Stiff matrix induces switch to pure β-cardiac myosin heavy chain expression in human ESC-derived cardiomyocytes

Differential impact of substrates on myosin heavy and light chain expression in human stem cell-derived cardiomyocytes at single-cell level

To fully exploit the potential of human pluripotent stem cell-derived cardiomyocytes, ideally they should acquire a mature, adult ventricular-like phenotype. Predominant expression of the β-isoform of myosin heavy chain (β-MyHC) and the ventricular isoform of myosin regulatory light chain 2 (MLC2v) is a marker of human adult cardiac ventricle. Yet predominant co-expression of these isoforms is rarely reported by current culture protocols. Here, we assessed the impact of different substrates on β-MyHC and MLC2v expression in single human embryonic stem cell-derived CMs (hESC-CMs). As substrates, surface materials with differing stiffness as defined by Young's modulus were combined with either laminin, a single-component coating, or Matrigel, a multi-component coating including growth factors. Semi-quantitative single-cell immunofluorescence analysis demonstrated that surfaces with supraphysiological stiffness in combination with laminin are sufficient for promotion of predominant β-MyHC expression, but not for predominant MLC2v expression in hESC-CMs. Accordingly, mechanical stimuli likely promote expression of β-MyHC in these cultures. Culture on matrices with a lower stiffness than glass in combination with growth factor-containing Matrigel led to only moderate increases in MLC2v expression, possibly more dependent on growth factors, suggesting different regulation of expression. Integrin-related downstream signal transducers, integrin-linked and cardiac troponin I-interacting kinase, as well as modulation of intracellular Ca2+-concentration and epigenetic signaling did not affect MyHC/MLC2 isoform expression. The data indicate that expression of adult ventricular markers β-MyHC and MLC2v depends on different stimuli like substrate stiffness and growth factors. To conclude, multiple stimuli appear to be necessary to promote an adult ventricular phenotype.

Patient-specific hiPSC-derived cardiomyocytes indicate allelic and contractile imbalance as pathogenic factor in early-stage Hypertrophic Cardiomyopathy

Hypertrophic Cardiomyopathy (HCM) is often caused by heterozygous mutations in β-myosin heavy chain (MYH7, β-MyHC). In addition to hyper- or hypocontractile effects of HCM-mutations, heterogeneity in contractile function (contractile imbalance) among individual cardiomyocytes was observed in end-stage HCM-myocardium. Contractile imbalance might be induced by burst-like transcription, leading to unequal fractions of mutant versus wildtype mRNA and protein in individual cardiomyocytes (allelic imbalance). Until now it is not known if allelic and contractile imbalance are present early in HCM-development or rather occur in response to disease-associated remodeling. To address this question, we used patient-specific human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) with heterozygous MYH7-mutations R723G and G741R as models of early-stage HCM without secondary adaptions upon disease progression. R723G-hiPSC-CMs showed typical HCM-markers like hypertrophy and myofibrillar disarray. Using RNA-FISH and allele-specific single-cell-PCR, we show for both cell lines that MYH7 is transcribed in bursts. Highly variable mutant vs. wildtype MYH7-mRNA fractions in individual HCM-hiPSC-CMs indicated allelic imbalance. HCM-hiPSC-CM-lines showed functional alterations like slowed twitch contraction kinetics and reduced calcium sensitivity of myofibrillar force generation. A significantly larger variability in force generation or twitch parameters of individual HCM-hiPSC-CMs compared to WT-hiPSC-CMs indicated contractile imbalance. Our results with early-stage hiPSC-CMs strongly suggest that burst-like transcription and allelic imbalance are general features of CMs, which together with mutation-induced changes of sarcomere contraction could induce contractile imbalance in heterozygous CMs, presumably aggravating development of HCM. Genetic or epigenetic approaches targeting functional heterogeneity in HCM could lead to promising future therapies, in addition to myosin modulation.

Protein-free media for cardiac differentiation of hPSCs in 2000 mL suspension culture

BACKGROUND

Commonly used media for the differentiation of human pluripotent stem cells into cardiomyocytes (hPSC-CMs) contain high concentrations of proteins, in particular albumin, which is prone to quality variations and presents a substantial cost factor, hampering the clinical translation of in vitro-generated cardiomyocytes for heart repair. To overcome these limitations, we have developed chemically defined, entirely protein-free media based on RPMI, supplemented with L-ascorbic acid 2-phosphate (AA-2P) and either the non-ionic surfactant Pluronic F-68 or a specific polyvinyl alcohol (PVA).

METHODS AND RESULTS

Both media compositions enable the efficient, directed differentiation of embryonic and induced hPSCs, matching the cell yields and cardiomyocyte purity ranging from 85 to 99% achieved with the widely used protein-based CDM3 medium. The protein-free differentiation approach was readily up-scaled to a 2000 mL process scale in a fully controlled stirred tank bioreactor in suspension culture, producing > 1.3 × 109 cardiomyocytes in a single process run. Transcriptome analysis, flow cytometry, electrophysiology, and contractile force measurements revealed that the mass-produced cardiomyocytes differentiated in protein-free medium exhibit the expected ventricular-like properties equivalent to the well-established characteristics of CDM3-control cells.

CONCLUSIONS

This study promotes the robustness and upscaling of the cardiomyogenic differentiation process, substantially reduces media costs, and provides an important step toward the clinical translation of hPSC-CMs for heart regeneration.

Noninvasive analysis of contractility during identical maturations revealed two phenotypes in ventricular but not in atrial iPSC-CM

Patient-derived induced pluripotent stem cells (iPSCs) can be differentiated into atrial and ventricular cardiomyocytes to allow for personalized drug screening. A hallmark of differentiation is the manifestation of spontaneous beating in a two-dimensional (2-D) cell culture. However, an outstanding observation is the high variability in this maturation process. We valued that contractile parameters change during differentiation serving as an indicator of maturation. Consequently, we recorded noninvasively spontaneous motion activity during the differentiation of male iPSC toward iPSC cardiomyocytes (iPSC-CMs) to further analyze similar maturated iPSC-CMs. Surprisingly, our results show that identical differentiations into ventricular iPSC-CMs are variable with respect to contractile parameters resulting in two distinct subpopulations of ventricular-like cells. In contrast, differentiation into atrial iPSC-CMs resulted in only one phenotype. We propose that the noninvasive and cost-effective recording of contractile activity during maturation using a smartphone device may help to reduce the variability in results frequently reported in studies on ventricular iPSC-CMs.NEW & NOTEWORTHY Differentiation of induced pluripotent stem cells (iPSCs) into iPSC-derived cardiomyocytes (iPSC-CMs) exhibits a high variability in mature parameters. Here, we monitored noninvasively contractile parameters of iPSC-CM during full-time differentiation using a smartphone device. Our results show that parallel maturations of iPSCs into ventricular iPSC-CMs, but not into atrial iPSC-CMs, resulted in two distinct subpopulations of iPSC-CMs. These findings suggest that our cost-effective method may help to compare iPSC-CMs at the same maturation level.

Rethinking replating

JGP study (In this issue, Osten et al. https://doi.org/10.1085/jgp.202313377) suggests that, by altering mechanosensitive signaling pathways, replating stem cell-derived cardiomyocytes changes myosin expression and contractile function.

Myosin expression and contractile function are altered by replating stem cell-derived cardiomyocytes

Myosin heavy chain (MyHC) is the main determinant of contractile function. Human ventricular cardiomyocytes (CMs) predominantly express the β-isoform. We previously demonstrated that ∼80% of human embryonic stem cell-derived cardiomyocytes (hESC-CMs) express exclusively β-MyHC after long-term culture on laminin-coated glass coverslips. Here, we investigated the impact of enzymatically detaching hESC-CMs after long-term culture and subsequently replating them for characterization of cellular function. We observed that force-related kinetic parameters, as measured in a micromechanical setup, resembled α- rather than β-MyHC-expressing myofibrils, as well as changes in calcium transients. Single-cell immunofluorescence analysis revealed that replating hESC-CMs led to rapid upregulation of α-MyHC, as indicated by increases in exclusively α-MyHC- and in mixed α/β-MyHC-expressing hESC-CMs. A comparable increase in heterogeneity of MyHC isoform expression was also found among individual human induced pluripotent stem cell (hiPSC)-derived CMs after replating. Changes in MyHC isoform expression and cardiomyocyte function induced by replating were reversible in the course of the second week after replating. Gene enrichment analysis based on RNA-sequencing data revealed changes in the expression profile of mechanosensation/-transduction-related genes and pathways, especially integrin-associated signaling. Accordingly, the integrin downstream mediator focal adhesion kinase (FAK) promoted β-MyHC expression on a stiff matrix, further validating gene enrichment analysis. To conclude, detachment and replating induced substantial changes in gene expression, MyHC isoform composition, and function of long-term cultivated human stem cell-derived CMs, thus inducing alterations in mechanosensation/-transduction, that need to be considered, particularly for downstream in vitro assays.

Multi-scale models reveal hypertrophic cardiomyopathy MYH7 G256E mutation drives hypercontractility and elevated mitochondrial respiration

Rationale

Over 200 mutations in the sarcomeric protein β-myosin heavy chain (MYH7) have been linked to hypertrophic cardiomyopathy (HCM). However, different mutations in MYH7 lead to variable penetrance and clinical severity, and alter myosin function to varying degrees, making it difficult to determine genotype-phenotype relationships, especially when caused by rare gene variants such as the G256E mutation.

Objective

This study aims to determine the effects of low penetrant MYH7 G256E mutation on myosin function. We hypothesize that the G256E mutation would alter myosin function, precipitating compensatory responses in cellular functions.

Methods

We developed a collaborative pipeline to characterize myosin function at multiple scales (protein to myofibril to cell to tissue). We also used our previously published data on other mutations to compare the degree to which myosin function was altered.

Results

At the protein level, the G256E mutation disrupts the transducer region of the S1 head and reduces the fraction of myosin in the folded-back state by 50.9%, suggesting more myosins available for contraction. Myofibrils isolated from hiPSC-CMs CRISPR-edited with G256E (MYH7 WT/G256E ) generated greater tension, had faster tension development and slower early phase relaxation, suggesting altered myosin-actin crossbridge cycling kinetics. This hypercontractile phenotype persisted in single-cell hiPSC-CMs and engineered heart tissues. Single-cell transcriptomic and metabolic profiling demonstrated upregulation of mitochondrial genes and increased mitochondrial respiration, suggesting altered bioenergetics as an early feature of HCM.

Conclusions

MYH7 G256E mutation causes structural instability in the transducer region, leading to hypercontractility across scales, perhaps from increased myosin recruitment and altered crossbridge cycling. Hypercontractile function of the mutant myosin was accompanied by increased mitochondrial respiration, while cellular hypertrophy was modest in the physiological stiffness environment. We believe that this multi-scale platform will be useful to elucidate genotype-phenotype relationships underlying other genetic cardiovascular diseases.

Gene-Edited Human-Induced Pluripotent Stem Cell Lines to Elucidate DAND5 Function throughout Cardiac Differentiation

(1) Background: The contribution of gene-specific variants for congenital heart disease, one of the most common congenital disabilities, is still far from our complete understanding. Here, we applied a disease model using human-induced pluripotent stem cells (hiPSCs) to evaluate the function of DAND5 on human cardiomyocyte (CM) differentiation and proliferation. (2) Methods: Taking advantage of our DAND5 patient-derived iPSC line, we used CRISPR-Cas9 gene-editing to generate a set of isogenic hiPSCs (DAND5-corrected and DAND5 full-mutant). The hiPSCs were differentiated into CMs, and RT-qPCR and immunofluorescence profiled the expression of cardiac markers. Cardiomyocyte proliferation was analysed by flow cytometry. Furthermore, we used a multi-electrode array (MEA) to study the functional electrophysiology of DAND5 hiPSC-CMs. (3) Results: The results indicated that hiPSC-CM proliferation is affected by DAND5 levels. Cardiomyocytes derived from a DAND5 full-mutant hiPSC line are more proliferative when compared with gene-corrected hiPSC-CMs. Moreover, parallel cardiac differentiations showed a differential cardiac gene expression profile, with upregulated cardiac progenitor markers in DAND5-KO hiPSC-CMs. Microelectrode array (MEA) measurements demonstrated that DAND5-KO hiPSC-CMs showed prolonged field potential duration and increased spontaneous beating rates. In addition, conduction velocity is reduced in the monolayers of hiPSC-CMs with full-mutant genotype. (4) Conclusions: The absence of DAND5 sustains the proliferation of hiPSC-CMs, which alters their electrophysiological maturation properties. These results using DAND5 hiPSC-CMs consolidate the findings of the in vitro and in vivo mouse models, now in a translational perspective. Altogether, the data will help elucidate the molecular mechanism underlying this human heart disease and potentiates new therapies for treating adult CHD.

Bioinspired two-in-one nanotransistor sensor for the simultaneous measurements of electrical and mechanical cellular responses

The excitation-contraction dynamics in cardiac tissue are the most important physiological parameters for assessing developmental state. We demonstrate integrated nanoelectronic sensors capable of simultaneously probing electrical and mechanical cellular responses. The sensor is configured from a three-dimensional nanotransistor with its conduction channel protruding out of the plane. The structure promotes not only a tight seal with the cell for detecting action potential via field effect but also a close mechanical coupling for detecting cellular force via piezoresistive effect. Arrays of nanotransistors are integrated to realize label-free, submillisecond, and scalable interrogation of correlated cell dynamics, showing advantages in tracking and differentiating cell states in drug studies. The sensor can further decode vector information in cellular motion beyond typical scalar information acquired at the tissue level, hence offering an improved tool for cell mechanics studies. The sensor enables not only improved bioelectronic detections but also reduced invasiveness through the two-in-one converging integration.

Cardiomyocyte Maturation-the Road is not Obstructed

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) represent one of the most promising ways to treat cardiovascular diseases. High-purity cardiomyocytes (CM) from different cell sources could be obtained at present. However, the immature nature of these cardiomyocytes hinders its further clinical application. From immature to mature state, it involves structural, functional, and metabolic changes in cardiomyocytes. Generally, two types of culturing (2D and 3D) systems have been reported to induce cardiomyocyte maturation. 2D culture mainly achieves the maturation of cardiomyocytes through long-term culture, co-culture, supplementation of small molecule compounds, and the application of biophysical cues. The combined use of biomaterial's surface topography and biophysical cues also facilitates the maturation of cardiomyocytes. Cardiomyocyte maturation is a complex process involving many signaling pathways, and current methods fail to fully reproduce this process. Therefore, analyzing the signaling pathway network related to the maturation and producing hPSC-CMs with adult-like phenotype is a challenge. In this review, we summarized the structural and functional differences between hPSC-CMs and mature cardiomyocytes, and introduced various methods to induce cardiomyocyte maturation.

Bioelectricity-coupling patches for repairing impaired myocardium

Cardiac abnormalities, which account for extensive burdens on public health and economy, drive necessary attempts to revolutionize the traditional therapeutic system. Advances in cardiac tissue engineering have expanded a highly efficacious platform to address cardiovascular events, especially cardiac infarction. Current efforts to overcome biocompatible limitations highlight the constructs of a conductive cardiac patch to accelerate the industrial and clinical landscape that is amenable for patient-accurate therapy, regenerative medicine, disease modeling, and drug delivery. With the notion that cardiac tissue synchronically contracts triggered by electrical pulses, the cardiac patches based on conductive materials are developed and treated on the dysfunctional heart. In this review, we systematically summarize distinct conductive materials serving as the most promising alternatives (conductive nanomaterials, conductive polymers, piezoelectric polymers, and ionic electrolytes) to achieve electric signal transmission and engineered cardiac tissues. Existing applications are discussed considering how these patches containing conductive candidates are fabricated into diverse forms with major strategies. Ultimately, we try to define a new concept as a bioelectricity-coupling patch that provides a favorable cardiac micro-environment for cardiac functional activities. Underlying challenges and prospects are presented regarding industrial processing and cardiovascular treatment of conductive patch progress. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Cardiovascular Disease.

The harder the climb the better the view: The impact of substrate stiffness on cardiomyocyte fate

The quest for novel methods to mature human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) for cardiac regeneration, modelling and drug testing has emphasized a need to create microenvironments with physiological features. Many studies have reported on how cardiomyocytes sense substrate stiffness and adapt their morphological and functional properties. However, these observations have raised new biological questions and a shared vision to translate it into a tissue or organ context is still elusive. In this review, we will focus on the relevance of substrates mimicking cardiac extracellular matrix (cECM) rigidity for the understanding of the biomechanical crosstalk between the extracellular and intracellular environment. The ability to opportunely modulate these pathways could be a key to regulate in vitro hiPSC-CM maturation. Therefore, both hiPSC-CM models and substrate stiffness appear as intriguing tools for the investigation of cECM-cell interactions. More understanding of these mechanisms may provide novel insights on how cECM affects cardiac cell function in the context of genetic cardiomyopathies.

How Localized Z-Disc Damage Affects Force Generation and Gene Expression in Cardiomyocytes

The proper function of cardiomyocytes (CMs) is highly related to the Z-disc, which has a pivotal role in orchestrating the sarcomeric cytoskeletal function. To better understand Z-disc related cardiomyopathies, novel models of Z-disc damage have to be developed. Human pluripotent stem cell (hPSC)-derived CMs can serve as an in vitro model to better understand the sarcomeric cytoskeleton. A femtosecond laser system can be applied for localized and defined damage application within cells as single Z-discs can be removed. We have investigated the changes in force generation via traction force microscopy, and in gene expression after Z-disc manipulation in hPSC-derived CMs. We observed a significant weakening of force generation after removal of a Z-disc. However, no significant changes of the number of contractions after manipulation were detected. The stress related gene NF-kB was significantly upregulated. Additionally, α-actinin (ACTN2) and filamin-C (FLNc) were upregulated, pointing to remodeling of the Z-disc and the sarcomeric cytoskeleton. Ultimately, cardiac troponin I (TNNI3) and cardiac muscle troponin T (TNNT2) were significantly downregulated. Our results allow a better understanding of transcriptional coupling of Z-disc damage and the relation of damage to force generation and can therefore finally pave the way to novel therapies of sarcomeric disorders.

Maturation of Stem Cell-Derived Cardiomyocytes: Foe in Translation Medicine

With the in-depth study of heart development, many human cardiomyocytes (CMs) have been generated in a laboratory environment. CMs derived from pluripotent stem cells (PSCs) have been widely used for a series of applications such as laboratory studies, drug toxicology screening, cardiac disease models, and as an unlimited resource for cell-based cardiac regeneration therapy. However, the low maturity of the induced CMs significantly impedes their applicability. Scientists have been committed to improving the maturation of CMs to achieve the purpose of heart regeneration in the past decades. In this review, we take CMs maturation as the main object of discussion, describe the characteristics of CMs maturation, summarize the key regulatory mechanism of regulating maturation and address the approaches to promote CMs maturation. The maturation of CM is gradually improving due to the incorporation of advanced technologies and is expected to continue.

S-nitrosoglutathione reductase (GSNOR) deficiency accelerates cardiomyocyte differentiation of induced pluripotent stem cells

Introduction

Induced pluripotent stem cells (iPSCs) provide a model of cardiomyocyte (CM) maturation. Nitric oxide signaling promotes CM differentiation and maturation, although the mechanisms remain controversial.

Aim

The study tested the hypothesis that in the absence of S-nitrosoglutathione reductase (GSNOR), a denitrosylase regulating protein S-nitrosylation, the resultant increased S-nitrosylation accelerates the differentiation and maturation of iPSC-derived cardiomyocytes (CMs).

Methods and Results

iPSCs derived from mice lacking GSNOR (iPSC^GSNOR-/-) matured faster than wildtype iPSCs (iPSC^WT) and demonstrated transient increases in expression of murine Snail Family Transcriptional Repressor 1 gene (Snail), murine Snail Family Transcriptional Repressor 2 gene (Slug) and murine Twist Family BHLH Transcription Factor 1 gene (Twist), transcription factors that promote epithelial-to-mesenchymal transition (EMT) and that are regulated by Glycogen Synthase Kinase 3 Beta (GSK3β). Murine Glycogen Synthase Kinase 3 Beta (Gsk3β) gene exhibited much greater S-nitrosylation, but lower expression in iPSC^GSNOR-/-. S-nitrosoglutathione (GSNO)-treated iPSC^WT and human (h)iPSCs also demonstrated reduced expression of GSK3β. Nkx2.5 expression, a CM marker, was increased in iPSC^GSNOR-/- upon directed differentiation toward CMs on Day 4, whereas murine Brachyury (t), Isl1, and GATA Binding Protein (Gata4) mRNA were decreased, compared to iPSC^WT, suggesting that GSNOR deficiency promotes CM differentiation beginning immediately following cell adherence to the culture dish-transitioning from mesoderm to cardiac progenitor.

Conclusion

Together these findings suggest that increased S-nitrosylation of Gsk3β promotes CM differentiation and maturation from iPSCs. Manipulating the post-translational modification of GSK3β may provide an important translational target and offers new insight into understanding of CM differentiation from pluripotent stem cells.

One sentence summary

Deficiency of GSNOR or addition of GSNO accelerates early differentiation and maturation of iPSC-cardiomyocytes.

Remdesivir induces persistent mitochondrial and structural damage in human induced pluripotent stem cell derived cardiomyocytes

AIMS

Remdesivir is a prodrug of an adenosine triphosphate analogue and is currently the only drug formally approved for the treatment of hospitalised COVID-19 patients. Nucleoside/nucleotide analogues have been shown to induce mitochondrial damage and cardiotoxicity, and this may be exacerbated by hypoxia, which frequently occurs in severe COVID-19 patients. Although there have been few reports of adverse cardiovascular events associated with remdesivir, clinical data are limited. Here, we investigated whether remdesivir induced cardiotoxicity using an in vitro human cardiac model.

METHODS AND RESULTS

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) were exposed to remdesivir under normoxic and hypoxic conditions to simulate mild and severe COVID-19 respectively. Remdesivir induced mitochondrial fragmentation, reduced redox potential and suppressed mitochondrial respiration at levels below the estimated plasma concentration under both normoxic and hypoxic conditions. Non-mitochondrial damage such as electrophysiological alterations and sarcomere disarray were also observed. Importantly, some of these changes persisted after the cessation of treatment, culminating in increased cell death. Mechanistically, we found that inhibition of DRP1, a regulator of mitochondrial fission, ameliorated the cardiotoxic effects of remdesivir, showing that remdesivir-induced cardiotoxicity was preventable and excessive mitochondrial fission might contribute to this phenotype.

CONCLUSIONS

Using an in vitro model, we demonstrated that remdesivir can induce cardiotoxicity in hiPSC-CMs at clinically relevant concentrations. These results reveal previously unknown potential side-effects of remdesivir and highlight the importance of further investigations with in vivo animal models and active clinical monitoring to prevent lasting cardiac damage to patients.

TRANSLATIONAL PERSPECTIVE

Adult cardiomyocytes have limited ability to regenerate, thus treatment-induced cardiotoxicity can potentially cause irreparable harm. Remdesivir is currently the only FDA approved treatment for COVID-19 but clinical safety data are limited. Using human pluripotent stem cell-derived cardiomyocytes, we revealed that remdesivir induced persistent mitochondrial and structural abnormalities at clinically relevant concentrations. We advise confirmatory experiments in in vivo animal models, investigations of cardioprotective strategies, and closer patient monitoring such that treatment-induced cardiotoxicity does not contribute to the long term sequelae of COVID-19 patients.

Cytotoxicity of snake venom Lys49 PLA2-like myotoxin on rat cardiomyocytes ex vivo does not involve a direct action on the contractile apparatus

Viperid snake venoms contain a unique family of cytotoxic proteins, the Lys49 PLA2 homologs, which are devoid of enzymatic activity but disrupt the integrity of cell membranes. They are known to induce skeletal muscle damage and are therefore named 'myotoxins'. Single intact and skinned (devoid of membranes and cytoplasm but with intact sarcomeric proteins) rat cardiomyocytes were used to analyze the cytotoxic action of a myotoxin, from the venom of Bothrops asper. The toxin induced rapid hypercontraction of intact cardiomyocytes, associated with an increase in the cytosolic concentration of calcium and with cell membrane disruption. Hypercontraction of intact cardiomyocytes was abrogated by the myosin inhibitor para-aminoblebbistatin (AmBleb). No toxin-induced changes of key parameters of force development were observed in skinned cardiomyocytes. Thus, although myosin is a key effector of the observed hypercontraction, a direct effect of the toxin on the sarcomeric proteins -including the actomyosin complex- is not part of the mechanism of cytotoxicity. Owing to the sensitivity of intact cardiomyocytes to the cytotoxic action of myotoxin, this ex vivo model is a valuable tool to explore in further detail the mechanism of action of this group of snake venom toxins.

A comprehensive analysis of gene expression changes in a high replicate and open-source dataset of differentiating hiPSC-derived cardiomyocytes

We performed a comprehensive analysis of the transcriptional changes occurring during human induced pluripotent stem cell (hiPSC) differentiation to cardiomyocytes. Using single cell RNA-seq, we sequenced > 20,000 single cells from 55 independent samples representing two differentiation protocols and multiple hiPSC lines. Samples included experimental replicates ranging from undifferentiated hiPSCs to mixed populations of cells at D90 post-differentiation. Differentiated cell populations clustered by time point, with differential expression analysis revealing markers of cardiomyocyte differentiation and maturation changing from D12 to D90. We next performed a complementary cluster-independent sparse regression analysis to identify and rank genes that best assigned cells to differentiation time points. The two highest ranked genes between D12 and D24 (MYH7 and MYH6) resulted in an accuracy of 0.84, and the three highest ranked genes between D24 and D90 (A2M, H19, IGF2) resulted in an accuracy of 0.94, revealing that low dimensional gene features can identify differentiation or maturation stages in differentiating cardiomyocytes. Expression levels of select genes were validated using RNA FISH. Finally, we interrogated differences in cardiac gene expression resulting from two differentiation protocols, experimental replicates, and three hiPSC lines in the WTC-11 background to identify sources of variation across these experimental variables.

Myosin-18B Regulates Higher-Order Organization of the Cardiac Sarcomere through Thin Filament Cross-Linking and Thick Filament Dynamics

MYO18B loss-of-function mutations and depletion significantly compromise the structural integrity of striated muscle sarcomeres. The molecular function of the encoded protein, myosin-18B (M18B), within the developing muscle is unknown. Here, we demonstrate that recombinant M18B lacks motor ATPase activity and harbors previously uncharacterized N-terminal actin-binding domains, properties that make M18B an efficient actin cross-linker and molecular brake capable of regulating muscle myosin-2 contractile forces. Spatiotemporal analysis of M18B throughout cardiomyogenesis and myofibrillogenesis reveals that this structural myosin undergoes nuclear-cytoplasmic redistribution during myogenic differentiation, where its incorporation within muscle stress fibers coincides with actin striation onset. Furthermore, this analysis shows that M18B is directly integrated within the muscle myosin thick filament during myofibril maturation. Altogether, our data suggest that M18B has evolved specific biochemical properties that allow it to define and maintain sarcomeric organization from within the thick filament via its dual actin cross-linking and motor modulating capabilities.

Maturation of human pluripotent stem cell derived cardiomyocytes in vitro and in vivo

Human pluripotent stem cell derived cardiomyocytes (hPSC-CMs) represent an inexhaustible cell source for in vitro disease modeling, drug discovery and toxicity screening, and potential therapeutic applications. However, currently available differentiation protocols yield populations of hPSC-CMs with an immature phenotype similar to cardiomyocytes in the early fetal heart. In this review, we consider the developmental processes and signaling cues involved in normal human cardiac maturation, as well as how these insights might be applied to the specific maturation of hPSC-CMs. We summarize the state-of-the-art and relative merits of reported hPSC-CM maturation strategies including prolonged duration in culture, metabolic manipulation, treatment with soluble or substrate-based cues, and tissue engineering approaches. Finally, we review the evidence that hPSC-CMs mature after implantation in injured hearts as such in vivo remodeling will likely affect the safety and efficacy of a potential hPSC-based cardiac therapy.

Cell states beyond transcriptomics: Integrating structural organization and gene expression in hiPSC-derived cardiomyocytes

Although some cell types may be defined anatomically or by physiological function, a rigorous definition of cell state remains elusive. Here, we develop a quantitative, imaging-based platform for the systematic and automated classification of subcellular organization in single cells. We use this platform to quantify subcellular organization and gene expression in >30,000 individual human induced pluripotent stem cell-derived cardiomyocytes, producing a publicly available dataset that describes the population distributions of local and global sarcomere organization, mRNA abundance, and correlations between these traits. While the mRNA abundance of some phenotypically important genes correlates with subcellular organization (e.g., the beta-myosin heavy chain, MYH7), these two cellular metrics are heterogeneous and often uncorrelated, which suggests that gene expression alone is not sufficient to classify cell states. Instead, we posit that cell state should be defined by observing full distributions of quantitative, multidimensional traits in single cells that also account for space, time, and function.

Maturation of Pluripotent Stem Cell-Derived Cardiomyocytes Enables Modeling of Human Hypertrophic Cardiomyopathy

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are a powerful platform for biomedical research. However, they are immature, which is a barrier to modeling adult-onset cardiovascular disease. Here, we sought to develop a simple method that could drive cultured hiPSC-CMs toward maturity across a number of phenotypes, with the aim of utilizing mature hiPSC-CMs to model human cardiovascular disease. hiPSC-CMs were cultured in fatty acid-based medium and plated on micropatterned surfaces. These cells display many characteristics of adult human cardiomyocytes, including elongated cell morphology, sarcomeric maturity, and increased myofibril contractile force. In addition, mature hiPSC-CMs develop pathological hypertrophy, with associated myofibril relaxation defects, in response to either a pro-hypertrophic agent or genetic mutations. The more mature hiPSC-CMs produced by these methods could serve as a useful in vitro platform for characterizing cardiovascular disease.

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Standard (glucose) cultured hiPSC-CMs demonstrate a blunted hypertrophic response

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A maturation method induces hiPSC-CM maturation and suppresses HIF1A expression

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Mature hiPSC-CMs demonstrate improved sarcomeric morphology and contractility

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Mature hiPSC-CMs respond to agonist- or mutation-induced hypertrophy

Generation of two iPSC clones (MHHi021-A and MHHi021-B) from a patient with hypertrophic cardiomyopathy with p.Arg723Gly mutation in the MYH7 gene

Hypertrophic cardiomyopathy (HCM) is the most common form of genetic heart disease and is characterized by abnormal thickening of the left ventricular wall and interventricular septum. Here we describe the generation of two induced pluripotent stem cell (iPSC) clones from a HCM patient, heterozygous for the p.Arg723Gly (c.2169C > G) mutation in the MYH7 gene. The generated iPSC clones may provide a useful resource for disease modelling to study the mechanisms underlying HCM pathogenesis in iPSC derived progenies, in particular cardiomyocytes.

Strategies and Challenges to Improve Cellular Programming-Based Approaches for Heart Regeneration Therapy

Limited adult cardiac cell proliferation after cardiovascular disease, such as heart failure, hampers regeneration, resulting in a major loss of cardiomyocytes (CMs) at the site of injury. Recent studies in cellular reprogramming approaches have provided the opportunity to improve upon previous techniques used to regenerate damaged heart. Using these approaches, new CMs can be regenerated from differentiation of iPSCs (similar to embryonic stem cells), the direct reprogramming of fibroblasts [induced cardiomyocytes (iCMs)], or induced cardiac progenitors. Although these CMs have been shown to functionally repair infarcted heart, advancements in technology are still in the early stages of development in research laboratories. In this review, reprogramming-based approaches for generating CMs are briefly introduced and reviewed, and the challenges (including low efficiency, functional maturity, and safety issues) that hinder further translation of these approaches into a clinical setting are discussed. The creative and combined optimal methods to address these challenges are also summarized, with optimism that further investigation into tissue engineering, cardiac development signaling, and epigenetic mechanisms will help to establish methods that improve cell-reprogramming approaches for heart regeneration.

Optimizing the Use of iPSC-CMs for Cardiac Regeneration in Animal Models

Simple Summary

In 2006, the first induced pluripotent stem cells were generated by reprogramming skin cells. Induced pluripotent stem cells undergo fast cell division, can differentiate into many different cell types, can be patient-specific, and do not raise ethical issues. Thus, they offer great promise as in vitro disease models, drug toxicity testing platforms, and for autologous tissue regeneration. Heart failure is one of the major causes of death worldwide. It occurs when the heart cannot meet the body’s metabolic demands. Induced pluripotent stem cells can be differentiated into cardiac myocytes, can form patches resembling native cardiac tissue, and can engraft to the damaged heart. However, despite correct host/graft coupling, most animal studies demonstrate an arrhythmogenicity of the engrafted tissue and variable survival. This is partially because of the heterogeneity and immaturity of the cells. New evidence suggests that by modulating induced pluripotent stem cells-cardiac myocytes (iPSC-CM) metabolism by switching substrates and changing metabolic pathways, you can decrease iPSC-CM heterogeneity and arrhythmogenicity. Novel culture methods and tissue engineering along with animal models of heart failure are needed to fully unlock the potential of cardiac myocytes derived from induced pluripotent stem cells for cardiac regeneration.

Abstract

Heart failure (HF) is a common disease in which the heart cannot meet the metabolic demands of the body. It mostly occurs in individuals 65 years or older. Cardiac transplantation is the best option for patients with advanced HF. High numbers of patient-specific cardiac myocytes (CMs) can be generated from induced pluripotent stem cells (iPSCs) and can possibly be used to treat HF. While some studies found iPSC-CMS can couple efficiently to the damaged heart and restore cardiac contractility, almost all found iPSC-CM transplantation is arrhythmogenic, thus hampering the use of iPSC-CMs for cardiac regeneration. Studies show that iPSC-CM cultures are highly heterogeneous containing atrial-, ventricular- and nodal-like CMs. Furthermore, they have an immature phenotype, resembling more fetal than adult CMs. There is an urgent need to overcome these issues. To this end, a novel and interesting avenue to increase CM maturation consists of modulating their metabolism. Combined with careful engineering and animal models of HF, iPSC-CMs can be assessed for their potential for cardiac regeneration and a cure for HF.

Contractility of Induced Pluripotent Stem Cell-Cardiomyocytes With an MYH6 Head Domain Variant Associated With Hypoplastic Left Heart Syndrome

Hypoplastic left heart syndrome (HLHS) is a clinically and anatomically severe form of congenital heart disease; however, its etiology remains largely unknown. We previously demonstrated that genetic variants in the MYH6 gene are significantly associated with HLHS. Additionally, induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from an HLHS-affected family trio (affected parent, unaffected parent, affected proband) carrying an MYH6-R443P head domain variant demonstrated dysmorphic sarcomere structure and increased compensatory MYH7 expression. Analysis of iPSC-CMs derived from the HLHS trio revealed that only beta myosin heavy chain expression was observed in CMs carrying the MYH6-R443P variant after differentiation day 15 (D15). Functional assessments performed between D20-D23 revealed that MYH6-R443P variant CMs contracted more slowly (40 ± 2 vs. 47 ± 2 contractions/min, P < 0.05), shortened less (5.6 ± 0.5 vs. 8.1 ± 0.7% of cell length, P < 0.05), and exhibited slower shortening rates (19.9 ± 1.7 vs. 28.1 ± 2.5 μm/s, P < 0.05) and relaxation rates (11.0 ± 0.9 vs. 19.7 ± 2.0 μm/s, P < 0.05). Treatment with isoproterenol had no effect on iPSC-CM mechanics. Using CRISPR/Cas9 gene editing technology, introduction of the R443P variant into the unaffected parent's iPSCs recapitulated the phenotype of the proband's iPSC-CMs, and conversely, correction of the R443P variant in the proband's iPSCs rescued the cardiomyogenic differentiation, sarcomere organization, slower contraction (P < 0.05) and decreased velocity phenotypes (P < 0.0001). This is the first report to identify that cardiac tissues from HLHS patients with MYH6 variants can exhibit sarcomere disorganization in atrial but not ventricular tissues. This new discovery was not unexpected, since MYH6 is expressed predominantly in the postnatal atria in humans. These findings demonstrate the feasibility of employing patient-derived iPSC-CMs, in combination with patient cardiac tissues, to gain mechanistic insight into how genetic variants can lead to HLHS. Results from this study suggest that decreased contractility of CMs due to sarcomere disorganization in the atria may effect hemodynamic changes preventing development of a normal left ventricle.

Advances in Stem Cell Modeling of Dystrophin-Associated Disease: Implications for the Wider World of Dilated Cardiomyopathy

Familial dilated cardiomyopathy (DCM) is mostly caused by mutations in genes encoding cytoskeletal and sarcomeric proteins. In the pediatric population, DCM is the predominant type of primitive myocardial disease. A severe form of DCM is associated with mutations in the DMD gene encoding dystrophin, which are the cause of Duchenne Muscular Dystrophy (DMD). DMD-associated cardiomyopathy is still poorly understood and orphan of a specific therapy. In the last 5 years, a rise of interest in disease models using human induced pluripotent stem cells (hiPSCs) has led to more than 50 original studies on DCM models. In this review paper, we provide a comprehensive overview on the advances in DMD cardiomyopathy disease modeling and highlight the most remarkable findings obtained from cardiomyocytes differentiated from hiPSCs of DMD patients. We will also describe how hiPSCs based studies have contributed to the identification of specific myocardial disease mechanisms that may be relevant in the pathogenesis of DCM, representing novel potential therapeutic targets.

Modeling Cardiovascular Diseases with hiPSC-Derived Cardiomyocytes in 2D and 3D Cultures

In the last decade, the generation of cardiac disease models based on human-induced pluripotent stem cells (hiPSCs) has become of common use, providing new opportunities to overcome the lack of appropriate cardiac models. Although much progress has been made toward the generation of hiPSC-derived cardiomyocytes (hiPS-CMs), several lines of evidence indicate that two-dimensional (2D) cell culturing presents significant limitations, including hiPS-CMs immaturity and the absence of interaction between different cell types and the extracellular matrix. More recently, new advances in bioengineering and co-culture systems have allowed the generation of three-dimensional (3D) constructs based on hiPSC-derived cells. Within these systems, biochemical and physical stimuli influence the maturation of hiPS-CMs, which can show structural and functional properties more similar to those present in adult cardiomyocytes. In this review, we describe the latest advances in 2D- and 3D-hiPSC technology for cardiac disease mechanisms investigation, drug development, and therapeutic studies.

Advanced Single-Cell Mapping Reveals that in hESC Cardiomyocytes Contraction Kinetics and Action Potential Are Independent of Myosin Isoform

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) represent an attractive model to investigate CM function and disease mechanisms. One characteristic marker of ventricular specificity of human CMs is expression of the ventricular, slow β-myosin heavy chain (MyHC), as opposed to the atrial, fast α-MyHC. The main aim of this study was to investigate at the single-cell level whether contraction kinetics and electrical activity of hESC-CMs are influenced by the relative expression of α-MyHC versus β-MyHC. For effective assignment of functional parameters to the expression of both MyHC isoforms at protein and mRNA levels in the very same hESC-CMs, we developed a single-cell mapping technique. Surprisingly, α- versus β-MyHC was not related to specific contractile or electrophysiological properties of the same cells. The multiparametric cell-by-cell analysis suggests that in hESC-CMs the expression of genes associated with electrical activity, contraction, calcium handling, and MyHCs is independently regulated.

Cardiomyocyte maturation: advances in knowledge and implications for regenerative medicine

Our knowledge of pluripotent stem cell (PSC) biology has advanced to the point where we now can generate most cells of the human body in the laboratory. PSC-derived cardiomyocytes can be generated routinely with high yield and purity for disease research and drug development, and these cells are now gradually entering the clinical research phase for the testing of heart regeneration therapies. However, a major hurdle for their applications is the immature state of these cardiomyocytes. In this Review, we describe the structural and functional properties of cardiomyocytes and present the current approaches to mature PSC-derived cardiomyocytes. To date, the greatest success in maturation of PSC-derived cardiomyocytes has been with transplantation into the heart in animal models and the engineering of 3D heart tissues with electromechanical conditioning. In conventional 2D cell culture, biophysical stimuli such as mechanical loading, electrical stimulation and nanotopology cues all induce substantial maturation, particularly of the contractile cytoskeleton. Metabolism has emerged as a potent means to control maturation with unexpected effects on electrical and mechanical function. Different interventions induce distinct facets of maturation, suggesting that activating multiple signalling networks might lead to increased maturation. Despite considerable progress, we are still far from being able to generate PSC-derived cardiomyocytes with adult-like phenotypes in vitro. Future progress will come from identifying the developmental drivers of maturation and leveraging them to create more mature cardiomyocytes for research and regenerative medicine.

Preclinical development of a miR-132 inhibitor for heart failure treatment

Despite proven efficacy of pharmacotherapies targeting primarily global neurohormonal dysregulation, heart failure (HF) is a growing pandemic with increasing burden. Treatments mechanistically focusing at the cardiomyocyte level are lacking. MicroRNAs (miRNA) are transcriptional regulators and essential drivers of disease progression. We previously demonstrated that miR-132 is both necessary and sufficient to drive the pathological cardiomyocytes growth, a hallmark of adverse cardiac remodelling. Therefore, miR-132 may serve as a target for HF therapy. Here we report further mechanistic insight of the mode of action and translational evidence for an optimized, synthetic locked nucleic acid antisense oligonucleotide inhibitor (antimiR-132). We reveal the compound’s therapeutic efficacy in various models, including a clinically highly relevant pig model of HF. We demonstrate favourable pharmacokinetics, safety, tolerability, dose-dependent PK/PD relationships and high clinical potential for the antimiR-132 treatment scheme.

miR-132 was shown to drive pathological cardiac remodeling, a hallmark of heart failure. Here, the authors show that an antisense inhibitor of miR-132 has favourable pharmacokinetics, safety-tolerability and preclinical efficacy in mouse and porcine models of heart failure.

Engineered Maturation Approaches of Human Pluripotent Stem Cell-Derived Ventricular Cardiomyocytes

Heart diseases such as myocardial infarction and myocardial ischemia are paroxysmal and fatal in clinical practice. Cardiomyocytes (CMs) differentiated from human pluripotent stem cells provide a promising approach to myocardium regeneration therapy. Identifying the maturity level of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) is currently the main challenge for pathophysiology and therapeutics. In this review, we describe current maturity indicators for cardiac microtissue and microdevice cultivation technologies that accelerate cardiac maturation. It may provide insights into regenerative medicine, drug cardiotoxicity testing, and preclinical safety testing.

Stem Cells in Cardiovascular Medicine: Historical Overview and Future Prospects

Cardiovascular diseases remain the leading cause of death in the developed world, accounting for more than 30% of all deaths. In a large proportion of these patients, acute myocardial infarction is usually the first manifestation, which might further progress to heart failure. In addition, the human heart displays a low regenerative capacity, leading to a loss of cardiomyocytes and persistent tissue scaring, which entails a morbid pathologic sequela. Novel therapeutic approaches are urgently needed. Stem cells, such as induced pluripotent stem cells or embryonic stem cells, exhibit great potential for cell-replacement therapy and an excellent tool for disease modeling, as well as pharmaceutical screening of novel drugs and their cardiac side effects. This review article covers not only the origin of stem cells but tries to summarize their translational potential, as well as potential risks and clinical translation.

A Microwell Cell Capture Device Reveals Variable Response to Dobutamine in Isolated Cardiomyocytes

Isolated ventricular cardiomyocytes exhibit substantial cell-to-cell variability, even when obtained from the same small volume of myocardium. In this study, we investigated the possibility that cardiomyocyte responses to β-adrenergic stimulus are also highly heterogeneous. To achieve the throughput and measurement duration desired for these experiments, we designed and validated a novel microwell system that immobilizes and uniformly orients isolated adult cardiomyocytes. In this configuration, detailed drug responses of dozens of cells can be followed for intervals exceeding 1 h. At the conclusion of an experiment, specific cells can also be harvested via a precision aspirator for single-cell gene expression profiling. Using this system, we followed changes in Ca²⁺ signaling and contractility of individual cells under sustained application of either dobutamine or omecamtiv mecarbil. Both compounds increased average cardiomyocyte contractility over the course of an hour, but responses of individual cells to dobutamine were significantly more variable. Surprisingly, some dobutamine-treated cardiomyocytes augmented Ca²⁺ release without increasing contractility. Other cells responded with increased contractility despite unchanged Ca²⁺ release. Single-cell gene expression analysis revealed significant co-expression of β-adrenergic pathway genes PKA regulatory subunit type I, PKA regulatory subunit type II, and Ca²⁺/calmodulin-dependent protein kinase II across cardiomyocytes. Other data supported a connection between the effects of dobutamine on relaxation rate and the expression of protein phosphatase 2. These findings suggest that variable drug responses among cells are not merely experimental artifacts. By enabling direct comparison of the functional behavior of an individual cell and the genes it expresses, this new system constitutes a unique tool for interrogating cardiomyocyte drug responses and discovering the genes that modulate them.

Comparing human iPSC-cardiomyocytes versus HEK293T cells unveils disease-causing effects of Brugada mutation A735V of NaV1.5 sodium channels

Loss-of-function mutations of the SCN5A gene encoding for the sodium channel α-subunit NaV1.5 result in the autosomal dominant hereditary disease Brugada Syndrome (BrS) with a high risk of sudden cardiac death in the adult. We here engineered human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) carrying the CRISPR/Cas9 introduced BrS-mutation p.A735V-NaV1.5 (g.2204C > T in exon 14 of SCN5A) as a novel model independent of patient´s genetic background. Recent studies raised concern regarding the use of hiPSC-CMs for studying adult-onset hereditary diseases due to cells' immature phenotype. To tackle this concern, long-term cultivation of hiPSC-CMs on a stiff matrix (27-42 days) was applied to promote maturation. Patch clamp recordings of A735V mutated hiPSC-CMs revealed a substantially reduced upstroke velocity and sodium current density, a prominent rightward shift of the steady state activation curve and decelerated recovery from inactivation as compared to isogenic hiPSC-CMs controls. These observations were substantiated by a comparative study on mutant A735V-NaV1.5 channels heterologously expressed in HEK293T cells. In contrast to mutated hiPSC-CMs, a leftward shift of sodium channel inactivation was not observed in HEK293T, emphasizing the importance of investigating mechanisms of BrS in independent systems. Overall, our approach supports hiPSC-CMs' relevance for investigating channelopathies in a dish.

Continuous WNT Control Enables Advanced hPSC Cardiac Processing and Prognostic Surface Marker Identification in Chemically Defined Suspension Culture

Aiming at clinical translation, robust directed differentiation of human pluripotent stem cells (hPSCs), preferentially in chemically defined conditions, is a key requirement. Here, feasibility of suspension culture based hPSC-cardiomyocyte (hPSC-CM) production in low-cost, xeno-free media compatible with good manufacturing practice standards is shown. Applying stirred tank bioreactor systems at increasing dimensions, our advanced protocol enables routine production of about 1 million hPSC-CMs/mL, yielding ∼1.3 × 10⁸ CM in 150 mL and ∼4.0 × 10⁸ CMs in 350–500 mL process scale at >90% lineage purity. Process robustness and efficiency is ensured by uninterrupted chemical WNT pathway control at early stages of differentiation and results in the formation of almost exclusively ventricular-like CMs. Modulated WNT pathway regulation also revealed the previously unappreciated role of ROR1/CD13 as superior surrogate markers for predicting cardiac differentiation efficiency as soon as 72 h of differentiation. This monitoring strategy facilitates process upscaling and controlled mass production of hPSC derivatives.

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Chemically defined hPSC cardiac differentiation applicable to stirred tank reactors

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Protocol generates >90% purity of ventricular-like cardiomyocytes

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Uninterrupted WNT pathway control enables superior cardiac mesoderm formation

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Novel ROR1/CD13 combination as superior, predictive marker of cardiomyogenesis

Comparison of Cultured Human Cardiomyocyte Clusters Obtained from Embryos/Fetuses or Derived from Human Embryonic Stem Cells

Cardiomyocytes (CMs) derived from human embryonic stem cells (hESCs) or induced pluripotent stem cells (iPSCs) are used to study cardiogenesis and mechanisms of heart disease, and are being used in methods for toxiological screening of drugs. The phenotype of stem-cell-derived CMs should ideally resemble native CMs. Here, we compare embryonic/fetal CMs with hESC-derived CMs according to function and morphology. CM clusters were obtained from human embryonic/fetal hearts from elective terminated pregnancies before gestational week 12, and separated into atrial and ventricular tissues. Specific markers for embryonic CMs and primary cilia were visualized using immunofluorescence microscopy analysis. Contracting human embryonic cardiomyocyte (hECM) clusters morphologically and phenotypically resemble CMs in the embryonic/fetal heart. In addition, the contracting hECM clusters expressed primary cilia similar to that of cells in the embryonic/fetal heart. The electrophysiological characteristics of atrial and ventricular CMs were established by recording action potentials (APs) using sharp electrodes. In contrast to ventricular APs, atrial APs displayed a marked early repolarization followed by a plateau phase. hESC-CMs displayed a continuum of AP shapes. In all embryonic/fetal clusters, both atrial and ventricular, AP duration was prolonged by exposure to the K_V11.1 channel inhibitor dofetilide (50 nM); however, the prolongation was not significant, possibly due to the relatively small number of experiments. This study provides novel information on APs and functional characteristics of atrial and ventricular CMs in first trimester hearts, and demonstrates that K_v11.1 channels play a functional role already at these early stages. These results provide information needed to validate methods being developed on the basis of in vitro-derived CMs from either hESC or iPSC, and although there was a good correlation between the morphology of the two types of CMs, differences in electrophysiological characteristics exist.

Femtosecond laser-based nanosurgery reveals the endogenous regeneration of single Z-discs including physiological consequences for cardiomyocytes

A highly organized cytoskeleton architecture is the basis for continuous and controlled contraction in cardiomyocytes (CMs). Abnormalities in cytoskeletal elements, like the Z-disc, are linked to several diseases. It is challenging to reveal the mechanisms of CM failure, endogenous repair, or mechanical homeostasis on the scale of single cytoskeletal elements. Here, we used a femtosecond (fs) laser to ablate single Z-discs in human pluripotent stem cells (hPSC) -derived CMs (hPSC-CM) and neonatal rat CMs. We show, that CM viability was unaffected by the loss of a single Z-disc. Furthermore, more than 40% of neonatal rat and 68% of hPSC-CMs recovered the Z-disc loss within 24 h. Significant differences to control cells, after the Z-disc loss, in terms of cell perimeter, x- and y-expansion and calcium homeostasis were not found. Only 14 days in vitro old hPSC-CMs reacted with a significant decrease in cell area, x- and y-expansion 24 h past nanosurgery. This demonstrates that CMs can compensate the loss of a single Z-disc and recover a regular sarcomeric pattern during spontaneous contraction. It also highlights the significant potential of fs laser-based nanosurgery to physically micro manipulate CMs to investigate cytoskeletal functions and organization of single elements.

Production of Cardiomyocytes from Human Pluripotent Stem Cells by Bioreactor Technologies

Cardiomyocytes from human pluripotent stem cells (hPSCs) have the ability to advance specificity of in vitro assays for drug discovery and safety pharmacology. They may also provide a superior cell source for envisioned cell therapies to repair damaged hearts. All applications will require the production of cardiomyocytes (CMs) by robust upscalable bioprocesses via industry-compliant technologies. This paper describes a detailed procedure for producing hPSC-CMs in stirred tank bioreactors in 100 ml process scale. The strategy combines both hPSC expansion in suspension culture and, directly followed by, cardiogenic differentiation using small molecule-Wnt pathway modulators. We also provide a protocol describing how to plan and expand the pluripotent stem cells to enable parallel inoculation of 4× 150 ml parallel bioreactor differentiations, potentially producing more than 240 × 10⁶ cardiomyocytes in 22 days.

Novel Adult-Onset Systolic Cardiomyopathy Due to MYH7 E848G Mutation in Patient-Derived Induced Pluripotent Stem Cells

A novel myosin heavy chain 7 mutation (E848G) identified in a familial cardiomyopathy was studied in patient-specific induced pluripotent stem cell-derived cardiomyocytes. The cardiomyopathic human induced pluripotent stem cell-derived cardiomyocytes exhibited reduced contractile function as single cells and engineered heart tissues, and genome-edited isogenic cells confirmed the pathogenic nature of the E848G mutation. Reduced contractility may result from impaired interaction between myosin heavy chain 7 and cardiac myosin binding protein C.

Scalable Cardiac Differentiation of Pluripotent Stem Cells Using Specific Growth Factors and Small Molecules

The envisioned routine application of human pluripotent stem cell (hPSC)-derived cardiomyocytes (CMs) for therapies and industry-compliant screening approaches will require efficient and highly reproducible processes for the mass production of well-characterized CM batches.On their way toward beating CMs, hPSCs initially undergo an epithelial-to-mesenchymal transition into a primitive-streak (PS)-like population that later gives rise to all endodermal and mesodermal lineages, including cardiovascular progenies (CVPs). CVPs are multipotent and possess the capability to give rise to all major cell types of the heart, including CMs, endothelial cells, cardiac fibroblasts, and smooth muscle cells. This article provides an historical overview and describes the stepwise development of protocols that typically result in the appearance of beating CMs within 7-12 days of hPSC differentiation.We describe the development of directed and closely controlled cardiomyogenic differentiation, which now enables the induction of >90% CM purity without further lineage enrichment. Although secreted lineage specifiers (revealed from developmental biology) were initially used, we outline the advantages of chemical pathway modulators, as defined by more recent screening approaches. Subsequently, we discuss the use of defined culture media for upscaling the production of hPSC-CMs in controlled bioreactors and how this, in principle, unlimited source of human CMs can be used to progress heart regeneration and stimulate the drug discovery pipeline. Graphical Abstract.

Human pluripotent stem cell models of cardiac disease: from mechanisms to therapies

It is now a decade since human induced pluripotent stem cells (hiPSCs) were first described. The reprogramming of adult somatic cells to a pluripotent state has become a robust technology that has revolutionised our ability to study human diseases. Crucially, these cells capture all the genetic aspects of the patient from which they were derived. Combined with advances in generating the different cell types present in the human heart, this has opened up new avenues to study cardiac disease in humans and investigate novel therapeutic approaches to treat these pathologies. Here, we provide an overview of the current state of the field regarding the generation of cardiomyocytes from human pluripotent stem cells and methods to assess them functionally, an essential requirement when investigating disease and therapeutic outcomes. We critically evaluate whether treatments suggested by these in vitro models could be translated to clinical practice. Finally, we consider current shortcomings of these models and propose methods by which they could be further improved.

A Cardiac Cell Outgrowth Assay for Evaluating Drug Compounds Using a Cardiac Spheroid-on-a-Chip Device

Three-dimensional (3D) models with cells arranged in clusters or spheroids have emerged as valuable tools to improve physiological relevance in drug screening. One of the challenges with cells cultured in 3D, especially for high-throughput applications, is to quickly and non-invasively assess the cellular state in vitro. In this article, we show that the number of cells growing out from human induced pluripotent stem cell (hiPSC)-derived cardiac spheroids can be quantified to serve as an indicator of a drug’s effect on spheroids captured in a microfluidic device. Combining this spheroid-on-a-chip with confocal high content imaging reveals easily accessible, quantitative outgrowth data. We found that effects on outgrowing cell numbers correlate to the concentrations of relevant pharmacological compounds and could thus serve as a practical readout to monitor drug effects. Here, we demonstrate the potential of this semi-high-throughput “cardiac cell outgrowth assay” with six compounds at three concentrations applied to spheroids for 48 h. The image-based readout complements end-point assays or may be used as a non-invasive assay for quality control during long-term culture.

Differences in Contractile Function of Myofibrils within Human Embryonic Stem Cell-Derived Cardiomyocytes vs. Adult Ventricular Myofibrils Are Related to Distinct Sarcomeric Protein Isoforms

Characterizing the contractile function of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) is key for advancing their utility for cellular disease models, promoting cell based heart repair, or developing novel pharmacological interventions targeting cardiac diseases. The aim of the present study was to understand whether steady-state and kinetic force parameters of β-myosin heavy chain (βMyHC) isoform-expressing myofibrils within human embryonic stem cell-derived cardiomyocytes (hESC-CMs) differentiated in vitro resemble those of human ventricular myofibrils (hvMFs) isolated from adult donor hearts. Contractile parameters were determined using the same micromechanical method and experimental conditions for both types of myofibrils. We identified isoforms and phosphorylation of main sarcomeric proteins involved in the modulation of force generation of both, chemically demembranated hESC-CMs (d-hESC-CMs) and hvMFs. Our results indicate that at saturating Ca²⁺ concentration, both human-derived contractile systems developed forces with similar rate constants (0.66 and 0.68 s⁻¹), reaching maximum isometric force that was significantly smaller for d-hESC-CMs (42 kPa) than for hvMFs (94 kPa). At submaximal Ca²⁺-activation, where intact cardiomyocytes normally operate, contractile parameters of d-hESC-CMs and hvMFs exhibited differences. Ca²⁺ sensitivity of force was higher for d-hESC-CMs (pCa₅₀ = 6.04) than for hvMFs (pCa₅₀ = 5.80). At half-maximum activation, the rate constant for force redevelopment was significantly faster for d-hESC-CMs (0.51 s⁻¹) than for hvMFs (0.28 s⁻¹). During myofibril relaxation, kinetics of the slow force decay phase were significantly faster for d-hESC-CMs (0.26 s⁻¹) than for hvMFs (0.21 s⁻¹), while kinetics of the fast force decay were similar and ~20x faster. Protein analysis revealed that hESC-CMs had essentially no cardiac troponin-I, and partially non-ventricular isoforms of some other sarcomeric proteins, explaining the functional discrepancies. The sarcomeric protein isoform pattern of hESC-CMs had features of human cardiomyocytes at an early developmental stage. The study indicates that morphological and ultrastructural maturation of βMyHC isoform-expressing hESC-CMs is not necessarily accompanied by ventricular-like expression of all sarcomeric proteins. Our data suggest that hPSC-CMs could provide useful tools for investigating inherited cardiac diseases affecting contractile function during early developmental stages.

Human Embryonic Stem Cell-Derived Cardiomyocytes Self-Arrange with Areas of Different Subtypes During Differentiation

The derivation of functional cardiomyocytes (CMs) from human embryonic stem cells (hESCs) represents a unique way of studying human cardiogenesis, including the development of CM subtypes. In this study, we investigated the development and organization of hESC-derived cardiomyocytes (hESC-CMs) and examined how the expression levels of CM subtypes correspond to human in vivo cardiogenesis. Beating clusters were used to determine cardiac differentiation, which was evaluated by the expression of cardiac genes GATA4 and TNNT2 and subcellular localization of GATA4 and NKX2.5. Sharp electrode recordings to determine action potentials (APs) further revealed spatial organization of intracluster CM subtypes (ie, complex clusters). Nodal-, atrial-, and ventricular-like AP morphologies were detected within distinct regions of complex clusters. The ability of different CM subtypes to self-organize was documented by immunohistochemical analyses and a differential spatial expression of β-III tubulin, myosin light chain 2v (MLC-2V), and α-smooth muscle actin (α-SMA). Furthermore, all hESC-CM subtypes formed expressed primary cilia, which are known to coordinate cellular signaling pathways during cardiomyogenesis and heart development. This study expands the foundation for studying regulatory pathways for spatial and temporal CM differentiation during human cardiogenesis.

CO2 permeability and carbonic anhydrase activity of rat cardiomyocytes

AIM

To determine the CO₂ permeability (P_CO2 ) of plasma membranes of cardiomyocytes. These cells were chosen because heart possesses the highest rate of O₂ consumption/CO₂ production in the body.

METHODS

Cardiomyocytes were isolated from rat hearts using the Langendorff technique. Cardiomyocyte suspensions exhibited a vitality of 2-14% and were studied by the previously described mass spectrometric ¹⁸ O-exchange technique deriving P_CO2 . We showed by mass spectrometry and by carbonic anhydrase (CA) staining that non-vital cardiomyocytes are free of CA and thus do not contribute to the mass spectrometric signal, which is determined exclusively by the fully functional vital cardiomyocytes.

RESULTS

Lysed cardiomyocytes yielded an intracellular CA activity for vital cells of 5070; that is, the rate of CO₂ hydration inside the cell is accelerated 5071-fold. Using this number, analyses of the mass spectrometric recordings from cardiomyocyte suspensions yield a P_CO2 of 0.10 cm s^-1 (SD ± 0.06, n = 15) at 37 °C.

CONCLUSION

In comparison with the P_CO2 of other cells, this value is quite high and about identical to that of the human red cell membrane. As no major protein CO₂ channels such as aquaporins 1 and 4 are present in rat cardiac sarcolemma, the high P_CO2 of this membrane is likely due to its low cholesterol content of about 0.2 (mol cholesterol)·(mol total membrane lipids)^-1 . Previous work predicted a P_CO2 of ≥0.1 cm s^-1 from this level of cholesterol. We conclude that the low cholesterol establishes a P_CO2 high enough to render the membrane resistance to CO₂ diffusion almost negligible, even under conditions of maximal O₂ consumption of the heart.