Genetic and Epigenetic Regulation of Human Cardiac Reprogramming and Differentiation in Regenerative Medicine

ELM2-SANT Domain-Containing Scaffolding Protein 1 Regulates Differentiation and Maturation of Cardiomyocytes Derived From Human-Induced Pluripotent Stem Cells

BACKGROUND

ELMSAN1 (ELM2-SANT domain-containing scaffolding protein 1) is a newly identified scaffolding protein of the MiDAC (mitotic deacetylase complex), playing a pivotal role in early embryonic development. Studies on Elmsan1 knockout mice showed that its absence results in embryo lethality and heart malformation. However, the precise function of ELMSAN1 in heart development and formation remains elusive. To study its potential role in cardiac lineage, we employed human-induced pluripotent stem cells (hiPSCs) to model early cardiogenesis and investigated the function of ELMSAN1.

METHODS AND RESULTS

We generated ELMSAN1-deficient hiPSCs through knockdown and knockout techniques. During cardiac differentiation, ELMSAN1 depletion inhibited pluripotency deactivation, decreased the expression of cardiac-specific markers, and reduced differentiation efficiency. The impaired expression of genes associated with contractile sarcomere structure, calcium handling, and ion channels was also noted in ELMSAN1-deficient cardiomyocytes derived from hiPSCs. Additionally, through a series of structural and functional assessments, we found that ELMSAN1-null hiPSC cardiomyocytes are immature, exhibiting incomplete sarcomere Z-line structure, decreased calcium handling, and impaired electrophysiological properties. Of note, we found that the cardiac-specific role of ELMSAN1 is likely associated with histone H3K27 acetylation level. The transcriptome analysis provided additional insights, indicating maturation reduction with the energy metabolism switch and restored cell proliferation in ELMSAN1 knockout cardiomyocytes.

CONCLUSIONS

In this study, we address the significance of the direct involvement of ELMSAN1 in the differentiation and maturation of hiPSC cardiomyocytes. We first report the impact of ELMSAN1 on multiple aspects of hiPSC cardiomyocyte generation, including cardiac differentiation, sarcomere formation, calcium handling, electrophysiological maturation, and proliferation.

Histone Modifications and miRNA Perturbations Contribute to Transcriptional Dysregulation of Hypertrophy in Obstructive Hypertrophic Cardiomyopathy

Background

Recently, we demonstrated transcriptional downregulation of hypertrophy pathways in myectomy tissue derived from patients with obstructive hypertrophic cardiomyopathy (HCM) despite translational activation of hypertrophy pathways. The mechanisms and modifiers of this transcriptional dysregulation in HCM remain unexplored. We hypothesized that miRNA and post-translational modifications of histones contribute to transcriptional dysregulation in HCM.

Methods

First, miRNA-sequencing and chromatin immunoprecipitation sequencing (ChIP-seq) were performed on HCM myectomy tissue and control donor hearts to characterize miRNA and differential histone marks across the genome. Next, the differential miRNA and histone marks were integrated with RNA-sequencing (RNA-seq) data. Finally, the effects of miRNA and histones were removed in silico to determine their necessity for transcriptional dysregulation of pathways.

Results

miRNA-analysis identified 19 differentially expressed miRNA. ChIP-seq analysis identified 2,912 (7%) differential H3K4me3 peaks, 23,339 (21%) differential H3K9ac peaks, 33 (0.05%) differential H3K9me3 peaks, 58,837 (42%) differential H3K27ac peaks, and 853 (3%) differential H3K27me3 peaks. Univariate analysis of concordance between H3K9ac with RNA-seq data showed activation of cardiac hypertrophy signaling, while H3K27me showed downregulation of cardiac hypertrophy signaling. Similarly, miRNAs were predicted to result in downregulation of cardiac hypertrophy signaling. In silico knock-out that effects either miRNA or histones attenuated transcriptional downregulation while knocking out both abolished downregulation of hypertrophy pathways completely.

Conclusion

Myectomy tissue from patients with obstructive HCM shows transcriptional dysregulation, including transcriptional downregulation of hypertrophy pathways mediated by miRNA and post-translational modifications of histones. Cardiac hypertrophy loci showed activation via changes in H3K9ac and a mix of activation and repression via H3K27ac.

Multi-lineage heart-chip models drug cardiotoxicity and enhances maturation of human stem cell-derived cardiovascular cells

Cardiovascular toxicity causes adverse drug reactions and may lead to drug removal from the pharmaceutical market. Cancer therapies can induce life-threatening cardiovascular side effects such as arrhythmias, muscle cell death, or vascular dysfunction. New technologies have enabled cardiotoxic compounds to be identified earlier in drug development. Human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes (CMs) and vascular endothelial cells (ECs) can screen for drug-induced alterations in cardiovascular cell function and survival. However, most existing hiPSC models for cardiovascular drug toxicity utilize two-dimensional, immature cells grown in static culture. Improved in vitro models to mechanistically interrogate cardiotoxicity would utilize more adult-like, mature hiPSC-derived cells in an integrated system whereby toxic drugs and protective agents can flow between hiPSC-ECs that represent systemic vasculature and hiPSC-CMs that represent heart muscle (myocardium). Such models would be useful for testing the multi-lineage cardiotoxicities of chemotherapeutic drugs such as VEGFR2/PDGFR-inhibiting tyrosine kinase inhibitors (VPTKIs). Here, we develop a multi-lineage, fully-integrated, cardiovascular organ-chip that can enhance hiPSC-EC and hiPSC-CM functional and genetic maturity, model endothelial barrier permeability, and demonstrate long-term functional stability. This microfluidic organ-chip harbors hiPSC-CMs and hiPSC-ECs on separate channels that can be subjected to active fluid flow and rhythmic biomechanical stretch. We demonstrate the utility of this cardiovascular organ-chip as a predictive platform for evaluating multi-lineage VPTKI toxicity. This study may lead to the development of new modalities for the evaluation and prevention of cancer therapy-induced cardiotoxicity.

Succinate dehydrogenase is essential for epigenetic and metabolic homeostasis in hearts

A hallmark of heart failure is a metabolic switch away from fatty acids β-oxidation (FAO) to glycolysis. Here, we show that succinate dehydrogenase (SDH) is required for maintenance of myocardial homeostasis of FAO/glycolysis. Mice with cardiomyocyte-restricted deletion of subunit b or c of SDH developed a dilated cardiomyopathy and heart failure. Hypertrophied hearts displayed a decrease in FAO, while glucose uptake and glycolysis were augmented, which was reversed by enforcing FAO fuels via a high-fat diet, which also improved heart failure of mutant mice. SDH-deficient hearts exhibited an increase in genome-wide DNA methylation associated with accumulation of succinate, a metabolite known to inhibit DNA demethylases, resulting in changes of myocardial transcriptomic landscape. Succinate induced DNA hypermethylation and depressed the expression of FAO genes in myocardium, leading to imbalanced FAO/glycolysis. Inhibition of succinate by α-ketoglutarate restored transcriptional profiles and metabolic disorders in SDH-deficient cardiomyocytes. Thus, our findings reveal the essential role for SDH in metabolic remodeling of failing hearts, and highlight the potential of therapeutic strategies to prevent cardiac dysfunction in the setting of SDH deficiency.

Cardiomyocyte proliferation is suppressed by ARID1A-mediated YAP inhibition during cardiac maturation

The inability of adult human cardiomyocytes to proliferate is an obstacle to efficient cardiac regeneration after injury. Understanding the mechanisms that drive postnatal cardiomyocytes to switch to a non-regenerative state is therefore of great significance. Here we show that Arid1a, a subunit of the switching defective/sucrose non-fermenting (SWI/SNF) chromatin remodeling complex, suppresses postnatal cardiomyocyte proliferation while enhancing maturation. Genome-wide transcriptome and epigenome analyses revealed that Arid1a is required for the activation of a cardiomyocyte maturation gene program by promoting DNA access to transcription factors that drive cardiomyocyte maturation. Furthermore, we show that ARID1A directly binds and inhibits the proliferation-promoting transcriptional coactivators YAP and TAZ, indicating ARID1A sequesters YAP/TAZ from their DNA-binding partner TEAD. In ischemic heart disease, Arid1a expression is enhanced in cardiomyocytes of the border zone region. Inactivation of Arid1a after ischemic injury enhanced proliferation of border zone cardiomyocytes. Our study illuminates the pivotal role of Arid1a in cardiomyocyte maturation, and uncovers Arid1a as a crucial suppressor of cardiomyocyte proliferation.

Novel biphasic mechanism of the canonical Wnt signalling component PYGO2 promotes cardiomyocyte differentiation from hUC-MSCs

Human umbilical cord-derived mesenchymal stem cells (hUC-MSCs) are used to regenerate the myocardium during cardiac repair after myocardial infarction. However, the regulatory mechanism underlying their ability to form mesodermal cells and differentiate into cardiomyocytes remains unclear. Here, we established a human-derived MSCs line isolated from healthy umbilical cords and established a cell model of the natural state to examine the differentiation of hUC-MSCs into cardiomyocytes. Quantitative RT-PCR, western blotting, immunofluorescence, flow cytometry, RNA Seq, and inhibitors of canonical Wnt signalling were used to detect the germ-layer markers T and MIXL1; the markers of cardiac progenitor cells MESP1, GATA4, and NKX2.5 and the cardiomyocyte-marker cTnT to identify the molecular mechanism associated with PYGO2, a key component of the canonical Wnt signalling pathway that regulates the formation of cardiomyocyte-like cells. We demonstrated that PYGO2 promotes the formation of mesodermal-like cells and their differentiation into cardiomyocytes through the hUC-MSC-dependent canonical Wnt signalling by promoting the early-stage entry of β-catenin into the nucleus. Surprisingly, PYGO2 did not alter the expression of the canonical-Wnt, NOTCH, or BMP signalling pathways during the middle-late stages. In contrast, PI3K-Akt signalling promoted hUC-MSCs formation and their differentiation into cardiomyocyte-like cells. To the best of our knowledge, this is the first study to demonstrate that PYGO2 uses a biphasic mechanism to promote cardiomyocyte formation from hUC-MSCs.

Significance of microRNA-targeted ErbB signaling pathway genes in cardiomyocyte differentiation

OBJECTIVE(S)

Cardiomyocyte differentiation is a complex process that follows the progression of gene expression alterations. The ErbB signaling pathway is necessary for various stages of cardiac development. We aimed to identify potential microRNAs targeting the ErbB signaling pathway genes by in silico approaches.

METHODS

Small RNA-sequencing data were obtained from GSE108021 for cardiomyocyte differentiation. Differentially expressed miRNAs were acquired via the DESeq2 package. Signaling pathways and gene ontology processes for the identified miRNAs were determined and the targeted genes of those miRNAs affecting the ErbB signaling pathway were determined.

RESULTS

Results revealed highly differentially expressed miRNAs were common between the differentiation stages and they targeted the genes involved in the ErbB signaling pathway as follows: let-7g-5p targets both CDKN1A and NRAS, while let-7c-5p and let-7d-5p hit CDKN1A and NRAS exclusively. let-7 family members targeted MAPK8 and ABL2. GSK3B was targeted by miR-199a-5p and miR-214-3p, and ERBB4 was targeted by miR-199b-3p and miR-653-5p. miR-214-3p, miR-199b-3p, miR-1277-5p, miR-21-5p, and miR-21-3p targeted CBL, mTOR, Jun, JNKK, and GRB1, respectively. MAPK8 was targeted by miR-214-3p, and ABL2 was targeted by miR-125b-5p and miR-1277-5p, too.

CONCLUSION

We determined miRNAs and their target genes in the ErbB signaling pathway in cardiomyocyte development and consequently heart pathophysiology progression.

Overexpression of PYGO1 promotes early cardiac lineage development in human umbilical cord mesenchymal stromal/stem cells by activating the Wnt/β-catenin pathway

Cardiovascular disease still has the highest mortality. Gene-modified mesenchymal stromal/stem cells could be a promising therapy. Pygo plays an important role in embryonic development and regulates life activities with a variety of regulatory mechanisms. Therefore, this study aimed to investigate whether the overexpression of the PYGO1 gene can promote the differentiation of human umbilical cord-derived mesenchymal stromal/stem cells (HUC-MSCs) into early cardiac lineage cells and to preliminary explore the relevant mechanisms. In this study, HUC-MSCs were isolated by the explant method and were identified by flow cytometry and differentiation assay, followed by transfected with lentivirus carrying the PYGO1 plasmid. In PYGO1 group (cells were incubated with lentiviral-PYGO1), the mRNA expressions of cardiac differentiation-specific markers (MESP1, NKX2.5, GATA4, MEF2C, ISL1, TBX5, TNNT2, ACTC1, and MYH6 genes) and the protein expressions of NKX2.5 and cTnT were significantly up-regulated compared with the NC group (cells were incubated with lentiviral-empty vector). In addition, the proportion of NKX2.5, GATA4, and cTnT immunofluorescence-positive cells increased with the inducement time. Overexpression of PYGO1 statistically significantly increased the relative luciferase expression level of Topflash plasmid, the protein expression level of β-catenin and the mRNA expression level of CYCLIND1. Compared with the control group, decreased protein levels of NKX2.5 and cTnT were detected in PYGO1 group after application of XAV-939, the specific inhibitor of the canonical Wnt/β-catenin pathway. Our study suggests that overexpression of PYGO1 significantly promotes the differentiation of HUC-MSCs into early cardiac lineage cells, which is regulated by the canonical Wnt/β-catenin signaling.

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.

Heart in a Dish: From Traditional 2D Differentiation Protocols to Cardiac Organoids

Human pluripotent stem cells (hPSCs) constitute a valuable model to study the complexity of early human cardiac development and investigate the molecular mechanisms involved in heart diseases. The differentiation of hPSCs into cardiac lineages in vitro can be achieved by traditional two-dimensional (2D) monolayer approaches or by adopting innovative three-dimensional (3D) cardiac organoid protocols. Human cardiac organoids (hCOs) are complex multicellular aggregates that faithfully recapitulate the cardiac tissue’s transcriptional, functional, and morphological features. In recent years, significant advances in the field have dramatically improved the robustness and efficiency of hCOs derivation and have promoted the application of hCOs for drug screening and heart disease modeling. This review surveys the current differentiation protocols, focusing on the most advanced 3D methods for deriving hCOs from hPSCs. Furthermore, we describe the potential applications of hCOs in the pharmaceutical and tissue bioengineering fields, including their usage to investigate the consequences of Severe Acute Respiratory Syndrome CoronaVirus 2 (SARS-CoV2) infection in the heart.

Stage-specific regulation of DNA methylation by TET enzymes during human cardiac differentiation

Changes in DNA methylation are associated with normal cardiogenesis, whereas altered methylation patterns can occur in congenital heart disease. Ten-eleven translocation (TET) enzymes oxidize 5-methylcytosine (5mC) and promote locus-specific DNA demethylation. Here, we characterize stage-specific methylation dynamics and the function of TETs during human cardiomyocyte differentiation. Human embryonic stem cells (hESCs) in which all three TET genes are inactivated fail to generate cardiomyocytes (CMs), with altered mesoderm patterning and defective cardiac progenitor specification. Genome-wide methylation analysis shows TET knockout causes promoter hypermethylation of genes encoding WNT inhibitors, leading to hyperactivated WNT signaling and defects in cardiac mesoderm patterning. TET activity is also needed to maintain hypomethylated status and expression of NKX2-5 for subsequent cardiac progenitor specification. Finally, loss of TETs causes a set of cardiac structural genes to fail to be demethylated at the cardiac progenitor stage. Our data demonstrate key roles for TET proteins in controlling methylation dynamics at sequential steps during human cardiac development.

miR-199a Overexpression Enhances the Potency of Human Induced-Pluripotent Stem-Cell-Derived Cardiomyocytes for Myocardial Repair

Mammalian cardiomyocytes exit the cell cycle during the perinatal period, and although cardiomyocytes differentiated from human induced-pluripotent stem cells (hiPSC-CMs) are phenotypically immature, their intrinsic cell-cycle activity remains limited. Thus, neither endogenous cardiomyocytes nor the small number of transplanted hiPSC-CMs that are engrafted by infarcted hearts can remuscularize the myocardial scar. microRNAs are key regulators of cardiomyocyte proliferation, and when adeno-associated viruses coding for microRNA-199a (miR-199a) expression were injected directly into infarcted pig hearts, measures of cardiac function and fibrosis significantly improved, but the treatment was also associated with lethal arrhythmia. For the studies reported here, the same vector (AAV6-miR-199a) was transduced into hiPSC-CMs, and the cells were subsequently evaluated in a mouse model of myocardial infarction. AAV6-mediated miR-199a overexpression increased proliferation in both cultured and transplanted hiPSC-CMs, and measures of left ventricular ejection fraction, fractional shortening, and scar size were significantly better in mice treated with miR-199a-overexpressing hiPSC-CMs than with hiPSC-CMs that had been transduced with a control vector. Furthermore, although this investigation was not designed to characterize the safety of transplanted AAV6-miR-199a-transduced hiPSC-CMs, there was no evidence of sudden death. Collectively, these results support future investigations of miR-199a-overexpressing hiPSC-CMs in large animals.

Loss of ten-eleven translocation 2 induces cardiac hypertrophy and fibrosis through modulating ERK signaling pathway

The ten-eleven translocation (Tet) family of dioxygenases convert 5-methylcytosine to 5-hydroxymethylcytosine (5hmC). Previous studies have shown that 5hmC-mediated epigenetic modifications play essential roles in diverse biological processes and diseases. Here, we show that Tet proteins and 5hmC display dynamic features during postnatal cardiac development and that Tet2 is the predominant dioxygenase present in heart. Tet2 knockout results in abnormal cardiac function, progressive cardiac hypertrophy and fibrosis. Mechanistically, Tet2 deficiency leads to reduced hydroxymethylation in the cardiac genome and alters the cardiac transcriptome. Mechanistically, Tet2 loss leads to a decrease of Hspa1b expression, a regulator of the extracellular signal-regulated protein kinase (Erk) signaling pathway, which leads to over-activation of Erk signaling. Acute Hspa1b knock down (KD) increased the phosphorylation of Erk and induced hypertrophy of cardiomyocytes, which could be blocked by Erk signaling inhibitor. Consistently, ectopic expression of Hspa1b was able to rescue the deficits of cardiomyocytes induced by Tet2 depletion. Taken together, our study's results reveal the important roles of Tet2-mediated DNA hydroxymethylation in cardiac development and function.

Mending a broken heart: In vitro, in vivo and in silico models of congenital heart disease

Birth defects contribute to ∼0.3% of global infant mortality in the first month of life, and congenital heart disease (CHD) is the most common birth defect among newborns worldwide. Despite the significant impact on human health, most treatments available for this heterogenous group of disorders are palliative at best. For this reason, the complex process of cardiogenesis, governed by multiple interlinked and dose-dependent pathways, is well investigated. Tissue, animal and, more recently, computerized models of the developing heart have facilitated important discoveries that are helping us to understand the genetic, epigenetic and mechanobiological contributors to CHD aetiology. In this Review, we discuss the strengths and limitations of different models of normal and abnormal cardiogenesis, ranging from single-cell systems and 3D cardiac organoids, to small and large animals and organ-level computational models. These investigative tools have revealed a diversity of pathogenic mechanisms that contribute to CHD, including genetic pathways, epigenetic regulators and shear wall stresses, paving the way for new strategies for screening and non-surgical treatment of CHD. As we discuss in this Review, one of the most-valuable advances in recent years has been the creation of highly personalized platforms with which to study individual diseases in clinically relevant settings.

Human iPSCs in Cardiovascular Research: Current Approaches in Cardiac Differentiation, Maturation Strategies, and Scalable Production

Manifestations of cardiovascular diseases (CVDs) in a patient or a population differ based on inherent biological makeup, lifestyle, and exposure to environmental risk factors. These variables mean that therapeutic interventions may not provide the same benefit to every patient. In the context of CVDs, human-induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) offer an opportunity to model CVDs in a patient-specific manner. From a pharmacological perspective, iPSC-CM models can serve as go/no-go tests to evaluate drug safety. To develop personalized therapies for early diagnosis and treatment, human-relevant disease models are essential. Hence, to implement and leverage the utility of iPSC-CMs for large-scale treatment or drug discovery, it is critical to (i) carefully evaluate the relevant limitations of iPSC-CM differentiations, (ii) establish quality standards for defining the state of cell maturity, and (iii) employ techniques that allow scalability and throughput with minimal batch-to-batch variability. In this review, we briefly describe progress made with iPSC-CMs in disease modelling and pharmacological testing, as well as current iPSC-CM maturation techniques. Finally, we discuss current platforms for large-scale manufacturing of iPSC-CMs that will enable high-throughput drug screening applications.

Cardiac Aging: From Basic Research to Therapeutics

With research progress on longevity, we have gradually recognized that cardiac aging causes changes in heart structure and function, including progressive myocardial remodeling, left ventricular hypertrophy, and decreases in systolic and diastolic function. Elucidating the regulatory mechanisms of cardiac aging is a great challenge for biologists and physicians worldwide. In this review, we discuss several key molecular mechanisms of cardiac aging and possible prevention and treatment methods developed in recent years. Insights into the process and mechanism of cardiac aging are necessary to protect against age-related diseases, extend lifespan, and reduce the increasing burden of cardiovascular disease in elderly individuals. We believe that research on cardiac aging is entering a new era of unique significance for the progress of clinical medicine and social welfare.

Hematopoietic Stem Cell Transcription Factors in Cardiovascular Pathology

Transcription factors as multifaceted modulators of gene expression that play a central role in cell proliferation, differentiation, lineage commitment, and disease progression. They interact among themselves and create complex spatiotemporal gene regulatory networks that modulate hematopoiesis, cardiogenesis, and conditional differentiation of hematopoietic stem cells into cells of cardiovascular lineage. Additionally, bone marrow-derived stem cells potentially contribute to the cardiovascular cell population and have shown potential as a therapeutic approach to treat cardiovascular diseases. However, the underlying regulatory mechanisms are currently debatable. This review focuses on some key transcription factors and associated epigenetic modifications that modulate the maintenance and differentiation of hematopoietic stem cells and cardiac progenitor cells. In addition to this, we aim to summarize different potential clinical therapeutic approaches in cardiac regeneration therapy and recent discoveries in stem cell-based transplantation.

Induced pluripotent stem cells as a platform to understand patient-specific responses to opioids and anaesthetics

Recent advances in human induced pluripotent stem cell (iPSC) technology may provide unprecedented opportunities to study patient-specific responses to anaesthetics and opioids. In this review, we will (1) examine the advantages and limitations of iPSC technology, (2) summarize studies using iPSCs that have contributed to our current understanding of anaesthetics and opioid action on the cardiovascular system and central nervous system (CNS), and (3) describe how iPSC technology can be used to further develop personalized analgesic and sedative pharmacotherapies with reduced or minimal detrimental cardiovascular effects.

Patient and Disease-Specific Induced Pluripotent Stem Cells for Discovery of Personalized Cardiovascular Drugs and Therapeutics

Human induced pluripotent stem cells (iPSCs) have emerged as an effective platform for regenerative therapy, disease modeling, and drug discovery. iPSCs allow for the production of limitless supply of patient-specific somatic cells that enable advancement in cardiovascular precision medicine. Over the past decade, researchers have developed protocols to differentiate iPSCs to multiple cardiovascular lineages, as well as to enhance the maturity and functionality of these cells. Despite significant advances, drug therapy and discovery for cardiovascular disease have lagged behind other fields such as oncology. We speculate that this paucity of drug discovery is due to a previous lack of efficient, reproducible, and translational model systems. Notably, existing drug discovery and testing platforms rely on animal studies and clinical trials, but investigations in animal models have inherent limitations due to interspecies differences. Moreover, clinical trials are inherently flawed by assuming that all individuals with a disease will respond identically to a therapy, ignoring the genetic and epigenomic variations that define our individuality. With ever-improving differentiation and phenotyping methods, patient-specific iPSC-derived cardiovascular cells allow unprecedented opportunities to discover new drug targets and screen compounds for cardiovascular disease. Imbued with the genetic information of an individual, iPSCs will vastly improve our ability to test drugs efficiently, as well as tailor and titrate drug therapy for each patient.

Cells, Materials, and Fabrication Processes for Cardiac Tissue Engineering

Cardiovascular disease is the number one killer worldwide, with myocardial infarction (MI) responsible for approximately 1 in 6 deaths. The lack of endogenous regenerative capacity, added to the deleterious remodelling programme set into motion by myocardial necrosis, turns MI into a progressively debilitating disease, which current pharmacological therapy cannot halt. The advent of Regenerative Therapies over 2 decades ago kick-started a whole new scientific field whose aim was to prevent or even reverse the pathological processes of MI. As a highly dynamic organ, the heart displays a tight association between 3D structure and function, with the non-cellular components, mainly the cardiac extracellular matrix (ECM), playing both fundamental active and passive roles. Tissue engineering aims to reproduce this tissue architecture and function in order to fabricate replicas able to mimic or even substitute damaged organs. Recent advances in cell reprogramming and refinement of methods for additive manufacturing have played a critical role in the development of clinically relevant engineered cardiovascular tissues. This review focuses on the generation of human cardiac tissues for therapy, paying special attention to human pluripotent stem cells and their derivatives. We provide a perspective on progress in regenerative medicine from the early stages of cell therapy to the present day, as well as an overview of cellular processes, materials and fabrication strategies currently under investigation. Finally, we summarise current clinical applications and reflect on the most urgent needs and gaps to be filled for efficient translation to the clinical arena.

Retinoic acid promotes metabolic maturation of human Embryonic Stem Cell-derived Cardiomyocytes

Cardiomyocytes differentiated from human embryonic stem cells (hESCs) represent a promising cell source for heart repair, disease modeling and drug testing. However, improving the differentiation efficiency and maturation of hESC-derived cardiomyocytes (hESC-CMs) is still a major concern. Retinoic acid (RA) signaling plays multiple roles in heart development. However, the effects of RA on cardiomyocyte differentiation efficiency and maturation are still unknown.

Methods: RA was added at different time intervals to identify the best treatment windows for cardiomyocyte differentiation and maturation. The efficiency of cardiomyocyte differentiation was detected by quantitative real-time PCR and flow cytometry. Cardiomyocytes maturation was detected by immunofluorescence staining, metabolic assays and patch clamp to verify structural, metabolic and electrophysiological maturation, respectively. RNA sequencing was used for splicing analysis.

Results: We found that RA treatment at the lateral mesoderm stage (days 2-4) significantly improved cardiomyocyte differentiation, as evidenced by the upregulation of TNNT2, NKX2.5 and MYH6 on day 10 of differentiation. In addition, flow cytometry showed that the proportion of differentiated cardiomyocytes in the RA-treated group was significantly higher than that in control group. RA treatment on days 15-20 increased cardiomyocyte area, sarcomere length, multinucleation and mitochondrial copy number. RNA sequencing revealed RA promoted RNA isoform switch to the maturation-related form. Meanwhile, RA promoted electrophysiological maturation and calcium handling of hESC-CMs. Importantly, RA-treated cardiomyocytes showed decreased glycolysis and enhanced mitochondrial oxidative phosphorylation, with the increased utilization of fatty acid and exogenous pyruvate but not glutamine.

Conclusion: Our data indicated that RA treatment at an early time window (days 2-4) promotes the efficiency of cardiomyocyte differentiation and that RA treatment post beating (days 15-20) promotes cardiomyocyte maturation. The biphasic effects of RA provide new insights for improving cardiomyocyte differentiation and quality.

Epigenetics in Cardiac Hypertrophy and Heart Failure

Heart failure (HF) is a complex syndrome affecting millions of people around the world. Over the past decade, the therapeutic potential of targeting epigenetic regulators in HF has been discussed extensively. Recent advances in next-generation sequencing techniques have contributed substantial progress in our understanding of the role of DNA methylation, post-translational modifications of histones, adenosine triphosphate (ATP)-dependent chromatin conformation and remodeling, and non-coding RNAs in HF pathophysiology. In this review, we summarize epigenomic studies on human and animal models in HF.

Convergences of Life Sciences and Engineering in Understanding and Treating Heart Failure

On March 1 and 2, 2018, the National Institutes of Health 2018 Progenitor Cell Translational Consortium, Cardiovascular Bioengineering Symposium, was held at the University of Alabama at Birmingham. Convergence of life sciences and engineering to advance the understanding and treatment of heart failure was the theme of the meeting. Over 150 attendees were present, and >40 scientists presented their latest work on engineering human functional myocardium for disease modeling, drug development, and heart failure research. The scientists, engineers, and physicians in the field of cardiovascular sciences met and discussed the most recent advances in their work and proposed future strategies for overcoming the major roadblocks of cardiovascular bioengineering and therapy. Particular emphasis was given for manipulation and using of stem/progenitor cells, biomaterials, and methods to provide molecular, chemical, and mechanical cues to cells to influence their identity and fate in vitro and in vivo. Collectively, these works are profoundly impacting and progressing toward deciphering the mechanisms and developing novel treatments for left ventricular dysfunction of failing hearts. Here, we present some important perspectives that emerged from this meeting.

Lack of Cardiac Improvement After Cardiosphere-Derived Cell Transplantation in Aging Mouse Hearts

RATIONALE

Aging is one of the most significant risk factors for cardiovascular diseases, and the incidence of myocardial ischemia increases dramatically with age. Some studies have reported that cardiosphere-derived cells (CDCs) could benefit the injured heart. Nevertheless, the convincing evidence on CDC-induced improvement of aging heart is still limited.

OBJECTIVE

In this study, we tested whether the CDCs isolated from neonatal mice could benefit cardiac function in aging mice.

METHODS AND RESULTS

We evaluated cardiac function of PBS- (n=15) and CDC-injected (n=19) aging mice. Echocardiography indicated that left ventricular (LV) ejection fraction (57.46%±3.57% versus 57.86%±2.44%) and LV fraction shortening (30.67%±2.41% versus 30.51%±1.78%) showed similar values in PBS- and CDC-injected mice. The diastolic wall thickness of LV was significantly increased after CDC injection, resulting in reduced diastolic LV volume. The pulse-wave Doppler and tissue Doppler imaging indicated that aging mice receiving PBS or CDC injection presented similar values of the peak early transmitral flow velocity, the peak late transmitral flow velocity, the ratio of the peak early transmitral flow velocity to the peak late transmitral flow velocity, and the ratio of the peak early transmitral flow velocity to the peak early diastolic mitral annular velocity, respectively. Pressure-volume loop experiment indicated that the LV end-diastolic pressure-volume relationship and end-systolic pressure-volume relationship were comparable in both PBS- and CDC-injected mice. Postmortem analysis of aging mouse hearts showed similar fibrotic degree in the 2 groups. In addition, the aging markers showed comparable expression levels in both PBS- and CDC-injected mice. The systemic aging performance measures, including exercise capacity, hair regrowth capacity, and inflammation, showed no significant improvement in CDC-injected mice. Finally, the telomere length was comparable between PBS- and CDC-injected mice.

CONCLUSIONS

Together, these results indicate that CDCs do not improve heart function and systemic performances in aging mice.

Metabolic Coordination of Physiological and Pathological Cardiac Remodeling

Metabolic pathways integrate to support tissue homeostasis and to prompt changes in cell phenotype. In particular, the heart consumes relatively large amounts of substrate not only to regenerate ATP for contraction but also to sustain biosynthetic reactions for replacement of cellular building blocks. Metabolic pathways also control intracellular redox state, and metabolic intermediates and end products provide signals that prompt changes in enzymatic activity and gene expression. Mounting evidence suggests that the changes in cardiac metabolism that occur during development, exercise, and pregnancy as well as with pathological stress (eg, myocardial infarction, pressure overload) are causative in cardiac remodeling. Metabolism-mediated changes in gene expression, metabolite signaling, and the channeling of glucose-derived carbon toward anabolic pathways seem critical for physiological growth of the heart, and metabolic inefficiency and loss of coordinated anabolic activity are emerging as proximal causes of pathological remodeling. This review integrates knowledge of different forms of cardiac remodeling to develop general models of how relationships between catabolic and anabolic glucose metabolism may fortify cardiac health or promote (mal)adaptive myocardial remodeling. Adoption of conceptual frameworks based in relational biology may enable further understanding of how metabolism regulates cardiac structure and function.

Epigenetic Regulation of Cardiac Development and Disease through DNA Methylation

Epigenetic control mechanisms play critical roles in organ development and tissue homeostasis. Increasing evidence suggests that cardiac lineage commitment and cardiovascular disease are tightly regulated by epigenetic mechanisms, controlling changes in DNA methylation, histone modifications, ATP-dependent chromatin remodeling, and expression levels for non-coding RNAs. This review summarizes our current understanding of epigenetic control mechanisms regulating cardiac development and disease, particularly focuses on the function of DNA methylation and demethylation through families of DNA methyltransferases and dioxygenases.

Transient induction of Cdk9 in the early stage of differentiation is critical for myogenesis

Cdk9 is a serine-threonine protein kinase that has been recognized as a regulator of cardiac differentiation. Recently, we have reported that transient induction of Cdk9 using noncoding RNA targeting Cdk9 sequences results in efficient cardiac differentiation. Concerning Cdk9 regulatory roles, here, we proposed whether constant overexpression of Cdk9 might influence the differentiation of myoblast C2C12 cells into myotubes. We overexpressed Cdk9 in mouse myoblast C2C12 cells to investigate its regulatory roles on myogenic differentiation. Upon Cdk9 overexpression, the expression level of myogenic regulatory factors was determined. Moreover, the expression profile of three important myomiRs consist of miR 1, 133 and 206 was examined during the differentiation process. Although Cdk9 expression is necessary for inducing differentiation in the early stage of myogenesis, continuous Cdk9 expression inhibits differentiation by modulating myomiRs and myogenic gene expression. Our results indicate that the transient induction of Cdk9 in the early stage of differentiation is critical for myogenesis.

Increasing the Efficacy of Stem Cell Therapy via Triple-Function Inorganic Nanoparticles

Stem cell therapy in heart disease is challenged by mis-injection, poor survival, and low cell retention. Here, we describe a biocompatible multifunctional silica-iron oxide nanoparticle to help solve these issues. The nanoparticles were made via an in situ growth of Fe₃O₄ nanoparticles on both the external surfaces and pore walls of mesocellular foam silica nanoparticles. In contrast to previous work, this approach builds a magnetic moiety inside the pores of a porous silica structure. These materials serve three roles: drug delivery, magnetic manipulation, and imaging. The addition of Fe₃O₄ to the silica nanoparticles increased their colloidal stability, T₂-based magnetic resonance imaging contrast, and superparamagnetism. We then used the hybrid materials as a sustained release vehicle of insulin-like growth factor-a pro-survival agent that can increase cell viability. In vivo rodent studies show that labeling stem cells with this nanoparticle increased the efficacy of stem cell therapy in a ligation/reperfusion model. The nanoparticle-labeled cells increase the mean left ventricular ejection fraction by 11 and 21% and the global longitudinal strain by 24 and 34% on days 30 and 60, respectively. In summary, this multifunctional nanomedicine improves stem cell survival via the sustained release of pro-survival agents.

Human embryonic stem cell-derived cardiomyocyte therapy in mouse permanent ischemia and ischemia-reperfusion models

Background

Ischemic heart diseases are still a threat to human health. Human pluripotent stem cell-based transplantation exhibits great promise in cardiovascular disease therapy, including heart ischemia. The purpose of this study was to compare the efficacy of human embryonic stem cell-derived cardiomyocyte (ESC-CM) therapy in two heart ischemia models, namely, permanent ischemia (PI) and myocardial ischemia reperfusion (IR).

Methods

Human embryonic stem cell-derived cardiomyocytes were differentiated from engineered human embryonic stem cells (ESC-Rep) carrying green fluorescent protein (GFP), herpes simplex virus-1 thymidine kinase (HSVtk), and firefly luciferase (Fluc). Two different heart ischemia models were generated by the ligation of the left anterior descending artery (LAD), and ESC-Rep-derived cardiomyocytes (ESC-Rep-CMs) were transplanted into the mouse hearts. Cardiac function was analyzed to evaluate the outcomes of ESC-Rep-CM transplantation. Bioluminescence signal analysis was performed to assess the cell engraftment. Finally, the inflammation response was analyzed by real-time PCR and ELISA.

Results

Cardiac function was significantly improved in the PI group with ESC-Rep-CM injection compared to the PBS-injected control, as indicated by increased left ventricular ejection fraction (LVEF) and left ventricular fractional shortening (LVFS), as well as reduced fibrotic area. However, minimal improvement by ESC-Rep-CM injection was detected in the IR mouse model. We observed similar engraftment efficiency between PI and IR groups after ESC-Rep-CM injection. However, the restricted inflammation was observed after the injection of ESC-Rep-CMs in the PI group, but not in the IR group. Transplantation of ESC-Rep-CMs can partially preserve the heart function via regulating the inflammation response in the PI model, while little improvement of cardiac function in the IR model may be due to the less dynamic inflammation response by the mild heart damage.

Conclusions

Our findings identified the anti-inflammatory effect of ESC-CMs as a possible therapeutic mechanism to improve cardiac function in the ischemic heart.

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1271-4) contains supplementary material, which is available to authorized users.

SETD7 Drives Cardiac Lineage Commitment through Stage-Specific Transcriptional Activation

Cardiac development requires coordinated and large-scale rearrangements of the epigenome. The roles and precise mechanisms through which specific epigenetic modifying enzymes control cardiac lineage specification, however, remain unclear. Here we show that the H3K4 methyltransferase SETD7 controls cardiac differentiation by reading H3K36 marks independently of its enzymatic activity. Through chromatin immunoprecipitation sequencing (ChIP-seq), we found that SETD7 targets distinct sets of genes to drive their stage-specific expression during cardiomyocyte differentiation. SETD7 associates with different co-factors at these stages, including SWI/SNF chromatin-remodeling factors during mesodermal formation and the transcription factor NKX2.5 in cardiac progenitors to drive their differentiation. Further analyses revealed that SETD7 binds methylated H3K36 in the bodies of its target genes to facilitate RNA polymerase II (Pol II)-dependent transcription. Moreover, abnormal SETD7 expression impairs functional attributes of terminally differentiated cardiomyocytes. Together, these results reveal how SETD7 acts at sequential steps in cardiac lineage commitment, and they provide insights into crosstalk between dynamic epigenetic marks and chromatin-modifying enzymes.

Epigenetics, cardiovascular disease, and cellular reprogramming

Under the seeming disorder of "junk" sequences the last decade has seen developments in our understanding of non-coding RNA's (ncRNAs). It's a complex revised order and nowhere is this more relevant than in the developing heart whereby old rules have been set aside to make room for new ones. The development of the mammalian heart has been studied at the genetic and cellular level for several decades because these areas were considered ideal control points. As such, detailed mechanisms governing cell lineages are well described. Emerging evidence suggests a complex new order regulated by epigenetic mechanisms mark cardiac cell lineage. Indeed, molecular cardiologists are in the process of shedding light on the roles played by ncRNAs, nucleic acid methylation and histone/chromatin modifications in specific pathologies of the heart. The aim of this article is to discuss some of the recent advances in the field of cardiovascular epigenetics that are related to direct cell reprogramming and repair. As such, we explore ncRNAs as nodes regulating signaling networks and attempt to make sense of regulatory disorder by reinforcing the importance of epigenetic components in the developmental program.

Use of human induced pluripotent stem cell-derived cardiomyocytes to assess drug cardiotoxicity

Cardiotoxicity has historically been a major cause of drug removal from the pharmaceutical market. Several chemotherapeutic compounds have been noted for their propensities to induce dangerous cardiac-specific side effects such as arrhythmias or cardiomyocyte apoptosis. However, improved preclinical screening methodologies have enabled cardiotoxic compounds to be identified earlier in the drug development pipeline. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) can be used to screen for drug-induced alterations in cardiac cellular contractility, electrophysiology, and viability. We previously established a novel 'cardiac safety index' (CSI) as a metric that can evaluate potential cardiotoxic drugs via high-throughput screening of hiPSC-CMs. This metric quantitatively examines drug-induced alterations in CM function, using several in vitro readouts, and normalizes the resulting toxicity values to the in vivo maximum drug blood plasma concentration seen in preclinical or clinical pharmacokinetic models. In this ~1-month-long protocol, we describe how to differentiate hiPSCs into hiPSC-CMs and subsequently implement contractility and cytotoxicity assays that can evaluate drug-induced cardiotoxicity in hiPSC-CMs. We also describe how to carry out the calculations needed to generate the CSI metric from these quantitative toxicity measurements.

Differentiation of brown adipose-derived stem cells into cardiomyocyte-like cells is regulated by a combination of low 5-azacytidine concentration and bone morphogenetic protein 4

Adipose-derived stem cells (ADSCs) could be an ideal candidate for seed cells to regenerate damaged heart tissue. This study examined and compared the cardio-myogenic differentiation efficacy of neonatal rat brown ADSCs (rbADSCs) treated with either 5-azacytidine (5-AZA), bone morphogenetic protein 4 (BMP4), or lower doses of both molecules. Briefly, by investigating the protein expression of cardiac-specific markers (i.e., cardiac troponin-I, α-sarcomeric actinin, sarcoplasmic reticulum Ca²⁺-ATPase, and connexin 43), our data indicated that rbADSCs could be differentiated into cardiomyocyte-like cells by all three treatments. By quantitatively measuring the number of cells with positive staining for the above markers, we found that the low-dose combined treatment showed higher differentiation efficiency compared to standard dose 5-AZA and BMP4 treatment. Similarly, the expression levels of these proteins as determined by western blotting were higher in the low-dose combination group than in the standard dose 5-AZA and BMP4 groups. Also, the combined strategy maintained the decreased cell viability caused by cytotoxicity of 5-AZA, probably through reducing the ratio of apoptotic rbADSCs. Furthermore, the extracellular regulated protein kinase (ERK) signaling pathways participate in the differentiation process, but the observed effects between the BMP4 and 5-Aza treatments are quite different.

Potential Strategies to Address the Major Clinical Barriers Facing Stem Cell Regenerative Therapy for Cardiovascular Disease: A Review

Importance

Although progress continues to be made in the field of stem cell regenerative medicine for the treatment of cardiovascular disease, significant barriers to clinical implementation still exist.

Objectives

To summarize the current barriers to the clinical implementation of stem cell therapy in patients with cardiovascular disease and to discuss potential strategies to overcome them.

Evidence Review

Information for this review was obtained through a search of PubMed and the Cochrane database for English-language studies published between January 1, 2000, and July 25, 2016. Ten randomized clinical trials and 8 systematic reviews were included.

Findings

One of the major clinical barriers facing the routine implementation of stem cell therapy in patients with cardiovascular disease is the limited and inconsistent benefit observed thus far. Reasons for this finding are unclear but may be owing to poor cell retention and survival, as suggested by numerous preclinical studies and a small number of human studies incorporating imaging to determine cell fate. Additional studies in humans using imaging to determine cell fate are needed to understand how these factors contribute to the limited efficacy of stem cell therapy. Treatment strategies to address poor cell retention and survival are under investigation and include the following: coadministration of immunosuppressive and prosurvival agents, delivery of cardioprotective factors packaged in exosomes rather than the cells themselves, and use of tissue-engineering strategies to provide structural support for cells. If larger grafts are achieved using these strategies, it will be imperative to carefully monitor for the potential risks of tumorigenicity, immunogenicity, and arrhythmogenicity.

Conclusions and Relevance

Despite important achievements to date, stem cell therapy is not yet ready for routine clinical implementation. Significant research is still needed to address the clinical barriers outlined herein before the next wave of large clinical trials is under way.

Pluripotent Stem Cell-Derived Cardiomyocytes as a Platform for Cell Therapy Applications: Progress and Hurdles for Clinical Translation

Cardiovascular diseases are the leading cause of morbidity and mortality worldwide. Regenerative therapy has been applied to restore lost cardiac muscle and cardiac performance. Induced pluripotent stem cells (iPSCs) can provide an unlimited source of cardiomyocytes and therefore play a key role in cardiac regeneration. Despite initial encouraging results from pre-clinical studies, progress toward clinical applications has been hampered by issues such as tumorigenesis, arrhythmogenesis, immune rejection, scalability, low graft-cell survival, and poor engraftment. Here, we review recent developments in iPSC research on regenerating injured heart tissue, including novel advances in cell therapy and potential strategies to overcome current obstacles in the field.

Human embryonic stem cell-derived cardiomyocytes restore function in infarcted hearts of non-human primates

Pluripotent stem cell–derived cardiomyocyte grafts can remuscularize substantial amounts of infarcted myocardium and beat in synchrony with the heart, but in some settings cause ventricular arrhythmias. It is unknown whether human cardiomyocytes can restore cardiac function in a physiologically relevant large animal model. Here we show that transplantation of ~750 million cryopreserved human embryonic stem cell–derived cardiomyocytes (hESC-CMs) enhances cardiac function in macaque monkeys with large myocardial infarctions. One month after hESC-CM transplantation, global left ventricular ejection fraction improved 10.6±0.9% vs. 2.5±0.8% in controls, and by 3 months there was an additional 12.4% improvement in treated vs. a 3.5% decline in controls. Grafts averaged 11.6% of infarct size, formed electromechanical junctions with the host heart and by 3 months contained ~99% ventricular myocytes. A subset of animals experienced graft-associated ventricular arrhythmias, shown by electrical mapping to originate from a point-source acting as an ectopic pacemaker. Our data demonstrate that remuscularization of the infarcted macaque heart with human myocardium provides durable improvement in left ventricular function.

Harnessing cell pluripotency for cardiovascular regenerative medicine

Human pluripotent stem cells (hPSCs), in particular embryonic stem cells and induced pluripotent stem cells, have received enormous attention in cardiovascular regenerative medicine owing to their ability to expand and differentiate into functional cardiomyocytes and other cardiovascular cell types. Despite the potential applications of hPSCs for tissue regeneration in patients suffering from cardiovascular disease, whether hPSC-based therapies can be safe and efficacious remains inconclusive, with strong evidence from clinical trials lacking. Critical factors limiting therapeutic efficacy are the degree of maturity and purity of the hPSC-derived differentiated progeny, and the tumorigenic risk associated with residual undifferentiated cells. In this Review, we discuss recent advances in cardiac-cell differentiation from hPSCs and in the direct reprogramming of non-myocyte cells for cardiovascular regenerative applications. We also discuss approaches for the delivery of cells to diseased tissue, and how such advances are contributing to progress in cardiac tissue engineering for tackling heart disease.

Human-Induced Pluripotent Stem Cell Technology and Cardiomyocyte Generation: Progress and Clinical Applications

Human-induced pluripotent stem cells (hiPSCs) are reprogrammed cells that have hallmarks similar to embryonic stem cells including the capacity of self-renewal and differentiation into cardiac myocytes. The improvements in reprogramming and differentiating methods achieved in the past 10 years widened the use of hiPSCs, especially in cardiac research. hiPSC-derived cardiac myocytes (CMs) recapitulate phenotypic differences caused by genetic variations, making them attractive human disease models and useful tools for drug discovery and toxicology testing. In addition, hiPSCs can be used as sources of cells for cardiac regeneration in animal models. Here, we review the advances in the genetic and epigenetic control of cardiomyogenesis that underlies the significant improvement of the induced reprogramming of somatic cells to CMs; the methods used to improve scalability of throughput assays for functional screening and drug testing in vitro; the phenotypic characteristics of hiPSCs-derived CMs and their ability to rescue injured CMs through paracrine effects; we also cover the novel approaches in tissue engineering for hiPSC-derived cardiac tissue generation, and finally, their immunological features and the potential use in biomedical applications.

Effect of densely ionizing radiation on cardiomyocyte differentiation from human-induced pluripotent stem cells

The process of human cardiac development can be faithfully recapitulated in a culture dish with human pluripotent stem cells, where the impact of environmental stressors can be evaluated. The consequences of ionizing radiation exposure on human cardiac differentiation are largely unknown. In this study, human-induced pluripotent stem cell cultures (hiPSCs) were subjected to an external beam of 3.7 MeV α-particles at low mean absorbed doses of 0.5, 3, and 10 cGy. Subsequently, the hiPSCs were differentiated into beating cardiac myocytes (hiPSC-CMs). Pluripotent and cardiac markers and morphology did not reveal differences between the irradiated and nonirradiated groups. While cell number was not affected during CM differentiation, cell number of differentiated CMs was severely reduced by ionizing radiation in a dose-responsive manner. β-adrenergic stimulation causes calcium (Ca²⁺) overload and oxidative stress. Although no significant increase in Ca²⁺ transient amplitude was observed in any group after treatment with 1 μmol/L isoproterenol, the incidence of spontaneous Ca²⁺ waves/releases was more frequent in hiPSC-CMs of the irradiated groups, indicating arrhythmogenic activities at the single cell level. Increased transcript expression of mitochondrial biomarkers (LONP1, TFAM) and mtDNA-encoded genes (MT-CYB, MT-RNR1) was detected upon differentiation of hiPSC-CMs suggesting increased organelle biogenesis. Exposure of hiPSC-CM cultures to 10 cGy significantly upregulated MT-CYB and MT-RNR1 expression, which may reflect an adaptive response to ionizing radiation. Our results indicate that important aspects of differentiation of hiPSCs into cardiac myocytes may be affected by low fluences of densely ionizing radiations such as α-particles.

Applications of genetically engineered human pluripotent stem cell reporters in cardiac stem cell biology

The advent of human pluripotent stem cells (hPSCs) has benefited many fields, from regenerative medicine to disease modeling, with an especially profound effect in cardiac research. Coupled with other novel technologies in genome engineering, hPSCs offer a great opportunity to delineate human cardiac lineages, investigate inherited cardiovascular diseases, and assess the safety and efficacy of cell-based therapies. In this review, we provide an overview of methods for generating genetically engineered hPSC reporters and a succinct synopsis of a variety of hPSC reporters, with a particular focus on their applications in cardiac stem cell biology.

Loss of microRNA-128 promotes cardiomyocyte proliferation and heart regeneration

The goal of replenishing the cardiomyocyte (CM) population using regenerative therapies following myocardial infarction (MI) is hampered by the limited regeneration capacity of adult CMs, partially due to their withdrawal from the cell cycle. Here, we show that microRNA-128 (miR-128) is upregulated in CMs during the postnatal switch from proliferation to terminal differentiation. In neonatal mice, cardiac-specific overexpression of miR-128 impairs CM proliferation and cardiac function, while miR-128 deletion extends proliferation of postnatal CMs by enhancing expression of the chromatin modifier SUZ12, which suppresses p27 (cyclin-dependent kinase inhibitor) expression and activates the positive cell cycle regulators Cyclin E and CDK2. Furthermore, deletion of miR-128 promotes cell cycle re-entry of adult CMs, thereby reducing the levels of fibrosis, and attenuating cardiac dysfunction in response to MI. These results suggest that miR-128 serves as a critical regulator of endogenous CM proliferation, and might be a novel therapeutic target for heart repair.

During early postnatal development in mammals, cardiomyocytes exit the cell cycle, losing their regenerative capacity. Here the authors show that, following myocardial infarction, loss of microRNA-128 promotes cardiomyocyte proliferation and cardiac regeneration in adult mice partly via enhancing the expression of the chromatin modifier SUZ12.

Distinct epigenetic programs regulate cardiac myocyte development and disease in the human heart in vivo

Epigenetic mechanisms and transcription factor networks essential for differentiation of cardiac myocytes have been uncovered. However, reshaping of the epigenome of these terminally differentiated cells during fetal development, postnatal maturation, and in disease remains unknown. Here, we investigate the dynamics of the cardiac myocyte epigenome during development and in chronic heart failure. We find that prenatal development and postnatal maturation are characterized by a cooperation of active CpG methylation and histone marks at cis-regulatory and genic regions to shape the cardiac myocyte transcriptome. In contrast, pathological gene expression in terminal heart failure is accompanied by changes in active histone marks without major alterations in CpG methylation and repressive chromatin marks. Notably, cis-regulatory regions in cardiac myocytes are significantly enriched for cardiovascular disease-associated variants. This study uncovers distinct layers of epigenetic regulation not only during prenatal development and postnatal maturation but also in diseased human cardiac myocytes.

Differentiation and Contractile Analysis of GFP-Sarcomere Reporter hiPSC-Cardiomyocytes

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) represent a powerful cellular platform for illuminating mechanisms of human cardiovascular disease and for pharmacological screening. Recent advances in CRISPR/Cas9-mediated genome editing technology underlie this profound utility. We have generated hiPSC-CMs harboring fluorescently-tagged sarcomeric proteins, which provide a tool to non-invasively study human sarcomere function and dysfunction. In this unit, we illustrate methods for conducting high-efficiency, small molecule-mediated differentiation of hiPSCs into cardiomyocytes, and for performing non-invasive contractile analysis through direct sarcomere tracking of GFP-sarcomere reporter hiPSC-CMs. We believe that this type of analysis can overcome sensitivity problems found in other forms of contractile assays involving hiPSC-CMs by directly measuring contractility at the fundamental contractile unit of the hiPSC-CM, the sarcomere. © 2018 by John Wiley & Sons, Inc.

Stem Cell Therapy for Congenital Heart Disease: A Systematic Review

BACKGROUND

Congenital heart disease (CHD) constitutes the most prevalent and heterogeneous group of congenital anomalies. Although surgery remains the gold standard treatment modality, stem cell therapy has been gaining ground as a complimentary or alternative treatment option in certain types of CHD. The aim of this study was to present the existing published evidence and ongoing research efforts on the implementation of stem cell-based therapeutic strategies in CHD.

METHODS

A systematic review was conducted by searching Medline, ClinicalTrials.gov, and the Cochrane library, along with reference lists of the included studies through April 23, 2017.

RESULTS

Nineteen studies were included in this review (8 preclinical, 6 clinical, and 5 ongoing trials). Various routes of cardiac stem cell delivery have been reported, including intracoronary, intramyocardial, intravenous, and epicardial. Depending on their origin and level of differentiation at which they are harvested, stem cells may exhibit different properties. Preclinical studies have mostly focused on modeling right ventricle dysfunction or failure and pulmonary artery hypertension by using pressure or volume overload in vitro or in vivo. Only a limited number of clinical trials on patients with CHD exist, and these primarily focus on hypoplastic left heart syndrome. Cell-based tissue engineering has recently been introduced, and research currently is focusing on developing cell-seeded grafts and patches that could potentially grow in parallel with whole body growth once implanted in the heart.

CONCLUSIONS

It seems that stem cell delivery to the diseased heart as an adjunct to surgical palliation may provide some benefits over surgery alone in terms of cardiac function, somatic growth, and quality of life. Despite encouraging preliminary results, stem cell therapies for patients with CHD should only be considered in the setting of well-designed clinical trials. More wet laboratory research experience is needed, and translation of promising findings to large clinical studies is warranted to clearly define the efficacy and safety profile of this alternative and potentially groundbreaking therapeutic approach.

Integrating cardiomyocytes from human pluripotent stem cells in safety pharmacology: has the time come?

Cardiotoxicity is a severe side effect of drugs that induce structural or electrophysiological changes in heart muscle cells. As a result, the heart undergoes failure and potentially lethal arrhythmias. It is still a major reason for drug failure in preclinical and clinical phases of drug discovery. Current methods for predicting cardiotoxicity are based on guidelines that combine electrophysiological analysis of cell lines expressing ion channels ectopically in vitro with animal models and clinical trials. Although no new cases of drugs linked to lethal arrhythmias have been reported since the introduction of these guidelines in 2005, their limited predictive power likely means that potentially valuable drugs may not reach clinical practice. Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) are now emerging as potentially more predictive alternatives, particularly for the early phases of preclinical research. However, these cells are phenotypically immature and culture and assay methods not standardized, which could be a hurdle to the development of predictive computational models and their implementation into the drug discovery pipeline, in contrast to the ambitions of the comprehensive pro-arrhythmia in vitro assay (CiPA) initiative. Here, we review present and future preclinical cardiotoxicity screening and suggest possible hPSC-CM-based strategies that may help to move the field forward. Coordinated efforts by basic scientists, companies and hPSC banks to standardize experimental conditions for generating reliable and reproducible safety indices will be helpful not only for cardiotoxicity prediction but also for precision medicine.

LINKED ARTICLES

This article is part of a themed section on New Insights into Cardiotoxicity Caused by Chemotherapeutic Agents. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.21/issuetoc.

Cell Type-Specific Chromatin Signatures Underline Regulatory DNA Elements in Human Induced Pluripotent Stem Cells and Somatic Cells

RATIONALE

Regulatory DNA elements in the human genome play important roles in determining the transcriptional abundance and spatiotemporal gene expression during embryonic heart development and somatic cell reprogramming. It is not well known how chromatin marks in regulatory DNA elements are modulated to establish cell type-specific gene expression in the human heart.

OBJECTIVE

We aimed to decipher the cell type-specific epigenetic signatures in regulatory DNA elements and how they modulate heart-specific gene expression.

METHODS AND RESULTS

We profiled genome-wide transcriptional activity and a variety of epigenetic marks in the regulatory DNA elements using massive RNA-seq (n=12) and ChIP-seq (chromatin immunoprecipitation combined with high-throughput sequencing; n=84) in human endothelial cells (CD31⁺CD144⁺), cardiac progenitor cells (Sca-1⁺), fibroblasts (DDR2⁺), and their respective induced pluripotent stem cells. We uncovered 2 classes of regulatory DNA elements: class I was identified with ubiquitous enhancer (H3K4me1) and promoter (H3K4me3) marks in all cell types, whereas class II was enriched with H3K4me1 and H3K4me3 in a cell type-specific manner. Both class I and class II regulatory elements exhibited stimulatory roles in nearby gene expression in a given cell type. However, class I promoters displayed more dominant regulatory effects on transcriptional abundance regardless of distal enhancers. Transcription factor network analysis indicated that human induced pluripotent stem cells and somatic cells from the heart selected their preferential regulatory elements to maintain cell type-specific gene expression. In addition, we validated the function of these enhancer elements in transgenic mouse embryos and human cells and identified a few enhancers that could possibly regulate the cardiac-specific gene expression.

CONCLUSIONS

Given that a large number of genetic variants associated with human diseases are located in regulatory DNA elements, our study provides valuable resources for deciphering the epigenetic modulation of regulatory DNA elements that fine-tune spatiotemporal gene expression in human cardiac development and diseases.

Improving cardiac reprogramming for heart regeneration

PURPOSE OF REVIEW

Cardiovascular disease is the leading cause of death in the world today, and the death rate has remained virtually unchanged in the last 20 years (American Heart Association). This severe life-threatening disease underscores a critical need for developing novel therapeutic strategies to effectively treat this devastating disease. Cell-based therapy represents an extremely promising approach. Generation of induced cardiomyocytes (iCMs) directly from fibroblasts offers an attractive novel strategy for in-situ heart regeneration. Major challenges of iCM reprogramming include the low conversion rate and heterogeneity of the iCMs. This review will summarize the major advancements in improving the iCM reprogramming efficiency and iCM maturation.

RECENT FINDINGS

Numerous studies have been published in the past 18 months to describe various strategies for achieving more efficient iCM reprogramming. These strategies are based on our understanding of the molecular mechanisms of cardiogenesis, which include transcriptional networks, signaling pathways and epigenetic cell fate change.

SUMMARY

Novel strategies for highly efficient iCM reprogramming will be required for applying iCM reprogramming to patients. Creative and combined methods based on our understanding of cardiogenesis will continue to contribute heavily in the advancement of iCM reprogramming. We are highly optimistic that iCM reprogramming-based heart therapy will restore the pumping function of damaged patient hearts.