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

Heart-derived collagen promotes maturation of engineered heart tissue

Although the extracellular matrix (ECM) plays essential roles in heart tissue engineering, the optimal ECM components for heart tissue organization have not previously been elucidated. Here, we focused on the main ECM component, fibrillar collagen, and analyzed the effects of collagens on heart tissue engineering, by comparing the use of porcine heart-derived collagen and other organ-derived collagens in generating engineered heart tissue (EHT). We demonstrate that heart-derived collagen induces better contraction and relaxation of human induced pluripotent stem cell-derived EHT (hiPSC-EHT) and that hiPSC-EHT with heart-derived collagen exhibit more mature profiles than those with collagens from other organs. Further, we found that collagen fibril formation and gel stiffness influence the contraction, relaxation, and maturation of hiPSC-EHT, suggesting the importance of collagen types III and type V, which are relatively abundant in the heart. Thus, we demonstrate the effectiveness of organ-specific collagens in tissue engineering and drug discovery.

Human-Engineered Atrial Tissue for Studying Atrial Fibrillation

This chapter details the generation of atrial fibrin-based engineered heart tissue (EHT) in standard 24-well format as a 3D model for the human atrium. Compared to 2D cultivation, human-induced pluripotent stem cells (hiPSCs)-derived atrial cardiomyocytes demonstrated a higher degree of maturation in 3D format. Furthermore, we have demonstrated in previous work that the model displayed atrial characteristics in terms of contraction and gene expression patterns, electrophysiology, and pharmacological response. Here, we describe how to embed atrial cardiomyocytes differentiated from hiPSCs in a fibrin hydrogel to form atrial EHT attached to elastic silicone posts, allowing auxotonic contraction. In addition, we describe how force and other contractility parameters can be derived from these beating atrial EHTs by video-optical monitoring. The presented atrial EHT model is suitable to study chamber-specific mechanisms, drug effects and to serve for disease modeling.

Incomplete Assembly of the Dystrophin-Associated Protein Complex in 2D and 3D-Cultured Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Human induced pluripotent stem cells derived cardiomyocytes (hiPSC-CM) are increasingly used to study genetic diseases on a human background. However, the lack of a fully mature adult cardiomyocyte phenotype of hiPSC-CM may be limiting the scope of these studies. Muscular dystrophies and concomitant cardiomyopathies result from mutations in genes encoding proteins of the dystrophin-associated protein complex (DAPC), which is a multi-protein membrane-spanning complex. We examined the expression of DAPC components in hiPSC-CM, which underwent maturation in 2D and 3D culture protocols. The results were compared with human adult cardiac tissue and isolated cardiomyocytes. We found that similarly to adult cardiomyocytes, hiPSC-CM express dystrophin, in line with previous studies on Duchenne’s disease. β-dystroglycan was also expressed, but, contrary to findings in adult cardiomyocytes, none of the sarcoglycans nor α-dystroglycan were, despite the presence of their mRNA. In conclusion, despite the robust expression of dystrophin, the absence of several other DAPC protein components cautions for reliance on commonly used protocols for hiPSC-CM maturation for functional assessment of the complete DAPC.

Cell Banking of hiPSCs: A Practical Guide to Cryopreservation and Quality Control in Basic Research

The reproducibility of stem cell research relies on the constant availability of quality-controlled cells. As the quality of human induced pluripotent stem cells (hiPSCs) can deteriorate in the course of a few passages, cell banking is key to achieve consistent results and low batch-to-batch variation. Here, we provide a cost-efficient route to generate master and working cell banks for basic research projects. In addition, we describe minimal protocols for quality assurance including tests for sterility, viability, pluripotency, and genetic integrity. © 2020 The Authors. Basic Protocol 1: Expansion of hiPSCs Basic Protocol 2: Cell banking of hiPSCs Support Protocol 1: Pluripotency assessment by flow cytometry Support Protocol 2: Thawing control: Viability and sterility Support Protocol 3: Potency, viral clearance, and pluripotency: Spontaneous differentiation and qRT-PCR Support Protocol 4: Identity: Short tandem repeat analysis.

Unfolded Protein Response as a Compensatory Mechanism and Potential Therapeutic Target in PLN R14del Cardiomyopathy

BACKGROUND

Phospholamban (PLN) is a critical regulator of calcium cycling and contractility in the heart. The loss of arginine at position 14 in PLN (R14del) is associated with dilated cardiomyopathy with a high prevalence of ventricular arrhythmias. How the R14 deletion causes dilated cardiomyopathy is poorly understood, and there are no disease-specific therapies.

METHODS

We used single-cell RNA sequencing to uncover PLN R14del disease mechanisms in human induced pluripotent stem cells (hiPSC-CMs). We used both 2-dimensional and 3-dimensional functional contractility assays to evaluate the impact of modulating disease-relevant pathways in PLN R14del hiPSC-CMs.

RESULTS

Modeling of the PLN R14del cardiomyopathy with isogenic pairs of hiPSC-CMs recapitulated the contractile deficit associated with the disease in vitro. Single-cell RNA sequencing revealed the induction of the unfolded protein response (UPR) pathway in PLN R14del compared with isogenic control hiPSC-CMs. The activation of UPR was also evident in the hearts from PLN R14del patients. Silencing of each of the 3 main UPR signaling branches (IRE1, ATF6, or PERK) by siRNA exacerbated the contractile dysfunction of PLN R14del hiPSC-CMs. We explored the therapeutic potential of activating the UPR with a small molecule activator, BiP (binding immunoglobulin protein) inducer X. PLN R14del hiPSC-CMs treated with BiP protein inducer X showed a dose-dependent amelioration of the contractility deficit in both 2-dimensional cultures and 3-dimensional engineered heart tissues without affecting calcium homeostasis.

CONCLUSIONS

Together, these findings suggest that the UPR exerts a protective effect in the setting of PLN R14del cardiomyopathy and that modulation of the UPR might be exploited therapeutically.

ESC working group on cardiac cellular electrophysiology position paper: relevance, opportunities, and limitations of experimental models for cardiac electrophysiology research

Cardiac arrhythmias are a major cause of death and disability. A large number of experimental cell and animal models have been developed to study arrhythmogenic diseases. These models have provided important insights into the underlying arrhythmia mechanisms and translational options for their therapeutic management. This position paper from the ESC Working Group on Cardiac Cellular Electrophysiology provides an overview of (i) currently available in vitro, ex vivo, and in vivo electrophysiological research methodologies, (ii) the most commonly used experimental (cellular and animal) models for cardiac arrhythmias including relevant species differences, (iii) the use of human cardiac tissue, induced pluripotent stem cell (hiPSC)-derived and in silico models to study cardiac arrhythmias, and (iv) the availability, relevance, limitations, and opportunities of these cellular and animal models to recapitulate specific acquired and inherited arrhythmogenic diseases, including atrial fibrillation, heart failure, cardiomyopathy, myocarditis, sinus node, and conduction disorders and channelopathies. By promoting a better understanding of these models and their limitations, this position paper aims to improve the quality of basic research in cardiac electrophysiology, with the ultimate goal to facilitate the clinical translation and application of basic electrophysiological research findings on arrhythmia mechanisms and therapies.

Genetic Variation in Enhancers Modifies Cardiomyopathy Gene Expression and Progression

BACKGROUND

Inherited cardiomyopathy associates with a range of phenotypes, mediated by genetic and nongenetic factors. Noninherited cardiomyopathy also displays varying progression and outcomes. Expression of cardiomyopathy genes is under the regulatory control of promoters and enhancers, and human genetic variation in promoters and enhancers may contribute to this variability.

METHODS

We superimposed epigenomic profiling from hearts and cardiomyocytes, including promoter-capture chromatin conformation information, to identify enhancers for 2 cardiomyopathy genes, MYH7 and LMNA. Enhancer function was validated in human cardiomyocytes derived from induced pluripotent stem cells. We also conducted a genome-wide search to ascertain genomic variation in enhancers positioned to alter cardiac expression and correlated one of these variants to cardiomyopathy progression using biobank data.

RESULTS

Multiple enhancers were identified and validated for LMNA and MYH7, including a key enhancer that regulates the switch from MYH6 expression to MYH7 expression. Deletion of this enhancer resulted in a dose-dependent increase in MYH6 and faster contractile rate in engineered heart tissues. We searched for genomic variation in enhancer sequences across the genome, with a focus on nucleotide changes that create or interrupt transcription factor binding sites. The sequence variant, rs875908, disrupts a T-Box Transcription Factor 5 binding motif and maps to an enhancer region 2 kilobases from the transcriptional start site of MYH7. Gene editing to remove the enhancer that harbors this variant markedly reduced MYH7 expression in human cardiomyocytes. Using biobank-derived data, rs875908 associated with longitudinal echocardiographic features of cardiomyopathy.

CONCLUSIONS

Enhancers regulate cardiomyopathy gene expression, and genomic variation within these enhancer regions associates with cardiomyopathic progression over time. This integrated approach identified noncoding modifiers of cardiomyopathy and is applicable to other cardiac genes.

Case Report on: Very Early Afterdepolarizations in HiPSC-Cardiomyocytes-An Artifact by Big Conductance Calcium Activated Potassium Current (Ibk,Ca)

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) represent an unlimited source of human CMs that could be a standard tool in drug research. However, there is concern whether hiPSC-CMs express all cardiac ion channels at physiological level and whether they might express non-cardiac ion channels. In a control hiPSC line, we found large, “noisy” outward K⁺ currents, when we measured outward potassium currents in isolated hiPSC-CMs. Currents were sensitive to iberiotoxin, the selective blocker of big conductance Ca²⁺-activated K⁺ current (I_BK,Ca). Seven of 16 individual differentiation batches showed a strong initial repolarization in the action potentials (AP) recorded from engineered heart tissue (EHT) followed by very early afterdepolarizations, sometimes even with consecutive oscillations. Iberiotoxin stopped oscillations and normalized AP shape, but had no effect in other EHTs without oscillations or in human left ventricular tissue (LV). Expression levels of the alpha-subunit (K_Ca1.1) of the BK_Ca correlated with the presence of oscillations in hiPSC-CMs and was not detectable in LV. Taken together, individual batches of hiPSC-CMs can express sarcolemmal ion channels that are otherwise not found in the human heart, resulting in oscillating afterdepolarizations in the AP. HiPSC-CMs should be screened for expression of non-cardiac ion channels before being applied to drug research.

3D Printed Hydrogel Multiassay Platforms for Robust Generation of Engineered Contractile Tissues

We present a method for reproducible manufacture of multiassay platforms with tunable mechanical properties for muscle tissue strip analysis. The platforms result from stereolithographic 3D printing of low protein-binding poly(ethylene glycol) diacrylate (PEGDA) hydrogels. Contractile microtissues have previously been engineered by immobilizing suspended cells in a confined hydrogel matrix with embedded anchoring cantilevers to facilitate muscle tissue strip formation. The 3D shape and mechanical properties of the confinement and the embedded cantilevers are critical for the tissue robustness. High-resolution 3D printing of PEGDA hydrogels offers full design freedom to engineer cantilever stiffness, while minimizing unwanted cell attachment. We demonstrate the applicability by generating suspended muscle tissue strips from C2C12 mouse myoblasts in a compliant fibrin-based hydrogel matrix. The full design freedom allows for new platform geometries that reduce local stress in the matrix and tissue, thus, reducing the risk of tissue fracture.

Disease-inspired tissue engineering: Investigation of cardiovascular pathologies

Once focused exclusively on the creation of tissues to repair or replace diseased or damaged organs, the field of tissue engineering has undergone an important evolution in recent years. Namely, tissue engineering techniques are increasingly being applied to intentionally generate pathological conditions. Motivated in part by the wide gap between 2D cultures and animal models in the current disease modeling continuum, disease-inspired tissue-engineered platforms have numerous potential applications, and may serve to advance our understanding and clinical treatment of various diseases. This review will focus on recent progress toward generating tissue-engineered models of cardiovascular diseases, including cardiac hypertrophy, fibrosis, and ischemia reperfusion injury, atherosclerosis, and calcific aortic valve disease, with an emphasis on how these disease-inspired platforms can be used to decipher disease etiology. Each pathology is discussed in the context of generating both disease-specific cells as well as disease-specific extracellular environments, with an eye toward future opportunities to integrate different tools to yield more complex and physiologically relevant culture platforms. Ultimately, the development of effective disease treatments relies upon our ability to develop appropriate experimental models; as cardiovascular diseases are the leading cause of death worldwide, the insights yielded by improved in vitro disease modeling could have substantial ramifications for public health and clinical care.

Engineering of human cardiac muscle electromechanically matured to an adult-like phenotype

The application of tissue-engineering approaches to human induced pluripotent stem (hiPS) cells enables the development of physiologically relevant human tissue models for in vitro studies of development, regeneration, and disease. However, the immature phenotype of hiPS-derived cardiomyocytes (hiPS-CMs) limits their utility. We have developed a protocol to generate engineered cardiac tissues from hiPS cells and electromechanically mature them toward an adult-like phenotype. This protocol also provides optimized methods for analyzing these tissues' functionality, ultrastructure, and cellular properties. The approach relies on biological adaptation of cultured tissues subjected to biomimetic cues, applied at an increasing intensity, to drive accelerated maturation. hiPS cells are differentiated into cardiomyocytes and used immediately after the first contractions are observed, when they still have developmental plasticity. This starting cell population is combined with human dermal fibroblasts, encapsulated in a fibrin hydrogel and allowed to compact under passive tension in a custom-designed bioreactor. After 7 d of tissue formation, the engineered tissues are matured for an additional 21 d by increasingly intense electromechanical stimulation. Tissue properties can be evaluated by measuring contractile function, responsiveness to electrical stimuli, ultrastructure properties (sarcomere length, mitochondrial density, networks of transverse tubules), force-frequency and force-length relationships, calcium handling, and responses to β-adrenergic agonists. Cell properties can be evaluated by monitoring gene/protein expression, oxidative metabolism, and electrophysiology. The protocol takes 4 weeks and requires experience in advanced cell culture and machining methods for bioreactor fabrication. We anticipate that this protocol will improve modeling of cardiac diseases and testing of drugs.

Myocardial tissue engineering strategies for heart repair: current state of the art

The current state-of-the-art treatment for heart failure patients is aimed at delaying disease progression, relieving symptoms, reducing morbidity and improving survival. Cardiac regeneration of the injured heart, however, is not achieved. Currently, numerous alternative treatment approaches aiming at cardiac regeneration are under investigation. Myocardial tissue engineering strategies follow the idea of in vitro generation of myocardium-like structures for epicardial transplantation. Recently, this field has made tremendous advances regarding in vitro optimization of tissue-engineered constructs, and valuable data have been generated in small animal models. This review summarizes the technical aspects of engineered human myocardium generation, lessons learned from preclinical in vitro and in vivo studies and the current state of this approach on its way to clinical translation.

Isogenic Pairs of hiPSC-CMs with Hypertrophic Cardiomyopathy/LVNC-Associated ACTC1 E99K Mutation Unveil Differential Functional Deficits

Hypertrophic cardiomyopathy (HCM) is a primary disorder of contractility in heart muscle. To gain mechanistic insight and guide pharmacological rescue, this study models HCM using isogenic pairs of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) carrying the E99K-ACTC1 cardiac actin mutation. In both 3D engineered heart tissues and 2D monolayers, arrhythmogenesis was evident in all E99K-ACTC1 hiPSC-CMs. Aberrant phenotypes were most common in hiPSC-CMs produced from the heterozygote father. Unexpectedly, pathological phenotypes were less evident in E99K-expressing hiPSC-CMs from the two sons. Mechanistic insight from Ca2+ handling expression studies prompted pharmacological rescue experiments, wherein dual dantroline/ranolazine treatment was most effective. Our data are consistent with E99K mutant protein being a central cause of HCM but the three-way interaction between the primary genetic lesion, background (epi)genetics, and donor patient age may influence the pathogenic phenotype. This illustrates the value of isogenic hiPSC-CMs in genotype-phenotype correlations.

Satellite cells delivered in their niche efficiently generate functional myotubes in three-dimensional cell culture

Biophysical/biochemical cues from the environment contribute to regulation of the regenerative capacity of resident skeletal muscle stem cells called satellites cells. This can be observed in vitro, where muscle cell behaviour is influenced by the particular culture substrates and whether culture is performed in a 2D or 3D environment, with changes including morphology, nuclear shape and cytoskeletal organization. To create a 3D skeletal muscle model we compared collagen I, Fibrin or PEG-Fibrinogen with different sources of murine and human myogenic cells. To generate tension in the 3D scaffold, biomaterials were polymerised between two flexible silicone posts to mimic tendons. This 3D culture system has multiple advantages including being simple, fast to set up and inexpensive, so providing an accessible tool to investigate myogenesis in a 3D environment. Immortalised human and murine myoblast lines, and primary murine satellite cells showed varying degrees of myogenic differentiation when cultured in these biomaterials, with C2 myoblasts in particular forming large multinucleated myotubes in collagen I or Fibrin. However, murine satellite cells retained in their niche on a muscle fibre and embedded in 3D collagen I or Fibrin gels generated aligned, multinucleated and contractile myotubes.

Blinded Contractility Analysis in hiPSC-Cardiomyocytes in Engineered Heart Tissue Format: Comparison With Human Atrial Trabeculae

Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) may serve as a new assay for drug testing in a human context, but their validity particularly for the evaluation of inotropic drug effects remains unclear. In this blinded analysis, we compared the effects of 10 indicator compounds with known inotropic effects in electrically stimulated (1.5 Hz) hiPSC-CM-derived 3-dimensional engineered heart tissue (EHT) and human atrial trabeculae (hAT). Human EHTs were prepared from iCell hiPSC-CM, hAT obtained at routine heart surgery. Mean intra-batch variation coefficient in baseline force measurement was 17% for EHT and 49% for hAT. The PDE-inhibitor milrinone did not affect EHT contraction force, but increased force in hAT. Citalopram (selective serotonin reuptake inhibitor), nifedipine (LTCC-blocker) and lidocaine (Na+ channel-blocker) had negative inotropic effects on EHT and hAT. Formoterol (beta-2 agonist) had positive lusitropic but no inotropic effect in EHT, and positive clinotropic, lusitropic, and inotropic effects in hAT. Tacrolimus (calcineurin-inhibitor) had a negative inotropic effect in EHTs, but no effect in hAT. Digoxin (Na+-K+-ATPase-inhibitor) showed a positive inotropic effect only in EHTs, but no effect in hAT probably due to short incubation time. Ryanodine (ryanodine receptor-inhibitor) reduced contraction force in both models. Rolipram and acetylsalicylic acid showed noninterpretable results in hAT. Contraction amplitude and kinetics were more stable over time and less variable in hiPSC-EHTs than hAT. HiPSC-EHT faithfully detected cAMP-dependent and -independent positive and negative inotropic effects, but limited beta-2 adrenergic or PDE3 effects, compatible with an immature CM phenotype.

Afterload promotes maturation of human induced pluripotent stem cell derived cardiomyocytes in engineered heart tissues

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) grown in engineered heart tissue (EHT) can be used for drug screening, disease modeling, and heart repair. However, the immaturity of hiPSC-CMs currently limits their use. Because mechanical loading increases during development and facilitates cardiac maturation, we hypothesized that afterload would promote maturation of EHTs. To test this we developed a system in which EHTs are suspended between a rigid post and a flexible one, whose resistance to contraction can be modulated by applying braces of varying length. These braces allow us to adjust afterload conditions over two orders of magnitude by increasing the flexible post resistance from 0.09 up to 9.2 μN/μm. After three weeks in culture, optical tracking of post deflections revealed that auxotonic twitch forces increased in correlation with the degree of afterload, whereas twitch velocities decreased with afterload. Consequently, the power and work of the EHTs were maximal under intermediate afterloads. When studied isometrically, the inotropy of EHTs increased with afterload up to an intermediate resistance (0.45 μN/μm) and then plateaued. Applied afterload increased sarcomere length, cardiomyocyte area and elongation, which are hallmarks of maturation. Furthermore, progressively increasing the level of afterload led to improved calcium handling, increased expression of several key markers of cardiac maturation, including a shift from fetal to adult ventricular myosin heavy chain isoforms. However, at the highest afterload condition, markers of pathological hypertrophy and fibrosis were also upregulated, although the bulk tissue stiffness remained the same for all levels of applied afterload tested. Together, our results indicate that application of moderate afterloads can substantially improve the maturation of hiPSC-CMs in EHTs, while high afterload conditions may mimic certain aspects of human cardiac pathology resulting from elevated mechanical overload.

Contractile Work Contributes to Maturation of Energy Metabolism in hiPSC-Derived Cardiomyocytes

Energy metabolism is a key aspect of cardiomyocyte biology. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are a promising tool for biomedical application, but they are immature and have not undergone metabolic maturation related to early postnatal development. To assess whether cultivation of hiPSC-CMs in 3D engineered heart tissue format leads to maturation of energy metabolism, we analyzed the mitochondrial and metabolic state of 3D hiPSC-CMs and compared it with 2D culture. 3D hiPSC-CMs showed increased mitochondrial mass, DNA content, and protein abundance (proteome). While hiPSC-CMs exhibited the principal ability to use glucose, lactate, and fatty acids as energy substrates irrespective of culture format, hiPSC-CMs in 3D performed more oxidation of glucose, lactate, and fatty acid and less anaerobic glycolysis. The increase in mitochondrial mass and DNA in 3D was diminished by pharmacological reduction of contractile force. In conclusion, contractile work contributes to metabolic maturation of hiPSC-CMs.

•

Higher mitochondrial mass, protein, and DNA content in 3D versus 2D hiPSC-CMs

•

Similarity of mitochondrial proteomes between 3D hiPSC-CMs and adult human heart

•

Preference for oxidative metabolism in favor of anaerobic glycolysis in 3D hiPSC-CMs

Myocardial Tissue Engineering for Regenerative Applications

PURPOSE OF REVIEW

This review provides an overview of the current state of tissue-engineered heart repair with a special focus on the anticipated modes of action of tissue-engineered therapy candidates and particular implications as to transplant immunology.

RECENT FINDINGS

Myocardial tissue engineering technologies have made tremendous advances in recent years. Numerous different strategies are under investigation and have reached different stages on their way to clinical translation. Studies in animal models demonstrated that heart repair requires either remuscularization by delivery of bona fide cardiomyocytes or paracrine support for the activation of endogenous repair mechanisms. Tissue engineering approaches result in enhanced cardiomyocyte retention and sustained remuscularization, but may also be explored for targeted paracrine or mechanical support. Some of the more advanced tissue engineering approaches are already tested clinically; others are at late stages of pre-clinical development. Process optimization towards cGMP compatibility and clinical scalability of contractile engineered human myocardium is an essential step towards clinical translation. Long-term allograft retention can be achieved under immune suppression. HLA matching may be an option to enhance graft retention and reduce the need for comprehensive immune suppression. Tissue-engineered heart repair is entering the clinical stage of the translational pipeline. Like in any effective therapy, side effects must be anticipated and carefully controlled. Allograft implantation under immune suppression is the most likely clinical scenario. Strategies to overcome transplant rejection are evolving and may further boost the clinical acceptance of tissue-engineered heart repair.

Real-Time Force and Frequency Analysis of Engineered Human Heart Tissue Derived from Induced Pluripotent Stem Cells Using Magnetic Sensing

Engineered heart tissues made from human pluripotent stem cell-derived cardiomyocytes have been used for modeling cardiac pathologies, screening new therapeutics, and providing replacement cardiac tissue. Current methods measure the functional performance of engineered heart tissue by their twitch force and beating frequency, typically obtained by optical measurements. In this article, we describe a novel method for assessing twitch force and beating frequency of engineered heart tissue using magnetic field sensing, which enables multiple tissues to be measured simultaneously. The tissues are formed as thin structures suspended between two silicone posts, where one post is rigid and another is flexible and contains an embedded magnet. When the tissue contracts it causes the flexible post to bend in proportion to its twitch force. We measured the bending of the post using giant magnetoresistive (GMR) sensors located underneath a 24-well plate containing the tissues. We validated the accuracy of the readings from the GMR sensors against optical measurements. We demonstrated the utility and sensitivity of our approach by testing the effects of three concentrations of isoproterenol and verapamil on twitch force and beating frequency in real-time, parallel experiments. This system should be scalable beyond the 24-well format, enabling greater automation in assessing the contractile function of cardiomyocytes in a tissue-engineered environment.

Ca(2+)-Currents in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes Effects of Two Different Culture Conditions

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) provide a unique opportunity to study human heart physiology and pharmacology and repair injured hearts. The suitability of hiPSC-CM critically depends on how closely they share physiological properties of human adult cardiomyocytes (CM). Here we investigated whether a 3D engineered heart tissue (EHT) culture format favors maturation and addressed the L-type Ca²⁺-current (I_Ca,L) as a readout. The results were compared with hiPSC-CM cultured in conventional monolayer (ML) and to our previous data from human adult atrial and ventricular CM obtained when identical patch-clamp protocols were used. HiPSC-CM were two- to three-fold smaller than adult CM, independently of culture format [capacitance ML 45 ± 1 pF (n = 289), EHT 45 ± 1 pF (n = 460), atrial CM 87 ± 3 pF (n = 196), ventricular CM 126 ± 8 pF (n = 50)]. Only 88% of ML cells showed I_Ca, but all EHT. Basal I_Ca density was 10 ± 1 pA/pF (n = 207) for ML and 12 ± 1 pA/pF (n = 361) for EHT and was larger than in adult CM [7 ± 1 pA/pF (p < 0.05, n = 196) for atrial CM and 6 ± 1 pA/pF (p < 0.05, n = 47) for ventricular CM]. However, ML and EHT showed robust T-type Ca²⁺-currents (I_Ca,T). While (−)-Bay K 8644, that activates I_Ca,L directly, increased I_Ca,Lto the same extent in ML and EHT, β₁- and β₂-adrenoceptor effects were marginal in ML, but of same size as (−)-Bay K 8644 in EHT. The opposite was true for serotonin receptors. Sensitivity to β₁ and β₂-adrenoceptor stimulation was the same in EHT as in adult CM (−logEC₅₀: 5.9 and 6.1 for norepinephrine (NE) and epinephrine (Epi), respectively), but very low concentrations of Rp-8-Br-cAMPS were sufficient to suppress effects (−logEC₅₀: 5.3 and 5.3 respectively for NE and Epi). Taken together, hiPSC-CM express I_Ca,L at the same density as human adult CM, but, in contrast, possess robust I_Ca,T. Increased effects of catecholamines in EHT suggest more efficient maturation.

Human Engineered Heart Tissue: Analysis of Contractile Force

Analyzing contractile force, the most important and best understood function of cardiomyocytes in vivo is not established in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM). This study describes the generation of 3D, strip-format, force-generating engineered heart tissues (EHT) from hiPSC-CM and their physiological and pharmacological properties. CM were differentiated from hiPSC by a growth factor-based three-stage protocol. EHTs were generated and analyzed histologically and functionally. HiPSC-CM in EHTs showed well-developed sarcomeric organization and alignment, and frequent mitochondria. Systematic contractility analysis (26 concentration-response curves) reveals that EHTs replicated canonical response to physiological and pharmacological regulators of inotropy, membrane- and calcium-clock mediators of pacemaking, modulators of ion-channel currents, and proarrhythmic compounds with unprecedented precision. The analysis demonstrates a high degree of similarity between hiPSC-CM in EHT format and native human heart tissue, indicating that human EHTs are useful for preclinical drug testing and disease modeling.

•

Engineered heart tissues (EHTs) from hiPSC-CM are generated with high reproducibility

•

EHTs show aligned cardiomyocytes with organized sarcomeres and immature t tubules

•

Spontaneous beating is regulated by both, membrane- and calcium-clock mechanisms

•

EHTs respond to physiological and pharmacological interventions like human heart tissue

Human engineered heart tissue as a model system for drug testing

Drug development is time- and cost-intensive and, despite extensive efforts, still hampered by the limited value of current preclinical test systems to predict side effects, including proarrhythmic and cardiotoxic effects in clinical practice. Part of the problem may be related to species-dependent differences in cardiomyocyte biology. Therefore, the event of readily available human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes (CM) has raised hopes that this human test bed could improve preclinical safety pharmacology as well as drug discovery approaches. However, hiPSC-CM are immature and exhibit peculiarities in terms of ion channel function, gene expression, structural organization and functional responses to drugs that limit their present usefulness. Current efforts are thus directed towards improving hiPSC-CM maturity and high-content readouts. Culturing hiPSC-CM as 3-dimensional engineered heart tissue (EHT) improves CM maturity and anisotropy and, in a 24-well format using silicone racks, enables automated, multiplexed high content readout of contractile function. This review summarizes the principal technology and focuses on advantages and disadvantages of this technology and its potential for preclinical drug screening.