Human pluripotent stem cells: a progress report

Developing defined culture systems for human pluripotent stem cells

Human pluripotent stem cells hold promising potential in many therapeutics applications including regenerative medicine and drug discovery. Over the past three decades, embryonic stem cell research has illustrated that embryonic stem cells possess two important and distinct properties: the ability to continuously self-renew and the ability to differentiate into all specialized cell types. In this article, we will discuss the continuing evolution of human pluripotent stem cell culture by examining requirements needed for the maintenance of self-renewal in vitro. We will also elaborate on the future direction of the field toward generating a robust and completely defined culture system, which has brought forth collaborations amongst biologists and engineers. As human pluripotent stem cell research progresses towards identifying solutions for debilitating diseases, it will be critical to establish a defined, reproducible and scalable culture system to meet the requirements of these clinical applications.

Correlation of expression and methylation of imprinted genes with pluripotency of parthenogenetic embryonic stem cells

Mammalian parthenogenetic embryos (pE) are not viable due to placental deficiency, presumably resulting from lack of paternally expressed imprinted genes. Pluripotent parthenogenetic embryonic stem (pES) cells derived from pE could advance regenerative medicine by avoiding immuno-rejection and ethical roadblocks. We attempted to explore the epigenetic status of imprinted genes in the generation of pES cells from parthenogenetic blastocysts, and its relationship to pluripotency of pES cells. Pluripotency was evaluated for developmental and differentiation potential in vivo, based on contributions of pES cells to chimeras and development to day 9.5 of pES fetuses complemented by tetraploid embryos (TEC). Consistently, pE and fetuses failed to express paternally expressed imprinted genes, but pES cells expressed those genes in a pattern resembling that of fertilized embryos (fE) and fertilized embryonic stem (fES) cells derived from fE. Like fE and fES cells, but unlike pE or fetuses, pES cells and pES cell-fetuses complemented by TEC exhibited balanced methylation of Snrpn, Peg1 and U2af1-rs1. Coincidently, global methylation increased in pE but decreased in pES cells, further suggesting dramatic epigenetic reprogramming occurred during isolation and culture of pES cells. Moreover, we identified decreased methylation of Igf2r, Snrpn, and especially U2af1-rs1, in association with increased contributions of pES cells to chimeras. Our data show that in vitro culture changes epigenetic status of imprinted genes during isolation of pES cells from their progenitor embryos and that increased expression of U2af1-rs1 and Snrpn and decreased expression of Igf2r correlate with pluripotency of pES cells.

Cardiomyogenic gene expression profiling of differentiating human embryonic stem cells

Human embryonic stem cells (hESCs) can differentiate into a variety of specialized cell types. Thus, they provide a model system for embryonic development to investigate the molecular processes of cell differentiation and lineage commitment. The development of the cardiac lineage is easily detected in mixed cultures by the appearance of spontaneously contracting areas of cells. We performed gene expression profiling of undifferentiated and differentiating hESCs and monitored 468 genes expressed during cardiac development and/or in cardiac tissue. Their transcription during early differentiation of hESCs through embryoid bodies (EBs) was investigated and compared with spontaneously differentiating hESCs maintained on feeders in culture without passaging (high-density (HD) protocol). We observed a larger variation in the gene expression between cells from a single cell line that were differentiated using two different protocols than in cells from different cell lines that were cultured according to the same protocol. Notably, the EB protocol resulted in more reproducible transcription profiles than the HD protocol. The results presented here provide new information about gene regulation during early differentiation of hESCs with emphasis on the cardiomyogenic program. In addition, we also identified regulatory elements that could prove critical for the development of the cardiomyocyte lineage.

Bone Matrix Formation in Osteogenic Cultures Derived from Human Embryonic Stem Cells in Vitro

Bone matrix production and mineralization involves sophisticated mechanisms, including the initial formation of an organic extracellular matrix into which inorganic hydroxyapatite crystals are later deposited. Human embryonic stem (hES) cells offer a potential to study early developmental processes and provide an unlimited source of cells. In this study, four different hES cell lines were used, and two different approaches to differentiate hES cells into the osteogenic lineage were taken. Undifferentiated cells were cultured either in suspension, facilitating the formation of embryoid bodies (EBs), or in monolayer, and both methods were in the presence of osteogenic supplements. Novel to our osteogenic differentiation study was the use of commercially available human foreskin fibroblasts to support the undifferentiated growth of the hES cell colonies, and their propagation in serum replacement-containing medium. Characterization of the osteogenic phenotype revealed that all hES cell lines differentiated toward the mesenchymal lineage, because T-Brachyury, Flt-1, and bone morphogenetic protein-4 could be detected. Main osteoblastic marker genes Runx2, osterix, bone sialoprotein, and osteocalcin were up-regulated. Alizarin Red S staining demonstrated the formation of bone-like nodules, and bone sialoprotein and osteocalcin were localized to these foci by immunohistochemistry. Cells differentiated in monolayer conditions exhibited greater osteogenic potential compared to those from EB-derived cells. We conclude that in vitro hES cells can produce a mineralized matrix possessing all the major bone markers, the differentiation of pluripotent hES cells to an osteogenic lineage does not require initiation via EB formation, and that lineage potential is not dependent on the mode of differentiation induction but on a cell line itself.

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.

Regrow or repair: potential regenerative therapies for the kidney

Regenerative medicine is being heralded in a similar way as gene therapy was some 15 yr ago. It is an area of intense excitement and potential, as well as myth and disinformation. However, with the increasing rate of end-stage renal failure and limited alternatives for its treatment, we must begin to investigate seriously potential regenerative approaches for the kidney. This review defines which regenerative options there might be for renal disease, summarizes the progress that has been made to date, and investigates some of the unique obstacles to such treatments that the kidney presents. The options discussed include in situ organ repair via bone marrow recruitment or dedifferentiation; ex vivo stem cell therapies, including both autologous and nonautologous options; and bioengineering approaches for the creation of a replacement organ.

Monitoring Differentiation of Human Embryonic Stem Cells Using Real-Time PCR

There is a general lack of rapid, sensitive, and quantitative methods for the detection of differentiating human embryonic stem cells (hESCs). Using light microscopy and immunohistochemistry, we observed that morphological changes of differentiating hESCs precede any major alterations in the expression of several commonly used hESC markers (SSEA-3, SSEA-4, TRA-1-60, TRA-1-81, Oct-4, and Nanog). In an attempt to quantify the changes during stochastic differentiation of hESCs, we developed a robust and sensitive multi-marker quantitative real-time polymerase chain reaction (QPCR) method. To maximize the sensitivity of the method, we measured the expression of up- and downregulated genes before and after differentiation of the hESCs. Out of the 12 genes assayed, we found it clearly sufficient to determine the relative differentiation state of the cells by calculating a collective expression index based on the mRNA levels of Oct-4, Nanog, Cripto, and alpha-fetoprotein. We evaluated the method using different hESC lines maintained in either feeder-dependent or feeder-free culture conditions. The QPCR method is very flexible, and by appropriately selecting reporter genes, the method can be designed for various applications. The combination of QPCR with hESC-based technologies opens novel avenues for high-throughput analysis of hESCs in, for example, pharmacological and cytotoxicity screening.

Evolving concepts on the pathogenic mechanisms of aniridia related keratopathy

Heterozygosity for PAX6 deficiency (PAX6+/-) results in aniridia. Corneal changes in aniridia-related keratopathy (ARK) include corneal vascular pannus formation, conjunctival invasion of the corneal surface, corneal epithelial erosions and epithelial abnormalities, which eventually result in corneal opacity and contribute to visual loss. Corneal changes in aniridia have been attributed to congenital deficiency of corneal limbal stem cells. The aim of this paper is to review the potential mechanisms that may underlie the pathogenesis of aniridia related keratopathy. Current evidence, based on clinical observations and an animal model of aniridia suggest that the proliferative potential of the corneal limbal stem cells may not primarily be impaired. The corneal changes in aniridia may be related to an abnormality within the limbal stem cell niche. The mechanisms underlying progressive corneal pathology in aniridia appear multi-factorial and include: (1) abnormal corneal healing responses secondary to anomalous extracellular matrix metabolism; (2) abnormal corneal epithelial differentiation leading to fragility of epithelial cells; (3) reduction in cell adhesion molecules in the PAX6 heterozygous state, rendering the cells susceptible to natural shearing forces; and (4) conjunctival and corneal changes leading to the presence of cells derived from conjunctiva on the corneal surface.

Enhanced Derivation of Osteogenic Cells from Murine Embryonic Stem Cells after Treatment with Ionic Dissolution Products of 58S Bioactive Sol–Gel Glass

Embryonic stem (ES) cells represent a potentially useful cell source for tissue regeneration. Previously, using factors known to enhance differentiation and mineralization of primary osteoblasts, we were able to generate cell populations enriched with osteoblasts from a murine ES cell source. Dexamethasone was a potent inducer of osteoblast differentiation and the timing of stimulation markedly increased the proportion of osteoblast lineage cells. This study examined whether inorganic stimuli derived from bioactive glasses could affect the differentiation of osteoblasts in an ES-cell based system. Previous work has demonstrated the ability of soluble ions released from bioactive glasses undergoing dissolution in vitro to stimulate gene expression characteristic of a mature phenotype in primary osteoblasts. We report here on the potential of soluble extracts prepared from 58S sol-gel bioactive glass to further enhance lineage-specific differentiation in murine ES cells. Differentiation of ES cells into osteogenic cells was characterized by the formation of multilayered, mineralized nodules. These nodules contained cells expressing the transcription factor runx2/cbfa-1, and deposition of osteocalcin in the extracellular matrix was detected by immunostaining. When differentiating cells were placed in an osteoblast maintenance medium supplemented with soluble extracts prepared from bioactive glass powders, we observed increased formation of mineralized nodules (98 +/- 6%, mean +/- SEM) and alkaline phosphatase activity (56 +/- 14%, mean +/- SEM) in a pattern characteristic of osteoblast differentiation. This effect of the glass extracts exhibited dose dependency, with alkaline phosphatase activity and nodule formation increasing with extract concentrations. Compared with medium supplemented with dexamethasone, which had previously been used to enhance osteoblast lineage derivation, the glass extracts were as effective at inducing formation of mineralized nodules by murine ES cells. When glass extracts were used in combination with dexamethasone, a further increase in the number of nodules was observed (110 +/- 16%; cf. 83 +/- 7% for dexamethasone alone). This study demonstrates the capacity of an entirely inorganic material to stimulate differentiation of ES cells toward a lineage with therapeutic potential in tissue-engineering applications.

Stem cell-derived angiogenic/vasculogenic cells: possible therapies for tissue repair and tissue engineering

1. The recent ability to isolate stem cells and study their specific capacity of self-renewal with the formation of different cell types has opened up exciting vistas to help the repair of damaged tissue and even the formation of new tissue. In the present review, we deal with the characteristics and sources that stem cells can be derived and cultured from. 2. We focus on the role that stem cell-derived vascular cells or endothelial progenitor cells (EPC) may play in (re)vascularization of ischaemic and engineered tissues. This so-called vasculogenesis resembles the embryological process in which 'haemangioblasts' differentiate in blood cells, as well as in primitive vessels. Although also derived from the blood-forming bone marrow, in adult life vasculogenic stem cells contribute only little to the regular vascular repair mechanisms: namely (i) angiogenesis (outgrowth of vessels from existing vessels); and (ii) arteriogenesis (monocyte-aided increase in the calibre of existing arteriolar collaterals). 3. Most attempts to increase vascular repair by stem cells involve the use of growth factors, which mobilize stem cells from bone marrow into the blood, sometimes combined with isolation and reinfusion of these cells after ex vivo expansion and differentiation into EPC. 4. Clear improved perfusion of ischaemic sites and new vasculature has been observed in vivo mostly in animal models. Specific homing or administration of these cells and regulated and quantitative expansion and (final) differentiation at these vascular (repair) sites are less studied, but are paramount for efficacy and safety. 5. In conclusion, the use of embryonic stem cells will still encounter ethical objections. Moreover, special attention and measures are needed to cope with the allogeneic barriers that these cells usually encounter. In general, the long and complicated ex vivo cultures to obtain sufficient offspring from the very small numbers of stem cells that can be obtained as starting material will be costly and cumbersome. Both basic research on conceptual matters and cost-effective development of the product itself will have to go a long way before the clinical use of some volume can be expected.

Derivation and characterization of monkey embryonic stem cells

Embryonic stem (ES) cell based therapy carries great potential in the treatment of neurodegenerative diseases. However, before clinical application is realized, the safety, efficacy and feasibility of this therapeutic approach must be established in animal models. The rhesus macaque is physiologically and phylogenetically similar to the human, and therefore, is a clinically relevant animal model for biomedical research, especially that focused on neurodegenerative conditions. Undifferentiated monkey ES cells can be maintained in a pluripotent state for many passages, as characterized by a collective repertoire of markers representing embryonic cell surface molecules, enzymes and transcriptional factors. They can also be differentiated into lineage-specific phenotypes of all three embryonic germ layers by epigenetic protocols. For cell-based therapy, however, the quality of ES cells and their progeny must be ensured during the process of ES cell propagation and differentiation. While only a limited number of primate ES cell lines have been studied, it is likely that substantial inter-line variability exists. This implies that diverse ES cell lines may differ in developmental stages, lineage commitment, karyotypic normalcy, gene expression, or differentiation potential. These variables, inherited genetically and/or induced epigenetically, carry obvious complications to therapeutic applications. Our laboratory has characterized and isolated rhesus monkey ES cell lines from in vitro produced blastocysts. All tested cell lines carry the potential to form pluripotent embryoid bodies and nestin-positive progenitor cells. These ES cell progeny can be differentiated into phenotypes representing the endodermal, mesodermal and ectodermal lineages. This review article describes the derivation of monkey ES cell lines, characterization of the undifferentiated phenotype, and their differentiation into lineage-specific, particularly neural, phenotypes. The promises and limitations of primate ES cell-based therapy are also discussed.

Selective ablation of human embryonic stem cells expressing a "suicide" gene

Over the past few years, technological procedures have been developed for utilizing stem cells in transplantation medicine. Human embryonic stem (ES) cells can produce an unlimited number of differentiated cells and are, therefore, considered a potential source of cellular material for use in transplantation medicine. However, serious clinical problems can arise when uncontrolled cell proliferation occurs following transplantation. To avoid these potential problems, we genetically engineered human ES cell lines to express the herpes simplex virus thymidine kinase (HSV-tk) gene. Expression of the HSV-tk protein renders the ES cells sensitive to the U.S. Food and Drug Administration-approved drug ganciclovir, inducing destruction of HSV-tk(+) cells at ganciclovir concentrations that are nonlethal to other cell types. The reversion rate of engineered cells was low even under prolonged selection with ganciclovir. The HSV-tk(+) clones retained a normal karyotype and the ability to differentiate to cells from all three germ layers. Most importantly, tumors that arose in mice following subcutaneous injection of HSV-tk(+) human ES cells could be ablated in vivo by administration of ganciclovir. By utilizing these cell lines, safety levels can be improved in transplantations involving tissues derived from human ES cells.

Stem cells today: A. Origin and potential of embryo stem cells

The study of embryo stem cells began in 1963, initially using disaggregates of cleaving rabbit and mouse embryos. Their differentiation in vitro was modest, and usually curtailed at best to the formation of trophectoderm cells, which attached to plastic. Rabbit morulae and blastocysts adhered more readily, trophectoderm forming a sheet of cells which was overgrown by stem cells from inner cell mass. Whole-blastocyst cultures on collagen-coated surfaces produced a pile of cells, and its outgrowths included neural, blood, neuronal, phagocytic and many other types of cell. When inner cell mass was freed and cultured intact or as cell disaggregates, lines of embryo stem cells (ES) were established which possessed good rates of cleavage, and immense stability in their secretion of enzymes, morphology and chromosomal complement. Developmental capacities of single mouse embryo stem cells were measured by injecting one or more into a recipient blastocyst, and extent of colonization in resulting chimaeras measured their pluripotency. In mouse, cell clumps were termed embryoid bodies, which produced similar outgrowths as in rabbit. Component cells again differentiated widely, depending to a limited extent on their exposure to various cytokines or substrates. Markers for differentiation or pluripotency were established, which revealed how neural, cardiac, haematological and other ES lines could be established in vitro. These have proved useful to study early differentiation and their use in grafting to sick recipients. Displaying similar properties, human ES cells emerged in the late 1990s. Models for the clinical use of ES cells showed how they colonized rapidly, travelled to target tissues via fetal pathways, differentiated and colonized target organs. No signs of inflammation or tissue damage were noted; injured tissues could be repaired including remyelination, and no cancers were formed. ES cells offer wide therapeutic potentials for humans, although extensive clinical trials are still awaited.

Human embryonic stem cells: mother of all cell and tissue types

Pluripotential embryonic stem (ES) cells have been derived very efficiently from spare human embryos produced by IVF and grown in culture to the nascent blastocyst stage. The inner cell mass (ICM) is isolated by immunosurgery and grown on selected embryonic fibroblast monolayer cultures. ICM cells lose their memory for axis during formation of ES cell colonies and are then unable to integrate tissue formation with a body plan. ES cells form teratomas in vivo with cells and tissues representative of the three major embryonic lineages (ectoderm, mesoderm, endoderm). The ES cells are continuously renewable and can be directed to differentiate into early progenitors of neural stem cells (Noggin cells) and from there into mature neurons and glia (astrocytes and oligodendrocytes). The neural stem cells formed from human ES cells repopulate the brains of newborn mice when injected into the lateral cerebral ventricles, forming astrocytes dominantly in the parenchyma. The human neural cells can be observed migrating from the subventricular areas along the rostral migratory stream. Human neurons can be found in the olfactory bulb. Human ES cells can also be directed into cardiomyocytes when co-cultured with visceral endoderm-like cells (END-2). These observations provide further scope to explore stem cell therapies, gene therapies and drug discovery. For compatible transplantation, ES may need to be derived with a range of HLA types or by nuclear transplantation or stem cell fusion.

From teratocarcinomas to embryonic stem cells

The recent derivation of human embryonic stem (ES) cell lines, together with results suggesting an unexpected degree of plasticity in later, seemingly more restricted, stem cells (so-called adult stem cells), have combined to focus attention on new opportunities for regenerative medicine, as well as for understanding basic aspects of embryonic development and diseases such as cancer. Many of the ideas that are now discussed have a long history and much has been underpinned by the earlier studies of teratocarcinomas, and their embryonal carcinoma (EC) stem cells, which present a malignant surrogate for the normal stem cells of the early embryo. Nevertheless, although the potential of EC and ES cells to differentiate into a wide range of tissues is now well attested, little is understood of the key regulatory mechanisms that control their differentiation. Apart from the intrinsic biological interest in elucidating these mechanisms, a clear understanding of the molecular process involved will be essential if the clinical potential of these cells is to be realized. The recent observations of stem-cell plasticity suggest that perhaps our current concepts about the operation of cell regulatory pathways are inadequate, and that new approaches for analysing complex regulatory networks will be essential.

The ribosomal S6 kinases, cAMP-responsive element-binding, and STAT3 proteins are regulated by different leukemia inhibitory factor signaling pathways in mouse embryonic stem cells

Mouse embryonic stem (ES) cells remain "pluripotent" in vitro in the continuous presence of leukemia inhibitory factor (LIF). In the absence of LIF, ES cells are irreversibly committed to differentiate into various lineages. In this study we have set up an in vitro assay based on the anti-apoptotic activity of LIF to distinguish pluripotent from "differentiation-committed" ES cells. We have examined the phosphorylation profiles of known (STAT3 and ERKs) and identified new (ribosomal S6 kinases (RSKs) and cAMP-responsive element-binding protein (CREB)) LIF-regulated targets in ES and in ES-derived neuronal cells. We have demonstrated that although STAT3, a crucial player in the maintenance of ES cell pluripotency, is induced by LIF in all cell types tested, the LIF-dependent activation of RSKs is restricted to ES cells. We have shown that LIF-induced phosphorylation of RSKs in ES cells is dependent on ERKs, whereas STAT3 phosphorylation is not mediated by any known MAPK activities. Our results also demonstrate that the LIF-dependent phosphorylation of CREB is partially under the control of the RSK2 kinase.