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

Transcription

1 Neuronal migration in the adult brain: are we there yet? H. Troy Ghashghaei*, Cary Lai and E. S. Anton* Abstract 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. Radial glial cells Cells that span the radial axis of the developing cortex and serve as precursors or guides for newly born postmitotic neurons on their way into the mantle zone. Ventricular zone Also known as the proliferative zone, this is the part of the neuroepithelium that faces the ventricular (inner) surface of the neural tube, where cells are proliferating. *UNC Neuroscience Center and the Department of Cell and Molecular Physiology, Room 7109B, 103 Mason Farm Road, The University of North Carolina School of Medicine, Chapel Hill, North Carolina , USA. Department of Molecular Biomedical Sciences, School of Veterinary Medicine, North Carolina State University, Raleigh, North Carolina 27606, USA. Molecular and Integrative Neuroscience Department, The Scripps Research Institute, North Torrey Pines Road, La Jolla, California 92037, USA. Correspondence to E.S.A. doi: /nrn2074 Neuronal position has a role in determining a neuron s function. Appropriate neuronal positioning is achieved through an active process of migration from the site of neuronal birth to the target location. Although the bulk of neuronal migration in the mammalian brain occurs during the embryonic period, a significant number of neurons migrate after birth, well into adulthood. Understanding the mechanisms that initiate, maintain and appropriately terminate the migration of newly generated neurons during the postnatal period is crucial to our understanding of the establishment of normal brain circuitry, and could provide insight into how alterations in the process might contribute to neurodevelopmental disorders. Furthermore, it may someday be possible to manipulate the migration process to permit the targeted delivery of newly generated cells to damaged or diseased sites in the brain. This review provides a summary of the principal underlying mechanisms and functional significance of neuronal migration in the postnatal brain. In particular, owing to its potential importance to our understanding of adult human CNS disorders, the mechanisms and relevance of neuronal migration that persists into adulthood are highlighted. Patterns of embryonic neuronal migration Appropriate migration and placement of neurons during development is essential for the construction of functional synaptic circuitry in the brain. In the developing CNS most, if not all, neurons undergo generation and differentiation in distinct locations. Specific classes of neurons migrate in distinctly oriented pathways and patterns, over distances of several hundred to thousands of cell lengths, to reach their target destinations in particular areas of the brain. Two main types of migration predominate during brain development: radial and tangential (FIG. 1). Radial migration is generally characterized by intimate, reciprocal interactions between migrating neurons and the processes of radial glial cells, which constitute a scaffold bridging the proliferative ventricular zone and pial surface. In the cerebral cortex, postmitotic neurons migrate radially from the ventricular zone towards the pial surface, past previously generated neuronal layers 1,2, to reach the top of the cortical plate, where their migration terminates and they assemble into layers with distinct patterns of connectivity. Radial migration of cortical neurons can occur in two distinct modes: somal translocation or locomotion 3 5. During somal translocation neurons first extend a long, basal process from the ventricular zone up to the pial surface, which is followed by rapid nucleokinesis and shortening of the basal process. Locomoting neurons have a free leading process and move in a saltatory manner 6. Somal translocation of neurons is prevalent during early stages of cortical development and, in contrast to locomoting neurons, these neurons rely less on the radial glial scaffold for their navigation to the cortical plate. Concurrently with the radial migration of neurons, subsets of neurons also migrate in tangential orientation. Tangential migration is defined as a mode of non-radial neuronal translocation that does not require specific interactions with radial glial cell processes 7 9. Neurons that eventually become pyramidal or glutamatergic cortical neurons tend to migrate radially, whereas GABA (γ-aminobutyric acid)-containing interneurons migrate tangentially. A combination of experimental embryology and analysis of distalless 1/2 (Dlx1/2) double-null mutant mice has unambiguously demonstrated that subpopulations of GABA-containing interneurons, originating from the ganglionic eminence, migrate tangentially into the neocortex 10. However, a notable exception to this trend might be the earliest cortical neurons that arrive in the cortical primordium by tangential migration, but do not seem to be of the interneuronal phenotype 11,12. NATURE REVIEWS NEUROSCIENCE VOLUME 8 FEBRUARY

2 Cortical plate The top of the developing cerebral cortex, where neurons end their migration and start to assemble into distinct neuronal layers. Leading process The process of a migrating neuron that is extended towards the direction of migration. Ganglionic eminence The part of the subpallium that gives rise to tangentially migrating interneurons. Rostral migratory stream Neural cells born in the subventricular zone migrate tangentially as cellular chains to the olfactory bulb. This organized stream of migrating neuroblasts forms the rostral migratory stream. Neurons can switch dynamically between tangential and radial modes of migration, as has been observed in cortical interneurons 13. Similarly, in the developing cerebellum, after tangential translocation in the superficial external granule cell layer, granule interneurons make a sharp 90 turn to migrate along the radial processes of Bergmann glia spanning the molecular layer, to reach the deep internal granule cell layer, where they settle and undergo terminal synaptic differentiation 14. This switch from a tangential to a radial mode of translocation is not unique to cortical interneurons or cerebellar granule neurons, and has also been observed for spinal cord dorsal column neurons 15. It is likely to reflect a basic property of migrating inhibitory interneurons. Distinct routes and modes of migration might not only serve the purpose of coordinated movement of neurons from their sites of origin to their final sites of differentiation, where they form functional synaptic connections, but might also function as an organizing mechanism to facilitate the orderly phenotypic differentiation of distinct classes of neurons that are only partly determined at their birth. Specific cell cell recognition and adhesive interactions between neurons, glia and the surrounding inductive cues during distinct modes of a b Figure 1 Radial and tangential migration of neurons in the developing cortex. Radially migrating neurons either use somal translocation with a long leading process (a) or migrate in close apposition (b) to a radial glial process (blue). Tangentially migrating neurons (purple) invade the cortex along the marginal zone or through the cerebral wall in a manner that is presumed to be independent of radial glia. These neurons are thought to use corticofugal fibres (green), marginal zone neurons (yellow) or the pial membrane (grey) as migratory guides. migration might trigger determinant signalling events, leading to different neuronal identities. Neuronal migration in the postnatal brain Commenting on the nature of post-embryonic brain plasticity, Ramón y Cajal noted that once development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably. In adult centres the nerve paths are something fixed, ended, immutable. Everything may die, nothing may be regenerated 16. Although most CNS structures change relatively little after early postnatal periods 17 19, there are restricted regions in the postnatal and adult brain where developmental processes such as neuronal generation and migration continue The functional relevance of the persistence of developmental processes in the adult brain remains to be fully explained, but prominent neuronal migration is evident in three particular regions of postnatal rodent brains: the cerebellum, hippocampus and rostral migratory stream (RMS) (FIG. 2). In addition, a very small number of neurons might migrate into the hypothalamus at around the time of birth. Development of two of the above systems, the cerebellum and hypothalamus, relies on neuronal migration during early postnatal periods. Multiple stages of early embryonic migration from cerebellar anlage and anterior rhombic lip lead to the assembly of basic components of nascent cerebellum namely granule neuron precursors, Purkinje neurons and cerebellar nuclei neurons by birth 170. The cerebellum has pivotal roles in postural balance, symmetry in movement, equilibrium and cognitive functions 27,28. The positioning of granule neurons, which results from appropriate postnatal migration (from about postnatal day (P) 3 to 21 in rodents, and up to year 2 in humans) 29 of these cells from the external granule cell layer to the internal granule cell layer along the Bergmann glial processes, is central to the cerebellar circuitry that underlies these functions (for reviews, see REFS 28,30 35). Similar, although more limited, neuronal migration occurs during very early postnatal periods in the hypothalamus. Remnants of gonadotropin-releasing hormone (GnRH + ) neurons, which are essential for autonomic functions relating to sexual behaviour 36, complete their journey to the hypothalamus at about the time of birth. After their birth in the olfactory placode during embryogenesis (at about embryonic day (E) 11 in rodents), the GnRH + neurons migrate along the caudal olfactory/vomeronasal nerves, invade the basal forebrain, detach from their guiding nerve fibres in the hypothalamic region and differentiate into neurons that express GnRH 37,38. In humans, failure of GnRH neuronal migration results in reproductive dysfunction and retarded pubertal maturation 37,38 (for reviews, see REFS 38 40). The limited extent of postnatal neurogenesis and migration in the cerebellum and hypothalamus suggests that the processes might merely be related to the completion of these structures functional circuitry. In contrast to the cerebellum and the GnRH system in the hypothalamus, extensive neurogenesis and migration continues into adulthood in the hippocampus and the RMS of the rodent olfactory bulb. Although early 142 FEBRUARY 2007 VOLUME 8

3 a IGL CB EGL Hipp DG Hyp SGZ V SVZ RMS b CB OB OE Figure 2 Neuronal migration in the postnatal and adult brain. a In rodent brains postnatal neuronal migration is evident in three main areas: the cerebellum (CB), the hippocampus (Hipp) and the rostral migratory stream (RMS). A small number of neurons also complete their migration into the hypothalamus (Hyp) at around the time of birth. Distinct germinal zones (green) give rise to neurons that migrate to adjacent target zones (red). Lighter shade indicates that migration in these regions occurs primarily during the very early postnatal period and does not persist into adulthood. Cells born in the anterior subventricular zone (SVZ, inset) initiate their migration from the SVZ (1) as chains (2) streaming towards the olfactory bulb (OB), where they end their migration (3). b A highly restricted pattern of neuronal migration is evident in the postnatal human brain. There are pools of neural precursors (green) around the walls of the lateral ventricles (V). Occasional, TUJ1 (beta III-tubulin)-positive, elongated neurons (red dots), reminiscent of actively migrating ones, have been identified adjacent to these precursor pools, but whether these represent migrating neurons is unclear. Although there is no RMS in humans, isolated new neurons have been reported in the olfactory bulb. In the hippocampus, subgranular zone precursors continue give rise to new neurons, which then migrate short distances to the adjacent dentate gyrus (DG). As in rodents, human cerebellar neuronal migration is limited to the very early postnatal period. EGL, external granule cell layer; IGL, internal granule cell layer; OE, olfactory epithelium; SGZ, subgranular zone. Anatomical image adapted, with permission, from REF. 169 (1996) Appleton & Lange. Subventricular zone (SVZ). The mitotically active region immediately adjacent to the rostral lining of the lateral ventricles where neural stem cells and restricted progenitor cells reside. DG Hipp 1 SVZ V 2 3 OB postnatal neurogenesis and migration in these structures may be growth related, the persistence of developmental processes during adulthood is probably related to the functional plasticity and maintenance of these systems. In the hippocampus, which is the organizing core for learning and memory 41, granule projection neurons are continuously generated in the subgranular zone and migrate short distances to be incorporated into the dentate gyrus During early postnatal ages in rodents, interneurons in the dentate gyrus can also traverse the granular layer in a radial-glial-independent manner while maintaining and acquiring synaptic contacts 46. OB The granule neurons of the dentate gyrus are the first recipients of input from the entorhinal cortex and form the mossy fibre system that is thought to be crucial to memory formation. The most extensive neuronal migration that continues into adult life, however, occurs in the RMS, which is situated in the anterior aspect of the forebrain. New interneurons born in the anterior subventricular zone (SVZ) migrate rostrally in a stream towards the olfactory bulb, where the young neurons either settle into the deep granule cell layer, or in the superficial periglomerular layer of the bulb and differentiate into GABA-containing and dopaminergic local circuit interneurons. Through this route, new interneurons are continuously fed into the olfactory bulb the first central relay station of olfactory sensory input which has an ongoing demand for functional rewiring as a result of continuous death and replacement of receptor neurons in the olfactory epithelium. Migrating chains of neuroblasts in mature telencephalic areas have been reported in adult rabbits 48 and some non-human primates 19,49,135, but such migration has not been observed in humans 50. The persistence of neuronal migration in several regions of the postnatal mammalian brain raises a number of fundamental questions regarding the functional significance and molecular control of this process in the maturing brain. First, what are the cellintrinsic and extracellular cues that sustain and promote appropriate neuronal migration in these sites that other sectors of the postnatal brain lack? Second, are the molecular and cellular mechanisms that are active during embryonic neuronal migration also operative in the postnatal brain? Third, what is the functional relevance of neuronal migration in the postnatal brain? And, fourth, can neurons generated from endogenous germinal zones of the postnatal brain be guided to sites of injury in a damaged or diseased postnatal brain? Analysis of these questions may elucidate some of the fundamental rules that guide neuronal migration in the postnatal brain and help to delineate its functional relevance to mature brain function. Extensive observations of neuronal migration in postnatal mammalian cerebral cortex, made during the past few decades, and recent molecular characterization of neuronal migration deficits have identified the RMS as a prototype model for the analysis of migration in the postnatal mammalian CNS. Therefore, we focus on this model to further evaluate the relevance and importance of neuronal migration in the mature brain. Neuronal migration in the RMS Neuronal migration in the RMS is unique in that the substrate for motility is provided by adjacent migrating cells themselves, as opposed to the glial-guided or axonal-guided modes of neuronal migration identified in the developing brain. After their generation and initial differentiation in the SVZ, neuroblasts migrate as neuronal chains, sliding along each other in the RMS, towards the olfactory bulb 51. This movement is highly directed, with no dispersion into the surrounding tissue, suggesting the presence of orienting cues in or around NATURE REVIEWS NEUROSCIENCE VOLUME 8 FEBRUARY

4 Neuronal chains Neuroblasts migrating in the rostral migratory stream slide along each other, forming interlinked chains of migrating neurons. Glial tube Neuroblasts in the rostral migratory stream migrate through tubes composed of astrocyte-like glial cells. These glial tubes might help to orient the migration of neuroblasts. Matrigel A solubulized extract of basement membrane proteins prepared from EHS mouse sarcoma cells. Initiation of migration SLIT ROBO MIA DCC the RMS. Oriented neuroblast migration in the adult RMS seems to depend on a combination of repulsive, motogenic and chemoattractive cues However, after ablation of the olfactory bulb, neuroblasts continue to migrate into the RMS, suggesting that long-distance attractive signals from the olfactory bulb might not be necessary for oriented migration, and, instead, that intrinsic and short-distance signals might create the crucial chemogradients needed for directing migration in the RMS 56. Furthermore, the lack of dispersion of migrating neuroblasts into surrounding tissues and the distinct channelling of neuroblasts into a stream towards the olfactory bulb might be due in part to astroglia, which encapsulate the migrating cells and form a barricadelike glial tube 57,58. The presence of these glial tubes that ensheath the migrating neuroblasts raises the possibility that migrating neurons might use them as a substrate for oriented migration. However, isolated RMS cells can migrate in vitro in the absence of glial cells 59. Moreover, it has been suggested that glial tubes might have a role in switching some of the neuroblasts from a motile phase into a mitotic phase, through contact-mediated mechanisms 60. When neuroblasts make contact with glial tubes, they may halt their migration and undergo a round of mitosis. This possibility is supported by the unique characteristic of some neuroblasts in the RMS to cycle back and forth between migratory and mitotic phases during their lifespan in the RMS 61. In addition to their potential role in modulating the mitotic or migratory state of neuroblasts through cell cell contact-mediated interactions, glial cells in the RMS have recently been implicated as the source of a secreted protein with migration-inducing activity (MIA) 52, although in vitro RMS neuroblasts show diminished speed of migration in the presence of glial cells 25,52,58. Furthermore, a three-dimensional Matrigel environment free of glial cells seems to be highly permissive for more rapid modes of migration 60. Together, these observations suggest that astroglial cells and the surrounding environment provide the secreted, substrate-bound and cell cell contact-mediated cues needed to guide the newly Netrin Glial tube Figure 3 Initiation of neuronal migration. The initiation of neuronal migration is regulated by a combination of motogenic (for example, migration-inducing activity, MIA, secreted by glial tubes surrounding the neuronal chains), chemoattractive (such as netrin and deleted in colorectal carcinoma, DCC) and chemorepulsive (for example, SLIT and ROBO) signals. Arrows indicate the direction of migration away from the subventricular zone towards the rostral migratory stream. generated, intrinsically polarized neuroblasts from the SVZ towards their target layers in the olfactory bulb of the mature forebrain. Initiation of migration The source of RMS neuroblasts are distinct glial fibrillary acidic protein (GFAP) positive (GFAP + ) or LeX/SSEA1 + (3-fucosyl-N-acetyl-lactosamine hapten) primary neural stem cells lining the SVZ. These cells give rise to rapidly proliferating intermediate nestin/dlx2-positive cell types, which, in turn, proliferate to generate the migrating neuroblasts of the stream The GFAP + population of astroglial cells adjacent to the ventricles is likely to be multifunctional, with some functioning as neuronal precursors and others helping to orient and guide initial neuroblast migration away from the SVZ. The radial processes of GFAP + cells in the anterior lining of the ventricles are highly polarized and are oriented away from the SVZ towards the RMS, and, therefore, might help to guide the newly generated neuroblasts in the direction of the RMS. Disruption of the polarized orientation of astroglial processes in the SVZ seems to affect the chain organization of migrating neuroblasts in this region 67. Although these specialized astroglial cells in the SVZ might have an orienting influence on the new neuro blasts, the mechanisms underlying the newly born neurons acquisition of the appropriate cellular polarity to initiate their migration in the direction of the olfactory bulb remains unclear. However, a dynamic balance of chemorepulsive and chemoattractive signals emanating from the SVZ milieu is likely to be a key determinant of this process (FIG. 3). SLIT proteins, which provide chemorepulsive cues for axonal growth and guidance, have been shown to repulse the newly generated cells from the SVZ, pushing them towards the olfactory bulb 55 (FIG. 3). SLIT1 and 2 are expressed and secreted by cells surrounding the SVZ that is, in the septum, striatum and the choroid plexus in the anterior horn of the lateral ventricles 55,68,69. The SLIT receptors ROBO1, 2 and 3 are expressed in the SVZ 68,69. Thus, on exposure to SLIT, new neuroblasts in the SVZ might orient away from the SVZ and the surrounding striatal/septal areas, towards the olfactory bulb. The results of recent studies by Sawamoto and colleagues 70 suggest that the activity of ependymal cell cilia, which line the lateral ventricles, might be essential in conveying orienting gradients of SLIT from the cerebrospinal fluid in the ventricles to migrating neuroblasts in the SVZ. The initiation and maintenance of neuroblast migration requires leading process extension, positioning of the centrosome ahead of the soma, and nuclear translocation. SLIT-mediated chemorepulsion has an essential role in modulating the polarity of SVZ neuroblasts during their migration, and the cell-polarity factors glycogen synthase kinase-3β (GSK3β) and protein kinase Cζ (PKCζ) are needed for centrosome reorientation and process stabilization during this process 71. In adult SLIT1-deficient mice, clusters of SVZ-derived neuroblasts migrate caudally into the corpus callosum, instead of into the RMS 69, and in Slit1/Slit2 double knockout mice the olfactory bulb the final destination of the SVZ neuroblasts is considerably smaller FEBRUARY 2007 VOLUME 8

5 Integrins ErbB4 Adhesion molecules Families of cell-surface or secreted molecules that mediate cell cell or cell substrate adhesion. MIA Neuregulin 1 PSA-NCAM Figure 4 Maintenance of neuronal migration. Neuroblasts migrate as chains, sliding along each other. Maintenance of chain migration is dependent on the continued motogenic activity of migration-inducing activity (MIA, orange), neuregulin 1 ErbB4 interactions (both purple), adhesion mediated by the polysialated form of neuronal cell adhesion molecule (PSA-NCAM, yellow) and extracellular matrix (ECM) integrin signalling (blue). Arrows indicate the direction of migration towards the olfactory bulb. ECM The nature of SLIT activity in the SVZ is contextdependent. For example, in the RMS both MIA, which is secreted by astroglial tubes, and heparan sulphate proteoglycans can modulate the chemorepulsive activities of SLIT 52,55,73,74. Furthermore, the recent observations that SLIT1 is expressed by the type A migrating neuroblasts and by the rapidly dividing, DLX2 + type C precursor cells in the SVZ and RMS suggest that SLIT1, in addition to its chemorepulsive function, could function cell autonomously in migrating neuroblasts 69. Although repulsive signals might act as the predominant orienting cues for the initiation of migration in the RMS 53,70 and long-distance attractive signals seem to be unnecessary for the initiation of neuroblast migration 56, localized chemoattractant cues might influence initiation of neuronal migration (FIG. 3). Chemoattractants such as netrin 1 and its receptor, deleted in colorectal carcinoma (DCC), have a modest attractive 52 or repulsive 54 effect on SVZ neuronal migration in vitro, even though in vivo the expression of netrin 1 and DCC are downregulated soon after birth 53. The function of other, yet to be characterized, context-dependent chemo attractants in the maturing SVZ/RMS cannot be ruled out. Once channelled away from the SVZ and appropriately polarized as a result of dynamic interactions between localized chemoguidance cues and cell-intrinsic mechanisms, new neuroblasts migrate as chains towards the olfactory bulb. Maintenance of oriented neuronal migration The maintenance and guidance of neuroblast migration through the mature forebrain requires a coordinated pattern of interactions between extracellular matrix (ECM) cues, secreted guidance or motogenic signals, cell adhesion molecules, and cell-surface tyrosine kinase or integrin signalling receptors 10,75. An essential signal for the organization of neuroblast chain migration in the RMS is the polysialated form of neuronal cell adhesion molecule (PSA-NCAM) (FIG. 4). The large, highly charged PSA moieties on NCAM modulate the homophilic interactions between NCAMs on neuro blasts, enabling them to maintain the optimal levels of adhesion needed to form chains and slide along each other within the glial tubes. Targeted deletion of NCAM or the enzymatic removal of PSA results in abnormal, diminished migration and build-up of neuroblasts in the RMS, leading to an enlarged RMS and smaller olfactory bulb containing fewer newly generated granule neurons These PSA-NCAM + migrating neuroblasts express the tyrosine kinase receptor ErbB4 and its ligands, neuregulins 1 and 2 (NRG1 and NRG2). Conditional deletion of ErbB4 in the CNS results in aberrant neuroblast chain organization, orientation and migration 83. In contrast to control neuroblasts, which have a long leading process oriented towards the olfactory bulb and a short trailing tail as they migrate, ErbB4-mutant neuroblasts lack these oriented processes and their leading edges are directed in multiple orientations. In vitro, the membrane-bound and secreted isoforms of NRG1 (type III and type I/II, respectively) modulate neuroblast migration through ErbB4 (REFS 67,83). NRG1 type III may provide a permissive, guidance substratum for neuroblast migration towards the olfactory bulb, whereas the secreted NRG1 isoform may function as a chemoattractant/motogen for these neuroblasts. In addition to ErbB4, the Eph tyrosine kinase receptors and their ephrin ligands have been identified as indirect modifiers of neuroblast migration in the RMS. The tyrosine kinase receptors EphA4 and EphB1 3, and their transmembrane ligands ephrin B2 and ephrin B3 are expressed by SVZ cells. Interruption of EphB2 ephrin B2 interactions in the SVZ, with the ectodomains of the respective receptor and ligand, have been found to enhance SVZ cell proliferation and retard neuroblast migration 84. The observed shift from neuroblasts migrating as chains to neuroblasts migrating as misoriented cell clusters or as individual cells in ErbB4 mutants, or following the disruption of Eph ephrin signalling, could be the result of changes in the ECM or in the adhesive properties of migrating cells. Gradients of ECM molecules in or around the route of migration can influence the direction of migration and provide the necessary levels of adhesion needed to propel cell motility 53,85. Integrins, which convey the ECM-derived signals to the neuroblasts, are expressed in a developmentally changing pattern in the RMS. Of the identified integrins in the RMS, the α1, β1 and β8 subunits are expressed mainly during early postnatal periods, whereas β3 and β6 integrin expression persists in the mature cortex. Function-blocking studies with various integrin-subunit-specific antibodies and the analysis of RMS in mouse integrin mutants indicate that α1, α5, α6, β1 and β4 integrins modulate neuronal migration in the RMS 53,86,87. The substrates for intergrin signalling in the RMS during neuro blast migration remain to be fully elucidated, but two laminin subunits, α5 and γ1, have been identified in the RMS 53. Another potential integrin ligand, tenascin-c, is expressed by the astroglial tubes and seems to fill the extracellular space between the migrating neurons and astrocytes in the RMS 53,57, Although tenascin-c is known to have anti-migratory effects and could potentially limit RMS neuroblasts from dispersing into the surrounding tissues 91, tenascin-c-null mice do not exhibit obvious adult neuronal migratory defects 90,92. NATURE REVIEWS NEUROSCIENCE VOLUME 8 FEBRUARY

6 The oriented migration of neuroblasts in the RMS relies on the coordinated modulation of adhesion or guidance cues 10,52. There is evidence for crosstalk between ErbB and integrin signalling pathways, as the integrins can switch NRG responsiveness in glioblasts from proliferation to differentiation 93. Similarly, bidirectional cooperative interactions between integrins and Eph ephrin signalling systems are also possible Therefore, the engagement of RMS-expressed integrins might provide neuroblasts with the competence to respond to NRG1 through ErbB4, permit Eph ephrin interactions, or vice versa. Such bidirectional signalling might be crucial for the guidance and maintenance of oriented migration of new neuroblasts in the mature forebrain. Even though environmentally derived cues exert a potent influence on the migratory behaviour of neuroblasts, new neurons in the mature brain have an intrinsic capacity to migrate, as is evidenced by their stereotypic migratory behaviour (FIG. 5). New neuroblasts polarize and extend a leading process in the direction of migration. Once the leading process is stabilized, the nucleus is translocated towards the leading process. Finally, nuclear translocation is arrested and the trailing process of the neuron is retracted towards the new position of the cell soma (FIG. 5). Repetition of these three stages is a fundamental characteristic of neuronal migration in the mature brain. The cell-intrinsic migratory mechanisms emanate from the neuron s polarity and cytoskeletal machinery. Although the polarity complexes that determine how the leading process is initiated and how the directional movement of the nucleus is organized remain to be fully delineated for migrating neurons in the adult brain, studies on migrating cerebellar neurons have been highly instructive 34. In postmitotic migrating neurons, clustering of centrosomes, the Golgi apparatus and endosomes opposite the plane of the last mitotic division might mark the area of the neuron from which the initial polarized process extension occurs 34,97,98 (FIG. 6). An actin Figure 5 Characteristic migratory behaviour of neuroblasts. The initiation of neuroblast migration requires a leading process (blue) to form in the direction of migration. Clustering of the centrosome (red) and the Golgi apparatus (green) seem to mark the area of a neuron from which the initial polarized process extension occurs. Once the leading process has been extended, forward movement of the centrosome allows the nucleus to translocate in the direction of migration. As the cell soma moves forwards the trailing process (purple) detaches and repositions itself. cortex lines the soma of these cells and their nuclei are surrounded by a perinuclear microtubule cage linked to a centrosome. Forward movement of the centrosome facilitates the microtubule-cage-enwrapped nucleus to move in the direction of migration. Centrosomes, which organize the microtubule network needed for directed nucleokinesis, are critically modulated by several members of the cell polarity complex: PAR6α (partitioning defective 6α homologue), GSK3β and PKCζ 34,99 (FIG. 6). In mice, PAR6α, which is expressed in cerebellar granule neurons just before their migration, localizes to the centrosome and modulates the forward movement of the centrosome that precedes cell somal movement. Depletion of PAR6α results in a stable, non-moving centrosome. Overexpression of Par6α disrupts the perinuclear microtubule cage, centrosomal motility and, therefore, oriented neuronal migration 34,99. The perinuclear microtubule network is also decorated with a dynein LIS1 (lissencephaly 1) NUDEL (nuclear distribution element-like) complex and focal adhesion kinase (FAK). Overexpression of cyclin-dependent kinase 5 (CDK5), nonphosphorylatable FAK or loss of dynein LIS1 NUDEL complex members disrupts centrosome nucleus coupling and the perinuclear microtubule organization needed for neuronal migration 98. Similar mechanisms might also be at work during adult neuronal migration. Inhibition of GSK3β or PKCζ in RMS neuroblasts disrupts their ability to reorient the centrosomes and stabilize processes, and so leads to failure of directed neuronal migration 71. In these neurons the microtubule cytoskeleton seems to be essential for the maintenance and growth of the leading processes, and the actin cytoskeleton is crucial for centrosomal reorientation. Recently, doublecortin (DCX), a microtubule-associated protein that predominantly localizes to the microtubule cage around the nucleus and to the leading processes was shown to have a direct role in the intrinsic mechanisms modulating RMS neuroblast migration. Dcx mutation did not affect the direction of migration or the ability of the new neurons to respond to guidance cues in the mature brain. However, it disrupted the velocity of migration, branching of leading processes, and nuclear translocation towards the centrosome in the direction of migration 71,100,101 (FIG. 3). Termination of migration in the forebrain On arrival in the mature olfactory bulb, migrating neuroblasts disperse from their chains and migrate radially in a radial-glial-independent manner towards their final positions in the granule and periglomerular layers (FIG. 7). At the junction between the end of the RMS and the olfactory bulb, neuroblasts require the expression of the transcription factor ARX (aristaless-related homeobox gene) for their entry into the olfactory bulb 102. In Arx mutants, neuroblasts accumulate at this entry point. Once neuroblasts enter the olfactory bulb, other molecular cues dictate their positioning and final differentiation fate. Reelin, an ECM protein critical for neuronal positioning in the developing brain, seems to function as a detachment signal for migrating neuroblasts 103,104. Reelin is expressed in the core of the olfactory bulb, where neurons exit the 146 FEBRUARY 2007 VOLUME 8

7 PAR6α PKCζ GSK3β DCX FAK Neurogenic niches Sites in the nervous system where neural precursors proliferate and give rise to new neural cells. Receptive fields The area of the sensory space in which stimulus presentation leads to the response of a particular sensory neuron. NUDEL Cell cell junctions Centrosome Golgi/endosomes LIS1 Dynein Actin cortex Microtubules Figure 6 Neuronal polarity of migrating neurons of the adult brain. Neurons polarity and cytoskeletal machinery regulate the stereotypic migratory behaviour of neuroblasts in the rostral migratory stream in rodents. The nucleus of the migrating neuron is surrounded by a perinuclear microtubule cage linked to a centrosome, and an actin cortex lines the cell soma. The centrosome, which organizes the microtubule network needed for oriented migration, is regulated by several members of the cellpolarity complex: cell division cycle 42 (Cdc42), partitioning defective protein 6α (PAR6α), glycogen synthase kinase-3β (GSK3β) and atypical protein kinase Cζ (PKCζ). Cdc42 can activate the Par6α/PKCζ complex, which can interact with GSK3β to facilitate centrosomal polarization and movement during neuronal migration. During this process, doublecortin (DCX), focal adhesion kinase (FAK), dynein, lissencephaly 1 (LIS1) and nuclear distribution element-like (NUDEL) associated with the perinuclear microtubule network may regulate the integrity of the perinuclear microtubule cage and nuclear translocation towards the centrosome. The forward movement of the nucleus and cell soma disrupts cell cell contacts formed with adjacent cells and allows the neuron to slide forwards. RMS and disperse into the bulb. The reelin receptor, apolipoprotein-e receptor 2 (ApoER2) but not the very low-density lipoprotein receptor (VLDLR) and its downstream signalling target, the adaptor protein disabled-1 (DAB1), are also present in RMS neuroblasts. Once chains of migrating neuroblasts reach the core of the olfactory bulb, reelin acts as a detachment signal to cause the radial dispersement of neuroblasts from the chains 105 (FIG. 7). Another ECM protein, tenascin-r, also seems to modulate the initiation of the detachment of neuroblasts from their chains and their radial migration 106. Tenascin-R is expressed in the deep layers of the olfactory bulb and around the most anterior end of the RMS, but not in the RMS proper. Importantly, expression of tenascin-r is olfactory sensory-activity dependent, suggesting that tenascin-r might be instrumental in recruiting new neurons to regions of the olfactory bulb where network activity demands incorporation and input from new neurons 106. Common migratory mechanisms A comparative analysis of molecular mechanisms underlying the generation and targeting of appropriate numbers and types of neurons to where they are needed in the developing and adult brain suggests that some common strategies are used in both the developing and adult brain. For example, during interneuronal migration to the embryonic cortex, motogenic factors in and around the medial ganglionic eminence (for example, hepatocyte growth factor, brain-derived neurotrophic factor and glial-cell-line derived neuro trophic factor), repulsive factors in the striatum and lateral ganglionic eminence (such as SLIT and semaphorins), permissive factors in migratory corridors in the ganglionic eminence (for example, NRG1 type III), as well as attractive signals in the cortex (such as NRG1 type I/II), cooperate in the coordination of long-distance interneuronal invasion of the developing cerebral cortex 13, Similarly, repulsive interactions mediated by SLIT, and permissive and motogenic signals such as NRG1 along the migratory route, in combination with netrin and other, yet to be characterized long-distance attractants, seem to channel newborn neurons in the adult forebrain towards their targets in the olfactory bulb. Reelin function in neuronal placement might also have been similarly preserved in the embryonic and adult brains. How and why these developmental signals are conserved and maintained into adulthood in restricted sites of the adult brain remains to be determined. Although different types of neuron (for example, GABA-containing inhibitory or glutamatergic projection neurons) can be generated in the adult brain 24, , whether these neurons use distinctly different mechanisms for migration to their target locations, as they do in the embryonic brain, is unclear. Functional significance of adult migration The neurogenic niches of the mammalian adult brain and the target locations of the newly generated neurons are separated by a complex and generally inhibitory environment. Adult neuronal migration might serve as a mechanism to get new neurons to where they are needed to maintain and modify functional neural circuitry in the mature brain. It is speculated that the generation, migration and incorporation of new neurons in the olfactory bulb are not merely rudiments of development, but subserve a demand for continuous remodelling of the olfactory bulb neural circuitry, engendered by olfactory receptor turnover. Newborn granule neurons are thought to be important in tuning the receptive fields of olfactory output neurons The tuning of these receptive fields may depend on the arborization of neurites and synaptic connectivity of new granule neurons. The enhanced receptive-field tuning and signal processing resulting from incorporation of adult-born granule neurons may facilitate olfactory learning and memory formation, as is evidenced by the interdependence between performance on memory and olfactory discrimination tasks and the level of neurogenesis Adult-born granule neurons participate in olfactory neural circuit activity shortly after their arrival in the bulb. They start to express GABA and glutamate receptors as they radially migrate to enter the olfactory bulb 123,124 and gradually increase their responsiveness to the neurotransmitters GABA and glutamate. The active granule neurons, in turn, can express factors such as tenascin-r, which may facilitate the continued arrival of new neurons to synaptically strengthen the functionally active sites of the olfactory bulb. NATURE REVIEWS NEUROSCIENCE VOLUME 8 FEBRUARY

8 Transitory amplifying progenitor cell Slow-dividing neural stem cells in the subventricular zone give rise to transitory amplifying progenitors (nestin + and DLX2 + ), which, in turn, rapidly proliferate to give rise to neuroblasts migrating in the rostral migratory stream. Arx DAB1 Tenascin-R ApoER2 Reelin Figure 7 Termination of neuronal migration. Once the neurons reach the end of the rostral migratory stream, the entrance of migrating cells into the olfactory bulb depends on the transcriptional activity of aristaless-related homeobox gene (Arx). Termination of migration in the olfactory bulb is regulated by the action of secreted reelin on its receptor, apolipoprotein-e receptor 2 (ApoER2), and by their downstream target, disabled 1 (DAB1). Tenascin-R also modulates this phase of neuroblast migration by initiating the detachment of neuroblasts from their chains and their radial migration into the olfactory bulb. Similarly, in the hippocampus, the survival and incorporation of new neurons is activity dependent and is regulated during a critical period soon after their birth by the activity of their own NMDA (N-methyld-aspartate)-type glutamate receptors 125. As these new neurons are incorporated and mature, their functional activity can change. For example, GABA is excitatory in immature, newly incorporated neurons but gradually becomes inhibitory as the excitatory glutamatergic synapses are formed 126. Furthermore, interneurons in the postnatal hippocampus can traverse from the molecular to the granular layer of the dentate gyrus while maintaining old synaptic connections and acquiring new ones. This type of local migration of synaptically connected interneurons might occur in response to local demands for synaptic circuitry modification 46. Together, this specific activity-driven survival, incorporation and functionality of new neurons into hippocampal circuitry might have an influential role in the encoding of learning and memory 118, Whether such remodelling of synaptic circuitry, based on incorporation of new neurons, occurs in other domains of the adult brain as part of processes such as learning and memory remains controversial 19, The results of recent studies indicate that induction of neurocytokines such as ciliary neurotrophic factor can trigger neural progenitors residing in the adult hypothalamic parenchyma to give rise to new neurons in the feeding centres of the hypothalamus 132. This addition of new neurons seems to regulate the neural circuitry that underlies energy balance and, therefore, changes in body weight. How these new neurons migrate locally in the hypothalamic parenchyma after their birth is currently unclear. Migration in ageing, injury and disease As animals age, the populations of neurons generated and how they migrate to their final destinations may change The number of new neurons generated, as well as the transitory amplifying progenitor cell population, decrease with increasing age. Although the number of both the SVZ neural-precursor-like astrocytes and the adjacent ependymal cells remains stable, their interactions appear to change more astrocytes were found to be interpolated with ependyma in older brains 133. In the olfactory bulb, the early-born, postnatal granule neurons are morphologically and functionally different from those produced later, in adulthood 136. Almost the entire subset of the early-born, postnatal granule neurons survive and are targeted to the external granule cell layer, whereas adult-born granule neurons are positioned in deeper layers and almost half are eliminated soon after reaching their target layers in the olfactory bulb 136. The electrophysiological properties of the superficial, early-born granule neurons differ significantly from the later-born, deeper layer neurons 124. The importance of these morphological and functional differences between neurons generated at different times in the mature brain is yet to be explained, however, it seems to parallel the general reduction of molecular cues that regulate neurogenesis and migration in the postnatal brain 53,83,137. Limited, localized neuronal injury and hypoxia have both been shown to induce neurogenesis and replacement of neurons in the adult cerebral cortex 111,112, but the extent of resultant functional recovery remains uncertain 138. These neurons might have been generated from local quiescent progenitors or in the neurogenic niches around the ventricular zone before migration to the site of injury. This process requires navigation through a complex and inhibitory adult cortical environment. New neurons or neural precursors can potentially migrate along myelinated fibres, blood vessels and astroglial processes towards the sites of injury using signals such as stromal-cell-derived factor-1α (SDF1α) released from neurons or glia at the sites of injury or by expressing matrix metalloproteinases (for example, MMP9) that can modulate components of the brain ECM in the path of migration It is also important to note that during tumorigenesis transformed tumour cells can migrate long distances in the adult human brain. Elucidating the differences in mechanisms used by tumour cells and normal newborn neurons to move within the adult human brain will be highly useful in understanding the molecular determinants that permit or limit longdistance neuronal migration in the adult brain. Molecules that are crucial for neuronal migration and placement in the embryonic and adult brain (such as reelin, NRG1, ErbB4 and disrupted in schizophrenia 1, DISC1) have also been implicated in human neurobehavioural disorders and epilepsy. NRG1 is a strong susceptibility gene for schizophrenia , and disrupted NRG1 ErbB4 signalling in the brain might enhance vulnerability to schizophrenia 149,150. DISC1 regulates neuronal migration and its loss of function may lead to schizophrenia 151. Similarly, changes in reelin expression and function have been observed in some cases of epilepsy, schizophrenia and autism The inability of newly generated neurons in the mature brain to migrate to their target locations when these molecular signals are disrupted might result in improper neural circuitry maintenance and function, and so might contribute to the emergence of these disorders. Furthermore, therapeutic agents such as antidepressants may increase hippocampal neurogenesis, and this seems to parallel behavioural changes in animal models FEBRUARY 2007 VOLUME 8

9 Conclusions and future directions Although there are pools of neural progenitors and a welldefined SVZ in the mature human brain, it is important to note that, in contrast to non-human models, evidence for organized, long-distance migration of newly generated neurons in the adult human brain is lacking 50, Furthermore, the results of recent studies using 14 C dating or BrdU birthdating indicate that although new neurons are added in the hippocampus, no new neurons are evident in human neocortex 17,163. Although there is no organized RMS in humans and the human SVZ might not provide new neurons to the olfactory bulb, some studies suggest that newborn neurons and their local precursors do exist in the adult human olfactory bulb 164,165 (FIG. 2B). The generally inhibitory environment of the adult human brain might retard long-distance migration-dependent addition of new neurons to the adult neural circuitry in the neocortex. However, localized, short-distance migration within specific niches, such as the hippocampus, or migration as individual neuroblasts within the forebrain may occur 166. Although the extent of neuronal migration in the adult brain has diminished during vertebrate evolution 167,168, neuronal migration has evolved as a mechanism to target appropriate numbers and types of neurons to where they are needed in the developing or mature brain. During development, interactions with the environment during the process of migration along distinct, permissive pathways facilitate the acquisition by the neurons of their final mature characteristics. Even though many areas of the adult human brain, including the neocortex, are generally resistant to the seamless incorporation of new neurons into the existing circuitry, the presence of neural progenitor pools in the adult brain suggests that an enhanced understanding of the mechanisms that can trigger and facilitate the recruitment and placement of correct numbers and types of neurons to where they are needed in an adult brain environment will be helpful in promoting functional recovery after brain injuries. Emerging evidence suggests that local, short-distance migration or migration as isolated neurons might be the predominant mechanism used to target new neurons in the adult human brain. Elucidating how this process is orchestrated and determining whether the substrates of this migration or molecular interactions that occur during this process influence the acquisition of desired functional phenotypes of new neurons in the adult brain will be crucial. Alternatively, quiescent neural stem cells might themselves migrate within the adult brain before giving rise to neurons locally. Repair strategies aimed at manipulating neural stem cells to generate the required types of neuron in injured adult brains should be supplemented with approaches that permit and promote the local navigation of neural stem cells or new neurons to their appropriate targets in the mature brain. 1. Sidman, R. L. & Rakic, P. Neuronal migration, with special reference to developing human brain: a review. Brain Res. 62, 1 35 (1973). 2. Mochida, G. H. & Walsh, C. A. Genetic basis of developmental malformations of the cerebral cortex. Arch. Neurol. 61, (2004). 3. Nadarajah, B. & Parnavelas, J. G. Modes of neuronal migration in the developing cerebral cortex. Nature Rev. Neurosci. 3, (2002). 4. Nadarajah, B., Alifragis, P., Wong, R. O. & Parnavelas, J. G. Ventricle-directed migration in the developing cerebral cortex. Nature Neurosci. 5, (2002). 5. 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Development 129, (2002). 34. Solecki, D. J., Govek, E. E., Tomoda, T. & Hatten, M. E. Neuronal polarity in CNS development. Genes Dev. 20, (2006). Outlines recent advances in our understanding of neuronal polarity and its relevance to developmental events, including neuronal migration. NATURE REVIEWS NEUROSCIENCE VOLUME 8 FEBRUARY