Regionalization and fate specification in neurospheres: the role of Olig2 and Pax6

Tenascin C in stem cell niches: redundant, permissive or instructive?

The stem cell niche provides the specialized environment that is able to sustain the lifelong maintenance of stem cells in their discrete locations within organs. The niche is usually composed of several different cell types and a specialized extracellular matrix consisting of many different constituents. Additionally, a variety of growth factors are secreted into the extracellular space and contribute to the functional organization of the niche. Here, I will concentrate on the multimodular extracellular matrix glycoprotein tenascin C (Tnc) and discuss it as an exemplary molecule that is present in several stem cell niches. In spite of its intuitively suggestive presence, it has been difficult to provide functional evidence for the importance of Tnc in the context of stem cells. In the nervous system, the careful analysis of Tnc-deficient mice has revealed that the developmental program neural stem cell pass-through is delayed due to changes in growth factor responsiveness. To gain further insight, we have employed the gene trap technology in neural stem cells to identify potential Tnc target genes. This approach has surfaced 2 interesting candidates that may contribute to a better understanding of the signal(s) elicited by Tnc in neural stem/progenitor cells in the niche.

The marginal zone/layer I as a novel niche for neurogenesis and gliogenesis in developing cerebral cortex

The cellular diversity of the cerebral cortex is thought to arise from progenitors located in the ventricular zone and subventricular zone in the telencephalon. Here we describe a novel source of progenitors located outside these two major germinative zones of the mouse cerebral cortex that contributes to neurogenesis and gliogenesis. Proliferating cells first appear in the preplate of the embryonic cerebral cortex and further increase in the marginal zone during mid and late neurogenesis. The embryonic marginal zone progenitors differ in their molecular characteristics as well as the size and identity of their clonal progeny from progenitors isolated from the ventricular zone and subventricular zone. Time-lapse video microscopy and clonal analysis in vitro revealed that the marginal zone progenitor pool contains a large fraction of oligodendrocyte or astrocyte progenitors, as well as neuronal and bipotent progenitors. Thus, marginal zone progenitors are heterogenous in regard to their fate specification, as well as in regard to their region of origin (pallial and subpallial) as revealed by in vivo fate mapping. The local environment in the marginal zone tightly regulates the size of this novel progenitor pool, because both basement membrane defects in laminin gamma1-/- mice or alterations in the cellular composition of the marginal zone in Pax6 Small Eye mutant mice lead to an increase in the marginal zone progenitor pool. In conclusion, we have identified a novel source of neuronal and glial progenitors in the marginal zone of the developing cerebral cortex with properties notably distinct from those of ventricular zone and subventricular zone progenitors.

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

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

The role of SVZ-derived neural precursors in demyelinating diseases: from animal models to multiple sclerosis

We will review the role of endogenous neural stem cells in myelin repair both in animal models of demyelination and multiple sclerosis. The mammalian sub-ventricular zone (SVZ) is the largest germinative zone of the adult brain, which contains a well characterized stem cell niche. While most studies highlight the neurogenic potential of SVZ progenitors, recent data indicate that SVZ cells become reactived in response to different pathological cues, like trauma, ischemia, neurodegeneration, inflammation and demyelination. Experimental models of demyelination in rodent demonstrate enhanced proliferation and recruitment of SVZ progenitors into myelin lesions, in response to demyelination. Moreover, cell lineage tracing experiments showed that SVZ progenitor cells can give rise to oligodendrocytes in demyelinated lesions, that could potentially contribute to remyelination. To examine the relevance of these studies in myelin diseases, we recently examined the human SVZ in post-mortem MS brains. The human SVZ is characterized by a ribbon of SVZ astrocytes lining the ependymal border of the lateral ventricles and which behave as multipotential progenitors in vitro. We showed that cellular density and proliferation were enhanced in MS SVZ compared to non-neurological controls. This high cellular density was correlated with the increased number of progenitor cells in MS SVZ, as well as in sub-ventricular lesions. Interestingly, some of these progenitors expressed transcription factors involved in oligodendrogenesis, such as Sox9, Olig2 and Sox10. These data indicate that gliogenesis occurs also in MS SVZ and suggest the recruitment of SVZ-derived oligodendrocyte precursors to peri-ventricular demyelinated lesions. Further investigation of adult neural stem cells and their progenitors in the brain of rodents and non-human primates should help to gain insights in their process of activation in response to demyelination and their role in myelin repair.

Functional properties of neurons derived from in vitro reprogrammed postnatal astroglia

With the exception of astroglia-like cells in the neurogenic niches of the telencephalic subependymal or hippocampal subgranular zone, astroglia in all other regions of the adult mouse brain do not normally generate neurons. Previous studies have shown, however, that early postnatal cortical astroglia in culture can be reprogrammed to adopt a neuronal fate after forced expression of Pax6, a transcription factor (TF) required for proper neuronal specification during embryonic corticogenesis. Here we show that also the proneural genes neurogenin-2 and Mash1 (mammalian achaete schute homolog 1) possess the ability to reprogram astroglial cells from early postnatal cerebral cortex. By means of time-lapse imaging of green fluorescent astroglia, we provide direct evidence that it is indeed cells with astroglial characteristics that give rise to neurons. Using patch-clamp recordings in culture, we show that astroglia-derived neurons acquire active conductances and are capable of firing action potentials, thus displaying hallmarks of true neurons. However, independent of the TF used for reprogramming, astroglia-derived neurons appear to mature more slowly compared with embryonic-born neurons and fail to generate a functional presynaptic output within the culturing period. However, when cocultured with embryonic cortical neurons, astroglia-derived neurons receive synaptic input, demonstrating that they are competent of establishing a functional postsynaptic compartment. Our data demonstrate that single TFs are capable of inducing a remarkable functional reprogramming of astroglia toward a truly neuronal identity.

Neural progenitor cell transplantation and imaging in a large animal model

To evaluate neural stem/progenitor cell (NPC) transplantation therapy in cat models of neurodegenerative diseases, we have isolated, expanded and characterized feline NPCs (fNPCs) from normal fetal cat brain. Feline NPCs responsive to both human epidermal growth factor (hEGF) and human fibroblast growth factor 2 (hFGF2) proliferated as neurospheres, which were able to differentiate to neurons and glial cells. The analysis of growth factors indicated that both hEGF and hFGF2 were required for proliferation of fNPCs. In contrast to the effect on human NPCs, human leukemia inhibitory factor (hLIF) enhanced differentiation of fNPCs. Expanded fNPCs were injected into the brains of normal adult cats. Immunohistochemical analysis showed that the majority of transplanted cells were located adjacent to the injection site and some fNPCs differentiated into neurons. The survival of transplanted fNPCs over time was monitored using non-invasive bioluminescent imaging technology. This study provided the first evidence of allotransplantation of fNPCs into feline CNS. Cats have heterogeneous genetic backgrounds and possess neurological diseases that closely resemble analogous human diseases. The characterization of fNPCs and exploration of non-invasive bioluminescent imaging to track transplanted cells in this study will allow evaluation of NPC transplantation therapy using feline models of human neurological diseases.

Polysialic acid-directed migration and differentiation of neural precursors are essential for mouse brain development

Polysialic acid, which is synthesized by two polysialyltransferases, ST8SiaII and ST8SiaIV, plays an essential role in brain development by modifying the neural cell adhesion molecule (NCAM). It is currently unclear how polysialic acid functions in different processes of neural development. Here we generated mice doubly mutant in both ST8SiaII and ST8SiaIV to determine the effects of loss of polysialic acid on brain development. In contrast to NCAM-deficient, ST8SiaII-deficient, or ST8SiaIV-deficient single mutant mice, ST8SiaII and ST8SiaIV double mutants displayed severe defects in anatomical organization of the forebrain associated with apoptotic cell death. Loss of polysialic acid affected both tangential and radial migration of neural precursors during cortical development, resulting in aberrant positioning of neuronal and glial cells. Glial cell differentiation was aberrantly increased in vivo and in vitro in the absence of polysialic acid. Consistent with these findings, polysialic acid-deficient mice exhibited increased expression of the glial cell marker glial fibrillary acidic protein and a decrease in expression of Pax6, a transcription factor regulating neural cell migration. These results indicate that polysialic acid regulates cell migration and differentiation of neural precursors crucial for brain development.

Directing neurotransmitter identity of neurones derived from expanded adult neural stem cells

In-vitro expanded neural stem cells (NSCs) of the adult subependymal zone (SEZ) may serve as a source for replacing degenerating neurones in disease and trauma. Crucial for the viability of this approach is the ability to selectively generate specific types of neurones from these cells. Here we show that NSCs derived from the adult mouse SEZ and expanded in vitro as neurosphere cells lose their in-vivo specification and generate a mixture of progeny comprising both GABAergic and also, surprisingly, glutamatergic neurones. When forced to express the pro-neural transcription factor neurogenin 2, virtually all progeny of in-vitro expanded adult NSCs acquire a glutamatergic identity, whereas only GABAergic neurones are generated upon expression of the transcription factor Mash1. Respecification of expanded NSCs from the adult SEZ by neurogenin 2 was accompanied by upregulation of the T-box transcription factor Tbr1, suggesting that their progeny had acquired a dorsal telencephalic identity. Thus, in-vitro expanded adult NSCs have the competence to become directed towards distinct functional neurotransmitter phenotypes when the appropriate transcriptional cues are provided.

Neuronal subtype specification in the cerebral cortex

In recent years, tremendous progress has been made in understanding the mechanisms underlying the specification of projection neurons within the mammalian neocortex. New experimental approaches have made it possible to identify progenitors and study the lineage relationships of different neocortical projection neurons. An expanding set of genes with layer and neuronal subtype specificity have been identified within the neocortex, and their function during projection neuron development is starting to be elucidated. Here, we assess recent data regarding the nature of neocortical progenitors, review the roles of individual genes in projection neuron specification and discuss the implications for progenitor plasticity.

Differential Regulation of Basic Helix-Loop-Helix Factors Mash1 and Olig2 by β-Amyloid Accelerates Both Differentiation and Death of Cultured Neural Stem/Progenitor Cells

Despite increased neurogenic differentiation markers in the hippocampal CA1 in Alzheimer disease, neurons are not replaced in CA1 and the neocortex in the disease. beta-Amyloid (Abeta) might cause deterioration of the brain microenvironment supporting neurogenesis and the survival of immature neurons. To test this possibility, we examined whether Abeta alters the expression of cell fate determinants in cerebral cortical cultures and in an Alzheimer disease mouse model (PrP-APP(SW)). Up-regulation of Mash1 and down-regulation of Olig2 were found in cerebral cortical cultures treated with Abeta-(1-42). Mash1 was expressed in nestin-positive immature cells. The majority of Mash1-positive cells in untreated cortical culture co-expressed Olig2. Abeta increased the proportion of Olig2-negative/Mash1-positive cells. A decrease in Olig2+ cells was also observed in the cerebral cortex of adult PrP-APP(SW) mice. Cotransfection experiments with Mash1 cDNA and Olig2 siRNA revealed that overexpression of Mash1 in neurosphere cells retaining Olig2 expression enhanced neural differentiation but accelerated death of Olig2-depleted cells. Growth factor deprivation, which down-regulated Olig2, accelerated death of Mash1-overexpressing neurosphere cells. We conclude that cooperation between Mash1 and Olig2 is necessary for neural stem/progenitor cells to develop into fully mature neurons and that down-regulation of Olig2 by Abeta in Mash1-overexpressing cells switches the cell fate to death. Maintaining Olig2 expression in differentiating cells could have therapeutic potential.

Chondroitin sulfate glycosaminoglycans control proliferation, radial glia cell differentiation and neurogenesis in neural stem/progenitor cells

Although the local environment is known to regulate neural stem cell (NSC) maintenance in the central nervous system, little is known about the molecular identity of the signals involved. Chondroitin sulfate proteoglycans (CSPGs) are enriched in the growth environment of NSCs both during development and in the adult NSC niche. In order to gather insight into potential biological roles of CSPGs for NSCs, the enzyme chondroitinase ABC (ChABC) was used to selectively degrade the CSPG glycosaminoglycans. When NSCs from mouse E13 telencephalon were cultivated as neurospheres, treatment with ChABC resulted in diminished cell proliferation and impaired neuronal differentiation, with a converse increase in astrocytes. The intrauterine injection of ChABC into the telencephalic ventricle at midneurogenesis caused a reduction in cell proliferation in the ventricular zone and a diminution of self-renewing radial glia, as revealed by the neurosphere-formation assay, and a reduction in neurogenesis. These observations suggest that CSPGs regulate neural stem/progenitor cell proliferation and intervene in fate decisions between the neuronal and glial lineage.

Revisiting the astrocyte–oligodendrocyte relationship in the adult CNS

The lineages of both astrocytes and oligodendrocytes have been popular areas of research in the last decade. The source of these cells in the mature CNS is relevant to the study of the cellular response to CNS injury. A significant amount of evidence exists to suggest that resident precursor cells proliferate and differentiate into mature glial cells that facilitate tissue repair and recovery. Additionally, the re-entry of mature astrocytes into the cell cycle can also contribute to the pool of new astrocytes that are observed following CNS injury. In order to better understand the glial response to injury in the adult CNS we must revisit the astrocyte-oligodendrocyte relationship. Specifically, we argue that there is a common glial precursor cell from which astrocytes and oligodendrocytes differentiate and that the microenvironment surrounding the injury determines the fate of the stimulated precursor cell. Ideally, better understanding the origin of new glial cells in the injured CNS will facilitate the development of therapeutics targeted to alter the glial response in a beneficial way.

The role of Pax genes in the development of tissues and organs: Pax3 and Pax7 regulate muscle progenitor cell functions

Pax genes play key roles in the formation of tissues and organs during embryogenesis. Pax3 and Pax7 mark myogenic progenitor cells and regulate their behavior and their entry into the program of skeletal muscle differentiation. Recent results have underlined the importance of the Pax3/7 population of cells for skeletal muscle development and regeneration. We present our current understanding of different aspects of Pax3/7 function in myogenesis, focusing on the mouse model. This is compared with that of other Pax proteins in the emergence of tissue specific lineages and their differentiation as well as in cell survival, proliferation, and migration. Finally, we consider the molecular mechanisms that underlie the function of Pax transcription factors, including the cofactors and regulatory networks with which they interact.

A Cre-lox approach for transient transgene expression in neural precursor cells and long-term tracking of their progeny in vitro and in vivo

Background

Neural precursor cells (NPCs) can be isolated from various regions of the postnatal central nervous system (CNS). Manipulation of gene expression in these cells offers a promising strategy to manipulate their fate both in vitro and in vivo. In this study, we developed a technique that allows the transient manipulation of single/multiple gene expression in NPCs in vitro, and the long-term tracking of their progeny both in vitro and in vivo.

Results

In order to combine the advantages of transient transfection with the long-term tracking of the transfected cells progeny, we developed a new approach based on the cre-lox technology. We first established a fast and reliable protocol to isolate and culture NPCs as monolayer, from the spinal cord of neonatal transgenic Rosa26-YFP cre-reporter mice. These cells could be reliably transfected with single/multiple plasmids by nucleofection. Nucleofection with mono- or bicistronic plasmids containing the Cre recombinase gene resulted in efficient recombination and the long-term expression of the YFP-reporter gene. The transient cre-expression was non-toxic for the transfected cells and did not alter their intrinsic properties. Finally, we demonstrated that cre-transfected cells could be transplanted into the adult brain, where they maintained YFP expression permitting long-term tracking of their migration and differentiation.

Conclusion

This approach allows single/multiple genes to be manipulated in NPCs, while at the same time allowing long-term tracking of the transfected cells progeny to be analyzed both in vitro and in vivo.

The on/off of Pax6 controls the tempo of neuronal differentiation in the developing spinal cord

During neurogenesis, complex networks of genes act sequentially to control neuronal differentiation. In the neural tube, the expression of Pax6, a paired-box-containing gene, just precedes the appearance of the first post-mitotic neurons. So far, its only reported function in the spinal cord is in specifying subsets of neurons. Here we address its possible function in controlling the balance between proliferation and commitment of neural progenitors. We report that increasing Pax6 level is sufficient to push neural progenitors toward cell cycle exit and neuronal commitment via Neurogenin 2 (Ngn2) upregulation. However, neuronal precursors maintaining Pax6(On) fail to perform neuronal differentiation. Conversely, turning off Pax6 function in these precursors is sufficient to provoke premature differentiation and the number of differentiated neurons depends of the amount of Pax6 protein. Moreover, we found that Pax6 expression involves negative feedback regulation by Ngn2 and this repression is critical for the proneural activity of Ngn2. We present a model in which the level of Pax6 activity first conditions the moment when a given progenitor will leave the cell cycle and second, the moment when a selected neuronal precursor will irreversibly differentiate.

Dorsal radial glia generate olfactory bulb interneurons in the postnatal murine brain

During embryogenesis, dorsal radial glia generate pyramidal cell neurons but not interneurons, and are thought to degenerate or transform into astrocytes in the postnatal brain. Ventral radial glia, in contrast, retract their processes to form GFAP+ subventricular zone (SVZ) astrocytes that continue to generate interneurons into adulthood. We sought to fate map dorsal radial glia by codelivering an adenovirus expressing Cre recombinase and a lentivirus expressing dsRedExpress to the dorsal cortical surface of ROSA26R-YFP mice. Whereas the adenovirus is retrogradely transported to the cell body, the VSV-G (vesicular stomatitis virus G) pseudotyped lentivirus is not and allows us to control for viral diffusion from the site of infection. We found that dorsal radial glia give rise to gray and white matter astrocytes and oligodendrocytes. In addition, the dorsal radial glia fate map to astrocytes lining the dorsal aspect of the SVZ that persist at least 8 weeks postnatally. Finally, we found that dorsal radial glial-derived cells generate granule cell and periglomerular interneurons in the olfactory bulb and continue to form interneuronal precursors into adulthood.

Chromatin organization and differentiation in embryonic stem cell models

Embryonic stem cells derived from mammalian embryos represent indispensable tools for mammalian genetics. Their key features--self-renewal and pluripotency--enable them, on the one hand, to be propagated in culture almost indefinitely and, on the other, to be used to study the molecular details of cell commitment and differentiation. In the past few years, it has become clear that chromatin and epigenetic modifications have a central role in maintaining the gene expression programs that are important for both self-renewal and cell commitment. Therefore, studies focused on the chromatin profiles of embryonic stem cells are likely to be very informative for understanding pluripotency and the process of differentiation, and ultimately for using embryonic stem cells as a tool for cell replacement therapy or as models for the study of genetic diseases, cancer progression or drug testing.

Conditional activation of Pax6 in the developing cortex of transgenic mice causes progenitor apoptosis

During development, Pax6 is expressed in a rostrolateral-high to caudomedial-low gradient in the majority of the cortical radial glial progenitors and endows them with neurogenic properties. Using a Cre/loxP-based approach, we studied the effect of conditional activation of two Pax6 isoforms, Pax6 and Pax6-5a, on the corticogenesis of transgenic mice. We found that activation of either Pax6 or Pax6-5a inhibits progenitor proliferation in the developing cortex. Upon activation of transgenic Pax6, specific progenitor pools with distinct endogenous Pax6 expression levels at different developmental stages show defects in cell cycle progression and in the acquisition of apoptotic or neuronal cell fate. The results provide new evidence for the complex role of Pax6 in mammalian corticogenesis.

Neural Stem/Progenitor Cells Express 20 Tenascin C Isoforms That Are Differentially Regulated by Pax6

Tenascin C (Tnc) is an alternatively spliced, multimodular extracellular matrix glycoprotein present in the ventricular zone of the developing brain. Pax6-deficient small eye (sey) mouse mutants show an altered Tnc expression pattern. Here, we investigated the expression of Tnc isoforms in neural stem/progenitor cells and their regulation by the paired-box transcription factor Pax6. Neural stem/progenitor cells cultured as neurospheres strongly expressed Tnc on the protein level. The Tnc isoform expression in neural stem/progenitor cells was analyzed by reverse transcriptase-PCR and dot blot Southern hybridization. In total, 20 different Tnc isoforms were detected in neurospheres derived from embryonic fore-brain cell suspensions. The Tnc isoform containing the fibronectin type III domains A1A4BD is novel and might be neural stem/progenitor cell-specific. Transient overexpression of Pax6 in neurospheres of the medial ganglionic eminence did not alter the total Tnc mRNA expression level but showed a pronounced regulative effect on different Tnc isoforms. The larger Tnc isoforms containing four, five, and six additional alternatively spliced fibronectin type III domains were up-regulated, whereas the small Tnc isoforms without any or with one additional domain were down-regulated. Thus, Pax6 is a homeodomain protein that also modulates the splicing machinery. We conclude that the combinatorial code of Tnc isoform expression in the neural stem/progenitor cell is complex and regulated by Pax6. These findings suggest a functional significance for individual Tnc isoforms in neural stem/progenitor cells.

Loss- and gain-of-function analyses reveal targets of Pax6 in the developing mouse telencephalon

Appropriate neurogenesis and patterning of the forebrain requires the transcription factor Pax6, yet it is largely unknown how Pax6 exerts its effects at the molecular level. To characterize Pax6-mediated regulation of gene expression during murine forebrain neurogenesis, we performed microarray analysis with tissue from the dorsal Pax6-dependent telencephalon and the ventral Pax6-negative telencephalon at the onset of neurogenesis (E12) and at mid-neurogenesis (E15) in wild-type and Pax6-deficient mutant littermates. In the Pax6-deficient cortex the expression levels of various transcription factors involved in neurogenesis (like Satb2, Nfia, AP-2gamma, NeuroD6, Ngn2, Tbr2, Bhlhb5) and the retinoic acid signalling molecule Rlbp1 were reduced. Regulation by Pax6 could be confirmed upon electroporation of a Pax6- and a dominant-negative Pax6-containing vector into embryonic cortex. Taken together, our data reveal novel insights into the molecular pathways regulated by Pax6 during cortical neurogenesis. Most intriguingly, this analysis revealed time- and region-specific differences in Pax6-mediated transcription, explaining the specific function of Pax6 at early and later stages of neurogenesis.

Embryonic cortical neural stem cells migrate ventrally and persist as postnatal striatal stem cells

Embryonic cortical neural stem cells apparently have a transient existence, as they do not persist in the adult cortex. We sought to determine the fate of embryonic cortical stem cells by following Emx1^IREScre; LacZ/EGFP double-transgenic murine cells from midgestation into adulthood. Lineage tracing in combination with direct cell labeling and time-lapse video microscopy demonstrated that Emx1-lineage embryonic cortical stem cells migrate ventrally into the striatal germinal zone (GZ) perinatally and intermingle with striatal stem cells. Upon integration into the striatal GZ, cortical stem cells down-regulate Emx1 and up-regulate Dlx2, which is a homeobox gene characteristic of the developing striatum and striatal neural stem cells. This demonstrates the existence of a novel dorsal-to-ventral migration of neural stem cells in the perinatal forebrain.

Transforming growth factor α promotes sequential conversion of mature astrocytes into neural progenitors and stem cells

An instability of the mature cell phenotype is thought to participate to the formation of gliomas, primary brain tumors deriving from astrocytes and/or neural stem cells. Transforming growth factor alpha (TGFalpha) is an erbB1 ligand overexpressed in the earliest stages of gliomas, and exerts trophic effects on gliomal cells and astrocytes. Here, we questioned whether prolonged TGFalpha exposure affects the stability of the normal mature astrocyte phenotype. We first developed astrocyte cultures devoid of residual neural stem cells or progenitors. We demonstrate that days of TGFalpha treatment result in the functional conversion of a population of mature astrocytes into radial glial cells, a population of neural progenitors. TGFalpha-generated radial glial cells support embryonic neurons migration, and give birth to cells of the neuronal lineage, expressing neuronal markers and the electrophysiological properties of neuroblasts. Lengthening TGFalpha treatment to months results in the delayed appearance of cells with neural stem cells properties: they form floating cellular spheres that are self-renewing, can be clonally derived from a single cell and differentiated into cells of the neuronal lineage. This study uncovers a novel population of mature astrocytes capable, in response to a single epigenetic factor, to regress progressively into a neural stem-like cell stage via an intermediate progenitor stage.

NG2 and Olig2 expression provides evidence for phenotypic deregulation of cultured central nervous system and peripheral nervous system neural precursor cells

Neural stem cells cultured with fibroblast growth factor 2 (FGF2)/epidermal growth factor (EGF) generate clonal expansions called neurospheres (NS), which are widely used for therapy in animal models. However, their cellular composition is still poorly defined. Here, we report that NS derived from several embryonic and adult central nervous system (CNS) regions are composed mainly of remarkable cells coexpressing radial glia markers (BLBP, RC2, GLAST), oligodendrogenic/neurogenic factors (Mash1, Olig2, Nkx2.2), and markers that in vivo are typical of the oligodendrocyte lineage (NG2, A2B5, PDGFR-alpha). On NS differentiation, the latter remain mostly expressed in neurons, together with Olig2 and Mash1. Using cytometry, we show that in growing NS the small population of multipotential self-renewing NS-forming cells are A2B5(+) and NG2(+). Additionally, we demonstrate that these NS-forming cells in the embryonic spinal cord were initially NG2(-) and rapidly acquired NG2 in vitro. NG2 and Olig2 were found to be rapidly induced by cell culture conditions in spinal cord neural precursor cells. Olig2 expression was also induced in astrocytes and embryonic peripheral nervous system (PNS) cells in culture after EGF/FGF treatment. These data provide new evidence for profound phenotypic modifications in CNS and PNS neural precursor cells induced by culture conditions.

Transduction of human embryonic stem cells by ecotropic retroviral vectors

The steadily increasing availability of human embryonic stem (hES) cell lines has created strong interest in applying available tools for gene transfer in murine cells to human systems. Here we present a method for the transduction of hES cells with ecotropic retroviral vectors. hES cells were transiently transfected with a construct carrying the murine retrovirus receptor mCAT1. Subsequently, the cells were exposed to replication-deficient Moloney murine leukemia virus (MoMuLV) derivatives or pseudotyped lentiviral vectors. With oncoretroviral vectors, this procedure yields overall transduction efficiencies of up to 20% and permits selection of permanently transduced clones with high frequency. Selected clones maintained expression of pluripotency-associated markers and exhibited multi-germ layer differentiation both in vitro and in vivo. HES cell-derived somatic cells including neural progeny maintained high levels of transgene expression. Lentiviral vectors pseudotyped with the MoMuLV envelope could be introduced in the same manner with efficiencies of up to 33%. Transgene expression of lentivirally transduced hES cells remained permanent after differentiation even without selection pressure. Bypassing the regulatory issues associated with the use of amphotropic retroviral systems and exploiting the large pool of existing murine vectors, this method provides a safe and versatile tool for gene transfer and lineage analysis in hES cells and their progeny.

Origin of oligodendrocytes in the subventricular zone of the adult brain

Glial fibrillary acidic protein (GFAP)-positive astrocytes (type B cells) in the subventricular zone (SVZ) generate large numbers of new neurons in the adult brain. SVZ stem cells can also generate oligodendrocytes in vitro, but it is not known whether these adult primary progenitors generate oligodendrocytes in vivo. Myelin repair and oligodendrocyte formation in the adult brain is instead associated with glial-restricted progenitors cells, known as oligodendrocyte progenitor cells (OPCs). Here we show that type B cells also generate a small number of nonmyelinating NG2-positive OPCs and mature myelinating oligodendrocytes. Some type B cells and a small subpopulation of actively dividing type C (transit-amplifying) cells expressed oligodendrocyte lineage transcription factor 2 (Olig2), suggesting that oligodendrocyte differentiation in the SVZ begins early in the lineage. Olig2-positive, polysialylated neural cell adhesion molecule-positive, PDGF receptor alpha-positive, and beta-tubulin-negative cells originating in the SVZ migrated into corpus callosum, striatum, and fimbria fornix to differentiate into the NG2-positive nonmyelinating and mature myelinating oligodendrocytes. Furthermore, primary clonal cultures of type B cells gave rise to oligodendrocytes alone or oligodendrocytes and neurons. Importantly, the number of oligodendrocytes derived from type B cells in vivo increased fourfold after a demyelinating lesion in corpus callosum, indicating that SVZ astrocytes participate in myelin repair in the adult brain. Our work identifies SVZ type B cells as progenitors of oligodendrocytes in normal and injured adult brain.

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

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

Olig gene function in CNS development and disease

Olig1 and Olig2 encode basic helix-loop-helix (bHLH) transcription factors that are expressed in both the developing and mature vertebrate central nervous system. While numerous studies have established critical functions for Olig genes during the formation of motor neurons and oligodendrocytes of the ventral neural tube, their roles at later stages of development and in adulthood have remained relatively obscure. Recent studies, however, reveal that in the fetal dorsal spinal cord and neural progenitor cells of the adult brain, Olig expression continues to mark, and may regulate, the formation of oligodendroglia. Studies of Olig expression in human brain tumors and repair of demyelinating lesions suggest the possibility of additional functions in a variety of neurological diseases.

Temporally restricted substrate interactions direct fate and specification of neural precursors derived from embryonic stem cells

It was, until now, not entirely clear how the nervous system attains its cellular phenotypic diversity and wired complexity during development. Here we describe how environmental interactions alone can modify the development of neurogenic precursor cells. Upon evaluating distinct growth-permissive substrates in an embryonic stem cell-neurogenesis assay, we found that laminin, fibronectin, and gelatin instruct neural fate and alter the functional specification of neurons when applied at distinct stages of development. Changes in phenotypic, electrophysiological, and molecular characteristics could resemble cellular events and interactions in the early embryonic brain and may explain why these extracellular matrix components transiently demarcate certain developing brain structures.

Mesodermal cell types induce neurogenesis from adult human hippocampal progenitor cells

Neurogenesis in the adult human brain occurs within two principle neurogenic regions, the hippocampus and the subventricular zone (SVZ) of the lateral ventricles. Recent reports demonstrated the isolation of human neuroprogenitor cells (NPCs) from these regions, but due to limited tissue availability the knowledge of their phenotype and differentiation behavior is restricted. Here we characterize the phenotype and differentiation capacity of human adult hippocampal NPCs (hNPCs), derived from patients who underwent epilepsy surgery, on various feeder cells including fetal mixed cortical cultures, mouse embryonic fibroblasts (MEFs) and PA6 stromal cells. Isolated hNPCs were cultured in clonal density by transferring the cells to serum-free media supplemented with FGF-2 and EGF in 3% atmospheric oxygen. These hNPCs showed neurosphere formation, expressed high levels of early neuroectodermal markers, such as the proneural genes NeuroD1 and Olig2, the NSC markers Nestin and Musashi1, the proliferation marker Ki67 and significant activity of telomerase. The phenotype was CD15low/-, CD34-, CD45- and CD133-. After removal of mitogens and plating them on poly D-lysine, they spontaneously differentiated into a neuronal (MAP2ab+), astroglial (GFAP+), and oligodendroglial (GalC+) phenotype. Differentiated hNPCs showed functional properties of neurons, such as sodium channels, action potentials and production of the neurotransmitters glutamate and GABA. Co-culture of hNPCs with fetal cortical cultures, MEFs and PA6 cells increased neurogenesis of hNPCs in vitro, while only MEFs and PA6 cells also led to a morphological and functional neurogenic maturation. Together we provide a first detailed characterization of the phenotype and differentiation potential of human adult hNPCs in vitro. Our findings reinforce the emerging view that the differentiation capacity of adult hNPCs is critically influenced by non-neuronal mesodermal feeder cells.

Signals from embryonic fibroblasts induce adult intestinal epithelial cells to form nestin-positive cells with proliferation and multilineage differentiation capacity in vitro

The intestinal epithelium has one of the greatest regenerative capacities in the body; however, neither stem nor progenitor cells have been successfully cultivated from the intestine. In this study, we applied an "artificial niche" of mouse embryonic fibroblasts to derive multipotent cells from the intestinal epithelium. Cocultivation of adult mouse and human intestinal epithelium with fibroblast feeder cells led to the generation of a novel type of nestin-positive cells (intestinal epithelium-derived nestin-positive cells [INPs]). Transcriptome analyses demonstrated that mouse embryonic fibroblasts expressed relatively high levels of Wnt/bone morphogenetic protein (BMP) transcripts, and the formation of INPs was specifically associated with an increase in Lef1, Wnt4, Wnt5a, and Wnt/BMP-responsive factors, but a decrease of BMP4 transcript abundance. In vitro, INPs showed a high but finite proliferative capacity and readily differentiated into cells expressing neural, pancreatic, and hepatic transcripts and proteins; however, these derivatives did not show functional properties. In vivo, INPs failed to form chimeras following injection into mouse blastocysts but integrated into hippocampal brain slice cultures in situ. We conclude that the use of embryonic fibroblasts seems to reprogram adult intestinal epithelial cells by modulation of Wnt/BMP signaling to a cell type with a more primitive embryonic-like stage of development that has a high degree of flexibility and plasticity.

Basic fibroblast growth factor endows dorsal telencephalic neural progenitors with the ability to differentiate into oligodendrocytes but not gamma-aminobutyric acidergic neurons

Basic fibroblast growth factor (bFGF) is commonly used to enrich and maintain neural stem cells in vitro. Olig2 is an essential transcription factor for oligodendrocyte lineage specification and is expressed predominantly in ventral neuroepithelial cells in the medial and lateral ganglionic eminence (GE), where oligodendrocyte progenitors originate. Here we report significant induction of Olig2 expression in dorsal neuroepithelium-derived cells cultured in the presence of bFGF, in which Olig2-expressing cells were initially negligible. Among Olig2-expressing cells appearing after a 5-day treatment with bFGF, 99.8% coexpressed nestin. There was no significant difference in proliferation or apoptosis in dorsal and ventral neuroepithelial cultures in the presence of bFGF, suggesting that bFGF induces ectopic expression of Olig2 in dorsal "cortical" neuroepithelial cells. Similarly, expression of Mash1, another ventral neuroepithelial cell marker gene, was also induced in cultured dorsal neuroepithelial cells in the presence of bFGF. Conversely, in this culture, expression of dorsal neuroepithelial cell markers, such as Neurogenin1, Neurogenin2, Pax6, and Emx2, was down-regulated. These results suggested a possible ventralizing activity of bFGF. In fact, bFGF-treated dorsal neuroepithelial cells acquired the potential to generate O4-positive oligodendrocytes with efficacy comparable to that observed with GE-derived cells. In marked contrast, bFGF did not enable dorsal neuroepithelial cells to generate gamma-aminobutyric acid (GABA) neurons, which normally develop only from GE in vivo. Thus, bFGF endows dorsal telencephalic neural progenitors with the ability to differentiate into oligodendrocytes but not GABAergic neurons, suggesting the presence of different mechanisms governing specification of dorsoventral cell identities of neuronal and glial cell lineages.

On neural plasticity, new neurons and the postischemic milieu: An integrated view on experimental rehabilitation

This review discusses actual and potential contributors to functional improvement after stroke injuries. Topics that will be covered are neuronal re-organization and sprouting, neural stem/progenitor cell activation and neuronal replacement, as well as the neuronal milieu defined by glia, inflammatory cells and blood vessel supply. It is well established that different types of neuronal plasticity ultimately lead to post-stroke recovery. However, an untapped potential which only recently has started to be extensively explored is neuronal replacement through endogenous or exogenous resources. Major experimental efforts are needed to achieve progress in this burgeoning area. The review stresses the importance of applying neurodevelopmental principles as well as performing a characterization of the role of the postischemic milieu when studying adult brain neural stem/progenitor cells. Integrated and multifaceted experimentation, incorporating actual and possible poststroke function modulators, will be necessary in order to determine future strategies that will ultimately enable considerable progress in the field of neurorehabilitation.

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.

Pax6-induced alteration of cell fate: Shape changes, expression of neuronal α tubulin, postmitotic phenotype, and cell migration

The transcription factor Pax6 plays an important role in the development of the central nervous system. To understand its mechanism of action, we transduced HeLa cells with a Pax6-expressing lentiviral vector. Upon transduction, HeLa cells markedly changed shape and formed neuritelike extensions. Pax6-transduced HeLa cells expressed high levels of neuronal alpha3 tubulin, demonstrating a partial transdifferentiation towards a neuronal phenotype. Neurons are postmitotic cells. Pax6-transduced HeLa cells became postmitotic through mechanisms involving up-regulation of p53 and cyclin-dependent kinase inhibitor p21. One of the most striking effects of Pax6 was observed by time-lapse videomicroscopy: cells started to dissociate from cell clusters and displayed intense migratory activity. Migration was accompanied by dynamic and reversible shape changes. Our results identified three elements of Pax6 action: (i) expression of neuron-specific genes; (ii) establishment of a postmitotic phenotype; and (iii) involvement in the regulation of cell shape and cell migration.

Forced expression of the motor neuron determinant HB9 in neural stem cells affects neurogenesis

In contrast to mouse embryonic stem cells and in spite of overlapping gene expression profiles, neural stem cells (NSCs) isolated from the embryonic spinal cord do not respond to physiological morphogenetic stimuli provided by Sonic hedgehog and retinoic acid and do not generate motor neurons upon differentiation. Transcription factors expressed in motor neuron progenitors during embryogenesis include Pax6, Ngn2, Nkx6.1 and Olig2, whose expression precedes that of factors specifying motor neuron fate, including HB9, Islet1 and LIM3. We showed that all these factors were present in neural progenitors derived from mouse ES cells, whereas NSCs derived from the rat embryonic spinal cord expressed neither HB9 nor Islet1 and contained low levels of Nkx6.1 and LIM3. We constructed a lentivirus vector to express HB9 and GFP in NSCs and examined the consequences of HB9 expression on other transcription factors and cell differentiation. Compared to cell expressing GFP alone, NSCs expressing GFP and HB9 cycled less rapidly, downregulated Pax6 and Ngn2 mRNA levels, produced higher proportions of neurons in vitro and lower numbers of neurons after transplantation in the spinal cord of recipient rats. Oligodendrocytic and astrocytic differentiations were not affected. HB9 expressing NSCs did not express Islet1 or upregulate LIM3. They neither responded to Sonic hedgehog and retinoic acid nor produced cholinergic neurons. We concluded that forced HB9 expression affected neurogenesis but was not sufficient to confer motor neuron fate to NSCs.

Complement: a novel factor in basal and ischemia-induced neurogenesis

Through its involvement in inflammation, opsonization, and cytolysis, the complement protects against infectious agents. Although most of the complement proteins are synthesized in the central nervous system (CNS), the role of the complement system in the normal or ischemic CNS remains unclear. Here we demonstrate for the first time that neural progenitor cells and immature neurons express receptors for complement fragments C3a and C5a (C3a receptor (C3aR) and C5a receptor). Mice that are deficient in complement factor C3 (C3(-/-)) lack C3a and are unable to generate C5a through proteolytic cleavage of C5 by C5-convertase. Intriguingly, basal neurogenesis is decreased both in C3(-/-) mice and in mice lacking C3aR or mice treated with a C3aR antagonist. The C3(-/-) mice had impaired ischemia-induced neurogenesis both in the subventricular zone, the main source of neural progenitor cells in adult brain, and in the ischemic region, despite normal proliferative response and larger infarct volumes. Thus, in the adult mammalian CNS, complement activation products promote both basal and ischemia-induced neurogenesis.

Pax6 is required for making specific subpopulations of granule and periglomerular neurons in the olfactory bulb

The subventricular zone (SVZ) produces different subclasses of olfactory bulb (OB) interneurons throughout life. Little is known about the molecular mechanisms controlling the production of different types of interneurons. Here we show that most proliferating adult SVZ progenitors express the transcription factor Pax6, but only a small subpopulation of migrating neuroblasts and new OB interneurons derived from these progenitors retains Pax6 expression. To elucidate the cell-autonomous role of Pax6 in OB neurogenesis, we transplanted green fluorescent protein-expressing embryonic forebrain progenitors of the dorsal lateral ganglionic eminence from Pax6 mutant Small Eye (Pax6(Sey/Sey)) mice into the SVZ of adult wild-type mice. Pax6(Sey/Sey) progenitors produce neuroblasts capable of migrating into the OB but fail to generate dopaminergic periglomerular and superficial granule cells. Interestingly, superficial granule neurons also express mRNA for tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis. Our data show that SVZ neuroblasts are heterogeneous and that Pax6 is required in a cell-autonomous manner for the production of cells in the dopaminergic lineage.

Migrating and myelinating potential of subventricular zone neural progenitor cells in white matter tracts of the adult rodent brain

Adult neural stem cells in the subventricular zone (SVZ) produce neuronal progenitors that migrate along the rostral migratory stream (RMS) and generate olfactory interneurons. Here, we evaluate the migratory potential of SVZ cells outside the RMS and their capacity to generate oligodendrocytes in the adult brain. We show that SVZ cells migrate long distances when grafted into white matter tracts such as the cingulum (Ci) and corpus callosum (CC). Furthermore, 22 days postinjection, most present morphologic and phenotypic characteristics of cells committed to the oligodendrocyte lineage. Cells grafted in shiverer CC and Ci become MBP-positive oligodendrocytes, abundantly myelinating these white matter tracts. Type A progenitors are involved in this myelinating process. Altogether, this study reveals the migrating and myelinating potential of SVZ cells in a new environmental context. Therefore, SVZ cells stand as interesting candidates for the development of novel therapeutic strategies for demyelinating diseases.

Extracellular nucleotide signaling in adult neural stem cells: synergism with growth factor-mediated cellular proliferation

We have previously shown that the extracellular nucleoside triphosphate-hydrolyzing enzyme NTPDase2 is highly expressed in situ by stem/progenitor cells of the two neurogenic regions of the adult murine brain: the subventricular zone (type B cells) and the dentate gyrus of the hippocampus (residual radial glia). We explored the possibility that adult multipotent neural stem cells express nucleotide receptors and investigated their functional properties in vitro. Neurospheres cultured from the adult mouse SVZ in the presence of epidermal growth factor and fibroblast growth factor 2 expressed the ecto-nucleotidases NTPDase2 and the tissue non-specific isoform of alkaline phosphatase, hydrolyzing extracellular ATP to adenosine. ATP, ADP and, to a lesser extent, UTP evoked rapid Ca(2+) transients in neurospheres that were exclusively mediated by the metabotropic P2Y(1) and P2Y(2) nucleotide receptors. In addition, agonists of these receptors and low concentrations of adenosine augmented cell proliferation in the presence of growth factors. Neurosphere cell proliferation was attenuated after application of the P2Y(1)-receptor antagonist MRS2179 and in neurospheres from P2Y(1)-receptor knockout mice. In situ hybridization identified P2Y(1)-receptor mRNA in clusters of SVZ cells. Our results infer nucleotide receptor-mediated synergism that augments growth factor-mediated cell proliferation. Together with the in situ data, this supports the notion that extracellular nucleotides contribute to the control of adult neurogenesis.

Pax6 is required for production and maintenance of progenitor cells in postnatal hippocampal neurogenesis

Neurogenesis is crucial for brain formation and continues to take place in certain regions of the postnatal brain including the subgranular zone (SGZ) of the hippocampal dentate gyrus (DG). Pax6 transcription factor is a key player for patterning the brain and promoting embryonic neurogenesis, and is also expressed in the SGZ. In the DG of wild-type rats, more than 90% of total BrdU-incorporated cells expressed Pax6 at 30 min time point after BrdU injection. Moreover, approximately 60% of Pax6+ cells in the SGZ exhibited as GFAP+ cells with a radial glial phenotype and about 30% of Pax6+ cells exhibited as PSA-NCAM+ cells in clusters. From BrdU labeling for 3 days, we found that cell proliferation was 30% decreased at postnatal stages in Pax6-deficient rSey2/+ rat. BrdU pulse/chase experiments combined with marker staining revealed that PSA-NCAM+ late progenitor cells increased at the expense of GFAP+ early progenitors in rSey2/+ rat. Furthermore, expression of Wnt ligands in the SGZ was markedly reduced in rSey2/+ rat. Taken all together, an appropriate dosage of Pax6 is essential for production and maintenance of the GFAP+ early progenitor cells in the postnatal hippocampal neurogenesis.

Striatal deafferentation increases dopaminergic neurogenesis in the adult olfactory bulb

Dopaminergic loss is known to be one of the major hallmarks of Parkinson disease (PD). In addition to its function as a neurotransmitter, dopamine plays significant roles in developmental and adult neurogenesis. Both dopaminergic deafferentation and stimulation modulate proliferation in the subventricular zone (SVZ)/olfactory bulb system as well as in the hippocampus. Here, we study the impact of 6-hydroxydopamine (6-OHDA) lesions to the medial forebrain bundle on proliferation and neuronal differentiation of newly generated cells in the SVZ/olfactory bulb axis in adult rats. Proliferation in the SVZ decreased significantly after dopaminergic deafferentation. However, the number of neural progenitor cells expressing the proneuronal cell fate determinant Pax-6 increased in the SVZ. Survival and quantitative cell fate analysis of newly generated cells revealed that 6-OHDA lesions induced opposite effects in the two different regions of neurogenesis in the olfactory bulb: a transient decrease in the granule cell layer contrasts to a sustained increase of newly generated neurons in the glomerular layer. These data point towards a shift in the ratio of newly generated interneurons in the olfactory bulb layers. Dopaminergic neurogenesis in the glomerular layer tripled after lesioning and consistent with this finding, the total number of tyrosine hydroxylase (TH)-positive cells increased. Thus, loss of dopaminergic input to the SVZ led to a distinct cell fate decision towards stimulation of dopaminergic neurogenesis in the olfactory bulb glomerular layer. This study supports the accumulating evidence that neurotransmitters play a crucial role in determining survival and differentiation of newly generated neurons.

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 cell biology of neurogenesis

During the development of the mammalian central nervous system, neural stem cells and their derivative progenitor cells generate neurons by asymmetric and symmetric divisions. The proliferation versus differentiation of these cells and the type of division are closely linked to their epithelial characteristics, notably, their apical-basal polarity and cell-cycle length. Here, we discuss how these features change during development from neuroepithelial to radial glial cells, and how this transition affects cell fate and neurogenesis.

Multipotent neural stem cells from the adult tegmentum with dopaminergic potential develop essential properties of functional neurons

Neurogenesis in the adult brain occurs within the two principal neurogenic regions: the hippocampus and the subventricular zone of the lateral ventricles. The occurrence of adult neurogenesis in non-neurogenic regions, including the midbrain, remains controversial, but isolation of neural stem cells (NSCs) from several parts of the adult brain, including the substantia nigra, has been reported. Nevertheless, it is unclear whether adult NSCs do have the capacity to produce functional dopaminergic neurons, the cell type lost in Parkinson's disease. Here, we describe the isolation, expansion, and in vitro characterization of adult mouse tegmental NSCs (tNSCs) and their differentiation into functional nerve cells, including dopaminergic neurons. These tNSCs showed neurosphere formation and expressed high levels of early neuroectodermal markers, such as the proneural genes NeuroD1, Neurog2, and Olig2, the NSC markers Nestin and Musashi1, and the proliferation markers Ki67 and BrdU (5-bromo-2-deoxyuridine). The cells showed typical propidium iodide-fluorescence-activated cell sorting analysis of slowly dividing cells. In the presence of selected growth factors, tNSCs differentiated into astroglia, oligodendroglia, and neurons expressing markers for cholinergic, GABAergic, and glutamatergic cells. Electrophysiological analyses revealed functional properties of mature nerve cells, such as tetrodotoxin-sensitive sodium channels, action potentials, as well as currents induced by GABA (gamma-aminobutyric acid), glutamate, and NMDA (N-methyl-D-aspartate). Clonal analysis demonstrated that individual NSCs retain the capacity to generate both glia and neurons. After a multistep differentiation protocol using co-culture conditions with PA6 stromal cells, a small number of cells acquired morphological and functional properties of dopaminergic neurons in culture. Here, we demonstrate the existence of adult tNSCs with functional neurogenic and dopaminergic potential, a prerequisite for future endogenous cell replacement strategies in Parkinson's disease.

Differential expression of cell fate determinants in neurons and glial cells of adult mouse spinal cord after compression injury

Cellular responses after spinal cord injury include activation of astrocytes, degeneration of neurons and oligodendrocytes, and reactions of the ependymal layer and meningeal cells. Because it has been suggested that tissue repair partially recapitulates morphogenesis, we have investigated the expression of several developmentally prominent molecules after spinal cord injury of adult mice where neurogenesis does not occur after injury. Cell fate determinants Numb, Notch-1, Shh and BMPs are abundantly expressed during development but mostly decline in the adult. In the present study, we investigated whether these genes are triggered by spinal cord injury as a sign of attempted recapitulation of development. Expression of Numb, Notch, Shh, BMP2/4 and Msx1/2 was analysed in the adult mouse spinal cord after compression injury by in situ hybridization up to 1 month after injury. The mRNA expression levels of Notch-1, Numb, Shh, BMP4 and Msx2 increased in the grey matter and/or white matter and in the ependyma rostral and caudal to the lesion site after injury. However, BMP2 and Msx1 were not up-regulated. Combining immunohistochemistry of cell type-specific markers with in situ hybridization we found that all the up-regulated genes were expressed in neurons. Moreover, Numb, BMP4 and Msx2 were also expressed by GFAP-positive astrocytes, while Shh was expressed by MBP-positive oligodendrocytes. In conclusion, the cell fate determinants Notch-1, Numb, Shh, BMP4 and Msx2 are expressed in neurons and/or glial cells after injury in a time-dependent manner, suggesting that these genes reflect to some extent an endogenous self-repair potential by recapitulating some features of development.

In vivo transcriptional profile analysis reveals RNA splicing and chromatin remodeling as prominent processes for adult neurogenesis

Neural stem cells and neurogenesis persist in the adult mammalian brain subventricular zone (SVZ). Cells born in the rodent SVZ migrate to the olfactory bulb (Ob) where they differentiate into interneurons. To determine the gene expression and functional profile of SVZ neurogenesis, we performed three complementary sets of transcriptional analysis experiments using Affymetrix GeneChips: (1) comparison of adult mouse SVZ and Ob gene expression profiles with those of the striatum, cerebral cortex, and hippocampus; (2) profiling of SVZ stem cells and ependyma isolated by fluorescent-activated cell sorting (FACS); and (3) analysis of gene expression changes during in vivo SVZ regeneration after anti-mitotic treatment. Gene Ontology (GO) analysis of data from these three separate approaches showed that in adult SVZ neurogenesis, RNA splicing and chromatin remodeling are biological processes as statistically significant as cell proliferation, transcription, and neurogenesis. In non-neurogenic brain regions, RNA splicing and chromatin remodeling were not prominent processes. Fourteen mRNA splicing factors including Sf3b1, Sfrs2, Lsm4, and Khdrbs1/Sam68 were detected along with 9 chromatin remodeling genes including Mll, Bmi1, Smarcad1, Baf53a, and Hat1. We validated the transcriptional profile data with Northern blot analysis and in situ hybridization. The data greatly expand the catalogue of cell cycle components, transcription factors, and migration genes for adult SVZ neurogenesis and reveal RNA splicing and chromatin remodeling as prominent biological processes for these germinal cells.

Expression pattern of the transcription factor Olig2 in response to brain injuries: implications for neuronal repair

Despite the presence of neural stem cells and ongoing neurogenesis in some regions of the adult mammalian brain, neurons are not replaced in most brain regions after injury. With the aim to unravel factors contributing to the failure of neurogenesis in the injured cerebral cortex, we examined the expression of cell fate determinants after acute brain injuries, such as stab wound or focal ischemia, and in a model of chronic amyloid deposition. Although none of the neurogenic factors, such as Pax6, Mash1, Ngn2, was detected in the injured parenchyma, we observed a strong up-regulation of the bHLH transcription factor Olig2, but not Olig1, upon acute and chronic injury. To examine the function of Olig2 in brain lesion, we injected retroviral vectors containing a dominant negative form of Olig2 into the lesioned cortex 2 days after a stab wound. Antagonizing Olig2 function resulted in a significant number of infected cells generating immature neurons that were not observed after injection of the control virus. These data, therefore, imply Olig2 as a repressor of neurogenesis in cells reacting to brain injury and open innovative perspectives toward evoking endogenous neuronal repair.

Cellular and molecular control of neurogenesis in the mammalian telencephalon

The mammalian telencephalon exhibits an amazing diversity of neuronal types. The generation of this diversity relies on multiple developmental strategies, including the regional patterning of progenitors, their temporal specification, and the generation of intermediate progenitor populations. Progress has recently been made in characterizing some of the mechanisms involved. In particular, intermediate progenitors have been shown to play important roles in the generation of neurons in the cerebral cortex, and the properties and lineage relationships between radial glial cells and these intermediate progenitors have recently been examined by elegant time-lapse microscopic studies. Multiple pathways control the progression of neural lineages from multipotent stem cells to intermediate progenitors, postmitotic precursors and finally mature neurons. The regulation of two essential steps, neuronal commitment and specification of subtype identities, is increasingly well understood. These two steps are clearly distinct but co-ordinately regulated by common transcription factors such as neurogenins and Pax6. As our knowledge of the mechanisms of subtype specification of telencephalic neurons progresses, it will become possible to direct stem cells into generating particular telencephalic neuronal populations, opening the way to efficient neuronal replacement therapies.

Expression of the transcription factor Pax 6 in the adult rat dentate gyrus

The transcription factor Pax 6 is expressed in precursor cells during embryonic CNS development, and it plays an important role in the regulation of cell proliferation and neuronal fate determination. Pax 6-expressing cells are also present in the adult hippocampal dentate gyrus and subventricular zone/rostral migratory stream, regions in which neuronal precursors exist during adult life. In the adult dentate gyrus, precursor cells are located in the innermost portion of the granule cell layer, and Pax 6-expressing nuclei are most abundant in this region. To examine the putative role of Pax 6 in adult hippocampal neurogenesis, we have studied the proliferative activity, distribution, and phenotype of Pax 6-expressing cells by using immunohistochemistry. Our results indicate that Pax 6 is intensely expressed in proliferating precursors of the adult dentate gyrus. Pax 6 is also expressed in nonproliferating cells, which may correspond to resting progenitor cells and to granule neurons in their very early developmental stages, because this transcription factor is strongly down-regulated during granule neuron differentiation. However, a small subpopulation of hilar mature neurons and certain astrocytes of the adult hippocampus also express Pax 6. Although the precise roles of this transcription factor in the adult brain remain to be determined, our findings support the idea that its function in the control of cell proliferation and neuronal fate determination during embryogenesis is also operative in the adult hippocampus. However, the expression of Pax 6 in astrocytes and certain mature neurons may indicate the existence of other roles for this transcription factor in this telencephalic region.

In vivo analysis of quiescent adult neural stem cells responding to Sonic hedgehog

Sonic hedgehog (Shh) has been implicated in the ongoing neurogenesis in postnatal rodent brains. Here we adopted an in vivo genetic fate-mapping strategy, using Gli1 (GLI-Kruppel family member) as a sensitive readout of Shh activity, to systematically mark and follow the fate of Shh-responding cells in the adult mouse forebrain. We show that initially, only a small population of cells (including both quiescent neural stem cells and transit-amplifying cells) responds to Shh in regions undergoing neurogenesis. This population subsequently expands markedly to continuously provide new neurons in the forebrain. Our study of the behaviour of quiescent neural stem cells provides in vivo evidence that they can self-renew for over a year and generate multiple cell types. Furthermore, we show that the neural stem cell niches in the subventricular zone and dentate gyrus are established sequentially and not until late embryonic stages.