Terminal Differentiation, Advanced Organotypic Maturation, and Modeling of Hypertrophic Growth in Engineered Heart Tissue

Engineered Tissue Patch for Cardiac Cell Therapy

OPINION STATEMENT

Cell therapy can be administered via injections delivered directly into the myocardium or as engineered cardiac tissue patches, which are the subject of this review. Engineered cardiac patches can be created from sheets of interconnected cells or by suspending the cells in a scaffold of material that is designed to mimic the native extracellular matrix. The sheet-based approach produces patches with well-aligned and electronically coupled cardiomyocytes, but cell-containing scaffolds are more readily vascularized by the host's circulatory system and, consequently, are currently more suitable for applications that require a thicker patch. Cell patches can also be modified for the co-delivery of peptides that may promote cell survival and activate endogenous repair mechanisms; nevertheless, techniques for controlling inflammation, limiting apoptosis, and improving vascular growth need continue to be developed to make it a therapeutic modality for patients with myocardial infarction.

Prospects for In Vitro Myofilament Maturation in Stem Cell-Derived Cardiac Myocytes

Cardiomyocytes derived from human stem cells are quickly becoming mainstays of cardiac regenerative medicine, in vitro disease modeling, and drug screening. Their suitability for such roles may seem obvious, but assessments of their contractile behavior suggest that they have not achieved a completely mature cardiac muscle phenotype. This could be explained in part by an incomplete transition from fetal to adult myofilament protein isoform expression. In this commentary, we review evidence that supports this hypothesis and discuss prospects for ultimately generating engineered heart tissue specimens that behave similarly to adult human myocardium. We suggest approaches to better characterize myofilament maturation level in these in vitro systems, and illustrate how new computational models could be used to better understand complex relationships between muscle contraction, myofilament protein isoform expression, and maturation.

S100A1 DNA-based Inotropic Therapy Protects Against Proarrhythmogenic Ryanodine Receptor 2 Dysfunction

Restoring expression levels of the EF-hand calcium (Ca(2+)) sensor protein S100A1 has emerged as a key factor in reconstituting normal Ca(2+) handling in failing myocardium. Improved sarcoplasmic reticulum (SR) function with enhanced Ca(2+) resequestration appears critical for S100A1's cyclic adenosine monophosphate-independent inotropic effects but raises concerns about potential diastolic SR Ca(2+) leakage that might trigger fatal arrhythmias. This study shows for the first time a diminished interaction between S100A1 and ryanodine receptors (RyR2s) in experimental HF. Restoring this link in failing cardiomyocytes, engineered heart tissue and mouse hearts, respectively, by means of adenoviral and adeno-associated viral S100A1 cDNA delivery normalizes diastolic RyR2 function and protects against Ca(2+)- and β-adrenergic receptor-triggered proarrhythmogenic SR Ca(2+) leakage in vitro and in vivo. S100A1 inhibits diastolic SR Ca(2+) leakage despite aberrant RyR2 phosphorylation via protein kinase A and calmodulin-dependent kinase II and stoichiometry with accessory modulators such as calmodulin, FKBP12.6 or sorcin. Our findings demonstrate that S100A1 is a regulator of diastolic RyR2 activity and beneficially modulates diastolic RyR2 dysfunction. S100A1 interaction with the RyR2 is sufficient to protect against basal and catecholamine-triggered arrhythmic SR Ca(2+) leak in HF, combining antiarrhythmic potency with chronic inotropic actions.

Physiologic force-frequency response in engineered heart muscle by electromechanical stimulation

A hallmark of mature mammalian ventricular myocardium is a positive force-frequency relationship (FFR). Despite evidence of organotypic structural and molecular maturation, a positive FFR has not been observed in mammalian tissue engineered heart muscle. We hypothesized that concurrent mechanical and electrical stimulation at frequencies matching physiological heart rate will result in functional maturation. We investigated the role of biomimetic mechanical and electrical stimulation in functional maturation in engineered heart muscle (EHM). Following tissue consolidation, EHM were subjected to electrical field stimulation at 0, 2, 4, or 6 Hz for 5 days, while strained on flexible poles to facilitate auxotonic contractions. EHM stimulated at 2 and 4 Hz displayed a similarly enhanced inotropic reserve, but a clearly diverging FFR. The positive FFR in 4 Hz stimulated EHM was associated with reduced calcium sensitivity, frequency-dependent acceleration of relaxation, and enhanced post-rest potentiation. This was paralleled on the cellular level with improved calcium storage and release capacity of the sarcoplasmic reticulum and enhanced T-tubulation. We conclude that electro-mechanical stimulation at a physiological frequency supports functional maturation in mammalian EHM. The observed positive FFR in EHM has important implications for the applicability of EHM in cardiovascular research.

Automated Video-Based Analysis of Contractility and Calcium Flux in Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes Cultured over Different Spatial Scales

Contractile motion is the simplest metric of cardiomyocyte health in vitro, but unbiased quantification is challenging. We describe a rapid automated method, requiring only standard video microscopy, to analyze the contractility of human-induced pluripotent stem cell-derived cardiomyocytes (iPS-CM). New algorithms for generating and filtering motion vectors combined with a newly developed isogenic iPSC line harboring genetically encoded calcium indicator, GCaMP6f, allow simultaneous user-independent measurement and analysis of the coupling between calcium flux and contractility. The relative performance of these algorithms, in terms of improving signal to noise, was tested. Applying these algorithms allowed analysis of contractility in iPS-CM cultured over multiple spatial scales from single cells to three-dimensional constructs. This open source software was validated with analysis of isoproterenol response in these cells, and can be applied in future studies comparing the drug responsiveness of iPS-CM cultured in different microenvironments in the context of tissue engineering.

miR-133a enhances the protective capacity of cardiac progenitors cells after myocardial infarction

miR-133a and miR-1 are known as muscle-specific microRNAs that are involved in cardiac development and pathophysiology. We have shown that both miR-1 and miR-133a are early and progressively upregulated during in vitro cardiac differentiation of adult cardiac progenitor cells (CPCs), but only miR-133a expression was enhanced under in vitro oxidative stress. miR-1 was demonstrated to favor differentiation of CPCs, whereas miR-133a overexpression protected CPCs against cell death, targeting, among others, the proapoptotic genes Bim and Bmf. miR-133a-CPCs clearly improved cardiac function in a rat myocardial infarction model by reducing fibrosis and hypertrophy and increasing vascularization and cardiomyocyte proliferation. The beneficial effects of miR-133a-CPCs seem to correlate with the upregulated expression of several relevant paracrine factors and the plausible cooperative secretion of miR-133a via exosomal transport. Finally, an in vitro heart muscle model confirmed the antiapoptotic effects of miR-133a-CPCs, favoring the structuration and contractile functionality of the artificial tissue.

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miR-1 and miR-133a have a role in adult cardiac progenitor cells (CPCs)

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miR-133a-CPCs protect cardiac function

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miR-133a-CPCs increase vascularization and protect against hypertrophy

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miR-133a is enriched in CPCs-derived exosomes

Biomechanical regulation of in vitro cardiogenesis for tissue-engineered heart repair

The heart is a continuously pumping organ with an average lifespan of eight decades. It develops from the onset of embryonic cardiogenesis under biomechanical load, performs optimally within a defined range of hemodynamic load, and fails if acutely or chronically overloaded. Unloading of the heart leads to defective cardiogenesis in utero, but can also lead to a desired therapeutic outcome (for example, in patients with heart failure under left ventricular assist device therapy). In light of the well-documented relevance of mechanical loading for cardiac physiology and pathology, it is plausible that tissue engineers have integrated mechanical stimulation regimens into protocols for heart muscle construction. To achieve optimal results, physiological principles of beat-to-beat myocardial loading and unloading should be simulated. In addition, heart muscle engineering, in particular if based on pluripotent stem cell-derived cardiomyocytes, may benefit from staggered tonic loading protocols to simulate viscoelastic properties of the prenatal and postnatal myocardial stroma. This review will provide an overview of heart muscle mechanics, summarize observations on the role of mechanical loading for heart development and postnatal performance, and discuss how physiological loading regimens can be exploited to advance myocardial tissue engineering towards a therapeutic application.

Mechanosensitive kinases regulate stiffness-induced cardiomyocyte maturation

Cells secrete and assemble extracellular matrix throughout development, giving rise to time-dependent, tissue-specific stiffness. Mimicking myocardial matrix stiffening, i.e. ~10-fold increase over 1 week, with a hydrogel system enhances myofibrillar organization of embryonic cardiomyocytes compared to static hydrogels, and thus we sought to identify specific mechanosensitive proteins involved. Expression and/or phosphorylation state of 309 unique protein kinases were examined in embryonic cardiomyocytes plated on either dynamically stiffening or static mature myocardial stiffness hydrogels. Gene ontology analysis of these kinases identified cardiogenic pathways that exhibited time-dependent up-regulation on dynamic versus static matrices, including PI3K/AKT and p38 MAPK, while GSK3β, a known antagonist of cardiomyocyte maturation, was down-regulated. Additionally, inhibiting GSK3β on static matrices improved spontaneous contraction and myofibril organization, while inhibiting agonist AKT on dynamic matrices reduced myofibril organization and spontaneous contraction, confirming its role in mechanically-driven maturation. Together, these data indicate that mechanically-driven maturation is at least partially achieved via active mechanosensing at focal adhesions, affecting expression and phosphorylation of a variety of protein kinases important to cardiomyogenesis.

Mimicking isovolumic contraction with combined electromechanical stimulation improves the development of engineered cardiac constructs

Electrical and mechanical stimulation have both been used extensively to improve the function of cardiac engineered tissue as each of these stimuli is present in the physical environment during normal development in vivo. However, to date, there has been no direct comparison between electrical and mechanical stimulation and current published data are difficult to compare due to the different systems used to create the engineered cardiac tissue and the different measures of functionality studied as outcomes. The goals of this study were twofold. First, we sought to directly compare the effects of mechanical and electrical stimulation on engineered cardiac tissue. Second, we aimed to determine the importance of the timing of the two stimuli in relation to each other in combined electromechanical stimulation. We hypothesized that delaying electrical stimulation after the beginning of mechanical stimulation to mimic the biophysical environment present during isovolumic contraction would improve construct function by improving proteins responsible for cell-cell communication and contractility. To test this hypothesis, we created a bioreactor system that would allow us to electromechanically stimulate engineered tissue created from neonatal rat cardiac cells entrapped in fibrin gel during 2 weeks in culture. Contraction force was higher for all stimulation groups as compared with the static controls, with the delayed combined stimulation constructs having the highest forces. Mechanical stimulation alone displayed increased final cell numbers but there were no other differences between electrical and mechanical stimulation alone. Delayed combined stimulation resulted in an increase in SERCA2a and troponin T expression levels, which did not happen with synchronous combined stimulation, indicating that the timing of combined stimulation is important to maximize the beneficial effect. Increases in Akt protein expression levels suggest that the improvements are at least in part induced by hypertrophic growth. In summary, combined electromechanical stimulation can create engineered cardiac tissue with improved functional properties over electrical or mechanical stimulation alone, and the timing of the combined stimulation greatly influences its effects on engineered cardiac tissue.

Bioreactor for modulation of cardiac microtissue phenotype by combined static stretch and electrical stimulation

We describe here a bioreactor capable of applying electrical field stimulation in conjunction with static strain and on-line force of contraction measurements. It consisted of a polydimethylsiloxane (PDMS) tissue chamber and a pneumatically driven stretch platform. The chamber contained eight tissue microwells (8.05 mm in length and 2.5 mm in width) with a pair of posts (2.78 mm in height and 0.8 mm in diameter) in each well to serve as fixation points and for measurements of contraction force. Carbon rods, stimulating electrodes, were placed into the PDMS chamber such that one pair stimulated four microwells. For feasibility studies, neonatal rat cardiomyocytes were seeded in collagen gels into the microwells. Following 3 days of gel compaction, electrical field stimulation at 3-4 V cm(-1) and 1 Hz, mechanical stimulation of 5% static strain or electromechanical stimulation (field stimulation at 3-4 V cm(-1), 1 Hz and 5% static strain) were applied for 3 days. Cardiac microtissues subjected to electromechanical stimulation exhibited elevated amplitude of contraction and improved sarcomere structure as evidenced by sarcomeric α-actinin, actin and troponin T staining compared to microtissues subjected to electrical or mechanical stimulation alone or non-stimulated controls. The expression of atrial natriuretic factor and brain natriuretic peptide was also elevated in the electromechanically stimulated group.

Functional improvement and maturation of rat and human engineered heart tissue by chronic electrical stimulation

Spontaneously beating engineered heart tissue (EHT) represents an advanced in vitro model for drug testing and disease modeling, but cardiomyocytes in EHTs are less mature and generate lower forces than in the adult heart. We devised a novel pacing system integrated in a setup for videooptical recording of EHT contractile function over time and investigated whether sustained electrical field stimulation improved EHT properties. EHTs were generated from neonatal rat heart cells (rEHT, n=96) or human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes (hEHT, n=19). Pacing with biphasic pulses was initiated on day 4 of culture. REHT continuously paced for 16-18 days at 0.5Hz developed 2.2× higher forces than nonstimulated rEHT. This was reflected by higher cardiomyocyte density in the center of EHTs, increased connexin-43 abundance as investigated by two-photon microscopy and remarkably improved sarcomere ultrastructure including regular M-bands. Further signs of tissue maturation include a rightward shift (to more physiological values) of the Ca(2+)-response curve, increased force response to isoprenaline and decreased spontaneous beating activity. Human EHTs stimulated at 2Hz in the first week and 1.5Hz thereafter developed 1.5× higher forces than nonstimulated hEHT on day 14, an ameliorated muscular network of longitudinally oriented cardiomyocytes and a higher cytoplasm-to-nucleus ratio. Taken together, continuous pacing improved structural and functional properties of rEHTs and hEHTs to an unprecedented level. Electrical stimulation appears to be an important step toward the generation of fully mature EHT.

Myocardial tissue engineering: in vitro models

Modeling integrated human physiology in vitro is a formidable task not yet achieved with any of the existing cell/tissue systems. However, tissue engineering is becoming increasingly successful at authentic representation of the actual environmental milieu of tissue development, regeneration and disease progression, and in providing real-time insights into morphogenic events. Functional human tissue units engineered to combine biological fidelity with the high-throughput screening and real-time measurement of physiological responses are poised to transform drug screening and predictive modeling of disease. In this review, we focus on the in vitro engineering of functional human myocardium that mimics heart tissue for analysis of myocardial function, in the context of physiological studies, drug screening for therapeutics, and safety pharmacology.

PDE2-mediated cAMP hydrolysis accelerates cardiac fibroblast to myofibroblast conversion and is antagonized by exogenous activation of cGMP signaling pathways

Recent studies suggest that the signal molecules cAMP and cGMP have antifibrotic effects by negatively regulating pathways associated with fibroblast to myofibroblast (MyoCF) conversion. The phosphodiesterase 2 (PDE2) has the unique property to be stimulated by cGMP, which leads to a remarkable increase in cAMP hydrolysis and thus mediates a negative cross-talk between both pathways. PDE2 has been recently investigated in cardiomyocytes; here we specifically addressed its role in fibroblast conversion and cardiac fibrosis. PDE2 is abundantly expressed in both neonatal rat cardiac fibroblasts (CFs) and cardiomyocytes. The overexpression of PDE2 in CFs strongly reduced basal and isoprenaline-induced cAMP synthesis, and this decrease was sufficient to induce MyoCF conversion even in the absence of exogenous profibrotic stimuli. Functional stress-strain experiments with fibroblast-derived engineered connective tissue (ECT) demonstrated higher stiffness in ECTs overexpressing PDE2. In regard to cGMP, neither basal nor atrial natriuretic peptide-induced cGMP levels were affected by PDE2, whereas the response to nitric oxide donor sodium nitroprusside was slightly but significantly reduced. Interestingly, despite persistently depressed cAMP levels, both cGMP-elevating stimuli were able to completely prevent the PDE2-induced MyoCF phenotype, arguing for a double-tracked mechanism. In conclusion, PDE2 accelerates CF to MyoCF conversion, which leads to greater stiffness in ECTs. Atrial natriuretic peptide- and sodium nitroprusside-mediated cGMP synthesis completely reverses PDE2-induced fibroblast conversion. Thus PDE2 may augment cardiac remodeling, but this effect can also be overcome by enhanced cGMP. The redundant role of cAMP and cGMP as antifibrotic meditators may be viewed as a protective mechanism in heart failure.

Robust T-tubulation and maturation of cardiomyocytes using tissue-engineered epicardial mimetics

Complex three-dimensional (3-D) heart structure is an important determinant of cardiac electrical and mechanical function. In this study, we set to develop a versatile tissue-engineered system that can promote important aspects of cardiac functional maturation and reproduce variations in myofiber directions present in native ventricular epicardium. We cultured neonatal rat cardiomyocytes within a 3-D hydrogel environment using microfabricated elastomeric molds with hexagonal posts. By varying individual post orientations along the directions derived from diffusion tensor magnetic resonance imaging (DTMRI) maps of human ventricle, we created large (2.5 × 2.5 cm(2)) 3-D cardiac tissue patches with cardiomyocyte alignment that replicated human epicardial fiber orientations. After 3 weeks of culture, the advanced structural and functional maturation of the engineered 3-D cardiac tissues compared to age-matched 2-D monolayers was evident from: 1) the presence of dense, aligned and electromechanically-coupled cardiomyocytes, quiescent fibroblasts, and interspersed capillary-like structures, 2) action potential propagation with near-adult conduction velocity and directional dependence on local cardiomyocyte orientation, and 3) robust formation of T-tubules aligned with Z-disks, co-localization of L-type Ca(2+) channels and ryanodine receptors, and accelerated Ca(2+) transient kinetics. This biomimetic tissue-engineered platform can enable systematic in vitro studies of cardiac structure-function relationships and promote the development of advanced tissue engineering strategies for cardiac repair and regeneration.

Cardiac Tissue Engineering

The engineering of 3-dimensional (3D) heart muscles has undergone exciting progress for the past decade. Profound advances in human stem cell biology and technology, tissue engineering and material sciences, as well as prevascularization and in vitro assay technologies make the first clinical application of engineered cardiac tissues a realistic option and predict that cardiac tissue engineering techniques will find widespread use in the preclinical research and drug development in the near future. Tasks that need to be solved for this purpose include standardization of human myocyte production protocols, establishment of simple methods for the in vitro vascularization of 3D constructs and better maturation of myocytes, and, finally, thorough definition of the predictive value of these methods for preclinical safety pharmacology. The present article gives an overview of the present state of the art, bottlenecks, and perspectives of cardiac tissue engineering for cardiac repair and in vitro testing.

The effect of cyclic stretch on maturation and 3D tissue formation of human embryonic stem cell-derived cardiomyocytes

The goal of cardiac tissue engineering is to restore function to the damaged myocardium with regenerative constructs. Human embryonic stem cell-derived cardiomyocytes (hESC-CMs) can produce viable, contractile, three-dimensional grafts that function in vivo. We sought to enhance the viability and functional maturation of cardiac tissue constructs by cyclical stretch. hESC-CMs seeded onto gelatin-based scaffolds underwent cyclical stretching. Histological analysis demonstrated a greater proportion of cardiac troponin T-expressing cells in stretched than non-stretched constructs, and flow sorting demonstrated a higher proportion of cardiomyocytes. Ultrastructural assessment showed that cells in stretched constructs had a more mature phenotype, characterized by greater cell elongation, increased gap junction expression, and better contractile elements. Real-time PCR revealed enhanced mRNA expression of genes associated with cardiac maturation as well as genes encoding cardiac ion channels. Calcium imaging confirmed that stretched constructs contracted more frequently, with shorter calcium cycle duration. Epicardial implantation of constructs onto ischemic rat hearts demonstrated the feasibility of this platform, with enhanced survival and engraftment of transplanted cells in the stretched constructs. This uniaxial stretching system may serve as a platform for the production of cardiac tissue-engineered constructs for translational applications.

Collagen-based engineered heart muscle

Cardiac muscle engineering has evolved over nearly 20 years from a scientific oddity to a mainstream technology with a wide range of applications. Of the many published methods it appears that hydrogels constitute the preferred scaffolds for myocardial tissue engineering and support of organotypic development. Here we describe a simple and highly robust protocol for the generation of engineered heart muscle using a collagen-based hydrogel method.

Generation of strip-format fibrin-based engineered heart tissue (EHT)

This protocol describes a method for casting fibrin-based engineered heart tissue (EHT) in standard 24-well culture dishes. In principle, a hydrogel tissue engineering method requires cardiomyocytes, a liquid matrix that forms a gel, a casting mold, and a device that keeps the developing tissue in place. This protocol refers to neonatal rat heart cells as the cell source; the matrix of choice is fibrin, and the tissues are generated in rectangular agarose-casting molds (12 × 3 × 3 mm) prepared in standard 24-well cell culture dishes, in which a pair of flexible silicone posts is suspended from above. A master mix of freshly isolated cells, medium, fibrinogen, and thrombin is pipetted into the casting mold and, over a period of 2 h, polymerizes and forms a fibrin cell block around two silicone posts. Silicone racks holding four pairs of silicone posts each are used to transfer the fresh fibrin cell blocks into new 24-well dishes with culture medium. Without further handling, the cells start to remodel the fibrin gel, form contacts with each other, elongate, and condense the gel to approximately ¼ of the initial volume. Spontaneous and rhythmic contractions start after 1 week. EHTs are viable and relatively stable for several weeks in this format and can be subjected to repeated measurements of contractile function and final morphological and molecular analyses.

Design and fabrication of biological wires

Cardiac tissue engineering using human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) has facilitated the creation of in vitro diagnostic platforms to study novel small molecules and cardiac disease at the tissue level. Yet, due to the immaturity of hPSC-CMs, there is a low fidelity between tissue-engineered cardiac tissues and adult cardiac tissues. To address this challenge, we have developed a platform that combines both physical and electrical cues to guide hPSC-CMs towards a more mature state in vitro.

Patching the heart: cardiac repair from within and outside

Transplantation of engineered tissue patches containing either progenitor cells or cardiomyocytes for cardiac repair is emerging as an exciting treatment option for patients with postinfarction left ventricular remodeling. The beneficial effects may evolve directly from remuscularization or indirectly through paracrine mechanisms that mobilize and activate endogenous progenitor cells to promote neovascularization and remuscularization, inhibit apoptosis, and attenuate left ventricular dilatation and disease progression. Despite encouraging results, further improvements are necessary to enhance current tissue engineering concepts and techniques and to achieve clinical impact. Herein, we review several strategies for cardiac remuscularization and paracrine support that can induce cardiac repair and attenuate left ventricular dysfunction from both within and outside the myocardium.

Modeling of arrhythmogenic right ventricular cardiomyopathy with human induced pluripotent stem cells

BACKGROUND

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a primary heart muscle disorder resulting from desmosomal protein mutations. ARVC is characterized pathologically by fibrofatty infiltration and clinically by arrhythmias and sudden cardiac death. We aimed to establish a patient-/disease-specific human induced pluripotent stem cell (hiPSC) model of ARVC.

METHODS AND RESULTS

Dermal fibroblasts were obtained from 2 patients with ARVC with plakophilin-2 (PKP2) mutations, reprogrammed to generate hiPSCs, coaxed to differentiate into cardiomyocytes (CMs), and then compared with healthy control hiPSC-derived CMs (hiPSC-CMs). Real-time polymerase chain reaction showed a significant decrease in the expression of PKP2 in the ARVC-hiPSC-CMs. Immunostainings revealed reduced densities of PKP2, the associated desmosomal protein plakoglobin, and the gap-junction protein connexin-43. Electrophysiological assessment demonstrated prolonged field potential rise time in the ARVC-hiPSC-CMs. Transmission electron microscopy identified widened and distorted desmosomes in the ARVC-hiPSC-CMs. Clusters of lipid droplets were identified in the ARVC-CMs that displayed the more severe desmosomal pathology. This finding was associated with upregulation of the proadipogenic transcription factor peroxisome proliferator-activated receptor-γ. Exposure of the cells to apidogenic stimuli augmented desmosomal distortion and lipid accumulation. The latter phenomenon was prevented by application of a specific inhibitor of glycogen synthase kinase 3β (6-bromoindirubin-3'-oxime).

CONCLUSIONS

This study highlights the unique potential of the hiPSC technology for modeling inherited cardiac disorders in general and ARVC specifically. The hiPSC-CMs were demonstrated to recapitulate the ARVC phenotype in the dish, provide mechanistic insights into early disease pathogenesis, and provide a unique platform for drug discovery and testing in this disorder.

Advancing functional engineered cardiac tissues toward a preclinical model of human myocardium

Cardiac experimental biology and translational research would benefit from an in vitro surrogate for human heart muscle. This study investigated structural and functional properties and interventional responses of human engineered cardiac tissues (hECTs) compared to human myocardium. Human embryonic stem cell-derived cardiomyocytes (hESC-CMs, >90% troponin-positive) were mixed with collagen and cultured on force-sensing elastomer devices. hECTs resembled trabecular muscle and beat spontaneously (1.18 ± 0.48 Hz). Microstructural features and mRNA expression of cardiac-specific genes (α-MHC, SERCA2a, and ACTC1) were comparable to human myocardium. Optical mapping revealed cardiac refractoriness with loss of 1:1 capture above 3 Hz, and cycle length dependence of the action potential duration, recapitulating key features of cardiac electrophysiology. hECTs reconstituted the Frank-Starling mechanism, generating an average maximum twitch stress of 660 μN/mm(2) at Lmax, approaching values in newborn human myocardium. Dose-response curves followed exponential pharmacodynamics models for calcium chloride (EC50 1.8 mM) and verapamil (IC50 0.61 μM); isoproterenol elicited a positive chronotropic but negligible inotropic response, suggesting sarcoplasmic reticulum immaturity. hECTs were amenable to gene transfer, demonstrated by successful transduction with Ad.GFP. Such 3-D hECTs recapitulate an early developmental stage of human myocardium and promise to offer an alternative preclinical model for cardiology research.

Scalable alignment of three-dimensional cellular constructs in a microfluidic chip

There have been considerable efforts to engineer three-dimensional (3D) microfluidic environments to enhance cellular function over conventional two-dimensional (2D) cultures in microfluidic chips, but few involve topographical features, such as micro/nano-grooves, which are beneficial for cell types of cardiac, skeletal and neuronal lineages. Here we have developed a cost-effective and scalable method to incorporate micro-topographical cues into microfluidic chips to induce cell alignment. Using commercially available optical media as molds for replica molding, we produced large surface areas of polydimethylsiloxane (PDMS) micro-grooved substrates and plasma-bonded them to multiple microfluidic chips. Besides aligning a 2D monolayer of cells, the micro-grooved substrate can align 3D cellular constructs on chip. C2C12 mouse myoblasts were cultured three-dimensionally in a microfluidic chip with incorporated PDMS micro-grooved substrate remodeled into an aligned 3D cellular construct, where the actin cytoskeleton and nuclei were preferentially oriented along the micro-grooves. Cells within the 3D cellular constructs can align without being in direct contact with the micro-grooves due to synergism between topography and fluid shear stress. Aligned C2C12 3D cellular constructs showed enhanced differentiation into skeletal muscles as compared to randomly aligned ones. This novel method enables the routine inclusion of micro-topographical cues into 2D or 3D microfluidic cultures to generate relevant physiological models for studying tissue morphogenesis and drug screening applications.

Gene expression profiles in engineered cardiac tissues respond to mechanical loading and inhibition of tyrosine kinases

Engineered cardiac tissues (ECTs) are platforms to investigate cardiomyocyte maturation and functional integration, the feasibility of generating tissues for cardiac repair, and as models for pharmacology and toxicology bioassays. ECTs rapidly mature in vitro to acquire the features of functional cardiac muscle and respond to mechanical load with increased proliferation and maturation. ECTs are now being investigated as platforms for in vitro models for human diseases and for pharmacologic screening for drug toxicities. We tested the hypothesis that global ECT gene expression patterns are complex and sensitive to mechanical loading and tyrosine kinase inhibitors similar to the maturing myocardium. We generated ECTs from day 14.5 rat embryo ventricular cells, as previously published, and then conditioned constructs after 5 days in culture for 48 h with mechanical stretch (5%, 0.5 Hz) and/or the p38 MAPK (p38 mitogen-activated protein kinase) inhibitor BIRB796. RNA was isolated from individual ECTs and assayed using a standard Agilent rat 4 × 44k V3 microarray and Pathway Analysis software for transcript expression fold changes and changes in regulatory molecules and networks. Changes in expression were confirmed by quantitative-polymerase chain reaction (q-PCR) for selected regulatory molecules. At the threshold of a 1.5-fold change in expression, stretch altered 1559 transcripts, versus 1411 for BIRB796, and 1846 for stretch plus BIRB796. As anticipated, top pathways altered in response to these stimuli include cellular development, cellular growth and proliferation; tissue development; cell death, cell signaling, and small molecule biochemistry as well as numerous other pathways. Thus, ECTs display a broad spectrum of altered gene expression in response to mechanical load and/or tyrosine kinase inhibition, reflecting a complex regulation of proliferation, differentiation, and architectural alignment of cardiomyocytes and noncardiomyocytes within ECT.

Bioengineering heart tissue for in vitro testing

A classical paradigm of tissue engineering is to grow tissues for implantation by using human stem cells in conjunction with biomaterial scaffolds (templates for tissue formation) and bioreactors (culture systems providing environmental control). A reverse paradigm is now emerging through microphysiological platforms for preclinical testing of drugs and modeling of disease that contain large numbers of very small human tissues. We discuss the biomimetic approach as a common underlying principle and some of the specifics related to the design and utilization of platforms with heart micro-tissues for high-throughput screening in vitro.

Modeling myocardial growth and hypertrophy in engineered heart muscle

The introduction of biomimetic culture paradigms has advanced myocardial tissue engineering fundamentally, enabling today the provision of engineered rodent and human heart muscle with features characteristically found in postnatal myocardium. This is in marked contrasts to "flat" cardiomyocyte cultures with their typically low degree of organotypic maturation. Here, we discuss the collagen hydrogel-based engineered heart muscle (EHM) technology and provide background information on its use in simulations of myocardial growth and disease.

Trichostatin A enhances differentiation of human induced pluripotent stem cells to cardiogenic cells for cardiac tissue engineering

Human induced pluripotent stem (iPS) cells are a promising source of autologous cardiomyocytes to repair and regenerate myocardium for treatment of heart disease. In this study, we have identified a novel strategy to enhance cardiac differentiation of human iPS cells by treating embryoid bodies (EBs) with a histone deacetylase inhibitor, trichostatin A (TSA), together with activin A and bone morphogenetic protein 4 (BMP4). Over a narrow window of concentrations, TSA (1 ng/ml) directed the differentiation of human iPS cells into a cardiomyocyte lineage. TSA also exerted an additive effect with activin A (100 ng/ml) and BMP4 (20 ng/ml). The resulting cardiomyocytes expressed several cardiac-specific transcription factors and contractile proteins at both gene and protein levels. Functionally, the contractile EBs displayed calcium cycling and were responsive to the chronotropic agents isoprenaline (0.1 μM) and carbachol (1 μM). Implanting microdissected beating areas of iPS cells into tissue engineering chambers in immunocompromised rats produced engineered constructs that supported their survival, and they maintained spontaneous contraction. Human cardiomyocytes were identified as compact patches of muscle tissue incorporated within a host fibrocellular stroma and were vascularized by host neovessels. In conclusion, human iPS cell-derived cardiomyocytes can be used to engineer functional cardiac muscle tissue for studying the pathophysiology of cardiac disease, for drug discovery test beds, and potentially for generation of cardiac grafts to surgically replace damaged myocardium.

Carbon nanotubes instruct physiological growth and functionally mature syncytia: nongenetic engineering of cardiac myocytes

Myocardial tissue engineering currently represents one of the most realistic strategies for cardiac repair. We have recently discovered the ability of carbon nanotube scaffolds to promote cell division and maturation in cardiomyocytes. Here, we test the hypothesis that carbon nanotube scaffolds promote cardiomyocyte growth and maturation by altering the gene expression program, implementing the cell electrophysiological properties and improving networking and maturation of functional syncytia. In our study, we combine microscopy, biological and electrophysiological methodologies, and calcium imaging, to verify whether neonatal rat ventricular myocytes cultured on substrates of multiwall carbon nanotubes acquire a physiologically more mature phenotype compared to control (gelatin). We show that the carbon nanotube substrate stimulates the induction of a gene expression profile characteristic of terminal differentiation and physiological growth, with a 2-fold increase of α-myosin heavy chain (P < 0.001) and upregulation of sarcoplasmic reticulum Ca(2+) ATPase 2a. In contrast, markers of pathological hypertrophy remain unchanged (β-myosin heavy chain, skeletal α-actin, atrial natriuretic peptide). These modifications are paralleled by an increase of connexin-43 gene expression, gap junctions and functional syncytia. Moreover, carbon nanotubes appear to exert a protective effect against the pathologic stimulus of phenylephrine. Finally, cardiomyocytes on carbon nanotubes demonstrate a more mature electrophysiological phenotype of syncytia and intracellular calcium signaling. Thus, carbon nanotubes interacting with cardiomyocytes have the ability to promote physiological growth and functional maturation. These properties are unique in the current vexing field of tissue engineering, and offer unprecedented perspectives in the development of innovative therapies for cardiac repair.

Transplantation of engineered heart tissue as a biological cardiac assist device for treatment of dilated cardiomyopathy

AIM

The aim of this study was to characterize an alternative treatment for dilated cardiomyopathy (DCM) using a novel cardiac biological assist device created from engineered heart tissue (EHT).

METHODS AND RESULTS

The EHTs were constructed in vitro from matrigel, collagen, and neonatal rat cardiomyocytes as small ring-like spontaneously contracting devices. DCM was induced in 50 rats by 6 weeks doxorubicin treatment (2.5 mg/kg/week). After 38 drug-free days, rats underwent either implantation of EHT (DCM-EHT, n = 13), which was sutured around the ventricles, or sham operation (DCM-Sham, n = 12). Eleven untreated healthy rats served as the control group. Rats were investigated using a Millar catheter for pressure-volume loop recording, and by echocardiography 30 days after operation. Thereafter, the hearts were excised and investigated functionally, histologically, and biochemically. Doxorubicin led to the development of DCM with reduced fractional shortening (FS), reduced dP/dt(max), increased systolic and diastolic LV diameters, and reduced response to dobutamine. In DCM-Sham, these changes were further enhanced, while in DCM-EHT we found improved FS, dP/dt(max), and dobutamine responsiveness. In isolated hearts, electrical multielectrode mapping revealed that EHT was electrically activated synchronously to the recipient heart. Histologically, we found increased vascularization in the EHT and the recipient heart, and EHT vessels connected to the coronary system.

CONCLUSIONS

Implantation of EHT improves LV performance in rats with doxorubicin-induced DCM.

Scalable cell alignment on optical media substrates

Cell alignment by underlying topographical cues has been shown to affect important biological processes such as differentiation and functional maturation in vitro. However, the routine use of cell culture substrates with micro- or nano-topographies, such as grooves, is currently hampered by the high cost and specialized facilities required to produce these substrates. Here we present cost-effective commercially available optical media as substrates for aligning cells in culture. These optical media, including CD-R, DVD-R and optical grating, allow different cell types to attach and grow well on them. The physical dimension of the grooves in these optical media allowed cells to be aligned in confluent cell culture with maximal cell-cell interaction and these cell alignment affect the morphology and differentiation of cardiac (H9C2), skeletal muscle (C2C12) and neuronal (PC12) cell lines. The optical media is amenable to various chemical modifications with fibronectin, laminin and gelatin for culturing different cell types. These low-cost commercially available optical media can serve as scalable substrates for research or drug safety screening applications in industry scales.

Cardiac differentiation of human embryonic stem cells and their assembly into engineered heart muscle

The advent of pluripotent human embryonic stem cells has created the unique opportunity for the development of a wide variety of humanized cellular tools for basic research, as well as industrial and clinical applications. It has, however, become apparent that embryonic stem cell derivatives in classical monolayer or embryoid body culture do not resemble bona fide tissues, mainly because of their limited organotypic organization and maturation in these culture formats. This shortcoming may be addressed by tissue engineering technologies aiming at the provision of a "natural" growth environment to facilitate organotypic tissue assembly. In this unit, we provide two harmonized basic protocols for (1) cardiac differentiation of human embryonic stem cells under serum-free conditions and (2) the assembly of the stem cell-derived cardiomyocytes into engineered heart muscle. This protocol can be easily adapted to bioengineer heart muscle also from other stem cell-derived cardiomyocytes, including cardiomyocytes from human-induced pluripotent stem cells.

Topological and electrical control of cardiac differentiation and assembly

Tissue engineering has developed many paradigms and techniques on how to best integrate cells and extracellular matrix to create in vitro structures that replicate native tissue. The strategy best suited for building these constructs depends mainly on the target cells, tissues, and organ of interest, and how readily their respective niches can be recapitulated in vitro with available technologies. In this review we examine engineered heart tissue and two techniques that can be used to induce tissue morphogenesis in artificial niches in vitro: engineered surface topology and electrical control of the system. For both the differentiation of stem cells into heart cells and further assembly of these cells into engineered tissues, these two techniques are effective in inducing in vivo like structure and function. Biophysical modulation through the control of topography and manipulation of the electrical microenvironment has been shown to have effects on cell growth and differentiation, expression of mature cardiac-related proteins and genes, cell alignment via cytoskeletal organization, and electrical and contractile properties. Lastly, we discuss the evolution and potential of these techniques, and bridges to regenerative therapies.

Microdomain heterogeneity in 3D affects the mechanics of neonatal cardiac myocyte contraction

Cardiac muscle cells are known to adapt to their physical surroundings, optimizing intracellular organization and contractile function for a given culture environment. A previously developed in vitro model system has shown that the inclusion of discrete microscale domains (or microrods) in three dimensions (3D) can alter long-term growth responses of neonatal ventricular myocytes. The aim of this work was to understand how cellular contact with such a domain affects various mechanical changes involved in cardiac muscle cell remodeling. Myocytes were maintained in 3D gels over 5 days in the presence or absence of 100-μm-long microrods, and the effect of this local heterogeneity on cell behavior was analyzed via several imaging techniques. Microrod abutment resulted in approximately twofold increases in the maximum displacement of spontaneously beating myocytes, as based on confocal microscopy scans of the gel xy-plane or the myocyte long axis. In addition, microrods caused significant increases in the proportion of aligned myofibrils (≤20° deviation from long axis) in fixed myocytes. Microrod-related differences in axial contraction could be abrogated by long-term interruption of certain signals of the RhoA-/Rho-associated kinase (ROCK) or protein kinase C (PKC) pathway. Furthermore, microrod-induced increases in myocyte size and protein content were prevented by ROCK inhibition. In all, the data suggest that microdomain heterogeneity in 3D appears to promote the development of axially aligned contractile machinery in muscle cells, an observation that may have relevance to a number of cardiac tissue engineering interventions.

Thinking inside the box: keeping tissue-engineered constructs in vitro for use as preclinical models

Tissue engineers have made great strides toward the creation of living tissue replacements for a wide range of tissue types and applications, with eventual patient implantation as the primary goal. However, an alternate use of tissue-engineered constructs exists: as in vitro preclinical models for purposes such as drug screening and device testing. Tissue-engineered preclinical models have numerous potential advantages over existing models, including cultivation in three-dimensional geometries, decreased cost, increased reproducibility, precise control over cultivation conditions, and the incorporation of human cells. Over the past decade, a number of researchers have developed and used tissue-engineered constructs as preclinical models for testing pharmaceuticals, gene therapies, stents, and other technologies, with examples including blood vessels, skeletal muscle, bone, cartilage, skin, cardiac muscle, liver, cornea, reproductive tissues, adipose, small intestine, neural tissue, and kidney. The focus of this article is to review accomplishments toward the creation and use of tissue-engineered preclinical models of each of these different tissue types.

Murine and human pluripotent stem cell-derived cardiac bodies form contractile myocardial tissue in vitro

AIMS

We explored the use of highly purified murine and human pluripotent stem cell (PSC)-derived cardiomyocytes (CMs) to generate functional bioartificial cardiac tissue (BCT) and investigated the role of fibroblasts, ascorbic acid (AA), and mechanical stimuli on tissue formation, maturation, and functionality.

METHODS AND RESULTS

Murine and human embryonic/induced PSC-derived CMs were genetically enriched to generate three-dimensional CM aggregates, termed cardiac bodies (CBs). Addressing the critical limitation of major CM loss after single-cell dissociation, non-dissociated CBs were used for BCT generation, which resulted in a structurally and functionally homogenous syncytium. Continuous in situ characterization of BCTs, for 21 days, revealed that three critical factors cooperatively improve BCT formation and function: both (i) addition of fibroblasts and (ii) ascorbic acid supplementation support extracellular matrix remodelling and CB fusion, and (iii) increasing static stretch supports sarcomere alignment and CM coupling. All factors together considerably enhanced the contractility of murine and human BCTs, leading to a so far unparalleled active tension of 4.4 mN/mm(2) in human BCTs using optimized conditions. Finally, advanced protocols were implemented for the generation of human PSC-derived cardiac tissue using a defined animal-free matrix composition.

CONCLUSION

BCT with contractile forces comparable with native myocardium can be generated from enriched, PSC-derived CMs, based on a novel concept of tissue formation from non-dissociated cardiac cell aggregates. In combination with the successful generation of tissue using a defined animal-free matrix, this represents a major step towards clinical applicability of stem cell-based heart tissue for myocardial repair.

Recapitulating long-QT syndrome using induced pluripotent stem cell technology

The generation of patient-specific stem cells by reprogramming somatic cells to induced pluripotent stem cells (iPSC) provides the basis for a promising new type of in vitro disease models. Patient-specific iPSC derived from individuals with hereditary disorders can be differentiated into somatic cells in vitro, thus allowing the pathophysiology of the diseases to be studied on a cellular level. Different types of long-QT syndrome have been successfully modeled using this approach, demonstrating that the iPSC-derived patient-specific cardiomyocytes recapitulated key features of the disease in vitro. This approach will likely serve to model other monogenetic or polygenetic cardiovascular disorders in the future. Moreover, test platforms based on patient-specific iPSC could be used to test the potential of drug candidates to induce QT-interval prolongation or other unwanted side effects, screen for novel cardiovascular drugs, or to tailor medical therapy to the specific needs of a single patient.

Comparative study of human-induced pluripotent stem cells derived from bone marrow cells, hair keratinocytes, and skin fibroblasts

AIMS

Induced pluripotent stem cells (iPSCs) provide a unique opportunity for the generation of patient-specific cells for use in disease modelling, drug screening, and regenerative medicine. The aim of this study was to compare human-induced pluripotent stem cells (hiPSCs) derived from different somatic cell sources regarding their generation efficiency and cardiac differentiation potential, and functionalities of cardiomyocytes.

METHODS AND RESULTS

We generated hiPSCs from hair keratinocytes, bone marrow mesenchymal stem cells (MSCs), and skin fibroblasts by using two different virus systems. We show that MSCs and fibroblasts are more easily reprogrammed than keratinocytes. This corresponds to higher methylation levels of minimal promoter regions of the OCT4 and NANOG genes in keratinocytes than in MSCs and fibroblasts. The success rate and reprogramming efficiency was significantly higher by using the STEMCCA system than the OSNL system. All analysed hiPSCs are pluripotent and show phenotypical characteristics similar to human embryonic stem cells. We studied the cardiac differentiation efficiency of generated hiPSC lines (n = 24) and found that MSC-derived hiPSCs exhibited a significantly higher efficiency to spontaneously differentiate into beating cardiomyocytes when compared with keratinocyte-, and fibroblast-derived hiPSCs. There was no significant difference in the functionalities of the cardiomyocytes derived from hiPSCs with different origins, showing the presence of pacemaker-, atrial-, ventricular- and Purkinje-like cardiomyocytes, and exhibiting rhythmic Ca2+ transients and Ca2+ sparks in hiPSC-derived cardiomyocytes. Furthermore, spontaneously and synchronously beating and force-developing engineered heart tissues were generated.

CONCLUSIONS

Human-induced pluripotent stem cells can be reprogrammed from all three somatic cell types, but with different efficiency. All analysed iPSCs can differentiate into cardiomyocytes, and the functionalities of cardiomyocytes derived from different cell origins are similar. However, MSC-derived hiPSCs revealed a higher cardiac differentiation efficiency than keratinocyte- and fibroblast-derived hiPSCs.

Myocyte-depleted engineered cardiac tissues support therapeutic potential of mesenchymal stem cells

The therapeutic potential of mesenchymal stem cells (MSCs) for restoring cardiac function after cardiomyocyte loss remains controversial. Engineered cardiac tissues (ECTs) offer a simplified three-dimensional in vitro model system to evaluate stem cell therapies. We hypothesized that contractile properties of dysfunctional ECTs would be enhanced by MSC treatment. ECTs were created from neonatal rat cardiomyocytes with and without bone marrow-derived adult rat MSCs in a type-I collagen and Matrigel scaffold using custom elastomer molds with integrated cantilever force sensors. Three experimental groups included the following: (1) baseline condition ECT consisting only of myocytes, (2) 50% myocyte-depleted ECT, modeling a dysfunctional state, and (3) 50% myocyte-depleted ECT plus 10% MSC, modeling dysfunctional myocardium with intervention. Developed stress (DS) and pacing threshold voltage (VT) were measured using 2-Hz field stimulation at 37°C on culture days 5, 10, 15, and 20. By day 5, DS of myocyte-depleted ECTs was significantly lower than baseline, and VT was elevated. In MSC-supplemented ECTs, DS and VT were significantly better than myocyte-depleted values, approaching baseline ECTs. Findings were similar through culture day 15, but lost significance at day 20. Trends in DS were partly explained by changes in the cell number and alignment with time. Thus, supplementing myocyte-depleted ECTs with MSCs transiently improved contractile function and compensated for a 50% loss of cardiomyocytes, mimicking recent animal studies and clinical trials and supporting the potential of MSCs for myocardial therapy.

Physiological aspects of cardiac tissue engineering

Cardiac tissue engineering aims at repairing the diseased heart and developing cardiac tissues for basic research and predictive toxicology applications. Since the first description of engineered heart tissue 15 years ago, major development steps were directed toward these three goals. Technical innovations led to improved three-dimensional cardiac tissue structure and near physiological contractile force development. Automation and standardization allow medium throughput screening. Larger constructs composed of many small engineered heart tissues or stacked cell sheet tissues were tested for cardiac repair and were associated with functional improvements in rats. Whether these approaches can be simply transferred to larger animals or the human patients remains to be tested. The availability of an unrestricted human cardiac myocyte cell source from human embryonic stem cells or human-induced pluripotent stem cells is a major breakthrough. This review summarizes current tissue engineering techniques with their strengths and limitations and possible future applications.

Induced pluripotent stem cells in cardiovascular research

The discovery that somatic cells can be reprogrammed to induced pluripotent stem cells (iPSC) by overexpression of a combination of transcription factors bears the potential to spawn a wealth of new applications in both preclinical and clinical cardiovascular research. Disease modeling, which is accomplished by deriving iPSC lines from patients affected by heritable diseases and then studying the pathophysiology of the diseases in somatic cells differentiated from these patient-specific iPSC lines, is the so far most advanced of these applications. Long-QT syndrome and catecholaminergic polymorphic ventricular tachycardia are two heart rhythm disorders that have been already successfully modeled by several groups using this approach, which will likely serve to model other mono- or polygenetic cardiovascular disorders in the future. Test systems based on cells derived from iPSC might prove beneficial to screen for novel cardiovascular drugs or unwanted drug side effects and to individualize medical therapy. The application of iPSC for cell therapy of cardiovascular disorders, albeit promising, will only become feasible if the problem of biological safety of these cells will be mastered.