Specification of midbrain dopamine neurons from primate pluripotent stem cells

Preparation, characterization, and banking of clinical-grade cells for neural transplantation: Scale up, fingerprinting, and genomic stability of stem cell lines

Parkinson's disease is a complex and progressive neurodegenerative condition that is characterized by the severe loss of midbrain dopaminergic (mDA) neurons, which innervate the striatum. Cell transplantation therapies to rebuild this dopaminergic network have been attempted for over 30 years. The most promising outcomes were observed when human fetal mesencephalic tissue was used as the source of cells for transplantation. However, reliance on terminations for a Parkinson's therapy presents significant logistical and ethical hurdles. An alternative source of transplantable mDA neurons is urgently needed, and the solution may come from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs). Protocols to differentiate hESCs/iPSCs toward mDA neurons are now robust and efficient, and upon grafting the cells rescue preclinical animal models of Parkinson's disease. The challenge now is to apply Good Manufacturing Practice (GMP) to the academic discoveries and protocols to produce clinical-grade transplantable mDA cells. Major technical and logistical considerations include (i) source of hESC or iPSC line, (ii) GMP compliance of the differentiation protocol and all reagents, (iii) characterization of the cell product in terms of identity, safety, and efficacy, (iv) characterization of genomic state and stability, and (v) banking of a transplantation-ready cell product. Approaches and solutions to these challenges are reviewed here.

A Knockin Reporter Allows Purification and Characterization of mDA Neurons from Heterogeneous Populations

Generation of midbrain dopaminergic (mDA) neurons from human pluripotent stem cells provides a platform for inquiry into basic and translational studies of Parkinson's disease (PD). However, heterogeneity in differentiation in vitro makes it difficult to identify mDA neurons in culture or in vivo following transplantation. Here, we report the generation of a human embryonic stem cell (hESC) line with a tyrosine hydroxylase (TH)-RFP (red fluorescent protein) reporter. We validated that RFP faithfully mimicked TH expression during differentiation. Use of this TH-RFP reporter cell line enabled purification of mDA-like neurons from heterogeneous cultures with subsequent characterization of neuron transcriptional and epigenetic programs (global binding profiles of H3K27ac, H3K4me1, and 5-hydroxymethylcytosine [5hmC]) at four different stages of development. We anticipate that the tools and data described here will contribute to the development of mDA neurons for applications in disease modeling and/or drug screening and cell replacement therapies for PD.

A novel floor plate boundary defined by adjacent En1 and Dbx1 microdomains distinguishes midbrain dopamine and hypothalamic neurons

The mesodiencephalic floor plate (mdFP) is the source of diverse neuron types. Yet, how this structure is compartmentalized has not been clearly elucidated. Here, we identify a novel boundary subdividing the mdFP into two microdomains, defined by engrailed 1 (En1) and developing brain homeobox 1 (Dbx1). Utilizing simultaneous dual and intersectional fate mapping, we demonstrate that this boundary is precisely formed with minimal overlap between En1 and Dbx1 microdomains, unlike many other boundaries. We show that the En1 microdomain gives rise to dopaminergic (DA) neurons, whereas the Dbx1 microdomain gives rise to subthalamic (STN), premammillary (PM) and posterior hypothalamic (PH) populations. To determine whether En1 is sufficient to induce DA neuron production beyond its normal limit, we generated a mouse strain that expresses En1 in the Dbx1 microdomain. In mutants, we observed ectopic production of DA neurons derived from the Dbx1 microdomain, at the expense of STN and PM populations. Our findings provide new insights into subdivisions in the mdFP, and will impact current strategies for the conversion of stem cells into DA neurons.

Predictive Markers Guide Differentiation to Improve Graft Outcome in Clinical Translation of hESC-Based Therapy for Parkinson's Disease

Stem cell treatments for neurodegenerative diseases are expected to reach clinical trials soon. Most of the approaches currently under development involve transplantation of immature progenitors that subsequently undergo phenotypic and functional maturation in vivo, and predicting the long-term graft outcome already at the progenitor stage remains a challenge. Here, we took an unbiased approach to identify predictive markers expressed in dopamine neuron progenitors that correlate with graft outcome in an animal model of Parkinson's disease through gene expression analysis of >30 batches of grafted human embryonic stem cell (hESC)-derived progenitors. We found that many of the commonly used markers did not accurately predict in vivo subtype-specific maturation. Instead, we identified a specific set of markers associated with the caudal midbrain that correlate with high dopaminergic yield after transplantation in vivo. Using these markers, we developed a good manufacturing practice (GMP) differentiation protocol for highly efficient and reproducible production of transplantable dopamine progenitors from hESCs.

Single-Cell Analysis Reveals a Close Relationship between Differentiating Dopamine and Subthalamic Nucleus Neuronal Lineages

Stem cell engineering and grafting of mesencephalic dopamine (mesDA) neurons is a promising strategy for brain repair in Parkinson's disease (PD). Refinement of differentiation protocols to optimize this approach will require deeper understanding of mesDA neuron development. Here, we studied this process using transcriptome-wide single-cell RNA sequencing of mouse neural progenitors expressing the mesDA neuron determinant Lmx1a. This approach resolved the differentiation of mesDA and neighboring neuronal lineages and revealed a remarkably close relationship between developing mesDA and subthalamic nucleus (STN) neurons, while also highlighting a distinct transcription factor set that can distinguish between them. While previous hESC mesDA differentiation protocols have relied on markers that are shared between the two lineages, we found that application of these highlighted markers can help to refine current stem cell engineering protocols, increasing the proportion of appropriately patterned mesDA progenitors. Our results, therefore, have important implications for cell replacement therapy in PD.

Efficiently Specified Ventral Midbrain Dopamine Neurons from Human Pluripotent Stem Cells Under Xeno-Free Conditions Restore Motor Deficits in Parkinsonian Rodents

Recent studies have shown evidence for the functional integration of human pluripotent stem cell (hPSC)‐derived ventral midbrain dopamine (vmDA) neurons in animal models of Parkinson’s disease. Although these cells present a sustainable alternative to fetal mesencephalic grafts, a number of hurdles require attention prior to clinical translation. These include the persistent use of xenogeneic reagents and challenges associated with scalability and storage of differentiated cells. In this study, we describe the first fully defined feeder‐ and xenogeneic‐free protocol for the generation of vmDA neurons from hPSCs and utilize two novel reporter knock‐in lines (LMX1A‐eGFP and PITX3‐eGFP) for in‐depth in vitro and in vivo tracking. Across multiple embryonic and induced hPSC lines, this “next generation” protocol consistently increases both the yield and proportion of vmDA neural progenitors (OTX2/FOXA2/LMX1A) and neurons (FOXA2/TH/PITX3) that display classical vmDA metabolic and electrophysiological properties. We identify the mechanism underlying these improvements and demonstrate clinical applicability with the first report of scalability and cryopreservation of bona fide vmDA progenitors at a time amenable to transplantation. Finally, transplantation of xeno‐free vmDA progenitors from LMX1A‐ and PITX3‐eGFP reporter lines into Parkinsonian rodents demonstrates improved engraftment outcomes and restoration of motor deficits. These findings provide important and necessary advancements for the translation of hPSC‐derived neurons into the clinic. Stem Cells Translational Medicine2017;6:937–948

Targeted Differentiation of Regional Ventral Neuroprogenitors and Related Neuronal Subtypes from Human Pluripotent Stem Cells

Embryoid body (EB) formation and adherent culture (AD) paradigms are equivalently thought to be applicable for neural specification of human pluripotent stem cells. Here, we report that sonic hedgehog-induced ventral neuroprogenitors under EB conditions are fated to medial ganglionic eminence (MGE), while the AD cells mostly adopt a floor-plate (FP) fate. The EB-MGE later on differentiates into GABA and cholinergic neurons, while the AD-FP favors dopaminergic neuron specification. Distinct developmental, metabolic, and adhesion traits in AD and EB cells may potentially account for their differential patterning potency. Gene targeting combined with small-molecule screening experiments identified that concomitant inhibition of Wnts, STAT3, and p38 pathways (3i) could largely convert FP to MGE under AD conditions. Thus, differentiation paradigms and signaling regulators can be integrated together to specify distinct neuronal subtypes for studying and treating related neurological diseases, such as epilepsy, Alzheimer's disease, and Parkinson's disease.

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EB and AD paradigms yield different ventral neuroprogenitors upon SHH patterning

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DA neurons of FP origin are generated in AD conditions upon SHH patterning

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Wnts/STAT3/p38 inhibition benefits MGE specification under AD conditions

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GABA and CHAT neurons of MGE origin are generated in EB or AD/3i conditions

Real-Time Intraoperative MRI Intracerebral Delivery of Induced Pluripotent Stem Cell-Derived Neurons

Induced pluripotent stem cell (iPSC)-derived neurons represent an opportunity for cell replacement strategies for neurodegenerative disorders such as Parkinson's disease (PD). Improvement in cell graft targeting, distribution, and density can be key for disease modification. We have previously developed a trajectory guide system for real-time intraoperative magnetic resonance imaging (RT-IMRI) delivery of infusates, such as viral vector suspensions for gene therapy strategies. Intracerebral delivery of iPSC-derived neurons presents different challenges than viral vectors, including limited cell survival if cells are kept at room temperature for prolonged periods of time, precipitation and aggregation of cells in the cannula, and obstruction during injection, which must be solved for successful application of this delivery approach. To develop procedures suitable for RT-IMRI cell delivery, we first performed in vitro studies to tailor the delivery hardware (e.g., cannula) and defined a range of parameters to be applied (e.g., maximal time span allowable between cell loading in the system and intracerebral injection) to ensure cell survival. Then we performed an in vivo study to evaluate the feasibility of applying the system to nonhuman primates. Our results demonstrate that the RT-IMRI delivery system provides valuable guidance, monitoring, and visualization during intracerebral cell delivery that are compatible with cell survival.

Advantages of nonhuman primates as preclinical models for evaluating stem cell-based therapies for Parkinson's disease

The derivation of dopaminergic neurons from induced pluripotent stem cells brings new hope for a patient-specific, stem cell-based replacement therapy to treat Parkinson's disease (PD) and related neurodegenerative diseases; and this novel cell-based approach has already proven effective in animal models. However, there are several aspects of this procedure that have yet to be optimized to the extent required for translation to an optimal cell-based transplantation protocol in humans. These challenges include pinpointing the optimal graft location, appropriately scaling up the graft volume, and minimizing the risk of chronic immune rejection, among others. To advance this procedure to the clinic, it is imperative that a model that accurately and fully recapitulates characteristics most pertinent to a cell-based transplantation to the human brain is used to optimize key technical aspects of the procedure. Nonhuman primates mimic humans in multiple ways including similarities in genomics, neuroanatomy, neurophysiology, immunogenetics, and age-related changes in immune function. These characteristics are critical to the establishment of a relevant model in which to conduct preclinical studies to optimize the efficacy and safety of cell-based therapeutic approaches to the treatment of PD. Here we review previous studies in rodent models, and emphasize additional advantages afforded by nonhuman primate models in general, and the baboon model in particular, for preclinical optimization of cell-based therapeutic approaches to the treatment of PD and other neurodegenerative diseases. We outline current unresolved challenges to the successful application of stem cell therapies in humans and propose that the baboon model in particular affords a number of traits that render it most useful for preclinical studies designed to overcome these challenges.

Differentiation and Characterization of Dopaminergic Neurons From Baboon Induced Pluripotent Stem Cells

: The progressive death of dopamine producing neurons in the substantia nigra pars compacta is the principal cause of symptoms of Parkinson's disease (PD). Stem cells have potential therapeutic use in replacing these cells and restoring function. To facilitate development of this approach, we sought to establish a preclinical model based on a large nonhuman primate for testing the efficacy and safety of stem cell-based transplantation. To this end, we differentiated baboon fibroblast-derived induced pluripotent stem cells (biPSCs) into dopaminergic neurons with the application of specific morphogens and growth factors. We confirmed that biPSC-derived dopaminergic neurons resemble those found in the human midbrain based on cell type-specific expression of dopamine markers TH and GIRK2. Using the reverse transcriptase quantitative polymerase chain reaction, we also showed that biPSC-derived dopaminergic neurons express PAX6, FOXA2, LMX1A, NURR1, and TH genes characteristic of this cell type in vivo. We used perforated patch-clamp electrophysiology to demonstrate that biPSC-derived dopaminergic neurons fired spontaneous rhythmic action potentials and high-frequency action potentials with spike frequency adaption upon injection of depolarizing current. Finally, we showed that biPSC-derived neurons released catecholamines in response to electrical stimulation. These results demonstrate the utility of the baboon model for testing and optimizing the efficacy and safety of stem cell-based therapeutic approaches for the treatment of PD.

SIGNIFICANCE

Functional dopamine neurons were produced from baboon induced pluripotent stem cells, and their properties were compared to baboon midbrain cells in vivo. The baboon has advantages as a clinically relevant model in which to optimize the efficacy and safety of stem cell-based therapies for neurodegenerative diseases, such as Parkinson's disease. Baboons possess crucial neuroanatomical and immunological similarities to humans, and baboon pluripotent stem cells can be differentiated into functional neurons that mimic those in the human brain, thus laying the foundation for the utility of the baboon model for evaluating stem cell therapies.

Chemical Control of Grafted Human PSC-Derived Neurons in a Mouse Model of Parkinson's Disease

Transplantation of human pluripotent stem cell (hPSC)-derived neurons is a promising avenue for treating disorders including Parkinson's disease (PD). Precise control over engrafted cell activity is highly desired, as cells do not always integrate properly into host circuitry and can cause suboptimal graft function or undesired outcomes. Here, we show tunable rescue of motor function in a mouse model of PD, following transplantation of human midbrain dopaminergic (mDA) neurons differentiated from hPSCs engineered to express DREADDs (designer receptors exclusively activated by designer drug). Administering clozapine-N-oxide (CNO) enabled precise DREADD-dependent stimulation or inhibition of engrafted neurons, revealing D1 receptor-dependent regulation of host neuronal circuitry by engrafted cells. Transplanted cells rescued motor defects, which could be reversed or enhanced by CNO-based control of graft function, and activating engrafted cells drives behavioral changes in transplanted mice. These results highlight the ability to exogenously and noninvasively control and refine therapeutic outcomes following cell transplantation.

Engineering human cells and tissues through pluripotent stem cells

The utility of human pluripotent stem cells (hPSCs) depends on their ability to produce functional cells and tissues of the body. Two strategies have been developed: directed differentiation of enriched populations of cells that match a regional and functional profile and spontaneous generation of three-dimensional organoids that resemble tissues in the body. Genomic editing of hPSCs and their differentiated cells broadens the use of the hPSC paradigm in studying human cellular function and disease as well as developing therapeutics.

Directed differentiation of basal forebrain cholinergic neurons from human pluripotent stem cells

BACKGROUND

Basal forebrain cholinergic neurons (BFCNs) play critical roles in learning, memory and cognition. Dysfunction or degeneration of BFCNs may connect to neuropathology, such as Alzheimer's disease, Down's syndrome and dementia. Generation of functional BFCNs may contribute to the studies of cell-based therapy and pathogenesis that is related to learning and memory deficits.

NEW METHOD

Here we describe a detail method for robust generation of BFCNs from human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs). In this method, BFCN progenitors are patterned from hESC or hiPSC-derived primitive neuroepithelial cells, with the treatment of sonic hedgehog (SHH) or combination with its agonist Purmorphamine, and by co-culturing with human astrocytes.

RESULTS

At day 20, ∼90% hPSC-derived progenitors expressed NKX2.1, which is a transcriptional marker for MGE. Moreover, around 40% of NKX2.1+ cells co-expressed OLIG2 and ∼15% of NKX2.1+ cells co-expressed ISLET1, which are ventral markers. At day 35, ∼40% neurons robustly express ChAT, most of which are co-labeled with NKX2.1, ISLET1 and FOXG1, indicating the basal forebrain-like identity. At day 45, these neurons express mature neuronal markers MAP2, Synapsin, and VAChT.

COMPARISON WITH EXISTING METHOD(S)

In this method, undefined conditions including genetic modification or cell-sorting are avoided. As a choice, feeder free conditions are used to avoid ingredients of animal origin. Moreover, Purmorphamine can be substituted for SHH to induce ventral progenitors effectively and economically.

CONCLUSION

We provide an efficient method to generate BFCNs from multiple hPSC lines, which offers the potential application for disease modeling and pharmacological studies.

Efficient derivation of dopaminergic neurons from SOX1⁻ floor plate cells under defined culture conditions

Background

Parkinson’s disease (PD) is a severe neurodegenerative disease associated with loss of dopaminergic neurons. Derivation of dopaminergic neurons from human embryonic stem cells (hESCs) could provide new therapeutic options for PD therapy. Dopaminergic neurons are derived from SOX⁻ floor plate (FP) cells during embryonic development in many species and in human cell culture in vitro. Early treatment with sonic hedgehog (Shh) has been reported to efficiently convert hESCs into FP lineages.

Methods

In this study, we attempted to utilize a Shh-free approach in deriving SOX1⁻ FP cells from hESCs in vitro. Neuroectoderm conversion from hESCs was achieved with dual inhibition of the BMP4 (LDN193189) and TGF-β signaling pathways (SB431542) for 24 h under defined culture conditions.

Results

Following a further 5 days of treatment with LDN193189 or LDN193189 + SB431542, SOX1⁻ FP cells constituted 70–80 % of the entire cell population. Upon treatment with Shh and FGF8, the SOX1⁻ FP cells were efficiently converted to functional Nurr1⁺ and TH⁺ dopaminergic cells (patterning), which constituted more than 98 % of the entire cell population. However, when the same growth factors were applied to SOX1⁺ cells, only less than 4 % of the cells became Nurr1⁺, indicating that patterning was effective only if SOX1 expression was down-regulated. After transplanting the Nurr1⁺ and TH⁺ cells into a hemiparkinsonian rat model, significant improvements were observed in amphetamine induced ipslateral rotations, apomorphine induced contra-lateral rotations and Rota rod motor tests over a duration of 8 weeks.

Conclusions

Our findings thus provide a convenient approach to FP development and functional dopaminergic neuron derivation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-016-0251-6) contains supplementary material, which is available to authorized users.

STB-HO, a novel mica fine particle, inhibits the teratoma-forming ability of human embryonic stem cells after in vivo transplantation

Although pluripotent stem cell (PSC) therapy has advantages for clinical applications because of the self-renewal and multi-lineage differentiation abilities of PSCs, it also has disadvantages in terms of the potential for PSCs to undergo malignant transformation or unexpected differentiation. The prevention of teratoma formation is the largest hurdle of all. Despite intensive studies that have investigated ways to block teratomas, such methods have yet to be further developed for clinical use. Here, a new approach has focused on exerting anti-tumorigenic effects using a novel mica fine particle (MFP) designated STB-HO. Treatment with STB-HO regulated pluripotency- and apoptosis-related genes in differentiating human embryonic stem (hES) cells, while there is no effects in undifferentiated hES cells. In particular, STB-HO blocked the anti-apoptotic gene BIRC5 and activated p53, p21 and the pro-apoptotic proteins Bim, Puma and p-Bad during early spontaneous differentiation. Moreover, STB-HO-pretreated differentiating hES cells did not give rise to teratomas following in vivo stem cell transplantation. Our in vitro and in vivo results suggest a method for teratoma prevention in the context of PSC-derived cell transplantation. This novel MFP could break through the limitations of PSC therapy.

Creation of a library of induced pluripotent stem cells from Parkinsonian patients

Induced pluripotent stem cells (iPSCs) are becoming an important source of pre-clinical models for research focusing on neurodegeneration. They offer the possibility for better understanding of common and divergent pathogenic mechanisms of brain diseases. Moreover, iPSCs provide a unique opportunity to develop personalized therapeutic strategies, as well as explore early pathogenic mechanisms, since they rely on the use of patients' own cells that are otherwise accessible only post-mortem, when neuronal death-related cellular pathways and processes are advanced and adaptive. Neurodegenerative diseases are in majority of unknown cause, but mutations in specific genes can lead to familial forms of these diseases. For example, mutations in the superoxide dismutase 1 gene lead to the motor neuron disease amyotrophic lateral sclerosis (ALS), while mutations in the SNCA gene encoding for alpha-synuclein protein lead to familial Parkinson's disease (PD). The generations of libraries of familial human ALS iPSC lines have been described, and the iPSCs rapidly became useful models for studying cell autonomous and non-cell autonomous mechanisms of the disease. Here we report the generation of a comprehensive library of iPSC lines of familial PD and an associated synucleinopathy, multiple system atrophy (MSA). In addition, we provide examples of relevant neural cell types these iPSC can be differentiated into, and which could be used to further explore early disease mechanisms. These human cellular models will be a valuable resource for identifying common and divergent mechanisms leading to neurodegeneration in PD and MSA.

Modeling Axonal Phenotypes with Human Pluripotent Stem Cells

Impaired axonal development and degeneration are implicated in many debilitating disorders, such as hereditary spastic paraplegia (HSP), amyotrophic lateral sclerosis (ALS), and periphery neuropathy. Human pluripotent stem cells (hPSCs) have provided researchers with an excellent resource for modeling human neuropathologic processes including axonal defects in vitro. There are a number of steps that are crucial when developing an hPSC-based model of a human disease, including generating induced pluripotent stem cells (iPSCs), differentiating those cells to affected cell types, and identifying disease-relevant phenotypes. Here, we describe these steps in detail, focusing on the neurodegenerative disorder HSP.

Generation of serotonin neurons from human pluripotent stem cells

Serotonin neurons located in the raphe nucleus of the hindbrain have crucial roles in regulating brain functions and have been implicated in various psychiatric disorders. Yet functional human serotonin neurons are not available for in vitro studies. Through manipulation of the WNT pathway, we demonstrate efficient differentiation of human pluripotent stem cells (hPSCs) to cells resembling central serotonin neurons, primarily those located in the rhombomeric segments 2-3 of the rostral raphe, which participate in high-order brain functions. The serotonin neurons express a series of molecules essential for serotonergic development, including tryptophan hydroxylase 2, exhibit typical electrophysiological properties and release serotonin in an activity-dependent manner. When treated with the FDA-approved drugs tramadol and escitalopram oxalate, they release or uptake serotonin in a dose- and time-dependent manner, suggesting the utility of these cells for the evaluation of drug candidates.

Cell Therapy for Parkinson's Disease: New Hope from Reprogramming Technologies

Parkinson's disease (PD) is a neurodegenerative disease with the major pathology being the progressive loss of dopaminergic (DA) midbrain neurons in the substantia nigra. As early as in the 1980s, open-label clinical trials employing fetal ventral mesencephalon (fVM) tissues have demonstrated significant efficacy for PD treatment, which led to two NIH-sponsored double-blind placebo-controlled clinical trials. However, both trials showed only mild outcome. Retrospective analysis revealed several possible reasons that include patient selection, heterogeneity of grafts, immune recognition of grafts, lack of standardization of transplantation procedure and uneven distribution of grafts. Recent years have seen advances in reprogramming technologies which may provide solutions to the problems associated with fVM tissues. Induced pluripotent stem cells (iPSCs) and induced neural stem cells (iNSCs) hold promise for generating clinical grade DA neural cells that are safe, homogeneous, scalable and standardizable. These new technologies may bring back clinical trials using cell therapy for PD treatment in the future.

A High-content screen identifies compounds promoting the neuronal differentiation and the midbrain dopamine neuron specification of human neural progenitor cells

Small molecule compounds promoting the neuronal differentiation of stem/progenitor cells are of pivotal importance to regenerative medicine. We carried out a high-content screen to systematically characterize known bioactive compounds, on their effects on the neuronal differentiation and the midbrain dopamine (mDA) neuron specification of neural progenitor cells (NPCs) derived from the ventral mesencephalon of human fetal brain. Among the promoting compounds three major pharmacological classes were identified including the statins, TGF-βRI inhibitors, and GSK-3 inhibitors. The function of each class was also shown to be distinct, either to promote both the neuronal differentiation and mDA neuron specification, or selectively the latter, or promote the former but suppress the latter. We then carried out initial investigation on the possible mechanisms underlying, and demonstrated their applications on NPCs derived from human pluripotent stem cells (PSCs). Our study revealed the potential of several small molecule compounds for use in the directed differentiation of human NPCs. The screening result also provided insight into the signaling network regulating the differentiation of human NPCs.

Excessive Wnt/beta-catenin signaling promotes midbrain floor plate neurogenesis, but results in vacillating dopamine progenitors

The floor plate (FP), a ventral midline structure of the developing neural tube, has differential neurogenic capabilities along the anterior-posterior axis. The midbrain FP, unlike the hindbrain and spinal cord floor plate, is highly neurogenic and produces midbrain dopaminergic (mDA) neurons. Canonical Wnt/beta-catenin signaling, at least in part, is thought to account for the difference in neurogenic capability. Removal of beta-catenin results in mDA progenitor specification defects as well as a profound reduction of neurogenesis. To examine the effects of excessive Wnt/beta-catenin signaling on mDA specification and neurogenesis, we have analyzed a model wherein beta-catenin is conditionally stabilized in the Shh+domain. Here, we show that the Foxa2+/Lmx1a+ domain is extended rostrally in mutant embryos, suggesting that canonical Wnt/beta-catenin signaling can drive FP expansion along the rostrocaudal axis. Although excess canonical Wnt/beta-catenin signaling generally promotes neurogenesis at midbrain levels, less tyrosine hydroxylase (Th)+, mDA neurons are generated, particularly impacting the Substantia Nigra pars compacta. This is likely because of improper progenitor specification. Excess canonical Wnt/beta-catenin signaling causes downregulation of net Lmx1b, Shh and Foxa2 levels in mDA progenitors. Moreover, these progenitors assume a mixed identity to that of Lmx1a+/Lmx1b+/Nkx6-1+/Neurog1+ progenitors. We also show by lineage tracing analysis that normally, Neurog1+ progenitors predominantly give rise to Pou4f1+ neurons, but not Th+ neurons. Accordingly, in the mutant embryos, Neurog1+ progenitors at the midline generate ectopic Pou4f1+ neurons at the expense of Th+ mDA neurons. Our study suggests that an optimal dose of Wnt/beta-catenin signaling is critical for proper establishment of the mDA progenitor character. Our findings will impact embryonic stem cell protocols that utilize Wnt pathway reagents to derive mDA neuron models and therapeutics for Parkinson's disease.

Selective Generation of Dopaminergic Precursors from Mouse Fibroblasts by Direct Lineage Conversion

Degeneration of midbrain dopaminergic (DA) neurons is a key pathological event of Parkinson’s disease (PD). Limited adult dopaminergic neurogenesis has led to novel therapeutic strategies such as transplantation of dopaminergic precursors (DPs). However, this strategy is currently restrained by a lack of cell source, the tendency for the DPs to become a glial-restricted state, and the tumor formation after transplantation. Here, we demonstrate the direct conversion of mouse fibroblasts into induced DPs (iDPs) by ectopic expression of Brn2, Sox2 and Foxa2. Besides expression with neural progenitor markers and midbrain genes including Corin, Otx2 and Lmx1a, the iDPs were restricted to dopaminergic neuronal lineage upon differentiation. After transplantation into MPTP-lesioned mice, iDPs differentiated into DA neurons, functionally alleviated the motor deficits, and reduced the loss of striatal DA neuronal axonal termini. Importantly, no iDPs-derived astroctyes and neoplasia were detected in mouse brains after transplantation. We propose that the iDPs from direct reprogramming provides a safe and efficient cell source for PD treatment.

Moving stem cells to the clinic: potential and limitations for brain repair

Stem cell-based therapies hold considerable promise for many currently devastating neurological disorders. Substantial progress has been made in the derivation of disease-relevant human donor cell populations. Behavioral data in relevant animal models of disease have demonstrated therapeutic efficacy for several cell-based approaches. Consequently, cGMP grade cell products are currently being developed for first in human clinical trials in select disorders. Despite the therapeutic promise, the presumed mechanism of action of donor cell populations often remains insufficiently validated. It depends greatly on the properties of the transplanted cell type and the underlying host pathology. Several new technologies have become available to probe mechanisms of action in real time and to manipulate in vivo cell function and integration to enhance therapeutic efficacy. Results from such studies generate crucial insight into the nature of brain repair that can be achieved today and push the boundaries of what may be possible in the future.

How to make a midbrain dopaminergic neuron

Midbrain dopaminergic (mDA) neuron development has been an intense area of research during recent years. This is due in part to a growing interest in regenerative medicine and the hope that treatment for diseases affecting mDA neurons, such as Parkinson's disease (PD), might be facilitated by a better understanding of how these neurons are specified, differentiated and maintained in vivo. This knowledge might help to instruct efforts to generate mDA neurons in vitro, which holds promise not only for cell replacement therapy, but also for disease modeling and drug discovery. In this Primer, we will focus on recent developments in understanding the molecular mechanisms that regulate the development of mDA neurons in vivo, and how they have been used to generate human mDA neurons in vitro from pluripotent stem cells or from somatic cells via direct reprogramming. Current challenges and future avenues in the development of a regenerative medicine for PD will be identified and discussed.

A Stem Cell-Derived Platform for Studying Single Synaptic Vesicles in Dopaminergic Synapses

The exocytotic release of dopamine is one of the most characteristic but also one of the least appreciated processes in dopaminergic neurotransmission. Fluorescence imaging has yielded rich information about the properties of synaptic vesicles and the release of neurotransmitters in excitatory and inhibitory neurons. In contrast, imaging-based studies for in-depth understanding of synaptic vesicle behavior in dopamine neurons are lagging largely because of a lack of suitable preparations. Midbrain culture has been one of the most valuable preparations for the subcellular investigation of dopaminergic transmission; however, the paucity and fragility of cultured dopaminergic neurons limits their use for live cell imaging. Recent developments in stem cell technology have led to the successful production of dopamine neurons from embryonic or induced pluripotent stem cells. Although the dopaminergic identity of these stem cell-derived neurons has been characterized in different ways, vesicle-mediated dopamine release from their axonal terminals has been barely assessed. We report a more efficient procedure to reliably generate dopamine neurons from embryonic stem cells, and it yields more dopamine neurons with more dopaminergic axon projections than midbrain culture does. Using a collection of functional measurements, we show that stem cell-derived dopamine neurons are indistinguishable from those in midbrain culture. Taking advantage of this new preparation, we simultaneously tracked the turnover of hundreds of synaptic vesicles individually using pH-sensitive quantum dots. By doing so, we revealed distinct fusion kinetics of the dopamine-secreting vesicles, which is consistent within both preparations.

The potential of alternate sources of cells for neural grafting in Parkinson's and Huntington's disease

Cell-based therapies for Parkinson's and Huntington's disease have provided mixed clinical outcomes and one of the reasons underlying this is the use of primary fetal tissue as the source of grafted cells. An alternate source of cells, such as stem cells, could overcome many of the issues associated with primary fetal tissue and would help bring forward cell replacement therapy as a reliable and effective treatment for these two neurodegenerative disorders. This review will discuss which stem cells are likely to go to clinic in the next generation of cells, based on trials for Parkinson's and Huntington's disease.

Generation and expansion of highly pure motor neuron progenitors from human pluripotent stem cells

Human pluripotent stem cells (hPSCs) have opened new opportunities for understanding human development, modelling disease processes and developing new therapeutics. However, these applications are hindered by the low efficiency and heterogeneity of cell types, such as motorneurons (MNs), differentiated from hPSCs as well as our inability to maintain the potency of lineage-committed progenitors. Here by using a combination of small molecules that regulate multiple signalling pathways, we develop a method to guide human embryonic stem cells to a near-pure population (>95%) of motor neuron progenitors (MNPs) in 12 days, and an enriched population (>90%) of functionally mature MNs in an additional 16 days. More importantly, the MNPs can be expanded for at least five passages so that a single MNP can be amplified to 1 × 10(4). This method is reproducible in human-induced pluripotent stem cells and is applied to model MN-degenerative diseases and in proof-of-principle drug-screening assays.

The role of microRNAs in human neural stem cells, neuronal differentiation and subtype specification

The impressive neuronal diversity found within the nervous system emerges from a limited pool of neural progenitor cells that proceed through different gene expression programs to acquire distinct cell fates. Here, we review recent evidence indicating that microRNAs (miRNAs) are critically involved in conferring neural cell identities during neural induction, neuronal differentiation and subtype specification. Several studies have shown that miRNAs act in concert with other gene regulatory factors and genetic switches to regulate the spatial and temporal expression profiles of important cell fate determinants. So far, most studies addressing the role of miRNAs during neurogenesis were conducted using animal models. With the advent of human pluripotent stem cells and the possibility to differentiate these into neural stem cells, we now have the opportunity to study miRNAs in a human context. More insight into the impact of miRNA-based regulation during neural fate choice could in the end be exploited to develop new strategies for the generation of distinct human neuronal cell types.

Differentiation of human epidermal neural crest stem cells (hEPI-NCSC) into virtually homogenous populations of dopaminergic neurons

Here we provide a protocol for the directed differentiation of hEPI-NCSC into midbrain dopaminergic neurons, which degenerate in Parkinson's disease. hEPI-NCSC are neural crest-derived multipotent stem cells that persist into adulthood in the bulge of hair follicles. The experimental design is distinctly different from conventional protocols for embryonic stem cells and induced pluripotent stem (iPS) cells. It includes pre-differentiation of the multipotent hEPI-NCSC into neural stem cell-like cells, followed by ventralizing, patterning, continued exposure to the TGFβ receptor inhibitor, SB431542, and at later stages of differentiation the presence of the WNT inhibitor, IWP-4. All cells expressed A9 midbrain dopaminergic neuron progenitor markers with gene expression levels comparable to those in normal human substantia nigra. The current study shows for the first time that virtually homogeneous populations of dopaminergic neurons can be derived ex vivo from somatic stem cells without the need for purification, with useful timeliness and high efficacy. This novel development is an important first step towards the establishment of fully functional dopaminergic neurons from an ontologically relevant stem cell type, hEPI-NCSC.

Catecholamine autotoxicity. Implications for pharmacology and therapeutics of Parkinson disease and related disorders

Several neurodegenerative diseases involve loss of catecholamine neurons-Parkinson disease is a prototypical example. Catecholamine neurons are rare in the nervous system, and why they are vulnerable in PD and related disorders has been mysterious. Accumulating evidence supports the concept of "autotoxicity"-inherent cytotoxicity of catecholamines and their metabolites in the cells in which they are produced. According to the "catecholaldehyde hypothesis" for the pathogenesis of Parkinson disease, long-term increased build-up of 3,4-dihydroxyphenylacetaldehyde (DOPAL), the catecholaldehyde metabolite of dopamine, causes or contributes to the eventual death of dopaminergic neurons. Lewy bodies, a neuropathologic hallmark of PD, contain precipitated alpha-synuclein. Bases for the tendency of alpha-synuclein to precipitate in the cytoplasm of catecholaminergic neurons have also been mysterious. Since DOPAL potently oligomerizes and aggregates alpha-synuclein, the catecholaldehyde hypothesis provides a link between alpha-synucleinopathy and catecholamine neuron loss in Lewy body diseases. The concept developed here is that DOPAL and alpha-synuclein are nodes in a complex nexus of interacting homeostatic systems. Dysfunctions of several processes, including decreased vesicular sequestration of cytoplasmic catecholamines, decreased aldehyde dehydrogenase activity, and oligomerization of alpha-synuclein, lead to conversion from the stability afforded by negative feedback regulation to the instability, degeneration, and system failure caused by induction of positive feedback loops. These dysfunctions result from diverse combinations of genetic predispositions, environmental exposures, stress, and time. The notion of catecholamine autotoxicity has several implications for treatment, disease modification, and prevention. Conversely, disease modification clinical trials would provide key tests of the catecholaldehyde hypothesis.

Combinatorial analysis of developmental cues efficiently converts human pluripotent stem cells into multiple neuronal subtypes

Specification of cell identity during development depends on exposure of cells to sequences of extrinsic cues delivered at precise times and concentrations. Identification of combinations of patterning molecules that control cell fate is essential for the effective use of human pluripotent stem cells (hPSCs) for basic and translational studies. Here we describe a scalable, automated approach to systematically test the combinatorial actions of small molecules for the targeted differentiation of hPSCs. Applied to the generation of neuronal subtypes, this analysis revealed an unappreciated role for canonical Wnt signaling in specifying motor neuron diversity from hPSCs and allowed us to define rapid (14 days), efficient procedures to generate spinal and cranial motor neurons as well as spinal interneurons and sensory neurons. Our systematic approach to improving hPSC-targeted differentiation should facilitate disease modeling studies and drug screening assays.

Directed dopaminergic neuron differentiation from human pluripotent stem cells

Dopaminergic (DA) neurons in the substantia nigra pars compacta (also known as A9 DA neurons) are the specific cell type that is lost in Parkinson's disease (PD). There is great interest in deriving A9 DA neurons from human pluripotent stem cells (hPSCs) for regenerative cell replacement therapy for PD. During neural development, A9 DA neurons originate from the floor plate (FP) precursors located at the ventral midline of the central nervous system. Here, we optimized the culture conditions for the stepwise differentiation of hPSCs to A9 DA neurons, which mimics embryonic DA neuron development. In our protocol, we first describe the efficient generation of FP precursor cells from hPSCs using a small molecule method, and then convert the FP cells to A9 DA neurons, which could be maintained in vitro for several months. This efficient, repeatable and controllable protocol works well in human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs) from normal persons and PD patients, in which one could derive A9 DA neurons to perform in vitro disease modeling and drug screening and in vivo cell transplantation therapy for PD.

Survival and Integration of Neurons Derived from Human Embryonic Stem Cells in MPTP-Lesioned Primates

A human embryonic stem cell (HESC) line, H1, was studied after differentiation to a dopaminergic phenotype in vitro in order to carry out in vivo studies in Parkinsonian monkeys. To identify morphological characteristics of transplanted donor cells, HESCs were transfected with a GFP lentiviral vector. Gene expression studies were performed at each step of a neural rosette-based dopaminergic differentiation protocol by RT-PCR. In vitro immunofluorescence revealed that >90% of the differentiated cells exhibited a neuronal phenotype by β-III-tubulin immunocytochemistry, with 17% of the cells coexpressing tyrosine hydroxylase prior to implantation. Biochemical analyses demonstrated dopamine release in culture in response to potassium chloride-induced membrane depolarization, suggesting that the cells synthesized and released dopamine. These characterized, HESC-derived neurons were then implanted into the striatum and midbrain of MPTP (1-methyl-4- phenyl-1,2,3,6-tetrahydropyridine)-exposed monkeys that were triple immunosuppressed. Here we demonstrate robust survival of transplanted HESC-derived neurons after 6 weeks, as well as morphological features consistent with polarization, organization, and extension of processes that integrated into the host striatum. Expression of the dopaminergic marker tyrosine hydroxylase was not maintained in HESC-derived neural grafts in either the striatum or substantia nigra, despite a neuronal morphology and expression of β-III-tubulin. These results suggest that dopamine neuronal cells derived from neuroectoderm in vitro will not maintain the correct midbrain phenotype in vivo in nonhuman primates, contrasted with recent studies showing dopamine neuronal survival using an alternative floorplate method.

Human foetal brain tissue as quality control when developing stem cells towards cell replacement therapy for neurological diseases

Human foetal brain tissue has been used in experimental and clinical trials to develop cell replacement therapy in neurodegenerative disorders such as Parkinson's disease and Huntington's disease. These pioneering clinical studies have shown proof of principle that cell replacement therapy can be effective and is worthwhile to develop as a therapeutic strategy for repairing the damaged brain. However, because of the limited availability of foetal brain material, and difficulties in producing standardized and quality-tested cell preparations from this source, there have been extensive efforts in investigating the potential use of alternative cell sources for generating a large number of transplantable, authentic neural progenitors and neurons. In this review, we highlight the value of using human foetal tissue as a reference material for quality control of acquired cell fate of in vitro generated neurons before and after transplantation.

Selection Based on FOXA2 Expression Is Not Sufficient to Enrich for Dopamine Neurons From Human Pluripotent Stem Cells

Human embryonic and induced pluripotent stem cells are potential cell sources for regenerative approaches in Parkinson disease. Inductive differentiation protocols can generate midbrain dopamine neurons but result in heterogeneous cell mixtures. Therefore, selection strategies are necessary to obtain uniform dopamine cell populations. Here, we developed a selection approach using lentivirus vectors to express green fluorescent protein under the promoter region of FOXA2, a transcription factor that is expressed in the floor plate domain that gives rise to dopamine neurons during embryogenesis. We first validated the specificity of the vectors in human cell lines against a promoterless construct. We then selected FOXA2-positive neural progenitors from several human pluripotent stem cell lines, which demonstrated a gene expression profile typical for the ventral domain of the midbrain and floor plate, but failed to enrich for dopamine neurons. To investigate whether this was due to the selection approach, we overexpressed FOXA2 in neural progenitors derived from human pluripotent stem cell lines. FOXA2 forced expression resulted in an increased expression of floor plate but not mature neuronal markers. Furthermore, selection of the FOXA2 overexpressing fraction also failed to enrich for dopamine neurons. Collectively, our results suggest that FOXA2 is not sufficient to induce a dopaminergic fate in this system. On the other hand, our study demonstrates that a combined approach of promoter activation and lentivirus vector technology can be used as a versatile tool for the selection of a defined cell population from a variety of human pluripotent stem cell lines.

Modeling ALS with iPSCs reveals that mutant SOD1 misregulates neurofilament balance in motor neurons

Amyotrophic lateral sclerosis (ALS) presents motoneuron (MN)-selective protein inclusions and axonal degeneration but the underlying mechanisms of such are unknown. Using induced pluripotent cells (iPSCs) from patients with mutation in the Cu/Zn superoxide dismutase (SOD1) gene, we show that spinal MNs, but rarely non-MNs, exhibited neurofilament (NF) aggregation followed by neurite degeneration when glia were not present. These changes were associated with decreased stability of NF-L mRNA and binding of its 3' UTR by mutant SOD1 and thus altered protein proportion of NF subunits. Such MN-selective changes were mimicked by expression of a single copy of the mutant SOD1 in human embryonic stem cells and were prevented by genetic correction of the SOD1 mutation in patient's iPSCs. Importantly, conditional expression of NF-L in the SOD1 iPSC-derived MNs corrected the NF subunit proportion, mitigating NF aggregation and neurite degeneration. Thus, NF misregulation underlies mutant SOD1-mediated NF aggregation and axonal degeneration in ALS MNs.

Isolation of human induced pluripotent stem cell-derived dopaminergic progenitors by cell sorting for successful transplantation

Human induced pluripotent stem cells (iPSCs) can provide a promising source of midbrain dopaminergic (DA) neurons for cell replacement therapy for Parkinson’s disease. However, iPSC-derived donor cells inevitably contain tumorigenic or inappropriate cells. Here, we show that human iPSC-derived DA progenitor cells can be efficiently isolated by cell sorting using a floor plate marker, CORIN. We induced DA neurons using scalable culture conditions on human laminin fragment, and the sorted CORIN⁺ cells expressed the midbrain DA progenitor markers, FOXA2 and LMX1A. When transplanted into 6-OHDA-lesioned rats, the CORIN⁺ cells survived and differentiated into midbrain DA neurons in vivo, resulting in significant improvement of the motor behavior, without tumor formation. In particular, the CORIN⁺ cells in a NURR1⁺ cell-dominant stage exhibited the best survival and function as DA neurons. Our method is a favorable strategy in terms of scalability, safety, and efficiency and may be advantageous for clinical application.

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DA neurons were highly efficiently induced from iPSCs on xeno-free laminin fragment

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DA progenitors were enriched by sorting of CORIN⁺ cells

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CORIN⁺ cell grafts resulted in good DA neuron survival without tumor formation

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NURR1⁺ cell-dominant stage exhibited the best survival and function as DA neurons

Mutant LRRK2 toxicity in neurons depends on LRRK2 levels and synuclein but not kinase activity or inclusion bodies

By combining experimental neuron models and mathematical tools, we developed a "systems" approach to deconvolve cellular mechanisms of neurodegeneration underlying the most common known cause of Parkinson's disease (PD), mutations in leucine-rich repeat kinase 2 (LRRK2). Neurons ectopically expressing mutant LRRK2 formed inclusion bodies (IBs), retracted neurites, accumulated synuclein, and died prematurely, recapitulating key features of PD. Degeneration was predicted from the levels of diffuse mutant LRRK2 that each neuron contained, but IB formation was neither necessary nor sufficient for death. Genetic or pharmacological blockade of its kinase activity destabilized LRRK2 and lowered its levels enough to account for the moderate reduction in LRRK2 toxicity that ensued. By contrast, targeting synuclein, including neurons made from PD patient-derived induced pluripotent cells, dramatically reduced LRRK2-dependent neurodegeneration and LRRK2 levels. These findings suggest that LRRK2 levels are more important than kinase activity per se in predicting toxicity and implicate synuclein as a major mediator of LRRK2-induced neurodegeneration.

Pluripotent stem cells in regenerative medicine: challenges and recent progress

After years of incremental progress, several recent studies have succeeded in deriving disease-relevant cell types from human pluripotent stem cell (hPSC) sources. The prospect of an unlimited cell source, combined with promising preclinical data, indicates that hPSC technology may be on the verge of clinical translation. In this Review, we discuss recent progress in directed differentiation, some of the new technologies that have facilitated the success of hPSC therapies and the remaining hurdles on the road towards developing hPSC-based cell therapies.

Direct lineage reprogramming of mouse fibroblasts to functional midbrain dopaminergic neuronal progenitors

The direct lineage reprogramming of somatic cells to other lineages by defined factors has led to innovative cell-fate-change approaches for providing patient-specific cells. Recent reports have demonstrated that four pluripotency factors (Oct4, Sox2, Klf4, and c-Myc) are sufficient to directly reprogram fibroblasts to other specific cells, including induced neural stem cells (iNSCs). Here, we show that mouse fibroblasts can be directly reprogrammed into midbrain dopaminergic neuronal progenitors (DPs) by temporal expression of the pluripotency factors and environment containing sonic hedgehog and fibroblast growth factor 8. Within thirteen days, self-renewing and functional induced DPs (iDPs) were generated. Interestingly, the inhibition of both Jak and Gsk3β notably enhanced the iDP reprogramming efficiency. We confirmed the functionality of the iDPs by showing that the dopaminergic neurons generated from iDPs express midbrain markers, release dopamine, and show typical electrophysiological profiles. Our results demonstrate that the pluripotency factors-mediated direct reprogramming is an invaluable strategy for supplying functional and proliferating iDPs and may be useful for other neural progenitors required for disease modeling and cell therapies for neurodegenerative disorders.

Improved cell therapy protocols for Parkinson's disease based on differentiation efficiency and safety of hESC-, hiPSC-, and non-human primate iPSC-derived dopaminergic neurons

The main motor symptoms of Parkinson's disease are due to the loss of dopaminergic (DA) neurons in the ventral midbrain (VM). For the future treatment of Parkinson's disease with cell transplantation it is important to develop efficient differentiation methods for production of human iPSCs and hESCs-derived midbrain-type DA neurons. Here we describe an efficient differentiation and sorting strategy for DA neurons from both human ES/iPS cells and non-human primate iPSCs. The use of non-human primate iPSCs for neuronal differentiation and autologous transplantation is important for preclinical evaluation of safety and efficacy of stem cell-derived DA neurons. The aim of this study was to improve the safety of human- and non-human primate iPSC (PiPSC)-derived DA neurons. According to our results, NCAM(+) /CD29(low) sorting enriched VM DA neurons from pluripotent stem cell-derived neural cell populations. NCAM(+) /CD29(low) DA neurons were positive for FOXA2/TH and EN1/TH and this cell population had increased expression levels of FOXA2, LMX1A, TH, GIRK2, PITX3, EN1, NURR1 mRNA compared to unsorted neural cell populations. PiPSC-derived NCAM(+) /CD29(low) DA neurons were able to restore motor function of 6-hydroxydopamine (6-OHDA) lesioned rats 16 weeks after transplantation. The transplanted sorted cells also integrated in the rodent brain tissue, with robust TH+/hNCAM+ neuritic innervation of the host striatum. One year after autologous transplantation, the primate iPSC-derived neural cells survived in the striatum of one primate without any immunosuppression. These neural cell grafts contained FOXA2/TH-positive neurons in the graft site. This is an important proof of concept for the feasibility and safety of iPSC-derived cell transplantation therapies in the future.

Patient-specific induced pluripotent stem cells in neurological disease modeling: the importance of nonhuman primate models

The development of the technology for derivation of induced pluripotent stem (iPS) cells from human patients and animal models has opened up new pathways to the better understanding of many human diseases, and has created new opportunities for therapeutic approaches. Here, we consider one important neurological disease, Parkinson's, the development of relevant neural cell lines for studying this disease, and the animal models that are available for testing the survival and function of the cells, following transplantation into the central nervous system. Rapid progress has been made recently in the application of protocols for neuroectoderm differentiation and neural patterning of pluripotent stem cells. These developments have resulted in the ability to produce large numbers of dopaminergic neurons with midbrain characteristics for further study. These cells have been shown to be functional in both rodent and nonhuman primate (NHP) models of Parkinson's disease. Patient-specific iPS cells and derived dopaminergic neurons have been developed, in particular from patients with genetic causes of Parkinson's disease. For complete modeling of the disease, it is proposed that the introduction of genetic changes into NHP iPS cells, followed by studying the phenotype of the genetic change in cells transplanted into the NHP as host animal, will yield new insights into disease processes not possible with rodent models alone.

Generation of integration-free and region-specific neural progenitors from primate fibroblasts

Postnatal and adult human and monkey fibroblasts were infected with Sendai virus containing the Yamanaka factors for 24 hr, then they were cultured in a chemically defined medium containing leukemia inhibitory factor (LIF), transforming growth factor (TGF)-β inhibitor SB431542, and glycogen synthase kinase (GSK)-3β inhibitor CHIR99021 at 39°C for inactivation of the virus. Induced neural progenitor (iNP) colonies appeared as early as day 13 and can be expanded for >20 passages. Under the same defined condition, no induced pluripotent stem cell (iPSC) colonies formed at either 37°C or 39°C. The iNPs predominantly express hindbrain genes and differentiate into hindbrain neurons, and when caudalized, they produced an enriched population of spinal motor neurons. Following transplantation into the forebrain, the iNP-derived cells retained the hindbrain identity. The ability to generate defined, integration-free iNPs from adult primate fibroblasts under a defined condition with predictable fate choices will facilitate disease modeling and therapeutic development.