Stem cell sources for vascular tissue engineering and regeneration

Three-Dimensional Bioprinting in Vascular Tissue Engineering and Tissue Vascularization of Cardiovascular Diseases

In the 21st century, significant progress has been made in repairing damaged materials through material engineering. However, the creation of large-scale artificial materials still faces a major challenge in achieving proper vascularization. To address this issue, researchers have turned to biomaterials and three-dimensional (3D) bioprinting techniques, which allow for the combination of multiple biomaterials with improved mechanical and biological properties that mimic natural materials. Hydrogels, known for their ability to support living cells and biological components, have played a crucial role in this research. Among the recent developments, 3D bioprinting has emerged as a promising tool for constructing hybrid scaffolds. However, there are several challenges in the field of bioprinting, including the need for nanoscale biomimicry, the formulation of hydrogel blends, and the ongoing complexity of vascularizing biomaterials, which requires further research. On a positive note, 3D bioprinting offers a solution to the vascularization problem due to its precise spatial control, scalability, and reproducibility compared with traditional fabrication methods. This paper aims at examining the recent advancements in 3D bioprinting technology for creating blood vessels, vasculature, and vascularized materials. It provides a comprehensive overview of the progress made and discusses the limitations and challenges faced in current 3D bioprinting of vascularized tissues. In addition, the paper highlights the future research directions focusing on the development of 3D bioprinting techniques and bioinks for creating functional materials.

Inhibition of glutaminolysis restores mitochondrial function in senescent stem cells

Mitochondrial dysfunction, a hallmark of aging, has been associated with the onset of aging phenotypes and age-related diseases. Here, we report that impaired mitochondrial function is associated with increased glutamine catabolism in senescent human mesenchymal stem cells (MSCs) and myofibroblasts derived from patients suffering from Hutchinson-Gilford progeria syndrome. Increased glutaminase (GLS1) activity accompanied by loss of urea transporter SLC14A1 induces urea accumulation, mitochondrial dysfunction, and DNA damage. Conversely, blocking GLS1 activity restores mitochondrial function and leads to amelioration of aging hallmarks. Interestingly, GLS1 expression is regulated through the JNK pathway, as demonstrated by chemical and genetic inhibition. In agreement with our in vitro findings, tissues isolated from aged or progeria mice display increased urea accumulation and GLS1 activity, concomitant with declined mitochondrial function. Inhibition of glutaminolysis in progeria mice improves mitochondrial respiratory chain activity, suggesting that targeting glutaminolysis may be a promising strategy for restoring age-associated loss of mitochondrial function.

Clinical Application for Tissue Engineering Focused on Materials

Cardiovascular-related medical conditions remain a significant cause of death worldwide despite the advent of tissue engineering research more than half a century ago. Although autologous tissue is still the preferred treatment, donor tissue is limited, and there remains a need for tissue-engineered vascular grafts (TEVGs). The production of extensive vascular tissue (>1 cm3) in vitro meets the clinical needs of tissue grafts and biological research applications. The use of TEVGs in human patients remains limited due to issues related to thrombogenesis and stenosis. In addition to the advancement of simple manufacturing methods, the shift of attention to the combination of synthetic polymers and bio-derived materials and cell sources has enabled synergistic combinations of vascular tissue development. This review details the selection of biomaterials, cell sources and relevant clinical trials related to large diameter vascular grafts. Finally, we will discuss the remaining challenges in the tissue engineering field resulting from complex requirements by covering both basic and clinical research from the perspective of material design.

Tissue engineered in-vitro vascular patch fabrication using hybrid 3D printing and electrospinning

Three-dimensional (3D) engineered cardiovascular tissues have shown great promise to replace damaged structures. Specifically, tissue engineering vascular grafts (TEVG) have the potential to replace biological and synthetic grafts. We aimed to design an in-vitro patient-specific patch based on a hybrid 3D print combined with vascular smooth muscle cells (VSMC) differentiation. Based on the medical images of a 2 months-old girl with aortic arch hypoplasia and using computational modelling, we evaluated the most hemodynamically efficient aortic patch surgical repair. Using the designed 3D patch geometry, the scaffold was printed using a hybrid fused deposition modelling (FDM) and electrospinning techniques. The scaffold was seeded with multipotent mesenchymal stem cells (MSC) for later maturation to derived VSMC (dVSMC). The graft showed adequate resistance to physiological aortic pressure (burst pressure 101 ​± ​15 ​mmHg) and a porosity gradient ranging from 80 to 10 ​μm allowing cells to infiltrate through the entire thickness of the patch. The bio-scaffolds showed good cell viability at days 4 and 12 and adequate functional vasoactive response to endothelin-1. In summary, we have shown that our method of generating patient-specific patch shows adequate hemodynamic profile, mechanical properties, dVSMC infiltration, viability and functionality. This innovative 3D biotechnology has the potential for broad application in regenerative medicine and potentially in heart disease prevention.

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This study combines multidisciplinary approach for bioprinting patient-specific.

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We create a 3D scaffold, printed using a hybrid fused deposition modelling and electrospinning techniques.

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The graft shows adequate resistance to physiological aortic pressure and a porosity gradient.

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Multipotent mesenchymal stem cells seeded in the scaffold are differentiated to derived vascular smooth muscle cells.

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dVSMC shows adequate endothelin- 1 induced Ca2+ increase associated with ETA overexpression.

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.

Current Progress in Vascular Engineering and Its Clinical Applications

Coronary heart disease (CHD) is caused by narrowing or blockage of coronary arteries due to atherosclerosis. Coronary artery bypass grafting (CABG) is widely used for the treatment of severe CHD cases. Although autologous vessels are a preferred choice, healthy autologous vessels are not always available; hence there is a demand for tissue engineered vascular grafts (TEVGs) to be used as alternatives. However, producing clinical grade implantable TEVGs that could healthily survive in the host with long-term patency is still a great challenge. There are additional difficulties in producing small diameter (<6 mm) vascular conduits. As a result, there have not been TEVGs that are commercially available. Properties of vascular scaffolds such as tensile strength, thrombogenicity and immunogenicity are key factors that determine the biocompatibility of TEVGs. The source of vascular cells employed to produce TEVGs is a limiting factor for large-scale productions. Advanced technologies including the combined use of natural and biodegradable synthetic materials for scaffolds in conjunction with the use of mesenchyme stem cells or induced pluripotent stem cells (iPSCs) provide promising solutions for vascular tissue engineering. The aim of this review is to provide an update on various aspects in this field and the current status of TEVG clinical applications.

Vascular Tissue Engineering: Challenges and Requirements for an Ideal Large Scale Blood Vessel

Since the emergence of regenerative medicine and tissue engineering more than half a century ago, one obstacle has persisted: the in vitro creation of large-scale vascular tissue (>1 cm³) to meet the clinical needs of viable tissue grafts but also for biological research applications. Considerable advancements in biofabrication have been made since Weinberg and Bell, in 1986, created the first blood vessel from collagen, endothelial cells, smooth muscle cells and fibroblasts. The synergistic combination of advances in fabrication methods, availability of cell source, biomaterials formulation and vascular tissue development, promises new strategies for the creation of autologous blood vessels, recapitulating biological functions, structural functions, but also the mechanical functions of a native blood vessel. In this review, the main technological advancements in bio-fabrication are discussed with a particular highlights on 3D bioprinting technologies. The choice of the main biomaterials and cell sources, the use of dynamic maturation systems such as bioreactors and the associated clinical trials will be detailed. The remaining challenges in this complex engineering field will finally be discussed.

The simpler, the better: tissue vascularization using the body's own resources

Tissue regeneration is crucially dependent on sufficient vascularization. In regenerative medicine, this can be effectively achieved by autologous vascularization strategies using the body's own resources. These strategies include the administration of blood-derived factor preparations, adipose tissue-based vascularization, and the in situ engineering of vascularized tissue. Due to their simplicity, the translation of these strategies into clinical practice is easier in terms of feasibility, safety requirements, and regulatory hurdles compared with complex and time-consuming procedures involving intensive cell manipulation. Hence, they are close to clinical application or are already being used to successfully treat patients by distinct personalized medicine concepts.

Immobilization of Jagged1 Enhances Vascular Smooth Muscle Cells Maturation by Activating the Notch Pathway

In Notch signaling, the Jagged1-Notch3 ligand-receptor pairing is implicated for regulating the phenotype maturity of vascular smooth muscle cells. However, less is known about the role of Jagged1 presentation strategy in this regulation. In this study, we used bead-immobilized Jagged1 to direct phenotype control of primary human coronary artery smooth muscle cells (HCASMC), and to differentiate embryonic multipotent mesenchymal progenitor (10T1/2) cell towards a vascular lineage. This Jagged1 presentation strategy was sufficient to activate the Notch transcription factor HES1 and induce early-stage contractile markers, including smooth muscle α-actin and calponin in HCASMCs. Bead-bound Jagged1 was unable to regulate the late-stage markers myosin heavy chain and smoothelin; however, serum starvation and TGFβ1 were used to achieve a fully contractile smooth muscle cell. When progenitor 10T1/2 cells were used for Notch3 signaling, pre-differentiation with TGFβ1 was required for a robust Jagged1 specific response, suggesting a SMC lineage commitment was necessary to direct SMC differentiation and maturity. The presence of a magnetic tension force to the ligand-receptor complex was evaluated for signaling efficacy. Magnetic pulling forces downregulated HES1 and smooth muscle α-actin in both HCASMCs and progenitor 10T1/2 cells. Taken together, this study demonstrated that (i) bead-bound Jagged1 was sufficient to activate Notch3 and promote SMC differentiation/maturation and (ii) magnetic pulling forces did not activate Notch3, suggesting the bead alone was able to provide necessary clustering or traction forces for Notch activation. Notch is highly context-dependent; therefore, these findings provide insights to improve biomaterial-driven Jagged1 control of SMC behavior.

Cellularized small-caliber tissue-engineered vascular grafts: looking for the ultimate gold standard

Due to the lack of efficacy of synthetic vascular substitutes in the replacement of small-caliber arteries, vascular tissue engineering (VTE) has emerged as a promising solution to produce viable small-caliber tissue-engineered vascular grafts (TEVG). Previous studies have shown the importance of a cellular intimal layer at the luminal surface of TEVG to prevent thrombotic events. However, the cellularization of a TEVG seems to be a critical approach to consider in the development of a TEVG. To date, no standard cellularization method or cell type has been established to create the ideal TEVG by promoting its long-term patency and function. In this review, advances in VTE are described and discussed with a particular focus on the construction approaches of cellularized small-caliber TEVGs, the cell types used, as well as their preclinical and clinical applications.

A 3-Layered Bioartificial Blood Vessel with Physiological Wall Architecture Generated by Mechanical Stimulation

The generation of cellularized bioartificial blood vessels resembling all three layers of the natural vessel wall with physiological morphology and cell alignment is a long pursued goal in vascular tissue engineering. Simultaneous culture of all three layers under physiological mechanical conditions requires highly sophisticated perfusion techniques and still today remains a key challenge. Here, three-layered bioartificial vessels based on fibrin matrices were generated using a stepwise molding technique. Adipose-derived stem cells (ASC) were differentiated to smooth muscle cells (SMC) and integrated in a compacted tubular fibrin matrix to resemble the tunica media. The tunica adventitia-equivalent containing human umbilical vein endothelial cells (HUVEC) and ASC in a low concentration fibrin matrix was molded around it. Luminal seeding with HUVEC resembled the tunica intima. Subsequently, constructs were exposed to physiological mechanical stimulation in a pulsatile bioreactor for 72 h. Compared to statically incubated controls, mechanical stimulation induced physiological cell alignment in each layer: Luminal endothelial cells showed longitudinal alignment, cells in the media-layer were aligned circumferentially and expressed characteristic SMC marker proteins. HUVEC in the adventitia-layer formed longitudinally aligned microvascular tubes resembling vasa vasorum capillaries. Thus, physiologically organized three-layered bioartificial vessels were successfully manufactured by stepwise fibrin molding with subsequent mechanical stimulation.

Umbilical Cord Pericytes Provide a Viable Alternative to Mesenchymal Stem Cells for Neonatal Vascular Engineering

Reconstructive surgery of congenital heart disease (CHD) remains inadequate due to the inability of prosthetic grafts to match the somatic growth of pediatric patients. Functionalization of grafts with mesenchymal stem cells (MSCs) may provide a solution. However, MSCs represent a heterogeneous population characterized by wide diversity across different tissue sources. Here we investigated the suitability of umbilical cord pericytes (UCPs) in neonatal vascular engineering. Explant outgrowth followed by immunomagnetic sorting was used to isolate neural/glial antigen 2 (NG2)+/CD31- UCPs. Expanded NG2 UCPs showed consistent antigenic phenotype, including expression of mesenchymal and stemness markers, and high proliferation rate. They could be induced to a vascular smooth muscle cell-like phenotype after exposure to differentiation medium, as evidenced by the expression of transgelin and smooth muscle myosin heavy chain. Analysis of cell monolayers and conditioned medium revealed production of extracellular matrix proteins and the secretion of major angiocrine factors, which conferred UCPs with ability to promote endothelial cell migration and tube formation. Decellularized swine-derived grafts were functionalized using UCPs and cultured under static and dynamic flow conditions. UCPs were observed to integrate into the outer layer of the graft and modify the extracellular environment, resulting in improved elasticity and rupture strain in comparison with acellular grafts. These findings demonstrate that a homogeneous pericyte-like population can be efficiently isolated and expanded from human cords and integrated in acellular grafts currently used for repair of CHD. Functional assays suggest that NG2 UCPs may represent a viable option for neonatal tissue engineering applications.

Application of tissue-engineered interventions for coronary artery bypass grafts

Coronary artery bypass graft is one of the extensively conducted procedures to release occlusion in the coronary vessel. Various biological grafts are used for this purpose, superiorly, saphenous vein graft, if unavailable, other vessels in the body, with likewise characteristics are exploited for the purpose. The choice of graft is yet under discovery that could impeccably meet all the requirements. Variation in perioperative and postoperative results have given uneven clinical inferences of these conduits. Alternatively, tissue-engineering is also being applied in this area for clinical improvements. This review underlines some of the commonly used grafts for coronary artery bypass graft and advancements in tissue engineering for this purpose.

Paracrine signalling from monocytes enables desirable extracellular matrix accumulation and temporally appropriate phenotype of vascular smooth muscle cell-like cells derived from adipose stromal cells

In vascular tissue engineering, the ability to obtain a robust and safe vascular tissue cell source (e.g. vascular smooth muscle cells (VSMCs)) and to promote vascular tissue-specific extracellular matrix (ECM) protein production is critically important. Mature blood vessel-derived VSMCs are not practical for in vitro vascular tissue regeneration. The authors have conceived a strategy to differentiate adipose derived stromal cells (ASCs) into VSMC-like cells (ASC-VSMCs) that were similar to mature umbilical artery VSMCs at the transcriptional, protein and contraction function levels. Monocytes/macrophages are known as important regulators of the inflammation and regeneration processes within different tissue types of the body. However, our understanding of the potential interactions between specific tissue-like cells differentiated from stem/stromal cells (e.g. ASC-VSMCs) and monocytes/macrophages (cued by specific biomaterial scaffolds) is still limited. In this study, indirect and direct ASC-VSMC-monocyte co-cultures were constructed within a porous polyurethane scaffold (D-PHI) previously shown to have an immunomodulatory character. The effects of monocytes/macrophages on the cellularity (cell number detected with DNA quantification assay), ECM (glycosaminoglycan (GAG), collagen, and elastin) accumulation as well as the maintenance of contractile VSMC markers (calponin and smoothelin) of the ASC-VSMCs after a month of co-culture were investigated. It was found that monocyte paracrine signalling in D-PHI positively affected the cellularity and ECM accumulation of ASC-VSMCs in co-culture. Cause-effect relationships were also identified between the release of pro-inflammatory/anti-inflammatory factors (i.e. IL6, TGF-β1) in co-culture and the expression of contractile proteins (calponin and smoothelin) by ASC-VSMCs. This study demonstrated the importance of combining an immune cell strategy with stromal cell derived VSMCs (i.e. ASC-VSMCs) to achieve a practical vascular tissue engineering outcome. STATEMENT OF SIGNIFICANCE: Adipose stromal cell derived-vascular smooth muscle cells (ASC-VSMCs) are a promising cell source for vascular tissue engineering. Monocytes/monocyte derived macrophages can be harnessed as an immune-assisted strategy to promote vascular tissue regeneration. This study demonstrated that the co-culture of human ASC-VSMCs with monocytes significantly enhanced the cellularity and extracellular matrix (ECM) accumulation within anionic polyurethane (D-PHI) scaffolds, partially mediated by monocyte paracrine signalling mechanisms. In addition, specific VSMC contractile markers (calponin and smoothelin) were still present in ASC-VSMCs when the cells were exposed to monocytes for a month in vitro. This study corroborated the potential selection of ASC-VSMCs for in vitro engineering of vascular tissue in an immunomodulatory biomaterial scaffold (e.g. D-PHI) based co-culture system containing monocytes.

Adipose-derived stem cells contribute to cardiovascular remodeling

Obesity is an independent risk factor for cardiovascular disease. Adipose tissue was initially thought to be involved in metabolism through paracrine. Recent researches discovered mesenchymal stem cells inside adipose tissue which could differentiate into vascular lineages in vitro and in vivo, participating vascular remodeling. However, there were few researches focusing on distinct characteristics and functions of adipose-derived stem cells (ADSCs) from different regions. This is the first comprehensive review demonstrating the variances of ADSCs from the perspective of their origins.

Autologous endothelialized small-caliber vascular grafts engineered from blood-derived induced pluripotent stem cells

An ideal cell source for human therapeutic and disease modeling applications should be easily accessible and possess unlimited differentiation and expansion potential. Human induced pluripotent stem cells (hiPSCs) derived from peripheral blood mononuclear cells (PBMCs) represent a promising source given their ease of harvest and their pluripotent nature. Previous studies have demonstrated the feasibility of using PBMC-derived hiPSCs for vascular tissue engineering. However, so far, no endothelialization of hiPSC-derived tissue engineered vascular grafts (TEVGs) based on fully biodegradable polymers without xenogenic matrix components has been shown. In this study, we have generated hiPSCs from PBMCs and differentiated them into αSMA- and calponin-positive smooth muscle cells (SMCs) as well as endothelial cells (ECs) positive for CD31, vWF and eNOS. Both cell types were co-seeded on PGA-P4HB starter matrices and cultured under static or dynamic conditions to induce tissue formation in vitro. The resulting small diameter vascular grafts showed abundant amounts of extracellular matrix, containing a thin luminal layer of vWF-positive cells and a subendothelial αSMA-positive layer approximating the architecture of native vessels. Our results demonstrate the successful generation of TEVGs based on SMCs and ECs differentiated from PBMC-derived hiPSC combined with a biodegradable polymer. These results pave the way for developing autologous PBMC-derived hiPSC-based vascular constructs for therapeutic applications or disease modeling. STATEMENT OF SIGNIFICANCE: We report for the first time the possibility to employ human peripheral blood mononuclear cell (PBMC)-derived iPSCs to generate biodegradable polymer-based tissue engineered vascular grafts (TEVG), which mimic the native layered architecture of blood vessels. hiPSCs from PBMCs were differentiated into smooth muscle cells as well as endothelial cells. These cells were co-seeded on a biodegradable PGA/P4HB scaffold and cultured in a bioreactor to induce tissue formation in vitro. The resulting small diameter TEVG showed abundant amounts of extracellular matrix, containing a αSMA-positive layer in the interstitium and a thin luminal layer of vWF-positive endothelial cells approximating the architecture of native vessels. Our findings improving the generation of autologous vascular replacements using blood as an easily accessible cell source.

The ins and outs of engineering functional tissues and organs: evaluating the in-vitro and in-situ processes

PURPOSE OF REVIEW

For many disorders that result in loss of organ function, the only curative treatment is organ transplantation. However, this approach is severely limited by the shortage of donor organs. Tissue engineering has emerged as an alternative solution to this issue. This review discusses the concept of tissue engineering from a technical viewpoint and summarizes the state of the art as well as the current shortcomings, with the aim of identifying the key lessons that we can learn to further advance the engineering of functional tissues and organs.

RECENT FINDINGS

A plethora of tissue-engineering strategies have been recently developed. Notably, these strategies put different emphases on the in-vitro and in-situ processes (i.e. preimplantation and postimplantation) that take place during tissue formation. Biophysical and biomechanical interactions between the cells and the scaffold/biomaterial play a crucial role in all steps and have started to be exploited to steer tissue regeneration.

SUMMARY

Recent works have demonstrated the need to better understand the in-vitro and in-situ processes during tissue formation, in order to regenerate complex, functional organs with desired cellular organization and tissue architecture. A concerted effort from both fundamental and tissue-specific research has the potential to accelerate progress in the field.

Vascular tissue engineering: Fabrication and characterization of acetylsalicylic acid-loaded electrospun scaffolds coated with amniotic membrane lysate

As the incidence of small-diameter vascular graft (SDVG) occlusion is considerably high, a great amount of research is focused on constructing a more biocompatible graft. The absence of a biocompatible surface in the lumen of the engineered grafts that can support confluent lining with endothelial cells (ECs) can cause thrombosis and graft failure. Blood clot formation is mainly because of the lack of an integrated endothelium. The most effective approach to combat this problem would be using natural extracellular matrix constituents as a mimic of endothelial basement membrane along with applying anticoagulant agents to provide local antithrombotic effects. In this study, we fabricated aligned and random electrospun poly-L-lactic acid (PLLA) scaffolds containing acetylsalicylic acid (ASA) as the anticoagulation agent and surface coated them with amniotic membrane (AM) lysate. Vascular scaffolds were structurally and mechanically characterized and assessed for cyto- and hemocompatibility and their ability to support endothelial differentiation was examined. All the scaffolds showed appropriate tensile strength as expected for vascular grafts. Lack of cytotoxicity, cellular attachment, growth, and infiltration were proved using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and scanning electron microscopy. The blood compatibilities of different scaffolds examined by in vitro hemolysis and blood coagulation assays elucidated the excellent hemocompatibility of our novel AM-coated ASA-loaded nanofibers. Drug-loaded scaffolds showed a sustained release profile of ASA in 7 days. AM-coated electrospun PLLA fibers showed enhanced cytocompatibility for human umbilical vein ECs, making a confluent endothelial-like lining. In addition, AM lysate-coated ASA-PLLA-aligned scaffold proved to support endothelial differentiation of Wharton's jelly-derived mesenchymal stem cells. Our results together indicated that AM lysate-coated ASA releasing scaffolds have promising potentials for development of a biocompatible SDVG.

Engineering blood vessels and vascularized tissues: technology trends and potential clinical applications

Vascular tissue engineering has the potential to make a significant impact on the treatment of a wide variety of medical conditions, including providing in vitro generated vascularized tissue and organ constructs for transplantation. Since the first report on the construction of a biological blood vessel, significant research and technological advances have led to the generation of clinically relevant large and small diameter tissue engineered vascular grafts (TEVGs). However, developing a biocompatible blood-contacting surface is still a major challenge. Researchers are using biomimicry to generate functional vascular grafts and vascular networks. A multi-disciplinary approach is being used that includes biomaterials, cells, pro-angiogenic factors and microfabrication technologies. Techniques to achieve spatiotemporal control of vascularization include use of topographical engineering and controlled-release of growth/pro-angiogenic factors. Use of decellularized natural scaffolds has gained popularity for engineering complex vascularized organs for potential clinical use. Pre-vascularization of constructs prior to implantation has also been shown to enhance its anastomosis after implantation. Host-implant anastomosis is a phenomenon that is still not fully understood. However, it will be a critical factor in determining the in vivo success of a TEVGs or bioengineered organ. Many clinical studies have been conducted using TEVGs, but vascularized tissue/organ constructs are still in the research & development stage. In addition to technical challenges, there are commercialization and regulatory challenges that need to be addressed. In this review we examine recent advances in the field of vascular tissue engineering, with a focus on technology trends, challenges and potential clinical applications.

Embryonic Mesenchymal Multipotent Cell Differentiation on Electrospun Biodegradable Poly(ester amide) Scaffolds for Model Vascular Tissue Fabrication

Vascular differentiation of stem cells and matrix component production on electrospun tubular scaffolds is desirable to engineer blood vessels. The mouse embryonic multipotent mesenchymal progenitor cell line (10T1/2) provides an excellent tool for tissue engineering since it shares similar differentiation characteristics with mesenchymal stem cells. Although 10T1/2 cells have been widely studied in the context of skeletal tissue engineering, their differentiation to smooth muscle lineage is less known. In this study, we fabricated tubular electrospun poly(ester amide) (PEA) fibers from L-phenylalanine-derived biodegradable biomaterials and investigated cell-scaffold interactions as well as their differentiation into vascular smooth muscle cell and subsequent elastin expression. PEA scaffolds fabricated under different collector speeds did not have an impact on the fiber directionality/orientation. 10T1/2 cytocompatibility and proliferation studies showed that PEA fibres were not cytotoxic and were able to support proliferation for 14 days. Furthermore, cells were observed infiltrating the fibrous scaffolds despite the small pore sizes (~ 5 µm). Vascular differentiation studies of 10T1/2 cells using qPCR, Western blot, and immunostaining showed a TGFβ1-induced upregulation of vascular smooth muscle cell (VSMC)-specific markers smooth muscle alpha-actin (SM-α-actin) and smooth muscle myosin heavy chain (SM-MHC). Differentiated 10T1/2 cells produced both elastin and fibrillin-1 suggesting the potential of fibrous PEA scaffolds to fabricate model vascular tissues.

Microvesicles (MIVs) secreted from adipose-derived stem cells (ADSCs) contain multiple microRNAs and promote the migration and invasion of endothelial cells

Extracellular vesicles (EVs) such as microvesicles (MIVs) play an important role in intercellular communications. MIVs are small membrane vesicles sized 100–1000 nm in diameter that are released by many types of cells, such as mesenchymal stem cells (MSCs), tumor cells and adipose-derived stem cells (ADSC). As EVs can carry out autocrine and paracrine functions by controlling multiple cell processes, it is conceivable that EVs can be used as delivery vehicles for treating several clinical conditions, such as to improve cardiac angiogenesis after myocardial infarction (MI). Here, we seek to investigate whether ADSC-derived MIVs contain microRNAs that regulate angiogenesis and affect cell migration of endothelial cells. We first characterized the ADSC-derived MIVs and found that the MIVs had a size range of 100–300 nm, and expressed the MIV marker protein Alix. We then analyzed the microRNAs in ADSCs and ADSC-derived MIVs and demonstrated that ADSC-derived MIVs selectively released a panel of microRNAs, several of which were related to angiogenesis, including two members of the let-7 family. Furthermore, we demonstrated that ADSC-derived MIVs promoted the cell migration and invasion of the HUVEC endothelial cells. The PKH26-labeled ADSC-derived MIVs were effectively uptaken into the cytoplasm of HUVEC cells. Collectively, our results demonstrate that the ADSC-derived MIVs can promote migration and invasion abilities of endothelial cells, suggesting pro-angiogenetic potential. Future studies should focus on investigating the roles and mechanisms through which ADSC-derived MIVs regulate angiogenesis.

Histomorphometric and immunohistochemical evaluation of angiogenesis in ischemia by tissue engineering in rats: Role of mast cells

The aim of this study was to find a proper method for improvement of ischemic condition in the rat hind limb and also to observe the efficacy of cell engraftment with alginate/gelatin three-dimensional scaffolds. Eighteen male Wistar rats weighing 200 to 250 g were randomly divided into three groups (n = 6) including a) ischemia group; in which femoral artery was removed after ligation at the distance of 5 mm, b) scaffold group; in which hydrogel scaffold was added to the site of transected femoral artery and c) test group; in which in addition to hydrogel scaffold, mast cells (MCs) were also added (1 × 106 cells). Analysis of capillary density, artery diameter, histomorphometric parameters and immunohistochemistry in transected location were done on day 14 after femoral artery transection. The average number of blood capillary was significantly higher in the test group than other groups. Also, the average number of medium and large blood vessels was significantly higher in the test group compared to ischemia and scaffold groups. Application of MCs through the use of hydrogel scaffolds (alginate/gelatin) can be considered as a new approach in the application of stem cells for therapeutic angiogenesis under ischemic conditions which can improve the angiogenesis process in patients with peripheral artery diseases.

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

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

Bone physiology as inspiration for tissue regenerative therapies

The development, maintenance of healthy bone and regeneration of injured tissue in the human body comprise a set of intricate and finely coordinated processes. However, an analysis of current bone regeneration strategies shows that only a small fraction of well-reported bone biology aspects has been used as inspiration and transposed into the development of therapeutic products. Specific topics that include inter-scale bone structural organization, developmental aspects of bone morphogenesis, bone repair mechanisms, role of specific cells and heterotypic cell contact in the bone niche (including vascularization networks and immune system cells), cell-cell direct and soluble-mediated contact, extracellular matrix composition (with particular focus on the non-soluble fraction of proteins), as well as mechanical aspects of native bone will be the main reviewed topics. In this Review we suggest a systematic parallelization of (i) fundamental well-established biology of bone, (ii) updated and recent advances on the understanding of biological phenomena occurring in native and injured tissue, and (iii) critical discussion of how those individual aspects have been translated into tissue regeneration strategies using biomaterials and other tissue engineering approaches. We aim at presenting a perspective on unexplored aspects of bone physiology and how they could be translated into innovative regeneration-driven concepts.

Human Pluripotent Stem Cells to Engineer Blood Vessels

Development of pluripotent stem cells (PSCs) is a remarkable scientific advancement that allows scientists to harness the power of regenerative medicine for potential treatment of disease using unaffected cells. PSCs provide a unique opportunity to study and combat cardiovascular diseases, which continue to claim the lives of thousands each day. Here, we discuss the differentiation of PSCs into vascular cells, investigation of the functional capabilities of the derived cells, and their utilization to engineer microvascular beds or vascular grafts for clinical application. Graphical Abstract Human iPSCs generated from patients are differentiated toward ECs and perivascular cells for use in disease modeling, microvascular bed development, or vascular graft fabrication.

Evaluation of recellularization on decellularized aorta scaffolds engineered by ultrasonication treatment

Aortic scaffolds prepared using sonication decellularization treatment has provided a successful medium for repopulation with vascular smooth muscle cells (VSMCs). The objective of this study is to explore the potential of tissue decellularization using ultrasonication treatment and its recellularization before implantation of the cell-seeded scaffolds into host. Aorta tissue samples are decellularized in 2% SDS with sonication for 10 hours and compared with the native tissues. The 4',6-diamidino-2-phenylindole (DAPI) staining was used to evaluate the decellularization and Hematoxylin-Eosin (H-E) staining was used to compare the VSMCs infiltrations onto the decellularized tissues at day-0 and day-6 after cell-seeding. The results histologically showed complete DNA removal from scaffolds after decellularization and subsequent recellularization resulted in successful VSMCs infiltration. Accordingly, the decellularized tissues treated with 2% SDS in sonication demonstrated successful VSMCs repopulation afterward and is speculated to have less toxicity and able to be effectively implanted into host.

Mechanochemical Effects on Extracellular Signal-Regulated Kinase Dynamics in Stem Cell Differentiation

Understanding how key signaling molecules are coregulated by biochemical agents and physical stimuli during stem cell differentiation is critical but often lacking. Due to the important role of extracellular signal-regulated kinase (ERK), this study has examined its temporal dynamics to determine the coregulation of mechanochemical cues on ERK phosphorylation for smooth muscle cell (SMC) differentiation. To assess ERK1/2 activity, a fluorescence resonance energy transfer-based biosensor was transfected into mesenchymal stem cells. The influences of nanopatterned substrates, growth factors, and drugs on ERK activities were related to their effects on SMC differentiation. Results revealed that nanopatterned substrates significantly increased ERK activity in cells, overriding ERK response from administered biochemical factors. The nanopatterned substrates reduced expression of SMC markers after a 48-h biochemical treatment, except for the combination with ERK inhibitor PD98059 treatment, which enhanced expression of mature SMC marker MYH11. Immunofluorescent staining for focal adhesion proteins, vinculin and zyxin, indicated no significant differences in vinculin cluster distribution or dimension, while the location of zyxin changed from adhesion sites of cell periphery on nonpatterned substrate to actin filaments on nanopatterned substrate. The zyxin-reinforced stress fibers likely enhanced the cytoskeletal tension to increase ERK dynamics. Collectively, results suggest that physical stimuli play a dominating role in initial ERK signaling and early-stage differentiation through focal adhesion changes, and the capability of monitoring signaling events in real time could be exploited to guide the engineering of cell microenvironment.

Building Blood Vessels with Vascular Progenitor Cells

Vascular progenitor cells have been identified from perivascular cell fractions and peripheral blood and bone marrow mononuclear fractions. These vascular progenitors share the ability to generate some of the vascular lineages, including endothelial cells, smooth muscle cells, and pericytes. The potential therapeutic uses for vascular progenitor cells are broad and relate to stroke, ischemic disease, and to the engineering of whole organs and tissues that require a vascular component. This review summarizes the best-characterized sources of vascular progenitor cells and discusses advances in 3D printing and electrospinning using blended polymers for the creation of biomimetic vascular grafts. These advances are pushing the field of regenerative medicine closer to the creation of small-diameter vascular grafts with long-term clinical utility.

Generating vascular conduits: from tissue engineering to three-dimensional bioprinting

Vascular disease - including coronary artery disease, carotid artery disease, and peripheral vascular disease - is a leading cause of morbidity and mortality worldwide. The standard of care for restoring patency or bypassing occluded vessels involves using autologous grafts, typically the saphenous veins or internal mammary arteries. Yet, many patients who need life- or limb-saving procedures have poor outcomes, and a third of patients who need vascular intervention have multivessel disease and therefore lack appropriate vasculature to harvest autologous grafts from. Given the steady increase in the prevalence of vascular disease, there is great need for grafts with the biological and mechanical properties of native vessels that can be used as vascular conduits. In this review, we present an overview of methods that have been employed to generate suitable vascular conduits, focusing on the advances in tissue engineering methods and current three-dimensional (3D) bioprinting methods. Tissue-engineered vascular grafts have been fabricated using a variety of approaches such as using preexisting scaffolds and acellular organic compounds. We also give an extensive overview of the novel use of 3D bioprinting as means of generating new vascular conduits. Different strategies have been employed in bioprinting, and the use of cell-based inks to create de novo structures offers a promising solution to bridge the gap of paucity of optimal donor grafts. Lastly, we provide a glimpse of our work to create scaffold-free, bioreactor-free, 3D bioprinted vessels from a combination of rat vascular smooth muscle cells and fibroblasts that remain patent and retain the tensile and mechanical strength of native vessels.

Generation of human umbilical cord vein CD146+ perivascular cell origined three-dimensional vascular construct

Small-diameter vascular grafts are needed for the treatment of coronary artery diseases in the case of limited accessibility of the autologous vessels. Synthetic scaffolds have many disadvantages so in recent years vascular constructs (VCs) made from cellularized natural scaffolds was seen to be very promising but number of studies comprising this area is very limited. In our study, our aim is to generate fully natural triple-layered VC that constitutes all the layers of blood vessel with vascular cells. CD146+ perivascular cells (PCs) were isolated from human umbilical cord vein (HUCV) and differentiated into smooth muscle cells (SMCs) and fibroblasts. They were then combined with collagen type I/elastin/dermatan sulfate and collagen type I/fibrin to form tunica media and tunica adventitia respectively. HUCV endothelial cells (ECs) were seeded on the construct by cell sheet engineering method after fibronectin and heparin coating. Characterization of the VC was performed by immunolabeling, histochemical staining and electron microscopy (SEM and TEM). Differentiated cells were identified by means of immunofluorescent (IF) labeling. SEM and TEM analysis of VCs revealed the presence of three histologic tunicae. Collagen and elastic fibers were observed within the ECM by histochemical staining. The vascular endothelial growth factor receptor expressing ECs in tunica intima; α-SMA expressing SMCs in tunica media and; the tenascin expressing fibroblasts in tunica adventitia were detected by IF labeling. In conclusion, by combining natural scaffolds and vascular cells differentiated from CD146+ PCs, VCs can be generated layer by layer. This study will provide a preliminary blood vessel model for generation of fully natural small-diameter vascular grafts.

Controlling Differentiation of Stem Cells for Developing Personalized Organ-on-Chip Platforms

Organ-on-chip (OOC) platforms have attracted attentions of pharmaceutical companies as powerful tools for screening of existing drugs and development of new drug candidates. OOCs have primarily used human cell lines or primary cells to develop biomimetic tissue models. However, the ability of human stem cells in unlimited self-renewal and differentiation into multiple lineages has made them attractive for OOCs. The microfluidic technology has enabled precise control of stem cell differentiation using soluble factors, biophysical cues, and electromagnetic signals. This study discusses different tissue- and organ-on-chip platforms (i.e., skin, brain, blood-brain barrier, bone marrow, heart, liver, lung, tumor, and vascular), with an emphasis on the critical role of stem cells in the synthesis of complex tissues. This study further recaps the design, fabrication, high-throughput performance, and improved functionality of stem-cell-based OOCs, technical challenges, obstacles against implementing their potential applications, and future perspectives related to different experimental platforms.

Organ-On-A-Chip Platforms: A Convergence of Advanced Materials, Cells, and Microscale Technologies

Significant advances in biomaterials, stem cell biology, and microscale technologies have enabled the fabrication of biologically relevant tissues and organs. Such tissues and organs, referred to as organ-on-a-chip (OOC) platforms, have emerged as a powerful tool in tissue analysis and disease modeling for biological and pharmacological applications. A variety of biomaterials are used in tissue fabrication providing multiple biological, structural, and mechanical cues in the regulation of cell behavior and tissue morphogenesis. Cells derived from humans enable the fabrication of personalized OOC platforms. Microscale technologies are specifically helpful in providing physiological microenvironments for tissues and organs. In this review, biomaterials, cells, and microscale technologies are described as essential components to construct OOC platforms. The latest developments in OOC platforms (e.g., liver, skeletal muscle, cardiac, cancer, lung, skin, bone, and brain) are then discussed as functional tools in simulating human physiology and metabolism. Future perspectives and major challenges in the development of OOC platforms toward accelerating clinical studies of drug discovery are finally highlighted.

Electrospun vascular scaffold for cellularized small diameter blood vessels: A preclinical large animal study

The strategy of vascular tissue engineering is to create a vascular substitute by combining autologous vascular cells with a tubular-shaped biodegradable scaffold. We have previously developed a novel electrospun bilayered vascular scaffold that provides proper biological and biomechanical properties as well as structural configuration. In this study, we investigated the clinical feasibility of a cellularized vascular scaffold in a preclinical large animal model. We fabricated the cellularized vascular construct with autologous endothelial progenitor cell (EPC)-derived endothelial cells (ECs) and smooth muscle cells (SMCs) followed by a pulsatile bioreactor preconditioning. This fully cellularized vascular construct was tested in a sheep carotid arterial interposition model. After preconditioning, confluent and mature EC and SMC layers in the scaffold were achieved. The cellularized constructs sustained the structural integrity with a high degree of graft patency without eliciting an inflammatory response over the course of the 6-month period in sheep. Moreover, the matured EC coverage on the lumen and a thick smooth muscle layer were formed at 6months after transplantation. We demonstrated that electrospun bilayered vascular scaffolds in conjunction with autologous vascular cells may be a clinically applicable alternative to traditional prosthetic vascular graft substitutes.

STATEMENT OF SIGNIFICANCE

This study demonstrates the utility of tissue engineering to provide platform technologies for rehabilitation of patients recovering from severe, devastating cardiovascular diseases. The long-term goal is to provide alternatives to vascular grafting using bioengineered blood vessels derived from an autologous cell source with a functionalized vascular scaffold. This novel bilayered vascular construct for engineering blood vessels is designed to offer "off-the-shelf" availability for clinical translation.

Deriving vascular smooth muscle cells from mesenchymal stromal cells: Evolving differentiation strategies and current understanding of their mechanisms

Vascular smooth muscle cells (VSMCs) play essential roles in regulating blood vessel form and function. Regeneration of functional vascular smooth muscle tissue to repair vascular diseases is an area of intense research in tissue engineering and regenerative medicine. For functional vascular smooth muscle tissue regeneration to become a practical therapy over the next decade, the field will need to have access to VSMC sources that are effective, robust and safe. While pluripotent stem cells hold good future promise to this end, more immediate translation is expected to come from approaches that generate functional VSMCs from adult sources of multipotent adipose-derived and bone marrow-derived mesenchymal stromal cells (ASCs and BMSCs). The research to this end is extensive and is dominated by studies relating to classical biochemical signalling molecules used to induce differentiation of ASCs and BMSCs. However, prolonged use of the biochemical induction factors is costly and can cause potential endotoxin contamination in the culture. Over recent years several non-traditional differentiation approaches have been devised to mimic defined aspects of the native micro-environment in which VSMCs reside to contribute to the differentiation of VSMC-like cells from ASCs and BMSCs. In this review, the promises and limitations of several non-traditional culture approaches (e.g., co-culture, biomechanical, and biomaterial stimuli) targeting VSMC differentiation are discussed. The extensive crosstalk between the underlying signalling cascades are delineated and put into a translational context. It is expected that this review will not only provide significant insight into VSMC differentiation strategies for vascular smooth muscle tissue engineering applications, but will also highlight the fundamental importance of engineering the cellular microenvironment on multiple scales (with consideration of different combinatorial pathways) in order to direct cell differentiation fate and obtain cells of a desired and stable phenotype. These strategies may ultimately be applied to different sources of stem cells in the future for a range of biomaterial and tissue engineering disciplines.

Bioprinting for vascular and vascularized tissue biofabrication

Bioprinting is a promising technology to fabricate design-specific tissue constructs due to its ability to create complex, heterocellular structures with anatomical precision. Bioprinting enables the deposition of various biologics including growth factors, cells, genes, neo-tissues and extra-cellular matrix-like hydrogels. Benefits of bioprinting have started to make a mark in the fields of tissue engineering, regenerative medicine and pharmaceutics. Specifically, in the field of tissue engineering, the creation of vascularized tissue constructs has remained a principal challenge till date. However, given the myriad advantages over other biofabrication methods, it becomes organic to expect that bioprinting can provide a viable solution for the vascularization problem, and facilitate the clinical translation of tissue engineered constructs. This article provides a comprehensive account of bioprinting of vascular and vascularized tissue constructs. The review is structured as introducing the scope of bioprinting in tissue engineering applications, key vascular anatomical features and then a thorough coverage of 3D bioprinting using extrusion-, droplet- and laser-based bioprinting for fabrication of vascular tissue constructs. The review then provides the reader with the use of bioprinting for obtaining thick vascularized tissues using sacrificial bioink materials. Current challenges are discussed, a comparative evaluation of different bioprinting modalities is presented and future prospects are provided to the reader.

STATEMENT OF SIGNIFICANCE

Biofabrication of living tissues and organs at the clinically-relevant volumes vitally depends on the integration of vascular network. Despite the great progress in traditional biofabrication approaches, building perfusable hierarchical vascular network is a major challenge. Bioprinting is an emerging technology to fabricate design-specific tissue constructs due to its ability to create complex, heterocellular structures with anatomical precision, which holds a great promise in fabrication of vascular or vascularized tissues for transplantation use. Although a great progress has recently been made on building perfusable tissues and branched vascular network, a comprehensive review on the state-of-the-art in vascular and vascularized tissue bioprinting has not reported so far. This contribution is thus significant because it discusses the use of three major bioprinting modalities in vascular tissue biofabrication for the first time in the literature and compares their strengths and limitations in details. Moreover, the use of scaffold-based and scaffold-free bioprinting is expounded within the domain of vascular tissue fabrication.

NANOG Restores Contractility of Mesenchymal Stem Cell-Based Senescent Microtissues

Mesenchymal stem cells (MSCs) have been extensively used in the field of tissue engineering as a source of smooth muscle cells (SMCs). However, recent studies showed deficits in the contractile function of SMCs derived from senescent MSCs and there are no available strategies to restore the contractile function that is impaired due to cellular or organismal senescence. In this study, we developed a tetracycline-regulatable system and employed micropost tissue arrays to evaluate the effects of the embryonic transcription factor, NANOG, on the contractility of senescent MSCs. Using this system, we show that expression of NANOG fortified the actin cytoskeleton and restored contractile function that was impaired in senescent MSCs. NANOG increased the expression of smooth muscle α-actin (ACTA2) as well as the contractile force generated by cells in three-dimensional microtissues. Interestingly, NANOG worked together with transforming growth factor-beta1 to further enhance the contractility of senescent microtissues. The effect of NANOG on contractile function was sustained for about 10 days after termination of its expression. Our results show that NANOG could reverse the effects of stem cell senescence and restore the myogenic differentiation potential of senescent MSCs. These findings may enable development of novel strategies to restore the function of senescent cardiovascular and other SMC-containing tissues.

The third annual BRDS on research and development of nucleic acid-based nanomedicines

The completion of human genome project, decrease in the sequencing cost, and correlation of genome sequencing data with specific diseases led to the exponential rise in the nucleic acid-based therapeutic approaches. In the third annual Biopharmaceutical Research and Development Symposium (BRDS) held at the Center for Drug Discovery and Lozier Center for Pharmacy Sciences and Education at the University of Nebraska Medical Center (UNMC), we highlighted the remarkable features of the nucleic acid-based nanomedicines, their significance, NIH funding opportunities on nanomedicines and gene therapy research, challenges and opportunities in the clinical translation of nucleic acids into therapeutics, and the role of intellectual property (IP) in drug discovery and development.

Human mesenchymal stem cells cultured on silk hydrogels with variable stiffness and growth factor differentiate into mature smooth muscle cell phenotype

Cell-matrix and cell-biomolecule interactions play critical roles in a diversity of biological events including cell adhesion, growth, differentiation, and apoptosis. Evidence suggests that a concise crosstalk of these environmental factors may be required to direct stem cell differentiation toward matured cell type and function. However, the culmination of these complex interactions to direct stem cells into highly specific phenotypes in vitro is still widely unknown, particularly in the context of implantable biomaterials. In this study, we utilized tunable hydrogels based on a simple high pressure CO2 method and silk fibroin (SF) the structural protein of Bombyx mori silk fibers. Modification of SF protein starting water solution concentration results in hydrogels of variable stiffness while retaining key structural parameters such as matrix pore size and β-sheet crystallinity. To further resolve the complex crosstalk of chemical signals with matrix properties, we chose to investigate the role of 3D hydrogel stiffness and transforming growth factor (TGF-β1), with the aim of correlating the effects on the vascular commitment of human mesenchymal stem cells. Our data revealed the potential to upregulate matured vascular smooth muscle cell phenotype (myosin heavy chain expression) of hMSCs by employing appropriate matrix stiffness and growth factor (within 72h). Overall, our observations suggest that chemical and physical stimuli within the cellular microenvironment are tightly coupled systems involved in the fate decisions of hMSCs. The production of tunable scaffold materials that are biocompatible and further specialized to mimic tissue-specific niche environments will be of considerable value to future tissue engineering platforms.

STATEMENT OF SIGNIFICANCE

This article investigates the role of silk fibroin hydrogel stiffness and transforming growth factor (TGF-β1), with the aim of correlating the effects on the vascular commitment of human mesenchymal stem cells. Specifically, we demonstrate the upregulation of mature vascular smooth muscle cell phenotype (myosin heavy chain expression) of hMSCs by employing appropriate matrix stiffness and growth factor (within 72h). Moreover, we demonstrate the potential to direct specialized hMSC differentiation by modulating stiffness and growth factor using silk fibroin, a well-tolerated and -defined biomaterial with an impressive portfolio of tissue engineering applications. Altogether, our study reinforce the fact that complex differentiation protocols may be simplified by engineering the cellular microenvironment on multiple scales, i.e. matrix stiffness with growth factor.

Patterning of Fibroblast and Matrix Anisotropy within 3D Confinement is Driven by the Cytoskeleton

Effects of 3D confinement on cellular growth and matrix assembly are important in tissue engineering, developmental biology, and regenerative medicine. Polydimethylsiloxane wells with varying anisotropy are microfabicated using soft-lithography. Microcontact printing of bovine serum albumin is used to block cell adhesion to surfaces between wells. The orientations of fibroblast stress fibers, microtubules, and fibronectin fibrils are examined 1 day after cell seeding using laser scanning confocal microscopy, and anisotropy is quantified using a custom autocorrelation analysis. Actin, microtubules, and fibronectin exhibit higher anisotropy coefficients for cells grown in rectangular wells with aspect ratios of 1:4 and 1:8, as compared to those in wells with lower aspect ratios or in square wells. The effects of disabling individual cytoskeletal components on fibroblast responses to anisotropy are then tested by applying actin or microtubule polymerization inhibitors, Rho kinase inhibitor, or by siRNA-mediated knockdown of AXL or cofilin-1. Latrunculin A decreases cytoskeletal and matrix anisotropy, nocodazole ablates both, and Y27632 mutes cellular polarity while decreasing matrix anisotropy. AXL siRNA knockdown has little effect, as does siRNA knockdown of cofilin-1. These data identify several specific cytoskeletal strategies as targets for the manipulation of anisotropy in 3D tissue constructs.

Fabrication of tissue-engineered vascular grafts with stem cells and stem cell-derived vascular cells

INTRODUCTION

Cardiovascular disease is the leading cause of mortality worldwide. Current surgical treatments for cardiovascular disease include vascular bypass grafting and replacement with autologous blood vessels or synthetic vascular grafts. However, there is a call for better alternative biological grafts.

AREAS COVERED

Tissue-engineered vascular grafts (TEVGs) are promising novel alternatives to replace diseased vessels. However, obtaining enough functional and clinically usable vascular cells for fabrication of TEVGs remains a major challenge. New findings in adult stem cells and recent advances in pluripotent stem cells have opened a new avenue for stem cell-based vascular engineering. In this review, recent advances on stem cell sourcing for TEVGs including the use of adult stem cells and pluripotent stem cells and advantages, disadvantages, and possible future implementations of different types of stem cells will be discussed. In addition, current strategies used during the fabrication of TEVGs will be highlighted.

EXPERT OPINION

The application of patient-specific TEVGs constructed with vascular cells derived from immune-compatible stem cells possesses huge clinical potential. Advances in lineage-specific differentiation approaches and innovative vascular engineering strategies will promote the vascular regeneration field from bench to bedside.

Stem cells: a promising source for vascular regenerative medicine

The rising and diversity of many human vascular diseases pose urgent needs for the development of novel therapeutics. Stem cell therapy represents a challenge in the medicine of the twenty-first century, an area where tissue engineering and regenerative medicine gather to provide promising treatments for a wide variety of diseases. Indeed, with their extensive regeneration potential and functional multilineage differentiation capacity, stem cells are now highlighted as promising cell sources for regenerative medicine. Their multilineage differentiation involves environmental factors such as biochemical, extracellular matrix coating, oxygen tension, and mechanical forces. In this review, we will focus on human stem cell sources and their applications in vascular regeneration. We will also discuss the different strategies used for their differentiation into both mature and functional smooth muscle and endothelial cells.

Stem cell-derived vasculature: A potent and multidimensional technology for basic research, disease modeling, and tissue engineering

Proper blood vessel networks are necessary for constructing and re-constructing tissues, promoting wound healing, and delivering metabolic necessities throughout the body. Conversely, an understanding of vascular dysfunction has provided insight into the pathogenesis and progression of diseases both common and rare. Recent advances in stem cell-based regenerative medicine - including advances in stem cell technologies and related progress in bioscaffold design and complex tissue engineering - have allowed rapid advances in the field of vascular biology, leading in turn to more advanced modeling of vascular pathophysiology and improved engineering of vascularized tissue constructs. In this review we examine recent advances in the field of stem cell-derived vasculature, providing an overview of stem cell technologies as a source for vascular cell types and then focusing on their use in three primary areas: studies of vascular development and angiogenesis, improved disease modeling, and the engineering of vascularized constructs for tissue-level modeling and cell-based therapies.

Patient-specific cardiovascular progenitor cells derived from integration-free induced pluripotent stem cells for vascular tissue regeneration

Tissue-engineered blood vessels (TEBVs) are promising in regenerating a live vascular replacement. However, the vascular cell source is limited, and it is crucial to develop a scaffold that accommodates new type of vascular progenitor cells and facilitates in vivo lineage specification of the cells into functional vascular smooth muscle cells (VSMCs) to regenerate vascular tissue. In the present study, integration-free human induced pluripotent stem cells (hiPSCs) were established from patient peripheral blood mononuclear cells through episomal vector nucleofection of reprogramming factors. The established hiPSCs were then induced into mesoderm-originated cardiovascular progenitor cells (CVPCs) with a highly efficient directed lineage specification method. The derived CVPCs were demonstrated to be able to differentiate into functional VSMCs. Subcutaneous implantation of CVPCs seeded on macroporous nanofibrous poly(l-lactide) scaffolds led to in vivo VSMC lineage specification and matrix deposition inside the scaffolds. In summary, we established integration-free patient-specific hiPSCs from peripheral blood mononuclear cells, derived CVPCs through directed lineage specification, and developed an advanced scaffold for these progenitor cells to further differentiate in vivo into VSMCs and regenerate vascular tissue in a subcutaneous implantation model. This study has established an efficient patient-specific approach towards in vivo regeneration of vascular tissue.

Union is strength: matrix elasticity and microenvironmental factors codetermine stem cell differentiation fate

Stem cells are an attractive cellular source for regenerative medicine and tissue engineering applications due to their multipotency. Although the elasticity of the extracellular matrix (ECM) has been shown to have crucial impacts in directing stem cell differentiation, it is not the only contributing factor. Many researchers have recently attempted to design microenvironments that mimic the stem cell niche with combinations of ECM elasticity and other cues, such as ECM physical properties, soluble biochemical factors and cell-cell interactions, thereby driving cells towards their preferred lineages. Here, we briefly discuss the effect of matrix elasticity on stem cell lineage specification and then summarize recent advances in the study of the combined effects of ECM elasticity and other cues on the differentiation of stem cells, focusing on two aspects: biophysical and biochemical factors. In the future, biomedical scientists will continue investigating the union strength of matrix elasticity and microenvironmental cues for manipulating stem cell fates.

Strategies and processes to decellularize and recellularize hearts to generate functional organs and reduce the risk of thrombosis

Heart failure is one of the leading causes of death in the United States. Current therapies, such as heart transplants and bioartificial hearts, are helpful, but not optimal. Decellularization of porcine whole hearts followed by recellularization with patient-specific human cells may provide the ultimate solution for patients with heart failure. Great progress has been made in the development of efficient processes for decellularization, and the design of automated bioreactors. Challenges remain in selecting and culturing cells, growing the cells on the decellularized scaffolds without contamination, characterizing the regenerated organs, and preventing thrombosis. Various strategies have been proposed to prevent thrombosis of blood-contacting devices, including reendothelization and the creation of nonfouling surfaces using surface modification technologies. This review discusses the progress and remaining challenges involved with recellularizing whole hearts, focusing on the prevention of thrombosis.

Engineering vascular tissue with functional smooth muscle cells derived from human iPS cells and nanofibrous scaffolds

Tissue-engineered blood vessels (TEBVs) are promising in the replacement of diseased vascular tissues. However, it remains a great challenge to obtain a sufficient number of functional smooth muscle cells (SMCs) in a clinical setting to construct patient-specific TEBVs. In addition, it is critical to develop a scaffold to accommodate these cells and retain their functional phenotype for the regeneration of TEBVs. In this study, human induced pluripotent stem cells (iPSCs) were established from primary human aortic fibroblasts, and characterized with the pluripotency markers expression and cells' capabilities to differentiate into all three germ layer cells. A highly efficient method was then developed to induce these human iPSCs into proliferative SMCs. After multiple times of expansion, the expanded SMCs retained the potential to be induced into the functional contractile phenotype of mature SMCs, which was characterized by the contractile response to carbachol treatment, up-regulation of specific collagen genes under transforming growth factor β1 treatment, and up-regulation of specific matrix metalloproteinase genes under cytokine stimulation. We also developed an advanced macroporous and nanofibrous (NF) poly(l-lactic acid) (PLLA) scaffold with suitable pore size and interpore connectivity to seed these human iPSC-derived SMCs and maintain their differentiated phenotype. Subcutaneous implantation of the SMC-scaffold construct in nude mice demonstrated vascular tissue formation, with robust collagenous matrix deposition inside the scaffold and the maintenance of differentiated SMC phenotype. Taken together, this study established an exciting approach towards the construction of patient-specific TEBVs. We established patient-specific human iPSCs, derived proliferative SMCs for expansion, turned on their mature contractile SMC phenotype, and developed an advanced scaffold for these cells to regenerate vascular tissue in vivo.

Synergism of matrix stiffness and vascular endothelial growth factor on mesenchymal stem cells for vascular endothelial regeneration

Mesenchymal stem cells (MSCs) hold tremendous potential for vascular tissue regeneration. Research has demonstrated that individual factors in the cell microenvironment such as matrix elasticity and growth factors regulate MSC differentiation to vascular lineage. However, it is not well understood how matrix elasticity and growth factors combine to direct the MSC fate. This study examines the combined effects of matrix elasticity and vascular endothelial growth factor (VEGF) on both MSC differentiation into endothelial lineage and MSC paracrine signaling. MSCs were seeded in soft nanofibrous matrices with or without VEGF, and in Petri dishes with or without VEGF. Only MSCs seeded in three-dimensional soft matrices with VEGF showed significant increases in the expression of endothelial markers (vWF, eNOS, Flt-1, and Flk-1), while eliminating the expression of smooth muscle marker (SM-α-actin). MSCs cultured in VEGF alone on two-dimensional dishes showed increased expression of both early-stage endothelial and smooth muscle markers, indicating immature vascular differentiation. Furthermore, MSCs cultured in soft matrices with VEGF showed faster upregulation of endothelial markers compared with MSCs cultured in VEGF alone. Paracrine signaling studies found that endothelial cells cultured in the conditioned media from MSCs differentiated in the soft matrix and VEGF condition exhibited increased migration and formation of capillary-like structures. These results demonstrate that VEGF and soft matrix elasticity act synergistically to guide MSC differentiation into mature endothelial phenotype while enhancing paracrine signaling. Therefore, it is critical to control both mechanical and biochemical factors to safely regenerate vascular tissues with MSCs.