Progressive lineage analysis by cell sorting and culture identifies FLK1+VE-cadherin+ cells at a diverging point of endothelial and hemopoietic lineages

Flk1-positive cells derived from embryonic stem cells serve as vascular progenitors

Interaction between endothelial cells and mural cells (pericytes and vascular smooth muscle) is essential for vascular development and maintenance. Endothelial cells arise from Flk1-expressing (Flk1+) mesoderm cells, whereas mural cells are believed to derive from mesoderm, neural crest or epicardial cells and migrate to form the vessel wall. Difficulty in preparing pure populations of these lineages has hampered dissection of the mechanisms underlying vascular formation. Here we show that Flk1+ cells derived from embryonic stem cells can differentiate into both endothelial and mural cells and can reproduce the vascular organization process. Vascular endothelial growth factor promotes endothelial cell differentiation, whereas mural cells are induced by platelet-derived growth factor-BB. Vascular cells derived from Flk1+ cells can organize into vessel-like structures consisting of endothelial tubes supported by mural cells in three-dimensional culture. Injection of Flk1+ cells into chick embryos showed that they can incorporate as endothelial and mural cells and contribute to the developing vasculature in vivo. Our findings indicate that Flk1+ cells can act as 'vascular progenitor cells' to form mature vessels and thus offer potential for tissue engineering of the vascular system.

Mouse flt-1 promoter directs endothelial-specific expression in the embyroid body model of embryogenesis

The endothelial-specific receptor tyrosine kinase flt-1 (VEGFR-1) is expressed early on during endothelial lineage commitment both in vivo and in vitro. However, the exact function of flt-1 in vascular development still remains unclear. Here we report that a 2.2-kb fragment 5' of the mouse flt-1 gene becomes transcriptionally active during endothelial cell differentiation in developing embryoid bodies derived from mouse ES cells. Reporter gene expression correlated well with PECAM-1 expression and mirrored the expression pattern of the endogenous flt-1 gene. The temporal and spatial activity of the 2.2-kb flt-1 promoter provides a means to (1) identify a living population of early committed endothelial/bipotential progenitors and (2) ectopically express biologically active genes during lineage commitment.

Building the vertebrate vasculature: research is going swimmingly

The vertebrate vasculature develops in remarkably similar fashion in all vertebrates. A cohort of unspecified mesodermal cells differentiates into primitive endothelial cells, which migrate to and occupy positions within the stereotypical blueprint of the primitive vasculature. Once in position, these cells coalesce and form cords, which lumenize and become ensheathed by supporting pericytes and smooth muscle cells. This primitive vascular network is extensively remodeled in some places, and expanded by sprouting in others. Various studies using the mouse, quail/chick, and frog have uncovered a number of signals that guide these complex processes but many gaps still exist in our understanding of the mechanisms by which the embryonic vasculature is built. Because many questions will require in vivo studies to be properly addressed, the zebrafish, with its unique accessibility to analysis by combined embryological, molecular, and genetic methods, should prove invaluable in identifying new molecules involved in blood vessel development and integrating pathways that influence embryonic blood vessel formation.

Induction of midbrain dopaminergic neurons from ES cells by stromal cell-derived inducing activity

We have identified a stromal cell-derived inducing activity (SDIA) that promotes neural differentiation of mouse ES cells. SDIA accumulates on the surface of PA6 stromal cells and induces efficient neuronal differentiation of cocultured ES cells in serum-free conditions without use of either retinoic acid or embryoid bodies. BMP4, which acts as an antineuralizing morphogen in Xenopus, suppresses SDIA-induced neuralization and promotes epidermal differentiation. A high proportion of tyrosine hydroxylase-positive neurons producing dopamine are obtained from SDIA-treated ES cells. When transplanted, SDIA-induced dopaminergic neurons integrate into the mouse striatum and remain positive for tyrosine hydroxylase expression. Neural induction by SDIA provides a new powerful tool for both basic neuroscience research and therapeutic applications.

Diversification of haematopoietic stem cells to specific lineages

Diverse types of blood cell (lineages) are produced from rare haematopoietic stem cells that reside in the bone marrow. This process, known as haematopoiesis, provides a valuable model for examining how genetic programs are established and executed in vertebrates, and also how homeostasis of blood formation is altered in leukaemias. So, how does an apparently small group of critical lineage-restricted nuclear regulatory factors specify the diversity of haematopoietic cells? Recent findings not only indicate how this may be achieved but also show the extraordinary plasticity of tissue stem cells in vivo.

Endothelial signal integration in vascular assembly

Regulated assembly of a highly specialized interconnecting network of vascular endothelial and supportive cells is fundamental to embryonic development and organogenesis, as well as to postnatal tissue repair in metazoans. This review advances an "endotheliocentric" model that defines tasks required of endothelial cells and describes molecular controls that regulate steps in activation, assembly, and maturation of new vessels. In addition to the classical assembly mechanisms--angiogenesis and vasculogenesis--endothelial cells are also recruited into vascular structures from the circulatory system in adult animals and from resident mesenchymally derived progenitors during organogenesis of kidney and other organs. Paracrine signaling cascades regulated by hypoxia initiate a sequentially coordinated series of endothelial responses, including matrix degradation, migration, proliferation, and morphogenetic remodeling. Surface receptors on committed endothelial lineage progenitors transduce cues from extracellular-matrix-associated proteins and cell-cell contact to direct migration, matrix attachment, proliferation, targeting and cell-cell assembly, and vessel maturation. Through their capacity to spatially segregate and temporally integrate a diverse range of extracellular signals, endothelial cells determine their migratory paths, cellular partners, and life-or-death responses to local cues.

The thin red line: angiogenesis in normal and malignant hematopoiesis

This review describes the current knowledge about cell subsets involved in vasculogenesis (i.e., differentiation of endothelial cells from mesodermal precursors) and angiogenesis (i.e., blood vessel generation from pre-existing vessels), together with recent findings about angiogenesis and antiangiogenic therapies in hematopoietic malignancies such as leukemia, lymphoma, myeloma, and myelodysplastic syndromes.

ESOP-1, a secreted protein expressed in the hematopoietic, nervous, and reproductive systems of embryonic and adult mice

To isolate soluble factors expressed in early phases of hematopoietic differentiation, we applied the signal sequence trap method to the in vitro murine hematopoietic differentiation system, in which ES cells are cocultured with OP-9 stroma cells. This strategy allowed us to isolate cDNA for a secreted protein, ESOP-1, of 160 amino acids, the sequence of which shows 64% identity with human ESOP-1/MD-2. ESOP-1 mRNA was highly expressed in the mouse embryos at 7.5 days after coitus. Expression of the ESOP-1 mRNA and protein was shown in the embryonic and adult hematopoietic system. In addition, the ESOP-1 protein was found in the yolk sac–blood islands, the developing nervous system, and the adult reproductive system. These results suggest that ESOP-1 may play some roles in the development or maintenance of hematopoietic, nervous, and reproductive systems.

Vascular endothelial growth factor(165) gene transfer augments circulating endothelial progenitor cells in human subjects

Preclinical studies in animal models and early results of clinical trials in patients suggest that intramuscular injection of naked plasmid DNA encoding vascular endothelial growth factor (VEGF) can promote neovascularization of ischemic tissues. Such neovascularization has been attributed exclusively to sprout formation of endothelial cells derived from preexisting vessels. We investigated the hypothesis that VEGF gene transfer may also augment the population of circulating endothelial progenitor cells (EPCs). In patients with critical limb ischemia receiving VEGF gene transfer, gene expression was documented by a transient increase in plasma levels of VEGF. A culture assay documented a significant increase in EPCs (219%, P<0.001), whereas patients who received an empty vector had no change in circulating EPCs, as was the case for volunteers who received saline injections (VEGF versus empty vector, P<0.001; VEGF versus saline, P<0.005). Fluorescence-activated cell sorter analysis disclosed an overall increase of up to 30-fold in endothelial lineage markers KDR (VEGF receptor-2), VE-cadherin, CD34, alpha(v)beta(3), and E-selectin after VEGF gene transfer. Constitutive overexpression of VEGF in patients with limb ischemia augments the population of circulating EPCs. These findings support the notion that neovascularization of human ischemic tissues after angiogenic growth factor therapy is not limited to angiogenesis but involves circulating endothelial precursors that may home to ischemic foci and differentiate in situ through a process of vasculogenesis.

A transitional stage in the commitment of mesoderm to hematopoiesis requiring the transcription factor SCL/tal-1

In this report, we describe the identification and characterization of an early embryoid body-derived colony, termed the transitional colony, which contains cell populations undergoing the commitment of mesoderm to the hematopoietic and endothelial lineages. Analysis of individual transitional colonies indicated that they express Brachyury as well as flk-1, SCL/tal-1, GATA-1, (beta)H1 and (beta)major reflecting the combination of mesodermal, hematopoietic and endothelial populations. This pattern differs from that found in the previously described hemangioblast-derived blast cell colonies in that they typically lacked Brachyury expression, consistent with their post-mesodermal stage of development (Kennedy, M., Firpo, M., Choi, K., Wall, C., Robertson, S., Kabrun, N. and Keller, G. (1997) Nature 386, 488–493). Replating studies demonstrated that transitional colonies contain low numbers of primitive erythroid precursors as well as a subset of precursors associated with early stage definitive hematopoiesis. Blast cell colonies contain higher numbers and a broader spectrum of definitive precursors than found in the transitional colonies. ES cells homozygous null for the SCL/tal-1 gene, a transcription factor known to be essential for development of the primitive and definitive hematopoietic systems, were not able to form blast colonies but did form transitional colonies. Together these findings suggest that the transitional colony represents a stage of development earlier than the blast cell colony and one that uniquely defines the requirement for a functional SCL/tal-1 gene for the progression to hematopoietic commitment.

Transplanted cord blood-derived endothelial precursor cells augment postnatal neovascularization

Endothelial precursor cells (EPCs) have been identified in adult peripheral blood. We examined whether EPCs could be isolated from umbilical cord blood, a rich source for hematopoietic progenitors, and whether in vivo transplantation of EPCs could modulate postnatal neovascularization. Numerous cell clusters, spindle-shaped and attaching (AT) cells, and cord-like structures developed from culture of cord blood mononuclear cells (MNCs). Fluorescence-trace experiments revealed that cell clusters, AT cells, and cord-like structures predominantly were derived from CD34-positive MNCs (MNC(CD34+)). AT cells and cell clusters could be generated more efficiently from cord blood MNCs than from adult peripheral blood MNCs. AT cells incorporated acetylated-LDL, released nitric oxide, and expressed KDR, VE-cadherin, CD31, and von Willebrand factor but not CD45. Locally transplanted AT cells survived and participated in capillary networks in the ischemic tissues of immunodeficient nude rats in vivo. AT cells thus had multiple endothelial phenotypes and were defined as a major population of EPCs. Furthermore, laser Doppler and immunohistochemical analyses revealed that EPC transplantation quantitatively augmented neovascularization and blood flow in the ischemic hindlimb. In conclusion, umbilical cord blood is a valuable source of EPCs, and transplantation of cord blood-derived EPCs represents a promising strategy for modulating postnatal neovascularization.

Basic fibroblast growth factor positively regulates hematopoietic development

Recently identified BLast Colony Forming Cells (BL-CFCs) from in vitro differentiated embryonic stem (ES) cells represent the common progenitor of hematopoietic and endothelial cells, the hemangioblast. Access to this initial cell population committed to the hematopoietic lineage provides a unique opportunity to characterize hematopoietic commitment events. Here, we show that BL-CFC expresses the receptor tyrosine kinase, Flk1, and thus we took advantage of the BL-CFC assay, as well as fluorescent activated cell sorter (FACS) analysis for Flk1(+) cells to determine quantitatively if mesoderm-inducing factors promote hematopoietic lineage development. Moreover, we have analyzed ES lines carrying targeted mutations for fibroblast growth factor receptor-1 (fgfr1), a receptor for basic fibroblast growth factor (bFGF), as well as scl, a transcription factor, for their potential to generate BL-CFCs and Flk1(+) cells, to further define events leading to hemangioblast development. Our data suggest that bFGF-mediated signaling is critical for the proliferation of the hemangioblast and that cells expressing both Flk1 and SCL may represent the hemangioblast.

The monoclonal antibody TER-119 recognizes a molecule associated with glycophorin A and specifically marks the late stages of murine erythroid lineage

The antigen specificity of a rat monoclonal antibody TER-119 was investigated. In adult mice, TER-119 reacted with mature erythrocytes, 20-25% of bone marrow cells and 2-3% of spleen cells but not with thymocytes nor lymph node cells. In fetal haematopoietic tissues, 30-40% of d 10 yolk sac cells, 80-90% of d 14 fetal liver cells and 40-50% of newborn liver cells were reactive with TER-119. TER-119+ cells in adult bone marrow expressed significant levels of CD45 but not myeloid (Mac-1, Gr-1) or B-cell (B220) markers. Morphological examination and haematopoietic colony-forming assays for isolated TER-119+ cells revealed that TER-119 reacts with erythroid cells at differentiation stages from early proerythroblast to mature erythrocyte, but not with cells showing typical erythroid blast-forming unit (BFU-E) and erythroid colony-forming unit (CFU-E) activities. Erythroleukaemia cell lines do not express the TER-119 antigen even after stimulation with dimethylsulphoxide. TER-119 immunoprecipitated protein bands with molecular masses of 110 kDa, 60 kDa, 52 kDa and 32 kDa from erythrocyte membrane, whereas only a 52-kDa band was detected by TER-119 in Western blot analysis. Further molecular and cellular analyses indicated that the TER-119 antigen is a molecule associated with cell-surface glycophorin A but not with glycophorin A itself.

Transplantation of ex vivo expanded endothelial progenitor cells for therapeutic neovascularization

Animal studies and preliminary results in humans suggest that lower extremity and myocardial ischemia can be attenuated by treatment with angiogenic cytokines. The resident population of endothelial cells that is competent to respond to an available level of angiogenic growth factors, however, may potentially limit the extent to which cytokine supplementation enhances tissue neovascularization. Accordingly, we transplanted human endothelial progenitor cells (hEPCs) to athymic nude mice with hindlimb ischemia. Blood flow recovery and capillary density in the ischemic hindlimb were markedly improved, and the rate of limb loss was significantly reduced. Ex vivo expanded hEPCs may thus have utility as a "supply-side" strategy for therapeutic neovascularization.

Vascular endothelial growth factor synergistically enhances bone morphogenetic protein-4-dependent lymphohematopoietic cell generation from embryonic stem cells in vitro

The totipotent mouse embryonic stem (ES) cell is known to differentiate into cells expressing the β-globin gene when stimulated with bone morphogenetic protein (BMP)-4. Here, we demonstrate that BMP-4 is essential for generating both erythro-myeloid colony-forming cells (CFCs) and lymphoid (B and NK) progenitor cells from ES cells and that vascular endothelial growth factor (VEGF) synergizes with BMP-4. The CD45+ myelomonocytic progenitors and Ter119+ erythroid cells began to be detected with 0.5 ng/mL BMP-4, and their levels plateaued at approximately 2 ng/mL. VEGF alone weakly elevated the CD34+ cell population though no lymphohematopoietic progenitors were induced. However, when combined with BMP-4, 2 to 20 ng/mL VEGF synergistically augmented the BMP-4-dependent generation of erythro-myeloid CFCs and lymphoid progenitors from ES cells, which were enriched in CD34+ CD31lo and CD34+CD45− cell populations, respectively, in a dose-dependent manner. Furthermore, during the 7 days of in vitro differentiation, BMP-4 was required within the first 4 days, whereas VEGF was functional after the action of BMP-4 (in the last 3 days). Thus, VEGF is a synergistic enhancer for the BMP-4-dependent differentiation processes, and it seems to be achieved by the ordered action of the 2 factors.

Quantitative expression of Oct-3/4 defines differentiation, dedifferentiation or self-renewal of ES cells

Cell fate during development is defined by transcription factors that act as molecular switches to activate or repress specific gene expression programmes. The POU transcription factor Oct-3/4 (encoded by Pou5f1) is a candidate regulator in pluripotent and germline cells and is essential for the initial formation of a pluripotent founder cell population in the mammalian embryo. Here we use conditional expression and repression in embryonic stem (ES) cells to determine requirements for Oct-3/4 in the maintenance of developmental potency. Although transcriptional determination has usually been considered as a binary on-off control system, we found that the precise level of Oct-3/4 governs three distinct fates of ES cells. A less than twofold increase in expression causes differentiation into primitive endoderm and mesoderm. In contrast, repression of Oct-3/4 induces loss of pluripotency and dedifferentiation to trophectoderm. Thus a critical amount of Oct-3/4 is required to sustain stem-cell self-renewal, and up- or downregulation induce divergent developmental programmes. Our findings establish a role for Oct-3/4 as a master regulator of pluripotency that controls lineage commitment and illustrate the sophistication of critical transcriptional regulators and the consequent importance of quantitative analyses.

The AML1 transcription factor functions to develop and maintain hematogenic precursor cells in the embryonic aorta-gonad-mesonephros region

We examined the role of the AML1 transcription factor in the development of hematopoiesis in the paraaortic splanchnopleural (P-Sp) and the aorta-gonad-mesonephros (AGM) regions of mouse embryos. The activity of colony-forming units of colonies from the P-Sp/AGM region was reduced severalfold by heterozygous disruption of the AML1 gene, indicating that AML1 functioned in a dosage-dependent manner to generate hematopoietic progenitors. In addition, no hematopoietic progenitor activity was detected in the P-Sp/AGM region of embryos with an AML1 null mutation. Similar results were obtained when a dispersed culture was first prepared from the P-Sp/AGM region before assay of the activity of the colony-forming units. In a culture of cells with the AML1(+/+) genotype, both hematopoietic and endothelial-like cell types emerged, but in a culture of cells with the AML1(-/-) genotype, only endothelial-like cells emerged. Interestingly, introduction of AML1 cDNA into the P-Sp/AGM culture with the AML1(-/-) genotype partially restored the production of hematopoietic cells. This restoration was observed for cultures prepared from 9.5-day postcoitum (dpc) embryos but not for cultures prepared from 11.5-dpc embryos. Therefore, the population of endothelial-like cells capable of growing in the AML1(-/-) culture would appear to contain inert but nonetheless competent hematogenic precursor cells up until at least the 9.5-dpc period. All these results support the notion that the AML1 transcription factor functions to develop and maintain hematogenic precursor cells in the embryonic P-Sp/AGM region.

Vascular endothelial growth factor synergistically enhances bone morphogenetic protein-4-dependent lymphohematopoietic cell generation from embryonic stem cells in vitro

The totipotent mouse embryonic stem (ES) cell is known to differentiate into cells expressing the β-globin gene when stimulated with bone morphogenetic protein (BMP)-4. Here, we demonstrate that BMP-4 is essential for generating both erythro-myeloid colony-forming cells (CFCs) and lymphoid (B and NK) progenitor cells from ES cells and that vascular endothelial growth factor (VEGF) synergizes with BMP-4. The CD45+ myelomonocytic progenitors and Ter119+ erythroid cells began to be detected with 0.5 ng/mL BMP-4, and their levels plateaued at approximately 2 ng/mL. VEGF alone weakly elevated the CD34+ cell population though no lymphohematopoietic progenitors were induced. However, when combined with BMP-4, 2 to 20 ng/mL VEGF synergistically augmented the BMP-4-dependent generation of erythro-myeloid CFCs and lymphoid progenitors from ES cells, which were enriched in CD34+ CD31lo and CD34+CD45− cell populations, respectively, in a dose-dependent manner. Furthermore, during the 7 days of in vitro differentiation, BMP-4 was required within the first 4 days, whereas VEGF was functional after the action of BMP-4 (in the last 3 days). Thus, VEGF is a synergistic enhancer for the BMP-4-dependent differentiation processes, and it seems to be achieved by the ordered action of the 2 factors.

Characterization of the vasculogenic block in the absence of vascular endothelial growth factor-A

Vascular endothelial growth factor (VEGF) signaling is required for both differentiation and proliferation of vascular endothelium. Analysis of differentiated embryonic stem cells with one or both VEGF-A alleles deleted showed that both the differentiation and the expansion of endothelial cells are blocked during vasculogenesis. Blood island formation was reduced by half in hemizygous mutant VEGF cultures and by 10-fold in homozygous mutant VEGF cultures. Homozygous mutant cultures could be partially rescued by the addition of exogenous VEGF. RNA levels for the endothelial adhesion receptors ICAM-2 and PECAM were reduced in homozygous mutant cultures, but ICAM-2 RNA levels decreased substantially, whereas PECAM RNA levels remained at hemizygous levels. The quantitative data correlated with the antibody staining patterns because cells that were not organized into vessels expressed PECAM but not ICAM-2. These PECAM+ cell clumps accumulated in mutant cultures as vessel density decreased, suggesting that they were endothelial cell precursors blocked from maturation. A subset of PECAM+ cells in clumps expressed stage-specific embryonic antigen-1 (SSEA-1), and all were ICAM-2(−) and CD34(−), whereas vascular endothelial cells incorporated into vessels were PECAM(+), ICAM-2(+), CD34(+), and SSEA-1(−). Analysis of flk-1 expression indicated that a subset of vascular precursor cells coexpressed PECAM and flk-1. These data suggest that VEGF signaling acts in a dose-dependent manner to affect both a specific differentiation step and the subsequent expansion of endothelial cells.

Genetically modified CD34+ cells as cellular vehicles for gene delivery into areas of angiogenesis in a rhesus model

To develop a cellular vehicle able to reach systemically disseminated areas of angiogenesis, we sought to exploit the natural tropism of circulating endothelial progenitor cells (EPCs). Primate CD34+ EPCs were genetically modified with high efficiency and minimal toxicity using a non-replicative herpes virus vector. These EPCs localized in a skin autograft model of angiogenesis in rhesus monkeys, and sustained the expression of a reporter gene for several weeks while circulating in the blood. In animals infused with autologous CD34+ EPCs transduced with a thymidine kinase-encoding herpes virus, skin autografts and subcutaneous Matrigel pellets impregnated with vascular growth factors underwent necrosis or accelerated regression after administration of ganciclovir. Importantly, the whole intervention was perfectly well tolerated. The accessibility, easy manipulation, lack of immunogenicity of the autologous CD34+ cell vehicles, and tropism for areas of angiogenesis render autologous CD34+ circulating endothelial progenitors as ideal candidates for exploration of their use as cellular vehicles when systemic gene delivery to those areas is required. Gene Therapy (2000) 7, 43-52.

Emergence of T, B, and Myeloid Lineage-Committed as well as Multipotent Hemopoietic Progenitors in the Aorta-Gonad- Mesonephros Region of Day 10 Fetuses of the Mouse

We investigated the developmental potential of hemopoietic progenitors in the aorta-gonad-mesonephros (AGM) region, where the definitive type hemopoietic progenitors have been shown to emerge before the fetal liver develops. By using an assay system that is able to determine the developmental potential of individual progenitors toward the T, B, and myeloid lineages, we show that not only multipotent progenitors but also progenitors committed to the T, B, or myeloid lineage already exist in this region of day 10 fetuses. Bipotent progenitors generating myeloid and T cells or those generating myeloid and B cells were also detected, suggesting that the commitment to T and B cell lineages is in progress in the AGM region. The numbers of these progenitors, however, were only 1/200–1/1000 of those in fetal liver of day 12 fetuses. Such small numbers of progenitors suggest that hemopoiesis has just started in the AGM region of day 10 fetuses. Although most of T cell lineage-committed progenitors in the AGM region generated only a small number of immature T cells, some were able to generate a large number of mature T cells. The detection of various types of lineage-committed progenitors strongly suggests that the AGM region is not only the site of stem cell emergence, but also the site of hemopoiesis, including lineage commitment. The T cell progenitors found in the AGM region may represent the first immigrants to the thymus anlage.

Identification of podocalyxin-like protein 1 as a novel cell surface marker for hemangioblasts in the murine aorta-gonad-mesonephros region

Recent studies with avian embryos and murine embryonic stem cells have suggested that hematopoietic cells are derived from hemangioblasts, the common precursors of hematopoietic and endothelial cells. We molecularly cloned podocalyxin-like protein 1 (PCLP1) as a novel surface marker for endothelial-like cells in the aorta-gonad-mesonephros (AGM) region of mouse embryos, where long-term repopulating hematopoietic stem cells (LTR-HSCs) are known to arise. PCLP1+ CD45 cells in the AGM region incorporated acetylated low-density lipoprotein and produced both hematopoietic and endothelial cells when cocultured with OP9 stromal cells. Moreover, multiple lineages of hematopoietic cells were generated in vivo when PCLP1 +CD45-cells were injected into neonatal liver of busulfan-treated mice. Thus, PCLP1 can be used to separate hemangioblasts that give rise to LTR-HSCs.

The orderly allocation of mesodermal cells to the extraembryonic structures and the anteroposterior axis during gastrulation of the mouse embryo

The prospective fate of cells in the primitive streak was examined at early, mid and late stages of mouse gastrula development to determine the order of allocation of primitive streak cells to the mesoderm of the extraembryonic membranes and to the fetal tissues. At the early-streak stage, primitive streak cells contribute predominantly to tissues of the extraembryonic mesoderm as previously found. However, a surprising observation is that the erythropoietic precursors of the yolk sac emerge earlier than the bulk of the vitelline endothelium, which is formed continuously throughout gastrula development. This may suggest that the erythropoietic and the endothelial cell lineages may arise independently of one another. Furthermore, the extraembryonic mesoderm that is localized to the anterior and chorionic side of the yolk sac is recruited ahead of that destined for the posterior and amnionic side. For the mesodermal derivatives in the embryo, those destined for the rostral structures such as heart and forebrain mesoderm ingress through the primitive streak early during a narrow window of development. They are then followed by those for the rest of the cranial mesoderm and lastly the paraxial and lateral mesoderm of the trunk. Results of this study, which represent snapshots of the types of precursor cells in the primitive streak, have provided a better delineation of the timing of allocation of the various mesodermal lineages to specific compartments in the extraembryonic membranes and different locations in the embryonic anteroposterior axis.

Vascular endothelial (VE)-cadherin: only an intercellular glue?

Data collected during the past years indicate that AJ- and more specifically VE-cadherin play an important role in endothelial cell biology. VE-cadherin may transfer information intracellularly through interaction with a complex network of cytoskeletal and signaling molecules. Expression of VE-cadherin is required for the control of vascular permeability and vascular integrity. In addition, the molecule may exert a morphogenetic role modulating the capacity of endothelial cells to organize into tubular-like structures. VE-cadherin presents many structural and sequence homologies to the other members of the family and apparently binds to the same intracellular molecules. However, remarkably, VE-cadherin may transfer specific signals to endothelial cells to modulate their functional reactivity.

KDR receptor: a key marker defining hematopoietic stem cells

Studies on pluripotent hematopoietic stem cells (HSCs) have been hindered by lack of a positive marker, comparable to the CD34 marker of hematopoietic progenitor cells (HPCs). In human postnatal hematopoietic tissues, 0.1 to 0.5% of CD34(+) cells expressed vascular endothelial growth factor receptor 2 (VEGFR2, also known as KDR). Pluripotent HSCs were restricted to the CD34+KDR+ cell fraction. Conversely, lineage-committed HPCs were in the CD34+KDR- subset. On the basis of limiting dilution analysis, the HSC frequency in the CD34+KDR+ fraction was 20 percent in bone marrow (BM) by mouse xenograft assay and 25 to 42 percent in BM, peripheral blood, and cord blood by 12-week long-term culture (LTC) assay. The latter values rose to 53 to 63 percent in LTC supplemented with VEGF and to greater than 95 percent for the cell subfraction resistant to growth factor starvation. Thus, KDR is a positive functional marker defining stem cells and distinguishing them from progenitors.

Increased hemangioblast commitment, not vascular disorganization, is the primary defect in flt-1 knock-out mice

We previously demonstrated the essential role of the flt-1 gene in regulating the development of the cardiovascular system. While the inactivation of the flt-1 gene leads to a very severe disorganization of the vascular system, the primary defect at the cellular level was unknown. Here we report a surprising finding that it is an increase in the number of endothelial progenitors that leads to the vascular disorganization in flt-1(−/−) mice. At the early primitive streak stage (prior to the formation of blood islands), hemangioblasts are formed much more abundantly in flt-1(−/−) embryos. This increase is primarily due to an alteration in cell fate determination among mesenchymal cells, rather than to increased proliferation, migration or reduced apoptosis of flt-1(−/−) hemangioblasts. We further show that the increased population density of hemangioblasts is responsible for the observed vascular disorganization, based on the following observations: (1) both flt-1(−/−) and flt-1(+/+) endothelial cells formed normal vascular channels in chimaeric embryos; (2) wild-type endothelial cells formed abnormal vascular channels when their population density was significantly increased; and (3) in the absence of wild-type endothelial cells, flt-1(−/−) endothelial cells alone could form normal vascular channels when sufficiently diluted in a developing embryo. These results define the primary defect in flt-1(−/−) embryos at the cellular level and demonstrate the importance of population density of progenitor cells in pattern formation.

Cbfa2 is required for the formation of intra-aortic hematopoietic clusters

Cbfa2 (AML1) encodes the DNA-binding subunit of a transcription factor in the small family of core-binding factors (CBFs). Cbfa2 is required for the differentiation of all definitive hematopoietic cells, but not for primitive erythropoiesis. Here we show that Cbfa2 is expressed in definitive hematopoietic progenitor cells, and in endothelial cells in sites from which these hematopoietic cells are thought to emerge. Endothelial cells expressing Cbfa2 are in the yolk sac, the vitelline and umbilical arteries, and in the ventral aspect of the dorsal aorta in the aorta/genital ridge/mesonephros (AGM) region. Endothelial cells lining the dorsal aspect of the aorta, and elsewhere in the embryo, do not express Cbfa2. Cbfa2 appears to be required for maintenance of Cbfa2 expression in the endothelium, and for the formation of intra-aortic hematopoietic clusters from the endothelium.

Role of vascular endothelial-cadherin in vascular morphogenesis

Vascular endothelial (VE)-cadherin is an adhesive transmembrane protein specifically expressed at interendothelial junctions. Its extracellular domain exhibits Ca2+-dependent homophilic reactivity, promoting cell-cell recognition. Mice deficient in VE-cadherin die at mid-gestation resulting from severe vascular defects. At the early phases of vascular development (E8.5) of VE-cadherin-deficient embryos, in situ differentiation of endothelial cells was delayed although their differentiation program appeared normal. Vascularization was defective in the anterior part of the embryo, while dorsal aortae and vitelline and umbilical arteries formed normally in the caudal part. At E9.25, organization of endothelial cells into large vessels was incomplete and angiogenesis was impaired in mutant embryos. Defects were more severe in extraembryonic vasculature. Blood islands of the yolk sac and clusters of angioblasts in allantois failed to establish a capillary plexus and remained isolated. This was not due to defective cell-cell recognition as endothelial cells formed intercellular junctions, as shown by electron microscopy. These data indicate that VE-cadherin is dispensable for endothelial homophilic adhesion but is required for vascular morphogenesis.

What guides early embryonic blood vessel formation?

Survival of vertebrate embryos depends on their ability to assemble a correctly patterned, integrated network of blood vessels to supply oxygen and nutrients to developing tissues. The arrangement of larger caliber intraembryonic vessels, specification of arterial-venous identity, and proper placement of major branch points and arterial-venous connections are all precisely determined. A number of recent studies in both mammalian and nonmammalian vertebrate species, reviewed here, have now begun to reveal the major role played by genetically predetermined extrinsic cues in guiding the formation of early embryonic blood vessels and determining the global pattern of the vasculature.

IL-7 receptor alpha+ CD3(-) cells in the embryonic intestine induces the organizing center of Peyer's patches

Peyer's patch (PP) organogenesis proceeds through three histologically distinct steps: formation of organizing centers expressing VCAM-1 and ICAM-1 in segregated regions of the intestine at 15.5 days post-coitus (d.p.c.) (step I), accumulation of blood cells expressing different sets of surface markers to this region at 16.5-17.0 d.p.c. (step II), and entry of CD3+ and B220+ lymphocytes just before birth (step III). PP formation of both Il7ra-/- and Lta-/- mice is impaired from step I, suggesting involvement of the two molecules at the same timing in PP organogenesis. Expression of lymphotoxin (LT) alpha and LTbeta in IL-7 receptor (IL-7R) alpha+ cells in the intestine indicates that defects of Il7ra-/- and Lta-/- mice are due to functional inability of IL-7Ralpha+ cells in the induction of PP anlage. Blocking of IL-7Ralpha function by a single injection of the antagonistic mAb in 15.5 d.p.c. embryos suppressed appearance of VCAM-1(+) spots and expression of LTalpha and LTbeta in the intestine, which eventually resulted in mice without PP but are otherwise normal. Intestinal IL-7Ralpha+ cells are lymphoid in morphology but CD3(-) and functional in both nu/nu and Rag2-/- mice. These results implicate IL-7Ralpha+ CD3(-) cells as the direct inducer of the organizing center of PP.

Differential expression of KDR/VEGFR-2 and CD34 during mesoderm development of the early human embryo

Recent findings on vertebrate embryos have provided compelling evidence for the existence of hemangioblasts, i.e. common precursors for endothelial and hematopoietic cells, characterized by expression of the VEGFR2/Flk1 receptor. We describe here a population of KDR+ CD34- mesoderm cells that emerges in early-somitic human embryos, by the beginning of the 4th week of gestation. In the developing blood vessels, KDR-expressing CD34- cells gradually coexpress increasing levels of CD34 antigen. Remarkably, as development proceeds, a KDR+ CD34- contingent persists in the paraaortic splanchnopleura until just prior to the emergence of aorta-associated hematopoietic cell clusters. These observations suggest that KDR+ CD34- mesodermal cells might represent the putative hemangioblastic precursor of human hematopoietic and endothelial lineages.

[Vascular endothelial factor (VEGF): therapeutic angiogenesis and vasculogenesis in the treatment of cardiovascular disease]

The formation of new blood vessel is essential for a variety of physiological processes like embryogenesis and the female reproduction as well as pathological processes like tumor growth, wound healing and neovascularization of ischemic tissue. Vasculogenesis and angiogenesis are the mechanisms responsible for the development of the blood vessels. While angiogenesis refers to the formation of capillaries from pre-existing vessels in the embryo and adult organism, vasculogenesis, the development of new blood vessels from in situ differentiating endothelial cells, has been previously considered restricted to embryogenesis. Recent investigations, however, show the existence of endothelial progenitor cells (EPCs) in the peripheral blood of the adult and their participation in ongoing neovascularization. Molecular and cell-biological experiments suggest that different cytokines and growth factors have a stimulatory effect on these bone-marrow derived EPCs. Results with GM-CSF (granulocyte macrophage-colony stimulating factor) and VEGF (vascular endothelial growth factor) open a new insight into the clinical use of cytokines and in particular the use of growth factors in gene therapy. The administration via protein or plasmid-DNA for neovascularization seems to enhance both pathways, angiogenesis and vasculogenesis.

Maturation of Embryonic Stem Cells Into Endothelial Cells in an In Vitro Model of Vasculogenesis

A primitive vascular plexus is formed through coordinated regulation of differentiation, proliferation, migration, and cell-cell adhesion of endothelial cell (EC) progenitors. In this study, a culture system was devised to investigate the behavior of purified EC progenitors in vitro. Because Flk-1+ cells derived from ES cells did not initially express other EC markers, they were sorted and used as EC progenitors. Their in vitro differentiation into ECs, via vascular endothelial-cadherin (VE-cadherin)+ platelet-endothelial cell adhesion molecule-1 (PECAM-1)+ CD34−to VE-cadherin+ PECAM-1+CD34+ stage, occurred without exogenous factors, whereas their proliferation, particularly at low cell density, required OP9 feeder cells. On OP9 feeder layer, EC progenitors gave rise to sheet-like clusters of Flk-1+ cells, with VE-cadherin concentrated at the cell-cell junction. The growth was suppressed by Flt-1-IgG1 chimeric protein and dependent on vascular endothelial growth factor (VEGF) but not placenta growth factor (PIGF). Further addition of VEGF resulted in cell dispersion, indicating the role of VEGF in the migration of ECs as well as their proliferation. Cell-cell adhesion of ECs in this culture system was mediated by VE-cadherin. Thus, the culture system described here is useful in dissecting the cellular events of EC progenitors that occur during vasculogenesis and in investigating the molecular mechanisms underlying these processes.

Expression of 4-Integrin Defines the Earliest Precursor of Hematopoietic Cell Lineage Diverged From Endothelial Cells

Embryonic stem cells can differentiate in vitro into hematopoietic cells through two intermediate stages; the first being FLK1+ E-cadherin− proximal lateral mesoderm and the second being CD45− VE-cadherin+endothelial cells. To further dissect the CD45−VE-cadherin+ cells, we have examined distribution of 4-integrin on this cell population, because 4-integrin is the molecule expressed on hematopoietic stem cells. During culture of FLK1+ E-cadherin− cells, CD45− VE-cadherin+4-integrin− cells differentiate first, followed by 4-integrin+ cells appearing in both CD45− VE-cadherin+ and CD45−VE-cadherin− cell populations. In the CD45−VE-cadherin+ cell population, 4-integrin+ subset but not 4-integrin− subset had the potential to differentiate to hematopoietic lineage cells, whereas endothelial cell progenitors were present in both subsets. The CD45−VE-cadherin− 4-integrin+ cells also showed hematopoietic potential. Reverse transcription-polymerase chain reaction analyses showed that differential expression of the Gata2 and Myb genes correlated with the potential of the 4-integrin+ cells to give rise to hematopoietic cell differentiation. Hematopoietic CD45−VE-cadherin+ 4-integrin+ cells were also present in the yolk sac and embryonic body proper of 9.5 day postcoitum mouse embryos. Our results suggest that the expression of 4-integrin is a marker of the earliest precursor of hematopoietic cell lineage that was diverged from endothelial progenitors.

Maturation of Embryonic Stem Cells Into Endothelial Cells in an In Vitro Model of Vasculogenesis

A primitive vascular plexus is formed through coordinated regulation of differentiation, proliferation, migration, and cell-cell adhesion of endothelial cell (EC) progenitors. In this study, a culture system was devised to investigate the behavior of purified EC progenitors in vitro. Because Flk-1+ cells derived from ES cells did not initially express other EC markers, they were sorted and used as EC progenitors. Their in vitro differentiation into ECs, via vascular endothelial-cadherin (VE-cadherin)+ platelet-endothelial cell adhesion molecule-1 (PECAM-1)+ CD34−to VE-cadherin+ PECAM-1+CD34+ stage, occurred without exogenous factors, whereas their proliferation, particularly at low cell density, required OP9 feeder cells. On OP9 feeder layer, EC progenitors gave rise to sheet-like clusters of Flk-1+ cells, with VE-cadherin concentrated at the cell-cell junction. The growth was suppressed by Flt-1-IgG1 chimeric protein and dependent on vascular endothelial growth factor (VEGF) but not placenta growth factor (PIGF). Further addition of VEGF resulted in cell dispersion, indicating the role of VEGF in the migration of ECs as well as their proliferation. Cell-cell adhesion of ECs in this culture system was mediated by VE-cadherin. Thus, the culture system described here is useful in dissecting the cellular events of EC progenitors that occur during vasculogenesis and in investigating the molecular mechanisms underlying these processes.

Segregation of the embryonic vascular and hemopoietic systems

The origin of endothelial cells and their subsequent assembly into the primary vascular system have been mostly analyzed in the avian embryo. Following the discovery of specific growth factors and their cognate receptors, the molecular mechanisms underlying these processes have been unraveled in both birds and mammals. In particular, experimental studies of the angiogenic vascular endothelial growth factor (VEGF) and its receptors, carried out in both vertebrate classes, have provided significant insight into the developmental biology of endothelial cells. The VEGF receptor VEGFR2 is the earliest marker known to be expressed by endothelial precursor cells of avian and mouse embryos. Based on the localization of VEGFR2+ cells in the avian embryo and on clonal culture experiments, two types of endothelial precursor cells can be distinguished from gastrulation stages onward: posterior mesodermal VEGFR2+ hemangioblasts, which have the capacity to differentiate into endothelial and hemopoietic cells, and anterior VEGFR2+ angioblasts, which can only give rise to endothelial cells.Key words: hemangioblast, endothelial cell, hemopoietic cell, embryo.

Hemangioblast development and regulation

Hematopoietic and endothelial cell lineages are the first to mature from mesoderm in the developing embryo. However, little is known about the molecular and (or) cellular events leading to hematopoietic commitment. The recent applications of technology utilizing gene targeted mice and the employment of many available in vitro systems have facilitated our understanding of hematopoietic establishment in the developing embryo. It is becoming clear that embryonic hematopoiesis occurs both in the extra-embryonic yolk sac and within the embryo proper in the mouse. The existence of the long pursued hemangioblast, a common progenitor of hematopoietic and endothelial cells, is now formally demonstrated. Based on this new information, many studies are being conducted to understand hematopoietic commitment events from mesoderm. In this review, we will first discuss the establishment of the hematopoietic system with special emphasis on the most primitive hematopoietic committed cells, the hemangioblast. We will then discuss mesoderm-inducing factors and their possible role in hematopoietic lineage commitment.Key words: hematopoietic commitment, hemangioblast, in vitro embryonic stem cell differentiation.

VEGF gene delivery to muscle: potential role for vasculogenesis in adults

Constitutive expression of VEGF after implantation of genetically engineered myoblasts into non-ischemic muscle led to an increase in vascular structures. Previously, effects of VEGF delivery to adult muscle have only been reported in ischemic tissues. The resulting vascular structures were reminiscent of those formed during embryonic vasculogenesis, rather than angiogenesis, sprouting from preexisting vessels. Initially, VEGF caused an accumulation of endothelial cells and macrophages, followed by networks of vascular channels and hemangiomas with locally high serum VEGF levels. No effects were evident in adjacent tissue or contralateral legs, where low serum VEGF was detected. These data suggest that the induction by VEGF of angiogenesis or vasculogenesis may be dose-dependent. Furthermore, VEGF expression must be carefully modulated, as overexpression is deleterious.

Hematopoiesis: progenitors and their genetic program

Recent advances in our understanding of blood cell development have included the identification of a hematopoietic progenitor derived from endothelial cells and the possibility that the embryonic and fetal environment can reprogram gene expression in adult hematopoietic stem cells.

In vitro generation of lymphohematopoietic cells from endothelial cells purified from murine embryos

We have investigated the lymphohematopoietic potentials of endothelial cells (EC) and hematopoietic cells (HPC) sorted from embryos. Expression of VE-cadherin, CD45, and Ter119 was used to distinguish EC (VE-cadherin+CD45-Ter119-) from HPC (VE-cadherin-CD45+). Thus defined, EC population takes up acetylated LDL and coexpresses CD31, Flk1, and CD34. In E9.5 embryos, EC from yolk sac (YS) and the embryo proper generate blood cells, including lymphocytes. Thus, lymphohematopoietic EC do exist in the embryo, and they are generated both in YS and the embryo proper. On the other hand, HPC with lymphopoietic potency appear first in the embryo proper. These findings implicate involvement of multiple environmental cues for acquiring lymphopoietic competency during differentiation of HPC.