Stem cell sources for vascular tissue engineering and regeneration

Regenerative implants for cardiovascular tissue engineering

A fundamental problem that affects the field of cardiovascular surgery is the paucity of autologous tissue available for surgical reconstructive procedures. Although the best results are obtained when an individual's own tissues are used for surgical repair, this is often not possible as a result of pathology of autologous tissues or lack of a compatible replacement source from the body. The use of prosthetics is a popular solution to overcome shortage of autologous tissue, but implantation of these devices comes with an array of additional problems and complications related to biocompatibility. Transplantation offers another option that is widely used but complicated by problems related to rejection and donor organ scarcity. The field of tissue engineering represents a promising new option for replacement surgical procedures. Throughout the years, intensive interdisciplinary, translational research into cardiovascular regenerative implants has been undertaken in an effort to improve surgical outcome and better quality of life for patients with cardiovascular defects. Vascular, valvular, and heart tissue repair are the focus of these efforts. Implants for these neotissues can be divided into 2 groups: biologic and synthetic. These materials are used to facilitate the delivery of cells or drugs to diseased, damaged, or absent tissue. Furthermore, they can function as a tissue-forming device used to enhance the body's own repair mechanisms. Various preclinical studies and clinical trials using these advances have shown that tissue-engineered materials are a viable option for surgical repair, but require refinement if they are going to reach their clinical potential. With the growth and accomplishments this field has already achieved, meeting those goals in the future should be attainable.

Serum-free spheroid suspension culture maintains mesenchymal stem cell proliferation and differentiation potential

There have been many clinical trials recently using ex vivo-expanded human mesenchymal stem cells (MSCs) to treat several disease states such as graft-versus-host disease, acute myocardial infarction, Crohn's disease, and multiple sclerosis. The use of MSCs for therapy is expected to become more prevalent as clinical progress is demonstrated. However, the conventional 2-dimensional (2D) culture of MSCs is laborious and limited in scale potential. The large dosage requirement for many of the MSC-based indications further exacerbates this manufacturing challenge. In contrast, expanding MSCs as spheroids does not require a cell attachment surface and is amenable to large-scale suspension cell culture techniques, such as stirred-tank bioreactors. In the present study, we developed and optimized serum-free media for culturing MSC spheroids. We used Design of Experiment (DoE)-based strategies to systematically evaluate media mixtures and a panel of different components for effects on cell proliferation. The optimization yielded two prototype serum-free media that enabled MSCs to form aggregates and proliferate in both static and dynamic cultures. MSCs from spheroid cultures exhibited the expected immunophenotype (CD73, CD90, and CD105) and demonstrated similar or enhanced differentiation potential toward all three lineages (osteogenic, chondrogenic, adipogenic) as compared with serum-containing adherent MSC cultures. Our results suggest that serum-free media for MSC spheroids may pave the way for scale-up production of MSCs in clinically relevant manufacturing platforms such as stirred tank bioreactors.

Biological and engineering design considerations for vascular tissue engineered blood vessels (TEBVs)

Considerable advances have occurred in the development of tissue-engineered blood vessels (TEBVs) to repair or replace injured blood vessels, or as in vitro systems for drug toxicity testing. Here we summarize approaches to produce TEBVs and review current efforts to (1) identify suitable cell sources for the endothelium and vascular smooth muscle cells, (2) design the scaffold to mimic the arterial mechanical properties and (3) regulate the functional state of the cells of the vessel wall. Initial clinical studies have established the feasibility of this approach and challenges that make TEBVs a viable alternative for vessel replacement are identified.

Differentiation of human hair follicle stem cells into endothelial cells induced by vascular endothelial and basic fibroblast growth factors

Hair follicle stem cells (HFSCs) possess powerful expansion and multi‑differentiation potential, properties that place them at the forefront of the field of tissue engineering and stem cell‑based therapy. The aim of the present study was to investigate the differentiation of human HFSCs (hHFSCs) into cells of an endothelial lineage. hHFSCs were expanded to the second passage in vitro and then induced by the addition of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) to the culture medium. The expression levels of endothelial cell (EC)‑related markers, including von Willebrand factor (vWF), vascular endothelial cadherin (VE)‑cadherin and cluster of differentiation (CD)31, were detected by immunofluorescence staining, flow cytometric analysis and reverse transcription‑polymerase chain reaction. The hHFSCs expressed vWF, VE‑cadherin and CD31 when exposed to a differentiation medium, similar to the markers expressed by the human umbilical vein ECs. More significantly, differentiated cells were also able to take up low‑density lipoprotein. The data of the present study demonstrated that an efficient strategy may be developed for differentiating hHFSCs into ECs by stimulation with VEGF and bFGF. Thus, hHFSCs represent a novel cell source for vascular tissue engineering and studies regarding the treatment of various forms of ischaemic vascular disease.

Vascular tissue engineering: from in vitro to in situ

Blood vessels transport blood to deliver oxygen and nutrients. Vascular diseases such as atherosclerosis may result in obstruction of blood vessels and tissue ischemia. These conditions require blood vessel replacement to restore blood flow at the macrocirculatory level, and angiogenesis is critical for tissue regeneration and remodeling at the microcirculatory level. Vascular tissue engineering has focused on addressing these two major challenges. We provide a systematic review on various approaches for vascular graft tissue engineering. To create blood vessel substitutes, bioengineers and clinicians have explored technologies in cell engineering, materials science, stem cell biology, and medicine. The scaffolds for vascular grafts can be made from native matrix, synthetic polymers, or other biological materials. Besides endothelial cells, smooth muscle cells, and fibroblasts, expandable cells types such as adult stem cells, pluripotent stem cells, and reprogrammed cells have also been used for vascular tissue engineering. Cell-seeded functional tissue-engineered vascular grafts can be constructed in bioreactors in vitro. Alternatively, an autologous vascular graft can be generated in vivo by harvesting the capsule layer formed around a rod implanted in soft tissues. To overcome the scalability issue and make the grafts available off-the-shelf, nonthrombogenic vascular grafts have been engineered that rely on the host cells to regenerate blood vessels in situ. The rapid progress in the field of vascular tissue engineering has led to exciting preclinical and clinical trials. The advancement of micro-/nanotechnology and stem cell engineering, together with in-depth understanding of vascular regeneration mechanisms, will enable the development of new strategies for innovative therapies.

Human hair follicle stem cell differentiation into contractile smooth muscle cells is induced by transforming growth factor-β1 and platelet-derived growth factor BB

Smooth muscle cells (SMCs) are important in vascular homeostasis and disease and thus, are critical elements in vascular tissue engineering. Although adult SMCs have been used as seed cells, such mature differentiated cells suffer from limited proliferation potential and cultural senescence, particularly when originating from older donors. By comparison, human hair follicle stem cells (hHFSCs) are a reliable source of stem cells with multi-differentiation potential. The aim of the present study, was to develop an efficient strategy to derive functional SMCs from hHFSCs. hHFSCs were obtained from scalp tissues of healthy adult patients undergoing cosmetic plastic surgery. The hHFSCs were expanded to passage 2 and induced by the administration of transforming growth factor-β1 (TGF-β1) and platelet-derived growth factor BB (PDGF-BB) in combination with culture medium. Expression levels of SMC-related markers, including α-smooth muscle actin (α-SMA), α-calponin and smooth muscle myosin heavy chain (SM-MHC), were detected by immunofluorescence staining, flow cytometry analysis and reverse transcription-polymerase chain reaction (RT-PCR). When exposed to differentiation medium, hHFSCs expressed early, mid and late markers (α-SMA, α-calponin and SM-MHC, respectively) that were similar to the markers expressed by human umbilical artery SMCs. Notably, when entrapped inside a collagen matrix lattice, these SM differentiated cells showed a contractile function. Therefore, the present study developed an efficient strategy for differentiating hHFSCs into contractile SMCs by stimulation with TGF-β1 and PDGF-BB. The high yield of derivation suggests that this strategy facilitates the acquisition of the large numbers of cells that are required for blood vessel engineering and the study of vascular disease pathophysiology.

Animal models for vascular tissue-engineering

Because of rise in cardiovascular disease throughout the world, there is increasing demand for small diameter blood vessels as replacement grafts. The present review focuses on the animal models that have been used to test small-diameter TEVs with emphasis on the attributes of each model. Small animal models are used to test short-term patency and address mechanistic hypotheses; and large, preclinical animal models are employed to test long-term patency, remodeling and function in an environment mimicking human physiology. We also discuss recent clinical trials that employed laboratory fabricated TEVs and showed very promising results. Ultimately, animal models provide a testing platform for optimizing vascular grafts before clinical use in patients without suitable autologous vessels.

Small-diameter vascular tissue engineering

Vascular occlusion remains the leading cause of death in Western countries, despite advances made in balloon angioplasty and conventional surgical intervention. Vascular surgery, such as CABG surgery, arteriovenous shunts, and the treatment of congenital anomalies of the coronary artery and pulmonary tracts, requires biologically responsive vascular substitutes. Autografts, particularly saphenous vein and internal mammary artery, are the gold-standard grafts used to treat vascular occlusions. Prosthetic grafts have been developed as alternatives to autografts, but their low patency owing to short-term and intermediate-term thrombosis still limits their clinical application. Advances in vascular tissue engineering technology-such as self-assembling cell sheets, as well as scaffold-guided and decellularized-matrix approaches-promise to produce responsive, living conduits with properties similar to those of native tissue. Over the past decade, vascular tissue engineering has become one of the fastest-growing areas of research, and is now showing some success in the clinic.

Vascular tissue engineering: biodegradable scaffold platforms to promote angiogenesis

The ability to understand and regulate human vasculature development and differentiation has the potential to benefit patients suffering from a variety of ailments, including cardiovascular disease, peripheral vascular disease, ischemia, and burn wounds. Current clinical treatments for vascular-related diseases commonly use the grafting from patients of autologous vessels, which are limited and often damaged due to disease. Considerable progress is being made through a tissue engineering strategy in the vascular field. Tissue engineering takes a multidisciplinary approach seeking to repair, improve, or replace biological tissue function in a controlled and predictable manner. To address the clinical need to perfuse and repair damaged, ischemic tissue, one approach of vascular engineering aims to understand and promote the growth and differentiation of vascular networks. Vascular tissue engineered constructs enable the close study of vascular network assembly and vessel interactions with the surrounding microenvironment. Scaffold platforms provide a method to control network development through the biophysical regulation of different scaffold properties, such as composition, mechanics, dimensionality, and so forth. Following a short description of vascular physiology and blood vessel biomechanics, the key principles in vascular tissue engineering are discussed. This review focuses on various biodegradable scaffold platforms and demonstrates how they are being used to regulate, promote, and understand angiogenesis and vascular network formation.

Functional vascular smooth muscle cells derived from human induced pluripotent stem cells via mesenchymal stem cell intermediates

AIMS

Smooth muscle cells (SMC) play an important role in vascular homeostasis and disease. Although adult mesenchymal stem cells (MSC) have been used as a source of contractile SMC, they suffer from limited proliferation potential and culture senescence, particularly when originating from older donors. By comparison, human induced pluripotent stem cells (hiPSC) can provide an unlimited source of functional SMC for autologous cell-based therapies and for creating models of vascular disease. Our goal was to develop an efficient strategy to derive functional, contractile SMC from hiPSC.

METHODS AND RESULTS

We developed a robust, stage-wise, feeder-free strategy for hiPSC differentiation into functional SMC through an intermediate stage of multipotent MSC, which could be coaxed to differentiate into fat, bone, cartilage, and muscle. At this stage, the cells were highly proliferative and displayed higher clonogenic potential and reduced senescence when compared with parental hair follicle mesenchymal stem cells. In addition, when exposed to differentiation medium, the myogenic proteins such as α-smooth muscle actin, calponin, and myosin heavy chain were significantly upregulated and displayed robust fibrillar organization, suggesting the development of a contractile phenotype. Indeed, tissue constructs prepared from these cells exhibited high levels of contractility in response to receptor- and non-receptor-mediated agonists.

CONCLUSION

We developed an efficient stage-wise strategy that enabled hiPSC differentiation into contractile SMC through an intermediate population of clonogenic and multipotent MSC. The high yield of MSC and SMC derivation suggests that our strategy may facilitate an acquisition of the large numbers of cells required for regenerative medicine or for studying vascular disease pathophysiology.