Robust functional vascular network formation in vivo by cooperation of adipose progenitor and endothelial cells

Functional Human Vascular Network Generated in Photocrosslinkable Gelatin Methacrylate Hydrogels

The generation of functional, 3D vascular networks is a fundamental prerequisite for the development of many future tissue engineering-based therapies. Current approaches in vascular network bioengineering are largely carried out using natural hydrogels as embedding scaffolds. However, most natural hydrogels present a poor mechanical stability and a suboptimal durability, which are critical limitations that hamper their widespread applicability. The search for improved hydrogels has become a priority in tissue engineering research. Here, the suitability of a photopolymerizable gelatin methacrylate (GelMA) hydrogel to support human progenitor cell-based formation of vascular networks is demonstrated. Using GelMA as the embedding scaffold, it is shown that 3D constructs containing human blood-derived endothelial colony-forming cells (ECFCs) and bone marrow-derived mesenchymal stem cells (MSCs) generate extensive capillary-like networks in vitro. These vascular structures contain distinct lumens that are formed by the fusion of ECFC intracellular vacuoles in a process of vascular morphogenesis. The process of vascular network formation is dependent on the presence of MSCs, which differentiate into perivascular cells occupying abluminal positions within the network. Importantly, it is shown that implantation of cell-laden GelMA hydrogels into immunodeficient mice results in a rapid formation of functional anastomoses between the bioengineered human vascular network and the mouse vasculature. Furthermore, it is shown that the degree of methacrylation of the GelMA can be used to modulate the cellular behavior and the extent of vascular network formation both in vitro and in vivo. These data suggest that GelMA hydrogels can be used for biomedical applications that require the formation of microvascular networks, including the development of complex engineered tissues.

Adipose-derived mesenchymal stromal/stem cells: tissue localization, characterization, and heterogeneity

Adipose tissue as a stem cell source is ubiquitously available and has several advantages compared to other sources. It is easily accessible in large quantities with minimal invasive harvesting procedure, and isolation of adipose-derived mesenchymal stromal/stem cells (ASCs) yields a high amount of stem cells, which is essential for stem-cell-based therapies and tissue engineering. Several studies have provided evidence that ASCs in situ reside in a perivascular niche, whereas the exact localization of ASCs in native adipose tissue is still under debate. ASCs are isolated by their capacity to adhere to plastic. Nevertheless, recent isolation and culture techniques lack standardization. Cultured cells are characterized by their expression of characteristic markers and their capacity to differentiate into cells from meso-, ecto-, and entodermal lineages. ASCs possess a high plasticity and differentiate into various cell types, including adipocytes, osteoblasts, chondrocytes, myocytes, hepatocytes, neural cells, and endothelial and epithelial cells. Nevertheless, recent studies suggest that ASCs are a heterogeneous mixture of cells containing subpopulations of stem and more committed progenitor cells. This paper summarizes and discusses the current knowledge of the tissue localization of ASCs in situ, their characterization and heterogeneity in vitro, and the lack of standardization in isolation and culture methods.

Endothelial progenitors as tools to study vascular disease

Endothelial progenitor cells (EPCs) have great clinical value because they can be used as diagnostic biomarkers and as a cellular therapy for promoting vascular repair of ischaemic tissues. However, EPCs also have an additional research value in vascular disease modelling to interrogate human disease mechanisms. The term EPC is used to describe a diverse variety of cells, and we have identified a specific EPC subtype called outgrowth endothelial cell (OEC) as the best candidate for vascular disease modelling because of its high-proliferative potential and unambiguous endothelial commitment. OECs are isolated from human blood and can be exposed to pathologic conditions (forward approach) or be isolated from patients (reverse approach) in order to study vascular human disease. The use of OECs for modelling vascular disease will contribute greatly to improving our understanding of endothelial pathogenesis, which will potentially lead to the discovery of novel therapeutic strategies for vascular diseases.

Fibroblast growth factor-2 facilitates rapid anastomosis formation between bioengineered human vascular networks and living vasculature

Many common diseases involve the injury, loss, or death of organ tissues. For these patients, organ transplantation is often the only viable solution. Nonetheless, organ transplantation is seriously limited by the relative scarcity of living and non-living donors, a situation that is worsening with aging of the world population. Tissue Engineering (TE) is a research discipline in regenerative medicine that aims to generate tissues in the laboratory that can replace diseased and damaged tissues in patients. Crucially, engineered tissues must have a vascular network that guarantees adequate nutrient supply, gas exchange, and elimination of waste products. Therefore, the search for clinically relevant sources of vasculogenic cells and the subsequent development of methods to achieve rapid vascularization is of utmost importance. We and others have previously shown that human blood-derived endothelial colony-forming cells (ECFCs) have the required vasculogenic capacity to form functional vascular networks in vivo. These studies demonstrated that, in the presence of an appropriate source of perivascular cells, ECFCs can self-assemble into microvascular networks and connect to the host vasculature, a process that takes approximately 7days in vivo. The prospect is to incorporate these vascular networks into future engineered tissues. However, engineered tissues must have a functional vasculature immediately after implantation in order to preserve viability and function. Thus, it is critical to further develop strategies for rapid formation of perfused vascular network in vivo. Here, we describe a methodology to deliver ECFCs and bone marrow-derived mesenchymal stem cells (MSCs) subcutaneously into immunodeficient mice in the presence of fibroblast growth factor-2 (FGF-2). This approach significantly reduces the time needed to achieve functional anastomoses between bioengineered human blood vessels and the host vasculature. This methodology includes (1) isolation, characterization and culture of ECFCs, (2) isolation, characterization and culture of MSCs, and (3) implantation of ECFCs and MSCs, in the presence of FGF-2, into immunodeficient mice to generate perfused vascular networks.

Changes in the frequency and in vivo vessel-forming ability of rhesus monkey circulating endothelial colony-forming cells across the lifespan (birth to aged)

INTRODUCTION

We have identified a novel hierarchy of human endothelial colony-forming cells (ECFCs) that are functionally defined by their proliferative and clonogenic potential and in vivo vessel-forming ability. The rhesus monkey provides an excellent model in which to examine the changes in circulating concentrations and functions of ECFCs since this nonhuman primate possesses a long lifespan and has been used extensively to model age-related processes that occur in humans.

RESULTS

Endothelial cells (ECs) derived from rhesus monkey ECFCs share a cell-surface phenotype similar to human cord blood ECFCs, rapidly form capillary-like structures in vitro, and form endothelial-lined vessels in vivo upon implantation in immunodeficient mice in an age-dependent manner. Of interest, although ECFCs from the oldest monkeys formed capillary-like structures in vitro, the cells failed to form inosculating vessels when implanted in vivo and displayed a deficiency in cytoplasmic vacuolation in vitro; a critical first step in vasculogenesis.

DISCUSSION

Utilizing previously established clonogenic assays for defining different subpopulations of human ECFCs, we have shown that a hierarchy of ECFCs, identical to human cells, can be isolated from the peripheral blood of rhesus monkeys, and that the frequency of the circulating cells varies with age. These studies establish the rhesus monkey as an important preclinical model for evaluating the role and function of circulating ECFCs in vascular homeostasis and aging.

METHODS

Peripheral blood samples were collected from 40 healthy rhesus monkeys from birth to 24 years of age for ECFC analysis including immunophenotyping, clonogenic assays, and in vivo vessel formation.

Mesenchymal stem cells and cardiovascular disease: a bench to bedside roadmap

In recent years, the incredible boost in stem cell research has kindled the expectations of both patients and physicians. Mesenchymal progenitors, owing to their availability, ease of manipulation, and therapeutic potential, have become one of the most attractive options for the treatment of a wide range of diseases, from cartilage defects to cardiac disorders. Moreover, their immunomodulatory capacity has opened up their allogenic use, consequently broadening the possibilities for their application. In this review, we will focus on their use in the therapy of myocardial infarction, looking at their characteristics, in vitro and in vivo mechanisms of action, as well as clinical trials.

Vascular incorporation of endothelial colony-forming cells is essential for functional recovery of murine ischemic tissue following cell therapy

OBJECTIVE

Cord blood-derived human endothelial colony-forming cells (ECFCs) bear a high proliferative capacity and potently enhance tissue neovascularization in vivo. Here, we investigated whether the leading mechanism for the functional improvement relates to their physical vascular incorporation or perivascular paracrine effects and whether the effects can be further enhanced by dual-cell-based therapy, including mesenchymal stem cells (MSCs).

METHODS AND RESULTS

ECFCs or MSCs were lentivirally transduced with thymidine kinase suicide gene driven by the endothelial-specific vascular endothelial growth factor 2 (kinase insert domain receptor) promoter and evaluated in a hindlimb ischemia model. ECFCs and MSCs enhanced neovascularization after ischemic events to a similar extent. Dual therapy using ECFCs and MSCs further enhanced neovascularization. Mechanistically, 3 weeks after induction of ischemia followed by cell therapy, ganciclovir-mediated elimination of kinase insert domain receptor(+) cells completely reversed the therapeutic effect of ECFCs but not that of MSCs. Histological analysis revealed that ganciclovir effectively eliminated ECFCs incorporated into the vasculature.

CONCLUSIONS

Endothelial-specific suicide gene technology demonstrates distinct mechanisms for ECFCs and MSCs, with complete abolishment of ECFC-mediated effects, whereas MSC-mediated effects remained unaffected. These data strengthen the notion that a dual-cell-based therapy represents a promising approach for vascular regeneration of ischemic tissue.

Human endothelial colony forming cells undergo vasculogenesis within biphasic calcium phosphate bone tissue engineering constructs

An important consideration in bone regeneration is the need for expedited neovascularization within the defect site. Formation of a vascular network is critical for cell viability and normal function leading to tissue regeneration, but spontaneous angiogenesis is too slow to yield sufficient vessel formation. In this pilot study, human umbilical cord blood (hUCB)-derived endothelial colony forming cells (ECFCs) were evaluated for in vivo vasculogenesis in the macropores of biphasic calcium phosphate (BCP)/bone morphogenetic protein-2 (BMP-2) bone tissue engineering constructs. Constructs were implanted on the abdominal wall of NOD/SCID mice for 4 weeks. This study demonstrated in vivo vasculogenesis by human ECFCs within the macropore space of BCP/BMP-2 constructs. The human ECFC-derived vessels anastomosed with the host vasculature and perfused vessels were visible in the very center of the 5mm diameter, 2.5mm tall scaffolds. Additionally, the vessels were evenly distributed throughout the construct. This study suggests that scaffolds containing ECFCs have significant potential for expedited neovascularization in bony defects.

Transplanted human bone marrow progenitor subtypes stimulate endogenous islet regeneration and revascularization

Transplanted murine bone marrow (BM) progenitor cells recruit to the injured pancreas and induce endogenous beta cell proliferation to improve islet function. To enrich for analogous human progenitor cell types that stimulate islet regeneration, we purified human BM based on high-aldehyde dehydrogenase activity (ALDH(hi)), an enzymatic function conserved in hematopoietic, endothelial, and mesenchymal progenitor lineages. We investigated the contributions of ALDH(hi) mixed progenitor cells or culture-expanded, ALDH-purified multipotent stromal cell (MSC) subsets to activate endogenous programs for islet regeneration after transplantation into streptozotocin-treated NOD/SCID mice. Intravenous injection of uncultured BM ALDH(hi) cells improved systemic hyperglycemia and augmented insulin secretion by increasing islet size and vascularization, without increasing total islet number. Augmented proliferation within regenerated endogenous islets and associated vascular endothelium indicated the induction of islet-specific proliferative and pro-angiogenic programs. Although cultured MSC from independent human BM samples showed variable capacity to improve islet function, and prolonged expansion diminished hyperglycemic recovery, transplantation of ALDH-purified regenerative MSC reduced hyperglycemia and augmented total beta cell mass by stimulating the formation of small beta cell clusters associated with the ductal epithelium, without evidence of increased islet vascularization or Ngn3(+) endocrine precursor activation. Thus, endogenous islet recovery after progenitor cell transplantation can occur via distinct regenerative mechanisms modulated by subtypes of progenitor cells administered. Further, understanding of how these islet regenerative and pro-angiogenic programs are activated by specific progenitor subsets may provide new approaches for combination cellular therapies to combat diabetes.

Bone marrow-derived mesenchymal stromal cells enhance chimeric vessel development driven by endothelial cell-coated microtissues

Adding bone marrow-derived mesenchymal stromal cells (bmMSCs) to endothelialized collagen gel modules resulted in mature vessel formation, presumably caused in part by the observed display of pericyte-like behavior for the transplanted GFP(+) bmMSCs. A previous study determined that rat aortic endothelial cells (RAECs) delivered on the surface of small (∼0.8 mm long×0.5 mm diameter) collagen gel cylinders (microtissues, modular tissue engineering) formed vessels after transplantation into immunosuppressed Sprague-Dawley (SD) rats. Although the RAECs formed vessels in this allogeneic transplant model, there was a robust inflammatory response and the vessels that formed were leaky as shown by microcomputed tomography (microCT) perfusion studies. In vitro assays showed that SD rat bmMSCs embedded into the collagen gel modules increased the extent of EC proliferation and enhanced EC sprouting. In vivo, although vessel number was not affected, the new vessels formed by the bmMSCs and RAECs were more stable and leaked less in the microCT perfusion analysis than vessels formed by implanted RAECs alone. Addition of the bmMSCs also decreased the total number of CD68(+) macrophages that infiltrated the implant and changed the distribution of CD163(+) (M2) macrophages so that they were found within the newly developed vascularized tissue. Most interestingly, the bmMSCs became smooth muscle actin positive and migrated to surround the EC layer of the vessel, which is the location typical of pericytes. The combination of these two effects was presumed to be the cause of improved vascularity when bmMSCs were embedded in the EC-coated modules. Further exploration of these observations is warranted to exploit modular tissue engineering as a means of forming large vascularized functional tissues using microtissue components.

Induction of erythropoiesis using human vascular networks genetically engineered for controlled erythropoietin release

For decades, autologous ex vivo gene therapy has been postulated as a potential alternative to parenteral administration of recombinant proteins. However, achieving effective cellular engraftment of previously retrieved patient cells is challenging. Recently, our ability to engineer vasculature in vivo has allowed for the introduction of instructions into tissues by genetically modifying the vascular cells that build these blood vessels. In the present study, we genetically engineered human blood-derived endothelial colony-forming cells (ECFCs) to express erythropoietin (EPO) under the control of a tetracycline-regulated system, and generated subcutaneous vascular networks capable of systemic EPO release in immunodeficient mice. These ECFC-lined vascular networks formed functional anastomoses with the mouse vasculature, allowing direct delivery of recombinant human EPO into the bloodstream. After activation of EPO expression, erythropoiesis was induced in both normal and anemic mice, a process that was completely reversible. This approach could relieve patients from frequent EPO injections, reducing the medical costs associated with the management of anemia. We propose this ECFC-based gene-delivery strategy as a viable alternative technology when routine administration of recombinant proteins is needed.

Tasisulam sodium, an antitumor agent that inhibits mitotic progression and induces vascular normalization

LY573636-sodium (tasisulam) is a small molecule antitumor agent with a novel mechanism of action currently being investigated in a variety of human cancers. In vitro, tasisulam induced apoptosis via the intrinsic pathway, resulting in cytochrome c release and caspase-dependent cell death. Using high content cellular imaging and subpopulation analysis of a wide range of in vitro and in vivo cancer models, tasisulam increased the proportion of cells with 4N DNA content and phospho-histone H3 expression, leading to G(2)-M accumulation and subsequent apoptosis. Tasisulam also blocked VEGF, epidermal growth factor, and fibroblast growth factor-induced endothelial cell cord formation but did not block acute growth factor receptor signaling (unlike sunitinib, which blocks VEGF-driven angiogenesis at the receptor kinase level) or induce apoptosis in primary endothelial cells. Importantly, in vivo phenocopying of in vitro effects were observed in multiple human tumor xenografts. Tasisulam was as effective as sunitinib at inhibiting neovascularization in a Matrigel plug angiogenesis assay in vivo and also caused reversible, non G(2)-M-dependent growth arrest in primary endothelial cells. Tasisulam also induced vascular normalization in vivo. Interestingly, the combination of tasisulam and sunitinib significantly delayed growth of the Caki-1 renal cell carcinoma model, whereas neither agent was active alone. These data show that tasisulam has a unique, dual-faceted mechanism of action involving mitotic catastrophe and antiangiogenesis, a phenotype distinct from conventional chemotherapies and published anticancer agents.

Core-shell cell bodies composed of human cbMSCs and HUVECs for functional vasculogenesis

Rapid induction and creation of functional vascular networks is essential for the success of treating ischemic tissues. The formation of mature and functional vascular networks requires the cooperation of endothelial cells (ECs) and perivascular cells. In the study, we used a thermo-responsive hydrogel system to fabricate core-shell cell bodies composed of cord-blood mesenchymal stem cells (cbMSCs) and human umbilical vascular ECs (HUVECs) for functional vasculogenesis. When seeded on Matrigel, the shelled HUVECs attempted to interact and communicate vigorously with the cored cbMSCs initially. Subsequently, HUVECs migrated out and formed tubular structures; cbMSCs were observed to coalesce around the HUVEC-derived tubes. With time progressing, the tubular networks continued to expand without regression, indicating that cbMSCs might function as perivascular cells to stabilize the nascent networks. In the in vivo study, cbMSC/HUVEC bodies were embedded in Matrigel and implanted subcutaneously in nude mice. At day 7, visible blood-filled vessels were clearly identified within the implant containing cbMSC/HUVEC bodies, indicating that the formed vessels anastomosed with the host vasculature. The cored cbMSCs were stained positive for smooth muscle actin, suggesting that they underwent smooth muscle differentiation and formed microvessels with the shelled HUVECs, as the role of perivascular cells. These data confirm that the formation of mature vessels requires heterotypic cooperation of HUVECs and MSCs. This study provides a new strategy for therapeutic vasculogenesis, by showing the feasibility of using cbMSC/HUVEC bodies to create functional vascular networks.

Bioengineering human microvascular networks in immunodeficient mice

The future of tissue engineering and cell-based therapies for tissue regeneration will likely rely on our ability to generate functional vascular networks in vivo. In this regard, the search for experimental models to build blood vessel networks in vivo is of utmost importance. The feasibility of bioengineering microvascular networks in vivo was first shown using human tissue-derived mature endothelial cells (ECs); however, such autologous endothelial cells present problems for wide clinical use, because they are difficult to obtain in sufficient quantities and require harvesting from existing vasculature. These limitations have instigated the search for other sources of ECs. The identification of endothelial colony-forming cells (ECFCs) in blood presented an opportunity to non-invasively obtain ECs (5-7). We and other authors have shown that adult and cord blood-derived ECFCs have the capacity to form functional vascular networks in vivo. Importantly, these studies have also shown that to obtain stable and durable vascular networks, ECFCs require co-implantation with perivascular cells. The assay we describe here illustrates this concept: we show how human cord blood-derived ECFCs can be combined with bone marrow-derived mesenchymal stem cells (MSCs) as a single cell suspension in a collagen/fibronectin/fibrinogen gel to form a functional human vascular network within 7 days after implantation into an immunodeficient mouse. The presence of human ECFC-lined lumens containing host erythrocytes can be seen throughout the implants indicating not only the formation (de novo) of a vascular network, but also the development of functional anastomoses with the host circulatory system. This murine model of bioengineered human vascular network is ideally suited for studies on the cellular and molecular mechanisms of human vascular network formation and for the development of strategies to vascularize engineered tissues.

An isoform of decorin is a resistin receptor on the surface of adipose progenitor cells

Adipose stromal cells (ASCs) serve as mesenchymal progenitors in white adipose tissue (WAT). Intercellular interactions involving ASCs have remained obscure. By merging phage display technology with fluorescence-activated cell sorting (FACS), we screened a combinatorial library for peptides that target mouse ASCs in vivo. We isolated peptide CSWKYWFGEC that specifically homes to ASCs, used it as bait to purify the corresponding ASC surface receptor, and identified it as a previously unreported cleavage product of decorin (DCN) lacking the glycanation site (termed ΔDCN). We demonstrate that ΔDCN is differentially expressed on ASC surface. In a screen for ΔDCN-binding proteins, we identified resistin, an adipokine for which the receptor has been unknown. Expression of ΔDCN in 3T3-L1 cells promoted proliferation and migration but suppressed lipid accumulation upon adipogenesis induction, which was resistin dependent. We conclude that ΔDCN serves as a functional receptor of resistin in adipocyte progenitors and may regulate WAT expansion.

A comparison of methods for quantifying angiogenesis in the Matrigel assay in vitro

Angiogenesis is of major interest because of its involvement in numerous pathologies or for promoting tissue repair. It is often assessed by the ability of endothelial cells to sprout, migrate, and form vascular tubules in Matrigel in vitro. Matrigel contains a mixture of basement membrane components, which stimulate endothelial cells to form capillary-like hexagonal structures, and is often preferred over other in vitro assays because of its ease of use, rapidity and the ability to measure key steps in angiogenesis, including migration, protease activity, and tubule formation. Various methods have been used to quantitate tubule formation, yet no consensus has been reached regarding the best quantification method for evaluating the efficacy of angiogenic stimulants or inhibitors in this Matrigel assay. Here, we have measured the ability of umbilical cord blood endothelial colony-forming cell-derived cells to form tubules in growth factor reduced Matrigel in the presence or absence of two angiogenic inhibitors, suramin and SU6668, to compare the benefits and limitations of two quantification methods-Angiosys and Wimasis. These comparative analyses revealed that both Angiosys and Wimasis are easy to use, accurately quantify angiogenesis, and will suit the needs of different types of users.

Adipose tissue-derived stem cells display a proangiogenic phenotype on 3D scaffolds

Ischemic heart disease is the leading cause of death worldwide. Recent studies suggest that adipose tissue-derived stem cells (ASCs) can be used as a potential source for cardiovascular tissue engineering due to their ability to differentiate along the cardiovascular lineage and to adopt a proangiogenic phenotype. To understand better ASCs' biology, we used a novel 3D culture device. ASCs' and b.END-3 endothelial cell proliferation, migration, and vessel morphogenesis were significantly enhanced compared to 2D culturing techniques. ASCs were isolated from inguinal fat pads of 6-week-old GFP+/BLI+ mice. Early passage ASCs cells (P3-P4), PKH26-labeled murine b.END-3 cells or a co-culture of ASCs and b.END-3 cells were seeded at a density of 1 × 10(5) on three different surface configurations: (a) a 2D surface of tissue culture plastic, (b) Matrigel, and (c) a highly porous 3D scaffold fabricated from inert polystyrene. VEGF expression, cell proliferation, and tubulization, were assessed using optical microscopy, fluorescence microscopy, 3D confocal microscopy, and SEM imaging (n = 6). Increased VEGF levels were seen in conditioned media harvested from co-cultures of ASCs and b.END-3 on either Matrigel or a 3D matrix. Fluorescence, confocal, SEM, bioluminescence revealed improved cell, proliferation, and tubule formation for cells seeded on the 3D polystyrene matrix. Collectively, these data demonstrate that co-culturing ASCs with endothelial cells in a 3D matrix environment enable us to generate prevascularized tissue-engineered constructs. This can potentially help us to surpass the tissue thickness limitations faced by the tissue engineering community today.

Isolating and defining cells to engineer human blood vessels

A great deal of attention has been recently focused on understanding the role that bone marrow-derived putative endothelial progenitor cells (EPC) may play in the process of neoangiogenesis. However, recent data indicate that many of the putative EPC populations are comprised of various haematopoietic cell subsets with proangiogenic activity, but these marrow-derived putative EPC fail to display vasculogenic activity. Rather, this property is reserved for a rare population of circulating viable endothelial cells with colony-forming cell (ECFC) ability. Indeed, human ECFC possess clonal proliferative potential, display endothelial and not haematopoietic cell surface antigens, and display in vivo vasculogenic activity when suspended in an extracellular matrix and implanted into immunodeficient mice. Furthermore, human vessels derived became integrated into the murine circulatory system and eventually were remodelled into arterial and venous vessels. Identification of this population now permits determination of optimal type I collagen matrix microenvironment into which the cells should be embedded and delivered to accelerate and even pattern number and size of blood vessels formed, in vivo. Indeed, altering physical properties of ECFC-collagen matrix implants changed numerous parameters of human blood vessel formation, in host mice. These recent discoveries may permit a strategy for patterning vascular beds for eventual tissue and organ regeneration.

Stromal Vascular Fraction From Adipose Tissue Forms Profound Vascular Network Through the Dynamic Reassembly of Blood Endothelial Cells

Objective—

Tremendous efforts have been made to establish effective therapeutic neovascularization using adipose tissue-derived stromal vascular fraction (SVF), but the efficiency is low, and underlying mechanisms and their interaction with the host in a new microenvironment are poorly understood.

Methods and Results—

Here we demonstrate that direct implantation of SVF derived from donor adipose tissue can create a profound vascular network through the disassembly and reassembly of blood endothelial cells at the site of implantation. This neovasculature successfully established connection with recipient blood vessels to form a functionally perfused circuit. Addition of vascular growth factors to the SVF implant improved the efficiency of functional neovasculature formation. In contrast, spheroid culture of SVF before implantation reduced the capacity of vasculature formation, possibly because of cellular alteration. Implanting SVF into the mouse ischemic hindlimb induced the robust formation of a local neovascular network and salvaged the limb. Moreover, the coimplantation of SVF prevented fat absorption in the subcutaneous adipose tissue graft model.

Conclusion—

Freshly isolated SVF can effectively induce new vessel formation through the dynamic reassembly of blood endothelial cells and could be applied to achieve therapeutic neovascularization for relieving ischemia and preventing fat absorption in an autologous manner.

Open innovation for phenotypic drug discovery: The PD2 assay panel

Phenotypic lead generation strategies seek to identify compounds that modulate complex, physiologically relevant systems, an approach that is complementary to traditional, target-directed strategies. Unlike gene-specific assays, phenotypic assays interrogate multiple molecular targets and signaling pathways in a target "agnostic" fashion, which may reveal novel functions for well-studied proteins and discover new pathways of therapeutic value. Significantly, existing compound libraries may not have sufficient chemical diversity to fully leverage a phenotypic strategy. To address this issue, Eli Lilly and Company launched the Phenotypic Drug Discovery Initiative (PD(2)), a model of open innovation whereby external research groups can submit compounds for testing in a panel of Lilly phenotypic assays. This communication describes the statistical validation, operations, and initial screening results from the first PD(2) assay panel. Analysis of PD(2) submissions indicates that chemical diversity from open source collaborations complements internal sources. Screening results for the first 4691 compounds submitted to PD(2) have confirmed hit rates from 1.6% to 10%, with the majority of active compounds exhibiting acceptable potency and selectivity. Phenotypic lead generation strategies, in conjunction with novel chemical diversity obtained via open-source initiatives such as PD(2), may provide a means to identify compounds that modulate biology by novel mechanisms and expand the innovation potential of drug discovery.

Vascularization of prevascularized and non-prevascularized fibrin-based human adipose tissue constructs after implantation in nude mice

Adipose regeneration strategies have been hampered by the inability to supply an adequate vascular supply following implantation. Vascularization in vitro, also called prevascularization, is a promising method that could promote the vascularization of engineered adipose tissue constructs upon implantation. In this study we compared the ability of prevascularized-to-non-prevascularized fibrin-based human adipose tissue to promote vascularization. Human adipose tissue-derived stromal cells (ASCs) and different mixtures (1:1, 1:2 and 1:5) of ASCs with human umbilical vein endothelial cells (HUVECs) were cultured in fibrin at two different densities (1.0 × 10(6) and 10 × 10(6) cells/ml) for 7 days. Histological analysis revealed that prevascular structures formed in 1:5 ASC/HUVEC fibrin-based constructs seeded with a total of 10 × 10(6) cells/ml. These constructs and ASC-only constructs were implanted subcutaneously in athymic mice for 7 days and generated lipid-containing grafts. The numbers and densities of blood vessels within the ASC/HUVEC constructs were similar to those of ASC-only constructs. Furthermore, immunostaining studies demonstrated human-derived vasculature within a few of the ASC/HUVEC and ASC-only constructs. A subset of this human-derived vasculature contained erythrocytes, indicating integration with the host vasculature. In conclusion, our study indicated no difference in the rate of vascularization of prevascularized ASC/HUVEC and non-prevascularized ASC-only fibrin-based constructs, suggesting that prevascularization of these fibrin-based constructs does not promote vascularization. Our results further indicated that not only endothelial cells, but also ASCs may contribute to the formation of vascular lumina upon implantation. This finding is interesting, since it demonstrates the possibility of vascularized adipose tissue engineering from a single cell source.

Endothelial progenitor cells: novel biomarker and promising cell therapy for cardiovascular disease

Bone-marrow-derived EPCs (endothelial progenitor cells) play an integral role in the regulation and protection of the endothelium, as well as new vessel formation. Peripheral circulating EPC number and function are robust biomarkers of vascular risk for a multitude of diseases, particularly CVD (cardiovascular disease). Importantly, using EPCs as a biomarker is independent of both traditional and non-traditional risk factors (e.g. hypertension, hypercholesterolaemia and C-reactive protein), with infused ex vivo-expanded EPCs showing potential for improved endothelial function and either reducing the risk of events or enhancing recovery from ischaemia. However, as the number of existing cardiovascular risk factors is variable between patients, simple EPC counts do not adequately describe vascular disease risk in all clinical conditions and, as such, the risk of CVD remains. It is likely that this limitation is attributable to variation in the definition of EPCs, as well as a difference in the interaction between EPCs and other cells involved in vascular control such as pericytes, smooth muscle cells and macrophages. For EPCs to be used regularly in clinical practice, agreement on definitions of EPC subtypes is needed, and recognition that function of EPCs (rather than number) may be a better marker of vascular risk in certain CVD risk states. The present review focuses on the identification of measures to improve individual risk stratification and, further, to potentially individualize patient care to address specific EPC functional abnormalities. Herein, we describe that future therapeutic use of EPCs will probably rely on a combination of strategies, including optimization of the function of adjunct cell types to prime tissues for the effect of EPCs.

Host myeloid cells are necessary for creating bioengineered human vascular networks in vivo

The recruitment of myeloid cells has been consistently associated with the formation of new blood vessels during pathological angiogenesis. However, the participation of myeloid cells in bioengineered vascular networks remains unclear. Therefore, we tested whether host myeloid cells play a role in the formation of bioengineered vascular networks that occurs in vivo upon coimplantation of blood-derived endothelial progenitor cells and bone-marrow-derived mesenchymal progenitor cells, suspended as single cells in Matrigel, into immune-deficient mice. We observed an influx of spatially organized host CD11b(+) myeloid cells into the Matrigel implant 1 to 3 days after implantation, which was shown to be cell mediated rather than a nonspecific response. Myeloid cells were significantly reduced once the implants were fully vascularized at days 6 and 7, suggesting an active role during steps that precede formation of functional anastomoses and perfused vessels. Importantly, depletion of circulating myeloid cells resulted in a significant reduction in microvessel density in the implants. In summary, the recruitment of myeloid cells occurs rapidly after coimplantation of endothelial and mesenchymal progenitor cells and is necessary for full vascularization in this model. This is the first demonstration of a role for recruited myeloid cells in the formation of bioengineered vascular networks.

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.

Type I collagen, fibrin and PuraMatrix matrices provide permissive environments for human endothelial and mesenchymal progenitor cells to form neovascular networks

The field of tissue engineering seeks to create metabolically demanding, functional tissues, which will require blood vessel networks capable of forming rapidly in a variety of extracellular matrix (ECM) environments. We tested whether human endothelial progenitor cells (EPCs) and mesenchymal progenitor cells (MPCs) could form microvascular networks in type I collagen, fibrin and an engineered peptide hydrogel, PuraMatrix, in 7 days in vivo in immune-deficient mice. These results are compared to those previously published, based on the Matrigel ECM. Perfused blood vessels formed in all three types of ECM within 7 days. Collagen at 5 and 6 mg/ml and 10 mg/ml fibrin supported vessel formation at 30-60 vessels/mm(2), and PuraMatrix enabled vessel formation to 160 vessels/mm(2), significantly greater than collagen or fibrin. Vessels were composed of EPCs with perivascular cells on their abluminal surfaces. EPCs injected alone formed a low density of blood vessels in collagen and PuraMatrix, while MPCs injected alone resulted in sparse vessel networks in all ECMs tested. A rheometer was used to determine whether the ECMs which supported vascularization had bulk physical properties similar to or distinct from Matrigel. Collagen and fibrin were the stiffest matrices to support extensive vascularization, with storage moduli in the range 385-510 Pa, while Matrigel, at 80 Pa, and PuraMatrix, at 5 Pa, were far more compliant. Thus, EPCs and MPCs were capable of vasculogenesis in environments having disparate physical properties, although vascular density was greater in more compliant ECMs. We propose that EPC/MPC-mediated vascularization is a versatile technology which may enable the development of engineered organs.

Adipose tissue progenitor cells directly interact with endothelial cells to induce vascular network formation

Adipose stromal cells (ASCs) express markers and functional properties of pericytes in vitro and, in combination with endothelial cells (ECs), are able to establish multilayer functional vessels in vivo. However, the factors that coordinate EC-ASC communications to promote migration of these cells toward one another, and their heterotypic assembly into vascular structures are not well defined. To understand the mechanisms of EC-ASC interaction, we developed an in vitro model of coculturing ECs with ASCs in a system containing serum but no additional exogenous cytokines or extracellular matrix (ECM) proteins. We demonstrated that ASCs have a profound potential to stimulate morphogenesis of ECs into branching networks of cord structures. The vascular networks developed in 6 days and were stable for at least 3 weeks. This process was associated with an increase in ECM protein production by ASCs and ECs, alpha-smooth muscle actin expression by ASCs, and increased CD31/platelet endothelial cell adhesion molecule-1 (PECAM-1) surface presentation by ECs. The vascular network formation (VNF) was dependent on matrix metalloproteinase activity and cell communications through vascular endothelial growth factor, hepatocyte growth factor, and platelet-derived growth factor-BB pathways. ASCs exhibited significantly higher potential to stimulate VNF than smooth muscle cells and fibroblasts. Media conditioned by ASCs promoted VNF by ECs cultured on smooth muscle cells and fibroblasts, but could not replace the presence of ASCs in coculture. The presence of ASCs in EC-fibroblast cocultures in a low fraction efficiently stimulated VNF. These findings demonstrate that the vasculogenesis-promoting potential of ASCs depends on interaction with ECs involving contact and likely bi-directional interaction, resulting in modulated secretion of cytokines and ECM proteins.

Fumagillin reduces adipose tissue formation in murine models of nutritionally induced obesity

The effect of fumagillin (a methionine aminopeptidase-type 2 (Met-AP2) inhibitor, with antiangiogenic properties) was investigated in murine models of diet-induced obesity. Eleven-week-old male C57Bl/6 mice (group 1) were given fumagillin by oral gavage at a dose of 1 mg/kg/day during 4 weeks while fed a high-fat diet (HFD) (20.1 kJ/g), and control mice (group 2) received solvent and were pair-fed. At the end of the experiment, body weights in group 1 were significantly lower as compared to group 2 (P < 0.0005). The subcutaneous (SC) and gonadal (GON) fat mass was also significantly lower in group 1 (P < 0.005 and P < 0.05, respectively). Adipocytes were smaller in adipose tissues of mice in group 1, associated with higher adipocyte density. Blood vessel density normalized to adipocyte density was lower in group 1 adipose tissues. However, in mice with established obesity monitored to maintain the same body weight and fat mass as controls, short-term fumagillin administration was also associated with adipocyte hypotrophy (P = 0.01) without affecting blood vessel size or density. Thus, treatment with fumagillin impaired diet-induced obesity in mice, associated with adipocyte hypotrophy but without marked effect on adipose tissue angiogenesis.

The therapeutic potential of engineered human neovessels for cell-based gene therapy

IMPORTANCE OF THE FIELD

Several works have shown the feasibility of engineering functional blood vessels in vivo using human endothelial cells and mural cells. In this context, the genetic modification of endothelial cells would ensure the secretion of a therapeutic protein into the systemic circulation for a prolonged period of time.

AREAS COVERED IN THIS REVIEW

We discuss the different strategies aimed at the formation of long-lasting neovessels in vivo, using human endothelial and mural cells. The main focus is the potential of these constructs in gene therapy strategies for the in vivo production of therapeutic proteins.

WHAT THE READER WILL GAIN

The reader will have an outline of the different types of cells that have been used for microvessel engineering in vivo, as well as scaffolds employed to seed these cells. We provide a critical review of their advantages and drawbacks, along with examples of their potential in cell-based gene therapy strategies.

TAKE HOME MESSAGE

There is a real potential for neovessels derived from human endothelial and mural cells to be incorporated in clinical interventions, either as a cell-based gene therapy to produce a therapeutic protein or as a component of engineered tissue constructs in regenerative medicine.

Adipose tissue-derived progenitor cells and cancer

Recruitment of stem cells and partially differentiated progenitor cells is a process which accompanies and facilitates the progression of cancer. One of the factors complicating the clinical course of cancer is obesity, a progressively widespread medical condition resulting from overgrowth of white adipose tissue (WAT), commonly known as white fat. The mechanisms by which obesity influences cancer risk and progression are not completely understood. Cells of WAT secret soluble molecules (adipokines) that could stimulate tumor growth, although there is no consensus on which cell populations and which adipokines are important. Recent reports suggest that WAT-derived mesenchymal stem (stromal) cells, termed adipose stem cells (ASC), may represent a cell population linking obesity and cancer. Studies in animal models demonstrate that adipokines secreted by ASC can promote tumor growth by assisting in formation of new blood vessels, a process necessary for expansion of tumor mass. Importantly, migration of ASC from WAT to tumors has been demonstrated, indicating that the tumor microenvironment in cancer may be modulated by ASC-derived trophic factors in a paracrine rather than in an endocrine manner. Here, we review possible positive and adverse implications of progenitor cell recruitment into the diseased sites with a particular emphasis on the role in cancer progression of progenitors that are expanded in obesity.

Functional endothelial progenitor cells from cryopreserved umbilical cord blood

Umbilical cord blood (UCB) is recognized as an enriched source of endothelial progenitor cells (EPCs) with potential therapeutic value. Because cryopreservation is the only reliable method for long-term storage of UCB cells, the clinical application of EPCs depends on our ability to acquire them from cryopreserved samples; however, the feasibility of doing so remains unclear. In this study we demonstrate that EPCs can be isolated from cryopreserved UCB-derived mononuclear cells (MNCs). The number of outgrowth EPC colonies that emerged in culture from cryopreserved samples was similar to that obtained from fresh UCB. Furthermore, EPCs obtained from cryopreserved MNCs were phenotypically and functionally indistinguishable from freshly isolated ones, including the ability to form blood vessels in vivo. Our results eliminate the necessity of performing cell isolation procedures ahead of future clinical needs and suggest that EPCs derived from cryopreserved UCB may be suitable for EPC-related therapies.

Rapid anastomosis of endothelial progenitor cell-derived vessels with host vasculature is promoted by a high density of cotransplanted fibroblasts

To ensure survival of engineered implantable tissues thicker than approximately 2-3 mm, convection of nutrients and waste products to enhance the rate of transport will be required. Creating a network of vessels in vitro, before implantation (prevascularization), is one potential strategy to achieve this aim. In this study, we developed three-dimensional engineered vessel networks in vitro by coculture of endothelial cells (ECs) and fibroblasts in a fibrin gel for 7 days. Vessels formed by cord blood endothelial progenitor cell-derived ECs (EPC-ECs) in the presence of a high density of fibroblasts created an interconnected tubular network within 4 days, compared with 5-7 days in the presence of a low density of fibroblasts. Vessels derived from human umbilical vein ECs (HUVECs) in vitro showed similar kinetics. Implantation of the prevascularized tissues into immune-compromised mice, however, revealed a dramatic difference in the ability of EPC-ECs and HUVECs to form anastomoses with the host vasculature. Vascular beds derived from EPC-ECs were perfused within 1 day of implantation, whereas no HUVEC vessels were perfused at day 1. Further, while almost 90% of EPC-EC-derived vascular beds were perfused at day 3, only one-third of HUVEC-derived vascular beds were perfused. In both cases, a high density of fibroblasts accelerated anastomosis by 2-3 days. We conclude that both EPC-ECs and a high density of fibroblasts significantly accelerate the rate of functional anastomosis, and that prevascularizing an engineered tissue may be an effective strategy to enhance convective transport of nutrients in vivo.

Topical delivery of mesenchymal stem cells and their function in wounds

Mesenchymal stem cells are a heterogeneous population of fibroblast-like cells found in most adult organs. However, most of our current knowledge is based on cells of bone marrow or interstitial adipose tissues. These cells are capable of differentiation along various mesenchymal lineages. In addition, they have demonstrated therapeutic characteristics in wounds and ischemic situations. The therapeutic characteristics of these cells are activated upon their entering wounds or other damaged tissues. A current problem is the development of strategies that ensure that these cells reach wound beds in a timely fashion and in sufficient numbers to maximize their therapeutic benefits. Currently, there are two basic delivery methods: systemic infusion of cells into the vascular circulation and direct application of therapeutic cells to wound sites. Skin wounds are optimal candidates for the topical delivery approach. However, the methods by which therapeutic cells are delivered to such wounds vary. This review outlines the basic methods used to deliver therapeutic cells to skin and other wounds. Upon entering wounds, therapeutic cells interact with other wound cells through paracrine mechanisms that are not yet well understood. Nonetheless, interactions with vascular endothelial cells and immunomodulation appear to play significant roles in accelerating wound healing and in reducing scar formation upon the completion of the healing process. Although the phenomenological body of evidence indicating the efficacy of therapeutic cells is substantial, considerable work is still required to better determine the molecular and cellular functions of these cells and to assess their fate and the long-term consequences of their application.

Human endothelial stem/progenitor cells, angiogenic factors and vascular repair

Neovascularization or new blood vessel formation is of utmost importance not only for tissue and organ development and for tissue repair and regeneration, but also for pathological processes, such as tumour development. Despite this, the endothelial lineage, its origin, and the regulation of endothelial development and function either intrinsically from stem cells or extrinsically by proangiogenic supporting cells and other elements within local and specific microenvironmental niches are still not fully understood. There can be no doubt that for most tissues and organs, revascularization represents the holy grail for tissue repair, with autologous endothelial stem/progenitor cells, their proangiogenic counterparts and the products of these cells all being attractive targets for therapeutic intervention. Historically, a great deal of controversy has surrounded the identification and origin of cells and factors that contribute to revascularization, the use of such cells or their products as biomarkers to predict and monitor tissue damage and repair or tumour progression and therapeutic responses, and indeed their efficacy in revascularizing and repairing damaged tissues. Here, we will review the role of endothelial progenitor cells and of supporting proangiogenic cells and their products, principally in humans, as diagnostic and therapeutic agents for wound repair and tissue regeneration.

Adipose stem cell treatment in mice attenuates lung and systemic injury induced by cigarette smoking

RATIONALE

Adipose-derived stem cells express multiple growth factors that inhibit endothelial cell apoptosis, and demonstrate substantial pulmonary trapping after intravascular delivery.

OBJECTIVES

We hypothesized that adipose stem cells would ameliorate chronic lung injury associated with endothelial cell apoptosis, such as that occurring in emphysema.

METHODS

Therapeutic effects of systemically delivered human or mouse adult adipose stem cells were evaluated in murine models of emphysema induced by chronic exposure to cigarette smoke or by inhibition of vascular endothelial growth factor receptors.

MEASUREMENTS AND MAIN RESULTS

Adipose stem cells were detectable in the parenchyma and large airways of lungs up to 21 days after injection. Adipose stem cell treatment was associated with reduced inflammatory infiltration in response to cigarette smoke exposure, and markedly decreased lung cell death and airspace enlargement in both models of emphysema. Remarkably, therapeutic results of adipose stem cells extended beyond lung protection by rescuing the suppressive effects of cigarette smoke on bone marrow hematopoietic progenitor cell function, and by restoring weight loss sustained by mice during cigarette smoke exposure. Pulmonary vascular protective effects of adipose stem cells were recapitulated by application of cell-free conditioned medium, which improved lung endothelial cell repair and recovery in a wound injury repair model and antagonized effects of cigarette smoke in vitro.

CONCLUSIONS

These results suggest a useful therapeutic effect of adipose stem cells on both lung and systemic injury induced by cigarette smoke, and implicate a lung vascular protective function of adipose stem cell derived paracrine factors.

Development of a porcine delayed wound-healing model and its use in testing a novel cell-based therapy

PURPOSE

A delayed full-thickness wound-healing model was developed and used for examining the capacity of adipose-derived stem cells (ASCs), either alone or in platelet-rich fibrin gels, to promote healing.

METHODS AND MATERIALS

Four pigs received electron beam radiation to the dorsal skin surface. Five weeks after radiation, subcutaneous fat was harvested from nonirradiated areas and processed to yield ASCs. Two weeks later, 28 to 30 full-thickness 1.5-cm(2) wounds were made in irradiated and nonirradiated skin. Wounds were treated with either saline solution, ASCs in saline solution, platelet-rich plasma (PRP) fibrin gel, ASCs in PRP, or non-autologous green fluorescence protein-labeled ASCs.

RESULTS

The single radiation dose produced a significant loss of dermal microvasculature density (75%) by 7 weeks. There was a significant difference in the rate of healing between irradiated and nonirradiated skin treated with saline solution. The ASCs in PRP-treated wounds exhibited a significant 11.2% improvement in wound healing compared with saline solution. Enhancement was dependent on the combination of ASCs and PRP, because neither ASCs nor PRP alone had an effect.

CONCLUSIONS

We have created a model that simulates the clinically relevant late radiation effects of delayed wound healing. Using this model, we showed that a combination of ASCs and PRP improves the healing rates of perfusion-depleted tissues, possibly through enhancing local levels of growth factors.

Combination of injectable multiple growth factor-releasing scaffolds and cell therapy as an advanced modality to enhance tissue neovascularization

OBJECTIVE

Vasculogenic progenitor cell therapy for ischemic diseases bears great potential but still requires further optimization for justifying its clinical application. Here, we investigated the effects of in vivo tissue engineering by combining vasculogenic progenitors with injectable scaffolds releasing controlled amounts of proangiogenic growth factors.

METHODS AND RESULTS

We produced biodegradable, injectable polylactic coglycolic acid-based scaffolds releasing single factors or combinations of vascular endothelial growth factor, hepatocyte growth factor, and angiopoietin-1. Dual and triple combinations of scaffold-released growth factors were superior to single release. In murine hindlimb ischemia models, scaffolds releasing dual (vascular endothelial growth factor and hepatocyte growth factor) or triple combinations improved effects of cord blood-derived vasculogenic progenitors. Increased migration, homing, and incorporation of vasculogenic progenitors into the vasculature augmented capillary density, translating into improved blood perfusion. Most importantly, scaffold-released triple combinations including the vessel stabilizer angiopoietin-1 enhanced the number of perivascular smooth muscle actin(+) vascular smooth muscle cells, indicating more efficient vessel stabilization.

CONCLUSIONS

Vasculogenic progenitor cell therapy is significantly enhanced by in vivo tissue engineering providing a proangiogenic and provasculogenic growth factor-enriched microenvironment. Therefore, combined use of scaffold-released growth factors and cell therapy improves neovascularization in ischemic diseases and may translate into more pronounced clinical effects.

Is Endothelium the Origin of Endothelial Progenitor Cells?

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

Therapeutic potential of adipose-derived stem cells in vascular growth and tissue repair

PURPOSE OF REVIEW

Adipose-derived stem cells (ASCs) are readily available from autologous adipose tissue and have been demonstrated to provide significant potential for tissue rescue from, or repair of, damage in multiple animal models. These include models of myocardial infarction, heart failure, hind limb ischemia, and inflammatory conditions. Early clinical studies have now extended testing of the effects of ASC into patients. This review highlights some of the key reports underlining the potential of ASCs, focusing particularly on diseases involving the cardiovascular system, vascular growth, and tissue repair.

RECENT FINDINGS

Clinical applications of ASCs have begun to show early safety results and promising possibility of efficacy in patients with a range of diseases, including acute myocardial infarction, peripheral vascular disease, and soft and bony tissue defects including cranial bone loss, Crohn's-related fistula, and skin wounds. These effects are importantly based on the secretion of trophic and survival factors by these cells and by their participations in the growth and remodeling of blood vessels. These results suggest that ASCs could be a valuable therapeutic option in vascular growth and tissue repair in various clinical settings.

SUMMARY

ASCs may ultimately represent a valuable therapeutic option in tissue rescue and repair based on their ready availability, proangiogenesis and antiapoptotic factor secretion, immunomodulatory effects, and capacity for multilineage differentiation and ready expansion.

Endothelial colony-forming cell role in neoangiogenesis and tissue repair

PURPOSE OF REVIEW

Patients suffering from vascular disease often have impaired angiogenic ability contributing to impaired tissue repair. One potential therapy is to deliver cells that can aid in angiogenesis. This review will discuss the ability of endothelial progenitor cells (EPCs), which have been reported to contribute to neoangiogenesis in both physiological and pathological conditions, to contribute to neoangiogenesis in tissue repair.

RECENT FINDINGS

In recent years, various reports have described conflicting roles for EPC in vessel formation. Currently there are three different assays for outgrowth of EPC all resulting in the isolation of different cell populations. This confusion is partially due to limited functional characterization of putative EPC populations. One population, endothelial colony-forming cell (ECFC), has been shown to possess all the characteristics of a true endothelial progenitor.

SUMMARY

The review overviews the role of putative EPC populations in angiogenesis and tissue repair. Whereas all EPC populations have been shown to play a role in angiogenesis, only ECFC have demonstrated the ability to form de-novo blood vessels in vivo. Additionally ECFC have been shown to play a role in neovascularization in several preclinical rodent models suggesting that it may be an excellent cell source for treatment of patients with diminished vascular function.

Fighting fat with fat: the expanding field of adipose stem cells

We are in the midst of a dire, unprecedented, and global epidemic of obesity and secondary sequelae, most prominently diabetes and hyperlipidemia. Underlying this epidemic is the most hated of cells, adipocytes, and their inherent dynamic ability to expand and renew. This capacity highlights a heretofore undefined stem compartment. Recent in vivo studies, relying upon lineage tracing and flow cytometry methods, have begun to unravel the identity of adipose stem cells, their niche, and the dynamism central to adipose expansion. Thus, the field is moving in a direction that may allow us to manipulate adipose stem cells to beneficial therapeutic ends.

Bone marrow derived pluripotent cells are pericytes which contribute to vascularization

Pericytes are essential to vascularization, but the purification and characterization of pericytes remain unclear. Smooth muscle actin alpha (alpha-SMA) is one marker [corrected] of pericytes. The aim of this study is to purify the alpha-SMA positive cells from bone marrow and study the characteristics of these cells and the interaction between alpha-SMA positive cells and endothelial cells. The bone marrow stromal cells were harvested from alpha-SMA-GFP transgenic mice, and the alpha-SMA-GFP positive cells were sorted by FACS. The proliferative characteristics and multilineage differentiation ability of the alpha-SMA-GFP positive cells were tested. A 3-D culture model was then applied to test their vascularization by loading alpha-SMA-GFP positive cells and endothelial cells on collagen-fibronectin gel. Results demonstrated that bone marrow stromal cells are mostly alpha-SMA-GFP positive cells which are pluripotent, and these cells expressed alpha-SMA during differentiation. The alpha-SMA-GFP positive cells could stimulate the endothelial cells to form tube-like structures and subsequently robust vascular networks in 3-D culture. In conclusion, the bone marrow derived pluripotent cells include [corrected] pericytes and can contribute to vascularization.

Altered adhesive properties of cord blood endothelial outgrowth cells expressing IL-1ra

The aim of this study was to examine the potential of endothelial outgrowth cells (EOCs) expanded from CD34(+) cord blood-derived cells (CB-EOCs) for overexpression of therapeutic transgenes. As proof of principle, we overexpressed icIL-1ra in CB-EOCs. Proinflammatory activation of CB-EOCs in response to cytokine stimulation (IL-1beta and tumor necrosis factor (TNF)) and during coculture with monocytes showed that icIL-1ra-expressing CB-EOCs express significantly reduced levels of ICAM-1, MCP-1 and thrombin receptor expression. Moreover, overexpression of icIL-1ra attenuated the IL-1beta-mediated proinflammatory activation by diminishing the expression of ICAM-1, SELE, MCP-1 and IL-1beta. Interestingly, overexpression of icIL-1ra also inhibited TNF-induced upregulation of ICAM-1. Expression of ICAM-1, VCAM-1, tissue factor and IL-1beta was also decreased on direct contact with monocytes. These changes in gene expression were accompanied by functional reduction in leukocyte rolling, adhesion of monocytes to CB-EOCs, as well as by a reduction in transendothelial migration of monocytes. Our findings show that CB-EOCs stably expressing transgenic icIL-1ra are protected against activation by not only IL-1beta but also TNFalpha-mediated proinflammatory stimuli and inhibit decisive pathomechanisms of inflammatory processes such as rolling, adhesion and transmigration of monocytes. Therefore, icIL-ra transgenic CB-EOCs may prove to be beneficial in the treatment of IL-1beta- and TNFalpha-mediated inflammatory vasculopathies.