Differentiation patterns of embryonic stem cells in two- versus three-dimensional culture

Pluripotent stem cells are attractive candidates as a cell source for regenerative medicine and tissue engineering therapies. Current methods of differentiation result in low yields and impure populations of target phenotypes, with attempts for improved efficiency often comparing protocols that vary multiple parameters. This basic science study focused on a single variable to understand the effects of two-dimensional (2D) versus three-dimensional (3D) culture on directed differentiation. We compared mouse embryonic stem cells (ESCs) differentiated on collagen type I-coated surfaces (SLIDEs), embedded in collagen type I gels (GELs), and in suspension as embryoid bodies (EBs). For a systematic analysis in these studies, key parameters were kept identical to allow for direct comparison across culture configurations. We determined that all three configurations supported differentiation of ESCs and that the kinetics of differentiation differed greatly for cells cultured in 2D versus 3D. SLIDE cultures induced overall differentiation more quickly than 3D configurations, with earlier expression of cytoskeletal and extracellular matrix proteins. For 3D culture as GELs or EBs, cells clustered similarly, formed complex structures and promoted differentiation towards cardiovascular phenotypes. GEL culture, however, also allowed for contraction of the collagen matrix. For differentiation towards fibroblasts and smooth muscle cells which actively remodel their environment, GEL culture may be particularly beneficial. Overall, this study determined the effects of dimensionality on differentiation and helps in the rational design of protocols to generate phenotypes needed for tissue engineering and regenerative medicine.

Fluid shear stress alters the hemostatic properties of endothelial outgrowth cells

Surface endothelialization is an attractive means to improve the performance of small diameter vascular grafts. While endothelial outgrowth cells (EOCs) are considered a promising source of autologous endothelium, the ability of EOCs to modulate coagulation-related blood activities is not well understood. The goal of this study was to assess the role of arterial flow conditions on the thrombogenic phenotype of EOCs. EOCs derived from baboon peripheral blood, as well as mature arterial endothelial cells from baboons, were seeded onto adsorbed collagen, then exposed to physiologic levels of fluid shear stress. For important hemostatic pathways, cellular responses to shear stress were characterized at the gene and protein level and confirmed with a functional assay for activated protein C (APC) activity. For EOCs, fluid shear stress upregulated gene and protein expression of anticoagulant and platelet inhibitory factors, including thrombomodulin, tissue factor pathway inhibitor, and nitric oxide synthase 3 (eNOS). Fluid shear stress significantly altered the functional activity of EOCs by increasing APC levels. This study demonstrates that fluid shear stress is an important determinant of EOC hemostatic properties. Accordingly, manipulation of EOC phenotype by mechanical forces may be important for the development of thrombo-resistant surfaces on engineered vascular implants.

Tissue engineering: from biology to biological substitutes

Tissue engineering is an emerging multidisciplinary and interdisciplinary field involving the development of bioartificial implants and/or the fostering of tissue remodeling with the purpose of repairing or enhancing tissue or organ function. Bioartificial constructs generally consist of cells and biomaterials, so tissue engineering draws from both cell and biomaterials science and technology. Successful applications require a thorough understanding of the environment experienced by cells in normal tissues and by cells in bioartificial devices before and after implantation. This paper reviews these topics, as well as the current status and future possibilities in the development of different bioartificial constructs, including bioartificial skin, cardiovascular implants, bioartificial pancreas, and encapsulated secretory cells. Issues that need to be addressed in the future are also discussed. These include, but are not limited to, the development of new cell lines and biomaterials, the evaluation of the optimal construct architecture, and the reproducible manufacture and preservation of bioartificial devices until ready for use.

Discovery of shear- and side-specific mRNAs and miRNAs in human aortic valvular endothelial cells

The role of endothelial cells (ECs) in aortic valve (AV) disease remains relatively unknown; however, disease preferentially occurs in the fibrosa. We hypothesized oscillatory shear (OS) present on the fibrosa stimulates ECs to modify mRNAs and microRNAs (miRNAs) inducing disease. Our goal was to identify mRNAs and miRNAs differentially regulated by OS and laminar shear (LS) in human AVECs (HAVECs) from the fibrosa (fHAVECs) and ventricularis (vHAVECs). HAVECs expressed EC markers as well as some smooth muscle cell markers and functionally aligned with the flow. HAVECs were exposed to OS and LS for 24 h, and total RNA was analyzed by mRNA and miRNA microarrays. We found over 700 and 300 mRNAs down- and upregulated, respectively, by OS; however, there was no side dependency. mRNA microarray results were validated for 26 of 28 tested genes. Ingenuity Pathway Analysis revealed thrombospondin 1 (Thbs1) and NF-κB inhibitor-α (Nfkbia) as highly connected, shear-sensitive genes. miRNA array analysis yielded 30 shear-sensitive miRNAs and 3 side-specific miRNAs. miRNA validation confirmed 4 of 17 shear-sensitive miRNAs and 1 of 3 side-dependent miRNAs. Using miRWalk and several filtering steps, we identified shear-sensitive mRNAs potentially targeted by shear-sensitive miRNAs. These genes and signaling pathways could act as therapeutic targets of AV disease.

Tissue engineering of a collagen-based vascular media: Demonstration of functionality

The property of vasoactivity is important for both resistance vessels and larger arteries. Evaluation of smooth muscle cell phenotype is often done in place of functional testing in engineered tissues, assuming a direct correlation between cell phenotype and tissue contractile force. In this study we look at a large panel of vasoactive agents to determine the functionality of our collagen-based tissue. The engineered vascular media elicited a measurable change in force in response to seven of the nine agents used. As part of this characterization, TGF-β1 and TNF-α were used to promote a more contractile and synthetic cell phenotype respectively. Both smooth muscle α-actin and vasoconstriction were evaluated in ring sections. Due to large differences in cell-compaction and cell distribution in the tissues, no correlation was found between α-actin expression and contractile strength. This highlights the need for functional testing of engineered tissue and the importance of cell-matrix interactions in vasoactivity.

In vitro human tissue models--moving towards personalized regenerative medicine

The field of tissue engineering is a rapidly growing interdisciplinary field within regenerative medicine involving biology, chemistry, physics, engineering and medical sciences, which focuses on the fabrication of replacement tissues and organs. Another major focus of tissue engineering is the creation of ex vivo-manufactured multi-organ test systems, in order to explore fundamental questions of cell, matrix and developmental biology. These ex-vivo manufactured systems can also be used to study drug delivery dynamics.

Differences in valvular and vascular cell responses to strain in osteogenic media

Calcification is the primary cause of failure of bioprosthetic and tissue-engineered vascular and valvular grafts. We used tissue-engineered collagen gels containing human aortic smooth muscle cells (HASMC) and human aortic valvular interstitial cells (HAVIC) as a model to investigate cell-mediated differences in early markers of calcification. The HASMCs and HAVICs were isolated from non-sclerotic human tissues. After 21 days of culture in either regular or osteogenic media with or without 10% cyclic strain at 1 Hz, the collagen gels were assessed for DNA content, collagen I, matrix metalloproteinase (MMP)-2 and glycosaminoglycan (GAG) content. The collagen gels containing HASMCs contained significantly greater amounts of collagen I and GAG compared to HAVICs. Although strain increased MMP-2 activity for both cell types, this trend was significant (p ≤ 0.05) only for HAVICs. Cultured gels were also assessed for osteogenic markers calcium content, alkaline phosphatase (ALP), and Runx2 and were present at greater amounts in gels containing HASMCs than HAVICs. Calcium content, Runx2 expression, and ALP activity were also modulated by mechanical strain. The results indicate that cell-mediated differences exist between the vascular and valvular calcification processes. Further investigation is necessary for improved understanding and to detect biomarkers for early detection or prevention of these diseases.

Cyclic strain improves strength and function of a collagen-based tissue-engineered vascular media

Tissue-engineered blood vessels may provide a solution to the lack of suitable blood vessels for coronary and peripheral vessel bypass grafting. Cyclic strain can be used to provide a more physiological environment that may result in tissue that more closely resembles native artery. In this study, cyclic strain is applied to a collagen-based, tissue-engineered vascular medium. An increased culture time was used to allow the tissue to adhere to the silastic sleeve and to eliminate longitudinal compaction. Cyclic strain improved tissue strength through increased collagen content as well as some radial tissue compaction. Mechanical stimulation promoted a more contractile phenotype and led to a greater contractile response to the vasoconstrictor endothelin-1.

Mesenchymal stem cells overexpressing ephrin-b2 rapidly adopt an early endothelial phenotype with simultaneous reduction of osteogenic potential

Restoration of the vascular supply to ischemic tissues is of high clinical relevance, and proangiogenic therapies aim to reduce morbidity and mortality rates associated with the onset of cardiovascular disease. Stem cell therapy has been proposed as a potentially useful proangiogenic therapy. Mesenchymal stem cells (MSCs) have been shown to be proangiogenic and produce a number of cytokines involved in vessel development and maturation. Preclinical studies have reported increased angiogenesis after MSC delivery to the heart, and similar outcomes have been reported in recent clinical trials. Stem-cell-mediated neovascularization has been augmented by genetic modification with overexpression of angiogenic cytokines, including vascular endothelial growth factor (VEGF) and platelet-derived growth factor, showing promising results. In this study we aimed to enhance the proangiogenic capability of MSCs. MSCs were genetically modified to overexpress a versatile molecule, Ephrin-B2, involved in tissue morphogenesis and vascular development to enhance inherent neovascularization potential. Using nucleofection, Ephrin-B2 was transiently overexpressed on the cell surface of MSCs to recapitulate embryonic signaling and promote neovascularization. Ephrin-B2-expressing MSCs adopted an early endothelial phenotype under endothelial cell culture conditions increasing expression of von Willebrand factor and VEGF-Receptor 2. The cells had an increased ability to form vessel-like structures, produce VEGF, and incorporate into newly formed endothelial cell structures. These data indicate that MSCs expressing Ephrin-B2 represent a novel proangiogenic cell source to promote neovascularization in ischemic tissues.

Regenerative medicine: the emergence of an industry

Over the last quarter of a century there has been an emergence of a tissue engineering industry, one that has now evolved into the broader area of regenerative medicine. There have been 'ups and downs' in this industry; however, it now appears to be on a track that may be described as 'back to the future'. The latest data indicate that for 2007 the private sector activity in the world for this industry is approaching $2.5 billion, with 167 companies/business units and more than 6000 employee full time equivalents. Although small compared with the medical device and also the pharmaceutical industries, these numbers are not insignificant. Thus, there is the indication that this industry, and the related technology, may still achieve its potential and address the needs of millions of patients worldwide, in particular those with needs that currently are unmet.

Fluid shear stress promotes an endothelial-like phenotype during the early differentiation of embryonic stem cells

Stem and progenitor cells are emerging as a potential source for cell-based therapies, in which large homogenous populations of differentiated cells are frequently deemed necessary for efficacy. Methods focused on biochemical cues have not yet yielded the numbers of endothelial cells thought necessary for cardiovascular applications. Interest in alternate methods has prompted the study of physical cues on stem and progenitor cell differentiation. In this study, fluid-based shear stress, at levels comparable to those experienced by endothelial cells in large vessels, was applied during the first few days of mouse embryonic stem cell differentiation. After 2 days of applied shear stress, there were increases in cell proliferation and in protein expression of endothelial markers (FLK1, VECAD, and PECAM). Further, treatment increased the number of FLK1+ cells from 1% to 40%, which were then capable of forming vessel-like structures in vitro. Thus, shear stress may be used to direct differentiation of embryonic stem cells toward an endothelial-like phenotype, helping to address the cell sourcing issue in cardiovascular regenerative medicine and tissue engineering.

The impact of biomechanics in tissue engineering and regenerative medicine

Biomechanical factors profoundly influence the processes of tissue growth, development, maintenance, degeneration, and repair. Regenerative strategies to restore damaged or diseased tissues in vivo and create living tissue replacements in vitro have recently begun to harness advances in understanding of how cells and tissues sense and adapt to their mechanical environment. It is clear that biomechanical considerations will be fundamental to the successful development of clinical therapies based on principles of tissue engineering and regenerative medicine for a broad range of musculoskeletal, cardiovascular, craniofacial, skin, urinary, and neural tissues. Biomechanical stimuli may in fact hold the key to producing regenerated tissues with high strength and endurance. However, many challenges remain, particularly for tissues that function within complex and demanding mechanical environments in vivo. This paper reviews the present role and potential impact of experimental and computational biomechanics in engineering functional tissues using several illustrative examples of past successes and future grand challenges.

Bone marrow-derived mesenchymal stem cells promote angiogenic processes in a time- and dose-dependent manner in vitro

Bone marrow-derived mesenchymal stem cells (MSCs) have received much attention as a potential treatment for myocardial infarction because of their potential to integrate into the host myocardium and repair the injured heart. The mode of action of stem cell-mediated cardiac repair is still somewhat unclear, including the potential role of MSCs in neovascularization. The objective of this study was to determine the in vitro effect of MSCs on angiogenesis-related endothelial cell (EC) behavior, including migration, monolayer permeability, and vessel formation and stabilization. In a noncontact coculture system, we found that MSCs increase EC proliferation and migration, promoting early events of angiogenesis, while also decreasing EC monolayer permeability. Further, in a time- and dose-dependent manner, MSCs in direct coculture with ECs on Matrigel can increase the persistence of preexisting vessels by greater than threefold, with complex vessels remaining stable for more than 10 days. The results demonstrate that MSCs play an active role in the cellular processes involved in the formation, stabilization, and maturation of newly formed vessels. Further, these outcomes are not governed solely by either paracrine or direct contact effects and are both time and dose dependent.

Endothelial Cell Monolayer Formation: Effect of Substrate and Fluid Shear Stress

The formation of an endothelial cell (EC) monolayer is one critical factor in the development of a tissue engineered vascular graft. One potential method of endothelialization is the migration of native ECs from the surrounding blood vessel onto the newly implanted graft. In the present study, in vitro experiments were performed to investigate the potential of ECs to migrate on a tissue-engineered blood vessel wall model (TEWM) and form a new monolayer. The TEWM was composed of a three-dimensional, type I collagen matrix seeded with smooth muscle cells. The potential of ECs to form new monolayer was evaluated in the absence and presence of fluid shear stress (10 dynes/cm(2)). The monolayer formation on the TEWM was compared to a control, two-dimensional substrate of glass coated with type I collagen. Results from these studies showed that EC monolayer formation was inhibited on the TEWM in comparison to glass coated with collagen. This correlated with an inhibition of proliferation on the TEWM. The application of shear stress to the EC on the glass coated with collagen also caused an inhibition of monolayer formation, with a corresponding inhibition of proliferation. Furthermore, blocking proliferation by incubation with mitomycin C caused a dose-response inhibition of monolayer formation. In contrast, stimulating proliferation with basic fibro- blast growth factor (bFGF) did not further increase monolayer formation on glass coated with collagen. These results suggest that proliferation is one necessary factor for monolayer formation, although not the only factor, where EC proliferation is regulated by its environment, including both substrate and the local shear stress. Continued investigation into the mechanism and stimulation of EC proliferation on the TEWM may lead to developing new strategies for the endothelialization of a tissue engineered vascular graft.

Poly(glycerol sebacate) supports the proliferation and phenotypic protein expression of primary baboon vascular cells

Poly(glycerol sebacate) (PGS) is a biodegradable and biocompatible elastomer specifically developed for soft tissue engineering. Vascular cells adhered to an elastomer may exhibit more physiological behavior because the substrate's mechanical properties more closely match those of the tissue. To investigate the feasibility of using PGS as a scaffold material for vascular tissue engineering, the authors examined the adhesion, proliferation, and phenotypic and morphologic properties of primary baboon endothelial progenitor cells (BaEPCs) and baboon smooth muscle cells (BaSMCs) cultured on PGS films and scaffolds. Tissue culture-treated polystyrene plates were used as controls. Phase contrast microscopy indicated that both types of cells showed normal morphology on PGS films. Immuofluorescent staining revealed that von Willebrand factor and alpha-smooth muscle actin were expressed by BaEPCs and BaSMCs, respectively. Both types of cells proliferated well on PGS surfaces. When cultured in PGS scaffolds, BaSMCs were distributed throughout the scaffolds and synthesized extracellular matrix, as indicated by histological evaluations. The distribution of the BaSMCs in the constructs was confirmed by scanning electron microscopy. Immunofluorescent staining of cocultured constructs indicated that the BaSMC-seeded constructs provided suitable surfaces for BaEPC adhesion, and both types of cells maintained their specific phenotypes. These results suggest that PGS is an appropriate scaffold material for blood vessel tissue engineering.

Vessel Wall–Embedded Dendritic Cells Induce T-Cell Autoreactivity and Initiate Vascular Inflammation

Human medium-sized and large arteries are targeted by inflammation with innate and adaptive immune responses occurring within the unique microspace of the vessel wall. How 3D spatial arrangements influence immune recognition and cellular response thresholds and which cell populations sense immunoactivating ligands and function as antigen-presenting cells are incompletely understood. To mimic the 3D context of human arteries, bioartificial arteries were engineered from collagen type I matrix, human vascular smooth muscle cells (VSMCs), and human endothelial cells and populated with cells implicated in antigen presentation and T-cell stimulation, including monocytes, macrophages, and myeloid dendritic cells (DCs). Responsiveness of wall-embedded antigen-presenting cells was probed with the Toll-like receptor ligand lipopolysaccharide, and inflammation was initiated by adding autologous CD4 + T cells. DCs colonized the outermost VSMC layer, recapitulating their positioning at the media–adventitia border of normal arteries. Wall-embedded DCs responded to the microbial product lipopolysaccharide by entering the maturation program and upregulating the costimulatory ligand CD86. Activated DCs effectively stimulated autologous CD4 T cells, which produced the proinflammatory cytokine interferon-γ and infiltrated deeply into the VSMC layer, causing matrix damage. Lipopolysaccharide-triggered macrophages were significantly less efficacious in recruiting T cells and promoting T-cell stimulation. CD14 + monocytes, even when preactivated, failed to support initial steps of vascular wall inflammation. Innate immune cells, including monocytes, macrophages, and DCs, display differential functions in the vessel wall. DCs are superior in sensing pathogen-derived motifs and are highly efficient in breaking T-cell tolerance, guiding T cells toward proinflammatory and tissue-invasive behavior.

Valvular endothelial cells and the mechanoregulation of valvular pathology

Endothelial cells are critical mediators of haemodynamic forces and as such are important foci for initiation of vascular pathology. Valvular leaflets are also lined with endothelial cells, though a similar role in mechanosensing has not been demonstrated. Recent evidence has shown that valvular endothelial cells respond morphologically to shear stress, and several studies have implicated valvular endothelial dysfunction in the pathogenesis of disease. This review seeks to combine what is known about vascular and valvular haemodynamics, endothelial response to mechanical stimuli and the pathogenesis of valvular diseases to form a hypothesis as to how mechanical stimuli can initiate valvular endothelial dysfunction and disease progression. From this analysis, it appears that inflow surface-related bacterial/thrombotic vegetative endocarditis is a high shear-driven endothelial denudation phenomenon, while the outflow surface with its related calcific/atherosclerotic degeneration is a low/oscillatory shear-driven endothelial activation phenomenon. Further understanding of these mechanisms may help lead to earlier diagnostic tools and therapeutic strategies.