Development of a Tissue-Engineered Bypass Graft Seeded with Stem Cells

The Antithrombotic Function of Sphingosine-1-Phosphate on Human Adipose-Stem-Cell-Recellularized Tissue Engineered Vascular Graft In Vitro

Adipose stem cells (ASCs) show potential in the recellularization of tissue engineerined vascular grafts (TEVGs). However, whether sphingosine-1-phosphate (S1P) could further enhance the adhesion, proliferation, and antithrombosis of ASCs on decellularized vascular scaffolds is unknown. This study investigated the effect of S1P on the recellularization of TEVGs with ASCs. Human ASCs were derived from lipoaspirate. Scaffolds were derived from human umbilical arteries (HUAs) with treatment of 0.1% sodium dodecyl sulfate (SDS) for 48 h (decellularized HUAs; DHUAs). The adhesion, proliferation, and antithrombotic functions (kinetic clotting time and platelet adhesion) of ASCs on DHUAs with S1P or without S1P were evaluated. The histology and DNA examination revealed a preserved structure and the elimination of the nuclear component more than 95% in HUAs after decellularizaiton. Human ASCs (hASCs) showed CD29(+), CD73(+), CD90(+), CD105(+), CD31(-), CD34(-), CD44(-), HLA-DR(-), and CD146(-) while S1P-treated ASCs showed marker shifting to CD31(+). In contrast to human umbilical vein endothelial cells (HUVECs), S1P didn't significantly increase proliferation of ASCs on DHUAs. However, the kinetic clotting test revealed prolonged blood clotting in S1P-treated ASC-recellularized DHUAs. S1P also decreased platelet adhesion on ASC-recellularized DHUAs. In addition, S1P treatment increased the syndecan-1 expression of ASCs. TEVG reconstituted with S1P and ASC-recellularized DHUAs showed an antithrombotic effect in vitro. The preliminary results showed that ASCs could adhere to DHUAs and S1P could increase the antithrombotic effect on ASC-recellularized DHUAs. The antithrombotic effect is related to ASCs exhibiting an endothelial-cell-like function and preventing of syndecan-1 shedding. A future animal study is warranted to prove this novel method.

Antishear Stress Bionic Carbon Nanotube Mesh Coating with Intracellular Controlled Drug Delivery Constructing Small-Diameter Tissue-Engineered Vascular Grafts

Small-diameter (<6 mm) tissue-engineered blood vessels (TEBVs) have a low patency rate due to chronic inflammation mediated intimal hyperplasia. Functional coating with drug release is a promising solution, but preventing the released drug from being rushed away by blood flow remains a great challenge. A single-walled carboxylic acid functionalized carbon nanotube (C-SWCNT) is used to build an irregular mesh for TEBV coating. However, an interaction between the released drug and the cells is still insufficient due to the blood flow. Thus, an intracellular drug delivery system mediated by macrophage cellular uptake is designed. Resveratrol (RSV) modified CNT is used for macrophage uptake. M1 macrophage uptakes CNT-RSV and then converts to the M2 phenotype upon intracellular RSV release. Prohealing M2 macrophage inhibits the chronic inflammation thus maintains the contractile phenotype of the vascular smooth muscle cell (VSMC), which reduces intimal hyperplasia. Additionally, RSV released from the mesh coating also directly protects the contractile VSMCs from being converted to a secretory phenotype. Through antishear stress coating and macrophage-based intracellular drug delivery, CNT-RSV TEBVs exhibit a long-term anti-intimal hyperplasia function. Animal transplantation studies show that the patency rate remains high until day 90 after grafting in rat carotid arteries.

Scaffolds and Cell-Based Tissue Engineering for Blood Vessel Therapy

The increasing morbidity of cardiovascular diseases in modern society has made it crucial to develop a small-caliber blood vessel. In the absence of appropriate autologous vascular grafts, an alternative prosthesis must be constructed for cardiovascular disease patients. The aim of this article is to describe the advances in making cell-seeded cardiovascular prostheses. It also discusses the combinations of types of scaffolds and cells, especially autologous stem cells, which are suitable for application in tissue-engineered vessels with the favorable properties of mechanical strength, antithrombogenicity, biocompliance, anti-inflammation, fatigue resistance and long-term durability. This article highlights the advancements in cellular tissue-engineered vessels in recent years.

Optimal Prosthetic Graft Design for Small Diameter Vascular Grafts

Autogenous vein and arterial grafts, such as great saphenous veins and internal mammary and radial arteries, remain the gold standard conduits for vascular reconstruction. Expanded polytetrafluoroethylene (PTFE) grafts, which exhibit little inflammatory and thrombogenic reactivity, are the most commonly used material of choice for small diameter vascular grafts when autogenous grafts are not available. Several modifications of the basic graft have been attempted to enhance graft healing of expanded PTFE grafts, and little but definite experimental and clinical improvement has been achieved so far. The technique of vascular tissue engineering, in combination with stem cell research, may hold the key for the creation of a practical and successful small diameter prosthetic graft.

Human adipose-derived stromal cells for the production of completely autologous self-assembled tissue-engineered vascular substitutes

There is a clinical need for small-diameter vascular substitutes, notably for coronary and peripheral artery bypass procedures since these surgeries are limited by the availability of grafting material. This study reports the characterization of a novel autologous tissue-engineered vascular substitute (TEVS) produced in 10weeks exclusively from human adipose-derived stromal cells (ASC) self-assembly, and its comparison to an established model made from dermal fibroblasts (DF). Briefly, ASC and DF were cultured with ascorbate to form cell sheets subsequently rolled around a mandrel. These TEVS were further cultured as a maturation period before undergoing mechanical testing, histological analyses and endothelialization. No significant differences were measured in burst pressure, suture strength, failure load, elastic modulus and failure strain according to the cell type used to produce the TEVS. Indeed, ASC- and DF-TEVS both displayed burst pressures well above maximal physiological blood pressure. However, ASC-TEVS were 1.40-fold more compliant than DF-TEVS. The structural matrix, comprising collagens type I and III, fibronectin and elastin, was very similar in all TEVS although histological analysis showed a wavier and less dense collagen matrix in ASC-TEVS. This difference in collagen organization could explain their higher compliance. Finally, human umbilical vein endothelial cells (HUVEC) successfully formed a confluent endothelium on ASC and DF cell sheets, as well as inside ASC-TEVS. Our results demonstrated that ASC are an alternative cell source for the production of TEVS displaying good mechanical properties and appropriate endothelialization.

A practical model for economic evaluation of tissue-engineered therapies

Tissue-engineered therapies are being developed across virtually all fields of medicine. Some of these therapies are already in clinical use, while others are still in clinical trials or the experimental phase. Most initial studies in the evaluation of new therapies focus on demonstration of clinical efficacy. However, cost considerations or economic viability are just as important. Many tissue-engineered therapies have failed to be impactful because of shortcomings in economic competitiveness, rather than clinical efficacy. Furthermore, such economic viability studies should be performed early in the process of development, before significant investment has been made. Cost-minimization analysis combined with sensitivity analysis is a useful model for the economic evaluation of new tissue-engineered therapies. The analysis can be performed early in the development process, and can provide valuable information to guide further investment and research. The utility of the model is illustrated with the practical real-world example of tissue-engineered constructs for corneal endothelial transplantation.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

Sphingosine-1-phosphate promotes the differentiation of adipose-derived stem cells into endothelial nitric oxide synthase (eNOS) expressing endothelial-like cells

BACKGROUND

Adipose tissue provides a readily available source of autologous stem cells. Adipose-derived stem cells (ASCs) have been proposed as a source for endothelial cell substitutes for lining the luminal surface of tissue engineered bypass grafts. Endothelial nitric oxide synthase (eNOS) is a key protein in endothelial cell function. Currently, endothelial differentiation from ASCs is limited by poor eNOS expression. The goal of this study was to investigate the role of three molecules, sphingosine-1-phosphate (S1P), bradykinin, and prostaglandin-E1 (PGE1) in ASC endothelial differentiation. Endothelial differentiation markers (CD31, vWF and eNOS) were used to evaluate the level of ASCs differentiation capability.

RESULTS

ASCs demonstrated differentiation capability toward to adipose, osteocyte and endothelial like cell phenotypes. Bradykinin, S1P and PGE were used to promote differentiation of ASCs to an endothelial phenotype. Real-time PCR showed that all three molecules induced significantly greater expression of endothelial differentiation markers CD31, vWF and eNOS than untreated cells. Among the three molecules, S1P showed the highest up-regulation on endothelial differentiation markers. Immunostaining confirmed presence of more eNOS in cells treated with S1P than the other groups. Cell growth measurements by MTT assay, cell counting and EdU DNA incorporation suggest that S1P promotes cell growth during ASCs endothelial differentiation. The S1P1 receptor was expressed in ASC-differentiated endothelial cells and S1P induced up-regulation of PI3K.

CONCLUSIONS

S1P up-regulates endothelial cell markers including eNOS in ASCs differentiated to endothelial like cells. This up-regulation appears to be mediated by the up-regulation of PI3K via S1P1 receptor. ASCs treated with S1P offer promising use as endothelial cell substitutes for tissue engineered vascular grafts and vascular networks.

Vascular extracellular matrix in atherosclerosis

The extracellular matrix (ECM) is an essential component of the human body that is responsible for the proper function of various organs. Changes in the ECM have been implicated in the pathogenesis of several cardiovascular conditions including atherosclerosis, restenosis, and heart failure. Matrix components, such as collagens and noncollagenous proteins, influence the function and activity of vascular cells, particularly vascular smooth muscle cells and macrophages. Matrix proteins have been shown to be implicated in the development of atherosclerotic complications, such as plaque rupture, aneurysm formation, and calcification. ECM proteins control ECM remodeling through feedback signaling to matrix metalloproteinases (MMPs), which are the key players of ECM remodeling in both normal and pathological conditions. The production of MMPs is closely related to the development of an inflammatory response and is subjected to significant changes at different stages of atherosclerosis. Indeed, blood levels of circulating MMPs may be useful for the assessment of the inflammatory activity in atherosclerosis and the prediction of cardiovascular risk. The availability of a wide variety of low-molecular MMP inhibitors that can be conjugated with various labels provides a good perspective for specific targeting of MMPs and implementation of imaging techniques to visualize MMP activity in atherosclerotic plaques and, most interestingly, to monitor responses to antiatheroslerosis therapies. Finally, because of the crucial role of ECM in cardiovascular repair, the regenerative potential of ECM could be successfully used in constructing engineered scaffolds and vessels that mimic properties of the natural ECM and consist of the native ECM components or composite biomaterials. These scaffolds possess a great promise in vascular tissue engineering.

eNOS transfection of adipose-derived stem cells yields bioactive nitric oxide production and improved results in vascular tissue engineering

This study evaluates the durability of a novel tissue engineered blood vessel (TEBV) created by seeding a natural vascular tissue scaffold (decellularized human saphenous vein allograft) with autologous adipose-derived stem cells (ASC) differentiated into endothelial-like cells. Previous work with this model revealed the graft to be thrombogenic, likely due to inadequate endothelial differentiation as evidenced by minimal production of nitric oxide (NO). To evaluate the importance of NO expression by the seeded cells, we created TEBV using autologous ASC transfected with the endothelial nitric oxide synthase (eNOS) gene to produce NO. We found that transfected ASC produced NO at levels similar to endothelial cell (EC) controls in vitro which was capable of causing vasorelaxation of aortic specimens ex vivo. TEBV (n = 5) created with NO-producing ASC and implanted as interposition grafts within the aorta of rabbits remained patent for two months and demonstrated a non-thrombogenic surface compared to unseeded controls (n = 5). Despite the xenograft nature of the scaffold, the TEBV structure remained well preserved in seeded grafts. In sum, this study demonstrates that upregulation of NO expression within adult stem cells differentiated towards an endothelial-like lineage imparts a non-thrombogenic phenotype and highlights the importance of NO production by cells to be used as endothelial cell substitutes in vascular tissue engineering applications.

Endogenous regeneration of critical-size chondral defects in immunocompromised rat xiphoid cartilage using decellularized human bone matrix scaffolds

Clinical efforts to repair cartilage defects delivering cells or engineered cartilage implants into the lesions have met with limited success. This study used a critical-size chondral defect model in immunocompromised rat xiphoid cartilage to test whether endogenous chondrogenesis could be achieved using human bone matrix scaffolds to deliver human cartilage particles and/or a variant isoform of fibroblast growth factor-2 (FGF2-variant). Seventy-two male athymic RNU rats were enrolled in this study with eight rats per experimental group. Decellularized and demineralized human bone matrix scaffolds loaded with human articular cartilage particles or heat-inactivated cartilage particles were combined with different doses of the FGF2-variant. Scaffolds were implanted into 3-mm-diameter critical-size defects prepared using a biopsy punch through the center of the xiphoid. The samples were evaluated 28 days postsurgery using X-ray, equilibrium partitioning of ionic contrast microcomputed tomography, and safranin O-stained histological sagittal sections. Scaffolds containing cartilage particles plus the FGF2-variant induced dose-dependent increases in the formation of neocartilage (p<0.05), which was distributed homogeneously throughout the defects in comparison to scaffolds containing only the FGF2-variant. These effects were less pronounced when scaffolds with heat-inactivated cartilage particles were used. These results demonstrate that endogenous repair of chondral defects can be achieved in the absence of exogenous cells or bone marrow, suggesting that a similar approach may be successful for treating chondral lesions clinically.

Biomaterials for vascular tissue engineering

Cardiovascular disease is the leading cause of mortality in the USA. The limited availability of healthy autologous vessels for bypass grafting procedures has led to the fabrication of prosthetic vascular conduits. While synthetic polymers have been extensively studied as substitutes in vascular engineering, they fall short of meeting the biological challenges at the blood-material interface. Various tissue engineering strategies have emerged to address these flaws and increase long-term patency of vascular grafts. Vascular cell seeding of scaffolds and the design of bioactive polymers for in situ arterial regeneration have yielded promising results. This article describes the advances made in biomaterials design to generate suitable materials that not only match the mechanical properties of native vasculature, but also promote cell growth, facilitate extracellular matrix production and inhibit thrombogenicity.

Linear shear conditioning improves vascular graft retention of adipose-derived stem cells by upregulation of the alpha5beta1 integrin

Use of adult adipose-derived stem cells (ASCs) as endothelial cell substitutes in vascular tissue engineering is attractive because of their availability. However, when seeded onto decellularized vascular scaffolding and exposed to physiological fluid shear force, ASCs are physically separated from the graft lumen. Herein we have investigated methods of increasing initial ASC attachment using luminal precoats and a novel protocol for the gradual introduction of shear stress to optimize ASC retention. Fibronectin coating of the graft lumen increased ASC attachment by nearly sixfold compared with negative controls. Gradual introduction of near physiological fluid shear stress using a novel bioreactor whereby flow rate was increased every second at a rate of 1.5 dynes/cm(2) per day resulted in complete luminal coverage compared with near complete cell loss following conventional daily abrupt increases. An upregulation of the alpha(5)beta(1) integrin was evinced following exposure to shear stress, which accounts for the observed increase in ASC retention on the graft lumen. These results indicated a novel method for seeding, conditioning, and retaining of adult stem cells on a decellularized vein scaffold within a high-shear stress microenvironment.

The role of hypoxia in stem cell differentiation and therapeutics

Stem cells differentiate into a variety of cell lines, making them attractive for tissue engineering and regenerative medicine. Specific microenvironmental cues regulate self-renewal and differentiation capabilities. Oxygen is an important component of the cellular microenvironment, serving as both metabolic substrate and signaling molecule. Oxygen has been shown to have a variety of effects on embryonic and adult stem cells. This review examines the role of hypoxia in regulating stem cell biology, specifically focusing on growth, maintenance of pluripotency, differentiation, and production of growth factors. Particular attention is paid to hypoxia and stem cells in relation to therapeutic angiogenesis. We conclude that further study is needed to optimize the use of hypoxia as a stimulus for various stem cell functions, including its potential role in therapeutic angiogenesis.

Polymeric materials for tissue engineering of arterial substitutes

Cardiovascular disease is the leading cause of mortality in the United States. The limited availability of healthy autologous vessels for bypass grafting procedures has led to the fabrication of prosthetic vascular conduits. Synthetic polymeric materials, while providing the appropriate mechanical strength, lack the compliance and biocompatibility that bioresorbable and naturally occurring protein polymers offer. Vascular tissue engineering approaches have emerged in order to meet the challenges of designing a vascular graft with long-term patency. In vitro culture techniques that have been explored with vascular cell seeding of polymeric scaffolds and the use of bioactive polymers for in situ arterial regeneration have yielded promising results. This review describes the development of polymeric materials in various tissue engineering strategies for the improvement in the mechanical and biological performance of an arterial substitute.

Human adipose stem cells: a potential cell source for cardiovascular tissue engineering

BACKGROUND/AIMS

A crucial step in providing clinically relevant applications of cardiovascular tissue engineering involves the identification of a suitable cell source. The objective of this study was to identify the exogenous and endogenous parameters that are critical for the differentiation of human adipose stem cells (hASCs) into cardiovascular cells.

METHODS

hASCs were isolated from human lipoaspirate samples, analyzed, and subjected to two differentiation protocols.

RESULTS

As shown by fluorescence-activated cell sorter (FACS) analysis, a population of hASCs expressed stem cell markers including CXCR4, CD34, c-kit, and ABCG2. Further, FACS and immunofluorescence analysis of hASCs, cultured for 2 weeks in DMEM-20%-FBS, showed the expression of smooth muscle cell (SMC)-specific markers including SM alpha-actin, basic calponin, h-caldesmon and SM myosin. hASCs, cultured for 2 weeks in endothelial cell growth medium-2 (EGM-2), formed a network of branched tube-like structures positive for CD31, CD144, and von Willebrand factor. The frequency of endothelial cell (EC) marker-expressing cells was passage number-dependent. Moreover, hASCs attached and formed a confluent layer on top of electrospun collagen-elastin scaffolds. Scanning electron microscopy and DAPI staining confirmed the integration of hASCs with the fibers and formation of a cell-matrix network.

CONCLUSION

Our results indicate that hASCs are a potential cell source for cardiovascular tissue engineering; however, the differentiation capacity of hASCs into SMCs and ECs is passage number- and culture condition-dependent.