Markers and methods for cell sorting of human embryonic stem cell-derived neural cell populations

Highly efficient differentiation and enrichment of spinal motor neurons derived from human and monkey embryonic stem cells

Background

There are no cures or efficacious treatments for severe motor neuron diseases. It is extremely difficult to obtain naïve spinal motor neurons (sMNs) from human tissues for research due to both technical and ethical reasons. Human embryonic stem cells (hESCs) are alternative sources. Several methods for MN differentiation have been reported. However, efficient production of naïve sMNs and culture cost were not taken into consideration in most of the methods.

Methods/Principal Findings

We aimed to establish protocols for efficient production and enrichment of sMNs derived from pluripotent stem cells. Nestin+ neural stem cell (NSC) clusters were induced by Noggin or a small molecule inhibitor of BMP signaling. After dissociation of NSC clusters, neurospheres were formed in a floating culture containing FGF2. The number of NSCs in neurospheres could be expanded more than 30-fold via several passages. More than 33% of HB9+ sMN progenitor cells were observed after differentiation of dissociated neurospheres by all-trans retinoic acid (ATRA) and a Shh agonist for another week on monolayer culture. HB9+ sMN progenitor cells were enriched by gradient centrifugation up to 80% purity. These HB9+ cells differentiated into electrophysiologically functional cells and formed synapses with myotubes during a few weeks after ATRA/SAG treatment.

Conclusions and Significance

The series of procedures we established here, namely neural induction, NSC expansion, sMN differentiation and sMN purification, can provide large quantities of naïve sMNs derived from human and monkey pluripotent stem cells. Using small molecule reagents, reduction of culture cost could be achieved.

Pericellular matrix of decidua-derived mesenchymal cells: a potent human-derived substrate for the maintenance culture of human ES cells

In routine culture, human embryonic stem (hES) cells are maintained on either feeder cells or special culture substrates such as Matrigel. However, to expand hES cells for clinical applications, it is desirable to minimize animal-derived materials in the culture for safety reasons. In this report, we show that the pericellular matrix prepared from human decidua-derived mesenchymal cells (PCM-DM) is a potent substrate material that supports the growth and pluripotency of hES cells as efficiently as Matrigel does. This supporting activity of PCM-DM is stable and can be preserved for several months in the refrigerator. PCM-DM-based culture is compatible with non-conditioned commercial defined medium, and with the maintenance of dissociated hES cells in the presence of ROCK inhibitor. Since decidual mesenchymal cells can be prepared and expanded in a large quantity, PCM-DM is a practical human-derived substitute for the animal-derived substrates for use in clinical-grade culture of hES cells.

Musings on genome medicine: is there hope for ethical and safe stem cell therapeutics?

Although most stem cell therapy has been non-controversial, therapy based on pluripotent stem cells has raised both ethical and safety concerns. Despite these concerns, the use of cells derived from pluripotent stem cells has recently been approved for clinical trials. We suggest that recent advances in the field have provided avenues to develop pluripotent cells that raise far fewer ethical concerns. Moreover, advances in cell sorting, gene modification and screening have allowed the development of safer therapeutic approaches. Continued advances in this rapidly evolving field are likely to allow therapy to be delivered in a safe and effective manner without socially divisive ethical controversy in the not-so-distant future.

Brain-specific 1B promoter of FGF1 gene facilitates the isolation of neural stem/progenitor cells with self-renewal and multipotent capacities

Fibroblast growth factor 1 (FGF1) has been shown to maintain proliferation and self-renewal capacities of neural stem/progenitor cells (NSPCs) in vitro. We have previously identified FGF1B as the major transcript of FGF1 gene expressed exclusively in brain areas that are known to be abundant for NSPCs in vivo. The 540-bp (-540 to +31) sequence upstream of the 1B transcription start site (F1B) is sufficient to drive the expression of a heterologous luciferase reporter in cultured cells. In this study, we report a direct genetic and functional approach to isolate F1B(+) NSPCs using green fluorescent protein (GFP) reporter gene under the control of human F1B promoter. The F1B-GFP reporter could facilitate the isolation of NSPCs with self-renewal and multipotent capacities from human glioblastoma tissues, developing or adult mouse brains by fluorescence-activated cell sorting. Future work elucidating the mechanisms that control FGF1B expression will help to identify new NSPC-related genes.

Identification of transplantable dopamine neuron precursors at different stages of midbrain neurogenesis

Protocols used for generation of mesencephalic dopamine (mesDA) neurons from stem cells, or fetal brain tissue, invariably result in cell preparations that are highly mixed in composition, containing mesDA neuron precursors in various states of fate commitment and differentiation. For further optimisation and refinement of these procedures it is essential to determine the optimal stage of development and phenotypic characteristics of cells used for grafting. We have used fluorescence-activated cell sorting procedures to isolate mesDA precursors in defined stages of differentiation from mouse ventral mesencephalon (VM), at embryonic day 10.5 (E10.5), when the mesDA neuron domain consists of proliferative radial glia-like cells expressing the mesDA neuron determinant Lmx1a and the floorplate marker Corin, and at E12.5, when the VM has expanded to comprise a mixture of proliferative progenitors, neuroblasts and young neurons. The sorted cells were transplanted to the striatum of 6-hydroxydopamine-lesioned rats. Results show that the Lmx1a/Corin-expressing ventricular zone progenitors, which are the source of mesDA neurons in grafts from E10.5 VM, had lost this capacity at E12.5. At this later stage all transplantable mesDA precursors resided in the intermediate zone as postmitotic Nurr1-expressing neuroblasts. The more differentiated, TH-expressing cells survived sorting and transplantation poorly. We also provide evidence that, during early mesDA neurogenesis, the progenitors for nigral mesDA neurons segregate to lateral parts of the Lmx1a-expressing domain and can be selectively isolated based on their level of Corin expression. These results have implications for current efforts to develop well-characterized stem cell-derived mesDA progenitor cell preparations for cell therapy.

High-efficiency transient transduction of human embryonic stem cell-derived neurons with baculoviral vectors

Transient genetic manipulation of human neurons without chromosomal integration of the transgene would be valuable but has been challenging due to the quiescent nature of these postmitotic cells. In this study, we developed a set of baculoviral vectors for transient transduction in nondividing neurons derived from human embryonic stem cells (hESCs). Using a baculoviral vector equipped with the woodchuck hepatitis virus posttranscriptional regulatory element (WPRE), we observed a quick onset of transgene expression as early as day 1 after baculoviral transduction and a high efficiency of up to 80%. Strong transgene expression in the cultured human neurons was observed for more than 1 month and the signal was easily detectable even after 3 months. Using two baculoviral vectors carrying different transgenes, we found that co-transduction at a single neuron level was possible. After transplantation into the brain of nude mice, the baculovirus-transduced human neurons were integrated into the mouse brain and maintained transgene expression for at least 4 weeks, portending the usefulness of this technique in assisting neural transplantation. Therefore, by mediating efficient transient gene expression, baculoviral vectors can provide useful tools for both basic gene function studies in human neurons and therapeutic applications of these cells.

Manufacturing neurons from human embryonic stem cells: biological and regulatory aspects to develop a safe cellular product for stroke cell therapy

Demographic trends, particularly those related to longer life expectancy, suggest that the demand for tissue and organ transplants will further increase since many disorders result from degeneration, injury or organ failure. The most urgent problem in transplantation medicine is the shortage or lack of suitable donor organs and tissue, leading to ethical and societal problems such as organ trafficking. The discovery of stem cells in the inner cell mass of developing embryos and in adult tissue has revolutionized the medical field by introducing new therapeutic dimensions to consider for previously untreatable diseases and injuries. The unlimited self-renewal ability and pluripotent capacity to become any cell type of the organism make human embryonic stem cells (hESCs) a compelling source of cells to study tissue histogenesis and to apply in a wide array of tissue engineering, cell transplantation therapy and drug discovery applications. In this article, we will focus on hESCs and address the derivation of therapeutic neural stem cell lines from hESCs, as well as the biological and regulatory aspects to developing a safe cellular product for stroke cell therapy.

Strategies for future histocompatible stem cell therapy

Stem cell therapy based on the safe and unlimited self-renewal of human pluripotent stem cells is envisioned for future use in tissue or organ replacement after injury or disease. A gradual decline of regenerative capacity has been documented among the adult stem cell population in some body organs during the aging process. Recent progress in human somatic cell nuclear transfer and inducible pluripotent stem cell technologies has shown that patient-derived nuclei or somatic cells can be reprogrammed in vitro to become pluripotent stem cells, from which the three germ layer lineages can be generated, genetically identical to the recipient. Once differentiation protocols and culture conditions can be defined and optimized, patient-histocompatible pluripotent stem cells could be directed towards virtually every cell type in the human body. Harnessing this capability to enrich for given cells within a developmental lineage, would facilitate the transplantation of organ/tissue-specific adult stem cells or terminally differentiated somatic cells to improve the function of diseased organs or tissues in an individual. Here, we present an overview of various experimental cell therapy technologies based on the use of patient-histocompatible stem cells, the pending issues needed to be dealt with before clinical trials can be initiated, evidence for the loss and/or aging of the stem cell pool and some of the possible uses of human pluripotent stem cell-derivatives aimed at curing disease and improving health.

Immune physiology in tissue regeneration and aging, tumor growth, and regenerative medicine

The immune system plays an important role in immunity (immune surveillance), but also in the regulation of tissue homeostasis (immune physiology). Lessons from the female reproductive tract indicate that immune system related cells, such as intraepithelial T cells and monocyte-derived cells (MDC) in stratified epithelium, interact amongst themselves and degenerate whereas epithelial cells proliferate and differentiate. In adult ovaries, MDC and T cells are present during oocyte renewal from ovarian stem cells. Activated MDC are also associated with follicular development and atresia, and corpus luteum differentiation. Corpus luteum demise resembles rejection of a graft since it is attended by a massive influx of MDC and T cells resulting in parenchymal and vascular regression. Vascular pericytes play important roles in immune physiology, and their activities (including secretion of the Thy-1 differentiation protein) can be regulated by vascular autonomic innervation. In tumors, MDC regulate proliferation of neoplastic cells and angiogenesis. Tumor infiltrating T cells die among malignant cells. Alterations of immune physiology can result in pathology, such as autoimmune, metabolic, and degenerative diseases, but also in infertility and intrauterine growth retardation, fetal morbidity and mortality. Animal experiments indicate that modification of tissue differentiation (retardation or acceleration) during immune adaptation can cause malfunction (persistent immaturity or premature aging) of such tissue during adulthood. Thus successful stem cell therapy will depend on immune physiology in targeted tissues. From this point of view, regenerative medicine is more likely to be successful in acute rather than chronic tissue disorders.

Separation of SSEA-4 and TRA-1-60 labelled undifferentiated human embryonic stem cells from a heterogeneous cell population using magnetic-activated cell sorting (MACS) and fluorescence-activated cell sorting (FACS)

A major concern in human embryonic stem cell (hESC)-derived cell replacement therapy is the risk of tumorigenesis from undifferentiated hESCs residing in the population of hESC-derived cells. Separation of these undifferentiated hESCs from the differentiated derivatives using cell sorting methods may be a plausible approach in overcoming this problem. We therefore explored magnetic activated cell sorting (MACS) and fluorescence activated cell sorting (FACS) to separate labelled undifferentiated hESCs from a heterogeneous population of hESCs and hepatocellular carcinoma cells (HepG2) deliberately mixed respectively at different ratios (10:90, 20:80, 30:70, 40:60 and 50:50) to mimic a standard in vitro differentiation protocol, instead of using a hESC-differentiated cell population, so that we could be sure of the actual number of cells separated. HES-3 and HES-4 cells were labelled in separate experiments for the stem cell markers SSEA-4 and TRA-1-60 using primary antibodies. Anti-PE magnetic microbeads that recognize the PE-conjugated SSEA-4 labelled hESCs was added to the heterogeneous cell mixture and passed through the MACS column. The cells that passed through the column ('flow-through' fraction) and those retained ('labelled' fraction') were subsequently analysed using FACS. The maximum efficacy of hESCs retention using MACS was 81.0 +/- 2.9% (HES-3) and 83.6 +/- 4.2% (HES-4). Using FACS, all the undifferentiated hESCs labelled with the two cell-surface markers could be removed by selective gating. Both hESCs and HepG2 cells in the 'flow-through' fraction following MACS separation were viable in culture whereas by FACS separation only the HepG2 cells were viable. FACS efficiently helps to eliminate the undifferentiated hESCs based on their cell-surface antigens expressed.

Molecular and cellular determinants for generating ES-cell derived dopamine neurons for cell therapy

Embryonic stem (ES) cells can generate midbrain dopaminergic (DA) neuronal phenotypes in vitro and have been successfully applied to restore function in animal models of Parkinson's disease (PD). How can we best integrate our growinginsight into the regulatory cascade of transcription factors guiding midbrain specification to further improve the in vitro differentiation of midbrain DA neurons for cell therapy of PD? To characterize the differentiation of authentic DA neurons in vitro, expression patterns of the numerous midbrain-characteristic markers need to be investigated. When using forced gene expression, such factors have to be closely monitored to avoid generation of nonphysiological cell types. Fluorescent markers such as Pitx3-GFP, TH-GFP, Sox1-GFP or surface antigens have proven useful for elimination of unwanted cell types by cell sorting, thereby averting tumors and increasing the DA fraction for transplantation studies. The importance of appropriate timing during application of extrinsic factors and the influence of cell-cell interactions in the dish has to be taken into account. This conceptual synopsis outlines current objectives, progress, but also challenges, in deriving midbrain DA neurons from pluripotent stem cells for clinical and scientific applications.

Transgenic reporter mice as tools for studies of transplantability and connectivity of dopamine neuron precursors in fetal tissue grafts

Cell therapy for Parkinson's disease (PD) is based on the idea that new midbrain dopamine (mDA) neurons, implanted directly into the brain of the patient, can structurally and functionally replace those lost to the disease. Clinical trials have provided proof-of-principle that the grafted mDA neurons can survive and function after implantation in order to provide sustained improvement in motor function for some patients. Nonetheless, there are a number of issues limiting the application of this approach as mainstream therapy, including: the use of human fetal tissue as the only safe and reliable source of transplantable mDA neurons, and variability in the therapeutic outcome. Here we review recent progress in this area from investigations using rodent models of PD, paying particular attention to the use of transgenic reporter mice as tools for neural transplantation studies. Cell type-specific expression of reporter genes, such as green fluorescent protein, affords valuable technical advantages in transplantation experiments, such as the ability to selectively isolate specific cell fractions from mixed populations prior to grafting, and the unambiguous visualization of graft-derived dopamine neuron fiber patterns after transplantation. The results from these investigations have given new insights into the transplantability of mDA precursors as well as their connectivity after grafting and have interesting implications for the development of stem cell based approaches for the treatment of PD.

Isolation and enrichment of stem cells

Stem cells have the potential to revolutionize tissue regeneration and engineering. Both general types of stem cells, those with pluripotent differentiation potential as well as those with multipotent differentiation potential, are of equal interest. They are important tools to further understanding of general cellular processes, to refine industrial applications for drug target discovery and predictive toxicology, and to gain more insights into their potential for tissue regeneration. This chapter provides an overview of existing sorting technologies and protocols, outlines the phenotypic characteristics of a number of different stem cells, and summarizes their potential clinical applications.

The embryonic midbrain directs neuronal specification of embryonic stem cells at early stages of differentiation

Specific neuronal differentiation of Embryonic Stem Cells (ESCs) depends on their capacity to interpret environmental cues. At present, it is not clear at which stage of differentiation ESCs become competent to produce multiple neuronal lineages in response to the niche of the embryonic brain. To unfold the developmental potential of ESC-derived precursors, we transplanted these cells into the embryonic midbrain explants, where neurogenesis occurs as in normal midbrain development. Using this experimental design, we show that the transition from ESCs to Embryoid Body (EB) precursors is necessary to differentiate into Lmx1a(+)/Ptx3(+)/TH(+) dopaminergic neurons around the ventral midline of the midbrain. In addition, EB cells placed at other dorsal-ventral levels of the midbrain give rise to Nkx6.1(+) red nucleus (RN) neurons, Nkx2.2(+) ventral interneurons and Pax7(+) dorsal neurons at the correct positions. Notably, differentiation of ESCs into Neural Precursor Cells (NPCs) prior to transplantation markedly reduces specification at the Lmx1a, Nkx6.1 and Pax7 expression domains, without affecting neuronal differentiation. Finally, exposure to Fgf8 and Shh in vitro promotes commitment of some ESC-derived NPCs to differentiate into putative Lmx1a(+) dopaminergic neurons in the midbrain. Our data demonstrate intrinsic developmental potential differences among ESC-derived precursor populations.

Elucidating the phenomenon of HESC-derived RPE: anatomy of cell genesis, expansion and retinal transplantation

Healthy Retinal Pigment Epithelium (RPE) cells are required for proper visual function and the phenomenon of RPE derivation from Human Embryonic Stem Cells (HESC) holds great potential for the treatment of retinal diseases. However, little is known about formation, expansion and expression profile of RPE-like cells derived from HESC (HESC-RPE). By studying the genesis of pigmented foci we identified OTX1/2-positive cell types as potential HESC-RPE precursors. When pigmented foci were excised from culture, HESC-RPE expanded to form extensive monolayers, with pigmented cells at the leading edge assuming a precursor role: de-pigmenting, proliferating, expressing keratin 8 and subsequently re-differentiating. As they expanded and differentiated in vitro, HESC-RPE expressed markers of both developing and mature RPE cells which included OTX1/2, Pax6, PMEL17 and at low levels, RPE65. In vitro, without signals from a developing retinal environment, HESC-RPE could produce regular, polarised monolayers with developmentally important apical and basal features. Following transplantation of HESC-RPE into the degenerating retinal environment of Royal College of Surgeons (RCS) dystrophic rats, the cells survived in the subretinal space, where they maintained low levels of RPE65 expression and remained out of the cell cycle. The HESC-RPE cells responded to the in vivo environment by downregulating Pax6, while maintaining expression of other markers. The presence of rhodopsin-positive material within grafted HESC-RPE indicates that in the future, homogenous transplants of this cell type may be capable of supporting visual function following retinal dystrophy.

CD marker expression profiles of human embryonic stem cells and their neural derivatives, determined using flow-cytometric analysis, reveal a novel CD marker for exclusion of pluripotent stem cells

Human embryonic stem cells (hESCs) are pluripotent cells that can differentiate into neural cell lineages. These neural populations are usually heterogeneous and can contain undifferentiated pluripotent cells that are capable of producing teratomas in cell grafts. The characterization of surface protein profiles of hESCs and their neural derivatives is important to determine the specific markers that can be used to exclude undifferentiated cells from neural populations. In this study, we analyzed the cluster of differentiation (CD) marker expression profiles of seven undifferentiated hESC lines using flow-cytometric analysis and compared their profiles to those of neural derivatives. Stem cell and progenitor marker CD133 and epithelial adhesion molecule marker CD326 were more highly expressed in undifferentiated hESCs, whereas neural marker CD56 (NCAM) and neural precursor marker (chemokine receptor) CD184 were more highly expressed in hESC-derived neural cells. CD326 expression levels were consistently higher in all nondifferentiated hESC lines than in neural cell derivatives. In addition, CD326-positive hESCs produced teratomas in SCID mouse testes, whereas CD362-negative neural populations did not. Thus, CD326 may be useful as a novel marker of undifferentiated hESCs to exclude undifferentiated hESCs from differentiated neural cell populations prior to transplantation.

[Human embryonic stem cells. Perspectives for the study and therapy of neurological disorders]

The remarkable capability of human embryonic stem cells (hES cells) to differentiate into all somatic cell types and tissues opens promising perspectives for the development of novel therapeutic approaches for neurological disorders. This article provides an overview on the current state of research in this field. We present strategies and results on the generation of selected neural subtypes (dopaminergic neurons, retinal progenitors, motoneurons, oligodendrocytes) and discuss problems and risks associated with a potential clinical application of this novel cell source.

PA6-induced human embryonic stem cell-derived neurospheres: a new source of human peripheral sensory neurons and neural crest cells

Human embryonic stem cells (hESC) have been directed to differentiate into CNS cells with clinical importance. However, for study of development and regeneration of the human PNS, and peripheral neuropathies, it would be useful to have a source of human PNS derivatives. We have demonstrated that peripheral sensory neuron-like cells (PSN) can also be derived from hESC via neural crest-like (NC) intermediates, and from neural progenitors induced from hESC using noggin. Here we report the generation of higher purity PSN from passagable neurospheres (NSP) induced by murine PA6 stromal cells. hESC were cultured with PA6, and colonies that developed a specific morphology were cut from the plates. Culture of these colonies under non-adhesive conditions yielded NSPs. Several NC marker genes were expressed in the NSP, and these were also detected in 3-5week gestation human embryos containing migrating NC. These NSPs passaged for 2-8weeks and re-plated on PA6 gave rise to many Brn3a+/peripherin+ cells, characteristic of early sensory-like neurons. Re-culturing PA6-induced NSP cells with PA6 resulted in about 25% of the human cells in the co-cultures differentiating to PSN after 1week, compared to only about 10% PSN obtained after 3 weeks when noggin-induced NSP were used. Two month adherent cultures of PA6-induced NSP cells contained neurons expressing several PSN neuropeptides, and voltage-dependent currents and action potentials were obtained from a molecularly identified PSN. hESC-derived PA6-induced NSP cells are therefore an excellent potential source of human PSN for study of differentiation and modeling of PNS disease.

Lineage selection of functional and cryopreservable human embryonic stem cell-derived neurons

A major prerequisite for the biomedical application of human embryonic stem cells (hESC) is the derivation of defined and homogeneous somatic cell types. Here we present a human doublecortin (DCX) promoter-based lineage-selection strategy for the generation of purified hESC-derived immature neurons. After transfection of hESC-derived neural precursors with a DCX-enhanced green fluorescent protein construct, fluorescence-activated cell sorting enables the enrichment of immature human neurons at purities of up to 95%. Selected neurons undergo functional maturation and are able to establish synaptic connections. Considering that the applicability of purified hESC-derived neurons would largely benefit from an efficient cryopreservation technique, we set out to devise defined freezing conditions involving caspase inhibition, which yield post-thaw recovery rates of up to 83%. Combined with our lineage-selection procedure this cryopreservation technique enables the generation of human neurons in a ready-to-use format for a large variety of biomedical applications.

Recent advances in cell-based therapy for Parkinson disease

In this review, the authors discuss recent advances in the field of cell therapy for Parkinson disease (PD). They compare and contrast recent clinical trials using fetal dopaminergic neurons. They attribute differences in cell preparation techniques, cell type specification, and immunosuppression as reasons for variable outcome and for some of the side effects observed in these clinical trials. To address ethical, practical, and technical issues related to the use of fetal cell sources, alternative sources of therapeutic dopaminergic neurons are being developed. The authors describe the progress in enrichment and purification strategies of stem cell-derived dopaminergic midbrain neurons. They conclude that recent advances in cell therapy for PD will create a viable long-term treatment option for synaptic repair for this debilitating disease.

Tlx3 exerts context-dependent transcriptional regulation and promotes neuronal differentiation from embryonic stem cells

The T cell leukemia 3 (Tlx3) gene has been implicated in specification of glutamatergic sensory neurons in the spinal cord. In cranial sensory ganglia, Tlx3 is highly expressed in differentiating neurons during early embryogenesis. To study a role of Tlx3 during neural differentiation, mouse embryonic stem (ES) cells were transfected with a Tlx3 expression vector. ES cells stably expressing Tlx3 were grown in the presence or absence of a neural induction medium. In undifferentiated ES cells, there was no significant difference in gene expression in the presence or absence of Tlx3, even after ES cells were cultured for an extensive time period. In contrast, expression levels of Mash1, Ngn1, and NeuroD were significantly higher in Tlx3-expressing cells after neural induction for 4 days compared with those in cells expressing the control vector. At 7 days after neural induction, whereas expression of the proneural genes was down-regulated, VGLUT2, GluR2, and GluR4 were significantly increased in ES cell-derived neurons expressing Tlx3. The sequential and coordinated expression of the proneural and neuronal subtype-specific genes identifies Tlx3 as a selector gene in ES cells undergoing neural differentiation. In addition, the differential effects of Tlx3 overexpression in undifferentiated ES cells compared with ES cell-derived neurons suggest that Tlx3 exerts context-dependent transcriptional signals on its downstream target genes. The context-dependent function of Tlx3 as a selector gene may be used to establish a novel strategy to conditionally generate excitatory glutamatergic neurons from ES cells to cure various types of neurodegenerative disorders.

Neurons derived from reprogrammed fibroblasts functionally integrate into the fetal brain and improve symptoms of rats with Parkinson's disease

The long-term goal of nuclear transfer or alternative reprogramming approaches is to create patient-specific donor cells for transplantation therapy, avoiding immunorejection, a major complication in current transplantation medicine. It was recently shown that the four transcription factors Oct4, Sox2, Klf4, and c-Myc induce pluripotency in mouse fibroblasts. However, the therapeutic potential of induced pluripotent stem (iPS) cells for neural cell replacement strategies remained unexplored. Here, we show that iPS cells can be efficiently differentiated into neural precursor cells, giving rise to neuronal and glial cell types in culture. Upon transplantation into the fetal mouse brain, the cells migrate into various brain regions and differentiate into glia and neurons, including glutamatergic, GABAergic, and catecholaminergic subtypes. Electrophysiological recordings and morphological analysis demonstrated that the grafted neurons had mature neuronal activity and were functionally integrated in the host brain. Furthermore, iPS cells were induced to differentiate into dopamine neurons of midbrain character and were able to improve behavior in a rat model of Parkinson's disease upon transplantation into the adult brain. We minimized the risk of tumor formation from the grafted cells by separating contaminating pluripotent cells and committed neural cells using fluorescence-activated cell sorting. Our results demonstrate the therapeutic potential of directly reprogrammed fibroblasts for neuronal cell replacement in the animal model.

Dopamine neurons implanted into people with Parkinson's disease survive without pathology for 14 years

Postmortem analysis of five subjects with Parkinson's disease 9-14 years after transplantation of fetal midbrain cell suspensions revealed surviving grafts that included dopamine and serotonin neurons without pathology. These findings are important for the understanding of the etiopathogenesis of midbrain dopamine neuron degeneration and future use of cell replacement therapies.

Embryonic stem cell-derived Pitx3-enhanced green fluorescent protein midbrain dopamine neurons survive enrichment by fluorescence-activated cell sorting and function in an animal model of Parkinson's disease

Both fetal ventral mesencephalic (VM) and embryonic stem (ES) cell-derived dopamine neurons have been used successfully to correct behavioral responses in animal models of Parkinson's disease. However, grafts derived from fetal VM cells or from ES cells contain multiple cell types, and the majority of these cells are not dopamine neurons. Isolation of ES cell-derived dopamine neurons and subsequent transplantation would both elucidate the capacity of these neurons to provide functional input and also further explore an efficient and safer use of ES cells for the treatment of Parkinson's disease. Toward this goal, we used a Pitx3-enhanced green fluorescent protein (Pitx3-eGFP) knock-in mouse blastocyst-derived embryonic stem (mES) cell line and fluorescence-activated cell sorting (FACS) to select and purify midbrain dopamine neurons. Initially, the dopaminergic marker profile of intact Pitx3-eGFP mES cultures was evaluated after differentiation in vitro. eGFP expression overlapped closely with that of Pitx3, Nurr1, Engrailed-1, Lmx1a, tyrosine hydroxylase (TH), l-aromatic amino acid decarboxylase (AADC), and vesicular monoamine transporter 2 (VMAT2), demonstrating that these cells were of a midbrain dopamine neuron character. Furthermore, postmitotic Pitx3-eGFP(+) dopamine neurons, which constituted 2%-5% of all live cells in the culture after dissociation, could be highly enriched to >90% purity by FACS, and these isolated neurons were viable, extended neurites, and maintained a dopaminergic profile in vitro. Transplantation to 6-hydroxydopamine-lesioned rats showed that an enriched dopaminergic population could survive and restore both amphetamine- and apomorphine-induced functions, and the grafts contained large numbers of midbrain dopamine neurons, which innervated the host striatum. Disclosure of potential conflicts of interest is found at the end of this article.

Therapeutic potentials of human embryonic stem cells in Parkinson's disease

The loss of dopaminergic neurons of the substantia nigra is the pathological hallmark characteristic of Parkinson's disease (PD). The strategy of replacing these degenerating neurons with other cells that produce dopamine has been the main approach in the cell transplantation field for PD research. The isolation, differentiation, and long-term cultivation of human embryonic stem cells and the therapeutic research discovery made in relation to the beneficial properties of neurotrophic and neural growth factors has advanced the transplantation field beyond dopamine-producing cells. The present review addresses recent advances in human embryonic stem cell experimentation in relation to treating PD, as well as cell transplantation techniques in conjunction with alternative therapeutics.

Human ES cell-derived neural rosettes reveal a functionally distinct early neural stem cell stage

Neural stem cells (NSCs) yield both neuronal and glial progeny, but their differentiation potential toward multiple region-specific neuron types remains remarkably poor. In contrast, embryonic stem cell (ESC) progeny readily yield region-specific neuronal fates in response to appropriate developmental signals. Here we demonstrate prospective and clonal isolation of neural rosette cells (termed R-NSCs), a novel NSC type with broad differentiation potential toward CNS and PNS fates and capable of in vivo engraftment. R-NSCs can be derived from human and mouse ESCs or from neural plate stage embryos. While R-NSCs express markers classically associated with NSC fate, we identified a set of genes that specifically mark the R-NSC state. Maintenance of R-NSCs is promoted by activation of SHH and Notch pathways. In the absence of these signals, R-NSCs rapidly lose rosette organization and progress to a more restricted NSC stage. We propose that R-NSCs represent the first characterized NSC stage capable of responding to patterning cues that direct differentiation toward region-specific neuronal fates. In addition, the R-NSC-specific genetic markers presented here offer new tools for harnessing the differentiation potential of human ESCs.

Critical issues of clinical human embryonic stem cell therapy for brain repair

Embryonic stem cells (ESCs) provide hope as a potential regenerative therapy for neurological conditions such as Parkinson's disease and spinal cord injury. Currently, ESC-based nervous system repair faces several problems. One major hurdle is related to problems in generating large and defined populations of the desired types of neurons from human ESCs (hESCs). Moreover, survival of grafted hESC-derived cells has varied and functional recovery in recipient animals has often been disappointing. Importantly, in clinical trials, adverse effects after surgery, including tumors or vigorous immune reactions, must be avoided. Here we highlight attempts to overcome these hurdles with hESCs intended for central nervous system repair. We focus on hESC-derived dopamine-producing neurons that can be grafted in Parkinson's disease and identify critical experiments that need to be conducted before clinical trials can occur.

ES cell-derived neuroepithelial cell cultures

ES cells have the potential to differentiate into cells from all germ layers, which makes them an attractive tool for the development of new therapies. In general, the differentiation of ES cells follows the concept to first generate immature progenitor cells, which then can be propagated and differentiated into mature cellular phenotypes. This also applies for ES cell-derived neurogenesis, in which the development of neural cells follows two major steps: First, the derivation and expansion of immature neuroepithelial precursors and second, their differentiation into mature neural cells. A common method to produce neural progenitors from ES cells is based on embryoid body (EB) formation, which reveals the differentiation of cells from all germ layers including neuroectoderm. An alternative and more efficient method to induce neuroepithelial cell development uses stromal cell-derived inducing activity (SDIA), which can be achieved by co-culturing ES cells with skull bone marrow-derived stromal cells. Both, EB formation and SDIA, reveal the development of rosette-like structures, which are thought to resemble neural tube- and/or neural crest-like progenitors. The neural precursors can be isolated, expanded and further differentiated into specific neurons and glia cells using defined culture conditions. Here, we describe the generation and isolation of such rosettes in co-culture experiments with the stromal cell line MS5 (2-5).