A chemically defined culture of VEGFR2+ cells derived from embryonic stem cells reveals the role of VEGFR1 in tuning the threshold for VEGF in developing endothelial cells

Effect of biochemical and biomechanical factors on vascularization of kidney organoid-on-a-chip

Kidney organoids derived from the human pluripotent stem cells (hPSCs) recapitulating human kidney are the attractive tool for kidney regeneration, disease modeling, and drug screening. However, the kidney organoids cultured by static conditions have the limited vascular networks and immature nephron-like structures unlike human kidney. Here, we developed a kidney organoid-on-a-chip system providing fluidic flow mimicking shear stress with optimized extracellular matrix (ECM) conditions. We demonstrated that the kidney organoids cultured in our microfluidic system showed more matured podocytes and vascular structures as compared to the static culture condition. Additionally, the kidney organoids cultured in microfluidic systems showed higher sensitivity to nephrotoxic drugs as compared with those cultured in static conditions. We also demonstrated that the physiological flow played an important role in maintaining a number of physiological functions of kidney organoids. Therefore, our kidney organoid-on-a-chip system could provide an organoid culture platform for in vitro vascularization in formation of functional three-dimensional (3D) tissues.

Vascular extracellular matrix and fibroblasts-coculture directed differentiation of human mesenchymal stem cells toward smooth muscle-like cells for vascular tissue engineering

Construction of an artificial vascular graft is widely considered a promising strategy in vascular tissue engineering. However, limited sources of functional vascular smooth muscle cells (VSMCs) remain a major obstacle in vascular tissue engineering. In this study, we innovatively developed an approach to obtain functional VSMCs by onsite differentiating human bone marrow-derived mesenchymal stem cells (MSCs) directed by decellularized extracellular matrix (ECM) and fibroblasts. The resulting cells and ECM-cells constructs were characterized by real time RT-PCR, immunofluorescence staining, cell contractile functions, and migration capacity. Our results showed both ECM and fibroblasts induced MSCs differentiation toward VSMC-like cells with increased transcription of marker genes, upregulated expression of contractile apparatus proteins, and enhanced functional activity of VSMC phenotype. Interestingly, our findings revealed that native ECM and fibroblasts-coculture had a higher potential to promote MSCs differentiation into VSMCs than growth factors cocktail (GFC) supplemented culture, thereby providing a potential source of VSMCs for blood vessel constitution.

Exploitation of receptor tyrosine kinases by viral-encoded growth factors

Receptor tyrosine kinases (RTKs) are essential components of cell communication pathways utilized from the embryonic to adult stages of life. These transmembrane receptors bind polypeptide ligands, such as growth factors, inducing signalling cascades that control cellular processes such as proliferation, survival, differentiation, motility and inflammation. Many viruses have acquired homologs of growth factors encoded by the hosts that they infect. Production of growth factors during infection allows viruses to exploit RTKs for entry and replication in cells, as well as for host and environmental dissemination. This review describes the genetic diversity amongst virus-derived growth factors and the mechanisms by which RTK exploitation enhances virus survival, then highlights how viral ligands can be used to further understanding of RTK signalling and function during embryogenesis, homeostasis and disease scenarios.

The Potential of Stem Cells and Stem Cell-Derived Exosomes in Treating Cardiovascular Diseases

In recent years, the cardiac protective mechanisms of stem cells have become a research focus. Increasing evidence has suggested that stem cells release vesicles, including exosomes and micro-vesicles. The content of these vesicles relies on an extracellular stimulus, and active ingredients are extensively being studied. Previous studies have confirmed that stem cell-derived exosomes have a cardiac protective function similar to that of stem cells, and promote angiogenesis, decrease apoptosis, and respond to stress. Compared to stem cells, exosomes are more stable without aneuploidy and immune rejection, and may be a promising and effective therapy for cardiovascular diseases. In this review, the biological functions and molecular mechanisms of stem cells and stem cell-derived exosomes are discussed.

Co-Emergence of Specialized Endothelial Cells from Embryonic Stem Cells

A well-formed and robust vasculature is critical to the health of most organ systems in the body. However, the endothelial cells (ECs) forming the vasculature can exhibit a number of distinct functional subphenotypes like arterial or venous ECs, as well as angiogenic tip and stalk ECs. In this study, we investigate the in vitro differentiation of EC subphenotypes from embryonic stem cells (ESCs). Using our staged induction methods and chemically defined mediums, highly angiogenic EC subpopulations, as well as less proliferative and less migratory EC subpopulations, are derived. Furthermore, the EC subphenotypes exhibit distinct surface markers, gene expression profiles, and positional affinities during sprouting. While both subpopulations contained greater than 80% VE-cad⁺/CD31⁺ cells, the tip/stalk-like EC contained predominantly Flt4⁺/Dll4⁺/CXCR4⁺/Flt-1^- cells, while the phalanx-like EC was composed of higher numbers of Flt-1⁺ cells. These studies suggest that the tip-specific EC can be derived in vitro from stem cells as a distinct and relatively stable EC subphenotype without the benefit of its morphological positioning in the sprouting vessel.

Multifactorial Optimizations for Directing Endothelial Fate from Stem Cells

Embryonic stem cells (ESC) and induced pluripotent stem (iPS) cells are attractive in vitro models of vascular development, therapeutic angiogenesis, and tissue engineering. However, distinct ESC and iPS cell lines respond differentially to the same microenvironmental factors. Developing improved/optimized differentiation methodologies tailored/applicable in a number of distinct iPS and ESC lines remains a challenge in the field. Currently published methods for deriving endothelial cells (EC) robustly generate high numbers of endothlelial progenitor cells (EPC) within a week, but their maturation to definitive EC is much more difficult, taking up to 2 months and requiring additional purification. Therefore, we set out to examine combinations/levels of putative EC induction factors—utilizing our stage-specific chemically-defined derivation methodology in 4 ESC lines including: kinetics, cell seeding density, matrix signaling, as well as medium treatment with vascular endothelial growth factor (VEGF), and basic fibroblast growth factor (bFGF). The results indicate that temporal development in both early and late stages is the most significant factor generating the desired cells. The generation of early Flk-1⁺/KDR⁺ vascular progenitor cells (VPC) from pluripotent ESC is directed predominantly by high cell seeding density and matrix signaling from fibronectin, while VEGF supplementation was NOT statistically significant in more than one cell line, especially with fibronectin matrix which sequesters autocrine VEGF production by the differentiating stem cells. Although some groups have shown that the GSK3-kinase inhibitor (CHIR) can facilitate EPC fate, it hindered the generation of KDR+ cells in our preoptimized medium formulations. The methods summarized here significantly increased the production of mature vascular endothelial (VE)-cadherin+ EC, with up to 93% and 57% purity from mouse and human ESC, respectively, before VE-cadherin+ EC purification.

Axitinib Has Antiangiogenic and Antitumorigenic Activity in Myxoid Liposarcoma

Myxoid liposarcoma is a rare form of soft-tissue sarcoma. Although most patients initially respond well to treatment, approximately 21% relapse, highlighting the need for alternative treatments. To identify novel treatment regimens and gain a better understanding of myxoid liposarcoma tumor biology, we screened various candidate and approved targeted therapeutics and chemotherapeutics against myxoid liposarcoma cell lines. Therapeutics that target angiogenesis showed antitumor activity. The small molecule inhibitor axitinib, which targets angiogenesis by inhibiting the VEGFR and PDGFR families and c-Kit, inhibited cell cycle progression and induced apoptosis in vitro, as well as having significant antitumor activity against MLS 1765 myxoid liposarcoma xenografts in mice. Axitinib also displayed synergistic antitumor activity in vitro when combined with the potassium channel ionophore salinomycin or the BH3 mimetic ABT-737. Another angiogenesis-targeting therapeutic, 4EGI-1, which targets the oncoprotein eIF4E, significantly decreased angiogenic ligand expression by myxoid liposarcoma cells and reduced tumor cell growth. To verify this oncogenic addiction to angiogenic pathways, we utilized VEGFR-derived ligand traps and found that autocrine VEGFR signaling was crucial to myxoid liposarcoma cell survival. Overall, these findings suggest that autocrine angiogenic signaling through the VEGFR family is critical to myxoid liposarcoma cell survival and that further study of axitinib as a potential anticancer therapy is warranted.

An in vitro model of hemogenic endothelium commitment and hematopoietic production

Adult-type hematopoietic stem and progenitor cells are formed during ontogeny from a specialized subset of endothelium, termed the hemogenic endothelium, via an endothelial-to-hematopoietic transition (EHT) that occurs in the embryonic aorta and the associated arteries. Despite efforts to generate models, little is known about the mechanisms that drive endothelial cells to the hemogenic fate and about the subsequent molecular control of the EHT. Here, we have designed a stromal line-free controlled culture system utilizing the embryonic pre-somitic mesoderm to obtain large numbers of endothelial cells that subsequently commit into hemogenic endothelium before undergoing EHT. Monitoring the culture for up to 12 days using key molecular markers reveals stepwise commitment into the blood-forming system that is reminiscent of the cellular and molecular changes occurring during hematopoietic development at the level of the aorta. Long-term single-cell imaging allows tracking of the EHT of newly formed blood cells from the layer of hemogenic endothelial cells. By modifying the culture conditions, it is also possible to modulate the endothelial cell commitment or the EHT or to produce smooth muscle cells at the expense of endothelial cells, demonstrating the versatility of the cell culture system. This method will improve our understanding of the precise cellular changes associated with hemogenic endothelium commitment and EHT and, by unfolding these earliest steps of the hematopoietic program, will pave the way for future ex vivo production of blood cells.

Establishment and characterization of a novel VEGF-producing HHV-8-unrelated PEL-like lymphoma cell line, OGU1

Primary effusion lymphoma (PEL) is a rare B-cell lymphoma subtype that is characterized by lymphomatous effusion without the presence of masses, and it typically occurs in human immunodeficiency virus (HIV)-infected individuals. Lymphoma cells are universally positive for human herpesvirus 8 (HHV-8). Recently, a cavity-based effusion lymphoma that is similar to PEL without HHV-8 infection, called HHV-8-unrelated PEL-like lymphoma, has been reported in non-HIV-infected individuals. However, the pathophysiology of this lymphoma is largely undefined. We established a novel B-cell line OGU1 derived from a patient with HHV-8-unrelated PEL-like lymphoma. Notably, OGU1 cells produced vascular endothelial growth factor (VEGF) and expressed VEGF receptor 1, whose inhibitors retarded cell growth. Because VEGF acts as a vascular permeability and growth factor, it could play a role, at least in part, in the pathogenesis of this unique lymphoma. Thus, the OGU1 cell line is useful for the investigation of HHV-8-unrelated PEL-like lymphoma.

Renal cells from spermatogonial germline stem cells protect against kidney injury

Spermatogonial stem cells reside in specific niches within seminiferous tubules and continuously generate differentiating daughter cells for production of spermatozoa. Although spermatogonial stem cells are unipotent, these cells are able to spontaneously convert to germline cell-derived pluripotent stem cells (GPSCs) in vitro. GPSCs have many properties of embryonic stem cells and are highly plastic, but their therapeutic potential in tissue regeneration has not been fully explored. Using a novel renal epithelial differentiation protocol, we obtained GPSC-derived tubular-like cells (GTCs) that were functional in vitro, as demonstrated through transepithelial electrical resistance analysis. In mice, GTCs injected after ischemic renal injury homed to the renal parenchyma, and GTC-treated mice showed reduced renal oxidative stress, tubular apoptosis, and cortical damage and upregulated tubular expression of the antioxidant enzyme hemeoxygenase-1. Six weeks after ischemic injury, kidneys of GTC-treated mice had less fibrosis and inflammatory infiltrate than kidneys of vehicle-treated mice. In conclusion, we show that GPSCs can be differentiated into functionally active renal tubular-like cells that therapeutically prevent chronic ischemic damage in vivo, introducing the potential utility of GPSCs in regenerative cell therapy.

Etv2/ER71 induces vascular mesoderm from Flk1+PDGFRα+ primitive mesoderm

Etv2 (Ets Variant 2) has been shown to be an indispensable gene for the development of hematopoietic cells (HPCs)/endothelial cells (ECs). However, how Etv2 specifies the mesoderm-generating HPCs/ECs remains incompletely understood. In embryonic stem cell (ESC) differentiation culture and Etv2-null embryos, we show that Etv2 is dispensable for generating primitive Flk-1(+)/PDGFRα(+) mesoderm but is required for the progression of Flk-1(+)/PDGFRα(+) cells into vascular/hematopoietic mesoderm. Etv2-null ESCs and embryonic cells were arrested as Flk-1(+)/PDGFRα(+) and failed to generate Flk-1(+)/PDGFRα(-) mesoderm. Flk-1(+)/Etv2(+) early embryonic cells showed significantly higher hemato-endothelial potential than the Flk-1(+)/Etv2(-) population, suggesting that Etv2 specifies a hemato-endothelial subset of Flk-1(+) mesoderm. Critical hemato-endothelial genes were severely down-regulated in Etv2-null Flk-1(+) cells. Among those genes Scl, Fli1, and GATA2 were expressed simultaneously with Etv2 in early embryos and seemed to be critical targets. Etv2 reexpression in Etv2-null cells restored the development of CD41(+), CD45(+), and VE-cadherin(+) cells. Expression of Scl or Fli1 alone could also restore HPCs/ECs in the Etv2-null background, indicating that these 2 genes are critical downstream targets. Furthermore, VEGF induced Etv2 potently and rapidly in Flk-1(+) mesoderm. We propose that Flk-1(+)/PDGFRα(+) primitive mesoderm is committed into Flk-1(+)/PDGFRα(-) vascular mesoderm through Etv2 and that up-regulation of Etv2 by VEGF promotes this commitment.

Purification and long-term expansion of multipotent endothelial-like cells with potential cardiovascular regeneration

Endothelial progenitor cells (EPC) represent a relatively rare cell population, and expansion of sufficient cell numbers remains a challenge. Nevertheless, human adipose-derived stem cells (hASC) can be easily isolated and possess the ability to differentiate into endothelial cells. Here, we propose the isolation and characterization of multipotent endothelial-like cells (ME-LC) with the capacity to maintain their vascular progenitor properties for long periods. hASC were isolated from lipoaspirates and cultured through distinct consecutive culture stages for 2 months to enrich ME-LC: first in Dulbecco's modified Eagle's medium-fetal bovine serum (stage I), followed by a stage of culture in absent of fetal bovine serum (stage II), a culture in SFO3 medium (stage III), and, finally, the culture of ME-LC into collagen IV-coated flasks in endothelial growth medium (EGM-2) (stage IV). ME-LC display increased expression levels of endothelial and hematopoietic lineage markers (CD45, KDR, and CXCR4) and EPC markers (CD34 and CD133), whereas the expression of CD31 was barely detectable. Reverse transcription (RT)-polymerase chain reaction assays showed expression of genes involved in early stages of EPC differentiation and decreased expression of genes associated to differentiated EPC (TIE-2, DLL4, and FLT-1). ME-LC formed capillary-like structures when grown on Matrigel, secreted increased levels of stromal cell-derived factor-1 (SDF-1), and showed the ability to migrate attracted by SDF-1, vascular endothelial growth factor, and hematopoietic growth factor cytokines. Importantly, ME-LC retained the capacity to differentiate into cardiomyocyte-like cells. We present a simplified and efficient method to generate large numbers of autologous ME-LC from lipoaspirates-derived hASC, opening up potential cell-based therapies for cardiovascular regenerative medicine.

CXCR4+ progenitors derived from bone mesenchymal stem cells differentiate into endothelial cells capable of vascular repair after arterial injury

Recent findings indicate that bone marrow mesenchymal stem cells (BMSCs) participate in the process of neovascularization in response to repair to injury and are involved in postinfarction myocardial repair. It is unclear what special characteristics the vascular progenitors of bone marrow origin has. CXCR4(+) stem/progenitor cells mobilized to the infarct area and improved the myocardial repair. In present study, we aimed to determine whether CXCR4(+)BMSCs contribute to the angiogenic capacity in vitro and in vivo. CXCR4(+)BMSCs were separated by using paramagnetic microbeads and cultured. RT-PCR and FACS analysis confirmed the gene expression phenotype. The uptake of acetylated low density lipoprotein (acLDL) and the tube formation evaluated the function of CXCR4(+)BMSCs. The effect of CXCR4(+)BMSCs transplantation on neovascularization was investigated in a murine model hindlimb ischemia. After induced by VEGF, CXCR4(+)BMSCs expressed the endothelial cells (ECs) phenotype. The expression of EC markers, PECAM-1, and von Willebrand factor (vWF) increased significantly at both the mRNA and protein levels. In addition, CXCR4(+)BMSCs enhanced the uptakes of Dil-acLDL and form capillary-like tubes in vitro. In vivo the local transfer of CXCR4(+)BMSCs increased neovascularization in ischemic hindlimb. These results demonstrate that CXCR4(+)BMSCs differentiate into ECs and contribute to neovascularization in the vascular lesion,, which indicate the important therapeutic implications for cardiovascular diseases and a new cell source for cell-based vascular engineering and repair in the future.

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

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

Different roles of Foxo1 and Foxo3 in the control of endothelial cell morphology

Foxo1, a member of the Foxo subfamily of forkhead box transcription factors, is known to be essential for progression of normal vascular development in the mouse embryos. In the cultures of endothelial cells derived from embryonic stem cells, Foxo1-deficient endothelial cells exhibit an abnormal morphological response to vascular endothelial growth factor-A (VEGF-A), which is characterized by a lack of cell elongation, yet the molecular mechanisms governing endothelial cell morphology under angiogenic stimulation remain unknown. Here, we report that transforming growth actor-beta also induces endothelial cell elongation in collaboration with Foxo1 and VEGF-A. Furthermore, tetracycline-regulated induction of Foxo3, another member of the Foxo subfamily, into Foxo1-null endothelial cells failed to restore abnormal morphological response to VEGF-A at an early differentiation stage. In contrast, Foxo1 and Foxo3 exerted the same function at a late differentiation stage, i.e. enhancement of VEGF responsiveness and promotion of cell elongation. Our results provide evidence that endothelial cell morphology is regulated by several mechanisms in which Foxo1 and Foxo3 express distinct functional properties depending on differentiation stages.

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

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

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

OBJECTIVE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

Tissue engineering of blood vessel

Vascular grafts are in large demand for coronary and peripheral bypass surgeries. Although synthetic grafts have been developed, replacement of vessels with purely synthetic polymeric conduits often leads to the failure of such graft, especially in the grafts less than 6 mm in diameter or in the areas of low blood flow, mainly due to the early formation of thrombosis. Moreover, the commonly used materials lack growth potential, and long-term results have revealed several material-related failures, such as stenosis, thromboembolization, calcium deposition and infection. Tissue engineering has become a promising approach for generating a bio-compatible vessel graft with growth potential. Since the first success of constructing blood vessels with collagen and cultured vascular cells by Weinberg and Bell, there has been considerable progress in the area of vessel engineering. To date, tissue- engineered blood vessels (TEBVs) could be successfully constructed in vitro, and be used to repair the vascular defects in animal models. This review describes the major progress in the field, including the seeding cell sources, the biodegradable scaffolds, the construction technologies, as well as the encouraging achievements in clinical applications. The remaining challenges are also discussed.

Stem cell-derived Sca-1+ progenitors differentiate into smooth muscle cells, which is mediated by collagen IV-integrin alpha1/beta1/alphav and PDGF receptor pathways

Embryonic stem (ES) cells can differentiate into smooth muscle cells (SMCs) that can be used for tissue engineering and repair of damaged organs. However, little is known about the molecular mechanisms of differentiation in these cells. In the present study, we found collagen IV can promote ES cells to differentiate into stem cell antigen-1-positive (Sca-1(+)) progenitor cells and SMCs. Pretreatment of ES cells with antibodies against collagen IV significantly inhibited SMC marker expression. To further elucidate the effect of collagen IV on the induction and maintenance of SMC differentiation, Sca-1(+) progenitor cells were isolated with magnetic beads, placed in collagen-IV-coated flasks, and cultured in differentiation medium with or without platelet-derived growth factor (PDGF)-BB for 6-90 days. Both immunostaining and fluorescence-activated cell sorter analyses revealed that the majority of these cells were positive for SMC-specific markers. Pretreatment of Sca-1(+) progenitors with antibodies against integrin alpha(1), alpha(v), and beta(1), but not beta(3), inhibited focal adhesion kinase (FAK) and paxillin phosphorylation and resulted in a marked inhibition of SMC differentiation. Various tyrosine kinase inhibitors, and specific siRNA for phosphatidylinositol 3-kinase (PI 3-kinase) and PDGF receptor-beta significantly inhibited SMC marker expression. Taken together, we demonstrate for the first time that collagen IV plays a crucial role in the early stage of SMC differentiation and that integrin (alpha(1), beta(1), and alpha(v))-FAK-PI 3-kinase-mitogen-activated protein kinase and PDGF receptor-beta signaling pathways are involved in SMC differentiation.

Sca-1+ progenitors derived from embryonic stem cells differentiate into endothelial cells capable of vascular repair after arterial injury

BACKGROUND

Embryonic stem cells possess the ability to differentiate into endothelium. The ability to produce large volumes of endothelium from embryonic stem cells could provide a potential therapeutic modality for vascular injury. We describe an approach that selects endothelial cells using magnetic beads that may be used therapeutically to treat arterial injury.

METHODS AND RESULTS

Large numbers of endothelial cells (ECs) with high purity were produced using Sca-1+ cells isolated with magnetic beads from predifferentiated embryonic stem cells (ESCs) cultured in alpha-MEM containing 10 ng/mL VEGF165 for a minimum of 21 days (esEC). The transcription regulator histone deacetylase (HDAC3) was essential for VEGF-induced EC differentiation. Immunofluorescence or fluorescence-activated cell sorter (FACS) analysis revealed that esECs expressed a full range of EC lineage-specific markers including CD31, CD106, CD144, Flk-1, Flt-1, and von Willebrand factor (vWF). FACS analysis confirmed that 99% of esECs were CD31-positive and 75% vWF-positive. Furthermore, almost all cells were positive for DiI-acLDL uptake. When matrigel containing esECs was subcutaneously implanted into mice, various vessel-like structures were observed indicating their endothelial cell like phenotype. In keeping with this, when esECs infected with adenovirus-LacZ were injected into denuded femoral arteries of mice, they were found to form a neo-endothelium that covered the injured areas (86%+/-13.6%), which resulted in a 73% decrease in neointimal area 2 weeks after injury.

CONCLUSIONS

We conclude that Sca-1+ cells can differentiate into functional ECs via activation of HDAC3, accelerating re-endothelialization of injured arteries and reducing neointima formation.

P19 Progenitor Cells Progress to Organized Contracting Myocytes After Chemical and Electrical Stimulation:Implications for Vascular Tissue Engineering

PURPOSE

To test the hypothesis that a level of chemical and electrical stimulation exists that allows differentiation of progenitor cells into organized contracting myocytes.

METHODS

A custom-made bioreactor with the capability of delivering electrical pulses of varying field strengths, widths, and frequencies was constructed. Individual chambers of the bioreactor allowed continuous electrical stimulation of cultured cells under microscopic observation. On day 0, 1% dimethylsulfoxide (DMSO), known to differentiate cells into myocytes, was added to P19 progenitor cells. Additionally, for the next 22 days, electrical pulses of varying field strengths (0-3 V/cm), widths (2-40 ms), and frequencies (10-25 Hz) were continuously applied. On day 5, the medium containing DMSO was exchanged with regular medium, and the electrical stimulation was continued. From days 6-22, the cells were visually assessed for signs of viability, contractility, and organization.

RESULTS

P19 cells remained viable with pulsed electrical fields <3 V/cm, pulse widths <40 ms, and pulse frequencies from 10 to 25 Hz. On day 12, the first spontaneous contractions were observed. For individual colonies, local synchronization and organization occurred; multiple colonies were synchronized with externally applied electrical fields.

CONCLUSION

P19 progenitor cells progress to organized contracting myocytes after chemical and electrical stimulation. Incorporation of such cells into existing methods of producing endothelial cells, fibroblasts, and scaffolds may allow production of improved tissue-engineered vascular grafts.

Vascular endothelial growth factor promotes cardiomyocyte differentiation of embryonic stem cells

Embryonic stem cells (ESCs) overexpressing the vascular endothelial growth factor (VEGF) improve cardiac function in mouse models of myocardial ischemia and infarction by mechanisms that are poorly understood. Here we studied the effects of VEGF on cardiomyocyte differentiation of mouse ESCs in vitro. We used flow cytometry to determine the expression of alpha-myosin heavy chain (alpha-MHC), cardiac troponin I (cTn-I), and Nkx2.5 in differentiated ESCs. VEGF (20 ng/ml) significantly enhanced alpha-MHC, cTn-I, and Nkx2.5 expression in differentiated ESCs. Western blot analysis confirmed these findings. We found that VEGF receptor FMS-like tyrosine kinase-1 (Flt-1) and fetal liver kinase-1 (Flk-1) expression increased during ESC differentiation. Antibodies against Flk-1 totally blocked and against Flt-1 partially blocked VEGF-induced NKx2.5-positive-stained cells. The ERK inhibitor PD-098059 abolished VEGF-induced cardiomyocyte differentiation of ESCs. Our results suggest that VEGF promotes cardiomyocyte differentiation predominantly by ERK-mediated Flk-1 activation and, to a lesser extent, by Flt-1 activation. These findings may be of significance for stem cell and growth factor therapies to regenerate failing cardiomyocytes.

Differentiation of lymphatic endothelial cells from embryonic stem cells on OP9 stromal cells

OBJECTIVE

The discovery of vascular endothelial growth factor C (VEGF-C) and VEGF receptor-3 (VEGFR-3) has started to provide an understanding of the molecular mechanisms of lymphangiogenesis. The homeobox gene prox1 has been proven to specify lymphatic endothelial cells (ECs) from blood ECs. We investigated the process of lymphatic EC (LEC) differentiation using embryonic stem (ES) cells.

METHODS AND RESULTS

VEGFR-2+ cells derived from ES cells differentiated into LECs at day 3 on OP9 stromal cells defined by the expression of prox1, VEGFR-3, and another lymphatic marker podoplanin. VEGFR-2+ cells gave rise to LYVE-1+ embryonic ECs, which were negative for prox1 on day 1 but turned to prox1+ LECs by day 3. VEGFR-3-Fc or Tie2-Fc, sequestering VEGF-C or angiopoietin1 (Ang1), suppressed colony formation of LECs on OP9 cells. However, addition of VEGF-C and Ang1 in combination with VEGF to the culture of VEGFR-2+ cells on collagen-coated dishes failed to induce LECs. LEC-inducing activity of OP9 cells was fully reproduced on paraformaldehyde-fixed OP9 cells with the conditioned medium.

CONCLUSIONS

We succeeded in differentiating LECs from ES cells and revealed the requirements of VEGF-C, Ang1, and other unknown factors for LEC differentiation.

A Novel Culture System Shows that Stem Cells Can be Grown in 3D and Under Physiologic Pulsatile Conditions for Tissue Engineering of Vascular Grafts

BACKGROUND

Currently available vascular grafts have been limited by variable patency rates, material availability, and immunological rejection. The creation of a tissue-engineered vascular graft (TEVG) from autologous stem cells would potentially overcome these limitations. As a first step in creating a completely autologous TEVG, our objective was to develop a novel system for culturing undifferentiated mouse embryonic stem cells (mESC) in a three-dimensional (3D) configuration and under physiological pulsatile flow and pressure conditions.

MATERIALS AND METHODS

A bioreactor was created to provide pulsatile conditions to a specially modified four-well Labtek Chamber-Slide culture system. Undifferentiated mESC were either suspended in a 3D Matrigel matrix or suspended only in cell-culture media within the culture system. Pulsatile conditions were applied to the suspended cells and visualized by video microscopy.

RESULTS

Undifferentiated mESC were successfully embedded in a 3D Matrigel matrix and could withstand physiological pulsatile conditions. Video microscopy demonstrated that the mESC in the 3D matrix were constrained to the wells of the culture system, moved in unison with the applied flows, and were not washed downstream; this was in contrast to the mESC suspended in media alone.

CONCLUSIONS

Undifferentiated mESC can be grown in 3D and under pulsatile conditions. We will use these results to study the effects of long-term pulsatile conditions on the differentiation of mESC into endothelial cells, smooth muscle cells, and fibroblast cells with the long-term goal of creating a completely autologous TEVG.

VEGF-A and FGF-2 synergistically promote neoangiogenesis through enhancement of endogenous PDGF-B-PDGFRbeta signaling

Combined stimulation with VEGF-A, FGF-2, or PDGF-BB has emerged as a potent strategy for therapeutic angiogenesis, although the mechanisms underlying the synergism of these factors are not well understood. In the present study, we investigated the mechanism of synergism between VEGF-A and FGF-2 by using Matrigel plug assay in vivo and embryonic stem cell (ESC)-derived VEGF receptor 2 (VEGFR2)-positive cells in vitro. Experiments in vitro revealed that, in addition to having direct mitogenic effects, these molecules enhance intercellular PDGF-B signaling in a cell-type specific manner: VEGF-A enhances endothelial PDGF-B expression, whereas FGF-2 enhances mural PDGF receptor beta (PDGFRbeta) expression. Co-stimulation with VEGF-A and FGF-2 caused significant mural cell recruitment in vitro and formation of functional neovasculature in vivo, compared with single-agent stimulation. These effects were abrogated not only by anti-PDGFRbeta neutralizing antibody, but also by exogenous PDGF-BB, which could overwhelm the endogenous PDGF-BB distribution. These findings indicated the importance of preservation of the periendothelial PDGF-BB gradient. Thus, we demonstrated that the directional enhancement of endogenous PDGF-B-PDGFRbeta signaling is indispensable for the synergistic effect of VEGF-A and FGF-2 on neoangiogenesis in adults. The findings provide insights into the mechanisms underlying the effects of co-stimulation by growth factors, which could lead to rational design of therapeutic angiogenic strategies.

Pleiotropic role of VEGF-A in regulating fetal pulmonary mesenchymal cell turnover

Tight regulation of VEGF-A production and signaling is important for the maintenance of lung development and homeostasis. VEGF null mice have provided little insight into the role of VEGF during the later stages of lung morphogenesis. Therefore, we examined the in vitro effects of autocrine and paracrine VEGF-A production and the inhibition of VEGF-A signaling on a Flk-1-negative subset of fetal pulmonary mesenchymal cells (pMC). We hypothesized that VEGF-A receptor signaling regulates turnover of fetal lung mesenchyme in a cell cycle-dependent manner. VEGF receptor blockade with SU-5416 caused cell spreading and decreased proliferation and bcl-2 localization. Nuclear expression of the cell cycle inhibitory protein, p21, was increased with SU-5416 treatment, and p27 was absent. Autocrine VEGF production by pMC resulted in proliferation and p21/p27-dependent contact inhibition. In contrast, exogenous VEGF-A increased cell progression through the cell cycle. Selective activation of Flt by placental growth factor demonstrated the importance of this receptor/kinase in the VEGF-A responsiveness of pMC. The expression and localization of the survival factor bcl-2 was dependent on VEGF. These results provide evidence that VEGF-A plays a critical role in the regulation of fetal pulmonary mesenchymal proliferation, survival, and the subsequent development of normal lung architecture through bcl-2 and p21/p27-dependent cell cycle control.

Thrombopoietin Regulates Differentiation of Rhesus Monkey Embryonic Stem Cells to Hematopoietic Cells

Thrombopoietin (TPO) is the primary regulator of megakaryocyte and platelet production in vitro and in vivo. It supports also survival and proliferation of hematopoietic stem cells. The TPO receptor, c-mpl, is a member of the protooncogene family. Our studies focused on the effect of TPO on proliferation and differentiation of rhesus monkey embryonic stem cells to hematopoietic cells. The rationale of the present investigations was the finding that patients with congenital amegakaryocytic thrombocytopenia (CAMT) reveal c-mpl mutations leading to the development of pancytopenia, suggesting that c-mpl is expressed in early hematopoiesis. Here we demonstrate that rhesus monkey embryonic stem (ES) cells are capable of differentiating into uncommitted progenitor cells, including hemangioblasts (hematopoietic and endothelial precursor cells). The combination of TPO and vascular endothelial growth factor (VEGF) significantly increases the number of hemangioblasts and promotes even differentiation to CD34(+) hematopoietic progenitor cells (up to 8%). In addition, analysis of gene expression during hemangioblast development demonstrates that TPO is capable of increasing the mRNA expression of the VEGF receptor (VEGFR) and its own receptor (c-mpl). The in vitro differentiation of rhesus monkey ES cells provides an opportunity to better understand the mechanism of TPO function in the early stage of hematopoietic development from ES cells to mature hematopoietic cells.

Nonhuman primate embryonic stem cells as a preclinical model for hematopoietic and vascular repair

Stem cell-based regenerative medicine therapies have been touted recently as a novel therapeutic approach to treat and cure a wide range of diseases. Both adult and embryonic stem (ES) cells can serve as important sources of precursor cells to derive more mature cells potentially utilized for clinical applications. Nonhuman primates have proven useful as a preclinical model, as demonstrated in studies of hematopoietic cell transplantation, gene therapy, and other areas. The derivation of nonhuman primate ES cells now provides an optimal resource to characterize and test ES cell-based therapies prior to trials with human ES cells. This review describes work to define strategies and mechanisms to derive blood and endothelial cells from nonhuman primate ES cells isolated from various species. Preclinical testing that solely relies on studies of putative therapeutic cells derived from mouse ES cells transplanted into other mice, or analyses of human ES cell-derived cells transplanted into immunodeficient or immunosuppressed rodents may not be predictive of efficacy in subsequent human trials. However, future testing using nonhuman primate ES cell-derived therapeutic cells done as an allogeneic transplant may best predict success for subsequent studies using human ES cells. Therefore, additional research on nonhuman primate ES cells, in addition to work on mouse and human ES cells, is greatly needed to facilitate clinical translation of new stem cell treatments.

Directed differentiation of telencephalic precursors from embryonic stem cells

We demonstrate directed differentiation of telencephalic precursors from mouse embryonic stem (ES) cells using optimized serum-free suspension culture (SFEB culture). Treatment with Wnt and Nodal antagonists (Dkk1 and LeftyA) during the first 5 d of SFEB culture causes nearly selective neural differentiation in ES cells ( approximately 90%). In the presence of Dkk1, with or without LeftyA, SFEB induces efficient generation ( approximately 35%) of cells expressing telencephalic marker Bf1. Wnt3a treatment during the late culture period increases the pallial telencephalic population (Pax6(+) cells yield up to 75% of Bf1(+) cells), whereas Shh promotes basal telencephalic differentiation (into Nkx2.1(+) and/or Islet1/2(+) cells) at the cost of pallial telencephalic differentiation. Thus, in the absence of caudalizing signals, floating aggregates of ES cells generate naive telencephalic precursors that acquire subregional identities by responding to extracellular patterning signals.

Fluid shear stress induces differentiation of Flk-1-positive embryonic stem cells into vascular endothelial cells in vitro

Pluripotent embryonic stem (ES) cells are capable of differentiating into all cell lineages, but the molecular mechanisms that regulate ES cell differentiation have not been sufficiently explored. In this study, we report that shear stress, a mechanical force generated by fluid flow, can induce ES cell differentiation. When Flk-1-positive (Flk-1(+)) mouse ES cells were subjected to shear stress, their cell density increased markedly, and a larger percentage of the cells were in the S and G(2)-M phases of the cell cycle than Flk-1(+) ES cells cultured under static conditions. Shear stress significantly increased the expression of the vascular endothelial cell-specific markers Flk-1, Flt-1, vascular endothelial cadherin, and PECAM-1 at both the protein level and the mRNA level, but it had no effect on expression of the mural cell marker smooth muscle alpha-actin, blood cell marker CD3, or the epithelial cell marker keratin. These findings indicate that shear stress selectively promotes the differentiation of Flk-1(+) ES cells into the endothelial cell lineage. The shear stressed Flk-1(+) ES cells formed tubelike structures in collagen gel and developed an extensive tubular network significantly faster than the static controls. Shear stress induced tyrosine phosphorylation of Flk-1 in Flk-1(+) ES cells that was blocked by a Flk-1 kinase inhibitor, SU1498, but not by a neutralizing antibody against VEGF. SU1498 also abolished the shear stress-induced proliferation and differentiation of Flk-1(+) ES cells, indicating that a ligand-independent activation of Flk-1 plays an important role in the shear stress-mediated proliferation and differentiation by Flk-1(+) ES cells.

SDF-1 involvement in endothelial phenotype and ischemia-induced recruitment of bone marrow progenitor cells

Chemokine stromal derived factor 1 (SDF-1) is involved in trafficking of hematopoietic stem cells (HSCs) from the bone marrow (BM) to peripheral blood (PB) and has been found to enhance postischemia angiogenesis. This study was aimed at investigating whether SDF-1 plays a role in differentiation of BM-derived c-kit(+) stem cells into endothelial progenitor cells (EPCs) and in ischemia-induced trafficking of stem cells from PB to ischemic tissues. We found that SDF-1 enhanced EPC number by promoting alpha(2), alpha(4), and alpha(5) integrin-mediated adhesion to fibronectin and collagen I. EPC differentiation was reduced in mitogen-stimulated c-kit(+) cells, while cytokine withdrawal or the overexpression of the cyclin-dependent kinase (CDK) inhibitor p16(INK4) restored such differentiation, suggesting a link between control of cell cycle and EPC differentiation. We also analyzed the time course of SDF-1 expression in a mouse model of hind-limb ischemia. Shortly after femoral artery dissection, plasma SDF-1 levels were up-regulated, while SDF-1 expression in the bone marrow was down-regulated in a timely fashion with the increase in the percentage of PB progenitor cells. An increase in ischemic tissue expression of SDF-1 at RNA and protein level was also observed. Finally, using an in vivo assay such as injection of matrigel plugs, we found that SDF-1 improves formation of tubulelike structures by coinjected c-kit(+) cells. Our findings unravel a function for SDF-1 in increase of EPC number and formation of vascular structures by bone marrow progenitor cells.

Differentiation and Diversification of Vascular Cells from Embryonic Stem Cells

Pluripotent embryonic stem (ES) cells are potent materials for both regenerative therapeutic approaches and developmental research. Recently, a novel ES cell differentiation system combined with 2-dimensional culture and flow cytometry-assisted cell sorting has been developed. In this system, endothelial, mural, and blood cells can be systematically induced from common progenitor vascular endothelial growth factor receptor-2 (Flk1)-expressing cells. This system is amenable for the in vitro observation of multiple steps of the vascular developmental process, such as vascular cell differentiation and diversification from progenitors, endothelial cell maturation and differentiation into arterial, venous, and lymphatic endothelium, and vascular formation. This constructive in vitro approach provides novel possibilities for elucidating the cellular and molecular mechanisms of vascular development. Vascular cell induction from primate ES cells reveals primate-specific vascular developmental mechanisms. ES cell research in developmental biology would be indispensable, especially in the human species for which a knock-out animal model is not available. ES cells should also contribute to regenerative medicine, not only as a cellular source for transplantation but also for the discovery of novel genes and drugs for regeneration. In this review, the significance of ES cell study in basic science and clinical medicine in the vascular field is discussed.

[Therapy-related changes of angiogenesis in Philadelphia chromosome positive chronic myelogenous leukemia]

To elucidate the effect of first-line treatment with interferon (IFN), hydroxyurea (HU) and the tyrosine kinase inhibitor imatinib (STI571) on angiogenesis, we studied bone marrow (BM) biopsies in 104 patients with chronic myelogenous leukemia (CML) and 138 patients before and after allogeneic BM transplantation (BMT). After immunostaining (CD34) and morphometric analysis in comparison with a control group, CML specimens showed an increased vascularity and conspicuous morphological abnormalities of microvessels. A close relationship between microvessels and fiber density was detectable in initial biopsies and also in repeatedly performed examinations following therapy. Monotherapy by imatinib and HU generated a significant reduction of microvessels and reticulin fibers in contrast to changes after IFN administration or combination regimens of IFN and HU. A persistence of numerical and structural anomalies of vasculature was observable even several months after BMT. These anomalies shed some light on disturbances of the stroma compartment after myeloablative therapy. The relationship between BM vascularity and fibers is probably dependent on concomitant changes of megakaryopoiesis as the source of various mediators involved in the development of myelofibrosis and neo-angiogenesis acting within a complex functional network.

Signal processing underlying extrinsic control of stem cell fate

PURPOSE OF REVIEW

Strategies to manipulate stem cells for therapeutic applications are limited by our inability to control or predict stem cell fate decisions in response to exogenous stimuli. This review focuses on the mechanisms by which exogenous stimuli influence cell fate.

RECENT FINDINGS

Limitations in our ability to control cell fate arises from our primarily qualitative understanding of stem cell regulation, which proposes straightforward cue-fate relationships that appear to be the exception rather than the rule. Alternatively, consideration of the underlying quantitative, temporal, and spatial mechanisms governing extrinsic regulation of stem cell fate may enable novel approaches to control stem cell output predictively.

SUMMARY

The authors review advances in the understanding of these underlying mechanisms, and discuss experimental and analytic tools necessary to investigate and use these mechanisms to control stem cell fate.

Identification of Flk-1 target genes in vasculogenesis: Pim-1 is required for endothelial and mural cell differentiation in vitro

The tyrosine kinase receptor fetal liver kinase 1 (Flk-1) plays a crucial role in vasculogenesis and angiogenesis, but its target genes remain elusive. Comparing Flk-1(+/+) with Flk-1(-/-) embryonic stem (ES) cells, we identified transcripts regulated by the vascular endothelial growth factor A (VEGF-A)/Flk-1 pathway at an early stage of their differentiation to endothelial and mural precursors. Further analysis of a number of these genes (Nm23-M1, Nm23-M2, Slug, Set, pp32, Cbp, Ship-1, Btk, and Pim-1) showed that their products were transiently up-regulated in vivo in endothelial cells (ECs) during angiogenesis of the ovary, and their mRNA was rapidly induced in vitro by VEGF-A in human umbilical cord vein endothelial cells (HUVECs). Functional analysis by RNA interference (RNAi) in ES cells induced to differentiate demonstrated that Pim-1 is required for their differentiation into ECs and smooth muscle cells (SMCs). In HUVECs, RNAi showed that Pim-1 is required in ECs for VEGF-A-dependent proliferation and migration. The identification of Flk-1 target genes should help in elucidating the molecular pathways that govern the vasculogenesis and angiogenesis processes.