Embryonic stem cell-derived embryoid bodies development in collagen gels recapitulates sprouting angiogenesis

Embryoid bodies as a model system for exploring early human embryonic development

This review article aims to summarize the existing concepts related to embryoid bodies (EBs) and explore their potential as a model system for studying various aspects of human embryonic development. The review involves the collection and analysis of information about the characteristics of EBs and the properties of stem cells that give rise to them. The results show that EBs derived from pluripotent stem cells are a promising model that closely replicates processes occurring in human embryos after implantation. The review also provides a comparative analysis of the advantages and limitations of models based on induced pluripotent stem cells (iPSCs) and embryonic stem cells (ESCs), with particular attention given to summarizing the results of limited studies on RNA sequencing in individual cells from human and mouse EBs. In conclusion, we would like to emphasize that embryoid bodies are an effective model system for studying early human embryogenesis. This opens up new possibilities for reproductive genetics and medicine.

Molecular mechanism of VE-cadherin in regulating endothelial cell behaviour during angiogenesis

Vascular endothelial (VE)-cadherin, an endothelium-specific adhesion protein, is found in the junctions between endothelial cells (ECs). It's crucial to maintain the homogeneity of ECs. Keeping and controlling the contact between ECs is essential. In addition to its adhesive function, VE-cadherin plays important roles in vascular development, permeability, and tumour angiogenesis. Signal transfer, cytoskeletal reconstruction, and contractile integrating, which are crucial for constructing and maintaining monolayer integrity as well as for repair and regeneration, are the foundation of endothelial cell (EC) junctional dynamics. The molecular basis of adhesion junctions (AJs), which are closely related and work with actin filaments, is provided by the VE-cadherin-catenin complex. They can activate intracellular signals that drive ECs to react or communicate structural changes to junctions. An increasing number of molecules, including the vascular endothelial growth factor receptor 2 (VEGFR2) and vascular endothelial protein tyrosine phosphatase (VE-PTP), have been connected to VE-cadherin in addition to the conventional VE-cadherin-catenin complex. This review demonstrates significant progress in our understanding of the molecular mechanisms that affect VE-cadherin's function in the regulation of EC behaviour during angiogenesis. The knowledge of the molecular processes that control VE-cadherin's role in the regulation of EC behaviour during angiogenesis has recently advanced, as shown in this review.

Multicellular Liver Organoids: Generation and Importance of Diverse Specialized Cellular Components

Over 40,000 patients in the United States are estimated to suffer from end-stage liver disease and acute hepatic failure, for which liver transplantation is the only available therapy. Human primary hepatocytes (HPH) have not been employed as a therapeutic tool due to the difficulty in growing and expanding them in vitro, their sensitivity to cold temperatures, and tendency to dedifferentiate following two-dimensional culture. The differentiation of human-induced pluripotent stem cells (hiPSCs) into liver organoids (LO) has emerged as a potential alternative to orthotropic liver transplantation (OLT). However, several factors limit the efficiency of liver differentiation from hiPSCs, including a low proportion of differentiated cells capable of reaching a mature phenotype, the poor reproducibility of existing differentiation protocols, and insufficient long-term viability in vitro and in vivo. This review will analyze various methodologies being developed to improve hepatic differentiation from hiPSCs into liver organoids, paying particular attention to the use of endothelial cells as supportive cells for their further maturation. Here, we demonstrate why differentiated liver organoids can be used as a research tool for drug testing and disease modeling, or employed as a bridge for liver transplantation following liver failure.

Bench-to-Bedside in Vascular Medicine: Optimizing the Translational Pipeline for Patients With Peripheral Artery Disease

Peripheral arterial disease is a growing worldwide problem with a wide spectrum of clinical severity and is projected to consume >$21 billion per year in the United States alone. While vascular researchers have brought several therapies to the clinic in recent years, few of these approaches have leveraged advances in high-throughput discovery screens, novel translational models, or innovative trial designs. In the following review, we discuss recent advances in unbiased genomics and broader omics technology platforms, along with preclinical vascular models designed to enhance our understanding of disease pathobiology and prioritize targets for additional investigation. Furthermore, we summarize novel approaches to clinical studies in subjects with claudication and ischemic ulceration, with an emphasis on streamlining and accelerating bench-to-bedside translation. By providing a framework designed to enhance each aspect of future clinical development programs, we hope to enrich the pipeline of therapies that may prevent loss of life and limb for those with peripheral arterial disease.

The growth of endothelial-like cells in zebrafish embryoid body culture

There is increasing interest in the possibility of culturing organ-like tissues (organoids) in vitro for biomedical applications. The ability to culture organoids would be greatly enhanced by having a functional circulation in vitro. The endothelial cell is the most important cell type in this context. Endothelial cells can be derived from pluripotent embryonic blastocyst cells in aggregates called embryoid bodies. Here, we examine the yield of endothelial-like cells in embryoid bodies (EBs) developed from transgenic zebrafish fli:GFP and kdrl:GFP blastocyst embryos. The isolated blastocyst cells developed into EBs within the first 24 h of culture and contained fli:GFP⁺ (putative endothelial, hematopoietic and other cell types); or kdrl:GFP⁺ (endothelial) cells. The addition of endothelial growth supplements to the media and culture on collagen type-I substratum increased the percentages of fli:GFP⁺ and kdrl:GFP⁺ cells in culture. We found that EBs developed in hanging-drop cultures possessed a higher percentage of fli:GFP⁺ (45.0 ± 3.1%) and kdrl:GFP⁺ cells (8.7 ± 0.7%) than those developed on conventional substrata (34.5 ± 1.4% or 5.2 ± 0.4%, respectively). The transcriptome analysis showed a higher expression of VEGF and TGFβ genes in EB cultures compared to the adherent cultures. When transferred to conventional culture, the percentage of fli:GFP⁺ or kdrl:GFP⁺ cells declined significantly over subsequent days in the EBs. The fli:GFP⁺ cells formed a monolayer around the embryoid bodies, while the kdrl:GFP⁺ cells formed vascular network-like structures in the embryoid bodies. Differences were observed in the spreading of fli:GFP⁺ cells, and network formation of kdrl:GFP⁺ cells on different substrates. The fli:GFP⁺ cells could be maintained in primary culture and sub-cultures. By contrast, kdrl:GFP⁺ cells were almost completely absent at 8d of primary culture. Our culture model allows real-time observation of fli:GFP⁺ and kdrl:GFP⁺ cells in culture. The results obtained from this study will be important for the development of vascular and endothelial cell culture using embryonic cells.

Local anti-angiogenic therapy by magnet-assisted downregulation of SHP2 phosphatase

Anti-angiogenic therapies are promising options for diseases with enhanced vessel formation such as tumors or retinopathies. In most cases, a site-specific local effect on vessel growth is required, while the current focus on systemic distribution of angiogenesis inhibitors may cause severe unwanted side-effects. Therefore, in the current study we have developed an approach for the local inhibition of vascularization, using complexes of lentivirus and magnetic nanoparticles in combination with magnetic fields. Using this strategy in the murine embryonic stem cell (ESC) system, we were able to site-specifically downregulate the protein tyrosine phosphatase SHP2 by RNAi technology in areas with active vessel formation. This resulted in a reduction of vessel development, as shown by reduced vascular tube length, branching points and vascular loops. The anti-angiogenic effect could also be recapitulated in the dorsal skinfold chamber of mice in vivo. Here, site-specific downregulation of SHP2 reduced re-vascularization after wound induction. Thus, we have developed a magnet-assisted, RNAi-based strategy for the efficient local inhibition of angiogenesis in ESCs in vitro and also in vivo.

Identification of RSK and TTK as Modulators of Blood Vessel Morphogenesis Using an Embryonic Stem Cell-Based Vascular Differentiation Assay

Blood vessels are formed through vasculogenesis, followed by remodeling of the endothelial network through angiogenesis. Many events that occur during embryonic vascular development are recapitulated during adult neoangiogenesis, which is critical to tumor growth and metastasis. Current antiangiogenic tumor therapies, based largely on targeting the vascular endothelial growth factor pathway, show limited clinical benefits, thus necessitating the discovery of alternative targets. Here we report the development of a robust embryonic stem cell-based vascular differentiation assay amenable to small-molecule screens to identify novel modulators of angiogenesis. In this context, RSK and TTK were identified as angiogenic modulators. Inhibition of these pathways inhibited angiogenesis in embryoid bodies and human umbilical vein endothelial cells. Furthermore, inhibition of RSK and TTK reduced tumor growth, vascular density, and improved survival in an in vivo Lewis lung carcinoma mouse model. Our study suggests that RSK and TTK are potential targets for antiangiogenic therapy, and provides an assay system for further pathway screens.

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Development of ESC-based vascular differentiation assay amenable to drug screening

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Screening a kinase library identified RSK and TTK as angiogenic modulators

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RSK and TTK inhibition disrupted angiogenesis in vitro

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RSK and TTK inhibition inhibited Lewis lung tumor growth and angiogenesis in vivo

High-Dose Fluoride Impairs the Properties of Human Embryonic Stem Cells via JNK Signaling

Fluoride is a ubiquitous natural substance that is often used in dental products to prevent dental caries. The biphasic actions of fluoride imply that excessive systemic exposure to fluoride can cause harmful effects on embryonic development in both animal models and humans. However, insufficient information is available on the effects of fluoride on human embryonic stem cells (hESCs), which is a novel in vitro humanized model for analyzing the embryotoxicities of chemical compounds. Therefore, we investigated the effects of sodium fluoride (NaF) on the proliferation, differentiation and viability of H9 hESCs. For the first time, we showed that 1 mM NaF did not significantly affect the proliferation of hESCs but did disturb the gene expression patterns of hESCs during embryoid body (EB) differentiation. Higher doses of NaF (2 mM and above) markedly decreased the viability and proliferation of hESCs. The mode and underlying mechanism of high-dose NaF-induced cell death were further investigated by assessing the sub-cellular morphology, mitochondrial membrane potential (MMP), caspase activities, cellular reactive oxygen species (ROS) levels and activation of mitogen-activated protein kinases (MAPKs). High-dose NaF caused the death of hESCs via apoptosis in a caspase-mediated but ROS-independent pathway, coupled with an increase in the phospho-c-Jun N-terminal kinase (p-JNK) levels. Pretreatment with a p-JNK-specific inhibitor (SP600125) could effectively protect hESCs from NaF-induced cell death in a concentration- and time-dependent manner. These findings suggest that NaF might interfere with early human embryogenesis by disturbing the specification of the three germ layers as well as osteogenic lineage commitment and that high-dose NaF could cause apoptosis through a JNK-dependent pathway in hESCs.

MiR-24 is required for hematopoietic differentiation of mouse embryonic stem cells

Overexpression of miRNA, miR-24, in mouse hematopoietic progenitors increases monocytic/ granulocytic differentiation and inhibits B cell development. To determine if endogenous miR-24 is required for hematopoiesis, we antagonized miR-24 in mouse embryonic stem cells (ESCs) and performed in vitro differentiations. Suppression of miR-24 resulted in an inability to produce blood and hematopoietic progenitors (HPCs) from ESCs. The phenotype is not a general defect in mesoderm production since we observe production of nascent mesoderm as well as mesoderm derived cardiac muscle and endothelial cells. Results from blast colony forming cell (BL-CFC) assays demonstrate that miR-24 is not required for generation of the hemangioblast, the mesoderm progenitor that gives rise to blood and endothelial cells. However, expression of the transcription factors Runx1 and Scl is greatly reduced, suggesting an impaired ability of the hemangioblast to differentiate. Lastly, we observed that known miR-24 target, Trib3, is upregulated in the miR-24 antagonized embryoid bodies (EBs). Overexpression of Trib3 alone in ESCs was able to decrease HPC production, though not as great as seen with miR-24 knockdown. These results demonstrate an essential role for miR-24 in the hematopoietic differentiation of ESCs. Although many miRNAs have been implicated in regulation of hematopoiesis, this is the first miRNA observed to be required for the specification of mammalian blood progenitors from early mesoderm.

Studies of mouse embryos and embryonic stem cells (ESCs) have defined the ontogeny of mammalian embryonic hematopoietic cells. The ESC differentiation system has been valuable for dissecting the molecular regulation of the development of mesoderm into HPCs. Extracellular signals regulate a complex network of transcription factors to direct embryonic hematopoietic development. Mammalian miRNAs have previously not been described to regulate this genetic network during embryonic hematopoiesis. However, a role for miRNAs in producing the hemangioblast, and hemogenic endothelium in Xenopus has been described. Our work with ESCs demonstrates a specific requirement for the miRNA, miR-24, in the development of hematopoietic progenitors cells (HPCs). Antagonizing miR-24 in ESCs does not affect generation of BL-CFCs, the in vitro equivalent of the hemangioblast, but does compromise the ability of those BL-CFCs to produced HPCs. Expression of transcription factors required for HPC production downstream of the hemangioblast, Scl, and Runx1, is greatly reduced by antagonizing miR-24. These results identify miR-24, as a mammalian miRNA required for the development of blood from newly formed mesoderm.

Sox7 is regulated by ETV2 during cardiovascular development

Vasculogenesis/angiogenesis is one of the earliest processes that occurs during embryogenesis. ETV2 and SOX7 were previously shown to play a role in endothelial development; however, their mechanistic interaction has not been defined. In the present study, concomitant expression of Etv2 and Sox7 in endothelial progenitor cells was verified. ETV2 was shown to be a direct upstream regulator of Sox7 that binds to ETV2 binding elements in the Sox7 upstream regulatory region and activates transcription. We observed that SOX7 over-expression can mimic ETV2 and increase endothelial progenitor cells in embryonic bodies (EBs), while knockdown of Sox7 is able to block ETV2-induced increase in endothelial progenitor cell formation. Angiogenic sprouting was increased by ETV2 over-expression in EBs, and it was significantly decreased in the presence of Sox7 shRNA. Collectively, these studies support the conclusion that ETV2 directly regulates Sox7, and that ETV2 governs endothelial development by regulating transcriptional networks which include Sox7.

ENDOGLIN is dispensable for vasculogenesis, but required for vascular endothelial growth factor-induced angiogenesis

ENDOGLIN (ENG) is a co-receptor for transforming growth factor-β (TGF-β) family members that is highly expressed in endothelial cells and has a critical function in the development of the vascular system. Mutations in Eng are associated with the vascular disease known as hereditary hemorrhagic telangiectasia type l. Using mouse embryonic stem cells we observed that angiogenic factors, including vascular endothelial growth factor (VEGF), induce vasculogenesis in embryoid bodies even when Eng deficient cells or cells depleted of Eng using shRNA are used. However, ENG is required for the stem cell-derived endothelial cells to organize effectively into tubular structures. Consistent with this finding, fetal metatarsals isolated from E17.5 Eng heterozygous mouse embryos showed reduced VEGF-induced vascular network formation. Moreover, shRNA-mediated depletion and pharmacological inhibition of ENG in human umbilical vein cells mitigated VEGF-induced angiogenesis. In summary, we demonstrate that ENG is required for efficient VEGF-induced angiogenesis.

Isolation and expansion of endothelial progenitor cells derived from mouse embryonic stem cells

The unlimited differentiation and proliferation capacity of embryonic stem cells represents a great resource for regenerative medicine. Here, we describe a method for differentiating, isolating, and expanding endothelial cells (ECs) from mouse embryonic stem cells (mESCs). First, mESCs are expanded on a mouse embryonic fibroblast (mEF) feeder layer and partially differentiated into embryoid bodies (EBs) by growing the cells in an ultra-low attachment plate for up to 5 days. The EBs are then differentiated along the endothelial lineage using endothelial growth medium supplemented with 40 ng/mL vascular endothelial growth factor (VEGF). The differentiated endothelial population expresses both Fetal Liver Kinase 1 (Flk-1) and VE-Cadherin on the cell surface which can be further purified using a fluorescence-activated cell sorting (FACS) system and subsequently expanded on 0.1 % gelatin-coated plates. The differentiated cells can be analyzed by real-time PCR and flow cytometry to confirm enrichment of EC-specific genes and proteins.

TR-644 a novel potent tubulin binding agent induces impairment of endothelial cells function and inhibits angiogenesis

TR-644 is a novel combretastatin A-4 (CA-4) analogue endowed with potent microtubule depolymerizing activity superior to that of the lead compound and it also has high affinity to colchicines binding site of tubulin. We tested TR-644 anti-angiogenic effects in human umbilical endothelial cells (HUVEC). It showed no significant effects on the growth of HUVEC cells at concentrations below 1,000 nM, but at much lower concentrations (10-100 nM) it induced inhibition of capillary tube formation, inhibition of endothelial cell migration and affected endothelial cell morphology as demonstrated by the disruption of the microtubule network. TR-644 also increased permeability of HUVEC cells in a time dependent manner. The molecular mechanism for the anti-vascular activity of TR-644 was investigated in detail. TR-644 caused G2/M arrest in endothelial cells and this effect correlated with downregulation of the expression of Cdc25C and Cdc2(Tyr15). Moreover TR-644 inhibited VEGF-induced phosphorylation of VE-cadherin but did not prevent the VEGF-induced phosphorylation of FAK. In chick chorioallantoic membrane in vivo assay, TR-644 (0.1-1.0 pmol/egg) efficiently counteracted the strong angiogenic response induced by FGF. Also CA-4, used as reference compound, caused an antagonistic effect, but in contrast, it induced per se, a remarkable angiogenic response probably due to an inflammatory reaction in the site of treatment. In a mice allogenic tumor model, immunohistochemical staining of tumors with anti-CD31 antibody showed that TR-644 significantly reduced the number of vessel, after 24 h from the administration of a single dose (30 mg/Kg).

α2-Macroglobulin enhances vasculogenesis/angiogenesis of mouse embryonic stem cells by stimulation of nitric oxide generation and induction of fibroblast growth factor-2 expression

α2-macroglobulin (α2M) is an acute-phase protein released upon challenges like cardiac hypertrophy and infarction. α2M signals via the low density lipoprotein receptor-related protein (LRP-1) and may induce stem cell activation. In the present study, the effects of α2M on vasculogenesis/angiogenesis and underlying signaling cascades were investigated in mouse embryonic stem (ES) cells. LRP-1 was expressed in ES cells and upregulated during differentiation. α2M dose dependently increased CD31-positive vascular structures in ES cell-derived embryoid bodies, the early cardiovascular markers isl-1, Nkx-2.5, and flk-1 as well as numbers of VE-cadherin and flk-1-positive cells, but downregulated α-smooth muscle actin. Enhancement of vasculogenesis/angiogenesis by α2M was abolished by the LRP-1 antagonist receptor-associated protein (RAP) and LRP-1 blocking antibody. Notably, α2M stimulated vascular growth in the chicken chorioallantois membrane assay, but not in a human umbilical vein endothelial cell spheroid model. α2M increased fibroblast growth factor-2 (FGF-2) protein expression, which was abolished by RAP, induced nitric oxide (NO) generation as determined by 4,5-diaminofluorescein diacetate microfluorometry, and activated nitric oxide synthase-3 (NOS-3) as well as extracellular-regulated kinase 1,2 (ERK1/2) and phosphatidyl inositol 3-kinase (PI3K). NO generation, the increase in FGF-2 expression, and the stimulation of vasculogenesis/angiogenesis by α2M were blunted by the NO synthase inhibitor L-NAME, the ERK1/2 inhibitor PD98059, and the PI3K inhibitor LY294002. Furthermore, vasculogenesis/angiogenesis by α2M was inhibited in the presence of the FGF receptor 1 antagonist SU5402. In conclusion, α2M stimulates endothelial and early cardiac, but not smooth muscle differentiation of ES cells through generation of NO, activation of ERK1/2 as well as PI3K, and induction of FGF-2 expression.

Neovascularization in tissue engineering

A prerequisite for successful tissue engineering is adequate vascularization that would allow tissue engineering constructs to survive and grow. Angiogenic growth factors, alone and in combination, have been used to achieve this, and gene therapy has been used as a tool to enable sustained release of these angiogenic proteins. Cell-based therapy using endothelial cells and their precursors presents an alternative approach to tackling this challenge. These studies have occurred on a background of advancements in scaffold design and assays for assessing neovascularization. Finally, several studies have already attempted to translate research in neovascularization to clinical use in the blossoming field of therapeutic angiogenesis.

Ex vivo reconstitution of arterial endothelium by embryonic stem cell-derived endothelial progenitor cells in baboons

There is an increasing need for an animal model that can be used to translate basic research into clinical therapy. We documented the differentiation and functional competence of embryonic stem cell (ESC)-derived endothelial cells in baboons. Baboon angioblasts were sequentially differentiated from embryoid body cultures for 9 days in an angioblast differentiation medium with varying concentrations of BMP-4, FLT-3 ligand, stem cell factor, thrombopoietin, basic fibroblast growth factor (FGF), vascular endothelial growth factor (VEGF), and knockout serum replacement. Real-time polymerase chain reaction results showed that ESC-derived angioblasts downregulated NANOG and OCT3/4, upregulated T-brachyury and GATA2, and moderately expressed CD34; they did not express CD144, TEK, or VWF, and varied in levels of CD31 expression. Several populations of putative angioblasts appeared 3 days and 9 days after differentiation, as identified by flow cytometry. Angioblasts at this stage exhibited dual paths of differentiation toward hematopoietic and vascular fates. To examine whether derived angioblasts could reconstitute the endothelium, we built an ex vivo culture system and seeded fluorescently labeled angioblast cultures onto a denuded segment of the femoral artery. We found that the seeded cells were able to grow into the endothelium on the interior surface of denuded artery segments within 5 days after seeding. After 14 days of ex vivo culture, the transplanted cells expressed CD31, an endothelial marker. The control arteries, seeded with vehicle only, did not harbor cells with endothelial markers. We conclude that ESC-derived angioblasts are promising therapeutic agents for repairing damaged vasculature, and that the baboon model will be vital for optimizing therapies for human clinical studies.

Role of the microRNA-17-92 cluster in the endothelial differentiation of stem cells

MicroRNAs (miRs) are small non-coding RNAs that recently emerged as potent regulators of gene expression. The members of the miR-17-92 cluster have been shown to control endothelial cell functions and neovascularization; however, the regulation and function of the cluster in endothelial cell lineage commitment has not been explored. This project aimed to test the role of the miR-17-92 cluster during endothelial differentiation. We demonstrate that miR-17, miR-18, miR-19 and miR-20 are increased upon the induction of endothelial cell differentiation of murine embryonic stem cells or induced pluripotent stem cells. In contrast, miR-92a and the primary miR-17-92 transcript were downregulated. The inhibition of each individual miR of the cluster by cholesterol-modified antagomirs did not affect endothelial marker gene expression. Moreover, the combination of all antagomirs had no effect. These findings illustrate that although the miR-17-92 cluster regulates vascular integrity and angiogenesis, none of the members has a significant impact on the endothelial differentiation of pluripotent stem cells.

The F-BAR protein NOSTRIN participates in FGF signal transduction and vascular development

F-BAR proteins are multivalent adaptors that link plasma membrane and cytoskeleton and coordinate cellular processes such as membrane protrusion and migration. Yet, little is known about the function of F-BAR proteins in vivo. Here we report, that the F-BAR protein NOSTRIN is necessary for proper vascular development in zebrafish and postnatal retinal angiogenesis in mice. The loss of NOSTRIN impacts on the migration of endothelial tip cells and leads to a reduction of tip cell filopodia number and length. NOSTRIN forms a complex with the GTPase Rac1 and its exchange factor Sos1 and overexpression of NOSTRIN in cells induces Rac1 activation. Furthermore, NOSTRIN is required for fibroblast growth factor 2 dependent activation of Rac1 in primary endothelial cells and the angiogenic response to fibroblast growth factor 2 in the in vivo matrigel plug assay. We propose a novel regulatory circuit, in which NOSTRIN assembles a signalling complex containing FGFR1, Rac1 and Sos1 thereby facilitating the activation of Rac1 in endothelial cells during developmental angiogenesis.

Mature endothelium and neurons are simultaneously derived from embryonic stem cells by 2D in vitro culture system

The connections existing between vessels and nerves go beyond the structural architecture of vascular and nervous systems to comprise cell fate determination. The analysis of functional/molecular links that interconnect endothelial and neural commitments requires a model in which the two differentiation programs take place at the same time in an artificial controllable environment. To this regard, this work presents an in vitro model to differentiate embryonic stem (ES) cells simultaneously into mature neurons and endothelial cells. Murine ES cells are differentiated within an artificial environment composed of PA6 stromal cells and a serum-free medium. Upon these basal culture conditions ES cells preferentially differentiate into neurons. The addition of basic fibroblast growth factor (FGF2) to the medium allows the simultaneous maturation of neurons and endothelial cells, whereas bone morphogenetic protein (BMP)4 drives endothelial differentiation to the disadvantage of neural commitment. The responsiveness of the system to exogenous cytokines was confirmed by genes expression analysis that revealed a significant up-regulation of endothelial genes in presence of FGF2 and a massive down-regulation of the neural markers in response to BMP4. Furthermore, the role played by single genes in determining endothelial and neural fate can be easily explored by knocking down the expression of the target gene with lentiviruses carrying the corresponding shRNA sequence. The possibility to address the neural and the endothelial fate separately or simultaneously by exogenous stimuli combined with an efficient gene silencing strategy make this model an optimal tool to identify environmental signals and genes pathways involved in both endothelial and neural specification.

In vitro imaging of angiogenesis using embryonic stem cell-derived endothelial cells

Angiogenesis is an important event during developmental processes, and it plays a key role in neovascularization. The development of an in vitro model that can be used for live imaging of vessel growth will facilitate the study of molecular and cellular mechanisms for the growth of blood vessels. Embryonic stem cells (ESCs) are considered to be a novel renewable source for the derivation of genetically manipulable endothelial cells (ECs). To derive green fluorescence protein (GFP)-expressing ECs, we used a transgenic ESC line in which a GFP reporter was driven by the endothelial-specific promoter fetal liver kinase 1. ESC-ECs were isolated from 11-day embryoid bodies by fluorescence-activated cell sorting. Embedding the aggregated ESC-ECs in a 3-dimensional collagen gel matrix resulted in ESC-EC migration out of the aggregates and coalescence into a capillary network. Time-lapse microscopy revealed EC migration, proliferation, lumen formation, and anastomosis to other capillary vessels during this process, which were reminiscent of angiogenic processes. Vascular endothelial growth factor plays major roles in the induction of ESC-EC angiogenesis in vitro. Blockage of the β1 integrin subunit severely impaired ESC-EC survival and migration. We demonstrate that our in vitro ESC-EC angiogenesis model represents a high-resolution dynamic video-image system for observing the cellular events underlying angiogenic cascades. We also consider this model as an image screening tool for the identification of pro-angiogenic and anti-angiogenic molecules.

Residual undifferentiated cells during differentiation of induced pluripotent stem cells in vitro and in vivo

Induced pluripotent stem (iPS) cells are a potential cell source for regenerative medicine. However, the tumorigenicity of iPS cells is a big concern for clinical application. In addition to the genetic manipulation of the reprogramming process and the greater risk of tumor formation, it is unclear whether iPS cells with normal development potential are still tumorigenic. Here, we investigated 3 mouse iPS cell lines, including one line that is able to generate full-term mice via tetraploid blastocyst complementation. We found that a small number of undifferentiated iPS cells could be steadily isolated and expanded after long-term differentiation of cells in vitro or in vivo. The residual undifferentiated iPS cells could be expanded and redifferentiated, and undifferentiated pluripotent stem cells could again be isolated after further rounds of differentiation, suggesting that residual undifferentiated iPS cells could not be eliminated by extended cell differentiation. The residual undifferentiated cells could form teratomas in vivo, indicating that they are a potential tumorigenic risk during transplantation. These findings prompt us to reconsider the strategies for solving the tumorigenic problem of iPS cells, not only focusing on improving the reprogramming process.

Enhancement of differentiation efficiency of hESCs into vascular lineage cells in hypoxia via a paracrine mechanism

Hypoxia is one way of inducing differentiation due to the activation of the key regulatory factor, Hypoxia-inducible factor 1 alpha (HIF-1α). However, the action of HIF-1α on the differentiation of hESCs was unclear until now. To investigate the effect of hypoxia on the differentiation of hESCs, we compared the differentiation efficacy into vascular lineage cells under normoxic and hypoxic conditions. We observed HIF-1α expression and the related expression of pro-angiogenic factors VEGF, bFGF, Ang-1 and PDGF in hEBs cultured under hypoxic conditions. Along with this, differentiation efficacy into vascular lineage cells was improved under hypoxic conditions. When HIF-1α was blocked by echinomycin, both angiogenic factors and the differentiation efficacy were down-regulated, suggesting that the enhancement of differentiation efficacy was caused by intrinsic up-regulation of HIF-1α and these pro-angiogenic factors under hypoxic condition. This response might be primarily regulated by the HIF-1α signal pathway, and hypoxia might be the key to improving the differentiation of hESCs into vascular lineage cells. Therefore, this study demonstrated that microenvironmental changes (i.e., hypoxia) can improve differentiation efficacy of hESCs into a vascular lineage without exogenous factors via cell-intrinsic up-regulation of angiogenic factors. These facts will contribute to the regulation of stem cell fate.

Glycosaminoglycans as regulators of stem cell differentiation

ES (embryonic stem) cell differentiation is dependent on the presence of HS (heparan sulfate). We have demonstrated that, during differentiation, the evolution of specific cell lineages is associated with particular patterns of GAG (glycosaminoglycan) expression. For example, different HS epitopes are synthesized during neural or mesodermal lineage formation. Cell lines mutant for various components of the HS biosynthetic pathway are selectively impaired in their differentiation, with lineage-specific effects observed for some lines. We have also observed that the addition of soluble GAG saccharides to cells, with or without cell-surface HS, can influence the pace and outcome of differentiation, again highlighting specific pattern requirements for particular lineages. We are combining this work with ongoing studies into the design of artificial cell environments where we have optimized three-dimensional scaffolds, generated by electrospinning or by the formation of hydrogels, for the culture of ES cells. By permeating these scaffolds with defined GAG oligosaccharides, we intend to control the mechanical environment of the cells (via the scaffold architecture) as well as their biological signalling environment (using the oligosaccharides). We predict that this will allow us to control ES cell pluripotency and differentiation in a three-dimensional setting, allowing the generation of differentiated cell types for use in drug discovery/testing or in therapeutics.

Musashi 1-positive cells derived from mouse embryonic stem cells can differentiate into neural and intestinal epithelial-like cells in vivo

Msi1 (Musashi 1) is regarded as a marker for neural and intestinal epithelial stem cells. However, it is still unclear whether Msi1-positive cells derived from mouse embryonic stem cells have the ability to differentiate into neural or intestinal epithelial cells. A pMsi1-GFP (green fluorescent protein) reporter plasmid was constructed in order to sort Msi1-positive cells out of the differentiated cell population. The GFP-positive cells (i.e. Msi1-positive cells) were sorted by FACS and were hypodermically engrafted into the backs of NOD/SCID (non-obese diabetic/severe combined immunodeficient) mice. The presence of neural and intestinal epithelial cells in the grafts was detected. Msi1 was highly expressed in the GFP-positive cells, but not in the GFP-negative cells. The markers for neural cells (Nestin and Tubulin β III) and intestinal epithelial cells [FABP2 (fatty acid binding protein 2), Lyz (lysozyme) and ChA (chromogranin A)] were more highly expressed in the grafts from Msi1-positive cells than those from Msi1-negative cells (P<0.05). The grafts from the Msi1-negative cells contained more mesodermal-like tissues than those from the Msi1-positive cells. The pMsi1-GFP vector can be used to sort Msi1-positive cells from a cell population derived from mouse embryonic stem cells. The Msi1-positive cells can differentiate into neural and intestinal epithelial-like cells in vivo.

SIRT1 deficiency compromises mouse embryonic stem cell hematopoietic differentiation, and embryonic and adult hematopoiesis in the mouse

SIRT1 is a founding member of a sirtuin family of 7 proteins and histone deacetylases. It is involved in cellular resistance to stress, metabolism, differentiation, aging, and tumor suppression. SIRT1(-/-) mice demonstrate embryonic and postnatal development defects. We examined hematopoietic and endothelial cell differentiation of SIRT1(-/-) mouse embryonic stem cells (ESCs) in vitro, and hematopoietic progenitors in SIRT1(+/+)(+/-), and (-/-) mice. SIRT1(-/-) ESCs formed fewer mature blast cell colonies. Replated SIRT1(-/-) blast colony-forming cells demonstrated defective hematopoietic potential. Endothelial cell production was unaltered, but there were defects in formation of a primitive vascular network from SIRT1(-/-)-derived embryoid bodies. Development of primitive and definitive progenitors derived from SIRT1(-/-) ESCs were also delayed and/or defective. Differentiation delay/defects were associated with delayed capacity to switch off Oct4, Nanog and Fgf5 expression, decreased β-H1 globin, β-major globin, and Scl gene expression, and reduced activation of Erk1/2. Ectopic expression of SIRT1 rescued SIRT1(-/-) ESC differentiation deficiencies. SIRT1(-/-) yolk sacs manifested fewer primitive erythroid precursors. SIRT1(-/-) and SIRT1(+/-) adult marrow had decreased numbers and cycling of hematopoietic progenitors, effects more apparent at 5%, than at 20%, oxygen tension, and these progenitors survived less well in vitro under conditions of delayed growth factor addition. This suggests a role for SIRT1 in ESC differentiation and mouse hematopoiesis.

Platelet endothelial cell adhesion molecule-1, stage-specific embryonic antigen-1, and Flk-1 mark distinct populations of mouse embryonic stem cells during differentiation toward hematopoietic/endothelial cells

Vascular endothelial cells (ECs) and most hematopoietic cells express platelet endothelial cell adhesion molecule-1 (PECAM-1), which is the cell surface protein also expressed in mouse embryonic stem (ES) cells. To better understand how PECAM-1(+) ES cells differentiate into PECAM-1(+) hematopoietic cells/ECs, 3 cell surface markers, PECAM-1, stage-specific embryonic antigen-1 (SSEA-1), and Flk-1, were utilized to dissect the developmental process during ES cell differentiation in vitro. Undifferentiated ES cells expressed PECAM-1, with a majority of them coexpressing SSEA-1. During ES cell differentiation, expression of PECAM-1 decreased to give rise to PECAM-1⁻/SSEA-1(+) cells, which represented epiblast stem cells. Subsequently, Flk-1-expressing cells developed from PECAM-1⁻/SSEA-1(+) cells, becoming SSEA-1⁻/Flk-1(+) through the downregulation of SSEA-1 expression. Following this, a second wave of PECAM-1 expression, which represented the mature hematopoietic cells/ECs, developed from Flk-1(+) cells. Also, a small portion of PECAM-1(+)/SSEA-1(+) cells, which represented the residual undifferentiated ES cells, were consistently observed in long-term differentiated embryoid bodies. This work revealed a sequential change in PECAM-1, SSEA-1, and Flk-1 expression during ES cell differentiation; therefore, they could be valuable cell surface markers for isolating cells at distinct developmental stages in ES cell differentiation.

Targeted Vezf1-null mutation impairs vascular structure formation during embryonic stem cell differentiation

OBJECTIVE

Vezf1 encodes an early zinc finger transcription factor that is essential for normal vascular development and functions in a dose-dependent manner. Here, we investigated the role of Vezf1 during processes of endothelial cell differentiation and maturation by studying mutant Vezf1 embryonic stem (ES) cells using the in vitro embryoid body differentiation model and the in vivo teratocarcinoma model.

METHODS AND RESULTS

Vezf1-/- ES cell-derived embryoid bodies failed to form a well-organized vascular network and showed dramatic vascular sprouting defects. Our results indicate that the retinol pathway is an important mediator of Vezf1 function and that loss of Vezf1 results in reduced retinol/vitamin A signaling and aberrant extracellular matrix (ECM) formation. Unexpectedly, we also uncovered defects during in vitro differentiation of Vezf1-/- ES cells along hematopoietic cell lineages. Vezf1-/- ES cell-derived teratocarcinomas were able to spontaneously differentiate into cell types of all 3 germ layers. However, histological and immunohistochemical examination of these tumors showed decreased cell proliferation, delayed differentiation, and large foci of cells with extensive deposition of ECM. Embryoid bodies and teratocarcinomas derived from heterozygous ES cells displayed an intermediate phenotype.

CONCLUSIONS

Together, these results suggest that Vezf1 is involved in early differentiation processes of the vasculature by regulating cell differentiation, proliferation, and ECM distribution and deposition.

Hypoxia influences the vascular expansion and differentiation of embryonic stem cell cultures through the temporal expression of vascular endothelial growth factor receptors in an ARNT-dependent manner

Adaptive responses to low oxygen (O(2)) tension (hypoxia) are mediated by the heterodimeric transcription factor hypoxia inducible factor (HIF). When stabilized by hypoxia, bHLH-PAS alpha- and beta- (HIF-1beta or ARNT) HIF complex regulate the expression of multiple genes, including vascular endothelial growth factor (VEGF). To investigate the mechanism(s) through which hypoxia contributes to blood vessel development, we used embryonic stem cell (ESC) differentiation cultures that develop into embryoid bodies (EBs) mimicking early embryonic development. Significantly, low O(2) levels promote vascular development and maturation in wild-type (WT) ESC cultures measured by an increase in the numbers of CD31(+) endothelial cells (ECs) and sprouting angiogenic EBs, but refractory in Arnt(-/-) and Vegf(-/-) ESC cultures. Thus, we propose that hypoxia promotes the production of ECs and contributes to the development and maturation of vessels. Our findings further demonstrate that hypoxia alters the temporal expression of VEGF receptors Flk-1 (VEGFR-2) and the membrane and soluble forms of the antagonistic receptor Flt-1 (VEGFR-1). Moreover, these receptors are distinctly expressed in differentiating Arnt(-/-) and Vegf(-/-) EBs. These results support existing models in which VEGF signaling is tightly regulated during specific biologic events, but also provide important novel evidence that, in response to physiologic hypoxia, HIF mediates a distinct stoichiometric pattern of VEGF receptors throughout EB differentiation analogous to the formation of vascular networks during embryogenesis.

Quantitative proteome analysis of pluripotent cells by iTRAQ mass tagging reveals post-transcriptional regulation of proteins required for ES cell self-renewal

Embryonic stem cells and embryonal carcinoma cells share two key characteristics: pluripotency (the ability to differentiate into endoderm, ectoderm, and mesoderm) and self-renewal (the ability to grow without change in an untransformed, euploid state). Much has been done to identify and characterize transcription factors that are necessary or sufficient to maintain these characteristics. Oct-4 and Nanog are necessary to maintain pluripotency; they are down-regulated at the mRNA level by differentiation. There may be additional regulatory genes whose mRNA levels are unchanged but whose proteins are destabilized during differentiation. We generated proteome-wide, quantitative profiles of ES and embryonal carcinoma cells during differentiation, replicating a microarray-based study by Aiba et al. (Aiba, K., Sharov, A. A., Carter, M. G., Foroni, C., Vescovi, A. L., and Ko, M. S. (2006) Defining a developmental path to neural fate by global expression profiling of mouse embryonic stem cells and adult neural stem/progenitor cells. Stem Cells 24, 889-895) who triggered differentiation by treatment with 1 μM all-trans-retinoic acid. We identified several proteins whose levels decreased during differentiation in both cell types but whose mRNA levels were unchanged. We confirmed several of these cases by RT-PCR and Western blot. Racgap1 (also known as mgcRacgap) was particularly interesting because it is required for viability of preimplantation embryos and hematopoietic stem cells, and it is also required for differentiation. To confirm our observation that RACGAP-1 declines during retinoic acid-mediated differentiation, we used multiple reaction monitoring, a targeted mass spectrometry-based quantitation method, and determined that RACGAP-1 levels decline by half during retinoic acid-mediated differentiation. We knocked down Racgap-1 mRNA levels using a panel of five shRNAs. This resulted in a loss of self-renewal that correlated with the level of knockdown. We conclude that RACGAP-1 is post-transcriptionally regulated during blastocyst development to enable differentiation by inhibiting ES cell self-renewal.

Hemato-endothelial differentiation from lentiviral-transduced human embryonic stem cells retains durable reporter gene expression under the control of ubiquitin promoter

Human embryonic stem (hES) cells are able to give rise to a variety of cell lineages under specific culture condition. An effective strategy for stable genetic modification in hES cells may provide a powerful tool for study of human embryogenesis and cell-based therapies. However, gene silences are documented in hES cells. In current study, we investigated whether genes controlled under ubiquitin promoter are expressed during hematopoietic-endothelial differentiation in hES cells. Undifferentiated hES cells (H1) were transduced by lentivirus encoding green fluorescent protein (GFP) gene under ubiquitin promoter. GFP-expressing hES cells (GFP-H1) were established after several rounds of mechanical selection under fluorescence microscope. GFP gene was stably expressed in hES cells throughout prolonged (> 50 passages) cultivation, and in differentiated embryo body (EB) and teratoma. Hematopoietic and endothelial markers, including KDR (VEGFR2), CD34, CD31, Tie-2, GATA-1 and GATA-2, were expressed at similar levels during hES cell differentiation in parent hES cells and GFP-H1 hES cells. CD34(+) cells isolated from GFP-H1 hES cells were capable to generate hematopoietic colony-forming cells and tubular structure-forming cells. Differentiated GFP-EB formed vasculature structures in a semi-solid sprouting EB model. These results indicated that a transgene under ubiquitin promoter in lentiviral transduced hES cells retained its expression in undifferentiated hES cells and in hES-derived hematopoietic and endothelial cells. With the view of embryonic mesodermal developing events in humans, genetic modification of hES cells by lentiviral vectors provides a powerful tool for study of hematopoiesis and vasculogenesis.

A role for Egfl7 during endothelial organization in the embryoid body model system

Epidermal growth factor-like domain 7, Egfl7, is a largely endothelial restricted gene which is thought to have a role during the differentiation of embryonic stem cells (ESCs) along the endothelial lineage. While it has been shown that Egfl7 knock-down in zebrafish impairs endothelial cord formation, the role of the gene in mammals has been unresolved. Interpretation of mouse knockout studies has been complicated by the fact that deletion of miR-126, an intronic microRNA located within Egfl7, results in vascular defects. Here we use an siRNA knock-down approach to target specific regions of Egfl7 without affecting miR-126 expression. Egfl7 was knocked down in mouse ESCs and the effect on vascular development was assessed using the in vitro embryoid body (EB) model after either 7 or 14 days of differentiation. Knock-down of Egfl7 resulted in the formation of abnormal sheet-like CD31+ structures that were abundant within EBs after 7 days of differentiation. Only up to 60% of these sheets co-expressed basement membrane and endothelial cell junction markers. Similar CD31+ sheets were also seen as outgrowths from 7 day EBs into collagen gels. A partial remodelling occurred by 14 days of differentiation when fewer CD31+ sheets were seen both within EBs, and as outgrowths from EBs. Formation of these sheets was due, at least in part, to increased proliferation specifically of CD31+ cells. Cell death within EBs was unaffected by Egfl7 knock-down. In conclusion, our work shows that knock-down of Egfl7 causes defects in early vascular cord formation, and results in the development of CD31+ sheet-like structures. This suggests that Egfl7 is vital for the formation of endothelial cell cords, and that the gene has an important role during both vasculogenesis and angiogenesis in mammalian cells.

Role of the microenvironment in the specification of endothelial progenitors derived from embryonic stem cells

Embryonic stem (ES) cells are pluripotent cells capable of differentiating in all the cell types present in a living organism. They derive from the inner cell mass of blastocysts of different species including humans. Given their unlimited potential, ES cells represent an invaluable resource of different cell types for transplantation and tissue engineering applications. However, in order to accomplish these therapeutic purposes, efficient and controlled in vitro systems of directing ES cell differentiation are mandatory. ES cell differentiation is strongly influenced by physical, chemical and cellular signals provided by the local microenvironment. Understanding the relationships occurring between differentiating cells and surrounding environment is pivotal for a successful ES cells-based therapy. This review describes three different methods of in vitro differentiation of ES cells by outlining the environmental elements required for endothelial fate specification. For each system, the efficiency of endothelial differentiation, the accessibility and the advantages are discussed. The main conclusion that arises from this analysis is that the knowledge of the role played by microenvironment in cell fate determination is essential to control and take advantage of ES cells potential.

Scaffolds with covalently immobilized VEGF and Angiopoietin-1 for vascularization of engineered tissues

The aim of this study was to engineer a biomaterial capable of supporting vascularization in vitro and in vivo. We covalently immobilized vascular endothelial growth factor (VEGF) and Angiopoietin-1 (Ang1) onto three-dimensional porous collagen scaffolds using 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC) chemistry. Over both 3 and 7 days in vitro, seeded endothelial cells (ECs) had increased proliferation on scaffolds with immobilized VEGF and/or Ang1 compared to unmodified scaffolds and soluble growth factor controls. Notably, the group with co-immobilized VEGF and Ang1 showed significantly higher cell number (P=0.0079), higher overall lactate production rate (P=0.0044) and higher overall glucose consumption rate (P=0.0034) at Day 3, compared to its corresponding soluble control for which growth factors were added to culture medium. By Day 7, hematoxylin and eosin, live/dead, CD31, and von Willebrand factor staining all showed improved tube formation by ECs when cultivated on scaffolds with co-immobilized growth factors. Interestingly, scaffolds with co-immobilized VEGF and Ang1 showed increased EC infiltration in the chorioallantoic membrane (CAM) assay, compared to scaffolds with independently immobilized VEGF/Ang1. This study presents an alternative method for promoting the formation of vascular structures, via covalent immobilization of angiogenic growth factors that are more stable than soluble ones and have a localized effect.

Controlled embryoid body formation via surface modification and avidin-biotin cross-linking

Cell-cell interaction is an integral part of embryoid body (EB) formation controlling 3D aggregation. Manipulation of embryonic stem (ES) cell interactions could provide control over EB formation. Studies have shown a direct relationship between EB formation and ES cell differentiation. We have previously described a cell surface modification and cross-linking method for influencing cell-cell interaction and formation of multicellular constructs. Here we show further characterisation of this engineered aggregation. We demonstrate that engineering accelerates ES cell aggregation, forming larger, denser and more stable EBs than control samples, with no significant decrease in constituent ES cell viability. However, extended culture >/=5 days reveals significant core necrosis creating a layered EB structure. Accelerated aggregation through engineering circumvents this problem as EB formation time is reduced. We conclude that the proposed engineering method influences initial ES cell-ES cell interactions and EB formation. This methodology could be employed to further our understanding of intrinsic EB properties and their effect on ES cell differentiation.

Transforming growth factor-beta 1 (TGF-beta1) induces angiogenesis through vascular endothelial growth factor (VEGF)-mediated apoptosis

VEGF and TGF-beta1 induce angiogenesis but have opposing effects on endothelial cells. VEGF protects endothelial cells from apoptosis; TGF-beta1 induces apoptosis. We have previously shown that VEGF/VEGF receptor-2 (VEGFR2) signaling mediates TGF-beta1 induction of apoptosis. This finding raised an important question: Does this mechanism stimulate or inhibit angiogenesis? Here we report that VEGF-mediated apoptosis is required for TGF-beta1 induction of angiogenesis. In vitro the apoptotic effect of TGF-beta1 on endothelial cells is rapid and followed by a long period in which the cells are refractory to apoptosis induction by TGF-beta1. Inhibition of VEGF/VEGFR2 signaling abrogates formation of cord-like structures by TGF-beta1 with an effect comparable to that of z-VAD, an apoptosis inhibitor. Similarly, genetic deficiency of VEGF abolishes TGF-beta1 upregulation of endothelial cell differentiation and formation of vascular structures in embryoid bodies. In vivo TGF-beta1 induces endothelial cell apoptosis as rapidly as in vitro. Inhibition of VEGF blocks TGF-beta1 induction of both apoptosis and angiogenesis, an effect similar to that of z-VAD. Thus, TGF-beta1 induction of angiogenesis requires a rapid and transient apoptotic effect mediated by VEGF/VEGFR2. This novel, unexpected role of VEGF and VEGFR2 indicates VEGF-mediated apoptosis as a potential target to control angiogenesis.

An optimized three-dimensional in vitro model for the analysis of angiogenesis

Angiogenesis is the formation of new blood vessels from the existing vasculature. It is a multistage process in which activated endothelial cells (EC) degrade basement membrane, sprout from the parent vessel, migrate, proliferate, align, undergo tube formation, and eventually branch and anastomose with adjacent vessels. Here we describe a three-dimensional in vitro assay that reproduces each of these steps. Human umbilical vein endothelial cells (HUVEC) are cultured on microcarrier beads, which are then embedded in a fibrin gel. Fibroblasts cultured on top of the gel provide factors that synergize with bFGF and VEGF to promote optimal sprouting and tube formation. Sprouts appear around day 2, lumen formation begins at day 4, and at day 10 an extensive anastomosing network of capillary-like tubes is established. The EC express a similar complement of genes as angiogenic EC in vivo and undergo identical morphologic changes during tube formation. This model, therefore, recapitulates in vivo angiogenesis in several critical aspects and provides a system that is easy to manipulate genetically, can be visualized in real time, and allows for easy purification of angiogenic EC for downstream analysis.

Contribution of host-derived tissue factor to tumor neovascularization

OBJECTIVE

The role of host-derived tissue factor (TF) in tumor growth, angiogenesis, and metastasis has hitherto been unclear and was investigated in this study.

METHODS AND RESULTS

We compared tumor growth, vascularity, and responses to cyclophosphamide (CTX) of tumors in wild-type (wt) mice, or in animals with TF levels reduced by 99% (low-TF mice). Global growth rate of 3 different types of transplantable tumors (LLC, B16F1, and ES teratoma) or metastasis were unchanged in low-TF mice. However, several unexpected tumor/context-specific alterations were observed in these mice, including: (1) reduced tumor blood vessel size in B16F1 tumors; (2) larger spleen size and greater tolerance to CTX toxicity in the LLC model; (3) aborted tumor growth after inoculation of TF-deficient tumor cells (ES TF(-/-)) in low-TF mice. TF-deficient tumor cells grew readily in mice with normal TF levels and attracted exclusively host-related blood vessels (without vasculogenic mimicry). We postulate that this complementarity may result from tumor-vascular transfer of TF-containing microvesicles, as we observed such transfer using human cancer cells (A431) and mouse endothelial cells, both in vitro and in vivo.

CONCLUSIONS

Our study points to an important but context-dependent role of host TF in tumor formation, angiogenesis and therapy.

The CMV early enhancer/chicken beta actin (CAG) promoter can be used to drive transgene expression during the differentiation of murine embryonic stem cells into vascular progenitors

Background

Mouse embryonic stem cells cultured in vitro have the ability to differentiate into cells of the three germ layers as well as germ cells. The differentiation mimics early developmental events, including vasculogenesis and early angiogenesis and several differentiation systems are being used to identify factors that are important during the formation of the vascular system. Embryonic stem cells are difficult to transfect, while downregulation of promoter activity upon selection of stable transfectants has been reported, rendering the study of proteins by overexpression difficult.

Results

CCE mouse embryonic stem cells were differentiated on collagen type IV for 4–5 days, Flk1⁺ mesodermal cells were sorted and replated either on collagen type IV in the presence of VEGFA to give rise to endothelial cells and smooth muscle cells or in collagen type I gels for the formation of vascular tubes. The activity of the CMV and β-actin promoters was downregulated during selection of stable transfectants and during differentiation to the Flk1 stage, while the CMV immediate enhancer/β-actin promoter in the pCAGIPuro-GFP vector led to 100% of stably transfected undifferentiated and differentiated cells expressing GFP. To further test this system we expressed syndecan-2 and -4 in these cells and demonstrated high levels of transgene expression in both undifferentiated cells and cells differentiated to the Flk1 stage.

Conclusion

Vectors containing the CAG promoter offer a valuable tool for the long term expression of transgenes during stem cell differentiation towards mesoderm, while the CMV and β-actin promoters lead to very poor transgene expression during this process.

Stem cells and scaffolds for vascularizing engineered tissue constructs

The clinical impact of tissue engineering depends upon our ability to direct cells to form tissues with characteristic structural and mechanical properties from the molecular level up to organized tissue. Induction and creation of functional vascular networks has been one of the main goals of tissue engineering either in vitro, for the transplantation of prevascularized constructs, or in vivo, for cellular organization within the implantation site. In most cases, tissue engineering attempts to recapitulate certain aspects of normal development in order to stimulate cell differentiation and functional tissue assembly. The induction of tissue growth generally involves the use of biodegradable and bioactive materials designed, ideally, to provide a mechanical, physical, and biochemical template for tissue regeneration. Human embryonic stem cells (hESCs), derived from the inner cell mass of a developing blastocyst, are capable of differentiating into all cell types of the body. Specifically, hESCs have the capability to differentiate and form blood vessels de novo in a process called vasculogenesis. Human ESC-derived endothelial progenitor cells (EPCs) and endothelial cells have substantial potential for microvessel formation, in vitro and in vivo. Human adult EPCs are being isolated to understand the fundamental biology of how these cells are regulated as a population and to explore whether these cells can be differentiated and reimplanted as a cellular therapy in order to arrest or even reverse damaged vasculature. This chapter focuses on advances made toward the generation and engineering of functional vascular tissue, focusing on both the scaffolds - the synthetic and biopolymer materials - and the cell sources - hESCs and hEPCs.

Simultaneous differentiation of endothelial and trophoblastic cells derived from human embryonic stem cells

Here we present a simple two-step in vitro model of vascularized trophoblastic tissue derived from human embryonic stem (hES) cells. The first step is the formation of cystic embryoid bodies (EBs) in suspension in a semisolid methyl cellulose medium, within which an endothelial platelet/endothelial cell adhesion molecule-1 (PECAM-1(+)) cell network develops. In a second step, deposition of these EBs on the bottom of nontreated, polystyrene tissue culture plates, leads by centrifugal outgrowth of the EB to the emergence of an adherent cell layer, with which a PECAM-1(+) network is associated. Cells of this adherent layer expressed VE-cadherin (CD144), PECAM-1 (CD31), and alpha-fetoprotein (alpha-FP). Trophoblastic differentiation was strongly suggested by the secretion of beta-human chorionic gonadotropin (beta-hCG) and by the presence of the cytotrophoblast and syncytiotrophoblast marker GB25. The INSL4 gene, a cyto and syncytio-trophoblast marker, was also highly expressed in the adherent layer, as well as other trophoblast genes such as CGA, CDX1, CDX2, and HAND1, compared to hES cell gene expression taken as reference. In contrast, expression of self-renewal genes, such as TERT, POU5F1, ZFP42, GDF3, and NODAL were decreased. No ectodermal or endodermal genes were expressed, but the mesodermal genes PECAM-1 and GATA2 were. The possibility of removing the EBs during the second step would permit analysis of their relative contribution to angiogenesis or possible hemangioblast formation, compared to that of the trophoblastic adherent layer. This primitive vascularized trophoblastic model could also provide a tool to study early steps of normal and pathological placental development.

Mouse lysocardiolipin acyltransferase controls the development of hematopoietic and endothelial lineages during in vitro embryonic stem-cell differentiation

The blast colony-forming cell (BL-CFC) was identified as an equivalent to the hemangioblast during in vitro embryonic stem (ES) cell differentiation. However, the molecular mechanisms underlying the generation of the BL-CFC remain largely unknown. Here we report the isolation of mouse lysocardiolipin acyltransferase (Lycat) based on homology to zebrafish lycat, a candidate gene for the cloche locus. Mouse Lycat is expressed in hematopoietic organs and is enriched in the Lin(-)C-Kit(+)Sca-1(+) hematopoietic stem cells in bone marrow and in the Flk1(+)/hCD4(+)(Scl(+)) hemangioblast population in embryoid bodies. The forced Lycat transgene leads to increased messenger RNA expression of hematopoietic and endothelial genes as well as increased blast colonies and their progenies, endothelial and hematopoietic lineages. The Lycat small interfering RNA transgene leads to a decrease expression of hematopoietic and endothelial genes. An unbiased genomewide microarray analysis further substantiates that the forced Lycat transgene specifically up-regulates a set of genes related to hemangioblasts and hematopoietic and endothelial lineages. Therefore, mouse Lycat plays an important role in the early specification of hematopoietic and endothelial cells, probably acting at the level of the hemangioblast.

No evidence for vasculogenesis regulation by angiostatin during mouse embryonic stem cell differentiation

During embryogenesis, the formation of blood vessels proceeds by both vasculogenesis and angiogenesis. Both processes appear to be finely regulated. To date, factors and genes involved in the negative regulation of embryonic vasculogenesis remain largely unknown. Angiostatin is a proteolytic fragment of plasminogen that acts as an inhibitor of angiogenesis. In this study, we analyzed the potential role of angiostatin during early stages of embryonic stem (ES) cell endothelial in vitro differentiation, as a model of vasculogenesis. We found an early expression of the known angiostatin binding sites (angiomotin, alphav integrin and c-met oncogene) during ES cell differentiation. Nevertheless, we did not detect any significant effect of angiostatin on mesoderm induction and on differentiation commitment into cells of the endothelial lineage. In both control and angiostatin-treated conditions, the temporal and extent of formation of the Flk1 positive and Flk-1/CD31 (PECAM-1) positive cell populations were not significantly different. Quantitative RT-PCR experiments of endothelial gene expression (Flk-1, PECAM-1 and tie-2) confirm a lack of interference with early steps of endothelial differentiation in embryoid bodies. No evidence for an angiostatin effect on endothelial cord-like formation could be detected at later differentiation stages. On the other hand, angiostatin inhibits vascular endothelial growth factor-induced endothelial sprouting from embryoid bodies cultured in three dimensional type I collagen gels. Taken together, these findings support a selective inhibitory effect on the sprouting angiogenesis response for angiostatin during embryonic vascular development.

Platelet-Derived Growth Factor Receptor-β Constitutive Activity Promotes Angiogenesis In Vivo and In Vitro

OBJECTIVE

Knockout studies have demonstrated crucial roles for the platelet-derived growth factor-B and its cognate receptor, platelet-derived growth factor receptor-beta (PDGFR-beta), in blood vessel maturation, that is, the coverage of newly formed vessels with mural cells/pericytes. This study describes the consequences of a constitutively activating mutation of the PDGFR-beta (Pdgfrb(D849V)) introduced into embryonic stem cells with respect to vasculogenesis/angiogenesis in vitro and in vivo.

METHODS AND RESULTS

Embryonic stem cells were induced to either form teratomas in vivo or embryoid bodies, an in vitro model for mouse embryogenesis. Western blotting studies on embryoid bodies showed that expression of a single allele of the mutant Pdgfrb led to increased levels of PDGFR-beta tyrosine phosphorylation and augmented downstream signal transduction. This was accompanied by enhanced vascular development, followed by exaggerated angiogenic sprouting with abundant pericyte coating as shown by immunohistochemistry/immunofluorescence. Pdgfrb(D849V/+) embryoid bodies were characterized by increased expression of vascular endothelial growth factor (VEGF)-A and VEGF receptor-2; neutralizing antibodies against VEGF-A/VEGF receptor-2 blocked vasculogenesis and angiogenesis in mutant embryoid bodies. Moreover, Pdgfrb(D849V/+) embryonic stem cell-derived teratomas in nude mice were more densely vascularized than wild-type teratomas.

CONCLUSIONS

Increased PDGFR-beta kinase activity is associated with elevated expression of VEGF-A and VEGF receptor-2, acting directly on endothelial cells and resulting in increased vessel formation.

In vitro assays of angiogenesis for assessment of angiogenic and anti-angiogenic agents

Blood vessels, either in insufficient numbers or in excess, contribute to the pathogenesis of many diseases. Agents that stimulate angiogenesis can improve blood flow in patients with ischemic diseases, whereas anti-angiogenic agents are used to treat disorders ranging from macular degeneration to cancer. In this review I describe in vitro assays that can be used to assess the activity of agents that affect angiogenesis. Means of quantifying endothelial cell matrix degradation, migration, proliferation, apoptosis and morphogenesis are discussed, as are embryoid body, aortic ring and metatarsal assays of vessel outgrowth. Strengths and limitations of these techniques are also addressed.

Methods for inducing embryoid body formation: in vitro differentiation system of embryonic stem cells

When cultured in suspension without antidifferentiation factors, embryonic stem (ES) cells spontaneously differentiate and form three-dimensional multicellular aggregates called embryoid bodies (EBs). EBs recapitulate many aspects of cell differentiation during early embryogenesis, and play an important role in the differentiation of ES cells into a variety of cell types in vitro. There are several methods for inducing the formation of EBs from ES cells. The three basic methods are liquid suspension culture in bacterial-grade dishes, culture in methylcellulose semisolid media, and culture in hanging drops. Recently, the methods using a round-bottomed 96-well plate and a conical tube are adopted for forming EBs from predetermined numbers of ES cells. For the production of large numbers of EBs, stirred-suspension culture using spinner flasks and bioreactors is performed. Each of these methods has its own peculiarity; thus, the features of formed EBs depending on the method used. Therefore, we should choose an appropriate method for EB formation according to the objective to be attained. In this review, we summarize the studies on in vitro differentiation of ES cells via EB formation and highlight the EB formation methods recently developed including the techniques, devices, and procedures involved.