DNA synthesis and multinucleation in embryonic stem cell-derived cardiomyocytes

Live cell screening identifies glycosides as enhancers of cardiomyocyte cell cycle activity

Promoting cardiomyocyte proliferation is a promising strategy to regenerate the heart. Yet, so far, it is poorly understood how cardiomyocyte proliferation is regulated, and no factor identified to promote mammalian cardiomyocyte proliferation has been translated into medical practice. Therefore, finding a novel factor will be vital. Here, we established a live cell screening based on mouse embryonic stem cell-derived cardiomyocytes expressing a non-functional human geminin deletion mutant fused to Azami Green (CM7/1-hgem-derived cardiomyocytes). We screened for a subset of compounds of the small molecule library Spectrum Collection and identified 19 potential inducers of stem cell-derived cardiomyocyte proliferation. Furthermore, the pro-proliferative potential of identified candidate compounds was validated in neonatal and adult rat cardiomyocytes as well as human induced pluripotent stem cell-derived cardiomyocytes. 18 of these compounds promoted mitosis and cytokinesis in neonatal rat cardiomyocytes. Among the top four candidates were two cardiac glycosides, peruvoside and convallatoxin, the flavonoid osajin, and the selective α-adrenoceptor antagonist and imidazoline I1 receptor ligand efaroxan hydrochloride. Inhibition of PTEN and GSK-3β enhanced cell cycle re-entry and progression upon stimulation with cardiac glycosides and osajin, while inhibition of IP3 receptors inhibited the cell cycle-promoting effect of cardiac glycosides. Collectively, we established a screening system and identified potential compounds to promote cardiomyocyte proliferation. Our data suggest that modulation of calcium handling and metabolism promotes cardiomyocyte proliferation, and cardiac glycosides might, besides increasing myocardial contraction force, contribute to cardiac repair by inducing cardiomyocyte proliferation.

Cited4 is related to cardiogenic induction and maintenance of proliferation capacity of embryonic stem cell-derived cardiomyocytes during in vitro cardiogenesis

Cardiac progenitor cells have a limited proliferative capacity. The CREB-binding protein/p300-interacting transactivator, with the Glu/Asp-rich carboxy-terminal domain (Cited) gene family, regulates gene transcription. Increased expression of the Cited4 gene in an adult mouse is associated with exercise-induced cardiomyocyte hypertrophy and proliferation. However, the expression patterns and functional roles of the Cited4 gene during cardiogenesis are largely unknown. Therefore, in the present study, we investigated the expression patterns and functional roles of the Cited4 gene during in vitro cardiogenesis. Using embryoid bodies formed from mouse embryonic stem cells, we evaluated the expression patterns of the Cited4 gene by quantitative reverse transcriptase-polymerase chain reaction. Cited4 gene expression levels increased and decreased during the early and late phases of cardiogenesis, respectively. Moreover, Cited4 gene levels were significantly high in the cardiac progenitor cell population. A functional assay of the Cited4 gene in cardiac progenitor cells using flow cytometry indicated that overexpression of the Cited4 gene significantly increased the cardiac progenitor cell population compared with the control and knockdown groups. A cell proliferation assay, with 5-ethynyl-2′-deoxyuridine incorporation and Ki67 expression during the late phase of cardiogenesis, indicated that the number of troponin T-positive embryonic stem cell-direived cardiomyocytes with proliferative capacity was significantly greater in the overexpression group than in the control and knockdown groups. Our study results suggest that the Cited4 gene is related to cardiac differentiation and maintenance of proliferation capacity of embryonic stem cell-derived cardiomyocytes during in vitro cardiogenesis. Therefore, manipulation of Cited4 gene expression may be of great interest for cardiac regeneration.

Concise Review: Measuring Physiological Responses of Human Pluripotent Stem Cell Derived Cardiomyocytes to Drugs and Disease

Cardiomyocytes from human pluripotent stem cells (hPSC) are of growing interest as models to understand mechanisms underlying genetic disease, identify potential drug targets and for safety pharmacology as they may predict human relevant effects more accurately and inexpensively than animals or other cell models. Crucial to their optimal use are accurate methods to quantify cardiomyocyte phenotypes accurately and reproducibly. Here, we review current methods for determining biophysical parameters of hPSC‐derived cardiomyocytes (hPSC‐CMs) that recapitulate disease and drug responses. Even though hPSC‐CMs as currently available are immature, various biophysical methods are nevertheless already providing useful insights into the biology of the human heart and its maladies. Advantages and limitations of assays currently available looking toward applications of hPSC‐CMs are described with examples of how they have been used to date. This will help guide the choice of biophysical method to characterize healthy cardiomyocytes and their pathologies in vitro. Stem Cells2016;34:2008–2015

The immune boundaries for stem cell based therapies: problems and prospective solutions

Stem cells have fascinated the scientific and clinical communities for over a century. Despite the controversy that surrounds this field, it is clear that stem cells have the potential to revolutionize medicine. However, a number of significant hurdles still stand in the way of the realization of this potential. Chiefly among these are safety concerns, differentiation efficiency and overcoming immune rejection. Here we review current progress made in this field to optimize the safe use of stem cells with particular emphasis on prospective interventions to deal with challenges generated by immune rejection.

Enhanced proliferation of monolayer cultures of embryonic stem (ES) cell-derived cardiomyocytes following acute loss of retinoblastoma

Background

Cardiomyocyte (CM) cell cycle analysis has been impeded because of a reliance on primary neonatal cultures of poorly proliferating cells or chronic transgenic animal models with innate compensatory mechanisms.

Methodology/Principal Findings

We describe an in vitro model consisting of monolayer cultures of highly proliferative embryonic stem (ES) cell-derived CM. Following induction with ascorbate and selection with puromycin, early CM cultures are >98% pure, and at least 85% of the cells actively proliferate. During the proliferative stage, cells express high levels of E2F3a, B-Myb and phosphorylated forms of retinoblastoma (Rb), but with continued cultivation, cells stop dividing and mature functionally. This developmental transition is characterized by a switch from slow skeletal to cardiac TnI, an increase in binucleation, cardiac calsequestrin and hypophosphorylated Rb, a decrease in E2F3, B-Myb and atrial natriuretic factor, and the establishment of a more negative resting membrane potential. Although previous publications suggested that Rb was not necessary for cell cycle control in heart, we find following acute knockdown of Rb that this factor actively regulates progression through the G1 checkpoint and that its loss promotes proliferation at the expense of CM maturation.

Conclusions/Significance

We have established a unique model system for studying cardiac cell cycle progression, and show in contrast to previous reports that Rb actively regulates both cell cycle progression through the G1 checkpoint and maturation of heart cells. We conclude that this in vitro model will facilitate the analysis of cell cycle control mechanisms of CMs.

The mouse as a model system to study cardiac regeneration

Cardiomyocytes exhibit robust proliferative activity during development. After birth, cardiomyocyte proliferation is markedly reduced. Consequently, regenerative growth in the postnatal heart via cardiomyocyte proliferation (including, by inference, via proliferation of stem cell-derived cardiomyocytes) is limited and often insufficient to effect repair following injury. Here we review methodologies which employ the mouse as a model system to study cardiac regeneration, and in particular cardiomyocyte replenishment, in health and disease.

Embryonic stem cells and cardiomyocyte differentiation: phenotypic and molecular analyses

Embryonic stem (ES) cell lines, derived from the inner cell mass (ICM) of blastocyst-stage embryos, are pluripotent and have a virtually unlimited capacity for self-renewal and differentiation into all cell types of an embryoproper. Both human and mouse ES cell lines are the subject of intensive investigation for potential applications in developmental biology and medicine. ES cells from both sources differentiate in vitro into cells of ecto-, endoand meso-dermal lineages, and robust cardiomyogenic differentiation is readily observed in spontaneously differentiating ES cells when cultured under appropriate conditions. Molecular, cellular and physiologic analyses demonstrate that ES cell-derived cardiomyocytes are functionally viable and that these cell derivatives exhibit characteristics typical of heart cells in early stages of cardiac development. Because terminal heart failure is characterized by a significant loss of cardiomyocytes, the use of human ES cell-derived progeny represents one possible source for cell transplantation therapies. With these issues in mind, this review will focus on the differentiation of pluripotent embryonic stem cells into cardiomyocytes as a developmental model, and the possible use of ES cell-derived cardiomyocytes as source of donor cells.

Formation of human myocardium in the rat heart from human embryonic stem cells

Human embryonic stem cells (hESCs) offer the opportunity to replenish cells lost in the postinfarct heart. We explored whether human myocardium could be generated in rat hearts by injecting differentiated cardiac-enriched hESC progeny into the left ventricular wall of athymic rats. Although initial grafts were predominantly epithelial, noncardiac elements were lost over time, and grafts consisted predominantly of cardiomyocytes by 4 weeks. No teratomatous elements were observed. Engrafted cardiomyocytes were glycogen-rich and expressed expected cardiac markers including beta-myosin heavy chain, myosin light chain 2v, and atrial natriuretic factor. Heat-shock treatment improved graft size approximately threefold. The cardiac implants exhibited substantial angiogenesis, both recipient and graft derived. Importantly, there was greater proliferation in human cardiomyocytes than previously seen in rodent-derived cardiomyocytes: 14.4% of graft cardiomyocytes expressed the proliferation marker Ki-67, and 2.7% incorporated the thymidine analog BrdU 4 weeks after transplantation. This proliferation was associated with a sevenfold increase in graft size over the 4-week interval. Thus, hESCs can form human myocardium in the rat heart, permitting studies of human myocardial development and physiology and supporting the feasibility of their use in myocardial repair.

Proliferation of cardiomyocytes derived from human embryonic stem cells is mediated via the IGF/PI 3-kinase/Akt signaling pathway

Cardiomyocytes from common experimental animals rapidly exit the cell cycle upon isolation, impeding studies of basic cell biology and applications such as myocardial repair. Here we examined proliferation of cardiomyocytes derived from human and mouse embryonic stem (ES) cells. While mouse ES cell-derived cardiomyocytes showed little proliferation, human cardiomyocytes were highly proliferative under serum-free conditions (15-25% BrdU+/sarcomeric actin+). The cells exhibited only a small serum dose-response, and proliferation gradually slowed with increasing differentiation of the cells. Neither cell density nor different matrix attachment factors affected cardiomyocyte proliferation. Blockade of phosphatidylinositol 3-kinase (PI 3-kinase) and Akt significantly reduced cardiomyocyte proliferation, whereas MEK inhibition had no effect. Antibody blocking of the insulin-like growth factor-1 (IGF-1) receptor significantly inhibited cardiomyocyte proliferation, while addition of IGF-1 or IGF-2 stimulated cardiomyocyte proliferation in a dose-dependent manner. Thus, cardiomyocytes derived from human ES cells proliferate extensively in vitro, and their proliferation appears to be mediated primarily via the PI 3-kinase/Akt signaling pathway, using the IGF-1 receptor as one upstream activator. This system should permit identification of regulatory pathways for human cardiomyocyte proliferation and may facilitate expansion of cardiomyocytes from human ES cells for therapeutic purposes.

Electrical and neurotransmitter activity of mature neurons derived from mouse embryonic stem cells by Sox-1 lineage selection and directed differentiation

Sx1TV2/16C is a mouse embryonic stem (ES) cell line in which one copy of the Sox1 gene, an early neuroectodermal marker, has been targeted with a neomycin (G418) selection cassette. A combination of directed differentiation with retinoic acid and G418 selection results in an enriched neural stem cell population that can be further differentiated into neurons. After 6-7 days post-plating (D6-7PP) most neurons readily fired tetrodotoxin (TTX)-sensitive action potentials due to the expression of TTX-sensitive Na(+) and tetraethylammonium (TEA)-sensitive K(+) channels. Neurons reached their maximal cell capacitance after D6-7PP; however, ion channel expression continued until at least D21PP. The percentage of cells receiving spontaneous synaptic currents (s.s.c.) increased with days in culture until 100% of cells received a synaptic input by D20PP. Spontaneous synaptic currents were reduced in amplitude and frequency by TTX, or upon exposure to a Ca(2+)-free, 2.5 mm Mg(2+) saline. S.s.c. of rapid decay time constants were preferentially blocked by the nonNMDA glutamatergic receptor antagonists CNQX or NBQX. Ca(2+) levels within ES cell-derived neurons increased in response to glutamate receptor agonists l-glutamate, AMPA, N-methyl-d-aspartate (NMDA) and kainic acid and to acetylcholine, ATP and dopamine. ES cell-derived neurons also generated cationic and Cl(-)-selective currents in response to NMDA and glycine or GABA, respectively. It was concluded that ES-derived neurons fire action potentials, receive excitatory and inhibitory synaptic input and respond to various neurotransmitters in a manner akin to primary central neurons.

Assessment of the ultrastructural and proliferative properties of human embryonic stem cell-derived cardiomyocytes

Assessment of early ultrastructural development and cell-cycle regulation in human cardiac tissue is significantly hampered by the lack of a suitable in vitro model. Here we describe the possible utilization of human embryonic stem cell (ES) lines for investigation of these processes. With the use of the embryoid body (EB) differentiation system, human ES cell-derived cardiomyocytes at different developmental stages were isolated and their histomorphometric, ultrastructural, and proliferative properties were characterized. Histomorphometric analysis revealed an increase in cell length, area, and length-to-width ratio in late-stage EBs (>35 days) compared with early (10–21 days) and intermediate (21–35 days) stages. This was coupled with a progressive ultrastructural development from an irregular myofibrillar distribution to an organized sarcomeric pattern. Cardiomyocyte proliferation, assessed by double labeling with cardiac-specific antibodies and either [3H]thymidine incorporation or Ki-67 immunolabeling, demonstrated a gradual withdrawal from cell cycle. Hence, the percentage of positively stained nuclei in early-stage cardiomyocytes ([3H]thymidine: 60 ± 10%, Ki-67: 54 ± 23%) decreased to 36 ± 7% and 9 ± 16% in intermediate-stage EBs and to <1% in late-stage cardiomyocytes. In conclusion, a reproducible temporal pattern of early cardiomyocyte proliferation, cell-cycle withdrawal, and ultrastructural maturation was noted in this model. Establishment of this unique in vitro surrogate system may allow to examine the molecular mechanisms underlying these processes and to assess interventions aiming to modify these properties. Moreover, the detailed characterization of the ES cell-derived cardiomyocyte may be crucial for the development of future cell replacement strategies aiming to regenerate functional myocardium.

Human embryonic germ cell derivatives express a broad range of developmentally distinct markers and proliferate extensively in vitro

Human pluripotent stem cells (hPSCs) have been derived from the inner cell mass cells of blastocysts (embryonic stem cells) and primordial germ cells of the developing gonadal ridge (embryonic germ cells). Like their mouse counterparts, hPSCs can be maintained in culture in an undifferentiated state and, upon differentiation, generate a wide variety of cell types. Embryoid body (EB) formation is a requisite step in the process of in vitro differentiation of these stem cells and has been used to derive neurons and glia, vascular endothelium, hematopoietic cells, cardiomyocytes, and glucose-responsive insulin-producing cells from mouse PSCs. EBs generated from human embryonic germ cell cultures have also been found to contain a wide variety of cell types, including neural cells, vascular endothelium, muscle cells, and endodermal derivatives. Here, we report the isolation and culture of cells from human EBs as well as a characterization of their gene expression during growth in several different culture environments. These heterogeneous cell cultures are capable of robust and long-term [>70 population doublings (PD)] proliferation in culture, have normal karyotypes, and can be cryopreserved, clonally isolated, and stably transfected. Cell cultures and clonal lines retain a broad pattern of gene expression including simultaneous expression of markers normally associated with cells of neural, vascular/hematopoietic, muscle, and endoderm lineages. The growth and expression characteristics of these EB-derived cells suggest that they are relatively uncommitted precursor or progenitor cells. EB-derived cells may be suited to studies of human cell differentiation and may play a role in future transplantation therapies.

Derivation of pluripotent stem cells from cultured human primordial germ cells

Human pluripotent stem cells would be invaluable for in vitro studies of aspects of human embryogenesis. With the goal of establishing pluripotent stem cell lines, gonadal ridges and mesenteries containing primordial germ cells (PGCs, 5-9 weeks postfertilization) were cultured on mouse STO fibroblast feeder layers in the presence of human recombinant leukemia inhibitory factor, human recombinant basic fibroblast growth factor, and forskolin. Initially, single PGCs in culture were visualized by alkaline phosphatase activity staining. Over a period of 7-21 days, PGCs gave rise to large multicellular colonies resembling those of mouse pluripotent stem cells termed embryonic stem and embryonic germ (EG) cells. Throughout the culture period most cells within the colonies continued to be alkaline phosphatase-positive and tested positive against a panel of five immunological markers (SSEA-1, SSEA-3, SSEA-4, TRA-1-60, and TRA-1-81) that have been used routinely to characterize embryonic stem and EG cells. The cultured cells have been continuously passaged and found to be karyotypically normal and stable. Both XX and XY cell cultures have been obtained. Immunohistochemical analysis of embryoid bodies collected from these cultures revealed a wide variety of differentiated cell types, including derivatives of all three embryonic germ layers. Based on their origin and demonstrated properties, these human PGC-derived cultures meet the criteria for pluripotent stem cells and most closely resemble EG cells.

Targeted inactivation of alphai2 or alphai3 disrupts activation of the cardiac muscarinic K+ channel, IK+Ach, in intact cells

Cardiac muscarinic receptors activate an inwardly rectifying K+ channel, IK+Ach, via pertussis toxin (PT)-sensitive heterotrimeric G proteins (in heart Gi2, Gi3, or Go). We have used embryonic stem cell (ES cell)-derived cardiocytes with targeted inactivations of specific PT-sensitive alpha subunits to determine which G proteins are required for receptor-mediated regulation of IK+Ach in intact cells. The muscarinic agonist carbachol increased IK+Ach activity in ES cell-derived cardiocytes from wild-type cells, in cells lacking alphao, and in cells lacking the PT-insensitive G protein alphaq. In cells with targeted inactivation of alphai2 or alphai3, channel activation by both carbachol and adenosine was blocked. Carbachol-induced channel activation was restored in the alphai2- and alphai3-null cells by reexpressing the previously targeted gene and guanosine 5'-[gamma-thio] triphosphate was able to fully activate IK+Ach in excised membranes patches from these mutants. In contrast, negative chronotropic responses to both carbachol and adenosine were preserved in cells lacking alphai2 or alphai3. Our results show that expression of two specific PT-sensitive alpha subunits (alphai2 and alphai3 but not alphao) is required for normal agonist-dependent activation of IK+Ach and suggest that both alphai2- and alphai3-containing heterotrimeric G proteins may be involved in the signaling process. Also the generation of negative chronotropic responses to muscarinic or adenosine receptor agonists do not require activation of IK+Ach or the expression of alphai2 or alphai3.

Genetically selected cardiomyocytes from differentiating embronic stem cells form stable intracardiac grafts

This study describes a simple approach to generate relatively pure cultures of cardiomyocytes from differentiating murine embryonic stem (ES) cells. A fusion gene consisting of the alpha-cardiac myosin heavy chain promoter and a cDNA encoding aminoglycoside phosphotransferase was stably transfected into pluripotent ES cells. The resulting cell lines were differentiated in vitro and subjected to G418 selection. Immunocytological and ultrastructural analyses demonstrated that the selected cardiomyocyte cultures (> 99% pure) were highly differentiated. G418 selected cardiomyocytes were tested for their ability to form grafts in the hearts of adult dystrophic mice. The fate of the engrafted cells was monitored by antidystrophin immunohistology, as well as by PCR analysis with primers specific for the myosin heavy chain-aminoglycoside phosphotransferase transgene. Both analyses revealed the presence of ES-derived cardiomyocyte grafts for as long as 7 wk after implantation, the latest time point analyzed. These studies indicate that a simple genetic manipulation can be used to select essentially pure cultures of cardiomyocytes from differentiating ES cells. Moreover, the resulting cardiomyocytes are suitable for the formation of intracardiac grafts. This selection approach should be applicable to all ES-derived cell lineages.