Tenascin-R mediates activity-dependent recruitment of neuroblasts in the adult mouse forebrain

The extracellular matrix glycoprotein tenascin-R affects adult but not developmental neurogenesis in the olfactory bulb

Neuronal precursors produced in the subventricular zone throughout an animal's life migrate tangentially along the rostral migratory stream and, once in the olfactory bulb (OB), turn to migrate radially to the bulbar layers, where they differentiate into interneurons. Despite extensive investigations, it has remained largely unknown whether the same molecular mechanisms control OB neurogenesis during early postnatal development and in adulthood. In this study, we show that the extracellular matrix glycoprotein tenascin-R (TNR) is produced in the granule cell layer of the OB and that its expression increases during postnatal development. Time-lapse video imaging and morphological analyses revealed that a lack of TNR decreases the radial migration of neuronal precursors in the adult, but not in the developing OB. A lack of TNR also reduces spine development of newborn neurons in adult mice. To understand the functional consequences of a lack of TNR, we performed electrophysiological and behavioral studies on young and adult mice. Electrophysiological recordings showed that mitral cells, the target cells of newly generated interneurons, receive reduced spontaneous and evoked inhibitory activity in adult, but not young, TNR knock-out mice. Moreover, the synchronized activity of mitral cells was decreased in the OB of adult TNR knock-out mice. Behavioral studies revealed that the lower numbers of newborn interneurons in the adult OB induce alterations in short-term odor memory. Our results indicate that TNR modulates adult but not developmental neurogenesis in the OB and also highlight that the regulation of OB neurogenesis can vary during an animal's lifetime.

Primary hippocampal neurons, which lack four crucial extracellular matrix molecules, display abnormalities of synaptic structure and function and severe deficits in perineuronal net formation

The extracellular matrix (ECM) of the brain plays crucial roles during the development, maturation, and regeneration of the CNS. In a subpopulation of neurons, the ECM condenses to superstructures called perineuronal nets (PNNs) that surround synapses. Camillo Golgi described PNNs a century ago, yet their biological functions remain elusive. Here, we studied a mouse mutant that lacks four ECM components highly enriched in the developing brain: the glycoproteins tenascin-C and tenascin-R and the chondroitin sulfate proteoglycans brevican and neurocan. Primary embryonic hippocampal neurons and astrocytes were cultivated using a cell insert system that allows for co-culture of distinct cell populations in the absence of direct membrane contacts. The wild-type and knock-out cells were combined in the four possible permutations. Using this approach, neurons cultivated in the presence of mutant astrocytes displayed a transient increase of synapses after 2 weeks. However, after a period of 3 weeks or longer, synapse formation and stabilization were compromised when either neuron or astrocyte cell populations or both were of mutant origin. The development of PNN structures was observed, but their size was substantially reduced on knock-out neurons. The synaptic activity of both wild-type and knock-out neurons was monitored using whole-cell patch clamping. The salient observation was a reduced frequency of IPSCs and EPSCs, whereas the amplitudes were not modified. Remarkably, the knock-out neuron phenotypes could not be rescued by wild-type astrocytes. We conclude that the elimination of four ECM genes compromises neuronal function.

Receptor-type protein-tyrosine phosphatase ζ is a functional receptor for interleukin-34

Interleukin-34 (IL-34) is highly expressed in brain. IL-34 signaling via its cognate receptor, colony-stimulating factor-1 receptor (CSF-1R), is required for the development of microglia. However, the differential expression of IL-34 and the CSF-1R in brain suggests that IL-34 may signal via an alternate receptor. By IL-34 affinity chromatography of solubilized mouse brain membrane followed by mass spectrometric analysis, we identified receptor-type protein-tyrosine phosphatase ζ (PTP-ζ), a cell surface chondroitin sulfate (CS) proteoglycan, as a novel IL-34 receptor. PTP-ζ is primarily expressed on neural progenitors and glial cells and is highly expressed in human glioblastomas. IL-34 selectively bound PTP-ζ in CSF-1R-deficient U251 human glioblastoma cell lysates and inhibited the proliferation, clonogenicity, and motility of U251 cells in a PTP-ζ-dependent manner. These effects were correlated with an increase in tyrosine phosphorylation of the previously identified PTP-ζ downstream effectors focal adhesion kinase and paxillin. IL-34 binding to U251 cells was abrogated by chondroitinase ABC treatment, and CS competed with IL-34 for binding to the extracellular domain of PTP-ζ and to the cells, indicating a dependence of binding on PTP-ζ CS moieties. This study identifies an alternate receptor for IL-34 that may mediate its action on novel cellular targets.

Neuronal restoration following ischemic stroke: influences, barriers, and therapeutic potential

Neurogenesis, the birth of new neurons, occurs throughout life in the subventricular zone and produces immature neurons that migrate tangentially through the rostral migratory stream to the olfactory bulb. This migration is tightly regulated by both structural and chemical influences. Interestingly, brain insults such as ischemic stroke increase neurogenesis and redirect neuroblast migration to the injury site. This injury-redirected neurogenesis and migration is coupled with angiogenic vasculature and is influenced by many of the factors that positively and negatively affect migration under developmental or normal adult conditions. Additionally, cytokines and chemokines such as stromal cell-derived factor-1 strongly influence neuronal migration poststroke. However, neuronal repopulation or brain regeneration is extremely limited. This limitation may potentially be due to the hostile poststroke microenvironment including the formation of the physical and chemical barriers of glial scar. Furthermore, interspecies differences in poststroke neurogenesis between rodents and humans complicate the translation of experimental results to humans. Despite these challenges, many drugs and other potential therapies have recently been evaluated for potential neurogenic properties poststroke. Improved understanding of poststroke neurorepair may lead to new and more effective neurorestorative therapies.

Plexin-B2 regulates the proliferation and migration of neuroblasts in the postnatal and adult subventricular zone

In the postnatal forebrain, the subventricular zone (SVZ) contains a pool of undifferentiated cells, which proliferate and migrate along the rostral migratory stream (RMS) to the olfactory bulb and differentiate into granule cells and periglomerular cells. Plexin-B2 is a semaphorin receptor previously known to act on neuronal proliferation in the embryonic brain and neuronal migration in the cerebellum. We show here that, in the postnatal and adult CNS, Plexin-B2 is expressed in the subventricular zone lining the telencephalic ventricles and in the rostral migratory stream. We analyzed Plxnb2(-/-) mice and found that there is a marked reduction in the proliferation of SVZ cells in the mutant. Plexin-B2 expression is downregulated in the olfactory bulb as interneurons initiate radial migration. BrdU labeling and GFP electroporation into postnatal SVZ, in addition to time-lapse videomicroscopy, revealed that neuroblasts deficient for Plexin-B2 migrate faster than control ones and leave the RMS more rapidly. Overall, these results show that Plexin-B2 plays a role in postnatal neurogenesis and in the migration of SVZ-derived neuroblasts.

The extracellular matrix glycoprotein tenascin-R regulates neurogenesis during development and in the adult dentate gyrus of mice

Abnormal generation of inhibitory γ-aminobutyric acid synthesizing (GABAergic) neurons is characteristic of neuropsychological disorders. We provide evidence that the extracellular matrix molecule tenascin-R (TNR) – being predominantly expressed, among neurons, by subpopulation of interneurons - plays a role in the generation of GABAergic and granule neurons in the murine dentate gyrus by regulating fate determination of neural stem/progenitor cells (NSCs). During development, absence of TNR in constitutively TNR-deficient (TNR−/−) mice results in increased numbers of dentate gyrus GABAergic neurons, being associated with decreased expression of its receptor β1 integrin, increased activation of p38 MAPK, and increased expression of the GABAergic specification gene ASCL1. Postnatally, increased GABAergic input to adult hippocampal NSCs in TNR−/− mice is associated not only with increased numbers of GABAergic and, particularly, parvalbumin-immunoreactive neurons, as seen during development, but also with increased numbers of granule neurons, thus contributing to the increased differentiation of NSCs into granule cells. These findings indicate the importance of TNR in the regulation of hippocampal neurogenesis and suggest that TNR acts through distinct direct and indirect mechanisms during development and in the adult.

Dynamic changes in the transcriptional profile of subventricular zone-derived postnatally born neuroblasts

The subventricular zone (SVZ) of the lateral ventricles is a major neurogenic region in the postnatal mammalian brain. Thousands of neuroblasts are generated daily throughout the life of an animal. Newly born neuroblasts migrate via the rostral migratory stream (RMS) into the olfactory bulb where they mature into distinct neuronal subtypes. Neuroblasts exiting the SVZ retain the ability to proliferate, however, proliferation declines in the course of migration to the olfactory bulb. While migrating in the RMS, neuroblasts receive a plethora of stimuli that modify transcription according to the local microenvironment, and eventually modulate neuroblast migration. In the target area, the olfactory bulb, neuroblasts develop into mature neurons. In this review, we discuss dynamic changes of the transcriptome that occur during the "lifetime" of a neuroblast, thereby governing the activation or inhibition of distinct genes/pathways that are responsible for proliferation, migration and differentiation.

Extracellular matrix functions during neuronal migration and lamination in the mammalian central nervous system

Extracellular matrix (ECM) glycoproteins are expressed in the central nervous system (CNS) in complex and developmentally regulated patterns. The ECM provides a number of critical functions in the CNS, contributing both to the overall structural organization of the CNS and to control of individual cells. At the cellular level, the ECM affects its functions by a wide range of mechanisms, including providing structural support to cells, regulating the activity of second messenger systems, and controlling the distribution and local concentration of growth and differentiation factors. Perhaps the most well known role of the ECM is as a substrate on which motile cells can migrate. Genetic, cell biological, and biochemical studies provide strong evidence that ECM glycoproteins such as laminins, tenascins, and proteoglycans control neuronal migration and positioning in several regions of the developing and adult brain. Recent findings have also shed important new insights into the cellular and molecular mechanisms by which reelin regulates migration. Here we will summarize these findings, emphasizing the emerging concept that ECM glycoproteins promote different modes of neuronal migration such as radial, tangential, and chain migration. We also discuss several studies demonstrating that mutations in ECM glycoproteins can alter neuronal positioning by cell nonautonomous mechanisms that secondarily affect migrating neurons.

Homozygous deletion of Tenascin-R in a patient with intellectual disability

Background

TNR encodes Tenascin-R, an extracellular matrix glycoprotein that is primarily expressed in the central nervous system. Loss of TNR impairs cognition, synaptic plasticity and motor abilities in mice, however its role in human neurodevelopment and cognition is less clear.

Methods and results

The authors present the case of a child with intellectual disability and transient choreoathetosis. Array genomic hybridisation revealed a homozygous deletion involving only two genes, including TNR. Sequencing TNR in a cohort of 219 patients with intellectual disability did not identify any potential pathogenic mutations.

Conclusion

This is the first report of a complete loss of TNR associated with intellectual disability. This study provides evidence of the important role of TNR in brain development and cognition in humans.

p53 Mutation suppresses adult neurogenesis in medaka fish (Oryzias latipes)

Tumor suppressor p53 negatively regulates self-renewal of neural stem cells in the adult murine brain. Here, we report that the p53 null mutation in medaka fish (Oryzias latipes) suppressed neurogenesis in the telencephalon, independent of cell death. By using 5-bromo-29-deoxyuridine (BrdU) immunohistochemistry, we identified 18 proliferation zones in the brains of young medaka fish; in situ hybridization showed that p53 was expressed selectively in at least 12 proliferation zones. We also compared the number of BrdU-positive cells present in the whole telencephalon of wild-type (WT) and p53 mutant fish. Immediately after BrdU exposure, the number of BrdU-positive cells did not differ significantly between them. One week after BrdU-exposure, the BrdU-positive cells migrated from the proliferation zone, which was accompanied by an increased number in the WT brain. In contrast, no significant increase was observed in the p53 mutant brain. Terminal deoxynucleotidyl transferase (dUTP) nick end-labeling revealed that there was no significant difference in the number of apoptotic cells in the telencephalon of p53 mutant and WT medaka, suggesting that the decreased number of BrdU-positive cells in the mutant may be due to the suppression of proliferation rather than the enhancement of neural cell death. These results suggest that p53 positively regulates neurogenesis via cell proliferation.

Identification of radial glia-like cells in the adult mouse olfactory bulb

Immature neurons migrate tangentially within the rostral migratory stream (RMS) to the adult olfactory bulb (OB), then radially to their final positions as granule and periglomerular neurons; the controls over this transition are not well understood. Using adult transgenic mice with the human GFAP promoter driving expression of enhanced GFP, we identified a population of radial glia-like cells that we term adult olfactory radial glia-like cells (AORGs). AORGs have large, round somas and simple, radially oriented processes. Confocal reconstructions indicate that AORGs variably express typical radial glial markers, only rarely express mouse GFAP, and do not express astroglial, oligodendroglial, neuronal, or tanycyte markers. Electron microscopy provides further supporting evidence that AORGs are not immature neurons. Developmental analyses indicate that AORGs are present as early as P1, and are generated through adulthood. Tracing studies show that AORGs are not born in the SVZa, suggesting that they are born either in the RMS or the OB. Migrating immature neurons from the adult SVZa are closely apposed to AORGs during radial migration in vivo and in vitro. Taken together, these data indicate a newly-identified population of radial glia-like cells in the adult OB that might function uniquely in neuronal radial migration during adult OB neurogenesis.

Astrocytes control the development of the migration-promoting vasculature scaffold in the postnatal brain via VEGF signaling

New neurons are constantly being generated in the postnatal subventricular zone. They have to migrate long distances via the rostral migratory stream (RMS) to reach their final destination in the olfactory bulb (OB). In adults, these neuronal precursors migrate in chains, ensheathed by astrocytic processes, and travel toward the OB along blood vessels (BVs) that topographically outline the RMS. The molecular and cellular mechanisms leading to the development of the RMS and the formation of the migration-promoting vasculature scaffold in the adult mice remain unclear. We now reveal that astrocytes orchestrate the formation and structural reorganization of the vasculature scaffold in the RMS and, during early developmental stages, the RMS contains only a few BVs oriented randomly with respect to the migrating neuroblasts. The first parallel BVs appeared at the outer border of the RMS, where vascular endothelial growth factor (VEGF)-expressing astrocytes are located. Gain-of-function and loss-of-function experiments revealed that astrocyte-derived VEGF plays a crucial role in the formation and growth of new BVs. Real-time videoimaging also showed that the migration of neuronal precursors in the developing RMS differs substantially from neuronal displacement in the adult migratory stream partially because of not yet fully developed vasculature scaffold. The downregulation of VEGF in vivo, specifically in the astrocytes of the developing RMS, affected the development of the vasculature scaffold and led to alterations in neuroblast migration. Altogether, our results demonstrate that astrocytes orchestrate the formation and growth of parallel BVs, crucial migration-promoting scaffolds in the adult migratory stream, via VEGF signaling.

Dermatan sulfotransferase Chst14/D4st1, but not chondroitin sulfotransferase Chst11/C4st1, regulates proliferation and neurogenesis of neural progenitor cells

Chondroitin sulfates (CSs) and dermatan sulfates (DSs) are enriched in the microenvironment of neural stem cells (NSCs) during development and in the adult neurogenic niche, and have been implicated in mechanisms governing neural precursor migration, proliferation and differentiation. In contrast to previous studies, in which a chondroitinaseABC-dependent unselective deglycosylation of both CSs and DSs was performed, we used chondroitin 4-O-sulfotransferase-1 (Chst11/C4st1)- and dermatan 4-O-sulfotransferase-1 (Chst14/D4st1)-deficient NSCs specific for CSs and DSs, respectively, to investigate the involvement of specific sulfation profiles of CS and DS chains, and thus the potentially distinct roles of CSs and DSs in NSC biology. In comparison to wild-type controls, deficiency for Chst14 resulted in decreased neurogenesis and diminished proliferation of NSCs accompanied by increased expression of GLAST and decreased expression of Mash-1, and an upregulation of the expression of the receptors for fibroblast growth factor-2 (FGF-2) and epidermal growth factor (EGF). By contrast, deficiency in Chst11 did not influence NSC proliferation, migration or differentiation. These observations indicate for the first time that CSs and DSs play distinct roles in the self-renewal and differentiation of NSCs.

Hepatocyte growth factor acts as a mitogen and chemoattractant for postnatal subventricular zone-olfactory bulb neurogenesis

Neural progenitor cells persist throughout life in the forebrain subventricular zone (SVZ). They generate neuroblasts that migrate to the olfactory bulb and differentiate into interneurons, but mechanisms underlying these processes are poorly understood. Hepatocyte growth factor/scatter factor (HGF/SF) is a pleiotropic factor that influences cell motility, proliferation and morphogenesis in neural and non-neural tissues. HGF and its receptor, c-Met, are present in the rodent SVZ-olfactory bulb pathway. Using in vitro neurogenesis assays and in vivo studies of partially HGF-deficient mice, we find that HGF promotes SVZ cell proliferation and progenitor cell maintenance, while slowing differentiation and possibly altering cell fate choices. HGF also acts as a chemoattractant for SVZ neuroblasts in co-culture assays. Decreased HGF signaling induces ectopic SVZ neuroblast migration and alters the timing of migration to the olfactory bulb. These results suggest that HGF influences multiple steps in postnatal forebrain neurogenesis. HGF is a mitogen for SVZ neural progenitors, and regulates their differentiation and olfactory bulb migration.

Abnormal neuronal migration changes the fate of developing neurons in the postnatal olfactory bulb

Neuronal precursors are continuously integrated into the adult olfactory bulb (OB). The vast majority of these precursor cells originates from the subventricular zone and migrates along the rostral migratory stream (RMS) en route to the OB. This process, called postnatal neurogenesis, results from intricate pathways depending both on cell-autonomous factors and extrinsic regulation provided by the local environment. Using electroporation in postnatal mice to label neuronal precursors with green fluorescent protein (GFP) and to reduce the expression levels of doublecortin (DCX) with short-hairpin (Sh) RNA, we investigated the consequences of impairing migration on the fate of postnatal-formed neurons. First, we showed that electroporation of Dcx ShRNA plasmid efficiently knocks down the expression of DCX and disrupts cells migration along the RMS. Second, we found misplaced anomalous migrating cells that displayed defects in polarity and directionality. Third, patch-clamp recordings performed at 5-7 days post-electroporation (dpe) revealed increased density of voltage-dependent Na(+) channels and enhanced responsiveness to GABA(A) receptor agonist. At later time points (i.e., 12 and 30 dpe), most of the Dcx ShRNA(+) cells developed in the core of the OB and displayed aberrant dendritic length and branching. Additional analysis revealed the formation of GABAergic and glutamatergic synaptic inputs on the mispositioned neurons. Finally, quantifying fate determination by numbering the proportion of GFP(+)/calretinin(+) newborn neurons revealed that Dcx ShRNA(+) cells acquire mature phenotype despite their immature location. We conclude that altering the pace of migration at early stages of postnatal neurogenesis profoundly modifies the tightly orchestrated steps of neuronal maturation, and unveils the influence of microenvironment on controlling neuronal development in the postnatal forebrain.

Strategies for regenerating striatal neurons in the adult brain by using endogenous neural stem cells

Currently, there is no effective treatment for the marked neuronal loss caused by neurodegenerative diseases, such as Huntington's disease (HD) or ischemic stroke. However, recent studies have shown that new neurons are continuously generated by endogenous neural stem cells in the subventricular zone (SVZ) of the adult mammalian brain, including the human brain. Because some of these new neurons migrate to the injured striatum and differentiate into mature neurons, such new neurons may be able to replace degenerated neurons and improve or repair neurological deficits. To establish a neuroregenerative therapy using this endogenous system, endogenous regulatory mechanisms that can be co-opted for efficient regenerative interventions must be understood, along with any potential drawbacks. Here, we review current knowledge on the generation of new neurons in the adult brain and discuss their potential for use in replacing striatal neurons lost to neurodegenerative diseases, including HD, and to ischemic stroke.

From progenitors to integrated neurons: role of neurotransmitters in adult olfactory neurogenesis

Adult neurogenesis is due to the persistence of pools of constitutive stem cells able to give rise to a progeny of proliferating progenitors. In rodents, adult neurogenic niches have been found in the subventricular zone (SVZ) along the lateral ventricles and in the subgranular zone of the dentate gyrus in the hippocampus. SVZ progenitors undergo a unique process of tangential migration from the lateral ventricle to the olfactory bulb (OB) where they differentiate mainly into GABAergic interneurons in the granule and glomerular layers. SVZ progenitor proliferation, migration and differentiation into fully integrated neurons, are strictly related processes regulated by complex interactions between cell intrinsic and extrinsic influences. Numerous observations demonstrate that neurotrasmitters are involved in all steps of the adult neurogenic process, but the understanding of their role is hampered by their intricate mechanism of action and by the highly complex network in which neurotransmitters work. By considering the three main steps of olfactory adult neurogenesis (proliferation, migration and integration), this review will discuss recent advances in the study of neurotransmitters, highlighting the regulatory mechanisms upstream and downstream their action.

Compartmentalization from the outside: the extracellular matrix and functional microdomains in the brain

The extracellular matrix (ECM) of the central nervous system is well recognized as a migration and diffusion barrier that allows for the trapping and presentation of growth factors to their receptors at the cell surface. Recent data highlight the importance of ECM molecules as synaptic and perisynaptic scaffolds that direct the clustering of neurotransmitter receptors in the postsynaptic compartment and that present barriers to reduce the lateral diffusion of membrane proteins away from synapses. The ECM also contributes to the migration and differentiation of stem cells in the neurogenic niche and organizes the polarized localization of ion channels and transporters at contacts between astrocytic processes and blood vessels. Thus, the ECM contributes to functional compartmentalization in the brain.

Prospects and limitations of using endogenous neural stem cells for brain regeneration

Neural stem cells (NSCs) are capable of producing a variety of neural cell types, and are indispensable for the development of the mammalian brain. NSCs can be induced in vitro from pluripotent stem cells, including embryonic stem cells and induced-pluripotent stem cells. Although the transplantation of these exogenous NSCs is a potential strategy for improving presently untreatable neurological conditions, there are several obstacles to its implementation, including tumorigenic, immunological, and ethical problems. Recent studies have revealed that NSCs also reside in the adult brain. The endogenous NSCs are activated in response to disease or trauma, and produce new neurons and glia, suggesting they have the potential to regenerate damaged brain tissue while avoiding the above-mentioned problems. Here we present an overview of the possibility and limitations of using endogenous NSCs in regenerative medicine.

Extracellular matrix: functions in the nervous system

An astonishing number of extracellular matrix glycoproteins are expressed in dynamic patterns in the developing and adult nervous system. Neural stem cells, neurons, and glia express receptors that mediate interactions with specific extracellular matrix molecules. Functional studies in vitro and genetic studies in mice have provided evidence that the extracellular matrix affects virtually all aspects of nervous system development and function. Here we will summarize recent findings that have shed light on the specific functions of defined extracellular matrix molecules on such diverse processes as neural stem cell differentiation, neuronal migration, the formation of axonal tracts, and the maturation and function of synapses in the peripheral and central nervous system.

Control of neuronal migration through rostral migration stream in mice

During the nervous system development, immature neuroblasts have a strong potential to migrate toward their destination. In the adult brain, new neurons are continuously generated in the neurogenic niche located near the ventricle, and the newly generated cells actively migrate toward their destination, olfactory bulb, via highly specialized migratory route called rostral migratory stream (RMS). Neuroblasts in the RMS form chains by their homophilic interactions, and the neuroblasts in chains continually migrate through the tunnels formed by meshwork of astrocytes, glial tube. This review focuses on the development and structure of RMS and the regulation of neuroblast migration in the RMS. Better understanding of RMS migration may be crucial for improving functional replacement therapy by supplying endogenous neuronal cells to the injury sites more efficiently.

Role of sensory activity on chemospecific populations of interneurons in the adult olfactory bulb

The olfactory bulb (OB) retains a remarkable capacity to renew its interneuronal populations throughout the lifespan of animals. Neuronal precursors giving rise to the bulbar interneurons are generated in the subventricular zone and have to migrate long distances before reaching the OB. In the adult OB these neuronal precursors differentiate into distinct neuronal types, including GABAergic cells located in the granule cell layer and a diverse set of neurons in the glomerular layer comprising GABAergic and dopaminergic interneurons, as well as other neuronal subtypes expressing calretinin and calbindin. While the role of sensory activity in the integration and/or survival of newly generated cells in the olfactory system is well established, very little is known about how odorant-induced activity affects fate specification of newborn cells as well as survival and fate maintenance of preexisting neuronal populations generated in adulthood. The present study demonstrates that sensory deprivation diminishes not only the number of newborn cells in the OB, but also reduces the density of granule and periglomerular cells generated before nostril occlusion. It also shows that sensory activity has an important influence on the development and expression of dopaminergic, but not GABAergic, calretinin or calbindin phenotypes. Our data reveal that odorant-induced activity is important for the survival of both newborn and preexisting OB interneurons generated at adulthood and suggests that these chemospecific populations are differentially affected by sensory deprivation.

Olfactory ensheathing cells: attractant of neural progenitor migration to olfactory bulb

Olfactory ensheathing cells (OECs) are the glial cells that derive from the olfactory placode, envelop olfactory axons in the course of migration from the olfactory epithelium to the olfactory bulb and reside primarily in the olfactory nerve layer. OECs transplantation as a promising experimental therapy for axonal injuries has been intensively studied; however, little is known about their roles in olfactory bulb development. In this study, we examined the effects of OECs on the migration of neural progenitors in rostral migratory stream (RMS). Initially, the neurosphere migration assay showed that OEC-conditioned medium promoted progenitors to migrate from RMS neurospheres in a concentration dependent manner. Moreover, co-culturing OECs nearby the RMS explants led to asymmetric migration of explants in different developing stages. However, OECs could influence the migration in a distance not further than 1.5 mm. Finally, slice assay that mimic the circumstance in vivo revealed that OECs had a chemoattractive activity on RMS neural progenitors. Together, these results demonstrate that OECs attract neural progenitors in RMS through the release of diffusible factors and it is likely that OECs mainly influence radial migration in the olfactory bulb but not tangential migration of the RMS invivo during development. This suggests a previously unknown function for OECs in olfactory development and a novel mechanism underlying the targeting of RMS cells.

Different astroglia permissivity controls the migration of olfactory bulb interneuron precursors

The rostral migratory stream (RMS) is a well defined migratory pathway for precursors of olfactory bulb (OB) interneurons. Throughout the RMS an intense astroglial matrix surrounds the migratory cells. However, it is not clear to what extent the astroglial matrix participates in migration. Here, we have analyzed the migratory behavior of neuroblasts cultured on monolayers of astrocytes isolated from areas that are permissive (RMS and OB) and nonpermissive (cortex and adjacent cortical areas) to migration. Our results demonstrate robust neuroblast migration when RMS-explants are cultured on OB or RMS-astrocytes, in contrast to their behavior on astroglia derived from nonpermissive areas. These differences, mediated by astrocyte-derived nonsoluble factors, are related to the overexpression of extracellular matrix and cell adhesion molecules, as revealed by real-time qRT-PCR. Our results show that astroglia heterogeneity could play a significant role in migration within the RMS and in cell detachment in the OB.

Should I stay or should I go? Becoming a granule cell

Cerebellar granule cells undergo profound and rapid morphological modifications during development while they migrate from their birthplace at the surface of the cerebellar cortex to its deepest layer. Post-mitotic granule cells extend bipolar axons and sequentially use the two main modes of migration, tangential and radial, to reach their final destinations. Recent studies show that protein degradation involving key cell-cycle regulators controls granule cell axon extension. The use of knockout mice deficient in different axon-guidance molecules combined with cutting-edge imaging methods has started to shed light on the molecular mechanisms that trigger granule cell migration. These studies suggest that a major reorganization of the cytoskeleton occurs as granule cells switch from tangential to radial migration.

A symphony of signals conducts early and late stages of adult neurogenesis

Neurogenesis is continually occurring in two regions within the mammalian central nervous system (CNS) and increasing evidence suggests that it is important for selective learning and memory. How this plasticity is maintained in isolated niches within mature networks has been extensively studied in recent years, and a large body of evidence has accumulated describing many different regulatory factors and points of regulation. In this review, we attempt to organize the current research by summarizing findings affecting early neurogenesis: during proliferation, fate commitment and migration, versus late neurogenesis: including dendritic development, synaptic integration, and survival. We discuss the roles of three different classes of factors regulating early and late phases of neurogenesis: intrinsic factors, extrinsic factors, and neurotransmitters. Finally, we suggest that neurotransmitters may act upstream from extracellular other factors and cell-intrinsic mechanisms by coupling network activity to the niche microenvironment and intracellular machinery to ultimately regulate neurogenesis.

Contributions of astrocytes to synapse formation and maturation - Potential functions of the perisynaptic extracellular matrix

The concept of the tripartite synapse proposes that in addition to the presynapse and the postsynaptic membrane closely apposed processes of astrocytes constitute an integral part of the synapse. Accordingly, astrocytes may influence synaptic activity by various ways. Thus glia- and neuron-derived neurotrophins, cytokines and metabolites influence neuronal survival, synaptic activity and plasticity. Beyond these facts, the past years have shown that astrocytes are required for synaptogenesis, the structural maintenance and proper functioning of synapses. In particular, astrocytes seem to play a key role in the organization of the brain's extracellular matrix (ECM) - most prominently the so-called perineuronal nets (PNNs), complex macromolecular assemblies of ECM components. Due to progress in cellular and molecular neurosciences, it has been possible to decipher the composition of ECM structures and to obtain insight into their function(s) and underlying mechanisms. It appears that PNN-related structures are involved in regulating the sprouting and pruning of synapses, which represents an important morphological correlate of synaptic plasticity in the adult nervous system. Perturbation assays and gene elimination by recombinant techniques have provided clear indications that astrocyte-derived ECM components, e.g. the tenascins and chondroitinsulfate proteoglycans (CSPGs) of the lectican family participate in these biological functions. The present review will discuss the glia-derived glycoproteins and CSPGs of the perisynaptic ECM, their neuronal and glial receptors, and in vitro assays to test their physiological functions in the framework of the synapse, the pivotal element of communication in the central nervous system.

Transcriptome profile and cytogenetic analysis of immortalized neuronally restricted progenitor cells derived from the porcine olfactory bulb

Recently, we established and phenotypically characterized an immortalized porcine olfactory bulb neuroblast cell line, OBGF400 (1). To facilitate the future application of these cells in studies of neurological dysfunctions and neuronal pathogen interactions, a comprehensive knowledge of their genomic variability and overall gene expression capacity was pursued. Accordingly, the OBGF400 cells were subjected to karyotyping and more extensive transcriptome analyses. Cytogenetic characterization of these cells revealed a genetic mosaicism of neuronal hyperdiploidy. A direct comparison of the OBGF400 cell transcriptome pattern, generated by utilizing the Affymetrix GeneChip(R) Porcine Genome Array, to that of a non-neural, porcine epithelial cell line facilitated the identification of 831 probe sets preferentially hybridized by the neuroblast transcripts. Subsequent functional annotation of these OBGF400 RNAs using the Database for Annotation, Visualization and Integrated Discovery 2008 enabled their allocation to the corresponding gene ontology biological process term, thereby assisting the recognition of key elements involved in the regulation of neuronal signal transduction and neurogenesis.

Making neurons from mature glia: a far-fetched dream?

The fact that cells with glial characteristics such as forebrain radial glia during development and astroglial stem cells in the adult neurogenic zones serve as neuronal precursors provokes the question why glia in most other areas of the adult central nervous system are apparently incapable of generating new neurons. Besides being of pivotal biological interest answers to this question may also open new avenues for cell-based therapies of neurodegenerative diseases that involve a permanent loss of neurons which are not replaced naturally. For if one could indeed instruct glia to generate neurons, such a strategy would carry the enormous advantage of making use of a large pool of endogenous, and hence autologous cells, thereby circumventing many of the problems associated with therapeutic strategies based on transplantation. Accordingly, the recent years have seen increasing effort in assessing the plasticity of astroglia and other types of resident non-neuronal cells as a potential source for new neurons in the injured brain or eye. For instance, following injury astroglia in the cerebral cortex and Müller glia in the retina can de-differentiate and acquire stem or precursor cell like properties. Moreover, it has been shown that astroglia can be reprogrammed in vitro by forced expression of neurogenic transcription factors to transgress their lineage restriction and stably acquire a neuronal identity. In this review I will discuss the status quo of these early attempts, the limitations currently encountered and the future challenges before the full potential of this approach can be weighed.

MRI visualization of endogenous neural progenitor cell migration along the RMS in the adult mouse brain: validation of various MPIO labeling strategies

The adult rodent brain contains neural progenitor cells (NPCs), generated in the subventricular zone (SVZ), which migrate along the rostral migratory stream (RMS) towards the olfactory bulb (OB) where they differentiate into neurons. The aim of this study was to visualize endogenous NPC migration along the RMS with magnetic resonance imaging (MRI) in adult healthy mice. We evaluated various in situ (in vivo) labeling approaches using micron-sized iron oxide particles (MPIOs) on their efficiency to label endogenous NPCs. In situ labeling and visualization of migrating NPCs were analyzed by a longitudinal MRI study and validated with histology. Here, we visualized endogenous NPC migration in the mouse brain by in vivo MRI and demonstrated accumulation of MPIO-labeled NPCs in the OB over time with ex vivo MRI. Furthermore, we investigated the influence of in situ injection of MPIOs on adult neurogenesis. Quantitative analysis of bromodeoxyuridine labeled cells revealed altered proliferation in the SVZ and NPC migration after in situ MPIO injection. From the labeling strategies presented in this report, intraventricular injection of a small number of MPIOs combined with the transfection agent poly-l-lysine hydrobromide was the best method as labeling of the NPCs was successful and proliferation in the SVZ was only marginally affected. While MRI visualization of endogenous NPC migration can provide insight into aberrant NPC migration in disease models, this work emphasizes the importance to carefully explore the impact on adult neurogenesis when new in situ labeling strategies are developed.

Adult neurogenesis and the olfactory system

Though initially described in the early 1960s, it is only within the past decade that the concept of continuing adult neurogenesis has gained widespread acceptance. Neuroblasts from the subventricular zone (SVZ) migrate along the rostral migratory stream (RMS) into the olfactory bulb, where they differentiate into interneurons. Neuroblasts from the subgranular zone (SGZ) of the hippocampal formation show relatively little migratory behavior, and differentiate into dentate gyrus granule cells. In sharp contrast to embryonic and perinatal development, these newly differentiated neurons must integrate into a fully functional circuit, without disrupting ongoing performance. Here, after a brief historical overview and introduction to olfactory circuitry, we review recent advances in the biology of neural stem cells, mechanisms of migration in the RMS and olfactory bulb, differentiation and survival of new neurons, and finally mechanisms of synaptic integration. Our primary focus is on the olfactory system, but we also contrast the events occurring there with those in the hippocampal formation. Although both SVZ and SGZ neurogenesis are involved in some types of learning, their full functional significance remains unclear. Since both systems offer models of integration of new neuroblasts, there is immense interest in using neural stem cells to replace neurons lost in injury or disease. Though many questions remain unanswered, new insights appear daily about adult neurogenesis, regulatory mechanisms, and the fates of the progeny. We discuss here some of the central features of these advances, as well as speculate on future research directions.

Activity-dependent extrinsic regulation of adult olfactory bulb and hippocampal neurogenesis

The adult mammalian brain continuously generates new neurons in the olfactory bulb and hippocampus throughout life. Adult neurogenesis, a highly dynamic process, has been shown to be exquisitely modulated by neuronal circuit activity at different stages, from proliferation of adult neural progenitors, to differentiation, maturation, integration, and survival of newborn neurons in the adult brain. Strategic activity-dependent addition of new neurons into the existing neuronal circuitry represents a prominent form of structural plasticity and may contribute to specific brain functions, such as learning, memory, and mood modulation. Here we review extrinsic mechanisms through which adult neurogenesis is regulated by environmental cues, physiological learning-related stimuli, and neuronal activities.

Cell migration in the normal and pathological postnatal mammalian brain

In the developing brain, cell migration is a crucial process for structural organization, and is therefore highly regulated to allow the correct formation of complex networks, wiring neurons, and glia. In the early postnatal brain, late developmental processes such as the production and migration of astrocyte and oligodendrocyte progenitors still occur. Although the brain is completely formed and structured few weeks after birth, it maintains a degree of plasticity throughout life, including axonal remodeling, synaptogenesis, but also neural cell birth, migration and integration. The subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampus are the two main neurogenic niches in the adult brain. Neural stem cells reside in these structures and produce progenitors that migrate toward their ultimate location: the olfactory bulb and granular cell layer of the DG respectively. The aim of this review is to synthesize the increasing information concerning the organization, regulation and function of cell migration in a mature brain. In a normal brain, proteins involved in cell-cell or cell-matrix interactions together with secreted proteins acting as chemoattractant or chemorepellant play key roles in the regulation of neural progenitor cell migration. In addition, recent data suggest that gliomas arise from the transformation of neural stem cells or progenitor cells and that glioma cell infiltration recapitulates key aspects of glial progenitor migration. Thus, we will consider glioma migration in the context of progenitor migration. Finally, many observations show that brain lesions and neurological diseases trigger neural stem/progenitor cell activation and migration toward altered structures. The factors involved in such cell migration/recruitment are just beginning to be understood. Inflammation which has long been considered as thoroughly disastrous for brain repair is now known to produce some positive effects on stem/progenitor cell recruitment via the regulation of growth factor signaling and the secretion of a number of chemoattractant cytokines. This knowledge is crucial for the development of new therapeutic strategies. One of these strategies could consist in increasing the mobilization of endogenous progenitor cells that could replace lost cells and improve functional recovery.

p27(KIP1) regulates neurogenesis in the rostral migratory stream and olfactory bulb of the postnatal mouse

Neuronal progenitor cells of the anterior subventricular zone (SVZa) migrate along the rostral migratory stream (RMS) to the olfactory bulb, where they exit the cell cycle and differentiate. The molecular mechanisms that regulate SVZa progenitor proliferation and cell-cycle exit are largely undefined. We investigated the role of p27(KIP1) in regulating cell proliferation and survival in the RMS and olfactory bulb between postnatal day 1 (P1) and P14, the peak period of olfactory bulb neuron generation. A large proportion of cells in the RMS and the olfactory bulb express cytoplasmic p27(KIP1), but a small percentage display high nuclear p27(KIP1) immunostaining, which exhibit a caudal(low)-rostral(high) gradient: lowest in the SVZa and highest in the glomerular layer of the olfactory bulb. p27(KIP1) is also present in the nucleus and/or the cytoplasm of neuron-specific type III beta-tubulin(+) cells. Cells with strong nuclear p27(KIP1) expression are BrdU(-) and Ki67(-). The percentage of BrdU(+) cells in the SVZa, RMS, and olfactory bulb is higher in p27(KIP1) null than wild-type (WT) mice at all ages analyzed. Consistent with these findings, p27(KIP1) overexpression in cultured p27(KIP1) null and WT SVZ cells reduced cell proliferation and self-renewal. Finally, in p27(KIP1) null mice, the diameter of the horizontal limb of the RMS is larger than in WT mice, and development of the olfactory bulb granule cell layer is delayed, together with increased apoptotic cell density. Our results indicate that in the postnatal brain, p27(KIP1) regulates the proliferation and survival of neuronal cells in the RMS and olfactory bulb.

Vasculature guides migrating neuronal precursors in the adult mammalian forebrain via brain-derived neurotrophic factor signaling

Adult neuronal precursors retain the remarkable capacity to migrate long distances from the posterior (subventricular zone) to the most anterior [olfactory bulb (OB)] parts of the brain. The knowledge about the mechanisms that keep neuronal precursors in the migratory stream and organize this long-distance migration is incomplete. Here we show that blood vessels precisely outline the migratory stream for new neurons in the adult mammalian forebrain. Real-time video imaging of cell migration in the acute slices demonstrate that neuronal precursors are retained in the migratory stream and guided into the OB by blood vessels that serve as a physical substrate for migrating neuroblasts. Our data suggest that endothelial cells of blood vessels synthesize brain-derived neurotrophic factor (BDNF) that fosters neuronal migration via p75NTR expressed on neuroblasts. Interestingly, GABA released from neuroblasts induces Ca(2+)-dependent insertion of high-affinity TrkB receptors on the plasma membrane of astrocytes that trap extracellular BDNF. We hypothesize that this renders BDNF unavailable for p75NTR-expressing migrating cells and leads to their entrance into the stationary period. Our findings provide new insights into the functional organization of substrates that facilitate the long-distance journey of adult neuronal precursors.

Polysialic acid and activity-dependent synapse remodeling

Polysialic acid (PSA) is a large carbohydrate added post-translationally to the extracellular domain of the Neural Cell Adhesion Molecule (NCAM) that influences its adhesive and other functional properties. PSA-NCAM is widely distributed in the developing nervous system where it promotes dynamic cell interactions, like those responsible for axonal growth, terminal sprouting and target innervation. Its expression becomes restricted in the adult nervous system where it is thought to contribute to various forms of neuronal and glial plasticity. We here review evidence, obtained mainly from hypothalamic neuroendocrine centers and the olfactory system, that it intervenes in structural synaptic plasticity and accompanying neuronal-glial transformations, making possible the formation and elimination of synapses that occur under particular physiological conditions. While the mechanism of action of this complex sugar is unknown, it is now clear that it is a necessary molecular component of various cell transformations, including those responsible for activity-dependent synaptic remodeling.