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

GABAergic neurons from mouse embryonic stem cells possess functional properties of striatal neurons in vitro, and develop into striatal neurons in vivo in a mouse model of Huntington's disease

Huntington's disease (HD) is a neurodegenerative disease where GABAergic medium spiny neurons (MSNs) in the striatum degenerate. Embryonic stem cell-derived neural transplantation may provide an appropriate therapy for HD. Here we aimed to develop a suitable protocol to obtain a high percentage of functional GABAergic neurons from mouse embryonic stem cells (mESCs), and then tested their differentiation potential in vivo. The monolayer method was compared with the embryoid body and five stage method for its efficiency in generating GABAergic neurons from mESCs. All three methods yielded a similar percentage of GABAergic neurons from mESCs. Monolayer method-derived GABAergic neurons expressed the MSN marker dopamine- and cyclic AMP-regulated phosphoprotein (DARPP32). The pluripotent stem cell population could be eliminated in vitro by treating cells with puromycin and retinoic acid. Using patch-clamp recordings, the functional properties of GABAergic neurons derived from mESCs were compared to GABAergic neurons derived from primary lateral ganglionic eminence. Both types of neurons showed active membrane properties (voltage-gated Na(+) and K(+) currents, Na(+)-dependent action potentials, and spontaneous postsynaptic currents) and possessed functional glutamatergic receptors and transporters. mESC-derived neural progenitors were transplanted into a mouse model of HD. Grafted cells differentiated to mature neurons expressing glutamate decarboxylase, dopamine type 1 receptors, and DARPP32. Also, neural precursors and dividing populations were found in the grafts. In summary, mESCs are able to differentiate efficiently into functional GABAergic neurons using defined in vitro conditions, and these survive and differentiate following grafting to a mouse model of HD.

Cellular programming and reprogramming: sculpting cell fate for the production of dopamine neurons for cell therapy

Pluripotent stem cells are regarded as a promising cell source to obtain human dopamine neurons in sufficient amounts and purity for cell replacement therapy. Importantly, the success of clinical applications depends on our ability to steer pluripotent stem cells towards the right neuronal identity. In Parkinson disease, the loss of dopamine neurons is more pronounced in the ventrolateral population that projects to the sensorimotor striatum. Because synapses are highly specific, only neurons with this precise identity will contribute, upon transplantation, to the synaptic reconstruction of the dorsal striatum. Thus, understanding the developmental cell program of the mesostriatal dopamine neurons is critical for the identification of the extrinsic signals and cell-intrinsic factors that instruct and, ultimately, determine cell identity. Here, we review how extrinsic signals and transcription factors act together during development to shape midbrain cell fates. Further, we discuss how these same factors can be applied in vitro to induce, select, and reprogram cells to the mesostriatal dopamine fate.

Teratocarcinoma Formation in Embryonic Stem Cell-Derived Neural Progenitor Hippocampal Transplants

Embryonic stem cells (ESCs) hold great therapeutic potential due to their ability to differentiate into cells of the three primary germ layers, which can be used to repopulate disease-damaged tissues. In fact, two cell therapies using ESC derivatives are currently in phase I clinical trials. A main concern in using ESCs and their derivatives for cell transplantation is the ability of undifferentiated ESCs to generate tumors in the host. Positive selection steps are often included in protocols designed to generate particular cell types from ESCs; however, the transition from ESC to progenitor cell or terminally differentiated cell is not synchronous, and residual undifferentiated cells often remain. In our transplants of ESC-derived neural progenitors (ESNPs) into the adult mouse hippocampus, we have observed the formation of teratocarcinomas. We set out to reduce teratocarcinoma formation by enrichment of ESNPs using fluorescence-activated cell sorting (FACS) and have found that, although enrichment prior to transplant reduces the overall rate of teratocarcinoma formation, the tumorigenicity of cell batches can vary widely, even after FACS enrichment to as much as 95% ESNPs. Our data suggest that this variability may be due to the percentage of residual ESCs remaining in the transplant cell population and to the presence of pluripotent epiblast-like cells, not previously identified in transplant batches. Our data emphasize the need for stringent characterization of transplant cell populations that will be used for cell replacement therapies in order to reduce the risk of tumor formation.

SFRP1 and SFRP2 dose-dependently regulate midbrain dopamine neuron development in vivo and in embryonic stem cells

Secreted Frizzled related proteins (sFRPs) are a family of proteins that modulate Wnt signaling, which in turn regulates multiple aspects of ventral midbrain (VM) and dopamine (DA) neuron development. However, it is not known which Wnt signaling branch and what aspects of midbrain DA neuron development are regulated by sFRPs. Here, we show that sFRP1 and sFRP2 activate the Wnt/planar-cell-polarity/Rac1 pathway in DA cells. In the developing VM, sFRP1 and sFRP2 are expressed at low levels, and sFRP1-/- or sFRP2-/- mice had no detectable phenotype. However, compound sFRP1-/-;sFRP2-/- mutants revealed a Wnt/PCP phenotype similar to that previously described for Wnt5a-/- mice. This included an anteroposterior shortening of the VM, a lateral expansion of the Shh domain and DA lineage markers (Lmx1a and Th), as well as an accumulation of Nurr1+ precursors in the VM. In vitro experiments showed that, while very high concentrations of SFRP1 had a negative effect on cell survival, low/medium concentrations of sFRP1 or sFRP2 promoted the DA differentiation of progenitors derived from primary VM cultures or mouse embryonic stem cells (ESCs), mimicking the effects of Wnt5a. We thus conclude that the main function of sFRP1 and sFRP2 is to enhance Wnt/PCP signaling in DA cells and to regulate Wnt/PCP-dependent functions in midbrain development. Moreover, we suggest that low-medium concentrations of sFRPs may be used to enhance the DA differentiation of ESCs and improve their therapeutic application.

Identification of embryonic stem cell-derived midbrain dopaminergic neurons for engraftment

Embryonic stem cells (ESCs) represent a promising source of midbrain dopaminergic (DA) neurons for applications in Parkinson disease. However, ESC-based transplantation paradigms carry a risk of introducing inappropriate or tumorigenic cells. Cell purification before transplantation may alleviate these concerns and enable identification of the specific DA neuron stage most suitable for cell therapy. Here, we used 3 transgenic mouse ESC reporter lines to mark DA neurons at 3 stages of differentiation (early, middle, and late) following induction of differentiation using Hes5::GFP, Nurr1::GFP, and Pitx3::YFP transgenes, respectively. Transplantation of FACS-purified cells from each line resulted in DA neuron engraftment, with the mid-stage and late-stage neuron grafts being composed almost exclusively of midbrain DA neurons. Mid-stage neuron cell grafts had the greatest amount of DA neuron survival and robustly induced recovery of motor deficits in hemiparkinsonian mice. Our data suggest that the Nurr1+ stage (middle stage) of neuronal differentiation is particularly suitable for grafting ESC-derived DA neurons. Moreover, global transcriptome analysis of progeny from each of the ESC reporter lines revealed expression of known midbrain DA neuron genes and also uncovered previously uncharacterized midbrain genes. These data demonstrate remarkable fate specificity of ESC-derived DA neurons and outline a sequential stage-specific ESC reporter line paradigm for in vivo gene discovery.

Successful derivation of EGFP-transgenic embryonic stem cell line from a genetically non-permissive FVB/N mouse

Derivation of embryonic stem (ES)-cell lines from genetically non-permissive mouse strains, such as FVB/N, has been difficult, despite this strain offering advantages for mouse transgenesis for developmental studies. We earlier generated β-actin promoter-driven enhanced green fluorescent protein (EGFP)-transgenic FVB/N mice, expressing EGFP in all cells. Here, by optimizing culture system and using RESGROTM ES-cell culture medium, we successfully derived EGFP-transgenic ES-cell line, 'GS-2' line, from F1 hybrid blastocysts, from wild-type 129/SvJ female X EGFP-transgenic homozygous FVB/N male. The GS-2 ES-cell line exhibited all defining criteria of a typical ES-cell line, including normal colony morphology and karyotype (40,XY), high replication-expansion efficiency (passages: >100), expression of pluripotent markers (Oct-4, Nanog, Sox-2, SSEA-1 and others) and, embryoid body (EB) development and EB differentiation to ecto-/meso-/endo-dermal cell types, expressing nestin, BMP-4 and α-fetoprotein, respectively. GS-2 ES-cells formed (i) teratoma containing three germ lineage-derived cell types, (ii) chimeric blastocysts and fetuses, following their aggregation with wild-type 8-cell embryos, (iii) functional cardiac clusters and (iv) predominantly neural cell types when EBs were developed in KOSR-supplemented medium. Taken together, we derived a robust EGFP-transgenic GS-2 ES-cell line, from a non-permissive transgenic (FVB/N) mouse by a single cross to 129/SvJ wild-type mouse. The GS-2 ES-cell line exhibited full differentiation potential, in vitro/in vivo, providing enormous opportunity for stem cell research, including experimental cell transplantation studies.

Clinical application of stem cell therapy in Parkinson's disease

Cell replacement therapies in Parkinson's disease (PD) aim to provide long-lasting relief of patients' symptoms. Previous clinical trials using transplantation of human fetal ventral mesencephalic (hfVM) tissue in the striata of PD patients have provided proof-of-principle that such grafts can restore striatal dopaminergic (DA-ergic) function. The transplants survive, reinnervate the striatum, and generate adequate symptomatic relief in some patients for more than a decade following operation. However, the initial clinical trials lacked homogeneity of outcomes and were hindered by the development of troublesome graft-induced dyskinesias in a subgroup of patients. Although recent knowledge has provided insights for overcoming these obstacles, it is unlikely that transplantation of hfVM tissue will become routine treatment for PD owing to problems with tissue availability and standardization of the grafts. The main focus now is on producing DA-ergic neuroblasts for transplantation from stem cells (SCs). There is a range of emerging sources of SCs for generating a DA-ergic fate in vitro. However, the translation of these efforts in vivo currently lacks efficacy and sustainability. A successful, clinically competitive SC therapy in PD needs to produce long-lasting symptomatic relief without side effects while counteracting PD progression.

Derivation of dopaminergic neurons from pluripotent stem cells

Midbrain dopamine neurons play a critical role in motor function and in reward-related motivational behaviors. The goal of developing a renewable source of human midbrain dopamine neurons was prompted by the pioneering studies on the use of human fetal dopamine neurons as an experimental therapy for the treatment of Parkinson's disease. More recently, dopamine neurons have also turned into an important tool for modeling of Parkinson's disease in patient-specific induced pluripotent stem cell lines. Protocols for the directed differentiation of mouse ESCs into midbrain dopamine neurons have been developed more than a decade ago and the successful derivation of human midbrain dopamine neurons was reported soon after. However, the initial human ESC reports were unable to demonstrate efficient in vivo dopamine neuron engraftment. Only very recently, those challenges have been overcome by using an alternative differentiation strategy that is based on deriving midbrain dopamine neurons via a distinct midbrain floor plate intermediate. With those novel tools in hand, it should now become possible to test the full potential of midbrain dopamine neurons in regenerative medicine and human disease modeling. However, several challenges remain such as the need to develop strategies that can enrich for selective subtypes of midbrain dopamine neurons, techniques to control postmitotic dopamine neuron maturation, and finally, clinical grade differentiation protocols that enable the production dopamine neurons suitable for human cell therapy.

Characterization and criteria of embryonic stem and induced pluripotent stem cells for a dopamine replacement therapy

Human pluripotent stem cells provide new choices for sources of A9-type dopaminergic (DA) neurons in clinical trials of neural transplantation for patients with Parkinson's disease (PD). For example, "self" and HLA-matched A9 DA neurons may improve the patient-to-patient variability observed in previous clinical trials using fetal DA neurons and obviate the need for long-term immunosuppression in the patient. Normal chromosomal structure and minimal somatic mutations in pluripotent stem cells are necessary criteria for assuring the safe and reproducible transplantation of differentiated DA neurons into patients with PD in clinical trials. However, with these new choices of cell source, the application of pluripotency assays as criteria to ensure pluripotent stem cell quality becomes less relevant. New more relevant standards of quality control, assurance, and function are required. We suggest that quality assurance measures for pluripotent stem cells need to focus upon readouts for authentic midbrain DA neurons, their integration and growth using in vivo assays, and their long-term functional stability.

Functional integration of dopaminergic neurons directly converted from mouse fibroblasts

Recent advances in somatic cell reprogramming have highlighted the plasticity of the somatic epigenome, particularly through demonstrations of direct lineage reprogramming of one somatic cell type to another by defined factors. However, it is not clear to what extent this type of reprogramming is able to generate fully functional differentiated cells. In addition, the activity of the reprogrammed cells in cell transplantation assays, such as those envisaged for cell-based therapy of Parkinson's disease (PD), remains to be determined. Here we show that ectopic expression of defined transcription factors in mouse tail tip fibroblasts is sufficient to induce Pitx3+ neurons that closely resemble midbrain dopaminergic (DA) neurons. In addition, transplantation of these induced DA (iDA) neurons alleviates symptoms in a mouse model of PD. Thus, iDA neurons generated from abundant somatic fibroblasts by direct lineage reprogramming hold promise for modeling neurodegenerative disease and for cell-based therapies of PD.

Cell-based therapies for Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease worldwide, classically characterized by a triad of motor features: bradykinesia, rigidity and resting tremor. Neurodegeneration in PD critically involves the dopaminergic neurons of the substantia nigra pars compacta, which results in a severe reduction in dopamine levels in the dorsal striatum. However, the disease also exhibits extensive non-nigral pathology and as many non-motor as motor features. Nevertheless, owing to the relatively circumscribed nature of the nigrostriatal lesion in PD, dopaminergic cell transplantation has emerged as a potentially reparative therapy for the disease. Sources for such cells are varied and include the developing ventral mesencephalon, several autologous somatic cell types, embryonic stem cells and induced pluripotent stem cells. In this article, we review the origins of dopaminergic transplantation for PD and the emergent hunt for a suitable long-term source of transplantable dopaminergic neurons.

Oct4-induced reprogramming is required for adult brain neural stem cell differentiation into midbrain dopaminergic neurons

Neural stem cells (NSCs) lose their competency to generate region-specific neuronal populations at an early stage during embryonic brain development. Here we investigated whether epigenetic modifications can reverse the regional restriction of mouse adult brain subventricular zone (SVZ) NSCs. Using a variety of chemicals that interfere with DNA methylation and histone acetylation, we showed that such epigenetic modifications increased neuronal differentiation but did not enable specific regional patterning, such as midbrain dopaminergic (DA) neuron generation. Only after Oct-4 overexpression did adult NSCs acquire a pluripotent state that allowed differentiation into midbrain DA neurons. DA neurons derived from Oct4-reprogrammed NSCs improved behavioural motor deficits in a rat model of Parkinson's disease (PD) upon intrastriatal transplantation. Here we report for the first time the successful differentiation of SVZ adult NSCs into functional region-specific midbrain DA neurons, by means of Oct-4 induced pluripotency.

Cell transplantation and gene therapy in Parkinson's disease

Parkinson's disease is a progressive neurodegenerative disorder affecting, in part, dopaminergic motor neurons of the ventral midbrain and their terminal projections that course to the striatum. Symptomatic strategies focused on dopamine replacement have proven effective at remediating some motor symptoms during the course of disease but ultimately fail to deliver long-term disease modification and lose effectiveness due to the emergence of side effects. Several strategies have been experimentally tested as alternatives for Parkinson's disease, including direct cell replacement and gene transfer through viral vectors. Cellular transplantation of dopamine-secreting cells was hypothesized as a substitute for pharmacotherapy to directly provide dopamine, whereas gene therapy has primarily focused on restoration of dopamine synthesis or neuroprotection and restoration of spared host dopaminergic circuitry through trophic factors as a means to enhance sustained controlled dopamine transmission. This seems now to have been verified in numerous studies in rodents and nonhuman primates, which have shown that grafts of fetal dopamine neurons or gene transfer through viral vector delivery can lead to improvements in biochemical and behavioral indices of dopamine deficiency. However, in clinical studies, the improvements in parkinsonism have been rather modest and variable and have been plagued by graft-induced dyskinesias. New developments in stem-cell transplantation and induced patient-derived cells have opened the doors for the advancement of cell-based therapeutics. In addition, viral-vector-derived therapies have been developed preclinically with excellent safety and efficacy profiles, showing promise in clinical trials thus far. Further progress and optimization of these therapies will be necessary to ensure safety and efficacy before widespread clinical use is deemed appropriate.

ES cell-derived renewable and functional midbrain dopaminergic progenitors

During early development, midbrain dopaminergic (mDA) neuronal progenitors (NPs) arise from the ventral mesencephalic area by the combined actions of secreted factors and their downstream transcription factors. These mDA NPs proliferate, migrate to their final destinations, and develop into mature mDA neurons in the substantia nigra and the ventral tegmental area. Here, we show that such authentic mDA NPs can be efficiently isolated from differentiated ES cells (ESCs) using a FACS method combining two markers, Otx2 and Corin. Purified Otx2(+)Corin(+) cells coexpressed other mDA NP markers, including FoxA2, Lmx1b, and Glast. Using optimized culture conditions, these mDA NPs continuously proliferated up to 4 wk with almost 1,000-fold expansion without significant changes in their phenotype. Furthermore, upon differentiation, Otx2(+)Corin(+) cells efficiently generated mDA neurons, as evidenced by coexpression of mDA neuronal markers (e.g., TH, Pitx3, Nurr1, and Lmx1b) and physiological functions (e.g., efficient DA secretion and uptake). Notably, these mDA NPs differentiated into a relatively homogenous DA population with few serotonergic neurons. When transplanted into PD model animals, aphakia mice, and 6-OHDA-lesioned rats, mDA NPs differentiated into mDA neurons in vivo and generated well-integrated DA grafts, resulting in significant improvement in motor dysfunctions without tumor formation. Furthermore, grafted Otx2(+)Corin(+) cells exhibited significant migratory function in the host striatum, reaching >3.3 mm length in the entire striatum. We propose that functional and expandable mDA NPs can be efficiently isolated by this unique strategy and will serve as useful tools in regenerative medicine, bioassay, and drug screening.

A small synthetic cripto blocking Peptide improves neural induction, dopaminergic differentiation, and functional integration of mouse embryonic stem cells in a rat model of Parkinson's disease

Cripto is a glycosylphosphatidylinositol-anchored coreceptor that binds Nodal and the activin type I (ALK)-4 receptor, and is involved in cardiac differentiation of mouse embryonic stem cells (mESCs). Interestingly, genetic ablation of cripto results in increased neuralization and midbrain dopaminergic (DA) differentiation of mESCs, as well as improved DA cell replacement therapy (CRT) in a model of Parkinson's disease (PD). In this study, we developed a Cripto specific blocking tool that would mimic the deletion of cripto, but could be easily applied to embryonic stem cell (ESC) lines without the need of genetic manipulation. We thus screened a combinatorial peptide library and identified a tetrameric tripeptide, Cripto blocking peptide (BP), which prevents Cripto/ALK-4 receptor interaction and interferes with Cripto signaling. Cripto BP treatment favored neuroectoderm formation and promoted midbrain DA neuron differentiation of mESCs in vitro and in vivo. Remarkably, Cripto BP-treated ESCs, when transplanted into the striatum of PD rats, enhanced functional recovery and reduced tumor formation, mimicking the effect of genetic ablation of cripto. We therefore suggest that specific blockers such as Cripto BP may be used to improve the differentiation of ESC-derived DA neurons in vitro and their engraftment in vivo, bringing us closer towards an application of ESCs in CRT.

Modulation of the generation of dopaminergic neurons from human neural stem cells by Bcl-X(L): mechanisms of action

Understanding the developmental mechanisms governing dopaminergic neuron generation and maintenance is crucial for the development of neuronal replacement therapeutic procedures, like in Parkinson's disease (PD), but also for research aimed at drug screening and pharmacology. In the present chapter, we review the present situation using stem cells of different origins (pluripotent and multipotent) and summarize current manipulations of stem cells for the enhancement of dopaminergic neuron generation, focusing on the actions of Bcl-X(L). Bcl-X(L) not only enhances dopaminergic neuron survival but also augments the expression of key developmental and maintenance genes, and, through the lengthening of the cell cycle early during differentiation, regulates cell fate decisions, producing a net enhancement of neurogenesis. The relevance of these findings is discussed in the context of basic neurogenesis and also for the development of efficient cell therapy in PD.

Dopamine cell transplantation in Parkinson's disease: challenge and perspective

BACKGROUND

Functional imaging provides a valuable adjunct to clinical evaluation for assessing the efficacy of cell-based restorative therapies in Parkinson's disease (PD).

SOURCES OF DATA

In this article, we review the latest advances on the use of positron emission tomography (PET) imaging in evaluating the surgical outcome of embryonic dopamine (DA) cell transplantation in PD patients.

AREAS OF AGREEMENT

These studies suggest long-term cell survival and clinical benefit following striatal transplantation of fetal nigral tissue in PD patients and in models of experimental parkinsonism.

AREAS OF CONTROVERSY

Adverse events subsequent to transplantation have also been noted and attributed to a variety of causes.

GROWING POINTS

Optimal outcomes of DA cell transplantation therapies are dependent on tissue composition and phenotype of DA neurons in the graft.

AREAS TIMELY FOR DEVELOPING RESEARCH

Given continued progress in DA neuron production from stem cells in recent years, transplantation of neural stem cells may be the next to enter clinical trials in patients.

CONCLUSION

The existing data from studies of embryonic DA transplantation for advanced PD have provided valuable insights for the design of new cell-based therapies for the treatment of this and related neurodegenerative disorders.

Cell transplantation for Huntington’s disease: practical and clinical considerations

Huntington’s disease is a dominantly inherited neurodegenerative disorder, usually starting in mid-life and leading to progressive disability and early death. There are currently no disease-modifying treatments available. Cell transplantation is being considered as a potential therapy, following proof of principle that cell transplantation can improve outcomes in another basal ganglia disorder, namely Parkinson’s disease. The principle aim is to replace the striatal medium spiny neurons lost in Huntington’s disease with new cells that are able to take over their function and reconnect the circuitry. This article reviews the experimental background and evidence from clinical studies that suggest that cell transplantation may improve function in Huntington’s disease, reviews the current status of the field and considers the current challenges to taking this experimental strategy forward to becoming a reliable therapeutic option.

VEGF-expressing human umbilical cord mesenchymal stem cells, an improved therapy strategy for Parkinson's disease

The umbilical cord provides a rich source of primitive mesenchymal stem cells (human umbilical cord mesenchymal stem cells (HUMSCs)), which have the potential for transplantation-based treatments of Parkinson's Disease (PD). Our pervious study indicated that adenovirus-associated virus-mediated intrastriatal delivery of human vascular endothelial growth factor 165 (VEGF 165) conferred molecular protection to the dopaminergic system. As both VEGF and HUMSCs displayed limited neuroprotection, in this study we investigated whether HUMSCs combined with VEGF expression could offer enhanced neuroprotection. HUMSCs were modified by adenovirus-mediated VEGF gene transfer, and subsequently transplanted into rotenone-lesioned striatum of hemiparkinsonian rats. As a result, HUMSCs differentiated into dopaminergic neuron-like cells on the basis of neuron-specific enolase (NSE) (neuronal marker), glial fibrillary acidic protein (GFAP) (astrocyte marker), nestin (neural stem cell marker) and tyrosine hydroxylase (TH) (dopaminergic marker) expression. Further, VEGF expression significantly enhanced the dopaminergic differentiation of HUMSCs in vivo. HUMSC transplantation ameliorated apomorphine-evoked rotations and reduced the loss of dopaminergic neurons in the lesioned substantia nigra (SNc), which was enhanced significantly by VEGF expression in HUMSCs. These findings present the suitability of HUMSC as a vector for gene therapy and suggest that stem cell engineering with VEGF may improve the transplantation strategy for the treatment of PD.

Ageing and neurodegenerative diseases

Ageing, which all creatures must encounter, is a challenge to every living organism. In the human body, it is estimated that cell division and metabolism occurs exuberantly until about 25 years of age. Beyond this age, subsidiary products of metabolism and cell damage accumulate, and the phenotypes of ageing appear, causing disease formation. Among these age-related diseases, neurodegenerative diseases have drawn a lot of attention due to their irreversibility, lack of effective treatment, and accompanied social and economical burdens. In seeking to ameliorate ageing and age-related diseases, the search for anti-ageing drugs has been of much interest. Numerous studies have shown that the plant polyphenol, resveratrol (3,5,4'-trihydroxystilbene), extends the lifespan of several species, prevents age-related diseases, and possesses anti-inflammatory, and anti-cancer properties. The beneficial effects of resveratrol are believed to be associated with the activation of a longevity gene, SirT1. In this review, we discuss the pathogenesis of age-related neurodegenerative diseases including Alzheimer's disease, Parkinson's disease and cerebrovascular disease. The therapeutic potential of resveratrol, diet and the roles of stem cell therapy are discussed to provide a better understanding of the ageing mystery.

Stem Cells for the Treatment of Neurodegenerative Diseases

Neurodegenerative diseases are characterized by neurodegenerative changes or apoptosis of neurons involved in networks, leading to permanent paralysis and loss of sensation below the site of the injury. Cell replacement therapy has provided the basis for the development of potentially powerful new therapeutic strategies for a broad spectrum of human neurological diseases. In recent years, neurons and glial cells have successfully been generated from stem cells, and extensive efforts by investigators to develop stem cell-based brain transplantation therapies have been carried out. We review here notable previously published experimental and preclinical studies involving stem cell-based cell for neurodegenerative diseases and discuss the future prospects for stem cell therapy of neurological disorders in the clinical setting. Steady and solid progress in stem cell research in both basic and preclinical settings should support the hope for development of stem cell-based cell therapies for neurological diseases.

Markers of pluripotency and differentiation in human neural precursor cells derived from embryonic stem cells and CNS tissue

Cell transplantation therapies for central nervous system (CNS) deficits such as spinal cord injury (SCI) have been shown to be effective in several animal models. One cell type that has been transplanted is neural precursor cells (NPCs), for which there are several possible sources. We have studied NPCs derived from human embryonic stem cells (hESCs) and human fetal CNS tissue (hfNPCs), cultured as neurospheres, and the expression of pluripotency and neural genes during neural induction and in vitro differentiation. mRNA for the pluripotency markers Nanog, Oct-4, Gdf3, and DNMT3b were downregulated during neural differentiation of hESCs. mRNA for these markers was found in nonpluripotent hfNPC at higher levels compared to hESC-NPCs. However, Oct-4 protein was found in hESC-NPCs after 8 weeks of culture, but not in hfNPCs. Similarly, SSEA-4 and CD326 were only found in hESC-NPCs. NPCs from both sources differentiated as expected to cells with typical features of neurons and astrocytes. The expressions of neuronal markers in hESC-NPCs were affected by the composition of cell culture medium, while this did not affect hfNPCs. Transplantation of hESC-NPC or hfNPC neurospheres into immunodeficient mouse testis or subcutaneous tissue did not result in tumor formation. In contrast, typical teratomas appeared in all animals after transplantation of hESC-NPCs to injured or noninjured spinal cords of immunodeficient rats. Our data show that transplantation to the subcutaneous tissue or the testes of immunodeficient mice is not a reliable method for evaluation of the tumor risk of remaining pluripotent cells in grafts.

Interactions of Wnt/beta-catenin signaling and sonic hedgehog regulate the neurogenesis of ventral midbrain dopamine neurons

Signaling mechanisms involving Wnt/beta-catenin and sonic hedgehog (Shh) are known to regulate the development of ventral midbrain (vMB) dopamine neurons. However, the interactions between these two mechanisms and how such interactions can be targeted to promote a maximal production of dopamine neurons are not fully understood. Here we show that conditional mouse mutants with region-specific activation of beta-catenin signaling in vMB using the Shh-Cre mice show a marked expansion of Sox2-, Ngn2-, and Otx2-positive progenitors but perturbs their cell cycle exit and reduces the generation of dopamine neurons. Furthermore, activation of beta-catenin in vMB also results in a progressive loss of Shh expression and Shh target genes. Such antagonistic effects between the activation of Wnt/beta-catenin and Shh can be recapitulated in vMB progenitors and in mouse embryonic stem cell cultures. Notwithstanding these antagonistic interactions, cell-type-specific activation of beta-catenin in the midline progenitors using the tyrosine hydroxylase-internal ribosomal entry site-Cre (Th-IRES-Cre) mice leads to increased dopaminergic neurogenesis. Together, these results indicate the presence of a delicate balance between Wnt/beta-catenin and Shh signaling mechanisms in the progression from progenitors to dopamine neurons. Persistent activation of beta-catenin in early progenitors perturbs their cell cycle progression and antagonizes Shh expression, whereas activation of beta-catenin in midline progenitors promotes the generation of dopamine neurons.

Human ES and iPS cells as cell sources for the treatment of Parkinson's disease: current state and problems

Cell therapy using human embryonic stem cells (hESCs) is a promising therapeutic option for Parkinson's disease (PD), an incurable neurodegenerative disease. A prerequisite for clinical application of hESCs for PD is an efficient and strict differentiation of hESCs into midbrain dopamine (mDA) neuron-like cells, which would be directly translated into high effectiveness of the therapy with minimum risk of undesirable side effects. Due to fruitful efforts from many laboratories, a variety of strategies for improving efficiency of dopaminergic differentiation from hESCs have been developed, mostly by optimizing culture conditions, genetic modification, and modulating intracellular signaling pathways. The rapid advances in the fields of dopaminergic differentiation of hESCs, prevention of tumor formation, and establishment of safe human induced pluripotent stem cells (hiPSCs) would open the door to highly effective, tumor-free, and immune rejection-free cell therapy for PD in the near future.

Stem cells and cell replacement therapy for Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder caused by a progressive degeneration of the midbrain dopamine (DA) neurons in the substantia nigra pars compacta (SNc) that predominantly affects the ventral population projecting to the dorsal striatum and leads to a gradual dysfunction of the motor system. There is currently no cure for PD. Pharmacological and surgical (e.g. deep brain stimulation) interventions can alleviate some of the symptoms, but lose their efficacy over time. The distinct loss of DA neurons in the SN offers the opportunity to assay neuronal cell replacement, and the clinical transplantation of fetal midbrain neuroblasts in PD patients has shown that this approach is feasible. However, there are multiple problems associated with the use of fetus-derived material, including limited availability. DA neurons derived from stem cells (SC) represent an alternative and unlimited cell source for cell replacement therapies. Currently, human pluripotent SC, such as embryonic (ES), and most recently, induced pluripotent stem cells (iPS), and multipotent (tissue-specific) adult SC are available, although the methodology for a reliable and efficient production of DA neurons necessary for biomedical applications is still underdeveloped. Here, we discuss some essentials for SC and SC-derived DA neurons to become therapeutic agents.

Stem cell-based neuroprotective and neurorestorative strategies

Stem cells, a special subset of cells derived from embryo or adult tissues, are known to present the characteristics of self-renewal, multiple lineages of differentiation, high plastic capability, and long-term maintenance. Recent reports have further suggested that neural stem cells (NSCs) derived from the adult hippocampal and subventricular regions possess the utilizing potential to develop the transplantation strategies and to screen the candidate agents for neurogenesis, neuroprotection, and neuroplasticity in neurodegenerative diseases. In this article, we review the roles of NSCs and other stem cells in neuroprotective and neurorestorative therapies for neurological and psychiatric diseases. We show the evidences that NSCs play the key roles involved in the pathogenesis of several neurodegenerative disorders, including depression, stroke and Parkinson’s disease. Moreover, the potential and possible utilities of induced pluripotent stem cells (iPS), reprogramming from adult fibroblasts with ectopic expression of four embryonic genes, are also reviewed and further discussed. An understanding of the biophysiology of stem cells could help us elucidate the pathogenicity and develop new treatments for neurodegenerative disorders. In contrast to cell transplantation therapies, the application of stem cells can further provide a platform for drug discovery and small molecular testing, including Chinese herbal medicines. In addition, the high-throughput stem cell-based systems can be used to elucidate the mechanisms of neuroprotective candidates in translation medical research for neurodegenerative diseases.

CD15, CD24, and CD29 define a surface biomarker code for neural lineage differentiation of stem cells

Identification and use of cell surface cluster of differentiation (CD) biomarkers have enabled much scientific and clinical progress. We identify a CD surface antigen code for the neural lineage based on combinatorial flow cytometric analysis of three distinct populations derived from human embryonic stem cells: (1) CD15(+)/CD29(HI)/CD24(LO) surface antigen expression defined neural stem cells; (2) CD15(-)/CD29(HI)/CD24(LO) revealed neural crest-like and mesenchymal phenotypes; and (3) CD15(-)/CD29(LO)/CD24(HI) selected neuroblasts and neurons. Fluorescence-activated cell sorting (FACS) for the CD15(-)/CD29(LO)/CD24(HI) profile reduced proliferative cell types in human embryonic stem cell differentiation. This eliminated tumor formation in vivo, resulting in pure neuronal grafts. In conclusion, combinatorial CD15/CD24/CD29 marker profiles define neural lineage development of neural stem cell, neural crest, and neuronal populations from human stem cells. We believe this set of biomarkers enables analysis and selection of neural cell types for developmental studies and pharmacological and therapeutic applications.

Stem cell-derived dopamine neurons for brain repair in Parkinson’s disease

One of the prospects for a curative treatment for Parkinson’s disease is to replace the lost dopaminergic neurons. Preclinical and clinical trials have demonstrated that dissected fetal dopaminergic neurons have the potential to markedly improve motor function in animal models and Parkinson’s disease patients. However, this source of cells will never be sufficient to use as a widespread therapy. Over the last 20 years, scientists have been searching for other reliable sources of midbrain dopamine neurons, and stem cells appear to be strong candidates. This article reviews the potential of different types of stem cells, from embryonic to adult to induced pluripotent stem cells, to see how well the cells can be differentiated into fully functional dopamine neurons, which cells might be the best candidates and how much more research is required before stem cell technology might be translated to a clinical therapy for Parkinson’s disease.

Introducing transcription factors to multipotent mesenchymal stem cells: making transdifferentiation possible

Multipotent mesenchymal stem cells (MSCs) represent a promising autologous source for regenerative medicine. Because MSCs can be isolated from adult tissues, they represent an attractive cell source for autologous transplantation. A straightforward therapeutic strategy in the field of stem cell-based regenerative medicine is the transplantation of functional differentiated cells as cell replacement for the lost or defective cells affected by disease. However, this strategy requires the capacity to regulate stem cell differentiation toward the desired cell fate. This therapeutic approach assumes the capability to direct MSC differentiation toward diverse cell fates, including those outside the mesenchymal lineage, a process termed transdifferentiation. The capacity of MSCs to undergo functional transdifferentiation has been questioned over the years. Nonetheless, recent studies support that genetic manipulation can serve to promote transdifferentiation. Specifically, forced expression of certain transcription factors can lead to reprogramming and alter cell fate. Using such a method, fully differentiated lymphocytes have been reprogrammed to become macrophages and, remarkably, somatic cells have been reprogrammed to become embryonic stem-like cells. In this review, we discuss the past and current research aimed at transdifferentiating MSCs, a process with applications that could revolutionize regenerative medicine.

In vitro and in vivo enhanced generation of human A9 dopamine neurons from neural stem cells by Bcl-XL

Human neural stem cells derived from the ventral mesencephalon (VM) are powerful research tools and candidates for cell therapies in Parkinson disease. Previous studies with VM dopaminergic neuron (DAn) precursors indicated poor growth potential and unstable phenotypical properties. Using the model cell line hVM1 (human ventral mesencephalic neural stem cell line 1; a new human fetal VM stem cell line), we have found that Bcl-X(L) enhances the generation of DAn from VM human neural stem cells. Mechanistically, Bcl-X(L) not only exerts the expected antiapoptotic effect but also induces proneural (NGN2 and NEUROD1) and dopamine-related transcription factors, resulting in a high yield of DAn with the correct phenotype of substantia nigra pars compacta (SNpc). The expression of key genes directly involved in VM/SNpc dopaminergic patterning, differentiation, and maturation (EN1, LMX1B, PITX3, NURR1, VMAT2, GIRK2, and dopamine transporter) is thus enhanced by Bcl-X(L). These effects on neurogenesis occur in parallel to a decrease in glia generation. These in vitro Bcl-X(L) effects are paralleled in vivo, after transplantation in hemiparkinsonian rats, where hVM1-Bcl-X(L) cells survive, integrate, and differentiate into DAn, alleviating behavioral motor asymmetry. Bcl-X(L) then allows for human fetal VM stem cells to stably generate mature SNpc DAn both in vitro and in vivo and is thus proposed as a helpful factor for the development of cell therapies for neurodegenerative conditions, Parkinson disease in particular.

Stem cells in human neurodegenerative disorders--time for clinical translation?

Stem cell-based approaches have received much hype as potential treatments for neurodegenerative disorders. Indeed, transplantation of stem cells or their derivatives in animal models of neurodegenerative diseases can improve function by replacing the lost neurons and glial cells and by mediating remyelination, trophic actions, and modulation of inflammation. Endogenous neural stem cells are also potential therapeutic targets because they produce neurons and glial cells in response to injury and could be affected by the degenerative process. As we discuss here, however, significant hurdles remain before these findings can be responsibly translated to novel therapies. In particular, we need to better understand the mechanisms of action of stem cells after transplantation and learn how to control stem cell proliferation, survival, migration, and differentiation in the pathological environment.

Lack of functional relevance of isolated cell damage in transplants of Parkinson's disease patients

Postmortem analyses from clinical neural transplantation trials of several subjects with Parkinson's disease revealed surviving grafted dopaminergic neurons after more than a decade. A subset of these subjects displayed isolated dopaminergic neurons within the grafts that contained Lewy body-like structures. In this review, we discuss why this isolated cell damage is unlikely to affect the overall graft function and how we can use these observations to help us to understand age-related neurodegeneration and refine our future cell replacement therapies.

Protective effects of neurotrophic factor-secreting cells in a 6-OHDA rat model of Parkinson disease

Stem cell-based therapy is a promising treatment for neurodegenerative diseases. In our laboratory, a novel protocol has been developed to induce bone marrow-derived mesenchymal stem cells (MSC) into neurotrophic factors- secreting cells (NTF-SC), thus combining stem cell-based therapy with the NTF-based neuroprotection. These cells produce and secrete factors such as brain-derived neurotrophic factor (BDNF) and glial cell-derived neurotrophic factor. Conditioned medium of the NTF-SC that was applied to a neuroblastoma cell line (SH-SY5Y) 1 h before exposure to the neurotoxin 6-hydroxydopamine (6-OHDA) demonstrated marked protection. An efficacy study was conducted on the 6-OHDA-induced lesion, a rat model of Parkinson's disease. The cells, either MSC or NTF-SC, were transplanted on the day of 6-OHDA administration and amphetamine-induced rotations were measured as a primary behavior index. We demonstrated that when transplanted posterior to the 6-OHDA lesion, the NTF-SC ameliorated amphetamine-induced rotations by 45%. HPLC analysis demonstrated that 6-OHDA induced dopamine depletion to a level of 21% compared to the untreated striatum. NTF-SC inhibited dopamine depletion to a level of 72% of the contralateral striatum. Moreover, an MRI study conducted with iron-labeled cells, followed by histological verification, revealed that the engrafted cells migrated toward the lesion. In a histological assessment, we found that the cells induced regeneration in the damaged striatal dopaminergic nerve terminal network. We therefore conclude that the induced MSC have a therapeutic potential for neurodegenerative processes and diseases, both by the NTFs secretion and by the migratory trait toward the diseased tissue.

Isolation and culture of ventral mesencephalic precursor cells and dopaminergic neurons from rodent brains

The ability to isolate ventral midbrain (VM) precursor cells and neurons provides a powerful means to characterize their differentiation properties and to study their potential for restoring dopamine (DA) neurons degenerated in Parkinson's disease (PD). Preparation and maintenance of DA VM in primary culture involves a number of critical steps to yield healthy cells and appropriate data. Here, we offer a detailed description of protocols to consistently prepare VM DA cultures from rat and mouse embryonic fetal-stage midbrain. We also present methods for organotypic culture of midbrain tissue, for differentiation as aggregate cultures, and for adherent culture systems of DA differentiation and maturation, followed by a synopsis of relevant analytical read-out options. Isolation and culture of rodent VM precursor cells and DA neurons can be exploited for studies of DA lineage development, of neuroprotection, and of cell therapeutic approaches in animal models of PD.

Emerging concepts in neural stem cell research: autologous repair and cell-based disease modelling

The increasing availability of human pluripotent stem cells provides new prospects for neural-replacement strategies and disease-related basic research. With almost unlimited potential for self-renewal, the use of human embryonic stem cells (ESCs) bypasses the restricted supply and expandability of primary cells that has been a major bottleneck in previous neural transplantation approaches. Translation of developmental patterning and cell-type specification techniques to human ESC cultures enables in vitro generation of various neuronal and glial cell types. The derivation of stably proliferating neural stem cells from human ESCs further facilitates standardisation and circumvents the problem of batch-to-batch variations commonly encountered in "run-through" protocols, which promote terminal differentiation of pluripotent stem cells into somatic cell types without defined intermediate precursor stages. The advent of cell reprogramming offers an opportunity to translate these advances to induced pluripotent stem cells, thereby enabling the generation of neurons and glia from individual patients. Eventually, reprogramming could provide a supply of autologous neural cells for transplantation, and could lead to the establishment of cellular model systems of neurological diseases.

Neuronal cell replacement in Parkinson's disease

Transplantation of foetal dopamine neurons into the striatum of Parkinson's disease patients can provide restoration of the dopamine system and alleviate motor deficits. However, cellular replacement is associated with several problems. As with pharmacological treatments, cell therapy can lead to disabling abnormal involuntary movements (dyskinesias). The exclusion of serotonin and GABA neurons, and enrichment of substantia nigra (A9) dopamine neurons, may circumvent this problem. Furthermore, although grafted foetal dopamine neurons can survive in Parkinson's patients for more than a decade, the occurrence of Lewy bodies within such transplanted cells and reduced dopamine transporter and tyrosine hydroxylase expression levels indicate that grafted cells are associated with pathology. It will be important to understand if such abnormalities are host- or graft induced and to develop methods to ensure survival of functional dopamine neurons. Careful preparation of cellular suspensions to minimize graft-induced inflammatory responses might influence the longevity of transplanted cells. Finally, a number of practical and ethical issues are associated with the use of foetal tissue sources. Thus, future cell therapy is aiming towards the use of embryonic stem cell or induced pluripotent stem cell derived dopamine neurons.

Cell-based therapies for Parkinson's disease: past, present, and future

Parkinson's disease (PD) researchers have pioneered the use of cell-based therapies (CBTs) in the central nervous system. CBTs for PD were originally envisioned as a way to replace the dopaminergic nigral neurons lost with the disease. Several sources of catecholaminergic cells, including autografts of adrenal medulla and allografts or xenografts of mesencephalic fetal tissue, were successfully assessed in animal models, but their clinical translation has yielded poor results and much controversy. Recent breakthroughs on cell biology are helping to develop novel cell lines that could be used for regenerative medicine. Their future successful clinical application depends on identifying and solving the problems encountered in previous CBTs trials. In this review, we critically analyze past CBTs' clinical translation, the impact of the host in graft survival, and the role of preclinical studies and emerging new cell lines. We propose that the prediction of clinical results from preclinical studies requires experimental designs that allow blind data acquisition and statistical analysis, assessment of the therapy in models that parallel clinical conditions, looking for sources of complications or side effects, and limiting optimism bias when reporting outcomes.

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.

Efficient production of mesencephalic dopamine neurons by Lmx1a expression in embryonic stem cells

Signaling factors involved in CNS development have been used to control the differentiation of embryonic stem cells (ESCs) into mesencephalic dopamine (mesDA) neurons, but tend to generate a limited yield of desired cell type. Here we show that forced expression of Lmx1a, a transcription factor functioning as a determinant of mesDA neurons during embryogenesis, effectively can promote the generation of mesDA neurons from mouse and human ESCs. Under permissive culture conditions, 75%-95% of mouse ESC-derived neurons express molecular and physiological properties characteristic of bona fide mesDA neurons. Similar to primary mesDA neurons, these cells integrate and innervate the striatum of 6-hydroxy dopamine lesioned neonatal rats. Thus, the enriched generation of functional mesDA neurons by forced expression of Lmx1a may be of future importance in cell replacement therapy of Parkinson disease.

Prospects of stem cell therapy for replacing dopamine neurons in Parkinson's disease

In Parkinson's disease (PD), the main pathology is a loss of nigrostriatal dopamine (DA) neurons. Clinical trials with intrastriatal transplantation of human embryonic mesencephalic tissue have shown that grafted DA neurons reinnervate the striatum, restore striatal DA release and, in some patients, induce major clinical benefit. Stem cells could provide an unlimited source of DA neurons for transplantation. Recent studies demonstrate that cells with properties of mesencephalic DA neurons can be produced from stem cells of different sources including reprogrammed somatic cells. However, as we discuss here, it remains to be shown that these cells can provide efficient functional reinnervation and behavioral recovery in animal PD models. Moreover, a clinically competitive cell therapy for PD will require better criteria for patient selection, improved functional efficacy of grafts by a tailor-made transplantation procedure providing optimum repair of the patient's DA system and strategies to prevent dyskinesias and tumor formation.

Medium spiny neurons for transplantation in Huntington's disease

Cell-replacement therapy for Huntington's disease is one of very few therapies that has reported positive outcomes in clinical trials. However, for cell transplantation to be made more readily available, logistical, standardization and ethical issues associated with the current methodology need to be resolved. To achieve these goals, it is imperative that an alternative cell source be identified. One of the key requirements of the cells is that they are capable of acquiring an MSN (medium spiny neuron) morphology, express MSN markers such as DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of 32 kDa), and function in vivo in a manner that replicates those that have been lost to the disease. Developmental biology has progressed in recent years to provide a vast array of information with regard to the key signalling events involved in the proliferation, specification and differentiation of striatal-specific neurons. In the present paper, we review the rationale for cell-replacement therapy in Huntington's disease, discuss some potential donor sources and consider the value of developmental markers in the identification of cells with the potential to develop an MSN phenotype.

Stem cells in development of therapeutics for Parkinson's disease: a perspective

Using Parkinson's disease as a prototype of neurodegenerative diseases, we propose applications of human stem cells in the development of therapeutics for neurodegenerative diseases. First, in vitro differentiation of human stem cells offers a versatile model for dissecting molecular interactions underlying human dopamine (DA) neuron specification, which may form a foundation for instigating regeneration of DA neurons from progenitors that reside in the brain. Second, stem cells derived from diseased cells or through genetic modification can serve as a platform for unraveling biochemical processes that lead to the cellular pathogenesis of degeneration. This may in turn serve as a template for identifying or developing therapeutics for slowing, stopping, or reversing the disease process. And finally, stem cells, particularly those induced from patients' own cells, provide a reliable source of DA neurons for cell-based therapy.

Pitx3 potentiates Nurr1 in dopamine neuron terminal differentiation through release of SMRT-mediated repression

In recent years, the meso-diencephalic dopaminergic (mdDA) neurons have been extensively studied for their association with Parkinson's disease. Thus far, specification of the dopaminergic phenotype of mdDA neurons is largely attributed to the orphan nuclear receptor Nurr1. In this study, we provide evidence for extensive interplay between Nurr1 and the homeobox transcription factor Pitx3 in vivo. Both Nurr1 and Pitx3 interact with the co-repressor PSF and occupy the promoters of Nurr1 target genes in concert. Moreover, in vivo expression analysis reveals that Nurr1 alone is not sufficient to drive the dopaminergic phenotype in mdDA neurons but requires Pitx3 for full activation of target gene expression. In the absence of Pitx3, Nurr1 is kept in a repressed state through interaction with the co-repressor SMRT. Highly resembling the effect of ligand activation of nuclear receptors, recruitment of Pitx3 modulates the Nurr1 transcriptional complex by decreasing the interaction with SMRT, which acts through HDACs to keep promoters in a repressed deacetylated state. Indeed, interference with HDAC-mediated repression in Pitx3(-/-) embryos efficiently reactivates the expression of Nurr1 target genes, bypassing the necessity for Pitx3. These data position Pitx3 as an essential potentiator of Nurr1 in specifying the dopaminergic phenotype, providing novel insights into mechanisms underlying development of mdDA neurons in vivo, and the programming of stem cells as a future cell replacement therapy for Parkinson's disease.

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.