Wnt, activin, and BMP signaling regulate distinct stages in the developmental pathway from embryonic stem cells to blood

Early cardiac development: a view from stem cells to embryos

From the 1920s, early cardiac development has been studied in chick and, later, in mouse embryos in order to understand the first cell fate decisions that drive specification and determination of the endocardium, myocardium, and epicardium. More recently, mouse and human embryonic stem cells (ESCs) have demonstrated faithful recapitulation of early cardiogenesis and have contributed significantly to this research over the past few decades. Derived almost 15 years ago, human ESCs have provided a unique developmental model for understanding the genetic and epigenetic regulation of early human cardiogenesis. Here, we review the biological concepts underlying cell fate decisions during early cardiogenesis in model organisms and ESCs. We draw upon both pioneering and recent studies and highlight the continued role for in vitro stem cells in cardiac developmental biology.

Activin A and BMP4 Signaling for Efficient Cardiac Differentiation of H7 and H9 Human Embryonic Stem Cells

The generation of cardiomyocytes from human embryonic stem cells (hESC) enables a variety of potential therapeutic and diagnostic applications. However, progress is challenged by the low efficiency of cardiomyocyte differentiation. Recently, Kattman et al., 2011 showed that individual hESC lines required proper balance of the Activin A and BMP4 signaling for efficient cardiac differentiation, presenting their differentiation protocols for several human and mouse ESC lines. However, two of the most utilized hESC lines, H7 and H9, were not included. Therefore, we set out to verify the published methodology for highly efficient cardiac specification and investigate the cardiac differentiation in the H7 and H9 ESC lines. Our studies examined a range of time points for the initial culture of hESC as embryoid bodies (EB) prior to transfer to monolayer culture, as well as, concentrations of Activin A and BMP4 in the medium formulations. The results highlight an efficient protocol for reproducibly generating cultures with approximately 50% cardiomyocytes from H7 and H9 ESC lines.

Targeting Wnt pathways in disease

Wnt-mediated signal transduction pathways have long been recognized for their roles in regulating embryonic development, and have more recently been linked to cancer, neurologic diseases, inflammatory diseases, and disorders of endocrine function and bone metabolism in adults. Although therapies targeting Wnt signaling are attractive in theory, in practice it has been difficult to obtain specific therapeutics because many components of Wnt signaling pathways are also involved in other cellular processes, thereby reducing the specificity of candidate therapeutics. New technologies, and advances in understanding the mechanisms of Wnt signaling, have improved our understanding of the nuances of Wnt signaling and are leading to promising new strategies to target Wnt signaling pathways.

In vitro modeling of paraxial mesodermal progenitors derived from induced pluripotent stem cells

Induced pluripotent stem (iPS) cells are generated from adult somatic cells by transduction of defined factors. Given their unlimited proliferation and differentiation potential, iPS cells represent promising sources for cell therapy and tools for research and drug discovery. However, systems for the directional differentiation of iPS cells toward paraxial mesodermal lineages have not been reported. In the present study, we established a protocol for the differentiation of mouse iPS cells into paraxial mesodermal lineages in serum-free culture. The protocol was dependent on Activin signaling in addition to BMP and Wnt signaling which were previously shown to be effective for mouse ES cell differentiation. Independently of the cell origin, the number of transgenes, or the type of vectors used to generate iPS cells, the use of serum-free monolayer culture stimulated with a combination of BMP4, Activin A, and LiCl enabled preferential promotion of mouse iPS cells to a PDGFR-α⁺/Flk-1⁻ population, which represents a paraxial mesodermal lineage. The mouse iPS cell-derived paraxial mesodermal cells exhibited differentiation potential into osteogenic, chondrogenic, and myogenic cells both in vitro and in vivo and contributed to muscle regeneration. Moreover, purification of the PDGFR-α⁺/KDR⁻ population after differentiation allowed enrichment of human iPS cell populations with paraxial mesodermal characteristics. The resultant PDGFR-α⁺/KDR⁻ population derived from human iPS cells specifically exhibited osteogenic, chondrogenic, and myogenic differentiation potential in vitro, implying generation of paraxial mesodermal progenitors similar to mouse iPS cell-derived progenitors. These findings highlight the potential of protocols based on the serum-free, stepwise induction and purification of paraxial mesodermal cell lineages for use in stem cell therapies to treat diseased bone, cartilage, and muscle.

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.

Getting blood from bone: an emerging understanding of the role that osteoblasts play in regulating hematopoietic stem cells within their niche

Blood and bone are dynamic tissues that are continuously renewed throughout life. Early observations based upon the proximity of bone and hematopoietic progenitor populations in marrow suggested that interactions between skeletal and hematopoietic elements are likely to be crucial in the development and function of each system. As a result of these morphologic observations, several groups have demonstrated that the osteoblasts play an important role in hematopoiesis by serving as a specific local microenvironment, or niche, for hematopoietic stem cells. Significant new developments in this area of active investigation have emerged since our last examination of this area in 2005. Here we discuss these new insights into the function and morphology of the hematopoietic stem cell niche, with a particular focus on cells of the osteoblastic lineage.

Efficient generation, purification, and expansion of CD34(+) hematopoietic progenitor cells from nonhuman primate-induced pluripotent stem cells

Induced pluripotent stem cell (iPSC) therapeutics are a promising treatment for genetic and infectious diseases. To assess engraftment, risk of neoplastic formation, and therapeutic benefit in an autologous setting, testing iPSC therapeutics in an appropriate model, such as the pigtail macaque (Macaca nemestrina; Mn), is crucial. Here, we developed a chemically defined, scalable, and reproducible specification protocol with bone morphogenetic protein 4, prostaglandin-E2 (PGE2), and StemRegenin 1 (SR1) for hematopoietic differentiation of Mn iPSCs. Sequential coculture with bone morphogenetic protein 4, PGE2, and SR1 led to robust Mn iPSC hematopoietic progenitor cell formation. The combination of PGE2 and SR1 increased CD34(+)CD38(-)Thy1(+)CD45RA(-)CD49f(+) cell yield by 6-fold. CD34(+)CD38(-)Thy1(+)CD45RA(-)CD49f(+) cells isolated on the basis of CD34 expression and cultured in SR1 expanded 3-fold and maintained this long-term repopulating HSC phenotype. Purified CD34(high) cells exhibited 4-fold greater hematopoietic colony-forming potential compared with unsorted hematopoietic progenitors and had bilineage differentiation potential. On the basis of these studies, we calculated the cell yields that must be achieved at each stage to meet a threshold CD34(+) cell dose that is required for engraftment in the pigtail macaque. Our protocol will support scale-up and testing of iPSC-derived CD34(high) cell therapies in a clinically relevant nonhuman primate model.

Hematopoietic stem cell development requires transient Wnt/β-catenin activity

Understanding how hematopoietic stem cells (HSCs) are generated and the signals that control this process is a crucial issue for regenerative medicine applications that require in vitro production of HSC. HSCs emerge during embryonic life from an endothelial-like cell population that resides in the aorta-gonad-mesonephros (AGM) region. We show here that β-catenin is nuclear and active in few endothelial nonhematopoietic cells closely associated with the emerging hematopoietic clusters of the embryonic aorta during mouse development. Importantly, Wnt/β-catenin activity is transiently required in the AGM to generate long-term HSCs and to produce hematopoietic cells in vitro from AGM endothelial precursors. Genetic deletion of β-catenin from the embryonic endothelium stage (using VE-cadherin-Cre recombinase), but not from embryonic hematopoietic cells (using Vav1-Cre), precludes progression of mutant cells toward the hematopoietic lineage; however, these mutant cells still contribute to the adult endothelium. Together, those findings indicate that Wnt/β-catenin activity is needed for the emergence but not the maintenance of HSCs in mouse embryos.

Stem cell sources for vascular tissue engineering and regeneration

This review focuses on the stem cell sources with the potential to be used in vascular tissue engineering and to promote vascular regeneration. The first clinical studies using tissue-engineered vascular grafts are already under way, supporting the potential of this technology in the treatment of cardiovascular and other diseases. Despite progress in engineering biomaterials with the appropriate mechanical properties and biological cues as well as bioreactors for generating the correct tissue microenvironment, the source of cells that make up the vascular tissues remains a major challenge for tissue engineers and physicians. Mature cells from the tissue of origin may be difficult to obtain and suffer from limited proliferative capacity, which may further decline as a function of donor age. On the other hand, multipotent and pluripotent stem cells have great potential to provide large numbers of autologous cells with a great differentiation capacity. Here, we discuss the adult multipotent as well as embryonic and induced pluripotent stem cells, their differentiation potential toward vascular lineages, and their use in engineering functional and implantable vascular tissues. We also discuss the associated challenges that need to be addressed in order to facilitate the transition of this technology from the bench to the bedside.

Primitive erythropoiesis is regulated by miR-126 via nonhematopoietic Vcam-1+ cells

Primitive erythropoiesis defines the onset of hematopoiesis in the yolk sac of the early embryo and is initiated by the emergence of progenitors assayed as colony-forming cells (EryP-CFCs). EryP-CFCs are detected for only a narrow window during embryonic development, suggesting that both their initiation and termination are tightly controlled. Using the embryonic stem differentiation system to model primitive erythropoiesis, we found that miR-126 regulates the termination of EryP-CFC development. Analyses of miR-126 null embryos revealed that this miR also regulates EryP-CFCs in vivo. We identified vascular cell adhesion molecule-1 (Vcam-1) expressed by a mesenchymal cell population as a relevant target of miR-126. Interaction of EryP-CFCs with Vcam-1 accelerated their maturation to ßh1-globin(+) and Ter119(+) cells through a Src family kinase. These findings uncover a cell nonautonomous regulatory pathway for primitive erythropoiesis that may provide insight into the mechanism(s) controlling the developmental switch from primitive to definitive hematopoiesis.

Mechanism of action of HOXB4 on the hematopoietic differentiation of embryonic stem cells

Pluripotent stem cells can be differentiated into hematopoietic lineages in vitro and hold promise for the future treatment of hematological disease. Differentiation strategies involving defined factors in serum-free conditions have been successful in producing hematopoietic progenitors and some mature cell types from mouse and human embryonic stem cells and induced pluripotent cells. However, these precisely defined protocols are relatively inefficient and have not been used successfully to produce hematopoietic stem cells capable of multilineage long-term reconstitution of the hematopoietic system. More complex differentiation induction strategies including coculture with stromal cells derived from sites of hematopoietic activity in vivo and enforced expression of reprogramming transcription factors, such as HOXB4, have been required to increase the efficiency of the differentiation procedure and to produce these most potent hematopoietic stem cells. We review the studies that have used HOXB4 to improve hematopoietic differentiation from pluripotent cells focusing on studies that have provided some insight into its mechanism of action. A better understanding of the molecular pathways involved in the action of HOXB4 might lead to more defined culture systems and safer protocols for clinical translation.

Activin A promotes hematopoietic fated mesoderm development through upregulation of brachyury in human embryonic stem cells

The development of the hematopoietic system involves multiple cellular steps beginning with the formation of the mesoderm from the primitive streak, followed by emergence of precursor populations that become committed to either the endothelial or hematopoietic lineages. A number of growth factors such as activins and fibroblast growth factors (FGFs) are known to regulate the early specification of hematopoietic fated mesoderm, notably in amphibians. However, the potential roles of these factors in the development of mesoderm and subsequent hematopoiesis in the human have yet to be delineated. Defining the cellular and molecular mechanisms by which combinations of mesoderm-inducing factors regulate this stepwise process in human cells in vitro is central to effectively directing human embryonic stem cell (hESC) hematopoietic differentiation. Herein, using hESC-derived embryoid bodies (EBs), we show that Activin A, but not basic FGF/FGF2 (bFGF), promotes hematopoietic fated mesodermal specification from pluripotent human cells. The effect of Activin A treatment relies on the presence of bone morphogenetic protein 4 (BMP4) and both of the hematopoietic cytokines stem cell factor and fms-like tyrosine kinase receptor-3 ligand, and is the consequence of 2 separate mechanisms occurring at 2 different stages of human EB development from mesoderm to blood. While Activin A promotes the induction of mesoderm, as indicated by the upregulation of Brachyury expression, which represents the mesodermal precursor required for hematopoietic development, it also contributes to the expansion of cells already committed to a hematopoietic fate. As hematopoietic development requires the transition through a Brachyury+ intermediate, we demonstrate that hematopoiesis in hESCs is impaired by the downregulation of Brachyury, but is unaffected by its overexpression. These results demonstrate, for the first time, the functional significance of Brachyury in the developmental program of hematopoietic differentiation from hESCs and provide an in-depth understanding of the molecular cues that orchestrate stepwise development of hematopoiesis in a human system.

The embryonic origins of erythropoiesis in mammals

Erythroid (red blood) cells are the first cell type to be specified in the postimplantation mammalian embryo and serve highly specialized, essential functions throughout gestation and postnatal life. The existence of 2 developmentally and morphologically distinct erythroid lineages, primitive (embryonic) and definitive (adult), was described for the mammalian embryo more than a century ago. Cells of the primitive erythroid lineage support the transition from rapidly growing embryo to fetus, whereas definitive erythrocytes function during the transition from fetal life to birth and continue to be crucial for a variety of normal physiologic processes. Over the past few years, it has become apparent that the ontogeny and maturation of these lineages are more complex than previously appreciated. In this review, we highlight some common and distinguishing features of the red blood cell lineages and summarize advances in our understanding of how these cells develop and differentiate throughout mammalian ontogeny.

Systematic analysis of embryonic stem cell differentiation in hydrodynamic environments with controlled embryoid body size

The sensitivity of stem cells to environmental perturbations has prompted many studies which aim to characterize the influence of mechanical factors on stem cell morphogenesis and differentiation. Hydrodynamic cultures, often employed for large scale bioprocessing applications, impart complex fluid shear and transport profiles, and influence cell fate as a result of changes in media mixing conditions. However, previous studies of hydrodynamic cultures have been limited in their ability to distinguish confounding factors that may affect differentiation, including modulation of embryoid body size in response to changes in the hydrodynamic environment. In this study, we demonstrate the ability to control and maintain embryoid body (EB) size using a combination of forced aggregation formation and rotary orbital suspension culture, in order to assess the impact of hydrodynamic cultures on ESC differentiation, independent of EB size. Size-controlled EBs maintained at different rotary orbital speeds exhibited similar morphological features and gene expression profiles, consistent with ESC differentiation. The similar differentiation of ESCs across a range of hydrodynamic conditions suggests that controlling EB formation and resultant size may be important for scalable bioprocessing applications, in order to standardize EB morphogenesis. However, perturbations in the hydrodynamic environment also led to subtle changes in differentiation toward certain lineages, including temporal modulation of gene expression, as well changes in the relative efficiencies of differentiated phenotypes, thereby highlighting important tissue engineering principles that should be considered for implementation in bioreactor design, as well as for directed ESC differentiation.

Heparan sulfate facilitates FGF and BMP signaling to drive mesoderm differentiation of mouse embryonic stem cells

Heparan sulfate (HS) has been implicated in regulating cell fate decisions during differentiation of embryonic stem cells (ESCs) into advanced cell types. However, the necessity and the underlying molecular mechanisms of HS in early cell lineage differentiation are still largely unknown. In this study, we examined the potential of EXT1(-/-) mouse ESCs (mESCs), that are deficient in HS, to differentiate into primary germ layer cells. We observed that EXT1(-/-) mESCs lost their differentiation competence and failed to differentiate into Pax6(+)-neural precursor cells and mesodermal cells. More detailed analyses highlighted the importance of HS for the induction of Brachyury(+) pan-mesoderm as well as normal gene expression associated with the dorso-ventral patterning of mesoderm. Examination of developmental cell signaling revealed that EXT1 ablation diminished FGF and BMP but not Wnt signaling. Furthermore, restoration of FGF and BMP signaling each partially rescued mesoderm differentiation defects. We further show that BMP4 is more prone to degradation in EXT1(-/-) mESCs culture medium compared with that of wild type cells. Therefore, our data reveal that HS stabilizes BMP ligand and thereby maintains the BMP signaling output required for normal mesoderm differentiation. In summary, our study demonstrates that HS is required for ESC pluripotency, in particular lineage specification into mesoderm through facilitation of FGF and BMP signaling.

Modeling human hematopoietic cell development from pluripotent stem cells

Understanding the steps and cues that allow hematopoietic cells to be generated during development holds great clinical as well as biological interest. Analysis of these events in mice has provided many important insights into the processes involved, but features that might be unique to humans remain challenging to elucidate because they cannot be studied directly in vivo. Human embryonic stem or induced pluripotent stem cells offer attractive in vitro alternatives to analyze the process. Here we review recent efforts to develop defined and quantitative systems to address outstanding developmental questions against a background of what we know about the development of hematopoietic cells in the fetus and derived from mouse embryonic stem cells.

Integration of BMP, Wnt, and notch signaling pathways in osteoblast differentiation

Bone marrow-derived mesenchymal stem cells (MSCs) are multipotent progenitors that can commit to osteoblast, chondrocyte, adipocyte, and several other lineages. The proper utilization of stem cells for clinical applications requires an integrated understanding of multiple signal inputs that control maintenance of stemness, proliferation, commitment, and differentiation. Various signaling pathways have been implicated in the regulation of MSC differentiation; however, complexities of pathway interactions, as well as seemingly contradictory results in the literature, create an often confusing and disjointed knowledge base. Several recent publications explore the integration of signaling pathways such as BMP, Wnt, Notch, Hedgehog, and Fibroblast Growth Factors in MSC osteoblast differentiation. The transcription factor Cbfa1/Runx2 has been implicated in these pathways as a potential focal point for signaling integration. This review will outline the current understanding of these pathways and indicate where both spatiotemporal effects during differentiation and comparable experimental conditions need to be considered in order to clarify the outcome(s) of differing regulatory levels of these signaling pathways.

BRACHYURY and CDX2 mediate BMP-induced differentiation of human and mouse pluripotent stem cells into embryonic and extraembryonic lineages

BMP is thought to induce hESC differentiation toward multiple lineages including mesoderm and trophoblast. The BMP-induced trophoblast phenotype is a long-standing paradox in stem cell biology. Here we readdressed BMP function in hESCs and mouse epiblast-derived cells. We found that BMP4 cooperates with FGF2 (via ERK) to induce mesoderm and to inhibit endoderm differentiation. These conditions induced cells with high levels of BRACHYURY (BRA) that coexpressed CDX2. BRA was necessary for and preceded CDX2 expression; both genes were essential for expression not only of mesodermal genes but also of trophoblast-associated genes. Maximal expression of the latter was seen in the absence of FGF but these cells coexpressed mesodermal genes and moreover they differed in cell surface and epigenetic properties from placental trophoblast. We conclude that BMP induces human and mouse pluripotent stem cells primarily to form mesoderm, rather than trophoblast, acting through BRA and CDX2.

High-throughput screening assay for embryoid body differentiation of human embryonic stem cells

Serum-free human pluripotent stem cell media offer the potential to develop reproducible clinically applicable differentiation strategies and protocols. The vast array of possible growth factor and cytokine combinations for media formulations makes differentiation protocol optimization both labor and cost-intensive. This unit describes a 96-well plate, 4-color flow cytometry-based screening assay to optimize pluripotent stem cell differentiation protocols. We provide conditions both to differentiate human embryonic stem cells (hESCs) into the three primary germ layers, ectoderm, endoderm, and mesoderm, and to utilize flow cytometry to distinguish between them. This assay exhibits low inter-well variability and can be utilized to efficiently screen a variety of media formulations, reducing cost, incubator space, and labor. Protocols can be adapted to a variety of differentiation stages and lineages.

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.

HOXB4 can enhance the differentiation of embryonic stem cells by modulating the hematopoietic niche

Hematopoietic differentiation of embryonic stem cells (ESCs) in vitro has been used as a model to study early hematopoietic development, and it is well documented that hematopoietic differentiation can be enhanced by overexpression of HOXB4. HOXB4 is expressed in hematopoietic progenitor cells (HPCs) where it promotes self-renewal, but it is also expressed in the primitive streak of the gastrulating embryo. This led us to hypothesize that HOXB4 might modulate gene expression in prehematopoietic mesoderm and that this property might contribute to its prohematopoietic effect in differentiating ESCs. To test our hypothesis, we developed a conditionally activated HOXB4 expression system using the mutant estrogen receptor (ER(T2)) and showed that a pulse of HOXB4 prior to HPC emergence in differentiating ESCs led to an increase in hematopoietic differentiation. Expression profiling revealed an increase in the expression of genes associated with paraxial mesoderm that gives rise to the hematopoietic niche. Therefore, we considered that HOXB4 might modulate the formation of the hematopoietic niche as well as the production of hematopoietic cells per se. Cell mixing experiments supported this hypothesis demonstrating that HOXB4 activation can generate a paracrine as well as a cell autonomous effect on hematopoietic differentiation. We provide evidence to demonstrate that this activity is partly mediated by the secreted protein FRZB.

Differentiation of definitive endoderm from mouse embryonic stem cells

Efficient production of definitive endoderm from embryonic stem (ES) cells opens doors to the possibilities of differentiation of endoderm-derived tissues such as the intestines, pancreas, and liver that could address the needs of people with chronic diseases involving these organs. The lessons learned from developmental biology have contributed significantly to in vitro differentiation of definitive endoderm. Gastrulation, a process that results in the production of all three embryonic germ cell layers, definitive endoderm, mesoderm, and ectoderm, is an important step in embryonic development. Gastrulation occurs as a result of the events that are orchestrated by the signaling pathways involving Nodal, FGF, Wnt, and BMP. Understanding these signaling pathways has led to the introduction of key ingredients such as Activin A, FGF, Wnt, and BMP to the differentiation protocols that have been able to produce definitive endoderm from ES cells. Efficient production of definitive endoderm needs to meet the specific criteria that include (a) increase in the production of markers of definitive endoderm such as Sox 17, FOXA2, GSC, and Mixl1; (b) decrease in the production of markers of primitive/visceral/parietal endoderm, Sox 7 and OCT4; and (c) decrease in the mesoderm markers (Brachyury, MEOX) and ectoderm markers (Sox1 and ZIC1).

The Notch pathway in hematopoietic stem cells

Hematopoiesis is the process that generates all the cell types of the blood, which are responsible for oxygen transport and immune defense. It has been now more than 50 years from the demonstration that blood cells derive from a common ancestor called Hematopoietic Stem Cell (HSC) McCulloch and Till (1960). Thus, the hematopoietic process relies on the unlimited and distinctive self-renewal ability of HSC, which in the adult mammalian organisms reside in the bone marrow, but their generation occurs during embryonic life. Questions still remain about how HSCs acquire and maintain the features of self-renewal and pluripotency that define stem-cell populations. Notch is a crucial signaling pathway involved in the generation of cell diversity and stem-cell maintenance in different systems. In some cases, Notch prevents differentiation, while in other contexts Notch directly participates in promoting cell differentiation. In the following sections, we will review what is known about the role of Notch in HSC establishment and hematopoietic cell lineage specification.

Lineage regulators direct BMP and Wnt pathways to cell-specific programs during differentiation and regeneration

BMP and Wnt signaling pathways control essential cellular responses through activation of the transcription factors SMAD (BMP) and TCF (Wnt). Here, we show that regeneration of hematopoietic lineages following acute injury depends on the activation of each of these signaling pathways to induce expression of key blood genes. Both SMAD1 and TCF7L2 co-occupy sites with master regulators adjacent to hematopoietic genes. In addition, both SMAD1 and TCF7L2 follow the binding of the predominant lineage regulator during differentiation from multipotent hematopoietic progenitor cells to erythroid cells. Furthermore, induction of the myeloid lineage regulator C/EBPα in erythroid cells shifts binding of SMAD1 to sites newly occupied by C/EBPα, whereas expression of the erythroid regulator GATA1 directs SMAD1 loss on nonerythroid targets. We conclude that the regenerative response mediated by BMP and Wnt signaling pathways is coupled with the lineage master regulators to control the gene programs defining cellular identity.

EVEN-SKIPPED HOMEOBOX 1 controls human ES cell differentiation by directly repressing GOOSECOID expression

TGFß signaling patterns the primitive streak, yet little is known about transcriptional effectors that mediate the cell fate choices during streak-like development in mammalian embryos and in embryonic stem (ES) cells. Here we demonstrate that cross-antagonistic actions of EVEN-SKIPPED HOMEOBOX 1 (EVX1) and GOOSECOID (GSC) regulate cell fate decisions in streak-like progenitors derived from human ES cells exposed to BMP4 and/or activin. We found that EVX1 repressed GSC expression and promoted formation of posterior streak-like progeny in response to BMP4, and conversely that GSC repressed EVX1 expression and was required for development of anterior streak-like progeny in response to activin. Chromatin immunoprecipitation assays showed that EVX1 bound to the GSC 5'-flanking region in BMP4 treated human ES cells, and band shift assays identified two EVX1 binding sites in the GSC 5'-region. Significantly, we found that intact EVX1 binding sites were required for BMP4-mediated repression of GSC reporter constructs. We conclude that BMP4-induced EVX1 repress GSC directly and the two genes form the core of a gene regulatory network (GRN) controlling cell fates in streak-like human ES cell progeny.

Simulated microgravity promoted differentiation of bipotential murine oval liver stem cells by modulating BMP4/Notch1 signaling

Faster growth and differentiation of liver stem cells to hepatocyte is one of the key factors during liver regeneration. In recent years, simulated microgravity, a physical force has shown to differentially regulate the differentiation and proliferation of stem cells. In the present work, we studied the effect of simulated microgravity on differentiation and proliferation of liver stem cells. The cells were subjected to microgravity, which was simulated using indigenously fabricated 3D clinostat. Proliferation, apoptosis, immunofluorescence assays and Western blot analysis were carried out to study the effects of simulated microgravity on liver stem cells. Microgravity treatment for 2 h enhanced proliferation of stem cells by twofold without inducing apoptosis and compromising cell viability. Analysis of hepatocyte nuclear factor 4-α (HNF4-α) expression after 2 h of microgravity treatment revealed that microgravity alone can induce the differentiation of stem cells within 2-3 days. Probing bone morphogenic protein 4 (BMP4) and Notch1 in microgravity treated stem cells elaborated downregulation of Notch1 and upregulation of BMP4 after 2 days of incubation. Further, blocking BMP4 using dorsomorphin and chordin conditioned media from chordin plasmid transfected cells attenuated microgravity mediated differentiation of liver stem cells. In conclusion, microgravity interplays with BMP4/Notch1 signaling in stem cells thus inducing differentiation of stem cells to hepatocytes. Present findings can be implicated in clinical studies where microgravity activated stem cells can regenerate the liver efficiently after liver injury.

Targeting SOX17 in human embryonic stem cells creates unique strategies for isolating and analyzing developing endoderm

Human embryonic stem cells (hESCs) can provide insights into development of inaccessible human tissues such as embryonic endoderm. Progress in this area has been hindered by a lack of methods for isolating endodermal cells and tracing fates of their differentiated progeny. By using homologous recombination in human ESCs, we inserted an enhanced green fluorescent protein (eGFP) transgene into the SOX17 locus, a postulated marker of human endoderm. FACS purification and gene expression profiling confirmed that SOX17(+)-hESC progeny expressed endodermal markers and unveiled specific cell surface protein combinations that permitted FACS-based isolation of primitive gut tube endodermal cells produced from unmodified human ESCs and from induced pluripotent stem cells (iPSC). Differentiating SOX17(+) endodermal cells expressed markers of liver, pancreas, and intestinal epithelium in vitro and gave rise to endodermal progeny in vivo. Thus, prospective isolation, lineage tracing, and developmental studies of SOX17(+) hESC progeny have revealed fundamental aspects of human endodermal biology.

TGFβ inhibition enhances the generation of hematopoietic progenitors from human ES cell-derived hemogenic endothelial cells using a stepwise strategy

Embryonic hematopoiesis is a complex process. Elucidating the mechanism regulating hematopoietic differentiation from pluripotent stem cells would allow us to establish a strategy to efficiently generate hematopoietic cells. However, the mechanism governing the generation of hematopoietic progenitors from human embryonic stem cells (hESCs) remains unknown. Here, on the basis of the emergence of CD43(+) hematopoietic cells from hemogenic endothelial (HE) cells, we demonstrated that VEGF was essential and sufficient, and that bFGF was synergistic with VEGF to specify the HE cells and the subsequent transition into CD43(+) hematopoietic cells. Significantly, we identified TGFβ as a novel signal to regulate hematopoietic development, as the TGFβ inhibitor SB 431542 significantly promoted the transition from HE cells into CD43(+) hematopoietic progenitor cells (HPCs) during hESC differentiation. By defining these critical signaling factors during hematopoietic differentiation, we can efficiently generate HPCs from hESCs. Our strategy could offer an in vitro model to study early human hematopoietic development.

Activin, BMP and FGF pathways cooperate to promote endoderm and pancreatic lineage cell differentiation from human embryonic stem cells

The study of how human embryonic stem cells (hESCs) differentiate into insulin-producing beta cells has twofold significance: first, it provides an in vitro model system for the study of human pancreatic development, and second, it serves as a platform for the ultimate production of beta cells for transplantation into patients with diabetes. The delineation of growth factor interactions regulating pancreas specification from hESCs in vitro is critical to achieving these goals. In this study, we describe the roles of growth factors bFGF, BMP4 and Activin A in early hESC fate determination. The entire differentiation process is carried out in serum-free chemically-defined media (CDM) and results in reliable and robust induction of pancreatic endoderm cells, marked by PDX1, and cell clusters co-expressing markers characteristic of beta cells, including PDX1 and insulin/C-peptide. Varying the combinations of growth factors, we found that treatment of hESCs with bFGF, Activin A and BMP4 (FAB) together for 3-4days resulted in strong induction of primitive-streak and definitive endoderm-associated genes, including MIXL1, GSC, SOX17 and FOXA2. Early proliferative foregut endoderm and pancreatic lineage cells marked by PDX1, FOXA2 and SOX9 expression are specified in EBs made from FAB-treated hESCs, but not from Activin A alone treated cells. Our results suggest that important tissue interactions occur in EB-based suspension culture that contribute to the complete induction of definitive endoderm and pancreas progenitors. Further differentiation occurs after EBs are embedded in Matrigel and cultured in serum-free media containing insulin, transferrin, selenium, FGF7, nicotinamide, islet neogenesis associated peptide (INGAP) and exendin-4, a long acting GLP-1 agonist. 21-28days after embedding, PDX1 gene expression levels are comparable to those of human islets used for transplantation, and many PDX1(+) clusters are formed. Almost all cells in PDX1(+) clusters co-express FOXA2, HNF1ß, HNF6 and SOX9 proteins, and many cells also express CPA1, NKX6.1 and PTF1a. If cells are then switched to medium containing B27 and nicotinamide for 7-14days, then the number of insulin(+) cells increases markedly. Our study identifies a new chemically defined culture protocol for inducing endoderm- and pancreas-committed cells from hESCs and reveals an interplay between FGF, Activin A and BMP signaling in early hESC fate determination.

Combined activin A/LiCl/Noggin treatment improves production of mouse embryonic stem cell-derived definitive endoderm cells

Induction of definitive endoderm (DE) cells is a prerequisite for the whole process of embryonic stem (ES) cells differentiating into hepatic or pancreatic progenitor cells. We have established an efficient method to induce mouse ES cell-derived DE cells in suspension embryonic body (EB) culture. Similar to previous studies, mouse ES cell-derived DE cells, which were defined as Cxcr4(+) c-Kit(+) , Cxcr4(+) E-cadherin(+) cells or Cxcr4(+) PDGFRa(-) cells, could be induced in the serum-free EBs at Day 4 of induction. The activations of Wnt, Nodal, and FGF signaling pathways in differentiating EBs promoted DE cell differentiation, while activation of BMP4 signaling inhibited the process. In the present study, we found that chemical activation of canonical Wnt signaling pathway by LiCl could synergize with Activin A-mediated Nodal signaling pathway to promote induction of DE cells, and inhibition of Bmp4 signaling by Noggin along with Activin A/LiCl further improved the efficiency of DE cell differentiation. The derived DE cells were proved for their capacities to become hepatic progenitor cells or pancreatic progenitor cells. In conclusion, we significantly improved the efficiency of generating mouse ES cell-derived DE cells by combined Activin A/LiCl/Noggin treatment. Our work will be greatly helpful to generate ES cell-derived hepatic cells and ES cell-derived pancreatic cells for future regenerative medicine.

A novel serum-free monolayer culture for orderly hematopoietic differentiation of human pluripotent cells via mesodermal progenitors

Elucidating the in vitro differentiation of human embryonic stem (ES) and induced pluripotent stem (iPS) cells is important for understanding both normal and pathological hematopoietic development in vivo. For this purpose, a robust and simple hematopoietic differentiation system that can faithfully trace in vivo hematopoiesis is necessary. In this study, we established a novel serum-free monolayer culture that can trace the in vivo hematopoietic pathway from ES/iPS cells to functional definitive blood cells via mesodermal progenitors. Stepwise tuning of exogenous cytokine cocktails induced the hematopoietic mesodermal progenitors via primitive streak cells. These progenitors were then differentiated into various cell lineages depending on the hematopoietic cytokines present. Moreover, single cell deposition assay revealed that common bipotential hemoangiogenic progenitors were induced in our culture. Our system provides a new, robust, and simple method for investigating the mechanisms of mesodermal and hematopoietic differentiation.

Role of mechanical factors in fate decisions of stem cells

Stem cells derived from adult tissues or from the inner cell mass of blastocyst-stage embryos can self-renew in culture and have the remarkable potential to undergo lineage-specific differentiation. Extensive studies have been devoted to achieving a better understanding of the soluble factors and the mechanism(s) by which they regulate the fate decisions of these cells, but it is only recently that a critical role has been revealed for physical and mechanical factors in controlling self-renewal and lineage specification. This review summarizes selected aspects of current work on stem cell mechanics with an emphasis on the influence of matrix stiffness, surface topography, cell shape and mechanical forces on the fate determination of mesenchymal stem cells and embryonic stem cells.

Modulation of TGF-β signaling by endoglin in murine hemangioblast development and primitive hematopoiesis

Endoglin (Eng), an accessory receptor for the transforming growth factor β (TGF-β) superfamily, is required for proper hemangioblast and primitive hematopoietic development. However the mechanism by which endoglin functions at this early developmental stage is currently unknown. Transcriptional analyses of differentiating eng(-/-) and eng(+/+) ES cells revealed that lack of endoglin leads to profound reductions in the levels of key hematopoietic regulators, including Scl, Lmo2, and Gata2. We also detected lower levels of phosphorylated Smad1 (pSmad1), a downstream target signaling molecule associated with the TGF-β pathway. Using doxycycline-inducible ES cell lines, we interrogated the TGF-β signaling pathway by expressing activated forms of ALK-1 and ALK-5, type I receptors for TGF-β. Our results indicate that ALK-1 signaling promotes hemangioblast development and hematopoiesis, as evidenced by colony assays, gene expression and FACS analyses, whereas signaling by ALK-5 leads to the opposite effect, inhibition of hemangioblast and hematopoietic development. In Eng(-/-) ES cells, ALK-1 rescued both the defective hemangioblast development, and primitive erythropoiesis, indicating that ALK-1 signaling can compensate for the absence of endoglin. We propose that endoglin regulates primitive hematopoiesis by modulating the activity of the Smad1/5 signaling pathway in early stages of development.

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

Efficient differentiation of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) to a variety of lineages requires step-wise approaches replicating the key commitment stages found during embryonic development. Here we show that expression of PdgfR-α segregates mouse ESC-derived Flk-1 mesoderm into Flk-1(+)PdgfR-α(+) cardiac and Flk-1(+)PdgfR-α(-) hematopoietic subpopulations. By monitoring Flk-1 and PdgfR-α expression, we found that specification of cardiac mesoderm and cardiomyocytes is determined by remarkably small changes in levels of Activin/Nodal and BMP signaling. Translation to human ESCs and iPSCs revealed that the emergence of cardiac mesoderm could also be monitored by coexpression of KDR and PDGFR-α and that this process was similarly dependent on optimal levels of Activin/Nodal and BMP signaling. Importantly, we found that individual mouse and human pluripotent stem cell lines require optimization of these signaling pathways for efficient cardiac differentiation, illustrating a principle that may well apply in other contexts.

Influencing hematopoietic differentiation of mouse embryonic stem cells using soluble heparin and heparan sulfate saccharides

Heparan sulfate proteoglycans (HSPG) encompass some of the most abundant macromolecules on the surface of almost every cell type. Heparan sulfate (HS) chains provide a key interaction surface for the binding of numerous proteins such as growth factors and morphogens, helping to define the ability of a cell to respond selectively to environmental cues. The specificity of HS-protein interactions are governed predominantly by the order and positioning of sulfate groups, with distinct cell types expressing unique sets of HS epitopes. Embryos deficient in HS-synthesis (Ext1(-/-)) exhibit pre-gastrulation lethality and lack recognizable organized mesoderm and extraembryonic tissues. Here we demonstrate that embryonic stem cells (ESCs) derived from Ext1(-/-) embryos are unable to differentiate into hematopoietic lineages, instead retaining ESC marker expression throughout embryoid body (EB) culture. However hematopoietic differentiation can be restored by the addition of soluble heparin. Consistent with specific size and composition requirements for HS:growth factor signaling, chains measuring at least 12 saccharides were required for partial rescue of hematopoiesis with longer chains (18 saccharides or more) required for complete rescue. Critically N- and 6-O-sulfate groups were essential for rescue. Heparin addition restored the activity of multiple signaling pathways including bone morphogenic protein (BMP) with activation of phospho-SMADs re-established by the addition of heparin. Heparin addition to wild-type cultures also altered the outcome of differentiation, promoting hematopoiesis at low concentrations, yet inhibiting blood formation at high concentrations. Thus altering the levels of HS and HS sulfation within differentiating ESC cultures provides an attractive and accessible mechanism for influencing cell fate.

Generation of transgene-free lung disease-specific human induced pluripotent stem cells using a single excisable lentiviral stem cell cassette

The development of methods to achieve efficient reprogramming of human cells while avoiding the permanent presence of reprogramming transgenes represents a critical step toward the use of induced pluripotent stem cells (iPSC) for clinical purposes, such as disease modeling or reconstituting therapies. Although several methods exist for generating iPSC free of reprogramming transgenes from mouse cells or neonatal normal human tissues, a sufficiently efficient reprogramming system is still needed to achieve the widespread derivation of disease-specific iPSC from humans with inherited or degenerative diseases. Here, we report the use of a humanized version of a single lentiviral "stem cell cassette" vector to accomplish efficient reprogramming of normal or diseased skin fibroblasts obtained from humans of virtually any age. Simultaneous transfer of either three or four reprogramming factors into human target cells using this single vector allows derivation of human iPSC containing a single excisable viral integration that on removal generates human iPSC free of integrated transgenes. As a proof of principle, here we apply this strategy to generate >100 lung disease-specific iPSC lines from individuals with a variety of diseases affecting the epithelial, endothelial, or interstitial compartments of the lung, including cystic fibrosis, α-1 antitrypsin deficiency-related emphysema, scleroderma, and sickle-cell disease. Moreover, we demonstrate that human iPSC generated with this approach have the ability to robustly differentiate into definitive endoderm in vitro, the developmental precursor tissue of lung epithelia.

Geminin promotes neural fate acquisition of embryonic stem cells by maintaining chromatin in an accessible and hyperacetylated state

Formation of the complex vertebrate nervous system begins when pluripotent cells of the early embryo are directed to acquire a neural fate. Although cell intrinsic controls play an important role in this process, the molecular nature of this regulation is not well defined. Here we assessed the role for Geminin, a nuclear protein expressed in embryonic cells, during neural fate acquisition from mouse embryonic stem (ES) cells. Whereas Geminin knockdown does not affect the ability of ES cells to maintain or exit pluripotency, we found that it significantly impairs their ability to acquire a neural fate. Conversely, Geminin overexpression promotes neural gene expression, even in the presence of growth factor signaling that antagonizes neural transcriptional responses. These data demonstrate that Geminin's activity contributes to mammalian neural cell fate acquisition. We investigated the mechanistic basis of this phenomenon and found that Geminin maintains a hyperacetylated and open chromatin conformation at neural genes. Interestingly, recombinant Geminin protein also rapidly alters chromatin acetylation and accessibility even when Geminin is combined with nuclear extract and chromatin in vitro. Together, these data support a role for Geminin as a cell intrinsic regulator of neural fate acquisition that promotes expression of neural genes by regulating chromatin accessibility and histone acetylation.

Stage-specific signaling through TGFβ family members and WNT regulates patterning and pancreatic specification of human pluripotent stem cells

The generation of insulin-producing β-cells from human pluripotent stem cells is dependent on efficient endoderm induction and appropriate patterning and specification of this germ layer to a pancreatic fate. In this study, we elucidated the temporal requirements for TGFβ family members and canonical WNT signaling at these developmental stages and show that the duration of nodal/activin A signaling plays a pivotal role in establishing an appropriate definitive endoderm population for specification to the pancreatic lineage. WNT signaling was found to induce a posterior endoderm fate and at optimal concentrations enhanced the development of pancreatic lineage cells. Inhibition of the BMP signaling pathway at specific stages was essential for the generation of insulin-expressing cells and the extent of BMP inhibition required varied widely among the cell lines tested. Optimal stage-specific manipulation of these pathways resulted in a striking 250-fold increase in the levels of insulin expression and yielded populations containing up to 25% C-peptide+ cells.

Stage-specific signaling through TGFβ family members and WNT regulates patterning and pancreatic specification of human pluripotent stem cells

The generation of insulin-producing β-cells from human pluripotent stem cells is dependent on efficient endoderm induction and appropriate patterning and specification of this germ layer to a pancreatic fate. In this study, we elucidated the temporal requirements for TGFβ family members and canonical WNT signaling at these developmental stages and show that the duration of nodal/activin A signaling plays a pivotal role in establishing an appropriate definitive endoderm population for specification to the pancreatic lineage. WNT signaling was found to induce a posterior endoderm fate and at optimal concentrations enhanced the development of pancreatic lineage cells. Inhibition of the BMP signaling pathway at specific stages was essential for the generation of insulin-expressing cells and the extent of BMP inhibition required varied widely among the cell lines tested. Optimal stage-specific manipulation of these pathways resulted in a striking 250-fold increase in the levels of insulin expression and yielded populations containing up to 25% C-peptide+ cells.

Single-lineage transcriptome analysis reveals key regulatory pathways in primitive erythroid progenitors in the mouse embryo

Primitive erythroid (EryP) progenitors are the first cell type specified from the mesoderm late in gastrulation. We used a transgenic reporter to image and purify the earliest blood progenitors and their descendants from developing mouse embryos. EryP progenitors exhibited remarkable proliferative capacity in the yolk sac immediately before the onset of circulation, when these cells comprise nearly half of all cells of the embryo. Global expression profiles generated at 24-hour intervals from embryonic day 7.5 through 2.5 revealed 2 abrupt changes in transcript diversity that coincided with the entry of EryPs into the circulation and with their late maturation and enucleation, respectively. These changes were paralleled by the expression of critical regulatory factors. Experiments designed to test predictions from these data demonstrated that the Wnt-signaling pathway is active in EryP progenitors, which display an aerobic glycolytic profile and the numbers of which are regulated by transforming growth factor-β1 and hypoxia. This is the first transcriptome assembled for a single hematopoietic lineage of the embryo over the course of its differentiation.

A sensitive and bright single-cell resolution live imaging reporter of Wnt/ß-catenin signaling in the mouse

BACKGROUND

Understanding the dynamic cellular behaviors and underlying molecular mechanisms that drive morphogenesis is an ongoing challenge in biology. Live imaging provides the necessary methodology to unravel the synergistic and stereotypical cell and molecular events that shape the embryo. Genetically-encoded reporters represent an essential tool for live imaging. Reporter strains can be engineered by placing cis-regulatory elements of interest to direct the expression of a desired reporter gene. In the case of canonical Wnt signaling, also referred to as Wnt/β-catenin signaling, since the downstream transcriptional response is well understood, reporters can be designed that reflect sites of active Wnt signaling, as opposed to sites of gene transcription, as is the case with many fluorescent reporters. However, even though several transgenic Wnt/β-catenin reporter strains have been generated, to date, none provides the single-cell resolution favored for live imaging studies.

RESULTS

We have placed six copies of a TCF/Lef responsive element and an hsp68 minimal promoter in front of a fluorescent protein fusion comprising human histone H2B to GFP and used it to generate a strain of mice that would report Wnt/β-catenin signaling activity. Characterization of developmental and adult stages of the resulting TCF/Lef:H2B-GFP strain revealed discrete and specific expression of the transgene at previously characterized sites of Wnt/β-catenin signaling. In support of the increased sensitivity of the TCF/Lef:H2B-GFP reporter, additional sites of Wnt/β-catenin signaling not documented with other reporters but identified through genetic and embryological analysis were observed. Furthermore, the sub-cellular localization of the reporter minimized reporter perdurance, and allowed visualization and tracking of individual cells within a cohort, so facilitating the detailed analysis of cell behaviors and signaling activity during morphogenesis.

CONCLUSION

By combining the Wnt activity read-out efficiency of multimerized TCF/Lef DNA binding sites, together with the high-resolution imaging afforded by subcellularly-localized fluorescent fusion proteins such as H2B-GFP, we have created a mouse transgenic line that faithfully recapitulates Wnt signaling activity at single-cell resolution. The TCF/Lef:H2B-GFP reporter represents a unique tool for live imaging the in vivo processes triggered by Wnt/β-catenin signaling, and thus should help the formulation of a high-resolution understanding of the serial events that define the morphogenetic process regulated by this signaling pathway.

Regulated expression of microRNAs-126/126* inhibits erythropoiesis from human embryonic stem cells

MicroRNAs (miRs) play an important role in cell differentiation and maintenance of cell identity, but relatively little is known of their functional role in modulating human hematopoietic lineage differentiation. Human embryonic stem cells (hESCs) provide a model system to study early human hematopoiesis. We differentiated hESCs by embryoid body (EB) formation and compared the miR expression profile of undifferentiated hESCs to CD34(+) EB cells. miRs-126/126* were the most enriched of the 7 miRs that were up-regulated in CD34(+) cells, and their expression paralleled the kinetics of hematopoietic transcription factors RUNX1, SCL, and PU.1. To define the role of miRs-126/126* in hematopoiesis, we created hESCs overexpressing doxycycline-regulated miRs-126/126* and analyzed their hematopoietic differentiation. Induction of miRs-126/126* during both EB differentiation and colony formation reduced the number of erythroid colonies, suggesting an inhibitory role of miRs-126/126* in erythropoiesis. Protein tyrosine phosphatase, nonreceptor type 9 (PTPN9), a protein tyrosine phosphatase that is required for growth and expansion of erythroid cells, is one target of miR-126. PTPN9 restoration partially relieved the suppressed erythropoiesis caused by miRs-126/126*. Our results define an important function of miRs-126/126* in negative regulation of erythropoiesis, providing the first evidence for a role of miR in hematopoietic differentiation of hESCs.

A high-throughput multiplexed screening assay for optimizing serum-free differentiation protocols of human embryonic stem cells

Serum-free differentiation protocols of human embryonic stem cells (hESCs) offer the ability to maximize reproducibility and to develop clinically applicable therapies. We developed a high-throughput, 96-well plate, four-color flow cytometry-based assay to optimize differentiation media cocktails and to screen a variety of conditions. We were able to differentiate hESCs to all three primary germ layers, screen for the effect of a range of activin A, BMP4, and VEGF concentrations on endoderm and mesoderm differentiation, and perform RNA-interference (RNAi)-mediated knockdown of a reporter gene during differentiation. Cells were seeded in suspension culture and embryoid bodies were induced to differentiate to the three primary germ layers for 6 days. Endoderm (CXCR4(+)KDR(-)), mesoderm (KDR(+)SSEA-3(-)), and ectoderm (SSEA-3(+)NCAM(+)) differentiation yields for H9 cells were 80 ± 11, 78 ± 7, and 41 ± 9%, respectively. Germ layer identities were confirmed by quantitative PCR. Activin A, BMP4, and bFGF drove differentiation, with increasing concentrations of activin A inducing higher endoderm yields and increasing BMP4 inducing higher mesoderm yields. VEGF drove lateral mesoderm differentiation. RNAi-mediated knockdown of constitutively expressed red fluorescent protein did not affect endoderm differentiation. This assay facilitates the development of serum-free protocols for hESC differentiation to target lineages and creates a platform for screening small molecules or RNAi during ESC differentiation.

Spatio-temporal regulation of Wnt and retinoic acid signaling by tbx16/spadetail during zebrafish mesoderm differentiation

Background

A complex network of signaling pathways and transcription factors regulates vertebrate mesoderm development. Zebrafish mutants provide a powerful tool for examining the roles of individual genes in such a network. spadetail (spt) is a mutant with a lesion in tbx16, a T-box transcription factor involved in mesoderm development; the mutant phenotype includes disrupted primitive red blood cell formation as well as disrupted somitogenesis. Despite much recent progress, the downstream targets of tbx16 remain incompletely understood. The current study was carried out to test whether any of the five major signaling pathways are regulated by tbx16 during two specific stages of mesoderm development: primitive red blood cell formation in the intermediate mesoderm and somite formation in the tail paraxial mesoderm. This test was performed using Gene Set Enrichment Analysis, which identifies coordinated changes in expression among a priori sets of genes associated with biological features or processes.

Results

Our Gene Set Enrichment Analysis results identify Wnt and retinoic acid signaling as likely downstream targets of tbx16 in the developing zebrafish intermediate mesoderm, the site of primitive red blood cell formation. In addition, such results identify retinoic acid signaling as a downstream target of tbx16 in the developing zebrafish posterior somites. Finally, using candidate gene identification and in situ hybridization, we provide expression domain information for 25 additional genes downstream of tbx16 that are outside of both pathways; 23 were previously unknown downstream targets of tbx16, and seven had previously uncharacterized expression in zebrafish.

Conclusions

Our results suggest that (1) tbx16 regulates Wnt signaling in the developing zebrafish intermediate mesoderm, the site of primitive red blood cell formation, and (2) tbx16 regulates retinoic acid signaling at two distinct embryonic locations and developmental stages, which may imply ongoing spatio-temporal regulation throughout mesoderm development.

Lessons for cardiac regeneration and repair through development

Cell-based regenerative strategies have the potential to revolutionize the way cardiovascular injury is treated, but successful therapies will require a precise understanding of the mechanisms that dictate cell fate, survival and differentiation. Recent advances in the study of cardiac development hold promise for unlocking the keys for successful therapies. Using mouse models and embryonic stem cells, researchers are uncovering cardiac progenitor cells in both embryonic and adult contexts. Furthermore, the signaling molecules and transcriptional regulators that govern these cells and their behavior are being revealed. Here, we focus on the recent advances in these areas of cardiac developmental research and their impact on the expanding field of regenerative medicine.

Wnt/beta-catenin signaling is involved in the induction and maintenance of primitive hematopoiesis in the vertebrate embryo

The formation of primitive (embryonic) blood in vertebrates is mediated by spatio-temporally restricted signaling between different tissue layers. In Xenopus, in which primitive blood originates in the ventral blood island, this involves the secretion of bone morphogenetic protein (BMP) ligands by the ectoderm that signal to the underlying mesoderm during gastrulation. Using novel transgenic reporter lines, we report that the canonical Wnt/β-catenin pathway is also activated in the blood islands in Xenopus. Furthermore, Wnt-reporter activity was also detected in the blood islands of the mouse yolk sac. By using morpholino-mediated depletion in Xenopus, we identified Wnt4 as the ligand that is expressed in the mesoderm of the ventral blood island and is essential for the expression of hematopoietic and erythroid marker genes. Injection of an inducible Wnt-interfering construct further showed that, during gastrulation, Wnt/β-catenin signaling is required both in the mesoderm and in the overlying ectoderm for the formation of the ventral blood island. Using recombination assays with embryonic explants, we document that ectodermal BMP4 expression is dependent on Wnt4 signals from the mesoderm. Our results thus reveal a unique role for Wnt4-mediated canonical signaling in the formation and maintenance of the ventral blood island in Xenopus.