Derivation of pre-X inactivation human embryonic stem cells under physiological oxygen concentrations

Molecular analyses of human induced pluripotent stem cells and embryonic stem cells

Recent work from our group and others has argued that human induced pluripotent stem cells (hiPSCs) generated by the introduction of four viruses bearing reprogramming factors differ from human embryonic stem cells (hESCs) at the level of gene expression (Chin et al., 2009). Many of the differences seen were common across independent labs and, at least to some extent, are thought to be a result of residual expression of donor cell-specific genes (Chin et al., 2009; Ghosh et al., 2010; Marchetto et al., 2009). Two new reports reanalyze similar expression data sets as those used in Chin et al. (2009) and come to different conclusions (Newman and Cooper, 2010; Guenther et al., 2010). We compare various approaches to perform gene expression meta-analysis that all support our original conclusions and present new data to demonstrate that polycistronic delivery of the reprogramming factors and extended culture brings hiPSCs transcriptionally closer to hESCs.

iPS cells: a source of cardiac regeneration

For the treatment of heart failure, a new strategy to improve cardiac function and inhibit cardiac remodeling needs to be established. Embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) are pluripotent cells that can differentiate into cell types from all three germ layers both in vitro and in vivo. The therapeutic effect of ES/iPS cell-derived progeny was reported in animal model. Mouse and human somatic cells can be reprogrammed to induced pluripotent stem cells (iPSCs) by the transduction of four transcription factors, Oct 3/4, Sox2, Klf4, and c-Myc. However, the low induction efficiency hinders the clinical application of iPS technology, and efforts have been made to improve the reprogramming efficiency. There are variations in the characteristics in ES/iPS cell lines, and the further understanding is necessary for the applications of ES/iPS cell technology. Some improvements were also made in the methods to induce cardiomyocytes from ES/iPS cells efficiently. This review article is focused on generation of iPS cells, cardiomyocyte differentiation from ES/iPS cells, and transplantation of derived cardiomyocytes.This article is part of a special issue entitled, "Cardiovascular Stem Cells Revisited".

Molecular mechanisms of pluripotency and reprogramming

Pluripotent stem cells are able to form any terminally differentiated cell. They have opened new doors for experimental and therapeutic studies to understand early development and to cure degenerative diseases in a way not previously possible. Nevertheless, it remains important to resolve and define the mechanisms underlying pluripotent stem cells, as that understanding will impact strongly on future medical applications. The capture of pluripotent stem cells in a dish is bound to several landmark discoveries, from the initial culture and phenotyping of pluripotent embryonal carcinoma cells to the recent induction of pluripotency in somatic cells. On this developmental time line, key transcription factors, such as Oct4, Sox2 or Nanog, have been revealed not only to regulate but also to functionally induce pluripotency. These early master regulators of development control developmental signalling pathways that affect the cell cycle, regulate gene expression, modulate the epigenetic state and repair DNA damage. Besides transcription factors, microRNAs have recently been shown to play important roles in gene expression and are embedded into the regulatory network orchestrating cellular development. However, there are species-specific differences in pluripotent cells, such as surface marker expression and growth factor requirements. Such differences and their underlying developmental pathways require clear definition and have major impacts on the preclinical test bed of pluripotent cells.

Vibrational spectroscopy differentiates between multipotent and pluripotent stem cells

Over the last few years, there has been an increased interest in the study of stem cells in biomedicine for therapeutic use and as a source for healing diseased or injured organs/tissues. More recently, vibrational spectroscopy has been applied to study stem cell differentiation. In this study, we have used both synchrotron based FTIR and Raman microspectroscopies to assess possible differences between human pluripotent (embryonic) and multipotent (adult mesenchymal) stem cells, and how O(2) concentration in cell culture could affect the spectral signatures of these cells. Our work shows that infrared spectroscopy of embryonic (pluripotent) and adult mesenchymal (multipotent) stem cells have different spectral signatures based on the amount of lipids in their cytoplasm (confirmed with cytological staining). Furthermore, O(2) concentration in cell culture causes changes in both the FTIR and Raman spectra of embryonic stem cells. These results show that embryonic stem cells might be more sensitive to O(2) concentration when compared to mesenchymal stem cells. While vibrational spectroscopy could therefore be of potential use in identifying different populations of stem cells further work is required to better understand these differences.

Female human iPSCs retain an inactive X chromosome

Generating induced pluripotent stem cells (iPSCs) requires massive epigenome reorganization. It is unclear whether reprogramming of female human cells reactivates the inactive X chromosome (Xi), as in mouse. Here we establish that human (h)iPSCs derived from several female fibroblasts under standard culture conditions carry an Xi. Despite the lack of reactivation, the Xi undergoes defined chromatin changes, and expansion of hiPSCs can lead to partial loss of XIST RNA. These results indicate that hiPSCs are epigenetically dynamic and do not display a pristine state of X inactivation with two active Xs as found in some female human embryonic stem cell lines. Furthermore, whereas fibroblasts are mosaic for the Xi, hiPSCs are clonal. This nonrandom pattern of X chromosome inactivation in female hiPSCs, which is maintained upon differentiation, has critical implications for clinical applications and disease modeling, and could be exploited for a unique form of gene therapy for X-linked diseases.

Variations of X chromosome inactivation occur in early passages of female human embryonic stem cells

X chromosome inactivation (XCI) is a dosage compensation mechanism essential for embryonic development and cell physiology. Human embryonic stem cells (hESCs) derived from inner cell mass (ICM) of blastocyst stage embryos have been used as a model system to understand XCI initiation and maintenance. Previous studies of undifferentiated female hESCs at intermediate passages have shown three possible states of XCI; 1) cells in a pre-XCI state, 2) cells that already exhibit XCI, or 3) cells that never undergo XCI even upon differentiation. In this study, XCI status was assayed in ten female hESC lines between passage 5 and 15 to determine whether XCI variations occur in early passages of hESCs. Our results show that three different states of XCI already exist in the early passages of hESC. In addition, we observe one cell line with skewed XCI and preferential expression of X-linked genes from the paternal allele, while another cell line exhibits random XCI. Skewed XCI in undifferentiated hESCs may be due to clonal selection in culture instead of non-random XCI in ICM cells. We also found that XIST promoter methylation is correlated with silencing of XIST transcripts in early passages of hESCs, even in the pre-XCI state. In conclusion, XCI variations already take place in early passages of hESCs, which may be a consequence of in vitro culture selection during the derivation process. Nevertheless, we cannot rule out the possibility that XCI variations in hESCs may reflect heterogeneous XCI states in ICM cells that stochastically give rise to hESCs.

Human embryonic stem cells with biological and epigenetic characteristics similar to those of mouse ESCs

Human and mouse embryonic stem cells (ESCs) are derived from blastocyst-stage embryos but have very different biological properties, and molecular analyses suggest that the pluripotent state of human ESCs isolated so far corresponds to that of mouse-derived epiblast stem cells (EpiSCs). Here we rewire the identity of conventional human ESCs into a more immature state that extensively shares defining features with pluripotent mouse ESCs. This was achieved by ectopic induction of Oct4, Klf4, and Klf2 factors combined with LIF and inhibitors of glycogen synthase kinase 3beta (GSK3beta) and mitogen-activated protein kinase (ERK1/2) pathway. Forskolin, a protein kinase A pathway agonist which can induce Klf4 and Klf2 expression, transiently substitutes for the requirement for ectopic transgene expression. In contrast to conventional human ESCs, these epigenetically converted cells have growth properties, an X-chromosome activation state (XaXa), a gene expression profile, and a signaling pathway dependence that are highly similar to those of mouse ESCs. Finally, the same growth conditions allow the derivation of human induced pluripotent stem (iPS) cells with similar properties as mouse iPS cells. The generation of validated "naïve" human ESCs will allow the molecular dissection of a previously undefined pluripotent state in humans and may open up new opportunities for patient-specific, disease-relevant research.