Human embryonic stem cells: 10 years on

Substantial advances in the biology of human embryonic stem (ES) cells, and the technology for working with them, have been made over the past 10 years. Regulatory frameworks for their study are well developed, although some countries remain particularly restrictive. Markers and criteria for characterising human ES cells are also generally agreed, and protocols for promoting their differentiation are being established, providing the groundwork for the development of applications over the next 10 years. The recent appearance of technology to convert somatic cells to 'induced Pluripotent Stem Cells' closely resembling ES cells will certainly speed up these developments.

New insights into the control of stem cell pluripotency

Nanog is a transcription factor that is expressed by mouse and human embryonic stem (ES) cells and by primordial germ cells. New research published recently in Nature (Chambers et al., 2007) points to an unexpected role for Nanog in the maintenance of pluripotency in mouse ES cells.

Stem cell therapy to treat diabetes mellitus

Transplantation of pancreatic islets offers a direct treatment for type 1 diabetes and in some cases, insulin-dependent type 2 diabetes. However, its widespread use is hampered by a shortage of donor organs. Many extant studies have focused on deriving beta-cell progenitors from pancreas and pluripotent stem cells. Efforts to generate beta-cells in vitro will help elucidate the mechanisms of beta-cell formation and thus provide a versatile in vivo system to evaluate the therapeutic potential of these cells to treat diabetes. Various successful experiments using beta-cells in animal models have generated extensive interest in using human embryonic stem cells to restore normoglycemia in diabetic patients. While new techniques are continually unveiled, the success of beta-cell generation rests upon successful manipulation of culture conditions and the induction of key regulatory genes implicated in pancreas development. In this review, we compare successfully conducted protocols, highlight essential steps and identify some of the remarkable shortfalls common to these methods. In addition, we discuss recent advancements in the derivation of patient-specific pluripotent stem cells that may facilitate the use of autologous beta-cells in stem cell therapy.

Heparin promotes the growth of human embryonic stem cells in a defined serum-free medium

A major limitation in developing applications for the use of human embryonic stem cells (HESCs) is our lack of knowledge of their responses to specific cues that control self-renewal, differentiation, and lineage selection. HESCs are most commonly maintained on inactivated mouse embryonic fibroblast feeders in medium supplemented with FCS, or proprietary replacements such as knockout serum-replacement together with FGF-2. These undefined culture conditions hamper analysis of the mechanisms that control HESC behavior. We have now developed a defined serum-free medium, hESF9, for the culture of HESCs on a type I-collagen substrate without feeders. In contrast to other reported media for the culture of HESCs, this medium has a lower osmolarity (292 mosmol/liter), l-ascorbic acid-2-phosphate (0.1 microg/ml), and heparin. Insulin, transferrin, albumin conjugated with oleic acid, and FGF-2 (10 ng/ml) were the only protein components. Further, we found that HESCs would proliferate in the absence of exogenous FGF-2 if heparin was also present. However, their growth was enhanced by the addition of FGF-2 up to 10 ng/ml although higher concentrations were deleterious in the presence of heparin.

Cell-cell signaling through NOTCH regulates human embryonic stem cell proliferation

Unlike pluripotent mouse embryonic stem (ES) cells, human ES cells and their malignant equivalents, embryonal carcinoma (EC) cells, require close cell-cell contact for efficient growth. Signaling through the NOTCH receptor, initiated by interaction with ligands of the DELTA/JAGGED family expressed on neighboring cells, plays a role in regulating the self-renewal of several stem cell systems. Members of the NOTCH and DELTA/JAGGED families are expressed by human EC and ES cells, and we have therefore investigated the possible role of NOTCH in the maintenance of these cells. Cleavage of both NOTCH1 and NOTCH2 to yield the intracellular domain responsible for the canonical signaling pathway of NOTCH was detected in several human EC and ES cell lines, suggesting that NOTCH signaling is active. Furthermore, the proliferation of human EC cells, as well as the expression of several downstream NOTCH target genes, was markedly reduced after small interfering RNA knockdown of NOTCH1, NOTCH2, and the canonical effector CBF-1 or after blocking NOTCH signaling with the gamma-secretase inhibitor L-685,458. The inhibitor also caused a reduction in the growth of human ES cells, although without evidence of differentiation. The results indicate that cell-cell signaling through the NOTCH system provides a critical cue for the proliferation of human EC and ES cell in vitro.

Characterization of human embryonic stem cell lines by the International Stem Cell Initiative

The International Stem Cell Initiative characterized 59 human embryonic stem cell lines from 17 laboratories worldwide. Despite diverse genotypes and different techniques used for derivation and maintenance, all lines exhibited similar expression patterns for several markers of human embryonic stem cells. They expressed the glycolipid antigens SSEA3 and SSEA4, the keratan sulfate antigens TRA-1-60, TRA-1-81, GCTM2 and GCT343, and the protein antigens CD9, Thy1 (also known as CD90), tissue-nonspecific alkaline phosphatase and class 1 HLA, as well as the strongly developmentally regulated genes NANOG, POU5F1 (formerly known as OCT4), TDGF1, DNMT3B, GABRB3 and GDF3. Nevertheless, the lines were not identical: differences in expression of several lineage markers were evident, and several imprinted genes showed generally similar allele-specific expression patterns, but some gene-dependent variation was observed. Also, some female lines expressed readily detectable levels of XIST whereas others did not. No significant contamination of the lines with mycoplasma, bacteria or cytopathic viruses was detected.

Culture adaptation of embryonic stem cells echoes germ cell malignancy

Teratocarcinomas are a subset of tumours that result from the neoplastic transformation of primordial germ cells. Such germ cell tumours (GCT) are histologically heterogeneous, reflecting a capacity for differentiation (pluripotency) of their embryonal carcinoma (EC) stem cells. However, malignant evolution of these tumours may ultimately correlate with a decrease in pluripotency, because this would tend to increase the propensity of EC cells for self-renewal. Human embryonic stem (ES) cells, derived from early blastocysts, closely resemble EC cells and, on prolonged culture in vitro, acquire progressive genetic changes that show striking similarity to those seen in GCT (e.g. gain of material from chromosome 12). In parallel, these abnormal ES cells show enhanced population growth rates and plating efficiencies, indicative of their adaptation to culture conditions. Understanding the mechanisms that drive such culture adaptation of ES cells may also provide insights into the development and progression of GCT.

Restriction landmark genome scanning identifies culture-induced DNA methylation instability in the human embryonic stem cell epigenome

Widespread provision of human embryonic stem cells (hESCs) for therapeutic use, drug screening and disease modelling will require cell lines sustainable over long periods in culture. Since the short-term, in vitro culture of mammalian embryos can result in DNA methylation changes, the epigenetic stability of hESCs warrants investigation. Existing hESC lines have been derived and cultured under diverse conditions, providing the potential for programming differential changes into the epigenome that may result in inter-line variability over and above that inherited from the embryo. By examining the DNA methylation profiles of > 2000 genomic loci by Restriction Landmark Genome Scanning, we identified substantial inter-line epigenetic distance between six independently derived hESC lines. Lines were found to inherit further epigenetic changes over time in culture, with most changes arising in the earliest stages post-derivation. The loci affected varied between lines. The majority of culture-induced changes (82.3-87.5%) were stably inherited both within the undifferentiated cells and post-differentiation. Adapting a line to a serum-free culture system resulted in additional epigenetic instability. Overall 80.5% of the unstable loci uncovered in hESCs have been associated previously with an adult tumour phenotype. Our study shows that current methods of hESC propagation can rapidly programme stable and unpredictable epigenetic changes in the stem cell genome. This highlights the need for (i) novel screening strategies to determine the experimental utility and biosafety of hESCs and (ii) optimization and standardization of procedures for the derivation and culture of hESC lines that minimize culture-induced instability.

Transient and stable transgene expression in human embryonic stem cells

Plasmid vectors remain a valuable yet capricious tool for the genetic manipulation of human embryonic stem (hES) cells. We have compared the efficacy of four promoters to mediate transient and stable transfection in hES and human embryonal carcinoma cell lines with the reporter enhanced green fluorescent protein (eGFP). In transient assays, the two mammalian promoters, UbiquitinC and Rosa26 (pUbiC and pR26), the human cytomegalovirus major immediate early promoter (HCMV-MIE; pCMV), and the HCMV-MIE/chicken beta-actin/rabbit beta-globin hybrid promoter (pCAGG) gave variable results that depended upon the cell line transfected but in an unpredictable way: each promoter supported strong transient expression in at least one cell line. The results for stable transfection were generally at variance with the transient assays. In each case, transgene silencing was quite marked, most notably with the pCMV, with which no eGFP-positive clones were obtained. An exception was the pCAG vector, in which the CAGG composite promoter is linked to the polyoma virus mutant enhancer PyF101; stable eGFP-positive transfectants were obtained, and these clones retained eGFP expression for over 120 passages, even in the absence of selection. However, if the PyF101 elements were removed, the resulting transfectants were also subjected to progressive gene silencing. Thus, the choice of promoter is critical for determining the desired effect of transgene expression in hES cells. Our data also demonstrate that pUbiC, pR26, pCAGG, and pCAG are more superior to the pCMV for generation of stable transfectants in hES cells. Disclosure of potential conflicts of interest is found at the end of this article.

Qualification of embryonal carcinoma 2102Ep as a reference for human embryonic stem cell research

As the number of human embryonic stem cell (hESC) lines increases, so does the need for systematic evaluation of each line's characteristics and potential. Comparisons between lines are complicated by variations in culture conditions, feeders, spontaneous differentiation, and the absence of standardized assays. These difficulties, combined with the inability of most labs to maintain more than a few lines simultaneously, compel the development of reference standards to which hESC lines can be compared. The use of a stable cell line as a reference standard offers many advantages. A line with a relatively unchanging hESC-like gene and protein expression pattern could be a positive control for developing assays. It can be used as a reference for genomics or proteomics studies, especially for normalizing results obtained in separate laboratories. Such a cell line should be widely available without intellectual property restraints, easily cultured without feeders, and resistant to spontaneous changes in phenotype. We propose that the embryonal carcinoma (EC) line 2102Ep meets these requirements. We compared the protein, gene, and microRNA expression of this cell line with those of hESC lines and alternative reference lines such as the EC line NTERA-2 and the karyotypically abnormal hESC line BG01V. The overall expression profiles of all these lines were similar, with exceptions reflecting the germ cell origins of EC. On the basis of global gene and microRNA expression, 2102Ep is somewhat less similar to hESC than the alternatives; however, 2102Ep expresses more hESC-associated microRNAs than NTERA-2 does, and fewer markers of differentiated fates.

Novel genomic aberrations in testicular germ cell tumors by array-CGH, and associated gene expression changes

Introduction: Testicular germ cell tumors of adolescent and young adult men (TGCTs) generally have near triploid and complex karyotypes. The actual genes driving the tumorigenesis remain essentially to be identified. Materials and Methods: To determine the detailed DNA copy number changes, and investigate their impact on gene expression levels, we performed an integrated microarray profiling of TGCT genomes and transcriptomes. We analyzed 17 TGCTs, three precursor lesions, and the embryonal carcinoma cell lines, NTERA2 and 2102Ep, by comparative genomic hybridization microarrays (array-CGH), and integrated the data with transcriptome profiles of the same samples. Results: The gain of chromosome arm 12p was, as expected, the most common aberration, and we found CCND2, CD9, GAPD, GDF3, NANOG, and TEAD4 to be the therein most highly over-expressed genes. Additional frequent genomic aberrations revealed some shorter chromosomal segments, which are novel to TGCT, as well as known aberrations for which we here refined boundaries. These include gains from 7p15.2 and 21q22.2, and losses of 4p16.3 and 22q13.3. Integration of DNA copy number information to gene expression profiles identified that BRCC3, FOS, MLLT11, NES, and RAC1 may act as novel oncogenes in TGCT. Similarly, DDX26, ERCC5, FZD4, NME4, OPTN, and RB1 were both lost and under-expressed genes, and are thus putative TGCT suppressor genes. Conclusion: This first genome-wide integrated array-CGH and gene expression profiling of TGCT provides novel insights into the genome biology underlying testicular tumorigenesis.

mGluR5 is involved in dendrite differentiation and excitatory synaptic transmission in NTERA2 human embryonic carcinoma cell-derived neurons

The pluripotent human embryonic carcinoma cell line NTERA2 readily differentiates into neurons when exposed to retinoic acid in vitro. These neurons show characteristic morphology with long processes and they express neuronal markers TUJ-1 and NeuN. NTERA2-derived neurons can regulate Ca2+ signalling through ionotropic glutamate (iGluR) and muscarinic receptors (mAChRs). Little is known, however, about the role of metabotropic glutamate receptors (mGluRs) in these neurons. Here we show that NTERA2-derived neurons express functional mGluR5, which is involved in Ca2+ signalling. Blocking mGluR5 activity at early stages of differentiation leads to fewer dendrites and a reduction in miniature excitatory postsynaptic currents (mEPSCs). Furthermore, cells cultured in the presence of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) show reduced N-methyl-D-aspartate (NMDA) receptor-mediated Ca2+ mobilisation but increased alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor Ca2+ permeability. During normal neuronal development, the edited GluR2 renders AMPARs Ca2+ impermeable. The increased Ca2+ permeability of AMPARs in MPEP-treated neurons is due to the reduced expression of GluR2 subunit protein. Thus, mGluR5 activity at early stages of differentiation is likely to play a role in the development of multipotent cell-derived neurons.

Adaptation to culture of human embryonic stem cells and oncogenesis in vivo

The application of human embryonic stem cells (HESCs) to provide differentiated cells for regenerative medicine will require the continuous maintenance of the undifferentiated stem cells for long periods in culture. However, chromosomal stability during extended passaging cannot be guaranteed, as recent cytogenetic studies of HESCs have shown karyotypic aberrations. The observed karyotypic aberrations probably reflect the progressive adaptation of self-renewing cells to their culture conditions. Genetic change that increases the capacity of cells to proliferate has obvious parallels with malignant transformation, and we propose that the changes observed in HESCs in culture reflect tumorigenic events that occur in vivo, particularly in testicular germ cell tumors. Further supporting a link between culture adaptation and malignancy, we have observed the formation of a chromosomal homogeneous staining region in one HESC line, a genetic feature almost a hallmark of cancer cells. Identifying the genes critical for culture adaptation may thus reveal key players for both stem cell maintenance in vitro and germ cell tumorigenesis in vivo.

A prospective on stem cell research

Stem cell research has stimulated considerable recent interest, but the concepts are old. Nevertheless, our understanding of the basic biology of different stem cell systems is poor. Many questions remain to be answered: How can we recognize stem cells? Are the underlying control mechanisms common to different types of stem cell, the so-called stemness concept, or is the control of self-renewal and commitment distinct in different stem cell types? What is the significance of differences in stem cells from different species? Do stem cells from somatic tissues really show plasticity with an ability to generate cells from distinct lineages, or are the observed examples consequences of experimental artifact, or rare events of no physiological significance? Do genetic mutations in the genes controlling stem cell self-renewal and differentiation lie at the heart of carcinogenesis? Answers to these and related questions now offer exciting future possibilities for both basic biology and medicine.

Characterisation of large-conductance calcium-activated potassium channels (BK(Ca)) in human NT2-N cells

Large-conductance calcium-activated potassium (BK(Ca)) channels were studied in inside-out patches of human NTERA2 neuronal cells (NT2-N). In symmetrical (140 mM) K(+) the channel mean conductance was 265 pS, the current reversing at approximately 0 mV. It was selective (P(K)/P(Na)=20:1) and blocked by internal paxilline and TEA. The open probability-voltage relationship for BK(Ca) was fitted with a Boltzmann function, the V((1/2)) being 76.3 mV, 33.6 mV and -14.1 mV at 0.1 muM, 3.3 muM and 10 muM [Ca(2+)](i), respectively. The relationship between open probability and [Ca(2+)](i) was fitted by the Hill equation (Hill coefficient 2.7, half maximal activation at 2.0 muM [Ca(2+)](i)). Open and closed dwell time histograms were fitted by the sum of two and three voltage-dependent exponentials, respectively. Increasing [Ca(2+)](i) produced both an increase in the longer open time constant and a decrease in the longest closed time constant, so increasing mean open time. "Intracellular" ATP evoked a concentration-dependent increase in NT2-N BK(Ca) activity. At +40 mV half-maximum activation occurred at an [ATP](i) of 3 mM (30 nM [Ca(2+)](i)). ADP and GTP were less potent, and AMP-PNP was inactive. This is the first characterisation of a potassium channel in NT2-N cells showing that it is similar to the BK(Ca) channel of other preparations.

Dysfunction of the mitotic:meiotic switch as a potential cause of neoplastic conversion of primordial germ cells

Germ cell tumours (GCT) are thought to arise as the result of a defect in early development, probably shortly after arrival of the migrating primordial germ cells (PGC) in the genital ridge when, if in a male genital ridge, the germ cells arrest in mitosis, but in a female genital ridge they enter meiosis. We suggest that dysfunction of the mitotic:meiotic switch, with cells aberrantly co-expressing functions pertinent to both states, might provide the genetic instability that could initiate tumour development. If this hypothesis is correct, GCT could arise because of disruption in the function of any one of a number of different genes involved in controlling mitosis and meiosis, rather than being dependent upon a single prominent susceptibility gene. The Notch signalling system is one candidate system for controlling the switch and we have identified expression of Notch2 and Notch4 in seminomas and carcinoma in situ. Thus those two members of the Notch family are candidates for proto-oncogenes that could play a role in GCT development. We have also identified a human homologue of the synaptonemal complex protein, SCP3, and have found its apparently aberrant expression in some established EC cell lines. One possibility is that abnormal regulation of such proteins involved in the synaptonemal complex could also lead to genetic instability in PGC and so also initiate tumour development.

Embryonic stem (ES) cells and embryonal carcinoma (EC) cells: opposite sides of the same coin

Embryonal carcinoma (EC) cells are the stem cells of teratocarcinomas, and the malignant counterparts of embryonic stem (ES) cells derived from the inner cell mass of blastocyst-stage embryos, whether human or mouse. On prolonged culture in vitro, human ES cells acquire karyotypic changes that are also seen in human EC cells. They also 'adapt', proliferating faster and becoming easier to maintain with time in culture. Furthermore, when cells from such an 'adapted' culture were inoculated into a SCID (severe combined immunodeficient) mouse, we obtained a teratocarcinoma containing histologically recognizable stem cells, which grew out when the tumour was explanted into culture and exhibited properties of the starting ES cells. In these features, the 'adapted' ES cells resembled malignant EC cells. The results suggest that ES cells may develop in culture in ways that mimic changes occurring in EC cells during tumour progression.

Cytotrophoblast stem cell lines derived from human embryonic stem cells and their capacity to mimic invasive implantation events

BACKGROUND

An effective embryonic-maternal interaction is crucial for successful human pregnancy. Failure of this process is a major cause of infertility and can lead to placental dysfunction resulting in recurrent miscarriage, fetal retardation and pre-eclampsia. Research is severely constrained by ethical and practical considerations; therefore, we aimed to generate cytotrophoblast stem (CTBS) cell lines from human embryonic stem cells (HESCs).

METHOD

Beta-HCG was used as a marker of viable trophoblast cells. In defined culture, embryoid bodies were generated from HESCs and selected for trophoblast enrichment by rounds of cellular aggregation and disaggregation. Distinct CTBS cell lines were isolated and characterized. Spheroid cytotrophoblast bodies were generated and their interaction with luteal-phase endometrial stroma was analysed by real-time image analysis.

RESULTS

Three CTBS cell lines were derived, which were maintained in the absence of residual HESCs, fibroblast feeder cells or extracellular matrix. CTBS cells displayed typical cytotrophoblast and syncytiotrophoblast characteristics and exhibited further differentiation to invasive endovascular cell phenotype. One cell line was generated with constitutive expression of enhanced green fluorescent protein (eGFP). Spheroid trophoblast bodies mimicked closely the early invasive stages of implantation when incubated with human endometrial stromal preparations in vitro.

CONCLUSION

These human CTBS cell lines are a significant new model for investigating human placentation and may have considerable potential in cell therapy applications.

The CDK inhibitor p27 enhances neural differentiation in pluripotent NTERA2 human EC cells, but does not permit differentiation of 2102Ep nullipotent human EC cells

Embryonal carcinoma (EC) cells, the stem cells of teratocarcinomas, are the malignant counterparts of pluripotent embryonic stem (ES) cells, but commonly exhibit a reduced ability to differentiate, presumably because of continual selection for genetic changes that alter the balance between self-renewal, differentiation and apoptosis in favour of self-renewal. To explore the nature of the genetic changes that promote nullipotency, we have compared two human EC cell lines, a 'nullipotent' line, 2102Ep, and a 'pluripotent' line, NTERA2. A hybrid derived by fusion of these cells differentiates in response to retinoic acid but, unlike the parental NTERA2 line, does not form terminally differentiated neurons. This implies that the nullipotent EC cell line, 2102Ep, differs in expression of at least two functions in comparison with the NTERA2 pluripotent line, one affecting commitment to differentiation, and one affecting terminal neural differentiation. We have now investigated the possible role of the CDK inhibitor, p27kip1 (p27) in commitment and terminal differentiation. In NTERA2, but not in 2102Ep cells, retinoic acid induces up-regulation of p27 expression, suggesting that 2102Ep cells lack this capacity. However, constitutive expression of a p27 transgene does not overcome the block to differentiation in the 2102Ep parental cells; commitment to differentiation must be blocked elsewhere. On the other hand, constitutive over-expression of p27 from a transgene enhances the neural differentiation of NTERA2 cells. Our results suggest that p27 plays a role in terminal neuronal differentiation of human EC cells, but not in their initial commitment to differentiation, and that other factors, possibly Cyclin D2, specifically limit its ability to promote neural differentiation.

Cellular differentiation hierarchies in normal and culture-adapted human embryonic stem cells

Human embryonic stem cell (HESC) lines vary in their characteristics and behaviour not only because they are derived from genetically outbred populations, but also because they may undergo progressive adaptation upon long-term culture in vitro. Such adaptation may reflect selection of variants with altered propensity for survival and retention of an undifferentiated phenotype. Elucidating the mechanisms involved will be important for understanding normal self-renewal and commitment to differentiation and for validating the safety of HESC-based therapy. We have investigated this process of adaptation at the cellular and molecular levels through a comparison of early passage (normal) and late passage (adapted) sublines of a single HESC line, H7. To account for spontaneous differentiation that occurs in HESC cultures, we sorted cells for SSEA3, which marks undifferentiated HESC. We show that the gene expression programmes of the adapted cells partially reflected their aberrant karyotype, but also resulted from a failure in X-inactivation, emphasizing the importance in adaptation of karyotypically silent epigenetic changes. On the basis of growth potential, ability to re-initiate ES cultures and global transcription profiles, we propose a cellular differentiation hierarchy for maintenance cultures of HESC: normal SSEA3+ cells represent pluripotent stem cells. Normal SSEA3- cells have exited this compartment, but retain multilineage differentiation potential. However, adapted SSEA3+ and SSEA3- cells co-segregate within the stem cell territory, implying that adaptation reflects an alteration in the balance between self-renewal and differentiation. As this balance is also an essential feature of cancer, the mechanisms of culture adaptation may mirror those of oncogenesis and tumour progression.

Differentiation of human embryonal carcinomas in vitro and in vivo reveals expression profiles relevant to normal development

Embryonal carcinoma is a histologic subgroup of testicular germ cell tumors (TGCTs), and its cells may follow differentiation lineages in a manner similar to early embryogenesis. To acquire new knowledge about the transcriptional programs operating in this tumor development model, we used 22k oligo DNA microarrays to analyze normal and neoplastic tissue samples from human testis. Additionally, retinoic acid-induced in vitro differentiation was studied in relevant cell lines. We identified genes characterizing each of the known histologic subtypes, adding up to a total set of 687 differentially expressed genes. Among these, there was a significant overrepresentation of gene categories, such as genomic imprinting and gene transcripts associated to embryonic stem cells. Selection for genes highly expressed in the undifferentiated embryonal carcinomas resulted in the identification of 58 genes, including pluripotency markers, such as the homeobox genes NANOG and POU5F1 (OCT3/4), as well as GAL, DPPA4, and NALP7. Interestingly, abundant expression of several of the pluripotency genes was also detected in precursor lesions and seminomas. By use of tissue microarrays containing 510 clinical testicular samples, GAL and POU5F1 were up-regulated in TGCT also at the protein level and hence validated as diagnostic markers for undifferentiated tumor cells. The present study shows the unique gene expression profiles of each histologic subtype of TGCT from which we have identified deregulated components in selected processes operating in normal development, such as WNT signaling and DNA methylation.