Sox2 regulatory sequences direct expression of a (beta)-geo transgene to telencephalic neural stem cells and precursors of the mouse embryo, revealing regionalization of gene expression in CNS stem cells

Pax6 directly modulate Sox2 expression in the neural progenitor cells

Pax6 is a key regulator in the neuronal fate determination as well as the proliferation of neural stem cells, but the mechanisms are still unknown. Our study shows that Pax6 regulate the proliferation of neural progenitor cells of cortical subventricular zone, through direct modulation of the Sox2 expression during the late developmental stage in mice. We found a dramatic decrease in the number of Sox2+ neural progenitor cells in the subventricular zone of E18.5 Pax6(-/-) mice. We confirmed that Pax6 could bind to the Sox2 promoter by chromatin immunoprecipitation assay and activate Sox2 expression by a luciferase reporter gene assay. Moreover, neural progenitors isolated from the Pax6(-/-) embryos showed a decreased neurosphere formation as well as proliferation.

Epigenetic modifications of SOX2 enhancers, SRR1 and SRR2, correlate with in vitro neural differentiation

SOX2 is a key neurodevelopmental gene involved in maintaining the pluripotency of stem cells and proliferation of neural progenitors and astroglia. Two evolutionally conserved enhancers, SRR1 and SRR2, are involved in controlling SOX2 expression during neurodevelopment; however, the molecular mechanisms regulating their activity are not known. We have examined DNA methylation and histone H3 acetylation at both enhancers in NT2-D1 progenitors, neurons and astrocytes, to establish the role of epigenetic mechanisms in cell-type-specific SOX2 expression. This study showed that 1) unmethylated DNA and acetylated histones at both enhancers correlated with a high level of SOX2 expression in proliferating neural progenitors and 2) reversible modifications of the SRR1 element were observed during gene reexpression in astrocytes, whereas permanent epigenetic marks on the SRR2 enhancer were seen in neurons where the gene was silenced. Taken together, these results are clear illustrations of cell-type-specific epigenomes and suggest mechanisms by which they may be created and maintained.

Pax6 promotes neurogenesis in human neural stem cells

During brain embryogenesis, transcription factors drive stem cells towards neuronal fate. Here we show that the transcription factor Pax6 increased in vitro generation of neurons from striatal but not cortical neural stem cells (NSCs), derived from 6 to 9 weeks old human fetuses, without affecting survival and proliferation. Overexpression of mouse Pax6 produced increased numbers of GABA+ and DARPP-32+ (characteristic of striatum) but not glutamate+ neurons (characteristic of cortex). Pax6-overexpressing cells survived and migrated to the same extent as control cells at 1 month after intrastriatal transplantation into newborn rats and generated more neuroblasts. Overexpression of mouse Pax6 in human NSCs also leads to altered levels of lineage-appropriate genes as revealed by Q-PCR. Our data suggest that Pax6 function is conserved between species since its overexpression activates similar genes in mouse and human NSCs. Also, that Pax6 overexpression in striatal NSCs increases the number of neurons but their region-specificity is maintained.

Temporal and epigenetic regulation of neurodevelopmental plasticity

The anticipated therapeutic uses of neural stem cells depend on their ability to retain a certain level of developmental plasticity. In particular, cells must respond to developmental manipulations designed to specify precise neural fates. Studies in vivo and in vitro have shown that the developmental potential of neural progenitor cells changes and becomes progressively restricted with time. For in vitro cultured neural progenitors, it is those derived from embryonic stem cells that exhibit the greatest developmental potential. It is clear that both extrinsic and intrinsic mechanisms determine the developmental potential of neural progenitors and that epigenetic, or chromatin structural, changes regulate and coordinate hierarchical changes in fate-determining gene expression. Here, we review the temporal changes in developmental plasticity of neural progenitor cells and discuss the epigenetic mechanisms that underpin these changes. We propose that understanding the processes of epigenetic programming within the neural lineage is likely to lead to the development of more rationale strategies for cell reprogramming that may be used to expand the developmental potential of otherwise restricted progenitor populations.

Secreted factors from olfactory mucosa cells expanded as free-floating spheres increase neurogenesis in olfactory bulb neurosphere cultures

Background

The olfactory epithelium is a neurogenic tissue comprising a population of olfactory receptor neurons that are renewed throughout adulthood by a population of stem and progenitor cells. Because of their relative accessibility compared to intra-cranially located neural stem/progenitor cells, olfactory epithelium stem and progenitor cells make attractive candidates for autologous cell-based therapy. However, olfactory stem and progenitor cells expand very slowly when grown as free-floating spheres (olfactory-spheres) under growth factor stimulation in a neurosphere assay.

Results

In order to address whether olfactory mucosa cells extrinsically regulate proliferation and/or differentiation of immature neural cells, we cultured neural progenitor cells derived from mouse neonatal olfactory bulb or subventricular zone (SVZ) in the presence of medium conditioned by olfactory mucosa-derived spheres (olfactory-spheres). Our data demonstrated that olfactory mucosa cells produced soluble factors that affect bulbar neural progenitor cell differentiation but not their proliferation when compared to control media. In addition, olfactory mucosa derived soluble factors increased neurogenesis, especially favouring the generation of non-GABAergic neurons. Olfactory mucosa conditioned medium also contained several factors with neurotrophic/neuroprotective properties. Olfactory-sphere conditioned medium did not affect proliferation or differentiation of SVZ-derived neural progenitors.

Conclusion

These data suggest that the olfactory mucosa does not contain factors that are inhibitory to neural stem/progenitor cell proliferation but does contain factors that steer differentiation toward neuronal phenotypes. Moreover, they suggest that the poor expansion of olfactory-spheres may be in part due to intrinsic properties of the olfactory epithelial stem/progenitor cell population.

Neural stem cell self-renewal

Two fundamental properties of stem cells are their ability to self-renew and to differentiate. Self-renewal is an integration of proliferation control with the maintenance of an undifferentiated state. Stem cell self-renewal is regulated by the dynamic interplay between transcription factors, epigenetic control, microRNA (miRNA) regulators, and cell-extrinsic signals from the microenvironment in which stem cells reside. Recent progress in defining specific roles for cell-intrinsic factors and extrinsic factors in regulating stem cell self-renewal starts to unfold the multilayered regulatory networks. This review focuses on cell-intrinsic regulators, including orphan nuclear receptor TLX, polycomb transcriptional repressor Bmi1, high-mobility-group DNA binding protein Sox2, basic helix-loop-helix Hes genes, histone modifying enzymes and chromatin remodeling proteins, and small RNA modulators, as well as cell-extrinsic signaling molecules, such as Wnt, Notch, Sonic hedgehog (Shh), TGFalpha, EGF, and FGF. Unraveling the mechanisms by which neural stem cells renew themselves will provide insights into both basic neurosciences and clinical applications of stem cell-based cell replacement therapies for neurodegenerative diseases.

Licensing regulators Geminin and Cdt1 identify progenitor cells of the mouse CNS in a specific phase of the cell cycle

Nervous system formation integrates control of cellular proliferation and differentiation and is mediated by multipotent neural progenitor cells that become progressively restricted in their developmental potential before they give rise to differentiated neurons and glial cells. Evidence from different experimental systems suggests that Geminin is a candidate molecule linking proliferation and differentiation during nervous system development. We show here that Geminin and its binding partner Cdt1 are expressed abundantly by neural progenitor cells during early mouse neurogenesis. Their expression levels decline at late developmental stages and become undetectable upon differentiation. Geminin and Cdt1 expressing cells also express Sox2 while no overlap is detected with cells expressing markers of a differentiated neuronal phenotype. A fraction of radial glial cells expressing RC2 and Pax6 are also immunoreactive for Geminin and Cdt1. The majority of the Geminin and Cdt1 expressing cell populations appears to be distinct from fate-restricted precursor cells expressing Mash1 or Neurogenin2. Bromo-deoxy-uridine (BrdU) incorporation experiments reveal a cell cycle specific expression in neural progenitor cells, with Geminin being present from S to M phase, while Cdt1 expression characterizes progenitor cells in G1 phase. Furthermore, in vitro differentiation of adult neurosphere cultures shows downregulation of Geminin/Cdt1 in the differentiated state, in line with our data showing that Geminin is present in neural progenitor cells of the CNS during mouse embryogenesis and adulthood and becomes downregulated upon cell fate specification and differentiation. This suggests a role for Geminin in the formation and maintenance of the neural progenitor cells.

Identification of self-replicating multipotent progenitors in the embryonic nervous system by high Notch activity and Hes5 expression

Discrimination of neural stem cells from other progenitors in the developing mammalian brain has been hampered by the lack of specific markers. Identifying the progenitor pools and signalling pathways that guide mammalian neurogenesis are central to understanding the complex mechanisms that govern development of the nervous system. Notch signalling plays a pivotal role in the development of the mammalian nervous system by maintaining multipotent neural stem cells and regulating their fate. In order to identify putative neural stem cells in situ, we generated transgenic mice that express Green Fluorescent Protein (GFP) and report Notch signalling activity in the developing CNS. Here we show the subdivision of progenitors within the neural tube of these mice. We purify progenitors from the neural tube and show that cells with the highest levels of Notch-reporter activity have self-renewal capability and multipotency, whereas those lacking Hes5 expression do not form neurospheres in vitro. Using marker protein co-expression and cell sorting, we show that both neuroepithelial cells as well as some radial glia at all axial levels of the embryonic neural tube display active Notch signalling. However, Tbr2-positive basal progenitors of the developing telencephalon and differentiating Islet1/2- and Lim1-positive motor neurons outside the ventricular zone do not express Hes5-GFP. Quantitative analysis showed that Hes5 expression correlates better with neural stem cell potential than expression of the related gene Hes1. Thus, Notch activity through Hes5 identifies multipotent progenitors with stem cell properties and subdivides the different progenitors into defined pools.

Isolation and characterization of murine neural stem/progenitor cells based on Prominin-1 expression

The identification of strategies for the isolation of neural stem cells (NSCs) has important implications for the understanding of their biology and the development of therapeutic applications. It has been previously described that human neural stem and progenitor cells (NSPCs) can be isolated from the central nervous system (CNS) using antibodies to prominin (CD133) and fluorescence-activated cell sorting (FACS). Although this antigen displayed an identical membrane topology in several human and murine tissues there was uncertainty as to the relationship between human and mouse prominin because of the low level of amino acid identity. Here we show that prominin expression can be used to identify and isolate also murine NSPCs from the developing or adult brain. Prominin is co-expressed with known neural stem markers like SOX 1-2, Musashi and Nestin. Moreover, neurosphere-forming cells with multipotency and self-renewal capacity reside within the prominin-positive fraction. Transplantation experiments show that CD133-positive cells give rise to neurons and glial cells in vivo, and that many neurons display appropriate phenotypic characteristics of the recipient tissues. The demonstration that CD133 is a stem cell antigen for murine NSPCs as it is for human NSPCs is useful for the investigation of mammal neurogenesis and development of preclinical tests of NSPCs transplantation in mouse analogues of human diseases.

The human subventricular zone: a source of new cells and a potential source of brain tumors

The mammalian brain has been perceived as a quiescent organ incapable of postnatal neurogenesis for many years. Most recently, several studies have demonstrated that the adult mammalian brain is indeed capable of neurogenesis and that the process is primarily confined to the subventricular zone (SVZ) of the forebrain and the subgranular zone (SGZ) of the hippocampus. Of these regions, the SVZ is the largest niche of neurogenesis in the adult mammalian brain. Within this niche resides a subpopulation of astrocytes with stem cell-like features of self-renewal and multipotentiality. Interestingly, there is also a subpopulation of cells within brain tumors that possess these same characteristics. Based on these findings, the emerging hypothesis is that brain tumor stem cells may be derived from neural stem cells and that both of these populations may originate from the SVZ. This possible connection stresses the importance of studying and understanding the role that the human SVZ plays in not only harboring neural and brain tumor stem cells, but how this microenvironment may support both neurogenesis and tumorigenesis. Furthermore, the obvious differences in the SVZ between humans and other animals make it important to understand the human model when studying human disease. Such an understanding may lead to novel therapeutic strategies for both neurodegenerative diseases and currently intractable brain tumors.

Human fetal cortical and striatal neural stem cells generate region-specific neurons in vitro and differentiate extensively to neurons after intrastriatal transplantation in neonatal rats

Human fetal brain is a potential source of neural stem cells (NSCs) for cell replacement therapy in neurodegenerative diseases. We explored whether NSCs isolated from cortex and striatum of human fetuses, aged 6-9 weeks post-conception, maintain their regional identity and differentiate into specific neuron types in culture and after intrastriatal transplantation in neonatal rats. We observed no differences between cortex- and striatum-derived NSCs expanded as neurospheres in proliferative capacity, growth rate, secondary sphere formation, and expression of neural markers. After 4 weeks of differentiation in vitro, cortical and striatal NSCs gave rise to similar numbers of GABAergic and VMAT2- and parvalbumin-containing neurons. However, whereas cortical NSCs produced higher number of glutamatergic and tyrosine hydroxylase- and calretinin-positive neurons, several-fold more neurons expressing the striatal projection neuron marker, DARPP-32, were observed in cultures of striatal NSCs. Human cortical and striatal NSCs survived and migrated equally well after transplantation. The two NSC types also generated similar numbers of mature NeuN-positive neurons, which were several-fold higher at 4 months as compared to at 1 month after grafting. At 4 months, the grafts contained cells with morphologic characteristics of neurons, astrocytes, and oligodendrocytes. Many of neurons were expressing parvalbumin. Our data show that NSCs derived from human fetal cortex and striatum exhibit region-specific differentiation in vitro, and survive, migrate, and form mature neurons to the same extent after intrastriatal transplantation in newborn rats.

In vitro characterization of a human neural progenitor cell coexpressing SSEA4 and CD133

The stage-specific embryonic antigen 4 (SSEA4) is commonly used as a cell surface marker to identify the pluripotent human embryonic stem (ES) cells. Immunohistochemistry on human embryonic central nervous system revealed that SSEA4 is detectable in the early neuroepithelium, and its expression decreases as development proceeds. Flow cytometry analysis of forebrain-derived cells demonstrated that the SSEA4-expressing cells are enriched in the neural stem/progenitor cell fraction (CD133(+)), but are rarely codetected with the neural stem cell (NSC) marker CD15. Using a sphere-forming assay, we showed that both subfractions CD133(+)/SSEA4(+) and CD133(+)/CD15(+) isolated from the embryonic forebrain are enriched in neurosphere-initiating cells. In addition CD133, SSEA4, and CD15 expression is sustained in the expanded neurosphere cells and also mark subfractions of neurosphere-initiating cells. Therefore, we propose that SSEA4 associated with CD133 can be used for both the positive selection and the enrichment of neural stem/progenitor cells from human embryonic forebrain.

Genetics of septo-optic dysplasia

Septo-optic dysplasia (SOD) is a highly heterogeneous condition comprising a variable phenotype of optic nerve hypoplasia, midline forebrain abnormalities and pituitary hypoplasia with consequent endocrine deficits. The majority of cases are sporadic and several aetiologies including drug and alcohol abuse have been suggested to account for the pathogenesis of the condition. However, a number of familial cases have been described and the identification of mutations in the key developmental gene HESX1 in patients with SOD and associated phenotypes suggests that a genetic causation is likely in the more common sporadic cases of the condition. More recently, we have implicated duplications of SOX3 and mutations of both SOX2 and SOX3 in the aetiology of variants of SOD. As with other developmental disorders such as holoprosencephaly, the precise aetiology is most likely multifactorial involving contributions from environmental factors in addition to an important role for crucial developmental genes. This potentially complex interaction between genetics and the environment is borne out by the variability of the penetrance and phenotypes in patients with genetic SOD, but at present, the understanding of these interactions is rudimentary. Further study of these critical factors may shed light on the aetiology of this complex disorder.

Sox-2 is expressed by glial and progenitor cells and Pax-6 is expressed by neuroblasts in the human subventricular zone

Transcription factors (TFs) are responsible for the specification and fate determination of cells as they develop from progenitor cells into specific types of cells in the brain. Sox-2 and Pax-6 are TFs with key functional roles in the developing brain, although less is known about TFs in the rudimentary germinal zones in the adult human brain. In this study we have investigated the distribution and characterization of Sox-2 and Pax-6 in the human subventricular zone (SVZ). Sox-2 immunoreactivity showed a nuclear labeling pattern and colocalised on GFAP immunoreactive cells as well as on bromodeoxyuridine (BrdU)-positive cells, whereas Pax-6 immunoreactivity was detectable in the nucleus and the cytoplasm of SVZ cells and colocalised with PSA-NCAM-positive progenitor cells. Thus, our data surprisingly reveal that these TFs are differentially expressed in the adult human SVZ where Sox-2 and Pax-6 specify a glial and neuronal fate, respectively.

Cancer stem cells and "stemness" genes in neuro-oncology

The main properties of stem cells include long-term self-renewal and the capacity to give rise to one or more types of differentiated progeny. Recently, much evidence was provided that leukemia and tumor maintenance and growth are sustained by a small proportion of cells exhibiting stem cell properties. In neural tumors, stem cells have been detected in glioblastoma, medulloblastoma and ependymoma. These observations imply that normal stem cells could be the origin of cancer stem cells; alternatively, a more differentiated progeny may revert to a "stem-like" status, and give rise to cancer stem cells. In adult brain residual stem cells are located in the hippocampus, the subventricular zone and possibly the cerebellum. However, evidence for the ability of more differentiated progeny (astroglia, oligodendroglia) to convert into "stem cells" in vitro has also been provided, thus greatly expanding the potential target of oncogenic mutations. In the framework of the cancer stem cell hypothesis, genes originally identified as important for normal neural stem cells may be essential to support cancer stem cells as well. Stem cell genes act in several ways: they stimulate stem cell self-replication, inhibit differentiation, control excessive replication that might lead to "exhaustion" of the stem cell pool. Mutations in man and mouse, in spontaneous or experimental brain tumors, often target stem cell genes or genes lying in their functional pathway, the main examples being the Sonic hedgehog and the Wnt pathways. Interestingly, several stem cell genes are often overexpressed in brain tumors, even if they are not mutated. This suggests that these genes may be important for the generation of cancer stem cells from more differentiated precursors, or for cancer stem cell maintenance. Cancer stem cells partially differentiate in vivo, and in vitro they also give rise to seemingly normal differentiated progeny, like normal stem cells: thus, their main defect, leading to cancer, may lie in the unbalance between self-replication and terminal differentiation of this minority cell population. Knowledge of extrinsic diffusible factors affecting the activity of stem cell genes may help identifying tools for inducing cancer stem cell differentiation, which might be of use in therapy.

Aging does not alter the number or phenotype of putative stem/progenitor cells in the neurogenic region of the hippocampus

To investigate whether dramatically waned dentate neurogenesis during aging is linked to diminution in neural stem/progenitor cell (NSC) number, we counted cells immunopositive for Sox-2 (a putative marker of NSCs) in the subgranular zone (SGZ) of young, middle-aged and aged F344 rats. The young SGZ comprised approximately 50,000 Sox-2+ cells and this amount did not diminish with aging. Quantity of GFAP+ cells and vimentin+ radial glia also remained stable during aging in this region. Besides, in all age groups, analogous fractions of Sox-2+ cells expressed GFAP (astrocytes/NSCs), NG-2 (oligodendrocyte-progenitors/NSCs), vimentin (radial glia), S-100beta (astrocytes) and doublecortin (new neurons). Nevertheless, analyses of Sox-2+ cells with proliferative markers insinuated an increased quiescence of NSCs with aging. Moreover, the volume of rat-endothelial-cell-antigen-1+ capillaries (vascular-niches) within the SGZ exhibited an age-related decline, resulting in an increased expanse between NSCs and capillaries. Thus, decreased dentate neurogenesis during aging is not attributable to altered number or phenotype of NSCs. Instead, it appears to be an outcome of increased quiescence of NSCs due to changes in NSC milieu.

Comparative Studies on the mRNA Expression of Development-Related Genes in an Individual Mouse Blastocyst with Different Developmental Potential

The evaluation of embryo morphology, widely used for selecting mammalian embryos before transfer, is not an adequate standard for selecting nuclear-transferred (NT) embryos. To search for markers useful for predicting the potential of NT embryos to develop into young, we examined the relation between the morphology of embryos with different developmental potential and gene expression of Oct 4, Nanog, Stat3, FGF4, Stella, and Sox2. In the present study, we examined pronuclear-exchanged blastocysts and morula blastomere, embryonic stem (ES) cell, and cumulus cell NT blastocysts, and in vivo-developed and in vitro-developed blastocysts. Based on the small variations in the gene expression levels among the in vivo-developed blastocysts, and the significant differences in gene expression between in vivo-developed (high developmental potential), and ES cell and cumulus cell NT blastocysts (low developmental potential), down-regulation of Sox2 and Oct4 genes is considered to be a candidate marker for the low potential of NT embryos to develop into young.

Pluripotent stem cells and their niches

The ability of stem cells to self-renew and to replace mature cells is fundamental to ontogeny and tissue regeneration. Stem cells of the adult organism can be categorized as mono-, bi-, or multipotent, based on the number of mature cell types to which they can give rise. In contrast, pluripotent stem cells of the early embryo have the ability to form every cell type of the adult body. Permanent lines of pluripotent stem cells have been derived from preimplantation embryos (embryonic stem cells), fetal primordial germ cells (embryonic germ cells), and malignant teratocarcinomas (embryonal carcinoma cells). Cultured pluripotent stem cells can easily be manipulated genetically, and they can be matured into adult-type stem cells and terminally differentiated cell types in vitro, thereby, providing powerful model systems for the study of mammalian embryogenesis and disease processes. In addition, human embryonic stem cell lines hold great promise for the development of novel regenerative therapies. To fully utilize the potential of these cells, we must first understand the mechanisms that control pluripotent stem cell fate and function. In recent decades, the microenvironment or niche has emerged as particularly critical for stem cell regulation. In this article, we review how pluripotent stem cell signal transduction mechanisms and transcription factor circuitries integrate information provided by the microenvironment. In addition, we consider the potential existence and location of adult pluripotent stem cell niches, based on the notion that a revealing feature indicating the presence of stem cells in a given tissue is the occurrence of tumors whose characteristics reflect the normal developmental potential of the cognate stem cells.

Early appearance of stem/progenitor cells with neural-like characteristics in human cord blood mononuclear fraction cultured in vitro

OBJECTIVE

The exposure of human umbilical cord blood mononuclear cells devoid of hematopoietic stem cells (HUCB-MNCsCD34-) to defined culture condition promotes their conversion into neural lineage. We have asked the question if observed fate change of HUCB-MNCsCD34- results from direct conversion of hematopoietic precursors into neural-like phenotypes due to expression of overlapping genetic program or, alternatively, these neural phenotypes arise from sequential differentiation of more primitive progenitors (embryonic-like cells) preexisting in HUCB-MNCsCD34- fraction.

MATERIALS AND METHODS

HUCB-MNCs negatively selected for CD34 antigens were cultured in vitro up to 14 days. Changes in stem/neural cell genes and proteins were successively evaluated during this period and after evoked neuronal differentiation of cells in the presence of RA or BDNF or cocultured with neonatal rat brain astrocytes.

RESULTS

Freshly isolated HUCB-MNCsCD34- expressed pluripotent cell markers: Oct3/4, Sox2, and Rex1 genes. During 24 hours of culture the frequency of Oct3/4 immunopositive cells increased markedly with parallel enlargement of "side population" and CD133+ cell appearance. Concomitantly, cultured cells start to form aggregates and express pro-neural genes, i.e., enhanced Sox2, OTX1, Nestin, GFAP, and NF-200. During the next days of culture immunoreactions for beta-tubulin III, MAP2, GFAP, S100beta, Doublecortin, and GalC were induced with reciprocal lowering of stem cell gene and protein markers. At this stage cells successively adhered to the bottom, dispersed, and decreased proliferation rate (Ki67 expression). Additional treatments with neuromorphogenes or coculturing with rat brain primary culture induced further differentiation of these neural precursors toward more advanced neuronal phenotypes.

CONCLUSIONS

HUCB-MNCs(CD34-) fraction contains embryonic-like stem/progenitor cells which increase rapidly but transiently in culture, then differentiate spontaneously after cell aggregate adhesion toward neural lineage. Neurally promoted cells from 10-14 DIV culture acquire three main neural-like phenotypes, i.e., neurons, astrocytes, and oligodendrocytes. In this respect they are promising candidates for experimental treatment of neuronal injury; however, the final proof for conversion of HUCB cells to neural cells can be obtained through transplantation experiments.

Neurogenesis in the adult central nervous system

Contrary to the long-held dogma, neurogenesis occurs throughout adulthood, and neural stem cells reside in the adult central nervous system (CNS) in mammals. The developmental process of the brain may thus never end, and the brain may be amenable to repair. Neurogenesis is modulated in a wide variety of physiological and pathological conditions, and is involved in processes such as learning and memory and depression. However, the relative contribution of newly generated neuronal cells to these processes, as well as to CNS plasticity, remains to be determined. Thus, not only neurogenesis contributes to reshaping the adult brain, it will ultimately lead us to redefine our knowledge and understanding of the nervous system.

Sox3 expression identifies neural progenitors in persistent neonatal and adult mouse forebrain germinative zones

Neural precursors persist throughout life in the rodent forebrain subventricular zone (SVZ) and hippocampal dentate gyrus. The regulation of persistent neural stem cells is poorly understood, in part because of the lack of neural progenitor markers. The Sox B1 subfamily of HMG-box transcription factors (Sox1-3) is expressed by precursors in the embryonic nervous system, where these factors maintain neural progenitors in an undifferentiated state while suppressing neuronal differentiation. Sox2 expression persists in germinative zones of the adult rodent brain, but Sox3 expression in the postnatal brain remains largely unexplored. Here we examine Sox3 expression in the neonatal and adult mouse brain to gain insight into its potential involvement in regulating persistent neural stem cells and neurogenesis. We also investigate Sox3 expression during expansion and neural differentiation of postnatal mouse SVZ neural stem cell and human embryonic stem cell (hESC) cultures. We find that Sox3 is expressed transiently by proliferating and differentiating neural progenitors in the SVZ-olfactory bulb pathway and dentate gyrus. Sox3 immunoreactivity also persists in specific postmitotic neuronal populations. In vitro, high Sox3 protein expression levels in undifferentiated, SVZ-derived neurospheres decline markedly with differentiation. Sox3 immunoreactivity in hESCs appears upon differentiation to neural progenitors and then decreases as cells differentiate further into neurons. These findings suggest that Sox3 labels specific stages of hESC-derived and murine neonatal and adult neural progenitors and are consistent with a role for Sox3 in neural stem cell maintenance. Persistent Sox3 expression in some mature neuronal populations suggests additional undefined roles for Sox3 in neuronal function.

SOX2 is a dose-dependent regulator of retinal neural progenitor competence

Approximately 10% of humans with anophthalmia (absent eye) or severe microphthalmia (small eye) show haploid insufficiency due to mutations in SOX2, a SOXB1-HMG box transcription factor. However, at present, the molecular or cellular mechanisms responsible for these conditions are poorly understood. Here, we directly assessed the requirement for SOX2 during eye development by generating a gene-dosage allelic series of Sox2 mutations in the mouse. The Sox2 mutant mice display a range of eye phenotypes consistent with human syndromes and the severity of these phenotypes directly relates to the levels of SOX2 expression found in progenitor cells of the neural retina. Retinal progenitor cells with conditionally ablated Sox2 lose competence to both proliferate and terminally differentiate. In contrast, in Sox2 hypomorphic/null mice, a reduction of SOX2 expression to <40% of normal causes variable microphthalmia as a result of aberrant neural progenitor differentiation. Furthermore, we provide genetic and molecular evidence that SOX2 activity, in a concentration-dependent manner, plays a key role in the regulation of the NOTCH1 signaling pathway in retinal progenitor cells. Collectively, these results show that precise regulation of SOX2 dosage is critical for temporal and spatial regulation of retinal progenitor cell differentiation and provide a cellular and molecular model for understanding how hypomorphic levels of SOX2 cause retinal defects in humans.

Notch signaling is required to maintain all neural stem cell populations--irrespective of spatial or temporal niche

Recently, Notch signaling has been reported to underscore the ability of neural stem cells (NSCs) to self-renew. Utilizing mice deficient in presenilin-1(PS1), we asked whether the function of Notch signaling in NSC maintenance was conserved. At embryonic day 14.5, all NSCs--both similar (cortex-, ganglionic eminence- and hindbrain-derived) and distinct (retinal stem cell)--require Notch signaling in a gene-dosage-sensitive manner to undergo expansionary symmetric divisions, as assessed by the clonal, in vitro neurosphere assay. Within the adult, however, Notch signaling modulates cell cycle time in order to ensure brain-derived NSCs retain their self-renewal property. At face value, the effects in the embryo and adult appear different. We propose potential hypotheses, including the ability of cell cycle to modify the mode of division, in order to resolve this discrepancy. Regardless, these findings demonstrate that PS1, and presumably Notch signaling, is required to maintain all NSCs.

FGF-4 regulates neural progenitor cell proliferation and neuronal differentiation

The FGF-4 (fibroblast growth factor 4, known as HST-1) protein is an important mitogen for a variety of cell types. However, only limited information is available concerning tissue distribution and the biological role of FGF-4 in the brain. In situ hybridization analysis revealed localization of mouse Fgf-4 mRNA in the normal postnatal mouse hippocampus, subventricular zone (SVZ), and the rostral migratory stream where new neurons generate, migrate, and become incorporated into the functional circuitry of the brain. We also investigated whether FGF-4 could promote both proliferation and differentiation of the neural progenitor cells by using an in vitro neurosphere assay. The addition of recombinant FGF-4 generated large proliferative spheres that have a multipotent differentiation ability. Furthermore, recombinant FGF-4 significantly promotes neuronal differentiation in attached clonal neurosphere culture. These findings suggest that FGF-4 has an ability to promote neural stem cell proliferation and neuronal differentiation in the postnatal brain.

Human neurospheres derived from the fetal central nervous system are regionally and temporally specified but are not committed

Proliferating single cells were isolated from various CNS regions (telencephalon, diencephalon, midbrain, cerebellum, pons and medulla, and spinal cord) of human fetal cadavers at 13 weeks of gestation and grown as neurospheres in long-term cultures. We investigated whether neural stem cells (NSCs) or progenitors within spheres have specific regional or temporal characteristics with regard to growth, differentiation, and region-specific gene expression, and whether these molecular specifications are reversible. Regardless of regional origin, all of the neurospheres were found to contain cells of different subtypes, which suggests that multipotent NSCs, progenitors or radial glial cells co-exist with restricted neuronal or glial progenitors within the neurospheres. Neurospheres from the forebrain grew faster and gave rise to significantly more neurons than did those from either the midbrain or hindbrain, and regional differences in neuronal differentiation appeared to be sustained during long-term passage of neurospheres in culture. There was also a trend towards a reduction in neuronal emergence from the respective neurospheres over time in culture, although the percentages of neurons generated from cerebellum-derived neurospheres increased dramatically. These results suggest that differences in neuronal differentiation for the various neurospheres are spatially and temporally determined. In addition, the properties of glial fibrillary acidic protein (GFAP)-, glutamate-, and gamma-aminobutyric acid (GABA)-expressing cells derived from neurospheres of the respective CNS regions appear to be regionally and temporally different. Isolated human neurospheres from different CNS compartments expressed distinctive molecular markers of regional identity and maintained these patterns of region-specific gene expression during long-term passage in vitro. To determine the potential of human neurospheres for regional fate plasticity, single spheres from the respective regions were co-cultured with embryonic day 16.5 (E16.5 d) mouse brain slices. Specific cues from the developing mouse brain tissues induced the human neurospheres to express different marker genes of regional identity and to suppress the expression of their original marker genes. Thus, even the early regional identities of human neurospheres may not be irreversible and may be altered by local inductive cues. These findings have important implications for understanding the characteristics of growth, differentiation, and molecular specification of human neurospheres derived from the developing CNS, as well as the therapeutic potential for neural repair.

An Shp2/SFK/Ras/Erk signaling pathway controls trophoblast stem cell survival

Little is known about how growth factors control tissue stem cell survival and proliferation. We analyzed mice with a null mutation of Shp2 (Ptpn11), a key component of receptor tyrosine kinase signaling. Null embryos die peri-implantation, much earlier than mice that express an Shp2 truncation. Shp2 null blastocysts initially develop normally, but they subsequently exhibit inner cell mass death, diminished numbers of trophoblast giant cells, and failure to yield trophoblast stem (TS) cell lines. Molecular markers reveal that the trophoblast lineage, which requires fibroblast growth factor-4 (FGF4), is specified but fails to expand normally. Moreover, deletion of Shp2 in TS cells causes rapid apoptosis. We show that Shp2 is required for FGF4-evoked activation of the Src/Ras/Erk pathway that culminates in phosphorylation and destabilization of the proapoptotic protein Bim. Bim depletion substantially blocks apoptosis and significantly restores Shp2 null TS cell proliferation, thereby establishing a key mechanism by which FGF4 controls stem cell survival.

Role of Sox2 in the development of the mouse neocortex

The mammalian neocortex is established from neural stem and progenitor cells that utilize specific transcriptional and environmental factors to create functional neurons and astrocytes. Here, we examined the mechanism of Sox2 action during neocortical neurogenesis and gliogenesis. We established a robust Sox2 expression in neural stem and progenitor cells within the ventricular zone, which persisted until the cells exited the cell cycle. Overexpression of constitutively active Sox2 in neural progenitors resulted in upregulation of Notch1, recombination signal-sequence binding protein-J (RBP-J) and hairy enhancer of split 5 (Hes5) transcripts and the Sox2 high mobility group (HMG) domain seemed sufficient to confer these effects. While Sox2 overexpression permitted the differentiation of progenitors into astroglia, it inhibited neurogenesis, unless the Notch pathway was blocked. Moreover, neuronal precursors engaged a serine protease(s) to eliminate the overexpressed Sox2 protein and relieve the repression of neurogenesis. Glial precursors and differentiated astrocytes, on the other hand, maintained Sox2 expression until they reached a quiescent state. Sox2 expression was re-activated by signals that triggered astrocytic proliferation (i.e., injury, mitogenic and gliogenic factors). Taken together, Sox2 appears to act upstream of the Notch signaling pathway to maintain the cell proliferative potential and to ensure the generation of sufficient cell numbers and phenotypes in the developing neocortex.

The Sox2 regulatory region 2 functions as a neural stem cell-specific enhancer in the telencephalon

Sox2 is expressed at high levels in neuroepithelial stem cells and persists in neural stem/progenitor cells throughout adulthood. We showed previously that the Sox2 regulatory region 2 (SRR2) drives strong expression in these cells. Here we generated transgenic mouse strains with the beta-geo reporter gene under the control of the SRR2 in order to examine the spatiotemporal function of this regulatory region. We show that the SRR2 functions specifically in neural stem/progenitor cells. However, unlike Nestin 2nd intronic enhancer, the SRR2 shows strong regional specificity functioning only in restricted areas of the telencephalon but not in any other portions of the central nervous system such as the spinal cord. We also show by in vitro clonogenic assay that at least some of these SRR2-functioning cells possess the hallmark properties of neural stem cells. In adult brains, we could detect strong beta-geo expression in the subventricular zone of the lateral ventricle and along the rostral migrating stream where actively dividing cells reside. Chromatin immunoprecipitation assays reveal interactions of POU and Sox factors with SRR2 in neural stem/progenitor cells. Our data also suggest that the specific recruitment of these proteins to the SRR2 in the telencephalon defines the spatiotemporal activity of the enhancer in the developing nervous system.

Role of SOX2 Mutations in Human Hippocampal Malformations and Epilepsy

PURPOSE

Seizures are noted in a significant proportion of cases of de novo, heterozygous, loss-of-function mutations in SOX2, ascertained because of severe bilateral eye malformations. We wished to determine the underlying cerebral phenotype in SOX2 mutation and to test the candidacy of SOX2 as a gene contributing to human epilepsies.

METHODS

We examined high-resolution MRI scans in four patients with SOX2 mutations, two of whom had seizures. We determined the Sox2 expression pattern in developing murine brain. We searched for SOX2 mutation in 24 patients with typical hippocampal sclerosis and for common variations in SOX2 in 655 patients without eye disease but with epilepsy, including 91 patients with febrile seizures, 93 with hippocampal sclerosis, and 258 with temporal lobe epilepsy.

RESULTS

Striking hippocampal and parahippocampal malformations were seen in all cases, with a history of febrile seizures or epilepsy in two of four cases. The Sox2 expression pattern in developing mouse brain supports the pattern of malformations observed. Mutation screening in patients with epilepsy did not reveal any abnormalities in SOX2. No associations were found between any clinical epilepsy phenotype and common variation in SOX2.

CONCLUSIONS

SOX2 haploinsufficiency causes mesial temporal malformation in humans, making SOX2 dysfunction a candidate mechanism for mesial temporal abnormalities associated with chronic epilepsy. However, although mutation of SOX2 in humans causes hippocampal malformation, SOX2 mutation or variation is unlikely to contribute commonly to mesial temporal lobe epilepsy or its structural (hippocampal sclerosis) or historic (febrile seizures) associations in humans.

Phenotypic and functional heterogeneity of GFAP-expressing cells in vitro: differential expression of LeX/CD15 by GFAP-expressing multipotent neural stem cells and non-neurogenic astrocytes

Recent findings show that the predominant multipotent neural stem cells (NSCs) isolated from postnatal and adult mouse brain express glial fibrillary acid protein (GFAP), a protein commonly associated with astrocytes, and that primary astrocyte cultures can contain GFAP-expressing cells that act as multipotent NSCs when transferred to neurogenic conditions. The relationship of GFAP-expressing NSCs to GFAP-expressing astrocytes is unclear, but has important implications. We compared the phenotype and neurogenic potential of GFAP-expressing cells derived from different CNS regions and maintained in vitro under different conditions. Multiple labeling immunohistochemistry revealed that both primary astrocyte cultures and adherent neurogenic cultures derived from postnatal or adult periventricular tissue contained subpopulations of GFAP-expressing cells that co-expressed nestin and LeX/CD15, two molecules associated with NSCs. In contrast, GFAP-expressing cells in similar cultures prepared from adult cerebral cortex did not express detectable levels of LeX/CD15, and exhibited no neurogenic potential. Fluorescence-activated cell sorting (FACS) of both primary astrocyte cultures and adherent neurogenic cultures for LeX/CD15 showed that GFAP-expressing cells competent to act as multipotent NSCs were concentrated in the LeX-positive fraction. Using neurosphere assays and a transgenic ablation strategy, we confirmed that the predominant NSCs in primary astrocyte and adherent neurogenic cultures were GFAP-expressing cells. These findings demonstrate that GFAP-expressing cells derived from postnatal and adult forebrain are heterogeneous in both molecular phenotype and neurogenic potential in vitro, and that this heterogeneity exists before exposure to neurogenic conditions. The findings provide evidence that GFAP-expressing NSCs are phenotypically and functionally distinct from non-neurogenic astrocytes.

Transcription factor protein expression patterns by neural or neuronal progenitor cells of adult monkey subventricular zone

The anterior subventricular zone of the adult mammalian brain contains progenitor cells which are upregulated after cerebral ischemia. We have previously reported that while a part of the progenitors residing in adult monkey anterior subventricular zone travels to the olfactory bulb, many of these cells sustain location in the anterior subventricular zone for months after injury, exhibiting a phenotype of either neural or neuronal precursors. Here we show that ischemia increased the numbers of anterior subventricular zone progenitor cells expressing developmentally regulated transcription factors including Pax6 (paired-box 6), Emx2 (empty spiracles-homeobox 2), Sox 1-3 (sex determining region Y-box 1-3), Ngn1 (neurogenin 1), Dlx1,5 (distalless-homeobox 1,5), Olig1,3 (oligodendrocyte lineage gene 1,3) and Nkx2.2 (Nk-box 2.2), as compared with control brains. Analysis of transcription factor protein expression by sustained neural or neuronal precursors in anterior subventricular zone revealed that these two cell types were positive for characteristic sets of transcription factors. The proteins Pax6, Emx2, Sox2,3 and Olig1 were predominantly localized to dividing neural precursors while the factors Sox1, Ngn1, Dlx1,5, Olig2 and Nkx2.2 were mainly expressed by neuronal precursors. Further, differences between monkeys and non-primate mammals emerged, related to expression patterns of Pax6, Olig2 and Dlx2. Our results suggest that a complex network of developmental signals might be involved in the specification of primate progenitor cells.

Embryonic stem-derived versus somatic neural stem cells: a comparative analysis of their developmental potential and molecular phenotype

Reliable procedures to induce neural commitment of totipotent undifferentiated embryonic stem (ES) cells have provided new tools for investigating the molecular mechanisms underlying cell fate choices. We extensively characterized the developmental potential of ES-induced neural cells obtained using an adaptation of the multistep induction protocol. We provided evidence that ES-derived neural proliferating cells are endowed with stem cell properties such as extensive self-renewal capacity and single-cell multipotency. In differentiating conditions, cells matured exclusively into neurons, astrocytes, and oligodendrocytes. All these features have been previously described in only somatic neural stem cells (NSCs). Therefore, we consider it more appropriate to rename our cells ES-derived NSCs. These similarities between the two NSC populations induced us to carefully compare their proliferation ability and differentiation potential. Although they were very similar in overall behavior, we scored specific differences. For instance, ES-derived NSCs proliferated at higher rate and consistently generated a higher number of neurons compared with somatic NSCs. To further investigate their relationships, we carried out a molecular analysis comparing their transcriptional profiles during proliferation. We observed a large fraction of shared expressed transcripts, including genes previously described to be critical in defining somatic NSC traits. Among the genes differently expressed, candidate genes possibly responsible for divergences between the two cell types were selected and further investigated. In particular, we showed that an enhanced MAPK (mitogen-activated protein kinase) signaling is acting in ES-induced NSCs, probably triggered by insulin-like growth factor-II. This may contribute to the high proliferation rate exhibited by these cells in culture.

The HMG box transcription factor Sox4 contributes to the development of the endocrine pancreas

To investigate the role of the Sry/hydroxymethylglutaryl box (Sox) transcription factors in the development of the pancreas, we determined the expression pattern of Sox factors in the developing mouse pancreas. By RT-PCR, we detected the presence of multiple Sox family members in both the developing pancreas and mature islets and then focused on two factors, Sox2 and Sox4. The expression field of Sox2, which plays a role in the maintenance of some stem cell populations, included the developing duodenum, but Sox2 was specifically excluded from the pancreatic buds. In contrast, Sox4 was detected broadly in the early pancreatic buds and eventually became restricted to the nuclei of all islet cells in the adult mouse. Mice homozygous for a null mutation of the sox4 gene showed normal pancreatic bud formation and endocrine cell differentiation up to embryonic day 12.5. Beyond that date, cultured pancreatic explants lacking sox4 failed to form normal islets. Instead, a markedly reduced number of endocrine cells were found scattered through the explant. We show here that several Sox transcription factors are expressed in the developing pancreas and in the islet, and that one of these factors, Sox4, is required for the normal development of pancreatic islets.

Alterations in cellular phenotypes differentiating from embryonic rat brain neurosphere cultures by immunoselection of neuronal progenitors

The neurosphere culture system is widely used to expand neural stem/progenitor cells in vitro and to provide a source of cells for transplantation approaches to CNS disorders. This study describes the populations of neurones, astrocytes and oligodendrocytes which differentiated from embryonic day (E) 14 rat cortical and striatal tissue grown as neurosphere cultures over three passages. The percentages of cells that adopted neuronal phenotypes decreased with passage, astrocytic percentages increased and oligodendrocytic percentages remained constant. In the second part of this study, immunomagnetic separation was used to positively select neuronal progenitor cells from E14 rat cortical and striatal tissue using an antibody, 2F7, which recognises an epitope on the cell surface of pre- and post-mitotic neurones. These immunomagnetically selected cells were grown as neurosphere cultures over three passages and gave rise to significantly different percentages of neurones, astrocytes and oligodendrocytes than those found in the baseline study. In particular, the percentage of neurones arising from the second and third passages was significantly higher following immunoselection. This indicates that neuronal progenitor cells can be isolated using immunomagnetic separation and then expanded using the neurosphere culture system, to generate enriched populations of neurones that can be used in CNS repair.

Isolation and characterization of neural precursor cells from theSox1-GFP reporter mouse

We have made use of a reporter mouse line in which enhanced green fluorescence protein (GFP) is inserted into the Sox1 locus. We show that the GFP reporter is coexpressed with the Sox1 protein as well as with other known markers for neural stem and progenitor cells, and can be used to identify and isolate these cells by fluorescence-activated cell sorting (FACS) from the developing or adult brain and from neurosphere cultures. All neurosphere-forming cells with the capacity for multipotency and self-renewal reside in the Sox1-GFP-expressing population. Thus, the Sox1-GFP reporter system is highly useful for identification, isolation and characterization of neural stem and progenitor cells, as well as for the validation of alternative means for isolating neural stem and progenitor cells. Further, transplantation experiments show that Sox1-GFP cells isolated from the foetal brain give rise to neurons and glia in vivo, and that many of the neurons display phenotypic characteristics appropriate for the developing brain region from which the Sox1-GFP precursors were derived. On the other hand, Sox1-GFP cells isolated from the adult subventricular zone or expanded neurosphere cultures gave rise almost exclusively to glial cells following transplantation. Thus, not all Sox1-GFP cells possess the same capacity for neuronal differentiation in vivo.

A roof plate-dependent enhancer controls the expression of Homeodomain only protein in the developing cerebral cortex

The smallest known homeodomain protein, Homeodomain only protein (Hop), was identified and described here as a temporally and spatially restricted gene in the neurogenic regions of the developing murine CNS including the cerebral cortex. Furthermore, an evolutionarily conserved 418 base pair upstream cis-regulatory DNA sequence was found to confine the Hop expression to the CNS of transgenic mice, but not to the heart which is the second major Hop expressing organ Chen, F., Kook, H., Milewski, R., Gitler, A.D., Lu, M.M., Li, J., Nazarian, R., Schnepp, R., Jen, K., Biben, C., Runke, G., Mackay, J.P., Novotny, J., Schwartz, R.J., Harvey, R.P., Mullins, M.C., Epstein, J.A., 2002. Hop is an unusual homeobox gene that modulates cardiac development. Cell 110, 713-723; Shin, C.H., Liu, Z.P., Passier, R., Zhang, C.L., Wang, D.Z., Harris, T.M., Yamagishi, H., Richardson, J.A., Childs, G., Olson, E.N., 2002. Modulation of cardiac growth and development by HOP, an unusual homeodomain protein. Cell 110, 725-735. The forebrain enhancer activity was successfully reproduced in vitro utilizing a combination of the electroporation and the organotypic brain culture method. Using this approach, the minimal requirement for the forebrain-specific enhancer sequence was delineated down to 200 base pairs. We further demonstrate that the Hop enhancer activity is inducible ectopically in a transgenic tissue by wild-type roof plate transplantation in vitro. Thus Hop is regulated in the forebrain by a so far unidentified paracrine signaling factor from the roof plate. Furthermore, the identified enhancer sequence provides an important tool for the targeted expression of transgenes in the medial cortex and the cortical hem.

Comparative analysis of development-related gene expression in mouse preimplantation embryos with different developmental potential

The potential of embryonic and somatic cell nuclear-transferred (NT) mouse oocytes to develop into young is low compared with bovine NT oocytes. To examine the reasons for the low developmental potential of NT mouse oocytes, we analyzed the gene expression patterns of six development-related genes (Oct4, Nanog, Stat3, stella, FGF4, and Sox2) during preimplantation development in manipulated oocytes with different potentials to develop into young using real-time polymerase chain reaction (PCR) methods. The manipulated oocytes were parthenogenetically activated oocytes and embryonic stem cell, cumulus cell, morula blastomere NT oocytes, and in vitro-cultured and in vivo-recovered embryos. The mRNA expression patterns in mouse NT-derived embryos markedly differed from in vivo and in vitro counterparts. Some transcript expression patterns in embryonic stem-cell NT oocytes resembled those of parthenogenetic oocytes. Of the six developmentally important transcripts examined in NT embryos, four had a downregulated expression pattern at the blastocyst stage. Our findings indicate that abnormal expression patterns of development-related genes during preimplantation development correlate with the low potential of NT oocytes to develop into young. Although more detailed information is required, Sox2 mRNA expression pattern in blastocysts seems to closely correlate with the developmental potential of NT embryos.

From stem cells to neurons and glia: a Soxist's view of neural development

During nervous system development, neural stem cells give rise to many different types of neurons and glia over an extended period. Little is known about the intrinsic factors that regulate stem-cell maintenance, decide whether neurons or glia are generated, or control terminal differentiation. Transcription factors of the Sox family provide important clues about the control of these events. In the central nervous system (CNS), Sox1, Sox2 and Sox3 are required for stem-cell maintenance, and their effects are counteracted by Sox21. Sox9, by contrast, alters the potential of stem cells from neurogenic to gliogenic, whereas Sox10 is essential for terminal oligodendrocyte differentiation. In the peripheral nervous system (PNS) the same Sox proteins have different functions, uncovering important developmental differences between the CNS and PNS.

Sox2 expression defines a heterogeneous population of neurosphere-forming cells in the adult murine brain

The identification of neural stem cells (NSCs) in situ has been prevented by the inability to identify a marker consistently expressed in all adult NSCs and is thus generally accomplished using the in vitro neurosphere-forming assay. The high-mobility group transcription factor Sox2 is expressed in embryonic neural epithelial stem cells; because these cells are thought to give rise to the adult NSC population, we hypothesized that Sox2 may continue to be expressed in adult NSCs. Using Sox2:EGFP transgenic mice, we show that Sox2 is expressed in neurogenic regions along the rostral-caudal axis of the central nervous system throughout life. Furthermore, all neurospheres derived from these neurogenic regions express Sox2, suggesting that Sox2 is indeed expressed in adult NSCs. We demonstrate that NSCs are heterogeneous within the adult brain, with differing capacities for cell production. In vitro, all neurospheres express Sox2, but the expression of markers common to early progenitor cells within individual neurospheres varies; this heterogeneity of NSCs is mirrored in vivo. For example, both glial fibrillary acidic protein and NG2 are expressed within individual neurospheres, but their expression is mutually exclusive; likewise, these two markers show distinct staining patterns within the Sox2+ regions of the brain's neurogenic regions. Thus, we propose that the expression of Sox2 is a unifying characteristic of NSCs in the adult brain, but that not all NSCs maintain the ability to form all neural cell types in vivo.

Temporal profiling of gene expression during neurogenesis and remodeling in the olfactory epithelium at short intervals after target ablation

Neurogenesis in the olfactory epithelium (OE) is induced by olfactory bulbectomy (OBX), which effectively axotomizes olfactory sensory neurons (OSNs) and removes their synaptic targets, resulting in apoptosis. We used Affymetrix high-density oligonucleotide arrays to investigate changes in gene expression during initiation of signaling in pathways that regulate apoptosis and neurogenesis in the murine OE at 2, 8, 16, and 48 hr after bilateral OBX compared to that in sham-operated controls. We focused on regulation of a defined set of genes associated with apoptosis, stem/progenitor cell regulation, and cell cycle progression because of the activation of these processes in OE degeneration and remodeling after OBX. After data scrubbing and categorical analysis, one-way analysis of variance identified 72 genes (4.9% of the present known genes) as being regulated significantly (P < 0.05) at one or more points; 50 were defined as regulated differentially with the false discovery rate at 10%. Significant changes in gene expression occurred in all categories as early as 2 hr post-OBX, with the greatest number of differentially regulated genes at 16 and 48 hr. Hierarchical cluster analysis and correlation coefficients were used to identify similarities in patterns of gene expression changes within and across categories. Validation was carried out with SuperArray macroarrays and real-time RT-PCR. Our results confirmed the participation of many genes in known signaling pathways and identified changes in the expression of 42 genes not identified previously as participating in apoptosis and neurogenesis in the OE. Additionally, our analyses indicated the early involvement of genes regulating cytoskeletal reorganization and angiogenesis in the response to OBX. These studies are an important first step in defining early time-dependent changes in gene expression after target ablation that lead to neurogenesis in the olfactory sensory epithelium.

Somatic mosaicism in neuronal precursor cells mediated by L1 retrotransposition

Revealing the mechanisms for neuronal somatic diversification remains a central challenge for understanding individual differences in brain organization and function. Here we show that an engineered human LINE-1 (for long interspersed nuclear element-1; also known as L1) element can retrotranspose in neuronal precursors derived from rat hippocampus neural stem cells. The resulting retrotransposition events can alter the expression of neuronal genes, which, in turn, can influence neuronal cell fate in vitro. We further show that retrotransposition of a human L1 in transgenic mice results in neuronal somatic mosaicism. The molecular mechanism of action is probably mediated through Sox2, because a decrease in Sox2 expression during the early stages of neuronal differentiation is correlated with increases in both L1 transcription and retrotransposition. Our data therefore indicate that neuronal genomes might not be static, but some might be mosaic because of de novo L1 retrotransposition events.

Long-term survival of dopamine neurons derived from parthenogenetic primate embryonic stem cells (cyno-1) after transplantation

Dopamine (DA) neurons can be derived from human and primate embryonic stem (ES) cells in vitro. An ES cell-based replacement therapy for patients with Parkinson's disease requires that in vitro-generated neurons maintain their phenotype in vivo. Other critical issues relate to their proliferative capacity and risk of tumor formation, and the capability of migration and integration in the adult mammalian brain. Neural induction was achieved by coculture of primate parthenogenetic ES cells (Cyno-1) with stromal cells, followed by sequential exposure to midbrain patterning and differentiation factors to favor DA phenotypic specification. Differentiated ES cells were treated with mitomycin C and transplanted into adult immunosuppressed rodents and into a primate (allograft) with out immunosuppression. A small percentage of DA neurons survived in both rodent and primate hosts for the entire term of the study (4 and 7 months, respectively). Other neuronal and glial populations derived from Cyno-1 ES cells showed, in vivo, phenotypic characteristics and growth and migration patterns similar to fetal primate transplants, and a majority of cells (>80%) expressed the forebrain transcription factor brain factor 1. No teratoma formation was observed. In this study, we demonstrate long-term survival of DA neurons obtained in vitro from primate ES cells. Optimization of differentiation, cell selection, and cell transfer is required for functional studies of ES-derived DA neurons for future therapeutic applications.

A bipotent neural progenitor cell line cloned from a cerebellum of an adultp53-deficient mouse generates both neurons and oligodendrocytes

Here we report developmental characteristics of a clonal cell line 2Y-3t established from a multifocal neoplasm that arose in a cerebellum of an adult p53-deficient mouse. The tumorigenicity of the line was not observed in soft agar assay or in nude mouse assay. In serum-containing medium, 2Y-3t cells were epithelial-like in morphology and were mitotic. When they were cultured in serum-free medium, the expressions of neural stem and/or progenitor cell markers were decreased. Concomitantly, the expressions of neuronal and oligodendrocyte markers were increased in concert with morphological differentiation, and DNA synthesis ceased. None of astrocyte markers were detected under these culture conditions. Double-labelling studies revealed that two cell populations coexisted, expressing neuronal or oligodendrocyte markers. Triiodothyronine (T3) increased the oligodendrocyte population when 2Y-3t cells were cultured in serum-free medium. Recloning of the line gave rise to three types of subclones. Sixteen subclones were capable of generating both neurons and oligodendrocytes, four subclones were capable of generating only neurons and one subclone was capable of generating only oligodendrocytes. Thus, 2Y-3t cells have characteristics of bipotent neural progenitor cells capable of generating both neurons and oligodendrocytes. In addition, the line expressed mRNA for Pax-2 and had GAD67-positive cells when cultured in serum-free medium. However, none of the mRNAs for Zic-1, Math1, zebrin or Calbindin-D28k were detected, suggesting that the 2Y-3t line might generate the GABAergic interneuron lineage of the mouse cerebellum.

Sox2 is required for sensory organ development in the mammalian inner ear

Sensory hair cells and their associated non-sensory supporting cells in the inner ear are fundamental for hearing and balance. They arise from a common progenitor, but little is known about the molecular events specifying this cell lineage. We recently identified two allelic mouse mutants, light coat and circling (Lcc) and yellow submarine (Ysb), that show hearing and balance impairment. Lcc/Lcc mice are completely deaf, whereas Ysb/Ysb mice are severely hearing impaired. We report here that inner ears of Lcc/Lcc mice fail to establish a prosensory domain and neither hair cells nor supporting cells differentiate, resulting in a severe inner ear malformation, whereas the sensory epithelium of Ysb/Ysb mice shows abnormal development with disorganized and fewer hair cells. These phenotypes are due to the absence (in Lcc mutants) or reduced expression (in Ysb mutants) of the transcription factor SOX2, specifically within the developing inner ear. SOX2 continues to be expressed in the inner ears of mice lacking Math1 (also known as Atoh1 and HATH1), a gene essential for hair cell differentiation, whereas Math1 expression is absent in Lcc mutants, suggesting that Sox2 acts upstream of Math1.

Cryopreservation Does Not Affect Proliferation and Multipotency of Murine Neural Precursor Cells

Stem cell research offers unique opportunities for developing new medical therapies for devastating diseases and a new way to explore fundamental questions of biology. Establishing an efficient freezing protocol for neural precursor cells (NPCs) is of great importance for advances in cell-based therapies. We used fluorescence-activated cell sorter-based cell death/survival analysis and Western blot analysis of proliferation markers (proliferating cell nuclear antigen) and prosurvival proteins (Bcl-2) to study the effect of a variety of cryoprotective agents on fetal mouse forebrain NPCs. Neurospheres frozen at -70 degrees C or in liquid nitrogen in a rate-controlled manner and thawed after 5 days retained viability of 60%-70% measured 24 hours after thawing. However, 1 week after thawing, viability dropped to 50%-60%. Using a clonogenic sphere formation assay, we showed that recovery rate of frozen NPCs was approximately 26% and did not significantly differ between dimethyl sulfoxide (DMSO)- and glycerol-supplemented samples. Application of the caspase inhibitor zVAD-fmk during freezing or in the first week after thawing resulted in protection of cryopreserved neurospheres after thawing but not during the freezing process, indicating that apoptosis limits recovery of NPCs. Cell survival was not reduced in cells that were enzymatically separated before cryopreservation. Optimal protection of NPCs was achieved when 10% DMSO alone or in a combination with 10% fetal calf serum (FCS) was used. However, 10% glycerol alone was equally effective. Using these protocols, NPCs retained their multipotency and differentiated into both glial (GFAP-positive) and neuronal (Tuj1-positive) cells. Percentage of Tuj1-positive cells in 5% and 10% DMSO, in 10% DMSO + 10% FCS, and in 10% glycerol remained at the same level as before freezing and varied from 5%-7%. We conclude that cryopreservation (up to 1 month at -70 degrees C and up to 1 year in liquid nitrogen) does not markedly alter the rate of proliferation and multipotency of murine neural precursor cells.

Molecular signals regulating proliferation of stem and progenitor cells in mouse olfactory epithelium

To understand how signaling molecules regulate the generation of neurons from proliferating stem cells and neuronal progenitors in the developing and regenerating nervous system, we have studied neurogenesis in a model neurogenic epithelium, the olfactory epithelium (OE) of the mouse. Our studies have employed a candidate approach to test signaling molecules of potential importance in regulating neurogenesis and have utilized methods that include tissue culture, in situ hybridization and mouse genetics. Using these approaches, we have identified three distinct stages of stem and transit amplifying progenitor cells in the differentiation pathway of olfactory receptor neurons (ORNs) and have identified mechanisms by which the development of each of these progenitor cell types is regulated by signals produced both within the OE itself and by its underlying stroma. Our results indicate that regulation of olfactory neurogenesis is critically dependent on multiple signaling molecules from two different polypeptide growth factor superfamilies, the fibroblast growth factors and the transforming growth factor beta (TGF-beta) group. In addition, they indicate that these signaling molecules interact in at least two important ways: first, opposing signals converge on cells at specific developmental stages in the ORN pathway to regulate proliferation and differentiation; and second, these signaling molecules--particularly the TGF-betas and their antagonists--play key roles in feedback loops that regulate the size of progenitor cell pools and thereby neuron number, during development and regeneration.

SOX2, a persistent marker for multipotential neural stem cells derived from embryonic stem cells, the embryo or the adult

Multipotent neural stem cells are present throughout the development of the central nervous system (CNS), persist into adulthood in defined locations and can be derived from more primitive embryonic stem cells. We show that SOX2, an HMG box transcription factor, is expressed in multipotent neural stem cells at all stages of mouse ontogeny. We have generated transgenic mice expressing enhanced green fluorescent protein (EGFP) under the control of the endogenous locus-regulatory regions of the Sox2 gene to prospectively identify neural stem/progenitor cells in vivo and in vitro. Fluorescent cells coexpress SOX2 protein, and EGFP fluorescence is detected in proliferating neural progenitor cells of the entire anterior-posterior axis of the CNS from neural plate stages to adulthood. SOX2-EGFP cells can form neurospheres that can be passaged repeatedly and can differentiate into neurons, astrocytes and oligodendrocytes. Moreover, prospective clonal analysis of SOX2-EGFP-positive cells shows that all neurospheres, whether isolated from the embryonic CNS or the adult CNS, express SOX2-EGFP. In contrast, the pattern of SOX2-EGFP expression using randomly integrated Sox2 promoter/reporter construct differs, and neurospheres are heterogeneous for EGFP expression. These studies demonstrate that SOX2 may meet the requirements of a universal neural stem cell marker and provides a means to identify cells which fulfill the basic criteria of a stem cell: self-renewal and multipotent differentiation.

Chromatin remodeling and histone modification in the conversion of oligodendrocyte precursors to neural stem cells

We showed previously that purified rat oligodendrocyte precursor cells (OPCs) can be induced by extracellular signals to convert to multipotent neural stem-like cells (NSLCs), which can then generate both neurons and glial cells. Because the conversion of precursor cells to stem-like cells is of both intellectual and practical interest, it is important to understand its molecular basis. We show here that the conversion of OPCs to NSLCs depends on the reactivation of the sox2 gene, which in turn depends on the recruitment of the tumor suppressor protein Brca1 and the chromatin-remodeling protein Brahma (Brm) to an enhancer in the sox2 promoter. Moreover, we show that the conversion is associated with the modification of Lys 4 and Lys 9 of histone H3 at the same enhancer. Our findings suggest that the conversion of OPCs to NSLCs depends on progressive chromatin remodeling, mediated in part by Brca1 and Brm.

Efficient identification of regulatory sequences in the chicken genome by a powerful combination of embryo electroporation and genome comparison

Recently expanded knowledge of gene regulation clearly indicates that the regulatory sequences of a gene, usually identified as enhancers, are widely distributed in the gene locus, revising the classical view that they are clustered in the vicinity of genes. To identify regulatory sequences for Sox2 expression governing early neurogenesis, we scanned the 50-kb region of the chicken Sox2 locus for enhancer activity utilizing embryo electroporation, resulting in identification of a number of enhancers scattered throughout the analyzed genomic span. The 'pan-neural' Sox2 expression in early embryos is actually brought about by the composite activities of five separate enhancers with distinct spatio-temporal specificities. These and other functionally defined enhancers exactly correspond to extragenic sequence blocks that are conspicuously conserved between the chicken and mammalian genomes and that are embedded in sequences with a wide range of sequence conservation between humans and mice. The sequences conserved between amniotes and teleosts correspond to subregions of the enhancer subsets which presumably represent core motifs of the enhancers, and the limited conservation partly reflects divergent expression patterns of the gene. The phylogenic distance between the chicken and mammals appears optimal for identifying a battery of genetic regulatory elements as conserved sequence blocks, and chicken embryo electroporation facilitates functional characterization of these elements.

Neural Stem Cells: Progenitors or Panacea?

Are neural stem cells (NSCs) maintained as totipotent precursors by the specialized environment within the stem cell niche or are they simply progenitors, which, while retaining their ability to proliferate, are parcellated and restricted along with their postmitotic brethren? In this review, we focus on what has been learned in recent years about endogenous populations of NSCs in the embryonic and adult brain. We compare the data garnered from in vitro analysis to what has been learned from the transplantation of NSCs into the developing, adult or lesioned brain.