Can Tissue Engineering Mend Broken Hearts?

Cell sheet tissue engineering for scaffold-free three-dimensional (3D) tissue reconstruction

Three-dimensional (3D) reconstruction of highly functional tissues is of great importance in advancing the clinical benefit of tissue engineering and regenerative medicine. In the last quarter century, many studies have found that by engineering a 3D microenvironment that resembles the in vivo tissue condition, cells exhibit behaviors and functions that reflect those of native tissue. Biomaterial scaffolds are a central technology for providing 3D microenvironments in vitro, and, in conjunction with diverse design and cell seeding advents, have produced highly functional and complex 3D tissues. Here, we describe a new approach to creating 3D cell-dense tissue-like constructs without a biomaterial scaffold. Cell sheet technology with cell sheet layering strategies generates highly cell dense, engineered tissue capable of direct crosstalk with the tissue-engraftment surface, in addition to paracrine-mediated signaling. In this chapter, we will introduce methods of reconstructing 3D tissue using cell sheet technology and the advantages of a scaffold-free design.

Strategies for the chemical and biological functionalization of scaffolds for cardiac tissue engineering: a review

The development of biomaterials for cardiac tissue engineering (CTE) is challenging, primarily owing to the requirement of achieving a surface with favourable characteristics that enhances cell attachment and maturation. The biomaterial surface plays a crucial role as it forms the interface between the scaffold (or cardiac patch) and the cells. In the field of CTE, synthetic polymers (polyglycerol sebacate, polyethylene glycol, polyglycolic acid, poly-l-lactide, polyvinyl alcohol, polycaprolactone, polyurethanes and poly(N-isopropylacrylamide)) have been proven to exhibit suitable biodegradable and mechanical properties. Despite the fact that they show the required biocompatible behaviour, most synthetic polymers exhibit poor cell attachment capability. These synthetic polymers are mostly hydrophobic and lack cell recognition sites, limiting their application. Therefore, biofunctionalization of these biomaterials to enhance cell attachment and cell material interaction is being widely investigated. There are numerous approaches for functionalizing a material, which can be classified as mechanical, physical, chemical and biological. In this review, recent studies reported in the literature to functionalize scaffolds in the context of CTE, are discussed. Surface, morphological, chemical and biological modifications are introduced and the results of novel promising strategies and techniques are discussed.

Cardiac tissue development for delivery of embryonic stem cell-derived endothelial and cardiac cells in natural matrices

The packaging and delivery of cells for cardiac regeneration has been explored using a variety biomaterials and delivery methods, but these studies often ignore one or more important design factors critical for rebuilding cardiac tissue. These include the biomaterial architecture, strength and stiffness, cell alignment, and/or incorporation of multiple cell types. In this article, we explore the combinatorial use of decellularized tissues, moldable hydrogels, patterned cell-seeding, and cell-sheet engineering and find that a combination of these methods is optimal in the recreation of transplantable cardiac-like tissue in vivo. We show that decellularized urinary bladder matrix (UBM), that is compliant and suturable, supports the survival of cell cultures but does not allow maintenance of cell-to-cell contacts of transferred cell-sheets (presumably, due to its rough surface). Moreover, the UBM material must be filled with hyaluronan (HA) hydrogels for smoothing rough surfaces and allowing the delivery of greater cell numbers. We additionally incorporated our previously developed "wrinkled" microchip for inducing alignment of cardiac cells with a laser-etched mask for co-seeding patterned "channels" of cells. This article also introduces a novel method of plasma coating for cell-sheet engineering that compares well with electron bean irradiation methods and may be combined with our "wrinkled" surfaces to facilitate the alignment of cardiac cells into sheets. Our data shows that an optimal design for generating cardiac tissue would include (1) decellularized matrix seeded with endothelial cells in a HA layered with (2) prealigned cardiac cell-sheets fabricated using our "wrinkled" microchips and thermo-responsive polymer [poly(N-isopropylacrylamide)] cell sheet transfer system.

Channelled scaffolds for engineering myocardium with mechanical stimulation

The characteristics of the matrix (composition, structure, mechanical properties) and external culture environment (pulsatile perfusion, physical stimulation) of the heart are important characteristics in the engineering of functional myocardial tissue. This study reports on the development of chitosan-collagen scaffolds with micropores and an array of parallel channels (~ 200 µm in diameter) that were specifically designed for cardiac tissue engineering using mechanical stimulation. The scaffolds were designed to have similar structural and mechanical properties of those of native heart matrix. Scaffolds were seeded with neonatal rat heart cells and subjected to dynamic tensile stretch using a custom designed bioreactor. The channels enhanced oxygen transport and facilitated the establishment of cell connections within the construct. The myocardial patches (14 mm in diameter, 1-2 mm thick) consisted of metabolically active cells that began to contract synchronously after 3 days of culture. Mechanical stimulation with high tensile stress promoted cell alignment, elongation, and expression of connexin-43 (Cx-43). This study confirms the importance of scaffold design and mechanical stimulation for the formation of contractile cardiac constructs.

Engineered heart tissue: a novel tool to study the ischemic changes of the heart in vitro

Background

Understanding the basic mechanisms and prevention of any disease pattern lies mainly on development of a successful experimental model. Recently, engineered heart tissue (EHT) has been demonstrated to be a useful tool in experimental transplantation. Here, we demonstrate a novel function for the spontaneously contracting EHT as an experimental model in studying the acute ischemia-induced changes in vitro.

Methodology/Principal Findings

EHT was constructed by mixing cardiomyocytes isolated from the neonatal rats and cultured in a ring-shaped scaffold for five days. This was followed by mechanical stretching of the EHT for another one week under incubation. Fully developed EHT was subjected to hypoxia with 1% O₂ for 6 hours after treating them with cell protective agents such as cyclosporine A (CsA) and acetylcholine (ACh). During culture, EHT started to show spontaneous contractions that became more synchronous following mechanical stretching. This was confirmed by the increased expression of gap junctional protein connexin 43 and improved action potential recordings using an optical mapping system after mechanical stretching. When subjected to hypoxia, EHT demonstrated conduction defects, dephosphorylation of connexin-43, and down-regulation of cell survival proteins identical to the adult heart. These effects were inhibited by treating the EHT with cell protective agents.

Conclusions/Significance

Under hypoxic conditions, the EHT responds similarly to the adult myocardium, thus making EHT a promising material for the study of cardiac functions in vitro.

Effect of thyroid hormone on the contractility of self-organized heart muscle

Tissue-engineered heart muscle may provide an alternative treatment modality for end-stage congestive heart failure. We have previously described a method to engineer contractile heart muscle in vitro (termed cardioids). This study describes a method to improve the contractile properties of cardioids utilizing thyroid hormone (T3) stimulation. Cardioids were engineered by promoting the self-organization of primary neonatal cardiac cells into a contractile tissue construct. Cardioids were maintained in standard cell culture media supplemented with varying concentrations of T3 in the range 1-5ng/ml. The contractile properties of the cardioids were evaluated 48h after formation. Stimulation with T3 resulted in an increase in the specific force of cardioids from an average value of 0.52 +/- 0.16kPa (N = 6) for control cardioids to 2.42 +/- 0.29kPa (N = 6) for cardioids stimulated with 3ng/ml T3. In addition, there was also an increase in the rate of contraction and relaxation in response to T3 stimulation. Cardioids that were stimulation with T3 exhibited improved pacing characteristics in response to electrical pacing at 1-5Hz and an increase in the degree of spontaneous contractility. Changes in the gene expression of SERCA2, phospholamban, alpha-myosin heavy chain, and beta-myosin heavy chain correlated with the changes in contractile properties. This study demonstrates the modulation of the contractile properties of tissue-engineered heart muscle using T3 stimulation.

Myocardial tissue engineering: a review

Myocardial tissue engineering, a concept that intends to overcome the obstacles to prolonging patients' life after myocardial infarction, is continuously improving. It comprises a biomaterial based 'vehicle', either a porous scaffold or dense patch, made of either natural or synthetic polymeric materials, to aid transportation of cells into the diseased region in the heart. Many different cell types have been suggested for cell therapy and myocardial tissue engineering. These include both autologous and embryonic stem cells, both having their advantages and disadvantages. Biomaterials suggested for this specific tissue-engineering application need to be biocompatible with the cardiac cells and have particular mechanical properties matching those of native myocardium, so that the delivered donor cells integrate and remain intact in vivo. Although much research is being carried out, many questions still remain unanswered requiring further research efforts. In this review, we discuss the various approaches reported in the field of myocardial tissue engineering, focusing on the achievements of combining biomaterials and cells by various techniques to repair the infarcted region, also providing an insight on clinical trials and possible cell sources in cell therapy. Alternative suggestions to myocardial tissue engineering, in situ engineering and left ventricular devices are also discussed.

Cell-based cardiac pumps and tissue-engineered ventricles

Mortalities resulting from cardiovascular disorders remain high, with an urgent need to develop novel treatment modalities. Tissue-engineering therapies aim to provide cell-based alternatives to conventional options. Significant technological advancements have occurred during the last decade towards the fabrication of functional 3D heart muscle in vitro. More recent research has focused on the development of cell-based cardiac pumps and tissue-engineered ventricles. The global objective of this collective work is to simulate the functional performance of the left ventricle, utilizing completely cell-based options. Current prototypes have shown several physiological performance metrics, including the ability of these devices to generate intraluminal pressure upon electrical stimulation. This review will highlight the transition from tissue engineering 3D heart muscle to cell-based cardiac pumps/ventricles.

Engineering the heart piece by piece: state of the art in cardiac tissue engineering

According to the National Transplant Society, more than 7000 Americans in need of organs die every year owing to a lack of lifesaving organs. Bioengineering 3D organs in vitro for subsequent implantation may provide a solution to this problem. The field of tissue engineering in its most rudimentary form is focused on the developed of transplantable organ substitutes in the laboratory. The objective of this article is to introduce important technological hurdles in the field of cardiac tissue engineering. This review starts with an overview of tissue engineering, followed by an introduction to the field of cardiovascular tissue engineering and finally summarizes some of the key advances in cardiac tissue engineering; specific topics discussed in this article include cell sourcing and biomaterials, in vitro models of cardiac muscle and bioreactors. The article concludes with thoughts on the utility of tissue-engineering models in basic research as well as critical technological hurdles that need to be addressed in the future.

Electrical coupling of cardiomyocyte sheets occurs rapidly via functional gap junction formation

Previously, we have successfully created pulsatile myocardial tissue grafts using our novel technology, "cell sheet engineering", that layers cell sheets fabricated on temperature-responsive culture dishes to form three-dimensional (3-D) structures. Electrical coupling is established between layered neonatal rat cardiomyocyte sheets, resulting in the synchronized beating of 3-D myocardial tissues. However, the mechanism by which these layered cardiomyocyte sheets communicate electrically is not well-understood. In this study, we used a multiple-electrode extracellular recording system and demonstrated that bilayer cardiomyocyte sheets coupled electrically with slight delays 34+/-2 min (mean+/-SEM) after layering. These delays gradually decreased and the electrical actions of layered cell sheets were completely coupled 46+/-3 min (mean+/-SEM) after initial layering. Immunohistological analysis showed that connexin43, a gap junction (GJ)-related protein, existed not only at cell-to-cell interfaces but also on the free cell membrane in the cardiomyocyte sheet. Additionally, neither connexin40 nor connexin45, but only connexin43 was detected between bilayer cardiomyocyte sheets within 30 min after layering. Dye transfer assay demonstrated that the exchange of small molecules via GJs occurred within 30 min. The cell sheet manipulation technique using the temperature-responsive culture dishes has substantial advances and the exciting potential in the fields of cell and tissue physiology, as well as tissue engineering.

Reconstitution of the Frank-Starling mechanism in engineered heart tissues

According to the Frank-Starling mechanism, as the heart is stretched, it increases its contraction force. Reconstitution of the Frank-Starling mechanism is an important milestone for producing functional heart tissue constructs. Spontaneously contracting engineered heart tissues (EHTs) were reconstituted by growing dissociated chicken embryo cardiomyocytes in collagen matrices. Twitch and baseline tensions were recorded at precisely controlled levels of tissue strain. The EHTs showed a steep increase in twitch tension from 0.47 +/- 0.02 to 0.91 +/- 0.02 mN/mm2 as they were stretched at a constant rate (2.67% per min) from 86% to 100% of the length at which maximum twitch force was exerted. In response to a sudden stretch (3.3%), the twitch tension increased gradually (approximately 60 s) in a Gd3+-sensitive manner, suggesting the presence of stretch-activated Ca2+ channels. A large difference in baseline tension between lengthening (loading) and shortening (unloading) was also recorded. Disruption of nonsarcomeric actin filaments by cytochalasin D and latrunculin B decreased this difference. A simple mechanical model interprets these results in terms of mechanical connections between myocytes and nonmuscle cells. The experimental results strongly suggest that regulation of twitch tension in EHTs is similar to that of natural myocardium.

A novel composite scaffold for cardiac tissue engineering

One approach to the engineering of functional cardiac tissue for basic studies and potential clinical use involves bioreactor cultivation of dissociated cells on a biomaterial scaffold. Our objective was to develop a scaffold that is (1) highly porous with large interconnected pores (to facilitate mass transport), (2) hydrophilic (to enhance cell attachment), (3) structurally stable (to withstand the shearing forces during bioreactor cultivation), (4) degradable (to provide ultimate biocompatibility of the tissue graft), and (5) elastic (to enable transmission of contractile forces). The scaffold of choice was made as a composite of poly(dl-lactide-co-caprolactone), poly(dl-lactide-co-glycolide) (PLGA), and type I collagen, with open interconnected pores and the average void volume of 80 +/- 5%. Neonatal rat heart cells suspended in Matrigel were seeded into the scaffold at a physiologically high density (1.35 x 10(8) cells/cm(3)) and cultivated for 8 d in cartridges perfused with culture medium or in orbitally mixed dishes (25 rpm); collagen sponge (Ultrafoam) and PLGA sponge served as controls. Construct cellularity, presence of cardiac markers, and contractile properties were markedly improved in composite scaffolds as compared with both controls.

Self-organization of rat cardiac cells into contractile 3-D cardiac tissue

The mammalian heart is not known to regenerate following injury. Therefore, there is great interest in developing viable tissue-based models for cardiac assist. Recent years have brought numerous advances in the development of scaffold-based models of cardiac tissue, but a self-organizing model has yet to be described. Here, we report the development of an in vitro cardiac tissue without scaffolding materials in the contractile region. Using an optimal concentration of the adhesion molecule laminin, a confluent layer of neonatal rat cardiomyogenic cells can be induced to self-organize into a cylindrical construct, resembling a papillary muscle, which we have termed a cardioid. Like endogenous heart tissue, cardioids contract spontaneously and can be electrically paced between 1 and 5 Hz indefinitely without fatigue. These engineered cardiac tissues also show an increased rate of spontaneous contraction (chronotropy), increased rate of relaxation (lusitropy), and increased force production (inotropy) in response to epinephrine. Cardioids have a developmental protein phenotype that expresses both alpha- and beta-tropomyosin, very low levels of SERCA2a, and very little of the mature isoform of cardiac troponin T.

Design of artificial myocardial microtissues

Cultivation technologies promoting organization of mammalian cells in three dimensions are essential for gene-function analyses as well as drug testing and represent the first step toward the design of tissue replacements and bioartificial organs. Embedded in a three-dimensional environment, cells are expected to develop tissue-like higher order intercellular structures (cell-cell contacts, extracellular matrix) that orchestrate cellular functions including proliferation, differentiation, apoptosis, and angiogenesis with unmatched quality. We have refined the hanging drop cultivation technology to pioneer beating heart microtissues derived from pure primary rat and mouse cardiomyocyte cultures as well as mixed populations reflecting the cell type composition of rodent hearts. Phenotypic characterization combined with detailed analysis of muscle-specific cell traits, extracellular matrix components, as well as endogenous vascular endothelial growth factor (VEGF) expression profiles of heart microtissues revealed (1). a linear cell number-microtissue size correlation, (2). intermicrotissue superstructures, (3). retention of key cardiomyocyte-specific cell qualities, (4). a sophisticated extracellular matrix, and (5). a high degree of self-organization exemplified by the tendency of muscle structures to assemble at the periphery of these myocardial spheroids. Furthermore (6). myocardial spheroids support endogenous VEGF expression in a size-dependent manner that will likely promote vascularization of heart microtissues produced from defined cell mixtures as well as support connection to the host vascular system after implantation. As cardiomyocytes are known to be refractory to current transfection technologies we have designed lentivirus-based transduction strategies to lead the way for genetic engineering of myocardial microtissues in a clinical setting.

Scaffold topography alters intracellular calcium dynamics in cultured cardiomyocyte networks

Structural and functional changes ensue in cardiac cell networks when cells are guided by three-dimensional scaffold topography. We report enhanced synchronous pacemaking activity in association with slow diastolic rise in intracellular Ca2+concentration ([Ca2+]i) in cell networks grown on microgrooved scaffolds. Topography-driven changes in cardiac electromechanics were characterized by the frequency dependence of [Ca2+]iin syncytial structures formed of ventricular myocytes cultured on microgrooved elastic scaffolds (G). Cells were electrically paced at 0.5–5 Hz, and [Ca2+]iwas determined using microscale ratiometric (fura 2) fluorescence. Compared with flat (F) controls, the G networks exhibited elevated diastolic [Ca2+]iat higher frequencies, increased systolic [Ca2+]iacross the entire frequency range, and steeper restitution of Ca2+transient half-width ( n = 15 and 7 for G and F, respectively, P < 0.02). Significant differences in the frequency response of force-related parameters were also found, e.g., overall larger total area under the Ca2+transients and faster adaptation of relaxation time to pacing rate ( P < 0.02). Altered [Ca2+]idynamics were paralleled by higher occurrence of spontaneous Ca2+release and increased sarcoplasmic reticulum load ( P < 0.02), indirectly assessed by caffeine-triggered release. Electromechanical instabilities, i.e., Ca2+and voltage alternans, were more often observed in G samples. Taken together, these findings 1) represent some of the first functional electromechanical data for this in vitro system and 2) demonstrate direct influence of the microstructure on cardiac function and susceptibility to arrhythmias via Ca2+-dependent mechanisms. Overall, our results substantiate the idea of guiding cellular phenotype by cellular microenvironment, e.g., scaffold design in the context of tissue engineering.

Cell sheet engineering for myocardial tissue reconstruction

Myocardial tissue engineering has now emerged as one of the most promising treatments for the patients suffering from severe heart failure. Tissue engineering has currently been based on the technology using three-dimensional (3-D) biodegradable scaffolds as alternatives for extracellular matrix. According to this most popular technique, several types of 3-D myocardial tissues have been successfully engineered by seeding cardiomyocytes into poly(glycolic acid), gelatin, alginate or collagen scaffolds. However, insufficient cell migration into the scaffolds and inflammatory reaction due to scaffold biodegradation remain problems to be solved. In contrast to these technologies, we now propose novel tissue engineering methodology layering cell sheets to construct 3-D functional tissues without any artificial scaffolds. Confluent cells on temperature-responsive culture surfaces can be harvested as a viable contiguous cell sheet only by lowering temperature without any enzymatic digestions. Electrical communications are established between layered cardiomyocyte sheets, resulting in simultaneous beating 3-D myocardial tissues. Layered cardiomyocyte sheets in vivo present long survival, macroscopic pulsation and characteristic structures of native heart tissue. Cell sheet engineering should have enormous potential for fabricating clinically applicable myocardial tissues and should promote tissue engineering research fields.