A simple, cost-effective but highly efficient system for deriving ventricular cardiomyocytes from human pluripotent stem cells

APLNR marks a cardiac progenitor derived with human induced pluripotent stem cells

Cardiomyocytes can be readily derived from human induced pluripotent stem cell (hiPSC) lines, yet its efficacy varies across different batches of the same and different hiPSC lines. To unravel the inconsistencies of in vitro cardiac differentiation, we utilized single cell transcriptomics on hiPSCs undergoing cardiac differentiation and identified cardiac and extra-cardiac lineages throughout differentiation. We further identified APLNR as a surface marker for in vitro cardiac progenitors and immunomagnetically isolated them. Differentiation of isolated in vitro APLNR+ cardiac progenitors derived from multiple hiPSC lines resulted in predominantly cardiomyocytes accompanied with cardiac mesenchyme. Transcriptomic analysis of differentiating in vitro APLNR+ cardiac progenitors revealed transient expression of cardiac progenitor markers before further commitment into cardiomyocyte and cardiac mesenchyme. Analysis of in vivo human and mouse embryo single cell transcriptomic datasets have identified APLNR expression in early cardiac progenitors of multiple lineages. This platform enables generation of in vitro cardiac progenitors from multiple hiPSC lines without genetic manipulation, which has potential applications in studying cardiac development, disease modelling and cardiac regeneration.

Single-cell RNA sequencing reveals maturation trajectory in human pluripotent stem cell-derived cardiomyocytes in engineered tissues

Cardiac in vitro models have become increasingly obtainable and affordable with the optimization of human pluripotent stem cell-derived cardiomyocyte (hPSC-CM) differentiation. However, these CMs are immature compared to their in vivo counterparts. Here we study the cellular phenotype of hPSC-CMs by comparing their single-cell gene expression and functional profiles in three engineered cardiac tissue configurations: human ventricular (hv) cardiac anisotropic sheet, cardiac tissue strip, and cardiac organoid chamber (hvCOC), with spontaneously aggregated 3D cardiac spheroids (CS) as control. The CM maturity was found to increase with increasing levels of complexity of the engineered tissues from CS to hvCOC. The contractile components are the first function to mature, followed by electrophysiology and oxidative metabolism. Notably, the 2D tissue constructs show a higher cellular organization whereas metabolic maturity preferentially increases in the 3D constructs. We conclude that the tissue engineering models resembling configurations of native tissues may be reliable for drug screening or disease modeling.

Human-Induced Pluripotent Stem Cell-Based Differentiation of Cardiomyocyte Subtypes for Drug Discovery and Cell Therapy

Drug attrition rates have increased over the past few years, accompanied with growing costs for the pharmaceutical industry and consumers. Lack of in vitro models connecting the results of toxicity screening assays with clinical outcomes accounts for this high attrition rate. The emergence of cardiomyocytes derived from human pluripotent stem cells provides an amenable source of cells for disease modeling, drug discovery, and cardiotoxicity screening. Functionally similar to to embryonic stem cells, but with fewer ethical concerns, induced pluripotent stem cells (iPSCs) can recapitulate patient-specific genetic backgrounds, which would be a huge revolution for personalized medicine. The generated iPSC-derived cardiomyocytes (iPSC-CMs) represent different subtypes including ventricular-, atrial-, and nodal-like cardiomyocytes. Purifying these subtypes for chamber-specific drug screening presents opportunities and challenges. In this chapter, we discuss the strategies for the purification of iPSC-CMs, the use of iPSC-CMs for drug discovery and cardiotoxicity test, and the current limitations of iPSC-CMs that should be overcome for wider and more precise cardiovascular applications.

Cellular and Engineered Organoids for Cardiovascular Models

An ensemble of in vitro cardiac tissue models has been developed over the past several decades to aid our understanding of complex cardiovascular disorders using a reductionist approach. These approaches often rely on recapitulating single or multiple clinically relevant end points in a dish indicative of the cardiac pathophysiology. The possibility to generate disease-relevant and patient-specific human induced pluripotent stem cells has further leveraged the utility of the cardiac models as screening tools at a large scale. To elucidate biological mechanisms in the cardiac models, it is critical to integrate physiological cues in form of biochemical, biophysical, and electromechanical stimuli to achieve desired tissue-like maturity for a robust phenotyping. Here, we review the latest advances in the directed stem cell differentiation approaches to derive a wide gamut of cardiovascular cell types, to allow customization in cardiac model systems, and to study diseased states in multiple cell types. We also highlight the recent progress in the development of several cardiovascular models, such as cardiac organoids, microtissues, engineered heart tissues, and microphysiological systems. We further expand our discussion on defining the context of use for the selection of currently available cardiac tissue models. Last, we discuss the limitations and challenges with the current state-of-the-art cardiac models and highlight future directions.

Regenerative Neurology and Regenerative Cardiology: Shared Hurdles and Achievements

From the first success in cultivation of cells in vitro, it became clear that developing cell and/or tissue specific cultures would open a myriad of new opportunities for medical research. Expertise in various in vitro models has been developing over decades, so nowadays we benefit from highly specific in vitro systems imitating every organ of the human body. Moreover, obtaining sufficient number of standardized cells allows for cell transplantation approach with the goal of improving the regeneration of injured/disease affected tissue. However, different cell types bring different needs and place various types of hurdles on the path of regenerative neurology and regenerative cardiology. In this review, written by European experts gathered in Cost European action dedicated to neurology and cardiology-Bioneca, we present the experience acquired by working on two rather different organs: the brain and the heart. When taken into account that diseases of these two organs, mostly ischemic in their nature (stroke and heart infarction), bring by far the largest burden of the medical systems around Europe, it is not surprising that in vitro models of nervous and heart muscle tissue were in the focus of biomedical research in the last decades. In this review we describe and discuss hurdles which still impair further progress of regenerative neurology and cardiology and we detect those ones which are common to both fields and some, which are field-specific. With the goal to elucidate strategies which might be shared between regenerative neurology and cardiology we discuss methodological solutions which can help each of the fields to accelerate their development.

Arrhythmic Risk Assessment of Hypokalaemia Using Human Pluripotent Stem Cell-Derived Cardiac Anisotropic Sheets

Introduction: Hypokalaemia, defined as an extracellular concentration of K⁺ below 3.5 mM, can cause cardiac arrhythmias by triggered or re-entrant mechanisms. Whilst these effects have been reported in animal and human stem cell-based models, to date there has been no investigation in more complex structures such as the human ventricular cardiac anisotropic sheet (hvCAS). Here, we investigated arrhythmogenicity, electrophysiological, and calcium transient (CaT) changes induced by hypokalaemia using this bioengineered platform.

Methods: An optical mapping technique was applied on hvCAS derived from human pluripotent stem cells to visualize electrophysiological and CaT changes under normokalaemic (5 mM KCl) and hypokalaemic (3 mM KCl) conditions.

Results: Hypokalaemia significantly increased the proportion of preparations showing spontaneous arrhythmias from 0/14 to 7/14 (Fisher’s exact test, p = 0.003). Hypokalaemia reduced longitudinal conduction velocity (CV) from 7.81 to 7.18 cm⋅s⁻¹ (n = 9, 7; p = 0.036), transverse CV from 5.72 to 4.69 cm⋅s⁻¹ (n = 12, 11; p = 0.030), prolonged action potential at 90% repolarization (APD₉₀) from 83.46 to 97.45 ms (n = 13, 15; p < 0.001), increased action potential amplitude from 0.888 to 1.195 ΔF (n = 12, 14; p < 0.001) and CaT amplitude from 0.76 to 1.37 ΔF (n = 12, 13; p < 0.001), and shortened effective refractory periods from 242 to 165 ms (n = 12, 13; p < 0.001).

Conclusion: Hypokalaemia exerts pro-arrhythmic effects on hvCAS, which are associated with alterations in CV, repolarization, refractoriness, and calcium handling. These preparations provide a useful platform for investigating electrophysiological substrates and for conducting arrhythmia screening.

SARS-CoV-2 exploits host DGAT and ADRP for efficient replication

Coronavirus Disease 2019 (COVID-19) is predominantly a respiratory tract infection that significantly rewires the host metabolism. Here, we monitored a cohort of COVID-19 patients’ plasma lipidome over the disease course and identified triacylglycerol (TG) as the dominant lipid class present in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-induced metabolic dysregulation. In particular, we pinpointed the lipid droplet (LD)-formation enzyme diacylglycerol acyltransferase (DGAT) and the LD stabilizer adipocyte differentiation-related protein (ADRP) to be essential host factors for SARS-CoV-2 replication. Mechanistically, viral nucleo capsid protein drives DGAT1/2 gene expression to facilitate LD formation and associates with ADRP on the LD surface to complete the viral replication cycle. DGAT gene depletion reduces SARS-CoV-2 protein synthesis without compromising viral genome replication/transcription. Importantly, a cheap and orally available DGAT inhibitor, xanthohumol, was found to suppress SARS-CoV-2 replication and the associated pulmonary inflammation in a hamster model. Our findings not only uncovered the mechanistic role of SARS-CoV-2 nucleocapsid protein to exploit LDs-oriented network for heightened metabolic demand, but also the potential to target the LDs-synthetase DGAT and LDs-stabilizer ADRP for COVID-19 treatment.

Model Systems for Addressing Mechanism of Arrhythmogenesis in Cardiac Repair

PURPOSE OF REVIEW

Cardiac cell-based therapy represents a promising approach for cardiac repair. However, one of the main challenges is cardiac arrhythmias associated with stem cell transplantation. The current review summarizes the recent progress in model systems for addressing mechanisms of arrhythmogenesis in cardiac repair.

RECENT FINDINGS

Animal models have been extensively developed for mechanistic studies of cardiac arrhythmogenesis. Advances in human induced pluripotent stem cells (hiPSCs), patient-specific disease models, tissue engineering, and gene editing have greatly enhanced our ability to probe the mechanistic bases of cardiac arrhythmias. Additionally, recent development in multiscale computational studies and machine learning provides yet another powerful tool to quantitatively decipher the mechanisms of cardiac arrhythmias. Advancing efforts towards the integrations of experimental and computational studies are critical to gain insights into novel mitigation strategies for cardiac arrhythmias in cell-based therapy.

Clofazimine broadly inhibits coronaviruses including SARS-CoV-2

The COVID-19 pandemic is the third outbreak this century of a zoonotic disease caused by a coronavirus, following the emergence of severe acute respiratory syndrome (SARS) in 2003¹ and Middle East respiratory syndrome (MERS) in 2012². Treatment options for coronaviruses are limited. Here we show that clofazimine-an anti-leprosy drug with a favourable safety profile³-possesses inhibitory activity against several coronaviruses, and can antagonize the replication of SARS-CoV-2 and MERS-CoV in a range of in vitro systems. We found that this molecule, which has been approved by the US Food and Drug Administration, inhibits cell fusion mediated by the viral spike glycoprotein, as well as activity of the viral helicase. Prophylactic or therapeutic administration of clofazimine in a hamster model of SARS-CoV-2 pathogenesis led to reduced viral loads in the lung and viral shedding in faeces, and also alleviated the inflammation associated with viral infection. Combinations of clofazimine and remdesivir exhibited antiviral synergy in vitro and in vivo, and restricted viral shedding from the upper respiratory tract. Clofazimine, which is orally bioavailable and comparatively cheap to manufacture, is an attractive clinical candidate for the treatment of outpatients and-when combined with remdesivir-in therapy for hospitalized patients with COVID-19, particularly in contexts in which costs are an important factor or specialized medical facilities are limited. Our data provide evidence that clofazimine may have a role in the control of the current pandemic of COVID-19 and-possibly more importantly-in dealing with coronavirus diseases that may emerge in the future.

A Heart-Breast Cancer-on-a-Chip Platform for Disease Modeling and Monitoring of Cardiotoxicity Induced by Cancer Chemotherapy

Cardiotoxicity is one of the most serious side effects of cancer chemotherapy. Current approaches to monitoring of chemotherapy-induced cardiotoxicity (CIC) as well as model systems that develop in vivo or in vitro CIC platforms fail to notice early signs of CIC. Moreover, breast cancer (BC) patients with preexisting cardiac dysfunctions may lead to different incident levels of CIC. Here, a model is presented for investigating CIC where not only induced pluripotent stem cell (iPSC)-derived cardiac tissues are interacted with BC tissues on a dual-organ platform, but electrochemical immuno-aptasensors can also monitor cell-secreted multiple biomarkers. Fibrotic stages of iPSC-derived cardiac tissues are promoted with a supplement of transforming growth factor-β 1 to assess the differential functionality in healthy and fibrotic cardiac tissues after treatment with doxorubicin (DOX). The production trend of biomarkers evaluated by using the immuno-aptasensors well-matches the outcomes from conventional enzyme-linked immunosorbent assay, demonstrating the accuracy of the authors' sensing platform with much higher sensitivity and lower detection limits for early monitoring of CIC and BC progression. Furthermore, the versatility of this platform is demonstrated by applying a nanoparticle-based DOX-delivery system. The proposed platform would potentially help allow early detection and prediction of CIC in individual patients in the future.

Human Pluripotent Stem Cell-Derived Cardiac Cells: Application in Disease Modeling, Cell Therapy, and Drug Discovery

Cardiac diseases are the leading cause of deaths worldwide; however, to date, there has been limited progress in the development of therapeutic options for these conditions. Animal models have been the most extensively studied methods to recapitulate a wide variety of cardiac diseases, but these models exhibit species-specific differences in physiology, metabolism and genetics, which lead to inaccurate and unpredictable drug safety and efficacy results, resulting in drug attrition. The development of human pluripotent stem cell (hPSC) technology in theory guarantees an unlimited source of human cardiac cells. These hPSC-derived cells are not only well suited for traditional two-dimensional (2-D) monoculture, but also applicable to more complex systems, such as three-dimensional (3-D) organoids, tissue engineering and heart on-a-chip. In this review, we discuss the application of hPSCs in heart disease modeling, cell therapy, and next-generation drug discovery. While the hPSC-related technologies still require optimization, their advances hold promise for revolutionizing cell-based therapies and drug discovery.

Dextran-based scaffolds for in-situ hydrogelation: Use for next generation of bioartificial cardiac tissues

In pursuit of a chemically-defined matrix for in vitro cardiac tissue generation, we present dextran (Dex)-derived hydrogels as matrices suitable for bioartificial cardiac tissues (BCT). The dextran hydrogels were generated in situ by using hydrazone formation as the crosslinking reaction. Material properties were flexibly adjusted, by varying the degrees of derivatization and the molecular weight of dextran used. Furthermore, to modulate dextran's bioactivity, cyclic pentapeptide RGD was coupled to its backbone. BCTs were generated by using a blend of modified dextran and human collagen (hColI) in combination with induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) and fibroblasts. These hColI + Dex blends with or without RGD supported tissue formation and functional maturation of CMs. Contraction forces (hColI + Dex-RGD: 0.27 ± 0.02 mN; hColI + Dex: 0.26 ± 0.01 mN) and frequencies were comparable to published constructs. Thus, we could demonstrate that, independent of the presence of RGD, our covalently linked dextran hydrogels are a promising matrix for building cardiac grafts.

iPSC-Cardiomyocyte Models of Brugada Syndrome-Achievements, Challenges and Future Perspectives

Brugada syndrome (BrS) is an inherited cardiac arrhythmia that predisposes to ventricular fibrillation and sudden cardiac death. It originates from oligogenic alterations that affect cardiac ion channels or their accessory proteins. The main hurdle for the study of the functional effects of those variants is the need for a specific model that mimics the complex environment of human cardiomyocytes. Traditionally, animal models or transient heterologous expression systems are applied for electrophysiological investigations, each of these models having their limitations. The ability to create induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs), providing a source of human patient-specific cells, offers new opportunities in the field of cardiac disease modelling. Contemporary iPSC-CMs constitute the best possible in vitro model to study complex cardiac arrhythmia syndromes such as BrS. To date, thirteen reports on iPSC-CM models for BrS have been published and with this review we provide an overview of the current findings, with a focus on the electrophysiological parameters. We also discuss the methods that are used for cell derivation and data acquisition. In the end, we critically evaluate the knowledge gained by the use of these iPSC-CM models and discuss challenges and future perspectives for iPSC-CMs in the study of BrS and other arrhythmias.

Adverse effects of hydroxychloroquine and azithromycin on contractility and arrhythmogenicity revealed by human engineered cardiac tissues

The coronavirus disease 2019 (COVID-19) outbreak caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has become a global pandemic as declared by World Health Organization (WHO). In the absence of an effective treatment, different drugs with unknown effectiveness, including antimalarial hydroxychloroquine (HCQ), with or without concurrent administration with azithromycin (AZM), have been tested for treating COVID-19 patients with developed pneumonia. However, the efficacy and safety of HCQ and/or AZM have been questioned by recent clinical reports. Direct effects of these drugs on the human heart remain very poorly defined. To better understand the mechanisms of action of HCQ +/− AZM, we employed bioengineered human ventricular cardiac tissue strip (hvCTS) and anisotropic sheet (hvCAS) assays, made with human pluripotent stem cell (hPSC)-derived ventricular cardiomyocytes (hvCMs), which have been designed for measuring cardiac contractility and electrophysiology, respectively. Our hvCTS experiments showed that AZM induced a dose-dependent negative inotropic effect which could be aggravated by HCQ; electrophysiologically, as revealed by the hvCAS platform, AZM prolonged action potentials and induced spiral wave formations. Collectively, our data were consistent with reported clinical risks of HCQ and AZM on QTc prolongation/ventricular arrhythmias and development of heart failure. In conclusion, our study exposed the risks of HCQ/AZM administration while providing mechanistic insights for their toxicity. Our bioengineered human cardiac tissue constructs therefore provide a useful platform for screening cardiac safety and efficacy when developing therapeutics against COVID-19.

Arrhythmia Mechanisms in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

ABSTRACT

Despite major efforts by clinicians and researchers, cardiac arrhythmia remains a leading cause of morbidity and mortality in the world. Experimental work has relied on combining high-throughput strategies with standard molecular and electrophysiological studies, which are, to a great extent, based on the use of animal models. Because this poses major challenges for translation, the progress in the development of novel antiarrhythmic agents and clinical care has been mostly disappointing. Recently, the advent of human induced pluripotent stem cell-derived cardiomyocytes has opened new avenues for both basic cardiac research and drug discovery; now, there is an unlimited source of cardiomyocytes of human origin, both from healthy individuals and patients with cardiac diseases. Understanding arrhythmic mechanisms is one of the main use cases of human induced pluripotent stem cell-derived cardiomyocytes, in addition to pharmacological cardiotoxicity and efficacy testing, in vitro disease modeling, developing patient-specific models and personalized drugs, and regenerative medicine. Here, we review the advances that the human induced pluripotent stem cell-derived-based modeling systems have brought so far regarding the understanding of both arrhythmogenic triggers and substrates, while also briefly speculating about the possibilities in the future.

Single-Cell Transcriptomics of Engineered Cardiac Tissues From Patient-Specific Induced Pluripotent Stem Cell-Derived Cardiomyocytes Reveals Abnormal Developmental Trajectory and Intrinsic Contractile Defects in Hypoplastic Right Heart Syndrome

Background

To understand the intrinsic cardiac developmental and functional abnormalities in pulmonary atresia with intact ventricular septum (PAIVS) free from effects secondary to anatomic defects, we performed and compared single‐cell transcriptomic and phenotypic analyses of patient‐ and healthy subject–derived human‐induced pluripotent stem cell–derived cardiomyocytes (hiPSC‐CMs) and engineered tissue models.

Methods and Results

We derived hiPSC lines from 3 patients with PAIVS and 3 healthy subjects and differentiated them into hiPSC‐CMs, which were then bioengineered into the human cardiac anisotropic sheet and human cardiac tissue strip custom‐designed for electrophysiological and contractile assessments, respectively. Single‐cell RNA sequencing (scRNA‐seq) of hiPSC‐CMs, human cardiac anisotropic sheet, and human cardiac tissue strip was performed to examine the transcriptomic basis for any phenotypic abnormalities using pseudotime and differential expression analyses. Through pseudotime analysis, we demonstrated that bioengineered tissue constructs provide pro‐maturational cues to hiPSC‐CMs, although the maturation and development were attenuated in PAIVS hiPSC‐CMs. Furthermore, reduced contractility and prolonged contractile kinetics were observed with PAIVS human cardiac tissue strips. Consistently, single‐cell RNA sequencing of PAIVS human cardiac tissue strips and hiPSC‐CMs exhibited diminished expression of cardiac contractile apparatus genes. By contrast, electrophysiological aberrancies were absent in PAIVS human cardiac anisotropic sheets.

Conclusions

Our findings were the first to reveal intrinsic abnormalities of cardiomyocyte development and function in PAIVS free from secondary effects. We conclude that hiPSC‐derived engineered tissues offer a unique method for studying primary cardiac abnormalities and uncovering pathogenic mechanisms that underlie sporadic congenital heart diseases.

NODAL inhibition promotes differentiation of pacemaker-like cardiomyocytes from human induced pluripotent stem cells

Directed cardiomyogenesis from human induced pluripotent stem cells (hiPSCs) has been greatly improved in the last decade but directed differentiation to pacemaking cardiomyocytes (CMs) remains incompletely understood. In this study, we demonstrated that inhibition of NODAL signaling by a specific NODAL inhibitor (SB431542) in the cardiac mesoderm differentiation stage downregulated PITX2c, a transcription factor that is known to inhibit the formation of the sinoatrial node in the left atrium during cardiac development. The resulting hiPSC-CMs were smaller in cell size, expressed higher pro-pacemaking transcription factors, TBX3 and TBX18, and exhibited pacemaking-like electrophysiological characteristics compared to control hiPSC-CMs differentiated from established Wnt-based protocol. The pacemaker-like subtype increased up to 2.4-fold in hiPSC-CMs differentiated with the addition of SB431542 relative to the control. Hence, Nodal inhibition in the cardiac mesoderm stage promoted pacemaker-like CM differentiation from hiPSCs. Improving the yield of human pacemaker-like CMs is a critical first step in the development of functional human cell-based biopacemakers.

Clofazimine is a broad-spectrum coronavirus inhibitor that antagonizes SARS-CoV-2 replication in primary human cell culture and hamsters

COVID-19 pandemic is the third zoonotic coronavirus (CoV) outbreak of the century after severe acute respiratory syndrome (SARS) in 2003 and Middle East respiratory syndrome (MERS) since 2012. Treatment options for CoVs are largely lacking. Here, we show that clofazimine, an anti-leprosy drug with a favorable safety and pharmacokinetics profile, possesses pan-coronaviral inhibitory activity, and can antagonize SARS-CoV-2 replication in multiple in vitro systems, including the human embryonic stem cell-derived cardiomyocytes and ex vivo lung cultures. The FDA-approved molecule was found to inhibit multiple steps of viral replication, suggesting multiple underlying antiviral mechanisms. In a hamster model of SARS-CoV-2 pathogenesis, prophylactic or therapeutic administration of clofazimine significantly reduced viral load in the lung and fecal viral shedding, and also prevented cytokine storm associated with viral infection. Additionally, clofazimine exhibited synergy when administered with remdesivir. Since clofazimine is orally bioavailable and has a comparatively low manufacturing cost, it is an attractive clinical candidate for outpatient treatment and remdesivir-based combinatorial therapy for hospitalized COVID-19 patients, particularly in developing countries. Taken together, our data provide evidence that clofazimine may have a role in the control of the current pandemic SARS-CoV-2, endemic MERS-CoV in the Middle East, and, possibly most importantly, emerging CoVs of the future.

Single-cell protein expression of hiPSC-derived cardiomyocytes using Single-Cell Westerns

The ability to reprogram human somatic cells into human induced pluripotent stem cells (hiPSCs) has enabled researchers to generate cell types in vitro that have the potential to faithfully recapitulate patient-specific disease processes and phenotypes. hiPSC-derived cardiomyocytes (hiPSC-CMs) offer the promise of in vitro patient- and disease-specific models for drug testing and the discovery of novel therapeutic approaches for treating cardiovascular diseases. While methods to differentiate hiPSCs into cardiomyocytes have been demonstrated, the heterogeneity and immaturity of these differentiated populations have restricted their potential in reproducing human disease and the associated target cell phenotypes. These barriers may be overcome through comprehensive single-cell characterization to dissect the rich heterogeneity of hiPSC-CMs and to study the source of varying cell fates. In this study, we optimized and validated a new Single-Cell Western method to assess protein expression in hiPSC-CMs. To better understand distinct subpopulations generated from cardiomyocyte differentiations and to track populations at single-cell resolution over time, we measured and quantified the expression of cardiomyocyte subtype-specific proteins (MLC2V and MLC2A) using Single-Cell Westerns. By understanding their heterogeneity through single-cell protein expression and quantification, we may improve upon current cardiomyocyte differentiation protocols, generate hiPSC-CMs that are more representative of in vivo derived cardiomyocytes for disease modeling, and utilize hiPSC-CMs for regenerative medicine purposes. Single-Cell Westerns provide a robust platform for protein expression analysis at single-cell resolution.

Inhibition of protein kinase D by CID755673 promotes maintenance of the pluripotency of embryonic stem cells

The identification of novel mechanisms to maintain embryonic stem cell (ESC) pluripotency is of crucial importance, because the currently used culture conditions are not suitable for ESCs from all species. In this study, we show that the protein kinase D (PKD) inhibitor CID755673 (CID) is able to maintain the undifferentiated state of mouse ESCs in combination with the mitogen-activated protein kinase kinase (MEK) inhibitor. The expression levels of PKD members, including PKD1, PKD2 and PKD3, were low in mouse ESCs but significantly increased under differentiation conditions. Therefore, depletion of three PKD genes was able to phenocopy PKD inhibition. Mechanistically, PKD inhibition activated PI3K/AKT signaling by increasing the level of AKT phosphorylation, and the addition of a PI3K/AKT signaling pathway inhibitor partially reduced the cellular response to PKD inhibition. Importantly, the self-renewal-promoting effect of CID was maintained in human ESCs. Simultaneous knockdown of the three human PKD isoforms enabled short-term self-renewal in human ESCs, whereas PI3K/AKT signaling inhibition eliminated this self-renewal ability downstream of the PKD inhibitor. These findings expand our understanding of the gene regulatory network of ESC pluripotency.

Modeling Cardiac Disease Mechanisms Using Induced Pluripotent Stem Cell-Derived Cardiomyocytes: Progress, Promises and Challenges

Cardiovascular diseases (CVDs) are a class of disorders affecting the heart or blood vessels. Despite progress in clinical research and therapy, CVDs still represent the leading cause of mortality and morbidity worldwide. The hallmarks of cardiac diseases include heart dysfunction and cardiomyocyte death, inflammation, fibrosis, scar tissue, hyperplasia, hypertrophy, and abnormal ventricular remodeling. The loss of cardiomyocytes is an irreversible process that leads to fibrosis and scar formation, which, in turn, induce heart failure with progressive and dramatic consequences. Both genetic and environmental factors pathologically contribute to the development of CVDs, but the precise causes that trigger cardiac diseases and their progression are still largely unknown. The lack of reliable human model systems for such diseases has hampered the unraveling of the underlying molecular mechanisms and cellular processes involved in heart diseases at their initial stage and during their progression. Over the past decade, significant scientific advances in the field of stem cell biology have literally revolutionized the study of human disease in vitro. Remarkably, the possibility to generate disease-relevant cell types from induced pluripotent stem cells (iPSCs) has developed into an unprecedented and powerful opportunity to achieve the long-standing ambition to investigate human diseases at a cellular level, uncovering their molecular mechanisms, and finally to translate bench discoveries into potential new therapeutic strategies. This review provides an update on previous and current research in the field of iPSC-driven cardiovascular disease modeling, with the aim of underlining the potential of stem-cell biology-based approaches in the elucidation of the pathophysiology of these life-threatening diseases.

Efficient differentiation of human pluripotent stem cells into cardiomyocytes on cell sorting thermoresponsive surface

The current differentiation process of human pluripotent stem cells (hPSCs) into cardiomyocytes to enhance the purity of hPSC-derived cardiomyocytes requires some purification processes, which are laborious processes. We developed cell sorting plates, which are prepared from coating thermoresponsive poly(N-isopropylacrylamide) and extracellular matrix proteins. After hPSCs were induced into cardiomyocytes on the thermoresponsive surface coated with laminin-521 for 15 days, the temperature of the cell culture plates was decreased to 8-9 °C to detach the cells partially from the thermoresponsive surface. The detached cells exhibited a higher cardiomyocyte marker of cTnT than the remaining cells on the thermoresponsive surface as well as the cardiomyocytes after purification using conventional cell selection. The detached cells expressed several cardiomyocyte markers, such as α-actinin, MLC2a and NKX2.5. This study suggested that the purification of hPSC-derived cardiomyocytes using cell sorting plates with the thermoresponsive surface is a promising method for the purification of hPSC-derived cardiomyocytes without conventional laborious processes.

Subtype-specific cardiomyocytes for precision medicine: Where are we now?

Patient-derived pluripotent stem cells (PSCs) have greatly transformed the current understanding of human heart development and cardiovascular disease. Cardiomyocytes derived from personalized PSCs are powerful tools for modeling heart disease and performing patient-based cardiac toxicity testing. However, these PSC-derived cardiomyocytes (PSC-CMs) are a mixed population of atrial-, ventricular-, and pacemaker-like cells in the dish, hindering the future of precision cardiovascular medicine. Recent insights gleaned from the developing heart have paved new avenues to refine subtype-specific cardiomyocytes from patients with known pathogenic genetic variants and clinical phenotypes. Here, we discuss the recent progress on generating subtype-specific (atrial, ventricular, and nodal) cardiomyocytes from the perspective of embryonic heart development and how human pluripotent stem cells will expand our current knowledge on molecular mechanisms of cardiovascular disease and the future of precision medicine.

Modeling the heart with Novoheart’s MyHeart™ platform

Reliable and predictive human-specific in vitro heart models can revolutionize drug discovery and development. With the advent of pluripotent stem cell technologies, human cardiomyocytes can now be readily produced in large quantities. Using tissue engineering techniques, they can be further assembled into cardiac tissues of specific 2D and 3D configurations, to create models that behave and function like the native human heart. Novoheart (BC, Canada) uniquely offers the MyHeartTM Platform of bioengineered human heart constructs, designed to provide researchers with effective models of either healthy or diseased human hearts. As in vitro, human-based assays become more widely accepted, the next decade could witness a shift away from animal testing towards more accurate and scalable human assays like the MyHeartTM Platform.

Combinatorial Treatment of Human Cardiac Engineered Tissues With Biomimetic Cues Induces Functional Maturation as Revealed by Optical Mapping of Action Potentials and Calcium Transients

Although biomimetic stimuli, such as microgroove-induced alignment (μ), triiodothyronine (T3) induction, and electrical conditioning (EC), have been reported to promote maturation of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs), a systematic examination of their combinatorial effects on engineered cardiac tissue constructs and the underlying molecular pathways has not been reported. Herein, human embryonic stem cell-derived ventricular cardiomyocytes (hESC-VCMs) were used to generate a micro-patterned human ventricular cardiac anisotropic sheets (hvCAS) for studying the physiological effects of combinatorial treatments by a range of functional, calcium (Ca²⁺)-handling, and molecular analyses. High-resolution optical mapping showed that combined μ-T3-EC treatment of hvCAS increased the conduction velocity, anisotropic ratio, and proportion of mature quiescent-yet-excitable preparations by 2. 3-, 1. 8-, and 5-fold (>70%), respectively. Such electrophysiological changes could be attributed to an increase in inward sodium current density and a decrease in funny current densities, which is consistent with the observed up- and downregulated SCN1B and HCN2/4 transcripts, respectively. Furthermore, Ca²⁺-handling transcripts encoding for phospholamban (PLN) and sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) were upregulated, and this led to faster upstroke and decay kinetics of Ca²⁺-transients. RNA-sequencing and pathway mapping of T3-EC-treated hvCAS revealed that the TGF-β signaling was downregulated; the TGF-β receptor agonist and antagonist TGF-β1 and SB431542 partially reversed T3-EC induced quiescence and reduced spontaneous contractions, respectively. Taken together, we concluded that topographical cues alone primed cardiac tissue constructs for augmented electrophysiological and calcium handling by T3-EC. Not only do these studies improve our understanding of hPSC-CM biology, but the orchestration of these pro-maturational factors also improves the use of engineered cardiac tissues for in vitro drug screening and disease modeling.

Intensive care for human hearts in pluripotent stem cell models

Successful drug discovery is ultimately contingent on the availability of workable, relevant, predictive model systems. Conversely, for cardiac muscle, the lack of human preclinical models to inform target validation and compound development has likely contributed to the perennial problem of clinical trial failures, despite encouraging non-human results. By contrast, human cardiomyocytes produced from pluripotent stem cell models have recently been applied to safety pharmacology, phenotypic screening, target validation and high-throughput assays, facilitating cardiac drug discovery. Here, we review the impact of human pluripotent stem cell models in cardiac drug discovery, discussing the range of applications, readouts, and disease models employed, along with the challenges and prospects to advance this fruitful mode of research further.

The Emergence of Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes (hiPSC-CMs) as a Platform to Model Arrhythmogenic Diseases

There is a need for improved in vitro models of inherited cardiac diseases to better understand basic cellular and molecular mechanisms and advance drug development. Most of these diseases are associated with arrhythmias, as a result of mutations in ion channel or ion channel-modulatory proteins. Thus far, the electrophysiological phenotype of these mutations has been typically studied using transgenic animal models and heterologous expression systems. Although they have played a major role in advancing the understanding of the pathophysiology of arrhythmogenesis, more physiological and predictive preclinical models are necessary to optimize the treatment strategy for individual patients. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have generated much interest as an alternative tool to model arrhythmogenic diseases. They provide a unique opportunity to recapitulate the native-like environment required for mutated proteins to reproduce the human cellular disease phenotype. However, it is also important to recognize the limitations of this technology, specifically their fetal electrophysiological phenotype, which differentiates them from adult human myocytes. In this review, we provide an overview of the major inherited arrhythmogenic cardiac diseases modeled using hiPSC-CMs and for which the cellular disease phenotype has been somewhat characterized.

Culture and differentiation of purified human adipose-derived stem cells by membrane filtration via nylon mesh filters

Human adipose derived stem cells purified by the membrane migration method through filter membranes coated with vitronectin showed high osteogenic differentiation.

Human Induced Pluripotent Stem Cell-Derived Non-Cardiomyocytes Modulate Cardiac Electrophysiological Maturation Through Connexin 43-Mediated Cell-Cell Interactions

The functional maturation status of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) has a notable impact upon their use in pharmacological studies, disease modeling, and therapeutic applications. Non-cardiomyocytes (non-CMs) produced in the differentiation process have previously been identified as having an extrinsic influence upon hiPSC-CM development, yet the underlying mechanisms are not fully understood. Herein, we aimed to modulate electrophysiological properties of hiPSC-CMs within co-cultures containing varied proportions of non-CMs and investigate the nature of interactions between these different cell types. Therefore, we sorted cardiac differentiations on day 10 and subsequently replated the cells at ratios of 7:3, 1:1, 3:7, and 1:9 non-CMs to CMs. After a month of co-culture, we evaluated electrophysiological properties through the genetically encoded voltage indicator ArcLight. We ultimately identified that co-cultures with approximately 70%–90% CM purity demonstrated the highest action potential (AP) amplitude and maximum upstroke velocity by day 40 of differentiation, indicative of enhanced electrophysiological maturation, as well as more ventricular-like AP morphologies. Notably, these findings were distinct from those observed for co-cultures of hiPSC-CMs and dermal fibroblasts. We determined that the co-culture phenotypes could not be attributed to paracrine effects of non-CMs due to the inability of conditioned media to recapitulate the observed effects. This led to the further observation of a distinctive expression pattern of connexin 43 (Cx43) at cell-cell interfaces between both CMs and non-CMs. Depletion of Cx43 by short hairpin RNA (shRNA) specifically in the non-CM population within a co-culture environment was able to recapitulate electrophysiological phenotypes of a purer hiPSC-CM population. Collectively, our data demonstrate that abundant non-CM content exerts a significant negative influence upon the electrophysiological maturation of hiPSC-CMs through Cx43-mediated cell-cell-contacts, and thus should be considered regarding the future production of purpose-built hiPSC-CM systems.

Episomal Induced Pluripotent Stem Cells: Functional and Potential Therapeutic Applications

The term episomal induced pluripotent stem cells (EiPSCs) refers to somatic cells that are reprogrammed into induced pluripotent stem cells (iPSCs) using non-integrative episomal vector methods. This reprogramming process has a better safety profile compared with integrative methods using viruses. There is a current trend toward using episomal plasmid reprogramming to generate iPSCs because of the improved safety profile. Clinical reports of potential human cell sources that have been successfully reprogrammed into EiPSCs are increasing, but no review or summary has been published. The functional applications of EiPSCs and their potential uses in various conditions have been described, and these may be applicable to clinical scenarios. This review summarizes the current direction of EiPSC research and the properties of these cells with the aim of explaining their potential role in clinical applications and functional restoration.

Sarco/endoplasmic reticulum Ca2+-ATPase is a more effective calcium remover than sodium-calcium exchanger in human embryonic stem cell-derived cardiomyocytes

Human pluripotent stem cell (hPSCs)-derived ventricular (V) cardiomyocytes (CMs) display immature Ca2+–handing properties with smaller transient amplitudes and slower kinetics due to such differences in crucial Ca2+-handling proteins as the poor sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) pump but robust Na+-Ca2+ exchanger (NCX) activities in human embryonic stem cell (ESC)-derived VCMs compared with adult. Despite their fundamental importance in excitation-contraction coupling, the relative contribution of SERCA and NCX to Ca2+-handling of hPSC-VCMs remains unexplored. We systematically altered the activities of SERCA and NCX in human embryonic stem cell-derived ventricular cardiomyocytes (hESC-VCMs) and their engineered microtissues, followed by examining the resultant phenotypic consequences. SERCA overexpression in hESC-VCMs shortened the decay of Ca2+ transient at low frequencies (0.5 Hz) without affecting the amplitude, SR Ca2+ content and Ca2+ baseline. Interestingly, short hairpin RNA-based NCX suppression did not prolong the transient decay, indicating a compensatory response for Ca2+ removal. Although hESC-VCMs and their derived microtissues exhibited negative frequency-transient/force responses, SERCA overexpression rendered them less negative at high frequencies (>2 Hz) by accelerating Ca2+ sequestration. We conclude that for hESC-VCMs and their microtissues, SERCA, rather than NCX, is the main Ca2+ remover during diastole; poor SERCA expression is the leading cause for immature negative-frequency/force responses, which can be partially reverted by forced expression. Combinatorial approach to mature calcium handling in hESC-VCMs may help shed further mechanistic insights.

NEW & NOTEWORTHY In this study of human pluripotent stem cell-derived cardiomyocytes, we studied the role of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and Na+-Ca2+ exchanger (NCX) in Ca2+ handling. Our data support the notion that SERCA is more effective in cytosolic calcium removal than the NCX.

Integrated transcriptomic and regulatory network analyses identify microRNA-200c as a novel repressor of human pluripotent stem cell-derived cardiomyocyte differentiation and maturation

Aims

MicroRNAs (miRNAs) are crucial for the post-transcriptional control of protein-encoding genes and together with transcription factors (TFs) regulate gene expression; however, the regulatory activities of miRNAs during cardiac development are only partially understood. In this study, we tested the hypothesis that integrative computational approaches could identify miRNAs that experimentally could be shown to regulate cardiomyogenesis.

Methods and results

We integrated expression profiles with bioinformatics analyses of miRNA and TF regulatory programs to identify candidate miRNAs involved with cardiac development. Expression profiling showed that miR-200c, which is not normally detected in adult heart, is progressively down-regulated both during cardiac development and in vitro differentiation of human embryonic stem cells (hESCs) to cardiomyocytes (CMs). We employed computational methodologies to predict target genes of both miR-200c and five key cardiac TFs to identify co-regulated gene networks. The inferred cardiac networks revealed that the cooperative action of miR-200c with these five key TFs, including three (GATA4, SRF and TBX5) targeted by miR-200c, should modulate key processes and pathways necessary for CM development and function. Experimentally, over-expression (OE) of miR-200c in hESC-CMs reduced the mRNA levels of GATA4, SRF and TBX5. Cardiac expression of Ca2+, K+ and Na+ ion channel genes (CACNA1C, KCNJ2 and SCN5A) were also significantly altered by knockdown or OE of miR-200c. Luciferase reporter assays validated miR-200c binding sites on the 3' untranslated region of CACNA1C. In hESC-CMs, elevated miR-200c increased beating frequency, and repressed both Ca2+ influx, mediated by the L-type Ca2+ channel and Ca2+ transients.

Conclusions

Our analyses demonstrate that miR-200c represses hESC-CM differentiation and maturation. The integrative computation and experimental approaches described here, when applied more broadly, will enhance our understanding of the interplays between miRNAs and TFs in controlling cardiac development and disease processes.

Correlation between frataxin expression and contractility revealed by in vitro Friedreich's ataxia cardiac tissue models engineered from human pluripotent stem cells

BACKGROUND

Friedreich's ataxia (FRDA) is an autosomal recessive disease caused by a non-coding mutation in the first intron of the frataxin (FXN) gene that suppresses its expression. Compensatory hypertrophic cardiomyopathy, dilated cardiomyopathy, and conduction system abnormalities in FRDA lead to cardiomyocyte (CM) death and fibrosis, consequently resulting in heart failure and arrhythmias. Murine models have been developed to study disease pathology in the past two decades; however, differences between human and mouse physiology and metabolism have limited the relevance of animal studies in cardiac disease conditions. To bridge this gap, we aimed to generate species-specific, functional in vitro experimental models of FRDA using 2-dimensional (2D) and 3-dimensional (3D) engineered cardiac tissues from FXN-deficient human pluripotent stem cell-derived ventricular cardiomyocytes (hPSC-hvCMs) and to compare their contractile and electrophysiological properties with healthy tissue constructs.

METHODS

Healthy control and FRDA patient-specific hPSC-hvCMs were derived by directed differentiation using a small molecule-based protocol reported previously. We engineered the hvCMs into our established human ventricular cardiac tissue strip (hvCTS) and human ventricular cardiac anisotropic sheet (hvCAS) models, and functional assays were performed on days 7-17 post-tissue fabrication to assess the electrophysiology and contractility of FRDA patient-derived and FXN-knockdown engineered tissues, in comparison with healthy controls. To further validate the disease model, forced expression of FXN was induced in FXN-deficient tissues to test if disease phenotypes could be rescued.

RESULTS

Here, we report for the first time the generation of human engineered tissue models of FRDA cardiomyopathy from hPSCs: FXN-deficient hvCTS displayed attenuated developed forces (by 70-80%) compared to healthy controls. High-resolution optical mapping of hvCAS with reduced FXN expression also revealed electrophysiological defects consistent with clinical observations, including action potential duration prolongation and maximum capture frequency reduction. Interestingly, a clear positive correlation between FXN expression and contractility was observed (ρ > 0.9), and restoration of FXN protein levels by lentiviral transduction rescued contractility defects in FXN-deficient hvCTS.

CONCLUSIONS

We conclude that human-based in vitro cardiac tissue models of FRDA provide a translational, disease-relevant biomimetic platform for the evaluation of novel therapeutics and to provide insight into FRDA disease progression.

A comprehensive, multiscale framework for evaluation of arrhythmias arising from cell therapy in the whole post-myocardial infarcted heart

Direct remuscularization approaches to cell-based heart repair seek to restore ventricular contractility following myocardial infarction (MI) by introducing new cardiomyocytes (CMs) to replace lost or injured ones. However, despite promising improvements in cardiac function, high incidences of ventricular arrhythmias have been observed in animal models of MI injected with pluripotent stem cell-derived cardiomyocytes (PSC-CMs). The mechanisms of arrhythmogenesis remain unclear. Here, we present a comprehensive framework for computational modeling of direct remuscularization approaches to cell therapy. Our multiscale 3D whole-heart modeling framework integrates realistic representations of cell delivery and transdifferentiation therapy modalities as well as representation of spatial distributions of engrafted cells, enabling simulation of clinical therapy and the prediction of emergent electrophysiological behavior and arrhythmogenensis. We employ this framework to explore how varying parameters of cell delivery and transdifferentiation could result in three mechanisms of arrhythmogenesis: focal ectopy, heart block, and reentry.

Modulation of chromatin remodeling proteins SMYD1 and SMARCD1 promotes contractile function of human pluripotent stem cell-derived ventricular cardiomyocyte in 3D-engineered cardiac tissues

Human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) have the ability of differentiating into functional cardiomyocytes (CMs) for cell replacement therapy, tissue engineering, drug discovery and toxicity screening. From a scale-free, co-expression network analysis of transcriptomic data that distinguished gene expression profiles of undifferentiated hESC, hESC-, fetal- and adult-ventricular(V) CM, two candidate chromatin remodeling proteins, SMYD1 and SMARCD1 were found to be differentially expressed. Using lentiviral transduction, SMYD1 and SMARCD1 were over-expressed and suppressed, respectively, in single hESC-VCMs as well as the 3D constructs Cardiac Micro Tissues (CMT) and Tissue Strips (CTS) to mirror the endogenous patterns, followed by dissection of their roles in controlling cardiac gene expression, contractility, Ca²⁺-handling, electrophysiological functions and in vitro maturation. Interestingly, compared to independent single transductions, simultaneous SMYD1 overexpression and SMARCD1 suppression in hESC-VCMs synergistically interacted to increase the contractile forces of CMTs and CTSs with up-regulated transcripts for cardiac contractile, Ca²⁺-handing, and ion channel proteins. Certain effects that were not detected at the single-cell level could be unleashed under 3D environments. The two chromatin remodelers SMYD1 and SMARCD1 play distinct roles in cardiac development and maturation, consistent with the notion that epigenetic priming requires triggering signals such as 3D environmental cues for pro-maturation effects.

Single-Cell RNA-Sequencing and Optical Electrophysiology of Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes Reveal Discordance Between Cardiac Subtype-Associated Gene Expression Patterns and Electrophysiological Phenotypes

The ability to accurately phenotype cells differentiated from human induced pluripotent stem cells (hiPSCs) is essential for their application in modeling developmental and disease processes, yet also poses a particular challenge without the context of anatomical location. Our specific objective was to determine if single-cell gene expression was sufficient to predict the electrophysiology of iPSC-derived cardiac lineages, to evaluate the concordance between molecular and functional surrogate markers. To this end, we used the genetically encoded voltage indicator ArcLight to profile hundreds of hiPSC-derived cardiomyocytes (hiPSC-CMs), thus identifying patterns of electrophysiological maturation and increased prevalence of cells with atrial-like action potentials (APs) between days 11 and 42 of differentiation. To profile expression patterns of cardiomyocyte subtype-associated genes, single-cell RNA-seq was performed at days 12 and 40 after the populations were fully characterized with the high-throughput ArcLight platform. Although we could detect global gene expression changes supporting progressive differentiation, individual cellular expression patterns alone were not able to delineate the individual cardiomyocytes into atrial, ventricular, or nodal subtypes as functionally documented by electrophysiology measurements. Furthermore, our efforts to understand the distinct electrophysiological properties associated with day 12 versus day 40 hiPSC-CMs revealed that ion channel regulators SLMAP, FGF12, and FHL1 were the most significantly increased genes at day 40, categorized by electrophysiology-related gene functions. Notably, FHL1 knockdown during differentiation was sufficient to significantly modulate APs toward ventricular-like electrophysiology. Thus, our results establish the inability of subtype-associated gene expression patterns to specifically categorize hiPSC-derived cells according to their functional electrophysiology, and yet, altered FHL1 expression is able to redirect electrophysiological maturation of these developing cells. Therefore, noncanonical gene expression patterns of cardiac maturation may be sufficient to direct functional maturation of cardiomyocytes, with canonical gene expression patterns being insufficient to temporally define cardiac subtypes of in vitro differentiation.

Structural and Mechanistic Bases of Nuclear Calcium Signaling in Human Pluripotent Stem Cell-Derived Ventricular Cardiomyocytes

The loss of nonregenerative, terminally differentiated cardiomyocytes (CMs) due to aging or diseases is generally considered irreversible. Human pluripotent stem cells (hPSCs) can self-renew while maintaining their pluripotency to differentiate into all cell types, including ventricular (V) cardiomyocytes (CMs), to provide a potential unlimited ex vivo source of CMs for heart disease modeling, drug/cardiotoxicity screening, and cell-based therapies. In the human heart, cytosolic Ca²⁺ signals are well characterized but the contribution of nuclear Ca²⁺ is essentially unexplored. The present study investigated nuclear Ca²⁺ signaling in hPSC-VCMs. Calcium transient or sparks in hPSC-VCMs were measured by line scanning using a spinning disc confocal microscope. We observed that nuclear Ca²⁺, which stems from unitary sparks due to the diffusion of cytosolic Ca²⁺ that are mediated by RyRs on the nuclear reticulum, is functional. Parvalbumin- (PV-) mediated Ca²⁺ buffering successfully manipulated Ca²⁺ transient and stimuli-induced apoptosis in hPSC-VCMs. We also investigated the effect of Ca²⁺ on gene transcription in hPSC-VCMs, and the involvement of nuclear factor of activated T-cell (NFAT) pathway was identified. The overexpression of Ca²⁺-sensitive, nuclear localized Ca²⁺/calmodulin-dependent protein kinase II δ_B (CaMKIIδ_B) induced cardiac hypertrophy through nuclear Ca²⁺/CaMKIIδB/HDAC4/MEF2 pathway. These findings provide insights into nuclear Ca²⁺ signal in hPSC-VCMs, which may lead to novel strategies for maturation as well as improved systems for disease modeling, drug discovery, and cell-based therapies.

Cardiovascular disease models: A game changing paradigm in drug discovery and screening

Cardiovascular disease is the leading cause of death worldwide. Although investment in drug discovery and development has been sky-rocketing, the number of approved drugs has been declining. Cardiovascular toxicity due to therapeutic drug use claims the highest incidence and severity of adverse drug reactions in late-stage clinical development. Therefore, to address this issue, new, additional, replacement and combinatorial approaches are needed to fill the gap in effective drug discovery and screening. The motivation for developing accurate, predictive models is twofold: first, to study and discover new treatments for cardiac pathologies which are leading in worldwide morbidity and mortality rates; and second, to screen for adverse drug reactions on the heart, a primary risk in drug development. In addition to in vivo animal models, in vitro and in silico models have been recently proposed to mimic the physiological conditions of heart and vasculature. Here, we describe current in vitro, in vivo, and in silico platforms for modelling healthy and pathological cardiac tissues and their advantages and disadvantages for drug screening and discovery applications. We review the pathophysiology and the underlying pathways of different cardiac diseases, as well as the new tools being developed to facilitate their study. We finally suggest a roadmap for employing these non-animal platforms in assessing drug cardiotoxicity and safety.

Human Cardiac Ventricular-Like Organoid Chambers and Tissue Strips From Pluripotent Stem Cells as a Two-Tiered Assay for Inotropic Responses

Traditional drug discovery is an inefficient process. Human pluripotent stem cell-derived cardiomyocytes can potentially fill the gap between animal and clinical studies, but conventional two-dimensional cultures inadequately recapitulate the human cardiac phenotype. Here, we systematically examined the pharmacological responses of engineered human ventricular-like cardiac tissue strips (hvCTS) and organoid chambers (hvCOC) to 25 cardioactive compounds covering various drug classes. While hvCTS effectively detected negative and null inotropic effects, the sensitivity to positive inotropes was modest. We further quantified the predictive capacity of hvCTS in a blinded screening, with accuracies for negative, positive, and null inotropic effects at 100%, 86%, and 80%, respectively. Interestingly, hvCOC, with a pro-maturation milieu that yields physiologically complex parameters, displayed enhanced positive inotropy. Based on these results, we propose a two-tiered screening system for avoiding false positives and negatives. Such an approach would facilitate drug discovery by leading to better overall success.

Disease modeling of cardiac arrhythmias using human induced pluripotent stem cells

INTRODUCTION

Inherited arrhythmias are an uncommon, but malignant family of cardiac diseases that result from genetic abnormalities in the ion channels and/or structural proteins within cardiomyocytes. Given the inherent differences between species and the limited reproducibility of in vitro heterologous cell models, progress in understanding the mechanisms underlying these malignant diseases has always languished far behind the clinical science and need. The ability to study human induced pluripotent stem cells (iPSCs) derived cardiomyocytes promises to change this paradigm as patient cells have the potential to become testing platforms for disease phenotyping or therapeutic discovery.

AREAS COVERED

This review will outline methods developed to genetically reprogram adult cells into iPSCs, differentiate iPSCs into ex vivo models of adult cardiac tissue and iPSCs-based progress in exploring the mechanisms underlying pro-arrhythmic disease phenotypes.

EXPERT OPINION

Despite being discovered less than 15 years ago, several studies have successfully leveraged iPSCs-derived cardiomyocytes to study malignant arrhythmogenic diseases. These models promise to increase our understanding of the pathophysiology underlying these complex diseases and may identify personalized approaches to treatment.

Physical stimulation by REAC and BMP4/WNT-1 inhibitor synergistically enhance cardiogenic commitment in iPSCs

It is currently known that pluripotent stem cells can be committed in vitro to the cardiac lineage by the modulation of specific signaling pathways, but it is also well known that, despite the significant increase in cardiomyocyte yield provided by the currently available conditioned media, the resulting cardiogenic commitment remains a highly variable process. Previous studies provided evidence that radio electric fields asymmetrically conveyed through the Radio Electric Asymmetric Conveyer (REAC) technology are able to commit R1 embryonic stem cells and human adipose derived stem cells toward a cardiac phenotype. The present study aimed at investigating whether the effect of physical stimulation by REAC in combination with specific chemical inductors enhance the cardiogenic potential in human induced pluripotent stem cells (iPSCs). The appearance of a cardiac-like phenotype in iPSCs cultured in the presence of a cardiogenic medium, based upon BMP4 and a WNT-inhibitor, was consistently increased by REAC treatment used only during the early fate differentiation for the first 72 hours. REAC-exposed iPSCs exhibited an upregulation in the expression of specific cardiogenic transcripts and morphologically in the number of beating clusters, as compared to cells cultured in the cardiogenic medium alone. Our results indicate that physical modulation of cellular dynamics provided by the REAC offers an affordable strategy to mimic iPSC cardiac-like fates in the presence of a cardiogenic milieu.

Stem cells as therapy for heart disease: iPSCs, ESCs, CSCs, and skeletal myoblasts

Heart Diseases are serious and global public health concern. In spite of remarkable therapeutic developments, the prediction of patients with Heart Failure (HF) is weak, and present therapeutic attitudes do not report the fundamental problem of the cardiac tissue loss. Innovative therapies are required to reduce mortality and limit or abolish the necessity for cardiac transplantation. Stem cell-based therapies applied to the treatment of heart disease is according to the understanding that natural self-renewing procedures are inherent to the myocardium, nonetheless may not be adequate to recover the infarcted heart muscle. Following the first account of cell therapy in heart diseases, examination has kept up to rapidity; besides, several animals and human clinical trials have been conducted to preserve the capacity of numerous stem cell population in advance cardiac function and decrease infarct size. The purpose of this study was to censoriously evaluate the works performed regarding the usage of four major subgroups of stem cells, including induced Pluripotent Stem Cells (iPSC), Embryonic Stem Cells (ESCs), Cardiac Stem Cells (CDC), and Skeletal Myoblasts, in heart diseases, at the preclinical and clinical studies. Moreover, it is aimed to argue the existing disagreements, unsolved problems, and prospect directions.

Genetic and Tissue Engineering Approaches to Modeling the Mechanics of Human Heart Failure for Drug Discovery

Heart failure is the leading cause of death in the western world and as such, there is a great need for new therapies. Heart failure has a variable presentation in patients and a complex etiology; however, it is fundamentally a condition that affects the mechanics of cardiac contraction, preventing the heart from generating sufficient cardiac output under normal operating pressures. One of the major issues hindering the development of new therapies has been difficulties in developing appropriate in vitro model systems of human heart failure that recapitulate the essential changes in cardiac mechanics seen in the disease. Recent advances in stem cell technologies, genetic engineering, and tissue engineering have the potential to revolutionize our ability to model and study heart failure in vitro. Here, we review how these technologies are being applied to develop personalized models of heart failure and discover novel therapeutics.

High-Content Electrophysiological Analysis of Human Pluripotent Stem Cell-Derived Cardiomyocytes (hPSC-CMs)

Considerable interest has been raised to develop human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) as a model for drug discovery and cardiotoxicity screening. High-content electrophysiological analysis of currents generated by transmembrane cell surface ion channels has been pursued to complement such emerging applications. Here we describe practical procedures and considerations for accomplishing successful assays of hPSC-CMs using an automated planar patch-clamp system.

Deep phenotyping of human induced pluripotent stem cell-derived atrial and ventricular cardiomyocytes

Generation of homogeneous populations of subtype-specific cardiomyocytes (CMs) derived from human induced pluripotent stem cells (iPSCs) and their comprehensive phenotyping is crucial for a better understanding of the subtype-related disease mechanisms and as tools for the development of chamber-specific drugs. The goals of this study were to apply a simple and efficient method for differentiation of iPSCs into defined functional CM subtypes in feeder-free conditions and to obtain a comprehensive understanding of the molecular, cell biological, and functional properties of atrial and ventricular iPSC-CMs on both the single-cell and engineered heart muscle (EHM) level. By a stage-specific activation of retinoic acid signaling in monolayer-based and well-defined culture, we showed that cardiac progenitors can be directed towards a highly homogeneous population of atrial CMs. By combining the transcriptome and proteome profiling of the iPSC-CM subtypes with functional characterizations via optical action potential and calcium imaging, and with contractile analyses in EHM, we demonstrated that atrial and ventricular iPSC-CMs and -EHM highly correspond to the atrial and ventricular heart muscle, respectively. This study provides a comprehensive understanding of the molecular and functional identities characteristic of atrial and ventricular iPSC-CMs and -EHM and supports their suitability in disease modeling and chamber-specific drug screening.

Probing flecainide block of INa using human pluripotent stem cell-derived ventricular cardiomyocytes adapted to automated patch-clamping and 2D monolayers

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) are emerging tools for applications such as drug discovery and screening for pro-arrhythmogenicity and cardiotoxicity as leading causes for drug attrition. Understanding the electrophysiology (EP) of hPSC-CMs is essential but conventional manual patch-clamping is highly laborious and low-throughput. Here we adapted hPSC-CMs derived from two human embryonic stem cell (hESC) lines, HES2 and H7, for a 16-channel automated planar-recording approach for single-cell EP characterization. Automated current- and voltage-clamping, with an overall success rate of 55.0 ± 11.3%, indicated that 90% of hPSC-CMs displayed ventricular-like action potential (AP) and the ventricular cardiomyocytes (VCMs) derived from the two hESC lines expressed similar levels of I_Na, I_CaL, I_kr and I_f and similarly lacked I_to and I_K1. These well-characterized hPSC-VCMs could also be readily adapted for automated assays of pro-arrhythmic drug screening. As an example, we showed that flecainide (FLE) induced I_Na blockade, leftward steady-state inactivation shift, slowed recovery from inactivation in our hPSC-VCMs. Since single-cell EP assay is insufficient to predict drug-induced reentrant arrhythmias, hPSC-VCMs were further reassembled into 2D human ventricular cardiac monolayers (hvCMLs) for multi-cellular electrophysiological assessments. Indeed, FLE significantly slowed the conduction velocity while causing AP prolongation. Our RNA-seq data suggested that cell-cell interaction enhanced the maturity of hPSC-VCMs. Taken collectively, a combinatorial approach using single-cell EP and hvCMLs is needed to comprehensively assess drug-induced arrhythmogenicity.

Human ISL1+ Ventricular Progenitors Self-Assemble into an In Vivo Functional Heart Patch and Preserve Cardiac Function Post Infarction

The generation of human pluripotent stem cell (hPSC)-derived ventricular progenitors and their assembly into a 3-dimensional in vivo functional ventricular heart patch has remained an elusive goal. Herein, we report the generation of an enriched pool of hPSC-derived ventricular progenitors (HVPs), which can expand, differentiate, self-assemble, and mature into a functional ventricular patch in vivo without the aid of any gel or matrix. We documented a specific temporal window, in which the HVPs will engraft in vivo. On day 6 of differentiation, HVPs were enriched by depleting cells positive for pluripotency marker TRA-1-60 with magnetic-activated cell sorting (MACS), and 3 million sorted cells were sub-capsularly transplanted onto kidneys of NSG mice where, after 2 months, they formed a 7 mm × 3 mm × 4 mm myocardial patch resembling the ventricular wall. The graft acquired several features of maturation: expression of ventricular marker (MLC2v), desmosomes, appearance of T-tubule-like structures, and electrophysiological action potential signature consistent with maturation, all this in a non-cardiac environment. We further demonstrated that HVPs transplanted into un-injured hearts of NSG mice remain viable for up to 8 months. Moreover, transplantation of 2 million HVPs largely preserved myocardial contractile function following myocardial infarction. Taken together, our study reaffirms the promising idea of using progenitor cells for regenerative therapy.

Bioengineering an electro-mechanically functional miniature ventricular heart chamber from human pluripotent stem cells

Tissue engineers and stem cell biologists have made exciting progress toward creating simplified models of human heart muscles or aligned monolayers to help bridge a longstanding gap between experimental animals and clinical trials. However, no existing human in vitro systems provide the direct measures of cardiac performance as a pump. Here, we developed a next-generation in vitro biomimetic model of pumping human heart chamber, and demonstrated its capability for pharmaceutical testing. From human pluripotent stem cell (hPSC)-derived ventricular cardiomyocytes (hvCM) embedded in collagen-based extracellular matrix hydrogel, we engineered a three-dimensional (3D) electro-mechanically coupled, fluid-ejecting miniature human ventricle-like cardiac organoid chamber (hvCOC). Structural characterization showed organized sarcomeres with myofibrillar microstructures. Transcript and RNA-seq analyses revealed upregulation of key Ca²⁺-handling, ion channel, and cardiac-specific proteins in hvCOC compared to lower-order 2D and 3D cultures of the same constituent cells. Clinically-important, physiologically complex contractile parameters such as ejection fraction, developed pressure, and stroke work, as well as electrophysiological properties including action potential and conduction velocity were measured: hvCOC displayed key molecular and physiological characteristics of the native ventricle, and showed expected mechanical and electrophysiological responses to a range of pharmacological interventions (including positive and negative inotropes). We conclude that such "human-heart-in-a-jar" technology could facilitate the drug discovery process by providing human-specific preclinical data during early stage drug development.