Human embryonic stem cells as an in vitro model for human vascular development and the induction of vascular differentiation

Not just fibrotic: endothelial-derived TGFβ maintains contractile function and lymphatic muscle phenotype during homeostasis

Cell-cell communication within the lymphatic vasculature during homeostasis is incompletely detailed. Although many discoveries highlight the pathological roles of transforming growth factor-beta (TGFβ) in chronic vascular inflammation and associated fibrosis, only a small amount is known surrounding the role of TGFβ-signaling in homeostatic lymphatic function. Here, we discovered that pharmacological blockade of TGFβ receptor 1 (TGFβR1) negatively impacts rat mesenteric lymphatic vessel pumping, significantly reducing vessel contractility and surrounding lymphatic muscle coverage. We have identified mesenteric lymphatic endothelial cells themselves as a source of endogenous vascular TGFβ and that TGFβ production is significantly increased in these cells via activation of a number of functional pattern recognition receptors they express. We show that a continuous supply of TGFβ is essential to maintain the contractile phenotype of neighboring lymphatic muscle cells and support this conclusion through in vitro analysis of primary isolated lymphatic muscle cells that undergo synthetic differentiation during 2-D cell culture, a phenomenon that could be effectively rescued by supplementation with recombinant TGFβ. Finally, we demonstrate that lymphatic endothelial production of TGFβ is regulated, in part, by nitric oxide in a manner we propose is essential to counteract the pathological over-production of TGFβ. Taken together, these data highlight the essential role of homeostatic TGFβ signaling in the maintenance of lymphatic vascular function and highlight possible deleterious consequences of its inhibition.NEW & NOTEWORTHY The growth factor TGFβ is commonly associated with its pathological overproduction during tissue fibrosis rather than its homeostatic functions. We expose the lymphatic endothelium as a source of endogenous TGFβ, the impact of its production on the maintenance of surrounding lymphatic muscle cell phenotype, and internally regulated mechanisms of its production. Overall, these results highlight the intricate balance of TGFβ-signaling as an essential component of maintaining lymphatic contractile function.

Human Induced Pluripotent Stem Cell-Derived Vascular Cells: Recent Progress and Future Directions

Human induced pluripotent stem cells (hiPSCs) hold great promise for cardiovascular regeneration following ischemic injury. Considerable effort has been made toward the development and optimization of methods to differentiate hiPSCs into vascular cells, such as endothelial and smooth muscle cells (ECs and SMCs). In particular, hiPSC-derived ECs have shown robust potential for promoting neovascularization in animal models of cardiovascular diseases, potentially achieving significant and sustained therapeutic benefits. However, the use of hiPSC-derived SMCs that possess high therapeutic relevance is a relatively new area of investigation, still in the earlier investigational stages. In this review, we first discuss different methodologies to derive vascular cells from hiPSCs with a particular emphasis on the role of key developmental signals. Furthermore, we propose a standardized framework for assessing and defining the EC and SMC identity that might be suitable for inducing tissue repair and regeneration. We then highlight the regenerative effects of hiPSC-derived vascular cells on animal models of myocardial infarction and hindlimb ischemia. Finally, we address several obstacles that need to be overcome to fully implement the use of hiPSC-derived vascular cells for clinical application.

Collagen IV differentially regulates planarian stem cell potency and lineage progression

Comprehensive assessment of matrisome genes identified collagen IV as one of the many extracellular matrix (ECM) proteins regulating the stem cell pool in planarian tissue homeostasis and regeneration. While collagen IV has been shown to be involved in stem cell biology, our finding links it to pluripotent stem cells in vivo, including self-renewal and differentiation into tissue-specific progenitors. We show a link between the ECM niches in the parenchyma/gut region and EGF/neuregulin-secreting neurons, thus providing mechanistic insight into interactions between cell niches. The conservation of basement membranes between planarian and mammalian gut niches suggests a similar interplay may exist in the mammalian systems, worthy of further investigation.

The extracellular matrix (ECM) provides a precise physical and molecular environment for cell maintenance, self-renewal, and differentiation in the stem cell niche. However, the nature and organization of the ECM niche is not well understood. The adult freshwater planarian Schmidtea mediterranea maintains a large population of multipotent stem cells (neoblasts), presenting an ideal model to study the role of the ECM niche in stem cell regulation. Here we tested the function of 165 planarian homologs of ECM and ECM-related genes in neoblast regulation. We identified the collagen gene family as one with differential effects in promoting or suppressing proliferation of neoblasts. col4-1, encoding a type IV collagen α-chain, had the strongest effect. RNA interference (RNAi) of col4-1 impaired tissue maintenance and regeneration, causing tissue regression. Finally, we provide evidence for an interaction between type IV collagen, the discoidin domain receptor, and neuregulin-7 (NRG-7), which constitutes a mechanism to regulate the balance of symmetric and asymmetric division of neoblasts via the NRG-7/EGFR pathway.

Cell seeding accelerates the vascularization of tissue engineering constructs in hypertensive mice

Rapid blood vessel ingrowth into transplanted constructs represents the key requirement for successful tissue engineering. Seeding three-dimensional scaffolds with suitable cells is an approved technique for this challenge. Since a plethora of patients suffer from widespread diseases that limit the capacity of neoangiogenesis (e.g., hypertension), we investigated the incorporation of cell-seeded poly-L-lactide-co-glycolide scaffolds in hypertensive (BPH/2J, group A) and nonhypertensive (BPN/3J, group B) mice. Collagen-coated scaffolds (A1 and B1) were additionally seeded with osteoblast-like (A2 and B2) and mesenchymal stem cells (A3 and B3). After implantation into dorsal skinfold chambers, inflammation and newly formed microvessels were measured using repetitive intravital fluorescence microscopy for 2 weeks. Apart from a weak inflammatory response in all groups, significantly increased microvascular densities were found in cell-seeded scaffolds (day 14, A2: 192 ± 12 cm/cm², A3: 194 ± 10 cm/cm², B2: 249 ± 19 cm/cm², B3: 264 ± 17 cm/cm²) when compared with controls (A1: 129 ± 10 cm/cm², B1: 185 ± 8 cm/cm²). In this context, hypertensive mice showed reduced neoangiogenesis in comparison with nonhypertensive animals. Therefore, seeding approved scaffolds with organ-specific or pluripotent cells is a very promising technique for tissue engineering in hypertensive organisms.

Combining Stem Cells and Biomaterial Scaffolds for Constructing Tissues and Cell Delivery

Combining stem cells with biomaterial scaffolds serves as a promising strategy for engineering tissues for both in vitro and in vivo applications. This updated review details commonly used biomaterial scaffolds for engineering tissues from stem cells. We first define the different types of stem cells and their relevant properties and commonly used scaffold formulations. Next, we discuss natural and synthetic scaffold materials typically used when engineering tissues, along with their associated advantages and drawbacks and gives examples of target applications. New approaches to engineering tissues, such as 3D bioprinting, are described as they provide exciting opportunities for future work along with current challenges that must be addressed. Thus, this review provides an overview of the available biomaterials for directing stem cell differentiation as a means of producing replacements for diseased or damaged tissues.

Aluminium foil as a single-use substrate for MALDI-MS fingerprinting of different melanoma cell lines

Herein, we present the intact cell matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for the fingerprinting of human melanoma cancer cell lines grown on aluminium foil.

Efficient generation of endothelial cells from human pluripotent stem cells and characterization of their functional properties

Although endothelial cells (ECs) have been derived from human pluripotent stem cells (hPSCs), large-scale generation of hPSC-ECs remains challenging and their functions are not well characterized. Here we report a simple and efficient three-stage method that allows generation of approximately 98 and 9500 ECs on day 16 and day 34, respectively, from each human embryonic stem cell (hESC) input. The functional properties of hESC-ECs derived in the presence and absence of a TGFβ-inhibitory molecule SB431542 were characterized and compared with those of human umbilical vein endothelial cells (HUVECs). Confluent monolayers formed by SB431542 ⁺ hESC-ECs, SB431542^- hESC-ECs, and HUVECs showed similar permeability to 10,000 Da dextran, but these cells exhibited striking differences in forming tube-like structures in 3D fibrin gels. The SB431542 ⁺ hESC-ECs were most potent in forming tube-like structures regardless of whether VEGF and bFGF were present in the medium; less potent SB431542^- hESC-ECs and HUVECs responded differently to VEGF and bFGF, which significantly enhanced the ability of HUVECs to form tube-like structures but had little impact on SB431542^- hESC-ECs. This study offers an efficient approach to large-scale hPSC-EC production and suggests that the phenotypes and functions of hPSC-ECs derived under different conditions need to be thoroughly examined before their use in technology development. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 678-687, 2016.

Dickkopf Homolog 3 Induces Stem Cell Differentiation into Smooth Muscle Lineage via ATF6 Signalling

Smooth muscle cells (SMCs) are a key component of healthy and tissue engineered vessels and play a crucial role in vascular development and the pathogenic events of vascular remodeling i.e. restenosis. However, the cell source from which they can be isolated is limited. Embryonic stem (ES) cells that have the remarkable capability to differentiate into vascular SMCs in response to specific stimuli provide a useful model for studying SMC differentiation. Previous studies suggested that dickkopf homolog 3 (DKK3) has a role in human partially induced pluripotent stem cell to SMC differentiation. Here, we demonstrate that the expression of DKK3 is essential for the expression of SMC markers and myocardin at both the mRNA and protein levels during mouse ES cell differentiation into SMCs (ESC-SMC differentiation). Overexpression of DKK3 leads to further up-regulation of the aforementioned markers. Further investigation indicates that DKK3 added as a cytokine activates activating transcription factor 6 (ATF6), leading to the increased binding of ATF6 on the myocardin promoter and increased its expression. In addition, inhibition of extracellular signal-regulated kinases 1/2 (ERK1/2) promotes the expression of ATF6 and leads to further increase of myocardin transcription. Our findings offer a novel mechanism by which DKK3 regulates ESC-SMC differentiation by activating ATF6 and promoting myocardin expression.

Thymopentin enhances the generation of T-cell lineage derived from human embryonic stem cells in vitro

Thymopentin is a group of biologically active peptide secreted mainly by the epithelial cells of thymic cortex and medulla. Whether it promotes T cells production from human embryonic stem cells(hESCs) in vitro remains an elusive issue. In the present study, we develop a novel strategy that enhances T-cell lineage differentiation of hESCs in collagen matrix culture by sequential cytokine cocktails treatment combined with thymopentin stimulation. We observed that approximately 30.75% cells expressed CD34 on day 14 of the cultures and expressed the surface markers of erythroid, lymphoid and myeloid lineages. The results of colony assays and gene expressions by RT-PCR analysis also demonstrated that hematopoietic progenitor cells (HPCs) derived from hESCs were capable of multi-lineage differentiation. Further study revealed that culturing with thymopentin treatment, the CD34(+)CD45RA(+)CD7(+) cells sorted from HPCs expressed T-cell-related genes, IKAROS, DNTT, TCRγ and TCRβ, and T-cell surface markers, CD3, cytoplasmic CD3, CD5, CD27, TCRγδ, CD4 and CD8. The differentiated cells produced the cytokines including IFN-γ, IL-2 and TNF-α in response to stimulation, providing the evidence for T-cell function of these cells. In conclusion, thymopentin enhances T-cell lineage differentiation from hESCs in vitro by mimicking thymus peptide environment in vivo.

Expression and function of the ID1 gene during transforming growth factor-β1-induced differentiation of human embryonic stem cells to endothelial cells

ID1 can mediate transforming growth factor-β (TGF-β)/activin receptor-like kinase-1 (ALK1)-induced (and Smad-dependent) migration in endothelial cells (ECs). However, the role that ID1 plays during differentiation of human embryonic stem cells (hESCs) into ECs induced by TGF-β1 remains unclear. In this study, a hESC differentiation model that recapitulates the developmental steps of vasculogenesis during the early stages of embryonic development was used to explore this question. We found that TGF-β1 increases endothelial cell differentiation and inhibits endothelial tube formation. Furthermore, at an early stage of differentiation, TGF-β1 may induce in vitro differentiation of hESCs into ECs by inhibiting expression of ID1, while at a later stage of differentiation, TGF-β1 may stimulate the proliferation and migration of ECs via the ALK1/Smad1/5/ID1 pathway. Downregulation of ID1 by gene silencing can lead to acceleration of TGF-β1-induced hESC differentiation into ECs and inhibition of proliferation and migration of ECs. This study may reveal some mechanisms of in vivo vasculogenesis in the early stages of embryonic development.

Engineering myocardial tissue patches with hierarchical structure-function

Complex hierarchical organization is a hallmark of tissues and their subsequent integration into organs. A major challenge in tissue engineering is to generate arrays of cells with defined structural organization that display appropriate functional properties. Given what is known about cellular responses to physiochemical cues from the surrounding environment, we can build tissue structures that mimic these microenvironments and validate these platforms using both experimental and computational approaches. Tissue generation encompasses many methods and tissue types, but here we review layering cell sheets to create scaffold-less myocardial patches. We discuss surgical criteria that can drive the design of myocardial cell sheets and the methods used to fabricate, mechanically condition, and functionally test them. We also focus on how computational and experimental approaches could be integrated to optimize tissue mechanical properties by using measurements of biomechanical properties and tissue anisotropy to create predictive computational models. Tissue anisotropy and dynamic mechanical stimuli affect cell phenotype in terms of protein expression and secretion, which in turn, leads to compositional and structural changes that ultimately impact tissue function. Therefore, a combinatorial approach of design, fabrication, testing, and modeling can be carried out iteratively to optimize engineered tissue function.

Scalable generation of universal platelets from human induced pluripotent stem cells

Human induced pluripotent stem cells (iPSCs) provide a potentially replenishable source for the production of transfusable platelets. Here, we describe a method to generate megakaryocytes (MKs) and functional platelets from iPSCs in a scalable manner under serum/feeder-free conditions. The method also permits the cryopreservation of MK progenitors, enabling a rapid “surge” capacity when large numbers of platelets are needed. Ultrastructural/morphological analyses show no major differences between iPSC platelets and human blood platelets. iPSC platelets form aggregates, lamellipodia, and filopodia after activation and circulate in macrophage-depleted animals and incorporate into developing mouse thrombi in a manner identical to human platelets. By knocking out the β2-microglobulin gene, we have generated platelets that are negative for the major histocompatibility antigens. The scalable generation of HLA-ABC-negative platelets from a renewable cell source represents an important step toward generating universal platelets for transfusion as well as a potential strategy for the management of platelet refractoriness.

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Large-scale production of platelets from human iPSCs under defined conditions

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iPSC platelets are functional both in vivo and in vitro

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Knockout of β2-microglobulin gene in iPSCs generates universal platelets

Molecular pathways governing development of vascular endothelial cells from ES/iPS cells

Assembly of complex vascular networks occurs in numerous biological systems through morphogenetic processes such as vasculogenesis, angiogenesis and vascular remodeling. Pluripotent stem cells such as embryonic stem (ES) and induced pluripotent stem (iPS) cells can differentiate into any cell type, including endothelial cells (ECs), and have been extensively used as in vitro models to analyze molecular mechanisms underlying EC generation and differentiation. The emergence of these promising new approaches suggests that ECs could be used in clinical therapy. Much evidence suggests that ES/iPS cell differentiation into ECs in vitro mimics the in vivo vascular morphogenic process. Through sequential steps of maturation, ECs derived from ES/iPS cells can be further differentiated into arterial, venous, capillary and lymphatic ECs, as well as smooth muscle cells. Here, we review EC development from ES/iPS cells with special attention to molecular pathways functioning in EC specification.

Role of extracellular matrix signaling cues in modulating cell fate commitment for cardiovascular tissue engineering

It is generally agreed that engineered cardiovascular tissues require cellular interactions with the local milieu. Within the microenvironment, the extracellular matrix (ECM) is an important support structure that provides dynamic signaling cues in part through its chemical, physical, and mechanical properties. In response to ECM factors, cells activate biochemical and mechanotransduction pathways that modulate their survival, growth, migration, differentiation, and function. This Review describes the role of ECM chemical composition, spatial patterning, and mechanical stimulation in the specification of cardiovascular lineages, with a focus on stem cell differentiation, direct transdifferentiation, and endothelial-to-mesenchymal transition. The translational application of ECMs is discussed in the context of cardiovascular tissue engineering and regenerative medicine.

Proteome array identification of bioactive soluble proteins/peptides in Matrigel: relevance to stem cell responses

Matrigel and similar commercial products are extracts of the Engelbreth-Holm-Swarm sarcoma that provide a basement-membrane-like attachment substrate or gel that is used to grow cells on or in, respectively. To ascertain further what proteins may be present in Matrigel, besides its major basement-membrane constituents, an analysis of the expressed liquid of gelled Matrigel was performed using proteome array technology. Among the growth factors/cytokines assayed, high positive detection was found for IGFBP1, IGFBP3, LIF, platelet factor 4, PlGF-2, and VEGF; moderate reactivity was found for cyr61, IGFBP2, IGFBP6, IL-1ra, and NOV; and low, but detectable, responses occurred for aFGF, IL-13, IL-23, M-CSF, and VEGF-B. Among the chemokines assayed, high positive detection was found for MIG and serpin E1; moderate reactivity was found for IP-10, MCP-1, and MCP-5, and low, but detectable, responses occurred for CXCL16, I-TAC, and MIP-1α. Among the other biologically active proteins assayed, high positive detection was found for adiponectin, C5a, endocan, lipocalin-2, sICAM-1, MMP-3, and TIMP-1; moderate reactivity was found for C-reactive protein, coagulation factor III, endoglin, endostatin/collagen XVIII, endothelin-1, ICAM-1, MMP-9, osteopontin, pentraxin-3, and RANTES; and low, but detectable, responses occurred for fetuin A, MMP-8, pentraxin-2, RBP4, resistin, and TIMP-4. The study found several growth factors, chemokines, and biologically active proteins not previously identified in Matrigel, and this may have significance to the interpretations of observed cellular responses when cells are grown on or in Matrigel.

Self-organized vascular networks from human pluripotent stem cells in a synthetic matrix

The success of tissue regenerative therapies is contingent on functional and multicellular vasculature within the redeveloping tissue. Although endothelial cells (ECs), which compose the vasculature's inner lining, are intrinsically able to form nascent networks, these structures regress without the recruitment of pericytes, supporting cells that surround microvessel endothelium. Reconstruction of typical in vivo microvascular architecture traditionally has been done using distinct cell sources of ECs and pericytes within naturally occurring matrices; however, the limited sources of clinically relevant human cells and the inherent chemical and physical properties of natural materials hamper the translational potential of these approaches. Here we derived a bicellular vascular population from human pluripotent stem cells (hPSCs) that undergoes morphogenesis and assembly in a synthetic matrix. We found that hPSCs can be induced to codifferentiate into early vascular cells (EVCs) in a clinically relevant strategy amenable to multiple hPSC lines. These EVCs can mature into ECs and pericytes, and can self-organize to form microvascular networks in an engineered matrix. These engineered human vascular networks survive implantation, integrate with the host vasculature, and establish blood flow. This integrated approach, in which a derived bicellular population is exploited for its intrinsic self-assembly capability to create microvasculature in a deliverable matrix, has vast ramifications for vascular construction and regenerative medicine.

Blood flow and stem cells in vascular disease

It is well known that the altered blood flow is related to vascular diseases, including atherosclerosis, restenosis, and arteriosclerosis, which preferentially located at areas with the disturbed blood flow, suggesting that altered biomechanical stress may exert their effect on the vascular disease. Recent evidence indicated the presence of abundant stem/progenitor cells in the vessel wall, in which laminar shear stress can stimulate these cells to differentiate towards endothelial lineage, while cyclic strain results in smooth muscle differentiation. In line with this, it was evidenced that altered biomechanical stress in stented vessels may lead to 'wrong' direction of vascular stem cell differentiation resulting in restenosis. However, the underlying mechanisms are not well understood. In this article, we will give an overview of the effect of the local flow pattern on stem/progenitor cell differentiation and the possible mechanism on how the blood flow influences stem cell behaviours in the development of vascular diseases.

Therapeutic potential of perivascular cells from human pluripotent stem cells

Vascularization of injured tissues or artificial grafts is a major challenge in tissue engineering, stimulating a continued search for alternative sources for vasculogenic cells and the development of therapeutic strategies. Human pluripotent stem cells (hPSCs), either embryonic or induced, offer a plentiful platform for the derivation of large numbers of vasculogenic cells, as required for clinical transplantations. Various protocols for generation of vasculogenic smooth muscle cells (SMCs) from hPSCs have been described with considerably different SMC derivatives. In addition, we recently identified hPSC-derived pericytes, which are similar to their physiological counterparts, exhibiting unique features of blood vessel-residing perivascular cells, as well as multipotent mesenchymal precursors with therapeutic angiogenic potential. In this review we refer to methodologies for the development of a variety of perivascular cells from hPSCs with respect to developmental induction, differentiation capabilities, potency and their dual function as mesenchymal precursors. The therapeutic effect of hPSC-derived perivascular cells in experimental models of tissue engineering and regenerative medicine are described and compared to those of their native physiological counterparts.

Vascular tissue engineering: biodegradable scaffold platforms to promote angiogenesis

The ability to understand and regulate human vasculature development and differentiation has the potential to benefit patients suffering from a variety of ailments, including cardiovascular disease, peripheral vascular disease, ischemia, and burn wounds. Current clinical treatments for vascular-related diseases commonly use the grafting from patients of autologous vessels, which are limited and often damaged due to disease. Considerable progress is being made through a tissue engineering strategy in the vascular field. Tissue engineering takes a multidisciplinary approach seeking to repair, improve, or replace biological tissue function in a controlled and predictable manner. To address the clinical need to perfuse and repair damaged, ischemic tissue, one approach of vascular engineering aims to understand and promote the growth and differentiation of vascular networks. Vascular tissue engineered constructs enable the close study of vascular network assembly and vessel interactions with the surrounding microenvironment. Scaffold platforms provide a method to control network development through the biophysical regulation of different scaffold properties, such as composition, mechanics, dimensionality, and so forth. Following a short description of vascular physiology and blood vessel biomechanics, the key principles in vascular tissue engineering are discussed. This review focuses on various biodegradable scaffold platforms and demonstrates how they are being used to regulate, promote, and understand angiogenesis and vascular network formation.

Endothelial cells derived from nuclear reprogramming

The endothelium plays a pivotal role in vascular homeostasis, regulating the tone of the vascular wall, and its interaction with circulating blood elements. Alterations in endothelial functions facilitate the infiltration of inflammatory cells and permit vascular smooth muscle proliferation and platelet aggregation. Therefore, endothelial dysfunction is an early event in disease processes including atherosclerosis, and because of its critical role in vascular health, the endothelium is worthy of the intense focus it has received. However, there are limitations to studying human endothelial function in vivo, or human vascular segments ex vivo. Thus, methods for endothelial cell (EC) culture have been developed and refined. Recently, methods to derive ECs from pluripotent cells have extended the scientific range of human EC studies. Pluripotent stem cells may be generated, expanded, and then differentiated into ECs for in vitro studies. Constructs for molecular imaging can also be employed to facilitate tracking these cells in vivo. Furthermore, one can generate patient-specific ECs to study the effects of genetic or epigenetic alterations on endothelial behavior. Finally, there is the opportunity to apply these cells for vascular therapy. This review focuses on the generation of ECs from stem cells; their characterization by genetic, histological, and functional studies; and their translational applications.

Modeling and rescue of the vascular phenotype of Williams-Beuren syndrome in patient induced pluripotent stem cells

Elastin haploinsufficiency in Williams-Beuren syndrome (WBS) leads to increased vascular smooth muscle cell (SMC) proliferation and stenoses. Our objective was to generate a human induced pluripotent stem (hiPS) cell model for in vitro assessment of the WBS phenotype and to test the ability of candidate agents to rescue the phenotype. hiPS cells were reprogrammed from skin fibroblasts of a WBS patient with aortic and pulmonary stenosis and healthy control BJ fibroblasts using four-factor retrovirus reprogramming and were differentiated into SMCs. Differentiated SMCs were treated with synthetic elastin-binding protein ligand 2 (EBPL2) (20 μg/ml) or the antiproliferative drug rapamycin (100 nM) for 5 days. We generated four WBS induced pluripotent stem (iPS) cell lines that expressed pluripotency genes and differentiated into all three germ layers. Directed differentiation of BJ iPS cells yielded an 85%-92% pure SMC population that expressed differentiated SMC markers, were functionally contractile, and formed tube-like structures on three-dimensional gel assay. Unlike BJ iPS cells, WBS iPS cells generated immature SMCs that were highly proliferative, showed lower expression of differentiated SMC markers, reduced response to the vasoactive agonists, carbachol and endothelin-1, impaired vascular tube formation, and reduced calcium flux. EBPL2 partially rescued and rapamycin fully rescued the abnormal SMC phenotype by decreasing the smooth muscle proliferation rate and enhancing differentiation and tube formation. WBS iPS cell-derived SMCs demonstrate an immature proliferative phenotype with reduced functional and contractile properties, thereby recapitulating the human disease phenotype. The ability of rapamycin to rescue the phenotype provides an attractive therapeutic candidate for patients with WBS and vascular stenoses.

Functional vascular smooth muscle cells derived from human induced pluripotent stem cells via mesenchymal stem cell intermediates

AIMS

Smooth muscle cells (SMC) play an important role in vascular homeostasis and disease. Although adult mesenchymal stem cells (MSC) have been used as a source of contractile SMC, they suffer from limited proliferation potential and culture senescence, particularly when originating from older donors. By comparison, human induced pluripotent stem cells (hiPSC) can provide an unlimited source of functional SMC for autologous cell-based therapies and for creating models of vascular disease. Our goal was to develop an efficient strategy to derive functional, contractile SMC from hiPSC.

METHODS AND RESULTS

We developed a robust, stage-wise, feeder-free strategy for hiPSC differentiation into functional SMC through an intermediate stage of multipotent MSC, which could be coaxed to differentiate into fat, bone, cartilage, and muscle. At this stage, the cells were highly proliferative and displayed higher clonogenic potential and reduced senescence when compared with parental hair follicle mesenchymal stem cells. In addition, when exposed to differentiation medium, the myogenic proteins such as α-smooth muscle actin, calponin, and myosin heavy chain were significantly upregulated and displayed robust fibrillar organization, suggesting the development of a contractile phenotype. Indeed, tissue constructs prepared from these cells exhibited high levels of contractility in response to receptor- and non-receptor-mediated agonists.

CONCLUSION

We developed an efficient stage-wise strategy that enabled hiPSC differentiation into contractile SMC through an intermediate population of clonogenic and multipotent MSC. The high yield of MSC and SMC derivation suggests that our strategy may facilitate an acquisition of the large numbers of cells required for regenerative medicine or for studying vascular disease pathophysiology.

Derivation and maturation of synthetic and contractile vascular smooth muscle cells from human pluripotent stem cells

AIMS

Embryonic vascular smooth muscle cells (vSMCs) have a synthetic phenotype; in adults, they commit to the mature contractile phenotype. Research shows that human pluripotent stem cells (hPSCs) differentiate into vSMCs, but nobody has yet documented their maturation into the synthetic or contractile phenotypes. This study sought to control the fate decisions of hPSC derivatives to guide their maturation towards a desired phenotype.

METHODS AND RESULTS

The long-term differentiation of hPSCs, including the integration-free-induced PSC line, in high serum with platelet-derived growth factor-BB (PDGF-BB) and transforming growth factor-β1, allowed us to induce the synthetic vSMC (Syn-vSMC) phenotype with increased extracellular matrix (ECM) protein expression and reduced expression of contractile proteins. By monitoring the expression of two contractile proteins, smooth muscle myosin heavy chain (SMMHC) and elastin, we show that serum starvation and PDGF-BB deprivation caused maturation towards the contractile vSMC (Con-vSMC) phenotype. Con-vSMCs differ distinctively from Syn-vSMC derivatives in their condensed morphology, prominent filamentous arrangement of cytoskeleton proteins, production and assembly of elastin, low proliferation, numerous and active caveolae, enlarged endoplasmic reticulum, and ample stress fibres and bundles, as well as their high contractility. When transplanted subcutaneously into nude mice, the human Con-vSMCs aligned next to the host's growing functional vasculature, with occasional circumferential wrapping and vascular tube narrowing.

CONCLUSION

We control hPSC differentiation into synthetic or contractile phenotypes by using appropriate concentrations of relevant factors. Deriving Con-vSMCs from an integration-free hiPSC line may prove useful for regenerative therapy involving blood vessel differentiation and stabilization.

Stem cell sources for vascular tissue engineering and regeneration

This review focuses on the stem cell sources with the potential to be used in vascular tissue engineering and to promote vascular regeneration. The first clinical studies using tissue-engineered vascular grafts are already under way, supporting the potential of this technology in the treatment of cardiovascular and other diseases. Despite progress in engineering biomaterials with the appropriate mechanical properties and biological cues as well as bioreactors for generating the correct tissue microenvironment, the source of cells that make up the vascular tissues remains a major challenge for tissue engineers and physicians. Mature cells from the tissue of origin may be difficult to obtain and suffer from limited proliferative capacity, which may further decline as a function of donor age. On the other hand, multipotent and pluripotent stem cells have great potential to provide large numbers of autologous cells with a great differentiation capacity. Here, we discuss the adult multipotent as well as embryonic and induced pluripotent stem cells, their differentiation potential toward vascular lineages, and their use in engineering functional and implantable vascular tissues. We also discuss the associated challenges that need to be addressed in order to facilitate the transition of this technology from the bench to the bedside.

Differentiation of multipotent adult germline stem cells derived from mouse testis into functional endothelial cells

Pluripotent stem cells hold great promise for the treatment of cardiovascular disease. We previously described multipotent adult germline stem cells (maGSCs) from mouse testis with differentiation potential similar to embryonic stem cells. The aim of this work was to differentiate maGSCs into functional endothelial cells and to study their potential for vasculogenesis. MaGSCs were cocultivated with OP9 stromal cells to induce differentiation into cardiovascular progenitors, i.e. fetal liver kinase 1-positive (Flk-1+) cells. Five days later, Flk-1+ cells were separated using fluorescence-activated cell sorting, followed by cultivation on collagen type IV under endothelial differentiation conditions. At different time points, maGSC-derived endothelial-like cells were characterized using RT-PCR, flow cytometry, immunofluorescence and functional assays. Cultivation of Flk-1+ cells resulted in the progressive upregulation of endothelial cell markers, including VE-cadherin, von Willebrand factor and endothelial nitric oxide synthase. Moreover, Flk-1+ maGSC-derived endothelial-like cells were able to branch and form networks in vitro and promoted functional blood vessel formation in vivo. Importantly, Flk-1+ cells retained their potential to proliferate and could be continuously expanded, while the ability of contact inhibition was preserved. Thus, maGSCs may provide a useful source of endothelial-like cells to study the basic mechanisms of vasculogenesis or endothelial differentiation.

Evaluation of endothelial cells differentiated from amniotic fluid-derived stem cells

Amniotic fluid holds great promise as a stem cell source, especially in neonatal applications where autologous cells can be isolated and used. This study examined chemical-mediated differentiation of amniotic fluid-derived stem cells (AFSC) into endothelial cells and verified the function of AFSC-derived endothelial cells (AFSC-EC). AFSC were isolated from amniotic fluid obtained from second trimester amnioreduction as part of therapeutic intervention from pregnancies affected with twin-twin transfusion syndrome. Undifferentiated AFSC were of normal karyotype with a subpopulation of cells positive for the embryonic stem cell marker SSEA4, hematopoietic stem cell marker c-kit, and mesenchymal stem cell markers CD29, CD44, CD73, CD90, and CD105. Additionally, these cells were negative for the endothelial marker CD31 and hematopoietic differentiation marker CD45. AFSC were cultured in endothelial growth media with concentrations of vascular endothelial growth factor (VEGF) ranging from 1 to 100 ng/mL. After 2 weeks, AFSC-EC expressed von Willebrand factor, endothelial nitric oxide synthase, CD31, VE-cadherin, and VEGF receptor 2. Additionally, the percentage of cells expressing CD31 was positively correlated with VEGF concentration up to 50 ng/mL, with no increase at higher concentrations. AFSC-EC showed a decrease in stem cells markers c-kit and SSEA4 and were morphologically similar to human umbilical vein endothelial cells (HUVEC). In functional assays, AFSC-EC formed networks and metabolized acetylated low-density lipoprotein, also characteristic of HUVEC. Nitrate levels for AFSC-EC, an indirect measure of nitric oxide synthesis, were significantly higher than undifferentiated controls and significantly lower than HUVEC. These results indicate that AFSC can differentiate into functional endothelial-like cells and may have the potential to provide vascularization for constructs used in regenerative medicine strategies.

Stem cell-derived vascular endothelial cells and their potential application in regenerative medicine

Although a 'vascular stem cell' population has not been identified or generated, vascular endothelial and mural cells (smooth muscle cells and pericytes) can be derived from currently known pluripotent stem cell sources including human embryonic stem cells and induced pluripotent stem cells. We review the vascular potential of these human pluripotent stem cells, the mechanisms by which they are induced to differentiate toward a vascular endothelial cell fate, and their applications in regenerative medicine.

Human embryonic stem cell-derived vascular smooth muscle cells in therapeutic neovascularisation

Ischemic diseases remain one of the major causes of morbidity and mortality throughout the world. In recent clinical trials on cell-based therapies, the use of adult stem and progenitor cells only elicited marginal benefits. Therapeutic neovascularisation is the Holy Grail for ischemic tissue recovery. There is compelling evidence from animal transplantation studies that the inclusion of mural cells in addition to endothelial cells (ECs) can enhance the formation of functional blood vessels. Vascular smooth muscle cells (SMCs) and pericytes are essential for the stabilisation of nascent immature endothelial tubes. Despite the intense interest in the utility of human embryonic stem cells (ESCs) for vascular regenerative medicine, ESC-derived vascular SMCs have received much less attention than ECs. This review begins with developmental insights into a range of smooth muscle progenitors from studies on embryos and ESC differentiation systems. We then summarise the methods of derivation of smooth muscle progenitors and cells from human ESCs. The primary emphasis is on the inherent heterogeneity of smooth muscle progenitors and cells and the limitations of current in vitro characterisation. Essential transplantation issues such as the type and source of therapeutic cells, mode of cell delivery, measures to enhance cell viability, putative mechanisms of benefit and long-term tracking of cell fate are also discussed. Finally, we highlight the challenges of clinical compatibility and scaling up for medical use in order to eventually realise the goal of human ESC-based vascular regenerative medicine.

Hemodynamic forces regulate embryonic stem cell commitment to vascular progenitors

Pluripotent embryonic stem can (ES) cells can differentiate into all cell lineages. During the process of embryonic development, ES cells are exposed to fluid flow or blood flow generated by the contracting heart. Absence of fluid flow results in the formation of abnormal cardiac chambers and valve formation. Thus, hemodynamic forces and ES cell differentiation to vascular progenitor cells (VPCs) are of emerging interests for restoring endothelial dysfunction, inducing angiogenesis, and forming blood vessel networks. Hemodynamic forces such as fluid shear stress increase the percentage of cells in the S and G₂-M phases, and induce decondensation of chromatin for gene transcription. Fluid shear stress further accelerates ES commitment to CD31⁺ VPC vascular progenitor cells. These ES-derived CD31⁺ cells express endothelial nitric oxide synthase (eNOS) and von Willebrand factor (vWF). They are also capable of LDL uptake and tubular network formation. In this context, understanding hemodynamic forces and ES cell kinetics of differentiation towards endothelial lineage has potential therapeutic applications for repairing vascular damage and engineering vascular graft. Multidisciplinary team approach will likely garner momentum and synergize expertise to address the current road blocks in basic stem cell research for engraftable, restorative, low immunogenic, and non-tumorigenic endothelial progenitors in high purity and stability.

Mechanical factors in the development of the vascular bed

During embryonic development, blood flow is needed not only to nourish the developing embryo but is also important for shaping the vascular network such that it becomes hemodynamically efficient. The first blood vessels form a network called the capillary plexus. After the onset of blood flow, the capillary plexus remodel into a more hierarchical tree-shaped network. Mechanical forces created by blood flow are required for remodelling to occur and these forces are believed to induce a maturation of the blood vessels that stabilizes the growing vascular network. The role of mechanical force has been extensively studied in the mature cardiovascular system. Though the events induced by blood flow during development are thought to be similar to what occurs in the adult, there are several important differences between the embryo and the adult. We therefore discuss what is known about the role of mechanical forces in vascular remodelling from the adult cardiovascular system and highlight how embryonic development differs from the adult. We consider the role of blood flow in altering branching morphology, arterial-venous identity and the formation of the blood vessel wall during early vascular development.

A defined, feeder-free, serum-free system to generate in vitro hematopoietic progenitors and differentiated blood cells from hESCs and hiPSCs

Human ESC and iPSC are an attractive source of cells of high quantity and purity to be used to elucidate early human development processes, for drug discovery, and in clinical cell therapy applications. To efficiently differentiate pluripotent cells into a pure population of hematopoietic progenitors we have developed a new 2-dimensional, defined and highly efficient protocol that avoids the use of feeder cells, serum or embryoid body formation. Here we showed that a single matrix protein in combination with growth factors and a hypoxic environment is sufficient to generate from pluripotent cells hematopoietic progenitors capable of differentiating further in mature cell types of different lineages of the blood system. We tested the differentiation method using hESCs and 9 iPSC lines generated from different tissues. These data indicate the robustness of the protocol providing a valuable tool for the generation of clinical-grade hematopoietic cells from pluripotent cells.

Towards the maturation and characterization of smooth muscle cells derived from human embryonic stem cells

In this study we demonstrate that CD34(+) cells derived from human embryonic stem cells (hESCs) have higher smooth muscle cell (SMC) potential than CD34(-) cells. We report that from all inductive signals tested, retinoic acid (RA) and platelet derived growth factor (PDGF(BB)) are the most effective agents in guiding the differentiation of CD34(+) cells into smooth muscle progenitor cells (SMPCs) characterized by the expression of SMC genes and proteins, secretion of SMC-related cytokines, contraction in response to depolarization agents and vasoactive peptides and expression of SMC-related genes in a 3D environment. These cells are also characterized by a low organization of the contractile proteins and the contractility response is mediated by Ca(2+), which involves the activation of Rho A/Rho kinase- and Ca(2+)/calmodulin (CaM)/myosin light chain kinase (MLCK)-dependent pathways. We further show that SMPCs obtained from the differentiation of CD34(+) cells with RA, but not with PDGF(BB,) can be maturated in medium supplemented with endothelin-1 showing at the end individualized contractile filaments. Overall the hESC-derived SMCs presented in this work might be an unlimited source of SMCs for tissue engineering and regenerative medicine.

Scaffolds in tissue engineering of blood vessels

Tissue engineering of small diameter (<5 mm) blood vessels is a promising approach for developing viable alternatives to autologous vascular grafts. It involves in vitro seeding of cells onto a scaffold on which the cells attach, proliferate, and differentiate while secreting the components of extracellular matrix that are required for creating the tissue. The scaffold should provide the initial requisite mechanical strength to withstand in vivo hemodynamic forces until vascular smooth muscle cells and fibroblasts reinforce the extracellular matrix of the vessel wall. Hence, the choice of scaffold is crucial for providing guidance cues to the cells to behave in the required manner to produce tissues and organs of the desired shape and size. Several types of scaffolds have been used for the reconstruction of blood vessels. They can be broadly classified as biological scaffolds, decellularized matrices, and polymeric biodegradable scaffolds. This review focuses on the different types of scaffolds that have been designed, developed, and tested for tissue engineering of blood vessels, including use of stem cells in vascular tissue engineering.

Engineering blood vessels using stem cells: innovative approaches to treat vascular disorders

Vascular disease is the leading cause of mortality in the USA, providing the impetus for new treatments and technologies. Current therapies rely on the implantation of stents or grafts to treat injured blood vessels. However, these therapies may be immunogenic or may incompletely recover the functional integrity of the vasculature. In light of these shortcomings, cell-based therapies provide new treatment options to heal damaged areas with more suitable substitutes. Current clinical trials employing stem cell-based therapies involve the transfusion of harvested endothelial progenitor cells. While the results from these trials have been encouraging, utilizing tissue-engineered approaches could yield technologically advanced solutions. This article discusses engineered stem cell-based therapies from three angles: the differentiation of adult stem cells, such as mesenchymal stem cells and endothelial progenitor cells, into vascular lineages; investigation of human embryonic stem cells and induced pluripotent stem cells as inexhaustible sources of vascular cells; and tissue-engineering approaches, which incorporate these vascular progenitor cells into biomimetic scaffolds to guide regeneration. The optimal solution to vascular disease lies at the interface of these technologies--embedding differentiated cells into engineered scaffolds to impart precise control over vascular regeneration.

Reconstructing the differentiation niche of embryonic stem cells using biomaterials

The biochemical cues and topographical architecture of the extracellular environment extensively influence ES cell fate. The microenvironment surrounding the developing embryo presents these instructive cues in a complex and interactive manner in order to guide cell fate decisions. Current stem cell research aims to reconstruct this multifaceted embryonic niche to recapitulate development in vitro. This review focuses on 2D and 3D differentiation niches created from natural and synthetic biomaterials to guide the differentiation of ES cells toward specific lineages. Biomaterials engineered to present specific physical constraints are also reviewed for their role in differentiation.

Transforming growth factor-beta and notch signaling mediate stem cell differentiation into smooth muscle cells

The differentiation of stem cells into smooth muscle cells (SMCs) plays an important role in vascular development and remodeling. In addition, stem cells represent a potential source of SMCs for regenerative medicine applications such as constructing vascular grafts. Previous studies have suggested that various biochemical factors, including transforming growth factor-beta (TGF-beta) and the Notch pathway, may play important roles in vascular differentiation. However, the interactions of these two signaling pathways in the differentiation of bone marrow mesenchymal stem cells (MSCs) have not been clearly defined. In this study, we profiled the gene expression in MSCs in response to TGF-beta, and showed that TGF-beta induced Notch ligand Jagged 1 (JAG1) and SMC markers, including smooth muscle alpha-actin (ACTA2), calponin 1 (CNN1), and myocardin (MYOCD), which were dependent on the activation of SMAD3 and Rho kinase. In addition, knocking down JAG1 expression partially blocked ACTA2 and CNN1 expression and completely blocked MYOCD expression, suggesting that JAG1 plays an important role in TGF-beta-induced expression of SMC markers. On the other hand, the activation of Notch signaling induced the expression of SMC markers in MSCs and human embryonic stem cells (hESCs). Notch activation in hESCs also resulted in an increase of neural markers and a decrease of endothelial markers. These results suggest that Notch signaling mediates TGF-beta regulation of MSC differentiation and that Notch signaling induces the differentiation of MSCs and hESCs into SMCs, which represents a novel mechanism involved in stem cell differentiation.

Hypoxia promotes efficient differentiation of human embryonic stem cells to functional endothelium

Early development of mammalian embryos occurs in an environment of relative hypoxia. Nevertheless, human embryonic stem cells (hESC), which are derived from the inner cell mass of blastocyst, are routinely cultured under the same atmospheric conditions (21% O(2)) as somatic cells. We hypothesized that O(2) levels modulate gene expression and differentiation potential of hESC, and thus, we performed gene profiling of hESC maintained under normoxic or hypoxic (1% or 5% O(2)) conditions. Our analysis revealed that hypoxia downregulates expression of pluripotency markers in hESC but increases significantly the expression of genes associated with angio- and vasculogenesis including vascular endothelial growth factor and angiopoitein-like proteins. Consequently, we were able to efficiently differentiate hESC to functional endothelial cells (EC) by varying O(2) levels; after 24 hours at 5% O(2), more than 50% of cells were CD34+. Transplantation of resulting endothelial-like cells improved both systolic function and fractional shortening in a rodent model of myocardial infarction. Moreover, analysis of the infarcted zone revealed that transplanted EC reduced the area of fibrous scar tissue by 50%. Thus, use of hypoxic conditions to specify the endothelial lineage suggests a novel strategy for cellular therapies aimed at repair of damaged vasculature in pathologies such as cerebral ischemia and myocardial infarction.

Therapeutic angiogenesis by transplantation of human embryonic stem cell-derived CD133+ endothelial progenitor cells for cardiac repair

Objective: This study aim to enhance endothelial differentiation of human embryonic stem cells (hESCs) by transduction of an adenovirus (Ad) vector expressing hVEGF165 gene (Ad-hVEGF165 ). Purified hESC-derived CD133+ endothelial progenitors were transplanted into a rat myocardial infarct model to assess their ability to contribute to heart regeneration. Methods: Optimal transduction efficiency with high cell viability was achieved by exposing differentiating hESCs to viral particles at a ratio of 1:500 for 4 h on three consecutive days. Results: Reverse transcription-PCR analysis showed positive upregulation of VEGF, Ang-1, Flt-1, Tie-2, CD34, CD31, CD133 and Flk-1 gene expression in Ad-hVEGF165 -transduced cells. Additionally, flow cytometric analysis of CD133, a cell surface marker, revealed an approximately fivefold increase of CD133 marker expression in Ad-hVEGF165 -transduced cells compared with the nontransduced control. Within a rat myocardial infarct model, transplanted CD133+ endothelial progenitor cells survived and participated, both actively and passively, in the regeneration of the infarcted myocardium, as seen by an approximately threefold increase in mature blood vessel density (13.62 ± 1.56 vs 5.11 ± 1.23; p < 0.01), as well as significantly reduced infarct size (28% ± 8.2% vs 76% ± 5.6%; p < 0.01) in the transplanted group compared with the culture medium-injected control. There was significant improvement in heart function 6 weeks post-transplantation, as confirmed by regional blood-flow analysis (1.72 ± 0.612 ml/min/g vs 0.8 ± 0.256 ml/min/g; p < 0.05), as well as echocardiography assessment of left ventricular ejection fraction (60.855% ± 7.7% vs 38.22 ± 8.6%; p < 0.05) and fractional shortening (38.63% ± 9.3% vs 25.2% ± 7.11%; p < 0.05). Conclusion: hESC-derived CD133+ endothelial progenitor cells can be utilized to regenerate the infarcted heart.

Adaptation to oxygen deprivation in cultures of human pluripotent stem cells, endothelial progenitor cells, and umbilical vein endothelial cells

Hypoxia plays an important role in vascular development through hypoxia-inducible factor-1alpha (HIF-1alpha) accumulation and downstream pathway activation. We sought to explore the in vitro response of cultures of human embryonic stem cells (hESCs), induced pluripotent stem cells (iPSCs), human endothelial progenitor cells (hEPCs), and human umbilical cord vein endothelial cells (HUVECs) to normoxic and hypoxic oxygen tensions. We first measured dissolved oxygen (DO) in the media of adherent cultures in atmospheric (21% O(2)), physiological (5% O(2)), and hypoxic oxygen conditions (1% O(2)). In cultures of both hEPCs and HUVECs, lower oxygen consumption was observed when cultured in 1% O(2). At each oxygen tension, feeder-free cultured hESCs and iPSCs were found to consume comparable amounts of oxygen. Transport analysis revealed that the oxygen uptake rate (OUR) of hESCs and iPSCs decreased distinctly as DO availability decreased, whereas the OUR of all cell types was found to be low when cultured in 1% O(2), demonstrating cell adaptation to lower oxygen tensions by limiting oxygen consumption. Next, we examined HIF-1alpha accumulation and the expression of target genes, including VEGF and angiopoietins (ANGPT; angiogenic response), GLUT-1 (glucose transport), BNIP3, and BNIP3L (autophagy and apoptosis). Accumulations of HIF-1alpha were detected in all four cell lines cultured in 1% O(2). Corresponding upregulation of VEGF, ANGPT2, and GLUT-1 was observed in response to HIF-1alpha accumulation, whereas upregulation of ANGPT1 was detected only in hESCs and iPSCs. Upregulation of BNIP3 and BNIP3L was detected in all cells after 24-h culture in hypoxic conditions, whereas apoptosis was not detectable using flow cytometry analysis, suggesting that BNIP3 and BNIP3L can lead to cell autophagy rather than apoptosis. These results demonstrate adaptation of all cell types to hypoxia but different cellular responses, suggesting that continuous measurements and control over oxygen environments will enable us to guide cellular responses.

Vascular differentiation of human embryonic stem cells in bioactive hydrogel-based scaffolds

The vascularization of tissue constructs remains a major challenge in regenerative medicine, as the diffusional supply of oxygen can support only 100-200 mum thick layers of viable tissue. The formation of a mature and functional vascular network requires communication between endothelial cells (ECs) and smooth muscle cells (SMCs). Potential sources of these cells that involve noninvasive methodologies are required for numerous applications including tissue-engineered vascular grafts, myocardial ischemia, wound healing, plastic surgery, and general tissue-engineering applications. Human embryonic stem cells (hESCs) can be an unlimited source of these cells. They can be expanded in vitro in an undifferentiated state without apparent limit, and hES-derived cells can be created in virtually unlimited amounts for potential clinical uses. Recently, vascular progenitor cells as well as endothelial and smooth muscle cells have been isolated from hESCs.

Human embryonic stem cell-derived vascular progenitor cells capable of endothelial and smooth muscle cell function

OBJECTIVE

Previous studies have demonstrated development of endothelial cells (ECs) and smooth muscle cells (SMCs) as separate cell lineages derived from human embryonic stem cells (hESCs). We demonstrate CD34(+) cells isolated from differentiated hESCs function as vascular progenitor cells capable of producing both ECs and SMCs. These studies better define the developmental origin and reveal the relationship between these two cell types, as well as provide a more complete biological characterization.

MATERIALS AND METHODS

hESCs are cocultured on M2-10B4 stromal cells or Wnt1-expressing M2-10B4 for 13 to 15 days to generate a CD34(+) cell population. These cells are isolated using a magnetic antibody separation kit and cultured on fibronectin-coated dishes in EC medium. To induce SMC differentiation, culture medium is changed and a morphological and phenotypic change occurs within 24 to 48 hours.

RESULTS

CD34(+) vascular progenitor cells give rise to ECs and SMCs. The two populations express respective cell-specific transcripts and proteins, exhibit intracellular calcium in response to various agonists, and form robust tube-like structures when cocultured in Matrigel. Human umbilical vein endothelial cells cultured under SMC conditions do not exhibit a change in phenotype or genotype. Wnt1-overexpressing stromal cells produced an increased number of progenitor cells.

CONCLUSIONS

The ability to generate large numbers of ECs and SMCs from a single vascular progenitor cell population is promising for therapeutic use to treat a variety of diseased and ischemic conditions. The stepwise differentiation outlined here is an efficient, reproducible method with potential for large-scale cultures suitable for clinical applications.

Vascular regeneration: engineering the stem cell microenvironment

Vascular diseases are a major threat to human health nowadays. While current treatments can cure some vascular diseases, their beneficial effects are only temporary; vascular regeneration holds the promise of permanent, effective treatments for many vascular diseases. Stem cells and endothelial progenitor cells can differentiate into vascular lineages and therefore have the potential to repair vascular systems. However, engineering appropriate microenvironments that will allow cell maturation and delivery remains the major challenge to the successful implementation of this treatment. This review introduces the cells that are being studied for vascular differentiation and regeneration; we then consider recent approaches to engineering microenvironments, including proper signaling cues and biodegradable scaffolds that will guide the development of these cells into vessels suitable for cell-based vascular therapy.