Stage-specific optimization of activin/nodal and BMP signaling promotes cardiac differentiation of mouse and human pluripotent stem cell lines

Human amniotic mesenchymal stem cell-derived induced pluripotent stem cells may generate a universal source of cardiac cells

Human amniotic mesenchymal stem cells (hAMSCs) demonstrated partially pluripotent characteristics with a strong expression of Oct4 and Nanog genes and immunomodulatory properties characterized by the absence of HLA-DR and the presence of HLA-G and CD59. The hAMSCs were reprogrammed into induced pluripotent stem cells (iPSCs) that generate a promising source of universal cardiac cells. The hAMSC-derived iPSCs (MiPSCs) successfully underwent robust cardiac differentiation to generate cardiomyocytes. This study investigated 3 key properties of the hAMSCs and MiPSCs: (1) the reprogramming efficiency of the partially pluripotent hAMSCs to generate MiPSCs; (2) immunomodulatory properties of the hAMSCs and MiPSCs; and (3) the cardiac differentiation potential of the MiPSCs. The characteristic iPSC colony formation was observed within 10 days after the transduction of the hAMSCs with a single integration polycistronic vector containing 4 Yamanaka factors. Immunohistology and reverse transcription-polymerase chain reaction assays revealed that the MiPSCs expressed stem cell surface markers and pluripotency-specific genes. Furthermore, the hAMSCs and MiPSCs demonstrated immunomodulatory properties enabling successful engraftment in the SVJ mice. Finally, the cardiac differentiation of MiPSCs exhibited robust spontaneous contractility, characteristic calcium transience across the membrane, a high expression of cardiac genes and mature cardiac phenotypes, and a contractile force comparable to cardiomyocytes. Our results demonstrated that the hAMSCs are reprogrammed with a high efficiency into MiPSCs, which possess pluripotent, immunomodulatory, and precardiac properties. The MiPSC-derived cardiac cells express a c-kit cell surface marker, which may be employed to purify the cardiac cell population and enable allogeneic cardiac stem cell therapy.

High-purity enrichment of functional cardiovascular cells from human iPS cells

AIMS

A variety of human inherited heart diseases affect the normal functions of cardiomyocytes (CMs), endothelial cells (ECs), or smooth muscle cells (SMCs). To study human heart disease and generate cardiac cells for basic and translational research, an efficient strategy is needed for production of cardiac lineages from human stem cells. In the present study, a highly reproducible method was developed that can simultaneously enrich a large number of CMs and cardiac SMCs and ECs from human induced pluripotent stem (iPS) cells with high purity.

METHODS AND RESULTS

Human multipotent cardiovascular progenitor cells were generated from human iPS cells, followed by selective differentiation of the multipotent cardiovascular progenitor cells into CMs, ECs, and SMCs. With further fluorescence-activated cell sorting, each of the three cardiovascular cell types could be enriched with high purity (>90%). These enriched cardiovascular cells exhibited specific gene expression signatures and normal functions when assayed both in vitro and in vivo. Moreover, CMs purified from iPS cells derived from a patient with LEOPARD syndrome, a disease characterized by cardiac hypertrophy, showed the expected up-regulated expression of genes associated with cardiac hypertrophy.

CONCLUSIONS

Overall, our technical advance provides the means for generating a renewable resource of pure human cardiovascular cells that can be used to dissect the mechanisms of human inherited heart disease and for the future development of drug and cell therapeutics for heart disease.

Cardiomyocytes derived from pluripotent stem cells recapitulate electrophysiological characteristics of an overlap syndrome of cardiac sodium channel disease

BACKGROUND

Pluripotent stem cells (PSCs) offer a new paradigm for modeling genetic cardiac diseases, but it is unclear whether mouse and human PSCs can truly model both gain- and loss-of-function genetic disorders affecting the Na(+) current (I(Na)) because of the immaturity of the PSC-derived cardiomyocytes. To address this issue, we generated multiple PSC lines containing a Na(+) channel mutation causing a cardiac Na(+) channel overlap syndrome.

METHOD AND RESULTS

Induced PSC (iPSC) lines were generated from mice carrying the Scn5a(1798insD/+) (Scn5a-het) mutation. These mouse iPSCs, along with wild-type mouse iPSCs, were compared with the targeted mouse embryonic stem cell line used to generate the mutant mice and with the wild-type mouse embryonic stem cell line. Patch-clamp experiments showed that the Scn5a-het cardiomyocytes had a significant decrease in I(Na) density and a larger persistent I(Na) compared with Scn5a-wt cardiomyocytes. Action potential measurements showed a reduced upstroke velocity and longer action potential duration in Scn5a-het myocytes. These characteristics recapitulated findings from primary cardiomyocytes isolated directly from adult Scn5a-het mice. Finally, iPSCs were generated from a patient with the equivalent SCN5A(1795insD/+) mutation. Patch-clamp measurements on the derivative cardiomyocytes revealed changes similar to those in the mouse PSC-derived cardiomyocytes.

CONCLUSION

Here, we demonstrate that both embryonic stem cell- and iPSC-derived cardiomyocytes can recapitulate the characteristics of a combined gain- and loss-of-function Na(+) channel mutation and that the electrophysiological immaturity of PSC-derived cardiomyocytes does not preclude their use as an accurate model for cardiac Na(+) channel disease.

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.

Physiological aspects of cardiac tissue engineering

Cardiac tissue engineering aims at repairing the diseased heart and developing cardiac tissues for basic research and predictive toxicology applications. Since the first description of engineered heart tissue 15 years ago, major development steps were directed toward these three goals. Technical innovations led to improved three-dimensional cardiac tissue structure and near physiological contractile force development. Automation and standardization allow medium throughput screening. Larger constructs composed of many small engineered heart tissues or stacked cell sheet tissues were tested for cardiac repair and were associated with functional improvements in rats. Whether these approaches can be simply transferred to larger animals or the human patients remains to be tested. The availability of an unrestricted human cardiac myocyte cell source from human embryonic stem cells or human-induced pluripotent stem cells is a major breakthrough. This review summarizes current tissue engineering techniques with their strengths and limitations and possible future applications.

Effects of substrate mechanics on contractility of cardiomyocytes generated from human pluripotent stem cells

Human pluripotent stem cell (hPSC-) derived cardiomyocytes have potential applications in drug discovery, toxicity testing, developmental studies, and regenerative medicine. Before these cells can be reliably utilized, characterization of their functionality is required to establish their similarity to native cardiomyocytes. We tracked fluorescent beads embedded in 4.4-99.7 kPa polyacrylamide hydrogels beneath contracting neonatal rat cardiomyocytes and cardiomyocytes generated from hPSCs via growth-factor-induced directed differentiation to measure contractile output in response to changes in substrate mechanics. Contraction stress was determined using traction force microscopy, and morphology was characterized by immunocytochemistry for α-actinin and subsequent image analysis. We found that contraction stress of all types of cardiomyocytes increased with substrate stiffness. This effect was not linked to beating rate or morphology. We demonstrated that hPSC-derived cardiomyocyte contractility responded appropriately to isoprenaline and remained stable in culture over a period of 2 months. This study demonstrates that hPSC-derived cardiomyocytes have appropriate functional responses to substrate stiffness and to a pharmaceutical agent, which motivates their use in further applications such as drug evaluation and cardiac therapies.

Human pre-implantation embryo development

Understanding human pre-implantation development has important implications for assisted reproductive technology (ART) and for human embryonic stem cell (hESC)-based therapies. Owing to limited resources, the cellular and molecular mechanisms governing this early stage of human development are poorly understood. Nonetheless, recent advances in non-invasive imaging techniques and molecular and genomic technologies have helped to increase our understanding of this fascinating stage of human development. Here, we summarize what is currently known about human pre-implantation embryo development and highlight how further studies of human pre-implantation embryos can be used to improve ART and to fully harness the potential of hESCs for therapeutic goals.

The notch ligand delta-like 4 regulates multiple stages of early hemato-vascular development

Background

In mouse embryos, homozygous or heterozygous deletions of the gene encoding the Notch ligand Dll4 result in early embryonic death due to major defects in endothelial remodeling in the yolk sac and embryo. Considering the close developmental relationship between endothelial and hematopoietic cell lineages, which share a common mesoderm-derived precursor, the hemangioblast, and many key regulatory molecules, we investigated whether Dll4 is also involved in the regulation of early embryonic hematopoiesis.

Methodology/Principal Findings

Using Embryoid Bodies (EBs) derived from embryonic stem cells harboring hetero- or homozygous Dll4 deletions, we observed that EBs from both genotypes exhibit an abnormal endothelial remodeling in the vascular sprouts that arise late during EB differentiation, indicating that this in vitro system recapitulates the angiogenic phenotype of Dll4 mutant embryos. However, analysis of EB development at early time points revealed that the absence of Dll4 delays the emergence of mesoderm and severely reduces the number of blast-colony forming cells (BL-CFCs), the in vitro counterpart of the hemangioblast, and of endothelial cells. Analysis of colony forming units (CFU) in EBs and yolk sacs from Dll4^+/− and Dll4^−/− embryos, showed that primitive erythropoiesis is specifically affected by Dll4 insufficiency. In Dll4 mutant EBs, smooth muscle cells (SMCs) were seemingly unaffected and cardiomyocyte differentiation was increased, indicating that SMC specification is Dll4-independent while a normal dose of this Notch ligand is essential for the quantitative regulation of cardiomyogenesis.

Conclusions/Significance

This study highlights a previously unnoticed role for Dll4 in the quantitative regulation of early hemato-vascular precursors, further indicating that it is also involved on the timely emergence of mesoderm in early embryogenesis.

Efficient derivation of purified lung and thyroid progenitors from embryonic stem cells

Two populations of Nkx2-1(+) progenitors in the developing foregut endoderm give rise to the entire postnatal lung and thyroid epithelium, but little is known about these cells because they are difficult to isolate in a pure form. We demonstrate here the purification and directed differentiation of primordial lung and thyroid progenitors derived from mouse embryonic stem cells (ESCs). Inhibition of TGFβ and BMP signaling, followed by combinatorial stimulation of BMP and FGF signaling, can specify these cells efficiently from definitive endodermal precursors. When derived using Nkx2-1(GFP) knockin reporter ESCs, these progenitors can be purified for expansion in culture and have a transcriptome that overlaps with developing lung epithelium. Upon induction, they can express a broad repertoire of markers indicative of lung and thyroid lineages and can recellularize a 3D lung tissue scaffold. Thus, we have derived a pure population of progenitors able to recapitulate the developmental milestones of lung/thyroid development.

Ghrelin promotes the differentiation of human embryonic stem cells in infarcted cardiac microenvironment

Ghrelin is broadly expressed in myocardial tissues, where it exerts different functions. It also has been found to have a wide variety of biological functions on cell differentiation and tissue development. The aim of this study was to investigate the effect of ghrelin on human embryonic stem cell (hESC) differentiation in infarcted cardiac microenvironment. The hESCs grown on feeder layers expressed several pluripotential markers including alkaline phosphatase (AKP). Four weeks after transplantation into rat infarcted hearts, the hESCs and their progeny cells survived and formed intracardiac grafts were 54.7% and 19.6% respectively in ghrelin- and phosphate-buffered saline (PBS)-treated groups. Double immunostaining with anti-human Sox9 and anti-HNA or anti-human fetal liver kinase-1 (Flk1) and anti β-tubulin showed that the human grafts were in development. However, double positive stains were only found in the ghrelin-treated group. In addition, the hESC injection protocol was insufficient to restore heart function of the acute myocardial infarction model. Our study, therefore, provides a new insight of ghrelin on promoting hESC survival and differentiation in rat infarcted cardiac microenvironment. This may give a clue for therapy for myocardial infarction by hESCs or progeny cells.

Differentiation and enrichment of cardiomyocytes from human pluripotent stem cells

Human cardiomyocytes derived from pluripotent stem cells hold great promise for cardiac cell therapy, disease modeling, drug discovery, and the study of developmental biology. Reaching these potentials fully requires the development of methods that enable efficient and robust generation of cardiomyocytes with expected characteristics. This review summarizes and discusses up-to-date methods that have been used to derive and enrich human cardiomyocytes from pluripotent stem cells, provides a brief overview of in vitro and in vivo characterization of these cardiomyocytes, and considers future advancement needed to further harness the power of these cells.

Prospective in vitro models of channelopathies and cardiomyopathies

An in vitro heart disease model is a promising model used for identifying the genes responsible for the disease, evaluating the effects of drugs, and regenerative medicine. We were interested in disease models using a patient-induced pluripotent stem (iPS) cell-derived cardiomyocytes because of their similarity to a patient's tissues. However, as these studies have just begun, we would like to review the literature in this and other related fields and discuss the path for future models of molecular biology that can help to diagnose and cure diseases, and its involvement in regenerative medicine. The heterogeneity of iPS cells and/or differentiated cardiomyocytes has been recognized as a problem. An in vitro heart disease model should be evaluated using molecular biological analyses, such as mRNA and micro-RNA expression profiles and proteomic analysis.

Derivation, expansion and differentiation of induced pluripotent stem cells in continuous suspension cultures

We describe derivation of induced pluripotent stem cells (iPSCs) from terminally differentiated mouse cells in serum- and feeder-free stirred suspension cultures. Temporal analysis of global gene expression revealed high correlations between cells reprogrammed in suspension and cells reprogrammed in adhesion-dependent conditions. Suspension culture-reprogrammed iPSCs (SiPSCs) could be differentiated into all three germ layers in vitro and contributed to chimeric embryos in vivo. SiPSC generation allowed for efficient selection of reprogramming factor-expressing cells based on their differential survival and proliferation in suspension culture. Seamless integration of SiPSC reprogramming and directed differentiation enabled scalable production of beating cardiac cells in a continuous single cell- and small aggregate-based process. This method is an important step toward the development of robust PSC generation, expansion and differentiation technology.

Mapping mouse hemangioblast maturation from headfold stages

The mouse posterior primitive streak at neural plate/headfold stages (NP/HF, ~7.5 dpc-8 dpc) represents an optimal window from which hemangioblasts can be isolated. We performed immunohistochemistry on this domain using established monoclonal antibodies for proteins that affect blood and endothelial fates. We demonstrate that HoxB4 and GATA1 are the first set of markers that segregate independently to endothelial or blood populations during NP/HF stages of mouse embryonic development. In a subset of cells, both proteins are co-expressed and immunoreactivities appear mutually excluded within nuclear spaces. We searched for this particular state at later sites of hematopoietic stem cell emergence, viz., the aorta-gonad-mesonephros (AGM) and the fetal liver at 10.5-11.5 dpc, and found that only a rare number of cells displayed this character. Based on this spatial-temporal argument, we propose that the earliest blood progenitors emerge either directly from the epiblast or through segregation within the allantoic core domain (ACD) through reduction of cell adhesion and pSmad1/5 nuclear signaling, followed by a stochastic decision toward a blood or endothelial fate that involves GATA1 and HoxB4, respectively. A third form in which binding distributions are balanced may represent a common condition shared by hemangioblasts and HSCs. We developed a heuristic model of hemangioblast maturation, in part, to be explicit about our assumptions.

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.

Efficient differentiation of human induced pluripotent stem cells generates cardiac cells that provide protection following myocardial infarction in the rat

Induced pluripotent stem (iPS) cells are being used increasingly to complement their embryonic counterparts to understand and develop the therapeutic potential of pluripotent cells. Our objectives were to identify an efficient cardiac differentiation protocol for human iPS cells as monolayers, and demonstrate that the resulting cardiac progenitors could provide a therapeutic benefit in a rodent model of myocardial infarction. Herein, we describe a 14-day protocol for efficient cardiac differentiation of human iPS cells as a monolayer, which routinely yielded a mixed population in which over 50% were cardiomyocytes, endothelium, or smooth muscle cells. When differentiating, cardiac progenitors from day 6 of this protocol were injected into the peri-infarct region of the rat heart; after coronary artery ligation and reperfusion, we were able to show that human iPS cell-derived cardiac progenitor cells engrafted, differentiated into cardiomyocytes and smooth muscle, and persisted for at least 10 weeks postinfarct. Hearts injected with iPS-derived cells showed a nonsignificant trend toward protection from decline in function after myocardial infarction, as assessed by magnetic resonance imaging at 10 weeks, such that the ejection fraction at 10 weeks in iPS treated hearts was 62%±4%, compared to that of control infarcted hearts at 45%±9% (P<0.2). In conclusion, we demonstrated efficient cardiac differentiation of human iPS cells that gave rise to progenitors that were retained within the infarcted rat heart, and reduced remodeling of the heart after ischemic damage.

Predictive microfluidic control of regulatory ligand trajectories in individual pluripotent cells

Local (cell-level) signaling environments, regulated by autocrine and paracrine signaling, and modulated by cell organization, are hypothesized to be fundamental stem cell fate control mechanisms used during development. It has, however, been challenging to demonstrate the impact of cell-level organization on stem cell fate control and to relate stem cell fate outcomes to autocrine and paracrine signaling. We address this fundamental problem using a combined in silico and experimental approach in which we directly manipulate, using laminar fluid flow, the local impact of endogenously secreted gp130-activating ligands and their activation of signal transducer and activator of transcription3 (STAT3) signaling in mouse embryonic stem cells (mESC). Our model analysis predicted that flow-dependent changes in autocrine and paracrine ligand binding would impact heterogeneity in cell- and colony-level STAT3 signaling activation and cause a gradient of cell fate determination along the direction of flow. Interestingly, analysis also predicted that local cell density would be inversely proportional to the degree to which endogenous secretion contributed to cell fate determination. Experimental validation using functional activation of STAT3 by secreted factors under microfluidic perfusion culture demonstrated that STAT3 activation and consequently mESC fate were manipulable by flow rate, position in the flow field, and local cell organization. As a unique demonstration of how quantitative control of autocrine and paracrine signaling can be integrated with spatial organization to elicit higher order cell fate effects, this work provides a general template to investigate organizing principles due to secreted factors.

Systematic engineering of 3D pluripotent stem cell niches to guide blood development

Pluripotent stem cells (PSC) provide insight into development and may underpin new cell therapies, yet controlling PSC differentiation to generate functional cells remains a significant challenge. In this study we explored the concept that mimicking the local in vivo microenvironment during mesoderm specification could promote the emergence of hematopoietic progenitor cells from embryonic stem cells (ESCs). First, we assessed the expression of early phenotypic markers of mesoderm differentiation (E-cadherin, brachyury (T-GFP), PDGFRα, and Flk1: +/-ETPF) to reveal that E-T+P+F+ cells have the highest capacity for hematopoiesis. Second, we determined how initial aggregate size influences the emergence of mesodermal phenotypes (E-T+P+F+, E-T-P+/-F+, and E-T-P+F-) and discovered that colony forming cell (CFC) output was maximal with ~100 cells per PSC aggregate. Finally, we introduced these 100-cell PSC aggregates into a low oxygen environment (5%; to upregulate endogenous VEGF secretion) and delivered two potent blood-inductive molecules, BMP4 and TPO (bone morphogenetic protein-4 and thrombopoietin), locally from microparticles to obtain a more robust differentiation response than soluble delivery methods alone. Approximately 1.7-fold more CFCs were generated with localized delivery in comparison to exogenous delivery, while combined growth factor use was reduced ~14.2-fold. By systematically engineering the complex and dynamic environmental signals associated with the in vivo blood developmental niche we demonstrate a significant role for inductive endogenous signaling and introduce a tunable platform for enhancing PSC differentiation efficiency to specific lineages.

Wnt inhibition correlates with human embryonic stem cell cardiomyogenesis: a structure-activity relationship study based on inhibitors for the Wnt response

Human embryonic stem cell-based high-content screening of 550 known signal transduction modulators showed that one "lead" (1, a recently described inhibitor of the proteolytic degradation of Axin) stimulated cardiomyogenesis. Because Axin controls canonical Wnt signaling, we conducted an investigation to determine whether the cardiogenic activity of 1 is Wnt-dependent, and we developed a structure-activity relationship to optimize the cardiogenic properties of 1. We prepared analogues with a range of potencies (low nanomolar to inactive) for Wnt/β-catenin inhibition and for cardiogenic induction. Both functional activities correlated positively (r(2) = 0.72). The optimal compounds induced cardiogenesis 1.5-fold greater than 1 at 30-fold lower concentrations. In contrast, no correlation was observed for cardiogenesis and modulation of transforming growth factor β (TGFβ)/Smad signaling that prominently influences cardiogenesis. Taken together, these data show that Wnt signaling inhibition is essential for cardiogenic activity and that the pathway can be targeted for the design of druglike cardiogenic molecules.

Bmp signaling exerts opposite effects on cardiac differentiation

RATIONALE

The importance for Bmp signaling during embryonic stem cell differentiation into myocardial cells has been recognized. The question when and where Bmp signaling in vivo regulates myocardial differentiation has remained largely unanswered.

OBJECTIVE

To identify when and where Bmp signaling regulates cardiogenic differentiation.

METHODS AND RESULTS

Here we have observed that in zebrafish embryos, Bmp signaling is active in cardiac progenitor cells prior to their differentiation into cardiomyocytes. Bmp signaling is continuously required during somitogenesis within the anterior lateral plate mesoderm to induce myocardial differentiation. Surprisingly, Bmp signaling is actively repressed in differentiating myocardial cells. We identified the inhibitory Smad6a, which is expressed in the cardiac tissue, to be required to inhibit Bmp signaling and thereby promote expansion of the ventricular myocardium.

CONCLUSION

Bmp signaling exerts opposing effects on myocardial differentiation in the embryo by promoting as well as inhibiting cardiac growth.

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.

A microfabricated platform to measure and manipulate the mechanics of engineered cardiac microtissues

Engineered myocardial tissues can be used to elucidate fundamental features of myocardial biology, develop organotypic in vitro model systems, and as engineered tissue constructs for replacing damaged heart tissue in vivo. However, a key limitation is an inability to test the wide range of parameters (cell source, mechanical, soluble and electrical stimuli) that might impact the engineered tissue in a high-throughput manner and in an environment that mimics native heart tissue. Here we used microelectromechanical systems technology to generate arrays of cardiac microtissues (CMTs) embedded within three-dimensional micropatterned matrices. Microcantilevers simultaneously constrain CMT contraction and report forces generated by the CMTs in real time. We demonstrate the ability to routinely produce ~200 CMTs per million cardiac cells (<1 neonatal rat heart) whose spontaneous contraction frequency, duration, and forces can be tracked. Independently varying the mechanical stiffness of the cantilevers and collagen matrix revealed that both the dynamic force of cardiac contraction as well as the basal static tension within the CMT increased with boundary or matrix rigidity. Cell alignment is, however, reduced within a stiff collagen matrix; therefore, despite producing higher force, CMTs constructed from higher density collagen have a lower cross-sectional stress than those constructed from lower density collagen. We also study the effect of electrical stimulation on cell alignment and force generation within CMTs and we show that the combination of electrical stimulation and auxotonic load strongly improves both the structure and the function of the CMTs. Finally, we demonstrate the suitability of our technique for high-throughput monitoring of drug-induced changes in spontaneous frequency or contractility in CMTs as well as high-speed imaging of calcium dynamics using fluorescent dyes. Together, these results highlight the potential for this approach to quantitatively demonstrate the impact of physical parameters on the maturation, structure, and function of cardiac tissue and open the possibility to use high-throughput, low volume screening for studies on engineered myocardium.

Cardiac regeneration

The heart is a pump that is comprised of cardiac myocytes and other cell types and whose proper function is critical to quality of life. The ability to trigger regeneration of heart muscle following injury eludes adult mammals, a deficiency of great clinical impact. Major research efforts are attempting to change this through advances in cell therapy or activating endogenous regenerative mechanisms that exist only early in life. In contrast with mammals, lower vertebrates like zebrafish demonstrate an impressive natural capacity for cardiac regeneration throughout life. This review will cover recent progress in the field of heart regeneration with a focus on endogenous regenerative capacity and its potential manipulation.

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.

Ascorbic acid enhances the cardiac differentiation of induced pluripotent stem cells through promoting the proliferation of cardiac progenitor cells

Generation of induced pluripotent stem cells (iPSCs) has opened new avenues for the investigation of heart diseases, drug screening and potential autologous cardiac regeneration. However, their application is hampered by inefficient cardiac differentiation, high interline variability, and poor maturation of iPSC-derived cardiomyocytes (iPS-CMs). To identify efficient inducers for cardiac differentiation and maturation of iPSCs and elucidate the mechanisms, we systematically screened sixteen cardiomyocyte inducers on various murine (m) iPSCs and found that only ascorbic acid (AA) consistently and robustly enhanced the cardiac differentiation of eleven lines including eight without spontaneous cardiogenic potential. We then optimized the treatment conditions and demonstrated that differentiation day 2-6, a period for the specification of cardiac progenitor cells (CPCs), was a critical time for AA to take effect. This was further confirmed by the fact that AA increased the expression of cardiovascular but not mesodermal markers. Noteworthily, AA treatment led to approximately 7.3-fold (miPSCs) and 30.2-fold (human iPSCs) augment in the yield of iPS-CMs. Such effect was attributed to a specific increase in the proliferation of CPCs via the MEK-ERK1/2 pathway by through promoting collagen synthesis. In addition, AA-induced cardiomyocytes showed better sarcomeric organization and enhanced responses of action potentials and calcium transients to β-adrenergic and muscarinic stimulations. These findings demonstrate that AA is a suitable cardiomyocyte inducer for iPSCs to improve cardiac differentiation and maturation simply, universally, and efficiently. These findings also highlight the importance of stimulating CPC proliferation by manipulating extracellular microenvironment in guiding cardiac differentiation of the pluripotent stem cells.

Regulated expression and role of c-Myb in the cardiovascular-directed differentiation of mouse embryonic stem cells

RATIONALE

c-myb null (knockout) embryonic stem cells (ESC) can differentiate into cardiomyocytes but not contractile smooth muscle cells (SMC) in embryoid bodies (EB).

OBJECTIVE

To define the role of c-Myb in SMC differentiation from ESC.

METHODS AND RESULTS

In wild-type (WT) EB, high c-Myb levels on days 0-2 of differentiation undergo ubiquitin-mediated proteosomal degradation on days 2.5-3, resurging on days 4-6, without changing c-myb mRNA levels. Activin-A and bone morphogenetic protein 4-induced cardiovascular progenitors were isolated by FACS for expression of vascular endothelial growth factor receptor (VEGFR)2 and platelet-derived growth factor receptor (PDGFR)α. By day 3.75, hematopoesis-capable VEGFR2+ cells were fewer, whereas cardiomyocyte-directed VEGFR2+/PDGFRα+ cells did not differ in abundance in knockout versus WT EB. Importantly, highest and lowest levels of c-Myb were observed in VEGFR2+ and VEGFR2+/PDGFRα+ cells, respectively. Proteosome inhibitor MG132 and lentiviruses enabling inducible expression or knockdown of c-myb were used to regulate c-Myb in WT and knockout EB. These experiments showed that c-Myb promotes expression of VEGFR2 over PDGFRα, with chromatin immunopreciptation and promoter-reporter assays defining specific c-Myb-responsive binding sites in the VEGFR2 promoter. Next, FACS-sorted VEGFR2+ cells expressed highest and lowest levels of SMC- and fibroblast-specific markers, respectively, at days 7-14 after retinoic acid (RA) as compared with VEGFR2+/PDGFRα+ cells. By contrast, VEGFR2+/PDGFRα+ cells cultured without RA beat spontaneously, like cardiomyocytes between days 7 and 14, and expressed cardiac troponin. Notably, RA was required to more fully differentiate SMC from VEGFR2+ cells and completely blocked differentiation of cardiomyocytes from VEGFR2+/PDGFRα+ cells.

CONCLUSIONS

c-Myb is tightly regulated by proteosomal degradation during cardiovascular-directed differentiation of ESC, expanding early-stage VEGFR2+ progenitors capable of RA-responsive SMC formation.

Induced pluripotent stem cells--opportunities for disease modelling and drug discovery

The ability to generate induced pluripotent stem cells (iPSCs) from patients, and an increasingly refined capacity to differentiate these iPSCs into disease-relevant cell types, promises a new paradigm in drug development - one that positions human disease pathophysiology at the core of preclinical drug discovery. Disease models derived from iPSCs that manifest cellular disease phenotypes have been established for several monogenic diseases, but iPSCs can likewise be used for phenotype-based drug screens in complex diseases for which the underlying genetic mechanism is unknown. Here, we highlight recent advances as well as limitations in the use of iPSC technology for modelling a 'disease in a dish' and for testing compounds against human disease phenotypes in vitro. We discuss how iPSCs are being exploited to illuminate disease pathophysiology, identify novel drug targets and enhance the probability of clinical success of new drugs.

Primitive cardiac cells from human embryonic stem cells

Pluripotent stem cell-derived cardiomyocytes are currently being investigated for in vitro human heart models and as potential therapeutics for heart failure. In this study, we have developed a differentiation protocol that minimizes the need for specific human embryonic stem cell (hESC) line optimization. We first reduced the heterogeneity that exists within the starting population of bulk cultured hESCs by using cells adapted to single-cell passaging in a 2-dimensional (2D) culture format. Compared with bulk cultures, single-cell cultures comprised larger fractions of TG30(hi)/OCT4(hi) cells, corresponding to an increased expression of pluripotency markers OCT4 and NANOG, and reduced expression of early lineage-specific markers. A 2D temporal differentiation protocol was then developed, aimed at reducing the inherent heterogeneity and variability of embryoid body-based protocols, with induction of primitive streak cells using bone morphogenetic protein 4 and activin A, followed by cardiogenesis via inhibition of Wnt signaling using the small molecules IWP-4 or IWR-1. IWP-4 treatment resulted in a large percentage of cells expressing low amounts of cardiac myosin heavy chain and expression of early cardiac progenitor markers ISL1 and NKX2-5, thus indicating the production of large numbers of immature cardiomyocytes (~65,000/cm(2) or ~1.5 per input hESC). This protocol was shown to be effective in HES3, H9, and, to a lesser, extent, MEL1 hESC lines. In addition, we observed that IWR-1 induced predominantly atrial myosin light chain (MLC2a) expression, whereas IWP-4 induced expression of both atrial (MLC2a) and ventricular (MLC2v) forms. The intrinsic flexibility and scalability of this 2D protocol mean that the output population of primitive cardiomyocytes will be particularly accessible and useful for the investigation of molecular mechanisms driving terminal cardiomyocyte differentiation, and potentially for the future treatment of heart failure.

SIRPA is a specific cell-surface marker for isolating cardiomyocytes derived from human pluripotent stem cells

To identify cell-surface markers specific to human cardiomyocytes, we screened cardiovascular cell populations derived from human embryonic stem cells (hESCs) against a panel of 370 known CD antibodies. This screen identified the signal-regulatory protein alpha (SIRPA) as a marker expressed specifically on cardiomyocytes derived from hESCs and human induced pluripotent stem cells (hiPSCs), and PECAM, THY1, PDGFRB and ITGA1 as markers of the nonmyocyte population. Cell sorting with an antibody against SIRPA allowed for the enrichment of cardiac precursors and cardiomyocytes from hESC/hiPSC differentiation cultures, yielding populations of up to 98% cardiac troponin T-positive cells. When plated in culture, SIRPA-positive cells were contracting and could be maintained over extended periods of time. These findings provide a simple method for isolating populations of cardiomyocytes from human pluripotent stem cell cultures, and thereby establish a readily adaptable technology for generating large numbers of enriched cardiomyocytes for therapeutic applications.

Engineered tissue grafts: opportunities and challenges in regenerative medicine

The human body has limited regenerative capacity in most of the major tissues and organs. This fact has spurred the field of regenerative medicine, promising to repair damage following traumatic injury or disease. Multiple therapeutic strategies are being explored including small molecules, gene delivery, and stem cells; however, tissue engineering remains a primary approach to achieving regeneration. Organ transplantation demonstrates that damaged tissues can be replaced, but technology to regenerate complex organs de novo is not yet available. Instead, tissue engineering can augment the body's own regenerative ability by replacing tissue sections and enhancing the regenerative cascade. As a consequence of these opportunities, it is timely to review the criteria and current status of engineered tissue grafts designed as patches to replace or regenerate damaged or diseased tissue and restore organ function. This topic will be explored starting from the biomaterials and cells incorporated into the engineered graft, the environment into which the graft is implanted and the integration of the engineered graft with the host. Common issues will be addressed that are relevant to regeneration in multiple tissue and organ systems. Specific examples will focus on engineered grafts for myocardial and corneal repair to illustrate the tissue-specific challenges and opportunities and highlight the innovation needed as the field moves forward.

The cardiogenic niche as a fundamental building block of engineered myocardium

Cardiac muscle engineering is evolving rapidly, aiming at the provision of innovative models for drug development and therapeutic myocardium. The progress in this field will depend crucially on the proper exploitation of stem cell technologies. Understanding the processes governing stem cell differentiation towards a desired phenotype and subsequent maturation in an organotypic manner will be key to ultimately providing realistic tissue models or therapeutics. Cardiogenesis is controlled by milieu factors that collectively constitute a so-called cardiogenic niche. The components of the cardiogenic niche are not yet fully defined but include paracrine factors and instructive extracellular matrix. Both are provided by supportive stromal cells under strict spatial and temporal control. Detailed knowledge on the exact composition and functionality of the dynamic cardiogenic niche during development will likely be instrumental to further advance cardiac muscle engineering. This review will discuss the concept of myocardial tissue engineering from the stem cell/developmental biology perspective and put forward the hypothesis of the cardiogenic niche as a fundamental building block of tissue-engineered myocardium.

Optimizing dynamic interactions between a cardiac patch and inflammatory host cells

Damaged heart muscle has only a minimal ability for regeneration following myocardial infarction in which cardiomyocytes are lost to ischemia. The most clinically promising approach to regeneration of cardiac muscle currently under investigation is that of injecting cardiogenic repair cells or implanting a preformed tissue-engineered patch. While major advances are being made in the derivation of functional human cardiomyocytes and the development of tissue-engineering modalities for cardiac repair, the host environment into which the repair cells are placed is largely overlooked. Within seconds of myocardial ischemia, hypoxia sets in in the myocardium and the inflammatory response starts, characterized by rapid deployment of circulating cells and the release of paracrine and autocrine signals. Therefore, the inflammatory conditions under which these interactions take place, the design of the scaffold material used, and the maturity of the implanted cells will determine the outcomes of any stem cell-based therapy. We discuss here the interactions between implanted and inflammatory cells of the host, which are critical for the design of effective heart repair therapies.

Collagen scaffolds with or without the addition of RGD peptides support cardiomyogenesis after aggregation of mouse embryonic stem cells

Embryonic stem (ES) cell-based cardiac muscle repair using tissue-engineered scaffolds is an attractive prospective treatment option for patients suffering from heart disease. In this study, our aim was to characterize mouse ES cell-derived cardiomyocytes growing on collagen I/III scaffolds, modified with the adhesion peptides arginine-glycine-aspartic acid (RGD). Mouse ES-derived embryoid bodies (EBs) differentiated efficiently into beating cardiomyocytes on the collagen scaffolds. QPCR analysis and immunofluorescent staining showed that cardiomyocytes expressed cardiac muscle-related transcripts and proteins. Analysis of cardiomyocytes by electron microscopy identified muscle fiber bundles and Z bands, typical of ES-derived cardiomyocytes. No differences were detected between the collagen + RGD and collagen control scaffolds. ES cells that were not differentiated as EBs prior to seeding on the scaffold, did not differentiate into cardiomyocytes. These results indicate that a collagen I/III scaffold supports cardiac muscle development and function after EB formation, and that this scaffold appears suitable for future in vivo testing. The addition of the RGD domain to the collagen scaffold did not improve cardiomyocyte development or viability, indicating that RGD signaling to integrins was not a rate-limiting event for cardiomyogenesis from EBs seeded on a collagen scaffold.

Biomaterials to Enhance Stem Cell Function in the Heart

Transplantation of stem cells into the heart can improve cardiac function after myocardial infarction and in chronic heart failure, but the extent of benefit and of reproducibility of this approach are insufficient. Survival of transplanted cells into myocardium is poor, and new strategies are needed to enhance stem cell differentiation and survival in vivo. In this review, we describe how biomaterials can enhance stem cell function in the heart. Biomaterials can mimic or include naturally occurring extracellular matrix and also instruct stem cell function in different ways. Biomaterials can promote angiogenesis, enhance engraftment and differentiation of stem cells, and accelerate electromechanical integration of transplanted stem cells. Biomaterials can also be used to deliver proteins, genes, or small RNAs together with stem cells. Furthermore, recent evidence indicates that the biophysical environment of stem cells is crucial for their proliferation and differentiation, as well as their electromechanical integration. Many approaches in regenerative medicine will likely ultimately require integration of molecularly designed biomaterials and stem cell biology to develop stable tissue regeneration.

Efficient differentiation of human pluripotent stem cells into mesenchymal stem cells by modulating intracellular signaling pathways in a feeder/serum-free system

Mesenchymal stem cells (MSCs) derived from human pluripotent stem cells (hPSC-derived MSCs) will be one promising alternative cell source for MSC-based therapies. Here, an efficient protocol is demonstrated for generating hPSC-derived MSCs under a feeder-free culture system by regulating signaling pathways. Simultaneous treatments with Activin A, BIO (6-bromoindirubin-3'-oxime), and bone morphogenetic protein 4 (ABB) activated the transcription of mesoderm-lineage genes such as T, MIXL1, and WNT3 in hPSCs. The ABB-treated hPSCs could develop into CD105(+) cells with a high efficiency of 20% in the MSC-induction medium. The properties of the hPSC-derived CD105(+) cells were similar to those of adult MSCs in terms of surface antigens. Also, hPSC-derived MSCs had the potential to differentiate into adipocytes, osteoblasts, and chondrocytes in vitro. The results demonstrated that functional MSCs could be generated efficiently from hPSCs by the combined modulation of signaling pathways.

Regenerative chemical biology: current challenges and future potential

The enthusiasm surrounding the clinical potential of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) is tempered by the fact that key issues regarding their safety, efficacy, and long-term benefits have thus far been suboptimal. Small molecules can potentially relieve these problems at major junctions of stem cell biology and regenerative therapy. In this review we will introduce recent advances in these important areas and the first generation of small molecules used in the regenerative context. Current chemical biology studies will provide the archetype for future interdisciplinary collaborations and improve clinical benefits of cell-based therapies.

Efficient and scalable purification of cardiomyocytes from human embryonic and induced pluripotent stem cells by VCAM1 surface expression

Rationale

Human embryonic and induced pluripotent stem cells (hESCs/hiPSCs) are promising cell sources for cardiac regenerative medicine. To realize hESC/hiPSC-based cardiac cell therapy, efficient induction, purification, and transplantation methods for cardiomyocytes are required. Though marker gene transduction or fluorescent-based purification methods have been reported, fast, efficient and scalable purification methods with no genetic modification are essential for clinical purpose but have not yet been established. In this study, we attempted to identify cell surface markers for cardiomyocytes derived from hESC/hiPSCs.

Method and Result

We adopted a previously reported differentiation protocol for hESCs based on high density monolayer culture to hiPSCs with some modification. Cardiac troponin-T (TNNT2)-positive cardiomyocytes appeared robustly with 30–70% efficiency. Using this differentiation method, we screened 242 antibodies for human cell surface molecules to isolate cardiomyocytes derived from hiPSCs and identified anti-VCAM1 (Vascular cell adhesion molecule 1) antibody specifically marked cardiomyocytes. TNNT2-positive cells were detected at day 7–8 after induction and 80% of them became VCAM1-positive by day 11. Approximately 95–98% of VCAM1-positive cells at day 11 were positive for TNNT2. VCAM1 was exclusive with CD144 (endothelium), CD140b (pericytes) and TRA-1-60 (undifferentiated hESCs/hiPSCs). 95% of MACS-purified cells were positive for TNNT2. MACS purification yielded 5−10×10⁵ VCAM1-positive cells from a single well of a six-well culture plate. Purified VCAM1-positive cells displayed molecular and functional features of cardiomyocytes. VCAM1 also specifically marked cardiomyocytes derived from other hESC or hiPSC lines.

Conclusion

We succeeded in efficiently inducing cardiomyocytes from hESCs/hiPSCs and identifying VCAM1 as a potent cell surface marker for robust, efficient and scalable purification of cardiomyocytes from hESC/hiPSCs. These findings would offer a valuable technological basis for hESC/hiPSC-based cell therapy.

Doing the dirty work: progress in the search for a reliable protocol for cardiomyogenesis

Cardiomyocytes generated from pluripotent stem cells have the potential to facilitate our understanding of cardiac diseases and help in their treatment. However, realizing the full potential of this technology is limited by the field's inability to efficiently and reliably differentiate pluripotent stem cells into cardiomyocytes. But now, due to a massive undertaking by Burridge and colleagues in a recent issue of PLOS One, we are one step closer to the reliable creation of cardiomyocytes. By systematically addressing 45 different variables, Burridge and colleagues were able to develop a protocol that consistently produced cardiomyocytes independent of the cell source. However, work still needs to be done to increase the efficiency and definitively determine the maturity of the myocytes generated through such a protocol.

Induction of cardiomyogenesis in human embryonic stem cells by human embryonic stem cell-derived definitive endoderm

We previously reported that chick anterolateral endoderm (AL endoderm) induces cardiomyogenesis in mouse embryoid bodies. However, the requirement to micro-dissect AL endoderm from gastrulation-stage embryos precludes its use to identify novel cardiomyogenic factors, or to scale up cardiomyocyte numbers for therapeutic experiments. To circumvent this problem we have addressed whether human definitive endoderm (hDE) cells, which can be efficiently generated in large numbers from human embryonic stem cells (hESCs), can mimic the ability of AL endoderm to induce cardiac myogenesis. Results demonstrate that both hDE cells and medium conditioned by them induce cardiac myogenesis in pluripotent hESCs, as indicated by rhythmic beating and immunohistochemical/quantitative polymerase chain reaction monitoring of marker gene expression. The cardiomyogenic effect of hDE is enhanced when pluripotent hESCs are preinduced to the mes-endoderm state. Because this approach is tractable and scalable, it may facilitate identification of novel hDE-secreted factors for inclusion in defined cardiomyogenic cocktails.

Small-molecule inhibitors of the Wnt pathway potently promote cardiomyocytes from human embryonic stem cell-derived mesoderm

RATIONALE

Human embryonic stem cells can form cardiomyocytes when cultured under differentiation conditions. Although the initiating step of mesoderm formation is well characterized, the subsequent steps that promote for cardiac lineages are poorly understood and limit the yield of cardiomyocytes.

OBJECTIVE

Our aim was to develop a human embryonic stem cell-based high-content screening assay to discover small molecules that drive cardiogenic differentiation after mesoderm is established to improve our understanding of the biology involved. Screening of libraries of small-molecule pathway modulators was predicted to provide insight into the cellular proteins and signaling pathways that control stem cell cardiogenesis.

METHODS AND RESULTS

Approximately 550 known pathway modulators were screened in a high-content screening assay, with hits being called out by the appearance of a red fluorescent protein driven by the promoter of the cardiac-specific MYH6 gene. One potent small molecule was identified that inhibits transduction of the canonical Wnt response within the cell, which demonstrated that Wnt inhibition alone was sufficient to generate cardiomyocytes from human embryonic stem cell-derived mesoderm cells. Transcriptional profiling of inhibitor-treated compared with vehicle-treated samples further indicated that inhibition of Wnt does not induce other mesoderm lineages. Notably, several other Wnt inhibitors were very efficient in inducing cardiogenesis, including a molecule that prevents Wnts from being secreted by the cell, which confirmed that Wnt inhibition was the relevant biological activity.

CONCLUSIONS

Pharmacological inhibition of Wnt signaling is sufficient to drive human mesoderm cells to form cardiomyocytes; this could yield novel tools for the benefit of pharmaceutical and clinical applications.

Chromatin insulator elements block transgene silencing in engineered human embryonic stem cell lines at a defined chromosome 13 locus

Lineage reporters of human embryonic stem cell (hESC) lines are useful for differentiation studies and drug screening. Previously, we created reporter lines driven by an elongation factor 1 alpha (EF1α) promoter at a chromosome 13q32.3 locus in the hESC line WA09 and an abnormal hESC line BG01V in a site-specific manner. Expression of reporters in these lines was maintained in long-term culture at undifferentiated state. However, when these cells were differentiated into specific lineages, reduction in reporter expression was observed, indicating transgene silencing. To develop an efficient and reliable genetic engineering strategy in hESCs, we used chromatin insulator elements to flank single-copy transgenes and integrated the combined expression constructs via PhiC31/R4 integrase-mediated recombination technology to the chromosome 13 locus precisely. Two copies of cHS4 double-insulator sequences were placed adjacent to both 5' and 3' of the promoter reporter constructs. The green fluorescent protein (GFP) gene was driven by EF1α or CMV early enhancer/chicken β actin (CAG) promoter. In the engineered hESC lines, for both insulated CAG-GFP and EF1α-GFP, constitutive expression at the chromosome 13 locus was maintained during prolonged culture and in directed differentiation assays toward diverse types of neurons, pancreatic endoderm, and mesodermal progeny. In particular, described here is the first normal hESC fluorescent reporter line that robustly expresses GFP in both the undifferentiated state and throughout dopaminergic lineage differentiation. The dual strategy of utilizing insulator sequences and integration at the constitutive chromosome 13 locus ensures appropriate transgene expression. This is a valuable tool for lineage development study, gain- and loss-of-function experiments, and human disease modeling using hESCs.

Distinct regulation of mitogen-activated protein kinase activities is coupled with enhanced cardiac differentiation of human embryonic stem cells

Improving cardiac differentiation of human pluripotent stem cells is mandatory to provide functional heart muscle cells for novel therapies. Here, we have investigated the enhancing effect of the small molecule SB203580, a p38 MAPK inhibitor, on cardiomyogenesis in hESC by monitoring the phosphorylation patterns of the major MAPK pathway components p38, JNK and ERK by western immunoblotting. A remarkable drop in phosphorylation levels of all three MAPK pathways was induced after overnight embryoid body (EB) formation. Upon further differentiation, phosphorylation dynamics in EBs were specifically altered by distinct inhibitor concentrations. At 5μM of SB203580, cardiomyogenesis was most efficient and associated with the expected p38 pathway inhibition. In parallel, JNK activation was observed suggesting a regulatory interlink between these pathways in hESC ultimately supporting cardiac differentiation. In contrast, moderately elevated SB203580 concentrations (15-30μM) resulted in complete disruption of cardiomyogenesis which was associated with prominent inhibition of ERK and further elevated JNK activity. We propose that a tightly-balanced pattern in MAPK phosphorylation is important for early mesoderm and subsequent cardiomyocyte formation. Our data provide novel insights into molecular consequences of small molecule supplementation in hESC differentiation, emphasizing the role of MAPK-signaling.

Harnessing the potential of induced pluripotent stem cells for regenerative medicine

The discovery of methods to convert somatic cells into induced pluripotent stem cells (iPSCs) through expression of a small combination of transcription factors has raised the possibility of producing custom-tailored cells for the study and treatment of numerous diseases. Indeed, iPSCs have already been derived from patients suffering from a large variety of disorders. Here we review recent progress that has been made in establishing iPSC-based disease models, discuss associated technical and biological challenges, and highlight possible solutions to overcome these barriers. We believe that a better understanding of the molecular basis of pluripotency, cellular reprogramming and lineage-specific differentiation of iPSCs is necessary for progress in regenerative medicine.