Coordination of sonic hedgehog and Wnt signaling determines ventral and dorsal telencephalic neuron types from human embryonic stem cells

Dorsal-ventral patterning of the neural tube: a tale of three signals

Development of the vertebrate nervous system begins with the acquisition of neural identity from the midline dorsal-ectodermal cells of the gastrulating embryos. The subsequent progressive specification of the neural plate along its anterior-posterior and dorsal-ventral (DV) axes allows the generation of the tremendous variety of neuronal and glial cells that compose the vertebrate central nervous system (CNS). Studies on the development of the spinal cord, the anatomically simplest part of the CNS, have generated most of our current knowledge on the signaling events and the genetic networks that orchestrate the DV patterning of the neural plate. In this review, we discuss the recent advances in our understanding of these events and highlight unresolved questions. We focused our attention on the activity and the integration of the three main instructive cues: Sonic hedgehog, the Wnts and the Bone Morphogenetic Proteins, giving particular attention to the less well understood dorsal signaling events.

Transcriptomine, a web resource for nuclear receptor signaling transcriptomes

The nuclear receptor (NR) superfamily of ligand-regulated transcription factors directs ligand- and tissue-specific transcriptomes in myriad developmental, metabolic, immunological, and reproductive processes. The NR signaling field has generated a wealth of genome-wide expression data points, but due to deficits in their accessibility, annotation, and integration, the full potential of these studies has not yet been realized. We searched public gene expression databases and MEDLINE for global transcriptomic datasets relevant to NRs, their ligands, and coregulators. We carried out extensive, deep reannotation of the datasets using controlled vocabularies for RNA Source and regulating molecule and resolved disparate gene identifiers to official gene symbols to facilitate comparison of fold changes and their significance across multiple datasets. We assembled these data points into a database, Transcriptomine (http://www.nursa.org/transcriptomine), that allows for multiple, menu-driven querying strategies of this transcriptomic "superdataset," including single and multiple genes, Gene Ontology terms, disease terms, and uploaded custom gene lists. Experimental variables such as regulating molecule, RNA Source, as well as fold-change and P value cutoff values can be modified, and full data records can be either browsed or downloaded for downstream analysis. We demonstrate the utility of Transcriptomine as a hypothesis generation and validation tool using in silico and experimental use cases. Our resource empowers users to instantly and routinely mine the collective biology of millions of previously disparate transcriptomic data points. By incorporating future transcriptome-wide datasets in the NR signaling field, we anticipate Transcriptomine developing into a powerful resource for the NR- and other signal transduction research communities.

Cellular reprogramming: a novel tool for investigating autism spectrum disorders

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impairment in reciprocal social interaction and communication, as well as the manifestation of stereotyped behaviors. Despite much effort, ASDs are not yet fully understood. Advanced genetics and genomics technologies have recently identified novel ASD genes, and approaches using genetically engineered murine models or postmortem human brain have facilitated understanding ASD. Reprogramming somatic cells into induced pluripotent stem cells (iPSCs) provides unprecedented opportunities in generating human disease models. Here, we present an overview of applying iPSCs in developing cellular models for understanding ASD. We also discuss future perspectives in the use of iPSCs as a source of cell therapy and as a screening platform for identifying small molecules with efficacy for alleviating ASD.

Sonic hedgehog signaling coordinates the proliferation and differentiation of neural stem/progenitor cells by regulating cell cycle kinetics during development of the neocortex

Sonic hedgehog (Shh) acts as a morphogen in normal development of various vertebrate tissues and organs. Shh signaling is essential for patterning and cell-fate specification, particularly in the central nervous system. Shh signaling plays different roles depending on its concentration, area, and timing of exposure. During the development of the neocortex, a low level of Shh is expressed in the neural stem/progenitor cells as well as in mature neurons in the dorsal telencephalon. Shh signaling in neocortex development has been shown to regulate cell cycle kinetics of radial glial cells and intermediate progenitor cells, thereby maintaining the proliferation, survival and differentiation of neurons in the neocortex. During the development of the telencephalon, endogenous Shh signaling is involved in the transition of slow-cycling neural stem cells to fast-cycling neural progenitor cells. It seems that high-level Shh signaling in the ventral telencephalon is essential for ventral specification, while low-level Shh signaling in the dorsal telencephalon plays important roles in the fine-tuning of cell cycle kinetics. The Shh levels and multiple functions of Shh signaling are important for proper corticogenesis in the developing brain. The present paper discusses the roles of Shh signaling in the proliferation and differentiation of neural stem/progenitor cells.

Generation of regionally specified neural progenitors and functional neurons from human embryonic stem cells under defined conditions

To model human neural-cell-fate specification and to provide cells for regenerative therapies, we have developed a method to generate human neural progenitors and neurons from human embryonic stem cells, which recapitulates human fetal brain development. Through the addition of a small molecule that activates canonical WNT signaling, we induced rapid and efficient dose-dependent specification of regionally defined neural progenitors ranging from telencephalic forebrain to posterior hindbrain fates. Ten days after initiation of differentiation, the progenitors could be transplanted to the adult rat striatum, where they formed neuron-rich and tumor-free grafts with maintained regional specification. Cells patterned toward a ventral midbrain (VM) identity generated a high proportion of authentic dopaminergic neurons after transplantation. The dopamine neurons showed morphology, projection pattern, and protein expression identical to that of human fetal VM cells grafted in parallel. VM-patterned but not forebrain-patterned neurons released dopamine and reversed motor deficits in an animal model of Parkinson's disease.

Producing striatal phenotypes for transplantation in Huntington's disease

Neural transplantation as a therapeutic strategy in neurodegenerative disorders offers to replace cells lost during the disease process, with the potential to reconstruct dysfunctional circuitry, thus alleviating associated disease symptoms. The focal loss of striatal cells, specifically medium-sized spiny neurons (MSN) in Huntington's disease (HD), makes transplantation a therapeutic option. Here, we review the progress made in generating striatal MSN phenotypes for transplantation in HD. We discuss the use of primary fetal tissue as a donor source in both preclinical and clinical studies and assess the options for renewable cell sources. We evaluate progress in directing the differentiation of renewable cells towards a striatal MSN phenotype for HD.

Human embryonic stem cell-derived GABA neurons correct locomotion deficits in quinolinic acid-lesioned mice

Degeneration of medium spiny GABA neurons in the basal ganglia underlies motor dysfunction in Huntington's disease (HD), which presently lacks effective therapy. In this study, we have successfully directed human embryonic stem cells (hESCs) to enriched populations of DARPP32-expressing forebrain GABA neurons. Transplantation of these human forebrain GABA neurons and their progenitors, but not spinal GABA cells, into the striatum of quinolinic acid-lesioned mice results in generation of large populations of DARPP32(+) GABA neurons, which project to the substantia nigra as well as receiving glutamatergic and dopaminergic inputs, corresponding to correction of motor deficits. This finding raises hopes for cell therapy for HD.

Differentiation and functional incorporation of embryonic stem cell-derived GABAergic interneurons in the dentate gyrus of mice with temporal lobe epilepsy

Cell therapies for neurological disorders require an extensive knowledge of disease-associated neuropathology and procedures for generating neurons for transplantation. In many patients with severe acquired temporal lobe epilepsy (TLE), the dentate gyrus exhibits sclerosis and GABAergic interneuron degeneration. Mounting evidence suggests that therapeutic benefits can be obtained by transplanting fetal GABAergic progenitors into the dentate gyrus in rodents with TLE, but the scarcity of human fetal cells limits applicability in patient populations. In contrast, virtually limitless quantities of neural progenitors can be obtained from embryonic stem (ES) cells. ES cell-based therapies for neurological repair in TLE require evidence that the transplanted neurons integrate functionally and replace cell types that degenerate. To address these issues, we transplanted mouse ES cell-derived neural progenitors (ESNPs) with ventral forebrain identities into the hilus of the dentate gyrus of mice with TLE and evaluated graft differentiation, mossy fiber sprouting, cellular morphology, and electrophysiological properties of the transplanted neurons. In addition, we compared electrophysiological properties of the transplanted neurons with endogenous hilar interneurons in mice without TLE. The majority of transplanted ESNPs differentiated into GABAergic interneuron subtypes expressing calcium-binding proteins parvalbumin, calbindin, or calretinin. Global suppression of mossy fiber sprouting was not observed; however, ESNP-derived neurons formed dense axonal arborizations in the inner molecular layer and throughout the hilus. Whole-cell hippocampal slice electrophysiological recordings and morphological analyses of the transplanted neurons identified five basic types; most with strong after-hyperpolarizations and smooth or sparsely spiny dendritic morphologies resembling endogenous hippocampal interneurons. Moreover, intracellular recordings of spontaneous EPSCs indicated that the new cells functionally integrate into epileptic hippocampal circuitry.

The potential of induced pluripotent stem cells as a translational model for neurotoxicological risk

An important goal of neurotoxicological research is to provide relevant and accurate risk assessment of environmental and pharmacological agents for populations and individuals. Owing to the challenges of human subject research and the real possibility of species specific toxicological responses, neuronal lineages derived from human embryonic stem cells (hESCs) and human neuronal precursors have been offered as a potential solution for validation of neurotoxicological data from model organism systems in humans. More recently, with the advent of induced pluripotent stem cell (iPSC) technology, there is now the possibility of personalized toxicological risk assessment, the ability to predict individual susceptibility to specific environmental agents, by this approach. This critical advance is widely expected to facilitate analysis of cellular physiological pathways in the context of human neurons and the underlying genetic factors that lead to disease. Thus this technology opens the opportunity, for the first time, to characterize the physiological, toxicological, pharmacological and molecular properties of living human neurons with identical genetic determinants as human patients. Furthermore, armed with a complete clinical history of the patients, human iPSC (hiPSC) studies can theoretically compare patients and at risk groups with distinct sensitivities to particular environmental agents, divergent clinical outcomes, differing co-morbidities, and so forth. Thus iPSCs and neuronal lineages derived from them may reflect the unique genetic blueprint of the individuals from which they are generated. Indeed, iPSC technology has the potential to revolutionize scientific approaches to human health. However, before this overarching goal can be reached a number of technical and theoretical challenges must be overcome. This review seeks to provide a realistic assessment of hiPSC technology and its application to risk assessment and mechanistic studies in the area of neurotoxicology. We seek to identify, prioritize, and detail the primary hurdles that need to be overcome if personalized toxicological risk assessment using patient-derived iPSCs is to succeed.

Pharmacological modulation of the Hedgehog pathway differentially affects dorsal/ventral patterning in mouse and human embryonic stem cell models of telencephalic development

A complex set of extrinsic and intrinsic signals acts in specific temporal and spatial orders to enable neural differentiation during development. These processes have been extensively studied in animal models, but human neural development remains much less understood. This lack of detailed information about human early neurogenesis is a hindrance for the differentiation of pluripotent stem cell lines into specific neuronal phenotypes. Therefore, it is important to strengthen the interspecies comparative approaches. We describe a novel model system in which in vitro differentiation of human and mouse embryonic stem (ES) cells are temporally aligned to each other and compared with mouse telencephalic neurogenesis in vivo. In this comparative model system, we tested the in vitro role of Hedgehog (Hh) signaling for ES cell-derived telencephalic differentiation. In vivo, Hh signaling mediates dorsal/ventral patterning during early stages of telencephalic development. We monitored the effect of pharmacological modulators of the Hh signaling pathway, purmorphamine-an agonist and cyclopamine-an antagonist of the Smoothened receptor (Smo), on the expression of region-specific transcription factors and signaling molecules relevant for telencephalic development in vivo. Purmorphamine strongly upregulated the expression of telencephalic ventral markers Nkx2.1, Nkx6.2, Lhx6, and Lhx8 in mouse and human cells, thus reflecting the in vivo process of the medial ganglionic eminence patterning and specification. Cyclopamine upregulated the expression of telencephalic dorsal markers, but at lower levels in human compared with mouse cells. Modulation of Smo in vitro differentially affected, in mouse and human cells, the expression of molecules of the Hh pathway, especially the Gli1 and Gli3 effectors, Sonic Hh ligand and Ptch receptors. These results provide evidence for the different default differentiation of mouse and human ES cells and prove the utility of the comparative system for optimizing the directed differentiation of human pluripotent stem cells.

Intracerebral transplantation for neurological disorders. Lessons from developmental, experimental, and clinical studies

The use of human pluripotent stem cells (PSCs) for cell therapy faces a number of challenges that are progressively answered by results from clinical trials and experimental research. Among these is the control of differentiation before transplantation and the prediction of cell fate after administration into the human brain, two aspects that condition both the safety and efficacy of the approach. For neurological disorders, this includes two steps: firstly, the identification of the optimal maturation stage for transplantation along the continuum that transforms PSCs into fully differentiated neural cell types, together with the derivation of robust protocols for large-scale production of biological products, and, secondly, the understanding of the effects of environmental cues and their possible interference with transplanted cells commitment. This review will firstly summarize our knowledge on developmental processes that have been applied to achieve robust in vitro differentiation of PSCs into neural progenitors. In a second part, we summarize results from experimental and clinical transplantation studies that help understanding the dialogue that establishes between transplanted cells and their host brain.

Using human pluripotent stem cells to study post-transcriptional mechanisms of neurodegenerative diseases

Post-transcriptional regulation plays a major role in the generation of cell type diversity. In particular, alternative splicing increases diversification of transcriptome between tissues, in different cell types within a tissue, and even in different compartments of the same cell. The complexity of alternative splicing has increased during evolution. With increasing sophistication, however, comes greater potential for malfunction of these intricate processes. Indeed, recent years have uncovered a wealth of disease-causing mutations affecting RNA-binding proteins and non-coding regions on RNAs, highlighting the importance of studying disease mechanisms that act at the level of RNA processing. For instance, mutations in TARDBP and FUS, or a repeat expansion in the intronic region of the C9ORF72 gene, can all cause amyotrophic lateral sclerosis. We discuss how interspecies differences highlight the necessity for human model systems to complement existing non-human approaches to study neurodegenerative disorders. We conclude by discussing the improvements that could further increase the promise of human pluripotent stem for cell-based disease modeling. This article is part of a Special Issue entitled "RNA-Binding Proteins".

Using iPSC-derived neurons to uncover cellular phenotypes associated with Timothy syndrome

Monogenic neurodevelopmental disorders provide key insights into the pathogenesis of disease and help us understand how specific genes control the development of the human brain. Timothy syndrome is caused by a missense mutation in the L-type calcium channel Ca(v)1.2 that is associated with developmental delay and autism. We generated cortical neuronal precursor cells and neurons from induced pluripotent stem cells derived from individuals with Timothy syndrome. Cells from these individuals have defects in calcium (Ca(2+)) signaling and activity-dependent gene expression. They also show abnormalities in differentiation, including decreased expression of genes that are expressed in lower cortical layers and in callosal projection neurons. In addition, neurons derived from individuals with Timothy syndrome show abnormal expression of tyrosine hydroxylase and increased production of norepinephrine and dopamine. This phenotype can be reversed by treatment with roscovitine, a cyclin-dependent kinase inhibitor and atypical L-type-channel blocker. These findings provide strong evidence that Ca(v)1.2 regulates the differentiation of cortical neurons in humans and offer new insights into the causes of autism in individuals with Timothy syndrome.

Directed differentiation of functional astroglial subtypes from human pluripotent stem cells

Regionally and functionally diverse types of astrocytes exist throughout the central nervous system and participate in nearly every aspect of normal and abnormal neural function. Therefore, human astrocyte subtypes are useful tools for understanding brain function, modulating disease processes and promoting neural regeneration. Here we describe a protocol for directed differentiation and maintenance of functional astroglia from human pluripotent stem cells in a chemically defined system. Human stem cells are first differentiated into neuroepithelial cells with or without exogenous patterning molecules (days 0-21). Regular dissociation of the neuroepithelial clusters in suspension, and in the presence of mitogens, permits generation of astroglial subtypes over a long-term expansion (days 21-90). Finally, the astroglial progenitors are either amplified for an extended time or differentiated into functional astrocytes on removal of mitogens and the addition of ciliary neurotrophic factor (days >90). This method generates robust populations of functionally diversified astrocytes with high efficiency.

Pluripotent stem cells for the study of CNS development

The mammalian central nervous system is a complex neuronal network consisting of a diverse array of cellular subtypes generated in a precise spatial and temporal pattern throughout development. Achieving a greater understanding of the molecular and genetic mechanisms that direct a relatively uniform population of neuroepithelial progenitors into diverse neuronal subtypes remains a significant challenge. The advent of pluripotent stem cell (PSC) technology allows researchers to generate diverse neural populations in vitro. Although the primary focus of PSC-derived neural cells has been their therapeutic potential, utilizing PSCs to study neurodevelopment is another frequently overlooked and equally important application. In this review, we explore the potential for utilizing PSCs to study neural development. We introduce the types of neurodevelopmental questions that PSCs can help to address, and we discuss the different strategies and technologies that researchers use to generate diverse subtypes of PSC-derived neurons. Additionally, we highlight the derivation of several thoroughly characterized neural subtypes; spinal motoneurons, midbrain dopaminergic neurons and cortical neurons. We hope that this review encourages researchers to develop innovative strategies for using PSCs for the study of mammalian, and specifically human, neurodevelopment.

Human pluripotent stem cell therapy for Huntington's disease: technical, immunological, and safety challenges human pluripotent stem cell therapy for Huntington's disease: technical, immunological, and safety challenges

Intra-striatal transplantation of homotypic fetal tissue at the time of peak striatal neurogenesis can provide some functional benefit to patients suffering from Huntington's disease. Currently, the only approach shown to slow down the course of this condition is replacement of the neurons primarily targeted in this disorder, although it has been transient and has only worked with a limited number of patients. Otherwise, this dominantly inherited neurodegenerative disease inevitably results in the progressive decline of motricity, cognition, and behavior, and leads to death within 15 to 20 years of onset. However, fetal neural cell therapy of Huntington's disease, as with a similar approach in Parkinson's disease, is marred with both technical and biological hurdles related to the source of grafting material. This heavily restricts the number of patients who can be treated. A substitute cell source is therefore needed, but must perform at least as well as fetal neural graft in terms of brain recovery and reconstruction, while overcoming its major obstacles. Human pluripotent stem cells (embryonic in origin or induced from adult cells through genetic reprogramming) have the potential to meet those challenges. In this review, the therapeutic potential in view of 4 major issues is identified during fetal cell therapy clinical trials: 1) logistics of graft procurement, 2) quality control of the cell preparation, 3) immunogenicity of the graft, and 4) safety of the procedure.

Anti-Aβ drug screening platform using human iPS cell-derived neurons for the treatment of Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is a neurodegenerative disorder that causes progressive memory and cognitive decline during middle to late adult life. The AD brain is characterized by deposition of amyloid β peptide (Aβ), which is produced from amyloid precursor protein by β- and γ-secretase (presenilin complex)-mediated sequential cleavage. Induced pluripotent stem (iPS) cells potentially provide an opportunity to generate a human cell-based model of AD that would be crucial for drug discovery as well as for investigating mechanisms of the disease.

METHODOLOGY/PRINCIPAL FINDINGS

We differentiated human iPS (hiPS) cells into neuronal cells expressing the forebrain marker, Foxg1, and the neocortical markers, Cux1, Satb2, Ctip2, and Tbr1. The iPS cell-derived neuronal cells also expressed amyloid precursor protein, β-secretase, and γ-secretase components, and were capable of secreting Aβ into the conditioned media. Aβ production was inhibited by β-secretase inhibitor, γ-secretase inhibitor (GSI), and an NSAID; however, there were different susceptibilities to all three drugs between early and late differentiation stages. At the early differentiation stage, GSI treatment caused a fast increase at lower dose (Aβ surge) and drastic decline of Aβ production.

CONCLUSIONS/SIGNIFICANCE

These results indicate that the hiPS cell-derived neuronal cells express functional β- and γ-secretases involved in Aβ production; however, anti-Aβ drug screening using these hiPS cell-derived neuronal cells requires sufficient neuronal differentiation.

Cortical network from human embryonic stem cells

The connection of embryonic stem cell technology and developmental biology provides valuable tools to decipher the mechanisms underlying human brain development and diseases, especially among neuronal populations, that are not readily available in primary cultures. It is obviously the case of neurons forming the human cerebral cortex. In the images that are presented, the neurons were generated in vitro from human embryonic stem cells via forebrain-like progenitors. Maintained in culture for prolonged time, they acquired a mainly glutamatergic phenotype and morphological characteristics of cortical pyramidal neurons, including dendritic spines, and formed spectacular networks.

Fezf2 regulates telencephalic precursor differentiation from mouse embryonic stem cells

The mechanisms by which transcription factors control stepwise lineage restriction during the specification of cortical neurons remain largely unknown. Here, we investigated the role of forebrain embryonic zinc finger like (Fezf2) in this process by generating Fezf2 knockdown and tetracycline-inducible Fezf2 overexpression mouse embryonic stem cell (mESC) lines. The overexpression of Fezf2 at early time points significantly increased the generation of rostral forebrain progenitors (Foxg1(+), Six3(+)) and inhibited the expression of transcription factors which are expressed by the midbrain and caudal diencephalon (En1(+), Irx(+)). This effect was partially achieved by the regulation of Wnt signaling during this critical early time window. The role of Fezf2 in regulating the rostrocaudal patterning was further confirmed by the significant decrease in the expression of Foxg1 and Six3 and the increase in the expression of En1 when Fezf2 was knocked down. In addition, Fezf2 overexpression at later time points had little effect on the expression of Foxg1 and Six3. Instead, Fezf2 promotes the generation of dorsal telencephalic progenitors and deep-layer cortical neurons at later stages. Collectively, our data suggest that Fezf2 controls the specification of telencephalic progenitors from mESCs through differentially regulating the expression of rostrocaudal and dorsoventral patterning genes.

Deriving excitatory neurons of the neocortex from pluripotent stem cells

The human cerebral cortex is an immensely complex structure that subserves critical functions that can be disrupted in developmental and degenerative disorders. Recent innovations in cellular reprogramming and differentiation techniques have provided new ways to study the cellular components of the cerebral cortex. Here, we discuss approaches to generate specific subtypes of excitatory cortical neurons from pluripotent stem cells. We review spatial and temporal aspects of cortical neuron specification that can guide efforts to produce excitatory neuron subtypes with increased resolution. Finally, we discuss distinguishing features of human cortical development and their translational ramifications for cortical stem cell technologies.

Development of patient-specific neurons in schizophrenia using induced pluripotent stem cells

Induced pluripotent stem cell (iPSC) technology has the potential to transform regenerative medicine. It also offers a powerful tool for establishing in vitro models of disease, in particular, for neuropsychiatric disorders where live human neurons are essentially impossible to procure. Using iPSCs derived from three schizophrenia (SZ) patients, one of whom has 22q11.2del (velocardiofacial syndrome; VCFS), the authors developed a culture system to study SZ on a molecular and cellular level. SZ iPSCs were differentiated into functional, primarily glutamatergic neurons that were able to fire action potentials after ∼8 weeks in culture. Early differentiating neurons expressed a number of transcription factors/chromatin remodeling proteins and synaptic proteins relevant to SZ pathogenesis, including ZNF804A, RELN, CNTNAP2, CTNNA2, SMARCA2, and NRXN1. Although a small number of lines were developed in this preliminary study, the SZ line containing 22q11.2del showed a significant delay in the reduction of endogenous OCT4 and NANOG expression that normally occurs during differentiation. Constitutive expression of OCT4 has been observed in Dgcr8-deficient mouse embryonic stem cells (mESCs); DGCR8 maps to the 22q11.2-deleted region. These findings demonstrate that the method of inducing neural differentiation employed is useful for disease modeling in SZ and that the transition of iPSCs with 22q11.2 deletions towards a differentiated state may be marked by subtle changes in expression of pluripotency-associated genes.

A targeted NKX2.1 human embryonic stem cell reporter line enables identification of human basal forebrain derivatives

We have used homologous recombination in human embryonic stem cells (hESCs) to insert sequences encoding green fluorescent protein (GFP) into the NKX2.1 locus, a gene required for normal development of the basal forebrain. Generation of NKX2.1-GFP(+) cells was dependent on the concentration, timing, and duration of retinoic acid treatment during differentiation. NKX2.1-GFP(+) progenitors expressed genes characteristic of the basal forebrain, including SHH, DLX1, LHX6, and OLIG2. Time course analysis revealed that NKX2.1-GFP(+) cells could upregulate FOXG1 expression, implying the existence of a novel pathway for the generation of telencephalic neural derivatives. Further maturation of NKX2.1-GFP(+) cells gave rise to γ-aminobutyric acid-, tyrosine hydroxylase-, and somatostatin-expressing neurons as well as to platelet-derived growth factor receptor α-positive oligodendrocyte precursors. These studies highlight the diversity of cell types that can be generated from human NKX2.1(+) progenitors and demonstrate the utility of NKX2.1(GFP/w) hESCs for investigating human forebrain development and neuronal differentiation.

Specification of neuronal and glial subtypes from human pluripotent stem cells

Human pluripotent stem cells (PSCs), including embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), provide a dynamic tool for revealing early embryonic development, modeling pathological processes, and developing therapeutics through drug discovery and potential cell replacement. The first step toward the utilities of human PSCs is directed differentiation to functionally specialized cell/tissue types. Following developmental principles, human ESCs, and lately iPSCs, have been effectively differentiated to region- and/or transmitter-specific neuronal and glial types, including cerebral glutamatergic, striatal γ-aminobutyric acid (GABA)-ergic, forebrain cholinergic, midbrain dopaminergic, and spinal motor neurons, as well as astrocytes and oligodendrocytes. These studies also reveal unique aspects of human cell biology, including intrinsically programmed developmental course, differential uses of transcription factors for neuroectoderm specification, and distinct responses to extracellular signals in regulating cell fate. Such information will be instrumental in translating biological findings to therapeutic development.

iPS cells to model CDKL5-related disorders

Rett syndrome (RTT) is a progressive neurologic disorder representing one of the most common causes of mental retardation in females. To date mutations in three genes have been associated with this condition. Classic RTT is caused by mutations in the MECP2 gene, whereas variants can be due to mutations in either MECP2 or FOXG1 or CDKL5. Mutations in CDKL5 have been identified both in females with the early onset seizure variant of RTT and in males with X-linked epileptic encephalopathy. CDKL5 is a kinase protein highly expressed in neurons, but its exact function inside the cell is unknown. To address this issue we established a human cellular model for CDKL5-related disease using the recently developed technology of induced pluripotent stem cells (iPSCs). iPSCs can be expanded indefinitely and differentiated in vitro into many different cell types, including neurons. These features make them the ideal tool to study disease mechanisms directly on the primarily affected neuronal cells. We derived iPSCs from fibroblasts of one female with p.Q347X and one male with p.T288I mutation, affected by early onset seizure variant and X-linked epileptic encephalopathy, respectively. We demonstrated that female CDKL5-mutated iPSCs maintain X-chromosome inactivation and clones express either the mutant CDKL5 allele or the wild-type allele that serve as an ideal experimental control. Array CGH indicates normal isogenic molecular karyotypes without detection of de novo CNVs in the CDKL5-mutated iPSCs. Furthermore, the iPS cells can be differentiated into neurons and are thus suitable to model disease pathogenesis in vitro.

The lineage contribution and role of Gbx2 in spinal cord development

Background

Forging a relationship between progenitors with dynamically changing gene expression and their terminal fate is instructive for understanding the logic of how cell-type diversity is established. The mouse spinal cord is an ideal system to study these mechanisms in the context of developmental genetics and nervous system development. Here we focus on the Gastrulation homeobox 2 (Gbx2) transcription factor, which has not been explored in spinal cord development.

Methodology/Principal Findings

We determined the molecular identity of Gbx2-expressing spinal cord progenitors. We also utilized genetic inducible fate mapping to mark the Gbx2 lineage at different embryonic stages in vivo in mouse. Collectively, we uncover cell behaviors, cytoarchitectonic organization, and the terminal cell fate of the Gbx2 lineage. Notably, both ventral motor neurons and interneurons are derived from the Gbx2 lineage, but only during a short developmental period. Short-term fate mapping during mouse spinal cord development shows that Gbx2 expression is transient and is extinguished ventrally in a rostral to caudal gradient. Concomitantly, a permanent lineage restriction boundary ensures that spinal cord neurons derived from the Gbx2 lineage are confined to a dorsal compartment that is maintained in the adult and that this lineage generates inhibitory interneurons of the spinal cord. Using lineage tracing and molecular markers to follow Gbx2-mutant cells, we show that the loss of Gbx2 globally affects spinal cord patterning including the organization of interneuron progenitors. Finally, long-term lineage analysis reveals that the presence and timing of Gbx2 expression in interneuron progenitors results in the differential contribution to subtypes of terminally differentiated interneurons in the adult spinal cord.

Conclusions/Significance

We illustrate the complex cellular nature of Gbx2 expression and lineage contribution to the mouse spinal cord. In a broader context, this study provides a direct link between spinal cord progenitors undergoing dynamic changes in molecular identity and terminal neuronal fate.

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.

Specification of transplantable astroglial subtypes from human pluripotent stem cells

Human pluripotent stem cells (hPSCs) have been differentiated efficiently to neuronal cell types. However, directed differentiation of hPSCs to astrocytes and astroglial subtypes remains elusive. In this study, hPSCs were directed to nearly uniform populations of immature astrocytes (>90% S100β(+) and GFAP(+)) in large quantities. The immature human astrocytes exhibit similar gene expression patterns as primary astrocytes, display functional properties such as glutamate uptake and promotion of synaptogenesis, and become mature astrocytes by forming connections with blood vessels after transplantation into the mouse brain. Furthermore, hPSC-derived neuroepithelia, patterned to rostral-caudal and dorsal-ventral identities with the same morphogens used for neuronal subtype specification, generate immature astrocytes that express distinct homeodomain transcription factors and display phenotypic differences of different astroglial subtypes. These human astroglial progenitors and immature astrocytes will be useful for studying astrocytes in brain development and function, understanding the roles of astrocytes in disease processes and developing novel treatments for neurological disorders.

Subregional specification of embryonic stem cell-derived ventral telencephalic tissues by timed and combinatory treatment with extrinsic signals

During early telencephalic development, the major portion of the ventral telencephalic (subpallial) region becomes subdivided into three regions, the lateral (LGE), medial (MGE), and caudal (CGE) ganglionic eminences. In this study, we systematically recapitulated subpallial patterning in mouse embryonic stem cell (ESC) cultures and investigated temporal and combinatory actions of patterning signals. In serum-free floating culture, the dorsal-ventral specification of ESC-derived telencephalic neuroectoderm is dose-dependently directed by Sonic hedgehog (Shh) signaling. Early Shh treatment, even before the expression onset of Foxg1 (also Bf1; earliest marker of the telencephalic lineage), is critical for efficiently generating LGE progenitors, and continuous Shh signaling until day 9 is necessary to commit these cells to the LGE lineage. When induced under these conditions and purified by fluorescence-activated cell sorter, telencephalic cells efficiently differentiated into Nolz1(+)/Ctip2(+) LGE neuronal precursors and subsequently, both in culture and after in vivo grafting, into DARPP32(+) medium-sized spiny neurons. Purified telencephalic progenitors treated with high doses of the Hedgehog (Hh) agonist SAG (Smoothened agonist) differentiated into MGE- and CGE-like tissues. Interestingly, in addition to strong Hh signaling, the efficient specification of MGE cells requires Fgf8 signaling but is inhibited by treatment with Fgf15/19. In contrast, CGE differentiation is promoted by Fgf15/19 but suppressed by Fgf8, suggesting that specific Fgf signals play different, critical roles in the positional specification of ESC-derived ventral subpallial tissues. We discuss a model of the antagonistic Fgf8 and Fgf15/19 signaling in rostral-caudal subpallial patterning and compare it with the roles of these molecules in cortical patterning.

Wnt5a is a transcriptional target of Dlx homeogenes and promotes differentiation of interneuron progenitors in vitro and in vivo

During brain development, neurogenesis, migration, and differentiation of neural progenitor cells are regulated by an interplay between intrinsic genetic programs and extrinsic cues. The Dlx homeogene transcription factors have been proposed to directly control the genesis and maturation of GABAergic interneurons of the olfactory bulb (OB), subpallium, and cortex. Here we provide evidence that Dlx genes promote differentiation of olfactory interneurons via the signaling molecule Wnt5a. Dlx2 and Dlx5 interact with homeodomain binding sequences within the Wnt5a locus and activate its transcription. Exogenously provided Wnt5a promotes GABAergic differentiation in dissociated OB neurons and in organ-type brain cultures. Finally, we show that the Dlx-mutant environment is unfavorable for GABA differentiation, in vivo and in vitro. We conclude that Dlx genes favor interneuron differentiation also in a non-cell-autonomous fashion, via expression of Wnt5a.

Isolation of MECP2-null Rett Syndrome patient hiPS cells and isogenic controls through X-chromosome inactivation

Rett syndrome (RTT) is a neurodevelopmental autism spectrum disorder that affects girls due primarily to mutations in the gene encoding methyl-CpG binding protein 2 (MECP2). The majority of RTT patients carry missense and nonsense mutations leading to a hypomorphic MECP2, while null mutations leading to the complete absence of a functional protein are rare. MECP2 is an X-linked gene subject to random X-chromosome inactivation resulting in mosaic expression of mutant MECP2. The lack of human brain tissue motivates the need for alternative human cellular models to study RTT. Here we report the characterization of a MECP2 mutation in a classic female RTT patient involving rearrangements that remove exons 3 and 4 creating a functionally null mutation. To generate human neuron models of RTT, we isolated human induced pluripotent stem (hiPS) cells from RTT patient fibroblasts. RTT-hiPS cells retained the MECP2 mutation, are pluripotent and fully reprogrammed, and retained an inactive X-chromosome in a nonrandom pattern. Taking advantage of the latter characteristic, we obtained a pair of isogenic wild-type and mutant MECP2 expressing RTT-hiPS cell lines that retained this MECP2 expression pattern upon differentiation into neurons. Phenotypic analysis of mutant RTT-hiPS cell-derived neurons demonstrated a reduction in soma size compared with the isogenic control RTT-hiPS cell-derived neurons from the same RTT patient. Analysis of isogenic control and mutant hiPS cell-derived neurons represents a promising source for understanding the pathogenesis of RTT and the role of MECP2 in human neurons.

Using human pluripotent stem cells to untangle neurodegenerative disease mechanisms

Human pluripotent stem cells, including embryonic (hES) and induced pluripotent stem cells (hiPS), retain the ability to self-renew indefinitely, while maintaining the capacity to differentiate into all cell types of the nervous system. While human pluripotent cell-based therapies are unlikely to arise soon, these cells can currently be used as an inexhaustible source of committed neurons to perform high-throughput screening and safety testing of new candidate drugs. Here, we describe critically the available methods and molecular factors that are used to direct the differentiation of hES or hiPS into specific neurons. In addition, we discuss how the availability of patient-specific hiPS offers a unique opportunity to model inheritable neurodegenerative diseases and untangle their pathological mechanisms, or to validate drugs that would prevent the onset or the progression of these neurological disorders.

Embryonic stem cell neurogenesis and neural specification

The prospect of using embryonic stem cell (ESC)-derived neural progenitors and neurons to treat neurological disorders has led to great interest in defining the conditions that guide the differentiation of ESCs, and more recently induced pluripotent stem cells (iPSCs), into neural stem cells (NSCs) and a variety of neuronal and glial subtypes. Over the past decade, researchers have looked to the embryo to guide these studies, applying what we know about the signaling events that direct neural specification during development. This has led to the design of a number of protocols that successfully promote ESC neurogenesis, terminating with the production of neurons and glia with diverse regional addresses and functional properties. These protocols demonstrate that ESCs undergo neural specification in two, three, and four dimensions, mimicking the cell-cell interactions, patterning, and timing that characterizes the in vivo process. We therefore propose that these in vitro systems can be used to examine the molecular regulation of neural specification.

Milestones of directed differentiation of mouse and human embryonic stem cells into telencephalic neurons based on neural development in vivo

Understanding the normal development of individual neural subtypes provides an essential framework for the design of rational approaches to embryonic stem cell differentiation for in vitro studies and cell replacement therapies. Of particular interest and a particular challenge are the cells that build-up the telencephalon. Recent research has unraveled key developmental mechanisms contributing to the generation of specific telencephalic cells. We focus on morphogens and transcription factors known to regulate distinct developmental processes. These include early anterior/posterior patterning, dorsal/ventral patterning, and generation of progenitor domains and neuronal specification into major classes of telencephalic cells: glutamatergic projection neurons, different subtypes of γ-aminobutyric acid-ergic interneurons and projection neurons, as well as cholinergic interneurons and projection neurons. Based on a comparison with in vivo telencephalic neurogenesis, we propose that the specific combinations of transcription factors expressed during development can serve as milestones for the in vitro differentiation of embryonic stem cells toward specific telencephalic neurons.

Impact of induced pluripotent stem cells on the study of central nervous system disease

The derivation of pluripotent stem cells from somatic tissues has provided researchers with a source of patient-specific stem cells. The potential applications of this technology are truly momentous, and include cellular modeling of disease processes, drug discovery, and cell-based therapy. Here, we review the use of induced pluripotent stem cells (iPSCs) to study CNS disease. Since the iPSC field is still in its infancy, we also discuss some of the challenges that will need to be overcome before the potential of this technology to study and to treat neurological and psychiatric disorders can be fully harnessed.

Modeling complex neuropsychiatric disease with induced pluripotent stem cells

The study of neuropsychiatric diseases and the development of effective treatments have been limited by a lack of appropriate models. Induced pluripotent stem cells (iPSCs) represent a potentially limitless supply of patient-specific cells for the study of neuropsychiatric disorders. In this review, we will discuss the potential and limitations of iPSCs for the development of cell-based models of neuropsychiatric diseases and the identification of novel therapeutics.

Programming embryonic stem cells to neuronal subtypes

Richness of neural circuits and specificity of neuronal connectivity depend on the diversification of nerve cells into functionally and molecularly distinct subtypes. Although efficient methods for directed differentiation of embryonic stem cells (ESCs) into multiple principal neuronal classes have been established, only a few studies systematically examined the subtype diversity of in vitro derived nerve cells. Here we review evidence based on molecular and in vivo transplantation studies that ESC-derived spinal motor neurons and cortical layer V pyramidal neurons acquire subtype specific functional properties. We discuss similarities and differences in the role of cell-intrinsic transcriptional programs, extrinsic signals and cell-cell interactions during subtype diversification of the two classes of nerve cells. We conclude that the high degree of fidelity with which differentiating ESCs recapitulate normal embryonic development provides a unique opportunity to explore developmental processes underlying specification of mammalian neuronal diversity in a simplified and experimentally accessible system.

Pax6 is a human neuroectoderm cell fate determinant

The transcriptional regulation of neuroectoderm (NE) specification is unknown. Here we show that Pax6 is uniformly expressed in early NE cells of human fetuses and those differentiated from human embryonic stem cells (hESCs). This is in contrast to the later expression of Pax6 in restricted mouse brain regions. Knockdown of Pax6 blocks NE specification from hESCs. Overexpression of either Pax6a or Pax6b, but not Pax6triangle upPD, triggers hESC differentiation. However, only Pax6a converts hESCs to NE. In contrast, neither loss nor gain of function of Pax6 affects mouse NE specification. Both Pax6a and Pax6b bind to pluripotent gene promoters but only Pax6a binds to NE genes during human NE specification. These findings indicate that Pax6 is a transcriptional determinant of the human NE and suggest that Pax6a and Pax6b coordinate with each other in determining the transition from pluripotency to the NE fate in human by differentially targeting pluripotent and NE genes.

Specification of region-specific neurons including forebrain glutamatergic neurons from human induced pluripotent stem cells

Background

Directed differentiation of human induced pluripotent stem cells (hiPSC) into functional, region-specific neural cells is a key step to realizing their therapeutic promise to treat various neural disorders, which awaits detailed elucidation.

Methodology/Principal Findings

We analyzed neural differentiation from various hiPSC lines generated by others and ourselves. Although heterogeneity in efficiency of neuroepithelial (NE) cell differentiation was observed among different hiPSC lines, the NE differentiation process resembles that from human embryonic stem cells (hESC) in morphology, timing, transcriptional profile, and requirement for FGF signaling. NE cells differentiated from hiPSC, like those from hESC, can also form rostral phenotypes by default, and form the midbrain or spinal progenitors upon caudalization by morphogens. The rostrocaudal neural progenitors can further mature to develop forebrain glutamatergic projection neurons, midbrain dopaminergic neurons, and spinal motor neurons, respectively. Typical ion channels and action potentials were recorded in the hiPSC-derived neurons.

Conclusions/Significance

Our results demonstrate that hiPSC, regardless of how they were derived, can differentiate into a spectrum of rostrocaudal neurons with functionality, which supports the considerable value of hiPSC for study and treatment of patient-specific neural disorders.

Differentiation of human ES and Parkinson's disease iPS cells into ventral midbrain dopaminergic neurons requires a high activity form of SHH, FGF8a and specific regionalization by retinoic acid

The cardinal motor symptoms of Parkinson's disease (PD) are caused by the vulnerability to dysfunction and degeneration of ventral midbrain (VM) dopaminergic (DA) neurons. A major limitation for experimental studies of current ES/iPS cell differentiation protocols is the lack of VM DA neurons with a stable phenotype as defined by an expression marker code of FOXA2/TH/β-tubulin. Here we demonstrate a combination of three modifications that were required to produce VM DA neurons. Firstly, early and specific exposure to 10(-)(8)M (low dose) retinoic acid improved the regional identity of neural progenitor cells derived from human ES cells, PD or healthy subject-specific iPS cells. Secondly, a high activity form of human sonic hedgehog established a sizeable FOXA2(+) neural progenitor cell population in vitro. Thirdly, early exposure to FGF8a, rather than Fgf8b, and WNT1 was required for robust differentiation of the FOXA2(+) floor plate-like human neural progenitor cells into FOXA2(+) DA neurons. FOXA2(+) DA neurons were also generated when this protocol was adapted to feeder-free conditions. In summary, this new human ES and iPS cell differentiation protocol using FGF8a, WNT1, low dose retinoic acid and a high activity form of SHH can generate human VM DA neurons that are required for relevant new bioassays, drug discovery and cell based therapies for PD.