Reelin, Disabled 1, and beta 1 integrins are required for the formation of the radial glial scaffold in the hippocampus

Reelin Induces Branching of Neurons and Radial Glial Cells during Corticogenesis

Newborn neurons migrate along the processes of radial glial cells (RGCs) to reach their final positions in the cortex. Here, we visualized individual migrating neurons and RGCs using in utero electroporation. We show that branching of migrating neurons and RGCs is closely correlated spatiotemporally with the distribution of Reelin. Time-lapse imaging revealed that the leading processes of migrating neurons gave rise to increasingly more branches once their growth cones contacted the Reelin-containing marginal zone. This was accompanied by translocation of the nucleus and gradual shortening of the leading process. Absence of Reelin in reeler mice altered these processes resulting in misorientation, loss of bipolarity, and aberrant migration of cortical neurons. Moreover, in reeler, the branching of the basal processes of RGCs in the marginal zone was severely disrupted. Consistent with previous reports, we show that in dissociated reeler cortical cultures, exposure to recombinant Reelin enhanced dendritic complexity and glial branching. Our results suggest that Reelin induces branching of the leading processes of migrating neurons and that of basal processes of RGCs when they arrive at the Reelin-containing marginal zone. Branching of these processes may be crucial for the termination of nuclear translocation during the migratory process and for correct neuronal positioning.

Distinctive population of Gfap-expressing neural progenitors arising around the dentate notch migrate and form the granule cell layer in the developing hippocampus

In the adult hippocampus, granule cells continue to be generated from astrocyte-like progenitors expressing glial fibrillary acidic protein (GFAP) that differ from embryonic neocortical progenitors. However, during the embryonic period, dentate granule neurons and neocortical pyramidal neurons are derived from the ventricular zone (VZ) of the pallium. Our question is when do GFAP+ progenitors of granule neurons appear in the developing hippocampus during the embryonic period, and how do they form the granule cell layer. The present analysis using Gfap-GFP transgenic mice shows that the GFP+ distinct cell population first appears in the VZ of the medial pallium at the dorsal edge of the fimbria on embryonic day 13.5. During the perinatal period, they form a migratory stream from the VZ to the developing dentate gyrus, and establish the germinal zones in the migratory stream, and the marginal and hilar regions in the developing dentate gyrus. GFP+ cells in these regions were positive for Sox2 and Ki67, but negative for BLBP. GFP+ cells with Neurogenin2 expression were largely distributed in the VZ, whereas GFP+ cells with Tbr2 and NeuroD expressions were seen in the migratory stream and developing dentate gyrus. Prox1-expressing GFP+ cells were restricted to the developing dentate gyrus. These results suggest that distinctive Gfap-expressing progenitors arising around the dentate notch form germinal regions in the migratory stream and the developing dentate gyrus where they differentiate into granule neurons, indicating that distinct astrocyte-like neural progenitors continue to generate granule neurons, from the beginning of dentate development and throughout life. J. Comp. Neurol. 522:261-283, 2014. © 2013 Wiley Periodicals, Inc.

Reelin induces Erk1/2 signaling in cortical neurons through a non-canonical pathway

Reelin is an extracellular protein that controls many aspects of pre- and postnatal brain development and function. The molecular mechanisms that mediate postnatal activities of Reelin are not well understood. Here, we first set out to express and purify the full length Reelin protein and a biologically active central fragment. Second, we investigated in detail the signal transduction mechanisms elicited by these purified Reelin proteins in cortical neurons. Unexpectedly, we discovered that the full-length Reelin moiety, but not the central fragment, is capable of activating Erk1/2 signaling, leading to increased p90RSK phosphorylation and the induction of immediate-early gene expression. Remarkably, Erk1/2 activation is not mediated by the canonical signal transduction pathway, involving ApoER2/VLDLR and Dab1, that mediates other functions of Reelin in early brain development. The activation of Erk1/2 signaling likely contributes to the modulation of neuronal maturation and synaptic plasticity by Reelin in the postnatal and adult brain.

Linking pathways in the developing and aging brain with neurodegeneration

The molecular and cellular mechanisms, which coordinate the critical stages of brain development to reach a normal structural organization with appropriate networks, are progressively being elucidated. Experimental and clinical studies provide evidence of the occurrence of developmental alterations induced by genetic or environmental factors leading to the formation of aberrant networks associated with learning disabilities. Moreover, evidence is accumulating that suggests that also late-onset neurological disorders, even Alzheimer's disease, might be considered disorders of aberrant neural development with pathological changes that are set up at early stages of development before the appearance of the symptoms. Thus, evaluating proteins and pathways that are important in age-related neurodegeneration in the developing brain together with the characterization of mechanisms important during brain development with relevance to brain aging are of crucial importance. In the present review we focus on (1) aspects of neurogenesis with relevance to aging; (2) neurodegenerative disease (NDD)-associated proteins/pathways in the developing brain; and (3) further pathways of the developing or neurodegenerating brains that show commonalities. Elucidation of complex pathogenetic routes characterizing the earliest stage of the detrimental processes that result in pathological aging represents an essential first step toward a therapeutic intervention which is able to reverse these pathological processes and prevent the onset of the disease. Based on the shared features between pathways, we conclude that prevention of NDDs of the elderly might begin during the fetal and childhood life by providing the mothers and their children a healthy environment for the fetal and childhood development.

Reelin, neuronal polarity and process orientation of cortical neurons

Deficient reelin signaling leads to characteristic layering malformations in the cerebral cortex and causes polarity defects of cortical neurons. Since the discovery of reelin much has been learned about the molecular mechanisms that underlie the characteristic defects of layering defects in the reeler mutant. More recent studies provided insights in the crosstalk between reelin signaling and molecular pathways that control polarity development of radially migrating neurons. The present review summarizes and discusses recent findings on the role of reelin in modulating polarization and process orientation of neurons in the neocortex and hippocampus.

Reelin delays amyloid-beta fibril formation and rescues cognitive deficits in a model of Alzheimer's disease

Reelin is an extracellular matrix protein that is crucial for neural development and adult brain plasticity. While the Reelin signalling cascade has been reported to be associated with Alzheimer's disease (AD), the role of Reelin in this pathology is not understood. Here we use an in vitro approach to show that Reelin interacts with amyloid-β (Aβ42) soluble species, delays Aβ42 fibril formation and is recruited into amyloid fibrils. Furthermore, Reelin protects against both the neuronal death and dendritic spine loss induced by Aβ42 oligomers. In mice carrying the APP(Swe/Ind) mutation (J20 mice), Reelin overexpression delays amyloid plaque formation and rescues the recognition memory deficits. Our results indicate that by interacting with Aβ42 soluble species, delaying Aβ plaque formation, protecting against neuronal death and dendritic spine loss and preventing AD cognitive deficits, the Reelin pathway deserves consideration as a therapeutic target for the treatment of AD pathogenesis.

Neuronal migration and its disorders affecting the CA3 region

In this review, we focus on CA3 neuronal migration disorders in the rodent. We begin by introducing the main steps of hippocampal development, and we summarize characteristic hippocampal malformations in human. We then describe various mouse mutants showing structural hippocampal defects. Notably, genes identified in human cortical neuronal migration disorders consistently give rise to a CA3 phenotype when mutated in the mouse. We successively describe their molecular, physiological and behavioral phenotypes that together contribute to a better understanding of CA3-dependent functions. We finally discuss potential factors underlying the CA3 vulnerability revealed by these mouse mutants and that may also contribute to other human neurological and psychiatric disorders.

Reelin in the Years: Controlling Neuronal Migration and Maturation in the Mammalian Brain

The extracellular protein Reelin was initially identified as an essential factor in the control of neuronal migration and layer formation in the developing mammalian brain. In the years following its discovery, however, it became clear that Reelin is a multifunctional protein that controls not only the positioning of neurons in the developing brain, but also their growth, maturation, and synaptic activity in the adult brain. In this review, we will highlight the major discoveries of the biological activities of Reelin and the underlying molecular mechanisms that affect the development and function of the mammalian brain, from embryonic ages to adulthood.

The stripe assay: studying growth preference and axon guidance on binary choice substrates in vitro

Stripe assays are frequently used for studying binary growth decisions of cells and axons towards surface-bound molecules in vitro. In particular in the fields of neurodevelopment and axon guidance, stripe assays have become a routine tool. Several variants of the stripe assay have been developed since its introduction by Bonhoeffer and colleagues in 1987 (Development 101:685-696, 1987). In all variants, however, the principle is the generation of a structured binary growth substrate, consisting of two sets of cues, arranged in alternating stripes. There are two major classes of stripe assays, mainly distinguished by the source material used for stripe pattern manufacturing: membrane stripe assays, where the stripe patterns are generated with membrane fractions isolated from tissue or cells, and stripe assays with purified proteins, also called modified stripe assays. In this chapter we describe in detail the classical membrane stripe assay, the commonly used modified stripe assay employing purified proteins, and a novel stripe assay for high-affinity interacting proteins, like receptor/ligand pairs.

The development of hippocampal cellular assemblies

The proper assembly of a cohort of distinct cell types is a prerequisite for building a functional hippocampus. In this review, we describe the major molecular events of the developmental program leading to the cellular construction of the hippocampus. Data from rodent studies are used here to elaborate on our understanding of these processes.

Expression of calcium-binding proteins in layer 1 reelin-immunoreactive cells during rat and mouse neocortical development

Cajal-Retzius cells in layer 1 of the developing cerebral cortex and their product of secretion, reelin, an extracellular matrix protein, play a crucial role in establishing the correct lamination pattern in this tissue. As many studies into reelin signaling routes and pathological alterations are conducted in murine models, we used double-labeling and confocal microscopy to compare the distribution of the cell-specific markers, calretinin and calbindin, in reelin-immunoreactive cells during postnatal rat and mouse neocortical development. In the rat, neither calretinin nor calbindin colocalized with reelin in Cajal-Retzius cells at P0-P2. From P5 to P14, the colocalization of reelin and calretinin was commonly found in presumptive rat subpial piriform cells. These cells progressively lacked calretinin expression and persisted into adulthood as part of the pool of layer 1 reelin-positive interneurons. Conversely, in the mouse, reelin-immunoreactive Cajal-Retzius cells colocalized with calretinin and/or calbindin. Subpial piriform cells containing reelin and calretinin were identified at P5-P7, but lacked calretinin expression at P14. In adult mice, as in the rat, reelin-immunoreactive cells did not colocalize with calcium-binding proteins. Our results reveal a complex neurochemical profile of layer 1 cells in the rat neocortex, which makes using a single calcium-binding protein as a marker of rodent reelin-immunoreactive cells difficult.

The structural role of radial glial endfeet in confining spinal motor neuron somata is controlled by the Reelin and Notch pathways

Neuronal migration is a fundamental biological process that enables the precise positioning of neurons to form functional circuits. Cortical neurons migrate along glial scaffolds formed by radial glia guided by Reelin ligand. However, it is unclear whether the Reelin-directed behavior of radial glia is also critical for positioning the spinal neurons. Here we demonstrate a novel role of radial glia that confines motor neurons within the neural tube and is promoted by Reelin and Notch signaling. Spinal radial glia express the Dab1 adaptor for Reelin signaling and are surrounded by Reelin. In reeler mice, in which Reelin is absent, ectopic motor neurons are found outside the neural tube, although they appear to maintain their identity. Boundary cap (BC) cells, Schwann cell precursors and the basal lamina at motor exit points are intact, whereas the glia limitans of radial glia are disorganized and detached from the basement membrane. The sparse and irregular radial scaffold is wide enough to allow motor somata to pass. Forced activation of Notch signaling rescued the structural defects in radial glia in reeler mice and the appearance of extraspinal neurons. In the absence of Reelin, Notch intracellular domain (NICD) protein level was reduced. In addition, disrupting the radial glia scaffold by destroying its polarity induced ectopic motor neurons in chick embryos. These findings suggest that activation of the Notch pathways by Reelin is required to establish the radial glial scaffold, a structure that actively constrains motor neuron somata and specifies the CNS-PNS boundary.

ApoER2 and VLDLr are required for mediating reelin signalling pathway for normal migration and positioning of mesencephalic dopaminergic neurons

The migration of mesencephalic dopaminergic (mDA) neurons from the subventricular zone to their final positions in the substantia nigra compacta (SNc), ventral tegmental area (VTA), and retrorubral field (RRF) is controlled by signalling from neurotrophic factors, cell adhesion molecules (CAMs) and extracellular matrix molecules (ECM). Reelin and the cytoplasmic adaptor protein Disabled-1 (Dab1) have been shown to play important roles in the migration and positioning of mDA neurons. Mice lacking Reelin and Dab1 both display phenotypes characterised by the failure of nigral mDA neurons to migrate properly. ApoER2 and VLDLr are receptors for Reelin signalling and are therefore part of the same signal transduction pathway as Dab1. Here we describe the roles of ApoER2 and VLDLr in the proper migration and positioning of mDA neurons in mice. Our results demonstrate that VLDLr- and ApoER2-mutant mice have both a reduction in and abnormal positioning of mDA neurons. This phenotype was more pronounced in VLDLr-mutant mice. Moreover, we provide evidence that ApoER2/VLDLr double-knockout mice show a phenotype comparable with the phenotypes observed for Reelin- and Dab1- mutant mice. Taken together, our results demonstrate that the Reelin receptors ApoER2 and VLDLr play essential roles in Reelin-mediated migration and positioning of mDA neurons.

Role of the postnatal radial glial scaffold for the development of the dentate gyrus as revealed by Reelin signaling mutant mice

During dentate gyrus development, the early embryonic radial glial scaffold is replaced by a secondary glial scaffold around birth. In contrast to neocortical and early dentate gyrus radial glial cells, these postnatal glial cells are severely altered with regard to position and morphology in reeler mice lacking the secreted protein Reelin. In this study, we focus on the functional impact of these defects. Most radial glial cells throughout the nervous system serve as scaffolds for migrating neurons and precursor cells for both neurogenesis and gliogenesis. Precursor cell function has been demonstrated for secondary radial glial cells but the exact function of these late glial cells in granule cell migration and positioning is not clear. No data exist concerning the interplay between granule neurons and late radial glial cells during dentate gyrus development. Herein, we show that despite the severe morphological defects in the reeler dentate gyrus, the precursor function of secondary radial glial cells is not impaired during development in reeler mice. In addition, selective ablation of Disabled-1, an intracellular adaptor protein essential for Reelin signaling, in neurons but not in glial cells allowed us to distinguish effects of Reelin signaling on radial glial cells from possible secondary effects based on defective granule cells positioning.

Exogenous Reelin modifies the migratory behavior of neurons depending on cortical location

Malformations of cortical development can arise when projection neurons generated in the germinal zones fail to migrate properly into the cortical plate. This process is critically dependent on the Reelin glycoprotein, which when absent leads to an inversion of cortical layers and blurring of borders. Reelin has other functions including supporting neuron migration and maintaining their trajectories; however, the precise role on glial fiber-dependent or -independent migration of neurons remains controversial. In this study, we wish to test the hypothesis that migrating cortical neurons at different levels of the cortical wall have differential responses to Reelin. We exposed neurons migrating across the cortical wall to exogenous Reelin and monitored their migratory behavior using time-lapse imaging. Our results show that, in the germinal zones, exogenous Reelin retarded neuron migration and altered their trajectories. This behavior is in contrast to the response of neurons located in the intermediate zone (IZ), possibly because Reelin receptors are not expressed in this zone. In the reeler cortex, Reelin receptors are expressed in the IZ and exposure to exogenous Reelin was able to rescue the migratory defect. These studies demonstrate that migrating neurons have nonequivalent responses to Reelin depending on their location within the cortical wall.

Reelin signaling in development, maintenance, and plasticity of neural networks

The developing brain is formed through an orchestrated pattern of neuronal migration, leading to the formation of heterogeneous functional regions in the adult. Several proteins and pathways have been identified as mediators of developmental neuronal migration and cell positioning. However, these pathways do not cease to be functionally relevant after the embryonic and early postnatal period; instead, they switch from guiding cells, to guiding synapses. The outcome of synaptic guidance determines the strength and plasticity of neuronal networks by creating a scalable functional architecture that is sculpted by cues from the internal and external environment. Reelin is a multifunctional signal that coordinates cortical and subcortical morphogenesis during development and regulates structural plasticity in adulthood and aging. Gain or loss of function in reelin or its receptors has the potential to influence synaptic strength and patterns of connectivity, with consequences for memory and cognition. The current review highlights similarities in the signaling cascades that modulate neuronal positioning during development, and synaptic plasticity in the adult, with a focus on reelin, a glycoprotein that is increasingly recognized for its dual role in the formation and maintenance of neural circuits.

Tbr2 expression in Cajal-Retzius cells and intermediate neuronal progenitors is required for morphogenesis of the dentate gyrus

The dentate gyrus (DG) is a unique cortical region whose protracted development spans the embryonic and early postnatal periods. DG development involves large-scale reorganization of progenitor cell populations, ultimately leading to the establishment of the subgranular zone neurogenic niche. In the developing DG, the T-box transcription factor Tbr2 is expressed in both Cajal-Retzius cells derived from the cortical hem that guide migration of progenitors and neurons to the DG, and intermediate neuronal progenitors born in the dentate neuroepithelium that give rise to granule neurons. Here we show that in mice Tbr2 is required for proper migration of Cajal-Retzius cells to the DG; and, in the absence of Tbr2, formation of the hippocampal fissure is abnormal, leading to aberrant development of the transhilar radial glial scaffold and impaired migration of progenitors and neuroblasts to the developing DG. Furthermore, loss of Tbr2 results in decreased expression of Cxcr4 in migrating cells, leading to a premature burst of granule neurogenesis during early embryonic development accompanied by increased cell death in mutant animals. Formation of the transient subpial neurogenic zone was abnormal in Tbr2 conditional knock-outs, and the stem cell population in the DG was depleted before proper establishment of the subgranular zone. These studies indicate that Tbr2 is explicitly required for morphogenesis of the DG and participates in multiple aspects of the intricate developmental process of this structure.

Reelin and the Cdc42/Rac1 guanine nucleotide exchange factor αPIX/Arhgef6 promote dendritic Golgi translocation in hippocampal neurons

In the cerebral cortex of reeler mutant mice lacking reelin expression, neurons are malpositioned and display misoriented apical dendrites. Neuronal migration defects in reeler have been studied in great detail, but how misorientation of apical dendrites is related to reelin deficiency is poorly understood. In wild-type mice, the Golgi apparatus transiently translocates into the developing apical dendrite of radially migrating neurons. This dendritic Golgi translocation has recently been shown to be promoted by reelin. However, the underlying signalling mechanisms are largely unknown. Here, we show that the Cdc42/Rac1 guanine nucleotide exchange factor αPIX/Arhgef6 promoted translocation of Golgi cisternae into developing dendrites of hippocampal neurons. Reelin treatment further increased the αPIX-dependent effect. In turn, overexpression of exchange activity-deficient αPIX or dominant-negative (dn) Cdc42 or dn-Rac1 impaired dendritic Golgi positioning, an effect that was not compensated by reelin treatment. Together, these data suggest that αPIX may promote dendritic Golgi translocation, as a downstream component of a reelin-modulated signalling pathway. Finally, we found that reelin promoted the translocation of the Golgi apparatus into the dendrite that was most proximal to the reelin source. The distribution of reelin may thus contribute to the selection of the process that becomes the apical dendrite.

A systemic transcriptome analysis reveals the regulation of neural stem cell maintenance by an E2F1-miRNA feedback loop

Stem cell fate decisions are controlled by a molecular network in which transcription factors and miRNAs are of key importance. To systemically investigate their impact on neural stem cell (NSC) maintenance and neuronal commitment, we performed a high-throughput mRNA and miRNA profiling and isolated functional interaction networks of involved mechanisms. Thereby, we identified an E2F1–miRNA feedback loop as important regulator of NSC fate decisions. Although E2F1 supports NSC proliferation and represses transcription of miRNAs from the miR-17∼92 and miR-106a∼363 clusters, these miRNAs are transiently up-regulated at early stages of neuronal differentiation. In these early committed cells, increased miRNAs expression levels directly repress E2F1 mRNA levels and inhibit cellular proliferation. In mice, we demonstrated that these miRNAs are expressed in the neurogenic areas and that E2F1 inhibition represses NSC proliferation. The here presented data suggest a novel interaction mechanism between E2F1 and miR-17∼92 / miR-106a∼363 miRNAs in controlling NSC proliferation and neuronal differentiation.

Roles of Integrins and Intracellular Molecules in the Migration and Neuritogenesis of Fetal Cortical Neurons: MEK Regulates Only the Neuritogenesis

The roles of integrin subunits and intracellular molecules in regulating the migration and neuritogenesis of neurons isolated from 16.5 gestation days rat fetal cortices were examined using in vitro assays. Results showed that laminin supported the migration of fetal cortical neurons better than fibronectin and that the fetal cortical neurons migrated on laminin using β1 and α3 integrin subunits which make up the α3β1 integrin receptor. On fibronectin, the migration was mediated by β1 integrin subunit. Perturbation of src kinase, phospholipase C, or protein kinase C activity, inhibition of IP3 receptor mediated calcium release, or chelation of intracellular calcium inhibited both migration and neuritogenesis, whereas inhibition of growth factor signaling via MEK inhibited only the neuritogenesis. The detection of α1 and α9 transcripts suggested that the migration of fetal cortical neurons may also be mediated by α1β1 and α9β1 integrin receptors. Results showed that calcium may regulate migration and neuritogenesis by maintaining optimum levels of microtubules in the fetal cortical neurons. It is concluded that the fetal cortical neurons are fully equipped with the integrin signaling cascade required for their migration and neuritogenesis, whereas crosstalk between the integrin and growth-factor signaling regulate only the neuritogenesis.

A 3D-matrigel/microbead assay for the visualization of mechanical tractive forces at the neurite-substrate interface of cultured neurons

Mechanical properties of neuronal processes contribute to neuronal function, to resistance of fiber tracts against mechanical trauma, and to morphological changes during development and neurodegeneration. Conventional in vitro cell culture systems on inflexible substrates do not allow for the visualization of changing mechanical stress between neurites and their substrate. To solve this problem, we adapted a three-dimensional gel matrix assay to visualize mechanical traction forces at the neurite-substrate interface. We chose matrigel as substrate because in this matrix various types of neurons initially adapt a bipolar morphology while migrating, similar to migrating neurons in vivo. To visualize emerging traction forces between neurites and their substrate, microbeads were embedded into the matrix as visible landmarks. We first analyzed mechanical distortion of matrigel by stepwise movements of a glass pipette tip under control of a micromanipulator to ensure reproducibility of induced bead displacement. The assay was then used to study the effect of the microtubule disrupting drug nocodazole on neuronal processes. By monitoring displacement of matrigel-embedded microbeads, we visualized here for the first time emerging mechanical traction forces between the leading process and the substrate during nocodazole-induced soma translocation. We did not observe bead displacement by processes of aged neurons.

Cell-autonomous inactivation of the reelin pathway impairs adult neurogenesis in the hippocampus

Adult hippocampal neurogenesis is thought to be essential for learning and memory, and has been implicated in the pathogenesis of several disorders. Although recent studies have identified key factors regulating neuroprogenitor proliferation in the adult hippocampus, the mechanisms that control the migration and integration of adult-born neurons into circuits are largely unknown. Reelin is an extracellular matrix protein that is vital for neuronal development. Activation of the Reelin cascade leads to phosphorylation of Disabled-1, an adaptor protein required for Reelin signaling. Here we used transgenic mouse and retroviral reporters along with Reelin signaling gain-of-function and loss-of-function studies to show that the Reelin pathway regulates migration and dendritic development of adult-generated hippocampal neurons. Whereas overexpression of Reelin accelerated dendritic maturation, inactivation of the Reelin signaling pathway specifically in adult neuroprogenitor cells resulted in aberrant migration, decreased dendrite development, formation of ectopic dendrites in the hilus, and the establishment of aberrant circuits. Our findings support a cell-autonomous and critical role for the Reelin pathway in regulating dendritic development and the integration of adult-generated granule cells and point to this pathway as a key regulator of adult neurogenesis. Moreover, our data reveal a novel role of the Reelin cascade in adult brain function with potential implications for the pathogenesis of several neurological and psychiatric disorders.

Reelin promotes microtubule dynamics in processes of developing neurons

The extracellular matrix protein reelin controls radial migration and layer formation of cortical neurons, in part by modulation of cytoskeletal dynamics. A stabilizing effect of reelin on the actin cytoskeleton has been described recently. However, it is poorly understood how reelin modulates microtubule dynamics. Here, we provide evidence that reelin increases microtubule assembly. This effect is mediated, at least in part, by promoting microtubule plus end dynamics in processes of developing neurons. Thus, we treated primary neuronal cultures with nocodazole to disrupt microtubules. After nocodazole washout, we found microtubule reassembly to be accelerated in the presence of reelin. Moreover, we show that reelin treatment promoted the formation of microtubule plus end binding protein 3 (EB3) comets in developing dendrites, and that EB3 immunostaining in the developing wild-type neocortex is most intense in the reelin-rich marginal zone where leading processes of radially migrating neurons project to. This characteristic EB3 staining pattern was absent in reeler. Also reassembly of nocodazole-dispersed dendritic Golgi apparati, which are closely associated to microtubules, was accelerated by reelin treatment, though with a substantially slower time course when compared to microtubule reassembly. In support of our in vitro results, we found that the subcellular distribution of α-tubulin and acetylated tubulin in reeler cortical sections differed from wild-type and from mice lacking the very low density lipoprotein receptor (VLDLR), known to bind reelin. Taken together, our results suggest that reelin promotes microtubule assembly, at least in part, by increasing microtubule plus end dynamics.

Neocortical neurogenesis and neuronal migration

The neocortex, the evolutionarily newest part of the cerebral cortex, controls nearly all aspects of behavior, including perception, language, and decision making. It contains an immense number of neurons that can be broadly divided into two groups, excitatory neurons and inhibitory interneurons. These neurons are predominantly produced through extensive progenitor cell divisions during the embryonic stages. Moreover, they are not randomly dispersed, but spatially organized into horizontal layers that are essential for neocortex function. The formation of this laminar structure requires exquisite control of neuronal migration from their birthplace to their final destination. Extensive research over the past decade has greatly advanced our understanding of the production and migration of both excitatory neurons and inhibitory interneurons in the developing neocortex. In this review, we aim to give an overview on the molecular and cellular processes of neocortical neurogenesis and neuronal migration.

Consensus paper: pathological role of the cerebellum in autism

There has been significant advancement in various aspects of scientific knowledge concerning the role of cerebellum in the etiopathogenesis of autism. In the current consensus paper, we will observe the diversity of opinions regarding the involvement of this important site in the pathology of autism. Recent emergent findings in literature related to cerebellar involvement in autism are discussed, including: cerebellar pathology, cerebellar imaging and symptom expression in autism, cerebellar genetics, cerebellar immune function, oxidative stress and mitochondrial dysfunction, GABAergic and glutamatergic systems, cholinergic, dopaminergic, serotonergic, and oxytocin-related changes in autism, motor control and cognitive deficits, cerebellar coordination of movements and cognition, gene-environment interactions, therapeutics in autism, and relevant animal models of autism. Points of consensus include presence of abnormal cerebellar anatomy, abnormal neurotransmitter systems, oxidative stress, cerebellar motor and cognitive deficits, and neuroinflammation in subjects with autism. Undefined areas or areas requiring further investigation include lack of treatment options for core symptoms of autism, vermal hypoplasia, and other vermal abnormalities as a consistent feature of autism, mechanisms underlying cerebellar contributions to cognition, and unknown mechanisms underlying neuroinflammation.

Layer 6 cortical neurons require Reelin-Dab1 signaling for cellular orientation, Golgi deployment, and directed neurite growth into the marginal zone

Background

The secreted ligand Reelin is believed to regulate the translocation of prospective layer 6 (L6) neocortical neurons into the preplate, a loose layer of pioneer neurons that overlies the ventricular zone. Recent studies have also suggested that Reelin controls neuronal orientation and polarized dendritic growth during this period of early cortical development. To explicitly characterize and quantify how Reelin controls this critical aspect of neurite initiation and growth we used a new ex utero explant model of early cortical development to selectively label a subset of L6 cortical neurons for complete 3-D reconstruction.

Results

The total neurite arbor sizes of neurons in Reelin-deficient (reeler mutant) and Dab1-deficient (Reelin-non-responsive scrambler mutant) cortices were quantified and unexpectedly were not different than control arbor lengths (p = 0.51). For each mutant, however, arbor organization was markedly different: mutant neurons manifested more primary processes (neurites emitted directly from the soma) than wild type, and these neurites were longer and displayed less branching. Reeler and scrambler mutant neurites extended tangentially rather than radially, and the Golgi apparatus that normally invests the apical neurite was compact in both reeler and scrambler mutants. Mutant cortices also exhibited a neurite “exclusion zone” which was relatively devoid of L6 neuron neurites and extended at least 15 μm beneath the pial surface, an area corresponding to the marginal zone (MZ) in the wild type explants. The presence of an exclusion zone was also indicated in the orientation of mutant primary neurite and neuronal somata, which failed to adopt angles within ~20˚ of the radial line to the pial surface. Injection of recombinant Reelin to reeler, but not scrambler, mutant cortices fully rescued soma orientation, Golgi organization, and dendritic projection defects within four hrs.

Conclusions

These findings indicate Reelin promotes directional dendritic growth into the MZ, an otherwise exclusionary zone for L6 neurites.

Foxg1 has an essential role in postnatal development of the dentate gyrus

Foxg1, formerly BF-1, is expressed continuously in the postnatal and adult hippocampal dentate gyrus (DG). This transcription factor (TF) is thought to be involved in Rett syndrome, which is characterized by reduced hippocampus size, indicating its important role in hippocampal development. Due to the perinatal death of Foxg1(-/-) mice, the function of Foxg1 in postnatal DG neurogenesis remains to be explored. Here, we describe the generation of a Foxg1(fl/fl) mouse line. Foxg1 was conditionally ablated from the DG during prenatal and postnatal development by crossing this line with a Frizzled9-CreER(TM) line and inducing recombination with tamoxifen. In this study, we first show that disruption of Foxg1 results in the loss of the subgranular zone and a severely disrupted secondary radial glial scaffold, leading to the impaired migration of granule cells. Moreover, detailed analysis reveals that Foxg1 may be necessary for the maintenance of the DG progenitor pool and that the lack of Foxg1 promotes both gliogenesis and neurogenesis. We additionally show that Foxg1 may be required for the survival and maturation of postmitotic neurons and that Foxg1 may be involved in Reelin signaling in regulating postnatal DG development. Last, prenatal deletion of Foxg1 suggests that it is rarely involved in the migration of primordial granule cells. In summary, we report that Foxg1 is critical for DG formation, especially during early postnatal stage.

Laminin/β1 integrin signal triggers axon formation by promoting microtubule assembly and stabilization

Axon specification during neuronal polarization is closely associated with increased microtubule stabilization in one of the neurites of unpolarized neuron, but how this increased microtubule stability is achieved is unclear. Here, we show that extracellular matrix (ECM) component laminin promotes neuronal polarization via regulating directional microtubule assembly through β1 integrin (Itgb1). Contact with laminin coated on culture substrate or polystyrene beads was sufficient for axon specification of undifferentiated neurites in cultured hippocampal neurons and cortical slices. Active Itgb1 was found to be concentrated in laminin-contacting neurites. Axon formation was promoted and abolished by enhancing and attenuating Itgb1 signaling, respectively. Interestingly, laminin contact promoted plus-end microtubule assembly in a manner that required Itgb1. Moreover, stabilizing microtubules partially prevented polarization defects caused by Itgb1 downregulation. Finally, genetic ablation of Itgb1 in dorsal telencephalic progenitors caused deficits in axon development of cortical pyramidal neurons. Thus, laminin/Itgb1 signaling plays an instructive role in axon initiation and growth, both in vitro and in vivo, through the regulation of microtubule assembly. This study has established a linkage between an extrinsic factor and intrinsic cytoskeletal dynamics during neuronal polarization.

Reelin regulates differentiation of neural stem cells by activation of notch signaling through Disabled-1 tyrosine phosphorylation

We have previously reported the cross-talk between Reelin and Notch-1 signaling pathways, which are 2 major pathways that regulate brain development. We found that Reelin activated Notch-1 signaling, leading to the expression of brain lipid binding protein (BLBP) and the formation of radial glial cells in human neural progenitor cells (hNPCs). In the current study, we investigated the molecular mechanisms by which Reelin activates Notch-1. We show that Reelin-stimulated Notch-1 activation is dependent on Reelin signaling. The induction of Disabled-1 (Dab-1) tyrosine phosphorylation, and the subsequent activation of Src family kinases, were found to be essential steps for the activation of Notch-1 signaling by Reelin. Reelin treatment increased the interaction between Dab-1 and Notch-1 intracellular domain (NICD), and enhanced NICD translocation to the nucleus. This study advances our knowledge of the regulation of Notch-1 activation by Reelin signaling in hNPCs, as an approach to understanding cell fate determination, differentiation, and neurogenesis during brain development.

Proper cerebellar development requires expression of β1-integrin in Bergmann glia, but not in granule neurons

β1-class integrins play essential roles both in developmental biology as well as in cancer. Particularly, a Nestin-driven deletion of β1-integrin receptors results in severe abnormalities of brain development including a laminar disorganization of cerebellar granule neurons. However, since Nestin is expressed in all kinds of neural precursors, these data do not allow conclusions to be drawn about the role of β1-integrins in distinct neuronal and glial cell types. By generating conditional knockout mice using granule cell-specific Math1-promoter sequences, we show here that the expression of β1-integrins in granule neurons is dispensable for the development of the cerebellum. Also, deletion of β1-integrin from tumors that arise in a mouse model of granule cell precursor-derived medulloblastoma did not result in a significant survival benefit. Last, expression levels of β1-integrin in human medulloblastoma samples did not predict patient's outcome. However, a β1-integrin knockout using hGFAP-promoter sequences led to cerebellar hypoplasia, inappropriate positioning of Bergmann glia cells in the molecular layer, undirected outgrowth of radial glia fibers, and granule cell ectopia. We therefore conclude that β1-integrin expression in cerebellar granule neurons is not essential during normal development or medulloblastoma formation. In fact, it is the expression of β1-integrin in glia that is crucial for the proper development of the cerebellar cortex.

Congruence of vascular network remodeling and neuronal dispersion in the hippocampus of reelin-deficient mice

In the hippocampus, neurons and fiber projections are strictly organized in layers and supplied with oxygen via a vascular network that also develops layer-specific characteristics in wild-type mice, as shown in the present study for the first time in a quantitative manner. By contrast, in the reeler mutant, well known for its neuronal migration defects due to the lack of the extracellular matrix protein reelin, emerging layer-specific characteristics of the vascular pattern were found to be remodeled during development of the dentate gyrus. Remarkably, in the first postnatal week, when a granule cell layer was still discernable in the reeler dentate gyrus, also the reeler vascular pattern resembled wild type. Thus, at postnatal day 6, unbranched microvessels traversed the granule cell layer and bifurcated when reaching the subgranular zone. Only after the first postnatal week vascular network remodeling in the reeler dentate gyrus became apparent, when the proportion of dispersed granule cells increased. Hence, vessel bifurcation frequency decreased in the maturing reeler dentate gyrus, but increased in wild type, resulting in significant differences (approx. 100%; p < 0.01) between adult wild type and reeler. Moreover, layer-specific vessel bifurcation frequencies disappeared in the maturing reeler dentate gyrus. Finally, a wild type-like vascular pattern was also found in the dentate gyrus of mice deficient for the reelin receptor very low density lipoprotein receptor (VLDLR), precluding a requirement of VLDLR for normal vascular pattern formation in the dentate gyrus. In sum, our findings show that vascular network remodeling in the reeler dentate gyrus is closely linked to the progression of granule cell dispersion.

Integrins as receptor targets for neurological disorders

This review focuses on the neurobiology of integrins, pathophysiological roles of integrins in neuroplasticity and nervous system disorders, and therapeutic implications of integrins as potential drug targets and possible delivery pathways. Neuroplasticity is a central phenomenon in many neurological conditions such as seizures, trauma, and traumatic brain injury. During the course of many brain diseases, in addition to intracellular compartment changes, alterations in non-cell compartments such as extracellular matrix (ECM) are recognized as an essential process in forming and reorganizing neural connections. Integrins are heterodimeric transmembrane receptors that mediate cell-ECM and cell-cell adhesion events. Although the mechanisms of neuroplasticity remain unclear, it has been suggested that integrins undergo plasticity including clustering through interactions with ECM proteins, modulating ion channels, intracellular Ca(2+) and protein kinase signaling, and reorganization of cytoskeletal filaments. As cell surface receptors, integrins are central to the pathophysiology of many brain diseases, such as epilepsy, and are potential targets for the development of new drugs for neurological disorders.

Reelin modulates cytoskeletal organization by regulating Rho GTPases

The correct positioning of postmitotic neurons in the developing neocortex and other laminated brain structures requires the activation of a Reelin-lipoprotein receptor-Dab1 signaling cascade. The large glycoprotein Reelin is secreted by Cajal-Retzius pioneer neurons and bound by the apolipoprotein E receptor family members Apoer2 and Vldl receptor on responsive neurons and radial glia. This leads to the tyrosine phosphorylation of the cytoplasmic protein Disabled-1 (Dab1) by non-receptor tyrosine kinases of the Src family. Various signaling pathways downstream of Dab1 connect Reelin to the actin and microtubule cytoskeleton. Despite this knowledge, a comprehensive view linking the different cell-biological and biochemical actions of Reelin to its diverse physiological roles not only during neurodevelopment but also in the maintenance and functioning of the adult brain is still lacking. In this review, we discuss our finding that Reelin activates Rho GTPases in neurons in the light of other recent studies, which demonstrate a role of Reelin in Golgi organization, and suggest additional roles of Cdc42 activation by Reelin in radial glial cells of the developing cortex.

Early and adult hippocampal TGF-β1 overexpression have opposite effects on behavior

TGF-β1 is an anti-inflammatory cytokine that is augmented in the brain of autistic patients and that can affect brain development. In this work, we studied the effects of overexpressing TGF-β1 in the dentate gyrus of adult or young mice on behavior. TGF-β1 overexpression during postnatal development led to a long-term decrease in social interaction and to long-term increases in self-grooming and depression-related behaviors. Our analysis shows that these behavioral changes correlate with the long-term downregulation of TGF-β1 and IL-6 expression in the dentate gyrus, as well as to decreases in the mRNA levels of the synaptic protein neuroligin 3 and in the number of Reelin-positive neurons in the dentate gyrus. In contrast, chronic expression of TGF-β1 during adulthood led to transient opposite effects on these behaviors. These results show a central role of hippocampal TGF-β1 in the programming and modulation of social interaction, repetitive behavior and depression-related behavior. Finally, our data suggest a role of hippocampal TGF-β1 and early-life neuroinflammation in the development of the behavioral alterations observed in autism spectrum disorders.

EphB2 tyrosine kinase-dependent forward signaling in migration of neuronal progenitors that populate and form a distinct region of the dentate niche

The dentate gyrus (DG) is one of two areas in the mature brain where stem cells reside to continuously produce new neurons throughout adulthood. While much research has focused on the DG for its roles in adult neurogenesis, little is known regarding how this key region of the brain initially develops to form its distinct architecture. We show here that the murine EphB2 receptor tyrosine kinase is critical for embryonic/postnatal development of a specific region of the DG known as the lateral suprapyramidal blade (LSB). Intracellular truncation and point mutants demonstrate that EphB2 catalytic activity is essential for LSB formation. This is consistent with expression of EphB2 in nestin-positive neural progenitor cells that migrate medially from the lateral ventricle dentate notch neuroepithelium to populate the tertiary matrix and form the DG near the midline of the brain. Animals lacking ephrin-B1 recapitulate loss of the receptor and show that this molecule acts as the ligand to stimulate EphB2 forward signaling and direct migration of the neural progenitors into the dorsal compartment of the tertiary matrix and form the LSB. Immunoreactivity against the extracellular matrix protein Reelin in a region directly above the developing LSB is dramatically reduced when EphB2 forward signaling is disrupted. Together, these results indicate ephrin-B1 interacting with EphB2 controls the migration of dentate progenitor cells into the dorsal half of the developing DG, perhaps in part by affecting Reelin expression in a key compartment directly above the LSB.

How neurons migrate: a dynamic in-silico model of neuronal migration in the developing cortex

Background

Neuronal migration, the process by which neurons migrate from their place of origin to their final position in the brain, is a central process for normal brain development and function. Advances in experimental techniques have revealed much about many of the molecular components involved in this process. Notwithstanding these advances, how the molecular machinery works together to govern the migration process has yet to be fully understood. Here we present a computational model of neuronal migration, in which four key molecular entities, Lis1, DCX, Reelin and GABA, form a molecular program that mediates the migration process.

Results

The model simulated the dynamic migration process, consistent with in-vivo observations of morphological, cellular and population-level phenomena. Specifically, the model reproduced migration phases, cellular dynamics and population distributions that concur with experimental observations in normal neuronal development. We tested the model under reduced activity of Lis1 and DCX and found an aberrant development similar to observations in Lis1 and DCX silencing expression experiments. Analysis of the model gave rise to unforeseen insights that could guide future experimental study. Specifically: (1) the model revealed the possibility that under conditions of Lis1 reduced expression, neurons experience an oscillatory neuron-glial association prior to the multipolar stage; and (2) we hypothesized that observed morphology variations in rats and mice may be explained by a single difference in the way that Lis1 and DCX stimulate bipolar motility. From this we make the following predictions: (1) under reduced Lis1 and enhanced DCX expression, we predict a reduced bipolar migration in rats, and (2) under enhanced DCX expression in mice we predict a normal or a higher bipolar migration.

Conclusions

We present here a system-wide computational model of neuronal migration that integrates theory and data within a precise, testable framework. Our model accounts for a range of observable behaviors and affords a computational framework to study aspects of neuronal migration as a complex process that is driven by a relatively simple molecular program. Analysis of the model generated new hypotheses and yet unobserved phenomena that may guide future experimental studies. This paper thus reports a first step toward a comprehensive in-silico model of neuronal migration.

The role of GABAergic system in neurodevelopmental disorders: a focus on autism and epilepsy

Autism spectrum disorders (ASD) and epilepsy are very common neurological disorders of childhood, with an estimated incidence of about 0.5 - 1 % in worldwide population. ASD and epilepsy are often associated, suggesting that common neurodevelopmental bases may exist for these two disorders. The neurodevelopmental bases of both ASD and epilepsy have been clearly showed by a number of genetic, neuroimaging and neuropathological studies. In recent years, dysfunction of inhibitory GABAergic circuits has been proposed as a cause for both disorders. Several studies performed on both animal models and postmortem human samples indicate that GABAergic neurons and circuits are altered in both ASD and epilepsy, suggesting that the excitation/inhibition imbalance resulting from neurodevelopmental defects in GABAergic circuitry might represent a common pathogenetic mechanism for these disorders. Here, we will review the most significant studies supporting this hypothesis.

The interesting interplay between interneurons and adult hippocampal neurogenesis

Adult neurogenesis is a unique form of plasticity found in the hippocampus, a brain region key to learning and memory formation. While many external stimuli are known to modulate the generation of new neurons in the hippocampus, little is known about the local circuitry mechanisms that regulate the process of adult neurogenesis. The neurogenic niche in the hippocampus is highly complex and consists of a heterogeneous population of cells including interneurons. Because interneurons are already highly integrated into the hippocampal circuitry, they are in a prime position to influence the proliferation, survival, and maturation of adult-generated cells in the dentate gyrus. Here, we review the current state of our understanding on the interplay between interneurons and adult hippocampal neurogenesis. We focus on activity- and signaling-dependent mechanisms, as well as research on human diseases that could provide better insight into how interneurons in general might add to our comprehension of the regulation and function of adult hippocampal neurogenesis.

Reelin regulates cadherin function via Dab1/Rap1 to control neuronal migration and lamination in the neocortex

Neuronal migration is critical for establishing neocortical cell layers and migration defects can cause neurological and psychiatric diseases. Recent studies show that radially migrating neocortical neurons use glia-dependent and glia-independent modes of migration, but the signaling pathways that control different migration modes and the transitions between them are poorly defined. Here, we show that Dab1, an essential component of the reelin pathway, is required in radially migrating neurons for glia-independent somal translocation, but not for glia-guided locomotion. During migration, Dab1 acts in translocating neurons to stabilize their leading processes in a Rap1-dependent manner. Rap1, in turn, controls cadherin function to regulate somal translocation. Furthermore, cell-autonomous neuronal deficits in somal translocation are sufficient to cause severe neocortical lamination defects. Thus, we define the cellular mechanism of reelin function during radial migration, elucidate the molecular pathway downstream of Dab1 during somal translocation, and establish the importance of glia-independent motility in neocortical development.

Role for Reelin in neurotransmitter release

The extracellular matrix molecule Reelin is known to control neuronal migration during development. Recent evidence suggests that it also plays a role in the maturation of postsynaptic dendrites and spines as well as in synaptic plasticity. Here, we aimed to address the question whether Reelin plays a role in presynaptic structural organization and function. Quantitative electron microscopic analysis of the number of presynaptic boutons in the stratum radiatum of hippocampal region CA1 did not reveal differences between wild-type animals and Reelin-deficient reeler mutant mice. However, additional detailed analysis showed that the number of presynaptic vesicles was significantly increased in CA1 synapses of reeler mutants. To test the hypothesis that vesicle fusion is altered in reeler, we studied proteins known to control transmitter release. SNAP25, a protein of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, was found to be significantly reduced in reeler mutants, whereas other SNARE complex proteins remained unaltered. Addition of recombinant Reelin to organotypic slice cultures of reeler hippocampi substantially rescued not only SNAP25 protein expression levels but also the number of vesicles per bouton area indicating a role for Reelin in presynaptic functions. Next, we analyzed paired-pulse facilitation, a presynaptic mechanism associated with transmitter release, and observed a significant decrease at CA1 synapses of reeler mutants when compared with wild-type animals. Together, these novel findings suggest a role for Reelin in modulating presynaptic release mechanisms.

Regulation of cortical neuron migration by the Reelin signaling pathway

Reeler is a mutant mouse with defects in layered structures of the central nervous system, such as the cerebral cortex, hippocampus, and cerebellum, and has been extensively examined for more than half a century. The full-length cDNA for the responsible gene for reeler, reelin, was serendipitously identified, revealing that Reelin encodes a large secreted protein. So far, two Reelin receptors, apolipoprotein E receptor 2 and very low-density lipoprotein receptor, and the cytoplasmic adaptor protein Disabled homolog 1 (Dab1) have been shown to be essential for Reelin signaling. Although a number of downstream cascades of Dab1 have also been reported using various experimental systems, the physiological functions of Reelin in vivo remain controversial. Here, we review recent advances in the understanding of the Reelin-Dab1 signaling pathway in the developing cerebral cortex.

Go or stop? Divergent roles of Reelin in radial neuronal migration

Neuronal migration is an essential step of brain development and is controlled by a variety of cellular proteins and extracellular matrix molecules. Reelin, an extracellular matrix protein, is required for neuronal migration. Over the past 10 years, the Reelin signaling cascade has been studied intensively. However, the role of Reelin in neuronal migration has remained unclear. Different Reelin fragments and different Reelin receptors suggest multiple functions of Reelin. In this review, the authors focus on Reelin effects on the actin cytoskeleton of migrating neurons.

Reelin signals through apolipoprotein E receptor 2 and Cdc42 to increase growth cone motility and filopodia formation

Lipoprotein receptor signaling regulates the positioning and differentiation of postmitotic neurons during development and modulates neuronal plasticity in the mature brain. Depending on the contextual situation, the lipoprotein receptor ligand Reelin can have opposing effects on cortical neurons. We show that Reelin increases growth cone motility and filopodia formation, and identify the underlying signaling cascade. Reelin activates the Rho GTPase Cdc42, known for its role in neuronal morphogenesis and directed migration, in an apolipoprotein E receptor 2-, Disabled-1-, and phosphatidylinositol 3-kinase-dependent manner. We demonstrate that neuronal vesicle trafficking, a Cdc42-controlled process, is increased after Reelin treatment and further provide evidence that the peptidergic VIP/PACAP38 system and Reelin can functionally interact to promote axonal branching. In conclusion, Reelin-induced activation of Cdc42 contributes to the regulation of the cytoskeleton of individual responsive neurons and converges with other signaling cascades to orchestrate Rho GTPase activity and promote neuronal development. Our data link the observation that defects in Rho GTPases and Reelin signaling are responsible for developmental defects leading to neurological and psychiatric disorders.

Origin, maturation, and astroglial transformation of secondary radial glial cells in the developing dentate gyrus

The dentate gyrus is a brain region where neurons are continuously born throughout life. In the adult, the role of its radial glia in neurogenesis has attracted much attention over the past years; however, little is known about the generation and differentiation of glial cells and their relationship to radial glia during the ontogenetic development of this brain structure. Here, we combine immunohistochemical phenotyping using antibodies against glial marker proteins with BrdU birthdating to characterize the development of the secondary radial glial scaffold in the dentate gyrus and its potential to differentiate into astrocytes. We demonstrate that the expression of brain lipid-binding protein, GLAST, and glial fibrillary acidic protein (GFAP) characterizes immature differentiating cells confined to an astrocytic fate in the early postnatal dentate gyrus. On the basis of our studies, we propose a model where immature astrocytes migrate radially through the granule cell layer to adopt their final positions in the molecular layer of the dentate gyrus. Time-lapse imaging of acute hippocampal slices from hGFAP-eGFP transgenic mice provides direct evidence for such a migration mode of differentiating astroglial cells in the developing dentate gyrus.

Emerging topics in Reelin function

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

Role for Reelin-induced cofilin phosphorylation in the assembly of sympathetic preganglionic neurons in the murine intermediolateral column

Sympathetic preganglionic neurons (SPNs) are located in the intermediolateral column (IMLC) of the spinal cord. This specific localization results from primary and secondary migratory processes during spinal cord development. Thus, following neurogenesis in the neuroepithelium, SPNs migrate first in a ventrolateral direction and then, in a secondary step, dorsolaterally to reach the IMLC. These migratory processes are controlled, at least in part, by the glycoprotein Reelin, which is known to be important for the development of laminated brain structures. In reeler mutants deficient in Reelin, SPNs initially migrate ventrolaterally as normal. However, most of them then migrate medially to become eventually located near the central canal. Here, we provide evidence that in wild-type animals this aberrant medial migration towards the central canal is prevented by Reelin-induced cytoskeletal stabilization, brought about by phosphorylation of cofilin. Cofilin plays an important role in actin depolymerization, a process required for the changes in cell shape during migration. Phosphorylation of cofilin renders it unable to depolymerize F-actin, thereby stabilizing the cytoskeleton. Using immunostaining for phosphorylated cofilin (p-cofilin), we demonstrate that SPNs in wild-type animals, but not in reeler mutants and other mutants of the Reelin signalling cascade, are immunoreactive for p-cofilin. These findings suggest that Reelin near the central canal induces cofilin phosphorylation in SPNs, thereby preventing them from aberrant migration towards the central canal. The results extend our previous studies on cortical neurons in which Reelin in the marginal zone was found to stabilize the leading processes of migrating neurons and terminate the migration process.

Altered speeds and trajectories of neurons migrating in the ventricular and subventricular zones of the reeler neocortex

The Reelin signaling pathway is essential for proper cortical development, but it is unclear to whether Reelin function is primarily important for cortical layering or neuron migration. It has been proposed that Reelin is perhaps required only for somal translocation but not glial-dependent locomotion. This implies that the location of neurons responding to Reelin is restricted to the outer regions of the cortical plate (CP). To determine whether Reelin is required for migration outside of the CP, we used time-lapse imaging to track the behavior of cells undergoing locomotion in the germinal zones. We focused on the migratory activity in the ventricular/subventricular zones where the first transition of bipolar to multipolar migration occurs and where functional Reelin receptors are known to be expressed. Despite Reelin loss, neurons had no difficulty in undergoing radial migration and indeed displayed greater migratory speed. Additionally, compared with the wild-type, reeler neurons displayed altered trajectories with greater deviation from a radial path. These results suggest that Reelin loss has early consequences for migration in the germinal zones that are portrayed as defective radial trajectories and migratory speeds. Together, these abnormalities can give rise to the increased cell dispersion observed in the reeler cortex.