Isolating and defining cells to engineer human blood vessels

Endothelial progenitor cells in the host defense response

Extensive injury of endothelial cells in blood vasculature, especially in the microcirculatory system, frequently occurs in hosts suffering from sepsis and the accompanied systemic inflammation. Pathological factors, including toxic components derived from invading microbes, oxidative stress associated with tissue ischemia/reperfusion, and vessel active mediators generated during the inflammatory response, are known to play important roles in mediating endothelial injury. Collapse of microcirculation and tissue edema developed from the failure of endothelial barrier function in vital organ systems, including the lung, brain, and kidney, are detrimental, which often predict fatal outcomes. The host body possesses a substantial capacity for maintaining vascular homeostasis and repairing endothelial damage. Bone marrow and vascular wall niches house endothelial progenitor cells (EPCs). In response to septic challenges, EPCs in their niche environment are rapidly activated for proliferation and angiogenic differentiation. In the meantime, release of EPCs from their niches into the blood stream and homing of these vascular precursors to tissue sites of injury are markedly increased. The recruited EPCs actively participate in host defense against endothelial injury and repair of damage in blood vasculature via direct differentiation into endothelial cells for re-endothelialization as well as production of vessel active mediators to exert paracrine and autocrine effects on angiogenesis/vasculogenesis. In recent years, investigations on significance of EPCs in host defense and molecular signaling mechanisms underlying regulation of the EPC response have achieved substantial progress, which promotes exploration of vascular precursor cell-based approaches for effective prevention and treatment of sepsis-induced vascular injury as well as vital organ system failure.

3D Tissue-Engineered Vascular Drug Screening Platforms: Promise and Considerations

Despite the efforts devoted to drug discovery and development, the number of new drug approvals have been decreasing. Specifically, cardiovascular developments have been showing amongst the lowest levels of approvals. In addition, concerns over the adverse effects of drugs to the cardiovascular system have been increasing and resulting in failure at the preclinical level as well as withdrawal of drugs post-marketing. Besides factors such as the increased cost of clinical trials and increases in the requirements and the complexity of the regulatory processes, there is also a gap between the currently existing pre-clinical screening methods and the clinical studies in humans. This gap is mainly caused by the lack of complexity in the currently used 2D cell culture-based screening systems, which do not accurately reflect human physiological conditions. Cell-based drug screening is widely accepted and extensively used and can provide an initial indication of the drugs' therapeutic efficacy and potential cytotoxicity. However, in vitro cell-based evaluation could in many instances provide contradictory findings to the in vivo testing in animal models and clinical trials. This drawback is related to the failure of these 2D cell culture systems to recapitulate the human physiological microenvironment in which the cells reside. In the body, cells reside within a complex physiological setting, where they interact with and respond to neighboring cells, extracellular matrix, mechanical stress, blood shear stress, and many other factors. These factors in sum affect the cellular response and the specific pathways that regulate variable vital functions such as proliferation, apoptosis, and differentiation. Although pre-clinical in vivo animal models provide this level of complexity, cross species differences can also cause contradictory results from that seen when the drug enters clinical trials. Thus, there is a need to better mimic human physiological conditions in pre-clinical studies to improve the efficiency of drug screening. A novel approach is to develop 3D tissue engineered miniaturized constructs in vitro that are based on human cells. In this review, we discuss the factors that should be considered to produce a successful vascular construct that is derived from human cells and is both reliable and reproducible.

Lipopolysaccharide increases exosomes secretion from endothelial progenitor cells by toll-like receptor 4 dependent mechanism

Endothelial progenitor cells (EPCs) can exert angiogenic effects by a paracrine mechanism, where exosomes work as an important mediator. Recent studies reported functional expression of toll-like receptor (TLR) 4 on human EPCs and dose-dependent effects of lipopolysaccharide (LPS) on EPC angiogenic properties. To study on the effects of TLR4/LPS signaling on EPC-derived exosomes (Exo) and involved mechanisms, we investigated the effect of LPS on exosomes secretion from human EPC and tested Exo functions by senescence-associated β-galactosidase activity assay and reactive oxygen species (ROS) related H₂ DCF-DA assay. To clarify the mechanism, we examined the changes in intracellular calcium levels and multivesicular bodies (MVBs) development in EPC. We employed the inhibitors of the plasma membrane Ca ²⁺ -ATPase (PMCA), endoplasmic reticulum Ca ²⁺ -ATPase (ERCA), PLC-IP₃ pathway and store-operated calcium entry to assess the effects of LPS on calcium signalings which critical for exosome secretion. LPS induced the release of Exo in a TLR4-dependent manner in vitro, which effect can be partly abrogated by the membrane-permeable IP ₃ R antagonist, 2-aminoethyl diphenylborinate (2-APB), but not PLC inhibitor, U-73122. The LPS can significantly delay the fallback of [Ca ²⁺ ]_i after isolating the cellular PMCA activity, and disturb PMCA 1/4 expression. The distribution of elevated intracellular calcium seemed coincident with the development of MVBs. Furthermore, the LPS-induced Exo maintained valid anti-oxidation/senescence properties. The PMCA and ER Ca ²⁺ release mechanism may contribute to the pro-exosomal effects of LPS on EPC, which is valuable for potential pro-regenerative application in future. This article is protected by copyright. All rights reserved.

Identification of Endothelial Cells and Their Progenitors

Blood vessel formation is a key feature in physiologic and pathologic processes. Once considered a homogeneous cell population that functions as a passive physical barrier between blood and tissue, endothelial cells (ECs) are now recognized to be quite "heterogeneous." While numerous attempts to enhance endothelial repair and replacement have been attempted using so called "endothelial progenitor cells" it is now clear that a better understanding of the origin, location, and activation of stem and progenitor cells of the resident vascular endothelium is required before attempting exogenous cell therapy approaches. This chapter provides an overview for performance of single-cell clonogenic studies of human umbilical cord blood circulating endothelial colony-forming cells (ECFC) that represent distinct precursors for the endothelial lineage with vessel forming potential.

Cellular Based Strategies for Microvascular Engineering

Vascularization is a major hurdle in complex tissue and organ engineering. Tissues greater than 200 μm in diameter cannot rely on simple diffusion to obtain nutrients and remove waste. Therefore, an integrated vascular network is required for clinical translation of engineered tissues. Microvessels have been described as <150 μm in diameter, but clinically they are defined as <1 mm. With new advances in super microsurgery, vessels less than 1 mm can be anastomosed to the recipient circulation. However, this technical advancement still relies on the creation of a stable engineered microcirculation that is amenable to surgical manipulation and is readily perfusable. Microvascular engineering lays on the crossroads of microfabrication, microfluidics, and tissue engineering strategies that utilize various cellular constituents. Early research focused on vascularization by co-culture and cellular interactions, with the addition of angiogenic growth factors to promote vascular growth. Since then, multiple strategies have been utilized taking advantage of innovations in additive manufacturing, biomaterials, and cell biology. However, the anatomy and dynamics of native blood vessels has not been consistently replicated. Inconsistent results can be partially attributed to cell sourcing which remains an enigma for microvascular engineering. Variations of endothelial cells, endothelial progenitor cells, and stem cells have all been used for microvascular network fabrication along with various mural cells. As each source offers advantages and disadvantages, there continues to be a lack of consensus. Furthermore, discord may be attributed to incomplete understanding about cell isolation and characterization without considering the microvascular architecture of the desired tissue/organ.

Design and biofabrication of dermal regeneration scaffolds: role of oligomeric collagen fibril density and architecture

Aim: To evaluate dermal regeneration scaffolds custom-fabricated from fibril-forming oligomeric collagen where the total content and spatial gradient of collagen fibrils was specified. Materials & methods: Microstructural and mechanical features were verified by electron microscopy and tensile testing. The ability of dermal scaffolds to induce regeneration of rat full-thickness skin wounds was determined and compared with no fill control, autograft skin and a commercial collagen dressing. Results: Increasing fibril content of oligomer scaffolds inhibited wound contraction and decreased myofibroblast marker expression. Cellular and vascular infiltration of scaffolds over the 14-day period varied with the graded density and orientation of fibrils. Conclusion: Fibril content, spatial gradient and orientation are important collagen scaffold design considerations for promoting vascularization and dermal regeneration while reducing wound contraction.

Presence of stromal cells in a bioengineered tumor microenvironment alters glioblastoma migration and response to STAT3 inhibition

Despite the increasingly recognized importance of the tumor microenvironment (TME) as a regulator of tumor progression, only few in vitro models have been developed to systematically study the effects of TME on tumor behavior in a controlled manner. Here we developed a three-dimensional (3D) in vitro model that recapitulates the physical and compositional characteristics of Glioblastoma (GBM) extracellular matrix (ECM) and incorporates brain stromal cells such as astrocytes and endothelial cell precursors. The model was used to evaluate the effect of TME components on migration and survival of various patient-derived GBM cell lines (GBM10, GBM43 and GBAM1) in the context of STAT3 inhibition. Migration analysis of GBM within the 3D in vitro model demonstrated that the presence of astrocytes significantly increases the migration of GBM, while presence of endothelial precursors has varied effects on the migration of different GBM cell lines. Given the role of the tumor microenvironment as a regulator of STAT3 activity, we tested the effect of the STAT3 inhibitor SH-4-54 on GBM migration and survival. SH-4-54 inhibited STAT3 activity and reduced 3D migration and survival of GBM43 but had no effect on GBM10. SH-4-54 treatment drastically reduced the viability of the stem-like line GBAM1 in liquid culture, but its effect lessened in presence of a 3D ECM and stromal cells. Our results highlight the interplay between the ECM and stromal cells in the microenvironment with the cancer cells and indicate that the impact of these relationships may differ for GBM cells of varying genetic and clinical histories.

iPSC-Derived Vascular Cell Spheroids as Building Blocks for Scaffold-Free Biofabrication

Recently a protocol is established to obtain large quantities of human induced pluripotent stem cells (iPSC)-derived endothelial progenitors, called endothelial colony forming cells (ECFC), and of candidate smooth-muscle forming cells (SMFC). Here, the suitability for assembling in spheroids, and in larger 3D cell constructs is tested. iPSC-derived ECFC and SMFC are labeled with tdTomato and eGFP, respectively. Spheroids are formed in ultra-low adhesive wells, and their dynamic proprieties are studied by time-lapse microscopy, or by confocal microscopy. Spheroids are also tested for fusion ability either in the wells, or assembled on the Regenova 3D bioprinter which laces them in stainless steel micro-needles (the "Kenzan" method). It is found that both ECFC and SMFC formed spheroids in about 24 h. Fluorescence monitoring indicated a continuous compaction of ECFC spheroids, but stabilization in those prepared from SMFC. In mixed spheroids, the cell distribution changed continuously, with ECFC relocating to the core, and showing pre-vascular organization. All spheroids have the ability of in-well fusion, but only those containing SMFC are robust enough to sustain assembling in tubular structures. In these constructs a layered distribution of alpha smooth muscle actin-positive cells and extracellular matrix deposition is found. In conclusion, iPSC-derived vascular cell spheroids represent a promising new cellular material for scaffold-free biofabrication.

Endothelial and smooth muscle cells derived from human cardiac explants demonstrate angiogenic potential and suitable for design of cell-containing vascular grafts

Background

Endothelial and smooth muscle cells are considered promising resources for regenerative medicine and cell replacement therapy. It has been shown that both types of cells are heterogeneous depending on the type of vessels and organs in which they are located. Therefore, isolation of endothelial and smooth muscle cells from tissues relevant to the area of research is necessary for the adequate study of specific pathologies. However, sources of specialized human endothelial and smooth muscle cells are limited, and the search for new sources is still relevant. The main goal of our study is to demonstrate that functional endothelial and smooth muscle cells can be obtained from an available source—post-surgically discarded cardiac tissue from the right atrial appendage and right ventricular myocardium.

Methods

Heterogeneous primary cell cultures were enzymatically isolated from cardiac explants and then grown in specific endothelial and smooth muscle growth media on collagen IV-coated surfaces. The population of endothelial cells was further enriched by immunomagnetic sorting for CD31, and the culture thus obtained was characterized by immunocytochemistry, ultrastructural analysis and in vitro functional tests. The angiogenic potency of the cells was examined by injecting them, along with Matrigel, into immunodeficient mice. Cells were also seeded on characterized polycaprolactone/chitosan membranes with subsequent analysis of cell proliferation and function.

Results

Endothelial cells isolated from cardiac explants expressed CD31, VE-cadherin and VEGFR2 and showed typical properties, namely, cytoplasmic Weibel-Palade bodies, metabolism of acetylated low-density lipoproteins, formation of capillary-like structures in Matrigel, and production of extracellular matrix and angiogenic cytokines. Isolated smooth muscle cells expressed extracellular matrix components as well as α-actin and myosin heavy chain. Vascular cells derived from cardiac explants demonstrated the ability to stimulate angiogenesis in vivo. Endothelial cells proliferated most effectively on membranes made of polycaprolactone and chitosan blended in a 25:75 ratio, neutralized by a mixture of alkaline and ethanol. Endothelial and smooth muscle cells retained their functional properties when seeded on the blended membranes.

Conclusions

We established endothelial and smooth muscle cell cultures from human right atrial appendage and right ventricle post-operative explants. The isolated cells revealed angiogenic potential and may be a promising source of patient-specific cells for regenerative medicine.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-017-1156-1) contains supplementary material, which is available to authorized users.

Oral Mucosa Harbors a High Frequency of Endothelial Cells: A Novel Postnatal Cell Source for Angiogenic Regeneration

Endothelial progenitor cells/endothelial cells (EPCs/ECs) have great potential to treat pathological conditions such as cardiac infarction, muscle ischemia, and bone fractures, but isolation of EPC/ECs from existing cell sources is challenging due to their low EC frequency. We have isolated endothelial progenitor (EP)-like cells from rat oral mucosa and characterized their yield, immunophenotype, growth, and in vivo angiogenic potential. The frequency of EP-like cells derived from oral mucosa is thousands of folds higher than EPCs derived from donor-match bone marrow samples. EP-like cells from oral mucosa were positive for EC markers CD31, VE-Cadherin, and VEGFR2. Oral mucosa-derived EP-like cells displayed robust uptake of acetylated low-density lipoprotein and formed stable capillary networks in Matrigel. Subcutaneously implanted oral mucosa-derived EP-like cells anastomosed with host blood vessels, implicating their ability to elicit angiogenesis. Similar to endothelial colony-forming cells, EP-like cells from oral mucosa have a significantly higher proliferative rate than human umbilical vein endothelial cells. These findings identify a putative EPC source that is easily accessible in the oral cavity, potentially from discarded tissue specimens, and yet with robust yield and potency for angiogenesis in tissue and organ regeneration.

Use of Statins to Augment Progenitor Cell Function in Preclinical and Clinical Studies of Regenerative Therapy: a Systematic Review

BACKGROUND

Mesenchymal stromal cells (MSCs) and endothelial progenitor cells (EPCs) are used in cell-based regenerative therapy. HMG CoA reductase inhibitors (statins) appear promising in blocking apoptosis, prolonging progenitor cell survival and improving their capacity to repair organ function.

METHODS

We performed a systematic review of preclinical and clinical studies to clarify whether statins can improve cell-based repair of organ injury. MEDLINE, EMBASE, and PUBMED databases were searched (1947 to June 25, 2013). Controlled clinical and pre-clinical studies were included that evaluated statin therapy used alone or in combination with MSCs or EPCs in patients or animals with organ injury.

RESULTS

After screening 771 citations, 100 records underwent full eligibility screening of which 38 studies met eligibility and were included in the review: Studies were grouped into pre-clinical studies that involved statin treatment in combination with cell therapy (18 studies), preclinical studies of statin therapy alone (13 studies) and clinical studies of statin therapy (7 studies). Studies addressed cardiac injury (14 studies), vascular disorders (15 studies), neurologic conditions (8 studies) and bone fractures (1 study). Pre-clinical studies of statins in combination with MSC infusion (15 studies) or EPC therapy (3 studies) were described and despite marked heterogeneity in reporting outcomes of cellular analysis and organ function, all of these cell-based pre-clinical studies reported improved organ recovery with the addition of statin therapy. Moreover, 13 pre-clinical studies involved the administration of a statin drug alone to animals. An increase in EPC number and/or function (no studies of MSCs) was reported in 11 of these studies (85 %) and improved organ function in 12 studies (92 %). We also identified 7 clinical studies and none involved the administration of cells but described an increased number and/or function of EPCs (no studies of MSCs) and improved organ function with statin therapy (1.2-fold to 35-fold improvement over controls) in all 7 studies.

CONCLUSION

Our systematic review provides a foundation of encouraging results that support further study of statins in regenerative therapy to augment the number and/or function of MSCs used in cell-based repair and to augment the number and function of EPCs in vivo to repair damaged tissues. Larger studies are needed to ensure safety and confirm clinical benefits.

Concise Review: Endothelial Progenitor Cells in Regenerative Medicine: Applications and Challenges

Endothelial progenitor cells (EPCs) are currently being studied as candidate cell sources for revascularization strategies. Significant advances have been made in understanding the biology of EPCs, and preclinical studies have demonstrated the vasculogenic, angiogenic, and beneficial paracrine effects of transplanted EPCs in the treatment of ischemic diseases. Despite these promising results, widespread clinical acceptance of EPCs for clinical therapies remains hampered by several challenges. The present study provides a concise summary of the different EPC populations being studied for ischemic therapies and their known roles in the healing of ischemic tissues. The challenges and issues surrounding the use of EPCs and the current strategies being developed to improve the harvest efficiency and functionality of EPCs for application in regenerative medicine are discussed.

SIGNIFICANCE

Endothelial progenitor cells (EPCs) have immense clinical value for cardiovascular therapies. The present study provides a concise description of the EPC subpopulations being evaluated for clinical applications. The current major lines of investigation involving preclinical and clinical evaluations of EPCs are discussed, and significant gaps limiting the translation of EPCs are highlighted. The present report could be useful for clinicians and clinical researchers with interests in ischemic therapy and for basic scientists working in the related fields of tissue engineering and regenerative medicine.

Tuning of collagen scaffold properties modulates embedded endothelial cell regulatory phenotype in repair of vascular injuries in vivo

Perivascularly implanted matrix embedded endothelial cells (MEECs) are potent regulators of inflammation and intimal hyperplasia following vascular injuries. Endothelial cells (ECs) in collagen scaffolds adopt a reparative phenotype with significant therapeutic potential. Although the biology of MEECs is increasingly understood, tuning of scaffold properties to control cell-substrate interactions is less well-studied. It is hypothesized that modulating scaffold degradation would change EC phenotype. Scaffolds with differential degradation are prepared by cross-linking and predegradation. Vascular injury increases degradation and the presence of MEECs retards injury-mediated degradation. MEECs respond to differential scaffold properties with altered viability in vivo, suppressed smooth muscle cell (SMC) proliferation in vitro, and altered interleukin-6 and matrix metalloproteinase-9 expression. When implanted perivascularly to a murine carotid wire injury, tuned scaffolds change MEEC effects on vascular repair and inflammation. Live animal imaging enables real-time tracking of cell viability, inflammation, and scaffold degradation, affording an unprecedented understanding of interactions between cells, substrate, and tissue. MEEC-treated injuries improve endothelialization and reduce SMC hyperplasia over 14 d. These data demonstrate the potent role material design plays in tuning MEEC efficacy in vivo, with implications for the design of clinical therapies.

Enhanced Viability of Endothelial Colony Forming Cells in Fibrin Microbeads for Sensor Vascularization

Enhanced vascularization at sensor interfaces can improve long-term function. Fibrin, a natural polymer, has shown promise as a biomaterial for sensor coating due to its ability to sustain endothelial cell growth and promote local vascularization. However, the culture of cells, particularly endothelial cells (EC), within 3D scaffolds for more than a few days is challenging due to rapid loss of EC viability. In this manuscript, a robust method for developing fibrin microbead scaffolds for long-term culture of encapsulated ECs is described. Fibrin microbeads are formed using sodium alginate as a structural template. The size, swelling and structural properties of the microbeads were varied with needle gauge and composition and concentration of the pre-gel solution. Endothelial colony-forming cells (ECFCs) were suspended in the fibrin beads and cultured within a perfusion bioreactor system. The perfusion bioreactor enhanced ECFCs viability and genome stability in fibrin beads relative to static culture. Perfusion bioreactors enable 3D culture of ECs within fibrin beads for potential application as a sensor coating.

Scarring, stem cells, scaffolds and skin repair

The treatment of full thickness skin loss, which can be extensive in the case of large burns, continues to represent a challenging clinical entity. This is due to an on-going inability to produce a suitable tissue engineered substrate that can satisfactorily replicate the epidermal and dermal in vivo niches to fulfil both aesthetic and functional demands. The current gold standard treatment of autologous skin grafting is inadequate because of poor textural durability, scarring and associated contracture, and because of a paucity of donor sites in larger burns. Tissue engineering has seen exponential growth in recent years with a number of 'off-the-shelf' dermal and epidermal substitutes now available. Each has its own limitations. In this review, we examine normal wound repair in relation to stem/progenitor cells that are intimately involved in this process within the dermal niche. Endothelial precursors, in particular, are examined closely and their phenotype, morphology and enrichment from multiple sources are described in an attempt to provide some clarity regarding the controversy surrounding their classification and role in vasculogenesis. We also review the role of the next generation of cellularized scaffolds and smart biomaterials that attempt to improve the revascularisation of artificial grafts, the rate of wound healing and the final cosmetic and functional outcome.

Dysfunction of endothelial progenitor cells from smokers and chronic obstructive pulmonary disease patients due to increased DNA damage and senescence

Cardiovascular disease (CVD) is a major cause of death in smokers, particularly in those with chronic obstructive pulmonary disease (COPD). Circulating endothelial progenitor cells (EPC) are required for endothelial homeostasis, and their dysfunction contributes to CVD. To investigate EPC dysfunction in smokers, we isolated and expanded blood outgrowth endothelial cells (BOEC) from peripheral blood samples from healthy nonsmokers, healthy smokers, and COPD patients. BOEC from smokers and COPD patients showed increased DNA double-strand breaks and senescence compared to nonsmokers. Senescence negatively correlated with the expression and activity of sirtuin-1 (SIRT1), a protein deacetylase that protects against DNA damage and cellular senescence. Inhibition of DNA damage response by silencing of ataxia telangiectasia mutated (ATM) kinase resulted in upregulation of SIRT1 expression and decreased senescence. Treatment of BOEC from COPD patients with the SIRT1 activator resveratrol or an ATM inhibitor (KU-55933) also rescued the senescent phenotype. Using an in vivo mouse model of angiogenesis, we demonstrated that senescent BOEC from COPD patients are dysfunctional, displaying impaired angiogenic ability and increased apoptosis compared to cells from healthy nonsmokers. Therefore, this study identifies epigenetic regulation of DNA damage and senescence as pathogenetic mechanisms linked to endothelial progenitors' dysfunction in smokers and COPD patients. These defects may contribute to vascular disease and cardiovascular events in smokers and could therefore constitute therapeutic targets for intervention.

miRNome traits analysis on endothelial lineage cells discloses biomarker potential circulating microRNAs which affect progenitor activities

Background

Endothelial progenitor cells (EPCs) play a fundamental role in not only blood vessel development but also post-natal vascular repair. Currently EPCs are defined as early and late EPCs based on their biological properties and their time of appearance during in vitro culture. Both EPC types assist angiogenesis and have been linked to ischemia-related disorders, including coronary artery disease (CAD).

Results

We found late EPCs are more mobile than early EPCs and matured endothelial cells (ECs). To pinpoint the mechanism, microRNA profiles of early EPCs late EPCs, and ECs were deciphered by small RNA sequencing. Obtained signatures made up of both novel and known microRNAs, in which anti-angiogenic microRNAs such as miR-221 and miR-222 are more abundant in matured ECs than in late EPCs. Overexpression of miR-221 and miR-222 resulted in the reduction of genes involved in hypoxia response, metabolism, TGF-beta signalling, and cell motion. Not only hamper late EPC activities in vitro, both microRNAs (especially miR-222) also hindered in vivo vasculogenesis in a zebrafish model. Reporter assays showed that miR-222, but not miR-221, targets the angiogenic factor ETS1. In contrast, PIK3R1 is the target of miR-221, but not miR-222 in late EPCs. Clinically, both miR-221-PIK3R1 and miR-222-ETS1 pairs are deregulated in late EPCs of CAD patients.

Conclusions

Our results illustrate EPCs and ECs exploit unique miRNA modalities to regulate angiogenic features, and explain why late EPC levels and activities are reduced in CAD patients. These data will further help to develop new plasma biomarkers and therapeutic approaches for ischemia-related diseases or tumor angiogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-802) contains supplementary material, which is available to authorized users.

Role of biomechanical forces in stem cell vascular lineage differentiation

Mechanical forces have long been known to play a role in the maintenance of vascular homeostasis in the mature animal and in developmental regulation in the fetus. More recently, it has been shown that stem cells play a role in vascular repair and remodeling in response to biomechanical stress. Laminar shear stress can directly activate growth factor receptors on stem/progenitor cells, initiating signaling pathways leading toward endothelial cell differentiation. Cyclic strain can stimulate stem cell differentiation toward smooth muscle lineages through different mechanisms. In vivo, blood flow in the coronary artery is significantly altered after stenting, leading to changes in biomechanical forces on the vessel wall. This disruption may activate stem cell differentiation into a variety of cells and cause delayed re-endothelialization. Based on progress in the research field, the present review aims to explore the role of mechanical forces in stem cell differentiation both in vivo and in vitro and to examine what this means for the application of stem cells in the clinic, in tissue engineering, and for the management of aberrant stem cell contribution to disease.

The transcription factor MEF2C negatively controls angiogenic sprouting of endothelial cells depending on oxygen

The MADS box transcription factor MEF2C has been detected by us to be upregulated by the angiogenic factors VEGF-A and bFGF in endothelial cells. We have here investigated its potential role for angiogenesis. MEF2C was surprisingly found to strongly inhibit angiogenic sprouting, whereas a dominant negative mutant rather induced sprouting. The factor mainly affected migratory processes of endothelial cells, but not proliferation. In gene profiling experiments we delineated the alpha-2-macroglobulin gene to be highly upregulated by MEF2C. Further data confirmed that MEF2C in endothelial cells indeed induces alpha-2-macroglobulin mRNA as well as the secretion of alpha-2-macroglobulin and that conditioned supernatants of cells overexpressing MEF2C inhibit sprouting. Alpha-2-macroglobulin mediates, at least to a large extent, the inhibitory effects of MEF2C as is shown by knockdown of alpha-2-macroglobulin mRNA by lentiviral shRNA expression which reduces the inhibitory effect. However, under hypoxic conditions the VEGF-A/bFGF-mediated upregulation of MEF2C is reduced and the production of alpha-2-macroglobulin largely abolished. Taken together, this suggests that the MEF2C/alpha-2-macroglobulin axis functions in endothelial cells as a negative feed-back mechanism that adapts sprouting activity to the oxygen concentration thus diminishing inappropriate and excess angiogenesis.

Differential gene expression in Lin-/VEGF-R2+ bone marrow-derived endothelial progenitor cells isolated from diabetic mice

BACKGROUND

Diabetes is known to impair the number and function of endothelial progenitor cells in the circulation, causing structural and functional alterations in the micro- and macro-vasculature. The aim of this study was to identify early diabetes-related changes in the expression of genes that have been reported to be closely involved in endothelial progenitor cell migration and function.

METHODS

Based on review of current literature, this study examined the expression level of 35 genes that are known to be involved in endothelial progenitor cell migration and function in magnetically sorted Lin-/VEGF-R2+ endothelial progenitor cells obtained from the bone marrow of Akita mice in the early stages of diabetes (18 weeks) using RT-PCR and Western blotting. We used the Shapiro-Wilk and D'Agostino & Pearson Omnibus tests to assess normality. Differences between groups were evaluated by Student's t-test for normally distributed data (including Welch correction in cases of unequal variances) or Mann-Whitney test for not normally distributed data.

RESULTS

We observed a significant increase in the number of Lin-/VEGF-R2+ endothelial progenitor cells within the bone marrow in diabetic mice compared with non-diabetic mice. Two genes, SDF-1 and SELE, were significantly differentially expressed in diabetic Lin-/VEGF-R2+ endothelial progenitor cells and six other genes, CAV1, eNOS, CLDN5, NANOG, OCLN and BDNF, showed very low levels of expression in diabetic Lin-/VEGF-R2+ progenitor cells.

CONCLUSION

Low SDF-1 expression may contribute to the dysfunctional mobilization of bone marrow Lin-/VEGF-R2+ endothelial progenitor cells, which may contribute to microvascular injury in early diabetes.

Matrix rigidity regulates spatiotemporal dynamics of Cdc42 activity and vacuole formation kinetics of endothelial colony forming cells

Recent evidence has shown that endothelial colony forming cells (ECFCs) may serve as a cell therapy for improving blood vessel formation in subjects with vascular injury, largely due to their robust vasculogenic potential. The Rho family GTPase Cdc42 is known to play a primary role in this vasculogenesis process, but little is known about how extracellular matrix (ECM) rigidity affects Cdc42 activity during the process. In this study, we addressed two questions: Does matrix rigidity affect Cdc42 activity in ECFC undergoing early vacuole formation? How is the spatiotemporal activation of Cdc42 related to ECFC vacuole formation? A fluorescence resonance energy transfer (FRET)-based Cdc42 biosensor was used to examine the effects of the rigidity of three-dimensional (3D) collagen matrices on spatiotemporal activity of Cdc42 in ECFCs. Collagen matrix stiffness was modulated by varying the collagen concentration and therefore fibril density. The results showed that soft (150 Pa) matrices induced an increased level of Cdc42 activity compared to stiff (1 kPa) matrices. Time-course imaging and colocalization analysis of Cdc42 activity and vacuole formation revealed that Cdc42 activity was colocalized to the periphery of cytoplasmic vacuoles. Moreover, soft matrices generated faster and larger vacuoles than stiff matrices. The matrix-driven vacuole formation was enhanced by a constitutively active Cdc42 mutant, but significantly inhibited by a dominant-negative Cdc42 mutant. Collectively, the results suggest that matrix rigidity is a strong regulator of Cdc42 activity and vacuole formation kinetics, and that enhanced activity of Cdc42 is an important step in early vacuole formation in ECFCs.

Preparation and characterization of circulating angiogenic cells for tissue engineering applications

Circulating angiogenic cells (CACs) are a heterogeneous cell population of bone marrow (BM) origin. These cells are most commonly derived from the peripheral blood, bone marrow, and cord blood, and are one of the leading candidates for promoting vascularization in tissue engineering therapies. CACs can be isolated by culturing peripheral blood mononuclear cells (PBMCs) on fibronectin or by flow cytometry to obtain more specific subpopulations. Here we will describe how to generate a population of CACs, and how to characterize the cells and confirm their phenotype. Also, we will provide select methods that can be used to assess the angiogenic and endothelial cell-like properties of the CACs.

Development and in vivo evaluation of small-diameter vascular grafts engineered by outgrowth endothelial cells and electrospun chitosan/poly(ε-caprolactone) nanofibrous scaffolds

Successful engineering of a small-diameter vascular graft is still a challenge despite numerous attempts for decades. The present study aimed at developing a tissue-engineered vascular graft (TEVG) using autologous outgrowth endothelial cells (OECs) and a hybrid biodegradable polymer scaffold. OECs were harvested from canine peripheral blood and proliferated in vitro, as well as identified by immunofluorescent staining. Electrospun hybrid chitosan/poly(ε-caprolactone) (CS/PCL) nanofibers were fabricated and served as vascular scaffolds. TEVGs were constructed in vitro by seeding OECs onto CS/PCL scaffolds, and then implanted into carotid arteries of cell-donor dogs (n=6). After 3 months of implantation, 5 out of 6 of TEVGs remained patent as compared with 1 out of 6 of unseeded grafts kept patent. Histological and immunohistochemical analyses of the TEVGs retrieved at 3 months revealed the regeneration of endothelium, and the presence of collagen and elastin. OECs labeled with fluorescent dye before implantation were detected in the retrieved TEVGs, indicating that the OECs participated in the vascular tissue regeneration. Biomechanical testing of TEVGs showed good mechanical properties that were closer to native carotid arteries. RT-PCR and western blot analysis demonstrated that TEVGs had favorable biological functional properties resembling native arteries. Overall, this study provided a new strategy to develop small-diameter TEVGs with excellent biocompatibility and regeneration ability.

How to utilize Ca²⁺ signals to rejuvenate the repairative phenotype of senescent endothelial progenitor cells in elderly patients affected by cardiovascular diseases: a useful therapeutic support of surgical approach?

Endothelial dysfunction or loss is the early event that leads to a host of severe cardiovascular diseases, such as atherosclerosis, hypertension, brain stroke, myocardial infarction, and peripheral artery disease. Ageing is regarded among the most detrimental risk factor for vascular endothelium and predisposes the subject to atheroscleorosis and inflammatory states even in absence of traditional comorbid conditions. Standard treatment to restore blood perfusion through stenotic arteries are surgical or endovascular revascularization. Unfortunately, ageing patients are not the most amenable candidates for such interventions, due to high operative risk or unfavourable vascular involvement. It has recently been suggested that the transplantation of autologous bone marrow-derived endothelial progenitor cells (EPCs) might constitute an alternative and viable therapeutic option for these individuals. Albeit pre-clinical studies demonstrated the feasibility of EPC-based therapy to recapitulate the diseased vasculature of young and healthy animals, clinical studies provided less impressive results in old ischemic human patients. One hurdle associated to this kind of approach is the senescence of autologous EPCs, which are less abundant in peripheral blood and display a reduced pro-angiogenic activity. Conversely, umbilical cord blood (UCB)-derived EPCs are more suitable for cellular therapeutics due to their higher frequency and sensitivity to growth factors, such as vascular endothelial growth factor (VEGF). An increase in intracellular Ca²⁺ concentration is central to EPC activation by VEGF. We have recently demonstrated that the Ca²⁺ signalling machinery driving the oscillatory Ca²⁺ response to this important growth factor is different in UCB-derived EPCs as compared to their peripheral counterparts. In particular, we focussed on the so-called endothelial colony forming cells (ECFCs), which are the only EPC population belonging to the endothelial lineage and able to form capillary-like structures in vitro and stably integrate with host vasculature in vivo. The present review provides a brief description of how exploiting the Ca²⁺ toolkit of juvenile EPCs to restore the repairative phenotype of senescent EPCs to enhance their regenerative outcome in therapeutic settings.

Human proangiogenic circulating hematopoietic stem and progenitor cells promote tumor growth in an orthotopic melanoma xenograft model

We previously identified a distinct population of human circulating hematopoietic stem and progenitor cells (CHSPCs; CD14(-)glyA(-)CD34(+)AC133(+/-)CD45(dim)CD31(+) cells) in the peripheral blood (PB) and bone marrow, and their frequency in the PB can correlate with disease state. The proangiogenic subset (pCHSPC) play a role in regulating tumor progression, for we previously demonstrated a statistically significant increase in C32 melanoma growth in NOD.Cg-Prkdc (scid) (NOD/SCID) injected with human pCHSPCs (p < 0.001). We now provide further evidence that pCHSPCs possess proangiogenic properties. In vitro bio-plex cytokine analyses and tube forming assays indicate that pCHSPCs secrete a proangiogenic profile and promote vessel formation respectively. We also developed a humanized bone marrow-melanoma orthotopic model to explore in vivo the biological significance of the pCHSPC population. Growth of melanoma xenografts increased more rapidly at 3-4 weeks post-tumor implantation in mice previously transplanted with human CD34(+) cells compared to control mice. Increases in pCHSPCs in PB correlated with increases in tumor growth. Additionally, to determine if we could prevent the appearance of pCHSPCs in the PB, mice with humanized bone marrow-melanoma xenografts were administered Interferon α-2b, which is used clinically for treatment of melanoma. The mobilization of the pCHSPCs was decreased in the mice with the humanized bone marrow-melanoma xenografts. Taken together, these data indicate that pCHSPCs play a functional role in tumor growth. The novel in vivo model described here can be utilized to further validate pCHSPCs as a biomarker of tumor progression. The model can also be used to screen and optimize anticancer/anti-angiogenic therapies in a humanized system.

Canonical transient receptor potential 3 channel triggers vascular endothelial growth factor-induced intracellular Ca2+ oscillations in endothelial progenitor cells isolated from umbilical cord blood

Endothelial colony-forming cells (ECFCs) are the only endothelial progenitor cells (EPCs) that are capable of acquiring a mature endothelial phenotype. ECFCs are mainly mobilized from bone marrow to promote vascularization and represent a promising tool for cell-based therapy of severe ischemic diseases. Vascular endothelial growth factor (VEGF) stimulates the proliferation of peripheral blood-derived ECFCs (PB-ECFCs) through oscillations in intracellular Ca(2+) concentration ([Ca(2+)]i). VEGF-induced Ca(2+) spikes are driven by the interplay between inositol-1,4,5-trisphosphate (InsP3)-dependent Ca(2+) release and store-operated Ca(2+) entry (SOCE). The therapeutic potential of umbilical cord blood-derived ECFCs (UCB-ECFCs) has also been shown in recent studies. However, VEGF-induced proliferation of UCB-ECFCs is faster compared with their peripheral counterpart. Unlike PB-ECFCs, UCB-ECFCs express canonical transient receptor potential channel 3 (TRPC3) that mediates diacylglycerol-dependent Ca(2+) entry. The present study aimed at investigating whether the higher proliferative potential of UCB-ECFCs was associated to any difference in the molecular underpinnings of their Ca(2+) response to VEGF. We found that VEGF induces oscillations in [Ca(2+)]i that are patterned by the interaction between InsP3-dependent Ca(2+) release and SOCE. Unlike PB-ECFCs, VEGF-evoked Ca(2+) oscillations do not arise in the absence of extracellular Ca(2+) entry and after pharmacological (with Pyr3 and flufenamic acid) and genetic (by employing selective small interference RNA) suppression of TRPC3. VEGF-induced UCB-ECFC proliferation is abrogated on inhibition of the intracellular Ca(2+) spikes. Therefore, the Ca(2+) response to VEGF in UCB-ECFCs is shaped by a different Ca(2+) machinery as compared with PB-ECFCs, and TRPC3 stands out as a promising target in EPC-based treatment of ischemic pathologies.

Comparison of Fibronectin and Collagen in Supporting the Isolation and Expansion of Endothelial Progenitor Cells from Human Adult Peripheral Blood

Background

Endothelial colony-forming cells (ECFCs), are circulating endothelial progenitor cells increasingly studied in various diseases because of their potential for clinical translation. Experimental procedures for their ex vivo culture still lack standardization. In particular two different extracellular matrix proteins, either fibronectin or collagen, are commonly used by different Authors for coating plastic plates, both allowing to obtain cells that have all the features of ECFCs. However, possible differences in the impact of each substrate on ECFCs have not been analysed, so far. Therefore, in this study we investigated whether fibronectin and collagen may differentially affect ECFC cultures.

Methodology/Principal Findings

ECFCs were isolated and cultured from peripheral blood mononuclear cells of healthy donors. The impact of fibronectin compared with collagen as the only variable of the experimental procedure was analysed separately in the phase of isolation of ECFC colonies and in the following phase of cell expansion. In the isolation phase, although similar frequencies of colonies were obtained on the two substrates, ECFC colonies appeared some days earlier when mononuclear cells were seeded on fibronectin rather than collagen. In the expansion phase, ECFCs cultured on collagen showed a longer lifespan and higher cell yields compared with ECFCs cultured on fibronectin, possibly related to the higher levels of IL-6 and IL-8 measured in their supernatants. ECFCs cultured on both substrates showed similar immunophenotype and ability for in vitro tube formation.

Conclusions/Significance

Overall, the results of this study indicate that, although both fibronectin and collagen efficiently sustain ECFC cultures, each of them brings some advantages within individual steps of the entire process. We suggest that colony isolation performed on fibronectin followed by cell expansion performed on collagen may represent a novel and the most efficient strategy to obtain ECFCs from adult peripheral blood samples.

Isolation and characterization of mouse bone marrow-derived Lin⁻/VEGF-R2⁺ progenitor cells

Circulating endothelial progenitor cells (EPCs) in the peripheral blood (PB) have physiological roles in the maintenance of the existing vascular beds and rescue of vascular injury. In this study, we have evaluated the properties of Lin⁻/VEGF-R2⁺ progenitor cells isolated from the mouse bone marrow (BM) and further studied their distribution and integration in an animal model of laser-induced retinal vascular injury. Lin⁻/VEGF-R2⁺ cells were enriched from C57BL/6 mice BM using magnetic cell sorting with hematopoietic lineage (Lin) depletion followed by VEGF-R2 positive selection. Lin⁻/VEGF-R2⁺ BM cells were characterized using flow cytometry and immunocytochemistry and further tested for colony formation during culture and tube formation on Matrigel®. Lin⁻/VEGF-R2⁺ BM cells possessed typical EPC properties such as forming cobble-stone shaped colonies after 3 to 4 weeks of culture, CD34⁺ expression, take up of Dil-acLDL and binding to Ulex europaeus agglutinin. However, they did not form tube-like structures on Matrigel®. The progenitor cells retained their phenotype over extended period of culture. After intravitreal transplantation in eyes subjected to the laser-induced retinal vascular injury, some Lin⁻/VEGF-R2⁺ cells were able to integrate into the damaged retinal vasculature but the level of cell integration seemed less efficient when compared with previous reports in which EPCs from the human PB were employed. Our results indicate that Lin⁻/VEGF-R2⁺ cells isolated from the mouse BM share some similarities to EPCs from the human PB but most of them are at a very early stage of maturation and remain quiescent during culture and after intravitreal transplantation.

Diabetes impairs mobilization of mouse bone marrow-derived Lin(-)/VEGF-R2(+) progenitor cells

Endothelial progenitor cells circulating in the peripheral blood (PB) contribute to vascular repair. This study aimed to evaluate the potential of a 'cocktail' consisting of erythropoietin, granulocyte colony-stimulating factor and tetrahydrobiopterin to mobilize hematopoietic lineage negative/vascular endothelial growth factor receptor 2 positive (Lin(-)/VEGF-R2(+)) cells from the bone marrow (BM) to PB in non-diabetic and diabetic mice. Diabetes was induced in mice by intraperitoneal injection of streptozotocin. Diabetic mice were studied after 16weeks of hyperglycemia. Half the mice in each group (non-diabetic and diabetic) received daily intraperitoneal injections of the cocktail for 6 consecutive days while the other half received vehicle buffer. Mobilization of Lin(-)/VEGF-R2(+) cells, which were expanded in MCP301 medium, was evaluated after isolating them from BM and PB and their phenotypic and morphological properties were studied. We found that 16weeks of diabetes affected neither the total number of BM mononucleated cells nor the number of Lin(-)/VEGF-R2(+) cells in BM compared with non-diabetic controls. In non-diabetic mice, cocktail treatment resulted in a significant decrease in BM Lin(-)/VEGF-R2(+) cells, paralleled by a significant increase of these cells in PB. Such changes in the number of Lin(-)/VEGF-R2(+) cells in BM and PB after the cocktail treatment were less marked in diabetic mice. In vitro studies of BM Lin(-)/VEGF-R2(+) cells from diabetic and non-diabetic mice did not reveal any differences in either phenotypes or colony forming potential. These findings indicate that diabetes impairs the mobilization of Lin(-)/VEGF-R2(+) cells from BM to PB. Impaired mobilization of BM Lin(-)/VEGF-R2(+) cells soon after the onset of diabetes may contribute to complications such as diabetic retinopathy.

Apolipoprotein (a) impairs endothelial progenitor cell-mediated angiogenesis

Improvement of blood flow and promotion of angiogenesis are important therapeutic measures for the treatment of ischemic peripheral vascular diseases. Since apolipoprotein (a) (apo (a)) is a glycoprotein with repetitive kringle domains exhibiting 75% to 98% structural homology with plasminogen (Plg), apo (a) may also have a negative effect on endothelial progenitor cell (EPC)-induced angiogenesis through Plg-like inhibitory effects on EPC proliferation, adhesion, migration, and angiogenesis. To evaluate the effect of apo (a) on EPCs-induced angiogenesis, EPCs were isolated from the bone marrow of apo (a) transgenic mice, wild-type litter mates, and normal mice. These cells were cultured without or with apo (a) before transplantation. Hindlimb ischemia models were surgically induced in mice, which then received an intravenous injection of 3×10(5) EPCs. At 3, 7, and 14 days post EPC transplantation, the adhesion, migration abilities, and capillary density in calf muscles were assessed. Results indicate that apo (a) significantly reduced the adhesion and migration abilities of EPCs. Furthermore, the tubule-like formation of EPCs on Matrigel gels was damaged. In vivo experiments showed the homing of EPCs to ischemic peripheral vascular, and the number of capillary vessels decreased significantly in apo(a) transgenic mice. This study demonstrated that apo (a) could attenuate the adhesion, migration, and homing abilities of EPCs and could impair the angiogenesis ability of EPCs.

Dynamic shear stress regulation of inflammatory and thrombotic pathways in baboon endothelial outgrowth cells

Endothelial outgrowth cells (EOCs) have garnered much attention as a potential autologous endothelial source for vascular implants or in tissue engineering applications due to their ease of isolation and proliferative ability; however, how these cells respond to different hemodynamic cues is ill-defined. This study investigates the inflammatory and thrombotic response of baboon EOCs (BaEOCs) to four hemodynamic conditions using the cone and plate shear apparatus: steady, laminar shear stress (SS); pulsatile, nonreversing laminar shear stress (PS); oscillatory, laminar shear stress (OS); and net positive, pulsatile, reversing laminar shear stress (RS). In summary, endothelial nitric oxide synthase (eNOS) mRNA was significantly upregulated by SS compared to OS. No differences were found in the mRNA levels of the inflammatory markers intercellular adhesion molecule-1 (ICAM-1), E-selectin, and vascular cell adhesion molecule-1 (VCAM-1) between the shear conditions; however, OS significantly increased the number of monocytes bound when compared to SS. Next, SS increased the anti-thrombogenic mRNA levels of CD39, thrombomodulin, and endothelial protein-C receptor (EPCR) compared to OS. SS also significantly increased CD39 and EPCR mRNA levels compared to RS. Finally, no significant differences were detected when comparing pro-thrombotic tissue factor mRNA or its activity levels. These results indicate that shear stress can have beneficial (SS) or adverse (OS, RS) effects on the inflammatory or thrombotic potential of EOCs. Further, these results suggest SS hemodynamic preconditioning may be optimal in increasing the efficacy of a vascular implant or in tissue-engineered applications that have incorporated EOCs.

Recruitment of human cord blood-derived endothelial colony-forming cells to sites of tumor angiogenesis

BACKGROUND AIMS

Endothelial progenitor cells (EPCs) specifically home to sites of malignant growth, rendering them attractive for anti-cancer therapies. Data are conflicting on the phenotype and quantitative contribution toward tumor angiogenesis based on differing culture assays to outgrow EPCs. To evaluate the origin and early phenotype of EPCs and to define a population with enhanced tumor-targeting capacity, we evaluated a hierarchy of cord blood-derived EPCs modeling the multi-step nature of tumor homing.

METHODS

CD34(+) mononuclear cells were isolated from fresh cord blood and cultured to derive endothelial colony-forming cells (ECFCs). Human umbilical vein endothelial cells (HUVECs) served as control. Using intra-vital microscopy, the recruitment was analyzed in mice bearing C6 xenografts. Adhesion, migration, transmigration and differentiation were further addressed.

RESULTS

Within the primary passage, ECFCs underwent a rapid maturation from a CD45(+) and CD31(+) phenotype to a CD45(-) and endothelial marker positive phenotype. Assessing in vivo tumor recruitment, ECFCs had the highest activity in all steps analyzed. In vitro, ECFCs demonstrated significantly higher adhesion under static and flow conditions. Similarly, ECFCs exhibited highest migratory and trans-migratory activity toward tumor-conditioned medium. On subcutaneous implantation, only ECFCs formed blood vessels covered with perivascular cells, similar to HUVECs.

CONCLUSIONS

Our study indicates that ECFCs emerge from a CD45(+) and CD31(+) progenitor and rapidly mature in culture. ECFCs have a significantly higher potential for tumor targeting than non-cultured CD34(+) cells and HUVECs. They are ideal candidates for future cell-based anti-cancer therapies.

Alginate-based strategies for therapeutic vascularization

Therapeutic stimulation of vessel growth to improve tissue perfusion has shown promise in many regenerative medicine and tissue engineering applications. Alginate-based biomaterial systems have been investigated for growth factor and/or cell delivery as tools for modulating vessel assembly. Growth factor encapsulation allows for a sustained release of protein and protection from degradation. Implantation of growth factor-loaded alginate constructs typically shows an increase in capillary density but without vascular stabilization. Delivery of multiple factors may improve these outcomes. Cell delivery approaches focus on stimulating vascularization either via cell release of soluble factors, cell proliferation and incorporation into new vessels or alginate prevascularization prior to implantation. These methods have shown some promise but routine clinical application has not been achieved. In this review, current research on the application of alginate for therapeutic neovascularization is presented, shortcomings are addressed and the future direction of these systems discussed.

Human endothelial colony-forming cells expanded with an improved protocol are a useful endothelial cell source for scaffold-based tissue engineering

One of the major challenges in tissue engineering is to supply larger three-dimensional (3D) bioengineered tissue transplants with sufficient amounts of nutrients and oxygen and to allow metabolite removal. Consequently, artificial vascularization strategies of such transplants are desired. One strategy focuses on endothelial cells capable of initiating new vessel formation, which are settled on scaffolds commonly used in tissue engineering. A bottleneck in this strategy is to obtain sufficient amounts of endothelial cells, as they can be harvested only in small quantities directly from human tissues. Thus, protocols are required to expand appropriate cells in sufficient amounts without interfering with their capability to settle on scaffold materials and to initiate vessel formation. Here, we analysed whether umbilical cord blood (CB)-derived endothelial colony-forming cells (ECFCs) fulfil these requirements. In a first set of experiments, we showed that marginally expanded ECFCs settle and survive on different scaffold biomaterials. Next, we improved ECFC culture conditions and developed a protocol for ECFC expansion compatible with 'Good Manufacturing Practice' (GMP) standards. We replaced animal sera with human platelet lysates and used a novel type of tissue-culture ware. ECFCs cultured under the new conditions revealed significantly lower apoptosis and increased proliferation rates. Simultaneously, their viability was increased. Since extensively expanded ECFCs could still settle on scaffold biomaterials and were able to form tubular structures in Matrigel assays, we conclude that these ex vivo-expanded ECFCs are a novel, very potent cell source for scaffold-based tissue engineering.

Differential expression of Tie2 receptor and VEGFR2 by endothelial clones derived from isolated bovine mononuclear cells

The purpose of these experiments was to evaluate the expression of endothelial markers, such as Tie2 and VEGFR2 in endothelial cells derived from blood mononuclear endothelial progenitor cells. Bovine mononuclear cells were isolated using separation by centrifugation and were grown in endothelial specific media supplemented with growth factors. Isolation of the whole cell population of mononuclear cells (MNC) from bovine peripheral blood gave rise to progenitor-like cells (CD45⁻) that, although morphologically similar, have different phenotypes revealed by expression of endothelial specific markers Tie2 and VEGFR2. Plating of MNCs on collagen and fibronectin gave rise to more colonies than non-coated dishes. Occasional colonies from MNC isolations had a mural cell phenotype, negative for Tie2 and VEGFR2 but positive for smooth muscle actin and PDGFRβ. Although cells expressing high levels of VEGFR2 and low levels of Tie2, and vice versa were both able to form cords on Matrigel, cells with higher expression of Tie2 migrate faster in a scratch assay than ones with lower expression of Tie2. When these different clones of cells were introduced in mice through tail vein injections, they retained an ability to home to angiogenesis occurring in a subcutaneous Matrigel plug, regardless of their Tie2/VEGFR2 receptor expression patterns, but cells with high VEGFR2/low Tie2 were more likely to be CD31 positive. Therefore, we suggest that active sites of angiogenesis (such as wounds, tumors, etc.) can attract a variety of endothelial cell precursors that may differentially express Tie2 and VEGFR2 receptors, and thus affect our interpretation of EPCs as biomarkers or therapies for vascular disease.

Meeting the need for regenerative therapies: translation-focused analysis of U.S. regenerative medicine opportunities in cardiovascular and peripheral vascular medicine using detailed incidence data

Cardiac and vascular diseases represent one of the most substantial medical areas for the applications of regenerative medicine. Despite advances in endovascular repair, surgical intervention, and disease management, atherosclerosis and heart failure continue to be prominent health problems. This report analyzes the regenerative medicine treatment opportunities in both cardiovascular and peripheral vascular repair, examining the treatment opportunities for tissue-engineered vascular grafts as well as cell-based therapies. U.S. hospital discharge data were used to generate a detailed estimate of the relative target populations for cardiac and vascular disease. Gap analyses were performed for vascular access, small caliber vascular grafts, and cell-based therapies for revascularization and heart failure. The analysis compared current alternatives, gaps in medical need, and what a tissue-engineered or regenerative alternative should achieve for optimum medical and commercial feasibility. Although the number of coronary bypass grafts vastly outnumbered peripheral grafts, a detailed consideration of re-grafts and the success of first grafts combined with gap analysis (GAP) leads us to conclude that peripheral vascular disease is the more commercially feasible and attractive target opportunity for engineered small caliber grafts for the foreseeable future. Cardiac bypass would need substantial long-term clinical experience, which could be a significant hurdle. Vascular access, often regarded as a first-in-man indication, is an excellent opportunity for an engineered graft as an alternative to arteriovenous fistula that could overcome complications associated with a prosthetic graft. The GAP also suggests that for heart failure, cellular therapies should link near-term changes in repair, such as improvement in cardiac output and reduced scarring with limiting progression of the disease, reducing the need for complex pharmacologic management, and reducing rates of hospitalization. Naturally, researchers must determine where their technology and know-how can be applied most effectively, but it is clear from our analysis that an astute strategy in the use of science and technology will be important to successful translation in this space.

The extracellular matrix is a novel attribute of endothelial progenitors and of hypoxic mature endothelial cells

Extracellular matrix (ECM) production is critical to preserve the function and integrity of mature blood vessels. Toward the engineering of blood vessels, studies have centered on ECM production by supporting cells, whereas few studies implicate endothelial cells (ECs) with ECM synthesis. Here, we elucidate variations between cultured human arterial, venous, and progenitor ECs with respect to ECM deposition assembly, composition, and response to biomolecular and physiological factors. Our studies reveal that progenitor ECs, endothelial colony-forming cells (ECFCs), deposit collagen IV, fibronectin, and laminin that assemble to an organized weblike structure, as confirmed by decellularized cultures. Mature ECs only express these ECM proteins intracellularly. ECFC-derived ECM is abrogated in response to TGFβ signaling inhibition and actin cytoskeleton disruption. Hypoxic (1%) and physiological (5%) O(2) tension stimulate ECM deposition from mature ECs. Interestingly, deposition of collagen I is observed only under 5% O(2) tension. ECM production from all ECs is found to be regulated by hypoxia-inducible factors 1α and 2α but differentially in the different cell lines. Collectively, we suggest that ECM deposition and assembly by ECs is dependent on maturation stage and oxygen supply and that these findings can be harnessed to advance engineered vascular therapeutics.

Induction of therapeutic neoangiogenesis using in vitro-generated endothelial colony-forming cells: an autologous transplantation model in rat

BACKGROUND

Accumulating evidence shows the potential of bone marrow-derived endothelial colony-forming cells (bmECFCs) as promising tools for vascular repair. However, knowledge about their in vitro expansion, characterization, and functional behavior is still controversial. We demonstrate the in vitro generation of rat bmECFCs and analyze their ability to promote tissue reperfusion in a chronic hind-limb ischemia model.

METHODS

Either in vitro-generated and characterized autologous bmECFCs or placebo was injected into ischemic hind limbs of Sprague-Dawley rats. Tissue perfusion was quantified by laser Doppler, in perfusion units (PU), at days 0, 15, and 30.

RESULTS

Rat bmECFCs acquire a typical phenotype (CD34(+)VEGFR2(+)CD133(+)CXCR4(+)CD45(-)), culture, and functional behavior (Dil-ac-LDL+) in vitro. Injection of autologous bmECFCs improves tissue perfusion in ischemic hind limbs (183.5 ± 3.29 PU(bmECFCs/day 30)versus 131 ± 3.9 PU(controls/day 30), P < 0.001).

CONCLUSIONS

We conclude that rat bmECFCs promote ischemic tissue reperfusion and their proangiogenic properties are a potential mechanism for this effect.

Phenotypic and functional characterization of endothelial colony forming cells derived from human umbilical cord blood

Longstanding views of new blood vessel formation via angiogenesis, vasculogenesis, and arteriogenesis have been recently reviewed. The presence of circulating endothelial progenitor cells (EPCs) were first identified in adult human peripheral blood by Asahara et al. in 1997 bringing an infusion of new hypotheses and strategies for vascular regeneration and repair. EPCs are rare but normal components of circulating blood that home to sites of blood vessel formation or vascular remodeling, and facilitate either postnatal vasculogenesis, angiogenesis, or arteriogenesis largely via paracrine stimulation of existing vessel wall derived cells. No specific marker to identify an EPC has been identified, and at present the state of the field is to understand that numerous cell types including proangiogenic hematopoietic stem and progenitor cells, circulating angiogenic cells, Tie2+ monocytes, myeloid progenitor cells, tumor associated macrophages, and M2 activated macrophages participate in stimulating the angiogenic process in a variety of preclinical animal model systems and in human subjects in numerous disease states. Endothelial colony forming cells (ECFCs) are rare circulating viable endothelial cells characterized by robust clonal proliferative potential, secondary and tertiary colony forming ability upon replating, and ability to form intrinsic in vivo vessels upon transplantation into immunodeficient mice. While ECFCs have been successfully isolated from the peripheral blood of healthy adult subjects, umbilical cord blood (CB) of healthy newborn infants, and vessel wall of numerous human arterial and venous vessels. CB possesses the highest frequency of ECFCs that display the most robust clonal proliferative potential and form durable and functional blood vessels in vivo. While the derivation of ECFC from adult peripheral blood has been presented, here we describe the methodologies for the derivation, cloning, expansion, and in vitro as well as in vivo characterization of ECFCs from the human umbilical CB.

Human umbilical cord is a unique and safe source of various types of stem cells suitable for treatment of hematological diseases and for regenerative medicine

Cord blood (CB) is a rich source of hematopoietic stem cells (HSCs) and for this reason CB transplantation has been used successfully for the treatment of some malignant and nonmalignant diseases. However, this technique is limited by the relatively low number of HSCs present in each CB unit and by the delayed engraftment of platelets and neutrophils. To bypass these obstacles efforts have been made to develop strategies to expand CB HSCs in vitro for transplantation. CB is also an important source of other stem cells, including endothelial progenitors, mesenchymal stem cells (MSCs), very small embryonic/epiblast-like (VSEL) stem cells, and unrestricted somatic stem cells (USSC), potentially suitable for use in regenerative medicine. For some of these stem cell populations, such as MSCs, clinical studies have been started and for other stem cell populations potential clinical applications have been identified and clinical studies will follow. In addition to CB, other parts of umbilical cord, such as the Wharton's jelly, or tissues strictly linked such as the placenta are also rich sources of stem cells.

Influence of substrate stiffness on circulating progenitor cell fate

In situ vascular tissue engineering (TE) aims at regenerating vessels using implanted synthetic scaffolds. An envisioned strategy is to capture and differentiate progenitor cells from the bloodstream into the porous scaffold to initiate tissue formation. Among these cells are the endothelial colonies forming cells (ECFCs) that can differentiate into endothelial cells and transdifferentiate into smooth muscle cells under biochemical stimulation. The influence of mechanical stimulation is unknown, but relevant for in situ vascular TE because the cells perceive a change in mechanical environment when captured inside the scaffold, where they are shielded from blood flow induced shear stresses. Here we investigate the effects of substrate stiffness as one of the environmental mechanical cues to control ECFC fate within scaffolds. ECFCs were seeded on soft (3.58±0.90 kPa), intermediate (21.59±2.91 kPa), and stiff (93.75±18.36 kPa) fibronectin-coated polyacrylamide gels, as well as on glass controls, and compared to peripheral blood mononuclear cells (PBMC). Cell behavior was analyzed in terms of adhesion (vinculin staining), proliferation (BrdU), phenotype (CD31, αSMA staining, and flow cytometry), and collagen production (col I, III, and IV). While ECFCs adhesion and proliferation increased with substrate stiffness, no change in phenotype was observed. The cells produced no collagen type I, but abundant amounts of collagen type III and IV, albeit in a stiffness-dependent organization. PBMCs did not adhere to the gels, but they did adhere to glass, where they expressed CD31 and collagen type III. Addition mechanical cues, such as cyclic strains, should be studied to further investigate the effect of the mechanical environment on captured circulating cells for in situ TE purposes.