Endothelial Progenitor Cells for the Vascularization of Engineered Tissues

Cell-based therapies for vascular regeneration: Past, present and future

Tissue vascularization remains one of the outstanding challenges in regenerative medicine. Beyond its role in circulating oxygen and nutrients, the vasculature is critical for organ development, function and homeostasis. Importantly, effective vascular regeneration is key in generating large 3D tissues for regenerative medicine applications to enable the survival of cells post-transplantation, organ growth, and integration into the host system. Therefore, the absence of clinically applicable means of (re)generating vessels is one of the main obstacles in cell replacement therapy. In this review, we highlight cell-based vascularization strategies which demonstrate clinical potential, discuss their strengths and limitations and highlight the main obstacles hindering cell-based therapeutic vascularization.

p300/Sp1-Mediated High Expression of p16 Promotes Endothelial Progenitor Cell Senescence Leading to the Occurrence of Chronic Obstructive Pulmonary Disease

OBJECTIVE

Chronic obstructive pulmonary disease (COPD) is a common chronic disease and develops rapidly into a grave public health problem worldwide. However, what exactly causes the occurrence of COPD remains largely unclear. Here, we are trying to explore whether the high expression of p16 mediated by p300/Sp1 can cause chronic obstructive pulmonary disease through promoting the senescence of endothelial progenitor cells (EPCs).

METHODS

Peripheral blood EPCs were isolated from nonsmoking non-COPD, smoking non-COPD, and smoking COPD patients. The expressions of p16, p300, and senescence-related genes were detected by RT-PCR and Western Blot. Then, we knocked down or overexpressed Sp1 and p300 and used the ChIP assay to detect the histone H4 acetylation level in the promoter region of p16, CCK8 to detect cell proliferation, flow cytometry to detect the cell cycle, and β-galactosidase staining to count the proportion of senescent cells.

RESULTS

The high expression of p16 was found in peripheral blood EPCs of COPD patients; the cigarette smoke extract (CSE) led to the increase of p16. The high expression of p16 in EPCs promoted cell cycle arrest and apoptosis. The CSE-mediated high expression of p16 promoted cell senescence. The expression of p300 was increased in peripheral blood EPCs of COPD patients. Moreover, p300/Sp1 enhanced the histone H4 acetylation level in the promoter region of p16, thereby mediating the senescence of EPCs. And knockdown of p300/Sp1 could rescue CSE-mediated cell senescence.

CONCLUSION

p300/Sp1 enhanced the histone H4 acetylation level in the p16 promoter region to mediate the senescence of EPCs.

Biofabrication of vasculature in microphysiological models of bone

Bone contains a dense network of blood vessels that are essential to its homoeostasis, endocrine function, mineral metabolism and regenerative functions. In addition, bone vasculature is implicated in a number of prominent skeletal diseases, and bone has high affinity for metastatic cancers. Despite vasculature being an integral part of bone physiology and pathophysiology, it is often ignored or oversimplified inin vitrobone models. However, 3D physiologically relevant vasculature can now be engineeredin vitro, with microphysiological systems (MPS) increasingly being used as platforms for engineering this physiologically relevant vasculature. In recent years, vascularised models of bone in MPSs systems have been reported in the literature, representing the beginning of a possible technological step change in how bone is modelledin vitro. Vascularised bone MPSs is a subfield of bone research in its nascency, however given the impact of MPSs has had inin vitroorgan modelling, and the crucial role of vasculature to bone physiology, these systems stand to have a substantial impact on bone research. However, engineering vasculature within the specific design restraints of the bone niche is significantly challenging given the different requirements for engineering bone and vasculature. With this in mind, this paper aims to serve as technical guidance for the biofabrication of vascularised bone tissue within MPS devices. We first discuss the key engineering and biological considerations for engineering more physiologically relevant vasculaturein vitrowithin the specific design constraints of the bone niche. We next explore emerging applications of vascularised bone MPSs, and conclude with a discussion on the current status of vascularised bone MPS biofabrication and suggest directions for development of next generation vascularised bone MPSs.

Generation of vascular chimerism within donor organs

Whole organ perfusion decellularization has been proposed as a promising method to generate non-immunogenic organs from allogeneic and xenogeneic donors. However, the ability to recellularize organ scaffolds with multiple patient-specific cells in a spatially controlled manner remains challenging. Here, we propose that replacing donor endothelial cells alone, while keeping the rest of the organ viable and functional, is more technically feasible, and may offer a significant shortcut in the efforts to engineer transplantable organs. Vascular decellularization was achieved ex vivo, under controlled machine perfusion conditions, in various rat and porcine organs, including the kidneys, liver, lungs, heart, aorta, hind limbs, and pancreas. In addition, vascular decellularization of selected organs was performed in situ, within the donor body, achieving better control over the perfusion process. Human placenta-derived endothelial progenitor cells (EPCs) were used as immunologically-acceptable human cells to repopulate the luminal surface of de-endothelialized aorta (in vitro), kidneys, lungs and hind limbs (ex vivo). This study provides evidence that artificially generating vascular chimerism is feasible and could potentially pave the way for crossing the immunological barrier to xenotransplantation, as well as reducing the immunological burden of allogeneic grafts.

3D Bioprinting Photo-Crosslinkable Hydrogels for Bone and Cartilage Repair

Three-dimensional (3D) bioprinting has become a promising strategy for bone manufacturing, with excellent control over geometry and microarchitectures of the scaffolds. The bioprinting ink for bone and cartilage engineering has thus become the key to developing 3D constructs for bone and cartilage defect repair. Maintaining the balance of cellular viability, drugs or cytokines' function, and mechanical integrity is critical for constructing 3D bone and/or cartilage scaffolds. Photo-crosslinkable hydrogel is one of the most promising materials in tissue engineering; it can respond to light and induce structural or morphological transition. The biocompatibility, easy fabrication, as well as controllable mechanical and degradation properties of photo-crosslinkable hydrogel can meet various requirements of the bone and cartilage scaffolds, which enable it to serve as an effective bio-ink for 3D bioprinting. Here, in this review, we first introduce commonly used photo-crosslinkable hydrogel materials and additives (such as nanomaterials, functional cells, and drugs/cytokine), and then discuss the applications of the 3D bioprinted photo-crosslinkable hydrogel scaffolds for bone and cartilage engineering. Finally, we conclude the review with future perspectives about the development of 3D bioprinting photo-crosslinkable hydrogels in bone and cartilage engineering.

Vascularization strategies for tissue engineering for tracheal reconstruction

Tissue engineering technology provides effective alternative treatments for tracheal reconstruction. The formation of a functional microvascular network is essential to support cell metabolism and ensure the long-term survival of grafts. Although several tracheal replacement therapy strategies have been developed in the past, the critical significance of the formation of microvascular networks in 3D scaffolds has not attracted sufficient attention. Here, we review key technologies and related factors of microvascular network construction in tissue-engineered trachea and explore optimized preparation processes of vascularized functional tissues for clinical applications.

Construction and a preliminary study of paracrine effect of bone marrow-derived endothelial progenitor cell sheet

The release of paracrine factors from endothelial progenitor cell (EPC) sheet is a central mechanism of tissue repair. The purpose of this study was to constuct the rat bone marrow derived-endothelial progenitor cell (BM-EPCs) sheet and investigate invest the role of stromal cell-derived factor-1α (SDF-1α)/CXCR4 axis in the biological function of BM-EPCs sheet. BM-EPC cells were identified by the cell-surface markers-CD34/CD133/VE-cadherin/KDR using flow cytometry and dual affinity for acLDL and UEA-1. After 7 days of incubation, the BM-EPC single-cell suspensions were seeded on thermo-sensitive plate to harvest the BM-EPC cell sheets. The expression levels of SDF-1α/CXCR4 axis-associated genes and proteins were examined using RT-qPCR and western blot analysis, and enzyme-linked immunosorbent assay (ELISA) was applied to determine the concentration of vascular endothelial growth factor (VEGF), epidermal growth factor (EGF) and SDF-1α in the cell culture medium. The BM-EPC cell sheets were successfully harvested. Moreover, BM-EPC cell sheets have superior migration and tube formation activity when compared with single cell suspension. When capillary-like tube were formed from EPCs sheets, the releasing of paracrine factors such as VEGF, EGF and SDF-1α were increased. To reveal the mechanism of tube formation of BM-EPCs sheets, our research showed that the activation of PI3K/AKT/eNOS pathway was involved in the process, because the phosphorylation of CXCR, PI3K, AKT and eNOS were increased. BM-EPC cell sheets have superior paracrine and tube formation activity than the BM-EPC single-cell. The strong ability to secrete paracrine factors was be potentially related to the SDF-1α/CXCR4 axis through PI3K/AKT/eNOS pathway.

Biocompatibility and Angiogenic Effect of Chitosan/Graphene Oxide Hydrogel Scaffolds on EPCs

Angiogenesis in the field of tissue engineering has attracted significant attention. Graphene oxide has become a promising nanomaterial in tissue engineering for its unique biochemical properties. Therefore, herein, a series of chitosan (CS)/graphene oxide (GO) hydrogel scaffolds were synthesized by crosslinking CS and GO at different concentrations (0.1, 0.5, and 1.0 wt.%) using genipin. Compared with the CS hydrogel scaffolds, the CS/GO hydrogel scaffolds have a better network structure and mechanical strength. Then, we used endothelial progenitor cells (EPCs) extracted from human umbilical cord blood and cocultured these EPCs with the as-prepared scaffolds. The scaffolds with 0.1 and 0.5 wt.%GO showed no considerable cytotoxicity, could promote the proliferation of EPCs and tube formation, and upregulated the expressions of CD34, VEGF, MMP9, and SDF-1 in EPCs compared to the case of the scaffold with 1.0 wt.%GO. This study shows that the addition of graphene oxide improves the structure of chitosan hydrogel and enhances the proliferation activity and angiogenic capacity of EPCs.

In vitro recellularization of decellularized bovine carotid arteries using human endothelial colony forming cells

BACKGROUND

Many patients suffering from peripheral arterial disease (PAD) are dependent on bypass surgery. However, in some patients no suitable replacements (i.e. autologous or prosthetic bypass grafts) are available. Advances have been made to develop autologous tissue engineered vascular grafts (TEVG) using endothelial colony forming cells (ECFC) obtained by peripheral blood draw in large animal trials. Clinical translation of this technique, however, still requires additional data for usability of isolated ECFC from high cardiovascular risk patients. Bovine carotid arteries (BCA) were decellularized using a combined SDS (sodium dodecyl sulfate) -free mechanical-osmotic-enzymatic-detergent approach to show the feasibility of xenogenous vessel decellularization. Decellularized BCA chips were seeded with human ECFC, isolated from a high cardiovascular risk patient group, suffering from diabetes, hypertension and/or chronic renal failure. ECFC were cultured alone or in coculture with rat or human mesenchymal stromal cells (rMSC/hMSC). Decellularized BCA chips were evaluated for biochemical, histological and mechanical properties. Successful isolation of ECFC and recellularization capabilities were analyzed by histology.

RESULTS

Decellularized BCA showed retained extracellular matrix (ECM) composition and mechanical properties upon cell removal. Isolation of ECFC from the intended target group was successfully performed (80% isolation efficiency). Isolated cells showed a typical ECFC-phenotype. Upon recellularization, co-seeding of patient-isolated ECFC with rMSC/hMSC and further incubation was successful for 14 (n = 9) and 23 (n = 5) days. Reendothelialization (rMSC) and partial reendothelialization (hMSC) was achieved. Seeded cells were CD31 and vWF positive, however, human cells were detectable for up to 14 days in xenogenic cell-culture only. Seeding of ECFC without rMSC was not successful.

CONCLUSION

Using our refined decellularization process we generated easily obtainable TEVG with retained ECM- and mechanical quality, serving as a platform to develop small-diameter (< 6 mm) TEVG. ECFC isolation from the cardiovascular risk target group is possible and sufficient. Survival of diabetic ECFC appears to be highly dependent on perivascular support by rMSC/hMSC under static conditions. ECFC survival was limited to 14 days post seeding.

NEAT1 promotes the repair of abdominal aortic aneurysms of endothelial progenitor cells via regulating miR-204-5p/Ang-1

PURPOSE

To clarify the regulatory effect of Nuclear-enriched abundant transcript 1 (NEAT1) on abdominal aortic aneurysm (AAA) model rats and isolated endothelial progenitor cells (EPCs).

METHODS

The AAA rat model was established by CaCl₂ stimulation, and overexpressed NEAT1 was injected into rats through tail vein. Abdominal aorta lesions and numbers of EPCs in tissues and peripheral blood were examined by hematoxylin-eosin, immunofluorescence and flow cytometry. The extracted EPCs were identified by microscopy, DiI-ac-LDL staining and flow cytometry. Effect of overexpressed/silencing NEAT1 on the viability, migration, tube formation and VEGF content of EPCs was investigated by MTT-, wound-healing, tube formation assays and ELISA, respectively. The expressions of NEAT1, miR-204-5p, Angiopoietin-1 (Ang-1)/ERK pathway were determined by qRT-PCR and Western blot as needed. The targeting relationships between NEAT1 and miR-204-5p, and miR-204-5p and Ang-1 were predicted on starBase, TargetScan and confirmed by dual-luciferase experiments. The mutual regulation effect was studied through rescue experiments.

RESULTS

Overexpressed NEAT1 not only reduced inflammatory infiltration and increased the number of EPCs in abdominal aorta and peripheral blood, but also promoted the viability, migration, tube formation of EPCs, increased VEGF content and upregulated the expression of the Ang-1/ERK pathway in EPCs. However, silencing NEAT1 produced opposite results. NEAT1 targeting miR-204-5p inhibited the functional effects of miR-204-5p on of EPCs. Overexpressed/silencing Ang-1 partially reversed the effects of NEAT1 or miR-204-5p on the characteristics of EPCs.

CONCLUSION

NEAT1 competitively binds with miR-204-5p and up-regulates Ang-1 expression in EPCs to effectively improve the proliferation, migration and angiogenesis of EPCs.

Down-regulation of miR-361-5p promotes the viability, migration and tube formation of endothelial progenitor cells via targeting FGF1

Transplantion of bone marrow-derived endothelial progenitor cells (EPCs) may be a novel treatment for deep venous thrombosis (DVT). The present study probed into the role of microRNA (miR)-361-5p in EPCs and DVT recanalization. EPCs were isolated from male Sprague-Dawley (SD) rats and identified using confocal microscopy and flow cytometry. The viability, migration and tube formation of EPCs were examined using MTT assay, wound-healing assay and tube formation assay, respectively. Target gene and potential binding sites between miR-361-5p and fibroblast growth factor 1 (FGF1) were predicted by StarBase and confirmed by dual-luciferase reporter assay. Relative expressions of miR-361-5p and FGF1 were detected using quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot as needed. A DVT model in SD rats was established to investigate the role of EPC with miR-361-5p antagomir in DVT by Hematoxylin-Eosin (H&E) staining. EPC was identified as 87.1% positive for cluster of difference (CD)31, 2.17% positive for CD133, 85.6% positive for von Willebrand factor (vWF) and 94.8% positive for vascular endothelial growth factor receptor-2 (VEGFR2). MiR-361-5p antagomir promoted proliferation, migration and tube formation of EPCs and up-regulated FGF1 expression, thereby dissolving thrombus in the vein of DVT rats. FGF1 was the target of miR-361-5p, and overexpressed FGF1 reversed the effects of up-regulating miR-361-5p on suppressing EPCs. Down-regulation of miR-361-5p enhanced thrombus resolution in vivo and promoted EPC viability, migration and angiogenesis in vitro through targeting FGF1. Therefore, miR-361-5p may be a potential therapeutic target for DVT recanalization.

Molecular Profiling and Gene Banking of Rabbit EPCs Derived from Two Biological Sources

Endothelial progenitor cells (EPCs) have been broadly studied for several years due to their outstanding regenerative potential. Moreover, these cells might be a valuable source of genetic information for the preservation of endangered animal species. However, a controversy regarding their characterization still exists. The aim of this study was to isolate and compare the rabbit peripheral blood- and bone marrow-derived EPCs with human umbilical vein endothelial cells (HUVECs) in terms of their phenotype and morphology that could be affected by the passage number or cryopreservation as well as to assess their possible neuro-differentiation potential. Briefly, cells were isolated and cultured under standard endothelial conditions until passage 3. The morphological changes during the culture were monitored and each passage was analyzed for the typical phenotype using flow cytometry, quantitative real–time polymerase chain reaction (qPCR) and novel digital droplet PCR (ddPCR), and compared to HUVECs. The neurogenic differentiation was induced using a commercial kit. Rabbit cells were also cryopreserved for at least 3 months and then analyzed after thawing. According to the obtained results, both rabbit EPCs exhibit a spindle-shaped morphology and high proliferation rate. The both cell lines possess same stable phenotype: CD14⁻CD29⁺CD31⁻CD34⁻CD44⁺CD45⁻CD49f⁺CD73⁺CD90⁺CD105⁺CD133⁻CD146⁻CD166⁺VE-cadherin⁺VEGFR-2⁺SSEA-4⁺MSCA-1⁻vWF⁺eNOS⁺AcLDL⁺ALDH⁺vimentin⁺desmin⁺α-SMA⁺, slightly different from HUVECs. Moreover, both induced rabbit EPCs exhibit neuron-like morphological changes and expression of neuronal markers ENO2 and MAP2. In addition, cryopreserved rabbit cells maintained high viability (>85%) and endothelial phenotype after thawing. In conclusion, our findings suggest that cells expanded from the rabbit peripheral blood and bone marrow are of the endothelial origin with a stable marker expression and interesting proliferation and differentiation capacity.

Angiogenesis: A Cellular Response to Traumatic Injury

ABSTRACT

The development of new vasculature plays a significant role in a number of chronic disease states, including neoplasm growth, peripheral arterial disease, and coronary artery disease, among many others. Traumatic injury and hemorrhage, however, is an immediate, often dramatic pathophysiologic insult that can also necessitate neovascularization to promote healing. Traditional understanding of angiogenesis involved resident endothelial cells branching outward from localized niches in the periphery. Additionally, there are a small number of circulating endothelial progenitor cells that participate directly in the process of neovessel formation. The bone marrow stores a relatively small number of so-called pro-angiogenic hematopoietic progenitor cells-that is, progenitor cells of a hematopoietic potential that differentiate into key structural cells and stimulate or otherwise support local cell growth/differentiation at the site of angiogenesis. Following injury, a number of cytokines and intercellular processes are activated or modulated to promote development of new vasculature. These processes initiate and maintain a robust response to vascular insult, allowing new vessels to canalize and anastomose and provide timely oxygen delivering to healing tissue. Ultimately as we better understand the key players in the process of angiogenesis we can look to develop novel techniques to promote healing following injury.

Phototunable interpenetrating polymer network hydrogels to stimulate the vasculogenesis of stem cell-derived endothelial progenitors

Vascularization of engineered scaffolds remains a critical obstacle hindering the translation of tissue engineering from the bench to the clinic. We previously demonstrated the robust micro-vascularization of collagen hydrogels with induced pluripotent stem cell (iPSC)-derived endothelial progenitors; however, physically cross-linked collagen hydrogels compact rapidly and exhibit limited strength. We have synthesized an interpenetrating polymer network (IPN) hydrogel comprised of collagen and norbornene-modified hyaluronic acid (NorHA) to address these challenges. This dual-network hydrogel combines the natural cues presented by collagen's binding sites and extracellular matrix (ECM)-mimicking fibrous architecture with the in situ modularity and chemical cross-linking of NorHA. We modulated the IPN hydrogel's stiffness and degradability by varying the concentration and sequence, respectively, of the NorHA peptide cross-linker. Rheological characterization of the photo-mediated gelation process revealed that the IPN hydrogel's stiffness increased with cross-linker concentration and was decoupled from the bulk NorHA content. Conversely, the swelling of the IPN hydrogel decreased linearly with increasing cross-linker concentration. Collagen microarchitecture remained relatively unchanged across cross-linking conditions, although the addition of NorHA delayed collagen fibrillogenesis. Upon iPSC-derived endothelial progenitor encapsulation, robust, lumenized microvascular networks developed in IPN hydrogels over two weeks. Subsequent computational analysis showed that an initial rise in stiffness increased the number of branch points and vessels, but vascular growth was suppressed in high stiffness IPN hydrogels. These results suggest that an IPN hydrogel consisting of collagen and NorHA is highly tunable, compaction resistant, and capable of supporting vasculogenesis.

An In Vitro Co-Culture Model of Bone Marrow Mesenchymal Stromal Cells and Peripheral Blood Mononuclear Cells Promotes the Differentiation of Myeloid Angiogenic Cells and Pericyte-Like Cells

Mesenchymal stromal cells (MSCs) are known to stimulate the survival and growth of endothelial cells (ECs) by producing paracrine signals, as well as to differentiate into pericytes and thereby support blood vessel formation and stability. On the other hand, cells with an EC-like phenotype have been found within the CD14⁺ and CD34⁺ cell populations of peripheral blood (PB) mononuclear cells (MNCs). The aim of this study was to investigate the proangiogenic differentiation potential of human MSC-MNC co-cultures. Bone marrow-derived MSCs (2,500 cells/cm²) were co-cultured with MNCs (50,000 cells/cm²), which were isolated from the PB of healthy donors. MSCs and MNCs cultured alone at same cell densities were used as controls. Cells in MNC fraction and in co-cultures were isolated for CD14, CD34, and CD31 surface markers with magnetic-activated cell sorting. Co-cultures were analyzed for cell proliferation and morphology, as well as for the expression of various hematopoietic, endothelial, and pericyte markers by immunocytochemistry, quantitative PCR (qPCR), and flow cytometry. Vascular endothelial growth factor (VEGF) expression and secretion was measured with qPCR and enzyme-linked immunosorbent assay, respectively. Our results show that in co-cultures with MSCs, CD14⁺CD45⁺ MNCs differentiated into spindle-shaped, nonproliferative, EC-like, myeloid angiogenic cells (MACs) expressing CD31, but also into pericyte-like cells expressing neural/glial antigen 2 (NG2) and CD146. Functionality of the isolated MACs was demonstrated in co-cultures with human umbilical vein endothelial cells, where they supported the formation of tube-like structures. NG2⁺ cells of MNC-origin were found among both CD34^-CD14⁺ and CD34^-CD14^- cell populations, indicating the existence of different subtypes of pericyte-like cells. In addition, VEGF was shown to be secreted in MSC-MNC co-cultures, mainly by MSCs. In conclusion, MSCs were shown to possess proangiogenic capacity in MSC-MNC co-cultures as they supported the differentiation of functional MACs, as well as the differentiation of pericyte-like cells of MNC origin. This phenomenon was mediated at least partially via secreted VEGF.

Selection of different endothelialization modes and different seed cells for tissue-engineered vascular graft

Tissue-engineered vascular grafts (TEVGs) have enormous potential for vascular replacement therapy. However, thrombosis and intimal hyperplasia are important problems associated with TEVGs especially small diameter TEVGs (<6 mm) after transplantation. Endothelialization of TEVGs is a key point to prevent thrombosis. Here, we discuss different types of endothelialization and different seed cells of tissue-engineered vascular grafts. Meanwhile, endothelial heterogeneity is also discussed. Based on it, we provide a new perspective for selecting suitable types of endothelialization and suitable seed cells to improve the long-term patency rate of tissue-engineered vascular grafts with different diameters and lengths.

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The material, diameter and length of tissue-engineered vascular graft are all key factors affecting its long-term patency.

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Endothelialization strategies should consider the different diameters and lengths of tissue-engineered vascular grafts.

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Cell heterogeneity and tissue heterogeneity should be considered in the application of seed cells.

Syntheses and characterization of anti-thrombotic and anti-oxidative Gastrodin-modified polyurethane for vascular tissue engineering

Vascular grafts must avoid negative inflammatory responses and thrombogenesis to prohibit fibrotic deposition immediately upon implantation and promote the regeneration of small diameter blood vessels (<6 mm inner diameter). Here, polyurethane (PU) elastomers incorporating anti-coagulative and anti-inflammatory Gastrodin were fabricated. The films had inter-connected pores with porosities equal to or greater than 86% and pore sizes ranging from 250 to 400 μm. Incorporation of Gastrodin into PU films resulted in desirable mechanical properties, hydrophilicity, swelling ratios and degradation rates without collapse. The released Gastrodin maintained bioactivity over 21 days as assessed by its anti-oxidative capability. The Gastrodin/PU had better anti-coagulation response (less observable BSA, fibrinogen and platelet adhesion/activation and suppressed clotting in whole blood). Red blood cell compatibility, measured by hemolysis, was greatly improved with 2Gastrodin/PU compared to other Gastrodin/PU groups. Notably, Gastrodin/PU upregulated anti-oxidant factors Nrf2 and HO-1 expression in H2O2 treated HUVECs, correlated with decreasing pro-inflammatory cytokines TNF-α and IL-1β in RAW 264.7 cells. Upon implantation in a subcutaneous pocket, PU was encapsulated by an obvious fibrous capsule, concurrent with a large amount of inflammatory cell infiltration, while Gastrodin/PU induced a thinner fibrous capsule, especially 2Gastrodin/PU. Further, enhanced adhesion and proliferation of HUVECs seeded onto films in vitro demonstrated that 2Gastrodin/PU could help cell recruitment, as evidenced by rapid host cell infiltration and substantial blood vessel formation in vivo. These results indicate that 2Gastrodin/PU has the potential to facilitate blood vessel regeneration, thus providing new insight into the development of clinically effective vascular grafts.

Engineering of axially vascularized bone tissue using natural coral scaffold and osteogenic bone marrow mesenchymal stem cell sheets

INTRODUCTION

Blood supply remains one of the obstacles to large bone tissue engineering. This study aimed to generate vascularized bone tissue by inducing axial vascularization into a construct combining natural coral scaffold and a bone marrow mesenchymal stem cells (BMSCs) sheet.

MATERIAL AND METHODS

Isolated BMSCs were cultured to form an osteogenic cell sheet using a continuous culture method. Natural coral scaffolds were prepared into customized shape with a cylinder of 20 mm length, 8 mm in outer diameter and 5 mm in inner diameter. Then, the freed superficial inferior epigastric vessel of rabbits was first wrapped with a cell sheet, and then inserted into the central passage of the scaffold, after being wrapped with another cell sheet, the complexes were implanted subcutaneously into a rabbit groin area. In contrast, the sheet-scaffold construct that implanted into groin subcutaneous area of the other side of the same rabbit with the distal end of the blood vessel was ligated, which was considered as control. New bone and vascularization formation were evaluated at 12 weeks postoperatively.

RESULTS

The volume of new bone formation and amount of capillary infiltration in the vascular circulation group were significantly greater than that in the vascular ligation group, which suggested that insertion of axial vessels could significantly promote angiogenesis and osteogenesis of the tissue-engineered bone.

CONCLUSIONS

These findings indicate that inserting an arteriovenous bundle into the constructs of mesenchymal stem cell sheet and coral has great potential for clinical applications to repair large bone defects.

Intercellular Communication by Vascular Endothelial Cell-Derived Extracellular Vesicles and Their MicroRNAs in Respiratory Diseases

Respiratory diseases and their comorbidities, such as cardiovascular disease and muscle atrophy, have been increasing in the world. Extracellular vesicles (EVs), which include exosomes and microvesicles, are released from almost all cell types and play crucial roles in intercellular communication, both in the regulation of homeostasis and the pathogenesis of various diseases. Exosomes are of endosomal origin and range in size from 50 to 150 nm in diameter, while microvesicles are generated by the direct outward budding of the plasma membrane in size ranges of 100-2,000 nm in diameter. EVs can contain various proteins, metabolites, and nucleic acids, such as mRNA, non-coding RNA species, and DNA fragments. In addition, these nucleic acids in EVs can be functional in recipient cells through EV cargo. The endothelium is a distributed organ of considerable biological importance, and disrupted endothelial function is involved in the pathogenesis of respiratory diseases such as chronic obstructive pulmonary disease, pulmonary hypertension, and acute respiratory distress syndrome. Endothelial cell-derived EVs (EC-EVs) play crucial roles in both physiological and pathological conditions by traveling to distant sites through systemic circulation. This review summarizes the pathological roles of vascular microRNAs contained in EC-EVs in respiratory diseases, mainly focusing on chronic obstructive pulmonary disease, pulmonary hypertension, and acute respiratory distress syndrome. Furthermore, this review discusses the potential clinical usefulness of EC-EVs as therapeutic agents in respiratory diseases.

Conditioned media from endothelial progenitor cells cultured in simulated microgravity promote angiogenesis and bone fracture healing

Background

Paracrine signaling from endothelial progenitor cells (EPCs) is beneficial for angiogenesis and thus promotes tissue regeneration. Microgravity (MG) environment is found to facilitate the functional potentials of various stem or progenitor cells. The present study aimed to elucidate the effects of MG on pro-angiogenic properties and fracture repair capacities of conditioned media (CM) from EPCs.

Methods

Human peripheral blood-derived EPCs were cultured under MG or normal gravity (NG) followed by analysis for angiogenic gene expression. Furthermore, the serum-free CM under MG (MG-CM) or NG (NG-CM) were collected, and their pro-angiogenic properties were examined in human umbilical vein endothelial cells (HUVECs). In order to investigate the effects of MG-CM on fracture healing, they were injected into the fracture gaps of rat models, and radiography, histology, and mechanical test were performed to evaluate neovascularization and fracture healing outcomes.

Results

MG upregulated the expression of hypoxia-induced factor-1α (HIF-1α) and endothelial nitric oxide synthase (eNOS) and promoted NO release. Comparing to NG-CM, MG-CM significantly facilitated the proliferation, migration, and angiogenesis of HUVECs through NO-induced activation of FAK/Erk1/2-MAPK signaling pathway. In addition, MG-CM were verified to improve angiogenic activities in fracture area in a rat tibial fracture model, accelerate fracture healing, and well restore the biomechanical properties of fracture bone superior to NG-CM.

Conclusion

These findings provided insight into the use of MG bioreactor to enhance the angiogenic properties of EPCs’ paracrine signals via HIF-1α/eNOS/NO axis, and the administration of MG-CM favored bone fracture repair.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-020-02074-y.

MiR-17-5p promotes the endothelialization of endothelial progenitor cells to facilitate the vascular repair of aneurysm by regulating PTEN-mediated PI3K/AKT/VEGFA pathway

The endothelialization of endothelial progenitor cells (EPCs) was proven to facilitate the vascular repair of aneurysm. MiR-17-5p regulated angiogenesis in various cancers. This research focused on exploring the effect of miR-17-5p on EPCs and the vascular repair of aneurysm. In vivo study: the aneurysm rat model was established and treated with AgomiR-17-5p; the histopathology of aneurysm tissues was examined by hematoxylin-eosin staining; and the level of EPCs in the aneurysm tissues and peripheral blood of rats were evaluated by immunofluorescence and flow cytometry, respectively. In vitro study: EPCs were cultured and identified using flow cytometry; the target of miR-17-5p was proven by dual-luciferase reporter assay; after transfection, the viability, migration, and tube formation of the EPCs were detected by MTT, wound healing, and tube formation assays, respectively; the expressions of VEGFA and factors related to PTEN-mediated PI3K/AKT pathway were detected by ELISA, qPCR, or Western blot as needed. In vivo study: miR-17-5p overexpression promoted the vascular repair in aneurysm rats and increased the level of EPCs in the aneurysm tissues and peripheral blood of the rats. In vitro study: miR-17-5p overexpression promoted the viability, migration, and tube formation of EPCs, up-regulated the expressions of VEGFA, p-PI3K, and p-AKT, and down-regulated the PTEN expression in EPCs; miR-17-5p silencing did the opposite; PTEN was targeted by miR-17-3p and further abrogated the effects of miR-17-5p overexpression on EPCs. MiR-17-5p promoted the endothelialization of EPCs to facilitate the vascular repair of aneurysm by regulating PTEN-mediated PI3K/AKT/VEGFA pathway.

Endothelial progenitor/stem cells in engineered vessels for vascular transplantation

BACKGROUND

Dysfunction of blood vessel leads to aneurysms, myocardial infarction and other thrombosis conditions. Current treatment strategies are transplantation of blood vessels from one part of the body to other dysfunction area, or allogenic, synthetic. Due to shortage of the donor, painful dissection, and lack of efficacy in synthetic, there is a need for alternative to native blood vessels for transplantation.

METHODS

Human umbilical-cord tissue obtained from the hospital with the informed consent. Umbilical-cord blood vessels were isolated for decellularization and to establish endothelial cell culture. Cultured cells were characterized by immunophenotype, gene expression and in vitro angiogenesis assay. Decellularized blood vessels were recellularized with the endothelial progenitors and Wharton jelly, CL MSCs (1:1), which was characterized by MTT, biomechanical testing, DNA content, SEM and histologically. Bioengineered vessels were transplanted into the animal models to evaluate their effect.

RESULTS

Cultured cells express CD31 and CD14 determining endothelial progenitor cells (EPCs). EPCs expresses various factors such as angiopoitin1, VWF, RANTES, VEGF, BDNF, FGF1, FGF2, HGF, IGF, GDNF, NGF, PLGF, NT3, but fail to express NT4, EGF, and CNTF. Pro and anti-inflammatory cytokine expressions were noticed. Functionally, these EPCs elicit in vitro tube formation. Negligible DNA content and intact ECM confirms the efficient decellularization of tissue. The increased MTT activity in recellularized tissue determines proliferating cells and biocompatibility of the scaffolds. Moreover, significant (P < 0.05) increase in maximum force and tensile of recellularized biomaterial as compared to the decellularized scaffolds. Integration of graft with host tissue, suggesting biocompatible therapeutic biomaterial with cells.

CONCLUSION

EPCs with stem cells in engineered blood vessels could be therapeutically applicable in vascular surgery.

Umbilical cord blood cells for the treatment of preterm white matter injury: Potential effects and treatment options

Preterm birth is a global public health problem. A large number of preterm infants survive with preterm white matter injury (PWMI), which leads to neurological deficits, and has multifaceted etiology, clinical course, monitoring, and outcomes. The principal upstream insults leading to PWMI initiation are hypoxia-ischemia and infection and/or inflammation and the key target cells are late oligodendrocyte precursor cells. Current PWMI treatments are mainly supportive, and thus have little effect in terms of protecting the immature brain or repairing injury to improve long-term outcomes. Umbilical cord blood (UCB) cells comprise abundant immunomodulatory and stem cells, which have the potential to reduce brain injury, mainly due to anti-inflammatory and immunomodulatory mechanisms, and also through their release of neurotrophic or growth factors to promote endogenous neurogenesis. In this review, we briefly summarize PWMI pathogenesis and pathophysiology, and the specific properties of different cell types in UCB. We further explore the potential mechanism by which UCB can be used to treat PWMI, and discuss the advantages of and potential issues related to UCB cell therapy. Finally, we suggest potential future studies of UCB cell therapy in preterm infants.

Enhanced Endothelial Cell Delivery for Repairing Injured Endothelium via Pretargeting Approach and Bioorthogonal Chemistry

Arterial wall injury often leads to endothelium cell activation, endothelial detachment, and atherosclerosis plaque formation. While abundant research efforts have been placed on treating the end stages of the disease, no cure has been developed to repair injured and denude endothelium often occurred at an early stage of atherosclerosis. Here, a pretargeting cell delivery strategy using combined injured endothelial targeting nanoparticles and bioorthogonal click chemistry approach was developed to deliver endothelial cells to replenish the injured endothelium via a two-step process. First, nanoparticles bearing glycoprotein 1b α (Gp1bα) proteins and tetrazine (Tz) were fabricated to provide a homogeneous nanoparticle coating on an injured arterial wall via the interactions between Gp1bα and von Willebrand factor (vWF), a ligand that is present on denuded endothelium. Second, transplanted endothelium cells bearing transcyclooctene (TCO) would be quickly immobilized on the surfaces of nanoparticles via TCO:Tz reactions. In vitro binding studies under both static and flow conditions confirmed that our novel Tz-labeled Gp1bα-conjugated poly(lactic-co-glycolic acid) (PLGA) nanoparticles can successfully pretargeted toward the injured site and support rapid adhesion of endothelial cells from the circulation. Ex vivo results also confirm that such an approach is highly efficient in mediating the local delivery of endothelial cells at the sites of arterial injury. The results support that this pretargeting cell delivery approach may be used for repairing injured endothelium in situ at its early stage.

Corneal endothelium tissue engineering: An evolution of signaling molecules, cells, and scaffolds toward 3D bioprinting and cell sheets

Cornea is an avascular and transparent tissue that focuses light on retina. Cornea is supported by the corneal-endothelial layer through regulation of hydration homeostasis. Restoring vision in patients afflicted with corneal endothelium dysfunction-mediated blindness most often requires corneal transplantation (CT), which faces considerable constrictions due to donor limitations. An emerging alternative to CT is corneal endothelium tissue engineering (CETE), which involves utilizing scaffold-based methods and scaffold-free strategies. The innovative scaffold-free method is cell sheet engineering, which typically generates cell layers surrounded by an intact extracellular matrix, exhibiting tunable release from the stimuli-responsive surface. In some studies, scaffold-based or scaffold-free technologies have been reported to achieve promising outcomes. However, yet some issues exist in translating CETE from bench to clinical practice. In this review, we compare different corneal endothelium regeneration methods and elaborate on the application of multiple cell types (stem cells, corneal endothelial cells, and endothelial precursors), signaling molecules (growth factors, cytokines, chemical compounds, and small RNAs), and natural and synthetic scaffolds for CETE. Furthermore, we discuss the importance of three-dimensional bioprinting strategies and simulation of Descemet's membrane by biomimetic topography. Finally, we dissected the recent advances, applications, and prospects of cell sheet engineering for CETE.

Chemerin enhances the adhesion and migration of human endothelial progenitor cells and increases lipid accumulation in mice with atherosclerosis

Background

The role of adipokines in the development of atherosclerosis (AS) has received increasing attention in recent years. This study aimed to explore the effects of chemerin on the functions of human endothelial progenitor cells (EPCs) and to investigate its role in lipid accumulation in ApoE-knockout (ApoE−/−) mice.

Methods

EPCs were cultured and treated with chemerin together with the specific p38 mitogen-activated protein kinase (MAPK) inhibitor SB 203580 in a time- and dose-dependent manner. Changes in migration, adhesion, proliferation and the apoptosis rate of EPCs were detected. ApoE−/− mice with high-fat diet-induced AS were treated with chemerin with or without SB 203580. Weights were recorded, lipid indicators were detected, and tissues sections were stained.

Results

The data showed that chemerin enhanced the adhesion and migration abilities of EPCs, and reduced the apoptosis ratio and that this effect might be mediated through the p38 MAPK pathway. Additionally, chemerin increased the instability of plaques. Compared with the control group and the inhibitor group, ApoE−/− mice treated with chemerin protein had more serious arterial stenosis, higher lipid contents in plaques and decreased collagen. Lipid accumulation in the liver and kidney and inflammation in the hepatic portal area were enhanced by treatment with chemerin, and the size of adipocytes also increased after chemerin treatment. In conclusion, chemerin can enhance the adhesion and migration abilities of human EPCs and reduce the apoptosis ratio. In animals, chemerin can increase lipid accumulation in atherosclerotic plaques and exacerbate plaques instability. At the same time, chemerin can cause abnormal lipid accumulation in the livers and kidneys of model animals. After specifically blocking the p38 MAPK pathway, the effect of chemerin was reduced.

Conclusions

In conclusion, this study showed that chemerin enhances the adhesion and migration abilities of EPCs and increases the instability of plaques and abnormal lipid accumulation in ApoE−/− mice. Furthermore, these effects might be mediated through the p38 MAPK pathway.

MRI tracing of ultrasmall superparamagnetic iron oxide nanoparticle‑labeled endothelial progenitor cells for repairing atherosclerotic vessels in rabbits

Endothelial progenitor cells (EPCs) have been discovered to be relevant to the prognosis of cardiovascular diseases. Previous research has demonstrated that EPCs serve vital roles in the occurrence and development of atherosclerosis. Significant improvements have been made in MRI technology and in the experimental use of EPCs for therapeutic angiogenesis and vascular repair. Nevertheless, the migratory, adhesive, proliferative and angiogenic properties of EPCs remain unknown. The aims of the present study were to investigate the potential of using non-invasive monitoring with ultrasmall superparamagnetic iron oxide nanoparticle (USPION)-labeled endothelial progenitor cells (EPCs) after transplantation, and to assess the treatment outcomes in an atherosclerotic rabbit model. EPCs derived from rabbit peripheral blood samples were labeled with USPION-poly-l-lysine (USPION-PLL). The morphology, proliferation, adhesive ability and labeling efficiency of the EPCs were determined by optical and electron microscopy. Moreover, biological activity was assessed by flow cytometry. In addition, T2-weighted image fast spin-echo MRI was used to detect cell labeling. USPION content in the labeled EPCs was determined by Prussian blue staining and scanning electron microscopy. Rabbit atherosclerosis model was established using a high-fat diet. USPION-labeled EPCs were transplanted into rabbits, and in vivo MRI was performed 1 and 7 days after transplantation. It was found that EPCs cultured on Matrigel formed capillary-like structures, and expressed the surface markers CD133, CD31, CD34 and vascular endothelial growth factor receptor 2 (VEGFR2). The optimal USPION concentration was 32 µg/ml, as determined by adhesion and proliferation assays. It was identified that USPION-PLL nanoparticles were 10–20 nm in diameter. Histopathological analysis results indicated that 1 day after transplantation of the labeled EPCs, blue-stained granules were observed in the intima of vascular lesions in rabbit models after Prussian blue staining. Therefore, the present results suggest that USPION-labeled EPCs may play a role in repairing endothelial injury and preventing atherosclerosis in vivo.

Repair of Bone Defects With Endothelial Progenitor Cells and Bone Marrow-Derived Mesenchymal Stem Cells With Tissue-Engineered Bone in Rabbits

PURPOSE

This study aimed to investigate the repair of bone defects in rabbits with tissue-engineered bones using cocultured endothelial progenitor cells (EPCs) and bone marrow mesenchymal stem cells (BMSCs) as seeding cells.

METHODS

Endothelial progenitor cells and BMSCs were isolated and purified from the peripheral blood and bone marrow, respectively, of New Zealand rabbits. The third passage of BMSCs was cultured alone or with EPCs. Cells were characterized using specific markers and then seeded on partially deproteinized biologic bones from pigs as a scaffold. The engineered bones were used to repair bone defects in rabbits. Hematoxylin and eosin and Masson staining were performed to examine vascularization and osteogenesis in the engineered bone.

RESULTS

The cocultured EPCs and BMSCs grew well on the surface of the scaffold. Compared with monocultured BMSCs, cocultured EPCs and BMSCs promoted the formation of blood vessels and bone on the scaffold, in addition to accelerating the repair of bone defects. The collagen content was significantly increased in the scaffold with cocultured EPCs and BMSCs, compared with the scaffold seeded with mono-cultured BMSCs.

CONCLUSIONS

Tissue-engineered bones seeded with cocultured EPCs and BMSCs may be used effectively for the repair of bone defects.

The Roles of Nanoparticles in Stem Cell-Based Therapy for Cardiovascular Disease

Cardiovascular disease (CVD) is currently one of the primary causes of mortality and morbidity worldwide. Nanoparticles (NPs) are playing increasingly important roles in regulating stem cell behavior because of their special features, including shape, size, aspect ratio, surface charge, and surface area. In terms of cardiac disease, NPs can facilitate gene delivery in stem cells, track the stem cells in vivo for long-term monitoring, and enhance retention after their transplantation. The advantages of applying NPs in peripheral vascular disease treatments include facilitating stem cell therapy, mimicking the extracellular matrix environment, and utilizing a safe non-viral gene delivery tool. However, the main limitation of NPs is toxicity, which is related to their size, shape, aspect ratio, and surface charge. Currently, there have been many animal models proving NPs’ potential in treating CVD, but no extensive applications of stem-cell therapy using NPs are available in clinical practice. In conclusion, NPs might have significant potential uses in clinical trials of CVD in the future, thereby meeting the changing needs of individual patients worldwide.

Identification of Potential Hub Genes and Signal Pathways Promoting the Distinct Biological Features of Cord Blood-Derived Endothelial Progenitor Cells Via Bioinformatics

Background: Numerous studies, ranging from the alleviation of tissue ischemia to the assessment of cancer prognosis, have demonstrated the fundamental biological differences between human umbilical cord blood-derived endothelial progenitor cells (CB-EPCs) and adult peripheral blood-derived endothelial progenitor cells (PB-EPCs). However, the underlying molecular mechanisms that produce these differences are not clear.The purpose of this study was to identify potential hub genes, key protein interactive networks, and correlated signal pathways unique to CB-EPC biology via bioinformatic methods. Materials and Methods: We selected the microarray dataset GSE39763 and identified the differentially expressed genes (DEGs) using the "limma" package in the RStudio software. These DEGs were annotated by gene ontology enrichment analyses and signal pathway analyses. A protein-protein interaction (PPI) analysis was then performed to construct PPI networks and identify a hub protein module. We further validated candidate DEGs from the selected module in the gene expression profiling interactive analysis (GEPIA) database because the DEGs were enriched in cancer pathways. Results: Setting an adjusted p-value <0.01 and |Log₂ fold change (FC)| ≥ 2 as cutoff criteria, a total of 346 DEGs, including 314 upregulated genes and 32 downregulated genes in CB-EPCs, were identified. Expression of the genes encoding the AT-Hook Containing Transcription Factor 1 (AHCTF1), the Cancer Susceptibility Candidate 5 (CASC5), the Centromere Protein C (CENPC), the Centromere Protein E (CENPE), the Centromere Protein F (CENPF), the NUF2 Component of NDC80 Kinetochore Complex (NUF2), the RAN-Binding Protein 2 (RANBP2), the Shugoshin-like 2 (SGOL2), the Structural Maintenance of Chromosomes 3 (SMC3), and the Spindle Apparatus Coiled-Coil Protein 1 (SPDL1) proteins were specifically associated with CB-EPCs. Except for CENPC, the other nine genes' expression are all associated with a poorer overall survival rate in cancers. The expression levels of the CENPF and NUF2 genes in tumor patients were significantly higher than those in the controls. Conclusion: The CB-EPCs express genes with greater potential for proliferation and increased migration compared to PB-EPCs; in this regard they are similar to cancer cells.

Combined Transplantation of Adipose Tissue-Derived Stem Cells and Endothelial Progenitor Cells Improve Diabetic Erectile Dysfunction in a Rat Model

Erectile dysfunction (ED) is a common complication in men suffered with diabetic mellitus. Stem cell transplantation is a promising strategy for the treatment of diabetic ED (DED). In this study, we evaluated whether combined transplantation of adipose tissue-derived stem cells (ADSCs) and endothelial progenitor cells (EPCs) could improve the erectile function of the DED rat model. DED rats were induced via intraperitoneal injection of streptozotocin (50 mg/kg), and ED was screened by apomorphine (100 mg/kg). DED rats were divided into 4 groups (n = 14 each): DED, ADSC, EPC, and ADSC/EPC group. Another 14 age-matched male SD rats with normal erectile function were served as the normal group. The normal group and the DED group were received intracavernous injection with phosphate-buffered saline (PBS). And the other groups were received intracavernous injection with ADSCs (1 × 10⁶), EPCs (1 × 10⁶), and ADSCs/EPCs (0.5 × 10⁶/0.5 × 10⁶), respectively. The total intracavernous pressure (ICP) and mean arterial pressure (MAP) were recorded at day 28 after injection. The endothelium, smooth muscle, and penile dorsal nerves were assessed within cavernoursal tissue. On day 28 after injection, the ADSC/EPC group displayed more significantly enhanced ICP and ICP/MAP than the DED or ADSC or EPC group (p < 0.05). Immunofluorescent analysis and western blot demonstrated that the improvement of erectile function in the ADSC/EPC5 group was associated with increased expression of endothelial marker (CD31) and the correction of eNOS-cGMP-NO signaling. More 5-ethynyl-2′-deoxyuridine- (EdU-) positive EPCs could be found lining in the cavernous endothelial layer in the ADSC/EPC group than the EPC group, which was attributed to the paracrine of vascular endothelial growth factor (VEGF) and stromal-derived factor-1 (SDF-1) by ADSCs. Combined transplantation of ADSCs and EPCs has a synergic effect in repairing the endothelial function of DED rats, and the underlying mechanism might be the paracrine of VEGF and SDF-1 by ADSCs, which improves the recruitment and proliferation of EPCs in the cavernosum.

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.

Endothelial cells and angiogenesis in the horse in health and disease-A review

The cardiovascular system is the first functional organ in the embryo, and its blood vessels form a widespread conductive network within the organism. Blood vessels develop de novo, by the differentiation of endothelial progenitor cells (vasculogenesis) or by angiogenesis, which is the formation of new blood vessels from existing ones. This review presents an overview of the current knowledge on physiological and pathological angiogenesis in the horse including studies on equine endothelial cells. Principal study fields in equine angiogenesis research were identified: equine endothelial progenitor cells; equine endothelial cells and angiogenesis (heterogeneity, markers and assessment); endothelial regulatory molecules in equine angiogenesis; angiogenesis research in equine reproduction (ovary, uterus, placenta and conceptus, testis); angiogenesis research in pathological conditions (tumours, ocular pathologies, equine wound healing, musculoskeletal system and laminitis). The review also includes a table that summarizes in vitro studies on equine endothelial cells, either describing the isolation procedure or using previously isolated endothelial cells. A particular challenge of the review was that results published are fragmentary and sometimes even contradictory, raising more questions than they answer. In conclusion, angiogenesis is a major factor in several diseases frequently occurring in horses, but relatively few studies focus on angiogenesis in the horse. The challenge for the future is therefore to continue exploring new therapeutic angiogenesis strategies for horses to fill in the missing pieces of the puzzle.

High expression of endothelial progenitor cell-induced angiogenic markers is associated with bile acid levels in HCC

Endothelial progenitor cell (EPC)-induced angiogenesis activity is enhanced in hepatocellular carcinoma (HCC); however, the contributing factors remain unknown. The present study aimed to investigate the factors influencing the number of EPCs and circulating progenitor cells (CPCs), as well as the expression levels of vascular endothelial growth factor receptor 2 (VEGFR-2) and CD34, in patients with HCC. The expression levels of VEGFR-2 and CD34 were assessed in 72 HCC tumor and matched adjacent tissue microarrays by immunohistochemistry. The associations between VEGFR-2 or CD34 expression in tumors, clinicopathological characteristics and overall survival rates were analyzed. The number of EPCs and CPCs were analyzed in the peripheral blood of patients with HCC. In this study, high expression levels of VEGFR-2 and CD34 were detected in the tumor tissues of 41 (56.9%) and 44 (61.1%) patients, respectively. VEGFR-2 expression was significantly associated with tumor size (P<0.001), bile acid level (P=0.014) and α-fetoprotein level (P=0.011). However, CD34 expression was associated with tumor size (P=0.009), recrudescence (P<0.001) and bile acid (P=0.009). Next, the expression levels of VEGFR-2 and CD34 in tumor and adjacent tissues were compared according to the bile acid level. VEGFR-2 and CD34 expression levels were both higher in the high bile acid group, whereas expression levels of the markers were higher in adjacent tissues compared with tumor tissues. Kaplan-Meier curve analysis identified that patients with low CD34 expression had a longer overall survival compared with patients with high CD34 expression (P=0.029). Multivariate analysis also indicated that both VEGFR-2 (P=0.020) and CD34 (P=0.035) were independent prognostic risk factors. Moreover, flow cytometry demonstrated that the number of EPCs and CPCs was negatively related with the bile acid levels in patients with HCC. In conclusion, in patients with HCC, bile acid promotes EPC-induced angiogenesis. Furthermore, EPCs and CPCs may be activated by bile acid in tumors but are more so in adjacent tissues.

Osteogenic and angiogenic lineage differentiated adipose-derived stem cells for bone regeneration of calvarial defects in rabbits

Cell sheet techniques are widely used in bone engineering. However, vascularization remains a challenge in fabricating vascularized engineered bone. The goal of this study was to induce adipose-derived stem cell (ADSC) osteogenic and angiogenic lineage differentiation and investigate the use of bidiretionally differentiated ADSCs for bone regeneration. ADSCs were cultured to form an osteogenic cell sheet. Other ADSCs were induced to differentiate into endothelial progenitor cells (EPCs), which were identified and characterized by morphological observation and CD31 immunofluorescent staining. Then, the ADSC sheet-EPC complexes were implanted subcutaneously into nude mice, while ADSC sheets alone were implanted as a control. After 8 weeks of transplantation, microcomputed tomography (micro-CT) and histological observation were used to assess bone formation. We then implanted the complexes in calvarial defects in rabbits and assessed bone repair by micro-CT and histological analysis. The ADSC sheets consisted of multiple layers of cells and extracellular matrix. The obtained EPCs formed capillary-like structures and expressed the specific antigen marker CD31. The osteogenic ADSC sheet-EPC complexes formed dense and well-vascularized new bone tissue at 8 weeks after implantation. Bone density was significantly lower in the control group than in the complex group (p < .05). In addition, the reconstruction of calvarial defects in rabbits in complex group was obviously greater than that in the control group (p < .05). These results suggested that the approach of engineering bone tissue with bidiretionally differentiated ADSCs enabled bone regeneration, thus offering a promising strategy for repairing bone defects.

Microvascular Fragments: More Than Just Natural Vascularization Units

Adipose tissue-derived microvascular fragments serve as natural vascularization units in angiogenesis research and tissue engineering due to their ability to rapidly reassemble into microvascular networks. Recent studies indicate that they exhibit additional unique properties that may be beneficial for a wide range of future biomedical applications. Their angiogenic activity can be increased during short-term cultivation as a means of adapting their vascularization capacity to patient-specific needs. Moreover, they are a source of endothelial progenitor cells, multipotent mesenchymal stromal cells, and lymphatic vessel fragments. Finally, they exert immunomodulatory effects, determining the tissue integration of implanted biomaterials. Hence, microvascular fragments represent versatile building blocks for the improvement of vascularization, organotypic tissue formation, lymphatic regeneration, and implant integration.

Biofabrication Strategies and Engineered In Vitro Systems for Vascular Mechanobiology

The vascular system is integral for maintaining organ-specific functions and homeostasis. Dysregulation in vascular architecture and function can lead to various chronic or acute disorders. Investigation of the role of the vascular system in health and disease has been accelerated through the development of tissue-engineered constructs and microphysiological on-chip platforms. These in vitro systems permit studies of biochemical regulation of vascular networks and parenchymal tissue and provide mechanistic insights into the biophysical and hemodynamic forces acting in organ-specific niches. Detailed understanding of these forces and the mechanotransductory pathways involved is necessary to develop preventative and therapeutic strategies targeting the vascular system. This review describes vascular structure and function, the role of hemodynamic forces in maintaining vascular homeostasis, and measurement approaches for cell and tissue level mechanical properties influencing vascular phenomena. State-of-the-art techniques for fabricating in vitro microvascular systems, with varying degrees of biological and engineering complexity, are summarized. Finally, the role of vascular mechanobiology in organ-specific niches and pathophysiological states, and efforts to recapitulate these events using in vitro microphysiological systems, are explored. It is hoped that this review will help readers appreciate the important, but understudied, role of vascular-parenchymal mechanotransduction in health and disease toward developing mechanotherapeutics for treatment strategies.

Vascularized Biomaterials to Study Cancer Metastasis

Cancer metastasis, the spread of cancer cells to distant organs, is responsible for 90% of cancer-related deaths. Cancer cells need to enter and exit circulation in order to form metastases, and the vasculature and endothelial cells are key regulators of this process. While vascularized 3D in vitro systems have been developed, few have been used to study cancer, and many lack key features of vessels that are necessary to study metastasis. This review focuses on current methods of vascularizing biomaterials for the study of cancer, and three main factors that regulate intravasation and extravasation: endothelial cell heterogeneity, hemodynamics, and the extracellular matrix of the perivascular niche.

Differentiation of Adipose Tissue-Derived CD34+/CD31- Cells into Endothelial Cells In Vitro

In this study, CD34+/CD31- progenitor cells were isolated from the stromal vascular fraction (SVF) of adipose tissue using magnetic activated cell sorting. The endothelial differentiation capability of these cells in vitro was evaluated by culturing them in vascular endothelial growth factor (VEGF) induced medium for 14 days. Viability, proliferation, differentiation and tube formation of these cells were evaluated. Cell viability study revealed that both undifferentiated and endothelial differentiated cells remained healthy for 14 days. However, the proliferation rate was higher in undifferentiated cells compared to endothelial differentiated ones. Upregulation of endothelial characteristic genes (Von Willebrand Factor (vWF) and VE Cadherin) was observed in 2D culture. However, PECAM (CD31) was only found to be upregulated after the cells had formed tube-like structures in 3D Matrigel culture. These results indicate that adipose derived CD34+/CD31- cells when cultured in VEGF induced medium, are capable differentiation into endothelial-like lineages. Tube formation of the cells started 3h after seeding the cells on Matrigel and formed more stable and connected network 24 h post seeding in presence of VEGF.

Targeting Purinergic Signaling and Cell Therapy in Cardiovascular and Neurodegenerative Diseases

Extracellular purines exert several functions in physiological and pathophysiological mechanisms. ATP acts through P2 receptors as a neurotransmitter and neuromodulator and modulates heart contractility, while adenosine participates in neurotransmission, blood pressure, and many other mechanisms. Because of their capability to differentiate into mature cell types, they provide a unique therapeutic strategy for regenerating damaged tissue, such as in cardiovascular and neurodegenerative diseases. Purinergic signaling is pivotal for controlling stem cell differentiation and phenotype determination. Proliferation, differentiation, and apoptosis of stem cells of various origins are regulated by purinergic receptors. In this chapter, we selected neurodegenerative and cardiovascular diseases with clinical trials using cell therapy and purinergic receptor targeting. We discuss these approaches as therapeutic alternatives to neurodegenerative and cardiovascular diseases. For instance, promising results were demonstrated in the utilization of mesenchymal stem cells and bone marrow mononuclear cells in vascular regeneration. Regarding neurodegenerative diseases, in general, P2X7 and A_2A receptors mostly worsen the degenerative state. Stem cell-based therapy, mainly through mesenchymal and hematopoietic stem cells, showed promising results in improving symptoms caused by neurodegeneration. We propose that purinergic receptor activity regulation combined with stem cells could enhance proliferative and differentiation rates as well as cell engraftment.

Therapeutic Potential of Endothelial Colony Forming Cells Derived from Human Umbilical Cord Blood

Endothelial progenitor cells (EPCs) are implicated in multiple biologic processes such as vascular homeostasis, neovascularization and tissue regeneration, and tumor angiogenesis. A subtype of EPCs is referred to as endothelial colony-forming cells (ECFCs), which display robust clonal proliferative potential and can form durable and functional blood vessels in animal models. In this review, we provide a brief overview of EPCs' characteristics, classification and origins, a summary of the progress in preclinical studies with regard to the therapeutic potential of human umbilical cord blood derived ECFCs (CB-ECFCs) for ischemia repair, tissue engineering and tumor, and highlight the necessity to select high proliferative CB-ECFCs and to optimize their recovery and expansion conditions.

Spatial localization of endothelial cells in heterotypic spheroids influences Notch signaling

Cell-based therapeutic approaches are an exciting strategy to replenish compromised endothelial cell (EC) populations that contribute to impaired vasculogenesis. Co-cultures of ECs and mesenchymal stromal cells (MSCs) can enhance neovascularization over ECs alone, but the efficacy of cells is limited by rapid cell death upon implantation. Co-culture spheroids exhibit improved survival compared with monodisperse cells, yet little is known about the influence of spatial regulation of ECs within co-culture spheroids. We hypothesized that EC sprouting from co-culture spheroids is a function of EC spatial localization. We formed co-culture spheroids containing ECs and MSCs in two formats: ECs uniformly distributed throughout the spheroid (i.e., mixed) or seeded on the perimeter of the MSC core (i.e., shell). Qualitative observations suggested increased vasculogenesis for mixed co-culture spheroids compared with shell conformations as early as day 3, yet quantitative metrics did not reveal significant differences in network formation between these 3D structures. Notch3 expression demonstrated significant increases in cell-cell communication in mixed conformations compared with shell counterparts. Furthermore, knockdown of Notch3 in MSCs abrogated the vasculogenic potential of mixed spheroids, supporting its role in promoting EC-MSC contacts. This study highlights the direct impact of EC-MSC contacts on sprouting and provides insight to improve the quality of network formation. KEY MESSAGES: • Endothelial cell (EC) localization can be controlled in co-culture EC-MSC spheroids. • Mixed spheroids exhibit consistent networks compared to shell counterparts. • Differences in NOTCH3 were observed between mixed and shell spheroids. • NOTCH3 may be a necessary target for improved vasculogenic potential.

Vascularization in tissue engineering: fundamentals and state-of-art

Vascularization is among the top challenges that impede the clinical application of engineered tissues. This challenge has spurred tremendous research endeavor, defined as vascular tissue engineering (VTE) in this article, to establish a pre-existing vascular network inside the tissue engineered graft prior to implantation. Ideally, the engineered vasculature can be integrated into the host vasculature via anastomosis to supply nutrient to all cells instantaneously after surgery. Moreover, sufficient vascularization is of great significance in regenerative medicine from many other perspectives. Due to the critical role of vascularization in successful tissue engineering, we aim to provide an up-to-date overview of the fundamentals and VTE strategies in this article, including angiogenic cells, biomaterial/bio-scaffold design and bio-fabrication approaches, along with the reported utility of vascularized tissue complex in regenerative medicine. We will also share our opinion on the future perspective of this field.

Endothelial progenitor cells promote viability and nerve regenerative ability of mesenchymal stem cells through PDGF-BB/PDGFR-β signaling

Denervation-induced erectile dysfunction (ED) is a prevailing health problem. Our previous study revealed that endothelial progenitor cells (EPCs) promoted the effect of mesenchymal stem cells (MSCs) on restoration of denervation-induced ED in rats. However, underling mechanisms are still largely elusive. In this study, EPCs and MSCs were co-cultured and resorted to co-EPCs and co-MSCs. EPCs-derived paracrine factors containing PDGF-BB (platelet-derived growth factor) were detected, and MSCs were pre-treated with PDGF-BB, while co-MSCs were pre-treated with PDGFR inhibitor AG1296. Either viability or nerve regenerative ability of MSCs was evaluated. In addition, inhibition of either PI3K/Akt or MEK/Erk pathway was performed to evaluate the role of PI3K/Akt and MEK/Erk pathway in PDGF-BB-induced viability of MSCs. The results revealed that PDGF-BB significantly increased the proportion of PDGFR-β⁺ MSCs, and promoted both in-vitro and in-vivo viability, as well as nerve regenerative capacity and erectile function restoration of MSCs in rats. Inhibition of PI3K/Akt, MEK/Erk pathway or mTOR led to decrease of PDGF-BB/PDGFR-β induced viability of MSCs. To our knowledge, our study first demonstrates that EPCs promote viability and potential nerve regenerative ability of MSCs through PDGF-BB/PDGFR-β signaling and its downstream PI3K/Akt and MEK/Erk pathways. mTOR acts as a co-mediator in PI3K/Akt and MEK/Erk pathways.

Endothelialized microrods for minimally invasive in situ neovascularization

Despite the significant advancements in fabricating various scaffolding systems over the past decades, generation of functional tissues towards vascularization remains challenging for the currently available biofabrication approaches. On the other hand, the applicability of traditional surgical transplantation of vascularized tissue constructs is sometimes limited due to the sophisticated surgical procedures, which are invasive, leading to increased risks of scar formation and infection. Considering these facts, we present an innovative platform, the angiogenic microrods composed of sodium alginate/gelatin harboring proliferating endothelial cells using a specially designed double T-junction microfluidic device with an expansion chamber, for achieving minimally invasive neovascularization in situ. Such vessel-like microarchitectures could be derived through controlled penetration of the crosslinker genepin for the gelatin phase, ensuing differential degrees in crosslinking of peripheral and central portions of the microstructures, thus resulting in the formation of vascular lumen-like hollow cavities via endothelial cell migration and proliferation during culture in vitro. Furthermore, in vivo performance of the microrods was explored. We believe that the development of these modular microrods for minimally invasive delivery is of great interest and offers a convenient approach for vascularization in situ.

Towards clinical application of tissue engineering for erectile penile regeneration

Penile wounds after traumatic and surgical amputation require reconstruction in the form of autologous tissue transfers. However, currently used techniques are associated with high infection rates, implant erosion and donor site morbidity. The use of tissue-engineered neocorpora provides an alternative treatment option. Contemporary tissue-engineering strategies enable the seeding of a biomaterial scaffold and subsequent implantation to construct a neocorpus. Tissue engineering of penile tissue should focus on two main strategies: first, correcting the volume deficit for structural integrity in order to enable urinary voiding in the standing position and second, achieving erectile function for sexual activity. The functional outcomes of the neocorpus can be addressed by optimizing the use of stem cells and scaffolds, or alternatively, the use of gene therapy. Current research in penile tissue engineering is largely restricted to rodent and rabbit models, but the use of larger animal models should be considered as a better representation of the anatomical and physiological function in humans. The development of a cell-seeded scaffold to achieve and maintain erection continues to be a considerable challenge in humans. However, advances in penile tissue engineering show great promise and, in combination with gene therapy and surgical techniques, have the potential to substantially improve patient outcomes.

Human Umbilical Vein Endothelial Cells (HUVECs) Co-Culture with Osteogenic Cells: From Molecular Communication to Engineering Prevascularised Bone Grafts

The repair of bone defects caused by trauma, infection or tumor resection is a major clinical orthopedic challenge. The application of bone grafts in orthopedic procedures is associated with a problem of inadequate vascularization in the initial phase after implantation. Meanwhile, the survival of cells within the implanted graft and its integration with the host tissue is strongly dependent on nutrient and gaseous exchange, as well as waste product removal, which are effectuated by blood microcirculation. In the bone tissue, the vasculature also delivers the calcium and phosphate indispensable for the mineralization process. The critical role of vascularization for bone healing and function, led the researchers to the idea of generating a capillary-like network within the bone graft in vitro, which could allow increasing the cell survival and graft integration with a host tissue. New strategies for engineering pre-vascularized bone grafts, that apply the co-culture of endothelial and bone-forming cells, have recently gained interest. However, engineering of metabolically active graft, containing two types of cells requires deep understanding of the underlying mechanisms of interaction between these cells. The present review focuses on the best-characterized endothelial cells-human umbilical vein endothelial cells (HUVECs)-attempting to estimate whether the co-culture approach, using these cells, could bring us closer to development and possible clinical application of prevascularized bone grafts.