Ligand-dependent development of the endothelial and hemopoietic lineages from embryonic mesodermal cells expressing vascular endothelial growth factor receptor 2

Differential adhesion molecule expression during murine embryonic stem cell commitment to the hematopoietic and endothelial lineages

Mouse embryonic stem cells (ESC) make cell fate decisions based on intrinsic and extrinsic factors. The decision of ESC to differentiate to multiple lineages in vitro occurs during the formation of embryoid bodies (EB) and is influenced by cell-environment interactions. However, molecular mechanisms underlying cell-environmental modulation of ESC fate decisions are incompletely understood. Since adhesion molecules (AM) influence proliferation and differentiation in developing and adult tissues, we hypothesized that specific AM interactions influence ESC commitment toward hematopoietic and endothelial lineages. Expression of AM in the adherens, tight and gap junction pathways in ESC subpopulations were quantified. E-cadherin (E-cad), Claudin-4 (Cldn4), Connexin-43 (Cx43), Zona Occludens-1 (ZO-1) and Zona Occludens-2 (ZO-2) transcript levels were differentially expressed during early stages of hematopoietic/endothelial commitment. Stable ESC lines were generated with reduced expression of E-cad, Cldn4, Cx43, ZO-1 and ZO-2 using shRNA technology. Functional and phenotypic consequences of modulating AM expression were assessed using hematopoietic colony forming assays, endothelial sprouting assays and surface protein expression. A decrease in E-cad, Cldn4, Cx43 and ZO-1 expression was associated with less commitment to the hematopoietic lineage and increased endothelial differentiation as evidenced by functional and phenotypic analysis. A reduction in ZO-2 expression did not influence endothelial differentiation, but decreased hematopoietic commitment two-fold. These data indicate that a subset of AM influence ESC decisions to commit to endothelial and hematopoietic lineages. Furthermore, differentially expressed AM may provide novel markers to delineate early stages of ESC commitment to hematopoietic/endothelial lineages.

[Neural crest and vertebrate evolution]

The neural crest (NC) is a remarkable structure of the Vertebrate embryo, which forms from the lateral borders of the neural plate (designated as neural folds) during neural tube closure. As soon as the NC is formed, its constitutive cells detach and migrate away from the neural primordium along definite pathways and at precise periods of time according to a rostro-caudal progression. The NC cells aggregate in definite places in the developing embryo, where they differentiate into a large variety of cell types including the neurons and glial cells of the peripheral nervous system, the pigment cells dispersed throughout the body and endocrine cells such as the adrenal medulla and the calcitonin producing cells. At the cephalic level only, in higher Vertebrates (but along the whole neural axis in Fishes and Amphibians), the NC is also at the origin of mesenchymal cells differentiating into connective tissue chondrogenic and osteogenic cells. Vertebrates belong to the larger group of Cordates which includes also the Protocordates (Cephalocordates and the Urocordates). All Cordates are characterized by the same body plan with a dorsal neural tube and a notochord which, in Vertebrates, exists only at embryonic stages. The main difference between Protocordates and Vertebrates is the very rudimentary development of cephalic structures in the former. As a result, the process of cephalization is one of the most obvious characteristics of Vertebrates. It was accompanied by the apparition of the NC which can therefore be considered as an innovation of Vertebrates during evolution. The application of a cell marking technique which consists in constructing chimeric embryos between two species of birds, the quail and the chicken, has led to show that the vertebrate head is mainly formed by cells originating from the NC, meaning that this structure was an important asset in Vertebrate evolution. Recent studies, described in this article, have strengthened this view by showing that the NC does not only provide the cells that build up the facial skeleton and most of the skull but plays a major role in early brain neurogenesis. It was shown that the cephalic NC cells produce signaling molecules able to regulate the activity of the two secondary organizing centers previously identified in the developing brain: the anterior neural ridge and the midbrain-hindbrain junction, which secrete Fgf8, a potent stimulator of early brain neurogenesis.

On the origin of hematopoietic stem cells: progress and controversy

Hematopoietic Stem Cells (HSCs) are responsible for the production and replenishment of all blood cell types during the entire life of an organism. Generated during embryonic development, HSCs transit through different anatomical niches where they will expand before colonizing in the bone marrow, where they will reside during adult life. Although the existence of HSCs has been known for more than fifty years and despite extensive research performed in different animal models, there is still uncertainty with respect to the precise origins of HSCs. We review the current knowledge on embryonic hematopoiesis and highlight the remaining questions regarding the anatomical and cellular identities of HSC precursors.

Methodological Development of a Clonogenic Assay to Determine Endothelial Progenitor Cell Potential

The precise and conceptual insight of circulating endothelial progenitor cell (EPC) kinetics is hampered by the absence of an assay system capable of evaluating the EPC differentiation cascade. An assay system for EPC colony formation was developed to delineate circulating EPC differentiation. EPC colony-forming assay using semisolid medium and single or bulk CD133 + cells from umbilical cord blood exhibited the formation of two types of attaching cell colonies made of small or large cells featuring endothelial lineage potential and properties, termed small EPC colony-forming units and large EPC colony-forming units, respectively. In vitro and in vivo assays of each EPC colony-forming unit cell revealed a differentiation hierarchy from small EPC to large EPC colonies, indicating a primitive EPC stage with highly proliferative activity and a definitive EPC stage with vasculogenic properties, respectively. Experimental comparison with a conventional EPC culture assay system disclosed EPC colony-forming unit cells differentiate into noncolony-forming early EPC. The fate analysis of single CD133 + cells into the endothelial and hematopoietic lineage was achieved by combining this assay system with a hematopoietic progenitor assay and demonstrated the development of colony-forming EPC and hematopoietic progenitor cells from a single hematopoietic stem cell. EPC colony-forming assay permits the determination of circulating EPC kinetics from single or bulk cells, based on the evaluation of hierarchical EPC colony formation. This assay further enables a proper exploration of possible links between the origin of EPC and hematopoietic stem cells, representing a novel and powerful tool to investigate the molecular signaling pathways involved in EPC biology.

Notch signaling in mammalian hematopoietic stem cells

Notch is a crucial cell signaling pathway in metazoan development. By means of cell-cell interactions, Notch signaling regulates cellular identity, proliferation, differentiation and apoptosis. Within the last decade, numerous studies have shown an important role for this pathway in the development and homeostasis of mammalian stem cell populations. Hematopoietic stem cells (HSCs) constitute a well-defined population that shows self-renewal and multi-lineage differentiation potential, with the clinically relevant capacity to repopulate the hematopoietic system of an adult organism. Here, we review the emergence, development and maintenance of HSCs during mammalian embryogenesis and adulthood, with respect to the role of Notch signaling in hematopoietic biology.

Embryonic origin of the adult hematopoietic system: advances and questions

Definitive hematopoietic stem cells (HSCs) lie at the foundation of the adult hematopoietic system and provide an organism throughout its life with all blood cell types. Several tissues demonstrate hematopoietic activity at early stages of embryonic development, but which tissue is the primary source of these important cells and what are the early embryonic ancestors of definitive HSCs? Here, we review recent advances in the field of HSC research that have shed light on such questions, while setting them into a historical context, and discuss key issues currently circulating in this field.

Stem cells in the treatment of patients with coronary heart disease. Part I

Heart failure (HF) is one of the leading death causes in patients with myocardial infarction (MI). The modern methods of reperfusion MI therapy, such as thrombolysis, surgery and balloon revascularization, even when performed early, could fail to prevent the development of large myocardial damage zones, followed by HF. Therefore, the researches have been searching for the methods which improve functional status of damaged myocardium. This review is focused on stem cell therapy, a method aimed at cardiac function restoration. The results of experimental and clinical studies on stem cell therapy in coronary heart disease are presented. Various types of stem cells, used for cellular cardiomyoplasty, are characterised. The methods of cell transplantation into myocardium and potential adverse effects of stem cell therapy are discussed.

Spry1 is expressed in hemangioblasts and negatively regulates primitive hematopoiesis and endothelial cell function

BACKGROUND

Development of the hematopoietic and endothelial lineages derives from a common mesodermal precursor, the Flk1(+) hemangioblast. However, the signaling pathways that regulate the development of hematopoietic and endothelial cells from this common progenitor cell remains incompletely understood. Using mouse models with a conditional Spry1 transgene, and a Spry1 knockout mouse, we investigated the role of Spry1 in the development of the endothelial and hematopoietic lineages during development.

METHODOLOGY/PRINCIPAL FINDINGS

Quantitative RT-PCR analysis demonstrates that Spry1, Spry2, and Spry4 are expressed in Flk1(+) hemangioblasts in vivo, and decline significantly in c-Kit(+) and CD41(+) hematopoietic progenitors, while expression is maintained in developing endothelial cells. Tie2-Cre-mediated over-expression of Spry1 results in embryonic lethality. At E9.5 Spry1;Tie2-Cre embryos show near normal endothelial cell development and vessel patterning but have reduced hematopoiesis. FACS analysis shows a reduction of primitive hematopoietic progenitors and erythroblastic cells in Spry1;Tie2-Cre embryos compared to controls. Colony forming assays confirm the hematopoietic defects in Spry1;Tie2-Cre transgenic embryos. Immunostaining shows a significant reduction of CD41 or CD71 and dpERK co-stained cells in Spry1;Tie2-Cre embryos compared to controls, whereas the number of VEC(+) and dpERK co-stained cells is comparable. Compared to controls, Spry1;Tie2-Cre embryos also show a decrease in proliferation and an increase in apoptosis. Furthermore, loss of Spry1 results in an increase of CD41(+) and CD71(+) cells at E9.5 compared with controls.

CONCLUSIONS/SIGNIFICANCE

These data indicate that primitive hematopoietic cells derive from Tie2-expressing hemangioblasts and that Spry1 over expression inhibits primitive hematopoietic progenitor and erythroblastic cell development and expansion while having no obvious effect on endothelial cell development.

An adult uterine hemangioblast: evidence for extramedullary self-renewal and clonal bilineage potential

Stem cells exhibit long-term self-renewal by asymmetric division and multipotent differentiation. During embryonic development, cell fate is determined with predictable orientation, differentiation, and partitioning to form the organism. This includes the formation of a hemangioblast from which 2 derivative cell clusters commit to either a hematopoietic or an endothelial lineage. Frequently, it is not clear whether tissue resident stem cells in the adult originate from the bone marrow. Here, we show that blast colony-forming cells exhibiting bilineage (hematopoietic and vascular) potential and long-term self-renewal originate from the uterus in the mouse. This is the first in vitro and in vivo evidence of an adult hemangioblast retained from development in the uterus. Our findings offer new understanding of uterine cell renewal and turnover and may provide insights and opportunities for the study of stem cell maintenance.

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.

Evaluation of mobilized peripheral blood CD34(+) cells from patients with severe coronary artery disease as a source of endothelial progenitor cells

BACKGROUND AIMS

The distinction between hematopoietic stem cells (HSC) and endothelial progenitor cells (EPC) is poorly defined. Co-expression of CD34 antigen with vascular endothelial growth factor (VEGF) receptor (VEGFR2) is currently used to define EPC ( 1 ).

METHODS

We evaluated the phenotypic and genomic characteristics of peripheral blood-derived CD34(+) cells in 22 granulocyte-colony-stimulating factor (G-CSF)-mobilized patients with severe coronary artery disease and assessed the influence of cell selection and storage on CD34(+) cell characteristics.

RESULTS

The median CD34(+) cell contents in the products before and after enrichment with the Isolex 300i Magnetic Cell Selection System were 0.2% and 82.5%, respectively. Cell-cycle analysis showed that 80% of CD34(+) cells were in G0 stage; 70% of the isolated CD34(+) cells co-expressed CD133, a marker for more immature progenitors. However, less than 5% of the isolated CD34(+) cells co-expressed the notch receptor Jagged-1 (CD339) and only 2% of the isolated CD34(+) population were positive for VEGFR2 (CD309). Molecular assessment of the isolated CD34(+) cells demonstrated extremely low expression of VEGFR2 and endothelial nitric oxide synthase (eNOS) and high expression of VEGF-A. Overnight storage at 4 degrees C did not significantly affect CD34(+) cell counts and viability. Storage in liquid nitrogen for 7 weeks did not affect the percentage of CD34(+) cells but was associated with a 26% drop in cell viability.

CONCLUSIONS

We have demonstrated that the majority of isolated CD34(+) cells consist of immature and quiescent cells that lack prototypic markers of EPC. High VEGF-A gene expression might be one of the mechanisms for CD34(+) cell-induced angiogenesis.

Tumor angiogenesis: insights and innovations

Angiogenesis is a vital process resulting in the formation of new blood vessels. It is normally a highly regulated process that occurs during human development, reproduction, and wound repair. However, angiogenesis can also become a fundamental pathogenic process found in cancer and several other diseases. To date, the inhibition of angiogenesis has been researched at both the bench and the bedside. While several studies have found moderate improvements when treating with angiogenesis inhibitors, greater success is being seen when the inhibition of angiogenesis is combined with other traditional forms of available therapy. This review summarizes several important angiogenic factors, examines new research and ongoing clinical trials for such factors, and attempts to explain how this new knowledge may be applied in the fight against cancer and other angiogenic-related diseases.

Evaluation of angiogenesis with vascular endothelial growth factor in patients with thalassemia major

BACKGROUND

Thalassemia major (TM) is an important cause of severe anemia that necessitates regular blood transfusion to prevent the profound weakness and cardiac decompensation caused by the anemia. However, iron overloading is an inevitable consequence of prolonged transfusion therapy. In addition, extramedullary hematopoiesis and hemosiderosis cause spleen, liver and marrow enlargement. In recent years the role of angiogenesis has been investigated in physiological and pathological conditions. However, it is known that angiogenetic factors, especially the vascular endothelial growth factor (VEGF), cause differentiation of the hemangioblast.

METHODS

The effect of angiogenesis hasn't been investigated in TM patients yet, and in this study, angiogenesis was researched in 43 thalassemic patients by serum VEGF measurement.

RESULTS

VEGF levels were not affected by hemoglobin levels, ferritin levels, or chelation type (P > 0.05). However, VEGF was positively affected by chelation starting age and negatively affected by yearly transfusion requirement of TM patients (P < 0.05). In addition, VEGF of patients who underwent splenectomy were higher than those who didn't undergo splenectomy (P < 0.05).

CONCLUSION

Early chelating age will negatively influence the VEGF level, which increases angiogenesis, however, early starting transfusion age and regular blood transfusion will positively influence the VEGF level, which decreases angiogenesis in thalassemic patients.

Angiogenesis inhibition in cancer therapy: platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF) and their receptors: biological functions and role in malignancy

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen in vitro and an angiogenic inducer in a variety of in vivo models. VEGF gene transcription is induced in particular in hypoxic cells. In developmental angiogenesis, the role of VEGF is demonstrated by the finding that the loss of a single VEGF allele results in defective vascularization and early embryonic lethality. Substantial evidence also implicates VEGF as a mediator of pathological angiogenesis. In situ hybridization studies demonstrate expression of VEGF mRNA in the majority of human tumors. Platelet-derived growth factor (PDGF) is mainly believed to be an important mitogen for connective tissue, and also has important roles during embryonal development. Its overexpression has been linked to different types of malignancies. Thus, it is important to understand the physiology of VEGF and PDGF and their receptors as well as their roles in malignancies in order to develop antiangiogenic strategies for the treatment of malignant disease.

Generation of immunogenic dendritic cells from human embryonic stem cells without serum and feeder cells

AIM

Dendritic cell (DC)-based vaccines have a potential utility for use in the treatment of malignancy. Human embryonic stem cells (hESCs) may provide a more cost-effective and reliable source of DCs for immunotherapy purposes, providing on-demand access for patients.

METHOD

We developed a protocol to generate DCs from hESCs in vitro in the absence of serum and feeder cells. This protocol uses growth factors bone morphogenetic protein-4, granulocyte macrophage-colony stimulating factor (GM-CSF), stem cell factor and VEGF in serum-free media to generate hESC-derived monocytic cells. These cells are further differentiated to hESC-derived immature DCs with GM-CSF and IL-4, and matured to hESC-derived mature DCs with a maturation cocktail consisting of GM-CSF, TNF-alpha, IL-1beta, IFN-gamma and PGE2.

RESULTS

This study demonstrates the applicability of our defined differentiation process in generating functional hESC-derived DCs from multiple hESC lines. We show that hESC-derived immature DCs phagocytose, process, and present antigen upon maturation. hESC-derived mature DCs express the maturation marker CD83, produce Th1-directing cytokine IL-12p70, migrate in response to chemokine, and activate both viral and tumor antigen-specific T-cell responses.

CONCLUSION

We developed a chemically defined system to generate unlimited numbers of DCs from hESCs. Our results demonstrate that hESC-derived DCs generated from this process are immunogenic and have the potential to be used for DC immunotherapy.

Enhancement of vascular progenitor potential by protein kinase A through dual induction of Flk-1 and Neuropilin-1

Fine tuning of vascular endothelial growth factor (VEGF) signaling is critical in endothelial cell (EC) differentiation and vascular development. Nevertheless, the system for regulating the sensitivity of VEGF signaling has remained unclear. Previously, we established an embryonic stem cell culture reproducing early vascular development using Flk1 (VEGF receptor-2)+ cells as common progenitors, and demonstrated that cyclic adenosine monophosphate (cAMP) enhanced VEGF-induced EC differentiation. Here we show that protein kinase A (PKA) regulates sensitivity of Flk1+ vascular progenitors to VEGF signaling for efficient EC differentiation. Blockade of PKA perturbed EC differentiation and vascular formation in vitro and ex vivo. Overexpression of constitutive active form of PKA (CA-PKA) potently induced EC differentiation and vascular formation. Expression of Flk1 and Neuropilin-1 (NRP1), which form a selective and sensitive receptor for VEGF(165), was increased only in CA-PKA-expressing progenitors, enhancing the sensitivity of the progenitors to VEGF(165) by more than 10 times. PKA activation induced the formation of a VEGF(165), Flk1, and NRP1 protein complex in vascular progenitors. These data indicate that PKA regulates differentiation potential of vascular progenitors to be endothelial competent via the dual induction of Flk1 and NRP1. This new-mode mechanism regulating "progenitor sensitivity" would provide a novel understanding in vascular development and regeneration.

Endothelial cells from cord blood CD133+CD34+ progenitors share phenotypic, functional and gene expression profile similarities with lymphatics

The existence of endothelial progenitor cells (EPC) with high cell-cycle rate in human umbilical cord blood has been recently shown and represents a challenging strategy for therapeutic neovascularization. To enhance knowledge for future cellular therapy, we compared the phenotypic, functional and gene expression differences between EPC-derived cells generated from cord blood CD34⁺ cells, and lymphatic and macrovascular endothelial cells (EC) isolated from human foreskins and umbilical veins, respectively. Under appropriate culture conditions, EPC developed into fully matured EC with expression of similar endothelial markers as lymphatic and macrovascular EC, including CD31, CD36, von Willebrand factor FVIII, CD54 (ICAM-1), CD105 (endoglin), CD144 (VE-cadherin), Tie-1, Tie-2, VEGFR-1/Flt-1 and VEGFR-2/Flk-1. Few EPC-derived cells became positive for LYVE-1, indicating their origin from haematopoietic stem cells. However they lacked expression of other lymphatic cell-specific markers such as podoplanin and Prox-1. Functional tests demonstrated that the cobblestone EPC-derived cells up-regulated CD54 and CD62E expression in response to TNF-α, incorporated DiI-acetylated low-density liproprotein and formed cord- and tubular-like structures with capillary lumen in three-dimensional collagen culture – all characteristic features of the vascular endothelium. Structures compatible with Weibel-Palade bodies were also found by electron microscopy. Gene microarray profiling revealed that only a small percentage of genes investigated showed differential expression in EPC-derived cells and lymphatic EC. Among them were adhesion molecules, extracellular matrix proteins and cytokines. Our data point to the close lineage relationship of both types of vascular cells and support the theory of a venous origin of the lymphatic system.

Regulation of cell migration during chick gastrulation

Gastrulation in chick starts with large-scale cell flows in the epiblast and hypoblast, which transport the mesendoderm into the midline of the embryo to form the primitive streak. Several mechanisms such as cell-cell intercalation, deformations of the extracellular matrix and directed cell movements in response to chemical gradients have been proposed to play a role in streak formation. In the streak the epiblast cells undergo an epithelial to mesenchymal transition (EMT), which involves the breakdown of apical junctions and changes in RhoA-dependent signalling to integrins that mediated contact with the basal lamina. The collective migration of the mesendoderm away from the streak appears to be controlled by gradients of growth factors of the FGF and VEGF and Wnt families and requires N-cadherin expression. The timing and order of ingression of epiblast cells appears to be controlled by temporal and spatial colinearity of Hox gene expression in the epiblast. The mechanisms by which Hox genes control these properties remain to be resolved.

Decoding the hemogenic endothelium in mammals

A collection of recent Nature papers examines the relationship between endothelial precursors and hematopoietic cells. Two of these studies (Eilken et al., 2009; Lancrin et al., 2009) use time-lapse imaging with live markers and genetic analysis of differentiating ESCs to reveal that even non-aortic-derived endothelial cells are hemogenic.

Discovery and characterization of novel vascular and hematopoietic genes downstream of etsrp in zebrafish

The transcription factor Etsrp is required for vasculogenesis and primitive myelopoiesis in zebrafish. When ectopically expressed, etsrp is sufficient to induce the expression of many vascular and myeloid genes in zebrafish. The mammalian homolog of etsrp, ER71/Etv2, is also essential for vascular and hematopoietic development. To identify genes downstream of etsrp, gain-of-function experiments were performed for etsrp in zebrafish embryos followed by transcription profile analysis by microarray. Subsequent in vivo expression studies resulted in the identification of fourteen genes with blood and/or vascular expression, six of these being completely novel. Regulation of these genes by etsrp was confirmed by ectopic induction in etsrp overexpressing embryos and decreased expression in etsrp deficient embryos. Additional functional analysis of two newly discovered genes, hapln1b and sh3gl3, demonstrates their importance in embryonic vascular development. The results described here identify a group of genes downstream of etsrp likely to be critical for vascular and/or myeloid development.

Sonic hedgehog is required for the assembly and remodeling of branchial arch blood vessels

Sonic hedgehog (Shh) is a morphogen involved in many developmental processes. Injection of cells (5E1) that produce a Shh-blocking antibody causes an attenuation of the Shh response, and this causes vascular malformations and impaired remodeling characterized by hemorrhages and protrusions of the anterior cardinal vein and outflow tract, delayed fusion of the dorsal aortae, impaired branching of the internal carotid artery, and delayed remodeling of the aortic arches. Distribution of smooth muscle cells in the vessel wall is unchanged. In 5E1-injected embryos, we also observed impaired assembly of endothelial cells into vascular tubes, particularly in the sixth branchial arch, around the anterior cardinal vein and around the dorsal aorta. In 5E1-treated embryos, increased numbers of macrophage-like cells, apoptotic cells, and a decreased level of proliferation were observed in head mesenchyme. Together, these observations show that Shh signaling is required at multiple stages for proper vessel formation and remodeling.

Human fetal aorta-derived vascular progenitor cells: identification and potential application in ischemic diseases

Vasculogenesis, the formation of blood vessels in embryonic or fetal tissue mediated by immature vascular cells (i.e., angioblasts), is poorly understood. Here we report a summary of our recent studies on the identification of a population of vascular progenitor cells (VPCs) in human fetal aorta. These undifferentiated mesenchymal cells co-express endothelial and myogenic markers (CD133+, CD34+, KDR+, desmin+) and are localized in outer layer of the aortic stroma of 11-12 weeks old human fetuses. Under stimulation with VEGF-A or PDGF-BB, VPCs give origin to a mixed population of mature endothelial and mural cells, respectively. When embedded in a three-dimensional collagen gel, VPCs organize into cohesive cellular cords that resembled mature vascular structures. The therapeutic efficacy of a small number of VPCs transplanted into ischemic limb muscle was demonstrated in immunodeficient mice. Investigation of the effect of VPCs on experimental heart ischemia and on diabetic ischemic ulcers in mice is in progress and seems to confirm their efficacy. On the whole, fetal aorta represents an important source for the investigation of phenotypic and functional features of human vascular progenitor cells.

Molecular and developmental biology of the hemangioblast

The hemangioblast hypothesis was proposed a century ago. The existence of hemangioblasts is now demonstrated in mouse and human embryonic stem cell (ESC)-derived embryoid bodies (EBs), in the mouse and zebrafish gastrula, and in adults. The hemangioblast is believed to derive from mesodermal cells, and is enriched in the Bry+Flk1+ and Flk1+Scl+ cell populations in EBs and in the posterior primitive streak of the mouse gastrula and in the ventral mesoderm of the zebrafish gastrula. However, recent studies suggest that the hemangioblast does not give rise to all endothelial and hematopoietic lineages in mouse and zebrafish embryos. Although several signaling pathways are known to involve the generation of hemangioblasts, it remains largely unknown how the hemangioblast is formed and what are the master genes controlling hemangioblast development. This review will summarize our current knowledge, challenges, and future directions on molecular and developmental aspects of the hemangioblast.

The Wnt signaling regulator R-spondin 3 promotes angioblast and vascular development

The vertebrate embryonic vasculature develops from angioblasts, which are specified from mesodermal precursors and develop in close association with blood cells. The signals that regulate embryonic vasculogenesis and angiogenesis are incompletely understood. Here, we show that R-spondin 3 (Rspo3), a member of a novel family of secreted proteins in vertebrates that activate Wnt/beta-catenin signaling, plays a key role in these processes. In Xenopus embryos, morpholino antisense knockdown of Rspo3 induces vascular defects because Rspo3 is essential for regulating the balance between angioblast and blood cell specification. In mice, targeted disruption of Rspo3 leads to embryonic lethality caused by vascular defects. Specifically in the placenta, remodeling of the vascular plexus is impaired. In human endothelial cells, R-spondin signaling promotes proliferation and sprouting angiogenesis in vitro, indicating that Rspo3 can regulate endothelial cells directly. We show that vascular endothelial growth factor is an immediate early response gene and a mediator of R-spondin signaling. The results identify Rspo3 as a novel, evolutionarily conserved angiogenic factor in embryogenesis.

Mechanisms of angiogenesis

Tissue activity of angiogenesis depends on the balance of many stimulating or inhibiting factors. The key signaling system that regulates proliferation and migration of endothelial cells forming the basis of any vessel are vascular endothelium growth factors (VEGF) and their receptors. The VEGF-dependent signaling system is necessary for formation of the embryonic vascular system. Neoangiogenesis during tumor growth is also associated with activation of this signaling system. The biological significance of the effect of such system on the cells depends on the content in tissue of various factors of the VEGF family and their receptors, while in the case of VEGFA it is defined by the ratio of different isoforms of this growth factor. A number of other signaling systems are also involved in regulation of the main steps of vessel formation. The signaling system Dll4/Notch regulates selection of endothelial cells for beginning of angiogenic expansion by endowing particular properties to endothelial cells leading in this process. An important step in vessel stabilization and maturation is vascular wall formation. Signaling system PDGFB/PDGFRbeta as well as angiopoietins Ang1, Ang2, and their receptor Tie2 are involved in recruiting mural cells (pericytes and smooth muscle cells). Identification of key molecules involved in the regulation of angiogenesis may provide new possibilities for development of drugs suitable for inhibition of angiogenesis or its stimulation in various pathologies.

Soluble Flt-1 regulates Flk-1 activation to control hematopoietic and endothelial development in an oxygen-responsive manner

Vascular endothelial growth factor (VEGF) and the vascular endothelial growth factor receptors (VEGFRs) regulate the development of hemogenic mesoderm. Oxygen concentration-mediated activation of hypoxia-inducible factor targets such as VEGF may serve as the molecular link between the microenvironment and mesoderm-derived blood and endothelial cell specification. We used controlled-oxygen microenvironments to manipulate the generation of hemogenic mesoderm and its derivatives from embryonic stem cells. Our studies revealed a novel role for soluble VEGFR1 (sFlt-1) in modulating hemogenic mesoderm fate between hematopoietic and endothelial cells. Parallel measurements of VEGF and VEGFRs demonstrated that sFlt-1 regulates VEGFR2 (Flk-1) activation in both a developmental-stage-dependent and oxygen-dependent manner. Early transient Flk-1 signaling occurred in hypoxia because of low levels of sFlt-1 and high levels of VEGF, yielding VEGF-dependent generation of hemogenic mesoderm. Sustained (or delayed) Flk-1 activation preferentially yielded hemogenic mesoderm-derived endothelial cells. In contrast, delayed (sFlt-1-mediated) inhibition of Flk-1 signaling resulted in hemogenic mesoderm-derived blood progenitor cells. Ex vivo analyses of primary mouse embryo-derived cells and analysis of transgenic mice secreting a Flt-1-Fc fusion protein (Fc, the region of an antibody which is constant and binds to receptors) support a hypothesis whereby microenvironmentally regulated blood and endothelial tissue specification is enabled by the temporally variant control of the levels of Flk-1 activation. Disclosure of potential conflicts of interest is found at the end of this article.

Synthesis and evaluation of heteroaryl-ketone derivatives as a novel class of VEGFR-2 inhibitors

We have discovered novel inhibitors of VEGFR-2 kinase with low nanomolar potency in both enzymatic and cell-based assays. Active series are heteroaryl-ketone compounds containing a central aromatic ring with either an indazolyl or indolyl keto group in the ortho orientation to the benzylic amine group (Fig. 1). The best compounds were demonstrated to be inactive against a small select panel of tyrosine and serine/threonine kinases with the exception of VEGFR-1 kinase, a close family member. In addition, the lead candidate 8 displayed acceptable exposure levels when administered orally to mice.

Differentiation of endothelial cells derived from mouse embryoid bodies: a possible in vitro vasculogenesis model

Mouse embryonic stem cells (mES cells), which are pluripotent and self-renewal cells, are derived from the inner cell mass of mouse blastocysts. The objective of this study was to construct more efficient mES cell-derived embryoid bodies (EBs) for use as a vasculogenesis model and as an in vitro vascular toxicity testing model. EBs were formed for 3 days using hanging drop cultures and plated on gelatin-coated plates in endothelial growth medium-2 (EGM-2) to promote vascular development. The differentiation of mES cell-derived EBs was confirmed by reverse transcription-polymerase chain reaction (RT-PCR), immunocytochemistry, and flow cytometry within 7 days after plating EBs. The mRNA and protein expressions of vascular endothelial growth factor receptors-2 (FLK-1), platelet endothelial cell adhesion molecule (PECAM), and vascular endothelial-cadherin (VE-cadherin) were observed in differentiated mES cells. When placed in matrigel, mES cell-derived endothelial like cells formed networks similar to vascular structures. mES cells were also exposed to 5-fluorouracil (5-FU), a strong inhibitor of vessel formation, and its cytotoxicity was determined using MTT assays. The inhibitory concentrations (IC50) of 5-FU for mES cells and C166 cells were 0.72 microM and 1.04 microM, respectively. These results demonstrate that mES cells can be used to study vasculogenesis and for cytotoxicity screening.

Analysis of the temporal and concentration-dependent effects of BMP-4, VEGF, and TPO on development of embryonic stem cell-derived mesoderm and blood progenitors in a defined, serum-free media

OBJECTIVE

To develop a robust serum-free (SF) system for generation of hemogenic mesoderm and blood progenitors from pluripotent cells.

MATERIALS AND METHODS

Embryonic stem cells (ESCs) maintained in N2B27 supplemented with leukemia inhibitory factor (LIF) and bone morphogenetic protein (BMP)-4 were induced to differentiate into Brachyury/T-expressing cells (measured using a green fluorescent protein reporter) and myeloid-erythroid colony-forming cells (ME-CFCs), by removing LIF, changing the base media formulation, and via the time- and concentration-dependent addition of other factors.

RESULTS

Presence of 10 ng/mL BMP-4 permitted the emergence of cells expressing T and the vascular endothelial growth factor receptor (VEGFR)-2, however, <5% of the cells were double-positive on day 4. Adjusting the SF media formulation allowed only 5 ng/mL BMP-4 to yield 24% +/- 4% Brachyury-green fluorescent protein VEGFR-2(+) cells by day 4. These cells could develop into ME-CFC, producing 4.4 +/- 0.8 CFC per 1000 cells at day 8. We also examined the timing and concentration sensitivity of BMP-4, VEGF, and thrombopoietin (TPO) during differentiation. BMP-4 with 50 ng/mL TPO generated 232 +/- 48 CFC per 5 x 10(4) cells, similar to the serum-control, and this response could be enhanced to 292 +/- 42 CFC per 5 x 10(4) cells by early (between day 0-5), but not late (after day 5) VEGF treatment.

CONCLUSION

Moving to SF systems facilitates directed differentiation by eliminating confounding signals. This article describes modifications to the N2B27 media that amplify mesoderm induction and extends earlier work defining blood progenitor cell induction from ESC with BMP-4, VEGF, and TPO.

Impaired embryonic haematopoiesis yet normal arterial development in the absence of the Notch ligand Jagged1

Specific deletion of Notch1 and RBPjkappa in the mouse results in abrogation of definitive haematopoiesis concomitant with the loss of arterial identity at embryonic stage. As prior arterial determination is likely to be required for the generation of embryonic haematopoiesis, it is difficult to establish the specific haematopoietic role of Notch in these mutants. By analysing different Notch-ligand-null embryos, we now show that Jagged1 is not required for the establishment of the arterial fate but it is required for the correct execution of the definitive haematopoietic programme, including expression of GATA2 in the dorsal aorta. Moreover, successful haematopoietic rescue of the Jagged1-null AGM cells was obtained by culturing them with Jagged1-expressing stromal cells or by lentiviral-mediated transduction of the GATA2 gene. Taken together, our results indicate that Jagged1-mediated activation of Notch1 is responsible for regulating GATA2 expression in the AGM, which in turn is essential for definitive haematopoiesis in the mouse.

Visualization and experimental analysis of blood vessel formation using transgenic zebrafish

The mechanisms of blood vessel formation have become a subject of enormous scientific and clinical interest. However, it is difficult to visualize the developing vasculature in most living animals due to the ubiquitous and deep localization of vessels within other tissues. The establishment of vascular-specific transgenic zebrafish with fluorescently "tagged" blood vessels has facilitated high-resolution imaging studies of developing blood and lymphatic vessels in vivo. Use of these transgenic lines for genetic and chemical screening, experimental manipulations, and time-lapse imaging has extended our knowledge of how complex networks of vessels assemble in vivo.

Constitutive over-expression of VEGF results in reduced expression of Hand-1 during cardiac development in Xenopus

During heart development, various signaling cascades are tightly regulated in a stage- and region-dependent manner. Vascular endothelial growth factor (VEGF) is one of the important molecules required for both vascular development and cardiac morphogenesis. VEGF receptors are present in the embryonic heart, so we focused on heart formation in VEGF-over-expressing Xenopus embryos. Over-expression of VEGF(170) caused disorganized vessels, while the expression of an endothelial marker, Tie-2, was increased. The embryo's heart was distinctly larger than that of control, and showed abnormal morphology. Histological analysis of these embryos showed failure of heart looping. In situ hybridization with Hand-1, which controls intrinsic morphogenetic pathways, revealed that the expression level of Hand-1 was decreased in the heart region. These results suggest that increased VEGF(170) levels disturb Hand-1 expression in the region required for normal heart morphogenesis. VEGF expression level may be important in heart morphology during embryonic development.

Advances in cell-based therapy for structural heart disease

Congestive heart failure and coronary artery disease are the leading causes of morbidity and mortality in the United States despite substantial therapeutic advances in the last half century. Only very recently have studies arisen that support possibility of regenerating tissue of damaged human organs including the heart. In this regard, there is growing pre-clinical and clinical evidence demonstrating the safety and efficacy of cell-based myocardial regeneration using a variety of cell lines. Although the data on the exact mechanism of action and the fate of the administered cells is controversial, there is consistent evidence for improved cardiac function and myocardial regeneration using different cell types. This extraordinarily exciting scientific advance has forced cardiovascular scientists to re-evaluate the long-held paradigm of cardiac myocyte terminal differentiation and life-long longevity of the cardiac myocytes that comprise the heart. Whereas, these new ideas originated with attempts to perform cellular transplantation using exogenous stem or precursor cells, mechanistic insights have rapidly evolved to the realization that adult organs harbor stem cells with significant plasticity, capable of repopulating their respective organ. Indeed these cells may be harnessed as a therapeutic agent or may represent the target of regenerative therapeutic strategies.

Involvement of marrow-derived endothelial cells in vascularization

Until recently, the adult neovasculature was thought to arise only through angiogenesis, the mechanism by which new blood vessels form from preexisting vessels through endothelial cell migration and proliferation. However, recent studies have provided evidence that postnatal neovasculature can also arise though vasculogenesis, a process by which endothelial progenitor cells are recruited and differentiate into mature endothelial cells to form new blood vessels. Evidence for the existence of endothelial progenitors has come from studies demonstrating the ability of bone marrow-derived cells to incorporate into adult vasculature. However, the exact nature of endothelial progenitor cells remains controversial. Because of the lack of definitive markers of endothelial progenitors, the in vivo contribution of progenitor cells to physiological and pathological neovascularization remains unclear. Early studies reported that endothelial progenitor cells actively integrate into the adult vasculature and are critical in the development of many types of vascular-dependent disorders such as neoplastic progression. Moreover, it has been suggested that endothelial progenitor cells can be used as a therapeutic strategy aimed at promoting vascular growth in a variety of ischemic diseases. However, increasing numbers of studies have reported no clear contribution of endothelial progenitors in physiological or pathological angiogenesis. In this chapter, we discuss the origin of the endothelial progenitor cell in the embryo and adult, and we discuss the cell's link to the primitive hematopoietic stem cell. We also review the potential significance of endothelial progenitor cells in the formation of a postnatal vascular network and discuss the factors that may account for the current lack of consensus of the scientific community on this important issue.

FGFs, Wnts and BMPs mediate induction of VEGFR-2 (Quek-1) expression during avian somite development

Regulation of VEGFR-2 (Quek1) is an important mechanism during blood vessel formation. In the paraxial mesoderm, Quek1 expression is restricted to the lateral portion of the somite and later to sclerotomal cells surrounding the neural tube. By implanting FGF 8b/8c or SU 5402 beads into the paraxial mesoderm, we show that FGF8 in addition to BMP4 from the intermediate mesoderm (IM) is a positive regulator of VEGFR-2 (Quek1) expression in the quail embryo. The expression of Quek1 in the medial somite half is normally repressed by the notochord and Sfrps-expression in the neural tube. Over-expression of Wnt 1/3a also results in an up-regulation of Quek1 expression in the somites. We also show that up-regulation of FGF8/Wnt 1/3a leads to an increase in the number of endothelial cells, whereas inhibition of FGF and Wnt signaling by SU 5402 and Sfrp-2 results in a loss of endothelial cells. Our results demonstrate that the regulation of Quek1 expression in the somites is mediated by the cooperative actions of BMP4, FGF8 and Wnt-signaling pathways.

Endothelial progenitor cell differentiation using cryopreserved, umbilical cord blood-derived mononuclear cells

AIM

To investigate the endothelial differentiation potentiality of umbilical cord blood (UCB), we induced the differentiation of endothelial progenitor cells (EPC) from cryopreserved UCB-derived mononuclear cells (MNC).

METHODS

MNC from cryopreserved UCB and peripheral blood (PB) were cultured in M199 medium with endothelial cell growth supplements for 14 d. EPC were characterized by RT-PCR, flow cytometry, and immunocytochemistry analysis. The proliferation of differentiated EPC was studied by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) assay, and vascular endothelial growth factor (VEGF) concentration was measured using an ELISA kit. Characteristics of UCB-derived EPC were compared with those of PB-derived EPC.

RESULTS

A number of round-shaped cells were loosely attached to the bottom after 24 h culture, and numerous spindleshaped cells began to appear from the round-shaped ones on d 7. Those cells expressed endothelial markers such as, Flt-1/VEGFR-1, ecNOS, VE-cadherin, von Willebrand factor, and secreted VEGF. The patterns of endothelial markers of EPC from PB and UCB did not show striking differences. The results of the proliferation and secretion of VEGF were also similar.

CONCLUSION

We successfully cultured UCB cells stored at -196 Celsius degree into cells with the quality of endothelial cells. Those EPC could be used for angiogenic therapeutics by activating adjacent endothelial cells and enhancing angiogenesis.

Hemangioblasts representing a functional endothelio-hematopoietic entity in ontogeny, postnatal life, and CML neovasculogenesis

The life-long interdependencies/interactions between hemato- and endotheliopoiesis suggest that they form a supplementary functional entity. This view is compatible with the concept of stem cell plasticity as a reversible continuum and is substantiated by the common hematopoietic-endothelial stem cell, i.e., hemangioblasts, with bidirectional, reversible gene transcription and persistence in postnatal life. Indeed, embryonal stem cells/hemangioblasts appear to form a reservior in the adult with the possibility of dedifferentiation of more differentiated progenitor cells back to hemangioblasts. The recent detection of BCR/ABL fusion proteins in endothelial cells during vascular neoangiogenesis in CML suggests that endothelial cells are part of the neoplastic clone, and extends the concept of a functional entity to include CML angiogenesis. Thus, hemangioblasts rather than committed hematopoietic stem cells appear to be target cells for the first oncogenic hit in CML, which could occur as early as during the first steps of embryonal stem cell differentiation towards hemato-endotheliopoiesis and/or in hemangioblasts persisting in adults. The relation of the other leukemias to hemangioblasts is not known.

Functional analysis of an established mouse vascular endothelial cell line

BACKGROUND

In vitrostudies using cell lines are useful for the understanding of cellular mechanisms. The purpose of our study is to develop a new immortalized aortic vascular endothelial cell (EC) line that retains endothelial characteristics and can facilitate the study of ECs.

METHODS

A mouse aortic vascular EC line (MAEC) was established from p53-deficient mouse aorta and cultured for over 100 passages. The expression of endothelial markers was assessed, and the function of this cell line was analyzed by tube formation and binding assays.

RESULTS

MAEC retained many endothelial properties such as cobblestone appearance, contact-inhibited growth, active uptake of acetylated low-density lipoprotein, existence of Weibel-Palade bodies and several EC markers. MAECs exhibited tube formation activity both in vitro and in vivo. Furthermore, crucially, tumor necrosis factor alpha, an inflammatory cytokine, promoted lymphocyte adhesion to MAECs, suggesting that MAECs may facilitate the study of atherosclerosis and local inflammatory reactions in vitro.

CONCLUSION

We describe the morphological and cell biological characteristics of MAEC, providing strong evidence that it retained endothelial properties. This novel cell line can be a useful tool for studying the biology of ECs.

Vascular endothelial growth factor pharmacogenetics: a new perspective for anti-angiogenic therapy

The pharmacogenetic approach to anti-angiogenic therapy should be considered a possible strategy for many pathological conditions with high incidence in Western countries, including solid tumors, age-related macular degeneration or endometriosis. While pharmacogenetic studies are building stronger foundations for the systematic investigations of phenotype–genotype relationships in many research and clinical fields of medicine, pharmacogenetic data regarding anti-angiogenic drugs are still lacking. Here we review preclinical and clinical genetic studies on angiogenic determinants such as vascular endothelial growth factor and vascular endothelial growth factor receptor-2. We suggest that pharmacogenetic profiling of patients who are candidates for the currently available anti-angiogenic agents targeting vascular endothelial growth factor and vascular endothelial growth factor receptor-2 may aid the selection of patients on the basis of their likelihood of responding to the drugs or suffering from toxicity.

Differential roles of vascular endothelial growth factor receptor-1 and receptor-2 in angiogenesis

Vascular endothelial growth factor (VEGF)-A, a major regulator for angiogenesis, binds and activates two tyrosine kinase receptors, VEGFR1 (Flt-1) and VEGFR2 (KDR/Flk-1). These receptors regulate physiological as well as pathological angiogenesis. VEGFR2 has strong tyrosine kinase activity, and transduces the major signals for angiogenesis. However, unlike other representative tyrosine kinase receptors which use the Ras pathway, VEGFR2 mostly uses the Phospholipase-Cgamma-Protein kinase-C pathway to activate MAP-kinase and DNA synthesis. VEGFR2 is a direct signal transducer for pathological angiogenesis including cancer and diabetic retinopathy, thus, VEGFR2 itself and the signaling appear to be critical targets for the suppression of these diseases. VEGFR1 plays dual role, a negative role in angiogenesis in the embryo most likely by trapping VEGF-A, and a positive role in adulthood in a tyrosine kinase-dependent manner. VEGFR1 is expressed not only in endothelial cells but also in macrophage-lineage cells, and promotes tumor growth, metastasis, and inflammation. Furthermore, a soluble form of VEGFR1 was found to be present at abnormally high levels in the serum of preeclampsia patients, and induces proteinurea and renal dysfunction. Therefore, VEGFR1 is also an important target in the treatment of human diseases. Recently, the VEGFR2-specific ligand VEGF-E (Orf-VEGF) was extensively characterized. Interestingly, the activation of VEGFR2 via VEGF-E in vivo results in a strong angiogenic response in mice with minor side effects such as inflammation compared with VEGF-A, suggesting VEGF-E to be a novel material for pro-angiogenic therapy.

The discovery of endothelial progenitor cells. An historical review

Although the earliest sites of hematopoietic cell and endothelial cell differentiation in the yolk sac blood islands were identified about 100 years ago, cells with hemangioblast properties have not yet been identified in vivo. Endothelial cells differentiate from angioblasts in the embryo and from endothelial progenitor cells, mesoangioblasts and multipotent adult progenitor cells in the adult bone marrow. Endothelial progenitor cells (EPC) were initially described by Asahara et al. [Asahara T, Murohara T, Sullivan A, et al. Isolation of putative progenitor endothelial cells for angiogenesis. Science 1997;275:964-7.], and the past few years have seen a rapid expansion of our knowledge of EPC biology. Prior to the discovery of this cell type, new vessel formation was believed to occur to proliferation of existing endothelial cells. These findings have overturned the previous dogma that vasculogenesis can only occur during embryogenesis. Questions persist regarding their functional characteristics, as well as the precise panel of cell surface markers that define this cell population.

Negative regulation of primitive hematopoiesis by the FGF signaling pathway

Hematopoiesis is controlled by multiple signaling molecules during embryonic and postnatal development. The function of the fibroblast growth factor (FGF) pathway in this process is unclear. Here we show that FGF plays a key role in the regulation of primitive hematopoiesis in chicks. Using hemoglobin mRNA expression as a sensitive marker, we demonstrate that timing of blood differentiation can be separated from that of initial mesoderm patterning and subsequent migration. High FGF activity inhibits primitive blood differentiation and promotes endothelial cell fate. Conversely, inhibition of FGFR activity leads to ectopic blood formation and down-regulation of endothelial markers. Expression and functional analyses indicate that FGFR2 is the key receptor mediating these effects. The FGF pathway regulates primitive hematopoiesis by modulating Gata1 expression level and activity. We propose that the FGF pathway mediates repression of globin gene expression and that its removal is essential before terminal differentiation can occur.

Wnt2 coordinates the commitment of mesoderm to hematopoietic, endothelial, and cardiac lineages in embryoid bodies

Our recent gene expression profiling analyses demonstrated that Wnt2 is highly expressed in Flk1(+) cells, which serve as common progenitors of endothelial cells, blood cells, and mural cells. In this report, we characterize the role of Wnt2 in mesoderm development during embryonic stem (ES) cell differentiation by creating ES cell lines in which Wnt2 was deleted. Wnt2(-/-) embryoid bodies (EBs) generated increased numbers of Flk1(+) cells and blast colony-forming cells compared with wild-type EBs, and had higher Flk1 expression at comparable stages of differentiation. Although Flk1(+) cells were increased, we found that endothelial cell and terminal cardiomyocyte differentiation was impaired, but hematopoietic cell differentiation was enhanced and smooth muscle cell differentiation was unchanged in Wnt2(-/-) EBs. Later stage Wnt2(-/-) EBs had either lower or undetectable expression of endothelial and cardiac genes compared with wild-type EBs. Consistently, vascular plexi were poorly formed and neither beating cardiomyocytes nor alpha-actinin-staining cells were detectable in later stage Wnt2(-/-) EBs. In contrast, hematopoietic cell gene expression was upregulated, and the number of hematopoietic progenitor colonies was significantly enhanced in Wnt2(-/-) EBs. Our data indicate that Wnt2 functions at multiple stages of development during ES cell differentiation and during the commitment and diversification of mesoderm: as a negative regulator for hemangioblast differentiation and hematopoiesis but alternatively as a positive regulator for endothelial and terminal cardiomyocyte differentiation.

Blood Cells and Blood Cell Development in the Animal Kingdom

Recent findings strongly suggest that the molecular pathways involved in the development and function of blood cells are highly conserved among vertebrates and various invertebrate phyla. This has led to a renewed interest regarding homologies between blood cell types and their developmental origin among different animals. One way to address these areas of inquiry is to shed more light on the biology of blood cells in extant invertebrate taxa that have branched off the bilaterian tree in between insects and vertebrates. This review attempts, in a broadly comparative manner, to update the existing literature that deals with early blood cell development. I begin by providing a brief survey of the different types of blood cell lineages among metazoa. There is now good reason to believe that, in vertebrates and invertebrates alike, blood cell lineages diverge from a common type of progenitor cell, the hemocytoblast. I give a synopsis of the origin and determination of the hematocytoblast, beginning with a look at the hematopoietic organs that house hemocytoblasts in adult animals, followed by a more detailed overview of the embryonic development of the hematopoietic organ. Finally, I compare the process of blood lineage diversification in vertebrates and Drosophila.