Deleted in colorectal carcinoma and differentially expressed integrins mediate the directional migration of neural precursors in the rostral migratory stream

Netrin-1: diversity in development

In 1990, the discovery of three Caenorhabditis elegans genes (unc5, unc6, unc40) involved in pioneer axon guidance and cell migration marked a significant advancement in neuroscience research [Hedgecock EM, Culotti JG, Hall DH. The unc-5, unc-6, and unc-40 genes guide circumferential migrations of pioneer axons and mesodermal cells on the epidermis in C. elegans. Neuron 1990;4:61-85]. The importance of this molecular guidance system was exemplified in 1994, when the vertebrate orthologue of Unc6, Netrin-1, was discovered to be a key guidance cue for commissural axons projecting toward the ventral midline in the rodent embryonic spinal cord [Serafini T, Kennedy TE, Galko MJ, Mirzayan C, Jessell TM, Tessier-Lavigne M. The netrins define a family of axon outgrowth-promoting proteins homologous to C. elegans UNC-6. Cell 1994;78:409-424]. Since then, Netrin-1 has been found to be a critical component of embryonic development with functions in axon guidance, cell migration, morphogenesis and angiogenesis. Netrin-1 also plays a role in the adult brain, suggesting that manipulating netrin signals may have novel therapeutic applications.

Poststroke neurogenesis: emerging principles of migration and localization of immature neurons

Stroke induces proliferation of newly born neurons in the subventricular zone, migration of these immature neurons away from the SVZ, and localization within peri-infarct tissues. These 3 processes of proliferation, migration, and localization constitute distinct spatial and temporal zones within poststroke neurogenesis with distinct molecular and cell-cell signaling environments. Immature neurons migrate after stroke in close association with blood vessels and astrocytic processes, in a process that involves matrix metalloproteinases. This poststroke migration shares similar features with normal neuroblast migration in the rostral migratory stream. Immature neurons localize in the peri-infarct cortex in a neurovascular niche where neurogenesis is causally linked to angiogenesis through the vascular factors SDF-1 and angiopoietin-1. Other vascular and neuronal growth factors have also been linked to poststroke neuroblast localization in peri-infarct tissue, including erythropoietin. Most data on poststroke neurogenesis derive from laboratory rodents, which may have an abnormal or blunted degree of neurogenesis and neuroplasticity compared to normal, wild rodents. This will likely affect translational application of the principles of poststroke neurogenesis from mouse to man.

BDNF/ TrkB interaction regulates migration of SVZ precursor cells via PI3-K and MAP-K signalling pathways

Neuroblasts born in the subventricular zone (SVZ) migrate along the rostral migratory stream, reaching the olfactory bulb (OB) where they differentiate into local interneurons. Several extracellular factors have been suggested to control specific steps of this process. The brain-derived neurotrophic factor (BDNF) has been demonstrated to promote morphological differentiation and survival of OB interneurons. Here we show that BDNF and its receptor TrkB are expressed in vivo throughout the migratory pathway, implying that BDNF might also mediate migratory signals. By using in vitro models we demonstrate that BDNF promotes migration of SVZ neuroblasts, acting both as inducer and attractant through TrkB activation. We show that BDNF induces cAMP response element-binding protein (CREB) activation in migrating neuroblasts via phosphatidylinositol 3-kinase (PI3-K) and mitogen-activated protein kinase (MAP-K) signalling. Pharmacological blockade of these pathways on SVZ explants significantly reduces CREB activation and impairs neuronal migration. This study identifies a function of BDNF in the SVZ system, which involves multiple protein kinase pathways leading to neuroblast migration.

Multiphoton microscope imaging: The behavior of neural progenitor cells in the rostral migratory stream

Neural progenitor cells (NPCs) in the subventricular zone (SVZ) travel a long distance along the rostral migratory stream (RMS) to give rise to interneurons in the olfactory bulb (OB). Using the multiphoton microscope and time-lapse recording techniques we here report the behavior of NPCs in the RMS under both intact and ischemic conditions in living brain slices. The NPCs were visualized in 3-week-old transgenic mice that carry the reporter gene, green fluorescent protein (GFP), driven by the nestin promoter. Cortical brain ischemia was induced by permanent occlusion of the right common carotid artery and the middle cerebral artery. We observed that the RMS contained two populations of NPCs: nonmigrating cells (bridge cells) and migrating cells. Bridge cells enabled migrating cells to travel and also produced new cells in the RMS. The direction of NPC migration in the RMS was bidirectional in both intact and ischemic conditions. Cortical ischemia impeded NPC travel in the RMS next to the lesion area during the early period of ischemia. Cell-cell contact was a prominent feature affecting NPC translocation and migratory direction. These data suggest that behavior and function of nestin-positive NPCs in the RMS are variable. Cell-cell contacts and microenvironmental changes influence NPC behavior in the RMS. This study may provide insights to help in understanding NPC biology.

Subventricular zone-derived neuroblast migration to the olfactory bulb is modulated by matrix remodelling

In the rodent brain neural progenitor cells are born in the subventricular zone and migrate along a pathway called the rostral migratory stream (RMS) into the olfactory bulb where they differentiate into several classes of interneurones. In the adult, tangential migration in the RMS takes place in 'chains' of cells contained within glial tubes. In contrast, neonatal neuroblasts along the RMS lack these defined glial tubes and chains, migrating instead as individual cells. Time-lapse confocal microscopy of neuroblasts at each of these ages shows that individual cells migrate in a saltatory manner with bursts of high speed followed by periods of slower speed. Tangential migration within a glial tube is 20% faster than migration as individual cells. Neuroblasts may also interact and modify the extracellular matrix during migration through expression of a family of proteins, the matrix metalloproteinases (MMPs). MMPs are present and active along the subventricular zone-olfactory bulb pathway. In the presence of inhibitors of MMPs, neuroblast migration rates were reduced only when cells migrate individually. Chain migration in the adult was unaffected by MMP inhibitors. Taken together, these data suggest that MMPs only influence migration as individual cells and not as chains.

Different networks, common growth factors: shared growth factors and receptors of the vascular and the nervous system

Growth factors and their respective receptors are key regulators during development and for homeostasis of the nervous system. In addition, changes in growth factor function, availability or downstream signaling is involved in many neuropathological disorders like Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis, stroke and brain tumours. Research of the recent years revealed that some growth factors, initially discovered as neural growth factors are also affecting blood vessels [e.g. nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF)]. Likewise, vascular growth factors, such as vascular endothelial growth factor (VEGF), which was previously described as an endothelial cell specific mitogen, also affect neural cells. The discovery of shared growth factors affecting the vascular and the nervous system is of relevance for potential therapies of vascular and neurological diseases. This review aims to give an overview about the growing field of common growth factors and receptors within the two different networks.

ApoER2/VLDL receptor and Dab1 in the rostral migratory stream function in postnatal neuronal migration independently of Reelin

Postnatal migration of interneuron precursors from the subventricular zone to the olfactory bulb occurs in chains that form the substrate for the rostral migratory stream. Reelin is suggested to induce detachment of neuroblasts from the chains when they arrive at the olfactory bulb. Here we show that ApoER2 and possibly very-low-density lipoprotein receptor (VLDLR) and their intracellular adapter protein Dab1 are involved in chain formation most likely independent of Reelin. F-spondin, which is present in the stream, may act as ligand for ApoER2 and VLDLR. In mice lacking either both receptors or Dab1 chain formation is severely compromised, and as a consequence the rostral migratory stream is virtually absent and neuroblasts accumulate in the subventricular zone. The mutant animals exhibit severe neuroanatomical defects in the subventricular zone and in the olfactory bulb. These data demonstrate a cell-autonomous function of ApoER2, and most likely VLDLR and Dab1, in postnatal migration of neuroblasts in the forebrain, which is suggested to depend on ligands other than Reelin.

Netrins: beyond the brain

Named after the Sanskrit word netr, which means 'one who guides', the netrin family of secreted proteins provides migrational cues in the developing central nervous system. Recently, netrins have also been shown to regulate diverse processes (such as cell adhesion, motility, proliferation, differentiation and, ultimately, cell survival) in a number of non-neuronal tissues. In some cases, netrins affect these functions through non-classic netrin receptors, prompting a renewed interest in these factors in and beyond the nervous system.

Neuronal migration in the adult brain: are we there yet?

The generation and targeting of appropriate numbers and types of neurons to where they are needed in the brain is essential for the establishment, maintenance and modification of neural circuitry. This review aims to summarize the patterns, mechanisms and functional significance of neuronal migration in the postnatal brain, with an emphasis on the migratory events that persist in the mature brain.

Subventricular zone cells remain stable in vitro after brain injury

Subventricular zone (SVZ) cells emigrate toward brain injury but relatively few survive. Thus, if they are to be used for repair, ex vivo expansion and autologous transplantation of SVZ cells may be necessary. Since it is unclear how brain injury alters SVZ cell culture, we studied neurosphere formation, differentiation, and migration, after cortical lesions. The number of neurosphere forming cells from lesioned mice was comparable to controls. Also, the proportion of astrocytes and neurons generated in vitro remained unchanged after cortical lesions. Cell emigration from neurospheres was characterized by increased cell-cell contact after injury in adults and neonates. However, neither molecules implicated in SVZ migration nor the extent of migration changed after injury. Thus, neurospheres can be successfully cultured after extensive brain damage, and they are remarkably stable in vitro, suggesting suitability for ex vivo expansion and autologous transplantation.

Characterization of neogenin-expressing neural progenitor populations and migrating neuroblasts in the embryonic mouse forebrain

Many studies have demonstrated a role for netrin-1-deleted in colorectal cancer (DCC) interactions in both axon guidance and neuronal migration. Neogenin, a member of the DCC receptor family, has recently been shown to be a chemorepulsive axon guidance receptor for the repulsive guidance molecule (RGM) family of guidance cues [Rajagopalan S, Deitinghoff L, Davis D, Conrad S, Skutella T, Chedotal A, Mueller B, Strittmatter S (2004) Neogenin mediates the action of repulsive guidance molecule. Nat Cell Biol 6:755-762]. Here we show that neogenin is present on neural progenitors, including neurogenic radial glia, in the embryonic mouse forebrain suggesting that neogenin expression is a hallmark of neural progenitor populations. Neogenin-positive progenitors were isolated from embryonic day 14.5 forebrain using flow cytometry and cultured as neurospheres. Neogenin-positive progenitors gave rise to neurospheres displaying a high proliferative and neurogenic potential. In contrast, neogenin-negative forebrain cells did not produce long-term neurosphere cultures and did not possess a significant neurogenic potential. These observations argue strongly for a role for neogenin in neural progenitor biology. In addition, we also observed neogenin on parvalbumin- and calbindin-positive interneuron neuroblasts that were migrating through the medial and lateral ganglionic eminences, suggesting a role for neogenin in tangential migration. Therefore, neogenin may be a multi-functional receptor regulating both progenitor activity and neuroblast migration in the embryonic forebrain.

Neurogenic astrocytes transplanted into the adult mouse lateral ventricle contribute to olfactory neurogenesis, and reveal a novel intrinsic subependymal neuron

Spatially and temporally restricted populations of neurogenic astrocytes can generate multipotent neurospheres in vitro. To examine the ability of neurogenic astrocytes to respond to in vivo differentiation cues within a germinal matrix, we provided cultured neonatal cerebellar astrocytes access to the subependymal zone (SEZ) by grafting them directly into the lateral ventricle of adult mice. Here we report three events that follow such transplants. 1) Donor cells attach to periventricular structures, and form "neoplastic-like" spheres that penetrate the ventricular wall. These attached spheres can persist for months, as they give rise to "clones" of cells that infiltrate forebrain parenchyma. 2) Many donor cells enter the rostral migratory stream and migrate into the olfactory bulb where a small percentage differentiates as olfactory interneurons. 3) Finally, within the SEZ, some donor cells formed cell clusters that appear to interact with the SEZ neuronal precursor chains, and some donor cells differentiate into distinctive neurons with extensive, beady projections precisely confined between the ependymal layer and the striatum. Further analysis of normal SEZ anatomy reveals indigenous neurons with identical morphologies--some of which are contacted by 5-HT+ fibers--that we propose represent a heretofore uncharacterized, intrinsic SEZ neuron of unknown function. These results suggest that cultured astrocytes derived from non-SEZ brain regions can respond in different ways to in vivo cues provided by the adult lateral ventricle and SEZ by differentiating into neurons that eventually inhabit both the olfactory bulb and SEZ proper.

The ADAMs family: coordinators of nervous system development, plasticity and repair

A disintegrin and metalloprotease (ADAM) transmembrane proteins have metalloprotease, integrin-binding, intracellular signaling and cell adhesion activities. In contrast to other metalloproteases, ADAMs are particularly important for cleavage-dependent activation of proteins such as Notch, amyloid precursor protein (APP) and transforming growth factor alpha (TGFalpha), and can bind integrins. Not surprisingly, ADAMs have been shown or suggested to play important roles in the development of the nervous system, where they regulate proliferation, migration, differentiation and survival of various cells, as well as axonal growth and myelination. On the eleventh anniversary of the naming of this family of proteins, the relatively unknown ADAMs are emerging as potential therapeutic targets for neural repair. For example, over-expression of ADAM10, one of the alpha-secretases for APP, can prevent amyloid formation and hippocampal defects in an Alzheimer mouse model. Another example of this potential neural repair role is the finding that ADAM21 is uniquely associated with neurogenesis and growing axons of the adult brain. This comprehensive review will discuss the growing literature about the roles of ADAMs in the developing and adult nervous system, and their potential roles in neurological disorders. Most excitingly, the expanding understanding of their normal roles suggests that they can be manipulated to promote neural repair in the degenerating and injured adult nervous system.

La neurogenèse bulbaire et son impact neurologique

Contrary to the long-held dogma according to which the adult mammalian brain does not produce neurons anymore, neuronal turnover has been reported in two discrete areas of the adult brain: the hippocampus and the olfactory bulb. Adult-generated neurons are produced from neural stem cells located in the hippocampal subgranular zone and the subventricular zone of the lateral ventricles. Recently, number of genetic and epigenetic factors that modulate proliferation of stem cells, migration, differentiation and survival of newborn neurons have been characterized. We know that neurogenesis increases in the diseased brain, after stroke or after traumatic brain injury. Importantly, progenitors from the subventricular zone, but not from the subgranular zone, are incorporated at the sites of injury, where they replace some of the degenerated neurons. Thus, the central nervous system has the capacity to regenerate itself after injury and, today, researchers develop strategies aimed at promoting neurogenesis in diseased areas. This basic research is attracting a lot of attention because of the hope that it will lead to regeneration and reconstruction therapy for the damaged brain. In this review, we discuss major findings concerning the organization of the neurogenic niche located in the subventricular zone and examine both intrinsic and extrinsic factors that regulate adult neurogenesis. Then, we present evidences for the intrinsic capability of the adult brain for cell replacement, and shed light on recent works demonstrating that one can greatly enhance appropriate brain cell replacement by using molecular cues known to endogenously control proliferation, migration, differentiation and/or survival of subventricular zone progenitors. Finally, we review some of the advantages and limits of strategies aimed at using endogenous progenitors and their relevance to human clinics.

Delayed onset of odor detection in neonatal mice lacking tenascin-C

The olfactory bulb is one of the few regions in the adult mammalian forebrain in which neurons are constitutively replaced throughout life. New neurons generated in the subventricular zone migrate long distances along the rostral migratory stream to the olfactory bulb where they differentiate into interneurons. Neuronal precursor generation, migration and incorporation into the bulbar network occur in an environment rich in extracellular matrix molecules. We investigated the potential role of one of the constituents of the extracellular matrix, tenascin-C (TNC), in bulbar neurogenesis and olfactory performance using TNC-deficient mice. We found that TNC deficiency resulted in a delayed onset of olfactory responses in neonatal animals. This delay normalized at around postnatal day 10. Interestingly, this delay in early olfactory performance was not due to impaired bulbar neurogenesis as proliferation, migration, incorporation and fate determination of newborn bulbar interneurons were normal in TNC-deficient animals. Thus, we conclude that a constitutive lack of TNC does not affect bulbar neurogenesis, but instead leads to ontogenetically early impairments in olfactory detection.

Position and time specify the migration of a pioneering population of olfactory bulb interneurons

We defined the cellular mechanisms for genesis, migration, and differentiation of the initial population of olfactory bulb (OB) interneurons. This cohort of early generated cells, many of which become postmitotic on embryonic day (E) 14.5, differentiates into a wide range of mature OB interneurons by postnatal day (P) 21, and a substantial number remains in the OB at P60. Their precursors autonomously acquire a distinct identity defined by their position in the lateral ganglionic eminence (LGE). The progeny migrate selectively to the OB rudiment in a pathway that presages the rostral migratory stream. After arriving in the OB rudiment, these early generated cells acquire cellular and molecular hallmarks of OB interneurons. Other precursors--including those from the medial ganglionic eminence (MGE) and OB--fail to generate neuroblasts with similar migratory capacity when transplanted to the LGE. The positional identity and migratory specificity of the LGE precursors is rigidly established between E12.5 and E14.5. Thus, the pioneering population of OB interneurons is generated from spatially and temporally determined LGE precursors whose progeny uniquely recognize a distinct migratory trajectory.

Doublecortin maintains bipolar shape and nuclear translocation during migration in the adult forebrain

The ability of the mature mammalian nervous system to continually produce neuronal precursors is of considerable importance, as manipulation of this process might one day permit the replacement of cells lost as a result of injury or disease. In mammals, the anterior subventricular zone (SVZa) region is one of the primary sites of adult neurogenesis. Here we show that doublecortin (DCX), a widely used marker for newly generated neurons, when deleted in mice results in a severe morphological defect in the rostral migratory stream and delayed neuronal migration that is independent of direction or responsiveness to Slit chemorepulsion. DCX is required for nuclear translocation and maintenance of bipolar morphology during migration of these cells. Our data identifies a critical function for DCX in the movement of newly generated neurons in the adult brain.

Analysis of Cell Migration Using Whole-Genome Expression Profiling of Migratory Cells in the Drosophila Ovary

Cell migration contributes to normal development and homeostasis as well as to pathological processes such as inflammation and tumor metastasis. Previous genetic screens have revealed signaling pathways that govern follicle cell migrations in the Drosophila ovary, but few downstream targets of the critical transcriptional regulators have been identified. To characterize the gene expression profile of two migratory cell populations and identify Slbo targets, we purified border cells and centripetal cells expressing the mouse CD8 antigen and carried out whole-genome microarray analysis. Genes predicted to control actin dynamics and the endocytic and secretory pathways were overrepresented in the migratory cell transcriptome. Mutations in five genes, including ttk, failed to complement previously isolated mutations that cause cell migration defects in mosaic clones. Functional analysis revealed a role for the Notch-activating protease Kuzbanian in border cell migration and identified Tie as a guidance receptor for the border cells.

Regulation of human neural precursor cells by laminin and integrins

Deciphering the factors that regulate human neural stem cells will greatly aid in their use as models of development and as therapeutic agents. The extracellular matrix (ECM) is a component of stem cell niches in vivo and regulates multiple functions in diverse cell types, yet little is known about its effects on human neural stem/precursor cells (NSPCs). We therefore plated human NSPCs on four different substrates (poly-L-ornithine, fibronectin, laminin, and matrigel) and compared their responses with those of mouse NSPCs. Compared with the other substrates, laminin matrices enhanced NSPC migration, expansion, differentiation into neurons and astrocytes, and elongation of neurites from NSPC-derived neurons. Laminin had a similar spectrum of effects on both human and mouse cells, highlighting the evolutionary conservation of NSPC regulation by this component of the ECM. Flow cytometry revealed that human NSPCs express on their cell surfaces the laminin-binding integrins alpha3, alpha6, alpha7, beta1, and beta4, and function-blocking antibodies to the alpha6 subunit confirmed a role for integrins in laminin-dependent migration of human NSPCs. These results define laminin and its integrin receptors as key regulators of human NSPCs.

The alpha6 integrin subunit in the developing mouse olfactory bulb

Integrins are heterodimeric cell surface receptors that mediate developmental events by binding extracellular matrix ligands. Several lines of evidence suggest a role for integrins, specifically the alpha 6 subunit, in neuronal migration, neurite outgrowth, and axon guidance during olfactory development. Therefore, we undertook an analysis of the expression of the alpha 6 subunit in the olfactory system of the embryonic and early postnatal mouse to understand the role it may play during neural development. In addition, as a functional assay we examined the developmental effects of the loss of this subunit on olfactory development by analyzing an alpha 6 knockout (alpha 6-/-). Immunohistochemical analyses and confocal microscopy were used to examine alpha 6 expression in the CD-1 embryonic and early postnatal olfactory system and also to examine the organization of the olfactory system in the alpha 6-/- mouse. In CD-1 mice from E13 to E17, alpha 6 localizes in radial patterns extending from the core of the olfactory bulb to the nerve layer and colocalizes with RC2, an antibody specific for radial glia. By the day of birth (P0; approximately E19), expression is limited to the external plexiform layer and the olfactory nerve layer, where it colocalizes with laminin and p75. In the alpha 6-/- mouse, areas of ectopic granule cells were observed in the mitral cell layer of the olfactory bulb. These ectopias coincided with areas of disorganization of the radial glial processes and breaks in the mitral cell layer. These observations suggest a role for alpha 6 integrin in neural migration during olfactory development, likely secondary to organization of the radial glial scaffold.

GDNF is a chemoattractant factor for neuronal precursor cells in the rostral migratory stream

Olfactory bulb (OB) interneurons are generated from neuroblast cells derived from the anterior subventricular zone (SVZa) of the forebrain. The mechanisms guiding the rostral migration of these neuronal precursors are not well understood. Here, we show that glial cell line-derived neurotrophic factor (GDNF) is produced in the olfactory bulb but distributed along the rostral migratory stream (RMS) in a pattern concordant with the expression of its GPI-anchored receptor GFRalpha1. We demonstrate that GDNF is a chemoattractant factor for RMS-derived neuronal precursors, but not for SVZa neuroblast cells. In agreement with this, GDNF increased Cyclin-dependent kinase 5 (Cdk5) activity in RMS cells, a kinase critically involved in neuronal migration and guidance. GDNF-mediated cell chemoattraction was abrogated in RMS explants treated with the Cdk5 inhibitor Roscovitine as well as in RMS explants isolated from Ncam mutant mice. Chemical cross-linking assays showed that 125I-GDNF is able to interact directly with NCAM in RMS-derived cells. Taken together, these data demonstrate that GDNF is a direct chemoattractant factor for neuroblast cells migrating along the RMS and support the participation of NCAM during this guidance process.

Subventricular zone neuroblasts emigrate toward cortical lesions

Adult subventricular zone (SVZ) neuroblasts migrate in the rostral migratory stream to the olfactory bulbs. Brain lesions generally increase SVZ neurogenesis or gliogenesis and cause SVZ cell emigration to ectopic locations. We showed previously that glia emigrate from the SVZ toward mechanical injuries of the somatosensory cerebral cortex in mice. Here we tested the hypotheses that SVZ neurogenesis increases, that neuroblasts emigrate, and that epidermal growth factor expression increases after cortical injuries. Using immunohistochemistry for phenotypic markers and BrdU, we show that newborn doublecortin-positive SVZ neuroblasts emigrated toward cerebral cortex lesions. However, the number of doublecortin-positive cells in the olfactory bulbs remained constant, suggesting that dorsal emigration was not at the expense of rostral migration. Although newborn neuroblasts emigrated, rates of SVZ neurogenesis did not increase after cortical lesions. Finally, we examined molecules that may regulate emigration and neurogenesis after cortical lesions and found that epidermal growth factor was increased in the SVZ, corpus callosum, and cerebral cortex. These results suggest that after injuries to the cerebral cortex, neuroblasts emigrate from the SVZ, that emigration does not depend either on redirection of SVZ cells or on increased neurogenesis, and that epidermal growth factor may induce SVZ emigration.

A disintegrin and metalloprotease 21 (ADAM21) is associated with neurogenesis and axonal growth in developing and adult rodent CNS

We have reported that alpha6beta1 integrin regulates the directed migration of neuroblasts from the adult rodent subventricular zone (SVZ) through the rostral migratory stream (RMS). ADAM (a disintegrin and metalloprotease) proteins bind integrins. Here, we show that ADAM21, but not ADAM2, -3, -9, -10, -12, -15, or -17, is expressed in adult rats and mice by ependyma and SVZ cells with long basal processes, and in radial glia at early postnatal times. ADAM21-positive processes projected into the RMS, contacted blood vessels, and were present within the RMS intermingled with neuroblasts up to where neuroblasts start their radial migration and differentiation in the olfactory bulb. Tissue inhibitors of metalloproteases (TIMPs) 1, 2, and 3 are present in the ependymal layer but not in the SVZ and RMS. Thus, ADAM21 could regulate neurogenesis and guide neuroblast migration through cleavage-dependent activation of proteins and integrin binding. ADAM21 is also present in growing axonal tracts during postnatal development and in growing primary olfactory axons in adults. In the olfactory nerve layer, ADAM21 often, but not always, colocalizes with OMP, a marker of mature olfactory neurons, but is not colocalized with the immature marker betaIII-tubulin. This suggests that ADAM21 is involved in the final axonal outgrowth phase and/or synapse formation. TIMP3 is present in periglomerular neurons, where it could restrict ADAM21-mediated axonal growth to the glomeruli. ADAM21's unique disintegrin and metalloprotease sequences and its restricted expression suggest that it might be a good target for influencing neurogenesis and neuronal plasticity.

The brain within the tumor: new roles for axon guidance molecules in cancers

Slits, semaphorins and netrins are three families of proteins that can attract or repel growing axons and migrating neurons in the developing nervous system of vertebrates and invertebrates. Recent studies have shown that they are widely expressed outside the nervous system and that they may play important roles in cancers. Several of the genes encoding these proteins are localized on chromosomal region associated with frequent loss-of-heterozygosity in tumors and cancer cell lines and there is also significant hypermethylation of their promoter suggesting that they may act as tumor suppressors. In addition, proteins in all these families and their receptors appear to control the vascularization of the tumors. Last, many axon guidance molecules also regulate cell migration and apoptosis in normal and tumorigenic tissues. Overall, this suggests that molecules that could mimick or block the activity of axon guidance molecules may be used as therapeutic agents for the treatment of malignancy.

Neuronal expression of keratinocyte-associated transmembrane protein-4, KCT-4, in mouse brain and its up-regulation by neurite outgrowth of Neuro-2a cells

One group of proteins that regulates neurite outgrowth and maintains neuronal networks is the immunoglobulin superfamily (IgSF). We previously identified a new member of the IgSF, keratinocyte-associated transmembrane protein-4 (KCT-4), by the signal sequence-trap method from primary cultured human keratinocytes. The KCT-4 mRNA has been found to be highly expressed in the adult human brain, although it is also distributed in various tissues. In the present study, to gain insight into the role of KCT-4 in the nervous system, we examined the expression profile and localization of KCT-4 mRNA in mouse brain. We also evaluated changes in KCT-4 mRNA expression in the differentiation of the neuroblastoma cell line Neuro-2a as the in vitro model of neurite outgrowth. KCT-4 mRNA was detected broadly in various regions of the adult mouse brain by RT-PCR. In situ hybridization revealed that it was expressed highly selectively by neurons but not by glial cells. Moreover, expression of KCT-4 mRNA was induced by neurite outgrowth of Neuro-2a. These data suggest that KCT-4 participates in the regulation of neurite outgrowth and maintenance of the neural network in the adult brain.

Adult neurogenesis in the mammalian central nervous system

Forty years since the initial discovery of neurogenesis in the postnatal rat hippocampus, investigators have now firmly established that active neurogenesis from neural progenitors continues throughout life in discrete regions of the central nervous systems (CNS) of all mammals, including humans. Significant progress has been made over the past few years in understanding the developmental process and regulation of adult neurogenesis, including proliferation, fate specification, neuronal maturation, targeting, and synaptic integration of the newborn neurons. The function of this evolutionarily conserved phenomenon, however, remains elusive in mammals. Adult neurogenesis represents a striking example of structural plasticity in the mature CNS environment. Advances in our understanding of adult neurogenesis will not only shed light on the basic principles of adult plasticity, but also may lead to strategies for cell replacement therapy after injury or degenerative neurological diseases.

Regulated formation and selection of neuronal processes underlie directional guidance of neuronal migration

Axon guidance and neuronal migration are critical features of neural development, and it is believed that extracellular gradients of secreted guidance cues play important roles in pathfinding. It has been well documented that the growth cones of extending axons respond to such extracellular gradients by growing toward or away from the source of the secreted cue via asymmetrical extension of a single growth cone. However, it is unclear whether migrating neurons change direction in response to guidance molecules using the same mode of turning as extending axons. In this study, we demonstrate that migrating neurons turn away from the chemo-repellent Slit through repeated rounds of process extension and retraction and do not turn through the reorientation of a single growth cone. We further show that Slit increases the rate of somal process formation and that these processes form preferentially on the side of the cell body furthest away from the Slit source. In addition, Slit causes cell turning through asymmetric process selection. Finally, we show that multiple types of migrating neurons employ this mode of cell turning in response to a variety of guidance cues. These results show that migrating neurons employ a unique type of turning when faced with secreted guidance cues that is distinct from the type employed by axons.

Chain formation and glial tube assembly in the shift from neonatal to adult subventricular zone of the rodent forebrain

The subventricular zone (SVZ) is regarded as an embryonic germinal layer persisting at the end of cerebral cortex neurogenesis and capable of generating neuronal precursors throughout life. The two distinct compartments of the adult rodent forebrain SVZ, astrocytic glial tubes and chains of migrating cells, are not distinguishable in the embryonic and early postnatal counterpart. In this study we analyzed the SVZ of mice and rats around birth and throughout different postnatal stages, describing molecular and morphological changes which lead to the typical structural arrangement of adult SVZ. In both species studied, most changes occurred during the first month of life, the transition being slightly delayed in mice, in spite of their earlier development. Important modifications affected the glial cells, eventually leading to glial tube assembly. These changes involved an overall reorganization of glial processes and their mutual relationships, as well as gliogenesis occurring within the SVZ which gives rise to glial cell subpopulations. The neuroblast cell population remained qualitatively quite homogeneous throughout all the stages investigated, changes being restricted to the relationships among cells and consequent formation of chains at about the third postnatal week. Electron microscopy showed that chain formation is not directly linked to glial tube assembly, generally preceding the occurrence of complete glial ensheathment. Moreover, chain and glial tube formation is asymmetric in the medial/lateral aspect of the SVZ, being inversely related. The attainment of an adult SVZ compartmentalization, on the other hand, seems linked to the pattern of expression of adhesion and extracellular matrix molecules.

Assays for measuring extracellular GABA levels and cell migration rate in acute slices

The postnatal subventricular zone (SVZ) contains the largest pool of dividing and migrating neural precursors in the adult rodent brain. Neuronal precursors migrate throughout the SVZ and along the rostral migratory stream (RMS) towards the olfactory bulb where they differentiate into interneurons. To facilitate the investigation of cell migration in the SVZ and RMS, an inexpensive migration assay was developed for use in acute brain slices. Acute sagittal slices were kept at 37 degrees C in 5% O2/95% CO2-saturated solution and migrating cells in the SVZ and RMS were visualized using an upright infrared-differential interference contrast microscope. Time-lapse movies were acquired to identify the direction and measure the speed of cell migration. The neurotransmitter GABA and inhibitors of GABA receptors or transporters can be bath applied to determine the function of endogenous GABA on the direction and speed of cell migration. In parallel, the levels of endogenous GABA released from acute SVZ or RMS explants were measured with mass spectrometry. Additional techniques such as electrophysiology and immunohistochemistry confirmed the identity of cells as neuronal precursors and characterized the expression of GABA receptors and transporters. This report describes how modulations in the direction and speed of neuronal precursor migration can be accurately monitored and how changes in local GABA levels can be measured. The described techniques can be used to identify the endogenous factors that regulate cell migration. Identifying such factors is essential for the future therapeutic use of SVZ cells to replace damaged or lost cells.

Planarian homologs of netrin and netrin receptor are required for proper regeneration of the central nervous system and the maintenance of nervous system architecture

Conserved axon guidance mechanisms are essential for proper wiring of the nervous system during embryogenesis; however, the functions of these cues in adults and during regeneration remain poorly understood. Because freshwater planarians can regenerate a functional central nervous system (CNS) from almost any portion of their body, they are useful models in which to study the roles of guidance cues during neural regeneration. Here, we characterize two netrin homologs and one netrin receptor family member from Schmidtea mediterranea. RNAi analyses indicate that Smed-netR (netrin receptor) and Smed-netrin2 are required for proper CNS regeneration and that Smed-netR may mediate the response to Smed-netrin2. Remarkably, Smed-netR and Smed-netrin2 are also required in intact planarians to maintain the proper patterning of the CNS. These results suggest a crucial role for guidance cues, not only in CNS regeneration but also in maintenance of neural architecture.

Regulation of neural progenitor proliferation and survival by beta1 integrins

Neural stem cells give rise to undifferentiated nestin-positive progenitors that undergo extensive cell division before differentiating into neuronal and glial cells. The precise control of this process is likely to be, at least in part, controlled by instructive cues originating from the extracellular environment. Some of these cues are interpreted by the integrin family of extracellular matrix receptors. Using neurosphere cell cultures as a model system, we show that beta1-integrin signalling plays a crucial role in the regulation of progenitor cell proliferation, survival and migration. Following conditional genetic ablation of the beta1-integrin allele, and consequent loss of beta1-integrin cell surface protein, mutant nestin-positive progenitor cells proliferate less and die in higher numbers than their wild-type counterparts. Mutant progenitor cell migration on different ECM substrates is also impaired. These effects can be partially compensated by the addition of exogenous growth factors. Thus, beta1-integrin signalling and growth factor signalling tightly interact to control the number and migratory capacity of nestin-positive progenitor cells.

Integrating new neurons into the adult olfactory bulb: joining the network, life–death decisions, and the effects of sensory experience

In contrast to the situation in the developing brain, neurons born during adulthood must integrate into established neuronal networks characterized by ongoing activity. For sensory systems, this neuronal activity is driven mainly by external stimuli that can lead to experience-dependent morpho-functional changes in adult circuits. Here, we describe new insights into the mechanisms by which sensory experience might govern the targeting of adult-generated neurons to appropriate regions, their differentiation into distinct neuronal subtypes, and finally their survival in the adult olfactory bulb. We propose not only that neurogenesis depends on the degree of sensory experience, but also that new neurons bring unique features to the operational network, allowing a continuous adjustment of information processing in response to an ever-changing external word.

Fgf10expression identifies parabronchial smooth muscle cell progenitors and is required for their entry into the smooth muscle cell lineage

Lineage formation in the lung mesenchyme is poorly understood. Using a transgenic mouse line expressing LacZ under the control of Fgf10 regulatory sequences, we show that the pool of Fgf10-positive cells in the distal lung mesenchyme contains progenitors of the parabronchial smooth muscle cells. Fgf10 gene expression is slightly repressed in this transgenic line. This allowed us to create a hypomorphic Fgf10 phenotype by expressing the LacZtransgene in a heterozygous Fgf10 background. Hypomorphic Fgf10 mutant lungs display a decrease inβ-galactosidase-positive cells around the bronchial epithelium associated with an accumulation of β-galactosidase-expressing cells in the distal mesenchyme. This correlates with a marked reduction of α smooth muscle actin expression, thereby demonstrating that FGF10 is mostly required for the entry of mesenchymal cells into the parabronchial smooth muscle cell lineage. The failure of exogenous FGF10 to phosphorylate its known downstream targets ERK and AKT in lung mesenchymal cultures strongly suggests that FGF10 acts indirectly on the progenitor population via an epithelial intermediate. We provide support for a role of epithelial BMP4 in mediating the formation of parabronchial smooth muscle cells.

Arx homeobox gene is essential for development of mouse olfactory system

The olfactory system provides an excellent model in which to study cell proliferation, migration, differentiation, axon guidance, dendritic morphogenesis, and synapse formation. We report here crucial roles of the Arx homeobox gene in the developing olfactory system by analyzing its mutant phenotypes. Arx protein was expressed strongly in the interneurons and weakly in the radial glia of the olfactory bulb, but in neither the olfactory sensory neurons nor bulbar projection neurons. Arx-deficient mice showed severe anatomical abnormalities in the developing olfactory system: (1) size reduction of the olfactory bulb, (2) reduced proliferation and impaired entry into the olfactory bulb of interneuron progenitors, (3) loss of tyrosine hydroxylase-positive periglomerular cells, (4) disorganization of the layer structure of the olfactory bulb, and (5) abnormal axonal termination of olfactory sensory neurons in an unusual axon-tangled structure, the fibrocellular mass. Thus, Arx is required for not only the proper developmental processes of Arx-expressing interneurons, but also the establishment of functional olfactory neural circuitry by affecting Arx-non-expressing sensory neurons and projection neurons. These findings suggest a likely role of Arx in regulating the expression of putative instructive signals produced in the olfactory bulb for the proper innervation of olfactory sensory axons.

Ventral migration of early-born neurons requires Dcc and is essential for the projections of primary afferents in the spinal cord

Neuronal migration and lamina-specific primary afferent projections are crucial for establishing spinal cord circuits, but the underlying mechanisms are poorly understood. Here, we report that in mice lacking Dcc (deleted in colorectal cancer), some early-born neurons could not migrate ventrally in the spinal cord. Conversely, forced expression of Dcc caused ventral migration and prevented dorsolateral migration of late-born spinal neurons. In the superficial layer of the spinal cord of Dcc-/- mutants, mislocalized neurons are followed by proprioceptive afferents, while their presence repels nociceptive afferents through Sema3a. Thus, our study has shown that Dcc is a key molecule required for ventral migration of early-born neurons, and that appropriate neuronal migration is a prerequisite for, and coupled to, normal projections of primary afferents in the developing spinal cord.

Specific beta1 integrins mediate adhesion, migration, and differentiation of neural progenitors derived from the embryonic striatum

Early inductive signals within the embryonic mammalian forebrain establish two major germinal regions along the dorsal-ventral axis. The dorsal germinal zone eventually forms the cerebral cortex while the ventral ganglionic eminence primarily forms the striatum and globus pallidus. The mechanisms leading to patterning of specific forebrain structures from these distinct germinal regions are not fully understood but may involve the adhesive and migratory properties of regionally specified cells and their interactions with the extracellular environments in which they reside. In the present study, we isolated ganglionic eminence neural progenitor cells (geNPC), precursors of the adult striatum, from the ventral forebrain germinal zone and analyzed adhesion, migration, and differentiation of geNPC on various extracellular matrix (ECM) substrates in vitro. Specifically, we evaluated the role of beta1 integrins, a family of cell surface receptors important in neural development, in mediating geNPC behavior on ECM molecules expressed in embryonic brain tissue. Adhesion and migration of geNPC were significantly enhanced on laminin (LN) and fibronectin (FN) relative to other ECM substrates. Antibody perturbation experiments revealed that although geNPC express several beta1 integrins (alpha1beta1, alpha2beta1, alpha3beta1, alpha5beta1, alpha6beta1, alphavbeta1), adhesion and migration on LN and FN were primarily mediated by alpha6beta1 and alpha5beta1, respectively, and these interactions were confirmed by biochemical cross-link/extraction procedures. Finally, neuronal differentiation of geNPC was enhanced on LN, indicating a role for LN in geNPC differentiation. beta1 integrin-ECM interactions may contribute to basic mechanisms of striatal development and may explain the potent migratory capacity of geNPC transplanted into the adult brain.

The Netrin family of guidance factors: emphasis on Netrin-1 signalling

During the development of the nervous system, neurons respond to the coordinated action of a variety of attractive and repulsive signals from the embryonic environment. Netrins form a family of extracellular proteins that regulate the migration of neurons and axonal growth cones. These proteins are bifunctional signals that are chemoattractive for some neurons and chemorepellent for others. Netrins mainly interact with the specific receptors DCC and UNC-5 family. To date, several Netrins have been described in mouse and humans: Netrin-1, -3/NTL2, -4/beta and G-Netrins. Netrin-1 is the most studied member of the family. It is involved in the development many projections of the nervous system. When Netrin-1 interacts with its specific receptors, a cascade of local cytoplasmic events is triggered. Several signal transduction pathways and effector molecules have been implicated in the response to Netrin-1: small Rho-GTPases, MAP-Kinases, second messengers and the Microtubule Associated Protein 1B (MAP1B).

The Corneal Epithelial Stem Cell Niche

In recent years, it has become generally accepted that the corneal epithelial stem cells are localized in the basal cell layer of the limbal epithelium. However, a number of questions remain regarding the number, markers, generation, and maintenance of the corneal epithelial stem cells. One of the key questions concerns what makes up the microenvironment or niche that is responsible for allowing the stem cells to remain and function throughout the life of the tissue. This review will consider the unique aspects of the limbus and compare these to what is known about other stem cell niches.

Receptor tyrosine kinase ErbB4 modulates neuroblast migration and placement in the adult forebrain

Neural progenitor proliferation, differentiation and migration are continually active in the rostral migratory stream of the adult brain. Here, we show that the receptor tyrosine kinase ErbB4 is expressed prominently by the neuroblasts present in the subventricular zone and the rostral migratory stream. The neuregulins (NRG1-NRG3), which have been identified as ErbB4 ligands, are detected either in the stream or in adjacent regions. Mice deficient in ErbB4 expressed under the control of either the nestin or the hGFAP promoter have altered neuroblast chain organization and migration and deficits in the placement and differentiation of olfactory interneurons. These findings suggest that ErbB4 activation helps to regulate the organization of neural chains that form the rostral migratory stream and influences the differentiation of olfactory interneuronal precursors.

Aging and neuronal replacement

Neural stem cells contribute to neurogenesis in both the embryonic and adult brain. However, while adult neural stem cells produce new neurons that populate the olfactory bulb and the granule cell layer of the hippocampus, they do not normally participate in reparative neurogenesis following injury or disease affecting regions distant from the subventricular zone or the dentate gyrus. Here we review differences between neural stem cells found in the embryo and the adult, and describe factors that enhance neuronal output from these cells in vivo. Additionally, we review evidence that neural stem cells can be transplanted into injured regions of the adult brain to enhance compensatory neurogenesis from endogenous precursors. Pre-differentiation of neural stem cells into immature neurons prior to transplantation can also aid in functional recovery following injury or disease.

Neonatal hypoxic-ischemic injury increases forebrain subventricular zone neurogenesis in the mouse

Neurogenesis persists throughout life in the rodent subventricular zone (SVZ)-olfactory bulb pathway and increases in the adult after brain insults. The influence of neonatal injury on SVZ neural precursors is unknown. We examined the effects of hypoxia-ischemia (HI) on neonatal mouse SVZ cell proliferation and neurogenesis. Postnatal day 10 (P10) mice underwent right carotid artery ligation followed by 10% O2 exposure for 45 min. The SVZ area and hemispheric injury were quantified morphometrically 1-3 weeks later. Bromodeoxyuridine (BrdU) was used to label proliferating cells, and cell phenotypes of the progeny were identified by immunohistochemistry. HI significantly enlarged the ipsilateral SVZ at P18, P24, and P31, and increases in the SVZ area correlated directly with the degree of hemispheric damage. HI also stimulated cell proliferation and neurogenesis in the SVZ and peri-infarct striatum. Some newborn cells expressed a neuronal phenotype at P24, but not at P31, indicating that neurogenesis was short-lived. These results suggest that augmenting SVZ neuroblast recruitment and survival may improve neural repair after neonatal brain injury.

Purification of neuronal precursors from the adult mouse brain: comprehensive gene expression analysis provides new insights into the control of cell migration, differentiation, and homeostasis

The progeny of neural stem cells in the subventricular zone (SVZ) of the adult mammalian brain consists in polysialylated NCAM-expressing immature neurons (PSA(+) cells), which migrate to the olfactory bulb (OB) to differentiate into GABAergic interneurons. We purified murine PSA(+) cells directly from the adult brain by FACS and analyzed their gene expression profile by SAGE. Comparative analyses led to the identification of precursor-enriched genes, including Survivin, Sox-4, Meis2, Dishevelled-2, C3aR1 and Riken 3110003A17, and many so far uncharacterized transcripts. Cluster analysis showed that groups of genes involved in axon guidance and gene clusters implicated in chemotaxis are strongly upregulated, indicating a role of both cues in the control of cell migration in the adult brain. Furthermore, genes involved in apoptosis and cell proliferation are co-expressed, suggesting that the amount of precursors that is present in the adult brain is a result of an equilibrium of these processes.

Identification and characterization of two novel brain-derived immunoglobulin superfamily members with a unique structural organization

We recently used a differential display PCR screen to identify secreted and transmembrane proteins that are highly expressed in the developing rat basilar pons, a prominent ventral hindbrain nucleus used as a model for studies of neuronal migration, axon outgrowth, and axon-target recognition. Here we describe cloning and characterization of one of these molecules, now called MDGA1, and a closely related homologue, MDGA2. Analyses of the full-length coding region of MDGA1 and MDGA2 indicate that they encode proteins that comprise a novel subgroup of the Ig superfamily and have a unique structural organization consisting of six immunoglobulin (Ig)-like domains followed by a single MAM domain. Biochemical characterization demonstrates that MDGA1 and MDGA2 proteins are highly glycosylated, and that MDGA1 is tethered to the cell membrane by a GPI anchor. The MDGAs are differentially expressed by subpopulations of neurons in both the central and peripheral nervous systems, including neurons of the basilar pons, inferior olive, cerebellum, cerebral cortex, olfactory bulb, spinal cord, and dorsal root and trigeminal ganglia. Little or no MDGA expression is detected outside of the nervous system of developing rats. The similarity of MDGAs to other Ig-containing molecules and their temporal-spatial patterns of expression within restricted neuronal populations, for example migrating pontine neurons and D1 spinal interneurons, suggest a role for these novel proteins in regulating neuronal migration, as well as other aspects of neural development, including axon guidance.

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

Neuroblasts arising in the adult forebrain that travel to the olfactory bulb use two modes of migration: tangentially, along the rostral migratory stream, and radially, in the core of the olfactory bulb where they start to ascend to the outer layers. Although the mechanisms of tangential migration have been extensively studied, the factors controlling radial migration remain unexplored. Here we report that the extracellular matrix glycoprotein tenascin-R, expressed in the adult mouse olfactory bulb, initiates both the detachment of neuroblasts from chains and their radial migration. Expression of tenascin-R is activity dependent, as it is markedly reduced by odor deprivation. Furthermore, grafting of tenascin-R-transfected cells into non-neurogenic regions reroutes migrating neuroblasts toward these regions. The identification of an extracellular microenvironment capable of directing migrating neuroblasts provides insights into the mechanisms regulating radial migration in the adult olfactory bulb and offers promising therapeutic venues for brain repair.

Migration patterns of subventricular zone cells in adult mice change after cerebral cortex injury

The subventricular zone (SVZ) generates the largest number of migratory cells in the adult brain. SVZ neuroblasts migrate to the olfactory bulbs (OB) in the adult, whereas during development, SVZ cells migrate into many adjacent nuclei. Previously, we showed that cerebral cortex injury in the adult causes molecular and cellular changes which may recapitulate the developmental migratory directions. Consistent with this, growth factors, as well as models of illness or injury can cause adult SVZ cells to migrate into non-olfactory bulb nuclei. Here, we tested the hypothesis that cerebral cortex injury in the adult mouse induces changes in migration, by labeling adult SVZ cells with a retroviral vector and examining the distribution of cells 4 days and 3 weeks later. Four days after cortical lesions, disproportionately fewer retrovirally-labeled cells had migrated to the olfactory bulb in lesioned mice than in controls. Conversely, the number of cells found in non-olfactory bulb regions (primarily the area of the lesion and the corpus callosum) was increased in lesioned mice. The morphology of these emigrated cells suggested that they were differentiating into glial cells. Three weeks after cortical injury, the majority of retrovirally-labeled cells in both groups of mice had migrated into the granule and periglomerular layers of the olfactory bulb. At 3 weeks, we still observed retrovirally-labeled glial cells in the corpus callosum and in the area of the injury in lesioned mice. These results suggest that cortical lesions cause a transient change in migration patterns of SVZ progeny, which is characterized by decreases in migration to the olfactory bulb but increased migration towards the injury. Our studies also suggest that cortical lesions induce the production of new glial cells which survive for at least 3 weeks after injury. The data support the concept that in the adult, SVZ cells can generate progeny that migrate towards injured areas and thus potentially be harnessed for neural repair.

Regulatory genes controlling cell fate choice in embryonic and adult neural stem cells

Neural stem cells are the most immature progenitor cells in the nervous system and are defined by their ability to self-renew by symmetric division as well as to give rise to more mature progenitors of all neural lineages by asymmetric division (multipotentiality). The interest in neural stem cells has been growing in the past few years following the demonstration of their presence also in the adult nervous system of several mammals, including humans. This observation implies that the brain, once thought to be entirely post-mitotic, must have at least a limited capacity for self-renewal. This raises the possibility that the adult nervous system may still have the necessary plasticity to undergo repair of inborn defects and acquired injuries, if ways can be found to exploit the potential of neural stem cells (either endogenous or derived from other sources) to replace damaged or defective cells. A full understanding of the molecular mechanisms regulating generation and maintenance of neural stem cells, their choice between different differentiation programmes and their migration properties is essential if these cells are to be used for therapeutic applications. Here, we summarize what is currently known of the genes and the signalling pathways involved in these mechanisms.

Ig superfamily cell adhesion molecules in the brain

Cell adhesion molecules of the immunoglobulin superfamily (IgSF CAMs) were discovered 25 years ago based on their role in cell-cell adhesion. Ever since, they have played a major role in developmental neuroscience research. The elucidation of IgSF CAM structure and function has been tightly linked to the establishment of new areas of research. Over the years, our view of the role of the IgSF CAMs has changed. First, they were thought to provide "specific glue" segregating subtypes of cells in the nervous system. Soon it became clear that IgSF CAMs can do much more. The focus shifted from simple adhesion to CAM-associated signaling that was shown to be involved in the promotion of axon growth and the regulation of cell migration. From there it was a small step to axon guidance, a field that has been given a lot of attention during the last decade. More recently, the involvement of IgSF CAMs in synapse formation and maturation has been discovered, although this last step in the formation of neural circuits was thought to be the domain of other families of cell adhesion molecules, such as the neuroligins, the neurexins, and the cadherins. Certainly, the most striking discovery in the context of IgSF CAMs has been the diversity of signaling mechanisms that are associated with them. The versatility of signals and their complexity make IgSF CAMs a perfect tool for brain development.

Cell migration in the forebrain

The forebrain comprises an intricate set of structures that are required for some of the most complex and evolved functions of the mammalian brain. As a reflection of its complexity, cell migration in the forebrain is extremely elaborated, with widespread dispersion of cells across multiple functionally distinct areas. Two general modes of migration are distinguished in the forebrain: radial migration, which establishes the general cytoarchitectonical framework of the different forebrain subdivisions; and tangential migration, which increases the cellular complexity of forebrain circuits by allowing the dispersion of multiple neuronal types. Here, we review the cellular and molecular mechanisms underlying each of these types of migrations and discuss how emerging concepts in neuronal migration are reshaping our understanding of forebrain development in normal and pathological situations.

A niche for adult neural stem cells

The adult mammalian brain harbors multipotent stem cells, which reside and participate in specialized niches that support self-renewal and differentiation. The first cellular and molecular elements of the stem cell niche in the adult brain have been identified and include cell-cell interactions and somatic cell signaling, the vasculature, the extracellular matrix and basal lamina. Furthermore, regulation at the epigenetic level via chromatin modification and remodeling is an integral aspect of stem cell biology. Understanding the in vivo stem cell niche will provide a framework for the elucidation of stem cell function in the adult brain.