Adult human hematopoietic cells provide functional hemangioblast activity

Wound Healing: A Cellular Perspective

Wound healing is one of the most complex processes in the human body. It involves the spatial and temporal synchronization of a variety of cell types with distinct roles in the phases of hemostasis, inflammation, growth, re-epithelialization, and remodeling. With the evolution of single cell technologies, it has been possible to uncover phenotypic and functional heterogeneity within several of these cell types. There have also been discoveries of rare, stem cell subsets within the skin, which are unipotent in the uninjured state, but become multipotent following skin injury. Unraveling the roles of each of these cell types and their interactions with each other is important in understanding the mechanisms of normal wound closure. Changes in the microenvironment including alterations in mechanical forces, oxygen levels, chemokines, extracellular matrix and growth factor synthesis directly impact cellular recruitment and activation, leading to impaired states of wound healing. Single cell technologies can be used to decipher these cellular alterations in diseased states such as in chronic wounds and hypertrophic scarring so that effective therapeutic solutions for healing wounds can be developed.

Mitoprotection attenuates myocardial vascular impairment in porcine metabolic syndrome

Metabolic syndrome (MetS) leads to cardiac vascular injury, which may reflect in increased retention of endothelial progenitor cells (EPCs). Coronary endothelial cell (EC) mitochondria partly regulate vascular function and structure. We hypothesized that chronic mitoprotection would preserve EC mitochondria and attenuate coronary vascular injury and dysfunction in swine MetS. Pigs were studied after 16 wk of diet-induced MetS, MetS treated for the last 4 wk with the mitochondria-targeted peptide elamipretide (ELAM; 0.1 mg/kg sc once daily), and lean controls ( n = 6 each). Cardiac remodeling and function were assessed in vivo by multidetector-computed tomography (CT), and coronary artery and sinus blood samples were collected. EC mitochondrial density, apoptosis, oxidative stress, endothelial nitric oxide synthase immunoreactivity, myocardial microvascular density (three-dimensional microcomputed tomography), and coronary endothelial function (organ bath) were assessed ex vivo. The number and arteriovenous gradient of CD34⁺/KDR⁺ EPCs were calculated by FACS (a negative net gradient indicating EPC retention). MetS and MetS + ELAM pigs developed similar MetS (obesity, hyperlipidemia, insulin resistance, and hypertension). EC mitochondrial density decreased in MetS animals compared with lean animals but normalized in MetS + ELAM animals. ELAM also attenuated EC oxidative stress and apoptosis and improved subendocardial microvascular density. ELAM-induced vasculoprotection was reflected by decreased coronary retention of EPCs. ELAM also partly improved endothelial nitric oxide synthase immunoreactivity, coronary endothelial function, and vessel maturity, whereas myocardial perfusion was unaffected. Chronic mitoprotection improved coronary EC mitochondrial density and decreased vascular remodeling and dysfunction. However, additional mitochondria-independent mechanisms likely contribute to MetS-induced cardiac vascular injury. NEW & NOTEWORTHY The present study shows that chronic mitoprotection preserved coronary endothelial cell mitochondria and decreased vascular injury, subendocardial microvascular loss, coronary retention of endothelial progenitor cells, and release of markers of vascular injury. However, myocardial perfusion remained blunted, suggesting that additional mitochondria-independent mechanisms likely contribute to metabolic syndrome-induced cardiac vascular injury.

Effect of Calcium-Infiltrated Hydroxyapatite Scaffolds on the Hematopoietic Fate of Human Umbilical Vein Endothelial Cells

Foamed hydroxyapatite offers a three-dimensional scaffold for the development of bone constructs, mimicking perfectly the in vivo bone structure. In vivo, calcium release at the surface is assumed to provide a locally increased gradient supporting the maintenance of the hematopoietic stem cells niche. We fabricated hydroxyapatite scaffolds with high surface calcium concentration by infiltration, and used human umbilical vein endothelial cells (HUVECs) as a model to study the effects on hematopoietic lineage direction. HUVECs are umbilical vein-derived and thus possess progenitor characteristics, with a prospective potential to give rise to hematopoietic lineages. HUVECs were cultured for long term on three-dimensional porous hydroxyapatite scaffolds, which were either infiltrated biphasic foams or untreated. Controls were cultured in two-dimensional dishes. The release of calcium into culture medium was determined, and cells were analyzed for typical hematopoietic and endothelial gene expressions, surface markers by flow cytometry, and hematopoietic potential using colony-forming unit assays. Our results indicate that the biphasic foams promoted a hematopoietic lineage direction of HUVECs, suggesting an improved in vivo-like scaffold for hematopoietic bone tissue engineering.

Endothelial cell derived angiocrine support of acute myeloid leukemia targeted by receptor tyrosine kinase inhibition

In acute myeloid leukemia (AML), refractory disease is a major challenge and the leukemia microenvironment may harbor refractory disease. Human AML cell lines KG-1 and HL-60 expressed receptors also found on endothelial cells (ECs) such as VEGFRs, PDGFRs, and cKit. When human AML cells were co-cultured with human umbilical vein endothelial cells (HUVECs) and primary bone marrow endothelial cell (BMECs), the AML cells were more resistant to cytarabine chemotherapy, even in transwell co-culture suggesting angiocrine regulation. Primary BMECs secreted significantly increased levels of VEGF-A and PDGF-AB after exposure to cytarabine. Pazopanib, a receptor tyrosine kinase inhibitor (RTKI) of VEGFRs, PDGFRs, and cKit, removed EC protection of AML cells and enhanced AML cell sensitivity to cytarabine. Xenograft modeling showed significant regression of AML cells and abrogation of BM hypervascularity in RTKI treated cohorts. Together, these results show direct cytotoxicity of RTKIs on AML cells and reversal of EC protection. Combining RTKIs with chemotherapy may serve as promising therapeutic strategy for patients with AML.

Functional integration of acute myeloid leukemia into the vascular niche

Vascular endothelial cells are a critical component of the hematopoietic microenvironment that regulates blood cell production. Recent studies suggest the existence of functional cross-talk between hematologic malignancies and vascular endothelium. Here, we show that human acute myeloid leukemia (AML) localizes to the vasculature in both patients and in a xenograft model. A significant number of vascular tissue-associated AML cells (V-AML) integrate into vasculature in vivo and can fuse with endothelial cells. V-AML cells acquire several endothelial cell-like characteristics, including the up-regulation of CD105, a receptor associated with activated endothelium. Remarkably, endothelial-integrated V-AML shows an almost 4-fold reduction in proliferative activity compared to non-vascular associated AML. Primary AML cells can be induced to down regulate the expression of their hematopoietic markers in vitro and differentiate into phenotypically and functionally-defined endothelial-like cells. After transplantation, these leukemia-derived endothelial cells are capable of giving rise to AML. Taken together, these novel functional interactions between AML cells and normal endothelium along with the reversible endothelial cell potential of AML suggest that vascular endothelium may serve as a previously unrecognized reservoir for acute myeloid leukemia.

Stem cell research: a novel boulevard towards improved bovine mastitis management

The dairy industry is a multi-billion dollar industry catering the nutritional needs of all age groups globally through the supply of milk. Clinical mastitis has a severe impact on udder tissue and is also an animal welfare issue. Moreover, it significantly reduces animal value and milk production. Mammary tissue damage reduces the number and activity of epithelial cells and consequently contributes to decreased milk production. The high incidence, low cure rate of this highly economic and sometimes deadly disease is an alarming for dairy sector as well as policy makers. Bovine mammary epithelial cells (MECs) and their stem cells are very important in milk production and bioengineering. The adult mammary epithelium consists of two main cell types; an inner layer of luminal epithelial cells, which produce the milk during lactation, and an outer layer of myoepithelial cells resting on a basement membrane, which are responsible for pushing the milk through the ductal network to the teat cistern. Inner layer of columner/luminal cells of bovine MECs, is characterized by cytokeratin18, 19 (CK18, CK19) and outer layer such as myoepithelial cells which are characterized by CK14, α-smooth muscle actin (α-SMA) and p63. Much work has been done in mouse and human, on mammary gland stem cell research, particularly in cancer therapy, but stem cell research in bovine is still in its infancy. Such stem/progenitor cell discoveries in human and mouse mammary gland bring some hope for application in bovines. These progenitors may be therapeutically adopted to correct the structural/cytological defects in the bovine udder due to mastitis. In the present review we focused on various kinds of stem/progenitor cells which can have therapeutic utility and their possibilities to use as a potential stem cell therapy in the management of bovine post-mastitis damage in orders to restore milk production. The possibilities of bovine mammary stem cell therapy offers significant potential for regeneration of tissues that can potentially replace/repair diseased and damaged tissue through differentiation into epithelial, myoepithelial and/or cuboidal/columnar cells in the udder with minimal risk of rejection and side effects.

The haematopoietic stem cell niche: new insights into the mechanisms regulating haematopoietic stem cell behaviour

The concept of the haematopoietic stem cell (HSC) niche was formulated by Schofield in the 1970s, as a region within the bone marrow containing functional cell types that can maintain HSC potency throughout life. Since then, ongoing research has identified numerous cell types and a plethora of signals that not only maintain HSCs, but also dictate their behaviour with respect to homeostatic requirements and exogenous stresses. It has been proposed that there are endosteal and vascular niches within the bone marrow, which are thought to regulate different HSC populations. However, recent data depicts a more complicated picture, with functional crosstalk between cells in these two regions. In this review, recent research into the endosteal/vascular cell types and signals regulating HSC behaviour are considered, together with the possibility of a single subcompartmentalised niche.

Angiogenesis in acute myeloid leukemia and opportunities for novel therapies

Acute myeloid leukemia (AML) arises from neoplastic transformation of hematopoietic stem and progenitor cells, and relapsed disease remains one of the greater challenges in treating this hematologic malignancy. This paper focuses on angiogenic aspects of AML including the significance and prognostic value of bone marrow microvessel density and circulating cytokine levels. We show three general mechanisms whereby AML exploits angiogenic pathways, including direct induction of angiogenesis, paracrine regulation, and autocrine stimulation. We also present early evidence that leukemia cells contribute directly to vascular endothelia. Novel treatment strategies are proposed, and a review of relevant antiangiogenic clinical trials is presented. By understanding how blood vessels can serve as a reservoir for refractory and relapsed AML, new diagnostics and promising treatment strategies can be developed.

The low viability of human CD34+ cells under acidic conditions is improved by exposure to thrombopoietin, stem cell factor, interleukin-3, or increased cyclic adenosine monophosphate levels

BACKGROUND

Transplanted hematopoietic progenitor cells (CD34+) have shown great promise in regenerative medicine. However, the therapeutic potential of transplanted cells is limited by their poor viability. It is well known that the microenvironment in which progenitors reside substantially affects their behavior. Because extracellular acidosis is a common feature of injured tissues or the tumor microenvironment and is a critical regulator of cell survival and activation, we evaluated the impact of acidosis on CD34+ cell biology.

STUDY DESIGN AND METHODS

Apoptosis was evaluated by fluorescence microscopy and binding of annexin V, hypodiploid cells, Bcl-xL expression, active caspase-3, and mitochondrial membrane potential was determined by flow cytometry. Colony-forming units were studied by clonogenic assays, and cell cycle was evaluated by flow cytometry.

RESULTS

Exposure of CD34+ cells to low pH (7.0-6.5) caused intracellular acidification, decreased cell proliferation, and triggered apoptosis via the mitochondrial pathway. Whereas exposure to thrombopoietin (TPO), stem cell factor (SCF), interleukin (IL)-3 or increases in cyclic adenosine monophosphate (cAMP) levels prevented CD34+ cell death induced by acidic conditions, granulocyte-macrophage-colony-stimulating factor, FMS-like tyrosine kinase 3-ligand, erythropoietin, and vascular endothelial growth factor had no effect. Despite their cytoprotective effect, CD34+ cell expansion triggered by TPO, SCF, or IL-3 was significantly impaired at low pH. However, a cocktail of these three cytokines synergistically supported proliferation, cell cycle progression, and colony formation.

DISCUSSION

Our findings indicate that an acidic milieu is deleterious for CD34+ cells and that a combination of certain cytokines and cAMP donors may improve cell viability and function. These data may be useful to develop new therapeutic strategies or to optimize protocols for regenerative medicine.

Clonal analysis reveals a common progenitor for endothelial, myeloid, and lymphoid precursors in umbilical cord blood

RATIONALE

several studies demonstrate that hematopoietic tissues are a source of endothelial progenitor cells, which contribute to newly formed blood vessels during tissue repair in adults. However, it is not clear which cell type in these hematopoietic tissues gives rise to endothelial progenitor cells.

OBJECTIVE

to identity the origin of endothelial progenitors within the hematopoietic hierarchy and to assess their in vivo revascularization potential.

METHODS AND RESULTS

using a single-cell sorting approach and in vitro multilineage differentiation assays, here we show that individual CD34(+)CD45(+)CD133(+)CD38(+) cells from cord blood uniquely have the ability to differentiate into T- and B-lymphoid, myeloid, and endothelial cells. The latter were characterized by the expression of VE-cadherin, KDR, von Willebrand factor, endothelial nitric oxide synthase, the lack of CD45, CD133, and c-fms (colony stimulating factor-1 receptor). Unexpectedly when transplanted into hindlimb ischemic NOD-scid IL2Rγ(null) mice, freshly isolated CD34(+)CD45(+)CD133(+)CD38(+) cells maintained their hematopoietic identity and were rarely found to integrate into host blood vessels. Nevertheless, they significantly improved perfusion, most likely through a paracrine mechanism. On the other hand, CD34(+)CD45(+)CD133(+)CD38(+) cells differentiated in vitro into endothelial cells were able to form vessels in vivo in both Matrigel plug and hindlimb ischemia transplantation assays.

CONCLUSIONS

these findings indicate that the CD34(+)CD45(+)CD133(+)CD38(+) cell fraction contains a common progenitor for the hematopoietic and vascular lineages and may represent a valuable cell source for therapeutic applications.

Coronary endothelial dysfunction in humans is associated with coronary retention of osteogenic endothelial progenitor cells

AIMS

Endothelial progenitor cells (EPC) may participate in the repair of injured coronary endothelium. We have recently identified EPC co-expressing the osteoblastic marker osteocalcin [OCN (+) EPC] and found that their numbers are increased in patients with early and late coronary atherosclerosis. The current study was designed to test the hypothesis that early coronary atherosclerosis is associated with the retention of osteogenic EPC within the coronary circulation.

METHODS AND RESULTS

Blood samples were taken simultaneously from the proximal aorta and the coronary sinus from 31 patients undergoing invasive coronary endothelial function testing. Using flow cytometry, peripheral blood mononuclear cells were analysed for EPC markers (CD133, CD34, KDR) and OCN. The net gradient of EPC was calculated by multiplying the coronary blood flow by the arteriovenous EPC gradient (a negative net gradient indicating retention of EPC). Similarly, serum samples were analysed for stromal cell-derived factor-1 alpha (SDF-1 alpha) and interleukin-8 (IL-8) and their net production calculated. Compared with controls (n = 17) patients with endothelial dysfunction (ED, n = 14) had a significant net retention of CD34+/CD133-/KDR+/OCN+ EPC [118.38 (0.00, 267.04) vs. -112.03 (838.36, 0.00), P = 0.004]. The retention of OCN (+) EPC correlated with the degree of ED. Patients with ED also showed a net retention of CD34+/CD133-/KDR+ EPC (P = 0.010). Net production of IL-8 was positive in ED [1540.80 (-300.40, 21744.10)pg/mL] but negative in controls [-3428.50 (-11225.00, 647.48), P = 0.025].

CONCLUSION

Our study demonstrates that patients with early coronary atherosclerosis are characterized by retention of OCN (+) EPC within the coronary circulation, potentially leading to progressive coronary calcification rather than normal repair.

Development of a network to test strategies in cardiovascular cell delivery: the NHLBI-sponsored Cardiovascular Cell Therapy Research Network (CCTRN)

The emerging sciences of stem cell biology and cellular plasticity have led to the development of cell-based therapies for advanced human disease. Pre-clinical studies which defined the potential of bone marrow-derived mononuclear cells to repair damaged and dysfunctional myocardium led to the rapid advancement of these strategies to the clinic. Such rapid advancement has led to controversy regarding the appropriate conduct of such studies. In the United States, the National Heart, Lung, and Blood Institute established the Cardiovascular Cell Therapy Research Network (CCTRN) to facilitate the early translation of clinical trials of cell therapy for left ventricular dysfunction. The premise upon which the CCTRN was established was that multiple clinical trial sites would interact effectively with a Data Coordinating Center to perform early phase 1 and 2 clinical trials within a highly coordinated network structure. In order to develop this network, the unmet needs of the community needed to be defined, the clinical trials identified, and the structure to perform the studies needed to be established. This manuscript highlights the challenges in the development of the CCTRN and the approaches faced to define a network to perform clinical trials in human cell therapy of cardiovascular disease.

Is Endothelium the Origin of Endothelial Progenitor Cells?

Endothelial cells provide the dynamic lining of blood vessels throughout the body and provide many tissue-specific functions, in addition to providing a nonthrombogenic surface for blood cells and conduit for oxygen and nutrient delivery. As might be expected, some endothelial cells are injured or become senescent and are sloughed into the bloodstream, and most circulating endothelial cells display evidence of undergoing apoptosis or necrosis. However, there are rare viable circulating endothelial cells that display properties consistent with those of a progenitor cell for the endothelial lineage. This article reviews historical and current literature to present some evidence that the endothelial lining of blood vessels may serve as a source for rare endothelial colony-forming cells that display clonal proliferative potential, self-renewal, and in vivo vessel forming ability. The article also discusses the current gaps in our knowledge to prove whether the colony-forming cells are in fact derived from vascular endothelium.

Bone marrow stem and progenitor cell contribution to neovasculogenesis is dependent on model system with SDF-1 as a permissive trigger

Adult bone marrow (BM) contributes to neovascularization in some but not all settings, and reasons for these discordant results have remained unexplored. We conducted novel comparative studies in which multiple neovascularization models were established in single mice to reduce variations in experimental methodology. In different combinations, BM contribution was detected in ischemic retinas and, to a lesser extent, Lewis lung carcinoma cells, whereas B16 melanomas showed little to no BM contribution. Using this spectrum of BM contribution, we demonstrate the necessity for site-specific expression of stromal-derived factor-1alpha (SDF-1alpha) and its mobilizing effects on BM. Blocking SDF-1alpha activity with neutralizing antibodies abrogated BM-derived neovascularization in lung cancer and retinopathy. Furthermore, secondary transplantation of single hematopoietic stem cells (HSCs) showed that HSCs are a long-term source of neovasculogenesis and that CD133(+)CXCR4(+) myeloid progenitor cells directly participate in new blood vessel formation in response to SDF-1alpha. The varied BM contribution seen in different model systems is suggestive of redundant mechanisms governing postnatal neovasculogenesis and provides an explanation for contradictory results observed in the field.

Endothelial progenitor cells: identity defined?

In the past decade, researchers have gained important insights on the role of bone marrow (BM)-derived cells in adult neovascularization. A subset of BM-derived cells, called endothelial progenitor cells (EPCs), has been of particular interest, as these cells were suggested to home to sites of neovascularization and neoendothelialization and differentiate into endothelial cells (ECs) in situ, a process referred to as postnatal vasculogenesis. Therefore, EPCs were proposed as a potential regenerative tool for treating human vascular disease and a possible target to restrict vessel growth in tumour pathology. However, conflicting results have been reported in the field, and the identification, characterization, and exact role of EPCs in vascular biology is still a subject of much discussion. The focus of this review is on the controversial issues in the field of EPCs which are related to the lack of a unique EPC marker, identification challenges related to the paucity of EPCs in the circulation, and the important phenotypical and functional overlap between EPCs, haematopoietic cells and mature ECs. We also discuss our recent findings on the origin of endothelial outgrowth cells (EOCs), showing that this in vitro defined EC population does not originate from circulating CD133⁺ cells or CD45⁺ haematopoietic cells.

Cell therapies for therapeutic angiogenesis: back to the bench

The discovery, over a decade ago, of endothelial progenitor cells that are able to participate in neovascularization of adult tissue has been greeted enthusiastically because of the potential for new cell-based therapies for therapeutic angiogenesis. Since that time, an ever-growing list of candidate cells has been proposed for cardiovascular regeneration. However, to date, pre-clinical and clinical studies evaluating the therapeutic potential of various cell therapies have reported conflicting results, generating controversy. Key issues within the field of cell therapy research include a lack of uniform cellular definitions, as well as inadequate functional characterization of the role of putative stem/progenitor cells in angiogenesis. Given the mixed results of initial clinical studies, there is now a scientific imperative to understand better the vascular biology of candidate cells in order to better translate cell therapy to the bedside. This review will provide a translationally relevant overview of the biology of candidate stem/progenitor cells for therapeutic angiogenesis.

Vascular wall resident progenitor cells: a review

The vessel wall has usually been thought to be relatively quiescent. But the discovery of progenitor cells in many tissues and in the vasculature itself has led to a reconsideration of the vascular biology. The presence of circulating endothelial and smooth muscle progenitors able to home to the injured vascular wall is a firm acquisition; less known is the notion, coming from embryonic and adult tissue studies, that stem cells able to differentiate into endothelial cells and smooth muscle cells also reside in the arterial wall. Moreover, the existence of a vasculogenic zone has recently been identified in adult human arteries; this niche-like zone is believed to act as a source of progenitors for postnatal vasculogenesis. From the literature it is already apparent that a complex interplay between circulating and resident vascular wall progenitors takes place during embryonal and postnatal life; a structural/functional disarray of these intimate stem cell compartments could hamper appropriate vascular repair, the development of vascular wall disease being the direct clinical consequence in adult life. This review gives an overview of adult large vessel progenitors established in the vascular wall during embryogenesis and their role in the maintenance of wall homeostasis.

Hemangioblasts from human embryonic stem cells generate multilayered blood vessels with functional smooth muscle cells

Background: The formation and regeneration of functional vasculatures require both endothelial cells (ECs) and vascular smooth muscle cells (SMCs). Identification and isolation of progenitors with potential for both EC and SMC lineage differentiation from an inexhaustible source, such as human embryonic stem (hES) or induced pluripotent stem cells, will be desirable for cell replacement therapy. Method: Recently, we have developed a serum-free and animal feeder-free differentiation system to generate blast cells (BCs) from hESCs. These cells possess the characteristics of hemangioblasts in vitro and are capable of repairing damaged retinal vasculatures, restoring blood flow in hind-limb ischemia and reducing the mortality rate after myocardial infarction in vivo. We demonstrate here that BCs express markers of SMCs and differentiate into smooth muscle-like cells (SMLCs), in addition to ECs and hematopoietic cells. Results: When BCs from individual blast colonies were cultured in SMC medium, they differentiated into both ECs and SMLCs, which formed capillary-vascular-like structures after replating on Matrigel™. The SMLCs expressed SMC-specific markers (α-SM actin and calponin) and contracted upon treatment with carbachol. When implanted in nude mice, these cells formed microvasculature with ECs in Matrigel plaques. The BCs differentiated into both ECs and SMLCs, and incorporated into blood vessels after injection into ischemic tissue. Conclusion: These results demonstrate that hemangioblasts (BCs) generated from hESCs are tripotential and can provide a potentially inexhaustible source of cells for the treatment of human blood and vascular diseases.

Isolation and enrichment of stem cells

Stem cells have the potential to revolutionize tissue regeneration and engineering. Both general types of stem cells, those with pluripotent differentiation potential as well as those with multipotent differentiation potential, are of equal interest. They are important tools to further understanding of general cellular processes, to refine industrial applications for drug target discovery and predictive toxicology, and to gain more insights into their potential for tissue regeneration. This chapter provides an overview of existing sorting technologies and protocols, outlines the phenotypic characteristics of a number of different stem cells, and summarizes their potential clinical applications.

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.

GeneChip analysis of human embryonic stem cell differentiation into hemangioblasts: an in silico dissection of mixed phenotypes

Transcriptional profiling of human embryonic stem cells differentiating into blast cells reveals that erythroblasts are the predominant cell type in the blast cell population. In silico comparisons with publicly available data sets revealed the presence of endothelia, cardiomyocytes and hematopoietic lineages.

Background

Microarrays are being used to understand human embryonic stem cell (hESC) differentiation. Most differentiation protocols use a multi-stage approach that induces commitment along a particular lineage. Therefore, each stage represents a more mature and less heterogeneous phenotype. Thus, characterizing the heterogeneous progenitor populations upon differentiation are of increasing importance. Here we describe a novel method of data analysis using a recently developed differentiation protocol involving the formation of functional hemangioblasts from hESCs. Blast cells are multipotent and can differentiate into multiple lineages of hematopoeitic cells (erythroid, granulocyte and macrophage), endothelial and smooth muscle cells.

Results

Large-scale transcriptional analysis was performed at distinct time points of hESC differentiation (undifferentiated hESCs, embryoid bodies, and blast cells, the last of which generates both hematopoietic and endothelial progenies). Identifying genes enriched in blast cells relative to hESCs revealed a genetic signature indicative of erythroblasts, suggesting that erythroblasts are the predominant cell type in the blast cell population. Because of the heterogeneity of blast cells, numerous comparisons were made to publicly available data sets in silico, some of which blast cells are capable of differentiating into, to assess and characterize the blast cell population. Biologically relevant comparisons masked particular genetic signatures within the heterogeneous population and identified genetic signatures indicating the presence of endothelia, cardiomyocytes, and hematopoietic lineages in the blast cell population.

Conclusion

The significance of this microarray study is in its ability to assess and identify cellular populations within a heterogeneous population through biologically relevant in silico comparisons of publicly available data sets. In conclusion, multiple in silico comparisons were necessary to characterize tissue-specific genetic signatures within a heterogeneous hemangioblast population.

Endothelial differentiation of hematopoietic stem and progenitor cells from patients with multiple myeloma

PURPOSE

Vasculogenesis is a physiologic process typical of fetal development in which new blood vessels develop from undifferentiated precursors (or angioblasts). In tumors, near angiogenesis, vasculogenesis contributes to the formation of the microvascular plexus that is important for diffusion. Here, we show that hematopoietic stem and progenitor cells (HSPC) of multiple myeloma (MM) patients are able to differentiate into cells with endothelial phenotype on exposure to angiogenic cytokines.

EXPERIMENTAL DESIGN

Circulating HSPCs were purified with an anti-CD133 antibody from patients with newly diagnosed MM before autologous transplantation and exposed to vascular endothelial growth factor (VEGF), fibroblast growth factor-2 and insulin-like growth factor in a 3-week culture.

RESULTS

HSPCs gradually lost CD133 expression and acquired VEGF receptor-2, factor VIII-related antigen, and vascular endothelial-cadherin expression. The expression pattern overlapped with paired MM endothelial cells (MMEC). During culture, cells adhered to fibronectin, spread, and acquired an endothelial cell shape. Differentiated HSPCs also became capillarogenic in the Matrigel assay with maximal activity at the third week of culture. Bone marrow biopsies revealed HSPCs inside the neovessel wall in patients with MM but not in those with monoclonal gammopathy of undetermined significance.

CONCLUSIONS

In patients with MM, but not in those with monoclonal gammopathy of undetermined significance, HSPCs contribute to the neovessel wall building together with MMECs. Therefore, besides angiogenesis, HSPC-linked vasculogenesis contributes to neovascularization in MM patients. Tentatively, we hypothesize that in HSPC cultures a multipotent cell population expressing low VEGF receptor-2 levels corresponds to the endothelial progenitor cell precursor and seems to be the MMEC precursor.

IFATS collection: Identification of hemangioblasts in the adult human adipose tissue

The stromal-vascular fraction (SVF) of human adipose tissue contains, among other cell types, mesenchymal stem cells and precursors of adipocyte and endothelial cells. Here we show that, in addition, the nonhematopoietic fraction of the SVF has hematopoietic activity, since all types of hematopoietic colony-forming units (CFUs) developed when cultured in methylcellulose-based medium. This hematopoietic activity was restricted to the CD45(-)CD105(+) cell subset, well correlated with KDR(+) cell content, and increased after culture with a combination of early-acting hematopoietic cytokines. Most of the CD45(-)KDR(+)CD105(+) cells were nonadherent and did not express CD31, and this subset included both CD34(-) and CD34(+) cells. Moreover, these nonadherent cells migrated in response to KDR gradient, and when they were cultured in the presence of both hematopoietic and endothelial growth factors, a wave of CFUs was followed by a wave of mixed colonies comprising adherent elongated and nonadherent round hematopoietic cells. These mixed hematopoietic-endothelial (Hem-End) colonies were able to generate secondary Hem-End colonies and exhibited both hematopoietic and endothelial activity, as demonstrated by in vitro functional assays. These findings demonstrate for the first time the existence of primitive mesodermal progenitors within the SVF of human adipose tissue that exhibit in vitro hematopoietic and hemangioblastic activities, susceptible to being used in cell therapy and basic cell research. Disclosure of potential conflicts of interest is found at the end of this article.

Injury mechanism dictates contribution of bone marrow-derived cells to murine hepatic vascular regeneration

Stem and progenitor cells derived from adult marrow have been shown to regenerate vascular cells in response to injury. However, it is unclear whether the type of injury dictates the contribution of such cells to neovascularization and which subpopulations of cells contribute to vascular regeneration. To address these questions, we determined the extent that hematopoietic stem cells (HSC) contributed to blood vessel formation in response to two types of liver injury, partial hepatectomy (PH) and toxin-induced injury. Lac-Z-labeled HSC were engrafted into lethally irradiated, genetically matched recipients. After 14 d, we identified transplanted cells engrafted within the vascular endothelium of toxin-damaged liver, but not in the vasculature of liver regenerated in response to PH. Engraftment of HSC-derived cells occurred in a gradient fashion with the highest activity in the severely injured areas. Although HSC-derived cells contributed to both microvessels and large vessels, the large caliber vessels trended toward higher engraftment levels. Thus, the contribution of marrow-derived cells to hepatic neovascularization is dependent upon the type of injury sustained. Furthermore, following toxin-induced liver injury, engraftment rates trended higher in large vessels compared with capillaries, suggesting that remodeling of existing vessels is a predominant mechanism of repair, relative to the formation of new microvasculature.

Neoangiogenesis with endothelial precursors for the treatment of ischemia

In the embryo, blood vessels and hematopoietic cells arise from the hemangioblast, a common precursor cell. Compelling evidence suggests that bone marrow from adult individuals contains endothelial cell precursors (EPCs), similar to embryonic hemangioblast. They are able to increase neovascularization of tissue after ischemia. Herein we have discussed the ontogeny of these cells, their phenotypes, and their isolation from various sources. We also have presented experimental studies indicating that EPCs are able to induce neovascularization and angiogenesis when transplanted into ischemic tissues. Furthermore, endogenous EPCs can be mobilized using factors that promote their homing to sites of tissue injury. We also have discussed the ongoing clinical trials using these cells to treat ischemic diseases.

Are postnatal hemangioblasts generated by dedifferentiation from committed hematopoietic stem cells?

Cell dedifferentiation occurs in different cell systems. In spite of a relative paucity of data it seems reasonable to assume that cell dedifferentiation exists in reversible equilibrium with differentiation, to which cells resort in response to intercellular signals. The current literature is indeed compatible with the concept that dedifferentiation is guided by structural rearrangements of nuclear chromatin, directed by epigenetic cell memory information available as silenced genes stored on heterochromatin, and that gene transcription exists in reversible "fluctuating continua" during parental cell cycles. Here, we review the molecular mechanisms of cell dedifferentiation and suggest for hematopoietic development that postnatal hemangioblasts are generated by dedifferentiation of committed hematopoietic stem cells.

Evidence that intracoronary-injected CD133+ peripheral blood progenitor cells home to the myocardium in chronic postinfarction heart failure

OBJECTIVE

To study the biodistribution of purified CD133(+) cells after intracoronary injection in patients with stable chronic postinfarction heart failure.

PATIENTS AND METHODS

Patients with longstanding myocardial infarction (>12 months prior to inclusion) and with an accessible left coronary artery were eligible. CD133(+) cells were mobilized with granulocyte colony-stimulating factor and purified with a CliniMACS device. Cells were labeled with (111)Indium and injected through a balloon catheter in a coronary artery feeding the necrotic or viable infarct-related region of the left ventricle during a standard coronary catheterization procedure. The total body biodistribution of (111)Indium was studied with a dual-head gamma camera in combination with (99m)Technetium-sestaMIBI cardiac distribution analysis.

RESULTS

The number of CD133(+) cells injected ranged between 5 and 10 x 10(6) cells (low dose, three patients) or between 18.5 and 50 x 10(6) cells (high dose, five patients). In the five patients receiving the higher cell doses, a clear residual radioactivity was observed at the level of the chronic injury at 2, 12, and up to 36 hours after injection. A detailed analysis in two patients showed 6.9% to 8.0% (after 2 hours) and 2.3% to 3.2% (after 12 hours) residual radioactivity at the heart. No adverse events were observed during the procedure and up to 3 months follow-up.

CONCLUSIONS

We demonstrate that CD133(+) progenitor cells are capable of homing to the postinfarction remodeling myocardium after intracoronary injections in patients with chronic postinfarction heart failure.

The hunt for cancer-initiating cells: a history stemming from leukemia

Conventional cancer therapies are plagued by disease relapses due to incomplete eradication of cancer-initiating cells. Evidence for cancer-initiating cells originally arose from studies in hematology and leukemia. Lessons learned from hematopoietic stem cells laid the bedrock for understanding how leukemic cells self-renew and remain in immature states. Decades later, leukemia-initiating cell techniques are now being applied to the field of solid tumors such as brain, breast, bone, colon, pancreas, lung and prostate cancer, with several cancer-initiating cell efforts led by hematologists. Different isolation techniques enriching for primitive cancer-initiating cells have been developed and are described in this review. Although the concept of cancer-initiating cells arose from studies in normal tissue stem cells, differences exist between neoplastic-initiating clones and their normal counterparts. Several efforts have uncovered aberrant molecular pathways and niche interactions unique to cancer-initiating cells. Efforts to exploit these pathways and interactions could ultimately lead to complete eradication of cancers.

Generation of functional hemangioblasts from human embryonic stem cells

Recent evidence suggests the existence of progenitor cells in adult tissues that are capable of differentiating into vascular structures as well as into all hematopoietic cell lineages. Here we describe an efficient and reproducible method for generating large numbers of these bipotential progenitors-known as hemangioblasts-from human embryonic stem (hES) cells using an in vitro differentiation system. Blast cells expressed gene signatures characteristic of hemangioblasts, and could be expanded, cryopreserved and differentiated into multiple hematopoietic lineages as well as into endothelial cells. When we injected these cells into rats with diabetes or into mice with ischemia-reperfusion injury of the retina, they localized to the site of injury in the damaged vasculature and appeared to participate in repair. Injection of the cells also reduced the mortality rate after myocardial infarction and restored blood flow in hind limb ischemia in mouse models. Our data suggest that hES-derived blast cells (hES-BCs) could be important in vascular repair.

Short-term endothelial progenitor cell colonies are composed of monocytes and do not acquire endothelial markers

BACKGROUND

The aim of this study was to identify circulating endothelial progenitor cells (EPC) with colony-forming capacity and compare them with the monocytic-macrophage lineage.

METHODS

Forty-two healthy donors were analyzed. EPC were cultured with VEGF and b-FGF. Sequential studies were performed on days +7 (colonies) +21 and +35. Monocytic cells were cultured using the same conditions as EPC until day +21 or alternatively by adding IGF.

RESULTS

The number of EPC colonies was higher in BM than in mobilized or steady-state PB. Using EPC medium, monocytic cells formed cord-like structures but no colonies. However, colonies grew when IGF was added to the medium. By immunocytochemistry, colonies showed CD45, CD31 and lysozyme but no vWF. Colonies were CD4+, CD13+dim, CD14+, CD15++, CD16-/+dim, CD31+dim, CD33+dim, CD45+, CD105-/+dim, lysozyme+ and VE-cadherin+, and constantly negative for CD34, CD133 and KDR, when flow cytometry was used. The immunophenotype of pre-cultured and cultured monocytes was similar to that described for EPC.

DISCUSSION

Our results suggest that the so-called 'EPC' obtained at 7 days of culture belong to the monocyte-macrophage lineage, as they share immunophenotypic and molecular features.

Working hypothesis to redefine endothelial progenitor cells

Since 1997, postnatal vasculogenesis has been purported to be an important mechanism for neoangiogenesis via bone marrow (BM)-derived circulating endothelial progenitor cells (EPCs). Based on this paradigm, EPCs have been extensively studied as biomarkers to assess severity of cardiovascular disease and as a cell-based therapy for several human cardiovascular disorders. In the majority of studies to date, EPCs were identified and enumerated by two primary methodologies; EPCs were obtained and quantified following in vitro cell culture, or EPCs were identified and enumerated by flow cytometry. Both methods have proven controversial. This review will attempt to outline the definition of EPCs from some of the most widely cited published reports in an effort to provide a framework for understanding subsequent studies in this rapidly evolving field. We will focus this review on studies that used cell culture techniques to define EPCs.

Cellular transplantation: future therapeutic options

PURPOSE OF REVIEW

Cardiac transplantation is a complex undertaking and an imperfect solution to end-stage heart failure. Cellular transplantation has been proposed as an alternative solution; however, clinical trials at present are small and show variable results. The mechanisms behind stem cell therapy have not yet been elucidated.

RECENT FINDINGS

Several large trials have been presented that address the question of bone marrow stem cells as therapy for acute myocardial infarction, and also the possible benefits of therapy with granulocyte colony-stimulating factor. Although some trials show a modest improvement in ejection fraction or reduction of infarct size, other trials show no change with treatment. Fewer clinical data are available on the treatment of chronic left ventricular systolic function. Many questions remain such as what cell type to use, dosing, the ideal timing for therapy, and the technique of cell delivery. Finally, further research continues on the cellular milieu, enhancement of cell engraftment, proliferation, and survival.

SUMMARY

This review briefly examines the background for stem cell therapy, as well as the larger clinical trials of stem cell therapy for acute myocardial infarction and chronic left ventricular systolic dysfunction, and possible pharmacologic enhancement options.

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.

Identification of Notch target genes in uncommitted T-cell progenitors: No direct induction of a T-cell specific gene program

Deregulated Notch signaling occurs in the majority of human T-ALL. During normal lymphoid development, activation of the Notch signaling pathway poses a T-cell fate on hematopoietic progenitors. However, the transcriptional targets of the Notch pathway are largely unknown. We sought to identify Notch target genes by inducing Notch signaling in human hematopoietic progenitors using two different methods: an intracellular signal through transfection of activated Notch and a Notch-receptor dependent signal by interaction with its ligand Delta1. Gene expression profiles were generated and evaluated with respect to expression profiles of immature thymic subpopulations. We confirmed HES1, NOTCH1 and NRARP as Notch target genes, but other reported Notch targets, including the genes for Deltex1, pre-T-cell receptor alpha and E2A, were not found to be differentially expressed. Remarkably, no induction of T-cell receptor gene rearrangements or transcription of known T-cell specific genes was found after activation of the Notch pathway. A number of novel Notch target genes, including the transcription factor TCFL5 and the HOXA cluster, were identified and functionally tested. Apparently, Notch signaling is essential to open the T-cell pathway, but does not initiate the T-cell program itself.

Biology of bone marrow-derived endothelial cell precursors

Over the past decade, the old idea that the bone marrow contains endothelial cell precursors has become an area of renewed interest. While some still believe that there are no endothelial precursors in the blood, even among those who do, there is no consensus as to what they are or what they do. In this review, we describe the problems in identifying endothelial cells and conclude that expression of endothelial nitric oxide synthase may be the most reliable antigenic indicator of the phenotype. The evidence for two different classes of endothelial precursors is also presented. We suggest that, though there is no single endothelial cell precursor, we may be able to use these phenotypic variations to our advantage in better understanding their biology. We also discuss how a variety of genetic, epigenetic, and methodological differences can account for the seemingly contradictory findings on the physiological relevance of bone marrow-derived precursors in normal vascular maintenance and in response to injury. Data on the impact of tumor type and location on the contribution of bone marrow-derived cells to the tumor vasculature are also presented. These data provide hope that we may ultimately be able to predict those tumors in which bone marrow-derived cells will have a significant contribution and design therapies accordingly. Finally, factors that regulate bone marrow cell recruitment to and function in the endothelium are beginning to be identified, and several of these, including stromal derived factor 1, monocyte chemoattractant factor-1, and vascular endothelial growth factor are discussed.

Bone marrow-derived cells home to and regenerate retinal pigment epithelium after injury

PURPOSE

To determine whether hematopoietic stem and progenitor cells (HSCs/HPCs) can home to and regenerate the retinal pigment epithelium (RPE) after induced injury.

METHODS

Enriched HSCs/HPCs from green fluorescent protein (gfp) transgenic mice were transplanted into irradiated recipient mice to track bone marrow-derived cells. Physical damage was induced by breaching Bruch's membrane and inducing vascular endothelial growth factor A (VEGFa) expression to promote neovascularization. RPE damage was also induced by sodium iodate injection (40 mg/kg) into wild-type or albino C57Bl/6 mice. Cell morphology, gfp expression, the presence of the Y chromosome, and the presence of melanosomes were used to determine whether the injured RPE was being repaired by the donor bone marrow.

RESULTS

Injury to the RPE recruits HSC/HPC-derived cells to incorporate into the RPE layer and differentiate into an RPE phenotype. A portion of the HSCs/HPCs adopt RPE morphology, express melanosomes, and integrate into the RPE without cell fusion.

CONCLUSIONS

HSCs/HPCs can migrate to the RPE layer after physical or chemical injury and regenerate a portion of the damaged cell layer.

Stem cell research

One of the most active areas of research in medicine today is stem cell biology. This review introduces the reader to the field of stem cell biology and its therapeutic potential. More importantly, the potential application of stem cell therapy in acute lung injury will be explored.

Transplantation of Human Hematopoietic Stem Cells into Ischemic and Growing Kidneys Suggests a Role in Vasculogenesis but Not Tubulogenesis

Transplantation of murine bone marrow-derived stem cells has been reported recently to promote regeneration of the injured kidney. We investigated the potential of human adult CD34(+) progenitor cells to undergo renal differentiation once xenotransplanted into ischemic and developing kidneys. Immunostaining with human-specific antibodies for tubular cells (broad-spectrum cytokeratin), endothelial cells (CD31, PECAM), stromal cells (vimentin), and hematopoietic cells (pan-leukocyte CD45) demonstrated that although kidney ischemia enhanced engraftment of human cells, they were mostly hematopoietic cells (CD45(+)) residing in the interstitial spaces. Few other engrafted cells demonstrated an endothelial phenotype (human CD31(+)in morphologically appearing peritubular capillaries), but no evidence of tubular or stromal cells of human origin was found. Upregulation of SDF1 and HIF1 transcript levels in the ischemic kidneys might explain the diffuse engraftment of CD45(+)cells following injury. Similarly, when embryonic kidneys rudiments were co-transplanted with human CD34(+)cells in mice, we found both human CD45(+)and CD31(+)cells in the periphery of the developing renal grafts, whereas parenchymal elements failed to stain. In addition, human CD34(+)cells had no effect on kidney growth and differentiation. This first demonstration of human CD34(+)stem cell transplantation into injured and developing kidneys indicates that these cells do not readily acquire a tubular phenotype and are restricted mainly to hematopoietic and, to a lesser extent, to endothelial lineages. Efforts should be made to identify additional stem cell sources applicable for kidney growth and regeneration.

Molecular Profile and Partial Functional Analysis of Novel Endothelial Cell-Derived Growth Factors that Regulate Hematopoiesis

Recent progress has been made in the identification of the osteoblastic cellular niche for hematopoietic stem cells (HSCs) within the bone marrow (BM). Attempts to identify the soluble factors that regulate HSC self-renewal have been less successful. We have demonstrated that primary human brain endothelial cells (HUBECs) support the ex vivo amplification of primitive human BM and cord blood cells capable of repopulating non-obese diabetic/severe combined immunodeficient repopulating (SCID) mice (SCID repopulating cells [SRCs]). In this study, we sought to characterize the soluble hematopoietic activity produced by HUBECs and to identify the growth factors secreted by HUBECs that contribute to this HSC-supportive effect. Extended noncontact HUBEC cultures supported an eight-fold increase in SRCs when combined with thrombopoietin, stem cell factor, and Flt-3 ligand compared with input CD34(+) cells or cytokines alone. Gene expression analysis of HUBEC biological replicates identified 65 differentially expressed, nonredundant transcripts without annotated hematopoietic activity. Gene ontology studies of the HUBEC transcriptome revealed a high concentration of genes encoding extracellular proteins with cell-cell signaling function. Functional analyses demonstrated that adrenomedullin, a vasodilatory hormone, synergized with stem cell factor and Flt-3 ligand to induce the proliferation of primitive human CD34(+)CD38(-)lin(-) cells and promoted the expansion of CD34(+) progenitors in culture. These data demonstrate the potential of primary HUBECs as a reservoir for the discovery of novel secreted proteins that regulate human hematopoiesis.

In vitro endothelial potential of human UC blood-derived mesenchymal stem cells

BACKGROUND

Human mesenchymal stem cells (MSC) possess powerful ex vivo expansion and versatile differentiation potential, placing themselves at the forefront of the field of stem cell-based therapy and transplantation. Of high clinical relevance is the endothelial differentiation potential of MSC, which can be used to treat various forms of ischemic vascular disease.

METHODS

We investigated whether human umbilical cord blood (UCB)-derived MSC are able to differentiate in vitro along an endothelial lineage, by using flow cytometry, RT-PCR and immunofluorescence analyzes, as well as an Ab array method.

RESULTS

When the cells were incubated for up to 3 weeks in the presence of VEGF, EGF and hydrocortisone, they began to express a variety of endothelial lineage surface markers, such as Flk-1, Flt-1, VE-Cadherin, vWF, VCAM-1, Tie-1 and Tie-2, and to secrete a specific set of cytokines. Differentiated cells were also found to be able to uptake low-density lipoprotein and form a tubular network structure.

DISCUSSION

These observations have led us to conclude that UCB-derived MSC retain endothelial potential that is suitable for basic and clinical studies aimed at the development of vasculature-directed regenerative medicine.

Evidence for the Hemangioblast

In recent years, numerous cellular and genetic studies have led to a better understanding of the developmental relationship between hematopoietic and endothelial cell lineages. Specifically, tracing cells expressing various genes such as Brachyury, Flk-1, or Scl has delineated the cellular sequence leading to hematopoietic and endothelial cell development from mesoderm. Moreover, in vitro as well as in vivo studies of invertebrate and vertebrate systems have established that hematopoietic and endothelial cells develop from a common progenitor, the hemangioblast. Finally, the presence of the hemangioblast has been confirmed in postnatal mice and humans. Further characterization of the hemangioblast, both embryo and postnatal, will be critical for a better understanding of the molecular events involved in hematopoietic and endothelial cell differentiation as well as for utilizing this cell population for clinical applications.

Hypothesis: Cell plasticity, linking embryonal stem cells to adult stem cell reservoirs and metastatic cancer cells?

Embryonal stem (ES) cells are the earliest ontogenetically identifiable stem cells of the embryo proper for all subsequent mesenchymal stem cells and for highly specialized differentiated cells. This review characterizes, in a working hypothesis, the role of reversible EMT/MET (epithelialmesenchymal transition) as a manifestation of cell plasticity 1) in the development of ES cells to adult stem cells (hematopoietic stem cells) and 2) in metastasizing cancer cells. Animal studies support the concept that EMT/MET is a key manifestation of cell plasticity in the development of ES cells to adult stem cells, and in conversion of localized to metastasizing cancer cells. In fact, ES cells may persist to postnatal life, in cytologically verifiable form and/or within the frame of EMT/MET, as ultimate reservoir for adult stem cells. Furthermore, EMT could possibly serve as a conceptional link between physiologic and pathologic signaling pathways. Clonal confirmation in humans is necessary.

The nitric oxide pathway modulates hemangioblast activity of adult hematopoietic stem cells

We have previously established a model inducing hematopoietic stem cell (HSC) production of circulating endothelial progenitor cells (EPCs) to revascularize ischemic injury in adult mouse retina. The unique vascular environment of the retina results in new blood vessel formation primarily from HSC-derived EPCs. Using mice deficient (–/–) in inducible nitric oxide synthase (iNOS) and endothelial nitric oxide synthase (eNOS), we show that vessel phenotype resulting from hemangioblast activity can be altered by modulation of the NO/NOS pathway. iNOS–/– or eNOS–/– animals were engrafted with wild-type (WT) HSCs expressing green fluorescence protein (gfp+) and subjected to our adult retinal ischemia model. WT hemangioblast activity in adult iNOS–/– recipients resulted in the formation of highly branched blood vessels of donor origin, which were readily perfused indicating functionality. In contrast, eNOS–/– recipients produced relatively unbranched blood vessels with significant donor contribution that were difficult to perfuse, indicating poor functionality. Furthermore, eNOS–/– chimeras had extensive gfp+ HSC contribution throughout their vasculature without additional injury. This neovascularization, via EPCs derived from the transplanted HSCs, reveals that the NO pathway can modulate EPC activity and plays a critical role in both blood vessel formation in response to injury and normal endothelial cell maintenance.

Mechanisms regulating the origins of the vertebrate vascular system

In order to sustain growth, differentiation, and organogenesis, vertebrate embryos must form a functional vascular system early in embryonic development. Intrinsic interest in this process as well as the promise of potential clinical applications has led to significant progress in understanding the mechanisms governing the formation of the vascular system, however the earliest stages of vascular development--the emergence of committed endothelial precursors from the mesoderm--remain unclear. A review of the current literature reveals an unexpected diversity and heterogeneity with respect to where vascular endothelial cells originate in the embryo, when they become committed and the mechanisms governing how endothelial cells acquire their identity. Spatially, a widespread region of the early mesoderm possesses the ability to give rise to vascular endothelial cells; temporally the process is not limited to a small window during embryogenesis, but rather, may continue throughout the lifespan of the organism. On the molecular level, recent findings point to several determinative pathways that regulate, modulate, and extend the scope of the Flk1/VEGF signaling system. An expanding array of novel gene products implicated in endothelial cell type determination appear to act synergistically, with different combinations of factors leading to diverse cellular responses, varying patterns of differentiation, and considerable heterogeneity of endothelial cell types during embryogenesis.

SDF-1 is both necessary and sufficient to promote proliferative retinopathy

Diabetic retinopathy is the leading cause of blindness in working-age adults. It is caused by oxygen starvation in the retina inducing aberrant formation of blood vessels that destroy retinal architecture. In humans, vitreal stromal cell-derived factor-1 (SDF-1) concentration increases as proliferative diabetic retinopathy progresses. Treatment of patients with triamcinolone decreases SDF-1 levels in the vitreous, with marked disease improvement. SDF-1 induces human retinal endothelial cells to increase expression of VCAM-1, a receptor for very late antigen-4 found on many hematopoietic progenitors, and reduce tight cellular junctions by reducing occludin expression. Both changes would serve to recruit hematopoietic and endothelial progenitor cells along an SDF-1 gradient. We have shown, using a murine model of proliferative adult retinopathy, that the majority of new vessels formed in response to oxygen starvation originate from hematopoietic stem cell-derived endothelial progenitor cells. We now show that the levels of SDF-1 found in patients with proliferative retinopathy induce retinopathy in our murine model. Intravitreal injection of blocking antibodies to SDF-1 prevented retinal neovascularization in our murine model, even in the presence of exogenous VEGF. Together, these data demonstrate that SDF-1 plays a major role in proliferative retinopathy and may be an ideal target for the prevention of proliferative retinopathy.

Stem cell plasticity

The central dogma in stem cell biology has been that cells isolated from a particular tissue can renew and differentiate into lineages of the tissue it resides in. Several studies have challenged this idea by demonstrating that tissue specific cell have considerable plasticity and can cross-lineage restriction boundary and give rise to cell types of other lineages. However, the lack of a clear definition for plasticity has led to confusion with several reports failing to demonstrate that a single cell can indeed differentiate into multiple lineages at significant levels. Further, differences between results obtained in different labs has cast doubt on some results and several studies still await independent confirmation. In this review, we critically evaluate studies that report stem cell plasticity using three rigid criteria to define stem cell plasticity; differentiation of a single cell into multiple cell lineages, functionality of differentiated cells in vitro and in vivo, robust and persistent engraft of transplanted cells.