Radial 'glial' progenitors: neurogenesis and signaling

SARM1 deletion inhibits astrogliosis and BBB damage through Jagged-1/Notch-1/NF-κB signaling to improve neurological function after ischemic stroke

Reactive astrogliosis is a critical process in the development of ischemic stroke. However, the precise mechanism by which reactive astrogliosis changes the pathogenesis of ischemic stroke remains elusive. Sterile alpha and TIR motif-containing 1 protein (SARM1) plays a key role in axonal degeneration and is involved in different cell death programs that regulate neuronal survival. The present study investigated the role of SARM1 in regulating reactive astrogliosis and neurological function after stroke in whole-body SARM1 knockout (SARM1-/-) mice. SARM1-/- mice showed significantly smaller infarction, slighter apoptosis, and fewer neurological function deficits 1-7 days after ischemic injury. Immunohistochemistry, western blot, and real-time PCR analyses revealed that compared with the wild-type (WT) mice, SARM1-/- mice exhibited reduced astrocytic proliferation, increased anti-inflammatory astrocytes, decreased glial scar formation in the infarct zone on day 7 after ischemic injury. SARM1 deletion also suppressed cerebral microvascular damage and blood-brain barrier (BBB) injury in ischemic brains. Mechanistically, SARM1 deletion inhibited the stroke-triggered activation of NF-κB signaling and decreased the expression of Jagged-1 and NICD in astrocytes. Overall, these findings provide the first line of evidence for a causative role of SARM1 protein in ischemia-induced reactive astrogliosis and ischemic neurovascular damage.

Brain stars take the lead during critical periods of early postnatal brain development: relevance of astrocytes in health and mental disorders

In the brain, astrocytes regulate shape and functions of the synaptic and vascular compartments through a variety of released factors and membrane-bound proteins. An imbalanced astrocyte activity can therefore have drastic negative impacts on brain development, leading to the onset of severe pathologies. Clinical and pre-clinical studies show alterations in astrocyte cell number, morphology, molecular makeup and astrocyte-dependent processes in different affected brain regions in neurodevelopmental (ND) and neuropsychiatric (NP) disorders. Astrocytes proliferate, differentiate and mature during the critical period of early postnatal brain development, a time window of elevated glia-dependent regulation of a proper balance between synapse formation/elimination, which is pivotal in refining synaptic connectivity. Therefore, any intrinsic and/or extrinsic factors altering these processes during the critical period may result in an aberrant synaptic remodeling and onset of mental disorders. The peculiar bridging position of astrocytes between synaptic and vascular compartments further allows them to "compute" the brain state and consequently secrete factors in the bloodstream, which may serve as diagnostic biomarkers of distinct healthy or disease conditions. Here, we collect recent advancements regarding astrogenesis and astrocyte-mediated regulation of neuronal network remodeling during early postnatal critical periods of brain development, focusing on synapse elimination. We then propose alternative hypotheses for an involvement of aberrancies in these processes in the onset of ND and NP disorders. In light of the well-known differential prevalence of certain brain disorders between males and females, we also discuss putative sex-dependent influences on these neurodevelopmental events. From a translational perspective, understanding age- and sex-dependent astrocyte-specific molecular and functional changes may help to identify biomarkers of distinct cellular (dys)functions in health and disease, favouring the development of diagnostic tools or the selection of tailored treatment options for male/female patients.

Endocannabinoids are potential inhibitors of glioblastoma multiforme proliferation

Globally, it is evident that glioblastoma multiforme (GBM) is an aggressive malignant cancer with a high mortality rate and no effective treatment options. Glioblastoma is classified as the stage-four progression of a glioma tumor, and its diagnosis results in a shortened life expectancy. Treatment options for GBM include chemotherapy, immunotherapy, surgical intervention, and conventional pharmacotherapy; however, at best, they extend the patient's life by a maximum of 5 years. GBMs are considered incurable due to their high recurrence rate, despite various aggressive therapeutic approaches which can have many serious adverse effects. Ceramides, classified as endocannabinoids, offer a promising novel therapeutic approach for GBM. Endocannabinoids may enhance the apoptosis of GBM cells but have no effect on normal healthy neural cells. Cannabinoids promote atypical protein kinase C, deactivate fatty acid amide hydrolase enzymes, and activate transient receptor potential vanilloid 1 (TRPV1) and TRPV2 to induce pro-apoptotic signaling pathways without increasing endogenous cannabinoids. In previous in vivo studies, endocannabinoids, chemically classified as amide formations of oleic and palmitic acids, have been shown to increase the pro-apoptotic activity of human cancer cells and inhibit cell migration and angiogenesis. This review focuses on the biological synthesis and pharmacology of endogenous cannabinoids for the enhancement of cancer cell apoptosis, which have potential as a novel therapy for GBM. Please cite this article as: Duzan A, Reinken D, McGomery TL, Ferencz N, Plummer JM, Basti MM. Endocannabinoids are potential inhibitors of glioblastoma multiforme proliferation. J Integr Med. 2023; Epub ahead of print.

An insight on established retinal injury mechanisms and prevalent retinal stem cell activation pathways in vertebrate models

Implementing different tools and injury mechanisms in multiple animal models of retina regeneration, researchers have discovered the existence of retinal stem/progenitor cells. Although they appear to be distributed uniformly across the vertebrate lineage, the reparative potential of the retina is mainly restricted to lower vertebrates. Regenerative repair post-injury requires the creation of a proliferative niche, vital for proper stem cell activation, propagation, and lineage differentiation. This seems to be lacking in mammals. Hence, in this review, we first discuss the many forms of retinal injuries that have been generated using animal models. Next, we discuss how they are utilized to stimulate regeneration and mimic eye disease pathologies. The key to driving stem cell activation in mammals relies on the information we can gather from these models. Lastly, we present a brief update about the genes, growth factors, and signaling pathways that have been brought to light using these models.

The potential of mechanistic information organised within the AOP framework to increase regulatory uptake of the developmental neurotoxicity (DNT) in vitro battery of assays

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Current in vivo DNT testing for regulatory purposes is not effective.

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In vitro assays anchored to key neurodevelopmental processes are available.

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Development of Adverse Outcome Pathways is required to increase mechanistic understanding of DNT effects.

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DNT Integrated Approaches to Testing and Assessment for various regulatory purposes should be developed.

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The OECD Guidance Document on use of in vitro DNT battery of assays is currently under development.

A major challenge in regulatory developmental neurotoxicity (DNT) assessment is lack of toxicological information for many compounds. Therefore, the Test Guidelines programme of the Organisation for Economic Cooperation and Development (OECD) took the initiative to coordinate an international collaboration between diverse stakeholders to consider integration of alternative approaches towards improving the current chemical DNT testing. During the past few years, a series of workshops was organized during which a consensus was reached that incorporation of a DNT testing battery that relies on in vitro assays anchored to key neurodevelopmental processes should be developed. These key developmental processes include neural progenitor cell proliferation, neuronal and oligodendrocyte differentiation, neural cell migration, neurite outgrowth, synaptogenesis and neuronal network formation, as well key events identified in the existing Adverse Outcome Pathways (AOPs). AOPs deliver mechanistic information on the causal links between molecular initiating event, intermediate key events and an adverse outcome of regulatory concern, providing the biological context to facilitate development of Integrated Approaches to Testing and Assessment (IATA) for various regulatory purposes. Developing IATA case studies, using mechanistic information derived from AOPs, is expected to increase scientific confidence for the use of in vitro methods within an IATA, thereby facilitating regulatory uptake. This manuscript summarizes the current state of international efforts to enhance DNT testing by using an in vitro battery of assays focusing on the role of AOPs in informing the development of IATA for different regulatory purposes, aiming to deliver an OECD guidance document on use of in vitro DNT battery of assays that include in vitro data interpretation.

Role of adherens junctions and apical-basal polarity of neural stem/progenitor cells in the pathogenesis of neurodevelopmental disorders: a novel perspective on congenital Zika syndrome

Radial glial cells (RGCs) are the neural stem/progenitor cells (NSPCs) that give rise to most of neurons and glial cells that constitute the adult central nervous system. A hallmark of RGCs is their polarization along the apical-basal axis. They extend a long basal process that contacts the pial surface and a short apical process to the ventricular surface. Adherens junctions (AJs) are organized as belt-like structures at the most-apical lateral plasma membrane of the apical processes. These junctional complexes anchor RGCs to each other and allow the recruitment of cytoplasmic proteins that act as apical-basal determinants. It has been proposed that disruption of AJs underlies the onset of different neurodevelopmental disorders. In fact, studies performed in different animal models indicate that loss of function of AJs-related proteins in NSPCs can disrupt cell polarity, imbalance proliferation and/or differentiation rates and increase cell death, which, in turn, lead to disruption of the cytoarchitecture of the ventricular zone, protrusion of non-polarized cells into the ventricles, cortical thinning, and ventriculomegaly/hydrocephalus, among other neuropathological findings. Recent Zika virus (ZIKV) outbreaks and the high comorbidity of ZIKV infection with congenital neurodevelopmental defects have led to the World Health Organization to declare a public emergency of international concern. Thus, noteworthy advances have been made in clinical and experimental ZIKV research. This review summarizes the current knowledge regarding the function of AJs in normal and pathological corticogenesis and focuses on the neuropathological and cellular mechanisms involved in congenital ZIKV syndrome, highlighting the potential role of cell-to-cell junctions between NSPCs in the etiopathogenesis of such syndrome.

De Novo Mutation in Genes Regulating Neural Stem Cell Fate in Human Congenital Hydrocephalus

Congenital hydrocephalus (CH), featuring markedly enlarged brain ventricles, is thought to arise from failed cerebrospinal fluid (CSF) homeostasis and is treated with lifelong surgical CSF shunting with substantial morbidity. CH pathogenesis is poorly understood. Exome sequencing of 125 CH trios and 52 additional probands identified three genes with significant burden of rare damaging de novo or transmitted mutations: TRIM71 (p = 2.15 × 10-7), SMARCC1 (p = 8.15 × 10-10), and PTCH1 (p = 1.06 × 10-6). Additionally, two de novo duplications were identified at the SHH locus, encoding the PTCH1 ligand (p = 1.2 × 10-4). Together, these probands account for ∼10% of studied cases. Strikingly, all four genes are required for neural tube development and regulate ventricular zone neural stem cell fate. These results implicate impaired neurogenesis (rather than active CSF accumulation) in the pathogenesis of a subset of CH patients, with potential diagnostic, prognostic, and therapeutic ramifications.

Role of drug transporters and heat shock proteins during ethanol exposure to human neural precursor cells and its lineages

INTRODUCTION

Ethanol exposure to developing brain may alter the growth and differentiation of neurological cells resulting in unfavorable pathologies. Earlier studies have provided very limited mechanistic insights of cellular and molecular mechanisms which do not mimic with human situation due to varying cell types and poses potential challenges for investigation. Therefore, the present study was undertaken to evaluate the role of ABC transporters and heat shock proteins mediated response in human neural precursor cells (NPCs) and its lineages during proliferation and lineage differentiation against ethanol exposure.

METHODS

Effect of ethanol exposure was examined for neuronal cell survival and variation in cellular phenotype during neurospheres development and lineage differentiation. Generation of reactive oxygen species, and variation in cell cycle was identified along with transcriptional profiling for pluripotent markers (Nestin, NCAM, Sox-2, and Notch-2), drug transporters (ABCB1 and ABCG2) and stress protein (HSP70) during ethanol exposure.

RESULTS

ABC transporters as well as HSP70 mRNA expression was higher during proliferation as compared to differentiation with chronic ethanol (1 M) exposure (p < 0.01). Ethanol exposure resulted in higher variability in size and shape of developing neurospheres and decreased ability to form new neurosphere colonies. Significant changes were observed in dendrite development due to late ethanol exposure (p < 0.0001).

CONCLUSION

The present study demonstrated significant role of ABC transporters and HSP70 proteins in providing defense against ethanol-induced damage in human neurological cells. However, the over-expression of ABC transporter and HSP-70 proteins during such pathological conditions do not provide complete defense and additional strategies are required to repair the damage.

Identification of Radial Glia Progenitors in the Developing and Adult Retina of Sharks

Neural stem cells give rise to transient progenitors termed neuroepithelial cells (NECs) and radial glial cells (RGCs). RGCs represent the major source of neurons, glia and adult stem cells in several regions of the central nervous system (CNS). RGCs are mostly transient in mammals, but they are widely maintained in the adult CNS of fishes, where they continue to be morphologically similar to RGCs in the mammalian brain and fulfill similar roles as progenitors and guide for migrating neurons. The retina of fishes offers an exceptional model to approach the study of adult neurogenesis because of the presence of constitutive proliferation from the ciliary marginal zone (CMZ), containing NECs, and from adult glial cells with radial morphology (the Müller glia). However, the cellular hierarchies and precise contribution of different types of progenitors to adult neurogenesis remain unsolved. We have analyzed the transition from NECs to RGCs and RGC differentiation in the retina of the cartilaginous fish Scyliorhinus canicula, which offers a particularly good spatial and temporal frame to investigate this process. We have characterized progenitor and adult RGCs by immunohistochemical detection of glial markers as glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS). We have compared the emergence and localization of glial markers with that of proliferating cell nuclear antigen (PCNA, a proliferation maker) and Doublecortin (DCX, which increases at early stages of neuronal differentiation). During retinal development, GFAP-immunoreactive NECs located in the most peripheral CMZ (CMZp) codistribute with DCX-immunonegative cells. GFAP-immunoreactive RGCs and Müller cells are located in successive more central parts of the retina and codistribute with DCX- and DCX/GS-immunoreactive cells, respectively. The same types of progenitors are found in juveniles, suggesting that the contribution of the CMZ to adult neurogenesis implies a transition through the radial glia (RG) state.

Emerging roles of Axin in cerebral cortical development

Proper functioning of the cerebral cortex depends on the appropriate production and positioning of neurons, establishment of axon–dendrite polarity, and formation of proper neuronal connectivity. Deficits in any of these processes greatly impair neural functions and are associated with various human neurodevelopmental disorders including microcephaly, cortical heterotopias, and autism. The application of in vivo manipulation techniques such as in utero electroporation has resulted in significant advances in our understanding of the cellular and molecular mechanisms that underlie neural development in vivo. Axin is a scaffold protein that regulates neuronal differentiation and morphogenesis in vitro. Recent studies provide novel insights into the emerging roles of Axin in gene expression and cytoskeletal regulation during neurogenesis, neuronal polarization, and axon formation. This review summarizes current knowledge on Axin as a key molecular controller of cerebral cortical development.

Cyp19a1 (aromatase) expression in the Xenopus brain at different developmental stages

Cytochrome P450 aromatase (P450arom; aromatase) is a microsomal enzyme involved in the production of endogeneous sex steroids by converting testosterone into oestradiol. Aromatase is the product of the cyp19a1 gene and plays a crucial role in the sexual differentiation of the brain and in the regulation of reproductive functions. In the brain of mammals and birds, expression of cyp19a1 has been demonstrated in neuronal populations of the telencephalon and diencephalon. By contrast, a wealth of evidence established that, in teleost fishes, aromatase expression in the brain is restricted to radial glial cells. The present study investigated the precise neuroanatomical distribution of cyp19a1 mRNA during brain development in Xenopus laevis (late embryonic to juvenile stages). For this purpose, we used in situ hybridisation alone or combined with the detection of a proliferative (proliferating cell nuclear antigen), glial (brain lipid binding protein, Vimentin) or neuronal (acetylated tubulin; HuC/D; NeuroβTubulin) markers. We provide evidence that cyp19a1 expression in the brain is initiated from the very early larval stage and remains strongly detected until the juvenile and adult stages. At all stages analysed, we found the highest expression of cyp19a1 in the preoptic area and the hypothalamus compared to the rest of the brain. In these two brain regions, cyp19a1-positive cells were never detected in the ventricular layers. Indeed, no co-labelling could be observed with radial glial (brain lipid binding protein, Vimentin) or dividing progenitors (proliferating cell nuclear antigen) markers. By contrast, cyp19a1-positive cells perfectly matched with the distribution of post-mitotic neurones as shown by the use of specific markers (HuC/D, acetylated tubulin and NeuroβTubulin). These data suggest that, similar to that found in other tetrapods, aromatase in the brain of amphibians is found in post-mitotic neurones and not in radial glia as reported in teleosts.

On the Genesis of Neuroblastoma and Glioma

As the emergence of cancer is most frequent in proliferating tissues, replication errors are considered to be at the base of this disease. This review concentrates mainly on two neural cancers, neuroblastoma and glioma, with completely different backgrounds that are well documented with respect to their ontogeny. Although clinical data on other cancers of the nervous system are available, usually little can be said about their origins. Neuroblastoma is initiated in the embryo at a moment when the nervous system (NS) is in full expansion and occasionally genomic damage can lead to neoplasia. Glioma, to the contrary, occurs in the adult brain supposed to be mostly in a postmitotic state. According to current consensus, neural stem cells located in the subventricular zone (SVZ) in the adult are thought to accumulate enough genomic mutations to diverge on a carcinogenic course leading to diverse forms of glioma. After weighing the pros and cons of this current hypothesis in this review, it will be argued that this may be improbable, yielding to the original old concept of glial origin of glioma.

Development of cerebellar neurons and glias revealed by in utero electroporation: Golgi-like labeling of cerebellar neurons and glias

Cerebellar cortical functions rely on precisely arranged cytoarchitectures composed of several distinct types of neurons and glias. Studies have indicated that cerebellar excitatory and inhibitory neurons have distinct spatial origins, the upper rhombic lip (uRL) and ventricular zone (VZ), respectively, and that different types of neurons have different birthdates. However, the spatiotemporal relationship between uRL/VZ progenitors and their final phenotype remains poorly understood due to technical limitations. To address this issue, we performed in utero electroporation (IUE) of fluorescent protein plasmids using mouse embryos to label uRL/VZ progenitors at specific developmental stages, and observed labeled cells at maturity. To overcome any potential dilution of the plasmids caused by progenitor division, we also utilized constructs that enable permanent labeling of cells. Cerebellar neurons and glias were labeled in a Golgi-like manner enabling ready identification of labeled cells. Five types of cerebellar neurons, namely Purkinje, Golgi, Lugaro and unipolar brush cells, large-diameter deep nuclei (DN) neurons, and DN astrocytes were labeled by conventional plasmids, whereas plasmids that enable permanent labeling additionally labeled stellate, basket, and granule cells as well as three types of glias. IUE allows us to label uRL/VZ progenitors at different developmental stages. We found that the five types of neurons and DN astrocytes were labeled in an IUE stage-dependent manner, while stellate, basket, granule cells and three types of glias were labeled regardless of the IUE stage. Thus, the results indicate the IUE is an efficient method to track the development of cerebellar cells from uRL/VZ progenitors facing the ventricular lumen. They also indicate that while the generation of the five types of neurons by uRL/VZ progenitors is regulated in a time-dependent manner, the progenitor pool retains multipotency throughout embryonic development.

BACE1 regulates hippocampal astrogenesis via the Jagged1-Notch pathway

BACE1 is the sole secretase for generating β-amyloid (Aβ) in vivo and is being actively pursued as a drug target for the treatment of Alzheimer's disease. Transmembrane BACE1 exerts its biological activity by cleaving its membrane-bound cellular substrates. Here, we reveal that BACE1 directly regulates the level of membrane-anchored full-length Jagged1 (Jag1), a signaling molecule important for the control of neurogenesis and astrogenesis, via interaction with its cognate Notch receptor. We show that shedding of Jag1 is reduced in BACE1 null mice and upregulated Jag1 enhances Notch signaling via cell-cell juxtacrine interactions. Additional biochemical assays confirmed that overexpression of BACE1 enhanced cleavage of Jag1. Consequently, BACE1 null mice exhibit a significant increase in astrogenesis with a corresponding decrease in neurogenesis in their hippocampi during early development. Hence, BACE1 appears to function as a signaling protease that controls the balance of neurogenesis and astrogenesis via the Jag1-Notch pathway.

Bergmann glia function in granule cell migration during cerebellum development

Granule cell migration influences the laminar structure of the cerebellum and thereby affects cerebellum function. Bergmann glia are derived from radial glial cells and aid in granule cell radial migration by providing a scaffold for migration and by mediating interactions between Bergmann glia and granule cells. In this review, we summarize Bergmann glia characteristics and the mechanisms underlying the effect of Bergmann glia on the radial migration of granule neurons in the cerebellum. Furthermore, we will focus our discussion on the important factors involved in glia-mediated radial migration so that we may elucidate the possible mechanistic pathways used by Bergmann glia to influence granule cell migration during cerebellum development.

Bergmann glia function in granule cell migration during cerebellum development

Granule cell migration influences the laminar structure of the cerebellum and thereby affects cerebellum function. Bergmann glia are derived from radial glial cells and aid in granule cell radial migration by providing a scaffold for migration and by mediating interactions between Bergmann glia and granule cells. In this review, we summarize Bergmann glia characteristics and the mechanisms underlying the effect of Bergmann glia on the radial migration of granule neurons in the cerebellum. Furthermore, we will focus our discussion on the important factors involved in glia-mediated radial migration so that we may elucidate the possible mechanistic pathways used by Bergmann glia to influence granule cell migration during cerebellum development.

Neurosteroidogenic enzymes and their regulation in the early brain of the protogynous grouper Epinephelus coioides during gonadal sex differentiation

The regulatory role of neurosteroids in the early brain during gonadal sex differentiation is unclear. The aim of this study was to investigate the expression and cellular localization of key steroidogenic enzymes in the early brain of the protogynous orange-spotted grouper Epinephelus coioides and the temporal expressions has been correlated with gonadal sex differentiation. In this study, we showed that peak neurosteroidogenesis occurs in the early brain during gonadal sex differentiation. The temporal expressions of key enzymes, cyp11a1 (cytochrome P450 side chain cleavage), hsd3b1 (3β-hydroxysteroid dehydrogenase) and cyp17a1 (cytochrome P450c17) were studied at different developmental ages (from 90- to 150-dah: days after hatching) using quantitative real-time PCR (q-PCR). q-PCR analysis indicated that the transcript expressions of cyp11a1, hsd3b1 and cyp17a1 were increased in the brain around the period of gonadal sex differentiation. Further, in situ hybridization (ISH) analysis showed that cyp11a1, hsd3b1 and cyp17a1 transcripts were widely expressed in several discrete brain regions, especially the intense expression in the forebrain, with an overall similar expression pattern. High density in the cyp19a1b/Cyp19a1b expression was detected in radial glial cells. Thus, the expression of grouper cyp19a1b/Cyp19a1b is restricted to radial glial cells, suggesting estrogens can modulate their activity. Next, by combining Cyp19a1b immunohistochemistry (IHC) with florescence ISH (FISH) of cyp11a1, hsd3b1 and cyp17a1, we showed that sub-cellular localization of cyp11a1, hsd3b1 and cyp17a1 transcripts, in partial, appeared to be in Cyp19a1b radial glial cell soma. Further, exogenous estradiol (E(2)) increased the expression of cyp17a1 and cyp19a1b/Cyp19a1b in the brain of grouper. Consequently, our results illustrated that the locally synthesized E(2) upregulated neurosteroidogenic enzymes in the early brain and suggest a role for these enzymes in the neurogenic process during gonadal sex differentiation.

In vitro expansion of fetal neural progenitors as adherent cell lines

In vitro studies of neural progenitors isolated from the developing mouse have provided important insights into intrinsic and extrinsic pathways that control their behavior. However, use of primary cultures or neurospheres established from fetal tissues in cell population-based assays can be compromised by cellular heterogeneity. A complementary approach that addresses this issue is the establishment of adherent clonal neural stem (NS) cell lines. Here I describe protocols and troubleshooting advice for establishing adherent NS cell lines from the mouse fetal forebrain. NS cells grow as pure cultures in defined serum-free conditions as adherent monolayers and are therefore amenable to chemical/genetic screens, biochemical studies, and population-based analysis of gene expression or transcriptional regulation (e.g. RNA-Seq and ChIP-Seq). NS cell lines therefore represent a tractable cellular model system to explore the molecular and cellular biology of neural stem cell self-renewal and differentiation. Similar protocols can be extended to rat and human embryos, as well as human brain tumors.

A cis-element in the Notch1 locus is involved in the regulation of gene expression in interneuron progenitors

Interneurons comprise approximately one third of the total cortical neurons in the mammalian cerebral cortex. Studies have revealed many details in the generation of this cell type. However, the mechanism that defines interneuron-lineage specific gene expression is not well understood. Gene regulatory elements, e.g., promoters, enhancers, and trans-acting factors, are essential for the proper control of gene expression. Here, we report that a novel evolutionarily conserved cis-element in the second intron of the Notch1 locus plays an important role in regulating gene expression in interneuron progenitors. The spatiotemporal activity of the cis-element in the developing central nervous system (CNS) was determined by both transient reporter expression in the developing chick and a transgenic mouse model. Its activity is well correlated with neurogenesis in both the chick and mouse and restricted to neural progenitor cells in the ganglionic eminence that are fated to differentiate into GABAergic interneurons of the neocortex. We further demonstrate that the cis-element activity requires the binding motif for trans-acting factors Gsh1/Barx2/Brn3. Deletion of this binding motif abolishes reporter gene expression. Together, these data provide new insights into the regulatory mechanisms of interneuron development in the vertebrate CNS.

Neuron-astroglial interactions in cell-fate commitment and maturation in the central nervous system

Neuron-astroglia interactions play a key role in several events of brain development, such as neuronal generation, migration, survival, and differentiation; axonal growth; and synapse formation and function. While there is compelling evidence of the effects of astrocyte factors on neurons, their effects on astrocytes have not been fully determined. In this review, we will focus on the role of neurons in astrocyte generation and maturation. Further, we highlight the great heterogeneity and diversity of astroglial and neural progenitors such as radial glia cells, and discuss the importance of the variety of cellular interactions in controlling the structural and functional organization of the brain. Finally, we present recent data on a new role of astrocytes in neuronal maturation, as mediators of the action of biolipids in the cerebral cortex. We will argue that the functional architecture of the brain depends on an intimate neuron-glia partnership, by briefly discussing the emerging view of how neuron-astrocyte dysfunctions might be associated with neurodegenerative diseases and neurological disorders.

Reprogramming of mesenchymal stem cells by the synovial sarcoma-associated oncogene SYT-SSX2

Cell identity is determined by its gene expression programs. The ability of a cell to change its identity and produce cell types outside its lineage is achieved by the activity of transcription controllers capable of reprogramming differentiation gene networks. The synovial sarcoma (SS)-associated protein, SYT-SSX2, reprograms myogenic progenitors and human bone marrow-derived mesenchymal stem cells (BMMSCs) by dictating their commitment to a pro-neural lineage. It fulfills this function by directly targeting an extensive array of neural-specific genes as well as genes of developmental pathway mediators. Concomitantly, the ability of both myoblasts and BMMSCs to differentiate into their normal myogenic and adipogenic lineages was compromised. SS is believed to arise in mesenchymal stem cells where formation of the t(X/18) translocation product, SYT-SSX, constitutes the primary event in the cancer. SYT-SSX is therefore believed to initiate tumorigenesis in its target stem cell. The data presented here allow a glimpse at the initial events that likely occur when SYT-SSX2 is first expressed, and its dominant function in subverting the nuclear program of the stem cell, leading to its aberrant differentiation, as a first step toward transformation. In addition, we identified the fibroblast growth factor receptor gene, Fgfr2, as one occupied and upregulated by SYT-SSX2. Knockdown of FGFR2 in both BMMSCs and SS cells abrogated their growth and attenuated their neural phenotype. These results support the notion that the SYT-SSX2 nuclear function and differentiation effects are conserved throughout sarcoma development and are required for its maintenance beyond the initial phase. They also provide the stem cell regulator, FGFR2, as a promising candidate target for future SS therapy.

RP58/ZNF238 directly modulates proneurogenic gene levels and is required for neuronal differentiation and brain expansion

Although neurogenic pathways have been described in the developing neocortex, less is known about mechanisms ensuring correct neuronal differentiation thus also preventing tumor growth. We have shown that RP58 (aka zfp238 or znf238) is highly expressed in differentiating neurons, that its expression is lost or diminished in brain tumors, and that its reintroduction blocks their proliferation. Mice with loss of RP58 die at birth with neocortical defects. Using a novel conditional RP58 allele here we show that its CNS-specific loss yields a novel postnatal phenotype: microencephaly, agenesis of the corpus callosum and cerebellar hypoplasia that resembles the chr1qter deletion microcephaly syndrome in human. RP58 mutant brains maintain precursor pools but have reduced neuronal and increased glial differentiation. Well-timed downregulation of pax6, ngn2 and neuroD1 depends on RP58 mediated transcriptional repression, ngn2 and neuroD1 being direct targets. Thus, RP58 may act to favor neuronal differentiation and brain growth by coherently repressing multiple proneurogenic genes in a timely manner.

Cobblestone lissencephaly: neuropathological subtypes and correlations with genes of dystroglycanopathies

Cobblestone lissencephaly represents a peculiar brain malformation with characteristic radiological anomalies, defined as cortical dysplasia combined with dysmyelination, dysplastic cerebellum with cysts and brainstem hypoplasia. Cortical dysplasia results from neuroglial overmigration into the arachnoid space, forming an extracortical layer, responsible for agyria and/or 'cobblestone' brain surface and ventricular enlargement. The underlying mechanism is a disruption of the glia limitans, the outermost layer of the brain. Cobblestone lissencephaly is pathognomonic of a continuum of autosomal recessive diseases with cerebral, ocular and muscular deficits, Walker-Warburg syndrome, muscle-eye-brain and Fukuyama muscular dystrophy. Mutations in POMT1, POMT2, POMGNT1, LARGE, FKTN and FKRP genes attributed these diseases to α-dystroglycanopathies. However, studies have not been able to identify causal mutations in the majority of patients and to establish a clear phenotype/genotype correlation. Therefore, we decided to perform a detailed neuropathological survey and molecular screenings in 65 foetal cases selected on the basis of histopathological criteria. After sequencing the six genes of α-dystroglycanopathies, a causal mutation was observed in 66% of cases. On the basis of a ratio of severity, three subtypes clearly emerged. The most severe, which we called cobblestone lissencephaly A, was linked to mutations in POMT1 (34%), POMT2 (8%) and FKRP (1.5%). The least severe, cobblestone lissencephaly C, was linked to POMGNT1 mutations (18%). An intermediary type, cobblestone lissencephaly B, was linked to LARGE mutations (4.5%) identified for the first time in foetuses. We conclude that cobblestone lissencephaly encompasses three distinct subtypes of cortical malformations with different degrees of neuroglial ectopia into the arachnoid space and cortical plate disorganization regardless of gestational age. In the cerebellum, histopathological changes support the novel hypothesis that abnormal lamination arises from a deficiency in granule cells. Our studies demonstrate the positive impact of histoneuropathology on the identification of α-dystroglycanopathies found in 66% of cases, while with neuroimaging criteria and biological values, mutations are found in 32-50% of patients. Interestingly, our morphological classification was central in the orientation of genetic screening of POMT1, POMT2, POMGNT1, LARGE and FKRP. Despite intensive research, one-third of our cases remained unexplained; suggesting that other genes and/or pathways may be involved. This material offers a rich resource for studies on the affected neurodevelopmental processes of cobblestone lissencephaly and on the identification of other responsible gene(s)/pathway(s).

JNK/stress-activated protein kinase associated protein 1 is required for early development of telencephalic commissures in embryonic brains

We previously reported that mice lacking JSAP1 (jsap1-/-) were lethal and the brain of jsap1-/- at E18.5 exhibited multiple types of developmental defects, which included impaired axon projection of the corpus callosum and anterior commissures. In the current study, we examined whether the early telencephalic commissures were formed abnormally from the beginning of initial development or whether they arose normally, but have been progressively lost their maintenance in the absence of JSAP1. The early corpus callosum in the brain of jsap1+/+ at E15.5-E16.5 was found to cross the midline with forming a distinct U-shaped tract, whereas the early axonal tract in jsap1-/- appeared to cross the midline in a diffuse manner, but the lately arriving axons did not cross the midline. In the brain of jsap1-/- at E17.5, the axon terminals of lately arriving collaterals remained within each hemisphere, forming an early Probst's bundle-like shape. The early anterior commissure in the brain of jsap1+/+ at E14.5-E15.5 crossed the midline, whereas the anterior commissure in jsap1-/- developed, but was deviated from their normal path before approaching the midline. The axon tracts of the corpus callosum and anterior commissure in the brain of jsap1-/- at E16.5-E17.5 expressed phosphorylated forms of FAK and JNK, however, their expression levels in the axonal tracts were reduced compared to the respective controls in jsap1+/+. Considering the known scaffolding function of JSAP1 for the FAK and JNK pathways, these results suggest that JSAP1 is required for the pathfinding of the developing telencephalic commissures in the early brains.

Effects of epidermal growth factor receptor blockade on ependymoma stem cells in vitro and in orthotopic mouse models

Some lines of evidence suggest that tumors, including ependymoma, might arise from a subpopulation of cells, termed cancer stem cells (CSCs), with self-renewal and tumor-initiation properties. Given the strict dependence of CSCs on epidermal growth factor (EGF) through EGF receptor (EGFR), we investigated the effects of EGFR inhibitors in ependymoma-stem cells (SCs) in vitro and in orthotopic mouse models. We established two ependymoma-SC lines from two recurrent pediatric ependymoma. Both lines expressed markers of radial glia--the candidate SCs of ependymoma--and showed renewal ability, multipotency, and tumorigenicity after orthotopic implantation, despite markedly different expression of CD133 (94 vs. 6%). High phosphorylated-EGFR/EGFR ratio was detected, which decreased after differentiation. EGFR inhibitors (gefitinib and AEE788) reduced clonogenicity, proliferation and survival of ependymoma-SC lines dose-dependently, and blocked EGF-induced activation of EGFR, Akt and extracellular signal-regulated kinase 1/2. Overall, AEE788 was more effective than gefitinib. EGFR blockade as well as differentiation strongly reduced CD133 expression. However, ex vivo treatment with AEE788 did not impair orthotopic tumor engraftment, whereas ex vivo differentiation did, suggesting that CD133 does not absolutely segregate for tumorigenicity in ependymoma-SCs. Orally administered AEE788 prolonged survival of mice bearing ependymoma-SC-driven orthotopic xenografts from 56 to 63 days, close to statistical significance (log-rank p=0.06). Our study describes for the first time EGFR signaling in ependymoma-SCs and the effects of EGFR blockade in complementary in vitro and in vivo systems. The experimental models we developed can be used to further investigate the activity of EGFR inhibitors or other antineoplastic agents in this tumor.

ErbB2 activation contributes to de-differentiation of astrocytes into radial glial cells following induction of scratch-insulted astrocyte conditioned medium

Radial glial cells play a significant role in the repair of spinal cord injuries as they exert critical role in the neurogenesis and act as a scaffold for neuronal migration. Our previous study showed that mature astrocytes of spinal cord can undergo a de-differentiation process and further transform into pluripotential neural precursors; the occurrence of these complex events arise directly from the induction of diffusible factors released from scratch-insulted astrocytes. However, it is unclear whether astrocytes can also undergo rejuvenation to revert to a radial glial progenitor phenotype after the induction of scratch-insulted astrocytes conditioned medium (ACM). Furthermore, the mechanism of astrocyte de-differentiation to the progenitor cells is still unclear. Here we demonstrate that upon treating mature astrocytes with ACM for 10 days, the astrocytes exhibit progressive morphological and functional conversion to radial glial cells. These changes include the appearance of radial glial progenitor cells, changes in the immunophenotypical profiles, characterized by the co-expression of nestin, paired homeobox protein (Pax6) and RC2 as well as enhanced capability of multipotential differentiation. Concomitantly, ErbB2 protein level was progressively up-regulated. Thereby these results provide a potential mechanism by which ACM could induce mature astrocytes to regain the profile of radial glial progenitors due to activating the ErbB2 signaling pathways.

How Notch establishes longitudinal axon connections between successive segments of the Drosophila CNS

Development of the segmented central nerve cords of vertebrates and invertebrates requires connecting successive neuromeres. Here, we show both how a pathway is constructed to guide pioneer axons between segments of the Drosophila CNS, and how motility of the pioneers along that pathway is promoted. First, canonical Notch signaling in specialized glial cells causes nearby differentiating neurons to extrude a mesh of fine projections, and shapes that mesh into a continuous carpet that bridges from segment to segment, hugging the glial surface. This is the direct substratum that pioneer axons follow as they grow. Simultaneously, Notch uses an alternate, non-canonical signaling pathway in the pioneer growth cones themselves, promoting their motility by suppressing Abl signaling to stimulate filopodial growth while presumably reducing substratum adhesion. This propels the axons as they establish the connection between successive segments.

Sonic hedgehog and notch signaling can cooperate to regulate neurogenic divisions of neocortical progenitors

BACKGROUND

Hedgehog (Hh) signaling is crucial for the generation and maintenance of both embryonic and adult stem cells, thereby regulating development and tissue homeostasis. In the developing neocortex, Sonic Hedgehog (Shh) regulates neural progenitor cell proliferation. During neurogenesis, radial glial cells of the ventricular zone (VZ) are the predominant neocortical progenitors that generate neurons through both symmetric and asymmetric divisions. Despite its importance, relatively little is known of the molecular pathways that control the switch from symmetric proliferative to differentiative/neurogenic divisions in neural progenitors.

PRINCIPAL FINDINGS

Here, we report that conditional inactivation of Patched1, a negative regulator of the Shh pathway, in Nestin positive neural progenitors of the neocortex leads to lamination defects due to improper corticogenesis and an increase in the number of symmetric proliferative divisions of the radial glial cells. Hedgehog-activated VZ progenitor cells demonstrated a concomitant upregulation of Hes1 and Blbp, downstream targets of Notch signaling. The Notch signaling pathway plays a pivotal role in the maintenance of stem/progenitor cells and the regulation of glial versus neuronal identity. To study the effect of Notch signaling on Hh-activated neural progenitors, we inactivated both Patched1 and Rbpj, a transcriptional mediator of Notch signaling, in Nestin positive cells of the neocortex.

CONCLUSIONS

Our data indicate that by mid neurogenesis (embryonic day 14.5), attenuation of Notch signaling reverses the effect of Patched1 deletion on neurogenesis by restoring the balance between symmetric proliferative and neurogenic divisions. Hence, our results demonstrate that correct corticogenesis is an outcome of the interplay between the Hh and Notch signaling pathways.

A tale of tailless

Drosophila Tailless(Tll) and its vertebrate homologue Tlx are conserved orphan nuclear receptors specifically expressed in the eye and the forebrain. Tll and Tlx act primarily as transcriptional repressors through their interactions with transcriptional corepressors, Atrophin family proteins, and histone-tail/chromatin-modifying factors such as lysine-specific histone demethylase 1 and histone deacetylases. The functional importance of Tll and Tlx is made apparent by the recent discovery that they are expressed in neural stem cells (NSCs) and are required for self-renewal of these cells in both Drosophila and the mouse. This review provides a snapshot of current knowledge about Tll and Tlx and their transcriptional network, which maintains NSCs in developing and adult animals.

Identification of the Tctex-1 regulatory element that directs expression to neural stem/progenitor cells in developing and adult brain

Previous studies showed that Tctex-1 immunoreactivity is selectively enriched in the germinal zones of adult brain. In this report we identify a regulatory region of the Tctex-1 gene that is capable of directing transgenic expression of green fluorescent protein (GFP) reporter that recapitulates the spatial and temporal expression pattern of endogenous Tctex-1. This construct specifically targeted expression to the nestin(+)/Pax6(+)/GLAST(+) radial glial cells and Tbr2(+) intermediate progenitors when the reporter construct was delivered to developing mouse neocortex via in utero electroporation. Characterization of mice transgenically expressing GFP under the same regulatory element showed that the GFP expression is faithful to endogenous Tctex-1 at the subgranular zone (SGZ) of dentate gyrus, ventricular/subventricular zone of lateral ventricles, and ependymal layer of 3rd ventricle of adult brains. Immunolocalization and bromodeoxyuridine incorporation studies of adult SGZ in four independent mouse lines showed that Tctex-1:GFP reporter selectively marks nestin(+)/GFAP(+)/Sox2(+) neural stem-like cells in two mouse lines (4 and 13). In two other mouse lines (17 and 18), Tctex-1:GFP is selectively expressed in Type-2 and Type-3 transient amplifying progenitors and a small subset of young neuronal progeny. The P/E-Tctex-1 reporter mouse studies independently confirmed the specific enrichment of Tctex-1 at adult SGZ stem/progenitor cells. Furthermore, these studies supported the notion that an analogous transcriptional program may be used to regulate neurogenesis in embryonic cerebral cortex and adult hippocampus. Finally, the genomic sequences and the reporter mouse lines described here provide useful experimental tools to advance adult neural stem cell research.

Neuronal differentiation distinguishes supratentorial and infratentorial childhood ependymomas

Ependymomas are glial neoplasms occurring in any location throughout the central nervous system and supposedly are derived from radial glia cells. Recent data suggest that these tumors may have different biological and clinical behaviors according to their location. Pediatric supratentorial and infratentorial ependymoma (SE and IE) were compared with respect to clinical and radiological parameters and immunohistochemistry (IHC). Neuronal markers were specifically assessed by IHC and quantitative PCR (qPCR). No single morphological or radiological characteristic was associated with location or any neuronal marker. However, there was a significant overexpression of neuronal markers in SE compared with IE: neurofilament light polypeptide 70 (NEFL)-positive tumor cells were found in 23 of 34 SE and in only 4 of 32 IE (P < .001). Among SE, 10 of 34 exhibited high expression of NEFL, defined as more than 5% positive cells. qPCR confirmed the upregulation of neuronal markers (NEFL, LHX2, FOXG1, TLX1, and NPTXR) in SE compared with IE. In addition, strong NEFL expression in SE was correlated with better progression-free survival (P = .007). Our results support the distinction of SE and IE. SEs are characterized by neuronal differentiation, which seems to be associated with better prognosis.

Emerging topics in Reelin function

Reelin signalling in the early developing cortex regulates radial migration of cortical neurons. Later in development, Reelin promotes maturation of dendrites and dendritic spines. Finally, in the mature brain, it is involved in modulating synaptic function. In recent years, efforts to identify downstream signalling events induced by binding of Reelin to lipoprotein receptors led to the characterization of novel components of the Reelin signalling cascade. In the present review, we first address distinct functions of the Reelin receptors Apoer2 and Vldlr in cortical layer formation, followed by a discussion on the recently identified downstream effector molecule n-cofilin, involved in regulating actin cytoskeletal dynamics required for coordinated neuronal migration. Next, we discuss possible functions of the recently identified Reelin-Notch signalling crosstalk, and new aspects of the role of Reelin in the formation of the dentate radial glial scaffold. Finally, progress in characterizing the function of Reelin in modulating synaptic function in the adult brain is summarized. The present review has been inspired by a session entitled 'Functions of Reelin in the developing and adult hippocampus', held at the Spring Hippocampal Research Conference in Verona/Italy, June 2009.

Cell cycle control of mammalian neural stem cells: putting a speed limit on G1

The potential to increase unlimitedly in number and to generate differentiated cell types is a key feature of somatic stem cells. Within the nervous system, cellular and environmental determinants tightly control the expansion and differentiation of neural stem cells. Importantly, a number of studies have indicated that changes in cell cycle length can influence development and physiopathology of the nervous system, and might have played a role during evolution of the mammalian brain. Specifically, it has been suggested that the length of G1 can directly influence the differentiation of neural precursors. This has prompted the proposal of a model to explain how manipulation of G1 length can be used to expand neural stem cells. If validated in non-neural systems, this model might provide the means to control the proliferation vs. differentiation of somatic stem cells, which will represent a significant advance in the field.

Ethanol inhibition of aspartyl-asparaginyl-beta-hydroxylase in fetal alcohol spectrum disorder: potential link to the impairments in central nervous system neuronal migration

Fetal alcohol spectrum disorder (FASD) is caused by prenatal exposure to alcohol and associated with hypoplasia and impaired neuronal migration in the cerebellum. Neuronal survival and motility are stimulated by insulin and insulin-like growth factor (IGF), whose signaling pathways are major targets of ethanol neurotoxicity. To better understand the mechanisms of ethanol-impaired neuronal migration during development, we examined the effects of chronic gestational exposure to ethanol on aspartyl (asparaginyl)-beta-hydroxylase (AAH) expression, because AAH is regulated by insulin/IGF and mediates neuronal motility. Pregnant Long-Evans rats were pair-fed isocaloric liquid diets containing 0, 8, 18, 26, or 37% ethanol by caloric content from gestation day 6 through delivery. Cerebella harvested from postnatal day 1 pups were used to examine AAH expression in tissue, and neuronal motility in Boyden chamber assays. We also used cerebellar neuron cultures to examine the effects of ethanol on insulin/IGF-stimulated AAH expression, and assess the role of GSK-3beta-mediated phosphorylation on AAH protein levels. Chronic gestational exposure to ethanol caused dose-dependent impairments in neuronal migration and corresponding reductions in AAH protein expression in developing cerebella. In addition, prenatal ethanol exposure inhibited insulin and IGF-I-stimulated directional motility in isolated cerebellar granule neurons. Ethanol-treated neuronal cultures (50mMx96h) also had reduced levels of AAH protein. Mechanistically, we showed that AAH protein could be phosphorylated on Ser residues by GSK-3beta, and that chemical inhibition of GSK-3beta and/or global Caspases increases AAH protein in both control- and ethanol-exposed cells. Ethanol-impaired neuronal migration in FASD is associated with reduced AAH expression. Because ethanol increases the activities of both GSK-3beta and Caspases, the inhibitory effect of ethanol on neuronal migration could be mediated by increased GSK-3beta phosphorylation and Caspase degradation of AAH protein.

Regulation of neural progenitor cell development in the nervous system

The mammalian telencephalon, which comprises the cerebral cortex, olfactory bulb, hippocampus, basal ganglia, and amygdala, is the most complex and intricate region of the CNS. It is the seat of all higher brain functions including the storage and retrieval of memories, the integration and processing of sensory and motor information, and the regulation of emotion and drive states. In higher mammals such as humans, the telencephalon also governs our creative impulses, ability to make rational decisions, and plan for the future. Despite its massive complexity, exciting work from a number of groups has begun to unravel the developmental mechanisms for the generation of the diverse neural cell types that form the circuitry of the mature telencephalon. Here, we review our current understanding of four aspects of neural development. We first begin by providing a general overview of the broad developmental mechanisms underlying the generation of neuronal and glial cell diversity in the telencephalon during embryonic development. We then focus on development of the cerebral cortex, the most complex and evolved region of the brain. We review the current state of understanding of progenitor cell diversity within the cortical ventricular zone and then describe how lateral signaling via the Notch-Delta pathway generates specific aspects of neural cell diversity in cortical progenitor pools. Finally, we review the signaling mechanisms required for development, and response to injury, of a specialized group of cortical stem cells, the radial glia, which act both as precursors and as migratory scaffolds for newly generated neurons.

Comparative analysis of proneural gene expression in the embryonic cerebellum

The embryonic cerebellum contains two germinative epithelia: the rhombic lip and the ventricular zone. While the lineage of glutamatergic neurons arising from the rhombic lip has been characterized, plenty remains to be learned about the factors giving rise to the array of ventricular zone-derived gamma-aminobutyric acid (GABA)ergic neurons. In the present study, we describe the expression of proneural genes Mash1/Ascl1, Ngn1/Neurog1, and Ngn2/Neurog2 in the cerebellar primordium at key stages of Purkinje cell and interneuron development, and compare them with the expression of other genes active in the same context. Our results indicate that Ngn1, Ngn2 and Mash1 are expressed at relevant stages of cerebellar neurogenesis in the prospective cerebellar nuclei and in the ventricular zone, excluding the Math1/Atoh1-positive rhombic lip. Their expression domains are only partially overlapping, suggesting that they may contribute selectively to ventricular zone regionalization, giving rise to the diversity of cerebellar GABA neurons and, possibly, Purkinje cell subtypes.

Notch2 is required for maintaining sustentacular cell function in the adult mouse main olfactory epithelium

Notch receptors are expressed in neurons and glia in the adult nervous system, but why this expression persists is not well-understood. Here we examine the role of the Notch pathway in the postnatal mouse main olfactory system, and show evidence consistent with a model where Notch2 is required for maintaining sustentacular cell function. In the absence of Notch2, the laminar nature of these glial-like cells is disrupted. Hes1, Hey1, and Six1, which are downstream effectors of the Notch pathway, are down-regulated, and cytochrome P450 and Glutathione S-transferase (GST) expression by sustentacular cells is reduced. Functional levels of GST activity are also reduced. These disruptions are associated with increased olfactory sensory neuron degeneration. Surprisingly, expression of Notch3 is also down-regulated. This suggests the existence of a feedback loop where expression of Notch3 is initially independent of Notch2, but requires Notch2 for maintained expression. While the Notch pathway has previously been shown to be important for promoting gliogenesis during development, this is the first demonstration that the persistent expression of Notch receptors is required for maintaining glial function in adult.

The POU homeobox protein Oct-1 regulates radial glia formation downstream of Notch signaling

Radial glia cells function as guide cells for neuronal migration and a source of neural progenitor cells, and play a crucial role for the development of the central nervous system. To date, several signals have been demonstrated to promote the formation of radial glia cells and Notch signaling is one such signal. However, the mechanism of the signaling hierarchy of radial glia developmental cascade promoted by Notch signaling still remains incomplete. Here we show that Notch signaling promotes Xenopus radial glia formation and that the Notch activation is sufficient for radial glia formation prior to neural tube closure. Moreover, we have identified Oct-1 (POU2f1), a POU transcription factor, as a downstream target of Notch signaling by microarray based screen. We demonstrate that the expression of Oct-1 in the brain is regulated by Notch signaling and that Oct-1 is sufficient and necessary for radial glia formation. Together, Oct-1 is a downstream effector of Notch signaling during radial glia formation.

Proliferation, neurogenesis and regeneration in the non-mammalian vertebrate brain

Post-embryonic neurogenesis is a fundamental feature of the vertebrate brain. However, the level of adult neurogenesis decreases significantly with phylogeny. In the first part of this review, a comparative analysis of adult neurogenesis and its putative roles in vertebrates are discussed. Adult neurogenesis in mammals is restricted to two telencephalic constitutively active zones. On the contrary, non-mammalian vertebrates display a considerable amount of adult neurogenesis in many brain regions. The phylogenetic differences in adult neurogenesis are poorly understood. However, a common feature of vertebrates (fish, amphibians and reptiles) that display a widespread adult neurogenesis is the substantial post-embryonic brain growth in contrast to birds and mammals. It is probable that the adult neurogenesis in fish, frogs and reptiles is related to the coordinated growth of sensory systems and corresponding sensory brain regions. Likewise, neurons are substantially added to the olfactory bulb in smell-oriented mammals in contrast to more visually oriented primates and songbirds, where much fewer neurons are added to the olfactory bulb. The second part of this review focuses on the differences in brain plasticity and regeneration in vertebrates. Interestingly, several recent studies show that neurogenesis is suppressed in the adult mammalian brain. In mammals, neurogenesis can be induced in the constitutively neurogenic brain regions as well as ectopically in response to injury, disease or experimental manipulations. Furthermore, multipotent progenitor cells can be isolated and differentiated in vitro from several otherwise silent regions of the mammalian brain. This indicates that the potential to recruit or generate neurons in non-neurogenic brain areas is not completely lost in mammals. The level of adult neurogenesis in vertebrates correlates with the capacity to regenerate injury, for example fish and amphibians exhibit the most widespread adult neurogenesis and also the greatest capacity to regenerate central nervous system injuries. Studying these phenomena in non-mammalian vertebrates may greatly increase our understanding of the mechanisms underlying regeneration and adult neurogenesis. Understanding mechanisms that regulate endogenous proliferation and neurogenic permissiveness in the adult brain is of great significance in therapeutical approaches for brain injury and disease.

Reinduction of ErbB2 in astrocytes promotes radial glial progenitor identity in adult cerebral cortex

Radial glial cells play a critical role in the construction of mammalian brain by functioning as a source of new neurons and by providing a scaffold for radial migration of new neurons to their target locations. Radial glia transform into astrocytes at the end of embryonic development. Strategies to promote functional recovery in the injured adult brain depend on the generation of new neurons and the appropriate guidance of these neurons to where they are needed, two critical functions of radial glia. Thus, the competence to regain radial glial identity in the adult brain is of significance for the ability to promote functional repair via neurogenesis and targeted neuronal migration in the mature brain. Here we show that the in vivo induction of the tyrosine kinase receptor, ErbB2, in mature astrocytes enables a subset of them to regain radial glial identity in the mature cerebral cortex. These new radial glial progenitors are capable of giving rise to new neurons and can support neuronal migration. These studies indicate that ErbB2 signaling critically modulates the functional state of radial glia, and induction of ErbB2 in distinct adult astrocytes can promote radial glial identity in the mature cerebral cortex.

The lateral line microcosmos

The lateral-line system is a simple sensory system comprising a number of discrete sense organs, the neuromasts, distributed over the body of fish and amphibians in species-specific patterns. Its development involves fundamental biological processes such as long-range cell migration, planar cell polarity, regeneration, and post-embryonic remodeling. These aspects have been extensively studied in amphibians by experimental embryologists, but it is only recently that the genetic bases of this development have been explored in zebrafish. This review discusses progress made over the past few years in this field.