Sinoatrial node cardiomyocytes derived from human pluripotent cells function as a biological pacemaker

Isolation and characterization of hESC-derived heart field-specific cardiomyocytes unravels new insights into their transcriptional and electrophysiological profiles

AIMS

We prospectively isolate and characterize first and second heart field- and nodal-like cardiomyocytes using a double reporter line from human embryonic stem cells. Our double reporter line utilizes two important transcription factors in cardiac development, TBX5 and NKX2-5. TBX5 expression marks first heart field progenitors and cardiomyocytes while NKX2-5 is expressed in nearly all myocytes of the developing heart (excluding nodal cells). We address the shortcomings of prior work in the generation of heart-field specific cardiomyocytes from induced pluripotent stem cells and provide a comprehensive early developmental transcriptomic as well as electrophysiological analyses of these three populations.

METHODS AND RESULTS

Transcriptional, immunocytochemical, and functional studies support the cellular identities of isolated populations based on the expression pattern of NKX2-5 and TBX5. Importantly, bulk and single-cell RNA sequencing analyses provide evidence of unique molecular signatures of isolated first and second heart-field cardiomyocytes, as well as nodal-like cells. Extensive electrophysiological analyses reveal dominant atrial action potential phenotypes in first and second heart fields in alignment with our findings in single-cell RNA sequencing. Lastly, we identify two novel surface markers, POPDC2 and CORIN, that enables purification of cardiomyocytes and first heart field cardiomyocytes, respectively.

CONCLUSIONS

We describe a high yield approach for isolation and characterization of human embryonic stem cell-derived heart field specific and nodal-like cardiomyocytes. Obtaining enriched populations of these different cardiomyocyte subtypes increases the resolution of gene expression profiling during early cardiogenesis, arrhythmia modeling, and drug screening. This paves the way for the development of effective stem cell therapy to treat diseases that affect specific regions of the heart or chamber-specific congenital heart defects.

TRANSLATIONAL PERSPECTIVE

Myocardial infarction leads to irreversible loss of cardiomyocytes and eventually heart failure. Human embryonic stem cells (hESCs) can be differentiated to cardiomyocytes and are considered a potential source of cell therapy for cardiac regeneration. However, current differentiation strategies yield a mixture of cardiomyocyte subtypes and safety concerns stemming from the use of a heterogenous population of cardiomyocytes have hindered its application. Here, we report generation of enriched heart field-specific cardiomyocytes using a hESC double reporter. Our study facilitates investigating early human cardiogenesis in vitro and generating chamber-specific cardiomyocytes to treat diseases that affect specific regions of the heart.

Bioengineering Clinically Relevant Cardiomyocytes and Cardiac Tissues from Pluripotent Stem Cells

The regenerative capacity of cardiomyocytes is insufficient to functionally recover damaged tissue, and as such, ischaemic heart disease forms the largest proportion of cardiovascular associated deaths. Human-induced pluripotent stem cells (hiPSCs) have enormous potential for developing patient specific cardiomyocytes for modelling heart disease, patient-based cardiac toxicity testing and potentially replacement therapy. However, traditional protocols for hiPSC-derived cardiomyocytes yield mixed populations of atrial, ventricular and nodal-like cells with immature cardiac properties. New insights gleaned from embryonic heart development have progressed the precise production of subtype-specific hiPSC-derived cardiomyocytes; however, their physiological immaturity severely limits their utility as model systems and their use for drug screening and cell therapy. The long-entrenched challenges in this field are being addressed by innovative bioengingeering technologies that incorporate biophysical, biochemical and more recently biomimetic electrical cues, with the latter having the potential to be used to both direct hiPSC differentiation and augment maturation and the function of derived cardiomyocytes and cardiac tissues by mimicking endogenous electric fields.

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

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

Genetic Cardiomyopathies: The Lesson Learned from hiPSCs

Genetic cardiomyopathies represent a wide spectrum of inherited diseases and constitute an important cause of morbidity and mortality among young people, which can manifest with heart failure, arrhythmias, and/or sudden cardiac death. Multiple underlying genetic variants and molecular pathways have been discovered in recent years; however, assessing the pathogenicity of new variants often needs in-depth characterization in order to ascertain a causal role in the disease. The application of human induced pluripotent stem cells has greatly helped to advance our knowledge in this field and enabled to obtain numerous in vitro patient-specific cellular models useful to study the underlying molecular mechanisms and test new therapeutic strategies. A milestone in the research of genetically determined heart disease was the introduction of genomic technologies that provided unparalleled opportunities to explore the genetic architecture of cardiomyopathies, thanks to the generation of isogenic pairs. The aim of this review is to provide an overview of the main research that helped elucidate the pathophysiology of the most common genetic cardiomyopathies: hypertrophic, dilated, arrhythmogenic, and left ventricular noncompaction cardiomyopathies. A special focus is provided on the application of gene-editing techniques in understanding key disease characteristics and on the therapeutic approaches that have been tested.

Tissue Chips and Microphysiological Systems for Disease Modeling and Drug Testing

Tissue chips (TCs) and microphysiological systems (MPSs) that incorporate human cells are novel platforms to model disease and screen drugs and provide an alternative to traditional animal studies. This review highlights the basic definitions of TCs and MPSs, examines four major organs/tissues, identifies critical parameters for organization and function (tissue organization, blood flow, and physical stresses), reviews current microfluidic approaches to recreate tissues, and discusses current shortcomings and future directions for the development and application of these technologies. The organs emphasized are those involved in the metabolism or excretion of drugs (hepatic and renal systems) and organs sensitive to drug toxicity (cardiovascular system). This article examines the microfluidic/microfabrication approaches for each organ individually and identifies specific examples of TCs. This review will provide an excellent starting point for understanding, designing, and constructing novel TCs for possible integration within MPS.

Decoding Genetics of Congenital Heart Disease Using Patient-Derived Induced Pluripotent Stem Cells (iPSCs)

Congenital heart disease (CHD) is the most common cause of infant death associated with birth defects. Recent next-generation genome sequencing has uncovered novel genetic etiologies of CHD, from inherited and de novo variants to non-coding genetic variants. The next phase of understanding the genetic contributors of CHD will be the functional illustration and validation of this genome sequencing data in cellular and animal model systems. Human induced pluripotent stem cells (iPSCs) have opened up new horizons to investigate genetic mechanisms of CHD using clinically relevant and patient-specific cardiac cells such as cardiomyocytes, endothelial/endocardial cells, cardiac fibroblasts and vascular smooth muscle cells. Using cutting-edge CRISPR/Cas9 genome editing tools, a given genetic variant can be corrected in diseased iPSCs and introduced to healthy iPSCs to define the pathogenicity of the variant and molecular basis of CHD. In this review, we discuss the recent progress in genetics of CHD deciphered by large-scale genome sequencing and explore how genome-edited patient iPSCs are poised to decode the genetic etiologies of CHD by coupling with single-cell genomics and organoid technologies.

Cellular and molecular landscape of mammalian sinoatrial node revealed by single-cell RNA sequencing

Bioelectrical impulses intrinsically generated within the sinoatrial node (SAN) trigger the contraction of the heart in mammals. Though discovered over a century ago, the molecular and cellular features of the SAN that underpin its critical function in the heart are uncharted territory. Here, we identify four distinct transcriptional clusters by single-cell RNA sequencing in the mouse SAN. Functional analysis of differentially expressed genes identifies a core cell cluster enriched in the electrogenic genes. The similar cellular features are also observed in the SAN from both rabbit and cynomolgus monkey. Notably, Vsnl1, a core cell cluster marker in mouse, is abundantly expressed in SAN, but is barely detectable in atrium or ventricle, suggesting that Vsnl1 is a potential SAN marker. Importantly, deficiency of Vsnl1 not only reduces the beating rate of human induced pluripotent stem cell - derived cardiomyocytes (hiPSC-CMs) but also the heart rate of mice. Furthermore, weighted gene co-expression network analysis (WGCNA) unveiled the core gene regulation network governing the function of the SAN in mice. Overall, these findings reveal the whole transcriptome profiling of the SAN at single-cell resolution, representing an advance toward understanding of both the biology and the pathology of SAN.

The spontaneous bioelectrical activity of pacemaker cells in sinoatrial node (SAN) triggers the heartbeats. Here, the authors perform single-cell RNA sequencing in the mouse SAN and identify molecular and cellular features of the SAN conserved in rabbit and cynomolgus monkey, identifying a new potential SAN marker.

Toward Biological Pacing by Cellular Delivery of Hcn2/SkM1

Electronic pacemakers still face major shortcomings that are largely intrinsic to their hardware-based design. Radical improvements can potentially be generated by gene or cell therapy-based biological pacemakers. Our previous work identified adenoviral gene transfer of Hcn2 and SkM1, encoding a "funny current" and skeletal fast sodium current, respectively, as a potent combination to induce short-term biological pacing in dogs with atrioventricular block. To achieve long-term biological pacemaker activity, alternative delivery platforms need to be explored and optimized. The aim of the present study was therefore to investigate the functional delivery of Hcn2/SkM1 via human cardiomyocyte progenitor cells (CPCs). Nucleofection of Hcn2 and SkM1 in CPCs was optimized and gene transfer was determined for Hcn2 and SkM1 in vitro. The modified CPCs were analyzed using patch-clamp for validation and characterization of functional transgene expression. In addition, biophysical properties of Hcn2 and SkM1 were further investigated in lentivirally transduced CPCs by patch-clamp analysis. To compare both modification methods in vivo, CPCs were nucleofected or lentivirally transduced with GFP and injected in the left ventricle of male NOD-SCID mice. After 1 week, hearts were collected and analyzed for GFP expression and cell engraftment. Subsequent functional studies were carried out by computational modeling. Both nucleofection and lentiviral transduction of CPCs resulted in functional gene transfer of Hcn2 and SkM1 channels. However, lentiviral transduction was more efficient than nucleofection-mediated gene transfer and the virally transduced cells survived better in vivo. These data support future use of lentiviral transduction over nucleofection, concerning CPC-based cardiac gene delivery. Detailed patch-clamp studies revealed Hcn2 and Skm1 current kinetics within the range of previously reported values of other cell systems. Finally, computational modeling indicated that CPC-mediated delivery of Hcn2/SkM1 can generate stable pacemaker function in human ventricular myocytes. These modeling studies further illustrated that SkM1 plays an essential role in the final stage of diastolic depolarization, thereby enhancing biological pacemaker functioning delivered by Hcn2. Altogether these studies support further development of CPC-mediated delivery of Hcn2/SkM1 and functional testing in bradycardia models.

Genome-Wide Analysis Identifies an Essential Human TBX3 Pacemaker Enhancer

RATIONALE

The development and function of the pacemaker cardiomyocytes of the sinoatrial node (SAN), the leading pacemaker of the heart, are tightly controlled by a conserved network of transcription factors, including TBX3 (T-box transcription factor 3), ISL1 (ISL LIM homeobox 1), and SHOX2 (short stature homeobox 2). Yet, the regulatory DNA elements (REs) controlling target gene expression in the SAN pacemaker cells have remained undefined.

OBJECTIVE

Identification of the regulatory landscape of human SAN-like pacemaker cells and functional assessment of SAN-specific REs potentially involved in pacemaker cell gene regulation.

METHODS AND RESULTS

We performed Assay for Transposase-Accessible Chromatin using sequencing on human pluripotent stem cell-derived SAN-like pacemaker cells and ventricle-like cells and identified thousands of putative REs specific for either human cell type. We validated pacemaker cell-specific elements in the SHOX2 and TBX3 loci. CRISPR-mediated homozygous deletion of the mouse ortholog of a noncoding region with candidate pacemaker-specific REs in the SHOX2 locus resulted in selective loss of Shox2 expression from the developing SAN and embryonic lethality. Putative pacemaker-specific REs were identified up to 1 Mbp upstream of TBX3 in a region close to MED13L harboring variants associated with heart rate recovery after exercise. The orthologous region was deleted in mice, which resulted in selective loss of expression of Tbx3 from the SAN and (cardiac) ganglia and in neonatal lethality. Expression of Tbx3 was maintained in other tissues including the atrioventricular conduction system, lungs, and liver. Heterozygous adult mice showed increased SAN recovery times after pacing. The human REs harboring the associated variants robustly drove expression in the SAN of transgenic mouse embryos.

CONCLUSIONS

We provided a genome-wide collection of candidate human pacemaker-specific REs, including the loci of SHOX2, TBX3, and ISL1, and identified a link between human genetic variants influencing heart rate recovery after exercise and a variant RE with highly conserved function, driving SAN expression of TBX3.

Development of the Cardiac Conduction System

The cardiac conduction system initiates and propagates each heartbeat. Specialized conducting cells are a well-conserved phenomenon across vertebrate evolution, although mammalian and avian species harbor specific components unique to organisms with four-chamber hearts. Early histological studies in mammals provided evidence for a dominant pacemaker within the right atrium and clarified the existence of the specialized muscular axis responsible for atrioventricular conduction. Building on these seminal observations, contemporary genetic techniques in a multitude of model organisms has characterized the developmental ontogeny, gene regulatory networks, and functional importance of individual anatomical compartments within the cardiac conduction system. This review describes in detail the transcriptional and regulatory networks that act during cardiac conduction system development and homeostasis with a particular emphasis on networks implicated in human electrical variation by large genome-wide association studies. We conclude with a discussion of the clinical implications of these studies and describe some future directions.

Enhancing Matured Stem-Cardiac Cell Generation and Transplantation: A Novel Strategy for Heart Failure Therapy

Heart failure (HF) remains one of the major causes of morbidity and mortality worldwide. Recent studies have shown that stem cells (SCs) including bone marrow mesenchymal stem (BMSC), embryonic bodies (EB), embryonic stem (ESC), human induced pluripotent stem (hiPSC)-derived cardiac cells generation, and transplantation treated myocardial infarction (MI) in vivo and in human. However, the immature phenotypes compromise their clinical application requiring immediate intervention to improve stem-derived cardiac cell (S-CCs) maturation. Recently, an unbiased multi-omic analysis involving genomics, transcriptomics, epigenomics, proteomics, and metabolomics identified specific strategies for the generation of matured S-CCs that may enhance patients' recovery processes upon transplantation. However, these strategies still remain undisclosed. Here, we summarize the recently discovered strategies for the matured S-CC generation. In addition, cardiac patch formation and transplantation that accelerated HF recuperation in clinical trials are discussed. A better understanding of this work may lead to efficient generation of matured S-CCs for regenerative medicine. Graphical abstract.

Pluripotent Stem Cells for Cardiac Regeneration - Current Status, Challenges, and Future Perspectives

Loss of myocardium permanently impairs cardiac function because the adult mammalian heart has limited regenerative capacity. Strategies to regenerate injured heart tissue include the transplantation of multiple types of stem cells. Among them, pluripotent stem cells (PSCs) are a promising option because of their unlimited self-renewal and unequivocal cardiomyogenic ability. To date, advances in stem cell biology allow generation of relatively homogeneous human PSC-derived cardiomyocytes (CMs). In this regard, preclinical studies of PSC-CM transplantation in rodents and larger animal models have provided convincing proof-of-concept results, triggering clinical studies in multiple countries. However, a few important uncertainties are yet to be addressed, warranting further investigation before clinical implementation of this novel therapy. An overview of the potential of stem cell therapy to provide new CMs for cardiac regeneration is presented.

hiPSC-Derived Cardiac Tissue for Disease Modeling and Drug Discovery

Relevant, predictive normal, or disease model systems are of vital importance for drug development. The difference between nonhuman models and humans could contribute to clinical trial failures despite ideal nonhuman results. As a potential substitute for animal models, human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes (CMs) provide a powerful tool for drug toxicity screening, modeling cardiovascular diseases, and drug discovery. Here, we review recent hiPSC-CM disease models and discuss the features of hiPSC-CMs, including subtype and maturation and the tissue engineering technologies for drug assessment. Updates from the international multisite collaborators/administrations for development of novel drug discovery paradigms are also summarized.

A Concise Review on Induced Pluripotent Stem Cell-Derived Cardiomyocytes for Personalized Regenerative Medicine

The induced pluripotent stem cells (iPSCs) are derived from somatic cells by using reprogramming factors such as Oct4, Sox2, Klf4, and c-Myc (OSKM) or Oct4, Sox2, Nanog and Lin28 (OSNL). They resemble embryonic stem cells (ESCs) and have the ability to differentiate into cell lineage of all three germ-layer, including cardiomyocytes (CMs). The CMs can be generated from iPSCs by inducing embryoid bodies (EBs) formation and treatment with activin A, bone morphogenic protein 4 (BMP4), and inhibitors of Wnt signaling. However, these iPSC-derived CMs are a heterogeneous population of cells and require purification and maturation to mimic the in vivo CMs. The matured CMs can be used for various therapeutic purposes in regenerative medicine by cardiomyoplasty or through the development of tissue-engineered cardiac patches. In recent years, significant advancements have been made in the isolation of iPSC and their differentiation, purification, and maturation into clinically usable CMs. Newer small molecules have also been identified to substitute the reprogramming factors for iPSC generation as well as for direct differentiation of somatic cells into CMs without an intermediary pluripotent state. This review provides a concise update on the generation of iPSC-derived CMs and their application in personalized cardiac regenerative medicine. It also discusses the current limitations and challenges in the application of iPSC-derived CMs. Graphical abstract.

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

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

Gene and Cell Therapy for Cardiac Arrhythmias

PURPOSE

In the last decade, interest in gene therapy as a therapeutic technology has increased, largely driven by an exciting yet modest number of successful applications for monogenic diseases. Setbacks in the use of gene therapy for cardiac disease have motivated efforts to develop vectors with enhanced tropism for the heart and more efficient delivery methods. Although monogenic diseases are the logical target, cardiac arrhythmias represent a group of conditions amenable to gene therapy because of focal targets (biological pacemakers, nodal conduction, or stem cell-related arrhythmias) or bystander effects on cells not directly transduced because of electrical coupling.

METHODS

This review provides a contemporary narrative of the field of gene therapy for experimental cardiac arrhythmias, including those associated with stem cell transplant. Recent articles published in the English language and available through the PubMed database and other prominent literature are discussed.

FINDINGS

The promise of gene therapy has been realized for a handful of monogenic diseases and is actively being pursued for cardiac applications in preclinical models. With improved vectors, it is likely that cardiac disease will also benefit from this technology. Cardiac arrhythmias, whether inherited or acquired, are a group of conditions with a potentially lower threshold for phenotypic correction and as such hold unique potential as targets for cardiac gene therapy.

IMPLICATIONS

There has been a proliferation of research on the potential of gene therapy for cardiac arrhythmias. This body of investigation forms a strong basis on which further developments, particularly with viral vectors, are likely to help this technology progress along its translational trajectory.

Human Pluripotent Stem Cell-Derived Cardiovascular Cells: From Developmental Biology to Therapeutic Applications

Advances in our understanding of cardiovascular development have provided a roadmap for the directed differentiation of human pluripotent stem cells (hPSCs) to the major cell types found in the heart. In this Perspective, we review the state of the field in generating and maturing cardiovascular cells from hPSCs based on our fundamental understanding of heart development. We then highlight their applications for studying human heart development, modeling disease-performing drug screening, and cell replacement therapy. With the advancements highlighted here, the promise that hPSCs will deliver new treatments for degenerative and debilitating diseases may soon be fulfilled.

Assessment of Cardiotoxicity With Stem Cell-based Strategies

PURPOSE

Adverse cardiovascular drug effects pose a substantial medical risk and represent a common cause of drug withdrawal from the market. Thus, current in vitro assays and in vivo animal models still have shortcomings in assessing cardiotoxicity. A human model for more accurate preclinical cardiotoxicity assessment is highly desirable. Current differentiation protocols allow for the generation of human pluripotent stem cell-derived cardiomyocytes in basically unlimited numbers and offer the opportunity to study drug effects on human cardiomyocytes. The purpose of this review is to provide a brief overview of the current approaches to translate studies with pluripotent stem cell-derived cardiomyocytes from basic science to preclinical risk assessment.

METHODS

A review of the literature was performed to gather data on the pathophysiology of cardiotoxicity, the current cardiotoxicity screening assays, stem cell-derived cardiomyocytes, and their application in cardiotoxicity screening.

FINDINGS

There is increasing evidence that stem cell-derived cardiomyocytes predict arrhythmogenicity with high accuracy. Cardiomyocyte immaturity represents the major limitation so far. However, strategies are being developed to overcome this hurdle, such as tissue engineering. In addition, stem cell-based strategies offer the possibility to assess structural drug toxicity (eg, by anticancer drugs) on complex models that more closely mirror the structure of the heart and contain endothelial cells and fibroblasts.

IMPLICATIONS

Pluripotent stem cell-derived cardiomyocytes have the potential to substantially change how preclinical cardiotoxicity screening is performed. To which extent they will replace or complement current approaches is being evaluated.

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.

Rapamycin Promotes Cardiomyocyte Differentiation of Human Induced Pluripotent Stem Cells in a Stage-Dependent Manner

Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) are a promising source for cardiac regenerative therapy, and ideal for in vitro cell modeling of cardiovascular diseases and drug screening. Recent studies have shown that rapamycin can promote cardiomyocyte differentiation in various stem cells. However, how rapamycin affects cardiomyocyte differentiation of iPSCs is still not fully understood. This study aimed to investigate the effect of rapamycin on cardiomyocyte differentiation based on embryoid body (EB) method. First, to determine the autophagy induction protocol, different concentrations of rapamycin were applied in hEBs on day 6. The autophagy was most significant when applying rapamycin at 1 μM for 48 h, demonstrating by the LC3II/LC3I ratio and p62 expression. Then, 1 μM rapamycin was applied for 48 h at different time points of cardiomyocyte differentiation to investigate the role of rapamycin in this process. Compared with control, rapamycin applied on days 0-4 of differentiation significantly decreased the proportion of beating EBs and expression of cardiomyocyte-specific genes, while rapamycin applied on days 4-14 significantly increased them. Among all groups, rapamycin applied on days 4-6 achieved highest cardiomyocyte differentiation efficiency. Furthermore, using autophagy inhibitor NH₄Cl and GSK-3β inhibitor CHIR-99021, we found rapamycin-induced autophagy promoted cardiomyocyte differentiation at middle stage by negatively regulating the Wnt/β-catenin signaling pathway. These results suggest that rapamycin regulates EB-based cardiomyocyte differentiation in a stage-dependent manner, and the negative regulation of Wnt/β-catenin signaling pathway by autophagy was involved in the prodifferentiation effect of rapamycin at middle stage.

Robust Cardiac Regeneration: Fulfilling the Promise of Cardiac Cell Therapy

PURPOSE

We review the history of cardiac cell therapy, highlighting lessons learned from initial adult stem cell (ASC) clinical trials. We present pluripotent stem cell-derived cardiomyocytes (PSC-CMs) as a leading candidate for robust regeneration of infarcted myocardium but identify several issues that must be addressed before successful clinical translation.

METHODS

We conducted an unstructured literature review of PubMed-listed articles, selecting the most comprehensive and relevant research articles, review articles, clinical trials, and basic or translation articles in the field of cardiac cell therapy. Articles were identified using the search terms adult stem cells, pluripotent stem cells, cardiac stem cell, and cardiac regeneration or from references of relevant articles, Articles were prioritized and selected based on their impact, originality, or potential clinical applicability.

FINDINGS

Since its inception, the ASC therapy field has been troubled by conflicting preclinical data, academic controversies, and inconsistent trial designs. These issues have damaged perceptions of cardiac cell therapy among investors, the academic community, health care professionals, and, importantly, patients. In hindsight, the key issue underpinning these problems was the inability of these cell types to differentiate directly into genuine cardiomyocytes, rendering them unable to replace damaged myocardium. Despite this, beneficial effects through indirect paracrine or immunomodulatory effects remain possible and continue to be investigated. However, in preclinical models, PSC-CMs have robustly remuscularized infarcted myocardium with functional, force-generating cardiomyocytes. Hence, PSC-CMs have now emerged as a leading candidate for cardiac regeneration, and unpublished reports of first-in-human delivery of these cells have recently surfaced. However, the cardiac cell therapy field's history should serve as a cautionary tale, and we identify several translational hurdles that still remain. Preclinical solutions to issues such as arrhythmogenicity, immunogenicity, and poor engraftment rates are needed, and next-generation clinical trials must draw on robust knowledge of mechanistic principles of the therapy.

IMPLICATIONS

The clinical transplantation of functional stem cell-derived heart tissue with seamless integration into native myocardium is a lofty goal. However, considerable advances have been made during the past 2 decades. Currently, PSC-CMs appear to be the best prospect to reach this goal, but several hurdles remain. The history of adult stem cell trials has taught us that shortcuts cannot be taken without dire consequences, and it is essential that progress not be hurried and that a worldwide, cross-disciplinary approach be used to ensure safe and effective clinical translation.

Overexpression of TBX3 in human induced pluripotent stem cells (hiPSCs) increases their differentiation into cardiac pacemaker-like cells

BACKGROUD

The TBX3(T-box 3)transcription factor is considered as an essential factor in sinoatrial node formation. While the effect of TBX3 in the differentiation of sinoatrial node cells from embryonic stem cells(ESCs) has been recognized, its role in human induced pluripotent stem cell derived cardiomyocytes(hiPSCMs) has not been addressed. Therefore, the purpose of the present study was to investigate whether overexpression of TBX3 in hiPSCs could increase their differentiation into pacemaker-like cells.

METHODS

The hiPSCs were transfected with TBX3 gene during differentiation into cardiomyocytes(CMs). The hiPSCMs were analyzed using immunofluorescence, RT-qPCR, flow cytometry, whole-cell patch clamp recording to identify the differentiation effect exerted by TBX3. We discovered that hiPSCs transfected with TBX3 showed more proportions of NKX2.5-cTNT + sinoatrial node cells and faster contracting rates.

RESULTS

The results showed increment in transcription factor TBX18, SHOX2; hyperpolarization-activated cyclic nucleotide (HCN) channel: HCN1, HCN2, HCN4, connexin 45(CX45), Na + Ca2+ exchanger(NCX) in TBX3 transfected hiPSCMs. Sinoatrial node cell specific If current and action potential were also confirmed by patch clamp in TBX3 transfected hiPSCMs and the pacemaker-like cells were able to pace hiPSCMs ex vivo.

CONCLUSION

In conclusion, the present study demonstrated that overexpression of TBX3 could increase the differentiation of hiPSCs into pacemaker-like cells. Our study provide new strategy to construct a biological pacemaker, however, further study is still needed to identify the efficacy and safety of using the pacemaker-like cells to produce biological pacemaker in vivo.

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.

From Human Pluripotent Stem Cells to 3D Cardiac Microtissues: Progress, Applications and Challenges

The knowledge acquired throughout the years concerning the in vivo regulation of cardiac development has promoted the establishment of directed differentiation protocols to obtain cardiomyocytes (CMs) and other cardiac cells from human pluripotent stem cells (hPSCs), which play a crucial role in the function and homeostasis of the heart. Among other developments in the field, the transition from homogeneous cultures of CMs to more complex multicellular cardiac microtissues (MTs) has increased the potential of these models for studying cardiac disorders in vitro and for clinically relevant applications such as drug screening and cardiotoxicity tests. This review addresses the state of the art of the generation of different cardiac cells from hPSCs and the impact of transitioning CM differentiation from 2D culture to a 3D environment. Additionally, current methods that may be employed to generate 3D cardiac MTs are reviewed and, finally, the adoption of these models for in vitro applications and their adaptation to medium- to high-throughput screening settings are also highlighted.

Single-Cell Mechanical Analysis of Human Pluripotent Stem Cell-Derived Cardiomyocytes for Drug Testing and Pathophysiological Studies

Current platforms for studying the mechanical properties of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) as single cells do not measure forces directly, require numerous assumptions, and cannot study cell mechanics at different loading conditions. We present a method for directly measuring the active and passive forces generated by single-cell hPSC-CMs at different stretch levels. Utilizing this technique, single hPSC-CMs exhibited positive length-tension relationship and appropriate inotropic, klinotropic, and lusitropic changes in response to pharmacological treatments (isoproterenol and verapamil). The unique potential of the approach for drug testing and disease modeling was exemplified by doxorubicin and omecamtiv mecarbil drug studies revealing their known actions to suppress (doxorubicin) or augment (omecamtiv mecarbil at low dose) cardiomyocyte contractility, respectively. Finally, mechanistic insights were gained regarding the cellular effects of these drugs as doxorubicin treatment led to cellular mechanical alternans and high doses of omecamtiv mecarbil suppressed contractility and worsened the cellular diastolic properties.

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A unique approach for evaluating the mechanical properties of single-cell hPSC-CMs

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Both active and passive forces can be directly measured at different stretch levels

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The new approach can be used to evaluate drug effects and pathological conditions

Cardiopharyngeal Progenitor Specification: Multiple Roads to the Heart and Head Muscles

During embryonic development, the heart arises from various sources of undifferentiated mesodermal progenitors, with an additional contribution from ectodermal neural crest cells. Mesodermal cardiac progenitors are plastic and multipotent, but are nevertheless specified to a precise heart region and cell type very early during development. Recent findings have defined both this lineage plasticity and early commitment of cardiac progenitors, using a combination of single-cell and population analyses. In this review, we discuss several aspects of cardiac progenitor specification. We discuss their markers, fate potential in vitro and in vivo, early segregation and commitment, and also intrinsic and extrinsic cues regulating lineage restriction from multipotency to a specific cell type of the heart. Finally, we also discuss the subdivisions of the cardiopharyngeal field, and the shared origins of the heart with other mesodermal derivatives, including head and neck muscles.

The Quantitative Relationship among the Number of the Pacing Cells Required, the Dimension, and the Diffusion Coefficient

The purpose of the paper is to derive a formula to describe the quantitative relationship among the number of the pacing cells required (NPR), the dimension i, and the diffusion coefficient D (electrical coupling or gap junction G). The relationship between NPR and G has been investigated in different dimensions, respectively. That is, for each fixed i, there is a formula to describe the relationship between NPR and G; and three formulas are required for the three dimensions. However, there is not a universal expression to describe the relationship among NPR, G, and i together. In the manuscript, surveying and investigating the basic law among the existed data, we speculate the preliminary formula of the relationship among the NPR, i, and G; and then, employing the cftool in MATLAB, the explicit formulas are derived for different cases. In addition, the goodness of fit (R ²) is computed to evaluate the fitting of the formulas. Moreover, the 1D and 2D ventricular tissue models containing biological pacemakers are developed to derive more data to validate the formula. The results suggest that the relationship among the NPR, i, and the G (D) could be described by a universal formula, where the NPR scales with the i (the dimension) power of the product of the square root of G (D) and a constant b which is dependent on the strength of the pacing cells and so on.

Heart regeneration using pluripotent stem cells

Pluripotent stem cells (PSCs), which include embryonic and induced pluripotent stem cells (ESCs and iPSCs, respectively), have great potential in regenerative medicine for heart diseases due to their virtually unlimited cardiogenic capacity. Many preclinical studies have described the functional benefits after transplantation of PSC-derived cardiomyocytes (PSC-CMs). However, transient ventricular arrhythmias were detected after injection into non-human primates and swine ischemic hearts; as engrafted PSC-CMs form an electrical coupling between host and graft, the immature characteristics of PSC-CMs may serve as an ectopic pacemaker. We are entering a critical time in the development of novel therapies using PSC-CMs, with the recent first clinical trial using human iPSC-CMs (hiPSC-CMs) being launched in Japan. In this review, we summarize the updated knowledge, perspectives, and limitations of PSC-CMs for heart regeneration.

Increased predominance of the matured ventricular subtype in embryonic stem cell-derived cardiomyocytes in vivo

Accumulating evidence suggests that human pluripotent stem cell-derived cardiomyocytes can affect “heart regeneration”, replacing injured cardiac scar tissue with concomitant electrical integration. However, electrically coupled graft cardiomyocytes were found to innately induce transient post-transplant ventricular tachycardia in recent large animal model transplantation studies. We hypothesised that these phenomena were derived from alterations in the grafted cardiomyocyte characteristics. In vitro experiments showed that human embryonic stem cell-derived cardiomyocytes (hESC-CMs) contain nodal-like cardiomyocytes that spontaneously contract faster than working-type cardiomyocytes. When transplanted into athymic rat hearts, proliferative capacity was lower for nodal-like than working-type cardiomyocytes with grafted cardiomyocytes eventually comprising only relatively matured ventricular cardiomyocytes. RNA-sequencing of engrafted hESC-CMs confirmed the increased expression of matured ventricular cardiomyocyte-related genes, and simultaneous decreased expression of nodal cardiomyocyte-related genes. Temporal engraftment of electrical excitable nodal-like cardiomyocytes may thus explain the transient incidence of post-transplant ventricular tachycardia, although further large animal model studies will be required to control post-transplant arrhythmia.

Trait-associated noncoding variant regions affect TBX3 regulation and cardiac conduction

Genome-wide association studies have implicated common genomic variants in the gene desert upstream of TBX3 in cardiac conduction velocity. Whether these noncoding variants affect expression of TBX3 or neighboring genes and how they affect cardiac conduction is not understood. Here, we use high-throughput STARR-seq to test the entire 1.3 Mb human and mouse TBX3 locus, including two cardiac conduction-associated variant regions, for regulatory function. We identified multiple accessible and functional regulatory DNA elements that harbor variants affecting their activity. Both variant regions drove gene expression in the cardiac conduction tissue in transgenic reporter mice. Genomic deletion from the mouse genome of one of the regions caused increased cardiac expression of only Tbx3, PR interval shortening and increased QRS duration. Combined, our findings address the mechanistic link between trait-associated variants in the gene desert, TBX3 regulation and cardiac conduction.

Enrichment differentiation of human induced pluripotent stem cells into sinoatrial node-like cells by combined modulation of BMP, FGF, and RA signaling pathways

BACKGROUND

Biological pacemakers derived from pluripotent stem cell (PSC) have been considered as a potential therapeutic surrogate for sick sinus syndrome. So it is essential to develop highly efficient strategies for enrichment of sinoatrial node-like cells (SANLCs) as seed cells for biological pacemakers. It has been reported that BMP, FGF, and RA signaling pathways are involved in specification of different cardiomyocyte subtypes, pacemaker, ventricular, and atrial cells. We aimed to investigate whether combined modulation of BMP, FGF, and RA signaling pathways could enrich the differentiation of SANLC from human pluripotent stem cell (hiPSC).

METHODS

During the differentiation process from human induced pluripotent stem cell to cardiomyocyte through small molecule-based temporal modulation of the Wnt signaling pathway, signaling of BMP, FGF, and RA was manipulated at cardiac mesoderm stage. qRT-PCR, immunofluorescence, flow cytometry, and whole cell patch clamp were used to identify the SANLC.

RESULTS

qRT-PCR results showed that manipulating each one of bone morphogenetic protein (BMP), fibroblast growth factor (FGF), and retinoid acid (RA) signaling was effective for the upregulation of SANLC markers. Moreover, combined modulation of these three pathways displayed the best efficiency for the expression of SANLC markers, which was further confirmed at protein level using immunofluorescence and flow cytometry. Finally, the electrophysiological characteristics of upregulated SANLC were verified by patch clamp method.

CONCLUSION

An efficient transgene-independent differentiation protocol for generating SANLC from hiPSC was developed, in which combined modulating BMP, FGF, and RA signaling at cardiac mesoderm stage generates SANLC at high efficiency. This may serve as a potential approach for biological pacemaker construction.

Transcriptomically Guided Mesendoderm Induction of Human Pluripotent Stem Cells Using a Systematically Defined Culture Scheme

Human pluripotent stem cells (hPSCs) are an essential cell source in tissue engineering, studies of development, and disease modeling. Efficient, broadly amenable protocols for rapid lineage induction of hPSCs are of great interest in the stem cell biology field. We describe a simple, robust method for differentiation of hPSCs into mesendoderm in defined conditions utilizing single-cell seeding (SCS) and BMP4 and Activin A (BA) treatment. BA treatment was readily incorporated into existing protocols for chondrogenic and endothelial progenitor cell differentiation, while fine-tuning of BA conditions facilitated definitive endoderm commitment. After prolonged differentiation in vitro or in vivo, BA pretreatment resulted in higher mesoderm and endoderm levels at the expense of ectoderm formation. These data demonstrate that SCS with BA treatment is a powerful method for induction of mesendoderm that can be adapted for use in mesoderm and endoderm differentiation.

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Single-cell seeding with BMP4/Activin A treatment supports hPSC mesendoderm induction

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The mesendoderm protocol is amenable to mesoderm and endoderm lineage differentiation

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Mesoderm/endoderm formation was enhanced in basal conditions in vitro and in vivo

Metabolic environment in vivo as a blueprint for differentiation and maturation of human stem cell-derived cardiomyocytes

Patient-derived human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) are increasingly being used for disease modeling, drug screening and regenerative medicine. However, to date, an immature, fetal-like, phenotype of hPSC-CMs restrains their full potential. Increasing evidence suggests that the metabolic state, particularly important for provision of sufficient energy in highly active contractile CMs and anabolic and regulatory processes, plays an important role in CM maturation, which affects crucial functional aspects of CMs, such as contractility and electrophysiology. During embryonic development the heart is subjected to metabolite concentrations that differ substantially from that of hPSC-derived cardiac cell cultures. A deeper understanding of the environmental and metabolic cues during embryonic heart development and how these change postnatally, will provide a framework for optimizing cell culture conditions and maturation of hPSC-CMs. Maturation of hPSC-CMs will improve the predictability of disease modeling, drug screening and drug safety assessment and broadens their applicability for personalized and regenerative medicine.

Canonical Wnt Signaling Sets the Pace

In this issue of Developmental Cell, Ren et al. (2019) identify the embryonic origin of cardiac pacemaker cells in zebrafish and implicate Wnt5b in promoting their differentiation. Furthermore, canonical Wnt activation in human stem cell-derived cardiac progenitors produces functional pacemaker cells in vitro, advancing the therapeutic potential of biological pacemakers.

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

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

Human Pluripotent Stem Cell-Derived Cardiomyocytes for Assessment of Anticancer Drug-Induced Cardiotoxicity

Cardiotoxicity is a major cause of high attrition rates among newly developed drugs. Moreover, anti-cancer treatment-induced cardiotoxicity is one of the leading reasons of mortality in cancer survivors. Cardiotoxicity screening in vitro may improve predictivity of cardiotoxicity by novel drugs, using human pluripotent stem cell (hPSC)-derived-cardiomyocytes. Anthracyclines, including Doxorubicin, are widely used and highly effective chemotherapeutic agents for the treatment of different forms of malignancies. Unfortunately, anthracyclines cause many cardiac complications early or late after therapy. Anthracyclines exhibit their potent anti-cancer effect primarily via induction of DNA damage during the DNA replication phase in proliferative cells. In contrast, studies in animals and hPSC-cardiomyocytes have revealed that cardiotoxic effects particularly arise from (1) the generation of oxidative stress inducing mitochondrial dysfunction, (2) disruption of calcium homeostasis, and (3) changes in transcriptome and proteome, triggering apoptotic cell death. To increase the therapeutic index of chemotherapeutic Doxorubicin therapy several protective strategies have been developed or are under development, such as (1) reducing toxicity through modification of Doxorubicin (analogs), (2) targeted delivery of anthracyclines specifically to the tumor tissue or (3) cardioprotective agents that can be used in combination with Doxorubicin. Despite continuous progress in the field of cardio-oncology, cardiotoxicity is still one of the major complications of anti-cancer therapy. In this review, we focus on current hPSC-cardiomyocyte models for assessing anthracycline-induced cardiotoxicity and strategies for cardioprotection. In addition, we discuss latest developments toward personalized advanced pre-clinical models that are more closely recapitulating the human heart, which are necessary to support in vitro screening platforms with higher predictivity. These advanced models have the potential to reduce the time from bench-to-bedside of novel antineoplastic drugs with reduced cardiotoxicity.

Cellular Reprogramming Approaches to Engineer Cardiac Pacemakers

PURPOSE OF REVIEW

The goal of this paper is to review present knowledge regarding biological pacemakers created by somatic reprogramming as a platform for mechanistic and metabolic understanding of the rare subpopulation of pacemaker cells, with the ultimate goal of creating biological alternatives to electronic pacing devices.

RECENT FINDINGS

Somatic reprogramming of cardiomyocytes by reexpression of embryonic transcription factor T-box 18 (TBX18) converts them into pacemaker-like. Recent studies take advantage of this model to gain insight into the electromechanical, metabolic, and architectural intricacies of the cardiac pacemaker cell across various models, including a surgical model of complete atrioventricular block (CAVB) in adult rats. The studies reviewed here reinforce the potential utility of TBX18-induced pacemaker myocytes (iPMS) as a minimally invasive treatment for heart block. Several challenges which must be overcome to develop a viable therapeutic intervention based on these observations are discussed.

Impact of Chronic Fetal Hypoxia and Inflammation on Cardiac Pacemaker Cell Development

Chronic fetal hypoxia and infection are examples of adverse conditions during complicated pregnancy, which impact cardiac myogenesis and increase the lifetime risk of heart disease. However, the effects that chronic hypoxic or inflammatory environments exert on cardiac pacemaker cells are poorly understood. Here, we review the current evidence and novel avenues of bench-to-bed research in this field of perinatal cardiogenesis as well as its translational significance for early detection of future risk for cardiovascular disease.

Stem cell therapy of myocardial infarction: a promising opportunity in bioengineering

Myocardial infarction (MI) is a life-threatening disease resulting from irreversible death of cardiomyocytes (CMs) and weakening of the heart blood-pumping function. Stem cell-based therapies have been studied for MI treatment over the last two decades with promising outcome. In this review, we critically summarize the past work in this field to elucidate the advantages and disadvantages of treating MI using pluripotent stem cells (PSCs) including both embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), adult stem cells, and cardiac progenitor cells. The main advantage of the latter is their cytokine production capability to modulate immune responses and control the progression of healing. However, human adult stem cells have very limited (if not 'no') capacity to differentiate into functional CMs in vitro or in vivo. In contrast, PSCs can be differentiated into functional CMs although the protocols for the cardiac differentiation of PSCs are mainly for adherent cells under 2D culture. Derivation of PSC-CMs in 3D, allowing for large-scale production of CMs via modulation of the Wnt/β-catenin signal pathway with defined chemicals and medium, may be desired for clinical translation. Furthermore, the technology of purification and maturation of the PSC-CMs may need further improvements to eliminate teratoma formation after in vivo implantation of the PSC-CMs for treating MI. In addition, in vitro derived PSC-CMs may have mechanical and electrical mismatch with the patient's cardiac tissue, which causes arrhythmia. This supports the use of PSC-derived cells committed to cardiac lineage without beating for implantation to treat MI. In this case, the PSC derived cells may utilize the mechanical, electrical, and chemical cues in the heart to further differentiate into mature/functional CMs in situ. Another major challenge facing stem cell therapy of MI is the low retention/survival of stem cells or their derivatives (e.g., PSC-CMs) in the heart for MI treatment after injection in vivo. This may be resolved by using biomaterials to engineer stem cells for reduced immunogenicity, immobilization of the cells in the heart, and increased integration with the host cardiac tissue. Biomaterials have also been applied in the derivation of CMs in vitro to increase the efficiency and maturation of differentiation. Collectively, a lot has been learned from the past failure of simply injecting intact stem cells or their derivatives in vivo for treating MI, and bioengineering stem cells with biomaterials is expected to be a valuable strategy for advancing stem cell therapy towards its widespread application for treating MI in the clinic.

Development of a sheep model of atrioventricular block for the application of novel therapies

Introduction

Sheep have been adopted as a pre-clinical large animal for scientific research as they are good models of cardiac anatomy and physiology, and allow for investigation of pathophysiological processes which occur in the large mammalian heart. There is, however, no defined model of atrioventricular block in sheep to allow for pre-clinical assessment of new cardiac treatment options. We therefore aimed to develop an adult sheep model of atrioventricular block with the focus on future novel applications.

Methods and results

We utilized six sheep to undergo two procedures each. The first procedure involved implantation of a single chamber pacemaker into the right ventricular apex, for baseline assessment over four weeks. The second procedure involved creating atrioventricular block by radiofrequency ablation of the His bundle, before holding for a further four weeks. Interrogation of pacemakers and electrocardiograms determined the persistence of atrioventricular block during the follow up period. Pacemakers were inserted, and atrioventricular block created in 6 animals using a conventional approach. One animal died following ablation of the His bundle, due to procedural complications. Four unablated sheep were assessed for baseline data over four weeks and showed 5.53 ± 1.28% pacing reliance. Five sheep were assessed over four weeks following His bundle ablation and showed continuous (98.89 ± 0.81%) ventricular pacing attributable to persistent atrioventricular block, with no major complications.

Conclusion

We have successfully developed, characterized and validated a large animal model of atrioventricular block that is stable and technically feasible in adult sheep. This model will allow for the advancement of novel therapies, including the development of cell and gene-based therapies.

The roles and regulation of TBX3 in development and disease

TBX3, a member of the ancient and evolutionary conserved T-box transcription factor family, is a critical developmental regulator of several structures including the heart, mammary glands, limbs and lungs. Indeed, mutations in the human TBX3 lead to ulnar mammary syndrome which is characterized by several clinical malformations including hypoplasia of the mammary and apocrine glands, defects of the upper limb, areola, dental structures, heart and genitalia. In contrast, TBX3 has no known function in adult tissues but is frequently overexpressed in a wide range of epithelial and mesenchymal derived cancers. This overexpression greatly impacts several hallmarks of cancer including bypass of senescence, apoptosis and anoikis, promotion of proliferation, tumour formation, angiogenesis, invasion and metastatic capabilities as well as cancer stem cell expansion. The debilitating consequences of having too little or too much TBX3 suggest that its expression levels need to be tightly regulated. While we have a reasonable understanding of the mutations that result in low levels of functional TBX3 during development, very little is known about the factors responsible for the overexpression of TBX3 in cancer. Furthermore, given the plethora of oncogenic processes that TBX3 impacts, it must be regulating several target genes but to date only a few have been identified and characterised. Interestingly, while there is compelling evidence to support oncogenic roles for TBX3, a few studies have indicated that it may also have tumour suppressor functions in certain contexts. Together, the diverse functional elasticity of TBX3 in development and cancer is thought to involve, in part, the protein partners that it interacts with and this area of research has recently received some attention. This review provides an insight into the significance of TBX3 in development and cancer and identifies research gaps that need to be explored to shed more light on this transcription factor.

Arrhythmogenic risks of stem cell replacement therapy for cardiovascular diseases

Ischemic heart disease and congestive heart failure are major contributors to high morbidity and mortality. Approximately 1.5 million cases of myocardial infarction occur annually in the United States; the yearly incidence rate is approximately 600 cases per 100,000 people. Although significant progress to improve the survival rate has been made by medications and implantable medical devices, damaged cardiomyocytes are unable to be recovered by current treatment strategies. After almost two decades of research, stem cell therapy has become a very promising approach to generate new cardiomyocytes and enhance the function of the heart. Along with clinical trials with stem cells conducted in cardiac regeneration, concerns regarding safety and potential risks have emerged. One of the contentious issues is the electrical dysfunctions of cardiomyocytes and cardiac arrhythmia after stem cell therapy. In this review, we focus on the cell sources currently used for stem cell therapy and discuss related arrhythmogenic risk.

A Brief History of Pacemaking

Cardiac pacemaking is a most fundamental cardiac function, thoroughly investigated for decades with a multiscale approach at organ, tissue, cell and molecular levels, to clarify the basic mechanisms underlying generation and control of cardiac rhythm. Understanding the processes involved in pacemaker activity is of paramount importance for a basic physiological knowledge, but also as a way to reveal details of pathological dysfunctions useful in the perspective of a therapeutic approach. Among the mechanisms involved in pacemaking, the "funny" (I_f) current has properties most specifically fitting the requirements for generation and control of repetitive activity, and has consequently received the most attention in studies of the pacemaker function. Present knowledge of the basic mechanisms of pacemaking and the properties of funny channels has led to important developments of clinical relevance. These include: (1) the successful development of heart rate-reducing agents, such as ivabradine, able to control cardiac rhythm and useful in the treatment of diseases such as coronary artery disease, heart failure and tachyarrhythmias; (2) the understanding of the genetic basis of disorders of cardiac rhythm caused by HCN channelopathies; (3) the design of strategies to implement biological pacemakers based on transfer of HCN channels or of stem cell-derived pacemaker cells expressing I_f, with the ultimate goal to replace electronic devices. In this review, I will give a brief historical account of the discovery of the funny current and the development of the concept of I_f-based pacemaking, in the context of a wider, more complex model of cardiac rhythmic function.

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

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

Toward the realization of cardiac regenerative medicine using pluripotent stem cells

Heart transplantation (HT) is the only radical treatment available for patients with end-stage heart failure that is refractory to optimal medical treatment and device therapies. However, HT as a therapeutic option is limited by marked donor shortage. To overcome this difficulty, regenerative medicine using human-induced pluripotent stem cells (hiPSCs) has drawn increasing attention as an alternative to HT.

Several issues including the preparation of clinical-grade hiPSCs, methods for large-scale culture and production of hiPSCs and cardiomyocytes, prevention of tumorigenesis secondary to contamination of undifferentiated stem cells and non-cardiomyocytes, and establishment of an effective transplantation strategy need to be addressed to fulfill this unmet medical need. The ongoing rapid technological advances in hiPSC research have been directed toward the clinical application of this technology, and currently, most issues have been satisfactorily addressed. Cell therapy using hiPSC-derived cardiomyocytes is expected to serve as an integral component of realistic medicine in the near future and is being potentially viewed as a treatment that would revolutionize the management of patients with severe heart failure.

HCN2 and TBX3 Reprogram Human-Induced Pluripotent Stem Cells-Derived Cardiomyocytes into Pacemaker-Like Cells

TBX3 reprograms cardiac myocytes into cells that possess sinoatrial node phenotype, but no specific funny current (If) was detected. We explore whether overexpression of TBX3 alone or combined with HCN2 can reprogram human-induced pluripotent stem cells-derived cardiomyocytes (hiPSC-CMs) into pacemaker-like cells. HiPSC-CMs were transfected with TBX3 and/or HCN2 in this study. Expression analysis showed that overexpression of TBX3 induces a reduced reduction expression profile of working cardiomyocytes into that of pacemaker cells. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and electrophysiological analyses showed a reduced expression of connexins subunits (CX40, CX43), the sodium current (SCN5A, INa), the inward rectified potassium channels (Kir2.1, IK1), and an increased expression of connexins subunits (CX30.2, CX45). No If was detected. The reduction of IK1 resulted in a more depolarized maximum diastolic potential together with an expression of If (generated by HCN2), which they work in synergy to generate spontaneous diastolic depolarization that was the most typical characteristic of pacemaker cells. In conclusion, overexpression of TBX3 and HCN2 could reprogram hiPSC-CMs into pacemaker-like cells. The ability to enable diastolic depolarization formation provides a new strategy for the construction of a biological pacemaker.