Heterozygous Dab1 Null Mutation Disrupts Neocortical and Hippocampal Development.

Loss-of-function mutations in Reelin and DAB1 signaling pathways disrupt proper neuronal positioning in the cerebral neocortex and hippocampus, but the underlying molecular mechanisms remain elusive. Here, we report that heterozygous yotari mice harboring a single autosomal recessive yotari mutation of Dab1 exhibited a thinner neocortical layer 1 than wild-type mice on postnatal day (P)7. However, a birth-dating study suggested that this reduction was not caused by failure of neuronal migration. In utero electroporation-mediated sparse labeling revealed that the superficial layer neurons of heterozygous yotari mice tended to elongate their apical dendrites within layer 2 than within layer 1. In addition, the CA1 pyramidal cell layer in the caudo-dorsal hippocampus was abnormally split in heterozygous yotari mice, and a birth-dating study revealed that this splitting was caused mainly by migration failure of late-born pyramidal neurons. Adeno-associated virus (AAV)-mediated sparse labeling further showed that many pyramidal cells within the split cell had misoriented apical dendrites. These results suggest that regulation of neuronal migration and positioning by Reelin-DAB1 signaling pathways has unique dependencies on Dab1 gene dosage in different brain regions.

Erratic and blood vessel-guided migration of astrocyte progenitors in the cerebral cortex.

Astrocytes are one of the most abundant cell types in the mammalian brain. They play essential roles in synapse formation, maturation, and elimination. However, how astrocytes migrate into the gray matter to accomplish these processes is poorly understood. Here, we show that, by combinational analyses of in vitro and in vivo time-lapse observations and lineage traces, astrocyte progenitors move rapidly and irregularly within the developing cortex, which we call erratic migration. Astrocyte progenitors also adopt blood vessel-guided migration. These highly motile progenitors are generated in the restricted prenatal stages and differentiate into protoplasmic astrocytes in the gray matter, whereas postnatally generated progenitors do not move extensively and differentiate into fibrous astrocytes in the white matter. We found Cxcr4/7, and integrin ß1 regulate the blood vessel-guided migration, and their functional blocking disrupts their positioning. This study provides insight into astrocyte development and may contribute to understanding the pathogenesis caused by their defects.

Dab1-deficient deep layer neurons prevent Dab1-deficient superficial layer neurons from entering the cortical plate.

The mammalian neocortex has a 6-layered cytoarchitecture, where early- and late-born neurons are positioned deeply and superficially, respectively. Inverted lamination has been observed in mice defective in the Reelin/Disabled-1 (Dab1) pathway. Considering that Dab1-deficient superficial layer neurons can migrate into the Dab1 +/+ cortical plate and that Dab1 is thought to function cell-autonomously, it is unclear why superficial layer neurons are positioned below deep layer neurons in Reelin/Dab1-deficient mice. Here, we reconfirmed that Dab1 -/- superficial layer neurons enter the cortical plate using in utero electroporation on embryonic day (E) 14.5 Dab1-floxed mice. Electroporation in E12.5 Dab1-floxed mice reconfirmed that many deep layer neurons were mispositioned below the subplate. We also found an accumulation of Dab1-deficient superficial layer neurons below the cortical plate in many of these brains, in which deep layer neurons below the subplate showed high cell density. These phenotypes were rescued by decreasing the knockout probability and by expressing Dab1 in deep layer neurons. These observations suggest that cell-dense Dab1 -/- deep layer neurons prevent Dab1 -/- superficial layer neurons from entering the cortical plate. This reflects a non-cell-autonomous function of Dab1 and may explain the preplate splitting failure and outside-in lamination observed in Reelin/Dab1-deficient mice.

Reelin-Nrp1 Interaction Regulates Neocortical Dendrite Development in a Context-Specific Manner.

Reelin plays versatile roles in neocortical development. The C-terminal region (CTR) of Reelin is required for the correct formation of the superficial structure of the neocortex; however, the mechanisms by which this position-specific effect occurs remain largely unknown. In this study, we demonstrate that Reelin with an intact CTR binds to neuropilin-1 (Nrp1), a transmembrane protein. Both male and female mice were used. Nrp1 is localized with very-low-density lipoprotein receptor (VLDLR), a canonical Reelin receptor, in the superficial layers of the developing neocortex. It forms a complex with VLDLR, and this interaction is modulated by the alternative splicing of VLDLR. Reelin with an intact CTR binds more strongly to the VLDLR/Nrp1 complex than to VLDLR alone. Knockdown of Nrp1 in neurons leads to the accumulation of Dab1 protein. Since the degradation of Dab1 is induced by Reelin signaling, it is suggested that Nrp1 augments Reelin signaling. The interaction between Reelin and Nrp1 is required for normal dendritic development in superficial-layer neurons. All of these characteristics of Reelin are abrogated by proteolytic processing of the six C-terminal amino acid residues of Reelin (0.17% of the whole protein). Therefore, Nrp1 is a coreceptor molecule for Reelin and, together with the proteolytic processing of Reelin, can account for context-specific Reelin function in brain development.SIGNIFICANCE STATEMENT Reelin often exhibits a context-dependent function during brain development; however, its underlying mechanism is not well understood. We found that neuropilin-1 (Nrp1) specifically binds to the CTR of Reelin and acts as a coreceptor for very-low-density lipoprotein receptor (VLDLR). The Nrp1/VLDLR complex is localized in the superficial layers of the neocortex, and its interaction with Reelin is essential for proper dendritic development in superficial-layer neurons. This study provides the first mechanistic evidence of the context-specific function of Reelin (>3400 residues) regulated by the C-terminal residues and Nrp1, a component of the canonical Reelin receptor complex.

Neuron-derived VEGF contributes to cortical and hippocampal development independently of VEGFR1/2-mediated neurotrophism.

Vascular endothelial growth factor (VEGF) is a potent mitogen critical for angiogenesis and organogenesis. Deletion or inhibition of VEGF during development not only profoundly suppresses vascular outgrowth, but significantly affects the development and function of various organs. In the brain, VEGF is thought to not only promote vascular growth, but also directly act on neurons as a neurotrophic factor by activating VEGF receptors. In the present study, we demonstrated that deletion of VEGF using hGfap-Cre line, which recombines genes specifically in cortical and hippocampal neurons, severely impaired brain organization and vascularization of these regions. The mutant mice had motor deficits, with lethality around the time of weaning. Multiple reporter lines indicated that VEGF was highly expressed in neurons, but that its cognate receptors, VEGFR1 and 2 were exclusive to endothelial cells in the brain. In accordance, mice lacking neuronal VEGFR1 and VEGFR2 did not exhibit neuronal deformities or lethality. Taken together, our data suggest that neuron-derived VEGF contributes to cortical and hippocampal development likely through angiogenesis independently of direct neurotrophic effects mediated by VEGFR1 and 2.

Proper Level of Cytosolic Disabled-1, Which Is Regulated by Dual Nuclear Translocation Pathways, Is Important for Cortical Neuronal Migration.

Disabled-1 (Dab1) is an essential intracellular protein in the Reelin pathway. It has a nuclear localization signal (NLS; hereafter referred to as "NLS1") and 2 nuclear export signals, and shuttles between the nucleus and the cytoplasm. In this study, we found that Dab1 has an additional unidentified NLS, and that the Dab1 NLS1 mutant could translocate to the nucleus in an unconventional ATP/temperature-dependent and cytoplasmic factor/RanGTP gradient-independent manner. Additional mutations in the NLS1 mutant revealed that K(67) and K(69) are important for the nuclear transport. Furthermore, an excess of the intracellular domain of the Reelin receptors inhibited the nuclear translocation of Dab1. An in utero electroporation study showed that a large amount of Dab1 in the cytoplasm in migrating neurons inhibited the migration, and that forced transport of Dab1 into the nucleus attenuated this inhibitory effect. In addition, rescue experiments using yotari, an autosomal recessive mutant of dab1, revealed that cells expressing Dab1 NLS1 mutant tend to distribute at more superficial positions than those expressing wild-type Dab1. Taken together, these findings suggest that Dab1 has at least 2 NLSs, and that the regulation of the subcellular localization of Dab1 is important for the proper migration of excitatory neurons.

The COUP-TFII/Neuropilin-2 is a molecular switch steering diencephalon-derived GABAergic neurons in the developing mouse brain.

The preoptic area (POa) of the rostral diencephalon supplies the neocortex and the amygdala with GABAergic neurons in the developing mouse brain. However, the molecular mechanisms that determine the pathway and destinations of POa-derived neurons have not yet been identified. Here we show that Chicken ovalbumin upstream promoter transcription factor II (COUP-TFII)-induced expression of Neuropilin-2 (Nrp2) and its down-regulation control the destination of POa-derived GABAergic neurons. Initially, a majority of the POa-derived migrating neurons express COUP-TFII and form a caudal migratory stream toward the caudal subpallium. When a subpopulation of cells steers toward the neocortex, they exhibit decreased expression of COUP-TFII and Nrp2. The present findings show that suppression of COUP-TFII/Nrp2 changed the destination of the cells into the neocortex, whereas overexpression of COUP-TFII/Nrp2 caused cells to end up in the medial part of the amygdala. Taken together, these results reveal that COUP-TFII/Nrp2 is a molecular switch determining the pathway and destination of migrating GABAergic neurons born in the POa.

Importance of Reelin C-terminal region in the development and maintenance of the postnatal cerebral cortex and its regulation by specific proteolysis.

During brain development, Reelin exerts a variety of effects in a context-dependent manner, whereas its underlying molecular mechanisms remain poorly understood. We previously showed that the C-terminal region (CTR) of Reelin is required for efficient induction of phosphorylation of Dab1, an essential adaptor protein for canonical Reelin signaling. However, the physiological significance of the Reelin CTR in vivo remains unexplored. To dissect out Reelin functions, we made a knock-in (KI) mouse in which the Reelin CTR is deleted. The amount of Dab1, an indication of canonical Reelin signaling strength, is increased in the KI mouse, indicating that the CTR is necessary for efficient induction of Dab1 phosphorylation in vivo. Formation of layer structures during embryonic development is normal in the KI mouse. Intriguingly, the marginal zone (MZ) of the cerebral cortex becomes narrower at postnatal stages because upper-layer neurons invade the MZ and their apical dendrites are misoriented and poorly branched. Furthermore, Reelin undergoes proteolytic cleavage by proprotein convertases at a site located 6 residues from the C terminus, and it was suggested that this cleavage abrogates the Reelin binding to the neuronal cell membrane. Results from ectopic expression of mutant Reelin proteins in utero suggest that the dendrite development and maintenance of the MZ require Reelin protein with an intact CTR. These results provide a novel model regarding Reelin functions involving its CTR, which is not required for neuronal migration during embryonic stages but is required for the development and maintenance of the MZ in the postnatal cerebral cortex.

Reelin receptors ApoER2 and VLDLR are expressed in distinct spatiotemporal patterns in developing mouse cerebral cortex.

In mammalian developing brain, neuronal migration is regulated by a variety of signaling cascades, including Reelin signaling. Reelin is a glycoprotein that is mainly secreted by Cajal-Retzius neurons in the marginal zone, playing essential roles in the formation of the layered neocortex via its receptors, apolipoprotein E receptor 2 (ApoER2) and very low density lipoprotein receptor (VLDLR). However, the precise mechanisms by which Reelin signaling controls the neuronal migration process remain unclear. To gain insight into how Reelin signaling controls individual migrating neurons, we generated monoclonal antibodies against ApoER2 and VLDLR and examined the localization of Reelin receptors in the developing mouse cerebral cortex. Immunohistochemical analyses revealed that VLDLR is localized to the distal portion of leading processes in the marginal zone (MZ), whereas ApoER2 is mainly localized to neuronal processes and the cell membranes of multipolar cells in the multipolar cell accumulation zone (MAZ). These different expression patterns may contribute to the distinct actions of Reelin on migrating neurons during both the early and late migratory stages in the developing cerebral cortex.

Characterization of the dipeptide repeat protein in the molecular pathogenesis of c9FTD/ALS.

The expansion of the GGGGCC hexanucleotide repeat in the non-coding region of the chromosome 9 open-reading frame 72 (C9orf72) gene is the most common cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) (c9FTD/ALS). Recently, it was reported that an unconventional mechanism of repeat-associated non-ATG (RAN) translation arises from C9orf72 expansion. Sense and anti-sense transcripts of the expanded C9orf72 repeat, i.e. the dipeptide repeat protein (DRP) of glycine-alanine (poly-GA), glycine-proline (poly-GP), glycine-arginine (poly-GR), proline-arginine (poly-PR) and proline-alanine (poly-PA), are deposited in the brains of patients with c9FTD/ALS. However, the pathological significance of RAN-translated peptides remains unknown. We generated synthetic cDNAs encoding 100 repeats of DRP without a GGGGCC repeat and evaluated the effects of these proteins on cultured cells and cortical neurons in vivo. Our results revealed that the poly-GA protein formed highly aggregated ubiquitin/p62-positive inclusion bodies in neuronal cells. In contrast, the highly basic proteins poly-GR and PR also formed unique ubiquitin/p62-negative cytoplasmic inclusions, which co-localized with the components of RNA granules. The evaluation of cytotoxicity revealed that overexpressed poly-GA, poly-GP and poly-GR increased the substrates of the ubiquitin-proteasome system (UPS), including TDP-43, and enhanced the sensitivity to a proteasome inhibitor, indicating that these DRPs are cytotoxic, possibly via UPS dysfunction. The present data indicate that a gain-of-function mechanism of toxic DRPs possibly contributes to pathogenesis in c9FTD/ALS and that DRPs may serve as novel therapeutic targets in c9FTD/ALS.

Reelin controls neuronal positioning by promoting cell-matrix adhesion via inside-out activation of integrin α5ß1.

Birthdate-dependent neuronal layering is fundamental to neocortical functions. The extracellular protein Reelin is essential for the establishment of the eventual neuronal alignments. Although this Reelin-dependent neuronal layering is mainly established by the final neuronal migration step called "terminal translocation" beneath the marginal zone (MZ), the molecular mechanism underlying the control by Reelin of terminal translocation and layer formation is largely unknown. Here, we show that after Reelin binds to its receptors, it activates integrin α5ß1 through the intracellular Dab1-Crk/CrkL-C3G-Rap1 pathway. This intracellular pathway is required for terminal translocation and the activation of Reelin signaling promotes neuronal adhesion to fibronectin through integrin α5ß1. Since fibronectin is localized in the MZ, the activated integrin α5ß1 then controls terminal translocation, which mediates proper neuronal alignments in the mature cortex. These data indicate that Reelin-dependent activation of neuronal adhesion to the extracellular matrix is crucial for the eventual birth-date-dependent layering of the neocortex.

Leucine-rich glioma inactivated 1 (Lgi1), an epilepsy-related secreted protein, has a nuclear localization signal and localizes to both the cytoplasm and the nucleus of the caudal ganglionic eminence neurons.

Leucine-rich glioma inactivated 1 (Lgi1) is a secreted synaptic protein that organizes a transsynaptic protein complex throughout the brain. Mutations in the Lgi1 gene have been found in patients with autosomal dominant lateral temporal lobe epilepsy (ADLTE). Although a large number of studies have focused on the expression and function of Lgi1 in the postnatal brain, information regarding its functions and distribution during development remains sparse. Here we report that Lgi1 mRNA is preferentially expressed in the caudal ganglionic eminence (CGE) of the early embryonic telencephalon, and LGI1 protein is unexpectedly localized in the nucleus of dissociated CGE neurons. Using bioinformatics analysis, we found that LGI1 contains a putative nuclear localization signal (NLS) in its leucine-rich repeat C-terminal domain. Furthermore, we show that the transient expression of Lgi1 in CGE neurons resulted in nuclear translocation of the LGI1 protein, and a mutation in the NLS led to the retention of LGI1 in the cytoplasm. We also confirmed that the NLS sequence of LGI1 had the ability to mediate the nuclear localization by using the NLS-containing fusion protein. Interestingly, when Lgi1 was expressed in neurons obtained from the medial ganglionic eminence or cerebral cortex, almost no nuclear localization of LGI1 was observed. These results raise the possibility of a novel role of Lgi1 within embryonic neurons through nuclear translocation and may provide insight into its potential effects on the development of the central nervous system and ADLTE pathogenesis.

The outermost region of the developing cortical plate is crucial for both the switch of the radial migration mode and the Dab1-dependent "inside-out" lamination in the neocortex.

Mammalian neocortex has a laminated structure that develops in a birth-date-dependent "inside-out" pattern. This layered structure is established by neuronal migration with sequential changes of the migratory mode regulated by several signaling cascades, including the Reelin-Disabled homolog 1 (Dab1) pathway. Although the importance of "locomotion," the major migratory mode, has been well established, the physiological significance of the mode change from locomotion to "terminal translocation," the final migratory mode, is unknown. In this study, we found that the outermost region of the mouse cortical plate has several histologically distinct features and named this region the primitive cortical zone (PCZ). Time-lapse analyses revealed that "locomoting" neurons paused transiently just beneath the PCZ before migrating into it by "terminal translocation." Furthermore, whereas Dab1-knockdown (KD) neurons could reach beneath the PCZ, they failed to enter the PCZ, suggesting that the Dab1-dependent terminal translocation is necessary for entry of the neurons into the PCZ. Importantly, sequential in utero electroporation experiments directly revealed that failure of the Dab1-dependent terminal translocation resulted in disruption of the inside-out alignment within the PCZ and that this disrupted pattern was still preserved in the mature cortex. Conversely, Dab1-KD locomoting neurons could pass by both wild-type and Dab1-KD predecessors beneath the PCZ. Our data indicate that the PCZ is a unique environment, passage of neurons through which involves molecularly and behaviorally different migratory mechanisms, and that the migratory mode change from locomotion to terminal translocation just beneath the PCZ is critical for the Dab1-dependent inside-out lamination in the mature cortex.

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.

Ectopic Reelin induces neuronal aggregation with a normal birthdate-dependent "inside-out" alignment in the developing neocortex.

Neurons in the developing mammalian neocortex form the cortical plate (CP) in an "inside-out" manner; that is, earlier-born neurons form the deeper layers, whereas later-born neurons migrate past the existing layers and form the more superficial layers. Reelin, a glycoprotein secreted by Cajal-Retzius neurons in the marginal zone (MZ), is crucial for this "inside-out" layering, because the layers are inverted in the Reelin-deficient mouse, reeler (Reln(rl)). Even though more than a decade has passed since the discovery of reelin, the biological effect of Reelin on individual migrating neurons remains unclear. In addition, although the MZ is missing in the reeler cortex, it is unknown whether Reelin directly regulates the development of the cell-body-sparse MZ. To address these issues, we expressed Reelin ectopically in the developing mouse cortex, and the results showed that Reelin caused the leading processes of migrating neurons to assemble in the Reelin-rich region, which in turn induced their cell bodies to form cellular aggregates around Reelin. Interestingly, the ectopic Reelin-rich region became cell-body-sparse and dendrite-rich, resembling the MZ, and the late-born neurons migrated past their predecessors toward the central Reelin-rich region within the aggregates, resulting in a birthdate-dependent "inside-out" alignment even ectopically. Reelin receptors and intracellular adaptor protein Dab1 were found to be necessary for formation of the aggregates. The above findings indicate that Reelin signaling is capable of inducing the formation of the dendrite-rich, cell-body-sparse MZ and a birthdate-dependent "inside-out" alignment of neurons independently of other factors/structures near the MZ.

The transcriptional repressor RP58 is crucial for cell-division patterning and neuronal survival in the developing cortex.

The neocortex and the hippocampus comprise several specific layers containing distinct neurons that originate from progenitors at specific development times, under the control of an adequate cell-division patterning mechanism. Although many molecules are known to regulate this cell-division patterning process, its details are not well understood. Here, we show that, in the developing cerebral cortex, the RP58 transcription repressor protein was expressed both in postmitotic glutamatergic projection neurons and in their progenitor cells, but not in GABAergic interneurons. Targeted deletion of the RP58 gene led to dysplasia of the neocortex and of the hippocampus, reduction of the number of mature cortical neurons, and defects of laminar organization, which reflect abnormal neuronal migration within the cortical plate. We demonstrate an impairment of the cell-division patterning during the late embryonic stage and an enhancement of apoptosis of the postmitotic neurons in the RP58-deficient cortex. These results suggest that RP58 controls cell division of progenitor cells and regulates the survival of postmitotic cortical neurons.

Dynactin is essential for growth cone advance.

Dynactin is a multi-subunit complex that serves as a critical cofactor of the microtubule motor cytoplasmic dynein. We previously identified dynactin in the nerve growth cone. However, the function of dynactin in the growth cone is still unclear. Here we show that dynactin in the growth cone is required for constant forward movement of the growth cone. Chromophore-assisted laser inactivation (CALI) of dynamitin, a dynactin subunit, within the growth cone markedly decreases the rate of growth cone advance. CALI of dynamitin in vitro dissociates another dynactin subunit, p150(Glued), from dynamitin. These results indicate that dynactin, especially the interaction between dynamitin and p150(Glued), plays an essential role in growth cone advance.

Mouse Disabled1 (DAB1) is a nucleocytoplasmic shuttling protein.

Disabled1 (DAB1) is an intracellular mediator of the Reelin-signaling pathway and essential for correct neuronal positioning during brain development. So far, DAB1 has been considered a cytoplasmic protein. Here, we show that DAB1 is subject to nucleocytoplasmic shuttling. In its steady state, DAB1 is mainly located in the cytoplasm. However, treatment with leptomycine B, a specific inhibitor of the CRM1 (chromosomal region maintenance 1)-RanGTP-dependent nuclear export, resulted in nuclear accumulation of DAB1. By using deletion or substitutional mutants of DAB1 fused with enhanced green fluorescent protein, we have mapped a bipartite nuclear localization signal and two CRM1-dependent nuclear export signals. These targeting signals were functional in both Neuro2a cells and primary cerebral cortical neurons. Using purified recombinant proteins, we have shown that CRM1 binds to DAB1 directly in a RanGTP-dependent manner. We also show that tyrosine phosphorylation of DAB1, which is indispensable for the layer formation of the brain, by Fyn tyrosine kinase or Reelin stimulation did not affect the subcellular localization of DAB1 in vitro. These results suggest that DAB1 is a nucleocytoplasmic shuttling protein and raise the possibility that DAB1 plays a role in the nucleus as well as in the cytoplasm.

Cellular and molecular mechanisms of neuronal migration in neocortical development.

The complicated mammalian brain structure arises from accurate movements of neurons from their birthplace to their final locations. Detailed observation of this migration process by various methods revealed that neuronal migration is highly motile and that there are different modes of migration. Moreover, mouse mutants or human disorders that disrupt normal migration have provided significant insights into molecular pathways that control the neuronal migration. Although our knowledge is still fragmentary, it is becoming clear that various molecules are participating in this process. In this review, we outline about the cellular and molecular mechanisms of neuronal migration in the cerebral cortex.

Developmental distribution of coxsackie virus and adenovirus receptor localized in the nervous system.

Mouse coxsackie virus and adenovirus receptor (mCAR), which was isolated from the nerve growth cone-enriched fraction of newborn mouse brains, is a member of immunoglobulin-super family, and functions as a homophilic adhesion molecule. We observed the expression of mCAR in embryos to adult tissues by means of immunohistochemical analysis with a peptide antibody. mCAR expression was first detected in the embryonic ectoderm in the uterus on embryonic day 6.5 (E6.5). Then it was strongly expressed in the neuroepithelium of the neural tube, the developing brain and the spinal cord from E8.5 to postnatal day 7 (P7), in the cranial motor nerves from E9.5 to E11.5, and in the optic nerve from E13.5 to P7, which agrees with periods of their respective morphogenetic peaks. This expression of mCAR decreased postnatally and was absent in adult tissues. We found that mCAR occurred in a few proliferating cells of the hippocampal dentate gyrus, the subventricular zone (SVZ) of the lateral ventricles, and the rostral migratory stream (RMS) over P21. These observations demonstrate that mCAR was expressed characteristically in the immature neuroepithelium including progenitor cells or radial cells derived from the neural tube and in immature cells in a selected germinal zone of the mature brain. Based on our findings, we propose that mCAR is involved in migration and fasciculation during a restricted period as an adhesion molecule.