Sox1 acts through multiple independent pathways to promote neurogenesis

Survival, migration, and differentiation of Sox1-GFP embryonic stem cells in coculture with an auditory brainstem slice preparation

The poor regeneration capability of the mammalian hearing organ has initiated different approaches to enhance its functionality after injury. To evaluate a potential neuronal repair paradigm in the inner ear and cochlear nerve we have previously used embryonic neuronal tissue and stem cells for implantation in vivo and in vitro. At present, we have used in vitro techniques to study the survival and differentiation of Sox1-green fluorescent protein (GFP) mouse embryonic stem (ES) cells as a monoculture or as a coculture with rat auditory brainstem slices. For the coculture, 300 microm-thick brainstem slices encompassing the cochlear nucleus and cochlear nerve were prepared from postnatal SD rats. The slices were propagated using the membrane interface method and the cochlear nuclei were prelabeled with DiI. After some days in culture a suspension of Sox1 cells was deposited next to the brainstem slice. Following deposition Sox1 cells migrated toward the brainstem and onto the cochlear nucleus. GFP was not detectable in undifferentiated ES cells but became evident during neural differentiation. Up to 2 weeks after transplantation the cocultures were fixed. The undifferentiated cells were evaluated with antibodies against progenitor cells whereas the differentiated cells were determined with neuronal and glial markers. The morphological and immunohistochemical data indicated that Sox1 cells in monoculture differentiated into a higher percentage of glial cells than neurons. However, when a coculture was used a significantly lower percentage of Sox1 cells differentiated into glial cells. The results demonstrate that a coculture of Sox1 cells and auditory brainstem present a useful model to study stem cell differentiation.

Eya1 and Six1 promote neurogenesis in the cranial placodes in a SoxB1-dependent fashion

Genes of the Eya family and of the Six1/2 subfamily are expressed throughout development of vertebrate cranial placodes and are required for their differentiation into ganglia and sense organs. How they regulate placodal neurogenesis, however, remains unclear. Through loss of function studies in Xenopus we show that Eya1 and Six1 are required for neuronal differentiation in all neurogenic placodes. The effects of overexpression of Eya1 or Six1 are dose dependent. At higher levels, Eya1 and Six1 expand the expression of SoxB1 genes (Sox2, Sox3), maintain cells in a proliferative state and block expression of neuronal determination and differentiation genes. At lower levels, Eya1 and Six1 promote neuronal differentiation, acting downstream of and/or parallel to Ngnr1. Our findings suggest that Eya1 and Six1 are required for both the regulation of placodal neuronal progenitor proliferation, through their effects on SoxB1 expression, and subsequent neuronal differentiation.

Inhibition of Activin/Nodal signaling promotes specification of human embryonic stem cells into neuroectoderm

Nodal, a member of the TGF-beta family of signaling molecules, has been implicated in pluripotency in human embryonic stem cells (hESCs) [Vallier, L., Reynolds, D., Pedersen, R.A., 2004a. Nodal inhibits differentiation of human embryonic stem cells along the neuroectodermal default pathway. Dev. Biol. 275, 403-421], a finding that seems paradoxical given Nodal's central role in mesoderm/endoderm specification during gastrulation. In this study, we sought to clarify the role of Nodal signaling during hESC differentiation by constitutive overexpression of the endogenous Nodal inhibitors Lefty2 (Lefty) and truncated Cerberus (Cerb-S) and by pharmacological interference using the Nodal receptor antagonist SB431542. Compared to wildtype (WT) controls, embryoid bodies (EBs) derived from either Lefty or Cerb-S overexpressing hESCs showed increased expression of neuroectoderm markers Sox1, Sox3, and Nestin. Conversely, they were negative for a definitive endoderm marker (Sox17) and did not generate beating cardiomyocyte structures in conditions that allowed mesendoderm differentiation from WT hESCs. EBs derived from either Lefty or Cerb-S expressing hESCs also contained a greater abundance of neural rosette structures as compared to controls. Differentiating EBs derived from Lefty expressing hESCs generated a dense network of beta-tubulin III positive neurites, and when Lefty expressing hESCs were grown as a monolayer and allowed to differentiate, they generated significantly higher numbers of beta-tubulin positive neurons as compared to wildtype hESCs. SB431542 treatments reproduced the neuralising effects of Lefty overexpression in hESCs. These results show that inhibition of Nodal signaling promotes neuronal specification, indicating a role for this pathway in controlling early neural development of pluripotent cells.

Differential control of Wnt target genes involves epigenetic mechanisms and selective promoter occupancy by T-cell factors

Canonical Wnt signaling and its nuclear effectors, beta-catenin and the family of T-cell factor (TCF) DNA-binding proteins, belong to the small number of regulatory systems which are repeatedly used for context-dependent control of distinct genetic programs. The apparent ability to elicit a large variety of transcriptional responses necessitates that beta-catenin and TCFs distinguish precisely between genes to be activated and genes to remain silent in a specific context. How this is achieved is unclear. Here, we examined patterns of Wnt target gene activation and promoter occupancy by TCFs in different mouse cell culture models. Remarkably, within a given cell type only Wnt-responsive promoters are bound by specific subsets of TCFs, whereas nonresponsive Wnt target promoters remain unoccupied. Wnt-responsive, TCF-bound states correlate with DNA hypomethylation, histone H3 hyperacetylation, and H3K4 trimethylation. Inactive, nonresponsive promoter chromatin shows DNA hypermethylation, is devoid of active histone marks, and additionally can show repressive H3K27 trimethylation. Furthermore, chromatin structural states appear to be independent of Wnt pathway activity. Apparently, cell-type-specific regulation of Wnt target genes comprises multilayered control systems. These involve epigenetic modifications of promoter chromatin and differential promoter occupancy by functionally distinct TCF proteins, which together determine susceptibility to Wnt signaling.

The amphioxus SoxB family: implications for the evolution of vertebrate placodes

Cranial placodes are regions of thickened ectoderm that give rise to sense organs and ganglia in the vertebrate head. Homologous structures are proposed to exist in urochordates, but have not been found in cephalochordates, suggesting the first chordates lacked placodes. SoxB genes are expressed in discrete subsets of vertebrate placodes. To investigate how placodes arose and diversified in the vertebrate lineage we isolated the complete set of SoxB genes from amphioxus and analyzed their expression in embryos and larvae. We find that while amphioxus possesses a single SoxB2 gene, it has three SoxB1 paralogs. Like vertebrate SoxB1 genes, one of these paralogs is expressed in non-neural ectoderm destined to give rise to sensory cells. When considered in the context of other amphioxus placode marker orthologs, amphioxus SoxB1 expression suggests a diversity of sensory cell types utilizing distinct placode-type gene programs was present in the first chordates. Our data supports a model for placode evolution and diversification whereby the full complement of vertebrate placodes evolved by serial recruitment of distinct sensory cell specification programs to anterior pre-placodal ectoderm.

Dual function of Sox1 in telencephalic progenitor cells

The transcription factor, Sox1 has been implicated in the maintenance of neural progenitor cell status, but accumulating evidence suggests that this is only part of its function. This study examined the role of Sox1 expression in proliferation, lineage commitment, and differentiation by telencephalic neural progenitor cells in vitro and in vivo, and further clarified the pattern of Sox1 expression in postnatal and adult mouse brain. Telencephalic neural progenitor cells isolated from Sox1 null embryos formed neurospheres normally, but were specifically deficient in neuronal differentiation. Conversely, overexpression of Sox1 in the embryonic telencephalon in vivo both expanded the progenitor pool and biased neural progenitor cells towards neuronal lineage commitment. Sox1 mRNA and protein were found to be persistently expressed in the postnatal and adult brain in both differentiated and neurogenic regions. Importantly, in differentiated regions Sox1 co-labeled only with neuronal markers. These observations, coupled with previous studies, suggest that Sox1 expression by early embryonic progenitor cells initially helps to maintain the cells in cell cycle, but that continued expression subsequently promotes neuronal lineage commitment.

Neuronal specificity of HSV/sleeping beauty amplicon transduction in utero is driven primarily by tropism and cell type composition

A novel bipartite vector system consisting of the herpes simplex virus (HSV) amplicon and the Sleeping Beauty(SB) transposon was previously shown to efficiently deliver a "transgenon" (integrating transgene) in utero. This vector platform facilitated long-term transgenon expression specifically within neurons and neuronal precursor cells of the rodent brain. However, the mechanism underlying the neurospecificity of the HSV/SB amplicon in the setting of mouse embryogenesis is unknown. We find that embryonic cells expressing the Sox1 "neurocompetence" transcription factor represent the primary targets for HSV amplicon transduction in utero. These cells, which comprise the ependymal and subventricular zones (SVZs), express significant levels of high-mobility-group protein B1 (HMGB1), a co-factor shown to facilitate SB-mediated transposition. Using a conventional, non-integrating amplicon expressing Cre recombinase to "tag" transduced cells embryonically in ROSA26 Cre indicator mice in utero, we found transduced cells were exclusively of the neuronal lineage but that in comparison to HSV/SB-mediated in utero delivery, staining patterns were less widespread and "tagged" neuroprogenitor cells were absent. Our findings demonstrate that in utero HSV/SB amplicon gene transfer is primarily neurospecific owing to viral tropism and target cell populations present embryonically, where multi-potent cells of the developing embryo are supportive of SB-driven transposition.

Integrated morphogen signal inputs in gammadelta versus alphabeta T-cell differentiation

Morphogens, a class of secreted proteins that regulate gene expression in a concentration-dependent manner, are responsible for directing nearly all lineage fate choices during embryogenesis. In the thymus, morphogen signal pathways consisting of WNT, Hedgehog, and the transforming growth factor-beta superfamily are active and have been implicated in various developmental processes including proliferation, survival, and differentiation of maturing thymocytes. Intriguingly, it has been inferred that some of these morphogen signal pathways differentially affect gammadelta and alphabeta T-cell development or maintenance, but their role in T-cell lineage commitment has not been directly probed. We have recently identified a modulator of morphogen signaling that significantly influences binary gammadelta versus alphabeta T-cell lineage diversification. In this review, we summarize functions of morphogens in the thymus and provide a highly speculative model of integrated morphogen signals, potentially directing the gammadelta versus alphabeta T-cell fate determination process.

Regulation of gammadelta versus alphabeta T lymphocyte differentiation by the transcription factor SOX13

alphabeta and gammadelta T cells originate from a common, multipotential precursor population in the thymus, but the molecular mechanisms regulating this lineage-fate decision are unknown. We have identified Sox13 as a gammadelta-specific gene in the immune system. Using Sox13 transgenic mice, we showed that this transcription factor promotes gammadelta T cell development while opposing alphabeta T cell differentiation. Conversely, mice deficient in Sox13 expression exhibited impaired development of gammadelta T cells but not alphabeta T cells. One mechanism of SOX13 function is the inhibition of signaling by the developmentally important Wnt/T cell factor (TCF) pathway. Our data thus reveal a dominant pathway regulating the developmental fate of these two lineages of T lymphocytes.

Neurocytoma is a tumor of adult neuronal progenitor cells

Central neurocytoma (CN) is a rare periventricular tumor, whose derivation, lineage potential, and molecular regulation have been mostly unexplored. We noted that CN cells exhibited an antigenic profile typical of neuronal progenitor cells in vivo, yet in vitro generated neurospheres, divided in response to bFGF (basic fibroblast growth factor), activated the neuroepithelial enhancer of the nestin gene, and gave rise to both neuron-like cells and astrocytes. When CN gene expression was compared with that of both normal adult VZ (ventricular zone) and E/nestin:GFP (green fluorescent protein)-sorted native neuronal progenitors, significant overlap was noted. Marker analysis suggested that the gene expression pattern of CN was that of a proneuronal population; glial markers were conspicuously absent, suggesting that the emergence of astroglia from CN occurred only with passage. The expression pattern of CN was distinguished from that of native progenitor cells by a cohort of differentially expressed genes potentially involved in both the oncogenesis and phenotypic restriction of neurocytoma. These included both IGF2 and several components of its signaling pathway, whose sharp overexpression implicated dysregulated autocrine IGF2 signaling in CN oncogenesis. Both receptors and effectors of canonical wnt signaling, as well as GDF8 (growth differentiation factor 8), PDGF-D, and neuregulin, were differentially overexpressed by CN, suggesting that CN is characterized by the concurrent overactivation of these pathways, which may serve to drive neurocytoma expansion while restricting tumor progenitor phenotype. This strategy of comparing the gene expression of tumor cells to that of the purified native progenitors from which they derive may provide a focused approach to identifying transcripts important to stem and progenitor cell oncogenesis.

Canonical Wnt signaling through Lef1 is required for hypothalamic neurogenesis

Although the functional importance of the hypothalamus has been demonstrated throughout vertebrates, the mechanisms controlling neurogenesis in this forebrain structure are poorly understood. We report that canonical Wnt signaling acts through Lef1 to regulate neurogenesis in the zebrafish hypothalamus. We show that Lef1 is required for proneural and neuronal gene expression, and for neuronal differentiation in the posterior hypothalamus. Furthermore, we find that this process is dependent on Wnt8b, a ligand of the canonical pathway expressed in the posterior hypothalamus, and that both Wnt8b and Lef1 act to mediate beta-catenin-dependent transcription in this region. Finally, we show that Lef1 associates in vivo with the promoter of sox3, which depends on Lef1 for its expression and can rescue neurogenesis in the absence of Lef1. The conserved presence of this pathway in other vertebrates suggests a common mechanism for regulating hypothalamic neurogenesis.

Expression of Sox1 during Xenopus early embryogenesis

Sox B1 group genes, Sox1, Sox2, and Sox3 (Sox1-3), are involved in neurogenesis in various species. Here, we identified the Xenopus homolog of Sox1, and investigated its expression patterns and neural inducing activity. Sox1 was initially expressed in the anterior neural plate of Xenopus embryos, with expression restricted to the brain and optic vesicle by the tailbud stage. Expression subsequently decreased in the eye region by the tadpole stage. Sox1 expression in animal cap explants was induced by inhibition of BMP signaling in the same manner as Sox2, Sox3, and SoxD. In addition, overexpression of Sox1 induced neural markers in ventral ectoderm and in animal caps. These results implicate Xenopus Sox1 in neurogenesis, especially brain and eye development.

Identification of Sox17 as a transcription factor that regulates oligodendrocyte development

Microarray analysis of oligodendrocyte lineage cells purified by fluorescence-activated cell sorting (FACS) from 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP)-enhanced green fluorescent protein (EGFP) transgenic mice revealed Sox17 (SRY-box containing gene 17) gene expression to be coordinately regulated with that of four myelin genes during postnatal development. In CNP-EGFP-positive (CNP-EGFP+) cells, Sox17 mRNA and protein levels transiently increased between postnatal days 2 and 15, with white matter O4+ preoligodendrocytes expressing greater Sox17 levels than Nkx2.2+ (NK2 transcription factor related, locus 2) NG2+, or GalC+ (galactocerebroside) cells. In spinal cord, Sox17 protein expression was undetectable in the primary motor neuron domain between embryonic days 12.5 and 15.5 but was evident in Nkx2.2+ and CC1+ cells. In cultured oligodendrocyte progenitor cells (OPCs), Sox17 levels were maximal in O4+ cells and peaked during the phenotypic conversion from bipolar to multipolar. Parallel increases in Sox17 and p27 occurred before MBP protein expression, and Sox17 upregulation was prevented by conditions inhibiting differentiation. Sox17 downregulation with small interfering RNAs increased OPC proliferation and decreased lineage progression after mitogen withdrawal, whereas Sox17 overexpression in the presence of mitogen had opposite effects. Sox17 overexpression enhanced myelin gene expression in OPCs and directly stimulated MBP gene promoter activity. These findings support important roles for Sox17 in controlling both oligodendrocyte progenitor cell cycle exit and differentiation.

Wnt signal pathways and neural stem cell differentiation

Self-renewal, migration and differentiation of neural progenitor cells are controlled by a variety of pleiotropic signal molecules. Members of the morphogen family of Wnt molecules play a crucial role for developmental and repair mechanisms in the embryonic and adult nervous system. A strategy of disclosure of the role of different canonical (glycogen synthase kinase-3beta/beta-catenin-dependent) and noncanonical (Ca2+- and JNK-dependent) signal pathways for progenitor cell expansion and differentiations is illustrated at the example of the rat striatal progenitor cell line ST14A that is immortalized by stable retroviral transfection with a temperature-sensitive mutant of the SV40 large T antigen. A shift from permissive 33 degrees C to nonpermissive 39 degrees C leads to proliferation stop and start of differentiation into glial and neuronal cells. Investigation of expression of Wnts, Wnt receptors and Wnt-dependent signal pathway assay point to a stage-dependent involvement of canonical and noncanonical signaling in proliferation and differentiation of ST14A cells, whereby a mutual suppression of pathway activities is likely. Canonical Wnt molecules are not detected in proliferating and differentiating ST14A cells except Wnt2. The noncanonical Wnt molecules Wnt4, Wnt5a and Wnt11 are expressed in proliferating cells and increase during differentiation, whereas cellular beta-catenin decreases in the early phase and is restored in the late phase of differentiation. Accumulation of beta-catenin at the membrane in undifferentiated proliferating cells and its nuclear localization in nondividing undifferentiated cells under differentiation conditions argues for a distinct spatially regulated role of the molecule in the proliferation and early differentiation phase. Ca2+-dependent and JNK-dependent noncanonical Wnt signaling is not detected during differentiation of ST14A cells. Complete exploration of the role of Wnt pathways, for differentiation of the neural progenitor cells ST14A will require Wnt overexpression and exposure of ST14A cells to exogenous Wnts either with purified Wnts or by co-cultures with Wnt producers.

Reorganization of actin filaments enhances chondrogenic differentiation of cells derived from murine embryonic stem cells

Differentiation of embryonic stem cells is of great interest to developmental biology and regenerative medicine. This study investigated the effects of cytochalasin D (CD) on the distribution of actin filaments in mouse embryoid body (EB)-derived cells. Furthermore, CD was applied to chondrogenic medium to examine its chondrogenic effect. CD at a concentration of 1 microg/ml disrupted stress fibers in EB-derived cells. Actin filaments in treated cells reorganized into a peripheral pattern, and type II collagen was detected by immunocytochemistry. The expression of type II collagen, Sox9, and at a later time point, aggrecan was up-regulated after CD treatment. In the CD-treated cells, Oct4 and Sox2, representing undifferentiation, were down-regulated as well as Sox1, AFP, and CTN-1, representing ectoderm, endoderm, and cardiogenesis, respectively. In conclusion, CD treatment enhances chondrogenesis of EB-derived cells. Moreover, it promotes a more complete stem cell differentiation toward chondrogenesis, when cultured in chondrogenic medium.

Stem cell marker expression in the Bergmann glia population of the adult mouse brain

Recent evidence suggests that the postnatal cerebellum contains cells with characteristics of neural stem cells, which had so far only been identified in the subventricular zone of the lateral ventricles and the subdentate gyrus of the hippocampus. In order to investigate the identity of these cells in the adult cerebellum, we have analyzed the expression of Sox1, a transcription factor from the SoxB1 subgroup and widely used marker of neural stem cells. In situ hybridization and the use of a transgenic mouse model show that, in the adult cerebellum, Sox 1 is only expressed in the Bergmann glia, a population of radial glia present in the Purkinje cell layer. Furthermore, another neural stem cell marker, Sox2 (also member of the SoxB1 subgroup), is also expressed in the Bergmann glia. We have previously shown that these same cells express Sox9, a member of the SoxE subgroup known for its role in glial development. Here we show that Sox9 is in fact also expressed in other regions harboring adult neural stem cells, suggesting that Sox9 represents a novel stem cell marker. Finally, using a Sox1-null mouse, we show that the formation of this Sox2/Sox9 positive Bergmann glia population does not require the presence of a functional Sox1. Our results identify these radial glia as a previously unreported Sox1/Sox2/Sox9 positive adult cell population, suggesting that these cells may represent the recently reported stem cells in the adult cerebellum.

Induction and specification of cranial placodes

Cranial placodes are specialized regions of the ectoderm, which give rise to various sensory ganglia and contribute to the pituitary gland and sensory organs of the vertebrate head. They include the adenohypophyseal, olfactory, lens, trigeminal, and profundal placodes, a series of epibranchial placodes, an otic placode, and a series of lateral line placodes. After a long period of neglect, recent years have seen a resurgence of interest in placode induction and specification. There is increasing evidence that all placodes despite their different developmental fates originate from a common panplacodal primordium around the neural plate. This common primordium is defined by the expression of transcription factors of the Six1/2, Six4/5, and Eya families, which later continue to be expressed in all placodes and appear to promote generic placodal properties such as proliferation, the capacity for morphogenetic movements, and neuronal differentiation. A large number of other transcription factors are expressed in subdomains of the panplacodal primordium and appear to contribute to the specification of particular subsets of placodes. This review first provides a brief overview of different cranial placodes and then synthesizes evidence for the common origin of all placodes from a panplacodal primordium. The role of various transcription factors for the development of the different placodes is addressed next, and it is discussed how individual placodes may be specified and compartmentalized within the panplacodal primordium. Finally, tissues and signals involved in placode induction are summarized with a special focus on induction of the panplacodal primordium itself (generic placode induction) and its relation to neural induction and neural crest induction. Integrating current data, new models of generic placode induction and of combinatorial placode specification are presented.

Knockdown of Sox4 expression by RNAi induces apoptosis in ACC3 cells

Microarray RNA gene expression profiling analysis has shown that Sox4 (Sry-related high mobility group (HMG) box 4) is one of the most upregulated genes in adenoid cystic carcinoma (ACC), relative to non-neoplastic tissue of origin. Here, we show that Sox4 protein is similarly upregulated in ACC by immunohistochemistry of 28 primary cancers and 20 normal tissues. To elucidate the functional significance of these findings, RNA interference (RNAi)-mediated RNA silencing was used to downregulate Sox4 expression in the ACC-derived cell line, ACC3. With confirmed knockdown of Sox4 protein, cell viability was reduced by 51%, with a corresponding increase of apoptosis to 85% as compared to 12% in controls. Apoptosis was confirmed by cell morphology, DNA fragmentation and flow cytometry. Cells could be rescued from the proapoptotic effects of Sox4 RNAi by co-transfection with a construct expressing functional Sox4. Microarray gene expression profiling of RNAi knockdown experiments shows that downregulation of Sox4-modulated expression of critical genes involved in apoptosis and cell cycle control. Overall, our findings suggest that Sox4 contributes to the malignant phenotype of ACC cells by promoting 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.

Telencephalic embryonic subtractive sequences: a unique collection of neurodevelopmental genes

The vertebrate telencephalon is composed of many architectonically and functionally distinct areas and structures, with billions of neurons that are precisely connected. This complexity is fine-tuned during development by numerous genes. To identify genes involved in the regulation of telencephalic development, a specific subset of differentially expressed genes was characterized. Here, we describe a set of cDNAs encoded by genes preferentially expressed during development of the mouse telencephalon that was identified through a functional genomics approach. Of 832 distinct transcripts found, 223 (27%) are known genes. Of the remaining, 228 (27%) correspond to expressed sequence tags of unknown function, 58 (7%) are homologs or orthologs of known genes, and 323 (39%) correspond to novel rare transcripts, including 48 (14%) new putative noncoding RNAs. As an example of this latter group of novel precursor transcripts of micro-RNAs, telencephalic embryonic subtractive sequence (TESS) 24.E3 was functionally characterized, and one of its targets was identified: the zinc finger transcription factor ZFP9. The TESS transcriptome has been annotated, mapped for chromosome loci, and arrayed for its gene expression profiles during neural development and differentiation (in Neuro2a and neural stem cells). Within this collection, 188 genes were also characterized on embryonic and postnatal tissue by in situ hybridization, demonstrating that most are specifically expressed in the embryonic CNS. The full information has been organized into a searchable database linked to other genomic resources, allowing easy access to those who are interested in the dissection of the molecular basis of telencephalic development.

Effects of Emx2 inactivation on the gene expression profile of neural precursors

Emx2 plays a crucial role in the development of the diencephalon and dorsal telencephalon. Thus, Emx2-null mutants have abnormal cortical lamination and a reduction in size of the caudal and medial areas of the prosencephalon. Emx2 is expressed in neural precursors of the subventricular zone in vivo and in cultured neurospheres in vitro where it controls the size of the transit-amplifying population, affecting proliferation and clonal efficiency of neural stem cells. To identify the cellular processes mastered by Emx2, and possibly the molecular mechanisms by which the gene exerts its action, we compared the expression profile of cultured neurospheres derived from wild-type and Emx2-null mouse embryos. The differential expression of several genes was also confirmed by semiquantitative RT-PCR, real-time PCR and cytofluorimetric analysis in different preparations of neurospheres, and by in situ hybridization. The gene expression profile suggested a role for Emx2 in regulating the differentiation and migration properties of neural precursor cells. This involvement was confirmed in vitro, where the altered clonogenicity and impaired migration of Emx2-null cells were partially corrected by transduction of the Emx2 gene. Taken together, our results indicate that Emx2 is indeed involved in the transition between resident early progenitors (perhaps stem cells) and more mature precursors capable of migrating out of the ventricular zone, becoming postmitotic and differentiating into the appropriate cell type, and help explain the alterations observed in the brains of knock-out mice.

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.

The Notch targets Esr1 and Esr10 are differentially regulated in Xenopus neural precursors

The HES family of bHLH repressors plays a key role in regulating the differentiation of neural precursors in the vertebrate embryo. Members of the HES gene family are expressed in neural precursors as targets of the Notch signaling pathway, but how this occurs in the context of neurogenesis is not known. Here, we address this issue by identifying enhancers driving Notch-dependent gene expression of two Hes5-like genes expressed in Xenopus called Esr1 and Esr10. Using frog transgenesis, we identify enhancer elements driving expression of Esr1 and Esr10 in neural precursors or in response to ectopic expression of the proneural protein, Xngnr1. Using deletion and mutation analysis, we define motifs required for enhancer activity of both genes, namely Notch-responsive elements and, in the case of Esr10, E-box motifs. We find that Esr1 and Esr10 are differentially regulated both in terms of Notch input and its interaction with heterologous factors. These studies reveal inputs required for proneural expression of genes encoding bHLH repressors in the developing vertebrate nervous system.

Evolutionary origins of vertebrate placodes: insights from developmental studies and from comparisons with other deuterostomes

Ectodermal placodes comprise the adenohypophyseal, olfactory, lens, profundal, trigeminal, otic, lateral line, and epibranchial placodes. The first part of this review presents a brief overview of placode development. Placodes give rise to a variety of cell types and contribute to many sensory organs and ganglia of the vertebrate head. While different placodes differ with respect to location and derivative cell types, all appear to originate from a common panplacodal primordium, induced at the anterior neural plate border by a combination of mesodermal and neural signals and defined by the expression of Six1, Six4, and Eya genes. Evidence from mouse and zebrafish mutants suggests that these genes promote generic placodal properties such as cell proliferation, cell shape changes, and specification of neurons. The common developmental origin of placodes suggests that all placodes may have evolved in several steps from a common precursor. The second part of this review summarizes our current knowledge of placode evolution. Although placodes (like neural crest cells) have been proposed to be evolutionary novelties of vertebrates, recent studies in ascidians and amphioxus have proposed that some placodes originated earlier in the chordate lineage. However, while the origin of several cellular and molecular components of placodes (e.g., regionalized expression domains of transcription factors and some neuronal or neurosecretory cell types) clearly predates the origin of vertebrates, there is presently little evidence that these components are integrated into placodes in protochordates. A scenario is presented according to which all placodes evolved from an adenohypophyseal-olfactory protoplacode, which may have originated in the vertebrate ancestor from the anlage of a rostral neurosecretory organ (surviving as Hatschek's pit in present-day amphioxus).

Sox3 expression in undifferentiated spermatogonia is required for the progression of spermatogenesis

Sox3, a member of the high mobility group (HMG) family of transcription factors, is expressed in neural progenitor cells and in the gonads. Targeted deletion of Sox3 in mice causes abnormal development of the diencephalon and Rathke's pouch, the progenitor of the anterior pituitary gland. Male and female mice are also infertile and exhibit a primary defect in gametogenesis. In this study, we examined the expression and function of Sox3 in C57BL/6 mice to better understand its role in spermatogenesis. Testis development was normal during embryogenesis. However, spermatogenesis failed to progress during the postnatal period, with germ cell loss beginning at postnatal day 10 (P10). By P14, Sox3 null mice were nearly agametic, retaining only Sertoli cells and undifferentiated spermatogonia. Pituitary gonadotropin and testosterone levels were normal, suggesting a defect in Sertoli cell and/or germ cell function. Immunostaining revealed that Sox3 was expressed in a subpopulation of germ cells localized at the base of the seminiferous tubules. Sox3 expression was restricted to proliferating germ cells and colocalized with neurogenin 3 (Ngn3), a helix-loop-helix transcription factor implicated in spermatogonial differentiation. The absence of Sox3 decreased Ngn3 and increased expression of Oct4, a marker of undifferentiated spermatogonia. We conclude that Sox3 is expressed in A(s), A(pr) and A(al) spermatogonia and is required for spermatogenesis through a pathway that involves Ngn3.

Gene expression signatures of seven individual human embryonic stem cell lines

Identification of molecular components that define a pluripotent human embryonic stem cell (hESC) provides the basis for understanding the molecular mechanisms regulating the maintenance of pluripotency and induction of differentiation. We compared the gene expression profiles of seven genetically independent hESC lines with those of nonlineage-differentiated cells derived from each line. A total of 8,464 transcripts were expressed in all hESC lines. More than 45% of them have no yet-known biological function, which indicates that a high number of unknown factors contribute to hESC pluripotency. Among these 8,464 transcripts, 280 genes were specific for hESCs and 219 genes were more than twofold differentially expressed in all hESC lines compared with nonlineage-differentiated cells. They represent genes implicated in the maintenance of pluripotency and those involved in early differentiation. The chromosomal distribution of these hESC-enriched genes showed over-representation in chromosome 19 and under-representation in chromosome 18. Although the overall gene expression profiles of the seven hESC lines were markedly similar, each line also had a subset of differentially expressed genes reflecting their genetic variation and possibly preferential differentiation potential. Limited overlap between gene expression profiles illustrates the importance of cross-validation of results between different ESC lines.

SOX2 functions in adult neural stem cells

Sox2 is expressed highly in the neuroepithelium of the developing CNS and has been shown to function in neural stem cells. Because Sox2-null mutant mice fail to develop beyond implantation, the role of SOX2 in the CNS has lacked validation. A new genetic model addresses the role of SOX2 in the adult brain and provides evidence that it is involved in the maintenance of neurons in specific regions, in the proliferation and/or maintenance of neural stem cells, and in neurogenesis.

Frizzled 5 signaling governs the neural potential of progenitors in the developing Xenopus retina

Progenitors in the developing central nervous system acquire neural potential and proliferate to expand the pool of precursors competent to undergo neuronal differentiation. The formation and maintenance of neural-competent precursors are regulated by SoxB1 transcription factors, and evidence that their expression is regionally regulated suggests that specific signals regulate neural potential in subdomains of the developing nervous system. We show that the frizzled (Fz) transmembrane receptor Xfz5 selectively governs neural potential in the developing Xenopus retina by regulating the expression of Sox2. Blocking either Xfz5 or canonical Wnt signaling within the developing retina inhibits Sox2 expression, reduces cell proliferation, inhibits the onset of proneural gene expression, and biases individual progenitors toward a nonneural fate, without altering the expression of multiple progenitor markers. Blocking Sox2 function mimics these effects. Rescue experiments indicate that Sox2 is downstream of Xfz5. Thus, Fz signaling can regulate the neural potential of progenitors in the developing nervous system.

Neuronal migration and ventral subtype identity in the telencephalon depend on SOX1

Little is known about the molecular mechanisms and intrinsic factors that are responsible for the emergence of neuronal subtype identity. Several transcription factors that are expressed mainly in precursors of the ventral telencephalon have been shown to control neuronal specification, but it has been unclear whether subtype identity is also specified in these precursors, or if this happens in postmitotic neurons, and whether it involves the same or different factors. SOX1, an HMG box transcription factor, is expressed widely in neural precursors along with the two other SOXB1 subfamily members, SOX2 and SOX3, and all three have been implicated in neurogenesis. SOX1 is also uniquely expressed at a high level in the majority of telencephalic neurons that constitute the ventral striatum (VS). These neurons are missing in Sox1-null mutant mice. In the present study, we have addressed the requirement for SOX1 at a cellular level, revealing both the nature and timing of the defect. By generating a novel Sox1-null allele expressing beta-galactosidase, we found that the VS precursors and their early neuronal differentiation are unaffected in the absence of SOX1, but the prospective neurons fail to migrate to their appropriate position. Furthermore, the migration of non-Sox1-expressing VS neurons (such as those expressing Pax6) was also affected in the absence of SOX1, suggesting that Sox1-expressing neurons play a role in structuring the area of the VS. To test whether SOX1 is required in postmitotic cells for the emergence of VS neuronal identity, we generated mice in which Sox1 expression was directed to all ventral telencephalic precursors, but to only a very few VS neurons. These mice again lacked most of the VS, indicating that SOX1 expression in precursors is not sufficient for VS development. Conversely, the few neurons in which Sox1 expression was maintained were able to migrate to the VS. In conclusion, Sox1 expression in precursors is not sufficient for VS neuronal identity and migration, but this is accomplished in postmitotic cells, which require the continued presence of SOX1. Our data also suggest that other SOXB1 members showing expression in specific neuronal populations are likely to play continuous roles from the establishment of precursors to their final differentiation.

SOX7 is an immediate-early target of VegT and regulates Nodal-related gene expression in Xenopus

In zebrafish, the divergent F-type SOX casanova acts downstream of Nodal signaling to specify endoderm. While no casanova orthologs have been identified in tetrapods, the F-type SOX, SOX7, is supplied maternally in Xenopus (Fawcett and Klymkowsky, 2004. GER 4, 29). Subsequent RT-PCR and section-based in situ hybridization analyses indicate that SOX7 mRNA is localized to the vegetal region of the blastula-stage embryo. Overexpression and maternal depletion studies reveal that the T-box transcription factor VegT, which initiates mesoendodermal differentiation, directly regulates SOX7 expression. SOX7, but not SOX17 (another F-type SOX), binds to sites within the Xnr5 promoter and SOX7, but not SOX17, induces expression of the Nodal-related genes Xnr1, Xnr2, Xnr4, Xnr5, and Xnr6, the homeodomain transcription factor Mixer, and the endodermal marker SOX17beta; both SOX7 and SOX17 induce expression of the pan-endodermal marker endodermin. SOX7's induction of Xnr expression in animal caps is independent of Mixer and Nodal signaling. In animal caps, VegT's ability to induce Mixer and Edd appears to depend upon SOX7 activity. Whole embryo experiments suggests that vegetal factors partially compensate for the absence of SOX7. Based on the antagonistic effects of SOX7 and SOX3 (Zhang et al., 2004. Dev. Biol. 273, 23) and their common binding sites in the Xnr5 promoter, we propose a model in which competitive interactions between these two proteins are involved in refining the domain of endodermal differentiation.

Diverse mechanisms regulate stem cell self-renewal

To what extent are the pathways that regulate self-renewal conserved between stem cells at different stages of development and in different tissues? Some pathways play a strikingly conserved role in regulating the self-renewal of diverse stem cells, whereas other pathways are specific to stem cells in certain tissues or at certain stages of development. Recent studies have highlighted differences between the self-renewal of embryonic, fetal and adult stem cells. By understanding these similarities and differences we may come to a molecular understanding of how stem cells replicate themselves and why aspects of this process differ between stem cells.

Sox2 induction by FGF and FGFR2 activating mutations inhibits Wnt signaling and osteoblast differentiation

Activating mutations in fibroblast growth factor receptor 2 (FGFR2) cause several craniosynostosis syndromes by affecting the proliferation and differentiation of osteoblasts, which form the calvarial bones. Osteoblasts respond to FGF with increased proliferation and inhibition of differentiation. We analyzed the gene expression profiles of osteoblasts expressing FGFR2 activating mutations (C342Y or S252W) and found a striking down-regulation of the expression of many Wnt target genes and a concomitant induction of the transcription factor Sox2. Most of these changes could be reproduced by treatment of osteoblasts with exogenous FGF. Wnt signals promote osteoblast function and regulate bone mass. Sox2 is expressed in calvarial osteoblasts in vivo and we show that constitutive expression of Sox2 inhibits osteoblast differentiation and causes down-regulation of the expression of numerous Wnt target genes. Sox2 associates with β-catenin in osteoblasts and can inhibit the activity of a Wnt responsive reporter plasmid through its COOH-terminal domain. Our results indicate that FGF signaling could control many aspects of osteoblast differentiation through induction of Sox2 and regulation of the Wnt–β-catenin pathway.

SOX genes and neural progenitor identity

Resident among the highly structured adult nervous system, a few cells, referred to as neural progenitors or stem cells, maintain the ability to self-renew or differentiate. From the time of their specification during neural induction and throughout the building of the nervous system, neural progenitor cells preserve their broad developmental potential and replicative capacity to be able to produce the vast array of neuronal and glial cell types of the mature nervous system as, and when, required. Recently, considerable attention has been focused on identifying the molecular mechanisms responsible for maintaining neural progenitor or stem cell fate throughout ontogeny. The expression of a subset of SOX transcription factors is initiated concomitant with the acquisition of neural progenitor identity and is then maintained in the entire progenitor population of the developing and adult nervous system. Strikingly, studies in the central and peripheral nervous system of chick and mouse have revealed that SOX factors are key regulators of neural progenitor identity, promoting self-renewal in a context-dependent manner by sustaining the undifferentiated state of progenitor cells and maintaining their ability to either proliferate or differentiate.

SoxB transcription factors specify neuroectodermal lineage choice in ES cells

Knowledge of lineage decision machinery in pluripotent embryonic stem (ES) cells may shed light on the process of germ layer segregation in the mammalian embryo and enable directed differentiation in vitro for biomedical applications. We have investigated the contribution of Class B1 Sox transcription factors to lineage choice during ES cell differentiation. We report that forced expression of Sox1 or Sox2 did not impair propagation of undifferentiated ES cells, but upon release from self-renewal promoted differentiation into neuroectoderm at the expense of mesoderm and endoderm. The efficient specification of a primary lineage by transcription factor manipulation provides a paradigm for instructing differentiation of ES cells for biopharmaceutical screening and cell therapy applications.