Dopaminergic differentiation of human embryonic stem cells

Stem cell therapy for Parkinson's disease

Transplantation of fetal dopaminergic (DA) neurons can produce symptomatic relief for patients with Parkinson's disease, but the technical and ethical difficulties have limited the application of this therapy. Neural precursor cells and embryonic stem cells (ESCs) are expected to be candidates of potential donor cells for transplantation. Human neural precursor cells obtained from the midbrain give rise to TH-positive neurons. The growth of the cells, however, is slow and the differentiation rate of DA neurons is still low for clinical application. Monkey ESCs give rise to midbrain DA neurons, and the transplanted ESC-derived neurospheres function as DA neurons, attenuating the neurological symptoms of the monkey Parkinson's disease model. These results suggest the possibility of using stem cells for the treatment of Parkinson's disease, but problems such as the low survival rate in vivo and tumor formation must be solved.

"NeuroStem Chip": a novel highly specialized tool to study neural differentiation pathways in human stem cells

Background

Human stem cells are viewed as a possible source of neurons for a cell-based therapy of neurodegenerative disorders, such as Parkinson's disease. Several protocols that generate different types of neurons from human stem cells (hSCs) have been developed. Nevertheless, the cellular mechanisms that underlie the development of neurons in vitro as they are subjected to the specific differentiation protocols are often poorly understood.

Results

We have designed a focused DNA (oligonucleotide-based) large-scale microarray platform (named "NeuroStem Chip") and used it to study gene expression patterns in hSCs as they differentiate into neurons. We have selected genes that are relevant to cells (i) being stem cells, (ii) becoming neurons, and (iii) being neurons. The NeuroStem Chip has over 1,300 pre-selected gene targets and multiple controls spotted in quadruplicates (~46,000 spots total). In this study, we present the NeuroStem Chip in detail and describe the special advantages it offers to the fields of experimental neurology and stem cell biology. To illustrate the utility of NeuroStem Chip platform, we have characterized an undifferentiated population of pluripotent human embryonic stem cells (hESCs, cell line SA02). In addition, we have performed a comparative gene expression analysis of those cells versus a heterogeneous population of hESC-derived cells committed towards neuronal/dopaminergic differentiation pathway by co-culturing with PA6 stromal cells for 16 days and containing a few tyrosine hydroxylase-positive dopaminergic neurons.

Conclusion

We characterized the gene expression profiles of undifferentiated and dopaminergic lineage-committed hESC-derived cells using a highly focused custom microarray platform (NeuroStem Chip) that can become an important research tool in human stem cell biology. We propose that the areas of application for NeuroStem microarray platform could be the following: (i) characterization of the expression of established, pre-selected gene targets in hSC lines, including newly derived ones, (ii) longitudinal quality control for maintained hSC populations, (iii) following gene expression changes during differentiation under defined cell culture conditions, and (iv) confirming the success of differentiation into specific neuronal subtypes.

Human embryonic stem cells: an in vitro model to study mechanisms controlling pluripotency in early mammalian development

The property of pluripotency confers the capacity for differentiation into a large number of cell types including extra-embryonic, somatic and germinal cells. During normal development, pluripotency is acquired by the cells of the early embryo, which shortly thereafter undergo differentiation, whereas embryonic stem cells (ESCs) uniquely maintain pluripotency while undergoing extensive in vitro proliferation. Studies using ESCs have begun to unravel the network of cytokines and transcription factors responsible for their maintenance of pluripotency. Surprisingly, mouse and human ESCs display significant differences in such mechanisms despite their similar embryonic origins. In this review, we compare the properties of pluripotent embryonic cells with those of ESCs to establish a general model for the mechanisms maintaining pluripotency. We first consider whether mouse and human ESCs represent comparable stages of early embryonic development. We then describe how human embryoid body (EB) differentiation could be used as a model of embryonic development. Finally, to concretely illustrate the discussion, we discuss our recent results concerning Nodal function in controlling cell fate at early stages of human EB development. With the new perspective of these findings, we suggest a previously unrecognized role of TGF-beta pathway signaling in maintaining pluripotency at early stages of mammalian embryonic development.

Tyrosine hydroxylase expression is unstable in a human immortalized mesencephalic cell line--studies in vitro and after intracerebral grafting in vivo

We have studied the stability of the dopaminergic phenotype in a conditionally immortalized human mesencephalic cell line, MESC2.10. Even though MESC2.10 cells exhibit features of dopaminergic neurons in vitro, none of the cells expressed tyrosine hydroxylase (TH) after transplantation into a rat model of Parkinson's disease. We examined whether this is caused by cell death or loss of transmitter phenotype. Cells were cultured in differentiation medium, then harvested and replated into the same medium where they continued to express TH, whereas replated cells fed medium lacking differentiation factors (dibutyryl cAMP and glial cell line-derived neurotrophic factor) did not. Interestingly, cultures grown in the absence of differentiation factors could regain TH expression once exposed to differentiation medium. Our data suggest that TH expression in vitro is inducible in neurons derived from the MESC2.10 cell line and that the dopaminergic phenotype of these cells in vivo might be unstable.

Efficient generation of dopamine neurons from human embryonic stem cells

In this chapter, we introduce a co-culture protocol for human embryonic stem (hES) cell differentiation in which dopamine (DA) neurons with midbrain-specific markers are efficiently derived. Human ES cells on a feeder layer of stromal cells are induced to differentiate into neuroepithelial or neural precursor cells with embryonic midbrain precursor properties. The resulting neural precursor cells are then selectively expanded and serially passaged to obtain a large, homogeneous population of these cells. Under the conditions for terminal differentiation, the majority of hES-derived neural precursors differentiate into neuronal cells that are positive for DA neuronal markers such as tyrosine hydroxylase (TH) and function in vitro as presynaptic DA neurons.

Human Embryonic Stem Cells

Human embryonic stem cells (hESCs) are stable in terms of their pluripotency, karyotype, global gene expression, ability to repair DNA and maintain telomerase levels, and growth characteristics. hESCs offer a renewable source of a wide range of cell types for use in research and cell-based therapies to treat disease. Characterization of cell populations that differentiate from them provides important information on early differentiation events and critical data for subsequent downstream manipulations. A strategy that has evolved in using cells is to develop a master bank of cells from which a working bank is generated, which is then used to generate appropriate cell types for screening, drug discovery, or therapeutic use. Appropriate cells are purified or enriched by one of several selection techniques, and such purified populations are used for most purposes. In this review, the authors discuss recent results and review the progress that has been made in the field, with a focus on using embryonic stem cells for neural targets.

Persistent dopamine functions of neurons derived from embryonic stem cells in a rodent model of Parkinson disease

The derivation of dopamine neurons is one of the best examples of the clinical potential of embryonic stem (ES) cells, but the long-term function of the grafted neurons has not been established. Here, we show that, after transplantation into an animal model, neurons derived from mouse ES cells survived for over 32 weeks, maintained midbrain markers, and had sustained behavioral effects. Microdialysis in grafted animals showed that dopamine (DA) release was induced by depolarization and pharmacological stimulants. Positron emission tomography measured the expression of presynaptic dopamine transporters in the graft and also showed that the number of postsynaptic DA D(2) receptors was normalized in the host striatum. These data suggest that ES cell-derived neurons show DA release and reuptake and stimulate appropriate postsynaptic responses for long periods after implantation. This work supports continued interest in ES cells as a source of functional DA neurons.

An in vitro model of human dopaminergic neurons derived from embryonic stem cells: MPP+ toxicity and GDNF neuroprotection

Human embryonic stem cells (hESCs) can proliferate indefinitely yet also differentiate in vitro, allowing normal human neurons to be generated in unlimited numbers. Here, we describe the development of an in vitro neurotoxicity assay using human dopaminergic neurons derived from hESCs. We showed that the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium (MPP(+)), which produces features of Parkinson's disease in humans, was toxic for hESC-derived dopaminergic neurons. Treatment with glial cell line-derived neurotrophic factor protected tyrosine hydroxylase-positive neurons against MPP(+)-induced apoptotic cell death and loss of neuronal processes as well as against the formation of intracellular reactive oxygen species. The availability of human dopaminergic neurons, derived from hESCs, therefore allows for the possibility of directly examining the unique features of human dopaminergic neurons with respect to their responses to pharmacological agents as well as environmental and chemical toxins.

A protocol for the differentiation of human embryonic stem cells into dopaminergic neurons using only chemically defined human additives: Studies in vitro and in vivo

Our ability to use human embryonic stem (hES) cells in cell replacement therapy for Parkinson's disease depends on the discovery of ways to simply and reliably differentiate a dopaminergic (DA) phenotype in these cells. Although several protocols exist for the differentiation of DA traits in hES, they involve the prolonged use of complex media with undefined components, cell conditioned media and/or co-culture with various cells, usually of animal origin. In this study, several well-characterized (H9, BG01) and several new uncharacterized (HUES7, HUES8) hES cell lines were studied for their capacity to differentiate into DA neurons in culture using a novel rapid protocol which uses only chemically-defined human-derived media additives and substrata. Within 3 weeks, cells from all 4 cell lines progressed from the undifferentiated state to beta-tubulin III positive cells expressing DA markers in vitro. Moreover, transplantation of these cells into the striata of 6-hydroxydopamine-treated rats at the neuronal progenitor stage resulted in the appearance of differentiated DA traits in vivo 2-3 weeks later.

A Scaleable and Defined System for Generating Neural Stem Cells from Human Embryonic Stem Cells

The ability to differentiate human ESCs (hESCs) to defined lineages in a totally controlled manner is fundamental to developing cell-based therapies and studying human developmental mechanisms. We report a novel, scaleable, and widely applicable system for deriving and propagating neural stem cells from hESCs without the use of animal products, proprietary formulations, or genetic manipulation. This system provides a definitive platform for studying human neural development and has potential therapeutic implications.

Neural conversion of human embryonic stem cell colonies in the presence of fibroblast growth factor-2

Pluripotency and the capability for unlimited self-renewal make human embryonic stem cells a promising tool for studying development and new cell replacement strategies. Here, we present a simple differentiation protocol, which permits the direct conversion of human embryonic stem cells into neurogenic precursors without formation of embryoid bodies or coculture with other cell types. In this protocol, human embryonic stem cells propagated as adherent cultures are induced to differentiate into the neural lineage in media containing fibroblast growth factor-2. The adherent cells are proliferated to form detaching neurospheres. Upon plating, these neurospheres give rise to a homogenous population of neural precursors capable of generating neurons, astrocytes and oligodendrocytes. Our findings suggest that fibroblast growth factor-2 exposure alone suffices to promote neural conversion of adherently growing human embryonic stem cell cultures.

Axonal growth regulation of fetal and embryonic stem cell-derived dopaminergic neurons by Netrin-1 and Slits

The physical restoration of dopamine circuits damaged or lost in Parkinson disease by implanting embryonic stem (ES)-derived cells may become a treatment. It is critical to understand responses of ES-derived dopamine (DA) neurons to guidance signals that determine axonal path and targeting. Using a collagen gel culture system, we examined effects of secreted molecules Netrin-1 and Slits on neurite outgrowth of fetal DA neurons and murine ES-differentiated DA neurons. We have previously shown that fetal DA neurons express DCC and Robo1/2 receptors and that Netrin-1 and Slit2 function as an attractant and a repellent for DA neurite outgrowth. In the present study, we observe that both Slit1 and Slit3 repel and inhibit neurite growth of fetal DA neurons. Here, we also demonstrate that ES-differentiated neurons including DA neurons express the Netrin receptor DCC and Slit receptor Robo proteins. In the gel culture system of ES cells, Netrin-1 promoted neurite outgrowth mediated by DCC receptor, and Slit1 and Slit3 were inhibitory for neurite outgrowth through Robo receptors. Slit2 appeared to exert inhibitory as well as repulsive effects in the coculture assay. However, unlike fetal DA neurites, no directed neurite outgrowth was observed in the cocultures of ES-derived DA neurons with Netrin-1-, Slit1-, and Slit3-producing cells. The findings suggest that ES-derived DA neurons generated by current protocols can respond to guidance cues in vitro in a similar manner to fetal cells but also exhibit distinct responses. This may result from developmental differences generated by present in vitro methods of cell patterning or conditioning during ES cell differentiation.

Enhanced yield of neuroepithelial precursors and midbrain-like dopaminergic neurons from human embryonic stem cells using the bone morphogenic protein antagonist noggin

It is currently not known whether dopamine (DA) neurons derived from human embryonic stem cells (hESCs) can survive in vivo and alleviate symptoms in models of Parkinson disease (PD). Here, we report the use of Noggin (a bone morphogenic protein antagonist) to induce neuroectodermal cell development and increase the yield of DA neurons from hESCs. A combination of stromal-derived inducing activity and Noggin markedly enhanced the generation of neuroepithelial progenitors that could give rise to DA neurons. In addition, Noggin diminished the occurrence of a fibroblast-like Nestin-positive precursor population that differentiated into myocytes. After transplantation of differentiated hESCs to a rodent model of PD, some grafts contained human midbrain-like DA neurons. This protocol demonstrates hESC derivation and survival of human DA neurons appropriate for cell therapy in PD.

Control of neurogenesis and tyrosine hydroxylase expression in neural progenitor cells through bHLH proteins and Nurr1

The production of dopamine (DA) neurons from neural progenitor cells (NPC) is of particular interest as these neurons degenerate in Parkinson's disease. Here, we report that the characteristics of NPC from the ventral midbrain (NPC(VM)) and the striatum (NPC(STR)) are intrinsically determined. A detailed analysis of the VM during development revealed Ngn2 and Mash1 expression in a DA progenitor domain. Interestingly, over-expression of either Ngn2 or Mash1 induced neurogenesis from expanded NPC(VM). Whereas Ngn2 inhibited cell division and the production of neurons even in the presence of mitogens, Mash1 allowed the progenitors to divide while retaining neurogenic potential. However, none of the new neurons derived by over-expressing Ngn2 or Mash1 were positive for DA neuronal markers such as tyrosine hydroxylase. Nurr1 over-expression increased TH levels in a dose-dependant manner within both neurons and glia, suggesting a non-neuronal-specific activation of this enzyme by Nurr1. Double infection with Nurr1 and either Ngn2 or Mash1 resulted in the production of small numbers of TH+ neurons, which were larger in size when derived from NPC(VM) compared to NPC(STR). These data provide proof of concept that over-expression of multiple transcription factors can drive the fate of NPC first towards neurons, and then towards the DA phenotype. However, further factors may be required to generate fully functional DA neurons.

Merging fields: stem cells in neurogenesis, transplantation, and disease modeling

Traditionally, applied stem cell research has been segregating into strategies aiming at endogenous repair and cell transplantation. Recent advances in both fields have unraveled unexpected potential for synergy between these disparate fields. The increasing dissection of the step-wise integration of adult-born neurons into an established brain circuitry provides a highly informative blueprint for the functional incorporation of grafted neurons into a host brain. On the other hand, in vitro recapitulation of developmental differentiation cascades permits the de novo generation of various neural cell types from pluripotent embryonic stem (ES) cells. Advanced tools in stem cell engineering enable not only genetic selection and instruction of disease-specific donor cells for neural replacement but also the exploitation of stem cells as transgenic cellular model systems for human diseases. In a comparative approach we here illuminate the functional integration of neurons derived from endogenous and transplanted stem cells, the evolving technologies for advanced stem cell engineering and the impact of cloned and mutated stem cells on disease modeling.

Improved development of human embryonic stem cell-derived embryoid bodies by stirred vessel cultivation

Human embryonic stem cells (hESCs) represent an important resource for novel cell-based regenerative medical therapies. hESCs are known to differentiate into mature cells of defined lineages through the formation of embryoid bodies (EBs) which are amenable to suspension culture for several weeks. However, EBs derived from hESCs in standard static cultures are typically non-homogeneous, leading to inefficient cellular development. Here, we systematically compare the formation, growth, and differentiation capabilities of hESC-derived EBs in stirred and static suspension cultures. A 15-fold expansion in total number of EB-derived cells cultured for 21 days in a stirred flask was observed, compared to a fourfold expansion in static (non-stirred) cultures. Additionally, stirred vessel mediated cultures have a more homogeneous EB morphology and size. Importantly, the EBs cultivated in spinner flasks retained comparable ability to produce hematopoietic progenitor cells as those grown in static culture. These results demonstrate the decoupling between EB cultivation method and EB-derived cells' ability to form hematopoietic progenitors, and will allow for improved production of scalable quantities of hematopoietic cells or other differentiated cell lineages from hESCs in a controlled environment.

Guiding embryonic stem cells towards differentiation: lessons from molecular embryology

Embryonic stem cells are uniquely endowed with the capacity of self-renewal and the potential to give rise to all possible cell types, including germ cells. These qualities have made mouse embryonic stem cells a valuable resource for genetic manipulation of the mouse genome. In addition, they present a powerful system for the in vitro dissection of mammalian embryonic development. The recent isolation of human embryonic stem cells has raised a lot of interest for the potential of transposing our knowledge of lineage-specific differentiation of embryonic stem cells to cell-based therapy of human disease. Recent reports have provided insights into the specific differentiation of embryonic stem cells to different cell types of the embryo. However, progress in this direction seems to depend on the knowledge of the mechanisms controlling lineage decisions during embryogenesis.

Automated mechanical passaging: a novel and efficient method for human embryonic stem cell expansion

There is a need for more standardized methods of maintenance and propagation of human embryonic stem cell (hESC) cultures. Enzymatic passaging currently represents the most widely used method for expansion of hESCs. Although rapid and straightforward, this technique results in variable-sized cell clusters and significant cellular trauma, which may apply selective pressure in long-term culture. Mechanical passaging has the potential advantages of defined colony fragment sizes, reduced cellular trauma, and the possibility of selecting undifferentiated colonies for transfer. However, manual dissection of individual colonies is a prohibitively time-consuming process unsuitable for maintaining large numbers of hESCs without the use of additional chemical means. In this study we report an efficient automated method for mechanically passaging hESCs. We have used this method exclusively to maintain hESCs in long-term undifferentiated culture without the use of enzymatic digestion for longer than 100 days. This automated technique can thus be used routinely to culture hESCs in the laboratory.

Stable expression of a neuronal dopaminergic progenitor phenotype in cell lines derived from human amniotic fluid cells

Cells from human amniotic fluid derived from the fetus are considered a source of multipotent cells. Their properties have not been fully exploited, partially because unlike other embryonic sources such as embryonic stem (ES) cells, cell lines from amniocentesis samples have not been generated. We have established and characterized the properties of eight individual cell lines. Flow cytometry using several cell surface markers showed that all cell lines generated consisted of homogeneous populations that lack HLAII antigenicity. Using a combination of immunocytochemistry, Western blotting, and RT-PCR, we found weak expression of Oct4 and nestin and strong expression of tubulin-betaIII, MAP2, and tau. Specific markers for cholinergic, (nor)adrenergic, and GABAergic neurons or glia were weakly expressed or absent, whereas expression of factors implicated in early induction of dopaminergic neurons, TGF-beta3 and beta-catenin were present. Further analysis showed strong expression of EN-1, c-RET, PTX3, and NURR1 essential for induction and survival of midbrain dopaminergic neurons, TH, AADC, and VMAT2 components of dopamine synthesis and secretion, and syntaxin1A and SNAP-25 necessary for neurotransmitter exocytosis. This phenotype was retained throughout passages and up to the current passage 36. Expression of neuronal and dopaminergic markers in individual AF cell lines was comparable to expression in neurons induced from ES cells and in IMR-32 and SH-SY5Y neuroblastomas. Our data show that cell lines can be derived from subcultures of amniocentesis, and are primarily composed of a population of progenitors with a phenotype similar to that of committed mesencephalic dopaminergic neurons.

Adult human bone marrow stromal spheres express neuronal traits in vitro and in a rat model of Parkinson's disease

Adult human bone marrow stromal cells (hMSCs) grown in suspension culture gave rise to spheres of neural progenitor (NP) cells, capable of expressing both dopaminergic (DA) and GABAergic (GABA) traits. After transplantation into the Parkinsonian rat, human NPs and neurons were present at 2 weeks. Although no DA neurons appeared to survive transplantation, there were abundant GABA neurons present in the graft. By 4 weeks, however, all cells had died. Finding ways to prolong survival and promote the appropriate neurotransmitter phenotype is essential if hMSCs are to be clinically useful.

Induction of Midbrain Dopaminergic Neurons from Primate Embryonic Stem Cells by Coculture with Sertoli Cells

The aim of this study was to produce dopaminergic neurons from primate embryonic stem (ES) cells following coculture with mouse Sertoli cells. After 3 weeks of induction, immunostaining revealed that 90% +/- 9% of the colonies contained tyrosine hydroxylase-positive (TH(+)) neurons, and 60% +/- 7% of the tubulin beta III-positive (Tuj III(+)) neurons were TH(+). Reverse transcription-polymerase chain reaction analyses showed that Sertoli-induced neurons expressed midbrain dopaminergic neuron markers, including TH, dopamine transporter, aromatic amino acid decarboxylase (AADC), receptors such as TrkB and TrkC, and transcription factors NurrI and Lmx1b. Neurons that had been differentiated on Sertoli cells were positive for Pax2, En1, and AADC, midbrain-related markers, and negative for dopamine-beta-hydroxylase, a marker of noradrenergic neurons. These Sertoli cell-induced dopaminergic cells can release dopamine when depolarized by high K(+). Sertoli cell-conditioned medium contained glial cell line-derived neurotrophic factor (GDNF) and supported neuronal differentiation. After pretreatment with anti-GDNF antibody, the percentage of Tuj III(+) colonies was reduced to 14%. Thus, GDNF contributed significantly to inducing primate ES cells into dopaminergic neurons. When transplanted into a 6-hydroxydopamine-treated Parkinson's disease model, primate-derived dopaminergic neurons integrated into the mouse striatum. Two weeks after transplantation, surviving TH(+) cells were present. These TH(+) cells survived for 2 months. Therefore, the induction method of coculture ES cells with Sertoli cells provides an unlimited source of primate cells for the study of pathogenesis and transplantation in Parkinson's disease.

Human embryonic stem cell technology: large scale cell amplification and differentiation

Embryonic stem cells (ESC) hold the promise of overcoming many diseases as potential sources of, for example, dopaminergic neural cells for Parkinson’s Disease to pancreatic islets to relieve diabetic patients of their daily insulin injections. While an embryo has the innate capacity to develop fully functional differentiated tissues; biologists are finding that it is much more complex to derive singular, pure populations of primary cells from the highly versatile ESC from this embryonic parent. Thus, a substantial investment in developing the technologies to expand and differentiate these cells is required in the next decade to move this promise into reality. In this review we document the current standard assays for characterising human ESC (hESC), the status of ‘defined’ feeder-free culture conditions for undifferentiated hESC growth, examine the quality controls that will be required to be established for monitoring their growth, review current methods for expansion and differentiation, and speculate on the possible routes of scaling up the differentiation of hESC to therapeutic quantities.

Concise review: recent advances on the significance of stem cells in tissue regeneration and cancer therapies

In this study, we report on recent advances on the functions of embryonic, fetal, and adult stem cell progenitors for tissue regeneration and cancer therapies. We describe new procedures for derivation and maturation of these stem cells into the tissue-specific cell progenitors. The localization of the adult stem cells and their niches, as well as their implication in the tissue repair after injuries and during cancer progression, are also described. The emphasis is on the interactions among certain developmental signaling factors, such as hormones, epidermal growth factor, hedgehog, Wnt/beta-catenin, and Notch. These factors and their pathways are involved in the stringent regulation of the self-renewal and/or differentiation of adult stem cells. Novel strategies for the treatment of both diverse degenerating disorders, by cell replacement, and some metastatic cancer types, by molecular targeting multiple tumorigenic signaling elements in cancer progenitor cells, are also illustrated.

Mesodermal cell types induce neurogenesis from adult human hippocampal progenitor cells

Neurogenesis in the adult human brain occurs within two principle neurogenic regions, the hippocampus and the subventricular zone (SVZ) of the lateral ventricles. Recent reports demonstrated the isolation of human neuroprogenitor cells (NPCs) from these regions, but due to limited tissue availability the knowledge of their phenotype and differentiation behavior is restricted. Here we characterize the phenotype and differentiation capacity of human adult hippocampal NPCs (hNPCs), derived from patients who underwent epilepsy surgery, on various feeder cells including fetal mixed cortical cultures, mouse embryonic fibroblasts (MEFs) and PA6 stromal cells. Isolated hNPCs were cultured in clonal density by transferring the cells to serum-free media supplemented with FGF-2 and EGF in 3% atmospheric oxygen. These hNPCs showed neurosphere formation, expressed high levels of early neuroectodermal markers, such as the proneural genes NeuroD1 and Olig2, the NSC markers Nestin and Musashi1, the proliferation marker Ki67 and significant activity of telomerase. The phenotype was CD15low/-, CD34-, CD45- and CD133-. After removal of mitogens and plating them on poly D-lysine, they spontaneously differentiated into a neuronal (MAP2ab+), astroglial (GFAP+), and oligodendroglial (GalC+) phenotype. Differentiated hNPCs showed functional properties of neurons, such as sodium channels, action potentials and production of the neurotransmitters glutamate and GABA. Co-culture of hNPCs with fetal cortical cultures, MEFs and PA6 cells increased neurogenesis of hNPCs in vitro, while only MEFs and PA6 cells also led to a morphological and functional neurogenic maturation. Together we provide a first detailed characterization of the phenotype and differentiation potential of human adult hNPCs in vitro. Our findings reinforce the emerging view that the differentiation capacity of adult hNPCs is critically influenced by non-neuronal mesodermal feeder cells.

Efficient generation of dopaminergic neurons from mouse embryonic stem cells enclosed in hollow fibers

Transplantation of dopamine neurons is a promising approach to treat Parkinson's disease. Embryonic stem (ES) cells are expected to be a cell source of the dopaminergic neurons. Various difficulties, however, need to be overcome to realize cell therapy of Parkinson's disease using dopaminergic neurons from ES cells. For example, they are highly sensitive to enzymatic treatment and physical dissociation, and the patient's immune system may attack the transplanted cells. In this study, we attempted to induce dopaminergic neurons from mouse ES cells enclosed in hollow fibers using conditioning medium from PA6 cells, the stromal cells derived from skull bone marrow. beta-tubulin type III positive cells and tyrosine hydroxylase positive cells were efficiently derived in hollow fibers after 16 days in culture, and dopamine release was observed when the hollow fibers containing cells were exposed to 56mm KCl for 15min to induce dopamine release through depolarization of the neurons. By our procedure, enclosure of dopaminergic neurons in hollow fibers was easily performed without loss of cells, and the hollow fiber membrane is expected to efficiently protect dopaminergic neurons from mechanical disturbances and attacks by the host immune system. Although there are many issues, especially related to immuno-isolation, that still remain to be addressed, we believe that differentiation of ES cells within hollow fibers is one of the crucial procedures so that cell therapy of Parkinson's disease can be realized.

Neural conversion of ES cells by an inductive activity on human amniotic membrane matrix

Here we report a human-derived material with potent inductive activity that selectively converts ES cells into neural tissues. Both mouse and human ES cells efficiently differentiate into neural precursors when cultured on the matrix components of the human amniotic membrane in serum-free medium [amniotic membrane matrix-based ES cell differentiation (AMED)]. AMED-induced neural tissues have regional characteristics (brainstem) similar to those induced by coculture with mouse PA6 stromal cells [a common method called stromal cell-derived inducing activity (SDIA) culture]. Like the SDIA culture, the AMED system is applicable to the in vitro generation of various CNS tissues, including dopaminergic neurons, motor neurons, and retinal pigment epithelium. In contrast to the SDIA method, which uses animal cells, the AMED culture uses a noncellular inductive material derived from an easily available human tissue; therefore, AMED should provide a more suitable and versatile system for generating a variety of neural tissues for clinical applications.

Directed differentiation of mouse embryonic stem cells into thyroid follicular cells

Elucidating the molecular mechanisms leading to the induction and specification of thyroid follicular cells is important for our understanding of thyroid development. To characterize the key events in this process, we previously established an experimental embryonic stem (ES) cell model system, which shows that wild-type mouse CCE ES cells can give rise to thyrocyte-like cells in vitro. We extend our analysis in this report by using a genetically manipulated ES cell line in which green fluorescent protein (GFP) cDNA is targeted to the TSH receptor (TSHR) gene, linking GFP expression to the transcription of the endogenous TSHR gene. The appearance of GFP-positive cells was dependent on the formation of embryoid bodies from undifferentiated ES cells and was greatly enhanced by TSH treatment during the first 2-4 d of differentiation. With the support of Matrigel, highly enriched ES cell-derived GFP-positive cells formed thyroid follicle-like clusters in a serum-free medium supplemented with TSH. Importantly, these clusters display the characteristics of thyroid follicular cells. Immunofluorescent studies confirmed the colocalization of TSHR with the Na+/I- symporter in the clusters and indicated that Na+/I- symporter was expressed exclusively in the plasma membrane. In addition, I- uptake activity was observed in these cells. Our results indicate that ES cells can be induced to differentiate into thyroid follicular cells, providing a powerful tool to study embryonic thyroid development and function.

Cell Replacement Therapy in the Inner Ear

The mammalian inner ear is vulnerable to genetic disorders and aging, as well as to injuries caused by overstimulation, ototoxic drugs, and viral infections. Due to the poor regeneration of the sensory epithelium and the spiral ganglion neurons in the adult mammalian inner ear, cell replacement therapy strategies have been proposed to compensate for degeneration and loss of sensory and neuronal cells. Transplantation of stem cells and embryonic neurons into the inner ear has revealed that exogenous cells can survive, migrate, differentiate, and extend neuritic projections in the auditory system of adult mammals. These results suggest that cell replacement therapy could provide an effective future treatment alternative for hearing loss and other inner ear disorders.

Transplantation of human neural precursor cells in the 6-OHDA lesioned rats: Effect of immunosuppression with cyclosporine A

Neural precursor cells (NPC) may provide a source for restaurative therapy. We wanted to study the immunogenic potential of human NPC. We transplanted human NPCs with or without cyclosporine A (10 mg/kg) expanded in serum-free conditions into the striatum of rats unilaterally lesioned with 6-hydroxydopamine. Four months after transplantation, there was significant improvement of amphetamine-induced rotational behavior 9 non-immunosuppressed (13.1+/-4.9 pre vs 8.5+/-4.0 after grafting) but nor for 11 animals immunosuppressed with CyA (12.3+/-1.7 vs 11.3+/-2.8). The number of TH-IR cells was comparable in both groups (1,580+/-700 vs 1,274+/-295). All grafted animals only showed mild activation of astrocytes and macrophages within the graft. There was no evidence for tumor formation. Immunosuppression of rats, xenotransplanted with human NPC did not improve graft survival or function.

Developmental Perspectives on Human Midbrain-Derived Neural Stem Cells

Regeneration or restoration of lost or damaged neurons is very likely to profoundly alter the disability and needs of many patients. The replacement of dopaminergic (DA) neurons in patients with Parkinson's disease via implantation of embryonic midbrain tissue was taken from animal experiments to clinical applications. Ethical concerns related to the use of fetal tissue derived from abortions further argue for the search for alternative tissue sources. Today, it seems possible to generate functional DA neurons from a variety of stem cells, including embryonic and neural stem cells. Bone marrow stromal cells are another source for cell replacement. Neural stem cells derived from human fetal midbrain tissue maintain a considerable capacity to self-renew and to differentiate into DA neurons. Therefore, these cells may be a promising source to generate functional human DA neurons.

A method for a more complete in vitro Parkinson's model: slice culture bioassay for modeling maintenance and repair of the nigrostriatal circuit

Slice culture model systems provide a unique opportunity to monitor and lesion brain circuits in a dish. Using a novel approach, we have generated parasagittal slices from mouse brains that preserve, throughout the culture process, the nigrostriatal circuit. These slices can be cultured for approximately 4 weeks with maintenance of normal neuronal cytoarchitecture. Application of the dopamine specific toxin 6-hydroxy dopamine (6-OHDA) induces a significant decline in tyrosine hydroxylase positive cell bodies and fibers. Using a transgenic mouse with green fluorescent protein (GFP) under the control of the tyrosine hydroxylase promoter, we have been able to visualize in real time the loss of GFP expression in the striatum of slices as a result of 6-OHDA exposure. Using these cultures we have demonstrated the feasibility of modeling cellular replacement strategies. GFP-positive embryonic stem cell-derived neuronal precursors can be tracked in real time throughout the experiment and are amenable to patch clamp recording within the slice environment. In addition, cell differentiation can be observed within these slices and the effects of morphogenetic proteins, like the extracellular matrix molecule laminin, drugs or small molecules can be observed. This unique culture system presents a new approach for modeling Parkinson's disease in vitro, and provides a potentially useful new method for screening cell and molecular therapies for neurodegenerative diseases.

Selection of neural differentiation-specific genes by comparing profiles of random differentiation

Differentiation of embryonic stem cells (ESCs) into neurons requires a high level of transcriptional regulation. To further understand the transcriptional regulation of neural differentiation of ESCs, we used oligonucleotide microarray to examine the gene expressions of the guided differentiation (GD) model for dopaminergic (DA) neurons from mouse ESCs. We also determined the gene expression profiles of the random differentiation (RD) model of mouse ESCs into embryoid bodies. From K-means clustering analysis using the expression patterns of the two models, most of the genes (1,282 of 1,884 genes [68.0%]) overlapped in their expression patterns. Six hundred twenty-two differentially expressed genes (DEGs) from the GD model by random variance F-test were classified by their critical molecular functions in neurogenesis and DNA replication (Gene Ontology analysis). However, 400 genes among GD-DEGs (64.3%) showed a high correlation with RD in Spearman's correlation analysis (Spearman's coefficient p(s) >or= .6). The genes showing marginal correlation (-.4 < p(s) < .6) were present in the early stages of differentiation of both GD and RD, which were non-specific to brain development. Finally, we distinguished 66 GD-specific genes based on p(s) <or= -.4, the molecular functions of which were related mainly to vesicle formation, neurogenesis, and transcription factors. From among these GD-specific genes, we confirmed the expression of Serpini1 and Rab33a in P19 differentiation models and adult brains. From these results, we identified the specific genes required for neural differentiation by comparing gene expressions of GD with RD; these would potentially be the highly specific candidate genes necessary for differentiation of DA neurons.

In vitro induction of neural differentiation of embryonic stem (ES) cells closely mimics molecular mechanisms of embryonic brain development

The capacity of pluripotent embryonic stem cells (ES cells) to proliferate and differentiate makes them promising tools in the field of cell therapy. In spite of the controversy surrounding the numerous ethical questions raised by this technology, it has been shown to have therapeutic potential for heart, lung, liver, bone and connective tissue regeneration. In addition, a very attractive aspect of this technology is its potential for the treatment of cerebral pathology. A number of studies using ES cell transplants report the differentiation of ES cells in the brain or spinal cord of rodents, and the improvement of locomotor and/or cognitive deficits caused by brain injury. This review offers a synthesis of recent advances in the field of both human and rodent stem cell manipulation to select populations of neurons, astrocytes and oligodendrocytes. In parallel, this review emphasizes the striking similarities that exist between genetically programmed embryonic development of the nervous system and the differentiation of ES cells in vitro.

The production and directed differentiation of human embryonic stem cells

Human embryonic stem cells (hESCs) are being rapidly produced from chromosomally euploid, aneuploid, and mutant human embryos that are available from in vitro fertilization clinics treating patients for infertility or preimplantation genetic diagnosis. These hESC lines are an important resource for functional genomics, drug screening, and, perhaps eventually, cell and gene therapy. The methods for deriving hESCs are well established and repeatable and are relatively successful with a ratio of 1:10 to 1:2 new hESC lines produced from 4- to 8-d-old morula and blastocysts and from isolated inner cell mass cell clusters of human blastocysts. The hESCs can be formed and maintained on human somatic cells in humanized serum-free culture conditions and for several passages in cell-free culture systems. The hESCs can be transfected with DNA constructs. Their gene expression profiles are being described and immunological characteristics determined. They may be grown indefinitely in vitro while maintaining their original karyotype and epigenetic status, but this needs to be confirmed from time to time in long-term cultures. hESCs spontaneously differentiate in the absence of the appropriate cell feeder layer, when overgrown in culture and when isolated from the ESC colony. All three major embryonic lineages are produced in differentiating flat attachment cultures and unattached embryoid bodies. Cell progenitors of interest can be identified by markers, expression of reporter genes, and characteristic morphology, and the cells thereafter enriched for progenitor types and further culture to more mature cell types. Directed differentiation systems are well developed for ectodermal pathways that result in neural and glial cells and the mesendodermal pathway for cardiac muscle cells and many other cell types including hematopoietic progenitors and endothelial cells. Directed differentiation into endoderm has been more difficult to achieve, perhaps because of the lack of markers of early progenitors in this lineage. There are reports of enriched cultures of keratinocytes, pigmented retinal epithelium, neural crest cells and motor neurons, hepatic progenitors, and cells that have some markers of gut tissue and pancreatic islet-like cells. The prospects for use of hESC derivatives in regenerative medicine are significant, and there is much optimism for their potential contributions to human regenerative medicine.

Neural stem cell systems: diversities and properties after transplantation in animal models of diseases

Currently available effective treatments of the diseased or damaged central nervous system (CNS) are restricted to a limited pharmacological relief of symptoms or those given to avoid further damage. Therefore the search is on for treatments that can restore function in the CNS. During recent years replacement of damaged neurons by cell transplantation is being enthusiastically explored as a potential treatment for many neurodegenerative diseases, stroke and traumatic brain injury. Several references in both scientific journals and popular newspapers concerning different types of cultured stem cells, potentially exploitable to treat pathological conditions of the brain, raise important questions pertinent to the fundamental and realistic differences between grafts of primary neural cells and the transplantation of in vitro expanded neural stem cells (NSCs). Our aim is to review the available information on the grafting of different NSC types into the adult rodent brain, focusing on critical aspects for the development of clinical therapies to replace damaged neurons.

Transplantation of human embryonic stem cell-derived cells to a rat model of Parkinson's disease: effect of in vitro differentiation on graft survival and teratoma formation

Human embryonic stem cells (hESCs) have been proposed as a source of dopamine (DA) neurons for transplantation in Parkinson's disease (PD). We have investigated the effect of in vitro predifferentiation on in vivo survival and differentiation of hESCs implanted into the 6-OHDA (6-hydroxydopamine)-lesion rat model of PD. The hESCs were cocultured with PA6 cells for 16, 20, or 23 days, leading to the in vitro differentiation into DA neurons. Grafted hESC-derived cells survived well and expressed neuronal markers. However, very few exhibited a DA neuron phenotype. Reversal of lesion-induced motor deficits was not observed. Rats grafted with hESCs predifferentiated in vitro for 16 days developed severe teratomas, whereas most rats grafted with hESCs predifferentiated for 20 and 23 days remained healthy until the end of the experiment. This indicates that prolonged in vitro differentiation of hESCs is essential for preventing formation of teratomas.

Human embryonic stem cell therapy for Parkinson's disease

Human embryonic stem cells (hESCs) may serve as the most enduring source of transplantable cells for Parkinson's disease patients. Accumulating experience in the transplantation of fetal midbrain tissue or cells into Parkinson's disease patients has set the stage for hESC therapy, but has also opened new controversies on the value and appropriate design of cell therapy. hESCs can be directed to differentiate into nigral dopaminergic neurons with high efficiency. The clinical use of hESCs will depend on their growth in controlled conditions, on whether safety can be proven, and on improving the survival of hESC-derived dopaminergic neurons in the host brain.

Characterization of a New NIH-Registered Variant Human Embryonic Stem Cell Line, BG01V: A Tool for Human Embryonic Stem Cell Research

Human embryonic stem cells (hESCs) offer a renewable source of a wide range of cell types for use in research and cell-based therapies. Characterizing these cells provides important information about their current state and affords relevant details for subsequent manipulations. For example, identifying genes expressed during culture, as well as their temporal expression order after passaging and conditions influencing the formation of all three germ layers may be helpful for the production of functional beta islet cells used in treating type I diabetes. Although several hESC lines have demonstrated karyotypic instability during extended time in culture, select variant lines exhibit characteristics similar to their normal parental lines. Such variant lines may be excellent tools and abundant sources of cells for pilot studies and in vitro differentiation research in which chromosome number is not a concern, similar to the role currently played by embryonal carcinoma cell lines. It is crucial that the cells be surveyed at a genetic and proteomic level during extensive propagation, expansion, and manipulation in vitro. Here we describe a comprehensive characterization of the variant hESC line BG01V, which was derived from the karyotypically normal, parental hESC line BG01. Our characterization process employs cytogenetic analysis, short tandem repeat and HLA typing, mitochondrial DNA sequencing, gene expression analysis using quantitative reverse transcription-polymerase chain reaction and microarray, assessment of telomerase activity, methylation analysis, and immunophenotyping and teratoma formation, in addition to screening for bacterial, fungal, mycoplasma, and human pathogen contamination.

Cooperative transcription activation by Nurr1 and Pitx3 induces embryonic stem cell maturation to the midbrain dopamine neuron phenotype

Midbrain dopamine (DA) neurons play a central role in the regulation of voluntary movement, and their degeneration is associated with Parkinson's disease. Cell replacement therapies, and in particular embryonic stem (ES) cell-derived DA neurons, offer a potential therapeutic venue for Parkinson's disease. We sought to identify genes that can potentiate maturation of ES cell cultures to the midbrain DA neuron phenotype. A number of transcription factors have been implicated in the development of midbrain DA neurons by expression analyses and loss-of-function knockout mouse studies, including Nurr1, Pitx3, Lmx1b, Engrailed-1, and Engrailed-2. However, none of these factors appear sufficient alone to induce the mature midbrain DA neuron phenotype in ES cell cultures in vitro, suggesting a more complex regulatory network. Here we show that Nurr1 and Pitx3 cooperatively promote terminal maturation to the midbrain DA neuron phenotype in murine and human ES cell cultures.

NTera2: a model system to study dopaminergic differentiation of human embryonic stem cells

NTera2, a human embryonal carcinoma (EC) stem cell line, shares many characteristics with human embryonic stem cells (hESCs). To determine whether NTera2 can serve as a useful surrogate for hESCs, we compared global gene expression between undifferentiated NTera2, multiple undifferentiated hESC cell lines, and their differentiated derivatives, and we showed that NTera2 cells share multiple markers with hESCs. Similar to hESCs, NTera2 cells differentiated into TH-positive cells that express dopaminergic markers including AADC, DAT, Nurr1, TrkB, TrkC, and GFRA1 when co-cultured with PA6 cells. Flow cytometry analysis showed that tyrosine hydroxylase (TH) and neural cell adhesion molecule (NCAM) expression increased, whereas SSEA4 expression decreased as cells differentiated. Medium conditioned by PA6 cells stimulated differentiation of NTera2 cells to generate TH-positive cells that expressed dopaminergic markers. Flow cytometry selected polysialylated (PSA-NCAM) cells responded to medium conditioned by PA6 cells by differentiating into TH-positive cells and expressed dopaminergic markers. Sorted cells differentiated for 4 weeks in PA6 cell conditioned media included functional neurons that responded to neurotransmitters and exhibited electronic excitability. Therefore, NTera2 cell dopaminergic neuronal differentiation and PSA-NCAM enrichment provides a useful system for the future study of hESCs.

Generation of graftable dopaminergic neuron progenitors from mouse ES cells by a combination of coculture and neurosphere methods

Parkinson's disease is characterized by a loss of midbrain dopamine (DA) neurons and is generally viewed as a potential target for stem cell therapy. Although several studies have reported the generation of postmitotic DA neurons from embryonic stem (ES) cells, it is unknown whether the proliferative progenitors of DA neurons can be isolated in vitro. To investigate this possibility, we have developed a combined approach in which ES cells are cocultured with PA6 stromal cells to expose them to stromal cell-derived inducing activity (SDIA) and are then cultured as neurospheres. Mouse ES cell colonies were detached from PA6 feeder cells after 8 days of SDIA treatment and then expanded as spheres for another 4 days in serum-free medium supplemented with fibroblast growth factor-2. The spheres exhibited neural stem cell characteristics and contained few DA neurons at this stage of culture. After being induced to differentiate on polyornithine/laminin-coated dishes for 7 days, these spheres generated DA neurons in vitro at a relatively low frequency. Intriguingly, addition of PA6 cell conditioned medium to the sphere culture medium significantly increased the percentage of DA neurons to 25-30% of the total number of neurons. Transplantation of conditioned medium-treated day 4 spheres, which contained DA neuron progenitors, into the mouse striatum resulted in the generation of a significant number of graft-derived DA neurons. These findings suggest that progenitors of DA neurons are generated and can proliferate in ES cell-derived neurospheres induced by serial SDIA and PA6 conditioned medium treatment.

A focused microarray to assess dopaminergic and glial cell differentiation from fetal tissue or embryonic stem cells

We designed oligonucleotide gene-specific probes to develop a focused array that can be used to discriminate between neural phenotypes, identify biomarkers, and provide an overview of the process of dopaminergic neuron and glial differentiation. We have arrayed approximately 100 genes expressed in dopaminergic neurons, oligodendrocytes, and astrocytes, an additional 200 known cytokines, chemokines, and their respective receptors, as well as markers for pluripotent and progenitor cells. The gene-specific 60-mer 3' biased oligonucleotides for these 281 genes were arrayed in a 25 x 12 format based on function. Using human adult brain substantia nigra, human embryonic stem cells (ESCs), and the differentiated progeny of pluripotent cells, we showed that this array was capable of distinguishing dopaminergic neurons, glial cells, and pluripotent cells by their gene expression profiles in a concentration-dependent manner. Using linear correlation coefficients of input RNA with output intensity, we identified a list of genes that can serve as reporting genes for detecting dopaminergic neurons, glial cells, and contaminating ESCs and progenitors. Finally, we monitored NTera2 differentiation toward dopaminergic neurons and have shown the ability of this array to distinguish stages of differentiation and provide important clues to factors regulating differentiation, the degree of contaminating populations, and stage of cell maturity. We suggest that this focused array will serve as a useful complement to other large-scale arrays in routine assessment of cell properties prior to their therapeutic use.

Generation, culture, and differentiation of human embryonic stem cells for therapeutic applications

Embryonic stem (ES) cells, derived from the inner cell mass of the mammalian blastocyst, can continuously proliferate in an undifferentiated state and can also be induced to differentiate into a desired cell lineage. These abilities make ES cells an appealing source for cell replacement therapies, the study of developmental biology, and drug/toxin screening studies. As compared to mouse ES cells, human ES cells have only recently been derived and studied. Although there are many differences in properties between mouse and human ES cells, the study of mouse ES cells has provided important insights into human ES cell research. In this review, we describe the advantages and disadvantages of methods used for human ES cell derivation, the expansion of human ES cells, and the current status of human ES cell differentiation research. In addition, we discuss the endeavor that scientists have undertaken toward the therapeutic application of these cells, which includes therapeutic cloning and the improvement of human ES cell culture conditions.

Directed differentiation of dopaminergic neuronal subtypes from human embryonic stem cells

How dopamine (DA) neuronal subtypes are specified remains unknown. In this study we show a robust generation of functional DA neurons from human embryonic stem cells (hESCs) through a specific sequence of application of fibroblast growth factor 8 (FGF8) and sonic hedgehog (SHH). Treatment of hESC-derived Sox1+ neuroepithelial cells with FGF8 and SHH resulted in production of tyrosine hydroxylase (TH)-positive neurons that were mostly bipolar cells, coexpression with gamma-aminobutyric acid, and lack of midbrain marker engrailed 1 (En1) expression. However, FGF8 treatment of precursor cells before Sox1 expression led to the generation of a similar proportion of TH+ neurons characteristic of midbrain projection DA neurons with large cell bodies and complex processes and coexpression of En1. This suggests that one mechanism of generating neuronal subtypes is temporal availability of morphogens to a specific group of precursors. The in vitro-generated DA neurons were electrophysiologically active and released DA in an activity-dependent manner. They may thus provide a renewable source of functional human DA neurons for drug screening and development of sustainable therapeutics for disorders affecting the DA system.

Stromal cell-derived inducing activity, Nurr1, and signaling molecules synergistically induce dopaminergic neurons from mouse embryonic stem cells

To induce differentiation of embryonic stem cells (ESCs) into specialized cell types for therapeutic purposes, it may be desirable to combine genetic manipulation and appropriate differentiation signals. We studied the induction of dopaminergic (DA) neurons from mouse ESCs by overexpressing the transcription factor Nurr1 and coculturing with PA6 stromal cells. Nurr1-expressing ESCs (N2 and N5) differentiated into a higher number of neurons (approximately twofold) than the naïve ESCs (D3). In addition, N2/N5-derived cells contained a significantly higher proportion (>50%) of tyrosine hydroxylase (TH)+ neurons than D3 (<30%) and an even greater proportion of TH+ neurons (approximately 90%) when treated with the signaling molecules sonic hedgehog, fibroblast growth factor 8, and ascorbic acid. N2/N5-derived cells express much higher levels of DA markers (e.g., TH, dopamine transporter, aromatic amino acid decarboxylase, and G protein-regulated inwardly rectifying K+ channel 2) and produce and release a higher level of dopamine, compared with D3-derived cells. Furthermore, the majority of generated neurons exhibited electrophysiological properties characteristic of midbrain DA neurons. Finally, transplantation experiments showed efficient in vivo integration/generation of TH+ neurons after implantation into mouse striatum. Taken together, our results show that the combination of genetic manipulation(s) and in vitro cell differentiation conditions offers a reliable and effective induction of DA neurons from ESCs and may pave the way for future cell transplantation therapy in Parkinson's disease.

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.

Characterization and culture of human embryonic stem cells

Human embryonic stem cells have been defined as self-renewing cells that can give rise to many types of cells of the body. How and whether these cells can be manipulated to replace cells in diseased tissues, used to screen drugs and toxins, or studied to better understand normal development, however, depends on knowing more about their fundamental properties. Many different human embryonic stem cell lines--which are pluripotent, proliferate indefinitely in vitro and maintain a normal, euploid karyotype over extended culture--have now been derived, but whether these cell lines are in fact equivalent remains unclear. It will therefore be important to define robust criteria for the assessment of both existing and newly derived cell lines and for the validation of new culture conditions.