Developmental potential of defined neural progenitors derived from mouse embryonic stem cells

Retinoic Acid-Mediated Differentiation of Mouse Embryonic Stem Cells to Neuronal Cells

The capability of pluripotent embryonic stem cells (ESCs) to proliferate and differentiate into specific lineages makes them an important avenue of research in the field of cell therapy as well as a useful model to study patterns of differentiation and gene expression, recapitulating many events that occur during the very early stages of development of the mammalian embryo. With striking similarities that exist between inherently programmed embryonic development of the nervous system in vivo and the differentiation of ESCs in vitro, they have already been used to treat locomotive and cognitive deficits caused by brain injury in rodents. A suitable differentiation model thus empowers us with all these opportunities. In this chapter, we describe a neural differentiation model from mouse embryonic stem cells using retinoic acid as the inducer. This method is among the most commonly used one to acquire a homogeneous population of neuronal progenitor cells or mature neurons as desired. The method is scalable, efficient, and results in production of ~70% neural progenitor cells within 4-6 days.

Identification of PAX6 and NFAT4 as the Transcriptional Regulators of the Long Noncoding RNA Mrhl in Neuronal Progenitors

The long noncoding RNA (lncRNA) Mrhl has been shown to be involved in coordinating meiotic commitment of mouse spermatogonial progenitors and differentiation events in mouse embryonic stem cells. Here, we characterized the interplay of Mrhl with lineage-specific transcription factors during mouse neuronal lineage development. Our results demonstrate that Mrhl is expressed in the neuronal progenitor populations in mouse embryonic brains and in retinoic acid-derived radial-glia-like neuronal progenitor cells. Depletion of Mrhl leads to early differentiation of neuronal progenitors to a more committed state. A master transcription factor, PAX6, directly binds to the Mrhl promoter at a major site in the distal promoter, located at 2.9 kb upstream of the transcription start site (TSS) of Mrhl. Furthermore, NFAT4 occupies the Mrhl-proximal promoter at two sites, at 437 base pairs (bp) and 143 bp upstream of the TSS. Independent knockdown studies for PAX6 and NFAT4 confirm that they regulate Mrhl expression in neuronal progenitors. We also show that PAX6 and NFAT4 associate with each other in the same chromatin complex. NFAT4 occupies the Mrhl promoter in PAX6-bound chromatin, implying possible coregulation of Mrhl. Our studies are crucial for understanding how lncRNAs are regulated by major lineage-specific transcription factors, in order to define specific development and differentiation events.

Screening for Best Neuronal-Glial Differentiation Protocols of Neuralizing Agents Using a Multi-Sized Microfluidic Embryoid Body Array

Stem cell technology and embryonic stem cell models are of great interest in biomedical research since they provide deeper insights into, e.g., neurogenesis and early mammalian brain development. Despite their great scientific potential, the reliable establishment of three-dimensional embryoid bodies (EBs) remains a major challenge, and the current lack of standardization and comparability is still limiting a broader application and translation of stem cell technology. Among others, a vital aspect for the reliable formation of EBs is optimizing differentiation protocols since organized differentiation is influenced by soluble inducers and EB size. A microfluidic biochip array was employed to automate cell loading and optimize directed neuronal and astrocytic differentiation protocols using murine P19 embryoid bodies to facilitate reliable embryonic stem cell differentiation. Our gravity-driven microfluidic size-controlled embryoid body-on-a-chip system allows (a) the robust operation and cultivation of up to 90 EBs in parallel and (b) the reproducible generation of five increasing sizes ranging from 300 µm to 1000 µm diameters. A comparative study adds two differentiation-inducers such as retinoic acid and EC23 to size-controlled embryoid bodies to identify the optimal differentiation protocol. Our study revealed a 1.4 to 1.9-fold higher neuron and astrocyte expression in larger embryoid bodies (above 750 µm) over smaller-sized EBs (below 450 µm), thus highlighting the importance of EB size in the establishment of robust neurodevelopmental in vitro models.

A PDMS-Based Microfluidic Hanging Drop Chip for Embryoid Body Formation

The conventional hanging drop technique is the most widely used method for embryoid body (EB) formation. However, this method is labor intensive and limited by the difficulty in exchanging the medium. Here, we report a microfluidic chip-based approach for high-throughput formation of EBs. The device consists of microfluidic channels with 6 × 12 opening wells in PDMS supported by a glass substrate. The PDMS channels were fabricated by replicating polydimethyl-siloxane (PDMS) from SU-8 mold. The droplet formation in the chip was tested with different hydrostatic pressures to obtain optimal operation pressures for the wells with 1000 μm diameter openings. The droplets formed at the opening wells were used to culture mouse embryonic stem cells which could subsequently developed into EBs in the hanging droplets. This device also allows for medium exchange of the hanging droplets making it possible to perform immunochemistry staining and characterize EBs on chip.

Combination of melatonin and Wnt-4 promotes neural cell differentiation in bovine amniotic epithelial cells and recovery from spinal cord injury

Although melatonin has been shown to exhibit a wide variety of biological functions, its effects on promoting differentiation of neural cells remain unknown. Wnt signaling mediates major developmental processes during embryogenesis and regulates maintenance, self-renewal, and differentiation of adult mammalian stem cells. However, the role of the noncanonical Wnt pathway during neurogenesis remains poorly understood. In this study, the amniotic epithelial cells ( AECs) were isolated from bovine amnion and incubated with various melatonin concentrations (0.01, 0.1, 1, 10, or 100 μm) and 5 × 10(-5) m all-trans retinoic acid (RA) for screening optimum culture medium of neural differentiation, compared with each groups, 1 μm melatonin and 5 × 10(-5) m RA were selected to induce neural differentiation of AECs, and then siMT1, siMT2, oWnt-4, and siWnt-4 were expressed in AECs to research role of these genes in neural differentiation. Efficiency of neural differentiation was evaluated after expressed above genes using flow cytometry. Cell function of neural cells was demonstrated in vivo using spinal cord injury model after cell transplantation, and damage repair of spinal cord was assessed using cell tracking and Basso, Beattie, Bresnahan Locomotor Rating Scale scores. Results demonstrated that melatonin stimulated melatonin receptor 1, which subsequently increased bovine amniotic epithelial cell vitality and promoted differentiation into neural cells. This took place through cooperation with Wnt-4. Additionally, following cotreatment with melatonin and Wnt-4, neurogenesis gene expression was significantly altered. Furthermore, single inhibition of melatonin receptor 1 or Wnt-4 expression decreased expression of neurogenesis-related genes, and bovine amniotic epithelial cell-derived neural cells were successfully colonized into injured spinal cord, which suggested participation in tissue repair.

Mapping gene regulatory circuitry of Pax6 during neurogenesis

Pax6 is a highly conserved transcription factor among vertebrates and is important in various aspects of the central nervous system development. However, the gene regulatory circuitry of Pax6 underlying these functions remains elusive. We find that Pax6 targets a large number of promoters in neural progenitors cells. Intriguingly, many of these sites are also bound by another progenitor factor, Sox2, which cooperates with Pax6 in gene regulation. A combinatorial analysis of Pax6-binding data set with transcriptome changes in Pax6-deficient neural progenitors reveals a dual role for Pax6, in which it activates the neuronal (ectodermal) genes while concurrently represses the mesodermal and endodermal genes, thereby ensuring the unidirectionality of lineage commitment towards neuronal differentiation. Furthermore, Pax6 is critical for inducing activity of transcription factors that elicit neurogenesis and repress others that promote non-neuronal lineages. In addition to many established downstream effectors, Pax6 directly binds and activates a number of genes that are specifically expressed in neural progenitors but have not been previously implicated in neurogenesis. The in utero knockdown of one such gene, Ift74, during brain development impairs polarity and migration of newborn neurons. These findings demonstrate new aspects of the gene regulatory circuitry of Pax6, revealing how it functions to control neuronal development at multiple levels to ensure unidirectionality and proper execution of the neurogenic program.

JNK-dependent gene regulatory circuitry governs mesenchymal fate

The epithelial to mesenchymal transition (EMT) is a biological process in which cells lose cell–cell contacts and become motile. EMT is used during development, for example, in triggering neural crest migration, and in cancer metastasis. Despite progress, the dynamics of JNK signaling, its role in genomewide transcriptional reprogramming, and involved downstream effectors during EMT remain largely unknown. Here, we show that JNK is not required for initiation, but progression of phenotypic changes associated with EMT. Such dependency resulted from JNK-driven transcriptional reprogramming of critical EMT genes and involved changes in their chromatin state. Furthermore, we identified eight novel JNK-induced transcription factors that were required for proper EMT. Three of these factors were also highly expressed in invasive cancer cells where they function in gene regulation to maintain mesenchymal identity. These factors were also induced during neuronal development and function in neuronal migration in vivo. These comprehensive findings uncovered a kinetically distinct role for the JNK pathway in defining the transcriptome that underlies mesenchymal identity and revealed novel transcription factors that mediate these responses during development and disease.

Endocrine, teratogenic and neurotoxic effects of cyanobacteria detected by cellular in vitro and zebrafish embryos assays

Cyanobacteria contain various types of bioactive compounds, which could cause adverse effects on organisms. They are released into surface waters during cyanobacterial blooms, but there is little information on their potential relevance for effects in vivo. In this study presence of bioactive compounds was characterized in cyanobacteria Microcystis aeruginosa (Chroococcales), Planktothrix agardhii (Oscillatoriales) and Aphanizomenon gracile (Nostocales) with selected in vitro assays. The in vivo relevance of detected bioactivities was analysed using transgenic zebrafish embryos tg(cyp19a1b-GFP). Teratogenic potency was assessed by analysis of developmental disorders and effects on functions of the neuromuscular system by video tracking of locomotion. Estrogenicity in vitro corresponded to 0.95-54.6 ng estradiol equivalent(g dry weight (dw))(-1). In zebrafish embryos, estrogenic effects could not be detected potentially because they were masked by high toxicity. There was no detectable (anti)androgenic/glucocorticoid activity in any sample. Retinoid-like activity was determined at 1-1.3 μg all-trans-retinoic acid equivalent(g dw)(-1). Corresponding to the retinoid-like activity A. gracile extract also caused teratogenic effects in zebrafish embryos. Furthermore, exposure to biomass extracts at 0.3 gd wL(-1) caused increase of body length in embryos. There were minor effects on locomotion caused by 0.3 gd wL(-1)M. aeruginosa and P. agardhii extracts. The traditionally measured cyanotoxins microcystins did not seem to play significant role in observed effects. This indicates importance of other cyanobacterial compounds at least towards some species or their developmental phases. More attention should be paid to activity of retinoids, estrogens and other bioactive substances in phytoplankton using in vitro and in vivo bioassays.

Critical role of Tet3 in neural progenitor cell maintenance and terminal differentiation

5-Hydroxymethylcytosine (5hmC), converted from 5-methylcytocine (5mC) by Tet family of dioxygenases (Tet1, Tet2, and Tet3), is enriched in the embryonic stem cells (ESCs) and in the brain. However, the role of 5hmC and Tet family in the process of ESC differentiation especially neuronal differentiation remains elusive. Here, we showed the evidence that Tet3 is critical in neural progenitor cell (NPC) maintenance and terminal differentiation of neurons. We found that Tet3 expression is basically undetectable in ESCs, but its level increases rapidly during neuronal differentiation. Tet3 knockout ESCs appear normal in self-renewal and maintenance but impaired in neuronal differentiation. NPCs could be induced efficiently from Tet3 knockout ESCs, as the expression of NPC marker Pax6 and nestin is comparable with NPCs from wild-type ESCs, but undergo apoptosis rapidly, and the terminal differentiation of neurons is greatly reduced. Our results indicate that Tet3 is important for NPC maintenance and terminal differentiation of neurons.

Retinoic acid receptors: from molecular mechanisms to cancer therapy

Retinoic acid (RA), the major bioactive metabolite of retinol or vitamin A, induces a spectrum of pleiotropic effects in cell growth and differentiation that are relevant for embryonic development and adult physiology. The RA activity is mediated primarily by members of the retinoic acid receptor (RAR) subfamily, namely RARα, RARβ and RARγ, which belong to the nuclear receptor (NR) superfamily of transcription factors. RARs form heterodimers with members of the retinoid X receptor (RXR) subfamily and act as ligand-regulated transcription factors through binding specific RA response elements (RAREs) located in target genes promoters. RARs also have non-genomic effects and activate kinase signaling pathways, which fine-tune the transcription of the RA target genes. The disruption of RA signaling pathways is thought to underlie the etiology of a number of hematological and non-hematological malignancies, including leukemias, skin cancer, head/neck cancer, lung cancer, breast cancer, ovarian cancer, prostate cancer, renal cell carcinoma, pancreatic cancer, liver cancer, glioblastoma and neuroblastoma. Of note, RA and its derivatives (retinoids) are employed as potential chemotherapeutic or chemopreventive agents because of their differentiation, anti-proliferative, pro-apoptotic, and anti-oxidant effects. In humans, retinoids reverse premalignant epithelial lesions, induce the differentiation of myeloid normal and leukemic cells, and prevent lung, liver, and breast cancer. Here, we provide an overview of the biochemical and molecular mechanisms that regulate the RA and retinoid signaling pathways. Moreover, mechanisms through which deregulation of RA signaling pathways ultimately impact on cancer are examined. Finally, the therapeutic effects of retinoids are reported.

In vivo opioid receptor heteromerization: where do we stand?

Opioid receptors are highly homologous GPCRs that modulate brain function at all levels of neural integration, including autonomous, sensory, emotional and cognitive processing. Opioid receptors functionally interact in vivo, but the underlying mechanisms involving direct receptor-receptor interactions, affecting signalling pathways or engaging different neuronal circuits, remain unsolved. Heteromer formation through direct physical interaction between two opioid receptors or between an opioid receptor and a non-opioid one has been postulated and can be characterized by specific ligand binding, receptor signalling and trafficking properties. However, despite numerous studies in heterologous systems, evidence for physical proximity in vivo is only available for a limited number of opioid heteromers, and their physiopathological implication remains largely unknown mostly due to the lack of appropriate tools. Nonetheless, data collected so far using endogenous receptors point to a crucial role for opioid heteromers as a molecular entity that could underlie human pathologies such as alcoholism, acute or chronic pain as well as psychiatric disorders. Opioid heteromers therefore stand as new therapeutic targets for the drug discovery field.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Gene regulation and priming by topoisomerase IIα in embryonic stem cells

Topoisomerases resolve torsional stress, while their function in gene regulation, especially during cellular differentiation, remains unknown. Here we find that the expression of topo II isoforms, topoisomerase IIα and topoisomerase IIβ, is the characteristic of dividing and postmitotic tissues, respectively. In embryonic stem cells, topoisomerase IIα preferentially occupies active gene promoters. Topoisomerase IIα inhibition compromises genomic integrity, which results in epigenetic changes, altered kinetics of RNA Pol II at target promoters and misregulated gene expression. Common targets of topoisomerase IIα and topoisomerase IIβ are housekeeping genes, while unique targets are involved in proliferation/pluripotency and neurogenesis, respectively. Topoisomerase IIα targets exhibiting bivalent chromatin resolve upon differentiation, concomitant with their activation and occupancy by topoisomerase IIβ, features further observed for long genes. These long silent genes display accessible chromatin in embryonic stem cells that relies on topoisomerase IIα activity. These findings suggest that topoisomerase IIα not only contributes to stem-cell transcriptome regulation but also primes developmental genes for subsequent activation upon differentiation.

Region-specific integration of embryonic stem cell-derived neuronal precursors into a pre-existing neuronal circuit

Enduring reorganization is accepted as a fundamental process of adult neural plasticity. The most dramatic example of this reorganization is the birth and continuously occurring incorporation of new neurons into the pre-existing network of the adult mammalian hippocampus. Based on this phenomenon we transplanted murine embryonic stem (ES)-cell derived neuronal precursors (ESNPs) into murine organotypic hippocampal slice cultures (OHC) and examined their integration. Using a precise quantitative morphological analysis combined with a detailed electrophysiology, we show a region-specific morphological integration of transplanted ESNPs into different subfields of the hippocampal tissue, resulting in pyramidal neuron-like embryonic stem cell-derived neurons (ESNs) in the Cornu Ammonis (CA1 and CA3) and granule neuron-like ESNs in the dentate gyrus (DG), respectively. Subregion specific structural maturation was accompanied by the development of dendritic spines and the generation of excitatory postsynaptic currents (EPSCs). This cell type specific development does not depend upon NMDA-receptor-dependent synaptic transmission. The presented integration approach was further used to determine the cell-autonomous function of the pan-neurotrophin receptor p75 (P75(NTR)), as a possible negative regulator of ESN integration. By this means we used p75(NTR)-deficient ESNPs to study their integration into a WT organotypic environment. We show here that p75(NTR) is not necessary for integration per se but plays a suppressing role in dendritic development.

Modeling of epigenome dynamics identifies transcription factors that mediate Polycomb targeting

Although changes in chromatin are integral to transcriptional reprogramming during cellular differentiation, it is currently unclear how chromatin modifications are targeted to specific loci. To systematically identify transcription factors (TFs) that can direct chromatin changes during cell fate decisions, we model the relationship between genome-wide dynamics of chromatin marks and the local occurrence of computationally predicted TF binding sites. By applying this computational approach to a time course of Polycomb-mediated H3K27me3 marks during neuronal differentiation of murine stem cells, we identify several motifs that likely regulate the dynamics of this chromatin mark. Among these, the sites bound by REST and by the SNAIL family of TFs are predicted to transiently recruit H3K27me3 in neuronal progenitors. We validate these predictions experimentally and show that absence of REST indeed causes loss of H3K27me3 at target promoters in trans, specifically at the neuronal progenitor state. Moreover, using targeted transgenic insertion, we show that promoter fragments containing REST or SNAIL binding sites are sufficient to recruit H3K27me3 in cis, while deletion of these sites results in loss of H3K27me3. These findings illustrate that the occurrence of TF binding sites can determine chromatin dynamics. Local determination of Polycomb activity by REST and SNAIL motifs exemplifies such TF based regulation of chromatin. Furthermore, our results show that key TFs can be identified ab initio through computational modeling of epigenome data sets using a modeling approach that we make readily accessible.

Retinoic acid signalling during development

Retinoic acid (RA) is a vitamin A-derived, non-peptidic, small lipophilic molecule that acts as ligand for nuclear RA receptors (RARs), converting them from transcriptional repressors to activators. The distribution and levels of RA in embryonic tissues are tightly controlled by regulated synthesis through the action of specific retinol and retinaldehyde dehydrogenases and by degradation via specific cytochrome P450s (CYP26s). Recent studies indicate that RA action involves an interplay between diffusion (morphogen-like) gradients and the establishment of signalling boundaries due to RA metabolism, thereby allowing RA to finely control the differentiation and patterning of various stem/progenitor cell populations. Here, we provide an overview of the RA biosynthesis, degradation and signalling pathways and review the main functions of this molecule during embryogenesis.

Target genes of Topoisomerase IIβ regulate neuronal survival and are defined by their chromatin state

Topoisomerases are essential for DNA replication in dividing cells, but their genomic targets and function in postmitotic cells remain poorly understood. Here we show that a switch in the expression from Topoisomerases IIα (Top2α) to IIβ (Top2β) occurs during neuronal differentiation in vitro and in vivo. Genome-scale location analysis in stem cell-derived postmitotic neurons reveals Top2β binding to chromosomal sites that are methylated at lysine 4 of histone H3, a feature of regulatory regions. Indeed Top2β-bound sites are preferentially promoters and become targets during the transition from neuronal progenitors to neurons, at a time when cells exit the cell cycle. Absence of Top2β protein or its activity leads to changes in transcription and chromatin accessibility at many target genes. Top2β deficiency does not impair stem cell properties and early steps of neuronal differentiation but causes premature death of postmitotic neurons. This neuronal degeneration is caused by up-regulation of Ngfr p75, a gene bound and repressed by Top2β. These findings suggest a chromatin-based targeting of Top2β to regulatory regions in the genome to govern the transcriptional program associated with neuronal differentiation and longevity.

Retinoic acid-dependent signaling pathways and lineage events in the developing mouse spinal cord

Studies in avian models have demonstrated an involvement of retinoid signaling in early neural tube patterning. The roles of this signaling pathway at later stages of spinal cord development are only partly characterized. Here we use Raldh2-null mouse mutants rescued from early embryonic lethality to study the consequences of lack of endogenous retinoic acid (RA) in the differentiating spinal cord. Mid-gestation RA deficiency produces prominent structural and molecular deficiencies in dorsal regions of the spinal cord. While targets of Wnt signaling in the dorsal neuronal lineage are unaltered, reductions in Fibroblast Growth Factor (FGF) and Notch signaling are clearly observed. We further provide evidence that endogenous RA is capable of driving stem cell differentiation. Raldh2 deficiency results in a decreased number of spinal cord derived neurospheres, which exhibit a reduced differentiation potential. Raldh2-null neurospheres have a decreased number of cells expressing the neuronal marker β-III-tubulin, while the nestin-positive cell population is increased. Hence, in vivo retinoid deficiency impaired neural stem cell growth. We propose that RA has separable functions in the developing spinal cord to (i) maintain high levels of FGF and Notch signaling and (ii) drive stem cell differentiation, thus restricting both the numbers and the pluripotent character of neural stem cells.

A chromatin-modifying function of JNK during stem cell differentiation

Signaling mediates cellular responses to extracellular stimuli. The c-Jun NH(2)-terminal kinase (JNK) pathway exemplifies one subgroup of the mitogen-activated protein (MAP) kinases, which, besides having established functions in stress response, also contribute to development by an unknown mechanism. We show by genome-wide location analysis that JNK binds to a large set of active promoters during the differentiation of stem cells into neurons. JNK-bound promoters are enriched with binding motifs for the transcription factor NF-Y but not for AP-1. NF-Y occupies these predicted sites, and overexpression of dominant-negative NF-YA reduces the JNK presence on chromatin. We find that histone H3 Ser10 (H3S10) is a substrate for JNK, and JNK-bound promoters are enriched for H3S10 phosphorylation. Inhibition of JNK signaling in post-mitotic neurons reduces phosphorylation at H3S10 and the expression of target genes. These results establish MAP kinase binding and function on chromatin at a novel class of target genes during stem cell differentiation.

Conditionally immortalised neural stem cells promote functional recovery and brain plasticity after transient focal cerebral ischaemia in mice

Cell therapy has enormous potential to restore neurological function after stroke. The present study investigated effects of conditionally immortalised neural stem cells (ciNSCs), the Maudsley hippocampal murine neural stem cell line clone 36 (MHP36), on sensorimotor and histological outcome in mice subjected to transient middle cerebral artery occlusion (MCAO). Adult male C57BL/6 mice underwent MCAO by intraluminal thread or sham surgery and MHP36 cells or vehicle were implanted into ipsilateral cortex and caudate 2 days later. Functional recovery was assessed for 28 days using cylinder and ladder rung tests and tissue analysed for plasticity, differentiation and infarct size. MHP36-implanted animals showed accelerated and augmented functional recovery and an increase in neurons (MAP-2), synaptic plasticity (synaptophysin) and axonal projections (GAP-43) but no difference in astrocytes (GFAP), oligodendrocytes (CNPase), microglia (IBA-1) or lesion volumes when compared to vehicle group. This is the first study showing a potential functional benefit of the ciNSCs, MHP36, after focal MCAO in mice, which is probably mediated by promoting neuronal differentiation, synaptic plasticity and axonal projections and opens up opportunities for future exploitation of genetically altered mice for dissection of mechanisms of stem cell based therapy.

Retinoic acid promotes neural conversion of mouse embryonic stem cells in adherent monoculture

Retinoic acid (RA) plays multiple roles in the nervous system, including induction of neural differentiation, axon outgrowth and neural patterning. Previously, RA for neural differentiation of embryonic stem (ES) cells always relies on embryoid bodies (EBs) formation. Here we report an in vitro adherent monoculture system to induce mouse ES cells into neural cells accompanied with RA. RA (1 μM) treatment, during initial 2 days of differentiation, can enhance the expression of neural markers, such as Nestin, Tuj1 and MAP2, and result in an earlier neural differentiation of ES cells. Furthermore, RA promotes a significant increase in neurite elongation of ES-derived neurons. Our study also implies that RA induced to express Wnt antagonist Dickkopf-1 (Dkk-1) for neural differentiation. However, the mechanisms of RA triggering neural induction remain to be determined. Our simple and efficient strategy is proposed to provide a basis for studying RA signaling pathways in neural differentiation in vitro.

Transplantation of mammalian embryonic stem cells and their derivatives to avian embryos

Xenografting of normal and transformed mammalian tissues and cells to chick embryos has been performed for almost 100 years. Embryonic stem cells, derived more than 25 years ago from murine, and more than 10 years ago from human blastocysts, have transformed many fields of biological research. There is a growing body of studies combining these two widely-used experimental systems. This review surveys those reports in which murine or human embryonic stem cells, or differentiated derivatives of these pluripotent stem cells, were transplanted to embryonated chick eggs. Many of these studies have utilized the unique characteristics of both experimental models to obtain answers to developmental questions that are difficult or impossible to approach with xenografting to adult rodents or tissue culture-only techniques.

Neuronal vs. glial fate of embryonic stem cell-derived neural progenitors (ES-NPs) is determined by FGF2/EGF during proliferation

Fate-specific differentiation of neural progenitors attracts keen interest in modern medicine due to its application in cell replacement therapy. Though various signaling pathways are involved in maintenance and differentiation of neural progenitors, the mechanism of development of lineage-restricted progenitors from embryonic stem (ES) cells is not clearly understood. Here, we have demonstrated that neuronal vs. glial differentiation potential of ES cell-derived neural progenitors (ES-NPs) are governed by the growth factors, exposed during their proliferation/expansion phase and cannot be significantly altered during differentiation phase. Exposure of ES-NPs to fibroblast growth factor-2 (FGF2) during proliferation triggered the expression of pro-neural genes that are required for neuronal lineage commitment, and upon differentiation, predominantly generated neurons. On the other hand, epidermal growth factor (EGF)-exposed ES-NPs are not committed to neuronal fate due to decreased expression of pro-neural genes. These ES-NPs further generate more glial cells due to expression of glial-restricted factors. Exposure of ES-NPs to the same growth factors during proliferation/expansion and differentiation phase augments the robust differentiation of neurons or glial subtypes. We also demonstrate that, during differentiation, exposure to growth factors other than that in which the ES-NPs were expanded does not significantly alter the fate of ES-NPs. Thus, we conclude that FGF2 and EGF determine the neural vs. glial fate of ES-NPs during proliferation and augment it during differentiation. Further modification of these protocols would help in generating fate-specified neurons for various regenerative therapies.

Neural stem cell systems: physiological players or in vitro entities?

Neural stem cells (NSCs) can be experimentally derived or induced from different sources, and the NSC systems generated so far are promising tools for basic research and biomedical applications. However, no direct and thorough comparison of their biological and molecular properties or of their physiological relevance and possible relationship to endogenous NSCs has yet been carried out. Here we review the available information on different NSC systems and compare their properties. A better understanding of these systems will be crucial to control NSC fate and functional integration following transplantation and to make NSCs suitable for regenerative efforts following injury or disease.

Differentiation of neurons from neural precursors generated in floating spheres from embryonic stem cells

Background

Neural differentiation of embryonic stem (ES) cells is usually achieved by induction of ectoderm in embryoid bodies followed by the enrichment of neuronal progenitors using a variety of factors. Obtaining reproducible percentages of neural cells is difficult and the methods are time consuming.

Results

Neural progenitors were produced from murine ES cells by a combination of nonadherent conditions and serum starvation. Conversion to neural progenitors was accompanied by downregulation of Oct4 and NANOG and increased expression of nestin. ES cells containing a GFP gene under the control of the Sox1 regulatory regions became fluorescent upon differentiation to neural progenitors, and ES cells with a tau-GFP fusion protein became fluorescent upon further differentiation to neurons. Neurons produced from these cells upregulated mature neuronal markers, or differentiated to glial and oligodendrocyte fates. The neurons gave rise to action potentials that could be recorded after application of fixed currents.

Conclusion

Neural progenitors were produced from murine ES cells by a novel method that induced neuroectoderm cells by a combination of nonadherent conditions and serum starvation, in contrast to the embryoid body method in which neuroectoderm cells must be selected after formation of all three germ layers.

Stem cells for ischemic brain injury: a critical review

No effective therapy is currently available to promote recovery following ischemic stroke. Stem cells have been proposed as a potential source of new cells to replace those lost due to central nervous system injury, as well as a source of trophic molecules to minimize damage and promote recovery. We undertook a detailed review of data from recent basic science and preclinical studies to investigate the potential application of endogenous and exogenous stem cell therapies for treatment of cerebral ischemia. To date, spontaneous endogenous neurogenesis has been observed in response to ischemic injury, and can be enhanced via infusion of appropriate cytokines. Exogenous stem cells from multiple sources can generate neural cells that survive and form synaptic connections after transplantation in the stroke-injured brain. Stem cells from multiple sources cells also exhibit neuroprotective properties that may ameliorate stroke deficits. In many cases, functional benefits observed are likely independent of neural differentiation, although the exact mechanisms remain poorly understood. Future studies of neuroregeneration will require the demonstration of function in endogenously born neurons following focal ischemia. Further, methods are currently lacking to demonstrate definitively the therapeutic effect of newly introduced neural cells. Increased plasticity following stroke may facilitate the functional integration of new neurons, but the loss of appropriate guidance cues and supporting architecture in the infarct cavity will likely impede the restoration of lost circuitry. Thus careful investigation of the mechanisms underlying trophic benefits will be essential. Evidence to date suggests that continued development of stem cell therapies may ultimately lead to viable treatment options for ischemic brain injury.

Molecular stages of rapid and uniform neuralization of human embryonic stem cells

Insights into early human development are fundamental for our understanding of human biology. Efficient differentiation of human embryonic stem cells (hESCs) into neural precursor cells is critical for future cell-based therapies. Here, using defined conditions, we characterized a new method for rapid and uniform differentiation of hESCs into committed neural precursor cells (designated C-NPCs). Dynamic gene expression analysis identified several distinct stages of ESC neuralization and revealed functional modules of coregulated genes and pathways. The first wave of gene expression changes, likely corresponding to the transition through primitive ectoderm, started at day 3, preceding the formation of columnar neuroepithelial rosettes. The second wave started at day 5, coinciding with the formation of rosettes. The majority of C-NPCs were positive for both anterior and posterior markers of developing neuroepithelium. In culture, C-NPCs became electrophysiologically functional neurons; on transplantation into neonatal mouse brains, C-NPCs integrated into the cortex and olfactory bulb, acquiring appropriate neuronal morphologies and markers. Compared to rosette-NPCs,(1) C-NPCs exhibited limited in vitro expansion capacity and did not express potent oncogenes such as PLAG1 or RSPO3. Concordantly, we never detected tumors or excessive neural proliferation after transplantation of C-NPCs into mouse brains. In conclusion, our study provides a framework for future analysis of molecular signaling during ESC neuralization.

Cotransplantation of Mouse Embryonic Stem Cells and Bone Marrow Stromal Cells following Spinal Cord Injury Suppresses Tumor Development

Embryonic stem (ES) cells are a potential source for treatment of spinal cord injury (SCI). Although one of the main problems of ES cell-based cell therapy is tumor formation, there is no ideal method to suppress tumor development. In this study, we examined whether transplantation with bone marrow stromal cells (BMSCs) prevented tumor formation in SCI model mice that received ES cell-derived grafts containing both undifferentiated ES cells and neural stem cells. Embryoid bodies (EBs) formed in 4-day hanging drop cultures were treated with retinoic acid (RA) at a low concentration of 5 × 10–9 M for 4 days, in order to allow some of the ES cells to remain in an undifferentiated state. RA-treated EBs were enzymatically digested into single cells and used as ES cell-derived graft cells. Mice transplanted with ES cell-derived graft cells alone developed tumors at the grafted site and behavioral improvement ceased after day 21. In contrast, no tumor development was observed in mice cotransplanted with BMSCs, which also showed sustained behavioral improvement. In vitro results demonstrated the disappearance of SSEA-1 expression in cytochemical examinations, as well as attenuated mRNA expressions of the undifferentiated markers Oct3/4, Utf1, Nanog, Sox2, and ERas by RT-PCR in RA-treated EBs cocultured with BMSCs. In addition, MAP2-immunopositive cells appeared in the EBs cocultured with BMSCs. Furthermore, the synthesis of NGF, GDNF, and BDNF was confirmed in cultured BMSCs, while immunohistochemical examinations demonstrated the survival of BMSCs and their maintained ability of neurotrophic factor production at the grafted site for up to 5 weeks after transplantation. These results suggest that BMSCs induce undifferentiated ES cells to differentiate into a neuronal lineage by neurotrophic factor production, resulting in suppression of tumor formation. Cotransplantation of BMSCs with ES cell-derived graft cells may be useful for preventing the development of ES cell-derived tumors.

CoCl(2) inhibits neural differentiation of retinoic acid-treated embryoid bodies

The effects of CoCl(2) on retinoic acid (RA)-treated embryoid bodies (EBs) were investigated. Four-day EBs were treated with 5x10(-6) M of RA for 4 d, then subjected to attached culturing for 7 d in the presence of CoCl(2) at 0, 20, and 100 microM. Differentiation into MAP2- and GFAP-immunopositive cells was inhibited by CoCl(2) in a dose-dependent manner. Next, RA-treated EBs were dissociated into single cells and cultured for 7 d at an initial cell density of 1x10(3)/cm(2). The number of cells increased in a CoCl(2)-dose dependent fashion. In cultures with 100 microM of CoCl(2), more than 90% of the cells were immunopositive for nestin and nestin-immunopositive cells formed clusters, while there were few cells immunopositive for MAP2 or GFAP. These results suggest that CoCl(2) inhibits neural differentiation of RA-treated EB cells and promotes the proliferation of nestin-immunopositive cells, i.e., embryonic stem (ES)-derived neural stem-like cells.

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

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

Lineage-specific polycomb targets and de novo DNA methylation define restriction and potential of neuronal progenitors

Cellular differentiation entails loss of pluripotency and gain of lineage- and cell-type-specific characteristics. Using a murine system that progresses from stem cells to lineage-committed progenitors to terminally differentiated neurons, we analyzed DNA methylation and Polycomb-mediated histone H3 methylation (H3K27me3). We show that several hundred promoters, including pluripotency and germline-specific genes, become DNA methylated in lineage-committed progenitor cells, suggesting that DNA methylation may already repress pluripotency in progenitor cells. Conversely, we detect loss and acquisition of H3K27me3 at additional targets in both progenitor and terminal states. Surprisingly, many neuron-specific genes that become activated upon terminal differentiation are Polycomb targets only in progenitor cells. Moreover, promoters marked by H3K27me3 in stem cells frequently become DNA methylated during differentiation, suggesting context-dependent crosstalk between Polycomb and DNA methylation. These data suggest a model how de novo DNA methylation and dynamic switches in Polycomb targets restrict pluripotency and define the developmental potential of progenitor cells.

Evidence that androgens regulate early developmental events, prior to sexual differentiation

Androgen signaling is critical for normal fetal development but is not thought to regulate events in early embryogenesis. Given the interest in factors controlling the differentiation of embryonic stem (ES) cells, we have explored the possibility that androgens may play a role. This study demonstrates expression of androgen receptor (AR) RNA and protein in four independent mouse ES (mES) cell lines, and shows that the AR is functional and can interact with transfected androgen response elements to promote green fluorescent protein expression. AR mRNA was detected throughout 10-d differentiation in embryoid bodies (EBs). Exposure of EBs to testosterone (T) or dihydrotestosterone, at doses of 1 and 0.1 mum, respectively, promoted formation of beating cardiomyocytes. Flow cytometric analyses demonstrated a significant increase in the number of alpha-actinin and tropomyosin (cardiac markers) positive cells after these treatments. Addition of flutamide (1 microM) to T-treated EBs inhibited the T-induced proliferation of cardiomyocytes, confirming that, in this instance, androgens act via the classical AR-mediated genomic pathway. We also report that mES cells express key steroidogenic enzymes, as detected by RT-PCR, and during 24-h incubations secrete T at concentrations of 1.38 +/- 0.22 nM, levels comparable to those secreted by cultured Leydig cells. These novel data demonstrate the capacity of androgens to stimulate increased differentiation of mouse ES cells to cardiomyocytes, and are in keeping with recent observations that AR-deficient mice exhibit cardiac impairment in adulthood.

Mechanisms that mediate stem cell self-renewal and differentiation

Stem cells have two common properties: the capacity for self-renewal and the potential to differentiate into one or more specialized cell types. In general, stem cells can be divided into two broad categories: adult (somatic) stem cells and embryonic stem cells. Recent evidence suggested that tumors may contain "cancer stem cells" with indefinite potential for self-renewal. In this review, we will focus on the molecular mechanisms regulating embryonic stem cell self-renewal and differentiation, and discuss how these mechanisms may be relevant in cancer cells.

Embryonic stem cells and retinal repair

In this review we examine the potential of embryonic stem cells (ESCs) for use in the treatment of retinal diseases involving photoreceptors and retinal pigment epithelium (RPE). We outline the ontogenesis of target retinal cell types (RPE, rods and cones) and discuss how an understanding of developmental processes can inform our manipulation of ESCs in vitro. Due to their potential for cellular therapy, special emphasis is placed upon the derivation and culture of human embryonic stem cells (HESCs) and their differentiation towards a retinal phenotype. In terms of achieving this goal, we suggest that much of the success to date reflects permissive in vitro environments provided by established protocols for HESC derivation, propagation and neural differentiation. In addition, we summarise key factors that may be important for enhancing efficiency of retinal cell-type derivation from HESCs. The retina is an amenable component of the central nervous system (CNS) and as such, diseases of this structure provide a realistic target for the application of HESC-derived cellular therapy to the CNS. In order to further this goal, the second component of our review focuses on the cellular and molecular cues within retinal environments that may influence the survival and behaviour of transplanted cells. Our analysis considers both the potential barriers to transplant integration in the retina itself together with the remodelling in host visual centres that is known to accompany retinal dystrophy.

Neurotrophin receptor-mediated death of misspecified neurons generated from embryonic stem cells lacking Pax6

Pax6-positive radial glial (RG) cells are the progenitors of most glutamatergic neurons in the cortex, a lineage that can be recapitulated in vitro using embryonic stem (ES) cells. We show here that ES cells lacking Pax6, a transcription factor long known to be essential for cortical development, generate Mash1-positive RG cells that differentiate in GABAergic neurons. These neurons express high levels of the neurotrophin receptor p75NTR causing their rapid death. Pax6 function was also investigated following transplantation of ES cells in the developing chick telencephalon and in mice lacking both Pax6 and p75NTR. Taken together, our results indicate that reliable predictions can be made with cultured ES cells when used to explore the role of genes impacting early aspects of mammalian neurogenesis. They also provide a novel opportunity to compare the molecular constituents of glutamatergic with those of GABA-ergic neurons and to explore the mechanisms of their generation.

Radial glia and neural stem cells

During the last decade, the role of radial glia has been radically revisited. Rather than being considered a mere structural component serving to guide newborn neurons towards their final destinations, radial glia is now known to be the main source of neurons in several regions of the central nervous system, notably in the cerebral cortex. Radial glial cells differentiate from neuroepithelial progenitors at the beginning of neurogenesis and share with their ancestors the bipolar shape and the expression of some molecular markers. Radial glia, however, can be distinguished from neuroepithelial progenitors by the expression of astroglial markers. Clonal analyses showed that radial glia is a heterogeneous population, comprising both pluripotent and different lineage-restricted neural progenitors. At late-embryonic and postnatal stages, radial glial cells give rise to the neural stem cells responsible for adult neurogenesis. Embryonic pluripotent radial glia and adult neural stem cells may be clonally linked, thus representing a lineage displaying stem cell features in both the developing and mature central nervous system.

Directing human embryonic stem cells to generate vascular progenitor cells

Pluripotent human embryonic stem cells (hESCs) differentiate into most of the cell types of the adult human body, including vascular cells. Vascular cells, such as endothelial cells and vascular smooth muscle cells (SMCs) are significant contributors to tissue repair and regeneration. In addition to their potential applications for treatment of vascular diseases and stimulation of ischemic tissue growth, it is also possible that endothelial cells and SMCs derived from hESCs can be used to engineer artificial vessels to repair damaged vessels and form vessel networks in engineered tissues. Here we review the current status of directing hESCs to differentiate to vascular cells.

The notch response inhibitor DAPT enhances neuronal differentiation in embryonic stem cell-derived embryoid bodies independently of sonic hedgehog signaling

During development of the neural tube, inhibition of the Notch response as well as the activation of the Sonic Hedgehog (Shh) response results in the formation of neuronal cell types. To determine whether Shh and Notch act independently, we tested the effects of the Notch inhibitor DAPT (N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester) on neuralized, embryonic stem (ES) cell-derived embryoid bodies (EBs), while varying the levels of Shh pathway activation. Shh-resistant EBs were derived from Smo null ES cells, while EBs with constitutive high level of Shh pathway activation were derived from Ptc1 null ES cells. Intermediate levels of Shh pathway activation was achieved by the addition of ShhN to the EB culture medium. It was found that DAPT-mediated inhibition of the Notch response resulted in enhanced neuronal differentiation. In the absence of Shh, more interneurons were detected, while the main effect of DAPT on EBs with an activated Shh response was the precocious loss of ventral neuronal precursor-specific markers.

Methods for inducing embryoid body formation: in vitro differentiation system of embryonic stem cells

When cultured in suspension without antidifferentiation factors, embryonic stem (ES) cells spontaneously differentiate and form three-dimensional multicellular aggregates called embryoid bodies (EBs). EBs recapitulate many aspects of cell differentiation during early embryogenesis, and play an important role in the differentiation of ES cells into a variety of cell types in vitro. There are several methods for inducing the formation of EBs from ES cells. The three basic methods are liquid suspension culture in bacterial-grade dishes, culture in methylcellulose semisolid media, and culture in hanging drops. Recently, the methods using a round-bottomed 96-well plate and a conical tube are adopted for forming EBs from predetermined numbers of ES cells. For the production of large numbers of EBs, stirred-suspension culture using spinner flasks and bioreactors is performed. Each of these methods has its own peculiarity; thus, the features of formed EBs depending on the method used. Therefore, we should choose an appropriate method for EB formation according to the objective to be attained. In this review, we summarize the studies on in vitro differentiation of ES cells via EB formation and highlight the EB formation methods recently developed including the techniques, devices, and procedures involved.

RPE transplantation and its role in retinal disease

Retinal pigment epithelial (RPE) transplantation aims to restore the subretinal anatomy and re-establish the critical interaction between the RPE and the photoreceptor, which is fundamental to sight. The field has developed over the past 20 years with advances coming from a large body of animal work and more recently a considerable number of human trials. Enormous progress has been made with the potential for at least partial restoration of visual function in both animal and human clinical work. Diseases that have been treated with RPE transplantation demonstrating partial reversal of vision loss include primary RPE dystrophies such as the merTK dystrophy in the Royal College of Surgeons (RCS) rat and in humans, photoreceptor dystrophies as well as complex retinal diseases such as atrophic and neovascular age-related macular degeneration (AMD). Unfortunately, in the human trials the visual recovery has been limited at best and full visual recovery has not been demonstrated. Autologous full-thickness transplants have been used most commonly and effectively in human disease but the search for a cell source to replace autologous RPE such as embryonic stem cells, marrow-derived stem cells, umbilical cord-derived cells as well as immortalised cell lines continues. The combination of cell transplantation with other modalities of treatment such as gene transfer remains an exciting future prospect. RPE transplantation has already been shown to be capable of restoring the subretinal anatomy and improving photoreceptor function in a variety of retinal diseases. In the near future, refinements of current techniques are likely to allow RPE transplantation to enter the mainstream of retinal therapy at a time when the treatment of previously blinding retinal diseases is finally becoming a reality.

Generation of a defined and uniform population of CNS progenitors and neurons from mouse embryonic stem cells

A detailed protocol is described allowing the generation of essentially pure populations of glutamatergic neurons from mouse embryonic stem (ES) cells. It is based on the culture of ES cells that are kept undifferentiated by repeated splitting and subsequently amplified as non-adherent cell aggregates. Treatment with retinoic acid causes these ES cells to essentially become neural progenitors with the characteristics of Pax6-positive radial glial cells. As they do in vivo, these progenitors differentiate in glutamatergic pyramidal neurons that form functional synaptic contacts and can be kept in culture for long periods of time. This protocol does not require the use of ES lines expressing resistance or fluorescent markers and can thus be applied in principle to any wild-type or mutant ES line of interest. At least 2 weeks are required from starting ES cell culture until plating progenitors and differentiating neurons establish synaptic transmission within about 10 days.

Adipogenesis of murine embryonic stem cells in a three-dimensional culture system using electrospun polymer scaffolds

A mechanistic understanding of adipose tissue differentiation is critical for the treatment and prevention of obesity and type 2 diabetes. Conventional in vitro models of adipogenesis are preadipocytes or freshly isolated adipocytes grown in two-dimensional (2D) cultures. Optimal results using in vitro tissue culture models can be expected only when adipocyte models closely resemble adipose tissue in vivo. Thus the design of an in vitro three-dimensional (3D) model which faithfully mimics the in vivo environment is needed to effectively study adipogenesis. Pluripotent embryonic stem (ES) cells are a self-renewing cell type that can readily be differentiated into adipocytes. In this study, a 3D culture system was developed to mimic the geometry of adipose tissue in vivo. Murine ES cells were seeded into electrospun polycaprolactone scaffolds and differentiated into adipocytes in situ by hormone induction as demonstrated using a battery of gene and protein expression markers along with the accumulation of neutral lipid droplets. Insulin-responsive Akt phosphorylation, and beta-adrenergic stimulation of cyclic AMP synthesis were demonstrated in ES cell-derived adipocytes. Morphologically, ES cell-derived adipocytes resembled native fat cells by scanning electron and phase contrast microscopy. This tissue engineered ES cell-matrix model has potential uses in drug screening and other therapeutic developments.

Directing the differentiation of embryonic stem cells to neural stem cells

Embryonic stem cells (ESCs) are a potential source of neural derivatives that can be used in stem cell-based therapies designed to treat neurological disorders. The derivation of specific neuronal or glial cell types from ESCs invariably includes the production of neural stem cells (NSCs). We describe the basic mechanisms of neural induction during vertebrate embryogenesis and how this information helped formulate several protocols used to generate NSCs from ESCs. We highlight the advantages and disadvantages of each approach and review what has been learned about the intermediate stages in the transition from ESC to NSC. Recent data describing how specific growth factors and signaling molecules regulate production of NSCs are described and a model synthesizing this information is presented.

Developmental potential of radial glia investigated by transplantation into the developing rat ventricular system in utero

During development there is a clear correlation between position of dividing progenitor cells, mode of division and developmental potential, suggesting that the local environment of progenitor cells may influence their cell fate [ 17 (6), 639-647]. The contribution of these conditions was investigated here by transplantation of radial glial progenitor cells into isotopic, isochronic, heterotopic and heterochronic environment conditions. Neuronal cells were removed from E14 spinal cords using negative immunoselection. The remaining radial glia were transplanted into the ventricular system of host embryos and pups. Distance of migration as well as morphological and antigenic phenotype of transplanted radial glia was examined after various survival times post transplantation. Host age clearly influenced migration and differentiation of transplant cells, with transplant cells migrating further in younger hosts and differentiating earlier in older aged host environments. Evidence is presented showing that most transplanted spinal cord radial glia give rise to astrocytes. In addition some transplanted radial glia were shown to give rise to neurons in spinal cord regions. Radial glia did not appear to generate neurons in the brains of host animals until postnatal ages, perhaps because transplanted radial glia were isolated from spinal cord and thus may not have been influenced to behave as endogenous radial glia in the brain which commonly produce neurons.

Embryonic stem cell-derived neural progenitors display temporal restriction to neural patterning

Neural stem cells have considerable therapeutic potential because of their ability to generate defined neuronal cell types for use in drug screening studies or cell-based therapies for neurodegenerative diseases. In this study, we differentiate mouse embryonic stem cells to neural progenitors with an initial forebrain identity in a defined system that enables systematic manipulation to generate more caudal fates, including motoneurons. We demonstrate that the ability to pattern embryonic stem cell-derived neural progenitors is temporally restricted and show that the loss of responsiveness to morphogenetic cues correlates with constitutive expression of the basic helix-loop-helix transcription factors Olig2 and Mash1, epidermal growth factor receptor, and vimentin and parallels the onset of gliogenesis. We provide evidence for two temporal classes of embryonic stem cell-derived putative radial glia that coincide with a transition from neurogenesis to gliogenesis and a concomitant loss of regional identity.

Concise review: scientific and ethical roadblocks to human embryonic stem cell therapy

Despite the identified therapeutic potential of embryonic stem cells for treating human disease and injury, a number of roadblocks, scientific and ethical, stand in the way of progress toward this goal. We identify six areas of particular interest: tumorigenicity, animal product contamination, genetic compatibility, funding, cell type for transplantation, "embryo-friendly" derivation methods and discuss avenues for moving beyond the difficulties.

Critical roles of Src family tyrosine kinases in excitatory neuronal differentiation of cultured embryonic stem cells

Embryonic stem (ES) cells have been tested for potential cell transplantation therapy for CNS disorders. Understanding their differentiation mechanism and identifying factors involved in driving excitatory and inhibitory neuron lineages should enhance the efficacy and efficiency of the cell transplantation therapy. We tested the hypothesis that selective expression of Src family tyrosine kinases is required for phenotype-specific differentiation and functional maturation of ES cell derived neurons. Cultured mouse pluripotent ES cells were treated with retinoic acid (RA) to induce neural differentiation. After RA induction, neurons derived from ES cells showed significant neurite growth, increased expression of Src, Fyn and Lck and an extension of Src kinase expression from cell body to neurite processes. ES cell derived neuron-like cells expressed neurofilament, synaptophysin, glutamate receptors, NMDA and kainate currents, became vulnerable to excitotoxicity and formed functional excitatory synapses. These developmental events were blocked or attenuated when cells were grown in the presence of Src family kinase inhibitor PP2. However, there was no change in the expression of GABAergic-specific protein GAD67 during PP2 treatment. Our data suggest that Src tyrosine kinases are involved in the terminal differentiation of excitatory neuronal phenotype during ES cell neural differentiation after RA induction.

Motoneurons derived from embryonic stem cells express transcription factors and develop phenotypes characteristic of medial motor column neurons

Embryonic stem (ES) cells differentiate into functional motoneurons when treated with a sonic hedgehog (Shh) agonist and retinoic acid (RA). Whether ES cells can be directed to differentiate into specific subtypes of motoneurons is unknown. We treated embryoid bodies generated from HBG3 ES cells with a Shh agonist and RA for 5 d in culture to induce motoneuron differentiation. Enhanced green fluorescent protein (eGFP) expression was used to identify putative motoneurons, because eGFP is expressed under the control of the Hb9 promoter in HBG3 cells. We found that 96 +/- 0.7% of the differentiated eGFP+ motoneurons expressed Lhx3, a homeobox gene expressed by postmitotic motoneurons in the medial motor column (MMCm), when the treated cells were plated on a neurite-promoting substrate for 5 d. When the treated embryoid bodies were transplanted into stage 17 chick neural tubes, the eGFP+ motoneurons migrated to the MMCm, expressed Lhx3, projected axons to the appropriate target for MMCm motoneurons (i.e., epaxial muscles), and contained synaptic vesicles within intramuscular axonal branches. In ovo and in vitro studies indicated that chemotropic factors emanating from the epaxial muscle and/or surrounding mesenchyme likely guide Lhx3+ motoneurons to their correct target. Finally, whole-cell patch-clamp recordings of transplanted ES cell-derived motoneurons demonstrated that they received synaptic input, elicited repetitive trains of action potentials, and developed passive membrane properties that were similar to host MMCm motoneurons. These results indicate that ES cells can be directed to form subtypes of neurons with specific phenotypic properties.

Transplantation of embryonic neuroectodermal progenitor cells into the site of a photochemical lesion: Immunohistochemical and electrophysiological analysis

GFP labeled/NE-4C neural progenitor cells cloned from primary neuroectodermal cultures of p53- mouse embryos give rise to neurons when exposed to retinoic acid in vitro. To study their survival and differentiation in vivo, cells were transplanted into the cortex of 6-week-old rats, 1 week after the induction of a photochemical lesion or into noninjured cortex. The electrophysiological properties of GFP/NE-4C cells were studied in vitro (8-10 days after differentiation induction) and 4 weeks after transplantation using the whole-cell patch-clamp technique, and immunohistochemical analyses were carried out. After transplantation into a photochemical lesion, a large number of cells survived, some of which expressed the astrocytic marker GFAP. GFP/GFAP-positive cells, with an average resting membrane potential (Vrest) of -71.9 mV, displayed passive time- and voltage-independent K+ currents and, additionally, voltage-dependent A-type K+ currents (KA) and/or delayed outwardly rectifying K+ currents (KDR). Numerous GFP-positive cells expressed NeuN, betaIII-tubulin, or 68 kD neurofilaments. GFP/betaIII-tubulin-positive cells, with an average Vrest of -61.6 mV, were characterized by the expression of KA and KDR currents and tetrodotoxin-sensitive Na+ currents. GFP/NE-4C cells also gave rise to oligodendrocytes, based on the detection of oligodendrocyte-specific markers. Our results indicate that GFP/NE-4C neural progenitors transplanted into the site of a photochemical lesion give rise to neurons and astrocytes with membrane properties comparable to those transplanted into noninjured cortex. Therefore, GFP/NE-4C cells provide a suitable model for studying neuro- and gliogenesis in vivo. Further, our results suggest that embryonic neuroectodermal progenitor cells may hold considerable promise for the repair of ischemic brain lesions.

Radial glial cells defined and major intermediates between embryonic stem cells and CNS neurons

Radial glial cells have been identified as a major source of neurons during development. Here, we review the evidence for the distinct "glial" nature of radial glial cells and contrast these cells with their progenitors, the neuroepithelial cells. Recent results also suggest that not only during neurogenesis in vivo, but also during the differentiation of cultured embryonic stem cells toward neurons, progenitors with clear glial antigenic characteristics act as cellular intermediates.