutant (ILK^R211A) resulted in much greater increase in the number of new cell aggregates as compared to overexpression of wild-type ILK (ILK^WT). The cardiomyogenic effects of ILK^R211A and ILK^WT were accompanied by concurrent activation of β-catenin (p<0.001) and increase expression of progenitor cell marker islet-1, which was also observed in lysates of transgenic mice with cardiac-specific over-expression of ILK^R211A and ILK^WT. Finally, endogenous ILK expression was shown to increase in concert with those of cardiomyogenic markers during directed cardiomyogenic differentiation in human embryonic stem cells (hESCs).

Conclusions/Significance

In the human fetal heart ILK activation is instructive to the specification of mesodermal precursor cells towards a cardiomyogenic lineage. Induction of cardiomyogenesis by ILK overexpression bypasses the requirement of proximal PI3K activation for transduction of growth factor- and β1-integrin-mediated differentiation signals. Altogether, our data indicate that ILK represents a novel regulatory checkpoint during human cardiomyogenesis.

Robust cardiomyocyte differentiation from human pluripotent stem cells via temporal modulation of canonical Wnt signaling

Human pluripotent stem cells (hPSCs) offer the potential to generate large numbers of functional cardiomyocytes from clonal and patient-specific cell sources. Here we show that temporal modulation of Wnt signaling is both essential and sufficient for efficient cardiac induction in hPSCs under defined, growth factor-free conditions. shRNA knockdown of β-catenin during the initial stage of hPSC differentiation fully blocked cardiomyocyte specification, whereas glycogen synthase kinase 3 inhibition at this point enhanced cardiomyocyte generation. Furthermore, sequential treatment of hPSCs with glycogen synthase kinase 3 inhibitors followed by inducible expression of β-catenin shRNA or chemical inhibitors of Wnt signaling produced a high yield of virtually (up to 98%) pure functional human cardiomyocytes from multiple hPSC lines. The robust ability to generate functional cardiomyocytes under defined, growth factor-free conditions solely by genetic or chemically mediated manipulation of a single developmental pathway should facilitate scalable production of cardiac cells suitable for research and regenerative applications.

Islet1-expressing cardiac progenitor cells: a comparison across species

Adult mammalian cardiac stem cells express the LIM-homeodomain transcription factor Islet1 (Isl1). They are considered remnants of Isl1-positive embryonic cardiac progenitor cells. During amniote heart development, Isl1-positive progenitor cells give rise mainly to the outflow tract, the right ventricle, and parts of the atria. This led to the hypothesis that the development of the right ventricle of the amniote heart depends on the recruitment of additional cells to the primary heart tube. The region from which these additional, Isl1-positive cells originate is called second heart field, as opposed to the first heart field whose cells form the primary heart tube. Here, we review the available data about Isl1 in different species, demonstrating that Isl1 is an important component of the core transcription factor network driving early cardiogenesis in animals of the two clades, deuterostomes, and protostomes. The data support the view of a single cardiac progenitor cell population that includes Isl1-expressing cells and which differentiates into the various cardiac lineages during embryonic development in vertebrates but not in other phyla of the animal kingdom.

Generation, characterization, and potential therapeutic applications of cardiomyocytes from various stem cells

Heart failure is one of the leading causes of death worldwide. Myocardial cell transplantation emerges as a novel therapeutic strategy for heart failure, but this approach has been hampered by severe shortage of human cardiomyocytes. We have recently induced mouse embryonic stem cells to differentiate into embryoid bodies and eventually, cardiomyocytes. Here, we address recent advancements in cardiomyocyte differentiation from cardiac stem cells and pluripotent stem cells. We highlight the methodologies, using growth factors, endoderm-like cell cocultures, small molecules, and biomaterials, in directing the differentiation of pluripotent stem cells into cardiomyocytes. The characterization and identification of pluripotent stem cell-derived cardiomyocytes by morphological, phenotypic, and functional features are also discussed. Notably, increasing evidence demonstrates that cardiomyocytes may be generated from the stem cells of several tissues outside the cardiovascular system, including skeletal muscles, bone marrow, testes, placenta, amniotic fluid, and adipose tissues. We further address the potential applications of cardiomyocytes derived from various kinds of stem cells. The differentiation of stem cells into functional cardiomyocytes, especially from an extra-cardiac stem cell source, would circumvent the scarcity of heart donors and human cardiomyocytes, and, most importantly, it would offer an ideal and promising cardiomyocyte source for cell therapy and tissue engineering in treating heart failure.

ISL1 common variant rs1017 is not associated with susceptibility to congenital heart disease in a Chinese population

BACKGROUND

ISL1, as a member of the LIM homeodomain transcription factor family, is expressed in a distinct population of undifferentiated cardiac progenitors and plays a pivotal role in cardiogenesis. Lacking ISL1 expression results in growth arrest or displays profound defects in heart development, including atria, ventricle, and the inflow and outflow tracts, which constitute a major form of congenital heart disease (CHD). Recently, an important study by Stevens et al. found that genetic variation in ISL1 is associated with risk of CHD in white and black/African American populations; this observation led us to hypothesize that ISL1 common variants might influence susceptibility to sporadic CHD in our Chinese population.

METHODS

We conducted a case-control study of CHD in Chinese to test our hypothesis by genotyping ISL1 common variant rs1017 in 1003 CHD cases and 1012 non-CHD controls.

RESULTS

We found that rs1017 was not associated with the risk of CHD (p=0.213). When we performed stratified analyses according to subjects' age, sex, and CHD classifications, we found no overall heterogeneity of risk in different subgroups.

CONCLUSIONS

This is the first study which indicates that ISL1 common variant rs1017 may not play a role in sporadic CHD susceptibility in the Chinese population.

Bioengineering heart muscle: a paradigm for regenerative medicine

The idea of extending the lifetime of our organs is as old as humankind, fueled by major advances in organ transplantation, novel drugs, and medical devices. However, true regeneration of human tissue has become increasingly plausible only in recent years. The human heart has always been a focus of such efforts, given its notorious inability to repair itself following injury or disease. We discuss here the emerging bioengineering approaches to regeneration of heart muscle as a paradigm for regenerative medicine. Our focus is on biologically inspired strategies for heart regeneration, knowledge gained thus far about how to make a "perfect" heart graft, and the challenges that remain to be addressed for tissue-engineered heart regeneration to become a clinical reality. We emphasize the need for interdisciplinary research and training, as recent progress in the field is largely being made at the interfaces between cardiology, stem cell science, and bioengineering.

Bone marrow-derived human mesenchymal stem cells express cardiomyogenic proteins but do not exhibit functional cardiomyogenic differentiation potential

Despite their paracrine activites, cardiomyogenic differentiation of bone marrow (BM)-derived mesenchymal stem cells (MSCs) is thought to contribute to cardiac regeneration. To systematically evaluate the role of differentiation in MSC-mediated cardiac regeneration, the cardiomyogenic differentiation potential of human MSCs (hMSCs) and murine MSCs (mMSCs) was investigated in vitro and in vivo by inducing cardiomyogenic and noncardiomyogenic differentiation. Untreated hMSCs showed upregulation of cardiac tropopin I, cardiac actin, and myosin light chain mRNA and protein, and treatment of hMSCs with various cardiomyogenic differentiation media led to an enhanced expression of cardiomyogenic genes and proteins; however, no functional cardiomyogenic differentiation of hMSCs was observed. Moreover, co-culturing of hMSCs with cardiomyocytes derived from murine pluripotent cells (mcP19) or with murine fetal cardiomyocytes (mfCMCs) did not result in functional cardiomyogenic differentiation of hMSCs. Despite direct contact to beating mfCMCs, hMSCs could be effectively differentiated into cells of only the adipogenic and osteogenic lineage. After intramyocardial transplantation into a mouse model of myocardial infarction, Sca-1(+) mMSCs migrated to the infarcted area and survived at least 14 days but showed inconsistent evidence of functional cardiomyogenic differentiation. Neither in vitro treatment nor intramyocardial transplantation of MSCs reliably generated MSC-derived cardiomyocytes, indicating that functional cardiomyogenic differentiation of BM-derived MSCs is a rare event and, therefore, may not be the main contributor to cardiac regeneration.

Towards regenerative therapy for cardiac disease

Development of regenerative therapeutic strategies to reverse the progression of advanced heart failure is one of the most urgent clinical needs of this century. Insights gained from clinical trials of adult stem cells, together with fundamental scientific advances in cardiac stem cell and regenerative biology, are beginning to yield potential new targets and strategies for regenerative therapies. Of particular importance are new scientific discoveries related to intrinsic cardiac regeneration, renewal factors that can trigger regeneration, and tissue-engineering technology, which are beginning to change the way investigators view the scientific and clinical position of cardiovascular regenerative therapy.

Regenerating functional heart tissue for myocardial repair

Heart disease is one of the leading causes of death worldwide and the number of patients with the disease is likely to grow with the continual decline in health for most of the developed world. Heart transplantation is one of the only treatment options for heart failure due to an acute myocardial infarction, but limited donor supply and organ rejection limit its widespread use. Cellular cardiomyoplasty, or cellular implantation, combined with various tissue-engineering methods aims to regenerate functional heart tissue. This review highlights the numerous cell sources that have been used to regenerate the heart as well as cover the wide range of tissue-engineering strategies that have been devised to optimize the delivery of these cells. It will probably be a long time before an effective regenerative therapy can make a serious impact at the bedside.

ER71 specifies Flk-1+ hemangiogenic mesoderm by inhibiting cardiac mesoderm and Wnt signaling

Two distinct types of Flk-1(+) mesoderm, hemangiogenic and cardiogenic, are thought to contribute to blood, vessel, and cardiac cell lineages. However, our understanding of how Flk-1(+) mesoderm is specified is currently limited. In the present study, we investigated whether ER71, an Ets transcription factor essential for hematopoietic and endothelial cell lineage development, could modulate the hemangiogenic or cardiogenic outcome of the Flk-1(+) mesoderm. We show that Flk-1(+) mesoderm can be divided into Flk-1(+)PDGFRα(-) hemangiogenic and Flk-1(+)PDGFRα(+) cardiogenic mesoderm. ER71-deficient embryonic stem cells produced only the Flk-1(+)PDGFRα(+) cardiogenic mesoderm, which generated SMCs and cardiomyocytes. Enforced ER71 expression in the wild-type embryonic stem cells skewed toward the Flk-1(+)PDGFRα(-) mesoderm formation, which generated hematopoietic and endothelial cells. Whereas hematopoietic and endothelial cell genes were positively regulated by ER71, cardiac and Wnt signaling pathway genes were negatively regulated by ER71. We show that ER71 could inhibit Wnt signaling in VE-cadherin-independent as well as VE-cadherin-dependent VE-cadherin/β-catenin/Flk-1 complex formation. Enforced β-catenin could rescue cardiogenic mesoderm in the context of ER71 overexpression. In contrast, ER71-deficient Flk-1(+) mesoderm displayed enhanced Wnt signaling, which was reduced by ER71 re-introduction. We provide the molecular basis for the antagonistic relationship between hemangiogenic and cardiogenic mesoderm specification by ER71 and Wnt signaling.

Pyrvinium, a potent small molecule Wnt inhibitor, increases engraftment and inhibits lineage commitment of mesenchymal stem cells (MSCs)

We and others have found that Wnt signaling inhibition is important in mesenchymal stem cell (MSC) self-renewal. Pyrvinium was identified as a potent Wnt inhibitor in a chemical screen for small molecules. In the present study, we hypothesized that pyrvinium will enhance MSC self-renewal to improve the clinical efficacy of MSC therapy. Pyrvinium increased MSC proliferation in vitro while inhibiting their osteogenic and chondrogenic lineage commitment by reducing cytoplasmic β-catenin. Although MSCs are a promising target for cell therapy, strategies to enhance their survival and maintain their stemness in the wounded area are essential. Using an in vivo model of granulation tissue formation, we demonstrated that pyrvinium enhanced long-term MSC engraftment. Pyrvinium-treated MSC-generated granulation tissue also demonstrated less ectopic differentiation into bone or cartilage. This study highlights the potential of using a therapeutic Wnt inhibitor to enhance MSC-driven regenerative therapy.

Nodal mutant eXtraembryonic ENdoderm (XEN) stem cells upregulate markers for the anterior visceral endoderm and impact the timing of cardiac differentiation in mouse embryoid bodies

Interactions between the endoderm and mesoderm that mediate myocardial induction are difficult to study in vivo because of the small size of mammalian embryos at relevant stages. However, we and others have demonstrated that signals from endodermal cell lines can influence myocardial differentiation from both mouse and human embryoid bodies (EBs), and because of this, assays that utilize embryonic stem (ES) cells and endodermal cell lines provide excellent in vitro models to study early cardiac differentiation. Extraembryonic endoderm (XEN) stem cells have a particular advantage over other heart-inducing cell lines in that they can easily be derived from both wild type and mutant mouse blastocysts. Here we describe the first isolation of a Nodal mutant XEN stem cell line. Nodal^−/− XEN cell lines were not isolated at expected Mendelian ratios, and those that were successfully established, showed an increase in markers for the anterior visceral endoderm (AVE). Since AVE represents the heart-inducing endoderm in the mouse, cardiac differentiation was compared in EBs treated with conditioned medium (CM) collected from wild type or Nodal^−/− XEN cells. EBs treated with CM from Nodal^−/− cells began beating earlier and showed early activation of myocardial genes, but this early cardiac differentiation did not cause an overall increase in cardiomyocyte yield. By comparison, CM from wild type XEN cells both delayed cardiac differentiation and caused a concomitant increase in overall cardiomyocyte formation. Detailed marker analysis suggested that early activation of cardiac differentiation by Nodal^−/− XEN CM caused premature differentiation and subsequent depletion of cardiac progenitors.

Production of de novo cardiomyocytes: human pluripotent stem cell differentiation and direct reprogramming

Cardiovascular disease is a leading cause of death worldwide. The limited capability of heart tissue to regenerate has prompted methodological developments for creating de novo cardiomyocytes, both in vitro and in vivo. Beyond uses in cell replacement therapy, patient-specific cardiomyocytes may find applications in drug testing, drug discovery, and disease modeling. Recently, approaches for generating cardiomyocytes have expanded to encompass three major sources of starting cells: human pluripotent stem cells (hPSCs), adult heart-derived cardiac progenitor cells (CPCs), and reprogrammed fibroblasts. We discuss state-of-the-art methods for generating de novo cardiomyocytes from hPSCs and reprogrammed fibroblasts, highlighting potential applications and future challenges.

From ontogenesis to regeneration: learning how to instruct adult cardiac progenitor cells

Since the first observations over two centuries ago by Lazzaro Spallanzani on the extraordinary regenerative capacity of urodeles, many attempts have been made to understand the reasons why such ability has been largely lost in metazoa and whether or how it can be restored, even partially. In this context, important clues can be derived from the systematic analysis of the relevant distinctions among species and of the pathways involved in embryonic development, which might be induced and/or recapitulated in adult tissues. This chapter provides an overview on regeneration and its mechanisms, starting with the lesson learned from lower vertebrates, and will then focus on recent advancements and novel insights concerning regeneration in the adult mammalian heart, including the discovery of resident cardiac progenitor cells (CPCs). Subsequently, it explores all the important pathways involved in regulating differentiation during development and embryogenesis, and that might potentially provide important clues on how to activate and/or modulate regenerative processes in the adult myocardium, including the potential activation of endogenous CPCs. Furthermore the importance of the stem cell niche is discussed, and how it is possible to create in vitro a microenvironment and culture system to provide adult CPCs with the ideal conditions promoting their regenerative ability. Finally, the state of clinical translation of cardiac cell therapy is presented. Overall, this chapter provides a new perspective on how to approach cardiac regeneration, taking advantage of important lessons from development and optimizing biotechnological tools to obtain the ideal conditions for cell-based cardiac regenerative therapy.

Mechanisms of cardiogenesis in cardiovascular progenitor cells

Self-renewing cells of the vertebrate heart have become a major subject of interest in the past decade. However, many researchers had a hard time to argue against the orthodox textbook view that defines the heart as a postmitotic organ. Once the scientific community agreed on the existence of self-renewing cells in the vertebrate heart, their origin was again put on trial when transdifferentiation, dedifferentiation, and reprogramming could no longer be excluded as potential sources of self-renewal in the adult organ. Additionally, the presence of self-renewing pluripotent cells in the peripheral blood challenges the concept of tissue-specific stem and progenitor cells. Leaving these unsolved problems aside, it seems very desirable to learn about the basic biology of this unique cell type. Thus, we shall here paint a picture of cardiovascular progenitor cells including the current knowledge about their origin, basic nature, and the molecular mechanisms guiding proliferation and differentiation into somatic cells of the heart.

FRS2α-mediated FGF signals suppress premature differentiation of cardiac stem cells through regulating autophagy activity

RATIONALE

Although the fibroblast growth factor (FGF) signaling axis plays important roles in heart development, the molecular mechanism by which the FGF regulates cardiogenesis is not fully understood.

OBJECTIVE

To investigate the mechanism by which FGF signaling regulates cardiac progenitor cell differentiation.

METHODS AND RESULTS

Using mice with tissue-specific ablation of FGF receptors and FGF receptor substrate 2α (Frs2α) in heart progenitor cells, we demonstrate that disruption of FGF signaling leads to premature differentiation of cardiac progenitor cells in mice. Using embryoid body cultures of mouse embryonic stem cells, we reveal that FGF signaling promotes mesoderm differentiation in embryonic stem cells but inhibits cardiomyocyte differentiation of the mesoderm cells at later stages. Furthermore, we also report that inhibiting FRS2α-mediated signals increases autophagy and that activating autophagy promotes myocardial differentiation and vice versa.

CONCLUSIONS

The results indicate that the FGF/FRS2α-mediated signals prevent premature differentiation of heart progenitor cells through suppressing autophagy. The findings provide the first evidence that autophagy plays a role in heart progenitor differentiation.

Highly efficient derivation of ventricular cardiomyocytes from induced pluripotent stem cells with a distinct epigenetic signature

Cardiomyocytes derived from pluripotent stem cells can be applied in drug testing, disease modeling and cell-based therapy. However, without procardiogenic growth factors, the efficiency of cardiomyogenesis from pluripotent stem cells is usually low and the resulting cardiomyocyte population is heterogeneous. Here, we demonstrate that induced pluripotent stem cells (iPSCs) can be derived from murine ventricular myocytes (VMs), and consistent with other reports of iPSCs derived from various somatic cell types, VM-derived iPSCs (ViPSCs) exhibit a markedly higher propensity to spontaneously differentiate into beating cardiomyocytes as compared to genetically matched embryonic stem cells (ESCs) or iPSCs derived from tail-tip fibroblasts. Strikingly, the majority of ViPSC-derived cardiomyocytes display a ventricular phenotype. The enhanced ventricular myogenesis in ViPSCs is mediated via increased numbers of cardiovascular progenitors at early stages of differentiation. In order to investigate the mechanism of enhanced ventricular myogenesis from ViPSCs, we performed global gene expression and DNA methylation analysis, which revealed a distinct epigenetic signature that may be involved in specifying the VM fate in pluripotent stem cells.