Making and repairing the mammalian brain--signaling toward neurogenesis and gliogenesis

Differentiation of neurosphere after transplantation into the damaged spinal cord

This study aimed to compare the differentiation and survival of human neural stem/progenitor cells of various origins in vitro and after transplantation into the injured spinal cord of laboratory animals. Rats with simulated spinal cord injury were transplanted with neurosphere cells obtained by directed differentiation of HUES6 cell lines. Fluorescence microscopy was used to visualize the obtained results. HUES6#1 and iPSC#1 neurospheres showed a wide range of markers associated with glial differentiation. The expression of the proliferation marker Ki67 did not exceed 25%, both in the lines of early and late neurospheres. Although neurospheres did not fully differentiate into astrocytes in vitro, they massively approached the GFAP+ astrocyte phenotype when exposed to the transplanted environment. PSC-derived neurospheres transplanted into the site of SM injury without additional growth factors showed only moderate survival, a significant degree of differentiation into astrocytes, and moderate differentiation into neurons. The difference in the survival and differentiation of HUES6#1 and iPSC#1 neurospheres, both in vitro and in vivo, can be explained by the difference in the regulatory behavior of signaling molecules corresponding to the source of origin of PSCs. Derivatives of human PSCs of various origins obtained according to the described differentiation protocol did not mature into astrocytic populations, nor did the glycogenic transition of PSC-derived NSCs occur in vitro. The study demonstrated the impact of the injured spinal cord microenvironment on the differentiation of transplanted HUES6#1 and iPSC#1 into astrocytes. The results showed that HUES6-derived neurospheres generated 90% of GFAP+ astrocytes and 5-10% of early neurons, while iPSC-derived neurospheres generated an average of 74% GFAP+ astrocytes and 5% of early neurons in vivo.

Histone deacetylase-3: Friend and foe of the brain

IMPACT STATEMENT

Brain development and degeneration are highly complex processes that are regulated by a large number of molecules and signaling pathways the identities of which are being unraveled. Accumulating evidence points to histone deacetylases and epigenetic mechanisms as being important regulators of these processes. In this review, we describe that histone deacetylase-3 (HDAC3) is a particularly crucial regulator of both neurodevelopment and neurodegeneration. In addition, HDAC3 regulates memory formation, synaptic plasticity, and the cognitive impairment associated with normal aging. Understanding how HDAC3 functions contributes to the normal development and functioning of the brain while also promoting neurodegeneration could lead to the development of therapeutic approaches for neurodevelopmental, neuropsychiatric, and neurodegenerative disorders.

Dynamic Methylation of an L1 Transduction Family during Reprogramming and Neurodifferentiation

The retrotransposon LINE-1 (L1) is a significant source of endogenous mutagenesis in humans. In each individual genome, a few retrotransposition-competent L1s (RC-L1s) can generate new heritable L1 insertions in the early embryo, primordial germ line, and germ cells. L1 retrotransposition can also occur in the neuronal lineage and cause somatic mosaicism. Although DNA methylation mediates L1 promoter repression, the temporal pattern of methylation applied to individual RC-L1s during neurogenesis is unclear. Here, we identified a de novo L1 insertion in a human induced pluripotent stem cell (hiPSC) line via retrotransposon capture sequencing (RC-seq). The L1 insertion was full-length and carried 5' and 3' transductions. The corresponding donor RC-L1 was part of a large and recently active L1 transduction family and was highly mobile in a cultured-cell L1 retrotransposition reporter assay. Notably, we observed distinct and dynamic DNA methylation profiles for the de novo L1 and members of its extended transduction family during neuronal differentiation. These experiments reveal how a de novo L1 insertion in a pluripotent stem cell is rapidly recognized and repressed, albeit incompletely, by the host genome during neurodifferentiation, while retaining potential for further retrotransposition.

Porcine Neural Progenitor Cells Derived from Tissue at Different Gestational Ages Can Be Distinguished by Global Transcriptome

The impact of gestational age on mammalian neural progenitor cells is potentially important for both an understanding of neural development and the selection of donor cells for novel cell-based treatment strategies. In terms of the latter, it can be problematic to rely entirely on rodent models in which the gestational period is significantly shorter and the brain much smaller than is the case in humans. Here, we analyzed pig brain progenitor cells (pBPCs) harvested at 2 different gestational ages (E45 and E60) using gene expression profiles, obtained by microarray analysis and quantitative polymerase chain reaction (qPCR), across time in culture. Comparison of the global transcriptome of pBPCs from age-matched transgenic green flourescent protein (GFP)-expressing fetuses versus non-GFP-expressing fetuses did not reveal significant differences between the 2 cell types, whereas comparison between E45 and E60 pBPCs did show separation between the data sets by principle component analysis. Further examination by qPCR showed evidence of relative downregulation of proliferation markers and upregulation of glial markers in the gestationally older (E60) cells. Additional comparisons were made. This study provides evidence of age-related changes in the gene expression of cultured fetal porcine neural progenitors that are potentially relevant to the role of these cells during development and as donor cells for transplantation studies.

Regionally-specified second trimester fetal neural stem cells reveals differential neurogenic programming

Neural stem/progenitor cells (NSC) have the potential for treatment of a wide range of neurological diseases such as Parkinson Disease and multiple sclerosis. Currently, NSC have been isolated only from hippocampus and subventricular zone (SVZ) of the adult brain. It is not known whether NSC can be found in all parts of the developing mid-trimester central nervous system (CNS) when the brain undergoes massive transformation and growth. Multipotent NSC from the mid-trimester cerebra, thalamus, SVZ, hippocampus, thalamus, cerebellum, brain stem and spinal cord can be derived and propagated as clonal neurospheres with increasing frequencies with increasing gestations. These NSC can undergo multi-lineage differentiation both in vitro and in vivo, and engraft in a developmental murine model. Regionally-derived NSC are phenotypically distinct, with hippocampal NSC having a significantly higher neurogenic potential (53.6%) over other sources (range of 0%–27.5%, p<0.004). Whole genome expression analysis showed differential gene expression between these regionally-derived NSC, which involved the Notch, epidermal growth factor as well as interleukin pathways. We have shown the presence of phenotypically-distinct regionally-derived NSC from the mid-trimester CNS, which may reflect the ontological differences occurring within the CNS. Aside from informing on the role of such cells during fetal growth, they may be useful for different cellular therapy applications.

VEGF induces neuroglial differentiation in bone marrow-derived stem cells and promotes microglia conversion following mobilization with GM-CSF

PURPOSE

Evaluation of potential tropic effects of vascular endothelial growth factor (VEGF) on the incorporation and differentiation of bone-marrow-derived stem cells (BMSCs) in a murine model of anterior ischemic optic neuropathy (AION).

METHODS

In the first approach, small-sized subset of BMCs were isolated from GFP donors mice by counterflow centrifugal elutriation and depleted of hematopoietic lineages (Fr25lin(-)). These cells were injected into a peripheral vein (1 × 10(6) in 0.2 ml) or inoculated intravitreally (2 × 10(5)) to syngeneic mice, with or without intravitreal injection of 5 μg/2μL VEGF, simultaneously with AION induction. In a second approach, hematopoietic cells were substituted by myelablative transplant of syngeseic GFP + bone marrow cells. After 3 months, progenitors were mobilized with granulocyte-macrophage colony-stimulating factor (GM-CSF) followed by VEGF inoculation into the vitreous body and AION induction . Engraftment and phenotype were examined by immunohistochemistry and FISH at 4 and 24 weeks post-transplantation, and VEGF receptors were determined by real time PCR.

RESULTS

VEGF had no quantitative effect on incorporation of elutriated cells in the injured retina, yet it induced early expression of neuroal markers in cells incorporated in the RGC layer and promoted durable gliosis, most prominent perivascular astrocytes. These effects were mediated by VEGF-R1/Flt-1, which is constitutively expresses in the elutriated fraction of stem cells. Mobilization with GM-CSF limited the differentiation of bone marrow progenitors to microglia, which was also fostered by VEGF.

CONCLUSIONS

VEGF signaling mediated by Flt-1 induces early neural and sustained astrocytic differentiation of stem cells elutriated from adult bone-marrow, with significant contribution to stabilization retinal architecture following ischemic injury.

Environmental regulation of the neural epigenome

There are numerous examples of enduring effects of early experience on gene transcription and neural function. We review the emerging evidence for epigenetics as a candidate mechanism for such effects. There is now evidence that intracellular signals activated by environmental events can directly modify the epigenetic state of the genome, including CpG methylation, histone modifications and microRNAs. We suggest that this process reflects an activity-dependent epigenetic plasticity at the level of the genome, comparable with that observed at the synapse. This epigenetic plasticity mediates neuronal differentiation and phenotypic plasticity, including that associated with learning and memory. Altered epigenetic states are also associated with the risk for and expression of mental disorders. In a broader context, these studies define a biological basis for the interplay between environmental signals and the genome in the regulation of individual differences in behavior, cognition and physiology, as well as the risk for psychopathology.

Mechanisms of neurotoxicity induced in the developing brain of mice and rats by DNA-damaging chemicals

It is not widely known how the developing brain responds to extrinsic damage, although the developing brain is considered to be sensitive to diverse environmental factors including DNA-damaging agents. This paper reviews the mechanisms of neurotoxicity induced in the developing brain of mice and rats by six chemicals (ethylnitrosourea, hydroxyurea, 5-azacytidine, cytosine arabinoside, 6-mercaptopurine and etoposide), which cause DNA damage in different ways, especially from the viewpoints of apoptosis and cell cycle arrest in neural progenitor cells. In addition, this paper also reviews the repair process following damage in the developing brain.

Epigenetics and the biological basis of gene x environment interactions

OBJECTIVE

Child and adolescent psychiatry is rife with examples of the sustained effects of early experience on brain function. The study of behavioral genetics provides evidence for a relation between genomic variation and personality and with the risk for psychopathology. A pressing challenge is that of conceptually integrating findings from genetics into the study of personality without regressing to arguments concerning the relative importance of genomic variation versus nongenomic or environmental influences.

METHOD

Epigenetics refers to functionally relevant modifications to the genome that do not involve a change in nucleotide sequence. This review examines epigenetics as a candidate biological mechanism for gene x environment interactions, with a focus on environmental influences that occur during early life and that yield sustained effects on neural development and function.

RESULTS

The studies reviewed suggest that epigenetic remodeling occurs in response to the environmental activation of cellular signalling pathways associated with synaptic plasticity, epigenetic marks are actively remodeled during early development in response to environmental events that regulate neural development and function, and epigenetic marks are subject to remodeling by environmental influences even at later stages in development.

CONCLUSION

Epigenetic remodeling might serve as an ideal mechanism for phenotypic plasticity--the process whereby the environment interacts with the genome to produce individual differences in the expression of specific traits.

TGFbeta1 stimulates the over-production of white matter astrocytes from precursors of the "brain marrow" in a rodent model of neonatal encephalopathy

Background

In children born prematurely and those surviving cerebral ischemia there are white matter abnormalities that correlate with neurological dysfunction. Since this injury occurs in the immature brain, when the majority of subventricular zone (SVZ) cells generate white matter oligodendrocytes, we sought to study the effect this injury has on gliogenesis from the SVZ. We hypothesized that there is aberrant glial cell generation from the SVZ after neonatal hypoxia ischemia (H/I) that contributes to an increased astrogliogenesis with concomitant oligodendroglial insufficiency. Mechanistically we hypothesized that an increase in specific locally produced cytokines during recovery from injury were modifying the differentiation of glial progenitors towards astrocytes at the expense of the more developmentally-appropriate oligodendrocytes.

Methodology/Principal Finding

For these studies we used the Vannucci H/I rat model where P6 rats are subjected to unilateral common carotid ligation followed by 75 min of systemic hypoxia. Retroviral lineage tracing studies combined with morphological and immunohistochemical analyses revealed the preferential generation of SVZ-derived white matter astrocytes instead of oligodendrocytes post hypoxia/ischemia. Microarray and QRT-PCR analyses of the damaged SVZ showed increased expression of several cytokines and receptors that are known to promote astrocyte differentiation, such as EGF, LIF and TGFß signaling components. Using gliospheres to model the neonatal SVZ, we evaluated the effects of these cytokines on signal transduction pathways regulating astrocyte generation, proliferation and differentiation. These studies demonstrated that combinations of EGF, LIF and TGFß1 reconstituted the increased astrogliogenesis. TGFß1-induced Smad 2/3 phosphorylation and the combination of EGF, LIF and TGFß1 synergistically increased STAT3 phosphorylation over single or double cytokine combinations. Pharmacologically inhibiting ALK5 signaling in vitro antagonized the TGFß1-induced increase in astrocyte generation and antagonizing ALK5 signaling in vivo similarly inhibited astrogliogenesis within the SVZ during recovery from H/I.

Conclusion/Significance

Altogether, these data indicate that there is aberrant specification of glial precursors within the neonatal SVZ during recovery from neonatal H/I that is a consequence of altered cytokine signaling. Our studies further suggest that antagonizing the ALK5 receptor will restore the normal pattern of cell differentiation after injury to the immature brain.

Bilirubin as a determinant for altered neurogenesis, neuritogenesis, and synaptogenesis

Elevated levels of serum unconjugated bilirubin (UCB) in the first weeks of life may lead to long-term neurologic impairment. We previously reported that an early exposure of developing neurons to UCB, in conditions mimicking moderate to severe neonatal jaundice, leads to neuritic atrophy and cell death. Here, we have further analyzed the effect of UCB on nerve cell differentiation and neuronal development, addressing how UCB may affect the viability of undifferentiated neural precursor cells and their fate decisions, as well as the development of hippocampal neurons in terms of dendritic and axonal elongation and branching, the axonal growth cone morphology, and the establishment of dendritic spines and synapses. Our results indicate that UCB reduces the viability of proliferating neural precursors, decreases neurogenesis without affecting astrogliogenesis, and increases cellular dysfunction in differentiating cells. In addition, an early exposure of neurons to UCB decreases the number of dendritic and axonal branches at 3 and 9 days in vitro (DIV), and a higher number of neurons showed a smaller growth cone area. UCB-treated neurons also reveal a decreased density of dendritic spines and synapses at 21 DIV. Such deleterious role of UCB in neuronal differentiation, development, and plasticity may compromise the performance of the brain in later life.

Subtractive hybridization identifies genes differentially expressed by olfactory ensheathing cells and neural stem cells

The in vitro differentiation of embryonic stem cells into glia has received relatively limited attention to date when compared with the interest in the generation of neurons. We are interested in a particular glial phenotype, the aldynoglia, and their differentiation from multipotential neural precursors (MNP), since this type of glia can promote neuronal regeneration. We constructed cDNA libraries from cultures of purified olfactory ensheathing cells (OEC), an aldynoglia cell type, and MNP to perform subtractive hybridization. As a result, we isolated four genes from the OEC: one tenascin C (Tn-C) isoform, Insulin-like growth factor binding protein 5 (Igfbp-5), cytochrome oxidase subunit I (COX1) and a phosphodiesterase for cyclic nucleotides (CNPase). With the exception of CNPase, these genes are expressed more strongly in the OEC than in the MNP and moreover, the expression of all four is induced when MNP were exposed to OEC conditioned media. The data suggest a role for these genes in MNP differentiation, and their products appear to represent characteristic proteins of the aldynoglia phenotype.

The basic helix-loop-helix transcription factor olig2 is critical for reactive astrocyte proliferation after cortical injury

The mechanisms underlying the formation of the glial scar after injury are poorly understood. In this report, we demonstrate that after cortical injury Olig2 is upregulated in reactive astrocytes coincident with proliferation of these cells. Short-term lineage tracing studies with glial subtype-restricted transgenic reporter lines indicate that Olig2-expressing cells in the astroglial but not the oligodendroglial lineage are the essential source of reactive astrocytes. In addition, cortical Olig2 ablation results in a decrease in proliferation of reactive astrocytes in response to injury. Cell-type-specific mutagenesis indicates that Olig2 ablation in GFAP+ astrocytes and their precursors rather than in neuronal or oligodendroglial cells is responsible for the reduction of reactive astrocyte proliferation. Thus, our studies suggest that Olig2 is critical for postinjury gliosis.

Characterization of cells recovered from the xenotransplanted NG97 human-derived glioma cell line subcultured in a long-term in vitro

BACKGROUND

In order to elucidate tumoral progression and drug resistance, cultured cell lines are valuable tools applied on tumor related assays provided they are well established and characterized. Our laboratory settled the NG97 cell line derived from a human astrocytoma grade III, which started to develop and express important phenotypical characteristics of an astrocytoma grade IV after injection in the flank of nude mice. Astrocytomas are extremely aggressive malignancies of the Central Nervous System (CNS) and account for 46% of all primary malignant brain tumors. Progression to worse prognosis occurs in 85% of the cases possibly due to changes in cell tumor microenvironment and through biological pathways that are still unclear.

METHODS

This work focused on characterizing the NG97 cell line specifically after being recovered from the xenotransplant, who maintained their undifferentiated characteristics along the following 60th passages in vitro. These cells were subcultivated to evaluate the possible contribution of these undifferentiated characteristics to the malignant progression phenotype. These characteristics were the expression of molecules involved in the processes of migration, dedifferentiation and chromosomal instability.

RESULTS

Results showed that NG97(ht) had an decrease in doubling time through sub cultivation, which was characterized by a converse modulation between the expression of glial fibrillary acidic protein (GFAP) and vimentin. In addition, beta1 integrins were present in intermediate levels while alpha5 integrins had a high expression profile as well as fibronectin and laminin. Cytogenetic analysis of NG97(ht) revealed several chromosomal abnormalities, 89% of the cells showed to be hyperdiploid and the modal number was assigned to be 63. Several acrocentric chromosomes were visualized and at least 30 figures were attributed to be murine. These findings suggest a possible fusion between the original NG97 cells with stromal murine cells in the xenotransplant.

CONCLUSION

In this study the NG97(ht) cells were characterized to embryonic recovery patterns of intermediate filaments, adhesion molecules expression, chromosomal imbalances and murine chromosomes. In the latter case, these presumably chromosomes were originated as fusions between murine stroma cells and NG97 cell lineage in the xenotransplant. Our results emphasize important queries about astrocytomas tumor progression.

Expression of Disabled 1 suppresses astroglial differentiation in neural stem cells

Disabled 1 (Dab1), a cytoplasmic adaptor protein expressed predominantly in the CNS, transduces a Reelin-initiated signaling that controls neuronal migration and positioning during brain development. To determine the role of Dab1 in neural stem cell (NSC) differentiation, we established a culture of neurospheres derived from the embryonic forebrain of the Dab1(-/-) mice, yotari. Differentiating Dab1(-/-) neurospheres exhibited a higher expression of GFAP, an astrocytic marker, at the expense of neuronal markers. Under Dab1-deficient condition, the expression of NeuroD, a transcription factor for neuronal differentiation, was decreased and the JAK-STAT pathway was evidently increased during differentiation of NSC, suggesting the possible involvement of Dab1 in astrocyte differentiation via JAK-STAT pathway. Notably, expression of neural and glial markers and the level of JAK-STAT signaling molecules were not changed in differentiating NSC by Reelin treatment, indicating that differentiation of NSC is Reelin-independent. Immunohistochemical analyses showed a decrease in the number of neurons and an increase in the number of GFAP-positive cells in developing yotari brains. Our results suggest that Dab1 participates in the differentiation of NSCs into a specific cell lineage, thereby maintaining a balance between neurogenesis and gliogenesis.

Setting the conditions for efficient, robust and reproducible generation of functionally active neurons from adult subventricular zone-derived neural stem cells

Although new culture conditions enable homogeneous and long-term propagation of radial glia-like neural stem (NS) cells in monolayer and serum-free conditions, the efficiency of the conversion of NS cells into terminally differentiated, functionally mature neurons is relatively limited and poorly characterized. We demonstrate that NS cells derived from adult mouse subventricular zone robustly develop properties of mature neurons when exposed to an optimized neuronal differentiation protocol. A high degree of cell viability was preserved. At 22 days in vitro, most cells (65%) were microtubule-associated protein 2(+) and coexpressed gamma-aminobutyric acid (GABA), GAD67, calbindin and parvalbumin. Nearly all neurons exhibited sodium, potassium and calcium currents, and 70% of them fired action potentials. These neurons expressed functional GABA(A) receptors, whereas activable kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartic acid receptors were present in approximately 80, 30 and 2% of cells, respectively. Antigenic and functional properties were efficiently and reliably reproduced across experiments and cell passages (up to 68). This is the first report showing a consistent and reproducible generation of large amounts of neurons from long-term passaged adult neural stem cells. Remarkably, the neuronal progeny carries a defined set of antigenic, biochemical and functional characteristics that make this system suitable for studies of NS cell biology as well as for genetic and chemical screenings.

Effects of estrogens on proliferation and differentiation of neural stem/progenitor cells

We investigated the effect of the female hormone 17beta-estradiol (E2) and the hormone mimic bisphenol A (BPA) on the proliferation and differentiation of rat neural stem/progenitors cells (NS/PCs) cultured from the telencephalon of embryonic day-15 rats. Basic fibroblast growth factor (FGF-2) is a potent mitogen of early generated NS/PCs, and is used for the proliferation of NS/PCs in vitro. Administration of E2 or BPA alone to the NS/PCs stimulated their proliferation in the absence but not in the presence of FGF-2. E2- or BPA-treatment increased the ratio of the oligodendrocytes generated from the NS/PCs to total cells; however, this ratio did not change when the cells were stimulated with platelet-derived growth factor (PDGF), a mitogen for oligodendrocyte precursors, or with neurotrophin-3, an oligogenic factor for glial progenitor cells. These results suggest that estrogens would influence the fate of NS/PCs when the cells are poorly supplied with mitogens or differentiation factors during the early stages of neurogenesis.

Neural stem cells in the mammalian brain

New fundamental results on stem cell biology have been obtained in the past 15 years. These results allow us to reinterpret the functioning of the cerebral tissue in health and disease. Proliferating stem cells have been found in the adult brain, which can be involved in postinjury repair and can replace dead cells under specific conditions. Numerous genomic mechanisms controlling stem cell proliferation and differentiation have been identified. The involvement of stem cells in the genesis of malignant tumors has been demonstrated. Neural stem cell tropism toward tumors has been shown. These findings suggest new lines of research on brain functioning and development. Stem cells can be used to develop radically new treatments of neurodegenerative and cancer diseases of the brain.

Neurons or glia? Can SHP2 know it all?

Normal development of the nervous system relies on the spatially and temporally well-controlled differentiation of neurons and glia. Here, we discuss the intra- and extracellular molecular mechanisms that underlie the sequential genesis of neurons and glia, emphasizing recent studies describing the role of a signaling molecule, the tyrosine phosphatase SHP2, in normal brain development. Activation of SHP2 simultaneously enhances downstream activation of the MEK-ERK pathway, which subsequently promotes neurogenesis, while inhibiting the JAK-STAT pathway, which is critical for astroglial differentiation. Mutations in SHP2 that increase its tyrosine phosphatase activity cause a mental retardation-related disorder, Noonan syndrome. An imbalance in neurogenesis versus gliogenesis due to SHP2 mutations may contribute to Noonan syndrome.

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.

Integrative genomic and functional analyses reveal neuronal subtype differentiation bias in human embryonic stem cell lines

The self-renewal and differentiation potential of human embryonic stem cells (hESCs) suggests that hESCs could be used for regenerative medicine, especially for restoring neuronal functions in brain diseases. However, the functional properties of neurons derived from hESC are largely unknown. Moreover, because hESCs were derived under diverse conditions, the possibility arises that neurons derived from different hESC lines exhibit distinct properties, but this possibility remains unexplored. To address these issues, we developed a protocol that allows stepwise generation from hESCs of cultures composed of approximately 70-80% human neurons that exhibit spontaneous synaptic network activity. Comparison of neurons derived from the well characterized HSF1 and HSF6 hESC lines revealed that HSF1- but not HSF6-derived neurons exhibit forebrain properties. Accordingly, HSF1-derived neurons initially form primarily GABAergic synaptic networks, whereas HSF6-derived neurons initially form glutamatergic networks. microRNA profiling revealed significant expression differences between the two hESC lines, suggesting that microRNAs may influence their distinct differentiation properties. These observations indicate that although both HSF1 and HSF6 hESCs differentiate into functional neurons, the two hESC lines exhibit distinct differentiation potentials, suggesting that they are preprogrammed. Information on hESC line-specific differentiation biases is crucial for neural stem cell therapy and establishment of novel disease models using hESCs.

A post-transcriptional regulatory switch in polypyrimidine tract-binding proteins reprograms alternative splicing in developing neurons

Many metazoan gene transcripts exhibit neuron-specific splicing patterns, but the developmental control of these splicing events is poorly understood. We show that the splicing of a large group of exons is reprogrammed during neuronal development by a switch in expression between two highly similar polypyrimidine tract-binding proteins, PTB and nPTB (neural PTB). PTB is a well-studied regulator of alternative splicing, but nPTB is a closely related paralog whose functional relationship to PTB is unknown. In the brain, nPTB protein is specifically expressed in post-mitotic neurons, whereas PTB is restricted to neuronal precursor cells (NPC), glia, and other nonneuronal cells. Interestingly, nPTB mRNA transcripts are found in NPCs and other nonneuronal cells, but in these cells nPTB protein expression is repressed. This repression is due in part to PTB-induced alternative splicing of nPTB mRNA, leading to nonsense-mediated decay (NMD). However, we find that even properly spliced mRNA fails to express nPTB protein when PTB is present, indicating contributions from additional post-transcriptional mechanisms. The PTB-controlled repression of nPTB results in a mutually exclusive pattern of expression in the brain, where the loss of PTB in maturing neurons allows the synthesis of nPTB in these cells. To examine the consequences of this switch, we used splicing-sensitive microarrays to identify different sets of exons regulated by PTB, nPTB, or both proteins. During neuronal differentiation, the splicing of these exon sets is altered as predicted from the observed changes in PTB and nPTB expression. These data show that the post-transcriptional switch from PTB to nPTB controls a widespread alternative splicing program during neuronal development.

A crucial role for Olig2 in white matter astrocyte development

The mechanisms underlying astrocyte heterogeneity in the developing mouse brain are poorly understood. The bHLH transcription factor Olig2 is essential for motoneuron and oligodendrocyte formation; however, its role in astrocyte development remains obscure. During cortical development, Olig2 is transiently expressed in immature developing astrocytes at neonatal stages and is progressively downregulated in astrocytes at late postnatal stages. To assess the function of Olig2 in astrocyte formation, we conditionally ablated Olig2 in a spatiotemporally controlled manner. In the Olig2-ablated cortex and spinal cord, the formation of astrocytes in the white matter is severely compromised. Temporally controlled mutagenesis revealed that postnatal Olig2 function is required for astrocyte differentiation in the cerebral white matter. By contrast, astrocytes in the cortical gray matter are formed, but with sustained GFAP upregulation in the superficial layers. Cell type-specific mutagenesis and fate-mapping analyses indicate that abnormal astrocyte formation is at least in part attributable to the loss of Olig2 in developing astrocytes and their precursors. Thus, our studies uncover a crucial role for Olig2 in white matter astrocyte development and reveal divergent transcriptional requirements for, and developmental sources of, morphologically and spatially distinct astrocyte subpopulations.

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

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

Glial cells more than support cells?

Glial cells are the most abundant cells in the human brain and have long been considered as passive supporting cells for neurons. In contrast to the extensive studies on various neuronal functions in the nervous system, we still have limited knowledge about glial cells. Recently a number of pioneering studies have provided convincing evidence that glia play active roles in development and function of the central nervous system. This review discusses recent advances in our understanding of the molecular mechanisms underlying glial cell differentiation. We then highlight some of the novel findings about glial function, i.e. the role of glia in synaptogenesis and the intricate relationship between astrocytes and adult neural stem cells. Finally, we summarize the emerging studies that implicate abnormalities in the formation or maintenance of glia leading to severe brain diseases, such as Alexander disease, glioblastoma and multiple sclerosis, and potential therapeutic strategies to tackle these diseases.

Truncated tyrosine kinase B brain-derived neurotrophic factor receptor directs cortical neural stem cells to a glial cell fate by a novel signaling mechanism

During development of the mammalian cerebral cortex neural stem cells (NSC) first generate neurons and subsequently produce glial cells. The mechanism(s) responsible for this developmental shift from neurogenesis to gliogenesis is unknown. Brain-derived neurotrophic factor (BDNF) is believed to play important roles in the development of the mammalian cerebral cortex; it enhances neurogenesis and promotes the differentiation and survival of newly generated neurons. Here, we provide evidence that a truncated form of the BDNF receptor tyrosine kinase B (trkB-t) plays a pivotal role in directing embryonic mouse cortical NSC to a glial cell fate. Expression of trkB-t promotes differentiation of NSC toward astrocytes while inhibiting neurogenesis both in cell culture and in vivo. The mechanism by which trkB-t induces astrocyte genesis is not simply the result of inhibition of full-length receptor with intrinsic tyrosine kinase activity signaling. Instead, binding of BDNF to trkB-t activates a signaling pathway (involving a G-protein and protein kinase C) that induced NSC to become glial progenitors and astrocytes. Thus, the increased expression of trkB-t in the embryonic cerebral cortex that occurs coincident with astrocyte production plays a pivotal role in the developmental transition from neurogenesis to gliogenesis. Our findings suggest a mechanism by which a single factor (BDNF) regulates the production of the two major cell types in the mammalian cerebral cortex.

Embryonic stem cell-derived neurons form functional networks in vitro

Embryonic stem (ES) cells provide a flexible and unlimited source for a variety of neuronal types. Because mature neurons establish neuronal networks very easily, we tested whether ES-derived neurons are capable of generating functional networks and whether these networks, generated in vitro, are capable of processing information. Single-cell electrophysiology with pharmacological antagonists demonstrated the presence of both excitatory and inhibitory synaptic connections. Extracellular recording with planar multielectrode arrays showed that spontaneous bursts of electrical activity are present in ES-derived networks with properties remarkably similar to those of hippocampal neurons. When stimulated with extracellular electrodes, ES-derived neurons fired action potentials, and the evoked electrical activity spread throughout the culture. A statistical analysis indicated that ES-derived networks discriminated between stimuli of different intensity at a single trial level, a key feature for an efficient information processing. Thus, ES-derived neurons provide a novel in vitro strategy to create functional networks with defined computational properties.

Overexpression of SOCS3 inhibits astrogliogenesis and promotes maintenance of neural stem cells

To investigate the effects of suppressors of cytokine signaling 3 (SOCS3) on neural stem cell fate, stem cells were infected with an adenoviral vector expressing SOCS3. Three days later, western blot analysis and immunocytochemical analysis revealed that the protein level of MAP2 and the number of MAP2-positive cells were significantly increased in SOCS3-transfected cells, whereas the protein level of GFAP and the number of GFAP-positive cells were significantly decreased. Furthermore, promoter assay revealed a significant reduction in the transcriptional level of signal transducer and activator of transcription 3 (Stat3) in the transfected cells. In addition, the mRNA levels of Notch family member (notch1) and inhibitory basic helix-loop-helix (bHLH) factors (hes5 and id3) were significantly up-regulated 1 day after overexpression of SOCS3. Three days after transfection, the mRNA level of hes5 was significantly decreased, whereas that of notch1 was still up-regulated. Moreover, all of SOCS3-positive cells expressed Nestin protein but did not express MAP2 or GFAP proteins. These data indicate that overexpression of SOCS3 induced neurogenesis and inhibited astrogliogenesis in neural stem cells. Our data also show that SOCS3 promoted maintenance of neural stem cells.

Molecular characterization of retinal stem cells and their niches in adult zebrafish

Background

The persistence in adult teleost fish of retinal stem cells that exhibit all of the features of true 'adult stem cells' – self-renewal, multipotency, and the capacity to respond to injury by mitotic activation with the ability to regenerate differentiated tissues – has been known for several decades. However, the specialized cellular and molecular characteristics of these adult retinal stem cells and the microenvironmental niches that support their maintenance in the differentiated retina and regulate their activity during growth and regeneration have not yet been elucidated.

Results

Our data show that the zebrafish retina has two kinds of specialized niches that sustain retinal stem cells: 1) a neuroepithelial germinal zone at the interface between neural retina and ciliary epithelium, called the ciliary marginal zone (CMZ), a continuous annulus around the retinal circumference, and 2) the microenvironment around some Müller glia in the differentiated retina. In the uninjured retina, scattered Müller glia (more frequently those in peripheral retina) are associated with clusters of proliferating retinal progenitors that are restricted to the rod photoreceptor lineage, but following injury, the Müller-associated retinal progenitors can function as multipotent retinal stem cells to regenerate other types of retinal neurons. The CMZ has several features in common with the neurogenic niches in the adult mammalian brain, including access to the apical epithelial surface and a close association with blood vessels. Müller glia in the teleost retina have a complex response to local injury that includes some features of reactive gliosis (up-regulation of glial fibrillary acidic protein, GFAP, and re-entry into the cell cycle) together with dedifferentiation and re-acquisition of phenotypic and molecular characteristics of multipotent retinal progenitors in the CMZ (diffuse distribution of N-cadherin, activation of Notch-Delta signaling, and expression of rx1, vsx2/Chx10, and pax6a) along with characteristics associated with radial glia (expression of brain lipid binding protein, BLBP). We also describe a novel specific marker for Müller glia, apoE.

Conclusion

The stem cell niches that support multi-lineage retinal progenitors in the intact, growing and regenerating teleost retina have properties characteristic of neuroepithelia and neurogenic radial glia. The regenerative capacity of the adult zebrafish retina with its ability to replace lost retinal neurons provides an opportunity to discover the molecular regulators that lead to functional repair of damaged neural tissue.

Exploring the regulation of human neural precursor cell differentiation using arrays of signaling microenvironments

Cells of a developing embryo integrate a complex array of local and long-range signals that act in concert with cell-intrinsic determinants to influence developmental decisions. To systematically investigate the effects of molecular microenvironments on cell fate decisions, we developed an experimental method based on parallel exposure of cells to diverse combinations of extracellular signals followed by quantitative, multi-parameter analysis of cellular responses. Primary human neural precursor cells were captured and cultured on printed microenvironment arrays composed of mixtures of extracellular matrix components, morphogens, and other signaling proteins. Quantitative single cell analysis revealed striking effects of some of these signals on the extent and direction of differentiation. We found that Wnt and Notch co-stimulation could maintain the cells in an undifferentiated-like, proliferative state, whereas bone morphogenetic protein 4 induced an ‘indeterminate' differentiation phenotype characterized by simultaneous expression of glial and neuronal markers. Multi-parameter analysis of responses to conflicting signals revealed interactions more complex than previously envisaged including dominance relations that may reflect a cell-intrinsic system for robust specification of responses in complex microenvironments.

Neural stem cell systems: diversities and properties after transplantation in animal models of diseases

Currently available effective treatments of the diseased or damaged central nervous system (CNS) are restricted to a limited pharmacological relief of symptoms or those given to avoid further damage. Therefore the search is on for treatments that can restore function in the CNS. During recent years replacement of damaged neurons by cell transplantation is being enthusiastically explored as a potential treatment for many neurodegenerative diseases, stroke and traumatic brain injury. Several references in both scientific journals and popular newspapers concerning different types of cultured stem cells, potentially exploitable to treat pathological conditions of the brain, raise important questions pertinent to the fundamental and realistic differences between grafts of primary neural cells and the transplantation of in vitro expanded neural stem cells (NSCs). Our aim is to review the available information on the grafting of different NSC types into the adult rodent brain, focusing on critical aspects for the development of clinical therapies to replace damaged neurons.

Cell cycle and cell death regulation of neural progenitor cells in the 5-azacytidine (5AzC)-treated developing fetal brain

In the developing brain, neural progenitor cells are susceptible to many extrinsic stresses, including DNA damage. We treated pregnant rats with 5-azacytidine (5AzC), a DNA demethylating and damaging agent, to investigate the cellular responses of the fetal brain, focusing on the regulation of proliferation and cell death. 5AzC first induced the accumulation of cells in abnormal mitosis, G2-phase accumulation, and then apoptosis of the neural progenitor cells. Most of the apoptotic cells were in G1 phase. Cell cycle transition studies suggested that G2/M progression was blocked, after which the cells moved to G1 phase or underwent apoptosis. p53, a key factor for response to DNA damage, and some of its target genes showed increased expression in Western blot and DNA microarray analyses. In 5AzC-treated fetal brains of p53-deficient mice, apoptosis did not occur, although G2/M accumulation was induced. These results suggest that, in the developing brain, apoptosis is p53-dependent but that another mechanism governs the G2/M checkpoint. The G2/M regulator, Cdc2, was activated by dephosphorylation through G2/M accumulation, suggesting accelerated entry into mitosis leading to accumulation of cells showing abnormal mitosis. Furthermore, some cells may have died due to mitotic catastrophe. Throughout brain development, various cell cycle and cell death regulation mechanisms provide neural progenitor cells with options for defense from DNA damage.

Brg1 is required for murine neural stem cell maintenance and gliogenesis

Epigenetic alterations in cell-type-specific gene expression control the transition of neural stem cells (NSCs) from predominantly neurogenic to predominantly gliogenic phases of differentiation, but how this switch occurs is unclear. Here, we show that brahma-related gene 1 (Brg1), an ATP-dependent chromatin remodeling factor, is required for the repression of neuronal commitment and the maintenance of NSCs in a state that permits them to respond to gliogenic signals. Loss of Brg1 in NSCs in conditional brg1 mutant mice results in precocious neuronal differentiation, such that cells in the ventricular zone differentiate into post-mitotic neurons before the onset of gliogenesis. As a result, there is a dramatic failure of astrocyte and oligodendrocyte differentiation in these animals. The ablation of brg1 in gliogenic progenitors in vitro also prevents growth-factor-induced astrocyte differentiation. Furthermore, proteins implicated in the maintenance of stem cells, including Sox1, Pax6 and Musashi-1, are dramatically reduced in the ventricular zones of brg1 mutant mice. We conclude that Brg1 is required to repress neuronal differentiation in NSCs as a means of permitting glial cell differentiation in response to gliogenic signals, suggesting that Brg1 regulates the switch from neurogenesis to gliogenesis.

Suppression of Stat3 promotes neurogenesis in cultured neural stem cells

To investigate the effects of signal transducer and activator of transcription 3 (Stat3) on neural stem cell fate, stem cells were inoculated with an adenovirus vector expressing dominant negative form of Stat3 (Stat3F). One day later, a promoter assay revealed significant reduction of the transcriptional level in the transfected cells. Three days later, Western blot analysis and immunocytochemical analysis revealed that the protein level of microtubule-associated protein (MAP)2 and the number of MAP2-positive cells were increased significantly in the transfected cells whereas the protein level of glial fibrillary acidic protein (GFAP) and the number of GFAP-positive cells were decreased significantly. In addition, mRNA levels of Notch family members (Notch1, 2, and 3) and of inhibitory basic helix-loop-helix (bHLH) factors (Hes5, Id2, and Id3) were significantly downregulated at 3 days after viral inoculation with Stat3F; however, mRNA levels of bHLH determination factors (Math1 and Neurogenin3) and bHLH differentiation factors (NeuroD1 and NeuroD2) were significantly upregulated. These data indicated that suppression of Stat3 directly induced neurogenesis and inhibited astrogliogenesis in neural stem cells.

Stem cells in postnatal myogenesis: molecular mechanisms of satellite cell quiescence, activation and replenishment

Satellite cells are the primary stem cells in adult skeletal muscle, and are responsible for postnatal muscle growth, hypertrophy and regeneration. In mature muscle, most satellite cells are in a quiescent state, but they activate and begin proliferating in response to extrinsic signals. Following activation, a subset of satellite cell progeny returns to the quiescent state during the process of self-renewal. Here, we review recent studies of satellite cell biology and focus on the key transitions from the quiescent state to the state of proliferative activation and myogenic lineage progression and back to the quiescent state. The molecular mechanisms of these transitions are considered in the context of the biology of the satellite cell niche, changes with age, and interactions with established pathways of myogenic commitment and differentiation.

DNA methylation controls the timing of astrogliogenesis through regulation of JAK-STAT signaling

DNA methylation is a major epigenetic factor that has been postulated to regulate cell lineage differentiation. We report here that conditional gene deletion of the maintenance DNA methyltransferase I (Dnmt1) in neural progenitor cells (NPCs) results in DNA hypomethylation and precocious astroglial differentiation. The developmentally regulated demethylation of astrocyte marker genes as well as genes encoding the crucial components of the gliogenic JAK-STAT pathway is accelerated in Dnmt1-/- NPCs. Through a chromatin remodeling process, demethylation of genes in the JAK-STAT pathway leads to an enhanced activation of STATs, which in turn triggers astrocyte differentiation. Our study suggests that during the neurogenic period, DNA methylation inhibits not only astroglial marker genes but also genes that are essential for JAK-STAT signaling. Thus, demethylation of these two groups of genes and subsequent elevation of STAT activity are key mechanisms that control the timing and magnitude of astroglial differentiation.

A positive autoregulatory loop of Jak-STAT signaling controls the onset of astrogliogenesis

During development of the CNS, neurons and glia are generated in a sequential manner. The mechanism underlying the later onset of gliogenesis is poorly understood, although the cytokine-induced Jak-STAT pathway has been postulated to regulate astrogliogenesis. Here, we report that the overall activity of Jak-STAT signaling is dynamically regulated in mouse cortical germinal zone during development. As such, activated STAT1/3 and STAT-mediated transcription are negligible at early, neurogenic stages, when neurogenic factors are highly expressed. At later, gliogenic periods, decreased expression of neurogenic factors causes robust elevation of STAT activity. Our data demonstrate a positive autoregulatory loop whereby STAT1/3 directly induces the expression of various components of the Jak-STAT pathway to strengthen STAT signaling and trigger astrogliogenesis. Forced activation of Jak-STAT signaling leads to precocious astrogliogenesis, and inhibition of this pathway blocks astrocyte differentiation. These observations suggest that autoregulation of the Jak-STAT pathway controls the onset of astrogliogenesis.

Neurodevelopmental Toxicology

The fields of neurotoxicology and developmental toxicology are exploding in research and interest. Much of the data currently known are from epidemiologic human studies or studies of animal models. Each of these modes is difficult to translate to individual clinical encounters. It is often difficult to state with certainty which of the numerous chemical or physical agents in our environment are neurotoxic. Basic scientists will help with advances in molecular biology and toxicology. Improved clinical understanding of these issues may help patients to understand the medical issues; allay feelings of anxiety, guilt, or fear; and avoid unnecessary testing. For exposures that manifest as threshold phenomena, such as lead, the risk to society is even greater than to an individual. Individual risk may be less of a concern than the population's risk because small elevations in the average BLL can cause profound shifts in the normative curve of intelligence, increasing the burden on our institutions and bankrupting the brain trust. Good scholarship and interpersonal judgement are vital when counseling patients on the potential consequences of chemical exposures and are no less important when making policy. The challenge for the clinician reading the research is to remain aware of the limitations and biases of our science.

Delivery systems for small molecule drugs, proteins, and DNA: the neuroscience/biomaterial interface

Manipulation of cellular processes in vivo by the delivery of drugs, proteins or DNA is of paramount importance to neuroscience research. Methods for the presentation of these molecules vary widely, including direct injection (either systemic or stereotactic), osmotic pump-mediated chronic delivery, or even implantation of cells engineered to indefinitely secrete a factor of interest. Biomaterial-based delivery systems represent an alternative to more traditional approaches, with the possibility of increased efficacy. Drug-releasing biomaterials, either as injectable microspheres or as three-dimensional implants, can deliver a molecule of interest (including small molecule drugs, biologically active proteins, or DNA) over a more prolonged period of time than by standard bolus injection, avoiding the need for repeated administration. Furthermore, sustained-release systems can maintain therapeutic concentrations at a target site, thus reducing the chance for toxicity. This review summarizes applications of polymer-based delivery of small molecule drugs, proteins, and DNA specifically relevant to neuroscience research. We detail the fabrication procedures for the polymeric systems and their utility in various experimental models. The biomaterial field offers unique experimental tools with downstream clinical application for the study and treatment of neurologic disease.

Mapping spatio-temporal activation of Notch signaling during neurogenesis and gliogenesis in the developing mouse brain

Notch1 plays various important roles including the maintenance of the stem cell state as well as the promotion of glial fates in mammalian CNS development. However, because of the very low amount of the activated form of Notch1 present in vivo, its precise activation pattern has remained unknown. In this study, we mapped the active state of this signaling pathway in situ in the developing mouse brain using a specific antibody that recognizes the processed form of the intracellular domain of Notch1 cleaved by presenilin/gamma-secretase activity. By using this antibody, active state of Notch1 came to be detectable with a higher sensitivity than using conventional antibody against Notch1. We found that activated Notch1 was mainly detected in the nuclei of a subpopulation of radial glial cells, the majority of proliferating precursor cells in the ventricular zone (VZ). However, Notch1 activation was not detected in neuronal precursor cells positive for neuronal basic helix-loop-helix proteins or in differentiating neurons in the embryonic forebrain. Interestingly, we found that Notch1 was transiently activated in the astrocytic lineage during perinatal CNS development. Taken together, the present method has enabled us to determine the timing, gradients, and boundaries of the activation of Notch signaling.

Progress in cerebral transplantation of expanded neuronal stem cells

OBJECT

Given the success and limitations of human fetal primary neural tissue transplantation, neuronal stem cells (NSCs) that can be adequately expanded in culture have been the focus of numerous attempts to develop a superior source of replacement cells for restorative neurosurgery. To clarify recent progress toward this goal, the transplantation into the adult brain of NSCs, expanded in vitro before grafting, was reviewed.

METHODS

Neuronal stem cells can be expanded from a variety of sources, including embryos, fetuses, adult bone marrow, and adult brain tissue. Recent investigations of each of these expanded stem cell types have generated a large body of information along with a great number of unanswered questions regarding the ability of these cells to replace damaged neurons. Expanded NSCs offer many advantages over their primary tissue predecessors, but also may exhibit different functional abilities as grafted cells. Because expanded NSCs will most likely ultimately replace primary tissue grafting in clinical trials, this review was undertaken to focus solely on this distinct body of work and to summarize clearly the existing preclinical data regarding the in vivo successes, limits, and unknowns of using each expanded NSC type when transplanted into the adult brain.

CONCLUSIONS

Embryonic stem cell-derived cells have demonstrated appropriate neuronal phenotypes after transplantation into nonneurogenic areas of the adult brain. Understanding the mechanisms responsible for this may lead to similar success with less studied adult neuronal progenitor cells, which offer the potential for autologous NSC transplantation with less risk of tumorigenesis.