Gene expression profiling of functional murine embryonic stem cell-derived cardiomyocytes and comparison with adult heart: profiling of murine ESC-derived cardiomyocytes

Experimental models of cardiac physiology and pathology

Experimental models of cardiac disease play a key role in understanding the pathophysiology of the disease and developing new therapies. The features of the experimental models should reflect the clinical phenotype, which can have a wide spectrum of underlying mechanisms. We review characteristics of commonly used experimental models of cardiac physiology and pathophysiology in all translational steps including in vitro, small animal, and large animal models. Understanding their characteristics and relevance to clinical disease is the key for successful translation to effective therapies.

A Comparative Assessment of Human and Chimpanzee iPSC-derived Cardiomyocytes with Primary Heart Tissues

Comparative genomic studies in primates have the potential to reveal the genetic and mechanistic basis for human specific traits. These studies may also help us better understand inter-species phenotypic differences that are clinically relevant. Unfortunately, the obvious limitation on sample collection and experimentation in humans and non-human apes severely restrict our ability to perform dynamic comparative studies in primates. Induced pluripotent stem cells (iPSCs), and their corresponding differentiated cells, may provide a suitable alternative system for dynamic comparative studies. Yet, to effectively use iPSCs and differentiated cells for comparative studies, one must characterize the extent to which these systems faithfully represent biological processes in primary tissues. To do so, we compared gene expression data from primary adult heart tissue and iPSC-derived cardiomyocytes from multiple human and chimpanzee individuals. We determined that gene expression in cultured cardiomyocytes from both human and chimpanzee is most similar to that of adult hearts compared to other adult tissues. Using a comparative framework, we found that 50% of gene regulatory differences between human and chimpanzee hearts are also observed between species in cultured cardiomyocytes; conversely, inter-species regulatory differences seen in cardiomyocytes are found significantly more often in hearts than in other primary tissues. Our work provides a detailed description of the utility and limitation of differentiated cardiomyocytes as a system for comparative functional genomic studies in primates.

The case for induced pluripotent stem cell-derived cardiomyocytes in pharmacological screening

The current drug screening models are deficient, particularly in detecting cardiac side effects. Human stem cell-derived cardiomyocytes could aid both early cardiotoxicity detection and novel drug discovery. Work over the last decade has generated human embryonic stem cells as potentially accurate sources of human cardiomyocytes, but ethical constraints and poor efficacy in establishing cell lines limit their use. Induced pluripotent stem cells do not require the use of human embryos and have the added advantage of producing patient-specific cardiomyocytes, allowing both generic and disease- and patient-specific pharmacological screening, as well as drug development through disease modelling. A critical question is whether sufficient standards have been achieved in the reliable and reproducible generation of 'adult-like' cardiomyocytes from human fibroblast tissue to progress from validation to safe use in practice and drug discovery. This review will highlight the need for a new experimental system, assess the validity of human induced pluripotent stem cell-derived cardiomyocytes and explore what the future may hold for their use in pharmacology.

Determination of appropriate stage of human-induced pluripotent stem cell-derived cardiomyocytes for drug screening and pharmacological evaluation in vitro

Human-induced pluripotent stem cell-derived cardiomyocytes (hiPS-CMs) at different stages (approximate days 30, 60, and 90) were used to determine the appropriate stage for functional and morphological assessment of drug effects in vitro. The hiPS-CMs had spontaneous beating activity, and β-adrenergic function was comparable in all stages of differentiation. Microelectrode array analyses using ion channel blockers indicated that the electrophysiological properties of these ion channels were comparable at all differentiation stages. Ultrastructural analysis using electron microscopy showed that myofibrillar structures at days 60 and 90 were similarly distributed and more mature than that at day 30. Analysis of motion vectors in contracting cells showed that the velocity of contraction was the highest at day 90 and was the most mature among the three stages. Gene expression analysis demonstrated that expression of some genes related to myofilament and sarcoplasmic reticulum increased with maturation of morphological and contractile properties. In conclusion, day 30 cardiomyocytes are useful for basic screening such as the assessment of electrophysiological properties, and days 60 and 90 are the appropriate differentiation stage for morphological assays. For the assay of contractile function associated with subcellular components such as sarcoplasmic reticulum, day 90 cardiomyocytes are the most suitable.

Contractile properties of early human embryonic stem cell-derived cardiomyocytes: beta-adrenergic stimulation induces positive chronotropy and lusitropy but not inotropy

Human embryonic stem cell-derived cardiomyocytes (hESC-CMs) provide the unique opportunity to study the very early development of the human heart. The aim of this study was to investigate the effect of calcium and beta-adrenergic stimulation on the contractile properties of early hESC-CMs. Beating clusters containing hESC-CMs were co-cultured in vitro with noncontractile slices of neonatal murine ventricles. After 5-7 days, when beating clusters had integrated morphologically into the damaged tissue, isometric force measurements were performed during spontaneous beating as well as during electrical field stimulation. Spontaneous beating stopped when extracellular calcium ([Ca²⁺](ec)) was removed or after administration of the Ca²⁺ channel blocker nifedipine. During field stimulation at a constant rate, the developed force increased with incremental concentrations of [Ca²⁺](ec). During spontaneous beating, rising [Ca²⁺](ec) increased beating rate and developed force up to a [Ca²⁺](ec) of 2.5 mM. When [Ca²⁺](ec) was increased further, spontaneous beating rate decreased, whereas the developed force continued to increase. The beta-adrenergic agonist isoproterenol induced a dose-dependent increase of the frequency of spontaneous beating; however, it did not significantly change the developed force during spontaneous contractions or during electrical stimulation at a constant rate. Force developed by early hESC-CMs depends on [Ca²⁺](ec) and on the L-type Ca²⁺ channel. The lack of an inotropic reaction despite a pronounced chronotropic response after beta-adrenergic stimulation most likely indicates immaturity of the sarcoplasmic reticulum. For cell-replacement strategies, further maturation of cardiac cells has to be achieved either in vitro before or in vivo after transplantation.

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.

Derivation of rat embryonic stem cells and generation of protease-activated receptor-2 knockout rats

One of the remarkable achievements in knockout (KO) rat production reported during the period 2008-2010 is the derivation of authentic embryonic stem (ES) cells from rat blastocysts using a novel culture medium containing glycogen synthase kinase 3 and mitogen-activated protein kinase kinase inhibitors (2i medium). Here, we report gene-targeting technology via homologous recombination in rat ES cells, demonstrating its use through production of a protease-activated receptor-2 gene (Par-2) KO rat. We began by generating germline-competent ES cells from Dark Agouti rats using 2i medium. These ES cells, which differentiate into cardiomyocytes in vitro, can produce chimeras with high ES cell contribution when injected into blastocysts. We then introduced a targeting vector with a neomycin-resistant gene driven by the CAG promoter to disrupt Par-2. After a 7-day drug selection, 489 neomycin-resistant colonies were obtained. Following screening by polymerase chain reaction (PCR) genotyping and quantitative PCR analysis, we confirmed three homologous recombinant clones, resulting in chimeras that transmitted the Par-2 targeted allele to offspring. Par-2 KO rats showed a loss of Par-2 messenger RNA expression in their stomach cells and a lack of PAR-2 mediated smooth muscle relaxation in the aorta as indicated by pharmacological testing. Compared with mice, rats offer many advantages in biomedical research, including a larger body size; consequently, they are widely used in scientific investigation. Thus, the establishment of a gene-targeting technology using rat ES cells will be a valuable tool in human disease model production and drug discovery.