Nitric oxide facilitates cardiomyogenesis in mouse embryonic stem cells

Ear mesenchymal stem cells (EMSC) can differentiate into spontaneously contracting muscle cells

We have previously shown that cells isolated from the outer ears of adult mice are a source of mesenchymal stem cells that can be induced to differentiate into adipo-, osteo-, and chondrocytes. In this study, we demonstrate that ear mesenchymal stem cells (EMSC) express stromal cell-associated markers (CD44, CD73) and stem cell marker Sca-1 and can be differentiated into spontaneously contracting muscle cells. Treatment of cells with epidermal growth factor (EGF) change their morphology from fibroblast shapes into stick-like structures that show repeated spontaneous contractions. Under conditions that promote myogenic differentiation, EMSC expressed mRNA for myoD and ventricular specific myosin light chain (MLC-2v) and protein for connexin 43, sarcomeric alpha-actinin, myocyte enhancer factor 2c (MEF2c), myosin heavy chain (MyHC), myogenin, and sarco-endoplasmic reticulum Ca(2+)ATPase (SERCA) 1. However, the cells were negative for Nkx2.5, GATA4, and ANP. Intracellular Ca(2+) transients in spontaneously beating EMSC, visualized by Fluo-3AM, showed a frequency of Ca(2+) oscillations ranging over 28-59/min (mean 41.17 +/- SEM 1.54). We also demonstrated that small pieces of ear tissues (ear punches) collected from live mice provide sufficient numbers of EMSC to isolate, culture and differentiate them into myocytes. Due to the ease of acquiring an expanding repertoire of differentiated EMSC cell types by a noninvasive surgical procedure, we conclude that the ear may prove to be a potential source of autologous cells for regenerative medicine, as supported by the fact that ears are one of the best sources of cells for somatic cell nuclear transfer (SCNT).

Human embryonic stem cells: current technologies and emerging industrial applications

The efficiency and accuracy of the drug development process is severely restricted by the lack of functional human cell systems. However, the successful derivation of pluripotent human embryonic stem (hES) cell lines in the late 1990s is expected to revolutionize biomedical research in many areas. Due to their growth capacity and unique developmental potential to differentiate into almost any cell type of the human body, hES cells have opened novel avenues both in basic and applied research as well as for therapeutic applications. In this review we describe, from an industrial perspective, the basic science that underlies the hES cell technology and discuss the current and future prospects for hES cells in novel and improved stem cell based applications for drug discovery, toxicity testing as well as regenerative medicine.

Embryonic stem cells as a source of models for drug discovery

Embryonic stem cells (ESCs) will become a source of models for a wide range of adult differentiated cells, providing that reliable protocols for directed differentiation can be established. Stem-cell technology has the potential to revolutionize drug discovery, making models available for primary screens, secondary pharmacology, safety pharmacology, metabolic profiling and toxicity evaluation. Models of differentiated cells that are derived from mouse ESCs are already in use in drug discovery, and are beginning to find uses in high-throughput screens. Before analogous human models can be obtained in adequate numbers, reliable methods for the expansion of human ESC cultures will be needed. For applications in drug discovery, involving either species, protocols for directed differentiation will need to be robust and affordable. Here, we explore current challenges and future opportunities in relation to the use of stem-cell technology in drug discovery, and address the use of both mouse and human models.

Bypassing heterogeneity: the road to embryonic stem cell-derived cardiomyocyte specification

Embryonic stem cells (ESCs) are a pluripotent cell type that may be considered for treatments in cell replacement therapies, such as for cardiovascular disease. The general premise is that ESCs may be differentiated in vitro into embryonic stem cell-derived cardiomyocytes (ESCMs). These ESCMs may then be directly injected into the damaged myocardium, which would facilitate the regeneration of the tissue. Indeed, multiple animal studies have shown this methodology to be promising. However, before these cells can be taken to clinical trials, several obstacles need to be overcome, including heterogeneity, which is a potential problem during ESC maintenance and differentiation. This review focuses on signaling pathways, such a Wnt/beta-catenin and bone morphogenic protein, (BMP), which influence cardiomyocyte specification from ESCs. By modifying signaling pathways in a temporal manner, we may be able to promote ESCM differentiation and reduce heterogeneity. Furthermore, we have recently found that by modifying the fibroblast growth factor receptor (FGFR)-Grb2-Ras-Mek-Erk pathway, we can effectively reduce the heterogeneity found during normal ESC maintenance. Such approaches will be beneficial in promoting the possibility of using ESCMs in transplantation therapies.

Time-dependence of cardiomyocyte differentiation disturbed by peroxisome proliferator-activated receptor alpha inhibitor GW6471 in murine embryonic stem cells in vitro

AIM

To investigate the possible roles of peroxisome proliferator-activated receptor alpha(PPAR alpha) and the signal pathway regulating the transcription of PPAR alpha in the cardiomyocyte differentiation course of murine embryonic stem (ES) cells in vitro.

METHODS

The expression of PPAR alpha during cardiomyocyte differentiation was analyzed using both Western blotting and immunofluorescence. Cardiac specific genes and sarcomeric proteins were evaluated when embryoid bodies were challenged with PPAR alpha specific inhibitor GW6471 at different time courses. The phosphorylation of p38 mitogen-activated protein kinase (MAPK) was studied in the differentiation process, and its specific inhibitor SB203580 was employed to study the function of p38 MAPK on cardiac differentiation and the expression of PPAR alpha.

RESULTS

The expression of PPAR alpha was observed to be at a low level in undifferentiated ES cells and markedly induced with the appearance of beating clusters. The inhibition of PPAR alpha by its specific inhibitor GW6471 (1X10(-5) mol/L) significantly prevented cardiomyocyte differentiation and resulted in the reduced expression of cardiac sarcomeric proteins (ie alpha-actinin, troponin-T) and specific genes (ie alpha-MHC, MLC2v) in a time-dependent manner. In the differentiation course, p-p38 MAPK was maintained at a high level from d 3 followed by a decrease from d 10. The inhibition of the p38 MAPK pathway by SB203580 between d 3 and d 7 efficiently prevented cardiomyocyte differentiation and resulted in the capture of the upregulation of PPAR alpha.

CONCLUSION

Taken together, these results showed a close association between PPAR alpha and cardiomyocyte differentiation in vitro, and p38 MAPK was partly responsible for the regulation of PPAR alpha.

Differential expression of nitric oxide signaling components in undifferentiated and differentiated human embryonic stem cells

Nitric oxide (NO) is an uncharged free-radical gas that is involved in a number of physiological and pathological events. We have examined the expression of various subunits of soluble guanylyl cyclase (sGC alpha (1), alpha (2), beta (1), beta (2)), nitric oxide synthase (s) (NOS-1, -2, -3), MLC2 (cardiac marker) and a cardiac-specific transcription factor (Nkx2.5) in human embryonic stem (hES) cells (H-9 cells) and differentiated cells subjected to differentiation in cell suspension using embryoid body (EB) formation. Our results demonstrate a time-dependent increase in the expression of sGC alpha (1) and beta (1) at the mRNA and protein levels in differentiated cells compared to undifferentiated H-9 cells as examined by real-time PCR and western blot analysis. mRNA for sGC alpha (2) also showed a time-dependent increase compared to undifferentiated cells. In contrast, there was a time-dependent decrease in sGC beta (2) mRNA expression in differentiated cells compared to undifferentiated H-9 cells. In contrast to undifferentiated H-9 cells, the maximum mRNA expression of cardiac marker MLC2 and the cardiac-specific transcription factor Nkx2.5 was observed at day 14 of the differentiated H-9 cells. The protein levels of MLC2 were stable up to day 25 compared to mRNA levels, which showed a sharp decline after day 15. Using immunofluorescence, we also demonstrate positive staining of cardiac markers such as troponin I, alpha-actinin, atrial natriuretic peptide, and SGC alpha (1) at days 8-37 post-differentiation. These results clearly demonstrate the role of NO signaling components in differentiation events or physiological processes of human ES or ES cell-derived cardiomyocytes.

Trichostatin A induces myocardial differentiation of monkey ES cells

Human embryonic stem (ES) cell lines are one of the possible sources of cardiac myocytes to be transplanted in patients with end-staged heart failure. However, prior to the application of human of ES cells for heart failure therapy, it is critical to validate their clinical use in large animals such as primates. Cynomolgus monkey ES cells have similar properties to human ES cells and can be used for primate studies. We demonstrate that 24-h stimulation by a histone deacetylase inhibitor, trichostatin A (TSA) facilitated myocardial differentiation of monkey ES cells with embryonic bodies that were seeded on gelatin-coated dishes. TSA-induced acetylating of histone-3/4 and expression of p300, one of the intrinsic histone acetyltransferases. Thus, such induction as well as inhibition of histone deacetylase may be involved in TSA-induced differentiation of cynomolgus monkey ES cells into cardiomyocytes.

Arginine vasopressin-mediated cardiac differentiation: insights into the role of its receptors and nitric oxide signaling

Despite the existence of a functional arginine vasopressin (AVP) system in the adult heart and evidence that AVP induces myogenesis, its significance in cardiomyogenesis is currently unknown. In the present study, we hypothesized a role for AVP in cardiac differentiation of D3 and lineage-specific embryonic stem (ES) cells expressing green fluorescent protein under the control of atrial natriuretic peptide (Anp) or myosin light chain-2V (Mlc-2V) promoters. Furthermore, we investigated the nitric oxide (NO) involvement in AVP-mediated pathways. AVP exposure increased the number of beating embryoid bodies, fluorescent cells, and expression of Gata-4 and other cardiac genes. V1a and V2 receptors (V1aR and V2R) differentially mediated these effects in transgenic ES cells, and exhibited a distinct developmentally regulated mRNA expression pattern. A NO synthase inhibitor, L-NAME, powerfully antagonized the AVP-induced effects on cardiogenic differentiation, implicating NO signaling in AVP-mediated pathways. Indeed, AVP elevated the mRNA and protein levels of endothelial NO synthase (eNOS) through V2R stimulation. Remarkably, increased beating activity was found in AVP-treated ES cells with down-regulated eNOS expression, indicating the significant involvement of additional pathways in cardiomyogenic effects of AVP. Finally, patch clamp recordings revealed specific AVP-induced changes of action potentials and increased L-type Ca2+ (ICa,L) current densities in differentiated ventricular phenotypes. Thus, AVP promotes cardiomyocyte differentiation of ES cells and involves Gata-4 and NO signaling. AVP-induced action potential prolongation appears likely to be linked to the increased ICa,L current in ventricular cells. In conclusion, this report provides new evidence for the essential role of the AVP system in ES cell-derived cardiomyogenesis.

Chibby, an antagonist of the Wnt/beta-catenin pathway, facilitates cardiomyocyte differentiation of murine embryonic stem cells

BACKGROUND

Embryonic stem cell (ESC)-derived cardiomyocytes are anticipated to serve as a useful source for future cell-based cardiovascular disease therapies. Research emphasis is currently focused on determining methods to direct the differentiation of ESCs to a large population of cardiomyocytes with high purity. To this aim, understanding the molecular mechanisms that control ESC-to-cardiomyocyte differentiation should play a critical role in the development of this methodology. The Wnt/beta-catenin signaling pathway has been implicated in both embryonic cardiac development and in vitro ESC differentiation into cardiomyocytes. Chibby is a recently identified nuclear protein that directly binds to beta-catenin and antagonizes its transcriptional activity.

METHODS AND RESULTS

Chibby was ubiquitously expressed in early stages of ESC differentiation but upregulated during cardiomyocyte specification. Of interest, the Chibby gene promoter has multiple binding sites for the cardiac-specific homeodomain protein Nkx2.5, and its promoter activity was indeed positively regulated by Nkx2.5. Furthermore, overexpression of Chibby increased cardiac differentiation of ESCs, whereas loss of Chibby by RNAi impaired cardiomyocyte differentiation.

CONCLUSIONS

These data illustrate the regulation and function of Chibby in facilitating cardiomyocyte differentiation from ESCs. By revealing molecular mechanisms that control ESC-to-cardiomyocyte differentiation, this study will allow for the future development of technologies to improve cardiomyocyte differentiation from ESCs.

Molecular and pharmacological properties of human embryonic stem cell-derived cardiomyocytes

Human embryonic stem cells (hESCs) can be coaxed to differentiate into specific cell types, including cardiomyocyte-like cells. These cells express cardiac-specific markers and display functional similarities to their adult counterparts. Based on these properties, hESC-derived cardiomyocytes have the potential to be extremely useful in various in vitro applications and to provide the opportunity for cardiac cell replacement therapies. However, before this can become a reality, the molecular and functional characteristics of these cells need to be investigated in more detail. In the present study we differentiate hESCs into cardiomyocyte-like cells via embryoid bodies (EBs). The fraction of spontaneously beating clusters obtained from the EBs averaged approximately 30% of the total number of EBs used. These cell clusters were isolated, dissociated into single-cell suspensions, and frozen for long-term storage. The cryopreserved cells could be successfully thawed and subcultured. Using electron microscopy, we observed Z discs and tight junctions in the hESC-derived cardiomyocytes, and by immunohistochemical analysis we detected expression of cardiac-specific markers (cTnI and cMHC). Notably, using BrdU labeling we also could demonstrate that some of the hESC-derived cardiomyocytes retain a proliferative capacity. Furthermore, pharmacological stimulation of the cells resulted in responses indicative of functional adrenergic and muscarinic receptor coupling systems. Taken together, these results lend support to the notion that hESCs can be used as a source for the procurement of cardiomyocytes for in vitro and in vivo applications.

Clinical hurdles for the transplantation of cardiomyocytes derived from human embryonic stem cells: role of molecular imaging

Over the past few years, human embryonic stem cells (hESCs) have gained popularity as a potentially ideal cell candidate for tissue regeneration. In particular, hESCs are capable of cardiac lineage-specific differentiation and confer improvement of cardiac function following transplantation into animal models. Although such data are encouraging, there remain significant hurdles before safe and successful translation of hESC-based treatment into clinical therapy, including the ability to assess cells following transplant. To this end, molecular imaging has proven a reliable methodology for tracking the long-term fate of transplanted cells. Imaging reporter genes that are introduced into the cells before transplantation enable non-invasive and longitudinal studies of cell viability, location and behaviour in vivo. Therefore, molecular imaging is expected to play an increasing role in characterizing the biology and physiology of hESC-derived cardiac cells in living subjects.

Improved cardiac function in infarcted mice after treatment with pluripotent embryonic stem cells

Because pluripotent embryonic stem cells (ESCs) are able to differentiate into any tissue, they are attractive agents for tissue regeneration. Although improvement of cardiac function has been observed after transplantation of pluripotent ESCs, the extent to which these effects reflect ESC-mediated remuscularization, revascularization, or paracrine mechanisms is unknown. Moreover, because ESCs may generate teratomas, the ability to predict the outcome of cellular differentiation, especially when transplanting pluripotent ESCs, is essential; conversely, a requirement to use predifferentiated ESCs would limit their application to highly characterized subsets that are available in limited numbers. In the experiments reported here, we transplanted low numbers of two murine ESC lines, respectively engineered to express a beta-galactosidase gene from either a constitutive (elongation factor) or a cardiac-specific (alpha-myosin heavy chain) promoter, into infarcted mouse myocardium. Although ESC-derived tumors formed within the pericardial space in 21% of injected hearts, lacZ histochemistry revealed that engraftment of ESC was restricted to the ischemic myocardium. Echocardiographic monitoring of ESC-injected hearts that did not form tumors revealed functional improvements by 4 weeks postinfarction, including significant increases in ejection fraction, circumferential fiber shortening velocity, and peak mitral blood flow velocity. These experiments indicate that the infarcted myocardial environment can support engraftment and cardiomyogenic differentiation of pluripotent ESCs, concomitant with partial functional recovery.

Nitric oxide signaling in oxytocin-mediated cardiomyogenesis

Oxytocin (OT), a hormone recently identified in the heart, induces embryonic and cardiac somatic stem cells to differentiate into cardiomyocytes (CM), possibly through nitric oxide (NO). We verified this hypothesis using P19 cells and P19 Clone 6 derivatives expressing a green fluorescent protein (GFP) reporter linked to cardiac myosin light chain-2v promoter. OT treatment of these cells induced beating cell colonies that were fully inhibited by N,G-nitro-L-arginine-methyl-ester (L-NAME), an inhibitor of NO synthases (NOS), partially reduced by 1400W, an inhibitor of inducible NOS, and ODQ, an inhibitor of NO-sensitive guanylyl cyclases. The NO generator S-nitroso-N-acetylpenicillamine (SNAP) reversed the L-NAME inhibition of cell beating and GFP expression. In OT-induced cells, L-NAME significantly decreased transcripts of the cardiac markers Nkx2.5, MEF2c, alpha-myosin heavy chain, and less, GATA4, endothelial NOS, and atrial natriuretic peptide, as well as the skeletal myocyte (SM) marker myogenin. Image analysis of OT-induced P19Cl6-GFP cells revealed ventricular CM coexpressing sarcomeric alpha-actinin and GFP, with some cells exclusively expressing alpha-actinin, most likely of the SM phenotype. The OT-mediated production of CM, but not SM, was diminished by L-NAME. In P19 cells, exogenously added OT stimulated the expression of its own transcript, which was reduced in the presence of L-NAME. Surprisingly, L-NAME alone decreased the expression of anti-stage specific embryonic antigen-1 marker of the undifferentiated state and induced some beating colonies as well as GFP in P19Cl6-GFP cells. Collectively, our data suggest that the pleiotropic action of NO is involved in the initiation of CM differentiation of P19 cells and maintenance of their undifferentiated state.

NO-cGMP signaling and regenerative medicine involving stem cells

Nitric oxide (NO) is a short lived diatomic free radical species synthesized by nitric oxide synthases (NOS). The physiological roles of NO depend on its local concentrations as well as availability and the nature of downstream target molecules. At low nanomolar concentrations, activation of soluble guanylyl cyclase (sGC) is the major event initiated by NO. The resulting elevation in the intracellular cyclic GMP (cGMP) levels serves as signals for regulating diverse cellular and physiological processes. The participation of NO and cGMP in diverse physiological processes is made possible through cell type specific spatio-temporal regulation of NO and cGMP synthesis and signal diversity downstream of cGMP achieved through specific target selection. Thus cyclic GMP directly regulates the activities of its downstream effectors such as Protein Kinase G (PKG), Cyclic Nucleotide Gated channels (CNG) and Cyclic nucleotide phosphodiesterases, which in turn regulate the activities of a number of proteins that are involved in regulating diverse cellular and physiological processes. Localization and activity of the NO-cGMP signaling pathway components are regulated by G-protein coupled receptors, receptor and non receptor tyrosine kinases, phosphatases and other signaling molecules. NO also serves as a powerful paracrine factor. At micromolar concentrations, NO reacts with superoxide anion to form reactive peroxinitrite, thereby leading to the oxidation of important cellular proteins. Extensive research efforts over the past two decades have shown that NO is an important modulator of axon outgrowth and guidance, synaptic plasticity, neural precursor proliferation as well as neuronal survival. Excessive NO production as that evoked by inflammatory signals has been identified as one of the major causative reasons for the pathogenesis of a number of neurodegenerative diseases such as ALS, Alzheimers and Parkinson diseases. Regenerative therapies involving transplantation of embryonic stem cells (ES cells) and ES cell derived lineage committed neural precursor cells have recently shown promising results in animal models of Parkinson disease (PD). Recent studies from our laboratory have shown that a functional NO-cGMP signaling system is operative early during the differentiation of embryonic stem cells. The cell type specific, spatio-temporally regulated NO-cGMP signaling pathways are well suited for inductive signals to use them for important cell fate decision making and lineage commitment processes. We believe that manipulating the NO-cGMP signaling system will be an important tool for large scale generation of lineage committed precursor cells to be used for regenerative therapies.

Myocardial regeneration by embryonic stem cell transplantation: present and future trends

Embryonic stem cells are a promising source for myocardial regeneration due to their pluripotency and plasticity. In theory, embryonic stem cells are capable of self-renewal in an unlimited fashion, and can differentiate into any cell type required for cell-based therapy, including cardiac myocytes. In recent years, embryonic stem cells have been transplanted for cardiac regeneration in animal models, and the results are encouraging. However, there are still many hurdles to be overcome for the clinical application of embryonic stem cells.

Lack of endothelial nitric oxide synthase decreases cardiomyocyte proliferation and delays cardiac maturation

We recently demonstrated that deficiency in endothelial nitric oxide synthase (eNOS) results in congenital septal defects and postnatal heart failure. The aim of this study was to investigate the role of eNOS in cardiomyocyte proliferation and maturation during postnatal development. Cultured eNOS knockout (eNOS(-/-)) cardiomyocytes displayed fewer cells and lower bromodeoxyuridine (BrdU) incorporation in vitro compared with wild-type (WT) cardiomyocytes (P < 0.05). Treatment with the nitric oxide (NO) donor diethylenetriamine NONOate increased BrdU incorporation and cell counts in eNOS(-/-) cardiomyocytes (P < 0.05). Inhibition of nitric oxide synthase activity using N(G)-nitro-L-arginine methyl ester decreased the level of BrdU incorporation and cell counts in WT cardiomyocytes (P < 0.05). Vascular endothelial growth factor (VEGF) increased the level of BrdU incorporation in cultured WT cardiomyocytes in a dose- and time-dependent manner (P < 0.05). Conversely, VEGF did not alter BrdU incorporation in eNOS(-/-) cardiomyocytes (P = not significant). Furthermore, deficiency in eNOS significantly decreased BrdU labeling indexes in neonatal hearts in vivo. Although WT hearts displayed a rapid decrease in atrial natriuretic peptide (ANP) expression in the first week of neonatal life, ANP expression in eNOS(-/-) hearts remain elevated. Our study demonstrated that NO production from eNOS is necessary for postnatal cardiomyocyte proliferation and maturation, suggesting that eNOS plays an important role during postnatal heart development.

The NADPH oxidase NOX4 drives cardiac differentiation: Role in regulating cardiac transcription factors and MAP kinase activation

Reactive oxygen species (ROS) generated by the NOX family of NADPH oxidases have been described to act as second messengers regulating cell growth and differentiation. However, such a function has hitherto not been convincingly demonstrated. We investigated the role of NOX-derived ROS in cardiac differentiation using mouse embryonic stem cells. ROS scavengers prevented the appearance of spontaneously beating cardiac cells within embryoid bodies. Down-regulation of NOX4, the major NOX isoform present during early stages of differentiation, suppressed cardiogenesis. This was rescued by a pulse of low concentrations of hydrogen peroxide 4 d before spontaneous beating appears. Mechanisms of ROS-dependent signaling included p38 mitogen-activated protein kinase (MAPK) activation and nuclear translocation of the cardiac transcription factor myocyte enhancer factor 2C (MEF2C). Our results provide first molecular evidence that the NOX family of NADPH oxidases regulate vertebrate developmental processes.

Stem cells as a source of regenerative cardiomyocytes

The realization of regenerative cardiac medicine depends on the availability of cardiomyocytes in sufficient numbers for transplantation of cardiac tissue and the accompanying blood vessels. Embryonic stem (ES) cells, bone marrow (BM) stem cells, and tissue-derived stem cells are all potential cell sources. Although ES cells are highly proliferative and suitable for mass production, an efficient protocol is yet to be established to ensure selective cardiomyocyte induction using these cells. Recent advances in developmental biology have clarified the involvement of critical factors in cardiomyocyte differentiation, including bone morphogenic protein and Wnt signaling proteins, and such factors have the potential to improve the efficiency of stem cell induction. Initial studies of the intracoronary administration of BM mononuclear cells after myocardial infarction has yielded promising results; however, intensive investigation of the underlying molecular mechanisms at play as well as double-blinded clinical trials will be necessary to establish the extent of both migration of the BM stem cells into the damaged cardiac tissue and their differentiation into cardiomyocytes. Several types of cardiac tissue stem cells have also been reported, but an accurate and extensive comparison of these cells with regard to their characteristics and multipotency remains to be done. An integrative study involving developmental biology, stem cell biology, and tissue engineering is required to achieve the full potential of cardiac regeneration.

NO-cGMP signaling in development and stem cells

Despite the recognition that the NO-cGMP signaling pathway is involved in so many physiological and pathological events, a clear understanding of many of the functions of this signaling pathway remains elusive. Because of its pleiotropic and often transient actions, its modulation for therapeutic purposes in multiple pathological states is a complex issue. Recent work that combines the areas of developmental and stem cell biology and NO-cGMP signaling in various models may help us elucidate some of these functions and even discover novel actions for this signaling paradigm. This review will discuss some of the recent work in these areas, with additional focus on the nitric oxide receptor, soluble guanylyl cyclase.

Icariin-mediated expression of cardiac genes and modulation of nitric oxide signaling pathway during differentiation of mouse embryonic stem cells into cardiomyocytes in vitro

AIM

To investigate effects of icariin on cardiac gene expression and the modulation of nitric oxide (NO) signal transduction during the differentiation of embryonic stem (ES) cells into cardiomyocytes in vitro.

METHODS

The expression levels of cardiac developmental-dependent genes were measured using reverse transcription-polymerase chain reaction (RT-PCR). The chronotropic responses of cardiomyocytes to b-adrenoceptor stimulation were determined. The levels of cAMP and cGMP in ES cells were measured using radioimmunoassay. Endogenous NO levels were measured by using the Griess reaction. Aminoguanidine (AG) was used to confirm the influence of icariin on the endogenous NO signal pathway.

RESULTS

Icariin significantly elevated mRNA levels of cardiac transcription factors GATA4 and Nkx2.5, and cardiac-specific alpha-MHC, MLC-2v and beta-AR genes in a concentration- and time-dependent manner (P<0.05). Cardiomyocytes derived from embryoid body (EB) treated with icariin were more sensitive to isoprenaline (P<0.01). Treatment of ES cells with icariin resulted in a continued elevation in the cAMP/cGMP ratio before a shift to the cardiomyocyte phenotype (P<0.05). AG decreased the NO level, and delayed and decreased the incidence of contracting EB to only approximately 35% on d 5+11, an effect that could be rescued by icariin. When cells were cocultured with icariin and AG, the percentage of beating EB reached a peak level of 73% on d 5+11 (P<0.05).

CONCLUSION

The inducible effects of icariin were partly related to increase in the expression of cardiac developmental-dependent genes, and elevation of the cAMP/cGMP ratio in ES cells, as well as upregulation of endogenous NO generation during the early stages of cardiac development.

Strategies for Developing Therapeutic Application of Human Embryonic Stem Cells

The ongoing debate on human embryonic stem cells (hESC) is fuelled by ethical concerns but also by the legitimate hope that hESC could one day be used for the cure of presently untreatable human diseases. Here we discuss current approaches to and constraints upon hESC differentiation and describe their potential application in clinical medicine.

Differentiation Pathways in Human Embryonic Stem Cell-Derived Cardiomyocytes

Human embryonic stem (hES) cells are pluripotent cell lines derived from the inner cell mass of the blastocyst-stage embryo. These unique cell lines can be propagated in the undifferentiated state in culture, while retaining the capacity to differentiate into derivatives of all three germ layers, including cardiomyocytes. The derivation of the hES cell lines presents a powerful tool to explore the early events of cardiac progenitor cell specification and differentiation, and it also provides a novel cell source for the emerging field of cardiovascular regenerative medicine. A spontaneous differentiation system of these stem cells to cardiomyocytes was established and the generated myocytes displayed molecular, structural, and functional properties of early-stage heart cells. In order to follow the in vitro differentiation process, the temporal expression of signaling molecules and transcription factors governing early cardiac differentiation was examined throughout the process. A characteristic pattern was noted recapitulating the normal in vivo cardiac differentiation scheme observed in other model systems. This review discusses the known pathways involved in cardiac specification and the possible factors that may be used to enhance cardiac differentiation of hES cells, as well as the steps required to fully harness the enormous potential of these unique cells.

Embryonic and adult stem cell-derived cardiomyocytes: lessons from in vitro models

For years, research has focused on how to treat heart failure by sustaining the overloaded remaining cardiomyocytes. Recently, the concept of cell replacement therapy as a treatment of heart diseases has opened a new area of investigation. In vitro-generated cardiomyocytes could be injected into the heart to rescue the function of a damaged myocardium. Embryonic and/or adult stem cells could provide cardiac cells for this purpose. Knowledge of fundamental cardiac differentiation mechanisms unraveled by studies on animal models has been improved using in vitro models of cardiogenesis such as mouse embryonal carcinoma cells, mouse embryonic stem cells and, recently, human embryonic stem cells. On the other hand, studies suggesting the existence of cardiac stem cells and the potential of adult stem cells from bone marrow or skeletal muscle to differentiate toward unexpected phenotypes raise hope and questions about their potential use for cardiac cell therapy. In this review, we compare the specificities of embryonic vs adult stem cell populations regarding their cardiac differentiation potential, and we give an overview of what in vitro models have taught us about cardiogenesis.

Prostanoid pattern and iNOS expression during chondrogenic differentiation of human mesenchymal stem cells

Availability of human chondrocytes is a major limiting factor regarding drug discovery projects and tissue replacement therapies. As an alternative human mesenchymal stem cells (hMSCs) from bone marrow are taken into consideration as they can differentiate along the chondrogenic lineage. However, it remains to be shown whether they could form a valid model for primary chondrocytes with regards to inflammatory mediator production, like nitric oxide (NO) and prostanoids. We therefore investigated the production of NO and prostanoids in hMSCs over the course of chondrogenic differentiation and in response to IL-1beta using primary OA chondrocytes as reference. Chondrogenic differentiation was monitored over 28 days using collagen I, collagen II, and collagen X expression levels. Expression levels of inducible nitric oxide synthase (iNOS), levels of NO, and prostanoids were assessed using PCR, Griess assay, and GC/MS/MS, respectively. The hMSCs collagen expression profile during course of differentiation was consistent with a chondrocytic phenotype. Contrary to undifferentiated cells, differentiated hMSCs expressed iNOS and produced NO following stimulation with IL-1beta. Moreover, this induction of iNOS expression was corticosteroid insensitive. The spectrum of prostanoid production in differentiated hMSCs showed similarities to that of OA chondrocytes, with PGE2 as predominant product. We provide the first detailed characterization of NO and prostanoid production in hMSCs in the course of chondrogenic differentiation. Our results suggest that differentiated hMSCs form a valid model for chondrocytes concerning inflammatory mediator production. Furthermore, we propose that IL-1beta stimulation, leading to corticosteroid-insensitive NO synthesis, can be used as a sensitive marker of chondrogenesis.

Cardiac repair by embryonic stem-derived cells

Cell transplantation approaches offer the potential to promote regenerative growth of diseased hearts. It is well established that donor cardiomyocytes stably engraft into recipient hearts when injected directly into the myocardial wall. Moreover, the transplanted donor cardiomyocytes participate in a functional syncytium with the host myocardium. Thus, transplantation of donor cardiomyocytes resulted in at least partial restoration of lost muscle mass. It is also well established that embryonic stem (ES) cells differentiate into cells of ecto-, endo-, and mesodermal lineages when cultured under appropriate conditions in vitro. Robust cardiomyogenic differentiation was frequently observed in spontaneously differentiating ES cultures. Cellular, molecular and physiologic analyses indicated that ES-derived cells were bona fide cardiomyocytes, with in vitro characteristics typical for cells obtained from early stages of cardiac development. Thus, ES-derived cardiomyocytes constitute a viable source of donor cells for cell transplantation therapies.

Reactive oxygen species as signaling molecules in cardiovascular differentiation of embryonic stem cells and tumor-induced angiogenesis

Besides the well known pathophysiological impact of oxidative stress in cardiovascular disease, reactive oxygen species (ROS) generated at low concentrations exert a role as signaling molecules that are involved in signal transduction cascades of numerous growth factor-, cytokine-, and hormone-mediated pathways, and regulate biological effects such as apoptosis, cell proliferation, and differentiation. Embryonic stem cells have the capacity to differentiate into the cardiovascular cell lineage. Furthermore, upon confrontation culture with tumor tissue, they form blood vessel-like structures that induce tumor-induced angiogenesis within tumor tissues. The role of ROS in cardiovascular differentiation of embryonic stem cells appears to be antagonistic. Whereas continuous exposure to ROS results in inhibition of cardiomyogenesis and vasculogenesis, pulse chase exposure to low-level ROS enhances differentiation toward the cardiomyogenic as well as vascular cell lineage. This review summarizes the current knowledge of ROS-induced cardiovascular differentiation of embryonic stem cells as well as the role of ROS in tumor-induced angiogenesis.

Differential expression of genes involved in cGMP-dependent nitric oxide signaling in murine embryonic stem (ES) cells and ES cell-derived cardiomyocytes

Nitric oxide (NO) performs multiple physiological roles as a biological signaling molecule. The role of NO and cGMP signaling in embryonic stem (ES) cell-derived cardiomyocytes (CM) has been investigated but many questions remain. In this study, we examined the expression of the NO signaling pathway components nitric oxide synthase (NOS-1, 2, 3), soluble guanylyl cyclase (sGCalpha(1) and beta(1)) and protein kinase G (PKG) genes and sGC activity in murine ES cells subjected to differentiation by embryoid body (EB) formation. We found that in undifferentiated ES cells, NOS-1, NOS-3, and sGCbeta(1) were detected while NOS-2, sGCalpha(1), and PKG were very low or undetectable. When ES cells were subjected to differentiation, NOS-1 abruptly decreased within one day, NOS-2 mRNA became detectable after several days, and NOS-3 increased after 7-10 days. Levels of sGCalpha(1), sGCbeta(1), and PKG all increased gradually over a several day time course of differentiation in EB outgrowths. Analysis of sGC activity in cell lysates derived from undifferentiated ES cells revealed that NO could not stimulate cGMP. However, lysates from differentiated EB outgrowths produced abundant cGMP levels after NO stimulation. Purification of ES-cell derived CM revealed that mRNA expression of all the NOS isoforms was very low to absent while sGCalpha(1) and beta(1) subunit mRNAs were abundant and sGC-mediated cGMP production was apparent in this population of cells. These data suggest that cGMP-mediated NO signaling may play a minor role, if any, in undifferentiated ES cells but could be involved in the early differentiation events or physiological processes of ES cells or ES cell-derived lineages.

Regenerating the heart

Cell-based cardiac repair offers the promise of rebuilding the injured heart from its component parts. Work began with committed cells such as skeletal myoblasts, but recently the field has expanded to explore an array of cell types, including bone marrow cells, endothelial progenitors, mesenchymal stem cells, resident cardiac stem cells, and both mouse and human embryonic stem cells. A related strategy for cardiac repair involves cell mobilization with factors such as cytokines. Translation of cell-based approaches to the clinic has progressed rapidly, and clinical trials using autologous skeletal myoblasts and bone marrow cells are under way. Many challenges remain before the vision of healing an infarct by muscle regeneration can be realized. Future research is likely to focus on improving our ability to guide the differentiation of stem cells, control their survival and proliferation, identify factors that mediate their homing and modulate the heart's innate inflammatory and fibrotic responses.

Focal adhesion kinase signaling regulates cardiogenesis of embryonic stem cells

The signaling steps that induce cardiac differentiation in embryonic stem (ES) cells are incompletely understood. We examined the effect of adhesion signaling including Src and focal adhesion kinase (FAK) on cardiogenesis in mouse ES cells using alpha-myosin heavy chain promoter-driven enhanced green fluorescent protein or luciferase as reporters. Cardiac transcription factors including Nkx2.5 and Tbx5 mRNA were first expressed at day 4 in hanging drop embryoid bodies, and adhesion of embryoid bodies to surfaces at or before that day strongly inhibited differentiation of ES cells to cardiomyocytes. Since adhesion signaling could suppress cardiogenesis through Src kinases, embryoid bodies were exposed to the small molecule PP2, known as a Src family kinase inhibitor. PP2 during embryoid body adhesion dramatically increased cardiomyocyte differentiation and decreased mRNA expression of neuronal cellular adhesion molecule and alpha-fetoprotein, neuroectodermal, and endodermal markers, respectively. Surprisingly, although there was an interaction between Src and FAK in cardiogenesis, the procardiogenic effect of PP2 appeared incompletely explained by Src kinase inhibition, since another Src family kinase inhibitor, SU6656, failed to induce cardiogenesis. Instead, PP2 specifically inhibited adhesion-induced FAK phosphorylation. In ES cells stably expressing FAK-related nonkinase, which functions as a dominant negative FAK, cell migration from embryoid bodies was inhibited, whereas alpha-myosin heavy chain expression and myosin-stained cardiomyocytes were increased, suggesting that reducing cell motility may contribute to cardiogenesis. These data indicate that FAK is a key regulator of cardiogenesis in mouse ES cells and that FAK signaling within embryoid bodies can direct stem cell lineage commitment.

Sequential development of hematopoietic and cardiac mesoderm during embryonic stem cell differentiation

The ability to generate a wide spectrum of differentiated cell types from ES cells in culture offers a powerful approach for studying lineage induction and specification and a promising source of progenitors for cell replacement therapy. Although significant efforts are being made to optimize culture conditions for the generation of different cell populations from ES cells, the identification and efficient isolation of specific progenitors for many lineages within these cultures remains a major challenge. By specifically tracking hematopoietic and cardiac development, we demonstrate here that these two lineages arise from distinct mesoderm subpopulations that develop in sequential waves from pre-mesoderm cells. Access to these populations provides a unique approach to isolate and characterize the earliest progenitors of these lineages.

Enhancement by growth factors of cardiac myocyte differentiation from embryonic stem cells: A promising foundation for cardiac regeneration

Cell transplantation is a promising, still novel, potentially therapeutic approach for the treatment of heart diseases. Clinical applications require generation of large number of donor cells. Embryonic stem (ES) cells are capable of self-renewal apparently in an unlimited fashion, in vitro. Theoretically, they can differentiate into any cell type required for cell transplantation, including cardiac myocytes. Diverse growth factors have been implicated in programming diverse cellular processes, including development of the embryonic heart, ES cell self-renewal, and cardiac myocyte differentiation from ES cells. This review addresses the current understanding of the role of growth factors in the differentiation of cardiac myocytes from ES-embryoid body cell systems in vitro as well as cardiac regeneration in vivo.

Regulation of the mammalian heart function by nitric oxide

The mammalian heart expresses all three isoforms of nitric oxide synthases (NOS) in diverse cell types of the myocardium. Despite their apparent promiscuity, the NOS isoforms support specific signaling because of their subcellular compartmentation with colocalized effectors and limited diffusibility of NO in muscle cells. eNOS and nNOS sustain normal EC coupling and contribute to the early and late phases of the Frank-Starling mechanism of the heart. They also attenuate the beta1-/beta2-adrenergic increase in inotropy and chronotropy, and reinforce the pre- and post-synaptic vagal control of cardiac contraction. By doing so, the NOS protect the heart against excessive stimulation by catecholamines, just as an "endogenous beta-blocker". In the ischemic and failing myocardium, induced iNOS further reinforces this effect, as does eNOS coupled to overexpressed beta3-adrenoceptors. nNOS expression also increases in the aging and infarcted heart, but its role (compensatory or deleterious) is less clear. In addition to their direct regulation of contractility, the NOS modulate oxygen consumption, substrate utilization, sensitivity to apoptosis, hypertrophy and regenerative potential, all of which illustrate the pleiotropic effects of this radical on the cardiac cell biology.

Embryonic stem cell differentiation: emergence of a new era in biology and medicine

The discovery of mouse embryonic stem (ES) cells >20 years ago represented a major advance in biology and experimental medicine, as it enabled the routine manipulation of the mouse genome. Along with the capacity to induce genetic modifications, ES cells provided the basis for establishing an in vitro model of early mammalian development and represented a putative new source of differentiated cell types for cell replacement therapy. While ES cells have been used extensively for creating mouse mutants for more than a decade, their application as a model for developmental biology has been limited and their use in cell replacement therapy remains a goal for many in the field. Recent advances in our understanding of ES cell differentiation, detailed in this review, have provided new insights essential for establishing ES cell-based developmental models and for the generation of clinically relevant populations for cell therapy.

Cardiomyocytes derived from stem cells

One way to restore failing heart function following myocardial infarction would be to replace lost or damaged cardiac cells by local or systemic injection. The sources of replacement cells presently discussed include embryonic stem cells, hematopoietic and non-hematopoietic stem cells from bone marrow or cord blood and small stem cell populations thought to reside in the heart itself or in skeletal muscle. Here we review this area of stem cell research with focus particularly on recent laboratory advances towards producing cardiomyocytes from embryonic stem cells. We conclude that embryonic stem cells and cardiac progenitors in the heart itself are the only proven sources of cardiomyocytes and that reported clinical effects of bone marrow stem currently undergoing validation are likely mediated by other mechanisms.