Electrophysiological properties of embryonic stem cells during differentiation into cardiomyocyte-like cell types

The method described here to differentiate mouse embryonic stem (ES) cells into cardiomyocytes is adapted from Maltsev et al. and results in a high percentage of spontaneously beating cardiomyocyte-like cells. In order to determine to what extent the differentiating ES cells resemble true cardiomyocytes, the cells were electrophysiologically characterized during differentiation, using the whole-cell variant of the patch-clamp technique. Action potentials (APs) and membrane currents were recorded and analyzed off-line to determine electrophysiological changes during development.

[Stem cells: biology and possible application to myocardial infarct]

If the heart fails to recover sufficient functionality following an infarct, then heart failure develops, an important cause of death in the western world. One obvious therapy is to create more muscle tissue to supplement the damaged myocardium with new functional contractile cells, together with neovasculogenesis. Stem cells repair recipient tissue by differentiating into tissue-specific cells or by creating an environment that stimulates the process of repair by the body's own cells at the site. In animal studies the heart function stabilised following an injection of stem cells in the infarcted area. In 3 non-randomised trials in humans, bone marrow stem cells were injected via the infarcted artery or round the infarcted area; the results indicated an improved heart function. There is currently still insufficient fundamental knowledge about the behaviour of multipotent cells, about the effects of using them for treatment, and about their long-term risk for these cells to be employed in the treatment of patients with a heart infarct.

Cardiomyocytes purified from differentiated embryonic stem cells exhibit characteristics of early chamber myocardium

Mouse embryonic stem (ES) cells easily differentiate towards the cardiac lineage making them suitable as an in vitro model to study cardiogenesis and as a potential source of transplantable cells. In this study, we show by in situ hybridisation that about 30% of the volume of cultures of differentiating ES cells consists of cardiomyocytes. RT-PCR analyses showed that the transcription factors Nkx2.5, Gata4, Mef2c and Irx4 were expressed at levels in the same order of magnitude as the levels observed in embryonic, neonatal and adult hearts. Atrial natriuretic factor and Connexin 40, associated with chamber formation in vivo, are expressed at relatively low levels, similar to those observed at early heart development in vivo. To facilitate the isolation of ES cell-derived cardiomyocytes, a cell line was constructed by stable transfection of the aminoglycoside phosphotransferase cDNA driven by the cardiac-specific distant upstream part of the Na(+)/Ca(2+) exchanger promoter. To accomplish single-copy integration, the construct was inserted into the hypoxanthine phosphoribosyltransferase locus of HM1 ES cells by homologous recombination. Cardiac-specific resistance to G418-sulphate (neomycin) allowed isolation of a pure population of cardiomyocytes. Genetically selected and unselected cell populations were characterised electrophysiologically using patch clamp. To explore whether clusters of cells have a similar differentiation profile, action potentials (APs) were measured in aggregates of differentiating ES cells, using a new method based on the voltage-dependent fluorescent dye di-4-ANEPPS. Both whole-cell recordings using patch-clamp and optical measurements with di-4-ANEPPS of the AP showed that upstroke velocity increases and AP duration decreases with differentiation time, accompanied by a decrease in AP interval, suggesting the initiation of the developmental programme underlying the formation of chamber myocardium.

Development of heart muscle-cell diversity: a help or a hindrance for phenotyping embryonic stem cell-derived cardiomyocytes

Despite the advances in cardiovascular treatment, cardiac disease remains a major cause of morbidity in all industrialized countries. The extraordinary potential of (embryonic) stem cells for therapeutic purposes has revolutionized ideas about cardiac repair of diseased cardiac muscle to exciting stages. This, in turn, has challenged research on cardiac differentiation of stem cells. For instance, cultures of mouse embryonic stem cells quite easily differentiate into the cardiogenic lineage, as assessed by their potential to beat spontaneously. However, repair of impaired cardiac muscle by spontaneously beating cardiac muscle cells might impose severe risks upon a human patient. Therefore, it is of crucial importance to understand the mechanisms that underlie the development of the distinct cardiac muscle cell types of the adult mammalian heart. In this review we tried to relate cardiac morphogenesis to the development of unique molecular phenotypes of cardiomyocytes. This relationship will provide a framework to assess the significance of the molecular phenotypes that are observed in embryonic stem cell-derived cardiomyocytes (ESDCs). Although for the phenotyping of ESDCs a comparison should be made with the phenotypes of the developing heart, so far none of the currently available markers allow unequivocal assignment of subtypes.

Cardiomyocytes derived from embryonic stem cells resemble cardiomyocytes of the embryonic heart tube

OBJECTIVE

After formation of the linear heart tube a chamber-specific program of gene expression becomes active that underlies the formation of the chamber myocardium. To assess whether this program is recapitulated in in vitro differentiated embryonic stem cells, we performed qualitative and quantitative analyses of cardiogenesis in vivo and in vitro.

METHODS

Gene expression profiles were made by in situ hybridisation and real-time PCR and electrophysiological profiles by patch clamp analyses of cardiomyocytes derived from time series of differentiating HM1 mouse embryonic stem cells and from embryonic and adult mouse hearts.

RESULTS

In embryoid bodies the in situ patterns of expression of alpha-myosin heavy chain, myosin light chain 2a and sarcoendoplasmic reticulum calcium ATPase 2a were similar to that of the heart muscle-specific marker gene cardiac troponin I. Myosin light chain 2v was expressed in part of the cardiac troponin I-expressing area, indicating heterogeneity within the cardiac cell population. Atrial natriuretic factor expression, indicative of the chamber-type program, could only very occasionally be detected by in situ hybridisation. Quantitative reverse transcriptase PCR showed that all cardiac genes, most notably atrial natriuretic factor, were expressed at relatively low levels, similar to those in embryonic hearts at embryonic day 8.75-9. Analysis of the electrophysiological characteristics of embryonic stem cell-derived cardiomyocytes showed an increase of the upstroke velocity and a shorter duration of the action potential during prolonged differentiation in vitro. When embryonic mouse heart compartments of embryonic day 12.5 were used as a reference, the electrophysiological characteristics of a substantial part of the embryonic stem cell-derived cardiomyocytes were most reminiscent to those observed in the embryonic outflow tract.

CONCLUSION

Together, these data suggest that most cardiomyocytes acquired by differentiation of embryonic stem cells maintain a phenotype reminiscent of that of the cardiomyocytes of the primary heart tube, and hardly any myocytes develop a chamber myocardial phenotype.

TBX5 overexpression stimulates differentiation of chamber myocardium in P19C16 embryonic carcinoma cells

In vitro differentiation of pluripotent embryonic cells is becoming a model system to study factors and genes involved in early developmental processes including cardiogenesis. An additional application involves the development of donor cells for treatment of diseases among which cardiac infarction. For this purpose differentiated cells should meet the functional characteristics of chamber myocardium, a requirement not convincingly reached as yet. The T-box transcription factor Tbx5 has been demonstrated to be crucial for heart formation. Using stably transfected clones of the P19C16 embryonic carcinoma cell line, reported to differentiate efficiently into the cardiac lineage, we investigated whether Tbx5 is sufficient to enhance cardiogenesis and differentiation of chamber myocardium. TBX5-transfected clones started to beat earlier, however, a relation between transgenic TBX5 mRNA levels and the number of beating foci or levels of Serca2a mRNA, a myocardial marker, could not be observed. However, TBX5-transfected clones displayed significantly higher levels of atrial natriuretic factor (Anf) and Connexin (Cx)40 mRNAs, which are associated with the formation of chamber myocardium. This indicates that Tbx5 enhances cardiac maturation within this system rather than cardiogenesis.

Non-radioactive in situ detection of mRNA in ES cell-derived cardiomyocytes and in the developing heart

Non-radioactive in situ hybridisation is an excellent method to visualise mRNA molecules within their topographical context. Recently we have reported a new non-radioactive in situ hybridisation procedure on tissue sections that is essentially based on the whole mount in situ hybridisation procedure. This method is superior in spatial resolution and sensitivity compared to the radioactive in situ hybridisation procedure. Generally, low levels of gene expression, such as found with the developmental onset of gene expression and in differentiating embryonic stem cells, are difficult to detect by in situ hybridisation. Here an application of the protocol is presented which is based on tyramide signal amplification, which enables the detection of very low abundant mRNAs. The significance of this method is two-fold: (1) the molecular phenotype of embryonic stem cell-derived cardiomyocytes can be examined at the cellular level with high sensitivity, and (2) the number of cells that express the gene of interest can be assessed.

Sensitivity and accuracy of quantitative real-time polymerase chain reaction using SYBR green I depends on cDNA synthesis conditions

The recent development of real-time PCR has offered the opportunity of sensitive and accurate quantification of mRNA levels that is crucial in biomedical research. Although reverse transcription (RT)-PCR is at present the most sensitive method available, many low abundant mRNAs are, although detectable, often not quantifiable. Here we report an improved two-step real-time RT-PCR procedure using SYBR green I and the LightCycler that better permits accurate quantification of mRNAs. Omission of dithiothreitol from the cDNA synthesis reaction was found to be crucial. This resulted in a lower cycle number at which the cDNA level is determined (C(T) value), steeper amplification curves, and removal of background fluorescence in the subsequent PCR. In addition, the choice of the cDNA priming oligo can improve detection sensitivity even further. In contrast to hexamer primer usage, both gene-specific and oligo-dT(VN) priming were very efficient and accurate, with gene-specific priming being the most sensitive. Finally, accurate quantification of mRNAs by real-time PCR using SYBR green I requires verification of the specificity of PCR by both melting curve and gel analysis.