Identification and selection of cardiomyocytes during human embryonic stem cell differentiation

Strategies for enrichment and selection of stem cell-derived tissue precursors

Human embryonic stem cells have the capacity for self-renewal and pluripotency and thus are a primary candidate for tissue engineering and regenerative therapies. These cells also provide an opportunity to study the development of human tissues ex vivo. To date, numerous human embryonic stem cell lines have been derived and characterized. In this review, we will detail the strategies used to direct tissue-specific differentiation of embryonic stem cells. We also will discuss how these strategies have produced new sources of tissue-specific progenitor cells. Finally, we will describe the next generation of methods being developed to identify and select stem cell-derived tissue precursors for experimental study and clinical use.

Stem cell therapy for cardiac disease

Congenital heart disease occurs in 1% of liveborn infants, making it the most common birth defect worldwide. Many of these children develop heart failure. In addition, both genetic and acquired forms of dilated cardiomyopathy are a significant source of heart failure in the pediatric population. Heart failure occurs when the myocardium is unable to meet the body's metabolic demands. Unlike some organs, the heart has limited, if any, capacity for repair after injury. Heart transplantation remains the ultimate approach to treating heart failure, but this is costly and excludes patients who are poor candidates for transplantation given their comorbidities, or for whom a donor organ is unavailable. Stem cell therapy represents the first realistic strategy for reversing the effects of what has until now been considered terminal heart damage. We will discuss potential sources of cardiac-specific stem cells, including mesenchymal, resident cardiac, embryonic, and induced pluripotent stem cells. We will consider efforts to enhance cardiac stem cell engraftment and survival in damaged myocardium, the incorporation of cardiac stem cells into tissue patches, and techniques for creating bioartificial myocardial tissue as well as whole organs. Finally, we will review progress being made in assessing functional improvement in animals and humans after cellular transplant.

Differentiation and enrichment of cardiomyocytes from human pluripotent stem cells

Human cardiomyocytes derived from pluripotent stem cells hold great promise for cardiac cell therapy, disease modeling, drug discovery, and the study of developmental biology. Reaching these potentials fully requires the development of methods that enable efficient and robust generation of cardiomyocytes with expected characteristics. This review summarizes and discusses up-to-date methods that have been used to derive and enrich human cardiomyocytes from pluripotent stem cells, provides a brief overview of in vitro and in vivo characterization of these cardiomyocytes, and considers future advancement needed to further harness the power of these cells.

Clinical applications of patient-specific induced pluripotent stem cells in cardiovascular medicine

The emergence of induced pluripotent stem cell (iPSC) technology has had a great impact on the field of medicine ever since the ground-breaking discovery in 2006 that overexpression of four specific transcription factors was able to turn back the developmental clock of somatic cells into an embryonic-like state. The resulting iPSCs carry the developmental potential of human embryonic stem cells (hESC) without the embryo and have been heralded as a powerful tool to study development and disease. This technology has made it possible for the first time for researchers to transform end-differentiated cells from a particular individual into another cell type that remains specific to that individual, paving the way for novel methods of in vitro disease modelling and therapeutic applications. This paper reviews some of the key areas in cardiovascular medicine in which iPSC technology has been applied and discusses the future directions and ongoing challenges ahead in this exciting field.

Human pluripotent stem cells for genetic disease modeling and drug screening

Considerable hope surrounds the use of disease-specific pluripotent stem cells, which can differentiate into any cell type, as starting materials to generate models of human disease that will allow exploration of pathological mechanisms and the search for new treatments. Disease-specific human embryonic stem cells have provided a useful source for studying certain disease states. However, reprogramming of human somatic cells that use readily accessible tissue, such as skin or blood, to generate embryonic-like induced pluripotent stem cells has opened new perspectives for modeling and understanding a larger number of human pathologies. Here, we examine the challenges in creating a disease model from human pluripotent stem cells, and describe their use to model both cell-autonomous and non-cell-autonomous mechanisms, the need for adequate control experiments and the genetic limitations of human induced pluripotent stem cells. Progress in these areas will substantially accelerate effective application of disease-specific human pluripotent stem cells for drug screening.

A novel purification method of murine embryonic stem cell- and human-induced pluripotent stem cell-derived cardiomyocytes by simple manual dissociation

Cardiomyocytes derived from embryonic stem cells (ES-CMs) and induced pluripotent stem cells (iPS-CMs) are useful for toxicity and pharmacology screening. In the present study, we found that cardiomyocyte-rich beating cell clusters (CCs) emerged from murine embryonic stem cell (mESC)-derived beating EBs and from human-induced pluripotent stem cell (hiPSC)-derived beating EBs dissociated by gentle pipetting with a thin glass pipette. The percentage of cardiac troponin T (cTnT)-positive cells in the beating CCs obtained from mESC-derived and hiPSC-derived beating EBs was higher (81.5% and 91.6%, respectively) than in beating-undissociated EBs (13.7% and 67.1%, respectively). For mESCs, the yield of cTnT-positive cells from beating CCs was estimated to be 1.6 times higher than that of beating EBs. The bromodeoxyuridine labeling index of mouse ES-CMs and human iPS-CMs in beating CCs was 1.5- and 3.2-fold, respectively, greater than those in beating EBs. To investigate the utility of the cells in toxicity assessment, we showed that doxorubicin, a cardiotoxic drug, induced myofilament disruption in cardiomyocytes isolated by this method. This simple method enables preparation of mouse ES-CMs and human iPS-CMs with better proliferative activity than beating EBs not dissociated by pipetting, and the cardiomyocytes are useful for drug-induced myocardial toxicity testing.

Production of de novo cardiomyocytes: human pluripotent stem cell differentiation and direct reprogramming

Cardiovascular disease is a leading cause of death worldwide. The limited capability of heart tissue to regenerate has prompted methodological developments for creating de novo cardiomyocytes, both in vitro and in vivo. Beyond uses in cell replacement therapy, patient-specific cardiomyocytes may find applications in drug testing, drug discovery, and disease modeling. Recently, approaches for generating cardiomyocytes have expanded to encompass three major sources of starting cells: human pluripotent stem cells (hPSCs), adult heart-derived cardiac progenitor cells (CPCs), and reprogrammed fibroblasts. We discuss state-of-the-art methods for generating de novo cardiomyocytes from hPSCs and reprogrammed fibroblasts, highlighting potential applications and future challenges.

Label-free identification and characterization of human pluripotent stem cell-derived cardiomyocytes using second harmonic generation (SHG) microscopy

Pluripotent stem cell-derived cardiomyocytes (PSC-CMs) are a potentially unlimited source of cardiomyocytes (CMs) for cardiac transplantation therapies. The establishment of pure PSC-CM populations is important for this application, but is hampered by a lack of CM-specific surface markers suitable for their identification and sorting. Contemporary purification techniques are either non-specific or require genetic modification. We report a second harmonic generation (SHG) signal detectable in PSC-CMs that is attributable to sarcomeric myosin, dependent on PSC-CM maturity, and retained while PSC-CMs are in suspension. Our study demonstrates the feasibility of developing a SHG-activated flow cytometer for the non-invasive purification of PSC-CMs.

Human pluripotent stem cells for disease modelling and drug screening

Considerable hope surrounds the use of disease-specific pluripotent stem cells to generate models of human disease allowing exploration of pathological mechanisms and search for new treatments. Disease-specific human embryonic stem cells were the first to provide a useful source for studying certain disease states. The recent demonstration that human somatic cells, derived from readily accessible tissue such as skin or blood, can be converted to embryonic-like induced pluripotent stem cells (hiPSCs) has opened new perspectives for modelling and understanding a larger number of human pathologies. In this review, we examine the opportunities and challenges for the use of disease-specific pluripotent stem cells in disease modelling and drug screening. Progress in these areas will substantially accelerate effective application of disease-specific human pluripotent stem cells for drug screening.

SIRPA is a specific cell-surface marker for isolating cardiomyocytes derived from human pluripotent stem cells

To identify cell-surface markers specific to human cardiomyocytes, we screened cardiovascular cell populations derived from human embryonic stem cells (hESCs) against a panel of 370 known CD antibodies. This screen identified the signal-regulatory protein alpha (SIRPA) as a marker expressed specifically on cardiomyocytes derived from hESCs and human induced pluripotent stem cells (hiPSCs), and PECAM, THY1, PDGFRB and ITGA1 as markers of the nonmyocyte population. Cell sorting with an antibody against SIRPA allowed for the enrichment of cardiac precursors and cardiomyocytes from hESC/hiPSC differentiation cultures, yielding populations of up to 98% cardiac troponin T-positive cells. When plated in culture, SIRPA-positive cells were contracting and could be maintained over extended periods of time. These findings provide a simple method for isolating populations of cardiomyocytes from human pluripotent stem cell cultures, and thereby establish a readily adaptable technology for generating large numbers of enriched cardiomyocytes for therapeutic applications.

Methods of cell purification: a critical juncture for laboratory research and translational science

Research in cell biology and the development of translational technologies are driven by competition, public expectations, and regulatory oversight, putting these fields at a critical juncture. Success in these fields is quickly becoming dependent on the ability of researchers to identify and isolate specific cell populations from heterogeneous mixtures accurately and efficiently. Many methods for cell purification have been developed, and each has advantages and disadvantages that must be considered in light of the intended application. Current cell separation strategies make use of surface proteins, genetic expression, and physics to isolate specific cells by phenotypic traits. Cell purification is also dependent on the cellular reagents available for use and the intended application, as these factors may preclude certain mechanisms used in the processes of labeling and sorting cells.

Human pluripotent stem cell-based approaches for myocardial repair: from the electrophysiological perspective

Heart diseases are a leading cause of mortality worldwide. Terminally differentiated adult cardiomyocytes (CMs) lack the innate ability to regenerate. Their malfunction or significant loss can lead to conditions from cardiac arrhythmias to heart failure. For myocardial repair, cell- and gene-based therapies offer promising alternatives to donor organ transplantation. Human embryonic stem cells (hESCs) can self-renew while maintaining their pluripotency. Direct reprogramming of adult somatic cells to become pluripotent hES-like cells (also known as induced pluripotent stem cells or iPSCs) has been achieved. Both hESCs and iPSCs have been successfully differentiated into genuine human CMs. In this review, we describe our current knowledge of the structure-function properties of hESC/iPSC-CMs, with an emphasis on their electrophysiology and Ca(2+) handling, along with the hurdles faced and potential solutions for translating into clinical and other applications (e.g., disease modeling, cardiotoxicity and drug screening).

Small molecule Wnt inhibitors enhance the efficiency of BMP-4-directed cardiac differentiation of human pluripotent stem cells

Human induced pluripotent stem (iPS) cells potentially provide a unique resource for generating patient-specific cardiomyocytes to study cardiac disease mechanisms and treatments. However, existing approaches to cardiomyocyte production from human iPS cells are inefficient, limiting the application of iPS cells in basic and translational cardiac research. Furthermore, strategies to accurately record changes in iPS cell-derived cardiomyocyte action potential duration (APD) are needed to monitor APD-related cardiac disease and for rapid drug screening. We examined whether modulation of the bone morphogenetic protein 4 (BMP-4) and Wnt/β-catenin signaling pathways could induce efficient cardiac differentiation of human iPS cells. We found that early treatment of human iPS cells with BMP-4 followed by late treatment with small molecule Wnt inhibitors led to a marked increase in production of cardiomyocytes compared to existing differentiation strategies. Using immunocytochemical staining and real-time intracellular calcium imaging, we showed that these induced cardiomyocytes expressed typical sarcomeric markers, exhibited normal rhythmic Ca(2+) transients, and responded to both β-adrenergic and electric stimulation. Furthermore, human iPS cell-derived cardiomyocytes demonstrated characteristic changes in action potential duration in response to cardioactive drugs procainamide and verapamil using voltage-sensitive dye-based optical recording. Thus, modulation of the BMP-4 and Wnt signaling pathways in human iPS cells leads to highly efficient production of cardiomyocytes with typical electrophysiological function and pharmacologic responsiveness. The use of human iPS cell-derived cardiomyocytes and the application of calcium- and voltage-sensitive dyes for the direct, rapid measurement of iPS cell-derived cardiomyocyte activity promise to offer attractive platforms for studying cardiac disease mechanisms and therapeutics.

Induced pluripotent stem cell-derived cardiomyocytes as models for genetic cardiovascular disorders

PURPOSE OF REVIEW

The development of induced pluripotent stem cell (iPSC) technology has led to many advances in the areas of directed cell differentiation and characterization. New methods for generating iPSC-derived cardiomyocytes provide an invaluable resource for the study of certain cardiovascular disorders. This review highlights the current technology in this field, its application thus far to the study of genetic disorders of the RAS/MAPK pathway and long-QT syndrome (LQTS), and future directions for the field.

RECENT FINDINGS

Enhanced methods increase the efficiency of generating and stringently purifying iPSC-derived cardiomyocytes. The use of cardiomyocytes derived from patients with LEOPARD syndrome and LQTS has shed light on the molecular mechanisms of disease and validated their use as reliable human disease models.

SUMMARY

The use of iPSC-derived cardiomyocytes to study genetic cardiovascular disorders will enable a deeper and more applicable understanding of the molecular mechanisms of human disease, as well as improving our ability to achieve successful cell-based therapies. Methods to efficiently generate these cells are improving and provide promise for future applications of this technology.

Derivation of genetically modified human pluripotent stem cells with integrated transgenes at unique mapped genomic sites

Many applications in human pluripotent stem cell (PSC) research require the genetic modification of PSCs to express a transgene in a stable and dependable manner. Random transgene integration commonly results in unpredictable and heterogeneous expression. We describe a protocol for the derivation of clonal populations of human embryonic stem cells or induced pluripotent stem cells (iPSCs) expressing a transgene from a single copy of an integrated lentiviral vector that is mapped to the genome. Using optimized transduction conditions, followed by single-cell subcloning and a round of antibiotic selection, we find that approximately half of the colonies retrieved contain a single vector copy. After expansion, the majority of these are confirmed to be clonal. The vector/genomic DNA junction is sequenced and the unique integration site is mapped to the genome. This protocol enables the efficient derivation of genetically modified PSCs containing an integrated transgene at a known genomic site in ∼7 weeks.

Human pluripotent stem cells in pharmacological and toxicological screening: new perspectives for personalized medicine

Human stem cells provide an important novel tool for generating in vitro pharmacological and toxicological test systems. In the development of new targeted therapies, as well as in critical safety issues, including hepato-, neuro- and cardio-toxicity, animal-based tests are mostly unsatisfactory, whereas the use of in vitro model systems is limited by the unavailability of relevant human tissues. Human embryonic stem cell lines may fill this gap and offer an advantage over primary cultures as well as tissue-derived (adult) stem cells. Human embryonic stem cells represent an unlimited source for the production of differentiated somatic progenies and allow various stable genetic manipulations. As a new opening in personalized medicine test systems, the generation of induced pluripotent stem cell lines and their derivatives can provide patient- and disease-specific cellular assays for drug development and safety assessments. This article reviews promising human stem cell applications in pharmacological and toxicological screenings, focusing on the implications for personalized medicine.

Cardiac differentiation of pluripotent stem cells

The ability of human pluripotent stem cells to differentiate towards the cardiac lineage has attracted significant interest, initially with a strong focus on regenerative medicine. The ultimate goal to repair the heart by cardiomyocyte replacement has, however, proven challenging. Human cardiac differentiation has been difficult to control, but methods are improving, and the process, to a certain extent, can be manipulated and directed. The stem cell-derived cardiomyocytes described to date exhibit rather immature functional and structural characteristics compared to adult cardiomyocytes. Thus, a future challenge will be to develop strategies to reach a higher degree of cardiomyocyte maturation in vitro, to isolate cardiomyocytes from the heterogeneous pool of differentiating cells, as well as to guide the differentiation into the desired subtype, that is, ventricular, atrial, and pacemaker cells. In this paper, we will discuss the strategies for the generation of cardiomyocytes from pluripotent stem cells and their characteristics, as well as highlight some applications for the cells.

Cardiomyocyte differentiation of pluripotent stem cells and their use as cardiac disease models

More than 10 years after their first isolation, human embryonic stem cells are finally 'coming of age' in research and biotechnology applications as protocols for their differentiation and undifferentiated expansion in culture become robust and scalable, and validated commercial reagents become available. Production of human cardiomyocytes is now feasible on a daily basis for many laboratories with tissue culture expertise. An additional recent surge of interest resulting from the first production of human iPSCs (induced pluripotent stem cells) from somatic cells of patients now makes these technologies of even greater importance since it is likely that (genetic) cardiac disease phenotypes can be captured in the cardiac derivatives of these cells. Although cell therapy based on replacing cardiomyocytes lost or dysfunctional owing to cardiac disease are probably as far away as ever, biotechnology and pharmaceutical applications in safety pharmacology and drug discovery will probably impact this clinical area in the very near future. In the present paper, we review the cutting edge of this exciting area of translational research.

Elimination of tumorigenic stem cells from differentiated progeny and selection of definitive endoderm reveals a Pdx1+ foregut endoderm stem cell lineage

Embryonic stem cell (ESC) derivatives offer promise for generating clinically useful tissues for transplantation, yet the specter of producing tumors in patients remains a significant concern. We have developed a simple method that eliminates the tumorigenic potential from differentiated ESC cultures of murine and human origin while purifying lineage-restricted, definitive endoderm-committed cells. A three-stage scheme utilizing magnetic bead sorting and specific antibodies to remove undifferentiated ESCs and extraembryonic endoderm cells, followed by positive selection of definitive endoderm cells on the basis of epithelial cell adhesion molecule (EpCAM) expression, was used to isolate a population of EpCAM(+)SSEA1(-)SSEA3(-) cells. Sorted cells do not form teratomas after transplantation into immunodeficient mice, but display gene and protein expression profiles indicative of definitive endoderm cells. Sorted cells could be subsequently expanded in vitro and further differentiated to express key pancreas specification proteins. In vivo transplantation of sorted cells resulted in small, benign tissues that uniformly express PDX1. These studies describe a straightforward method without genetic manipulation that eliminates the risk of teratoma formation from ESC differentiated derivatives. Significantly, enriched populations isolated by this method appear to be lineage-restricted definitive endoderm cells with limited proliferation capacity.

Present state and future perspectives of using pluripotent stem cells in toxicology research

The use of novel drugs and chemicals requires reliable data on their potential toxic effects on humans. Current test systems are mainly based on animals or in vitro–cultured animal-derived cells and do not or not sufficiently mirror the situation in humans. Therefore, in vitro models based on human pluripotent stem cells (hPSCs) have become an attractive alternative. The article summarizes the characteristics of pluripotent stem cells, including embryonic carcinoma and embryonic germ cells, and discusses the potential of pluripotent stem cells for safety pharmacology and toxicology. Special attention is directed to the potential application of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) for the assessment of developmental toxicology as well as cardio- and hepatotoxicology. With respect to embryotoxicology, recent achievements of the embryonic stem cell test (EST) are described and current limitations as well as prospects of embryotoxicity studies using pluripotent stem cells are discussed. Furthermore, recent efforts to establish hPSC-based cell models for testing cardio- and hepatotoxicity are presented. In this context, methods for differentiation and selection of cardiac and hepatic cells from hPSCs are summarized, requirements and implications with respect to the use of these cells in safety pharmacology and toxicology are presented, and future challenges and perspectives of using hPSCs are discussed.

Noninvasive detection and imaging of molecular markers in live cardiomyocytes derived from human embryonic stem cells

Raman microspectroscopy (RMS) was used to detect and image molecular markers specific to cardiomyocytes (CMs) derived from human embryonic stem cells (hESCs). This technique is noninvasive and thus can be used to discriminate individual live CMs within highly heterogeneous cell populations. Principal component analysis (PCA) of the Raman spectra was used to build a classification model for identification of individual CMs. Retrospective immunostaining imaging was used as the gold standard for phenotypic identification of each cell. We were able to discriminate CMs from other phenotypes with >97% specificity and >96% sensitivity, as calculated with the use of cross-validation algorithms (target 100% specificity). A comparison between Raman spectral images corresponding to selected Raman bands identified by the PCA model and immunostaining of the same cells allowed assignment of the Raman spectral markers. We conclude that glycogen is responsible for the discrimination of CMs, whereas myofibril proteins have a lesser contribution. This study demonstrates the potential of RMS for allowing the noninvasive phenotypic identification of hESC progeny. With further development, such label-free optical techniques may enable the separation of high-purity cell populations with mature phenotypes, and provide repeated measurements to monitor time-dependent molecular changes in live hESCs during differentiation in vitro.

An engineered cardiac reporter cell line identifies human embryonic stem cell-derived myocardial precursors

Unlike some organs, the heart is unable to repair itself after injury. Human embryonic stem cells (hESCs) grow and divide indefinitely while maintaining the potential to develop into many tissues of the body. As such, they provide an unprecedented opportunity to treat human diseases characterized by tissue loss. We have identified early myocardial precursors derived from hESCs (hMPs) using an α-myosin heavy chain (αMHC)-GFP reporter line. We have demonstrated by immunocytochemistry and quantitative real-time PCR (qPCR) that reporter activation is restricted to hESC-derived cardiomyocytes (CMs) differentiated in vitro, and that hMPs give rise exclusively to muscle in an in vivo teratoma formation assay. We also demonstrate that the reporter does not interfere with hESC genomic stability. Importantly, we show that hMPs give rise to atrial, ventricular and specialized conduction CM subtypes by qPCR and microelectrode array analysis. Expression profiling of hMPs over the course of differentiation implicate Wnt and transforming growth factor-β signaling pathways in CM development. The identification of hMPs using this αMHC-GFP reporter line will provide important insight into the pathways regulating human myocardial development, and may provide a novel therapeutic reagent for the treatment of cardiac disease.

Cardiac cell therapy: where we've been, where we are, and where we should be headed

INTRODUCTION

Stem cell therapy has emerged as a promising strategy for the treatment of ischemic cardiomyopathy.

SOURCES OF DATA

Multiple candidate cell types have been used in preclinical animal models and in clinical trials to repair or regenerate the injured heart either directly (through formation of new transplanted tissue) or indirectly (through paracrine effects activating endogenous regeneration).

AREAS OF AGREEMENT

(i) Clinical trials examining the safety and efficacy of bone marrow derived cells in patients with heart disease are promising, but results leave much room for improvement. (ii) The safety profile has been quite favorable. (iii) Efficacy has been inconsistent and, overall, modest. (iv) Tissue retention of cells after delivery into the heart is disappointingly low. (v) The beneficial effects of adult stem cell therapy are predominantly mediated by indirect paracrine mechanisms.

AREAS OF CONTROVERSY

The cardiogenic potential of bone marrow-derived cells, the mechanism whereby small numbers of poorly-retained cells translate to measurable clinical benefit, and the overall impact on clinical outcomes are hotly debated. GROWING POINTS/AREAS TIMELY FOR DEVELOPING RESEARCH: This overview of the field leaves us with cautious optimism, while motivating a search for more effective delivery methods, better strategies to boost cell engraftment, more apt patient populations, safe and effective 'off the shelf' cell products and more potent cell types.

Stem cell therapy for ischemic heart disease

Stem cell transplantation has emerged as a novel treatment option for ischemic heart disease. Different cell types have been utilized and the recent development of induced pluripotent stem cells has generated tremendous excitement in the regenerative field. Bone marrow-derived multipotent progenitor cell transplantation in preclinical large animal models of postinfarction left ventricular remodeling has demonstrated long-term functional and bioenergetic improvement. These beneficial effects are observed despite no significant engraftment of bone marrow cells in the myocardium and even lower differentiation of these cells into cardiomyocytes. It is thought to be related to the paracrine effect of these stem cells, which secrete factors that lead to long-term gene expression changes in the host myocardium, thereby promoting neovascularization, inhibiting apoptosis, and stimulating resident cardiac progenitor cells. Future studies are warranted to examine the changes in the recipient myocardium after stem cell transplantation and to investigate the signaling pathways involved in these effects.

Adult stem cells: from new cell sources to changes in methodology

Cardiovascular diseases constitute the first cause of mortality and morbidity worldwide. Alternative treatments like transplantation of (stem) cell populations derived from several adult tissue sources, like the bone marrow, skeletal muscle, or even adipose tissue, have been already employed in diverse clinical trials. Results from these studies and previous animal studies have reached to the conclusion that stem cells induce a benefit in the treated hearts, which is exerted mainly through paracrine mechanisms and not through direct differentiation as it was initially expected. However, a strong technical limitation for the stem cell therapy, which is the low level of cell survival and engraftment, diminishes their potential. Thus, new strategies like combination of the cells with bioengineering techniques have been developed and are being subject of intense research, suggesting that new strategies may improve the efficacy of these therapies. In this review, we will discuss the different therapeutic approaches, drawbacks, and future expectations of new regenerative therapies for cardiovascular diseases.

Isoflurane preconditioning elicits competent endogenous mechanisms of protection from oxidative stress in cardiomyocytes derived from human embryonic stem cells

BACKGROUND

Human embryonic stem cell (hESC)-derived cardiomyocytes potentially represent a powerful experimental model complementary to myocardium obtained from patients that is relatively inaccessible for research purposes. We tested whether anesthetic-induced preconditioning (APC) with isoflurane elicits competent protective mechanisms in hESC-derived cardiomyocytes against oxidative stress to be used as a model of human cardiomyocytes for studying preconditioning.

METHODS

H1 hESC cell line was differentiated into cardiomyocytes using growth factors activin A and bone morphogenetic protein-4. Living ventricular hESC-derived cardiomyocytes were identified using a lentiviral vector expressing a reporter gene (enhanced green fluorescent protein) driven by a cardiac-specific human myosin light chain-2v promoter. Mitochondrial membrane potential, reactive oxygen species production, opening of mitochondrial permeability transition pore, and survival of hESC-derived cardiomyocytes were assessed using confocal microscopy. Oxygen consumption was measured in contracting cell clusters.

RESULTS

Differentiation yielded a high percentage (∼85%) of cardiomyocytes in beating clusters that were positive for cardiac-specific markers and exhibited action potentials resembling those of mature cardiomyocytes. Isoflurane depolarized mitochondria, attenuated oxygen consumption, and stimulated generation of reactive oxygen species. APC protected these cells from oxidative stress-induced death and delayed mitochondrial permeability transition pore opening.

CONCLUSIONS

APC elicits competent protective mechanisms against oxidative stress in hESC-derived cardiomyocytes, suggesting the feasibility to use these cells as a model of human cardiomyocytes for studying APC and potentially other treatments/diseases. Our differentiation protocol is very efficient and yields a high percentage of cardiomyocytes. These results also suggest a promising ability of APC to protect and improve engraftment of hESC-derived cardiomyocytes into the ischemic heart.

Non-cardiomyocytes influence the electrophysiological maturation of human embryonic stem cell-derived cardiomyocytes during differentiation

Various types of cardiomyocytes undergo changes in automaticity and electrical properties during fetal heart development. Human embryonic stem cell-derived cardiomyocytes (hESC-CMs), like fetal cardiomyocytes, are electrophysiologically immature and exhibit automaticity. We used hESC-CMs to investigate developmental changes in mechanisms of automaticity and to determine whether electrophysiological maturation is driven by an intrinsic developmental clock and/or is regulated by interactions with non-cardiomyocytes in embryoid bodies (EBs). We isolated pure populations of hESC-CMs from EBs by lentivirus-engineered Puromycin resistance at various stages of differentiation. Using pharmacological agents, calcium (Ca(2+)) imaging, and intracellular recording techniques, we found that intracellular Ca(2+)-cycling mechanisms developed early and contributed to dominant automaticity throughout hESC-CM differentiation. Sarcolemmal ion channels evolved later upon further differentiation within EBs and played an increasing role in controlling automaticity and electrophysiological properties of hESC-CMs. In contrast to the development of intracellular Ca(2+)-handling proteins, ion channel development and electrophysiological maturation of hESC-CMs did not occur when hESC-CMs were isolated from EBs early and maintained in culture without further interaction with non-cardiomyocytes. Adding back non-cardiomyocytes to early-isolated hESC-CMs rescued the arrest of electrophysiological maturation, indicating that non-cardiomyocytes in EBs drive electrophysiological maturation of early hESC-CMs. Non-cardiomyocytes in EBs contain most cell types present in the embryonic heart that are known to influence early cardiac development. Our study is the first to demonstrate that non-cardiomyocytes influence electrophysiological maturation of early hESC-CMs in cultures. Defining the nature of these extrinsic signals will aid in the directed maturation of immature hESC-CMs to mitigate arrhythmogenic risks of cell-based therapies.

Na+/Ca2+ exchanger is a determinant of excitation-contraction coupling in human embryonic stem cell-derived ventricular cardiomyocytes

In adult cardiomyocytes (CMs), the Na(+)/Ca(2+) exchanger (NCX) is a well-defined determinant of Ca(2+) homeostasis. Developmentally, global NCX knockout in mice leads to abnormal myofibrillar organization, electrical defects, and early embryonic death. Little is known about the expression and function of NCX in human heart development. Self-renewable, pluripotent human embryonic stem cells (hESCs) can serve as an excellent experimental model. However, hESC-derived CMs are highly heterogeneous. A stably lentivirus-transduced hESC line (MLC2v-dsRed) was generated to express dsRed under the transcriptional control of the ventricular-restricted myosin light chain-2v (MLC2v) promoter. Electrophysiologically, dsRed+ cells differentiated from MLC2vdsRed hESCs displayed ventricular action potentials (AP), exclusively. Neither atrial nor pacemaker APs were observed. While I(Ca-L), I(f), and I(Kr) were robustly expressed, I(Ks) and I(K1) were absent in dsRed+ ventricular hESCCMs. Upon differentiation (7+40 to +90 days), the basal [Ca(2+)](i), Ca(2+) transient amplitude, maximum upstroke, and decay velocities significantly increased (P < 0.05). The I(Ca-L) antagonizer nifedipine (1 microM) decreased the Ca(2+) transient amplitude (to approximately 30%) and slowed the kinetics (by approximately 2-fold), but Ca(2+) transients could still be elicited even after complete ICa-L blockade, suggesting the presence of additional Ca(2+) influx(es). Indeed, Ni(2+)-sensitive INCX could be recorded in 7+40- and +90-day dsRed+ hESC-CMs, and its densities increased from -1.2 +/- 0.6 pA/pF at -120 mV and 3.6 +/- 1.0 pA/pF at 60 mV by 6- and 2-folds, respectively. With higher [Ca(2+)](i), 7+90-day ventricular hESC-CMs spontaneously but irregularly fired transients upon a single stimulus under an external Na(+)-free condition; however, without extracellular Na(+), nifedipine could completely inhibit Ca(2+) transients. We conclude that I(NCX) is functionally expressed in developing ventricular hESC-CMs and contributes to their excitation-contraction coupling.

Differential gene expressions in atrial and ventricular myocytes: insights into the road of applying embryonic stem cell-derived cardiomyocytes for future therapies

Myocardial infarction has been the leading cause of morbidity and mortality in developed countries over the past few decades. The transplantation of cardiomyocytes offers a potential method of treatment. However, cardiomyocytes are in high demand and their supply is extremely limited. Embryonic stem cells (ESCs), which have been isolated from the inner cell mass of blastocysts, can self-renew and are pluripotent, meaning they have the ability to develop into any type of cell, including cardiomyocytes. This suggests that ESCs could be a good source of genuine cardiomyocytes for future therapeutic purposes. However, problems with the yield and purity of ESC-derived cardiomyocytes, among other hurdles for the therapeutic application of ESC-derived cardiomyocytes (e.g., potential immunorejection and tumor formation problems), need to be overcome before these cells can be used effectively for cell replacement therapy. ESC-derived cardiomyocytes consist of nodal, atrial, and ventricular cardiomyocytes. Specifically, for treatment of myocardial infarction, transplantation of a sufficient quantity of ventricular cardiomyocytes, rather than nodal or atrial cardiomyocytes, is preferred. Hence, it is important to find ways of increasing the yield and purity of specific types of cardiomyocytes. Atrial and ventricular cardiomyocytes have differential expression of genes (transcription factors, structural proteins, ion channels, etc.) and are functionally distinct. This paper presents a thorough review of differential gene expression in atrial and ventricular myocytes, their expression throughout development, and their regulation. An understanding of the molecular and functional differences between atrial and ventricular myocytes allows discussion of potential strategies for preferentially directing ESCs to differentiate into chamber-specific cells, or for fine tuning the ESC-derived cardiomyocytes into specific electrical and contractile phenotypes resembling chamber-specific cells.

Cardiac regeneration using human embryonic stem cells: producing cells for future therapy

Directed differentiation of human embryonic stem cells (hESCs) has generated much interest in the field of regenerative medicine. Because of their ability to differentiate into any cell type in the body, hESCs offer a novel therapeutic paradigm for myocardial repair by furnishing a supply of cardiomyocytes (CMs) that would ultimately restore normal myocardial function when delivered to the damaged heart. Spontaneous CM differentiation of hESCs is an inefficient process that yields very low numbers of CMs. In addition, it is not clear that fully differentiated CMs provide the benefits sought from cell transplantation. The need for new methods of directed differentiation of hESCs into functional CMs and cardiac progenitors has led to an explosion of research utilizing chemical, genetic, epigenetic and lineage selection strategies to direct cardiac differentiation and enrich populations of cardiac cells for therapeutic use. Here, we review these approaches and highlight their increasingly important roles in stem cell biology and cardiac regenerative medicine.

Evaluating the utility of cardiomyocytes from human pluripotent stem cells for drug screening

Functional cardiomyocytes can now be derived routinely from hPSCs (human pluripotent stem cells), which collectively include embryonic and induced pluripotent stem cells. This technology presents new opportunities to develop pharmacologically relevant in vitro screens to detect cardiotoxicity, with a view to improving patient safety while reducing the economic burden to industry arising from high drug attrition rates. In the present article, we consider the need for human cardiomyocytes in drug-screening campaigns and review the strategies used to differentiate hPSCs towards the cardiac lineage. During early stages of differentiation, hPSC-cardiomyocytes display gene expression profiles, ultra-structures, ion channel functionality and pharmacological responses reminiscent of an embryonic phenotype, but maturation during extended time in culture has been demonstrated convincingly. Notably, hPSC-cardiomyocytes have been shown to respond in a highly predictable manner to over 40 compounds that have a known pharmacological effect on the human heart. This suggests that further development and validation of the hPSC-cardiomyocyte model as a tool for assessing cardiotoxicity is warranted.

Differentiation of human embryonic stem cells to cardiomyocytes for in vitro and in vivo applications

The ability of human embryonic stem cells to differentiate into spontaneously contracting cardiomyocyte-like cells has attracted substantial interest from the scientific community over the last decade. From having been difficult to control, human cardiomyogenesis in vitro is now becoming a process which, to a certain extent, can be effectively manipulated and directed. Although much research remains, new and improved protocols for guiding pluripotent stem cells to the cardiomyocyte lineage are accumulating in the scientific literature. However, the stem cell derived cardiomyocytes described to date, generally resemble immature embryonic/fetal cardiomyocytes, and they are in some functional and structural aspects different from adult cardiomyocytes. Thus, a future challenge will be to design strategies that eventually may allow the cells to reach a higher degree of maturation in vitro. Nevertheless, the cells which can be prepared using current protocols still have wide spread utility, and they have begun to find their way into the drug discovery platforms used in the pharmaceutical industry. In addition, stem cell derived cardiomyocytes and cardiac progenitors are anticipated to have a tremendous impact on how heart disease will be treated in the future. Here, we will discuss recent strategies for the generation of cardiomyocytes from human embryonic stem cells and recapitulate their features, as well as highlight some in vitro applications for the cells. Finally, opportunities in the area of cardiac regenerative medicine will be illustrated.

Injectable cardiac tissue engineering for the treatment of myocardial infarction

Heart disease is a leading cause of morbidity and mortality worldwide. Myocardial infarction leads to permanent loss of cardiac tissue and ultimately heart failure. However, current therapies could only stall the progression of the disease. Thus, new therapies are needed to regenerate damaged hearts to overcome poor prognosis of patients with heart failure. The shortage of heart donors is also a factor for innovating new therapies. Although the cardiac performance by cell-based therapy has improved, unsatisfactory cell retention and transplant survival still plague this technique. Because biomaterials can improve the cell retention, survival and differentiation, cardiac tissue engineering is now being explored as an approach to support cell-based therapies and enhance their efficacy for cardiac disease. In the last decade, cardiac tissue engineering has made considerable progress. Among different kinds of approaches in the cardiac tissue engineering, the approach of injectable cardiac tissue engineering is more minimally invasive than that of in vitro engineered tissue or epicardial patch implantation. It is therefore clinically appealing. In this review, we strive to describe the major progress in the flied of injectable cardiac tissue engineering, including seeding cell sources, biomaterials and novel findings in preclinical studies and clinical applications. The remaining problems will also be discussed.

Genetic manipulation of human embryonic stem cells

One of the great advantages of embryonic stem (ES) cells over other cell types is their accessibility to genetic manipulation. They can easily undergo genetic modifications while remaining pluripotent and can be selectively propagated, allowing the clonal expansion of genetically altered cells in culture. Since the first isolation of ES cells in mice, many effective techniques have been developed for gene delivery and manipulation of ES cells. These include transfection, electroporation, and infection protocols, as well as different approaches for inserting, deleting, or changing the expression of genes. These methods proved to be extremely useful in mouse ES cells, for monitoring and directing differentiation, discovering unknown genes and studying their function, and are now being initiated in human ES (HESC) cells. This chapter describes the different approaches and methodologies that have been applied for the genetic manipulation of HESCs and their applications. Detailed protocols for generating clones of genetically modified HESCs by transfection, electroporation, and infection will be described, with special emphasis on the important technical details that are required for this purpose.

Methods for human embryonic stem cells derived cardiomyocytes cultivation, genetic manipulation, and transplantation

A decade has passed since the initial derivation of human embryonic stem cells (hESC). The ensuing years have witnessed a significant progress in the development of methodologies allowing cell cultivation, differentiation, genetic manipulation, and in vivo transplantation. Specifically, the potential to derive human cardiomyocytes from the hESC lines, which can be used for several basic and applied cardiovascular research areas including in the emerging field of cardiac regenerative medicine, attracted significant attention from the scientific community. This resulted in the development of protocols for the cultivation of hESC and their successful differentiation toward the cardiomyocyte lineage fate. In this chapter, we will describe in detail methods related to the cultivation, genetic manipulation, selection, and in vivo transplantation of hESC-derived cardiomyocytes.

Prediction of drug-induced cardiotoxicity using human embryonic stem cell-derived cardiomyocytes

Recent withdrawals of prescription drugs from clinical use because of unexpected side effects on the heart have highlighted the need for more reliable cardiac safety pharmacology assays. Block of the human Ether-a-go go Related Gene (hERG) ion channel in particular is associated with life-threatening arrhythmias, such as Torsade de Pointes (TdP). Here we investigated human cardiomyocytes derived from pluripotent (embryonic) stem cells (hESC) as a renewable, scalable, and reproducible system on which to base cardiac safety pharmacology assays. Analyses of extracellular field potentials in hESC-derived cardiomyocytes (hESC-CM) and generation of derivative field potential duration (FPD) values showed dose-dependent responses for 12 cardiac and noncardiac drugs. Serum levels in patients of drugs with known effects on QT interval overlapped with prolonged FPD values derived from hESC-CM, as predicted. We thus propose hESC-CM FPD prolongation as a safety criterion for preclinical evaluation of new drugs in development. This is the first study in which dose responses of such a wide range of compounds on hESC-CM have been generated and shown to be predictive of clinical effects. We propose that assays based on hESC-CM could complement or potentially replace some of the preclinical cardiac toxicity screening tests currently used for lead optimization and further development of new drugs.

Nongenetic method for purifying stem cell-derived cardiomyocytes

Several applications of pluripotent stem cell (PSC)-derived cardiomyocytes require elimination of undifferentiated cells. A major limitation for cardiomyocyte purification is the lack of easy and specific cell marking techniques. We found that a fluorescent dye that labels mitochondria, tetramethylrhodamine methyl ester perchlorate, could be used to selectively mark embryonic and neonatal rat cardiomyocytes, as well as mouse, marmoset and human PSC-derived cardiomyocytes, and that the cells could subsequently be enriched (>99% purity) by fluorescence-activated cell sorting. Purified cardiomyocytes transplanted into testes did not induce teratoma formation. Moreover, aggregate formation of PSC-derived cardiomyocytes through homophilic cell-cell adhesion improved their survival in the immunodeficient mouse heart. Our approaches will aid in the future success of using PSC-derived cardiomyocytes for basic and clinical applications.

Cardiac applications for human pluripotent stem cells

Human embryonic stem cells (hESCs) and induced pluripotent stem cells (hiPSCs) can self-renew indefinitely, while maintaining the capacity to differentiate into useful somatic cell types, including cardiomyocytes. As such, these stem cell types represent an essentially inexhaustible source of committed human cardiomyocytes of potential use in cell-based cardiac therapies, high-throughput screening and safety testing of new drugs, and modeling human heart development. These stem cell-derived cardiomyocytes have an unambiguous cardiac phenotype and proliferate robustly both in vitro and in vivo. Recent transplantation studies in preclinical models have provided exciting proof-of-principle for their use in infarct repair and in the formation of a "biological pacemaker". While these successes give reason for cautious optimism, major challenges remain to the successful application of hESCs (or hiPSCs) to cardiac repair, including the need for preparations of high cardiac purity, improved methods of delivery, and approaches to overcome immune rejection and other causes of graft cell death. In this review, we describe the phenotype of hESC- and hiPSC-derived cardiomyocytes, the state of preclinical transplantation studies with these cells, and potential approaches to overcome the aforementioned hurdles.

Extracellular matrix formation after transplantation of human embryonic stem cell-derived cardiomyocytes

Transplantation of human embryonic stem cell-derived cardiomyocytes (hESC-CM) for cardiac regeneration is hampered by the formation of fibrotic tissue around the grafts, preventing electrophysiological coupling. Investigating this process, we found that: (1) beating hESC-CM in vitro are embedded in collagens, laminin and fibronectin, which they bind via appropriate integrins; (2) after transplantation into the mouse heart, hESC-CM continue to secrete collagen IV, XVIII and fibronectin; (3) integrin expression on hESC-CM largely matches the matrix type they encounter or secrete in vivo; (4) co-transplantation of hESC-derived endothelial cells and/or cardiac progenitors with hESC-CM results in the formation of functional capillaries; and (5) transplanted hESC-CM survive and mature in vivo for at least 24 weeks. These results form the basis of future developments aiming to reduce the adverse fibrotic reaction that currently complicates cell-based therapies for cardiac disease, and to provide an additional clue towards successful engraftment of cardiomyocytes by co-transplanting endothelial cells.

In vivo selection of human embryonic stem cell-derived cells expressing methotrexate-resistant dihydrofolate reductase

Human embryonic stem cells (hESCs) provide a novel source of hematopoietic and other cell populations suitable for gene therapy applications. Preclinical studies to evaluate engraftment of hESC-derived hematopoietic cells transplanted into immunodeficient mice demonstrate only limited repopulation. Expression of a drug resistance gene, such as Tyr22-dihydrofolate reductase (Tyr22-DHFR), coupled to methotrexate (MTX) chemotherapy has the potential to selectively increase engraftment of gene-modified hESC-derived cells in mouse xenografts. Here, we describe the generation of Tyr22-DHFR – GFP expressing hESCs that maintain pluripotency, produce teratomas and can differentiate into MTXr-hemato-endothelial cells. We demonstrate that MTX administered to nonobese diabetic/severe combined immunodeficient/IL-2Rγc^null (NSG) mice after injection of Tyr22-DHFR-derived cells significantly increases human CD34⁺ and CD45⁺ cell engraftment in the bone marrow (BM) and peripheral blood of transplanted MTX-treated mice. These results demonstrate that MTX treatment supports selective, long-term engraftment of Tyr22-DHFR-cells in vivo, and provides a novel approach for combined human cell and gene therapy.

Cardiac cell repair therapy: a clinical perspective

From bone marrow transplants 5 decades ago to the most recent stem cell-derived organ transplants, regenerative medicine is increasingly recognized as an emerging core component of modern practice. In cardiovascular medicine, innovation in stem cell biology has created curative solutions for the treatment of both ischemic and nonischemic cardiomyopathy. Multiple cell-based platforms have been developed, harnessing the regenerative potential of various natural and bioengineered sources. Clinical experience from the first 1000 patients (approximately) who have received stem cell therapy worldwide indicates a favorable safety profile with modest improvement in cardiac function and structural remodeling in the setting of acute myocardial infarction or chronic heart failure. Further investigation is required before early adoption and is ongoing. Broader application in practice will require continuous scientific advances to match each patient with the most effective reparative phenotype, while ensuring optimal cell delivery, dosing, and timing of intervention. An interdisciplinary effort across the scientific and clinical community within academia, biotechnology, and government will drive the successful realization of this next generation of therapeutic agents for the "broken" heart.

Cardiac cell repair therapy: a clinical perspective

From bone marrow transplants 5 decades ago to the most recent stem cell-derived organ transplants, regenerative medicine is increasingly recognized as an emerging core component of modern practice. In cardiovascular medicine, innovation in stem cell biology has created curative solutions for the treatment of both ischemic and nonischemic cardiomyopathy. Multiple cell-based platforms have been developed, harnessing the regenerative potential of various natural and bioengineered sources. Clinical experience from the first 1000 patients (approximately) who have received stem cell therapy worldwide indicates a favorable safety profile with modest improvement in cardiac function and structural remodeling in the setting of acute myocardial infarction or chronic heart failure. Further investigation is required before early adoption and is ongoing. Broader application in practice will require continuous scientific advances to match each patient with the most effective reparative phenotype, while ensuring optimal cell delivery, dosing, and timing of intervention. An interdisciplinary effort across the scientific and clinical community within academia, biotechnology, and government will drive the successful realization of this next generation of therapeutic agents for the "broken" heart.