Analysis of stem cell lineage progression in the neonatal subventricular zone identifies EGFR+/NG2- cells as transit-amplifying precursors

Growth/differentiation factor 15 controls ependymal and stem cell number in the V-SVZ

The expression of growth/differentiation factor (GDF) 15 increases in the ganglionic eminence (GE) late in neural development, especially in neural stem cells (NSCs). However, GDF15 function in this region remains unknown. We report that GDF15 receptor is expressed apically in the GE and that GDF15 ablation promotes proliferation and cell division in the embryonic GE and in the adult ventricular-subventricular zone (V-SVZ). This causes a transient generation of additional neuronal progenitors, compensated by cell death, and a lasting increase in the number of ependymal cells and apical NSCs. Finally, both GDF15 receptor and the epidermal growth factor receptor (EGFR) were expressed in progenitors and mutation of GDF15 affected EGFR signaling. However, only exposure to exogenous GDF15, but not to EGF, normalized proliferation and the number of apical progenitors. Thus, GDF15 regulates proliferation of apical progenitors in the GE, thereby affecting the number of ependymal cells and NSCs.

Targeting epidermal growth factor receptor to recruit newly generated neuroblasts in cortical brain injuries

BACKGROUND

Neurogenesis is stimulated in the subventricular zone (SVZ) of mice with cortical brain injuries. In most of these injuries, newly generated neuroblasts attempt to migrate toward the injury, accumulating within the corpus callosum not reaching the perilesional area.

METHODS

We use a murine model of mechanical cortical brain injury, in which we perform unilateral cortical injuries in the primary motor cortex of adult male mice. We study neurogenesis in the SVZ and perilesional area at 7 and 14 dpi as well as the expression and concentration of the signaling molecule transforming growth factor alpha (TGF-α) and its receptor the epidermal growth factor (EGFR). We use the EGFR inhibitor Afatinib to promote neurogenesis in brain injuries.

RESULTS

We show that microglial cells that emerge within the injured area and the SVZ in response to the injury express high levels of TGF-α leading to elevated concentrations of TGF-α in the cerebrospinal fluid. Thus, the number of neuroblasts in the SVZ increases in response to the injury, a large number of these neuroblasts remain immature and proliferate expressing the epidermal growth factor receptor (EGFR) and the proliferation marker Ki67. Restraining TGF-α release with a classical protein kinase C inhibitor reduces the number of these proliferative EGFR+ immature neuroblasts in the SVZ. In accordance, the inhibition of the TGF-α receptor, EGFR promotes migration of neuroblasts toward the injury leading to an elevated number of neuroblasts within the perilesional area.

CONCLUSIONS

Our results indicate that in response to an injury, microglial cells activated within the injury and the SVZ release TGF-α, activating the EGFR present in the neuroblasts membrane inducing their proliferation, delaying maturation and negatively regulating migration. The inactivation of this signaling pathway stimulates neuroblast migration toward the injury and enhances the quantity of neuroblasts within the injured area. These results suggest that these proteins may be used as target molecules to regenerate brain injuries.

Role of Running-Activated Neural Stem Cells in the Anatomical and Functional Recovery after Traumatic Brain Injury in p21 Knock-Out Mice

Traumatic brain injury (TBI) represents one of the most common worldwide causes of death and disability. Clinical and animal model studies have evidenced that TBI is characterized by the loss of both gray and white matter, resulting in brain atrophy and in a decrease in neurological function. Nowadays, no effective treatments to counteract TBI-induced neurological damage are available. Due to its complex and multifactorial pathophysiology (neuro-inflammation, cytotoxicity and astroglial scar formation), cell regeneration and survival in injured brain areas are strongly hampered. Recently, it has been proposed that adult neurogenesis may represent a new approach to counteract the post-traumatic neurodegeneration. In our laboratory, we have recently shown that physical exercise induces the long-lasting enhancement of subventricular (SVZ) adult neurogenesis in a p21 (negative regulator of neural progenitor proliferation)-null mice model, with a concomitant improvement of olfactory behavioral paradigms that are strictly dependent on SVZ neurogenesis. On the basis of this evidence, we have investigated the effect of running on SVZ neurogenesis and neurorepair processes in p21 knock-out mice that were subject to TBI at the end of a 12-day session of running. Our data indicate that runner p21 ko mice show an improvement in numerous post-trauma neuro-regenerative processes, including the following: (i) an increase in neuroblasts in the SVZ; (ii) an increase in the migration stream of new neurons from the SVZ to the damaged cortical region; (iii) an enhancement of new differentiating neurons in the peri-lesioned area; (iv) an improvement in functional recovery at various times following TBI. All together, these results suggest that a running-dependent increase in subventricular neural stem cells could represent a promising tool to improve the endogenous neuro-regenerative responses following brain trauma.

Tumor Spheroids of an Aggressive Form of Central Neurocytoma Have Transit-Amplifying Progenitor Characteristics with Enhanced EGFR and Tumor Stem Cell Signaling

Central neurocytoma (CN) has been known as a benign neuronal tumor. In rare cases, CN undergoes malignant transformation to glioblastomas (GBM). Here we examined its cellular origin by characterizing differentiation potential and gene expression of CN-spheroids. First, we demonstrate that both CN tissue and cultured primary cells recapitulate the hierarchal cellular composition of subventricular zone (SVZ), which is comprised of neural stem cells (NSCs), transit amplifying progenitors (TAPs), and neuroblasts. We then derived spheroids from CN which displayed EGFR+/MASH+ TAP and BLBP+ radial glial cell (RGC) characteristic, and mitotic neurogenesis and gliogenesis by single spheroids were observed with cycling multipotential cells. CN-spheroids expressed increased levels of pluripotency and tumor stem cell genes such as KLF4 and TPD5L1, when compared to their differentiated cells and human NSCs. Importantly, Gene Set Enrichment Analysis showed that gene sets of GBM-Spheroids, EGFR Signaling, and Packaging of Telomere Ends are enriched in CN-spheroids in comparison with their differentiated cells. We speculate that CN tumor stem cells have TAP and RGC characteristics, and upregulation of EGFR signaling as well as downregulation of eph-ephrin signaling have critical roles in tumorigenesis of CN. And their ephemeral nature of TAPs destined to neuroblasts, might reflect benign nature of CN.

Epidermal Growth Factor in the CNS: A Beguiling Journey from Integrated Cell Biology to Multiple Sclerosis. An Extensive Translational Overview

This article reviews the wealth of papers dealing with the different effects of epidermal growth factor (EGF) on oligodendrocytes, astrocytes, neurons, and neural stem cells (NSCs). EGF induces the in vitro and in vivo proliferation of NSCs, their migration, and their differentiation towards the neuroglial cell line. It interacts with extracellular matrix components. NSCs are distributed in different CNS areas, serve as a reservoir of multipotent cells, and may be increased during CNS demyelinating diseases. EGF has pleiotropic differentiative and proliferative effects on the main CNS cell types, particularly oligodendrocytes and their precursors, and astrocytes. EGF mediates the in vivo myelinotrophic effect of cobalamin on the CNS, and modulates the synthesis and levels of CNS normal prions (PrP^Cs), both of which are indispensable for myelinogenesis and myelin maintenance. EGF levels are significantly lower in the cerebrospinal fluid and spinal cord of patients with multiple sclerosis (MS), which probably explains remyelination failure, also because of the EGF marginal role in immunology. When repeatedly administered, EGF protects mouse spinal cord from demyelination in various experimental models of autoimmune encephalomyelitis. It would be worth further investigating the role of EGF in the pathogenesis of MS because of its multifarious effects.

The Orphan Nuclear Receptor TLX Represses Hes1 Expression, Thereby Affecting NOTCH Signaling and Lineage Progression in the Adult SEZ

In the adult subependymal zone (SEZ), neural stem cells (NSCs) apically contacting the lateral ventricle on activation generate progenitors proliferating at the niche basal side. We here show that Tailless (TLX) coordinates NSC activation and basal progenitor proliferation by repressing the NOTCH effector Hes1. Consistent with this, besides quiescence Hes1 expression also increases on Tlx mutation. Since HES1 levels are higher at the apical SEZ, NOTCH activation is increased in Tlx^−/− NSCs, but not in surrounding basal progenitors. Underscoring the causative relationship between higher HES1/NOTCH and increased quiescence, downregulation of Hes1 only in mutant NSCs normalizes NOTCH activation and resumes proliferation and neurogenesis not only in NSCs, but especially in basal progenitors. Since pharmacological blockade of NOTCH signaling also promotes proliferation of basal progenitors, we conclude that TLX, by repressing Hes1 expression, counteracts quiescence and NOTCH activation in NSCs, thereby relieving NOTCH-mediated lateral inhibition of proliferation in basal progenitors.

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TLX autonomously controls quiescence in apical NSCs by repressing Hes1

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TLX controls basal progenitor proliferation via NOTCH-mediated lateral inhibition

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Downregulation of Hes1 in apical Tlx^−/− NSCs resumes proliferation and neurogenesis

Regulation of Myelination by Exosome Associated Retinoic Acid Release from NG2-Positive Cells

In the CNS, oligodendrocytes are responsible for myelin formation and maintenance. Following spinal cord injury, oligodendrocyte loss and an inhibitory milieu compromise remyelination and recovery. Here, we explored the role of retinoic acid receptor-beta (RARβ) signaling in remyelination. Using a male Sprague Dawley rat model of PNS-CNS injury, we show that oral treatment with a novel drug like RARβ agonist, C286, induces neuronal expression of the proteoglycan decorin and promotes myelination and differentiation of oligodendrocyte precursor cells (NG2⁺ cells) in a decorin-mediated neuron–glia cross talk. Decorin promoted the activation of RARα in NG2⁺ cells by increasing the availability of the endogenous ligand RA. NG2⁺ cells synthesize RA, which is released in association with exosomes. We found that decorin prevents this secretion through regulation of the EGFR–calcium pathway. Using functional and pharmacological studies, we further show that RARα signaling is both required and sufficient for oligodendrocyte differentiation. These findings illustrate that RARβ and RARα are important regulators of oligodendrocyte differentiation, providing new targets for myelination.

SIGNIFICANCE STATEMENT This study identifies novel therapeutic targets for remyelination after PNS-CNS injury. Pharmacological and knock-down experiments show that the retinoic acid (RA) signaling promotes differentiation of oligodendrocyte precursor cells (OPCs) and remyelination in a cross talk between neuronal RA receptor-beta (RARβ) and RARα in NG2⁺ cells. We show that stimulation of RARα is required for the differentiation of OPCs and we describe for the first time how oral treatment with a RARβ agonist (C286, currently being tested in a Phase 1 trial, ISRCTN12424734) leads to the endogenous synthesis of RA through retinaldehyde dehydrogenase 2 (Raldh2) in NG2 cells and controls exosome-associated-RA intracellular levels through a decorin–Ca²⁺ pathway. Although RARβ has been implicated in distinct aspects of CNS regeneration, this study identifies a novel function for both RARβ and RARα in remyelination.

Bone Morphogenetic Protein Promotes Lewis X Stage-Specific Embryonic Antigen 1 Expression Thereby Interfering with Neural Precursor and Stem Cell Proliferation

The glycoprotein Prominin-1 and the carbohydrate Lewis X stage-specific embryonic antigen 1 (LeX-SSEA1) both have been extensively used as cell surface markers to purify neural stem cells (NSCs). While Prominin-1 labels a specialized membrane region in NSCs and ependymal cells, the specificity of LeX-SSEA1 expression and its biological significance are still unknown. To address these issues, we have here monitored the expression of the carbohydrate in neonatal and adult NSCs and in their progeny. Our results show that the percentage of immunopositive cells and the levels of LeX-SSEA1 immunoreactivity both increase with postnatal age across all stages of the neural lineage. This is associated with decreased proliferation in precursors including NSCs, which accumulate the carbohydrate at the cell surface while remaining quiescent. Exposure of precursors to bone morphogenetic protein (BMP) increases LEX-SSEA1 expression, which promotes cell cycle withdrawal by a mechanism involving LeX-SSEA1-mediated interaction at the cell surface. Conversely, interference with either BMP signaling or with LeX-SSEA1 promotes proliferation to a similar degree. Thus, in the postnatal germinal niche, the expression of LeX-SSEA1 increases with age and exposure to BMP signaling, thereby downregulating the proliferation of subependymal zone precursors including NSCs. Stem Cells 2017;35:2417-2429.

Electromagnetic Fields for the Regulation of Neural Stem Cells

Localized magnetic fields (MFs) could easily penetrate the scalp, skull, and meninges, thus inducing an electrical current in both the central and peripheral nervous systems, which is primarily used in transcranial magnetic stimulation (TMS) for inducing specific effects on different regions or cells that play roles in various brain activities. Studies of repetitive transcranial magnetic stimulation (rTMS) have led to novel attractive therapeutic approaches. Neural stem cells (NSCs) in adult human brain are able to self-renew and possess multidifferential ability to maintain homeostasis and repair damage after acute central nervous system. In the present review, we summarized the electrical activity of NSCs and the fundamental mechanism of electromagnetic fields and their effects on regulating NSC proliferation, differentiation, migration, and maturation. Although it was authorized for the rTMS use in resistant depression patients by US FDA, there are still unveiling mechanism and limitations for rTMS in clinical applications of acute central nervous system injury, especially on NSC regulation as a rehabilitation strategy. More in-depth studies should be performed to provide detailed parameters and mechanisms of rTMS in further studies, making it a powerful tool to treat people who are surviving with acute central nervous system injuries.

Accelerating bioelectric functional development of neural stem cells by graphene coupling: Implications for neural interfacing with conductive materials

In order to govern cell-specific behaviors in tissue engineering for neural repair and regeneration, a better understanding of material-cell interactions, especially the bioelectric functions, is extremely important. Graphene has been reported to be a potential candidate for use as a scaffold and neural interfacing material. However, the bioelectric evolvement of cell membranes on these conductive graphene substrates remains largely uninvestigated. In this study, we used a neural stem cell (NSC) model to explore the possible changes in membrane bioelectric properties - including resting membrane potentials and action potentials - and cell behaviors on graphene films under both proliferation and differentiation conditions. We used a combination of single-cell electrophysiological recordings and traditional cell biology techniques. Graphene did not affect the basic membrane electrical parameters (capacitance and input resistance), but resting membrane potentials of cells on graphene substrates were more strongly negative under both proliferation and differentiation conditions. Also, NSCs and their progeny on graphene substrates exhibited increased firing of action potentials during development compared to controls. However, graphene only slightly affected the electric characterizations of mature NSC progeny. The modulation of passive and active bioelectric properties on the graphene substrate was accompanied by enhanced NSC differentiation. Furthermore, spine density, synapse proteins expressions and synaptic activity were all increased in graphene group. Modeling of the electric field on conductive graphene substrates suggests that the electric field produced by the electronegative cell membrane is much higher on graphene substrates than that on control, and this might explain the observed changes of bioelectric development by graphene coupling. Our results indicate that graphene is able to accelerate NSC maturation during development, especially with regard to bioelectric evolvement. Our findings provide a fundamental understanding of the role of conductive materials in tuning the membrane bioelectric properties in a graphene model and pave the way for future studies on the development of methods and materials for manipulating membrane properties in a controllable way for NSC-based therapies.

γ-Aminobutyric A receptor (GABA(A)R) regulates aquaporin 4 expression in the subependymal zone: relevance to neural precursors and water exchange

Activation of γ-aminobutyric A receptors (GABA(A)Rs) in the subependymal zone (SEZ) induces hyperpolarization and osmotic swelling in precursors, thereby promoting surface expression of the epidermal growth factor receptor (EGFR) and cell cycle entry. However, the mechanisms underlying the GABAergic modulation of cell swelling are unclear. Here, we show that GABA(A)Rs colocalize with the water channel aquaporin (AQP) 4 in prominin-1 immunopositive (P(+)) precursors in the postnatal SEZ, which include neural stem cells. GABA(A)R signaling promotes AQP4 expression by decreasing serine phosphorylation associated with the water channel. The modulation of AQP4 expression by GABA(A)R signaling is key to its effect on cell swelling and EGFR expression. In addition, GABA(A)R function also affects the ability of neural precursors to swell in response to an osmotic challenge in vitro and in vivo. Thus, the regulation of AQP4 by GABA(A)Rs is involved in controlling activation of neural stem cells and water exchange dynamics in the SEZ.

Traumatic brain injury reveals novel cell lineage relationships within the subventricular zone

The acute response of the rodent subventricular zone (SVZ) to traumatic brain injury (TBI) involves a physical expansion through increased cell proliferation. However, the cellular underpinnings of these changes are not well understood. Our analyses have revealed that there are two distinct transit-amplifying cell populations that respond in opposite ways to injury. Mash1+ transit-amplifying cells are the primary SVZ cell type that is stimulated to divide following TBI. In contrast, the EGFR+ population, which has been considered to be a functionally equivalent progenitor population to Mash1+ cells in the uninjured brain, becomes significantly less proliferative after injury. Although normally quiescent GFAP+ stem cells are stimulated to divide in SVZ ablation models, we found that the GFAP+ stem cells do not divide more after TBI. We found, instead, that TBI results in increased numbers of GFAP+/EGFR+ stem cells via non-proliferative means-potentially through the dedifferentiation of progenitor cells. EGFR+ progenitors from injured brains only were competent to revert to a stem cell state following brief exposure to growth factors. Thus, our results demonstrate previously unknown changes in lineage relationships that differ from conventional models and likely reflect an adaptive response of the SVZ to maintain endogenous brain repair after TBI.

Growth/differentiation factor 15 promotes EGFR signalling, and regulates proliferation and migration in the hippocampus of neonatal and young adult mice

The activation of epidermal growth factor receptor (EGFR) affects multiple aspects of neural precursor behaviour, including proliferation and migration. Telencephalic precursors acquire EGF responsiveness and upregulate EGFR expression at late stages of development. The events regulating this process and its significance are still unclear. We here show that in the developing and postnatal hippocampus (HP), growth/differentiation factor (GDF) 15 and EGFR are co-expressed in primitive precursors as well as in more differentiated cells. We also provide evidence that GDF15 promotes responsiveness to EGF and EGFR expression in hippocampal precursors through a mechanism that requires active CXC chemokine receptor (CXCR) 4. Besides EGFR expression, GDF15 ablation also leads to decreased proliferation and migration. In particular, lack of GDF15 impairs both processes in the cornu ammonis (CA) 1 and only proliferation in the dentate gyrus (DG). Importantly, migration and proliferation in the mutant HP were altered only perinatally, when EGFR expression was also affected. These data suggest that GDF15 regulates migration and proliferation by promoting EGFR signalling in the perinatal HP and represent a first description of a functional role for GDF15 in the developing telencephalon.

Proliferation and cilia dynamics in neural stem cells prospectively isolated from the SEZ

Neural stem cells (NSCs) generate new neurons in vivo and in vitro throughout adulthood and therefore are physiologically and clinically relevant. Unveiling the mechanisms regulating the lineage progression from NSCs to newborn neurons is critical for the transition from basic research to clinical application. However, the direct analysis of NSCs and their progeny is still elusive due to the problematic identification of the cells. We here describe the isolation of highly purified genetically unaltered NSCs and transit-amplifying precursors (TAPs) from the adult subependymal zone (SEZ). Using this approach we show that a primary cilium and high levels of epidermal growth factor receptor (EGFR) at the cell membrane characterize quiescent and cycling NSCs, respectively. However, we also observed non-ciliated quiescent NSCs and NSCs progressing into the cell cycle without up-regulating EGFR expression. Thus, the existence of NSCs displaying distinct molecular and structural conformations provides more flexibility to the regulation of quiescence and cell cycle progression.

Receptor tyrosine kinase (RTK) signalling in the control of neural stem and progenitor cell (NSPC) development

Important developmental responses are elicited in neural stem and progenitor cells (NSPC) by activation of the receptor tyrosine kinases (RTK), including the fibroblast growth factor receptors, epidermal growth factor receptor, platelet-derived growth factor receptors and insulin-like growth factor receptor (IGF1R). Signalling through these RTK is necessary and sufficient for driving a number of developmental processes in the central nervous system. Within each of the four RTK families discussed here, receptors are activated by sets of ligands that do not cross-activate receptors of the other three families, and therefore, their activation can be independently regulated by ligand availability. These RTK pathways converge on a conserved core of signalling molecules, but differences between the receptors in utilisation of signalling molecules and molecular adaptors for intracellular signal propagation become increasingly apparent. Intracellular inhibitors of RTK signalling are widely involved in the regulation of developmental signalling in NSPC and often determine developmental outcomes of RTK activation. In addition, cellular responses of NSPC to the activation of a given RTK may be significantly modulated by signal strength. Cellular propensity to respond also plays a role in developmental outcomes of RTK signalling. In combination, these mechanisms regulate the balance between NSPC maintenance and differentiation during development and in adulthood. Attribution of particular developmental responses of NSPC to specific pathways of RTK signalling becomes increasingly elucidated. Co-activation of several RTK in developing NSPC is common, and analysis of co-operation between their signalling pathways may advance knowledge of RTK role in NSPC development.

Kv3.1 channels stimulate adult neural precursor cell proliferation and neuronal differentiation

Adult neural stem/precursor cells (NPCs) play a pivotal role in neuronal plasticity throughout life. Among ion channels identified in adult NPCs, voltage-gated delayed rectifier K(+) (KDR) channels are dominantly expressed. However, the KDR channel subtype and its physiological role are still undefined. We used real-time quantitative RT-PCR and gene knockdown techniques to identify a major functional KDR channel subtype in adult NPCs. Dominant mRNA expression of Kv3.1, a high voltage-gated KDR channel, was quantitatively confirmed. Kv3.1 gene knockdown with specific small interfering RNAs (siRNA) for Kv3.1 significantly inhibited Kv3.1 mRNA expression by 63.9% (P < 0.001) and KDR channel currents by 52.2% (P < 0.001). This indicates that Kv3.1 is the subtype responsible for producing KDR channel outward currents. Resting membrane properties, such as resting membrane potential, of NPCs were not affected by Kv3.1 expression. Kv3.1 knockdown with 300 nm siRNA inhibited NPC growth (increase in cell numbers) by 52.9% (P < 0.01). This inhibition was attributed to decreased cell proliferation, not increased cell apoptosis. We also established a convenient in vitro imaging assay system to evaluate NPC differentiation using NPCs from doublecortin-green fluorescent protein transgenic mice. Kv3.1 knockdown also significantly reduced neuronal differentiation by 31.4% (P < 0.01). We have demonstrated that Kv3.1 is a dominant functional KDR channel subtype expressed in adult NPCs and plays key roles in NPC proliferation and neuronal lineage commitment during differentiation.

Newborn cortical neurons: only for neonates?

Despite a century of debate over the existence of adult cortical neurogenesis, a consensus has not yet been reached. Here, we review evidence of the existence, origin, migration, and integration of neurons into the adult and neonatal cerebral cortex. We find that the lack of consensus likely stems from the low rate of postnatal cortical neurogenesis that has been observed, the fact that neurogenesis may be limited to subtypes of interneurons, and variability in other conditions, both physiological and environmental. We emphasize that neurogenesis occurs in the neonatal cortex and that neural stem cells are present into adulthood; it is possible that these progenitors are dormant, but they may be reactivated, for example, following injury.

Functions of neurotrophins and growth factors in neurogenesis and brain repair

The identification and isolation of multipotent neural stem and progenitor cells in the brain, giving rise to neurons, astrocytes, and oligodendrocytes initiated many studies in order to understand basic mechanisms of endogenous neurogenesis and repair mechanisms of the nervous system and to develop novel therapeutic strategies for cellular regeneration therapies in brain disease. A previous review (Trujillo et al., Cytometry A 2009;75:38-53) focused on the importance of extrinsic factors, especially neurotransmitters, for directing migration and neurogenesis in the developing and adult brain. Here, we extend our review discussing the effects of the principal growth and neurotrophic factors as well as their intracellular signal transduction on neurogenesis, fate determination and neuroprotective mechanisms. Many of these mechanisms have been elucidated by in vitro studies for which neural stem cells were isolated, grown as neurospheres, induced to neural differentiation under desired experimental conditions, and analyzed for embryonic, progenitor, and neural marker expression by flow and imaging cytometry techniques. The better understanding of neural stem cells proliferation and differentiation is crucial for any therapeutic intervention aiming at neural stem cell transplantation and recruitment of endogenous repair mechanisms.

Bioelectric state and cell cycle control of Mammalian neural stem cells

The concerted action of ion channels and pumps establishing a resting membrane potential has been most thoroughly studied in the context of excitable cells, most notably neurons, but emerging evidences indicate that they are also involved in controlling proliferation and differentiation of nonexcitable somatic stem cells. The importance of understanding stem cell contribution to tissue formation during embryonic development, adult homeostasis, and regeneration in disease has prompted many groups to study and manipulate the membrane potential of stem cells in a variety of systems. In this paper we aimed at summarizing the current knowledge on the role of ion channels and pumps in the context of mammalian corticogenesis with particular emphasis on their contribution to the switch of neural stem cells from proliferation to differentiation and generation of more committed progenitors and neurons, whose lineage during brain development has been recently elucidated.

Expression of Tlx in both stem cells and transit amplifying progenitors regulates stem cell activation and differentiation in the neonatal lateral subependymal zone

Niche homeostasis in the postnatal subependymal zone of the lateral ventricle (lSEZ) requires coordinated proliferation and differentiation of neural progenitor cells. The mechanisms regulating this balance are scarcely known. Recent observations indicate that the orphan nuclear receptor Tlx is an intrinsic factor essential in maintaining this balance. However, the effect of Tlx on gene expression depends on age and cell-type cues. Therefore, it is essential to establish its expression pattern at different developmental ages. Here, we show for the first time that in the neonatal lSEZ activated neural stem cells (NSCs) and especially transit-amplifying progenitors (TAPs) express Tlx and that its expression may be regulated at the posttranscriptional level. We also provide evidence that in both cell types Tlx affects gene expression in a positive and negative manner. In activated NSCs, but not in TAPs, absence of Tlx leads to overexpression of negative cell cycle regulators and impairment of proliferation. Moreover, in both cell types, the homeobox transcription factor Dlx2 is downregulated in the absence of Tlx. This is paralleled by increased expression of Olig2 in activated NSCs and glial fibrillary acidic protein in TAPs, indicating that in both populations Tlx decreases gliogenesis. Consistent with this, we found a higher proportion of cells expressing glial makers in the neonatal lSEZ of mutant mice than in the wild type counterpart. Thus, Tlx playing a dual role affects the expression of distinct genes in these two lSEZ cell types.

GABA Not Only a Neurotransmitter: Osmotic Regulation by GABA(A)R Signaling

Mature macroglia and almost all neural progenitor types express γ-aminobutyric (GABA) A receptors (GABA_ARs), whose activation by ambient or synaptic GABA, leads to influx or efflux of chloride (Cl⁻) depending on its electro-chemical gradient (E_Cl). Since the flux of Cl⁻ is indissolubly associated to that of osmotically obliged water, GABA_ARs regulate water movements by modulating ion gradients. In addition, since water movements also occur through specialized water channels and transporters, GABA_AR signaling could affect the movement of water by regulating the function of the channels and transporters involved, thereby affecting not only the direction of the water fluxes but also their dynamics. We will here review recent observations indicating that in neural cells GABA_AR-mediated osmotic regulation affects the cellular volume thereby activating multiple intracellular signaling mechanisms important for cell proliferation, maturation, and survival. In addition, we will discuss evidence that the osmotic regulation exerted by GABA may contribute to brain water homeostasis in physiological and in pathological conditions causing brain edema, in which the GABAergic transmission is often altered.

GABAA receptor signaling induces osmotic swelling and cell cycle activation of neonatal prominin+ precursors

Signal-regulated changes in cell size affect cell division and survival and therefore are central to tissue morphogenesis and homeostasis. In this respect, GABA receptors (GABA(A)Rs) are of particular interest because allowing anions flow across the cell membrane modulates the osmolyte flux and the cell volume. Therefore, we have here investigated the hypothesis that GABA may regulate neural stem cell proliferation by inducing cell size changes. We found that, besides neuroblasts, also neural precursors in the neonatal murine subependymal zone sense GABA via GABA(A) Rs. However, unlike in neuroblasts, where it induced depolarization-mediated [Ca(2+)](i) increase, GABA(A) Rs activation in precursors caused hyperpolarization. This resulted in osmotic swelling and increased surface expression of epidermal growth factor receptors (EGFRs). Furthermore, activation of GABA(A) Rs signaling in vitro in the presence of EGF modified the expression of the cell cycle regulators, phosphatase and tensin homolog and cyclin D1, increasing the pool of cycling precursors without modifying cell cycle length. A similar effect was observed on treatment with diazepam. We also demonstrate that GABA and diazepam responsive precursors represent prominin(+) stem cells. Finally, we show that as in in vitro also in in vivo a short administration of diazepam promotes EGFR expression in prominin(+) stem cells causing activation and cell cycle entry. Thus, our data indicate that endogenous GABA is a part of a regulatory mechanism of size and cell cycle entry of neonatal stem cells. Our results also have potential implications for the therapeutic practices that involve exposure to GABA(A) Rs modulators during neurodevelopment.

PDGFRα expression distinguishes GFAP-expressing neural stem cells from PDGF-responsive neural precursors in the adult periventricular area

Jackson et al. (2006) have reported that adult glial fibrillary acid protein (GFAP)-expressing neural stem cells (NSCs) also express platelet-derived growth factor (PDGF) receptor-α (PDGFRα), and that their stimulation by PDGF induced the formation of a glioma-like mass. Here, we reexamined the relationship between PDGFRα and GFAP expression within the three-dimensional organization of the adult periventricular area. Using four independent PDGFRα antibodies, we found that adult mouse GFAP-expressing NSCs and PDGFRα-expressing cells represent two distinct populations of neural precursors. Examination of the adult periventricular area in a mouse line that expresses nuclear-localized enhanced green fluorescent protein under the control of the PDGFRα promoter confirmed that GFAP-expressing NSCs do not express PDGFRα. Furthermore, PDGF-responsive neural precursors were found at least one cell layer subjacent to the ependymal layer, and were evenly distributed across the lateral ventricular wall, which contrasts with the reported patchy and often ependymal localization of adult GFAP-expressing NSCs. Adult human PDGFRα-expressing neural precursors were also found not to express GFAP. PDGF-responsive neural precursors, but not GFAP-expressing NSCs, responded to infusions of PDGF by generating glioma-like masses. Our results do not support the view that GFAP-expressing NSCs are the origin of glioma-like masses that form after intraventricular PDGF infusion.

Cross-disorder analysis of bipolar risk genes: further evidence of DGKH as a risk gene for bipolar disorder, but also unipolar depression and adult ADHD

Recently, several genome-wide association studies (GWAS) on bipolar disorder (BPD) suggested novel risk genes. However, only few of them were followed up and further, the specificity of these genes is even more elusive. To address these issues, we genotyped SNPs in ANK3, CACNA1C, CMTM8, DGKH, EGFR, and NPAS3, which were significantly associated with BPD in previous GWAS, in a sample of 380 BPD patients. Replicated SNPs were then followed up in patients suffering from unipolar depression (UPD; n=387) or adult attention-deficit/hyperactivity disorder (aADHD; n=535). While we could not confirm an association of ANK3, CACNA1C, and EGFR with BPD, 10 SNPs in DGKH, CMTM8, and NPAS3 were nominally associated with disease, with two DGKH markers surviving correction for multiple testing. When these were followed up in UPD and aADHD, seven DGKH SNPs were also associated with UPD, while one SNP each in NPAS3 and CMTM8 and four in DGKH were linked to aADHD. Furthermore, a DGKH haplotype consisting of rs994856/rs9525580/rs9525584 GAT was associated with all disorders tested, while the complementary AGC haplotype was protective. The corresponding haploblock spans a 27-kb region covering exons coding for amino acids 65-243, and thus might include functional variants yet to be identified. We demonstrate an association of DGKH with BPD, UPD, and aADHD by applying a two-stage design. These disorders share the feature of mood instability, so that this phenotype might be associated with genetic variation in DGKH.

Vascular endothelial growth factor receptor 3 directly regulates murine neurogenesis

Neural stem cells (NSCs) are slowly dividing astrocytes that are intimately associated with capillary endothelial cells in the subventricular zone (SVZ) of the brain. Functionally, members of the vascular endothelial growth factor (VEGF) family can stimulate neurogenesis as well as angiogenesis, but it has been unclear whether they act directly via VEGF receptors (VEGFRs) expressed by neural cells, or indirectly via the release of growth factors from angiogenic capillaries. Here, we show that VEGFR-3, a receptor required for lymphangiogenesis, is expressed by NSCs and is directly required for neurogenesis. Vegfr3:YFP reporter mice show VEGFR-3 expression in multipotent NSCs, which are capable of self-renewal and are activated by the VEGFR-3 ligand VEGF-C in vitro. Overexpression of VEGF-C stimulates VEGFR-3-expressing NSCs and neurogenesis in the SVZ without affecting angiogenesis. Conversely, conditional deletion of Vegfr3 in neural cells, inducible deletion in subventricular astrocytes, and blocking of VEGFR-3 signaling with antibodies reduce SVZ neurogenesis. Therefore, VEGF-C/VEGFR-3 signaling acts directly on NSCs and regulates adult neurogenesis, opening potential approaches for treatment of neurodegenerative diseases.

Age-dependent fate and lineage restriction of single NG2 cells

NG2-expressing glia (NG2 cells, polydendrocytes) appear in the embryonic brain, expand perinatally, and persist widely throughout the gray and white matter of the mature central nervous system. We have previously reported that NG2 cells generate oligodendrocytes in both gray and white matter and a subset of protoplasmic astrocytes in the gray matter of the ventral forebrain and spinal cord. To investigate the temporal changes in NG2 cell fate, we generated NG2creER™BAC transgenic mice, in which tamoxifen-inducible Cre is expressed in NG2 cells. Cre induction at embryonic day 16.5, postnatal day (P) 2, P30 and P60 in mice that were double transgenic for NG2creER™BAC and the Cre reporter revealed that NG2 cells in the postnatal brain generate only NG2 cells or oligodendrocytes, whereas NG2 cells in the embryonic brain generate protoplasmic astrocytes in the gray matter of the ventral forebrain in addition to oligodendrocytes and NG2 cells. Analysis of cell clusters from single NG2 cells revealed that more than 80% of the NG2 cells in the P2 brain give rise to clusters consisting exclusively of oligodendrocytes, whereas the majority of the NG2 cells in the P60 brain generate clusters that contain only NG2 cells or a mixture of oligodendrocytes and NG2 cells. Furthermore, live cell imaging of single NG2 cells from early postnatal brain slices revealed that NG2 cells initially divide symmetrically to produce two daughter NG2 cells and that differentiation into oligodendrocytes occurred after 2-3 days.

Rostral migratory stream neuroblasts turn and change directions in stereotypic patterns

Neuroblasts generated in the adult subventricular zone (SVZ) migrate through the rostral migratory stream (RMS) to the olfactory bulb (OB). Previous work uncovered motility ranging from straight to complex, but it was unclear if directional changes were stochastic or exhibited stereotypical patterns. Here, we provide the first in-depth two-photon time-lapse microscopy study of morphological and dynamic features that accompany turning and direction reversals in the RMS. We identified three specific kinds of turning (30-90 degrees): bending of the leading process proximal to the cell body (P-bending 47% of cases), bending of the distal leading process (D-bending 30%) or branching of the leading process or lamellipodium (23%). Bending and branching angles were remarkably constrained and were significantly different from one another. Cells reversed direction (> 90 degrees) through D-bendings (54%), branching (11%) or de novo growth of processes from the soma (23%), but not P-bending. Direction reversal was often composed of several iterations of D-bending or branching as opposed to novel modalities. Individual neuroblasts could turn or change direction in multiple patterns suggesting that the patterns are not specific for different lineages. These findings show that neuroblasts in the RMS use a limited number of distinct and constrained modalities to turn or reverse direction.

Imaging and recording subventricular zone progenitor cells in live tissue of postnatal mice

The subventricular zone (SVZ) is one of two regions where neurogenesis persists in the postnatal brain. The SVZ, located along the lateral ventricle, is the largest neurogenic zone in the brain that contains multiple cell populations including astrocyte-like cells and neuroblasts. Neuroblasts migrate in chains to the olfactory bulb where they differentiate into interneurons. Here, we discuss the experimental approaches to record the electrophysiology of these cells and image their migration and calcium activity in acute slices. Although these techniques were in place for studying glial cells and neurons in mature networks, the SVZ raises new challenges due to the unique properties of SVZ cells, the cellular diversity, and the architecture of the region. We emphasize different methods, such as the use of transgenic mice and in vivo electroporation that permit identification of the different SVZ cell populations for patch clamp recording or imaging. Electroporation also permits genetic labeling of cells using fluorescent reporter mice and modification of the system using either RNA interference technology or floxed mice. In this review, we aim to provide conceptual and technical details of the approaches to perform electrophysiological and imaging studies of SVZ cells.

Dose-dependent effect of EGF on migration and differentiation of adult subventricular zone astrocytes

Adult neural stem cells (NSCs) are located in the subventricular zone (SVZ), a specialized brain niche located on the walls of the lateral ventricle. Under physiological conditions, NSCs generate a large number of young neurons and some oligodendrocytes, however the mechanisms controlling cell proliferation and migration are unclear. In vitro, epidermal growth factor (EGF) signaling has been shown to be an important mediator of cell proliferation and migration in the adult brain; however, the primary SVZ progenitors that respond to EGF are not well known. In this study, we isolated SVZ type-B astrocytes and cultured them under different EGF concentrations. We found a dose-dependent effect of EGF on proliferation rates and migration of SVZ type-B astrocytes. We found that GFAP+ type-B astrocytes gave rise to highly migratory and proliferating cells that expressed Olig2 and NG2. After EGF withdrawal, a significant number of EGF-stimulated cells differentiated into S100beta+/O4+ oligodendrocytes. This study provides new insights about the production of oligodendrocytes derived from the astrocyte NSCs residing in the adult SVZ. To be able to manipulate the endogenous adult progenitors, it is crucial to identify and isolate the responding primary precursors and determine the extracellular signals that regulate their cell division, migration, and fate.

Characterization of novel monoclonal antibodies able to identify neurogenic niches and arrest neurosphere proliferation and differentiation

Two monoclonal antibodies (Nilo1 and Nilo2) were generated after immunization of hamsters with E13.5 olfactory bulb-derived mouse neurospheres. They are highly specific for neural stem and early progenitor cell surface antigens. Nilo positive cells present in the adult mouse subventricular zone (SVZ) were able to initiate primary neural stem cell cultures. Moreover, these antibodies added to neurosphere cultures induced proliferation arrest and interfered with their differentiation. In the lateral ventricles of adult mice, Nilo1 stained a cell subpopulation lining the ventricle and cells located in the SVZ, whereas Nilo2 stained a small population associated with the anterior horn of the SVZ at the beginning of the rostral migratory stream. Co-staining of Nilo1 or Nilo2 and neural markers demonstrated that Nilo1 identifies an early neural precursor subpopulation, whereas Nilo2 detects more differentiated neural progenitors. Thus, these antibodies identify distinct neurogenic populations within the SVZ of the lateral ventricle.

Physiological roles of ion channels in adult neural stem cells and their progeny

Elucidation of the machinery of adult neurogenesis is indispensable for the treatment of neurodegenerative diseases by therapeutic drugs and/or by neural stem cell (NSC) transplantation. It is well known that membrane ion channels play a critical role in cell function, including proliferation, apoptosis and migration in a wide range of cells. In NSC research, interdisciplinary collaboration between cell biologists and membrane physiologists has been pursued principally to monitor ion channel and synaptic currents as a hallmark of neuronal differentiation and maturation of NSC progeny. Nevertheless, less attention had been paid to a functional role of ion channels in NSCs or their immature progeny. Recently, however, evidence regarding their functional relevance has started to accumulate. In focusing on the early stages of the neurogenic process during which NSCs give rise to neuroblasts, this review highlights the latent ability of ion channels to act as functional regulators of adult neurogenesis.

NG2 cells: Properties, progeny and origin

The NG2 proteoglycan is a type 1-transmembrane protein expressed by a range of cell types within and outside the mammalian nervous system. NG2-expressing (NG2) cells are found in grey and white matter tracts of the developing and adult CNS and have previously been assumed to represent oligodendrocyte precursor cells: new work using transgenic mice has shown that NG2 cells generate oligodendrocytes, protoplasmic astrocytes and in some instances neurons in vivo. NG2 cells express GABAA receptors and the AMPA subtype of glutamate receptors. They make intimate contact to neurons prior to myelinating axons and also form electron-dense synaptic specialisations with axons in the cerebellum, cortex and hippocampus and with non-myelinated axons in the corpus callosum. These synaptic NG2 cells respond to neuronal release of glutamate and GABA. This neuron-glia interaction may thus regulate the differentiation and proliferation of NG2 cells. The C-terminal PDZ-binding motif of the NG2 protein binds several PDZ proteins including Mupp1, Syntenin and the Glutamate Receptor Interacting Protein (GRIP). Since GRIP can bind subunits of the AMPA receptors expressed by NG2 cells, the interaction between GRIP and NG2 may orientate the glial AMPA receptors towards sites of neuronal glutamate release. The origin, heterogeneity and function of NG2 cells as modulators of the neuronal network are important incompletely resolved questions.

Multipotent precursors in the anterior and hippocampal subventricular zone display similar transcription factor signatures but their proliferation and maintenance are differentially regulated

Precursors within the subventricular zone (SVZ) exhibit regional variations in the expression of transcription factors important for the regulation of their proliferation and differentiation. In the anterior SVZ (aSVZ) the homeobox transcription factor distalless (Dlx)2 modulates both processes by promoting neural stem cell (NSC) activation as well as neurogenesis. Activated NSCs and transit-amplifying precursors (TAPs) in the aSVZ both express high levels of epidermal growth factor receptor (EGFR(high)) and form clones in response to exogenous EGF. EGF-responsive cells are also present in the hippocampal subependyma (hSVZ). However, it is not clear whether they represent NSCs or TAPs and whether their proliferation and differentiation are regulated as in the aSVZ. Here we have purified EGFR(high) cells from both the aSVZ and hSVZ at different ages. When isolated from perinatal tissue both populations were enriched in multipotent clonogenic precursors, which generated GABAergic neurons. Although they differed in absolute expression levels, activated NSCs and TAPs in both regions displayed similar signatures of transcription factor expression. However, activated NSCs were less frequent in the hSVZ than in the aSVZ. Furthermore, increasing age had a greater inhibitory effect on NSC proliferation in the hSVZ than in the aSVZ. This suggests that NSC activation is differentially regulated in the two regions. Consistent with this hypothesis, we found that in hippocampal precursors Dlx2 promoted neurogenesis but not NSC activation. Thus, most clonogenic EGFR(high) precursors in the hSVZ represent TAPs and NSC proliferation in the aSVZ and hSVZ is regulated by different mechanisms.

Neurotransmitter signaling in postnatal neurogenesis: The first leg

Like the liver or other peripheral organs, two regions of the adult brain possess the ability of self-renewal through a process called neurogenesis. This raises tremendous hope for repairing the damaged brain, and it has stimulated research on identifying signals controlling neurogenesis. Neurogenesis involves several stages from fate determination to synaptic integration via proliferation, migration, and maturation. While fate determination primarily depends on a genetic signature, other stages are controlled by the interplay between genes and microenvironmental signals. Here, we propose that neurotransmitters are master regulators of the different stages of neurogenesis. In favor of this idea, a description of selective neurotransmitter signaling and their functions in the largest neurogenic zone, the subventricular zone (SVZ), is provided. In particular, we emphasize the interactions between neuroblasts and astrocyte-like cells that release gamma-aminobutyric acid (GABA) and glutamate, respectively. However, we also raise several limitations to our knowledge on neurotransmitters in neurogenesis. The function of neurotransmitters in vivo remains largely unexplored. Neurotransmitter signaling has been viewed as uniform, which dramatically contrasts with the cellular and molecular mosaic nature of the SVZ. How neurotransmitters are integrated with other well-conserved molecules, such as sonic hedgehog, is poorly understood. In an effort to reconcile these differences, we discuss how specificity of neurotransmitter functions can be provided through their multitude of receptors and intracellular pathways in different cell types and their possible interactions with sonic hedgehog.

The modulatory effects of bHLH transcription factors with the Wnt/beta-catenin pathway on differentiation of neural progenitor cells derived from neonatal mouse anterior subventricular zone

The subventricular zone (SVZ) located adjacent to the lateral ventricles is the major site where neural progenitor cells (NPCs) are concentrated in the adult brain. NPCs in the anterior subventricular zone (SVZa) generate neuronal precursors and migrate along a highly localized pathway--the rostral migratory stream (RMS) to the olfactory bulb (OB), where they differentiate into interneurons. To investigate the modulatory effects of basic helix-loop-helix (bHLH) transcription factors on differentiation from SVZa NPCs, we firstly examined the distribution of bHLH family members (Mash1, Id2, and Hes1) in cultured mouse SVZa NPCs and evaluated their regulatory effects on differentiation by transfection with Mash1, Id2, or Hes1 eukaryotic expression plasmid. Furthermore, we assessed the effects of bHLH transcription factors on the expression of downstream molecules of the Wnt/beta-catenin pathway, beta-catenin and (Glycogen synthase kinase-3beta). Our results demonstrated that Mash1, Id2, Hes1 were all widely expressed in in vitro progenies from mouse SVZa NPCs. Analyses of SVZa NPCs transfected with eukaryotic expression plasmids showed that Mash1 promoted neuronal differentiation from SVZa NPCs, while Id2 and Hes1 repressed neuronal differentiation. In addition, we found that Id2 and Hes1 simulated expression of beta-catenin and GSK-3beta, while Mash1 inhibited their expression. Our results suggest that the classic bHLH transcription factors, Mash1, Id2 and Hes1, play important roles in the regulation of differentiation from SVZa NPCs. This modulation is possibly mediated by a coordination of bHLH and Wnt/beta-catenin signaling.

Chromosome 7 and 19 trisomy in cultured human neural progenitor cells

Background

Stem cell expansion and differentiation is the foundation of emerging cell therapy technologies. The potential applications of human neural progenitor cells (hNPCs) are wide ranging, but a normal cytogenetic profile is important to avoid the risk of tumor formation in clinical trials. FDA approved clinical trials are being planned and conducted for hNPC transplantation into the brain or spinal cord for various neurodegenerative disorders. Although human embryonic stem cells (hESCs) are known to show recurrent chromosomal abnormalities involving 12 and 17, no studies have revealed chromosomal abnormalities in cultured hNPCs. Therefore, we investigated frequently occurring chromosomal abnormalities in 21 independent fetal-derived hNPC lines and the possible mechanisms triggering such aberrations.

Methods and Findings

While most hNPC lines were karyotypically normal, G-band karyotyping and fluorescent in situ hybridization (FISH) analyses revealed the emergence of trisomy 7 (hNPC⁺⁷) and trisomy 19 (hNPC⁺¹⁹), in 24% and 5% of the lines, respectively. Once detected, subsequent passaging revealed emerging dominance of trisomy hNPCs. DNA microarray and immunoblotting analyses demonstrate epidermal growth factor receptor (EGFR) overexpression in hNPC⁺⁷ and hNPC⁺¹⁹ cells. We observed greater levels of telomerase (hTERT), increased proliferation (Ki67), survival (TUNEL), and neurogenesis (β_III-tubulin) in hNPC⁺⁷ and hNPC⁺¹⁹, using respective immunocytochemical markers. However, the trisomy lines underwent replicative senescence after 50–60 population doublings and never showed neoplastic changes. Although hNPC⁺⁷ and hNPC⁺¹⁹ survived better after xenotransplantation into the rat striatum, they did not form malignant tumors. Finally, EGF deprivation triggered a selection of trisomy 7 cells in a diploid hNPC line.

Conclusions

We report that hNPCs are susceptible to accumulation of chromosome 7 and 19 trisomy in long-term cell culture. These results suggest that micro-environmental cues are powerful factors in the selection of specific hNPC aneuploidies, with trisomy of chromosome 7 being the most common. Given that a number of stem cell based clinical trials are being conducted or planned in USA and a recent report in PLoS Medicine showing the dangers of grafting an inordinate number of cells, these data substantiate the need for careful cytogenetic evaluation of hNPCs (fetal or hESC-derived) before their use in clinical or basic science applications.